Hot-Carrier Organic Synthesis via the Near-Perfect Absorption of Light

Article abstract of DOI:10.1021/acscatal.8b03486

Photocatalysis enables the synthesis of valuable organic compounds by exploiting photons as a chemical reagent. Although light absorption is an intrinsic step, existing approaches rely on poorly absorbing catalysts that require high illumination intensities to afford enhanced efficiencies. Here, we demonstrate that a plasmonic metamaterial capable of near-perfect light absorption (94%) readily catalyzes a model organic reaction with a 29-fold enhancement in conversion relative to controls. The oxidation of benzylamine proceeds via a reactive iminium intermediate with high selectivity at ambient temperature and pressure, using only low-intensity visible irradiation. Control experiments demonstrated that only hot charge carriers produced following photoexcitation facilitate the formation of superoxide radicals, which, in turn, leads to iminium formation. Modeling shows that hot holes with energies that overlap with the highest-occupied molecular orbital (HOMO) of the reactant can participate and initiate the photocatalytic conversion. These results have important implications for hot-carrier photocatalysis and plasmon-hot-carrier extraction.

Full text of DOI:10.1021/acscatal.8b03486

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Article
Hot-Carrier Organic Synthesis via the Near-Perfect Absorption of Light
Qi Xiao, Timothy U. Connell, Jasper Cadusch, Ann Roberts,
Anthony Sidney Richard Chesman, and Daniel Gomez
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03486 • Publication Date (Web): 26 Sep 2018
Downloaded from http://pubs.acs.org on September 27, 2018
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HotꢀCarrier Organic Synthesis via the NearꢀPerfect
Absorption of Light
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*
,†
‡
§
ǁ
†,∆
Qi Xiao, Timothy U. Connell, Jasper J. Cadusch, Ann Roberts, Anthony S. R. Chesman,
*,‡,∆
and Daniel E. Gómez
†
CSIRO Manufacturing, Bayview Ave, Clayton, VIC 3168, Australia
‡
RMIT University, Melbourne, VIC 3000, Australia
§
Department of Electrical and Electronic Engineering, The University of Melbourne, Parkville,
VIC 3010, Australia
ǁ
School of Physics, The University of Melbourne, Parkville, VIC 3010, Australia
∆
Melbourne Centre for Nanofabrication, Australian National Fabrication Facility, Clayton, VIC
3
168, Australia
Corresponding Author
*
Email: Q. X. kevin.xiao@csiro.au; and D. E. G. daniel.gomez@rmit.edu.au
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ABSTRACT: Photocatalysis enables the synthesis of valuable organic compounds by exploiting
photons as a chemical reagent. Although light absorption is an intrinsic step, existing approaches
rely on poorly absorbing catalysts requiring high illumination intensities to afford preferable
efficiency. Here, we demonstrate that a plasmonic metamaterial capable of nearꢀperfect light
absorption (94%) readily catalyzes a model organic reaction with a 29ꢀfold enhancement in
conversion relative to controls. The oxidation of benzylamine proceeds via a reactive iminium
intermediate with high selectivity at ambient temperature and pressure, using only lowꢀintensity
visible irradiation. Control experiments demonstrated that only hot chargeꢀcarriers produced
following photoꢀexcitation facilitate the formation of superoxide radicals, which in turn leads to
iminium formation. Modelling shows that hot holes with energies that overlap with the HOMO
of the reactant can participate and initiate the photocatalytic conversion. These results have
important implications for the hotꢀcarrier photocatalysis and plasmonꢀhotꢀcarrier extraction.
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KEYWORDS: surface plasmons; nearꢀperfect absorber; hotꢀcarriers; photocatalysis;
benzylamine oxidation; charge separation
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INTRODUCTION
Solar energy provides a renewable and near limitless resource compared to carbonꢀbased fuels.
The discovery of chemical processes driven directly by visible solar light therefore stands to
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[
1ꢀ3]
replace existing industrial processes and significantly reduce future carbon emissions.
