Optical Control of Selectivity of High Rate CO2 Photoreduction Via Interband- or Hot Electron Z-Scheme Reaction Pathways in Au-TiO2 Plasmonic Photonic Crystal Photocatalyst

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Applied Catalysis B: Environmental xxx (xxxx) xxx-xxx
Contents lists available at ScienceDirect
Applied Catalysis B: Environmental
journal homepage: www.elsevier.com
Optical control of selectivity of high rate CO⁠2 photoreduction via interband- or hot
electron Z-Scheme reaction pathways in Au-TiO⁠2 plasmonic photonic crystal
photocatalyst
Sheng Zeng ⁠a, Ehsan Vahidzadeh ⁠a, Collin G. VanEssen ⁠a, Piyush Kar ⁠a, Ryan Kisslinger ⁠a, Ankur Goswami ⁠a⁠, ⁠c,
Yun Zhang ⁠a, Najia Mahdi ⁠a, Saralyn Riddell ⁠a, Alexander E. Kobryn ⁠b, Sergey Gusarov ⁠b, Pawan Kumar ⁠a⁠, ⁎,
Karthik Shankar ⁠a⁠,
⁎
a
Department of Electrical and Computer Engineering, University of Alberta, 9211 - 116 St, Edmonton, AB T6G 1H9, Canada
b
Nanotechnology Research Centre, National Research Council, 11421 Saskatchewan Drive, Edmonton, AB, T6G 2M9, Canada
c
Department of Materials Science and Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India
A R T I C L E I N F O
Keywords:
Heterogeneous catalysis
Light trapping
Plasmonic photocatalysis
Density functional theory
FDTD simulations
A B S T R A C T
Photonic crystals consisting of TiO⁠2 nanotube arrays (PMTiNTs) with periodically modulated diameters were fab-
ricated using a precise charge-controlled pulsed anodization technique. The PMTiNTs were decorated with gold
nanoparticles (Au NPs) to form plasmonic photonic crystal photocatalysts (Au-PMTiNTs). A systematic study of
CO⁠2 photoreduction performance on as-prepared samples was conducted using different wavelengths and illu-
mination sequences. A remarkable selectivity of the mechanism of CO⁠2 photoreduction could be engineered by
merely varying the spectral composition of the illumination sequence. Under AM1.5G simulated sunlight (path-
way#1), the Au-PMTiNTs produced methane (302µmol h⁠−1) from CO⁠2 with high selectivity (89.3 %). When
also illuminated by a UV-poor white lamp (pathway#2), the Au-PMTiNTs produced formaldehyde (420µmol
h⁠−1) and carbon monoxide (323µmol h⁠−1) with almost no methane evolved. We confirmed the photore-
duction results by ⁠13C isotope labeling experiments using GC MS. These results point to optical control of the
selectivity of high-rate CO⁠2 photoreduction through selection of one of two different mechanistic pathways. Path-
way#1 implicates electron-hole pairs generated through interband transitions in TiO⁠2 and Au as the primary
active species responsible for reducing CO⁠2 to methane. Pathway#2 involves excitation of both TiO⁠2 and surface
plasmons in Au. Hot electrons produced by plasmon damping and photogenerated holes in TiO⁠2 proceed to re-
duce CO⁠2 to HCHO and CO through a plasmonic Z-scheme.
1. Introduction
In August 2018, the oldest and thickest sea ice in the Arctic broke
and drifted away from the coastline. The thinning and retreat of arc-
tic sea ice presents unambiguous evidence of global climate change
[1]. Global warming due to anthropogenic carbon emissions is a po-
tential environmental catastrophe and the research proposed in this ap-
plication is to generate a solution to this vexing problem by treating
CO⁠2 not merely as a waste product but as a feedstock to synthesize
value-added chemicals and fuel using sunlight. The first demonstration
of gas-phase CO⁠2 photoreduction was reported by Hemminger et al. in
1978 using water vapour and carbon dioxide to form methane on Sr-
TiO⁠3 as a catalyst [2]. In 1979, Inoue and co-workers first reported
the photoelectrocatalytic transformation of liquid CO⁠2 on catalytic semi-
conductors, producing organic compounds such as formic acid,
formaldehyde, methanol, and methane [3]. Subsequently, TiO⁠2 has been
regarded as one of the best photocatalysts insofar as activity and sta-
bility are concerned, especially for carbon dioxide photoreduction, al-
beit limited by its poor visible light absorption [4]. For decades, sci-
entists have shown enormous interest in improving the light harvest-
ing capability of TiO⁠2 in order to enhance photocatalytic yield through
compositional doping to narrow the bandgap [5–8], engineering visi-
ble light absorbing sub-bandgap defect states [9,10], using molecular
sensitizers [11] and incorporating nanophotonic light trapping effects
[12]. Simultaneously, efforts have been made to make the most effi-
cient use of harvested light, by decreasing the recombination of photo
⁎ Corresponding authors.
Email addresses: zeng1@ualberta.ca (P. Kumar); kshankar@ualberta.ca (K. Shankar)
https://doi.org/10.1016/j.apcatb.2020.118644
Received 29 October 2019; Received in revised form 28 December 2019; Accepted 13 January 2020
Available online xxx
0926-3373/ © 2020.

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S. Zeng et al. Applied Catalysis B: Environmental xxx (xxxx) xxx-xxx
generated carriers using for example, mixed-phase TiO⁠2 [13,14], vari-
ous morphologies [10,15–17], and decorating with metal or metal ox-
ides on TiO⁠2 [18–20]. Two general directions, which aim to overcome
the limitations of TiO⁠2 and to enhance photocatalytic performance, are
modification of material composition and the formation of heterojunc-
tions with visible light absorbers.
Several industrially relevant catalytic chemical reactions require ei-
ther high temperature or high pressure or both. A particularly energy in-
tensive reaction is the transformation of CO⁠2 into value-added products
[21,22]. Achieving sustainability involves making these reactions more
energy efficient by reducing the thermal budget required. Plasmonic
photocatalysis offers the promise of using light as the energy source to
drive a variety of chemical reactions close to room temperature [23,24].
