Influence of surface density on the CO2 photoreduction activity of a DC magnetron sputtered TiO2 catalyst

Advancing in the photocatalyst scale-up is crucial for the development of highly efficient solar fuels production at industrial scale. Here, we report DC-magnetron sputtering as a suitable technique to produce photocatalytic TiO 2 coatings for CO 2 reduction with a view on process scalability. The crystallinity of the obtained TiO 2 coatings varies with surface density, with amorphous or quasi-amorphous coatings obtained with very low densities, while UV light absorption coefficients show the opposite trend, which has been related to the proportionally higher abundace of surface defects and grain boundaries associated to the small crystal size and/or amorphicity of the lightest coatings. The as-prepared samples lead to the reduction of CO 2 as demonstrated by 13 C isotope tracing. An optimum catalyst area density of 1 g/m 2 (by geometric area) is obtained in terms of CO 2 photoreduction production, which is ascribed to a compromise situation between crystallinity and absorption coefficient. Selectivity to the different reaction products also varies with the coating characteristics, with amorphous or quasi-amorphous light coatings favouring methanol formation, in contrast with the preferred CO evolution in heavier, crystalline ones. Raman spectroscopy reveals the formation of peroxo and peroxocarbonate species on the photocatalyst surface as oxidation products during the CO 2 reduction, the accummulation of which is proposed to be related to the observed catalyst deactivation.