Invited Progress Report

Efficiency and bond-selectivity in plasmon-induced photochemistry

Emiliano Cortes

Department of Physics, Imperial College London, SW7 2AZ London, UK

e.cortes@imperial.ac.uk

ABSTRACT

Light-induced chemical reactions on bulk metal surfaces have been explored for more than 50

years. Light absorption in the metal surface plays a key role in inducing chemical transformations

of adsorbed molecules. Our current ability to control both the absorption cross-sections and the

energy of absorbed light by metal plasmonic nanoparticles opens completely new pathways for

photochemical reactions. Plasmon modes, enhanced surface states, and field-confinement in and

around metal nanoparticles forces us to revisit our traditional understanding of photochemical

reactions at metal surfaces. Long standing goals in the field – such as bond-selectivity and

increased efficiency of photo-catalytic processes – might now be achievable, assisted by

plasmonic nanoparticles. This Progress Report intends to examine some of the most recent

advancements in the fields of plasmonic chemistry, charge transfer at the nanoscale, and surface

photochemistry.

Keywords: plasmonic chemistry; hot-electrons; photocatalysis; photoabsorption; hot-carriers

Introduction

Plasmonics and chemistry have been linked since long before Faraday performed the first

controlled synthesis of gold colloids [1]; we can travel back to the 4th century and find the Romans

utilizing small metal nanoparticles in glassware, unwittingly fascinated by their plasmonic

properties [2]. However, it was the initial discovery of the surface-enhanced Raman scattering

(SERS) effect [3-5] that triggered the advent of the field of nanoplasmonics, devoted to the

control of light and light-matter interactions at the nanoscale.

For many years, the possibility of focusing and enhancing light in nanoscale volumes eclipsed the

attention of researchers belonging to a vast number of different disciplines [6-8]. In chemistry in

particular, many different branches have contributed to the expanding field of nanoplasmonics:

surface chemistry, photochemistry, electrochemistry, photocatalysis, and inorganic synthesis,

amongst others. Plasmonic chemistry hence emerged as a new area of chemistry, mixing light,

plasmons, and molecules. However, until recently, the chemical interaction between these

components was mainly passive in nature. As such, plasmonic nanoantennas have been widely

used to explore the surrounding chemical environments, to couple with nearby emitters, or to

produce heat in nanoscale regions [7-10].

In parallel to the evolution of plasmonic chemistry, the ability and understanding in using light to

trigger chemical reactions at bulk metal surfaces also evolved tremendously. Photo-excited states

at the bulk metal-molecule interface have been studied by a vast number of techniques for many

years; surface photochemistry is a much older field compared to plasmonic chemistry and, for

many years, has been closely associated with other areas of research such as heterogeneous

(photo)catalysis and femtosecond chemistry [11-14].

Recently, the possibility to actively induce photochemical reactions by using plasmonic metal

nanoparticles opened new avenues for both the plasmonic chemistry and surface

photochemistry communities [15]. It is not the intention of this Progress Report to cover areas

recently reviewed nicely by other authors [16-21], but to offer a more fundamental point of view

of hot-carriers in the broader context of surface photochemistry and plasmonic chemistry. As

such, I will start briefly describing the traditional uses of plasmonic nanoantennas, emphasizing

the role of energy losses within metal nanoparticles, and the recent appearance of high-refractive

index dielectric antennas as powerful tools for enhancing electric and magnetic fields with

minimal losses. I then move forward to introduce the basis of molecular reactivity in

photochemical reactions on both bulk metal surfaces and metal plasmonic nanoparticles before

highlighting the new possibilities of plasmon-driven photochemistry regarding bond-selectivity,

enhanced (quantum and chemical) efficiency, and spatial distribution of reactivity.

Complementary studies studying charge-transfer processes at the metal-molecule interface are

briefly touched upon. Finally, a road map of challenges and possible routes to be explored is

provided.

Metal and dielectric nanoantennas: the role of losses

When light interacts with a metal nanoparticle (NP), its conduction electrons can be driven by

the incident electric field in collective oscillations known as localized surface plasmon resonances

(LSPRs) [8]. In this way, nanostructured materials that present plasmonic resonances enable

intense light focusing, mediating electromagnetic (EM) energy transfer from the far- to the near-

field. Furthermore, LSPRs can also couple to the EM fields emitted by molecules placed in the

vicinity of the NP, in turn leading to a strong near- to far-field coupling and re-emission of light.

Metal NPs actively collect light from areas larger than their physical size [22]. Thus, these

elements can be considered as optical nanoantennas and are key elements in the conversion of

free-space light to nanometre-scale volumes below the diffraction limit.

For metals such as Au and Ag, localized surface plasmon resonances (LSPRs) in nanoantennas fall

within the optical regime. As such, these elements have been widely used in order to

fabricate/synthesise nanomaterials capable of supporting LSPRs in the visible range. By changing

their size, shape, and arrangement, exciting opportunities for fine-tuning the spectral position of

these LSPRs have been achieved. High-field nanoscale-confinement at desired wavelengths is not

a challenge anymore in the field of plasmonics and its realization has enabled countless

applications in many different fields such as enhanced infrared, Raman and fluorescence

spectroscopies, harmonics generation, nanoscale waveguiding, optical trapping and

manipulation of nano-objects, and imaging. Most of these applications are based on their

resonant behaviour and on the interaction of the sub-diffraction fields produced by the

plasmonic antenna with surrounding molecules or nanomaterials.

