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Nanoplasmonics for chemistry

Cite this: Chem. Soc. Rev., 2014,
Guillaume Baffoua and Romain Quidant*bc
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43, 3898
Noble metal nanoparticles supporting plasmonic resonances behave as efficient nanosources of light,
heat and energetic electrons. Owing to these properties, they offer a unique playground to trigger
chemical reactions on the nanoscale. In this tutorial review, we discuss how nanoplasmonics can benefit
Received 15th October 2013 chemistry and review the most recent developments in this new and fast growing field of research.
DOI: 10.1039/c3cs60364d

www.rsc.org/csr

Key learning points

 Optical and thermal properties of noble metal nanoparticles.
 Generation of hot electrons in plasmonic nanoparticles.
 Mechanisms involved in plasmon-assisted chemistry.
 Application of plasmonic nanoparticles to different chemical reactions.

Introduction trend is to explore possible frameworks that could gain from

the unique properties of metal NPs. So far, biology has been
Over the last two decades, noble metal nanoparticles (NPs) have one of the areas of science that has benefited the most from
been the subject of extensive research in the frame of nano- nanoplasmonics. For instance, gold NPs as nanosources of heat
technology, mainly owing to their unique optical properties. are already at the basis of applications ranging from photo-
Indeed, the free electron gas of such NPs features a resonant thermal cancer therapy,2–4 bio-imaging,5 drug delivery6 and
oscillation upon illumination in the visible part of the spectrum. nanosurgery.7 Chemistry is another field of science that can
The spectral properties of this resonance depend on the con- potentially greatly profit from plasmonic NPs. Indeed, heat is a
stitutive material, the geometry of the NP and its environment. major parameter in any chemical reaction, light can be used to
This resonant electronic oscillation is called localized surface achieve high selectivity in chemical mechanisms thanks to
plasmon (LSP), and the field of research that studies the funda- quantum selection rules and adjustable photon energies, and
mentals and applications of LSP is known as nanoplasmonics.1 electron transfer is the basis of redox reactions. Hence, the idea
LSPs are accompanied by valuable physical effects such as to use metal NPs as efficient nanosources of heat, light and
optical near-field enhancement, heat generation and excitation electrons appears to be an appealing concept to both boost
of hot-electrons. Hence, plasmonic NPs can behave as efficient the yield of chemical reactions and improve their spatial and
nanosources of heat, light or energetic electrons, remotely temporal control.
controllable by light. In nanoplasmonics, these properties have In this tutorial review, we survey recent advances in plasmon-
stimulated extensive basic research and already led to a wide assisted chemistry. The first section gives the readers the basics
range of applications in nanotechnologies. of nanoplasmonics. We successively describe the optical and
Light and heat are physical quantities involved in many thermal properties of metal NPs as well as the process of hot
mechanisms in physics, chemistry and biology. Hence, a natural electron generation. In the second section we review the first
class of studies in which the intense light field concentrated at
the metal NP surface drives photochemical reactions. The third
a
CNRS, Aix Marseille université, Centrale Marseille, Institut Fresnel, UMR 7249, section focuses on exploiting the heat generated by metal NPs to
13013 Marseille, France
b
control the yield of chemical reactions. Finally, the last section
ICFO-Institut de Ciènces Fotòniques, Mediterranean Technology Park,
08860 Castelldefels, Barcelona, Spain
discusses how energetic electrons can be extracted from the NP
c
ICREA-Institució Catalana de Recerca i Estudis Avançats, 08010 Barcelona, Spain. to trigger chemical transformation of neighbouring reactants or
E-mail: romain.quidant@icfo.es enhance the efficiency of photocatalysts.

