PtPd nanowire arrays supported on reduced graphene oxide as advanced electrocatalysts for methanol oxidation
Introduction
Pt nanowires, as one-dimensional (1D) nanostructure, have been demonstrated as high performance catalyst for various catalysis applications, benefiting from their anisotropic structure and unique surface properties [1], [2]. For practical applications, Pt nanowires can be grown on various supports, e.g. metal gauzes [3], [4], carbon paper [5], [6], carbon black [7], carbon fiber [8] or nanotubes [9], or even directly in water [2]. Recently, graphene has also attracted tremendous attention for the utilization in electrochemical conversion and storage applications, in particular as catalyst support in fuel cells, due to its high conductivity (103–104 S m−1), large surface area (calculated value, 2630 m2 g−1), unique graphitized basal plane structure, and potentially low manufacturing cost [10], [11], [12]. To improve the Pt utilization and enhance the catalytic activity, Pt nanoparticles and nanodendrites have been dispersed on graphene surfaces to achieve high performance fuel cell catalysts [13]. However, with these graphene-based hybrids, the drawback from Pt nanoparticle or nanodendrite themselves can still not be overcome, e.g. Ostwald ripening and dissolution [14], [15], [16], in addition with the serious stacking and folding of graphene nanosheets in catalyst preparation caused by their 2-dimensional morphology and soft feature [10], [17], thus the poor catalyst performance and stability can still not be effectively improved in practical applications. To address these issues, the synthesis of graphene/Pt nanowires has been demonstrated with reduced graphene oxide (RGO) as initial supports [10], [11], [18]. However, although some functional groups existed on GO surface, the inert surface properties of the framework make it very difficult to grow uniform Pt nanowires to produce a high performance catalyst. Fig. 1 shows SEM and TEM images of Pt nanowires grown directly on RGO surface with formic acid as the reducing agent at room temperature, by an approach as reported in the literature [18]. It can be seen that, similar to the reported results, Pt nanowires grew only on some RGO nanosheets, and due to the limited nucleation sites on the surface, most Pt nanowires assembled each other to form large superstructures with a size of 50–500 nm, and even some huge aggregates with a diameter of 1 μm or larger (Fig. 1d). Recently, Chen and coworkers have demonstrated the synthesis of Pt nanowire arrays on sulfur-doped graphene which was achieved by heating graphene with phenyl disulfide (PDS) at a high temperature of 1000 °C [12]. But it still remains a grand challenge to synthesize graphene nanosheet/Pt nanowire hybrids via a mild chemical route.
Motivated by need, in this work, we demonstrate a new facile wet-chemical approach for the synthesis of uniform Pt nanowire arrays supported on reduced graphene oxide nanosheets. The whole preparation strategy is displayed in Fig. 2. After partially reducing and a surface functionalization of graphene oxide nanosheets, Pd nanoparticles were first introduced as growing seeds on the nanosheet surface. Then by controlling the growth reaction kinetics of the chemical synthetic process, single crystal Pt nanowires were grown with Pd nanoparticles as seeds, thus a hybrid nanostructure of RGO/PtPd nanowires were achieved. Formic acid was used as reducing agent for the deposition of Pd nanoparticles and the growth of Pt nanowires. Both deposition processes were achieved in aqueous solution at room temperature, without using any template, surfactant or organic solvent.
Section snippets
Materials
Poly(N-vinyl-2-pyrrolidone) (PVP·K30, molecular weight = 30,000−40,000), H2PtCl6·6H2O, PdCl2, HCOOH, ascorbic acid (AA), and ethanol were purchased from Sigma–Aldrich UK and used as received without further purification. Single layer graphene oxide water dispersion (thickness 0.43–1.23 nm, diameter 1.5–5.5 μm, dispersed in water with 2 wt%) was purchased from US Research Nanomaterials, Inc. The state-of-the-art Johnson-Matthey HiSpec™ 1000 Pt black was purchased from Alfa Aesar for comparison with
Results and discussion
Fig. 3 shows typical TEM images and the corresponding diffraction pattern of Pd nanoparticles loaded on PVP-functionalized RGO nanosheets. It is observed that small Pd nanoparticles have a size of ca. 5 nm in average. The high-resolution (HR) TEM image in Fig. 3c indicates that they are single crystal with many (1 1 1) facets. Due to the very small lattice mismatch (only 0.77%) between Pd and Pt, these Pd nanoparticles on RGO nanosheets would work as seeds to direct the growth of Pt nanowires upon
Conclusions
We have developed a facile wet-chemical approach to prepare RGO nanosheet/PtPd-nanowire hybrid nanstructures. The whole deposition process was achieved in aqueous solution at room temperature. Uniform Pt-nanowire arrays were successfully grown on RGO nanosheet surfaces, led to a large ECSA close to the conventional Pt black. Most importantly, the hybrid nanostructures exhibited a higher catalytic activity and stability toward the methanol oxidation reaction (MOR), where a mass activity of 0.51 A
Acknowledgements
YX Lu was sponsored by a joint Li Siguang Scholarship from the University of Birmingham (UoB) and the China Scholarship Council (CSC). We gratefully acknowledge our colleague, Dr Surbhi Sharma for her support on early feasibility experiments for growing Pt nanowires on multi-layer graphene oxide nanosheets. X-ray photoelectron spectra were obtained at the National EPSRC XPS User’s Service (NEXUS) at Newcastle University, an EPSRC Mid-Range Facility (NEXUS). TEM analysis was performed at Leeds
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