New insights into electronic and geometric effects in the enhanced photoelectrooxidation of ethanol using ZnO nanorod/Ultrathin au nanowire hybrids

Article abstract of DOI:10.1021/ja5059444

Oxidation of small organic molecules in a fuel cell is a viable method for energy production. However, the key issue is the development of suitable catalysts that exhibit high efficiencies and remain stable during operation. Here, we demonstrate that amine-modified ZnO nanorods on which ultrathin Au nanowires are grown act as an excellent catalyst for the oxidation of ethanol. We show that the modification of the ZnO nanorods with oleylamine not only modifies the electronic structure favorably but also serves to anchor the Au nanowires on the nanorods. The adsorption of OH^? species on the Au nanowires that is essential for ethanol oxidation is facilitated at much lower potentials as compared to bare Au nanowires leading to high activity. While ZnO shows negligible electrocatalytic activity under normal conditions, there is significant enhancement in the activity under light irradiation. We demonstrate a synergistic enhancement in the photoelectrocatalytic activity of the ZnO/Au nanowire hybrid and provide mechanistic explanation for this enhancement based on both electronic as well as geometric effects. The principles developed are applicable for tuning the properties of other metal/semiconductor hybrids with potentially interesting applications beyond the fuel cell application demonstrated here.

Full text of DOI:10.1021/ja5059444

Article
pubs.acs.org/JACS
New Insights into Electronic and Geometric Effects in the Enhanced
Photoelectrooxidation of Ethanol Using ZnO Nanorod/Ultrathin Au
Nanowire Hybrids
†
‡
,§
Annamalai Leelavathi, Giridhar Madras, and Narayanan Ravishankar*
†
‡
§
Centre for Nanoscience and Engineering, Department of Chemical Engineering, and Materials Research Centre, Indian Institute of
Science, Bangalore-560012, India
*
S Supporting Information
ABSTRACT: Oxidation of small organic molecules in a fuel
cell is a viable method for energy production. However, the key
issue is the development of suitable catalysts that exhibit high
efficiencies and remain stable during operation. Here, we
demonstrate that amine-modified ZnO nanorods on which
ultrathin Au nanowires are grown act as an excellent catalyst for
the oxidation of ethanol. We show that the modification of the
ZnO nanorods with oleylamine not only modifies the electronic
structure favorably but also serves to anchor the Au nanowires
−
on the nanorods. The adsorption of OH species on the Au
nanowires that is essential for ethanol oxidation is facilitated at
much lower potentials as compared to bare Au nanowires
leading to high activity. While ZnO shows negligible electro-
catalytic activity under normal conditions, there is significant
enhancement in the activity under light irradiation. We demonstrate a synergistic enhancement in the photoelectrocatalytic
activity of the ZnO/Au nanowire hybrid and provide mechanistic explanation for this enhancement based on both electronic as
well as geometric effects. The principles developed are applicable for tuning the properties of other metal/semiconductor hybrids
with potentially interesting applications beyond the fuel cell application demonstrated here.
1
5
INTRODUCTION
synthesis of Au nanowires for applications. However, the
inherently low stability of these nanowires precludes harsh
■
Rising energy demands and the depletion of nonrenewable
resources have motivated research and development of liquid
fuel cells. Particularly, efforts have been geared toward the
oxidation of small organic molecules (SOM) that are
recognized as the richest source of fuels. Among the SOM,
ethanol has considerable advantages in terms of its high energy
capacity, no toxicity and possibility of deriving it from
14
cleaning treatments and hence limits their application. The
capping agent on the Au nanowires may also modify the surface
properties and possibly hinder the electron transfer during
14
electrocatalytic reactions and thus slow down the rates.
A
possible way to overcome the aforementioned issues is to
anchor the wires on a specific support that increases the
stability of the wires in polar medium and also provides
flexibility to remove undesired surfactants. While there are a
few methods available to anchor nanowires on substrates,
there is no general method that may be applicable for a variety
of substrates because the nanowires grow via a nonclassical
1
,2
biomass. Nevertheless, direct ethanol fuel cells (DEFC)
have limitations for anode materials owing to complex reactions
16
17−19
3
that involve breaking of C−C bonds. Traditionally Pt-based
anodes have been shown to exhibit high efficiency toward
alcohol oxidation; however, they suffer from the reaction
2
0,21
4
oriented attachment process.
intermediate CO poisoning during operation. Numerous
Among all the possible support materials, the distinct
property of metal oxide semiconductors makes them an ideal
platform to anchor electroactive materials with potential
enhancement in multifunctionality. The possibility of
minimizing CO poisoning via bifunctional mechanism by
transfer of lattice oxygen and an enhancement of electro-
catalytic activity via photoexcitation are two attractive features
associated with semiconducting oxide supports.
studies have focused on engineering Pt-free nanostructured
catalysts for improved durability and enhancement in electro-
5
−7
chemical activity.
Ultrathin nanowires are attractive as electrocatalysts because
2
2
8
−11
of their inherent dimensional confinement.
have been shown to adopt exotic structures with enhanced
chemical reactivity toward oxidation reactions. For instance,
Au nanowires displayed negative shift in the redox potential of
ferri/ferrocyto-c relative to Au thin films. Wet chemical
reduction with oleylamine as both the reducing agent as well as
the capping agent is a promising method for high yield
Au nanowires
23
12
13
24−27
Consid-
14
Received: June 13, 2014
©
XXXX American Chemical Society
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Figure 1. Schematic illustration of the procedure to synthesize ZnO/Au nanowire hybrids. Conventional method to grow ultrathin Au nanowires on
ZnO leads to the formation of particles with a very few wires. Modification of ZnO with oleylamine aids in the growth of nanowires.
erable progress has been achieved to synergistically enhance the
electrochemical properties for exciting photofuel cell applica-
electrochemical activity via photoexcitation. Our experimental
study demonstrates that ultrathin nanowires are potentially
interesting systems for catalysis applications provided they are
stabilized by a suitable support. In the present case, the support,
in addition to providing stability, also promotes the activity of
Au nanowires at lower potential in addition to imparting
significant improvement in activity in the presence of light. We
discuss the origin of this synergistic enhancement that provides
key insights into the development of multifunctional hybrids
with potential solutions for energy and environment-related
issues.
