Plasmon-Mediated Photochemical Reactions

Abstract

The Cu2O-based photocathode is considered as one of the best performing photocathode materials for solar water reduction. However, the relatively negative onset potential for H2 production of these photocathodes impedes further optimization of the solar-to-fuel conversion efficiency. An important reason is the inability to achieve meaningful photovoltages with Cu2O. Here, the photovoltage barrier can be readily broken by replacing the semiconductor/water interface with a semiconductor/semiconductor one. A n-Cu2O layer was found to form high-quality buried junction with p-Cu2O to increase the photovoltage and thus shift the turn-on voltage positively. Mott−Schottky Measurements confirmed that the improvement was benefited from a thermodynamic shift of the flatband potentials. An extremely positive onset potential in n-Cu2O/AuAg/p-Cu2O was further obtained, which is comparable to what has been measured using water reduction catalysts. The alloy nanoparticles sandwiched between the homojunction of n-Cu2O and p-Cu2O play an important role in enhancing the photocatalytic performance. First, the alloy nanoparticles not only serve as an electron relay, but also promote electron-hole pair generation in nearby semiconductors, which facilitates the charge transfer between n-Cu2O and p-Cu2O since the sandwich structure is measured by X-ray absorption spectroscopy. Second, the alloy nanoparticles act as a plasmonic photosensitizer, which enables the solar-to-hydrogen conversion at wavelengths longer than the band edge of homojunction structure, extending the incident photo-to-current conversion efficiency wavelength. Finally, the plasmonic energy-transfer mechanism is identified as direct transfer of the plasmonic hot carriers, and the interfacial Schottky barrier height is shown to modulate the plasmonic hot electron transfer. This facile sandwich structure combines both the electrical and the optical functions of alloy nanoparticles into a single structure, which has implications for the design of efficient solar-energy-harvesting devices.