Synthesis of the visible-light-driven Ag₃VO₄/Ag₃PO₄/Ag photocatalysts with enhanced photocatalytic activity

Abstract

A novel visible-light-driven Ag₃VO₄/Ag₃PO₄/Ag photocatalyst was successfully synthesized via an anion-exchange reaction and characterized by X-ray diffraction, field emission scanning electron microscopy, X-ray photoelectron spectroscopy, UV-vis diffuse-reflectance spectroscopy and Fourier transform infrared spectroscopy. The photocatalytic activities of prepared catalysts were evaluated by degradation of acid blue 92 (AB92) aqueous solutions under visible light irradiation. The photocatalytic results indicated that the synthesized Ag₃VO₄/Ag₃PO₄/Ag hybrid displays a significantly enhanced activity in degradation of acid blue 92 under visible light compared with pure Ag₃VO₄. The improved photocatalytic activity can be attributed to the efficient separation of photo-induced electrons and holes in the composite. A possible mechanism has been proposed for this improvement.

---

1. Introduction

In the past few decades, semiconductor photocatalysis has been widely studied for applications in environmental pollution mediation and solar energy conversion.^1–4 The visible region (λ > 400 nm) covers the largest proportion of the solar spectrum. In order to transform this technology into practical applications it is essential to develop high-efficiency visible-light-driven photocatalysts. The Ag-based materials have good photocatalytic activity in visible light and hence, they have received much attention as photocatalysts. In recent years, numerous efforts have been devoted to investigate the photocatalytic activity of Ag-based photocatalysts, such as Ag₂CO₃,⁵ Ag₂O,⁶ Ag₃PO₄ (ref. 7) and AgX^8–10 (X = I, Br and Cl). Due to their high visible light sensitivity, facile syntheses and availability of raw materials¹¹ they have become interesting to researchers. Among them, fabrication of a semiconductor heterojunction by coupling of two Ag-based materials is an attractive approach to improve their photocatalytic efficiency. The AgI/Ag₃PO₄,¹² AgBr/Ag₂CO₃,¹³ Ag₃PO₄/AgBr/Ag¹⁴ and Ag₃VO₄/AgBr/Ag¹⁵ photocatalysts have been successfully synthesized for toxic chemical degradation. Among this photocatalyst Ag₃VO₄ has become an attractive material after Konta et al.¹⁶ reported its photocatalytic activity under light irradiation. Ag₃VO₄ with a band-gap energy of 2.2 eV,¹⁷ is a visible-light-responsive catalyst; however, its photocatalytic activity is limited due to its high recombination rate of photo-induced charge carriers. Therefore, many efforts have been directed towards enhancing its photo-induced electron separation efficiency and improving its photocatalytic activity. For example, BiOI/Ag₃VO₄,¹⁸ g-C₃N₄/Ag₃VO₄ (ref. 17) and ZnFe₂O₄/Ag₃VO₄,¹⁹ have been developed with enhanced photocatalytic efficiency in visible light irradiation compare to the individual photocatalyst.

A novel photocatalyst is introduced by combining Ag₃PO₄, a semiconductor with a band gap of around 2.4 eV,²⁰ with Ag₃VO₄. The Ag₃VO₄/Ag₃PO₄/Ag hybrid synthesized by an anion-exchange reaction and characterized by XRD, FESEM, XPS, DRS and FT-IR spectrometer. The visible-light-response photocatalytic ability of this composite studied by degradation of AB92 in aqueous solution. In addition, photocatalytic mechanism of Ag₃VO₄/Ag₃PO₄/Ag also discussed.

2. Experimental

2.1. Synthesis of Ag₃VO₄

20 ml solution of NaOH and V₂O₅ with mole ratio of 6 : 1 was prepared, then 60 ml AgNO₃ solution was added dropwise into the above solution under vigorously stirring (Ag/V mole ratio of 3 : 1).²¹ After stirring for 30 min at room temperature, the suspension was transferred into a 200 ml Teflon-lined stainless autoclave and heated for 12 h at 140 °C. Finally, the brownish yellow products were collected, washed several times with distilled water and dried in air at 70 °C overnight.

