Gram-Scale Synthesis of Pt-Cu Nanowires with Enhanced Electrocatalytic Activity towards Methanol Oxidation Reaction

A facile method to prepare Pt-Cu nanowires (NWs) was introduced. Structural characterization such as high-resolution transmission electron microscope (HR-TEM), selected-area electron diffraction (SAED), EDS element mapping, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and inductively coupled plasma mass spectrometry (ICP-MS) showed the formation of Pt-Cu alloy, with a width of 4.5 nm on average. The formation process of Pt-Cu NWs was studied; it was found that bromine ion, who has preferential adsorption on Pt (100) face, served as a growth-directing agent; Brij58 not only served as a protector but also played an important role in forming Pt-Cu NWs; the mechanism was proposed. Their electrocatalytic activity towards methanol oxidation was investigated; we found that the current density of Pt-Cu NWs was 295mA·mg when the ratio of Pt/Cu is 1 : 1, which is 11.5 and 2.35 times higher than that of pure Pt (26mA·mg) and commercial Pt/C (126mA·mg). The high electrocatalytic activity is attributed to the presence of abundant structural defects and surface active sites on the synthesized Pt-Cu NWs.


Introduction
Direct methanol fuel cell (DMFC) technology is promising in energy conversion because of the rich resources, high energy density, and low pollution [1,2]. Platinum nanomaterials are commonly used as the anode catalysts in the methanol oxidation reaction (MOR) on account of their high electrocatalytic activity [3,4]. Nevertheless, the commercial application of DMFC is still limited by the high cost and low antipoisoning ability of Pt. To address these issues, substantial efforts have been devoted by partially replacing Pt with transition element (Cu, Co, Fe, etc.) [5,6]. Among various Pt/nonnoble metal alloys, Pt and Cu have a significantly favorable synergistic effect [7,8].
As we know, the tuning of the structure and morphology of nanomaterials can further improve the catalytic activity. For instance, Li and coworkers synthesized Pt-Cu nanotubes, which showed attractive catalytic activity of 6.09 mA cm -2 and high stability towards MOR compared with commercial Pt black [9]. Zhang et al. prepared diverse Pt-Cu alloy nanocrystals (dendrite, yolk-cage, and box) by changing the concentration of glycine; these catalysts with different morphology exhibited enhanced electrocatalytic performance for MOR (the highest specific activity from yolk-cage dendrite reached 2.8 mA cm -2 ) in comparison with commercial Pt black [10]. Lin's group reported an efficient synthetic method of porous Pt 78 Cu 22 nanodendrites with superior electrocatalytic activity towards the oxygen reduction reaction [11]. Up to now, various Pt-Cu alloyed nanostructures have been reported, including nanocage, nanoconcave, nanocube, nanosheet, hollow nanosphere, and nanowire (NW); they all showed super catalytic properties [12][13][14].
Ultrathin Pt-based alloy NWs with uncommon onedimensional (1D) morphology and high surface area usually exhibit excellent performance, due to their inherent anisotropy characteristic, higher structural stability, reactive surface, and easy electron transport. Accordingly, the Pt-based nanomaterials with such attractive structure have been generally regarded as excellent electrocatalysts. Recently, Dai et al. reported the preparation of ultrathin Pt NWs by a solvothermal method, and these Pt NWs showed remarkable electrocatalytic activity and excellent stability in comparison to Pt/C catalyst towards the oxidation of methanol and formic acid as well as the oxygen reduction reaction [15]. Xia's group successfully synthesized Pt-Sn NWs by a general wetchemical method, which indicated Pt 6 Sn 3 with big surface area and countless defects exhibited superior activity towards MOR and ethanol oxidation reaction (EOR) [16]. Song's group reported that ultrathin PtCu 3 alloy NWs were successfully obtained by introducing CO in the reaction. Compared with the Pt-Ru and Pt black catalysts, the electrocatalytic activity of PtCu 3 for MOR increased by 1.4-fold and 2.0-fold, respectively [17]. Hong and coworkers reported a novel synthesis of ultrafine Pt-Cu NWs by a template-assistant method. The Pt-Ru NWs were applied in MOR and EOR, which showed excellent activity in alkaline conditions [18].
In general, Pt-based NWs were synthesized by the reduction of metal precursors in the organic compounds. Some of these solvents are not environment friendly, and some of them are difficult to remove when they attach to the surface of NWs, which could have negative influences on their catalytic activity. Consequently, it is essential to explore a simple and environment-friendly approach to synthetize Pt-based NWs in an aqueous system. Meanwhile, gram-scale synthesis     Journal of Nanomaterials with high yield is necessary to promote the commercial application of these catalysts. To address the above issues, we developed a facile scalable synthetic strategy for the preparation of Pt-Cu NWs in an aqueous solution without any organic solvent involved. Brand Brij58 were added to form NWs; the formation mechanism was investigated. Moreover, owing to the abundant surface atoms and substantial numbers of surface active sites, the as-prepared Pt-Cu NWs displayed enhanced electrocatalytic activity as well as stability for MOR.

Synthesis of Ultrafine Pt-Cu NWs.
In a typical synthesis, 160 mg of Brij58 and 154.8 mg of NaBr were added into 10 mL of deionized water with stirring at room temperature until Brij58 was completely dissolved. After that, 2.5 mL of H 2 PtCl 6 ·6H 2 O (20 mM) and 2.5 mL of CuCl 2 ·2H 2 O (20 mM) solutions were introduced. The mixed solution was continuously stirred for 15 minutes, then 680 μL of formaldehyde (40%) was added. The reaction mixture was transferred into a 20 mL Teflon-lined stainless-steel autoclave. The sealed vessel was heated under 200°C for 6 h, and it was cooled naturally to room temperature. Finally, the resulting products were collected by centrifugation (8000 rpm for 10 min) and washed with water/ethanol for several times. The obtained samples were dispersed in ethanol for further use.

Material Characterization.
Transmission electron microscopy (TEM) images were determined with an H-7650 microscope operating at 200 kV. HRTEM images and EDS elemental mappings were recorded using an FEI Tecnai F20 FEG-TEM microscope operated at 200 kV. X-ray diffraction (XRD) data were determined by a D-MAX 2500/PC diffractometer. X-ray photoelectron spectroscopy (XPS) analysis was acquired on an ESCALAB 250 spectrometer with Al Kα X-ray excitation. ICP-MS analysis of samples was performed on NexION 350D. The electrochemical tests were carried out by using a CHI 660E electrochemical analyzer at room temperature.     the formation of NWs networks, indicating their highly flexible feature. The average diameter of the NWs was around 4.5 nm. The Pt/Cu atomic ratio determined by the ICP-MS measurement was 45 : 55 (Table S1), which was in accord with the Pt/Cu atomic ratio in the initial reaction system.

