Electronic excitation and dynamic promotion of a surface reaction

Site-Selective Excitation of Ti3+ Ions in Rutile TiO2 via Anisotropic Intra-Atomic 3d → 3d Transition

Site-selective excitation (SSE), which is usually realized by tuning the wavelength of absorbed light, is an ideal way to study bond-selective chemistry, analyze the crystal structure, investigate protein conformation, etc., eventually leading to active manipulation of desired processes. Herein, SSE has been explored in (110)-, (100)-, and (011)-faced rutile TiO2, a prototypical material in both surface science and photocatalysis fields. Using ultraviolet photoelectron spectroscopy and photon energy-, substrate orientation-, and laser polarization-dependent two-photon photoemission spectroscopy (2PPE), intra-atomic 3d → 3d transition from the split Ti3+ 3d orbitals, i.e., band gap states and excited states at ∼1.00 eV below and ∼2.40 eV above the Fermi level, respectively, has been proven for all of the samples, suggesting that it is a common property of this material. The distinct structure of rutile TiO2 results in the anisotropic 3d → 3d transitions with the transition dipole moment along the long axes ([110] and [11̅0]) of TiO6 blocking units. This anisotropy facilitates the selective excitation of Ti3+ ions in the two types of TiO6, which cannot be realized by conventional wavelength tuning, via polarization alignment of the excitation source. Discovery in this work builds the foundation for future investigation of site-selective photophysical and photochemical processes and eventually possible active manipulation in this material at the atomic level.

Disentangling the role of electrons and phonons in the photoinduced CO desorption and CO oxidation on (O,CO)-Ru(0001)

The role played by electronic and phononic excitations in the femtosecond laser induced desorption and oxidation of CO coadsorbed with O on Ru(0001) is investigated using ab initio molecular dynamics with electronic friction. To this aim, simulations that account for both kind of excitations and that only consider electronic excitations are performed. Results for three different surface coverages are obtained. We unequivocally demonstrate that CO desorption is governed by phononic excitations. In the case of oxidation the low statistics does not allow to give a categorical answer. However, the analysis of the adsorbates kinetic energy gain and displacements strongly suggest that phononic excitations and surface distortion also play an important role in the oxidation process.

Investigation of the Plasmon-Activated C-C Coupling Reactions by Liquid-State SERS Measurement

The implementation of plasmonic materials in heterogeneous catalysis was limited due to the lack of experimental access in managing the plasmonic hot carriers. Herein, we propose a liquid-state surface-enhanced Raman scattering (SERS) technique to manipulate and visualize heterogeneous photocatalysis with transparent plasmonic chips. The liquid-state measurement conquers the difficulties that arise from the plasmon-induced thermal effects, and thus the plasmon based strategies can be extended to investigate a wider range of catalytic reactions. We demonstrated the selection of reaction products by modulating the plasmonic hot carriers and explored the mechanisms in several typical C-C coupling reactions with 4-bromothiophenol (4-BTP) as reactants. The real-time experimental results suggest brand new mechanisms of the formation of C-C bonds on plasmonic metal nanoparticles (NPs): the residue of 4-BTP, but not thiophenol (TP), is responsible for the C-C coupling. Furthermore, this technique was extended to study the evolution of the Suzuki-Miyaura reaction on nonplasmonic palladium metals by establishing the charge transfer channels between palladium and Au NPs. The cleavage and formation of chemical bonds in each individual reaction step were discerned, and the corresponding working mechanisms were clarified.

