Controllable Cu₂O–Cu nanoparticle electrodeposition onto carbon paper and its superior photoelectrochemical performance

Abstract

Cu₂O–Cu was successfully electrodeposited onto a carbon paper substrate through a potentiostatic procedure with a three-electrode configuration. The results demonstrate that the Cu₂O–Cu, electrodeposited at a potential of −0.4 V, evenly and uniformly grows on the carbon paper and displays octahedron shaped nanoparticles with an average edge length of 100 nm. Moreover, the photocurrent density of Cu₂O–Cu electrodeposited at a potential of −0.4 V can reach up to 1.2 mA cm⁻². In this Cu₂O–Cu, the interface between the Cu₂O and Cu is suggested as the location of photoinduced electron–hole pair separation, and thus it hinders the rapid surface and bulk recombination of the photoinduced electron–hole pairs. Consequently, the photogenerated electrons can be easily transferred through the Cu metal .

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1. Introduction

Cuprous oxide (Cu₂O), due to its narrow band gap (2.0–2.2 eV), high absorption efficiency, non-toxicity and low preparation cost, has attracted significant attention and become a promising candidate for potential applications as a photocatalyst, and in solar energy conversion and biosensors.^1–7 Cu₂O is typically prepared as a p-type material due to the presence of Cu vacancies.^8,9 At the same time, n-type Cu₂O has been seldom reported except under special synthetic conditions.^10–12 The theoretical photoelectric absorption efficiency of Cu₂O reaches up to 18%, however, its actual efficiency is only 2%, which is attributed to its poor stability due to self-photocorrosion in an electrolyte solution and imperfect morphology control technology.^13–15

As is well known, the photoelectrochemical properties of a semiconductor can be greatly affected by particle size and morphology. When the particle size decreases to nanometer and micron levels, due to an increase of quantum effects and specific surface areas, the electronic conduction of the semiconductor increases significantly, and thus the photoelectrochemical performance is obviously improved.⁸ Up to now, there have been many studies about the shape-controlled synthesis of nano- and micro-Cu₂O at home and abroad.^16–25 Among numerous preparation methods, electrochemical deposition is the most promising one, due to its low cost, high deposition rate, availability for large-scale production, etc.^26–28 It is generally known that the typical reaction of Cu₂O in a cathodic electrodeposition process is as shown in eqn (1):

2Cu²⁺ + H₂O + 2e⁻ → Cu₂O + 2H⁺ (1)

An important feature of a photocatalyst is its quantum efficiency. In general, a bare semiconductor can show some shortcomings in the photoelectrochemical reaction process, such as photocorrosion, and easy recombination between photoelectrons and holes, thus exhibiting a very low quantum efficiency and limiting its application.^9,29 Adding a noble metal (e.g., Cu, Pt, Au, Ag) to the bare semiconductor can quickly transfer photogenerated electrons and effectively prevent the recombination of electron–hole pairs, thus improving the quantum efficiency and making the semiconductor–metal heterojunction exhibit a higher photocatalytic activity.^30–34 In recent years, there have been a few reports regarding the synthesis and photocatalytic properties of Cu₂O–Cu.^35,36 In a cuprous (II) solution, we know that another electrodeposition reaction at the cathode also usually occurs on the substrate according to the following eqn (2):

Cu²⁺ + 2e⁻ → Cu (2)

In this work, Cu₂O–Cu was co-electrodeposited onto a carbon paper electrode substrate in a copper acetate (Cu(CH₃COO)₂) solution. We have also clarified the influence of the electrochemical deposition potentials on particle size and morphology, tested the photoelectrochemical performance and proposed a mechanism for the photoelectrochemical reaction processes of the Cu₂O–Cu film under visible light irradiation.

2. Experimental

Typically, Cu₂O–Cu was electrodeposited onto a carbon paper substrate in the as-prepared 0.05 M Cu(CH₃COO)₂ aqueous solution using a potentiostatic procedure with a three-electrode configuration. The carbon paper substrate (1.0 × 1.0 cm²) was used as the working electrode (WE), a Pt sheet (1.0 × 1.0 cm²) as the counter electrode (CE), and a saturated calomel electrode (SCE) as the reference electrode (RE). The electrodeposition time was set at 2000 s, and the electrodeposition of Cu₂O–Cu was conducted at potentials of −0.2 V, −0.3 V, −0.4 V and −0.5 V, respectively. After electrodeposition, the working electrode was taken out from the aqueous solution, washed with ethanol and deionized water, and then dried under air for further characterization.

