Light-Irradiated Electrochemical Direct Construction of Cu₂O/CuO Bilayers by Switching Cathodic/Anodic Polarization...

p-CuO with a band gap energy of 1.5 eV, p-Cu₂O with a band gap energy of 2.05 eV, and their bilayers were prepared by controlling the potential of anodic and cathodic polarization in a copper(II)-tartrate complex aqueous solution containing copper(II) sulfate hydrate and tartaric acid in the dark and under light irradiation. Electrochemical characteristics of the electrodeposition and the resultant CuO and Cu₂O layers were investigated with cyclic voltammetry, chronoamperometry, and Mott-Schottky plots, and the structural and optical characterizations were performed with X-ray diffraction, scanning electron microscopy, and optical absorption spectra measurements. The CuO layer prepared at 0.4-0.7 V was composed of aggregates of granular grains with the monoclinic lattice, and the Cu₂O layer composed of coarse grains with the cubic lattice was deposited at -0.4 to 0.6 V. The flat-band potentials were estimated to be 0.145 and -0.1 V (vs Ag/AgCl) for the CuO and Cu₂O layers, respectively. The 0.4 μm CuO/1.1 μm Cu₂O bilayers could be prepared by switching the electrodeposition potentials of 0.4 and -0.4 V, irrespective of the presence of light irradiation. The photoelectrodeposition under light irradiation enabled the preparation of continuous and dense 1.1 μm Cu₂O/0.4 μm CuO bilayer by controlling the potential, while electrodeposition in the dark led to sparse, isolated, and coarse Cu₂O grains being deposited. The mechanism for the photoelectrodeposition of the bilayers was discussed based on the energy band alignment at the heterointerface to the Cu-tartrate complex solution.