Hot Electron and Surface Plasmon-Driven Catalytic Reaction in Metal–Semiconductor Nanostructures

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.

Manipulation of hot electron flow on plasmonic nanodiodes fabricated by nanosphere lithography

Energy conversion to generate hot electrons through the excitation of localized surface plasmon resonance (LSPR) in metallic nanostructures is an emerging strategy in photovoltaics and photocatalytic devices. Important factors for surface plasmon and hot electron generation are the size, shape, and materials of plasmonic metal nanostructures, which affect LSPR excitation, absorbance, and hot electron collection. Here, we fabricated the ordered structure of metal-semiconductor plasmonic nanodiodes using nanosphere lithography and reactive ion etching. Two types of hole-shaped plasmonic nanostructures with the hole diameter of 280 and 115 nm were fabricated on Au/TiO₂Schottky diodes. We show that hot electron flow can be manipulated by changing the size of plasmonic nanostructures on the Schottky diode. We show that the short-circuit photocurrent changes and the incident photon-to-electron conversion efficiency results exhibit the peak shift depending on the structures. These phenomena are explicitly observed with finite difference time domain simulations. The capability of tuning the morphology of plasmonic nanostructure on the Schottky diode can give rise to new possibilities in controlling hot electron generation and developing novel hot-electron-based energy conversion devices.

Growth of Nanostructured Silver Flowers by Metal-Mediated Catalysis for Surface-Enhanced Raman Spectroscopy Application

Metallic flowers with nanoscale surface roughness can provide a platform for highly sensitive and reproductive surface-enhanced Raman spectroscopy (SERS). Here, we present a method to grow a nanostructured silver flower (NSF) at the apex of a plasmonic tip based on metal-mediated catalysis, where the NSF was rapidly generated in no more than 1 min. The NSF was used as the SERS substrate under linear polarization beam (LPB) excitation to achieve a 10^-9 M detection sensitivity for the malachite green analyte. The reproducibility for SERS is examined to have been guaranteed by comparing Raman intensity enhanced by different NSFs. Compared with the LPB, the azimuthal vector beam (AVB) excitation can further improve the SERS activity of the NSF, which is consistent with the simulation result that the gap mode can be effectively generated between two adjacent Ag nanoparticles (NPs) and between the NPs and the Ag pyramids on the surface of the NSF under AVB illumination. This work makes it promising for plasmonic tip-mediated catalysis to be applied in nanofabrication, the products of which can be further exploited in nanostructure-based ultrasensitive detection.

Cu-Ag Bimetal Alloy Decorated SiO2@TiO2 Hybrid Photocatalyst for Enhanced H2 Evolution and Phenol Oxidation under Visible Light

With a broader objective to replace visible light driven Pt-based photoelectrochemical/catalytic hydrogen evolution, a series of cost-effective bimetallic nanoalloys of Cu-Ag have been deposited on core-shell nanostructured SiO₂@TiO₂ through a facile reduction route. The physicochemical properties, i.e. crystal structure, morphology, chemical environment, and optical properties of Cu-Ag bimetal alloy decorated SiO₂@TiO₂ hybrid photocatalyst, have been thoroughly investigated through X-ray diffraction, high resolution transmission electron microscopy, field emission scanning electron microscopy, X-ray photoelectron spectroscopy, UV-vis diffuse reflectance spectroscopy, and photoluminescence spectroscopy, respectively. TEM study confirms the coating of an ultrathin layer of TiO₂ shell on 100 nm sized SiO₂ core, and about 4.5 nm of Ag-Cu nanoalloys are uniformly distributed on the core-shell nanostructure. The higher light absorption throughout the visible range and better separation of charge carrier by Ag-Cu (1:3) deposited SiO₂@TiO₂ hybrid compared to other counterparts is confirmed from UV-vis, diffuse reflectance spectroscopy, photoluminescence, and electrochemical impedance studies. Eightfold higher photocurrent enhancements, threefold enhanced photocatalytic hydrogen generation, and twofold higher phenol oxidation activities of Ag-Cu (1:3) deposited SiO₂@TiO₂ hybrid compared to those of the monometallic plasmonic catalyst may be attributed to the synergetic effect of enriched light harvesting and surface plasmon induced hot electron transfer from the nanoalloy to the TiO₂ interface, resulting in efficient charge transfer.

