Catalytic Activity of BiVO₄-graphene Nanocomposites for the Reduction of Nitrophenols and the Photocatalytic Degradation of Organic Dyes

Catalytic Activity of BiVO ₄ -graphene Nanocomposites for the Reduction of Nitrophenols and the Photocatalytic Degradation of Organic Dyes

Jiulong Li ^* , Jeong Won Ko ^* , and Weon Bae Ko ^*,**,†

* Department of Convergence Science, Graduate School, Sahmyook University, Seoul 139-742, South Korea

** Department of Chemistry, Sahmyook University, Seoul 139-742, South Korea (Received September 5, 2016, Revised September 19, 2016, Accepted September 19, 2016)

Abstract: BiVO

nanomaterial was synthesized from bismuth (III) nitrate pentahydrate [Bi(NO

)

·5H

O] and ammonium vanadate (V) [NH

VO

]. The BiVO

-graphene nanocomposite was fabricated by calcining the BiVO

nanomaterial and graphene under an oxygen-free atmosphere at 700

C. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed to characterize structural and morphological properties of samples. The catalytic activity of the BiVO

- graphene nanocomposite was studied for the reduction of 4-nitrophenol, 3-nitrophenol and 2-nitrophenol by sodium boro- hydride [NaBH

]. The photocatalytic activity of the BiVO

-graphene nanocomposite was demonstrated by the degradation of organic dyes like BG, MB, MO and RhB under irradiation at 365 nm. The catalytic and photocatalytic activity were stud- ied by UV-vis spectrophotometry.

Keywords: BiVO

-graphene Nanocomposites, catalytic activities, photocatalytic activities, UV-vis spectrophotometer

Introduction

Progress in industrialization has gradually improved the quality of human life, but has also brought upon a series of environmental problems. In recent decades, wastewater treat- ment techniques have attracted much attention. ¹ Aromatic nitro compounds represented by nitrophenols and synthetic dyes are major water pollution sources, owing to their sol- ubility, and biological and chemical stability in water. ^2-5

Aminophenol is an important intermediate in the synthesis of analgesic and antipyretic drugs such as phenacetin, acet- anilide and acetaminophen. ⁶ A reducing agent, aminophenol is also utilized as a photographic developer and as an anti- corrosion lubricant in fuels. ⁷ Therefore, it is necessary to con- vert nitrophenols in wastewater to aminophenols, which can then be used for industrial production. Many studies have been focused on the reduction by NaBH ₄ in the presence of a suitable catalyst to solve this problem. ^1,7 Due to their unique electrical, optical, and catalytic performances in com- parison to bulk particles, metal and metal oxide nanoparticles such as Ag, ⁸ Au, ⁹ Cu, ¹⁰ Ni ¹¹ and CuO ¹² have been developed as catalysts for the reduction of nitrophenols. A large number

of individual studies of catalysts have been reported for the reduction of nitrophenols and their use as photocatalysts for the degradation of organic dyes; however, the study of com- mon catalysts for both reduction and photocatalytic degra- dation is scarce. We fabricated BiVO ₄ nanoparticles and decorated them on graphene to obtain a BiVO ₄ -graphene nanocomposite. The catalytic reduction of 4-nitrophenol to 4- aminophenol in the presence of NaBH ₄ was carried out on the BiVO ₄ -graphene nanocomposite in an aqueous solution.

Other nitrophenols like 3-nitrophenol and 2-nitrophenol were also tested under the same conditions.

