Asymmetric Plasmon-Mediated CO2 Photoreduction Boosts Ethane Selectivity

Asymmetric Multipole Plasmon-Mediated Catalysis Shifts the
Product Selectivity of CO2 Photoreduction toward C2+ Products
Ehsan Vahidzadeh,* Sheng Zeng, Ajay P. Manuel, Saralyn Riddell, Pawan Kumar, Kazi M. Alam,
and Karthik Shankar*
Cite This: ACS Appl. Mater. Interfaces 2021, 13, 7248−7258 Read Online
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sı Supporting Information
ABSTRACT: Cu/TiO2 is a well-known photocatalyst for the photocatalytic
transformation of CO2 into methane. The formation of C2+ products such as ethane
and ethanol rather than methane is more interesting due to their higher energy density
and economic value, but the formation of C−C bonds is currently a major challenge in
CO2 photoreduction. In this context, we report the dominant formation of a C2
product, namely, ethane, from the gas-phase photoreduction of CO2 using TiO2
nanotube arrays (TNTAs) decorated with large-sized (80−200 nm) Ag and Cu
nanoparticles without the use of a sacrificial agent or hole scavenger. Isotope-labeled
mass spectrometry was used to verify the origin and identity of the reaction products.
Under 2 h AM1.5G 1-sun illumination, the total rate of hydrocarbon production
(methane + ethane) was highest for AgCu-TNTA with a total CxH2x+2 rate of 23.88
μmol g−1
h−1
. Under identical conditions, the CxH2x+2 production rates for Ag-TNTA
and Cu-TNTA were 6.54 and 1.39 μmol g−1
h−1
, respectively. The ethane selectivity
was the highest for AgCu-TNTA with 60.7%, while the ethane selectivity was found to be 15.9 and 10% for the Ag-TNTA and Cu-
TNTA, respectively. Adjacent adsorption sites in our photocatalyst develop an asymmetric charge distribution due to quadrupole
resonances in large metal nanoparticles and multipole resonances in Ag−Cu heterodimers. Such an asymmetric charge distribution
decreases adsorbate−adsorbate repulsion and facilitates C−C coupling of reaction intermediates, which otherwise occurs poorly in
TNTAs decorated with small metal nanoparticles.
KEYWORDS: multistep electron transfer, reaction pathway tuning, n-type TiO2, Schottky junction, plasmonic catalysis,
hot electron-mediated photocatalysis
1. INTRODUCTION
The photoreduction of CO2 to value-added products is
currently a subject of significant research interest. The primary
work of Inoue et al. in 1979 showed that CO2
1
can be
converted into useful products such as CO, CH4, C2H6,
HCOOH, CH3OH, etc. utilizing semiconductor photocatalysts
such as TiO2, ZnO, CdS, and GaP. The lack of product
selectivity in CO2 photoreduction is a major challenge that
limits practical applications and industrial adoption of this
technology.2
The major products of this reaction are typically
C1 products such as CO and/or CH4 and/or HCOO−
(formate), but it has been reported that C2+ products (such
as C2H6) can also be produced during the photoreduction
process.3−6
C2+ products are more interesting since they have a
higher energy density and serve as precursors for a greater
variety of industrial reactions, thus generating more market
value. For instance, ethane can be transformed into liquid fuels
more easily than methane.5,7
The reaction mechanism involved
in the production of C2+ products is more complex and usually
involves the C−C coupling reaction, which has a high
activation energy barrier. It is noteworthy that the success
achieved in the electrochemical CO2 reduction reaction
(CO2RR) field in generating C2+ products such as ethylene
and ethanol at yields of 60−90% has not translated thus far
into CO2 photoreduction using similar material systems.
