Mechanistic Insights into Photocatalyzed Hydrogen Desorption from Palladium Surfaces Assisted by Localized Surface Plasmon Resonances

Strategies to Obtain Reliable Energy Landscapes from Embedded Multireference Correlated Wavefunction Methods for Surface Reactions

Embedded correlated wavefunction (ECW) theory is a powerful tool for studying ground- and excited-state reaction mechanisms and associated energetics in heterogeneous catalysis. Several factors are important to obtaining reliable ECW energies, critically the construction of consistent active spaces (ASs) along reaction pathways when using a multireference correlated wavefunction (CW) method that relies on a subset of orbital spaces in the configuration interaction expansion to account for static electron correlation, e.g., complete AS self-consistent field theory, in addition to the adequate partitioning of the system into a cluster and environment, as well as the choice of a suitable basis set and number of states included in excited-state simulations. Here, we conducted a series of systematic studies to develop best-practice guidelines for ground- and excited-state ECW theory simulations, utilizing the decomposition of NH3 on Pd(111) as an example. We determine that ECW theory results are relatively insensitive to cluster size, the aug-cc-pVDZ basis set provides an adequate compromise between computational complexity and accuracy, and that a fixed-clean-surface approximation holds well for the derivation of the embedding potential. Additionally, we demonstrate that a merging approach, which involves generating ASs from the molecular fragments at each configuration, is preferable to a creeping approach, which utilizes ASs from adjacent structures as an initial guess, for the generation of consistent potential energy curves involving open-d-shell metal surfaces, and, finally, we show that it is essential to include bands of excited states in their entirety when simulating excited-state reaction pathways.

Quantifying Ultrafast Energy Transfer from Plasmonic Hot Carriers for Pulsed Photocatalysis on Nanostructures

Photocatalysis with plasmonic nanostructures has lately emerged as a transformative paradigm to drive and alter chemical reactions using light. At the surface of metallic nanoparticles, photoexcitation results in strong near fields, short-lived high-energy "hot" carriers, and light-induced heating, thus creating a local environment where reactions can occur with enhanced efficiencies. In this context, it is critical to understand how to manipulate the nonequilibrium processes triggered by light, as their ultrafast (femto- to picoseconds) relaxation dynamics compete with the process of energy transfer toward the reactants. Accurate predictions of the plasmon photocatalytic activity can lead to optimized nanophotonic architectures with enhanced selectivity and rates, operating beyond the intrinsic limitations of the steady state. Here, we report on an original modeling approach to quantify, with space, time, and energy resolution, the ultrafast energy exchange from plasmonic hot carriers (HCs) to molecular systems adsorbed on the metal nanoparticle surface while consistently accounting for photothermal bond activation. Our analysis, illustrated for a few typical cases, reveals that the most energetic nonequilibrium carriers (i.e., with energies well far from the Fermi level) may introduce a wavelength-dependence of the reaction rates, and it elucidates on the role of the carriers closer to the Fermi energy and the photothermally heated lattice, suggesting ways to enhance and optimize each contribution. We show that the overall reaction rates can benefit strongly from using pulsed illumination with the optimal pulse width determined by the properties of the system. Taken together, these results contribute to the rational design of nanoreactors for pulsed catalysis, which calls for predictive modeling of the ultrafast HC-hot adsorbate energy transfer.

Thermal-effect dominated plasmonic catalysis on silver nanoislands

Plasmonic metal nanostructures with the intrinsic property of localized surface plasmon resonance can effectively promote energy conversion in many applications such as photocatalysis, photothermal therapy, seawater desalinization, etc. It is known that not only are plasmonically excited hot electrons generated from metal nanostructures under light irradiation, which can effectively trigger chemical reactions, but also plasmonically induced heating simultaneously occurs. Although plasmonic catalysis has been widely explored in recent years, the underlying mechanisms for distinguishing the contribution of hot electrons from thermal effects are not fully understood. Here, a simple and efficient self-assembly system using silver nanoislands as plasmonic substrates is designed to investigate the photo-induced azo coupling reaction of nitro- and amino-groups at various temperatures. In the experiments, surface-enhanced Raman spectroscopy is employed to monitor the time and temperature dependence of plasmon-induced catalytic reactions. It was found that a combination of hot electrons and thermal effects contribute to the reactivity. The thermal effects play the dominant role in the plasmon-induced azo coupling reaction of nitro-groups, which suggests that the localized temperature must be considered in the development of photonic applications based on plasmonic nanomaterials.

Effects of Nanowire Doping on Plasmon-Enhanced N2 Dissociation

Doping a transition metal element into plasmonic systems can tune the optical properties of the system, which will potentially facilitate the plasmon-enhanced catalytic process. In this study, we applied the linear-response time-dependent density functional theory (LR-TDDFT) method with real-time electron dynamics and mean-field Ehrenfest dynamics methods to computationally investigate the effects of doping silver nanowires on plasmon-enhanced N2 dissociation. We calculated the absorption spectra for different doped systems, applied an external electric field to the system, and performed mean-field Ehrenfest dynamics to examine how plasmonic excitation will affect the N2 activation or dissociation. In addition, we also studied how the transition metal dopant affects the system's electronic structure and potential energy surface.

