Synthesis of surfactant-free Cu–Pt dendritic heterostructures with highly electrocatalytic performance for methanol oxidation reaction

ABSTRACT A facile and free surfactant strategy is explored to synthesize Cu–Pt bimetallic nano-heterostructures with dendritic exterior. For comparison, the Cu–Pt coral-like nanoparticles are fabricated by using CTAC as a surfactant. The well-designed Cu–Pt dendritic spherical heterostructures exhibit superior enhanced electrocatalytic activity and stability toward methanol oxidation reaction in alkaline media, compared to the Cu–Pt coral-like nanoparticles and the commercial Pt/C, respectively. The advanced technique for fabricating Cu–Pt dendritic spherical heterostructures could pave a way to pursue low-cost Pt-based catalysts, maintaining highly promoted electrocatalytic performance and durability. IMPACT STATEMENT The well-designed Cu–Pt dendritic spherical heterostructures are synthesized without any surfactant, which demonstrate highly promoted electrocatalytic performance and durability toward methanol oxidation reaction. GRAPHICAL ABSTRACT


Introduction
Very recently, metallic nano-heterostructures with distinctive architecture and size have been motivated to tailor catalytic performance and thermal stability for achieving more potential applications such as oxygen reduction, [1][2][3] methanol oxidation [4,5] and formic acid oxidation. [6] Considerable attention has been focused on platinum (Pt)-based nanostructures with various morphologies, which could be applied for electrocatalytic methanol oxidation reaction (MOR) because of its slow CONTACT  kinetics and the demand for fuel-cell cathodes. [7][8][9] The promoted catalytic activity is usually explained and influenced by synergistic effect, [10] bifunctional mechanism and electronic effects (include lattice strain effects, surface ligand effects and geometric effects). [11][12][13][14] Notably, the morphology plays a significant role in determining the selectivity and active sites. Plenty of Ptbased nano-architectures with typical shape and size have been successfully synthesized, exhibiting high catalytic activity. [15][16][17] Such as Pt cubes, tetrahedrons, truncated octahedrons, single-crystal Pt nanowires and Pt nanoalloys covered by high-index facets. [18][19][20][21][22][23] Numerous examples of adatoms to Pt have been conducted including Ru, [24,25] Co, [26] Cu [5,27,28] and Pd. [29] Recently, our group developed a facile strategy to synthesize well-designed Au/Ag/Pt trimetallic heterogeneous nanocrystals with highly effective catalytic properties and stability. [30] In conventional wet chemical methods, the surfactants are frequently used in colloidal synthesis to inhibit nanoparticles extreme overgrowth and aggregation, as well as to control the structural characteristics of the resulted nanoparticles in a precise manner. However, these surfactants such as polyvinylpyrrolidone (PVP) could occupy surface activity sites due to their strong adsorption, which would be difficult to be removed, dramatically declining the catalytic activity of nanoparticles. [31] Therefore, it is essential to develop high techniques for synthesis of green and distinctive morphological Pt-based nanostructures without any surfactant. [32] Herein, we engage in exploiting facial and universal techniques to synthesize Cu-Pt bimetallic nanoheterostructures with dendritic morphology. As expected, the well-designed Cu-Pt dendritic spherical heterostructures exhibit superior electrocatalytic functions. Simply, the Cu-Pt dendritic spherical heterostructures are synthesized via a one-step wet chemical strategy, by simultaneous reduction of potassium tetrachloroplatinate (II) (K 2 PtCl 4 ) and copper sulfate (CuSO 4 ) with 3:1 molar ratio in ultrapure water as solvent at 60°C for 2 h, l-ascorbic acid (AA) as the reducing agent. Neither surfactants nor dispersion agents are introduced in the above fabrication process. For comparison, Cu-Pt coral-like nanoparticles are synthesized with highly uniform size and morphology, using cetyl trimethyl ammonium chloride (CTAC) as a surfactant. The schematic procedures of Cu-Pt dendritic spherical heterostructures and Cu-Pt coral-like nanoparticles are shown in Figure 1. Several experimental parameters on growth of Cu-Pt alloys are investigated through systematic orthogonal methods. The Cu-Pt dendritic spherical heterostructures exhibit highly promoted electrochemical activity and durability for MOR in alkaline media compared to Cu-Pt coral-like nanoparticles and commercial Pt/C, respectively.

Synthesis of Cu-Pt dendritic spherical heterostructures
About 4.5 mL ultrapure water, 0.3 mL of aqueous K 2 PtCl 4 (10 mM) and 1 μL of aqueous CuSO 4 (1 M) are added in a 10 mL vial in turn. After the mixture is stirred for around 2 min, 0.2 mL of aqueous AA (0.1 M) is added into the mixture rapidly. The resulting homogeneous mixture is transferred into a water bath and heated at 60°C for 2 h with stirring. The products are collected by centrifugation at 12,000 rpm for 10 min, and then washed with ultrapure water and ethanol three times.

