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Cite This: J. Phys. Chem. C 2017, 121, 21383-21389
Structure and Reactivity of a Model Oxide Supported Silver
Nanocluster Catalyst Studied by Near Ambient Pressure X‑ray
Photoelectron Spectroscopy
Michael Wagstaffe,† Hadeel Hussain,‡,⊥ Matthew J. Acres,‡ Rosemary Jones,§ Karen L. Syres,*,¶
and Andrew G. Thomas*,‡,§
†
School of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester, M13 9PL, U.K.
School of Materials, The University of Manchester, Sackville Street, Manchester, M13 9PL, U.K.
§
Photon Science Institute, The University of Manchester, Oxford Road, Manchester, M13 9PL, U.K.
¶
Jeremiah Horrocks Institute, The University of Central Lancashire, Fylde Road, Preston, PR1 2HE, U.K.
Downloaded by SCIENCE & TECHNOLOGY FACLTY CNCL at 09:09:16:232 on June 14, 2019
from https://pubs.acs.org/doi/10.1021/acs.jpcc.7b05818.
‡
S Supporting Information
*
ABSTRACT: The photocatalytic activity of anatase TiO2
decorated with metal clusters has been widely documented, but
the nature of the metal−metal oxide interface and reaction
intermediates in catalytic processes are still not well understood.
This in part is due to the fact that use of photoelectron
spectroscopy to deduce the surface chemistry of catalytic systems
has long been hampered by the huge pressure difference between
real-world operation and the requirement of high vacuum for
electron detection. Here, the in situ growth of silver nanoparticles
on a model metal-oxide catalyst support and their reactivity with a
CO/H2O gas mixture has been investigated in detail. Using
synchrotron X-ray photoelectron spectroscopy, near-ambient
pressure X-ray photoelectron spectroscopy, and scanning
tunneling microscopy, the interaction of Ag with the anatase TiO2 surface leads to metal-surface charge transfer and low
mobility of Ag on the surface. Upon exposure to a 1.5 mbar CO/1.5 mbar H2O gas mixture, partial oxidation of the Ag clusters is
observed. There is also evidence suggesting that a Ag−carbonyl species is formed during exposure of the Ag/TiO2 surface to a
CO/H2O gas mixture.
■
TiO2 is an extremely versatile metal oxide: it is abundant, has
low manufacturing costs, is chemically stable, and is both nontoxic and highly resistant to corrosion. The anatase polymorph
of TiO2 is widely used in catalysis due to its high photocatalytic
activity, with the (101) facet being the most thermodynamically
stable.25 Often rutile and anatase TiO2 powders are mixed, as in
P25,26 which increases the catalytic activity further due to the
slight differences in the band gap energies of the two
polymorphs. Ag is also non-toxic, catalytically active and has
the highest thermal and electrical conductivity of all the noble
metals.27 In addition, Ag is not as costly as other noble metals.9
Catalyst systems based on Ag/TiO2 have been shown to
efficiently catalyze reactions at solid−gas interfaces and in
aqueous solutions. They have also been found to substantially
enhance the photocatalytic capacity of TiO2 for renewable
energy generation and degradation of environmental pollutants.20,28 Density functional theory (DFT+U) calculations of
INTRODUCTION
Metal clusters supported on well-ordered metal oxide surfaces
are of great interest due to their application in a wide variety of
fields, including photovoltaics, photocatalysis and transition
metal catalysis.1−6 Carbon monoxide (CO), in particular, is a
major air pollutant which causes serious environmental and
health concerns. As such, the catalytic oxidation of CO, to form
CO2, is an important research area.7−9 Noble metals supported
on metal oxide ceramic supports have long been used for the
catalytic conversion of CO to CO2, but the high cost of these
metals remains a major issue. Noble metal clusters on oxide
substrates are also of interest since they can produce high
Schottky barriers which facilitate the capture of electrons.10 As
a result, such systems have been widely investigated, both
experimentally and theoretically5,11−19 as potential photocatalytic materials which rely on rapid separation of photogenerated electrons and holes. The structural, chemical, and
electronic properties at the interface between the metal clusters
and the oxide support have all been shown to have a strong
influence on catalytic and photocatalytic activity.1,2,6,8,17−24
© 2017 American Chemical Society
Received: June 14, 2017
Revised: August 30, 2017
Published: September 7, 2017
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Figure 1. (a) Ag 3d XPS core-level spectra (hν = 1 keV) recorded from the anatase TiO2(101) single crystal following Ag deposition for 110 min
(top) and the clean anatase TiO2(101) surface (bottom). (b) Corresponding Ti 2p spectra with (top) and without (bottom) Ag (hν = 1 keV). The
spectra are normalized to the height of the Ti 2p3/2 Ti4+ peak at 459.2 eV. (c) Plot of relative Ti3+ concentration vs Ag exposure. (d) Expanded view
of the Ti 2p XPS core-level spectra, highlighting the increase in the Ti3+ oxidation state as Ag coverage is increased. (e) Plot of the variation in full
width half-maximum of the Ag 3d5/2 peak as Ag coverage is increased. (f) Plot of the variation in binding energy of the Ag 3d5/2 peak vs Ag coverage.
