On the Detection of Chemically-Induced Hot Electrons in Surface Processes: from X-ray Edges to Schottky Barriers

Plasmon of Au nanorods activates metal-organic frameworks for both the hydrogen evolution reaction and oxygen evolution reaction

Electrocatalytic water splitting holds great promise for renewable energy conversion and storage systems. However, it usually suffers from sluggish kinetics, which greatly hinders its real application. Here, we demonstrate the utilization of the localized surface plasmon resonance (LSPR) of Au nanorods (AuNRs) to significantly improve the electroactivity of both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) at Co-MOF nanosheets (Co-MOFNs) under different polarizations. Theoretical calculations suggest that the HER enhancement can be largely attributed to the injection of hot electrons from plasmonic AuNRs to Co-MOFN catalysts, which upraises the Fermi level of Co-MOFNs, increasing their reductive activity towards the HER. Regarding the promotion of the OER, it is indicated that the formed holes in Co-MOFNs should majorly locate on the surface oxygen atoms, which may also serve as active positions working jointly with neighboring Co atoms in oxidizing OH-. The plasmon enhanced HER and OER electrocatalysis could also be observed over AuNR/Ni-MOFN and AuNR/NiCo-MOFN catalysts, suggesting the generality of this strategy. This study highlights the possibility of accelerating both the HER and OER efficiency by AuNR plasmonic excitation and provides a new route towards the design of more efficient water splitting systems with the assistance of light energy.

Direct Imaging of Surface Plasmon-Driven Hot Electron Flux on the Au Nanoprism/TiO2

Direct measurement of hot electron flux from a plasmonic Schottky nanodiode is important for obtaining fundamental insights explaining the mechanism for electronic excitation on a surface. Here, we report the measurement of photoinduced hot electrons on a triangular Au nanoprism on TiO₂ under incident light with photoconductive atomic force microscopy (pc-AFM), which is direct proof of the intrinsic relation between hot electrons and localized surface plasmon resonance. We find that the local photocurrent measured on the boundary of the Au nanoprism is higher than that inside the Au nanoprism, indicating that field confinement at the boundary of the Au nanoprism acts as a hot spot, leading to the enhancement of hot electron flow at the boundary. Under incident illumination with a wavelength near the absorption peak (645 nm) of a single Au nanoprism, localized surface plasmon resonance resulted in the generation of a higher photoinduced hot electron flow for the Au nanoprism/TiO₂, compared with that at a wavelength of 532 nm. We show that the application of a reverse bias results in a higher photocurrent for the Au nanoprism/TiO₂, which is associated with a lowering of the Schottky barrier height caused by the image force. These nanoscale measurements of hot electron flux with pc-AFM indicate efficient photon energy transfer mediated by surface plasmons in hot electron-based energy conversion.

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.

Hot Electrons at Solid-Liquid Interfaces: A Large Chemoelectric Effect during the Catalytic Decomposition of Hydrogen Peroxide

The study of energy and charge transfer during chemical reactions on metals is of great importance for understanding the phenomena involved in heterogeneous catalysis. Despite extensive studies, very little is known about the nature of hot electrons generated at solid-liquid interfaces. Herein, we report remarkable results showing the detection of hot electrons as a chemicurrent generated at the solid-liquid interface during decomposition of hydrogen peroxide (H2 O2 ) catalyzed on Schottky nanodiodes. The chemicurrent reflects the activity of the catalytic reaction and the state of the catalyst in real time. We show that the chemicurrent yield can reach values up to 10(-1) electrons/O2 molecule, which is notably higher than that for solid-gas reactions on similar nanodiodes.

The effect of hot electrons and surface plasmons on heterogeneous catalysis

Hot electrons and surface-plasmon-driven chemistry are amongst the most actively studied research subjects because they are deeply associated with energy dissipation and the conversion processes at the surface and interfaces, which are still open questions and key issues in the surface science community. In this topical review, we give an overview of the concept of hot electrons or surface-plasmon-mediated hot electrons generated under various structural schemes (i.e. metals, metal-semiconductor, and metal-insulator-metal) and their role affecting catalytic activity in chemical reactions. We highlight recent studies on the relation between hot electrons and catalytic activity on metallic surfaces. We discuss possible mechanisms for how hot electrons participate in chemical reactions. We also introduce controlled chemistry to describe specific pathways for selectivity control in catalysis on metal nanoparticles.

