*3.2.2. The plasmonic metal case: plasmonic-metal/semiconductor heterodimer nanocrystals*

The harvesting and conversion of solar energy become a renewed in recent years. Among various technologies, the direct conversion of solar to chemical energy using photocatalysts has received significant attention. Although heterogeneous photocatalysts are almost exclu‐ sively semiconductors, it has been demonstrated recently that plasmonic nanostructures of noble metals (mainly silver and gold) also show significant promise [1]. Herein, the recent progress in using plasmonic metallic nanostructures in the field of photocatalysis is reviewed. We focus on plasmon-enhanced water splitting on composite photocatalysts containing semiconductor and plasmonic-metal building blocks and recently reported plasmon-mediated photocatalytic reactions on plasmonic nanostructures of noble metals.

Plasmonic metallic nanostructures are characterized by their strong interaction with resonant photons through an excitation of surface plasmon resonance (SPR). SPR can be described as the resonant photon-induced collective oscillation of valence electrons, established when the frequency of photons matches the natural frequency of surface electrons oscillating against the restoring force of positive nuclei (Figure 22A). The resonant photon wavelength is different for different metals. For example, gold, silver, and copper nanostructures exhibit resonant behavior when interacting with ultraviolet (UV) and visible (Vis) photons. The utilization of the localized SPR (LSPR) effect of nanostructured Au, Ag, and Cu was examined for the potential photocatalysis [97–99], using Au or Ag nanoparticles (NPs) supported on ZrO2, AgCl, or TiO2 for the unselective degradation of organic species under visible light irradiation. As shown in Figure 22 A, LSPR is the resonant photon-induced coherent oscillation of charge at the metal-dielectric interface, established when the photon frequency matches the natural frequency of metal surface electrons oscillating against the restoring force of their positive nuclei [100]. The frequency of the surface plasmon absorption is highly dependent on the type of metal, size, shape, surrounding dielectric medium, distance between neighboring objects, and configuration their ensemble [101]. A wide range of metal/semiconductor heterostruc‐ tures, including Au/TiO2, Ag/TiO2, Au/CdS, and Au/Fe2O3, have been explored to achieve enhanced photocatalytic activity [102–110].

Although the exact nature of LSPR effect on enhanced photocatalytic activity is not entirely understood, three possible enhancement mechanisms have been proposed: (1) near-field enhancement, (2) SPR-induced electron transfer from metal to semiconductor, and (3) scatter‐ ing [103, 104] (Figure 22A–C). The strong SPR-induced electric field of plasmonic metal NPs can interact with the adjacent semiconductor (Figure 22B-a), this interaction may increase the rate of exciton formation and the concentration of the charge carriers generated in this part of the semiconductor [104]. If only the metal excited, the metallic plasmonic NPs can absorb resonant photons and transfer energetic photogenerated charge carrier to the semiconductor during the decay of the LSPR (Figure 22B-b). As a result of the plasmonic sensitization process, a wide band gap semiconductor could perform catalytic reduction reactions under visible light. Plasmonic structures of size larger than 50 nm are efficient in scattering the resonant photons, which increases the path length of photons in semiconductor/plasmonic metal nanostructures. Therefore, the resonant photons that are not absorbed by semiconductor photocatalysts could be scattered by the bigger plasmonic metal particles, ultimately increasing the number of electron/hole pairs. The above-mentioned three mechanisms are governed by the metal/ semiconductor configurations and their arrangements in the hybrid system.

*3.2.2. The plasmonic metal case: plasmonic-metal/semiconductor heterodimer nanocrystals*

photocatalytic reactions on plasmonic nanostructures of noble metals.

320 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

enhanced photocatalytic activity [102–110].

The harvesting and conversion of solar energy become a renewed in recent years. Among various technologies, the direct conversion of solar to chemical energy using photocatalysts has received significant attention. Although heterogeneous photocatalysts are almost exclu‐ sively semiconductors, it has been demonstrated recently that plasmonic nanostructures of noble metals (mainly silver and gold) also show significant promise [1]. Herein, the recent progress in using plasmonic metallic nanostructures in the field of photocatalysis is reviewed. We focus on plasmon-enhanced water splitting on composite photocatalysts containing semiconductor and plasmonic-metal building blocks and recently reported plasmon-mediated