2.2. Preparation of Ag₃VO₄/Ag₃PO₄/Ag photocatalyst

Ag₃VO₄/Ag₃PO₄/Ag photocatalysts were prepared through an anion-exchange method in dark condition. 0.1 g of Ag₃VO₄ was dispersed in 20 ml of ethylene glycol. Subsequently, 20 ml of an ethylene glycol solution of NaH₂PO₄ was added drop wise to above solution, after stirring in dark for 40 min, the reaction was continued under visible light for 20 min in order to convert some silver ions on the surfaces of Ag₃VO₄/Ag₃PO₄ to silver nanoparticles. Then, the resulting sample was washed with deionized water and dried in air at 60 °C for 12 h.

2.3. Characterization of photocatalysts

X-ray diffraction (XRD) analysis of the samples were carried out using a Philips X'pert instrument with a Cu Kα irradiation (λ = 0.15406 nm) at 40 kV/40 mA. Fourier-transform infrared (FT-IR) spectra were recorded with an ABB BOMER MB series spectrophotometer. The morphologies of the samples were observed using an XL30 Field emission scanning electron microscopy (FESEM). UV-vis diffuse-reflectance spectroscopy (UV-vis DRS) of the obtained samples were done via a GBC Cintra 40 instrument.

2.4. Evaluation of photocatalytic performance

The photocatalytic activity of the Ag₃VO₄/Ag₃PO₄/Ag was evaluated by decomposition of AB92 under visible light irradiation at the natural pH value. Typically, 0.02 g of catalyst was dispersed in 100 ml of 2.5 g l⁻¹ AB92 aqueous solution in a reactor that was surrounded by a running water jacket to keep the temperature unchanged. After stirring in dark for 30 min, to reach the adsorption–desorption equilibrium of AB92 molecules on the surface of catalyst, the suspension was illuminated by a 125 W mercury lamp with a UV cut-off filter as visible light source (λ ≥ 420 nm). At given time interval (20 min), about 3 ml of the mixture was withdrawn and centrifuged to remove the photocatalyst particles. The photocatalytic degradation process of AB92 was monitored by measuring its characteristic absorption at 571 nm with an UV-vis spectrophotometer.

3. Results and discussion

3.1. Characterization of photocatalysts

The crystallographic structure of the as-prepared samples was analyzed by XRD measurements. Fig. 1 shows the XRD patterns of Ag₃VO₄ and Ag₃VO₄/Ag₃PO₄/Ag products. All of the diffraction peaks of the Ag₃VO₄ samples can be indexed to the monoclinic phase of Ag₃VO₄ (JCPDS: 43-0542). The composites exhibit several additional peaks which are indexed to Ag₃PO₄ (JCPDS: 06-0505), indicating that Ag₃PO₄ nanocrystals were generated and verifying co-existence of Ag₃VO₄ and Ag₃PO₄ in Ag₃VO₄/Ag₃PO₄/Ag hybrid. However no obvious characteristic diffraction peaks of Ag⁰ species were detected in the Ag₃VO₄/Ag₃PO₄/Ag patterns, because of the low contents of Ag⁰.

Fig. 1 XRD patterns of as prepared Ag₃VO₄ (a) and Ag₃VO₄/Ag₃PO₄/Ag (b) samples.

The microstructures of the samples were obtained by field emission scanning electron microscope. As shown in Fig. 2a and b pure Ag₃VO₄ displays irregular polyhedral shapes. It is obvious that the hybrid catalysts (Fig. 2c and d) still maintain the morphology and structure of the original shape of Ag₃VO₄ crystals. Fig. 2c and d demonstrate that the uniform Ag₃PO₄ covered the surface of Ag₃VO₄ crystals after ion-exchange process. Typical SEM image of Ag₃VO₄/Ag₃PO₄/Ag hybrid (Fig. 2e) indicate that some small Ag nanoparticles existed randomly on the surface of Ag₃VO₄/Ag₃PO₄ heterocrystals.

Fig. 2 Typical FESEM images of the Ag₃VO₄ (a and b), Ag₃VO₄/Ag₃PO₄ (c and d), and Ag₃VO₄/Ag₃PO₄/Ag (e).

Fig. 3 depicts the Fourier transform infrared spectra of Ag₃VO₄/Ag₃PO₄/Ag nanocomposite, pure Ag₃VO₄ and Ag₃PO₄. In FT-IR spectrum of composite, absorption peaks at 740 and 864 cm⁻¹ correspond to the stretching vibrations of the VO₄ unit.²² The peaks at 567 and 1016 cm⁻¹ can be attributed to the stretching vibrations of the PO₄ group²³ which verify the existence of Ag₃PO₄ in the sample, and the peaks around 1600 and 3200 cm⁻¹ represent the physically adsorbed water on the catalyst surface.

