Bulk and Surface Chemistry of Photoactive Electrodeposited Cu₂O Films Using X-Ray Absorption Spectroscopy

Cu2O is an attractive photocathode for important renewable energy reactions such as water splitting and CO2 reduction. A variety of synthesis methods for Cu2O films have been reported such as physical vapour deposition techniques1,2, chemical vapour deposition3,4, hydrothermal methods5 and electrodeposition6–8. Electrodeposition is a particularly popular method due its low cost and versatility, which appears to get consistent and even film deposition on conductive substrates. However, very little is known on the effects of the synthesis conditions on structure and properties. Majority of works in the literature often show vastly different photoelectrochemical activity, morphology and conductivity type with seemingly slight variation in the synthesis conditions. These variations can be in the electrolyte bath composition, deposition temperature, time, solution pH and electrochemical conditions. While commonly used characterisation methods like XRD, SEM, XPS and Raman spectroscopy are necessary to provide basic information of Cu2O formation, they are insufficient in providing a further understanding of the composition in terms of the Cu species present in each film. In this work, we take a systematic look into the bulk and surface chemistry of electrochemically deposited Cu2O using high resolution XANES and EXAFS spectroscopy. By using HERFD-XAS, we were able to see that changing the deposition parameters altered the Cu(II) amounts. A series of quantitative analyses revealed that none of films were purely composed of Cu(I) species, with amounts of Cu(II) varying from 0.7% to 9.35% across samples. Photoelectrochemical measurements appear to show a correlation in the amount of Cu(II) and the activity of the photocathodes. Highest photocurrent generation was observed in the film with the highest Cu(II) content, however, this was at the expense of stability due to reductive decomposition to Cu metal. We propose that the Cu(II) species initially reduce to Cu metal, forming Schottky barriers at the newly formed metal semiconductor contacts, hence the increase in photocurrent. Nevertheless, this is limited by other factors including film thickness and crystallite grain size. The most stable film was consistent with the quantitative analyses from XAS, which indicated that this film had the lowest amount of Cu(II) in its structure. We concluded that there must be a compromise between activity and stability when optimising deposition parameters for electrodeposition of Cu2O as a photocathode. This work shows the importance of advanced characterisation techniques for gaining a more thorough understanding of structure and composition, and how this may correlate to the performance or functionality of a material. It serves as a guide for future development of more active Cu2O based films for photoelectrochemical processes as well as for solar cell applications. Manuscript in preparation. References 1 D. S. Darvish and H. A. Atwater, J. Cryst. Growth, 2011, 319, 39–43. 2 S. Dolai, S. Das, S. Hussain, R. Bhar and A. K. Pal, Vacuum, 2017, 141, 296–306. 3 M. Ottosson and J.-O. Carlsson, Surf. Coatings Technol., 1996, 78, 263–273. 4 C. R. Crick and I. P. Parkin, J. Mater. Chem., 2011, 21, 14712–14716. 5 A. D. Handoko, C. W. Ong, Y. Huang, Z. G. Lee, L. Lin, G. B. Panetti and B. S. Yeo, J. Phys. Chem. C, 2016, 120, 20058–20067. 6 A. E. Rakhshani and J. Varghese, Sol. Energy Mater., 1987, 15, 237–248. 7 A. E. Rakhshani and J. Varghese, J. Mater. Sci., 1988, 23, 3847–3853. 8 W. Siripala, L. D. R. . Perera, K. T. L. De Silva, J. K. D. S. Jayanetti and I. M. Dharmadasa, Sol. Energy Mater. Sol. Cells, 1996, 44, 251–260.