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Epigrammatic progress and perspective on the photocatalytic properties of BiVO4-based photocatalyst in photocatalytic water treatment technology: A review
Accepted Manuscript Epigrammatic progress and perspective on the photocatalytic properties of BiVO4-based photocatalyst in photocatalytic water treatment technology: A review
Mohamad Fakhrul Ridhwan Samsudin, Suriati Sufian, B.H. Hameed PII: DOI: Reference:
S0167-7322(18)31127-9 doi:10.1016/j.molliq.2018.07.051 MOLLIQ 9372
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
5 March 2018 4 July 2018 13 July 2018
Please cite this article as: Mohamad Fakhrul Ridhwan Samsudin, Suriati Sufian, B.H. Hameed , Epigrammatic progress and perspective on the photocatalytic properties of BiVO4-based photocatalyst in photocatalytic water treatment technology: A review. Molliq (2018), doi:10.1016/j.molliq.2018.07.051
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ACCEPTED MANUSCRIPT Epigrammatic Progress and Perspective on the Photocatalytic Properties of BiVO4-based Photocatalyst in Photocatalytic Water Treatment Technology: A Review Mohamad Fakhrul Ridhwan Samsudina, Suriati Sufiana,*, B. H. Hameedb Chemical Engineering Department, Universiti Teknologi PETRONAS, 32610, Bandar Seri
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Iskandar, Perak, Malaysia.
School of Chemical Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal,
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Penang, Malaysia.
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Corresponding author: [email protected]
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Abstract In recent years, Advanced Oxidation Processes (AOPs) via photocatalyst has shown great promises as a new frontier environmental friendly and sustainable wastewater treatment
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technology in replacing the current conventional techniques. At present, bismuth vanadate
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(BiVO4) has garnered much attention as one of the promising photocatalysts in environmental
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remediation applications under visible light irradiation. This is due to intrinsic physical and chemical features of BiVO4; appealing electronic band structure, high physicochemical stability
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and environmental friendliness. This review aims to provide an insight into the most recent
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progress in synthesis, properties and photocatalytic applications of BiVO4, particularly in wastewater treatment. Special emphasis is also placed on the discussion on the major limitations
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exists within the BiVO4 photocatalyst such as poor charge carrier transfer, slow water oxidation
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kinetics and fast recombination rate. Subsequently, an insight into the most recent strategies to circumvent the major limitations is detailed. Finally, invigorating perspectives and future
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directions of this BiVO4 photocatalyst will be presented. It is anticipated that this review will
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deliver a scientific and technical overview to the researchers who work in this field to facilitate the next generation of BiVO4 photocatalyst with profound performances.
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Keyword: Bismuth vanadate, photocatalysis, advanced oxidation processes, wastewater treatment.
1 Introduction In the recent years, environmental pollutions such as global warming and water contamination due to the excessive consumption of fossil fuels and release of toxic and harmful 2
ACCEPTED MANUSCRIPT chemicals, such as dyes [1], heavy metals [2], ionic liquid [3], pesticides and surfactants [4] owing to the rapid development of industrialization has been recognized as one of the biggest threats facing the society. For example, the excessive release of the carcinogenic chemicals may cause severe effects not only on aquatic organisms but also to the human being. According to the
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Malato et al., [5] approximately around millions of people globally reported to die due to the
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serious waterborne diseases owing to the consumption of polluted water and 2.6 billion of people
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has no access to a clean water source. Moreover, these unfortunate phenomena may continue to
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grow if there is no solution being employed in solving this crisis.
These environmental problems breeze in as the presence of the organic pollutant is not fully
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degraded by the conventional methods [6–8]. Chlorination, bio-flocculation, and membrane
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technologies [9–12] are some of the many conventional solutions that are currently being utilized. However, these methods have several immedicable issues such as multiple stage
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processes, expensive and incomplete treatment which lead to deprived of the overall
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performance [13,14]. For an example, flocculation and coagulation have been one of the most common wastewater technology used in many industries [15]. In this technology, chemical
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flocculants or coagulants such as inorganic metal salts are added during the treatment, resulting
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in the increasing of the concentration of metal ion in the treated water [16]. The increasing of metal ion concentration in the treated water can cause severe harm to the human health. Moreover, the major limitation of the coagulation technology is its low performance in treating a very fine particle presence in the wastewater and the efficiency of the coagulation treatment significantly deteriorate in the presence of the cold water [17,18]. On the other hand, membrane technology has been considered as one of the promising emerging technology which is expected to overcome the current limitations facing in the wastewater treatment. Albeit the higher 3
ACCEPTED MANUSCRIPT chemical oxygen demand (COD) removal efficiency performance obtained via membrane technology, the applicability of this emerging technology is still being confined to its membrane fouling issues [16]. To date, advanced oxidation processes (AOPs) has garnered more attention in providing an
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alternative for the current limitation facing in the conventional wastewater treatment. The AOPs
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has been denoted as a current innovative water treatment technologies with intrinsic features such as environmental friendliness [19], economically feasible [20] and simple processes [21]. In
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this system, the in-situ generation of highly reactive oxidant species is capable of fully treating and removing most of the presence of the organic pollutants in the wastewater [22–24]. These
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reactive oxidant species, commonly superoxide (•O2¯) and superhydroxyl (•OH) radicals are
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known as one of the strongest radicals after fluorine owing to its high standard reduction potential of E° (•OH/H2O) = 2.80 V standard hydrogen electrode (SHE) [25]. Moreover, the
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during the AOPs treatment.
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short lifetime of these generated radicals within nanoseconds allows it to be self-eliminated
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Generally, there are four different type of AOPs can be employed which are photocatalysis, ozone treatment, direct decomposition of water and electrochemical processes [13].
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Photocatalysis via photocatalyst materials uses light energy to generate electron-hole pairs which consequently produce highly reactive oxidant species for treating the recalcitrant pollutant [25]. Meanwhile, ozone treatment is a process in which ozone gas is introduced into the wastewater. The ozone gas will directly or indirectly interact with the presence of the compounds in the wastewater, resulting in the decomposition reaction [22]. On the other hand, Fenton oxidation process uses a strong chemical such as hydrogen peroxide (H2O2) and Iron (II) (Fe2+) in 4
ACCEPTED MANUSCRIPT generating the reactive oxidant species. However, the use of H2O2 in the treatment cause a setback in AOPs performance as Fenton oxidation process cannot be operated in an alkaline condition [26]. Among these AOPs, photocatalysis technology has intrigued much attention due to its relative ease of process and capability of treating the recalcitrant pollutant without
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producing any secondary harmful by-product [27].
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For the past decades, titanium dioxide (TiO2) is the most frequent photocatalyst used owing to its outstanding chemical and physical stability, economically viable, non-toxicity and
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abundance in comparison to others type of photocatalysts such as zinc oxide (ZnO) and copper (II) oxide (CuO) [2,28–30]. In lieu of TiO2 good effectuation, TiO2 has a relatively large band
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gap energy of 3.2 eV, consequently abnegate its performance in harnessing more visible light
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energy that occupies the astronomical part of the solar light [3,31]. Moreover, the fast recombination rates within 10 nanoseconds between the photogenerated electron-hole pairs of
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TiO2, abstain its photocatalytic degradation performance [31]. Hence, many efforts have been
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devoted to excavating a new type of photocatalysts that are capable of harnessing the visible light energy such as zinc sulphide (ZnS), cadmium sulphide (CdS), tungsten oxide (WO3) and
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BiVO4 [3,20,31–44].
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Among these photocatalysts, bismuth vanadate (BiVO4) have garnered tremendous scientific interest as a frontier photocatalyst in AOPs technologies [45–56]. BiVO4 is a non-toxic metal free photocatalyst that possesses outstanding chemical and photonic properties [56,57]. In addition, the lower band gap energy of ~ 2.4 eV allows BiVO4 to correspond to the visible light spectrum which accounts up to 48% of total solar light energy [20]. Additionally, the presence of O 2p and Bi 6s hybrid orbitals in the electronic structure of BiVO4 in comparison to TiO2 which 5
ACCEPTED MANUSCRIPT solely made up of O 2p orbitals helped the BiVO4 to be able to absorb visible light energy by decreasing the band gap energy [58]. Nevertheless, the photocatalytic degradation performance of BiVO4 is substantially low owing to the poor electron-hole pair transport and mobility, and slow water oxidation kinetics [59,60]. Given of the aforementioned issues, various strategies
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have been adopted such as the development of the heterostructure system [61,62], coupling with
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carbonaceous materials [8,51,63], introducing electron-hole scavengers [21,53,64], and
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additional of bias energy [49,65] show promises to enhance the efficiency of BiVO4.
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In recognition of the great potential of BiVO4 as a frontier visible light driven photocatalyst, the work accounted here is aiming to give an overview of the recent related studies on the
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development of the BiVO4-based photocatalyst in photocatalytic wastewater treatment
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application. The basic principle and mechanism of photocatalyst system are detailed to promote a better understanding of the photocatalyst specifically on BiVO4. Next, the topical reported
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literature on the photocatalytic performance of BiVO4-based photocatalyst is summarized.
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Afterwards, the limiting factors affecting the photocatalytic performance of BiVO4 and the current strategies to alleviate the aforementioned limiting factors are thoroughly reviewed.
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Subsequently, current challenge on the photoactivity of BiVO4 and future directions of BiVO4
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are included in this review.
2 General Overview on BiVO4 Photocatalyst Initially, the use of photocatalyst as an environmental sound solution has been extensively investigated since the first idea that was triggered by Honda and Fujishima [66]. They had successfully demonstrated the overall water splitting process using photoelectrochemical water 6
ACCEPTED MANUSCRIPT splitting technique employing rutile-TiO2 and Pt cathode under ultraviolet (UV) radiation along with the presence of external bias. However, the photocatalytic effectiveness of TiO2 photocatalyst remains a boundless challenge in this field as it can only be excited by UV irradiation which accounts up to 4% of solar light energy. TiO2 has a wide band gap which is
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approximate ~3.2 eV, thus limiting the practical application of TiO2 in sunlight [30]. Henceforth,
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the development of an efficient visible-light-driven photocatalyst has been a topic of focus in
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photocatalysis.
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Up to date, various type of visible-light-driven photocatalyst has been discovered and developed as an alternative for conventional TiO2 photocatalyst such as iron oxide (Fe2O3) [67–
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72], BiVO4 [34,56,57,73], WO3 [74–78], ZnO [79–82], CuO [83–85] and graphitic carbon nitride
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(g-C3N4) [86–89] photocatalyst. Figure 1 depicts the currently reported photocatalyst with their band gap energy. Most of the reported photocatalysts in Figure 1 have a thermodynamically
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favorable of valence band for the water oxidation reactions to occur, resulting in a more available
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electron-hole pair for producing •O2¯ and •OH radicals. Having an appropriate band gap energy which is favorable for producing •O2¯ and •OH radicals is a significant factor in developing
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highly functionalized photocatalysts for photocatalytic degradation application. Among these,
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BiVO4 is a promising candidate that struck a balance between various intrinsic features, due to its suitable band gap, proper band location, great stability and environment friendliness.
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Synthesis of BiVO4 Photocatalysts
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Figure 1: Summary of the current reported photocatalyst with its band gap energy.
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There are several representative synthesis methods that have been widely used for the
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production of BiVO4 ranging from a microwave-assisted method, hydrothermal-assisted method, solid-liquid state reaction, a sol-gel method, co-precipitation method and solution combustion
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method. The microwave-assisted method is generally known as a method that escalates the
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reaction rates and often occurs at a lower temperature compared to other conventional heating methods [90]. In microwave-assisted approach, the photocatalyst materials can be uniformly heated resulting in a homogeneous nucleation and thus produce the smaller size of photocatalyst structure which is highly pure and greater yield of productions [91]. In 2008, Ming et al. [90] reported the successful fabrication of BiVO4 via this method in which highly crystalline phase was converted irreversibly from tetragonal to monoclinic phase. The sodium metavanadate (NaVO3) and bismuth (III) nitrate pentahydrate (Bi (NO3)3.5H2O) were used as starting 8
ACCEPTED MANUSCRIPT precursors with the addition of cetyl trimethyl ammonium bromide (CTAB) as a shape controller. However, the addition of CTAB leads to some drawbacks in term of complex process and presence of heterogeneous impurities as a by-product. With regard to this issue, Weidong et al. [92] demonstrated the successful fabrication of
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monoclinic BiVO4 via a facile organic-free microwave-assisted method in which Bi
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(NO3)3.5H2O and ammonium metavanadate (NH4VO3) were used as starting materials. Similarly, Abraham et al. [91] reported the development of BiVO4 photocatalyst via this
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approach which utilized the same starting materials dissolved in a mixture of diethylene glycol and deionized water. The XRD analysis verified the sharp peaks splitting observed indicates a
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good crystalline structure was developed and can be ascribed to monoclinic structure (JCPDS
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No. 75-1866). Recently, Liu et al. [93] also reported the fabrication of monoclinic BiVO4 via the microwave-assisted method with an addition of PEG 10000 materials as a morphology-directing
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agent which can confine the crystal growth in certain directions. Along with an additional of
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hydrothermal reaction time, the solid BiVO4 sheets were observed to be in sandwich-like sheets
activities.
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that have a large BET surface area which is beneficial to the enhancement of photocatalytic
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On the other hand, hydrothermal synthesis method has been widely used for synthesizing BiVO4 photocatalyst owing to its simple process which can produce a perfect crystal structure at a relatively low temperature [94–96]. The hydrothermal process uses aqueous and/or nonaqueous systems as the reaction medium and promotes environmental friendly procedure as the reactions are carried out in a closed system. Generally, there are three variations on preparing BiVO4 via this method, by adding (i) organic and inorganic additives, or (ii) surfactants and (iii) 9
ACCEPTED MANUSCRIPT without adding any additives. Malathi et al. [97] successfully synthesized BiVO4 nanoparticles via the additive-free hydrothermal process in which Bi (NO3)3.5H2O and NH4VO3 were mixed under continuous agitation before heated at a temperature of 180 °C for 12 hours. The XRD peaks signify the prepared BiVO4 was in a monoclinic structure (JCPDS No. 83-1699) with a
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band gap energy of 2.33 eV as calculated from a Tauc plot graph. Similarly, Wang et al. [98]
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using the same initial precursors which successfully developed monoclinic BiVO4, evidently
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from the peak splitting observed in XRD analysis are in a good agreement with the standard
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monoclinic phase (JCPDS No. 14-0688).
On the other hand, Lu et al. [99] prepared different morphological structure of BiVO4 via a
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facile one-step hydrothermal method with the addition of sodium citrate as a chelating agent.
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Likewise, Thalluri et al. [100] using ammonium carbonate a structural directing agent for growing the selectivity of {040} crystal facet of monoclinic BiVO4. According to their group,
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the addition of this structural directing agent has a significant impact on growing the {040}
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crystal planes, evidently from the increase in the XRD peak intensities. The selectivity growth of {040} crystal planes is known to have positive impacts towards the photocatalytic water
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oxidation activities. Besides that, Jian et al. [101] reported the successful fabrication of BiVO4
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via hybridizing the reverse microemulsion and hydrothermal methods. In addition, the BiVO4 photocatalyst can also be prepared via the one-pot hydrothermal method as reported by Guang et al. [102]. Their group claimed that this approached can easily tailor the morphological structure of as-synthesized photocatalyst into 3D four-petaled flower-like, cube-like and rod-like structure by varying the amount of Br:V molar ratios.
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ACCEPTED MANUSCRIPT Besides that, solid-liquid state reaction has been reported as one of the synthesizing methods for preparing BiVO4. The problem existed within the conventional synthesizing method of BiVO4 is the undesirable by-products such as nitric acid. With regard to this problem, Iwase et al. [103] proposed an environmental processing approach by utilizing oxide starting materials. In
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this approach, bismuth (III) oxide (Bi2O3) and vanadium pentoxide (V2O5) were used as starting
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precursors and mixed with different concentrations of aqueous nitric acid solution. Recently, Tan
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et al. and Samsudin et al. [59,104–106] also prepared the monoclinic BiVO4 photocatalyst using the same methods. Apart from these, sol-gel method also one of the common method for
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preparing the BiVO4 photocatalyst [107]. The advantages of using this method are owing to its
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relatively low production cost high reliability, the simplicity of process and ease of morphological control [108]. Moreover, co-precipitation [109–111] and solution combustion
Properties of BiVO4 Photocatalysts
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2.2
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synthesis are one of the fabrication technique that has been reported previously [99,112–114].
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Initially, BiVO4-based materials have been extensively explored as a promising alternative for toxic-pigments based materials used in the coating and plastic industry [115]. However, for
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the past few decades, the exploration of this promising materials has been shifted to the applicability of BiVO4 in photocatalytic application owing to its intrinsic crystal structure [116]. Naturally, the crystallographic structure of BiVO4 exists in an orthorhombic structure originate from mineral pucherite [116]. Nevertheless, the efforts for mimicking BiVO4 orthorhombic nature structure lead to the unsuccessful outcome [115]. Most of the preparation method for synthesizing BiVO4 failed to adopt the pucherite structure, but the structure of BiVO4 was 11
ACCEPTED MANUSCRIPT crystallized in a scheelite or a zircon-like structure [117].
Generally, there are three different
types of BiVO4 structure reported in the literature [6,101,118,119]. The three crystal structures of BiVO4 are monoclinic scheelite, tetragonal scheelite and tetragonal zircon. As mentioned earlier, the photocatalytic properties of BiVO4 are strongly dependent on its crystal structure [116].
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Previously, Kudo et al. [120] reported that the monoclinic scheelite structure with the
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presence of electron scavengers showed a better photocatalytic water oxidation performance in comparison to the tetragonal structure. The better photocatalytic performance observed here was
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owing to the better optical absorption properties of the monoclinic structure with a band gap energy of 2.4 eV in comparison to the tetragonal structure with 2.9 – 3.1 eV band gap energy
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[121]. According to Kudo et al., [120] the presence of an additional Bi 6s orbitals in the valence
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band of monoclinic BiVO4 allows a short transition of photoexcited electron to the V 3d orbitals in the conduction band in comparison to the tetragonal BiVO4 which only consist of O 2p
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orbitals, resulting in a smaller band gap energy. Figure 2 illustrates the schematic diagram of the
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tetragonal zircon and monoclinic scheelite BiVO4 structure.
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Figure 2: Difference in a band structure of tetragonal zircon and monoclinic scheelite BiVO4. This postulation make by Kudo et al. was further confirmed by Walsh et al. [122] in which
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their Density functional theory (DFT) calculations suggested that the monoclinic BiVO4 was a
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direct band gap semiconductor which the conduction band composed of O 2p and Bi 6p orbitals whereas the valence band composed of O 2p and Bi 6s orbitals. According to their report, the
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monoclinic structure of BiVO4 (space group 14, C2h6) consists of four unique lattice sites: Bi
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(4e), V (4e), O1 (8f) and O2 (8f). With respect to the crystal inversion symmetry, a primitive cell of two formula units can be developed as shown in Figure 3. The layered structure of Bi-V-O
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units stacked parallel to the c axis draws originates from Bi2O3 and V2O5 binary analogues [123]. The arrangement of Bi atom is in a distorted oxygen octahedron with nearest neighbour distances ranging from 2.35 to 2.53 Å, whereas its centre is made up of V species with 2 x 1.74 Å and 2 x 1.75 Å bond lengths. In addition, the O1 is coordinated to one Bi and V, whereas O2 is coordinated to two Bi and a single V. The asymmetric coordination system observed within the
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ACCEPTED MANUSCRIPT monoclinic structure is attributed to the 6s2 electronic configuration of Bi which is associated to
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the active lone pair commonly present with ns2 valence cations [124].
Figure 3: BiVO4 crystal structure with blue, red and green represented bismuth, oxygen and
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vanadium tetrahedral, respectively. Reproduced with permission from Ref. [122].
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In contrary, a recent report by Cooper and co-workers [125] suggested that the monoclinic BiVO4 was not a direct band gap semiconductor. They performed a thorough DFT calculation
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along with the experimental evidence obtained via a combination of X-ray absorption
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spectroscopy (XAS), X-ray emission spectroscopy, resonant inelastic X-ray spectroscopy (RIXS) and X-ray photoelectron spectroscopy. According to Cooper et al., [125] the conduction band of BiVO4 was mainly dominated by the O 2p orbitals whereas the Bi 6p – O sp2 and V 3d – O sp2 orbitals were located at the lowest and middle regions (as shown in Figure 4). Besides that, the inequivalent pairs of oxygen atoms surrounding V atoms reduce the symmetry to C 2, which consequently splits the d-orbitals into triplet bands. In addition, the predicted triplet d-manifold splitting of V 3d conduction band states with splitting energies of 1.00 and 1.91 eV, arising from 14
ACCEPTED MANUSCRIPT lone pair-induced lattice distortions, is quantified by V-L and O K-edge XAS. Furthermore, the localization of V d-orbitals at the conduction band minimum (CBM) due to poor overlap with Bi
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6p orbital would result in poor electron mobility.
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Figure 4: Energy level diagram for the electronic structure of ms-BiVO4 calculated from density
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functional theory (DFT) and experimental analysis. Reproduced with permission from Ref. [125].
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Similarly, Zhao et al. [126] suggested that the valence band of BiVO4 was mainly characterized by non-bonding O 2pΠ orbitals and there are non-bonding states both at the top
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and bottom of valence and conduction band, respectively. Moreover, Zhao et al. postulated that the small band gap energy observed in BiVO4 was attributed to the lone-pair distortion in Bi 6s, consequently shifted the O 2p orbitals to a lower energy level. Furthermore, Zhao et al. and Walsh et al. recommend that the effective masses of both electrons and holes in BiVO4 in monoclinic scheelite structure were much smaller in comparison to other oxide-based photocatalysts. This smaller effective masses obtained via DFT calculations signify the unique 15
ACCEPTED MANUSCRIPT features of the monoclinic structure of BiVO4 in which it will facilitate the electron-hole pair separation and migration, resulting in better photocatalytic performance [116].