Studies of semiconductor photocatalysts reveal important results that could help to solve the problem of pollution caused by synthetic dyes. TiO ₂ is a well-known early semi- conductor photocatalyst used for the degradation of organic dyes. ¹³ However, the photocatalytic activity of TiO ₂ is limited because of its relatively wide band gap (3.2 eV). ^14,15 Many efforts have been made to improve this deficiency by mod- ifying TiO ₂ with other semiconductors; many novel semi- conductor materials with narrow band gaps have been explored. ¹³ Among the photocatalysts explored, metal oxide semiconductors such as WO ₃ , ¹⁶ Fe ₂ O ₃ , ¹⁷ Bi ₂ O ₃ , ¹⁸ Ag ₃ VO ₄ , ¹⁹ and BiVO ₄ , ²⁰ have been developed. Particularly, BiVO ₄ has attracted interest due to its non-toxicity, non-corrosiveness,

†

Corresponding author E-mail: [email protected]

low cost and excellent photocatalytic activity. ²¹ BiVO ₄ crys- tallizes in three crystal forms: scheelite-monoclinic (m), scheelite-tetragonal (s-t), and zircon-tetragonal (z-t). ²² Among the different crystal structures, only the scheelite-monoclinic (m-BiVO ₄ ) is suitable for photocatalytic water splitting and organic dye decomposition under visible-light irradiation due to its narrow band gap of 2.41 eV. ²³ However, it is still necessary to improve the photocatalytic activity of m-BiVO ₄ because the photogenerated electron-hole pairs formed under irradiation recombine easily on its surface. ²⁴ Further, m-BiVO ₄ was found to have a poor adsorptive performance. Therefore, many studies have been performed to prevent the recombi- nation of photogenerated electron-hole pairs and enhance the adsorptive ability of the photocatalyst. ²³ Similar to the mod- ification of TiO ₂ with other semiconductors, metals such as Ag, ²³ Sm, ²⁴ Au ²⁵ and semiconductors such as CeO ₂ , ^20,26 Co ₃ O ₄ ²⁷ and Bi ₂ O ₃ ²⁸ have been combined with BiVO ₄ to enhance its photocatalytic activity. In addition, an m-BiVO ₄ on a carbon photocatalyst has been developed and reported by Lee et al.. ^21,29 In the study of carbon materials, graphene with its unique two-dimensional (2D) sheet structure has received growing attention due to its excellent physical and chemical properties. ³⁰ The combinations of graphene with several semiconductors such as CdS, ³¹ ZnS, ³² Bi ₂ WO ₆ , ³³ and ZnO ³⁴ have been reported as photocatalysts for the degra- dation of organic pollutants. The abundance of delocalized electrons in graphene expedites efficient transfer of photo- generated electron-hole pairs. ³⁵

In this study, the photocatalytic activity of the BiVO ₄ - graphene nanocomposite was verified by the degradation of organic dyes (BG, MB, MO and RhB) under irradiation at 365 nm. The catalytic and photocatalytic activities were eval- uated by UV-vis spectrophotometry.

Experimental

1. Materials

Bismuth (III) nitrate pentahydrate [Bi (NO ₃ ) ₃ ·5H ₂ O], eth- ylene glycol [(CH ₂ OH) ₂ ], 2-nitrophenol, 4-nitrophenol, bril- liant green (BG) and methyl orange (MO) were purchased from Sigma-Aldrich. Ammonium vanadate (V) [NH ₄ VO ₃ ] was supplied by Junsei Chemical Co., Ltd. Graphene was supplied by ENano Tec. Tetrahydrofuran (THF), methylene blue (MB), and rhodamine B (RhB) were obtained from Samchun Chemicals (Korea). 3-Nitrophenol was supplied by

Tae Jin Chemical. Sodium borohydride (NaBH ₄ ) was purchased from Kanto Chemical Co., Inc.

2. Synthesis

Bismuth (III) nitrate pentahydrate [Bi(NO ₃ ) ₃ ·5H ₂ O; 0.01 mol] and ammonium vanadate (V) [NH ₄ VO ₃ , 0.01 mol] were used as raw materials to prepare the BiVO ₄ nanomaterial.