thaw cycles to remove dissolved gases. All photoelectron
spectra are recorded at normal emission with the crystal at
room temperature.
The structure and size of the clusters deposited on the
TiO2(101) surface were studied using an Omicron atomic force
microscope-scanning tunnelling microscope (AFM-STM), in a
chamber equipped with both Auger electron spectroscopy
(AES), and low-energy electron diffraction (LEED). Sample
preparation was carried out in an identical way to the
photoemission work. STM data were typically acquired in the
constant current mode at sample biases of 1.5−2 V and
tunnelling currents 0.1−0.5 nA. Image processing was
performed using a combination of Nanotec Electronica
WSxM36 and Gwyddion.37 For both XPS and STM measurements, the anatase TiO2(101) single crystal (5 mm ×5 mm,
Pikem Ltd.) was cleaned by repeated 1 keV Ar+ ion
bombardment and 700 °C anneal cycles until a sharp (1 ×
1) LEED pattern was obtained and XPS/AES showed the
surface to be free of contamination.
For all experimental systems the Ag evaporator consisted of 5
μm diameter silver wire (99.99%; BDH Chemicals Ltd.)
wrapped around a tungsten filament and was thoroughly
degassed prior to use. Ag dosing was carried out at chamber
pressures below 10−9 mbar. Fitting of all core level spectra was
carried out using CasaXPS, utilizing a Shirley background,
Gaussian:Lorentzian(G:L) curves (0.7:0.3), for nonmetallic
species, and a modified Gaussian:Lorentzian curve (0.7:0.3),
with an asymmetric tail to higher binding energy, for metallic
species.38 This 0.7:0.3, G:L line shape is widely used in the
fitting of photoemission peaks recorded using monochromatic
X-rays and produces a good fit to the data. The choice of 70:30
G:L peak shape is empirical and based on the shape which gave
the best fit to the data.
O2 and CO activation on the Ag/anatase TiO2 (101) surface
indicated that the atomic and electronic structures of the metal
clusters and the synergistic effect of adjacent reactants and
oxygen defects on the semiconductor support all have an
important role in catalytic O2 activation and CO oxidation.29
Here we present a detailed study of the growth of Ag clusters
deposited on the anatase TiO2(101) surface. Near-ambient
pressure X-ray photoelectron spectroscopy (NAP-XPS) has
developed rapidly over the previous 10 years. The technique
allows X-ray photoemission spectroscopy (XPS) measurements
to be made at pressures of tens of millibars, and has led to the a
wealth of publications relevant to catalysis, electrochemistry,
material degradation, and gas capture for example.30−35 NAPXPS is therefore employed to study the in situ reaction of H2O
and CO on an Ag cluster decorated anatase TiO2(101) surface
in order to determine the mechanism of CO oxidation.
■
EXPERIMENTAL METHOD
UHV synchrotron photoelectron spectroscopy was carried out
on the bending magnet soft X-ray beamline, D1011 (photon
energy range 30 eV < hν < 1600 eV) at MAX-lab, Sweden. The
NAP-XPS system employed in this work is located at The
University of Manchester and is equipped with a monochromated Al Kα source (hν = 1486.6 eV) and a SPECS 150 mm
Phoibos 150 NAP electron energy analyzer, fitted with a threestage, differentially pumped electrostatic lens. The TiO2(101)
crystal with Ag nanoclusters was analyzed at 10−9 mbar and
under exposure to 3 mbar of H2O and a 1.5 mbar H2O/1.5
mbar CO gas mixture in a high-pressure cell. The CO gas
(99.9995%, CK gases) is further purified by means of a
scrubber (SAES MicroTorr). Water vapor is produced from
doubly distilled water, which is subjected to multiple freeze
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Figure 2. 100 × 100 nm2 STM images of (a) the as-prepared anatase TiO2(101)(1 × 1) surface and the TiO2(101)(1 × 1) surface after dosing Ag
for (b) 1 min (c) 5 min, and (d) 20 min. (e) A 50 × 50 nm2 image of surface b with an accompanying line profile (f).