3 Photocatalytic
Principles
and
Mechanism
BiVO4
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Photocatalyst
over
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Photocatalyst can be described as a combination of catalysis and photochemistry in which absorption of photon energy from light via catalyst is the key towards photoreaction process
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[13]. The difference of photocatalyst in comparison to conventional catalyst is the mode of
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activation. In conventional catalyst technology, thermal energy is required for catalyst activation whereas in photocatalyst system, photons energy is required for the activation of photocatalyst
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[25]. The use of photon energy as a source of activation make photocatalyst technology as one of
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the frontier renewable energy technology in which it can utilize the solar energy.
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Figure 5: Schematic illustration of photocatalytic degradation principle and mechanism of BiVO4 photocatalyst.
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BiVO4 is a type of semiconductor having a small band gap energy (2.4 – 2.5 eV) which fall
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within visible-light region [104,105]. It is a promising photocatalyst that has intrinsic features such as smaller band gap energy, ease of production, and potential for promoting greener
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technologies. Figure 5 illustrates the basic photocatalytic degradation mechanism of BiVO4
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photocatalyst. Initially, BiVO4 photocatalyst will absorb photon energy with an energy equivalent or greater than its band gap energy, causing an electron (е¯) in the valence band (VB)
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to be excited and migrate to the conduction band (CB). BiVO4 + hv → е¯ CB (BiVO4) + h+ VB (BiVO4)
(1)
The migration of this photoexcited electron will leave photogenerated holes (h+) in the valence band. The photogenerated electron-hole pairs will migrate to the surface of the photocatalyst and trapped there. The photogenerated holes then react with adsorbed water to 17
ACCEPTED MANUSCRIPT produce strong oxidizing •OH radicals whereas the photoexcited electrons react with adsorbed oxygen to generate •O2¯ radicals. (2)
h+ VB (BiVO4) + OH¯ → •OH
(3)
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е¯ CB (BiVO4) + O2 → •O2¯
degraded the recalcitrant pollutant.
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•O2¯ + recalcitrant pollutants → degradation of pollutant
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The formation of these radicals will further react with recalcitrant pollutant and subsequently
(5)
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•OH + recalcitrant pollutants → degradation of pollutant
(4)
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Apart from these major photocatalytic degradation reaction described earlier, there are some weaker photocatalytic reaction could presumably occur during the reaction which can produce
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some other intermediate species such as H2O2 and HO2• which can be summarized as follow
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[25]:
(6)
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•O2¯ + H+ → HO2• 2HO2• → H2O2 + O2
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(7)
Nevertheless, the major limiting factor in BiVO4 photocatalyst system is the fast recombination rate within nanoseconds. Congruously, with the absence of an appropriate electron-hole acceptor or donor, the photoexcited electrons and photogenerated holes will recombine and dissipate the energy as heat [13]. Moreover, some of the unstable photoexcited electrons could be adsorbed by •OH, consequently produced OH¯. 18
ACCEPTED MANUSCRIPT е¯ CB (BiVO4) + h+ VB (BiVO4) → Catalyst + heat (energy)
(8)
е¯ CB (BiVO4) + •OH → •OH¯
(9)
This recombination of electron-hole pair phenomenon is one of the significant factor and
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drawback in hindering the photocatalytic degradation performance of BiVO4. Furthermore, the
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poor performance of BiVO4 photocatalysts is hampered by its sluggish water oxidation kinetics
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[115] and poor photogenerated charge carrier separation and migration [116]. With regards to these limiting factors exist within the BiVO4 photocatalyst, the forthcoming section is devoted to
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enhance the overall photocatalytic performance.
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briefly discuss the aforementioned limitation and current strategies that recently been adopted to
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4 Limitations of BiVO4 Photocatalyst
BiVO4 photocatalyst can be considered as one of the most intrigued visible-light-driven
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photocatalyst owing to its smaller band gap energy of 2.4 eV. In light of the small band gap
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energy of BiVO4 photocatalyst, it was estimated that up to 11% of the standard AM1.5 solar spectrum can be absorbed, corresponding to the maximum photon to photocurrent conversion
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efficiency of 7.6 mA cm-2 [115]. Nevertheless, to date, the actual energy conversion obtained was still far from its theoretical value which makes the BiVO4 photocatalyst was still far from debuting into practical industrial applications. The major limiting factors that govern the BiVO4 photocatalyst performance were poor photocharge carrier transfer [20,116], slow water oxidation kinetics [115,127], and fast recombination rate of electron-hole pairs [37,52,128, 129].
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ACCEPTED MANUSCRIPT The density functional theory (DFT) study performed by Zhou et al. [130] indicated that the poor photocharge carrier transfer within BiVO4 photocatalyst was due to the geometrical structure of tetrahedral VO4 species which was disconnected between each other in the V 3d orbitals, consequently abnegate the smooth movement of photoexcited electrons between
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tetrahedral VO4 species and substandard solar conversion efficiency. The disconnected of
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tetrahedral VO4 species caused a further in the distance of photocharge carrier movement,
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consequently promote the photocharge recombination rate (as shown in Figure 6). According to the Xie et al. [131], the disconnected of tetrahedral VO4 species in monoclinic BiVO4 observed
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caused bigger disadvantages in photocharge carrier separation and migration if the
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heterostructure strategies were employed to circumvent the limitations facing by pristine BiVO4.
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It is known that the enhancement of photocharge separation and migration within the heterostructure system are strongly dependent on the connection and lattice matching between
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the photocatalyst structures. For example, the integration of monoclinic BiVO4 and tetragonal
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anatase TiO2 would result in weaker photocharge carrier transfer owing to the low degree of lattice matching, consequently hampered the overall photocatalytic activity of heterostructure
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[132,133]. Thus, it is a great interest among researcher recently to introduce an electron mediator
separation
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as a bridge at the interface of the heterostructure system for facilitating the photocharge carrier and
migration
which
will
be
[38,43,67,78,88,134].
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further
discussed
in
the
next
section
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Figure 6: (a) electronic band structure, (b) electronic density of states, plotted of (c) minimum
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conduction band and (d) maximum valence band at certain point and (e) atomic supercell
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structures of pristine BiVO4. Reproduced with permission from Ref. [130]. Slow water oxidation kinetics occurs owing to its high kinetic barriers for water oxidation
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reaction to occurs [129]. In water oxidation process, a bulk of electron and holes involved during
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the separation and migration reaction, consequently a significantly high over potential are required for the water oxidation reaction to occurs [116]. Similarly, Tan et al. [60] observed a
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saturated BiVO4 photocurrent at a low potential of -0.30 V vs Ag/AgCl owing to its relatively
AC
bigger particulate size. Additionally, the BiVO4 photocatalyst has a higher onset potential of 0.65 V vs Ag/AgCl which indicated a higher energy input was required for the reaction to occur [133,135]. Moreover, Tan et al. [60] concluded that the efficiency of BiVO4 was still dominantly limited by its intrinsically low carrier mobility despite the various successful effort demonstrated previously in circumventing the respective limitations.
21
ACCEPTED MANUSCRIPT Meanwhile, Xia et al. [19] postulated that the sluggish water oxidation kinetics of BiVO4 would cause a highly concentrated of photogenerated holes residing on the surface of the photocatalyst, consequently promote the recombination of the electron-hole pair. Generally, the photogenerated holes will migrate towards the surface of photocatalyst upon the continuous light
T
illumination until a steady state concentration is reached, resulting in a dense of photogenerated
IP
holes accumulated at the photocatalyst surface. According to the Xiao et al. [136] the slow water
CR
oxidation kinetics at the photocatalyst surface rich with the holes density while the negative transient of photocurrent observed indicated the recombination of the photoexcited electron with
US
the accumulated photogenerated holes at the photocatalyst surface.
AN
Another performance limiting factor for BiVO4 photocatalyst is fast recombination rate of
M
the electron-hole pair. This fast recombination rate of electron-hole pair occurred within a few nanoseconds [104,105]. Generally, the photogenerated carrier charge recombination rate can be
ED
explored via photoluminescence spectra characterization by examined the released energy of the
PT
photocatalyst. According to Huang et al., [53] the faster the photogenerated charge carrier recombination rate would result in a higher detection of photoluminescence intensity,
CE
consequently affecting the overall photocatalytic performance.
AC
In addition, Yu et al. [137] observed prominent photoluminescence peaks at the region of 525 nm, indicating a detection of recombination center within BiVO4 photocatalyst originated from the V 3d and hybridization of Bi 6s and O 2p orbitals. Subsequently, Song et al. [50] revealed that the poor photocatalytic degradation performance of BiVO4 photocatalyst in treating the gaseous ethylene under visible light irradiation despite being a visible-light-driven
22
ACCEPTED MANUSCRIPT photocatalyst was due to fast recombination rate and poor photocharge carrier separation and migration.
5 Strategies for Improving Photocatalytic Activities of BiVO4
Morphological Control and Facet Dependent Engineering
US
5.1
CR
IP
T
Photocatalyst
Morphological control and facet-dependent engineering are among strategy can be used for
AN
overcoming the major limitation facing by the BiVO4 photocatalyst. Some reported studies claimed that the photocatalytic performance of photocatalyst was majorly dependent on the
M
degree of facet interface exposure [115,116]. This postulation was due to the fact that most of the
ED
photocatalytic reaction occurred on the surface of the photocatalyst in which coordination and
PT
arrangement of the atomic surface was the vital key towards the development of highly functionalized photocatalyst [115]. To date, numerous method of fabricating the BiVO4
CE
photocatalyst has been developed including sonochemical method [94], spray pyrolysis [138],
AC
microwave [45] and solid-state reaction [104,105]. Different in fabricating method would result in different morphological and crystal structure of BiVO4. Thus, it is crucial for selecting an effective method for fabricating BiVO4 which can produce a highly crystalline structure with a perfect morphological structure. Previously, Zeng et al. [139] reported the successful fabrication of BiVO4 microcrystals prepared via surfactant-assisted hydrothermal method using sodium dodecylbenzenesulfonate 23
ACCEPTED MANUSCRIPT (SDBS). The result showed a significant improvement in photocatalytic activity of BiVO4 owing to its enhancement in photocharge carrier separation and migration. Nonetheless, the fabrication of BiVO4 photocatalyst with selective crystal facet via surfactant-assisted hydrothermal method still caused a setback in photocatalytic performance as well as increase the production cost.
T
Moreover, the used of certain surfactant in synthesizing BiVO4 is hardly removed after
IP
synthesizing process which would result in unwanted surfactants molecules attached in the
CR
crystal structure of BiVO4 photocatalyst. The presence of this unwanted surfactant molecules would limit the absorption rate between BiVO4 photocatalyst and targeted recalcitrant pollutant,
US
thus reduce the photocatalytic activities. With regard to this problem, Wu et al. [140]
AN
successfully reported the fabrication of the different morphological structure of BiVO4 prepared via a surfactant-free hydrothermal method. They observed that the BiVO4 morphological
M
structure can be simply modulated via varying the molar ratio of starting precursor, resulting in
ED
several structures including dumbbell, rod, ellipsoid sphere and cake-like structure as depicted in Figure 7. In addition, the as-developed BiVO4 photocatalyst exhibited different photocatalytic
AC
CE
PT
performance associated with their surface area and morphological structure.
24
AN
US
CR
IP
T
ACCEPTED MANUSCRIPT
M
Figure 7: BiVO4 photocatalyst prepared via surfactant-free hydrothermal method with different
ED
molar ratio of starting precursor. Reproduced with permission from Ref. [140]. On the other hand, Tan et al. [60] suggested the use of nanoscaling approached in improving
PT
the photocharge separation and migration via reducing the size of the photocatalyst in both
CE
BiVO4 photoelectrochemical cell (PEC) and powder suspension system (PS). Their group claimed that the mechanism for the photocharge separation and migration in both systems are
AC
different (as shown in Figure 8). In powder suspension, the transfer of photogenerated holes and photoexcited electrons to the electrolyte and bulk, respectively were dependent on the thickness of space charge layer formed at the interface of the photocatalyst. Meanwhile, the photocharge separation and migration were smoothly transferred via the addition of an external applied potential in the PEC system. According to the Tan et al., the photocurrent density of BiVO4 with a smaller particle size significantly improved in comparison to larger particle size. Furthermore, 25
ACCEPTED MANUSCRIPT the onset potential result showed a significant negative potential shifted with respect to the decrease in particle size from -0.65 to 0.35 V vs Ag/AgCl. The low onset potential was favorable in the photocatalytic application as the only small amount of energy were require to initiate the
CE
PT
ED
M
AN
US
CR
IP
T
reaction.
AC
Figure 8: Influence of different particle size on photoactivity of BiVO4 in PEC and PS system. Reproduced with permission from Ref. [60].
Despite the successful enhancement of photocatalytic performance via tailoring the morphological structure of BiVO4, the fabricating of BiVO4 photocatalyst with the specific growth of crystal facet has garnered much attention recently [59,100,141–144]. Thalluri et al. [100] reported the green synthesized of selective {040} crystal facet of BiVO4 via sustainable hydrothermal synthesis method with an additional of ammonium carbonate as a structural 26
ACCEPTED MANUSCRIPT directing agent. The XRD result showed an enhancement of {040} crystal facet which signified the help of ammonium carbonate in helping the preferential growth of {040} crystal facet, thereby remarkably enhanced the photocatalytic activity by 20 and 100 times in comparison to pristine BiVO4. Moreover, the smaller difference in the BiVO4 crystallite size associated with the
T
different dopant loading indicated the insignificant effect of crystallite size on photocatalytic
CR
IP
performance.
On the contrary, Tan et al. [59] suggested that the photocatalytic performance of BiVO4 were
US
mainly governed by the {010} crystal planes which known as most active facet. The BiVO4 photocatalyst with a large surface area of {010} crystal facet showed a superior photocatalytic
AN
performance in comparison to BiVO4 photocatalyst with a large surface area of {110} crystal
M
facet. Moreover, the photoluminescence measurement of {010}-BiVO4 photocatalyst showed significant quenched in response to hole-scavengers while inverse phenomenon was observed
ED
with {110}-BiVO4 photocatalyst. Besides that, the time-resolved photoluminescence and Mott-
PT
Schottky study revealed a major extent of electron trapping and flat band potential were detected in {110}-BiVO4 photocatalyst. These phenomenon observed were ascribed to the longer distance
CE
of electrons transfer from the bulk BiVO4 to the {010} crystal facet as shown in Figure 9. In
AC
addition, the smaller size of {010} crystal facet in {110}-BiVO4 photocatalyst limits the electron density accumulated on the {010} crystal facet, consequently promote the photocharge carrier recombination and inefficient photocatalytic performance.
27
CR
IP
T
ACCEPTED MANUSCRIPT
Figure 9: The schematic illustration on the influence of {010} and {110} crystal facet dependent
US
on BiVO4 photocatalytic activity. Reproduced with permission from Ref. [59].
AN
Likewise, Zhao et al. [145] performed a crystal modulation study by transforming the standard decahedron BiVO4 structure prepared via hydrothermal method into short rod-like
M
particles which specifically tailoring the proportion of {010}/{011} crystal facet. It was found
ED
that the modified BiVO4 structure showed a significant photocatalytic activity with 50 times better than standard BiVO4. The significant results obtained was owing to the synergistic effect
PT
of {010} crystal facet act as an active reduction site while {011} crystal facet serves as an active
CE
oxidation site. Similarly, inspired by the feasibility of smooth photogenerated charge carrier separation and migration between {010} and {110} crystal facet owing to the valence band and
AC
conduction band of {110} is higher than {010} crystal facet, Li et al. [142] exploited this advantages by incorporating Ag and Fe2O3 nanoparticles for producing a highly performing photocatalytic activity. As a result, remarkable photocurrent density with the smallest impedance was observed with regard to the aforementioned strategy. The remarkable photocatalytic performance was owing to the synergistic effect of n-n and Z-scheme mechanism associated to
28
ACCEPTED MANUSCRIPT the manipulation of crystal facet which smoothly accedes the electron-hole pair separation and migration.
Elemental Doping
T
5.2
IP
Elemental doping with a metal or non-metal materials is one of the alternative strategies used
CR
to improve the photocatalytic performance of BiVO4 photocatalyst. Elemental doping is a process of incorporating an additional element or impurities in a photocatalyst material which
US
consequently tailoring the photocatalyst physical properties such as electrical and optical
AN
properties [146]. There are few advantages of these elemental doping strategies such as (i) enhancement of the photocatalyst electrical conductivity , (ii) modulation of photocatalyst band
M
gap energy, (iii) enhancement in photocatalyst active surface site area and (iv) allows a smooth
ED
electron-hole separation and migration [116,146]. Nevertheless, this elemental doping strategy may not always be favorable in enhancing the photocatalytic performance of photocatalyst. This
PT
is because the improper doping of metal or non-metal materials may also lead to the facilitation
CE
of recombination rate of electron-hole pairs and hamper the photocharge carrier transportation and migration by creating scattering center [116]. Hence, identifying a suitable elemental and its
AC
total amount of doping is crucial in elemental doping strategy for enhancing the overall photocatalytic performance of BiVO4. In lieu of the beneficial effects of elemental doping, Quinonero et al [129] explored the effect of La and Ce doping on the photocatalytic performance of pristine BiVO4. The better photocatalytic performance of doped BiVO4 was ascribed to the beneficial substitution of Bi3+ species with La3+ and Ce3+/Ce4+ ions. The introduction of these two impurities altered the 29
ACCEPTED MANUSCRIPT crystallographic structure of monoclinic BiVO4, resulting in better photocatalytic performance. Moreover, the Ce3+ ions also act as electron donors within the BiVO4, thereby increasing the concentration of photocharge carriers within the BiVO4 photocatalyst. As a result, BiVO4 gained benefits from the creation of an additional n-type conductivity, resulting in better photocatalytic
IP
T
performance. Meanwhile, Zhou et al. [130] demonstrated the design of 3D macro-mesoporous Mo: BiVO4
CR
via controllable colloidal crystal template method. The substitution of V5+ species with Mo6+ ion
US
promotes the deformation of BiVO4 crystal structure from monoclinic to the tetragonal owing to the larger tetrahedral structure of Mo6+ ion, resulting in enhancement of the photocharge carrier
AN
separation and migration. In addition, the extra one valence electron of Mo6+ ion residing at V4+
M
species may interchange between the adjacent V5+ species, resulting in concentrated localized polarons. As a result, the activation energy of polarons hopping can be reduced, thereby
Formation of BiVO4-based Heterostructure
CE
5.3
PT
ED
improved the transfer of electron-hole pair.
AC
Apart from morphological control, facet-dependent and elemental doping, the formation of BiVO4-based heterostructure has elicited more attention as alternative strategies for enhancing the overall photocatalytic performance [38,56,104,128]. Generally, the BiVO4-based heterostructure is engineered by hybridizing BiVO4 photocatalyst with a different type of photocatalyst or co-catalyst. In this heterostructure systems, the valence band and conduction band of the coupled photocatalyst or co-catalyst should be lower or higher in comparison to that BiVO4 photocatalyst. The hybridization of two different type of photocatalyst with a difference 30
ACCEPTED MANUSCRIPT in electronic structure and band gap locations will resulting in a formation of band bending at the interface of the heterostructure [146]. This band bending observed is the aftereffect of the potential difference between two photocatalysts. As a result, a new electric field within a space charge region will be developed, resulting in spatial photocharge carrier separation and
ED
M
AN
US
CR
IP
T
migration.
heterostructure.
CE
PT
Figure 10: Schematic illustration of three different type of BiVO4-based band edge alignment
Generally, the formation of the BiVO4-based heterostructure with a different type of
AC
photocatalyst can be tailored to three different type of heterostructure band alignment. The three different types of heterostructure band alignment are Type I (straddling gap), Type II (staggered gap) and Type III (broken gap) [127,147]. Each of these types would result in difference photogenerated holes and photoexcited electron separation and migration between adjacent photocatalyst (as shown in Figure 10). From Figure 10, the band gap energy of coupled photocatalyst in Type I heterostructure are larger in comparison to the BiVO4 band gap energy, 31
ACCEPTED MANUSCRIPT resulting in a straddling band alignment. Thus, when the heterostructure photocatalyst absorbs the photon energy equivalent to or greater than its band gap energy, resulting in the separation and migration of electron-hole pairs in a single component within the heterostructure photocatalyst. However, there is no concrete evidence in the enhancement of electron-hole pair
IP
CR
photocatalyst upon separation and migration process [146].
T
separation and migration as the photogenerated charge carriers are commonly accumulated in a
Meanwhile, the band gap alignment between BiVO4 and coupled photocatalyst is staggered
US
between each other in Type II, resulting in the formation of the upward or downward band bending (as shown in Figure 10) [127]. The observed phenomenon causes the separation and
AN
migration of electron-hole pair in the heterostructure system to be in the opposite direction. As a
M
result, the smooth electron-hole pair separation and migration is expected to occur and the fast recombination rate of the electron-hole pair is significantly reduced owing to prolong the lifetime
ED
of photogenerated charge carriers [39]. On the other hand, for the type III heterostructure, both
PT
conduction and valence band of BiVO4 and coupled photocatalyst are not cross-linked with each other causing a gap between both band alignments in the heterostructure photocatalyst. It is hard
CE
to postulate that the separation and migration of electron-hole pair in this type III heterostructure
AC
will smoothly occur and thus limit the overall photocatalytic performance [146]. With regard to these different types of heterostructure band edge alignment, it is important to properly select the appropriate coupled photocatalyst which will result in not only better light absorption capability, but also smoothly facilitate the separation and migration of electron-hole pair via the proper band edge interface. Table 1 enlists some of the recent examples of the formation of BiVO4-heterostructure system along with their proposed mechanism. For example, 32
ACCEPTED MANUSCRIPT Xia et al. [19] performed a studied on the heterostructure of α-Fe2O3/BiVO4 photocatalyst prepared via spin-coating-based-successive ionic layer adsorption and reaction (SILAR) method on the phenol degradation. The photocurrent density and incident-to-current conversion efficiency (IPCE) of α-Fe2O3/BiVO4 photocatalyst were 1.63 mA cm-2 at 1.23 V vs. a reversible
T
hydrogen electrode (RHE) and above 27%, respectively which was 2.14 times and triple in
IP
comparison to pristine BiVO4. Furthermore, the incorporation of α-Fe2O3 in the heterostructure
CR
photocatalyst caused a cathodic shifted for both onset potentials and over potential of water oxidation in comparison to pristine BiVO4. This cathodic shifted observed signified the
US
beneficial of α-Fe2O3 in limiting the recombination of the electron-hole pair within the
AN
heterostructure photocatalyst and hence improve the overall photocatalytic performance. In light of the good photocatalytic performance of Fe2O3/BiVO4 photocatalyst, Zhang et al. [6] further
M
optimized the aforementioned heterostructure photocatalyst by synergizing the Fe2O3 with the
ED
three-dimensionally ordered macroporous (3DOM) BiVO4 in degrading the 4-nitrophenol. The profound performance was attributed to excellent architecture porosity, high surface area and
AC
CE
PT
smooth photogenerated charge carrier separation and migration.
