Confined Au-Pd ensembles in mesoporous TiO2 spheres for the photocatalytic oxidation of acetaldehyde

Article abstract of DOI:10.1002/cctc.201300584

An ensemble cast: The confinement of ensembles of Au-Pd inside unique mesoporous TiO₂ spheres contributes to their superior photocatalytic activity. Direct spectroscopic evidence reveals that the addition of palladium could suppress the growth of gold nanoclusters and prolong the lifetime of the excited electrons on these nanoclusters, thereby resulting in enhanced photoactivity. Copyright

Full text of DOI:10.1002/cctc.201300584

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DOI: 10.1002/cctc.201300584
Confined Au-Pd Ensembles in Mesoporous TiO2 Spheres
for the Photocatalytic Oxidation of Acetaldehyde
Fenglong Wang,[a] Yijiao Jiang,*[a] Xiaoming Wen,[b] Junhai Xia,[c] Gang Sha,[c] and
Rose Amal*[a]
Titanium dioxide (TiO2) is the most widely used photocatalyst
for environmental remediation and solar-fuel production,
owing to its low cost, nontoxicity, and abundance.[1] In TiO2-
based photocatalysis, the photogenerated electrons and holes
separate and migrate to the surface to be involved in the sur-
face redox reactions. However, a high recombination rate of
the photogenerated charge carriers leads to low photocatalytic
efficiency. Various noble metals have been introduced to trap
photogenerated electrons across the interface and, thus, im-
prove the charge separation within the TiO2 semiconductor.[2]
In the case of metal/TiO2 composites, the photogenerated elec-
trons are trapped by the metal, owing to the formation of
a Schottky barrier at the metal/TiO2 interface, which inhibits
electron–hole recombination and, thus, prolongs the lifetime
of the charge carriers. In addition, the dispersed noble metal
can also enhance reactant adsorption, as well as act as a co-
catalyst to help the redox process by lowering the over-poten-
tial that arises from strong metal–support interactions.[3] As
a consequence, enhanced photocatalytic activities have been
observed on the metal/TiO2 composites.[2a,4]
particles that were sonochemically immobilized onto P25 TiO2
were studied for the photocatalytic generation of hydrogen
from an EtOH/water solution. The highest activity was ach-
ieved for an Au-core/Pd-shell structure with an Au/Pd molar
ratio of 25:75.[7] Very recently, the Au-Pd/TiO2 photocatalyst
was also applied to water purification by using phenol as
a probe molecule and it was suggested that the metal nano-
particles could suppress undesired redox reactions that ineffec-
tively consumed photogenerated radicals, thus improving the
photo-oxidation efficiency.[8] However, to maximize their per-
formance, the role of bimetallic nanoparticles in photocatalysis
still needs further study.
Mesoporous TiO2 provides the possibility of precisely con-
trolling particle sizes and compositions, owing to its high sur-
face area and narrow pore-size distribution.[9] To the best of
our knowledge, studies on bimetallic photocatalysts that are
supported on mesoporous TiO2 spheres have never been re-
ported. Herein, we report that a significant enhancement of
acetaldehyde (ACE) removal under UV-A irradiation has been
achieved on Au-Pd ensembles that were confined in mesopo-
rous TiO2 by optimizing the bimetallic ratio. We found that bi-
metallic catalysts that were prepared through a simultaneous-
deposition procedure exhibited superior activities. For the first
time, steady-state fluorescence spectroscopy, in combination
with time-correlated single photon counting (TCSPC), revealed
that the addition of palladium could stabilize the gold nano-
clusters and prolong the lifetime of the excited electrons on
these clusters.