2
8
tions. Among the large family of photoactive materials, ZnO
has been extensively investigated because of its outstanding
electron mobility that results in high photoactivity besides low
material cost and nontoxicity. Specific geometries of the crystal
facets exhibit different photocatalytic activities and thus there
have been significant efforts to design the morphology of
2
9
nanomaterials. In particular, high aspect ratio 1D nanostruc-
ture retains the advantages of superior photogenerated carrier
30
transport with low rate of recombination. Furthermore, it was
found that the separation efficiency of photogenerated charge
carriers was enhanced by post-treatment of metal oxides
RESULTS AND DISCUSSION
■
31
32,33
surfaces with either organic or inorganic ligands
results in the elimination of midtrap states.
that
Passivation of ZnO by Oleylamine: Implications for
Hybrid Formation and Properties. A schematic illustration
of the methodology employed for the preparation of ZnO/Au
nanowire hybrid is shown in Figure 1 and involves the
modification of ZnO by oleylamine (OA) followed by the
growth of ultrathin Au nanowires on mZnO. High aspect ratio
ZnO (1 μm length) was initially prepared according to our
Based on the above considerations, we have developed a
simple in situ strategy to grow ultrathin Au nanowires on
oleylamine (OA) passivated ZnO nanorods (designated as
mZnO) for electrocatalytic oxidation of ethanol. The strong
grafting of ZnO with OA is the key for providing Au nucleation
sites, followed by subsequent growth of nanowires through
oriented attachment mechanism. In this study, the insight
gained through cyclic voltammetry (CV) measurements leads
to useful information about versatile role of ZnO; mZnO not
only serves as support to stabilize the ultrathin nanowires
against disintegration in ethanol fuel cells while in operation
but also promotes the electrocatalytic activity via electronic
effect. This effect is related with electron donation to Au
nanowires from ZnO that determines the formation of incipient
hydrous gold oxides at low potential. These oxide species serve
as redox mediators for electrocatalytic oxidation of ethanol at
low potential; repeated high potential cycling is needed to
promote the oxidation reaction in the case of bare Au
nanowires. We also demonstrate a significant increase in the
30
earlier report. ZnO nanorods were mixed with oleylamine
(OA) using a mortar and pestle. The mixture was finally
washed with hexane and ethanol mixture to remove the excess
unbound amine. The amine groups bound to the surface of
ZnO facilitate the growth of Au nanowire on the ZnO
34
nanorods. This is evident from the experiments carried out
without surface modification that resulted in very few ultrathin
nanowires on support and primarily leads to the formation of
larger size Au nanoparticles in both solution as well as the
support (Figure S1, Supporting Information). In our earlier
report, we had shown that Au nanowires can be grown on
metal oxide but the areal density of nanowires was very much
35
limited, which hampers potential applications. Our modified
scheme of passivating the ZnO by direct mixing of the
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nanorods with OA rather than dispersing the rods in a solvent
containing OA clearly results in much better binding of the
amine on the ZnO surface and is responsible for the growth of
high areal density ultrathin Au nanowire on ZnO. Solid state
mixing resulted in higher concentration of OA immobilized on
ZnO with respect to mixing in hexane solvent (Figure S2).
The high angle annular dark field (HAADF) image (Figure
nanowires are well separated; the distance of separation is
approximately equal to OA chain length (2 nm).
The binding of amine to the surface of ZnO has been verified
using X-ray photoelectron spectroscopy (XPS) and Fourier
transform infrared (FT-IR) spectroscopy. High resolution N 1s
spectra of mZnO can be deconvoluted into two peaks as shown
in Figure 3A, peaks at 398.8 and 400.2 eV which are assigned to
36
2A) reveals a uniform distribution of ultrathin Au wires on
nitrogen atom in oleylamine and oleylammonium cation. This
strongly suggests the adsorption of nitrogen species on ZnO
surface. The deconvoluted oxygen spectra confirm the existence
of oxygen defects (Figure S4). Figure S5 (Supporting
Information) represents the FTIR spectra of ZnO and
mZnO, which shows the existence of amine species after
modification. The N−H stretching vibration of OA was merged
with O−H vibration. The bands observed at 2923 and 2853
−1
cm are attributed to C−H antisymmetric and symmetric
3
7
stretching vibration of CH , respectively. In mZnO, we
2
−1
observed strong absorption at 1064 cm corresponding to
shifted C−N bands of OA emphasizing that OA is grafted on
15
surface of ZnO. In addition, we see drastic variations in the
density of states after OA modification on ZnO as seen in the
UPS valence spectra (Figure S6).
Figure 2. STEM-HAADF images of as-synthesized mZnO/Au
nanowires. (A) Image displaying dense growth of ultrathin wires on
mZnO and (B) higher magnification image showing wires aligned
parallel to the long axis of the nanorods.
To examine the electronic interaction of OA with ZnO
nanorods, we measured current before and after modification of
ZnO. Previous studies have shown that amine grafting on ZnO
can modulate the electrical conductivity that was explained on
38
mZnO. Bright-field TEM images show that, in some instances,
the grown Au nanowires are aligned along the entire length of
the nanorods to form longitudinal bands (Figure S3). The
higher magnification HAADF image (Figure 2B) suggests that
the basis of the dipolar fields offered by the amine groups. In
contrast, I−V measurements in our present work displayed
negligible change in the conductivity after OA modification
(Figure S7). However, in the presence of light, we observed a 2-
Figure 3. (A) Deconvoluted N 1s XPS spectrum of amine-modified ZnO (mZnO), displaying the existence of nitrogen species. (B) Photocurrent of
ZnO (a), mZnO (b), and mZnO/Au (c) nanowires upon solar light (xenon lamp) illumination. (C) Photoluminescence spectra (PL) of ZnO and
oleylamine-modified ZnO. Oleylamine modification results in marked enhancement of band-edge emission peak (excited at 325 nm). (D) Current−
voltage characteristic of mZnO and mZnO/Au nanowire hybrid measured in ambient, showing enhanced electrical conductivity of the hybrid.
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Figure 4. Cyclic voltammograms (CV) of mZnO/Au nanowires. (A) (a) 1 M NaOH (without ethanol); the electrochemical surface area was
calculated from the backward reduction peak area (from −0.2 to 0.2 V vs SCE) (for 0.06 mg) and (b) with 1.5 M ethanol in 1 M NaOH showing
characteristic oxidation peak of ethanol. (B) CV with different molar concentration of NaOH containing 1.5 M ethanol. (C) CVs of bare ultrathin
Au nanowires in 1 M NaOH containing 1.5 M ethanol with different positive potential limits. (a) Cycling up to 0.6 V (enlarged in the inset) showing
featureless voltammogram. (b) Characteristic oxidation and reduction peak of Au appeared after cycling to 1 V. (c) Ethanol oxidation peak is
observed in the next cycle after initial cycling to 1 V. All the scan rates are at 40 mV/s. (D) Deconvoluted XPS Au 4f spectra of mZnO/Au nanowires
showing the existence of Au anionic species.
4
2,43
fold increase in the generated photocurrent from mZnO with
an applied voltage of 0.5 V as shown in Figure 3B.
