Performance Enhancement Strategies of Bi-based Photocatalysts: A Review on Recent Progress

Journal Pre-proofs Review Performance Enhancement Strategies of Bi-based Photocatalysts: A Review on Recent Progress Min Xu, Jingkai Yang, Chaoyang Sun, Lu Liu, Yan Cui, Bo Liang PII: DOI: Reference:

S1385-8947(20)30393-4 https://doi.org/10.1016/j.cej.2020.124402 CEJ 124402

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Please cite this article as: M. Xu, J. Yang, C. Sun, L. Liu, Y. Cui, B. Liang, Performance Enhancement Strategies of Bi-based Photocatalysts: A Review on Recent Progress, Chemical Engineering Journal (2020), doi: https:// doi.org/10.1016/j.cej.2020.124402

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Performance Enhancement Strategies of Bi-based Photocatalysts: A Review on Recent Progress Min Xu1, Jingkai Yang1,2*, Chaoyang Sun1, Lu Liu1, Yan Cui1, Bo Liang1* 1 State Key Laboratory of Metastable Materials Science and Technology, College of Materials Science and Engineering, Yanshan University, Qinhuangdao 066004, China 2 Key Laboratory of Green Construction and Intelligent Maintenance for Civil Engineering of Hebei Province, Yanshan University, Qinhuangdao 066004, China Corresponding authors: [email protected] (Jingkai Yang); [email protected] (Bo Liang)

Abstract: Bi-based photocatalysts are important visible-light-driven photocatalysts for their great potential to solve the energy and environmental issues under visible light. However, the photocatalytic performance of pristine Bi-based photocatalysts remains unsatisfactory due to their inherent drawbacks. In the past few years, the enormous efforts have been made to improve the performance and understand the related mechanism. In this review, several well-known Bi-based photocatalysts and their synthesis methods have been introduced, and then the recent advances and performance-enhancing strategies for Bi-based photocatalysts have been discussed in detail, including structural design, microstructure control, fabrication of Bi-based composites. By summarizing the enhancement mechanism such as light absorption, modulation of energy band, and utilization of photogenerated charge, we highlight the relationship between structural characteristics and photocatalytic performance. Finally, future prospects for the development of Bi-based photocatalysts as well as

1

challenges are also discussed and summarized. Keywords: Bi-based

photocatalysts;

Synthesis

methods;

Structural

design;

Composites; Photocatalytic mechanism

1 Introduction Photocatalysis is reported as a promising technique to solve environmental and energy challenges[1-3]. It is a green and environmentally friendly technique that could utilize the abundant solar energy for water splitting for H2 evolution, harmful pollutants removal, selective organic transformations and CO2 photoreduction to synthesize carbon-bearing fuels[4-7]. However, the conventional TiO2 photocatalyst is responds only to ultraviolet (UV) light[8]. Thus, to develop more efficient photocatalysts with reasonable photocatalytic performance for practical applications, construction of high efficiency visible-light-driven photocatalysts have drawn considerable attention. In recent years, Bi-based photocatalysts have drawn increasing attention, should benefit from their good chemical stability under visible light irradiation, especially their unique electronic band structure and controlled morphology[9]. The well-dispersed valence band (VB) of Bi-based photocatalysts is composited by the hybridization of O 2p and Bi 6s orbitals. Moreover, the lone-pair distortion of Bi 6s orbitals can cause the pronounced overlap of O 2p and Bi 6s orbitals, which is beneficial for increasing the mobility of charge carriers and decreasing the band gap[10,11]. Therefore, the band gaps of the Bi-based photocatalysts are usually less

2

than 3.0 eV. Considering the stability of Bi3+, most studies have focused on Bi3+-containing compounds, such as Bi2O3[12], BiVO4[13], Bi2MO6 (M=Mo, W)[14], BiOX (X=Cl, Br, I)[15], (BiO)2CO3[16] and so on. However, it should be noted that the advanced environmental application over bulk Bi-based photocatalysts is still unsatisfactory under visible light due to their low conduction band (CB) levels and low charge carriers separation efficiencies. For further development, various strategies such as structural design, microstructure control and fabrication of Bi-based composites, should be made to enhance their photocatalytic performance under visible light. At the same time, more attention should be paid to the environmental pollution and the safe use of hazards reagents during the real production process of photocatalysts. Thus, the main purpose of this review is to summarize the synthesis of Bi-based photocatalysts and their modification strategies for efficient photocatalytic applications. The review begins with the well-known Bi-based photocatalysts and their synthesis methods at this stage, along with discussing the some key challenges of process paths, such as environmental safety and economic implications. Then, the efforts to enhance the photocatalytic performance of Bi-based photocatalysts will be discussed in the context of heteroatom doping, solid solution, stoichiometry alteration, vacancy/defect introduction, ultrathin structures, hierarchical structures, hollow and porous structures, crystal facet engineering, heterojunction and electron transporting material loading. We emphasize the impacts of each of these strategies on the relationship between the structural characteristics and photocatalytic performance.

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Finally, the conclusions and perspectives on the future exploration of Bi-based photocatalysts are provided. 2 Well-known Bi-based photocatalysts and synthesis methods 2.1 Bi-based photocatalysts Photocatalytic performance is closely related to the crystal and electronic structures. In this section, the crystal and electronic structures of well-known Bi-based photocatalysts and the effect on photocatalytic performance are discussed. This is very important to guide and perfect the strategies of obtaining efficient photocatalysts. 2.1.1 Bi2O3 and Bi2S3 Bi2O3 with a band gap of 2.1-2.8 eV and has a good absorption in the visible light range. There are mainly four phase reported for Bi2O3: monoclinic phase (α-Bi2O3), tetragonal phase (β-Bi2O3), body centered cubic (γ-Bi2O3) and face cubic phase

(δ-Bi2O3)[17].

Among

these

polymorphs,

α-(low-temperature)

and

δ-(high-temperature) phases are stable, while others are all metastable phases[16]. It is reported that β-Bi2O3 exhibits relatively excellent photocatalytic performance compared to that of α-Bi2O3. It can be attributed to the unique structure of β-Bi2O3, which can provide channels for the transfer of the charge carriers to prevent their recombination in the photocatalytic process[18]. However, β-Bi2O3 is a metastable state, it is still a challenge to develop facile routes to prepare pure β-Bi2O3, especially in the nanoscale[19]. The current research on Bi2O3 is mainly focused on the strategies to improve the photocatalytic performance of α-Bi2O3 and the synthesis method of β-Bi2O3[20,21].