Determining the role of Pd catalyst morphology and deposition criteria over large area plasmonic metasurfaces during light-enhanced electrochemical oxidation of formic acid

The use of metal composites based on plasmonic nanostructures partnered with catalytic counterparts has recently emerged as a promising approach in the field of plasmon-enhanced electrocatalysis. Here, we report on the role of the surface morphology, size, and anchored site of Pd catalysts coupled to plasmonic metasurfaces formed by periodic arrays of multimetallic Ni/Au nanopillars for formic acid electro-oxidation reaction (FAOR). We compare the activity of two kinds of metasurfaces differing in the positioning of the catalytic Pd nanoparticles. In the first case, the Pd nanoparticles have a polyhedron crystal morphology with exposed (200) facets and were deposited over the Ni/Au metasurfaces in a site-selective fashion by limiting their growth at the electromagnetic hot spots (Ni/Au-Pd@W). In contrast, the second case consists of spherical Pd nanoparticles grown in solution, which are homogeneously deposited onto the Ni/Au metasurface (Ni/Au-Pd@M). Ni/Au-Pd@W catalytic metasurfaces demonstrated higher light-enhanced FAOR activity (61%) in comparison to the Ni/Au-Pd@M sample (42%) for the direct dehydrogenation pathway. Moreover, the site-selective Pd deposition promotes the growth of nanoparticles favoring a more selective catalytic behavior and a lower degree of CO poisoning on Pd surface. The use of cyclic voltammetry, energy-resolved incident photon to current conversion efficiency, open circuit potential, and electrochemical impedance spectroscopy highlights the role of plasmonic near fields and hot holes in driving the catalytic enhancement under light conditions.

Why Ultrafast Photoinduced CO Desorption Dominates over Oxidation on Ru(0001)

CO oxidation on Ru(0001) is a long-standing example of a reaction that, being thermally forbidden in ultrahigh vacuum, can be activated by femtosecond laser pulses. In spite of its relevance, the precise dynamics of the photoinduced oxidation process as well as the reasons behind the dominant role of the competing CO photodesorption remain unclear. Here we use ab initio molecular dynamics with electronic friction that account for the highly excited and nonequilibrated system created by the laser to investigate both reactions. Our simulations successfully reproduce the main experimental findings: the existence of photoinduced oxidation and desorption, the large desorption to oxidation branching ratio, and the changes in the O K-edge X-ray absorption spectra attributed to the initial stage of the oxidation process. Now, we are able to monitor in detail the ultrafast CO desorption and CO oxidation occurring in the highly excited system and to disentangle what causes the unexpected inertness to the otherwise energetically favored oxidation.

Quantitative molecular simulations

All-atom simulations can provide molecular-level insights into the dynamics of gas-phase, condensed-phase and surface processes. One important requirement is a sufficiently realistic and detailed description of the underlying intermolecular interactions. The present perspective provides an overview of the present status of quantitative atomistic simulations from colleagues' and our own efforts for gas- and solution-phase processes and for the dynamics on surfaces. Particular attention is paid to direct comparison with experiment. An outlook discusses present challenges and future extensions to bring such dynamics simulations even closer to reality.

The Regulation of O2 Spin State and Direct Oxidation of CO at Room Temperature Using Triboelectric Plasma by Harvesting Mechanical Energy

Oxidation reactions play a critical role in processes involving energy utilization, chemical conversion, and pollutant elimination. However, due to its spin-forbidden nature, the reaction of molecular dioxygen (O₂) with a substrate is difficult under mild conditions. Herein, we describe a system that activates O₂ via the direct modulation of its spin state by mechanical energy-induced triboelectric corona plasma, enabling the CO oxidation reaction under normal temperature and pressure. Under optimized reaction conditions, the activity was 7.2 μmol h^-1, and the energy consumption per mole CO was 4.2 MJ. The results of kinetic isotope effect, colorimetry, and density functional theory calculation studies demonstrated that electrons generated in the triboelectric plasma were directly injected into the antibonding orbital of O₂ to form highly reactive negative ions O₂^-, which effectively promoted the rate-limiting step of O₂ dissociation. The barrier of the reaction of O₂^- ions and CO molecular was 3.4 eV lower than that of O₂ and CO molecular. This work provides an effective strategy for using renewable and green mechanical energy to realize spin-forbidden reactions of small molecules.