Photoluminescence studies were performed to investigate the
impact of OA on observed photoresponse. We note that OA
modification significantly increased the UV band edge emission
of ZnO (by ∼1.5 times) as shown in Figure 3C, thus confirms
an electronic interaction of the amine with ZnO. This suggests
that the nonradiative emission associated with uncoordinated
bonds is effectively removed which is consistent with the
grown on mZnO displayed excellent electrical conductivity
(Figure 3D), signifying that high aspect ratio ultrathin Au
nanowire can, in principle, be used for electrocatalytic
applications. The hybrids exhibit a further sharp increase in
photocurrent, revealing that the photoexcited electron can
possibly transfer from ZnO to Au nanowire leading to a better
separation of the photogenerated charge carriers. Although
nanowires utilized in this study absorb across a range of visible
light wavelength (Figure S10), we do not observe any
measurable photocurrent generation by bare nanowires as
shown in Figure S11. This is also validated by the wavelength-
dependent photocurrent measurements on mZnO/Au as
depicted in Figure S12. These results highlight that surface
modification of ZnO either with suitable passivation groups
and/or metal nanostructures is a viable method to enhance the
electro- and photoconductivity.
Electrocatalysis Using mZnO/Au Nanowires. Inspired
by the photo and electrical properties of mZnO/Au nanowires,
we investigated them as possible photoassisted electrocatalysts
for alcohol oxidation reactions. To gain insights into the
photoelectrocatalytic activity, we first studied the ethanol
oxidation (EOX) on mZnO/Au (without light) in alkaline
medium in order to evaluate the electrocatalytic activity of the
electrode. The electrochemical surface area (ECSA) of the
electrode was measured using cyclic voltammetry (CV) in 1 M
NaOH electrolyte solution without ethanol at a scan rate of 40
mV/s (Figure 4A). During forward scan, the slight increase in
the current at very low potential is attributed to the charge
transfer process associated with OH− group adsorption
(reaction 1, below) that plays a key role in alcohol oxidation.
44
39
intensified band-edge emission. However, the visible emission
remains almost same. In principle, visible emission originates
from defect states, serves as scavenger, and facilitates the
separation of photogenerated charge carriers, consequently
40
increasing the carrier lifetimes. As the degree of passivation
increases with increase in the concentration of OA relative to
ZnO, an increase in the photocurrent was observed consistent
with the proposal that amine interacts strongly with the ZnO
surface (Figure S8). By decreasing the OA concentration, the
band edge emission decreases as shown in Figure S9.
Although, ligand-passivated ZnO has been investigated
4
1,39
earlier,
our results differ from many of the existing
observations in the following ways. First, the passivation is
seen to affect only the nonradiative recombination and
accounts for the best possible separation of photogenerated
charge carriers. Second, the passivation preserves the electronic
defect states that are responsible for suppressing the
recombination of carriers. In addition to these electronic
effects, the passivation presents strong anchoring sites for the
nucleation of Au nanoparticles that subsequently assist in the
growth of ultrathin wires. Furthermore, ultrathin Au nanowires
D
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The characteristic peak for monolayer oxide formation on Au
oxidation peak shifted negatively that unambiguously confirms
that hydroxyl adsorption has a promoting effect on EOX
(Figure 4B).
The measured electrocatalytic activity of mZnO/Au nano-
wires for EOX was compared with the activity of unsupported
nanowires. Scanning and transmission electron microscopy
(SEM and TEM, respectively) micrographs of the unsupported
OA capped Au nanowires are shown in Figure 5A and B. The
45
takes place during the forward scan. The cathodic peaks
observed during the reverse scan correspond to the reduction
16
of Au oxide and to the desorption of hydroxyl groups. The
electrochemical actual surface area (ECSA) was estimated by a
calculation of the oxide reduction peak area from the CV scans.
The oxidation current was normalized to ECSA for the
respective electrodes for further comparison of the catalyst
activities (Supporting Information), current density obtained
for nanowire hybrid is higher when compared to polycrystalline
4
6,47
Au electrode.
The obtained oxidation current was
normalized with mass of the gold to calculate current density.
The voltammetric response in the presence of ethanol (1.5 M
of CH CH OH in 1 M NaOH) for synthesized mZnO/Au
3
2
shows that the beginning of the EOX reaction coincides with
−
the chemisorption of OH process; current density increases at
very slow rate below 0 V and with increasing potential the
current increases indicating the initiation of EOX (Figure 4A).
At 0.18 V, the oxidation current reaches a maximum and then
decreases with further increasing potential, which can be
attributed to the formation of an inactive compact oxide layer
on Au surface. In the reverse scan, the peak appeared at −0.05
V corresponds to the removal of incompletely oxidized
carbonate species absorbed on the gold surface. This peak
onset potential coincides with reduction peak of gold in the
ethanol-free NaOH solution, which can be attributed to the
47
reactivation of the Au surface.
The oxidation of alcohols in the alkaline medium occurs only
when the gold electrode surface is partially covered with
oxygenated species. Several studies have indicated that alcohol
oxidation reactions take place even on gold surfaces that do not
have significant chemisorbing capacity only if an interfacial
cyclic redox mediator forms during the reaction as exemplified
Figure 5. SEM and TEM bright field images of unsupported ultrathin
Au nanowires (A,B) before and (C,D) after EOX reaction.
Unsupported nanowires are not stable, indicating that the nanowires
disintegrate and aggregate to form nanoparticles during reaction.
48
by the incipient hydrous oxide/adatom mediator model. The
driving force for the formation of such species is more favorable
−
46
in alkaline medium through adsorption of OH , and the
oxidation reaction has been suggested to follow the steps
shown below. It is well-documented that C is the preferred
2
electrocatalytic oxidation current was negligible in the case of
bare Au nanowires (curve a, Figure 4C) under similar
conditions as those used for mZnO/Au. It has been shown
that the presence of an amine capping layer on the Au can lead
to suppression of redox processes, resulting in a featureless
pathway for EOX on gold; the initial step in this case involves
46
the formation of aldehyde.
−
(1−λ)−
+ λe−
Au + OH → Au − (OH)
(1)
ads
14
−
−
voltammogram. We observe that when the potential is swept
to higher values (above 0.6 V), an anodic oxidation feature was
observed around 0.9 V attributed to the formation of oxide
species on gold surface accompanied by a reversible reduction
of formed oxide layer observed at −0.05 V in negative scan (as
shown in Figure 4C, curve b). For the subsequent scans,
intriguing changes are observed as suggested by the appearance
of the sharp quasi-coincidence of forward and reverse anodic
oxidation features at 0.18 V and −0.1 V and can be ascribed to
the oxidation of ethanol and the formed intermediates
respectively (Figure 4C, curve c). These results suggest that
scanning at higher potential is observed to have a significant
influence on the formation of Au oxygenated mediators and
that these formed products promote redox reactions, resulting
in the oxidation of ethanol in the following cycles.