Peeking into the Femtosecond Hot-Carrier Dynamics Reveals Unexpected Mechanisms in Plasmonic Photocatalysis

Plasmonic-driven photocatalysis may lead to reaction selectivity that cannot be otherwise achieved. A fundamental role is played by hot carriers, i.e., electrons and holes generated upon plasmonic decay within the metal nanostructure interacting with molecular species. Understanding the elusive microscopic mechanism behind such selectivity is a key step in the rational design of hot-carrier reactions. To accomplish that, we present state-of-the-art multiscale simulations, going beyond density functional theory, of hot-carrier injections for the rate-determining step of a photocatalytic reaction. We focus on carbon dioxide reduction, for which it was experimentally shown that the presence of a rhodium nanocube under illumination leads to the selective production of methane against carbon monoxide. We show that selectivity is due to a (predominantly) direct hole injection from rhodium to the reaction intermediate CHO. Unexpectedly, such an injection does not promote the selective reaction path by favoring proper bond breaking but rather by promoting bonding of the proper molecular fragment to the surface.

Solvent-induced local environment effect in plasmonic catalysis

Solvents are known to affect the local surface plasmon resonance of metal nanoparticles; however, how solvents can be used to manipulate the interfacial charge and energy transfer in plasmonic catalysis remains to be explored. Here, using NH3 decomposition on a Ru-doped Cu surface as an example, we report density functional theory (DFT) and delta self-consistent field (SCF) calculations, through which we investigate the effect of different protic solvent molecules on interfacial charge transfer by calculating excitation energy of an electronic transition between the metal and the molecular reactant. We find that the H-bonds between water and NH3 can alter the direct interfacial charge transfer due to the shift of the molecular frontier orbitals with respect to the metal Fermi level. These effects are also observed when the H-bonds are formed between methanol (or phenol) and ammonia. We show that the solvent possessing stronger basicity induces a more pronounced effect on the excitation energy. This work thus provides valuable insights for tuning the excitation energy and controlling different routes to channel the photon energy into plasmonic catalysis.

Recent Advances in Real-Time Time-Dependent Density Functional Theory Simulations of Plasmonic Nanostructures and Plasmonic Photocatalysis

Plasmonic catalysis provides a possible means for driving chemical reactions under relatively mild conditions. Rational design of these systems is impeded by the difficulty in understanding the electron dynamics and their interplay with reactions. Real-time, time-dependent density functional theory (RT-TDDFT) can provide dynamic information on excited states in plasmonic systems, including those relevant to plasmonic catalysis, at time scales and length scales that are otherwise out of reach of many experimental techniques. Here, we discuss previous RT-TDDFT studies of plasmonic systems, focusing on recent work that gains insight into plasmonic catalysis. These studies provide insight into plasmon dynamics, including size effects and the role of specific electronic states. Further, these studies provide significant insight into mechanisms underlying plasmonic catalysis, showing the importance of charge transfer between metal and adsorbate states, as well as local field enhancement, in different systems.

Energy Transfer to Molecular Adsorbates by Transient Hot Electron Spillover

Hot electron (HE) photocatalysis is one of the most intriguing fields of nanoscience, with a clear potential for technological impact. Despite much effort, the mechanisms of HE photocatalysis are not fully understood. Here we investigate a mechanism based on transient electron spillover on a molecule and subsequent energy release into vibrational modes. We use state-of-the-art real-time Time Dependent Density Functional Theory (rt-TDDFT), simulating the dynamics of a HE moving within linear chains of Ag or Au atoms, on which CO, N₂, or H₂O are adsorbed. We estimate the energy a HE can release into adsorbate vibrational modes and show that certain modes are selectively activated. The energy transfer strongly depends on the adsorbate, the metal, and the HE energy. Considering a cumulative effect from multiple HEs, we estimate this mechanism can transfer tenths of an eV to molecular vibrations and could play an important role in HE photocatalysis.

Active Site Engineering on Plasmonic Nanostructures for Efficient Photocatalysis

Plasmonic nanostructures have shown immense potential in photocatalysis because of their distinct photochemical properties associated with tunable photoresponses and strong light-matter interactions. The introduction of highly active sites is essential to fully exploit the potential of plasmonic nanostructures in photocatalysis, considering the inferior intrinsic activities of typical plasmonic metals. This review focuses on active site-engineered plasmonic nanostructures with enhanced photocatalytic performance, wherein the active sites are classified into four types (i.e., metallic sites, defect sites, ligand-grafted sites, and interface sites). The synergy between active sites and plasmonic nanostructures in photocatalysis is discussed in detail after briefly introducing the material synthesis and characterization methods. Active sites can promote the coupling of solar energy harvested by plasmonic metal to catalytic reactions in the form of local electromagnetic fields, hot carriers, and photothermal heating. Moreover, efficient energy coupling potentially regulates the reaction pathway by facilitating the excited state formation of reactants, changing the status of active sites, and creating additional active sites using photoexcited plasmonic metals. Afterward, the application of active site-engineered plasmonic nanostructures in emerging photocatalytic reactions is summarized. Finally, a summary and perspective of the existing challenges and future opportunities are presented. This review aims to deliver some insights into plasmonic photocatalysis from the perspective of active sites, expediting the discovery of high-performance plasmonic photocatalysts.

