Plasmon resonant amplification of hot electron-driven photocatalysis

Abstract

We report plasmon resonant excitation of hot electrons in a metal based photocatalyst in the oxygen evolution half reaction in aqueous solution. Here, the photocatalyst consists of a 100-nm thick Au film deposited on a corrugated silicon substrate. In this configuration, hot electrons photoexcited in the metal are injected into the solution, ultimately reversing the water oxidation reaction (O2 + 4H+ + 4e− ⇋ 2H2O) and producing a photocurrent. In order to amplify this process, the gold electrode is patterned into a plasmon resonant grating structure with a pitch of 500 nm. The photocurrent (i.e., charge transfer rate) is measured as a function of incident angle using 633 nm wavelength light. We observe peaks in the photocurrent at incident angles of ±9° from normal when the light is polarized parallel to the incident plane (p-polarization) and perpendicular to the lines on the grating. Based on these peaks, we estimate an overall plasmonic gain (or amplification) factor of 2.1× in the charge transfer rate. At these same angles, we also observe sharp dips in the photoreflectance, corresponding to the condition when there is wavevector matching between the incident light and the plasmon mode in the grating. No angle dependence is observed in the photocurrent or photoreflectance when the incident light is polarized perpendicular to the incident plane (s-polarization) and parallel to the lines on the grating. Finite difference time domain simulations also predict sharp dips in the photoreflectance at ±9°, and the electric field intensity profiles show clear excitation of a plasmon-resonant mode when illuminated at those angles with p-polarized light.