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

• WO3- NiO with 6.2 at.% Ni shows highest photoelectrochemical water splitting efficiency of 2.5 mA cm-2at 405 nm
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• Tungsten oxide. nickel oxide thin film material library was prepared by vapour phase co-deposition
• Wide compositional spread from 3 to 62 at.% Ni was investigated by XRD, SEM; XPS
• Photoelectrochemical properties of entire library were mapped using PE-SDCM

A combinatorial thin film library containing WO3 and NiO was produced using thermal co-evaporation of WO3 and Ni followed by high temperature reoxidation. Microstructure and crystallographic particularities of the WO3-NiO library investigated by SEM and XRD revealed three distinct compositional zones: A low Ni concentration zone containing crystalline WO3 and amorphous NiO, a high Ni concentration zone containing crystalline NiO and amorphous WO3 and a middle range amorphous zone connecting the extremes. Photo Electrochemical Scanning Droplet Cell Microscopy PE-SDCM was used for locally investigating the photoelectric response of the library as a function of Ni concentration. A substantial photocurrent peak was identified at 6.2 at.% Ni with values in excess of 2.5 mA cm−2 while Ni amounts above 9 at.% resulted in a completely photoinactive thin film. XPS surface analysis indicated a surface composition different than the bulk with WO3 being present up to 45 at.% Ni in the bulk. For bulk Ni amounts above this composition, the surface contained no WO3 anymore, only an amorphous Ni sub-oxide being suggested.

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