Understanding the Enhancement Mechanisms of Surface Plasmon-Mediated Photoelectrochemical Electrodes: A Case Study on Au Nanoparticle Decorated TiO₂ Nanotubes

Surface plasmon resonance (SPR) enhancement in photocatalyst and photovoltaics has been widely studied and different enhancement mechanisms have been established based on different heterostructure interface configurations. This work is intended to unveil the mechanisms behind charge or energy transfer in different plasmonic configurations of metal particle-semiconductor interfaces, especially with a dielectric layer. For this purpose, a series of composite photoelectrodes based on anodic TiO₂ nanotube (TONT) backbones coated with Au, Al₂O_3, or both are designed and characterized systematically. In conjunction with both experimental measurements and numerical simulations, it is revealed that in the TONT-Al₂O₃-Au electrode system (i.e., a thin nonconductive spacer between semiconductor and metal), the enhancement is dominantly governed by SPR-mediated hot-electron injection rather than conventional-thought near-field electromagnetic enhancement. Among all configurations, the TONT-Au-Al₂O₃ electrode shows the best photoresponse in both UV and visible regions. The superior visible light response of the TONT-Au-Al₂O₃ electrode is ascribed to the Al₂O₃ intensified local electromagnetic field that enhances the hot-electron injection through the TiO₂-Au interface, and an effective surface passivation by the Al₂O₃ coating. With the presence of a nonconductive spacer, the enhancement mechanism of TONT (TiO₂ nanotubes)-Al₂O₃-Au plasmonic electrodes is dominantly governed by surface plasmon resonance (SPR)-mediated hot-electron injection rather than near-field electromagnetic enhancement. The superior visible light response of the TONT-Au-Al₂O₃ electrode is ascribed to the Al₂O₃ intensified local electromagnetic field that enhances the hot-electron injection process, and an effective surface passivation by the Al₂O₃ coating.