A g-C₃N₄/WO₃ photoanode with exceptional ability for photoelectrochemical water splitting

Abstract

Herein, we report a photoanode of g-C₃N₄/WO₃ heterojunctions with exceptional ability and stability for photoelectrochemical (PEC) water splitting which achieved a high photocurrent density of 1.92 mA cm⁻² at +1.23 V versus (vs.) RHE which is about 2 times higher than that of the pristine WO₃ photoanode (0.71 mA cm⁻²).

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With the increasingly serious environmental pollution and the exhaustion of fossil energy resources, developing clean and reproducible energy to satisfy the growing global demand for energy has become increasingly inevitable and imperative.^1–4 Under such a circumstance, hydrogen is believed to be the most promising clean energy and photoelectrochemical (PEC) water splitting is one of the most important pathways to produce hydrogen from water.^5–7 Among the numerous photoanode materials, n-type semiconductors, WO₃ has been widely studied due to its high chemical stability, environmental friendliness, low cost and natural abundance.^8–10 However, WO₃ suffers from low solar light absorption due to its large bandgap (E_g 2.5–2.8 eV) and relatively fast electron–hole recombination, which limit its photoconversion efficiency.^11,12

To overcome these drawbacks, several strategies have been attempted, which include: (i) impurity element doping,^13–15 (ii) surface passivation^16,17 and (iii) semiconductor heterojunctions.^18–20 What is noteworthy is that the semiconductor-based heterostructure construction, which can tremendously improve the PEC performance by enhancing the absorption of solar radiation and improving the separation of photo-generated carriers, has attracted widespread attention in recent years. For example, several reports have demonstrated that WO₃/BiVO₄ heterojunction films can enhance the charge separation thus improving the overall photocurrent conversion efficiency,^21–23 and Sivula et al. reported that depositing Fe₂O₃ on WO₃ can obviously improve the PEC performance due to an increase in the absorbed photon conversion efficiency.²⁴ Hence, developing WO₃ with other semiconductors as a photoanode for PEC water splitting is highly desirable.

The graphite-like carbon nitride (g-C₃N₄) is a novel and appealing metal-free photocatalyst with various outstanding physicochemical properties, such as excellent electrical conductivity, high chemical and thermal stability, nontoxicity, suitable band-edge position and well-matched band-structure.^25,26 In addition, owing to the suitable band-gap (E_g = 2.7 eV), g-C₃N₄ shows a good response to solar radiation whose wavelength can reach 460 nm. By virtue of the above-mentioned properties, g-C₃N₄ has recently received considerable attention and has been widely used as a photocatalyst, especially exhibiting brilliant photocatalytic performance for PEC water splitting using solar radiation.^27,28 Without doubt, g-C₃N₄ is an ideal semiconductor material which can be used to improve the PEC properties of WO₃. What is more satisfactory is that combining with WO₃ to form the heterostructure construction can also effectively overcome the drawbacks of g-C₃N₄ like the high combination rate of photogenerated charge, low specific surface area and absence of active sites.²⁹ For example, Zhan et al. synthesized g-C₃N₄/WO₃ heterojunction plate array films with enhanced PEC performance via a combination of hydrothermal and dipping–annealing methods, which achieved a maximum photocurrent density of 2.10 mA cm⁻² at +2.0 V (vs. RHE), and was almost 2-fold higher than that of the pure WO₃ film (0.78 mA cm⁻²) under illumination.³⁰ And Hou et al. reported the design and fabrication of hybrid three-dimensionally branched WO₃ nanosheet array and C₃N₄ nanosheet (3DB WO₃-NA/C₃N₄-NS) heterojunctions with higher photocurrent density for efficient PEC water oxidation, which can be ascribed to both the enhanced light harvesting from the “window effect”, and the efficient charge separation at the WO₃-NA/C₃N₄-NS heterojunction.⁸

