Ultrathin Au nanowires supported on rGO/TiO2 as an efficient photoelectrocatalyst

Article abstract of DOI:10.1039/c5ta03988f

We report the synthesis of stable rGO/TiO₂/Au nanowire hybrids showing excellent electrocatalytic activity for ethanol oxidation. Phase-pure anatase TiO₂ nanoparticles (～3 nm) were grown on GO sheets followed by the growth of ultrathin Au nanowires leading to the formation of a multidimensional ternary structure (0-D TiO₂ and 1-D Au on 2-D graphene oxide). The oleylamine used for the synthesis of the Au nanowires not only leads to stable Au nanowires anchored on the GO sheets but also leads to the functionalization and room temperature reduction of GO. Using control experiments, we delineate the role of the three components in the hybrid and show that there is a significant synergy. We show that the catalytic activity for ethanol oxidation primarily stems from the Au nanowires. While TiO₂ triggers the formation of oxygenated species on the Au nanowire surface at a lower potential and also imparts photoactivity, rGO provides a conducting support to minimize the charge transfer resistance in addition to stabilizing the Au nanowires. Compared with nanoparticle hybrids, the nanowire hybrids display a much better electrocatalytic performance. In addition to high efficiency, the nanowire hybrids also show a remarkable tolerance towards H₂O₂. While our study has a direct bearing on fuel cell technology, the insights gained are sufficiently general such that they provide guiding principles for the development of multifunctional ternary hybrids.

Full text of DOI:10.1039/c5ta03988f

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Journal of Material Chemistry A
Paper
Ultrathin Au nanowires supported on rGO/TiO2 as an efficient
photoelectrocatalyst
A Leelavathi,a Giridhar Madrasb and N Ravishankar*c
Received 00th January 20xx,
Accepted 00th January 20xx
We report the synthesis of stable graphene oxide rGO/TiO2/Au nanowire hybrids showing excellent electrocatalytic activity
for ethanol oxidation. Phase-pure anatase TiO2 nanoparticles (~ 3 nm) were grown on GO sheets followed by the growth of
ultrathin Au nanowires leading to the formation of a multidimensional ternary structure (0-D TiO2 and 1-D Au on 2-D
graphene oxide). The oleylamine used for the synthesis of the Au nanowires not only leads to stable Au nanowires
anchored on the GO sheets but also leads to the functionalization and room temperature reduction of GO. Using control
experiments, we delineate the role of the three components in the hybrid and show that there is a significant synergy. We
show that the catalytic activity for ethanol oxidation primarily stems from the Au nanowires. While TiO2 triggers the
formation of oxygenated species on the Au nanowire surface at a lower potential and also imparts photoactivity, rGO
provides a conducting support to minimize the charge transfer resistance in addition to stabilizing the Au nanowires.
Compared with nanoparticle hybrids, the nanowire hybrids display a much better electrocatalytic performance. In addition
to high efficiency, the nanowire hybrids also show a remarkable tolerance towards H2O2. While our study has a direct
bearing on fuel cell technology, the insights gained are sufficiently general such that they provide guiding principles for the
development of multifunctional ternary hybrids.
DOI: 10.1039/x0xx00000x
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leads to compositional changes and is detrimental for the long-
term stability of the catalyst.4 To solve the aforementioned
issues, oxide-supported monometal nanostructures are ideal,
Introduction
based on both electronic and bifunctional effects of support.10,
Gold and its alloys have been extensively used as catalysts for
wide range of reactions for energy generation and
11
a
Furthermore, the strong metal-support interaction
environmental protection.1 For these applications, controlling
the size,2 shape3 and composition of the catalyst4 is critical for
enhancing the catalytic performance. The activity of Au
catalysts is usually explained in terms of an electronic effect (d-
band position), that directly influences the interaction of the
reactant molecules with the catalyst surface.5 However, an
inherently low binding energy of oxygen species on Au surface
is a major limiting factor for various oxidation reactions.6 The
electrocatalytic oxidation of alcohols is regarded to be ideal for
portable power applications owing to the high energy
conversion efficiency; however, this reaction requires an
optimum adsorption of oxygenated species on Au catalyst.7 To
achieve this, the most widely used strategy is to design a
bimetallic catalyst of Au alloyed with oxophilic metals that may
significantly enhance the reaction kinetics.8, 9 The key problem
involved in these bimetallic catalysts is the dissolution of the
lesser noble metal in the electrolyte during operation that
promotes the oxidation reaction, due to the modified
electronic structure of Au.12 Recently, it has been proposed
that the metal oxide support could enhance the performance
13
It is
of ethanol fuel cell by weakening the C-C bond.7,
generally accepted that the oxide-gold interface plays a
14
TiO2 has
significant role in the catalytic performance.12,
emerged as a promising substrate because of inherent
optoelectronic properties that is expected to further stimulate
the performance of the catalyst via photoexcitation.15
However, the low conductivity of TiO2 and high rates of charge
recombination is a disadvantage for practical applications and
thus the design of a different type of support is needed.16, 17
The toolbox to tackle the above-mentioned problems is
rich and includes the modification of the metal oxide with
carbon supports18,19 that are capable of providing high
conductivity and good dispersion of the oxide. Over the past
few years, reduced graphene oxide (rGO) has emerged as a
potential support because of its anchoring ability towards
catalyst particles arising from the functional groups on its
surface.20,
a.Centre for Nanoscience and Engineering, Indian Institute of Science, Bangalore-
560012, India.