Surface plasmons are quantized collective oscillations of the electrons
in noble metal nanostructures that strongly interact with visible and
near-infrared photons. Nanoparticles (NPs) of Ag, Au, Al and Cu can ex-
hibit localized surface plasmon resonances at visible wavelengths, and
the dephasing of particle plasmons through Landau damping creates hot
electron-hole pairs [25], that in principle, can be used to drive a chemi-
cal reaction and perform work. However, the hot electrons lose their ex-
cess energy thermal equilibrium over the picosecond timescale through
a sequence of relaxation processes [26]. Hot carrier pairs in metal NPs
can be separated by a built-in electric field at a Schottky junction or
through direct excitation of charge transfer states due to chemical in-
terface damping. Consequently, metallic NP decoration has been widely
used for increasing the photocatalytic performance of TiO⁠2 [27–29] Yui
and co-workers used palladium decorated TiO⁠2 to enhance the photocat-
alytic performance due to the removal of organic adsorbates by partial
oxidation of palladium [30]. Hou et al. demonstrated plasmon enhanced
photocatalytic conversion of CO⁠2 in 2011 [19].
One of the most critical elements to boost photocatalytic efficiency
is to enhance photon capture. Bottom-up fabricated photonic crystals
combining high surface area, mass-producibility and scalability, have
been recognized as promising structures to facilitate photon capture in
solar cells and photocatalysts [12,31–33], wherein periodic modulation
of structure or refractive index produces unique optical properties. Ozin
et al. claimed that the slow photon effect at the edge of photonic stop-
band gap can enhance weak bandgap absorption [34]. Liu et al. used a
Pt-loaded hierarchical TiO⁠2 photonic crystal to double the rate of hydro-
gen generation in photocatalytic water splitting [35]. Moreover, pho-
tonic crystals are widely employed in dye degradation. For instance,
Chen et al. reported the practical significance of the photonic crystal ar-
chitecture in enhancing methylene blue photodegradation on TiO⁠2 in-
verse opal film [36]. The aforementioned reports demonstrate the util-
ity of better light management for improved photocatalytic performance
[37].
The highlight of our present work is the demonstration of extreme
selectivity in product formation during CO⁠2 photoreduction, through
wavelength control of the incident illumination with the secondary ben-
efit of plasmon-mediated visible light harvesting. To appreciate why
such high selectivity is of paramount importance, consider that the
separation and purification of dissimilar hydrocarbon products (e.g. al-
cohols, aldehydes, short chain alkanes and alkenes, etc) themselves
constitute expensive and energy-intensive processes. A related find-
ing is that photocatalysts containing one or more distinct plasmon
resonances may allow manipulation of the reaction pathway depend-
ing on resonance/off-resonance excitation. Herein, we report highly
efficient CO⁠2 to hydrocarbon conversion to achieve variant products
through two different reaction pathways based on different illumina-
tion sequences. Both pathways were verified in isotope labeled exper-
iments using gas chromatography with a mass spectrometric detector
to eliminate contamination and to confirm the origin of reaction prod-
ucts. Experimentation was combined with modelling to achieve a holis
tic understanding. We used Lumerical FDTD to simulate the periodically
modulated TiO⁠2 nanotubes (PMTiNTs) with and without Au NP decora-
tion, and density functional theory (DFT) to understand the electronic
structure which together provided a systematic understanding of the op-
tical and electronic properties of this photocatalyst system.
2. Materials and methods
2.1. Materials
Titanium Foil (99 % purity, 0.89mm thickness) and aluminum foil
(99.99 % purity, 0.25mm thickness) were procured from Alfa Aesar.
Ethylene glycol (> 99 %), ammonium fluoride (> 98 %), aqueous hy-
drofluoric acid (49 %), nitric acid (70 %) and acetic acid (> 99 %) were
obtained from Fisher Scientific and used in their as-received form with-
out any further purification. De-ionized water was used throughout all
experiments. ⁠12
CO⁠2 was of 99 % purity and was obtained from Praxair
(Canada). ⁠13
CO⁠2 of 99 % purity was obtained from Cambridge Isotope
Laboratories, Inc. (USA).
2.2. Catalyst preparation
Ti foil was cut into 2.5×2.5cm pieces and washed using soap water,
followed by sonication in DI water, acetone, and methanol successively
for 10min each. After being dried under a nitrogen stream, the Ti pieces
were chemically polished by dipping into a mixed acid solution of 2:1:1
HNO⁠3: HF: CH⁠3COOH (v/v). They were then rinsed in DI water and
stored in a methanol bath. Before anodization, foils were dried under a
N⁠2 stream and then adhered to 2.5×4.5cm aluminum foils to improve
uniformity by ensuring constant potential across the back of the foil.
The Ti foil was anodized using charge-controlled pulse anodization in
a two-electrode cell using a 2.5×4.5cm Ti counter electrode. Anodiza-
tion was performed using a 3-step charge-controlled pulse for 30–100
periods in an ethylene glycol (EG) based electrolyte (0.3wt% NH⁠4F,
4vol% DI) at 65°C. After anodization, the as-formed PMTiNT on Ti foil
was detached from the aluminum backing and soaked in methanol and
hexane for 5 and 3min respectively, to clean off the electrolyte and en-
sure low surface-tension for drying. It was found that using high sur-
face-tension solvents to clean the PMTiNT caused it to crack and delam-
inate from the Ti substrate. Finally, the as-prepared PMTiNT were an-
nealed in a 3-zone tube furnace (STF55666C-1, Thermo Scientific Lind-
berg/Blue M), increasing from room temperature to 525°C in 4h and
held at 525°C for an additional 4h. The 3-step anodization pulse con-
sisted of a positive charge-controlled 60V period, followed by 2s of
−4V, and 20s of 0V. The charge was kept constant across pulses by in-
tegrating current over time using a 0.1O shunt resistor in series with
the cell, which was measured using an Arduino ATmega328P, and shut
off once it reached the desired value. During the 20s of 0V, the anode
and cathode were shorted to quickly remove any lingering static poten-
tial between the electrodes. For Au nanoparticle decoration, as-prepared
PMTiNT was loaded in a DC magnetron sputter system (Kurt J. Lesker
Co.). The base pressure was 7 mTorr, the power applied to the Au tar-
get was 75W and the duration of sputtering was 1min. Gold nanopar-
ticles (Au NPs) were deposited by magnetron sputtering without sub-
strate heating. The distance between the sputtering target and the sam-
ples was > 1m, due to which radiative heating of the samples by the
Ar plasma could be safely ignored. Therefore, the samples were at room
temperature during the whole sputtering process. The sputter deposition
rate was 7.4nm/min at a chamber pressure of 7 mTorr. The sample area
was 6.25cm⁠2 and the density of gold was 19.3gcm⁠−3. The loading of Au
NPs was estimated to be 85µg per sample. Following sputtering, sam-
ples were annealed in the 3-zone tube furnace again at the same tem-
perature (525°C) for 30min.
