Aqueous Solution-Gel Synthesis of Copper-Based Ternary Oxides As Efficient Photocathodes in Photoelectrochemical Cells
Authors: Elen Ken , Joos Bjorn , Debusschere Nele , Talukdar Kristy , Hardy An , Van Bael Marlies K. 
Published: December 2024

The efficient valorization of solar energy will be vital for sustainable energy and fuel production while mitigating atmospheric pollution. Photoelectrochemical (PEC) systems present a promising avenue for converting light into chemical energy, by using semiconducting photoanodes and/or -cathodes to initiate redox reactions via electron-hole pair generation. The choice of semiconductor materials plays a critical role in PEC performance, requiring materials that meet specific criteria such as efficient absorption of visible light, composition of earth-abundant elements, and stability in aqueous solutions under irradiation. CuO and Cu2O have attracted attention because their appropriate band edges are favorable to both photo-electrochemical water splitting and CO2 conversion, alongside their reasonable abundance and low toxicity. However, substantial photocorrosion limits their functionality as photo-electrodes, motivating the exploration of copper-based ternary oxides, which exhibit greater stability under operating conditions. [1]
With over 60 elements available for potential photo-electrode compositions, a comprehensive and accessible synthesis method would be valuable for identifying suitable photoactive materials. Here, the citrate-based aqueous solution-gel method emerges as a versatile strategy for formulating stable aqueous metal ion precursor solutions, facilitating the synthesis of (doped) multi-metal oxides. [2] Nonetheless, achieving phase-pure copper-based ternary oxides presents challenges due to copper's distinctive redox chemistry. This study focuses on synthesizing copper-based photocathode materials, specifically copper bismuth oxide (CBO), copper ferrite (CFO), and copper niobate (CNO). The formulation of precursor solutions and their thermal processing are highlighted. Thin-film photocathodes are deposited on transparent conductive substrates via spin coating. The phase purity of resulting copper-based oxides is assessed using X-ray diffraction and Raman spectroscopy, while UV-Vis spectroscopy is used to determine their optical bandgap. Furthermore, the photocathodes are subjected to linear sweep voltammetry in both dark and illuminated conditions to evaluate their photoelectrochemical performance.
The aqueous solution-gel method enables the deposition of various functional oxide coatings as photocathodes, allowing for the optimization of coating thickness and phase purity. [3] Interestingly, this approach can also be used to establish a non-stoichiometric gradient in the coatings, which is demonstrated to enhance electron-hole separation. Additionally, the solution-gel deposition of a suitable hole transfer layer improves the overall PEC performance even further. These observations emphasize the importance of combining materials design and innovative synthesis strategies for exploring efficient photoactive materials and optimizing device architectures.
[1] Wang et al. “Insights into the development of Cu-based photocathodes for carbon dioxide conversion” Green Chem. (2021) 23, 3207
[2] Van Bael et al. “Aqueous Precursor Systems” in “Chemical Solution Deposition of Functional Oxide Thin Films” Schneller, T., Waser, R., Kosec, M., Payne, D. (eds) 2013, Springer, Vienna.
[3] Joos et al. “Facile Aqueous Solution-Gel route toward Thin Film CuBi2O4 Photocathodes for Solar Hydrogen Production” Advanced Sustainable Systems (2023) 7, 2300083

This work has received financial support from the European Fund for Regional Development through the Interreg Vlaanderen-Nederland project FOTON, from the Belgian federal government through the ETF project T-REX,
from
the Flemish government under the program ‘Vlaamse Veerkracht’ and from the VLAIO network “Flanders Innovation & Entrepreneurship” through the Catalisti Moonshot project SYN-CAT.