Hot-electron-based solar energy conversion with metal-semiconductor nanodiodes

Energy dissipation at metal surfaces or interfaces between a metal and a dielectric generally results from elementary excitations, including phonons and electronic excitation, once external energy is deposited to the surface/interface during exothermic chemical processes or an electromagnetic wave incident. In this paper, we outline recent research activities to develop energy conversion devices based on hot electrons. We found that photon energy can be directly converted to hot electrons and that hot electrons flow through the interface of metal-semiconductor nanodiodes where a Schottky barrier is formed and the energy barrier is much lower than the work function of the metal. The detection of hot electron flow can be successfully measured using the photocurrent; we measured the photoyield of photoemission with incident photons-to-current conversion efficiency (IPCE). We also show that surface plasmons (i.e. the collective oscillation of conduction band electrons induced by interaction with an electromagnetic field) are excited on a rough metal surface and subsequently decay into secondary electrons, which gives rise to enhancement of the IPCE. Furthermore, the unique optical behavior of surface plasmons can be coupled with dye molecules, suggesting the possibility for producing additional channels for hot electron generation.

Chemical energy dissipation at surfaces under UHV and high pressure conditions studied using metal–insulator–metal and similar devices

Thin film metal heterostructures have allowed new light to be shed on the dissipation of chemical energy into electric excitations on metal surfaces.

Amplification of hot electron flow by the surface plasmon effect on metal-insulator-metal nanodiodes

Au-TiO2-Ti nanodiodes with a metal-insulator-metal structure were used to probe hot electron flows generated upon photon absorption. Hot electrons, generated when light is absorbed in the Au electrode of the nanodiode, can travel across the TiO2, leading to a photocurrent. Here, we demonstrate amplification of the hot electron flow by (1) localized surface plasmon resonance on plasmonic nanostructures fabricated by annealing the Au-TiO2-Ti nanodiodes, and (2) reducing the thickness of the TiO2. We show a correlation between changes in the morphology of the Au electrodes caused by annealing and amplification of the photocurrent. Based on the exponential dependence of the photocurrent on TiO2 thickness, the transport mechanism for the hot electrons across the nanodiodes is proposed.

Hot carrier-driven catalytic reactions on Pt-CdSe-Pt nanodumbbells and Pt/GaN under light irradiation

Hybrid nanocatalysts consisting of metal nanoparticle-semiconductor junctions offer an interesting platform to study the role of metal-oxide interfaces and hot electron flows in heterogeneous catalysis. Here, we report that hot carriers generated upon photon absorption significantly impact the catalytic activity of CO oxidation. We found that Pt-CdSe-Pt nanodumbbells exhibit a higher turnover frequency by a factor of 2 during irradiation by light with energy higher than the bandgap of CdSe, while the turnover rate on bare Pt nanoparticles did not depend on light irradiation. We found that Pt nanoparticles deposited on a GaN substrate under light irradiation exhibit changes in catalytic activity of CO oxidation that depends on the type of doping of the GaN. We suppose that hot electrons are generated upon the absorption of photons by the semiconducting nanorods or substrates, whereafter the hot electrons are injected into the Pt nanoparticles, resulting in the change in catalytic activity. The results imply that hot carrier flows generated during light irradiation significantly influence the catalytic activity of CO oxidation, leading to potential applications as a hot electron-based catalytic actuator.

Surface plasmon-driven hot electron flow probed with metal-semiconductor nanodiodes

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 (TiO(2)) 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 plasmon resonance.

Electronic excitations induced by hydrogen surface chemical reactions on gold

Associated with chemical reactions at surfaces energy may be dissipated exciting surface electronic degrees of freedom. These excitations are detected using metal-insulator-metal (MIM) heterostructures (Ta-TaOx-Au) and the reactions of H with and on a Au surface are probed. A current corresponding to 5×10(-5) electrons per adsorbing H atom and a marked isotope effect are observed under steady-state conditions. Analysis of the current trace when the H atom flux is intermitted suggests that predominantly the recombination reaction creates electronic excitations. Biasing the front versus the back electrode of the MIM structure provides insights into the spectrum of electronic excitations. The observed spectra differ for the two isotopes H and D and are asymmetric when comparing negative and positive bias voltages. Modeling indicates that the excited electrons and the concurrently created holes differ in their energy distributions.