Bi(NO ₃ ) ₃ ·5H ₂ O and NH ₄ VO ₃ were dissolved in a 1:9 (v/v) mixture of ethylene glycol and deionized water. After stirring for 1 h at room temperature, the mixture was ultrasonicated for 2 h. The precipitate was aged for 12 h and then washed with ethanol and deionized water. The BiVO ₄ nanomaterial was obtained by drying the washed precipitate at 100 ^o C for 6 h. The BiVO ₄ -graphene nanocomposites were fabricated by stirring the BiVO ₄ nanomaterial with graphene (mass ratio 1:1) in THF (10 mL) for 30 min and then calcining at 700

°C for 2 h in an electric furnace (Ajeon Heating Industry Co., Ltd) under an argon atmosphere.

3. Characterization

The crystal phases of the BiVO ₄ -graphene nanocomposite were analyzed by X-ray diffraction (XRD, Bruker, D8 Advance) with Cu-K _α radiation (λ = 1.54178 Å) in the range of 5-90°, employing a scanning rate of 0.02 s ^-1 . The mor- phology of the BiVO ₄ -graphene nanocomposite was inves- tigated by scanning electron microscopy (SEM, JEOL Ltd, JSM-6510) at an accelerating voltage between 0.5 and 30 kV.

A UV-vis spectrophotometer (Shimazu, UV-1619 PC) with a wavelength range of 200 to 800 nm was used to determine the optical properties in the kinetics experiments.

4. Measurements of the catalytic properties

Aqueous solutions of nitrophenols (2-nitrophenol, 3-nitro-

phenol and 4-nitrophenol) at a certain concentration were

prepared to investigate the effect of BiVO ₄ -graphene nano-

composites as a catalyst. NaBH ₄ powder (5 mg) was added

in a glass vial and nitrophenol solution (10 mL) was injected

subsequently. The solution was stirred with a magnetic

stirrer to dissolve the NaBH ₄ powder. The BiVO ₄ -graphene

nanocomposite (2 mg) was added as a catalyst in the solu-

tion. The progress of the catalytic reduction of nitro-

phenols was followed by UV-vis spectrophotometry at 5 min

intervals.

5. Measurements of the photocatalytic properties

A UV lamp (8 W, 365 nm, 77202 Marne La Valee-cedex 1 France) was used as the light source for the photocatalytic reactions. The concentrations of the organic dyes BG, MB, MO, and RhB in water were manipulated to get the absor- bance value of the aqueous solution in the range of 0.9-1.1 at λ _max .

A 10 mL glass vial was used as a reactor and BiVO ₄ - graphene nanocomposite (5 mg) was added after the organic dye (10 mL) was injected. The reactor was placed in the dark for 30 min to ensure adsorption-desorption equili- brium between the photocatalyst and the organic dye.

Subsequently, the suspension was exposed to radiation at 365 nm. The concentration change of the organic dye was determined using the UV-vis spectrophotometer at 5 min intervals.

Results and Discussion

1. Structural analysis of BiVO ₄ -graphene nanocomposite

Figure 1 shows the X-ray diffraction (XRD) pattern and the crystal structure of the BiVO ₄ -graphene nanocomposites.

The peaks at 18.65°, 28.65°, 34.42°, 34.80°, 39.58°, 42.56°, 50.12° and 59.32° are assigned to the (011), (-121), (200),

(002), (211), (051), (202) and (123) crystal planes of the BiVO ₄ crystal (JCPDS No. 14-0688). ¹⁹ The peak observed at 26.49° was concluded to be the diffraction peak of graph- ene. It can be seen from Figure 1 that the samples were not pure BiVO ₄ -graphene nanocomposites; appendant peaks were present in the XRD pattern.

2. Morphology of BiVO ₄ -graphene nanocomposite

Figure 2 shows the SEM image of the as-prepared BiVO ₄ - graphene nanocomposite. BiVO ₄ nanoparticles showed a globular structure in the SEM image ³⁶ with the BiVO ₄

Figure 1. XRD pattern of BiVO

-graphene nanocomposites.

Figure 2. SEM image of BiVO

-graphene nanocomposites.

nanoparticles gathered together. ³⁷ Pores were distributed un- equally on the surface of the gathered BiVO ₄ nanoparticles.