Synthesis of Cu-Pt Coral-like Nanoparticles
In a typical synthesis of well-organized Cu-Pt coral-like nanoparticles, 4.4 mL ultrapure water, 0.1 mL of aqueous CTAC (0.1 M), 0.3 mL of aqueous K 2 PtCl 4 (10 mM) and 1 μL of aqueous CuSO 4 (1 M) are added in a 10 mL vial in turn. After the mixture is stirred for around 2 min, 0.2 mL of aqueous AA (0.1 M) is added into above mixture rapidly. The resulting homogenous mixture is transferred into a water bath and heated at 60°C for 2 h with stirring. The products are collected by centrifugation at 12,000 rpm for 10 min, and then washed with ultrapure water three times.

X-ray diffraction (XRD) is performed on a Bruker D8
Advance with Cu Kα radiation (λ = 1.54178 Å). Transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) images are prepared with the FEI Tecnai G2 F20 S-Twin microscope at an acceleration voltage of 200 kV. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and EDS mapping images are carried out on the FEI Titan3 G2 80-300 microscope at an acceleration voltage of 300 kV. Inductively coupled plasma optimal emission spectrometry (ICP-OES) is used to measure the contents of the Cu-Pt alloy.

Electrochemical measurements
Electrochemical measurements are conducted with a three electrode system on a CHI 660D electrochemical workstation (Shanghai Chenhua Instruments). The working electrode is a glassy-carbon electrode (GCE, CHI104, diameter: 3 mm, area: 0.071 cm 2 ), a Pt wire is used as the counter electrode, an Ag/AgCl is used as the reference electrode in 3 M KCl solution. The Pt loading of Cu-Pt coral-like nanoparticles, Cu-Pt dendritic spherical heterostructures and Pt/C are all 4 μg. The electrochemical active surface area (ECSA) measurements are determined by integrating the hydrogen adsorption charge on the cyclic voltammetry (CV) at room temperature in argon-saturated 0.5 M H 2 SO 4 solution. The potential scan rate is 50 mV/s for the CV measurement.   (Figure 2(c-f)). The merged mappings provide clear evidence for the elements distribution, the presence of Cu (green) and Pt (red) appears in the individual bimetallic nanostructure, further illustrating a completely overlapping structure of two elements.