In this study, we reported the synthesis and characterization of the heterojunction of g-C₃N₄ nanosheet/WO₃ nanorod composites to enhance the PEC water splitting ability of the WO₃ photoanode.³¹ The photocurrent density of the g-C₃N₄-NS/WO₃-NR photoanode achieved 1.92 mA cm⁻² at +1.23 V versus (vs.) RHE, which is much higher than that of the pristine WO₃ photoanode (0.71 mA cm⁻² at +1.23 V vs. RHE). The photocurrent of the g-C₃N₄-NS/WO₃-NR photoanode only decreases to 1.85 mA cm⁻² within 400 min of illumination, indicating that the g-C₃N₄-NS/WO₃-NR photoanode is very stable during a long irradiation time at 1.23 V vs. RHE. The incident photon-to-electron conversion efficiency (IPCE) value of the g-C₃N₄-NS/WO₃-NR photoanode reaches 57.79% at 330 nm and 8.38% at 430 nm, which is much higher than that of the pristine WO₃-NR photoanode (43.76% at 330 nm and 2.26% at 430 nm). This work suggests that introducing g-C₃N₄ can effectively improve the PEC performance of WO₃ PEC photoanodes.

The g-C₃N₄-NS/WO₃ photoanode was prepared on FTO substrates via a two-step process, as schematically illustrated in Fig. 1 (Experimental section). These g-C₃N₄/WO₃ composites were characterized by X-ray diffraction (XRD). Fig. 1b shows the XRD patterns of the prepared samples in comparison to g-C₃N₄/WO₃ and WO₃. All the diffraction peaks can be readily indexed to WO₃. The XRD pattern of g-C₃N₄/WO₃ exhibits typical peaks corresponding to C₃N₄ and WO₃, indicating the formation of g-C₃N₄/WO₃ composites. Furthermore, the IR-FT studies also clearly show that the peaks in the range of 1250 cm⁻¹ to 1600 cm⁻¹ are detected for g-C₃N₄/WO₃, which belong to g-C₃N₄ (Fig. 1c). In order to investigate the chemical composition of products, X-ray photoelectron spectroscopy (XPS) analysis was performed. The XPS survey spectra recorded for the WO₃ NR and g-C₃N₄-NS/WO₃-NR samples are shown in Fig. 2d. Besides the Sn and Si signals originating from FTO and the C signals originating from adventitious carbon, only W, O and N are detected on the sample surface, confirming that the samples were pure. All the results above showed that g-C₃N₄/WO₃ composites were successfully fabricated on FTO.

Fig. 1 (a) Schematic illustration of the fabrication procedures for g-C₃N₄/WO₃ composites on FTO substrates. (b) XRD patterns of WO₃ and g-C₃N₄-NSs/WO₃-NRs. (c) FT-IR spectra of the WO₃, C₃N₄, g-C₃N₄-NS/WO₃-NR samples. (d) XPS survey spectra of the WO₃ and g-C₃N₄-NS/WO₃-NR samples.

Fig. 2 SEM images of (a) WO₃, and (b) g-C₃N₄/WO₃ (inset in (a and b) are the corresponding magnified images). (c) Low magnification TEM images of g-C₃N₄/WO₃. (d) High-resolution TEM image of g-C₃N₄/WO₃. (e–i) EDS elemental mapping of the same region, indicating spatial distribution of O, W, N, and C, respectively.

The morphology and crystalline properties of the as-prepared samples were also investigated by SEM and TEM. The morphology of bulk g-C₃N₄ powder was investigated by SEM, which appeared to have amorphous solid blocks or aggregated particles containing many smaller crystals (Fig. S2, ESI†). Fig. 2a is the SEM of WO₃ NRs consisting of a 2 µm length structure. After being impregnated in g-C₃N₄ solution, some changes in morphology and many nanosheets were observed on the surface of WO₃ NRs (Fig. 2b). As shown in the low magnification TEM images (Fig. 2c), the size of g-C₃N₄/WO₃ is about 2 µm, which is very close to that of WO₃-NRs and g-C₃N₄-NSs. Fig. 2d shows the high-resolution TEM image of g-C₃N₄/WO₃. Some clear lattice fringes of WO₃ nanocrystals are detected; by measuring these lattice fringes, the resolved interplanar distance is 0.33 nm, corresponding to the (101) plane of WO₃. Note that the g-C₃N₄-NSs are non-crystalline, which were on the surface of WO₃. In Fig. 2e–i, the signals of O, W, N and C elements are clearly observed, respectively, implying that g-C₃N₄-NSs covered the WO₃-NR surface uniformly and closely. These results suggest that the g-C₃N₄/WO₃ sample manifests itself as a composite of WO₃ and g-C₃N₄ nanostructures.

