(Invited) Plasmon Resonant Amplification of Hot Electron-Driven Photocatalysis

Abstract

We report measurements of photocatalytic water splitting using Au films with and without TiO2 coatings. In these structures, a thin (3–10 nm) film of TiO2 is deposited using atomic layer deposition (ALD) on top of a 100 nm thick Au film. We utilize an AC lock-in technique, which enables us to detect the relatively small photocurrents (~mA) produced by the short-lived hot electrons that are photoexcited in the metal. Under illumination, the bare Au film produces a small AC photocurrent (<1µA) for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) due to hot electrons and hot holes, respectively, that are photoexcited in the Au film. The samples with TiO2 produce a larger AC photocurrent indicating that hot electrons are being injected from the metal into the TiO2 semiconductor where they then reduce hydrogen ions in solution forming H2 (i.e., 2H+ + 2e- → H2). The AC photocurrent exhibits a narrow peak when plotted as a function of reference potential, which is a signature of hot electrons. Here, we photoexcite a monoenergetic source of hot electrons, which produces a peak in the photocurrent, as the electrode potential is swept through the resonance with the redox potential of the desired half-reaction. This stands in contrast to conventional bulk semiconductor photocatalysts, whose AC photocurrent saturates beyond a certain potential (i.e., light limited photocurrent). The photocurrents produced at the metal–liquid interface are smaller than those of the metal–semiconductor system, mainly because, in the metal–semiconductor system, there is a continuum of energy and momentum states that each hot electron can be injected into, while for an ion in solution, the number of energy and momentum states are very small.1 2, 3 We also report plasmon resonant excitation of hot electrons in a metal-based photocatalyst in contact with ferrocene redox couple in acetonitrile solution. Here the photocatalyst consists of a 50nm thick Au film and a top thin (5nm) film of Al2O3 deposited using atomic layer deposition. In this configuration, hot electrons, photo-excited in the metal, jump over the oxide barrier ultimately reducing ferrocenium in solution (i.e., Fe(C5H5)2 + + e- → Fe(C5H5)2) producing a photocurrent. In order to amplify this process, the bottom gold electrode is patterned into a plasmon resonant grating structure with a pitch of 500nm. The photocurrent (i.e., charge transfer rate) is measured as a function of incident angle using 633nm wavelength light. We observe peaks in the photocurrent at incident angles of ±10o 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 2.3X in the charge transfer rate. At these same angles, we also observe sharp dips in the photo-reflectance, 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 (FDTD) simulations also predict sharp dips in the photoreflectance at ±10o, and the electric field intensity profiles show clear excitation of a plasmon-resonant mode when illuminated at those angles with p-polarized light.