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2.3. CO⁠2 photoreduction experiments
The photocatalyst PMTiNT or Au-PMTiNT was loaded in a cylindri-
cal reactor (32ml internal volume) having a quartz window, as men-
tioned in our previous report [12]. One piece of Au-PMTiNT photocat-
alyst was placed in the reactor, which was charged with 200µl DI wa-
ter and 50psi of CO⁠2. Prior to introducing CO⁠2, the reactor was purged
several times with N⁠2 gas followed by CO⁠2 and finally the CO⁠2 pres-
sure was maintained at 50psi. Subsequently, the reactor was placed
on a hot plate heated to 80°C to evaporate water droplets. To inves-
tigate CO⁠2 to CH⁠4 photocatalytic reaction, the loaded reactor was il-
luminated under collimated AM 1.5G irradiation from a Class A so-
lar simulator (Newport-Oriel instrument USA) for 2h. The power den-
sity at the surface of the samples was 100mW cm⁠−2. The sample was
also irradiated with 50W white cold LED light (λ>400nm, max. inten-
sity at 510nm, Model: GT-FL001-50W, Power density 1mW cm⁠−2
) for
2h, followed by solar simulator for 2h. Further, to investigate the ef-
fect of wavelength on the product selectivity and probe the Z-scheme
photoreduction mechanism, the photocatalyst loaded reactor was illu-
minated by 365nm (UV light) and 505nm (green light) LEDs, simul-
taneously for 2h. The power density on the surface of samples was 5
and 10 mWcm⁠−2
for 365nm and 505nm LEDs respectively. Subsequent
to each irradiation step, the obtained gas samples were analyzed by a
Shimadzu gas chromatograph (GC-2014) equipped with a Porapak Q
column and a molecular sieve column, and a pulsed discharge detec-
tor (PDD). Conditions: He carrier flow rate: 0.5ml min⁠−1
, detector tem-
perature: 160°C, oven temp: started from 60°C and raised up to 160°C
to expel water vapor. In addition, isotope labeling experiments using
⁠13CO⁠2 instead of ⁠12CO⁠2 were carried out under identical conditions
to rule out contamination from ⁠12C source. Gas chromatography-mass
spectrometry (GC–MS) was employed to analyze the obtained gas sam-
ples and verify that the products were generated from the photocatalytic
reaction.
3. Results and discussion
As evident from FESEM imaging of PMTiNT, the pulsed anodiza-
tion method forms a periodically modulated nanotube structure with
an average diameter of 80nm±5nm, the repeating long segment is
194.5nm±14nm in length, and the short segment is 54.8nm±5.8nm
in length (Fig. 1B and C). The periodicity of the TiO⁠2 phase results
in oscillatory dielectric constants that influence photon propagation.
Thus, desired photonic band gaps (PBGs) can be obtained by modify-
ing the structure's periodicity, which is controlled by the quantity of
constant charge used during pulsed anodization. To achieve the de-
sired photonic band gap, the length of low-refractive index periods is
controlled via tuning the quantity of anodization charge. TEM images
of Au-PMTiNT indicate Au NPs with a diameter ranging from 3−7nm
are deposited throughout the nanotubes (Fig. 1D and E). Fig. 1F and
G, show HRTEM images of Au-PMTiNT. It can be seen from Fig. 1F
that Au NPs were homogeneously distributed on the walls of TiO⁠2
nanotubes. TEM images at high magnification distinctly show lattice
fringes of Au nanoparticles with 0.24nm d-spacing corresponding to
the (111) plane of metallic Au (Fig. 1F). The Au NPs had an average
diameter of 4.9nm with a standard deviation of 1.9nm. The Au NP
Fig. 1. (A) Schematic illustration of the preparation process of PMTiNT arrays (B, C) Top-view and cross-sectional view FESEM images of as-prepared PMTiNT (D, E) TEM images of Au
loaded PMTiNT (F, G) HRTEM images of gold nanoparticles and lattice fringes (H) Variation of photonic crystal pitch (length of repeating periodic segment) and optical reflectance peak
vs. anodization charge in charge-controlled anodization for the design of photonic bandgaps.
3

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size distribution was determined by measuring the diameter of 100 in-
dividual nanoparticles. Fig. 1H presents the relationship between an-
odization charge, period length, and PBG wavelength. The X-ray diffrac-
tion (XRD) pattern for Au-PMTiNT split into its component signals ex-
hibited various specific peaks corresponding to anatase TiO⁠2 (JCPDS#
21–1272), rutile TiO⁠2 (JCPDS# 21–1276), fcc metallic Au (JCPDS#
04-0784) and Ti respectively (Fig. 2a). The rutile phase signal intensity
was too low, indicating anatase to be the predominant phase of TiO⁠2 in
Au-PMTiNT samples.
The interaction with light is critical for photocatalysts, which was
examined using diffuse reflectance UV–vis spectroscopy and Lumeri-
cal® FDTD simulations. The UV–vis diffuse reflectance spectrum of
PMTiNT (black line in Fig. 2b) shows a broad photonic stopband from
350 to 550nm, which is supported by simulated reflectance of PMTiNT
(blue dash line in Fig. 2b) showing three sharp PBGs at 350nm, 400nm,
and 550nm. After Au sputtering (red line in Fig. 2b), the reflectance
of the photonic stopband is damped drastically. The absorption band
edge is shifted to 386nm which is characteristic of the anatase bandgap.
Additionally, the reflectance valley is shifted at 550nm due to the
absorption of the localized surface plasmon resonance (LSPR) of Au
NPs. The propagation of photons in PMTiNT was investigated through
a cross-sectional view of the spatial distribution of Poynting vector
field map obtained from Lumerical® FDTD. Clear evidence of peri-
odic high intensity local electric fields is seen in Fig. 2c, which sup-
ported the aforementioned theory that oscillatory dielectric constants
forbid the propagation of photons of particular wavelengths. In Fig.
2d, a simulated top view of the absorption caused electric field in-
dicates a strong electric field surrounding the Au NPs under plas
mon resonance excitation. These plasmonic Au NPs induced over 10
times higher local field compared to the electric field at the air-TiO⁠2 in-
terface. The aforementioned plasmonic response and 1D photonic crys-
tal architecture are critical effects that make the Au-PMTiNT nanostruc-
tured heterojunction a unique photocatalyst.
