Role of Hot Electrons and MetalâOxide Interfaces in Surface

Review pubs.acs.org/CR

Role of Hot Electrons and Metal−Oxide Interfaces in Surface Chemistry and Catalytic Reactions Jeong Young Park,*,†,‡ L. Robert Baker,§ and Gabor A. Somorjai*,∥,⊥ †

Center for Nanomaterials and Chemical Reactions, Institute for Basic Science, Daejeon 305-701, South Korea Graduate School of EEWS, KAIST, Daejeon 305-701, South Korea § Department of Chemistry and Biochemistry, The Ohio State University, Columbus, Ohio 43210, United States ∥ Department of Chemistry, University of California, Berkeley, Berkeley, California 94720, United States ⊥ Materials Sciences and Chemical Sciences Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, Berkeley, California 94720, United States ‡

5.1. Concept of Catalytic Nanodiodes 5.2. Fabrication and Characterization of Metal− Semiconductor Nanodiodes 5.3. Chemicurrent and Catalytic Activity of M/ TiO2 Nanodiodes (M = Pt, Rh, and Pd) 5.4. Chemicurrent and Catalytic Activity of Nanodiodes under Hydrogen Oxidation and NO/ CO Catalytic Reactions 5.5. Nanoparticle−Catalytic Nanodiode Hybrid System 5.6. Differentiating Hot Electron Current from Thermal Background 5.7. Theoretical Considerations for Electronic Excitation and Chemicurrent 6. Influence of Hot Electrons on Surface Chemistry and Heterogeneous Catalysis 6.1. Influence of Hot Electrons on Atomic and Molecular Processes 6.2. Electronic Coupling of Surface Electrons to Adsorbate Vibration 6.3. Solid-State Device for Electronic Control of Surface Chemistry 6.4. Hot Electron Effect on Metal−Oxide Hybrid Nanocatalysts 7. Future Perspectives and Concluding Remarks 7.1. Hot Electron-Based Solar Energy Conversion 7.2. Acid−Base Catalysis 7.3. Beyond Catalytic Nanodiodes 7.4. Conclusion Author Information Corresponding Authors Notes Biographies Acknowledgments References

CONTENTS 1. Introduction 2. Energy Dissipation at Surfaces 2.1. Energy Dissipation Mechanism 2.1.1. Phonons or Quantized Lattice Vibration 2.1.2. Excitation of Electron−Hole Pairs 2.2. Born−Oppenheimer Approximation and Its Breakdown in Fast Atomic Processes 2.3. Concept of Hot Electrons Generated on a Metallic Surface 3. Detection of Hot Electrons 3.1. Hot Electron Generation by Photons 3.2. Hot Electrons Generation by Transfer of Energetic Molecules 3.3. Hot Electron Generation by Electron Beams 3.3.1. Hot Electrons Generated by a Metal− Insulator−Metal Junctions and Ballistic Emission Electron Microscopy 3.3.2. Hot Electron Detection Using a Metal− Semiconductor Schottky Diode 4. Electron Transfer at Interfaces 4.1. Electronic Structure and Catalysis at the Metal−Support Interface 4.2. Chemisorption on Supported Metal Clusters 4.3. Effect of Oxide Support on Metal Cluster Morphology 4.4. Active Sites at the Metal−Support Interface 4.5. Molecular Origins of Metal−Support Interactions 4.6. Role of Dopants of Oxides as an Electronically Active Support for Pt 5. Catalytic Nanodiodes: Probing Hot Electrons from Exothermic Catalytic Reactions

© XXXX American Chemical Society

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1. INTRODUCTION Fundamental mechanisms of energy dissipation and conversion at solid−gas interfaces are key issues for the surface science community. Exothermic reactions lead to the emission of

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Received: June 6, 2013

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DOI: 10.1021/cr400311p Chem. Rev. XXXX, XXX, XXX−XXX

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the form of ballistic hot electrons.14,20,21 In still other experiments, electrical bias,22 photoexcitation,22,23 or catalyst doping24,25 can each change the charge state of a surface adsorbate, which changes the activity and selectivity of the catalyst. Although, in each case, the experimental parameters of interest are unique, the common thread is the prevalent role of ion chemistry on surfaces. The role of covalent surface bonding in dissociating and activating reaction intermediates is also important for many catalytic processes. In fact, in real catalysts, complex architecture gives rise to tandem reactions involving both ionic and covalent pathways; a complete understanding of the gradations between these two mechanisms is still lacking. Of the two types of surface chemistry, covalent surface chemistry is better understood and has dominated much of surface science and catalysis research over the past 70 years. However, in the past decade, strong experimental techniques have been developed and applied to understanding the role of charge transport during surface catalytic reactions. Consequently, this review focuses exclusively on the role of charge transfer and ion chemistry to mediate the activity and selectivity of heterogeneous catalysts. In section 2, we review various energy dissipation mechanisms on the surface and the concept of hot electron generation by nonadiabatic electron excitation. We review various detection schemes for hot electrons produced as a result of exothermic chemical processes, including exoelectron detection, metal− insulator−metal diodes, and metal−semiconductor Schottky diodes (section 3). Experimental and theoretical results on hot electron generation by photons, transfer of vibrational energy from molecules, or electron beams will be also shown in section 3. Section 4 outlines recent results on the role of charge transfer in metal−oxide heterogeneous catalysis. Experimental chemicurrent measurements using a metal−semiconductor Schottky diode will be highlighted in section 5, including descriptions of various structures of nanodiodes and chemicurrent measurements during exothermic catalytic reactions. The correlation between hot electron flow and catalytic activity will also be discussed. In section 6, experimental results indicating the influence of hot electrons on atomic and molecular processes (e.g., desorption, surface roughness, isomerization, and change of catalytic reactivity) will be discussed. The influence of hot electron flow on catalytic activity will be shown for catalytic nanodiodes and metal−semiconductor hybrid catalysts. Finally, section 7 examines prospects for future research on hot electron physics and chemistry and outlines the conclusions from this review.

particles during nonadiabatic gas/surface reactions. These nonthermal excitations include exoelectron emission, surface chemiluminescence, vibrational state populations, and ion/atom emission (abstraction).1,2 Haber and Just found that the reaction of gases on alkali metals and alloys leads to electron emission.3,4 Since the development of high vacuum technology, studies on gas adsorption (e.g., magnesium−oxygen reaction and related light emission on clean metallic surfaces) revealed electron-, ion-, and photon-emission during gas−solid reactions.5−7 Later, Norskov et al. employed a correlation diagram that originated from gas-phase experiments to explain surface chemiluminescence.8 Energy dissipation at surfaces and interfaces is mediated by excitation of elementary processes, including phonons and electron excitation, once the external energy is transferred to the surface.9−12 Electron excitation in exothermic chemical reactions or the irradiation of photons at metal surfaces leads to the flow of high-energy electrons with an energy of 1−3 eV. Most of the chemical or photon energy is converted to electron flow on a short (femtosecond) time scale before atomic vibrations adiabatically dissipate the energy (picoseconds). Energy transfer to the surface generates energetic electrons that are not in thermal equilibrium with the metal atoms, which are called “hot electrons”.13−16 Hot electrons move into the bulk of the metal catalyst and turn into low-energy electrons through lattice atom relaxation within a lifetime of 10 fs, and within the length scale of the electron mean free path, which is in the 5−10 nm range. There are a number of studies demonstrating the influence of hot electrons on atomic and molecular processes. The detection of hot electron flow under atomic or molecular processes and understanding its role in chemical reactions are major topics in surface chemistry. Electron excitation created during atomic or molecular processes at the surface was demonstrated by recent experimental and theoretical studies. Theoretical models have been suggested that nonadiabatic energy transfer during a surface chemical process is dominated by the excitation of electron−hole pairs.17,18 In this review, we outline recent findings on fundamental mechanisms of energy dissipation and conversion occurring on the surface and at interfaces. We discuss the possible mechanism for hot electron generation in surface reactions. We highlight recent studies on the detection of hot electrons and the relationship between hot electrons and catalytic activity on metallic surfaces. We will show recent experiments demonstrating the influence of hot electrons on atomic and molecular processes and we will discuss the implication of hot electron flow in reactive surface chemical processes. The experimental observations discussed in this review show that a close correlation exists between charge transfer and surface chemistry in catalytic systems. This correlation seems to point toward a new class of surface chemical reactions where the breaking of strong surface bonds leads to activation of reaction intermediates and controls activity and selectivity of the catalyst system. Charge transfer with a static nature may occur via several mechanisms, each showing a dependence on unique catalyst parameters. For example, in strong metal−support interactions (SMSI), enhanced catalytic activity at the oxide−metal interface correlates with the oxidation state of the oxide support, and charge transfer is the result of chemical bonding at O vacancy sites that act as active redox centers.19 In contrast, in nanodiode systems, nonadiabatic transitions on the metal surface alone can drive a continuous and dynamic flow of charge across the buried metal−support interface as exothermic reactions dissipate heat in

2. ENERGY DISSIPATION AT SURFACES 2.1. Energy Dissipation Mechanism

The basic mechanism for energy dissipation involves phonons and nonadiabatic processes (e.g., electron−hole excitation, plasmons, exoemission, and chemiluminescence). Figure 1 illustrates the concept of energy dissipation at surfaces, after deposition of external energy, that is mediated by phonons, electron−hole excitation, and photons. 2.1.1. Phonons or Quantized Lattice Vibration. The energy of phonons is in tens of meV, which is 2 orders of magnitude smaller than chemical energies. The direct transfer of energy into the phonon system of the metal requires multiple excitations of phonons during chemical processes through adiabatic energy transfer, eventually creating “heat”.26−28 Measuring the lifetimes of vibrationally excited adsorbates often shows the significance of this channel of energy dissipation. B


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Figure 1. Illustration of energy dissipation at surfaces mediated by basic elements including phonons, electrons, and photons.

For example, infrared emission studies of CO from NaCl showed energy transfer from high-frequency CO stretching to lowfrequency phonons. It was found that energy transfer in this physisorbed system takes place on the ms time scale or slower.29,30 2.1.2. Excitation of Electron−Hole Pairs. The excitation of electron−hole pairs leads to a hot electron with energy above the Fermi level and to a hot hole with energy below the Fermi level. The excitation of an electron−hole pair takes place via a nonadiabatic process. Emission of charged particles, exoelectrons,6,31 and light, surface chemiluminescence,5,7,8 during gas adsorption or reaction is generally associated with a nonadiabatic charge transfer process.

Figure 2. (a) One-dimensional potential energy curves for a system that can exist in two electronic states, A and B, which illustrates a nonadiabatic process. If the system starts in state A and moves toward the left, the system approaches a crossing point, where the system has some probability of existing in either state A or B. In the Born− Oppenheimer case, the motion along the nuclear coordinates is slow enough that the electron has enough time to transition to state B (dashed line), which is the lower energy state past the crossing point; thus, crossing is “avoided”. In a nonadiabatic situation, nuclear motion is fast enough that the system is still in state A past the crossing point and no longer in the electronic ground state. If relaxation then occurs, an amount of energy, ΔE, is released by the system. (b) If we start in the lower energy state A (metal (M) and an adsorbate molecule (A)) at an infinite distance, the reaction coordinate decreases. At short distances, state B (metal (M+) and an adsorbate molecule (A−) after charge transfer) is lower in energy. In the nonadiabatic situation where the nuclei move too fast for the electrons to respond, the crossing, which involves the energy transfer, takes places. The curve continua represent the continuum of conduction band states of the metal.

2.2. Born−Oppenheimer Approximation and Its Breakdown in Fast Atomic Processes

The phenomena of chemical dynamics often rely on the Born− Oppenheimer approximation (BOA). The approximation relies on the fact that electrons move on a much shorter time scale than nuclei because of the considerable difference in mass.32 As the nuclei positions change, electrons have ample time to sample the space available to them and find the energy minimum, preventing nuclear motion from causing electron excitation. This makes it considerably easier to describe the potential energy landscape of polyatomic systems; each electronic level has its own distinct potential surface where the nuclear coordinates are the only variables. A system that behaves this way is described as adiabatic. Several theoretical models have been developed to explain these phenomena that involve an electronic friction approach or a weak coupling approximation.33−35 For many gas-phase reactions, the BOA, particularly at low total energies, gives rise to excellent agreement between experimental and theoretical results, confirming that the BOA is valid for many systems.36−38 For certain reactions at metal surfaces, however, different nonadiabatic coupling plays a crucial role in energy transfer between the adsorbate and the substrate, and the BOA breaks down.39−42 Along a given reaction coordinate, two potential energy surfaces, corresponding to two different electronic states (A and B), may cross, as shown in Figure 2a. At the crossing point, an electronic transition requires no change in energy. The superposition of both states A and B implies a nonzero probability that the transition will occur. Figure 2b shows another view of this process for the case of molecular adsorption. Let us assume that we start in the lower energy state A (metal (M) and an adsorbate molecule (A)) at an infinite distance and decrease the reaction coordinate. At short distances, state B (metal (M+) and an adsorbate molecule (A−) after charge transfer) is lower in energy. In the adiabatic situation, motion along the reaction coordinate is slow enough that the system will transition to B, the lower energy state, when it reaches

the crossing point; crossing is “avoided” and the Born− Oppenheimer approximation holds. This is no longer true if the nuclei move too fast for the electrons to respond. Landau, Zener, Stückelberg, and Majorana all independently derived the expression for the probability, pLZ, of transitioning at the crossover region in the case of a one-dimensional system9,43−45 ⎛ 4πVAB 2 ⎞ ⎟ pLZ = exp⎜ − ⎝ hν|sA − sB| ⎠

(1)

where ν is the velocity along the reaction coordinate at the crossing point, sA and sB are the slopes of the potential energy curves for A and B at the crossing point, respectively, and VAB is half the difference in energy between the two curves at the crossing point. In the adiabatic approximation, ν approaches 0 and the transition probability approaches 1. The transition may also occur, with some delay, after the system has gone beyond the crossover region. The system would then be in an excited electronic state, and if relaxation were to occur, an amount of energy, ΔE, would have to be dissipated. If A is a solid surface and B a molecule impinging on it, creation of an electron−hole pair within the solid is one possible channel for dissipating this excess energy. A nonadiabatic process is especially likely for an atom or molecule reacting with a solid C


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flow of hot electrons only takes place in the near-surface region and is spatially confined by the length scale characterized by the electron mean free path, as shown in the energy diagram in Figure 3b. Because hot electrons have short lifetimes (10 fs) and are present at the localized near-surface region, the detection of hot electrons has been rather challenging. However, if the spatial extent of metal catalysts is several nanometers (e.g., nanoparticles 1−10 nm in size) and the metal catalyst is in contact with the oxide, hot electrons can reach the interface between the metal and the oxide, overcome the potential barrier at the interface, and then be transported into the oxide.

surface. The energy levels of the solid form a continuous band, as shown in Figure 2b, which implies a continuum of crossing points. In the region where the energy of the impinging molecule overlaps with one of these bands, the VAB term in pLZ approaches zero and the probability of transition approaches 1. In section 3, we will further discuss experimental evidence of the molecule− surface interaction that indicates the breakdown of BOA and theoretical approaches to explain the phenomena.12,42 2.3. Concept of Hot Electrons Generated on a Metallic Surface

Another view justifying the generation of hot electrons is related to the difference between the heat capacities of electrons and phonons. Via electron−phonon interactions, hot electrons thermalize within picoseconds. We can rationalize the phenomenon of hot electron creation as follows: The electronic heat capacity in thermal equilibrium, Celectron, of most metals is much smaller than the lattice heat capacity, Clattice. For example, for copper at 26 °C Celectron γT 0.70 × 10−3 × 300 ≈ = = 0.008 C lattice 3R 3 × 8.3 −1

3. DETECTION OF HOT ELECTRONS In atomic and molecular processes, a pulse of electrons of high kinetic energy (1−3 eV) in metals can be generated due to nonadiabatic electron excitation. Detection of hot electron flow has been an active research topic in the surface chemistry and catalysis communities. Showing that nuclear motion couples to electron excitation is more difficult if the excitation is too small to cause emission. Relaxation of these hot electrons happens on their mean free path (i.e., on the order of 10 nm) and has a femtosecond to picosecond lifetime. This implies two detection strategies: The first is to obtain sufficient time resolution to observe these excitations. The second consists of reducing the size of the setup to below the mean free path. In this section, we show various detection schemes for hot electrons generated by photonic, ionic, and electronic chemical processes.