The gathered BiVO ₄ nanoparticles were scattered on the sur- face of the graphene.

3. Catalytic reduction of nitrophenols

The catalytic activity of the prepared BiVO ₄ -graphene nanocomposite was measured by the aqueous reduction of

Figure 3. UV-vis spectra of (a) 4-nitrophenol, (b) 3-nitrophenol and (c) 2-nitrophenol reduction with BiVO

-graphene nanocomposites

as a catalyst in the present of NaBH

.

nitrophenols in the presence of NaBH ₄ . Figure 3(a) shows the catalytic reduction processes of 4-nitrophenol. As shown in Figure 3(a), pure 4-nitrophenol absorbs at 317 nm; with the addition of NaBH ₄ , the peak was red-shifted to 400 nm. After the BiVO ₄ -graphene nanocomposite catalyst was added, the

yellow color of the solution faded away. The peak at 400 nm decreased and a new peak at 300 nm simultaneously ap- peared. The decrease of the peak at 400 nm signified the reduction of 4-nitrophenolate ions and the appearance of the peak at 300 nm substantiated the formation of 4-aminophe- Figure 3. Continued.

Figure 4. Kinetic studies for reduction of various nitrophenols using BiVO

-graphene nanocomposites as a catalyst.

nol. Reduction reactions of other nitrophenols by NaBH ₄ were also studied on BiVO ₄ -graphene nanocomposites. ³⁸ As shown in Figure 3(b), the absorbance spectrum of 3-nitro- phenolate ions showed a peak at 390 nm. When the yellow color was discharged, the peak at 390 nm also decreased. In

addition, the peak at 291 nm was blue-shifted and the peak at 251 nm eventually disappeared. ³⁹ Figure 3(c) shows the catalytic reduction of 2-nitrophenol. As can be seen in Figure 3(c), 2-nitrophenolate ions show a peak at 415 nm in the presence of NaBH ₄ . With the addition of the catalyst, the yel-

Figure 5. UV-vis spectra of the degradation of (a) BG, (b) MB, (c) MO and (d) RhB with the BiVO

-graphene nanocomposites as a

photocatalyst under ultraviolet irradiation at 365 nm.

low color of the aqueous solution started fading, the peak at 415 nm started decreasing, and the peak at 282 nm was red- shifted. ³⁹

The concentration of NaBH ₄ was controlled and regarded to be constant during the course of the reaction. The kinetics of nitrophenol-reduction can be considered a pseudo-first-

order reaction. ⁸ The kinetic equation for the reduction of nitrophenol with a BiVO ₄ -graphene nanocomposite in the presence of NaBH ₄ can be written as follows:

ln(C/C ₀ ) = −kt

C 0 is the initial concentration of nitrophenol, C is the con-

Figure 5. Continued.

centration at real time t, and k is the rate constant. The result of kinetics for reduction was shown in Figure 4, which indi- cated that the order of reactivity of nitrophenols was 4- nitrophenol > 2-nitrophenol > 3-nitrophenol.

4. Photocatalytic degradation of organic dyes

The photocatalytic performance of the BiVO ₄ -graphene nanocomposite was evaluated by degradation of organic dyes with irradiation at 365 nm. Figure 5 shows the photocatalytic degradation of the organic dyes (a) BG, (b) MB, (c) MO, and (d) RhB on BiVO ₄ -graphene nanocomposite. Before irradi- ation, the reactor was placed in the dark for 30 min to estab- lish an adsorption-desorption equilibrium between the photo- catalyst and the organic dye. Under ultraviolet light irradi- ation, the electrons (e ⁻ ) in the valence band of BiVO ₄ nanoparticles are transferred to the conduction band and leave holes (h ⁺ ) in the valence band. The separation of the photogenerated electron-hole pairs was improved in graph- ene. Simultaneously, the photogenerated electrons (e ⁻ ) in the conduction band of graphene crystals are transferred to the conduction band of BiVO ₄ nanoparticles because the con- duction band of BiVO ₄ nanoparticles is lower than that of graphene crystal. ⁴⁰ The photogenerated holes (h ⁺ ) are trans- ferred from the valence band of BiVO ₄ nanoparticles to the