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Basics of nanoplasmonics where I is the light intensity (power per unit area). The
Optical near field enhancement associated temperature increase dTNP does not only depend on
P, but also on the NP morphology and the thermal conductivity
Upon illumination, the free electron gas of a metal NP oscillates at of the surrounding medium. For a spherical NP in a uniform
the frequency of the incident electric field. This oscillation is further medium of thermal conductivity k, there exists a simple analytical
enhanced when the frequency of the incoming light matches its LSP expression of dTNP, which reads
resonance. Due to this electronic oscillation, the NP behaves as an
electromagnetic dipole re-emitting light coherently at the same dTNP = P/(4pkR) (2)
frequency. While part of this emitted light is scattered to the far
where R is the radius of the NP. As an example, illuminating a
field, the other is concentrated at the metal surface. Depending on
20 nm spherical gold NP in water (k = 0.6 W m 1 K 1) with a
the morphology of the NP, the enhancement of the optical near field
green laser of 1 mW focused over 1 mm2 leads to a temperature
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can be gigantic. Fig. 1 presents numerical simulations, performed

increase of dTNP = 5 K. The temperature profile outside the NP,
using the boundary element method (BEM),8,9 of the optical near
in the surrounding medium, varies as 1/r (where r is the radial
field around NPs of various geometries immersed in water while
coordinate, see Fig. 2a) according to:
illuminated at their LSP resonance. As the result of the strong
gradient of surface charges, optical intensity enhancements of dT(r) = P/(4pkr) (3)
several orders of magnitude are achieved either at the apex of tips,
or by approaching two NPs very close together to form a nanometric It is important to notice that for non-spherical NPs, retrieving
insulating gap (55 for a sphere, 1800 for a dimer, 3000 for an dTNP from the knowledge of P is not straightforward, and
ellipsoid and 33 000 for a dimer of ellipsoids). The dramatic increase numerical simulations are required.11,12
of the available number of photons per unit volume at these optical When using NPs, the heated region is so small that the
‘‘hot spots’’ is expected to increase the yield of photochemical thermal inertia of the system is very weak leading to a very fast
reactions close to the surface of the metal. Some of the main heating and cooling dynamics. If L is the size of the heated region
developments based on this concept will be presented in the section and D the thermal diffusivity of the surroundings, the time scale t
Near-field assisted photochemistry. governing the temperature dynamics of the system reads:

t = L2/D (4)
Heat generation
The resonant oscillation of the electronic gas of the NP is also For instance, for L = 1 mm in water, temperature dynamics
responsible for energy dissipation into the metal via the Joule as fast as a few microseconds can be achieved. Such a fast
effect that results in heat generation and eventually an increase dynamics, which is prohibited on the macroscopic scale, offers
of the NP temperature.10 opportunities to achieve fast dynamic control of chemical
The total heat power P absorbed (and subsequently delivered reactions. Furthermore, an interesting consequence of this very
to the environment) by a metal NP under continuous (cw) fast dynamics is the further confinement of the temperature
illumination can be directly estimated from its absorption profile at the vicinity of the NP upon short pulse illumination.
cross-section sabs: Fig. 2 illustrates this effect by comparing the spatial extension
of the temperature profile around a spherical NP under cw and
P = sabsI (1) pulsed illumination. While a steady state temperature profile in

Guillaume Baffou studied physics Romain Quidant received his PhD

at the Ecole Normale Supérieure in Physics in 2002 from the
de Cachan and University Paris University of Dijon (France).
XI (France). He got his PhD in Since then he has worked in
2007 from the University Paris XI. Barcelona at ICFO in the field of
He then spent 3 years at ICFO, nanoplasmonics. In 2006, he was
the Institute of Photonic Science appointed junior Professor and a
(Castelldefels, Spain), as a post- group leader of the Plasmon
doc in the Plasmon Nanooptics NanoOptics group at ICFO and
group led by Prof. Romain became a tenure professor in
Quidant. Since 2010, he has been 2009 both at ICFO and ICREA.
a research scientist at the Institut His research focuses on the study
Guillaume Baffou Fresnel in Marseille, France. Romain Quidant of the optical properties of metal
His research interests include nano-structures and their use in
plasmonics and associated the fabrication of future miniaturized optical functionalities and
thermal effects. devices. Current activities include biosensing, quantum optics and
optomechanics, optical manipulation and nanochemistry.

