Novel visible-light driven g-C₃N₄/Zn_0.25Cd_0.75S composite photocatalyst for efficient degradation of dyes and reduction of Cr(VI) in water

Abstract

A facile and template free in situ precipitation method was developed for the synthesis of a g-C₃N₄/Zn_0.25Cd_0.75S photocatalyst. The obtained products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and ultraviolet-visible diffuse reflection spectroscopy (DRS). The DRS results showed that with the increase of the g-C₃N₄ content, the light absorption edge for g-C₃N₄/Zn_0.25Cd_0.75S was blue shifted in the visible light region. The TEM images showed that the Zn_0.25Cd_0.75S particles had been finely distributed on the surfaces of the g-C₃N₄ sheets. The HRTEM images showing clear lattice fringes proved the formation of a heterojunction structure at the interfaces of g-C₃N₄ and Zn_0.25Cd_0.75S. In the photocatalytic degradation of dyes, the g-C₃N₄/Zn_0.25Cd_0.75S composite exhibited significantly enhanced activities, and the optimal g-C₃N₄ content was 20 wt%. A controlled experiment showed that the high charge separation efficiency of the photo-generated electron–hole pairs and the suitable energy band structures result in the high performance of g-C₃N₄/Zn_0.25Cd_0.75S under visible light irradiation. The g-C₃N₄/Zn_0.25Cd_0.75S sample also possessed a superior activity in the photocatalytic reduction of Cr(VI) in neutral solution. The photoelectrochemical measurements confirmed that the interface charge separation efficiency was greatly improved by coupling g-C₃N₄ with Zn_0.25Cd_0.75S. A terephthalic acid photoluminescence probing technique has been performed to detect the generation of ˙OH in the reaction system.

---

Introduction

Nowadays, toxic organic pollutants composed of synthetic textile dyes and other industrial dyestuffs have become one of the main sources of water pollution. All those pollutants discharged from factories are harmful to the environment and human health. More severely, they are difficult to remove by traditional treatment methods because of the high stability.^1–3 Most of the azo dyes and fluorine dyes, such as aromatic amines, are suspected to have carcinogenic effects, so the international environmental standards are becoming more and more stringent, correspondingly the treatment of organic pollutants discharged into the environment has also been developed recently.^4,5 As we all know, the photocatalytic oxidation (PCO) technology was an effective method for the environment remediation.^6–10 Titanium dioxide (TiO₂) has been widely studied as a photocatalyst for pollutant degradation and solar energy conversion.^11–14 However, its large band gap (3.2 eV for anatase) and low quantum yield determine that it is only active under ultraviolet light irradiation.¹⁵ Therefore, great efforts have been devoted to suppressing the recombination rate of photo-generated carriers and exploring visible-light driven photocatalyst.^16,17

Recently, great efforts have been done in our group to explore visible-light responsive photocatalysts. For example, Li et al., has reported the synthesis and visible light photocatalytic activity of Zn_xCd_1−xS solid solutions.¹⁸ Through changing the molar ratio of Zn/Cd in the preparation process, Zn_xCd_1−xS nanocrystals with different band-gaps have been obtained. The molar ratio of Zn/Cd in Zn_xCd_1−xS has great influences on its activity under visible light illumination. And it was found that the sample with molar ratio of Zn/Cd = 1 : 3 (Zn_0.25Cd_0.75S) displayed the best activity. In order to further improve the activity of Zn_0.25Cd_0.75S, it has been used to combine with TiO₂ to synthesize Zn_0.25Cd_0.75S/TiO₂ composite photocatalyst for improving the carrier separation efficiency, but the results were not satisfactory.¹⁹

In this paper, we have used graphitic carbon nitride (g-C₃N₄) to combine with Zn_0.25Cd_0.75S to prepare efficient visible-light responsive g-C₃N₄/Zn_0.25Cd_0.75S composite photocatalyst. Similar with graphene,^20–22 g-C₃N₄ has received considerable attention due to its wide applications. The photocatalytic performance of g-C₃N₄ was first reported by Wang et al., but its activity was really low under visible light irradiation.²³ In this work, we have demonstrated a facile in situ precipitation method to synthesize g-C₃N₄/Zn_0.25Cd_0.75S composite with high photocatalytic activity toward methyl orange (MO) and rhodamine B (RhB) under visible light irradiation. It was found that the g-C₃N₄ content had great influences on the photocatalytic activity, and the optimum g-C₃N₄ mass ratio was 20%. Moreover, g-C₃N₄/Zn_0.25Cd_0.75S photocatalyst also possessed a superior activity in the photocatalytic reduction of Cr(VI) in neutral solution without addition of any sacrifice reagents.

