Recent advances in photocatalytic treatment of pollutants in aqueous media

Environment International 91 (2016) 94–103

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Environment International journal homepage: www.elsevier.com/locate/envint

Review article

Recent advances in photocatalytic treatment of pollutants in aqueous media P. Anil Kumar Reddy a, P. Venkata Laxma Reddy b, Eilhann Kwon c, Ki-Hyun Kim d,⁎, Tahmina Akter e, Sudhakar Kalagara e a

Indian Institute of Chemical Technology, Tarnaka, Hyderabad, Telangana 500076, India Program in Environmental Science and Engineering, University of Texas El Paso, El Paso, TX 799038, USA Department of Environment and Energy, Sejong University, Seoul 143-747, Republic of Korea d Department of Civil & Environmental Engineering, Hanyang University, 222, Wangsimni-Ro, Seoul 133-791, Republic of Korea e Department of Chemistry, University of Texas at El Paso, El Paso, TX 79968, USA b c

a r t i c l e

i n f o

Article history: Received 18 December 2015 Received in revised form 7 February 2016 Accepted 7 February 2016 Available online 23 February 2016 Keywords: Photocatalysis Water decontamination Hazardous Pollutants

a b s t r a c t Photocatalysis can be an excellent solution for resolving the world's energy and environmental problems. It has a wide range of applications for the decontamination of diverse hazardous pollutants in aqueous media. Technological progress in this research field has been achieved toward the improvement of the solar sensitivity to enhance the efficiency of pollutant decontamination. As a result, various strategies have been introduced to upgrade photocatalytic performance with the modification of prototypical photocatalyst such as doping, dye sensitization, semiconductor coupling, mesoporous supports, single site, and nano-based catalysts. In this review, a brief survey is presented to describe those strategies based on the evaluation made against various pollutants (such as pharmaceuticals, pesticides, heavy metals, detergents, and dyes) in aqueous media. © 2016 Elsevier Ltd. All rights reserved.

Contents 1. 2. 3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Semiconductor oxide based photocatalysts . . . . . . . . . . . . . . . Modifications to improve solar sensitivity of semiconductor photocatalysis 3.1. Dye sensitization . . . . . . . . . . . . . . . . . . . . . . . 3.2. Semiconductor doping . . . . . . . . . . . . . . . . . . . . . 3.3. Quantum dot sensitization . . . . . . . . . . . . . . . . . . . 3.4. Solid solutions . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Plasmon based photocatalysts . . . . . . . . . . . . . . . . . 3.6. Semiconductor combinations (coupling) . . . . . . . . . . . . 4. Approaches to enhance the yield of photocatalytic degradation of pollutants 4.1. Mesoporous materials supported photocatalysts . . . . . . . . . 4.2. Single site catalysts . . . . . . . . . . . . . . . . . . . . . . 4.3. Semiconductor modification of size and morphology . . . . . . . 4.4. Photocatalyst coupled enzymes . . . . . . . . . . . . . . . . . 5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⁎ Corresponding author. E-mail address: [email protected] (K.-H. Kim).

http://dx.doi.org/10.1016/j.envint.2016.02.012 0160-4120/© 2016 Elsevier Ltd. All rights reserved.

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P.A.K. Reddy et al. / Environment International 91 (2016) 94–103

1. Introduction The industrial revolution has brought many benefits to mankind. On the other hand, it has also resulted in adverse impacts on our ecosystem. Among the most critical contemporary global issues, the conservation of water resources in association with global climate change is of high environmental importance (Trenberth et al., 2014). Disposal of unwanted (or waste) anthropogenic chemicals has led to the contamination of rivers, lakes, groundwater aquifers, and oceans (Ayrault et al., 2014; Yuan et al., 2014; Miao et al., 2015). In the aquatic environment, diverse hazardous pollutants are identified to include textile dyes, surfactants, insecticides, pesticides, and heavy metals (Belenguer et al., 2014; Fujita et al., 2014; Sherman and Gaal, 2015). Among various treatment options for those pollutants, photocatalysis has been regarded as one of the most realistic solutions due to its proven potential in environmental clean-up of wide range of pollutants or generation of renewable energy (i.e., H2). To further illustrate this point, a number of surveys on photocatalytic research have been introduced in the recent years (e.g., Reddy and Kim, 2015). Despite its proven effectiveness, its capacity is not yet proven in scaling up from the laboratory to meet global demands. As such, there are still various challenges that need to be resolved to pursue the further progress of the technology (Zhong and Haghighat, 2015). Photocatalytic applications based on TiO2 have been used most extensively for the treatment of pollutants due to their well-known efficiency and near-universal applications. However, many drawbacks are also reported for such applications (e.g., recombination of generated redox environment, low solar sensitivity, and selectivity) (Ozawa et al., 2014). On this note, extensive research has been carried out to improve its efficiency. For instance, enhancement of the spectral properties (e.g., solar sensitivity) became an active research area (Schultz and Yoon, 2014; Banerjee et al., 2014). Much effort has also been directed to improve the performance of the semiconductor oxide based photocatalysts alone or through modifications. In this work, a comprehensive review is provided to describe the important aspects of the photocatalytic approaches for environmental applications. To this end, we carried out evaluation of their performance to cover semiconductor based photocatalysts and many other options with or without modifications. 2. Semiconductor oxide based photocatalysts As aforementioned, semiconductor-based photocatalysis has been one of the prominent tools for the environmental applications. Photocatalysis is generally initiated by the production of electron-hole pairs after bandgap excitation. The photocatalyst is illuminated by light with appropriate wavelength with energy equal to or greater than bandgap energy; hence, the electrons are excited from valence band to the conduction band to leave a positive hole in the valence band. If the excited electron-hole pairs are recombined, the energy will be released as heat. However, if the electrons and holes migrate to the surface of the semiconductor without recombination, then they are capable of participating in redox reactions with water, oxygen, and other organic (or inorganic) species (pollutants) (Gaya, 2014). These redox reactions are the fundamental mechanisms of photocatalytic (water/air) remediation. For the photocatalytic treatment of pollutants in water (or air), valence band holes are the important components that instigate the oxidative decomposition in a given media. The positive hole can oxidize pollutants either directly or through reaction with water to produce the hydroxyl radical (•OH) at an oxidation potential of 2.8 eV (NHE). These hydroxyl radicals can then rapidly attack pollutants at the surface (and in solution) to mineralize them (e.g., CO2, H2O, etc.). Various semiconductors (like TiO2, ZnO, ZnS, ZrO2, MoS2, WO3, SnO2, Fe2O3, CdS, etc.) can be used as a photocatalyst for pollutant control in aqueous phase. Among them, TiO2 (i.e., titania) has been the most commonly utilized due to its well-known advantages (like cost-effectiveness,