Controlling Selectivity in Plasmonic Catalysis: Switching Reaction Pathway from Hydrogenation to Homocoupling Under Visible-Light Irradiation

Plasmonic catalysis enables the use of light to accelerate molecular transformations. Its application to the control reaction selectivity is highly attractive but remains challenging. Here, we have found that the plasmonic properties in AgPd nanoparticles allowed different reaction pathways for tunable product formation under visible-light irradiation. By employing the hydrogenation of phenylacetylene as a model transformation, we demonstrate that visible-light irradiation can be employed to steer the reaction pathway from hydrogenation to homocoupling. Our data showed that the decrease in the concentration of H species at the surface due to plasmon-enhanced H₂ desorption led to the control in selectivity. These results provide important insights into the understanding of reaction selectivity with light, paving the way for the application of plasmonic catalysis to the synthesis of 1,3-diynes, and bringing the vision of light-driven transformations with target selectivity one step closer to reality.

Plasmonic Photocatalysis with Chemically and Spatially Specific Antenna-Dual Reactor Complexes

Plasmonic antenna-reactor photocatalysts have been shown to convert light efficiently to chemical energy. Virtually all chemical reactions mediated by such complexes to date, however, have involved relatively simple reactions that require only a single type of reaction site. Here, we investigate a planar Al nanodisk antenna with two chemically distinct and spatially separated active sites in the form of Pd and Fe nanodisks, fabricated in 90° and 180° trimer configurations. The photocatalytic reactions H₂ + D₂ → 2HD and NH₃ + D₂ → NH₂D + HD were both investigated on these nanostructured complexes. While the H₂-D₂ exchange reaction showed an additive behavior for the linear (180°) nanodisk complex, the NH₃ + D₂ reaction shows a clear synergistic effect of the position of the reactor nanodisks relative to the central Al nanodisk antenna. This study shows that light-driven chemical reactions can be performed with both chemical and spatial control of the specific reaction steps, demonstrating precisely designed antennas with multiple reactors for tailored control of chemical reactions of increasing complexity.

Plasmonic enhanced piezoelectric photoresponse with flexible PVDF@Ag-ZnO/Au composite nanofiber membranes

The coordination of piezoelectric and plasmonic effects to regulate the separation and migration of photo-generated carriers is still a significant method to improve the performance of visible-light photoresponse. Herein, we propose the PVDF@Ag-ZnO/Au composite nanofiber membranes utilizing the piezoelectric and plasmonic effects to promote the photocatalytic degradation of organic dyes. Here, ZnO nanorods can generate a built-in electric field under vibration to separate electron-hole pairs. The Schottky junction formed by noble metal/semiconductor can not only inhibit the recombination of photo-generated carriers and accelerate the migration of carriers, but also enhance the utilization of visible light. In addition, the structure has excellent flexibility and easy recycling characteristics. We demonstrate that the plasmonic effect of noble metal can enhance the light response of membranes and broaden light absorption from ultraviolet to visible light region. With the help of the surface-enhanced Raman scattering (SERS), modulation effects of the piezoelectric effect on light response is proved. For catalytic processes, rhodamine B (98.8%) can be almost completely degraded using PVDF@Ag-ZnO/Au within 120 minutes in the piezoelectric photocatalysis process, which is 2.2 and 2.8 times higher than photocatalysis and piezoelectric catalysis, respectively. This work provides a promising strategy for harnessing solar and mechanical energy.

Thermo-photo catalysis: a whole greater than the sum of its parts

Thermo-photo catalysis, which is the catalysis with the participation of both thermal and photo energies, not only reduces the large energy consumption of thermal catalysis but also addresses the low efficiency of photocatalysis. As a whole greater than the sum of its parts, thermo-photo catalysis has been proven as an effective and promising technology to drive chemical reactions. In this review, we first clarify the definition (beyond photo-thermal catalysis and plasmonic catalysis), classification, and principles of thermo-photo catalysis and then reveal its superiority over individual thermal catalysis and photocatalysis. After elucidating the design principles and strategies toward highly efficient thermo-photo catalytic systems, an ample discussion on the synergetic effects of thermal and photo energies is provided from two perspectives, namely, the promotion of photocatalysis by thermal energy and the promotion of thermal catalysis by photo energy. Subsequently, state-of-the-art techniques applied to explore thermo-photo catalytic mechanisms are reviewed, followed by a summary on the broad applications of thermo-photo catalysis and its energy management toward industrialization. In the end, current challenges and potential research directions related to thermo-photo catalysis are outlined.