Experimental section

Synthesis of g-C₃N₄

All the reagents are analytical-grade and purchased from Sinopharm Chemical Reagent Co., Ltd. The g-C₃N₄ powders were synthesized by heating melamine in a muffle furnace according to the literatures with small modification.^24,25 Briefly, 5 g of melamine was put into a semi-closed alumina crucible with a cover. The crucible was then placed in a muffle furnace and heated to 500 °C at a heating rate of 5 °C min⁻¹, which has been held for 3 h at this temperature. When the temperature of alumina crucible was cooled down after reaction, the obtained g-C₃N₄ products would be ground into powders for use.

Synthesis of g-C₃N₄/Zn_0.25Cd_0.75S

The facile in situ synthesis routes are as follows: first, an appropriate amount of analytical grade Zn(Ac)₂·2H₂O and Cd(Ac)₂·2H₂O (Zn/Cd = 1 : 3) were dissolved in deionized water to form a solution. Then a certain amount of g-C₃N₄ powders were added into this solution, which was then poured into Teflon-lined container (100 mL) and ultrasonicated for 30 min to completely disperse the g-C₃N₄. After that, a certain amount of 0.5 M Na₂S solution was slowly added into the suspension with continuous stirring. After being stirred for 12 h at room temperature, the Teflon-lined container was then sealed in a stainless steel autoclave and maintained at 160 °C for 16 h. After reaction, the autoclave was naturally cooled to ambient temperature, and the obtained solid products were centrifuged, washed and dried in air at 80 °C overnight. According to this method, g-C₃N₄/Zn_0.25Cd_0.75S composite photocatalysts with different mass ratios of 5, 10, 20, 30, 40, 50, and 60 wt% have been synthesized and named as CNZS-5, CNZS-10, CNZS-20, CNZS-30, CNZS-40, CNZS-50, and CNZS-60, respectively. PM-g-C₃N₄/Zn_0.25Cd_0.75S is the abbreviation for the g-C₃N₄/Zn_0.25Cd_0.75S composite prepared by the physical mixing method with a same composition as CNZS-20 (20 wt% g-C₃N₄ and 80 wt% Zn_0.25Cd_0.75S physical mixed without any treatment). Pure CdS, g-C₃N₄/ZnS (20 wt%), and g-C₃N₄/CdS (20 wt%) composites have also be fabricated as references.

Characterization of photocatalysts

X-ray diffraction (XRD) patterns of the obtained products were recorded on a Bruker D8 Advance X-ray diffractometer under the conditions of generator voltage = 40 kV; generator current = 40 mA; divergence slit = 1.0 mm; Cu Kα (λ = 1.5406 Å); and polyethylene holder. The TEM and HRTEM images were obtained on a JEOL model JEM 2010 EX transmission electron microscope. Diffuse reflectance ultraviolet-visible light (UV-vis) spectra (DRS) were measured at room temperature in the range of 200–700 nm on a UV-vis spectrophotometer (Cary 500 Scan Spectrophotometers, Varian, and U.S.A) equipped with an integrating sphere attachment. CHI-660D electrochemical workstation (CH Instruments, USA) has been used to perform the photoelectrochemical experiment. A three-electrode cell equipped with a quartz window has been used for the photoelectrochemical current response measurements. The sample deposited FTO glass has been used as the working electrode. The counter and reference electrodes were platinum wire and Ag/AgCl electrode, respectively. The electrolyte used in the experiment was 0.02 M Na₃PO₄ solution.