(2) −2

where γ is the Sommerfeld constant (0.70 mJ mol K ) for Cu and R is the gas constant. When external energy is deposited as a result of exothermic surface reactions or photon flux, the electrons heat up much faster (femtoseconds) than the lattice (picoseconds) because of this significant difference in heat capacity. Electron excitation in exothermic catalytic reactions involves the flow of hot electrons, assuming that most of the chemical energy is converted to electron flow. Thermalization of hot electrons in metals and at metal surfaces occurs via electron− electron (e−e) scattering with the conduction-band electrons. Because of efficient e−e scattering, hot electron thermalization occurs at femtosecond time scales. The electron gas equilibrates with the lattice by electron−phonon (e−p) scattering on a subpicosecond time scale. Through nonadiabatic electron excitation, the electrons on the metal surface become energetic (or hot) and these energetic electrons migrate into the bulk of the metal catalyst. After the energy dissipates, hot electrons eventually turn into low-energy electrons through inelastic scattering within the electron mean free path (3−10 nm range), as shown in Figure 3a. Because the net charge should be conserved, these low-energy electrons should move back to the surface, filling vacancies left after hot electron migration. The

3.1. Hot Electron Generation by Photons

Pump−probe experiments carried out on femtosecond time scales could detect hot electrons with 1−3 eV kinetic energies on metal surfaces. The scheme of hot electron detection via the twophoton photoemission experiment is described in Figure 4a. Optical excitation and probing of metal surfaces by ultrafast laser pulses enables us to detect well-defined nonequilibrium hot electron distributions and to determine time-resolved relaxation.47−49 Time-resolved two-photon photoemission spectroscopy has been applied for direct measurement of charge carrier dynamics in semiconductors, hot electron relaxation in metal surfaces,48,49 and surface state dynamics of adsorbate molecules on metal surfaces.50,51 Two-photon photoemission (2PPE) is a variation of photoelectron spectroscopy that allows for the investigation of occupied and unoccupied electronic states in a metal or semiconductor.52,53 These states are located below the vacuum level at an interval defined by the pump photon energy. As shown in Figure 4a, a first (pump) laser pulse perturbs the equilibrium population and a second laser pulse, irradiated after time t, probes the transient electron distribution by photoemitting electrons. Measurement of their kinetic energy permits us to determine the binding energy of the perturbed state with respect to the vacuum level. The difference between the user-controlled optical paths for the pump and probe laser pulses gives temporal resolution in the electronic states being investigated. Ogawa et al. investigated the lifetimes of hot electrons on Cu(100), Cu(110), and Cu(111) surfaces using 2PPE. Figure 4b shows the lifetimes of hot electrons with 1.3−3.2 eV energy, obtained by fitting two-pulse correlation measurements.52 The inset in Figure 4b shows typical two-pulse correlation measurements of hot electron dynamics at Ei (the energy of the intermediate states) of 1.6 and 2.8 eV for Cu(110), revealing a slower decay for the 1.6 eV electrons. A numerical calculation of the e−e scattering rates was obtained based on the Cu band

Figure 3. (a) Schematic and (b) energy diagram of nonadiabatic electron excitation and generation of hot electrons during an exothermic reaction on a metallic surface. Hot electrons are generated on the surface, dissipate energy, and turn into low-energy electrons within the length scale of the electron mean free path. (Reprinted with permission from ref 46. Copyright 2007, American Chemical Society.) D


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Figure 4. (a) Scheme of two-photon photoemission. Hot electron dynamics are measured with a time-delayed probe pulse. A first laser pulse perturbs the equilibrium population, and a subsequent second laser pulse, irradiated after time t, probes the transient electron distribution by induced photoemitting electrons. (b) Hot electron lifetimes measured for Cu(100), Cu(110), and Cu(111) surfaces. Lifetimes calculated by the band structure model (dashed line), scaled band structure model (solid line), and the Fermi liquid theory using a free-electron model (dotted line) are also shown. The inset shows typical two-pulse correlation measurements for 1.6 and 2.8 eV hot electrons measured at the Cu(110) surface. (Reprinted with permission from ref 52. Copyright 1997, American Physical Society.)

Figure 5. Experimental scheme (left) of NO vibration on Au or Li is shown. Loss of about 1.5 eV within 100 fs on Au implies an electron-mediated process and almost no vibrational relaxation on the LiF insulating surface.39 Plots of vibrational energy loss to (a) an insulator and (b) a metal surface for collisions of highly vibrationally excited NO as a function of vibration states. When NO is prepared in v = 15, collisions with Au(111) transfer significant energy (average of 1.3 eV) to the surface, while energy transfer to LiF is approximately vibrationally elastic. (Reprinted with permission from ref 39. Copyright 2000, AAAS.)

structure. The result reproduces the energy dependence of the

3.2. Hot Electrons Generation by Transfer of Energetic Molecules

hot electron lifetimes for the Cu(110) surface with relatively

Earlier experiments by Amirav and Cardillo showed a way of directly measuring the excitation of hot electrons by xenon impact.54,55 The experiments involve scattering a beam of

good agreement with experimental results. E


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experiment by Hellberg et al., the measurement was performed in an ultrahigh vacuum (UHV) chamber at 10−10 Torr base pressure by impinging a beam of Cl2 molecules onto a potassium surface at 130 K. The potassium surface was prepared by depositing a 50 nm thick film on the silicon surface. The total electron yield was determined by measuring the current change in the sample. Figure 6a shows the total measured exoelectron

hyperthermal xenon atoms from the surface of a Ge or InP p−n junction. In this experiment, a reverse bias was applied across the p−n junction so that the electron−hole pairs excited in the depletion region of the diode lead to a flow of hot electrons. In this way, Amirav and Cardillo obtained the product of the electron−hole pair excitation probability and the collection efficiency on the order of 10−4 for xenon atoms with 9 eV energy. Huang et al.39,56 studied the energy transfer of highly vibrationally excited NO molecules at LiF and Au(111) surfaces. Efficient excitation of electrons (where the efficiency was higher than 50%) was observed at the gold surface by NO molecules prepared in high vibrational states (n = 15) impinging on the metal surface. They found that vibrational energy losses to metals and insulators are quite different. Figure 5 shows that, on a metal, NO (v = 15) loses about half of its vibrational energy, on average, in a direct scattering collision (subpicosecond time scale), while on an insulator, NO (v = 12) loses ignorable vibrational energy even when trapping/desorption dominates (piconanosecond time scale). They showed that other forms of molecular energy transfer processes (e.g., vibration−rotation and vibration− translation) have a very low probability and become important only on picosecond time scales, while the time scale of molecular vibration metal electron energy transfer is on a 10 fs time scale. Similar results of multiquantum relaxation were also observed for NO scattering from Cu(111) and O/Cu(111).57,58 The mechanism for multiquantum relaxation is explained by a vibrational autodetachment mechanism. When NO approaches the surface in a high vibration state, an electron can be transferred from the Fermi level of the surface to the lowest unoccupied molecular orbital (LUMO) of the molecule to form NO− near the outer turning point of the vibration. As the bond readily compresses, the potential energy of the LUMO goes up (2−4 eV), giving rise to the finite probability of electron ejection from the LUMO to the unoccupied orbitals of the metal’s conduction band.41,59 If the electron jumps back to the surface, the vibrational states will be populated for the final neutral NO.60,61 Shenvi et al. came up with a more refined model that takes into account the quantum-mechanical description of vibrational motion on the NO and NO−.18,62,63 If the coupling between NO and NO− adiabats are considered a perturbation, electron hopping between the metal and the molecule ensues. The coupled electron−nuclear dynamics were calculated using independent electron surface hopping (IESH). Shenvi et al. found that vibrational relaxation is absent in scattering from a surface without nonadiabatic coupling and that most of the vibrational energy is transferred in multiquantum transitions when the nonadiabatic coupling is turned on. The predicted distribution of vibration states was in good agreement with that of experimental values. Another demonstration of hot electron generation by energetic molecules or ions was made by Hellberg et al.64 In this experiment, an exothermic surface reaction (K + Cl2) induced electron transfer from the metal to the molecular orbital of the adsorbed diatomic molecule. Experimental evidence suggests that electron excitation is involved in atomic/molecular processes, such as adsorption/ desorption or molecular dissociation.65,66 Hellberg et al.64 detected the emission of energetic electrons into a vacuum when a beam of chlorine molecules with varying kinetic energy (0.08−0.68 eV) reacted with a potassium thin film. In the process of Cl2 dissociation and Cl− embedding, two consecutive electron transfers occur, resulting in nonadiabatic electron excitation and emission of photons, exoelectrons, and Cl− ions. In the

Figure 6. Measurement of an exoelectron when Cl2 molecules with kinetic energy impinge on a potassium surface. (a) Measured exoelectron current as a function of Cl2 velocity. (b) Schematic picture of the model of charge transfer, mentioned in the text. (Reprinted with permission from ref 64. Copyright 1997, American Physical Society.)

current as a function of the velocity of the Cl2. Also, the energy distribution of the electrons was measured by a planar three-grid analyzer. Hellberg et al. used a model that has four key steps: (1) Around 4 Å from the surface, an electron transfer takes place from the K surface to the antibonding orbital of Cl2. The Cl2− ion immediately dissociates or forms a pair made up of a temporary negative ion and a hole. (2) A resonant electron transfers from the metal to fill the hole or (3) the hole survives until its affinity level is shifted below the bottom of the conduction band of potassium. Subsequently, (4) the charge de-excites by an Auger transition or radiative decay, producing a measurable electron. These processes are illustrated in Figure 6b. In another set of experiments where oxygen molecules reacted with a cesium metal film, Bottcher et al.31 observed hot electrons emitted at the surface into a vacuum. A theoretical model for electronically nonadiabatic effects was suggested by Gadzuk to account for the observed hot electron generation.17 3.3. Hot Electron Generation by Electron Beams

3.3.1. Hot Electrons Generated by a Metal−Insulator− Metal Junctions and Ballistic Emission Electron Microscopy. A nonadiabatic event on a surface produces an electron that is out of thermal equilibrium with the other metal electrons. If the energy of this hot electron is greater than the work function of the metal, it can be ejected into the vacuum and detected. At lower energies, the electron stays confined within the metal and quickly thermalizes. A hot electron detector must therefore be able to do two things: its size must be small enough that hot F


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Figure 7. (a) Scheme and (b) energy diagram of BEEM where a scanning tunneling microscope tip injects hot electrons into the thin metal layer.

Figure 8. (a) Scheme drawing of a Ag−AlOx−Al thin film tunnel junction for the detection of hot carriers induced by multiply charged argon ions. (b) Plot of tunneling yield induced by multiply charged argon ions in a Ag−AlOx−Al thin film as a function of the total potential energy (Epot (eV)). The solid line is a linear fit to the experimental data. (Reprinted with permission from ref 71. Copyright 1997, American Physical Society.)

Figure 9. (a) Energy and (b) schematic diagrams of Schottky diodes for chemicurrent measurement. Hydrogen atoms react with the metal surface (1) creating electron−hole pairs, followed by (2) ballistic transport of the hot carriers through the metal film. The hot carriers go over the Schottky barrier and move into the semiconductor where the chemicurrent is measured. (c) Plot of chemicurrent on Ag and Cu surfaces as a function of hydrogen exposure time. (Reprinted with permission from ref 86. Copyright 1999, American Physical Society.)

electrons are collected before they thermalize and it must be able to distinguish high-energy electrons excited by the nonadiabatic event from other electrons in the crystal. The thin, tunneling

junction in ballistic emission electron microscopy (BEEM), metal−insulator−metals (MIM), and Schottky diodes fulfill these requirements. G


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Ag and Cu thin films deposited onto n- and p-doped Si wafers with ohmic contacts on both sides of the wafer. When these diodes were exposed to a beam of H atoms, a current on the order of 0.01−1 nA was observed. This chemicurrent decayed exponentially to a steady-state value, in agreement with the kinetics of impinging particles competing for surface sites.11,86,87 Figure 9c shows a plot of chemicurrent on Ag and Cu surfaces as a function of hydrogen exposure time. Nienhaus et al. found that the number of electrons detected/adsorption events (or chemicurrent yield) is 4.5 × 10−3 for Ag and 1.5 × 10−4 for Cu. This sensitivity difference can be attributed to the shorter mean free path of electrons in Cu films compared with that in Ag films.88 The difference in sensitivity was also attributed to the possible formation of silicide between the copper and silicon, giving rise to a higher roughness and thus, more scattering centers in the Cu/Si. Chemicurrent also showed an exponential decrease with film thickness, as predicted for hot charge carriers thermalizing in a solid. The chemicurrent yield, which is defined as the number of hot electrons collected for each adsorption event, was compared for atomic deuterium and hydrogen adsorption on Ag/n-Si. Deuterium produced 6 times less current than hydrogen, in agreement with a first-principles model of chemicurrent from hydrogen and deuterium on Cu.89 The same scheme was used for different types of diodes and adsorbates: H on Ag/n-Si and Fe/nSi, O2 on Ag/n-Si,87 NO and O2 on Ag/n-Si,90 or Mg on Mg/pSi.91,92 The chemicurrent yields varied from 10−6 to 5 × 10−3, depending on the diode and adsorbate. The yield and chemicurrent kinetics were found to closely correlate with the adsorption energy of many different species on Ag/n-Si.10 Theoretical aspects of chemically induced nonadiabatic excitation of the hot charge carriers was recently described by Trail et al.89 and Mizielinski et al.93 Based on the first-principle theory of chemicurrents, they showed that nonadiabatic dissipation of chemical energy leads to Boltzmann-like energy distributions of hot electrons above the Fermi level. We note that hot electron detection is an active ongoing area of research. For example, two-photon time-resolved photoemission spectroscopy and ballistic emission electron microscopy have not been used under chemical reaction (or operando) conditions. If these techniques can be utilized under catalytic reaction conditions, it would give another insight into energy dissipation during chemical reactions. The scheme of hot electron detection by adsorption or catalytic reaction using a metal−semiconductor diode is an in situ characterization, indicating the flow of hot electrons under chemical reactions and, therefore, nonadiabatic electron excitation.

In BEEM, as shown in the scheme and energy diagram in Figure 7, a scanning tunneling microscope (STM) tip injects hot electrons into the thin metal layer;67,68 thus, the STM tip serves as the source of hot electrons. When hot electrons injected from a STM tip are scattered inside the metal, they lose energy and are unable to cross the interface between the metal and insulator or semiconductor. If an electron has sufficient energy, it can cross the interface, get collected in the back contact, and be measured as a BEEM current.67−70 The detection of photon- or chemically induced electron excitation became possible with MIM tunnel junctions as well as with Schottky devices. If excited carriers have enough energy to overcome either a tunnel or a Schottky barrier, hot carriers can be collected. In this case, the metal film acts both as a substrate for the photon-adsorbing layer and the reaction, and as an emitter of hot carriers. There have been many experimental attempts to elucidate the nature of hot carriers using the MIM junction structure.71−82 Additional experiments found that hot electrons injected in MIM structures influence the surface reactivity.83−85 Figure 8a shows a typical scheme of a MIM junction for this experiment. As Ar ions bombard the top Ag film, hot charge carriers are generated by dissipation of the kinetic and potential energies of the projectiles, travel into the bottom Al electrode, and are detected as an ion-induced internal emission current. In this experiment, the MIM samples were irradiated under normal incidence with multiply charged Arq+ ions produced by a 14.5 GHz electron cyclotron resonance (ECR) ion source. The samples were bombarded with ions of constant kinetic energy while varying the charge state by changing the extraction potential of the ECR source. Figure 8b shows the tunneling yield, induced by multiply charged Arq+ ions with a kinetic energy of 1 keV, plotted as a function of the potential energy, Epot, of the projectile. The internal emission yield shows an approximately linear dependence on the potential energy of the projectile, as shown in Figure 8b.71 It was found that at low potential energy, a bias voltage applied between the two metal films strongly influences the internal emission current, whereas this influence of the bias voltage becomes much weaker as the projectile charge state increases. 3.3.2. Hot Electron Detection Using a Metal−Semiconductor Schottky Diode. Nonadiabatic electron excitation leads to hot electron generation in the metal that can be emitted into a vacuum if the kinetic energy of the electron is higher than the work function. Alternatively, hot electrons can be collected using a Schottky diode configuration when the electrons’ energy is higher than the Schottky barrier height. A Schottky diode is a type of metal−semiconductor contact. The energy diagram and scheme of Schottky diodes for chemicurrent measurement are shown in Figure 9a,b, respectively. When two different solids are brought into contact, electrons in the higher chemical potential solid tend to flow into the solid with the lower chemical potential until the potentials equilibrate. A negative charge forms where the electrons migrate to and a positive charge forms where the nuclei are left behind. This gives rise to an electric field in the semiconductor that can be represented by a bending of the energy bands. If electrons are the majority carrier in the semiconductor (an n-type semiconductor) and if the metal’s chemical potential was higher than that of the metal, the bands bend downward, leaving no barrier to electron flow. This is referred to as an ohmic contact, since current through this interface is proportional to voltage. Nienhaus et al. were the first to successfully measure chemicurrent on a nanodiode.11,86 Their devices consisted of