CH CH OH + OH → CH CH O + H O + e
(2)
3
2
3
2
2
−
CH CH O + Au(OH) → Au(CH CHO) + H O + e
3
2
ads
3
2
(
3)
Au(OH)ads + CH CHO + OH
3
−
→
Au + CH COOH + H O + e
(4)
3
2
The chemisorption of hydroxyl groups will be favored on the
gold surface that leads to partial oxidation of gold (reaction 1).
The first deprotonation of ethanol is base catalyzed (reaction
4
7
2
), and hence, the formation of alkoxide will be enhanced
with increasing concentration of NaOH. In fact, the second
−
hydrogen elimination from alkoxide involves an OH adsorbed
Au surface; this dehydrogenation step leads to aldehyde
formation. The aldehyde further reacts with OH to produce
Although the exact mechanism is not known, we hypothesize
that OA capping on nanowires masks the formation of Au
oxygen mediators at low potential. It has been proposed that
the amine can form a complex with Au with applied potential in
basic solution and that the complex formed at the interface can
4
9
−
acetic acid. Almost all the above steps are greatly influenced by
−
OH ions either in solution (reaction 2) or chemisorbed on the
5
0,51
Au surface (reaction 3 and 4).
As expected from the above
48
reactions, an increase in the concentration of NaOH, the
retard the formation of oxygenated gold species. Scanning to
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Scheme 1. Charge Transfer Scenarios between ZnO and Au Nanowires during (A) Electro- and (B) Photoelectrocatalytic
Ethanol Oxidation
Figure 6. CV of mZnO/Au nanowires (A) stability test, in 5 M NaOH containing 2 M ethanol, first cycle and after 60th cycle; inset displays the peak
current density versus cycle number at a scan rate of 10 mV/s, and (B) 3 M NaOH containing 1 M ethanol at different scan rates. Inset of (B) shows
a linear relationship of current density with the square root of scan rate. (C) CVs at a scan rate of 40 mV/s in 1 M NaOH solutions with different
concentrations of ethanol. (D) Electrochemical impedance spectra: Nyquist plot of the oxidation of ethanol over Au nanostructures supported on
ZnO in 1 M NaOH and 1.5 M C H OH. The applied electrode potentials are shown as figure legends. Recorded from 100 kHz to 10 MHz with the
2
5
amplitude of AC signal at 10 mV. Hollow and solid symbols are representative of mZnO/Au nanowires and ZnO/Au nanoparticles, respectively.
F
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a higher potential can lead to oxidation of the formed amine
complex, thus favoring the formation of incipient hydrous oxide
mediators on Au in the successive cycles. However, the amine
layer oxidation destabilizes the nanowires significantly during
reaction, resulting in the disintegration of the nanowires to
nanoparticles (as shown in Figure 5C and D). Regardless of the
morphological change, requirement of high potential to activate
the gold surface is not viable for fuel cell applications.
electrode. At the end of the 60 cycles, there is no significant
decrease in oxidation current, indicating the stability of the
hybrid (Figure 6A). The inset depicts that peak current density
versus respective cycle number. In addition, we note that there
is no significant change in the microstructure of the mZnO/Au
hybrid after electrocatalysis (Figure S14). Our method of direct
growth on the substrate thus overcomes the stability issues of
nanowires in ethanol medium and thus opens up new
14
The above results highlights that the formation of interfacial
hydrous oxide mediators during the positive scan is different for
unsupported and for ZnO supported Au nanowires. This
finding is crucial because it implies that the mZnO support has
an electronic effect that is important for the formation of
hydrous gold oxide at low potential. To verify further the
conjecture about the electronic effect induced by mZnO, we
carried out detailed XPS studies on mZnO/Au (Figure 4D).
We observe peaks at binding energy of 82.8 and 86.8 eV
possibilities for various potential applications.
The impact of scan rate on the electrocatalytic performance
on mZnO/Au, as shown in Figure 6B, shows that the current
density is linearly proportional to the square root of scan rates
(inset of Figure 6B). This is characteristic of a diffusion-
controlled process limited by the diffusion of either ethanol or
the reaction intermediates. Moreover, from Figure 6C, we
observe that, upon increasing the ethanol concentration,
oxidation current linearly increased and a subsequent decrease
in gold oxide layer reduction cathodic current occurred. This
can be rationalized due to ample availability of ethanol
molecules to interact with formed gold oxide layer during
δ‑
corresponding to Au 4f7/2 and 4f5/2 of Au , respectively, and
also a peak at 83.7 eV that can be rationalized by the presence
0
δ−
of Au state. The formation of Au indicates that there is
electron transfer to Au from the substrate. This can happen in
two ways. Recent investigations have shown that capping agents
like OA could cause a change in the Au electronic structure by
back-donating electron to Au nanostructures which can cause
5
0
initial period of electrochemical reduction process.
To compare the role of the morphology of Au on the
electrocatalytic activity, we compare the activity of mZnO/Au
nanowires with the activity of ZnO/Au nanoparticles
synthesized by microwave synthesis. Optical absorption and
TEM micrographs of the synthesized Au nanoparticle hybrids
are shown in Figure S10 and S15, respectively. The hybrid Au
nanoparticles (Figure S16) displayed current density that is
approximately 1.5 times lower than obtained with hybrid Au
nanowires (Figure 4A). The mass specific current density of
mZnO/Au nanowires shows a clear enhancement in activity in
the case of nanowires. The possible reason for the higher
activity of mZnO/Au nanowires can be attributed to the
following geometric effects. First, 1D nanostructures that are
single crystalline inherently possess less defects as compared to
52
Au core level binding energy shift. This possibility was
discarded by an XPS study on unsupported OA capped
0
δ+
nanowires that showed that Au and small amount of Au are
present in the system (Figure S13), indicating that a charge
transfer between amine and Au may be insignificant in our
system. The second possibility is the transfer of electrons from
the ZnO to the Au. However, it may not be favorable to
transfer electrons from the valence band of ZnO to Au. The
presence of F-centers lead to the existence of electronic states
located at 0.05 eV below the conduction band of ZnO that
53
could favor electron transfer to Au (Scheme 1A). According
to work function levels, an electron from F-center of ZnO (4.4
eV) can migrate toward Au which has work function of 5 eV
61
nanoparticles. Generally, defect sites are irreversibly oxidized
under applied potential that degrades the electrocatalytic
performance. Second, diffusion of electrochemical species
toward catalyst surfaces are significantly improved in high
aspect ratio nanowires as has been demonstrated experimen-
54−56
and can lead to electron-rich Au structures.
