Synthesis of highly visible light active TiO₂-2-naphthol surface complex and its application in photocatalytic chromium(VI) reduction

Abstract

Photocatalysis is an effective approach for the removal of heavy metal ions present in the aquatic bodies. In this report, TiO₂ nanoparticles were successfully functionalized with 2-naphthol (2-NAP) using simple and scalable condensation reaction. The prepared photocatalyst was demonstrated as superior visible light photocatalyst for the effective reduction of Cr(VI). The 2-NAP functionalized TiO₂ displayed a remarkable enhancement in the photocatalytic reduction of Cr(VI) under visible light irradiation (λ > 400 nm). The maximum Cr(VI) reduction of about 100% (7 fold higher activity than bare TiO₂) was achieved within 3 h. The discernible enhancement in the photocatalytic reduction of TiO₂-2-NAP can be ascribed to improved optical absorption in visible region, high crystallinity of TiO₂ and high surface area. In addition, the photogenerated electron transfer from 2-NAP to TiO₂ (ligand to metal transfer) can significantly improved the photocatalytic performance than bare TiO₂ counterparts. Therefore, the functionalization of metal oxides with organic ligands can open new directions to overcome the existing limitations in photocatalysis.

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1. Introduction

The heavy metals present in the aquatic system create adverse effects to the environment due to its highly toxic nature. Among the existing heavy metals, chromium (Cr) is the most commonly used metal for a variety of applications such as alloys, steel manufacturing, chrome plating, leather tanning and wood processing industries etc.^1,2 In general, Cr exists in two variable oxidation states which are Cr(III) and Cr(VI). Compared to two Cr ions with different oxidation states, Cr(VI) is tremendously toxic and carcinogenic. Nonetheless, Cr(VI) is the most common contaminant present in the wastewater system which are directly discharged from the industries to the environment. As reported by the World Health Organization (WHO), the maximum tolerable concentration of Cr(VI) in drinking water is 0.05 mg L⁻¹.³ Thus, it is a paramount important to reduce the highly toxic Cr(VI) to less harmful Cr(III). The techniques available for the removal and reduction of Cr(VI) are adsorption, ion-exchange, reverse osmosis, chemical precipitation, reduction with sodium sulfide and reduction by using ferrous sulfate, etc.^4–9 However, most of these experimental techniques need sophisticated instrumentation, large quantity of chemicals and also it can produce the secondary wastes during the removal process.

In recent years, photocatalytic reduction of Cr(VI) using semiconductor photocatalysts has been considered as a simple and an alternative technique for the efficient reduction of Cr(VI) to Cr(III). TiO₂ based materials are the most promising photocatalysts for oxidation of organic pollutants and the removal of heavy metal ions from the aquatic bodies. However, TiO₂ cannot be activated under visible light because of its wide bandgap which lies in the UV light region. The modification of TiO₂ with visible light active metals, metal oxides, polymers and organic dye molecules are attracted as methods for design of efficient photocatalysts in the effective reduction of Cr(VI) as well as degradation of organic pollutants.^10–17 Apart from these available methods, functionalization of TiO₂ with organic ligands is a simple, alternative and scalable process for the reduction reaction. The condensation reaction between aromatic hydroxyl group and TiO₂ surface hydroxyl group leads to the formation of ligand to metal charge transfer (LMCT) complex, which further excites electron from the highest occupied molecular orbital (HOMO) of aromatic ligand to the conduction band of TiO₂ under visible light irradiation.^18–21

Several similar strategies have been employed to improve the photocatalytic ability of TiO₂ photocatalyst. Zhang et al. synthesized phenolic resin functionalized TiO₂ by using the grafting method for hydrogen production in the presence of visible light.²² Yamashita et al. reported that the hydroxynaphthalene functionalized TiO₂ showed successful reduction of 4-nitrophenol to 4-aminophenol.¹⁸ Koliyat and his co-workers reported that the TiO₂–carbon based hybrid ligand to metal charge transfer complex prepared by N₂ pyrolysis at 800 °C exhibited much higher photocatalytic activity than the bare TiO₂ towards hydrogen production.²³ Some research groups prepared visible light responsive amino-functionalised TiO₂ by solvothermal method for both hydrogen production and nitrophenol reduction.^24,25 Fu et al. synthesized visible light active amine functionalized TiO₂ photocatalyst for CO₂ reduction.²⁴ Chen and co-workers modified the TiO₂ surface with 2,4-diisocynate by using a surface chemical modification process for degradation of 2,4-dichlorophenol under visible light illumination.²⁶ Jiang et al. synthesized the TiO₂–phenolic resol hybrid material photocatalyst by solvothermal method for the degradation of methyl orange dye.²⁷