Strategies used in CO2RR to optimize C−C coupling and
enhance the yield of C2+ products such as higher alkali-metal
cation concentrations in the electrolyte, higher pH near the
electrode, higher electrolyte concentration of CO2, running the
reaction at higher overpotentials, etc. do not have direct
counterparts in photocatalytic CO2 reduction.8−10
The presence of a cocatalyst (mostly nanoparticles (NPs) of
transition metals such as Au, Ag, Cu, Pt, Pd, etc.) in a
photocatalyst system offers several benefits including increas-
ing the light-harvesting ability of the photocatalyst through
interband and intraband transitions in the metal nanoparticle,
improving the separation of photogenerated charge carriers
Received: November 26, 2020
Accepted: January 25, 2021
Published: February 4, 2021
Research Article
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© 2021 American Chemical Society
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through the formation of a heterojunction interface, providing
new sites for adsorption of reactant molecules on the surface of
the photocatalyst, etc. Transition metals are known for
improving the hydrogenation reaction rather than C−C
coupling, due to which decoration of semiconductor photo-
catalysts with transition-metal nanoparticles favor the
production of methane compared to C2+ hydrocarbons.7
Carbon dioxide anion radical (CO2
•−
), electronically excited
state of carbon monoxide (CO*), formyl radical (•
CHO), and
carboxyl radical (COOH) ﬁgure prominently as reaction
intermediates in multiple pathways proposed for CO2
photoreduction.11,12
Interadsorbate steric and dipole repulsion
forces hinder the adsorption and C−C coupling of these
reaction intermediates on adjacent reaction sites of metal
atoms.13
The interplay between long-range attractive forces
and short-range Pauli repulsion and repulsive dipole−dipole
coupling is well studied for CO monolayers of various
coverages on Cu(100), Cu(110), and Cu(111) surfaces.14−16
Nanoparticles of Ag, Au, Cu, and Al exhibit strong localized
surface plasmon resonances at visible wavelengths, which
consist of coherent and collective charge density oscillations of
conduction-band electrons. Nanoparticles smaller than ∼50
nm primarily exhibit a symmetric dipole resonance wherein
optical excitation displaces the center of negative charge with
respect to the center of positive charge due to the polarization
of the electron cloud by the incident ﬁeld.17
The combination
of adsorbate−adsorbate repulsion between reaction intermedi-
ates and a symmetric distribution of surface charge due to
dipolar localized surface plasmon resonance (LSPR) excitation
makes C−C coupling more unlikely on such metal nano-
particle surfaces. This may be because vapor-phase CO2
photoreduction on TiO2 nanotubes surface-decorated by <10
nm-sized nanoparticles of Cu, Pt, Ru, Au, Pd, AuPd, ZnPd, Ag,
etc. has primarily been reported to result in the dominant
Figure 1. TEM analysis of the AgCu-TNTA sample. (a, b) Cross-sectional view, (c, d) HR-TEM analysis of AgCu nanoparticles and corresponding
d-spacings, and (e) elemental mapping of the AgCu-TNTA sample.
ACS Applied Materials & Interfaces www.acsami.org Research Article
7249

formation (>95% of all product molecules formed) of C1
products such as methane, carbon monoxide, methanol, and
formaldehyde, with C2 products such as ethane and ethylene
being formed as minor byproducts (<5% of all products).18−25
In metal particles larger than 50 nm, the quasi-static
approximation is no longer valid and the quadrupole plasmon
mode becomes significant, which produces an antisymmetric
distribution of charge density oscillations on the surface of the
metal nanoparticle when excited.26,27
In even larger nano-
particles and aggregates of dissimilar nanoparticles, multipole
plasmon excitations with rapidly alternating surface charge
density oscillations dominate.28
Multipolar plasmon resonance
modes and symmetry breaking in heterodimers29,30
afford the
possibility of closely lying reaction sites having opposite
charge, thus enabling a reduction in the dipole repulsion of
adsorbed C1 reaction intermediates and facilitating their C−C
coupling to form C2 products.
There are a few reports on the production of C2+
hydrocarbons (more specifically ethane) in the CO2 photo-
reduction process. Recently, Sorcar et al. reported production
of ethane with 28% selectivity at a rate of 77 μmol g−1
over a
Pt-sensitized graphene-wrapped titania photocatalyst. Fu et al.2
reported C−C coupling reaction on Au NPs with almost 40%
selectivity for ethane production.3
Our previous work in
Angew. Chem. Int. Ed. on a Cu−Pt nanoparticle-supported
titania nanotube array photocatalyst exhibited a maximum 14%
selectivity toward ethane formation.4
In all of the reported
literature so far, the cocatalyst typically consists of one or more
precious metals such as Pt, Au, Ru, and Pd.18,21,31
Herein, we
successfully synthesized large-sized, less expensive Ag, Cu, and
AgCu nanoparticles and successfully hosted these nano-
particles on a TiO2 nanotube array (TNTA) scaffold. The
results from our experiments demonstrate that the synergetic
effect of large (>80 nm) Ag and Cu nanoparticles as cocatalysts
in CO2 photoreduction increases both the efficiency and
selectivity of CO2 photoreduction; a C2 product, namely,
ethane, is generated with an unprecedented selectivity as high
as 60%. The key conceptual innovation consists of the use of
hybrid quadrupolar and multipolar plasmon resonances in
large plasmonic nanoparticles and their aggregates to achieve
an asymmetric photoexcited charge distribution on the
photocatalyst surface.