Photocatalytic tests

The visible light activity of g-C₃N₄/Zn_0.25Cd_0.75S composites were evaluated by the photocatalytic degradation of dyes (MO and RhB) in aqueous solution. A 500 W halogen lamp (Philips Electronics) has been used as the visible light source, which was placed in a cylindrical glass vessel equipped with water cooling system. The temperature of the reaction system has been maintained at 25 °C. Two cut-off filters have been used to completely remove any radiation below 420 nm, ensuring only visible light (420 nm < λ < 800 nm) could penetrate and irradiate the reaction system. 80 mL of MO aqueous solution (10 ppm) was first added into a 100 mL Pyrex glass vessel, and then 0.08 g of the obtained sample was introduced to form a suspension. Before visible-light irradiation, the suspension was stirred for 60 min for the dye and catalyst to reach adsorption–desorption equilibrium. During the reaction process under visible light irradiation, 3 mL aliquots were sampled at given time intervals. The acquired suspension was centrifuged to remove the catalyst, and the resulting clear liquor was analyzed using a Perkin-Elmer UV-vis spectrophotometer to monitor the dye concentrations. The percentage for dye degradation has been reported as C/C₀. C is the absorption intensity for the main spectrum peak of dye at each irradiated time interval. And C₀ is the absorption intensity of the starting concentration when adsorption–desorption equilibrium was achieved. For comparison, N-doped TiO₂, In₂S₃, and CdS were utilized as references of visible-light driven photocatalysts. N-doped TiO₂ was synthesized by heating commercial P25 under NH₃ atmosphere at 600 °C for 3 h.²⁶ CdS and In₂S₃ also have been respectively prepared according to previous literatures.^27,28 The experiment conditions for the photocatalytic reduction of Cr(VI) over g-C₃N₄/Zn_0.25Cd_0.75S was nearly the same as that used in decomposing dyes except for the amount of catalyst added (40 mg L⁻¹) and the concentration of K₂Cr₂O₇ (50 mg L⁻¹).

Results and discussion

Structural characterization

Fig. 1 showed the XRD patterns of the as-prepared g-C₃N₄, Zn_0.25Cd_0.75S, and g-C₃N₄/Zn_0.25Cd_0.75S composites with different mass ratios. As we can see, The pure g-C₃N₄ sample has two distinct peaks located at 27.4° and 13.1° respectively, which are indexed to the (0 0 2) and (1 0 0) diffraction planes of g-C₃N₄ (JCPDS 87-1526). As for pure Zn_0.25Cd_0.75S, the sharp and narrow diffraction peaks demonstrate the good crystallization. However, for the g-C₃N₄/Zn_0.25Cd_0.75S composites with different mass ratios, only broadened diffraction peaks for Zn_0.25Cd_0.75S have been observed. The overlapping of the tow closely peaks for Zn_0.25Cd_0.75S (26.7°) and g-C₃N₄ (27.4°) had made it difficult to distinguish the peak for g-C₃N₄. Ge et al. has reported a very similar phenomenon in fabricating g-C₃N₄/Bi₂WO₆ composite.²⁹

Fig. 1 XRD patterns of pure g-C₃N₄, Zn_0.25Cd_0.75S, and g-C₃N₄/Zn_0.25Cd_0.75S composite, (a) pure Zn_0.25Cd_0.75S, (b) CNZS-40, (c) CNZS-30, (d) CNZS-20, (e) CNZS-10, (f) CNZS-5, and (g) pure g-C₃N₄.

The morphologies of g-C₃N₄, Zn_0.25Cd_0.75S, and g-C₃N₄/Zn_0.25Cd_0.75S composites are demonstrated in Fig. 2. Fig. 2a revealed that the as-prepared g-C₃N₄ appears to be aggregated large sheets. Fig. 2b showed that the as-prepared Zn_0.25Cd_0.75S exhibited irregular particle morphology with a uniform diameter of 15 nm. Fig. 2c illustrated that the Zn_0.25Cd_0.75S particles had been finely distributed on the surfaces of g-C₃N₄ sheets. Fig. 2d showed the HRTEM image of CNZS-20 sample. Two different kinds of lattice fringes were clearly exhibited, one of d = 0.323 nm matched (0 0 2) crystallographic plane of g-C₃N₄ (JCPDS 87-1526), the other of d = 0.351 nm matched the crystallographic plane of Zn_0.25Cd_0.75S, which was just between d = 0.362 nm for (1 0 0) planes (CdS in PDF no. 41-1049) and d = 0.311 nm for (1 1 1) planes (ZnS in PDF no. 65-0309). This finding suggested that the heterojunction structure was indeed formed in the g-C₃N₄/Zn_0.25Cd_0.75S composites. The heterojunction interfaces would presumably provide good potential for carrier transport.

