Comprehensive Interfacial Study of Potentio-Dynamically Synthesized Copper Oxide Thin Films for Photoelectrochemical Applications

Ambient synthesis of intrinsic p-type copper oxide through potentio-dynamic technique under neutral pH led to the formation of transparent, crystalline and cubic cuprous oxide (Cu2O) film in less than five minutes. On sintering at 450°C, the film transformed to transparent monoclinic cupric oxide (CuO). The optical bandgap for Cu2O and CuO obtained from UV-Visible spectra was 2.41 eV and 1.66 eV, respectively. The p-type nature was confirmed from Hall measurement and Mott-Schottky analysis. The flatband potential obtained was 0.02 and 0.05 V, respectively for Cu2O and CuO and the acceptor density was 3.62 × 1021 and 4.25 × 1022 cm−3. The charge transfer process at the copper oxide-electrolyte (0.1 M Na2SO4) interface was studied under illumination in the frequency range of 1 MHz to 1 Hz under biasing from 0 V to −0.2 V using electrochemical impedance spectroscopy (EIS). Performance of these transparent metal oxides in photoelectrochemical water splitting was checked with linear sweep voltammetric studies. The solar to hydrogen efficiency was calculated from chronoamperometric analysis for Cu2O and CuO was 0.5 and 0.2%, respectively. Present studies indicate that cuprous oxide is more photoactive but cupric oxide is more stable. The poor photocurrent density of CuO, despite its higher acceptor density and conductivity, can be attributed to its indirect bandgap, less carrier mobility and shorter carrier lifetime under illumination. The chemical reaction involved in the corrosion of CuO is a two step process, which explains its stability.