4

Bi2S3 with a narrow band gap of 1.3-1.7 eV is easily excited to generate photogenerated carriers in visible and near-IR light, and has been widely used as stability sensitizer. The Bi2S3 crystals usually exist in an orthorhombic phase and have a layered structure. However, the easy recombination of the photogenerated electron-holes seriously affects its photocatalytic performance when being employed alone[22]. In this case, many studies have reported about coupling Bi2S3 with other materials to form heterojunction[23]. 2.1.2 BiVO4 There are three polymorphs of BiVO4: monoclinic scheelite (mBiVO4) , tetragonal scheelite (tBiVO4) and tetragonal zircon structures (t-z BiVO4). Among the three structures, t-z BiVO4 with a band gap of 2.9 eV possesses relatively low photocatalytic performance under visible light irradiation[24]. The mostly studied BiVO4 photocatalysts at present is the scheelite BiVO4 with a band gap of 2.4 eV. For mBiVO4 and tBiVO4, the positions of CB minimum and VB maximum are similar, so the thermodynamic limitations for the photocatalytic reaction should be similar. However, mBiVO4 shows higher photocatalytic performance[25]. Thus, the charge kinetics will play a more important role, because the charge carriers transport dominates the quantity of the photogenerated electron-holes participating in the reaction[26]. The disparity of photocatalytic reaction rate between the two BiVO4 is derived from the difference in the distortion of the local environment[25]. Specifically, the distortion of the Bi–O polyhedron in mBiVO4 are more distorted by a 6s2 lone pair of Bi3+ than those in tBiVO4, which is responsible for the effective

5

charge transport and separation[25]. Electronic structure predictions based on density functional theory (DFT) calculations shows that the VB of mBiVO4 is chiefly consisted of O 2p and Bi 6s orbitals, while the CB is majorly dominated by V 3d orbitals[27]. 2.1.3 Bi2MO6 (M=Mo, W) There are only two crystalline phases in Bi2MO6 (M=Mo, W): orthorhombic and monoclinic structures. Orthorhombic structure of Bi2MO6 exists at low temperatures (T < 960°C) and monoclinic structure is a high temperature phase (T > 960°C). The presently studied Bi2MO6 (M=Mo, W) photocatalysts are focused on the orthorhombic structure. Bi2MO6 (M=Mo, W) are the simplest members in Aurivillius type oxides family, and consist of [MO6] octahedral layers sandwiched between [Bi2O2]2+ layers[28]. In particular, the local structures of M and Bi ions are also distorted in the orthorhombic Bi2MO6 (M=Mo, W). Theoretical calculations have shown that the VB of Bi2MO6 (M=Mo, W) is mainly consisted of O 2p and Bi 6s orbitals and the CB is composed of X nd (Mo 4d, W 5d) orbitals. The band gaps of Bi2MoO6 and Bi2WO6 are about 2.6 eV and 2.8 eV, respectively[29]. 2.1.4 BiOX (X = Cl, Br, I) BiOX (X = Cl, Br, I) crystallizes usually in the tetragonal matlockite structure, a layer structure characterized by [Bi2O2]2+ slabs interleaved by two slabs of X － along the z-axis direction. The open-layer crystalline structure can offer enough space to polarize the related atoms and orbitals, which induces the internal static electric fields perpendicular to the [Bi2O2]2+ slabs and X- slabs[30]. Furthermore, the internal

6

electric field that formed between the layers can promote effective separation of photogenerated electron-holes, which plays a critical role in improving the photocatalytic performance of BiOX (X = Cl, Br, I)[31]. Moreover, for BiOX (X = Cl, Br, I), the VB is mainly comprised of O 2p and X np (n = 3, 4 and 5 for X = Cl, Br and I, respectively) orbitals, the CB is majorly dominated by Bi 6p orbitals. It also should be noted that with the increase of atomic number of Cl, Br, and I elements, the contribution of X ns orbitals increases significantly, and the dispersive characteristic of band energy level becomes more and more striking[32]. As a result, the band gaps of BiOCl (3.2 eV), BiOBr (2.7 eV) and BiOI (1.7 eV) become narrower[33]. 2.2 Synthesis methods It is well known that the synthesis routes can affect the morphology, size and specific surface areas of the photocatalysts, which plays a decisive role in the adsorption properties and photocatalytic performance. Also, it have an impact on the environment, synthesis scale, production cost, and safety issues[34,35]. Currently, the hydrothermal/solventthermal, solid reaction, and template methods are the widely used routes for preparation of Bi-based photocatalysts. The Bi source for the synthesis of Bi-based photocatalysts mainly include Bi(NO3)3·5H2O, NaBiO3·2H2O, BiCl3, Bi2O3 and Bi. 2.2.1 Hydrothermal/solvothermal method The hydrothermal/solvothermal method is a primary approach for the synthesis of Bi-based photocatalysts. Facet, size, surface defects, morphology and dimensionality of the Bi-based photocatalysts can be performed by adjusting pH

7

value, solvent, reaction time and temperature. In general, the photocatalysts prepared by hydrothermal/solvothermal method possess better performance in terms of quality and the nanoparticles are better suitable for specialized applications than those produced by dry methods[36]. However, low production rate is an obvious disadvantage of hydrothermal/solvothermal method. The main reason is the long production time and batch character of production caused by the use of special autoclaves. There is a risk of leakage of nanoparticle mainly to the water for the hydrothermal/solvothermal method, and there is also a risk of hazardous solvent emissions for the solvothermal method[37]. Recently, Lin et al. successfully synthesized mBiVO4 photocatalyst via the hydrothermal method[38]. As shown in Fig. 1, mBiVO4 prepared at different pH conditions exhibit distinct morphology and size. More importantly, pH value has a significant influence on the microstructure and photocatalytic performance of BiVO4. Under visible light irradiation, the coralloid particles synthesized at pH=7 can display an outstanding photocatalytic degradation performance toward Rhodamine B (RhB), and its high photocatalytic performance can be attributed to the effective solar energy harvesting and promoted charge carriers separation capability. Moreover, the key role of pH value for the control of facet exposure, which can affect the photocatalytic performance, has been discussed. For example, BiVO4 reported by Colon group shows a clear evolution from ball structure to needle like morphology with different pH values[39]. In other study, Sarkar et al. synthesized various sizes and shapes of the Bi2S3 nanoparticles by the solvothermal method using different solvents and surfactants[40]. The Bi2S3 nanoparticles

8

synthesized from the mixture of oleylamine and trioctylphosphine oxide exhibits highest photocatalytic performance than others due to the large specific surface areas. Besides this, the surface defects in Bi-based photocatalysts can also be achieved through reducing solvents. Olive-green few-layered BiOI with expanded spacing of the (001) facets and oxygen vacancy can enhance the photoreduction of CO2, which is synthesized by the hydrothermal method using ethylene glycol as solvent[41]. 2.2.2 Solid reaction method Many advantages have been demonstrated for the synthesis of Bi-based photocatalysts by hydrothermal/solvothermal method, especially controllable nanostructure. If the commercial Bi-based photocatalysts is prepared by this method, a large amount of water is required. In this regard, solid reaction method without the utilization of water which is suitable for large-scale production has potential advantages. Futhermore, there is also no need for expensive organic solvents as is the case for solvothermal. However, solid reaction method ranks among those methods with a high risk of the release of nanoparticles to the air, so solid reaction method is not fully environmentally friendly[42]. In general, there is a broader size distribution in the case of solid reaction synthesis than that prepared with wet methods because of the greater difficulties in controlling the production process[36]. For instance, Bi3O4ClxBr1−x is synthesized by calcination of Bi2O3 and BiOX (Cl, Br) at 400°C[43]. This can be attributed the relatively weak van der Waals interaction between the halogen atoms, which are easily replaced in Bismuth oxyhalide. Moreover, the Bi2O3 and MoO3 is used to prepare γ-Bi2MoO6 powder by calcination at 550°C[44], and

