Proceedings of nanoGe September Meeting 2017 (NFM17)
Publication date: 20th June 2016
Scalable technology for efficient solar-to-chemical energy conversion is one of the most sought after processes for storage of intermittently available renewable energy. Photocatalytic water splitting can potentially address this issue. However, efficiency of most photocatalytic systems remains very low due to the mismatch between short lifetimes of photogenerated charge carriers (fs–ns) and slow kinetics of the redox reactions (s–min). In some instances trapping of photogenerated electrons and/or holes can substantially increase their lifetimes enabling the desired redox reactions. Therefore, an in-depth understanding of charge carrier trapping and the influence of surface groups and adsorbates on this process is key for rational material optimization.
In this work we used rapid scan time-resolved diffuse reflectance Fourier transform mid-infrared spectroscopy (DRIFTS) to study dynamics of long-living photogenerated electrons (PGE) in TiO2 P25 at subsecond to minutes timescale. We found that the signal of PGE was substantially stronger and their decay rates were slower in the presence of associated adsorbed water on the nanoparticle surface as compared to TiO2 samples dehydrated at elevated temperatures. Temperature-dependent measurements revealed that the PGE decay rates in hydrated TiO2 follow Arrhenius-type behavior in the 293–423 K temperature range with Ea = (0.06 – 0.17 eV) but became temperature independent at T > 473 K and T < 273 K. TiO2 dehydrated at high temperature exhibited hysteresis of PGE decay rates which were temperature-independent even when the material was cooled below 473 K. Materials behavior was restored upon rehydration. Temperature-dependent measurements performed with the TiO2 samples subjected to different pretreatments revealed a prominent dependence of the apparent activation energy with materials hydration. Based on the experimental results and a theoretical DFT+U analysis we conclude that hydrogen-bonding between associated adsorbed water and the oxide surface stabilizes surface-trapped holes and slows down charge carrier recombination rates.
published as: A. Litke et al., J. Phys. Chem. C 121, 7514, 2017.
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