Direct photocatalysis for organic synthesis by using plasmonic-metal nanoparticles irradiated with visible light.

FOCUS REVIEW DOI: 10.1002/asia.201402310

Direct Photocatalysis for Organic Synthesis by Using Plasmonic-Metal Nanoparticles Irradiated with Visible Light Qi Xiao, Esa Jaatinen, and Huaiyong Zhu*[a]

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Abstract: Recent advances in direct-use plasmonic-metal nanoparticles (NPs) as photocatalysts to drive organic synthesis reactions under visible-light irradiation have attracted great interest. Plasmonic-metal NPs are characterized by their strong interaction with visible light through excitation of the localized surface plasmon resonance (LSPR). Herein, we review recent developments in direct photocatalysis using plasmonic-metal NPs and their applications. We focus on the role played by the LSPR of the metal NPs in catalyzing organic transformations and, more broadly, the role that light irradiation plays in catalyzing the reactions. Through this, the reaction mechanisms that these light-excited energetic electrons promote will be highlighted. This review will be of particular interest to researchers who are designing and fabricating new plasmonic-metal NP photocatalysts by identifying important reaction mechanisms that occur through light irradiation. Keywords: nanoparticles · organic synthesis · photochemistry · surface plasmon resonance · visible light

Introduction

mately 52 % infrared (l > 800 nm) in addition to the UV component. Given that visible and infrared light constitute most of the solar emission,[8] one of the more interesting photocatalysis challenges is to devise new photocatalysts that better utilize solar energy but still exhibit high activity and selectivity when illuminated by sunlight at ambient conditions. Several recent discoveries have demonstrated that it is possible to drive highly selective photocatalytic synthetic reactions with visible light when suitable photocatalysts are used under mild reaction conditions. Albini and MacMillan et al. reported recent comprehensive reviews of homogeneous photocatalysts for a series of organic transformations under visible-light irradiation,[1, 9] and some studies of organic transformations through heterogeneous visible light photocatalysis have been reported by Zhao et al.[2] Therefore, in this review, these topics will not be covered here and readers are referred to those reviews for an in-depth understanding of those systems and processes. Plasmonic-metal nanoparticles (NPs) are recognized as a new form of medium that is particularly efficient in harvesting light energy for chemical processes due to their strong light absorption over a wide range of the visible and UV regions of the solar spectrum.[10–13] A characteristic of plasmonic-metallic nanostructures is their strong interaction with resonant incident photons through excitation of the localized surface plasmon resonance (LSPR). The LSPR is the photon-induced collective oscillation of conduction electrons, established when the light frequency is resonant with the natural oscillation frequency of metals free electrons in response to the restoring force of the positive nuclei. In turn, this charge oscillation creates an intense electromagnetic field concentrated in close proximity (< 100 nm) to the metal nanostructures surface.[12] It is this intense localized field associated with the plasmonic-metal nanostructures that has resulted in their use in many diverse applications, such as solar cells,[11] surface-enhanced Raman spectroscopy,[14, 15] molecular sensing in biological systems,[16] singlemolecule spectroscopy,[15, 17, 18] and many others.[19] Because the plasmonic and optical properties of the metal nanostructures are strongly affected by the size and geometry of the

One of the challenges in chemistry is to develop pollutionfree technologies and chemical transformations that are driven by clean renewable energy sources, to limit environmental impact. Sunlight stands out as the most promising choice to meet our energy demand through its abundant, clean, and renewable features and shows great potential for driving environmentally benign chemical transformations.[1] The use of light irradiation would shift the thermodynamic emphasis from performing high-temperature chemical reactions to instead favoring chemical synthesis at ambient temperatures, which thus avoids unwanted byproducts formed at higher temperatures.[2] During the last decade, the field of heterogeneous photocatalysis has expanded rapidly due to various developments, particularly in the design and realization of new photocatalysts and their applications.[2, 3] However, organic synthesis through photocatalytic reactions that use heterogeneous photocatalysts irradiated with visible light have received much less attention. In addition, traditional semiconductor photocatalysts capable of efficient pollutant photodegradation and water splitting usually have high oxidation potentials.[4–7] This limits their application for the catalytic synthesis of fine chemicals, for which partially oxidized products are often preferred. Furthermore, because ultraviolet (UV) light is usually required to drive reactions that use semiconductor photocatalysts, the UV radiation can activate chemical bonds in many molecules and have undesirable consequences. The involvement of free-radical intermediates in UV-induced chemical reactions results in unsatisfactory selectivity for the desired product. In terms of available energy, UV light (l = 200–400 nm) accounts for less that 5 % of the total solar energy. Sunlight consists of approximately 43 % visible (l = 400–800 nm), and approxi[a] Q. Xiao, Prof. Dr. E. Jaatinen, Prof. Dr. H. Zhu School of Chemistry, Physics and Mechanical Engineering Science and Engineering Faculty Queensland University of Technology 2 George St. Brisbane, QLD 4001 (Australia) Fax: (+ 61) 07-3138-1804 E-mail: [email protected]

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structure, much research is devoted to these attributes.[20, 21] For example, the vivid colors observed when colloidal solutions of spherical metal nanocrystals are illuminated is dependent on their diameter because this determines the plasmon resonant wavelength at which light is strongly absorbed or scattered.[12] Initially, plasmonic-metal NPs were utilized exclusively to improve the performance of semiconductor photocatalysts, in which the strong absorption that results from the LSPR is used to transfer photon energy to nearby semiconductors or molecular photocatalysts to drive reactions.[22] Readers interested in detailed discussions on these plasmonic-metal/semiconductor composite photocatalytic processes mediated by LSPR excitation should refer to specific reviews on that topic.[23, 24] From these origins, a recent conceptual breakthrough showed that the role of the plasmonic NP in the photocatalytic process was much more than simply a benign energy harvester. Studies on light-driven reactions catalyzed by gold (Au) or silver (Ag) NPs supported on photocatalytically inactive hosts (insulating solids with very wide bandgaps) demonstrated that these materials are efficient photocatalysts that function through very different mechanisms from those that occur in semiconductor photocatalysts.[25] A recent summary of these Au and Ag NPs photocatalysts irradiated with UV or visible light demonstrated that these noble-metal NPs act directly as visible-light photocatalysts,[26] and that in these systems no electron transfer between the NPs and support material was observed. In this case, because the metallic NPs serve as both a light absorber and host to the catalytic sites, we can use many functional materials to support them instead of the semiconductors that were thought vital for photocatalysis. Thus, porous materials, carbon materials, and even polymers, could be used to produce more optimal and superior photocatalysts for chemical synthesis. Coupling the light-harvesting and catalysis functionalities to the metal NPs themselves opens up a significant new direction in catalysis.[25–28] In this Focus Review, we provide a comprehensive introduction to direct plasmonic-metal photocatalysis for organic synthesis reactions. To provide a contextual backdrop for this review, we begin with a brief introduction of plasmonicmetal NPs and focus on the unique characteristic features of the LSPR effect in the plasmonic metals. This will be followed by a summary of recent applications of plasmonicmetal NP photocatalysts in organic transformations under visible-light irradiation, with the mechanisms behind these observations briefly discussed. Finally, we extend the recent advances in single plasmonic-metal photocatalysts to bimet-

allic (alloy) photocatalyst systems and highlight the mechanisms of direct light induced energetic electron transfer from the metal NP surface to the adsorbed reactant molecules. Throughout the review we attempt to identify the critical design parameters that need to be considered for optimal photocatalytic performance with plasmonic-metal NPs and determine plausible photocatalytic reaction pathways when they are irradiated with light. An outlook for the future of this exciting new field will be provided at the end of this review. It should be noted that there have already been many excellent review articles that summarize the research on plasmonic-metal catalysts, including, but not limited to, their fabrication and characterization, plasmonic properties, and photocatalytic applications.[12, 26, 29–31] Therefore, we will mainly highlight the very recent research progress in organic synthesis by using direct plasmonic-metal NP

Qi Xiao received his BS in light–chemical engineering from the Nanjing University of Technology in 2008, at which he also completed his MSc in Organic chemistry in 2011. In August 2011, he became a full-time PhD student under the guidance of Professor Huaiyong Zhu at the Queensland University of Technology studying materials chemistry. His research interests focus on exploring novel nanoparticle photocatalysts and using visible light to drive the synthesis of fine organic chemicals. Esa Jaatinen received his PhD in Physics from the Australian National University in 1994. From 1994 to 2002, he worked as a senior research scientist at Australias National Measurement Laboratory where he investigated light–matter interactions and applied them to metrology. He left there in 2002 as project leader to take up a lectureship at the Queensland University of Technology, and was promoted to senior lecturer in 2009 and to associate professor in 2012. His research interests include linear and nonlinear optical interactions in nanostructures and the application of spectroscopy for sensors. Huaiyong Zhu received his PhD in Chemistry from the University of Antwerp in 1994. He worked as a research associate at Hiroshima University, then moved to the University of Queensland in 1996 and was granted a Queen Elizabeth II research fellowship (2000–2004). He joined the University of Sydney as a Lecturer in 2002, and the Queensland University of Technology as an Associate Professor in 2005, where he became a full Professor in 2009. His current research interests include new visible-light photocatalysts for organic synthesis reactions and advanced adsorbents and filtration membranes of ceramic nanofibers.