To this
end, recent years have seen a rapid increase in reports of synthetic photochemistry, although the
design of materials that combine high light absorption with efficient photocatalysis remains
challenging. Wide bandꢀgap semiconductors, such as TiO , enable some chemical
2
[4]
transformations, but typically require high energy UV radiation. Homogeneous photocatalysts
based on transitionꢀmetal organometallic complexes, particularly of iridium(III) and
[
5,6]
ruthenium(II), can facilitate a wider range of complex chemical transformations.
However,
while they exhibit absorption crossꢀsections that extend into the visible range, they still typically
require relatively high energy photons (λ < 500 nm), which only account for a small fraction of
total solar irradiance (< ~20%).
In contrast, plasmonic metal nanostructures exhibit greater absorption crossꢀsections with
[7ꢀ11]
visible light through the excitation of localized surface plasmon resonances.
Furthermore,
absorption frequencies are readily tuned across a wide wavelength range through the
[
12]
manipulation of nanostructure composition, size and geometry.
These structures facilitate
photochemical transformations via the nonꢀradiative relaxation of their surface charge
[
9ꢀ11,13]
oscillations (surface plasmon resonances), resulting in the emission of hot chargeꢀcarriers,
which are a transient electron population with an average energy higher than that of a thermallyꢀ
[14ꢀ16]
equilibrated electron population at room temperature.
Hot chargeꢀcarrier extraction can be
achieved through metalꢀsemiconductor (Schottky) contacts, creating chargeꢀseparated states with
[
11,17,18]
a sufficient energy to drive chemical transformations.
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Efforts to reduce photocatalyst loadings of precious metals are conventionally impeded by a
[
19ꢀ21]
proportional decrease in light absorption.
By instead employing an architecture that creates
a plasmonic nearꢀperfect absorption of light, a strong interaction with incident radiation is still
present while achieving a low catalyst loading. One approach to realizing these materials builds
upon the “Salisbury screen”, originally invented in 1952 to assist in avoiding RADAR
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[
22]
detection.
These structures typically consist of a supporting reflector and an absorptive
metallic nanostructure layer separated by a spacer of fixed distance. In this configuration, the
perfect absorption of light occurs due to optical impedance matching, which has been postulated
to occur due to the renormalization of the polarizability of the metal nanostructures through their
[
23]
interactions with the mirror, and is also due to a magnetic optical response that arises from
antiparallel and outꢀofꢀphase currents taking place on both the mirror and the nanostructures
(
Figure 1a): the dipolar radiation of the nanoparticles (NPs) is cancelled by its mirror image
[
23ꢀ26]
originating from the reflector.
The mirror suppresses the transmission of light and from the
principle of conservation of energy, the low transmission and reflection of light translates into
high absorption of light. Nearꢀperfect absorption of light can be achieved at frequencies where
the optical crossꢀsection for absorption of light by each NP is comparable to the interꢀparticle
[
27]
spacing in the monolayer.
Incorporating nanostructured metal particles into nearꢀperfect
absorbing metamaterials dramatically enhances their optical absorption properties, improves hotꢀ
[
19ꢀ21,28]
carrier generation rate, and facilitates hotꢀcarrierꢀinduced chemistry.
However, the
reaction mechanism remains in dispute, and has never been observed in complex organic
chemical transformations.
Recently, photocatalytic transformations have become an important tool for organic
[
29ꢀ31]
synthesis.
The selective oxidation of primary amines to imines is an essential laboratory and
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[32,33]
commercial procedure.
Industrially, imines are synthesized by the direct oxidation of
[
34]
amines or the condensation of amines with aldehydes,
processes that require stoichiometric
amounts of harmful or explosive reagents, often in a homogeneous phase, which later require
complex separation and purification strategies. Therefore, the development of new catalytic
systems using environmentally benign reagents, such as molecular oxygen, to drive the reaction
efficiently under mild conditions is highly desirable. In order to supersede the existing
approaches implemented by industry, and hence eliminate the wasteful processes involved in
synthesizing and subsequently removing catalysts, novel photocatalytic systems must
demonstrate substantial increases in efficiency, ease of use, and reusability.