of various metal clusters, including Co, Ni, and Pt.4,10,39 Au, on
the other hand has been shown to remove Ti3+ from the
surface.40 The degree of reduction of Ti is characteristic of the
relative strength of the interaction between the metal clusters
and the titania surface.4,10 At saturation we find a 6% increase in
the concentration of Ti3+ compared to the clean surface. In
contrast, following deposition of silver clusters on the rutile
TiO2 (110) surface, no reduction was reported. This suggests
that the interfacial interaction of Ag with the anatase (101) face
is stronger than with the rutile (110) surface of TiO2.1
The Ag 3d spectrum recorded from this surface is composed
of spin−orbit peaks at binding energies of 368.7 eV (Ag 3d5/2)
and 374.7 eV (Ag 3d3/2). These binding energies are consistent
with Ag in the metallic state.24 As the surface coverage of Ag on
TiO2 increases, a small downward shift in BE of the Ag 3d peak
is observed, as shown in Figure 1f. Similar shifts have been seen
with Ag and other transition metal clusters on various metal
oxide surfaces including the rutile TiO2(110) surface and are
indicative of an increasing cluster size.1,41−44 Figure 1e
illustrates the decrease in full width half-maximum (fwhm) as
coverage is increased and is further evidence of an increasing
RESULTS AND DISCUSSION
Ag Cluster Growth on Anatase(101). Core level
photoelectron spectra for Ag 3d and Ti 2p (hν = 1 keV) for
the clean anatase TiO2(101) surface and the maximum Ag
coverage are shown in Figure 1, parts a and b, respectively. A
full set of Ti 2p and Ag 3d spectra, as a function of Ag exposure,
are shown in Figure S1 of the Supporting Information. In each
case, the Ag spectra are normalized to the peak intensity of the
corresponding Ti 2p3/2 feature.
The Ti 2p spectrum is composed of spin orbit split peaks at
binding energies of 459.2 eV (Ti 2p3/2) and 464.9 eV (Ti
2p1/2), consistent with Ti4+. Upon exposure to Ag a small
shoulder appears on the lower binding energy (BE) side of the
main Ti 2p3/2 peak, consistent with the reduction of Ti4+ to
Ti3+ (457.7 eV), suggesting charge transfer from the Ag to the
TiO2 surface. Parts c and d of Figure 1 show how the relative
amount of Ti3+ changes as a function of Ag deposition. As the
Ag coverage increases, the proportion of Ti3+ also increases up
to an exposure time of around 20 min. For deposition times of
over 20 min, the Ti3+ concentration reaches a plateau.
Reduction of titania has been reported following deposition
■
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Figure 3. Histograms detailing the average diameter of the clusters populating the surface after dosing Ag for (a) 1 min (average diameter of 1.2 nm
with a standard deviation of 0.5), with inset relative percentage of larger clusters (>1.5 nm) that reside on terraces (T) or step edges (S), (b) 5 min
(average diameter of 2.4 nm with a standard deviation of 0.6), with inset relative percentage of larger clusters (>1.5 nm) that reside on terraces (T)
or step edges (S), and (c) 20 min (average diameter of 3.8 nm with a standard deviation of 1.3).
cluster size.1,42,44 Initial and final state effects will occur to
varying degrees for Ag clusters of different sizes. This then
produces a distribution of BEs manifested as a broadening of
the peak. This broadening effect becomes less prominent as the
Ag clusters become more bulk like.1 Peak broadening for small
cluster sizes may also be a result of the shortened lifetime of the
core hole, due to faster relaxation times.44
STM images for the as-prepared anatase TiO2(101)(1 × 1)
surface and three different Ag exposures, are shown in Figure
2a−d. The characteristic trapezoidal island structures of
anatase(101) are clearly observed in Figure 2a, separated by
monatomic steps of height around 3 Å. Assuming that the
clusters adopt a FCC packing structure45 with interatomic Ag−
Ag distance equivalent to that of bulk Ag, i.e., 2.9 Å,21 and are
hemispherical with a radius equivalent to the apparent height, it
is possible to extract a qualitative estimate for the number of Ag
atoms per cluster at exposures up to one monolayer (1 ML).