33
ACCEPTED MANUSCRIPT
Table 1: Recent development of BiVO4-based heterostructure with the proposed mechanism. Photocatalyst
Model
T P
Proposed Mechanism
Remarks
I R
Pollutant
Ag/Ag2CO3/BiVO4
The
Tetracycline
C S
addition
of
Ag
nanoparticles
Ref
enhanced
the Reproduced
U N
photogenerated electrons and hinder recombination rate of
A
electron-hole pairs. However, excessive amount of Ag
M
with permission from Ref.
nanoparticles caused a light shielding effect which [148]
D E
RGO-TiO2/BiVO4
Methylene
E C
C A
blue &
T P
consequently reduced the photocatalytic performance.
The construction of RGO-TiO2/BiVO4 promote better Reproduced crystallinity structure. The intimate contact between the
Industrial
petroleum
wastewater
with permission
developed photocatalyst allows smooth electron-hole pair
from Ref.
separation and migration, resulting more available radicals [149]
34
ACCEPTED MANUSCRIPT for the degradation reaction.
T P
I R
BiVO4/Polydopami
C S
U N
The amino and catechol groups in the PDA have a strong Reproduced
Glyphosate
ne/g-C3N4
A
and irreversible binding ability which is suitable for
M
formation of heterostructure. The BET specific surface area
D E
T P
C A
E C
with permission from Ref.
plays an important role within this study as larger BET [150] surface area is observed in BiVO4/Polydopamine/g-C3N4 photocatalyst. However, the uniqueness of PDA properties in enhancing the photocatalyst is briefly explained by the authors.
35
ACCEPTED MANUSCRIPT
Fe2O3/3DOM BiVO4
The
4Nitrophenol
good
photocatalytic
performance
observed
are Reproduced
attributed to its unique porous architecture, high surface
with permission
T P
area, good light-harvesting ability, high adsorbed oxygen
from Ref.
I R
species concentration and excellent electron-hole pair [6] separation.
Bi4V2O11/BiVO4
C S
U N
The high photocatalytic activity observed attributed to its Reproduced
Methylene
A
Blue
M
multiple reflections of their special multi-shell hollow
D E
T P
C A
E C
spheres. The authors claimed that the surface area is not a
with permission from Ref.
major factors that govern the reactions while the synergistic [55] effects of morphological and microstructure characteristics provides better performance.
36
ACCEPTED MANUSCRIPT
Fe2O3 modified Ag-
Methyl
010BiVO4
Orange
The crystal facet dependence approach is used to optimize Reproduced the photocatalytic performance. Moreover, the synergistic
with permission
T P
effect of n-n and Z-scheme system in the different facet
from Ref.
I R
helps to facilitate the photocharge carrier separation and [142] migration.
GQD/BiVO4
C S
U N
The excessive amount GQD contents resulting in blockage Reproduced
Carbamazep
A
ine
of an active sites, hence reducing the catalytic capacity. The
D E
T P
M
degradation of Carbamazepine pathways involved
with permission
(1)
from Ref.
hydrogen abstraction and hydroxylation of heterocycle ring [151] and
E C
C A
37
(2) cleavage of amide group.
ACCEPTED MANUSCRIPT
TiO2/BiVO4/Co-Pi
The BiVO4 nanoflakes are vertically attached to the side Reproduced
Water Splitting
faces of rutile TiO2 and have a selective growth direction
with permission
T P
along {112}. The use of 1D nanoarray configuration helps
from Ref.
I R
to promote better photocatalytic performance via increasing [39] the light absorption and chemical reaction sites owing to the
C S
built-in electric field at the heterojunction.
BiVO4/P25
U N
A
The good performance observed owing to the multiphase Reproduced
Ethylene
D E
T P
C A
E C
M
nanosystem was developed which consist of monoclinic,
with permission
anatase and rutile phases. The light absorption of developed
from Ref.
heterostructure is improved and n-N heterojunction is [50] constructed. The photocatalyst has a stable performance via recyclability study.
38
ACCEPTED MANUSCRIPT
Biochar/Fe3O4/BiV O4
The addition of Biochar as an adsorbent helps to promote Reproduced
Methylpara ben
better photocatalytic performance. It is suggested that
with permission
T P
reactive oxygen species are responsible for the degradation
from Ref.
I R
activities and addition of H2O2 does escalates the overall [152] performance.
BiVO4/Bi2S3/MoS2
C S
U N
A
The developed photocatalyst was fabricated by coupling Reproduced
Rhodamine B,
D E
Methylene
T P
Blue, Malachite
E C
Green
C A
M
diamond –shaped BiVO4 with layered MoS2 which allowed
with permission
photocharge carrier separation and migration as well as
from Ref.
increased the light absorption capabilities. The h+ species [98] play a major roles in photocatalytic degradation reaction in comparison to •O2- and •OH radicals.
39
ACCEPTED MANUSCRIPT
m-BiVO4/t-BiVO4
The type II heterostructure is observed which led to an Reproduced
Methylene Blue
increase in the charge carrier life time. The trapping
with permission
analysis suggested that the h+, •O2- and •OH radicals are the
T P
I R
from Ref.
main active species that contribute to the overall [153] photocatalytic performance.
C S
A
U N
D E
M
T P
E C
C A
40
ACCEPTED MANUSCRIPT On the other hand, Zhang et al. [154] explored the photocatalytic performance of TiO2/BiVO4 heterostructure photocatalyst prepared via a one-step microwave hydrothermal method. Similarly, Ma et al. and Song et al. [50,155] examined the benefit of TiO2/BiVO4 heterostructure prepared via biomorphic templates in degrading the methylene blue and a
T
modified citric acid complexing in degrading gaseous ethylene, respectively. Despite the
IP
enhancement of photocatalytic performance over TiO2/BiVO4 photocatalyst, the photocatalytic
CR
mechanism observed within this heterostructure was ascribed to Type I heterostructure
US
mechanism in which the transfer of photogenerated holes from BiVO4 to the TiO2 surface were obstructed. This impeded movement of holes observed was owing to the mismatched position of
AN
TiO2 valence band that was lower than BiVO4. As discussed earlier, having a Type II heterostructure system is more favorable for developing a highly functionalized heterostructure
AC
CE
PT
ED
M
photocatalyst with a superior photocatalytic performance.
Figure 11: Schematic illustration of the TiO2/BiVO4 heterostructure system with and without hydrogen treatment. Reproduced with permission from Ref. [127].
41
ACCEPTED MANUSCRIPT Inspired by the advantages of type II heterostructure system, Singh et al. [127] converted the conventional Type I TiO2/BiVO4 heterostructure into Type II heterostructure via hydrogen treatment on the top layer of TiO2 (as shown in Figure 11). Based on their density functional theory calculation, the conversion of Type I to Type II heterostructure was feasible to ameliorate
T
the distortion of oxygen sublattice existed in TiO2. This band edge realignment resulting in a
IP
better photocurrent density up to 4.44 mA cm-2 at 1.23 V vs reversible hydrogen electrode (RHE)
CR
and low onset potential of – 0.14 V vs RHE. In addition, this band edge realignment allowed the smooth transfer of photogenerated holes to the surface of the electrode and improved the
US
absorption rate of TiO2.
AN
Meanwhile, the profound photocatalytic performance was observed over the use of WO3 and
M
lanthanum vanadate (LaVO4) photocatalyst in the BiVO4-based heterostructure system in photocatalytic application performed by Liu et al. [36] and Veldurthi et al. [112]. On the other
ED
hand, Baek et al. [21] explored the beneficial effect of BiVO4-based heterostructure system by
PT
fabricating a double-heterojunction over BiVO4/WO3/SnO2. Furthermore, Zhao et al. [156] investigated the effect of BiVO4@MoS2 prepared via a facile in-situ hydrothermal method for
CE
degrading the Cr6+ and oxidation of crystal violet under visible light irradiation. Inspired by the
AC
intrinsic features of Se such as prolong photocharge carrier lifetime, high conductivity and better photo absorption capability, Nasir et al. [157] examined the beneficial of Se/BiVO4 heterostructure in photoelectrochemical water splitting. All of the abovementioned BiVO4-based heterostructure system shown a remarkable photocatalytic performance owing to their intrinsic feature.
42
ACCEPTED MANUSCRIPT In lieu of the Type II heterostructure great photocatalytic performance discussed earlier, its relatively low photocharge carrier oxidation and reduction power upon electron-hole pair separation and migration, consequently abnegate its photocatalytic performance. The key underlying this problematic phenomenon lies within the migration electron-hole pair to a less
T
negative conduction band of coupled photocatalyst and a less positive valence band of the
CR
IP
adjacent photocatalyst.
Up to date, a new type of heterostructure system, namely Z-scheme heterostructure has been
US
reported which can help to overcome the limitation facing the conventional Type II heterostructure system [8,67,158,159]. Generally, in this Z-scheme heterostructure mechanism,
AN
the photoexcited electron from the less negative conduction band of photocatalyst will migrate
M
and interact with the less positive valence band of adjacent photocatalyst, resulting in the formation of holes in the valence band of the photocatalyst itself and photoexcited electron in the
ED
conduction band of the adjacent photocatalyst (as shown in Figure 12).
PT
Furthermore, more recent studies reported the introducing of intermediate bridge materials
CE
such as reduced graphene oxide (RGO), silver (Ag), copper (Cu) and gold (Au) in the interface of the heterostructure structure system would significantly smooth and facilitate the separation
AC
and migration of electron-hole pair between the photocatalyst in the heterostructure samples [8,31,43,160–162]. The advantage of this Z-scheme heterostructure mechanism over the conventional Type II heterostructure system is it has a superior photoredox ability owing to its ability for nurturing a highly positive and negative valence band and conduction band, respectively [163]. In addition, the efficient of the photocharge carrier separation and migration
43
ACCEPTED MANUSCRIPT between the photocatalyst is one of the advantages of this Z-scheme heterostructure system
M
AN
US
CR
IP
T
[144,164,165].
Figure 12: Schematic illustration of the Z-scheme heterostructure mechanism (a) without and (b)
ED
with a bridge electron mediator.
PT
For the past few years, the studies on the BiVO4-based heterostructure which employ the new solid-state Z-scheme mechanism to optimize the overall photocatalytic performance has
CE
been reported [112,134,166,167]. Although the BiVO4-based heterostructure photocatalyst
AC
studies are still dominated by current conventional Type II heterostructure, with respect to the great advantage of Z-scheme heterostructure mechanism, it is greatly anticipated that the employ of the Z-scheme heterostructure mechanism in the photocatalyst system will be encyclopaedically studied and grow significantly in the future. Table 2 enlists some of the recent examples of the Z-Scheme BiVO4-heterostructure system along with their proposed mechanism.
44
ACCEPTED MANUSCRIPT
Table 2: Recent development of Z-Scheme BiVO4-based heterostructure system with the proposed mechanism. Photocatalyst
AgI/BiVO4
Model Pollutant
T P
Proposed Mechanism
Remarks
I R
AgI used as a photosensitizers owing to its suitable
E-Coli &
C S
Oxytetracycline
energy-band position. The incorporation of AgI in
U N
Hydrochloride
the as-prepared heterostructure significantly inhibit
A
D E
M
T P
E C
C A
45
the fast recombination rate of electron-hole pair evidently from PL spectra analysis.
Ref
Reproduced with permission from Ref. [134]
ACCEPTED MANUSCRIPT
Porous-doped
g-
The as-developed heterostructure was prepared via
Tetracycline
C3N4/BiVO4
one-pot impregnated precipitation method. The
Reproduced with permission
T P
improved photocatalytic performance of this Z-
I R
scheme is attributed to the enhanced photocharge
from Ref. [86]
carrier separation and improved of charge carrier
C S
lifetime (1.65 ns).
Graphene bridged
A
U N
Tetracycline
Ag3PO4/Ag/BiVO4
D E
(040)
M
The addition of metallic Ag improved the photocharge carrier transfer ability within the
Reproduced with permission
composite while graphene serve as a good support
T P
to make the loaded nanoparticles to achieve
E C
uniform distribution without aggregation. The •O2-
C A
and h+ are the predominant active species that contribute to the overall photocatalytic reaction.
46
from Ref. [8]
ACCEPTED MANUSCRIPT
Se/BiVO4
The Se/BiVO4 has boosted the photocurrent density
Water splitting
by 3-fold in comparison to pristine BiVO4. The
Reproduced with permission
T P
DFT studies show that Se makes a direct Z-scheme
I R
with BiVO4 where the photoexcited electron of
from Ref. [157]
BiVO4 recombines with the valance band of Se. In
C S
addition, the existence of dual absorption layers of
U N
Se and BiVO4 which significantly increased the
A
BiVO4/PyridineDoped g-C3N4
D E
Phenol and
M
The
T P
Methyl Orange
light absorption.
composite
is
prepared
via
an
in-situ
hydrothermal method. The profound performance is
Reproduced with permission
E C
attributed to the construction of the direct Z-scheme system which is free from any mediator. The •O2
C A
-
and •OH radicals plays significant roles in photocatalytic reaction.
47
from Ref. [168]
ACCEPTED MANUSCRIPT
BiVO4-BiFeO3-
The Z-schematic charge carrier transfer are
4-Nitrophenol
CuInS2
enhanced via inserting a ferroelectric material with
Reproduced with permission
T P
downward band bending between transfer PS II and
I R
PS I. The photocurrent density showed an improved performance on as-developed composite with the
C S
polarization-induced electric field oriented from
U N
CuInS2 to BiVO4.
A
D E
M
T P
E C
C A
48
from Ref. [38]
ACCEPTED MANUSCRIPT To date, Zou et al. [61] fabricated a Z-scheme CdS/BiVO4 heterostructure system via lowtemperature water bath system. The Z-scheme system showed a better photodegradation performance in treating Rhodamine B (RhB) in comparison to pure photocatalyst. The better photocatalytic performance of this Z-scheme system was attributed to the quasi-continuous effect
T
owing to the concentrated number of defects aggregated at the Z-scheme interface. Meanwhile,
IP
drawing inspiration from the recent advantages of 1 D nanostructures, Zhou et al. [169]
for
a
Z-schematic
study
on
photocatalytic
hydrogen
evolution
and
US
nanoparticles
CR
manipulating the 1D nanostructure of BiVO4 nanowires and decorated with the hierarchical CdS
photodegradation of Rhodamine B. The CdS/BiVO4 nanowires exhibited higher photocurrent
AN
responses in comparison to pristine BiVO4, indicating the photoexcited electron were smoothly transfer to substrate whereas the photogenerated holes were trapped by the electrolyte. Moreover,
M
the photoluminescence spectra of CdS/BiVO4 nanowires at the region of 495 and 535 nm which
ED
attributed to BiVO4 nanowires and CdS, respectively depicted weakened emission peaks in
PT
comparison to pristine samples. The weakened peaks spectrum observed were strongly justified the efficient photocharge separation and migration.
CE
In light of the beneficial effect of introducing electron bridge mediator in the Z-scheme
AC
mechanism as discussed earlier, Wu et al. [43] explored the advantage of incorporating carbon dots as a solid state electron mediator over BiVO4/CDs/CdS Z-scheme structure without the addition of any sacrificial agent or co-catalyst. The Nyquist plot of BiVO4/CDs/ CdS showed a high-frequency semicircle which response to the migration of photoexcited electron at the interface of Z-scheme system and electrolyte. In addition, a smaller semicircle radius was detected from EIS study which signifies the efficiency of photocharge carrier transfer. Moreover, 49
ACCEPTED MANUSCRIPT the substandard photoluminescence spectra were observed within the as-developed z-scheme photocatalyst in comparison to pristine samples. This phenomenon observed was the key towards explaining the superior photocatalytic performance observed within BiVO4/CDs/CdS
US
CR
IP
T
photocatalyst.
AN
Figure 13: The fabrication process of the g-C3N4@Ag/BiVO4 Z-scheme heterostructure system.
M
Reproduced with permission from Ref. [144]. On the other hand, Ou et al. [144] established the Z-scheme mechanism over g-
ED
C3N4@Ag/BiVO4 {040} photocatalyst. In their study, the {040} crystal facet was preferential
PT
growth and hybrid with g-C3N4 and Ag nanoparticles before the photocatalytic oxidation was performed (as shown in Figure 13). The remarkable photocurrent density of g-C3N4@Ag/BiVO4
CE
{040} photocatalyst which is 6.3 times greater than pristine photocatalyst justified the superior
AC
photocatalytic performance observed, indicating the smooth interfacial photocharge carrier separation and migration. Similarly, Chen et al. [8] studied the impact of Ag nanoparticles hybridized with reduced graphene oxide as a solid state electron mediator in the photodegradation of tetracycline over Ag/Ag3PO4/BiVO4/RGO Z-scheme heterostructure. The photodegradation performance result of as-prepared z-scheme heterostructure showed a prominent removal of tetracycline and 50
ACCEPTED MANUSCRIPT photocatalyst stability after four times recycling in comparison to pristine photocatalyst. The photoluminescence
study
revealed
the
lowest
spectrum
signal
obtained
by
Ag/Ag3PO4/BiVO4/RGO samples, indicated the remarkable photocharge carrier separation and migration. Evidently, the highest photocurrent density observed by Ag/Ag3PO4/BiVO4/RGO
T
photocatalyst justified the effectiveness incorporating of solid-state electron mediator as a bridge
CR
IP
within the Z-scheme system.
US
6 Conclusion and Future Directions
AN
The growing concern on the scarcity of clean water sources due to a fast development of industrialization has force a rapid breakthrough dedicated to the development of Advanced
M
Oxidation Processes (AOPs) using photocatalyst system. Over the past few years, the studies on
ED
BiVO4-based photocatalyst in AOPs system has witnessed auspicious potential promises by BiVO4-based photocatalyst in photocatalytic degradation applications. To this end, the
PT
outstanding photocatalytic activity showed by the BiVO4-based photocatalyst is mainly govern
CE
by its intrinsic features such as suitable band gap, proper band location, great stability and environment friendliness. Recognition of the great potential of the BiVO4-based photocatalyst in
AC
AOPs system, the whole framework of this review was aimed to cover a recent scientific literature and progress related to BiVO4-based photocatalyst in this emerging field. Table 3 summarizes the recent reports on the photocatalytic applications of BiVO4-based photocatalyst. Firstly, this review discussed the fundamental properties of BiVO4–based photocatalyst in the view of experimental and computational perspective. Following that, the photocatalytic degradation principles over BiVO4-based photocatalyst along with the possible photocatalytic 51
ACCEPTED MANUSCRIPT mechanism was well discussed. Understanding the fundamental properties and photocatalytic principles is indispensable to elucidate a firm knowledge of the nature factor which will govern the photocatalytic performance of photocatalyst. Despite the state-of-the-art BiVO4 photocatalyst shows promises in this emerging field, BiVO4 photocatalyst also suffers from a major setback
T
due to its limiting factor which including poor electrical conductivity and charge transfer, slow
IP
water oxidation kinetics and fast recombination rate of electron-hole pairs. With regard to the
CR
aforementioned shortcomings, several strategies such as morphological control, facet engineering, elemental doping and formation of BiVO4-based heterostructure system has been
US
recently suggested and shown a remarkably heightened photocatalytic performance. The insight
AN
of all of these limitations and strategies presented here is imperative for intensifying and advancing the future directions of the BiVO4-based photocatalyst in AOPs technology before
M
debuting into practical industrial applications. Nonetheless, knowledge gap still exists within the
ED
abovementioned discussion where the further in-depth study is required to reveal the true
PT
potential of the BiVO4-based photocatalyst. By running through the various section of this review, the following perspectives and
CE
challenges arise for the future. Most of the current reported literature were only evaluating the
AC
modified BiVO4-based photocatalyst using the synthetic wastewater water such as methylene blue, tetracycline, bisphenol A and methyl orange. However, the report on the evaluation of modified BiVO4-based photocatalyst using real industrial wastewater is relatively scarce. It is known that the real industrial wastewater contains various type of compound which are challenging to be removed in comparison to the synthetic wastewater which focussed on a specific type of compound. In addition, the studies on the intermediates product formed during the photocatalytic reaction are necessary for evaluating the quality of the treated wastewater. 52
ACCEPTED MANUSCRIPT Moreover, the use of natural sunlight throughout the photocatalytic analysis instead of using simulated solar light is one of the perspectives to be highlighted. Most of the reported literature usually used simulated solar light as a light source such as xenon lamp and a halogen lamp. Thus, it is imperative to evaluate the developed photocatalyst materials under the natural sunlight
IP
T
illumination for a future full-fledged commercialization.
CR
Afterward, the photostability of the BiVO4-based photocatalyst is one of the main challenges for photocatalytic applications. Despite the claim on the high photocatalytic efficiency of the
US
BiVO4-based photocatalyst, the lifetime of this materials is too short. The photocorrosion phenomenon observed within this system is hard to avoid. Although some of the literature has
AN
claimed the extended photocatalyst stability are successfully achieved, but these often require a
M
very complex synthetic process. With regard to this matter, a new design toward producing a highly efficient BiVO4-based photocatalyst in complying with the photocorrosion stability is
ED
highly desirable towards practical applications. On the other hand, further pilot plant analysis
PT
with different reactor configurations is highly desirable to ensure that this AOPs via BiVO4based photocatalyst system is well established. In summation, a large scale AOPs technology
CE
design with highly efficient BiVO4-based photocatalyst material can be debuted in the short
AC
future if the abovementioned parameters, limitations and challenges can be circumvented.