However, exciting LSPRs in metal NPs for focusing, enhancing and/or re-emitting light in

nanoscale volumes comes at a price. The kinetic energy stored in the free-electron movement

ends up being dissipated as heat within some nanoseconds (see Figure 1a-d)[17]. Non-negligible

absorption of metals at optical frequencies severely limits the amount of power that can be

delivered to the antenna before melting/re-shaping. Highly-confined heat can also vaporize the

surrounding media of the NP [23], affect stability of emitters or molecules nearby, or even create

strong repulsive forces between nano-objects [24]. In recent years several applications, such as

photothermal cancer therapy [25], photothermal imaging [26], and photohermal biosensing [27],

among many others, have been proposed in order to take advantage of this highly-localized heat

generated in metallic-based plasmonic nanoantennas. Although heat-dissipation strategies may

help to mitigate temperature increase in metal plasmonic NPs, real-world applications so far have

been strongly limited due to these drawbacks [28].

In recent years, all-dielectric nanoantennas have been proposed as strategy to overcome the

aforementioned problems [29]. Employing nanostructured high-refractive index dielectrics,

excited above their bandgap energies, allows high field confinement in the nanoscale with

negligible temperature increase (Figure 1e) [30]. Due to charge displacements and internal

currents, electric and magnetic resonances can be achieved in these materials [31]. Recently,

applications such as second or third harmonic generation as well as surface enhanced Raman or

fluorescence spectroscopy have been explored by exploiting the ability of high-refractive index

nanoantennas to highly confine the electric field at sub-wavelength volumes. Different dielectrics

have been investigated, such as Si, AlGaAs, Ge, GaP, amongst others. In particular GaP, whose

bandgap lies at approximately 550 nm, could become an interesting alternative for Au and Ag in

the visible regime [32]. As an example, GaP scatters more than 99% of the light that receives at

optical frequencies thus highlighting its ultralow loss characteristics (i.e. less than 1% of the

energy is being absorbed).

Figure 1: Absorption processes in metal and dielectric nanoantennas. a–d) Photoexcitation and subsequent relaxation processes following the illumination of a metal nanoparticle. a) First, the excitation of a LSPR redirects the flow of light (Poynting vector) towards and into the nanoparticle. b–d) Schematic representations of the population of the electronic states (grey) following plasmon excitation: hot electrons are represented by the red areas above the Fermi energy EF and hot hole distributions are represented by the blue area below EF. b) Following Landau damping, the athermal distribution of electron–hole pairs decays either through re-emission of photons or through carrier multiplication caused by electron–electron interactions (1fs to 100 fs). c) The hot carriers will redistribute their energy by electron–electron scattering processes (100 fs to 1 ps). d) Finally, heat is transferred to the surroundings of the metallic structure via thermal conduction (100 ps to 10 ns). e-g) Average temperature (T) measured by fluorescence induced photo-thermal quenching for e) Si and f) Au disk-dimer nanoantennas, excited at resonance. The inset in each figure shows the calculated temperature map, excited at resonance, around the disks for P = 5 mW μm−2 in both cases. Scale bar, 100 nm. g) Extracted temperature in the gap (hot-spot) for Si (cyan) and Au (magenta) nanoantennas as a function of the heating (resonant) laser intensity.

Dashed lines show the numerical calculations for the expected temperature at the hot-spot. Figures (adapted), reprinted with permission from: a-d) ref. [17] © 2015 and e) ref. [30] © 2015 Nature Publishing Group.

If metals are intrinsically lossy and high-refractive index dielectrics, when nanostructured, can

perform as nanoantennas without absorption/heat generation and with outstanding scattering

possibilities (even in the optical regime), then the natural question is: do metal NPs have a future

in the context of plasmonics? As previously discussed, their role as nanoscale heat sources may

well be desirable. An alternative method of exploiting the lossy character of metals at optical

frequencies has been very recently proposed. The key to this exciting “new” area is based on a

time scale inspection: as shown in Figure 1b-d, between plasmon excitation and thermal

dissipation there is a time window, of some femto- to picoseconds, where excited carriers live.

The population of the electronic states in the metal is far from that at thermal equilibrium as

highly energetic electron-hole pairs are initially created in the plasmon decay process. Being able

to efficiently transfer these “hot” electrons or holes to molecules nearby may open the door to

chemically modify the surrounding environment of metal NPs, more than to inspect it using the

enhanced near-field of the antennas.

In this way, losses in metal NPs may now present an exciting opportunity for light-into-chemical

energy conversion. Plasmonics provides ways to manipulate light absorption with nanometre-

scale precision and – at sub-femtosecond timescales – enables new levels of control of hot-carrier

processes [17]. In order to understand the interplay between plasmonic NPs, plasmon decay, and

hot carrier molecular processes, it is first necessary to revisit some concepts from the

longstanding field of surface photochemistry.

Molecular reactivity in surface photochemistry

In 1952, Fukui et al. proposed that molecular reactivity is often dominated by the frontier orbitals

(HOMO, LUMO and nearby) [33]. This concept and the subsequent theory behind it have enabled

a major advancement in our current understanding and ability to predict chemical reactions. For

the particular case of the metal-molecule interface, only the HOMO and LUMO bands lying in the

range of several electron volts around the Fermi level can participate in the adsorption of

molecules and surface reactions on metals [34]. In order to activate a chemical reaction at a metal

surface, an external energy-source is usually involved (temperature, light, etc.). Of special

interest in this Report are photo-induced reactions of molecules on metal surfaces.

Initial studies of light-induced reactions on metal surfaces were mainly devoted to photo-

desorption processes. Initially, these laser-driven reactions were rationalized as new kind of

phonon-driven reaction, similar to those triggered by temperature increase. In this mechanism,

the reaction takes place through coupling the phonons of the metal with excited vibrational

states of the molecules. This coupling enables the evolution of the reaction along the potential

energy surface and the subsequent formation of products. In this mechanism, the vibrational

states of the molecule are more important than the electronic states as there is no associated

charge-transfer and the potential energy surface is not modified (Figure 2a).