ZnO (Ag-N) Nanorods Films Optimized for Photocatalytic Water Purification

ZnO nanorods (NRs) films, nitrogen-doped (ZnO:N), and ZnO doped with nitrogen and decorated with silver nanostructures (ZnO:N-Ag) NRs films were vertically supported on undoped and N doped ZnO seed layers by a wet chemical method. The obtained films were characterized structurally by X-ray diffraction. Morphological and elemental analysis was performed by scanning electron microscopy, including an energy dispersive X-ray spectroscopy facility and their optical properties by Ultraviolet-Visible Spectroscopy. Analysis performed in the NRs films showed that the nitrogen content in the seed layer strongly affected their structure and morphology. The mean diameter of ZnO NRs ranged from 70 to 190 nm. As the nitrogen content in the seed layer increased, the mean diameter of ZnO:N NRs increased from 132 to 250 nm and the diameter dispersion decreased. This diameter increase occurs simultaneously with the incorporation of nitrogen into the ZnO crystal lattice and the increase in the volume of the unit cell, calculated using the X-ray diffraction patterns and confirmed by a slight shift in the XRD angle. The diffractograms indicated that the NRs have a hexagonal wurtzite structure, with preferential growth direction along the c axis. The SEM images confirmed the presence of metallic silver in the form of nanoparticles dispersed on the NRs films. Finally, the degradation of methyl orange (MO) in an aqueous solution was studied by UV-vis irradiation of NRs films contained in the bulk of aqueous MO solutions. We found a significant enhancement of the photocatalytic degradation efficiency, with ZnO:N-Ag NRs film being more efficient than ZnO:N NRs film, and the latter better than the ZnO NRs film.

Configuration-Modulated Hot Electron Dynamics of Gold Nanorod Assemblies

Comprehension and modulation of hot electron dynamics at an ultrafast time scale are crucial for exploring the hot electron-assisted energy transfer processes. Here, we report the hot electron dynamics of dispersed gold nanorods and their controlled assemblies measured by time-resolved pump-probe spectroscopy. Both assembly configurations are shown to accelerate the hot electron decay in comparison with dispersed nanorods. The hot electron dynamics exhibit different variations with aspect ratio in transverse and longitudinal polarizations. The hot electron lifetime and the spectral signature of the induced absorption modification are found to be highly sensitive to photon energy as well as assembly configuration and aspect ratios, showing different contributions of plasmon coupling and electron-surface scattering. This work not only improves the understanding of the underlying mechanisms of hot electron dynamics but also paves the way to optimize performance characteristics of hot carrier-assisted photocatalysis, photovoltaics, and all-optical high-rate photonic processing applications.

Plasmons of hollow nanobar oligomers

Assembling metal nano-objects into well-defined configurations is an effective way to create hybrid plasmonic structures with unusual functionalities.

Optical properties of aluminum intercalated magnesium nanoparticle square array: a computational study

Magnesium nanostructures have recently emerged as a vivid and amazing plasmonic material.

Exciton-plasmon hybrids for surface catalysis detected by SERS

Surface plasmons (SPs), in which the free electrons are collectively excited on the metal surface, have been successfully used in chemical analysis and signal detection. Generally, SPs possess two types of decay channels. SPs decay either nonradiatively via the generation of hot electrons or radiatively through re-emitted photons, which can trigger surface chemical reactions when the molecules are adsorbed on the surface of metal nanoparticles. An excitation light with a special wavelength is irradiated on the surface of the plasmonic nanostructure, the strong coupling interaction between electrons and light will then occur on this, and this is followed by the development of a series of unique properties. 2D materials have been a hot topic of research for more than a decade, since graphene was found in 2004. Recently, the combination of graphene with metal NPs has been shown to possess many supernormal advantages, such as high stability and catalytic activity, which have been successfully applied in plasmon-exciton co-driven chemical reactions.

