Enhancing photoelectrochemical activity of CdS quantum dots sensitized WO₃ photoelectrodes by Mn doping

Abstract

Mn-doped CdS quantum dots sensitized WO₃ photoelectrodes were successfully synthesized by a combination of hydrothermal and chemical bath deposition (CBD) methods. To improve the stability of the photoelectrodes in an alkaline environment, the electrodes were treated with TiCl₄ to form a nano-TiO₂ buffer layer on the WO₃ plate surface before depositing CdS quantum dots (QDs). The resulting electrodes were applied as photoanodes in the photoelectrochemical cell for water splitting. The photoelectrochemical (PEC) properties were investigated by the photocurrent density curves and incident photon-to-current conversion efficiency (IPCE). The as-prepared Mn-CdS QDs sensitized WO₃ plate-like photoelectrodes exhibit a significant improvement in their photoelectrochemical performance compared with undoped photoelectrodes. To better understand the enhanced PEC properties, the electron transport properties and efficient electron lifetime were studied in detail using electrochemical impedance spectroscopy (EIS), transient photocurrent spectroscopy and intensity modulated photocurrent spectroscopy (IMPS). The results show that the Mn-CdS QDs/TiO₂/WO₃ photoelectrodes exhibit a higher electron transit rate and a longer electron lifetime. This is most likely due to the existence of electronic states in the mid-gap region of the Mn-CdS QD.

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1. Introduction

Hydrogen gas is considered to be a potential candidate for one of the most clean energy sources due to its renewable and environmental friendly characteristics.¹ Since the first report on the photoelectrochemical water splitting for hydrogen production using a titanium dioxide (TiO₂) photoelectrode in 1971,² many studies have been dedicated to develop suitable photoelectrode materials, such as ZnO,^3,4 Fe₂O₃,^5,6 Cu₂O^7,8 and WO₃, for converting solar energy into hydrogen.^9,10 Among these materials, tungsten trioxide (WO₃), which exhibits high photo-catalytic activity, chemical stability in acidic solutions and a nontoxic nature, is known as a promising material.^10,11 Unfortunately, its large band gap (E_g ≥ 2.6 eV) limits the energy absorption in the near UV region.¹² A large number of approaches have been taken to improve the activity of WO₃ in the visible light region.^13–15 A good solution could be the construction of a heterostructure junction between the narrow bandgap semiconductor and the wide bandgap semiconductor, which provides significant advantages for light absorption.^16–19 Therefore, several types of heterostructures, such as BiVO₄/WO₃,²⁰ Fe₂O₃/WO₃,²¹ NiWO₃/WO₃,²² InVO₄/WO₃ (ref. 23) and CdS/WO₃,^24,25 have been demonstrated to be potential candidates to improve the photocatalytic activity of WO₃.

Among the various narrow bandgap semiconductor materials, CdS quantum dots (QDs) are a well-known semiconductor material that have been widely used as photosensitizers for various wide bandgap semiconductor photoanodes.^26–28 CdS QDs have been paid much attention due to their specific advantages, including the appropriate band gap (E_g ≈ 2.4 eV) and suitable valence band position.^29,30 CdS QDs sensitized wide bandgap semiconductor electrodes, such as CdS QDs/TiO₂,^31,32 CdS QDs/ZnO,^33,34 CdS QDs/BiVO₄ (ref. 35) and CdS QDs/WO₃,³⁶ showed enhanced visible light absorption and improved water splitting performance. Many efforts, including the use of bilayer electrodes^37–39 and co-sensitization together with infrared QDs (PbS,⁴⁰ CdSe²⁷ and Ag₂S⁴¹), have been devoted to further improve the performance of CdS QDs-based photoelectrodes. Recently, the synthesis of Mn doped CdS (Mn-CdS) has been proven to be a powerful strategy to extend the lifetime of charge carriers to boost the efficiency of QDs-sensitized solar cells.⁴²

In this study, we first report an investigation on the Mn-CdS QDs sensitized WO₃ plate-like photoelectrodes and their performance for PEC hydrogen generation. The high-density arrays of monoclinic single crystalline WO₃ plates were grown directly on FTO by a hydrothermal method. The conformal coating of a nano-TiO₂ buffer layer on WO₃ plates was carried out by the thermal hydrolysis of TiCl₄. The Mn-CdS QDs were deposited on the plate-like film by the CBD technique. The structural, morphological and photoelectrochemical properties of the photoelectrodes were evaluated.