2. RESULTS AND DISCUSSION
2.1. Structure and Chemical Composition of the
Samples. Electrochemical anodization was used to synthesize
TiO2 nanotube arrays (TNTAs),32,33
and photodeposition
from Ag(acac) and Cu(acac)2 solutions34,35
was used to form
monometallic and bimetallic nanoparticles (NPs) decorating
and infiltrating the TNTAs (see the Materials and Methods
section in the Supporting Information). The fabrication
process is schematically depicted in Figure S1 in the
Supporting Information. The morphologies of TNTAs were
investigated using high-resolution transmission electron
microscopy (HR-TEM) and field emission scanning electron
microscopy (FESEM). The cross-sectional images of the
TNTAs (Figures 1a,b and S2 in the Supporting Information)
clearly show an array of hollow, vertically oriented nanotubes
with an average length of ∼6 μm and an average outer
diameter of 110 nm. The top view of the AgCu-TNTA sample
(Figures 1a and 2b) shows 80−200 nm polydispersed
nanoparticles of Ag and Cu agglomerated on the top of the
TiO2 nanotube arrays. From Figure 1b, it is evident that in
addition to the aggregates of larger Ag and Cu nanoparticles on
top of the nanotubes, there are smaller nanoparticles with
diameters 25−80 nm comparable to the inner diameter of the
Figure 2. (a, b) Top-view FESEM images of the TNTA sample before (a) and after (b) photodeposition of metal naoparticles; (b, c) X-ray
diffraction (XRD) patterns of TNTA, Cu-TNTA, Ag-TNTA, and AgCu-TNT samples; and (d) Raman spectra of TNTA, Cu-TNTA, Ag-TNTA,
and AgCu-TNTA samples.
7250

nanotubes, which penetrate along the length of the TNTAs.
Previous studies reported that bulk Cu and Ag exhibit lattice d-
spacings of 0.208 and 0.240 nm, respectively, and the alloy of
AgCu exhibits a d-spacing in the range 0.210 < d < 0.240 nm.36
d-spacings of 0.350 and 0.240 nm were determined from
selected area electron diffraction (SAED) patterns collected
from higher-magnification HR-TEM images (Figure 1c,d) and
correspond to the (101) plane of anatase TiO2 and FCC
Ag(111). In some cases, a d-spacing of 0.22 nm was observed,
indicative of the formation of AgCu bimetallic alloy through
galvanic replacement.36
The elemental map analysis of the area
of interest in the AgCu-TNTA sample is shown in Figure 1e,
which further confirms infiltration of the TiO2 nanotubes with
both Ag and Cu nanoparticles. Figure 2a shows the pristine
TiO2 nanotubes before photodeposition, while Figure 2b
confirms the TEM micrograph and shows a layer of densely
aggregated metal nanoparticles on top of the TNTAs.
The X-ray diffractograms of TNTAs, Ag-TNTAs, and AgCu-
TNTAs are shown in Figure 2c. The bare TNTA sample
exhibited reflections corresponding to the pure anatase phase
(JCPDS No. 21-1272). This is in agreement with prior reports
that show the dominance of the anatase phase when anodic
TiO2 nanotubes are annealed at temperatures lower than 550
°C. The temperature of 450 °C used in this work to crystallize
the nanotubes (see the Materials and Methods section in the
Supporting Information) is not high enough to induce the
anatase-to-rutile phase transition. In addition to anatase peaks,
other peaks originating from pure Ti could be detected in the
XRD pattern of TNTA samples, which correspond to the
portion of the titanium foil substrate that was not anodized.
For the Ag-TNTA and AgCu-TNTA samples, additional peaks
located at 2θ values of 44.08 and 64.3° are present in the
diffractograms, which were indexed to face-centered cubic
silver (JCPDS No. 04-0783). No peaks related to Cu could be
detected in the XRD patterns of AgCu-TNTA and Cu-TNTA
samples, which indicates that the Cu nanoparticles were highly
scattered, and the amount of Cu was extremely small
compared to Ag, Ti, and anatase TiO2.4
Consequently, it is
reasonable to assume that the aggregated layer of large metal
NPs seen in Figure 2b consists of a mixture of monometallic
Ag and bimetallic AgCu NPs. However, the UV−vis spectra
(discussed in Section 2.2) strongly suggest bimetallic AgCu
NPs.
The Raman spectra of the TNTA, Cu-TNTA, Ag-TNTA,
and AgCu-TNTA samples (Figure 2d) exhibit one intense and
three less intense peaks, which confirm the phonon modes in
tetragonal anatase TiO2. The prominent peak at 145 cm−1
is
due to the Eg mode, while less intense peaks at 395, 516, and
640 cm−1
correspond to the B1g, A2g, and Eg modes.35
An
expected shift and broadening for the main Eg modes in the
Raman spectra of Cu-TNTA, Ag-TNTA, and AgCu-TNTA has
been observed, which is related to the changes induced in the
structure of TNTAs by the presence of Ag and Cu
nanoparticles.35
The chemical composition of the catalyst
surface and subsurface was determined using X-ray photo-
electron microscopy (XPS) (Figure 3). The signature peaks of
Ti, O, Ag, and Cu are present in the XPS elemental survey scan
(Figure 3a), confirming the successful photodeposition of Cu
and Ag on the surface of the TNTAs. The deconvoluted core-
level high-resolution XPS (HR-XPS) spectra of TNTA and
AgCu-TNTA samples in the Ti 2p region exhibited two
symmetric peaks located at binding energies (BE) of 458.96
Figure 3. (a) XPS elemental survey scan of compact TiO2 nanotubes arrays (black) and AgCu-TNTA (red) and core-level HR-XPS spectra of
TNTA and AgCu-TNTA in the (b) Ti 2p region, (c) O 1s region (d) Cu 2p region, and (e) Ag 3d region.