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In this work, we present the realization of hotꢀcarrier driven organic synthesis enabled by a
plasmonic nearꢀperfect absorber for the selective oxidation of benzylamine. The hybrid
mirror/semiconductor/NPs system absorbs up to 94% of incident light, resulting in a ~29 fold
rate increase in a model synthetic reaction relative to the nonꢀnearꢀperfect absorbing analogue
(
semiconductor/NPs). Mechanistic studies confirm the generation of an important class of
reactive chemical intermediates, namely iminium ions, in the photocatalytic oxidation of
benzylamine into the corresponding imine.
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Figure 1. (a) The induced dipole in the metal NP (arrow in absorptive layer) can interact with its
mirror image (arrow in reflective layer), producing a magnetic mode (circular arrows in
semiconductor layer). Reflection of light is suppressed when these dipoles oscillate out of phase.
ꢁꢂ
ꢀ
ꢁꢂ
: Electric field, ꢃ: momentum of incident light. (b) Schematic structure with TiO positioned in
2
between the Au mirror and Au NPs. Inset: the TiO /Au NPs contact energy diagram. (c)
2
Simulation of electric field spatial distribution throughout the metamaterial during irradiation at
600 nm. The values are shown on a logarithmic scale of the absolute magnitude square of the
electric field relative to the incident field. (d) SEM image. Inset: size histogram of the NPs. (e)
Optical absorption spectra. Also shown is the (normalized) spectrum of the light source (see the
reaction setup in Figure S1). (f) Digital photograph of the nearꢀperfect absorber on a silicon
wafer.
RESULTS AND DISCUSSION
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Near-Perfect Absorber Preparation and Characterization. In constructing the nearꢀperfect
absorber, we employ Au NPs, TiO as the spacer layer, and a continuous Au thin film as the rear
2
reflective layer (Figure 1b). The TiO /Au NPs interface acts as a Schottky contact with a height
2
[
35]
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of approximately 1.0 eV, through which hot electrons with energies in excess of this barrier
[36]
can be emitted (Figure 1b inset). The resulting chargeꢀseparated states (electrons in TiO and
2
holes in Au) can be subsequently used for reductive and oxidative chemical transformations,
[
37]
respectively.
Numerical fullꢀwave simulations indicate a strong (and subꢀwavelength)
localization of the electric field at the spatial location of the metal NPs when irradiated at 600 nm
(Figure 1c, Figure S2).
Nearꢀperfect absorber substrates were prepared using an approach that simplifies a
[28]
previously developed protocol.
The previous procedure required multistep deposition
(temperature and bombardment of oxygen); here we developed a new oneꢀbatch deposition
technique, followed by a single annealing step. Specifically, substrates were prepared on cleaned
glass slides via the sequential electron beam vapor deposition of a Au mirror (100 nm thick),
TiO spacer (30 nm thick) and finally a thin film of Au (4 nm thick). Thermal annealing (500 °C,
2
3
1
0 mins) caused the top layer to coalesce into NPs with a mean size distribution of 35 nm (Figure
2
d, Figures S3,4). The loading of Au NPs on the top layer was estimated to be 7.72 ꢁg/cm . The
thickness of the TiO layer affects the optical properties and efficiency of hotꢀcarrier extraction,
2
[
28]
and the metamaterial was optimized with the assistance of fullꢀwave numerical simulations to
ensure a high absorption crossꢀsection of the light source to be used in photocatalytic
experiments (Figure 1e). Absorption spectra of TiO and the Au mirror exhibit the expected line
2
shapes (Figure 1e). A sample of Au NPs on top of a glassꢀsupported TiO film revealed a single
2
absorption band centered around 650 nm, characteristic of localized surface plasmon resonances
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[38]
on a high refractive index support. The mirror/semiconductor/NPs sample exhibits an intense
absorption band with a peak at 580 nm with nearꢀperfect (∼94%) absorbance, an approximately
5ꢀfold increase compared to the control sample with no mirror (Figure 1e, Figure S5). The use of
physical vapor deposition techniques allow for facile scaleꢀup, with the fabrication of waferꢀ
sized nearꢀperfect absorbers easily achieved (Figure 1f).
