Journal of Materials Chemistry C · 2019. 3. 28. · 1008 | . Mater. Chem. C, 2017, 5 , 1008--1021 This journal is ' The Royal Society of Chemistry 2017 Cite this J.Mater. Chem. C,

1008 | J. Mater. Chem. C, 2017, 5, 1008--1021 This journal is©The Royal Society of Chemistry 2017

Cite this: J.Mater. Chem. C, 2017,

5, 1008

Plasmonic nanostructures in solarenergy conversion

Wei Ye, Ran Long,* Hao Huang and Yujie Xiong*

Photocatalysis and photovoltaics are two major approaches sharing similar processes (including light

absorption, and charge generation and separation) for solar energy conversion with semiconductors.

Various strategies have been proposed to improve the efficiency of solar energy conversion due to

limited light absorption and rapid charge recombination in semiconductors. Integrating semiconductors

with plasmonic nanostructures has been proven as an effective way to greatly enhance the performance

in photocatalysis and photovoltaic devices. This review outlines the fundamental mechanisms, including

hot electron injection, local electromagnetic field enhancement and resonant energy transfer, which are

responsible for both plasmonics-enhanced photocatalysis and photovoltaics. Furthermore, we review

some recent progress in practical applications such as photocatalytic water splitting, artificial photo-

synthesis, photodegradation of organic pollutants and solar cells integrated with plasmonic nanostructures.

In specific cases, the possible working mechanisms for the enhancement of photocatalytic or photovoltaic

performance by plasmonics are clarified together with materials design. Finally, the existing challenges and

future prospects for the utilization of plasmonics in solar energy conversion are discussed.

1. Introduction

The current energy crisis spurs us on to find a new pollution-free,sustainable energy source. Solar energy is the most promisingone to meet the future demand as the energy from the Sun is in

limitless supply. Photocatalysis and photovoltaics are the mainprocesses for the conversion of solar energy, which transformand/or store photon energy to chemical bonds and electricity,respectively.1,2 For example, photocatalytic hydrogen evolutionand carbon dioxide reduction into organic molecules such asmethane, methanol or formic acid can readily achieve thetransformation and storage of solar energy.3,4 This transforma-tion not only produces fuel molecules, but also provides analternative carbon-neutral process for the world.

Although the photocatalytic and photovoltaic approachesshow great potential in solar energy conversion, the conversion

Hefei National Laboratory for Physical Sciences at the Microscale, iChEM

(Collaborative Innovation Center of Chemistry for Energy Materials), School of

Chemistry and Materials Science, Hefei Science Center (CAS), and National

Synchrotron Radiation Laboratory, University of Science and Technology of China,

Hefei, Anhui 230026, P. R. China. E-mail: [email protected],

[email protected]

Wei Ye

Wei Ye was born in Anhui, China,in 1987. He received his BS inapplied chemistry from GuangxiUniversity in 2010, and his PhD ininorganic chemistry from ShandongUniversity in 2015. Currently he is apostdoctoral fellow working withProf. Yujie Xiong at the Universityof Science and Technology of China(USTC). His research interests focuson the controlled synthesis andapplications of noble metal nano-crystals. Ran Long

Ran Long was born in Anhui,China, in 1987. She received herBS in chemistry in 2009, and herPhD in inorganic chemistry underthe tutelage of Professor YujieXiong in 2014, both from theUniversity of Science and Techno-logy of China (USTC). After a two-year postdoctoral training withProfessors Yujie Xiong and LiSong, she is currently a ResearchAssociate Professor at the USTC.Her research interests focus on thecontrolled synthesis and catalyticapplications of metal nanocrystals.

Received 8th November 2016,Accepted 30th December 2016

DOI: 10.1039/c6tc04847a

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efficiencies from solar energy to chemical energy or electricityin many systems have yet to be improved upon for practicalapplications. Looking into the working mechanisms, one canrecognize that two common bottlenecks often exist in bothsolar utilization schemes – the limited light absorption andthe rapid charge recombination in semiconductors.5 Somesemiconductors that have been explored for photocatalyticand photovoltaic applications possess relatively low lightabsorption coefficients. In particular, semiconductors withwide bandgaps such as TiO2 (Eg = 3.2 eV) can only absorb lightin the ultraviolet region so as to waste the visible and near-infrared photons that account for a large portion of the fullsolar spectrum.6 Moreover, the semiconductors often havelarge refractive indices, which would further exacerbate lightreflection. On the other hand, the photoexcited electrons andholes tend to recombine before they can reach the catalystsurface for chemical reactions or electrodes for electricitycollection.7 When the apparent physical sizes of semiconduc-tors are larger than the carrier transmission distance, thephotoexcited electrons and holes tend to recombine in the bulkphase rather than transport to the surface. Thus the twobottlenecks largely limit the utilization and conversion efficienciesof solar photons. For this reason, it is imperative to developsolutions to broaden light absorption and suppress charge recom-bination toward efficient solar energy conversion.

The integration of semiconductors with plasmonic metalnanostructures has been proven to be an effective solution toovercome the limitations as the plasmonic nanostructures canact as subwavelength antennae to concentrate light and theresulting enhanced local electromagnetic field can promote theseparation of electrons from holes.8,9 In addition, the plasmonicnanostructures absorb the light via the collective oscillations ofsurface electrons, so the free electrons can be excited and directlyinjected into the conduction band of semiconductors forchemical reactions or electricity generation.10 Taken together,

these merits designate the implementation of plasmonic nano-structures into photocatalysis and photovoltaics as a promisingstrategy for solar energy conversion. In this review article, we willoutline the recent progress in plasmonics-enhanced photo-catalytic (including photocatalytic water splitting, artificialphotosynthesis and photodegradation of organic pollutant)and photovoltaic systems, together with the fundamentalmechanisms behind each application. From the workingmechanisms, the readers would recognize that many applica-tions share similar fundamentals, and are able to designplasmonic nanostructures and their interfacial structures withsemiconductors for specific applications. We aim to provide adifferent overview on the entire plasmonic solar energy conversionfield from the existing reviews that have been mainly focusedon the fundamental physics of interfacial structures or a singleapplication.11–17

2. Surface plasmon of metalnanostructures

Plasmonic metal nanostructures offer high capabilities for lighttrapping and electromagnetic field enhancement as the freeelectrons on the metal surface can have strong interactionswith incident photons. When the frequency of the incidentphotons matches well with the inherent frequency of the freeelectrons on the surface of metal nanostructures, it will lead toa collective oscillation commonly known as localized surfaceplasmon resonance (LSPR) as shown in Fig. 1.18,19 The collectiveoscillation of electrons results in an enhanced local electro-magnetic field, which is mainly confined to the metal surface.Upon this resonant excitation, the plasmon relaxes within tens offemtoseconds. The relaxation partially takes place through radia-tive photon emission (i.e., scattering). Meanwhile, it generatesenergetic charge-carriers at the nanoparticle surface through

