Electrochemical fabrication of Hydrangea macrophylla flower-like Pt hierarchical nanostructures and their properties for methanol electrooxidation

Abstract

The electrochemical preparation of non-carbon supported catalysts is a favourable way for fuel cells to keep catalysts from agglomerating during the electrocatalytic process and provide the catalyst layer with higher mechanical stability than the established drop-coating deposition of a mixture of catalysts and carbon powder. Herein, one-step current-directed approach is proposed to fabricate 3-dimensional (3D) Pt hierarchical nanostructures (3DPHNs) without any capping agents. The resulting 3DPHNs were composed of Hydrangea macrophylla flower-like Pt microspheres, and each microsphere consisted of several nano-petals. The size, the number density and the exposed facets of Pt microspheres in 3DPHNs can be adjusted by changing the current density of Pt deposition. High-resolution transmission electron microscopy (HRTEM) revealed that Pt nano-petals obtained at deposition current densities higher or lower than 3.5 mA cm⁻² contained Pt{200} and Pt{111} facets. However, Pt nano-petals obtained at 3.5 mA cm⁻² were single crystals with {111} orientation that showed better specific catalytic activity and stability to methanol electrooxidation compared to commercial Pt/C catalyst due to its resistance to catalyst agglomeration and the exposure of specific facets and the specific nanostructure.

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1. Introduction

Platinum always attracts the particular attention of scientists due to its broad catalytic applications in organic reactions¹ and cleaning of exhaust gas² and fuel cells.^3,4 Although Pt is an indispensable metal in the catalytic field, its reserves on earth are very limited and Pt catalyst is apt to be poisoned by CO-like intermediates. To maintain sustainable development, Pt-based catalysts, such as binary and ternary alloys^3,5 and core–shell structures have been widely studied.^6–8 Studies have demonstrated^9,10 that Pt electrocatalytic activity (EA) or tolerance towards CO-like poisoning molecules can be enhanced through the synergistic effect between Pt and other metallic elements such as Au,¹¹ Cu¹² or Pb,¹³ which is a significant alternative to solve the problems caused by pure Pt catalysts. However, to effectively design binary or ternary metal catalysts, it is essential to first improve our understanding of the catalytic mechanism of the pure Pt metal.

The catalytic performance of Pt is mainly dependent on two aspects: the size and shape of Pt particle. To date, the effect of particle size has been well elucidated: Pt particles less than 10 nm are preferable. To conveniently manipulate catalysts and effectively exploit Pt atoms, commercial Pt catalyst particles usually are ∼3 nm. The surfaces of spherical Pt nanoparticles commonly consist of mixed crystallographic facets such as {111}, {100} and {110} facets.¹⁴ Different facets have different catalytic effects on specified reactions.¹⁵ Methanol electrooxidation on platinum takes place in acidic electrolytes involving the following adsorbed intermediates such as CO_ads, HCOO_ads, formaldehyde and formic acid.^16–18 CO_ads has been identified as a key poisoning molecule in the process of methanol oxidation. CO_ads molecules adsorbed on Pt{100} and Pt{111} were considered to be produced by an indirect and direct pathway, respectively.^16,19 Thus, Pt{111} facets are expected to facilitate the methanol oxidation via the direct pathway and exhibit a good tolerance to CO poisoning.¹⁶

To efficiently improve Pt catalytic activity, specific facets of Pt are exposed by optimizing the synthetic method so that Pt particles exhibit a certain shape in appearance. All types of shapes were successfully synthesized in solution by reducing Pt precursors such as Pt polyhedral,²⁰ nanowires,²¹ nanorods,²² nanotubes,²³ nanocubes,^24,25 nanosheets,²⁶ fivefold stars²⁷ and flower-like nanostructures.²⁸ Pt concave nanocubes exhibited 2.5 times better specific electrocatalytic activity than Pt/C catalysts for formic acid electrooxidation.²⁵ Compared with the wet-chemical synthesis mentioned above, electrochemical preparation is a much easier and faster protocol, and the final morphology and size of electrodeposited target materials can be controlled simply by varying the applied potential, the current density, the type of the electrochemical technique, the precursor concentration the pH value, and electrodeposition time.²⁹ To date Pt nanostructures with a variety of shapes, such as cubic nanobox,³⁰ nanowire array,³¹ cauliflower-like,³² needle-like,³² honeycomb³³ and nanosheet³⁴ have been electrochemically synthesized and these unusually shaped Pt nanostructures exhibited better catalytic activity towards alcohol oxidation.³⁵ For example, tetrahexahedral Pt nanocrystals bound by high-index facets {730} were first electrochemically prepared by the combination of Sun's group and Wang's group,³⁶ and showed excellent catalytic activities: 2 times and 3.3 times better than that of commercial Pt/C catalysts towards electrooxidation of formic acid and ethanol, respectively. The shape control is mainly achieved by the template-directed growth³¹ or potential-interfered growth (e.g. potentiostatic method,³⁸ pulse electrodeposition,³⁷ square wave voltammetry³⁶ and staircase voltammetry³²). To the best of our knowledge, one-step current-directed growth methods were rarely used to control the morphology of Pt nanostructure.^39,40

Herein, a one-step current-directed approach is proposed to fabricate capping-agent-free three-dimensional Pt hierarchical structure (3DPHN) composed of Hydrangea macrophylla flower-like Pt microspheres, and each sphere consisted of many entangled nano-petals. Moreover, the number density and exposed facets of nano-petals on the electrode substrate could be adjusted by changing the magnitude of the applied current. The optimized 3DPHN showed better electrocatalytic activity (EA) and stability towards methanol oxidation than the commercial Pt/C catalyst.