Plasmonic metallic nanostructures are characterized by their strong interaction with resonant photons through an excitation of surface plasmon resonance (SPR). SPR can be described as the resonant photon-induced collective oscillation of valence electrons, established when the frequency of photons matches the natural frequency of surface electrons oscillating against the restoring force of positive nuclei (Figure 22A). The resonant photon wavelength is different for different metals. For example, gold, silver, and copper nanostructures exhibit resonant behavior when interacting with ultraviolet (UV) and visible (Vis) photons. The utilization of the localized SPR (LSPR) effect of nanostructured Au, Ag, and Cu was examined for the potential photocatalysis [97–99], using Au or Ag nanoparticles (NPs) supported on ZrO2, AgCl, or TiO2 for the unselective degradation of organic species under visible light irradiation. As shown in Figure 22 A, LSPR is the resonant photon-induced coherent oscillation of charge at the metal-dielectric interface, established when the photon frequency matches the natural frequency of metal surface electrons oscillating against the restoring force of their positive nuclei [100]. The frequency of the surface plasmon absorption is highly dependent on the type of metal, size, shape, surrounding dielectric medium, distance between neighboring objects, and configuration their ensemble [101]. A wide range of metal/semiconductor heterostruc‐ tures, including Au/TiO2, Ag/TiO2, Au/CdS, and Au/Fe2O3, have been explored to achieve

Although the exact nature of LSPR effect on enhanced photocatalytic activity is not entirely understood, three possible enhancement mechanisms have been proposed: (1) near-field enhancement, (2) SPR-induced electron transfer from metal to semiconductor, and (3) scatter‐ ing [103, 104] (Figure 22A–C). The strong SPR-induced electric field of plasmonic metal NPs can interact with the adjacent semiconductor (Figure 22B-a), this interaction may increase the rate of exciton formation and the concentration of the charge carriers generated in this part of the semiconductor [104]. If only the metal excited, the metallic plasmonic NPs can absorb resonant photons and transfer energetic photogenerated charge carrier to the semiconductor during the decay of the LSPR (Figure 22B-b). As a result of the plasmonic sensitization process, a wide band gap semiconductor could perform catalytic reduction reactions under visible light. Plasmonic structures of size larger than 50 nm are efficient in scattering the resonant photons, which increases the path length of photons in semiconductor/plasmonic metal nanostructures. Therefore, the resonant photons that are not absorbed by semiconductor photocatalysts could be scattered by the bigger plasmonic metal particles, ultimately increasing the number of

**Figure 22.** Schematic illustrating (A) localized surface plasmon resonance [102]. Copyright: America Chemistry Soci‐ ety, 2011. (B) The mechanisms for plasmon-enhanced chemical reactions with metal/semiconductor hybrid nanostruc‐ tures: (a) plasmonic enhancement of light absorption; (b) hot-electron effect [103], with permission. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. (C) The scattering mechanism [104]. Copyright: Royal Society of Chemistry, 2013.

In the case of SPR-mediated charge injection from metal to semiconductor, charge carriers are directly injected from excited plasmonic-metal nanostructures into the semiconductor surface. The metallic plasmonic nanoparticles essentially act as a sensitizer, absorbing resonant photons and transferring the energetic electron, formed in the process of the SPR excitation, to the nearby semiconductor (Figure 23) [1, 111]. Furthermore, the ability to tune the SPR resonance wavelength and intensity by changing the size or shape of Au or Ag nanostructures suggests that plasmon enhancement could be more dramatic when the interface between Au (Ag) and semiconductor, such as TiO2, CdS, etc. The charge injection mechanism was func‐ tional in composite photocatalysts where the plasmonic nanoparticles and semiconductor are in direct contact with each other, allowing a rapid transfer of charge carriers. These composite systems are geometrically similar to the conventional cocatalyst/semiconductor photocatalysts that are often synthesized by an incipient wetness deposition of metal precursors and their subsequent thermal treatment on a semiconductor surface. Therefore, the defect in the heterointerface should be decreased to get more efficient charge separation and injection to be "chemically useful" energetic charge carriers for photoreduction. Otherwise, too many defects on the interface will trap the photoinduced charge.

**Figure 23.** Mechanism of SPR-induced charge transfer with approximate energy levels on the NHE scale. Dashed red lines refer to the water-splitting redox potentials. (i) Electrons near the metal Fermi level, Ef , are excited to surface plas‐ mon (SP) states; (ii) the electrons transfer to a nearby semiconductor particle; (iii) this activates electron-driven process‐ es such as the hydrogen evolution half-reaction [1]. Copyright of Nature group 2011, Macmillan Publishers Limited.