Abstract in Chinese:

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photocatalysts, although some related work before will also be mentioned to aid coherence and completeness.

1. Plasmonic-Metal Nanoparticles 1.1. The Role of the LSPR Effect A localized surface plasmon is an optical phenomena that occurs when light is incident on a conductive NP that is smaller than the wavelength of incident light, which produces a strong interaction between the incident electric field and the free conduction electrons of the metal NPs (Figure 1). At resonance, when the frequency of the free-

Figure 1. Localized surface plasmons (LSPs) of a metal nanosphere.[34]

electron oscillation is the same as that of the incident light, constructive interference results in the strongest oscillation and localized field strength. This interaction produces coherent localized plasmon oscillations with a resonant frequency that can be tuned by varying the NP size, shape, material, and proximity to other nanoparticles. As the shape and/or size of the NP changes, the intensity of the electromagnetic field produced by the oscillating charges at the NP surface also changes, which leads to not only a shift in the oscillation frequency of the conduction electrons but also to strong enhancement of the local electromagnetic field near the surface of noble metal NPs.[32, 33] In addition, the frequency and strength of the plasmon resonance also depend on the intrinsic dielectric properties of the metal and the surrounding medium and the surface polarization, which depends on particle size and shape.[34] This makes it possible to tailor the LSPR of metal NPs simply by synthesizing the desired NP size and shape. As shown in Figure 2, the calculated LSPR spectra of various Ag NPs suspended in water is significantly influenced by particle geometry. There are several generic guiding principles that apply in tuning the LSPR absorption of the plasmonic NPs through manipulating particle size/shape. First, dipole resonance peaks redshift with increasing corner sharpness and particle anisotropy. Second, the LSPR peak intensity increases with particle symmetry. In addition, the number of LSPR peaks observed depends on the number of unique polarization modes possessed by the nanostructure.[34] The LSPR absorbance spectra of Au, Ag, and Cu spherical NPs with diameters of 20 nm exhibit maxima at l 530, 400, and 580 nm, respectively (Figure 3), which are all within the visible range of light. The interaction of resonant

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Figure 2. Calculated UV/Vis extinction (black), absorption (red), and scattering spectra (blue) of Ag nanocrystals, which illustrates the effect of NP shape on spectral characteristics: a) sphere, b) cube, c) tetrahedron, d) octahedron, e) triangular plate, and f) rectangular bars with aspect ratios of 2 (gray), 3 (red), and 4 (blue).[34]

Figure 3. Surface plasmon absorption bands for Au, Ag, and Cu NPs.[39]

photons and surface conduction electrons initially results in the coherent oscillation of electrons that can persist for nearly a picosecond, which produces a corresponding intense oscillating electric field near the NP surface with a high concentration of energetic electrons.[35–37] The combination of energized surface electrons, intense local electric fields, and the known catalytic properties of NPs suggest that plasmonic NPs are highly suited for direct photocatalysis.[38] It has been reported that the localized electric fields near the NP surfaces that are produced through light absorption,

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and the corresponding high concentration of energetic surface electrons, can induce chemical transformations.[25, 40–42] Strong evidence that the LSPR plays an important role in the enhancement of photocatalytic reaction rates was found when reaction rates photocatalyzed by plasmonic metals irradiated with light of different wavelength and intensity were compared. As shown in Figure 4, the correlation be-

which is observed in metal/semiconductor photocatalysts, is not required for catalysis to occur. As recently summarized by Christopher et al.,[44] three processes can transfer light energy into the adsorbed reactants in direct plasmonic-metal photocatalysis: 1) elastic radiative re-emission of photons, 2) non-radiative Landau damping, which results in the excitation of energetic electrons and holes in the metal particle, and 3) the interaction of excited surface plasmons with unpopulated adsorbate acceptor states, inducing direct electron injection into the adsorbate, called chemical interface damping (CID; Figure 5).

Figure 4. a) Wavelength- and b) intensity-dependent photoactivity.[28]

tween the measured photocatalytic rate and the calculated plasmon absorption intensity indicates that the photocatalysis was plasmonically enhanced. Furthermore, the observed linear dependence of the photoinduced rate on the source intensity is a signature of an electron-driven chemical process, which is apparently different from a traditional thermal heating process.[28] The dependence of photocatalytic activity on light intensity and wavelength indicate that the LSPR plays a significant role in the observed photoactivity.

Figure 5. Schematic showing the three dephasing mechanisms of oscillating surface plasmons.[44]

The magnitude of the field enhancement, the resonant wavelength, and the proportion of plasmon excitations that decay through processes 1)–3) depend on nanostructure geometry, composition, and local environment.[35, 36] Despite the different underlying mechanisms, all three plasmon decay processes can transfer light energy into the adsorbates. Here we will mainly discuss energy transfer from surface plasmons to adsorbates and will describe how this can result in photocatalytic reactions directly. Two factors that will be discussed that impact on the photocatalytic activity of the plasmonic-metal NPs are the enhanced electromagnetic field of neighboring NPs and the direct interaction of the LSPR excited energetic electrons with adsorbates.

1.2. Direct Photocatalysis on Plasmonic-Metal NPs We will focus on how these plasmonic-metals NPs act simultaneously as light absorbers and catalytic sites when irradiated with visible light. Direct plasmonic-metal photocatalysis was initially regarded as improbable due to the short lifetimes of plasmon-derived charge carriers and extremely fast quenching of electronically excited adsorbates on metal surfaces.[43] However, this view is somewhat challenged because it is known that high-energy electrons can induce chemical reactions and that excited adsorbates on metal surfaces are important intermediates in heat-driven thermal catalysis processes. The role the energetic surface electrons play in the catalytic process hinges on their energy but not on the source of the energy. Thus, there is no reason why the electrons of NPs excited by light cannot induce reactions. The first experimental observation of direct photocatalysis with plasmonic-metal NPs was reported in 2008.[25] Since that time, direct plasmon-driven photocatalysis has been demonstrated in numerous systems, which is producing an understanding of the underlying mechanisms that result in very efficient utilization of photons to overcome activation barriers. A distinct feature of these direct plasmonic photocatalysis systems is that both light absorption and activation of the reactants take place on the NPs. This means that charge transfer between the plasmonic metal and solid support,

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1.2.1. Local Electric Field Enhancement Irradiating metal NPs near their plasmon resonance frequency produces an oscillating electric dipole, which generates an intense local electric field that is confined to near the NP surface. If two or more NPs are in close proximity, they can couple to produce a local plasmonic field that is greater than that produced by one NP in isolation. Finitedifference time-domain (FDTD) simulations have shown that the electric field intensity of local plasmonic hot spots can reach as much as 1000 times that of the incident electric field.[45, 46] In these hot-spot regions, the electron–hole pair generation rate is 1000 times greater than that achieved by the incident electromagnetic field.[45] Linics group also

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interband absorption.[54] These energetic electrons at the Au NP surface remain in an excited hot state for up to 0.5 to 1 ps,[36] and, therefore, can interact directly with reactant molecules to activate them for chemical reactions.[25, 27, 41, 54] Given that there are energetic electrons distributed across the surface of the NPs, the energy of the incoming photons is predominantly transferred to the NP surface. This allows the energetic electrons to interact directly with adsorbed molecules and activate them for reactions (Figure 7). The

