Elucidating the role of adsorption during artificial photosynthesis: H2O and CO2 adsorption isotherms over TiO2 reveal thermal effects under UV illumination.

Adsorption measurements of CO2 and H2O over TiO2 surfaces in dark and under illumination were carried out to reveal the ensuing bottlenecks of the initial steps of the artificial photosynthesis reaction. When the adsorption isotherms of both CO2 and H2O were measured under illumination, the results were comparable to isotherms measured at higher temperatures in dark. This evidence is interpreted as the presence of hot spots, due to charge carrier recombination reactions. Differential heat of adsorption measurements revealed that H2O adsorption on TiO2 is stronger, and with a higher coverage than that of CO2. Dissociation of water is an energetically uphill reaction, and the local hot spots due to charge carrier recombination in indirect bandgap semiconductors can enhance the reaction probability. At higher temperatures, higher reaction probabilities are expected and estimated by a thermodynamic analysis for water splitting reaction. The potential role of these hot spots during natural and artificial photosynthetic reactions is discussed.