95

non-toxicity, regeneration ability, photocatalytic efficiency, and high chemical stability). Lachheb et al. (2002) reported that the UV irradiated titania was very effective in dye degradation (e.g., Alizarin S (AS), azoic (Crocein Orange G (OG), Methyl Red (MR), and Congo Red (CR)) or heteropolyaromatic (Methylene Blue (MB)) in wastewater, while it helped enhance the decoloration rate of the dyes with 100% mineralization. In another work, the degradation of two selected dyes (such as acridine orange and ethidium bromide) was also achieved by titania under UV irradiation. The results of this work also indicated that the maximum degradation of dye pollutants was achieved only at certain (optimum) concentrations (Faisal et al., 2007). A major drawback of pure TiO2 is its relatively large bandgap, as it can only absorb a small portion of the UV radiation spectrum. In some experiments, other semiconductor oxides (such as ZnO) proved to be a better alternative to TiO2. For instance, the degradation of acid red 14 was achieved successfully by the ZnO-based photocatalyst in the presence of hydrogen peroxide. It was reported that optimal level of photocatalyst was 160 ppm at the dye concentration of 20 ppm with a few drawbacks (e.g., low stability in aqueous media and susceptibility to photocorrosion) (Daneshvar et al., 2004). In addition, WO3 exhibited superior absorptivity in visible light radiation with a reduced bandgap (2.8 eV) relative to TiO2 and ZnO (3.2 eV). However, due to the low reduction potential, its light conversion efficiency is smaller than that of TiO2. Hence, it is sometimes desirable to carry out the modifications to enhance their photolysis potential of the semiconductor. The direct application of semiconductor-based photocatalysts has certain drawbacks pertaining to their stability, visible light absorption, degradation kinetics, and efficiency. The modification of existing semiconductors has been an attractive research subject in the recent years along with the exploration of new novel semiconducting photocatalysts. The criteria for an ideal photocatalyst (e.g., high surface area, appropriate bandgap, efficient visible light absorption, high carrier mobility, and correct band-edge positions) should be taken into consideration to overcome the limitations of prototype semiconductor photocatalyst (Reddy et al., 2011). In this review, we briefly discussed the two essential approaches which can improve the solar sensitivity of the photocatalysis technology on one end, and other hand the strategies that could enhance the degradation efficiency of wide range of organic pollutants. Either goals can be achieved by the proper selection and/or modification of the semiconductor oxide based photocatalysts. In Table 1, summary of various photocatalytic approaches is provided along with the performance evaluation in terms of degradation efficiency (DE) and the time (T) for each reaction. 3. Modifications to improve solar sensitivity of semiconductor photocatalysis The need for external light source like UV lamps for the excitation of an electron from valence band to conduction band is the major impediment for the improvement of photocatalysis. In this respect, various technical approaches have been proposed to optimize the bandgap structure as well as surface area of photocatalysts. 3.1. Dye sensitization Photosensitization was widely used to extend the photo-response of semiconductor photocatalysts into the visible region (Cho et al., 2001). The mechanism of the dye sensitized photocatalysis is highly contingent on the transfer (or injection) of an electron from an excited state of dye molecule to the semiconductor surface (Fig. 1). For example, the visible light excites the sensitizer molecules adsorbed on semiconductor and subsequently inject electrons to the conduction band (CB) of semiconductor. The valence band (VB) remains unaffected in a typical photosensitization, while the CB acts as a mediator for transferring electrons from the sensitizer to substrate (electron acceptors) on the surface of semiconductor. In other words, as the dye molecule absorbs visible light,