According to the Norskov d-band model, the relatively deep
d-band below Fermi energy is the primary reason for the
inertness of gold toward chemisorption of oxygenated
62
tally earlier. Third, interconnected wires could also facilitate
5
7
63
species. It is well know that particle size reduction can shift
easier charge transport, thus enhancing the electrocatalytic
58
the d-band state and reduce the barrier for chemisorption.
activity. Thus, the morphology of the Au on the ZnO support
plays a key role in its electrocatalytic activity.
Electrochemical impedance spectroscopy (EIS) is a funda-
mental tool for characterizing the interfacial charge transfer
properties of electrodes. Generally, the diameter of the
semicircle in the Nyquist plot is equal to the charge transfer
Alternatively, in the hybrid, a charge transfer from the F-center
ZnO would better explain the upward shift of the Au Fermi
5
9
level. The increased electron density in Au would be
beneficial for hydroxyl adsorption due to preferred electron
donation from Au to highly electronegative adsorbates. For
60
instance, Koper et al. performed a detailed investigation
regarding electrochemical oxidation of CO on Au in alkaline
medium and proposed as “self-promotion mechanism” due to
tendency to enhance adsorption of hydroxyl groups. In the
present case, a consequence of the upward shift in Fermi leve−l
of Au enhances the adsorption of electronegative OH
adsorbates, thereby driving the formation of hydrous oxide
mediators on gold at very low potential. Thus, the ZnO support
favors for the formation of incipient hydrous oxides on the gold
surface that act as mediators for EOX; however, in the case of
bare nanowires, repeated high positive potential cycling is
needed to activate the gold surface to promote oxidation
reactions.
resistance (R ) of the electrode surface. Figure 6D indicates the
ct
representative complex plane of the Nyquist plot for
synthesized hybrids in 1 M NaOH and 1.5 M ethanol at
different applied potentials over the frequency range of 100
kHz to 10 mHz (with AC signal amplitude of 10 mV). It is
found that ZnO/Au nanowires have a low Rct value as
compared to ZnO/Au nanoparticles, that is in accordance with
the above results of CV. The calculated values of R at 0.17 V
ct
are 130 Ω and 2.25 kΩ for mZnO/Au nanowires and ZnO/Au
nanoparticles, respectively. The profiles indicate that R
ct
decreases upon increasing the potential (from −0.15 to 0.17
V) and then increases at higher potential (0.3 V) for the
nanowire hybrid. This is due to the formation of monolayer
surface oxides at higher potential and consistent with the CV
results.
To evaluate the long-term performance of the mZnO/Au
nanowires, we carried out repeated scans with the same
G
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Figure 7. (A) Electrocurrent (a) and photocurrent (b) of mZnO electrode. Inset shows the ON/OFF switching photoresponse of mZnO electrode
upon solar irradiation (xenon lamp). (B) CVs of mZnO/Au nanowires without (a) and with (b) solar light irradiation in 1 M NaOH containing 1.5
M ethanol. (C) Summary of the synergetic effect of photogenerated charge carrier on current density evaluated by CV measurements. (D) PL
spectra of TAOH formed after 5 CV cycles of potential sweep from −0.3 to 0.6 V for the electrolyte (6 mM TA in 0.1 M NaOH) of the irradiated
mZnO (a) and mZnO/Au nanowires (b) electrode at a scan rate of 40 mV/s.
Electrooxidation of ethanol did not take place with just
mZnO as the catalyst that confirms that the observed
electrocatalytic activity in the mZnO/Au hybrid primarily
stems from the Au. However, upon irradiation, a substantial
increase in current density was observed for mZnO at different
applied potentials, as shown in Figure 7A. In the case of ZnO,
the enhanced photoresponse at different potential is
pronounced, leading to merging of EOX peak with generated
photocurrent. It is understood that under illumination electron
and hole pairs are created in ZnO (reaction 5). Typically, the
photogenerated holes in alkaline medium are effectively
current increase and knowledge that we have gained from the
studies with the proposed mechanism by the model of
66
Morrison and Freund, we are postulating that “current
doubling” phenomenon occurs due to two step oxidation.
Therefore, for single photon absorption, two electrons are
injected into the conduction band, one by photoexcited
electrons and other from intermediate radicals that leads to
the increase in current. As shown earlier, Au nanowires are
responsible for the observed electrocatalytic oxidation current
in the synthesized mZnO/Au nanowires. We have demon-
strated that mZnO also oxidizes ethanol under solar light and
thus an effective coupling of these two processes (reaction 1 to
8) can boost the performance of the fuel cell.
Synergistic Photoelectrooxidation of Ethanol. The
electrooxidation current response of the mZnO/Au nanowire
hybrids under illumination is shown in Figure 7B. The
characteristic EOX peak was shifted to lower onset potential
under irradiation. Similarly, the oxidation of carbonate species
that occurred during the reverse scan was shifted to more
positive potential, as marked in the figure as dotted line. The
reason for the reduced overpotential in the electrocatalytic
−
scavenged by OH groups that protect the ZnO from
dissolution, which leads to the formation of surface bound
6
4
•
OH radicals (reaction 6) or direct oxidization of ethanol to
form intermediate oxidized radical.
ZnO + hν → eCB− + hVB+
(5)
(6)
+
ZnO−OH + h → ZnO + •OH
C H OH + •OH → •CH(OH)CH + H O
(7)
(8)
2
5
3
2
reaction could arise from the enhanced hydrophilicity of
•
CH(OH)CH + 11•OH → 2CO + 8H O
3
2
2
67,68
photoinduced defect sites on ZnO,
which leads to increased
The energy levels of intermediate (reaction 7) α-hydroxyethyl
concentration of ethanol on the active site of electrode
surface. Furthermore, the magnitude of the current density
was enhanced by 2.2 times and is higher than the algebraic
27
radicals •CH(OH)CH are favorable for injecting an electron
3
65,44
to conduction band of ZnO.
By combining the observed
H
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Journal of the American Chemical Society
Article
summation of photooxidation of mZnO and electrocatalytic
activity of mZnO/Au nanowires as shown in Figure 7C. This is
attributed to additional photooxidation and electrooxidation
reactions of the formed byproduct acetic acid (reaction 4) and
photogenerated electrons, respectively. This observation
emphasizes the synergistic effect of photoassisted electro-
it was revealed that hybrids upon photoirradiation accelerate
the activity of Au, by interfacial migration of photoelectron
which suppresses the recombination of photogenerated carriers
thus leading to synergy between electro and photooxidation
current. Although the scope of this work is limited to fuel cell,
interfacing ultrathin nanowire with semiconductor can enhance
various physic-chemical properties. We believe that the research
community will leverage the synthesized nanowire hybrids for
many exciting applications in the fields of nanoelectronics,
sensors, and catalysis.
28
catalytic activity, which intensifies the oxidation current.
