Controllable synthesis of CdS/WO₃ heterostructures with enhanced photoelectrochemical performance

Abstract

With the increasing energy and environmental problems, photoelectrochemical (PEC) water splitting has recently attracted a great deal of attention. Herein, we report the facile synthesis of CdS/WO₃ heterostructures as high-performance photoanodes for PEC water splitting. CdS/WO₃ heterostructures are successfully prepared on F doped SnO₂ glass substrates by a two-step process, which involves a hydrothermal growth and electrochemical deposition process. In comparision to pristine WO₃ nanostructures, CdS/WO₃ heterostructures exhibit substantially high harvesting ability. The CdS/WO₃ heterostructures yield a significant photocurrent density of 1.8 mA cm⁻² at 0 V vs. Ag/AgCl, which is substantially higher than that of bare WO₃ and CdS electrodes. These findings will open up new opportunities to develop other efficient solar-light-driven photocatalytic cells.

---

Introduction

As the energy crisis and environmental pollution are getting more and more serious, hydrogen has attracted an enormous amount of research attention as a clean energy source because its product of combustion is just water and it does not bring about any greenhouse gases.^1–3 Among the various methods to produce hydrogen, one of the most promising and environmentally friendly ways is photoelectrochemical (PEC) water splitting over semiconductors.^4–9 Tungsten trioxide (WO₃), with a band gap of around 2.6 eV, is of great interest due to its excellent intrinsic stability, environmental compatibility and excellent solar-to-chemical performance under visible light irradiation.^10–15 However, some key drawbacks of WO₃ photoanodes, such as relatively high recombination of the photo-generated electron–hole pairs and low absorption coefficient of visible light, have severely limited their practical applications.^10,16,17 Several strategies have been explored to reduce electron–hole recombination in WO₃, including nanostructuring and doping with transition elements.^{11,12,16,18,19} However, the efficiency is still not satisfactory and it is urgently required to find suitable approaches to enhance the PEC performance of WO₃ photoanodes.

Sulfides are emerging as attractive visible-light-driven photocatalysts since they have narrow band gaps with suitable conduction bands at relatively high potentials.^20–24 Among them, CdS is one of the most attractive II–VI group semiconductors with a direct band gap of 2.4 eV, which has been extensively studied in photocatalytic hydrogen production and the photo-degradation due to its strong ability for harvesting visible light.^23,25–29 Considerable efforts have been devoted to enhance the photo–catalytic activity of WO₃ by combining it with the CdS, and some achievements have been successfully made.^30–33 However, there are few reports about the PEC performance of the CdS/WO₃ heterostructures. Thus, it is highly desirable to investigate the properties of CdS/WO₃ heterostructures for their PEC water splitting. Herein, we demonstrated the design and synthesis of CdS/WO₃ heterostructures on F doped SnO₂ (FTO) glass substrates, following by studying their PEC water splitting. The CdS/WO₃ heterostructures exhibited a significant photocurrent density of 1.8 mA cm⁻² at 0 V vs. Ag/AgCl with good stability. These findings will open up new opportunities to develop other efficient solar-light-driven photocatalytic cells.

Results and discussion

X-ray powder diffraction (XRD) analysis was conducted to identify the crystalline phase of the products. Fig. 1a shows the typical XRD spectra of the WO₃ and CdS/WO₃ samples. All of the reflections of the XRD pattern for WO₃ sample could be indexed to monoclinic WO₃ with a lattice constant a = 7.297 Å, b = 7.539 Å and c = 7.688 Å, compatible with JCPDS Card no. 43-1035. The diffraction peaks rose at diffraction angle of 23.1°, 23.6°, 24.4°, 33.3°, 34.2°, 55.6° and 56.0° corresponding to (002), (020), (200), (022), (202), (140) and (420) planes were observed, indicating good crystallinity of the as-prepared WO₃ sample. The strong diffraction peaks of SnO₂ at 26.7°, 33.9°, 37.9° and 51.7° come from FTO substrate. No other obvious peaks were obtained, demonstrating that high purity of the WO₃. The SEM image of a typical WO₃ nanostructure is presented in Fig. 1b. Many nanosheets constituted with some small nanoparticles were observed. The width of the nanosheets was around 500 nm. After the electro-deposition at the potential of −1.0 V vs. Ag/AgCl for 300 s at room temperature, the CdS/WO₃ heterostructures were prepared. No characteristic peaks corresponding to CdS were observed, which may be due to the amorphous structures and/or the low intensity of the CdS. It is noteworthy that the width of the CdS/WO₃ heterostructures is also closed to 500 nm, suggesting that a thin CdS layer was constructed on the surface of WO₃ (Fig. 1c and d).