The Regulation of O2 Spin State and Direct Oxidation of CO at Room Temperature Using Triboelectric Plasma by Harvesting Mechanical Energy

Oxidation reactions play a critical role in processes involving energy utilization, chemical conversion, and pollutant elimination. However, due to its spin-forbidden nature, the reaction of molecular dioxygen (O2) with a substrate is difficult under mild conditions. Herein, we describe a system that activates O2 via the direct modulation of its spin state by mechanical energy-induced triboelectric corona plasma, enabling the CO oxidation reaction under normal temperature and pressure. Under optimized reaction conditions, the activity was 7.2 μmol h-1, and the energy consumption per mole CO was 4.2 MJ. The results of kinetic isotope effect, colorimetry, and density functional theory calculation studies demonstrated that electrons generated in the triboelectric plasma were directly injected into the antibonding orbital of O2 to form highly reactive negative ions O2-, which effectively promoted the rate-limiting step of O2 dissociation. The barrier of the reaction of O2- ions and CO molecular was 3.4 eV lower than that of O2 and CO molecular. This work provides an effective strategy for using renewable and green mechanical energy to realize spin-forbidden reactions of small molecules.

Photoinduced Desorption Dynamics of CO from Pd(111): A Neural Network Approach

Modeling the ultrafast photoinduced dynamics and reactivity of adsorbates on metals requires including the effect of the laser-excited electrons and, in many cases, also the effect of the highly excited surface lattice. Although the recent ab initio molecular dynamics with electronic friction and thermostats, (T_e,T_l)-AIMDEF [AlducinM.;Phys. Rev. Lett.2019, 123, 246802]31922860, enables such complex modeling, its computational cost may limit its applicability. Here, we use the new embedded atom neural network (EANN) method [ZhangY.;J. Phys. Chem. Lett.2019, 10, 496231397157] to develop an accurate and extremely complex potential energy surface (PES) that allows us a detailed and reliable description of the photoinduced desorption of CO from the Pd(111) surface with a coverage of 0.75 monolayer. Molecular dynamics simulations performed on this EANN-PES reproduce the (T_e,T_l)-AIMDEF results with a remarkable level of accuracy. This demonstrates the outstanding performance of the obtained EANN-PES that is able to reproduce available density functional theory (DFT) data for an extensive range of surface temperatures (90–1000 K); a large number of degrees of freedom, those corresponding to six CO adsorbates and 24 moving surface atoms; and the varying CO coverage caused by the abundant desorption events.

Plasmonic enhancement of molecular hydrogen dissociation on metallic magnesium nanoclusters

Light-driven plasmonic enhancement of chemical reactions on metal catalysts is a promising strategy to achieve highly selective and efficient chemical transformations. The study of plasmonic catalyst materials has traditionally focused on late transition metals such as Au, Ag, and Cu. In recent years, there has been increasing interest in the plasmonic properties of a set of earth-abundant elements such as Mg, which exhibit interesting hydrogenation chemistry with potential applications in hydrogen storage. This work explores the optical, electronic, and catalytic properties of a set of metallic Mg nanoclusters with up to 2057 atoms using time-dependent density functional tight-binding and density functional theory calculations. Our results show that Mg nanoclusters are able to produce highly energetic hot electrons with energies of up to 4 eV. By electronic structure analysis, we find that these hot electrons energetically align with electronic states of physisorbed molecular hydrogen, occupation of which by hot electrons can promote the hydrogen dissociation reaction. We also find that the reverse reaction, hydrogen evolution on metallic Mg, can potentially be promoted by hot electrons, but following a different mechanism. Thus, from a theoretical perspective, Mg nanoclusters display very promising behaviour for their use in light promoted storage and release of hydrogen.

Flow and extraction of energy and charge carriers in hybrid plasmonic nanostructures

Strong interactions of electromagnetic fields with plasmonic nanomaterials have been exploited in various applications. These applications have centred on plasmon-enhanced scattering rates in nearby molecules or plasmon-induced heating. A question that has emerged recently is whether it is possible to use plasmonic nanostructures in a range of hot electron (hole) applications, including photocatalysis, photovoltaics and photodetection. These applications require coupling of a plasmonic component, which amplifies the interaction of light with the material, to an attached non-plasmonic component that extracts this energy in the form of electronic excitations to perform a function. In this Perspective, we discuss recent work in the emerging field of hybrid plasmonics. We focus on fundamental questions related to the nanoscopic flow of energy and excited charge carriers in these multicomponent materials. We also address critical misconceptions, challenges and opportunities that require more attention.