In the present study, first time we have synthesized visible light active 2-naphthol (2-NAP) functionalized TiO₂ using simple condensation reaction at ambient conditions. The synthesized TiO₂-2-NAP showed effective reduction of Cr(VI) upon visible light irradiation. Hence, the present research work was mainly focused on synthesis, characterization studies and photocatalytic reduction properties of TiO₂-2-NAP photocatalysts.

2. Experimental details

2.1. Materials

The titanium tetraisopropoxide was purchased from spectrochem chemicals (India) and potassium dichromate (K₂Cr₂O₇), 2-naphthol analytical grade was purchased from SRL chemicals, India. All other solvents and reagents (analytical grade) were purchased and used without any further purification.

2.2. Synthesis of mesoporous TiO₂

The mesoporous TiO₂ was synthesized as reported earlier.²⁸ In brief, 0.032 mol titanium isopropoxide and 0.016 mol glacial acetic acid were dissolved in 20 mL of absolute ethanol. The mixture was allowed to stirr for 1 h. After stirring, the resultant suspension was added dropwise to a 100 mL deionised water under sonication. Subsequently, the whole colloidal mixture was sonicated for 3 h by low frequency ultrasonicator (3 s on, 1 s off, pulse mode, amplitude 35%) using a 13 mm diameter high intensity probe (Sonics and Materials UC 750, 20 KHz). The powder was collected by centrifugation, washed several times with deionised water and dried at 100 °C. The synthesized powder was calcined in air at 400 °C for 1 h.

2.3. Synthesis of TiO₂-2-NAP complex

The TiO₂-2-NAP surface complex was synthesized using simple condensation reaction. 500 mg of prepared TiO₂ was suspended in 10 mL of acetone. To the above suspension, required amount (different wt%) of 2-NAP was added drop by drop under continuous stirring. The resulting suspension was allowed to stirr for 1 h at room temperature. Then, the color of the reaction mixture was changed to light yellow, which indicating the formation of TiO₂-2-NAP complex. Finally, the product was collected by centrifugation and washed several times with acetone to remove residual organic moiety and dried at 40 °C.

2.4. Characterization studies

The synthesized TiO₂-2-NAP complex was characterized using various spectroscopic techniques. X-ray diffraction (XRD) patterns were recorded from PANalticalX'pert powder diffractometer using Cu Kα radiation. Diffuse Reflectance Spectra (DRS) were monitored by UV-2600 spectrophotometer (Shimadzu) in DRS mode. The morphological studies of the prepared photocatalysts were carried out by using Field Emission-Scanning Electron Microscopy (FEI Quanta FEG 200 HR-SEM). The surface area was obtained by using a BEL-SORP max (BEL Japan, Inc.). Fourier Transform Infrared (FT-IR) spectra were obtained using Perkin-Elmer-USA.

2.5. Photocatalytic reduction studies

The photocatalyst powder (100 mg) was suspended in 100 mL of 30 ppm standard Cr(VI) solution, which is prepared from potassium dichromate crystals. Before the addition of photocatalyst, the pH of the solution was adjusted to 7 by the addition of 0.1 M NaOH. The resultant mixture was allowed to stirr for 30 min at dark to reach the adsorption–desorption equilibrium. After stirring, the suspension was irradiated with 150 W visible lights (λ > 400 nm) with constant stirring at room temperature. The visible lamp was placed into a quartz double-wall water circulation jacket to avoid overheating and the whole setup was immersed into the chromium solution taken in a 250 mL beaker. However, The intensity of light was measured using Lux meter and the intensity was 38 000 Lux. During the photocatalytic reaction, at a fixed interval of time, 3 mL sample was collected, centrifuged and analyzed by UV-visible spectrophotometer (Specord 200 plus, Analytik Jena, Germany) to quantify the concentration of Cr(VI). Presence of Cr ions before and after the photocatalytic reaction was estimated using atomic absorption spectroscopy (AAS) which is recorded using Agilent Verne – 210, USA.