valence band of the graphene crystal because the valence band of graphene crystal is lower than that of the BiVO ₄ nanoparticles. The photogenerated electrons (e ⁻ ) were trapp- ed by oxygen (O ₂ ) dissolved in the aqueous solution to pro- duce superoxide anion radicals (O ₂ ^−• ) and the photogenerated holes (h ⁺ ) were captured by hydroxide ion (OH ^- ) to yield hydroxyl radicals (OH ^• ). As a result, the organic dyes were degraded into carbon dioxide (CO ₂ ) and H ₂ O by hydroxyl radicals (OH ^• ) and superoxide anion radicals (O ₂ ^−• ), which resulted in the color change of the aqueous solution. ⁴¹

The Langmuir-Hinshelwood model was used to investigate the kinetics of photocatalytic degradation; the equation is as follows:

ln(C/C 0 ) = −Kap·t

C/C 0 is the ratio of the concentration of organic dye after intervals of time t to its initial concentration. Kap is the apparent degradation rate constant. ⁴² The investigation indi- cated that the order of photocatalytic activity for the BiVO ₄ - graphene nanocomposite was MB > BG > RhB > MO (Figure 6).

Conclusion

BiVO ₄ -graphene nanocomposites were fabricated by cal-

Figure 6. Kinetic studies of the photocatalytic degradation of the organic dyes using BiVO

-graphene nanocomposites as a photocatalyst

under ultraviolet irradiation at 365 nm.

cination with graphene and BiVO ₄ nanomaterial. The BiVO ₄ nanomaterial was synthesized from bismuth (III) nitrate pen- tahydrate and ammonium vanadate (V). X-ray diffraction (XRD) and scanning electron microscopy (SEM) were em- ployed to structurally and morphologically characterize the properties of the samples. As a catalyst, the catalytic activity of BiVO ₄ -graphene nanocomposite was confirmed by the reduction of various nitrophenols (4-nitrophenol, 3-nitrophe- nol, and 2-nitrophenol) by sodium borohydride. The exper- imental results indicate that BiVO ₄ -graphene nanocomposites present excellent catalytic activity, and kinetic studies show- ed the order of reactivity of nitrophenols was 4-nitrophenol

> 2-nitrophenol > 3-nitrophenol. BiVO ₄ -graphene nanocom- posites also exhibited good photocatalytic performance for the photo-degradation of organic dyes like BG, MB, MO, and RhB at 365 nm. Kinetic studies indicated that the order of the photocatalytic activity of BiVO ₄ -graphene nanocompos- ites for the degradation of the organic dyes was MB > BG

> RhB > MO.

Acknowledgments

This study was supported by Sahmyook University funding in Korea.

References

1. A. Goyal, S. Bansal, and S. Singhal, “Facile Reduction of Nitrophenols: Comparative Catalytic Efficiency of MFe ₂ O ₄ (M = Ni, Cu, Zn) Nano Ferrites”, Int. J. Hydrogen Energ., 39, 4895 (2014).

2. H. Liu, T. Lv, X. H. Wu, C. K. Zhu, and Z. F. Zhu, “Prepa- ration and Enhanced Photocatalytic Activity of CdS@RGO Core-Shell Structural Microspheres”, Appl. Sulf. Sci., 305, 242 (2014).

3. S. Ameen, M. S. Akhtar, M. Nazim, and H. S. Shin, “Rapid Photocatalytic Degradation of Crystal Violet Dye over ZnO Flower Nanomaterials”, Mater. Lett., 96, 228 (2013).