2. Experimental section

2.1 Synthesis and characterization

The WO₃ plate-like films were synthesized by a hydrothermal method according to our previous study.⁴³ 0.231 g of Na₂WO₄·2H₂O was dissolved in 30 mL of ultrapure water under constant stirring at room temperature. Then, 10 mL of 3 M HCl was added to the solution, followed by the addition of 0.2 g of (NH₄)₂C₂O₄. After five minutes of stirring, 30 mL of ultrapure water was added into the solution with continual stirring for 30 min. The as-prepared precursor was transferred into a 100 mL of Teflon-lined autoclave. The FTO substrates with the conducting side facing down were immersed and leaned against the wall of the Teflon vessel. The hydrothermal synthesis was conducted at 140 °C for 3 h. The as-prepared films were annealed at 450 °C for 2 h. The conformal coating of the nano-TiO₂ buffer layer on WO₃ plates was carried out by the thermal hydrolysis of TiCl₄, which was described in the reported literature.⁴⁴ Mn-CdS QDs were deposited on the plate-like film by the CBD technique. Firstly, the substrates with WO₃ plate-like films were immersed in an ethanol solution containing 5 mM Cd(NO₃)₂ and 3.75 mM manganese acetate for 5 min for the adsorption of Cd²⁺ and Mn²⁺ on the WO₃ plates surface and then thoroughly washed with anhydrous ethanol. Then, they were dipped in a 5 mM methanol solution of Na₂S for 5 min and washed with methanol at least three times. This two-step dipping procedure was repeated ten times. The as-prepared films were calcined at 200 °C for 1 h.

Scanning electron microscopy (SEM, Nova NanoSEM 230) was utilized to observe the morphology and size of the products. High-resolution transmission electron microscopy (HR-TEM, TECNAI G2 F20, FEI) was operated at 200 kV to observe the crystallinity and distribution of WO₃, TiO₂ and Mn-CdS. The crystalline phases of the synthesized products were characterized by an X-ray diffractometer (XRD, D/Max2250, Rigaku Corporation, Japan). The surface electronic states of the films were analyzed using X-ray photoelectron spectroscopy (XPS, K-Alpha 1063, Thermo Fisher scientific). A spectrophotometer (DR-UVS, Shimadzu 2450 spectrophotometer) was used to obtain the UV-vis spectra in the 300–800 nm range.

2.2 Electrical and photoelectrochemical measurements

The photoelectrochemical response of the samples was recorded with an electrochemical analyzer (Zennium, Zahner, Germany) using a typical three-electrode electrochemical cell. An Ag/AgCl/satd. KCl electrode and a platinum foil were employed as the reference electrode and the counter electrode, respectively. An aqueous solution containing 0.35 M Na₂SO₃ and 0.25 M Na₂S (pH ≈ 11.5) was used as electrolyte and sacrificial reagent to maintain the stability of CdS QDs, respectively. A 150 W Xe lamp (CHF-XM35, Beijing Trusttech Co. Ltd) with an ultraviolet filter (λ > 400 nm) to remove UV irradiation was used as the visible light source and positioned 10 cm away from the photoelectrochemical cell. The light intensity was adjusted to 100 mW cm⁻². Intensity modulated photocurrent spectroscopy (IMPS) measurements were obtained using a Zahner CIMPS-2 system. A white light lamp emitting diode (λ = 520 nm) driven by a PP210 was used as a lamp.