Of particular relevance to this discussion, in 1988 Buntin and co-workers identified the role of

hot-electrons in molecular excitation processes at metal surfaces [35]. These authors

demonstrated that NO(ad) desorption from a Pt(111) surface took place before thermalization of

the carriers in the metal, thus accounting for a faster process that the phonon-induced reaction.

Femtosecond-lifetime hot-carriers belonging to the non-thermal photo-excited distribution in

the metal, as those shown in Figure 1b, had to be involved in photochemical reactions [35]. The

dependence of photodesorption quantum yields on the polarization of the excitation light with

respect to the surface plane further supported the role of hot-carriers involved in surface

photochemistry [36].

In order to rule out the role of the HOMO-LUMO bands and the Fermi level of the metal in photo-

induced reactions at surfaces, let us begin by describing this scenario qualitatively: direct photo-

excitation of adsorbed molecules is, in most cases, overwhelmed by excitations in the substrate.

Because electronic excitation in the substrate can be transferred to the adsorbate, e.g. via the

attachment of photo excited substrate electrons, the dominance of substrate photo-absorption

has contributed to the observation of surface photochemistry in a great number of systems [13].

Light can be absorbed at a metal surface through the dipole excitation within the bulk metal,

through nonlocal interactions related to the discontinuity of the optical field at the surface, or

through localized molecular states. Once absorbed, the distribution of excited carriers (electron-

hole pairs) in energy, momentum, and space depends on the photon energy, the band structure

of the substrate, and the coupling between the occupied and unoccupied electronic states by the

external field [37]. From this point onwards, a series of ultrafast processes regulate the hot-

carriers dynamics at the metal surface [38]. The energy deposited in the electronic system by

light is dissipated to secondary electrons through hot carrier multiplication over a time scale of a

few femtoseconds. This depletes the density of hot electrons with sufficient energy to initiate

surface photochemistry. The primary photo-excited hot electron distribution evolves through

electron-electron and electron-phonon scattering into a thermal distribution that is equilibrated

first within the electronic system, and subsequently also with the lattice [37]. However, before

thermalization but after hot-carrier multiplication, some of these excited electrons (i.e. with

energies above the Fermi level) can be transferred to adsorbed molecules (see Figure 2b).

After accepting the electron, the virtual LUMO plays a critical role in the metal-to-molecule

electron transfer reactions given its responsibility for the existence of transient anionic states

[39]. These transient (i.e. excited electronic) states on metal surfaces are characterized by

ultrashort lifetimes. This is due to the ease with which an electron in an excited molecular orbital

can elastically transfer to the vast number of resonant electronic states (band) in the metal or

can inelastically scatter with the large population of cold electrons at the Fermi sea [40].

Once the adsorbate becomes a transient ion species on the metal surface, it can undergo a series

of relaxation processes (depending on the energy landscape of the adsorbed species) enabling a

vast number of photochemical reactions including photo-desorption, photo-dissociation, or

photo-electrochemical redox. In other words, to induce photochemistry – that is, to convert

electronic excitation energy into energy of nuclear motion – an optical excitation has to bring the

molecule concerned to a potential energy surface with large slope in the Franck-Condon region,

such that the atoms can be accelerated along it [41]. This simplified description involves a single

excitation process. However, multiple cycles of excitation and relaxation may be necessary

before formation of the final product. In these cases, the molecule is sequentially excited by

populating it vibrational states, as shown in Figure 2b.

Figure 2: Illustration of substrate–adsorbate coupling mechanisms in surface photochemistry. a)

Phonon mediation of a surface reaction proceeding adiabatically on the electronic ground state via

vibrational ladder climbing. b) DIMET picture of electron mediation involving (multiple) electronic

transitions: the high-energy tail of the electronic occupation distribution transiently populates unoccupied

molecular orbitals of the adsorbate–substrate complex (e.g. the LUMO). After relaxation back to the

ground state, vibrational energy has been acquired and accordingly repeated excitation/deexcitation

cycles lead to desorption. Adapted from reference [42] © 2008 IOP Publishing. Reproduced with

permission. All rights reserved.

As a summary so far, photochemical reactions at metal surfaces are mainly governed by photon

absorption within the metal and subsequent electron-transfer to unoccupied molecular orbitals

a) b)

(or anti-bonding molecular states in the case of photo-induced dissociation reactions). A similar

approach can be applied to hot-hole transfer between the metal-molecule interface [43]. These

processes occur in a highly non-thermal regime far from equilibrium within femtosecond

timescales after absorption within the metal. Transient molecular states can then undergo

relaxation of their new potential energy surface leading to desorption, dissociation, or even redox

reactions. This field of research has been widely explored under the formalism initially introduced

by Menzel, Gomer and Readhead [44, 45] and it is usually known as desorption induced by

electronic transitions (DIET) [12]. When multiple excitations occurs within the relaxation time for

the adsorbate-metal vibration, the DIET concept can be extended to desorption induced by

multiple electronic transitions (DIMET) [46]. In the next section we turn our attention to

photochemical reactions driven by the highly-enhanced absorption of metal NPs relative to bulk

(flat) metal surfaces, where the LSPR plays a dominant role [10, 15, 18, 41, 47, 48].