Hao Huang

Hao Huang was born in Jiangxi,China, in 1994. He received his BSin chemistry (Special Class for theGifted Young) in 2013 from theUniversity of Science and Techno-logy of China (USTC). Since thenhe has been studying as a PhDcandidate under the tutelage ofProfessor Yujie Xiong at theUSTC. His research interestsfocus on the controlled synthesisand catalytic applications ofmetal nanostructures. Yujie Xiong

Yujie Xiong received his BS inchemical physics in 2000 andPhD in inorganic chemistry in2004 (with Professor Yi Xie),both from the University ofScience and Technology of China(USTC). After a four-year trainingwith Professors Younan Xia andJohn A. Rogers, he joined the NSF-NNIN at Washington Universityin St Louis as the PrincipalScientist and Lab Manager.Starting from 2011, he is aProfessor of Chemistry at the

USTC. He has published more than 130 papers with over 10 000citations (H-index 48). His research interests include the synthesis,fabrication and assembly of inorganic materials for energy andenvironmental applications.

Review Journal of Materials Chemistry C


non-radiative Landau damping (i.e., absorption). The energeticelectron–hole pairs are commonly called hot electrons/holes,whose energy matches the resonant photon energy. The hotelectrons can interact with other charge carriers and nanoparticlelattices, inducing the cooling of hot electrons. The cooling willtransfer the energy of hot electrons to the phonon modes ofnanoparticles, observed as a photothermal process. The photo-thermal conversion increases the temperature of the metal surfacedozens of degrees Celsius.

Enabled by the working mechanisms, surface plasmonprovides a powerful tool to concentrate light at the metal–dielectric interface. Here the dielectric layer includes all thepossible types of matters that surround metal nanostructuressuch as air, water, organic species or inorganic coating. As anantenna, the surface plasmon shows great application prospectsin surface-enhanced Raman scattering (SERS),20 sensing,21 photo-catalysis22 and photovoltaics.23 Among various plasmonic metalnanostructures, Au and Ag nanocrystals are most widely used asantennae to concentrate light in photocatalysis and photo-voltaics.24,25

2.1 Manipulation of LSPR in metal nanostructures

There are two basic parameters for LSPR-mediated applica-tions: the coefficient and wavelength of maximum extinctionwhere the extinction is the sum of scattering and absorption.The extinction coefficient reflects the ability to concentrate lightat the metal–dielectric interface. Relative to the coefficient, thewavelength of maximum extinction may play a more importantrole in solar light utilization as it determines the band of lightharvesting. The excitation of wide-bandgap semiconductorsrequires the high photon energy mainly located in the ultravioletregion, which fails to utilize the visible and infrared light thatoccupies 95% of photons in the full solar spectrum. Thusharvesting visible and infrared light is a central task for solarenergy conversion, towards which surface plasmons can makemajor contributions. In certain plasmon-mediated applications(e.g., local electromagnetic field enhancement), it also requires asubstantial spectral overlap between semiconductors and plasmonicnanostructures, highlighting the importance of tuning plasmonicbands. For this reason, it is highly important to tune theplasmonic features toward solar energy conversion applications.

Previous reports indicate that the LSPR features have strongcorrelations with the compositions and morphologies of metalnanostructures as well as the dielectric layers on the metalsurface. The correlations provide a great opportunity for tuning

the coefficient and wavelength of maximum extinction. Forinstance, the wavelengths of maximum extinction for silver,gold and copper nanospheres are located in the near-ultravioletand visible regions (420 nm for 38 � 12 nm Ag nanospheres,520 nm for 25 � 5 nm Au nanospheres, and 610 nm for133 � 23 nm Cu nanospheres), respectively (Fig. 2a).

As the surface plasmon may propagate along the surfacewith different directions, the morphologies of metal nano-structures determine the number of extinction peaks (Fig. 2b).In the case of one-dimensional structures, the surface plasmonpropagation takes place along the longitudinal direction. Forexample, Au nanorods exhibit two resonance peaks which areattributed to longitudinal and transverse plasmon resonancebands, respectively. The longitudinal LSPR peak is sensitive tothe aspect ratios (ARs) of Au nanorods which can be regulatedvia selective chemical etching.26 The wavelength of maximumextinction can also be tuned via the anisotropic growth ofanother element with the assistance of selective surfactantadsorption on the specific surface or via underpotential deposi-tion (UPD) mechanism.27 Longitudinal LSPR maxima can bered-shifted from 700 nm to 1200 nm by increasing aspect ratios(Fig. 2c), while the transverse LSPR peak remains unchanged.28

In addition to the intrinsic parameters, extinction spectraare also associated with the dielectric layer around metalnanostructures. In a typical case, Au nanodisks with increasingsizes show totally different wavelengths and coefficients of maxi-mum extinction when they are immersed in water (n = 1.33) andbiomedium (n = 1.41), respectively (Fig. 2d).29 The wavelength

Fig. 1 Localized surface plasmon for a spherical gold nanoparticle.Adapted with permission from ref. 18. Copyright 2007 Annual Reviews.

Fig. 2 Optical properties of plasmonic nanostructure. (a) Normalizedextinction spectra for different metal elements including Ag (38 � 12 nmdiameter), Au (25 � 5 nm) and Cu (133 � 23 nm) nanospheres, respectively.(b) Normalized extinction spectra for Ag nanostructures with differentmorphologies. Adapted with permission from ref. 22. Copyright 2011Nature Publishing Group. (c) Normalized extinction spectra for Au nano-rods with increasing aspect ratios. Adapted with permission from ref. 28.Copyright 2013 American Chemical Society. (d) The extinction spectra forAu nanodisks of different sizes in water (blue, n = 1.33) and biomoleculemedia (red, n = 1.41). Adapted with permission from ref. 29. Copyright2005 American Chemical Society.

Journal of Materials Chemistry C Review


of maximum extinction is red-shifted with the increase in theirextinction intensities as the dielectric constant (n) is increased.This highlights that we should pay attention to the actualenvironment of plasmonic nanoparticles in real plasmon-related applications.

