Surface-Plasmon-Driven Hot Electron Photochemistry - Chemical

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Cite This: Chem. Rev. 2018, 118, 2927−2954

Surface-Plasmon-Driven Hot Electron Photochemistry Yuchao Zhang, Shuai He, Wenxiao Guo, Yue Hu, Jiawei Huang, Justin R. Mulcahy, and Wei David Wei* Department of Chemistry and Center for Catalysis, University of Florida, Gainesville, Florida 32611, United States ABSTRACT: Visible-light-driven photochemistry has continued to attract heightened interest due to its capacity to efficiently harvest solar energy and its potential to solve the global energy crisis. Plasmonic nanostructures boast broadly tunable optical properties coupled with catalytically active surfaces that offer a unique opportunity for solar photochemistry. Resonant optical excitation of surface plasmons produces energetic hot electrons that can be collected to facilitate chemical reactions. This review sums up recent theoretical and experimental approaches for understanding the underlying photophysical processes in hot electron generation and discusses various electrontransfer models on both plasmonic metal nanostructures and plasmonic metal/ semiconductor heterostructures. Following that are highlights of recent examples of plasmon-driven hot electron photochemical reactions within the context of both cases. The review concludes with a discussion about the remaining challenges in the field and future opportunities for addressing the low reaction efficiencies in hot-electron-induced photochemistry.

CONTENTS 1. Introduction 2. Physical Understanding of Plasmon-Mediated Hot Electron Generation and Transfer 2.1. Hot Electron Generation 2.2. Hot Electron Transfer from Metal Nanostructures to Adsorbate Molecules 2.2.1. Indirect Electron Transfer 2.2.2. Direct Electron Transfer 2.3. Hot Electron Transfer from Metal Nanostructures to Semiconductors 2.3.1. Indirect Electron Transfer 2.3.2. Direct Electron Transfer 2.3.3. Physical Locations of Trapped Hot Electrons on Semiconductors 3. Hot Electron Photochemistry on Metal Nanostructures 3.1. Indirect Electron Transfer 3.1.1. H2 Dissociation 3.1.2. O2 Dissociation 3.1.3. Growth of Metal Nanostructures 3.2. Direct Electron Transfer 3.3. Hot Electron Photochemistry on Bimetallic Nanostructures 3.3.1. Alloy Nanostructures 3.3.2. Bimetallic Heterostructures 4. Hot Electron Photochemistry on Metal/Semiconductor Heterostructures 4.1. Electron−Hole Separation Enhancement 4.2. Hot Electron Energy-Level Manipulation 4.3. Hot-Electron-Induced Adsorbate Activation 5. Conclusions and Outlook Author Information Corresponding Author © 2017 American Chemical Society

ORCID Notes Biographies Acknowledgments References

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1. INTRODUCTION The growing energy crisis and the current rate of pollution have made the efficient utilization of renewable solar energy in chemical transformations extremely important for modern industry. Plants naturally do this through photosynthesis, and carefully designed photocatalysts must be implemented to reproduce similar photochemical processes in artificial systems. Fujishima and Honda1 focused on such photocatalysis in their groundbreaking work in 1972, in which electrochemical H2O splitting was facilitated by photoexcited charge carriers in a TiO2 electrode using UV-light irradiation. Since then, photocatalytic materials based on semiconductors (e.g., TiO2 and ZnO) have been extensively explored for photochemical applications, such as solar H2O splitting2−4 and organic transformations.5−7 However, technologies solely based on semiconductors suffer from low efficiency due to inherent limitations; the most commonly used semiconductors have wide band gaps (>3.1 eV) and can only absorb UV light,7−9 which counts for 4% of all incoming solar radiation.10 Therefore, numerous photochemical studies have attempted to discover new materials and develop new strategies in order to efficiently convert visible light (which constitutes 42% of solar radiation)10 into chemical energy.

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Special Issue: Plasmonics in Chemistry Received: July 18, 2017 Published: November 30, 2017 2927

DOI: 10.1021/acs.chemrev.7b00430 Chem. Rev. 2018, 118, 2927−2954

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Figure 1. Localized SPR excitation on spherical NPs. (a) EM-field-induced coherent localized oscillation of the electron cloud. (b) Hot carrier generation and the corresponding absorption spectrum of plasmonic metals. e− and h+ represent hot electrons and holes, respectively, which are generated after SPR dephasing. The purple curve represents the absorption spectrum of the plasmonic metal. Panel a adapted with permission from ref 49. Copyright 2011 American Chemical Society.

Plasmonic nanostructures possess broadly tunable optical properties coupled with catalytically active surfaces that benefit solar photochemistry.11−31 Surface plasmon resonance (SPR) is a unique optical property associated with certain metallic nanomaterials (e.g., Au, Ag, Cu).15,16,32 The localized SPR excitation of a metal nanoparticle (NP) can be visualized as an electromagnetic (EM) field coupled to the coherent oscillation of all conduction electrons in a conducting medium (Figure 1a).18,33 It has been reported that the absorption cross section of metallic NPs can be up to 5 orders of magnitude larger than that of typical dye-sensitizer molecules.22 More importantly, the optical properties of these plasmonic nanomaterials are tunable (by varying size and shape) across the entire visible spectrum.32,34 Thus, incorporating plasmonic metal NPs into photocatalytic systems holds great promise for dramatically improving the efficiency of sunlight absorption and solar energy conversion. The resonant optical excitation of surface plasmons produces energetic hot carriers that can participate in photochemical reactions. Upon SPR excitation, the oscillation of free electrons quickly dephases and leads to the generation of energetic hot electrons and holes (Figure 1b) on a time scale ranging from 1 to 100 fs.35−38 This review is targeted toward hot electron rather than hot hole photochemistry; as a result, only hot electron dynamics will be discussed in most cases. Furthermore, despite the numerous examples of hot-electron-driven processes, few examples have been reported regarding the dynamics of plasmon-generated hot holes.39−42 Most works in plasmonic photocatalysts purposely used sacrificial reagents to scavenge hot holes in order to enhance charge carrier separation for more efficient hot-electron-induced photochemical processes.43−48 For hot electron photochemistry, it has been shown that the plasmon-generated hot electrons can directly interact with molecules adsorbed on the metal surface.12,35,50−54 Moreover, hot electrons can also be transferred to an adjacent semiconductor support via a plasmon-mediated electron-transfer (PMET) pathway.55−63 In the plasmonic metal/semiconductor systems, the SPR excitation significantly increases the yield of hot electrons with high potential energy on the plasmonic metals and induces fast and efficient transfer of hot electrons to the semiconductors (Figure 2). The Schottky barrier (ϕSB) at the

Figure 2. PMET strategy in a plasmonic metal/semiconductor heterostructure for efficiently capturing and converting visible light to facilitate the surface reduction reactions. SPR-generated hot electrons with sufficient energy can transfer to the CB of the semiconductor. The ϕSB at the interface helps trap the transferred hot electrons, prolong their lifetimes, and make them available for facilitating surface chemical reactions. Dox and Dred represent the oxidation and reduction states, respectively, of an electron donor in solution; Aox and Ared represent the oxidation and reduction states, respectively, of an electron acceptor in solution. ECB stands for the bottom of CB, EVB stands for the top of valence band (VB), and EF stands for Fermi level.

metal/semiconductor interface helps trap the transferred hot electrons in the conduction band (CB) of semiconductors by delaying them from traveling back to the plasmonic metals.64 This PMET strategy effectively prolongs the lifetimes of hot electrons transferred to the CB of semiconductors and therefore makes them capable of fostering various surface chemical reactions (Figure 2), such as H2O splitting,8,55,62,63,65 CO2 reduction,66−70 and organic transformations71 including aerobic oxidation72 and aldehyde hydrogenation.73 To date, a detailed molecular-level description of hot-carrierassisted photochemical processes remains elusive. In the case of heterostructures, though the ultrafast dynamics (time t approximately femtoseconds to picoseconds) of PMET have been extensively studied,64,74 the ensuing electronic landscape eventually established on plasmonic photocatalysts under steadystate conditions (t approximately milliseconds to minutes) is worth exploring. Since the interfacial electronic structure of metal/semiconductor heterostructures exerts a profound influence on their catalytic activity,55,56,58,60,62,63 precise knowl2928


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Figure 3. Plasmon-induced hot carrier generation and hot electron transfer/back-transfer processes in clean metal, metal/adsorbate, and metal/ semiconductor systems. In each case, the left component (relative to energy y-axis) displays the Fermi level (EF) of plasmonic metals and the right component (relative to energy y-axis) illustrates the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) for adsorbates, or the CB and VB positions for semiconductors. The gray parts represent the population of electronic states.

Figure 4. Time scales of plasmon-induced hot carrier generation, hot electron transfer, and thermalization processes with/without an adsorbate or a semiconductor. From top to bottom: a series of time scales corresponding to the fate of hot carriers in a clean metal NP, additional events in a metal NP capped with an adsorbate, and the processes involved in a metal NP loaded on a semiconductor support via a Schottky contact are shown. Note: el stands for electron and ph stands for phonon. Data were adapted from refs 35, 53, 64, 74, 87, and 90.

edge of the interfacial electronic structure and the spatial distribution of hot electrons over time scales that coincide with chemical reactions is required to fully understand the related processes. On the other hand, for photochemical reactions on metal nanostructures where PMET is absent and does not contribute its effect of extending the average hot electron lifetime, the mechanism that circumvents the mismatch between the lifetimes of hot electrons (t approximately femtoseconds to nanoseconds)35,54,55 and the time scale of chemical reactions (t approximately microseconds to seconds)75−81 still requires further investigation. Specifically motivated by a desire to explore this question, researchers have paid special attention to the interaction between metal NPs and adsorbates on the molecular level, such as binding modes of reactants on the surfaces of metal NPs, orbital couplings, and the influence of such

orbital coupling on the generation and transfer of hot electrons.82−85 A molecular-level understanding of these issues is crucial to gaining a full picture of photochemical reactions driven by plasmon-induced hot electrons, which is a prerequisite for further modifying the nanostructures and manipulating the hot electrons to achieve full optimization of plasmonic photochemical systems. This review focuses on the recent progress of photochemical reactions driven by plasmon-generated hot electrons. The discussion begins with a physical understanding of plasmoninduced hot electron generation, and the influence of molecular adsorbates is also considered (section 2.1). For both singlecomponent (section 2.2) and heterostructure (section 2.3) plasmonic photocatalysts, the possible electron-transfer pathways across the related interfaces are comprehensively reviewed. In the following sections, we highlight important recent 2929


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Figure 5. Theoretical studies of plasmon-induced hot carrier generation. (a) Number of hot electrons (red distribution) and hot holes (blue distribution) generated per unit of time and volume as a function of their energy for (i) 15 nm and (ii) 25 nm Ag NPs. The distributions are simulated by use of different hot carrier lifetimes (τ) ranging from 0.05 to 1 ps. Zero energy refers to the Fermi level. (b) Hot carrier energy distributions for (i) Al, (ii) Ag, (iii) Cu, and (iv) Au. Zero energy refers to the Fermi level. (c) Comparison of resistive, geometry-assisted, phonon-assisted, and direct-transition contributions to the absorption as a function of frequency for a 10 nm Au sphere. The resistive loss arises from the intrinsic lifetimes of the electronic states comprising the collective oscillation. Such loss results in heating rather than hot electron generation. The geometry-assisted intraband transition is the direct decay of plasmon (below the interband threshold energy) into an electron−hole pair with net crystal momentum. Panel a adapted with permission from ref 93. Copyright 2014 American Chemical Society. Panel b adapted with permission from ref 96. Copyright 2014 Macmillan Publishers Limited. Panel c reprinted with permission from ref 91. Copyright 2016 American Chemical Society.