3. Results and discussion

3.1 XRD, SEM, EDS, HR-TEM and XPS analysis

The crystal structures of the sample electrodes were examined by XRD analysis. Fig. 1 shows the XRD patterns of WO₃ film, TiO₂/WO₃ film, CdS QDs/TiO₂/WO₃ film and Mn-CdS QDs/TiO₂/WO₃ film. The patterns of all the films present the same peaks, corresponding to monoclinic WO₃ (JCPDS no. 83-0950). Unfortunately, there are no other obvious peaks in the patterns, which is likely due to the low TiO₂, CdS and Mn-CdS content. The morphologies of the sample films were examined by field emission scanning electron microscopy (FESEM). Fig. 2 displays the typical top-view FESEM images of the WO₃ plate-like film at low and high magnification. It can be seen that the high density and uniform vertical alignment of WO₃ plates are grown on a FTO substrate. All the WO₃ plates exhibit an edge length in the range of 0.5–1.5 μm and a thickness of 50–200 nm. As compared to pristine WO₃ plates with a smooth surface, the TiO₂/WO₃ plates exhibit a rough surface (Fig. 2a and b). This is because a number of nano-TiO₂ flakes and dots were aggregated to form a thin film on the surface of the plates after TiCl₄ treatment. After loading CdS QDs and Mn-CdS QDs, the number of dots obviously increased, which can be observed in Fig. 2c and d. The TEM image of a Mn-CdS QDs/TiO₂/WO₃ plate in Fig. 3a shows that the Mn-CdS QDs are uniformly distributed on the entire TiO₂/WO₃ plate surface to form a high-quality WO₃/Mn-CdS core–shell nanoplate. The spatial chemical compositions of Mn-CdS QDs/TiO₂/WO₃ plate were identified by the EDS spectrum, which shows the existence of Mn, Cd, S, and Ti elements on the WO₃ plates, as shown in Fig. 3b. The composition of the Mn-CdS QDs/TiO₂/WO₃ film was further detected by XPS, as shown in Fig. 4. The survey XPS spectrum is shown in Fig. 4a, which contains the peaks of W_4f, S_2p, Cd_3d, Ti_2p and O_1s. The peaks centered at 41.3 eV, 37.6 eV and 35.5 eV are attributed to W⁶⁺ (Fig. 4b), and the peaks at 464.3 eV and 458.6 eV are attributed to Ti_2p (Fig. 4e). In addition, the peaks at 405.3 eV and 411.9 eV correspond to Cd_3d and at 161.7 eV and 162.5 eV correspond to S_2p (Fig. 4c and d). Importantly, the Mn_2p peak can be observed at 641.5 eV (Fig. 4f), which is in good agreement with the reported study.⁴⁵ On the basis of the abovementioned results, it can be confirmed that Mn has been successfully doped into the CdS QDs.

Fig. 1 XRD patterns of the film samples.

Fig. 2 SEM images of (a) WO₃ film, (b) TiO₂/WO₃ film, (c) CdS QDs/TiO₂/WO₃ film and (d) Mn-CdS QDs/TiO₂/WO₃ film.

Fig. 3 (a) TEM images and (b) EDS spectra of Mn-CdS QDs/TiO₂/WO₃ film.

Fig. 4 XPS spectra analysis of Mn-CdS QDs/TiO₂/WO₃ film, (a) wide scan for Mn-CdS QDs/TiO₂/WO₃ film, narrow scan for (b) W element, (c) Cd element, (d) S element, (e) Ti element (f) Mn element.

3.2 UV-visible optical absorption

Fig. 5 illustrates the UV-visible optical absorption spectra of the TiO₂/WO₃, QDs CdS/TiO₂/WO₃ and Mn-QDs CdS/TiO₂/WO₃ film. The spectrum obtained from the TiO₂/WO₃ film shows that the adsorption edge of the TiO₂/WO₃ film is located at ∼455 nm. By the in situ growth of CdS QDs in the WO₃ plate-like film, the absorption spectrum was extended into the visible light region with a band edge of more than 530 nm due to the CdS QDs absorption. Relative to the absorption edge of the QDs CdS/TiO₂/WO₃ film, the absorption edge of Mn-QDs CdS/TiO₂/WO₃ film shows a slight red-shift and a little enhancement in the visible light region, which is consistent with what is reported in previous study.⁴⁶ The images of the sample films shown in Fig. S1† also support the abovementioned result. There is no obvious difference in color between the pristine WO₃ film and the TiO₂/WO₃ film. Compared with the former two samples, the color of the WO₃ films sensitized by QDs evidently changed to orange. The Mn-QDs CdS/TiO₂/WO₃ film is slightly darker than the QDs CdS/TiO₂/WO₃ film.