7251

and 464.84 eV, assigned, respectively, to the Ti 2p3/2 and Ti
2p1/2 peak components of Ti4+
ions present in the TiO2 crystal
lattice (Figure 3b).37−39
The obtained peak splitting between
Ti 2p3/2 and Ti 2p1/2 peak components was found to be 5.88
eV, suggesting O2−
coordinated with Ti4+
in the TiO2 lattice
structure.40,41
The HR-XPS of TNTAs in the O 1s region can
be deconvoluted into two chemically shifted components
centered at BEs of 530.33 and 532.17 eV. The strong peak
component at BE ∼ 530.33 eV originated from the Ti-bonded
oxygens present in the crystal lattice (Ti−O−Ti), while the
weak shoulder peak at 532.17 eV was attributed to surface-
bounded −OH and nonlattice adventitious oxygens (Figure
3c).42−44
The binding energy of the O 1s peak components in
AgCu-TNTA remains almost identical to TNTA, demonstrat-
ing that decoration of Ag and AgCu nanoparticles on TiO2
nanotubes does not induce chemical changes on the surface.
Two well-resolved Cu 2p peaks for AgCu-TNTA at BEs of
932.63 and 952.65 eV (Figure 3d) originated due to Cu 2p3/2
and Cu 2p1/2 peak components of metallic Cu(0),
respectively.45,46
The absence of any shoulder or satellite
peak indicates the absence of any Cu1+
or Cu2+
oxides. The Ag
3d spectra (Figure 3e) of AgCu-TNTA showed two peaks at
368.08 and 374.13 eV with 6.08 eV peak splitting corroborated
to Ag(0) and the absence of shoulder peaks verified phase-pure
metallic silver.47,48
Integration and subsequent comparison of
the peak areas of the Ag 3d and Cu 2p peaks followed by
normalization to their respective photoelectron yields
indicated a Ag:Cu ratio of 43.4:1 (see Figure S11 in the
Figure 4. (a) Schematic illustration of Ag@Cu core−shell NP homodimers on top of adjacent TiO2 nanotubes. (b) Simulated optical absorption of
TNTAs decorated with Ag@Cu homodimer NPs with a core diameter of 100 nm and different shell thicknesses. (c, d) Electric field profile for a
TNTA/100 nm Ag core−5 nm Cu shell homodimer with a 3 nm edge-to-edge spacing in the substrate (xy) plane (c) and xz plane (d). The
incident plane wave is polarized in the x-direction and propagates along the z-direction.
Figure 5. (a) DRS spectra of TNTA, Ag-TNTA, and AgCu-TNTA samples. (b) Comparison between simulated absorption cross section (obtained
via FDTD modeling of small metal NPs) of AgCu-TNTA sample and its DRS spectrum. While the peak positions in the experimental and
simulated spectra show correspondence, the measured amplitude of the extinction is much higher than expected from FDTD modeling.
7252

Supporting Information), which confirms the small quantity of
Cu relative to Ag in the AgCu-TNTA samples.
2.2. Plasmon Excitation and Optical Properties. The
galvanic displacement of Ag atoms by Cu atoms on the surface
of silver nanoparticles mentioned earlier results in Ag@Cu
core−shell-type nanoparticles. We used finite-difference time
domain (FDTD) electromagnetic simulations to study the
interaction of light with TiO2 nanotubes decorated with a
single layer of Ag@Cu NP homodimers perched on top of the
pore openings. The simulated optical properties of the dimers
were used to obtain more general insight into the behavior of
the aggregated nanoparticles on the surface of TNTAs seen in
Figure 2b. The diameter of the silver core was kept constant at
100 nm, and the thickness of the copper shell was varied from
5 to 20 nm. The results of these simulations are shown in
Figure 4. Due to the strong interband damping-mediated
absorption of Cu, TNTAs decorated with Ag@Cu homo-
dimers exhibit significant visible light absorption. The strong
damping due to the presence of Cu also ensures that the
highest electric field intensities are found not in-between the
two particles constituting the dimer but rather at Ag−TiO2 and
Ag−air interfaces. The above analysis ignores the effect of
smaller Ag@Cu NPs located deeper in the pores of the TiO2
nanotubes.
The optical properties of the TNTA, Cu-TNTA, Ag-TNTA,
and AgCu-TNTA samples were investigated by diffuse-
reflectance UV−vis spectroscopy (DRS), as well as finite-
difference time domain (FDTD) electromagnetic simulations.
The DRS spectra of TNTA sample (black line in Figure 5a)
exhibits a band edge (dashed line in Figure 5a) around 380
nm, which is in accordance with its band gap (3.2 eV).49
The
TNTA samples also exhibit a broad absorption feature peaking
around 700 nm, which arises due to defects (Ti3+
states, Ti2+
states, O vacancies) as well as absorption by the Ti substrate.
