The discovery of the photoelectric effect by Heinrich Hertz in 1887 set the foundation for over 125 years of hot carrier science and technology. In the early 1900s it played a critical role in the development of quantum mechanics, but even today the unique properties of these energetic, hot carriers offer new and exciting opportunities for fundamental research and applications. Measurement of the kinetic energy and momentum of photoejected hot electrons can provide valuable information on the electronic structure of materials. The heat generated by hot carriers can be harvested to drive a wide range of physical and chemical processes. Their kinetic energy can be used to harvest solar energy or create sensitive photodetectors and spectrometers. Photoejected charges can also be used to electrically dope two-dimensional materials. Plasmon excitations in metallic nanostructures can be engineered to enhance and provide valuable control over the emission of hot carriers. This Review discusses recent advances in the understanding and application of plasmon-induced hot carrier generation and highlights some of the exciting new directions for the field.

Plasmon-induced hot carrier science and technology

Optical generation of hot electrons in metallic structures and its potential as an alternative to conventional electron–hole separation in semiconductor devices are reviewed. The possibilities for realizing high conversion efficiencies with low fabrication costs are discussed along with challenges in terms of the materials, architectures and fabrication methods

Plasmon-induced hot-electron generation at nanoparticle/metal-oxide interfaces for photovoltaic and photocatalytic devices

Band bending in semiconductors: chemical and physical consequences at surfaces and interfaces.

Heterogeneous catalysis is of paramount importance in chemistry and energy applications. Catalysts that couple light energy into chemical reactions in a directed, orbital-specific manner would greatly reduce the energy input requirements of chemical transformations, revolutionizing catalysis-driven chemistry. Here we report the room temperature dissociation of H2 on gold nanoparticles using visible light. Surface plasmons excited in the Au nanoparticle decay into hot electrons with energies between the vacuum level and the work function of the metal. In this transient state, hot electrons can transfer into a Feshbach resonance of an H2 molecule adsorbed on the Au nanoparticle surface, triggering dissociation. We probe this process by detecting the formation of HD molecules from the dissociations of H2 and D2 and investigate the effect of Au nanoparticle size and wavelength of incident light on the rate of HD formation. This work opens a new pathway for controlling chemical reactions on metallic catalysts.

Hot Electrons Do the Impossible: Plasmon-Induced Dissociation of H2 on Au

Metal nanoparticles stabilized on a support material catalyse many major industrial reactions. Metal-support interactions in these nanomaterials can have a substantial influence on the catalysis, making metal-support interaction modulation one of the few tools able to enhance catalytic performance. This topic has received much attention in recent years, however, a systematic rationalization of the field is lacking due to the great diversity in catalysts, reactions and modification strategies. In this review, we cover and categorize the recent progress in metal-support interaction tuning strategies to enhance catalytic performance for various reactions. Furthermore, we quantify the productivity enhancements resulting from metal-support interaction control that have been achieved in C1 chemistry in recent years. Our analysis shows that up to fifteen-fold productivity enhancement has been achieved, and that metal-support interaction is most impactful for metal nanoparticles smaller than four nanometres. These findings demonstrate the importance of metal-support interaction to improve performance in catalysis. Methods to control the performance of heterogeneous catalysts are extremely relevant to the success of industrial processes. This review provides a rationalization of the effects that metal support interactions have on the reactivity of different catalytic systems, emphasizing strategies to tune such effects.

Control of metal-support interactions in heterogeneous catalysts to enhance activity and selectivity

A continuous flow of hot electrons that are not at thermal equilibrium with the surrounding metal atoms is generated by the absorption of photons. Here we show that hot electron flow generated on a gold thin film by photon absorption (or internal photoemission) is amplified by localized surface plasmon resonance. This was achieved by direct measurement of photocurrent on a chemically modified gold thin film of metal-semiconductor (TiO2) Schottky diodes. The short-circuit photocurrent obtained with low-energy photons is consistent with Fowler’s law, confirming the presence of hot electron flows. The morphology of the metal thin film was modified to a connected gold island structure after heating such that it exhibits surface plasmon. Photocurrent and optical measurements on the connected island structures revealed the presence of a localized surface plasmon at 550 ± 20 nm. The results indicate an intrinsic correlation between the hot electron flow generated by internal photoemission and localized surface p...

