Strategies for Hot Electron-Mediated Catalytic Reactions: Catalytronics

Hot Electron Phenomena at Solid-Liquid Interfaces

Understanding the role of energy dissipation and charge transfer under exothermic chemical reactions on metal catalyst surfaces is important for elucidating the fundamental phenomena at solid-gas and solid-liquid interfaces. Recently, many surface chemistry studies have been conducted on the solid-liquid interface, so correlating electronic excitation in the liquid-phase with the reaction mechanism plays a crucial role in heterogeneous catalysis. In this review, we introduce the detection principle of electron transfer at the solid-liquid interface by developing cutting-edge technologies with metal-semiconductor Schottky nanodiodes. The kinetics of hot electron excitation are well correlated with the reaction rates, demonstrating that the operando method for understanding nonadiabatic interactions is helpful in studying the reaction mechanism of surface molecular processes. In addition to the detection of hot electrons excited by a catalytic reaction, we highlight recent results on how the transfer of the hot electrons influences surface chemical and photoelectrochemical reactions.

Chemistry and Charge Trapping at the Interface of Silver and Ultrathin Layers of Zinc Oxide

Zinc oxide, a wide-band-gap semiconductor, shows intriguing optoelectronic properties when coupled with Ag. Specifically, an absorbance band in the visible range that is not apparent in the separated materials emerges when the interface is formed. Interestingly, photoexcitation of this "interface band" or band-to-band results in a counterintuitive photovoltaic response when a supra/sub-band-gap light is shone. To investigate the origin of this absorbance band and photovoltaic response, we studied in detail the energy-band alignment of ultrathin layers of ZnO (3-60 nm) with Ag. Our analysis indicated that an 'electrostatic potential cliff' is formed within the first 1-2 nm of ZnO. In addition, oxygen vacancies, presumably generated by Ag_xO-Zn bonds, form mid-gap acceptor states within these first few nm. Both effects facilitate a valence band-to-defect state optical transition that is confined to the interface region. The second type of defects-hole-trap states associated with zinc hydroxide-are spread throughout the ZnO layer and dominate the supra-band-gap photovoltaic response. These findings have potential implications in emerging technologies such as photocatalytic Ag/ZnO heterostructures that will utilize the long-lived charges for chemical work or other optoelectronic applications.

Electronic Control of Hot Electron Transport Using Modified Schottky Barriers in Metal-Semiconductor Nanodiodes

Hot electron flux, generated by both incident light energy and the heat of the catalytic reaction, is a major element for energy conversion at the surface. Controlling hot electron flux in a reversible manner is extremely important for achieving high energy conversion efficiency. Here we demonstrate that hot electron flux can be controlled by tuning the Schottky barrier height. This phenomenon was monitored by using a Schottky nanodiode composed of a metal-semiconductor. The formation of a Schottky barrier at a nanometer scale inevitably accompanies an intrinsic image potential between the metal-semiconductor junction, which lowers the effective Schottky barrier height. When a reverse bias is applied to the nanodiode, an additional image potential participates in a secondary barrier lowering, leading to the increased hot electron flow. Besides, a decrease of tunneling width results in facile electron transport through the barrier. The increased hot electron flux by the chemical reaction (chemicurrent) and by the photon absorption (photocurrent) indicates hot electrons are captured more effectively by modifying the Schottky barrier. This study can shed light on a quantitative understanding and application of charge behavior at metal-semiconductor interfaces, in solar energy conversion, or in a catalytic reaction.

Charge Transfer during the Aluminum-Water Reaction Studied with Schottky Nanodiode Sensors

The aluminum-water reaction is a promising source for hydrogen production. However, experimental studies of this reaction are difficult because of the highly concentrated alkaline solution used to activate the surface of aluminum. Here, we show that the reaction kinetics can be monitored in real time by a Schottky diode sensor, consisting of an ultrathin aluminum film deposited on a semiconductor substrate. Charge resulting from the corrosion of the aluminum film causes an electrical signal in the sensor, which is proportional to the rate of the chemical process. We discuss the possible mechanisms for the reaction-induced charge generation and transfer, as well as the use of Schottky diode based sensors for operando studies of the aluminum-water reaction and similar reactions on metals in concentrated alkaline solutions.

Hot electron effects during reactive scattering of H2 from Ag(111): assessing the sensitivity to initial conditions, coupling magnitude, and electronic temperature

We use an analytical representation of electronic friction for H₂ on Ag(111) to assess the validity and robustness of the MDEF method based on TDPT.

Enhancing hot electron collection with nanotube-based three-dimensional catalytic nanodiode under hydrogen oxidation

A novel three-dimensional catalytic nanodiode composed of a Pt thin film on TiO₂ nanotubes was designed for the efficient detection of the flux of hot electrons, or chemicurrent, under hydrogen oxidation.