Molecular studies of model surfaces of metals from single crystals to nanoparticles under catalytic reaction conditions. Evolution from prenatal and postmortem studies of catalysts

Molecular level studies of metal crystal and nanoparticle surfaces under catalytic reaction conditions at ambient pressures during turnover were made possible by the use of instruments developed at the University of California at Berkeley. Sum frequency generation vibrational spectroscopy (SFGVS), owing to its surface specificity and sensitivity, is able to identify the vibrational features of adsorbed monolayers of molecules. We identified reaction intermediates, different from reactants and products, under reaction conditions and for multipath reactions on metal single crystals and nanoparticles of varying size and shape. The high-pressure scanning tunneling microscope (HP-STM) revealed the dynamics of a catalytically active metallic surface by detecting the mobility of the adsorbed species during catalytic turnover. It also demonstrated the reversible and adsorbate-driven surface restructuring of platinum when exposed to molecules such as CO and ethylene. Ambient pressure X-ray photoelectron spectroscopy (AP-XPS) detected the reversible changes of surface composition in rhodium-palladium, platinum-palladium, and other bimetallic nanoparticles as the reactant atmosphere changed from oxidizing to reducing. It was found that metal nanoparticles of less than 2 nm in size are present in higher oxidation states, which alters and enhances their catalytic activity. The catalytic nanodiode (CND) confirmed that a catalytic reaction-induced current flow exists at oxide-metal interfaces, which correlates well with the reaction turnover.

The Interplay of Energy Disposal and Reaction Rates in Exoergic Processes at Metal Surfaces: Desorption Rates in Vibrationally Excited Adlayers

We report on the desorption kinetics of adsorbates from metal surfaces in the presence of exoergic-processes, either atom recombination or non-dissociative adsorption. The modeling takes into account the dissipation of the adatom energy to both the solid and the adlayer and permits one to investigate the effect of these processes on the desorption rate. The model kinetics, which is solved analytically for two recombination channels under non steady state conditions, shows that the desorption rate is ruled by the ratio between the rate constant for recombination and for energy loss to the solid. The behavior of the vibrational distribution function of the adatoms on this control parameter is analyzed and compared with Boltzmann equilibrium distributions. Rate coefficients for energy disposal to a metal surface, evaluated for a relaxation process involving electron-hole pair excitation, are shown to be linked to the electron density, which than becomes a control parameter of reaction rates at metal surfaces.

Hydrogen oxidation-driven hot electron flow detected by catalytic nanodiodes

Hydrogen oxidation on platinum is shown to be a surface catalytic chemical reaction that generates a steady state flux of hot (>1 eV) conduction electrons. These hot electrons are detected as a steady-state chemicurrent across Pt/TiO(2) Schottky diodes whose Pt surface is exposed to hydrogen and oxygen. Kinetic studies establish that the chemicurrent is proportional to turnover frequency for temperatures ranging from 298 to 373 K for P(H2) between 1 and 8 Torr and P(O2) at 760 Torr. Both chemicurrent and turnover frequency exhibit a first order dependence on P(H2).

Density functional theory for transition metals and transition metal chemistry

We introduce density functional theory and review recent progress in its application to transition metal chemistry. Topics covered include local, meta, hybrid, hybrid meta, and range-separated functionals, band theory, software, validation tests, and applications to spin states, magnetic exchange coupling, spectra, structure, reactivity, and catalysis, including molecules, clusters, nanoparticles, surfaces, and solids.