53
ACCEPTED MANUSCRIPT
Table 3: Summary on the most recent BiVO4-based photocatalyst performance in photocatalytic degradation applications. Photocatalyst
Model
T P
Irradiation
Experimental Conditions
I R
Pollutant
C S
BiVO4 microstructures
Rhodamine B
U N
BiVO4 microstructures were prepared via a
A
M
Performance
Ref.
time
(%)
(min)
180
99.5
[140]
150
100
[150]
surfactant-free hydrothermal method. Different
D E
amount of Bi3+/V5+ molar ratios was study. A
T P
350 W Xenon lamp with a 400 nm cut-off filter was used as the light source.
BiVO4/PDA/g-C3N4
C A
E C
Glyphosate
BiVO4 was prepared via chemical precipitation method while g-C3N4 powder was fabricated by heating urea powder. A 125 W high pressure
54
ACCEPTED MANUSCRIPT mercury lamp, which the UV light portion was filtered by a 2 M NaNO2 solution. The concentration of glyphosate is 0.1 mM.
Co-Pd/BiVO4
Phenol
I R
T P
BiVO4 support with a leaf-like morphology was
180
90
[170]
150
94.9
[148]
C S
prepared via the hydrothermal method. The
U N
composite photocatalyst was prepared via chemical reduction method with polyvinyl
A
M
alcohol as protecting agent and NaBH4 as a
D E
reducing agent. A 300 W Xenon lamp with an
T P
optical cut-off filter was used a visible-light source. The concentration of phenol is 0.2
E C
Ag/Ag2CO3/BiVO4
C A
Tetracycline
mmol/L.
The sphere BiVO4 sample was prepared via a facile
hydrothermal
55
method
whereas
the
ACCEPTED MANUSCRIPT composite photocatalyst was prepared via a precipitation-photoreduction reaction. A 500 W Xe lamp was used a light source. The initial
T P
concentration of tetracycline is 20 mg/L.
AgI/BiVO4
I R
C S
Oxytetracyclin
The composite photocatalyst was prepared via a
e
deposition-precipitation method. A 300 W
hydrochloride
Xenon lamp with a 420 nm cut-off filter was used
a
D E
80
[134]
60
96.95
[86]
U N
A
M
visible-light
60
source.
The
initial
concentration of OTC-HCl is 20 mg/L.
PCNS/BiVO4
Tetracycline
T P
The phosphorus doped ultrathin graphitic
E C
carbon nitride (PCNS) was prepared via thermal
C A
polycondensation with some modifications. Different weight of PCNS in the composite structure was analysed. A 300 W Xenon lamp
56
ACCEPTED MANUSCRIPT with a UV cut-off filter was used a visible-light source. The initial concentration of tetracycline is 10 mg/L.
RGO-TiO2/BiVO4
Methylene blue
The
RGO-TiO2/BiVO4
prepared
via
Different
amount
photocatalyst
U N
RGO
was
120
100
[149]
210
66.87
[68]
C S
wet-impregnation of
I R
T P
method.
loading
was
analysed. A 500 W halogen lamp was used a
A
M
visible-light source. The initial concentration of
D E
methylene blue is 10 mg/L.
BiVO4/α-Fe2O3
T P
Gaseous
The composite materials was prepared via
Benzene
hydrothermal-calcinations
C A
E C
method.
The
degradation of gaseous benzene was measured in a tubular photo-reactor. A 8 W 365 nm UV lamp was used as light source. 100 mg/m3 of
57
ACCEPTED MANUSCRIPT gaseous benzene was injected into the reactor with micro-syringe.
RGO/g-C3N4/BiVO4
Tetracycline
The new composite photocatalyst was prepared
Hydrochloride
using a facile hydrothermal method. The CN nanosheets
were
prepared
T P 150
72.5
[171]
150
98.95
[172]
I R
C S
via
the
U N
polymerization of urea while RGO was prepared according to the modified Hummers’
A
M
method. A 500 W tungsten light lamp was used
D E
a light source. The initial concentration of TC is
T P
35 mg/L.
BiVO4-rGO
E C
Methyl Orange The composite materials was prepared via a
C A
simple one-step hydrothermal method. The photocatalytic reaction were evaluated under simulated sunlight in a homemade reactor with
58
ACCEPTED MANUSCRIPT a cooling water circulator assembled to keep the reactor at a constant temperature. The initial concentration of MO is 10 mg/L.
BiVO4/Ag/rGO
I R
T P
Methylene
The composite photocatalyst was prepared via
Blue
one-pot synthesis using a facile and cost-
150
-
[173]
90
95
[97]
C S
U N
effective hydrothermal method. A 300 W Xe lamp
was
used
a
light
A
M
source.
The
photocatalytic reaction was performed in a
D E
custom water-cooled borosilicate glass reactor
T P
which capable for maintaining the reaction temperature at ~25°C. The initial concentration
E C
BiFeWO6/BiVO4
C A
Methylene Blue
of MB is 10 ppm.
The BiVO4 nanoparticles were synthesized via additive-free
hydrothermal
59
method
while
ACCEPTED MANUSCRIPT BiFeWO6 was prepared via a facile coprecipitation method. The nanocomposite was synthesized
via
simple
additive-free
wet
chemical process. A 250 W tungsten halogen
I R
lamp was use a light source. The initial
C S
concentration of MB is 2 x 10-5 M.
Sandwich-like BiVO4
U N
Methyl Orange The photocatalyst was prepared via a facile (MO)
A
M
T P 150
86.5
[93]
180
98.5
[105]
microwave-assisted method. A 200 W xenon
D E
lamp with 420 nm cut-off filter was used as a
T P
light source. The initial concentration of MO is 2 x 10-5 M.
m-BiVO4
E C
C A
Methylene
The photocatalyst was prepared via solid-liquid
Blue
state reaction. A 500 W halogen lamp was used a light source. The initial concentration of MB
60
ACCEPTED MANUSCRIPT is 10 mg/L.
BiVO4/Bi2S3/MoS2
Rhodamine B
The heterojunction photocatalyst was fabricated
97
T P
via a facile in-situ hydrothermal method based
I R
Methylene
on the formation of the intermediate Bi2S3 by Blue
300
C S
photocatalytic analysis was done using natural
Green
sunlight at a fixed reaction time with an
[98]
94
hybridizing BiVO4 and MoS2 precursor. The Malachite
93
U N
A
M
ambient temperature of 30°C.
BiVO4/TiO2
Rhodamine B
D E
The photocatalysts were prepared in the form of
T P
films by a simple wet chemical methods.
E C
(RhB)
Various layered configuration of the developed
C A
heterostructures were studied. Different type of irradiation such as UVA, metal-halogenide lamp and visible light was used a light source.
61
180
-
[174]
ACCEPTED MANUSCRIPT The initial concentration of RhB is 10-5 M.
Palygorskite-BiVO4
Tetracycline
The
photocatalyst
was
prepared
via
a
Hydrochloride
hydrothermal method. A 500 W Xe lamp with a
(TC)
cut-off filter was used a light source. The initial
240
91
[175]
97.4
[152]
T P
I R
C S
concentration of TC is 30 mg/L with a constant
U N
stirring at 500 rpm.
Biochar/Fe3O4/BiVO4
Methylparaben
A
The biochar was collected from Himalayan Pine
D E
M
forests. The composite was prepared via addition of biochar and Fe3O4 during the
T P
synthesis of BiVO4. The initial concentration of
E C
methyl paraben is 5 mg/L and 0.01 g of
C A
photocatalyst was used during the reaction study. The solution was kept in the double wall cylinder equipped with a water circulation
62
120
ACCEPTED MANUSCRIPT placed on a magnetic stirrer.
BiVO4/BiOCl
Norfloxacin
The photocatalyst was prepared via an in-situ
60
100
[176]
98.48
[177]
T P
transformation method and the morphological
I R
structure of 2-D BiOCl was facilely tuned via
C S
controlling the concentration of Cl- ions in the
U N
reaction solution. 0.05 g of photocatalyst was used and the initial concentration of norfloxacin
A
M
is 5 mg/L. The distance between the light
D E
source and the liquid level was constant at 10
T P
cm. A circulating water system was employed to maintain the operation temperature at 25°C.
Ag4V2O7/BiVO4
E C
C A
Methylene Blue
The composite photocatalyst was prepared via a facile
sodium
polyphosphate-assisted
hydrothermal method. A 300 W Xe lamp with a
63
60
ACCEPTED MANUSCRIPT visible light filter was used a light source and the distance between the lamp and samples was constant at 10 cm. A 50 mg of photocatalyst was used and the initial concentration of MB is
I R
5 mg/L.
BiOCl/BiVO4@AgNWs
T P
C S
U N
Methylene
The sheet like structure composite photocatalyst
Blue
was prepared via a simple sol-gel method with
180
A
M
97.67
[178]
96.14
the presence of deep eutectic solvents. The Xe Rhodamine B
D E
arc lamp was used a light source. A 10 mg of
T P
photocatalyst
was
used
and
the
initial
concentration of MB and RhB is 10 mg/L.
BiOCl/BiVO4
C A
E C
Rhodamine B
The hierarchical microspheres heterostructure was prepared via a co-precipitation process followed by hydrothermal treatment. For a
64
240
100
[179]
ACCEPTED MANUSCRIPT comparative study, pure BiVO4 was prepared under the same conditions with the addition of nitric acid instead of hydrochloric acid while BiOCl was prepared via simple precipitation
I R
method without the addition of NHVO3, and
C S
without the hydrothermal treatment.
BiVO4/ZnO
Rhodamine B
U N
The Au coated magnetic cilia film was prepared
A
M
T P 120
100
[180]
180
92.01
[181]
via magnetron sputtering under vacuum for 20
nanosheet/Au
D E
s. A 300 W Xenon lamp with cut-off filer was
T P
used with a distant between lamp and solution was constant at 25 cm. The initial concentration
E C
BiVO4/WO3
C A
2-
Chlorophenol
of RhB is 0.01 g/L.
The
heterostructure
nanocomposites
were
synthesized via hydrothermal method. A 150 W
65
ACCEPTED MANUSCRIPT Xe arc lamp with an UV cut-off filter was used as a light source. The analysis was conducted in a cylindrical immersion type photoreactor equipped with a circulating water jacket to
I R
maintain a constant temperature.
In2O3-BiVO4 {010}
Rhodamine B
T P
C S
U N
The composite was prepared via a simple two-
180
91
[182]
100
99
[183]
step hydrothermal method with a different
{110}
A
M
molar ratios. A 500 W Xe arc lamp through a
D E
10-cm IR water filter and a cut-off filter was
T P
used a light source. A 50 mg of photocatalyst was used and the initial concentration of RhB is
E C
BiVO4/MWCNT/Ag@ AgCl
C A
Rhodamine B
5 mg/L.
The heterostructure photocatalyst was prepared via
hydrothermal
and
66
in-situ
oxidization
ACCEPTED MANUSCRIPT method. A 500 W Xe arc lamp with a 420 cutoff filter was used a light source. A 50 mg of photocatalyst
was
used
and
the
initial
T P
concentrations of RhB is 5 mg/L.
m-BiVO4/BiOBr
Rhodamine B
I R
C S
The photocatalysts was prepared via a simple
30
98.9
[102]
180
~55
[111]
U N
one-pot hydrothermal method. A 500 W Xe lamp with a 420 nm cut-off filter was used as a
A
M
light source. A 50 mg of photocatalyst was used
D E
and the initial concentration of RhB is 10 mg/L.
BiVO4
Glyphosate
T P
The photocatalyst was prepared via a co-
E C
precipitation method. A 125 W high-pressure
C A
mercury lamp which the UV light portion was filtered by a 2.0 mol/L NaNO2 was used a light source. A 0.5 g of photocatalyst was used and
67
ACCEPTED MANUSCRIPT the initial concentration of glyphosate is 10-4 mol/L.
Ni-BiVO4
Ibuprofen
T P
The photocatalyst was prepared using a
90
80
[184]
200
-
[185]
I R
microwave hydrothermal method. A 150 W
C S
short arc lamp with a 420 nm cut-off filter was
U N
used a light source. The incident light intensity was 150 mWcm-2 and the total energy per
A
M
second was 20 W. A 0.1 g of photocatalyst was
D E
used and the initial concentration of ibuprofen
T P
is 20 mg/L.
Ce-BiVO4
E C
Rhodamine B
C A
The composite photocatalyst was prepared via a simple precipitation-impregnation method. A 4 W LED light was used a light source and the distance between the lamp and solution was set
68
ACCEPTED MANUSCRIPT to 11 cm. A 0.2 g of photocatalyst was used and the initial concentration of RhB is 5 mg/L.
Ag@g-C3N4@BiVO4
Tetracycline
T P
The g-C3N4, BiVO4 and Ag@g-C3N4@BiVO4
60
90.76
[167]
50
96
[107]
I R
were prepared via modified two-step thermal
C S
treatment method, a facile hydrothermal method and
a
facile
U N
photodeposition
method,
respectively. A 300 W Xe lamp with different
A
M
wavelength band-pass filters was used a light
D E
source. A 0.03 g of photocatalyst was used and
T P
the initial concentration of TC is 20 mg/L.
xCu-BiVO4
E C
Rhodamine B
C A
The photocatalyst was prepared via ethylene glycol solvothermal method with EDTA as the chelating agent. A 250 W halogen lamp was used a light source. A 30 mg of photocatalyst
69
ACCEPTED MANUSCRIPT was used and the initial concentration of RhB is 15 mg/L.
Graphene/BiVO4/TiO2
Methylene
The ternary nanocomposites was synthesized
Blue
via a facile, ultrasonic wave-assisted one pot
T P 10
100
[186]
150
92
[142]
I R
C S
hydrothermal method. A 500 W halogen lamp
U N
was used a light source. 0.1 g of photocatalyst was used and the initial concentration of MB is
A
10 ppm.
Fe2O3 modified Ag-
D E
M
Methyl Orange The n-n-Z-scheme photocatalyst was prepared
T P
via impregnation-calcination method. A 0.02 g
010BiVO4
E C
of photocatalyst was dispersed in 20 ml of MO
C A
solution with an initial concentration of 0.01 g/L.
70
ACCEPTED MANUSCRIPT
BiVO4/Ag+
Methylene Blue
The
photocatalyst
was
prepared
via
a
25
180
-
[73]
240
-
[62]
T P
I R
dispersed in 50 ml of MB solution with an initial concentration of 10 ppm.
Rhodamine B
[53]
hydrothermal method. A Xe lamp was used a light source. 0.05 g of photocatalyst was
Nb-modified BiVO4
100
C S
U N
The photocatalyst was prepared via a simple
A
sol-gel method. A metal halogenide lamp was Stearic Acid
M
used a light source. The active surface area of
D E
thin film is 1.4 cm x 1.4 cm and the initial
T P
concentration of RhB is 10-5 M.
Bi2O3/BiVO4
E C
Methylene
The heterostructure photocatalyst was prepared
Blue
via hydrothermal treatment. The sample was
C A
exposed using six lamps in a homemade photoreactor maintained at 18°C. 10 mg of 71
ACCEPTED MANUSCRIPT photocatalyst was used in a 20 mL of 5 mg/L concentration of MB.
BiVO4/Pyridine-Doped
Phenol
pyridine
g-C3N4
T P
The sample was synthesized via organic doping
approach
and
I R
thermal
Methyl Orange
150
C S
92
[168]
97
copolymerization treatment while the composite
U N
photocatalyst was constructed via an in-situ hydrothermal
method.
A
M
A
0.05g
of
the
photocatalyst was used and dispersed into 150
D E
mL of the solution with the initial concentration
T P
of 10 mg/L.
g-C3N4/BiVO4
E C
Bisphenol A
C A
(BPA)
The composite photocatalyst was used via a facile electro-spinning technique. A Xe lamp with a 420 nm optical filter was used a light source. The initial concentration of BPA is 5
72
120
93
[187]
ACCEPTED MANUSCRIPT mg/L with the presence of 10 mM of H2O2.
CuOx/BiVO4
Bisphenol A
The composite materials was prepared via a
180
used
a
light
-
[38]
I R
as reductant. A 100 W of Xenon ozone-free was
[37]
T P
polyol-reduction method using ethylene glycol
lamp
85
C S
source.
The
U N
photocatalytic experiments were performed in a glass, cylindrical vessel at a liquid holdup of
A
120 mL.
BiVO4-BiFeO3-CuInS2
4-Nitrophenol
D E
M
The composite photocatalyst was prepared via
T P
depositing BiVO4, BiFeO3 and CuInS2 in
E C
2,4-
sequence. A high-pressure xenon short arc lamp
Dichloropheno
C A
with a visible light cut-off filter was used as a
l
light source. The light intensity was set to 100 mW cm-2 using a digital radiometer. A 20 mL
73
180
ACCEPTED MANUSCRIPT of solution with initial concentration of 5 mg/L was used.
Nd/Er co-doped BiVO4
Rhodamine B
T P
The photocatalyst was synthesized using a
150
96
[52]
150
88.58
[155]
I R
microwave hydrothermal method. A 500 W
C S
Xenon lamp was used a light source. The reaction
was
photochemical
U N
conducted reactor
with
A
M
in
an
50
XPA-7
mg
of
photocatalyst dispersed into 50 mL of RhB
D E
solution (10 mg/L).
TiO2/BiVO4
T P
Methylene
The microfiber heterojunctions was prepared
Blue
using cotton as biomorphic templates. A 500 W
C A
E C
xenon lamp was used a light source at a room temperature. 100 mg of photocatalyst was used in 100 mL of 10 mg/L MB concentration.
74
ACCEPTED MANUSCRIPT
BiVO4/TiO2(N2) NTs
Methylene
The TiO2 nanotubes (NTs) were prepared via a
Blue
template method in which ZnO nanowires were
80
91.8
[188]
98
[6]
T P
transformed during a liquid phase deposition
I R
process. The heterostructure photocatalyst was prepared using spin coating techniques. A 350
C S
W Xe lamp with light intensity of 100 mW cm-2 was
used
a
light
U N
source.
The
initial
A
concentration of MB is 10 mg/L.
Fe2O3/3DOM BiVO4
4-Nitrophenol
D E
M
The photocatalyst was prepared using the
T P
ascorbic acid-assisted polymethyl methacrylate
E C
templating and incipient wetness impregnation
C A
methods. A 300 W Xe lamp with an optical cutoff filter was used a light source. 40 mg of photocatalyst and 0.6 mL of H2O2 were added to 100 mL of 4-NP with an initial concentration 75
30
ACCEPTED MANUSCRIPT of 0.4 mmol/L.
GQD/m-BiVO4/t-BiVO4 Carbamazepine The BiVO4 was prepared via hydrothermal (CBZ)
180
96.1
[151]
94.96
[8]
T P
process. A 350 W Xenon lamp was used a light
I R
source. 0.01 g of photocatalyst was dispersed in
C S
50 mL of 10 mg/L of CBZ solution at a room
U N
temperature.
Graphene-bridged
Tetracycline
A
The heterostructure photocatalyst was prepared
D E
M
Ag3PO4/Ag/BiVO4
using a facile in-situ deposition method
(040)
followed by photo-reduction. A 300 W Xe lamp
T P
with a 420 nm cut-off filter was used a light
E C
source. 50 mg of photocatalyst was dispersed in
C A
the 100 mL of TC solution with an initial concentration of 10 mg/L.
76
60
ACCEPTED MANUSCRIPT
BiVO4 nanotubes
Cr(VI)
The BiVO4 nanotubes was synthesized via single-spinneret
electro-spinning
80
95.3
[47]
92.98
[45]
without
T P
template. A 300 W Xe lamp with a cut-off filter
I R
was used as a light source in the presence of
citric acid. 0.05 g of photocatalyst was added
C S
into 100 mL of solution with an initial
U N
concentration of 10 mg/L.
Yb+3, Er+3 and Tm+3 doped BiVO4
Methylene Blue
A
M
The photocatalyst was prepared via microwave
D E
hydrothermal
method
using
T P
ethylenediaminetetraacetic acid (EDTA). A 100
E C
W of infrared lamp was used as an NIR light
C A
source while UV cut-off filter was used a visible light source. 0.1 g of photocatalyst was dispersed in 70 mL of MB solution with an
77
180
ACCEPTED MANUSCRIPT initial concentration of 20 ppm.
Bi4V2O11/BiVO4
Methylene
A modified carbonaceous spheres sacrificial
Blue
template growth technique was employ to
80
100
[55]
98
[7]
T P
I R
prepare the photocatalyst. A 300 W Xe lamp
C S
with cut-off filter was used a light source. 0.1 g
U N
of photocatalyst was added into 100 mL of MB solution with an initial concentration of 10-5
A
mol/L.
CuOx/BiVO4
Ethyl Paraben (EP)
The
D E
M
heterostructure
T P
photocatalyst
was
synthesized via a polyol-reduction method
E C
using ethylene glycol as reductant. A 100 W of
C A
Xenon ozone-free lamp and Air Mass 1.5 Global filter simulating solar radiation was used as a light source. A 120 mL of the aqueous
78
60
ACCEPTED MANUSCRIPT solution containing the desired concentration of EP were loaded in an open-glass, cylindrical reaction vessel at ambient conditions under
T P
continuous stirring.
BiVO4/P25
Ethylene
I R
C S
The nanocomposites was prepared using a
U N
modified citric acid complexing method. An ultrahigh-pressure Xe lamp with a UV cut-off
A
M
filter was used as light source. 3 mL of gaseous
D E
ethylene was injected into the airtight reactor
T P
containing catalyst film.
