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Accepted Manuscript
Title: Enhanced Electronic and Electrochemical Properties of
Core-Shelled V2 O5 -Pt Nanowires
Authors: Ko-Ying Pan, Da-Hua Wei
PII:
DOI:
Reference:
S0169-4332(17)32648-X
http://dx.doi.org/10.1016/j.apsusc.2017.09.033
APSUSC 37112
To appear in:
APSUSC
Received date:
Revised date:
Accepted date:
6-6-2017
1-9-2017
5-9-2017
Please cite this article as: Ko-Ying Pan, Da-Hua Wei, Enhanced Electronic and
Electrochemical Properties of Core-Shelled V2O5-Pt Nanowires, Applied Surface
Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.09.033
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Enhanced Electronic and Electrochemical Properties of
Core-Shelled V2O5-Pt Nanowires
Ko-Ying Pan and Da-Hua Wei*
Institute of Manufacturing Technology and Department of Mechanical Engineering, National
Taipei University of Technology (TAIPEI TECH), Taipei 10608, Taiwan, R.O.C.
*Corresponding author: dhwei@ntut.edu.tw
Highlights
1. We adopted thermal evaporation without using precursors for V2O5 NWs and Pt@
V2O5 NWs.
2. The electrical resistivity of V2O5 NWs and three kinds of Pt@V2O5 NWs (50, 100
and 150 cycles) are 1.716x107, 9.091x105, 4.456x104 and 1.961x103 Ω,
respectively.
3. The value of IF/IR of Pt@V2O5-150 cycles NWs is the largest of these four
samples, so the Pt@ V2O5-150 cycles is the best material for fuel cell.
ABSTRACT
Platinum nanoparticles (Pt NPs) were decorated on vanadium pentoxide
nanowires (V2O5 NWs) to form the core-shelled vanadium-platinum nanowires
(Pt@V2O5 NWs) and their electrochemical activities for methanol oxidation were
investigated. The synthetic procedure involved the synthesis of abundant vanadium
pentoxide nanowires (V2O5 NWs) by a direct vapor-solid growth process (VS
method), followed by atomic layer depositions (ALD) of platinum nanoparticles (Pt
NPs) onto the V2O5 NWs. After the physical examinations, three designed deposition
parameters (50, 100 and 150 cycles) of Pt NPs onto the V2O5 NWs by ALD process
were successful. From the measurements of current-voltage (I-V) and cyclic
voltammetry (CV) curves respectively, both the conductivity and the ratio of the
forward anodic peak current (IF) to the reverse anodic peak current (IR) are enhancing
proportionately to the deposition cycles of ALD process, which denotes that coating
Pt atomic layers onto V2O5 nanowires indeed improves the catalytic performances
than that of pure V2O5 nanowires.
Keywords: Pt@V2O5 nanowires, VS method, Atomic layer deposition, I-V curve, CV
curve, Catalyst
1. Introduction
For four decades, in the field of fuel cell technology and electro-catalysis, various
oxidation states of vanadium have received considerable interest, including VO, V2O3,
VO2 and V2O5 [1-6]. Because of the compositions of single and mixed valence states
are used to obtain highly desirable manufactures and plans to reduce pollution, and
vanadium is one of the abundant metallic elements within our planet’s crust. Most of
all, vanadium pentoxide (V2O5) is widely adopted in actuators, sensors, field-emitters,
lithium ion batteries, catalysis, electrochromic devices and supercapacitors, owing to
its intriguing microstructures and electronic properties [3, 7-13]. To our best
knowledge, V2O5 is an n-type semiconductor with a bandgap of 2.4 eV, and it owns a
special layered structure, which is composed of layers of VO5 square pyramids; also,
these VO5 square pyramids regularly link at edges or corners. Various approaches
have been developed to fabricate V2O5 nanostructure, such as thermal chemical vapor
evaporation, hydrothermal treatment, sol-gel process, pulsed laser deposition, and so
on [1-15]. Mostly, the solution-based methods such as in-situ hydrothermal growth
[16] and procedure of binder-free V2O5 as a cathode for rechargeable aluminum
battery [17] have been widely used to prepare V2O5 nanocomposites. However, the
previously published papers demonstrate that two big issues diminish the advantages
of V2O5; one condition is the low conductivity; the other is poor microstructure
stability. Above two severe disadvantages cause poor retention capacity while the long
term cycling is working in the cell. In order to improve the poor retention capacity
resulted from features of V2O5, many manufactures for fabricating core-shelled
metal@V2O5
[18],
oxide
semiconductor@V2O5
nanocomposites [20] have been still developing rapidly.