The morphologies, microstructures and compositions of the obtained Cu₂O–Cu films were characterized by X-ray diffraction (XRD, Rigaku, D/max-RB), and field emission scanning electron microscopy (FESEM, JEOL, JSM-6701F).

A photoelectrochemical study of the Cu₂O–Cu electrode was performed in a three-electrode electrochemical cell in which the Cu₂O–Cu, the Pt gauze and the SCE served as the WE, CE and RE, respectively. The electrolyte was a 0.5 M Na₂SO₄ aqueous solution. A 300 W Xe lamp (PLS-SXE300) was used as the light source. The photoelectrochemical current was recorded in the dark and light in an open circuit potential using an electrochemical workstation (Solartron 1287).

The photocatalytic activity of the samples was evaluated by the degradation of methylene blue (MB) at a concentration of 10 mg L⁻¹. The as-prepared Cu₂O–Cu electrode was put in 50 mL of dye solution. Then, the photocatalytic test was conducted by irradiation with a 300 W Xe lamp. The concentration of MB was determined at the characteristic peak wavelength (λ = 660 nm) by a spectrophotometer (UNICO, UV-2000).

3. Results and discussion

The crystallographic structures of the samples obtained at different potentials were analyzed by XRD, as shown in Fig. 1. All of the deposits show five peaks at 2θ values of 29.8°, 36.7°, 42.32°, 61.4° and 73.6° corresponding to Cu₂O (110), (111), (200), (220) and (311) respectively. It can be found that the diffraction peaks are consistent with what is expected from a Cu₂O phase (JCPDS 05-0667). The diffraction peaks at 2θ values of 43.5°, 50.6° and 74.2° can be indexed to Cu (111), (200) and (220) respectively (JCPDS 85-1326). Moreover, the relative intensity of the Cu peaks increases with decreasing electrodeposition potentials, which indicates a higher Cu content under more negative potentials. Notably, the peak at around 55° is ascribed to the carbon paper substrate. As a result, the composition of the Cu₂O–Cu can be changed by adjusting the electrodeposition potential.

Fig. 1 XRD patterns of Cu₂O–Cu electrodeposited for 2000 s at potentials of −0.2 V (a), −0.3 V (b), −0.4 V (c) and −0.5 V (d).

The Cu₂O–Cu electrodeposited at different potentials was characterized by FESEM, as shown in Fig. 2. It can be noted from Fig. 2a and b that the Cu₂O–Cu films consist of tiny octahedron shaped nanoparticles with some combined into larger agglomerated particles on the surface of the carbon paper substrate. In addition, the Cu₂O–Cu film electrodeposited at a potential of −0.4 V demonstrates rather a different surface morphology. It can be seen from Fig. 2c that the Cu₂O–Cu is evenly distributed on the carbon paper surface. As the electrodeposition potential decreases to −0.5 V, it can be observed from Fig. 2d that the obtained Cu₂O–Cu has completely agglomerated into larger particles. It is clear from the XRD and FESEM results that both Cu₂O and Cu can be electrodeposited together onto a carbon paper substrate.

Fig. 2 FESEM images of Cu₂O–Cu electrodeposited for 2000 s at potentials of −0.2 V (a), −0.3 V (b), −0.4 V (c) and −0.5 V (d).

It is worth pointing out that the stability of a given photocatalyst during the photoelectrochemical process is a crucial factor for its practical applications. A 0.5 M Na₂SO₄ aqueous solution was used as the electrolyte, which doesn't contain any redox species that can be photoelectrochemically reduced or oxidized other than H⁺, OH⁻, and H₂O. The photoelectrochemical current of the Cu₂O–Cu electrode was measured at an open circuit potential under pulse visible light irradiation, as shown in Fig. 3a. It can be found that the more negative the electrodeposition potential is, the higher the photocurrent density of the electrodeposited Cu₂O–Cu electrode is. The photocurrent density of Cu₂O–Cu electrodeposited at potentials of −0.2 V, −0.3 V, −0.4 V and −0.5 V remains stable at around 0.11 mA cm⁻², 0.50 mA cm⁻², 1.20 mA cm⁻² and 0.60 mA cm⁻² under visible light irradiation, respectively. Notably, the photocurrent density of Cu₂O–Cu electrodeposited at a potential of −0.4 V was the highest, which shows a significantly superior photocurrent in comparison to previously reported Cu₂O.^7,8,13,37 We believe that this superior photocurrent property can be attributed to the uniform particle size and morphology, and the promotion of the transmission of photogenerated electrons. The Cu₂O–Cu deposit can be deemed as an attractive candidate as a visible-light-driven photocatalyst.