Fig. 2 TEM micrographs of the samples, (a) pure g-C₃N₄, (b) pure Zn_0.25Cd_0.75S, (c) CNZS-20, and (d) HRTEM image of CNZS-20.

Optical characterization

The UV-vis diffuse reflectance spectra of g-C₃N₄, Zn_0.25Cd_0.75S, and g-C₃N₄/Zn_0.25Cd_0.75S composites were shown in Fig. 3. As we can see, the light absorption of the samples is different. The absorption edge of g-C₃N₄ was estimated at 450 nm, while that for Zn_0.25Cd_0.75S was located at 510 nm. As for the g-C₃N₄/Zn_0.25Cd_0.75S composites, their visible light absorption red-shifted which might result in generation of more electron–hole pairs and therefore enhancement of photocatalytic activity.

Fig. 3 UV-vis diffuse reflectance spectra of the as-prepared samples: (a) pure Zn_0.25Cd_0.75S, (b) CNZS-5, (c) CNZS-10, (d) CNZS-20, (e) CNZS-30, (f) CNZS-40, (g) CNZS-50, (h) CNZS-60, and (i) pure g-C₃N₄.

Photocatalytic activity tests

The activities of g-C₃N₄/Zn_0.25Cd_0.75S composites were mainly evaluated by the visible light photocatalytic degradation for dyes and reduction of Cr(VI). The temporal concentration changes of dye, such as MO, were monitored by examining the variations in maximal absorption in UV-vis spectra at 464 nm. To investigate the influences of g-C₃N₄ content on the photocatalytic activity, g-C₃N₄/Zn_0.25Cd_0.75S composites with different g-C₃N₄ contents were used to decompose MO under the same conditions, and the results were shown in Fig. 4.As we can see, pure Zn_0.25Cd_0.75S showed a poor activity, that the degradation ratio of MO was only 33% after 75 min of visible light irradiation, while pure g-C₃N₄ could hardly decompose MO. However, when a small amount of g-C₃N₄ was combined with Zn_0.25Cd_0.75S, the activity would be remarkably enhanced. We estimate the optimum g-C₃N₄ mass ratio was 20%. When the g-C₃N₄ mass ratio gradually increased from 5 to 20%, the photocatalytic degradation ratio of MO was correspondingly improved from 66 to 99% within 45 min of reaction. However, when the g-C₃N₄ content was further increased, the photocatalytic activities of g-C₃N₄/Zn_0.25Cd_0.75S composites decreased gradually. For example, when the g-C₃N₄ content increased to 60%, the MO degradation ratio decreased to 82%. It was noteworthy that the activity of CNZS-20 was indeed higher than that for g-C₃N₄/CdS (20 wt%).

Fig. 4 Degradation rates of MO under visible light irradiation using (a) pure Zn_0.25Cd_0.75S, (b) CNZS-5, (c) CNZS-10, (d) CNZS-20, (e) CNZS-30, (f) CNZS-40, (g) CNZS-50, (h) CNZS-60, (i) pure g-C₃N₄, (j) g-C₃N₄/CdS(20 wt%), and (k) g-C₃N₄/ZnS(20 wt%).

To evaluate the photocatalytic activities of g-C₃N₄/Zn_0.25Cd_0.75S composites, other photocatalyst such as TiO_2−xN_x, CdS, and In₂S₃ was respectively used as comparison to decompose MO under visible light irradiation. Fig. 5 showed that when the suspension containing CNZS-20 catalyst was stirred in the dark for 45 min, no obvious concentration change of MO has been observed. However, under visible light irradiation, the photocatalytic activity of CNZS-20 was greatly higher than that of CdS, In₂S₃, and TiO_2−xN_x. After 45 min of reaction, the decomposition ratio of MO over CNZS-20 was up to 99%, while CdS, In₂S₃, and TiO_2−xN_x only decomposed 29, 40, and 12% of MO dye respectively.

Fig. 5 Comparisons of photodegradation of MO on CNZS-20, TiO_2−xN_x, CdS, and In₂S₃ under identical conditions.