### *3.2.3. The cocatalystic metal on semiconductor nanocrystals with well-defined crystal face exposure*

Based on the photoreduction reaction of Pt4+ (Pt4+ + e− → Pt) and photooxidation reaction of Pb2+ (Pb2+ + H2O + h+ → PbO2), it is clearly indicated that rutile {110} and {011} facets provide reduction and oxidation sites, respectively (Figure 24Aa, c) [112–113]. The obvious separation of reduction and oxidation sites on faceted rutile crystals is attributed to photoexcited electron and hole transfer between {011} and {110} facets, which is driven by the higher electronic energy levels of {011}. Although the selective distribution of photodeposited Pt and PbO2 particles on anatase {101} and {001} facets (Figure 24 Ab, d) is not as obvious as that on different rutile facets, anatase {101} and {001} facets can still be considered to be more reductive and more oxidative, respectively [113].

resonance wavelength and intensity by changing the size or shape of Au or Ag nanostructures suggests that plasmon enhancement could be more dramatic when the interface between Au (Ag) and semiconductor, such as TiO2, CdS, etc. The charge injection mechanism was func‐ tional in composite photocatalysts where the plasmonic nanoparticles and semiconductor are in direct contact with each other, allowing a rapid transfer of charge carriers. These composite systems are geometrically similar to the conventional cocatalyst/semiconductor photocatalysts that are often synthesized by an incipient wetness deposition of metal precursors and their subsequent thermal treatment on a semiconductor surface. Therefore, the defect in the heterointerface should be decreased to get more efficient charge separation and injection to be "chemically useful" energetic charge carriers for photoreduction. Otherwise, too many defects

**Figure 23.** Mechanism of SPR-induced charge transfer with approximate energy levels on the NHE scale. Dashed red

mon (SP) states; (ii) the electrons transfer to a nearby semiconductor particle; (iii) this activates electron-driven process‐ es such as the hydrogen evolution half-reaction [1]. Copyright of Nature group 2011, Macmillan Publishers Limited.

*3.2.3. The cocatalystic metal on semiconductor nanocrystals with well-defined crystal face exposure*

Based on the photoreduction reaction of Pt4+ (Pt4+ + e− → Pt) and photooxidation reaction of Pb2+ (Pb2+ + H2O + h+ → PbO2), it is clearly indicated that rutile {110} and {011} facets provide reduction and oxidation sites, respectively (Figure 24Aa, c) [112–113]. The obvious separation of reduction and oxidation sites on faceted rutile crystals is attributed to photoexcited electron and hole transfer between {011} and {110} facets, which is driven by the higher electronic energy levels of {011}. Although the selective distribution of photodeposited Pt and PbO2 particles on anatase {101} and {001} facets (Figure 24 Ab, d) is not as obvious as that on different rutile

, are excited to surface plas‐

lines refer to the water-splitting redox potentials. (i) Electrons near the metal Fermi level, Ef

on the interface will trap the photoinduced charge.

322 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

The spatial separation of reduction and the oxidation sites in TiO2 crystals with appropriate different facets are well known now [114]. Consequently, the corresponding spatial separation of photoexcited charge carriers on different facets improves the photocatalytic activities of photocatalysts, as illustrated in Figure 24B and C. Liu et al. reported that anatase crystals with {101} and {001} facets, with selective deposition of Pt particles by a photoreduction process only on the reductive {101} facets show a much higher photocatalytic hydrogen evolution (reduction reaction) from a mixture of H2O and methanol than do the same crystals with Pt particles on both {101} and {001} facets (Figure 24D). It is now clear that the difference in spatial distribution of cocatalysts on photocatalysts is an important but unfortunately neglected factor. The spatial separation of photoexcited electrons and holes on different facets was also seen in brookite nanosheets surrounded with four {210} and two {101} facts as reduction sites, and two {201} facets as oxidation sites. Consequently, such spatial separation produces an excellent photocatalytic activity of brookite, which is conventionally inactive [113–116].