In comparison to the bare TNTA sample, the band edge(s) for
the Ag-TNTA and AgCu-TNTA samples were blue-shifted to
lower wavelengths. This blue shift is due to the Mie scattering
of ultraviolet photons by metal nanoparticles.50
On the other
hand, the Cu-TNTA samples do not exhibit this blueshift due
to the Mie scattering attenuated by strong interband damping
in Cu. In Figure 5, the optical spectra of Ag-TNTA and AgCu-
TNTA samples do not resemble each other at all in spite of the
similarity in the XRD and Raman spectra. For the Ag-TNTA
sample, there is a broad, featureless localized surface plasmon
resonance (LSPR) peak centered at 450 nm, which agrees with
the polydispersed nature of Ag NPs fabricated in this study.
The AgCu-TNTA sample exhibits two LSPR peaks at 425 and
525 nm. The 425 nm peak is due to the hybrid quadrupolar
resonance of larger Ag NPs of size >75 nm27,30
that is likely
red-shifted by 50 nm from the corresponding resonance of Ag
nanoislands due to the higher permittivity of the TiO2
nanotube array scaffold.51
The 525 nm peak corresponds to
the dipolar resonance of AgCu NPs in the vicinity of TiO2 and
weakened in amplitude by the interband damping of Cu.27
The
optical spectra indicate the AgCu-TNTA samples to be less
polydispersed than the Ag-TNTA samples, which we attribute
to the nucleation of Cu on Ag NPs and the etching of already
agglomerated particles by galvanic replacement. The much
lower scattering amplitude for AgCu NPs (green curve in
Figure 4) due to damping provides persuasive evidence for the
dominant presence of bimetallic AgCu nanoparticles over
monometallic Ag NPs. The galvanic replacement by Cu2+
ions
reduces the size of the Ag nanoparticles and also makes either
an alloy hybrid of the two materials or a core−shell structure
consisting of an Ag core and a Cu shell.36
Considering that
further elevated annealing treatments were not used, the core−
Figure 6. (a, c) Electric field profile for a Ag−Cu heterodimer with a 3 nm edge-to-edge spacing in the substrate (xy) plane and corresponding
schematic illustration showing the surface charge distribution. (b, d) Electric field profile in the xz plane and schematic illustration of simulated
geometry showing large (120 nm diameter) spherical Ag and Cu nanoparticles perched on top of the nanotubes covering their mouth as in Figure
1a. The incident plane wave is polarized in the x-direction and propagates along the z-direction.
7253

shell scenario is more likely. The simulated absorption spectra
of large Ag@Cu core−shell NP homodimers as a function of
Cu shell thickness shown in Figure 4 and discussed in the
previous paragraph have some similarities and differences with
the experimentally measured spectrum (green curve in Figure
5a). Unlike the simulated homodimers, the experimental
spectra do not exhibit any peaks at red and near-infrared peaks.
However, the experimental spectra do show a significant albeit
decreasing longer-wavelength absorption similar to the
homodimers. The experimental spectrum shows two broad
peaks at visible wavelengths similar to the homodimer
simulated spectra, but the peak positions do not match.
FDTD simulations of smaller nanoparticles (intended to
capture the interactions of smaller bimetallic nanoparticles
found deeper in the nanotube pore) captured the key peak
locations (∼425 and 525 nm) in the optical spectra of AgCu-
TNTA samples, but could not capture the large amplitude of
the extinction at visible wavelengths seen in Figure 4b because
the simulation geometry of well-separated metal nanoparticles
infiltrated into TNTAs (Figure S3 in the Supporting
Information) did not capture the multipolar resonances
expected in the layer of aggregated large nanoparticles on
top of the nanotubes (Figures 1a and 2b).
The simulated spectra of TNTAs decorated with Ag@Cu
core−shell homodimers (Figure 4b) and small, separated Ag
and Cu NPs (Figure 5b) do not fully explain the huge
differences observed between the CO2 photoreduction
performance of TNTAs decorated with Ag NPs, Cu NPs,
and AgCu NPs, as will be discussed in the next section. If Ag
NPs deep in the nanotube pores are the species dominating
light−matter interactions (since chemical interactions indicate
the overall amount of Cu to be quite small), then the
electromagnetics of Ag-TNTA samples and AgCu-TNTA
samples would be similar. If the Cu shells coating large Ag
particles are responsible for the catalytic activity and light−
matter interactions, then the CO2 photoreduction performance
of AgCu-TNTA and Cu-TNTA samples ought to be similar
(but they are not). Therefore, we next considered the
possibility of heterodimers consisting of Ag and Cu nano-
particles (which also approximates the situation of Ag and
Ag@Cu nanoparticles). To further investigate the behavior of
the aggregated layer of large metal NPs on top of the TNTAs,
we performed FDTD simulations of close-lying Ag−Cu
heterodimers (Figure 6).