The concentration of surface adsorbed CO⁠2 is of utmost importance
to afford maximum product yield and water molecules always com-
pete with CO⁠2 to occupy active sites. Since CO⁠2 is weakly acidic in na-
ture, the availability of basic catalytic sites is essential to achieve a
higher surface concentration. CO⁠2 temperature programmed desorption
(CO⁠2-TPD) was employed on both PMTiNT and Au-PMTiNT samples in
order to investigate the nature of catalytic sites and the physicochemical
interaction of CO⁠2 interaction with the material surface (see Support-
ing Information, Fig. S6). The TPD spectra of bare PMTiNT displayed
three desorption peaks located at 111°C, 293°C and 335°C assigned to
the desorption of chemisorbed CO⁠2 from Ti⁠3+, Ti⁠4+, and oxygen va-
cancy sites respectively. The intense peaks suggest plenty of basic sites
were present on the surface, favorable for excellent chemisorption of
weakly acidic CO⁠2. On the other hand, in the TPD spectra of Au-PMTiNT
sample, the intensities of these three peaks decreased significantly and
shifted toward lower temperatures (106°C, 171°C and 190°C), suggest-
ing sputtered Au acts as an adsorption barrier layer due to the block-
ing of available surface sites and promotes faster desorption of CO⁠2
at relatively lower temperature. The TPD data unambiguously points
to poor-to-negligible adsorption of CO⁠2 on the surface of Au NPs. The
most significant difference between the PMTiNT and Au-PMTiNT sur-
faces is that Ti⁠3+
states are not significant in the Au-PMTiNT samples.
Even TiO⁠2 nanotubes that are not subjected to a special reducing treat-
ment possess Ti⁠3+
states due to the incorporation of fluorine atoms dur
Fig. 2. (a) XRD pattern of Au-PMTiNT, illustrating material components and TiO⁠2 phase by separating signals. (b) Diffuse reflectance spectra of PMTiNT (black line), Au-PMTiNT (red
line) and FDTD simulated PMTiNT (blue dash line) (c) Cross-section view of simulated Poynting vector plot for PMTiNT (d) Top view simulated electric field map for Au-PMTiNT. (For
interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
4

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ing the anodization process and incomplete oxidation of Ti [38,39]. In
Au-PMTiNT, electron paramagnetic resonance spectra (not shown) did
not detect Ti⁠3+ peaks [40]. Likewise, XPS spectra (Fig. S9) did not show
the presence of Ti⁠3+ states since the Ti 2p⁠3/2 and Ti 2p⁠1/2 states pre-
sent in Au-PMTiNT at 458.4eV and 464.2eV respectively, correspond
to Ti⁠4+ states; in previous work too, Ti⁠3+ states were noticeably ab-
sent for TiO⁠2 nanotubes sputter-coated with Au NPs [41]. This is cor-
roborated by the FTIR spectra of PMTiNT and Au-PMTiNT samples ex-
posed to CO⁠2 alone, and to both CO⁠2 and water vapor (Fig. S15 in Sup-
porting Information) wherein the symmetric stretching mode of mon-
odentate carbonate species adsorbed on defect sites (ν⁠S ≈ 1325cm⁠−1)
is prominent only in bare PMTiNT, while being weak or absent in
Au-PMTiNT [42]. Other prominent features corresponding to bicarbon-
ate species (1410cm⁠−1
and 1630cm⁠−1
) and linearly adsorbed CO⁠2 mol-
ecules (2346cm⁠−1
) are seen in both PMTiNT and Au-PMTiNT samples
[42–44]; however, the amplitude of these features is stronger in bare
PMTiNT compared to Au-PMTiNT. This suggests that while a smaller
amount of CO⁠2 is adsorbed on Au-PMTiNT (confirming the TPD result)
and Ti⁠3+
sites are far fewer, the adsorption modes of CO⁠2 on Ti⁠4+
-O⁠2-
Lewis acid-base pairs in Au-PMTiNT are not fundamentally different
from those in bare PMTiNT. The equilibration between TiO⁠2 surfaces
and Au NPs through transfer of negative charge from TiO⁠2 to Au due to
Schottky junction formation described in the next section, may have re-
sulted in the disappearance of Ti⁠3+
states in Au-PMTiNT.
The surface chemical composition and nature of chemical bonding
of Au-PMTiNT and anatase thin film TiO⁠2 (ATF) was evaluated using
X-ray photoelectron spectroscopy (XPS) (See Supporting Information,
Fig. S7-S8). To understand the charge generation and separation mecha-
nisms which might be leading to specific product distributions, the elec-
tronic band structure of ATF and Au-PMTiNT was evaluated with work
function and valence band spectra using ultraviolet photoelectron spec-
troscopy (UPS) (See Supporting Information, Fig. S8d). Based on our
calculation for both ATF and Au-PMTiNT, WF were respectively 4.06eV
and 3.64eV; while the valence band maxima of ATF and Au-PMTiNT
were 3.12eV and 2.72eV lower than Fermi level. In CO⁠2 reduction stud-
ies, the band positions are usually expressed at NHE scale. So, the ob-
tained band positions were converted to NHE scale at pH 0 by using
expression E⁠NHE(V)= E⁠vac(eV) +4.5eV followed by conversion to E⁠NHE
at pH-7 using Nernst equation E⁠0 (at desired pH) = E⁠0 (pH⁠0)-0.06×pH.
The Fermi level and valence band positions for pristine ATF were found
to be -0.86V and +2.26V while for Au-PMTiNT these values were cal-
culated to be −1.26 and +1.44V vs NHE at pH-7 respectively. The up-
shifting of the Fermi level and band position demonstrate upward band
bending and formation of Schottky junction via Fermi level alignment
between metallic gold and n-type semiconductor TiO⁠2. The band energy
diagram of TiO⁠2, Au-PMTiNT, and CO⁠2 reduction potential vs NHE at
pH-0 before and after the formation of Schottky junction are given in
Fig. 3.
Fig. 3. Proposed energy band diagram(s) of Au-PMTiNT obtained from UPS WF and VB spectra showing (a) Band energies of pristine TiO⁠2 and gold before contact (b) Formation of
Schottky junction between Au and TiO⁠2 via Fermi level alignment and reduction of CO⁠2 to methane under solar simulated light (UV; TiO⁠2 and UV+Vis; Au LSPR) leading to conventional
SPR assisted scheme and the reduction potential of various CO⁠2 reduction products is given in the blue text (c) Overall schematic representation of methane formation on Au-PMTiNT
system under AM1.5G solar illumination (d) Hot electron injection from Au NPs across the Schottky barrier into E⁠CB of TiO⁠2 under visible irradiation via LSPR originated electrons on
Au under visible light resulting in a build-up of excess electrons in TiO⁠2 (e) Recombination of accumulated electrons in TiO⁠2 with LSPR originated holes in Au NPs results in a plasmonic
Z-scheme wherein hot electrons on Au generated via LSPR in visible light are used for CO⁠2 reduction to CH⁠2O and CO and photogenerated holes in E⁠VB of TiO⁠2 are used for the water
oxidation to furnish required protons for the photoreduction (f) Overall schematic illustration of Z-scheme photocatalytic conversion of CO⁠2 to CH⁠2O. (For interpretation of the references
to colour in this figure legend, the reader is referred to the web version of this article).
