In Situ Electronic Probing of Photoconductive Trap States for the Catalytic Reduction of CO2 by In2O3- xOH y Nanorods.

We use photoconductivity time, optical absorption, and electron quantum efficiency measurements under in situ reactant CO2 + H2 atmospheres to determine the role of surface trap states during photoreduction of CO2 to CO using In2O3- xOH y nanorods of varied annealing times. Photocurrent decay trends show an asymmetric energy distribution of surface barrier potentials with increased asymmetry from vacuum to CO2 + H2. Urbach analysis shows crystalline disorder parameters of 0.35-0.40 under vacuum and 0.45-0.6 under CO2 + H2. Quantum efficiency spectra show that under H2 + CO2 average tail state energies are similar to those under vacuum but with increased densities of photoconductive gap states. Photoelectro-paramagnetic-resonance measurements show the creation of new paramagnetic centers. Overall, enhanced activity is associated with a lower maximum barrier potential of 0.39 eV than that of 0.41 eV, lower average trap energies of 2.58 eV compared to 2.69 eV, with higher disorder due to increased surface state densities. These techniques pave the way for facile in situ probing of gas-phase photocatalysts, providing simple macrolevel understanding of adsorbed reactants on surface band bending, thus correlating to catalytic efficacy.