Results and Discussion
The selected-area electron diffraction (SAED) image of an individual Pt-Cu NWs (Figure 1(c), inset) displayed that the NWs were polycrystals and the diffraction rings were in good agreement with (111), (200), (220), and (311) planes of the Pt-Cu alloy phase. The high-resolution TEM (HRTEM) image of crystal grains with their lattice orientation (selected from the square area in Figure 1(c)) is shown in Figure 1(d). The interplanar spacing of 0.22 nm was clearly observed, which was smaller than the (111) lattice spacing (0.226 nm) of Pt crystal but larger than the (111) lattice spacing (0.208) of Cu. This confirmed the production of a Pt-Cu alloy. In addition, there were abundant grain boundaries along with the Pt-Cu NWs networks, as indicated by red arrows in Figure 1(c). Moreover, many structural defects (such as atomic steps, corner and edge atoms) were observed on the surface (marked with red circles in Figure 1(d)). It has been researched that these structural defects could serve as active catalytic sites to enhance the catalytic activity effectively [19,20]. The elemental mapping showed these NWs were composed of Pt and Cu, and the two elements were homogeneously distributed over the whole NWs (Figures 1(e)-1(h)).
In order to investigate the surface composition and valence state of Pt-Cu NWs, XPS measurements were used for further characterization (Figure 2). The Pt 4f spectrum exhibits peaks at 74.7 eV and 71.4 eV, which could be originated in Pt 4f 7/2 and Pt 4f 5/2 of metallic Pt 0 [21], respectively. The Cu 2p spectrum exhibited that metallic Cu 0 (932.2 eV) was predominant while a weak signal of Cu 2+ (∼934.5) also exists. The existence of tiny amount of Cu 2+ could be further confirmed by a satellite peak at ∼943 eV, which was attributed to the oxidation of surface Cu atoms in air. An obvious positive shifted (+0.2 eV) in the binding energy of Pt 4f 7/2 was presented compared to pure Pt (71.2 eV). The shift was probably originated in the changes of the electronic structure by the charge transfer between Pt and Cu.
In addition, different Pt-Cu alloy NWs were obtained by adjusting the ratio of Pt 4+ to Cu 2+ (Fig. S1). The width of well-dispersed wavy NWs slightly increased from 4.0 to   Journal of Nanomaterials 4.9 nm with the increase of the Pt/Cu ratio. Shevchenko's group has demonstrated the size of CoPt 3 nanocrystals shrank with the decrease of the Pt/Co ratio, which could be attributed to an increase of the nucleation rate [22]. Similarly, under the current system, the decrease of the Pt/Cu ratio led to the reduction of the width of Pt-Cu NWs, which might be explained by the decrease of nucleation rate. With the low content of Cu precursor, fewer nucleation seeds were generated and therefore the wider Pt-Cu NWs were formed. Additionally, the uniform nanowire network could not be formed in the absence of Cu 2+ and nanoparticles with obvious aggregation were obtained, as shown in Fig. S2. The results demonstrated a key role of Cu precursor in facilitating the formation of Pt-Cu NWs. The crystalline structures of these Pt-Cu NWs were detected by X-ray diffraction (XRD). As shown in Figure 3, the typical diffraction peaks of a face-centered-cubic structure could be observed. Three diffraction peaks were originated from the (111), (200), and (220) planes of Pt-Cu NWs; all peaks were situated between the standard peaks of Pt (JCPDS no. 04-0802) and Cu (JCPDS no. 04-0836), which was indicative of the formation of an alloyed structure [23,24]. The diffraction peaks of all samples were shifted to smaller angle with the increase of the ratio of Pt/Cu, further confirming the formation of alloyed Pt-Cu nanocrystals [25]. Moreover, all diffraction peaks were widened due to the ultrathin feature of NWs.
In order to demonstrate the influence of bromine ions, a series of control experimentation was performed. When NaBr was absent while the other experimental conditions were maintained, as exhibited in Figure 4(a), irregular particles with severe aggregation were obtained. When Brwas replaced by F -, Cl -, and I -, discontinuous and inhomogeneous nanochains were formed with an average size of 14 nm, 7 nm, and 10 nm, respectively, as shown in Figures 4(b)-4(d). The result further confirmed the essentiality of Brin this experiment system. The different morphologies could be ascribed to the following reasons. Firstly, when H 2 PtCl 6 ·6H 2 O precursor was introduced as a solution containing bromine ions, [PtBr 6 ] 2− would become foremost ingredients due to the stronger coordination between Brand Pt 4+ [26][27][28], thus improving the stability and reducing the reaction rate. Secondly, the preferential adsorption of bromine ion on Pt (100) face would restrict 5 Journal of Nanomaterials the growth of this face [29,30], and this bonding between platinum and bromine ions benefited the anisotropic growth of the Pt-Cu NWs.
The surfactant of Brij58 micelles also played an important role in the formation of NWs. In our system, metalaqua complexes could be formed when metallic precursors were coordinated by water molecules. These complexes were adsorbed on the external hydrophilic CH 2 CH 2 O region of the Brij58 micelles owing to the interaction between the CH 2 CH 2 O group and the coordinated water molecules [31]. While there was no surfactant involved in the reaction system, the nanoparticles obtained were heavily agglomerated with indefinite shapes, as shown in Fig. S3a. These results demonstrated that a role of Brij58 was played to prevent the crystal from agglomeration. PVP is a very common surface protector in order to well disperse noble metal nano-structures [32][33][34]. When PVP was used, nanoparticles with a diameter of about 3 nm were obtained without NWs (Fig. S3b). It confirmed that Brij58 not only served as a stabilizing agent but also had a crucial effect on the formation of the Pt-Cu NWs.