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Performing Photocatalysis with a Near-Perfect Absorber. To evaluate the photocatalytic
properties of the nearꢀperfect absorber, we investigated the selective oxidation of benzylamine to
the imine product Nꢀbenzylidenebenzylamine as a model reaction (Figure 2). Imines are an
[
32,33]
important class of intermediate in both fine chemical and pharmaceutical synthesis.
Recent
reports using plasmonic photocatalysts for imine synthesis afford only modest conversions (<
[39]
5
%),
which we aimed to improve upon through incorporation of the catalyst into a nearꢀ
perfect absorbing architecture. The light source used for these experiments was a 100 W white
LED (Figure S1), which minimized both thermal heating from infrared radiation (as caused by
high power halogen lamps) and the direct generation of electronꢀhole pairs in the semiconductor
layer by high energy radiation (i.e. UV, the TiO is transparent to the visible light LED source as
2
shown in Figure 1e). Reactions were performed at ambient laboratory conditions under an
atmosphere of molecular oxygen, and conversion was assessed by the integration and
comparison of diagnostic peaks of both the reactant (δ 3.86 ppm) and product (δ 4.82, 8.39 ppm)
1
in H NMR spectra (Figure S6).
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Figure 2. Photocatalytic oxidation of benzylamine, calculated reaction rate and TON for the
reactions under control conditions: (i) the reaction with the nearꢀperfect absorber in the dark, (ii)
the reaction with the nearꢀperfect absorber under illumination, (iii) a thin film of TiO supported
2
by an Au mirror, and (iv) a thin film of TiO with Au NPs. Reaction conditions: photocatalyst
2
(size 10 × 25 mm, containing 19.3 ꢁg Au); benzylamine, 0.05 mmol; solvent, 4 mL of
acetonitrile; 1 atm of O ; room temperature; reaction time, 20 h; and LED light intensity, 100
2
2
mW/cm .
We measured a catalytic rate of 37 mmolproduct/gAu catalyst/h (conversion 28 %, TON of 146)
with the use of the nearꢀperfect absorber. This compares very favorably to the control substrates
that omit either the mirror or nanoparticle layer, which returned only trace amounts of product
(Figure 2). Furthermore, a bare Au film was unable to drive the photocatalytic reaction (Figure
S7). Importantly, no product was observed for reactions performed in the dark, even when the
solution was heated at 70 °C (Figure S8), confirming light is required for this transformation. No
product was detected when the reaction was performed under an inert atmosphere, confirming
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the active oxidant derives from molecular oxygen (Figure S9). Calculation of the reaction
turnover number (TON = moles of desired product formed/moles of Au NPs) revealed a 29ꢀfold
increase in reactivity with the nearꢀperfect absorbing substrates relative to those with no mirror
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(
TiO /Au NPs).
2
The activity of the photocatalyst with several substituted derivatives of benzylamine was also
evaluated (Table 1). Benzylamines functionalized with either electron withdrawing and donating
groups were included, with varying degrees of conversion observed, although no clear trend
between the electronic properties of the reagent and the final conversion could be elucidated.
Secondary amines (entries 7 and 8, Table 1) also yielded the corresponding imines with
moderate conversions.
Table 1. Reaction scope of photocatalytic oxidation of various amines by nearꢀperfect absorbers.
Entry
Amine
Product
Conversion
Rate
TON
(%)
(mmolproduct/gAu
catalyst/h)
1
2
3
28
29
12
37
37
16
146
147
62
4
10
13
52
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Reaction conditions: nearꢀperfect absorber (size 10 × 25 mm); reactant, 0.05 mmol; solvent, 4
mL of acetonitrile; 1 atm of O ; room temperature; reaction time, 20 h; and LED light intensity,
2
2
1
100 mW/cm . Product conversion was determined by crude H NMR analysis.