Figure 6. a) Average electric field enhancement around an Ag cube with an edge length of 120 nm as a function of the distance (d) from the cube, as calculated by using FDTD simulations. Inset: Local enhancement of the electric field calculated from an FDTD simulation of a 120 nm Ag cube in water.[47] b) An illustration of a hot spot for a NP dimer and the rapid change in SERS enhancement factors with respect to relative position.[53]

showed the FDTD-calculated field enhancements around a 120 nm Ag cube in water falls off quickly as the distance (d) from the cube increases (Figure 6a).[47] Such findings are of particular interest in surface-enhanced Raman spectroscopy (SERS) because an enhanced Raman signal will occur for molecules adsorbed to the surface of NPs or in the vicinity of hot spots, such as spaces between NPs (Figure 6b) or at the sharp edges or tips of individual NPs. Due to this enhanced field, these hot-spot regions will have high concentrations of energetic electrons and thus higher rates of direct plasmonic photocatalysis are expected at these sites. However, to date, the experimental observations to confirm this are limited. The light-induced surface plasmons will ultimately decay and produce energetic charge carriers in the NPs.[48, 49] These energetic carriers can in turn be transferred to the surroundings[50] or relax by heating the nanostructure.[51, 52] The enhanced electric field itself may also accelerate the transfer of charge carriers from the NP. Here, both the plasmonic local heating and the plasmon-assisted transfer of energetic electrons into the adsorbates can facilitate chemical reactions. Furthermore, the interaction of a reactant molecule with a NP surface may lead to perturbations in electronic structure and produce a change in polarizability, which may also facilitate the chemical reactions.[53]

Figure 7. a) The conduction electrons of Au NPs (such as 6sp electrons) exist at the NP surface and a small fraction of the conduction electrons distribute in the energy levels above the Fermi level at ambient temperature. b) Au NPs strongly absorb the visible light mainly due to the LSPR effect and interband electron transitions, by which the conduction electrons gain the energy of light irradiation (more conduction electrons distribute to high energy levels). It follows that the light flux to the Au NPs predominantly results in a surface (indicated in red) with high-energy electrons, which is desirable for activating molecules on the NPs for chemical reactions.[26]

obvious advantage of such a process is that the incident light energy is economically utilized because it is efficiently channeled into the reactant molecules, with only a small fraction dispersed to other components of the reaction system, such as the container, solvent, and the material that supports the NPs. In the light-excited energetic electron transfer process, initially, the addition of adsorbates to the surface of plasmonic NPs can induce a new, ultrafast dephasing pathway that occurs on the scale of about 5 fs through the so-called CID.[44] The energetic electrons generated by plasmon excitation are injected directly into an available molecular orbital, which is usually the lowest unoccupied molecular orbital (LUMO), to form transient anions (Figure 8).[12, 28, 55] The adsorbates here may experience a large change in electron

1.2.2. Light-Excited Energetic Electron Transfer The conduction electrons of the plasmonic-metal NPs gain the incident light energy through either LSPR absorption or

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a simple substrate; it can provide large surfaces to allow metal NPs to be adequately dispersed and enable the recovery and recycling of the catalysts. For example, graphene can stabilize Cu NPs but oxide supports cannot.[59] The lightexcited metal electrons can migrate to the support if the conduction band energy of a semiconductor support is sufficiently low. Thus, the choice of support material may change the reaction pathway. Mesoporosity of the support may affect product selectivity due to steric restriction. Metal oxides also possess surface acid–base properties, which can facilitate the formation of products for heterogeneous reaction processes. To comprehensively understand the functionality of a plasmonic photocatalyst, both the properties of plasmonic NPs and the support should be considered.

Figure 8. Proposed mechanism of direct charge injection from metal to molecular states that occur during plasmon dephasing in the CID mechanism.[30, 44]

2. Plasmonic Photocatalytic Reactions

energy distribution depending on whether the energy transfer occurs instantly or some time after dephasing.[44] Brus noted that the injection of energetic electrons into the LUMO of the adsorbate molecules results in the weakening and stretching of their original chemical bond, and thus lowers the energy of the formed transient anions.[14] The subsequent decay of the transient anions and the transfer of electrons back into the metal NP deposits energy into the molecular vibrational modes.[37] The average time for molecular vibrational energy to dissipate on metal NP surfaces is of the order of picoseconds, which is long enough to allow for typical chemical transformations.[56] If the deposited energy is sufficient to overcome the activation barrier required for the dissociation of the adsorbate molecules, chemical transformations can then occur.[12, 28] In addition, electron transfer in the opposite direction can also occur if a hot hole in the metal NPs attacks an occupied molecular orbital.[37, 57, 58] Finally, this electronic energy will dissipate in the form of heat, which may also enhance the rate of photocatalysis.

2.1. Au-NP-Catalyzed Reactions Direct photocatalysis by Au NPs through LSPR light absorption that was independent of any other system component (such as TiO2 or Ag halide) was first observed when the volatile organic compound (VOC) HCHO was photocatalytically degraded by irradiation with visible light at room temperature.[25] Soon after, it was recognized that the application of Au NP photocatalysts is not limited to the degradation of organic pollutants. This opened the prospect of driving catalytic chemical transformations with sunlight, an environmentally friendly alternative compared with the traditional thermally driven heterogeneous catalysis. However, such a possibility is not without its challenges. Intensive light absorption is required to drive chemical synthesis because sunlight has a relatively low energy density at the earths surface ( 0.1 W cm 2). Most known semiconductor photocatalysts have insufficient visible-light absorption to meet this requirement. However, the absorption cross-section area of the plasmonic NPs, which is indicative of the absorbing efficiency of a particle, can be much larger than their geometrical area.[60] This LSPR-enhanced light absorption efficiency is what allows plasmonic-metal NPs to absorb visible sunlight with high efficiency. Thus, it is a logical extension to hypothesize that Au NPs are likely to be efficient at driving a variety of organic synthesis reactions with visible light. Recently, we applied Au NPs supported on ZrO2 (Au@ZrO2) to catalyze reductive coupling of nitro-aromatic compounds to produce azo compounds under both visible and UV irradiation, and found high photocatalytic activity at ambient temperatures (Figure 9).[41] The Au@ZrO2 cata-

1.2.3. The Effect of the Support Although free-standing plasmonic NPs without support are also capable of catalyzing organic transformations under visible-light irradiation, they are not stable under typical photocatalytic conditions.[2] Therefore, for realistic applications most plasmonic photocatalysts are supported on a solid inert support. This makes the overall structure of these direct plasmonic photocatalysts appear quite similar to semiconductor photocatalysts modified with metal NPs, in which plasmonic-metal NPs are incorporated to improve the photocatalytic performance of the semiconductor. However, although similar in appearance, the sites for photocatalysis are different, which means that the underlying mechanisms are not the same. For example, electron transfer between the NP and the support is not a prerequisite for direct plasmonic-metal photocatalysts. Because the metallic NPs serve as both light absorber and catalytic sites, many functional materials can be used as supports, such as porous materials, carbon materials, and even polymers. The support material can further assist functionality and go beyond being

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Figure 9. Photocatalytic selective reduction of nitroaromatic compounds to produce azo compounds by using Au NPs under visible-light irradiation.

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lyst was prepared by reduction deposition of HAuCl4 on ZrO2 with NaBH4 in the presence of lysine in H2O. The resulting Au NPs were about 6 nm in size. In the reductive coupling process, the surface hydrogen species (H Au NP) is formed by the abstraction of a hydrogen atom of Au NP from the solvent isopropyl alcohol. As shown in Figure 10,

Figure 11. Photocatalytic deoxygenation of epoxides to alkenes, reduction of ketones to alcohols, and hydrogenation of azobenzene to hydroazobenzene with Au NPs under visible-light irradiation.

kenes, reduction of ketones to alcohols, and hydrogenation of azobenzene to hydroazobenzene (Figure 11).[62] Au NPs on CeO2 were found to be an effective visible-light photocatalyst for these reductions. A significant finding of this work was the ability to control the reduction power through a choice of irradiation wavelength. For example, all three reactions occurred when irradiated with visible light (l = 400– 800 nm). When light with wavelengths longer than l = 550 nm was used (by filtering out wavelengths shorter than l = 550 nm) the reduction of styrene oxide did not take place. However, the reduction of azobenzene did still occur, albeit with a lower conversion yield (from 40 % to 22 %). Similarly, the hydrogenation of acetophenone was also still observed although the conversion decreased from 31 to 9 %. When the same systems were irradiated with light with wavelengths of l = 600 nm or greater, the reduction of azobenzene was still observed (20 % conversion), but acetophenone and styrene oxide could not be reduced. These observations are a direct consequence of the light wavelength determining the amount of energy received by the conduction electrons. The shorter the wavelength, the higher the energy of the photoexcited electrons and, therefore, the greater the proportion of electrons in higher energy states. From this finding, it is apparent that the wavelength used to irradiate the Au NPs is an important consideration with regards to the overall catalytic ability. In addition to reduction reactions, Au NPs can also act as efficient photocatalysts for various oxidation reactions under visible-light irradiation. Zheng et al. developed a facile in situ method for the preparation of noble-metal photocatalysts M@TiO2 (M = Au, Pt, Ag), and used them for the oxidation of benzene to phenol in aqueous phenol.[63] Au NPs exhibit the best performance of the three photocatalysts, and the electron-depleted Au oxidizes phenoxy anions to form phenoxy radicals that oxidize benzene to phenol.[63] Tanaka et al. examined the oxidation of benzyl alcohols in aqueous and aerated suspensions of Au supported on CeO2 when irradiated with green LED light (l = 530 nm; Figure 12).[64] A linear correlation was observed between the external surface area of Au NPs loaded on CeO2 and the rate of photocatalytic benzaldehyde formation, which indicated that the external surface area of Au NPs on CeO2 rather than the amount of Au loaded has greater influence on the photocatalytic activity of Au@CeO2 under visible-