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Fig. 1 Influence of the NP geometry on the optical confinement and spectral features of the LSP resonance: (a) four geometries of gold NPs are
considered: an isolated 50 nm sphere, a plasmonic dimer formed by two adjacent 40 nm NPs separated by 5 nm, an isolated oblate NP (100 nm long,
aspect ratio of 3) and a dimer of oblate NPs (82 nm long, aspect ratio 3) separated by 5 nm. All four structures feature the same volume of gold. We
consider an illumination from the top by a plane wave polarized horizontally. Sphere: 50 nm in diameter. (b) Simulations of the corresponding scattering
cross sections showing the evolution of the LSP resonance (c) schematic of the charge distribution at a given time during the period of the charge
oscillation. (d) Simulations of the distribution of the optical enhancement around the NPs at the respective LSP resonance wavelengths (simulations
performed using the Boundary Element Method8,9).

So far, the unique photothermal properties of plasmonic

NPs have mostly benefited bio-related applications, such as
cancer therapy, drug and gene delivery, photoacoustic imaging
and nanosurgery.10 Yet, since any area of science features
thermal-induced processes, one can envision a broad applic-
ability of this concept. Our focus of interest in the section
Thermoplasmonics for chemistry is to show how this effect can
be beneficial to chemical synthesis.

Hot electron injection

Following the absorption of a photon, a free electron of the
metal NP gets promoted to a higher energy level. This electronic
transition will occur with a constant probability from below the
Fermi level, creating a uniform probability to find the excited
electron at energies between Ef and Ef + hn. Then, within a time
scale of around 10 fs, this electron looses energy through
electron–electron scattering, leading to a cascading process
redistributing the energy of the primary electron to the electronic
Fig. 2 Temperature profile around a spherical gold nanoparticle for both
gas, creating a non-equilibrium Fermi–Dirac electron distribu-
cw and pulsed illuminations: (a) comparison of the temperature profiles tion. During this excitation and energy redistribution process,
under cw and pulsed illumination (dash line: temperature at different times energetic electron transfer from the metal to nearby acceptors
after the pulse absorption). (b and c) Corresponding 3-D renderings. can occur via two different pathways. (1) A coherent process
Reproduced with permission from ref. 13. where the injected electron is the primary electron that absorbed
a photon and did not interact with other electrons. (2) An
1/r is observed under cw illumination, femtosecond-pulsed incoherent process where the excited hot electron primarily
illumination leads to a much more confined profile in 1/r3.13 undergoes electron–electron interaction.14 In the latter case,

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can be involved in different mechanisms triggering chemical

reactions. The six main mechanisms originating from these
three elementary processes are listed in Fig. 4. In Mechanism A,
a thermal-induced reaction is enhanced by the temperature
increase around the plasmonic NP. In Mechanism B, the light
concentration around the NP enhances the incident photon
rate experienced by the nearby reactant. Mechanism C involves
a hot electron created by photon absorption and transferred
from the NP to an adjacent reactant. Another family of mechan-
isms involves a photocatalyst: the photocatalytic activity can
Fig. 3 Hot electron transfer from a metallic NP to an adjacent electron be enhanced either by the NP temperature increase (Mecha-
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acceptor: (left) electronic energy diagram of a metal nanoparticle at the

nism D) or the optical field enhancement at the catalyst location
vicinity of a molecule (Ef and Ev are the Fermi energy and the vacuum
energy, respectively). Under illumination, a photo-excited electron is (Mechanism E). Finally, in Mechanism F, the catalyst is further
generated in the metal NP and tunnels to an unoccupied energy level in activated by hot electron transfer from the plasmonic NP to
the molecule. (right) Electronic energy diagram of the junction between a the catalyst. Note that in the literature about plasmon-assisted
metal and a semiconductor (Schottky junction). Under illumination, a chemistry, the identification of the involved mechanism(s)
photo-excited electron is generated in the metal NP and tunnels to the
conduction band (CB) of the semiconductor.
remains sometimes unclear (especially when using a photo-
catalyst), as discussed in the last section of this review.