Metal-Organic Framework Hybrids Aid Metabolic Profiling for Colorectal Cancer

Colorectal cancer (CRC) is the third most common fatal cancer worldwide, accounting for ≈10% of cancer-related mortality. Metabolic shift occurs from the very early stage during the development of CRC, which is of significant etiological and diagnostic importance toward precision medicine. Here, an advanced molecular tool to characterize the metabolic alterations in CRC, based on metal-organic framework (MOF) hybrids is reported. Consuming only 500 nL of plasma without any sample pretreatment, MOF hybrids yield direct metabolic fingerprints by laser desorption/ionization mass spectrometry in seconds. A diagnostic prediction model by a machine learning algorithm is constructed, to discriminate CRC patients from normal controls with an average area under the curve of 0.947 for the discovery cohort and 0.912 for the independent validation cohort. In addition, CRC-specific metabolic signature consisting of 34 potential biomarkers, based on the aforementioned diagnostic model is identified. The results advance the design of nanomaterial-based platforms for metabolic analysis and establish a new liquid biopsy tool for CRC screening compatible with the current clinical workflow in practice.

On-Surface Chemistry Induced by Long-Lived Excitons: (NO)2 Dissociation on C60

Solid-state excitonic excitations play an increasingly important role in optoelectronic and light harvesting processes due to their ubiquitous presence in dipolar two-dimensional materials. Here we show that long-lived solid-state excitons induce chemical reactions in adsorbed molecules and thus convert light into chemical energy. For the model system (NO)₂ dimer adsorbed on ordered c(4×4) C₆₀ films, time-of-flight measurements following UV laser excitation reveal a slow and a fast dissociative desorption channel, which are assigned to intersystem crossing and internal conversion, respectively, by time-dependent density functional theory calculations.

In Situ ATR-SEIRAS of Carbon Dioxide Reduction at a Plasmonic Silver Cathode

Illumination of a voltage-biased plasmonic Ag cathode during CO2 reduction results in a suppression of the H2 evolution reaction while enhancing CO2 reduction. This effect has been shown to be photonic rather than thermal, but the exact plasmonic mechanism is unknown. Here, we conduct an in situ ATR-SEIRAS (attenuated total reflectance-surface-enhanced infrared absorption spectroscopy) study of a sputtered thin film Ag cathode on a Ge ATR crystal in CO2-saturated 0.1 M KHCO3 over a range of potentials under both dark and illuminated (365 nm, 125 mW cm-2) conditions to elucidate the nature of this plasmonic enhancement. We find that the onset potential of CO2 reduction to adsorbed CO on the Ag surface is -0.25 VRHE and is identical in the light and the dark. As the production of gaseous CO is detected in the light near this onset potential but is not observed in the dark until -0.5 VRHE, we conclude that the light must be assisting the desorption of CO from the surface. Furthermore, the HCO3- wavenumber and peak area increase immediately upon illumination, precluding a thermal effect. We propose that the enhanced local electric field that results from the localized surface plasmon resonance (LSPR) is strengthening the HCO3- bond, further increasing the local pH. This would account for the decrease in H2 formation and increase the CO2 reduction products in the light.

Electrons and Phonons Cooperate in the Laser-Induced Desorption of CO from Pd(111)

Femtosecond laser induced desorption of CO from a CO-covered Pd(111) surface is investigated with ab initio molecular dynamics with electronic friction that incorporates effects due to the excited electronic and phononic systems, as well as out-of-phase coadsorbate interactions. Our simulations show evidence of an important electron-phonon synergy in promoting CO desorption that has largely been neglected in other similar systems. At the saturated coverage of 0.75 ML, effects due to CO-CO interadsorbate energy exchange are also important. Our dynamics simulations, in concert with site-specific desorption energy calculations, allow us to understand the large coverage dependence of the desorption yields observed in experiments.

