Facile synthesis of Pt nanoparticles supported on anatase TiO2 nanotubes with good photo-electrocatalysis performance for methanol

Article abstract of DOI:10.1039/c7ra11564d

A simple, high-yield and rinsing-free method is used in this work to prepare Pt nanoparticles with a size of approximately 2 nm, which are successfully supported on free-standing TiO₂ nanotubes (TNTs) synthesized by a simple two-step anodization method with titanium metal foil. Field-emission scanning electron microscopy images indicate that in the case of 30 V for 90 min, the inner diameter of as-synthesized TNTs is 60-70 nm and the length is approximately 1.9 μm. Electrochemical experimental results clarify that the Pt/TNTs/C catalyst has higher electrochemically active specific surface areas and better catalytic activity toward methanol electro-oxidation compared with the Pt/C and Pt/TNTs catalysts. What's more important, catalytic activity of Pt/TNTs/C is drastically enhanced under light illumination, with the mass activity of 525.0 mA mg_Pt^-1, 1.47 times higher than that in the dark (357.4 mA mg_Pt^-1). The improved activity is mainly derived from the synergistic effects between the electro-catalysis on Pt nanoparticles and the photo-catalysis of TiO₂ nanotubes. In this work, the improved photo-electrocatalysis performance of the Pt/TNTs/C catalyst is further analyzed and the findings here are of significance in providing a new strategy to fabricate Pt-based photo-electro catalysts for the methanol oxidation reaction.

Full text of DOI:10.1039/c7ra11564d

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PAPER
Facile synthesis of Pt nanoparticles supported on
anatase TiO2 nanotubes with good photo-
electrocatalysis performance for methanol†
Cite this: RSC Adv., 2017, 7, 56194
abc
Jianbo Zhang,‡abc Nan Su,‡abc Xiulan Hu,
Faquan Zhu,abc Yawei Yuabc
*
and Hui Yangabc
A simple, high-yield and rinsing-free method is used in this work to prepare Pt nanoparticles with a size of
approximately 2 nm, which are successfully supported on free-standing TiO2 nanotubes (TNTs) synthesized
by a simple two-step anodization method with titanium metal foil. Field-emission scanning electron
microscopy images indicate that in the case of 30 V for 90 min, the inner diameter of as-synthesized
TNTs is 60–70 nm and the length is approximately 1.9 mm. Electrochemical experimental results clarify
that the Pt/TNTs/C catalyst has higher electrochemically active specific surface areas and better catalytic
activity toward methanol electro-oxidation compared with the Pt/C and Pt/TNTs catalysts. What's more
important, catalytic activity of Pt/TNTs/C is drastically enhanced under light illumination, with the mass
activity of 525.0 mA mgPtꢀ1, 1.47 times higher than that in the dark (357.4 mA mgPtꢀ1). The improved
activity is mainly derived from the synergistic effects between the electro-catalysis on Pt nanoparticles
and the photo-catalysis of TiO2 nanotubes. In this work, the improved photo-electrocatalysis
performance of the Pt/TNTs/C catalyst is further analyzed and the findings here are of significance in
providing a new strategy to fabricate Pt-based photo-electro catalysts for the methanol oxidation reaction.
Received 19th October 2017
Accepted 6th December 2017
DOI: 10.1039/c7ra11564d
rsc.li/rsc-advances
result in higher catalytic activities and stabilities of Pt-based
catalysts during the MOR process. Due to the abundant –OH
1. Introduction
species adsorbed on the metal oxides and the strong support-
metal interaction, these oxides can mitigate the poisoning of
Pt catalysts by CO-like intermediate species during the MOR
process and further enhance the MOR activity.