The photocatalyst showed no loss of activity over successive reaction cycles providing a
straightforward catalyst regeneration step was employed (thermal annealing, 250 °C, 30 mins,
Figures S10ꢀS12). Reusing the substrate without regeneration resulted in a significant activity
drop, likely caused by the deposition of organic material on the surface preventing further amine
[
39]
binding. This simple treatment and the ease of handling glassꢀslideꢀbased catalysts are in stark
contrast to the recovery and recycling of traditional heterogeneous catalysts, which typically
involves repeated centrifugation and washing steps.
Study of the Photocatalytic Mechanism. The reaction requires oxygen to occur, so in order
to determine the active oxygen species generated, we evaluated the photocatalytic reaction in the
1
presence of various scavengers (Figure 3a). The introduction of the singlet oxygen ( O )
2
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scavenger βꢀcarotene caused no change in the reaction rate. Addition of the radical scavenger
(2,2,6,6ꢀtetramethylpiperidinꢀ1ꢀyl)oxyl (TEMPO) partially suppressed the rate, strongly
suggesting a radical based pathway. Finally, upon addition of pꢀbenzoquinone (pꢀBQ), a
•−
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superoxide (O ) scavenger, the reaction was effectively stopped, therefore providing
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compelling evidence that superoxide is the reactive species generated during photocatalysis.
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Figure 3. (a) The photocatalytic oxidation of benzylamine by a nearꢀperfect absorber in the
presence of specific scavengers. Scavenger (0.05 mmol) otherwise all other reaction conditions
were identical as in Figure 2. (b) The proposed reaction mechanism.
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Based on these results, we propose a reaction mechanism that begins with the absorption of
visible light by the nearꢀperfect absorber, leading to a transient population of hot electrons and
holes upon nonꢀradiative relaxation of the localized surface plasmons in the Au NPs (Figure 3b).
Both benzylamine and the product exhibit no absorption bands in the visible spectrum (Figure
S13), and hence a direct chemical transformation in the absence of the nearꢀperfect absorber is
not possible. Reduction of molecular oxygen to the superoxide radical occurs at the TiO ꢀsolvent
2
interface following excited electron injection into the semiconductor support, a step previously
[40,41]
supported by electron spin resonance spectroscopy.
The resulting hole residing in the Au
NPs then abstracts an electron from benzylamine (1) (adsorbed on the Au NP surface) resulting
[
41ꢀ44]
•−
in a benzylamine radical cation (2).
The generated superoxide (O ) is capable of
2
abstracting an αꢀH from the radical cation to generate the iminium cation intermediate (3). The
iminium cation undergoes a condensation reaction with another benzylamine molecule to the
aminal intermediate (4) before finally undergoing hydrogen atom transfer (HAT) and elimination
of ammonia to afford the product. Control experiments with Au mirror/SiO /Au NPs (a similar
2
architecture that substitutes the TiO with insulating SiO , Figure S14) yielded only trace amount
2
2
of product, confirming the role of the Schottky barrier in the proposed chargeꢀseparation step
(Figure 1b).
To confirm the presence of this reactive iminium intermediate, we performed the
photocatalytic reaction with an alternative nucleophile. Using nitromethane as the solvent
yielded two new chemical species as the major products, as determined by liquid
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chromatographyꢀmass spectrometry (Figure 3b, Figure S15). The imine (6) is the product of the
AzaꢀHenry reaction between benzylamine and nitromethane, while the Nꢀbenzylꢀ2ꢀnitroꢀ1ꢀ
phenylethanꢀ1ꢀamine (7) likely results from further nucleophilic attack of benzylamine. Light
irradiation of the nearꢀperfect absorber significantly enhanced the production of the iminium
intermediate (Figure S15). Iminium ions are potent electrophiles capable of installing new bonds
at the αꢀposition of amines and greatly expand the reaction scope achievable using plasmonic
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[
5,45]
nearꢀperfect absorbers.
The carbonꢀcarbon bond forming AzaꢀHenry reaction exemplifies the
expanded scope that iminium intermediates afford and we demonstrated this with some
additional substrates (Table 2).
Table 2. Photocatalytic AzaꢀHenry reactions with the nearꢀperfect absorbers.