5

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PROOF
The CO⁠2 photoreduction performance of Au-PMTiNT was investi-
gated under two different irradiation conditions leading to two differ-
ent reaction pathways 1) Conventional direct route under AM1.5G sim-
ulated sunlight and 2) Z-scheme under UV-poor visible illumination fol-
lowed by AM1.5G simulated sunlight. The CO⁠2 photoreduction experi-
ments using Au-PMTiNT as a photocatalyst were performed in a stain-
less steel gas-tight reactor with a quartz window in the presence of gas
phase CO⁠2 and vapor phase H⁠2O [10]. For the first conventional re-
action pathway, the reactor was irradiated under AM1.5G simulated
sunlight for 2h. The gaseous products were analyzed by using GC-PDD
and GC–MS, and the obtained product yield in ppm was converted into
µmol h⁠−1
, as mentioned in our previous report [10]. Under simu-
lated sunlight, methane was observed as the dominant CO⁠2 photore-
duction product with a methane formation rate (R⁠CH4) of 302µmol
h⁠−1. Extremely high selectivity manifested in the product compo-
sition which consisted of 89.3 % methane, 8.5 % carbon monoxide,
1.6 % hydrogen and 0.6 % ethane (Fig. 4a). To manipulate the reac-
tion into following the second reaction pathway, the experiment was
carried out under identical conditions with merely the illumination se-
quence changed, by first irradiating the reactor with a 50W white
LED (P⁠max at 520nm) for 2h followed by irradiating the reactor with
AM1.5G simulated sunlight for 2h respectively. Interestingly, follow-
ing this illumination sequence, the composition of the photoreduction
product changed drastically compared to conventional CO⁠2/CH⁠4 pho-
tocatalysis; formaldehyde (CH⁠2O) and carbon monoxide (CO) were ob-
served as the primary reaction products (Fig. 4a). From GC analysis,
the CO and CH⁠2O formation rate (R⁠CO and R⁠CH2O) were extracted to
be 323 and 420µmol h⁠−1 respectively. To validate the optical wave-
length-directed product selectivity of CO⁠2 photoreduction on
Au-PMTiNT, the photoreaction was also carried out under identical con-
ditions by illuminating with UV light (365nm) and green light (505nm)
LEDs simultaneously. As anticipated, formaldehyde and CO were ob-
served as major reaction products and no trace of methane was de-
tected. The rate of photocatalytic CO and CH⁠2O generation were found
to be 364 and 123µmol h⁠−1 respectively (Fig. 4a). To discern if the
observed reaction products were truly a result of CO⁠2 photoreduction
and not due to carbonaceous impurities, a series of sanity tests were per-
formed under identical conditions but with absence of one element in
control experiments: 1) absence of photocatalyst; 2) absence of water; 3)
absence of light; 4) absence of CO⁠2. No CO⁠2 reduction product was ob-
served in any of the four control experiments which proved the photo-
catalytic origin of products. Additionally, experiments using isotopically
labeled ⁠13
CO⁠2 were also conducted to confirm the origin of observed
products and to eliminate possible carbon contamination. The GC–MS
spectra of reaction product exhibited ion-chromatogram peaks at m/z
value of 17 for ⁠13
CH⁠4, 29 for ⁠13
CO and 31 for ⁠13
CH⁠2O respectively (Fig.
4b).
To understand the charge separation mechanism and carrier dynam-
ics in Au-PMTiNT, the surface potential (SP) under different illumina-
tion conditions was determined using Kelvin probe force microscopy
(KPFM) (Fig. 4c). Since the gold was sputtered on to the PMTiNTs, the
maximum number of Au NPs are found on the top surface of the PMT-
iNTs, and the charge on these Au NPs determines the surface poten-
tial of the Au-PMTiNT samples. Under dark conditions, the SP of Au-
Fig. 4. (a) Product distribution chart from CO⁠2 photoreduction on Au-PMTiNT for two different illumination conditions, showing clearly that the dominating products varied under dif-
ferent illumination conditions (b) Ion-chromatogram confirming the existence of ⁠13
C isotopic labeled products (c) Surface potential shift under different illumination conditions measured
by KPFM with the opposite shifts under 365nm and 532nm illumination suggesting two different reaction pathways at work (d) Absorption spectra of as-prepared Au-PMTiNT before and
after irradiation of white 50W, LED lamp (filtered-out UV) (Both collected in the backward scan) After forwarding scan of LED illuminated samples the enhanced absorption peak in NIR
region corresponded to free electrons was disappeared and spectra were exactly identical as the non-irradiated sample.
6

UNCORRECTED
PROOF
PMTiNT was found to be −50mV (black curve in Fig. 4c) which demon-
strates a slightly electron-rich surface of Au-PMTiNT, which confirmed
the presence of a Schottky junction at equilibrium due to transfer of
electrons from TiO⁠2 to Au, and validated the UPS results. Under UV ir-
radiation by 365nm photons, a large negative shift of 300mV with re-
spect to dark was observed (blue curve in Fig. 4c). Owing to the large
amount of TiO⁠2 in Au-PMTiNT system and the UV-induced photogen-
eration in TiO⁠2, a flat band or accumulation region is expected to pre-
vail at the Au-TiO⁠2 interface (e.g. Fig. 3e) with the concomitant trans-
fer of a larger amount of negative charge to the Au NPs. The Au NPs
thus act as electron sinks for the large excess carrier concentration in
TiO⁠2 under UV illumination, a behaviour that is well-documented in
prior reports [45,46]. On the other hand, under 532nm illumination,
Au nanoparticles are the major absorbers due to the LSPR. Under visi-
ble illumination, a small positive shift (25mV) was observed which in-
dicates the transfer of hot electrons into E⁠CB of TiO⁠2 across the Schot-
tky junction following absorption and plasmon dephasing (as illustrated
in Fig. 3d), thus leaving positively charged holes on the Au NPs. The
electrons transferred to TiO⁠2 following LSPR excitation and damping,
have extremely long lifetimes (seconds to hours) due to two major rea-
sons: (1) The extremely low residual hole concentration in TiO⁠2 that
discourages recombination and (2) The high dielectric relaxation time
in TiO⁠2 due to which thermal equilibration of excess carrier concentra-
tions is a slow process [47,48]. Consequently, the remnant photogen-
erated holes in Au also have a long lifetime. Two further experiments
were used to confirm these inferences. First, the terephthalic acid photo-
luminescence test [49] was used to detect the presence of hydroxyl rad-
icals in Au-PMTiNT following UV-filtered white light illumination (Fig.