Fig. 5 UV-visible optical absorption spectral of the film samples.

3.3 Photoelectrochemical study

To confirm the potential advantages of the Mn-CdS QDs/TiO₂/WO₃ film as a photoanode, four types of photoanodes with Mn-CdS QDs/TiO₂/WO₃, CdS QDs/TiO₂/WO₃, TiO₂/WO₃ and WO₃ were fabricated on FTO glass for comparison. The linear sweep voltammograms of the four photoanodes are compared in Fig. 6a. These measurements were carried out in dark and under illumination of AM 1.5 (with UV cutoff) at 100 mW cm⁻². Under light illumination, the TiO₂/WO₃ and WO₃ photoanodes show negligible photocurrent generation, while Mn-CdS QDs/TiO₂/WO₃ and CdS QDs/TiO₂/WO₃ photoanodes exhibit significant photocurrent generation. This indicates that the large photocurrents can be attributed to electrons generated from CdS QDs. Among them, the maximum photocurrent density of 2.51 mA cm⁻² was obtained with Mn-CdS QDs/TiO₂/WO₃ photoanodes at 0 V vs. Ag/AgCl, which is about 10 times larger than that of the bare WO₃ photoanodes (0.24 mA cm⁻²) and about 7 times larger than that of the TiO₂/WO₃ photoanodes (0.34 mA cm⁻²). Furthermore, the Mn-CdS QDs/TiO₂/WO₃ photoanodes present better photoelectrochemical properties than those of CdS QDs/TiO₂/WO₃ photoanodes (1.40 mA cm⁻²).

Fig. 6 (a) The photocurrent density and (b) IPCE of the photoelectrodes.

The incident photon-to-current conversion efficiencies (IPCE) of the photoelectrodes were measured at an applied voltage of −0.4 V vs. Ag/AgCl, and the results are shown in Fig. 6b. As expected from the absorption edge of TiO₂/WO₃ films, the TiO₂/WO₃ electrodes show the photoresponse only at the wavelength of ∼452 nm with a lower IPCE value of ∼15%. Compared to TiO₂/WO₃ electrodes, the electrodes sensitized by QDs show substantially enhanced IPCE in a wide range (from 350 to 550 nm) due to the increased light absorption by the QDs. More importantly, the IPCE value of the Mn-CdS QDs/TiO₂/WO₃ electrodes is the highest, which is in accordance with the photocurrent results.

To better understand the enhanced PEC properties in the Mn-CdS QDs/TiO₂/WO₃ photoelectrodes, the electron transport properties and the efficient electron lifetime were assessed using electrochemical impedance spectroscopy (EIS). EIS is an effective method to investigate electrochemical behavior, especially for charge transfer processes. The EIS Nyquist plots are related to the charge transfer resistance and the separation efficiency of the photoelectron–hole pairs.⁴⁷ Fig. 7 shows the EIS Nyquist plots of CdS QDs/TiO₂/WO₃ and Mn-CdS QDs/TiO₂/WO₃ photoelectrodes at the applied potential of −0.2 V vs. Ag/AgCl under visible light illumination. The EIS Nyquist plots is fitted by the equivalent circuit (the inset in Fig. 7) composed of a charge transfer resistance (R₁), a chemical capacitance (CPE₁) and a series resistance (R_S). R₁ corresponds to the complex charge transfer resistance in the photoelectrode, and R_S represents the sheet resistance of the FTO glass, contact resistance, and wire resistance.⁴⁸ The electron lifetimes (τ_e) could be estimated using the following formula: τ_e = R₁CPE₁.²⁸ A larger τ_e means that the electrons have a longer lifetime and a faster diffusion rate in the electrode.⁴⁹ As listed in Table 1, the R_S values of the two samples are similar, while the R₁ values are very different. The R₁ value of the Mn-CdS QDs/TiO₂/WO₃ photoelectrode is only 1/2 of that of the CdS QDs/TiO₂/WO₃ photoelectrodes, implying that Mn-doped-CdS greatly promotes the charge transport and suppresses the electron–hole recombination, which is consistent with the improved PEC property. Moreover, the Mn-CdS QDs/TiO₂/WO₃ photoelectrodes (497.3 ms) have a 2-fold longer τ_e compared with CdS QDs/TiO₂/WO₃ (1101 ms). This is due to the existence of electronic states in the midgap region of the Mn-CdS QD, resulting in the long-lived charge carriers and the retarded charge pair recombination.⁴² It was also supported by the transient photocurrent plots (Fig. S2†), which indicates that the addition of Mn²⁺ into a crystal lattice of CdS QDs prolongs the lifetime of the photogenerated electron–hole pairs.