Intermolecular Forces Dictate Vibrational Energy Transfer in Plasmonic-Molecule Systems

Plasmonic materials are a promising category of photocatalysts for solar energy harvesting and conversion. However, there are some significant obstacles that need to be overcome to make plasmonic catalysts commercially available. One major challenge is to obtain a systematic understanding of how to design and optimize plasmonic systems from the perspective of both plasmonic materials and reagent molecules to achieve highly efficient and selective catalysis. It is well-known that the contributions of plasmon-molecule interactions such as plasmon-induced resonant energy transfer and charge transfer to the catalytic mechanism are rather complicated and possibly multifold. Observation of these phenomena is challenging due to the highly heterogeneous nature of plasmonic substrates as well as the large difference in sizes and optical cross sections between plasmonic materials and molecules. In this work, we use a molecular perspective to examine the crucial process of energy transfer between plasmons and molecules, with the goal of determining which experimental parameters can be used to control this energy flow. We employ ultrafast surface-enhanced anti-Stokes and Stokes Raman spectroscopy to investigate vibrational energy transfer in plasmonic-molecule systems. By comparing the energy transfer kinetics of five different aromatic thiols on the picosecond time scale, we find that intermolecular forces play an important role in energy distribution in molecules adsorbed to plasmonic materials, which changes the amount of energy deposited onto the molecule and the lifetime of the energy deposited. Our work implies that careful consideration of catalyst loading and molecule adsorption geometry is crucial for enhancing or suppressing the rate and efficiency of plasmon-driven energy transfer.

Reaction of H2 with polyoxometalate supported Rhodium(0) and Iridium(0) nanoparticles in aqueous suspensions: a kinetic study

The mechanism of the reaction between Rh0 and Ir0 NPs with H2 was measured in the absence of an electrical bias via monitoring the catalytic reduction of PW12O403− and it was compared to the previous results of Pt0 NPs.

Localized Orbital Excitation Drives Bond Formation in Plasmonic Catalysis

Localized surface plasmons generated on metallic nanostructures can be used to accelerate molecular transformations; however, the efficiency is limited by the challenge to control the energy/charge transfer at the interfaces. Here, we combine density functional theory (DFT) calculations and experiments to reveal the mechanism of nitrophenol reduction on Au nanoparticles under visible-light irradiation and propose a strategy to further enhance the reaction rates. DFT calculations show a reduced activation barrier under electronic excitation on Au(111), thus explaining the measured higher rates under visible-light irradiation. Furthermore, we propose a heterostructure with Au nanoparticles covered by a thin film of hexagonal boron nitride; the latter is used to decouple the molecular orbitals from the metal to enable charge localization in the molecule. DFT calculations show that by this electronic decoupling, the activation barrier can be lowered by a factor of five. This work thus provides a valuable strategy for optimizing catalytic efficiency in plasmonic photocatalysis.

Advancing Plasmon-Induced Selectivity in Chemical Transformations with Optically Coupled Transmission Electron Microscopy

Nanoparticle photocatalysts are essential to processes ranging from chemical production and water purification to air filtration and surgical instrument sterilization. Photochemical reactions are generally mediated by the illumination of metallic and/or semiconducting nanomaterials, which provide the necessary optical absorption, electronic band structure, and surface faceting to drive molecular reactions. However, with reaction efficiency and selectivity dictated by atomic and molecular interactions, imaging and controlling photochemistry at the atomic scale are necessary to both understand reaction mechanisms and to improve nanomaterials for next-generation catalysts. Here, we describe how advances in plasmonics, combined with advances in electron microscopy, particularly optically coupled transmission electron microscopy (OTEM), can be used to image and control light-induced chemical transformations at the nanoscale. We focus on our group's research investigating the interaction between hydrogen gas and Pd nanoparticles, which presents an important model system for understanding both hydrogenation catalysis and hydrogen storage. The studies described in this Account primarily rely on an environmental transmission electron microscope, a tool capable of circumventing traditional TEM's high-vacuum requirements, outfitted with optical sources and detectors to couple light into and out of the microscope. First, we describe the H₂ loading kinetics of individual Pd nanoparticles. When confined to sizes of less than ∼100 nm, single-crystalline Pd nanoparticles exhibit coherent phase transformations between the hydrogen-poor α-phase and hydrogen-rich β-phase, as revealed through monitoring the bulk plasmon resonance with electron energy loss spectroscopy. Next, we describe how contrast imaging techniques, such as phase contrast STEM and displaced-aperture dark field, can be employed as real-time techniques to image phase transformations with 100 ms temporal resolution. Studies of multiply twinned Pd nanoparticles and high aspect ratio Pd nanorods demonstrate that internal strain and grain boundaries can lead to partial hydrogenation within individual nanoparticles. Finally, we describe how OTEM can be used to locally probe nanoparticle dynamics under optical excitation and in reactive chemical environments. Under illumination, multicomponent plasmonic photocatalysts consisting of a gold nanoparticle "antenna" and a Pd "reactor" show clear α-phase nucleation in regions close to electromagnetic "hot spots" when near plasmonic antennas. Importantly, these hot spots need not correspond to the traditionally active, energetically preferred sites of catalytic nanoparticles. Nonthermal effects imparted by plasmonic nanoparticles, including electromagnetic field enhancement and plasmon-derived hot carriers, are crucial to explaining the site selectivity observed in PdH_x phase transformations under illumination. This Account demonstrates how light can contribute to selective chemical phenomena in plasmonic heterostructures, en route to sustainable, solar-driven chemical production.