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excitation of an electron proceeds from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of the dye. Consequently, the excited dye molecule transfers LUMO electrons into the conduction band of semiconductor photocatalyst, while the dye itself is converted into its cationic radical. The function of a photocatalyst as a mediator for transport electron (from the sensitizer to the adsorbed substrate (electron acceptors such as O2 on the photocatalyst surface and the valence band of the photocatalyst) is unaltered (Bae and Choi, 2003). However, this process takes place only when the LUMO of the dye molecules is more negative than that of the conduction band for the corresponding semiconductor photocatalyst. The injected electrons on the surface of photocatalyst are quickly accepted by molecular oxygen to give superoxide radicals (O2•−) and hydrogen peroxide radicals (•OOH) which initiate the radical chain reactions to degrade the pollutants in wastewater. The environmental applications of the dye sensitized photocatalysts have been investigated by many researchers. For instance, nanoscale ZnO (nanoZnO*), sensitized with the Alizarin Red S dye, was employed to treat toxic metal Cr+6. For nanoZnO* (1.0 g/L) at neutral pH, removal efficiency of Cr6+ was about 68 and 90%, if irradiated by household fluorescence lamps for 6 h and 17 h, respectively. However, if a visiblelight source was used, the Cr6+ removal efficiency was reduced to 50% and 75%, respectively. It was interesting to note that the nanoZnO* has yielded the highest removal efficiency around 90% (Yang and Chan, 2009). In another study, the degradation of phenol was carried out using Eosin Y dye as the sensitizer of the TiO2 catalyst. A small amount of platinum and triethanolamine was used as a metallic catalyst and sacrificial electron donor, respectively. Under optimal experimental conditions, 40 ppm phenol solution was degraded in b90 min with 93% efficiency (Chowdhury et al., 2012). In recent years, the superior efficiency of dye sensitized photocatalysts has been demonstrated; however, the application of dye sensitized photocatalysis has been limited in practical sense due to its instability. 3.2. Semiconductor doping In general, various strategies have been employed to modify semiconductors. For such purpose, doping has been used as a promising option. Numerous benefits have been reported for the photocatalyst applications with the external doping. If semiconductor oxides (such as TiO2) can be doped with the appropriate amount of transition metal ions, it can facilitate the formation of electron capture centers (Fig. 2). Nonetheless, such treatment can alter the crystallinity to minimize electron/hole recombination. Accordingly, the doping of metal ions is an efficient way to enhance the performance of a photocatalyst. The photocatalytic efficiency of TiO2 nanoparticles doped with silver was greatly improved the degradation of chloramphenicol (CAP). Accordingly, at 900 mg/L of Ag/TiO2, the maximal removal of chloramphenicol (at initial concentration of 20 mg/L) was achieved. Under such optimum conditions, the highest removal efficiency of CAP (∼ 100%) was achieved in 20 min (Shokri et al., 2013). It must be noted that the concentration of dopant can directly affect the performance of the photocatalyst. For instance, Nitoi et al. (2015) investigated removal of nitrobenzene (NB) under UV\\VIS irradiation with and without doping of TiO2. The effect of diverse metal dopants was then examined as a function of dopant (Fe, Co, and Ni) and TiO2 amount (0.5–5 wt%). It was concluded that the photocatalytic degradation of nitrobenzene proceeded more efficiently at the lowest Fe content (0.5 wt%) (relative to high Fe content) or when it was doped with Co (or Ni). Accordingly, the overall degradation efficiency of 99% was achieved for nitrobenzene. Despite many known advantages, doping strategy was proven to induce the thermal instability of the TiO2 crystal structure. To resolve this issue, the non-metal doping was also investigated as an alternative to metal-based doping. Such modification generally helped facilitate the upward movement of valence band as well as the formation of oxygen deficient sites (Fig. 3). Note that the latter can