Control of Selectivity through a New Hydrogen-Transfer Mechanism in Photocatalytic Reduction Reactions: Electronically Relaxed Neutral H and the Role of Electron-Phonon Coupling

Controlling the fate of hydrogen in photocatalytic synthesis reactions has been an ongoing challenge in CO₂ reduction by H₂O and nitrogen fixation efforts. Our studies have identified catalysts (SiC) that exhibit dramatically improved selectivity toward hydrogenation and a photocatalytically active ground-state neutral H that is transferred via vibrational excitation through electronic-vibrational coupling with excited states. This new species and mechanism have been directly connected to the fate of H by comparing GaN and SiC and purposefully manipulated over a single catalyst (SiC) to illustrate generality. Studies included surface reaction modeling using density functional theory (DFT), experimental performance, ¹H NMR spectroscopy, and deuterium kinetic isotope effect. The discovery of this mechanism may have considerable impact on the direction of photocatalytic synthesis, the understanding of the coupling of thermal and photoelectrochemical reaction steps, and electronic-vibrational spectrum coupling in energy sequestration.

Photocatalytic CO2 reduction by H2O: insights from modeling electronically relaxed mechanisms

Understanding of the ground-state surface reaction mechanism for photocatalytic CO₂ reduction and new connections between catalyst surface reactivity and experimentally observed activity and selectivity are presented to facilitate the development of catalysts that exhibit improved activity, controlled product distributions, and enhanced quantum yield.

Gas phase 1H NMR studies and kinetic modeling of dihydrogen isotope equilibration catalyzed by Ru-nanoparticles under normal conditions: dissociative vs. associative exchange

Exposure of surface H-containing Ru-nanoparticles to D₂ gas produces HD via associative adsorption, surface H-transfer and associative desorption.

Investigation of charge transfer at the TiO2–MBA–Au interface based on surface-enhanced Raman scattering: SPR contribution

Investigation of SPR contribution to interfacial charge transfer by tuning the SPR of the assemblies and excitation wavelength.

Photon Energy Threshold in Direct Photocatalysis with Metal Nanoparticles: Key Evidence from the Action Spectrum of the Reaction

By investigating the action spectra (the relationship between the irradiation wavelength and apparent quantum efficiency of reactions under constant irradiance) of a number of reactions catalyzed by nanoparticles including plasmonic metals, nonplasmonic metals, and their alloys at near-ambient temperatures, we found that a photon energy threshold exists in each photocatalytic reaction; only photons with sufficient energy (e.g., higher than the energy level of the lowest unoccupied molecular orbitals) can initiate the reactions. This energy alignment (and the photon energy threshold) is determined by various factors, including the wavelength and intensity of irradiation, molecule structure, reaction temperature, and so forth. Hence, distinct action spectra were observed in the same type of reaction catalyzed by the same catalyst due to a different substituent group, a slightly changed reaction temperature. These results indicate that photon-electron excitations, instead of the photothermal effect, play a dominant role in direct photocatalysis of metal nanoparticles for many reactions.

Adlayer structure dependent ultrafast desorption dynamics in carbon monoxide adsorbed on Pd (111)