4. A. Niaz, J. Fischer, J. Barek, B. Yosypchuk, Sirajuddin, and M.I. Bhanger, “Voltammetric Determination of 4-Nitrophenol Using a Novel Type of Silver Amalgam Paste Electrode”, Electroanal., 21, 1786 (2009).

5. S. Saha, A. Pal, S. Kundu, S. Basu, and T. Pal, “Photochem- ical Green Synthesis of Calcium-Alginate-Stabilized Ag and Au Nanoparticles and Their Catalytic Application to 4-Nitro- phenol Reduction”, Langmuir 26, 2885 (2010).

6. C. V. Rode, M. J. Vaidya, and R. V. Chaudhari, “Synthesis of

p-Aminophenol by Catalytic Hydrogenation of Nitroben- zene”, Org. Process Res. Dev., 3, 465 (1999).

7. V. K. Gupta, M. L. Yola, T. Eren, F. Kartal, M. O. Caglayan, and N. Atar, “Catalytic Activity of Fe@Ag Nanoparticle Involved Calcium Alginate Beads for the Reduction of Nitro- phenols”, J. Mol. Liq., 190, 133 (2014).

8. K. S. Shin, Y. K. Cho, J. Y. Choi, and K. Kim, “Facile Syn- thesis of Silver-deposited Silanized Magnetite Nanoparticles and Their Application for Catalytic Reduction of Nitrophe- nols”, Appl. Catal. A: Gen., 413, 170 (2012).

9. M. Haruta and M. Date, “Advances in the Catalysis of Au Nanoparticles”, Appl. Catal. A: Gen., 222, 427 (2001).

10. X. M. Gao, F. Fu, and W. H. Li, “Photocatalytic Degradation of Phenol over Cu Loading BiVO ₄ Metal Composite Oxides under Visible Light Irradiation”, Phys. B., 412, 26 (2013).

11. S. K. Ghosh, M. Mandal, S. Kundu, S. Nath, and T. Pal,

“Bimetallic Pt-Ni Nanoparticles Can Catalyze Reduction of Aromatic Nitro Compounds by Sodium Borohydride in Aqueous Solution”, Appl. Catal. A: Gen., 268, 61 (2004).

12. W. R. Zhao, Y. Wang, Y. Yang, J. Tang, and Y. N. Yang, “Car- bon Spheres Supported Visible-Light-Driven CuO-BiVO ₄ Heterojunction: Preparation, Characterization, and Photocata- lytic Propertie”, Appl. Catal. B: Environ., 115, 90 (2012).

13. W. Zhang, X. Xiao, L. Zheng, and C. Wan, “Fabrication of TiO ₂ /MoS ₂ @zeolite Photocatalyst and its Photocatalytic Activ- ity for Degradation of Methyl Orange under Visible Light”, Appl. Surf. Sci., 358, 468 (2015).

14. L. Zhang, M. S. Tse, and O. K. Tan, “Controlled Deposition and Enhanced Visible Light Photocatalytic Performance of Pt-Modified TiO ₂ Nanotube Arrays”, J. Environ. Chem. Eng., 2, 1214 (2014).

15. G. Xiao, X. Zhang, W. Y. Zhang, S. Zhang, H. J. Su, and T. W.

Tan, “Visible-Light-Mediated Synergistic Photocatalytic Anti- microbial Effects and Mechanism of Ag-Nanoparticles@chi- tosan-TiO ₂ Organic-Inorganic Composites for Water Disin- fection”, Appl. Catal. B: Environ., 170, 255 (2015).

16. T. Arai, M. Yanagida, Y. Konishi, A. Ikura, Y. Iwasaki, H.

Sugihara, and K. Sayama, “The Enhancement of WO ₃ -Cata- lyzed Photodegradation of Organic Substances Utilizing the Redox Cycle of Copper Ions”, Appl. Catal. B, 84, 42 (2008).

17. A. Duret and M. Gratzel, “Visible Light-Induced Water Oxi- dation on Mesoscopic α-Fe ₂ O ₃ Films Made by Ultrasonic Spray Pyrolysis”, J. Phys. Chem. B, 109, 17184 (2005).