Synergistic effect in the hybrid arises from efficient interfacial
charge transfer and prominent separation of photogenerated
holes and electrons. As the Fermi level of Au is lower than that
4
4
of ZnO, the photogenerated electron from conduction band
of mZnO can migrate toward Au nanowires grown on mZnO
leading to the net separation of photogenerated holes and
electron pairs, as illustrated in Scheme 1B, which is consistent
with observed photocurrent measurement. Subsequently, the
photomigrated electrons further stimulate hydroxyl binding on
Au nanowires that further enhance the electrocatalytic EOX
EXPERIMENTAL SECTION
■
Synthesis of ZnO Nanorods. ZnO nanorods (∼1 μm length)
were synthesized according to our earlier report. Zinc acetate (14.5 g)
was dissolved in 62.5 mL of methanol, and then KOH (4 M, 32.5 mL
methanol) was added to the above solution under vigorous stirring at
6
7 °C. After the mixture was aged for 2 h to evaporate half the volume
(
reaction 8), as seen above. The isolated holes can react with
hydroxyl groups on the ZnO and produce an •OH radical that
of solvent, the concentrated solution was incubated in a stainless steel
autoclave for 12 h at 120 °C. Finally, the obtained product was washed
several times with distilled water and dried at 100 °C.
leads to photooxidation of ethanol (reaction 9).
Oleylamine (OA) Modified ZnO (mZnO). ZnO was passivated
with oleylamine by means of simple solid state reaction. In a typical
synthesis, 100 mg of ZnO nanorods was taken in a mortar and mixed
with 50 μL of OA using a pestle at room temperature that resulted in
the formation of amine grafted ZnO. The product was repeatedly
washed with hexane and ethanol mixture to remove unbound OA and
then dried in ambient conditions.
−
−
CH CH OH + 5Au(OH ) → CH COO + 4H O + 4e
3
2
3
2
(8)
+
CH CH OH + 12•OH(mZnO...OH + h )
3
2
→
2CO + 9H O
(9)
2
2
Preparation of mZnO/Au Nanowires. To produce the hybrid
material, as prepared mZnO (10 mg), 3 mg of HAuCl , 100 μL of OA,
The photogenerated charge separation efficiency of hybrid
can be indirectly qualified by the amount of produced •OH
radicals. Fluorescence measurement with terephthalic acid
4
and 150 μL of triisopropylsilane(TIPS) were added into 2.5 mL of
1
5
hexane following the method of Xing et al. The mixture was
sonicated for 2 min and then kept without stirring for 6 h until the
color of the solution changed to dark red. The obtained precipitate was
washed thrice with an ethanol/hexane mixture using centrifugation,
and the final products were used for further photo- and electrocatalytic
study.
(
TA) is employed to analyze the •OH radicals (Supporting
Information). PL measurements (Figure 7D) give direct
evidence for the formation of 2-hydroxyterephthalic acid as a
result of interaction between terephthalic acid and produced
•
OH, resulting in PL emission around 422 nm (excited at 325
Preparation of ZnO/Au Nanoparticles. ZnO/Au nanoparticles
nm). The PL intensity increased for mZnO/Au nanowires as
compared to mZnO as shown in Figure 7D, indicating
enhanced separation of holes and electrons in mZnO/Au
nanowires. These results highlight the direct consequence of
separated photogenerated holes and electrons by interfacial
charge migration between mZnO and Au nanowires. At the first
step, the hybrid structure minimizes the recombination of
photogenerated holes and electrons and thus enhances the
production of •OH, which not only increases the photo-
oxidation of ethanol but it can also participate in the
photooxidization of the product formed during reaction 4.
Meanwhile, the photomigrated electron induces negatively
were prepared through a microwave assisted synthesis route, according
3
0
to our previous study. An amount of 50 mg of synthesized ZnO
nanorods was sonicated in 30 mL of water for 10 min, followed by the
addition of HAuCl (3 mg). This was subject to microwave radiation
4
(
2.45 GHz, 800 W) for 3 min to obtain ZnO/Au nanoparticles hybrid.
Characterizations. TEM micrographs were acquired using a FEI
Tecnai T20 instrument operated at an accelerating voltage of 200 kV,
and HAADF images were obtained using a FEI Tecnai T30 instrument
with an accelerating voltage of 300 kV. XPS measurements were
performed on an Axis Ultra instrument using a monochromatic source
of Al Kα radiation, and charge correction was done with respect to
graphitic carbon at 284.5 eV. FT-IR spectra were recorded using a
UATR accessory on a Frontier PerkinElmer spectrometer. The
composition quantification of Au was found using inductively coupled
plasma-mass spectroscopy (ICP-MS, Thermo X Series II). Photo-
luminescence spectra for both solid catalyst and liquid electrolyte
samples were measured using a LabRAM HR spectrophotometer with
the help of a CCD detector.
Photoconductivity Measurements. Photocurrent measure-
ments were done using a Class 3A solar simulator (Oriel Sol 3A)
comprising a xenon lamp 450 W, with a 1.5 air mass filter and a
Keithley (2420 model) source meter. The samples were drop casted
between ITO contact pads coated on glass and separated at a distance
of 50 μm. The electrical measurements were performed using an
Agilent Device analyzer B1500A.
charged species in addition to F-center electrons around the Au−
surface, which increases the electrocatalytic activity via OH
adsorption.
In summary, we have developed a method for the synthesis
of high dense ultrathin Au wires on ZnO nanorods for
electrocatalytic applications. Oleylamine modified ZnO shows
an enhancement in the photocurrent by passivated surface
defects. Support is mandatory to circumvent the disintegration
of nanowires especially in ethanol medium. In addition to the
stability, the support induces electronic effect on Au nanowires,
which facilitates redox process at low potential. Considerably,
high positive potential sweeps are needed to initiate the
electrochemical reaction on unsupported oleylamine capped
nanowires. These informative results are significant for
understanding the key steps involved in the electrocatalytic
activity of surfactant capped nanostructures. Most importantly,
Photoelectrochemical Measurements. A conventional three
electrode system was adopted to perform cyclic voltammetry
experiments using saturated calomel electrode (SCE) and platinum
foil auxiliary as reference and counter electrode, respectively. The
working electrode of nanowire hybrids was fabricated by loading
hexane dispersed hybrids on toray carbon paper. The catalyst ink of
I
dx.doi.org/10.1021/ja5059444 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
nanoparticle hybrids was made by mixing 15 mg of nanoparticle hybrid
with 30 μL of Nafion in isopropanol solvent, and then the prepared
ink was drop casted on toray paper. CVs of the prepared electrodes
were measured with and without light irradiation. A xenon lamp of 100
W was employed as an excitation source. All the electrochemical
measurements were conducted at room temperature and performed by
using a potentiostat (PG26250, Techno Science Instrument).