**Figure 24.** .(A) SEM images of (a) a rutile particle and (b) an anatase particle on which Pt fine particles were deposited by UV-irradiation in a solution of 1.0 mM H2PtCl6; SEM images of (c) a rutile particle and (d) an anatase particle show‐ ing PbO2 deposits, which were loaded on the particles by UV irradiation of the Pt-deposited TiO2 powder in a solution of 0.1 M Pb(NO3)2. Reprinted with permission from Ohno et al. [113]. Copyright 2002 Royal Society of Chemistry. (B) Schematic of the spatial separation of redox sites on anatase crystals with {101} and {001} facets and rutile TiO2 particle with {110} and {011} facets. Reprinted with permission from Murakami et al. [115]. Copyright 2009 American Chemical Society. (C) Hydrogen production after 6 h irradiation time using TiO2 nanocrystals (NCs) with different exposed {001} facets as photocatalysts. Reprinted with permission from Liu et al. [116]. Copyright 2013 Wiley-VCH. (D) (a) Hydrogen production amounts of Pt deposited TiO2 NCs by means of photochemical-reduction (■) and chemical reduction routes (▲). SEM images of TiO2 NCs with 0.5% Pt loading amount prepared by (b) chemical reduction and (c) photo‐ chemical reduction, respectively. Reprinted with permission from Liu et al. [116]. Copyright 2013 Wiley-VCH.

## **3.3. Metal–semiconductor core/shell nanocrystals and photocatalysis application**

## *3.3.1. Metal core–semiconductor shell: large lattice mismatch-induced interface control and photocatalytic applications*

In metal-semiconductor hetero structures, the contact interface between metal and semicon‐ ductor plays a critical role in transfer and separation of charge carriers [117–118]. Our precise symmetry control of Au–CdX can enable different contact interface between metal and semiconductor components at the nanoscale, thus offering a new dimension to control optical and electronic properties of hybrid nanostructures in a highly control manner. Figure 25A highlights evolution of plasmon–exciton coupling in Au–CdS enabled by their symmetry control. As compared with the SPR feature of a pure metal nanoparticles, optical absorption of Au–CdS is significantly broadened in the visible regime (500–700 nm), which can be attributed to the existence of plasmon–exciton coupling [119, 3]. They further performed finitedifferent time-domain (FDTD) simulation to evaluate optical response in Au–CdS with different structural symmetry and summarize the results in Figure 25B. The FDTD simulation reproduces symmetry dependent optical property very well. The strong SPR-induced visible light absorption of Au/CdS colloids can overlap most of the solar spectrum, and particularly important, its tunability makes it feasible to optimize photorelated processes, including solar water splitting.

It is well known that CdS or CdSe possesses conduction and valence bands at potentials appropriate for water reduction, namely, the bottoms of conduction bands locate at a more negative potential than the reduction potential of H+ to H2. In order to get high efficiency of H2 evolution by sunlight irradiation, more visible light should be harvested, and then photo‐ excited electron/hole should be separated and migrated to the surface without recombination [120, 121]. By integrating with plasmonic Au nanoparticle in a hybrid nanostructure, its tunable optical response achieved in Figure 25A and B should make it feasible to mediate the visible light-induced electron/hole separation in CdX and electron transfer from CdX to Au surface due to the plasmon–exciton coupling [3,122]. As shown in Figure 25C and D, for comparison, photocatalysis with pure 4.5 nm CdS QDs and the CdS/Au heterodimers prepared by reported in situ deposition are also presented [123–124]. The rate of CdS QDs alone is 0.1 mmol h−1 g −1.The concentric core/shell Au–CdS has the least H2 evolution activity (0.009 mmol h−1 g−1) and the heterodimer structured Au/CdS has better H2 evolution activity (7.3 mmol h−1 g−1) that manifest a dramatic photoactivity enhancement of 730 times. However, the CdS/Au hetero‐ dimers has less H2 evolution activity (0.359 mmol h−1 g−1). Figure 25C and D can be understood by the mechanism of SPR enhanced electron–hole separation and collection, as illustrated in the Figure 25E. For an asymmetric Au–CdS heterodimer, under visible light illumination, the energy band alignment between CdS and Au suggests a rapid electron transfer from conduc‐ tion band of CdS to Au nanoparticle. In the meantime, concentrated electric field due to SPR of metal constituent can significantly enhance light absorption of CdS semiconductor and promote charge separation near the Au–CdS interface. Such synergetic effect enriches the electron (e-) on the Au tip for the efficient water reduction and leads to high efficient H2 evolution activity. The bad H2 evolution activity of the CdS/Au heterodimers mainly attributes to the weak SPR and consequent visible light harvest of smaller sized Au (2–3 nm). On the other hand, in a concentric Au–CdS core–shell nanostructure with high structure symmetry, the electrons are retained in the Au core after photo excitation and charge separation and cannot participate in photocatalytic reaction; thus, the photon absorption of CdS is suppressed. As a result, almost no H2 evolution is observed with concentric Au–CdS.