2.2 Spatial distribution of electromagnetic field

As we mentioned above, the free electrons on the metalnanostructure surface will achieve a collective oscillation underincident light with an appropriate frequency. This oscillation isaccompanied by the enhanced local electromagnetic field inspace as shown in Fig. 3a. Theoretical simulation is a powerfultool to reveal the plasmonic behaviour of metal nanostructuresunder incident light. In recent years, a number of theorieshave been developed, and in turn, plentiful cases have beenanalyzed.30,31 In particular, the Mie theory is thought to be themost efficient theory to predict the plasmonic behavior underincident light with diverse frequencies.32 The discrete dipoleapproximation (DDA) and finite difference time-domain (FDTD)calculations based on the Mie theory are powerful tools toinvestigate the spatial distribution of the electromagnetic fieldfor complicated nanostructures.33

It has been proven that the spatial distribution of theelectromagnetic field is strongly dependent on the geometrical

shape of metal nanostructures.34–36 The electromagnetic near-field enhancement for an isolated nanosphere is unevenlydistributed, and it becomes more complicated when the nano-structure has an irregular shape. It is commonly consideredthat the field enhancement is concentrated on sharp cornersand edges known as ‘‘hot spots’’. Taking Au concave nanocubesas an example, the electromagnetic field at sharp corners isobviously stronger than that on faces (Fig. 3b and c).37 Specifically,the enhanced electromagnetic field intensity at the corners can be1000 times higher than the incident light. Similarly, Au nanorodsoffer the localization of electromagnetic field enhancement attheir endpoints. This indicates that the metal nanostructures withmore complicated corners may possess a stronger capability oflight concentration.

When two nanoparticles with different morphologies orcompositions get closer to form a symmetry-breaking dimer,their local electromagnetic fields will be coupled to induce astrong Fano resonance. Tian et al. investigated electromagneticfield coupling by assembling Au nanospheres and Ag nano-cubes with an appropriate gap distance. As the Au nanosphereand the Ag nanocube form a dimer with a gap distance, abroad scattering peak can be generated when polarization isperpendicular to the dimer axis in the simulated scatteringspectra (Fig. 3d). The broad peak can be split into two peaks,

Fig. 3 Spatial distribution of the electromagnetic field for metallic nanostructures. (a) The electromagnetic field intensely distributed around a 25 nm Aunanoparticle with an incident light wavelength of 850 nm. Adapted with permission from ref. 9. Copyright 2010 Nature Publishing Group. (b) Scanningelectron microscopy (SEM) image of a concave Au nanocube. (c) Theoretical simulations of the near-field enhancements (E2) plotted for a single concaveAu nanocube. Adapted with permission from ref. 37. Copyright 2012 American Chemical Society. (d) The simulated scattering spectra and electric fielddistributions of Au (nanosphere)–Ag (nanocube) dimer, in which electric polarization is parallel (black curve) and perpendicular (red curve) to the dimeraxis. Adapted with permission from ref. 38. Copyright 2016 Royal Society of Chemistry. (e) Charge plots for Au cluster assembly. Adapted with permissionfrom ref. 39. Copyright 2013 American Chemical Society. (f) Optical absorption map for a Au–TiO2 Janus particle. Adapted with permission from ref. 40.Copyright 2012 John Wiley & Sons, Inc.



which are assigned to the dipole–dipole coupling mode, whenpolarization is parallel to the dimer axis. The strong coupling isreflected by the substantially higher field enhancement at thehot spots of 1 and 2.38 For a more complicated Au nanoclusterassembly, abundant Au–Au interfaces are formed to couplethe localized surface plasmon. As a result, charge plots showmore complicated field distribution, concentrated at the clusterinterfaces (Fig. 3e).39

In a different system, light can be concentrated and con-fined to the metal–oxide interface when a metal nanoparticle isin contact with the oxide. For a Au–TiO2 Janus particle, the Aunanosphere is partly surrounded by the TiO2 layer with a highrefractive index and the solvent with a low refractive index.As such, a local electromagnetic field is mainly confined to theAu–TiO2 interface as shown in Fig. 3f. Based on this importantfeature, the light concentrated at the interface couples with thesemiconductor to more easily transfer solar energy to TiO2 inpractical applications owing to the reduced operation distanceand the enhanced local electromagnetic field.40

In brief, light concentration and confinement can be achievedat the surface of plasmonic nanostructures via plasmonic effects.The plasmonic properties can be tuned by controlling the morpho-logies, constituents and physical arrangements of plasmonic metalnanostructures, harvesting visible and near-infrared light. Thetunable optical properties can in turn meet the demand forharvesting solar energy in a specific spectral range towardapplications in photocatalysis and photovoltaics.

3. Plasmonic nanostructures forphotocatalysis

Photocatalysis is a process converting solar energy to chemicalenergy that can be stored in chemical bonds by virtue of semi-conductors. In a typical photocatalytic process, semiconductorsabsorb light with appropriate energy to excite their electronsfrom the valence band to the conduction band, which generatesphotoexcited electrons and holes. The photo-generated electron–hole pairs are separated and transferred to the surface for redoxreactions such as hydrogen and oxygen evolution during watersplitting. The integration of semiconductors with plasmonicnanostructures can make multiple contributions to the improve-ment of photocatalytic performance. For instance, metal nano-structures with a plasmonic band in the visible region can offercomplementary light absorption to wide-bandgap semiconductorssuch as TiO2, which are generally more active in photocatalysis;plasmonic nanostructures can enhance charge separation insemiconductors using different methods.

3.1 Mechanisms

When plasmonic metal nanostructures are integrated withsemiconductors, the charge kinetics in photocatalysis can bemaneuvered. In such a metal–semiconductor hybrid system, aSchottky barrier is formed at the metal–semiconductor inter-face. The Schottky junction traps photoexcited electrons on themetal, which designates the metal as a cocatalyst for surface

reduction reactions.41 More importantly, plasmonic nanostruc-tures can act as antennae to concentrate light to vastly promotelight utilization efficiency. In principle, plasmonic nanostructuresfacilitate the photocatalytic process by means of the followingthree mechanisms: hot electron injection, local electromagneticfield enhancement and resonant energy transfer.

Hot electron injection. Plasmon decay takes place throughradiative photon re-emission and non-radiative Landau damping.The non-radiative decay generates energetic electrons and holes,which are commonly referred to as hot electrons and hot holes,respectively (Fig. 4a). When energetic incident photons areabsorbed by the free electrons of the metal nanostructure, theelectrons will leap over the Fermi level to a higher energy level(Ef + hn) (Fig. 4b). The majority of energetic electrons aredissipated by electron–electron scattering in about 100 femto-seconds (fs). In the meantime, a small number of energeticelectrons will be transferred to adjacent species such as organicmolecules and semiconductors (Fig. 4c).42

Driven by the Schottky junction, the electrons in the con-duction band of semiconductors have a tendency to flow to themetal surface. In this case, considerable attention should bepaid to the Fermi level equilibrium after the accumulation ofcarriers on the metal nanostructures (Fig. 4d). Since electronaccumulation can shift the Fermi level of metallic nanostructures

Fig. 4 (a) Plasmon decay through radiative photon re-emission and non-radiative Landau damping. (b) Generation of plasmonic hot electrons:electrons on the d-band are excited and come across the Fermi level viaa non-radiative process. (c) Injection of hot electrons into the conductionband of semiconductors. Adapted with permission from ref. 10. Copyright2014 Nature Publishing Group. (d) Fermi level equilibration of semicon-ductor–metal nanocomposites before and after UV irradiation. Adaptedwith permission from ref. 43. Copyright 2004 American Chemical Society.



to a more negative potential, the resulting apparent Fermi level ofthe hybrid structure would eventually reach an equilibrium, whichis closer to the conduction band of semiconductors. The shiftingof the Fermi level thus improves the energetics of the semicon-ductor system, which can be regulated by controlling the sizeof metal nanoparticles.43 The Fermi level equilibration effectfacilitates the extraction of carriers and suppression of chargerecombination, and contributes to the enhanced photocatalyticperformance.44 Meanwhile, the hot electrons with energy levelabove the Schottky barrier can be reversibly injected into theconduction band of semiconductors (Fig. 5a). In the extreme case,a small portion of hot electrons can tunnel across the energybarrier. This process would reduce the electron density of metalnanostructures, and the surrounding solution and other mediamay transfer electrons to the metal nanostructures to maintainelectrical neutrality in the entire system.

