Oxalic acid photooxidation on rutile nanowire electrodes.

A combined photoelectrochemical and IR spectroscopic approach was used to study the photooxidation of oxalic acid on thin films consisting of oriented rutile TiO(2) nanowires (diameter: approximately 2 nm). Anodic spikes appear in the photocurrent transients particularly at low oxalic acid concentrations and point to mass transport limitations due to the presence of narrow pores (< or = 1 nm) in the nanowire films. Accordingly, IR measurements reveal that the photocurrent decrease at low concentrations is accompanied by an adsorbate depletion as tracked by the decrease of IR bands at 1699 cm(-1) (adsorbate species A) and 1720 cm(-1) (adsorbate species B). The relative intensities of the two bands are found to change with illumination time. Based on the comparison of the equilibrium adsorption constants as deduced independently from photocurrent transients and from IR spectra, the photocatalytic oxidation of oxalate seems to proceed on the surface of rutile nanowires mainly via species A, which is also the one with the largest adsorption constant. The strong decrease of the IR band at 1720 cm(-1) is rationalized by a fast replenishment of photooxidized species A by species B. In a more general vein, the challenges and prospects of combining photoelectrochemical and spectroscopic measurements are discussed.