E C
C A
79
360
11.2
[50]
ACCEPTED MANUSCRIPT
7 Acknowledgement The authors would like to express their appreciation to the Chemical Engineering
T
Department, Universiti Teknologi PETRONAS and Centre of Innovative Nanostructures &
CR
IP
Nanodevices (COINN), Universiti Teknologi PETRONAS for the support.
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Heterojunction Photoanode for Highly Efficient Photoelectrocatalytic Applications, Nano-
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ACCEPTED MANUSCRIPT Epigrammatic Progress and Perspective on the Photocatalytic Properties of BiVO4-based Photocatalyst in Photocatalytic Water Treatment Technology: A Review Highlights:
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1. Latest progress and development of BiVO4-based photocatalyst
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2. Current limitations and strategies to improve the BiVO4-based photocatalyst.
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3. An invigorating perspectives and future directions of the BiVO4-based photocatalyst.
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Mohamad Fakhrul Ridhwan Samsudin, Suriati Sufian, B.H. Hameed PII: DOI: Reference:
S0167-7322(18)31127-9 doi:10.1016/j.molliq.2018.07.051 MOLLIQ 9372
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
5 March 2018 4 July 2018 13 July 2018
Please cite this article as: Mohamad Fakhrul Ridhwan Samsudin, Suriati Sufian, B.H. Hameed , Epigrammatic progress and perspective on the photocatalytic properties of BiVO4-based photocatalyst in photocatalytic water treatment technology: A review. Molliq (2018), doi:10.1016/j.molliq.2018.07.051
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ACCEPTED MANUSCRIPT Epigrammatic Progress and Perspective on the Photocatalytic Properties of BiVO4-based Photocatalyst in Photocatalytic Water Treatment Technology: A Review Mohamad Fakhrul Ridhwan Samsudina, Suriati Sufiana,*, B. H. Hameedb Chemical Engineering Department, Universiti Teknologi PETRONAS, 32610, Bandar Seri
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b
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Iskandar, Perak, Malaysia.
School of Chemical Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal,
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Penang, Malaysia.
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Corresponding author: [email protected]
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Abstract In recent years, Advanced Oxidation Processes (AOPs) via photocatalyst has shown great promises as a new frontier environmental friendly and sustainable wastewater treatment
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technology in replacing the current conventional techniques. At present, bismuth vanadate
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(BiVO4) has garnered much attention as one of the promising photocatalysts in environmental
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remediation applications under visible light irradiation. This is due to intrinsic physical and chemical features of BiVO4; appealing electronic band structure, high physicochemical stability
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and environmental friendliness. This review aims to provide an insight into the most recent
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progress in synthesis, properties and photocatalytic applications of BiVO4, particularly in wastewater treatment. Special emphasis is also placed on the discussion on the major limitations
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exists within the BiVO4 photocatalyst such as poor charge carrier transfer, slow water oxidation
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kinetics and fast recombination rate. Subsequently, an insight into the most recent strategies to circumvent the major limitations is detailed. Finally, invigorating perspectives and future
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directions of this BiVO4 photocatalyst will be presented. It is anticipated that this review will
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deliver a scientific and technical overview to the researchers who work in this field to facilitate the next generation of BiVO4 photocatalyst with profound performances.
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Keyword: Bismuth vanadate, photocatalysis, advanced oxidation processes, wastewater treatment.
1 Introduction In the recent years, environmental pollutions such as global warming and water contamination due to the excessive consumption of fossil fuels and release of toxic and harmful 2
ACCEPTED MANUSCRIPT chemicals, such as dyes [1], heavy metals [2], ionic liquid [3], pesticides and surfactants [4] owing to the rapid development of industrialization has been recognized as one of the biggest threats facing the society. For example, the excessive release of the carcinogenic chemicals may cause severe effects not only on aquatic organisms but also to the human being. According to the
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Malato et al., [5] approximately around millions of people globally reported to die due to the
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serious waterborne diseases owing to the consumption of polluted water and 2.6 billion of people
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has no access to a clean water source. Moreover, these unfortunate phenomena may continue to
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grow if there is no solution being employed in solving this crisis.
These environmental problems breeze in as the presence of the organic pollutant is not fully
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degraded by the conventional methods [6–8]. Chlorination, bio-flocculation, and membrane
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technologies [9–12] are some of the many conventional solutions that are currently being utilized. However, these methods have several immedicable issues such as multiple stage
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processes, expensive and incomplete treatment which lead to deprived of the overall
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performance [13,14]. For an example, flocculation and coagulation have been one of the most common wastewater technology used in many industries [15]. In this technology, chemical
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flocculants or coagulants such as inorganic metal salts are added during the treatment, resulting
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in the increasing of the concentration of metal ion in the treated water [16]. The increasing of metal ion concentration in the treated water can cause severe harm to the human health. Moreover, the major limitation of the coagulation technology is its low performance in treating a very fine particle presence in the wastewater and the efficiency of the coagulation treatment significantly deteriorate in the presence of the cold water [17,18]. On the other hand, membrane technology has been considered as one of the promising emerging technology which is expected to overcome the current limitations facing in the wastewater treatment. Albeit the higher 3
ACCEPTED MANUSCRIPT chemical oxygen demand (COD) removal efficiency performance obtained via membrane technology, the applicability of this emerging technology is still being confined to its membrane fouling issues [16]. To date, advanced oxidation processes (AOPs) has garnered more attention in providing an
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alternative for the current limitation facing in the conventional wastewater treatment. The AOPs
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has been denoted as a current innovative water treatment technologies with intrinsic features such as environmental friendliness [19], economically feasible [20] and simple processes [21]. In
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this system, the in-situ generation of highly reactive oxidant species is capable of fully treating and removing most of the presence of the organic pollutants in the wastewater [22–24]. These
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reactive oxidant species, commonly superoxide (•O2¯) and superhydroxyl (•OH) radicals are
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known as one of the strongest radicals after fluorine owing to its high standard reduction potential of E° (•OH/H2O) = 2.80 V standard hydrogen electrode (SHE) [25]. Moreover, the
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during the AOPs treatment.
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short lifetime of these generated radicals within nanoseconds allows it to be self-eliminated
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Generally, there are four different type of AOPs can be employed which are photocatalysis, ozone treatment, direct decomposition of water and electrochemical processes [13].
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Photocatalysis via photocatalyst materials uses light energy to generate electron-hole pairs which consequently produce highly reactive oxidant species for treating the recalcitrant pollutant [25]. Meanwhile, ozone treatment is a process in which ozone gas is introduced into the wastewater. The ozone gas will directly or indirectly interact with the presence of the compounds in the wastewater, resulting in the decomposition reaction [22]. On the other hand, Fenton oxidation process uses a strong chemical such as hydrogen peroxide (H2O2) and Iron (II) (Fe2+) in 4
ACCEPTED MANUSCRIPT generating the reactive oxidant species. However, the use of H2O2 in the treatment cause a setback in AOPs performance as Fenton oxidation process cannot be operated in an alkaline condition [26]. Among these AOPs, photocatalysis technology has intrigued much attention due to its relative ease of process and capability of treating the recalcitrant pollutant without
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producing any secondary harmful by-product [27].
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For the past decades, titanium dioxide (TiO2) is the most frequent photocatalyst used owing to its outstanding chemical and physical stability, economically viable, non-toxicity and
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abundance in comparison to others type of photocatalysts such as zinc oxide (ZnO) and copper (II) oxide (CuO) [2,28–30]. In lieu of TiO2 good effectuation, TiO2 has a relatively large band
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gap energy of 3.2 eV, consequently abnegate its performance in harnessing more visible light
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energy that occupies the astronomical part of the solar light [3,31]. Moreover, the fast recombination rates within 10 nanoseconds between the photogenerated electron-hole pairs of
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TiO2, abstain its photocatalytic degradation performance [31]. Hence, many efforts have been
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devoted to excavating a new type of photocatalysts that are capable of harnessing the visible light energy such as zinc sulphide (ZnS), cadmium sulphide (CdS), tungsten oxide (WO3) and
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BiVO4 [3,20,31–44].
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Among these photocatalysts, bismuth vanadate (BiVO4) have garnered tremendous scientific interest as a frontier photocatalyst in AOPs technologies [45–56]. BiVO4 is a non-toxic metal free photocatalyst that possesses outstanding chemical and photonic properties [56,57]. In addition, the lower band gap energy of ~ 2.4 eV allows BiVO4 to correspond to the visible light spectrum which accounts up to 48% of total solar light energy [20]. Additionally, the presence of O 2p and Bi 6s hybrid orbitals in the electronic structure of BiVO4 in comparison to TiO2 which 5
ACCEPTED MANUSCRIPT solely made up of O 2p orbitals helped the BiVO4 to be able to absorb visible light energy by decreasing the band gap energy [58]. Nevertheless, the photocatalytic degradation performance of BiVO4 is substantially low owing to the poor electron-hole pair transport and mobility, and slow water oxidation kinetics [59,60]. Given of the aforementioned issues, various strategies
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have been adopted such as the development of the heterostructure system [61,62], coupling with
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carbonaceous materials [8,51,63], introducing electron-hole scavengers [21,53,64], and
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additional of bias energy [49,65] show promises to enhance the efficiency of BiVO4.
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In recognition of the great potential of BiVO4 as a frontier visible light driven photocatalyst, the work accounted here is aiming to give an overview of the recent related studies on the
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development of the BiVO4-based photocatalyst in photocatalytic wastewater treatment
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application. The basic principle and mechanism of photocatalyst system are detailed to promote a better understanding of the photocatalyst specifically on BiVO4. Next, the topical reported
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literature on the photocatalytic performance of BiVO4-based photocatalyst is summarized.
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Afterwards, the limiting factors affecting the photocatalytic performance of BiVO4 and the current strategies to alleviate the aforementioned limiting factors are thoroughly reviewed.
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Subsequently, current challenge on the photoactivity of BiVO4 and future directions of BiVO4
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are included in this review.
2 General Overview on BiVO4 Photocatalyst Initially, the use of photocatalyst as an environmental sound solution has been extensively investigated since the first idea that was triggered by Honda and Fujishima [66]. They had successfully demonstrated the overall water splitting process using photoelectrochemical water 6
ACCEPTED MANUSCRIPT splitting technique employing rutile-TiO2 and Pt cathode under ultraviolet (UV) radiation along with the presence of external bias. However, the photocatalytic effectiveness of TiO2 photocatalyst remains a boundless challenge in this field as it can only be excited by UV irradiation which accounts up to 4% of solar light energy. TiO2 has a wide band gap which is
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approximate ~3.2 eV, thus limiting the practical application of TiO2 in sunlight [30]. Henceforth,
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the development of an efficient visible-light-driven photocatalyst has been a topic of focus in
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photocatalysis.
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Up to date, various type of visible-light-driven photocatalyst has been discovered and developed as an alternative for conventional TiO2 photocatalyst such as iron oxide (Fe2O3) [67–
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72], BiVO4 [34,56,57,73], WO3 [74–78], ZnO [79–82], CuO [83–85] and graphitic carbon nitride
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(g-C3N4) [86–89] photocatalyst. Figure 1 depicts the currently reported photocatalyst with their band gap energy. Most of the reported photocatalysts in Figure 1 have a thermodynamically
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favorable of valence band for the water oxidation reactions to occur, resulting in a more available
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electron-hole pair for producing •O2¯ and •OH radicals. Having an appropriate band gap energy which is favorable for producing •O2¯ and •OH radicals is a significant factor in developing
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highly functionalized photocatalysts for photocatalytic degradation application. Among these,
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BiVO4 is a promising candidate that struck a balance between various intrinsic features, due to its suitable band gap, proper band location, great stability and environment friendliness.
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Synthesis of BiVO4 Photocatalysts
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Figure 1: Summary of the current reported photocatalyst with its band gap energy.
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There are several representative synthesis methods that have been widely used for the
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production of BiVO4 ranging from a microwave-assisted method, hydrothermal-assisted method, solid-liquid state reaction, a sol-gel method, co-precipitation method and solution combustion
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method. The microwave-assisted method is generally known as a method that escalates the
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reaction rates and often occurs at a lower temperature compared to other conventional heating methods [90]. In microwave-assisted approach, the photocatalyst materials can be uniformly heated resulting in a homogeneous nucleation and thus produce the smaller size of photocatalyst structure which is highly pure and greater yield of productions [91]. In 2008, Ming et al. [90] reported the successful fabrication of BiVO4 via this method in which highly crystalline phase was converted irreversibly from tetragonal to monoclinic phase. The sodium metavanadate (NaVO3) and bismuth (III) nitrate pentahydrate (Bi (NO3)3.5H2O) were used as starting 8
ACCEPTED MANUSCRIPT precursors with the addition of cetyl trimethyl ammonium bromide (CTAB) as a shape controller. However, the addition of CTAB leads to some drawbacks in term of complex process and presence of heterogeneous impurities as a by-product. With regard to this issue, Weidong et al. [92] demonstrated the successful fabrication of
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monoclinic BiVO4 via a facile organic-free microwave-assisted method in which Bi
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(NO3)3.5H2O and ammonium metavanadate (NH4VO3) were used as starting materials. Similarly, Abraham et al. [91] reported the development of BiVO4 photocatalyst via this
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approach which utilized the same starting materials dissolved in a mixture of diethylene glycol and deionized water. The XRD analysis verified the sharp peaks splitting observed indicates a
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good crystalline structure was developed and can be ascribed to monoclinic structure (JCPDS
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No. 75-1866). Recently, Liu et al. [93] also reported the fabrication of monoclinic BiVO4 via the microwave-assisted method with an addition of PEG 10000 materials as a morphology-directing
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agent which can confine the crystal growth in certain directions. Along with an additional of
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hydrothermal reaction time, the solid BiVO4 sheets were observed to be in sandwich-like sheets
activities.
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that have a large BET surface area which is beneficial to the enhancement of photocatalytic
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On the other hand, hydrothermal synthesis method has been widely used for synthesizing BiVO4 photocatalyst owing to its simple process which can produce a perfect crystal structure at a relatively low temperature [94–96]. The hydrothermal process uses aqueous and/or nonaqueous systems as the reaction medium and promotes environmental friendly procedure as the reactions are carried out in a closed system. Generally, there are three variations on preparing BiVO4 via this method, by adding (i) organic and inorganic additives, or (ii) surfactants and (iii) 9
ACCEPTED MANUSCRIPT without adding any additives. Malathi et al. [97] successfully synthesized BiVO4 nanoparticles via the additive-free hydrothermal process in which Bi (NO3)3.5H2O and NH4VO3 were mixed under continuous agitation before heated at a temperature of 180 °C for 12 hours. The XRD peaks signify the prepared BiVO4 was in a monoclinic structure (JCPDS No. 83-1699) with a
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band gap energy of 2.33 eV as calculated from a Tauc plot graph. Similarly, Wang et al. [98]
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using the same initial precursors which successfully developed monoclinic BiVO4, evidently
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from the peak splitting observed in XRD analysis are in a good agreement with the standard
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monoclinic phase (JCPDS No. 14-0688).
On the other hand, Lu et al. [99] prepared different morphological structure of BiVO4 via a
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facile one-step hydrothermal method with the addition of sodium citrate as a chelating agent.
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Likewise, Thalluri et al. [100] using ammonium carbonate a structural directing agent for growing the selectivity of {040} crystal facet of monoclinic BiVO4. According to their group,
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the addition of this structural directing agent has a significant impact on growing the {040}
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crystal planes, evidently from the increase in the XRD peak intensities. The selectivity growth of {040} crystal planes is known to have positive impacts towards the photocatalytic water
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oxidation activities. Besides that, Jian et al. [101] reported the successful fabrication of BiVO4
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via hybridizing the reverse microemulsion and hydrothermal methods. In addition, the BiVO4 photocatalyst can also be prepared via the one-pot hydrothermal method as reported by Guang et al. [102]. Their group claimed that this approached can easily tailor the morphological structure of as-synthesized photocatalyst into 3D four-petaled flower-like, cube-like and rod-like structure by varying the amount of Br:V molar ratios.
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this approach, bismuth (III) oxide (Bi2O3) and vanadium pentoxide (V2O5) were used as starting
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precursors and mixed with different concentrations of aqueous nitric acid solution. Recently, Tan
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et al. and Samsudin et al. [59,104–106] also prepared the monoclinic BiVO4 photocatalyst using the same methods. Apart from these, sol-gel method also one of the common method for
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preparing the BiVO4 photocatalyst [107]. The advantages of using this method are owing to its
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relatively low production cost high reliability, the simplicity of process and ease of morphological control [108]. Moreover, co-precipitation [109–111] and solution combustion
Properties of BiVO4 Photocatalysts
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synthesis are one of the fabrication technique that has been reported previously [99,112–114].
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Initially, BiVO4-based materials have been extensively explored as a promising alternative for toxic-pigments based materials used in the coating and plastic industry [115]. However, for
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the past few decades, the exploration of this promising materials has been shifted to the applicability of BiVO4 in photocatalytic application owing to its intrinsic crystal structure [116]. Naturally, the crystallographic structure of BiVO4 exists in an orthorhombic structure originate from mineral pucherite [116]. Nevertheless, the efforts for mimicking BiVO4 orthorhombic nature structure lead to the unsuccessful outcome [115]. Most of the preparation method for synthesizing BiVO4 failed to adopt the pucherite structure, but the structure of BiVO4 was 11
ACCEPTED MANUSCRIPT crystallized in a scheelite or a zircon-like structure [117].
Generally, there are three different
types of BiVO4 structure reported in the literature [6,101,118,119]. The three crystal structures of BiVO4 are monoclinic scheelite, tetragonal scheelite and tetragonal zircon. As mentioned earlier, the photocatalytic properties of BiVO4 are strongly dependent on its crystal structure [116].
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Previously, Kudo et al. [120] reported that the monoclinic scheelite structure with the
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presence of electron scavengers showed a better photocatalytic water oxidation performance in comparison to the tetragonal structure. The better photocatalytic performance observed here was
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owing to the better optical absorption properties of the monoclinic structure with a band gap energy of 2.4 eV in comparison to the tetragonal structure with 2.9 – 3.1 eV band gap energy
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[121]. According to Kudo et al., [120] the presence of an additional Bi 6s orbitals in the valence
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band of monoclinic BiVO4 allows a short transition of photoexcited electron to the V 3d orbitals in the conduction band in comparison to the tetragonal BiVO4 which only consist of O 2p
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orbitals, resulting in a smaller band gap energy. Figure 2 illustrates the schematic diagram of the
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tetragonal zircon and monoclinic scheelite BiVO4 structure.
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Figure 2: Difference in a band structure of tetragonal zircon and monoclinic scheelite BiVO4. This postulation make by Kudo et al. was further confirmed by Walsh et al. [122] in which
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their Density functional theory (DFT) calculations suggested that the monoclinic BiVO4 was a
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direct band gap semiconductor which the conduction band composed of O 2p and Bi 6p orbitals whereas the valence band composed of O 2p and Bi 6s orbitals. According to their report, the
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monoclinic structure of BiVO4 (space group 14, C2h6) consists of four unique lattice sites: Bi
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(4e), V (4e), O1 (8f) and O2 (8f). With respect to the crystal inversion symmetry, a primitive cell of two formula units can be developed as shown in Figure 3. The layered structure of Bi-V-O
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units stacked parallel to the c axis draws originates from Bi2O3 and V2O5 binary analogues [123]. The arrangement of Bi atom is in a distorted oxygen octahedron with nearest neighbour distances ranging from 2.35 to 2.53 Å, whereas its centre is made up of V species with 2 x 1.74 Å and 2 x 1.75 Å bond lengths. In addition, the O1 is coordinated to one Bi and V, whereas O2 is coordinated to two Bi and a single V. The asymmetric coordination system observed within the
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the active lone pair commonly present with ns2 valence cations [124].
Figure 3: BiVO4 crystal structure with blue, red and green represented bismuth, oxygen and
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vanadium tetrahedral, respectively. Reproduced with permission from Ref. [122].
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In contrary, a recent report by Cooper and co-workers [125] suggested that the monoclinic BiVO4 was not a direct band gap semiconductor. They performed a thorough DFT calculation
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along with the experimental evidence obtained via a combination of X-ray absorption
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spectroscopy (XAS), X-ray emission spectroscopy, resonant inelastic X-ray spectroscopy (RIXS) and X-ray photoelectron spectroscopy. According to Cooper et al., [125] the conduction band of BiVO4 was mainly dominated by the O 2p orbitals whereas the Bi 6p – O sp2 and V 3d – O sp2 orbitals were located at the lowest and middle regions (as shown in Figure 4). Besides that, the inequivalent pairs of oxygen atoms surrounding V atoms reduce the symmetry to C 2, which consequently splits the d-orbitals into triplet bands. In addition, the predicted triplet d-manifold splitting of V 3d conduction band states with splitting energies of 1.00 and 1.91 eV, arising from 14
ACCEPTED MANUSCRIPT lone pair-induced lattice distortions, is quantified by V-L and O K-edge XAS. Furthermore, the localization of V d-orbitals at the conduction band minimum (CBM) due to poor overlap with Bi
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6p orbital would result in poor electron mobility.
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Figure 4: Energy level diagram for the electronic structure of ms-BiVO4 calculated from density
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functional theory (DFT) and experimental analysis. Reproduced with permission from Ref. [125].
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Similarly, Zhao et al. [126] suggested that the valence band of BiVO4 was mainly characterized by non-bonding O 2pΠ orbitals and there are non-bonding states both at the top
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and bottom of valence and conduction band, respectively. Moreover, Zhao et al. postulated that the small band gap energy observed in BiVO4 was attributed to the lone-pair distortion in Bi 6s, consequently shifted the O 2p orbitals to a lower energy level. Furthermore, Zhao et al. and Walsh et al. recommend that the effective masses of both electrons and holes in BiVO4 in monoclinic scheelite structure were much smaller in comparison to other oxide-based photocatalysts. This smaller effective masses obtained via DFT calculations signify the unique 15
ACCEPTED MANUSCRIPT features of the monoclinic structure of BiVO4 in which it will facilitate the electron-hole pair separation and migration, resulting in better photocatalytic performance [116].