Until now, mono-metal-deposited TiO2 materials have been
intensively studied, whereas research on the photocatalysis of
bimetallic systems has been essentially limited to depositions
on commercial P25 TiO2 (Degussa). Rosseler et al. reported that
bimetallic Pd-Pt/TiO2 nanoparticles exhibited superior activity
for the simultaneous photo-oxidation of CO and acetone in
humid conditions, compared to metal-free and mono-metal-
deposited TiO2 composites.[5] Tsukamoto et al. reported an effi-
cient Au-Ag/TiO2-promoted production of H2O2 from an O2-sa-
turated EtOH/water mixture.[6] Bimetallic materials efficiently
promoted the formation of H2O2 and suppressed the photoca-
talytic decomposition of the as-formed H2O2. Bimetallic Au/Pd
Figure 1a shows a SEM image of the mesoporous TiO2
spheres with diameters of about 300 nm. TEM (Figure 1a,
inset) indicated that the mesopores originated from the ag-
glomeration of primary TiO2 particles with sizes of about 30–
50 nm. N2-adsorption/desorption isotherms confirmed that the
mesoporous nature of the materials was maintained upon
metal deposition (see the Supporting Information, Figure S1
and Table S1). A high-resolution TEM (HR-TEM) image of the
Au0.25-Pd0.25/TiO2 catalyst (Figure 1b) showed that the TiO2
spheres were highly crystalline and the lattice spacing was
consistent with the anatase phase. A crystalline Pd-containing
Au particle (size: about 8 nm), with exposed (111) facets, was
observed. Notably, continuity of the TiO2 lattice on the dark
Au-Pd particles revealed that these metal particles were con-
fined inside the TiO2 mesopores. Moreover, some tiny bright
spots were also observed, as shown in Figure 1c; however,
owing to their instability under electron-beam irradiation, it
was impossible to identify them by using TEM.
[a] F. Wang, Dr. Y. Jiang, Prof. R. Amal
ARC Centre of Excellence for Functional Nanomaterials
School of Chemical Engineering
University of New South Wales
Sydney, NSW 2052 (Australia)
E-mail: yijiao.jiang@unsw.edu.au
r.amal@unsw.edu.au
[b] Dr. X. Wen
School of Photovoltaics and Renewable Energy Engineering
University of New South Wales
Sydney, NSW 2052 (Australia)
[c] Dr. J. Xia, Dr. G. Sha
School of Aerospace
Mechanical & Mechatronic Engineering
University of Sydney
Sydney, NSW 2006 (Australia)
A bright metal particle with a size of about 6 nm was ob-
served in the high-angle annular dark-field scanning TEM
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300584.
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Figure 1. a) SEM image of the mesoporous TiO2 spheres; inset shows a low-
magnification TEM image. b) HR-TEM image of Au0.25-Pd0.25/TiO2, which
shows the presence of a gold-rich nanoparticle. c) TEM image of Au0.25
Pd0.25/TiO2, which shows the surface features of the TiO2 particles. d) HAADF-
STEM image of Au0.25-Pd0.25/TiO2. e) compositional line-scanning profile
through the metal particle in (d) along the arrow and f) EDS analysis on and
away from the metal particle.
Figure 2. Photocatalytic activity in the oxidation of acetaldehyde on bare
and metal-deposited mesoporous TiO2 spheres under UV-A irradiation with
a) the simultaneous introduction of the metal precursor and b) different
loading sequences. Initial acetaldehyde concentration: (183Æ10) ppm.
-
face caused surface defects, which might be favorable for the
photocatalytic activity (see the Supporting Information,
Figure S3).[11]
(HAADF-STEM) image of Au0.25-Pd0.25/TiO2 (Figure 1d). As shown
in Figure 1e, the compositional line-scanning profile through
the metal particle (along the direction shown in Figure 1d) in-
dicates that the Au-Pd ensembles exist as a solid solution of ar-
bitrary composition and it is clear that the content of Au is
higher than that of Pd. The energy-dispersive X-ray spectra
(EDS) of the metal particle (bottom, Figure 1 f) corroborates
the coexistence of Au and Pd, thus confirming the formation
of Au-Pd ensembles. In contrast, EDS analysis of several ran-
domly selected sites away from the metal particle, such as the
one marked by the spot in Figure 1d, showed no signals that
corresponded to Au and Pd (Figure 1 f, top). Wang and co-
workers reported that the Au-core/Pd-shell was destroyed and
a uniform alloy structure was formed after annealing at
5278C.[10] Powder X-ray diffraction (XRD) patterns only show
the diffractions of anatase TiO2 and no diffractions of Au or Pd
(see the Supporting Information, Figure S2), owing to the low
loading (0.5 wt%) and high dispersion of the noble metals. The
metallic particles at such a low content did not cause any mor-
phological or textural changes of TiO2. However, the Raman
spectra showed that the deposition of the metals on the sur-
Figure 2 shows the ACE effluent under the irradiation of UV-
A light; the initial content of ACE was (183Æ10) ppm. From
Figure 2a, it was clear that the content of ACE in the effluent
decreased dramatically upon irradiation for all the studied cat-
alysts; 58% of ACE was degraded over the bare TiO2. After illu-
mination for 5 h, 93.5% removal of ACE on the Au0.5/TiO2 cata-
lyst was achieved, whereas Pd0.5/TiO2 only degraded 66% ACE.