the electron transfer relies on the interaction between the athermal

Fermi–Dirac electron distribution with a nearby acceptor. In 2014, Near-field assisted photochemistry
Govorov et al.15 have theoretically shown that generation of hot
electrons with large energies is favoured in small nanocrystals with In this section, we review recent advances in the use of plasmonic
sizes around 10–20 nm. Fig. 3 sketches the mechanisms of hot NPs to enhance the yield of photo-chemical processes. The
electron transfer for two different nearby reactants: a molecule and capability of plasmonic nanostructures to enhance the light
a semiconductor. In the first case, hot electrons from the NP can intensity at the metal surface enables both (i) to dramatically
tunnel into the LUMO (lowest unoccupied molecular orbital) of an increase the efficiency of photochemical reactions by providing
adsorbed molecular reactant located nearby the metal NP. In the more photons per unit volume and (ii) to control these reactions
second mechanism, the NP is in contact with a semiconductor. in small volumes down to the nanometre scale. The idea of using
In this case, the equilibration of the Fermi level causes the bending a rough metallic surface to enhance the yield of surface reactions
of the conduction band of the semiconductor, and the possible was first proposed by Nitzan and Brus16 in 1981 and experimen-
formation of a Schottky barrier. Some recent studies based on this tally implemented two years later by Chen and Osgood.17 In their
effect will be reviewed in the section Hot electron injection from pioneer experiment, the authors demonstrated enhanced UV
plasmonic NPs. photo-dissociation of organometallic molecules at the surface
of UV resonant cadmium nanoparticles. Following this proof
of concept, many different experiments have been reported in
Overview of the main mechanisms in plasmon-assisted the literature. Hereafter we classify the main contributions into
chemistry four categories.
The three processes described above, namely optical near-field In the first family of experiments, NPs were used to control
enhancement, temperature increase and hot electron transfer, photo-polymerization on the nanometre scale. In this approach

Fig. 4 Main physical mechanisms involved in plasmon-assisted chemistry: Mechanism A: the photo-induced temperature increase of the NP provides
heat to an adjacent reactant. Mechanism B: the enhancement of the optical near field at the vicinity of the NP increases the photon rate seen by an
adjacent reactant. Mechanism C: a photo-induced hot electron is transferred to a nearby reactant. Mechanism D: the electron–hole (e –h+) generation
rate in a photocatalyzer is enhanced by heat generated by the NP. Mechanism E: the electron–hole generation rate in a photocatalyzer is enhanced by
the strong optical near-field of the plasmonic NP. Mechanism F: the photocatalyst adjacent to the NP is activated by hot electron transfer from the
plasmonic NP.

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Fig. 5 (a) Nanoscale photopolymerization of SU8: the SEM image of an