Figure 10. Mechanism for the photocatalytic reduction of nitroaromatic compounds. The H Au NP reacts with N O bonds to produce a HO Au NP species, which subsequently decomposes to produce oxygen molecules and H Au NP species.[41]

these H Au NP species can combine with the oxygen atoms of N O bonds to give HO Au NP species. The electrons that are excited when the Au NPs absorb light can provide the activation energy that is required for the cleavage of the N O bond. The HO Au NP species can then release O2 and transform into the H Au NP species and the subsequent reaction process can be recycled. It was quite a remarkable finding that in addition to the aromatic azo compounds and acetone, O2 was concomitantly produced as the co-product in the photocatalytic process. This may serve as a model system to deepen our understanding of the oxidation of water to O2 during artificial photosynthesis driven by plasmonic photocatalysts. This process is viewed as the only example of azobenzene synthesis by direct reductive coupling of nitroaromatic compounds under mild reaction conditions.[61] Unlike the conditions required to thermally catalyze the reactions, the photocatalytic reduction process with supported Au NP photocatalysts requires much lower temperatures and pressures. These much more moderate environmental conditions make it possible to isolate a metastable intermediate as the desired product. Thus not only does this new class photocatalysts enable a cleaner sustainable use of solar energy for the catalytic production of fine chemicals, but the moderate experimental conditions required for these processes allow for products to be created that cannot be obtained with conventional thermal catalysts. In an attempt to identify common features of visible-lightdriven reduction processes with supported Au NP photocatalysts, we further investigated three light-driven reductions under ambient conditions: deoxygenation of epoxides to al-

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sponding carbonyl compounds with H2O2 under l = 530 nm LED irradiation.[67] Colloidal Au NPs have been successfully employed in the plasmon-mediated oxidation of both secphenethyl and benzyl alcohol through irradiation with either l = 532 nm laser or l = 530 nm LED radiation (Figure 15). A Figure 12. Photocatalytic selective oxidation of alcohol with Au@CeO2 under l = 530 nm green LED light irradiation.[64]

light irradiation.[64] Increasing the average Au particle size to > 30 nm results in a very active visible-light plasmonic photocatalyst. Multistep photodeposition methods can be used to create larger Au NPs in Au@CeO2 composites than those obtained with a single-step method.[65] Photocatalytic oxidation of benzyl alcohol with an amino group revealed that the Au@CeO2 photocatalyst exhibited high chemoselectivity toward the hydroxyl group of the alcohol. For example, the Au@CeO2 photocatalyst can almost quantitatively convert 4-aminobenzyl alcohol to 4-aminobenzaldehyde with a 99 % yield and without any oxidation of the amino group (Figure 13). Maldotti et al. proposed a pathway for

Figure 15. Photocatalytic oxidation of benzylic alcohols by using colloidal Au NPs in the presence of H2O2.

mechanism for the plasmon-mediated Au NP alcohol oxidation was also proposed that involved both photoexcitation and photothermal effects. Single-electron transfer (SET) from the Au NP and ketyl radical formation are initiated primarily through interaction of the NP surface with the light incident on the sample, followed by sequential back electron transfer and proton loss. Furthermore, O2 evolves from both photothermal and photocatalytic decomposition of excess H2O2 present in the reaction mixture. The paper highlights the first examples of colloidal Au NP employed in plasmon-mediated organic reactions, specifically alcohol oxidations. Recently, we found that a photocatalytic process that uses visible light and Au NP catalysts supported by various materials, including ZrO2, Al2O3, and nitrogen-doped TiO2, is very effective for the synthesis of imine at ambient temperatures.[68] Because the supporting material itself does not show any photocatalytic activity, it confirms that the Au NPs are the active centers for this photocatalytic reaction. Imines can be produced by reacting alkynes with anilines to give hydroamination products, with Au NPs supported on nitrogen-doped TiO2 (Au@TiO2-N), which display the best catalytic activity. In this process, Au NPs absorb light and activate aniline molecules on the NP surface and the support can contribute to the activation of alkynes. The nitrogen-doped TiO2 support was found to produce better results than other supports. The presence of Ti3 + from TiO2, arising from nitrogen doping, provides more coordination sites for the alkyne and thus prompts a better performance than that found with pure TiO2 (Figure 16). Similarly, some more complex pharmaceutical compounds, such as propargylamines, have been produced by a one-pot synthesis procedure with the introduction of aldehyde to the amine and alkyne mixture, with Au@ZnO catalysts under l = 530 nm LED irradiation (Figure 17).[69] The high yields of propargylamines result from the interaction of alkyne with the ZnO support. In this reaction, the alkyne groups are first adsorbed on the Au NP surface due to the alkynophilicity displayed by the gold species[70, 71] and on ZnO. The enamine formed between the aldehyde and the amine then interacts with the alkynyl–Au NP species to produce the desired propargylamine. It is proposed that both

Figure 13. Photocatalytic selective oxidation of aminobenzyl alcohol to aminobenzaldehyde with Au@CeO2 under l = 530 nm green LED light irradiation.[65]

photocatalytic alcohol oxidation by using a Au@CeO2 photocatalyst.[66] They conducted electron paramagnetic resonance (EPR) spin trapping experiments to confirm that the H Au NPs species are important intermediates that govern alcohol oxidation by Au NPs, and that the photoexcitation influences the stability of Au H bonds. Zhang et al. reported that Au NPs supported on zeolite Y can efficiently catalyze the oxidation of aromatic alcohols to aldehydes under visible-light irradiation at room temperature (Figure 14).[27] The zeolite supports could concentrate

Figure 14. Photocatalytic selective oxidation of alcohol with Au@zeolite under visible-light irradiation.

reactants at ambient temperature and the conversions of aromatic alcohols depended on their molecular polarities. Experimental results showed that the catalytic activity of Au@zeolite catalysts are influenced by the adsorptive properties of the zeolite supports, the size of Au particles, the LSPR effect, and the specific surface areas of the Au NPs. Free-standing Au NPs without support can also act as photocatalysts for the oxidation of alcohols to the corre-

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electrons with O2 molecules on the Ag NP surface was studied in detail by Linics group very recently. This work showed that the production rate of ethylene oxide at 200 8C in the dark was identical to that of a catalyst operating at 160 8C under 250 mW cm 2 visible irradiation (Figure 19a), which indicates that the plasmonic activity of the Ag cata-

Figure 16. The proposed mechanism for the photocatalytic hydroamination of alkynes on supported Au NPs.[68] Figure 19. a) The rate of ethylene epoxidation in the dark and with visible-light illumination. A significant enhancement in the rate caused by visible-light illumination was observed. b) DFT-calculated potential energy surface for O2 and O2 on AgACHTUNGRE(100).[28]

lysts allows a unique route towards enhancing the energy efficiency of important selective chemical reactions. It was also shown that the rates of CO oxidation and NH3 oxidation by O2 over the Ag nanocube catalyst were significantly enhanced due to visible-light illumination of the catalyst. In the epoxidation of ethylene to ethylene oxide, the rate-determining step was the dissociation of molecular O2 on silver, and the energetic electrons of Ag NPs excited by the LSPR effect were proposed to accelerate this step.[28] Kinetic studies using 18O2-labeled oxygen showed a large KIE value of 1.19 0.01 for the photocatalytic process. The linear dependence of the photoinduced reaction on the light-source intensity in addition to the large KIE value indicates an energetic electron-assisted O2-dissociation process. The results of the DFT calculation on the potential energy surface for O2 and O2 on the Ag surface indicate that excitation of silver surface plasmons allows for the transfer of an excited electron to O2, which forms O2 . The O2 becomes negatively charged and nuclear motion is induced along the O2 potential energy surface. When energetic electrons are excited into the antibonding orbital of adsorbed O2, the O O bond is stretched, that is, the lowest energy configuration for the charged O2 state is characterized by a larger O O bond length (Figure 19b). The average time required for vibrationally excited adsorbates to dissipate vibrational energy on metal surfaces is sufficiently long to allow reactions to occur before the vibrational energy is dissipated. The net effect of the light-induced electron-transfer process is that the O2 adsorbate is activated at a reduced temperature compared with that of the traditional thermal reactions. This report shows convincing evidence that the direct plasmondriven catalytic cycle was electronic in nature. We explored the use of Ag NPs photocatalysts supported on photocatalytically inert oxides (Ag@zeolite Y, Ag@ZrO2, and Ag@SiO2) for dye degradations, such as the oxidation