10 ps) that result in a higher lattice temperature.35,53,87 The heat then slowly dissipates to the environment (100 ps−10 ns).35,64 In the presence of an electron-accepting orbital of a nearby adsorbate or semiconductor, electrons with sufficient energy can transfer from a metal NP into these species (Figures 3 and 4). In general, two pathways exist for interfacial electron transfer, namely, the conventional indirect transfer and the recently proposed direct transfer.74,82,84,88 For indirect electron transfer, the electrons are transferred to either the adsorbate or the semiconductor (PMET) after hot carrier generation. On the other hand, in the presence of strongly interacting adsorbate molecules or closely contacted semiconductors, an additional dephasing channel, namely, chemical interface damping (CID), can induce the direct generation of hot electrons in the electronaccepting orbitals of the adsorbate or the semiconductor, in a process that is termed direct electron transfer.53,82,84,89 Thermalization and back-transfer processes always compete with the transfer processes at comparable time scales, which are major concerns for the efficient use of hot electrons in plasmonic photochemistry.

achievements in photocatalysis on metal nanostructures and metal/semiconductor heterostructures. For photocatalysis on metal nanostructures (section 3), several representative photocatalytic processes are covered, with a particular focus on the relationship between ultrafast hot electron transfer and outcome of photochemical reactions. The activation of reactants by the formation of transient negative ion (TNI) species as a result of the rapid back-transfer of hot electrons and the recently discovered surfactant effect that mediates anisotropic metal nanostructure growth are highlighted. Selectivity control on photochemistry achieved through the construction of bimetallic nanostructures is also discussed. For photocatalysis on metal/ semiconductor heterostructures (section 4), we highlight the strategies for enhancing electron−hole separation and manipulating hot electron energy levels and the mechanisms of hotelectron-induced adsorbate activation on the semiconductor surfaces. This review concludes with the current challenges facing the field of hot-electron-induced photochemistry and the opportunities for addressing the low reaction efficiencies (section 5).

2.1. Hot Electron Generation

2. PHYSICAL UNDERSTANDING OF PLASMON-MEDIATED HOT ELECTRON GENERATION AND TRANSFER As shown in Figures 3 and 4, upon SPR excitation on clean metal surfaces, the coherent electron oscillation nonradiatively dephases and generates hot carriers (1−100 fs).35,53,86 The hot electrons and hot holes are initially in a nonthermal distribution. It rapidly thermalizes to a Fermi−Dirac distribution via electron−electron and electron−phonon scatterings (100 fs−

In metals, the absorption of photons can generate hot carriers via electron transitions, with the momentum being conserved.36 This conservation requirement can be naturally met above interband threshold energies (e.g., from d band to sp band).91 Below this threshold, transitions (i.e., intraband) must be assisted with the necessary momentum provided by other quasiparticles (e.g., phonons).36 In plasmonic metals, Landau damping (i.e., damping of the collective mode of oscillations in a plasma) offers an additional pathway for the hot carrier generation process,37,38 2930


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H2O-capped Ag clusters (Figure 6). Covalent chemical bonds were formed between the quinone and the Ag cluster, whereas

in which momentum conservation is satisfied due to the excessive momentum contained in the surface plasmon when the effective size of the surface plasmon is much smaller than the halfwavelength of incident light.36 Theoretical models have been proposed to describe the hot carrier generation process.92−95 For instance, Nordlander and co-workers93 calculated the energy distribution for hot electrons and holes in a Ag NP using a free electron model and verified the results with density functional theory (DFT). The simulation revealed that larger NPs increased the production rate of hot carriers but decreased their energy (Figure 5a). Later, Atwater and co-workers96 developed a quantized plasmon model with a detailed electronic structure deduced from DFT to predict the promoted distribution of hot electrons and holes generated by direct electron transitions. They found that the energy distribution of hot carriers depended on the electronic band structure of metal NPs. For example, Au and Cu produced electrons with lower energy than that of holes by 1−2 eV, while energy was equitably distributed between electrons and holes for Ag and Al (Figure 5b). These results were further confirmed by subsequent theoretical studies using the many-body perturbation theory.97 In a later work, Atwater and co-workers91 used ab initio calculations to provide a more detailed picture that accounted for all relevant microscopic mechanisms of plasmon decay, which consisted of phonon-assisted electron transitions, geometryassisted electron transitions, and direct electron transitions, together with the classic resistive effects (Figure 5c). The ab initio calculation revealed that the contributions of different loss mechanisms were highly influenced by the size and band structure of the material. Furthermore, they predicted that the lifetimes of plasmon-generated hot carriers were on the order of 10 fs.91 Under the perturbation of adsorbed molecules, another plasmon dephasing pathway, namely, CID, also contributes to hot electron generation, in which hot electrons are directly generated in the electron-accepting states on the adsorbate, leaving hot holes at the electron-donating states on metal surfaces.53,89,98−100 CID is induced by the coupling between unpopulated adsorbate states and excited surface plasmons. This coupling modifies the natural dephasing mechanism and accelerates the overall plasmon dephasing.33,101 The existence of CID was evidenced by peak broadening and a decline in intensity of the SPR band within optical absorption spectra of the plasmonic metal NPs.98−101 For instance, the adsorption of SO2 on Ag NPs produced observable line broadening of the Ag SPR band.98 The optical spectra measurement on 2 nm Ag clusters showed that after the Ag clusters were embedded into a SiO2 matrix, the full width at half-maximum (fwhm) of the SPR feature increased from ∼0.2 to ∼1.0 eV.99 It has been reported that efficient CID requires strong orbital hybridization between the adsorbates and the surface of a metal NP.101 In a sulfate-covered Au NP system, the electronic structure of hybridized orbitals involved in CID consisted of a σ donation and a π* backdonation.90 Surface-adsorbed sulfates were suggested to donate electrons from the occupied σ orbital to the metal surface while accepting a back-donation of electrons from the metal to its unoccupied π* orbital. This orbital hybridization caused the coupling of Au electrons and sulfate vibrational modes, leading to a strong CID. Recently, an atomic-level insight into CID on a system with complex adsorbates was achieved by use of timedependent density functional theory (TDDFT).101 In this study, the electronic population migration from Ag sp bands to the LUMOs of adsorbates was calculated for quinone-capped and

Figure 6. Strong interaction of adsorbates and metal nanoclusters for facilitation of hot electron generation through CID. (a) Ag 309 nanocluster/H2O and (b) Ag309 nanocluster/quinone molecular structures and corresponding calculated optical absorption spectra. Optical spectra were calculated by fitting a time-dependent dipole moment equation. Strong orbital hybridization between quinone molecules and Ag nanoclusters results in band broadening of the SPR. Furthermore, broadening of the SPR bands of Ag clusters increased with the number of quinone molecules. Adapted with permission from ref 101. Copyright 2016 American Chemical Society.

similar strong orbital hybridization was not generated at the interface of Ag/H2O. As a result of the different binding strengths between Ag/H2O and Ag/quinone, the chemical interface exhibited distinct degrees of electronic reorganization. The broadening of the Ag SPR band was more significant with the adsorption of quinone (Figure 6).101 These results not only confirmed that CID accelerated the generation of hot electrons but also demonstrated that a strong interfacial interaction between adsorbates and metals was necessary to increase the efficiency of CID.101 It is known that plasmon-excited electrons thermalize to form a Fermi−Dirac distribution via electron−electron scattering and electron−phonon scattering.33,35 In the presence of neighboring phases (e.g., surface adsorbates), apart from the classical thermalization pathway, the excited electrons can also relax through chemical interface scattering (CIS), which portrays how the electrons resonantly and repeatedly scatter into and out of the nearby empty electronic states, as introduced by coupling with the LUMOs of surface adsorbates.90,102−104 In metal/ adsorbate systems, hot electrons can transiently transfer into the adsorbates and deposit a portion of their energy to the adsorbates when they return to the metal NPs. The deposited energy causes the adsorbates to be vibrationally excited, turning them into hot adsorbates. These hot adsorbates can be viewed as transient energy reservoirs of plasmon energy. The hot electrons transfer back and forth between the metal and adsorbates repeatedly. This process results in an accumulation of energy within the asgenerated hot adsorbates, which allows the hot electrons later 2931


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Figure 7. Hot electron transfer from metals to adsorbates. (a) Depiction of a two-step process for indirect electron transfer. (i) First, the nonthermalized distribution of electron−hole pairs is generated on metal surfaces; (ii) second, a Fermi−Dirac distribution of thermalized hot carriers forms. Hot electrons with enough energy can transfer to surface-adsorbed molecules. (b) Depiction of a direct-electron-transfer process. Here, some of the nonthermalized electrons temporarily transfer into the LUMOs of adsorbates and then they rapidly transfer back to the metal, as shown by the red arrows. Electrons can deposit energy into the adsorbates. Note that red areas represent hot electrons and blue areas represent hot holes. Adapted with permission from ref 35. Copyright 2015 Macmillan Publishers Limited, part of Springer Nature.

adsorbates. Studies have shown that even for physically adsorbed molecules, such as H2 on Au, orbital hybridization (though relatively weak) induced by van der Waals forces is still required to prompt electron transfer.85,121 2.2.1. Indirect Electron Transfer. The indirect-electrontransfer mechanism assumes that hot electrons are first generated within metal NPs (as described in section 2.1). Subsequently, hot electrons with suitable energy can transfer into the LUMOs of adsorbates (Figure 7a). The key parameter determining indirect electron-transfer efficiency is the number of photoexcited carriers at the energy of the LUMOs of the adsorbed molecules (εmol):92

transferred into the adsorbates to gain back part of the energy previously deposited there. Such a cycling process effectively prolongs the time required for hot electron thermalization. The time scale of this process has been estimated to be in the picosecond regime, which is much longer than the original lifetimes of plasmon-excited electrons.90,102 Experimental evidence for this increase in lifetimes was gained by use of transient absorption (TA) spectroscopy in a Au/sulfate system.90 In addition, Bauer et al.102 performed TA measurements using 4 nm Au NPs covered with mercaptosuccinic acid and TiO2. They observed nonclassical broadening of the SPR band and found that the thermalization process could last 800 fs for electrons at 0.4 eV above the Fermi level. Compared with the thermalization of hot electrons in metal NPs with clean surfaces, CIS enabled by the adsorbate retards the overall thermalization process.90,102 It must be noted that all of the aforementioned hot electron dynamics (femtoseconds to nanoseconds)35,53,64,74,87,90 are much faster than typical chemical reaction kinetics (microseconds to seconds).75−81 Therefore, additional dynamic processes should exist that elongate the lifetimes of plasmongenerated hot electrons and make them available to participate in chemical reactions. In order to explore what these processes are, our discussion focuses on the manipulation of hot electrons via electron transfer from metal to adsorbate molecules (section 2.2) and from metal to semiconductor supports (section 2.3).

eff(ω) =

ratee(h)(ω) (σNCI0/ℏω)