3. Results and discussion

3.1. XRD Pattern

The XRD patterns of P25 (Degussa TiO₂), TiO₂ and TiO₂-2-NAP are depicted in Fig. 1. The commercial P25 shows the characteristic diffraction peaks at 25 and 28°, indicating that the P25 is composed of both anatase and rutile phase structures.²⁹ The mesoporous TiO₂ prepared using ultrasound exhibits the characteristic diffraction peaks at 25.1, 37.8, 47.9, 53.9, 55.3, 62.6, 68.8, 70.2, 75.2 and 82.8° correspond to (101), (004), (200), (105), (211), (204), (116), (220), (215) and (224) planes, respectively. These observed XRD peaks clearly indicate the formation of well crystalline anatase phase TiO₂ structure (JCPDS card no. 89-4921).³⁰ However, no additional peaks or shift in the existing peaks of anatase phase of TiO₂ was observed with the 2-NAP functionalized TiO₂ photocatalyst, which indicates that the functionalization of TiO₂ with 2-NAP did not affect or induce any new crystalline phase in the TiO₂ structure.³¹

Fig. 1 XRD Patterns of P25, meso-TiO₂ and TiO₂-2-NAP.

3.2. SEM and EDX

The morphology of the prepared TiO₂ and TiO₂-2-NAP photocatalysts were investigated by SEM analysis. The SEM micrographs are displayed in Fig. 2a & b. It can be clearly seen that the irregular shaped particles are obtained for both TiO₂ and TiO₂-2-NAP. However, the 2-NAP functionalized TiO₂ has no specific influence on the morphological change of TiO₂. Fig. 2c & d gives the EDX profile of TiO₂ and TiO₂-2-NAP photocatalysts. The prepared TiO₂-2-NAP is composed of Ti, O and C respectively. Therefore, the existence of C in the present result clearly confirms the functionalization of 2-NAP with TiO₂.

Fig. 2 SEM image of (a) TiO₂, (b) TiO₂-2-NAP and EDX spectra of (c) TiO₂ [inset: elemental weight percentage table] (d) TiO₂-2-NAP [inset: elemental weight percentage table].

3.3. UV-vis-DRS spectra

Fig. 3 displays the UV-vis diffuse reflectance spectra of the P25, TiO₂ and 2-NAP functionalized TiO₂. The absorption edge for P25, TiO₂ and TiO₂-2-NAP are observed at 379, 389 and 407 nm, respectively. Compared to P25 and TiO₂, the red-shift in the absorption edge of TiO₂-2-NAP is obtained due to the formation of TiO₂-2-NAP LMCT complex. As shown in Fig. 3, the band gap energy was calculated from the corresponding absorption edges of P25, TiO₂ and TiO₂-2-NAP. The bandgap energies of P25, TiO₂ and TiO₂-2-NAP was estimated to be 3.2, 3.1 and 3.0 eV, respectively. The slight red-shift in the bandgap is mainly due to the formation of the LMCT complex between surface hydroxyl group of TiO₂ and 2-NAP. Moreover, TiO₂-2-NAP has a clear optical response in the visible light area ranging from 400 nm to 550 nm, which further demonstrates that TiO₂-2-NAP is more capable to drive the photocatalytic reaction under visible light irradiation compared to that of bare P25 and TiO₂ alone.

Fig. 3 Diffuse reflectance UV-vis spectra of P25, meso-TiO₂ and TiO₂-2-NAP photocatalysts.

3.4. BET-surface area analysis

Fig. 4 gives the N₂ adsorption–desorption isotherms of P25, TiO₂ and TiO₂-2-NAP photocatalyst. The surface areas of photocatalysts are given in Table 1. As can be seen from Table 1, the surface area of P25, TiO₂ and TiO₂-2-NAP was observed to be 46.1, 133 and 130 m² g⁻¹, respectively. This clearly reveals that the photocatalyst prepared in the presence of ultrasound have more surface area, whereas the P25 has the less surface area. Similar result has been reported by Neppolian et al.³² However, there is no significant change in surface area was observed with the 2-NAP functionalized TiO₂. The pore diameters of meso-TiO₂ and TiO₂-2-NAP were 8.79 and 8.77 nm. Larger surface area and smaller pore diameter can make this photocatalyst more efficient for the effective reduction of Cr(VI).