Figure 17. Propargylamine synthesis on supported Au NPs.

plasmonic heating and plasmonic excited charge transfer could be responsible for the observed enhanced catalytic activity. Pineda et al. reported that Au@SiO2 irradiated with a l = 532 nm laser can catalyze the production of 4-benzoylmorpholine from benzaldehyde and morpholine through amide formation, for which quantitative yields of the target amide were obtained after 3–5 h of reaction (Figure 18).[72] The

Figure 18. Laser-catalyzed formation of 4-benzoylmorpholine from benzaldehyde and morpholine by using Au@SiO2.

protocol could also be extended to a tandem oxidation/amidation process, which shows the potential of the proposed approach for the promotion of liquid-phase organic reactions at room temperature. They proposed that the laser-excited hot electron can transfer into the reactants electronic states and thus facilitate the reactions. 2.2. Reactions Catalyzed by Ag and Cu NPs Selective oxidation (epoxidation) of ethylene to form ethylene oxide (EO) is a commercially important reaction that has also been successfully catalyzed by Ag@Al2O3 under visible-light irradiation.[28] The interaction of photogenerated

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of sulforhodamine B, under visible-light illumination.[73] In addition, there have been some studies on Ag NPs on insulating solid supports. Most reports on plasmonic Ag NPs photocatalysts focus on Ag supported on silver halide, such as AgCl and AgBr. These are another extensively studied class of semiconductor support with a different photocatalytic mechanism,[31] which is beyond the scope of this review. The strong light absorption of Ag NPs is reflected by their absorption cross section, which is 50 times their geometrical sectional area, and one of the reasons that Ag NP photocatalysts continue to be of future interest. The application of Cu NPs as catalysts has been impeded because of the ease at which the metallic Cu surface can be oxidized. One of the first uses of Cu NPs as plasmonic photocatalysts was by Marimuthu et al.[74] They reported that steady-state selectivity in propylene epoxidation with Cu NPs increases sharply when the catalyst is illuminated with visible light (Figure 20a). The selectivity increase is accom-

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stance it was shown that light can be used to switch the oxidation state from an oxide to a metallic state. The copper atoms newfound ability to shake off oxygen attached to its surface under light irradiation could allow it to act as a potential catalyst for many other reactions. Very recently, Guo et al. reported that Cu NPs on graphene supports are efficient photocatalysts for controllably catalyzing the coupling reactions of aromatic nitro compounds to the corresponding azoxy or azo compounds under visible-light irradiation (Figure 21).[59] The coupling of nitro-

Figure 21. Cu NPs on graphene used for catalyzing the coupling reactions of aromatic nitro compounds to the corresponding azoxy or azo compounds under visible-light irradiation.[59]

benzene produces azoxybenzene with a yield of 90 % at 60 8C and azobenzene with a yield of 96 % at 90 8C. They proposed that the electrons gain the energy of the incident light through the LSPR of the Cu NPs. The excited energetic electrons at the surface of the Cu NPs facilitate the cleavage of the N O bonds in the aromatic nitro compounds. This work highlights that graphene can be used as support to stabilize nanoparticles susceptible to oxidation, which may be further extended to other nanoparticle catalysts.

Figure 20. a) Selectivity to PO for thermal (light off) and photothermal (light on) processes by using Cu NPs as a function of the reaction rate. b) The UV/Vis extinction spectra of the Cu NP catalyst reduced with hydrogen and exposed to propylene oxidation under photothermal conditions.[74]

panied by light-induced reduction of the Cu atoms at the surface, which is brought about through the LSPR of the Cu NPs.[74] Cu NPs with an average size of (41 9) nm supported on SiO2 (Cu/SiO2) can tune the selectivity for the lightdriven epoxidation of propylene. The selectivity enhancement observed under Xe lamp irradiation is attributed to the LSPR of Cu NPs, which weakens the Cu O bond and thereby promotes the reduction of Cu2O to Cu0. When the same experiment was performed in the dark, the copper oxidized and only 20 % of the propylene converted to propylene oxide. When irradiated, the copper converted to and stayed in the metallic state and converted 50 % of the propylene into propylene oxide. As shown in Figure 20b, the UV/Vis extinction spectrum of the Cu catalyst changes from that associated with metallic Cu to that corresponding to an oxidized Cu shell covering the metallic core. Subsequent illumination of the functioning Cu catalyst with light results in a dramatic transformation of the catalyst, which manifests itself as the reduction of the Cu oxide shell to Cu0. The metallic copper under the oxidized surface absorbs the light, which frees electrons from copper atoms that then break the bonds between the copper and oxygen. This was the first in-

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2.3. Reactions Catalyzed by Bimetallic NPs It should be noted that the number of chemical reactions that can be catalyzed by the three plasmonic metals are relatively few when compared with the range that can be catalyzed by non-plasmonic transition metals. Therefore, although they are relatively poor light absorbers, their broad applicability has led to these metals being widely used as catalysts for the synthesis of important organic compounds. To develop visible-light photocatalytic processes that are applicable to a wide range of chemical reactions, some effective approach is required to couple the light-absorbing ability of the plasmonic-metal nanostructures to the proven inherent catalytic behavior of the transition metals. For example, an alloy NP of a plasmonic metal with a transition metal could efficiently harvest the light energy through the plasmonic-metal component and then enhance the intrinsic catalytic ability of the transition metal. In such a unique structure, plasmonic metals can act as an antenna that can

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ratio of the alloy NPs has an important impact on performance of the catalysts because it determines both the electronic heterogeneity and the distribution of Pd sites at the NP surface, with these two factors playing key roles in the catalytic activity. Irradiating with light produces the most profound enhancement in the catalytic performance of the NPs. Light absorption due to the LSPR effect of gold nanocrystals plays an important role in enhancing catalyst performance. The catalytic activity of the alloy NPs depends on the intensity and wavelength of incident light. We believe that the conduction electrons of the NPs gain the absorbed light energy and produce energetic electrons at the surface Pd sites, which enhances the sites intrinsic catalytic ability.[75] DFT simulations suggest that the charge heterogeneity at the surface of the alloy NPs is responsible for the enhanced catalytic activity. The charge heterogeneity is strongly influenced by the Au/Pd ratio, and changing this ratio will alter the charge distributions in the alloy NPs. The increased charge heterogeneity means that interactions between the alloy NPs and reactant molecules is enhanced.[41, 54] When the alloy NPs are irradiated with light, the conduction electrons are excited, which increases the NPs ability to induce reactions of the adsorbed reactant molecules. As shown in Figure 23, the comparison of simulation results without irradiation to the results with irradiation suggests that light irradiation also promotes the charge heterogeneity in Au–Pd alloy NPs. In addition to the selective oxidation of aromatic alcohols driven by visible light in oxygen atmosphere, we also find that irradiation of Au–Pd alloy NPs supported on ZrO2 significantly enhances their catalytic activity for oxidant-free dehydrogenation of aromatic alcohols to the corresponding aldehydes at ambient temperatures.[76] Because the aromatic alcohol is the sole reactant in the dehydrogenation process, it is convenient to understand the reaction mechanism. The first step of the dehydrogenation is the abstraction of a-H atoms from the alcohol molecules. When irradiated with light, the light-excited electrons are available to Pd sites at the alloy NP surface because of electron–electron collisions and electron redistribution between Au and Pd. The Pd sites have good affinity to the aromatic alcohol molecules and the interaction between the alcohol molecules and the NP surface is enhanced by the surface charge heterogeneity of the alloy NPs. The strong interaction facilitates the transfer of the light-excited electrons of the NPs to the adsorbed alcohol molecules. DFT simulation indicates that the electron injection to the alcohol molecules on the metal NPs can facilitate the abstraction of a-H atoms from the alcohol molecules. Once this abstraction is completed, the subsequent abstraction of the hydrogen atom from the hydroxyl group of the transient anion species proceeds readily to produce aldehyde as the final product whereas the negative charge of the transient anions returns to the alloy NPs according to the theoretical predication.[14] In addition to Au–Pd alloy NPs, Pd NPs deposited on the surface of Au nanorods were also reported to efficiently