To study the effect of g-C₃N₄-NSs on WO₃-NRs for PEC water splitting, PEC measurements were performed on the pristine WO₃-NR photoanode and the g-C₃N₄-NS/WO₃-NR photoanode in a three-electrode electrochemical cell. The advantages of the g-C₃N₄-NSs grown on the WO₃ photoanode were demonstrated by linear sweep voltammogram (LSV) curves in 0.1 M KH₂PO₄ solution (pH = 7) at a scan rate of 25 mV s⁻¹ under AM 1.5 G illumination (Fig. 3a). The photocurrent density of the g-C₃N₄-NS/WO₃-NR photoanode achieved 1.92 mA cm⁻² at +1.23 V vs. RHE, which is much higher than that of the pristine WO₃ photoanode (0.71 mA cm⁻² at +1.23 V vs. RHE). And the effect of C₃N₄ loading amounts on the PEC performance was determined by impregnating WO₃·0.33H₂O into g-C₃N₄-NS dispersions for different time periods (0.25, 0.5, 1.0, 1.5, 2.0 h) before annealing, and their PEC performances are shown in Fig. S3 (ESI†). As the impregnation time was prolonged, the amount of g-C₃N₄ loading on WO₃ increased, which had a great effect on the PEC performance. When the impregnation time extended from 0 h to 1 h, the photocurrent density of g-C₃N₄/WO₃ increased from 0.71 mA cm⁻² to 1.92 mA cm⁻² at +1.23 V vs. RHE due to an increase in the amount of g-C₃N₄/WO₃ heterojunctions with increasing g-C₃N₄ loading amounts. However, the excessive g-C₃N₄ loading on the surface of WO₃ will hinder it from absorbing visible light, and thus the photocurrent density decreased after impregnating into g-C₃N₄-NS dispersions for more than 1 h.

Fig. 3 (a) Linear sweep voltammograms of bare WO₃ and g-C₃N₄-NSs/WO₃-NRs collected at a scan rate of 25 mV s⁻¹ at applied potentials from 0.5 to 1.5 V vs. RHE in 0.1 M KH₂PO₄ solution in the dark and under visible light. (b) The current–time (i–t) curves for the pristine WO₃-NR and g-C₃N₄-NS/WO₃-NR photoanodes are collected at 1.23 V vs. RHE. (c) IPCE spectra of bare WO₃ and g-C₃N₄-NS/WO₃-NR photoanodes measured at different bias (0.4 V and +0.4 V) vs. Ag/AgCl and the photo of reaction is inserted in (c).

Furthermore, the stability of the g-C₃N₄-NS/WO₃-NR photoanode was also evaluated and remarked. The current–time (i–t) curves for the pristine WO₃-NR and g-C₃N₄-NS/WO₃-NR photoanodes are collected at 1.23 V vs. RHE and shown in Fig. 3b. The photocurrent of the g-C₃N₄-NS/WO₃-NR photoanode only decreases to 1.85 mA cm⁻² within 400 min of illumination, indicating that the g-C₃N₄-NS/WO₃-NR photoanode is very stable during the long irradiation time at 1.23 V vs. RHE. This result clearly shows that the g-C₃N₄-NSs will greatly enhance the ability of WO₃-NRs for photoelectrochemical (PEC) water splitting by forming a g-C₃N₄-NS/WO₃-NR heterojunction.