= γtun

δne(h)(εmol , ω) nDOS(εmol)(σNCI0/ℏω)

where δne(h) is the average number of excited electrons at a given energy, εmol is the energy of LUMOs of the adsorbed molecule, γtun is the tunneling rate, σNC is the absorption cross section of the metal NP, and nDOS(εmol) is the density of states of the metal NP at εmol. It is expected that indirect electron-transfer efficiency would increase with the incident photon energy, as a higher energy would generate more electrons with enough potential to overcome the interfacial barrier between metal and adsorbate.35,122 2.2.2. Direct Electron Transfer. Recent studies have shown that, in addition to the conventional indirect electron transfer, SPR excitation on metal NPs also prompts the direct transfer of hot electrons from the metal NPs to the adsorbates (Figure 7b).53,100 This direct transfer originates from CID. As mentioned in section 2.1, the occurrence of CID requires the formation of hybridized surface states between metal NPs and adsorbates, and it directly leads to electron transitions within the hybridized states.84,90 In the case where electron-donating states of the hybridized states are centered at the metal and electronaccepting states are centered at the adsorbate, the transitions resulting from CID can be treated as a direct electron transfer from metal to adsorbates.82,84 Compared with the indirect electron transfer that occurs after hot electron generation, direct electron transfer is completed during the dephasing of a plasmon.33 Therefore, as the indirectelectron-transfer pathway suffers from significant energy loss due to electron−electron scattering, the direct-electron-transfer pathway is expected to have higher electron-transfer efficiency and lower energy loss.100 The photon energy required to induce electron transfer is expected to change according to the extent of molecular orbital

2.2. Hot Electron Transfer from Metal Nanostructures to Adsorbate Molecules

The transfer of plasmon-excited hot electrons to molecules adsorbed on the metal surface has been corroborated by numerous surface-enhanced Raman spectroscopic (SERS) studies.105−114 Evidence from a number of investigations has verified that the electron-transfer process increases the electron density in empty molecular orbitals and produces new features in the Raman spectra.106,115,116 Theoretical and experimental studies have been conducted to elucidate the hot-electroninduced chemical transformation of adsorbates on plasmonic metal nanostructures.12,13,50,51,53,82,84,105,107−114,117−120 Conventional models treat this process as indirect electron transfer, in which the hot electrons are generated on metal surfaces and then transfer into the LUMOs of adsorbed molecules, followed by the thermalization process (Figure 7a).35,54,92 However, recent works reported the discovery of a direct-electron-transfer pathway stemming from CID (Figure 7b).53,82,84,90,99−101 It should be pointed out that orbital overlap is a prerequisite for both direct and indirect electron transfer from metals to 2932


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Figure 8. Three electron-transfer mechanisms in nonplasmonic metal NP systems. (a) Indirect electron transfer, where the electrons are first generated in a metal via photoexcitation and then transfer to the LUMO of an adsorbate. (b) Intramolecular photoexcitation of an adsorbate physically adsorbed on a metal surface. (c) Photoexcitation of electrons in the hybridized orbitals formed between metal and adsorbate chemically adsorbed to the metal surface. Since the HOMOs of the hybridized bond have a larger metal characteristic, and the LUMOs have a larger adsorbate characteristic, the HOMO−LUMO transition can be viewed as a direct electron transfer from metal to adsorbates. Adapted with permission from ref 125. Copyright 2014 American Chemical Society.

Figure 9. Wavelength-dependent H2O splitting on the surface of Au as a result of direct electron transfer. (a) TDDFT calculations showing that cleavage of the H−O bond in H2O occurred in tens of femtoseconds. (i) Schematic of H2O splitting process; (ii) maximum laser field that reached the Au NPs at ∼6.6 fs; (iii) change in bond length of two H−O bonds in H2O. The dissociation of H2O started immediately after absorption of the laser pulse. (b) Calculated time-evolved oscillating electron density in a 1.9 nm Au NP under excitation at (i) 2.36 and (ii) 2.62 eV. The oscillating feature excited at 2.36 eV had an odd mode, and the one excited at 2.62 eV had an even mode. (iii) As a result of the odd-mode excitation, the oscillating electron density would have a large spatial overlap with the electron-accepting state of H2O, making efficient direct electron transfer possible. (c) Time-evolved energy distribution in Au NPs relative to energy of the electron-accepting state of H2O at (i) 2.62 and (ii) 2.36 eV. The energy of the oscillating electrons matched with the electron-accepting state of H2O (AB state) only when the odd mode (2.36 eV) was excited. Repinted with permission from ref 24. Copyright 2016 American Chemical Society.

electron-transfer efficiency varies with different adsorbates due to the various extents of orbital hybridization.124,125 These insights suggest that the efficiency of indirect and direct electron transfer in plasmonic metal/adsorbate systems should exhibit different wavelength dependences. Generally speaking, in the case of indirect electron transfer, optimum efficiency can be achieved when the incident wavelength is resonant with the metal SPR. Direct electron transfer, however, requires not only the excitation of SPR but also an energy match between the SPR and the HOMO−LUMO transition of hybridized surface states to maximize the electron-transfer efficiency.126 Recently, Meng and co-workers24 conducted a theoretical study of plasmoninduced H2O splitting on Au NPs, and their results showed significant energy and spatial overlap between the oscillating electron density within Au NPs and the electron-accepting orbitals of H2O. More importantly, they found that direct electron transfer from Au to H2O occurred only when a specific plasmon mode of Au NPs was excited at a designated wavelength (Figure 9).24

hybridization. Although well-established experimental evidence is still limited for plasmonic photocatalysis, analogous situations have been explored and summarized in photocatalytic studies on nonplasmonic metal NPs, in which electrons are generated via direct photoexcitation rather than through SPR decay. Figure 8 shows three possible electron-transfer mechanisms in a nonplasmonic metal/adsorbate system,123,124 which are expected to have different wavelength dependences.124,125 Maximum electron-transfer efficiency is expected to be achieved when the energy of the incident photon is resonant with the absorption of the metal NP, in the case shown in Figure 8a, or matches with the HOMO−LUMO gap of the molecules, as shown in Figure 8b. Special attention should be paid to the case displayed in Figure 8c, in which the highest electron-transfer efficiency is achieved when the energy of the incident light matches the transition within the new hybridized surface states formed between adsorbate molecules and metal surfaces. Orbital hybridization also exhibits the additional effect that significantly reduces the energy needed for HOMO−LUMO transitions. Hence, the 2933


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height. Theoretical frameworks have been further built to develop Fowler theory of plasmonic effects.92,136,137 For instance, Govorov et al.92 employed quantum theory to simulate the distribution of hot carriers in a metal NP. They used the quantum solution of the steady-state electron distribution under optical excitation to evaluate the electron-transfer efficiencies from plasmonic nanostructures to semiconductors and found that the analytical equation for plasmon-induced photocurrent followed the Fowler law. Fowler law reveals that the electron-transfer efficiency is highly determined by ϕSB. Both theoretical and experimental efforts have been carried out to develop strategies for manipulating ϕSB in the pursuit of improving photocatalytic performance.138 However, quantitatively measuring ϕSB in a metal/semiconductor nanostructure remains a challenge in the field. Previously, ϕSB has been estimated by use of the Schottky− Mott equation:

The strong metal/adsorbate interaction required for surface orbital hybridization is usually not easily achieved for plasmonic photocatalysts. Au, which is the most commonly used plasmonic metal, is known to be chemically inert in various environments, so the challenge of forming strong molecular attachments remains.127 This greatly limits the contribution of direct electron transfer to the overall photochemistry. Moreover, both direct and indirect electron-transfer processes occur simultaneously in real photocatalytic reactions, so quantitatively distinguishing the individual contributions remains a challenging area of investigation.100 A recent ultrafast SERS study identified that indirect electron transfer was dominant in the hot-electron-induced coupling of 4-nitrobenzenethiol (4-NBT), regardless of its strong adsorption on Ag NPs.120 2.3. Hot Electron Transfer from Metal Nanostructures to Semiconductors

The CB of the attached semiconductors can accept plasmongenerated hot electrons via the PMET process. Moreover, significantly different from the metal/adsorbate molecule system, the ϕSB formed at the metal/semiconductor interface assists in the trapping of hot electrons transferred to the CB of semiconductors by delaying them from traveling back to the metal nanostructure (Figure 2). Since there are no holes generated in the VB of wide-band-gap semiconductors under visible light, the hot electrons transferred to semiconductors have a much longer lifetime and are available to foster surface reactions.44,56,128 In an early photoelectrochemical (PEC) study, Kozuka and co-workers129 showed that an anodic photocurrent was generated on a TiO2 electrode modified with Au or Ag NPs under visible-light illumination, suggesting the functionality behind PMET. Since then, numerous studies have been reported to confirm the PMET processes in the metal/semiconductor nanostructures.20,48,55−57,61−63,130−132 For instance, Tian and Tatsuma48,61 observed that the incident photon-to-electron conversion efficiency (IPCE) spectra exactly followed the extinction spectra of metal NPs, conclusively demonstrating that SPR was responsible for producing the photocurrent. Further work reported by Halas and co-workers59 showed that plasmon-induced hot electron generation was utilized to drive a Au/Si photovoltaic device. With SPR excitation, the hot electrons can be injected into the semiconductor, cross the interfacial ϕSB, and contribute to a photocurrent. In the following subsections, we outline various theoretical and experimental approaches that address the calculation of electron-transfer efficiency, as well as the major factors that influencing it. 2.3.1. Indirect Electron Transfer. The indirect transfer of plasmon-generated hot electrons to a semiconductor occurs via a two-step pathway (Figure 2):59,91,133 first, hot electron−hole pairs are generated in the metal on the order of femtoseconds;91 second, the hot electrons are transferred into the CB of the adjacent semiconductor to participate in chemical reactions on photocatalysts or produce photocurrents in photovoltaic devices.16,92,134 Experiments have shown that the excitation energy dependence of the photocurrent through the indirect process followed the Fowler equation that was originally developed to understand the probability of generating photoelectrons near the threshold of metal:59,132,133,135

ϕSB = ϕM − χSM

where ϕM is the work function of the metal and χSM is the electron affinity of the semiconductor. The values of both ϕM and χSM were obtained via photoemission spectroscopy.136,139 Recent studies, however, have shown that the values of ϕSB not only are determined by the electronic parameters of metals and semiconductors (i.e., ϕM and χSM) but also are significantly influenced by their interfacial chemistry.140,141 It is known that the orbital overlap between metal and semiconductor forms chemical bonds during construction of the metal/semiconductor interface. These chemical bonds cause charge redistribution at the interface, and the resulting charge arrangement creates additional interfacial dipoles.140,141 The current Schottky−Mott models are not able to accurately estimate ϕSB without considering the complicated interfacial chemical effects.140 New methods are required to experimentally measure ϕSB. For instance, Pettersson and co-workers142 used high-resolution electron-beam lithography (HREBL) to fabricate selective electric contacts for directly measuring the interfacial energy barrier. They observed a reduced pinning state density as well as newly formed interfacial electric dipole layers at the epitaxial contact between Au−In alloy catalytic particles and GaAs nanowires. More importantly, their measurement showed a low value for ϕSB.142 Their measurement approaches could be implemented in direct measurement of ϕSB for conventional metal/semiconductor photocatalysts, such as Au/TiO2, which serves as a criterion for the rational design of a metal/ semiconductor junction. The key to having the greatest possible quantity of electrons that can transfer into the semiconductor is to build a suitable ϕSB at the metal/semiconductor interface. The Fowler equation suggests that a lower ϕSB allows a larger amount of electrons to transfer from metal into semiconductor; however, in that case, these electrons quickly transfer back to the metal and recombine with the remaining holes. Alternatively, a high ϕSB would block the electron transfer from metal to semiconductor. Therefore, there should exist a maximum electron-transfer efficiency when the balance between these two effects is considered. White and Catchpole136 have conducted a theoretical calculation by assuming full optical absorption and achieved a maximum electron-transfer efficiency of 8% in a typical plasmonic-based photovoltaic device, which is consistent with the theoretical value later obtained by Govorov et al.92 in a Au/Si and Au/TiO2 system. However, it must be noted that the commonly reported quantum efficiencies of plasmon-enhanced photovoltaics are

I = c(hν − E B)n /hν

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Figure 10. Plasmon-mediated direct electron transfer. (a) TA data for electron transfer in three different systems. Electron transfer in the Au/TiO2 system was completed within 240 fs. (b) Calculated electron density (represented by yellow regions) under SPR excitation in the Au/TiO2 system. Delocalized electron density on TiO2 during the occurrence of Au SPR resulted in a 50% probability of direct electron transfer across the interface. (c) Schematic of direct-electron-transfer process proposed in the Au/CdSe system. (d) Wavelength-dependent QYs obtained experimentally (red circles) and predicted theoretically (dashed lines). Panel a reprinted with permission from ref 64. Copyright 2007 American Chemical Society. Panel b adapted with permission from ref 88. Copyright 2014 American Chemical Society. Panels c and d reprinted with permission from ref 74. Copyright 2015 American Association for the Advancement of Science.

