One-pot synthesis of lotus-shaped Pd–Cu hierarchical superstructure crystals for formic acid oxidation

Abstract

In this study, bimetallic Pd–Cu hierarchical superstructure crystals were successfully synthesized using an aqueous solution method. SEM, TEM, XRD and element mapping analysis were used to characterize the structural features, and the composition was determined by ICP-OES. The electrocatalytic activity of these Pd–Cu hierarchical superstructure crystals exhibited enhanced electrocatalytic activity for formic acid oxidation compared with that of commercial Pd black and commercial Pt black. The peak current densities and mass current value of the Pd₅₃Cu₄₇ nanoalloys were 5.3 mA cm⁻² and 0.43 A mg⁻¹. For commercial Pd black, these values were 1.18 mA cm⁻² and 0.28 A mg⁻¹, and for commercial Pt black, they were 0.85 mA cm⁻² and 0.21 A mg⁻¹. Moreover, the current–time tests showed that the Pd–Cu hierarchical superstructure nanoalloys were more stable than commercial Pd black.

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Palladium nanocrystals are well known because of their unique physical and chemical properties and wide range of applications,^1–8 including in catalysis. Pure Pd is better suited to direct formic acid fuel cells (DFAFCs) than pure Pt. However, pure Pd nanocrystals are a limited resource and are costly. By combining less expensive transition metals, such as Cu, the formation of an alloy not only reduces the consumption of Pd but also enhances the overall performance because of its synergetic effects originating from neighboring Pd and Cu.^9–12

The structure, size, shape and hierarchies of metal crystals are key factors in catalytic performance.^13–16 As a result, a variety of synthetic strategies for preparing metal crystals with different structures have been successfully developed.^17–20 Complex metal crystal superstructures are more difficult to develop than metal oxides, such as ZnO,^21–23 CuO,^24,25 Fe₂O₃^26–28 and TiO₂.^29–31 To date, the template method,^32,33 the seed-mediated method^34,35 or the technique of aligning nanoparticle building blocks³⁶ are used to fabricate crystal superstructures. However, these methods often require multiple steps or complicated synthetic routines. As a consequence, the development of a simple and direct method for fabrication of metallic superstructures is highly desired. Pd–Cd bimetallic dendritic superstructures³⁷ have already been prepared through the hydrothermal method grown on Ti substrates. Recently, Pt–Cu bimetallic superstructures^38–40 were successfully prepared using a one-pot, hydrothermal method. These Pt–Cu bimetallic superstructures enhanced the catalytic performance on methanol or formic acid oxidation better than modern Pt/C catalysts. There are recent examples of anisotropic Pd–Cu bimetallic alloys;^41,42 however, to the best of our knowledge, Pd–Cu bimetallic alloys with lotus-shaped hierarchical superstructures have not been developed thus far.

In this study, we developed a simple aqueous method to prepare hierarchical Pd–Cu bimetallic superstructures. The synthesized hierarchical Pd–Cu bimetallic superstructures have a lotus-like shape. Further, the electrocatalytic properties of the lotus-shaped Pd₅₃Cu₄₇ superstructure exhibited superior activity and stability towards HCOOH oxidation compared to those of commercial Pd black.

Fig. 1a and b and S1, ESI† show the representative scanning electron microscope (SEM) of the as-synthesized Pd₅₃Cu₄₇ sample (the feeding metal molar percentage of Na₂PdCl₄ and CuCl₂ is 50 : 50). As can be seen, the as-synthesized particles looked like a lotus-shaped hierarchical superstructure assembled with many petals. The diameter of the whole hierarchical superstructure crystal is of ∼800 nm. The ICP-OES analysis showed that the atomic percentages of Pd and Cu were 53 : 47, which is very in accord with the feed ratio of metal precursors. And it implies the metal precursors are almost quantitatively converted. The selectivity of lotus-shaped hierarchical superstructure is approximately 90 percent (Fig. S1, ESI†). Fig. 1b represents one single Pd₅₃Cu₄₇ hierarchical superstructure particle with many petals, and these petals are interconnected. The surface of the petal is very rough and has many corners and edges, all of these sites can act as highly active catalytic sites in catalytic reactions. The TEM images further demonstrate the hierarchical lotus-shaped superstructure and the rough surface of the petals (Fig. 1c and d and S2, ESI†).

Fig. 1 SEM (a and b) and TEM (c and d) images of as-synthesized Pd₅₃Cu₄₇ hierarchical superstructure crystals.