Interestingly, the simultaneously deposited bimetallic Au0.25
-
Pd0.25/TiO2 catalyst showed the highest activity with a degrada-
tion rate of 99.9% after irradiation for 5 h, which could be at-
tributed to a synergistic effect between the two metals. How-
ever, the simultaneously deposited Au0.4-Pd0.1/TiO2 and Au0.1-
Pd0.4/TiO2 materials showed much lower activities, that is,
76.5% and 63% ACE removal, respectively, which indicated
that the Au/Pd weight ratio played a crucial role in achieving
this synergistic effect.
Next, the effects of simultaneous and successive metal depo-
sition on the photocatalytic activity were investigated. As
shown in Figure 2b, the removal of 99.1% of ACE was ach-
ieved over Au0.25Pd0.25/TiO2 (successive deposition of Au and
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Pd), which was comparable to that of simultaneously deposit-
ed Au0.25-Pd0.25/TiO2 (99.9%); the discrepancy between the reac-
tivities could be attributed to differences in the metal-deposi-
tion efficiency. As shown by inductively coupled plasma atomic
emission spectroscopy, the deposition efficiency of Au re-
mained at about 85%, whereas that of Pd varied within the
range 20–58% with different deposition sequences. This result
implied that the affinity of Au to the TiO2 surface was stronger
than that of Pd. Therefore, as expected, a lower degradation
rate of 87.5% was obtained over Pd0.25Au0.25/TiO2 (successive
deposition of Pd and Au), which could be explained in that the
interface between Pd and TiO2 presumably made a smaller
contribution to the charge migration from the conduction
band (CB) of TiO2 into the metal particles, thereby resulting in
minimal enhancement. This result agreed well with the previ-
ous study on bimetallic Au-Ag/TiO2 for photocatalytic H2O2
production, probably owing to the lack of a homogeneous
mixture of the two metals, which led to inefficient electron–
hole separation.[6] For all of the catalysts studied herein, com-
plete stoichiometric oxidation into CO2 was observed, as
shown in the Supporting Information, Figures S4 and S5. The
activity measurements of the catalysts were performed over-
night and no decline in the conversion of ACE was observed.
The variation in activity was checked by a couple of repetitive
runs and the errors were less than 1%.
To understand the photocatalytic activity, X-ray photoelec-
tron spectroscopy (XPS) was employed as one of the most ap-
propriate methods for detecting the charge transfer between
a metal and the surface of the TiO2 support.[12] As shown in
Figure 3a, all of the gold in the Au-containing TiO2 existed in
metallic states, as indicated by the Au 4f spectrum, which was
composed of Au 4f7/2 and Au 4f5/2 peaks. Compared to the XPS
data of bulk gold materials (84.0 eV), the Au 4f7/2 peaks in all of
these Au-containing TiO2 samples were slightly shifted toward
lower binding energies (about 83.5 eV), thus indicating strong
interactions between the Au and the TiO2 support. As a conse-
quence, the gold particles on the surface may be negatively
charged, owing to electron migration from the CB of the TiO2
supports.[13]
Figure 3. XPS spectra of the metal-deposited mesoporous TiO2 spheres:
a) Au-containing materials; b) Pd-containing materials.
cited charge carriers and enhanced activity.[3,15] In the bimetal-
lic system, the formation of a unique Au-Pd alloy implies band
re-hybridization resulting in a suitable height of the Schottky
barrier, which favors smooth electron transfer from TiO2 to the
alloy nanoparticles. However, in this study, it was problematic
to precisely establish the correlation between the extent of
binding-energy shifts and photocatalytic activity, owing to the
extremely low content of the metals.