Direct methanol fuel cells (DMFCs) have attracted much
attention as power sources for portable devices and fuel-cell
vehicles due to their good power efficiency, low operation
temperature, and easy transportation and storage of methanol
compared with H2 gas fuel cells.1 Pt-based catalysts supported
on carbons play a signicantly important role in catalyzing
methanol oxidation reaction (MOR).2 However, there are still
a lot of challenges facing the wide commercialization of the
applications of Pt-based catalysts, such as high cost, low cata-
lytic efficiency and poisoning effect.3–5 Recently, various kinds of
electrocatalysts were developed to enhance the MOR perfor-
mance, among which, metal oxides promoted Pt-based catalysts
have been considered to be quite active. It was reported that the
involvement of metal oxides (such as TiO2,6–14 ZnO,15–17
SnO2,18–22 WO3,1,23–26 Fe2O3 (ref. 27–29) and Co3O4,30,31 etc.) can
The photoresponsive semiconductor metal oxides, among
these oxides mentioned above, are of great inter-
ests.15,16,21,23,26,29,32–44 The anodic catalysts incorporating photo-
responsive semiconductor evidently improve MOR peak current
density in the presence of UV irradiation compared with those
traditional Pt catalysts. Hybrid TiO2/CF/Pt–Ru catalyst (CF,
carbon ber) for methanol electro-oxidation was developed to
boost the performance of DMFCs in the presence of UV.32 The
atomic layer deposition (ALD), a kind of in situ synthesis
method which can cause strong interface interaction, was used
to combine the ZnO nanorods with Pt nanoparticles on the
carbon cloth. And the Pt@ZnO/C electrode exhibited 90%
higher methanol oxidation activity than commercial Pt cata-
lysts.15 Especially, the graphene nanosheets (GNs) or reduced
graphene oxide (RGO) were considered as effective materials to
extend light adsorption range and enhance photoresponsive
effect of these semiconductor metal oxides by the formation of
graphene-semiconductor nanostructure.16,21,37,38,40,45 But the
increasing activity of the catalysts is at the expense of the
cumbersome and risky synthesis process and high cost of
aCollege of Materials Science and Engineering, Nanjing Tech University, Xin-Mo-Fan
Road No. 5, 210009, Nanjing, Jiangsu, China. E-mail: whoxiulan@163.com; Tel:
+86 152 4022 7230
bThe Synergetic Innovation Center for Advanced Materials, Xin-Mo-Fan Road No. 5,
210009, Nanjing, Jiangsu, China
cJiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Xin-
Mo-Fan Road No. 5, 210009, Nanjing, Jiangsu, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra11564d
‡ Nan Su and Jianbo Zhang are co-rst authors.
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ꢂ
graphene materials, which might be relatively unfavourable in residual ethylene glycol and NH4F, and then dried at 60 C in
the application.
air. Therefore, the as-obtained TiO2 nanotubes powder was
It was reported that anodization of titanium metal foil to dened as TNTs.
synthesis TiO2 nanotubes is a simple way to obtain a high yield
of products with vertical orientation arranged nanotube arrays.
Nanostructured TiO2 has great applicative potential in photo-
electrochemical devices46–51 due to the large effective light-
harvesting interface area and wide band gap. As is known to
all, the TiO2 is a kind of excellent photoresponsive material.
What's more, the nanotube-like TiO2 materials with larger
specic surface area and abundant surface oxygen locus to
generate –OH species were of great hot topics of photo-
electrocatalyst. In this paper, TiO2 nanotubes were chosen as
the support material of Pt nanoparticles, in order to enhance
their MOR activity under the light illumination.
Noble metal nanoparticles are widely used as electrocatalysts
and photocatalysts, and recent years of intense efforts have seen
an increasing interest in heterogeneous nanocomposites, which
usually exhibit enhanced catalytic properties due to the syner-
gistic effect among their different domains.52 The heteroge-
neously structured nanomaterials usually exhibit enhanced
2.2 Synthesis of Pt nanoparticles
The method, synthesis of Pt nanoparticles by plasma sputtering
in water, was similar to our previous works.55–57,60 Fig. S1b†
shows the experiment setup for this method. Generally
speaking, the Pt metal wires (F ¼ 1.0 mm) were served as
opposite electrodes, and immersed into deionized water. The
gap between two opposite electrodes was kept at about 0.3 mm.
Discharge was generated using a high voltage pulsed DC power
supply (repetition frequency: 10–20 kHz, pulse width: 1–2 ms,
Kurita Co. Ltd., Japan). The water gradually changed from being
colorless to dark grey with increasing discharge time. The yields
of Pt nanoparticle can be controlled by adjusting the discharge
time in this study. The as-synthesized Pt nanoparticles were
well-dispersed in water.
2.3 Preparation of Pt/TNTs/C catalyst
catalytic properties in comparison with each one of the The Pt-based catalysts were prepared by step-wise mechanical
constituent materials due to the synergistic effect among their mixing process in deionized water. Firstly, 16 mg TNTs were put
different domains.53
into the suspension containing 8 mg Pt nanoparticles with
In this present work, the free-standing TiO2 nanotubes were ultrasonic dispersion for 90 min to obtain homogeneous
synthesized by a simple two-step anodization method with suspension of Pt/TNTs mixture. Secondly, a well-dispersed XC-
titanium metal foils and NH4F in ethylene glycol. A simple, 72 suspension was put into the homogeneous suspension of
high-yield and rinsing-free method, preparation of Pt nano- Pt/TNTs mixture with stirring vigorously for 30 min and
particles with plasma sputtering in water,54–58 was used. The subsequently treated with ultrasound bath for 60 min to obtain
electrochemical results indicated that TiO2 nanotubes sup- homogeneous suspension of Pt/TNTs/C mixture. Then this
porting Pt catalysts (Pt/C/TNTs) expressed a promoted activity mixture was dried at 45 ꢂC in air to obtain Pt/TNTs/C composite.
toward methanol electro-oxidation under light illumination.