9

then γ-Bi2MoO6 film with superior photocatalytic performance is formed from γ-Bi2MoO6 powder via a decomposition/evaporation sequential process. 2.2.3 Template method Among the various synthesis methods, the template method is an effective pathway for the controlled synthesis of Bi-based photocatalysts with highly anisotropic or hollow structures or highly ordered multidimensional forms, which is extremely difficult to obtain with direct synthesis methods[45]. Depending on the type of templates, the template method can be divided into hard template, soft template, and self-template method. For template method, the high cost is due to template synthesis and removal, as well as the time intensiveness of the template method. Although templates such as SiO2 are low cost, easy to prepare and modify, there is a need for strenuous measures to protect the environment as removal of the templates usually requires extremely corrosive acid or base[46]. Moreover, most research to date has focused on choosing templates for produce nanostructures of functional materials, which are often poor choices because they are chosen for functionality and not economic implications. In particular, self-template method eliminates the need of additional templates, making it more convenient for practical applications due to the significantly reduced production cost and simplified synthesis processes[47]. Xiao et al. presented a self-template method to synthesize Bi2WO6 rod-like hollow hierarchical structure[48]. The hydrolysis of Bi(NO3)3 in water could swiftly produce Bi precursor with microrods which can be served as sacrificial template. The hydrolysis reaction is described as :

10

6Bi(NO3)3+11H2O → Bi6O5(OH)3(NO3)5•3H2O + 13HNO3. As shown in Fig. 2, the transformation of the microrods into the hollow hierarchical structures is achieved by employing the Kirkendall effect. The WO42– anions firstly exchange with the NO3– anions in the Bi precursor microrods via an anion exchange process. Subsequently, the Bi6O5(OH)35+ polycations reacting with WO42– anions will produce Bi2WO6 under the hydrothermal condition. Consequently, the initially formed Bi2WO6 nuclei on the surface will serve as the nucleate sites on which the subsequently formed Bi2WO6 specie inside will diffuse to their surface. Finally, Bi2WO6 microrods are formed due to the mass transport discrepancy. 3 Structural design of Bi-based photocatalysts 3.1 Heteroatom doping Heteroatom doping is always considered as an efficient method to optimize the photocatalytic performance by tuning the electronic structure, which greatly affects light

absorption

response

and

charge

carriers

separation

efficiency

of

photocatalysts[49,50]. The effect of heteroatom doping on the electronic structure of photocatalysts is the presence of dispersed doping levels. Specifically, orbital hybridization occurs between the dopant orbital and the molecular orbital of photocatalysts, resulting in dispersed doping levels above VB or below CB[51]. In general, heteroatoms doping can tune the electronic structure of Bi-based photocatalysts for enhancing visible light response[52]. However, the low utilization efficiency of the photogenerated charge is the primary issue to be solved for these Bi-based photocatalysts. For this reason, one of the focuses is that doping can

11

effectively improve the efficiency of charge carriers separation by introducing doping level. For example, Cu-doped BiVO4 has shown higher photocatalytic degradation performance toward Methylene Blue (MB) than that of pure BiVO4[53]. After the substitution of Bi3+ by Cu2+ in crystal structure, the in-gap level appears from the localized Cu 3d orbitals hybridized with O 2p orbitals. And then the formation of in-gap doping level just above the VB can serve as the electron trap sites, which can promote the separation of the photogenerated electron-holes. Moreover, the energy span over the CB width of Cu-doped BiVO4 is narrowed by 0.47 eV compared to that of BiVO4. The effective electron mass will increase with the narrowing of the energy of the density of states which reduces the electron mobility in the excited states. Also, the effective mass of carriers (m*) can be used to explain the photocatalytic performance, and it can be calculated as follows:

 d2E  m* = h  2   dk  2

where m* is the effective mass of carriers, and E, k and h represents band energy, wave vector and reduced Planck constant, respectively. Thus, the diffusion length of charge carriers increases, and a higher amount of electrons and holes can participate in the photocatalytic reaction. All the above process will be conducive to the improvement of photocatalytic performance in BiVO4. Wang et al. investigated, both theoretically and experimentally, the effect of Co doping on the electronic structure of BiOCl nanosheets[54]. It’s well proved that Co element has replaced some Bi in [Bi2O2]2+ layers after Co-doping modification. An in-gap level in the band gap of Co-BiOCl is mainly consisted of Co 3d orbitals and a little O 2p orbitals (Fig. 3). As a 12

consequence, the electrons in the VB of Co-BiOCl can firstly transport into the Co doping level by the interconversion between Co2+ and Co3+, then be excited by visible light irradiation and finally injected to the CB. Benefiting from the formed Co doping level, Co-BiOCl exhibits greatly accelerated photogenerated charge carriers separation efficiency, as compared with the pure BiOCl, thus leading to a desirable performance toward bisphenol A (BPA) degradation (0.35 mg m−2 min−1) under visible light irradiation. In addition, the efficient carriers separation can also be achieved to establish redox centers in the crystal structure. Li et al. demonstrated the influence of various Ln13+4ƒ7+x/Ln23+4ƒ7−x (Ln1/Ln2 = Tb/Eu, Dy/Sm, Er/Nd; x = 1, 2, 4) co-doping on the photocatalytic performance over Bi2MoO6[55]. After co-doping with the same proportion of Ln1 and Ln2 ions in Bi2MoO6, the redundant electrons of Ln1 4ƒ1 orbital can be transferred to the unoccupied orbital of Ln2 4ƒ2, then the coupling-half-filled in the 4ƒ1/4ƒ2 orbital that both holds 7 ƒ-electrons are formed, and this configuration is relatively stable. Due to the longer lasting electrons-transferring in heterodinuclear ions, Ln1/Ln2-Bi2MoO6 exhibits excellent capability of charge carriers separation. Moreover, the photogenerated electrons in CB are captured by the Ln1/Ln2 redox couple, and then the trapped electrons can easily transfer to Bi2MoO6 surface. The trapping and releasing process of electrons suppress the charge carriers recombination. Therefore, Ln1/Ln2-Bi2MoO6 can display enhanced photocatalytic performance for RhB degradation, compared with pure Bi2MoO6. However, the effect of doping can not always be advantageous. In some systems, a deep level may serve as a recombination center, leading to a decreased quantum efficiency[56].