4. ELECTRON TRANSFER AT INTERFACES High activity and selectivity in heterogeneous catalysis is often achieved on the energy landscapes of excited electronic states because these states often have lower kinetic barriers compared with ground state reaction pathways.94 Consequently, catalysis is more efficient when, rather than thermally pushing a reaction along its ground state pathway, the system is placed on a more favorable energy landscape by charge transfer between the catalyst and a reaction intermediate. This understanding is the basis for treating hot electrons, electric fields, acids and bases, reactive ions, and surface catalysis as a single topic. In heterogeneous catalysis, it is desirable to reduce the size of metal nanoparticle catalysts to maximize the surface area to volume ratio of the metal. Because only the surface of a metal catalyst is active, a fixed amount of material can yield the highest H


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Figure 10. (a) Plot of conversion during the CO oxidation reaction for Ag, ZnO, and ZnO/Ag catalysts. The results show that the combined activity of ZnO and Ag make it a much more active catalyst at low temperature than either component alone. (b) Band diagram, proposed by Schwab, suggesting how Ag acts as an electronic promoter for ZnO by donating electrons to the conduction band in the boundary layer. (Reprinted with permission from ref 100. Copyright 1968, American Chemical Society.)

catalytic deactivation. This was the first observation suggesting a relationship between electronic structure and catalytic activity. In the same study, Schwab also investigated the intermetallic phases of Ag−Sb and Cu−Sn. Within a given phase, the activation energy scaled with electron concentration, as described above. However, in Cu−Sn the γ phase showed an especially high activation energy. He rationalized this observation using the theory by Mott and Jones.103 According to Mott and Jones, a phase is stable until the electron concentration becomes sufficiently high that the electrons at the Fermi level have a wavelength approaching the spacing of the lattice planes. At this point, the electrons will satisfy the Bragg condition for resonant reflection and the phase becomes unstable. This can be pictured as the Fermi sphere in k-space expanding until it touches the edges of the Brillouin zone. This is the maximum electron concentration allowed for a given phase. The degree to which the Fermi sphere fills the entire area of the Brillouin zone determines the conductivity of the alloy or the intermetallic phase. A perfect insulator results when the Fermi sphere entirely fills the Brillouin zone. For most phases, this is impossible because the shape of the Brillouin zone is nonspherical. However, the γ phase of the Cu−Sn alloy is a deformed, body-centered cubic lattice with up to 52 atoms in the unit cell, so its Brillouin zone is nearly spherical. This means that, at solute saturation, the γ phase is the most insulating. This confirms the correlation, stated above, between electrical resistivity and activation energy. Based on these correlations, Schwab concluded that chemical activation scales with the conductivity of a metal catalyst and that catalysis occurs via charge transfer between an adsorbed intermediate and the metal catalyst. This is the first indirect prediction of a surface ion as an active reaction intermediate. Schwab also investigated the role of Ag as a promoter for methanol oxidation over ZnO catalysts.100 In this study, four catalysts were investigated: (1) pure Ag, (2) pure ZnO, (3) Ag and ZnO (placed separately in the reactor), and (4) a Ag/ZnO mixture. The mixed Ag/ZnO catalyst was much more active than the other three, showing CO2 formation at temperatures as low as 120 °C, while the other catalysts did not produce CO2 until temperatures above 200 °C. The following mechanism is proposed: O atoms chemisorbed on ZnO represent active sites for sequential H atom abstractions from methanol that eventually produce CO and H2O. Electron transfer from the ZnO electronically mediates the activity of this surface O, so activity scales with the electron concentration in the boundary layer of

activity at very high dispersion where nearly every metal atom is at the surface and the bulk metal almost disappears. This typically occurs when nanoparticle sizes are just below 1 nm; however, such small clusters are not sufficiently stable to withstand hightemperature reaction conditions.95 Consequently, most heterogeneous catalysts consist of metal nanoparticles finely dispersed on a porous high-surface-area support.96 In these metal−support systems, the metal is traditionally considered to be the active catalyst while the support or carrier is used only to increase the dispersion and thermal stability of the finely divided metal nanoparticles. However, this assumption neglects the effects of chemical and electronic interactions between the catalytic metal cluster and the supporting oxide; numerous studies during the past five decades have shown that dramatic catalytic effects often originate from a complex series of interactions between finely divided metal nanoparticles and their supporting metal oxide. 4.1. Electronic Structure and Catalysis at the Metal−Support Interface

From the 1940s to the 1960s, Schwab performed a series of landmark experiments in which he correlated catalytic activity with the electronic structure of a metal catalyst.97−100 From his experiments, he concluded that the activity of a catalyst is largely determined by its ability to transfer charge to and from surface reaction intermediates. Schwab studied both alloy and mixedphase catalysts. To explain his results on alloys, he referred to the Hume−Rothery theory that had previously been used to predict solubility limits for metal atoms in alloy solutions. Hume− Rothery phases are alloy phases for a solute/solvent combination where the solubility is determined by the net electron concentration.101,102 For any given solute, the solubility limit is fixed at a given electron concentration such that higher valency atoms are less soluble than lower valency atoms. In Schwab’s catalytic reaction studies on alloys, he found that, for solute atoms moving across a row of the periodic table, the rate of deactivation with electron concentration increased with the square of the electron valency.97 This means that the same electron concentration caused by a few high-valency atoms has a greater effect than many low-valency atoms. Previously, the Hume−Rothery theory had accurately predicted solute effects on melting points and lattice spacings, but these properties showed linear, rather than quadratic, scaling with solute valency (i.e., they depended only on the net electron concentration). However, Schwab noted that the effect of the solute on electrical resistance scales with the square of solute valency in perfect correlation to I


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acceptors) and a range of possible donor ions, including V, Cr, Mn, Fe, Co, Ru, Rh, Ir, and Pt. The observation that Ti cations can strongly bind to other transition metals inspired Tauster, who was working on heterogeneous catalysis at Exxon Corporate Research Laboratories, to investigate metal−support interactions in noble metal catalysts supported on TiO2. Tauster used H2 and CO chemisorption measurements to characterize the catalytic properties of noble metal catalysts supported on TiO2.106 He found that, following reduction of the catalyst at 200 °C in H2, all metals used in his study (i.e., Ru, Rh, Pd, Ox, Ir, and Pt) showed high levels of H2 and CO uptake consistent with high dispersion of the metal nanoparticles. However, following reduction of the catalyst at 500 °C in H2, chemisorption decreased to nearly zero in all cases. Such low uptakes of H2 would predict a loss of dispersion consistent with the formation of metal agglomerates with an average size >25 nm. However, both transmission electron microscopy (TEM) and Xray diffraction (XRD) measurements failed to reveal any such agglomerates. Collapse of the TiO2 support caused by hightemperature reduction was also quickly eliminated as a possible explanation by Brunauer−Emmett−Teller theory (BET) measurements, which showed no drop in the TiO2 surface area. Additionally, Tauster showed that H2 and CO chemisorption could be completely restored by oxidation of the catalyst followed by low-temperature reduction. All this evidence seemed to point to a complete change in the catalytic properties of the dispersed metal catalyst induced by strong bonding with titanium cations in the support following removal of surface O by hightemperature reduction. A subsequent study was performed using Ir supported on many different metal oxides.107 The same experiment, described above, was repeated and chemisorption values were plotted relative to reduction temperature for each Ir support system, as shown in Figure 11. A direct correlation was observed between suppression of H2 and chemisorption of CO on Ir and the reducibility of the supporting oxide. Accordingly, reducible metal oxides were classified as SMSI active (e.g., Nb2O5 and TiO2) because Ir supported on these oxides showed the characteristic loss of H2 and CO chemisorption following high-temperature reduction. Other metal oxides were classified as SMSI inactive

ZnO. Ag is an electronic promoter because it causes band bending in the boundary region of ZnO, which results in an increased electron concentration and enhanced O activity, as shown in Figure 10. Conductivity was also measured across pressed pellets of several metal oxides with and without Ag. It was found that Ag enhances the conductivity of n-type metal oxides but decreases the conductivity of p-type metal oxides at temperatures from 300 to 700 °C. This was interpreted as the effect of band bending at the boundary region of the metal oxide: Electrons transfer from Ag into the metal oxide such that an n-type metal oxide becomes more n-type (i.e., more conductive) and a p-type metal oxide becomes less p-type (i.e., less conductive). This seemed to explain why Ag acted as an electronic promoter for O activation on n-type metal oxides. Additional work by Schwab and Steinbach supported this original theory.99 They showed that the activation energy for CO oxidation on ZnO is 20.6 kcal/mol but is lowered to only 9.0 kcal/mol when the ZnO is supported on a Ag film. However, illumination of the Ag/ZnO catalyst increases the activation energy back to 20.6 kcal/mol. They propose the following explanation: Light generates electron−hole pairs in the ZnO, which leads to charge transport across the Ag/ZnO interface. Because of the band bending in the boundary layer of ZnO, electrons move to the Ag support while the holes remain in the ZnO. This decreases the electron concentration in the ZnO boundary layer, leading to decreased O activity. 4.2. Chemisorption on Supported Metal Clusters

Early studies in the 1970s investigated the role of metal−support interactions on the catalytic activity of Pt in zeolites. Boudart observed that Pt clusters prepared inside of Y zeolite showed remarkably high activity for hydrogenation, hydrogenolysis, and isomerization reactions.104 This was initially attributed to an electronic effect resulting from charge transfer between the Pt cluster and zeolite support. Because of the strong acidity of zeolites, it was supposed that an electron transferred from the Pt to the zeolite in a Lewis acid−base interaction that left the Pt cluster with a net positive charge and the cationic Pt clusters were highly active for the observed reactions. However, subsequent investigations by Gallezot105 showed that contact between the Pt and acidic sites in the zeolite were not crucial for the observed activity enhancement; it was concluded that a Pt size effect resulting from the small pores in the Y zeolite were responsible for the activity enhancement. Still the question of metal−support interactions remained an important consideration in heterogeneous catalytic systems. Inspired by a model for metal−metal bonding in solid-state physics, Tauster approached this question from a different perspective.106 The phenomenon of metal−metal bonding refers to covalent bonding between metal cations in a transition metal oxide resulting from d-orbital overlap of metal ions in adjacent lattice sites. This type of bonding was already known to give rise to a distorted rutile structure in Mo, W, V, Nb, and Re oxides and to exist in various other mixed transition metal oxides. In most cases, metal−metal bonding occurs between similar cations in adjacent lattice sites. However, hexagonal barium titanates represent an interesting example of metal−metal bonding where bonding occurs between dissimilar metal ions. Barium titanate takes the general formula of BaM1/3Ti2/3O3−x where M refers to a donor cation and x depends on the valency of M. The hexagonal structure of this compound is stabilized by a strong metal−metal bond that forms between Ti cations (known as

Figure 11. H2 chemisorption results for Ir supported on 12 different metal oxide supports as a function of reduction temperature. This illustrates the classical test for an SMSI oxide in which loss of chemisorption by a supported noble metal catalyst following hightemperature reduction was interpreted as an indication of strong bonding at the metal−support interface. This figure also illustrates the correlation between SMSI activity of an oxide and reducibility. (Reprinted with permission from ref 107. Copyright 1981, AAAS.) J


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Early evidence for decoration of a metal with a reduced support was provided by Simoens et al., who prepared model two-dimensional catalysts of Ni supported on TiO2 and SiO2 for surface investigations.112 In these studies, TiO2 was deposited on a substrate. A well-defined pattern of SiO2 was then deposited on top of the TiO2, leaving sections of the TiO2 exposed and sections of the TiO2 coated with a thin layer of SiO2. Then, a monolayer of Ni was deposited on top of the entire sample, which was subsequently exposed to H2 at increasing temperatures. The surface composition of these samples was then analyzed by Auger electron spectroscopy as a function of reduction temperature. Before any reduction, the Auger spectrum of the sample showed only Ni, C, and O, regardless of location on the sample. However, following reduction at elevated temperature, the surface showed dramatic reconstruction only where the Ni directly contacted the TiO2. In this region, there was a nearly complete loss of Ni signal and the Ti signal appeared. In regions of the sample where SiO2 prevented contact between the Ni and the TiO2 layer, however, no reconstruction was observed in the Auger spectrum. High-resolution electron microscopy in this study showed that, following reduction at 150 °C, the TiO2 is predominately anatase. However, diffraction pattern analysis revealed that this structure changes gradually as the temperature increases until 800 °C, when the TiO2 is completely converted to Ti4O7. From these data, the authors concluded that high-temperature reduction of Ni catalysts supported on TiO2 results in conversion of the TiO2 to Ti4O7 and that mobile Ti atoms in the reduced oxide migrate over the Ni catalyst, resulting in a major surface reconstruction and the observed loss of H2 chemisorption. With improvements in electron microscopy, it is possible to directly observe decoration and encapsulation of metal nanoparticles by the supporting oxide following high-temperature reduction. Komaya and co-workers demonstrated this for the Rh/TiO2 system, showing that, following reduction at temperatures as low as 300 °C, Rh nanoparticles were partially encapsulated by a layer of amorphous TiO2.110 Bernal et al. reported unquestionable TEM evidence for migration of reduced supports over metal nanoparticles (see Figure 12).111 However, this report also concluded that decoration alone could not be responsible for the loss of chemisorption previously attributed to metal−support interactions.

(e.g., SiO2, Al2O3, Sc2O3, Y2O3, HfO2, and ZrO2) because Ir supported on these oxides showed no change in chemisorption behavior with respect to reduction temperature. Ta2O5, which has reducibility between that of the active and inactive oxides, also showed a slight loss of chemisorption but only after highertemperature reduction than was used to activate Nb2O5 and TiO2. This correlation between reducibility and SMSI activity was the first convincing evidence that O vacancies play an important role in metal−support interactions. 4.3. Effect of Oxide Support on Metal Cluster Morphology

Using TEM, Baker et al. investigated the effects of reduction at temperatures from 152 to 802 °C on Pt morphology for Pt supported on TiO2, SiO2, Al2O3, and carbon.108 While sintering occurred on all supports, Pt on TiO2 showed much higher dispersion after sintering, compared with Pt on SiO2, Al2O3, or carbon. TEM images show that, in the presence of Pt, TiO2 was reduced to Ti4O7 at high temperature and that Pt formed thin, raft-like structures (also referred to as “pill-box” structures) on this reduced support. It was hypothesized that the raft-like structures form to maximize the interface between the Pt and Ti4O7 as a result of a strong binding interaction and that this strong binding is also responsible for the loss of H2 and CO chemisorption. Calculations to understand the nature of strong bonding between Pt and reduced TiO2 were performed using molecular orbital theory of metal−ligand complexes. These calculations predict that a Pt atom forms a partly ionic bond with the Ti3+ cation at an O vacancy site in a [TiO5]7− cluster.109 This bond is predicted to be much stronger than a Pt−O bond on a fully oxidized [TiO6]8− hexagonal cluster. It is suggested that this strong bonding explains the tendency of Pt to form thin, raft-like structures on reduced TiO2 as Pt atoms in a cluster rearrange themselves to occupy O vacancy sites in the reduced support. A model is also suggested, based on molecular orbital theory, to explain how Pt bonding at O vacancy sites results in a loss of H2 chemisorption.109 Based on this model, H2 dissociation occurs when occupied metal−ligand antibonding orbitals donate electron density to the σ* orbital of an adsorbed H2 molecule. However, it is shown that Pt atoms occupying O vacancy sites in reduced TiO2 lack the metal−ligand orbitals necessary to catalyze H2 dissociation. Because of the dramatic effect on chemisorption when a metal−support system is placed in its SMSI state, significant research has focused on characterizing structural changes of the catalyst caused by high-temperature reduction. Tauster initially ruled out the possibility of encapsulation of the metal catalyst by a collapse of the supporting oxide based on two observations: First, BET measurements showed no loss of surface area of the supporting oxide following high-temperature reduction. Second, the suppression of chemisorption was reversible following moderate heating in O2.106 Neither of these observations seems consistent with a collapse of the porous support. Consequently, Tauster contended that strong bonding at the metal−support interface causes a change in the electronic and catalytic properties of the supported metal. However, since that time, electron microscopy studies have shown that, following high-temperature reduction, metal suboxides from the reduced support can migrate over supported metal nanoparticles.110,111 This effect is known as decoration or, in extreme cases, encapsulation and is believed to contribute to the observed loss of H2 and CO chemisorption following high-temperature reduction of a metal−support system.