Although the heterodimers expected in our samples are
closely lying Ag NPs and Cu-coated Ag NPs, Ag−Cu
heterodimers constitute a more general case that captures
the largest range of possibilities based on the structure and
composition data discussed in Section 2.1. We also simulated
the behavior of isolated Ag, Cu NPs, as well as Ag−Ag and
Cu−Cu homodimers (Figures S4 and S5 in the Supporting
Information). At very few spots at the right edge of the Ag NP
directly opposite the Cu NP on top of TiO2, an absolute local
field intensity enhancement factor as high as 90 was obtained
due to the well-known electrodynamic coupling of plasmon
modes in closely lying metal nanoparticles.52
Absolute local
field intensity enhancement factors of 15 and higher were
obtained at a number of spots all along the Ag−TiO2 interface
due to the low loss of Ag and the additional field concentration
effect at the high-index interface with TiO2 nanotubes. The
electric field profiles were scaled using a jet colormap from
dark blue (low field amplitude) to dark red (large field
amplitude), respectively. While the scale factor does not
change the intensity values, the purpose of scaling is to
Figure 7. (a) Reaction products under irradiation with AM1.5G 1-sun simulated sunlight. (b) Comparison of ethane selectivities using Cu-TNTA,
Ag-TNTA, and AgCu-TNTA photocatalysts. (c) Reusability test of AgCu-TNTA sample for CO2 photoreduction reaction under standard
condition.
7254

normalize the colors in the monitor so that field intensity
colors that are otherwise too small to see can be clearly seen
for comparison. In Figure 6a, the relatively low field intensity at
Cu−TiO2 and Cu−air interfaces occurs due to the large
interband damping in Cu. In Figure 6b, regions of highest
electric field intensity are to the right of the Ag particle
member of the heterodimer, closer to its interface with the Cu
particle, which can be explained by a multipolar asymmetric
surface charge distribution as in Figure 6c. Both the
homodimer and heterodimer simulations confirmed a high
density of hotspots in the Ag-TNTA and AgCu-TNTA
samples, where the local field intensity was enhanced by a
factor of at least 4.
2.3. CO2 Photoreduction Performance. The photo-
catalytic activities of the Cu-TNTA, Ag-TNTA, and AgCu-
TNTA samples were investigated under identical conditions
involving 2 h of illumination by AM1.5G simulated sunlight at
50 °C. Immediately following illumination, the reaction
products were analyzed by gas chromatography (Figures S6−
S8 in the Supporting Information). The amounts of reaction
products are reported in this work as micromoles of evolute
per gram of photocatalyst per duration of the test (μmol g−1
h−1
). The results of the photoreduction tests are shown in
Figure 7a whose inset shows the total hydrocarbon production
by the samples. The main reaction product was ethane (C2H6)
for AgCu-TNTA samples, while methane was the main
product for the Ag-TNTA and Cu-TNTA samples. The total
rate of hydrocarbon production (methane + ethane) of 23.88
μmol g−1
h−1
was highest for AgCu-TNTA samples. The
CxH2x+2 production rates for the Ag-TNTA and Cu-TNTA
samples were 6.54 and 1.8 μmol g−1
h−1
, respectively. The
ethane selectivity for these three samples is shown in Figure
7b. The selectivity was highest for the AgCu-TNT sample
(60.7%), while the ethane selectivities were calculated to be
15.9 and 10% for Ag-TNTA and Cu-TNTA, respectively. The
highest selectivity of ethane for the AgCu-TNTA sample and
the fact that ethane was the primary product of CO2
photoreduction for this sample indicate that the synergistic
effect between the Cu and Ag cocatalysts was responsible for
the dominant production of ethane.