Thanks to the tunable plasmonic features of metal nano-structures, this scheme enables the maximum utilization ofsolar energy in visible and even infrared light by integratingmetal nanostructures with semiconductors. One classical exampleis the hybrid system between Au/Ag nanoparticles and TiO2

nanostructures. In 2004, Tatsuma et al. found that the incidentphoton-to-current conversion efficiency (IPCE) was greatlyenhanced under visible-light irradiation by loading Au/Agnanoparticles onto the TiO2 nanofilm. It is known that theenergy of visible light is insufficient to excite the TiO2 semi-conductor. It was thus concluded that IPCE enhancement wasattributed to the charge transfer that took place from thesurface plasmon of Au/Ag nanoparticles excited by visible light

to the conduction band of TiO2. The IPCE was a functionof incident light wavelength, clearly demonstrating that thesurface plasmon of metal nanostructures was the direct reasonfor electron–hole generation.45,46 Although the specific mechanismremained unclear at that time, the huge enhancement of IPCEboosted the research based on the injection of hot electrons. Apartfrom the Au–TiO2 system, the hot electron injection could beapplied to many other hybrid systems such as Au–ZnO nanowirearrays,47 and Au/Pt/C3N4 nanocomposites,48 which achievedsignificant photocatalytic performance enhancement. In a typicalcase, the performance of photocatalytic hydrogen evolution wasenhanced 64-fold for TiO2 nanosheets by integrating them withAu nanoparticles.49

Local electromagnetic field enhancement. According to theprinciple of surface plasmons, one can recognize that an intenselyenhanced local electromagnetic field would be generated nearplasmonic nanostructures under light irradiation. Previousstudies have proven that the local electromagnetic field canenhance photocatalytic performance, in which the contributionof direct electron transfer (DET) is excluded by covering metalnanostructures with an insulation layer.50 The insulation layercompletely prevents the injection of hot electrons into semi-conductors, so the performance enhancement should beascribed to the field effect. The local electromagnetic fieldfavours the generation and separation of electron–hole pairsin semiconductors (Fig. 5b), whose rates are proportional tofield intensities.23 This situation is quite similar to the acceleratedmotion of ions in an electromagnetic field. However, it should benoted that the local electromagnetic field would not directly excitesemiconductors to generate electron–hole pairs. The local electro-magnetic field enhancement (LEMF)-induced charge separationonly works for the incident photons with energies above thebandgap of semiconductors.

In the LEMF mechanism, it is worth mentioning that themetal–semiconductor interface can provide an enhanced elec-tromagnetic field. The electromagnetic field rapidly decays withthe distance, and the working distance is few nanometers.50

The direct contact between the metal and the semiconductorsubstantially reduces the distance and enhances photocatalyticperformance. In addition to the distance, increasing the con-tact area between the metal and the semiconductor is anothereffective approach for a good coupling of the electromagneticfield with the semiconductor, which further enhances theelectron–hole generation.51 In the case of direct contact, theeffects from hot electrons and local electromagnetic fieldenhancement are inevitably entangled in a photocatalyticsystem, which forms an obstacle to justifying the mechanismthat plays a leading role.

Resonant energy transfer. Apart from hot electron injection(Fig. 5a) and local electromagnetic field enhancement (Fig. 5b),surface plasmons can also work for semiconductor-basedphotocatalysis through resonant energy transfer (RET) as longas the plasmonic band overlaps the spectrum of semiconductorlight absorption. As mentioned above, the local electromagneticfield enhancement mechanism only acts at the energy level abovethe bandgap of the semiconductor by improving charge separation.

Fig. 5 Charge separation mechanisms at the semiconductor–plasmonicnanostructure interface: (a) direct electron transfer (DET) of plasmonic hotelectrons to the conduction band of the semiconductor; (b) local electro-magnetic field enhancement (LEMF) of semiconductor charge separation;and (c) resonant energy transfer (RET) from the plasmonic dipole to theelectron–hole pair in the semiconductor. Adapted with permission fromref. 52. Copyright 2012 American Chemical Society. (d) Schematic illustra-tion for the working mechanism in Au nanoparticles-confined N-dopedTiO2 bowl nanoarrays under full-spectrum irradiation. (e) Chart of photo-catalytic hydrogen and oxygen evolution performance using Aunanoparticles-confined N-doped TiO2 bowl nanoarrays as catalysts.Adapted with permission from ref. 55. Copyright 2016 Elsevier.



As compared with the LEMF, RET is a process which directlygenerates electron–hole pairs in semiconductors through therelaxation of localized surface plasmon dipoles.52,53

Here we take metal@SiO2@TiO2 (metal refers to Au or Agnanosphere) core–shell nanostructures as an example to illustratethe possible mechanism. To resolve the concentration changes ofcarriers created by surface plasmon in semiconductors, transientabsorption spectroscopy was employed to examine the system. Inthis system, the plasmonic band of Ag was partly overlapped withthe absorption band of TiO2 while there was no overlap betweenAu and TiO2. The SiO2 interlayer was used to effectively shield thehot electron injection from the metal surface to TiO2. It turned outthat Ag@SiO2@TiO2 enabled resonance energy transfer usingvisible light irradiation. In sharp contrast, no energy transferwas observed for Au@SiO2@TiO2 core–shell nanoparticles dueto the lack of optical overlap.54

As compared with LEMF, light harvesting via the RETmechanism can be extended to longer wavelengths, making itpossible to utilize visible and near-infrared light. For instance,Xiong and co-workers designed a N-doped TiO2 bowl array–Auhybrid system, in which the light absorption of TiO2 was tuned byN doping to overlap the plasmonic band of Au nanoparticles.55

With charge generation and separation enhanced by RET,the hybrid system achieved the hydrogen evolution rate of0.637 mmol g�1 h�1 in the full spectrum, which well exceeded thoseof bare TiO2 bowl arrays in the full spectrum (0.07 mmol g�1 h�1)and N-doped TiO2–Au arrays under ultraviolet irradiation(0.256 mmol g�1 h�1) (Fig. 5e).