For the Pd-containing materials (Figure 3b), PdO was the
dominant Pd surface species; Pd 3d5/2 peaks were detected at
about 336.7 eV, which agreed well with the reported value of
bulk PdO (336.8 eV).[14] From the shift in binding energy shown
in Figure 3, the magnitude of the shift of the Au peak was
larger than that of the Pd peak, thus indicating that electron
transfer from the CB of TiO2 to the Au particles was more effi-
cient than that of Pd. TiO2 has a work function of 4.7 eV and
gold and palladium have work functions of 5.42 and 5.12 eV,
respectively. Electron transfer from the CB of TiO2 to the Au
particles is expected to be more efficient than that to the Pd
particles, although a higher Schottky barrier exists between Au
and TiO2, which agrees well with our observations from the
XPS studies.[6,13] This result can be understood as a conse-
quence of the fact that noble metals, with larger work func-
tions, that is, lower Fermi levels, than TiO2, could more effi-
ciently trap the photogenerated electrons from the CB of TiO2,
thereby leading to highly efficient separation of the photoex-
To explore the role of bimetallic Au-Pd ensembles in enhanc-
ing photocatalytic performance, the materials were character-
ized by using TEM (Figure 1c). Tiny bright spots were captured,
but the identification of the detailed structural properties of
these spots failed, as mentioned above. Steady-state fluores-
cence spectroscopy, in combination with time-correlated single
photon counting (TCSPC), can be used overcome this obstacle
of characterizing gold nanoclusters, even with sizes of less
than 2 nm. Atomically precise metal nanoclusters that com-
prise a few to tens of atoms possess discrete energy levels and
molecule-like properties in their absorption and fluorescence
features, owing to enhanced quantum-confinement effects.[16]
These nanoclusters are highly luminescent for fluorescence de-
tection. Moreover, fluorescence spectroscopy is capable of dis-
tinguishing large plasmonic nanoparticles from small nanoclus-
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ters, because the nanoclusters are too small to support surface
plasmon resonance.[16a,17] The emission wavelength of Au clus-
ters that were obtained by fluorescence spectroscopy could be
used to calculate the number of atoms in each gold cluster.[18]
It has been observed that Au8 and Au10 emit fluorescence in
the blue- and green-light regions, respectively.[19] Larger Au
clusters, such as Au30–Au50, emit in the near-infrared region.[20]
On the contrary, large metallic nanoparticles do not exhibit dis-
crete energy level and are non-luminescent.
this type of ensemble. Interestingly, no fluorescence peaks
were observed in the infrared region for both Au0.25-Pd0.25/TiO2
and Au0.25Pd0.25/TiO2, which indicated that no large nanoclus-
ters had formed. Thus, the formation of Au-Pd clusters inhibit-
ed further aggregation of the gold clusters.
To exclude the effects of metal content on the cluster size
and to verify the stabilizing role of Pd on the Au nanoclusters,
the fluorescence spectra of Au0.25/TiO2 and Au0.5-Pd0.25/TiO2 as
reference samples were also recorded, as shown in the Sup-
porting Information, Figure S6. Au0.25/TiO2 showed a much
weaker fluorescence peak in the near-infrared region (about
850 nm) compared to Au0.5/TiO2, because, at such low Au con-
tent, the amount of Au nanoclusters may not be sufficient to
form large nanoclusters. It seemed that the uniform dispersion
of Au promoted the activity, because Au0.25/TiO2 achieved
95.5% removal of ACE under identical conditions. Similar to
Au0.25/TiO2, Au0.25-Pd0.25/TiO2 showed a weak fluorescence peak,
irrespective of the presence of Pd. However, from a comparison
of Au0.5/TiO2 and Au0.5-Pd0.25/TiO2, the fluorescence peak that
arose from the latter sample was much weaker, comparable
with that of Au0.25/TiO2. The differences between the fluores-
cence spectra in the near-infrared region confirm that the addi-
tion of Pd may suppress the growth of Au nanoclusters, thus
leading to superior photocatalytic performance.
Figure 4a shows the steady-state fluorescence spectra of the
catalysts. No luminescence was observed for either bare TiO2
or the Pd0.5/TiO2 sample in the visible-light and near-infrared
regions. For the Au0.5/TiO2 sample, a weak fluorescence peak
was observed in the green-light region (505 nm), which
matched the fluorescence of Au8 and Au10 clusters. This result
suggested that Au8 and/or Au10 nanoclusters were formed and
adsorbed onto TiO2. A distinct fluorescence peak was also ob-
served in the near-infrared region (about 850 nm), which, in
turn, indicated the formation of larger nanoclusters, owing to
aggregation. In the case of Au0.25-Pd0.25/TiO2 and Au0.25Pd0.25
/
TiO2, the fluorescence peaks exhibited a slight red-shift with re-
spect to that of Au0.5/TiO2, most likely owing to the formation
of Au-Pd clusters.[21] Bright spots with sizes of less than 2 nm in
the TEM image in Figure 1c further confirmed the formation of
In addition, the lifetimes of the excited electrons on the
gold nanoclusters were measured at 510 nm by using TCSPC,
as shown in Figure 4b. Electron evolution exhibited bi-expo-
nential decay. Notably, lifetimes of (2.43Æ0.02), (1.96Æ0.02),
and (1.51Æ0.02) ns were obtained on Au0.25-Pd0.25/TiO2,
Au0.25Pd0.25/TiO2, and Au0.5/TiO2, respectively. A longer lifetime
essentially indicates a greater opportunity of contributing ex-
cited electrons to TiO2. Kamat and co-workers recently report-
ed that small metal clusters with relatively long lifetimes could
serve as electron donors to sensitize TiO2.[22] The fact that the
simultaneously deposited Au-Pd ensembles has longer elec-
tron lifetimes might be another reason why Au0.25-Pd0.25/TiO2
showed superior activity to Au0.5/TiO2.