Electrochemical Impedance Spectroscopy (EIS) Measure-
ments. EIS was performed using a CHI 660D electrochemical
workstation. The spectra were recorded from 100 kHz to 10 MHz at
an AC signal of 10 mV.
(16) Choi, Y.; Gu, M.; Park, J.; Song, H. K.; Kim, B. S. Adv. Energy
Mater. 2012, 2, 1510−1518.
(17) Cui, H.; Hong, C.; Ying, A.; Yang, X.; Ren, S. ACS Nano 2013,
7, 7805−7811.
(18) Chandni, U.; Kundu, P.; Singh, A. K.; Ravishankar, N.; Ghosh,
A. ACS Nano 2011, 5, 8398−8403.
(19) Kundu, P.; Chandni, U.; Ghosh, A.; Ravishankar, N. Nanoscale
2012, 4, 433−437.
(
20) Halder, A.; Ravishankar, N. Adv. Mater. 2007, 19, 1854−1858.
(21) Kundu, P.; Anumol, E.; Nethravathi, C.; Ravishankar, N. Phys.
Chem. Chem. Phys. 2011, 13, 19256−19269.
22) Kulesza, P. J.; Pieta, I. S.; Rutkowska, I. A.; Wadas, A.; Marks,
D.; Klak, K.; Stobinski, L.; Cox, J. A. Electrochim. Acta 2013, 110, 474−
83.
23) Liu, B.; Chia, Z.-W.; Lee, Z.-Y.; Cheng, C.-H.; Lee, J.-Y.; Liu, Z.-
L. J. Power Sources 2012, 206, 97−102.
24) Li, W.; Bai, Y.; Li, F.; Liu, C.; Chan, K.-Y.; Feng, X.; Lu, X. J.
Mater. Chem. 2012, 22, 4025−4031.
25) Georgieva, J.; Valova, E.; Armyanov, S.; Philippidis, N.; Poulios,
(
ASSOCIATED CONTENT
Supporting Information
■
S
4
(
*
Procedure for terephthalic test and ECSA calculation. Figures
S1−S16. This material is available free of charge via the Internet
at http://pubs.acs.org.
(
(
AUTHOR INFORMATION
Corresponding Author
nravi@mrc.iisc.ernet.in
■
I.; Sotiropoulos, S. J. Hazard. Mater. 2012, 211, 30−46.
(26) Antoniadou, M.; Lianos, P. Appl. Catal., B 2010, 99, 307−313.
(27) Su, C.-Y.; Hsueh, Y.-C.; Kei, C.-C.; Lin, C.-T.; Perng, T.-P. J.
Phys. Chem. C 2013, 117, 11610−11618.
Notes
(
28) Drew, K.; Girishkumar, G.; Vinodgopal, K.; Kamat, P. V. J. Phys.
Chem. B 2005, 109, 11851−11857.
29) Tian, Z. R.; Voigt, J. A.; Liu, J.; Mckenzie, B.; Mcdermott, M. J.;
The authors declare no competing financial interest.
(
ACKNOWLEDGMENTS
■
Rodriguez, M. A.; Konishi, H.; Xu, H. Nat. Mater. 2003, 2, 821−826.
(30) Leelavathi, A.; Madras, G.; Ravishankar, N. Phys. Chem. Chem.
Phys. 2013, 15, 10795−10802.
We thank Prof. Praveen C. Ramamurthy for providing chemical
workstation facilities for impedance measurements. The authors
thank Prof. Ramananda Chakrabarti and his students for access
to the ICP-MS facility. The Tecnai T20 and F30 microscopes
are part of Advanced facility for Microscopy and Microanalysis
at IISc. The XPS, solar simulator, and PL spectrophotometer
are part of MNCF, CeNSE, IISc. The authors thank Subhajit
Kundu and Akash for their help with TEM imaging. N.R.
thanks DST Nanomission and TUE, DST for financial support.
(31) Koleilat, G. I.; Levina, L.; Shukla, H.; Myrskog, S. H.; Hinds, S.;
Pattantyus-Abraham, A. G.; Sargent, E. H. ACS Nano 2008, 2, 833−
40.
32) Lan, X.; Masala, S.; Sargent, E. H. Nat. Mater. 2014, 13, 233−
40.
33) Tang, J.; Kemp, K. W.; Hoogland, S.; Jeong, K. S.; Liu, H.;
Levina, L.; Furukawa, M.; Wang, X.; Debnath, R.; Cha, D. Nat. Mater.
011, 10, 765−771.
34) Mourdikoudis, S.; Liz-Marzan
1465−1476.
35) Kundu, P.; Halder, A.; Viswanath, B.; Kundu, D.; Ramanath, G.;
Ravishankar, N. J. Am. Chem. Soc. 2009, 132, 20−21.
36) Loubat, A.; Imperor-Clerc, M.; Pansu, B.; Meneau, F.; Raquet,
B.; Viau, G.; Lacroix, L.-M. Langmuir 2014, 30, 4005−4012.
37) Kumar, D. R.; Manoj, D.; Santhanalakshmi, J. Anal. Methods
014, 6, 1011−1020.
38) Belagodu, T.; Azhar, E. A.; Yu, H. Nanoscale 2012, 4, 7330−
333.
39) Soares, J. W.; Whitten, J. E.; Oblas, D. W.; Steeves, D. M.
Langmuir 2008, 24, 371−374.
40) Wang, J.; Liu, P.; Fu, X.; Li, Z.; Han, W.; Wang, X. Langmuir
008, 25, 1218−1223.
41) Guo, L.; Yang, S.; Yang, C.; Yu, P.; Wang, J.; Ge, W.; Wong, G.
8
(
2
(
2
(
́
, L. M. Chem. Mater. 2013, 25,
REFERENCES
■
(
(
1) Kowal, A.; Li, M.; Shao, M.; Sasaki, K.; Vukmirovic, M.; Zhang, J.;
Marinkovic, N.; Liu, P.; Frenkel, A.; Adzic, R. Nat. Mater. 2009, 8,
(
́
3
25−330.
2) Antolini, E. J. Power Sources 2007, 170, 1−12.
3) He, Q.; Shyam, B.; Macounova, K.; Krtil, P.; Ramaker, D.;
Mukerjee, S. J. Am. Chem. Soc. 2012, 134, 8655−8661.
4) Beden, B.; Lamy, C.; De Tacconi, N.; Arvia, A. Electrochim. Acta
990, 35, 691−704.
5) Wang, A.-L.; Xu, H.; Feng, J.-X.; Ding, L.-X.; Tong, Y.-X.; Li, G.-
R. J. Am. Chem. Soc. 2013, 135, 10703−10709.
6) Xu, C.; Wang, H.; Shen, P. K.; Jiang, S. P. Adv. Mater. 2007, 19,
256−4259.