In 2015, Lian and co-workers74 reported the first experimental evidence showing direct electron transfer in a Au/CdSe nanorod (NR) heterostructure (Figure 10c). Their ultraviolet−visible (UV−vis) spectra of the heterostructure showed a broadened absorption peak with an onset wavelength in the near-IR region.74 This strong damping of Au SPR suggested the existence of a substantial orbital coupling between CdSe and Au. Kinetic analysis of the TA study revealed an electron-transfer time scale of 20 ± 10 fs.74 They also recorded a quantum yield (QY) as high as 24% and found that QY values were independent of the incident light wavelength in the range of Au SPR (Figure 10d).74 Their results highlighted that, in addition to energy alignment, the chemical nature of the metal/semiconductor interfacesuch as the strength of orbital couplingshould also be seriously considered for fully understanding and manipulating the electron-transfer process. Previous studies have validated the use of QYs to indicate different electron-transfer mechanisms. For instance, low QYs of electron transfer have been reported for Au/graphene (∼10%) and Au/CdS (∼2.75%),149,150 suggesting that those systems followed the Fowler theory and that electrons transferred from Au to graphene or CdS via the indirect pathway. Nonetheless, it should be pointed out that to date a systematic differentiation of electron-transfer mechanisms in different heterostructure systems has not been thoroughly developed. 2.3.3. Physical Locations of Trapped Hot Electrons on Semiconductors. After transferring into the semiconductor from the metal via PMET, those hot electrons subsequently diffuse and become trapped at various electronic states within the semiconductor. Knowing the physical locations of those hot electrons is essential for a better understanding of photochemistry on plasmonic metal/semiconductor heterostructures,

much lower. For instance, IPCEs for Au NPs on TiO2 have been reported to be around 3%,133,143,144 suggesting that there is plenty of room for further optimizing metal/semiconductor interfacial structures in order to achieve higher electron-transfer efficiencies. 2.3.2. Direct Electron Transfer. The conventional indirectelectron-transfer model suggests that the time scale of the PMET process should be on the order of picoseconds.54 However, Furube et al.64 used femtosecond TA spectroscopy to investigate electron transfer within a Au/TiO2 heterostructure and found the electron-transfer time scale to be less than 240 fs (Figure 10a). In their follow-up, convolution analyses were carried out and showed that electron transfer was accomplished within ∼50 fs.45,145 Similar results have also been reported by other groups.146−148 Additionally, in the work of Furube et al.,64 the electron-transfer efficiency was determined to be ∼40% via comparison with N3-sensitized TiO2. This value is much larger than the theoretical value (∼8%) predicted for the two-step indirect transfer model, as mentioned in section 2.3.1.92,136 Collectively, this evidence implies the existence of other underlying electron-transfer channels in plasmonic metal/ semiconductor heterostructures. Indeed, Long and Prezhdo88 developed theoretical models to describe that the electron transfer occurred in a direct pathway with a high probability (∼50%) in addition to the conventional indirect pathway: the plasmon decay in metal/semiconductor heterostructures directly excited electrons to acceptor states in the semiconductor and left the holes in the metal (Figure 10b). The process taking place here is analogous to the CID process (see section 2.1) in the metal/adsorbate system. This direct electron transfer is believed to be more efficient than indirect electron transfer, as it circumvents the energy loss of hot electrons that occurs in the electron−electron and electron−phonon scattering of Au NPs.74 2935


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Figure 11. Hot-electron-induced H2 dissociation. (a) Schematic of hot-electron-induced H2 dissociation mechanism. B and AB are bonding and antibonding states of adsorbed H2, respectively. (i) Hot electrons are generated through SPR excitation of Au. (ii) After thermalization, a Fermi−Dirac distribution is formed. Hot electrons with suitable energy can transfer into the antibonding state of H2, creating a TNI, H2δ−. (iii) TNI travels on the excited-state potential energy surface (PES) with its bond length being extended. Then the electron from the TNI transfers back to the Au NP surface and the H2 molecule dissociates. (b) Plasmon-induced H/D exchange on Au/SiO2 and Au/TiO2 photocatalysts. (i) Transmission electron microscopic (TEM) image of Au/SiO2 heterostructure. (ii) Schematic of hot-electron-induced H2 dissociation occurring over the surface of Au within a Au/SiO2 system. (iii, iv) H/D exchange rates of (iii) Au/SiO2 and (iv) Au/TiO2 catalysts as a function of time. (c) Transition-state structures for H2 dissociation at the perimeter of Au/TiO2. Panel a reprinted with permission from ref 85. Copyright 2012 American Chemical Society. Panel b reprinted with permission from ref 13. Copyright 2013 American Chemical Society. Panel c adapted with permission from ref 169. Copyright 2014 American Chemical Society.

an extremely long lifetime (∼10 min) for the trapped hot electrons,43 which is consistent with what has been observed by high-resolution X-ray absorption spectroscopy (XAS).44 It is noted that those long-lived hot electrons at different trapping sites could affect surface chemical reactions distinctively.151 EPR spectroscopic results have revealed that, in Au/TiO2 heterostructures, hot electrons trapped at Ti3+ suppressed the H2 production rate, while hot electrons localized at surface oxygen vacancies enhanced this activity.151

as the trapping sites are always the active sites for surface chemical reactions.151−153 Hot electrons in the CB of semiconductors have been extensively studied by femtosecond TA spectroscopy and X-ray absorption fine structure (XAFS) spectroscopy.45,57,64,145−147 TA spectroscopic results showed that the electrons transferred back from semiconductor to metal NPs in the nanosecond regime (e.g., by tunneling through ϕSB),57 and the decay time scale was believed to be sensitive to the synthesis methods of the samples, possibly suppressed under the influence of electrolyte.45,64,145−147 Apart from being accumulated in the CB, hot electrons also have the opportunity to diffuse to the defect states in the semiconductor, whose energy levels are lower than that of the CB.152−154 For instance, previous studies have proposed that at least eight types of electron-trapping states exist below the CB minimum of rutile single crystals.153,155 The commonly recognized trapping states are known as oxygen vacancies, which are formed by the removal of oxygen atoms from the surface or bulk.152,153,156−161 For TiO2, the removal of bridging oxygen atoms mainly creates oxygen vacancies on the surface.43,151,153,157 Brückner and co-workers43,151 employed in situ electron paramagnetic resonance (EPR) spectroscopy to show that, within Au/TiO2 heterostructures, hot electrons were trapped at oxygen vacancies and Ti sites (Ti3+). They further analyzed the double integrals of EPR signals that reflect the number of hot electrons trapped at oxygen vacancies and found

3. HOT ELECTRON PHOTOCHEMISTRY ON METAL NANOSTRUCTURES Photochemical reactions driven by plasmon-generated hot electrons provide opportunities for efficiently using visible light, which constitutes nearly 42% of the solar spectrum.10 Moreover, the fact that the absorption properties of plasmonic metal NPs are shape- and size-dependent makes it feasible to design full-spectrum photocatalysts by combining nanostructures of different morphologies.21,22,28−31,162 Hot-electrondriven photochemical reactions are known to occur on either plasmonic metal nanostructures or metal/semiconductor heterostructures. Compared with the lifetimes of hot electrons on metal/semiconductor heterostructures, which are prolonged by the interfacial ϕSB, the lifetimes of hot electrons on metal surfaces are relatively short, limiting their participation in 2936


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SiO2 was 2 orders of magnitude higher than that on Au/TiO2.13 The enhancement in dissociation efficiency was attributed to the increased number of hot electrons on the Au/SiO2 due to the absence of Au-to-substrate electron transfer, which further supported the theory that H2 dissociation indeed took place on Au but not on TiO2. Additionally, Borguet and co-workers165 reported the observation of a metastable Au hydride (AuHx) after H2 dissociation, further suggesting the dissociation of H2 on plasmon-excited Au surface. As discussed in section 2, the electron-transfer efficiency from metal to adsorbate strongly depends on orbital hybridization. Thus, the weak adsorption of H2 on Au that was previously reported would not favor the proposed explanation of plasmoninduced H2 dissociation on Au.166 Recently, a possible chemical adsorption of H2 on Au clusters was reported in which the H2 σ* orbitals were suggested to overlap with Au 5d orbitals and thereby cause a strong interaction between H2 molecule and Au atoms.167 In such a case, the plasmon-generated hot electrons on Au would transfer to the H2 σ* orbitals to activate the chemically adsorbed H2 and lead to dissociation,117 which is consistent with a theoretical study of H2 dissociation on a seven-atom Au cluster.168 It is noted that the Au clusters are fundamentally different from the Au NPs utilized by Halas and co-workers.85 Au NPs are more chemically inert and should have H2 adsorption properties that are distinct from those of Au clusters. Study of a Au/Al2O3 model catalyst showed that, upon H2 adsorption, there was a change in the X-ray absorption near-edge structures (XANES) spectra of Au L2 and L3 edges,167 suggesting that H2 was chemically bonded on the corner or edge of Au NPs. However, several studies later demonstrated that, instead of Au surfaces, the perimeter sites between Au and oxide supports became platforms for dissociative H2 chemisorption.169−171 For instance, a DFT calculation study using Hückel theory determined O2−-H+-H−-Au to be the dominant transition-state structure for H2 adsorption on Au/ TiO2, as shown in Figure 11c.169 This H2 heterolysis effect has also been reported in adsorbate/Au NP systems, where the proposed cause of heterolytic dissociation of H2 was the Lewis pair effect induced by adsorbed nitrogen-containing molecules on Au NPs.172 In addition, it has been reported that formation of the O 2− -H + -H − -Au transition structure reduced the H 2 dissociation energy barrier to only 0.27 eV, which is much lower than the 0.76 eV energy barrier on Au NP surfaces.169 Moreover, studies of the H/D exchange rate and turnover frequencies (TOFs) on Au/TiO2 with different sizes of Au NP also revealed that the perimeter sites in Au/TiO2 should be responsible for H2 dissociation.170 Study of the H/D exchange reactions on TiOx islands dispersed on Au(111) showed similar HD formation and TOFs, strongly suggesting Auδ+-Oδ−-Ti to be the active sites for H2 dissociation.171 Obviously, further investigations on the support effect169−171 and ligand effect173 are needed to clarify these controversial results and to identify the exact active sites for plasmon-induced H2 dissociation on metal nanostructures. Plasmonic materials aside from Au have also been reported as photocatalysts for H2 dissociation.11,118,164 For instance, Halas and co-workers118 have demonstrated Al/Al2O3 to be an efficient plasmonic photocatalyst for H2 dissociation and suggested a mechanism similar to the one proposed for Au catalysts. They believed that the hot electrons generated via both SPR decay and direct excitation of interband transitions contributed to the H2 dissociation process. Similar to the aforementioned O2−-H+-H−Au transition structure, the adsorption sites of H2 molecules on

chemical reactions. However, recent studies have demonstrated photocatalysis on plasmonic metal nanostructures to make more efficient use of hot electrons by circumventing the energy loss during PMET in metal/semiconductor heterostructures.12,13,50,51,53,54,75,85,118 Hot-electron-induced chemical transformation on plasmonic metal NPs was first demonstrated in surface-enhanced Raman spectroscopic (SERS) studies of p-aminothiophenol (pATP).105,106 Two significant peaks that were absent in the typical Raman spectra of p-ATP were observed when the molecules were attached to Ag substrates.106 Both EM and chemical enhancement mechanisms failed to explain this phenomenon.105 These two peaks were eventually attributed to the formation of a new chemical species, p,p′-dimercaptoazobenzene, as a result of hot-electron-driven coupling of adjacent p-ATP molecules.105,107−114 The microscopic processes involved in hot-electron-induced photocatalysis on metal nanostructures are still subject to debate.12,50,51,53,82,84,163 Apart from the mismatch between lifetimes of hot electrons and time scales of chemical transformations, they are further complicated by the two possible electron-transfer pathways (indirect and direct electron transfer). In this section, we categorize the recently reported hot-electroninduced reactions on Au or Ag NPs based on their tentatively assigned underlying electron-transfer pathways. However, it must be noted that most examples of reactions did not have a criterion for differentiating between indirect and direct electrontransfer pathways in their original reports. 3.1. Indirect Electron Transfer