[19]
and
polymer@V2O5
On the other hand, atomic layer deposition (ALD) is a cutting-edge technology,
which has been considered a powerful coating skill to form three-dimensional
uniformity. Also, it depends on serial self-terminating gas-solid reactions, which has
been applied for coating sequential atomic layers of organic metal on the basic
materials. Moreover, ALD is generally adopted in thin-film process of semiconductors
manufacturing technology due to its good performance of controllable thickness and
complicated shape of atomic monolayer [21]. Although V2O5 has been widely
discovered experimentally, there have been rare literatures on the function and
application of Pt@V2O5 NWs for the catalysts.
In this task, in advance, thermal evaporation was demonstrated to synthesize
pristine V2O5 nanowires via VS method. These nanostructures, V2O5 nanowires, have
been grown on the silicon chips without a catalyst. Then, Pt nanoparticles - known to
be a high conductivity and good catalytic material- have been grown uniformly onto
the surface of V2O5 NWs by ALD process. The samples were characterized by
scanning electron microscopy (SEM), energy-dispersive X-ray spectrometer (EDS)
and high-resolution transmission electron microscope (HRTEM). As to electronic and
electrochemical properties, the synthesized V2O5 and Pt@V2O5 nanowires have been
investigated and compared via using current-voltage (I-V) curves and cyclic
voltammetry (CV) curves, respectively.
2. Experimental Method
The vanadium pentoxide nanowires (V2O5 NWs) used here were fabricated on
silicon (100) substrates, by thermal evaporation, similar to that reported for GaN
nanorods synthesis [22, 23]. The whole preparation tactics for constructing pristine
V2O5 nanowires and Pt@V2O5 nanowires are sketched in the Figure 1. High purity
V2O5 powders (Alfa, 99.5%, 325 mesh) in an alumina boat and several silicon
substrates were situated in the other alumina oxide (Al2O3) boat. Then, authors placed
these two Al2O3 boats and a space length is 5 cm between them in the heating area of
a quartz tube, as illustrated in Fig. 1(a). The commercial Si (001) substrates were
prepared without coating metallic precursors, chemical etchings or physical damages
but purged ultrasonically in acetone solution. During the growth of V2O5 nanowires,
the pressure inside the quartz tube was kept up at 1 Torr by a rotary pump. The
working conditions of this vapor-solid process (VS) were as follows: (1) The working
temperature had gone up to 850°C with 20°C/min at a constant flow rate of 20 sccm
Ar gas. (2) Maintaining the working temperature at 850°C for 1 hour was at a
continual flow rate of mixed gas of 20 sccm Ar and 10 sccm O2. (3) Cooling the
working temperature to the room temperature was under a fixed flow rate of 20 sccm
of Ar gas.
After the formations of pristine V2O5 nanowires, Pt nanoparticles were deposited
on the nanowires surface to form Pt@ V2O5 nanowires by ALD of Pt after 50, 100
and 150 cycles using alternative exposures to MeCpPtMe3 (Aldrich, 98%) and oxygen
(99.99%) at 250°C, as sketched in Fig. 1(b). In the ALD process, the receptacle of
MeCpPtMe3 was kept at 45°C, suppling a stable flux of MeCpPtMe3 at a fixed
pressure of 3 mTorr to the reactor. In every single cycle, 1 second of MeCpPtMe 3
pulse and 5 seconds of O2 pulse were separated by 20 seconds of N2 (99.99%) purge
[21].