Fig. 3 FT-IR spectra of Ag₃VO₄/Ag₃PO₄/Ag composite, pure Ag₃VO₄ and Ag₃PO₄.

To investigate the metallic state of Ag⁰ on the surface of photocatalyst, we carried out X-ray photoelectron spectroscopy (XPS), and the results are shown in Fig. 4. In the survey spectrum (Fig. 4a) the obvious peaks of Ag, C, O, V and P can be clearly detected. The higher resolution XPS data for Ag 3d peaks, shows two individual peaks at approximately 373.1 (Ag 3d_3/2) and 367.1 (Ag 3d_5/2) eV.²⁴ The Ag 3d_3/2 and Ag 3d_5/2 peaks can be divided into two different bands. The peaks at 373.6 and 367.6 are assigned to Ag⁰, and those at 373.1 and 367.1 are attributed to Ag⁺ species.²⁵

Fig. 4 XPS analysis of Ag₃VO₄/Ag₃PO₄/Ag: (a) the survey XPS spectrum and (b) the high-resolution spectrum for the Ag 3d states.

The optical response of the Ag₃VO₄/Ag₃PO₄/Ag composite at different light wavelength was determined by UV-vis diffuse reflectance spectroscopy. As seen in Fig. 5. Ag₃VO₄ has an adsorption edge at around 610 nm. The Ag₃VO₄/Ag₃PO₄/Ag hybrid displays a much stronger response in visible region than pure Ag₃VO₄ and the visible absorption intensity of the nanocomposite was enhanced, which can attributed to the SPR of the Ag NPs on the surfaces of Ag₃VO₄/Ag₃PO₄/Ag.^14,15 Remarkable absorption enhancement leads to good visible light photocatalytic activity.

Fig. 5 UV-vis diffuse-reflectance spectra of Ag₃VO₄ and Ag₃VO₄/Ag₃PO₄/Ag.

3.2. Photocatalytic activity of Ag₃VO₄/Ag₃PO₄/Ag

The photocatalytic activities of prepared samples for degradation of AB92 were evaluated under visible light irradiation. The photocatalytic degradation efficiency (X%) of AB92 was obtained using formula (1):where C₀ and C_t are the initial concentration and the concentration of AB92 after irradiation for t min, respectively. The degradation efficiency of dye under visible light without the catalyst and with the present of pure Ag₃VO₄, Ag₃VO₄/Ag₃PO₄ and Ag₃VO₄/Ag₃PO₄/Ag hybrid photocatalysts, are shown in Fig. 6. It is obvious that in the absence of any catalyst, AB92 is stable while Ag₃VO₄/Ag₃PO₄/Ag hybrid exhibited much higher photocatalytic activity than pure Ag₃VO₄ and Ag₃VO₄/Ag₃PO₄. It can be seen that 84.0% of the AB92 is photocatalytically degraded after 80 min for Ag₃VO₄/Ag₃PO₄/Ag hybrid. Whereas, for the pure Ag₃VO₄ and Ag₃VO₄/Ag₃PO₄ photocatalyst only 22.0% and 76% of AB92 dye molecule is degraded under same condition, respectively.

Fig. 6 The degradation efficiency of AB92 in the presence of different samples under visible light irradiation.

Linear relationship between ln(C₀/C_t) and irradiation time, propose that the photocatalytic reaction obeys pseudo-first-order kinetics. The plot of ln(C₀/C_t) versus t is shown in Fig. 7 and according to the Langmuir–Hinshelwood model (ln(C₀/C_t) = kt), the rate coefficient of AB92 degradation over the samples are demonstrated in Fig. 8. The photodegradation rate coefficient of Ag₃VO₄/Ag₃PO₄/Ag hybrid (0.022 min⁻¹) is higher by a factor of 11 compare to pure Ag₃VO₄ (0.002 min⁻¹).

Fig. 7 Linear-log plot for AB92 degradation in the presence of as synthesized photocatalysts under visible light illumination.

Fig. 8 The degradation rate constant under visible light irradiation.

These results clearly indicate that the coupling of Ag₃PO₄ with Ag₃VO₄ greatly enhances the photocatalytic activity which is obvious from changes in UV-visible absorption spectra of AB92 solution at different times using pure Ag₃VO₄ and Ag₃VO₄/Ag₃PO₄/Ag hybrid under visible light (Fig. 9).