Reactant
Product (Conversion %)
6
3%
6%
11%
30%
3
1%
Reaction conditions: nearꢀperfect absorber (size 10 × 25 mm); reactant, 0.05 mmol; solvent, 4
mL of nitromethane; 1 atm of O ; room temperature; reaction time, 20 h; and LED light
2
2
intensity, 100 mW/cm . Product conversion was determined by UPLCꢀMS analysis.
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Finally, we measured the apparent quantum yield (AQY) of the reaction as a function of the
incident irradiation wavelength (Figure 4a, S16). The AQY is a wavelengthꢀdependent quantity
that is given by the ratio of the number of molecules N produced to the number of incident
photons n (as opposed to the number of absorbed photons). By definition, this is a quantity that is
independent of the intensity of the incident radiation, and its spectrum provides physical insight
on the mechanism accounting for the conversion of photonic energy into chemical potential
energy.
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In Figure 4a, the measured AQY exhibits a monotonic increase with increasing photon
energy with little overlap with the measured optical absorption spectrum, a situation similar to
[46,47]
reported AQYs in other nanostructured metal photocatalysts.
In order to explain this
observed AQY lineꢀshape, we have developed a microscopic model that accounts for the
conversion of incident photon energy into plasmonic hotꢀcarrier energy which drives the
formation of iminium radicals.
Following photoꢀexcitation, plasmon nonꢀradiative relaxation leads to a population of hot
carriers (electrons and holes) with energy distributions above (electrons) and below (holes) the
metal’s Fermi level. Only those holes with energies that match the highestꢀoccupied molecular
orbitals (HOMO) of adsorbed benzylamine molecules can be injected across the metalꢀadsorbate
interface, initiating the oxidation process (The case shown in Figure 4b,d,e). The reduction of O2
•−
to superoxide (O ) by the hot electron must also coexist in the overall process. The reaction
2
cannot take place without either of these two processes (Figure 4d,e). It is possible to estimate
the photonꢀenergy dependence of these steps, by calculating the energy distribution of hot
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carriers within a simple freeꢀelectron model for the electronic structure of metals (see inset of
[
36]
Figure 4a).
Within this level of approximation, the probability of benzylamine oxidation is
proportional to the overlap between the hotꢀhole energy distribution and the (broadened) HOMO
[
48]
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(further details of the role of the HOMO are shown in Figure S17).
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Figure 4. (a) The AQY for the photocatalytic reaction is plotted as a function of the calculated
hot carrierꢀreactant HOMO overlap. The inset schematic shows the theoretical model for the
energy overlap between the hot hole and the reactant HOMO level. E is the Fermi level, E is
F
h
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the energy of the hot hole, and ℏω is the maximum hot carrier energy with respect to the Fermi
level. (bꢀe) Energy diagrams of the possible hot carrier mediated mechanisms for the
photocatalytic benzylamine (BA) oxidation.
The line shape of the experimental AQY spectrum is well matched with the calculated hot
carrier–reactant HOMO overlap. With sufficient light energy (ℏω ) irradiation, the
1
photocatalytic reaction will take place when both kinds of hot carriers have energies that
coincide with the potentials for charge transfer to occur (Figure 4d). The observed monotonic
increase of AQY with photon energy originates from the fact that shorter wavelengths with
higher energy (ℏω ) excite hot electrons and holes to higher energy levels (Figure 4e), leading to
2
a much higher hotꢀcarrier injection probability (higher reaction activity, and consequently, higher
AQY). The role of perfect light absorption in this model is to increase the rate of excitation of
hot carriers.
CONCLUSION
In summary, we have demonstrated the hotꢀcarrier driven synthesis of imines via a reactive
iminium intermediate generated using a plasmonic nearꢀperfect absorber of light. The chemical
transformations took place under ambient laboratory conditions powered by only a visible light
[
49,50]
source. Compared to traditional heterogeneous photocatalysts,
for which large active
specific surface areas are essential, the nearꢀperfect absorber metamaterial is limited to a
monolayer of Au NPs whilst maintaining excellent photocatalytic efficiency. Importantly, the
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mechanistic studies presented provide unambiguous evidence that these reactions are photoꢀ
initiated and occur via the formation of radical species, which must result from charge separation
following plasmonic nonꢀradiative relaxation.