Fig. 7 Electrochemical impedance spectra of the photoelectrodes.

Table 1 Equivalent circuit parameters of EIS for CdS QDs/TiO₂/WO₃ and Mn-CdS QDs/TiO₂/WO₃ photoelectrodes

Samples	Rs (Ω cm^2)	R1 (Ω cm^2)	CPE1 (μF)	τe (ms)
CdS QDs/TiO2/WO3	34.45	1318	377.3	497.3
Mn-CdS QDs/TiO2/WO3	27.30	617.1	1784	1101

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In order to further characterize the electron transport properties and support the abovementioned ideas, intensity-modulated photocurrent spectroscopy (IMPS) measurements were employed. During the IMPS measurements, the photoelectrodes were biased at −0.2 V (vs. Ag/AgCl) and illuminated with a white light emitting diode (λ = 520 nm) at 2.19 mW cm⁻². The IMPS plots of the sample photoelectrodes are shown in Fig. 8. The electron transport time (τ_d) can be estimated from the IMPS plots, according to the following formula: τ_d = (2πf_min)⁻¹, where f_min is the characteristic minimum frequency of the imaginary component of IMPS.^50,51 The electron transport times calculated for Mn-CdS QDs/TiO₂/WO₃ and CdS QDs/TiO₂/WO₃ electrodes are 2.98 ms and 6.45 ms, respectively. This result suggests that the Mn-CdS QDs/TiO₂/WO₃ photoelectrodes exhibit a higher electron transit rate than CdS QDs/TiO₂/WO₃ photoelectrodes, which supports the EIS results very well.

Fig. 8 Complex plane plot of the IMPS response of the photoelectrodes.

The schematic for the Mn-CdS QDs/TiO₂/WO₃ electrode illustrates the electron transfer process, which can explain the possible mechanism for the enhanced photoelectrochemical properties under visible light irradiation (Fig. 9). The conduction band value of the CdS QDs is −0.4 eV, which is above that of TiO₂ (−0.1 eV) and WO₃ (+0.4 eV), according to previous works.⁵² In the CdS QDs/TiO₂/WO₃ system, when the electron–hole pairs are generated in CdS QDs under visible light illumination, the photoexcited electrons can be separated from the holes by transfer to TiO₂ and then to WO₃ along the potential gradient. In the Mn-CdS QDs/TiO₂/WO₃ system, the electronic states in the midgap region of the CdS QD can be created by Mn doping. The photoexcited electrons can easily migrate to the electronic states in the midgap region of the Mn-CdS QD, and then to TiO₂ and WO₃ (as shown in the schematic in Fig. 9).⁴² The electronic states in the midgap region of the Mn-CdS QD can be used as the buffer level state. This induces long-lived charge carriers and retards the recombination of charge pairs, thereby resulting in enhancing PEC performance.

Fig. 9 Sketch showing the architecture of the Mn-CdS QDs/TiO₂/WO₃ photoelectrodes and electron-transfer processes.

4. Conclusion

In summary, we have synthesized Mn-doped CdS quantum dots sensitized WO₃ photoelectrodes by a combination of hydrothermal and CBD methods. The results of EDS and XPS demonstrated that Mn was indeed incorporated into the crystal lattice of CdS. The SEM and TEM images showed that the Mn-CdS QDs were uniformly deposited on the surface of the TiO₂/WO₃ plates. The results show that the Mn-CdS QDs/TiO₂/WO₃ electrodes exhibit excellent PEC activities, which is attributed to the existence of electronic states in the midgap region of the Mn-CdS QD, resulting in the long-lived charge carriers and the retarded charge pair recombination.