absorb light and excite the conduction electrons in the metals; these energetic electrons can migrate to and improve the catalytic activity of the transition metal in the catalysts under light irradiation. Recent studies on Au and Pd alloy NPs confirmed that, when irradiated with visible light, they could photocatalyze several reactions conventionally catalyzed thermally by Pd NPs. Thus, alloying in this way can significantly enhance the intrinsic catalytic activity of the transition metal at ambient temperatures. Alloy photocatalyst structures of this type are likely to be efficient in driving various chemical reactions with sunlight, especially for reactions of organic molecules that can be typically driven by various transition metals. A combination of two kinds of metals has been widely applied in various materials to enhance the performance and reliability of the materials, and various strategies for combination, such as physical mixing, doping, alloying, and core-shell forming, have been proposed. Here, we summarize several examples of the latest reported bimetallic photocatalysts. 2.3.1. Au–Pd Nanostructure Photocatalysts Palladium (Pd) is known to be catalytically active for many reactions of important organic synthesis because of its affinity to many organic molecules. As we found in our recent research, the coupling of the light absorption of alloy NPs and the intrinsic catalytic activity of Pd can significantly enhance the intrinsic catalytic performance of Pd in several reactions, such as Suzuki–Miyaura coupling, oxidative addition of benzylamine to form imine, selective oxidation of aromatic alcohols, and phenol degradation (Figure 22). The Au/Pd molar

Figure 22. Dependence of Au–Pd@ZrO2 performance on the Au/Pd molar ratio of the alloy NPs in the light-enhanced reaction (top left) and in the dark reaction (top right) for the three reactions in the present study.[75]

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Figure 23. The optimized geometry and the natural charge distributions of a) the Au cluster and b) the Au–Pd cluster in the ground state and considered excited state.[75]

drive Suzuki coupling under light irradiation.[77] The integration of plasmonic Au nanorods with catalytic Pd NPs through seeded growth enabled efficient light harvesting for catalytic reactions on the nanostructures. Upon plasmon excitation, catalytic reactions were induced and accelerated through both plasmonic photocatalysis and photothermal effects. Under illumination with an l = 809 nm laser at 1.68 W, the yield of the Suzuki coupling reaction was approximately two times that obtained when the reaction was thermally heated to the same temperature (Figure 24). The overall contribution of plasmonic photocatalysis became larger when laser illumination at the plasmon resonance wavelength of the Au nanorods was applied, and it further increased when the power of the incident laser was raised. The reaction can even be driven under direct solar irradiation by using this unique Au–Pd nanostructure. Theoretically, as shown by DFT calculations, it is probable that hot

electrons are injected from the Pd surface to the adsorbed bromobenzene molecules. Overall, it was found that the plasmonic photocatalysis contribution was found to depend on the incident laser power, the plasmonic wavelength, the separating layer between the Au core and the Pd NPs, the environmental temperature, and the size of the nanostructures.[77] Here, the light absorption by the Au nanorods produced excited electrons that migrate to the Pd NPs on the surface of the Au nanorods to facilitate the reaction. In addition, the enhanced electromagnetic field in the close proximity to the Au nanorods should also contribute to the catalysis, but this is difficult to quantify without precise geometrical data. Huang et al. reported that bimetallic Au–Pd nanowheels can utilize solar energy to drive catalytic reactions, such as the oxidation of benzyl alcohol and Suzuki coupling reactions (Figure 25), at low temperatures and with much higher

Figure 24. Photocatalyst composed of Pd NPs deposited on Au nanorods for Suzuki reactions.[77]

Figure 25. Bimetallic Au–Pd nanowheels for benzyl alcohol oxidation and Suzuki reactions.[78]

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performance than conventional heating processes.[78] The bimetallic nanowheels exhibit well-defined LSPR properties and the LSPR peaks are tunable in the one-pot synthesis process. They proposed that the enhanced activity may be attributed to the LSPR of the Au–Pd nanowheels under light irradiation and the interface interaction between Au and Pd segments in the nanowheels.[78] In addition, the contribution from the enhanced electromagnetic field in close proximity to the Au nanowheels should also be taken into account.

tion band. It is also probable that the LSPR of Cu may play a role by energizing the light-excited electrons that facilitate the reaction. They proposed that the positive charges formed on the NPs oxidize substrates and the conduction band electrons reduce molecular oxygen, which promotes photocatalytic cycles. Cu alloying with Pt decreases the work function of the alloy NPs and the height of the Schottky barrier created at the NP/anatase heterojunction. This promotes efficient electron transfer from the photoactivated metal NPs to anatase and results in enhanced photocatalytic activity. The Pt–Cu alloy catalyst has been successfully activated by sunlight and enables efficient and selective aerobic oxidation of aromatic alcohols at ambient temperature. The efficient charge separation at the Pt–Cu alloy NP/anatase interface plays an important role in the enhanced photocatalytic activity.[80] In this study, anatase TiO2 is necessary because the support itself does not display any activity for this reaction.

2.3.2 Pd–Au Core-Shell Structure Photocatalysts Newly functionalized core-shell Au–Pd NPs supported on TiO2 can quantitatively convert chlorobenzene and 2-propanol to benzene and acetone under visible-light irradiation (Figure 26). Control of the thickness of the Pd shell was

2.3.4. Au–Cu Alloy NP Photocatalysts Au Cu alloy NPs supported on P25 have been found to be able to successfully promote aerobic oxidation under visible-light irradiation without catalyst deactivation. This is triggered by visible-light absorption of the surface Au atoms through the LSPR. Collective oscillation of the conduction electrons of the Au NPs reduces the oxidized surface Cu atoms and maintains the Au–Cu alloying effect. The enhanced activity of the Au–Cu alloy catalyst is due to visible light reducing the surface Cu atoms that have been oxidized by O2. Sunlight irradiation can also facilitate activity regeneration and promote the photocatalytic reaction at room temperature.[81] However, in this case, whether the light-excited electrons originate from the Au species, the Cu species, or the alloy NPs is still questionable, although they showed good correlation between the LSPR band of the catalyst and the apparent quantum yield for the reaction. The protection of surface Cu atoms from oxidation is a challenge for efficient aerobic oxidation reactions. As shown in Figure 28, plasmon-excited surface Au atoms promote the intraband transition of 6sp electrons. The electrons are transferred to the adjacent oxidized Cu atoms to reduce

Figure 26. Au–Pd core-shell nanoparticles catalyze chlorobenzene and 2propanol to benzene and acetone under visible-light irradiation.[79]

very important for both a satisfactory co-catalyst effect and absorption due to LSPR of Au NPs. It is proposed that electrons are transferred from the Au particles to the Pd shell, with the reactant chlorobenzene reduced by the Pd electrons, which results in the formation of benzene and elimination of chloride ions. The resultant electron-deficient Au particles are reduced by electrons that originate from 2propanol and return to the original metallic state along with formation of acetone.[79] 2.3.3. Pt–Cu Alloy NP Photocatalysts Visible-light irradiation of Pt–Cu bimetallic alloy NPs supported on anatase TiO2 was reported to efficiently promote aerobic oxidation (Figure 27).[80] This is facilitated through the interband excitation of Pt atoms by visible light followed by the transfer of excited electrons to the anatase conduc-

Figure 27. Pt–Cu bimetallic alloy NPs for selective aerobic oxidation of alcohols.[80]

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Figure 28. Possible energy diagram (vs. NHE) for plasmon-activated Au– Cu/P25 catalysts under visible-light irradiation.[81]

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several physical intertwined mechanisms behind photocatalytic processes with plasmonic-metal NPs, unlike that found for semiconductor photocatalysts for which temperature and heating are not important factors, and for conventional thermal catalysis processes with metal catalysts. In reality, both the light-excited energized electrons and the photothermal processes will play a role in photocatalysis with metal NPs. Analysis of the influence of the incident light intensity and wavelength may help to distinguish between the contributions from the two effects. Studies of this type will help us to answer questions regarding the real driving force of plasmonic-metal NP photocatalysis and its underlying mechanisms.

them. The plasmon-excited electrons on the surface of the alloy NPs transfer to the TiO2 conduction band and reduce the O2. As shown in the figure, the potential of fB for TiO2 is more negative than the reduction potentials of Cu + and Cu2 + . Plasmon-excited electrons are, therefore, preferentially transferred to oxidized surface Cu atoms. These results suggest that electron transfer from plasmon-excited surface Au atoms facilitates the successful reduction of oxidized surface Cu atoms. In summary, the detailed reaction mechanisms proposed for the reported organic transformations catalyzed by plasmonic-metal NPs or bimetallic NP under light irradiation may vary under different conditions. However, all of the reported reactions involve transfer of light-excited energetic electrons, which is a unique feature in the direct plasmonicmetal NP photocatalysis process.

