Single Pt Nanowire Electrode: Preparation, Electrochemistry, and

Article pubs.acs.org/ac

Single Pt Nanowire Electrode: Preparation, Electrochemistry, and Electrocatalysis Yongxin Li,*,†,‡ Qingqing Wu,† Shoufeng Jiao,† Chaodi Xu,† and Lun Wang† †

College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, China Western Transportation Institute, College of Engineering, Montana State University, Bozeman 59717-4250, United States

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S Supporting Information *

ABSTRACT: A single Pt nanowire electrode (SPNE) was fabricated through HF etching process from Pt disk nanoelectrode and an underpotential deposition (UPD) redox replacement technique. The electrochemical experiments showed that SPNE had steady-state electrochemical responses at redox species solution and the mass transfer rates were affected by the lengths and radii of SPNEs. The prepared SPNEs were utilized to examine the oxygen-reduction reaction in a KOH solution to explore the feasibility of electrocatalytic activity of single Pt nanowire and the results showed that the electrocatalytic activity of SPNE was dependent on the surface position of single Pt nanowire: the tip end position is more active than the sidewall position. Meanwhile, the electrocatalytic activity of SPNE was related to the radius of nanowire. These observations are not only important to understand the structure−function relationship in single nanowire level but have significant implications for the synthesis and selection of novel catalysts with high efficiency used in electrochemistry, energy, bioanalysis, etc.

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underpotential deposition (UPD) redox replacement process.10b,12 Electrochemical experiments showed that the SPNE still had steady-state responses, which is in accordance with theoretical discussion.13 It also found that the mass transfer rates were affected by the lengths and radii of SPNEs. Electrocatalytic experiments toward oxygen reduction reaction (ORR) have revealed that active of SPNE is surface positiondependent and size-dependent: the tip end position and small radius are more active than the sidewall position and large radius. These observations can help us understand the structure−function relationship in single nanowire level, and have significant implications for the synthesis and selection of novel catalysts with high efficiency used in electrochemistry, energy, bioanalysis, etc.

ne-dimensional (1D) nanomaterials have stimulated great interest recently because of their importance in basic research and potential applications.1 In particular, 1D platinum nanostructures, such as nanowires, have received more attention because they can be used as a high-efficiency elecrocatalyst for fuel cells and biosensors.2 To date, many methodologies have been developed for preparing 1D platinum nanowires, such as template synthesis,3 wet-chemical method,4 galvanic displacement,2a,5 and the electrospinning method.2b,6 Despite many methodologies listed above have been developed, it is still interesting to develop a simple and mild route to fabricate 1D platinum nanowires. Moreover, the electrocatalytic process of platinum nanowires in-depth is still unclear. Recently, Dai et al.,7 Chen et al.,8 and Miller et al.9 have found that the tips and sidewalls of carbon nanotubes have different electrocatalytic activity and electron-transfer rate, which inspires us to investigate the electrocatalytic properties of Pt nanowires at different area positions, such as tips and sidewalls. Most importantly, traditional measurements,2a−c,10 which analyze the electrocatalytic reaction through the ensembles of Pt nanowires, often only obtain their averaged catalytic behaviors, whereas individuality of Pt nanowire is lost. Therefore, developing a method capable of resolving the individual behaviors of Pt nanowires is thus highly desired. Recently, Yang et al. reported the performance of a single platinum nanowire for the detection of H2 in air.2f Herein, a simple method has been developed to fabricate single Pt nanowire electrode (SPNE) with different diameter and length. This method includes two steps: one is the etching process through immersing a single Pt disk nanoelectrode prepared previously11 into HF solution; another is a copper © 2013 American Chemical Society

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EXPERIMENTAL SECTION Chemicals and Materials. Ferrocene (Fc, Fluka), ferrocenemethanol (FcCH2OH, 97%, Aldrich), potassium ferricyanide (K3Fe(CN)6, Acros Organics), tetra-n-butylammoniumhexafluorophosphate (TBAPF6, Aldrich), potassium chloride (KCl, Mallinckrodt Baker), acetonitrile (ACN, Mallinckrodt Baker) were of reagent grade quality or better and used without further purification. Aqueous and organic solutions were prepared from deionized water (Barnstead Nanopure Systems) and acetontrile, respectively. Pt wire with a diameter of 25 μm (purity 99.95%; hard) was purchased from Alfa-Aesar. The used quartz-glass capillaries (o.d. = 1.2 mm, i.d. = 0.40 mm) were Received: January 31, 2013 Accepted: March 18, 2013 Published: March 18, 2013 4135

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supplied by Sutter Instrument Co. The silver-filled epoxy glue (DuPont) was used to contact the Pt-wire with a Tungsten wire. Alumina polishing suspension with different particle sizes, 1.0, 0.3, and 0.05 μm, and finer grit sandpaper with different types, 400, 600, and 800 grits, were purchased from Buehler. Fabrication of SPNEs. The fabrication of Pt disk nanoelectrode using laser-assisted pulling method involves four-step process as previously described.11 Briefly, an ultrasharp Pt nanowire tip ( 100 nm), a ∼150 mV potential shift can be found. It is well-known that the electrocatalytic activity toward ORR is related to the reduction potential: the more positive reduction potential, the higher electrocatalytic activity of catalysts.16 From Figure 5B, it

Figure 5. (A) Voltammetric responses of an oxygen-saturated 0.10 M KOH solution using a bare 8 nm diameter Pt nanoelectrode (black), and the same electrode etched in 1:4 HF (v/v) solution for 3 s (red), 5 s (green), 15 s (blue), 40 s (cyan), 1 min (magenta), and 2 min (purple). The scan rate was 10 mV/s and the corresponding lengths of SPNEs were 0, 2, 7, 18, 75, 270, and 950 nm, respectively. (B) After normalization of the current to the limiting value.

can be concluded that the tip end positions of SPNE should be more active than side wall positions, which is similar to the results obtained from carbon nanotubes.7−9 It is well-known that the mass transport rate is always affected by the size of electrodes or particles.21 To further discuss the electrocatalytic process of ORR using different length or radius of SPNEs, the relationships between the limiting current density/E1/2 and the length of Pt nanowire have been obtained. Figure 6A shows that both the limiting current density in Fc solution and oxygen-saturated solution are decreased when the length of Pt nanowire increases, which means the mass transport rate is affected by the length of Pt nanowire. However, if normalizing the limiting current density in Figure 6A, it can be found that the current density obtained from ORR decreases more quickly than that from Fc oxidation (shown in Figure 6B). Moreover, the E1/2 in ORR has an obvious decrease whereas the value in Fc oxidation almost keeps constant (see Supporting Information Figure S3). Thus, we can conclude that ORR on the surface of Pt nanowire is an electrocatalytic process, not only mass transport effect. The electrocatalytic activity toward ORR is also affected by the radii of SPNEs, which can be observed from the relationship between the normalized current density and the lengths of SPNEs with different radii (shown in Supporting Information Figures S4 and S5). Supporting Information Figure S4 gives the relationship between the normalized current of SPNE scanning in 5 mM Fc + ACN solution and the lengths of SPNEs with different radii, whereas Supporting Information Figure S5 shows the relationship in oxygen-saturated 0.10 M KOH aqueous solution. It can be seen that the normalized current density from all the SPNEs with different radii is 4138

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technique. Electrochemical experiments showed that the SPNE had steady-state responses, which is in accordance with theoretical discussion.13 From the shift of half-wave potentials (E1/2) of different lengths of SPNEs, it could be obtained that the SPNE with short length had faster mass transfer rate. Experimental and simulation results showed the electrocatalytic activity of SPNE was affected by the lengths and the radii of single Pt nanowire. With the increase of wire length, the limiting current density collected from Fc oxidation and ORR is decreased greatly, and the E1/2 for ORR has negative shift, whereas the value for Fc oxidation is almost keeping constant. From the obvious E1/2 shift and the more limiting current density decrease for ORR, it can be obtained that the electrocatalytic activity at the tip end position of SPNE is higher than the side wall position. Moreover, the radius of Pt nanowire is another issue that can affect its electrocatalytic activity. These observations can help us understand the structure−function relationship in single nanowire level and have significant implications for the synthesis and selection of novel catalysts with high efficiency used in electrochemistry, energy, bioanalysis, etc.

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Figure 6. (A) Relationship between the limiting current density (limiting current value divided by surface area) and the length of Pt nanowire scanning in a 5 mM Fc ACN solution (black dots) and an oxygen-saturated aqueous solution containing 0.10 M KOH (red dots). (B) Relationship between the normalized limiting current density in panel A and the length of Pt nanowire scanning in a 5 mM Fc ACN solution (black dots) and an oxygen-saturated aqueous solution containing 0.10 M KOH (red dots). Radius of the SPNE ≈ 4 nm.

ASSOCIATED CONTENT

S Supporting Information *

Additional materials as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 1-406-994-6719. Fax: 1-406-994-1697. E-mail: [email protected], [email protected].

decreased with the length increase of SPNEs. However, the normalized current density of ORR and the oxidation of Fc from the small radius SPNEs decreases greatly, indicating the fast mass transport rate from small radius electrodes. Moreover, from the lower decreased ratio of normalized current density with different radii (e.g., 3 and 44 nm) obtained from ORR and the oxidation of Fc (∼0.32 vs 0.53), it can be obtained that the SPNEs with small radii have higher electrcatalytic activity, which can also be observed from the change of E1/2 of ORR shown in Supporting Information Figure S6 (the E1/2 of ORR has larger shift with the decrease of radii of SPNEs). Previous researchers have reported that 1D nanostructures such as Pt nanowires display highly active low energy crystalline facets and relatively few defect sites.22 Diao et al.23 proved that surface stresses could cause gold nanowires to transform from a face-centered-cubic structure to a body-centered-tetragonal structure, which was also controlled by wire size, initial orientation, boundary conditions, temperature, and initial cross-sectional shape. For our case, the catalytic activity of SPNE at different surface positions and different sizes is probably ascribed to the contribution of surface stress or other issues, which can make the phase transformation of SPNE at different surface position and different sizes. Currently, we’re trying to get the HRTEM images of SPNE and other bimetallic single nanowire, which can help us to investigate the reaction mechanism of ORR using SPNE.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Bo Zhang (University of Washington) for his useful suggestion and discussion. This work is financially supported by the National Natural Science Foundation of China (No.20975002), the Key Project of Chinese Ministry of Education (No. 212083), and Anhui Normal University. Parts of the experiments were done at University of Washington.

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CONCLUSIONS In summary, a simple method has been established to fabricate SPNE through HF etching process from Pt disk nanoelectrode and an underpotential deposition (UPD) redox replacement 4139

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