The mass ratio of Pt nanoparticles, TNTs and XC-72 carbon was
xed to 1 : 2 : 4 (Pt loading of 14.3 wt%). And the Pt/TNTs and
Pt/C composites with the Pt loading of 20 wt% were prepared as
the same procedure mentioned above.
2. Experimental
2.1 Preparation of TiO2 nanotubes
A two-step anodization method59 was used in this work to
prepare well-organized free-standing TiO2 nanotubes
(Fig. S1a†). Prior to carrying out anodization, Ti foils (2 ꢁ 3 ꢁ
0.05 cm) were degreased by ultrasonic treatment in acetone,
ethanol and deionized water for 15 min, respectively, and then
dried at 60 ꢂC in air. Anodization was carried out in a two-
electrode system with a counter of stainless steel sheet.
0.5 wt% NH4F (Sinopharm Chemical Reagent Co., Ltd) in
ethylene glycol (Sinopharm Chemical Reagent Co., Ltd) with
3 vol% H2O was used as electrolyte for anodization. The rst
step of anodization on Ti foils was conducted under 30 V for
90 min. Aer anodization, samples were washed with deionized
water and dried at 60 ꢂC in air, then annealed at 450 ꢂC for 2 h in
air atmosphere and cooled slowly together with the furnace to
2.4 Characterization
The X-ray diffraction (XRD) measurements were carried out by
a Rigaka Smartlab system with Cu Ka (l ¼ 1.5418 A) radiation
˚
operated at 30 kV and 40 mA to evaluated the crystal phase and
the scanning rate of measurements was 8ꢂ minꢀ1. The
morphology and structure of samples were studied by a eld-
emission scanning electron microscopy (FE-SEM, ZEISS Ultra
55) and a transmission electron microscopy (TEM, JEOL JEM-
2100F). X-ray photoelectron spectroscopy (XPS, Kratos AXIS
ULTR DLD) with an Al X-ray source was carried out to detect the
surface properties of the samples.
2.5 Electro- and photo-electrochemical measurements
room temperature. The second step of anodization on the 6 mg Pt/TNTs/C composite was ultrasonically dispersed in 2 mL
annealed Ti foils was carried out under 30 V for 30 min in the deionized water containing 20 mL Naon® solution (5 wt%,
same electrolyte, in order to obtain free-standing TiO2 nanotube DuPont, USA) for at least 30 min to form a homogeneous
membrane. Aer the second-step anodization, the TiO2 nano- catalyst ink. The Pt/TNTs and Pt/C catalysts ink were also
tube membrane detached from the Ti foil was treated with an prepared with 4.3 mg composites and 2 mL deionized water
ultrasonic bath for 5 min. Subsequently the free-standing TNTs containing 20 mL Naon® solution. Electrochemical measure-
were cleaned with centrifugation for three times to remove the ments were carried out in a three-electrode electrochemical cell
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at 25 ꢂC. A piece of Pt foil (1 cm2) and an Ag/AgCl electrode in situ light emission spectra, via solution plasma sputtering.
(3.5 mol dmꢀ3 KCl, 0.205 V relative to standard hydrogen elec- Briey, Pt atoms were ejected into the plasma region from their
trode, SHE) were used as the counter and reference electrodes, electrodes by the bombardment of highly energetic electrons
respectively. The working electrode was prepared as follows. A and radical particles (such as cOH, cH, O, O2ꢀ, and HO2). Since
glassy carbon electrode (GCE, F ¼ 3 mm, geometrical area of the surfaces of Pt electrodes pair were bombarded continuously,
0.07 cm2) which was polished with 0.03 mm alumina suspen- Pt nanoparticles were formed in water by diffusion, growth, and
sions to a mirror surface was served as an underlying substrate condensation. And the size of as-prepared Pt nanoparticles is
for the working electrode. 6 mL of three kinds of catalyst inks approximately 2–5 nm, on the basis of our previous TEM
was pipetted onto the GCE, respectively. The loading of Pt on measurements.55,61
the GCE was 2.57 mg. A solution of 0.5 mol dmꢀ3 H2SO4 purged
Fig. S2† shows the XRD patterns of two Ti foils, which were
with nitrogen gas for 40 min to remove dissolved oxygen served directly obtained aer the rst anodization step and obtained
as the electrolyte for cyclic voltammetry (CV) measurements. aer annealing process, respectively. For both patterns (a) and
The CV measurements were carried out in the solution of (b) in Fig. S2,† the peaks at 35.1ꢂ, 38.4ꢂ, 40.1ꢂ, 53.0ꢂ, 62.9ꢂ, 70.6ꢂ,
0.5 mol dmꢀ3 H2SO4 (N2 saturated) within a potential range of 76.1ꢂ and 77.4ꢂ can be assigned to the (100), (002), (101), (102),
ꢀ0.2 to 1.2 V at 50 mV sꢀ1. The electro- and photo- (110), (103), (112) and (201) planes of Ti metal (JCPDS card no.
electrocatalytic oxidization of methanol was monitored with 85-2959), respectively. For the pattern (b) only, the peaks at
CV measurements in a solution of 0.5 mol dmꢀ3 H2SO4 con- 25.3ꢂ, 37.8ꢂ, 48.1ꢂ, 53.8ꢂ, 55.0ꢂ and 68.7ꢂ are corresponding to
taining 1 mol dmꢀ3 CH3OH within the potential range of 0– the (101), (004), (200), (105), (211), (116) planes of anatase TiO2
1.0 V. Chronoamperometry (CA) was carried out in 0.5 mol (JCPDS no. 84-1285), respectively. While no signicant diffrac-
dmꢀ3 H2SO4 containing 1.0 mol dmꢀ3 CH3OH at 0.6 V. A 50 W tion peaks of TiO2 were detected in the pattern (a) except for the
Xe lamp (PLS-SXE300UV, Peking Perfectlight Co., China, wave peaks of Ti metal. These results claried only amorphous TiO2
length $ 200 nm) was used in the test of photo-electrocatalytic was formed aer the rst-step anodization. Crystalline anatase
oxidization of methanol. The distance between illuminant with TiO2 was formed aer annealing at 450 ꢂC for 2 h.59 These sharp
working electrode was kept at approximately 15 cm. The cata- characteristic peaks of anatase TiO2 further indicated their
lysts supported on GCEs were pre-conditioned by CV within the good crystallinity.
potential range of ꢀ0.2 to 1.2 V at 10 mV sꢀ1 for 8 full potential
Fig. 2 shows the SEM images of the top view and cross
cycles in 0.5 mol dmꢀ3 H2SO4 (N2 saturated). The all potentials section of as-synthesized TNTs on Ti foil before detaching by
in the present work were expressed versus Ag/AgCl (3.5 mol the second-step anodization. From the top view in Fig. 2a, the
dmꢀ3 KCl). All the electro- and photo-electrochemical TNTs is highly self-organized and with the inner diameter of 60–
measurements were performed on
a Zahner Zennium 70 nm (the inset of Fig. 2a). The gaps between the individual
(ZAHNER-Elektrik GmbH & Co. KG, Germany) electrochemical tubes are formed and nanotubes are incompact and well-
workstation at room temperature.
arranged. The aligned TNTs length shown in Fig. 2b is
approximately 1.9 mm (Fig. 2b) and the external diameter of
TNTs is 78–85 nm (Fig. 2c).
Aer the second anodization step, the as-prepared TNTs
membrane detached from the Ti foil substrate. Fig. 3 exhibits
the XRD patterns of the TNTs powder and the Pt/TNTs
composite. For these two samples, the peaks at 25.3ꢂ, 37.0ꢂ,
3. Results and discussion
3.1 Characterization
Based on the results of our previous work,55 the formation
mechanism of Pt nanoparticles is shown in Fig. 1, indicated by
Fig. 1 Schematic diagram of the formation of Pt nanoparticles by solution plasma sputtering.
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Fig. 2 SEM images of TNTs on Ti foil before detaching by the second-step anodization. (a) Top view, (b) cross section, and (c) high magnification
of the cross section. The inset of (a) is high magnification image of the TNTs top view.
37.8ꢂ, 48.0ꢂ, 53.8ꢂ, 55.0ꢂ, 62.7ꢂ, 68.8ꢂ, 70.3ꢂ, 75.0ꢂ and 82.7ꢂ are 46.2ꢂ, 67.5ꢂ and 81.3ꢂ) were detected. These results agreed
corresponding to the JCPDS card no. 99-0008 of anatase TiO2. In extremely well with those of the JCPDS card no. 65-2868 for Pt,
pattern (b), four relatively weaker diffraction peaks (2q ¼ 39.8ꢂ, demonstrating that the black products formed during solution
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Fig. 3 XRD patterns for the TNTs powder (a) and the Pt/TNTs
composite (b).
plasma sputtering are denitely Pt metal particles. And these
broaden diffraction peaks imply the relatively smaller size of Pt
particles.
Fig. S3† shows the TEM images of the as-obtained TNTs. It
can be clearly seen from the typical TEM image in Fig. S3a† that
the TiO2 sample synthesized by anodization has an obvious
tubular appearance. In the middle part of a single TNT (Fig.-
S3b†), the regular crystal lattice fringes can be clearly detected.
And in the inset of Fig. S3b,† it can be found that the lattice
plane spacing is 0.35 nm, which is highly consistent with the
(101) plane of the anatase TiO2. From the edge part of the TNT
in Fig. S3c,† the disordered structure with the lattice of different
orientations can be clearly observed. Furthermore, the corre-
sponding selected area electron diffraction (SAED) analysis in
the inset of Fig. S3c† indicates the different orientations of TiO2
crystals in this edge part, as well. The disordered small-particle-
like morphology can be further obviously observed at a broken
tip of a single TNT in Fig. S3d,† which could be advantageous to
store electrolyte, providing a quick supply and short diffusion
distance to accelerate the electrocatalytic kinetics.
The morphology of the Pt/TNTs composite was further
determined by TEM in Fig. 4. From the bright eld TEM image
in Fig. 4a, the Pt nanoparticles are arranged in line on the
surface of TNT, while the Pt nanoparticles with the size of 2 nm
could be well-dispersed on the surface of XC-72 carbon without
aggregation.55 Sui et al. have also reported this phenomenon
that the Pt nanoparticles would tend to be arranged in line on
the TNTs' surface.10 In Fig. 4b, it can be distinctly observed that
the arrangement of Pt nanoparticles on TNTs' surface presents
an extension trend from the surface to outward. Furthermore,
Fig. 4 TEM images of the Pt/TNTs composite. (a) Bright field TEM
from the HR-TEM image in Fig. 4c, the Pt nanoparticles on image of Pt/TNTs composite; (b) typical TEM image of the Pt nano-
structure on TNT's surface; (c) HR-TEM image of the Pt nanoparticles.
TNTs' surface with a size range of 2–5 nm present the different
lattice orientations of the (111) plane with the lattice plane
spacing of 0.226 nm. It could be assumed that these Pt nano-
XPS is an efficient method to analyze the chemical state
information of elements. The XPS spectra of Pt/C and Pt/TNTs/C
are shown in Fig. S4.† Fig. S4a† compares the wide XPS surveys
particles in line on TNTs' surface would exhibit outstanding
catalytic ability, but it should be further proved by electro-
chemical measurements.
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for the Pt/C and Pt/TNTs/C composites. The peaks of C 1s and O where QH (millicoulomb, mC) represents the charge due to the
1s can be found at 284.4 eV and 532.4 eV, respectively. The Ti hydrogen adsorption/desorption in the hydrogen region of the
element is clearly detected in the Pt/TNTs/C sample. Further- CVs. The correlation constant of 0.21 (mC cmꢀ2) represents the
more, the peak at approximately 71.3 eV and the paired peaks at charge required to oxidize a monolayer of hydrogen on
315.5 eV & 331.8 eV are corresponded to the Pt 4f and Pt 4d, a smooth surface of polycrystal Pt. MPt (g) represents the Pt
respectively. In the comparison with Pt/C, the distinctly enhanced loading on the working electrode. The ECSA is 266 cm2 mgPt
ꢀ1
Pt signal of Pt/TNTs/C was detected, and the surface Pt to C mass for the Pt/TNTs/C, 1.27 times higher than the value of Pt/C
ratios calculated from XPS spectrum for the Pt/C and Pt/TNTs/C (210 cm2 mgPtꢀ1), demonstrating that the addition of TNTs
samples are 0.09 and 0.183, respectively, indicating that the results in the increase of ECSA of these catalysts. As is in
addition of TNTs causes the Pt nanoparticles to be distributed agreement with the analysis of Pt signals in XPS wide survey
more uniformly on the surface of supporting materials. As (Fig. S4†), this nding could be related to the more amount of
depicted in Fig. S4b,† the binding energies of Pt 4f for the Pt/C exposed Pt nanoparticles at the surface of supporting materials
and Pt/TNTs/C samples are 71.4 eV & 71.55 eV (Pt 4f7/2) and resulted from the tubular structure of TNTs. However, the
74.7 eV & 74.9 eV (Pt 4f5/2), respectively. Compared with the Pt/TNTs sample can hardly show any ECSA for HAD, which may
binding energies of Pt 4f for Pt/C, the values for Pt/TNTs/C present be caused by the poor electric conductivity of bare TiO2.
a slightly positive shi, on account of the relatively strong inter-
Fig. 5b shows the catalytic activity toward methanol electro-
action between Pt and TNTs. And according to the spectra of Pt 4f, oxidation of the Pt/C and Pt/TNTs/C catalysts with CVs in
the intensity of Pt 4f peak for Pt/TNTs/C is denitely stronger than a solution of 0.5 mol dmꢀ3 H2SO4 containing 1.0 mol dmꢀ3
the value of Pt/C, which is in agreement with the observation of CH3OH. The CV curves of the Pt-based catalysts for methanol
wide survey. The Ti 2p XPS spectra of the Pt/TNTs/C sample is electro-oxidation basically includes two anodic peaks, the
shown in Fig. S4c.† The two peaks at 459.6 eV and 465.3 eV can be forward peak and backward peak. The forward peak current
assigned to Ti 2p3/2 and Ti 2p1/2, respectively. The peak separation density (normalized by the loading of Pt metal) dened as the
between Ti 2p3/2 and Ti 2p1/2 lines is 5.7 eV, which is consistent mass activity that is normally used to evaluate the catalytic
with the +4 oxidation state of Ti.62
activity. It can be seen that the value for Pt/TNTs/C is
357.4 mA mgPtꢀ1, 1.4 times higher than the value of Pt/C
(252.8 mA mgPtꢀ1), suggesting that the catalytic activity of the
Pt/TNTs/C catalyst is much better than that of Pt/C catalysts.
What is noteworthy is that this relatively lower ECSA increase
(1.27 times) causes a larger increase of mass activity for meth-
anol electro-oxidation (1.4 times). And it can be easily found
that the onset oxidation potential of forward anodic peak
presents a slightly negative shi with the addition of TNTs
indicating that the MOR is easier to perform on the Pt/TNTs/C
than on the Pt/C catalysts. These two phenomena imply the
metal-support electronic interaction and bifunctional effect
between Pt nanoparticles and TNTs, which is in agreement with
the observation in XPS measurements.
3.2 Electro- and photo-electrochemical measurements
CVs for the Pt/C, Pt/TNTs and Pt/TNTs/C samples were carried
out in 0.5 mol dmꢀ3 H2SO4 and the results are shown in Fig. 5.
All the curves in Fig. 5a, except the sample of Pt/TNTs, present
the typical hydrogen adsorption–desorption peaks (within the
potential range from ꢀ0.2 V to 0.1 V) and reduction peaks for
platinum oxide (within the potential range from 0.3 V to 0.7 V).
Electrochemically active specic surface areas (ECSA) of Pt-
based catalysts are obtained with the measurements of the
hydrogen adsorption–desorption (HAD) integrals. The ECSA of
as-prepared catalysts can be easily calculated via the equation
below with coulombic charges accumulated during HAD aer
correcting for the double-layer charging current from the CVs:
From the all above, the enhanced catalytic performance of
the Pt/TNTs/C catalysts may result from the following factors.
Firstly, this Pt/TNTs/C catalyst has more electrochemical active
sites than Pt/C catalyst, as evidenced by the larger ECSA of the
QH
ECSA ¼
(1)
0:21 ꢁ MPt
Fig. 5 (a) CV curves measured on the Pt/C, Pt/TNTs/C and Pt/TNTs catalysts in 0.5 mol dmꢀ3 H2SO4 at 50 mV sꢀ1. (b) CV curves measured on Pt/
C and Pt/TNTs/C catalysts in 0.5 mol dmꢀ3 H2SO4 containing 1.0 mol dmꢀ3 CH3OH at 50 mV sꢀ1
.
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former catalyst.12 Secondly, the relatively strong metal-support catalyst under light illumination, CA measurement was
interaction between Pt and TNTs support, as evidenced by the carried out on bare TNTs sample, and CV measurements were
positively shied binding energies of Pt 4f in Pt/TNTs/C catalyst, also carried out on Pt/TNTs catalyst without any carbon mate-
could result in lower bonding strength between Pt and CO-like rials. CA curve measured on the TNTs sample at 0.6 V by turning
species and thus enhance MOR activity on Pt surface.14 Third, the light on/off in 0.5 mol dmꢀ3 H2SO4 is shown in Fig. 7a. As we
TiO2 can supply the rich active –OH species, which are able to all know, TiO2 is a type of corrosion-proof oxides which could
remove the CO-like intermediate species adsorbed on Pt surface stably exist in dilute sulfuric acid. The observed current under
and thus enhance the MOR kinetics through the bifunctional light illumination in CA measurement on TNTs could be
mechanism.63
attributed to the double layer charging current and the photo-
Given the fact that TiO2 is an effective photo-responsive generated current, as the current increased sharply when the
semiconductor with a wide band gap of 3.0–3.2 eV, the light light was turned on and dived when the light was turned off. It
illumination effects on Pt/TNTs/C toward methanol electro- can be found that the average photo-generated current density
oxidation were also investigated in this work. Fig. 6a shows is approximately 0.3 mA, which is extremely smaller compared
that the mass activity toward methanol electro-oxidation of the with the current of methanol electro-oxidation. Fig. 7b shows
Pt/TNTs/C catalyst increase drastically under light illumination. the CV curves measured on the Pt/TNTs catalyst under light
The Pt/TNTs/C catalyst exhibits remarkably enhanced mass illumination and in dark in 0.5 mol dmꢀ3 H2SO4 containing
activity of 525.0 mA mgPtꢀ1 under light illumination, 1.47 times 1.0 mol dmꢀ3 CH3OH at 50 mV sꢀ1. The background current of
higher than the value of the same catalyst and 2.07 times higher these two CV curves exhibit extremely larger difference because
than the value of Pt/C catalyst in dark, suggesting that the Pt/ of light illumination. With the existence of the Pt nanoparticles
TNTs/C exhibits higher catalytic activity toward MOR under on the TNTs, the photo-generated electron/hole (eꢀ/h+) pairs
light illumination. CA measurements on the Pt/C and Pt/TNTs/ can be split apart more efficiently, which can increase the
C catalysts at 0.6 V in a solution of 0.5 mol dmꢀ3 H2SO4 con- photo-generated current. Anyhow, according to the observa-
taining 1.0 mol dmꢀ3 CH3OH were also carried out to further tions described above, the increase of mass activity of the Pt/
evaluate the light illumination effects for MOR. The corre- TNTs/C catalyst under light illumination (Fig. 6a) is partly
sponding CA curves are shown in Fig. 6b. The current of all the resulted from the photo-generated current which caused by the
catalysts decay rapidly during the earlier minutes due to the TNTs under light illumination. From the CV curve of Pt/TNTs
accumulation of CO-like intermediate poisonous species. As catalyst in dark in Fig. 7b, it can be clearly detected that there
shown in Fig. 6b, the current declining rate of decay section for is neither evident forward peak nor backward peak, which can
the Pt/TNTs/C catalyst under light illumination is the smallest be attributed to the low electrical conductivity of the as-
among these three curves, which indicates that Pt/TNTs/C synthesized TiO2 sample. However, under light illumination,
under light illumination shows the best tolerance perfor- the CV curve of the Pt/TNTs catalyst shows two characteristic
mance toward CO-like intermediate poisonous species. It is anodic peaks of methanol electro-oxidation, which could be
noteworthy that, without light illumination, the Pt/TNTs/C also attributed to the improvement of conductivity of TNTs caused
possesses a relatively slower current declining rate than Pt/C by optical excitation and positive applied bias on Pt/TNTs
during the earlier minutes, demonstrating that Pt/TNTs/C has catalyst.29
better electro-catalytic stability toward MOR and enhanced
To sum up the analysis of photo-electrochemical measure-
tolerance performance toward CO-like species in dark ments, the improvement in the methanol electro-oxidation
compared with the Pt/C catalyst, which agrees well with the performance under light illumination can be partly attributed
analysis of CVs curves in Fig. 5b.
to the photo-generated current caused by TNTs support.
For the sake of guring out the reasons for the improvement Furthermore, it is supposed that the enhancement of methanol
of methanol electro-oxidation performance of the Pt/TNTs/C electro-oxidation performance depends greatly on the
Fig. 6 (a) CV curves measured on Pt/TNTs/C catalyst under light illumination and in dark in 0.5 mol dmꢀ3 H2SO4 containing 1 mol dmꢀ3 CH3OH
at 50 mV sꢀ1. (b) CA curves measured on Pt/C and Pt/TNTs/C catalysts at 0.6 V in 0.5 mol dmꢀ3 H2SO4 with 1 mol dmꢀ3 CH3OH.