Fig. 4 Nitrogen adsorption–desorption isotherms of P25, meso-TiO₂ and TiO₂-2-NAP.

Table 1 BET surface area, pore volume and pore size of the photocatalysts

Entry	Name of samples	SBET (m^2 g^−1)	Vp (cm^3 g^−1)	Pore size (nm)
1	TiO2-2-NAP	130	0.286	8.79
2	TiO2	133	0.291	8.77
3	P25	46.1	0.357	31.0

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3.5. FT-IR

To further confirm the LMCT surface complex formation between 2-NAP and TiO₂, FTIR spectra were recorded in attenuated total reflectance (ATR) mode. The FTIR spectra of free 2-NAP and TiO₂-2-NAP are illustrated in Fig. 5a & b. As shown in Fig. 5a, the peaks centered at 1632, 1601, 1583, 1512, 1466 and 1408 cm⁻¹ are attributed to the stretching vibrations of the aromatic ring (C–C) and (C=C) bonds. Similarly, the observed bands at 1118, 1138, 1172, 1217, 1242, 1277, 1323, 1363 and 1379 cm⁻¹ correspond to the stretching or bending vibration of the phenolic C–OH group. In the case of TiO₂-2-NAP, the intensity of phenolic vibration peaks at 1138, 1172, 1217, 1242, 1277, 1323, 1363 and 1379 cm⁻¹ are lower than pure 2-NAP (Fig. 5b).^18,33,34 This observation clearly revealed the formation of surface complexes between the hydroxyl groups of both TiO₂ and 2-NAP through simple dehydration reaction.

Fig. 5 ATR-FTIR spectra of (a) free 2-NAP and (b) TiO₂-2-NAP.

3.6. Photocatalytic reduction of Cr(VI)

The photocatalytic reduction of Cr(VI) was carried out to evaluate the photoreduction ability of 2-NAP functionalized TiO₂ photocatalyst. Fig. 6a & b shows the decrease in the absorption spectrum of Cr(VI) in the presence of bare TiO₂ and TiO₂-2-NAP upon visible light irradiation (λ > 400 nm). As shown in Fig. 6a, the bare meso TiO₂ exhibits lower photocatalytic reduction of Cr(VI), whereas the 2-NAP functionalized TiO₂ photocatalyst shows complete reduction (100%) of Cr(VI) within 3 h of visible light irradiation [Fig. 6b]. Recently, similar reduction efficiency was observed by Mondal et al. using SnS₂ photocatalyst.³⁵ For a comparison, the photocatalytic reduction reaction was carried out with P-25 and its exhibited only 12% reduction under similar conditions. Hence, the substantial enhancement in the photoreduction behavior of TiO₂-2-NAP, can be justified that the 2-NAP induces the charge transfer (CT) between the HOMO of 2-NAP to conduction band of TiO₂.

Fig. 6 UV-vis absorption spectra of Cr(VI) reduction using (a) meso-TiO₂ and (b) TiO₂-2-NAP (1 wt%).

As shown in Fig. 7, our preliminary tests demonstrated that no reduction reaction is observed in the absence of either light or photocatalyst alone, suggesting that the reaction was mainly driven by photocatalysis, whereas around 20% of adsorption is noted in dark condition after 3 h. In order to study the effect of 2-NAP in Cr(VI) reduction, different weight percents (0.5, 1, 1.5 and 2.0 wt%) of 2-NAP was loaded and their photocatalytic activity was evaluated under identical experimental conditions. As shown in Fig. 7, the photocatalytic reduction efficiency of 0.5, 1, 1.5 and 2.0 wt% of 2-NAP loaded TiO₂ are 67, 100, 85, and 76%, respectively. It can be clearly seen that 1 wt% 2-NAP loaded photocatalyst exhibits remarkable enhancement in the Cr(VI) reduction than the other composites. However, the higher concentration of 2-NAP loaded TiO₂ decreases its photocatalytic activity. This might be due steric hindrance and hence it may suppress the electron transfer from the aromatic ring of 2-NAP to TiO₂. However, the excellent photocatalytic activity of TiO₂-2-NAP composites can be ascribed due to the direct electron ejection from the HOMO of 2-NAP to the conduction band of TiO₂. Moreover, it also an evidence for the LMCT complex formation between the TiO₂ and 2-NAP as proven by FT-IR spectrum.

