Combinatorial electrochemical synthesis and characterization of tungsten-based mixed-metal oxides.

Automated systems for electrochemical synthesis and high-throughput screening of photoelectrochemical materials were developed and used to prepare tungsten-based mixed-metal oxides, W(n)O(m)M(x) [M = Ni, Co, Cu, Zn, Pt, Ru, Rh, Pd, and Ag], specifically for hydrogen production by photoelectrolysis of water. Two-dimensional arrays (libraries) of diverse metal oxides were synthesized by automated cathodic electrodeposition of the oxides on Ti foil substrates. Electrolytes for the mixed oxides were prepared from various metal salts added to a solution containing tungsten stabilized as a peroxo complex. Electrodeposition of the peroxo-stabilized cations gave rise to three distinguishable oxide groups: (1) mixed-metal oxides [Ni], (2) metal-doped tungsten oxides [Pt, Ru, Rh, Pd, Ag], and (3) metal-metal oxide composites [Co, Cu, Zn]. The oxides typically showed n-type semiconducting behavior. Automated measurement of photocurrent using a scanning photoelectrochemical cell showed the W-Ni mixed oxide had the largest relative zero bias photocurrent, particularly at a low Ni concentration (5-10 atomic percent Ni). Pt and Ru were also found to increase the photoactivity of bulk tungsten oxide at relatively low concentrations; however, at concentrations above 5 atomic percent, crystallization of WO(3) was inhibited and photoactivity was diminished. Addition of Co, Cu, and Zn to WO(3) was not found to improve the photoelectrochemical activity.