We report our ultrafast photoinduced desorption investigation of the coverage dependence of substrate-adsorbate energy transfer in carbon monoxide adlayers on the (111) surface of palladium. As the CO coverage is increased, the adsorption site population shifts from all threefold hollows (up to 0.33 ML), to bridge and near bridge (>0.5 to 0.6 ML) and finally to mixed threefold hollow plus top site (at saturation at 0.75 ML). We show that between 0.24 and 0.75 ML this progression of binding site motifs is accompanied by two remarkable features in the ultrafast photoinduced desorption of the adsorbates: (i) the desorption probability increases roughly two orders magnitude, and (ii) the adsorbate-substrate energy transfer rate observed in two-pulse correlation experiments varies nonmonotonically, having a minimum at intermediate coverages. Simulations using a phenomenological model to describe the adsorbate-substrate energy transfer in terms of frictional coupling indicate that these features are consistent with an adsorption-site dependent electron-mediated energy coupling strength, ηel, that decreases with binding site in the order: three-fold hollow > bridge and near bridge > top site. This weakening of ηel largely counterbalances the decrease in the desorption activation energy that accompanies this progression of adsorption site motifs, moderating what would otherwise be a rise of several orders of magnitude in the desorption probability. Within this framework, the observed energy transfer rate enhancement at saturation coverage is due to interadsorbate energy transfer from the copopulation of molecules bound in three-fold hollows to their top-site neighbors.

Disentangling Multidimensional Nonequilibrium Dynamics of Adsorbates: CO Desorption from Cu(100)

Hot carriers at metal surfaces can drive nonthermal reactions of adsorbates. Characterizing nonequilibrium statistics among various degrees of freedom in an ultrafast time scale is crucial to understand and develop hot carrier-driven chemistry. Here we demonstrate multidimensional vibrational dynamics of carbon monoxide (CO) on Cu(100) along hot-carrier induced desorption studied by using time-resolved vibrational sum-frequency generation with phase-sensitive detection. Instantaneous frequency and amplitude of the CO internal stretching mode are tracked with a subpicosecond time resolution that is shorter than the vibrational dephasing time. These experimental results in combination with numerical analysis based on Langevin simulations enable us to extract nonequilibrium distributions of external vibrational modes of desorbing molecules. Superstatistical distributions are generated with mode-dependent frictional couplings in a few hundred femtoseconds after hot-electron excitation, and energy flow from hot electrons and intermode anharmonic coupling play crucial roles in the subsequent evolution of the non-Boltzman distributions.

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.

Nanometals for Solar-to-Chemical Energy Conversion: From Semiconductor-Based Photocatalysis to Plasmon-Mediated Photocatalysis and Photo-Thermocatalysis

Nanometal materials play very important roles in solar-to-chemical energy conversion due to their unique catalytic and optical characteristics. They have found wide applications from semiconductor photocatalysis to rapidly growing surface plasmon-mediated heterogeneous catalysis. The recent research achievements of nanometals are reviewed here, with regard to applications in semiconductor photocatalysis, plasmonic photocatalysis, and plasmonic photo-thermocatalysis. As the first important topic discussed here, the latest progress in the design of nanometal cocatalysts and their applications in semiconductor photocatalysis are introduced. Then, plasmonic photocatalysis and plasmonic photo-thermocatalysis are discussed. A better understanding of electron-driven and temperature-driven catalytic behaviors over plasmonic nanometals is helpful to bridge the present gap between the communities of photocatalysis and conventional catalysis controlled by temperature. The objective here is to provide instructive information on how to take the advantages of the unique functions of nanometals in different types of catalytic processes to improve the efficiency of solar-energy utilization for more practical artificial photosynthesis.

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.

Photochemical transformations on plasmonic metal nanoparticles

The strong interaction of electromagnetic fields with plasmonic nanomaterials offers opportunities in various technologies that take advantage of photophysical processes amplified by this light-matter interaction. Recently, it has been shown that in addition to photophysical processes, optically excited plasmonic nanoparticles can also activate chemical transformations directly on their surfaces. This potentially offers a number of opportunities in the field of selective chemical synthesis. In this Review we summarize recent progress in the field of photochemical catalysis on plasmonic metallic nanostructures. We discuss the underlying physical mechanisms responsible for the observed chemical activity, and the issues that must be better understood to see progress in the field of plasmon-mediated photocatalysis.