Fig. 9 Changes in UV-visible absorption spectra of AB92 solution using pure Ag₃VO₄ (a) and Ag₃VO₄/Ag₃PO₄/Ag (b) hybrid at different times under visible light irradiation.

3.3. Stability of photocatalyst

Stability of Ag-containing photocatalysts is an important issue for their practical applications. To investigate the stability of Ag₃VO₄/Ag₃PO₄/Ag, crystallinity of the fresh and used composite was compared by using XRD patterns. As shown in Fig. 10, no obvious crystalline structure changing can be observed in XRD patterns, demonstrating that Ag₃VO₄/Ag₃PO₄/Ag photocatalyst posses high stability. Furthermore the photocatalyst activity after first run was determined about 77%, confirming that Ag₃VO₄/Ag₃PO₄/Ag nanocomposite is stable enough during the photocatalytic process.

Fig. 10 XRD patterns of the Ag₃VO₄/Ag₃PO₄/Ag after photocatalytic experiment.

3.4. Discussion of the effects of the reactive species on the photodegradation of AB92

It is generally accepted that semiconductor photocatalytic process involves generation of electrons and holes in the conduction band and valance band of semiconductor under light irradiation. Subsequently, in addition to the active hole and electron process, photo-generated charge carriers can react with preadsorbed H₂O/OH⁻ on the catalyst surface and dissolved oxygen in the solution to produce reactive species^26–28

e_CB⁻ + O₂ → O₂˙⁻ (2)

O₂˙⁻ + H⁺ → HO^˙₂ (3)

HO^˙₂ + HO^˙₂ → H₂O₂ (4)

O₂˙⁻ + HO^˙₂ + H⁺ → H₂O₂ + O₂ (5)

h_VB⁺ + H₂O → OH˙ (6)

h_VB⁺ + OH⁻ → OH˙ (7)

To consider the effect of these reactive species on the photo degradation of AB92, different scavengers (iodide ion, t-BuOH and p-benzoquinone) were introduced into the AB92 degradation process. As shown in Fig. 11, when p-benzoquinone was used as O₂˙⁻ scavengers, the degradation of AB92 significantly decreased, demonstrate that molecular oxygen play a major role in the photodegradation of dye molecules. In the present of t-BuOH, the degradation of AB92 remarkably decreased, it can be caused by reaction of t-BuOH with OH˙ in the solution, this result implying that the OH˙ play an important role in the reaction. The degradation was almost unchanged in the presence of iodide ion illustrating that h_VB⁺ and/or surface-adsorbed OH˙ have very little influence in the degradation of AB92. The above results indicate that both OH˙ and O₂˙⁻ are the predominant reactive species in the photocatalytic degradation of AB92 under visible light irradiation.

Fig. 11 Effect of different scavengers on degradation of AB92 by Ag₃VO₄/Ag₃PO₄/Ag.

3.5. Possible mechanism

Since Ag₃VO₄ and Ag₃PO₄ have the band gap energies of 2.2 eV (ref. 17) and 2.45 eV (ref. 7) respectively, they should be active in visible light region. Photo-induced electrons on the Ag₃VO₄ particles can easily transfer to the surface of Ag₃PO₄ and photo-induced holes of Ag₃PO₄ can also migrate to the surface of the Ag₃VO₄. These transformation are due to the fact that the CB and VB of Ag₃VO₄ (CB = 0.04 eV, VB = 2.24 eV)¹⁷ are more negative than that of Ag₃PO₄ (CB = 0.45 eV, VB = 2.87 eV).¹² Therefore, the efficiency of the photo-induced charge-separation process was enhanced, and more charge carriers can increase the degradation of AB92. When Ag NPs was deposited on the surface of the Ag₃VO₄/Ag₃PO₄ composite, enhancement of photocatalytic activity was observed. Under visible light irradiation, Ag NPs act as electron acceptors, and electrons on the CB of the Ag₃PO₄ can be easily transferred into Ag NPs because CB of Ag₃PO₄ is more negative than the Fermi level of deposited Ag NPs through the Schottky barrier.^29,30