3.2 Applications in photocatalysis

The integration of semiconductors with plasmonic nanostruc-tures for photocatalysis recently has made remarkable progressin water splitting and artificial photosynthesis. Metal nano-particles greatly boosted photocatalytic efficiencies through hotelectron injection and field-related mechanisms.

Water splitting. Water splitting driven by solar energy is apromising approach to solve the current energy crisis andenvironmental pollution.1 In this system, semiconductors arephotoexcited to generate electron–hole pairs which can beseparated and used for hydrogen and oxygen evolution, respec-tively. In the past years, the semiconductors integrated withplasmonic nanostructures have shown enhanced photocatalyticperformance in hydrogen or oxygen evolution,8 includingAu–TiO2 dumbbell-like nanorods,56 Au–ZnO nanorods arrays,57

Au-decorated Ta2O5/Ta3N5 nanoparticles,58 Ag-modified BiOClnanosheets,59 Au–CdS hierarchical structures,60 and Au@Cu2ZnSnS4

core–shell nanoparticles.61 For instance, anatase TiO2 nanosheetswith exposed (001) facets achieved 64-fold enhancement insolar-to-hydrogen conversion efficiency by incorporating 6 nmAu nanoparticles.49 Zhao et al. reported an all-solid-stateg-C3N4/Au/P25 Z-scheme hybrid system for photocatalytichydrogen evolution, which exhibited a 30 times higher activitythan pristine g-C3N4.62 Yu et al. modified ZnS nanoflowers withAu nanoparticles at 0.4 wt% to achieve an impressive hydrogenevolution rate of 3306 mmol h�1 g�1.63

Overall, the photocatalytic performance enhancement byplasmonic metal nanoparticles can be attributed to the threemechanisms outlined in Section 3.1. Specifically, the advantagescan be summarized: (1) the plasmonic nanostructures extend thelight absorption of photocatalysts to visible and near-infraredregions; (2) the local electromagnetic field or dipole–dipole inter-action enhances the generation and separation of electron–holepairs in semiconductors; (3) the hot electrons derived fromsurface plasmon resonance are injected into the conductionband of semiconductors and provide additional carriers forphotocatalytic reactions; and (4) the Schottky barrier naturallyformed at the metal–semiconductor interface traps photoexcitedelectrons on metal for surface reduction reactions. In a specialcase, whispering gallery mode (WGM) resonances can extendlight absorption to the entire visible region by carefully arrangingAu nanoparticles.64

Indeed, harvesting visible and near-infrared light is essentiallyimportant for photocatalysis as it occupies a large proportion ofsolar photons. Taking TiO2 as an example, the bandgap of 3.2 eVlimits its light absorption to the ultraviolet region. The imple-mentation of plasmonic metal nanostructures into photocatalysiscan greatly improve the utilization efficiency of long-wavelengthlight. Xiong et al. integrated multiple plasmonic and co-catalystmetal nanostructures with TiO2 nanosheets for photocatalytichydrogen evolution. Au nanocubes and nanocages endow thehybrid system with the capability of harvesting visible and near-infrared light, respectively. Together with the Pd nanocube as acocatalyst, the hybrid system showed dramatically higher activitythan its counterparts.65

The plasmonic effects not only work for the hydrogenevolution reaction, but are also used for the other half reactionin water splitting (i.e., oxygen evolution). The oxygen evolutionis the bottleneck for water splitting due to its ultraslow reactionkinetics. Xiong et al. designed a system by integrating Ag andPd nanocubes on the different facets of BiOCl nanosheets(Fig. 6a). In this system, carrier trapping by the Schottkyjunction was synergized with plasmonic hot carrier injectionfor photocatalytic performance enhancement by selecting twodifferent semiconductor facets for interfacing with metals. Furtherintegrated with the local electromagnetic field enhancement ofcharge separation in semiconductor by plasmonics, the hybridsystem performance achieved an about 7-fold enhancement in

Fig. 6 (a) Schematic illustrating the band alignments and charge flow attwo metal–semiconductor interfaces. (b) Photocatalytic O2 evolution fromwater with BiOCl, BiOCl(110)-Pd, Ag-(001)BiOCl and Ag-(001)BiOCl(110)-Pdas catalysts under full-spectrum irradiation. Adapted with permission fromref. 59. Copyright 2015 John Wiley & Sons, Inc.



photocatalytic oxygen evolution (Fig. 6b).59 Kominami et al.designed a Pt/Au/WO3 photocatalyst, in which the surfaceplasmon was coupled with the excitation of WO3, whichenhanced both oxygen and hydrogen evolution reactions undervisible light irradiation.66 Plasmonic effects can also contributeto photoelectrochemical water oxidation.67 Moskovits et al.designed TiO2-coated Au nanorod arrays as photoanodes forwater splitting using visible light. They achieved 20-foldproduction efficiency enhancement under visible light ascompared with UV irradiation.68 The noble metals (Au, Ag)could be replaced by cheap metals such as Al in the core-multishell nanowires that were used as photoelectrodes forwater splitting, in which Al plasmonics enhanced visible lightabsorption.69

Carbon dioxide reduction. The photo-generated electronscan also be used for carbon dioxide reduction. The reduction ofCO2 to hydrocarbon fuels is highly important for the develop-ment of sustainable energy as well as for providing an approachto carbon neutrality. As compared with proton reduction, CO2

reduction is a more complicated process which may produceCO, formic acid, methanol or methane by multiple-electrontransfer reactions as listed below:

CO2 + 2H2O - CH4 + 2O2 (1)

2CO2 + 4H2O - 2CH3OH + 3O2 (2)

2CO2 + 2H2O - 2CHOOH + O2 (3)

Apparently the selectivity of products is the key to CO2

reduction. It is extremely desirable to obtain fuel moleculessuch as methanol and methane with high selectivity. In the pastyears, it has been demonstrated that integration of plasmonicnanostructures with semiconductors can effectively improvethe photocatalytic performance. Here we still take the classicalAu–TiO2 hybrid system as an example, whose energy leveldiagram is shown in Fig. 7a–c. As compared with the widelyused TiO2, 24-fold enhancement has been achieved by thishybrid system for the reduction of photocatalytic CO2 tomethane (Fig. 7d), which can be attributed to the intenselyenhanced local electromagnetic field offered by Au nanostruc-tures. The field enhancement greatly improved the sub-bandgapabsorption in TiO2. As incident light was switched to the ultra-violet region with high intensity, the d-band electrons wereexcited and possessed a higher redox potential. Yet it wasinsufficient to drive CO2 reduction with a light irradiationwavelength of 365 nm (Fig. 7b). When the light wavelengthwas further shifted to 254 nm, the reduction products such asCH3OH or C2H6 were obtained because the d-band electrons onAu nanostructures were excited and possessed higher energythan the redox potentials for CH3OH and C2H6 (Fig. 7c).70

