Automated electrochemical synthesis and photoelectrochemical characterization of Zn_1-xCo_xO thin films for solar hydrogen production

High-throughput electrochemical methods have been developed for the investigation of Zn_1-xCo_xO films for photoelectrochemical hydrogen production from water. A library of 120 samples containing 27 different compositions (0 ≤ x ≤ 0.068) was synthesized by automated serial electrochemical deposition. High-throughput photoelectrochemical screening revealed improved solar hydrogen production for the cobaltdoped films, with Zn_0.956Co_0.044O exhibiting a 4-fold improvement over pure ZnO with no external bias. Flat-band potential, bias-dependent photocurrent, and action spectra were also measured automatically with the high-throughput screening system. The 200-nm-thick films were subsequently characterized by numerous techniques, including SEM, XRD, XPS, and UV-vis, which show that the depositions are well-controlled. Zn/Co stoichiometry in the films was controlled by the ratio of the Zn and Co precursors in each deposition bath. All films exhibited the wurtzite structure typical of pure ZnO, and the Co²⁺ appears to substitute Zn²⁺, forming a single-phase solid solution. Band gaps of the solid solutions were systematically lower than the 3.2-eV band gap typical of ZnO.