The CB level of Ag₃PO₄ is more negative than the standard redox potential of O₂/H₂O₂ (0.682 eV vs. NHE),³¹ hence, electrons on the surface of Ag₃PO₄ can be trapped by molecular oxygen in the solution to form H₂O₂. H₂O₂ in turn reacts with electrons to produce active OH˙ in solution, and subsequently, these active radical get involved in photocatalytic degradation of AB92. However, the standard redox potential of O₂/O₂˙⁻ (−0.33 eV vs. NHE)³² is so negative that electrons in the CB of Ag₃PO₄ cannot reduce O₂ to O₂˙⁻ or HO^˙₂. Upon visible light irradiation photo-exited electrons in Ag₃PO₄ are transferred to Ag NPs. This accumulated electrons in the Ag NPs react whit O₂ molecules to produce O₂˙⁻.^33,34 The VB position of Ag₃VO₄ is less positive than the standard redox potential of OH˙, H⁺/H₂O (2.72 eV vs. NHE),³⁵ suggesting that the accumulated holes in the VB of Ag₃VO₄ cannot oxidize H₂O to form adsorbed OH˙¹⁷ while they can oxidize OH⁻ to form OH˙ in solution because the VB of Ag₃VO₄ is more positive than the standard redox potential of OH˙/OH⁻ (1.99 eV vs. NHE).³⁶ These enriched holes on the VB of Ag₃VO₄ can also directly decompose the absorbed AB92 on the surface of the catalysts. Based on the above results and the mentioned discussion, the possible photocatalytic mechanism of the Ag₃VO₄/Ag₃PO₄/Ag can be proposed as shown in Scheme 1.

Scheme 1 Schematic illustration of photocatalytic mechanism and charge transfer of the Ag₃VO₄/Ag₃PO₄/Ag under visible light irradiation.

Under visible light irradiation, both Ag₃VO₄ and Ag₃PO₄ can be exited. Photo-induced electrons in the CB of the Ag₃VO₄ can be transferred to that of Ag₃PO₄. These electrons can be trapped by molecular oxygen in solution to form OH˙ and other oxidative species or migrate to the surface of Ag NPs and then participate in photocatalytic reactions. Photo-induced holes in the VB of Ag₃PO₄ can migrate to the surface of Ag₃VO₄ and can directly oxidize the adsorbed dye on the surface of the catalysts.

4. Conclusion

Novel visible-light-driven Ag₃VO₄/Ag₃PO₄/Ag photocatalyst has been successfully prepared via an in situ anion-exchange process, followed by light reduction. The Ag₃VO₄/Ag₃PO₄/Ag nanocomposite show much higher photocatalytic activity than Ag₃VO₄/Ag₃PO₄ heterocatalyst and pure Ag₃VO₄ for degradation of AB92 under visible light irradiation. Photocatalytic activity of Ag₃VO₄ increased with coupling by Ag₃PO_4, due to improving the transfer of photo-induced electron–hole pairs between Ag₃PO₄ and Ag₃VO₄. Furthermore, Ag NPs promote migration efficiency of the photogenerated electrons, and thus enhanced photocatalytic activity of Ag₃VO₄/Ag₃PO₄.

Acknowledgements

Authors would like to express their special thanks to Dr Mohammad Mahdi Ahadian for his kind supports.