Certainly the product selectivity is not only related to theenergy level of electrons, but also depends on the compositionsof reaction sites. In a metal–semiconductor hybrid structure,metal nanostructures may also act as co-catalysts by trappingelectrons. For this reason, the metal composition (e.g., Au, Ag orCu) significantly affects the selectivity of CO2 reduction.71–76

Surface plasmons can also be adapted for other hybrid systems,such as Au–Cu alloy-loaded SrTiO3/TiO2 coaxial nanotubearrays,77 Ag–SrTiO3 nanoparticles,78 Au–ZnO nanoparticles,79

and Ag-modified ZnGa2O4,80 to enhance CO2 reduction perfor-mance. In a special case, the Ag-loaded ALa4Ti4O15 (A = Ca, Sr,and Ba) layered perovskite structures yielded the majorityconsisting of CO, accompanied by a small amount of HCOOHrather than H2 in an aqueous medium. This manifests apromising catalyst for CO2 reduction.81

Degradation of the organic pollutant. Photocatalytic H2

evolution and CO2 reduction both utilized the photoexcitedelectrons for reduction reactions, while the holes were consumedby H2O oxidation or sacrificial agents. In the past decades, thephotoexcited holes have also been used for the degradation oforganic pollutants owing to their strong oxidation ability. Inaddition to the Schottky junction, the integration of semicon-ductors with plasmonic nanostructures offers an intense localelectromagnetic field to accelerate interband transition and topromote the separation of electron-hole pairs, which greatlyenhances the performance for organic pollutant degradation.For example, Li et al. reported that TiO2 nanotubes decoratedwith Au nanoparticles showed enhanced photocatalytic degra-dation ability for methyl orange. The Au nanoparticles not onlyoffer visible light absorption, but also promote electron–holeseparation.82 Balakumar et al. designed a hybrid system byassembling ZnO–Ag core–shell nanoparticles on graphenenanosheets, in which Ag shells provide plasmonic propertiesin favour of charge separation. As a result, the hybrid systemdemonstrated tremendous enhancement in the photocatalyticdegradation of acridine orange.83

Overall, the photocatalytic performance enhancement byplasmonic nanostructures can be attributed to the three majormechanisms. However, it is very challenging to assign a case toa specific working mechanism as the complex structure, energylevel, irradiation wavelength and many other factors often

Fig. 7 Energy level diagrams for TiO2, Au and redox potentials for CO2

and H2O at various radiation wavelengths: (a) l = 532 nm, (b) l = 365 nm,and (c) l = 254 nm. (d) Photocatalytic CH4 production yield using TiO2,Au/TiO2 and Au as catalysts with l = 532 nm. Adapted with permissionfrom ref. 70. Copyright 2011 American Chemical Society.



make several mechanisms mixed together. In order to establishthe corresponding relationship between specific structures andworking mechanisms, we outline the materials design for sur-face plasmon enhancement in various photocatalytic reactionsas well as the corresponding mechanisms in Table 1.

4. Plasmonic nanostructures forphotovoltaics

Photovoltaics (PV), another approach to solar energy conver-sion, is a promising technique to generate electricity from solarenergy. Thus far, photovoltaics based on monocrystallinesilicon, quantum dot84 and perovskite structure85 devices hasmade significant progress. Currently, a low-cost approach usesvery thin layers (typically 1–2 mm) of absorber materials in PVcells, referred to as ‘‘thin-film PV’’. However, such reduction inthe absorber thickness greatly compromises the efficiency ofthe device due to relatively poor light absorption, in particular,for indirect-bandgap semiconductor crystalline silicon. For this

reason, it is crucial to develop solutions to efficiently harvestor trap light for the continued applications of low-cost PVmodules. As a matter of fact, harvesting light in a broadspectrum is also highly important for all PV devices. A varietyof approaches have been developed to improve power conversionefficiency, among which the integration of photovoltaic deviceswith plasmonic nanostructures has proven to be an efficient way.

4.1 Mechanisms

In nature, the photovoltaic devices integrated with plasmonicnanostructures may work similarly to the cases of photo-catalysis. The working mechanisms for photocatalysis andphotovoltaic devices require two common steps in the followingprocesses – light harvesting, and charge generation and separa-tion. In both applications, the integration of plasmonic nano-structures with semiconductors can extend light absorption tothe visible and near-infrared regions. Moreover, the enhancedlocal electromagnetic field by surface plasmon can effectivelysuppress the recombination of photoexcited electrons andholes in semiconductors. However, the surface plasmon may

Table 1 Materials design for plasmonics enhancement in various photocatalytic reactions and the corresponding mechanisms

Semiconductor/meal Material structure Photocatalytic reaction

Enhancementfold Working mechanism Ref.

ZnO/Au or Ag ZnO nanowire arrays/metal particle Degradation of MB 2.4–3.2 Hot electrons/electromagnetic field

47

g-C3N4/Au/Pt g-C3N4/metal particle Degradation of TC-HCl 3.4 Hot electrons 48TiO2/Au TiO2 nanosheet/Au particle Photocatalytic hydrogen

evolution64 Hot electrons/

electromagnetic field49

TiO2/Ag Thin film Degradation of MB 2 Electromagnetic field 50TiO2/Au Sandwich structure Degradation of RhB 3.7 Electromagnetic field 51Cu2O/Au Au@SiO2@Cu2O core–multishell Degradation of MO 5.7 Resonant energy transfer 52TiO2/Au or Ag Au/Ag@SiO2@TiO2 core–multishell Resonant energy transfer/

hot electrons54

TiO2/Au TiO2 nanobowl arrays/Au particle Photocatalytic hydrogenevolution

9.1 Resonant energy transfer 55

TiO2/Au Au nanorod-TiO2 dumbbell-like structure Photocatalytic hydrogenevolution

5.8 Hot electrons 56

ZnO/Au ZnO nanowire arrays/Au particle Photocatalytic hydrogenevolution

2 Hot electrons 57

Ta2O5/Ta3N5/Au Nanoparticle Photocatalytic hydrogenevolution

Hot electrons/electromagnetic field

58

BiOCl/Ag/Pd BiOCl nanosheet/Ag cube/Pd cube Photocatalytic oxygenevolution

B7.5 Hot electrons/electromagnetic field

59

CdS/Au CdS@SiO2/Au@SiO2 particle Photocatalytic hydrogenevolution

1.4 electromagnetic field 60

Cu2ZnSnS4/Au Au@Cu2ZnSnS4 core–shell nanosphere Photocatalytic hydrogenevolution

7 Hot electrons 61

g-C3N4/P25/Au Z-scheme Photocatalytic hydrogenevolution

30 Hot electrons/electromagnetic field

62

ZnS/Au ZnS nanoflower/Au particle Photocatalytic hydrogenevolution

2.0 Electromagnetic field/narrowed bandgap

63

TiO2/Au TiO2 particle/Au particle Photocatalytic hydrogenevolution

2–4 Hot electrons/electromagnetic field

64

TiO2/Au/Pd TiO2 nanosheet/Au cube and cage Photocatalytic hydrogenevolution

Hot electrons 65

TiO2/Au/IrO2 TiO2 nanowire arrays/Au particle Photoelectrochemicaloxygen evolution