Extraction of absorption and scattering contribution of metallic nanoparticles toward rational synthesis and application

Noble metal nanoparticles have unique localized surface plasmon resonance (LSPR), leading to their strong absorption and scattering in the visible light range. Up to date, the common practice in the selection of nanoparticles for a specific application is still based on the measured extinction spectra. This practice may be erroneous, because the extinction spectra contain both absorption and scattering contribution that may play different roles in different applications. It would be highly desirable to develop an efficient way to obtain the absorption and scattering spectra simultaneously. Herein, we develop a method to use the experimentally measured extinction and scattering signals to extract the absorption and scattering spectra that is in excellent agreement with that simulated by discrete dipole approximation (DDA). The heating curve measurement on the three types of gold nanorods, with almost the same extinction spectra but different absorption and scattering contribution, convincingly reveals an excellent correlation between the heating effect and the absorption strength rather than the extinction strength. The result demonstrates the importance to obtain the scattering and absorption spectra to predict the potential application for different types of nanoparticles, which in turn will screen efficiently nanoparticles for a specific application.

Concept of a single temperature for highly nonequilibrium laser-induced hydrogen desorption from a ruthenium surface

Laser-induced condensed phase reactions are often interpreted as nonequilibrium phenomena that go beyond conventional thermodynamics. Here, we show by Langevin dynamics and for the example of femtosecond-laser desorption of hydrogen from a ruthenium surface that light adsorbates thermalize rapidly due to ultrafast energy redistribution after laser excitation. Despite the complex reaction mechanism involving hot electrons in the surface region, all desorption product properties are characterized by equilibrium distributions associated with a single, unique temperature. This represents an example of ultrahot chemistry on the subpicosecond time scale.

Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy

Recent years have seen a renewed interest in the harvesting and conversion of solar energy. Among various technologies, the direct conversion of solar to chemical energy using photocatalysts has received significant attention. Although heterogeneous photocatalysts are almost exclusively semiconductors, it has been demonstrated recently that plasmonic nanostructures of noble metals (mainly silver and gold) also show significant promise. Here we review recent progress in using plasmonic metallic nanostructures in the field of photocatalysis. We focus on plasmon-enhanced water splitting on composite photocatalysts containing semiconductor and plasmonic-metal building blocks, and recently reported plasmon-mediated photocatalytic reactions on plasmonic nanostructures of noble metals. We also discuss the areas where major advancements are needed to move the field of plasmon-mediated photocatalysis forward.

The effects of electron-hole pair coupling on the infrared laser-controlled vibrational excitation of NO on Au(111)

In this work, we present theoretical simulations of laser-driven vibrational control of NO adsorbed on a gold surface. Our goal is to tailor laser pulses to selectively excite specific modes and vibrational eigenstates, as well as to favor photodesorption of the adsorbed molecule. To this end, various control schemes and algorithms are applied. For adsorbates at metallic surfaces, the creation of electron-hole pairs in the substrate is known to play a dominant role in the transfer of energy from the system to the surroundings. These nonadiabatic couplings are included perturbatively in our reduced density matrix simulations using a generalization of the state-resolved position-dependent anharmonic rate model we recently introduced. An extension of the reduced density matrix is also proposed to provide a sound model for photodesorption in dissipative systems.

Visible-light-enhanced catalytic oxidation reactions on plasmonic silver nanostructures

Catalysis plays a critical role in chemical conversion, energy production and pollution mitigation. High activation barriers associated with rate-limiting elementary steps require most commercial heterogeneous catalytic reactions to be run at relatively high temperatures, which compromises energy efficiency and the long-term stability of the catalyst. Here we show that plasmonic nanostructures of silver can concurrently use low-intensity visible light (on the order of solar intensity) and thermal energy to drive catalytic oxidation reactions--such as ethylene epoxidation, CO oxidation, and NH₃ oxidation--at lower temperatures than their conventional counterparts that use only thermal stimulus. Based on kinetic isotope experiments and density functional calculations, we postulate that excited plasmons on the silver surface act to populate O₂ antibonding orbitals and so form a transient negative-ion state, which thereby facilitates the rate-limiting O₂-dissociation reaction. The results could assist the design of catalytic processes that are more energy efficient and robust than current processes.