**3.3. Metal–semiconductor core/shell nanocrystals and photocatalysis application**

*3.3.1. Metal core–semiconductor shell: large lattice mismatch-induced interface control and*

In metal-semiconductor hetero structures, the contact interface between metal and semicon‐ ductor plays a critical role in transfer and separation of charge carriers [117–118]. Our precise symmetry control of Au–CdX can enable different contact interface between metal and semiconductor components at the nanoscale, thus offering a new dimension to control optical and electronic properties of hybrid nanostructures in a highly control manner. Figure 25A highlights evolution of plasmon–exciton coupling in Au–CdS enabled by their symmetry control. As compared with the SPR feature of a pure metal nanoparticles, optical absorption of Au–CdS is significantly broadened in the visible regime (500–700 nm), which can be attributed to the existence of plasmon–exciton coupling [119, 3]. They further performed finitedifferent time-domain (FDTD) simulation to evaluate optical response in Au–CdS with different structural symmetry and summarize the results in Figure 25B. The FDTD simulation reproduces symmetry dependent optical property very well. The strong SPR-induced visible light absorption of Au/CdS colloids can overlap most of the solar spectrum, and particularly important, its tunability makes it feasible to optimize photorelated processes, including solar

It is well known that CdS or CdSe possesses conduction and valence bands at potentials appropriate for water reduction, namely, the bottoms of conduction bands locate at a more

H2 evolution by sunlight irradiation, more visible light should be harvested, and then photo‐ excited electron/hole should be separated and migrated to the surface without recombination [120, 121]. By integrating with plasmonic Au nanoparticle in a hybrid nanostructure, its tunable optical response achieved in Figure 25A and B should make it feasible to mediate the visible light-induced electron/hole separation in CdX and electron transfer from CdX to Au surface due to the plasmon–exciton coupling [3,122]. As shown in Figure 25C and D, for comparison, photocatalysis with pure 4.5 nm CdS QDs and the CdS/Au heterodimers prepared by reported in situ deposition are also presented [123–124]. The rate of CdS QDs alone is 0.1 mmol h−1 g −1.The concentric core/shell Au–CdS has the least H2 evolution activity (0.009 mmol h−1 g−1) and the heterodimer structured Au/CdS has better H2 evolution activity (7.3 mmol h−1 g−1) that manifest a dramatic photoactivity enhancement of 730 times. However, the CdS/Au hetero‐ dimers has less H2 evolution activity (0.359 mmol h−1 g−1). Figure 25C and D can be understood by the mechanism of SPR enhanced electron–hole separation and collection, as illustrated in the Figure 25E. For an asymmetric Au–CdS heterodimer, under visible light illumination, the energy band alignment between CdS and Au suggests a rapid electron transfer from conduc‐ tion band of CdS to Au nanoparticle. In the meantime, concentrated electric field due to SPR of metal constituent can significantly enhance light absorption of CdS semiconductor and promote charge separation near the Au–CdS interface. Such synergetic effect enriches the electron (e-) on the Au tip for the efficient water reduction and leads to high efficient H2 evolution activity. The bad H2 evolution activity of the CdS/Au heterodimers mainly attributes

to H2. In order to get high efficiency of

*photocatalytic applications*

324 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

water splitting.

negative potential than the reduction potential of H+

**Figure 25.** (A) Evolution of experimental UV-Vis absorption spectra with symmetry of Au/CdS colloid. (B) The normal‐ ized FDTD simulation of the SPR in these Au/CdS heteronanostructures. (C) Comparison of H2 evolution activities of different Au–CdS photocatalysts. (D) Time evolution of photocatalytic generation of H2 evolution amount versus irra‐ diation time for them. (E) Schematic illustrations of visible light-induced electron/hole separation and transfer differ‐ ence in heterodimer and core/shell Au/CdS when dispersed in aqueous solution containing sacrificial reagents. From Zhao et al. [32] with permission; copyright Wiley-VCH Verlag GmbH & Co. KGaA.

In conclusion, based on the series of metal-semiconductor hybrid nanostructures from concentric core–shell to nonconcentric heterodimer with precisely controlled separation between metal and semiconductor constituents, the gradual symmetry evolution has led to novel control of optical response (plasmon–exciton coupling) and photocatalytic activity in hybrid nanostructures, which highlight the importance of nanoscale interface control for both fundamental understanding and technology applications. Thus, it can allow us to evaluate a possible upper limit of a photocatalytic reaction and guide the design toward high efficient hybrid nanostructures. Due to the plasmon intensity difference from different sized Au NPs, the larger sized Au, such as tens of nanometers to hundreds of nanometers size, the plasmon enhancement for photocatalysis may become strong.