Fig. 1 (a) XRD patterns of pure WO₃ and CdS/WO₃ heterostructures. SEM images of: (b) pure WO₃, (c and d) CdS/WO₃ heterostructures.

In order to better understand the microstructure of CdS/WO₃ heterostructures, transmission electron microscopy (TEM) was conducted and displayed in Fig. 2. It shows that the WO₃ was covered by a shell of CdS. SAED analysis clearly demonstrate that both the CdS and WO₃ have high degree of crystallinity (Fig. 2b). Fig. 2c is the high-resolution TEM (HR-TEM) image taken from the rectangle part of the CdS/WO₃ (Fig. 2a). The lattice spacing of 0.37 nm and 0.38 nm match well with the (020) and (002) lattice spacing of monoclinic structure WO₃ (JCPDS Card no. 43-1035). In addition, the well-resolved lattice space of 0.24 nm corresponds to the interplanar spacing of (102) plane of hexagonal CdS (JCPDS Card no. 65-3414). Therefore, CdS/WO₃ heterostructures are successfully synthesized.

Fig. 2 (a) TEM image, (b) SAED pattern and (c) HR-TEM image of the CdS/WO₃ heterostructures.

As presented in Fig. 3a, Raman spectra of WO₃ sample showed well defined bands at 715 and 808 cm⁻¹ corresponding to the monoclinic phase of tungsten oxide. Both these two characteristic peaks originate from the O–W–O bending modes and stretching modes.³⁴ No obvious peaks can be assigned to CdS. The UV-vis absorption spectra of bare WO₃ and CdS/WO₃ heterostructures were measured in order to reveal the influence of the CdS. As shown in Fig. 3b, the WO₃ have a shape absorption edge around 450 nm, which is consistent with its band gap of 2.6 eV.¹⁶ On the other hand, it has a visible light absorption from 450 to 560 nm due to the strong absorption of CdS in the visible region. The absorption edge of the CdS/WO₃ heterostructures shifts to 580 nm due to the strong absorption of the CdS in the visible region. Hence, the surface of WO₃ nanostructures was successfully coated with a visible-light absorption material of CdS.

Fig. 3 (a) Raman and (b) UV-vis spectra of pure WO₃ and CdS/WO₃ heterostructures.

X-ray photoelectron spectroscopy (XPS) analysis was also conducted to study the composition of pure WO₃ and CdS/WO₃ heterostructures, which is presented in Fig. 4. Fig. 4a compares the XPS survey spectra of the pure WO₃ and CdS/WO₃ heterostructures. In comparison to pure WO₃, the XPS survey spectrum of CdS/WO₃ heterostructure shows additional peaks of S and Cd, which demonstrates the formation of CdS. Additionally, the atom ratio of Cd, S, W and O in these CdS/WO₃ heterostructures is about 4.51%, 3.74%, 13.13% and 37.94%, respectively. The ratio of W to O is closed to the chemical formula of WO₃, while the ratio of Cd to S is higher than 1, suggesting that Cd is slightly rich than S in our sample. Fig. 4b displays the W 3d core level XPS spectra. After coating with CdS, the intensity of the W was decreased, indicating the CdS is generated at the surface of the WO₃ nanostructures. The Cd 3d core level XPS spectrum of CdS/WO₃ sample is showed in Fig. 4c. Two obvious peaks located at 404.9 and 411.7 eV are observed, which are consistent with the reported values for Cd²⁺.³⁵ As shown in Fig. 4d, the S 2p core level XPS spectrum can be deconvolved to two peaks. The peaks centered at 161.3 and 162.4 eV are attributed to the S 2p_3/2 and S 2p_2/1 of S²⁻.³⁶ All these results clearly confirm that the successful preparation of the CdS/WO₃ heterostructures, which consists with the XRD and TEM analyses.