21, 22
A judicious combination of TiO2 and rGO is
as it could potentially suppress the
attractive,23,
24
b.Department of Chemical Engineering, Indian Institute of Science, Bangalore-
560012, India.
recombination of photogenerated charge carriers and can
serve as a highly conducting support material for fuel cell.25
c. Materials Research Centre, Indian Institute of Science, Bangalore-560012, India.
Email: nravi@mrc.iisc.ernet.in
†Electronic Supplementary Information (ESI) available. See
DOI: 10.1039/x0xx00000x
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The electrocatalytic alcohol oxidation using Au as the by using Raman spectroscopy, the characteristic vibration peak
catalyst is sensitive to the structure of the gold surface.3 In for anatase was observed at 150 (Eg(1)), 396 (B1g(1)), 513
case of ultrathin nanowires, a significant fraction of the atoms (A1g+B1g(2)) and 639 cm-1 (Eg(2)) (Fig. S3),35 along with D and G
reside on the surface and contribute to subtle changes in the bands of GO. The average size of the TiO2 particles is around 3
electronic structure due to relaxation.26 Thus, it would be nm and is much smaller compared to the size of TiO2
interesting to investigate the use of Au nanowire as a potential nanoparticles reported earlier.36, 37
28
candidate for electrocatalytic reactions.27,
Indeed, recent
work using Au nanowires (AuNW) has demonstrated
a
negative shift in the redox potential of ferri/ferro-cyto-c as
well as exceptional performance for electrocatalysis.29, 30 In
this study, we use ultrathin Au nanowires synthesized using
wet chemical means31, 32, 33 as a catalyst for alcohol oxidation
reactions. We use TiO2 nanoparticles supported on reduced
graphene oxide as the support to grow the ultrathin AuNW
and demonstrate
a synergistic coupling effect in the
multidimensional ternary catalyst involving 0-D TiO2, 1-D
AuNW and 2-D rGO. We also synthesize the binary
counterparts to delineate the contribution of each of the
components to the overall performance of the hybrid. The
ternary
hybrid
architecture
exhibited
promising
electrochemical activity due to the synergy by utilizing
incipient oxide formation on Au nanowires coupled with the
higher electrical conductivity from rGO. The supported AuNW
exhibited higher electrocatalytic activity than nanoparticle
hybrids and also better tolerance against H2O2 crossover
effects. To further improve the performance, we have coupled
Fig. 1 Scheme for growth of ultrathin Au nanowires on rGO/TiO2. (a) GO prepared by
Hummer’s method (b) nucleation of TiO2 nanoparticles (~ 3 nm) on GO sheets via
microwave synthesis (c) room temperature reduction and in-situ amine
the optoelectronic properties of TiO2 with electrocatalytic functionalization of GO, (d) functionalized substrates facilitates the growth of Au
nanowires and forms multidimensional ternary hybrid rGO/TiO2/AuNW. The hexagonal
properties of AuNW and demonstrate an improvement in the
pattern in the schematic is only shown to represent rGO and is not drawn to scale.
photo-assisted electrocatalytic activity in the ternary hybrid.
Considering its superior performance, particularly photo-
The as-synthesized GO/TiO2 composite was added to Au
assisted electrocatalytic activity, tolerance and stability,
nanowire growth solution (for details, see Experimental
rGO/TiO2/AuNW appears to be a promising candidate for
Section). The solution mixture was sealed and undisturbed for
photofuel cells.
6 h at room temperature.38 During this process, the presence
of oleylamine (OA) leads to intercalation39 and
functionalization of GO followed by direct nucleophilic attack
Results and discussion
of the amine on epoxy group that leads to deoxygenation or
reduction of GO.40 The resulting amine-modified reduced GO
Growth of ultrathin Au nanowires on supports
(rGO) loaded with TiO2 favors the growth of nanowires on the
surface as depicted schematically in Fig. 1. TEM micrographs
(Fig. 2C) of the samples display dense growth of high-aspect-
ratio ultrathin Au nanowires on the rGO/TiO2 sheets; the
nanowires are well-separated and dispersed on the substrate
(Fig. S4).
The preparation of ternary rGO/TiO2/AuNW involves two
steps: deposition of TiO2 nanoparticles on GO sheets, followed
by one-pot GO reduction and growth of ultrathin high single-
crystalline Au nanowires. In the first step, graphitic oxide was
prepared according to modified Hummer’s method,34 and TiO2
nanoparticles (~ 3 nm diameter) were uniformly nucleated on
GO through a simple, rapid microwave technique as illustrated
in Fig. 1. The XRD pattern of as-synthesized GO/TiO2 shows
peaks corresponding to pure anatase TiO2 in addition to the
intense peak of GO (Fig. S1). The morphology of the as-
synthesized GO/TiO2 was examined using electron microscopy.