H2 and O2 dissociations are the most important elementary steps in various photochemical organic transformations, and a great deal of effort has been made to use plasmonic metal nanostructures to prompt those reactions.13,85,50,51 Both H2 and O2 are known to weakly adsorb on the surfaces of commonly used plasmonic Au and Ag.13,50,51,85 The fluctuation of electric dipole moments generates a weak physical attraction between adsorbates and surfaces,121 which renders this environment suitable to employ the indirect-electron-transfer mechanism to describe hot electron transfer in these systems. 3.1.1. H2 Dissociation. In plasmon-induced H2 dissociation, Au NPs function not only as efficient light absorbers but also as active catalytic sites for the reaction.11,13,85,118,164 In a recent study from Halas and co-workers,85 both DFT calculations (Figure 11a) and H/D exchange experiments were provided to support the following hypothesis: thermalized hot electrons from the Au NPs transferred to the antibonding orbital of H2 molecules as they approached the Au surface and created a negatively charged ion, H2δ−, by following the indirect-electrontransfer pathway as suggested in section 2.2.1. Due to the short lifetimes of hot electrons in the antibonding orbital of H2, H2δ− was only a TNI and the hot electrons quickly transferred back to the Au surface. Finally, the H2 molecule returned to its ground electronic state with a stretched H−H bond (i.e., accumulation of vibration energy) that led to the final H2 dissociation.85 However, it is noted that this study used Au NPs on TiO2 support and did not exclude the possibility that plasmon-generated hot electrons could transfer across ϕSB at the Au/TiO2 interface and lead to H2 dissociation occurring on TiO2. In their follow-up, Halas and co-workers13 replaced the TiO2 substrate with SiO2 and Al2O3, as shown in Figure 11b. They compared Au/TiO2 and Au/SiO2 by measuring the H/D exchange rate and found that the HD formation rate on Au/ 2937


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Al/Al2O3 were suggested to be pairs of Al and O atoms at the interface between Al NPs and Al2O3.118 Moreover, recently Halas and co-workers11,164 have designed Al/Pd heterodimers in which the light absorber (Al NP) was physically separated from the catalyst for H2 dissociation (Pd NP). In their strategy, the Al SPR induced forced plasmons on the Pd NPs and generated hot electrons within the Pd NPs for subsequent dissociation of adsorbed H2.11,164 3.1.2. O2 Dissociation. O2 dissociation is the ratedetermining step (RDS) for partial oxidation reactions on noble metal (e.g., Ag and Au) surfaces.174−176 In the conventional approach, high operating temperatures were required to drive this process,50 with highly thermostable catalysts being employed, which typically resulted in low energy efficiency.50 Facilitating O2 dissociation with visible light has recently attracted significant attention, as such an approach exhibits the potential to achieve effective O2 dissociation at lower operating temperatures and with more efficient energy utilization. Linic and co-workers50 observed a 4-fold enhancement in the reaction rate of partial oxidation of ethylene on Ag nanocubes compared to that in the dark. They proposed that plasmongenerated hot electrons from Ag transferred to the antibonding orbital of adsorbed O2 and generated O2−.12,50,51 Since the lifetime of O2− (∼1−10 fs)50,177,178 is much shorter than the time scale of O2 dissociation (∼500 fs),179 the transferred electrons quickly returned to Ag but deposited energy into the O2 vibrational modes. Their DFT calculations showed that the Fermi level of Ag was very close to the center of the energy states of partially occupied 2π* antibonding orbitals of O2 (Figure 12a).50 As a result, they proposed that the formed TNI O2− then activated the O2 molecules along the vibrational coordinate and that the process kept repeating since the energy dissipation occurred in the range of picoseconds (Figure 12b).50,180 Eventually, enough vibrational energy accumulated to overcome the activation energy barrier, leading to O2 dissociation (Figure 12c). After O2 was dissociated into O atoms, the oxametallacycle intermediate was generated on Ag via connecting ethylene with the O atoms, and this intermediate step underwent isomerization reaction to form ethylene oxide.181 Recent studies of plasmon-induced selective oxidation of pATP to p,p′-dimercaptoazobenzene (DMAB) on the surfaces of Ag and Au suggested that the plasmon-generated hot electrons resided in the antibonding orbital of O2 and formed strongly adsorbed, long-lived O2−, which directly dissociated into O atoms (Figure 12d).119,182 Their TDDFT studies adopted M20O2− (M = Au or Ag) as model systems to show that the complex formed a stable closed-shell electronic configuration that prevented the transfer of hot electrons from O2− back to the metal clusters.183−189 Meanwhile, the pH-dependent p-ATP oxidation and X-ray photoemission spectroscopic (XPS) results also indicated that p-ATP was oxidized to DMAB by atomic oxygen species in the form of silver or gold oxides or hydroxides.182 3.1.3. Growth of Metal Nanostructures. If plasmongenerated hot holes are quickly removed, hot electrons pile up and cause an increase of the electron quasi-Fermi level of metal NPs, further lowering the activation energy of associated reduction reactions. In this scenario, metal NPs behave in such a way that they can be viewed as cathodically polarized nanoelectrodes ready to drive reduction reactions. For instance, a recent study showed that, for the plasmon-induced reduction of K3[Fe(CN)6] to K4[Fe(CN)6] on 13 nm Au NPs with ethanol as the hole scavenger, the electron quasi-Fermi level of Au NPs was

Figure 12. Hot-electron-induced O2 dissociation. (a) DFT-calculated density of states for O2 adsorbed on Ag (100) surface. Dashed line depicts the Fermi level of Ag. Red and blue curves represent antibonding and bonding orbitals of O2, respectively. (b) DFT-calculated PES for O2 and O2− on Ag(100) surface. The energy of the hot electrons was deposited into the O2 vibrational mode once the hot electrons transferred back to Ag, activating the O2 molecule. (c) Schematic of O2 dissociation induced by vibrationally excited states. (d) Schematic of hot-electron-induced aerobic oxidation of p-ATP. Hot electrons resided in the antibonding orbital of the O2 molecule, generating the long-lived O2− species that oxidized p-ATP. Panels a−c reprinted with permission from ref 50. Copyright 2011 Macmillan Publishers Limited. Panel d reprinted with permission from ref 119. Copyright 2015 American Chemical Society.

found to increase by >200 mV, while the activation energy decreased by 23 kJ/mol (Figure 13a).190 The plasmon-induced growth of metal nanostructures is a typical hot-electron-driven photocatalytic process, in which the products are directly deposited on the plasmonic metal surfaces. In 2001, Mirkin and co-workers191 demonstrated the fabrication of Ag triangular nanoprisms using low-intensity white light. Since then, this method has been extensively used to synthesize various Ag anisotropic nanostructures including nanorods,192 triangular bipyramids,193 and nanocubes.194 Brus and co-workers195−198 investigated the role of hot electrons in Ag nanoprism growth by measuring the photovoltage on Ag seed electrodes (Figure 13b). Their results showed that plasmon-generated hot holes oxidized the surface-adsorbed citrate anions and hot electrons then accumulated on the Ag NP surfaces, facilitating the reduction of Ag+ to form Ag nanoprisms,195−197 which is consistent with the nuclear magnetic resonance (NMR) study from Mirkin and co-workers.199 Although those studies on anisotropic Ag nanostructures confirmed the growth to be a photocatalytic process driven by plasmon-generated hot electrons, none of them has established a clear picture of how the hot electrons were involved in anisotropic growth. Considering the slow kinetics of Ag+ reduction,197 the accumulated hot electrons should be evenly distributed along the Ag NP surfaces and lead to isotropic growth. Recently, our group75 reported the plasmon-mediated synthesis of Au nanoprisms and discovered that the surfactant, poly(vinylpyrrolidone) (PVP), functioned as an electron relay to 2938


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Figure 13. Hot-electron-induced metal ion reduction and metal NP growth. (a) Schematic of plasmon-mediated K3[Fe(CN)6] reduction. Hot electron accumulation increased the electron quasi-Fermi level of Au (Au NP → Au NP*), leading to a lower activation energy (ΔH⧧dark → ΔH⧧light) of the reduction reaction. (b) Photovoltage results of Ag NP electrodes in an aqueous solution of 500 μM trisodium citrate in the absence (blue curve) and presence (black curve) of 250 μM AgNO3. The light-on periods are shaded in gray. The negative shifts in potential indicate the accumulation of hot electrons. A decrease in the photovoltage was observed when AgNO3 was present, indicating the consumption of hot electrons by Ag+ reduction. (c) Schematic depicting the PVP-controlled growth of Au nanoprisms under visible light. The PVP molecules acted as an electron relay to stabilize and accumulate hot electrons at perimeter sites, which promoted the anisotropic growth of Au nanoprisms in the presence of AuCl4−. (d) Scanning electron microscopic (SEM) images of individual planar-twinned and pentatwinned Au nanostructures (i) before and (ii) after plasmon-induced growth (excitation wavelength ∼500 nm, 30 min); (iii) ion-induced SEM image of Au nanostructures obtained from 2 h plasmon-mediated growth. (iv, v) Corresponding NanoSIMS images obtained from panel iii, mapping the spatial distribution of (iv) Au and (v) PVP by measuring the elemental distributions of 197Au− (red) and 12C14N− (green). Panel a reprinted with permission from ref 190. Copyright 2016 American Chemical Society. Panel b reprinted with permission from ref 195. Copyright 2007 American Chemical Society. Panel d reprinted with permission from ref 75. Copyright 2016 Macmillan Publishers Limited.