Fig. 4 (a) XPS survey spectrum, (b) W 4f, (c) Cd 3d, (c) O 1s, and (d) S 2p core level XPS spectrum of the pure WO₃ and CdS/WO₃ heterostructures.

Because the nanostructure can provide high pathway for the electron transport, well-aligned nanostructure with narrow band gap is very promising and efficient for the conversion of solar energy.⁸ Therefore, the as-prepared CdS/WO₃ heterostructures are used as photoanodes in a PEC cell. For better comparison, pristine CdS electrode was prepared by directly using FTO as substrate at the same synthetic condition with CdS/WO₃ electrode. The PEC activities of bare WO₃, CdS and CdS/WO₃ photoelectrodes were firstly characterized by measuring the linear sweep voltammogram (LSW) curves with or without light irradiation. As presented in Fig. 5a, the scans for all the samples collected in the potential range from −1.0 V to 0.2 V vs. Ag/AgCl showed a negligible background current without light irradiation. In contrast, under the light illumination, all the samples showed pronounced photocurrent. It is noteworthy that the CdS/WO₃ electrode achieved a remarkable photocurrent density of 1.8 mA cm⁻² at the potential of 0 V vs. Ag/AgCl, which is substantially higher than that of bare WO₃ (0.8 mA cm⁻²) and CdS (0.6 mA cm⁻²) electrodes at the same potential. Such the enhancement of photocurrent for the CdS/WO₃ heterostructures can be attributed to the improvement of light-harvesting (Fig. 3b).

Fig. 5 (a) LSV curves collected at 50 mV s⁻¹ under dark and light irradiation and (b) IPCE curves at the incident wavelength ranging from 300 to 600 nm for the WO₃, CdS and CdS/WO₃ samples in a solution of 0.1 M Na₂S and 0.1 Na₂SO₃. (c) Photocurrent density of the CdS, WO₃ and CdS@WO₃ photoanodes as a function of irradiation time collected at 0 V vs. Ag/AgCl. Inset is its corresponding temporal photocurrent responses at 0 V vs. Ag/AgCl. (d) Nyquist plots of the WO₃, CdS and CdS/WO₃ electrodes under light irradiation.

The incident photon-to-current conversion efficiency (IPCE) of the samples was measured in the solution of 0.1 M Na₂S and 0.1 M Na₂SO₃ at −0.2 V vs. Ag/AgCl in order to quantitatively investigate the relation between the light absorption and the PEC performance. As shown in Fig. 5b, the maximum IPCE value of the bare WO₃ electrode is approximately 23% at the wavelength of 340 nm and then it gradually decrease ranging from 350 to 450 nm and to almost zero at λ > 500 nm last, indicating that the band gap of WO₃ is ca. 2.6 eV. As for the bare CdS electrode, its maximum IPCE is about 19% and it showed a higher IPCE value than that of the WO₃ electrode at the wavelength of 380–550 nm, revealing the superior photoactivity of CdS under visible light. Significantly, the IPCE value of the CdS/WO₃ heterostructures was up to ca. 67%, much higher than the WO₃ and CdS samples. It also shows remarkable photo response of 23% at 450 nm and gradually decreases to zero at 550 nm. All these results above fully confirm that the synergistic effect of CdS layer and WO₃ nanostructures can significantly enhance the photoelectrochemical activity of the CdS/WO₃ heterostructures.