S9 in Supporting Information). Since these hydroxyl radicals arise from
the reaction of photogenerated holes with surface adsorbed water mole-
cules, the presence of long-lived holes on the Au NPs (TiO⁠2 was not ex-
cited by the light source used) was confirmed. Secondly, UV–vis spectra
of Au-PMTiNT were measured before and after a 2h white lamp illumi-
nation (λ>420nm) (Fig. 4d). Fig. 4d clearly shows the surface plasmon
resonance peak at ca. 550nm. The white LED lamp had maximum in-
tensity around 510nm; following illumination and LSPR excitation, the
transfer of hot electrons into TiO⁠2 as described above, significantly in-
creased the concentration of free electrons in TiO⁠2 which manifested it-
self as an increased absorbance in the near-infrared (NIR) region in a
backward scan (longer wavelength to shorter wavelength) due to en-
hanced free carrier absorption of NIR photons (red curve in Fig. 4d). In-
terestingly, after illumination when the sample was scanned in the for-
ward direction (from shorter wavelength to longer wavelengths), no in-
crease in absorbance was observed in the NIR region (black curve in Fig.
4d) which suggests that free electrons were used up during UV irradia-
tion. This interesting result provided us the first indication that a plas-
monic Z-scheme might be operational in the Au-PMTiNT photocatalyst
as indicated in Fig. 3f. Taken together, the picture that emerges from the
gas evolute, KPFM and UV–vis data is that illumination by near-LSPR
visible photons alone produces transfer of hot electrons from Au to TiO⁠2
and illumination by UV photons alone produces transfer of photogener-
ated electrons from TiO⁠2 to Au through the electron sink effect of noble
metal NPs. Under AM1.5G one sun illumination following visible illu-
mination, an accumulation-type interfacial band alignment exists (Fig.
3e) which promotes a large flux of electrons (both photogenerated elec-
trons and transferred hot electrons) to move from TiO⁠2 to Au, which in
turn recombine with holes in Au generated by plasmon damping. This
leaves hot electrons in Au NPs and photogenerated holes in TiO⁠2 avail-
able to drive photocatalytic reactions (Fig. 3f). Although hot electrons
in Au experience thermalization at picosecond timescales, our gas evo-
lute results clearly indicate that charge transfer to adsorbed CO⁠2 species
occurs within this timeframe.
To the best of our knowledge, the optical control of product selec-
tivity has never been previously observed. The key question that arises
is what factors govern the controllable product selectivity under differ-
ent wavelength illumination conditions and the underlying mechanism
involved. Highly selective methane formation on Au/TiO⁠2 systems un-
der solar illumination has been previously observed and explained on
the basis of energy level positions i.e. the E⁠CB position of TiO⁠2 which lies
above reduction potential of CO⁠2/CH⁠4 but remains below the reduction
potential of CO⁠2/CO and CO⁠2/CH⁠2O [19]. However, conversion of band
positions values on NHE scale at pH 7 actually shows that TiO⁠2 can re-
duce CO⁠2 to CO and CH⁠2O [50–53]. Further, we calculated that E⁠CB of
TiO⁠2 in Au-PMTiNT to be situated at -0.88V vs NHE at pH 7 suggest-
ing that TiO⁠2 can reduce CO⁠2 to CO and CH⁠2O. Such an energy level
alignment is depicted in Fig. 3b for instance. Therefore, the energy lev-
els of TiO⁠2 are insufficient to explain the controllable product selectivity
observed by us. The excellent selectivity of methane formation under
AM1.5G simulated sunlight can be explained based on 1) Photonic band
gaps [54–56] and 2) Carbene pathway. As mentioned earlier, PBGs of
PMTiNT were observed at 350, 400, and 550nm (also see Fig. S3 in Sup-
porting Information), which allowed the propagation of selected wave-
lengths in the PMTiNT photonic crystal and enhanced the selectivity of
products. The electronic band gap of anatase phase TiO⁠2 is 3.2eV which
corresponds to a band-edge of 386nm and only photons with wave-
lengths shorter than 386nm can induce a direct transition of electrons
from the valence band to conduction band. While the conduction band
is constituted of Ti⁠4+
3d overlaps, the presence of defect states (Ti⁠3+
,
O vacancies etc.) creates sub-gap states just below the conduction band
and longer wavelengths can also stimulate electronic transitions. Due to
the presence of photonic stop-bands in the PMTiNT at 350 and 400nm,
only light in a narrow range of 355−386nm remains available for the
transition. Photons with wavelengths in the range 386−410nm, which
are primarily responsible for the sub-bandgap excitation of defect states,
were attenuated. Due to this filtering effect, only mobile, high energy
electrons are excited to the conduction band and react with surface ad-
sorbed CO⁠2 to form methane.
Our infrared spectroscopy results for adsorbed CO⁠2 and H⁠2O on
PMTiNT and Au-PMTiNT surfaces (Fig. S15) are indicative but not ex-
haustive since they were obtained in specular reflectance mode wherein
only the adsorption modes of CO⁠2 and H⁠2O on the top surface of the
nanostructured three-dimensional films are detected. Anodically synthe-
sized TiO⁠2 nanotubes can exhibit a strong crystallographic texture with
(001) facets on top surfaces and (101) facets on the walls [57]. In such
a case, the adsorption modes of CO⁠2 on the nanotube walls could be
different from that on the top surface. With this caveat in place, we
briefly discuss how CO⁠2 photoreduction might proceed along the car-
bene pathway, which is the leading route for methane formation on
the Ti⁠4+
centres. Yongfei Ji et al. describe how CO can form through
concurrent transfer of protons and electrons to linear CO⁠2 molecules
(which are clearly observed in Fig. S15), on defect-free anatase (101)
surfaces [58]. Once CO is formed, it can be easily photoreduced to
other organic molecules [58]. Previous studies also suggest that biden-
tate coordination of CO⁠2 is favored on Ti⁠4+
centers (strong Ti⁠4+
Lewis
base sites in conjunction with strong O⁠−
Lewis acid sites), while the
monodentate configuration is achieved on Ti⁠3+
sites (strong Ti⁠3+
Lewis
base sites in conjunction with weak Lewis acid sites (oxygen vacan-
cies) [59–61]. The bidentate coordination of CO⁠2 through two oxygens
on two Ti⁠4+ centers followed by bonding of carbon with on TiO⁠2 oxy-
gen centers form adsorbed carbonate species which promotes facile re-
duction of CO⁠2 to CO⁠2 − anion radical bonded in a bidentate manner
followed by the subsequent reduction to methane via carbene pathway
[58,62–65]. While neither bidentate coordinated CO⁠2 nor Ti⁠3+ states
were observed in our study, it is important to keep an open mind re-
garding CO⁠2 coordination due to the possibility of crystallographic tex
7

UNCORRECTED
PROOF
turing obscuring adsorption modes. Monodentate coordination of CO⁠2
on TiO⁠2 through one oxygen atom facilitates the formation of carboxyl
(hydroxyformyl) radical ( COOH) which is transformed into formalde-
hyde and subsequently into methane through the formaldehyde path-
way [51,62,66]. The observation of methane as a sole reaction product
(with minor CO) under AM1.5G simulated sunlight indicates that the di-
rect transition of sufficiently energetic electrons to the conduction band
of TiO⁠2 is favoured, and the filtering effect of the photonic bandgaps
prevent defect mediated low energy transitions which might lead to
other CO⁠2 reduction products (schematically illustrated in Fig. 3c). Ad-
ditionally, due to the lower reduction potential of CO⁠2/CH⁠4 (-0.24V
vs NHE at pH7) in comparison to CO⁠2/CO (-0.53V) and CO⁠2/CH⁠2O
(-0.48V), the formation of methane was thermodynamically more fea-
sible. The phase of water also affects the product distribution on the
surface of TiO⁠2 during CO⁠2 photoreduction, and methane is observed as
the dominant product in vapor phase reactions such as the one reported
here [67,68].