In conclusion, Au-Pd ensembles that were confined in meso-
porous TiO2 structures were clearly identified and promoted
the stoichiometric conversion of gaseous acetaldehyde
through photocatalytic oxidation under UV-A irradiation. We
found that the Au/Pd ratio and their deposition sequence
played important roles in governing the activity of the ensem-
bles. Based on spectroscopic studies on interfacial electron
transfer in the Au-Pd/TiO2 nanocomposites, it was evident that
the electron transfer from TiO2 to Au was more efficient than
that to Pd. Steady-state fluorescence and time-correlated
single photon counting confirmed that the formation of the
Au-Pd ensembles suppressed the growth of gold nanoclusters
and also resulted in increased lifetimes of the excited elec-
trons, thus enhancing the photocatalytic activity.
Experimental Section
Preparation of the photocatalysts
Figure 4. a) Steady-state fluorescence spectra of bare TiO2 and the metal-de-
posited TiO2 material. b) Transient decay curves at 510 nm on Au0.5/TiO2,
Au0.25Pd0.25/TiO2, and Au0.25-Pd0.25/TiO2.
Spherical particles of mesoporous TiO2 were synthesized by using
a combination of sol-gel and solvothermal processes, as reported
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by Zhao and co-workers.[23] Au, Pd, and Au-Pd with different metal
weight ratios were deposited onto mesoporous TiO2 through
a wet-impregnation method to afford a total metal content of
about 0.5 wt%. The subscripts denote the nominal metal loadings:
For example, a sample with a nominal Au loading of 0.5 wt% is de-
noted as Au0.5/TiO2 and a sample with a nominal Pd loading of
0.5 wt% is denoted as Pd0.5/TiO2. On the simultaneous addition of
two metal salts, Au0.1-Pd0.4/TiO2 denotes a sample with a nominal
Au loading of 0.1 wt% and a nominal Pd loading of 0.4 wt% and
Keywords: charge transfer · fluorescence spectroscopy · gold ·
mesoporous materials · palladium
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Au0.25-Pd0.25/TiO2 denotes
a sample with nominal loadings of
0.25 wt% Au and 0.25 wt% Pd. To investigate the effects of differ-
ent metal-deposition sequences on the activity of the materials,
a successive deposition approach was also used. For example, if
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The activities of these samples were evaluated in the photocatalyt-
ic oxidation of gaseous ACE in a continuous-flow packed-bed an-
nular glass reactor that was illuminated by four surrounding UV-A
lamps, as described elsewhere.[24] A mixture of the fresh photocata-
lyst (25 mg) and acid-washed silica beads (250 mg) were prepared
and held in position by quartz wool plugs. Prior to the reaction,
the catalyst was pre-illuminated with UV light for about 1 h to
“burn off” any organic contaminants on the catalyst surface. Then,
a stream of an ACE/air mixture was passed through the reactor
until adsorption saturation was achieved (about 5 h); subsequently,
the photocatalytic oxidation of ACE was started by turning all four
lamps on. The effluent gases were sampled every 10 min and ana-
lyzed by on-line gas chromatography (GC, Shimadzu 2010) with
a flame-ionization detector and methanizer. The GC was equipped
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Acknowledgements
Financial support by the ARC Discovery Early Career Researcher
Award is gratefully acknowledged. We also acknowledge the use
of the facilities and technical assistance at the Mark Wainwright
Analytical Centre at University of New South Wales and the
AMMRF of the University of Sydney.
Received: July 19, 2013
Published online on October 7, 2013
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2013, 5, 3557 – 3561 3561