Figure 2b shows the anatase TiO2(101) surface following the
deposition of Ag on the anatase TiO2(101) surface for 1 min.
Bright protrusions are assumed to be individual atoms of Ag
with a height of ∼0.2 nm and an average lateral diameter of 1.2
nm. At the lowest coverage there appears to be no preferential
adsorption site for the Ag atoms, although at larger
magnification as shown in Figure 2e, the formation of some
larger clusters, particularly at step edges, is observed (56% of
clusters >1.5 nm exist on step edges). These clusters appear to
straddle the steps i.e. they are located partly on both the upper
and lower terraces and have been calculated to contain up to 8
Ag atoms. A few larger clusters are also seen on the terraces,
which we assume to nucleate at surface O-vacancies. Figure 2f
contains a line profile showing the formation of a cluster
straddling a step edge (with an apparent height, measured from
the top of the step, of 0.4 nm) and on a terrace (with an
apparent height of 0.3 nm).
Figure 2c shows an image recorded following 5 min of Ag
deposition. At this coverage the step edges are almost
completely saturated with Ag clusters (65% of all “large”
clusters can be seen occupying step edges) and individual Ag
atoms can no longer be resolved. At this coverage a number of
clusters are observed on the terraces. This coverage
corresponds roughly to 1 ML Ag by comparison to deposition
on the rutile (110) surface.46 Patches of bare substrate are still
visible at this coverage. The clusters have an average height of
0.7 nm with an average lateral cluster diameter of 2.4 nm, which
corresponds to approximately three Ag atomic layers and a total
of 40 atoms of silver per cluster. Looking at the lateral height of
the largest clusters we find a range of between 5 and 100 Ag
atoms per cluster. The 3D growth mode observed here has
been attributed to the low bonding strength of Ag−O
compared to Ti−O and also the higher free surface energy of
Ag compared to the oxide substrate.47 It has been suggested
that on the rutile TiO2(110) surface, Ag atoms remain highly
mobile and diffuse across the surface until they reach a step
edge.46 Figure 2d shows an image following the exposure of the
anatase TiO2(101) surface to the Ag source for 20 min. At this
point, the substrate surface is no longer visible, but the Ag still
exists as nanoclusters. A measure of cluster diameters gives a
mean Ag cluster diameter of 3.8 nm. For this coverage, an
accurate height above the TiO2 surface and, therefore, the
number of atoms per cluster cannot be readily extracted due to
the fact that the clusters are too closely packed to measure the
height from the clean surface.
The histogram plots in Figure 3 show the spread of cluster
diameters for the different Ag exposures. Although tip
convolution effects tend to lead to an overestimate of cluster
diameters,21,39 the measurements still allow a qualitative
measure of the cluster growth. The histogram plots show an
increase in average cluster size as a function of deposition time/
coverage and that the distribution of the diameters increases
with a standard deviation of 1.3 nm at maximum coverage.
The narrow range of cluster heights and diameters found for
the 5 min (ca. 1 ML) deposition of Ag on anatase TiO2 is
consistent with the growth of Ag on the rutile TiO2(110)
surface,1,46 although the average cluster height and diameter on
the anatase surface are somewhat smaller. The observed 3D
growth mode is as expected from surface free energies of the Ag
and TiO2 and is common to most transition metal growth
modes on metal oxide surfaces.47 On the rutile surface, the
average cluster heights and diameters at the equivalent coverage
are 1.4 and 7.5 nm, respectively. The observation of individual
atoms at the lowest exposure studied here, and the smaller
cluster sizes suggest that Ag diffusion to step edges and defect
sites is much slower than on the rutile surface. For the rutile
TiO2(110) surface an increase in the surface temperature,
which increases the diffusion rate, leads to larger cluster sizes.46
This slower diffusion on the anatase surface suggests a stronger
interaction between Ag and anatase TiO2 and is consistent with
the observation of charge transfer from the Ag to surface Ti
ions. This growth mode also accounts for the observed initial
increase and then plateau observed in the surface Ti3+
concentration in photoelectron spectra. The STM data confirm
that the observed downward shift in binding energy of the Ag
3d peaks is related to the cluster size; i.e., as the clusters grow,
they become more metal-like until the binding energy reaches
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Figure 4. (a) Ag 3d and (b) O 1s core level photoelectron spectra around the high binding edge of the main oxide derived O 1s peak. The full O 1s
spectrum is shown in Figure S2. The spectra are recorded at hν = 1486.6 eV under exposure to 3 mbar H2O and coexposure to 1.5 mbar H2O/1.5
mbar CO.
that of bulk metal.1,39 The smaller clusters observed on the
anatase relative to rutile and also the charge transfer observed is
likely to account for the enhanced catalytic activity of anatase
relative to rutile TiO2, since for a similar Ag content the surface
area/volume ratio of the clusters will be larger.