add the blocking effect on reoxidation. For instance, C-doped TiO2 photocatalyst was able to efficiently degrade a stable organic dye such as Remazol Brilliant Blue® (RBB) relative to titania P25 (Mattle and Thampi, 2013). As per the experimental observations, the doped catalyst was able to decrease total organic carbon content by more than 70% within 6.5 h. Likewise, the C\\N\\S tridoped TiO2 (using thiourea as precursor) was also seen to efficiently degrade tetracycline pollutants in water. The narrowing of the bandgap made C\\N\\S tridoping and carbonaceous species dopant serve as an efficient photosensitizer (Wang et al., 2011). In addition to the metal and non-metal doping, there are other emerging strategies such as co-doping and self-doping. According to Li et al. (2011), N/Fe co-doped TiO2 exhibited enhanced performance in the degradation of methyl orange relative to the undoped (or nonmetal (or metal) doped) TiO2. The overall degradation efficiency of methyl orange was 96.4% within 1 h, which was superior to TiO2 P25. In the recent years, self-doped TiO2 using Ti3 + was successfully employed to prove the enhanced performance in environmental applications, while demanding further research (Xing et al., 2013). A wide range of the dopants have been indeed employed based on their cation or anion charge (Daghrir et al., 2013). Different doping strategies can also be taken as physical- (such as metal ion implantation) or chemical-based methods (like sol gel method, hydrothermal method, wet impregnation, chemical vapour deposition, and plasma based doping). Although doping can be used to enhance the overall efficiency of the photocatalyst, many factors need to be considered (e.g., the dopant concentration, type of dopant, and preparation method). The combined effects of such factors are likely to play a vital role in the enhancement of overall efficiency of photocatalysis. 3.3. Quantum dot sensitization In the past decade, quantum dots (QDs) have attracted a great deal of interest as a novel fluorescence nanomaterial. QDs are generally described as nanoscaled particles that are subject to the quantum confinement effect with several advantages: (i) The bandgap of QDs can be tailored via varying the size to tune the visible response, (ii) They can utilize hot electrons or generate multiple charge carriers with a single high energy photon, and (iii) Their high surface area to volume ratio (relative to their bulk counterparts) can enhance the absorption capability on the photocatalyst surface. In addition, high bandgap energy of QDs can help produce a high redox potential in the system. In light of such promising properties, QDs have been used as sensitizers for photocatalytic applications. To efficiently transfer an electron from a visible light-active QD to a semiconductor photocatalyst, its conduction band edge should be located above that of other semiconductor materials (Lee et al., 2014). It must be noted that electron transfer can also greatly facilitate the charge separation, while inhibiting the recombination through an interface potential gradient. Previously, the applications of Cd and Zn-based QDs have been made for photocatalytic degradation of organic pollutants (Li et al., 2008). The ZnO QDs were grown in situ on SiO2 nanotubes with high photocatalytic activity to degrade Rhodamine B. The enhanced activity was attributed to both the high dispersion of ZnO QDs over SiO2 nano tubes and high separation efficiency of photogenerated electron–hole pairs which were trapped between the interface of ZnO QDs and SiO2 (Zhang et al., 2012). In another work, the CdS sensitized with Bi2WO6 was employed to treat phenol. It was observed that this catalyst exhibited enhanced activity under visible irradiation with the overall degradation rate of 97% at 3.7% CdS-Bi2WO6 over 3 h. 3.4. Solid solutions To control the band structure of the photocatalysts, a method was developed to form solid solutions between the wide and narrow bandgaps of semiconductor. This is a very useful modification to

Table 1 Performance of photocatalysts for treating various pollutants in aqueous media. Order

Photocatalyst

Target Pollutant

Concentration Photocatalyst (g/L)

B. Sensitizer-induced photocatalysts 12 Tris(4,4′-dicarboxy-2,2′-bipyridyl)ruthenium(II) complex sensitized-TiO2 13 Eosin Y dye-TiO2 14 Porphyrin-TiO2 15 Novel Bi2S3-sensitized BiOCl C. Nanomaterial 16 Nano-TiO2 17 18 19 20 Nano-ZnO 21 22 23 24 Nano-WO3

s-Triazine (herbicide) Acid blue 80 dye Bisphenol A 17β-Estradiol Organic sulfides Methyl orange 2-Polyphenol Azo dye Remazol Red RR Acridine orange 2,4-Dichlorophenol

0.1 1 1 1 1.5

Carbon tetrachloride

0.5

Phenol Acid chrome blue K Rhodamine B

0.8

Rhodamine 4-Chlorophenol Azure and sudan dye Nitrobenezenes Methylene blue Diazinon Phenol Alizarin yellow Safranin-O dye

1

Pollutant ppm

(g/L)

M

(%)

(h)

References

ppm

0.01 0.02 0.04 1 × 10−6 50

90 85 100 99 80

1.5 5 0.5 2.5 1

Konstantinou et al. (2001) Bianco Prevot et al. (2001) Ohko et al. (2001) Ohko et al. (2002) Habibi and Vosooghian (2005) Al-Qaradawi and Salman (2002) Khodja et al. (2001) Sakthivel et al. (2003) Akyol et al. (2004) Pare et al. (2008) Ba-Abbada et al. (2010)

2

Cho et al. (2001)

93 94 98

1.5 0.25 2

Chowdhury et al. (2012) Li et al. (2008) Cao et al. (2012)

96.80

1 8

Aarthi and madras (2007) Venkatachalam et al. (2007) Aarthi et al. (2007) Priya and Madras (2006) Jang et al. (2006) Daneshvar et al. (2007) Pardeshi and Patil (2008) Hayat et al. (2010) Hayat et al. (2011)