Hot electrons 67

TiO2/Au/Co Au nanowire arrays Photoelectrochemicaloxygen evolution

20 Hot electrons 68

TiO2/Au Thin film Artificial photosynthesis 24 for CH4 Electromagnetic field 70TiO2/Au Au@TiO2 yolk–shell Artificial photosynthesis 1.9 for CH4 Electromagnetic field 71TiO2/Au TiO2 macroporous arrays/Au particle Artificial photosynthesis 2.8 for CH4 Hot electrons 72TiO2 /AuCu Nanoparticle Artificial photosynthesis 44 for CH4 Hot electrons 33ZnO/Au Nanoparticle Artificial photosynthesis Plasmonic heating effect 79



play additional roles in solid-state photovoltaic devices suchas scattering for light trapping. Here we will outline thepossible working mechanisms for plasmonic nanostructuresin photovoltaics.

Light trapping. Light trapping is an effective approach toimprove the light absorption of solar cells. For conventional Sisolar cells with thick absorbers, light trapping is usuallyachieved using a surface texture method which can changethe light traveling directions into solar cells.86 However, thisapproach is not suitable for thin-film photovoltaic devices dueto the increased charge recombination that occurs in the conven-tional surface texture method. The integration of plasmonics intoPV technologies, which uses nanostructured metal materials toguide and localize light at the nanoscale, can increase lightabsorption and therefore PV cell efficiency without the need toradically redesign devices.

When metallic nanoparticles are placed on the surface ofsemiconductors (Fig. 8a), part of the incident light will bescattered into the semiconductor as the semiconductor haslarger permittivity.87 The incident light scattered into thesemiconductor will be scattered again when it strikes onthe back side of metal nanoparticles, until the light reachesthe critical angle of 161 for the Si–air interface. In somephotovoltaic devices equipped with reflecting metal on theback, the reflected light will be scattered via the same mechanism.As a result, the optical path length will be increased to trapincident light similarly to the surface texture method. For anon-spherical plasmonic nanostructure, the orientation of the

nanoparticle would have an impact on light scattering, which maymaximize the amount of light scattered into semiconductors andthus improve the photovoltaic efficiency.88

Light trapping by scattering of metal nanostructures willincrease the photocurrents of PV devices. For example, Stuartand Hall designed Ag nanoparticle arrays to scatter light into Siphotodetectors, which achieved a nearly 20-fold enhancementof photocurrents in the near-infrared region.89 This strategybased on light scattering achieves a grand success for varioussystems including a narrow-bandgap organic solar cell.18

Light trapping can also be achieved by the metallic nano-particles embedded in semiconductors, which concentrateincident light in the form of surface plasmons. As shown inFig. 8b, the metal nanoparticle works as an antenna to intenselyabsorb incident light and localize the sunlight energy on themetal nanoparticle surface with a time scale of 10–50 fs. Thisworking mechanism works well especially for small nano-particles with a diameter of 5–20 nm as the small nanoparticleshave low reflectivity and the carriers are collected in a timelyway by a current collector. This light trapping by a localizedsurface plasmon has been proven to be effective in polymersolar cells,90 thin-film Si solar cells and other systems.

The third light trapping mechanism by a plasmon convertsincident light into surface plasmon polaritons (SPPs) with thehelp of periodic corrugated nanostructures on the back metalas shown in Fig. 8c. SPPs are electromagnetic waves whichtransverse propagation along the metal–semiconductor inter-face. At a frequency near the plasmon resonance, the SPP isexcited to turn the incident solar flux by 901, allowing lightabsorption along the lateral direction of the PV cell. Thisincreases the light propagation length, finally enhancing thePV device performance.

Hot electrons. As discussed in Section 3.1, surface plasmondecay can generate and inject hot electrons into the conductionband of the semiconductor and enhance photocatalytic perfor-mance. For photovoltaic devices, power conversion efficiency islargely restricted by limited light absorption in the visible andinfrared regions. Metal nanostructures such as Au and Ag thatoffer plasmonic bands in visible and near-infrared regions canserve as antennae for light harvesting and convert it to hotelectrons. Combined with the semiconductor, the plasmonicnanostructures make it possible to fully utilize solar energy.91

However, it should be noted that the efficiency of solar cellsbased on plasmonic hot electron injection is far below thetheoretical efficiency. For instance, the maximum efficiency fordye-sensitized solar cells based on hot electron generation isonly 10.6%.92 Lian et al. found that the low efficiency wascaused by the loss of hot electrons via ultrafast electron-electron scattering. The bottleneck can be overcome by directlyexciting the electrons of the metal to the semiconductor via aclose-knit metal–semiconductor interface. This plasmon-inducedinterfacial charge transfer greatly improved the efficiency to24%.93 Xiong’s group proposed a strategy for integrating Sinanowire arrays with Ag nanoplates which absorbed and convertednear-infrared light into hot electrons (Fig. 9). The hot electronswere then injected into the conduction band of Si nanowires.

Fig. 8 Schematics illustrating the plasmonic light trapping for thin-filmsolar cells. (a) Light trapping by the scattering from metal nanoparticles.(b) Light trapping by the excitation of a localized surface plasmon in metalnanoparticles embedded in the semiconductor. (c) Light trapping by theexcitation of surface plasmon polaritons at the metal–semiconductorinterface. Adapted with permission from ref. 9. Copyright 2010 NaturePublishing Group.



This approach effectively utilized the near-infrared light for PVdevices, and external quantum efficiency was improved to 59%.94

Local electromagnetic field enhancement. Like the case ofphotocatalysis, the generation of photoexcited electrons andholes can be promoted by coupling solar cells with the localelectromagnetic field, which is confined on the surface of metalnanoparticles. For example, in the system in which CdSe quantumdots and Ag nanoparticles were separated by an Al2O3 layer, a localelectromagnetic field substantially restrained the recombination ofelectrons and holes.95 For photovoltaic devices, the enhanced localelectromagnetic field can contribute to the enhancement of photo-currents and power conversion efficiencies.

4.2 Applications in photovoltaics

Various PV devices have been integrated with plasmonic nano-structures for improved PV performance. Here we name a fewexamples to demonstrate the functions of surface plasmons inPV technologies.