18. T. Saison, N. Chemin, C. Chaneac, O. Durupthy, V. Ruaux, L.

Mariey, F. Mauge, P. Beaunier, and J. P. Jolive, “Bi ₂ O ₃ , BiVO ₄ , and Bi ₂ WO ₆ : Impact of Surface Properties on Photocatalytic Activity under Visible Light”, J. Phys. Chem. C, 115, 5657 (2011).

19. Q. Yu, Z. R. Tang, and Y. J. Xu., “Synthesis of BiVO ₄ Nano-

Sheets-Graphene Composites Toward Improved Visible Light Photoactivity”, J. Energy Chem., 23, 564 (2014).

20. J. Xu, W. Wang, J. Wang, and Y. Liang, “Controlled Fabrication and Enhanced Photocatalystic Performance of BiVO ₄ @CeO ₂ Hollow Microspheres for the Visible-Light-Driven Degrada- tion of Rhodamine B”, Appl. Surf. Sci., 349, 529 (2015).

21. M. Niu, R. Zhu, F. Tian, K. Song, G. Cao, and F. Ouyang,

“The Effects of Precursors and Loading of Carbon on the Photocatalytic Activity of C-BiVO ₄ for the Degradation of High Concentrations of Phenol under Visible Light Irradia- tion”, Catal. Tod., 258, 585 (2015).

22. H. M. Fan, T. F. Jiang, H. Y. Li, D. J. Wang, L. L. Wang, J. L.

Zhai, D. Q. He, P. Wang, and T. F. Xie, “Effect of BiVO ₄ Crystalline Phases on the Photoinduced Carriers Behavior and Photocatalytic Activity”, J. Phys. Chem. C, 116, 2425 (2012).

23. S. Kohtani, M. Tomohiro, K. Tokumura, and R. Nakagaki,

“Photooxidation Reactions of Polycyclic Aromatic Hydrocar- bons over Pure and Ag-Loaded BiVO ₄ Photocatalysts”, Appl.

Catal. B: Environ., 58, 265 (2005).

24. M. Wang, C. Niu, J. Liu, Q. Wang, C. Yang, and H. Zheng,

“Characterization and Photocatalytic Properties of N-Doped BiVO ₄ Synthesized via a Sol-Gel Method”, J. Alloys Comp., 548, 70 (2013).

25. S. W. Cao, Z. Yin, J. Barber, F. Y. C. Boey, S. C. J. Loo, and C. Xue, “Preparation of Au-BiVO ₄ Heterogeneous Nano- structures as Highly Efficient Visible-Light Photocatalysts”, ACS Appl. Mater. Interfaces, 4, 418 (2012).

26. N. Wetchakum, S. Chaiwichain, B. Inceesungvorn, K. Ping- muang, S. Phanichphant, A. I. Minett, and J. Chen, “BiVO ₄ / CeO ₂ Nanocomposites with High Visible-Light-Induced Pho- tocatalytic Activity”, ACS Appl. Mater. Interfaces, 4, 3718 (2012).

27. M. C. Long, W. M. Cai, J. Cai, B. X. Zhou, X. Y. Chai, and Y.

H. Wu, “Efficient Photocatalytic Degradation of Phenol over Co ₃ O ₄ /BiVO ₄ Composite under Visible Light Irradiation”, J.

Phys. Chem. B, 110, 20211 (2006).

28. L. Z. Li and B. Yan, “BiVO ₄ /Bi ₂ O ₃ Submicrometer Sphere Composite: Microstructure and Photocatalytic Activity under Visible-Light Irradiation”, J. Alloys Compd., 476, 624 (2009).

29. D. K. Lee, I. S. Cho, S. Lee, S. T. Bae, J. H. Noh, D. W. Kim, and K. S. Hong, “Effects of Carbon Content on the Photocat- alytic Activity of C/BiVO ₄ Composites under Visible Light Irradiation”, Mater. Chem. Phys., 119, 106 (2010).