IPCE measurements were further performed to investigate the PEC performances of the pristine WO₃-NR photoanode and the g-C₃N₄-NSs/WO₃-NRs photoanode and measured at +1.23 V vs. RHE in 0.1 M KH₂PO₄ solution (pH = 7). Fig. 3c shows the IPCE spectra of the pristine WO₃-NRs photoanode and the g-C₃N₄-NS/WO₃-NR photoanode, which are in accord with their current density–voltage (J–V) characteristics. The IPCE value of the g-C₃N₄-NS/WO₃-NR photoanode reaches 57.79% at 330 nm, which is much higher than that of the pristine WO₃-NR photoanode (43.76% at 330 nm). For visible light (>420 nm), the IPCE value of the g-C₃N₄-NS/WO₃-NR photoanode reaches 8.38% at 430 nm, which is much higher than that of the pristine WO₃-NR photoanode (2.26% at 430 nm). These results confirmed that the g-C₃N₄-NSs can improve the photoactive range of the WO₃-NRs and enhance the separation of electron–hole pairs. This brilliant photocatalytic activity and stability prove the g-C₃N₄/WO₃ photoanode to be a promising new kind of photoanode material for solar water splitting.

The evolution of O₂ was measured by gas chromatography. After adjustment and calculation, the Faradaic efficiency of the g-C₃N₄/WO₃ photoanode was determined to be 81.8% under 4500 s light irradiation (calculated O₂ evolution 2.85 µmol, measured O₂ evolution 2.33 µmol, measured H₂ evolution : measured O₂ evolution = 4.50 µmol : 2.33 µmol = 1.93 : 1, Fig. S4, ESI†), which indicates that the amount of O₂ evolved is slightly less than expected. This number is lower than expected (100%), and it may be as a result of a gas leak during the experiment. On the other hand, previous studies on WO₃ electrodes reported that oxidation of water to O₂ was not the only photo-oxidation reaction that occurred under light irradiation. The incomplete oxidation of water to peroxide species can easily occur.^32,33 In addition, photo-oxidation of anions in the solution present as supporting electrolytes can also compete with the photooxidation of water.³³ Compared with previously reported data, the efficiency of a similar photoanode of 3DB WO₃-NA/C₃N₄-NS//CoO_x was calculated to be 82.8%, which is close to that of the g-C₃N₄/WO₃ photoanode.⁸ Moreover, the Faradaic efficiency of Ag-Bi/WO₃³⁴ and Au/WO₃³⁵ photoanodes can reach 91.3% and 94.0%, respectively.

It is known that both the light absorption of the semiconductors and the separation efficiency of photoexcited electron–hole pairs have an important influence on the PEC properties. Diffuse reflectance UV-visible spectra were collected for WO₃, C₃N₄, and g-C₃N₄-NS/WO₃-NR samples to understand the influence of C₃N₄ on the light harvesting capability. As shown in Fig. 4a, the band edges of all samples were almost similar. These results reveal that the C₃N₄ coating has no influence on the light absorption ability of WO₃, which means that such an enhancement of PEC performance should not be ascribed to the light absorption ability. Their band gap energy (E_g) was evaluated using the following equation:³⁶

α(hν) = A(hν − E_g)n/2

where α, ν, E_g, and A are the absorption coefficient, light frequency, band gap energy, and a constant, respectively, and n is determined by the type of optical transition in the semiconductor. According to the literature, the band gap energies (E_g values) of these samples were estimated from a plot of (αhν)^1/2 as a function of the photon energy (hν) to be approximately 2.48, 2.62, and 2.39 eV for WO₃, C₃N₄, and g-C₃N₄-NS/WO₃-NR samples, respectively. The band positions of the as-synthesized WO₃ and C₃N₄ were calculated using the following empirical equation:³⁷

E_VB = X − E^e + 0.5E_g

where E_VB is the valence band edge potentials, X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms, E^e is the energy of the free electrons on the hydrogen scale (approximately 4.5 eV), E_g is the band gap energy of the semiconductor, and E_CB (conductance band edge potentials) can be determined by E_CB = E_VB − E_g. The results are shown in Fig. 4b. Therefore, the photogenerated electrons in the C₃N₄ layer can be easily transferred to the surface of WO₃, leaving the holes on the surface; however, the photoinduced holes on the surface of WO₃ can migrate to the C₃N₄ surface, which promotes the effective separation of photoexcited electrons and holes and hence decreases the probability of electron–hole recombination. Based on the results above, the enhanced activity of g-C₃N₄-NS/WO₃-NR composites can be attributed to the separation of photo-generated carriers.