array of closely packed gold NPs after 3 h exposure with a cw source
polarized linearly along the direction indicated by the arrow. The dashed
circles highlight the polymerized regions (scale bar = 100 nm). Reprinted
with permission from ref. 18 (Copyright 2008 American Chemical Society).
(b) Four-photon absorption lithography in PMMA enables us to map
successive odd resonant modes in gold nanorods: (top) SEM images with Fig. 6 Isomer structures of a diarylethene molecule along with the
exposed PMMA (highlighted in red) around l/2-, 3l/2-, and 5l/2-antennas. absorption spectra. Reprinted with permission from ref. 20 (Copyright
All antennas are resonant at 860 nm. For the three exposures, the incident 2009 American Chemical Society).
power on the antennas is 4, 31, and 103 mW, respectively. (bottom)
Normalized electric near-field intensities computed in the half-plane of
the different rods. Reprinted with permission from ref. 19 (Copyright 2012 resonant plasmonic nanoparticles provides high enough inten-
American Chemical Society). sity to boost this otherwise very inefficient mechanism. This
work enabled the mapping of the optical near field of various
plasmonic modes in gold nanostructures with an unprecedented
a pattern of plasmonic nanostructures is covered with a spatial resolution.
thin layer of polymers. The degree of photo-polymerization is The third family of experiments focuses on isomerisation,
contrasted according to the non-uniform optical field intensity which is the re-arrangement of the atoms of a molecule.
surrounding the NPs. Hence, imaging the regions where poly- Isomerisation is certainly the easiest way to illustrate the
merization occurs makes it possible to map the optical near- concept of plasmon-induced photochemistry as it involves only
field of NPs. In 2008, Ueno et al.18 used a SU8 polymer layer one reactant and one product and many isomerizations can be
deposited on a pattern of lithographic gold nanostructures and photo-induced. The most classical example is the photo-
they observed the local polymerization in the hot spot regions isomerization of diarylethene (DE) molecules. Tsuboi et al.20
induced by a two-photon absorption (Fig. 5). Besides its interest evidenced in 2009 the ring opening of DE molecules in solution
to map the near field around plasmonic structures, this experi- assisted by cw NIR illumination of gold NPs at l = 800 nm
ment is conceptually important since it demonstrates that (Fig. 6). Since this excitation wavelength l did not match the
chemical reactions induced by two-photon absorption can be absorption of the molecule, two-photon absorption was
initiated even under cw illumination. This may enable cost- and proposed as the involved mechanism. This assumption was
energy-effective nanolithography, microscopy, as well as exploita- corroborated by a quadratic dependence of the reaction yield as
tion of nonlinear absorption in solar energy conversion. a function of the laser power. This work is another illustration
In 2012, Volpe et al.19 employed the opposite strategy that of the ability of plasmonic NPs to trigger non-linear processes
consists in optically breaking PMMA polymer chains on the even under moderate cw illumination. It is worth mentioning a
nanoscale. The exposed regions can be specifically dissolved direct application of plasmonic-assisted photo-isomerisation to
forming holes in the polymer layer. This mechanism is quite high-resolution optical lithography. Azobenzene (AB) molecules
similar to e-beam lithography where the PMMA chains get are known to undergo trans–cis isomerisation under photo-
broken by accelerated electrons instead of photons. In practice excitation with visible light. When grafted to PMMA, the use of
PMMA has an absorption peak in the UV range of the spectrum polarized light enables molecular rotation through thermal
(centred at 213 nm) but is transparent to visible or infrared diffusion within the PMMA matrix. AB molecules thus play
light where plasmonic nanostructures resonate. To compensate the role of molecular motors, pushing or pulling the polymeric
for this wavelength mismatch, the authors rely on the absorp- host as reorientation occurs. This results in a mass transport
tion of four near infrared (NIR) photons. The high brightness of from high to low local intensity regions that provide a very

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high-resolution snapshot of the illumination field. When combined