Metal-to-Ligand Charge-Transfer Spectrum of a Ru-Bipyridine-Sensitized TiO2 Cluster from Embedded Multiconfigurational Excited-State Theory

Understanding optical properties of the dye molecule in dye-sensitized solar cells (DSSCs) from first-principles quantum mechanics can contribute to improving the efficiency of such devices. While density functional theory (DFT) and time-dependent DFT have been pivotal in simulating optoelectronic properties of photoanodes used in DSSCs at the atomic scale, questions remain regarding DFT's adequacy and accuracy to furnish critical information needed to understand the various excited-state processes involved. Here, we simulate the absorption spectra of a dye-sensitized solar cell analogue, comprised of a Ru-bipyridine (Ru-bpy) dye molecule and a small TiO₂ cluster via DFT and via an accurate embedded correlated wavefunction (CW) theory. We generated CW spectra for the adsorbed Ru-bpy dye via a recently introduced capped density functional embedding theory or capped-DFET (to generate the embedding potential that accounts for the interaction of the molecule and the TiO₂ cluster). We then combined capped-DFET with the accurate but expensive multiconfigurational complete active space second-order perturbation theory (CASPT2)-embedded CASPT2. Because the CW theory is conducted on only a portion of the total system in the presence of an embedding potential that describes that portion's interaction with its environment, we efficiently obtain CW-quality predictions that reflect local properties of the entire system. Specifically, for example, with capped-DFET and embedded CW theory, we can simulate accurately a plethora of metal-to-ligand charge-transfer excited properties at a manageable computational cost. Here, we predict detailed electronic spectra within the visible region, featuring the lowest three singlet and triplet excited states, along with predictions of the singlets' lifetimes. We illustrated these results using a Jablonski diagram that show the relative energy position of the singlet and longer-lived triplet excited states and analyzed and proposed relaxation paths for the excited state corresponding to the most intense but short-lived absorption (interconversion, intersystem crossing, fluorescence, and phosphorescence) that may lead to longer-lived excited states necessary for efficient charge separation required to generate current in solar cells.

Revisiting Understanding of Electrochemical CO2 Reduction on Cu(111): Competing Proton-Coupled Electron Transfer Reaction Mechanisms Revealed by Embedded Correlated Wavefunction Theory

Copper (Cu) electrodes, as the most efficacious of CO₂ reduction reaction (CO₂RR) electrocatalysts, serve as prototypes for determining and validating reaction mechanisms associated with electrochemical CO₂ reduction to hydrocarbons. As in situ electrochemical mechanism determination by experiments is still out of reach, such mechanistic analysis typically is conducted using density functional theory (DFT). The semilocal exchange-correlation (XC) approximations most often used to model such catalysis unfortunately engender a basic error: predicting the wrong adsorption site for CO (a key CO₂RR intermediate) on the most ubiquitous facet of Cu, namely, Cu(111). This longstanding inconsistency casts lingering doubt on previous DFT predictions of the attendant CO₂RR kinetics. Here, we apply embedded correlated wavefunction (ECW) theory, which corrects XC functional error, to study the CO₂RR on Cu(111) via both surface hydride (*H) transfer and proton-coupled electron transfer (PCET). We predict that adsorbed CO (*CO) reduces almost equally to two intermediates, namely, hydroxymethylidyne (*COH) and formyl (*CHO) at -0.9 V vs the RHE. In contrast, semilocal DFT approximations predict a strong preference for *COH. With increasing applied potential, the dominance of *COH (formed via potential-independent surface *H transfer) diminishes, switching to the competitive formation of both *CHO and *COH (both formed via potential-dependent PCET). Our results also demonstrate the importance of including explicitly modeled solvent molecules in predicting electron-transfer barriers and reveal the pitfalls of overreliance on simple surface *H transfer models of reduction reactions.

First-Principles Insights into Plasmon-Induced Catalysis

The size- and shape-controlled enhanced optical response of metal nanoparticles (NPs) is referred to as a localized surface plasmon resonance (LSPR). LSPRs result in amplified surface and interparticle electric fields, which then enhance light absorption of the molecules or other materials coupled to the metallic NPs and/or generate hot carriers within the NPs themselves. When mediated by metallic NPs, photocatalysis can take advantage of this unique optical phenomenon. This review highlights the contributions of quantum mechanical modeling in understanding and guiding current attempts to incorporate plasmonic excitations to improve the kinetics of heterogeneously catalyzed reactions. A range of first-principles quantum mechanics techniques has offered insights, from ground-state density functional theory (DFT) to excited-state theories such as multireference correlated wavefunction methods. Here we discuss the advantages and limitations of these methods in the context of accurately capturing plasmonic effects, with accompanying examples.