Carbene pathway:
The selective CH⁠4 formation under AM1.5G simulated sunlight, and
the photoreduction of CO⁠2 under white LED illumination followed by
AM1.5G solar illumination to form formaldehyde and carbon monox-
ide as major products strongly suggests the presence of a second mech-
anism. The selective formation of CH⁠2O and CO can be explained by
the Z-scheme mechanism (Fig. 3e-f). The irradiation of Au-PMTiNT sam-
ples with white cold LED light (λ>420nm) which has maximum pho-
ton flux at 520nm cannot excite electrons from the E⁠VB to E⁠CB of TiO⁠2
while Au can absorb most of the light in this region due to LSPR and
inject the generated hot electrons into the E⁠CB of TiO⁠2 leaving behind
poorly oxidizing holes. The poorly oxidizing hole on Au does not have
adequate energy to drive water splitting. Hence, visible light irradiation
resulted in the accumulation of negatively charged electrons in E⁠CB of
TiO⁠2 and positive holes on Au as described previously. Despite the capa-
bility of photogenerated hot electrons in gold to perform proton assisted
CO⁠2 reduction, the absence of water spitting to furnish the required pro-
tons, suppresses the reaction. Once the solar simulator is turned on, elec-
tron-hole pairs are generated in E⁠CB and E⁠VB of TiO⁠2. As previously de-
scribed, the electrons in E⁠CB of TiO⁠2 move rapidly toward Au and re-
combine with holes there while holes left in the E⁠VB of TiO⁠2, oxidize
water to release required electrons and protons (Z-scheme) [69]. Sev-
eral reports demonstrate transfer of electrons from semiconductors to
the gold nanoparticles and Z-scheme catalysis [70–74]. As the photon
flux of the white LED was concentrated near the gold LSPR excitation,
a large fraction of photons from the white LED was used for genera-
tion of hot electrons and low energy holes. On the other hand, due to
only 5 % of energy in sunlight residing at ultraviolet wavelengths, the
number of photogenerated electrons-holes pairs in TiO⁠2 was much lower
than Au. The large population of less energetic holes on the Au sur-
face recombined with photogenerated electrons in E⁠CB on TiO⁠2 to ac-
tuate a Z-scheme mechanism. Due to the low numbers of holes in the
E⁠VB of TiO⁠2, the rate of proton generation from water was poor and
the surface concentration of protons was the rate-determining step. The
low surface concentration of protons favored proton deficient CO⁠2 re-
duction products CH⁠2O and CO (4 and 2 proton reduction products)
via the formaldehyde pathway. Further, previous reports suggest that
selective formation of carbonyl reduction products [75,76] occurs due
to effective overlap between metal d orbital (donor) and π* orbital (ac-
ceptor) of carbonyl compounds that stabilize transition states on its
surface [77–79]. To validate the Z-scheme mechanism, the CO⁠2 reduc-
tion reaction was carried out by simultaneously illuminating the sam-
ple with UV (365nm) and green (505nm) LEDs having identical inten
sity on the surface of the sample. Again, CH⁠2O and CO were observed
as major products. The observed results suggest that the recombination
rate in TiO⁠2 was higher which reduced the effective concentration of
available protons for complete CO⁠2 reduction and a Z-scheme mecha-
nism operated.
Formaldehyde Pathway:
4. DFT modelling
To gain more insight into the electronic structure of the Au-TiO⁠2
heterojunction, density functional theory (DFT) calculations were con-
ducted on Au cluster decorated (101) anatase TiO⁠2 for imitating the
Au-PMTiNT photocatalyst [80,81]. DFT calculations with Hubbard U
correction method were performed using OpenMx software (an open
source package for Material eXplorer) [82], which allowed us to ob-
tain the projected density of states (PDOS), which is shown in Fig. 5,
and which was calculated from pseudoatomic orbitals and relativistic
pseudopotentials adopted by LDA. DFT calculations were done for two
cases respectively: (1) When the distance of Au nanoparticles and the
anatase surface was large (several angstroms away) and (2) When they
were close. In Fig. 6, it is shown that both the HOMO and LUMO are
localized on the Au nanoparticles, which reflected the case that the in-
teraction of Au atoms and TiO⁠2 surface was weak when they were away.
However, an interesting result emerged when Au atoms approached the
surface of TiO⁠2 closely. In Fig. 6b, it is seen that the HOMO orbitals lay
on TiO⁠2 while the LUMO orbitals were mostly located on Au nanopar-
ticles while slightly penetrating into the TiO⁠2 surface. It is worth not-
ing that the spatial and energetic overlap of HOMO and LUMO fulfills
the condition for the construction of a Z-scheme as mentioned in the
main article. This HOMO and LUMO diagram demonstrated electron
flow from E⁠CB of TiO⁠2 to the gold Fermi level, and is consistent with ex-
perimental results. Fig. 6c is the second level positions of HOMO and
LUMO in our system. It is clear that the second LUMO level is spread
out on both gold cluster and TiO⁠2, which is supportive of our claim in
the DFT section of the main article.
Combining the DFT results with the aforementioned experimen-
tal results, the PDOS diagram not only supported our hypothesis but
Fig. 5. Projected density of states diagram of selected gold atom (yellow line), titanium
atom (red line), oxygen atom (blue line). the Fermi energy is at 0eV. (For interpretation
of the references to colour in this figure legend, the reader is referred to the web version
of this article).