−4

2 2 5 5 2

5 × 10 2 × 10−4

98.6 95 90 98

0.05 2 × 10−5 0.05 1 × 10−3 40 10 10

0.07 0.03

8.2 250 21

0.2 1 1 1.5

15 20 70

150 250 3 1

Tb

3.234 × 10−4 5.7 × 10−4

0.5 0.5 20 0.5 1.5

95.60 86–99 80 95.87 90 94

1 1.3 8 0.83 0.16

P.A.K. Reddy et al. / Environment International 91 (2016) 94–103

A. Semiconductor oxides 1 TiO2 2 3 4 5 6 7 ZnO 8 9 10 11

g

DEa

(continued on next page)

97

98

Table 1 (continued) Order

Photocatalyst

Target Pollutant

Concentration Photocatalyst (g/L)

25 26 27

ZnO/CdO nanocomposite MoO3/ZnO nanorod composites ZnO nanopowder

Methylene blue 2,4-Dichlorophenoxyacetic acid Acid blue 1

Monocrotophos Methylene blue Cl reactive blue 4 Methylene blue Ethylene glycol Bisphenol A Trichloroethylene Mecocrop and clopyralid Azo dye Acetaldehyde Benzene and methyl orange Methyl orange

E. Semiconductor coupling 41 ZnO/SnO2 42 CdSe/TiO2 43 CuO–SnO2

Methyl orange 4-Chlorphenol Acid blue 62

F. Mesoporous materials 44 WO3-ZY 45 Titania on MCM-41

Pollutant ppm

(g/L)

M

(%)

(h)

20

97.80 99.20

40 40 20

74 80 91.25

0.05

0.001 0.1 0.1

0.3

200

100

30

90 98–100

−3

2

1 × 10 0.03 0.1 0.3

100 × 10−6 L 250 40

2

2.5 1.2

64.60

20 2 ± 10−4 0.25

50

99 95.40

Rhodamine B Hexanol

0.025

10

95

G. Single site photocatalysts/MOF 46 Fe(III) ions on HY zeolite 47 Cerium dispersed Al-MCM-41 48 Fe3O4@MIL-100

Phenol Phenol Methylene blue

0.025 0.12 0.01

H. Quantum dot sensitization 49 ZnO QDs on Si nanotubes 50 CdSe QDs with magnetite

Rhodamine B Methylene blue

0.01

I. Plasmon enhanced photocatalysis 51 Au nanoparticles onto the TiO2

Methylene blue

0.002

J. Enzyme coupled photocatalysis 52 TiO2-laccase

2,4-Dichlorophenol

Superscripts a and b denote degradation efficiency (DE) and time (T) for reaction or irradiation, respectively.

References

ppm

3 × 10−5 1 0.5

Tb

6 2 16

2 4 1.5

5 4 2.5 2.28 23 0.35

3 4.17 2

3

Saravanan et al. (2011) Lam et al. (2013) Chen et al. (2008)

Anandan et al. (2007) Sun et al. (2011) Zhou et al. (2009) Umebayashi et al. (2003) Tachikawa et al. (2006) Venkatachalam et al. (2006) Yokosuka et al. (2009) Šojić et al. (2010) Mirkhani et al. (2009) Yang et al. (2007) Xu et al. (2010) Shi et al. (2008)

Wang et al. (2004) Lo et al. (2004) Xia et al. (2007)

Nor et al. (2015) 1 × 10−4 1 × 10−4 10 × 10−3

99 100 99

1 4 1.7

Noorjahan et al. (2005) Reddy et al. (2008) Zhang et al. (2013)

0.83 0.75

Zhang et al. (2012)

99.10

88

8

90

2

10 40–160

10 × 10−6

10

50

1 × 10−4

Jia et al. (2012)

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D. Doped photocatalysts 29 La-doped ZnO 30 Sn-doped ZnO 31 Nd-doped ZnO 32 S-doped TiO2 33 N-doped TiO2 34 35 36 37 Ag-doped TiO2 38 V-doped TiO2 39 CNT doped-TiO2 40 Sm-doped TiO2

g

DEa

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99

Fig. 3. The upward movement of valence band in non-metal doping.

Fig. 1. The primary electron pathway in dye sensitized semiconductor photocatalyst.

develop visible-sensitive photocatalysts. For this approach, the position and the bandgap are tunable by the variation of the ratio in compositions between the narrow and wide bandgap semiconductors (in the solid solution). According to Li et al. (2008), nanocrystal ZnxCd1 − xS solid solutions were prepared by using inorganic salts Cd(Ac)2, Zn(Ac)2, and Na2S. This nanocrystal was able to degrade 96% of methyl orange dye under visible light irradiation. In another work, nanosheetbased Bi2Mo0.25W0.75O6 solid solutions with adjustable bandgaps were prepared. This nano sheet was also able to degrade methylene blue very efficiently under visible light illumination (Zhou et al., 2010). 3.5. Plasmon based photocatalysts Among all the modifications, the addition of metal nanoparticles (NPs) is particularly meaningful to enhance the photocatalytic capacity (Fig. 4). This is because the noble metal NPs (e.g., Au and Ag) have the ability to absorb the visible light via surface plasmon resonance (SPR). SPR is described as the collective oscillation of conduction band electrons in a metal particle driven by the electromagnetic field of incident light. In recent years, notable progress has been made on the synthesis and assembly of plasmonic nanocomposites, e.g., through coupling of NPs with semiconductor substrates of high surface area and active sites. Thus, such coupling exerts the synergistic effect on the photocatalytic activity. The use of plasmonic based metal nanostructures for solar-light harvesting and the energy-conversion has also been investigated. It is observed that the presence of plasmonic NPs can lead to an increase in