Cr(VI) remediation from aqueous environment through modified-TiO2-mediated photocatalytic reduction

Cr(VI) exhibits cytotoxic, mutagenic and carcinogenic properties; hence, effluents containing Cr(VI) from various industrial processes pose threat to aquatic life and downstream users. Various treatment techniques, such as chemical reduction, ion exchange, bacterial degradation, adsorption and photocatalysis, have been exploited for remediation of Cr(VI) from wastewater. Among these, photocatalysis has recently gained considerable attention. The applications of photocatalysis, such as water splitting, CO2 reduction, pollutant degradation, organic transformation reactions, N2 fixation, etc., towards solving the energy crisis and environmental issues are briefly discussed in the Introduction of this review. The advantages of TiO2 as a photocatalyst and the importance of its modification for photocatalytic reduction of Cr(VI) has also been addressed. In this review, the photocatalytic activity of TiO2 after modification with carbon-based advanced materials, metal oxides, metal sulfides and noble metals towards reduction of Cr(VI) was evaluated and compared with that of bare TiO2. The photoactivity of dye-sensitized TiO2 for reduction of Cr(VI) was also discussed. The mechanism for enhanced photocatalytic activity was highlighted and attributed to the resultant properties, namely, effective separation of photoinduced charge carriers, extension of the light absorption range and intensity, increase of the surface active sites, and higher photostability. Advantages and limitations for photoreduction of Cr(VI) over modified TiO2 are depicted in the Conclusion. The various challenges that restrict the technology from practical applications in remediation of Cr(VI) from wastewater were addressed in the Conclusion section as well. The future perspectives of the field presented in this review are focused on the development of whole-solar-spectrum responsive, TiO2-coupled photocatalysts which provide efficient photocatalytic reduction of Cr(VI) along with their good recoverability and recyclability.

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.

Optical properties of magnesium nanorods using time dependent density functional theory calculations

Plasmonic nanostructures made of Earth-abundant and low-cost metals such as aluminum and magnesium have recently emerged as a potential alternative candidate to conventional plasmonic metals such as gold and silver.

Coupling of Crumpled-Type Novel MoS2 with CeO2 Nanoparticles: A Noble-Metal-Free p-n Heterojunction Composite for Visible Light Photocatalytic H2 Production

In terms of solar hydrogen production, semiconductor-based photocatalysts via p-n heterojunctions play a key role in enhancing future hydrogen reservoir. The present work focuses on the successful synthesis and characterization of a novel p-MoS₂/n-CeO₂ heterojunction photocatalyst for excellent performance toward solar hydrogen production. The synthesis involves a simple in situ hydrothermal process by varying the wt % of MoS₂. The various characterization techniques support the uniform distribution of CeO₂ on the surface of crumpled MoS₂ nanosheets, and the formation of p-n heterojunction is further confirmed by transmission electron microscopy and Mott-Schottky analysis. Throughout the experiment, it is demonstrated that 2 wt % MoS₂ in the MoS₂/CeO₂ heterojunction photocatalyst exhibits the highest rate of hydrogen evolution with a photocurrent density of 721 μA cm^-2. The enhanced photocatalytic activity is ascribed to the formation of the p-n heterojunction that provides an internal electric field to facilitate the photogenerated charge separation and transfer.

Optimization of Nanoparticle-Based SERS Substrates through Large-Scale Realistic Simulations

Surface-enhanced Raman scattering (SERS) has become a widely used spectroscopic technique for chemical identification, providing unbeaten sensitivity down to the single-molecule level. The amplification of the optical near field produced by collective electron excitations -plasmons- in nanostructured metal surfaces gives rise to a dramatic increase by many orders of magnitude in the Raman scattering intensities from neighboring molecules. This effect strongly depends on the detailed geometry and composition of the plasmon-supporting metallic structures. However, the search for optimized SERS substrates has largely relied on empirical data, due in part to the complexity of the structures, whose simulation becomes prohibitively demanding. In this work, we use state-of-the-art electromagnetic computation techniques to produce predictive simulations for a wide range of nanoparticle-based SERS substrates, including realistic configurations consisting of random arrangements of hundreds of nanoparticles with various morphologies. This allows us to derive rules of thumb for the influence of particle anisotropy and substrate coverage on the obtained SERS enhancement and optimum spectral ranges of operation. Our results provide a solid background to understand and design optimized SERS substrates.

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.