Beside photocurrent, stability is another one of important parameters to evaluate the performance of the photoanode. The current–time (i–t) chronoamperometric response of the WO₃, CdS and CdS/WO₃ photoanodes at a potential of 0 V vs. Ag/AgCl were collected in Fig. 5c. Obviously, the CdS/WO₃ electrode exhibited remarkably higher photocurrent than that of the bare WO₃ and CdS electrodes during the long-term test, again showing the outstanding photoelectrochemical performance of CdS/WO₃ heterostructures. The bare CdS electrode remained only 69.8% of its initial photocurrent after 60 min under light irradiation. In contrast, the CdS/WO₃ electrode showed enhanced stability that can retain more than 80.1% of its initial photocurrent density. This indicates that the construction of heterostructures can improve the stability of CdS. On the other hand, we also collected the temporal photocurrent response of all photoanodes at the potential of 0 V vs. Ag/AgCl. It can be seem from the inset in Fig. 5c that the current of the CdS/WO₃ electrode suffered from a relatively sharp decrease at the initial short time and then retained stable. This temporal decrease might be due to the recombination of photo-generated charge carries by the surface (or defect) states of the CdS/WO₃ heterostructures.³⁷

Electrochemical impedance spectroscopy (EIS) spectra of the WO₃, CdS and CdS/WO₃ electrodes under light irradiation were collected to further understand the charge-transfer and separation properties of the hetero-structures, as shown in Fig. 5d. The radius of the arc is correlated with the charge-transfer ability at the corresponding electrode/electrolyte interface. A smaller radius correlated with a lower charge-transfer resistance and an effective separation of photo-generated electron–hole pairs.³⁹ The arc radius of CdS/WO₃ heterostructures is obviously reduced compared to those of both the CdS and WO₃ under light irradiation, which means an effective interfacial charge transfer and charge separation.

Conclusions

In summary, we have designed and synthesized the CdS/WO₃ heterostructures for the PEC water splitting. The images of SEM and TEM indicated that the CdS/WO₃ heterostructures were successfully prepared, and the UV-vis absorption spectra showed an obvious red-shift of the absorption edge. Under the light illumination, the CdS/WO₃ heterostructures achieved a significant photocurrent density of 1.8 mA cm⁻² at the potential 0 V vs. Ag/AgCl, which is much larger than that of the bare WO₃ and CdS electrodes at the same potential. The IPCE spectra also show the enhanced photo-response in the visible region. These results suggest that the heterostructures can greatly enhance the photoactivity of the WO₃ nanostructures. This achievement makes it possible to design composite photoelectrodes to more effectively utilize the solar spectrum.

Experimental

Preparation of WO₃ nanostructures

All the reagents used in this study were of analytical grade and were used directly without any purification. The FTO glass was cleaned ultrasonically in ethanol, distilled water, and acetone and finally rinsed in ethanol. The preparation of the WO₃ nanostructures can be found in our previous work.³⁸ In general, 1.8 g H₂WO₄ was dissolved into 35 mL H₂O and 5 mL H₂O₂ (30%) under strong stirring for 4 h. Then, the clear solution was heated at 95 °C with stirring, and further diluted into 40 mL with deionized water. Then, around 4 mL of the precursor was mixed with 16 mL of ethanol and NH₄Cl, and transferred to Teflon-lined stainless steel autoclave. A piece of FTO-glass substrate was sealed into the solution and the autoclave maintained at 180 °C for 10 h. After reaction, the autoclave cooled down naturally to room temperature, and the FTO were washed with deionized water and ethanol for 3 times to remove ions that possibly remained at the final products, following by dried at 60 °C for 12 h.

Synthesis of the CdS quantum dots (CdS QDs)

Cadmium acetate and thioacetamide were dissolved separately at a concentration of 10 mM in deionized water at room temperature as precursor solutions. CdS QDs was obtained by mixing precursors with the same volume and aging 30 min before electrochemical deposition.

Synthesis of the CdS/WO₃ heterostructures

The CdS/WO₃ heterostructures were prepared by a facile electrochemical deposition. In a typical synthesis route, chronopotentiometry was carried out in a conventional three-electrode cell with a Pt wire as the counter electrode and a saturated calomel electrode (SCE) serving as the reference electrode. The working electrode was the WO₃ nanostructures on the FTO substrate. The deposition was carried out by applying the potential of −1.0 V vs. Ag/AgCl for 300 s at room temperature. Pristine CdS/FTO electrode was also prepared by using FTO as substrate at the same synthetic condition.