Molecular reactivity in plasmon-induced photochemistry

As previously described, metal NPs have been widely used as nanoantennas due to their

scattering properties, and have opened interesting scientific pathways in both the near and the

far fields by focusing and enhancing light at the nanoscale [7, 8]. However, only part of the energy

received by the NP is scattered; the remainder is absorbed. Depending on the size, shape,

material, and the wavelength, the ratio between scattering and absorption in metal NPs can be

modified and tuned [49]. As an example, calculated LSPRs, extinction coefficients, and

scattering/absorption cross-sections ratios for Au nanospheres as a function of their diameter

(D) are illustrated in Figure 3. Au nanospheres approximately 40 nm in diameter exhibit an

absorption cross-section 5 orders higher (at their LSPR maxima) than conventional absorbing

dyes, while the magnitude of light scattering by 80 nm Au nanospheres is 5 orders higher than

the light emission from strongly fluorescent dyes [49]. The larger absorption cross-section of a

NP relative to absorbing molecules turns highly improbable the excitation of HOMO-LUMO

transitions (i.e. molecular photoabsorption) in the adsorbate once bounded to the NP, as occurs

in traditional solution based photochemistry. On the contrary, the electronic excitation of the

metal has to be taken into account to explain surface photochemistry in these systems.

The enhanced absorption of photons in metal NPs in comparison to bulk metal surfaces and the

subsequent excitation and non-radiative decay of surface plasmon resonances sets a new

scenario for surface photochemical reactions at metal-NP surfaces [15, 18, 50-52]. Let us now

describe the role of the LSPR and absorption process in metal NPs to elucidate the role of hot-

carriers in photon-driven chemical reactions at the surface of plasmonic NPs.

Figure 3: Properties of metal nanoparticles and plasmon-induced photochemical reactions. a-c) Calculated variation of a) the LSPR maxima, b) extinction coefficient (Cext) and c) scattering/absorption cross-section ratio (Csca/Cabs) for Au nanospheres as a function of the nanoparticle’s diameter (D). d) Schematic of hot electron excitation in a Au nanoparticle showing: d-band electron−hole pair excited above the Fermi level upon plasmon decay. The narrow bonding and broad antibonding states of adsorbed H2 are denoted as B and AB, respectively. e) Schematic of Fermi−Dirac type distribution of hot electrons permitting hot electron transfer into the antibonding state of H2. f) Proposed mechanism of hot-electron induced dissociation of H2 on AuNP surface. Figures (adapted), reprinted with permission from a-c) Ref [49] © (2006) American Chemical Society, d-f) from Ref [52] © (2013) American Chemical Society.

For metals such as Au and Ag, LSPRs of nanoantennas fall within the optical regime. Due to their

sub-wavelength character, the electrical energy density is significantly higher than the magnetic

counterpart for these modes. Self-sustaining electromagnetic oscillations then require an

additional energy term, found in the form of a kinetic energy density of the free carriers of the

metal [28, 53]. Sub-diffraction electric field concentration at visible wavelengths in metals is only

possible due to the existence of these energetic carriers, highlighting the mixed light/matter

modal nature of LSPRs [28].

When light impinges on a metallic nanoantenna, electrons may be promoted to energies above

the Fermi level. The final energy of the carriers will vary depending on the specific absorption

process that takes place – potentially phonon-assisted absorption, direct interband transitions,

or Landau damping [54, 55]. In particular, Landau damping is responsible for the generation of

the most energetic holes and electrons in metals. After being generated, these hot-carriers will

lose their energy on a timescale of just a few tens of femtoseconds via a series of ultrafast

processes such as electron-electron scattering, thermalization, and the emission of acoustic

phonons [55]. This points towards the notion that losses in metallic plasmonic materials at visible

wavelengths are inevitable, and that the energy of these plasmons will be lost within

femtoseconds of excitation.

Light-induced chemical transformations due solely to heating within NPs have been reported. In

these cases – usually referred as phonon-driven reactions – high intensity laser powers were

employed [56, 57]. However, before thermalization occurs, there exists the possibility to transfer

these hot-carriers to uncopied (or anti-bonding) molecular orbitals of adsorbed species (see

Figure 3d-e). Indeed, absorption on metals and the possibility to excite carriers over the Fermi

level has been the core that triggers photochemical reactions on bulk metal surfaces, as

described previously. This charge transfer process between the NPs and the adsorbed molecule

can then re-create the transient anion species described in the previous section for surface

photochemical reactions on bulk metal surfaces. The system (molecule-metal complex) may then

evolve through a different potential energy surface, inducing forces in a given (activated)

molecular bond according to the reaction coordinates of the system as shown in Figure 3f.

Nuclear motion of atoms can take place and a chemical reaction can occur [18, 39]. Thus far, the

process can be described very well with the established mechanisms from surface

photochemistry (DIET and DIMET). Plasmonic particles, however, present major advantages

compared to bulk metal surfaces in order to induce surface photochemical reactions, as

described next [17, 18].