8

UNCORRECTED
PROOF
Fig. 6. Images from left to right of each row are visualizations of optimized geometries, HOMO and LUMO positions respectively. Three cases are displayed: (a) Gold atoms are far away
from TiO⁠2 anatase surface (several angstroms) (b) first HOMO/LUMO positions and (c) second HOMO/LUMO positions, when gold atoms are close to TiO⁠2 anatase surface.
also displayed the hidden charge flow mechanism from an electronic
structure perspective. In Fig. 5, although the Au sp hybridized orbital
extended out to -1.2eV, the dominant filled electronic orbitals in this
system are those corresponding to O 2p and Au 3d. O 2p is well known
as the main contributor to the valence band in TiO⁠2 [83]. Once the sam-
ple is illuminated by photons exceeding the bandgap of TiO⁠2, electrons
are excited to the Ti 3d orbital that constitutes the conduction band of
TiO⁠2 [84]. An Au 3d interband transition overlaps with the O 2p → Ti 3d
transition. Practically, considering the much larger proportion of TiO⁠2
in the fabricated Au NP decorated PMTiNT structure, the O 2p → Ti 3d
transition is expected to dominate photogeneration following illumina-
tion by ultraviolet photons. It is noteworthy that the PDOS diagram in
Fig. 5 also indicates a potential intraband transition that excites elec-
trons from +0.1eV Au sp orbital to +0.8eV Au sp orbital [85]. This
intraband transition is stimulated following dephasing of the Au surface
plasmons excited by visible light illumination to create a nonequilib-
rium distribution of carriers (hot electrons). It is also important to note
that the Ti 3d orbital at +0.1eV is isoenergetic with the +0.1eV Au sp
orbital (also showed as orbital visualization in Fig. 5
). Therefore, if ultraviolet illumination immediately follows or is coin-
cident with visible illumination close to the LSPR peak, photogenerated
electrons populating the Ti 3d orbital would be energetically inclined to
recombine with holes on the Au sp orbital that were created due to the
intraband transition [70,72].
An unexpected and interesting feature in the PDOS (Fig. 5) is that
the next energy level higher than the LUMO of the Au-TiO⁠2 system is at
+0.8eV (sometimes called S⁠2 in a molecular system), and mainly con-
sists of Ti 3d orbital isoenergetic with the Au sp orbital as shown in Fig.
6. As earlier discussed, the majority of CO⁠2 is adsorbed on TiO⁠2 based on
our TPD analysis. The coexistence of second LUMO indicates that Ti 3d
orbital is strongly coupled with Au sp orbital and serves as a shuttle to
deliver the LSPR electrons to the CO⁠2 photoreduction reaction [86,87].
5. Conclusions
In summary, Au nanoparticle-decorated periodically modulated TiO⁠2
nanotube arrays for photocatalytic CO⁠2 reduction were pre
9

UNCORRECTED
PROOF
pared using precise charge-controlled pulse anodization method with re-
markable photoactivity. The unique optical property of photonic crystal
to regulate the propagation of photons and the surface plasmon reso-
nance of Au-PMTiNT leads to remarkable wavelength-controlled prod-
uct selectivity. The spectral composition of incident light could be used
to switch the photoreduction pathways freely between a methane gen-
erating pathway and a plasmonic Z-scheme that generated formalde-
hyde and carbon monoxide. This is the first time that a true plasmon-in-
volved Z-scheme based on Au/TiO⁠2 heterojunction has been reported.
CH⁠4 (302μmol/g/h with 89.3 % selectivity) was observed as the domi-
nant CO⁠2 photoreduction product under simulated sunlight while CH⁠2O
(420μmol/g/h) and CO (323μmol/g/h) were produced predominantly
under visible+UV light. Isotopic labeling experiments using ⁠13
CO⁠2 au-
thenticated the light-driven generation of reaction products.
Author contributions
Sheng Zeng led this research project and contributed to fabrica-
tion, data collection and interpretation, and manuscript writing. Ehsan
Vahidzadeh collected BET, TPD data and assisted in CO⁠2 photoreduc-
tion experiments. Collin VanEssen conceived pulsed charge anodization
and fabricated PMTiNT samples. Piyush Kar was involved in CO⁠2 pho-
toreduction experiments. Ryan Kisslinger collected TEM images. Ankur
Goswami performed KPFM measurement and interpretation. Yun Zhang
helped in UV-VIS data collection. Najia Mahdi helped in sample fabrica-
tion. Saralyn Riddell acquired new HRTEM images and analyzed lattice
fringe and gold nanoparticle size distribution. Alexander E. Kobryn and
Sergey Gusarov conducted DFT modelling. Pawan Kumar contributed to
paper writing and manuscript editing. Karthik Shankar supervised the
research and edited the manuscript.
CRediT authorship contribution statement
Sheng Zeng: Conceptualization, Methodology, Validation, Formal
analysis, Investigation, Writing - original draft, Visualization. Ehsan
Vahidzadeh: Investigation. Collin G. VanEssen: Conceptualization,
Software, Methodology. Piyush Kar: Methodology. Ryan Kisslinger:
Investigation, Writing - review & editing. Ankur Goswami:
Investigation. Yun Zhang: Investigation. Najia Mahdi: Investigation.
Saralyn Riddell: Investigation, Formal analysis. Alexander E.
Kobryn: Methodology, Investigation, Visualization. Sergey Gusarov:
Methodology, Investigation, Visualization, Funding acquisition. Pawan
Kumar: Writing - review & editing, Visualization. Karthik Shankar:
Conceptualization, Writing - review & editing, Visualization,
Supervision, Project administration, Funding acquisition, Resources.
Declaration of Competing Interest
The authors have no conflicts to declare.
Acknowledgements
All authors thank NSERC, NRC-NINT, CFI, CMC Microsystems, Al-
berta Innovates and Future Energy Systems CFREF for direct and in-
direct (equipment-related) financial support. The characterization as-
sistance provided by Dr. Kai Cui (NRC-NINT), Dr. Steve Launspach
(NRC-NINT), Dr. Mike Xia (NRC-NINT), Ms. Lisa Nikolai, and the staff at
the UofA nanoFab is acknowledged. S.Z., E.V. and K.S offer their heart-
felt thanks to Prof. Arvind Rajendran and James Sawada for assistance
with gas chromatography, to Prof. Thomas Thundat for access to KPFM,
and to Prof. Natalia Semagina for access to temperature-programmed
desorption.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.apcatb.2020.118644.
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