Figure 12. High-resolution TEM image showing complete encapsulation of a Rh nanoparticle by a CeTbOx support following reduction at 900 °C. This clearly shows the ability of a reduced oxide support to migrate over supported metal nanoparticles. It is concluded that this effect, known as decoration or encapsulation, contributes to the loss of chemisorption previously observed by Tauster and others. However, it is not possible that decoration alone can explain all the observations associated with metal−support interactions. (Reprinted with permission from ref 111. Copyright 2003, Elsevier.) K


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than on the metal alone.117 Second, it was alternatively suggested that metal−support systems may promote CO dissociation at the perimeter of a supported metal nanoparticle by formation of a bridging CO intermediate where the O atom is adsorbed to an O vacancy site in the support and the C atom is bonded to a metal site.118 Because dissociation of CO is often rate-limiting for this reaction, formation of this predissociative bridging bond at the metal−support interface could explain the large rate enhancement observed for certain oxide supports.

Here we note that it is important to clearly define strong metal−support interactions. Tauster initially used the term SMSI to refer to a strong bonding interaction between a metal nanoparticle catalyst and its supporting oxide. He contended that this strong bonding at the metal−support interface electronically altered the catalytic and chemisorption properties of the metal nanoparticle. Although early work by Baker et al. showed that this interaction led to spreading of the metal nanoparticles to form thin raft-like structures,108 the classical test for determining if such strong bonding existed in any given metal−support system became suppression of H2 and CO chemisorption. Consequently, chemisorption suppression following high-temperature reduction quickly became synonymous with SMSI. However, later evidence became available that showed decoration of a metal nanoparticle by a reduced support. Encapsulation of metal particles by a migrating support also led to suppressed chemisorption, and it became unclear to what extent the SMSI phenomenon was even related to bonding at the metal−support interface. The present authors believe that ample evidence exists to show that both phenomena (i.e., strong bonding between a metal nanoparticle and its support and decoration of a metal nanoparticle by a reduced suboxide) exist and that each contributes to the observed loss of chemisorption. Certain studies have shown that careful control of the reduction temperature can resolve between the onset of SMSI, as determined by chemisorption measurements, and the onset of decoration, as observed by electron microscopy.111,113,114 However, to take a larger view of this important issue, it is necessary to consider the effect of metal−support interactions not only on chemisorption suppression but on activity and selectivity of these complex catalytic systems. Metal−support interactions have been shown to selectively enhance the rate of certain catalytic reactions. This observation raises many questions regarding the nature of the active site in metal− support systems and the role of strong bonding and charge transfer at the metal−support interface to drive surface chemical reactions. The definition of SMSI has expanded to broadly refer to support-induced changes in catalytic activity and selectivity of metal nanoparticles, mainly because the metal−support interaction is closely linked to the catalytic behavior of the metal. In fact, the ability of metal−support interactions to dramatically enhance the activity of nanoparticle catalysts for selective catalytic reactions continues to make SMSI an important area of research. Metal−support interactions have been shown to selectively enhance the rate of specific surface reaction pathways. The first report of enhanced catalysis by an SMSI system dealt with the Fischer−Tropsch activity of Ni catalysts supported on TiO2.115 Vannice and co-workers observed that Ni supported on TiO2 is 10 times more active for Fischer−Tropsch synthesis than Ni supported on SiO2, Al2O3, or carbon. TiO2 also had an effect on Ni selectivity, increasing the formation of higher molecular weight hydrocarbons, relative to other supports. TiO2 was subsequently shown to have similar effects on the activity of Pt, Pd, Rh, and Ir.116 Two theories were initially proposed to explain the enhanced activity of SMSI catalysts for CO bond hydrogenation: First, because SMSI had already been correlated with suppression of CO chemisorption and because CO was known to block H2 dissociation on noble metal catalysts, it was proposed that H2 adsorption was more competitive with CO on SMSI systems

4.4. Active Sites at the Metal−Support Interface

Traditional catalyst fabrication by insipient wetness impregnation of a high surface area oxide allows little or no control of the interface between the metal nanoparticle catalysts and the oxide carrier. To investigate the important role of the metal−support interface in catalytic reactions, it is possible to fabricate twodimensional model catalysts by depositing submonolayer films of a metal oxide on top of a planar metal catalyst. This geometry of oxide islands on a metal support is referred to as an inverse catalyst and allows a high degree of control over the oxide−metal interface. Because the surface energy of the oxide is lower than the metal, the oxide layer grows as single monolayer islands on the metal without any multilayer formation until reaching 95% surface coverage.119,120 This preparation method allows for precise control of the relative concentration of oxide and metal surface sites, compared to previous SMSI catalysts prepared by high-temperature reduction. Because this type of model catalyst is two-dimensional, it also allows for sample characterization by a wide range of surface analytical techniques. Boffa et al. conducted a series of insightful experiments that suggest that enhanced catalytic activity by an SMSI system may be due to highly active sites at the boundary between the metal catalyst and the oxide. In an initial study,119 they performed a series of submonolayer depositions of vanadia onto a Rh foil. Using Auger spectroscopy, ion scattering spectroscopy, and CO temperature-programmed desorption (TPD) measurements, they confirmed that the vanadia grew as islands one monolayer tall. Using XPS spectroscopy, they investigated the oxidation state of the vanadia islands. An oxidizing pretreatment initially ensured that the vanadia was entirely in the V3+ state. The vanadia was then partly reduced to V2+ by CO exposure; XPS measurements revealed both the absolute amount of V2+ on the surface as well as the relative concentrations of V3+ and V2+ sites following reduction by CO. It was found that the relative concentration of V2+ to V3+ was 34% for low vanadia coverages and decreased monotonically to 50-fold, corresponding to about a 90% change in selectivity for this catalyst following PVP removal. Figure 14a−c show the SFG spectra of these three systems (i.e., Pt/SiO2, TiO2, and Pt/TiO2), respectively, obtained under reaction conditions for furfuraldehyde hydrogenation. The vibrational spectra are interpreted to represent specific surface reaction intermediates, as shown above the spectra. It can be seen that, on TiO2 without Pt, a furfuryl-oxy intermediate forms as a result of a surface bond between the oxide and the carbonyl O of the aldehyde molecule. This intermediate is the precursor for furfuryl alcohol. However, no turnover is observed on TiO2 without Pt. On the Pt/SiO2 catalyst, a very different spectrum indicates the presence of furan on the Pt. Furan is the major product of furfuraldehyde hydrogenation on the Pt/SiO2 catalyst and results from decarbonylation of the aldehyde. A small amount of propylene is also observed, which is a secondary product of furan hydrogenation. The Pt/TiO2 catalyst shows all of the main resonant features of the TiO2 and the Pt/SiO2 spectra. However, in this case, the spectrum is inverted with the resonant features appearing as dips against a high, nonresonant background. This effect, which is common in SFG, is the result of phase mismatch between the resonant and nonresonant components of the spectrum. The high, nonresonant background, which exists only for the Pt/TiO2 catalyst, is the effect of H spillover from the Pt to the TiO2, resulting in the formation of O vacancies. Comparison of the surface spectra with kinetic measurements reveals that enhanced furfuryl alcohol production on the Pt/ TiO2 catalyst is the result of a furfuryl-oxy intermediate that forms on the TiO2 and that the Pt/TiO2 interface is required primarily for H spillover. In this study, density functional N


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The theory that charge transfer between a metal catalyst and a semiconductor support alters the electron structure of the metal catalyst has been investigated by XPS.153−157 Fung performed one of the first XPS studies looking at charge transfer in a Pt/ TiO2 catalyst.153 In this study, he investigated three samples: (1) a Pt film on SiO2, (2) a Pt film on TiO2, and (3) impregnated Pt on TiO2 powder. For Pt on a SiO2 film, the Pt 4f binding energy is 71.5 eV for both the as-deposited film and after reduction at 300 °C. However, the Pt film, as deposited on TiO2, shows a binding energy of 72.7 eV that shifts to 71.1 eV following reduction at 350 °C. The authors explain that the binding energy decreases with increasing particle size due to screening of the core−hole excited state by electrons from neighboring Pt atoms. This may explain the relatively high initial binding energy for the Pt, as deposited on TiO2, if the particles are smaller on the TiO2 than on the SiO2. The authors further explain that the large shift of the Pt to lower binding energy following reduction of the Pt/TiO2 sample is indicative of electron transfer from the TiO2 to the Pt when the sample is placed in its SMSI state. They claim that the Pt/SiO2 shows no change in binding energy because SiO2 is not an active SMSI support. For Pt supported on TiO2 powder, they show similar behavior in the XPS to the Pt/TiO2 thin film sample: following reduction of the Pt at 200 °C, the binding energy is 72.5 eV. However, the binding energy shifts to 71.1 eV following reduction at 550 °C. The shift in binding energy corresponds to the loss in H2 chemisorption. However, H2 chemisorption is recoverable following oxidation at 500 °C and reduction at 110 °C. The correlation of charge transfer from TiO2 to Pt with the loss of H2 chemisorption would be convincing if the shifts in binding energy were also reversible, but they are not. The authors explain the lack of reversibility in the binding energy measurements using XRD data. These data show that Pt on the TiO2 powder aggregates to form large particles during the first high-temperature oxidation step. This means that the same electron transfer observed by the second high-temperature reduction step is occurring on a large particle, so it represents many fewer electrons per Pt particle and is consequently no longer visible as a large shift in binding energy. However, this report has been viewed skeptically by many because it naively assigns the entire binding energy change as a chemical shift without accounting for environmental effects and the reported shift of 1.6 eV is not reproducible.154,165 A more careful study was performed by Sexton et al.154 In this study, Rh supported on TiO2, V2O3, Ta2O5, Nb2O5, and HfO2 were studied using XPS. Additionally, Pt supported on TiO2 was also studied for continuity with Fung’s work. XPS spectra of the supported metal were taken under reducing conditions at 200 and 550 °C. For the reducible oxides, the supported metal showed an initial shift to lower binding energy between 0.4 and 0.6 eV at 550 °C. For the nonreducible oxides, no shift in binding energy was observed. This is analogous to Fung’s work except that the shift observed for Pt/TiO2 was only 0.5 eV in this work, compared with Fung’s reported value of 1.6 eV. In the case of the Rh/TiO2 sample, this experiment was repeated over many cycles, which included a necessary oxidation step at 350 °C to remove the catalyst from the SMSI state. Similar to Fung, it was observed that the high binding energy observed on the initial catalyst was never restored. However, the authors did observe a 0.2 eV shift to a lower binding energy at 550 °C, which was reversible over many cycles. Accordingly, the authors give 0.2 electrons/Rh atom (based on an estimated 1 eV shift/ electron/atom) as a lower limit for charge transfer between the

Figure 14. (a) SFG spectra of Pt/SiO2 catalyst, (b) TiO2 support without Pt, and (c) Pt/TiO2 catalyst under reaction conditions for furfuraldehyde hydrogenation. The schematics in the insets depict the surface intermediates represented in the spectrum; red dots on the molecules above the spectra show the C−H bonds responsible for each vibrational mode. (Reprinted with permission from ref 19. Copyright 2012, American Chemical Society.)

calculations were also performed to investigate the nature of the furfuryl-oxy formation on TiO2. The results of these calculations show that furfuraldehyde binds to Ti3+ cations at O vacancy sites in TiOx. This surface bond results in an electron transfer from the Ti3+ cation to the adsorbed aldehyde in which Ti3+ is oxidized to Ti4+ and a negative charge localizes around the carbonyl C of furfuraldehyde, activating it for H addition. These combined experimental and theoretical results explain the molecular origin of highly selective catalysis at the metal−support interface and draw a strong analogy between SMSI and acid−base catalysis. Current work is underway that appears to show that a similar mechanism applies to a range of aldehyde hydrogenation reactions on Pt/TiO2 catalysts as well as to Pt supported on other SMSI-active oxides.163,164 O


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Rh and Ti3+ sites when the catalyst is placed in its SMSI state by high-temperature reduction. The authors actually predict that the charge transfer is more likely 0.5−0.7 electrons/Rh atom. They explain that, as observed by Baker et al.,108 noble metal clusters form monolayer, raft-like structures during hightemperature reduction. XPS of a monolayer raft shows a shift to higher binding energy due to decreased screening. This relaxation shift makes determination of true chemical shifts resulting from charge transfer between the metal and the support difficult, and Fung made no effort to decouple these effects. However, the authors estimate that the observed −0.2 eV shift is actually offset by an approximately +0.5 eV relaxation shift.156 This predicts that the actual chemical shift due to charge transfer is as high as 0.7 eV, corresponding to approximately 0.5−0.7 electrons/Rh atom. Although not as high as predicted by Fung for Pt on TiO2,153 this chemical shift is still significant, representing a strongly anionic Rh cluster, and this value agrees closely with the theoretical value of 0.6 electrons/metal atom predicted by Horsley et al.109

highly n-type supports where free electron density is in the actual conduction band rather than a reduced suboxide band, as illustrated in Figure 15.

4.6. Role of Dopants of Oxides as an Electronically Active Support for Pt

Figure 15. Band diagram depicting the mechanism for enhanced oxidation activity by Pt catalysts supported on highly n-type TiO2. Highly n-type TiO2 fabricated by F doping increases the electron concentration in the actual conduction band. This is in contrast to O vacancy doping that creates a suboxide band that pins the Fermi energy 0.5−1.0 eV below the conduction band edge. Consequently, electrons from F-doped TiO2 can spillover to activate surface O, making this support electronically active, while electrons from the suboxide band cannot spillover due to their lower energy. (Reprinted with permission from ref 24. Copyright 2011, American Chemical Society.)

Traditional, n-type doping of metal oxides is achieved by O vacancy doping. However, O vacancies result in partially filled defect states in the bandgap of a reduced metal oxide. These defect states give rise to high conductivity, but they also pin the Fermi energy of the metal oxide semiconductor well below the conduction band edge. Consequently, a high Fermi energy is seldom possible by O vacancy doping. However, it was recently shown that F acts as an n-type donor in TiO2.166 F also binds to Ti3+ cations at O vacancy sites. Accordingly, F doping raises the Fermi energy of TiO2 by two mechanisms: first, F passivates O vacancy defect states that cause Fermi energy pinning, and second, F increases the free electron concentration in the actual conduction band. Recent studies have shown that this highly ntype metal oxide acts as an electronically active support for Pt in oxidation reactions.24,25 It is hypothesized that electron spillover from the highly n-type Pt support electronically activates surface O. Two reactions have been reported using F-doped TiO2 as an active support for Pt: CO oxidation24 and methanol oxidation.25 In the case of CO oxidation by Pt nanofilms supported on undoped TiO2 substrates, it was found that the Pt catalyst quickly deactivated due to the buildup of surface O species, as evidenced by postreaction XPS analysis. The observation that catalyst activity is completely restored by CO pretreatment but is poisoned again by treatment in pure O2 also suggests that O is responsible for the poisoning. Comparing the activity of Pt on undoped and F-doped TiO2 supports, the F-doped supports increase the activity of Pt for CO oxidation by a factor of 2 relative to undoped supports. The activity of Pt on the highly ntype F-doped support also increases with time under reaction conditions, in stark contrast to the deactivation behavior observed for the undoped catalysts. It is also important to note that the effect of F is only significant for fully oxidized TiO2 supports. When O vacancies are present, F has little or no effect on the catalytic activity of supported Pt. This is because the presence of a reduced TiOx suboxide pins the Fermi energy well below the conduction band edge, and F is unable to completely passivate these states. The excellent correlation of kinetic results for CO oxidation on supported Pt with the electronic structure of O vacancies and F-doped TiO2 shows that O activation by electron spillover only occurs for

The exact nature of electron spillover is not yet clear. At least three possible mechanisms exist: (1) The electrons in the doped support may activate lattice O in TiO2 such that CO oxidation occurs at the Pt/TiO2 interface as CO adsorbed on the Pt reduces the TiO2 along the Pt perimeter in a Mars−van Krevelen mechanism. (2) Electron transfer may activate O adsorbed on the TiO2 followed by spillover of a reactive O− intermediate onto the Pt catalyst. (3) Finally, the interface potential at the Schottky junction between Pt and a highly n-type support may result in anionic Pt clusters that cause increased activity of O chemisorbed directly on the Pt. Similar catalysts were also investigated for methanol oxidation. Methanol oxidation is a multipath reaction and reveals the effect of O activation on reaction selectivity. Methanol oxidation on Pt produces both CO2 and formaldehyde. It is shown that selectivity of Pt nanofilms toward the partial oxidation of methanol to formaldehyde shows the same correlation with electronic structure as was observed for CO oxidation activity. These results strongly suggest that two separate (rather than sequential) pathways exist for formaldehyde and CO2 formation from methanol and that electronically activated O favors the formaldehyde pathway. This theory is consistent with the results of early studies by Schwab on Ag/ZnO catalysts for methanol oxidation.98,100 Work in progress shows that similar effects are observed for supported Pt catalysts during liquid-phase electrocatalysis, suggesting that the electronic structure of the support also controls the overpotential required for charge transport.167 Few heterogeneous catalysts are capable of partially oxidizing primary alcohols to aldehydes at high selectivity using molecular O2 as the oxidizer, and stoichiometric O donors are often used to increase reaction selectivity.168 However, molecular O2 is P


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Figure 16. Scheme of catalytic nanodiodes (left) and energy diagram of catalytic nanodiodes (right). Hot electrons can be generated by exothermic catalytic reactions.