Silicon solar cells. Silicon solar cells are the first generationof photovoltaic devices as silicon has high natural abundanceand excellent reliability. Integrating silicon PVs with plasmonicnanostructures can substantially improve light absorption invisible and near-infrared regions. For example, Green et al.found that the entire spectral absorption of thin-film Si solar

cells was enhanced by the surface plasmon of Ag nanoparticles.It achieved about 7-fold and 16-fold enhancement when theplasmonic bands were overlapped with the Si bandgap at1200 nm and 1050 nm, respectively.96 Yang et al. investigatedthe impact of surface plasmons on photovoltaic performanceusing a single nanowire of monocrystalline silicon as a modelwhich was decorated with octahedral Ag nanoparticles. Thesingle-nanowire model largely simplified the case. As such, theyrevealed that photocurrents increased at the wavelength closeto the surface plasmon resonance of Ag nanoparticles, to whichthe interaction of the semiconductor with dipolar and quadru-polar plasmon resonances made contributions.97

Perovskite-structured solar cells. Organic–inorganic perov-skite structures have shown promising prospects in PV technologiesowing to their high charge transport and quantum efficiencies.Plasmonic nanostructures have been proven to be useful inenhancing the power conversion efficiency for perovskite solarcells. Snaith et al. designed perovskite solar cells for integrationwith Ag@TiO2 core–shell nanoparticles, which boosted the powerconversion efficiency to 16.3% as the metal nanoparticles couldenhance the radiative decay and re-absorb the emitted photons.98

Gu et al. developed perovskite solar cells by integrating Ag nano-particles on the back surface of the perovskite layer. This inte-grated system achieved 4-fold photocurrent enhancement on a10 nm ultrathin perovskite layer. The Ag nanoparticles offeredlight trapping by scattering, and their local electromagnetic fieldalso contributed to the photocurrent enhancement.99

Quantum-dot solar cells. The power conversion efficiency ofquantum-dot solar cells can also be improved using plasmonicnanostructures. Wang et al. incorporated multispiked Aunanostructures into InAs quantum-dot solar cells to dramati-cally boost the PV performance, which should be attributed toscattering by Au nanostars in a broad spectrum.100 In order tofully utilize near-infrared light, Ag nanocubes were implementedin the solar cells that were fabricated with PdS quantum dots andZnO nanowire arrays. The Ag nanocubes offered a plasmonicband complementary to the light absorption of PdS quantumdots. As a result, the power conversion efficiency was improvedfrom 4.45% to 6.03%.101

Fig. 9 Flexible near-infrared PV devices based on plasmonic hot electroninjection. (a) The schematic illustrating the injection of plasmonic hotelectrons from Ag nanoplates (NPs) to n-type Si nanowires (NWs) in aSi-poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonate) (PEDOT:PSS)hybrid PV device. (b) The photograph of a Ag nanoplates-mediatedSi–PEDOT:PSS hybrid PV device with mechanical flexibility. Adapted withpermission from ref. 94. Copyright 2016 John Wiley & Sons, Inc.

Table 2 Materials design for plasmonics enhancement in various photovoltaic devices and the corresponding mechanisms

Semiconductor/metal Material structure Form of PV deviceEnhancement ofquantum efficiency Working mechanism Ref.

Si/Ag Si film/Ag particle array Silicon solar cells Light trapping 86Si/Ag Thin film Silicon solar cells 20 Light trapping 89Polymer/Au Thin film Polymer solar cells 13% Light trapping 90Si/Ag Si nanowire arrays/

Ag nanoplateSilicon solar cells 59% Hot electrons 94

Si/Ag Si thin film Silicon solar cells 16 Light trapping 96Si/Ag Single Si nanowire/

Ag octahedronSilicon solar cells Electromagnetic field 97

Perovskite/Ag@TiO2 Thin film Perovskite-structuresolar cells

16.3% Light trapping 98

Perovskite/Ag Thin film Perovskite-structuresolar cells

4 Light trapping 99

GaAs/Au GaAs thin film/Au nanostar

Quantum-dot solar cells 400% for short-wavelength region,10–50% for long-wavelength region

Light trapping 100

PbS/ZnO/Ag PbS quantum-dot/ZnOnanowire array/Ag cube

Quantum-dot solar cells 1.36 Light trapping 101



In order to emphasize the relationship of materials designand the working mechanism for PV device applications, thematerial structures and PV device performance enhancementin specific cases are summarized in Table 2, together with theircorresponding mechanisms.

5. Conclusions and future prospects

Plasmonic nanostructures can serve as antennae to concentratelight for photocatalysis and photovoltaics, which become moreefficient in converting solar light into chemical energy and elec-tricity, respectively. In this article, we reviewed recent progress inthe use of plasmonic nanostructures for enhanced photocatalyticand photovoltaic performance, with the mechanisms behindpractical applications including light trapping, hot electron injec-tion, local electromagnetic field enhancement and resonant energytransfer. The surface plasmon used for photocatalysis and photo-voltaics shares most working mechanisms but shows a slightdifference due to their different detailed systems and limitations.

Despite the grand progress, there still remain many chal-lenges in this research field. First of all, it is a very complicatedsystem for the integration of semiconductors with plasmonicmetals. In this system, the physical parameters of metal nanos-tructures (e.g., work function, plasmonic band, surface facet)and semiconductors (e.g., band structure, surface facet) and themanner of integration all have a great impact on the plasmon-enhanced performance. Tailoring all the parameters is a highlysystematic task which calls for advanced synthesis and fabrica-tion techniques. In particular, the interface between semicon-ductors and plasmonic nanostructures holds the key to theenergy or electron transfer between two components.

Until now, the mechanism that is dominant in certain situationsstill eludes us, and there is a lack of powerful techniques or idealmodel systems to distinguish one from the other. It is thusimperative to perform research at the intersection of controlledsynthesis, advanced characterization and theoretical simulation.Specifically, we need characterization techniques to resolve theinvolved processes at high spatial and temporal resolutions and atelectronic/molecular levels.

The future studies will enable a better understanding of thenature of surface plasmon effects on the enhancement ofphotocatalysis and photovoltaic performance. Nevertheless,toward practical applications, scalability and cost would be twokey factors that urge us to develop synthesis and fabricationmethods at the large scale and with high uniformity. Theremaining challenges mean that many opportunities are aroundthe corner; the research community would find it interesting tofurther exploit plasmonic nanostructures for photocatalytic andphotovoltaic applications from both fundamental and technicalperspectives.

Acknowledgements

This work was financially supported by the CAS Key ResearchProgram of Frontier Sciences (QYZDB-SSW-SLH018), 973 Program

(No. 2014CB848900), NSFC (No. 21471141, 21101145, 91123010,U1532135), Hefei Science Center CAS (2015HSC-UP009), Recruit-ment Program of Global Experts, CAS Hundred Talent Program,Specialized Research Fund for the Doctoral Program of HigherEducation (No. 20123402110050), and Fundamental ResearchFunds for the Central Universities (No. WK2060190025 andWK2310000035).

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Journal of Materials Chemistry C · 2019. 3. 28. · 1008 | . Mater. Chem. C, 2017, 5 , 1008--1021 This journal is ' The Royal Society of Chemistry 2017 Cite this J.Mater. Chem. C,