3 Photocatalytic
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BiVO4
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Photocatalyst
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Photocatalyst can be described as a combination of catalysis and photochemistry in which absorption of photon energy from light via catalyst is the key towards photoreaction process
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[13]. The difference of photocatalyst in comparison to conventional catalyst is the mode of
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activation. In conventional catalyst technology, thermal energy is required for catalyst activation whereas in photocatalyst system, photons energy is required for the activation of photocatalyst
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[25]. The use of photon energy as a source of activation make photocatalyst technology as one of
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the frontier renewable energy technology in which it can utilize the solar energy.
16
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Figure 5: Schematic illustration of photocatalytic degradation principle and mechanism of BiVO4 photocatalyst.
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BiVO4 is a type of semiconductor having a small band gap energy (2.4 – 2.5 eV) which fall
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within visible-light region [104,105]. It is a promising photocatalyst that has intrinsic features such as smaller band gap energy, ease of production, and potential for promoting greener
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technologies. Figure 5 illustrates the basic photocatalytic degradation mechanism of BiVO4
CE
photocatalyst. Initially, BiVO4 photocatalyst will absorb photon energy with an energy equivalent or greater than its band gap energy, causing an electron (е¯) in the valence band (VB)
AC
to be excited and migrate to the conduction band (CB). BiVO4 + hv → е¯ CB (BiVO4) + h+ VB (BiVO4)
(1)
The migration of this photoexcited electron will leave photogenerated holes (h+) in the valence band. The photogenerated electron-hole pairs will migrate to the surface of the photocatalyst and trapped there. The photogenerated holes then react with adsorbed water to 17
ACCEPTED MANUSCRIPT produce strong oxidizing •OH radicals whereas the photoexcited electrons react with adsorbed oxygen to generate •O2¯ radicals. (2)
h+ VB (BiVO4) + OH¯ → •OH
(3)
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е¯ CB (BiVO4) + O2 → •O2¯
degraded the recalcitrant pollutant.
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•O2¯ + recalcitrant pollutants → degradation of pollutant
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The formation of these radicals will further react with recalcitrant pollutant and subsequently
(5)
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•OH + recalcitrant pollutants → degradation of pollutant
(4)
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Apart from these major photocatalytic degradation reaction described earlier, there are some weaker photocatalytic reaction could presumably occur during the reaction which can produce
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some other intermediate species such as H2O2 and HO2• which can be summarized as follow
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[25]:
(6)
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•O2¯ + H+ → HO2• 2HO2• → H2O2 + O2
AC
(7)
Nevertheless, the major limiting factor in BiVO4 photocatalyst system is the fast recombination rate within nanoseconds. Congruously, with the absence of an appropriate electron-hole acceptor or donor, the photoexcited electrons and photogenerated holes will recombine and dissipate the energy as heat [13]. Moreover, some of the unstable photoexcited electrons could be adsorbed by •OH, consequently produced OH¯. 18
ACCEPTED MANUSCRIPT е¯ CB (BiVO4) + h+ VB (BiVO4) → Catalyst + heat (energy)
(8)
е¯ CB (BiVO4) + •OH → •OH¯
(9)
This recombination of electron-hole pair phenomenon is one of the significant factor and
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drawback in hindering the photocatalytic degradation performance of BiVO4. Furthermore, the
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poor performance of BiVO4 photocatalysts is hampered by its sluggish water oxidation kinetics
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[115] and poor photogenerated charge carrier separation and migration [116]. With regards to these limiting factors exist within the BiVO4 photocatalyst, the forthcoming section is devoted to
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enhance the overall photocatalytic performance.
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briefly discuss the aforementioned limitation and current strategies that recently been adopted to
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4 Limitations of BiVO4 Photocatalyst
BiVO4 photocatalyst can be considered as one of the most intrigued visible-light-driven
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photocatalyst owing to its smaller band gap energy of 2.4 eV. In light of the small band gap
CE
energy of BiVO4 photocatalyst, it was estimated that up to 11% of the standard AM1.5 solar spectrum can be absorbed, corresponding to the maximum photon to photocurrent conversion
AC
efficiency of 7.6 mA cm-2 [115]. Nevertheless, to date, the actual energy conversion obtained was still far from its theoretical value which makes the BiVO4 photocatalyst was still far from debuting into practical industrial applications. The major limiting factors that govern the BiVO4 photocatalyst performance were poor photocharge carrier transfer [20,116], slow water oxidation kinetics [115,127], and fast recombination rate of electron-hole pairs [37,52,128, 129].
19
ACCEPTED MANUSCRIPT The density functional theory (DFT) study performed by Zhou et al. [130] indicated that the poor photocharge carrier transfer within BiVO4 photocatalyst was due to the geometrical structure of tetrahedral VO4 species which was disconnected between each other in the V 3d orbitals, consequently abnegate the smooth movement of photoexcited electrons between
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tetrahedral VO4 species and substandard solar conversion efficiency. The disconnected of
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tetrahedral VO4 species caused a further in the distance of photocharge carrier movement,
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consequently promote the photocharge recombination rate (as shown in Figure 6). According to the Xie et al. [131], the disconnected of tetrahedral VO4 species in monoclinic BiVO4 observed
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caused bigger disadvantages in photocharge carrier separation and migration if the
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heterostructure strategies were employed to circumvent the limitations facing by pristine BiVO4.
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It is known that the enhancement of photocharge separation and migration within the heterostructure system are strongly dependent on the connection and lattice matching between
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the photocatalyst structures. For example, the integration of monoclinic BiVO4 and tetragonal
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anatase TiO2 would result in weaker photocharge carrier transfer owing to the low degree of lattice matching, consequently hampered the overall photocatalytic activity of heterostructure
CE
[132,133]. Thus, it is a great interest among researcher recently to introduce an electron mediator
separation
AC
as a bridge at the interface of the heterostructure system for facilitating the photocharge carrier and
migration
which
will
be
[38,43,67,78,88,134].
20
further
discussed
in
the
next
section
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Figure 6: (a) electronic band structure, (b) electronic density of states, plotted of (c) minimum
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conduction band and (d) maximum valence band at certain point and (e) atomic supercell
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structures of pristine BiVO4. Reproduced with permission from Ref. [130]. Slow water oxidation kinetics occurs owing to its high kinetic barriers for water oxidation
ED
reaction to occurs [129]. In water oxidation process, a bulk of electron and holes involved during
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the separation and migration reaction, consequently a significantly high over potential are required for the water oxidation reaction to occurs [116]. Similarly, Tan et al. [60] observed a
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saturated BiVO4 photocurrent at a low potential of -0.30 V vs Ag/AgCl owing to its relatively
AC
bigger particulate size. Additionally, the BiVO4 photocatalyst has a higher onset potential of 0.65 V vs Ag/AgCl which indicated a higher energy input was required for the reaction to occur [133,135]. Moreover, Tan et al. [60] concluded that the efficiency of BiVO4 was still dominantly limited by its intrinsically low carrier mobility despite the various successful effort demonstrated previously in circumventing the respective limitations.
21
ACCEPTED MANUSCRIPT Meanwhile, Xia et al. [19] postulated that the sluggish water oxidation kinetics of BiVO4 would cause a highly concentrated of photogenerated holes residing on the surface of the photocatalyst, consequently promote the recombination of the electron-hole pair. Generally, the photogenerated holes will migrate towards the surface of photocatalyst upon the continuous light
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illumination until a steady state concentration is reached, resulting in a dense of photogenerated
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holes accumulated at the photocatalyst surface. According to the Xiao et al. [136] the slow water
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oxidation kinetics at the photocatalyst surface rich with the holes density while the negative transient of photocurrent observed indicated the recombination of the photoexcited electron with
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the accumulated photogenerated holes at the photocatalyst surface.
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Another performance limiting factor for BiVO4 photocatalyst is fast recombination rate of
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the electron-hole pair. This fast recombination rate of electron-hole pair occurred within a few nanoseconds [104,105]. Generally, the photogenerated carrier charge recombination rate can be
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explored via photoluminescence spectra characterization by examined the released energy of the
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photocatalyst. According to Huang et al., [53] the faster the photogenerated charge carrier recombination rate would result in a higher detection of photoluminescence intensity,
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consequently affecting the overall photocatalytic performance.
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In addition, Yu et al. [137] observed prominent photoluminescence peaks at the region of 525 nm, indicating a detection of recombination center within BiVO4 photocatalyst originated from the V 3d and hybridization of Bi 6s and O 2p orbitals. Subsequently, Song et al. [50] revealed that the poor photocatalytic degradation performance of BiVO4 photocatalyst in treating the gaseous ethylene under visible light irradiation despite being a visible-light-driven
22
ACCEPTED MANUSCRIPT photocatalyst was due to fast recombination rate and poor photocharge carrier separation and migration.
5 Strategies for Improving Photocatalytic Activities of BiVO4
Morphological Control and Facet Dependent Engineering
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5.1
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Photocatalyst
Morphological control and facet-dependent engineering are among strategy can be used for
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overcoming the major limitation facing by the BiVO4 photocatalyst. Some reported studies claimed that the photocatalytic performance of photocatalyst was majorly dependent on the
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degree of facet interface exposure [115,116]. This postulation was due to the fact that most of the
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photocatalytic reaction occurred on the surface of the photocatalyst in which coordination and
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arrangement of the atomic surface was the vital key towards the development of highly functionalized photocatalyst [115]. To date, numerous method of fabricating the BiVO4
CE
photocatalyst has been developed including sonochemical method [94], spray pyrolysis [138],
AC
microwave [45] and solid-state reaction [104,105]. Different in fabricating method would result in different morphological and crystal structure of BiVO4. Thus, it is crucial for selecting an effective method for fabricating BiVO4 which can produce a highly crystalline structure with a perfect morphological structure. Previously, Zeng et al. [139] reported the successful fabrication of BiVO4 microcrystals prepared via surfactant-assisted hydrothermal method using sodium dodecylbenzenesulfonate 23
ACCEPTED MANUSCRIPT (SDBS). The result showed a significant improvement in photocatalytic activity of BiVO4 owing to its enhancement in photocharge carrier separation and migration. Nonetheless, the fabrication of BiVO4 photocatalyst with selective crystal facet via surfactant-assisted hydrothermal method still caused a setback in photocatalytic performance as well as increase the production cost.
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Moreover, the used of certain surfactant in synthesizing BiVO4 is hardly removed after
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synthesizing process which would result in unwanted surfactants molecules attached in the
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crystal structure of BiVO4 photocatalyst. The presence of this unwanted surfactant molecules would limit the absorption rate between BiVO4 photocatalyst and targeted recalcitrant pollutant,
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thus reduce the photocatalytic activities. With regard to this problem, Wu et al. [140]
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successfully reported the fabrication of the different morphological structure of BiVO4 prepared via a surfactant-free hydrothermal method. They observed that the BiVO4 morphological
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structure can be simply modulated via varying the molar ratio of starting precursor, resulting in
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several structures including dumbbell, rod, ellipsoid sphere and cake-like structure as depicted in Figure 7. In addition, the as-developed BiVO4 photocatalyst exhibited different photocatalytic
AC
CE
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performance associated with their surface area and morphological structure.
24
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Figure 7: BiVO4 photocatalyst prepared via surfactant-free hydrothermal method with different
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molar ratio of starting precursor. Reproduced with permission from Ref. [140]. On the other hand, Tan et al. [60] suggested the use of nanoscaling approached in improving
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the photocharge separation and migration via reducing the size of the photocatalyst in both
CE
BiVO4 photoelectrochemical cell (PEC) and powder suspension system (PS). Their group claimed that the mechanism for the photocharge separation and migration in both systems are
AC
different (as shown in Figure 8). In powder suspension, the transfer of photogenerated holes and photoexcited electrons to the electrolyte and bulk, respectively were dependent on the thickness of space charge layer formed at the interface of the photocatalyst. Meanwhile, the photocharge separation and migration were smoothly transferred via the addition of an external applied potential in the PEC system. According to the Tan et al., the photocurrent density of BiVO4 with a smaller particle size significantly improved in comparison to larger particle size. Furthermore, 25
ACCEPTED MANUSCRIPT the onset potential result showed a significant negative potential shifted with respect to the decrease in particle size from -0.65 to 0.35 V vs Ag/AgCl. The low onset potential was favorable in the photocatalytic application as the only small amount of energy were require to initiate the
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reaction.
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Figure 8: Influence of different particle size on photoactivity of BiVO4 in PEC and PS system. Reproduced with permission from Ref. [60].
Despite the successful enhancement of photocatalytic performance via tailoring the morphological structure of BiVO4, the fabricating of BiVO4 photocatalyst with the specific growth of crystal facet has garnered much attention recently [59,100,141–144]. Thalluri et al. [100] reported the green synthesized of selective {040} crystal facet of BiVO4 via sustainable hydrothermal synthesis method with an additional of ammonium carbonate as a structural 26
ACCEPTED MANUSCRIPT directing agent. The XRD result showed an enhancement of {040} crystal facet which signified the help of ammonium carbonate in helping the preferential growth of {040} crystal facet, thereby remarkably enhanced the photocatalytic activity by 20 and 100 times in comparison to pristine BiVO4. Moreover, the smaller difference in the BiVO4 crystallite size associated with the
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different dopant loading indicated the insignificant effect of crystallite size on photocatalytic
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performance.
On the contrary, Tan et al. [59] suggested that the photocatalytic performance of BiVO4 were
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mainly governed by the {010} crystal planes which known as most active facet. The BiVO4 photocatalyst with a large surface area of {010} crystal facet showed a superior photocatalytic
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performance in comparison to BiVO4 photocatalyst with a large surface area of {110} crystal
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facet. Moreover, the photoluminescence measurement of {010}-BiVO4 photocatalyst showed significant quenched in response to hole-scavengers while inverse phenomenon was observed
ED
with {110}-BiVO4 photocatalyst. Besides that, the time-resolved photoluminescence and Mott-
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Schottky study revealed a major extent of electron trapping and flat band potential were detected in {110}-BiVO4 photocatalyst. These phenomenon observed were ascribed to the longer distance
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of electrons transfer from the bulk BiVO4 to the {010} crystal facet as shown in Figure 9. In
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addition, the smaller size of {010} crystal facet in {110}-BiVO4 photocatalyst limits the electron density accumulated on the {010} crystal facet, consequently promote the photocharge carrier recombination and inefficient photocatalytic performance.
27
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Figure 9: The schematic illustration on the influence of {010} and {110} crystal facet dependent
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on BiVO4 photocatalytic activity. Reproduced with permission from Ref. [59].
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Likewise, Zhao et al. [145] performed a crystal modulation study by transforming the standard decahedron BiVO4 structure prepared via hydrothermal method into short rod-like
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particles which specifically tailoring the proportion of {010}/{011} crystal facet. It was found
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that the modified BiVO4 structure showed a significant photocatalytic activity with 50 times better than standard BiVO4. The significant results obtained was owing to the synergistic effect
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of {010} crystal facet act as an active reduction site while {011} crystal facet serves as an active
CE
oxidation site. Similarly, inspired by the feasibility of smooth photogenerated charge carrier separation and migration between {010} and {110} crystal facet owing to the valence band and
AC
conduction band of {110} is higher than {010} crystal facet, Li et al. [142] exploited this advantages by incorporating Ag and Fe2O3 nanoparticles for producing a highly performing photocatalytic activity. As a result, remarkable photocurrent density with the smallest impedance was observed with regard to the aforementioned strategy. The remarkable photocatalytic performance was owing to the synergistic effect of n-n and Z-scheme mechanism associated to
28
ACCEPTED MANUSCRIPT the manipulation of crystal facet which smoothly accedes the electron-hole pair separation and migration.
Elemental Doping
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5.2
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Elemental doping with a metal or non-metal materials is one of the alternative strategies used
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to improve the photocatalytic performance of BiVO4 photocatalyst. Elemental doping is a process of incorporating an additional element or impurities in a photocatalyst material which
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consequently tailoring the photocatalyst physical properties such as electrical and optical
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properties [146]. There are few advantages of these elemental doping strategies such as (i) enhancement of the photocatalyst electrical conductivity , (ii) modulation of photocatalyst band
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gap energy, (iii) enhancement in photocatalyst active surface site area and (iv) allows a smooth
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electron-hole separation and migration [116,146]. Nevertheless, this elemental doping strategy may not always be favorable in enhancing the photocatalytic performance of photocatalyst. This
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is because the improper doping of metal or non-metal materials may also lead to the facilitation
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of recombination rate of electron-hole pairs and hamper the photocharge carrier transportation and migration by creating scattering center [116]. Hence, identifying a suitable elemental and its
AC
total amount of doping is crucial in elemental doping strategy for enhancing the overall photocatalytic performance of BiVO4. In lieu of the beneficial effects of elemental doping, Quinonero et al [129] explored the effect of La and Ce doping on the photocatalytic performance of pristine BiVO4. The better photocatalytic performance of doped BiVO4 was ascribed to the beneficial substitution of Bi3+ species with La3+ and Ce3+/Ce4+ ions. The introduction of these two impurities altered the 29
ACCEPTED MANUSCRIPT crystallographic structure of monoclinic BiVO4, resulting in better photocatalytic performance. Moreover, the Ce3+ ions also act as electron donors within the BiVO4, thereby increasing the concentration of photocharge carriers within the BiVO4 photocatalyst. As a result, BiVO4 gained benefits from the creation of an additional n-type conductivity, resulting in better photocatalytic
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performance. Meanwhile, Zhou et al. [130] demonstrated the design of 3D macro-mesoporous Mo: BiVO4
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via controllable colloidal crystal template method. The substitution of V5+ species with Mo6+ ion
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promotes the deformation of BiVO4 crystal structure from monoclinic to the tetragonal owing to the larger tetrahedral structure of Mo6+ ion, resulting in enhancement of the photocharge carrier
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separation and migration. In addition, the extra one valence electron of Mo6+ ion residing at V4+
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species may interchange between the adjacent V5+ species, resulting in concentrated localized polarons. As a result, the activation energy of polarons hopping can be reduced, thereby
Formation of BiVO4-based Heterostructure
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5.3
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improved the transfer of electron-hole pair.
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Apart from morphological control, facet-dependent and elemental doping, the formation of BiVO4-based heterostructure has elicited more attention as alternative strategies for enhancing the overall photocatalytic performance [38,56,104,128]. Generally, the BiVO4-based heterostructure is engineered by hybridizing BiVO4 photocatalyst with a different type of photocatalyst or co-catalyst. In this heterostructure systems, the valence band and conduction band of the coupled photocatalyst or co-catalyst should be lower or higher in comparison to that BiVO4 photocatalyst. The hybridization of two different type of photocatalyst with a difference 30
ACCEPTED MANUSCRIPT in electronic structure and band gap locations will resulting in a formation of band bending at the interface of the heterostructure [146]. This band bending observed is the aftereffect of the potential difference between two photocatalysts. As a result, a new electric field within a space charge region will be developed, resulting in spatial photocharge carrier separation and
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migration.
heterostructure.
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Figure 10: Schematic illustration of three different type of BiVO4-based band edge alignment
Generally, the formation of the BiVO4-based heterostructure with a different type of
AC
photocatalyst can be tailored to three different type of heterostructure band alignment. The three different types of heterostructure band alignment are Type I (straddling gap), Type II (staggered gap) and Type III (broken gap) [127,147]. Each of these types would result in difference photogenerated holes and photoexcited electron separation and migration between adjacent photocatalyst (as shown in Figure 10). From Figure 10, the band gap energy of coupled photocatalyst in Type I heterostructure are larger in comparison to the BiVO4 band gap energy, 31
ACCEPTED MANUSCRIPT resulting in a straddling band alignment. Thus, when the heterostructure photocatalyst absorbs the photon energy equivalent to or greater than its band gap energy, resulting in the separation and migration of electron-hole pairs in a single component within the heterostructure photocatalyst. However, there is no concrete evidence in the enhancement of electron-hole pair
IP
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photocatalyst upon separation and migration process [146].
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separation and migration as the photogenerated charge carriers are commonly accumulated in a
Meanwhile, the band gap alignment between BiVO4 and coupled photocatalyst is staggered
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between each other in Type II, resulting in the formation of the upward or downward band bending (as shown in Figure 10) [127]. The observed phenomenon causes the separation and
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migration of electron-hole pair in the heterostructure system to be in the opposite direction. As a
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result, the smooth electron-hole pair separation and migration is expected to occur and the fast recombination rate of the electron-hole pair is significantly reduced owing to prolong the lifetime
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of photogenerated charge carriers [39]. On the other hand, for the type III heterostructure, both
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conduction and valence band of BiVO4 and coupled photocatalyst are not cross-linked with each other causing a gap between both band alignments in the heterostructure photocatalyst. It is hard
CE
to postulate that the separation and migration of electron-hole pair in this type III heterostructure
AC
will smoothly occur and thus limit the overall photocatalytic performance [146]. With regard to these different types of heterostructure band edge alignment, it is important to properly select the appropriate coupled photocatalyst which will result in not only better light absorption capability, but also smoothly facilitate the separation and migration of electron-hole pair via the proper band edge interface. Table 1 enlists some of the recent examples of the formation of BiVO4-heterostructure system along with their proposed mechanism. For example, 32
ACCEPTED MANUSCRIPT Xia et al. [19] performed a studied on the heterostructure of α-Fe2O3/BiVO4 photocatalyst prepared via spin-coating-based-successive ionic layer adsorption and reaction (SILAR) method on the phenol degradation. The photocurrent density and incident-to-current conversion efficiency (IPCE) of α-Fe2O3/BiVO4 photocatalyst were 1.63 mA cm-2 at 1.23 V vs. a reversible
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hydrogen electrode (RHE) and above 27%, respectively which was 2.14 times and triple in
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comparison to pristine BiVO4. Furthermore, the incorporation of α-Fe2O3 in the heterostructure
CR
photocatalyst caused a cathodic shifted for both onset potentials and over potential of water oxidation in comparison to pristine BiVO4. This cathodic shifted observed signified the
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beneficial of α-Fe2O3 in limiting the recombination of the electron-hole pair within the
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heterostructure photocatalyst and hence improve the overall photocatalytic performance. In light of the good photocatalytic performance of Fe2O3/BiVO4 photocatalyst, Zhang et al. [6] further
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optimized the aforementioned heterostructure photocatalyst by synergizing the Fe2O3 with the
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three-dimensionally ordered macroporous (3DOM) BiVO4 in degrading the 4-nitrophenol. The profound performance was attributed to excellent architecture porosity, high surface area and
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smooth photogenerated charge carrier separation and migration.