The electronic effect is mainly associated with charge transport between the metal and the oxide support. Elucidation of the origin of the metal−support interaction requires measurement of the charge transport through the oxide−metal interface. To detect this charge transport or flow of hot electrons under catalytic reaction conditions, metal−semiconductor Schottky diodes have been developed.11,21 In this section, we outline the experimental and theoretical efforts to elucidate the behavior of hot electrons generated under catalytic reaction conditions, including the influence of hot electrons on catalytic activity. This was enabled by hot electron detection using metal−semiconductor Schottky diodes under catalytic reactions.

preferred for cost and environmental concerns. In addition to the work on F-doped TiO2, it has been recently demonstrated that Au−Pd nanoparticles on TiO2 can produce formaldehydes from primary alcohols at high selectivity using O2 as the oxidizer.159 These Au−Pd/TiO2 systems are also shown to be highly selective for H2O2 synthesis from H2 and O2.169 In addition, it is known that halogen-induced electronic effects in Ag/AgO catalysts dramatically increase the activity and selectivity for ethylene epoxidation.170 Based on the correlation of all these studies, it appears that negatively charged O may represent an important reaction intermediate that could be present on the surface of many partial oxidation catalysts and that the metal− support interface plays an important role in the charge transport responsible for electronic O activation.

5.1. Concept of Catalytic Nanodiodes

If the size of the metal catalyst is close to the electron mean free path (about 10 nm), it is possible to collect hot electrons as they are transported across the metal−semiconductor interface without collision, as shown in Figure 16. Hot electrons can be detected as a chemicurrent if their excess energy is larger than the effective Schottky barrier. Due to energy dissipation, once hot electrons arrive at the oxide, they cannot go back to the metal, which leads to the irreversible, one-way charge transport of hot electrons from the metal to the semiconductor, as depicted in the energy diagram in Figure 16. It was found that the hot electron flows correlate with the turnover rate of CO oxidation.20,179 Photon energy has been observed being converted into hot electron flows with metal−semiconductor diodes.180,181 The detection of hot electrons may lead to a fundamental understanding of energy dissipation and conversion processes, which would introduce new opportunities for energy conversion.

5. CATALYTIC NANODIODES: PROBING HOT ELECTRONS FROM EXOTHERMIC CATALYTIC REACTIONS One of the key issues in heterogeneous catalysis is the role of metal−oxide interfaces in altering catalytic activity and selectivity. The smart design of nanocatalysts can improve the catalytic activity of transition metals on reducible oxide supports (e.g., Pt nanoparticles or nanowires on a TiO2 substrate) by engineering the metal−oxide interfaces or the SMSI effect. The SMSI effect refers to changes in the catalytic activity when group VIII metals (i.e., Fe, Ni, Rh, Pt, Pd, and Ir) are supported on certain oxides (e.g., TiO2, TaO5, CeO2, and NbO), as described in section 4. For example, methane formation from CO or CO2 and H2 is enhanced by 3 orders of magnitude.107,171−173 The significant role of the metal−oxide interface and the Schottky barrier between them in enhancing catalytic activity was first suggested by Schwab and others, who performed oxidation of carbon monoxide on Ag/NiO.174,175 Later, Hayek and others found that the reaction rate in the oxide−metal model system depends on the free metal surface area, the oxidation state of the supporting oxide, as well as the number of sites at the interface between the metal and the support.176−178 The origin of such metal−oxide interactions is attributed to either geometric or electronic effects. For the geometric effect, it is assumed that the active surface area of the metal catalysts changes during the reduction process.

5.2. Fabrication and Characterization of Metal−Semiconductor Nanodiodes

Fabrication of metal−semiconductors requires several criteria: (1) Metal thin film: catalytically reactive metal film or nanostructures (e.g., Pt, Rh, Ru, and Pd) should be used. (2) Schottky barrier height (SBH): SBH is the energy difference between the work function of the metal and the electron affinity of the semiconductor; thus, SBH can be tuned by changing the metal film or the semiconducting substrate. For example, the SBH of Pt/GaN is 1.0−1.4 eV because the electron affinity of GaN is 4.1 eV and the work function of Pt is 5.1−5.5 eV. Table 1 shows a list of work functions of various metal catalysts and the electron Q


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Table 1. Work of Various Metal Catalysts and the Electron Affinity of Semiconductors That Can Be Catalytic Nanodiode Components metal catalysts

work function (eV)

Pt Ru Pd Ni Au Rh Cu Fe

5.1−5.5 5.0 5.1 4.6 4.8−5.1 5.0 4.5 4/6

semiconductor

electron affinity (eV)

TiO2 GaN GaAs GaP InAs ZnO BN (hexagonal) Si Ge

3.9 4.1 4.1 3.8 4.9 4.3−4.5 4.5 4.05 4.0

affinity of the semiconductor. We note that the SBH of a nanostructured diode shows a deviation from this value because of the change of work function for nanostructured materials. (3) Ohmic contact to the semiconductor: For the Pt/TiO2 diode, Ti/Au ohmic contacts were deposited to make the ohmic contact to the TiO2. In the case of Pt/GaN, a multilayer of Al and Ti were used for the ohmic contact. Diffusion of Al and Ti toward GaN and subsequent formation of AlN or TiN alloys at the interface leads to ohmic contact to the GaN layer. The fabrication of metal/TiO2 nanodiodes is described elsewhere.20,179 Briefly, vertically oriented metal/TiO2 Schottky diodes were fabricated on an insulating silicon oxide film on a Si(100) wafer, as shown in Figure 17a. A Pt/GaN diode was fabricated on an n-type GaN substrate, which has a wurtzite structure and the orientation of the (0001) c-axis Ga face (misorientation is less than 0.5°). A 200 nm SiO2 insulating layer, which prevents the low-work-function contact pads from shortcircuiting the devices, was formed on the GaN surface using electron beam evaporation, as shown in the scheme in Figure 17a. Next, a Pt layer with variable thickness was deposited using electron beam evaporation to form the Schottky contact. Finally, Ti/Al layers were deposited for the ohmic contacts. The formation of a Schottky barrier between Pt and TiO2 or GaN and subsequent electrical characterization were previously reported. Ali et al. fabricated gas sensors with a Pt/GaN Schottky diode with a Schottky barrier height of 0.92 eV.182 Based on the thermionic emission theory, Wang et al. reported barrier heights of 1.1 and 0.96 eV on Pt/GaN and Pd/GaN diodes, respectively.183 Dittrich et al. showed Schottky barrier heights of 1.2−1.3 eV on Pt/TiO2 diodes.184 Because this energy barrier is much higher than the thermal energy of electrons, the contribution of electron flow from thermal excitation of electrons can be suppressed. To determine the barrier heights and ideality factors of the Schottky diodes, the current−voltage (I−V) curves of our devices were fit to the thermionic emission equation. For thermionic emission over the Schottky barrier, the current (I) of Schottky contacts as a function of applied voltage (V) is given by171 ⎤ ⎛ Φ ⎞⎡ ⎛ e (Va − RSI ) ⎞ I = FA * T 2 exp⎜ − n ⎟⎢exp⎜ 0 ⎟ − 1⎥ ⎥⎦ ηkBT ⎝ kBT ⎠⎢⎣ ⎝ ⎠

Figure 17. (a) Scheme of Pt/TiO2 and Pt/GaN nanodiodes (b) I−V curve measured on the Pt/TiOx diode and fitted to the thermionic emission equation. Based on the fitting, a Schottky barrier height of 1.0 eV, ideality factor of 1.9, and series resistance of 780 ohms were obtained. The Pt/GaN diode showed a similar ideality factor (1.9) and a higher barrier height (1.19 eV) compared with the Pt/TiO2 diode. (Reprinted with permission from ref 46. Copyright 2007, American Chemical Society.)

Richardson constant for TiO2 is 24 A/cm2 K. Figure 17b shows the I−V curve measured on Pt/TiO2 diodes. 5.3. Chemicurrent and Catalytic Activity of M/TiO2 Nanodiodes (M = Pt, Rh, and Pd)

A batch reaction system, equipped with an electrical measurement capability, was used to carry out the gas-phase reaction on catalytic nanodiodes.179 The design of the reaction cell (base pressure of 5 × 10−8 Torr) is shown in Figure 18. A ceramic boron nitride heater was used to change the temperature of the sample. The temperature fluctuations were kept below 0.5 °C using a temperature controller that provided feedback to the current applied to the heater. The reaction rates were continuously measured with a gas chromatograph (GC) from the reactant and product concentrations. The hot electron current between the two gold electrodes was measured using a current preamplifier during the chemical reaction. When measuring chemicurrent during a chemical reaction, the current signal between the two electrodes was measured with an applied voltage of 0 V. An exothermic catalytic reaction generates a continuous flow of ballistic charge carriers, which is detected using various

(3)

where F is area, A* is the effective Richardson constant, Φn is Schottky barrier height, η is the ideality factor, and Rs is the series resistance, respectively. The effective mass of the conduction electrons in GaN, m*, is 0.22 m0, which gives an effective Richardson constant of 2.64 × 104 A/cm2 K. The effective R


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consistent with the variation in thermoelectric current measured on the catalytic nanodiodes. Heat transfer from the exothermic chemical reaction to the Pt surface can lead to an increase in the local sample temperature. The increase in temperature caused by the exothermic reaction in our experimental range (up to a turnover rate of 103 molecules/ Pt site/second) was estimated based on the thermal transport equation, and it was shown that it can be minor effect (less than 10−3 K).186 The small increase in local temperature is attributed to the planar structure of the diode: millimeter-scale spatial size and nanometer-scale thickness of the layers. Figure 19a shows the turnover rate and chemicurrent measured as a function of temperature under CO oxidation

Figure 18. (a) Scheme and (b) a photograph of the batch reaction cell with electrical contacts for measurement of chemicurrent and catalytic activity.

catalytic metal−semiconductor Schottky diodes. Ji et al. found that hot electron currents were generated at steady state for several hours using two types of catalytic nanodiodes, a Pt/TiO2 and a Pt/GaN Schottky nanodiode, under carbon monoxide oxidation catalyzed by the platinum.185 This experiment clearly showed that the Schottky diode converts chemical reaction energy directly into hot electrons. This is in contrast to prior work by Nienhaus et al. where they generated transient currents utilizing adsorption reactions in UHV at 130 K. The current signal was measured between two gold contacts on the Pt/TiO2 diodes under reaction conditions and under pure He to investigate thermal effects. During current measurement, zero bias was applied between the two Au contacts. When the diode was under 1 atm He, thermoelectric current due to the high temperature was observed. The difference in electrical potential between the two electrodes, due to the Seebeck effect, caused the thermoelectric current. Therefore, the thermoelectric current was mainly influenced by the Seebeck coefficient of each layer of diodes. For the Pt/TiO2 and Pd/TiO2 diodes, the direction of the thermoelectric current was opposite to the chemicurrent. In the case of the Pt/GaN diode, the thermoelectric current moved in the same direction as the chemicurrent and the current value was much smaller than that of the Pt/TiO2 and Pd/TiO2 diodes. This indicates that the thermoelectric current is mainly influenced by thermoelectric properties, such as the Seebeck coefficient of the semiconductor layer. The higher thermoelectric current of the TiOx-based diode is associated with the high Seebeck coefficient of TiO2; the Seebeck coefficient of TiO2 is 0.4 mV/K while that of GaN is −0.05 mV/K at 300 K. The trend of the Seebeck coefficient of the semiconducting layer is

Figure 19. (a) Plot of chemicurrent and catalytic turnover rate measured on Pt/TiO2 under CO oxidation as a function of temperature. (b) Chemicurrent detected on a Pt/TiO2 catalytic nanodiode as the sample temperature changed. The chemicurrent clearly shows reversibility. (Reprinted with permission from ref 187. Copyright 2006, American Chemical Society.)

(100 Torr oxygen and 40 Torr CO). The chemicurrent began to appear at 140 °C. While current flow between 140−200 °C is clearly detected, the accumulation of CO2 in that temperature range was beyond the detection range of the GC. This suggests that detection of chemicurrent can be used as a sensitive chemical probe that is more sensitive than gas chromatography. The number of electrons per reaction event (number of CO2) is estimated to be 1−2 × 10−4. We note that this value depends on surface cleanness, Pt thickness, and conductivity of the TiO2 layer. The slope of the Arrhenius plot of turnover rate and chemicurrent were used to determine the activation energy of carbon monoxide oxidation on the Pt/TiO2 nanodiode. The activation energy obtained from the chemicurrent measurement was 21 kcal/mol, which is very close to that derived from gas chromatography (about 22 kcal/mol).14,20 This excellent correlation between hot electron flux and turnover rate suggests that chemicurrent measurement can indeed be used to monitor the chemical reaction in a quantitative and sensitive manner. S


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attenuation length. Using the logarithmic fit shown in the inset of Figure 20a, we obtain an attenuation length of 3.0 nm. This value is comparable to other values in the literature (e.g., the attenuation length (5.4 nm) in Mg/Si(111) that was measured after adsorption of oxygen188,189 or other values for attenuation of chemically induced hot charge carriers).11,86,87

The stability and reversibility of the chemicurrent are crucial issues for applications including chemical sensors or energy conversion devices. As the hot electron effects are confined to the mean free path of electrons, surface reconstruction and change of surface composition during the chemical reactions interfere rectifying behavior of metal−semiconductor interfaces. As the potential on the diodes under elevated temperature is applied, enhanced diffusion of vacancies causes instability of metal− semiconductor interfaces which is assisted by the strong electrostatic field at the interface, resulting in diode failure. Therefore, the chemicurrent measurement was carried out at zero bias under catalytic reactions and below certain temperatures, keeping the interface stable under reaction conditions. Figure 19b shows the plot of chemicurrent versus time while changing the temperature back and forth between 160 and 220 °C.187 The chemicurrent changes reversibly with changing temperature, clearly demonstrating the reversibility of the chemicurrent measurement in a catalytic nanodiode. The dependence of the Pt film thickness on chemicurrent was observed using Pt/TiO2 nanodiodes. Figure 20a shows a plot of

