PLASMA-SPRAYED SEMICONDUCTOR ELECTRODES - PHOTOELECTROCHEMICAL CHARACTERIZATION AND NH3 PHOTOPRODUCTION BY SUBSTOICHIOMETRIC TUNGSTEN-OXIDES

Two substoichiometric tungsten oxide coatings have been obtained by plasma spray of WO3 powder on Ti substrates. The films are 40 ± 20 μm thick and are yellow (WO2.99) or dark blue (WO2.97). WO2.99 coatings show a highly textured surface with a specific area 27.9 times the geometrical one. X-ray diffraction pattern reveals that their structure is a mixture of monoclinic and triclinic phases. The yellow films have been characterized photoelectrochemically in regenerative cells by using O2/H2O redox at pH 2.0. Under anodic polarization of 1.5 V (SCE) their quantum yield is between 10% and 20% in the wavelength range comprised between 270 and 430 nm with an indirect bandgap of 2.55 eV and a flatband potential of -0.1 V. WO2.99 films have been tested for NH3 photoproduction. Small amounts of NH3 are detected under white light illumination but it is demonstrated with the help of X-ray absorption spectroscopy (XANES and EXAFS) that the reaction is not photocatalytic. It is proposed that N2 is reduced by hydrogen released from the reaction of adsorbed water with oxygen vacancies. Light absorbed by the oxide is only necessary to release adsorbed NH3 from the 6.5 × 1014 sites available/cm2 on the surface of the coating. A total amount between 4.5 × 1015 and 5.8 × 1015 ionized vacancies/cm2 are able to diffuse toward the surface of the coating to participate in the reaction. WO2.97 coatings which are a mixture of amorphous and unidentified crystalline phases are characterized by very weak photoelectrochemical activity and no NH3 photoproduction. © 1990 American Chemical Society.