Fig. 7 Photocatalytic reduction of Cr(VI) using different wt% 2-NAP loaded TiO₂.

3.7. Effect of initial concentration of Cr(VI) and different amount of catalyst loading

The effect of initial concentration of Cr(VI) on photocatalytic reduction was carried out by varying the initial concentration of Cr(VI) from 20 to 50 ppm and the catalyst loading was kept constant. The change in Cr(VI) concentration with respect to the irradiation time is shown in Fig. 8a. As can be seen from Fig. 8a, the photocatalytic reduction rate of Cr(VI) is gradually decreasing from 100 to 64% by increasing the initial concentration from 20 to 50 ppm. The rate constant values for the photoreduction of Cr(VI) with different concentrations (20–50 ppm) using TiO₂-2-NAP photocatalyst were calculated from Fig. 8a and the results are summarized in Table 2. As can be seen from Table 2 that the rate constant decreases with increase in the initial concentration of Cr(VI). This result clearly reveals that the photocatalytic reduction of Cr(VI) using TiO₂-2-NAP photocatalyst depends on the initial concentration of Cr(VI) and follows pseudo first order reaction kinetics. However, 20 ppm concentration is very high concentration in comparison with the naturally available chromium or present in different industrial effluents. This result clearly emphasizing that the initial concentration of Cr(VI) plays a significant role in the photoreduction process. Since the reaction time and the amount of catalyst are constant, the photo-generated electrons on the surface of the catalyst is also constant. Therefore, the relative number of electrons reacting with Cr(VI) decreases with increasing concentration of the chromium. Moreover, the reduced Cr ions can occupy the catalytic active sites of photocatalysts, which results in lower photocatalytic activity.^36,37 The another possible reason is that the increase in the initial concentration of Cr(VI) can easily prevent the penetration of light to the surface of photocatalysts.³⁸

Fig. 8 (a) Effect of different initial concentration of Cr(VI) and (b) different catalyst loading.

Table 2 Effect of initial concentration of Cr(VI) on the photocatalytic reduction using TiO₂-2-NAP photocatalyst under visible light irradiation

Entry	Initial concentration of Cr(VI) (ppm)	k′ (min^−1)	R^2
1	20	0.046	0.9470
2	30	0.013	0.9860
3	40	0.010	0.9665
4	50	0.007	0.9732

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To investigate the effect of photocatalyst loading on the photocatalytic reduction of Cr(VI), the catalyst loading amount was varied from 50 to 125 mg without changing the initial concentration of Cr(VI). It is clearly seen from Fig. 8b, the photocatalytic reduction rate of Cr(VI) is gradually increased from 56, 77 and 100% with increase in catalyst dosage (50 to 125 mg). This phenomenon can be clearly explained that the increase in catalyst loading can absorb more incident light to enhance the charge transfer between aromatic rings of 2-NAP and the conduction band of TiO₂, which further facilitate the high photocatalytic activity.

3.8. Atomic absorption spectroscopy

In order to confirm the reduction of Cr(VI) to Cr(III) by this photocatalytic route, AAS measurements were carried out before and after the photocatalytic reaction (Fig. 9). For this purpose, 20 ppm Cr(VI) solution at neutral pH was irradiated with visible light in the presence of TiO₂-2-NAP photocatalyst. The initial and final solution was tested using AAS to estimate the Cr concentration and was found to be 19.72 and 18.68 ppm, respectively. This result confirmed the presence of Cr in the final solution, after the visible light driven photoreaction. However, the formation of Cr(III) ion was authenticated by adjusting the pH of the initial and final reaction medium to 10.5 using 1 M NaOH solution. After adjusting the pH to 10.5, the Cr(III) ion present in the final solution precipitated as Cr(OH)₃ which was removed by filtration (using 0.22 micron filter paper) before AAS analysis. The AAS results suggested the presence of 19.91 ppm of Cr ions in the initial solution at pH = 10.5 whereas the final solution showed only 0.65 ppm. This substantiated the reduction of Cr(VI) ions by this photocatalytic system in presence of TiO₂-2-NAP and also strongly proved the presence of Cr(III) ions.

Fig. 9 Atomic absorption spectroscopic results of Cr solution.

