PHOTOELECTROCHEMICAL CHARACTERISTICS OF FERRIC TUNGSTATE

Active ferric tungstate was prepared by fusing an equimolar mixture of tungsten oxide and ferric oxide at 1100 degrees C and annealing at 80 0 degrees C for 20. h. Analysis of the electrode material by X-ray dif fraction showed that its composition was Fe2WO6. When this material wa s illuminated by visible light in 0.1 M NaOH solution, an anodic photo current at a positive potential of 0.5 V (SCE) was obtained. Therefore , this material is considered as an n-type semiconductor. The d.c. con ductivity of this material at 25 degrees C was 4 x 10(-6) Omega(-1) cm (-1). In the dark: unexpectedly high anodic currents were observed at positive potentials of 0.8 V (SCE) in 0.1 M NaOH. These currents are a ttributed to the existence of a high density of electron-hole recombin ation centers within the band-gap of ferric tungstate. When dimethyl v iologen (DMV) was used as an electroactive compound in the electrolyte , the anodic photocurrents increased significantly. The oxidation of D MV is thus expected to compete with the electron-hole recombination pr ocess. Furthermore, the process of electron-hole recombination was als o predicted from the shape of the photocurrent transients under interr upted illumination. These transients exhibited first-order relaxation effects in the region of the onset time of the photocurrents. The band gap energy of Fe2WO6 was found to be about 1.5 eV and its flat-band po tential in 0.1 M NaOH was about -0.3 V (SCE). The photoelectrochemical properties of ferric tungstate are explained according to the formali sm of the band model of the semiconductor/electrolyte interface.