Computational approaches to dissociative chemisorption on metals: towards chemical accuracy

We review the state-of-the-art in the theory of dissociative chemisorption (DC) of small gas phase molecules on metal surfaces, which is important to modeling heterogeneous catalysis for practical reasons, and for achieving an understanding of the wealth of experimental information that exists for this topic, for fundamental reasons. We first give a quick overview of the experimental state of the field. Turning to the theory, we address the challenge that barrier heights (E_b, which are not observables) for DC on metals cannot yet be calculated with chemical accuracy, although embedded correlated wave function theory and diffusion Monte-Carlo are moving in this direction. For benchmarking, at present chemically accurate E_b can only be derived from dynamics calculations based on a semi-empirically derived density functional (DF), by computing a sticking curve and demonstrating that it is shifted from the curve measured in a supersonic beam experiment by no more than 1 kcal mol^-1. The approach capable of delivering this accuracy is called the specific reaction parameter (SRP) approach to density functional theory (DFT). SRP-DFT relies on DFT and on dynamics calculations, which are most efficiently performed if a potential energy surface (PES) is available. We therefore present a brief review of the DFs that now exist, also considering their performance on databases for E_b for gas phase reactions and DC on metals, and for adsorption to metals. We also consider expressions for SRP-DFs and briefly discuss other electronic structure methods that have addressed the interaction of molecules with metal surfaces. An overview is presented of dynamical models, which make a distinction as to whether or not, and which dissipative channels are modeled, the dissipative channels being surface phonons and electronically non-adiabatic channels such as electron-hole pair excitation. We also discuss the dynamical methods that have been used, such as the quasi-classical trajectory method and quantum dynamical methods like the time-dependent wave packet method and the reaction path Hamiltonian method. Limits on the accuracy of these methods are discussed for DC of diatomic and polyatomic molecules on metal surfaces, paying particular attention to reduced dimensionality approximations that still have to be invoked in wave packet calculations on polyatomic molecules like CH₄. We also address the accuracy of fitting methods, such as recent machine learning methods (like neural network methods) and the corrugation reducing procedure. In discussing the calculation of observables we emphasize the importance of modeling the properties of the supersonic beams in simulating the sticking probability curves measured in the associated experiments. We show that chemically accurate barrier heights have now been extracted for DC in 11 molecule-metal surface systems, some of which form the most accurate core of the only existing database of E_b for DC reactions on metal surfaces (SBH10). The SRP-DFs (or candidate SRP-DFs) that have been derived show transferability in many cases, i.e., they have been shown also to yield chemically accurate E_b for chemically related systems. This can in principle be exploited in simulating rates of catalyzed reactions on nano-particles containing facets and edges, as SRP-DFs may be transferable among systems in which a molecule dissociates on low index and stepped surfaces of the same metal. In many instances SRP-DFs have allowed important conclusions regarding the mechanisms underlying observed experimental trends. An important recent observation is that SRP-DFT based on semi-local exchange DFs has so far only been successful for systems for which the difference of the metal work function and the molecule's electron affinity exceeds 7 eV. A main challenge to SRP-DFT is to extend its applicability to the other systems, which involve a range of important DC reactions of e.g. O₂, H₂O, NH₃, CO₂, and CH₃OH. Recent calculations employing a PES based on a screened hybrid exchange functional suggest that the road to success may be based on using exchange functionals of this category.

Light Driven Mechanism of Carbon Dioxide Reduction Reaction to Carbon Monoxide on Gold Nanoparticles: A Theoretical Prediction

Insightful understanding of the light driven CO₂ reduction reaction (CO₂RR) mechanism on gold nanoparticles is one of the important issues in the plasmon mediated photocatalytic study. Herein, time-dependent density functional theory and reduced two-state model are adopted to investigate the photoinduced charge transfer in interfaces. According to the excitation energy and orbital coupling, the light driven mechanism of CO₂RR on gold nanoparticles can be described as follows: the light induces electron excitation and then transfers to the physisorbed CO₂, and CO₂ can relax to a bent structure adsorbed on gold nanoparticles, and the adsorbed C-O bonds are dissociated finally. Moreover, our calculated results demonstrate that the s, p, and d electron excitations of gold nanoparticles are the major contribution for the CO₂ adsorption and the C-O dissociation process, respectively. This work would promote the understanding of the light driven electron transfer and photocatalytic CO₂RR on the noble metal.

Driving energetically unfavorable dehydrogenation dynamics with plasmonics

Nanoparticle surface structure and geometry generally dictate where chemical transformations occur, with higher chemical activity at sites with lower activation energies. Here, we show how optical excitation of plasmons enables spatially modified phase transformations, activating otherwise energetically unfavorable sites. We have designed a crossed-bar Au-PdH _x antenna-reactor system that localizes electromagnetic enhancement away from the innately reactive PdH _x nanorod tips. Using optically coupled in situ environmental transmission electron microscopy, we track the dehydrogenation of individual antenna-reactor pairs with varying optical illumination intensity, wavelength, and hydrogen pressure. Our in situ experiments show that plasmons enable new catalytic sites, including dehydrogenation at the nanorod faces. Molecular dynamics simulations confirm that these new nucleation sites are energetically unfavorable in equilibrium and only accessible through tailored plasmonic excitation.

Fundamentals and applications of photo-thermal catalysis

Photo-thermal catalysis has recently emerged as an alternative route to drive chemical reactions using light as an energy source.

