Hot Carrier Transfer-Induced Photodegradation at a Thiolated Au/TiO2 Interface under X-ray Irradiation

This work presents a mechanistic interpretation of hot carrier-driven photoreduction of Au NPs coupled with the oxidative photodegradation of thiol-containing capping ligands on Au/TiO2 catalysts under X-ray and visible-light irradiation. The exposure of X-rays leads to the photoexcitation and transfer of hot electrons from TiO2 to Au nanoparticles (NPs) across the Au/TiO2 heterojunction and the photoreduction of Au NPs. In addition, driven by the dominant electron-withdrawing groups in the L-glutathione capping ligand, the energized hot electrons on Au were found to delocalize and deposit onto neighboring S atoms. While the quantity of the Au-S bonds remained constant, a photoredox cycle was completed by oxidative photodegradation of the dangling carboxylic and amine groups via means of deamination and deprotonation. Control experiments on bare anatase TiO2 and Au NPs found no changes in terms of the surface compositional and electronic structures. Photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectroscopy were employed to investigate the carrier dynamics on Au NP and Au/TiO2 and revealed that the presence of TiO2 largely avoids the recombination of hot carriers by extending their lifetime. Kinetic analysis of the photodriven interfacial chemistry suggests a two-stage flux-dependent hot carrier transfer dynamics, corresponding to single- and multiple-electron harvesting processes on TiO2. Further introduction of visible-light pumps hot electrons from Au NP to neighboring S atoms, revealing the plasmonic nature of the Au NPs. The enhanced band bending at the Au/TiO2 heterojunction effectively prevents the back-flow of hot electrons to TiO2 and decreases the probability of hot carrier recombination. The present study in a model catalytic system, thiolated Au/TiO2, outlines the transfer of hot carriers under photoexcitation and the redistribution of electron density under the influence of capping ligands. Implications of the analysis of photosensitive materials using spectroscopic methodologies are also discussed.