Fig. 3 (a) Photoelectrochemical current profiles in an open circuit potential under pulse visible light irradiation of Cu₂O–Cu electrodeposited for 2000 s at different potentials. (b) The enlarged FESEM image of Cu₂O–Cu electrodeposited for 2000 s at a potential of −0.4 V.

To further understand the best photoelectrochemical current property of Cu₂O–Cu electrodeposited for 2000 s at a potential of −0.4 V, a magnified FESEM image is shown in Fig. 3b. It can be seen that the Cu₂O–Cu evenly and uniformly grows on the carbon paper, showing octahedron shaped nanoparticles with an average edge length of 100 nm. A schematic illustration of the electron–hole separation and transport within the Cu₂O–Cu in the photoelectrochemical reaction process is shown in Fig. 4. The interface between the Cu₂O and Cu is suggested as the location of the photoinduced electron–hole pair separation, and thus it hinders the rapid surface and bulk recombination of the photoinduced electron–hole pairs. Consequently, the photogenerated electrons can be easily transferred through the Cu metal.

Fig. 4 Schematic illustration of the electron–hole separation and transport within the Cu₂O–Cu in the photoelectrochemical reaction process.

The photocatalytic activity of Cu₂O–Cu electrodeposited for 2000 s at different potentials was measured by monitoring the changes in optical absorption of a MB aqueous solution during its photocatalytic decomposition process, as shown in Fig. 5. As can be seen, the photocatalytic activity of Cu₂O–Cu electrodeposited at a potential of −0.4 V was the highest. Moreover, it can be found that the photocatalytic activities of all of the samples are not high, because the electrodeposited Cu₂O–Cu films have very low masses resulting in the insufficient degradation of MB.

Fig. 5 The photocatalytic degradation of a MB aqueous solution under visible light irradiation for Cu₂O–Cu electrodeposited for 2000 s at different potentials.

4. Conclusion

In summary, Cu₂O–Cu was successfully electrodeposited onto a carbon paper substrate through a potentiostatic procedure with a three-electrode configuration. The XRD results demonstrate that all of the deposits at various potentials consist of Cu₂O and Cu, displaying a higher content of Cu under a more negative potential. The photocurrent density of Cu₂O–Cu electrodeposited at a potential of −0.2 V, −0.3 V, −0.4 V and −0.5 V remains stable at around 0.11 mA cm⁻², 0.50 mA cm⁻², 1.20 mA cm⁻² and 0.60 mA cm⁻² under visible light irradiation, respectively. It can be noted that the photocurrent density of Cu₂O–Cu electrodeposited at a potential of −0.4 V is the highest, which is attributed to its uniform particle size and morphology, and the promotion of the transmission of photogenerated electrons. Moreover, the presence of Cu is also beneficial for the transport of the photogenerated electrons in the photoelectrochemical process. Based on the above discussion, we believe that this Cu₂O–Cu is an attractive candidate as a visible-light-driven photocatalyst.

Acknowledgements

This work was supported by the National Basic Research Program of China (no. 2013CB632404).