Hybrid theoretical models for molecular nanoplasmonics

The multidisciplinary nature of the research in molecular nanoplasmonics, i.e., the use of plasmonic nanostructures to enhance, control, or suppress properties of molecules interacting with light, led to contributions from different theory communities over the years, with the aim of understanding, interpreting, and predicting the physical and chemical phenomena occurring at molecular- and nano-scale in the presence of light. Multiscale hybrid techniques, using a different level of description for the molecule and the plasmonic nanosystems, permit a reliable representation of the atomistic details and of collective features, such as plasmons, in such complex systems. Here, we focus on a selected set of topics of current interest in molecular plasmonics (control of electronic excitations in light-harvesting systems, polaritonic chemistry, hot-carrier generation, and plasmon-enhanced catalysis). We discuss how their description may benefit from a hybrid modeling approach and what are the main challenges for the application of such models. In doing so, we also provide an introduction to such models and to the selected topics, as well as general discussions on their theoretical descriptions.

Morphology-Dependent Reactivity of a Plasmonic Photocatalyst

The shape of a plasmonic nanoparticle strongly controls its light-matter interaction, which in turn affects how specific morphologies may be used in applications such as sensing, photodetection, and active pixel displays. Here, we show that particle shape also controls plasmonic photocatalytic activity. Three different Al nanocrystal morphologies, octopods, nanocubes, and nanocrystals, all with very similar plasmon resonance frequencies, were used as photocatalysts for the H₂ dissociation reaction. We observe widely varying reaction rates for the three different morphologies. Octopods show a 10 times higher reaction rate than nanocrystals and a 5 times higher rate than nanocubes, with lower apparent activation energies than either nanocubes or nanocrystals by 45% and 49%, respectively. A theoretical model of hot electron direct transfer from photoexcited Al nanoparticles to H₂ molecules is consistent with this observed morphological dependence. This research strongly suggests that nanoparticle geometry, in addition to plasmon resonance energy, is a critical factor in plasmonic photocatalyst design.

Site-Selective Nanoreactor Deposition on Photocatalytic Al Nanocubes

Photoactivation of catalytic materials through plasmon-coupled energy transfer has created new possibilities for expanding the scope of light-driven heterogeneous catalysis. Here we present a nanoengineered plasmonic photocatalyst consisting of catalytic Pd islands preferentially grown on vertices of Al nanocubes. The regioselective Pd deposition on Al nanocubes does not rely on complex surface ligands, in contrast to site-specific transition-metal deposition on gold nanoparticles. We show that the strong local field enhancement on the sharp nanocube vertices provides a mechanism for efficient coupling of the plasmonic Al antenna to adjacent Pd nanoparticles. A substantial increase in photocatalytic H₂ dissociation on Pd-bound Al nanocubes relative to pristine Al nanocubes can be observed, incentivizing further engineering of heterometallic antenna-reactor photocatalysts. Controlled growth of catalytic materials on plasmonic hot spots can result in more efficient use of the localized surface plasmon energy for photocatalysis, while minimizing the amount and cost of precious transition-metal catalysts.

Enhanced Piezo-Photoelectric Catalysis with Oriented Carrier Migration in Asymmetric Au-ZnO Nanorod Array

Current photocatalytic semiconductors often have low catalytic performance due to limited light utilization and fast charge carrier recombination. Formation of Schottky junction between semiconductors and plasmonic metals can broaden the light absorption and facilitate the photon-generated carriers separation. To further amplify the catalytic performance, herein, an asymmetric gold-zinc oxide (Asy-Au-ZnO) nanorod array is rationally designed, which realizes the synergy of piezocatalysis and photocatalysis, as well as spatially oriented electron-hole pairs separation, generating a significantly enhanced catalytic performance. In addition to conventional properties from noble metal/semiconductor Schottky junction, the rationally designed heterostructure has several additional advantages: 1) The piezoelectric ZnO under light and mechanical stress can directly generate charge carriers; 2) the Schottky barrier can be reduced by ZnO piezopotential to enhance the injection efficiency of hot electrons from Au nanoparticles to ZnO; 3) the unique asymmetric nanorod array structure can achieve a spatially directed separation and migration of the photon-generated carriers. When ultrasound and all-spectrum light irradiation are exerted simultaneously, the Asy-Au-ZnO reaches the highest catalytic efficiency of 95% in 75 min for dye degradation. It paves a new pathway for designing unique asymmetric nanostructures with the synergy of photocatalysis and piezocatalysis.

Surface-Plasmon-Induced Ammonia Decomposition on Copper: Excited-State Reaction Pathways Revealed by Embedded Correlated Wavefunction Theory

Ammonia is a promising hydrogen storage medium; however, its decomposition via conventional thermal catalysis requires a significant amount of thermal energy input in order to overcome the reaction barriers. Here, we use embedded correlated wavefunction (ECW) theory to quantify reaction pathways and energetics for ammonia decomposition (N-H bond dissociation and N₂ and H₂ associative desorption) on copper (Cu) nanoparticles using a Cu (111) surface model. We predict that surface plasmon excitations will be able to facilitate ammonia decomposition by substantially reducing the effective barriers along excited-state pathways. We estimate the reductions in reaction barriers for breaking the first N-H bond and for recombinative desorption of surface-bound nitrogen and hydrogen atoms to be approximately 1.7, 0.8, and 0.5 eV, respectively. Further, by using the experimental N₂ desorption barrier as a reference, we compare the accuracy of various theoretical methods, including plane-wave Kohn-Sham density functional theory calculations with commonly used exchange-correlation functionals, embedded complete active space second-order perturbation theory, and embedded multiconfiguration pair-density functional theory. This work offers further confirmation that the ECW theoretical framework is the most robust for treating highly correlated local electronic structures of solids.

