Controlled Synthesis of Pt Nanowires with Ordered Large Mesopores for Methanol Oxidation Reaction

Catalysts for methanol oxidation reaction (MOR) are at the heart of key green-energy fuel cell technology. Nanostructured Pt materials are the most popular and effective catalysts for MOR. Controlling the morphology and structure of Pt nanomaterials can provide opportunities to greatly increase their activity and stability. Ordered nanoporous Pt nanowires with controlled large mesopores (15, 30 and 45 nm) are facilely fabricated by chemical reduction deposition from dual templates using porous anodic aluminum oxide (AAO) membranes with silica nanospheres self-assembled in the channels. The prepared mesoporous Pt nanowires are highly active and stable electrocatalysts for MOR. The mesoporous Pt nanowires with 15 nm mesopores exhibit a large electrochemically active surface area (ECSA, 40.5 m2 g−1), a high mass activity (398 mA mg−1) and specific activity (0.98 mA cm−2), and a good If/Ib ratio (1.15), better than the other mesoporous Pt nanowires and the commercial Pt black catalyst.

membrane and self-assembled silica or polymer spheres) techniques (referred to as hard/hard-templating method). The dual templating method provides a satisfactory way to control the pore size and the surface structure of the wires. The diameter of the porous metal wires can be confined by the channels of the porous membrane and the pore structure can be controlled by the mesoporous template (such as LLCs as a soft template [17][18][19] , or self-assembled spheres as a hard template [20][21][22] ) priorly deposited in the channels of the porous membrane. Compared to the hard/soft-templating method, the hard/hard-templating method based on 1D porous membrane and self-assembled nanospheres (< 50 nm) would have great advantage in producing nanoporous metal nanowires with controlled large mesopores (> 10 nm), which are important for efficient diffusion of guest species during the reactions. The nanospheres (e.g., silica nanospheres) used to fill in the porous membrane can be facilely prepared by the published method 23,24 and their easily controlled diameters of 10-50 nm can define the mesopores of the metals 25 . Also, the long-range order of the mesoporous structures in the final metal replicas can be obtained from the close-packed nanospheres with high thermal stability.
Li et al. 20 first reported the preparation of porous metal (Au and Ni) wires with controllable morphology from directed assemblies of spheres in 1D porous membranes, but the macroscale of the pores limits their applications. Bechelany et al. 22 reported a similar dual templating method to fabricate Co nanowires with controlled porosity. However, in the previous reports, the systematic study of the fabrication of metal nanowires with various large mesopores (10-50 nm) and their properties has not been described. Furthermore, electrochemical deposition was typically used to grow the metals inside the dual templates in these reports. By contrast, chemical reduction deposition offers a simple and cost-effective way to replicate the structure of the porous template in the whole thickness range, without the need of using external electric field sources and electronically conductive substrates 26,27 .
In this study, we first report the chemical reduction fabrication of the mesoporous Pt nanowires with ordered large mesopores from AAO membranes with silica nanospheres assembled in the channels. Also, the electrocatalytic properties of the mesoporous Pt nanowires for the methanol oxidation reaction (MOR) are investigated. The Pt nanowires with ordered large mesopores would be desirable to improve the electrocatalytic activity due to the efficient transport of molecules and ions from the interconnected ordered large mesopores for increasing accessibility to the active sites, as well as enhance the durability owing to their longitudinal axis structures. Figure 1 illustrates the synthetic procedure for the preparation of the Pt nanowires with large mesopores (see Methods for more details). First, silica nanospheres were self-assembled in the channels of the AAO membrane to form the AAO-silica composite. Then Pt was deposited in the void space of the AAO-silica composite by chemical reduction deposition method. Subsequent removal of the AAO membrane and silica nanospheres with a dilute HF solution produced the freestanding mesoporous Pt nanowires. In order to obtain well-defined interconnected mesoporous structure over the whole nanowire length, the ratio of the AAO pore size to silica nanosphere diameter was selected larger than 4. In this work, we prepare the mesoporous Pt nanowires produced from 15, 30 and 45 nm silica nanospheres in 200 nm pore AAO membranes (abbreviated as 15/200-Pt, 30/200-Pt and 45/200-Pt nanowires, respectively). Figure 2A shows a typical scanning electron microscopy (SEM) image of the nanowires composed of 45-nm diameter silica nanospheres, obtained by etching away the AAO template from the AAO-silica composite using a phosphoric acid solution. The silica nanospheres are arranged in a hexagonal close-packing, which is essential for the formation of ordered mesoporous Pt nanowires. Figure 2B reveals the SEM image of the 45/200-Pt nanowires. The average diameter of the Pt nanowires is ca. 300 nm that disagrees with the surface pore size (~200 nm) of the AAO membrane, because the diameter of the inside channels of the AAO membrane is larger than that of the surface pores. Ordered spherical mesopores can be clearly seen from the inset of Fig. 2B. The porous Pt nanowires exhibit well-ordered mesoporous structure and the mesopore size is ca. 45 nm that is consistent with the particle size of the original silica nanospheres, indicating negligible shrinkage of the metallic structure during removal of the silica template. The corresponding transmission electron microscopy (TEM) image of the 45/200-Pt nanowires with ordered mesopores is shown in Fig. 2C. The interconnected ordered mesoporous structure is important for the facile diffusion of reactants during the electrocatalytic electrode reactions. The selected area electron diffraction (SAED) pattern exhibits polycrystalline face-centered cubic (fcc) Pt feature with homogenous diffraction rings (Fig. 2D), which corresponds to the presence of many small Pt nanoparticles in the mesopore walls. The detailed structural characterization of the 15/200-Pt nanowires is presented in Fig. S1 (Supplementary Information), also revealing the well-ordered mesoporous structure of the Pt nanowires. Figure Fig. 3A) reveals the lattice fringes with a spacing of 0.23 nm in the mesopore walls, which is in agreement with the d-spacing of adjacent (111) crystallographic planes in face-centered cubic (fcc) Pt, indicating a high crystallinity of the Pt nanoparticles. These mesoporous Pt nanowires have the same diameter, but with different mesopore sizes. As can be seen in the images, the mesopore walls are composed of small Pt nanoparticles. With the decrease of the mesopore size, the number of Pt nanoparticles aggregated in the mesopore walls reduces.