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Consequently, a proper dopant with optimum concentration is significant for enhancing the performance of Bi-based photocatalysts. 3.2 Solid solution Different from heteroatoms doping, which leads to dispersed doping levels in the band gap, the formation of solid solution can allow one to tune the band gaps of photocatalysts continuously[57,58]. In solid solution photocatalysts, the anions or cations of the host semiconductor are selectively replaced by introducing ones over the whole range of the composition, and thus their band gaps usually fall in the region between those of original semiconductors, subsequently tailoring the electronic structure to optimize the photocatalytic performance[59,60]. For instance, Bi13-xTexMo4-xV1+xO34 (0 ≤ x < 2.5) solid-solutions is synthesized by the modified sol-gel method[61]. As shown in Fig. 4, with the x value in the range of 0–2.5, the appearance colors of Bi13-xTexMo4-xV1+xO34 nanoparticles change from bluish white, to pale yellow, deep yellow, and yellowish orange. In accordance with the continuous changes in colors, the band gaps of solid solutions narrowed from 2.88 eV to 2.05 eV. Evaluated by RhB under visible light irradiation, the degradation rate can be increased from 40% to 90% in 120 min. In another report, BixY1−xVO4 solid solution can achieve the overall water splitting under visible light irradiation[62]. This can be attributed to the contribution of the Bi 6s and Y 4d orbitals to CB, where the position of CB to satisfy the H2O/H2 potential. Bismuth oxyhalide solid solutions have been extensively studied because of their similar

layered

structure

and

atomic

14

arrangements.

Nanosheet-assembled

three-dimensional (3D) BiOCl1−xBrx hierarchical microspheres exhibits better performance than single BiOCl and BiOBr[63]. In the study, the continuous band gaps of BiOCl1−xBrx are obtained by gradually increasing the composition molar ratio of Br-to-Cl atom, accompanying with the potential of VB maximum increases gradually. It should be mentioned that the balance between band gap and potential of VB in BiOCl1−xBrx plays an essential role in the photocatalytic performance. The decreased band gaps of BiOCl1−xBrx can increase the amount of photogenerated electron-holes for photocatalytic reaction. However, the upward shift of VB maximum

reduces

the

oxidation

capacity

to

p-nitrophenol

(PNP)

and

tetrabromobisphenol-A (TBBPA). Thus, the fabricated BiOCl0.3Br0.7 with proper band gap structures can displays the best photocatalytic performance. Similarly, when the three kinds of halogens are introduced during fabrication, ternary solid solutions BiOClxBryIz (x + y + z = 1) can be obtained, and the obtained solid solutions also possess a tunable band gap and excellent photocatalytic performance[64]. 3.3 Vacancy/defect introduction Vacancies and defects play an important role in photocatalytic processes as they can modify the involved electronic structures of Bi-based photocatalysts, then controll the photocatalytic performance. For example, Zou et al. have reported the excellent photocatalytic degradation performance toward RhB in the oxygen vacancy rich BiOCl under visible light, where the oxygen vacancies on the surface of BiOCl are detected by the X-ray photoelectron spectroscopy (XPS) and Electron spin resonance (ESR) measurements[65]. As shown in Fig. 5, the role of vacancy in promoting

15

photocatalytic performance can be described as follows: oxygen vacancies build the new photoexcitation processes. The electrons are excited up to defect states from the VB under visible light irradiation. Furthermore, the photogenerated electrons on defect states can not recombine easily with photogenerated holes due to the oxygen vacancies served as traps for electrons. As a consequence, the life of electrons in electron traps is longer than that in the CB. Thus, the electrons in the defect states can react with oxygen adsorbed by oxygen vacancies to produce superoxide radicals (·O2−) and subsequently photocatalytic reaction can be generated by ·O2− and holes (h+). Metal atom vacancy also would profoundly influence the light absorption, surface reactive sites and charge transport. Recently, Bi vacancies have been engineered in monolayered Bi2WO6 nanosheets with the thickness of around 1.0 nm[66]. In-depth investigations show that the formed Bi defects are favor of the adsorption and activation of reactant molecules, which lowers the energy barrier of the photocatalytic reaction. Besides, the existence of Bi vacancies can also effectively promote the charge carriers separation due to the strong capture of the photogenerated electrons by the electron-deficient protonated hydroxyl groups around Bi vacancies. Such effects result in a 32 times increase in photocatalytic performance of Bi2WO6 nanosheets with Bi vacancies for the removal of gaseous toluene than that by pure Bi2WO6 nanosheets under visible light irradiation. 3.4 Stoichiometry alteration Generally speaking, the redox potentials of VB and CB play an important role in determining the photocatalytic performance. Futhermore, the photocatalytic

16

performance of Bi-based photocatalysts are restricted in unsuitable band structure (especially the poor reduction ability of the photogenerated electrons from the less negative CB edge). Therefore, the stoichiometry alteration strategy is used to tune the band structure of Bi-based photocatalysts with a moderate upshift of CB and downshift of VB[67,68]. In this field, bismuth oxyhalides are the most frequently studied. Theoretical calculations show that VB and CB of BiOX are mainly determained by p orbitals or sp orbitals origined from Bi or O element[6]. As a result, the increase of the Bi and O content can adjust the CB and VB edges and change the bandgap energy. For instance, it’s observed that hollow Bi4O5Br2 achieved by bismuth rich strategy can selectively reduce CO2 into CO and CH4 under visible light due to the suitable CB edge[69]. Ultrasmall few-layer Bi4O5I2 nanosheets exhibit a higher performance than BiOI in the photocatalytic degradation of RhB and colourless ciprofloxacin (CIP)[70]. The improvement comes from the more positive VB of Bi4O5I2 which leads to a stronger oxidation capacity. A series of bismuth oxyhalides (including BiOCl, Bi3O4Cl, BiOBr, Bi3O4Br, Bi4O5Br2, BiOI, Bi5O7I, Bi7O9I3) has been reported by Xiao et al.[71]. Under visible light irradiation, the as-prepared O-rich BiOX exhibits efficiently photocatalytic performance for the degradation of bisphenol-A (BPA). According to the UV-vis diffuse reflection spectra (DRS) spectra and DFT calculations, the kinds of halogen and O:X ratio can tailor the VB composition and potentials of the BiOX. The band structure of other Bi-based photocatalysts can also be altered by varying the stoichiometry for performance enhancement. Bi4V2O11 nanosheets synthesized by the hydrothermal reaction possess

17

effective light harvesting and suitable CB edge for water splitting in comparison with BiVO4[72]. 4 Microstructure control of Bi-based photocatalysts 4.1 Ultrathin structures The ultrathin structures with thickness of several nanometers are considered as promising architecture for solar energy utilization, due to their unique structure and physicochemical property, in comparison with the bulk counterparts. First of all, photocatalytic reaction mainly takes place on the photocatalysts surface, the average diffusion time (τ) of charge carriers from the interior to the surface can be expressed by the formula τ = r2/π2D, where r is the particle size and D is the diffusion coefficient of the charge carriers. As a consequence, when the grain radius decreases, a large amount of photogenerated electron-holes will migrate to the surface for photocatalytic reaction[73]. Moreover, photocatalysts with a ultrathin structure possess relatively higher specific surface areas than bulk counterparts. For instance, a series of BiOCl nanosheets with different sizes have been prepared to explore the relationship between size and performance of the photocatalysts[74]. The increased specific surface areas and the efficient separation of charge carriers can enhance greatly the photocatalytic performance, with the decrease in the average thickness of the BiOCl nanosheets. Bi2WO6 layers with single-unit-cell thickness has been reported, where the thickness of about 1.65 nm is identified from atomic force microscopic (AFM)[75]. Benefiting from the advantages of ultrathin structure, these Bi2WO6 atomic-layers

can

achieve

higher

CO2

18

adsorption

capacity

and

stronger

photoabsorption, increase the carriers lifetime, thus exhibit a methanol generation rate of 75 μmol g−1 h−1 under simulated solar light irradiation, which is 125-times higher than that of bulk Bi2WO6. Following the same strategy, it is observed that the photocatalytic performance of the ultrathin square-like BiOCl nanosheets with 3–7 nm thickness are augmented along with a reduction in thickness[76]. The larger specific surface area in BiOCl nanosheets plays crucial roles for photocatalytic degradation of RhB. Also, when the thickness decreases to atomic scale, the atomic-escape energy becomes relatively small, and thus atom vacancies with surrounding dangling bonds will easily appear at surface, which greatly influences the photocatalytic performance of Bi-based photocatalysts[77,78]. Ultrathin 2D BiVO4 nanosheets with a thickness of less than 3 nm but a diameter larger than 1.2 μm (Fig. 6) have been synthesized via a simple two-phase method by Dong et al.[79]. In their study, the widely distributed oxygen vacancies (VO) have been well proved by XPS, Raman, ESR, and extended X-ray absorption fine structure spectroscopy (EXAFS) measurements at the V K-edge. According to the experimental and theoretical investigations, the presence of VO not only increases the density of state at CB minimum and VB maximum, ensuring higher redox ability and photogenerated electron-holes separation efficiency, but also contributes to stronger interaction between H2O and BiVO4, easier charge transfer from H2O to BiVO4, and lower dissociation energy of H2O. Ultrathin structures also have an effect on the interlayer interaction of materials. The Coulomb interactions between photogenerated electron-holes in ultrathin

19

structure photocatalysts, as compared with those bulk counterpart, tend to be significantly promoted, because of the reduced electronic screening effects and increased structural confinement, which plays critical roles for modulate photocatalytic performance[80]. Bi2WO6 monolayer with a sandwiched structure of [BiO]+–[WO4]2-–[BiO]+ has been exploited[81]. During the formation process, the stacking of the monolayers is blocked by Coulomb repulsion forces and the hydrophobic chains of CTA+ ions. Significantly, the sandwich structure like the heterojunction interface with space charge can promote the separation of the charge carriers. Under visible light excitation, holes is directly generated on the active surfaces, while electrons in the middle layer, resulting in the superior photocatalytic performance for the RhB degradation. In addition, when the size of Bi-based photocatalysts is below its Bohr radius, the band gap structure is dependent on the particle size owing to a quantum confinement effect. BiVO4 quantum tubes with a diameter of about 5 nm have been prepared and first utilized for H2 production without any additional sacrificial reagents[82]. In detail, the total yield of H2 after 24 h is 5.3 μmol. This work shows that the band gap widening induced by the quantum confinement effect can result in a more negative reduction potential than H2O/H2, thereby achieving water splitting under simulated solar light irradiation. In another study[83], BiOI with hollow flower-like structure consisting of ultrathin nanosheet (h-BiOI) exhibits a more positive VB than that of bulk BiOI, indicating that a stronger oxidation capacity of the former. Moreover, the theoretical investigations have been carried out for the

20

electronic and photocatalytic properties of monolayer phase (MP), double-layer phase (DP) and bulk phase (BP) Bi2WO6 by means of DFT calculations[84]. The results reveal that MP has a similar band gap when compared with its BP counterpart, because the lattice changes partly offset the quantum confinement effect on the band gap. Furthermore, in all kinds of Bi2WO6 structures, MP can be expected to the highest oxidation capacity with the furthest downward shift of VB maximum. Meanwhile, DP can be expected to possess the strongest reduction ability due to the increase of CB minimum induced by the quantum confinement effect. 4.2 Hierarchical structures Bi-based photocatalysts with hierarchical structures usually exhibit better performance than bulk photocatalysts. Such superiority is due to: i) hierarchical structures with interconnected pore networks can bring more efficient channels for the transport of reactant molecules to reactive sites, which facilitates the diffusion kinetics of reactants and products[85,86]; ii) hierarchically porous core-shell and hollow structures created during the formation of photocatalysts can promote light scattering, which can lead to the enhancement of light absorption efficiency[85,87]; iii) typically, hierarchical structures with high surface to volume ratios also furnish adequate reaction sites and contribute to the distribution uniformity of active sites in photocatalysts[85]. For example, interface-rich Bi24O31Br10 hierarchical hexagonal nanoplates with a size of 10–20 μm assembled by approximately 5 nm nanosheets (Fig. 7) display superior photocatalytic performance, which can deliver 94% of Cr(VI) reduction and 90% of oxytetracycline hydrochloride degradation in 20 min

21

and 30 min, respectively[87]. That is because that plenty of interfaces in such hierarchical structure endow the photocatalyst with strong charge storage capability, and suppress carrier recombination effectively. Bi2Ti2O7/γ-Bi2O3 (BT/γ-Bi2O3) hierarchical heterojunction structure (Fig. 8) via a simple in-situ transformation method exhibits extremely high performance in photocatalytic removal of various high concentration pollutants[88]. This is mainly because the formed hierarchical structure can benefit the charge carriers separation, and facilitate the charge transport and subsequent surface reactions. 4.3 Hollow and porous structures Constructing hollow and porous structures of Bi-based photocatalysts is another appealing strategy to enhance photocatalytic performance. The multiple reflections or scattering from specific hollow structures can enhance the utilization rate of the solar light and subsequent more charge carriers, which are the advantage for photocatalytic process.

BiOI

hollow

microspheres

via

a

reactable

ionic

liquid

1-butyl-3-methylimidazolium iodine ([Bmim]I)-assisted microemulsion method at room temperature have been synthesized, and the formation process can be attributed to the growth mechanisms of self-assembly and inside-out Ostwald ripening (Fig. 9) [89]. Hollow structure in BiOI hollow microspheres is demonstrated to enhance the RhB photodegradation from three aspects. First of all, the hollow structure of BiOI microspheres can bring better visible light absorption by repeatedly refracting the incident light. Longer optical path length gives rise to a higher amount of charge carriers, which is available to participate in the photocatalytic reaction. Secondly, the

22

hollow structure can bring about the low internal resistance and the tight interface contact of outer parts, building a more efficient photogenerated charge carriers separation. Thirdly, the BiOI hollow microspheres with larger specific surface areas can absorb more active species and reactant molecules on the surface. Nanosliced BiOBr hollow microspheres synthesized through a facile template-free route possess high specific surface areas due to the highly hollow architecture, and exhibit desirable photocatalytic performance for the degradation of RhB and MB[90]. At the meantime, hollow structures are not restricted to hollow spheres. Bi2O3 nanotubes with a uniform pore size of about 2.2 nm is achieved by UV light induction (Fig. 10)[91]. The UV light irradiation promotes the dehydration and condensation of amorphous bismuth hydroxide to α-Bi2O3. They have found that the formation of special tubular morphology is derived from the ultrathin nanoparticle morphology of precursor and oriented

attachment

mechanism.

Profiting

from

the

hollow

interior

and

nanoparticle-built structure, the hollow Bi2MoO6 nanorods via a hydrothermal route using Ag2CrO4 nanorods as a hard template exhibit a much stronger visible light absorption and larger surface areas in comparison with bulk Bi2MoO6, leading to superior photocatalytic performance with the RhB completely removed after 60 min visible light irradiation[92]. Similarly, compared with bulk Bi2WO6, Bi2WO6 HHRs (a hollow hierarchical rod frame work composed of Bi2WO6 nanosheets) can deliver a CH4 production rate of up to 2.6 μmol g−1 h−1 after 6 h under visible light irradiation[48], 8 times higher than that of bulk Bi2WO6 (0.33 μmol g−1 h−1), and displays a much larger specific surface areas and better charge transfer kinetics.

23

Apart from the hollow structures, the porous structures also display unique features. For example, Wan et al. prepared mesoporous nanoplate multi-directional assembled Bi2WO6 architecture via a hydrothermal method and calcining process[93]. In their study, Bi2WO6-160 possess the biggest specific surface areas and best light absorption as well as promoted charge carriers separation capability, however, it did not exhibit the best photocatalytic performance for NO oxidation. In general, large specific surface areas and efficient photogenerated electron-holes separation can improve photocatalytic performance by enhancing the adsorption kinetics and charge transfer kinetics, and the porous structures facilitate mass loading and diffusion. In the light of the above discussions, the excellent photocatalytic performance for NO oxidation of Bi2WO6-180-C can be attributed to its special hierarchical mesoporous structure with an suitable pore size and interconnected porous network, which is benificial for gas diffusion and transport during the photocatalytic reaction. In another study[94], the remarkably enhanced photocatalytic performance has been achieved by 1D mesoporous BiVO4 nanofibers via a foaming-assisted electrospinning technique, which is due to the connected 1D mesoporous architecture and large surface areas. Especially, the interconnected mesochannels can provide efficient paths for transport of the reaction intermediates and final products. 4.4 Crystal facet engineering As surface structure is critical to the photocatalytic behaviors, the crystal facet of photocatalysts is closely related to the photocatalytic performance. This is because different crystal facets have different atomic arrangements and surface structures,

24

exhibiting intrinsic surface physicochemical properties. Generally speaking, exposed facets with higher surface energies support higher photocatalytic performance. Hence, the synthesis of crystals exposed with highly reactive facets is an effective method for regulating the performance of Bi-based photocatalysts. Following the above strategy, BiOBr nanosheets with fully exposed (001) facet (B001) and (010) facet (B010) have been prepared by adjusting pH values of the hydrothermal system[95]. B001 nanosheets exhibit greatly higher photocatalytic inactivation towards Escherichia coli (E. coli) K-12 than those of B010 nanosheets (Fig. 11) due to the more efficient separation of charge carriers as well as more oxygen vacancies of B001 nanosheets. Moreover, by controlling the exposed specific facet ratios of BiOI using pH-induced transformation method, the relationships between selectively exposed facets and photocatalytic performance of the nanoplate BiOI have been studied[96]. The higher photocatalytic oxidation performance on gasphase mercury goes to BiOI with exposed (110) facets because it can generate more ·O2− and hydroxyl radicals (·OH) than BiOI with exposed (001) facets. In another study, Bi2MoO6 nanosheets with exposed (010) facets, hydrothermally synthesized with the aid of Cetyltrimethyl Ammonium Bromide (CTAB) as the coating agent, show a faster degradation of RhB, oxytetracycline, and tetracycline than that of Bi2MoO6 nanoparticles[97]. Aside from the control of exposed facets, the synergetic utilization of diverse crystal facets has also attracted great interest to further enhance the photocatalytic performance. Using mBiVO4 as a model photocatalyst, Li et al.[98] have demonstrated that the photogenerated electron-holes can be driven to different crystal

25

facets. Furthermore, a series of BiVO4-based photocatalysts by selective deposition of reduction and oxidation cocatalysts on the (010) and (110) facets of BiVO4, respectively is designed (Fig. 12). The enhanced photocatalytic performance is attributed to the intrinsic nature of charge separation between the (010) and (110) facets of BiVO4, and the synergetic effect of dual-cocatalysts deposited on different facets of BiVO4[99]. Very recently, Ag@AgBr/BiVO4/Co3O4 has been synthesized by selective deposition of Ag@AgBr and Co3O4 on (010) and (110) facets of BiVO4, respectively[100]. In their study, BiVO4 can work as a ‘pre-separation channel’ to achieve spatial charge separation owing to the relatively lower CB and VB energy levels of the (010) facets. Profiting from the synergistic effect of pre-separation channel and deriving-hole-type co-catalysts, Ag@AgBr/BiVO4/Co3O4 exhibits superior photocatalytic degradation performance toward RhB. Therefore, the optimized separation of photogenerated electron-holes can be achieved between different crystal facets due to the potential difference in the CB and VB levels, which finally contributes to the enhancement of photocatalytic performance. 5 Fabrication of Bi-based composites 5.1 Heterojunction construction It is well known that an efficient photocatalyst typically requires the semiconductor with suitable band gaps for light harvesting, effective charge carriers separation capability, and appropriate VB and CB edge potential. At present, it is difficult to satisfy above these harsh terms simultaneously on a single Bi-based photocatalyst. Constructing semiconductor heterojunction may be an effective

26

pathway to overcome the problem of the individual Bi-based photocatalysts, owing to the tunable band structure and efficient photogenerated electron-holes separation, which endows them with superior properties[101]. In this section, two main types of heterojunctions, including conventional Type II and direct Z-scheme heterojunction, will be discussed for improving the performance of Bi-based photocatalysts. Subjecting to the band structure alignment, conventional heterojunctions can be divided into three types: straddling gap (Type I), staggered gap (Type II) and broken gap (Type III), as shown in Fig. 13[44]. Among the different types of heterojunctions, the conventional Type II heterojunction is the most reported one. For instance, AgI coupled with 3D flower-like BiVO4 spheres have been constructed to match the energy band structure[102]. During the photocatalytic reaction (Fig. 14), photogenerated electrons at the CB of AgI tend to migrate toward the CB of BiVO4 and the holes transfer from the VB of BiVO4 to that of AgI, resulting in an effective photogenerated electron-hole separation. Correspondingly, AgI/BiVO4 shows high photocatalytic performance for both TC oxidation and Cr(VI) reduction. In a conventional Type-II heterojunction, the effective charge carriers separation can be achieved

in

composites,

such

as

Fe2O3/Bi2S3[103],

BiOBr/CdS[104],

BiOCl/g-C3N4[105], BiOI@(BiO)2CO3[106], Bi2WO6/Fe3O4[107]. However, these advantages are based on the expense of redox ability of each components. The photogenerated electrons will accumulate on the negative CB, while the holes on the less positive VB, leading to a weakened redox ability of charge carriers. Compared

to

the

conventional

Type-II

27

heterojunction,

the

Z-scheme

heterojunction possesses significantly different charge transfer mechanism. As shown in Fig. 15, the photogenerated electrons in PSII with lower reduction ability can recombine with the holes in PSI with lower oxidation ability. As a result, the Z-scheme heterojunction can achieve efficient charge carriers separation and preserve their high reduction and oxidation ability simultaneously, which is more favorable for the photocatalytic reactions (especially for H2 evolution). There are two typical forms of Z-scheme mechanism: (i) direct Z-scheme heterojunction (Fig. 15a) without electron mediator, and (ii) indirect Z-scheme heterojunction (Fig. 15b) with electron mediator (conductor)[108]. For the direct Z-scheme heterojunction with electron mediator, it can inevitably induce some undesirable backward reactions and dissipate the charge carriers that participate in the intended pathway. The electron mediator will compete with the reactants for redox reaction during the photocatalytic processes. In addition, the direct Z-scheme heterojunction can be operated only in solution systems. For the indirect Z-scheme heterojunction with conductor, the use of noble metals as conductors will increase the cost, and the light-shielding effect can occur. On the contrary, electron mediator (conductor) is not required in a direct Z-scheme heterojunction, and the photogenerated charge can directly migrate using the built-in electric field at the interfaces between PSI and PSII. Based on this, the Bi-based direct Z-scheme heterojunction has been widely studied for various photocatalytic applications. For example, Wang et al.[109] developed a direct Z-scheme g-C3N4/(010)BiVO4 heterojunction to enhance the photocatalytic performance for the RhB degradation.

28

The layered g-C3N4 is anchored on the (010) facets of BiVO4 through strong interface electrostatic interaction. Under visible light irradiation, the Z-scheme charge transfers with the electrons in the CB of (010) facets of BiVO4 combining with the holes in the VB of g-C3N4 to ensure the strong redox ability in the photocatalytic reaction. The direct Z-scheme photocatalytic mechanism is verified by active species trapping experiments and photoluminescence technology using terephthalic acid as a probe molecule. Similarly, a direct Z-scheme heterojunction made up by black phosphorus (BP) and Bi2WO6 nanosheets (MBWO) exhibits an effective charge separation and high redox ability[110]. The optimized BP/MBWO shows a 9.15 times yield of H2 (21042 μmolg-1) compared with pure MBWO and the NO removal ratio is up to 67%. Moreover, the direct Z-scheme charge transfer mechanism is confirmed based on active radicals, intermediates, and final products (Fig. 16). Other Bi-based direct Z-scheme hetorojunction are summarized in Table 1. 5.2 Electron transporting material loading Although heterojunctions are excellent for the photogenerated electron-holes separation, the challenges remain in the enhancement of photocatalytic performance, such as the selection of suitable semiconductors with appropriate bandgap and surface structures. The use of electron transporting material as electron capture agents can accelerate the separation of photogenerated electron-holes, leading to a significantly enhancement of photocatalytic performance. Carbon, metals, some metal oxides and polymers have been extensively loaded onto Bi-based photocatalyst[147]. In this regards, various types of carbons, including

29

active carbon and biochar, carbon quantum dots (CQDs), carbon nanotubes (CNTs) and graphene, play important roles in enhancing the performance of Bi-based photocatalysts. Reduced graphene oxide (rGO) is very suitable for fabricating efficient

composites

with

Bi-based

photocatalysts.

In

the

rGO/Bi2MoO6

photocatalysts, layered rGO plays crucial roles for O2 production and RhB degradation under visible light irradiation[148]. Specifically, rGO serve as an electron collector caused by the abundance of delocalized electrons from the conjugated sp2 bonded carbon network for separating the charge carriers by the interaction between layered rGO and Bi2MoO6 with 3D hierarchical microspheres, which is evidenced by the photocurrent response and photoluminescence spectra. Noble metal NPs (such as Au, Ag, Pt, and Pd) loading on Bi-based photocatalysts is also an effective strategy to achieve outstanding photocatalytic performance. On one hand, since the metal NPs serve as traps for photogenerated electrons, they would promote the charge carriers separation. On the other hand, the surface plasmon resonance (SPR) effect of metal NPs provides wider light absorption region. The intimate anchoring of Au-Pd NPs on Bi2WO6 ultrathin nanosheets creates a Schottky barrier to prevent the drift of photogenerated electrons from the metal back to Bi2WO6 (Fig. 17)[149]. Furthermore, when AuPd-Bi2WO6 is irradiated by the visible light, the oscillating hot electrons from Au can be transferred into Pd due to the localized SPR effect. The hot electrons injection from Au can further enrich the electron density on Pd surface. Whereafter, O2 molecules dissolved in water can be combined with photogenerated electrons accumulated on the Pd surface for generation

30

of ·O2−. While the alkoxide anions react with the holes on the Bi2WO6 surface and subsequently deprotonate to form carbon radicals. As a result, AuPd-Bi2WO6 can achieve the synergetic utilization of the Schottky junction and the SPR effect to enhance photocatalytic activity. In addition, multi-component electron transport materials have been reported. In Ag/BiVO4/rGO[150], rGO play a critical role in raising photogenerated charge transfer kinetics because of the excellent charge mobility. At the same time, Ag NPs can serve as electron capture agents to improve the separation efficiency of photogenerated electron-holes. Besides, the SPR effect of Ag NPs and the sensitizing effect of rGO can enhance the absorption ability of visible light. Benefiting from these features, Triclosan is completely removed after 100 min under visible light irradiation. 6 Conclusions and perspectives By

virtue

of

the

unique

electronic

structure,

crystal

structure

and

physicochemical properties, Bi-based photocatalysts hold great potentials in solving the environmental remediation and energy conversion under visible light. Herein, we mainly review the recent significant progress of Bi-based photocatalysts based on structural design, microstructure control and fabrication of Bi-based composites, along with exploring the role of various strategies in the photocatalytic process. Moreover, the performance efficiency related to Bi-based photocatalyst also have been summarized in Table S1. Although significant enhancement in photocatalytic efficiency has been achieved, there still remains many opportunities and challenges in Bi-based photocatalyst.