5.4. Chemicurrent and Catalytic Activity of Nanodiodes under Hydrogen Oxidation and NO/CO Catalytic Reactions

Other exothermic chemical reactions (e.g., hydrogen oxidation or NO/CO reactions) lead to the generation of hot electron flow. Hervier et al. detected hot electron flows during hydrogen oxidation using Pt/TiO2 catalytic nanodiodes at atmospheric pressure. Hydrogen oxidation has a heat of reaction (ΔhH2O) of −2.5 eV per product (water) molecule.190 Distinct reaction pathways for hydrogen oxidation may be followed at different conditions.190−193 The rate-limiting step of hydrogen oxidation follows the Langmuir−Hinshelwood mechanism: O(a) + H(a) → OH(a).192,194 The reactive desorption of H2O may involve H(a) + OH(a) → H2O(g)191,193 or 2OH(a) → H2O(g) + O(a), which exhibit activation energies of 16 and 18 kcal/mol in the low coverage regime, respectively.195 As the surface water catalyzes hydroxyl formation, the activation energy is substantially lowered, compared with the water desorption temperature.193 Studies on hydrogen oxidation on oxygen-covered Pt(111) in UHV show that another channel, H2O(g) + O(a) → 2OH(a), is open above −123 °C, and reactive desorption, 2OH(a) → H2O(g) + O(a), begins at −58 °C and continues above 27 °C.196 However, much less is known about the mechanism of hydrogen oxidation at atmospheric pressure. Chemicurrent measured at different temperatures on a Pt/ TiO2 diode in 6 Torr H2 and 760 Torr O2 are shown in Figure 21a. Because the TiO2 is reduced by atomic hydrogen and the subsequent diffusion of hydrogen through the platinum film197 as well as the explosion limit of the hydrogen/oxygen mixture at certain conditions, excess oxygen (>99%) is used for the reaction experiment. We found that the rectifying behavior of the diodes is sustained under hydrogen oxidation conditions where there is a large excess of oxygen.21 This suggests an absence of the spillover effect because the oxygen inhibits the reduction of TiO2 and prevents the hydrogen from diffusing through the platinum, thus making the Schottky barrier remain constant during the reaction experiments. Figure 21a shows the current measured with a Pt/TiO2 diode under hydrogen oxidation conditions with temperatures of 25− 100 °C. We observed that the current increased with temperature and was stable at fixed temperatures. The thermoelectric current was measured separately under 1 atm He and was found to be lower than 0.3 nA at 100 °C. The turnover number of product molecules is simultaneously monitored to determine the chemicurrent yield. Figure 21b shows the turnover number, which is the number of water molecules per platinum site produced on the Pt/TiO2 diode. Figure 21c shows Arrhenius plots of the turnover rate and chemicurrent on the same graph. The activation energy of the chemicurrent (7.4 ± 0.3 kcal/mol) agrees with the turnover rate (7.6 ± 0.6 kcal/mol), which implies a strong correlation between the chemicurrent and the catalytic activity in the case of hydrogen oxidation. The thermoelectric current is negligible (less than 0.3 nA) in the range of temperatures (25−100 °C) used in this study. Earlier work on CO oxidation has shown that the effect of an increase in sample temperature (i.e., due to the heat of reaction) on the chemicurrent is negligible.20 The partial pressure dependence

Figure 20. (a) Thickness dependence of chemicurrent for Pt/TiO2 under CO oxidation measured at 240 °C. (b) AFM images of Pt disks on TiO2 (60 × 60 μm2).

chemicurrent and activation energy as a function of film thickness. The film thickness of Pt was measured with AFM imaging using Pt disks fabricated by deposition of the Pt film through the transmission electron microscopy grid. Figure 20b shows an AFM image of a Pt disk on TiO2. The exact film thickness was estimated by taking the line profile of the AFM image. The inset shows a logarithmic plot of the chemicurrent as a function of film thickness. If the chemicurrent, I, follows an exponential dependence on the film thickness because of inelastic scattering in the metallic layer, we can assume that I = A exp [−d/λ ] where d is the film thickness and λ is the T


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Figure 21. (a) Chemicurrent measured at different temperatures on a Pt/TiO2 diode, under 6 Torr H2 and 760 Torr O2. The inset shows the energy diagram of a Pt/TiO2 Schottky diode. (b) Plot of turnover number for the hydrogen oxidation reaction as a function of time, measured at four different temperatures. (c) Arrhenius plots obtained from chemicurrent and turnover frequency measurements on a Pt/TiO2 diode. The error is the standard deviation obtained from multiple measurements of chemicurrent and turnover rates. (Reprinted with permission from ref 21. Copyright 2009, American Chemical Society.)

For the Rh/GaN nanodiode, the reaction was run under 8 Torr NO, 8 Torr CO, and 744 Torr He between 200 and 250 °C. Arrhenius plots of turnover and chemicurrent data are shown in Figure 22b. The activation energies of turnover frequency and chemicurrent were 32.8 and 31.7 kcal/mol, respectively. The chemicurrent yield for the Rh/GaN nanodiode was 2.8 × 10−4 electrons/event at 220 °C. We note that the chemicurrent yield does not directly reflect the number of hot electrons generated because the catalytic nanodiode scheme relies on the collection of hot electrons through Schottky barriers. Therefore, the apparent chemicurrent yield is subject to the roughness of the interfaces, thickness of the metal film, and concentration of impurties localized at the metal−oxide interface.

revealed a linear relationship between the H2 partial pressure and water formation, which is consistent with a Langmuir− Hinshelwood mechanism. The apparent chemicurrent yield is (1.1 ± 0.1) × 10−4, which is the ratio of the chemicurrent to the turnover rate for hydrogen oxidation, measured at several different temperatures. The chemicurrent yield for H2 oxidation is lower than that for CO oxidation on the same Pt/TiO2 diode (2.3 ± 0.5 × 10−4).14 This result could be associated with the lower heat of reaction for H2 oxidation (2.5 eV per H2O molecule198,199) compared with that for CO oxidation (2.9 eV per CO2 molecule200). These values of chemicurrent yield for H2 oxidation and CO oxidation are close to values seen in the chemisorption of hydrogen on Ag/nSi(111) and Cu/n-Si(111) diodes, which showed chemicurrent yields of 4.5 × 10−3 and 1.5 × 10−4, respectively.86 The hot electron current on 5 nm Rh/TiOx catalytic nanodiodes was also measured during oxidation of CO by NO under 8 Torr NO, 8 Torr CO, and 744 Torr He.201 Gas chromatography and chemicurrent measurements were taken between 190 and 220 °C, as shown in Figure 22a. At 220 °C, the measured TOF, calculated as the production of CO2 per Rh site per s, was 0.54 molecules/site/s, and the measured chemicurrent was 7.9 nA. The activation energies calculated from the turnover and chemicurrent measurements were 26.1 and 25.4 kcal/mol, respectively, indicating a good correlation between hot electron generation and catalytic activity, which is consistent with other reactions (e.g., CO oxidation and H2 oxidation). The chemicurrent yield at 220 °C was 5.5 × 10−4 electrons/event.

5.5. Nanoparticle−Catalytic Nanodiode Hybrid System

A novel scheme for detecting hot electrons generated on colloid nanoparticles under catalytic reaction conditions was demonstrated by Park et al. using Au/TiO2 diodes.202 The nanoparticle−nanodiode hybrid system is composed of a platinum nanoparticle array and a Au/TiO2 Schottky diode. The energy diagram and schematic of the nanoparticle−Au/TiO2 diodes are shown in Figure 23. The interface between the Au and TiO2 forms a Schottky barrier with a height of about 1.0 eV. The exothermic catalytic CO oxidation reaction leads to the generation of hot electrons on the surface of the platinum metal nanoparticles. These energetic electrons go over the energy barrier between the Au and TiO2 and are collected as the chemicurrent. For the ballistic transport of hot electrons through the metal, the overall thickness of the metal assembly (the U


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nanoparticles and the Au thin film) should be comparable to the electron mean free path. Platinum colloid nanoparticles with four types of capping layers were deposited on the Au/TiO2 diodes: citrate, tetradecyltrimethylammonium bromide (TTAB), hexadecylamine (HDA), and hexadecylthiol (HDT).202,203 Figure 23c shows a plot of chemicurrent, turnover frequency, and chemicurrent yield measured on citrate-capped Pt nanoparticles on a Au/TiO2 diode measured at five different temperatures. Interestingly, unlike thin film Pt/GaN and Pt/TiO2 diodes, the activation energy estimated with the measurement of turnover frequency is 28 kcal/mol, significantly higher than that of the chemicurrent (14 kcal/mol). The difference in activation energy is attributed to a charging effect taking place at the organic capping layers that are present at the metal−oxide interface, which influences the total chemicurrent by the trapped charges. 5.6. Differentiating Hot Electron Current from Thermal Background

An important question pertaining to the accurate measurement of hot electrons with a catalytic nanodiode is the ability to differentiate between thermal and nonthermal contributions to the net current measured by a catalytic nanodiode. In addition to chemically driven hot electron flow, other sources can give rise to current across a metal−semiconductor interface. Thermoelectric current and thermionic emissions, in particular, represent a thermal background current that is difficult to differentiate experimentally from hot electron flow. Several groups have made significant attempts to address this issue and to quantify the thermal and nonthermal contributions to the total current measured across a catalytic nanodiode during an exothermic reaction. To date, these studies have yielded conflicting results and quantitative differentiation remains a challenge. Creighton and Coltrin did a careful study of CO oxidation on Pt/GaN devices.204 In their work, they gave particular attention to measuring the surface temperature of the device during reaction. Using a mid-IR pyrometer, they measured the timedependent temperature of the device surface during reaction with a probe depth of approximately 50 μm and found a temperature rise on the order of several °C that scaled with the reaction rate. They also used thermocouples to probe the lateral temperature gradient between the ohmic contacts of the device during reaction and found a rise in the lateral temperature gradient on the order of several tenths of a °C that also scaled with the reaction rate. Applying these measured temperature gradients to calculations based on the Seebeck effect, they suggest that these gradients are sufficient to explain nearly all of the current measured across the nanodiode during the reaction, with minimal contribution from hot electrons. Karpov et al. have worked on catalytic nanodiodes with particular emphasis on chemical-to-electrical energy conversion.205−207 Recently, they have also made several efforts to distinguish between thermal and nonthermal contributions to current produced by a nanodiode during catalytic reaction.208,209 In one study, they performed calculations to predict the magnitude of thermoelectric and thermionic currents across a metal/n-Si Schottky junction.208 For these calculations, they assume a fixed backside temperature and a constant rate of surface heating, determined by the turnover frequency of the exothermic reaction. They conclude that the dominant source of thermal current is thermoelectric current resulting from the Seebeck effect in the semiconductor layer and that thermionic emission can be neglected. They also show that, at low reaction temperatures, the expected thermoelectric current is much lower

Figure 22. Chemicurrent and TOF measurements on (a) Rh/TiO2 and (b) Rh/GaN under the NO/CO reaction. (Reprinted with permission from ref 201. Copyright 2010, American Chemical Society.)

Figure 23. (a) Energy diagram and (b) scheme of hot electron generation in Pt nanoparticles on a Au/TiO2 Schottky diode. (c) Plot of chemicurrent, turnover frequency (TOF), and chemicurrent yield as a function of temperature measured on a citrate-capped Pt nanoparticle array on a Au/TiO2 diode. (Reprinted with permission from ref 202. Copyright 2008, American Chemical Society.)

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resulting in greater surface cooling and a steeper temperature gradient. Depending on experimental conditions, this result has also been previously reported.14,20 The mechanism for the current flow observed in the nonthermal direction, as observed by Renzas et al.,210 is not entirely clear. However, we hypothesize that it represents hot electron flow induced by O2 or CO adsorption. This would occur if the exothermic adsorption is nonadiabatic, as shown by Nienhaus et al., and if the endothermic desorption process occurs thermally, resulting in hot electron flow induced by dynamic adsorption/desorption equilibrium on the surface. In this case, the activation energy for current flow would likely correspond to the heat of adsorption for the reactant on the metal surface. However, proving this hypothesis is still complicated by the effect of gas composition on the device barrier height, making a comparison between reactants with different heats of adsorption difficult. In addition, thickness dependence of chemicurrent shown in Figure 20 is consistent with the chemical nature of the measured current flow. In any case, the experimental observation of a reverse current in the strict absence of reaction-induced surface heating seems to support the conclusion that a nonthermal contribution to chemicurrent does exist, even if exactly quantifying the thermal and nonthermal components remains difficult.

than experimental current measurements under similar conditions, suggesting that these experimental results primarily reflect hot electron flow. However, at reaction temperatures above about 125 °C, the predicted thermoelectric current becomes comparable in magnitude to experimental measurements, and the authors conclude that, for high-temperature reactions, thermal effects need to be considered. In another work, Karpov et al. claim to differentiate hot electron flow from thermal background sources using a unique, resistively heated nanodiode.209 The resistively heated device eliminates the need for backside substrate heating and is intended to create a unidirectional temperature gradient. Using this new heating method for a Pt/GaP device for the H2 oxidation reaction, the authors perform the same analysis as was previously used by Somorjai and co-workers where a background current is measured in an inert gas mixture (O2/N2), and is used as a correction to the current measured under reaction conditions (O2/H2). Consistent with previous results, this work affirms that the current is higher under reaction conditions than in the inert gas environment and attributes this difference to hot electron flow. However, it is important to consider an additional challenge to accurately differentiating between a thermal background current and a true hot electron current. Here, we refer to chemical changes that occur on the device under reaction conditions. The changes that affect barrier height are often reversible with gas composition.210 In general, a reducing atmosphere decreases the barrier height, relative to an oxidizing atmosphere. This is especially dramatic for the case of a Pt/TiO2 device in the presence of H2 where the barrier is reduced to zero by reduction of the TiO2 surface to a suboxide via H spillover unless the device is stabilized by a large excess of O2.21 Because of the contribution of a thermal current to the total current, it is common to subtract a thermal baseline, measured in an inert gas mixture, from the current measured under reaction conditions to determine the actual hot electron flow, as described above. However, determining an appropriate gas mixture to act as a background is more challenging than has been previously realized because removing one or another reactant often changes the barrier height, which exponentially affects the perceived thermal background. Consequently, care must be taken to ensure accurate interpretation of thermal and nonthermal currents; continued work is necessary to make this differentiation more quantitative. In light of the present question, we now comment on previous results by Somorjai et al., which have been reported briefly but not yet thoroughly discussed. Analysis of previous results reveals a reverse current (i.e., electron emission from the metal to the semiconductor) measured on Rh/TiO2 and Rh/GaN devices in pure O2 or pure CO, which increases exponentially with increasing temperature.210 This is not consistent with a thermal mechanism for current flow. In these experiments, the device is heated from the back (i.e., semiconductor side) and cooled by a flow of gas across the top (metal side). By measuring current in the absence of one or the other reactants, the question of local heating of the metal by the exothermic reaction is removed, and the temperature gradient is described by a cooler metal surface relative to the warmer semiconductor substrate. In this case, the expected thermal current would represent electron emission from the warmer semiconductor to the cooler metal, but the observed current is in the opposite direction. Here, however, we note that the expected thermal current can be observed only if the gas flow is high and directed exactly onto the device surface,

5.7. Theoretical Considerations for Electronic Excitation and Chemicurrent

Maximoff and Head-Gordon theoretically investigated the chemicurrent effect, giving particular emphasis to the question of universality.211 Their results indicate that chemicurrent is a general phenomenon for numerous reactions on multiple catalysts. They show that three components are fundamentally necessary for hot electron generation during a surface chemical reaction: (1) localization of conduction electrons from the catalyst to the surface via reductive adsorption, (2) delocalization of the surface electrons back to the conduction band of the catalyst via subsequent oxidative desorption, and (3) energy dissipation by the exothermic reaction must exceed the product adsorption energy. They generalize this series of events to represent a class of reactions where reductive adsorption of a gasphase reactant onto a metal catalyst localizes electron density to the surface, resulting in a negatively charged reaction intermediate. When this charged intermediate reacts exothermically, the product, which initially contains the localized negative charge, quickly desorbs in a concerted step, leaving the localized charge as a hot electron in the metal. This fast desorption is irreversible and is the nonadiabatic step, which makes chemicurrent a general phenomenon. To show that hot electron generation by this method is realistic and experimentally observable, the authors investigate a microscopic reaction coordinate for CO oxidation on Pt, which, they show, acts as a pump to produce hot electrons. These hot electrons are predicted to have kinetic energy in excess of 1 eV, which is sufficient for detection using Pt/TiO2 nanodiodes, as previously discussed. In this case, O2 dissociative adsorption is shown to occur reductively (i.e., O orbitals that are unfilled prior to adsorption exist below the Fermi level following adsorption, resulting in localization of conduction band electrons in the Pt via the formation of a negatively charged O surface intermediate). Reaction of this charged O with CO proceeds via a CO2−≠ transition state and results in a local minimum, which is represented by a CO2− intermediate. However, the barrier to CO2 desorption is not sufficiently high to contain this W


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electron flows and catalytic reactions, furthermore suggesting that hot electron generation plays a part in the catalytic reaction. There were interesting experimental results demonstrating that hot electron flows can influence atomic or molecular processes, including desorption,214 diffusion,215 and isomerization of molecules.216,217

intermediate, which moves quickly along the desorption coordinate following the exothermic reaction between CO and Oδ−. This results in a concerted reaction/desorption step. The important question is what becomes of the localized negative charge from the O. During early stages of the reaction, the O−CO bond distance is long and the OCO bond angle is bent. At this point the negative charge is stable on the CO2− complex. In fact, the authors show that before CO2−≠, multiple negatively charged states exist below the ionization continuum. However, as the reaction proceeds, the O−CO bond distance shortens and the complex becomes linear. As this occurs, the discrete state rises quickly into the continuum, eventually reaching an energy greater than 2 eV above the Fermi energy of the Pt. An energy diagram and density of states along this reaction coordinate are shown in Figure 24. Somewhere along this coordinate, the

6.1. Influence of Hot Electrons on Atomic and Molecular Processes

Incident hot electrons can induce atomic-scale hydrogen desorption through electronic and vibrational excitation mechanisms. Shen et al.214 utilized hot electrons released from STM to desorb hydrogen from hydrogen-terminated Si(100) surfaces. Two atomic-scale mechanisms were suggested in this study: multiple vibrational excitation by tunneling electrons at low tip voltage and direct electronic excitation of the Si−H bond by high-energy field-emitted hot electrons. The hot electrons also promote other molecular processes, such as diffusion and isomerization. Fomin et al.215 investigated the diffusion of isolated H2O and D2O molecules on Pd(111) using STM at low temperature (about −233 °C). They showed that diffusion was thermally activated at low tunneling voltage. On the other hand, when the tunneling electron had enough energy to excite the vibrational “‘scissor’” mode of the molecule, the diffusion rate was enhanced by several orders of magnitude. Figure 25a shows a plot of the hopping rate as a function of tip voltage for isolated H2O and D2O molecules. The hopping rate

Figure 24. Diagram showing the density of states in the valence region of the O−CO complex along the reaction coordinate from CO(a) + O(a) (left) to CO2(g) (right). The red horizontal line is the Fermi energy. The positive binding energies correspond to vacant states. The color gradient from blue to white to red represents an increasing density of states. The red lines show the molecular orbital energies of O−CO decoupled from Pt(111). (Reprinted with permission from ref 211. Copyright 2009, National Academy of Sciences.)

localized charge is dumped as a hot electron back into the Pt as the CO2 product irreversibly desorbs. Derivations based on nonadiabatic coupling parameters predict that the hot electron yield, Y, for this process should have a power law dependence, Y = (u − χ)α, where u is the energy of a hot electron and χ is the minimum work that is required to free a gas molecule or the maximum work that is produced upon localizing it to the active interface. The exponent, α, is 2.66 for hot electrons generated during chemical reactions, which is in close agreement with 2.7, as measured by Gergen et al. for adsorption-induced chemicurrent on Ag/n-Si diodes.10 This value is significantly higher than the 2.0 scaling based on Fowler’s law for photoemission, showing that chemically induced electron emission belongs to a unique class of universality.