Chemistry of fast electrons

A chemicurrent is a flux of fast (kinetic energy approximately > 0.5-1.3 eV) metal electrons caused by moderately exothermic (1-3 eV) chemical reactions over high work function (4-6 eV) metal surfaces. In this report, the relation between chemicurrent and surface chemistry is elucidated with a combination of top-down phenomenology and bottom-up atomic-scale modeling. Examination of catalytic CO oxidation, an example which exhibits a chemicurrent, reveals 3 constituents of this relation: The localization of some conduction electrons to the surface via a reduction reaction, 0.5 O(2) + deltae(-) --> O(delta(-)) (Red); the delocalization of some surface electrons into a conduction band in an oxidation reaction, O(delta(-)) + CO --> CO(2)(delta-) --> CO(2) + deltae(-) (Ox); and relaxation without charge transfer (Rel). Juxtaposition of Red, Ox, and Rel produces a daunting variety of metal electronic excitations, but only those that originate from CO(2) reactive desorption are long-range and fast enough to dominate the chemicurrent. The chemicurrent yield depends on the universality class of the desorption process and the distribution of the desorption thresholds. This analysis implies a power-law relation with exponent 2.66 between the chemicurrent and the heat of adsorption, which is consistent with experimental findings for a range of systems. This picture also applies to other oxidation-reduction reactions over high work function metal surfaces.

Probing hot electron flow generated on Pt nanoparticles with Au/TiO2 Schottky diodes during catalytic CO oxidation

Hot electron flow generated on colloid platinum nanoparticles during exothermic catalytic carbon monoxide oxidation was directly detected with Au/TiO2 diodes. Although Au/TiO2 diodes are not catalytically active, platinum nanoparticles on Au/TiO2 exhibit both chemicurrent and catalytic turnover rate. Hot electrons are generated on the surface of the metal nanoparticles and go over the Schottky energy barrier between Au and TiO2. The continuous Au layer ensures that the metal nanoparticles are electrically connected to the device. The overall thickness of the metal assembly (nanoparticles and Au thin film) is comparable to the mean free path of hot electrons, resulting in ballistic transport through the metal. The chemicurrent and chemical reactivity of nanoparticles with citrate, hexadecylamine, hexadecylthiol, and TTAB (tetradecyltrimethylammonium bromide) capping agents were measured during catalytic CO oxidation at pressures of 100 Torr O2 and 40 Torr CO at 373-513 K. We found that chemicurrent yield varies with each capping agent but always decreases with increasing temperature. We suggest that this inverse temperature dependence is associated with the influence of charging effects due to the organic capping layer during hot electron transport through the metal-oxide interface.

Ab initio simulation of the spin transition during chemisorption: H/Al(111)

Electronically nonadiabatic effects during the chemisorption of hydrogen atoms on an Al(111) surface are simulated ab initio using time-dependent density-functional theory for the electrons in combination with Ehrenfest dynamics for the nuclei. Strongly nonadiabatic effects close to the spin transition of the H atom are identified, and the dissipated energy as well as the electron-hole pair excitation spectra are calculated. The recent Newns-Anderson-model approach by Mizielinski et al. is confirmed. The simulations illustrate the physical processes that contribute to internal exoelectron emission.

The Catalytic Nanodiode: Detecting Continous Electron Flow at Oxide-Metal Interfaces Generated by a Gas-Phase Exothermic Reaction

Continuous flow of ballistic charge carriers is generated by an exothermic chemical reaction and detected using the catalytic metal-semiconductor Schottky diode. We obtained a hot electron current for several hours using two types of catalytic nanodiodes, Pt/TiO2 or Pt/GaN, during carbon monoxide oxidation at pressures of 100 Torr of O2 and 40 Torr of CO at 413-573 K. This result reveals that the chemical energy of an exothermic catalytic reaction is directly converted into hot electrons flux in the catalytic nanodiode. By heating the nanodiodes in He, we could measure the thermoelectric current which is in the opposite direction to the flow of the hot electron current. The chemicurrent is well correlated with the turnover rate of CO oxidation, which is separately measured with gas chromatography. The influence of the flow of hot charge carriers on the chemistry at the oxide-metal interface, and the turnover rate in the chemical reaction are discussed.

Theoretical evidence for nonadiabatic vibrational deexcitation in H2(D2) state-to-state scattering from Cu(100)

Dynamical calculations are presented for electronically nonadiabatic vibrational deexcitation of H2 and D2 in scattering from Cu(111). Both the potential energy surface and the nonadiabatic coupling strength were obtained from density functional calculations. The theoretically predicted magnitude of the deexcitation and its dependence on incident energy and isotope are all in agreement with state-to-state scattering experiments [on Cu(100)], and this gives indirect evidence for a nonadiabatic mechanism of the observed deexcitation. Direct evidence could be obtained by measuring the chemicurrent associated with the deexcitation, and its properties have been predicted.