Figure 14. Comparison between indirect and direct electron transfer. (a) Plasmon-induced electron transfer on an adsorbate-covered Ag NP surface. (i) Direct charge excitation from Ag to unoccupied orbital of the adsorbate; (ii) following the thermalization process, indirect charge transfer of hot electrons from Ag to adsorbate orbitals. (b) Raman spectra of methylene blue at (i) 532 and (ii) 785 nm excitation wavelengths. Anti-Stokes shifts were observed at 785 nm excitation, indicating that the methylene blue molecules were more activated at longer excitation wavelength. Reprinted with permission from ref 82. Copyright 2016 Macmillan Publishers Limited.

photochemistry. The function of PVP in electron trapping,

stabilize plasmon-generated hot electrons once the hot holes were quickly scavenged by methanol (Figure 13c). Further single-NP-level investigations using nanoscale secondary ion mass spectrometry (NanoSIMS) showed that PVP molecules were preferentially adsorbed on the edges of Au seed NPs (Figure 13d), prompting the selective accumulation of highdensity hot electrons over prolonged time scales and enabling the anisotropic growth of Au nanoprisms.75 This newly discovered role for PVP in manipulating plasmon-generated hot electrons provides a general strategy for plasmon-driven hot electron

stabilization, and accumulation is similar to that of ϕSB in the metal/semiconductor heterostructures, as mentioned in section 2.3: it significantly ameliorates the time-scale mismatch between hot electron lifetime (approximately femtoseconds to picoseconds) and HAuCl4 reduction kinetics (approximately microseconds to milliseconds).75,76 2939


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Figure 15. Direct electron transfer in nonplasmonic metal systems. (a) Selective photocatalytic oxidization of CO on Pt NPs. (i) Schematic of selective CO oxidation. When Pt NPs were excited at 470 nm, CO was more efficiently oxidized compared with H2. (ii) Absorption spectra (solid lines, right axis) of Pt NPs with CO adsorption and CO oxidation QY (dots, left axis) for different sizes of Pt NPs. An additional absorption peak at ∼470 nm was observed for Pt NPs due to the surface-state hybridization between Pt and CO, and the CO oxidation QY reached a maximum at the same wavelength. (iii) Absorption spectra of Pt NPs in the presence of H2 and the oxidation QY of H2. No additional absorption peak was observed and the QY was relatively low. (iv) Direct comparison between the two oxidation reactions. Selectivity on reactants showed a wavelength dependence with a maximum at ∼470 nm. (b) Visible-light-induced cleavage of dimethyl disulfide molecules caused by direct electron transfer between hybridized surface states. (i) Scanning tunneling microscopic (STM) images depicting cleavage of the disulfide bond of dimethyl disulfide. (ii) Schematic description of electron transfer from HOMOs strongly hybridized with the metal to weakly hybridized LUMOs. (iii−v) Simulated (iii) LUMOs and (iv) HOMOs of the surface hybridized states compared with (v) HOMO and LUMO of the free dimethyl disulfide molecule, which shows the extent of hybridization. Panel a reprinted with permission from ref 125. Copyright 2014 American Chemical Society. Panel b reprinted with permission from ref 124. Copyright 2017 American Chemical Society.

3.2. Direct Electron Transfer

only when the energy gap of the HOMO−LUMO transition was resonant with the Ag SPR at 785 nm.82,84 The direct-electron-transfer pathway offers an opportunity to selectively activate target reactants in a mixture by choosing a wavelength that is resonant with the transition between the hybridized states.82,84 Although this hybridized-state-induced selectivity was only reported in few plasmonic systems,82,84 it has been well documented in nonplasmonic systems.124,125 For instance, Christopher and co-workers125 reported the selective photocatalytic oxidization of CO in a mixture of CO and H2 on Pt NPs under visible-light illumination (Figure 15a). Their DFT calculations suggested that direct electron transition in the surface-hybridized Pt-CO orbitals led to efficient activation of adsorbed CO molecules. In addition, although the short lifetimes of hot electrons residing in hybridized orbitals prevented them from directly participating in the redox reactions,50,51,82 a recent study demonstrated that the lifetimes of photoexcited electrons in the hybridized LUMOs of dimethyl disulfide molecules on Cu(111) and Ag(111) were significantly extended due to weak coupling of adsorbate LUMOs with metal orbitals, leading to efficient cleavage of the disulfide bond (Figure 15b).124

Although indirect electron transfer is commonly recognized as the dominant mechanism contributing to hot-electron-induced photochemical processes on metal surfaces, recent studies suggested the possibility of direct electron transfer in CID to activate the adsorbed molecules for subsequent chemical reactions.82,84 For instance, Linic and co-workers82,84 observed that methylene blue molecules degraded more efficiently under 785 nm irradiation than under 532 nm irradiation on the Ag nanocube photocatalyst that possessed SPR absorption at the two wavelengths. Meanwhile, in the SERS measurement, they also found the methylene blue molecules to be more activated at 785 nm than at 532 nm, based on an evaluation of the anti-Stokes to Stokes signal ratio (Figure 14b).82 These results strongly suggested that plasmon-generated hot electrons were more efficiently transferred to methylene blue molecules under longerwavelength irradiation, contradicting with what was predicted from the conventional two-step electron-transfer mechanism (section 2.2.1). Therefore, Linic and co-workers82 proposed that a direct-electron-transfer mechanism should be considered for the plasmon-induced methylene blue degradation on Ag (Figure 14a). In their model, strong hybridization occurred at the methylene blue/Ag interface, with electron-accepting states (LUMOs) centered at methylene blue and electron-donating states (HOMOs) centered on Ag. The Ag SPR dephasing directly generated hot electrons in the electron-accepting states

3.3. Hot Electron Photochemistry on Bimetallic Nanostructures

The inherent chemical inertness of plasmonic noble metals (e.g., Au and Ag) limits their wide application in catalyzing chemical reactions.174 A simple solution is to integrate the plasmonic metal with another metal that has high catalytic activities for the 2940


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Figure 16. Hot electron transfer in plasmonic alloy nanostructures. (a) Strong adsorption of nitrobenzene on Au/Cu alloy NPs and resulting selective formation of aminobenzene. (b) Plasmon-mediated promotion of H2O production in O2 reduction on Ag/Pt alloy NPs. (i) Schematic showing electron transfer from Ag to Pt under SPR excitation. The electrons populated the Pt5d states, which would decrease the relative vacancy on Pt and cause a decrease in XANES spectra of the Pt L3 edge. (ii) Relative vacancy (ΔA/A) in the Pt 5d state measured from XANES. Decreases in the relative vacancy indicated electron transfer from Ag to Pt. Lines in different colors represent the spectra from Ag/Pt alloy NPs with different Pt contents. The numbers stand for the volume of Pt precursor used during the synthesis of alloy NPs. (iii). Schematic of O2 reduction with and without SPR excitation. While H2O2 was mainly produced in the dark condition, H2O was the major product when Ag SPR was excited. Panel a reprinted with permission from ref 83. Copyright 2016 American Chemical Society. Panel b reprinted with permission from ref 202. Copyright 2017 American Chemical Society.

Figure 17. Spectroscopic study of Au−Pd bimetallic heterostructures. (a) Schematic of electron-transfer process and chemical reaction site in a Au−Pd bimetallic heterostructure. (b) PL mapping of individual (i) Au−Pd bimetallic heterostructures and (ii) Au nanorods. Their corresponding TEM images are shown as insets in this figure with their particle numbers indicated. (c) Single-particle PL spectra for particles (i) 2, (ii) 4, (iii) 5, and (iv) 7. PL signals from Au−Pd bimetallic heterostructures (particles 2 and 4) were significantly quenched compared with those from Au nanorods (particles 5 and 7). Reprinted with permission from ref 203. Copyright 2014 American Chemical Society.

desired reactions.83,123,200−206 It has been demonstrated that inclusion of a second metal not only creates new active sites but also provides additional pathways to facilitate charge separation and further improves the photocatalytic activities.83,123,200−206 3.3.1. Alloy Nanostructures. The incorporation of Pd atoms into Au NPs has been demonstrated to significantly improve plasmon-driven CO2 reduction and Suzuki−Miyaura cross-coupling on Au NPs,123,200,201,207 as Pd is known to have high catalytic activities for these reactions. Additionally, the introduced metal also substantially changes the electronic structures of the constituent metals.83,123,201,208 The different electronegativities of metals cause partial electron transfer

between both components and form an inherent charge heterogeneity within the alloy metal NPs,83,123,201,207−209 which has been demonstrated to improve reaction selectivity by changing the adsorption strength of the reactant molecules on active sites.83,208,210,211 For instance, for hot-electron-induced reduction of p-nitrobenzene, azobenzene (99%) was formed on Au NPs while aminobenzene and azoxybenzene were identified as the dominant products on Au/Cu and Ag/Cu alloy NPs, respectively.83,208,210,211 It was suggested that the strong interaction between Cu and N atoms in p-nitrobenzene was responsible for selectively yielding aminobenzene or azoxybenzene on the alloy NPs compared to the Au NPs (Figure 16a). 2941


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Figure 18. Spectroscopic and theoretical study of Ag−Pt bimetallic heterostructures. (a) Dark-field scanning transmission electron microscopic (STEM) images and energy-dispersive X-ray spectroscopic (EDS) images of a Ag−Pt core@shell nanocube. (i) STEM image. (ii−iv) EDS images showing distributions of (ii) Ag, (iii) Pt, and (iv) Ag−Pt overlay. (b) Optical spectra of Ag−Pt nanocubes. (i) Extinction spectra of both pure Ag and Ag@Pt nanocubes. (ii, iii) Extinction, scattering, and adsorption spectra of (ii) pure Ag and (iii) Ag−Pt nanocubes. (c) Energy flow from Ag to Pt. (i) Simulated mapping of the power dissipation per volume of either pure Ag (upper cube) or Ag−Pt nanocube (lower cube) under SPR excitation. (ii) Simulated portions of light absorbed by the core section and outermost layer of a pure Ag nanocube. (iii) Simulated portions of light absorbed by the core section (Ag) and outmost layer (Pt) of a Ag−Pt nanocube. A larger portion of light was absorbed by materials at the outermost layer for the Ag−Pt nanocube. Reprinted with permission from ref 213. Copyright 2017 Macmillan Publishers Limited, part of Springer Nature.