Surface Plasmon-Driven Hot Electron Flow Probed with Metal-Semiconductor Nanodiodes

Atomic-scale defects on carbon nanostructures have been considered as detrimental factors and critical problems to be eliminated in order to fully utilize their intrinsic material properties such as ultrahigh mechanical stiffness and electrical conductivity. However, defects that can be intentionally controlled through chemical and physical treatments are reasonably expected to bring benefits in various practical engineering applications such as desalination thin membranes, photochemical catalysts, and energy storage materials. Herein, we report a defect-engineered self-assembly procedure to produce a three-dimensionally nanohole-structured and palladium-embedded porous graphene hetero-nanostructure having ultrahigh hydrogen storage and CO oxidation multifunctionalities. Under multistep microwave reactions, agglomerated palladium nanoparticles having diameters of ∼10 nm produce physical nanoholes in the basal-plane structure of graphene sheets, while much smaller palladium nanoparticles are readily impreg...

Nanohole-Structured and Palladium-Embedded 3D Porous Graphene for Ultrahigh Hydrogen Storage and CO Oxidation Multifunctionalities

We report the catalytic activity of Au/TiO2 and Pt/TiO2 nanocatalysts under CO oxidation fabricated by arc plasma deposition (APD), which is a facile dry process involving no organic materials. Using APD, the catalyst nanoparticles were well dispersed on the TiO2 powder with an average particle size of 2–4 nm, well below that of nanoparticles prepared by the sol–gel method (10 nm). We found that the average particle size of the dispersed gold nanoparticles can be controlled by changing the plasma discharge voltage of APD. Accordingly, the amount of gold loaded on the TiO2 powder increased as the discharge voltage increased, but the specific surface area of the Au/TiO2 samples decreased. As for catalytic reactivity, Au/TiO2 showed a higher catalytic activity than Pt/TiO2 in CO oxidation. The catalytic activity of the Au/TiO2 samples showed size dependence where higher catalytic activity occurred on smaller gold nanoparticles. This study suggests that APD is a simple way to fabricate catalytically active nanocatalysts.

Catalytic activity of Au/TiO2 and Pt/TiO2 nanocatalysts prepared with arc plasma deposition under CO oxidation

Hierarchical N-doped TiO2 nanostructured catalysts with micro-, meso-, and macroporosity are synthesized by a facile self-formation route using ammonia and titanium isopropoxide precursor. UV–vis diffuse reflectance spectra confirm the red shift and band gap narrowing due to the doping of N species in the TiO2 nanoporous catalyst. Hierarchical macroporosity with fibrous channel patterning is observed and well preserved even after calcination at 800 °C, indicating thermal stability, whereas micro- and mesoporosity are lost after calcination at 500 °C. The photocatalytic activity of hiearchical N doped TiO2 catalysts loaded with Au is evaluated for H2 production reaction in visible light. The enhanced photocatalytic activity is attributed to the combined synergetic effect of N doping for visible light absorption, micro- and mesoporosity for an increase of effective surface area and light harvestation, and hierarchical macroporous fibrous structure for multiple reflection and effective charge transfer.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/admi.201300018

Enhanced H2 Generation of Au‐Loaded, Nitrogen‐Doped TiO2 Hierarchical Nanostructures under Visible Light

We report the catalytic activity of Pt/SiO2 nanocatalysts synthesized via the ultrasonic spray pyrolysis (USP) process under CO oxidation. We found that the average particle size of the dispersed platinum nanoparticles can be controlled by changing the concentration of the Pt precursor and the calcination conditions. The amount of loaded platinum on the SiO2 powder increased as the precursor concentration increased, while the specific surface area of the Pt/SiO2 samples decreased. As the calcination temperature and time increased, the size of the platinum particles on the SiO2 increased. As for catalytic reactivity, high loading of Pt/SiO2 showed a higher conversion of CO. The turnover rate of the Pt/SiO2 catalysts increased after calcination at 600 °C, then decreased after calcination at 750 °C, mainly due to agglomeration at the high temperature and partly because of severe oxidation. The catalytic activity of the Pt/SiO2 nanocatalysts synthesized using USP exhibited higher catalytic activity compared with Pt/SiO2 synthesized via wet chemical synthesis or wetness impregnation. It is attributed to better dispersion of the nanoparticles on the SiO2 as well as the removal of hydrocarbon impurities during calcination. This work demonstrated the successful use of the spray pyrolysis process for synthesis of oxide-supported metal catalysts with high thermal stability and activity.

Chan Ho Jung

Papers

Surface Plasmon-Driven Hot Electron Flow Probed with Metal-Semiconductor Nanodiodes

Nanohole-Structured and Palladium-Embedded 3D Porous Graphene for Ultrahigh Hydrogen Storage and CO Oxidation Multifunctionalities

Catalytic activity of Au/TiO2 and Pt/TiO2 nanocatalysts prepared with arc plasma deposition under CO oxidation

Enhanced H2 Generation of Au‐Loaded, Nitrogen‐Doped TiO2 Hierarchical Nanostructures under Visible Light

Catalytic activity of Pt/SiO2 nanocatalysts synthesized via ultrasonic spray pyrolysis process under CO oxidation