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Our work demonstrates one of the many potential pathways for the generation of reactive
organic radical species via the efficient conversion of photons into hot carriers afforded by
plasmonic nearꢀperfect absorbers. An aspect of our ongoing investigation is the application of
this conversion mechanism to other C–C bond formation reactions. One of the greatest
challenges to be overcome before the deployment of our concept to industrial applications is the
issue of scale; in general, the most promising strategy for upꢀscaling the use of our substrates
[
51,52]
would be their deployment catalysts in a photoꢀredox flowꢀchemistry approach.
Overall, in
the context of recent research into photocatalytic organic synthesis, we believe that the
underlying physical and chemical concepts discussed here will motivate the rational design of
other multicomponent systems with nearꢀperfect light absorbers, resulting in the application of
optimized photonic energy harvesting to a broad range of organic syntheses with increasing
molecular complexity.
EXPERIMENTAL SECTION
Materials. Cleanroom glass slides (SCHOTT Nexterion® slide glass B) were used for the
preparation of the nearꢀperfect absorber. All chemical reagents and solvents were used as
obtained from the suppliers without further purification. βꢀCarotene (≥95%, HPLC), (2,2,6,6ꢀ
Tetramethylpiperidinꢀ1ꢀyl)oxyl (98%), pꢀbenzoquinone (Reagent grade, ≥98%), benzylamine
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(
ReagentPlus®, 99%), and all the other substituted benzylamines were purchased from Sigmaꢀ
Aldrich. Acetonitrile (SeccoSolv®) was purchased from Merck.
Substrate Preparation. All films were deposited in an electron beam evaporation system
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(
Intlvac, Nanochrome II) using a 10 kV power supply in a Class 10,000 cleanroom. Cleanroom
glass slides (SCHOTT, Nexterion® slide glass B) were used as the substrate without additional
cleaning and were not removed from the cleanroom once opened. [Note: The cleanliness of the
glass slides is critical for achieving a uniform nearꢀperfect absorber surface. When a standard
microscope slide was used, the surface showed a mottled appearance post annealing, suggesting
nonꢀuniformity in the layers (see Figure S18). This occurred even after extensive cleaning].
First, a thin layer of Cr (5 nm, 0.5 Å/s) was deposited prior to the Au mirror to ensure good
adhesion to the glass substrate. Then a Au mirror (100 nm, 1.0 Å/s), TiO spacer (30 nm, 0.5
2
Å/s) final layer of Au (4 nm, 0.1–0.4 Å/s) were deposited sequentially without exposing
substrates to vacuum. To finish, the substrate was annealed on a hotplate in air (500 °C for 30
min, heating rate 10 °C/min) to generate a layer of small Au NPs (mean diameter approximately
3
5 nm).
The thickness of the Au thin film (h) could be finely controlled during the electron beam
evaporation deposition. To estimate the loading amount of Au NPs, we can simply measure the
size of the substrate and then calculate the area of the covered Au NPs layer (S), the loading
amount of Au NPs = S × h × d, where d is the density of Au.
Substrate Characterization. Scanning electron microscope (SEM) images of the Au NPs
were obtained using Field Emission Scanning Electron Microscope (Zeiss, Merlin FESEM). The
samples were mounted on an aluminium stub with doubleꢀsided conductive carbon tape. These
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samples were then iridium coated using a Cressington 208HRD sputter coater. The thickness of
the iridium coating was approximately 4 nm (60 mA for 30 seconds). The samples were imaged
with the microscope operated in secondary electron mode to highlight topographical features. An
accelerating voltage of 3 kV was used for imaging using the In lens detector. The particle size
distribution histogram was calculated based on 500 isolated particles.
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The diffuse and specular reflectance (R) and transmittance (T) spectra were measured using a
UV−visible spectrophotometer equipped with an integrating sphere (Perkin Elmer, Lambda 1050
and 150 mm InGaAs Integrating Sphere). With these two measurements, the absorbance (A) was
calculated as A = 1 − R − T.