Moreover, the electronegativity of Au (Pauling scale 2.54),212 being much higher than that of Cu (Pauling scale 1.90)212 and Ag (Pauling scale 1.93),212 signified a more pronounced charge heterogeneity in Au/Cu NPs, leading to stronger adsorption of pnitrobenzene on Cu compared with that on Ag/Cu NPs. The immobilization of p-nitrobenzene accounted for the selective formation of aminobenzene over azoxybenzene. A recent study showed that SPR excitation could further magnify the charge heterogeneity within Ag/Pt alloy NPs.202 In this study, the appearance of the Pt L3 edge XANES spectra indicated the accumulation of Ag plasmon-generated electrons on Pt atoms (Figure 16b).202 This enhanced charge heterogeneity was believed to strengthen the adsorption of O2, populate its antibonding states, and lead to cleavage of the O−O bond and formation of H2O over H2O2 on the Ag/Pt alloy NPs.202 3.3.2. Bimetallic Heterostructures. In attempts to further enhance charge heterogeneity, recent studies adopted plasmonic bimetallic heterostructures to facilitate the separation of plasmon-generated hot electrons and holes across the interface.206,203−205 For instance, Yan and co-workers206 reported that when the two ends of Au nanorods were coated with Pd, the heterostructures exhibited significantly improved reaction activity for Suzuki coupling under visible-light irradiation, while the photocatalytic activity was greatly reduced when a layer of TiOx was inserted between Au and Pd. Furthermore, Majima and co-workers203−205 measured the photoluminescence (PL) of single Au nanorods decorated with Pd or Pt and found that the PL signals were effectively quenched (Figure 17), strongly suggesting that efficient electron−hole separation occurred at the Au−Pd(Pt) interface. Meanwhile, they also observed significantly higher efficiency for both H2O splitting and formic acid dehydrogenation on the bimetallic heterostructures.203 The energy flow within bimetallic nanostructures has recently been shown to influence the plasmon dephasing pathway and subsequent generation of hot carriers.213 Linic and co-workers213 reported that introduction of a Pt shell on a Ag nanocube core (Figure 18a) increased the absorption-to-scattering ratio for plasmon decay (Figure 18b) due to the larger imaginary

dielectric function of Pt. The enhanced light absorption and energy dissipation primarily occurred within the Pt shell, which led to the formation of energetic electron−hole pairs on the Pt surfaces (Figure 18c) and eventually contributed to the preferential oxidation of CO in excess H2 on the hybrid core− shell heterostructures.213

4. HOT ELECTRON PHOTOCHEMISTRY ON METAL/SEMICONDUCTOR HETEROSTRUCTURES Metal/semiconductor heterostructures have been recognized as typical platforms for hot-electron-induced photochemistry mainly due to the kinetic advantages associated with forming an interfacial ϕSB to facilitate plasmon-driven charge separation.56,58 The lifetimes of these hot electrons are sustained over extended time scales by providing a pathway for spatially separating hot electrons from hot holes across the metal/ semiconductor interface.43,44,56 Furthermore, the depletion region established within the semiconductors serves to sweep hot electrons away from the metal/semiconductor interface and further suppresses their recombination with the hot holes left behind on the metal NPs.64,66,214 The accumulation of hot electrons in the semiconductors thus offers opportunities for manipulating the energy levels of these hot electrons. In addition, semiconductor surfaces exhibit distinct characteristics from the metal surfaces and provide additional active sites for prompting various photocatalytic reactions, including H 2 O splitting,40,55−58,62,63,65,132,215−224 CO2 reduction,67−70 and organic transformations.73,225−227 4.1. Electron−Hole Separation Enhancement

Due to the slow kinetics of chemical reactions (e.g., H2O splitting, approximately milliseconds to seconds),77−81,228 the recombination of transferred hot electrons in the semiconductors with hot holes remaining on the plasmonic metals significantly limits photocatalytic efficiency. For instance, to date, the reported reaction efficiencies of plasmon-driven H2O splitting on Au/TiO2 are on the order of microamperes per square centimeter in typical PEC systems.56,57,65,215 In addition, 2942


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Figure 19. Strategies for suppression of hot carrier recombination within metal/semiconductor heterostructures. (a) Proposed mechanism for removal of hot holes within the Au/TiO2 heterostructures through application of Fe2+ as an electron donor. (b) Depiction of an autonomous photosynthetic device for overall H2O splitting. (c) Energy diagram of hot-electron-induced H2O splitting. Hot holes are transported to the counter electrode via the NiOx layer, which suppresses hot carrier recombination for facilitation of H2O splitting. Panel a reprinted with permission from ref 48. Copyright 2005 American Chemical Society. Panel b reprinted with permission from ref 218. Copyright 2013 Macmillan Publishers Limited, part of Springer Nature. Panel c reprinted with permission from ref 234. Copyright 2015 American Chemical Society.

Figure 20. Anisotropic flow of hot electrons within metal/semiconductor heterostructures. (a) Time-dependent profiles of H2 generation on Au/ MesoTiO2 and Au,Pt/mesoTiO2 samples. (b) Schematic of anisotropic flow of hot electrons from the basal surface of mesoTiO2 to the lateral surface. (c) TRDRS measurements of basal Au/mesoTiO2 (red, labeled as Au/mesoTiO2), lateral Au/MesoTiO2 (dark yellow, labeled as Au/mesoTiO2-PD), and Au/TiO2 NPs (blue, labeled as Au/P25) after 530 nm laser irradiation. Reprinted with permission from ref 243. Copyright 2013 American Chemical Society.

also efficiently helped the transfer of hot holes to the counter electrode, and the remaining hot electrons on Au induced H2O splitting (Figure 19c). Ye and co-workers224 utilized Ni(OH)2 to trap hot holes generated on Au NPs to promote electron transfer to the glassy carbon (GC) substrate, and they reported a high photocurrent density together with a low overpotential for PEC H2O splitting. Recently, the commonly used Z-scheme strategy in semiconductor photochemistry235,236 was applied to plasmonic photocatalysts in order to enhance their electron−hole separation.237 Majima and co-workers237 developed a ternary structure of black phosphorus/Au/La2Ti2O7 and demonstrated that the photogenerated hot electrons in black phosphorus transferred to Au to capture the plasmon-generated hot holes, and hot electrons on Au subsequently transferred to La2Ti2O7, resulting in efficient electron−hole separation and high H2Osplitting activity. SPR-enhanced EM fields might also facilitate electron−hole separation in plasmonic metal/semiconductor heterostructures.238,239 Han and co-workers238 selectively deposited Cu2O at the high-curvature vertexes of hexoctahedral Au NPs and found that such heterostructures exhibited a much higher H2 generation rate under visible light than those with Cu2O

the fast electron−hole recombination also causes a photovoltage of only ∼0.2 V in the Au/TiO2 photoanode under SPR excitation,56,57 which is far from the required 1.23 V for overall H2O splitting.229,230 Molecular hole scavengers (e.g., Fe2+, Figure 19a) have been commonly used to remove plasmon-generated hot holes on metals to effectively suppress electron−hole recombination.45−48 Another alternative strategy is to integrate hole-transport materials with metals for rapid transfer of hot holes away from the metals. This has been well-documented in the research on plasmonic solar cells231−233 but has only recently been demonstrated in plasmonic photocatalysis.57,218 For instance, Moskovits and co-workers218 developed an autonomous photosynthetic device consisted of Au nanorods, TiO2, and Co-based oxygen evolution catalyst (Co-OEC) as a hole-transport material for wireless overall H2O splitting (Figure 19b) and detected continuous H2 evolution under visible-light irradiation (λ > 410 nm) in the absence of molecular hole scavengers. Later, Liu and co-workers57 deposited an OEC IrOx on a Au/TiO2 photoanode to enhance electron−hole separation by trapping the hot holes in IrOx, and they reported an increase in photocurrent density from 2 to 4 μA/cm2 in PEC H2O splitting. Moreover, Thomann and co-workers234 reported that a p-type NiOx layer deposited on Au 2943


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Figure 21. Manipulating energy levels of hot electrons in Au/TiO2 heterostructures. (a) Difference in Au NP size promoting a change in hot electron energy level. Large Au NPs induced a higher hot electron energy level in the TiO2 CB compared with small Au NPs. (b) H2O-splitting activities of Au/ TiO2 photocatalysts under visible irradiation (λ > 435 nm). Reprinted with permission from ref 58. Copyright 2014 American Chemical Society.

Figure 22. Schematic of O2 activation on semiconductor surfaces. (a) Mechanism of hot-electron-induced aerobic oxidation of alcohol on Au/P25 heterostructure. (i) Hot electrons from Au NPs first transfer into the rutile CB and then (ii) diffuse into the anatase CB to (iii) generate surface O2−. (iv) The O2− species abstracts an H atom from the alcohol and generates the Au−alcoholate species. (v) Further removal of the H atom from Au−alcoholate species produces a carbonyl product. (b) Schematic of 1O2 formation on Au/TiO2 heterostructures under visible-light irradiation. Hot electrons transfer into TiO2, producing O2− species. Then O2− further donates an electron to the electron-deficient Au surface to generate 1O2. Panel a reprinted with permission from ref 225. Copyright 2012 American Chemical Society. Panel b reprinted with permission from ref 251. Copyright 2014 American Chemical Society.

deposited on the flat faces of the Au NPs. Their finite difference time domain (FDTD) simulation results indicated that the EM field enhancement was more pronounced at the vertexes.238 Direct manipulation of hot electrons in semiconductors is another way to suppress electron−hole recombination.39,240−244 Similar to the way in which hole-storage materials were used, electron-storage materials such as Pt were deposited on semiconductors to serve as electron sinks, thus delaying the hot electrons from transferring back to the metals.240 Meanwhile, Pt is known to have high catalytic activity and to function as a cocatalyst to promote various chemical reactions.240 For instance, Pt has been applied in the aforementioned autonomous photosynthetic device developed by Moskovits and co-workers218 to trap hot electrons and catalyze H2 production (Figure 19b). Majima and co-workers243 also reported that selective deposition of Pt on the Au/MesoTiO2 (nanoplate anatase TiO2 mesocrystal) induced significant enhancement in H2 production (Figure 20a). In addition, Au layers,39 bimodal Au NPs,241 Pd

layers,242 and Ag NPs244 have also been constructed on Au/TiO2 heterostructures as electron sinks and cocatalysts. Using the intrinsic electronic heterogeneity existing in semiconductors to control the migration of hot electrons also helps electron−hole separation. It has been reported that the lateral and basal surfaces of MesoTiO2 have different electron affinities and that this crystal-face-dependent electron-trapping property caused the electrons to migrate from the basal surfaces to the lateral surfaces.245,246 Majima and co-workers243 demonstrated that when Au NPs were selectively deposited on the basal surfaces of MesoTiO2, the transferred hot electrons flowed toward the lateral surfaces and left the hot holes on Au NPs (Figure 20b). Their time-resolved diffuse reflectance spectroscopy (TRDRS) study showed that a higher concentration of hot electrons with longer lifetimes accumulated on the semiconductors (Figure 20c).243 Furthermore, the introduced defect states such as oxygen vacancies in semiconductors would also help to trap hot electrons and promote electron−hole 2944


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separation. Zhang and co-workers72 created oxygen vacancies at the Au/BiOCl interface and found a remarkable enhancement of surface photovoltage (SPV) and hot-electron-induced generation of O2−, indicating more hot electrons were accumulated on the BiOCl surfaces. Brückner and co-workers151 also reported that surface vacancies and surface OH groups on Au/TiO2 benefited electron−hole separation via in situ EPR measurement in the H2O-splitting reaction. 4.2. Hot Electron Energy-Level Manipulation

By suppressing electron−hole recombination, the lifetimes of transferred hot electrons are substantially extended. This results in the accumulation of hot electrons in the CB of semiconductors.56,58 The naturally formed repulsive force among them then causes the pile-up of hot electrons and raises their potential energy levels, which greatly enhances their capability for driving various reduction reactions. Recently, our group58 has demonstrated that the potential energy level of hot electrons was simply tuned in Au/TiO2 heterostructures by varying the size of plasmonic metal NPs. We observed that, compared to small Au NPs, large Au NPs had a larger optical absorption cross section, generated more hot electrons that were transferred to TiO2, and established electron energy levels high enough for H2O splitting, while almost no H2 production was detected over small Au NPs (Figure 21b).58 In addition, Pradhan and co-workers25 also demonstrated that the photocatalytic activity of methylene blue reduction was enhanced by increasing Au NP sizes in the Au/SnS heterostructure. As the reduction potential of H2O is quite close to the CB of TiO2,55,58,247 the H2O-splitting reaction rate was significantly affected by the change in electron energy level in the TiO2 CB (Figure 21a). This result renders the H2O-splitting reaction as a suitable thermodynamic ruler to inspect the energy level of hot electrons.

Figure 23. Plasmon-induced selective hydrogenation of cinnamaldehyde to cinnamyl alcohol in the presence of 2-propanol over Au/SiC heterostructures. The hot electrons and steric effects at the interface facilitate the activation of α,β-unsaturated aldehydes to unsaturated alcohols. Reprinted with permission from ref 73. Copyright 2016 American Chemical Society.