A comparison was conducted between the photocatalytic
activity of AgCu-TNTAs reported in this work and some of the
previously reported studies on TiO2 photocatalysts decorated
with monometallic Ag or Cu nanoparticles (Table S2 in the
Supporting Information). It is evident from this table that the
AgCu-TNTA sample reported here outperforms most of the
previously reported photocatalysts. Figure 7c exhibits the
reusability of the best performing photocatalysts (AgCu-
TNTA) during three consecutive runs, and the AgCu-TNTA
photocatalyst still maintained a high level of photocatalytic
activity during the second and third runs with a slight decrease
in performance. The shows that the higher selectivity toward
ethane than to methane can be explained by looking into the
carbene reaction pathway, which is widely agreed to be the
mechanistic route for CO2 photoreduction on nanoporous/
nanotubular TiO2 surfaces.49,53,54
Depending on the CO2
adsorption mode and the amount of energy provided by the
charge carriers, CO2 reduction can go through two different
reaction pathwaysthe carbene pathway, in which the main
product is methane and/or ethane, and the formaldehyde
pathway, in which formaldehyde itself is one of the main
reaction products/intermediates.55−57
The carbene pathway can be written as follows
l
m
o
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o
o
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CO CO CO OH CO
C OH CH CH
CH
CH
C H
2
e
2
e H e
e H e H e H
2
e H
3
e H
4
CH
2 6
3
→ ⎯ →
⎯⎯⎯⎯⎯ + →
⎯ →
⎯⎯⎯⎯⎯ + ⎯ →
⎯⎯⎯⎯⎯ ⎯ →
⎯⎯⎯⎯⎯
⎯ →
⎯⎯⎯⎯⎯
⎯ →
⎯⎯⎯⎯⎯
⎯ →
⎯⎯⎯⎯
− −
• + − − −
•
+ − + • +
*
+
*
•
+
− +
− + − + − +
− +
− +
*
•
The overall reaction for methane and ethane can be written
as
CO 8H 8e CH 2H O
2 4 2
+ + → +
+ −
2CO 14H 14e C H 4H O
2 2 6 2
+ + → +
+ −
The photocatalytic conversion of CO2 to hydrocarbons
through the carbene pathway is a complex reaction with several
intermediates. The methyl radical (•
CH3*) plays an important
role in the selectivity of the reaction. It requires 14 electrons to
produce ethane from CO2, while it requires eight electrons to
produce methane, thus making the ethane production
significantly more complicated. A photocatalyst, which can
stabilize methyl radical and provide a lot of electron−hole
pairs, is more successful in the production of ethane. If a •
CH3*
reacts with a proton, the product is methane. On the other
hand, ethane can be formed by dimerization of two •
CH3*
radicals. It has been suggested that the Cu+
sites on a
photocatalyst can stabilize the •
CH3* radicals and make the
formation of ethane more favorable.50,58,59
Interestingly, our
results show that ethane can be produced with higher
selectivity through CO2 photoreduction by Ag-TNTA samples
compared to Cu-TNTA samples, which provides empirical
evidence that Ag also enables the production of ethane by
stabilizing the methyl radical. Since the selectivity is higher
(Figure 7b), we deduced that the Ag cocatalyst can stabilize
methyl radicals even better than Cu. We note here that Ag
cocatalyst is known to lower CH4 production and increase C2+
selectivity in the Fischer−Tropsch reaction.60
The highest
selectivity of ethane for AgCu-TNTA samples proves that the
synergistic effect between the Ag and Cu in AgCu bimetallic
cocatalyst can boost methyl radical stabilization and pave the
way for the dominant production of ethane in the CO2
photoreduction process. Furthermore, the dipole repulsive
forces between adsorbed reaction intermediates, particularly
CO2
•−
and CO*, are weakened by the asymmetric surface
charge distribution explained in Section 2.2, which in turn
promotes C−C coupling.
Ultraviolet photoelectron spectra (UPS) were obtained to
determine the energy-level alignment at the metal−semi-
conductor interface in the heterojunction photocatalyst
(Figure S9a,b in the Supporting Information). The resulting
determination of the interfacial band structure confirmed the
presence of a Schottky junction in AgCu-TNTA with a
concomitant depletion region and built-in potential exceeding
1.2 V (Figure S9c,d in the Supporting Information). Since the
number of electron transfer steps required for ethane
formation is more than double that required for methane
generation, carrier recombination losses are known to cripple
ethane production and shift the product selectivity toward C1
products.7
The strong built-in electric field at the interface
reduces recombination losses through facilitating the separa-
tion of photogenerated charge carriers.35,61
This, in turn,
enables the high photocatalytic activity observed in this work
7255

and preserves the selectivity for ethane formation (i.e., C2
product). Hot electrons produced in the metal NP by plasmon
decay are injected over the Schottky barrier into TiO2, where
they possess unusually long lifetimes due to the long dielectric
relaxation time and low hole concentration of TiO2.17,62,63
The
resulting positively charged metal NP (due to the residual
hole) is a powerful stabilizer of the methyl radical.64
The Cu−
CH3
+
metal-ion stabilized methyl radical has a measured
lifetime of at least 50 μs, and metal−carbon bond dissociation
energies of 288 and 177 kJ mol−1
have been calculated for Cu−
CH3 and Ag−CH3, respectively.64,65
Hot spots with a high
local electric field have been suggested to favor the formation
of C2+ products in CO2RR.8
The plasmonic architecture of
AgCu-TNTA samples enables such hot spots, where CO2
molecules can be polarized by intense local electric fields66
(as
pointed our earlier in our discussion of the optical simulations
and spectra), and we surmise that these hot spots favor C2+
products in CO2 photoreduction as well.
To confirm that the reaction products stem from CO2 and
not from any source of carbon contamination, a series of sanity
tests were conducted. The sanity tests performed included
conducting the reaction in a reactor filled with nitrogen gas
(instead of CO2), conducting the reaction in the dark (with
and without photocatalyst), conducting the experiment
without heating the reactor, illuminating the reactor filled
with only water and CO2 (no photocatalyst), and conducting
the reaction using a titanium foil instead of the photocatalyst.