As illustrated in Figure 3, there are many tuning parameters (size, shape, and material for

example) to adjust within the fabrication of plasmonic NPs allowing a high degree of control of

the metal’s absorption. Notably, the resonant energy condition (i.e. the wavelength at which the

LSPR is excited) can be decided beforehand. The LSPR enhances the production of hot-carriers at

the NP’s surface relative to the bulk case. As discussed later on in this Report, this may present

one of the most exciting opportunities for plasmon-driven chemistry; the same system (i.e. a

given NP-molecule interface) can be very efficiently excited at different wavelengths by tuning

the LSPR. As a consequence, this allows also the tuning of the carrier’s energy distribution [58],

which could, in turn, affect certain pre-selected bonds within the adsorbed molecule (see

discussion later on in the text regarding bond-selectivity). This can be thought as an additional

tool in selecting the potential energy surface to couple with. Selectivity has been a holy grail in

heterogeneous catalysis and related fields, as it would allow full control of the reaction paths,

the prediction of formation product, and the enhancement of the efficiency of a given chemical

transformation. Furthermore, crystal faces in metal NPs can also add another degree of freedom

in the search of specific reaction pathways. Heterogeneous catalysis and electrochemical studies

on different crystal faces in nano-materials have shown differential reactivity for a given crystal-

facet [59]. Finally, electronic surface states are confined upon the NP surface, which may also

play an important role in enhancing the efficiency of chemical transformations compared to bulk

surfaces, where dissipation of the energy is favoured [18]. Quantum efficiency (photons to hot-

carrier conversion) and chemical efficiency (reactants to products) can be enhanced through the

high degree of control of the metal NP-molecule system. In the next section we discuss some

new developments and examples of important aspects of plasmon-driven reactions using NPs.

Efficiency, bond-selectivity and reactive-sites in plasmon-driven photochemistry

In the last few years exciting examples of plasmon-driven photochemistry have appeared in the

literature [15, 48, 51, 52, 60, 61]. Christopher and co-workers furthered our understanding of the

phenomenon with the demonstration of ethylene epoxidation, CO oxidation, and NH3 oxidation

on Ag NPs [15]. It is not the intention of this Report to cover all of the examples of this type of

reaction that have been demonstrated thus far, but to emphasise some of the interesting aspects

of the mechanism, highlighting the role of the non-radiative plasmon decay, the charge transfer

mechanism at the interface, and the localization of the reactions.

One important point not yet fully addressed is the actual mechanism of hot-carriers injection into

unoccupied molecular orbitals of the adsorbed species. As previously described, both

temperature (phonons) and excited-electronic state (DIET-based, transient-anions) mechanisms

could be responsible for the observed light-induced chemical transformations at the interface of

plasmonic metal NP and adsorbed species. Monitoring the power-dependence of product

formation (from linear to superlinear in the case of DIET) or through the kinetic isotope effect, it

is possible to infer which of these two main pathways is responsible for the observed reaction

[18, 52]. It is likely that a combination of the two in different proportions is always at work.

However, to date, plasmon-induced photochemical reactions have shown limited efficiency

(approximately 1%) that renders any impending industrial application unlikely [51, 52].

Very recently, Lian and co-workers proposed that when a strongly-coupled acceptor (a

semiconductor in their case) is used to collect the hot-carriers, there is a direct and instantaneous

highly-efficient charge-transfer mechanism that successfully explains their observed quantum

efficiencies of over 24% [62]. The plasmon-induced interfacial charge-transfer transition (PICTT)

demands that the decay of a plasmon directly excites an electron from the metal to a strongly

coupled acceptor. As a consequence, this interfacial electron transfer process strongly damps the

plasmon. These results not only highlight the importance of the interface in the efficiency of

these processes, but also open new perspectives about the microscopic mechanism of hot-carrier

transfer processes [62].

Linic and co-workers have recently proposed that the injection of the hot-carriers into molecules

can occur via two different paths: direct and indirect charge excitation mechanisms [18, 63, 64].

In this way, molecules can be seen as strongly-coupled acceptors and chemical damping of the

plasmon can also take place. Both of these mechanisms are subject to the same basic principles

and can be rationalized as DIET mechanisms. However, there are some substantial differences

between the two in the manner that the transient adsorbed species is achieved via the excitation

mechanism of the carrier from the metal surface. Let us start by describing briefly the indirect

mechanism (Figure 4b).

The indirect mechanism – as discussed in the previous sections – relies on the formation of a

carrier energy distribution through the plasmon decay process. Following Landau damping (1-

100 fs), the electron-hole pairs in the metal can decay through either the re-emission of a photon

or through carrier multiplication caused by electron-electron scattering interactions (100 fs to 1

ps) [17]. This latter non-radiative mechanism is responsible for the Fermi-Dirac distribution of

carriers observed in Figure 4b. In this scenario, it is only the carriers with adequate energy to

transfer to an unoccupied molecular orbital of the system that become the transient species that

can lead to a photochemical reaction.

On the other hand, the direct transfer mechanism assumes that the direct LSPR-induced electron

excitation from occupied to unoccupied orbitals of the molecule-NP complex is not mediated by

the formation of an excited electron distribution within the metal nanoparticle (Figure 4a).

Instead, the decay of an oscillating surface plasmon results in the excitation of an electron directly

between adsorbate states into an unoccupied orbital of matching energy [63, 64]. Direct photo-

induced electron-transfer to hybridized (metal-molecule) states has been recently also shown for

non-plasmonic small (approximately 5 nm) Pt NPs, where the influence of the adsorbed species

on the electronic structure of the system in much greater than in the bulk metal case [65]. Further

evidence of a direct mechanism, also in bulk metal surfaces, have been recently proposed [37,

66].

Figure 4: Direct and indirect photo-excitation processes in plasmon-induced photochemistry. Incident photons excite the surface plasmons of the metal nanoparticle. These surface plasmon oscillations decay through the formation of energetic electron–hole pairs. a) In the direct process, the electron is excited directly into an unoccupied orbital of matching energy within the adsorbate. b) In the indirect process, the energetic electrons formed by the non-radiative decay of the plasmons form a distribution within the metal nanoparticle. Electrons with proper energy can then scatter into available adsorbate orbitals. Because of the nature of the electron distribution formed in the indirect mechanism, more electrons will scatter into lower energy orbitals (II) and chemical transformation will preferentially proceed through that lower energy activated pathway. In the direct mechanism, however, the electrons can be potentially excited into higher energy orbitals (III) when that energy matches the incident photon energy. This opens the possibility for selective chemical pathway targeting that impossible in the indirect mechanism. Reproduced (adapted) from reference [63] © 2016 Nature Publishing Group.