Acknowledgements

This study was supported by the National Nature Science Foundation of China (no. 21171175), China Scholarship Council (CSC File no. 201406370157) and the Fundamental Research Funds for the Central Universities of Central South University (2014zzts015).

References

1. J. A. Turner, Science, 2004, 305, 972–974.
2. A. Fujishima and K. Honda, Nature, 1972, 238, 37–38.
3. A. Wolcott, W. A. Smith, T. R. Kuykendall, Y. Zhao and J. Z. Zhang, Adv. Funct. Mater., 2009, 19, 1849–1856.
4. H. Chen, Z. Wei, K. Yan, Y. Bai, Z. Zhu, T. Zhang and S. Yang, Small, 2014, 10, 4760–4769.
5. A. A. Tahir, K. G. U. Wijayantha, S. Saremi-Yarahmadi, M. Mazhar and V. McKee, Chem. Mater., 2009, 21, 3763–3772.
6. H. Dotan, K. Sivula, M. Grätzel, A. Rothschild and S. C. Warren, Energy Environ. Sci., 2011, 4, 958.
7. Y.-K. Hsu, C.-H. Yu, Y.-C. Chen and Y.-G. Lin, Electrochim. Acta, 2013, 105, 62–68.
8. Y.-K. Hsu, C.-H. Yu, Y.-C. Chen and Y.-G. Lin, J. Power Sources, 2013, 242, 541–547.
9. G. Hodes, D. Cahen and J. Manassen, Nature, 1976, 260, 312–313.
10. X. Liu, F. Wang and Q. Wang, Phys. Chem. Chem. Phys., 2012, 14, 7894–7911.
11. D. V. Esposito, J. G. Chen, R. W. Birkmire, Y. Chang and N. Gaillard, Int. J. Hydrogen Energy, 2011, 36, 9632–9644.
12. B. Cole, B. Marsen, E. Miller, Y. Yan, B. To, K. Jones and M. Al-Jassim, J. Phys. Chem. C, 2008, 112, 5213–5220.
13. Y. Sun, C. J. Murphy, K. R. Reyes-Gil, E. A. Reyes-Garcia, J. M. Thornton, N. A. Morris and D. Raftery, Int. J. Hydrogen Energy, 2009, 34, 8476–8484.
14. W. Li, J. Li, X. Wang and Q. Chen, Appl. Surf. Sci., 2012, 263, 157–162.
15. Y. C. Nah, I. Paramasivam, R. Hahn, N. K. Shrestha and P. Schmuki, Nanotechnology, 2010, 21, 105704.
16. B. Wang, J. Zhang, Y. Hu, S. Wang, R. Liu, C. He, X. Wang and H. Wang, Int. J. Electrochem. Sci., 2013, 8, 7175–7186.
17. Y. J. Hwang, A. Boukai and P. Yang, Nano Lett., 2008, 9, 410–415.
18. J. Schrier, D. O. Demchenko, L. W. Wang and A. P. Alivisatos, Nano Lett., 2007, 7, 2377–2382.
19. N. S. Lewis, Science, 2007, 315, 798–801.
20. J. Su, L. Guo, N. Bao and C. A. Grimes, Nano Lett., 2011, 11, 1928–1933.
21. K. Sivula, F. L. Formal and M. Grätzel, Chem. Mater., 2009, 21, 2862–2867.
22. J. Zhu, W. Li, J. Li, Y. Li, H. Hu and Y. Yang, Electrochim. Acta, 2013, 112, 191–198.
23. F. Zhang, W. Zhao and K. Zhang, React. Kinet., Mech. Catal., 2013, 108, 253–261.
24. H.-i. Kim, J. Kim, W. Kim and W. Choi, J. Phys. Chem. C, 2011, 115, 9797–9805.
25. Z.-G. Zhao, Z.-F. Liu and M. Miyauchi, Adv. Funct. Mater., 2010, 20, 4162–4167.
26. L. M. Peter, D. J. Riley, E. J. Tull and K. G. U. Wijayantha, Chem. Commun., 2002, 1030–1031.
27. G. Wang, X. Yang, F. Qian, J. Z. Zhang and Y. Li, Nano Lett., 2010, 10, 1088–1092.