6. INFLUENCE OF HOT ELECTRONS ON SURFACE CHEMISTRY AND HETEROGENEOUS CATALYSIS In the previous section, we focused on hot electron generation by various mechanisms. In this section, we highlight the influence of hot electrons on various chemical reactions. Nonadiabatic electron excitation in exothermic catalytic reactions involves the flow of hot electrons (1−3 eV).11,17,187 A catalytic nanodiode permitted us to detect a steady-state flux of hot (>1 eV) electrons generated during a gas-phase exothermic reaction on the surface of the catalyst metal.21,179,212,213 Earlier studies on Pt/TiO2 catalytic nanodiodes showed that the activation energy of chemicurrent and catalytic activity are quite similar (about 21 kcal/mol for CO oxidation,20,179 7.4−7.6 kcal/mol for H2 oxidation21), which indicates an intrinsic relation between hot

Figure 25. (a) Semilog plot of the hopping rates for H2O and D2O molecules as a function of voltage on a Pd(111) sample at −233 °C with a 40 pA tunnel current. Hopping rates rise rapidly for bias voltages higher than 200 mV for H2O and 150 mV for D2O. (Reprinted with permission from ref 215. Copyright 2006, Springer.) (b) Illustration depicting the concept of electron-induced isomerization of an azobenzene molecule adsorbed on a metallic surface with hot electrons injected from STM. (Reprinted with permission from ref 216. Copyright 2006, American Physical Society.) (Right) Plot of the isomerization yield, Y, versus bias voltage, U, for trans−cis isomerization. (Reprinted with permission from ref 217. Copyright 2006, Wiley.) X


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Figure 26. (a) Experimental scheme for measurement of the hot electron-induced change in chemical activity. (b) SERS spectral changes of PNBA on the Ag/AlOx/Al tunnel junction under positive bias voltage to the Ag electrode. (Reprinted with permission from ref 85. Copyright 2006, Elsevier.)

decomposition of 2-methyl-1,4-naphthoquinone was observed by Wadayama et al. on the MIM tunnel junction.84 Wadayama et al. used surface-enhanced Raman spectroscopy (SERS) to uncover the hot electron-induced surface reaction of p-nitrobenzoic acid (PNBA) adsorbed on a Ag top electrode in a Ag/AlOx/Al tunnel junction. The experimental scheme is shown in Figure 26a. The MIM tunnel junction that consists of a Ag/ AlOx/Al junction can serve as a template for an anisotropic surface and enhances the surface reactivity by injecting hot electrons. Changes in the SERS spectra of PNBA on the Ag surface as a function of positive bias voltage are shown in Figure 26b. The spectrum recorded at 0 eV reveals strong bands of COO− (1350 cm−1), NO2 (1380 cm−1), and phenyl groups (1580 cm−1) of the adsorbed PNBA−. The presence of these intermediates indicates that PNBA dissociatively adsorbs on the Ag electrode surface. At voltages >2.2 V, a new band emerges near 1452 cm−1, which is more prominent as the bias voltage increases. The PNBA− adsorbed on the Ag substrate reacts in a reductive coupling reaction to generate azodibenzoate. The results demonstrate that the hot electrons can induce the reductive coupling reaction of the adsorbed PNBA−, which leads to generation of azodibenzoate at the top Ag electrode.

was obtained from all of the hopping events, normalized to the total number of molecules in the series of STM images or movies and to total imaging time. The hopping rate was independent of bias voltage but increased very rapidly above 200 mV for H2O and 150 mV for D2O. Fomin et al. also studied the current dependence of tip-induced hopping for H2O and D2O. They found that the tip-induced hopping rate was linear in tunnel current up to 1000 pA above the vibrational excitation threshold. They concluded that this linearity was consistent with singleelectron vibrational excitation of the scissor mode. Choi et al.216 and Henzl et al.217 reported a trans−cis conformational change of isolated azobenzene (AB) molecules induced by the hot electrons from the STM tips. On the basis of the spatially resolved STM or scanning tunneling spectroscopy (STS), Choi et al. observed that the reversible conformational change in AB can take place, as illustrated in Figure 25b. Tunneling electrons with a specific bias in the STM geometry triggers this switching behavior. The electronic structures of the two potential energy minima were measured using spatially resolved STS. Henzl et al. also carried out switching of the AB molecules on Au(111) more than 70 times between two bent forms and one elongated form.217 The manipulation yield per electron was calculated for the different electron energies, and it was found that the threshold depends on the configuration: about 650 meV for changes from the elongated (trans) configuration and about 640 meV for changes from the bent (cis) configuration. The different threshold voltages indicate that the electronic structure of the molecule changes after isomerization. More and more experimental and theoretical evidence is showing that hot electrons can affect surface reactivity. For example, theoretical work by Gadzuk et al. on hot electrons at the metal−insulator−metal electrode suggests the concept of “hot electron chemistry”.17,218 Diesing et al. found that the hydrogen evolution reaction is promoted when a bias voltage is applied to the metal−insulator−metal junction.72 The promotion of decomposition of chlorinated hydrocarbons by electron emission was observed by Sharpe et al.83 Bias voltage-induced

6.2. Electronic Coupling of Surface Electrons to Adsorbate Vibration

Electronic coupling of surface electrons to adsorbate vibration holds great promise for catalysis, since it offers a key parameter for tuning rationally designed catalysts. This is made clear by Bonn et al. in a study of CO oxidation on Ru(0001).94 If CO and O are coadsorbed on a Ru(0001) single crystal and the crystal is heated, CO and O desorb before any CO2 is produced. Instead of conventional heating, the authors used an infrared femtosecond laser to increase the surface temperature. Two pulses were fired at the surface, separated by a variable delay at picosecond scale. The laser pulses initially raise the electronic temperature, up to several thousand °C, since the heat capacity of electrons is quite low, while the lattice temperature remains almost unaffected. The electrons eventually equilibrate with the phonons in the Y


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Figure 27. (a) Calculated electron and phonon temperatures (dashed lines) of Rh metal after excitation by a femtosecond laser pulse, along with calculated reaction rates for CO oxidation and CO desorption. (b) Schematic mechanism for desorption of CO2 and CO. The activation energy for desorption is much lower than for oxygen activation for CO oxidation, and no CO2 is formed from heating. The laser pulse makes it possible to activate the oxygen while the lattice temperature is low enough to prevent CO desorption and CO2 is formed. (Reprinted with permission from ref 94. Copyright 1999, AAAS.)

Figure 28. (a) Architecture of the Pt/n-Si catalytic nanodiode. The device was fabricated by depositing 4 nm Pt films onto n-type Si substrates. Ohmic contacts were made to the n-Si and to the Pt using Al and Au layers, respectively. (b) Band diagrams of the device in open circuit and during reverse bias. Bias allows the Pt/n-Si interface potential to be reversibly controlled. (Reprinted with permission from ref 22. Copyright 2012, American Chemical Society.)

6.3. Solid-State Device for Electronic Control of Surface Chemistry

lattice, with a characteristic time of 300 fs, and, after several picoseconds, the surface is at thermal equilibrium. Figure 26 shows the electronic and lattice temperatures calculated based on a previously published model.219 A single laser pulse produces no CO2. Two laser pulses separated by more than 10 ps are also unable to lead to turnover, since in that time, the lattice has equilibrated with the electrons (see Figure 27). However, if the second laser pulse occurs before this equilibration takes place, CO2 is observed desorbing from the surface. With a short delay, the surface electrons reach a temperature high enough that a previously inaccessible, nonthermal reaction pathway is opened. Density functional theory (DFT) calculations show an antibonding orbital for O adsorbed on Ru 1.7 eV above the Fermi level. The electronic heating caused by the laser pulse creates a population of electrons at such an energy in the metal surface that may populate the antibonding orbital, activating the O atom for reaction with CO. In addition, Morin et al. measured the vibrational lifetime of CO molecules adsorbed on a Cu(100) surface and found it to be 2.0 ± 1.0 ps, in agreement with theory.9,36 For CO on NaCl(100), Chang et al. measured a lifetime of 4.3 ms, over 9 orders of magnitude higher.29 The lifetime is comparably long for a CO molecule in the gas phase.36 This enormous difference is explained by the possibility for the vibrational motion of the CO molecule to couple to the electron excitation on the Cu. In the gas phase, or on an insulating surface like NaCl with a large bandgap, there are no states for CO to couple with, and the CO molecule is left to stretch and compress, on average, 106 times before finally relaxing.

The ability to electronically activate a surface chemical reaction using a solid-state device is a concept with important implications for chemical processing and energy conversion and has been the goal of considerable research.10,218,220,221 A number of attempts have been made to achieve this goal using a metal−insulator− metal device as a tunnel junction.222−228 This type of device allows for the emission of hot electrons with tunable energy from one metal layer to the other. If the cathode layer of the device is sufficiently thin, then the emitted electrons may reach the surface before thermalizing. As discussed above, hot electron emitters based on MIM devices have demonstrated the ability to induce a chemical reaction involving a preadsorbed monolayer of reactants. However, another important goal is to electronically control a catalytic surface reaction operating at steady state. This was recently realized using a Pt/n-Si Schottky junction to reversibly control the rate of CO oxidation during steady-state kinetics.22 As described in section 4, the electronic structure of the metal−support interface has a dominant effect on the kinetics of the supported metal catalysts. Fabrication of a Pt/n-Si catalytic nanodiode allows the electronic structure at this interface to be reversibly tuned by applying an external bias or by generating photoexcited charge carriers, as shown in Figure 28a. The device was fabricated by depositing 4 nm Pt films onto n-type Si substrates. This formed an electronically continuous Pt layer. However, the film contained small cracks and holes resulting in a Pt/n-Si interface that was exposed to the gas-phase reactants. Ohmic contacts were made to the n-Si and to the Pt using Al and Au layers, respectively. Z


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Figure 29. (a) Effect of applied bias and (b) photoinduced current and reaction rate for CO oxidation on the Pt/n-Si catalytic nanodiode. (Reprinted with permission from ref 22. Copyright 2012, American Chemical Society.)

by CO desorption.230,231 CO covers the surface of Pt single crystals, preventing dissociative adsorption of O2. This is reflected in the reaction order being negative in CO and positive in O2. In this regime, the reaction rate would be increased by activating CO desorption. However, electron back-donation from Pt to the π* orbital of CO increases the Pt−C bond strength. This is because the π* orbital, although antibonding with respect to the CO bond, is bonding with respect to the Pt−C bond. Consequently, the red shift in the CO bond frequency observed during reverse bias of a Pt/TiO2 catalyst indicates a stronger Pt−C bond and would result in a rate decrease rather than a rate increase for a CO-poisoned surface. Based on this understanding, it is concluded that the biasinduced rate enhancement is better understood in light of an O activation mechanism, as has been suggested previously.24,94,100

In open circuit, the interface potential of the device represents a negative charge on the Pt catalyst and a positive charge on the Si support. The built-in potential of the interface is about 0.2 eV in open circuit. However, applying a bias across the device can increase the interface potential. On the other hand, illuminating the device with visible light can decrease the interface potential. During illumination, photoexcited charge carriers, generated in the Si layer, separate; the positively charged holes then migrate to the Pt surface and the negatively charge electrons migrate to the Si bulk. A band diagram of the device in open circuit and during reverse bias is shown in Figure 28b. This has the effect of offsetting the negative surface potential of the Pt catalysts and eliminating the interface potential. Both the applied bias and light have a reversible effect on the reaction kinetics of CO oxidation on the Pt/n-Si catalyst, as shown in Figure 29a,b, respectively. Increasing the interface potential via applied bias significantly increases the reaction rate at low temperature; decreasing the interface potential with light nearly stops the reaction. It appears that the negative charge on the Pt surface promotes the reaction, suggesting that a negatively charged intermediate or transition state is involved in the reaction pathway. This is consistent with early studies on metal− support interactions by Schwab98,100 and more recent studies using F-doped TiO2 as an electronically active catalyst support for oxidation reactions.24,25 The correlation of all these studies suggests that the interface potential plays a key role in the electronic activation of surface O. Vibrational spectroscopy has been performed on CO adsorbed on similar diodes to investigate the effect of interface potential on CO adsorption.229 These studies reveal that the bias-induced surface charge affects CO bonding. In this case, a reverse bias increases the interface potential, resulting in a reversible red shift in the CO stretching frequency. This is explained as the signature for increased back-donation from the negatively charged Pt catalyst to the π* orbital of the adsorbed CO molecule. Careful analysis of the spectra shows that this effect is strongest along cracks and holes in the Pt layer where the adsorbed CO is essentially bridging the Pt/TiO2 interface. In light of these spectral results, it may seem that the bias effect observed for CO oxidation on Pt/n-Si devices is a result of CO bond activation rather than O activation. However, kinetic experiments suggest otherwise. The Pt/n-Si catalyst shows kinetics that are positive order in CO and negative order in O2, suggesting that O activation is rate-limiting on this catalyst. This is different than Pt single crystals where CO oxidation is limited

6.4. Hot Electron Effect on Metal−Oxide Hybrid Nanocatalysts

Hybrid nanostructures, such as metal−semiconductors232−234 and semiconductor−semiconductors,235,236 have been used to uncover the role of the metal−semiconductor junction on heterogeneous catalysis. Controlling the interface between the metal and the semiconductor, which plays an important role in determining the type of barrier and in generating hot electron flows, is a key issue in synthesizing these nanostructures. Recently, Kim et al. demonstrated that the catalytic activity of the CO oxidation reaction on Pt−CdSe−Pt nanodumbbells and Pt nanoparticles on GaN substrates is influenced by light irradiation, which generates the flow of hot electrons across the metal−semiconductor junctions.237 Figure 30a shows schematic drawings (left) of the flow of hot electrons formed on a Pt−CdSe−Pt nanodumbbell under light irradiation. Electron−hole pairs can be generated on the semiconducting region upon absorption of photons, and the hot electrons can be injected onto the surface of the Pt nanoparticles and participate in the CO oxidation reaction, as illustrated in the energy band diagram (right). Figure 30b shows representative TEM images of the Pt−CdSe−Pt nanodumbbells, which show both-sided growth of Pt nanoparticles (average size 2.2 nm) onto the CdSe quantum rods (15 nm length and 4 nm diameter). CO oxidation on the Pt−CdSe−Pt nanodumbbells was carried out to identify the role of these hot electrons generated by light irradiation in chemical reactions. Figure 31a shows measurement of the catalytic reactivity of CO oxidation on the nanodumbbells with (+hν) and without light. During light irradiation, the AA


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Figure 30. (a) Illustration representing the flow of hot electrons generated on a Pt−CdSe−Pt nanodumbbell under light irradiation (left), and the energy band diagram between the CdSe semiconductor and the Pt metal tips (right). (b) TEM images (left) and high resolution TEM image (right) of the Pt−CdSe−Pt nanodumbbells. Pt tips at the rod edges and the CdSe lattice for the rod in the center can be identified, as marked. (Reprinted with permission from ref 237. Copyright 2013, American Chemical Society.)

