Design of Non-Ammoniacal Solution for Electrodeposition of CuO on Cu2o Substrate
Authors: Miura Ryutaro , Kitada Atsushi , Fukami Kazuhiro , Murase Kuniaki 
Published: December 2020

Semiconductor materials are attracting increasing attention as photoelectrodes and photovoltaic devices. Cuprous oxide (Cu2O), which has been intensively studied due to its high absorption coefficient, low cost, and low toxicity, is a p-type semiconductor with a bandgap energy of approximately 2.1 eV. However, Cu2O has a narrow solar absorption region due to its high bandgap energy. CuO (bandgap energy of 1.4 eV) has been combined with Cu2O to increase the absorption wavelength range [1].
Thermal oxidation of Cu2O surfaces for forming CuO/Cu2O bi-layer films has been reported [2]. However, this thermal oxidation creates nanopores and voids in the films and degrades the film performance. In this study, an electrochemical formation of CuO on a Cu2O surface is investigated. Previously reported methods of electrochemical CuO formation include an anodic electrodeposition using an ammoniacal aqueous solution [3]. In ammoniacal solutions, however, the Cu2O surface is unstable and can be dissolved as Cu(I)-ammine complexes. In this study, suitable non-ammoniacal ligands, which can be used to stabilize Cu2O during CuO electrodeposition, are examined through thermodynamic calculations, and the electrodeposition behavior of CuO films on Cu2O is experimentally investigated.
An appropriate ligand, 2-aminoethanol, i.e., mono-ethanolamine (MEA), was obtained by drawing the pH speciation diagram using thermodynamic data [4, 5]. It is difficult to use MEAs to form complexes with Cu(I) and easy to form with Cu(II), and MEAs are relatively inexpensive and versatile. Figure 1 shows that CuO is stable at a pH region lower than the region where the Cu(II)-MEA complex is dominant, suggesting that it is feasible to achieve CuO electrodeposition using a local pH drop, through, for example, anodic oxygen evolution reaction.
In a 0.05 M Cu(II) + 0.125 M MEA aqueous solution without NaOH addition (pH = 8.5), precipitates − probably hydroxide − were formed (Figure 2(b)). In contrast, when the pH was increased to 12.5, a deep blue aqueous solution was obtained without any precipitates (Figure 2(c)). While a set of existing stability constants [5] indicated the presence of CuO or Cu(OH)2 precipitates in the 0.05 M Cu(II) + 0.125 M MEA aqueous solution at a pH value of 12.5, a clear solution was obtained at [Cu(II)] = 0.05 M, [MEA] = 0.125 M, and pH = 12.5. A Cu2O substrate was immersed in this solution for 5 h to confirm the stability of Cu2O in the solution. From the results, almost no weight change was observed, unlike the case of ammoniacal solutions, when nitrogen degassing was carried out for 30 min before performing the immersion test to prevent the oxidation of Cu2O by dissolved oxygen.
Potentiostatic electrodeposition of CuO was conducted on +0.45 V vs. Ag/AgCl reference electrode at 313 K. Cu2O/FTO —Cu2O was electrodeposited on fluorine-doped tin oxide (FTO) in advance [2]— and Pt plates were used as both the working electrode (WE) and the counter electrode. Figure 3 shows the X-ray diffraction (XRD) profiles of the Cu2O/FTO WE before and after the electrodeposition. Along with cubic Cu2O, monoclinic CuO was detected. Additionally, the cross-sectional field-emission scanning electron microscopy (FE-SEM) images depicted in Fig. 4 indicate a bi-layer structure. Therefore, we conclude that CuO was stacked, keeping the Cu2O layer intact.
Figure 5 shows the visible spectroscopy near-infrared region (vis-NIR) absorbance spectra of the Cu2O and CuO/Cu2O films. The absorption edge of the Cu2O films was 630 nm, while that of the CuO/Cu2O films shifted to 900 nm, demonstrating that the CuO/Cu2O films had a broader absorption wavelength range than pure Cu2O or pure CuO films.
In summary, a non-ammoniacal bath is surveyed from the thermodynamics database, and CuO/Cu2O bi-layer films are electrochemically fabricated.
References
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Figure 1