3.9. Plausible photocatalytic reaction mechanism

On the basis of the above results and discussion, a plausible mechanism of visible light induced LMCT is proposed to explain the photocatalytic activity of the TiO₂-2-NAP surface complex as illustrated in the Fig. 10. Generally, visible light activation of wide bandgap semiconductors (like TiO₂, ZnO, etc.) can be achieved by using surface adsorbates (either dyes or complexes) as sensitizers other than coupling with narrow bandgap semiconductors. The photochemical process of dye sensitization is usually initiated by HOMO to LUMO photoexcitation of dye molecules, followed by electron transfer from the LUMO level of the excited dye to TiO₂ conduction band (CB). On the other hand, the visible light mediated charge transfer occurs from the HOMO of adsorbates to the CB of TiO₂, which is referred to as the ligand-to-metal charge transfer (LMCT) process.³⁹ Hence, unlike the case of dye sensitization where the sensitizer itself should absorb the visible light, a variety of organic or inorganic compounds (that do not absorb visible light) can be a potential LMCT sensitizer. In the present study, when visible light strikes on the TiO₂-2-NAP complex, the electrons present in the HOMO of 2-NAP are directly excited to the conduction band of TiO₂. Hence, the population of electron increases in the conduction band of TiO₂, which is subsequently induce the Cr(VI) to Cr(III) reduction. Recently, Tachan et al. proposed similar LMCT mechanism for visible light active catechol functionalized TiO₂ photocatalyst.⁴⁰ Moreover, the similar LMCT mechanism has also been reported by other researchers for the different photocatalytic applications.^41–43

Fig. 10 A possible mechanism of photocatalytic Cr(VI) reduction under visible light irradiation using TiO₂-2-NAP.

4. Conclusion

A visible light sensitive TiO₂-2-NAP surface complex was successfully synthesized via simple condensation reaction method. The synthesized 2-NAP functionalized TiO₂ exhibited 100% photocatalytic activity for the Cr(VI) reduction. The higher photocatalytic performance was mainly attributed due to the functionalization of 2-NAP on TiO₂. The optimal amount of 2-NAP functionalization was determined to be 1 wt%. A plausible mechanism of the photocatalytic Cr(VI) reduction process was proposed. This study opens a new possibility in the investigation of TiO₂-2-NAP surface complex and promotes their practical applications in environmental issues.

Acknowledgements

We gratefully acknowledge SERB-DST (DST no. SR/FT/CS-127/2011), New Delhi, India for the financial support.