28. H. Chen, L. Zhu, H. Liu and W. Li, Electrochim. Acta, 2013, 105, 289–298.
29. H. Chen, L. Zhu, Q. Hou, W. Liang, H. Liu and W. Li, J. Mater. Chem., 2012, 22, 23344–23347.
30. J. Deng, M. Wang, X. Song, Y. Shi and X. Zhang, J. Colloid Interface Sci., 2012, 388, 118–122.
31. Y. Hu, B. Wang, J. Zhang, T. Wang, R. Liu, J. Zhang, X. Wang and H. Wang, Nanoscale Res. Lett., 2013, 8, 1–5.
32. F. Su, J. Lu, Y. Tian, X. Ma and J. Gong, Phys. Chem. Chem. Phys., 2013, 15, 12026–12032.
33. H. Chen, W. Li, H. Liu and L. Zhu, Sol. Energy, 2010, 84, 1201–1207.
34. M. Thambidurai, N. Muthukumarasamy, N. Sabari Arul, S. Agilan and R. Balasundaraprabhu, J. Nanopart. Res., 2011, 13, 3267–3273.
35. J. Jiang, M. Wang, R. Li, L. Ma and L. Guo, Int. J. Hydrogen Energy, 2013, 38, 13069–13076.
36. C. Liu, Y. Li, W. Li, J. Zhu, J. Li, Q. Chen and Y. Yang, Mater. Lett., 2014, 120, 170–173.
37. S. Buhbut, S. Itzhakov, E. Tauber, M. Shalom, I. Hod, T. Geiger, Y. Garini, D. Oron and A. Zaban, ACS Nano, 2010, 4, 1293–1298.
38. S. Buhbut, S. Itzhakov, D. Oron and A. Zaban, J. Phys. Chem. Lett., 2011, 2, 1917–1924.
39. B. Wang, H. Ding, Y. Hu, H. Zhou, S. Wang, T. Wang, R. Liu, J. Zhang, X. Wang and H. Wang, Int. J. Hydrogen Energy, 2013, 38, 16733–16739.
40. H. J. Lee, P. Chen, S. J. Moon, F. Sauvage, K. Sivula, T. Bessho, D. R. Gamelin, P. Comte, S. M. Zakeeruddin, S. I. Seok, M. Gratzel and M. K. Nazeeruddin, Langmuir, 2009, 25, 7602–7608.
41. I. Hwang, M. Seol, H. Kim and K. Yong, Appl. Phys. Lett., 2013, 103, 023902.
42. P. K. Santra and P. V. Kamat, J. Am. Chem. Soc., 2012, 134, 2508–2511.
43. J. Yang, W. Li, J. Li, D. Sun and Q. Chen, J. Mater. Chem., 2012, 22, 17744.
44. H. Zheng, Y. Tachibana and K. Kalantar-zadeh, Langmuir, 2010, 26, 19148–19152.
45. D. S. Kim, Y. J. Cho, J. Park, J. Yoon, Y. Jo and M.-H. Jung, J. Phys. Chem. C, 2007, 111, 10861–10868.
46. J. Luo, H. Wei, Q. Huang, X. Hu, H. Zhao, R. Yu, D. Li, Y. Luo and Q. Meng, Chem. Commun., 2013, 49, 3881–3883.
47. L. Wu, J. Li, S. Zhang, L. Long, X. Li and C. Cen, J. Phys. Chem. C, 2013, 117, 22591–22597.
48. X. Miao, K. Pan, Y. Liao, W. Zhou, Q. Pan, G. Tian and G. Wang, J. Mater. Chem. A, 2013, 1, 9853–9861.
49. C. Zha, L. Shen, X. Zhang, Y. Wang, B. A. Korgel, A. Gupta and N. Bao, ACS Appl. Mater. Interfaces, 2013, 6, 122–129.
50. H. Chen, Z. Wei, K. Yan, Y. Bai and S. Yang, J. Phys. Chem. Lett., 2014, 5, 2890–2896.
51. J. Su, L. Guo, N. Bao and C. A. Grimes, Nano Lett., 2011, 11, 1928–1933.
52. D. Robert, Catal. Today, 2007, 122, 20–26.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra01787d

‡ Contributed equally to this work.

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