References

1. M. Hara, T. Kondo, M. Komoda, S. Ikeda, K. Shinohara, A. Tanaka, J. N. Kondo and K. Domen, Chem. Commun., 1998, 357–358.
2. J. T. Zhang, J. F. Liu, Q. Peng, X. Wang and Y. D. Li, Chem. Mater., 2006, 18, 867–871.
3. Y. W. Tan, X. Y. Xue, Q. Peng, H. Zhao, T. H. Wang and Y. D. Li, Nano Lett., 2007, 7, 3723–3728.
4. F. Zewge, R. van de Krol and P. W. Appel, Bull. Chem. Soc. Ethiop., 2008, 22, 27–40.
5. S. J. Guo, Y. X. Fang, S. J. Dong and E. K. Wang, Inorg. Chem., 2007, 46, 9537–9539.
6. Y. Gu, X. Su, Y. L. Du and C. M. Wang, Appl. Surf. Sci., 2010, 256, 5862–5866.
7. L. B. Xiong, S. Huang, X. Yang, M. Q. Qiu, Z. H. Chen and Y. Yu, Electrochim. Acta, 2011, 56, 2735–2739.
8. C. M. McShane and K. S. Choi, J. Am. Chem. Soc., 2009, 131, 2561–2569.
9. Z. H. Zhang, R. B. Dua, L. B. Zhang, H. B. Zhu, H. N. Zhang and P. Wang, ACS Nano, 2013, 7, 1709–1717.
10. X. F. Han, K. H. Han and M. Tao, Electrochem. Solid-State Lett., 2009, 12, H89–H91.
11. B. Balamurugan, I. Aruna and B. R. Mehta, Phys. Rev. B: Condens. Matter Mater. Phys., 2004, 69, 165419.
12. L. C. Wang and M. Tao, Electrochem. Solid-State Lett., 2007, 10, H248–H250.
13. R. M. Liang, Y. M. Chang, P. W. Wu and P. Lin, Thin Solid Films, 2010, 518, 7191–7195.
14. A. D. Tang, Y. Xiao, J. Ouyang and S. Nie, J. Alloys Compd., 2008, 457, 447–451.
15. A. Mittiga, E. Salza, F. Sarto, M. Tucci and R. Vasanthi, Appl. Phys. Lett., 2006, 88, 163502–163503.
16. Y. Zhang, B. Deng, T. R. Zhang, D. M. Gao and A. W. Xu, J. Phys. Chem. C, 2010, 114, 5073–5079.
17. K. X. Yao, X. M. Yin, T. H. Wang and H. C. Zeng, J. Am. Chem. Soc., 2010, 132, 6131–6144.
18. Y. Chang and H. C. Zeng, Cryst. Growth Des., 2004, 4, 273–278.
19. H. W. Zhang, X. Zhang, H. Y. Li, Z. K. Qu, S. Fan and M. Y. Ji, Cryst. Growth Des., 2007, 7, 820–824.
20. H. L. Xu, W. Z. Wang and L. Zhou, Cryst. Growth Des., 2008, 8, 3486–3489.
21. Y. M. Sui, W. Y. Fu, H. B. Yang, Y. Zeng, Y. Y. Zhang, Q. Zhao, Y. G. Li, X. M. Zhou, Y. Leng, M. H. Li and G. T. Zou, Cryst. Growth Des., 2010, 10, 99–108.
22. M. J. Siegfried and K. S. Choi, J. Am. Chem. Soc., 2006, 128, 10356.
23. C. H. Kuo and M. H. Huang, J. Am. Chem. Soc., 2008, 130, 12815–12820.
24. C. H. Kuo and M. H. Huang, J. Phys. Chem. C, 2008, 112, 18355–18360.
25. H. J. Qiu, L. Lu, X. R. Huang and Y. B. Qu, Electrochim. Acta, 2010, 56, 291–296.
26. L. L. Ma, Y. L. Lin, Y. Wang, J. L. Li, E. K. Wang, M. Q. Qiu and Y. Yu, J. Phys. Chem. C, 2008, 112, 18916–18922.
27. S. Joseph and P. V. Kamath, Solid State Sci., 2008, 10, 1215–1221.
28. A. Paracchino, V. Laporte, K. Sivula, M. Grätzel and E. Thimsen, Nat. Mater., 2011, 10, 456–461.
29. H. Gerischer, J. Electroanal. Chem., 1977, 82, 133–143.
30. T. Sreethawong and S. Yoshikawa, Catal. Commun., 2005, 6, 661–668.
31. F. X. Zhang, R. C. Jin, J. X. Chen, C. Z. Shao, W. L. Gao, L. D. Li and N. J. Guan, J. Catal., 2005, 232, 424–431.
32. V. Iliev, D. Tomova, R. Todorovska, D. Oliver, L. Petrov, D. Todorovsky and M. Uzunova-Bujnova, Appl. Catal., A, 2006, 313, 115–121.
33. S. W. Lam, K. Chiang, T. M. Lim, R. Amal and G. K. C. Low, Appl. Catal., B, 2007, 72, 363–372.
34. B. Cheng, Y. Le and J. G. Yu, J. Hazard. Mater., 2010, 177, 971–977.
35. S. U. Son, I. K. Park, J. Park and T. Hyeon, Chem. Commun., 2004, 778–779.
36. B. Zhou, Z. G. Liu, H. X. Wang, Y. Q. Yang and W. H. Su, Catal. Lett., 2009, 132, 75–80.
37. J. Q. Tian, H. Y. Li, Z. C. Xing, L. Wang, Y. L. Luo, A. M. Asiri, A. O. Al-Youbi and X. P. Sun, Catal. Sci. Technol., 2012, 2, 2227–2230.

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