31

(1) The preparation for desired structure and morphology in a sustainable way still a main challenge. Environmental, safety and economic implications should be taken into account when locating the "sustainable energy trilemma" during the design and synthesis of Bi-based photocatalysts. (2) Few studies have been performed on the application of Bi-based photocatalysts in the energy photocatalysis field, such as H2 generation, CO2 reduction and selective organic transformation, which is mainly due to the poor reduction ability of the photogenerated electrons from the less negative CB edge. The construction of direct Z-scheme hetorojunctions between Bi-based photocatalysts and semiconductors with more negative CB is one of the most effective approaches to explore reduction applications. Meantime, the stability and recyclability of Bi-based photocatalysts in practical application should also be studied. (3) The accurate reaction mechanism of Bi-based photocatalysts remains largely unclear and must be extensively researched. For instance, there are some arguments about active species of some Bi-based photocatalysts. Some experimental work and theoretical investigations indicate that there should be no reactive oxygen species generated due to the band structure[151]. But EPR measurements and active species trapping experiments have shown that there are a few reactive oxygen species generated[152,153]. Therefore, the band width mechanism and discrete VB mechanism are suggested to explain it[119,151]. Also, we can often find unusual phenomenon

in

some

studies

of

Bi-based

photocatalysts,

such

as

the

nonstoichiometric ratio of H2 and O2 in photocatalytic water splitting of BiVO4

32

quantum dots[82]. Therefore, the interrelationship between experimental work and theoretical investigations has important guiding significance for the further optimization of the photocatalytic performance. Specially, some key issues that account for the high photocatalytic performance, such as reactant adsorption sites, charge transfer dynamics, and molecular orbitals, should receive more attention in future studies. Furthermore, the characterization techniques with high spatial, temporal, and spectral resolutions such as aberration-corrected scanning transmission electron microscopy (STEM), in situ X-ray absorption fine structure (XAFS), ultrafast spectroscopy, are indispensable for in-depth understanding the relationship between microstructure and performance. (4) It is significantly important to understand the effects of reaction conditions on the introduction of specific structural characteristics. In this case, more attention should be paid to the fundamental principles of the assembly, growth, and processing of Bi-based photocatalysts with the multifunctional coupling, which is beneficial for fine tuning of reaction conditions. Acknowledgments This work was financially supported by National Natural Science Foundation of China (No. 51602278), the Hebei Province Department of Higher Education Science and Technology Plan of Young Talents (No. BJ2018004). References: [1] P. Lanzafame, G. Centi, S. Perathoner, Catalysis for biomass and CO2 use through solar energy: opening new scenarios for a sustainable and low-carbon chemical production, CHEM SOC REV. 43

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Figures:

Fig. 1.

SEM images of BiVO4 powders produced at pH＝ (a) 0.5, (b)

2, (c) 7, (d) 12. Reproduced with permission from Ref.[38]. (License Number 4687480133599).

56

Fig. 2.

(a–d) SEM images of the intermediate products formed during

the synthesis of Bi2WO6 HHRs at under different reaction time (from a to d: 0, 5, 10 and 20 h); (e) Schematic illustration of the evolution process from Bi precursor microrods to Bi2WO6 HHRs. Reproduced with permission from Ref.[48]. (License Number 4687420600825).

57

Fig. 3.

(a) Calculated electronic energy band structures and (b) DOS

of BiOCl and Co-BiOCl. Reproduced with permission from Ref.[54]. (License Number 4687060119902).

Fig. 4.

Absorption spectra (a), energy calculations for band gap for k

= 2 (b) of Bi13-xTexMo4-xV1+xO34. Inset (a) is the pictures showing the appearance colors of the samples. Reproduced with permission from Ref.[61]. (License Number 4687411319228).

Fig. 5.

Proposed photocatalytic mechanism of oxygen vacancy rich 58

BiOCl under visible light irradiation. Reproduced with permission from [65].

Fig. 6.

(a) TEM, (b) HRTEM images of ultrathin 2D BiVO4 NSs, (c)

AFM image and (d) the corresponding height profiles of ultrathin 2D BiVO4 NSs. (e) Dissociation energies of H2O molecule: (1) dissociation of free H2O molecule; (2) dissociation of H2O molecule on the BiVO4 surface of (010) plane; (3, 4) dissociation of H2O molecule on the BiVO4 surface of (010) plane with VO. (f) Calculated DOS of BiVO4 without and with VO on the surface of the (010) plane. (g) Charge 59

density distribution at the Fermi level of BiVO4 with VO on the surface of the (010) plane. Reproduced with permission from Ref.[79].

Fig. 7.

SEM and TEM characterizations of Bi24O31Br10 HPs (a, b)

SEM images, (c, d) TEM images, (e) HRTEM, (f) SAED pattern. Reproduced with permission from Ref.[87]. (License Number 4687430623235).

Fig. 8.

SEM images of (a) γ-Bi2O3, (b, e) 0.5% BT/γ-Bi2O3, (c, f) 3%

BT/γ-Bi2O3, and (d, g) 7% BT/γ-Bi2O3. Reproduced with permission 60

from Ref. [88]. (License Number 4687490034176).

Fig. 9.

Schematic illustration of the formation process for BiOI

hollow microspheres. Reproduced with permission from Ref.[89]. (License Number 4687080380701).

Fig. 10.

(a)SEM, (b)TEM and (c, d) HRTEM images of α-Bi2O3.

Reproduced with permission from Ref.[91]. (License Number 4687491412950).

61

Fig. 11.

Photocatalytic inactivation efficiency of E. coli K-12 (1 × 107

CFU mL-1 ) in the presence of B001 and B010 nanosheets under VL irradiation. Reproduced with permission from Ref.[95]. (License Number 4687390978289).

62

Fig. 12.

(a) Selective deposition of dual redox cocatalysts on specific

facets of BiVO4 (b) SEM images of (i) MnOx/BiVO4; (ii) IrO2/BiVO4. Reproduced with permission from Ref.[99]. (License Number 4687410862967).

Fig. 13.

Schematic illustration of photogenerated charge transfer for:

(a) Type I heterojunction, (b) Type II heterojunction and (c) Type III heterojunction.

Fig. 14.

(a) The FESEM image of 0.3-AgI/BiVO4, (b) comparison of 63

AgI, BiVO4 and AgI/BiVO4 for photocatalytic TC oxidation with Cr(VI), (c) a schematic diagram showing the charge transfer of a BMO/BOI

heterostructure.

Reproduced

with

permission

from

Ref.[102]. (License Number 4687090853873).

Fig. 15.

Schematic illustration of photogenerated charge transfer for

(a) direct Z-scheme heterojunction and (b) indirect Z-scheme heterojunction.

Fig. 16. by

Photocatalytic mechanism of NO removal and water splitting

BP/MBWO

heterojunction

under 64

visible

light

irradiation.

Reproduced with permission from Ref.[110]. (License Number 4687100231249).

Fig. 17.

(a) TEM image and the estimated AuPd particle size

distribution, (b) HRTEM image of AuPd-Bi2WO6 sample, (c) Band diagram and charge transfer in bimetallic AuPd-Bi2WO6 photocatalyst. (d) A possible pathway for photocatalytic selective oxidation of aromatic alcohols to aromatic aldehydes on AuPd-Bi2WO6. Reproduced with permission from Ref.[149]. (License Number 4687100991542).

65