Furthermore, CNZS-20 has also exhibited excellent photocatalytic activity in decomposing RhB (2 × 10⁻⁵ M) under visible light irradiation. The temporal concentration changes of RhB were monitored by examining the variations in maximal absorption at 554 nm, and the degradation results were shown in Fig. 6. As we can see, RhB could be quickly decomposed under visible light illumination over CNZS-20, and about 99% could be degraded within 45 min. As a comparison, the degradation of RhB over PM-g-C₃N₄/Zn_0.25Cd_0.75S was also investigated under the same conditions, and the degradation results were also shown in Fig. 6. It can be clearly seen that the activity of PM-g-C₃N₄/Zn_0.25Cd_0.75S is getting worse compared with pure Zn_0.25Cd_0.75S. That is to say, even if g-C₃N₄ and Zn_0.25Cd_0.75S was physically mixed together in the ratio of 2 : 8, the activity was not enhanced because there no heterojunction interface was formed with simply physical mixing.

Fig. 6 Degradation rates of RhB under visible light irradiation using (a) pure g-C₃N₄, (b) PM-g-C₃N₄/Zn_0.25Cd_0.75S, (c) pure Zn_0.25Cd_0.75S, and (d) CNZS-20.

Fig. 7 showed the temporal evolution of the spectral changes of RhB mediated by CNZS-20 and Zn_0.25Cd_0.75S, respectively. It exhibited that the degradation processes of RhB over CNZS-20 and Zn_0.25Cd_0.75S were different. In the presence of Zn_0.25Cd_0.75S, the absorption peak of RhB located at 554 nm gradually blue shifted and decreased in the whole degradation process. However, when CNZS-20 was mediated, the absorption peak of RhB quickly shifted from 554 to 499 nm within 30 min of reaction. This hypsochromic shift of absorption maximum was caused by the N-deethylation of RhB during irradiation, which has been confirmed by Watanabe and co-workers.^30,31

Fig. 7 The absorption spectra of RhB in the presence of (a) pure Zn_0.25Cd_0.75S and (b) CNZS-20 under exposure to visible light (420 nm < λ < 800 nm).

The photocatalyst's lifetime is very important for its practical application. The repeated degradation experiments toward MO over CNZS-20 photocatalyst have been performed. In the cycle experiments, the photocatalyst was centrifuged and dried after each run, which was then weighed again to add the lost portion and used for the next run. The results were shown in Fig. 8. It showed that there was no obvious loss of photocatalytic properties after the 4^th run. The XRD patterns of the fresh and used CNZS-20 samples were also shown in Fig. 9. It illustrated that the crystal structure of the CNZS-20 did not change after the photocatalytic reaction. Therefore, the CNZS-20 photocatalyst was stable enough and photo-corrosion hardly occurred in the reaction.

Fig. 8 Cycling run in the photocatalytic degradation of MO over CNZS-20 sample.

Fig. 9 Comparison of XRD patterns between the fresh and used CNZS-20 sample.

In addition, CNZS-20 photocatalyst also exhibited efficient activity for photocatalytic reduction of Cr(VI) to Cr(III) in neutral solution without any addition of sacrifice reagents. As shown in Fig. 10, the maximum absorption band of the Cr(VI) solution located at 371 nm decreased gradually with the increase of irradiation time. After 25 min of reaction the absorption peak completely disappeared, and the reduction ratio of Cr(VI) was up to 99%. That is to say, because of the formation of heterojunction structure and high separation efficiency of carriers, large amount of electrons would be provided to deoxidize Cr(VI), while the holes might oxidize water to produce H⁺ and O₂ when no extra hole scavengers were added in the system.

Fig. 10 Photocatalytic reduction of aqueous Cr(VI) over CNZS-20 under visible light irradiation. (Inset) UV-visible spectral changes of Cr(VI) in aqueous CNZS-20 dispersions as a function of irradiation time.

Because g-C₃N₄ was an effective electron transporter, it would facilitate the charge migration and reduce the recombination of electron–hole pairs for the g-C₃N₄ based photocatalysts. When heterojunction interface was formed between g-C₃N₄ and Zn_0.25Cd_0.75S, the photo-generated electrons should be quickly transfer from g-C₃N₄ to Zn_0.25Cd_0.75S, and consequently enhance the overall activity. To better understand this standpoint, photoelectrochemical technique was used to characterize g-C₃N₄, Zn_0.25Cd_0.75S, and CNZS-20 sample.