56200 | RSC Adv., 2017, 7, 56194–56203
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Fig. 7 (a) CA curve measured on the TNTs at 0.6 V by turning the light on/off in 0.5 mol dmꢀ3 H2SO4. (b) CV curves measured on the Pt/TNTs
catalyst under light illumination and in dark in 0.5 mol dmꢀ3 H2SO4 containing 1 mol dmꢀ3 CH3OH at 50 mV sꢀ1
.
improvement in charge transport properties within the catalyst illumination. What's more, the methanol not only acts as
under light illumination. The photo-induced improvements a sacricial electron donor to scavenge the h+s but also can
that occur at the Pt/TNTs/C interface are illustrated in Fig. 8. promote the formation of methoxy radicals, which could further
Firstly, as TiO2 is a kind of n-type semiconductor, under light inject electrons to increase the current, known as the “current
illumination, the Fermi level would move to assisting charge doubling effect”.15 The photocatalytic oxidation of methanol if
separation with a interaction of Pt nanoparticles and a positive adopted in a fuel cell should provide a good additive effect to
applied bias,64 which causes a greater fraction of the photon boost the current generation. Thirdly, the as-prepared TNTs'
generated eꢀ/h+ pairs participate in surface redox chemical surface is disordered and the electrolyte would be stored in the
reaction. This effect can as well translate into an improvement lacunes between these disordered structures, which provides
in the conductivity of catalyst and further enhance the charge a quick supply and short diffusion distance to accelerate the
transfer while decreasing carrier recombination. Secondly, it is electrocatalytic activity. Generally speaking, it is reasonable that
known to all that the Pt-based catalysts would be poisoned the enhancement of methanol electro-oxidation performance
easily by CO-like intermediate products (such as CO and COH) by TNTs support with disordered surface.
during the methanol electro-oxidation process. Under light
illumination, the photo-generated h+s can migrate to the
4. Conclusion
surface of TiO2 and participate in redox reactions, transforming
the adsorbed –OH species to cOH radicals65 that can easily
In summary, Pt nanoparticles with the particle size of ca. 3 nm
transform the CO-like species on Pt surface to –COOH species.29
were successfully prepared by plasma sputtering in water in this
Therefore the methanol electro-oxidation performance of the
work, which is a simple, high-yield and rinsing-free method.
Pt/TNTs/C catalyst would increase
a
lot under light
And the well-organized free-standing TiO2 nanotubes with the
inner diameter of 60–70 nm were synthesized by a simple two-
step anodization method with titanium metal foil. The lining-
arranged Pt nanoparticles on TNTs' surface, the strong metal-
support interaction between Pt and TiO2, the disordered
surface structure of TNTs and the higher ECSA of Pt/TNTs/C
result in a much higher catalytic activity and stability toward
methanol electro-oxidation. What's more, the synergistic effects
of photo-assisted electro-catalysis toward methanol electro-
oxidation are realized successfully under light illumination,
drastically improving the catalytic performance on Pt/TNTs/C
for methanol oxidation. To sum up the analysis of photo-
electrocatalysis performance, the improvement in the conduc-
tivity of catalyst, the alleviating COads poisoning role of the
photo-generated h+s and the photo-generated current from TiO2
cause the drastic improvement of activity toward methanol
electro-oxidation under light illumination. The nding here will
be of signicance in attaining basic knowledge of photo-
assisted electrocatalysis and provides a new strategy to fabri-
cate photo-electro Pt-based catalysts for DMFCs anodic
reaction.
Fig. 8 Schematic illustration of the electro-photo synergistic catalysis
of methanol on Pt/TNTs/C.
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19 H. J. Wang, X. H. Wang, J. D. Zheng, F. Peng and H. Yu, J.
Nanosci. Nanotechnol., 2015, 15, 3662–3669.
Conflicts of interest
20 G. P. Lu, X. B. Ma, H. F. Yang, D. S. Kong and Y. Y. Feng, Int. J.
Hydrogen Energy, 2015, 40, 5889–5896.
There are no conicts to declare.
21 F. L. Lei, Z. S. Li, L. T. Ye, Y. L. Wang and S. Lin, Int. J.
Hydrogen Energy, 2016, 41, 255–264.
Acknowledgements
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This work was supported by the National Natural Science
Foundation of China (Grant No. 51372113), Jiangsu National
Synergetic Innovation Center for Advanced Materials (SICAM),
Program for New Century Excellent Talents in University (NCET-
12-0733, China), the scientic research foundation for Returned
Overseas Students, and Project Funded by the Priority Academic
Program Development of Jiangsu Higher Education Institutions
(PAPD).
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