with plasmonics, it becomes possible to control photo-isomerisation
on the true nanometre scale as only the AB molecules at the vicinity
of the hot spots will undergo an isomerisation. This one step
rearrangement of the polymer topography has applications in
photo-lithography as it enables nanoscale resolution with visible
light and it results to be another simple and powerful way to
map with a nanoscale resolution the near field optical response
of plasmonic nanoscale systems.21–23
Another promising concept proposed in 2012 consists in
imprinting with nanometre accuracy a specific chemical func-
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tional group at predefined locations of metallic nanostructures. Fig. 7 Light assisted molecular immobilization in the gap of gold dimers:
The non-uniform distribution of the optical near field intensity (a) schematic of the chemical modification used to bind a gold colloid to
bound to the nanostructure controls the site and the extent of the the immobilized protein. (b and c) SEM images showing binding of a single
gold nanoparticle in the gap of homogeneous gold dimers. (d) Series of
photochemical reaction at the metal surface. A first illustration of SEM images that illustrates self-alignment of the gold colloid in the hot
this concept relies on cleaving the nitroveratryloxycarbonyl spot despite the morphological irregularities of the hosting dimers
(NVOC) group of an organosilane upon pulsed NIR illumina- (scale bar = 150 nm). Reprinted with permission from ref. 28 (Copyright
tion.24 The absorption of two NIR photons by the NVOC group 2013 American Chemical Society).
(absorption maximum of around 350 nm) leads to the formation
of free amine groups at the gold surface where the plasmonic
field is more intense, i.e. at the tips of the crescent nanostruc- surprisingly, their use as enhanced nanosources of heat to
tures. After exposure, the sample was incubated with COOH control the yield of chemical reactions is much more recent
functionalized gold colloids to create an amide bond between as its first implementation dates from 2007.29 Several factors
the amines at the exposed regions and the carboxylic groups at could explain this delay. On the one hand, thermoplasmonics is
the surface of the gold nanoparticles. Scanning Electron Micro- a very recent field of research that was born in 2002 within a
scopy (SEM) indicates that the gold colloids preferentially bind to medical frame.30,31 Before this date, heat generation by plasmonic
the crescent tips. Another implementation of this concept relies NPs was mostly considered as a side effect. On the other hand, it
on the so-called Light Assisted Molecular Immobilization took some time to identify what heating by plasmonic NPs could
(LAMI).25–27 LAMI exploits an inherent natural property of pro- further provide beyond conventional macroscopic heating using a
teins and peptides whereby a disulfide bridge is disrupted upon hot plate. Hereafter are listed the main advantages of using metal
absorption of UV photons by the nearby aromatic amino acids. NPs as nanosources of heat:
The created free thiol groups are subsequently used to immobi- – Reducing the heated region makes it possible to improve
lize the protein or the peptide to a thiol-reactive substrate. In the heating dynamics due to a reduced thermal inertia of
2013, Galloway et al.28 used a plasmonics approach to achieve the system.
protein immobilization with nanometre accuracy in the nanogap – Heating a micrometric area makes it possible to achieve
between two adjacent gold nanoparticles. The immobilized solvent superheating (above the boiling point) under ambient
protein was subsequently used as a scaffold to attach a single pressure conditions, which would otherwise require the use of a
gold nanocolloid (Fig. 7). This universal technique is envisioned bulky apparatus, such as an autoclave.32
to benefit a wide range of applications and more especially – Finally, like in near-field assisted photochemistry, heating
biochemical sensing and enhanced spectroscopy whose perfor- based on the illumination of metallic NPs could make it
mance strongly depends on the relative position of the analyte possible to control a chemical reaction down to the nanometre
and the optical hot spot. scale, thus enabling the formation of products at a specific
location.
In 2007, Cao et al.29 reported on the first illustration of the
Photothermal chemistry potential of thermoplasmonics in chemistry. The authors
reported on the plasmon-assisted growth of semiconductor
The rate K of most chemical reactions follows the Arrhenius law nanowires by flowing a precursor gas (SiH4 or GeH4) over gold
K(T) = A exp( Ea/RT) (5) NPs shined with a cw laser. Growth of nanowires is known to
occur at temperature above 380 1C. Here, such temperature
where T is the temperature of the reaction medium, A a constant, could be achieved without macroscopic heating by simply
Ea the activation energy and R the ideal gas constant. According to illuminating a gold NP pattern with 20 mW during less than
this law, K increases with the temperature T. By exploiting the one minute. This approach enabled the authors to control
capability of plasmonic NPs to efficiently heat upon illumination, the growth of single nanowires at predefined locations of the
one can locally control the yield of chemical reactions. patterned surface. In 2009, Yen and El-Sayed33,34 reported on the
While using plasmonic NPs as enhanced nanosources of reduction of a cyanoferrate complex assisted by gold NP illumina-
light in photochemistry is an idea that was born in 1981,16 tion. While several possible mechanisms were considered, the

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authors concluded that the chemical transformation was solely