Results and discussion
Simultaneously, the achieved Cu-Pt coral-like nanoparticles are characterized by TEM, HAADF-STEM and elemental mapping (Figure S1A † ). The overall Cu-Pt coral-like nanoparticles exhibit highly uniform morphology and size of 16.2 ± 2.0 nm. The Cu-Pt corallike nanoparticle consist of irregular dendritic structures as shown in Figure S1B † , Cu and Pt atoms distribute evenly among each particle ( Figure S1D-F  † ). These corallike nano-architectures dramatically promote the specific surface area, which result in a slight enhancement of catalytic activity investigated by MOR measurements.
The low-angle XRD is carried out in order to confirm the crystalline structures of Cu-Pt dendritic spherical heterostructures and Cu-Pt coral-like nanoparticles (Figure S2  † ). The patterns demonstrate that the typical peaks of both nanoparticles can be referenced to a facecentered cubic (fcc) structure, corresponding to the pure fcc Cu (JCPDS no. 85-1,326) and the pure fcc Pt (JCPDS no.87-0,646). According to Vegard's law, the Cu-Pt dendritic spherical nanostructures and the Cu-Pt coral-like nanoparticles are estimated as Pt3Cu, [33,34] which is equal to the ratio of Pt/Cu measured by ICP-OES. In addition, the individual Cu-Pt dendritic spherical heterostructures and the Cu-Pt coral-like nanoparticles are further characterized by HRTEM (Figure 3, Figure  S3  Interestingly, considering all the experimental parameters, it is found that the molar ratio of K 2 PtCl 4 /CuSO 4 is the crucial actor for successful synthesizing of Cu-Pt dendritic spherical heterostructures. Figure S4  As the molar ratio of K 2 PtCl 4 /CuSO 4 is varied to be 3:1, the obtained Cu-Pt coral-like nanoparticles are in uniform shape and size. The Cu-Pt dendritic spherical heterostructures and the Cu-Pt coral-like nanoparticles are expected to exhibit enhanced electrochemical activity for MOR. For comparison, commercial Pt/C is investigated in the same condition. The ECSA is evaluated by integrating the charge in the hydrogen desorption region after correction for double layer contribution between −0.3 and 1.0 V Ag/AgCl . According to the CV curves shown in Figure 4, the ECSAs is calculated as 41.09 m 2 g −1 Pt for the Cu-Pt coral-like nanoparticles, 35.66 m 2 g −1 Pt for the Cu-Pt dendritic spherical heterostructures and 67.10 m 2 g −1 Pt for the commercial Pt/C. The ECSA of Pt/C is the highest than those of Cu-Pt coral-like nanoparticles and Cu-Pt dendritic spherical heterostructures, due to the Pt nanoparticles with smaller size possessing much higher active surface area. The highly branched structures of Cu-Pt coral-like nanoparticles provide a reasonably higher surface area compared to  the Cu-Pt dendritic spherical heterostructures with large overall particle size relatively. The MOR measurements are performed in an Ar 2purged 1.0 M KOH and 1.0 M CH 3 OH solution from −0.8 to 0.1 V at a scan rate of 50 mV s −1 . The CVs for specific activities and mass activities of Cu-Pt coral-like, Cu-Pt dendritic spherical heterostructures and commercial Pt/C are investigated, as shown in Figure 5(a-c). The peak current density of Cu-Pt dendritic spherical heterostructures is much higher than that of Cu-Pt coral-like particles and commercial Pt/C. For specific activities, the current density of Cu-Pt dendritic spherical heterostructures (2.49 mA cm −2 ) is around four times that of Cu-Pt coral-like nanoparticles (0.64 mA cm −2 ), and 10 times that of Pt/C (0.25 mA cm −2 ), respectively. Regard to mass activities, it obviously reveals that the function of Cu-Pt dendritic spherical heterostructures is the optimal among three catalysts. Moreover, the catalytic activities illustrate as following: Cu-Pt dendritic spherical heterostructures > Cu-Pt coral-like nanoparticles > commercial Pt/C, with current-density values of 888.0, 263.2, and 172.3 mA mg −1 , respectively. Furthermore, we evaluate the long-term stability of Cu-Pt dendritic spherical heterostructures, Cu-Pt corallike nanoparticles and commercial Pt/C catalysts through accelerating durability tests by continuous CV measurements performed from −0.8 to 0.1 V at a scan rate of 50 mV s −1 for 1,500 cycles ( Figure S6  † ). As shown in Figure 5(d), it demonstrates that the value of initial forward current density of Cu-Pt dendritic spherical heterostructures and Cu-Pt coral-like nanoparticles still remains 70% and 65% after 1,500 cycles, whereas the commercial Pt/C loses 73% of its initial current density comparably. Additionally, TEM characterizations confirm that most of Cu-Pt heterostructures maintain their dendritic nanospherical morphology after continuous CV evaluation for 1,500 cycles ( Figure S7A  † ), further evidencing the long-term durability. Cu-Pt coral-like nanoparticles partially dissolve and aggregate during the durability investigation ( Figure S7B  † ).
The mechanism of superior catalytic performance for Cu-Pt dendritic spherical heterostructures can be attributed to the typical geometrical morphology and heterostructures. The dendritic Cu-Pt alloys provide plenty of Pt stepped atoms and edge atoms on surface shown by the HRTEM image Figure S8 † , which could supply abundant activated sites for methanol oxidation. Simultaneously, the synergistic effect between Cu and Pt caused by lattice strain effects and geometrical variation can dramatically enhance their electrocatalytic functions and durability. Possibly, Pt catalyzes methanol oxidation generating as Pt-CO, while Cu promotes water dissociating as Cu-OH, then Pt-CO and Cu-OH interact to generate CO 2 , thus sufficient Pt active sites are released for maintaining continuous MOR. In addition, the efficiency of Cu-Pt electrons transfer is significantly promoted during the fabrication of Cu-Pt dendritic spherical heterostructures without introducing any surfactant. In Cu-Pt coral-like nanoparticles with inferior electrocatalytic activities because it is impossible to wash off the surfactant thoroughly results in the residual CTAC molecules still adsorbed on the surface of nanostructures, which could severely influence their catalytic functions. It should be pointed out that Cu as a supporting element is essential to reduce the amount of Pt particles while maintaining the high catalytic activity.
Meanwhile, we conducted the catalytic efficiency of MOR in acidic environment to compare with those of alkaline media. As shown in Figure S9A-C † , the catalytic properties of Cu-Pt dendritic spherical heterostructures, Cu-Pt coral-like nanoparticles and commercial Pt/C are much lower than those of the alkaline condition. Even though the Cu-Pt dendritic spherical heterostructures exhibit pretty much low value of current density after 1,500 cycles, the value of current density of Cu-Pt corallike nanoparticles only retain 16.7%. The possible reason caused is that the adsorption strength of the intermediate products in an alkaline solution is much weaker than that of acidic solution, which results in highly enhanced electrocatalytic activity for MOR in former media.

Conclusion
In summary, the well-designed Cu-Pt dendritic spherical heterostructures are synthesized successfully without introducing any surfactants or organic solvents as protective agent, and the Cu-Pt coral-like nanoparticles are prepared using CTAC for comparison. It reveals that the well-designed Cu-Pt dendritic spherical heterostructures exhibit highly enhanced electrocatalytic property and durability toward MOR, compared with the obtained Cu-Pt coral-like nanoparticles and commercial Pt/C. Moreover, the highly promoted electrocatalytic activity and stability can be attributed to these unique dendrite structures of Cu-Pt through the modified synthesis approach. This advanced facile and green synthesis technique could scale up bimetallic heterostructures with high yield, and it provides a concept for fabricating other trimetallic or multimetallic nanoarchitechtures with tunable morphology and size.

Disclosure statement
No potential conflict of interest was reported by the authors.