the absorption cross-section of semiconductors through the process of strong field enhancement and the extension of light absorption to longer wavelengths. It was also proven to enhance electron–hole charge separation in semiconductor medium. All these factors have indeed contributed to the maximization of the photocatalytic efficiency. As such, the integration of plasmonic Au nanoparticles onto the TiO2 semiconductor has been proven to improve the efficiency of photocatalytic degradation of the methyl orange under the solar visible illumination (Hou et al., 2011). 3.6. Semiconductor combinations (coupling) To improve photocatalytic capacity of semiconductor, it is undeniably important to develop visibly active photocatalysts that can harvest a wide range of photons in the solar spectrum. In general, a decrease in the bandgap of the photocatalyst increases the absorption of visible photons. Then, the redox potentials of conduction (CB) and valance bands (VB) are compromised in the process to delay the recombination of electron. This aspect affects the yield of photocatalytic degradation of organic contaminants. Hence, it is desirable to develop photocatalytic systems capable of absorbing visible light without compromising on the redox potentials of CB and VB. Recently, considerably enhanced activities of diode structures made of n- and p-type semiconductors were demonstrated compared to individual semiconductor. The improved performance of the such coupled systems was attributed to: (i) coupling of semiconductors with distinctive bandgaps (low and high) for the extension of the spectral range of light absorption, (ii) the formation of p–n junction between coupled photocatalysts to facilitate the separation of electron–hole (for the least recombination of photoexcited electron–hole) and (iii) the retaining of the high redox potentials for the individual semiconductors in the semiconductor combinations; here their reactions can occur simultaneously at different

Fig. 2. Bandgap modification in semiconductor photocatalyst by the formation of intermediate band levels through metal doping.

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4. Approaches to enhance the yield of photocatalytic degradation of pollutants To enhance the overall yield of the photocatalysis, some researchers focused on the selectivity to increase the rate of interaction between the pollutant and photocatalyst. 4.1. Mesoporous materials supported photocatalysts

Fig. 4. Plasmon enhanced process of semiconductor photocatalyst.

semiconductor surfaces to activate redox reactions (Zhang et al., 2012) (Fig. 5). The applicability of coupled semiconductors with various combinations has been explored to facilitate visible light absorption and efficient charge carrier separation. A low bandgap semiconductor (CaFe2O4) can thus be coupled with a high bandgap semiconductor (such as TaON, ZnO, TiO2, and Ag3VO4) to assist the separation of charge carriers. More examples of coupled semiconductors can be found as CoFe2O4/ TiO2, BiOI/TiO2, WO3/TiO2, ZnFe2O4/TiO2, ZnO/CuO, Sm2Ti2O7/SmCrO3, BiOI/BiPO4, ZnO/SnO2, and CaBi6O10/Bi2O3 (Lin et al., 2007). In addition to the binary coupling, ternary coupling of heterostructured composite photocatalysts (such as AgIO3/AgI/TiO2, AgBr–Ag–Bi2WO6, MgO/ZnO/ In2O3, Pt/TiO2 − xNx/SrTiO3, and BiVO4–Cu2O–TiO2) has also been explored to seek for the improvement of degradation activity or synergic effects such as enhanced light absorption and charge transfer along with the least electron–hole recombination. The SnO2/TiO2 coupled semiconductor thin film was in fact employed successfully in the degradation of azo dye (Vinodgopal and Kamat, 1995). Likewise, Bansal et al. (2014) used a heterostructured ZnO/TiO2 nanophotocatalysts in the degradation of Acid orange 7 to achieve 81% degradation which was far superior to ZnO (40.0%) and Degussa TiO2 (69.1%).