Enhancement of interface charge separation efficiency

Fig. 11 showed the photocurrent transient response for the electrodes of bare g-C₃N₄, Zn_0.25Cd_0.75S, and CNZS-20 under visible light irradiation. As it can be seen, with the light switched -on and -off cycles, the CNZS-20 sample exhibited the highest photocurrent transient response under visible light irradiation, which was greatly larger than that for bare g-C₃N₄ and Zn_0.25Cd_0.75S. It suggested the remarkably enhanced carrier separation ratio over CNZS-20, which was in agreement with the enhancement of photo-activity for CNZS-20.

Fig. 11 Photocurrent response of (a) pure g-C₃N₄, (b) Zn_0.25Cd_0.75S, and (c) CNZS-20 under visible light irradiation.

The high separation efficiency of photo-generated carriers should be ascribed to the suitable band potentials of g-C₃N₄ and Zn_0.25Cd_0.75S. According to a previous report, Zn_0.25Cd_0.75S was n-type semiconductor with V_fb of −0.8 V.¹⁹ The conduction band potential (V_CB) of Zn_0.25Cd_0.75S was about −0.9 V, while the valence band potential (V_VB) was 1.51 V. As for g-C₃N₄, Wang et al., reported that the V_CB and V_VB of polymeric g-C₃N₄ were determined at −1.3 and +1.4 V,²³ respectively. A scheme for the separation and transport of photo-generated electron–hole pairs on the g-C₃N₄/Zn_0.25Cd_0.75S interface was shown in Fig. 12. The photo-induced electrons in g-C₃N₄ could move freely towards the surface of Zn_0.25Cd_0.75S while the holes could transfer to the VB of g-C₃N₄ conveniently. As a result, the photo-generated electrons and holes were efficiently separated between g-C₃N₄ and Zn_0.25Cd_0.75S thereby enhanced the photocatalytic activity. Hydroxyl radicals (˙OH) were commonly considered as the possible key active species in the degradation process.^29,32,33 Fig. 13 showed the ˙OH-trapping PL spectra of suspensions containing CNZS-20 and terephthalic acid. It exhibited that the fluorescence intensity increased steadily along with the irradiation time. It elucidated that ˙OH in CNZS-20 system was really produced under visible light irradiation.

Fig. 12 Proposed mechanism for the photocatalytic degradation of dyes on CNZS-20 under visible light irradiation.

Fig. 13 ˙OH-trapping PL spectra of suspensions containing CNZS-20 and TA.

Conclusions

Novel visible-light induced g-C₃N₄/Zn_0.25Cd_0.75S photocatalysts have been successfully synthesized via a facile in situ precipitation method. After introduction of g-C₃N₄, the g-C₃N₄/Zn_0.25Cd_0.75S photocatalyst possessed a significantly enhanced visible light activity in decomposing dyes. The highest degradation efficiency was observed for the CNZS-20 sample. The activity enhancement was mainly attributed to the high separation efficiency of electron–hole pairs on their heterojunction interfaces. This method was expected to be extended for other g-C₃N₄ loaded materials, which might have potential applications in removing pollutants.

Acknowledgements

This work was financially supported by the Scientific Research Reward Fund for Excellent Young and Middle-Aged Scientists of Shandong Province (BS2012HZ001), National Natural Science Foundation of China (no. 21103069, 21075052, 21175057, and 40672158), and Scientific Research Foundation for Doctors of University of Jinan (XBS1037 and XKY1043).