Liquid chromatographyꢀmass spectrometry analysis was performed using an ultraꢀ
performance liquid chromatography system (Waters, Acquity UPLC iꢀClass) equipped with an
adjustmentꢀfree atmospheric pressure ionization electrospray mass detector (Waters, Acquity
QDa). Positive and negative ions were recorded simultaneously with full scan analysis in m/z
range 50 to 1000. The separating conditions used were: reverse phase C18 column (Waters,
Acquity UPLC BEH C18, 50 × 2.1 mm, 1.7 µm particle size); mobile phase A: 100% MilliꢀQ
water with 0.1% formic acid; mobile phase B: 100% acetonitrile with 0.1% formic acid.
Gradient: 95% A to 100% B over 4.50 minutes. Hold at 100% B for 1 minute. Change to 95% A
over 0.5 minute, then hold for 1 minute. MS was collecting data for the complete 7 minute run.
Spectral analysis was from 190 to 350 nm with chromatograms extracted using a wavelength of
254 nm. (Flow rate: 0.400 ml/min; column temperature: 30°C; sample injection volume: 1 µl)
Standard Protocol for Photocatalytic Reactions. The photocatalytic oxidation of
benzylamine was performed in a glass culture tube. A cut piece of nearꢀperfect absorber slide (10
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×
25 mm) was placed in the tube. Then a solution of benzylamine in acetonitrile (12.5 mM) was
added. The tube was capped with a rubber septum, sealed with parafilm, and sparged with
oxygen for 5 minutes and maintained under a positive O atmosphere. The reaction was
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irradiated with a white LED (100 W, distance from the reaction tube to the LED was 10 cm,
2
measured light intensity ~100 mW/cm ) for 20 h. At the conclusion of the reaction, the catalyst
was removed and rinsed with CHCl . All solvent (reaction and washing solution) was combined
3
and removed under reduced pressure. The crude product was dissolved in CDCl and the yield
3
1
determined using H NMR spectroscopy to compare the integration of diagnostic peaks in both
the product and reactant.
Action Spectrum Experiments. The reaction was irradiated by monowavelength LED light
sources (100 W, wavelength: 395 nm, 460 nm, 520 nm, 590 nm, 620 nm, 730 nm). The light
intensities at each wavelength were measured with a digital optical power and energy meter
(Thorlabs, PM100D). All the other reaction conditions and procedures were identical to those in
the standard protocol.
The apparent quantum yield (AQY), a wavelengthꢀdependent quantity, was calculated as:
ꢅ
AQY(λ) =ꢄ
× 100%, where N is the number of product molecules formed in the reaction (N =
ꢆ(ꢇ)
mole of products formed × Avogadro's constant), and n is the number of incident photons
ꢈ
involved in the reaction. n = , where E is the total energy input involved in the reaction, and e is
ꢉ
the energy of one incident photon at the specific wavelength , the energy of one photon e = hν =
ꢊꢋ
,
h is the Planck constant, c is the speed of light in vacuum and λ is the photon's wavelength.
ꢇ
ASSOCIATED CONTENT
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Supporting Information.
The Supporting Information is available free of charge on the ACS Publications website.
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Detailed reaction setꢀup, characterization data, additional catalytic data, theoretical
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modelling, H NMR, and LCꢀMS spectra (PDF)
AUTHOR INFORMATION
Corresponding Authors
*
kevin.xiao@csiro.au
*
daniel.gomez@rmit.edu.au
Author Contributions
Q.X. and D.G. developed the project. Q.X. prepared the samples and performed the experiments.
T.U.C. provided the mechanism analysis. J.J.C. and A.R. performed the numerical simulations.
A.S.R.C. performed the reflectance and transmittance spectra measurements. The manuscript
was written through contributions of all authors. All authors have approved the final manuscript.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the
Victorian Node of the Australian National Fabrication Facility (ANFF). The authors would like
to thank Milena Czyz for her feedback regarding the photocatalytic mechanism. Q.X.
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acknowledges CSIRO for an OCE Fellowship. We acknowledge the ARC for support through a
Discovery Project (DP160100983) and a Future Fellowship (FT140100514).
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