7) Tian, N.; Zhou, Z.-Y.; Yu, N.-F.; Wang, L.-Y.; Sun, S.-G. J. Am.
Chem. Soc. 2010, 132, 7580−7581.
8) Koenigsmann, C.; Santulli, A. C.; Gong, K.; Vukmirovic, M. B.;
Zhou, W.-p.; Sutter, E.; Wong, S. S.; Adzic, R. R. J. Am. Chem. Soc.
011, 133, 9783−9795.
9) Lu, Y.; Tu, J.-p.; Gu, C.-d.; Xia, X.-h.; Wang, X.-l.; Mao, S. X. J.
Mater. Chem. 2011, 21, 4843−4849.
10) Ruan, L.; Zhu, E.; Chen, Y.; Lin, Z.; Huang, X.; Duan, X.;
Huang, Y. Angew. Chem., Int. Ed. 2013, 52, 12577−12581.
11) Koenigsmann, C.; Sutter, E.; Adzic, R. R.; Wong, S. S. J. Phys.
(
(
2
(
7
(
́
(
1
(
(
(
2
(
(
4
(
K. Chem. Mater. 2000, 12, 2268−2274.
(
(42) Yang, W.; Qu, L.; Zheng, R.; Liu, Z.; Ratinac, K. R.; Shen, L.;
Yu, D.; Yang, L.; J. Barrow, C.; Ringer, S. P.; Dai, L.; Braet, F. Chem.
Mater. 2011, 23, 2760−2765.
2
(
(43) Wang, C.; Hu, Y.; Lieber, C. M.; Sun, S. J. Am. Chem. Soc. 2008,
130, 8902−8903.
(
(44) Subramanian, V.; Wolf, E. E.; Kamat, P. V. J. Phys. Chem. B
2003, 107, 7479−7485.
(
(45) Yan, S.; Zhang, S.; Lin, Y.; Liu, G. J. Phys. Chem. C 2011, 115,
6986−6993.
Chem. C 2012, 116, 15297−15306.
̈
̈
(46) Lai, S.; Kleijn, S. E.; Ozturk, F. T.; van Rees Vellinga, V. C.;
(12) Kondo, Y.; Takayanagi, K. Science 2000, 289, 606−608.
(13) An, W.; Pei, Y.; Zeng, X. Nano Lett. 2008, 8, 195−202.
(14) Koposova, E.; Kisner, A.; Shumilova, G.; Ermolenko, Y.;
Koning, J.; Rodriguez, P.; Koper, M. Catal. Today 2010, 154, 92−104.
(47) Huang, J.; Han, X.; Wang, D.; Liu, D.; You, T. ACS Appl. Mater.
Interfaces 2013, 5, 9148−9154.
Offenhau
3951.
15) Feng, H.; Yang, Y.; You, Y.; Li, G.; Guo, J.; Yu, T.; Shen, Z.; Wu,
T.; Xing, B. Chem. Commun. 2009, 1984−1986.
̈
sser, A.; Mourzina, Y. J. Phys. Chem. C 2013, 117, 13944−
1
(48) Burke, L.; Nugent, P. Gold Bull. 1998, 31, 39−50.
(49) Zope, B. N.; Hibbitts, D. D.; Neurock, M.; Davis, R. J. Science
2010, 330, 74−78.
(
J
dx.doi.org/10.1021/ja5059444 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
(
(
50) De Lima, R.; Varela, H. Gold Bull. 2008, 41, 15−22.
51) Tremiliosi-Filho, G.; Gonzalez, E.; Motheo, A.; Belgsir, E.;
Leg
(
(
́
er, J.-M.; Lamy, C. J. Electroanal. Chem. 1998, 444, 31−39.
52) Zhang, G.-R.; Xu, B.-Q. Nanoscale 2010, 2, 2798−2804.
53) Schmidt-Mende, L.; MacManus-Driscoll, J. L. Mater. Today
2
(
(
007, 10, 40−48.
54) Wang, J.; Hammer, B. Top. Catal. 2007, 44, 49−56.
55) Radnik, J.; Mohr, C.; Claus, P. Phys. Chem. Chem. Phys. 2003, 5,
1
(
72−177.
56) Arrii, S.; Morfin, F.; Renouprez, A.; Rousset, J. J. Am. Chem. Soc.
2
(
(
004, 126, 1199−1205.
57) Hammer, B.; Norskov, J. Nature 1995, 376, 238−240.
58) Lopez, N.; Janssens, T.; Clausen, B.; Xu, Y.; Mavrikakis, M.;
Bligaard, T.; Nørskov, J. K. J. Catal. 2004, 223, 232−235.
59) Sanchez, A.; Abbet, S.; Heiz, U.; Schneider, W.-D.; Hak
H.; Barnett, R.; Landman, U. J. Phys. Chem. A 1999, 103, 9573−9578.
60) Rodriguez, P.; Kwon, Y.; Koper, M. T. Nat. Chem. 2012, 4,
77−182.
61) Koenigsmann, C.; Zhou, W.-p.; Adzic, R. R.; Sutter, E.; Wong, S.
S. Nano Lett. 2010, 10, 2806−2811.
62) Wu, P.; Zhang, H.; Qian, Y.; Hu, Y.; Zhang, H.; Cai, C. J. Phys.
Chem. C 2013, 117, 19091−19100.
63) Sanchez-Iglesias, A.; Rivas-Murias, B.; Grzelczak, M.; Per
Juste, J.; Liz-Marzan, L. M.; Rivadulla, F.; Correa-Duarte, M. A. Nano
Lett. 2012, 12, 6066−6070.
64) Fujishima, A.; Kato, T.; Maekawa, E.; Honda, K. Bull. Chem. Soc.
Jpn. 1981, 54, 1671−1674.
65) Lu, H.; Zhao, J.; Li, L.; Gong, L.; Zheng, J.; Zhang, L.; Wang, Z.;
(
̈
kinen,
(
1
(
(
(
́
́
ez-
́
(
(
Zhang, J.; Zhu, Z. Energy Environ. Sci. 2011, 4, 3384−3388.
(
66) Morrison, S. R.; Freund, T. J. Chem. Phys. 1967, 47, 1543−1551.
(
67) Feng, X.; Feng, L.; Jin, M.; Zhai, J.; Jiang, L.; Zhu, D. J. Am.
Chem. Soc. 2004, 126, 62−63.
68) Sun, R.-D.; Nakajima, A.; Fujishima, A.; Watanabe, T.;
Hashimoto, K. J. Phys. Chem. B 2001, 105, 1984−1990.
(
K
dx.doi.org/10.1021/ja5059444 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX