Single Ag Nanowire Electrodes and Single Pt@Ag Nanowire Electrodes: Fabrication, Electrocatalysis, and Surface-Enhanced Raman Scattering Applications

Article abstract of DOI:10.1021/acs.analchem.8b04610

Silver nanowires (AgNWs) have received much attention due to their excellent optical, electrical, and conductive properties. In this work, we provided a new perspective for investigating the property of AgNWs from a single nanowire level. Single Ag nanowire electrodes and single Pt@Ag nanowire electrodes were fabricated by a laser-assisted pulling technique and galvanic replacement reaction (GRR). The radius, length, and metal ratio of the nanowires are tunable as needed. The prepared nanowire exhibited excellent electrocatalytic activity toward the methanol electro-oxidation and high sensitivity for monitoring the reduction of 4-nitrothiophenol by recording SERS spectra. This work will help us deeply understand the catalytic performance at the single nanowire level and open a new perspective for nanomaterials research.

Full text of DOI:10.1021/acs.analchem.8b04610

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Letter
Single Ag Nanowire Electrodes and Single Pt@Ag Nanowire
Electrodes: Fabrication, Electrocatalysis and SERS Applications
Dongmei Wang, Hongmei Hua, YONG LIU, Haoran Tang, and Yongxin Li
Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b04610 • Publication Date (Web): 11 Mar 2019
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Single Ag Nanowire Electrodes and Single Pt@Ag Nanowire Electrodes:
Fabrication, Electrocatalysis and SERS Applications
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Dongmei Wang,†‡ Hongmei Hua,† Yong Liu,† Haoran Tang, † and Yongxin Li,*,†
†Anhui Key Laboratory of Chemo/Biosensing, College of Chemistry and Materials
Science, Anhui Normal University, Wuhu, 241000, P.R. China
‡
College of Chemistry and Material Engineering, Chaohu University, Chaohu Anhui
238000, P.R. China
*corresponding author. Email: yongli@mail.ahnu.edu.cn
Phone: 86-553-386-9302; Fax: 86-553-386-9303
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Silver nanowires (AgNWs) have received much attention due to their excellent optical,
electrical and conductive properties. In this work, we provided a new perspective for
investigating the property of AgNWs from a single nanowire level. Single Ag nanowire
electrodes and single Pt@Ag nanowire electrodes were fabricated by laser-assisted
pulling technique and galvanic replacement reaction (GRR). The radius, length and
metal ratio of the nanowires are tunable as needed. The prepared nanowire exhibited
excellent electrocatalytic activity towards the methanol electro-oxidation and high
sensitivity for monitoring the reduction of 4-nitrothiophenol by recording SERS spectra.
This work will help us deeply understand the catalytic performance at single nanowire
level, and open a new perspective for nanomaterials research.
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Metal one-dimensional (1D) nanostructures are expected to play a valuable role in the
fabrication of electronic devices due to their unique electrical, thermal and optical
properties.1-3 Among 1D nanomaterials, silver nanowires (AgNWs) have received the
much attention because bulk silver exhibits excellent properties, thus AgNWs have
been used extensively in solar cell, flexible optoelectronics, electrocatalysis, and
surface-enhanced Raman scattering (SERS).1 Many methods have been developed
to synthesize AgNWs, including hydrothermal synthesis, wet chemical process, and
polyol reduction process.4,5 Despite the methods mentioned above have achieved
huge success, challenge still exist because most of the obtained AgNWs are irregular
morphology, poor conductivity and low aspect ratio, and template is always needed to
direct the growth of nanowire during the synthesis process.1,4 On the other hand, due
to the remarkable advantages, such as smaller RC constant, faster mass-transport
rate, and the reduced effect of solution resistance, single nanoelectrode have attracted
wide interest, especially at fundamental research, electrocatalysis and sensing.6,7
Single nanoelectrodes can be served as a platform for the research of electrochemistry
and electrocatalysis at single nanoparticle/nanoelectrode level, which have overcome
the interaction between nanoparticles/nanowires.8
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Herein, we present a simple method for fabricating single AgNWs by laser-
assisted pulling technique.2,9-11 Single Pt/Au nanoelectrodes have been fabricated by
our lab2,3,9 and other groups10,11 using laser-assisted pulling technique but challenge
still exist for fabricating single Ag nanoelectrodes due to the specific property of Ag.12
Ag nanodisk electrodes were fabricated successfully by adjusting the pulling
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parameters and protocols.12 Considering the lower melting point of silver, the whole
process consisted of four successive steps, which were presented as Scheme S1.10
The first two steps were aimed at sealing the silver wire in glass and heating them in
order to obtain a complete seal between Ag and capillary. The Ag microwire was
inserted into a 1-cm long silica capillary (o.d., 350 μm; i.d., 80 μm) and then the
capillary containing Ag microwire was inserted into an 8-cm long silica capillary (i.d.,
0.4 mm, o.d., 1.2 mm). The prepared Ag/capillary was heated and sealed tighter with
the parameters: heat = 440 and filament = 5. After that, the sealed Ag/capillary was
pulled into two ultra-sharp tips and the tips were sealed into glass tube using a
hydrogen flame. A tungsten wire was used to make electrical contact with the Ag wire
using conductive silver paste, and then the electrode was polished to expose the Ag
nanodisk tip under the monitor of a home-made tester.9 Finally the AgNW was obtained
through immersing the nanodisk electrode into a HF aqueous solution (1:4, v/v) to etch
the silicate sheath for different time.3
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The morphologies of single AgNW before and after HF etching were checked by
transmission electron microscopy (TEM, shown in Figure S1 and Figure 1A), which
clearly showed that a single Ag wire was exposed from capillaries with a radius of ~30
nm and a length of ~ 150 nm. The surface of nanowire was smooth and no obvious
gap was observed between the Ag wire and glass sheath. From the EDS result (Figure
1B) and the cyclic voltammogram (CV) in 0.5 M NaOH solution (Figure 1C), the typical
silver energy peaks and the characteristic Ag oxidation peak (0.32 V) and reduction
peak (0.01 V) validated the AgNW was fabricated successfully. It is possible that tiny
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gap between silver wire and glass sheath may be existed during HF etching process,
but from the well-defined sigmoidal-shaped CVs (Figure S2) using single AgNWE at
different scan rate (5 mV/s vs. 5 V/s), it can be obtained that no obvious leakage
existed at Ag/glass surface.2,3
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The electrochemical properties of single AgNWEs were investigated by recording
CVs in a 5 mM Ru(NH3)6Cl3 solution (Figure 1D). From Figure 1D, four well-shaped
steady-state voltammograms could be observed using single AgNWEs with different
lengths (0 nm, 30 nm, 81 nm, 154 nm), respectively, indicating single AgNWEs were
well prepared. According to Bard’s derivation,13 the radius of nanodisk electrode and
the length of nanowire electrode can be calculated from the following equation 1 and
equation 2, respectively.
id = 4nFDCba
(1)
(2)
2푛퐹퐴퐷퐶푏
=
푟0푙푛휏
푖푞푠푠
where id and iqss are the steady-state limiting current from nanodisk electrode and
nanowire electrode, respectively, F is Faraday’s constant, D is the diffusion coefficient,
Cb is the bulk concentration of redox species, a is the radius of disk nanoelectrode, A
is the electrode geometric area, n is the number of electrons transferred per molecule,
2
r0 is the radius of nanoelectrode. τ (τ = 4Dt/r0 ) can be obtained from the time
component t (t = RT/Fv), where v is the scan rate. Moreover, the electrochemistry
diffusion process of Ru(NH3)63+ can be simulated by COMSOL software and the radius
and length of nanowire electrode can be estimated by comparing the experimental
data and the COMSOL simulation result. (Figure S3).2,3 We normalized the forward
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scan curve of CVs shown in Figure 1D, and found that the voltammetric half-wave
potential, E1/2, was shifted gradually towards positive potentials with the decrease of
the wire length of single AgNWEs (Figure S4), indicating more fast mass-transport
rate, which was in good agreement with the results from Pt nanowire and Au
nanowire.2,3
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It is well-known that bimetallic nanostructures have specific properties by adding
a second metal component, which have wide applications in fuel cells, electrocatalysis,
and sensing and bioanalysis.3,14,15 To explore the applications of prepared single
AgNWs, we prepared single platinum-coated Ag nanowire electrodes (Pt@AgNWEs)
through a galvanic replacement reaction (GRR).14,15 The process is simply carried out
by immersing a AgNW into a solution containing 0.01 M K2PtCl6. The concentration of
Pt salt used here is higher than previous reports due to the low reaction rate without
stirring.16,17 The ratio of Pt/Ag and the morphology of the AgNWs are depended on the
immersion time, K2PtCl6, and solution temperature, etc. The GRR was monitored by
recording the CVs of obtained Pt@AgNWEs in a 0.5M H2SO4 solution (Figure 2A).
From Figure 2A, it could be observed that with the immersion time increase (from 0 to
30 min), the oxidation peak of Ag at ~0.47V decreased due to the consumption of Ag,
and the reduction peak increased and shifted negatively (0.41V0.33V) (the reduction
peak of Pt at ~0.30 V vs. SCE), which could be attributed to the rough surface of
Pt@AgNW (shown in Figure S5) and overlay of the reduction peak of Pt.7 Therefore,
the peak current ratios of oxidation/reduction peaks were smaller and smaller in Figure
2A due to more and more AgCl and Pt nanodots coated on the AgNW surface with the
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increase of immersion time during GRR (Table S1). Moreover, the hydrogen
adsorption/ desorption peaks appeared in the potential range -0.4 V - 0V (Figure 2A
inset) with the immersion time increased. Therefore, Pt should be immobilized on the
surface of AgNWs through GRR process,2,3 which can also be confirmed by the EDS
result shown in Figure 2B and the atomic ratio of Pt/Ag is ~ 0.12. From TEM images
showed in Figure 2C, 2D and Figure S5, it is clearly seen that the lattice spacing of
0.229 nm are found in the surface of Pt@AgNW, which is located between the Ag (111)
planes and Pt (111) planes, and can be indexed as (111) planes of Ag-Pt alloy18.
Moreover, the lattice spacing of 0.236 nm is considered as Ag (111) planes. It can be
observed in Figure 2D that the outward profiles of such novel heterostructure is
constituted by nanoproducts (nanoparticles) which is considered to be Pt/Ag or Pt/AgCl
19. According to equation 3 and above discussion, single Pt@AgNW was fabricated
successfully. Moreover, the ratio of Pt/Ag is controllable by adjusting the concentration
of Pt salt and immersion time. From Figure 2D, it can be observed that un-continuous
Pt nanodots are formed on the surface of AgNW, which is favorable for
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electrocatalysis.20
2-
4Ag(s)+PtCl6 (aq)Pt(s)+4AgCl(s)+2Cl-(aq)
(3)
We first investigated the electrocatalytic performance of the prepared single
Pt@AgNW for methanol oxidation reaction (MOR). Before that, the electrochemically
activated surface areas (ECSAs) of NWs were calculated (see Supporting information).
The catalytic efficiencies of the Pt@AgNWs were measured in an aqueous solution
containing 0.5 M CH3OH. For comparison, the catalytic properties of PtNW with the
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similar size were also investigated under the same conditions (shown in Figure 3).
Figure 3A shows the CVs of 0.5 M CH3OH solution using a single Pt@AgNWE and Pt
NWE, respectively. Both of the CVs for MOR showed two anodic peaks, a well-defined
peak appeared at ~ -0.32 V vs. SCE in the forward scan and another peak appeared
at ~ - 0.40 V vs. SCE in the backward scan, which was a typical feature of MOR.20,21
The forward peak is due to the methanol oxidation and the second peak may be
attributed to the removal of incompletely oxidized carbonaceous species formed during
the forward scan.20 From Figure 3A, it could be obtained that the peak current density
of the forward scan using single Pt@AgNW was 108.7 mA cm-2, which was nearly 1.4
times higher than the value using PtNW catalyst (78.6 mA cm-2). It is well-known that
the ratio of forward anodic peak current to the backward anodic peak current (If:Ib) is
often used as an important factor to compare the tolerance of catalysts to poisoning.22
For the single Pt@AgNW catalyst, the ratio of If:Ib is estimated to be 8.9, which is 4
times higher than that using PtNW catalyst (2.1). Furthermore, the onset potential of
the single Pt@AgNW (-0.69 V vs. SCE) is obviously more negative than these of the
PtNW (-0.5 V vs. SCE). Therefore, from the larger peak current density, higher If:Ib
ratio and lower onset potential, it reveals that the Pt@AgNW has excellent catalytic
activity towards MOR, which is much better than other Pt@Ag catalysts (Table S2).
Furthermore, single Pt@AgNW also shows good long-term catalytic activities and
stability for MOR (Figure S6 and S7). Experiments revealed that the element ratio of
Pt and Ag and the size of Pt@AgNW played important roles for MOR (Figure S8 and
Figure S9). The high performance for MOR of Pt@AgNW may be attributed to the
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following two reasons: First, the rough surface of Pt@AgNW can provide more active
sites, which was proved by the larger current density. Second, the synergistic effect
between the two metals in Pt@AgNW23,24 which can be proved by CO stripping, the
adsorbed CO molecules can be oxidized at lower potential on Pt@AgNW than on
PtNW (Figure S10).
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Developing multifunctional metallic substrates for in situ SERS monitoring catalytic
reaction have received significant attention.25 We investigated the feasibility of using
the single Pt@AgNWs for in situ SERS monitoring the reduction of 4-nitrothiophenol
(4-NTP) by NaBH4. The single Pt@AgNW was incubated with a 4-NTP solution for 2
hours, which resulted in the absorption of 4-NTP on NW.26 Then NaBH4 solution was
dropped on the tip of single Pt@AgNW to initiate the catalytic reaction. At initial stage,
Figure 3B shows the SERS spectrum of 4-NTP with four characteristic vibrational
bands at 857, 1108, 1336, and 1572 cm-1, corresponding to the C-H wagging, C-S
stretching, symmetric nitro stretching, and phenyl ring modes, respectively.25 After the
addition of NaBH4, the four bands decreased gradually, and new bands at 1143, 1388,
1430 cm-1 appeared, which attributed to the C-N symmetric stretching, N=N stretching,
and C-H in-plane bending modes of 4,4’-dimercapto-azobenzene(4,4’-DMAB).25 As
the reaction progressed, 4-aminothiophenol (4-ATP) is the final reaction product and
the corresponding band emerged at 1591 cm-1. From above discussion, it suggested
that the reduction of 4-NTP to 4-ATP using single Pt@AgNW had two stages, and 4,4’-
DMAB was the intermediate which was consistent with previously reports.25. To further
investigate the reduction process of 4-NTP, the control experiments were taken place:
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first, the SERS spectra of single AgNW and single PtNW after adding NaBH4 at 0 and
30 min were recorded (shown in Figure S11), the SERS spectra remained the same
after 30 min of dropping NaBH4 on Ag NW, indicating that the AgNW has no obvious
catalytic activity in the reduction of 4-NTP.27 and no SERS spectra was recorded on Pt
NW indicating that single PtNW is not SERS-active substrate28. To exclude the
possibility of photoinduced reduction of 4-NTP to 4-ATP, the SERS spectra of 4-NTP-
functionalized single Pt@AgNW were recorded in the absence of NaBH4. After 30 min,
the SERS remained unchanged (shown in Figure S12). Additionally, we noticed that
the intensity of SERS spectra on the single Pt@AgNW declined only ~ 20% compared
with that on the bare single AgNW, which indicated the less disturbance of Pt on the
surface of AgNW for SERS test.
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In summary, we have developed a simple approach for the fabrication of single
AgNWE and single Pt@AgNWE by laser-assisted pulling technique. Single Pt@AgNW
exhibited excellent electrocatalytic activity towards MOR and high sensitivity for
monitoring the reduction of 4-NTP by recording SERS spectra. The catalytic activity of
single Pt@AgNW is depended on the ratio of Pt/Ag and the length of nanowire. This is
the first attempt to investigate the MOR and SERS properties by nanomaterials at
single nanowire level, which will open a new perspective for nanomaterials research.
Acknowledgements
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We thank Prof. Bo Zhang (University of Washington) for his useful suggestions and
discussion. This work is financially supported by the National Natural Science
Foundation of China (No.21775003, No.21375002).
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Supporting Information
Detailed Experimental Section, Figures, and Supporting results are described. The
supporting information is available free of charge via the internet at http://pubs.acs.org
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Figure 1. (A) TEM image of a single AgNWE with a radius of ~30 nm and a length of
~150 nm (B) EDS spectrum of AgNWE (C) CV of AgNWE in 0.1M KOH solution.
Starting Radius: 50 nm Length: 180 nm. Scan rate：20 mV/s; Potential range: -0.8 ~
0.6 V. (D) CVs of 5 mM Ru(NH3)6Cl3 in 0.1M KCl aqueous solution at a single AgNWE.
Scan rate：20mV/s; Potential range: 0.1 ~ -0.4V.
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Figure 2. (A) CVs of four AgNWEs measured in a 0.5 M H2SO4 with different
immersing time. 0 min：radius 60 nm, length 190 nm; 10 min: radius 54 nm, length
185 nm; 20 min: radius 68 nm, length 192 nm; 30 min: radius 63 nm, length 188 nm.
The Inset figure in (A) is the magnification of circled section of CVs from 0.05 to -0.4
V. (B) EDS spectrum of Pt@AgNW. (C, D) HRTEM images of the Pt@AgNW.
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Figure 3. (A) CVs of single Pt@AgNW (black curve) and PtNW (red curve) in a 0.5 M
CH3OH + 0.5 M KOH. Pt@AgNW：radius, 90 nm, length, 288 nm, Pt NW: radius, 93
nm, length, 291 nm. Scan rate: 20 mV/s; Potential range: -0.8 ~ 0.1 V. (B) SERS
spectra for monitoring the reduction of 4-NTP to 4-ATP using single Pt@AgNW as
substrate. Single Pt@AgNW: radius, 105 nm; length, 840 nm.
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