References

1. S. Rengaraj, S. Venkataraj, J. W. Yeon, Y. Kim, X. Z. Li and G. K. H. Pang, Appl. Catal., B, 2007, 77, 157.
2. J. S. Hu, L. L. Ren, Y. G. Guo, H. P. Liang, A. M. Cao, L. J. Wan and C. L. Bai, Angew. Chem., Int. Ed., 2005, 44, 1269.
3. D. Z. Li, Z. X. Chen, Y. L. Chen, W. J. Li, H. J. Huang, Y. H. He and X. Z. Fu, Environ. Sci. Technol., 2008, 42, 2130.
4. H. Lachheb, E. Puzenat, A. Houas, M. Ksibi, E. Elaloui, C. Guillard and J. M. Herrmann, Appl. Catal., B, 2002, 39, 75.
5. Z. X. Chen, D. Z. Li, W. J. Zhang, Y. Shao, T. W. Chen, M. Sun and X. Z. Fu, J. Phys. Chem. C, 2009, 113, 4433.
6. U. G. Akpan and B. H. Hameed, J. Hazard. Mater., 2009, 170, 520.
7. N. S. Foster, R. D. Noble and C. A. Koval, Environ. Sci. Technol., 1993, 27, 350.
8. R. Vinu and G. Madras, Environ. Sci. Technol., 2008, 42, 913.
9. J. J. Testa, M. A. Grela and M. I. Litter, Environ. Sci. Technol., 2004, 38, 1589.
10. S. Luo, Y. Xiao, L. Yang, C. Liu, F. Su and Y. Li, Sep. Purif. Technol., 2011, 79, 85.
11. L. M. Wang, N. Wang, L. H. Zhu, H. W. Yu and H. Q. Tang, J. Hazard. Mater., 2008, 152, 93.
12. C. R. Chenthamarakshan and K. Rajeshwar, Langmuir, 2000, 16, 2715.
13. H. B. Yu, S. Chen, X. Quan, H. M. Zhao and Y. B. Zhang, Environ. Sci. Technol., 2008, 42, 3791.
14. N. Shaham-Waldmann and Y. Paz, J. Phys. Chem. C, 2010, 114, 18946.
15. Y. C. Zhang, J. Li, M. Zhang and D. D. Dionysiou, Environ. Sci. Technol., 2011, 45, 9324.
16. M. Sun, D. Z. Li, Y. Zheng, W. J. Zhang, Y. Shao, Y. B. Chen, W. J. Li and X. Z. Fu, Environ. Sci. Technol., 2009, 43, 7877.
17. M. Sun, D. Z. Li, Y. B. Chen, W. Chen, W. J. Li, Y. H. He and X. Z. Fu, J. Phys. Chem. C, 2009, 113, 13825.
18. W. J. Li, D. Z. Li, Z. X. Chen, H. J. Huang, M. Sun, Y. H. He and X. Z. Fu, J. Phys. Chem. C, 2008, 112, 14943.
19. W. J. Li, D. Z. Li, S. G. Meng, W. Chen, X. Z. Fu and S. Yu, Environ. Sci. Technol., 2011, 45, 2987.
20. X. R. Wang, X. L. Li, L. Zhang, Y. K. Yoon, P. K. Weber, H. L. Wang, J. Guo and H. J. Dai, Science, 2009, 324, 768.
21. X. S. Li, W. W. Cai, J. H. An, S. Kim, J. Nah, D. X. Yang, R. Piner, A. Velamakanni, I. Jung, E. Tutuc, S. K. Banerjee, L. Colombo and R. S. Ruoff, Science, 2009, 324, 1312.
22. Y. B. Zhang, T. T. Tang, C. Girit, Z. Hao, M. C. Martin, A. Zettl, M. F. Crommie, Y. R. Shen and F. Wang, Nature, 2009, 459, 820.
23. X. C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, K. Domen and M. Antonietti, Nat. Mater., 2009, 8, 76.
24. G. Zhang, J. Zhang, M. Zhang and X. Wang, J. Mater. Chem., 2012, 22, 8083.
25. J. Liu, T. Zhang, Z. Wang, G. Dawson and W. Chen, J. Mater. Chem., 2011, 21, 14398.
26. H. Irie, Y. Watanabe and K. Hashimoto, J. Phys. Chem. B, 2003, 107, 5483.
27. S. L. Xiong, B. J. Xi and Y. T. Qian, J. Phys. Chem. C, 2010, 114, 14029.
28. X. Q. An, J. C. Yu, F. Wang, C. H. Li and Y. C. Li, Appl. Catal., B, 2013, 129, 80.
29. L. Ge, C. C. Han and J. Liu, Appl. Catal., B, 2011, 108–109, 100.
30. T. Watanabe, T. Takizawa and K. Honda, J. Phys. Chem., 1977, 81, 1845.
31. T. Takizawa, T. Watanabe and K. Honda, J. Phys. Chem., 1978, 82, 1391.
32. W. Liu, M. L. Wang, C. X. Xu and S. F. Chen, Chem. Eng. J., 2012, 209, 386.
33. S. C. Yan, Z. S. Li and Z. G. Zou, Langmuir, 2010, 26(6), 3894.

---