Light-Irradiated Electrochemical Direct Construction of Cu2O/CuO Bilayers by Switching Cathodic/Anodic Polarization in Copper(II)–Tartrate Complex Aqueous Solution

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.