Energy transfer in plasmonic photocatalytic composites

Among the many novel photocatalytic systems developed in very recent years, plasmonic photocatalytic composites possess great potential for use in applications and are one of the most intensively investigated photocatalytic systems owing to their high solar energy utilization efficiency. In these composites, the plasmonic nanoparticles (PNPs) efficiently absorb solar light through localized surface plasmon resonance and convert it into energetic electrons and holes in the nearby semiconductor. This energy transfer from PNPs to semiconductors plays a decisive role in the overall photocatalytic performance. Thus, the underlying physical mechanism is of great scientific and technological importance and is one of the hottest topics in the area of plasmonic photocatalysts. In this review, we examine the very recent advances in understanding the energy transfer process in plasmonic photocatalytic composites, describing both the theoretical basis of this process and experimental demonstrations. The factors that affect the energy transfer efficiencies and how to improve the efficiencies to yield better photocatalytic performance are also discussed. Furthermore, comparisons are made between the various energy transfer processes, emphasizing their limitations/benefits for efficient operation of plasmonic photocatalysts.

Photocatalytic H2 generation on macro-mesoporous oxide-supported Pt nanoparticles

Photocatalysts with high surface area and crystalline walls are synthesized through a dual-templating strategy. The role of ordered macro-mesoporous oxides with crystalline walls was studied for photocatalytic water splitting to generate H₂.

Gold nanoparticle incorporation into nanoporous anatase TiO2mesocrystal using a simple deposition–precipitation method for photocatalytic applications

Schottky junction plasmonic photocatalysts were synthesized by the modification of spindle-shaped TiO₂mesocrystals with Au nanoparticles, and their photocatalysis degradation mechanisms are proposed.

A Stationary Reaction Current Effect in Mesoporous Pt/ZrO2 System Under H2/O2 Environment

There have been many works analyzing thermionic currents and chemicurrents generated on various electrolyte-free metal/semiconductor nanostructures. More recently, the chemicurrent phenomenon was reported for mesoporous Pt/semiconductor systems adept at converting surface-released chemical energy into a stationary electrical signal at room-temperature conditions. The present work points out the existence of an entire class of such surface-driven functional nanosystems. Here, the reaction current generation of Pt/ZrO2 systems was studied at room temperature under exposure to oxyhydrogen environments for mesoporous zirconia; this nanostructure was capable of the continuous oxidation of hydrogen, producing a long-standing stationary current. Synthesis parameters during the anodization process were manipulated to control sample pore density and the average pore diameter. Deposited via wide-angle PVD sputtering, the Pt phase forms an electrically continuous topographical nanomesh layer, and thus the Pt/ZrO2/gas interface is regulated through the manipulation of zirconia porosity. We observed reaction current enhancements with increasing porosity due to the lengthening of the Pt/ZrO2 interface. The most porous sample was significantly more sensitive to initial hydrogen additions, pointing toward the spillover of positive ionic charge across the Pt/ZrO2 interface as the origin of the observed electromotive force.

Direct Plasmon-Driven Photoelectrocatalysis

Harnessing the energy from hot charge carriers is an emerging research area with the potential to improve energy conversion technologies.1-3 Here we present a novel plasmonic photoelectrode architecture carefully designed to drive photocatalytic reactions by efficient, nonradiative plasmon decay into hot carriers. In contrast to past work, our architecture does not utilize a Schottky junction, the commonly used building block to collect hot carriers. Instead, we observed large photocurrents from a Schottky-free junction due to direct hot electron injection from plasmonic gold nanoparticles into the reactant species upon plasmon decay. The key ingredients of our approach are (i) an architecture for increased light absorption inspired by optical impedance matching concepts,4 (ii) carrier separation by a selective transport layer, and (iii) efficient hot-carrier generation and injection from small plasmonic Au nanoparticles to adsorbed water molecules. We also investigated the quantum efficiency of hot electron injection for different particle diameters to elucidate potential quantum effects while keeping the plasmon resonance frequency unchanged. Interestingly, our studies did not reveal differences in the hot-electron generation and injection efficiencies for the investigated particle dimensions and plasmon resonances.

Interfacial band alignment for photocatalytic charge separation in TiO2 nanotube arrays coated with CuPt nanoparticles

The band-alignment at heterojunctions formed by photodepositing CuPt nanoparticles on anatase-phase TiO₂ nanotubes is of the Schottky type and significantly different from heterojunctions where the CuPt nanoparticles are coated on the nanotubes by sputtering.

Sun-light-driven photocatalytic activity by ZnO/Ag heteronanostructures synthesized via a facile thermal decomposition approach

ZnO/Ag heteronanostructures with good photocatalytic activity towards photodegradation of methylene blue have been synthesized using a facile thermal decomposition approach.