30. X. Men, H. Chen, K. Chang, X. Fang, C. Wu, W. Qin, and S.

Yin, “Three-Dimensional Free-Standing ZnO/Graphene Com- posite Foam for Photocurrent Generation and Photocatalytic

Activity”, Appl. Catal. B: Environ., 187, 367 (2016).

31. N. Zhang, Y. Zhang, X. Pan, M. Q. Yang, and Y. J. Xu, “Con- structing Ternary CdS-Graphene-TiO ₂ Hybrids on the Flat- land of Graphene Oxide with Enhanced Visible-Light Photo- activity for Selective Transformation”, J. Phys. Chem. C, 116, 180233 (2012).

32. S. Pan and X. Liu, “ZnS-Graphene Nanocomposite: Synthe- sis, Characterization and Optical Properties”, J. Sol. Sta.

Chem., 191, 51 (2012).

33. Y. L. Min, K. Zhang, Y. C. Chen, and Y. G. Zhang, “Enhanced Photocatalytic Performance of Bi ₂ WO ₆ by Graphene Sup- porter as Charge Transfer Channel”, Separ. Purif. Technol., 86, 98 (2012).

34. T. Xu, L. Zhang, H. Cheng, and Y. Zhu, “Significantly En- hanced Photocatalytic Performance of ZnO via Graphene Hybridization and the Mechanism Study”, Appl. Catal. B:

Environ., 101, 382 (2011).

35. S. Y. Yin, X. J. Men, H. Sun, P. She, W. Zhang, C. F. Wu, W.

P. Qin, and X. D. Chen, “Enhanced Photocurrent Generation of Bio-Inspired Graphene/ZnO Composite Films”, J. Mater.

Chem. A, 3, 12016 (2015).

36. M. Sangareswari and M. M. Sundaram, “A Comparative Study on Photocatalytic Efficiency of TiO ₂ and BiVO ₄ Nanomate- rial for Degradation of Methylene Blue Dye under Sunlight Irradiation”, J. Avd. Chem. Sci., 1, 75 (2015).

37. S. Sarkar and K. K. Chattopadhyay, “Visible Light Photoca- talysis and Electron Emission from Porous Hollow Spherical BiVO ₄ Nanostructures Synthesized by a Novel Route”, Phys- ica. E, 58, 52 (2014).

38. S. Wunder, F. Polzer, Y. Lu, Y. Mei, and M. Ballauff, “Kinetic Analysis of Catalytic Reduction of 4-Nitrophenol by Metallic Nanoparticles Immobilized in Spherical Polyelectrolyte Brushes”, J. Phys. Chem. C, 114, 8814 (2010).

39. J. Feng, L. Su, Y. Ma, C. Ren, Q. Guo, and X. Chen, “CuFe ₂ O ₄ Magnetic Nanoparticles: A Simple and Efficient Catalyst for the Reduction of Nitrophenol”, Chem. Eng. J., 221, 16 (2013).

40. H. Liu, T. Lv, C. Zhu, X. Su, and Z. Zhu, “Efficient Synthesis of MoS ₂ Nanoparticles Modified TiO ₂ Nanobelts with Enhanced Visible-Light-Driven Photocatalytic Activity”, J. Mol. Catal.

A: Chem., 396, 136 (2015).

41. Y. Geng, P. Zhang, N. Li, and Z. Sun, “Synthesis of Co Doped BiVO ₄ with Enhanced Visible-Light Photocatalytic Activi- ties”, J. Alloys Compd., 651, 744 (2015).

42. K. Dai, G. Dawson, S. Yang, Z. Chen, and L. Lu, “Large Scale

Preparing Carbon Nanotube/Zinc Oxide Hybrid and its Appli-

cation for Highly Reusable Photocatalyst”, Chem. Eng. J.,

191, 571 (2012).