Fig. 2A is a representative transmission electron microscope
(TEM) image that reveals a homogeneous dispersion of TiO2
nanoparticles on the GO sheets as is also evident from the SEM
image (Fig. S2). High-resolution TEM micrographs from several
regions show that the titania particles are interconnected (Fig.
2B); the d-spacing of 3.5 Å matches well with the (101) spacing
of the anatase phase of TiO2 and is also consistent with XRD
analysis. The formation of anatase TiO2 was further confirmed
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In particular, based on mechanisms proposed in the literature,
nucleophilic substitution between amine and GO leads to
deoxygenation and reduction of GO.40 Interestingly, a shift of
Au binding energy towards lower values was observed for TiO2
(Fig. S9) and rGO/TiO2 (Fig. 3C) supported ultrathin Au
nanowires as compared to the Au nanowires on bare GO (Fig.
3D). We propose that electron transfer from TiO2 to Au
contributes to the observed shift, leading to electron
accumulation in the Au wire.12 From the relative position of
the Au work function, intrinsic defect states (F-center electron)
and conduction band of TiO2,46 an electron from F-center of
TiO2 can migrate to Au contributing to the reduction in the
binding energy as observed by XPS. On the contrary, no shift in
Au 4f was observed for rGO/AuNW (Fig. 3D) validating the
hypothesis of electron transfer from TiO2 to Au.
Fig. 2 (A) TEM image shows the presence of TiO2 nanoparticles (~ 3 nm) on GO sheets
synthesized using titanyl nitrate precursors. (B) HRTEM micrograph of GO/TiO2,
showing lattice spacing corresponds to the (101) anatase phase of TiO2. (C) bright-field
image showing ultrathin Au nanowires (~ 2 nm diameter) on TiO2/rGO hybrid support
(D) Low magnification SEM micrograph displays the dense and uniform growth of Au
nanowires on rGO over a large area.
As control samples, binary hybrids such as rGO/AuNW and
TiO2/AuNW were also synthesized under similar conditions for
a better understanding of electrochemical properties of the
ternary hybrid. Incubating GO in the nanowire growth solution
leads to a dense growth of ultrathin AuNW on GO sheets as
seen in the SEM image (Fig. 2D). However, functionalization
was required on TiO2 for the growth of nanowires30 (Fig. S5).
TEM image (Fig. S6) illustrates the growth of nanowires on
amine-modified TiO2 nanoparticles.
X-ray photoelectron spectroscopy (XPS) analysis of
rGO/TiO2/AuNW samples was performed to confirm the
oxidation states and the possible binding mechanism between
the Au nanowires and the substrate. The presence of spectral
peaks corresponding to N 1s (Fig. S7) confirms chemical
modification of GO by nitrogen.41 It is known that OA strongly
adsorbs on the GO sheets.39 Furthermore, C 1s spectrum of
rGO/TiO2/AuNW can be deconvoluted into several peaks at
284.5 eV (sp2C), 285.4 eV (sp3C or C-OH or C-N), 286.5 eV
(epoxy C-O), 287.7 eV (carbonyl –C=O) and 288.8 eV
(carboxylate –COO-) as shown in Fig. 3A.42, 43, 44 Compared with
the C 1s of GO/TiO2 (as shown in Fig. 3B), the oxygenated
functional groups are significantly reduced in rGO/TiO2/AuNW,
indicating the room temperature reduction of GO by OA. This
is also evident from XRD analysis that shows a reduction in the
basal spacing resulting from the reduction of GO (Fig. S1).45
Additionally, similar GO reduction (without TiO2) was observed
(Fig. S8) under incubation of GO in Au nanowire growth
solution suggesting simultaneous room temperature reduction
of GO and Au with OA. Therefore, it can be deduced that the
room temperature reduction of GO is facilitated by the amine.
Fig. 3 XPS spectra of as-synthesized samples. High-resolution C 1s spectra of GO/TiO2
(A) after and (B) before growth of AuNW. The least-square fitted peaks reveals that the
contributions from –C-O, C-OH –C=O and –COO- groups decreased after nanowire
growth, indicating reduction of GO. (C) Deconvoluted Au 4f of rGO/TiO2/AuNW
showing the presence of Auδ− species possibly due to electron transfer from TiO2 to Au.
(D) No Auδ- was observed in case of rGO/Au nanowires. Legend representation: GO-
graphitic oxide, G- rGO, T- TiO2 and A- Au.
Support-dependent electrocatalytic activity
We carried out alcohol oxidation reaction to test the
electrochemical properties of the ternary and binary hybrids.
On the basis of both theoretical calculation and experimental
observations, electrocatalytic oxidation of alcohols requires
OH¯ adsorption or pre-oxidation species on the catalyst gold
surface.47,
48
These species act as an oxidant, leading to a
significant reduction in the activation energies for
electrocatalytic alcohol oxidation. Indeed, several studies have
validated that the appearance of redox features are
47, 49
mandatory for alcohol oxidation on gold surface.30,
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Therefore, we have initially evaluated the electrodes for the curve of TiO2/AuNW electrode was recorded as shown in Fig.
redox features in 1 M NaOH at a scan rate of 40 mV/s (Fig. 4A). 4A which shows oxidation and reduction features of Au at
Cyclic voltammograms of rGO/TiO2/AuNW exhibit a broad much lower potential values similar to that seen in
peak around 0.3 V that corresponds to partial oxidation of Au rGO/TiO2/AuNW. We rationalize that the observed redox
surface accompanied by the reduction of formed oxide at -0.01 feature is possibly due to the migration of electrons from F-
46
V in the reverse scan. Based on the IHOAM model,47 the center of TiO2 to Au that could be beneficial for adsorption
observed characteristic gold redox features, suggests that the of hydroxyl groups. This is also evident from the Au 4f spectra
ternary hybrid could be active towards ethanol oxidation. We in the samples containing TiO2 (Fig. 3C, for instance). This
note that the inherent inertness of Au due to deep d-band implies that TiO2 has an electronic effect on Au that drives the
position6 towards chemisorption of oxygenated species is by- formation of incipient hydrous oxide at low potentials that can
passed here with the presence of TiO2 in the support. To act as a mediator for ethanol oxidation (EOX).
validate the role of TiO2, rGO/Au nanowires with comparable
In the case of rGO/AuNW, active Au redox surfaces are only
ratio of Au to carbon was employed for comparison. The obtained after higher positive potential scan, which can be
voltammograms in this case are featureless while cycling up to considered to be a kind of activation in order to form
0.6 V as shown in Fig. 4A. Moreover, when repeatedly oxygenated species on the Au surface. The presence of
scanning between -0.3 V to 0.6 V for multiple cycles, there is oleylamine bound to the nanowires surface hampers the
no reduction peak of Au oxide in the reverse scan indicating adsorption of OH¯. The amine forms a complex with Au in
that absence of Au redox reactions in this case. However, basic solution that can be oxidized only at high positive
increasing the cycling potential range to +1 V for a few cycles potential.47 Also, the inherent hydrophobic nature of
leads to redox features as manifested in the form oxidation oleylamine prevents the effective adsorption of OH¯ from the
and reduction peaks at 0.3 V and -0.01 V, respectively. Once electrolyte.50 The origin of the activation at a higher potential
the redox features start to appear, they remain even when the is possibly due to the oxidation of the undesirable oleylamine
subsequent cycling range is reduced back to 0.6 V. This complex that leads to the appearance of the redox features in
observation implies that high positive potential is initially the following cycles.47, 30
needed to possibly pre-activate the Au surface.
High resolution XPS was used to investigate the chemical
change on the rGO/AuNW electrodes scanned to different final
positive potentials. The O 1s XPS spectrum of rGO/AuNW
dipped in the electrolyte (1 M NaOH and 1.5 M ethanol
solution) before any electrochemical cycling is displayed in Fig.
S10. In this case, the oxygen spectrum displays a slightly higher
content of -C=O species as compared to the as-synthesized
hybrid nanowires. The oxygen containing amide (carbonyl)
group, dramatically increased by scanning the electrode to 0.6
V. Further increase in the potential to 1 V resulted in a
decrease in the amide groups while the relative ratios of the
other oxygen species are similar to the as-synthesized hybrid.
The change in the amount of the amide after scanning to 1 V
accounts for the very different catalytic behavior and redox
process of the hybrid (Fig. S10). The presence of the amide
may block OH¯ adsorption and impede the redox process of
Au. The oxidation of the amide by sweeping to high positive
potential allows the OH¯ adsorption on Au and is the key
mechanism for the activation of the hybrid.
Ethanol oxidation (EOX) efficiencies of the hybrids were
investigated in an alkaline medium. The ethanol oxidation
peak on rGO/TiO2/AuNW reaches a maximum at 0.17 V (Fig.
4B) accompanied by a peak in the reverse scan at -0.04 V that
corresponds to the removal of carbonaceous species that are
formed due to the incomplete oxidation in the forward scan.51
As shown in Fig. 4B, significant differences in the oxidation
current density between the ternary and binary (TA) catalysts
are observed highlighting the role of supports. Compared with
TiO2/AuNW, the rGO-supported AuNW showed remarkably
higher oxidation current, which is directly related to the
improvement in the conductivity. To further validate this, we
carried out electrochemical impedance spectroscopy (EIS) to
determine the interfacial charge resistance influenced by the
Fig. 4 (A) Cyclic voltammograms in 1 M NaOH of rGO/TiO2/AuNW (GTA), rGO/AuNW
(GA), TiO2/AuNW (TA) electrodes. Nanowires without TiO2 nanoparticles (GA) needs
high positive potential to exhibit characteristics Au redox features. (B) CVs of supported
nanowires measured in 1 M NaOH containing 1.5 M ethanol at a scan rate of 40 mV/s
showing support-dependent activity. The presence of conducting rGO support leads to
a higher oxidation current. (C) Nyquist plot at different applied potentials showing that
charge transfer resistances are relatively low for AuNW grown on conducting support
in 1.5 M ethanol and 1 M NaOH. Hollow and solid symbols represent GA and TA,
respectively. (D) CVs measured in 1 M NaOH containing 1.5 M ethanol for pre-activated
GA electrode showing negligible EOX current after dipping the electrode in hexane
containing oleylamine. After cycling to high positive potential for several cycles, the
same electrode exhibited activity. Legend representation: G-rGO, T-TiO2 and A-Au.
To obtain further insights into the role played by TiO2 in the
ternary catalyst, we prepared TiO2/AuNW hybrids. The CV
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different supports during ethanol oxidation. Fig. 4C shows the negligible drop in activity over 100 cycles of test (Fig. S12). It
representative Nyquist complex plane impedance spectra at should be pointed out that our method to grow ultrathin
different applied potentials; charge transfer resistance is nanowires on support overcomes the inherent instability of
notably lower for rGO-supported nanowires (275
Ω
for wires in polar medium like ethanol.29 One of the other key
rGO/TiO2/AuNW as compared to 1050 Ω for TiO2/AuNW). This drawbacks in EOX is the dramatic decrease in the kinetics of
result indicates the beneficial role of rGO integration in the oxidation owing to the presence of intermediates like CO that
catalyst as confirmed from the CV results as well. The absence compete with ethanol for adsorption. An experiment was
of ethanol oxidation peak with just rGO/TiO2 support validates conducted to evaluate the electrocatalytic oxidation of CO by
that the observed electrocatalytic activity stems from the Au means of CO stripping test (Fig. S13). Most of the previous
nanowires (Fig. S11). Thus both TiO2 nanoparticles and rGO studies suggest that the presence of TiO2 nanoparticles would
add their specific beneficial effect to the AuNW, leading to a increase OH¯ species near the vicinity of the Au surface, which
superior ternary system. Further tests showed that the activity leads to an increased CO oxidation activity via bifunctional
of rGO/AuNW (without TiO2 nanoparticles) was negligible mechanism.52, 53 This is apparently not the case in our present
without activation at a high positive potential. The activated systems, as our CO stripping experiments (Fig. S13) clearly
samples show a dramatic reduction in the current density after revealed that supported Au nanowires exhibited less current
dipping the electrode in hexane containing oleylamine further density when compared to reported Au nanoparticles.54, 55 This
confirming the detrimental role of OA for electrooxidation (Fig. observation coupled with the fact that the EOX current does
4D). Upon potential cycling to 1 V for several sweeps, the not decrease implies that CO adsorption does not poison the
catalyst exhibited EOX oxidation activity (Fig. 4D). We note catalyst in the present case.
that there is no morphological change in the supported wires
that was observed even after high potential scan in contrast to
ultrathin Au nanowires that were drop-cast on the substrate
(Fig. 5). This clearly indicates the beneficial role of the direct
growth of the wires on the substrate that leads to good
binding and higher stability of the nanowires.
Fig. 6 (A) CVs of rGO/TiO2/Au nanoparticles (GTA NP) and TiO2/Au nanoparticles (TA
NP) in 1 M NaOH containing 1.5 M ethanol at the scan rate of 40 mV/s showing that
the current density that are significantly lower as compared with nanowire hybrids.
(B) Tolerance test to avoid cross-over effects. CV curves of rGO/AuNW recorded in 1
M NaOH containing 1.5 M ethanol in the absence (a) and presence (b) of H2O2 (0.4
M).
The cross-over effect of supported nanowires with
respect to H2O2 oxidant was assessed through CV
Fig. 5 TEM and SEM micrographs of rGO/AuNW (A) and bare AuNW (B) recorded after
measurements. Membrane fuel cells employing H2O2 as
oxidant are received increasing attention because of their high
theoretical voltages. It is well-documented that H2O2 can
oxidize on Au surface through a free-radical chain reaction.56
Considering the fact that interaction of H2O2 with AuNW is
likely to decrease ethanol oxidation current in the presence of
H2O2, we examined the crossover effects of H2O2 during
ethanol oxidation reaction. Surprisingly, we observed an
increase in ethanol oxidation current for rGO/AuNW in the
cycling to 1 V in 1 M NaOH containing 1.5 M ethanol indicating that supported
nanowires are stable during electrocatalysis reactions while drop-cast nanowires
disintegrate into nanoparticles.
The impact of Au morphology (wire) on electrocatalytic
activity was studied; the activity of rGO/TiO2/Au nanoparticles,
and TiO2/Au nanoparticles was compared with the observed
activity of the supported nanowire hybrids. The supported
nanoparticles displayed current density that was atleast 50%
smaller than that of nanowire hybrids as shown in Fig. 6A.
Moreover, as was the case with the nanowire hybrids, it is
observed that the rGO-supported nanoparticles possessed
better activity compared to the nanoparticle hybrids without
rGO.
57
presence of H2O2 (Fig. 6B), coupled with existence of
oxidation peak of H2O2 at -0.11 V in the forward scan. This
result demonstrates that nanowire hybrid possesses enhanced
EOX with negligible poisoning effects, which is crucial for
developing direct ethanol-hydrogen peroxide fuel cells.
