Photoelectrochemical water splitting in a tungsten oxide - nickel oxide thin film material library

tA combinatorial thin film library containing WO3and NiO was produced using thermal co-evaporation ofWO3and Ni followed by high temperature reoxidation. Microstructure and crystallographic particular-ities of the WO3-NiO library investigated by SEM and XRD revealed three distinct compositional zones:A low Ni concentration zone containing crystalline WO3and amorphous NiO, a high Ni concentrationzone containing crystalline NiO and amorphous WO3and a middle range amorphous zone connectingthe extremes. Photo Electrochemical Scanning Droplet Cell Microscopy PE-SDCM was used for locallyinvestigating the photoelectric response of the library as a function of Ni concentration. A substantialphotocurrent peak was identified at 6.2 at.% Ni with values in excess of 2.5 mA cm−2while Ni amountsabove 9 at.% resulted in a completely photoinactive thin film. XPS surface analysis indicated a surface com-position different than the bulk with WO3being present up to 45 at.% Ni in the bulk. For bulk Ni amountsabove this composition, the surface contained no WO3anymore, only an amorphous Ni sub-oxide beingsuggested.