In a simplified view, this direct mechanism can be thought as an HOMO-LUMO transition of the

hybrid system (molecule adsorbed on the surface of the nanoparticle). The energy of the excited

carrier will depend then on the incident-photon energy, allowing for the population of higher

(than LUMO) unoccupied metal-molecule (hybrid) orbitals. The reactivity of adsorbed molecules

under this direct mechanism would no longer be dominated by the frontier orbitals as higher

unoccupied states might become available for population. The role of the LSPR in this mechanism

is then highly related to the field-enhancement capabilities of the nanoantenna and, once more,

the LSPR can be tuned so as to target specific transitions. In this way, the possibility to populate

orbitals of higher energy than the LUMO can open interesting possibilities for bond-selectivity in

plasmon-induced photochemistry [18, 63-65]. Moreover, this direct, ultrafast, momentum-

transition should be enhanced on the NPs relative to the bulk case due to the increased

proportion of surface states influenced by the adsorbed molecules.

It may be possible that one mechanism dominates over the other depending on the energy

barriers of the surface complex. For instance, if the unoccupied orbitals that accept the electron

are closer to the Fermi level of the metal NP or if the reaction involves more than a single

excitation process, then the indirect mechanism may have an increased number of chances to

occur given the quantity of low-energy electrons (i.e. just above the Fermi level) derived from

the plasmon decay. On the other hand, occupation of higher-energy orbitals (with respect to the

Fermi level) is more likely through a direct mechanism [64].

We should also note that with any plasmon-induced photochemical reaction mechanism that

involves the net transfer of an electron – that is, for photo-induced oxidation and reduction

reactions – there must be a counter reaction closing the circuit [67]. The global energy equation

of the reaction involves the counter reaction and the possibility that such a reaction will occur

will also determine the reactivity of the whole system. Furthermore, the Fermi level of a metal

NPs strongly depends on its size, its local environment (i.e. solvent, capping layer, etc.) and

reaction conditions, setting another degree of freedom for photo-induced reactions and their

efficiencies [68]. Finally, mechanisms of injection of electrons into molecules through a metal

contact have been largely explored in the fields of molecular electronics and electrochemistry

[69-71]. Hopping and tunnelling mechanisms, among others, have been identified within these

systems. Although not identical, these systems share a number of common traits with light-

induced redox reactions and future connections between the fields may open new perspectives

on the molecular basis of light-induced electron transfer pathways, as shown in the next section

of this Report.

Another important point to take into account when discussing the increase in efficiency of

plasmon-driven chemical reactions is the localization of reactive-spots in plasmonic antennas,

that is, the spatial localization of the reactive regions of the metal nanoparticles [72-74]. This

could permit intelligent design of plasmonic materials in order to also increase the efficiency of

these types of reactions. In this regard, we have recently shown a strong spatial-energy

dependence of the generated carriers and their extraction, both from first principle calculations

and experiments (Figure 5 a-c). Reactions requiring highly energetic carriers will proceed only

upon a very small fraction of the antenna’s surface. Equally, reactions involving high density of

electrons will be strongly localized [75]. Thus these results can open new avenues for the design

of much more efficient nanoscale plasmonic systems for hot-carrier-driven chemical reactions

[72].

Recently, Zhai and co-workers highlighted the importance of the localization of hot-carriers in

order to disentangle the mechanisms of plasmon-guided synthesis of NPs [76-78]. The spatial

distribution of the surfactant PVP (polyvinylpyrrolidone) has a major role in the edge-reactivity

of the excited carriers, as shown in Figure 5d. PVP may act as a hot-electron reservoir guiding the

reduction of metal ions from solution around the edges of the NPs (where PVP is located), in turn

guiding their growth into various nanoprism geometries [78]. Metal ion reduction on the surface

of NPs may involve a more complex mechanism than simple reduction once nearby the surface,

as diffusion times are longer than hot-electron lifetime. In this way, either surfactants or small

metal clusters in solution can play an important role [78, 79]. Site-selective etching or metal

deposition as well as polymerization have also been achieved with hot-carriers [73, 74, 80].

Figure 5: Spatial distribution of reactivity in plasmon-induced photochemical reactions. a-c) Mapping

hot-electron conversion in Ag bow-tie antennas. a) Top panel shows the FDTD simulated near-field

distribution of the antenna at 633 nm (parallel polarization). Middle panel shows representative SEM

image of 15 nm Au reporter nanoparticles bound at the locations at which photochemical reactions have

occurred after one minute of resonant illumination. Scale bars: 100 nm. Bottom panel indicates the

collapsed localizations over 100 antennas. Colour bar indicates the number of particles localized in the

whole array. b) As a) but in this case for 2 minutes illumination. c) First-principles predictions of spatial

and energy-resolved probabilities of plasmonic hot carriers that reach the surface of a Ag bow-tie antenna

under 633 nm illumination. d) SEM image of Au triangular nanoprisms obtained after 2 hours of irradiation

with the addition of iodide (I−) to the growth solution following the seed separation method. The insets

show (i) a high-magnification SEM image of a single triangular nanoprism and (ii) a NanoSIMS image

showing the elemental distribution of 12C 14N signals (green) and 127I signals (blue) from a triangular

nanoprism. The scale bars in all insets represent 200 nm. a-c) Reproduced (adapted) from reference [72]

© 2017 Nature Publishing Group. d) Reproduced (adapted) from reference [78] © 2016 Nature Publishing

Group.