Materials characterizations

The surface morphology, microstructure and the composition of the samples were analyzed by scanning electron microscope (SEM, Quanta 400), transmission electron microscopy (TEM, JEM2010-HR), X-ray diffraction (XRD, D8 ADVANCE) with Cu Kα radiation (λ = 1.5418 Å), and X-ray Photoelectron Spectroscopy (XPS, ESCALab250) and the Raman spectroscopy (Raman, RenishawinVia). The optical properties of the samples were measured with a UV-vis-NIR Spectrophotometer (UV, Shimadzu UV-3150).

Photoelectrochemical characterizations

PEC measurements were carried out in a home-made three-electrode cell with a quartz window to facilitate illumination in a mixed solution of 0.1 M Na₂S and 0.1 Na₂SO₃. The working electrode is the as-prepared samples, while a Pt wire and a saturated Ag/AgCl electrode was used as the counter electrode and reference electrode, respectively. All photoelectrochemical experiments were measured by a CHI 760D electrochemical station. The illumination source was a 150 W xenon lamp (Newport 69907) with intensity of about 100 mW cm⁻². The incident-photon-to-current-conversion efficiencies (IPCE) were measured with a CHI 760D electrochemical station with a solar simulator (Newport 69911, 300 W xenon lamp) coupled with an aligned monochromator (Oriel Cornerstone 260 1/4 m).

Acknowledgements

This work was supported by the Science and technology project of Guangdong Province (2015A090905013), Guangzhou Yuexiu District Science and Technology Projects Funds (2014-WS-029), Science and technology project of self financing in Guangdong Province.