Plasmon-Driven Catalysis on Molecules and Nanomaterials

As a new class of photocatalysts, plasmonic noble metal nanoparticles with the unique ability to harvest solar energy across the entire visible spectrum and produce effective energy conversion have been explored as a promising pathway for the energy crisis. The resonant excitation of surface plasmon resonance allows the nanoparticles to collect the energy of photons to form a highly enhanced electromagnetic field, and the energy stored in the plasmonic field can induce hot carriers in the metal. The hot electron-hole pairs ultimately dissipate by coupling to phonon modes of the metal nanoparticles, resulting in a higher lattice temperature. The plasmonic electromagnetic field, hot electrons, and heat can catalyze chemical reactions of reactants near the surface of the plasmonic metal nanoparticles. This Account summarizes recent theoretical and experimental advances on the excitation mechanisms and energy transfer pathways in the plasmonic catalysis on molecules. Especially, current advances on plasmon-driven crystal growth and transformation of nanomaterials are introduced. The efficiency of the chemical reaction can be dramatically increased by the plasmonic electromagnetic field because of its higher density of photons. Similar to traditional photocatalysis, energy overlap between the plasmonic field and the HOMO-LUMO gap of the reactant is needed to realize resonant energy transfer. For hot-carrier-driven catalysis, hot electrons generated by plasmon decay can be transferred to the reactant through the indirect electron transfer or direct electron excitation process. For this mechanism, the energy of hot electrons needs to overlap with the unoccupied orbitals of the reactant, and the particular chemical channel can be selectively enhanced by controlling the energy distribution of hot electrons. In addition, the local thermal effect following plasmon decay offers an opportunity to facilitate chemical reactions at room temperature. Importantly, surface plasmons can not only catalyze chemical reactions of molecules but also induce crystal growth and transformation of nanomaterials. As a new development in plasmonic catalysis, plasmon-driven crystal transformation reveals a more powerful aspect of the catalysis effect, which opens the new field of plasmonic catalysis. We believe that this Account will promote clear understanding of plasmonic catalysis on both molecules and materials and contribute to the design of highly tunable catalytic systems to realize crystal transformations that are essential to achieve efficient solar-to-chemical energy conversion.

Photocatalytic Hydrogenation of Graphene Using Pd Nanocones

Plasmonic photocatalytic processes typically use the interaction of light with metallic nanoparticles to drive chemical reactions on their surfaces. Here we show that a plasmonic photocatalyst can also induce a reaction on an adjacent material. A combination of spontaneous H₂ dissociation and plasmon-induced H desorption from tilted palladium (Pd) nanocones yields reactive H atoms which, in the direct vicinity of a graphene monolayer, results in its local hydrogenation. The conversion of pristine to hydrogenated graphene, a semiconductor, is detectable by visible local fluorescence of the hydrogenated regions of the graphene sheet, as well as by Raman spectroscopic analysis. These results may lead to new approaches for local, light-driven functionalization of graphene and other 2D materials and for precision patterning of functional devices.

PdZn alloy nanoparticles encapsulated within a few layers of graphene for efficient semi-hydrogenation of acetylene

PdZn alloy NPs encapsulated within a few layers of graphene exhibit enhanced acetylene selectivity and robust long-term stability in selective hydrogenation of acetylene, compared with that of traditional supported Pd and PdZn alloy catalysts.

In-situ observation of plasmon-controlled photocatalytic dehydrogenation of individual palladium nanoparticles

Plasmonic nanoparticle catalysts offer improved light absorption and carrier transport compared to traditional photocatalysts. However, it remains unclear how plasmonic excitation affects multi-step reaction kinetics and promotes site-selectivity. Here, we visualize a plasmon-induced reaction at the sub-nanoparticle level in-situ and in real-time. Using an environmental transmission electron microscope combined with light excitation, we study the photocatalytic dehydrogenation of individual palladium nanocubes coupled to gold nanoparticles with sub-2 nanometer spatial resolution. We find that plasmons increase the rate of distinct reaction steps with unique time constants; enable reaction nucleation at specific sites closest to the electromagnetic hot spots; and appear to open a new reaction pathway that is not observed without illumination. These effects are explained by plasmon-mediated population of excited-state hybridized palladium-hydrogen orbitals. Our results help elucidate the role of plasmons in light-driven photochemical transformations, en-route to design of site-selective and product-specific photocatalysts.

Quantifying hot carrier and thermal contributions in plasmonic photocatalysis

Photocatalysis based on optically active, “plasmonic” metal nanoparticles has emerged as a promising approach to facilitate light-driven chemical conversions under far milder conditions than thermal catalysis. However, an understanding of the relation between thermal and electronic excitations has been lacking. We report the substantial light-induced reduction of the thermal activation barrier for ammonia decomposition on a plasmonic photocatalyst. We introduce the concept of a light-dependent activation barrier to account for the effect of light illumination on electronic and thermal excitations in a single unified picture. This framework provides insight into the specific role of hot carriers in plasmon-mediated photochemistry, which is critically important for designing energy-efficient plasmonic photocatalysts.