induced by heating of the entire reaction medium. This con-
clusion is somehow disappointing since it means that similar
results would have been obtained using a hot plate; none of the
advantages listed above was here evidenced. Adleman et al.35
reported on plasmon-assisted catalysis (PAC). The authors
induced the formation of a gas phase (bubbles) in a liquid
reaction medium by illuminating gold NPs lying on the sub-
strate. For a liquid environment composed of a mixture of water
and ethanol, the authors evidenced the formation of H2, CO
and CO2 molecules coming from the reforming of ethanol. The
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work illustrates the fact that gold NPs can act as catalysts that
can be activated when heated up upon illumination. Fig. 8 Plasmon-assisted decomposition of methylene blue using an
In 2008, Chen et al.36 evidenced the synergistic effect of a Ag–TiO2 system: (a) TEM view of the cross section of a TiO2 film deposited
on silver NPs lying on a SiO2 substrate. (b) Decomposition rate of methylene
mixture of gold NPs and metal oxide powders for the oxidation
blue as a function of time. (i) A TiO2 film on a SiO2 substrate. (ii) A TiO2 film
of volatile organic compounds such as formaldehyde (HCHO) on a Ag–SiO2 core–shell structure on a SiO2 substrate. SiO2 thickness of
into CO2. The authors explained the enhancement of the (i) and (ii) was 20 nm. (iii) A TiO2 film on a Ag–SiO2 core–shell structure
photocatalytic activity by a plasmonic increase of the catalyst where SiO2 thickness was 5 nm on a SiO2 substrate. Reprinted with
temperature (Fig. 4, Mechanism D). The authors also evoked permission from ref. 46 (Copyright 2008 American Chemical Society).
the interaction between the local electromagnetic field and the
dipolar moment of the molecules as a possible additional effect
responsible for the reported observations. In a pioneer communication in 2004, Tian and Tatsuma44
In 2010, Hung et al.37 studied the cooperative effect of plas- studied for the first time a system composed of nanoporous
monic gold NPs and metal oxides (Fe2O3) on the reaction rate of TiO2 and gold NPs upon visible illumination, as a potential
the oxidation of carbon monoxide CO to carbon dioxide CO2. improved photocatalyst based on photo-induced electron transfer
Their data show that metal oxide acts a photocatalyst whose from the NPs to TiO2 (Fig. 4, Mechanism F). A few months later,
efficiency is enhanced by the heat delivered by the NPs. Remark- the same authors reported the experimental evidence of their
ably, their data underline that the plasmon driven catalytic prediction by demonstrating the catalytic oxidation of methanol
reaction rate was several orders of magnitude higher than the and ethanol by oxygen.45
one obtained under conventional uniform heating. In several In 2008, Awazu et al.46 reported on a similar experiment in
reported studies, NP heating is often not the only effect respon- which they examined the decomposition of dye molecules
sible for the enhanced reaction yield. More complex mechanisms (methylene blue) on Ag–SiO2 core–shell NPs coated with a thin
sensitive to heating but involving transient electron transfer have TiO2 layer (Fig. 8). The chemical yield was enhanced seven folds
been proposed,38 as detailed in the next section. by the presence of the Ag NPs. However, a different mechanism
was proposed to explain the enhanced catalytic activity of the
TiO2. The authors concluded that the enhanced optical field
Hot electron injection from plasmonic experienced by TiO2 leads to a higher rate in the electron–hole
NPs generation (Mechanism E). In 2010, Christopher, Ingram and
Linic47 re-examined this experiment with the aim of unravelling
Heterogeneous catalysis is of critical importance in chemical, the actual involved mechanism. Various plasmonic NP materials
environmental and energy conversion processes. Among cata- and geometries were investigated. They ended up with the same
lytic mechanisms, photocatalysis aims at converting photons mechanism originally proposed by Awazu and evidenced a
into electricity or fuels, or to degrade chemical waste and strong influence of the NP shape and size on the photocatalytic
pollutants. Ideally, such photoconversion processes would be activity (Fig. 9): while gold NPs feature a low activity due to the
triggered by clean and inextinguishable sunlight. So far, however, mismatch between the plasmonic resonance of the Au NP and
most photocatalytic systems have been operated with UV illumi- the absorption band of TiO2, silver nano-cubes featured the best
nation and show low efficiency in the visible range, limiting thus catalytic enhancement.
the use of solar energy. In this context, plasmonic NPs have Several studies have recently demonstrated that plasmonic
recently raised great promise.39–41 In 2003, it was evidenced that photocatalysis can also dramatically improve the yield of the
the presence of noble metal NPs could increase the efficiency water splitting reaction, which is one of the most attractive
of neighbouring photocatalysts via a hot electron injection reactions in the quest for clean energy.
mechanism.42 Since then, this approach has been applied to
a large variety of catalytic reactions, such as water splitting, 2H2O - 2H2 + O2
hydrogen splitting, CO2 reduction to organic compounds or
molecular degradation.43 In the following, we review a selection Efficient and cost effective water splitting would be a key
of important contributions to this field. technology towards the so-called hydrogen economy in which