Notes and references

1. X. Lu, S. Xie, H. Yang, Y. Tong and H. Ji, Chem. Soc. Rev., 2014, 43, 7581–7593.
2. M. Li, X. He, Y. Zeng, M. Chen, Z. Zhang, H. Yang, P. Fang, X. Lu and Y. Tong, Chem. Sci., 2015, 6, 6799–6805.
3. C. Yao, B. Wei, H. Ma, L. Meng, X. Zhang and Q. Gong, J. Power Sources, 2013, 237, 295–299.
4. G. Wang, Y. Ling, H. Wang, L. Xihong and Y. Li, J. Photochem. Photobiol., C, 2014, 19, 35–51.
5. S. Xie, T. Zhai, W. Li, M. Yu, C. Liang, J. Gan, X. Lu and Y. Tong, Green Chem., 2013, 15, 2434–2440.
6. J.-J. Park, D.-Y. Kim, J.-G. Lee, Y.-H. Cha, M. T. Swihart and S. S. Yoon, RSC Adv., 2014, 4, 8661–8670.
7. G. Wang, H. Wang, Y. Ling, Y. Tang, X. Yang, R. C. Fitzmorris, C. Wang, J. Z. Zhang and Y. Li, Nano Lett., 2011, 11, 3026–3033.
8. S. Xie, M. Li, W. Wei, T. Zhai, P. Fang, R. Qiu, X. Lu and Y. Tong, Nano Energy, 2014, 10, 313–321.
9. H. Li, J. Yang, J. Zhang and M. Zhou, RSC Adv., 2012, 2, 6258–6261.
10. S. S. Kalanur, Y. J. Hwang, S. Y. Chae and O. S. Joo, J. Mater. Chem. A, 2013, 1, 3479–3488.
11. C. Santato, M. Odziemkowski, M. Ulmann and J. Augustynski, J. Am. Chem. Soc., 2001, 123, 10639–10649.
12. A. Tanaka, K. Hashimoto and H. Kominami, J. Am. Chem. Soc., 2013, 136, 586–589.
13. G. Hodes, D. Cahen and J. Manassen, Nature, 1976, 260, 312–313.
14. J. Su, L. Guo, N. Bao and C. A. Grimes, Nano Lett., 2011, 11, 1928–1933.
15. J. Lin, P. Hu, Y. Zhang, M. Fan, Z. He, C. K. Ngaw, J. S. C. Loo, D. Liao and T. T. Y. Tan, RSC Adv., 2013, 3, 9330–9336.
16. G. Wang, Y. Ling, H. Wang, X. Yang, C. Wang, J. Z. Zhang and Y. Li, Energy Environ. Sci., 2012, 5, 6180–6187.
17. W. Li, J. Li, X. Wang, J. Ma and Q. Chen, Int. J. Hydrogen Energy, 2010, 35, 13137–13145.
18. F. Zhan, R. Xie, W. Li, J. Li, Y. Yang, Y. Li and Q. Chen, RSC Adv., 2015, 5, 69753–69760.
19. B. Cole, B. Marsen, E. Miller, Y. Yan, B. To, K. Jones and M. Al-Jassim, J. Phys. Chem. C, 2008, 112, 5213–5220.
20. W. Li, S. Xie, M. Li, X. Ouyang, G. Cui, X. Lu and Y. Tong, J. Mater. Chem. A, 2013, 1, 4190–4193.
21. G. Wang, X. Yang, F. Qian, J. Z. Zhang and Y. Li, Nano Lett., 2010, 10, 1088–1092.
22. S. Yang, X. Wen, W. Zhang and S. Yang, J. Electrochem. Soc., 2005, 152, G220–G226.
23. S. Xie, X. Lu, T. Zhai, J. Gan, W. Li, M. Xu, M. Yu, Y.-M. Zhang and Y. Tong, Langmuir, 2012, 28, 10558–10564.
24. H. Li, C. Chen, X. Huang, Y. Leng, M. Hou, X. Xiao, J. Bao, J. You, W. Zhang, Y. Wang, J. Song, Y. Wang, Q. Liu and G. A. Hope, J. Power Sources, 2014, 247, 915–919.
25. W. Li, M. Li, S. Xie, T. Zhai, M. Yu, C. Liang, X. Ouyang, X. Lu, H. Li and Y. Tong, CrystEngComm, 2013, 15, 4212–4216.
26. W. Kim, M. Seol, H. Kim, J. B. Miller, A. J. Gellman and K. Yong, J. Mater. Chem. A, 2013, 1, 9587–9589.
27. W.-T. Sun, Y. Yu, H.-Y. Pan, X.-F. Gao, Q. Chen and L.-M. Peng, J. Am. Chem. Soc., 2008, 130, 1124–1125.
28. H. Tada, T. Mitsui, T. Kiyonaga, T. Akita and K. Tanaka, Nat. Mater., 2006, 5, 782–786.
29. H. Li, C. Yao, L. Meng, H. Sun, J. Huang and Q. Gong, Electrochim. Acta, 2013, 108, 45–50.
30. Z.-G. Zhao, Z.-F. Liu and M. Miyauchi, Adv. Funct. Mater., 2010, 20, 4162–4167.
31. H.-I. Kim, J. Kim, W. Kim and W. Choi, J. Phys. Chem. C, 2011, 115, 9797–9805.
32. H. Kim, Y. Tak, K. Senthil, J. Joo, S. Jeon and K. Yong, J. Vac. Sci. Technol., B: Microelectron. Nanometer Struct.–Process., Meas., Phenom., 2009, 27, 2182–2186.
33. H. Li, Y. Zhou, L. Chen, W. Luo, Q. Xu, X. Wang, M. Xiao and Z. Zou, Nanoscale, 2013, 5, 11933–11939.
34. M. Boulova, N. Rosman, P. Bouvier and G. Lucazeau, J. Phys.: Condens. Matter, 2002, 14, 5849.
35. C. Wang, L. Sun, H. Yun, J. Li, Y. Lai and C. Lin, Nanotechnology, 2009, 20, 295601.
36. C. Z. Yao, B. H. Wei, H. X. Ma, H. Li, L. X. Meng, X. S. Zhang and Q. J. Gong, J. Power Sources, 2013, 237, 295–299.
37. Y. Mao, J. He, X. Sun, W. Li, X. Lu, J. Gan, Z. Liu, L. Gong, J. Chen, P. Liu and Y. Tong, Electrochim. Acta, 2012, 62, 1–7.
38. Y. Liu, S. Xie, C. Liu, J. Li, X. Lu and Y. Tong, J. Power Sources, 2014, 269, 98–103.
39. H. Li, Y. Zhou, L. Chen, W. Luo, Q. Xu, X. Wang, M. Xiao and Z. Zou, Nanoscale, 2013, 5, 11933–11939.

---