The long-term stability, the minimization of cross-over and
poisoning effects are critical for the use of the hybrids for real
applications. The ternary rGO/TiO2/AuNW exhibited good
stability in ethanol oxidation over multiple cycles with a
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Photo-assisted electrocatalytic activities
Considering that TiO2 is a traditional photocatalyst, we coupled
the effect of solar light irradiation on TiO2 with the
electrocatalytic activity of ultrathin nanowire. The effect of
irradiation on the electrooxidation current using TiO2 was
investigated. A significant increase in the current with solar
light illumination (as shown in Fig. 7A), indicates that the
photogenerated charge carriers are consumed for ethanol
oxidation. Furthermore, as anticipated, a higher oxidation
current was observed for TiO2/AuNW under illumination as
shown in Fig. 7B. Importantly, the magnitude of the
photocurrent density is higher than the algebraic sum of
ethanol photo-oxidation current of TiO2 and electrocatalytic
current of TiO2/AuNW. It is well documented that under
illumination electron and hole pairs are generated in TiO2
(reaction 1)
TiO2 + h
ꢀ
ꢀ
ꢀ
ꢀ
eCB¯ + hVB
+
(1)
(2)
(3)
C2H5OH + •OH (OH + h+)
•CH(OH)CH3 +11•OH
•CH(OH)CH3 + H2O
2 CO2 + 8 H2O
Fig. 7 CVs of synthesized catalyst recorded in 1 M NaOH containing 1.5 M ethanol at a
scan rate of 40 mV/s. TiO2 electrode showing (A) significant increase in the
electrooxidation current under irradiation using a xenon lamp. (B) rGO/TiO2/AuNW
exhibits higher photo-assisted electroactivity than TiO2/AuNW showing the influence of
the conducting GO substrate. TA and GTA indicate TiO2/AuNW and rGO/TiO2/AuNW,
respectively. (C) Au nanoparticles nucleated on unmodified TiO2 (TA NP) displays
marked enhancement in photocurrent with considerable potential shift while Au
nanoparticles nucleated on oleylamine modified TiO2 (mTA NP) exhibit low
photoactivity. (D) Summary of the electro and photo activity of the hybrids evaluated
by CV measurements. Amine effect was nullified by incorporated rGO, which facilitates
the fast electron transfer.
Since the electron affinity of TiO2 is lower than the Fermi
level of Au, photo-excited electron transfer from conduction
band of TiO2 to Au is energetically favorable (Scheme 1)58 and
an effective charge separation is achieved. The photo-
generated holes promote further oxidation of ethanol via
hydroxyl radicals (reaction 2).59 Another frequently occurring
phenomenon in alcohol photo-oxidation is that of ‘current
doubling’ (reaction 3) whereby oxidation of α-hydroxyethyl
intermediate radical proceeds with constant injection of
electron to conduction band of TiO2.60 These electrons along
with F-center electrons stimulate the adsorption of hydroxyl
group on the Au surface (Scheme 1) that enhances the
electrocatalytic activity of nanowires. Thus, the enhanced
current density is attributed to synergetic effect of photo-
assisted electrocatalytic activity of nanowires.
To examine the role of morphology of Au on
photoelectrocatalytic activity, we compare the activity of
TiO2/AuNW with TiO2/Au nanoparticles. The Au nanoparticle
hybrid was synthesized without surface modification of TiO2
with amine, unlike the growth of ultrathin Au nanowires on
amine-modified TiO2. TEM micrographs of supported Au
nanoparticles are shown in Fig. S15. The electrocatalytic
oxidation current density of TiO2/Au nanoparticles is only 1.7
mA/cm2, but under illumination shows a 3-fold increase (Fig.
7C) and is higher than the photoresponse of nanowire hybrids.
In addition, the characteristic oxidation EOX peak for TiO2/Au
nanoparticles was shifted to much lower potential with light as
marked in the Fig. 7C, the smallest potential reported so far.51
Similarly, the oxidation of intermediate carbonates also
occurred at higher positive potential in the reverse scan. We
hypothesize the observed inferior photoresponse in the case
of hybrid nanowire, may be due to the modification of the TiO2
support with the amine. Indeed, investigations have
demonstrated that the charge injection between metal oxide
and metal is directly affected by the presence of interface
dipoles induced by ligands like amine.64, 65
The introduction of rGO support significantly increases the
generated photocurrent throughout the potential region
(Fig.7B). The higher photo-electrocatalytic activity of ternary
rGO/TiO2/AuNW observed in this study can be attributed to
the enhancement of charge separation in the presence of rGO
that extends the lifetime of the active species61,62 and a
possible photosensitizing effect of rGO on TiO2 that can
increase the photon flux by absorbing a wide range of the light
spectrum63 (Fig. S14). Thus, the TiO2 and rGO add their specific
benefits to AuNW, leading to a superior performance.
To inspect the electronic interaction between TiO2 and
amine, we nucleated Au nanoparticles on an amine-modified
TiO2 similar to that used for nanowire growth (for detailed
synthesis steps, please see Experimental Section) and
employed this as the photoanode (Fig. 7C). The electro and
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photo-catalytic activity towards ethanol oxidation was rGO opens up path for the transfer of photogenerated charge
dramatically reduced as compared to unmodified TiO2/Au carriers either to electrolyte or Au surface through rGO (as
nanoparticles (as shown in Fig. 7D). In addition, this is also shown in Scheme 1). Considering the work function levels, the
reflected in I-V measurements, which displays drastic decrease presence of rGO suggests that there would be a dynamic
in photo and electrocurrent of TiO2 after amine modification electron (photogenerated) transfer from TiO2 to Au,20 leading
(as shown in Fig. 8A). Further, the amine grafting was verified to net separation of photogenerated charge carriers as shown
with XPS analysis; deconvoluted N 1s spectra of modified TiO2 in Scheme 1. These findings highlight that direct interference
displays peaks at 398.8 and 400.3 eV (Fig. 8B), which are of rGO, involved in the interfacial charge transport is likely to
associated with nitrogen atoms in oleylamine and boost the photoassisted electrocatalytic performance (as
oleylammonium cation.41 These findings highlight that amine- shown in the Scheme 1).
grafted TiO2 reduce the electro and photoconductivity unlike
the increase that happens in the case of ZnO nanorods.30 The
above results highlight that while amine modification of TiO2 is
essential to grow ultrathin AuNW, the presence of the amine
reduces the overall photoresponse of the hybrid.
Fig.
8 (A) Photocurrent of TiO2 with and without amine modification upon solar
Scheme 1. Illustration depicts the effect of amine and rGO on interfacial photo-induced
charge transfer in both binary (TiO2/AuNW) and ternary system (rGO/TiO2/AuNW).
illumination (Xenon lamp), amine grafting deactivates the electro and photo
conductivities of TiO2. (B) Deconvoluted N 1s spectra of oleylamine modified TiO2,
showing the existence of nitrogen species.
Conclusions
In our earlier report, we have shown that amine modifies
the surface of ZnO nanorods by passivating the surface
dangling bonds that in turn enhances the photo-current and
catalytic activity.30 However, the amine has a detrimental
effect on the photocurrent and catalytic activity of TiO2. The
above result suggests that the charge transfer scenario is
different in ZnO nanorods and TiO2 nanoparticles after amine
modification. Based on the literature reports for the effects of
surface modification on metal oxides, it appears that long
chain ligand grafting leads to two counter effects.66 Firstly, the
linked ligands can permanently passivate the surface
defect/shield the recombination centers, resulting in an
enhancement in the photoactivity by suppressing non-
radiative recombination.67 The second effect is a dipole effect
wherein long chain molecules at the oxide surface create fields
that hinder the charge transfer process at the interface. The
former effect dominates in 1-D ZnO because single crystalline
long nanorods are known for efficient charge transporter, so it
overwhelms the later effect.68, 69 However, in the present case,
the TiO2 nanoparticles are not interconnected and thus the
interfacial charge transfer is deteriorated. As complied in Fig.
7D, the comparative analysis of photo and electrocatalytic
activity between the different binary and ternary hybrids
provides interesting information. The effect of amine is not so
pronounced in the ternary hybrid when rGO is present, as
shown in Fig. 7B. This clearly indicates that the presence of
In conclusion, we have developed
a new type of
multidimensional 0-D TiO2, 1-D AuNW and 2-D rGO ternary
hybrid electrocatalyst by controlled growth of ultrathin Au
nanowires on rGO sheet in contact with TiO2 nanoparticles.
The amine molecules involved in the reaction not only assist
the dense growth of ultrathin Au nanowires on TiO2 /GO
sheets but also stimulate the room temperature reduction of
GO. Our investigation reveals that the electrochemical activity
rests on the synergy between integral constituents of the
catalyst. Specifically, the change in Au electronic structure by
the migrated electrons from TiO2 favors generation of
oxygenated species at less positive potential that promotes
the alcohol oxidation. Systematic study of support reveals that
rGO reduces the charge transfer resistance of ternary
electrode and also stabilizes the nanowires during
performance. In addition to the stability, inherent
optoelectronic properties of the support synergistically
enhance the performances of nanowires. The incorporation of
rGO is also mandatory to nullify the negative effect of amine
on TiO2 especially during photo-assisted electrocatalytic
oxidation. The above-mentioned insights clearly point the
direction for synthesizing hybrids with enhanced performance
for various applications beyond fuel cell applications.
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Amine grafted TiO2 (30 mg) was taken in 5 mL of ethanol, 5
mg of HAuCl4 was added. This was followed by drop-wise
addition of sodium borohydride (dissolved in ethanol) to the
above solution under mild stirring. Then the solution was
centrifuged and washed several times with ethanol.
Experimental
Synthesis of GO/TiO2
The graphene oxide (GO) was first prepared based on the
modified Hummer’s method.22 For the microwave synthesis of
GO/TiO2 hybrid, 10 mg of GO was sonicated in 30 mL of de-
ionized water for 20 min to achieve a homogeneous
dispersion. Titanyl nitrate (TiO(NO3)2) which was prepared by
dissolving titanium isopropoxide (250 μL) in a few drops of
concentrated nitric acid was added to the above GO
dispersion, followed by addition of 1 M NaOH (1 mL). Finally,
the suspension was sonicated for 5 min and then subjected to
microwave (2.45 GHz, 300 W) for 3 min. After the reaction, the
suspension was cooled down to room temperature in ambient
conditions. The products were centrifuged and washed with
Characterizations
The microstructure of the synthesized samples were
characterized by transmission electron microscopy (TEM, FEI
Technai T20) operated at accelerating voltages of 200 kV. The
samples were dispersed in ethanol and drop casted on carbon
coated Cu-grids. The scanning electron microscopy (SEM)
images were acquired using field emission SEM, Ultra55 Carl
Zeiss. XRD was performed on a Phillips X’pert diffractometer
using Cu Kα radiation. The chemical analysis were analyzed by
using X-ray photoelectron spectroscopy (XPS, Axis Ultra) with
monochromatic source of Al Kα radiation and graphitic carbon
peak at 284.5 eV was used for charge correction.
distilled water for several times, finally dried at 100 ⁰C.