33
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Table 1: Recent development of BiVO4-based heterostructure with the proposed mechanism. Photocatalyst
Model
T P
Proposed Mechanism
Remarks
I R
Pollutant
Ag/Ag2CO3/BiVO4
The
Tetracycline
C S
addition
of
Ag
nanoparticles
Ref
enhanced
the Reproduced
U N
photogenerated electrons and hinder recombination rate of
A
electron-hole pairs. However, excessive amount of Ag
M
with permission from Ref.
nanoparticles caused a light shielding effect which [148]
D E
RGO-TiO2/BiVO4
Methylene
E C
C A
blue &
T P
consequently reduced the photocatalytic performance.
The construction of RGO-TiO2/BiVO4 promote better Reproduced crystallinity structure. The intimate contact between the
Industrial
petroleum
wastewater
with permission
developed photocatalyst allows smooth electron-hole pair
from Ref.
separation and migration, resulting more available radicals [149]
34
ACCEPTED MANUSCRIPT for the degradation reaction.
T P
I R
BiVO4/Polydopami
C S
U N
The amino and catechol groups in the PDA have a strong Reproduced
Glyphosate
ne/g-C3N4
A
and irreversible binding ability which is suitable for
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formation of heterostructure. The BET specific surface area
D E
T P
C A
E C
with permission from Ref.
plays an important role within this study as larger BET [150] surface area is observed in BiVO4/Polydopamine/g-C3N4 photocatalyst. However, the uniqueness of PDA properties in enhancing the photocatalyst is briefly explained by the authors.
35
ACCEPTED MANUSCRIPT
Fe2O3/3DOM BiVO4
The
4Nitrophenol
good
photocatalytic
performance
observed
are Reproduced
attributed to its unique porous architecture, high surface
with permission
T P
area, good light-harvesting ability, high adsorbed oxygen
from Ref.
I R
species concentration and excellent electron-hole pair [6] separation.
Bi4V2O11/BiVO4
C S
U N
The high photocatalytic activity observed attributed to its Reproduced
Methylene
A
Blue
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multiple reflections of their special multi-shell hollow
D E
T P
C A
E C
spheres. The authors claimed that the surface area is not a
with permission from Ref.
major factors that govern the reactions while the synergistic [55] effects of morphological and microstructure characteristics provides better performance.
36
ACCEPTED MANUSCRIPT
Fe2O3 modified Ag-
Methyl
010BiVO4
Orange
The crystal facet dependence approach is used to optimize Reproduced the photocatalytic performance. Moreover, the synergistic
with permission
T P
effect of n-n and Z-scheme system in the different facet
from Ref.
I R
helps to facilitate the photocharge carrier separation and [142] migration.
GQD/BiVO4
C S
U N
The excessive amount GQD contents resulting in blockage Reproduced
Carbamazep
A
ine
of an active sites, hence reducing the catalytic capacity. The
D E
T P
M
degradation of Carbamazepine pathways involved
with permission
(1)
from Ref.
hydrogen abstraction and hydroxylation of heterocycle ring [151] and
E C
C A
37
(2) cleavage of amide group.
ACCEPTED MANUSCRIPT
TiO2/BiVO4/Co-Pi
The BiVO4 nanoflakes are vertically attached to the side Reproduced
Water Splitting
faces of rutile TiO2 and have a selective growth direction
with permission
T P
along {112}. The use of 1D nanoarray configuration helps
from Ref.
I R
to promote better photocatalytic performance via increasing [39] the light absorption and chemical reaction sites owing to the
C S
built-in electric field at the heterojunction.
BiVO4/P25
U N
A
The good performance observed owing to the multiphase Reproduced
Ethylene
D E
T P
C A
E C
M
nanosystem was developed which consist of monoclinic,
with permission
anatase and rutile phases. The light absorption of developed
from Ref.
heterostructure is improved and n-N heterojunction is [50] constructed. The photocatalyst has a stable performance via recyclability study.
38
ACCEPTED MANUSCRIPT
Biochar/Fe3O4/BiV O4
The addition of Biochar as an adsorbent helps to promote Reproduced
Methylpara ben
better photocatalytic performance. It is suggested that
with permission
T P
reactive oxygen species are responsible for the degradation
from Ref.
I R
activities and addition of H2O2 does escalates the overall [152] performance.
BiVO4/Bi2S3/MoS2
C S
U N
A
The developed photocatalyst was fabricated by coupling Reproduced
Rhodamine B,
D E
Methylene
T P
Blue, Malachite
E C
Green
C A
M
diamond –shaped BiVO4 with layered MoS2 which allowed
with permission
photocharge carrier separation and migration as well as
from Ref.
increased the light absorption capabilities. The h+ species [98] play a major roles in photocatalytic degradation reaction in comparison to •O2- and •OH radicals.
39
ACCEPTED MANUSCRIPT
m-BiVO4/t-BiVO4
The type II heterostructure is observed which led to an Reproduced
Methylene Blue
increase in the charge carrier life time. The trapping
with permission
analysis suggested that the h+, •O2- and •OH radicals are the
T P
I R
from Ref.
main active species that contribute to the overall [153] photocatalytic performance.
C S
A
U N
D E
M
T P
E C
C A
40
ACCEPTED MANUSCRIPT On the other hand, Zhang et al. [154] explored the photocatalytic performance of TiO2/BiVO4 heterostructure photocatalyst prepared via a one-step microwave hydrothermal method. Similarly, Ma et al. and Song et al. [50,155] examined the benefit of TiO2/BiVO4 heterostructure prepared via biomorphic templates in degrading the methylene blue and a
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modified citric acid complexing in degrading gaseous ethylene, respectively. Despite the
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enhancement of photocatalytic performance over TiO2/BiVO4 photocatalyst, the photocatalytic
CR
mechanism observed within this heterostructure was ascribed to Type I heterostructure
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mechanism in which the transfer of photogenerated holes from BiVO4 to the TiO2 surface were obstructed. This impeded movement of holes observed was owing to the mismatched position of
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TiO2 valence band that was lower than BiVO4. As discussed earlier, having a Type II heterostructure system is more favorable for developing a highly functionalized heterostructure
AC
CE
PT
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photocatalyst with a superior photocatalytic performance.
Figure 11: Schematic illustration of the TiO2/BiVO4 heterostructure system with and without hydrogen treatment. Reproduced with permission from Ref. [127].
41
ACCEPTED MANUSCRIPT Inspired by the advantages of type II heterostructure system, Singh et al. [127] converted the conventional Type I TiO2/BiVO4 heterostructure into Type II heterostructure via hydrogen treatment on the top layer of TiO2 (as shown in Figure 11). Based on their density functional theory calculation, the conversion of Type I to Type II heterostructure was feasible to ameliorate
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the distortion of oxygen sublattice existed in TiO2. This band edge realignment resulting in a
IP
better photocurrent density up to 4.44 mA cm-2 at 1.23 V vs reversible hydrogen electrode (RHE)
CR
and low onset potential of – 0.14 V vs RHE. In addition, this band edge realignment allowed the smooth transfer of photogenerated holes to the surface of the electrode and improved the
US
absorption rate of TiO2.
AN
Meanwhile, the profound photocatalytic performance was observed over the use of WO3 and
M
lanthanum vanadate (LaVO4) photocatalyst in the BiVO4-based heterostructure system in photocatalytic application performed by Liu et al. [36] and Veldurthi et al. [112]. On the other
ED
hand, Baek et al. [21] explored the beneficial effect of BiVO4-based heterostructure system by
PT
fabricating a double-heterojunction over BiVO4/WO3/SnO2. Furthermore, Zhao et al. [156] investigated the effect of BiVO4@MoS2 prepared via a facile in-situ hydrothermal method for
CE
degrading the Cr6+ and oxidation of crystal violet under visible light irradiation. Inspired by the
AC
intrinsic features of Se such as prolong photocharge carrier lifetime, high conductivity and better photo absorption capability, Nasir et al. [157] examined the beneficial of Se/BiVO4 heterostructure in photoelectrochemical water splitting. All of the abovementioned BiVO4-based heterostructure system shown a remarkable photocatalytic performance owing to their intrinsic feature.
42
ACCEPTED MANUSCRIPT In lieu of the Type II heterostructure great photocatalytic performance discussed earlier, its relatively low photocharge carrier oxidation and reduction power upon electron-hole pair separation and migration, consequently abnegate its photocatalytic performance. The key underlying this problematic phenomenon lies within the migration electron-hole pair to a less
T
negative conduction band of coupled photocatalyst and a less positive valence band of the
CR
IP
adjacent photocatalyst.
Up to date, a new type of heterostructure system, namely Z-scheme heterostructure has been
US
reported which can help to overcome the limitation facing the conventional Type II heterostructure system [8,67,158,159]. Generally, in this Z-scheme heterostructure mechanism,
AN
the photoexcited electron from the less negative conduction band of photocatalyst will migrate
M
and interact with the less positive valence band of adjacent photocatalyst, resulting in the formation of holes in the valence band of the photocatalyst itself and photoexcited electron in the
ED
conduction band of the adjacent photocatalyst (as shown in Figure 12).
PT
Furthermore, more recent studies reported the introducing of intermediate bridge materials
CE
such as reduced graphene oxide (RGO), silver (Ag), copper (Cu) and gold (Au) in the interface of the heterostructure structure system would significantly smooth and facilitate the separation
AC
and migration of electron-hole pair between the photocatalyst in the heterostructure samples [8,31,43,160–162]. The advantage of this Z-scheme heterostructure mechanism over the conventional Type II heterostructure system is it has a superior photoredox ability owing to its ability for nurturing a highly positive and negative valence band and conduction band, respectively [163]. In addition, the efficient of the photocharge carrier separation and migration
43
ACCEPTED MANUSCRIPT between the photocatalyst is one of the advantages of this Z-scheme heterostructure system
M
AN
US
CR
IP
T
[144,164,165].
Figure 12: Schematic illustration of the Z-scheme heterostructure mechanism (a) without and (b)
ED
with a bridge electron mediator.
PT
For the past few years, the studies on the BiVO4-based heterostructure which employ the new solid-state Z-scheme mechanism to optimize the overall photocatalytic performance has
CE
been reported [112,134,166,167]. Although the BiVO4-based heterostructure photocatalyst
AC
studies are still dominated by current conventional Type II heterostructure, with respect to the great advantage of Z-scheme heterostructure mechanism, it is greatly anticipated that the employ of the Z-scheme heterostructure mechanism in the photocatalyst system will be encyclopaedically studied and grow significantly in the future. Table 2 enlists some of the recent examples of the Z-Scheme BiVO4-heterostructure system along with their proposed mechanism.
44
ACCEPTED MANUSCRIPT
Table 2: Recent development of Z-Scheme BiVO4-based heterostructure system with the proposed mechanism. Photocatalyst
AgI/BiVO4
Model Pollutant
T P
Proposed Mechanism
Remarks
I R
AgI used as a photosensitizers owing to its suitable
E-Coli &
C S
Oxytetracycline
energy-band position. The incorporation of AgI in
U N
Hydrochloride
the as-prepared heterostructure significantly inhibit
A
D E
M
T P
E C
C A
45
the fast recombination rate of electron-hole pair evidently from PL spectra analysis.
Ref
Reproduced with permission from Ref. [134]
ACCEPTED MANUSCRIPT
Porous-doped
g-
The as-developed heterostructure was prepared via
Tetracycline
C3N4/BiVO4
one-pot impregnated precipitation method. The
Reproduced with permission
T P
improved photocatalytic performance of this Z-
I R
scheme is attributed to the enhanced photocharge
from Ref. [86]
carrier separation and improved of charge carrier
C S
lifetime (1.65 ns).
Graphene bridged
A
U N
Tetracycline
Ag3PO4/Ag/BiVO4
D E
(040)
M
The addition of metallic Ag improved the photocharge carrier transfer ability within the
Reproduced with permission
composite while graphene serve as a good support
T P
to make the loaded nanoparticles to achieve
E C
uniform distribution without aggregation. The •O2-
C A
and h+ are the predominant active species that contribute to the overall photocatalytic reaction.
46
from Ref. [8]
ACCEPTED MANUSCRIPT
Se/BiVO4
The Se/BiVO4 has boosted the photocurrent density
Water splitting
by 3-fold in comparison to pristine BiVO4. The
Reproduced with permission
T P
DFT studies show that Se makes a direct Z-scheme
I R
with BiVO4 where the photoexcited electron of
from Ref. [157]
BiVO4 recombines with the valance band of Se. In
C S
addition, the existence of dual absorption layers of
U N
Se and BiVO4 which significantly increased the
A
BiVO4/PyridineDoped g-C3N4
D E
Phenol and
M
The
T P
Methyl Orange
light absorption.
composite
is
prepared
via
an
in-situ
hydrothermal method. The profound performance is
Reproduced with permission
E C
attributed to the construction of the direct Z-scheme system which is free from any mediator. The •O2
C A
-
and •OH radicals plays significant roles in photocatalytic reaction.
47
from Ref. [168]
ACCEPTED MANUSCRIPT
BiVO4-BiFeO3-
The Z-schematic charge carrier transfer are
4-Nitrophenol
CuInS2
enhanced via inserting a ferroelectric material with
Reproduced with permission
T P
downward band bending between transfer PS II and
I R
PS I. The photocurrent density showed an improved performance on as-developed composite with the
C S
polarization-induced electric field oriented from
U N
CuInS2 to BiVO4.
A
D E
M
T P
E C
C A
48
from Ref. [38]
ACCEPTED MANUSCRIPT To date, Zou et al. [61] fabricated a Z-scheme CdS/BiVO4 heterostructure system via lowtemperature water bath system. The Z-scheme system showed a better photodegradation performance in treating Rhodamine B (RhB) in comparison to pure photocatalyst. The better photocatalytic performance of this Z-scheme system was attributed to the quasi-continuous effect
T
owing to the concentrated number of defects aggregated at the Z-scheme interface. Meanwhile,
IP
drawing inspiration from the recent advantages of 1 D nanostructures, Zhou et al. [169]
for
a
Z-schematic
study
on
photocatalytic
hydrogen
evolution
and
US
nanoparticles
CR
manipulating the 1D nanostructure of BiVO4 nanowires and decorated with the hierarchical CdS
photodegradation of Rhodamine B. The CdS/BiVO4 nanowires exhibited higher photocurrent
AN
responses in comparison to pristine BiVO4, indicating the photoexcited electron were smoothly transfer to substrate whereas the photogenerated holes were trapped by the electrolyte. Moreover,
M
the photoluminescence spectra of CdS/BiVO4 nanowires at the region of 495 and 535 nm which
ED
attributed to BiVO4 nanowires and CdS, respectively depicted weakened emission peaks in
PT
comparison to pristine samples. The weakened peaks spectrum observed were strongly justified the efficient photocharge separation and migration.
CE
In light of the beneficial effect of introducing electron bridge mediator in the Z-scheme
AC
mechanism as discussed earlier, Wu et al. [43] explored the advantage of incorporating carbon dots as a solid state electron mediator over BiVO4/CDs/CdS Z-scheme structure without the addition of any sacrificial agent or co-catalyst. The Nyquist plot of BiVO4/CDs/ CdS showed a high-frequency semicircle which response to the migration of photoexcited electron at the interface of Z-scheme system and electrolyte. In addition, a smaller semicircle radius was detected from EIS study which signifies the efficiency of photocharge carrier transfer. Moreover, 49
ACCEPTED MANUSCRIPT the substandard photoluminescence spectra were observed within the as-developed z-scheme photocatalyst in comparison to pristine samples. This phenomenon observed was the key towards explaining the superior photocatalytic performance observed within BiVO4/CDs/CdS
US
CR
IP
T
photocatalyst.
AN
Figure 13: The fabrication process of the g-C3N4@Ag/BiVO4 Z-scheme heterostructure system.
M
Reproduced with permission from Ref. [144]. On the other hand, Ou et al. [144] established the Z-scheme mechanism over g-
ED
C3N4@Ag/BiVO4 {040} photocatalyst. In their study, the {040} crystal facet was preferential
PT
growth and hybrid with g-C3N4 and Ag nanoparticles before the photocatalytic oxidation was performed (as shown in Figure 13). The remarkable photocurrent density of g-C3N4@Ag/BiVO4
CE
{040} photocatalyst which is 6.3 times greater than pristine photocatalyst justified the superior
AC
photocatalytic performance observed, indicating the smooth interfacial photocharge carrier separation and migration. Similarly, Chen et al. [8] studied the impact of Ag nanoparticles hybridized with reduced graphene oxide as a solid state electron mediator in the photodegradation of tetracycline over Ag/Ag3PO4/BiVO4/RGO Z-scheme heterostructure. The photodegradation performance result of as-prepared z-scheme heterostructure showed a prominent removal of tetracycline and 50
ACCEPTED MANUSCRIPT photocatalyst stability after four times recycling in comparison to pristine photocatalyst. The photoluminescence
study
revealed
the
lowest
spectrum
signal
obtained
by
Ag/Ag3PO4/BiVO4/RGO samples, indicated the remarkable photocharge carrier separation and migration. Evidently, the highest photocurrent density observed by Ag/Ag3PO4/BiVO4/RGO
T
photocatalyst justified the effectiveness incorporating of solid-state electron mediator as a bridge
CR
IP
within the Z-scheme system.
US
6 Conclusion and Future Directions
AN
The growing concern on the scarcity of clean water sources due to a fast development of industrialization has force a rapid breakthrough dedicated to the development of Advanced
M
Oxidation Processes (AOPs) using photocatalyst system. Over the past few years, the studies on
ED
BiVO4-based photocatalyst in AOPs system has witnessed auspicious potential promises by BiVO4-based photocatalyst in photocatalytic degradation applications. To this end, the
PT
outstanding photocatalytic activity showed by the BiVO4-based photocatalyst is mainly govern
CE
by its intrinsic features such as suitable band gap, proper band location, great stability and environment friendliness. Recognition of the great potential of the BiVO4-based photocatalyst in
AC
AOPs system, the whole framework of this review was aimed to cover a recent scientific literature and progress related to BiVO4-based photocatalyst in this emerging field. Table 3 summarizes the recent reports on the photocatalytic applications of BiVO4-based photocatalyst. Firstly, this review discussed the fundamental properties of BiVO4–based photocatalyst in the view of experimental and computational perspective. Following that, the photocatalytic degradation principles over BiVO4-based photocatalyst along with the possible photocatalytic 51
ACCEPTED MANUSCRIPT mechanism was well discussed. Understanding the fundamental properties and photocatalytic principles is indispensable to elucidate a firm knowledge of the nature factor which will govern the photocatalytic performance of photocatalyst. Despite the state-of-the-art BiVO4 photocatalyst shows promises in this emerging field, BiVO4 photocatalyst also suffers from a major setback
T
due to its limiting factor which including poor electrical conductivity and charge transfer, slow
IP
water oxidation kinetics and fast recombination rate of electron-hole pairs. With regard to the
CR
aforementioned shortcomings, several strategies such as morphological control, facet engineering, elemental doping and formation of BiVO4-based heterostructure system has been
US
recently suggested and shown a remarkably heightened photocatalytic performance. The insight
AN
of all of these limitations and strategies presented here is imperative for intensifying and advancing the future directions of the BiVO4-based photocatalyst in AOPs technology before
M
debuting into practical industrial applications. Nonetheless, knowledge gap still exists within the
ED
abovementioned discussion where the further in-depth study is required to reveal the true
PT
potential of the BiVO4-based photocatalyst. By running through the various section of this review, the following perspectives and
CE
challenges arise for the future. Most of the current reported literature were only evaluating the
AC
modified BiVO4-based photocatalyst using the synthetic wastewater water such as methylene blue, tetracycline, bisphenol A and methyl orange. However, the report on the evaluation of modified BiVO4-based photocatalyst using real industrial wastewater is relatively scarce. It is known that the real industrial wastewater contains various type of compound which are challenging to be removed in comparison to the synthetic wastewater which focussed on a specific type of compound. In addition, the studies on the intermediates product formed during the photocatalytic reaction are necessary for evaluating the quality of the treated wastewater. 52
ACCEPTED MANUSCRIPT Moreover, the use of natural sunlight throughout the photocatalytic analysis instead of using simulated solar light is one of the perspectives to be highlighted. Most of the reported literature usually used simulated solar light as a light source such as xenon lamp and a halogen lamp. Thus, it is imperative to evaluate the developed photocatalyst materials under the natural sunlight
IP
T
illumination for a future full-fledged commercialization.
CR
Afterward, the photostability of the BiVO4-based photocatalyst is one of the main challenges for photocatalytic applications. Despite the claim on the high photocatalytic efficiency of the
US
BiVO4-based photocatalyst, the lifetime of this materials is too short. The photocorrosion phenomenon observed within this system is hard to avoid. Although some of the literature has
AN
claimed the extended photocatalyst stability are successfully achieved, but these often require a
M
very complex synthetic process. With regard to this matter, a new design toward producing a highly efficient BiVO4-based photocatalyst in complying with the photocorrosion stability is
ED
highly desirable towards practical applications. On the other hand, further pilot plant analysis
PT
with different reactor configurations is highly desirable to ensure that this AOPs via BiVO4based photocatalyst system is well established. In summation, a large scale AOPs technology
CE
design with highly efficient BiVO4-based photocatalyst material can be debuted in the short
AC
future if the abovementioned parameters, limitations and challenges can be circumvented.