In all of these sanity tests, no product could be detected by the
gas chromatograph, which provides direct evidence that the
simultaneous presence of photocatalyst, water vapor, heat, and
light (solar simulator) is essential for driving the chemical
reaction. In addition, we performed a 13
CO2 isotope labeling
test, in which instead of introducing the regular 12
CO2 gas into
the reactor, 13
CO2 alone was introduced into the reactor while
keeping all other reaction conditions identical. The gaseous
products of the isotope labeling test were performed by gas
chromatography−mass spectrometry (GC−MS), and the
resulting ion chromatogram is shown in Figure S10 in the
Supporting Information. The isotope labeling test confirmed
the presence of 13
CH4 at an e/m value of 17 and 13
C2H6 at an
e/m value of 32.
3. CONCLUSIONS
The next frontier in CO2 photoreduction research is improving
product selectivity. The research focus is shifting from larger
yield numbers to higher-purity and more valuable products. A
whole gamut of chemicals including C1 products (CO, CH4,
and CH3OH) and heavier C2 hydrocarbons (C2H6, C2H5OH)
can be produced during the CO2 photoreduction process.
Heavier hydrocarbons are preferred because of their higher
energy density and higher economic value. Obtaining heavier
hydrocarbons is extremely difficult due to lower yields
(recombination and other losses in more numerous electron
transfer steps), poorer selectivity (more intermediates and
byproducts), interadsorbate repulsion inhibiting C−C cou-
pling, and less kinetic and thermodynamic preference. Herein,
we presented a method toward the preferential formation of C2
products during CO2 photoreduction using TiO2 nanotube
arrays (TNTAs) decorated with large-sized (80−200 nm)
photodeposited AgCu nanoparticles (NPs). Large, close-lying
plasmonic NPs exhibit an asymmetric surface charge
distribution following photoexcitation due to antisymmetric
quadrupole resonances and hybrid multipole plasmonic modes
with electric fields that vary rapidly in space. AgCu-TNTA
exhibited higher absolute product yields (14.5 μmol g−1
h−1
of
ethane and 9.38 μmol g−1
h−1
of methane) and higher
selectivity (60.7% selectivity toward ethane production) for C2
product compared to TNTAs decorated with monometallic Ag
or Cu NPs. We attribute the superior performance to the (i)
improved C−C coupling due to lowered dipole repulsion
between adsorbed reaction intermediates, (ii) Schottky barrier-
mediated long-lived charge separation following excitation and
plasmon decay, (iii) local electric field enhancement at hot
spots, and (iv) enhanced stabilization of methyl radicals by
hole-rich AgCu NPs. To date, the best selectivity for ethane
reported in the literature was achieved through expensive
cocatalysts such as Pt and Au, with the highest reported
selectivity of ∼40% using Au NP cocatalysts. In this report, we
utilized a relatively cheap cocatalyst consisting of AgCu NPs
with an unprecedented selectivity of 60% toward ethane. Our
results show that bimetallic cocatalysts made of relatively cheap
materials have the potential to replace the typical expensive
cocatalysts used in the literature and even outperform them in
selectivity.
■ ASSOCIATED CONTENT
*
sı Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acsami.0c21067.
Materials and methods; schematic illustration of
fabrication process; additional FDTD simulated electric
field profiles; gas chromatograms of CO2 photo-
reduction experiments collected using a gas chromato-
graph equipped with a pulse discharge detector; ion
chromatograms of isotope-labeled CO2 photoreduction
experiments collected using a gas chromatography−mass
spectrometry system (GC−MS); elemental chemical
maps of Cu-TNTA sample; and ultraviolet photon
spectra and band alignment at AgCu-TNTA interface
(PDF)
■ AUTHOR INFORMATION
Corresponding Authors
Ehsan Vahidzadeh − Department of Electrical and Computer
Engineering, University of Alberta, Edmonton, AB T6G 1H9,
Canada; Email: vahidzad@ualberta.ca
Karthik Shankar − Department of Electrical and Computer
Canada; orcid.org/0000-0001-7347-3333;
Email: kshankar@ualberta.ca
Authors
Sheng Zeng − Department of Electrical and Computer
Canada
Ajay P. Manuel − Department of Electrical and Computer
Canada
Saralyn Riddell − Department of Electrical and Computer
Canada
Pawan Kumar − Department of Electrical and Computer
Canada; orcid.org/0000-0003-2804-9298
7256

Kazi M. Alam − Department of Electrical and Computer
Canada; National Research Council Nanotechnology
Research Centre, Edmonton, AB T6G 2M9, Canada;
orcid.org/0000-0001-5075-5928
Complete contact information is available at:
https://pubs.acs.org/10.1021/acsami.0c21067
Notes
The authors declare no competing ﬁnancial interest.
■ ACKNOWLEDGMENTS
All authors thank the Natural Sciences and Engineering
Research Council of Canada (NSERC), the National Research
Council Canada (NRC), and Future Energy Systems CFREF
for direct and indirect (equipment use) ﬁnancial support.
Scholarship support to APM from the NSERC CGSD program
and to EV from the MITACS Globalink program is
acknowledged. The authors acknowledge use of the deposition
and characterization facilities at the University of Alberta
nanoFAB.
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