It is desirable to improve the mass transfer, active surface area, and recycling efficiency of photocatalysts, while minimizing charge carrier recombination. Hence, researchers have been pursuing to synthesize a variety of supports to photocatalysts. The developed supports are diverse enough to include clay minerals, zeolites, mesoporous materials, carbon-nanotubes, and activated carbons. In a work by Ménesi et al. (2008), the positively charged TiO2 nanoparticles were prepared to bind the surface of the negatively charged montmorillonite layers (via heterocoagulation). The clay mineral was employed as adsorbent and the support for the photooxidation of phenol. Phenol was then degraded by irradiation with UV\\VIS light on 40–60% TiO2/Ca-montmorillonite compositions. Here, the overall degradation rate of phenol increased in proportion to TiO2 loading. The mixture containing 25% TiO2 was able to degrade 70% of the phenol over 60 min, whereas those with 80% TiO2 achieved 95–98% of phenol removal over 40 min. Likewise, those with zeolites can also be employed as supports to increase such efficiency. For instance, the photocatalytic activity of the WO3-supported zeolite catalyst (WO3-ZY) was tested for the degradation of Rhodamine B (RhB) under various irradiation (visible, UV, and solar light) conditions. Accordingly, the overall photocatalytic degradation under UV light irradiation was about 86% (140 min), whereas it was 95% (180 min) under solar irradiation. In summary, photocatalytic activity of the WO3 was dramatically improved with utilisation of zeolite support (Jothivenkatachalam et al., 2014). Furthermore, a wide range of porous materials were used as supports in the degradation of pollutants. According to Bhattacharyya et al. (2004), the TiO2 photocatalyst was impregnated with three different kinds of adsorbents, i.e., mesoporous (MCM-41), microporous (β-zeolite), and pillared structure (montmorillonite). It was reported that all supported catalysts were efficient in degradation of orange dye in comparison to bare TiO2 and P25 Degussa. In fact, there are a wide range of supports that can be used to convert poor photocatalysts into excellent agents. For instance, photocatalytically poor nickel ferrite nanocrystal was able to completely and efficiently degrade phenols upon the addition of the multi-walled carbon nanotubes as support (Xiong et al., 2012). In addition, the natural wastes (like fly ash and saw dust) were also employed as supports in degradation experiments. For instance, Shi et al. (2011) showed that the fly ash supported titanium dioxide (TiO2/fly ash) was effective in removing 94.4% of phenol from aqueous solution. In summary, various supports (e.g., zeolites and mesoporous materials) have been used favorably due to their high surface area, unique micro (or mesoporous) structure, and ion exchange properties. The localized excitation of the highly dispersed semiconductors over these supports allowed to have unique photocatalytic properties relative to unsupported ones. In the case of the latter, the photogenerated electrons and holes should be rapidly separated from each other at relatively large distance, making reactions occur separately at different surface sites. The transport of molecules within the intracrystalline cage network of porous materials is thus an important parameter in the design of photocatalysts. 4.2. Single site catalysts

Fig. 5. Electron transfer process and electron/hole separation in the semiconductor coupled modification.

Recently, the widely used term “single site photocatalysts” has drawn much attention with their efficient and selective photocatalytic properties (relative to bulk photocatalysts). In general, the single site photocatalysts are referred to as the isolated polyhedral semiconductor metal oxides (such as Ti, V, Cr, Nb, and Mo) that are evenly dispersed

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Fig. 6. The principle of single site photocatalyst.

within the supports (Fig. 6). Their photocatalytic properties could be ascribed to the ligands to metal charge transfer (LMCT) process within the isolated semiconductor metal oxide. The isolated metal oxides ([Mn+–O2−]) are excited by light irradiation to form charge-transferexcited triplet states ([M(n − 1)+–O−]⁎) through one electron transfer from an oxygen atom in ligands (O2−) to the central metal (Mn+). In this state, the M(n − 1)+ and O− pair is considered the electron-hole pair which is localized quite next to each other as compared to the electron hole pairs that are generated in bulk semiconductors. This characteristic feature of “single site photocatalyst” has been favorably adopted to a variety of environmental applications such as degradation of methylene blue (Gomez and Rodriguez-Paez, 2015). The introduction of transition metal ions into the framework of micro or mesoporous materials is known to facilitate the modification of their catalytic properties. For example, Noorjahan et al. (2005) immobilized Fe(III) ions on HY zeolite to achieve improved photocatalytic degradation of phenol in aqueous solutions under UV. As such, incorporation of oxides of transition metals (Ti, V, Mo, Cr, etc.) into zeolites was used to upgrade photocatalytic reactivity as single-site photocatalysts. They were indeed employed to remove various noxious components in gaseous media (Anpo et al., 2009). Under photoirradiation conditions, the single-site metal atoms (within the micropores or the framework of zeolites) can facilitate the formation of the charge transfer excited state to improve the photocatalytic performance of the material. Semiconductor metal cations have also been employed to support micro and mesoporous siliceous materials as single site photocatalysts. As silica is transparent to UV–visible light, it has been functionalized with some metallic components (Ti, Cr, V, Mo, and W). For example, Ce-dispersed Al-MCM-41 was tested to degrade phenol under UV irradiation (Reddy et al., 2008). The 0.3 wt% Ce on Al-MCM-41 yielded 100% degradation of phenol in duration of 4 h. Metal–organic frameworks (MOFs) are also a new type of porous materials. The photocatalytic activities of the MOFs can be attributed to the metallic components either as isolated metal centers or as metal clusters connected through the organic linkers. Due to the presence of metalactive sites, MOFs can also be considered the single-site photocatalysts. MOFs are one of the most exciting research fields in the recent years due to their potential applicabilities for industrial and environmental purposes. The advantageous properties of MOFs in heterogeneous

