Photo-Induced Enhanced Raman Spectroscopy (PIERS) is a new surface enhanced Raman spectroscopy (SERS) modality with an order-of-magnitude Raman signal enhancement of adsorbed analytes over that of typical SERS substrates. Despite the impressive PIERS enhancement factors and explosion in recent demonstrations of its utility, the detailed enhancement mechanism remains undetermined. Using a range of optical and X-ray spectroscopies, supported by density functional theory calculations, we elucidate the chemical and atomic-scale mechanism behind the PIERS enhancement. Stable PIERS substrates with enhancement factors of 10^6 were fabricated using self-organized hexagonal arrays of TiO2 nanotubes that were defect-engineered via annealing in inert atmospheres, and silver nanoparticles were deposited by magnetron sputtering and subsequent thermal dewetting. We identified the key source of the enhancement of PIERS vs. SERS in these structures as an increase in the Raman polarizability of the adsorbed probe molecule upon photo-induced charge transfer. A balance between crystallinity, which enhances charge transfer due to higher electron mobility in anatase-rutile heterostructures but decreases visible light absorption, and oxygen vacancy defects, which increase visible light absorption and photo-induced electron transfers, was critical to achieve high PIERS enhancements.
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