Efficiency and bond-selectivity in plasmon-induced ... · photochemical reactions on both bulk...
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Invited Progress Report
Efficiency and bond-selectivity in plasmon-induced photochemistry
Emiliano Cortes
Department of Physics, Imperial College London, SW7 2AZ London, 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