Reaction of Ag/TiO2 with 3 mbar H2O and 1.5 mbar
H2O + 1.5 mbar CO. In order to investigate the reactivity of
the Ag/TiO2 surface Ag clusters were deposited upon a freshly
prepared anatase TiO2(101) crystal for 5 min, corresponding to
the intermediate coverage shown in the STM image in Figure
2c. This sample was exposed to 3 mbar H2O followed by a
mixed gas composition of pH2O = 1.5 mbar and pCO = 1.5 mbar,
while XPS spectra were recorded in situ.
Exposure of the Ag/TiO2 surface to a water partial pressure
(pH2O) of 3 mbar has no effect on Ag and Ti core level spectra.
The H2O/CO mixture also has no noticeable effect on the Ti
2p spectrum. Exposure to the mixed CO/H2O gas, however,
leads to the appearance of new features in the Ag 3d spectrum
as shown in Figure 4b. These occur, as spin−orbit split peaks, at
binding energies of 367.6 and 373.7 eV and at and 371.4 and
377.5 eV. The lower binding energy peaks in the Ag spectra are
consistent with the oxidation of some Ag moieties48 and have
been observed following CO oxidation on an Ag(001)
surface.49 This downward shift appears anomalous, since an
increase in oxidation state is usually associated with an increase
in binding energy for metal cations in ionic systems. Although
the downward shift in binding energy of oxidized silver was first
reported over 40 years ago by Schön et al.,50 the origin of the
shift is still unclear. Various explanations based on the initial
state energy of the emitting cation have been proposed,
including covalency of the Ag−O bond combined with the
effect of the Madelung potential on the BE51 or a shift in the
Fermi energy.48 A recent density functional theory (DFT) and
high resolution core level spectroscopy study assigns the
downward shift to a final-state effect, where the core-hole is
screened by s-derived states of bonding character.48 The precise
oxidation state of the Ag can not be determined from these
measurements, but the binding energy agrees well with the
formation of Agδ+.49
The origin of the peaks at 371.4 and 377.5 eV is less clear.
The binding energies are consistent with Ag plasmon loss
peaks, but if this were the case then one would also expect to
observe them on the Ag clusters before exposure to H2O/CO.
The presence of these peaks, in conjunction with the lower
binding energy peaks are also consistent with the presence of
Ag(III) species, where they have been assigned as satellites.
However, previous work suggests Ag(III) is not particularly
stable in vacuum51 so this is unlikely. In addition, although the
peaks appear at the same time as the oxide peaks the intensity is
almost equivalent to that of the main oxide peaks so this too
suggests these peaks are not satellites related to Ag (III). These
higher energy features were not observed in spectra recorded
from an Ag(001) exposed to 5 mbar CO and subjected to
“post-mortem” analysis.49 Figure 4b shows that removal of the
CO/H2O gas in order to return to UHV conditions leads to the
disappearance of both sets of doublets introduced by the
exposure to the gas mixture. These observations suggest that
these higher binding energy peaks are associated with the
interaction of CO with the Ag clusters, but their origin cannot
be unambiguously determined from these data. Further work
using synchrotron radiation resonant near-ambient pressure
photoemission, which would allow us to carry out depth
profiling by tuning the kinetic energy and emission angle of the
photoemitted electrons, may prove useful in determining the
origin of these peaks. Detailed density functional theory
calculations would also be useful. The loss of the “new” Ag
3d related features, however, indicates that the adsorption
process is transient and only occurs in the presence of relatively
high pressures of CO at room temperature. In situ IR
measurements for CO adsorption on Ag/SiO2 catalysts have
shown a similar result.52 During exposure to 26 mbar CO,
linearly adsorbed Agδ+−CO species were identified through the
appearance of a new band. This band subsequently disappeared
following evacuation of the gas cell.