photocatalysis are attributed to their unique porous structure along with ample physicochemical properties; for instance, Du et al. (2011) successfully decolorized methylene blue (MB) dye using MIL-53(Fe) under UV and visible light. Likewise, Zhang et al. (2013) also developed a very efficient magnetic recyclable MOF (Fe3O4@MIL-100 (Fe)) and completed 99.8% of degradation of MB dye in 200 min. These photocatalysts can be magnetically separated and recycled without significant loss of photocatalytic activity. 4.3. Semiconductor modification of size and morphology The size and morphology of the semiconductors play a vital role in controlling the electronic and optical properties of the photocatalysts. Numerous efforts have been made to control the size and morphology of the semiconductor materials to minimize recombination of the electron hole. Due to high surface-volume ratios, nanomaterials emerged as efficient photocatalysts. As their charge carriers can be generated near the surface, they can be used efficiently for photocatalysis. As such, nanomaterials have great advantage in modifying the electronic properties with the aid of various factors (e.g., quantum size effects, surface reconstruction, and surface curvature). These effects may also significantly contribute to the improvement of the reaction/interaction between a device and the surrounding media. In addition to the size, attempts have also been made to modify the shapes of semiconductors to attain favourable morphologies to control the photocatalytic efficiency. One dimensional nanostructure materials (such as nanorods, nanowires, and nanotubes) have been advantageous to improve the photocatalytic activity of the semiconductor materials. For instance, the Zn2GeO4 nanorods exhibited a much higher photocatalytic activity than TiO2 in the degradation of organic pollutants (such as methyl orange, salicylic acid, and 4-chlorophenol) (Huang et al., 2008). These qualities boosted efforts to synthesize and modify the morphologies of the semiconductor with various dimensions. Several semiconductor photocatalysts with different morphologies have been introduced continuously in the form of nanorods, nanotubes, and nanowires of several metal oxides (TiO2, ZnO, WO3, and others) (Lee et al., 2015). In addition, two-dimensional (2D) metal oxide nanosheets have also been developed as photocatalysts. In general, the 2D nanostructures (with thin and high surface area properties) helped shorten the distance for photogenerated holes to diffuse the surfaces for efficient light

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harvesting. For instance, 2-D nanosheets (such as BiOI) were proven as an excellent photocatalyst for the degradation of sodium pentachlorophenate (PCP-Na). It was reported that more than 90% of the PCP-Na was removed under solar illumination for the duration of one hour (Chang et al., 2009). In fact, a 3-D nanomaterial has also been demonstrated for diverse applicabilities in the environmental decontamination. ZnO NPs were employed successfully to remove ∼ 95% of the rhodamine-B dye under UV illumination within 70 min (Rahman et al., 2013). 4.4. Photocatalyst coupled enzymes Enzymes are known as biocatalysts with catalytic selectivity (such as hydrolysis, dehydration, and isomerisation) even under moderate reaction conditions. Several enzymes are identified to have the capability to perform redox reaction analogous to photocatalysts. Therefore, coupling of semiconductor photocatalysts with enzymes has been proposed as an effective strategy to improve the overall efficiency of the photocomposites. A variety of hybrid methods have been suggested to couple photochemical and biochemical methods to treat hazardous pollutants. In this respect, Jia et al. (2012) employed a simultaneous photochemical– enzymatic process with TiO2 nanoparticles (as photocatalyst) and laccase (as biocatalyst) to degrade 2,4-dichlorophenol (2,4-DCP). After immobilization, the degradation rate of 2,4-DCP was improved as high as 90% (in duration of 2 h) with the stabilization of laccose. It was reported that this hybrid method was more efficient than photochemical (or enzymatic) methods which were applied separately. 5. Conclusions Photocatalysis has an excellent prospectus for handling various contemporary environmental issues. Despite its predicted role as an elixir, its effectiveness or efficiency for use in large scale applications yet needs to be validated. The major limitations are often designated as its lack of solar sensitivity and poor efficiency. In this respect, strategies for the development of numerous photocatalysts have been proposed to overcome those hindering issues (such as recombination and low solar sensitivity) and to further expand their treatment capacities. This review was organized to offer a detailed account on the importance of photocatalysts with or without modification in all those fronts and to properly comply with the future demand for potent photocatalysts. The performance of modified photocatalysts has been dramatically improved compared to prototype of semiconductor oxides. Although there is a significant progress in this research, the society is still quite far to adapt this technology at a large scale. In this respect there is still a need for further research to advance this technology with the enhancement of its economic feasibility as well as sustainability. As such, we call up on for more research in the development of efficient photocatalysts. Acknowledgements This study was supported by a grant from the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) (No. 2009-0093848). The third author also acknowledges the support made by a National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MSIP) (No. 2914RA1A004893). References Aarthi, T., Madras, G., 2007. Photocatalytic degradation of rhodamine dyes with nanoTiO2. Ind. Eng. Chem. Res. 46, 7–14. Aarthi, T., Narahari, P., Madras, G., 2007. Photocatalytic degradation of Azure and Sudan dyes using nano TiO2. J. Hazard. Mater. 149, 725–734. Akyol, A., Yatmaz, H.C., Bayramoglu, M., 2004. Photocatalytic decolorization of Remazol Red (RR) in aqueous ZnO suspensions. Appl. Catal. B Environ. 54, 19–24.

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