Fig. 4 (a) UV-Vis absorption spectra of WO₃, C₃N₄, g-C₃N₄-NS/WO₃-NR samples and the (αhν)^1/2vs. photon energy (h) of WO₃, C₃N₄, g-C₃N₄-NS/WO₃-NR are inserted in (a). (b) A schematic diagram for the charge-transfer process in g-C₃N₄-NSs/WO₃-NRs.

In summary, we demonstrated that the g-C₃N₄-NS/WO₃-NR composites exhibited significantly enhanced PEC activity under visible light irradiation. The photocurrent density of the g-C₃N₄-NS/WO₃-NR sample achieved 1.92 mA cm⁻² at 1.23 V vs. RHE, which is about 2 times higher than that of pure WO₃-NRs (0.71 mA cm⁻² at +1.23 V vs. RHE). The IPCE value of the g-C₃N₄-NS/WO₃-NR photoanode reaches 57.79% at 330 nm and 8.38% at 430 nm, which is much higher than that of the pristine WO₃-NR photoanode (43.76% at 330 nm and 2.26% at 430 nm). The enhanced activity can be attributed to the separation of photo-generated carriers. Furthermore, the g-C₃N₄-NS/WO₃-NR composites are very stable during a long irradiation time. The photocurrent of the g-C₃N₄/WO₃ photoanode only decreases to 1.85 mA cm⁻² within 400 min of illumination at 1.23 V vs. RHE. These findings could open up new opportunities for the design of high-performance anodes.

This work was preliminarily supported by the Natural Science Foundation of China (21425627 and 21476271), the Science and Technology Plan Project (2013B090600036) and Natural Science Foundation (2014A030308012 and 2014KTSCX004) of Guangdong Province, China.