2.4.1. Impact of Light Intensity and Wavelength As discussed in Section 1.1, the photocatalytic reaction rates strongly depend on the photon flux (light intensity) and wavelength, which is a unique characteristic of plasmonicmetal NP photocatalysts. Figure 29 shows the light intensity dependence of photocatalytic activity using Au–Pd alloy NPs for the transformation from benzyl alcohol to benzaldehyde at two different temperatures. The light contribution and the thermal effect were also calculated. When the light intensity was increased, the conversion rate increased linearly up to an intensity of 0.8 W cm 2. Further increase in the light intensity resulted in a much greater rate increase, and the relationship between light intensity and reaction rate became nonlinear. This is a feature of the chemical processes driven by the light-excited electrons of metals.[28] It is also possible that when the light intensity is very high, multiphoton absorption occurs, which increases the number of excit-

2.4. The Role of Light Irradiation in Plasmonic Photocatalysis Although many examples of chemical reactions driven by direct plasmonic-metal NPs under light irradiation been reported, there has also been some questioning of what fundamentally produces the observed catalytic enhancement. Does the enhanced catalytic activity originate through a simple photothermal effect in which light absorption heats the metal NPs and the surrounding environment and thereby facilitates the reactions? Indeed, metals have continuous electronic energy levels and are able to effectively absorb both photonic energy and thermal energy simultaneously to drive chemical reactions. This means that there are probably

Figure 29. Light-intensity-dependent photocatalytic activity of Au–Pd alloy NPs for the transformation from benzyl alcohol to benzaldehyde at a, b) 45 and c, d) 60 8C.[76]

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ed metal electrons with sufficient energy to drive the reactions. The results also show that the light-induced contribution depends linearly on the irradiation intensity for the reactions catalyzed at different temperatures and demonstrates that the photoexcitation of the metal electrons is the dominant factor for the light-enhanced activity. Additionally, the light contribution is higher at low temperatures, which confirms that the dominant contribution to the photocatalytic activity of the metal NPs is from photoexcited electrons rather than photothermal effects. The study of Wang et al. also suggested that at low light intensity the photothermal effect dominates, whereas at high intensity and lower reaction temperature, photoexcitation becomes more important.[77] Increasing light intensity increases the number of the light-excited electrons populating high energy levels. This will result in the transfer of more electrons from excited states with sufficient energy to reactant molecules and thus enhance the photocatalytic activity. The increase in light intensity also enhances photothermal effects because the number of the excited electrons with insufficient energy for direct photoexcitation may also increase with the light intensity. These electrons will relax back to low energy states and heat the metal particles. Wavelength-dependent photocatalytic reactions have classically been used to relate photon-absorption properties of photocatalysts to their photocatalytic performance.[28] It is anticipated that if plasmon excitation occurs through a direct electron injection into the adsorbate, the wavelength dependence would resemble an overlap of the LSPR absorption spectrum and the photocatalytic reaction rate. The action spectrum shows that there is a reasonable agreement between the wavelength dependence of the photocatalytic rate and the plasmon absorption intensity (Figure 30a). The similarity between the wavelength-dependent rates of photocatalysis and absorption spectrum of the plasmonic photocatalysts provides strong evidence that LSPR excitation was responsible for the photocatalytic activity. As shown in Figure 30b, under irradiation with shorter wavelengths at ambient temperatures, photoexcitation plays a much more important role than photothermal effects, whereas at longer wavelengths, the photothermal heating plays a more important role in driving the reactions. Such arguments are also in line with the recognized fact that longer wavelengths result in more pronounced photothermal heating than shorter wavelength irradiation. Because the rate of the catalyzed reactions is expected to depend on the population of electrons with sufficient energy to initiate reactions of the reactant molecules, one can increase the number of energetic electrons by applying high light intensity or light with shorter wavelengths. The electron energy sufficient to initiate reactions of the molecules on the metal NPs is dependent on the actual reaction in question. Tuning the irradiation wavelength can increase the number of the electrons with sufficient energy to initiate a reaction. Combining these wavelength-dependent results with a linear dependence of photocatalytic rate on intensity, as a few studies have done, indicates that the LSPR excita-

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Figure 30. Wavelength-dependent photocatalytic activity of Au–Pd alloy NPs for the transformation from benzyl alcohol to benzaldehyde.[76]

tion is driving photocatalysis mainly through an electronic (non-thermal) mechanism. 2.4.2. Impact of Photothermal Effect During the light excitation of the plasmonic-metals, excited electrons can relax through electron–phonon relaxation, which refers to the collision of electrons with the ionic lattice of the metal NP, over timescales that are typically of the order of 0.7 ps.[82] This causes an increase in the temperature of the metal NP. The phonon–phonon relaxation also couples the vibrations of the metal lattice and the support lattice, which heats the NPs and the surrounding local environment. It should be noted that such photothermal effects work more efficiently in small NPs (typically below 30 nm)[83–85] Furthermore, during the process of light-excited-electron transfer, low-energy electrons with insufficient energy to overcome the activation barrier couple to phonon modes, which thus heat up the metal lattice on a timescale of approximately 1 ps, followed by the dissipation of this heat to the surrounding environment on a timescale of 10– 100 ps.[86] Although such a photothermal effect does not produce active electrons or holes, it can increase the rate of the catalysis process[84] in the same way that heat-driven catalysis reactions go faster at a higher temperature. The plasmonmediated nanostructure heating could result in energy transfer to adsorbates, which would drive chemical transformations through the Arrhenius dependence of reaction rate on surface temperature. The photothermal effect is not negligi-

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ble in most of the photocatalysis of metal NPs, particularly when a high light intensity is applied. The plasmonic photothermal effect can be used to reduce the operating temperature of reactions that are conventionally conducted at high temperatures, as demonstrated recently by Linic et al. for the oxidations of ethylene, CO, and NH3.[28] The catalysts were Ag nanocubes supported on aAl2O3 particles, which could absorb visible light through the LSPR of Ag NPs. For ethylene oxidation at 430 K, the reaction rate with visible-light irradiation (250 mW cm 2) was increased by three to eight times compared with that in the dark. To reach the same rate of reaction in the dark, it would need to be performed at 470 K. The decrease in the reaction temperature was seen to contribute significantly to the stability and lifetime of the catalysts. It should be mentioned that over and above temperature increase, this work finally attributed the enhanced oxidation rate to the transfer of energetic electrons to the adsorbed O2 based on the observed linear dependence of reaction rate on light intensity.[28] However, no matter what mechanism is really at work, plasmonic enhancement of thermocatalytic reactions does exist. Experimental and theoretical analyses of plasmon-mediated nanostructure heating have shown that under illumination at solar intensities (100 mW cm 2), maximum transient temperature increases of only about 10 2 K can be achieved.[28, 87] Assuming that the rate of a thermal reaction doubles with a 10 K increase in operating temperature (apparent activation barrier of 100 kJ mol 1), an illumination intensity of 106 mW cm 2 would be necessary to produce a twofold increase in the rate of reaction due to photothermal effects. Although the plasmonic heating mechanism should be considered when analyzing photocatalytic reaction mechanisms on illuminated plasmonic NPs, under low-intensity illumination this mechanism is unlikely to play a substantial role in inducing chemical reactions, particularly when reactions are run in continuous-flow, isothermal environments. To confirm that the accelerated rate of the reactions conducted under light irradiation was due to the photoexcitation of energetic electrons, Huang et al. studied the dependences of the light-irradiation enhancement factor on the irradiation intensity for reactions at various temperatures and thus demonstrated that photoexcitation of the metal electrons is the primary factor responsible for the light-enhanced activity.[78] They found that the conversion enhancement decreased as a function of temperature from about ninefold at lower temperatures to about 2.3-fold at higher temperatures, with the light enhancement significantly higher at low temperatures (Figure 31). The same trend was also observed in the work of Linics group, the rate enhancement of photocatalytic epoxidation of ethylene decreased as a function of temperature from about eightfold at lower temperatures to about threefold at higher temperatures.[28] All these results confirm that the dominant contribution to the photocatalytic activity of the metal NPs is from light-excited electrons with photothermal effects playing a more minor role. The study of Wang et al. on plasmon-