The intermediates at different growth stages were characterized by TEM in order to investigate the growth mechanism of Pt-Cu NWs (Figure 5). At the early stage of the reaction (30 min), [PtBr 6 ] 2− and Cu 2+ were reduced quickly by formaldehyde, and substantial sub-4 nm nanoparticles were observed. After 1 h, some nanoparticles began to coalesce with each other and some short NWs formed. After 2 h, a large number of Pt-Cu alloy NWs were obtained in virtue of continuous attachment. The pleasant surprise was the width of NWs remained roughly constant in the process of reaction. Furthermore, the obvious nodes were present in  Journal of Nanomaterials the Pt-Cu NWs (Figure 1(c)) which further indicated that the lattice-matched oriented attachment growth mechanism is appropriate in the reaction system [35]. Figure 5(d) showed the nanocrystals after 8 h. Compared with the production of 6 h (Figure 1(b)), there was no obvious variations in morphology and size, demonstrating that the reaction finished when the reaction time was extended to 6 h. The synthesis strategy was general and scalable. We tried to scale it up by 10 times, and the Pt-Cu NWs were well-dispersed with the same morphology (Fig. S4). The as-prepared NWs with different compositions of Pt 1 Cu 4 , Pt 1 Cu 1 , and Pt 4 Cu 1 were conducted to evaluate their catalytic performances of MOR. For comparison, pure Pt nanoparticles and commercial Pt/C were also tested as the reference catalysts. The cyclic voltammogram (CV) was firstly carried out in 0.5 M H 2 SO 4 from −0.25 V to 0.2 V (Figure 6(a)). The electrochemically surface active area (ECSA) of Pt 1 Cu 4 , Pt 1 Cu 1 , Pt 4 Cu 1 , Pt, and commercial Pt/C after calculation were 86.8, 93.7, 66.3, 11.2, and 52.5 m 2 g -1 , respectively. Figure 6(b) presented the CV curves of five catalysts in a mixture of 0.5 M H 2 SO 4 and 1 M CH 3 OH. Apparently, the CV curves displayed two anodic peaks which were the methanol oxidation peak (at the higher potential) and the intermediate carbonaceous species oxidation peak (at the lower potential). The mass activity of electrocatalysts was usually taken as an index to evaluate their activity. The peak current density of Pt 1 Cu 4 , Pt 1 Cu 1 , Pt 4 Cu 1 , Pt, and commercial Pt/C were 235, 295, 164, 127, and 29 mA·mg -1 , respectively. As displayed in Figure 6(c), the current density at lower potential 0.45 and 0.6 V (vs. RHE) of Pt 1 Cu 1 NWs was the highest (11 and 57 mA·mg -1 , respectively), which was higher than that of Pt 1 Cu 4 (10.16 and 52.02 mA·mg -1 ), Pt 4 Cu 1 (5.92 and 29.54 mA·mg -1 ), Pt (1.23 and 5.89 mA·mg -1 ), and Pt/C (5.69 and 28.99 mA·mg -1 ). Compared with pure Pt, Pt 1 Cu 1 NWs had lower onset potential, which was beneficial for MOR on the surface of Pt-Cu NWs (Figure 7(a)). The stability of the catalysts was further measured by repeating the potential for 0.6 V (vs. RHE). Pt 1 Cu 1 catalyst showed the best stability among the five catalysts during a 2000 s test, as shown in Figure 6(d). Compared with Pt black and pure Pt, the Pt-Cu NWs displayed bigger oxidation current and slower current decay rate, demonstrating a good stability. Additionally, the same electrode was saved for a week for reuse, and its current density presented 91% of the initial value (Figure 7(b)), which further demonstrates their excellent reusability.
The activity of the catalyst decreased with the time in the stability test. There might be two reasons [36,37]: (1) as this process goes on continuously, the catalyst morphology changed; (2) at the beginning of the potential scan, a rapid accumulation of strongly adsorbed species (CO, oxygenate, etc.) occurred on the catalyst surface, which caused the number of active sites to decrease rapidly. Taking Pt 1 Cu 1 NWs as an example, their TEM characterization after the stability test showed that there was no obvious change in the morphology of the catalyst (Fig. S5a), which proved that the decrease of catalyst activity was not caused by the change of catalyst morphology. Then, the CO stripping experiment was carried out on the electrode after the stability test (Fig. S5b). It was found that the catalyst had obvious CO stripping peak (870 mV), indicating the accumulation of CO on the surface of Pt 1 Cu 1 NWs during the methanol oxidation reaction, which led to the decrease of the activity of the catalyst.
The high activity and stability of Pt-Cu NWs for MOR are rationally attributed to their catalytic composition and unique structure, which could be explained as follows. (1) It is universally known that representative procedures for MOR can produce poisonous intermediates (e.g., CO), which has a strong adsorption on Pt surface and reduces the catalytic performance of the catalyst. Owing to the d-band center theory [38], the Cu atoms in the Pt-Cu alloy change the electronic energy level structure of Pt. Such change weakens the adsorption strength between Pt and intermediates, which is beneficial to the MOR. Meanwhile, the ability of CO adhering to Cu takes precedence over Pt which release more Pt atom active sites than the one without Cu. (2) 1D NWs provide 7 Journal of Nanomaterials abundant surface atoms, structural defects (e.g., grain boundaries), and substantial numbers of surface active sites, which increase the availability of Pt atoms and enhance catalytic performance. (3) The NW's inherent anisotropic structure makes the catalysts less accessible to Ostwald ripening as well as aggregation and thus preserves their enhanced stability.

Conclusions
In conclusion, 1D Pt-Cu NWs have been acquired by an environmentally friendly and facile aqueous solution method in gram scale. The employments of Brand Brij58 as growthdirection and stabilization reagents were considered vital for achieving Pt-Cu NWs. Compared with commercial Pt black, the as-prepared Pt 1 Cu 1 NWs showed excellent activity (295 mA·mg -1 ) and stability towards MOR. These remarkable catalytic performances were mainly attributed to the exceptional structure with abundant structural defects and surface active sites.

Data Availability
The data used to support the findings of this study are available from the corresponding authors upon request.

Conflicts of Interest
The authors declare that they have no conflicts of interest.