Figure 31. (a) Turnover frequency of CO oxidation measured on Pt− CdSe−Pt nanodumbbells with (+hν) and without light at 270, 280, and 290 °C. (b) Catalytic activity of Pt−CdSe−Pt nanodumbbells without light, with light having low photon energy (1.0 eV < hν < 2.0 eV) and with light having high photon energy (2.0 eV < hν < 3.0 eV), measured at 280 °C. (Reprinted with permission from ref 237. Copyright 2013, American Chemical Society.)

nanodumbbells exhibit a higher catalytic activity than when in the dark, implying that hot electrons generated by light irradiation influence the CO oxidation reaction on the Pt−CdSe−Pt nanodumbbells. CO oxidation carried out on bare Pt nanoparticles did not show any change with light irradiation. The influence of photon energy on catalytic enhancement was studied by using two types of filter, a short-wave pass filter (2.0 eV < hν < 3.0 eV) and a long-wave pass filter (1.0 eV < hν < 2.0 eV). It was observed that the Pt−CdSe−Pt nanodumbbells irradiated by light with a higher photon energy (2.0 eV < hν < 3.0 eV) were two times more active for CO oxidation, compared with when irradiated by light with a lower photon energy (1.0 eV < hν < 2.0 eV), as shown in Figure 30b. In the case of a lower photon energy, the catalytic activity was very similar to CO oxidation in the dark. Therefore, this additional experiment shows that hot electrons are generated only when irradiated with light whose energy is higher than the bandgap energy (∼1.9 eV) of the CdSe rods. The type of hot carrier is a key factor that influences catalytic activity. Schafer et al. observed that the type of gallium nitride (GaN) doping affected the reactivity of the Pt nanoparticles. They suggest that this is due to the strong electronic interaction between the nanoparticles and the n- or p-type GaN substrates, which has a large influence on the chemical composition and oxygen affinity of the supported nanoparticles under X-ray irradiation.238 To elucidate the effect of the type of hot carrier in a catalytic chemical reaction, the change in catalytic activity was studied on colloidal Pt nanoparticles on p- and n-type GaN under light irradiation. Kim et al. carried out the CO oxidation reaction on Pt colloid nanoparticles on n- and p-doped GaN substrates with and without light to identify the role of hot carriers in the chemical reaction.237 Furthermore, capping-layer-free Pt nanoparticles

deposited with arc plasma deposition239,240 on the GaN substrate were also used as model catalysts to understand the role of capping layers. It is expected that recombination of holes from the n-type semiconductor under light irradiation with electrons from the nanoparticles takes place with the given energetic alignment illustrated in the energy band diagram (Figure 32a). Likewise, electrons can be transferred to the Pt nanoparticles with p-type supports. Such an interfacial charge transport process would lead to an increased net charge on the p-type GaN and a reduced net charge of the Pt nanoparticles on the n-type GaN. This change in net charge that depends on the type of GaN doping will influence catalytic activity under light irradiation. The catalytic activity of the Pt nanoparticles supported on GaN exhibited a clear change that was dependent on the type of doping. The catalytic activity of the Pt nanoparticles on the pdoped GaN wafer increased by 17−33% under light irradiation compared with no irradiation (i.e., in the dark), while the Pt nanoparticles on the n-doped GaN wafer decreased by 8−15% under light irradiation, compared with no irradiation, as shown in Figure 31b. The change in catalytic activity increased on the smaller Pt nanoparticles compared with that on larger nanoparticles. This result can be explained by the effect of the mean free path of the hot carriers. In other words, the injection of hot carriers on the Pt surface is more facile for the smaller nanoparticles. Also, the capping-layer-free Pt nanoparticles showed the same trend, with increased activity on the p-doped GaN wafer, suggesting that the observation is general. The change in catalytic effect influenced by hot electron flow is due to the electron−hole pair generated by the absorption of AB


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Figure 32. (a) Band diagram of Pt/GaN substrates showing how the direction of the hot carrier influences catalytic activity as the type of semiconductor doping changes. (b) Plot of percentage change of turnover rate on Pt nanoparticle/n-type GaN and Pt/p-type GaN for two different nanoparticle sizes (2.1 and 4.3 nm). (Reprinted with permission from ref 237. Copyright 2013, Royal Society of Chemistry.)

photons on CdSe or GaN, causing the ballistic transport of hot electrons to the Pt surface, which then affects the catalytic reaction. After accepting electrons from the semiconducting components (CdSe or GaN), the negatively charged CO2− or O2− transition states can be formed on the Pt surfaces. In the case of Pt−CdSe−Pt nanodumbbells, the hole generated in CdSe can move to the silicon substrate to complete the closed cycle. For Pt nanoparticles on GaN, the hole can move into the GaN bulk. Earlier theoretical studies, based on DFT calculations, showed that hot electron generation is mediated via a negatively charged CO2− transition state, showing the important role of negatively charged reaction intermediates.13 Activation of Pt−O bonds by hot electrons, resulting in their reaction with coadsorbed CO might be another plausible mechanism.94 Nevertheless, these results imply that hot electron generation on metal−semiconductor hybrid nanocatalysts is of significant importance for the reaction.

Figure 33. (a) Schemes for hot electron flow detection from photon absorption on a modified metal−semiconductor diode. (b) Incident photon-to-current conversion efficiency measured as a function of photons on a Au/TiO2 diode with connected Au island structures. The inset depicts the formation of the metallic nanoscale domains. (Reprinted with permission from ref 181. Copyright 2011, American Chemical Society.)

The steady-state current from the continuous flow of ballistic charge carriers was measured by absorption of photons in the visible range.180 Recently, Lee et al.181 proposed a scheme for enhanced light absorption with localized surface plasmon resonance and therefore for enhanced hot electron generation by utilizing Au/TiO2 Schottky diodes with Au island structures, as illustrated in Figure 33b. The nanometer-scale domains in the connected gold island structure were electrically connected to the ohmic pad, ensuring the presence of surface plasmons, and providing a means for measurement of the continuous flow of hot electrons. The photocurrent as a function of photon energy was measured to determine the correlation between surface plasmons and hot electron flows. An enhancement of photocurrent in internal photoemission was observed by changing the surface morphology,181,242 or depositing dye molecules243 or nanowires244 in such a way that the surface exhibits nanoscale domains with a strong localized surface plasmon resonance.

7. FUTURE PERSPECTIVES AND CONCLUDING REMARKS 7.1. Hot Electron-Based Solar Energy Conversion

The earlier concept of energy conversion from photon absorption to hot electron flow was suggested by McFarland and Tang.241 In this scheme, photons are effectively absorbed by dye molecules adsorbed on a Au/TiO2 Schottky diode, as shown in Figure 33a. Electrons are released from the dye molecules and injected into the conduction band of the metal layer upon absorption of light on the dye layer. These electrons need enough energy to travel over the Schottky barrier and into the TiO2 conduction band and are then collected as the current in the internal photoemission process.

7.2. Acid−Base Catalysis

Like SMSI, acid−base catalysis is often characterized by the selective enhancement of a single reaction pathway. This is because the generation of ionic reaction intermediates during AC


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acid−base catalysis determines the reaction rate and selectivity, and the charged intermediates are often highly active for a specific pathway.124,245,246 In the case of acid catalysis, charge is transferred to an organic molecule, usually by H+ addition or H− abstraction.247 Because of the active nature of the resulting carbocations, strong acids are catalysts for isomerization,248 alkylation,249,250 polymerization,251,252 and carbonylation reactions.253 The first superacids studied were anhydrous liquids. Addition of a strong Lewis acid, such as SbF5, to Bronsted acids, like H2SO4 or HFSO3, increases their acidic strength dramatically. This is because the SbF5 destabilizes the proton on the Bronsted acid by an electron-withdrawing interaction. The acidic strength of these mixed Lewis−Bronsted acids is sufficiently high to protonate hydrocarbons, and carbocations are stable in these acids and have been observed by infrared, Raman, nuclear magnetic resonance, and X-ray photoelectron spectroscopies.254 Because of the industrial advantages of heterogeneous catalysis, compared with homogeneous catalysis, preparation of solid superacids has become an important area of research.247 Several classes of solid superacids exist, including liquid acids mounted on a solid support,255 sulfate-activated metal oxides,256,257 mixed salts,258,259 and zeolites.260 Acid catalysis represents a unique chemistry that relies on charge transport via protons and molecular ions, and many similarities appear to exist between SMSI and acid−base catalysis. The evolution of hot electron work that began by measuring chemically induced charge flow at the oxide−metal interface20,21,211 is quickly leading to a molecular-level study of acid-catalyzed chemistry with a focus on understanding the close relationship between charge flow and catalytic selectivity. Specifically, it is clear that a fundamental understanding of catalyst selectivity is closely related to understanding the flow of charge between various active sites on a catalyst. This charge flow occurs both by the migration of molecular ions and by electrical conduction. Additionally, by controlling the flow of charge at a catalyst interface, novel reaction pathways will become accessible and these processes are central to the future of chemical energy conversion.

Figure 34. (a) Possible scheme of photocatalytic devices and (b) catalytic actuator based on hot electrons.

mediated by hot electrons may have benefits over fuel cells that rely on ion transport by liquid- or solid-phase diffusion. Incorporating nanostructures on the nanodiode using nanolithography262,263 or self-assembly techniques264 would permit us to make electrical contacts to nanowires on the oxide surface, and maximize the hot electron flow due to the larger area of the metal−oxide interfaces. The energy conversion efficiency can be altered, for example, by tailoring the diode materials or the nature of the oxide or other wide bandgap semiconductors, such as SiC, ZnO, or doped diamond. A catalytic actuator with configurations of source, drain, and gate (Figure 34b) may bring about new possibilities for electronic control of catalytic activity via changes of the electric field in the pathway of the hot electrons. 7.4. Conclusion

In this review, we discussed recent studies of the role of hot electron flows in surface chemistry and heterogeneous catalysis. We reviewed the various energy dissipation mechanisms and detection schemes for hot electrons. We discussed SMSI effects for various catalytic systems to address the role of the metal− oxide interface in heterogeneous catalysis. Hot electron flows generated with a catalytic nanodiode provide insights into the role of electron excitation leading to energy conversion processes. We found that hot electron flux is well correlated with the turnover rates of catalytic reactions, which suggests possible applications for chemical sensors and novel energy conversion. We showed that hot electrons could affect heterogeneous catalysis for various catalytic systems ranging from catalytic nanodiodes to metal−semiconductor hybrid nanocatalysts. We showed that the hot electron is the main mediator of energy conversion between chemical, photon, and electrical energies, suggesting a new energy conversion mechanism based on hot electrons. Together, these findings demonstrate the prevalence of surface ion chemistry across a wide range of heterogeneous reactions and catalyst systems and show that charge transfer and ion chemistry represent one of the fundamental mechanisms of chemical activation on surfaces. In real catalyst systems, tandem reactions occur between the products of covalent and ionic activation pathways. This review focused on the major advances of the past decade that provide understanding of electronic activation and

7.3. Beyond Catalytic Nanodiodes

For the application of catalytic nanodiodes as chemical sensors or energy conversion devices, a crucial challenge is the cost of device nanofabrication. Currently, the cost of nanodiode fabrication is approximated as several hundreds of dollars for a six-inch wafer of diodes, which is mainly limited by the thin film deposition of noble metals. Obviously, preparation of these devices is more costly than preparation of standard metal−supported catalysts. One approach to overcome this issue is to make diodes out of porous materials,261 which may increase the surface area of the metal catalysts and, thus, chemical reactivity. Nevertheless, catalytic nanodiodes permit us to measure hot electron flow, which is important for understanding the basic mechanisms of energy dissipation at surfaces. Hot electrons on a metallic surface can be created by external energy deposition in the form of photons, ions, electrons, and chemical reactions. Therefore, we can consider hot electrons to be a major mediator for general energy conversion. The scheme of energy conversion from chemical energy (catalytic reactions) and photon energy to electrical energy (hot electron current) may give insight into other types of energy sources, including solar cell and photocatalytic devices, as illustrated in Figure 34a. Using direct conversion of solar energy to chemical energy AD


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charge flow by hot electrons during surface catalytic reactions. This new understanding of the ionic aspect of surface chemical processes, in addition to the already understood covalent mechanisms of surface activation, represents the basis for the design of new catalyst systems for high selectivity and energy conversion applications of the future.

2008. He received his Ph.D. from Gabor Somorjai at the University of California, Berkeley in 2012 where his dissertation research focused on electronic activation of surface chemical reactions at the metal−support interface. From 2012 to 2014 he performed postdoctoral research with Stephen Leone studying ultrafast electron dynamics in metal oxides using transient X-ray absorption spectroscopy.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies

Gabor A. Somorjai is a Professor of Chemistry at the University of California, Berkeley and University Professor of the University of California System. He was born in Budapest, Hungary in 1935, and received his Ph.D. degree in Chemistry from the University of California, Berkeley in 1960. Professor Somorjai is the recipient of numerous awards, including the BBVA Foundation Frontiers of Knowledge Award in Basic Sciences (2011), the ENI New Frontiers of Hydrocarbons Prize (2011), the Priestley Medal (2008), the Langmuir Prize (2007), the National Medal of Science (2002), and the Wolf Prize (1998). He has educated more than 130 Ph.D. students and 250 postdoctoral fellows. He has written three textbooks and is the author of more than 1200 scientific papers in the fields of surface chemistry, heterogeneous catalysis, and solid-state chemistry.

Jeong Young Park is currently a group leader of the Center for Nanomaterials and Chemical Reactions, Institute for Basic Science, and an associate professor at the Graduate School of EEWS, Korea Advanced Institute of Science and Technology, Republic of Korea. He received his B.S. and Ph.D. from the Department of Physics, Seoul National University in 1993 and 1999, respectively. After spending three years at the University of Maryland as a postdoctoral associate, he joined Lawrence Berkeley National Laboratory where he worked as a staff scientist. His research focuses on surface phenomena covering energy dissipation and conversion at surfaces, nanotribology, catalysis, and hot electron transport through metal−oxide interfaces. He has published 120 peer-reviewed articles and book chapters in the fields of surface science and catalysis.

ACKNOWLEDGMENTS This work was supported by IBS-R004-G4, Republic of Korea, and by the Director, Office of Science, Office of Basic Energy Sciences, Division of Chemical Science, Geological and Biosciences of the U.S. Department of Energy under Contract DE-AC02-05CH11231. REFERENCES (1) Greber, T. Surf. Sci. Rep. 1997, 28, 1−64. (2) Mantell, D. A.; Ryali, S. B.; Halpern, B. L.; Haller, G. L.; Fenn, J. B. Chem. Phys. Lett. 1981, 81, 185−187. (3) Haber, F.; Just, G. Ann. Phys. 1909, 30, 411. (4) Haber, F.; Just, G. Ann. Phys. 1911, 30, 308. (5) Kasemo, B. Phys. Rev. Lett. 1974, 32, 1114−1117. (6) Kasemo, B.; Tornqvist, E.; Norskov, J. K.; Lundqvist, B. I. Surf. Sci. 1979, 89, 554−565. (7) Kasemo, B.; Wallden, L. Surf. Sci. 1975, 53, 393−407. (8) Norskov, J. K.; Newns, D. M.; Lundqvist, B. I. Surf. Sci. 1979, 80, 179−188. (9) Majorana, E. Nuovo Cimento 1932, 9, 43−50. (10) Gergen, B.; Nienhaus, H.; Weinberg, W. H.; McFarland, E. W. Science 2001, 294, 2521−2523. (11) Nienhaus, H. Surf. Sci. Rep. 2002, 45, 3−78. (12) Tully, J. C. Annu. Rev. Phys. Chem. 2000, 51, 153−178. (13) Lee, H.; Nedrygailov, I. I.; Lee, C.; Somorjai, G. A.; Park, J. Y. Angew. Chem., Int. Ed. 2015, 54, 2340−2344. (14) Kim, S. M.; Lee, H.; Park, J. Y. Catal. Lett. 2015, 145, 299−308. (15) Shenvi, N.; Cheng, H. Z.; Tully, J. C. Phys. Rev. A 2006, 74, 10. (16) Somorjai, G. A.; Park, J. Y. Catal. Lett. 2007, 115, 87−98.

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