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Figure 31. Reaction rate enhancement (left axis, N) as calculated by dividing the conversion of the light-irradiation process by the conversion of the conventional heating process as a function of temperature for a) the benzyl alcohol oxidation and b) the Suzuki coupling reaction.[78]

ic photocatalysts also suggested that the photothermal effect is negligible at around room temperature.[77] Thus, even though plasmonic NPs are excellent photon-induced heat generators under resonant excitation, owing to their very high absorption coefficients, it is not expected that the exponential dependence of reaction rate on light intensity will be observed through heating. Furthermore, the use of the plasmonic localized heating mechanism to drive chemical reactions is analogous to externally heating the system and does not provide a unique pathway to control reaction selectivity. In addition, photothermal effects cannot play a dominant role in direct plasmonic photocatalysis under irradiation with solar intensities at room temperature. In all direct photocatalysis observations using plasmonicmetal NPs, the photothermal effect and photoexcitation coexist but their contributions vary with the reaction conditions. At relatively low temperatures (compared with those used for traditional thermal catalysis processes) used in most present studies, the visible-light-excited electrons play a dominant role.

3. Conclusions and Outlook Integration of LSPR into catalysis allows for the design of new photocatalytic systems. The exciting developments in this active field have opened up new pathways for the efficient transformation of solar energy into chemical energy. In plasmonic-metal NP photocatalysis, the light-harvesting and catalyzing reaction can be effectively coupled on the NPs. The light irradiation produces energetic conduction electrons at the surface of the NPs that are able to directly

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sive, earth-abundant elements, such as aluminum and copper. Their LSPR properties and chemical stability for catalytic chemical reactions are important issues to be considered. In addition, several types of nonplasmonic-metal nanomaterials have recently been found to exhibit interesting inter-band light absorption properties.[88] Exploration of the NPs of metals other than plasmonic metals for enhancement of chemical reactions under light irradiation will be interesting and worthwhile. Overall, direct plasmonic-metal photocatalysis is a new developing area for chemical transformations. Although some significant progress has been made in this area of research, great challenges are still to be resolved. More exciting discoveries can be conceived in the pursuit of a green and renewable chemical future. Hopefully, this review can stimulate new insights and creative solutions, thereby expediting progress in this field and leading to many more exciting discoveries in visible-light-induced selective organic synthesis reactions by plasmonic-metal photocatalysis.

induce reactions of adsorbed reactant molecules. The number of energetic electrons can be controlled by applying high light intensity and by tuning the irradiation wavelength to optimize the reaction efficiency. Given the high conduction electron density at the NP surface, the natural affinity of the NP surface to many reactants, and the ability of the NPs to couple the stimuli of light and heat to excite conduction electrons, these materials are a new class of photocatalyst that, in many cases, are superior to semiconductor or composite photocatalysts for the synthesis of organic compounds. The reports of direct plasmonic photocatalysis to date show conclusively that excitation of LSPR through low-intensity continuous-wave photon illumination can induce significant enhancement in rates of reactions that are conventionally achieved by heating. This means that in these reactions the LSPR excitation channels photon energy to speed up the rate-limiting step. Furthermore, some examples in which catalytic selectivity was modulated by LSPR excitation provide initial evidence that direct plasmon-driven photocatalysis may allow for unique routes to control catalytic selectivity. Mechanistic studies have suggested that the majority of reported direct plasmonic-metal photocatalysis occurs through the transient transfer of energetic electrons to adsorbate orbitals.[28, 44] In this process, adsorbates gain vibrational energy from vibronic energy exchanges that drive adsorbates over activation barriers. It has been proposed that the nature of the adsorbate may have a significant impact on the efficiency of this process and potentially allow for a unique method to control selectivity in plasmon-driven reactions, although this has not yet been conclusively demonstrated. The development of relationships between adsorbate electronic structure and plasmonic photocatalytic signatures is of significant importance for designing plasmonic systems that allow for unique control of reaction selectivity. More experimental and theoretical efforts on the elaboration of these processes are strongly desired because the understanding of these will greatly help to optimize the plasmonic enhancement effects in the further application of these new photocatalysts. Another area of research in direct plasmon-driven photocatalysis that is still in its infancy is the development of structure–function relationships based on the size and shape of the plasmonic photocatalysis. A known feature of plasmonic nanostructures is their tunable LSPR wavelength based on particle geometry, such as composition, shape, and size. This means it is possible in principle to design nanostructures that can absorb the entire solar spectrum more efficiently by manipulating these properties in catalyst preparation. The dependence of direct plasmon-driven photocatalytic characteristics (e.g., efficiency, wavelength dependence, reaction selectivity) on the structure of the plasmonic NPs is expected to be a focus of future research. Many experiments so far on plasmon-enhanced chemical reactions have mainly relied on the plasmonic properties of gold and silver. Practical implementation of plasmon-enhanced chemical reactions will require the use of inexpen-

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Acknowledgements The authors gratefully acknowledge financial support from the Australian Research Council (ARC DP110104990).

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Huaiyong Zhu et al.

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Huaiyong Zhu et al.

FOCUS REVIEW Photocatalysis Qi Xiao, Esa Jaatinen, Huaiyong Zhu*

Making changes with visible light: Recent developments in the direct photocatalysis of plasmonic-metal nanoparticles are described, with a focus on the role of the localized surface plasmon resonance (LSPR) effect in plasmonic metals and their applications in organic transformations (see figure). The role of light irradiation in the catalyzed reactions and the lightexcited energetic electron reaction mechanisms will be highlighted.

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Direct Photocatalysis for Organic Synthesis by Using Plasmonic-Metal Nanoparticles Irradiated with Visible Light

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Heterogeneous visible light photocatalysis for selective organic transformations.

Solar synthesis: prospects in visible light photocatalysis.

Correction: Enhancing optical absorption of metal-organic frameworks for improved visible light photocatalysis.

[2+2] cycloaddition of 1,3-dienes by visible light photocatalysis.

Kinetic study of acetaminophen degradation by visible light photocatalysis.

Conducting polymer nanostructures for photocatalysis under visible light.

Electronic Reconstruction of α-Ag2WO4 Nanorods for Visible-Light Photocatalysis.

Highly Visible Light Responsive, Narrow Band gap TiO2 Nanoparticles Modified by Elemental Red Phosphorus for Photocatalysis and Photoelectrochemical Applications.

Advances in Photocatalysis: A Microreview of Visible Light Mediated Ruthenium and Iridium Catalyzed Organic Transformations.

Intimate Coupling of Photocatalysis and Biodegradation for Degrading Phenol Using Different Light Types: Visible Light vs UV Light.

Decomposition of hydrazine by an organic fullerene-phthalocyanine p-n bilayer photocatalysis system over the entire visible-light region.

Black and yellow anatase titania formed by (H,N)-doping: strong visible-light absorption and enhanced visible-light photocatalysis.

[3+2] Photooxygenation of aryl cylopropanes via visible light photocatalysis.

Nanogold plasmonic photocatalysis for organic synthesis and clean energy conversion.

Constructing Quaternary Carbons from N-(Acyloxy)phthalimide Precursors of Tertiary Radicals Using Visible-Light Photocatalysis.

Metastable β-Bi2O3 Nanoparticles with Potential for Photocatalytic Water Purification Using Visible Light Irradiation.

Metastable β-Bi2O3 Nanoparticles with Potential for Photocatalytic Water Purification Using Visible Light Irradiation.

Photocatalysis by 3,6-disubstituted-s-tetrazine: visible-light driven metal-free green synthesis of 2-substituted benzimidazole and benzothiazole.

One-step synthesis of metal@titania core-shell materials for visible-light photocatalysis and catalytic reduction reaction.

Solvothermal synthesis of CdIn2S4 photocatalyst for selective photosynthesis of organic aromatic compounds under visible light.

Nanospace-enhanced photoreduction for the synthesis of copper(I) oxide nanoparticles under visible-light irradiation.

Efficient visible light driven photocatalytic hydrogen production from water using attapulgite clay sensitized by CdS nanoparticles.

Light-harvesting photocatalysis for water oxidation using mesoporous organosilica.

Electronic and optical properties of N-doped Bi2O3 polymorphs for visible light-induced photocatalysis.