Role of Adsorbed Water on Charge Carrier Dynamics in Photoexcited TiO₂

Abstract

Overall photocatalytic water splitting is one of the most sought after processes for sustainable solar-to-chemical energy conversion. The efficiency of this process strongly depends on charge carrier recombination and interaction with surface adsorbates at different time scales. Here, we investigated how hydration of TiO2 P25 affects dynamics of photogenerated electrons at the millisecond to minute time scale characteristic for chemical reactions. We used rapid scan diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS). The decay of photogenerated electron absorption was substantially slower in the presence of associated water. For hydrated samples, the charge carrier recombination rates followed an Arrhenius-type behavior in the temperature range of 273–423 K; these became temperature-independent when the material was dehydrated at temperatures above 423 K or cooled below 273 K. A DFT+U analysis revealed that hydrogen bonding with adsorbed water stabilizes surface-trapped holes at anatase TiO2(101) facet and lowers the barriers for hole migration. Hence, hole mobility should be higher in the hydrated material than in the dehydrated system. This demonstrates that adsorbed associated water can efficiently stabilize photogenerated charge carriers in nanocrystalline TiO2 and suppress their recombination at the time scale up to minutes.

The influence of Al doping on the photocatalytic activity of nanostructured ZnO: The role of adsorbed water

“…Notably, these findings justify both the results of photoactivity and adsorption of IC in the dark. In the relevant literature the hydrophilicity of the surface has been often reported as a key parameter strongly affecting the activation energy [48], the dynamics of the charge carriers [49], the conversion and selectivity towards target compounds [50] of photocatalytic reactions. The higher affinity of the Al doped samples for water implies a higher degree of surface hydroxylation.…”

Water-Assisted Hole Trapping at the Highly Curved Surface of Nano-TiO2 Photocatalyst

“…Transient absorption spectroscopy has been applied in a wide time scale and wavelength range. 10 − 21 In particular, it is well-known that photogenerated electrons in the conduction band and in shallow trap states reveal a Drude-like broad absorption band in the mid infrared. 16 , 21 Trapped carriers have been also investigated theoretically, although most of the calculations were done using slab-type models of anatase TiO 2 , mimicking a single-crystal surface.…”

Probing the Interfaces in Nanosilica‐Supported TiO2 Photocatalysts by Solid‐State NMR and In Situ FTIR

“…The IR signal for OH was not visible in the DFNS/Ti‐2.5‐OD sample at 30 °C due to masking by signals of adsorbed water molecules (Figure a). However, on heating, we started observing the signal at 3732 cm −1 that can be due to the both silanols as well as the titanols (Figure b) . On heating at higher temperatures the intensity of the OH signals increased till 600 °C.…”