Understanding the Chemical Mechanism behind Photoinduced Enhanced Raman Spectroscopy.

Photoinduced enhanced Raman spectroscopy (PIERS) is a new surface enhanced Raman spectroscopy (SERS) modality with a 680% Raman signal enhancement of adsorbed analytes over that of SERS. Despite the explosion in recent demonstrations, the PIERS mechanism remains undetermined. Using X-ray and time-resolved optical spectroscopies, electron microscopy, cyclic voltammetry, and density functional theory simulations, we elucidate the atomic-scale mechanism behind PIERS. Stable PIERS substrates were fabricated using self-organized arrays of TiO2 nanotubes with controlled oxygen vacancy doping and size-controlled silver nanoparticles. The key source of PIERS vs SERS enhancement is an increase in the Raman polarizability of the adsorbed analyte upon photoinduced charge transfer. A balance between improved crystallinity, which enhances charge transfer due to higher electron mobility but decreases light absorption, and increased oxygen vacancy defect concentration, which increases light absorption, is critical. This work enables the rational design of PIERS substrates for sensing.

Enhancing photocatalytic activities of titanium dioxide via well-dispersed copper nanoparticles.

The modification of titanium dioxide (TiO2) using noble metal nanoparticles is considered as a promising technique to make electrode with outstanding photocatalytic performance. In this paper, self-organized anodic TiO2 nanotube arrays were decorated with well-distributed small Cu nanoparticles through a novel technique that combines magnetron sputtering and thermal dewetting. The obtained nanocomposite catalyst exhibited 4-fold increase in the photodegradation rate of methylene blue aqueous solution under solar light irradiation than anatase TiO2 prepared with same anodization conditions. The enhanced photocatalytic activity was attributed to the synergistic effect of Schottky barrier and Surface plasmon resonance. The influence of post annealing process, sputtering time and thermal dewetting temperature on photocatalytic performance was studied and the optimal preparation conditions were proposed. The results of this study may provide a new strategy to improve photocatalytic efficiency of TiO2 without using high-cost noble metals.