Results
Powder X-ray diffraction (XRD) was employed to further investigate the Pt particle size of the catalysts (Fig. 4). The Pt nanoparticle sizes of the 15/200-Pt, 30/200-Pt and 45/200-Pt nanowires, as well as the commercial Pt black catalyst estimated by the Scherrer equation are 3.8 nm, 4.9 nm, 5.2 nm, and 5.5 nm, respectively. In overall trend, the Pt particle size increases with the mesopore size of the Pt nanowires. Figure 5 compares the MOR performances of the mesoporous Pt nanowires and the commercial Pt black catalyst. The electrocatalytic results of the four catalysts are summarized in Table 1. Figure 5A   may be attributed to the smallest interconnected mesopores resulting in highly dispersed Pt nanoparticles 17 . In addition, the linear sweep voltammograms (LSVs) and the corresponding Tafel plots (Supplementary Fig. S2) also suggest that the 15/200-Pt nanowire catalyst is promising as a potential practical electrocatalyst for methanol oxidation.
Since the agglomeration of Pt particles on the mesopore walls becomes more severe with the increase of the pore size of Pt nanowires, the 45/200-Pt nanowires exhibit smaller ECSA and lower mass activity than the commercial Pt black catalyst even though the Pt particle size of the 45/200-Pt nanowires is a bit smaller than that of the commercial Pt black catalyst. However, the 45/200-Pt nanowires have improved specific activity compared with the Pt black catalyst, which might arise from the 1D structure that could facilitate the reaction kinetics and improve the diffusion of reactants due to the path directing effects of the structural anisotropy 35,36 .
From Fig. 5B, the ratio of I f (the peak current of the forward scan) to I b (the peak current of the backward scan) of the 15/200-Pt nanowires catalyst is 1.15, which is higher than that of the 30/200-Pt nanowires (1.08), 45/200-Pt nanowires (1.02), and commercial Pt black (0.90). A higher I f /I b is indicative of better tolerance for poisoning of carbonaceous species 37 . The I f /I b value of the 15/200-Pt nanowire catalyst is comparable to that of the reported mesoporous Pt nanoparticles (1.07) 34 , macro-/mesoporous Pt (1.25) 12 , and mesoporous Pt nanorods (1.25) 16 .
The chronoamperomograms (Fig. 5D) recorded at 0.8 V indicated that the current density of the 15/200-Pt nanowires was higher than that of the 30/200-Pt and 45/200-Pt nanowires, and the commercial Pt black catalyst over the entire time range, also demonstrating the improved electrocatalytic performance of the 15/200-Pt nanowires for the MOR. In addition, the long-term poisoning rate (δ ) is calculated by measuring the linear decay of the current for a period of more than 5000 s from Fig. 5D 19,30 . An increase in the mesopore size of Pt nanowires results in a decrease in the anti-poisoning performance and stability, which may arise from the more severe aggregation of Pt particles in the mesopore walls ( Fig. 3A-C) that deteriorates the mass transfer efficiency and makes the Pt particles grow rapidly in the long-time reaction. The 45/200-Pt nanowires show better poisoning tolerance in the first CV curve but worse stability in the long-time reaction than the commercial Pt black, which is probably attributed to that the agglomerated Pt nanoparticles on the mesopore walls tend to grow faster than the dispersed Pt nanoparticles of the commercial Pt black.

Discussion
In summary, mesoporous Pt nanowires with ordered uniform large mesopores (15 nm, 30 nm, and 45 nm) have been successfully synthesized by chemical reduction deposition method from a dual-templating technique using the porous AAO membranes with silica nanospheres self-assembled in the channels as the templates. The mesopore walls of the Pt nanowires are composed of polycrystalline Pt nanoparticles, and as the mesopore size increases, more Pt nanoparticles aggregate in the mesopore walls. Overall, the prepared mesoporous Pt nanowires exhibit enhanced electrocatalytic activity, and improved poisoning tolerance and durability for the methanol oxidation reaction (MOR) compared with the commercial Pt black catalyst, which may be related to the interconnected ordered mesopores that allow the facile transport of the reactant and product molecules and ions to and from the catalyst and the 1D structure that could facilitate the reaction kinetics as well as improve the diffusion of reactants. Due to the smaller and more dispersed Pt nanoparticles in the mesopore walls, the mesoporous Pt nanowires with 15 nm mesopores have better MOR performance than the other mesoporous Pt nanowires. It is expected that the present method could be extended to prepare other ordered mesoporous metal (e.g., Ni, and Pd) or alloy (e.g., Pt-Ni, and Pt-Ru alloys) nanowires, which may be of technological importance in various fields, such as nanodevices, fuel cells, and catalysis.   Synthesis of silica nanospheres. The silica nanospheres of about 15, 30, and 45 nm diameters were prepared by hydrolysis of TEOS in the presence of L-lysine 11 . To fabricate the silica nanospheres with diameter of ~15 nm, 0.19 g of L-lysine, 12.00 g of ethanol and 180 mL of water were mixed in a three-necked flask and stirred for 20 min. Then the flask was transferred into an oil bath and stirred at 90 °C, after which 13.50 g of TEOS was added under stirring at 500 rpm. The silica nanosphere (~15 nm in diameter) sol was obtained after continuous magnetic stirring for 48 h. The silica nanospheres of ~30 nm diameter were fabricated through a seeded growth approach with the ~15 nm nanospheres used as seeds. Subsequent repeated addition of TEOS (each time 27.00 g), followed by stirring (500 rpm) at 90 °C for 48 h, was carried out twice to increase the particle size to ~30 nm. The ~45 nm silica nanospheres were produced following the same steps using the ~15 nm nanospheres as seeds, but each repeated addition of TEOS is 47.25 g in mass.

Synthesis of AAO-silica template.
To get the AAO-silica template, a piece of AAO membrane was put in the bottom of a 50 mL beaker containing 10 mL silica nanosphere sol (5 wt.%), and then the beaker was placed in 40 °C oven. After evaporation the solvent for 2 days, the silica nanospheres were self-assembled in the channels of  Characterization. Scanning electron microscopy (SEM) images were obtained on a Hitachi S-4700 FEG scanning electron microscope. Transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) were carried out on a Hitachi H800 and a JEOL JEM-3010 transmission electron microscope, respectively (both operating at 200 kV). Powder X-ray diffraction (XRD) data were collected on a Shimadzu XRD-6000 diffractometer with Cu Kα radiation (λ = 1.5418 Å).
Electrochemical experiments. Cyclic voltammetry (CV) and chronoamperometry measurements were carried out in a potentiostat/galvanostat (Reference 600, Gamry Instruments) using a conventional three-electrode cell consisting of a glassy carbon (GC) electrode (5 mm in diameter) as the work electrode, a double junction Ag/AgCl (saturated KCl) electrode as the reference electrode, and a Pt flag as the counter electrode. All electrode potentials were measured against the Ag/AgCl reference electrode and converted to the normal hydrogen electrode (NHE) potentials by using relationship of E NHE = E Ag/AgCl; sat. KCl + 0.197 V. To prepare the working electrode, the GC electrode was polished to a mirror finish using 50 nm alumina suspensions, and cleaned by ultrasonication in nitric acid, ethanol, and deionized water. The catalyst (5 mg) was ultrasonically dispersed in 5 mL ethanol by ultrasonication for 1 h to obtain a 1.0 mg mL −1 homogeneous ink-like suspension. A volume of 5 μ L of the suspension was then pipetted onto the GC substrate. After drying at room temperature, 5 μ L of Nafion solution (5 wt.%) was pipetted on the GC substrate and dried completely. The CV tests were performed in argon-purged 0.5 M H 2 SO 4 solution with or without 1 M CH 3 OH at room temperature.