The structure and composition of the as-synthesized hierarchical superstructure Pd₅₃Cu₄₇ bimetallic particles were determined by X-ray diffraction (XRD), energy dispersive X-ray (EDX) spectroscopy and an elemental mapping analysis. The EDX spectra (Fig. S3, ESI†) revealed the product consisted of Pd and Cu. The XRD pattern (Fig. 2a) of the as-synthesized product showed four peaks corresponding to (111), (200), (220) and (222) of fcc Pd/Cu (Pd: JCPDS-65-2867; Cu: JCPDS-04-0836). The XRD peaks shifted to a higher angle because of the incorporation of Cu atoms into the Pd fcc lattice,^43,44 and the peaks between the Pd standard value and the Cu standard value showed no single Pd and Cu peaks, which indicated the lone presence of Pd₅₃Cu₄₇ nanoalloy crystals. Further, the Pd₅₃Cu₄₇ hierarchical superstructure nanoalloys were also confirmed by the results of the elemental mapping analysis. Fig. 2c–e indicate that the Cu and Pd were distributed throughout the whole particle. In our synthetic approach, glycine and CuCl₂ were necessary, since in the absence of glycine, the product consisted of an irregular polyhedral structure and no hierarchical superstructure (Fig. 3a), and in the absence of CuCl₂, the product consist of uniformly-sized spheres, the size of the sphere is about 70 ± 10 nm (Fig. 3b). However, PVP-8000 was also necessary for the reduction to form the hierarchical superstructure. In the absence of PVP-8000, no product was produced. In addition, the molar ratio of Na₂PdCl₄ to CuCl₂ was also important, because when the molar ratio of Cu in Na₂PdCl₄ to CuCl₂ changed from 1 : 2 (Fig. 3c) and 2 : 1 (Fig. 3d), the product consisted of many polyhedral structures and a small number of hierarchical superstructures.

Fig. 2 XRD pattern (a), the red and blue lines are the standard peaks for Pd (red) and Cu (blue), HADDF-STEM image (b) and the corresponding elemental maps of the as-synthesized Pd₅₃Cu₄₇ nanoalloys. Cu (green) (c), Pd (red) (d) and (e) reconstructed overlay image of the maps shown in (c) and (d).

Fig. 3 Without glycine (a), without CuCl₂ (b), Pd : Cu atom ratio is 1 : 2 (c), (d) Pd : Cu atom ratio is 2 : 1.

The electrochemical characterization was determined in a standard three-electrode cell at a temperature. The working electrode was a glassy carbon electrode loaded with Pd₅₃Cu₄₇ nanoalloys, commercial Pd black and commercial Pt black. The electrode was immersed in a nitrogen-saturated solution, and the potential was scanned from −0.25 to 0.9 V (vs. Ag/AgCl) at a scan rate of 50 mV s⁻¹ to obtain cyclic voltammetric curves (CVs). Fig. 4a shows the CVs Pd₅₃Cu₄₇ bimetallic nanoalloys, commercial Pd black and commercial Pt black recorded in the 0.1 M HClO₄ solution. The specific current was normalized by the electrochemical surface area (ECSA) calculated by measuring the charge collected in the hydrogen adsorption–desorption region and assuming a value of 210 μC cm⁻². The Pd₅₃Cu₄₇ nanoalloys showed a different CV shape compared to that of commercial Pd black. Fig. 4b shows CVs of Pd₅₃Cu₄₇ nanoalloys, commercial Pd black and Pt black in a 0.5 M HCOOH + 0.1 M HClO₄ solution at 50 mV s⁻¹. The current was normalized by the ECSA to obtain the current density (j). As such, it was directly compared for different samples. The well-defined peak that appeared at approximately 0.5 V was attributed to the formic acid oxidation in both the forward and backward scans. The peak current density value of Pd₅₃Cu₄₇ nanoalloys were 5.31 mA cm⁻² and 0.43 A mg⁻¹. For commercial Pd black, these values were 1.18 mA cm⁻² and 0.28 A mg⁻¹, and for commercial Pt black, they were 0.85 mA cm⁻² and 0.21 A mg⁻¹. The highest catalytic activity and its electrocatalytic improved about four times compared to that of Pd black and about five times compared to that of Pt black. The curves for the mass activity per unit mass of Pd of the catalysts are shown in Fig. 4c. The anodic potential of formic acid oxidation for the catalyst based on the Pd₅₃Cu₄₇ hierarchical superstructure nanoalloys peaked at 0.3 V, and the corresponding peak mass activity was 0.43 A mg⁻¹, while the commercial Pd black positioned at 0.27 V had mass activities of 0.28 A mg⁻¹, and the commercial Pt black positioned at 0.78 V had mass activities of 0.21 A mg⁻¹. Both of the Pd₅₃Cu₄₇ hierarchical superstructure alloys performed better than commercial Pd black and commercial Pt black. The current value of the Pd₅₃Cu₄₇ hierarchical superstructure nanoalloys showed higher catalytic activity than commercial Pd black and commercial Pt black at 0.3 V. The electrochemical stability of Pd₅₃Cu₄₇ hierarchical superstructure nanoparticles was determined by chronoamperometry at 0.3 V for 1 hour (Fig. 4d). We found that the Pd₅₃Cu₄₇ nanoalloys exhibited a slower current decay and much higher current densities than the Pd black catalysts after 1 hour. The enhancement of catalytic activity and stability may be ascribed to the rough surface and the possible synergistic effect between the Pd and Cu.^{9–12,45–47} This was supported by the XPS analysis. Fig. S4, ESI† shows the XPS spectra for Pd 3d and Cu 2p for the as-synthesized Pd₅₃Cu₄₇ nanoalloys, which shows metallic Pd and Cu atoms coexist in the surface of Pd₅₃Cu₄₇ nanoalloys. Besides, the hierarchical superstructure will not only develop a good electrical connection between the sites of oxidation–reduction but also be beneficial for electron transfer in the reaction and lead to better catalytic property.^38–40,48 Moreover, the PdCu nanoalloys still maintained lotus-shaped hierarchical superstructure after current–time test (Fig. S5, ESI†). However, the sharp tops of petal got dull after current–time test.