The O 1s spectra before and after exposure to both H2O and
H2O/CO are composed of multiple components. An expanded
view of the higher binding energy shoulder is shown in the
Supporting Information, Figure S2. The O 1s spectrum prior to
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exposure to H2O or CO is fitted with 4 components: Two
components at binding energies of 530.5 and 531.4 eV, arising
from O2− in anatase TiO2(101) and two components at 531.8
eV, and 532.7 eV, which are associated with the presence of
surface defects and surface hydroxyls.53 Following exposure to 3
mbar H2O and 3 mbar H2O/CO, additional peaks due to
photoemission from gas phase H2O (535.5 eV) and gas phase
CO (538.2 eV) are introduced. Upon exposure to 3 mbar H2O
an increase in intensity is observed in peaks at binding energies
of 531.8 eV, 532.7 and 533.5 eV. This observation is in
agreement with our previous work studying the interaction of
water with the anatase TiO2(101) surface. The peaks are
assigned to the interaction of physisorbed OH, chemisorbed
OH and physisorbed molecular water, respectively.53 For the
H2O/CO mixture, the peak at a binding energy of 532.7 eV is
seen to increase in intensity, and therefore is assigned to the
adsorption of CO on the Ag nanoclusters. Leaving the sample
for 24 h in order to attain ultrahigh vacuum conditions results
in some residual oxygen species at 532.7 eV, due to
chemisorbed OH.53 It is, however, clear that the adsorbed
molecular water peak at 533.5 eV is lost and the peak at 532.7
eV is reduced in intensity upon the return to UHV. The
reduction in intensity of the 532.7 eV peak is in agreement with
the loss of the peaks tentatively associated with CO adsorption
in the Ag 3d spectra.
The observation of the extra features in the Ag 3d spectra,
only at higher pressure serves to show the potential of NAPXPS in bridging the pressure gap between ultrahigh vacuum
and the pressures at which “real” catalytic reactions occur.
Although NAP-XPS measurements are still performed at a few
orders of magnitude below the pressures involved in some
catalytic reactions, subtle chemical changes that have hitherto
been unattainable can now be resolved.
■
CONCLUSIONS
■
ASSOCIATED CONTENT
■
AUTHOR INFORMATION
Corresponding Authors
*(A.G.T.) E-mail: andrew.g.thomas@manchester.ac.uk.
*(K.L.S.) E-mail: ksyres@uclan.ac.uk.
ORCID
Michael Wagstaffe: 0000-0002-2795-829X
Karen L. Syres: 0000-0001-7439-475X
Andrew G. Thomas: 0000-0002-1900-6686
Present Address
⊥
Current address: Diamond Light Source, Harwell Science and
Innovation Campus, Didcot, Oxfordshire, OX11 0DE, UK.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors would like to thank Alexei Preobrajenski and
Nikolay Vinogradov of MAX-lab for beamline support and
MAX-lab for the beamtime award. We are also grateful to
EPSRC UK for support through overseas travel Grant No. EP/
J015075/1. M.W. would like to thank EPSRC UK for award of
a studentship. M.A. and H.H. acknowledge financial support of
the BP ICAM.
■
■
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The nucleation and subsequent growth of Ag clusters on
anatase TiO2(101) have been investigated using a combination
of NAP-XPS and STM. Ag growth on the anatase TiO2(101)
occurs via a 3D self-limiting growth mechanism, predominantly
at step edges and at defects on terrace sites to form clusters.
Reduction of Ti4+ to Ti3+ is observed for the anatase surface via
charge transfer from the Ag clusters, suggesting a stronger
interaction between the Ag nanoclusters and the anatase
TiO2(101) surface than for the rutile TiO2(110) surface. This
strong interaction leads to the formation of smaller clusters and
the presence of individual Ag atoms at very low coverage. We
also find a chemisorption reaction between a mixed CO/H2O
vapor mixture which indicates oxidation of Ag species on the
clusters by reaction with CO to form Agδ+−CO. This species is
only observed during exposure to CO/H2O at pressures
achievable in a NAP system.
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jpcc.7b05818.
Core level photoelectron spectra for Ag 3d and Ti 2p
(Figure S1), core level NAP-XPS spectra for O 1s
(Figure S2), and binding energy and assignment of Ti 2p,
O 1s, and Ag 3d XPS core-level peaks. (Table S1) (PDF)
21388
DOI: 10.1021/acs.jpcc.7b05818
J. Phys. Chem. C 2017, 121, 21383−21389
The Journal of Physical Chemistry C
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