The morphology, crystal structure and chemical composition of the V 2O5 and
Pt@V2O5 nanowires were analyzed by a field emission scanning electron microscope
(FESEM, JEOL, JSM-6500F), a high-resolution transmission electron microscope
(HRTEM, JEOL, JEM-2010), and energy-dispersive X-ray spectroscopy (EDS,
Oxford), respectively. Regarding the electronic property of V2O5 and Pt@V2O5
nanowires, the current-voltage (I-V) curves were measured between -10V and 10V by
Keithley 4200. As for the electrochemical property of V2O5 and Pt@V2O5 nanowires,
the cyclic voltammetric (CV) curve was plotted by an Electrochemical Workstation
(CHI 627E) to investigate the potential of oxidation peak and reduction peak. The test
cell includes three kinds of electrodes and one electrolyte in a beaker. A saturated
calomel electrode (SCE) and a platinum foil were adopted as the reference electrode
and counter electrode, separately. The working electrodes were made by mixing 80
wt% V2O5 or Pt@V2O5 nanowires, 10 wt% polyvinylidene difluoride (PVdF) and 10
wt% carbon black in N-methyl pyrrolidinone (NMP) solvent to become a slurry. The
slurry was spread onto an Al2O3 foil and evenly distributed on the Al2O3 foil, then the
sample was placed in the oven to transform slurry into a solid. 1 M H2SO4+ 1M
CH3OH solution was prepared as the electrolyte in the beaker-style three-electrode
test cell. The cyclic voltammetry (CV) was conducted in a voltage range between -0.1
and 1 V at 50 mVS-1 scan rate at room temperature.
3. Results and Discussion
Figure 2(a) exhibits the SEM image of abundant V2O5 nanowires; it is very clear
that pure V2O5 nanowires are with diameter about 100 nm and length more than 10
μm, which have been directly obtained by VS process. Figures 2(b), 2(c) and 2(d)
show the SEM images of three conditions (50, 100 and 150 cycles) of Pt@V2O5
nanowires, respectively. Judging from these three SEM images of Pt@V2O5
nanowires, three kinds of ALD process in this work are highly successful because
every final product still keeps the same shape as the initial V2O5 nanowires. The
phenomena definitely prove that Pt monolayers were extremely deposited uniformly
and well-ordered onto pure V2O5 nanowires via the surface-control process of ALD.
Furthermore, the micromorphology and microstructure of one single Pt@V2O5
nanowire were inspected by a classical observation of TEM. In Fig. 2(e), the
SEM-EDS result of spot 1 on the surface indicates the co-existence of V and O, and
the percentages of V and O atoms are 27.89 and 72.11, which is approaching to the
composition ratio of V2O5. To observe the surfaces of V2O5 nanowires exactly, the
low-magnification and high-resolution TEM images of V2O5 samples were taken in
Fig. 3. Figures 3(a) and 3(b) are typical low-magnification TEM images of an
individual V2O5 and Pt@V2O5 (50 cycles) NWs, which demonstrate the diameters are
103 nm and 108.54 nm, respectively. The high resolution TEM images of Pt@V2O5
(50 cycles) NWs are shown in Figs. 3(c) and 3(d), and Fig. 3(c) reveals that the
thickness of Pt layer is 2.77 nm. From Fig. 3(d), the HRTEM image shows the
combination of Pt NPs and V2O5 NW very well with highly ordered and continuous
states. The lattice spacing between two adjacent lattice planes is 0.22 nm,
corresponding to the (111) plane of face-centered cubic (fcc) Pt. Figures 3(e) and 3(f)
are the TEM-EDS results on the spots 2 and 3 marked in Figure 3(b), respectively.
The TEM-EDS acquires from an individual Pt@V2O5 NW (50 cycles) indicates that
the elements of the product consists entirely V, O and Pt. In the next analysis, authors
would certify the Pt is certain whether metallic or oxidative states via doing XPS. The
TEM images of a single Pt@V2O5 NW (100 cycles and 150 cycles) are presented in
Figs. 3(g) and 3(h), respectively. From the two photos, the thicknesses of Pt layer are
6.24 nm and 8.89 nm, respectively. The XPS patterns of V2O5 and Pt@V2O5
nanowires are shown in Fig. 4, and Fig. 4(a) is the survey scan for V2O5 and
Pt@V2O5 samples. From Fig. 4(b), three peaks at 517.7, 525.2 and 530.5 eV are
associated with orbits of V2p3/2, V2p1/2 and O1s, respectively. It should be noted that
the binding energy difference (ΔE) between orbits of V2p3/2 and O1s is 12.8 eV,
which is consisted with the previous literature [24], so the as-synthesized nanowires
prepared by thermal evaporation could be completely verified as vanadium pentoxide
nanowires (V2O5 NWs). In Fig. 4(a), three peaks from Pt 4p, 4d and 4f orbitals mean
that Pt coatings indeed were deposited onto the surface of V2O5 nanowires. The above
results reveal that the Pt@V2O5 nanowires are able to fabricate via using a procedure
consists of the thermal evaporation and the subsequent ALD processes of Pt. Figure
4(c) is Pt 4f spectrum and Pt 4f7/2 located at 71.2 eV implies that the
atomic-layer-deposited Pt on the V2O5 nanowires is the form of metallic Pt. The
electric properties of the V2O5 NWs and three kinds of Pt@V2O5 NWs (50, 100 and
150 cycles) are also evaluated and compared. A schematic drawing of a fabricated
V2O5 NWs or Pt@V2O5 NWs- based device for electric test is shown in Fig. 5(a).
Figure 5(b) displays the current-voltage (I-V) curves of all V2O5 and Pt@V2O5
samples, and all curves appear to be linear with the applied voltage, also indicating
that Ohmic contact forms are between these nanostructures and the Ag electrodes,
confirming that the V2O5 NWs and Pt@V2O5 NWs- based devices are well connected
and have low contact resistance. The electrical resistivity can be expressed as
R=ρ×L/A, and charge transfer between the nanostructures can be ignored. The
electrical resistivity of V2O5 NWs and three kinds of Pt@V2O5 NWs (50, 100 and 150
cycle) are 1.716x107 Ω, 9.091x105 Ω, 4.456x104 Ω and 1.961x103 Ω, respectively.
That ALD of numerous Pt particles on V2O5 NWs causes the formation of Pt films
that cover the V2O5 NWs surface and upgrade the charge transfer would be the root
cause of this phenomenon [25].
For making a comparison of the electrochemical properties among V2O5 NWs and
Pt@V2O5 NWs, the CV technique was used to measure the electrocatalytic activities.
The typical CV curves of V2O5 nanowires and Pt@V2O5 NWs (50, 100 and 150
cycles) in 1 M H2SO4+ 1M CH3OH solution at the scan rate of 50 mVs-1 at room
temperature is plotted in Fig. 6, performed in a potential range of -0.1 V to 1 V at the
100th cycle [26, 27]. As shown in Fig. 6, the oxidation peak and the reduction peak of
pristine V2O5 nanowires along the wine dashed line are at 0.608 and 0.372 V,
respectively. The onset potentials for methanol oxidation are 0.477, 0.302, 0.133 and
0.126 V, corresponding with pristine V2O5 nanowires and Pt@V2O5 nanowires (50,
100 and 150 cycles), respectively. There is no doubt that the better dispersion of Pt
NPs deposited by ALD process on the pristine V2O5 NWs surface do the trick for the
methanol oxidation owing to their high surface area and good conductivity.
As given in Fig. 6 and Table 1, the IF/IR values of the Pt@V2O5 nanowires (50,100
and 150 cycles) are 1.574, 1.859 and 1.886, respectively. The ratios of the forward
anodic peak current (IF) to the reverse anodic peak current (IR) are used to evaluate the
catalyst tolerance to the poisoning species on the surfaces of the electrodes. The
higher IF/IR value signifies that the removal of poisoning species on the catalysts
surfaces is more energy efficient, so Pt@V2O5 NWs get better catalytic activity
toward methanol oxidation than that of pristine V2O5 nanowires [28-32].
The demonstration of catalytic activity for Pt@V2O5 NWs is similar to that of
some bifunctional compositions derived from published literatures [30-32]. At the
first step of the improvement of the catalytic activity between Pt NPs and V2O5 NWs,
the methanol shows favor to binding with Pt atoms on surface and then is produced
CO absorbed species by dehydrogenation. The products, COad intermediates, are
deemed to be the major poisoning species while the electro-oxidation of methanol is
in progress [28, 30]. During the oxidation reaction, the related schematic mechanism
of methanol adsorption turns out in four steps as shown in Fig. 7, making various
products owing to dissociation of the molecule that could be formulated as following
reactions and can be expressed as (Eqs. (1)–(4))
Pt-(CH3OH)ad→ Pt-(CH2OH)ad + H+ + e-
(1)
Pt-(CH2OH)ad→ Pt-(CHOH)ad + H+ + e-
(2)
Pt-(CHOH)ad→ Pt-(COH)ad + H+ + e-
(3)
Pt-(COH)ad→ Pt-(CO)ad + H+ + e-
(4)
Second, thanks to the better affinity of V2O5 NWs being oxygen-containing
species, a large amount of OHad has been made to provide rational CO oxidation rates
on V2O5 NWs than Pt NPs. The OHad species are essential for the oxidative removal
of COad intermediates, which causes the better catalytic activity and longer lifespan
for the entire methanol oxidation on Pt@V2O5 NWs. The above chemical reaction in
methanol electro-oxidation can be illustrated as following (Eqs. (5)–(8)) [30, 32],
V2O5 +2H| → 2VO2 + H2O
(5)
4VO2 + 4H| → 4VO2+ + O2 + 2H2O
(6)
VO2+ + H2O → VOOH- + H+
(7)
COad + VOOH → CO2 + VO2 | + H- + e
(8)
For instance, in our designed core-shelled nanostructures, the CV curve shows that
Pt@V2O5 NWs (150 cycles) in the solution containing methanol at the oxidation peak,
take place Pt-CO bonds on themselves surfaces. After the negative sweep, the CV
curve reveals that CO molecules are reduced to CO2 at the oxidation peak of negative
sweep, as drawn in Fig. 7. The above results show that Pt@V2O5 nanowires exhibit
higher catalytic activity to methanol oxidation with the increasing Pt deposition cycles
of ALD process, on account of the following two reasons. First, metallic Pt
nanoparticles are deposited on as-prepared V2O5 NWs uniformly, which can
additionally lessen the effect of aggregation derived from V2O5 NWs and make plenty
of accessible immobilization sites inV2O5 NWs during the interaction of methanol
oxidation. Second, increasing metallic Pt nanolayers enhances the conductivity and
then enlarges the capacity of final products during methanol oxidation for fuel cell.
Conclusions
In summary, after confirmation with SEM, EDS, XPS and HRTEM analyses, the
V2O5 NWs and Pt@V2O5 NWs (50 cycles, 100 cycles and 150 cycles) were
successfully synthesized on commercial Si substrates via direct VS process and
subsequent ALD process. Both examinations of current-voltage (I-V) curve and cyclic
voltammetric (CV) curve of the different V2O5 and Pt@V2O5 NWs were investigated,
which demonstrated that the Pt@V2O5 NWs (150 cycles) obtain the superior catalytic
activity toward methanol oxidation for fuel cell.
Acknowledgment
The authors acknowledge financial support of the main research projects of the
Ministry of Science and Technology (MOST) under Grant Nos. 105-2731-M-027-001
and 105-2221-E-027-047-MY3.
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Figure Captions
Figure 1 (a) Schematic diagram illustrating the thermal evaporation set-up with three
main processes including evaporation, reaction and condensation. (b) Sketched
picture of ALD process and its working conditions.
Figure 2 SEM image of (a) V2O5 nanowires, (b) Pt@V2O5 nanowires-50 cycles, (c)
Pt@V2O5 nanowires-100cycles, (d) SEM image of Pt@V2O5 nanowires-150 cycles;
(e) SEM-EDS analysis at spot 1 of V2O5 nanowires. Inset: SEM image of spot 1.
Figure 3 Low-magnification TEM image of (a) a V2O5 nanowire, (b) a Pt@V2O5
nanowire (50 cycles). HRTEM images of (c) and (d) a Pt@V2O5 nanowire (50 cycles).
TEM-EDS analysis of (e) and (f) on the spot 2 and 3 in Figure 3(b).
Low-magnification TEM images of (g) a Pt@V2O5 nanowire (100 cycles) and (h) a
Pt@V2O5 nanowire (150 cycles).
Figure 4 XPS spectra of the core-shelled nanowires: (a) Survey scan, (b) V 2p and O
1s, and (c) Pt 4f.
Figure 5 (a) Schematic illustration of the fabricated device for electric measurements.
(b) The measured I-V curves of V2O5 NWs and Pt@V2O5 NWs (50,100 and 150
cycles).
Figure 6 The measured CV curves of V2O5 NWs and Pt@V2O5 NWs (50,100 and
150 cycles).
Figure 7 Schematic diagram illustrating the oxidation and reduction reactions in CV
characterization.
(a)
V2O5 NWs
Pt 4f
Pt@V2O5 NWs-50 cycle
Pt@V2O5 NWs-100 cycle
Intensity (a.u.)
Pt@V2O5 NWs-150 cycle
Pt 4d5/2
Pt 4d3/2
V(LMM)
O(KVV)
O1s
V2p
V2s
N1s
C1s
V(LML)
O2s
0
100
200
300
400
500
600
700
800
900
1000
1100
Binding energy (eV)
(b)
V2p3/2
ΔE=12.8 eV
O1s
530.5 eV
(c)
517.7 eV
Pt 4f7/2
Intensity (a.u.)
71.2 eV
V2p1/2
525.2 eV
Intensity (a.u.)
Pt 4f5/2
515
520
525
530
535
Binding energy (eV)
70
75
80
Binding energy (eV)
85
6.0x10-4
4.0x10-4
Current (A)
2.0x10-4
7
V O NWs (R=1.176x10 Ω)
2 5
5
Pt@V O NWs- 50 cycles (R=9.091x 10 Ω)
2 5
4
Pt@V O NWs- 100 cycles (R=4.546x 10 Ω)
2 5
3
Pt@V O NWs- 150 cycles (R=1.961x 10 Ω)
2 5
0.0
-2.0x10-4
-4.0x10-4
-6.0x10-4
-10
-5
0
Voltage (V)
5
10
12
V2O5 NWs
10
Pt@V2O5 NWs-50 cycles
Pt@V2O5 NWs-100 cycles
Pt@V2O5 NWs-150 cycles
2
Current (mA/cm )
8
6
4
2
0
-2
-0.2
0.0
0.2
0.4
0.6
Potential(V/SCE)
0.8
1.0
Table Caption
Table 1 Comparison of electrocatalytic activity of Pt@V2O5 NWs (50,100 and 150
cycles).
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