C bond is more thermodynamically and kinetically favorable than hydrogenation of the CO bond;73,253 thus, the results indicate that the plasmon-driven hot-electron-induced adsorbate activation exhibits additional advantages of controlling reaction selectivity over traditional thermal-induced organic transformations. Reducible semiconductors (e.g., Cu2O) provide on-site tunability of adsorbate activation on the metal/semiconductor heterostructure surface under SPR excitation.254,255 Garcia and co-workers254 reported plasmon-induced selective CO2 reduction on (Au−Cu)/TiO2 heterostructures (Cu initially in the oxide form) as shown in Figure 24a. While hot electrons on the TiO2 surface favored proton reduction to produce H2, the hotelectron-induced reduction of Cu2O to metallic Cu promoted selective activation of CO2 and made CH4 the dominant product (Figure 24a).254 In addition, Linic and co-workers255 also achieved control of selective propylene epoxidation on Cu/ Cu2O heterostructures via on-site switching of the catalyst surface layer from Cu2O to Cu under SPR excitation (Figure 24b).

4.3. Hot-Electron-Induced Adsorbate Activation

Semiconductors are known to have distinct catalytic characteristics from the metal surfaces and provide additional active sites to facilitate the transfer of accumulated hot electrons to adsorbed molecules to form active intermediate species (e.g., O2 to O2−).7,9,225−227,248−250 Different from metals, semiconductors have fewer electron-accepting states for the hot electrons to transfer back from the activated intermediates, leading to their significantly extended lifetimes on the semiconductor surfaces.153 For instance, Shiraishi and co-workers225 used EPR to detect O2− formation on Au/anatase/rutile TiO2 heterostructures under visible light (Figure 22a). Similar results were also reported on Au/CeO2 heterostructures.248 Moreover, studies showed that when the long-lived O2− diffused on metal/semiconductor heterostructures, one electron in the antibonding orbital of O2− transferred back to the electrondeficient Au NPs and produced singlet O2 (1O2) (Figure 22b).251 It has been shown that both O2− and 1O2 are active in selective oxidation of aromatic alcohols.227,252 The PMET process locally redistributes charge density across metal/semiconductor heterostructures, and the induced charge heterogeneities enable the transferred hot electrons to selectively activate the polar organic adsorbates on semiconductor surfaces.71,73 A recent study from Guo and co-workers73 reported that in a Au/SiC heterostructure, a high-electron-density region was formed on SiC to selectively adsorb and activate the CO bonds over the CC bonds of cinnamaldehyde [(2E)-3phenylprop-2-enal], leading to 100% hydrogenation of CO bonds to produce cinnamyl alcohol with a high turnover frequency (Figure 23). It is noted that hydrogenation of the C

5. CONCLUSIONS AND OUTLOOK The research on plasmon-induced hot electron photochemistry has been steadily expanding, and the recent progress covered in this review is only the tip of the iceberg. Despite the current exciting results, the overall reaction efficiencies remain low, even though very large plasmon-to-electron conversion efficiencies have been proved. For instance, Furube et al.64 have demonstrated that the electron-transfer efficiency of PMET in Au/TiO2 heterostructures can be as high as 40%. However, to date the reported reaction efficiencies on Au/TiO2 are far below this value.133,143,144 The mismatch between the short lifetimes of hot electrons and the slow kinetics of surface reactions is the major limitation to the efficiencies of plasmonic photochemical reactions. Thus, the key issue that needs to be addressed in the area of plasmonic photochemistry is how to effectively capture, stabilize, and accumulate the plasmon-generated hot electrons and make them available for participating in the subsequent chemical reactions. While the ultrafast dynamics (t approximately femtoseconds to picoseconds) of electron transfer has been extensively studied,64,74,256 the ensuing electronic landscape eventually established on plasmonic photocatalysts under steady-state 2945


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Figure 24. On-site tunability of reactant activation and product selectivity achieved by constructing heterostructures with the reducible semiconductor Cu2O. (a) Schematic showing two adsorbate-activation pathways upon Au SPR excitation in (Au−Cu)/TiO2 heterostructures. The hot electrons transferred to TiO2 drive H2 generation. The hot electrons remaining on Au−Cu reduced the semiconductor Cu2O to metallic Cu, which facilitated the reduction of CO2 to CH4. (b) Tunable selectivity in SPR-induced propylene epoxidation achieved by reducing the Cu2O on Cu surface. (i) X-ray diffraction (XRD) spectra of Cu/Cu2O heterostructures with (red line) and without (blue line) intense visible-light illumination. Disappearance of the Cu2O (111) peak after illumination confirms reduction of the Cu2O layer. (ii) Plots showing change in rate of propylene consumption (red squares) and selectivity of propylene oxide (blue diamonds) along with light intensity. A significant decrease in total propylene consumption and an increase in propylene epoxide selectivity occurred when the light intensity reached 550 mW·cm−2, indicating the altered adsorbate-activation pathways were due to reduction of Cu2O to metallic Cu. Panel a reprinted with permission from ref 254. Copyright 2014 American Chemical Society. Panel b reprinted with permission from ref 255. Copyright 2013 American Association for the Advancement of Science.

capable of generating electron-rich surfaces that favored the selective adsorption and activation of electron-deficient reactants.264−266 Moreover, adsorbates also acted as chemical relays (proton relays with designated pKa values) that served to capture targeting reactants and enhance the selectivity of chemical reactions.267 Apart from adsorbates, the construction of bimetallic nanostructures also circumvents the limitation of noble metal NPs (e.g., Au, Ag, and Cu) for chemical transformations by introducing more active catalytic materials (e.g., Pt and Pd), while providing an additional plasmon decay pathway for hot electron generation on the reactive surface sites.213 Electron-transfer processes locally redistribute electron densities across metal/semiconductor or metal/adsorbate interfaces as hot electrons are transferred to the semiconductor support or the adsorbed molecules while leaving behind charge vacancies (i.e., hot holes) on the metal NPs.8,35,55,57,216−218 Despite numerous examples of hot-electron-driven processes, little is known about these plasmon-generated hot holes.39−42 To date, most studies of PMET have been inherently incapable of identifying hot holes in plasmonic photocatalysts: they either solely monitored the occupation of states in the semiconductor support57,132 or purposely used sacrificial reagents to scavenge hot holes.43−48 In future investigations, it is necessary to put in the same effort to understand the role of plasmon-generated hot holes in photochemistry. We hope that all these fundamental studies will illustrate a unique approach for guiding the further development of advanced plasmonic photocatalysts that are capable of fully utilizing both hot electrons and hot holes for enhancing a variety of reactions.

conditions (t approximately milliseconds to minutes) remains largely unexplored. After PMET in the metal/semiconductor heterostructures, the spatiotemporal evolution of transferred hot electrons on the semiconductors over extended time scales is poorly understood.35 Recently, it has been shown that EPR43 and XAS44 were able to detect hot electrons on the time scale of approximately minutes and to reveal their energy levels and trapping sites. In addition, TA microscopy has been demonstrated to reveal the spatially resolved relaxation dynamics of free carrier and exciton populations in methylammonium lead iodide perovskites (MAPbI3) at room temperature.257 Therefore, the use of these techniques would help to obtain precise knowledge of the spatial distribution of plasmon-generated hot electrons over time scales that coincide with reaction kinetics to elucidate their respective roles in plasmonic photochemistry. Acquiring insights into the precise physical locations of active sites on the plasmonic photocatalysts would greatly assist in the design and optimization of plasmonic photochemical reactions. The identification of active sites can be achieved by implementing advanced techniques developed from nonplasmonic systems. For example, electrochemical impedance spectroscopy (EIS) has been used to measure hematite and anatase TiO2 photoanodes to confirm that the surface states were essential for interfacial charge transfer.258−261 These results suggest that PEC could be expanded to the plasmonic metal/ semiconductor heterostructures for identifying their active sites. Furthermore, operando technologies have been developed to map the active sites on catalysts with high spatial and temporal resolution.257,262,263 For instance, Gross and co-workers263 showed that synchrotron-radiation-based infrared nanospectroscopy provided operando information on active sites on Pt and Au NPs with spatial resolution of 25 nm. Our recent work75 on the PVP-assisted growth of Au nanoprisms highlighted the essential role of surfactants in stabilizing and accumulating hot electrons for photocatalysis on metal NPs, which might have been overlooked in the previous studies of plasmonic photochemistry. Further investigations are needed to achieve a detailed molecular-level description of surfactant-assisted photocatalysis. Recent studies suggested that adsorbates with strong electron-donating properties were

AUTHOR INFORMATION Corresponding Author

*E-mail [email protected]fl.edu. ORCID

Yuchao Zhang: 0000-0003-3215-1033 Shuai He: 0000-0002-4624-4110 Wei David Wei: 0000-0002-3121-5798 2946


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The authors declare no competing financial interest. Biographies Yuchao Zhang received his B.S. in environmental science from Peking University in 2011 and his Ph.D. in physical chemistry from the Institute of Chemistry, Chinese Academy of Sciences (ICCAS) under the direction of Professors Jincai Zhao and Chuncheng Chen in 2016. Currently, he is a postdoctoral researcher at the University of Florida in the group of Professor Wei David Wei. His research interests include plasmon-induced hot hole photochemistry and photoelectrochemical water oxidation. Shuai He received his B.S. in chemistry from Beihang University in 2013. Currently, he is a Ph.D. candidate in physical chemistry , working under the direction of Professor Wei David Wei at the University of Florida. His research interests are in the areas of photonics and magnetism. Wenxiao Guo received her B.S. in chemical technology at The Hong Kong Polytechnic University in 2014. Currently, she is a Ph.D. candidate in physical chemistry at the University of Florida, working under the direction of Prof. Wei David Wei. Her research focuses on controlling the photocatalytic activity of plasmonic metal nanostructures with functional surfactants and adsorbates. Yue Hu received her B.S. in material science at Sun Yat-sen University in 2015. Currently, she is a Ph.D. candidate in physical chemistry at the University of Florida, working under the direction of Professor Wei David Wei. Her research focuses on developing functional surfactants to manipulate the catalytical properties of plasmonic nanostructures. Jiawei Huang received his bachelor’s degree in engineering from Xi’an Jiaotong University in 2015. Currently, he is a Ph.D. candidate in physical chemistry at the University of Florida, working under the direction of Professor Wei David Wei. His research interest is in situ observation of interfacial electron transfer within plasmonic metal/ semiconductor heterostructures. Justin R. Mulcahy received his B.S. in chemistry from the University of California, Davis, in 2014. He is currently a Ph.D. candidate in inorganic chemistry at the University of Florida, working under the direction of Professor Wei David Wei. His research interest focuses on probing the surface and photophysical dynamics of metal and semiconductor nanostructures. Wei David Wei currently is an associate professor in the Department of Chemistry of the University of Florida. He received his Ph.D. from the University of Texas at Austin with Mike White and trained as a postdoctoral researcher at Northwestern University with Chad Mirkin. His research interests are the novel electronic and optical properties of metallic and semiconductor nanomaterials and their applications in solar energy harvesting, conversion, and storage; visible-light photocatalysis; and chemical and biological detection.

ACKNOWLEDGMENTS We acknowledge support from the National Science Foundation [Grants DMR-1352328 and CHE-1308644 and the CCI Center for Nanostructured Electronic Materials (CHE-1038015)] and the Air Force Office of Scientific Research (FA9550-14-1-0304). W.G. especially appreciates the support of a graduate school fellowship from the University of Florida. REFERENCES (1) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37−38. 2947


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