Formation of supported nanowire hybrids
To obtain ternary rGO/TiO2/AuNW, the above synthesized
GO/TiO2 was introduced into the gold nanowires growth
solution (containing appropriate amounts of HAuCl4 (3 mg),
oleylamine (100 μL) and tetraisopropylsilane (150 μL) in 2.5 mL
of hexane).38 The mixture was sonicated for 1 min and then
incubated for 6 h at room temperature. The end product was
cleaned with hexane and ethanol to removal undesired loosely
bound ultrathin nanowires along with unreacted precursors, if
present.
Photoelectrochemical measurements
The electrochemical properties were examined by cyclic
voltammetry (CV) as well as with CO stripping voltammetry
using three electrode cell. A platinum foil was used as a
counter electrode and saturated calomel electrode (SCE) as
reference. The working electrodes were prepared by drop
casting known concentration of hexane stored supported
nanowires on Toray paper. The prepared electrodes were
dried overnight at room temperatures before electrocatalytic
tests. All the experiments were carried out at room
temperatures in NaOH (1 M) either with or without ethanol as
electrolyte solution. CVs of the electrodes were measured with
and without light irradiation. A Xenon lamp of 100 W was used
as light source. For CO stripping experiments, the electrolyte
solution was saturated with CO (99.99%, CHEMIX gases-
Bangalore, India) by bubbling CO for 30 min through the
solution. Subsequently, the electrodes were scanned to obtain
the corresponding CO stripping CVs. All the experiments were
conducted at room temperature and performed by
potentiostat (PG26250, Techno Science Instrument).
rGO/AuNW was prepared by adding GO into gold
nanowires growth solution and left undisturbed for 24 h. The
obtained product was washed with hexane and ethanol, to
remove unreacted reagents. As-synthesized rGO/AuNW were
stored in hexane for future characterization and
electrochemical tests.
To synthesize TiO2/AuNW, initially anatase TiO2
nanoparticles were prepared by sol-gel combustion method
based on our earlier report.70 To grow ultrathin AuNW on TiO2,
solid state mixing was adopted to modify the surface of TiO2
with oleylamine. 50 μL of oleylamine was mixed with TiO2 (100
mg) using mortar and pestle, the obtained product was
cleaned with hexane and ethanol mixture to remove unbound
oleylamine.30 Finally, the amine modified TiO2 was dried and
introduced into the Au nanowires growth solution. TEM
micrographs revealed that without surface modification of
TiO2 leads to primarily larger Au nanoparticles, which implies
that amine modification is required for the growth of
nanowires on TiO2 substrate.
Electrochemical impedance spectroscopy (EIS) measurements
EIS measurements were carried out using three electrode
system, CHI 660D electrochemical workstation. The spectra
were collected using an AC signal of 10 mV from 100 kHz to 10
MHz.
Photoconductivity measurements
Synthesis of nanoparticle hybrid
Photocurrent I-V measurements, were performed using 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. An ITO coated glass with a non-conducting gap
(50 µm) was adopted as an electrode. The samples were
dispersed in ethanol and drop casted on the gap between ITO
contacts.
30 mg of TiO2 was sonicated for 15 min in 30 mL of water to
obtain homogeneous dispersion and 5 mg of HAuCl4 was
added. Then the reaction mixture was subjected to microwave
for subjected to microwave (2.45 GHz, 800 W) for 3 min.71 The
resulting products were washed several times with distilled
water and then dried at 100 ⁰C for further use. Under identical
conditions instead of TiO2, GO/TiO2 was used to obtain
rGO/TiO2/Au nanoparticles.
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ARTICLE
27. C. Koenigsmann, A. C. Santulli, K. Gong, M. B. Vukmirovic,
W.-p. Zhou, E. Sutter, S. S. Wong and R. R. Adzic, J. Am.
Chem. Soc., 2011, 133, 9783-9795.
28. C. Koenigsmann, W.-p. Zhou, R. R. Adzic, E. Sutter and S. S.
Wong, Nano Lett., 2010, 10, 2806-2811.
29. E. Koposova, A. Kisner, G. Shumilova, Y. Ermolenko, A.
Acknowledgements
We thank Prof. Praveen C. Ramamurthy for impedance
measurements facility. The Tecnai T20 is a part of Advanced
facility for Microscopy and Microanalysis at IISc. The XPS and
solar simulator are part of MNCF, CeNSE, IISc. NR
acknowledges financial support from the TUE and
Swarnajayanti Fellowship from DST.
Offenhäusser and Y. Mourzina, J. Phys. Chem. C, 2013, 117
13944-13951.
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Page 11 of 11
Journal of Materials Chemistry A
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DOI: 10.1039/C5TA03988F
Graphical Abstract
A method to grow ultrathin Au nanowires and metal oxide on reduced graphene
oxide (rGO) to produce a hybrid that exhibits good activity for ethanol oxidation.