53
ACCEPTED MANUSCRIPT
Table 3: Summary on the most recent BiVO4-based photocatalyst performance in photocatalytic degradation applications. Photocatalyst
Model
T P
Irradiation
Experimental Conditions
I R
Pollutant
C S
BiVO4 microstructures
Rhodamine B
U N
BiVO4 microstructures were prepared via a
A
M
Performance
Ref.
time
(%)
(min)
180
99.5
[140]
150
100
[150]
surfactant-free hydrothermal method. Different
D E
amount of Bi3+/V5+ molar ratios was study. A
T P
350 W Xenon lamp with a 400 nm cut-off filter was used as the light source.
BiVO4/PDA/g-C3N4
C A
E C
Glyphosate
BiVO4 was prepared via chemical precipitation method while g-C3N4 powder was fabricated by heating urea powder. A 125 W high pressure
54
ACCEPTED MANUSCRIPT mercury lamp, which the UV light portion was filtered by a 2 M NaNO2 solution. The concentration of glyphosate is 0.1 mM.
Co-Pd/BiVO4
Phenol
I R
T P
BiVO4 support with a leaf-like morphology was
180
90
[170]
150
94.9
[148]
C S
prepared via the hydrothermal method. The
U N
composite photocatalyst was prepared via chemical reduction method with polyvinyl
A
M
alcohol as protecting agent and NaBH4 as a
D E
reducing agent. A 300 W Xenon lamp with an
T P
optical cut-off filter was used a visible-light source. The concentration of phenol is 0.2
E C
Ag/Ag2CO3/BiVO4
C A
Tetracycline
mmol/L.
The sphere BiVO4 sample was prepared via a facile
hydrothermal
55
method
whereas
the
ACCEPTED MANUSCRIPT composite photocatalyst was prepared via a precipitation-photoreduction reaction. A 500 W Xe lamp was used a light source. The initial
T P
concentration of tetracycline is 20 mg/L.
AgI/BiVO4
I R
C S
Oxytetracyclin
The composite photocatalyst was prepared via a
e
deposition-precipitation method. A 300 W
hydrochloride
Xenon lamp with a 420 nm cut-off filter was used
a
D E
80
[134]
60
96.95
[86]
U N
A
M
visible-light
60
source.
The
initial
concentration of OTC-HCl is 20 mg/L.
PCNS/BiVO4
Tetracycline
T P
The phosphorus doped ultrathin graphitic
E C
carbon nitride (PCNS) was prepared via thermal
C A
polycondensation with some modifications. Different weight of PCNS in the composite structure was analysed. A 300 W Xenon lamp
56
ACCEPTED MANUSCRIPT with a UV cut-off filter was used a visible-light source. The initial concentration of tetracycline is 10 mg/L.
RGO-TiO2/BiVO4
Methylene blue
The
RGO-TiO2/BiVO4
prepared
via
Different
amount
photocatalyst
U N
RGO
was
120
100
[149]
210
66.87
[68]
C S
wet-impregnation of
I R
T P
method.
loading
was
analysed. A 500 W halogen lamp was used a
A
M
visible-light source. The initial concentration of
D E
methylene blue is 10 mg/L.
BiVO4/α-Fe2O3
T P
Gaseous
The composite materials was prepared via
Benzene
hydrothermal-calcinations
C A
E C
method.
The
degradation of gaseous benzene was measured in a tubular photo-reactor. A 8 W 365 nm UV lamp was used as light source. 100 mg/m3 of
57
ACCEPTED MANUSCRIPT gaseous benzene was injected into the reactor with micro-syringe.
RGO/g-C3N4/BiVO4
Tetracycline
The new composite photocatalyst was prepared
Hydrochloride
using a facile hydrothermal method. The CN nanosheets
were
prepared
T P 150
72.5
[171]
150
98.95
[172]
I R
C S
via
the
U N
polymerization of urea while RGO was prepared according to the modified Hummers’
A
M
method. A 500 W tungsten light lamp was used
D E
a light source. The initial concentration of TC is
T P
35 mg/L.
BiVO4-rGO
E C
Methyl Orange The composite materials was prepared via a
C A
simple one-step hydrothermal method. The photocatalytic reaction were evaluated under simulated sunlight in a homemade reactor with
58
ACCEPTED MANUSCRIPT a cooling water circulator assembled to keep the reactor at a constant temperature. The initial concentration of MO is 10 mg/L.
BiVO4/Ag/rGO
I R
T P
Methylene
The composite photocatalyst was prepared via
Blue
one-pot synthesis using a facile and cost-
150
-
[173]
90
95
[97]
C S
U N
effective hydrothermal method. A 300 W Xe lamp
was
used
a
light
A
M
source.
The
photocatalytic reaction was performed in a
D E
custom water-cooled borosilicate glass reactor
T P
which capable for maintaining the reaction temperature at ~25°C. The initial concentration
E C
BiFeWO6/BiVO4
C A
Methylene Blue
of MB is 10 ppm.
The BiVO4 nanoparticles were synthesized via additive-free
hydrothermal
59
method
while
ACCEPTED MANUSCRIPT BiFeWO6 was prepared via a facile coprecipitation method. The nanocomposite was synthesized
via
simple
additive-free
wet
chemical process. A 250 W tungsten halogen
I R
lamp was use a light source. The initial
C S
concentration of MB is 2 x 10-5 M.
Sandwich-like BiVO4
U N
Methyl Orange The photocatalyst was prepared via a facile (MO)
A
M
T P 150
86.5
[93]
180
98.5
[105]
microwave-assisted method. A 200 W xenon
D E
lamp with 420 nm cut-off filter was used as a
T P
light source. The initial concentration of MO is 2 x 10-5 M.
m-BiVO4
E C
C A
Methylene
The photocatalyst was prepared via solid-liquid
Blue
state reaction. A 500 W halogen lamp was used a light source. The initial concentration of MB
60
ACCEPTED MANUSCRIPT is 10 mg/L.
BiVO4/Bi2S3/MoS2
Rhodamine B
The heterojunction photocatalyst was fabricated
97
T P
via a facile in-situ hydrothermal method based
I R
Methylene
on the formation of the intermediate Bi2S3 by Blue
300
C S
photocatalytic analysis was done using natural
Green
sunlight at a fixed reaction time with an
[98]
94
hybridizing BiVO4 and MoS2 precursor. The Malachite
93
U N
A
M
ambient temperature of 30°C.
BiVO4/TiO2
Rhodamine B
D E
The photocatalysts were prepared in the form of
T P
films by a simple wet chemical methods.
E C
(RhB)
Various layered configuration of the developed
C A
heterostructures were studied. Different type of irradiation such as UVA, metal-halogenide lamp and visible light was used a light source.
61
180
-
[174]
ACCEPTED MANUSCRIPT The initial concentration of RhB is 10-5 M.
Palygorskite-BiVO4
Tetracycline
The
photocatalyst
was
prepared
via
a
Hydrochloride
hydrothermal method. A 500 W Xe lamp with a
(TC)
cut-off filter was used a light source. The initial
240
91
[175]
97.4
[152]
T P
I R
C S
concentration of TC is 30 mg/L with a constant
U N
stirring at 500 rpm.
Biochar/Fe3O4/BiVO4
Methylparaben
A
The biochar was collected from Himalayan Pine
D E
M
forests. The composite was prepared via addition of biochar and Fe3O4 during the
T P
synthesis of BiVO4. The initial concentration of
E C
methyl paraben is 5 mg/L and 0.01 g of
C A
photocatalyst was used during the reaction study. The solution was kept in the double wall cylinder equipped with a water circulation
62
120
ACCEPTED MANUSCRIPT placed on a magnetic stirrer.
BiVO4/BiOCl
Norfloxacin
The photocatalyst was prepared via an in-situ
60
100
[176]
98.48
[177]
T P
transformation method and the morphological
I R
structure of 2-D BiOCl was facilely tuned via
C S
controlling the concentration of Cl- ions in the
U N
reaction solution. 0.05 g of photocatalyst was used and the initial concentration of norfloxacin
A
M
is 5 mg/L. The distance between the light
D E
source and the liquid level was constant at 10
T P
cm. A circulating water system was employed to maintain the operation temperature at 25°C.
Ag4V2O7/BiVO4
E C
C A
Methylene Blue
The composite photocatalyst was prepared via a facile
sodium
polyphosphate-assisted
hydrothermal method. A 300 W Xe lamp with a
63
60
ACCEPTED MANUSCRIPT visible light filter was used a light source and the distance between the lamp and samples was constant at 10 cm. A 50 mg of photocatalyst was used and the initial concentration of MB is
I R
5 mg/L.
BiOCl/BiVO4@AgNWs
T P
C S
U N
Methylene
The sheet like structure composite photocatalyst
Blue
was prepared via a simple sol-gel method with
180
A
M
97.67
[178]
96.14
the presence of deep eutectic solvents. The Xe Rhodamine B
D E
arc lamp was used a light source. A 10 mg of
T P
photocatalyst
was
used
and
the
initial
concentration of MB and RhB is 10 mg/L.
BiOCl/BiVO4
C A
E C
Rhodamine B
The hierarchical microspheres heterostructure was prepared via a co-precipitation process followed by hydrothermal treatment. For a
64
240
100
[179]
ACCEPTED MANUSCRIPT comparative study, pure BiVO4 was prepared under the same conditions with the addition of nitric acid instead of hydrochloric acid while BiOCl was prepared via simple precipitation
I R
method without the addition of NHVO3, and
C S
without the hydrothermal treatment.
BiVO4/ZnO
Rhodamine B
U N
The Au coated magnetic cilia film was prepared
A
M
T P 120
100
[180]
180
92.01
[181]
via magnetron sputtering under vacuum for 20
nanosheet/Au
D E
s. A 300 W Xenon lamp with cut-off filer was
T P
used with a distant between lamp and solution was constant at 25 cm. The initial concentration
E C
BiVO4/WO3
C A
2-
Chlorophenol
of RhB is 0.01 g/L.
The
heterostructure
nanocomposites
were
synthesized via hydrothermal method. A 150 W
65
ACCEPTED MANUSCRIPT Xe arc lamp with an UV cut-off filter was used as a light source. The analysis was conducted in a cylindrical immersion type photoreactor equipped with a circulating water jacket to
I R
maintain a constant temperature.
In2O3-BiVO4 {010}
Rhodamine B
T P
C S
U N
The composite was prepared via a simple two-
180
91
[182]
100
99
[183]
step hydrothermal method with a different
{110}
A
M
molar ratios. A 500 W Xe arc lamp through a
D E
10-cm IR water filter and a cut-off filter was
T P
used a light source. A 50 mg of photocatalyst was used and the initial concentration of RhB is
E C
BiVO4/MWCNT/Ag@ AgCl
C A
Rhodamine B
5 mg/L.
The heterostructure photocatalyst was prepared via
hydrothermal
and
66
in-situ
oxidization
ACCEPTED MANUSCRIPT method. A 500 W Xe arc lamp with a 420 cutoff filter was used a light source. A 50 mg of photocatalyst
was
used
and
the
initial
T P
concentrations of RhB is 5 mg/L.
m-BiVO4/BiOBr
Rhodamine B
I R
C S
The photocatalysts was prepared via a simple
30
98.9
[102]
180
~55
[111]
U N
one-pot hydrothermal method. A 500 W Xe lamp with a 420 nm cut-off filter was used as a
A
M
light source. A 50 mg of photocatalyst was used
D E
and the initial concentration of RhB is 10 mg/L.
BiVO4
Glyphosate
T P
The photocatalyst was prepared via a co-
E C
precipitation method. A 125 W high-pressure
C A
mercury lamp which the UV light portion was filtered by a 2.0 mol/L NaNO2 was used a light source. A 0.5 g of photocatalyst was used and
67
ACCEPTED MANUSCRIPT the initial concentration of glyphosate is 10-4 mol/L.
Ni-BiVO4
Ibuprofen
T P
The photocatalyst was prepared using a
90
80
[184]
200
-
[185]
I R
microwave hydrothermal method. A 150 W
C S
short arc lamp with a 420 nm cut-off filter was
U N
used a light source. The incident light intensity was 150 mWcm-2 and the total energy per
A
M
second was 20 W. A 0.1 g of photocatalyst was
D E
used and the initial concentration of ibuprofen
T P
is 20 mg/L.
Ce-BiVO4
E C
Rhodamine B
C A
The composite photocatalyst was prepared via a simple precipitation-impregnation method. A 4 W LED light was used a light source and the distance between the lamp and solution was set
68
ACCEPTED MANUSCRIPT to 11 cm. A 0.2 g of photocatalyst was used and the initial concentration of RhB is 5 mg/L.
Ag@g-C3N4@BiVO4
Tetracycline
T P
The g-C3N4, BiVO4 and Ag@g-C3N4@BiVO4
60
90.76
[167]
50
96
[107]
I R
were prepared via modified two-step thermal
C S
treatment method, a facile hydrothermal method and
a
facile
U N
photodeposition
method,
respectively. A 300 W Xe lamp with different
A
M
wavelength band-pass filters was used a light
D E
source. A 0.03 g of photocatalyst was used and
T P
the initial concentration of TC is 20 mg/L.
xCu-BiVO4
E C
Rhodamine B
C A
The photocatalyst was prepared via ethylene glycol solvothermal method with EDTA as the chelating agent. A 250 W halogen lamp was used a light source. A 30 mg of photocatalyst
69
ACCEPTED MANUSCRIPT was used and the initial concentration of RhB is 15 mg/L.
Graphene/BiVO4/TiO2
Methylene
The ternary nanocomposites was synthesized
Blue
via a facile, ultrasonic wave-assisted one pot
T P 10
100
[186]
150
92
[142]
I R
C S
hydrothermal method. A 500 W halogen lamp
U N
was used a light source. 0.1 g of photocatalyst was used and the initial concentration of MB is
A
10 ppm.
Fe2O3 modified Ag-
D E
M
Methyl Orange The n-n-Z-scheme photocatalyst was prepared
T P
via impregnation-calcination method. A 0.02 g
010BiVO4
E C
of photocatalyst was dispersed in 20 ml of MO
C A
solution with an initial concentration of 0.01 g/L.
70
ACCEPTED MANUSCRIPT
BiVO4/Ag+
Methylene Blue
The
photocatalyst
was
prepared
via
a
25
180
-
[73]
240
-
[62]
T P
I R
dispersed in 50 ml of MB solution with an initial concentration of 10 ppm.
Rhodamine B
[53]
hydrothermal method. A Xe lamp was used a light source. 0.05 g of photocatalyst was
Nb-modified BiVO4
100
C S
U N
The photocatalyst was prepared via a simple
A
sol-gel method. A metal halogenide lamp was Stearic Acid
M
used a light source. The active surface area of
D E
thin film is 1.4 cm x 1.4 cm and the initial
T P
concentration of RhB is 10-5 M.
Bi2O3/BiVO4
E C
Methylene
The heterostructure photocatalyst was prepared
Blue
via hydrothermal treatment. The sample was
C A
exposed using six lamps in a homemade photoreactor maintained at 18°C. 10 mg of 71
ACCEPTED MANUSCRIPT photocatalyst was used in a 20 mL of 5 mg/L concentration of MB.
BiVO4/Pyridine-Doped
Phenol
pyridine
g-C3N4
T P
The sample was synthesized via organic doping
approach
and
I R
thermal
Methyl Orange
150
C S
92
[168]
97
copolymerization treatment while the composite
U N
photocatalyst was constructed via an in-situ hydrothermal
method.
A
M
A
0.05g
of
the
photocatalyst was used and dispersed into 150
D E
mL of the solution with the initial concentration
T P
of 10 mg/L.
g-C3N4/BiVO4
E C
Bisphenol A
C A
(BPA)
The composite photocatalyst was used via a facile electro-spinning technique. A Xe lamp with a 420 nm optical filter was used a light source. The initial concentration of BPA is 5
72
120
93
[187]
ACCEPTED MANUSCRIPT mg/L with the presence of 10 mM of H2O2.
CuOx/BiVO4
Bisphenol A
The composite materials was prepared via a
180
used
a
light
-
[38]
I R
as reductant. A 100 W of Xenon ozone-free was
[37]
T P
polyol-reduction method using ethylene glycol
lamp
85
C S
source.
The
U N
photocatalytic experiments were performed in a glass, cylindrical vessel at a liquid holdup of
A
120 mL.
BiVO4-BiFeO3-CuInS2
4-Nitrophenol
D E
M
The composite photocatalyst was prepared via
T P
depositing BiVO4, BiFeO3 and CuInS2 in
E C
2,4-
sequence. A high-pressure xenon short arc lamp
Dichloropheno
C A
with a visible light cut-off filter was used as a
l
light source. The light intensity was set to 100 mW cm-2 using a digital radiometer. A 20 mL
73
180
ACCEPTED MANUSCRIPT of solution with initial concentration of 5 mg/L was used.
Nd/Er co-doped BiVO4
Rhodamine B
T P
The photocatalyst was synthesized using a
150
96
[52]
150
88.58
[155]
I R
microwave hydrothermal method. A 500 W
C S
Xenon lamp was used a light source. The reaction
was
photochemical
U N
conducted reactor
with
A
M
in
an
50
XPA-7
mg
of
photocatalyst dispersed into 50 mL of RhB
D E
solution (10 mg/L).
TiO2/BiVO4
T P
Methylene
The microfiber heterojunctions was prepared
Blue
using cotton as biomorphic templates. A 500 W
C A
E C
xenon lamp was used a light source at a room temperature. 100 mg of photocatalyst was used in 100 mL of 10 mg/L MB concentration.
74
ACCEPTED MANUSCRIPT
BiVO4/TiO2(N2) NTs
Methylene
The TiO2 nanotubes (NTs) were prepared via a
Blue
template method in which ZnO nanowires were
80
91.8
[188]
98
[6]
T P
transformed during a liquid phase deposition
I R
process. The heterostructure photocatalyst was prepared using spin coating techniques. A 350
C S
W Xe lamp with light intensity of 100 mW cm-2 was
used
a
light
U N
source.
The
initial
A
concentration of MB is 10 mg/L.
Fe2O3/3DOM BiVO4
4-Nitrophenol
D E
M
The photocatalyst was prepared using the
T P
ascorbic acid-assisted polymethyl methacrylate
E C
templating and incipient wetness impregnation
C A
methods. A 300 W Xe lamp with an optical cutoff filter was used a light source. 40 mg of photocatalyst and 0.6 mL of H2O2 were added to 100 mL of 4-NP with an initial concentration 75
30
ACCEPTED MANUSCRIPT of 0.4 mmol/L.
GQD/m-BiVO4/t-BiVO4 Carbamazepine The BiVO4 was prepared via hydrothermal (CBZ)
180
96.1
[151]
94.96
[8]
T P
process. A 350 W Xenon lamp was used a light
I R
source. 0.01 g of photocatalyst was dispersed in
C S
50 mL of 10 mg/L of CBZ solution at a room
U N
temperature.
Graphene-bridged
Tetracycline
A
The heterostructure photocatalyst was prepared
D E
M
Ag3PO4/Ag/BiVO4
using a facile in-situ deposition method
(040)
followed by photo-reduction. A 300 W Xe lamp
T P
with a 420 nm cut-off filter was used a light
E C
source. 50 mg of photocatalyst was dispersed in
C A
the 100 mL of TC solution with an initial concentration of 10 mg/L.
76
60
ACCEPTED MANUSCRIPT
BiVO4 nanotubes
Cr(VI)
The BiVO4 nanotubes was synthesized via single-spinneret
electro-spinning
80
95.3
[47]
92.98
[45]
without
T P
template. A 300 W Xe lamp with a cut-off filter
I R
was used as a light source in the presence of
citric acid. 0.05 g of photocatalyst was added
C S
into 100 mL of solution with an initial
U N
concentration of 10 mg/L.
Yb+3, Er+3 and Tm+3 doped BiVO4
Methylene Blue
A
M
The photocatalyst was prepared via microwave
D E
hydrothermal
method
using
T P
ethylenediaminetetraacetic acid (EDTA). A 100
E C
W of infrared lamp was used as an NIR light
C A
source while UV cut-off filter was used a visible light source. 0.1 g of photocatalyst was dispersed in 70 mL of MB solution with an
77
180
ACCEPTED MANUSCRIPT initial concentration of 20 ppm.
Bi4V2O11/BiVO4
Methylene
A modified carbonaceous spheres sacrificial
Blue
template growth technique was employ to
80
100
[55]
98
[7]
T P
I R
prepare the photocatalyst. A 300 W Xe lamp
C S
with cut-off filter was used a light source. 0.1 g
U N
of photocatalyst was added into 100 mL of MB solution with an initial concentration of 10-5
A
mol/L.
CuOx/BiVO4
Ethyl Paraben (EP)
The
D E
M
heterostructure
T P
photocatalyst
was
synthesized via a polyol-reduction method
E C
using ethylene glycol as reductant. A 100 W of
C A
Xenon ozone-free lamp and Air Mass 1.5 Global filter simulating solar radiation was used as a light source. A 120 mL of the aqueous
78
60
ACCEPTED MANUSCRIPT solution containing the desired concentration of EP were loaded in an open-glass, cylindrical reaction vessel at ambient conditions under
T P
continuous stirring.
BiVO4/P25
Ethylene
I R
C S
The nanocomposites was prepared using a
U N
modified citric acid complexing method. An ultrahigh-pressure Xe lamp with a UV cut-off
A
M
filter was used as light source. 3 mL of gaseous
D E
ethylene was injected into the airtight reactor
T P
containing catalyst film.
E C
C A
79
360
11.2
[50]
ACCEPTED MANUSCRIPT
7 Acknowledgement The authors would like to express their appreciation to the Chemical Engineering
T
Department, Universiti Teknologi PETRONAS and Centre of Innovative Nanostructures &
CR
IP
Nanodevices (COINN), Universiti Teknologi PETRONAS for the support.
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ACCEPTED MANUSCRIPT Epigrammatic Progress and Perspective on the Photocatalytic Properties of BiVO4-based Photocatalyst in Photocatalytic Water Treatment Technology: A Review Highlights:
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1. Latest progress and development of BiVO4-based photocatalyst
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3. An invigorating perspectives and future directions of the BiVO4-based photocatalyst.
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