Photoelectrochemical behavior of nanostructured WO3 thin-film electrodes:: The oxidation of formic acid

Nanostructured tungsten trioxide thin-film electrodes are prepared on conducting glass substrates by either potentiostatic electrodepostion from aqueous solutions of peroxotungstic acid or direct deposition of WO3 slurries. Once treated thermally in air at 450 degrees C, the electrodes are found to be composed of monoclinic WO3 grains with a particle size around 30-40 nm. The photoelectrochemical behaviour of these electrodes in 1 M HClO4 apparently reveals a low degree of electron-hole recombination. Upon addition of formic acid, the electrode showed the current multiplication phenomenon together with a shift of the photocurrent onset potential toward less positive values. Photoelectrochemical experiments devised on the basis of a kinetic model reported recently [I. Mora-Sero, T. Lana-Villarrel, J. Bisquert, A. Pitarch, R. Gomez, P. Salvador, J. Phys. Chem. B 2005, 109, 3371] showed that an interfacial mechanism of inelastic, direct hole tranfer takes place in the photooxidation of formic acid. This behaviour is attributed to the tendency of formic acid molecules to be specifically adsorbed on the WO3 nanoparticles as evidenced by attenuated total reflection infrared spectroscopy.