These examples highlight the importance of the spatial localization of these plasmon-induced

reactions; through accurate prediction of the location of these highly reactive spots, we can

expect to greatly enhance the final efficiency of such systems. As shown before, a small fraction

of the molecules attached to a nanoparticle might be located at a position where the plasmon-

induced reaction can takes place. Novel methods to position, locate, and access molecules into

the reactive-spots can dramatically enhance the efficiency of these reactions [72, 75]. Moreover,

by renewing the molecules at the reactive-spot (i.e. by releasing the molecules after the reaction

has taken place) can further help in this regard. Bimetallic approaches of materials where the

reaction’s product is weakly adsorbed can be implemented in conjunction with the plasmonic

particles [81].

Complementary studies of electron-transfer and electronic transitions in

molecules adsorbed onto metal-nanoparticle surfaces

As previously stated within this Progress Report, the reactivity of the metal-molecule system in

plasmon-driven photochemistry is governed mainly by the frontier molecular orbitals of the

metal-molecule complex. The energy landscape – after molecular adsorption on the surface of

the nanoparticle – forms the basis of the energy-requirements for the excited electrons to

become reactive in these systems. Major advances have been achieved in such other fields as

electrochemistry, molecular electronics, and enhanced spectroscopies regarding similar metal-

molecule charge-transfer processes. Although these methods may or may not be guided by light,

they can offer interesting insights into the charge-transfer processes at the NP-molecule

interface.

Electrochemical methods are an interesting complementary tool to disentangle the metal-

molecule energy landscape [71]. By applying a voltage scan, the Fermi level of the metal

nanoparticles can be tuned over a wide range of energies within the electrochemical potential

window offered by the system (i.e. before solvent degradation, metal oxidation, etc.). In this way,

the simplified view of charge-transfer once the energetic electron crosses the frontier molecular

orbitals can be inspected in a systematic way. Furthermore, new electronic states originating

from the hybridization of d metal orbitals and HOMO/LUMO molecular states can be

experimentally taken into account. However, linking the redox potentials derived from

electrochemical methods with the plasmonic experiments is not straightforward. Contrary to

electrochemical measurements, in plasmon-guided redox chemistry: the counter reaction occurs

on the same particle (i.e. electrode potential is not defined in plasmonic systems); there is a

distinct lack of the electric double layer; and mass-transport (molecular diffusion) is not

electrically biased. Increasing effort over the past few years has been devoted to linking UHV

(ultra-high vacuum) heterogeneous catalysis to electrochemical-environment experiments.

Similar approaches can be applied to the plasmon-guided hot-carrier redox reactions.

Indeed, the connection between plasmonics and electrochemistry returns to the initial discovery

of SERS and has recently made the study of charge-transfer processes at the single-molecule level

(SMSERS) possible, as shown in Figure 6a-c [82]. Differences in the redox potential of a weakly-

adsorbed Nile Blue molecule on Ag NPs along two consecutive voltammetric cycles have shown

that the energy requirements to perform the charge transfer process can vary significantly

depending on the particular orientation of the molecule [82-84]. Surface-site heterogeneity in

the potentials required to perform the redox reaction have been also elucidated by SMSERS (see

Figure 6d) [59]. SERS has also recently served as a tool to explore single molecule hot-electron

reactivity (Figure 6e) [85]. Additionally, tip-enhanced Raman spectroscopy (TERS) has been

successfully implemented to study electron-transfer reactions and catalytic processes of just a

few molecules, with nanometre spatial resolution [86-89]. In all of these examples, the molecules

being investigated can be considered as the most reactive examples for light induced processes.

Both direct and indirect photo-excitation processes should be enhanced at the hot-spot (as

shown in Figures 4 and 5). Thus, these examples highlight the notion that plasmon-driven

reactions will face similarly broad energy distributions from molecule-to molecule and site to site

while exploring bond-selectivity pathways.

Figure 6: Single molecule electron-transfer followed by SERS. Temporal evolution of a) many molecules

and b-c) single-molecule SERS (SMSERS) spectra of Nile blue A adsorbed on Ag nanoparticles along a

potential scan. SERS can be used as an amplifier of the electron-transfer events, both from and to the

metal surface in order to figure out the reduction and oxidation potentials of a single molecule [82]. d)

Calculated adsorption energy (number below each configuration) for different metal-molecule motifs on

a defect-rich surface (i.e. the surface of a nanoparticle). Site-specific behaviour is expected for charge-

transfer processes at the nanoscale [59]. e) A similar concept as the one shown in a-c) was recently

extended for light-induced redox reactions catalysed by metal nanoparticles [85]. Reprinted (adapted)

with permission from a-c) ref. [82] © 2010 American Chemical Society, d) ref. [59] and e) ref. [85] © 2016

American Chemical Society.