Notes and references

1. X. Chang, T. Wang, P. Zhang, J. Zhang, A. Li and J. Gong, J. Am. Chem. Soc., 2015, 137, 8356.
2. Y. Huang, B. Long, H. Li, M. S. Balogun, Z. Rui, Y. Tong and H. Ji, Adv. Mater. Interfaces, 2015, 2, 1500249.
3. Y. Huang, H. Li, M.-S. Balogun, W. Liu, Y. Tong, X. Lu and H. Ji, ACS Appl. Mater. Interfaces, 2014, 6, 22920.
4. K.-H. Ye, J.-Y. Wang, N. Li, Z.-Q. Liu, S.-H. Guo, Y.-P. Guo and Y.-Z. Su, Inorg. Chem. Commun., 2014, 45, 116.
5. W. Bi, C. Ye, C. Xiao, W. Tong, X. Zhang, W. Shao and Y. Xie, Small, 2014, 10, 2820.
6. F. Lei, Y. Sun, K. Liu, S. Gao, L. Liang, B. Pan and Y. Xie, J. Am. Chem. Soc., 2014, 136, 6826.
7. Y. Zhao, F. Zhao, X. Wang, C. Xu, Z. Zhang, G. Shi and L. Qu, Angew. Chem., Int. Ed., 2014, 53, 13934.
8. Y. Hou, F. Zuo, A. P. Dagg, J. Liu and P. Feng, Adv. Mater., 2014, 26, 5043.
9. B. M. Klepser and B. M. Bartlett, J. Am. Chem. Soc., 2014, 136, 1694.
10. A. J. Rettie, K. C. Klavetter, J.-F. Lin, A. Dolocan, H. Celio, A. Ishiekwene, H. L. Bolton, K. N. Pearson, N. T. Hahn and C. B. Mullins, Chem. Mater., 2014, 26, 1670.
11. J. Guo, Y. Li, S. Zhu, Z. Chen, Q. Liu, D. Zhang, W.-J. Moon and D.-M. Song, RSC Adv., 2012, 2, 1356.
12. I. Aslam, C. Cao, M. Tanveer, M. H. Farooq, W. S. Khan, M. Tahir, F. Idrees and S. Khalid, RSC Adv., 2015, 5, 6019.
13. D. W. Hwang, J. Kim, T. J. Park and J. S. Lee, Catal. Lett., 2002, 80, 53.
14. Y. Liu, J. Li, W. Li, Y. Yang, Y. Li and Q. Chen, J. Phys. Chem. C, 2015, 119, 14834.
15. C. Liu, H. Tang, J. Li, W. Li, Y. Yang, Y. Li and Q. Chen, RSC Adv., 2015, 5, 35506.
16. T. Singh, R. Müller, J. Singh and S. Mathur, Appl. Surf. Sci., 2015, 347, 448.
17. Y. O. Kim, S.-H. Yu, K.-S. Ahn, S. K. Lee and S. H. Kang, J. Electroanal. Chem., 2015, 752, 25.
18. C. Ng, A. Iwase, Y. H. Ng and R. Amal, J. Phys. Chem. Lett., 2012, 3, 913.
19. F. Zhan, J. Li, W. Li, Y. Liu, R. Xie, Y. Yang, Y. Li and Q. Chen, Int. J. Hydrogen Energy, 2015, 40, 6512.
20. X. Cui, Y. Wang, G. Jiang, Z. Zhao, C. Xu, Y. Wei, A. Duan, J. Liu and J. Gao, RSC Adv., 2014, 4, 15689.
21. J. Su, L. Guo, N. Bao and C. A. Grimes, Nano Lett., 2011, 11, 1928.
22. R. Saito, Y. Miseki and K. Sayama, Chem. Commun., 2012, 48, 3833.
23. Y. Pihosh, I. Turkevych, K. Mawatari, T. Asai, T. Hisatomi, J. Uemura, M. Tosa, K. Shimamura, J. Kubota and K. Domen, Small, 2014, 10, 3692.
24. K. Sivula, F. L. Formal and M. Grätzel, Chem. Mater., 2009, 21, 2862.
25. X. Zhang, T. Peng, L. Yu, R. Li, Q. Li and Z. Li, ACS Catal., 2014, 5, 504.
26. F. He, G. Chen, Y. Yu, S. Hao, Y. Zhou and Y. Zheng, ACS Appl. Mater. Interfaces, 2014, 6, 7171.
27. Y. Shiraishi, S. Kanazawa, Y. Sugano, D. Tsukamoto, H. Sakamoto, S. Ichikawa and T. Hirai, ACS Catal., 2014, 4, 774.
28. Y. He, J. Cai, L. Zhang, X. Wang, H. Lin, B. Teng, L. Zhao, W. Weng, H. Wan and M. Fan, Ind. Eng. Chem. Res., 2014, 53, 5905.
29. Y. Chen, W. Huang, D. He, Y. Situ and H. Huang, ACS Appl. Mater. Interfaces, 2014, 6, 14405.
30. F. Zhan, R. Xie, W. Li, J. Li, Y. Yang, Y. Li and Q. Chen, RSC Adv., 2015, 5, 69753.
31. Y. Liu, S. Xie, C. Liu, J. Li, X. Lu and Y. Tong, J. Power Sources, 2014, 269, 98.
32. Q. Mi, A. Zhanaidarova, B. S. Brunschwig, H. B. Gray and N. S. Lewis, Energy Environ. Sci., 2012, 5, 5694.
33. J. A. Seabold and K.-S. Choi, Chem. Mater., 2011, 23, 1105.
34. Q. Zhao, Z. Yu, W. Yuan and J. Li, Int. J. Hydrogen Energy, 2012, 37, 13249.
35. D. Hu, P. Diao, D. Xu and Q. Wu, Nano Res., 2016, 9, 1735.
36. I. M. Szilágyi, B. Fórizs, O. Rosseler, Á. Szegedi, P. Németh, P. Király, G. Tárkányi, B. Vajna, K. Varga-Josepovits and K. László, J. Catal., 2012, 294, 119.
37. B. Weng, J. Wu, N. Zhang and Y.-J. Xu, Langmuir, 2014, 30, 5574.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6qm00009f

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