References

1. A. Fujishima and K. Honda, Nature, 1972, 238, 37–38.
2. X. Zhang, L. Zhang, T. Xie and D. Wang, J. Phys. Chem. C, 2009, 113, 7371–7378.
3. H. Wang, L. Zhang, Z. Chen, J. Hu, S. Li, Z. Wang, J. Liu and X. Wang, Chem. Soc. Rev., 2014, 43, 5234–5244.
4. N. Wetchakun, S. Chaiwichain, B. Inceesungvorn, K. Pingmuang, S. Phanichphant, A. I. Minett and J. Chen, ACS Appl. Mater. Interfaces, 2012, 4, 3718–3723.
5. H. Dong, G. Chen, J. Sun, C. Li, Y. Yu and D. Chen, Appl. Catal., B, 2013, 134–135, 46–54.
6. W. Zhou, H. Liu, J. Wang, D. Liu, G. Du and J. Cui, ACS Appl. Mater. Interfaces, 2010, 2(8), 2385–2392.
7. P. Wang, P. Shi, Y. Hong, X. Zhou and W. Yao, Mater. Res. Bull., 2015, 62, 24–29.
8. H. Yu, L. Liu, X. Wang, P. Wang, J. Yu and Y. Wang, Dalton Trans., 2012, 41, 10405–10411.
9. B. Tian, T. Wang, R. Dong, S. Bao, F. Yang and J. Zhang, Appl. Catal., B, 2014, 147, 22–28.
10. L. Sun, R. Zhang, Y. Wang and W. Chen, ACS Appl. Mater. Interfaces, 2014, 6(17), 14819–14826.
11. M. Long and W. Cai, Nanoscale, 2014, 6, 7730.
12. Z. Chen, W. Wang, Z. Zhang and X. Fang, J. Phys. Chem. C, 2013, 117, 19346–19352.
13. H. Xu, J. Zhu, Y. Song, W. Zhao, Y. Xu, Y. Song, H. Ji and H. Li, RSC Adv., 2014, 4, 9139–9147.
14. W. Wang, H. Du, R. Wang, T. Wen and A. Xu, Nanoscale, 2013, 5, 3315–3321.
15. Q. Zhu, W. Wang, L. Lin, G. Gao, H. Guo, H. Du and A. Xu, J. Phys. Chem. C, 2013, 117, 5894–5900.
16. R. Konta, H. Kato, H. Kobayashi and A. Kudo, Phys. Chem. Chem. Phys., 2003, 5, 3061–3065.
17. S. Wang, D. Li, C. Sun, S. Yang, Y. Guan and H. He, Appl. Catal., B, 2014, 144, 885–892.
18. S. Wang, Y. Guan, L. Wang, W. Zhao, H. He, J. Xiao, S. Yang and C. Sun, Appl. Catal., B, 2015, 168, 448–457.
19. L. Zhang, Y. He, P. Ye, Y. Wu and T. Wu, J. Alloys Compd., 2013, 549, 105–113.
20. Z. Yang, G. Huang, W. Huang, J. Wei, X. Yan, Y. Liu, C. Jiao, Z. Wan and A. Pan, J. Mater. Chem. A, 2013, 2, 1750–1756.
21. C. Shifu, Z. Wei, L. Wei, Z. Huaye, Y. Xiaoling and C. Yinghao, J. Hazard. Mater., 2009, 172, 1415–1423.
22. N. Tian, H. Huang, Y. He, Y. Guo, T. Zhang and Y. Zhang, Dalton Trans., 2015, 44, 4297–4307.
23. N. Mohaghegh, E. Rahimi and M. R. Gholami, Mater. Sci. Semicond. Process., 2015, 39, 506–514.
24. Y. Sang, Y. Huang, W. Wang, Z. Fang and B. Geng, RSC Adv., 2015, 5, 39651–39656.
25. H. Cheng, B. Huang, P. Wang, Z. Wang, Z. Lou, J. Wang, X. Qin, X. Zhang and Y. Dai, Chem. Commun., 2011, 47, 7054–7056.
26. Z. He, Y. She, C. Gao, L. Wen, J. Chen and S. Song, J. Phys. Chem. C, 2014, 118, 389–398.
27. D. D. Zhang, R. L. Qui, L. Song, B. Eric, Y. Q. Mo and X. F. Huang, J. Hazard. Mater., 2009, 163, 843–847.
28. I. K. Konstantinou and T. A. Albanis, Appl. Catal., B, 2004, 49, 1–14.
29. Z. Cheng, F. Bing, Q. Liu, Z. Zhang and X. Fang, J. Mater. Chem. A, 2014, 1–3.
30. H. Katsumata, T. Sakai, T. Suzuki and S. Kaneco, Ind. Eng. Chem. Res., 2014, 53, 8018–8025.
31. M. Antoniadou and P. Lianos, Appl. Catal., B, 2010, 99, 307–313.
32. W. Liu, M. Wang, C. Xu, S. Chen and X. Fu, Mater. Res. Bull., 2013, 48(1), 106–113.
33. H. Hu, Z. Jiao, T. Wang, J. Ye, G. Lu and Y. Bi, J. Mater. Chem. A, 2013, 1, 10612–10616.
34. J. Xiong, Z. Li, J. Chen, S. Zhang, L. Wang and S. Dou, ACS Appl. Mater. Interfaces, 2014, 6(18), 15716–15725.
35. G. Li, K. H. Wong, X. Zhang, C. Hu, J. C. Yu, R. C. Y. Chan and P. K. Wong, Chemosphere, 2009, 76, 1185–1191.
36. M. Xu, L. Han and S. Dong, ACS Appl. Mater. Interfaces, 2013, 5, 12533–12540.

---