Fig. 4 Cyclic voltammetric curves (CVs) of as-synthesized Pd₅₃Cu₄₇ nanoalloys, commercial Pd black, commercial Pt black. Electrochemical characterization in 0.1 M HClO₄ solution (a). Specific activity and mass activity in 0.5 M HCOOH + 0.1 M HClO₄ solution (b and c). Current–time curves of formic acid oxidation on Pd₅₃Cu₄₇ nanoalloys and commercial Pd black in 0.5 M HCOOH + 0.1 M HClO₄ solution at 0.3 V for 1 hour (d).

Fig. 5a shows the CV plots of formic acid electrooxidation on a Pd₅₃Cu₄₇ hierarchical superstructure nanoparticles modified electrode at different scan rates, ranging from 10 mV s⁻¹ to 100 mV s⁻¹. These plots indicated that the peak current potential shifted positively and the peak current density increased when the scan rate increased. A linear relationship was found between the square root of the scan rate (v^1/2) and the forward peak current value (Fig. 5b). These results indicated that formic acid electrooxidation on the Pd₅₃Cu₄₇ hierarchical superstructure nanoparticles followed a diffusion-controlled process. Fig. 5c illustrates the CV curves of formic acid electrooxidation on the Pd modified electrode at different scan rates. A linear relationship also existed between the current value and v^1/2 for the Pd catalyst (Fig. 5d). The higher slope value of the Pd₅₃Cu₄₇ hierarchical superstructure nanoparticles relative to that of Pd suggested the improved electrooxidation kinetics of the hierarchical superstructure, which accounted for the high catalytic activity of the Pd₅₃Cu₄₇ hierarchical superstructure alloys. The improved electrooxidation kinetics of the Pd₅₃Cu₄₇ hierarchical superstructure alloys originated from their bimetallic Pd₅₃Cu₄₇ composition and their special structures.

Fig. 5 CV plots of formic acid electrooxidation on Pd₅₃Cu₄₇ hierarchical superstructure alloys modified electrodes at different scan rates (a) and the corresponding plot of forward peak current versus the square root of the scan rate (v^1/2) (b). CV plots of formic acid electro oxidation on Pd black modified electrodes at different scan rates (c) and the corresponding plot of j_p versus the v^1/2 (d).

Conclusions

In summary, we developed a simple method to synthesize bimetallic Pd–Cu hierarchical superstructure crystals. The glycine, CuCl₂ and metal precursor feed ratio were crucial to the synthesis. The Pd–Cu hierarchical structure crystals exhibited enhanced catalytic activity and stability toward formic acid oxidation in the acid medium when compared to those in commercial Pd black and commercial Pt black. The catalytic activities of the Pd–Cu hierarchical superstructure crystals were approximately 4.5 times greater than that of commercial Pd black and about 6 times greater than that of commercial Pt black, and the mass activity was approximately 1.5 times greater than commercial Pd black and 2 times greater than commercial Pt black. The chronoamperometric measurements showed that the Pd–Cu hierarchical superstructure crystals were more stable than commercial Pd black after 1 hour. These bimetallic Pd–Cu hierarchical superstructure crystals have promising use in the future development of synthesizing hierarchical superstructures and direct formic acid fuel cells.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21361005 and 21571038).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, TEM, SEM, EDX spectra and XPS. See DOI: 10.1039/c6ra21823g

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