References and Notes

1. L. B. Khalil, W. E. Mourad and M. W. Rophae, Appl. Catal., B, 1998, 17, 267.
2. N. Shevchenko, V. Zaitsev and A. Walcarius, Environ. Sci. Technol., 2008, 42, 6922.
3. WHO, Geneva 3/e, 2004, 334.
4. M. Dakiky, M. Khamis, A. Manassra and M. Mereb, Adv. Environ. Res., 2002, 6, 533.
5. M. Emine, N. Yasar and D. Murat, J. Hazard. Mater., 2006, 138, 142.
6. S. A. Cavaco, S. Fernandes, M. M. Quina and L. M. Ferreira, J. Hazard. Mater., 2007, 144, 634.
7. S. A. M. Rad, S. A. Mirbagheri and T. Mohammadi, Using reverse osmosis membrane for chromium removal from aqueous solution, World Academy of Science, Engineering and Technology, 2009, vol. 3, p. 28.
8. L. L. Skovbjerg, S. L. S. Stipp, S. Utsunomiy and R. C. Ewing, Geochim. Cosmochim. Acta, 2006, 70, 3582.
9. R. D. Ludwig, C. Su, T. R. Lee, R. T. Wilkin, S. D. Acree, R. R. Rose and A. Keeley, Environ. Sci. Technol., 2007, 41, 5299.
10. D. Zhang, X. Li, H. Tan, G. Zhang, Z. Zhao, H. Shi, L. Zhang, W. Yub and Z. Sun, RSC Adv., 2014, 4, 44322.
11. S. G. Babu, R. Vinoth, B. Neppolian, D. D. Dionysiou and M. Ashokkumar, J. Hazard. Mater., 2015, 291, 83.
12. S. Kim, S. J. Hwang and W. Choi, J. Phys. Chem. B, 2005, 109, 24260.
13. J. Yu, G. Dai and B. Huang, J. Phys. Chem. C, 2009, 113, 16394.
14. H. Park and W. Choi, Langmuir, 2006, 22, 2906.
15. J. Zhang, J. H. Bang, C. Tang and P. V. Kamat, ACS Nano, 2010, 4, 387.
16. H. Yamashita, M. Harada, J. Misaka, M. Takeuchi, B. Neppolian and M. Anpo, Catal. Today, 2003, 84, 191.
17. B. Neppolian, H. Jung and H. Choi, J. Adv. Oxid. Technol., 2007, 10, 2.
18. T. Kamegawa, H. Seto, S. Matsuura and H. Yamashita, ACS Appl. Mater. Interfaces, 2012, 4, 6635.
19. J. Moser, S. Punchihewa, P. P. Infelta and M. Gratzel, Langmuir, 1991, 7, 3012.
20. P. Persson, R. Bergstrom and S. Lunell, J. Phys. Chem. B, 2000, 104, 10348.
21. I. A. Jankovic, Z. V. Saponjic, M. I. Comor and J. M. Nedeljkovic, J. Phys. Chem. C, 2009, 113, 12645.
22. G. Zhang and W. Choi, Chem. Commun., 2012, 48, 10621.
23. S. K. Parayil, H. S. Kibombo and R. T. Koodali, Catal. Today, 2013, 199, 8.
24. Y. Fu, D. Sun, Y. Chen, R. Huang, Z. Ding, X. Fu and Z. Li, Angew. Chem., Int. Ed., 2012, 51, 3364.
25. T. Toyao, M. Saito, Y. Horiuchi, K. Mochizuki, M. Iwata, H. Higashimura and M. Matsuoka, Catal. Sci. Technol., 2013, 3, 2092.
26. F. Chen, W. Zou, W. Qu and J. Zhang, Catal. Commun., 2009, 10, 1510.
27. Y. Jiang, L. Meng, X. Mu, X. Li, H. Wang, X. Chen, X. Wang, W. Wang, F. Wu and X. Wang, J. Mater. Chem., 2012, 22, 23642.
28. J. C. Yu, L. Zhanga and J. Yua, New J. Chem., 2002, 26, 416.
29. H. Zhang, X. Lv, Y. Li, Y. Wang and J. Li, ACS Nano, 2010, 4, 380.
30. B. Neppolian, Q. Wang, H. Jung and H. Choi, Ultrason. Sonochem., 2008, 15, 649.
31. D. Jianga, Y. Xua, D. Wua and Y. Suna, J. Solid State Chem., 2008, 181, 593.
32. B. Neppolian, E. Celik, M. Anpo and H. Choi, Catal. Lett., 2008, 125, 183.
33. T. D. Savi, Z. V. Saponjic, M. I. Comor, J. M. Nedeljkovi, M. D. D. Canin, M. G. N. Dusan, Z. Veljkovi, S. D. Zari and I. A. Jankovi, Nanoscale, 2013, 5, 7601.
34. T. D. Savi, I. A. Jankovi, Z. V. Saponji, M. I. Comor, M. G. N. Dusan, Z. Veljkovi, S. D. Zari and J. M. Nedeljkov, Nanoscale, 2012, 4, 1612.
35. C. Mondal, M. Ganguly, J. Pal, A. Roy, J. Jana and T. Pal, Langmuir, 2014, 30, 4157.
36. A. Akyol, H. C. Yatmaz and M. Bayramoglu, Appl. Catal., B, 2004, 54, 19.
37. M. S. Siboni, M. Farrokhi, R. D. C. Soltani, A. Khataee and S. Tajassosi, Ind. Eng. Chem. Res., 2014, 53, 1079.
38. M. Qamara, M. A. Gondala and Z. H. Yamania, J. Mol. Catal. A: Chem., 2011, 341, 83.
39. V. H. Houlding and M. Gratzel, J. Am. Chem. Soc., 1983, 105, 5695.
40. Z. Tachan, I. Hod and A. Zaban, Adv. Energy Mater., 2014, 4, 1301249.
41. X. Li, C. Chen and J. Zhao, Langmuir, 2001, 17, 4118.
42. G. Kim and W. Choi, Appl. Catal., B, 2010, 100, 77.
43. G. Zhang, G. Kim and W. Choi, Energy Environ. Sci., 2014, 7, 954.

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