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Fig. 9 Influence of the structural parameters of the plasmonic NPs on

plasmon-assisted decomposition of methylene blue: (a) evolution of
reactant concentration (methylene blue molecules) as a function of time
for different catalytic systems. The highest photocatalytic activity was
observed for silver nanocubes. (b–d) SEM micrographs of Ag nanocubes/
TiO2 composites (b), Ag nanospheres/TiO2 composites (c), Ag nanowires/
TiO2 composites (d). Reproduced with permission from ref. 47 (Copyright
2010 American Chemical Society).

hydrogen would be used as a clean source for most of our

energy needs. It is also worth mentioning that water splitting is
also the initial step of photosynthesis; hence mastering it
would contribute to the development of synthetic photosynthesis
Fig. 10 Plasmon-assisted water splitting: (a) description of the plasmonic
for the generation of chemical energy through the synthesis of photocathode used in ref. 50. (b) Evolution of the measured photocurrent
carbohydrates and sugars. In practice plasmon-assisted water as a function of the wavelength of the incident light. The response
splitting can be achieved by decorating TiO2 powder with metallic matches the extinction spectrum of the device (red line). Reprinted with
NPs.48 In certain circumstances, silver was shown to be more permission from ref. 50.

efficient than gold due to better matching of its LSPR resonance

with the semiconductor absorption spectrum.49 In the 2013, the
Moskovits’ group reported on plasmon-enhanced solar water solely 21 K would give exactly the same enhancement factor.
splitting using a carpet of gold nanorods (Fig. 10).50 Vertically The question was addressed but the insignificance of photo-
orientated gold nanorods were grown by electrodeposition in a thermal effects was not convincingly established at this stage.
porous anodic aluminium oxide template. The nanorods were The same group published in 2012 a follow-up work where they
first coated with a thin TiO2 layer for charge separation. TiO2 was explained how the linear dependence of the reaction rate as a
then covered by tiny Pt NPs, which trigger the reduction of function of the laser power turns into a super-linear dependence
hydrogen ions, after capturing the hot electrons. Finally, a cobalt at relatively low intensity if the NPs are re-arranged in a close
based catalyst was used to feed the metal back with electrons. packed manner.51 This observation tends to demonstrate the
The fabricated composite shows enhanced responsivity across dominant role of transient electron transfer in the underlying
the plasmon band of the gold nanorods. Furthermore, the system mechanism to the detriment of a photothermal process. This work
features long operational stability with no decrease in activity was intended to demonstrate that plasmonic NPs stand for
over tens of hours of solar illumination. valuable alternatives to commonly used metal oxides as efficient
In 2011, Christopher et al.38 proposed an alternative strategy photocatalyzers. It also illustrates how hard it usually is to
in which metal NPs illuminated at their plasmonic resonance discriminate among different possible involved mechanisms.
can play the role of photocatalysts by themselves, without metal Nearly at the same time, Halas’ group reported the H2 dissocia-
oxides. The authors evidenced the oxidation of molecules such tion on gold NPs.52 Dissociation of H2 is one of the most important
as ethylene, CO and NH3 solely by illuminating silver NPs. but also one of the most challenging reactions in heterogeneous
The proposed mechanism is based on the generation of hot catalysis. Indeed, the energy of the H2 bond is too high (4.5 eV) to
electrons and their direct transfer to adsorbed O2 molecules to cleave the H2 molecules by thermal heating. So far H2 dissociation
form O2 radicals that facilitate the O2 dissociation and the has been performed at the surface of metals, using very strong
associated oxidation of the molecules of interest. In this study, oxidants or upon very strong laser fields. In this experiment, the
standard macroscopic heating of the sample at around 450 K was sample consisted of small gold NPs in a TiO2 matrix. According to
required to induce the reaction. The illumination contributed to the authors, here TiO2 does not act as an electron donor as in
increase the yield by a factor of 4. The question was raised previously reported studies but aims at retaining H2 a sufficiently
whether the illumination was not just further heating the sample long time near the gold surface. Interestingly, no temperature
by optical absorption, since a rise of the sample temperature by increase seems to be required in this approach, which differs

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