Another method that investigates the (light-induced) HOMO-LUMO transition in molecules is

ultraviolet-visible (UV-vis) absorption spectroscopy. Wavelength scans are used in order to find

the resonant energy at which a transition from the HOMO to the LUMO takes place. In the same

way, LSPRs in metal NPs can be investigated by UV-vis, accounting for the simplest and fastest

method to determine the extinction spectra of NPs in solution. However, when both molecules

and plasmonic NPs are combined, the much stronger LSPR response conceals the HOMO-LUMO

transitions within the molecules surrounding the NPs. Recently, Le Ru and co-workers succeeded

in disentangling both contributions [90]. Once more, as this measurement explores the coupled

system (molecules adsorbed on the surface of the plasmonic NPs), it becomes an interesting

opportunity to explore the frontier orbitals in the context of hot-carrier reactivity. As shown in

Figure 7, measurable changes in the optical resonant condition of molecules can be detected

once adsorbed onto the metal NP surface. Significantly, the experiments were performed with

low molecular coverage and using molecules whose absorption is far from the LSPR, thus

emphasising the strong molecule-metal interaction over molecule-molecule or molecule-

plasmon interactions [90]. Although the transitions in these examples do not lead to

photochemical reactions (the metal’s absorption is decoupled from the HOMO-LUMO excitation

here), it could serve as method to explore the energy of optically permitted transitions in the

molecule-NP interface. These transitions should be avoided in order to increase the efficiency of

photochemical reactions and to reduce photobleaching. Plasmon-engineering to enhance

absorption at certain energies (i.e. through Fano resonances) and block scattering for given

wavelengths (i.e. dark-modes) could potentially utilise this valuable information.

Figure 7: Differential absorbance spectra of common dyes adsorbed on Ag nanoparticles. a) Crystal

Violet. b) Nile Blue A. c) Rhodamine 6G. The colloid concentration is 8 pM. The dye concentrations are low

enough (10, 10 and 2.5 nM, respectively) to avoid any effects from dye–dye interactions. Dashed lines are

the reference spectra in water that would be measured at the same concentration, scaled for easier

visualization. The dye chemical formulae are reproduced at the top of each panel for reference. Reprinted

(adapted) with permission from ref. [90] © 2016 Nature Publishing Group.

The examples mentioned here are just a few of the many that could potentially assist advancing

the field of plasmon-induced photochemistry. Stronger interactions between many other

scientific communities and the development of techniques to explore the energy landscape of

the metal NP-molecule interface could help to experimentally access to the information that we

a) b) c)

are missing in order to efficiently target reactions by light. Some ideas in this regard and current

challenges in the field are provided within the next section.

Challenges and opportunities for plasmon-enhanced photochemistry

Despite surface-photochemistry being a traditional and well established field of research, recent

advancements in plasmon-induced photochemistry demonstrated that long-standing goals in the

field may now be accessible. As briefly described here, plasmonic NPs may open new avenues for

enhanced photochemical and photocatalytic reactions. Two of the major challenges in the field

are related to bond-selective reactions and enhanced efficiency (both quantum and chemical)

[91] compared to traditional bulk metal surfaces. In a very simplified picture, this system can be

broken down into four major components: the metal NPs, the plasmon modes, the adsorbed

molecules, and the metal NP-molecule interface. As highlighted in Figure 8, there is room for

improvement in all of them.

Figure 8: Challenges. Road map for plasmon-enhanced photochemistry.

Our advanced abilities in NP fabrication permits a high degree of control of the nanostructures’

size, shape, and composition. This advanced control of their plasmon modes, their LSPRs, and

their absorption/scattering ratios, coupled with subsequent surface modification with molecules,

expands tremendously the possibility for systematic study of plasmon-induced chemical

reactions and the influence of these parameters in their final photo-conversion efficiencies.

Bimetallic and porous NPs, dark-plasmon modes, and Fano resonances are some of the possible

routes ripe for exploration. In-situ ultrafast spectroscopic studies [75], photo-driven electron-

transport experiments [92], and single-molecule charge-transfer approaches [82] should help us

to disentangle the photo-induced chemical mechanisms, reaction pathways and intermediate’s

formation. Further studies to clarify the conditions under which each of the proposed

mechanisms (direct, indirect, phonon-assisted) dominate should be enhanced by these

approaches. Theoretical approaches capable of describing the hybrid electronic structure of the

interfaces are also needed for a full understanding of the mechanisms and future opportunities

in terms of bond-selectivity.

Photosynthesis remains one of the most efficient and selective processes on earth for energy

conversion. Increasing the efficiency and ruling out any possible bond-selective mechanisms of

light-into-chemical energy conversion are some of the missing puzzle-pieces that are needed in

order to understand and mimic plants [51]. In this sense, plasmonic nanoparticles can serve us

as tools for unprecedented efficient manipulation of the photochemical reaction pathways.

ACKNOWLEDGEMENTS

E.C. acknowledges financial support from a Marie Curie Fellowship of the European Commission

and a 2016 Royal Society Challenge Grant (CH 160100). E.C. thanks Thomas Brick for fruitful

discussions.

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AUTHOR’S BIOGRAPHY

Emiliano Cortes received his PhD in 2013 from Universidad Nacional de La

Plata, Argentina where he studied self-assembled monolayers onto planar,

rough and nanoparticle surfaces. He did a research stay at Victoria University

of Wellington, New Zealand, where he studied single-molecule SERS

electrochemistry. After a postdoc in optical printing at the Center for

Bionanosciences in Buenos Aires, he moved to the Experimental Solid State

Physics group at Imperial College London where he is since 2015 a Marie Curie

Fellow. His current research lines are devoted to study losses in plasmonic

nanoantennas, novel dielectric antennas, plasmon-based super-resolution

approaches and plasmon-induced photochemistry.

TABLE OF CONTENTS

Plasmon-induced photochemistry might allow us to perform

bond-selective reactions and to increase efficiency in light-into-

chemical energy conversion processes. Metal nanoparticles,

plasmon-resonances and metal-molecule interactions can be

tuned and engineered with a high degree of control thus allowing

unprecedented manipulation of the photochemical reaction

pathways. This Progress Report article intent to describe this

scenario.

Keyword: Plasmonic-chemistry Author: Dr. Emiliano Cortes* Tittle: Efficiency and bond-selectivity in plasmon-induced photochemistry