2. Materials and methods

2.1 Chemicals

Potassium hexachloroplatinate(VI) (K₂PtCl₆, ≥99.79%), perchloric acid (HClO₄, ≥72.0%), methanol (CH₃OH, ≥99.5%), and sulfuric acid (H₂SO₄, 95–98%) were supplied by Shanghai Chemical Reagent Company (China). Pt/C (20 wt% Pt) commercial catalyst was purchased from Johnson Matthey Company. Milli-Q water was used in all experiments.

2.2 Electrochemical preparation of the three-dimensional Pt hierarchical nanostructures (3DPHNs)

Electrodeposition was conducted on a homemade inverted three-electrode cell. The so-called ‘inverted’ means that the working electrode (glassy carbon, GC, 3 mm in diameter) was put into the cell from the bottom, which was at the same level as the bottom of the cell. A saturated calomel electrode (SCE) and a platinum wire were used as the reference electrode and the counter electrode, respectively. All potentials mentioned below were referred to SCE unless otherwise specified.

Before electrodeposition, the GC disc surface was polished successively with 1.0 μm-, 0.3 μm- and 0.05 μm-alumina slurry, and then ultrasonically rinsed twice with copious amounts of water and ethanol. The freshly cleaned GC electrode was used for the Pt electrodeposition in 50 mL of aqueous solution containing 2.0 mmol L⁻¹ K₂PtCl₆ and 1.0 mol L⁻¹ HClO₄. The electrodeposition was carried out galvanostatically at room temperature and the applied cathodic current densities ranged from 0.14 mA cm⁻² to 7.0 mA cm⁻². The deposition time was 40 min. After electrodeposition was completed, the as-prepared Pt nanostructures were rinsed with water for characterization and methanol electrooxidation.

2.3 Methanol electrooxidation

Methanol electrooxidation was conducted in a conventional three-electrode configuration. The working electrode was a GC disc covered with Pt nanostructure (GC/Pt), which was further cleaned before electrooxidizing methanol by cycling potentials from −0.25 V to 1.2 V in 0.5 mol L⁻¹ H₂SO₄ aqueous solution until the potential–current pattern remained unchanged. The measurement of methanol electrooxidation was carried out in 50 mL of 0.5 mol L⁻¹ H₂SO₄ + 1.0 mol L⁻¹ CH₃OH solution that was aerated with N₂ for 15 min to eliminate dissolved O₂ before measurement. For comparison, the commercial Pt/C electrode was prepared as well: 1.0 mL of 0.05 wt% Nafion ethanol solution containing 5 mg of Pt/C was pipetted onto a GC disc (diameter, 3 mm) and dried at room temperature.

2.4 Characterization

All electrochemical experiments were performed using a CHI 660D workstation (Chenhua, Shanghai). Scanning electron microscopic (SEM) images were taken on a scanning electron microscope (Hitachi-S4800) operating at 10 kV. Energy dispersive X-ray spectroscopy (EDS) and high resolution transmission electron microscopy (HRTEM) were performed on a T20 (EEI, America) microscope with an accelerating voltage of 200 kV.

3. Results and discussion

The current density of electrodeposition has a critical influence on the morphology of the resultant Pt nanostructures. When the current density was 0.14 mA cm⁻², the as-prepared 3D Pt nanostructure was composed of inter-connected Pt solid spheres, and these spheres had a large size ranging from 260 nm to 470 nm (Fig. 1A). Large Pt solid spheres are not sustainable for catalytic reactions because of their limited surface areas. However, when the current density was 0.7 mA cm⁻², larger Pt spheres were produced with a diameter ranging from 460 nm to 920 nm (Fig. 1B). Interestingly, these larger Pt spheres had hierarchical structures composed of many nano-petals, just like Hydrangea macrophylla flowers (colorful inset in Fig. 1B). The average thickness of the nano-petals was about 10 nm according to the estimation from SEM images with a high magnification (inset in Fig. 1B). With the increase of the current density from 0.7 mA cm⁻² to 3.5 mA cm⁻² and then to 7.0 mA cm⁻², all the resultant Pt nanostructures were constructed by Pt nano-petals (Fig. 1C and D). The only difference was the number density of Pt nano-petals on GC surface. At 3.5 mA cm⁻², the whole GC surface was covered with inter-connected Pt hierarchical spheres, the outline of which was distinguishable, and some Pt hierarchical spheres were sparsely decorated on the top of the Pt nanostructure, forming the-second-layer-Pt nanostructure. At 7.0 mA cm⁻², what we observed was 3D dense nano-petal structure and single Pt hierarchical spheres were nearly undistinguishable.

Fig. 1 SEM images of the 3D Pt hierarchical nanostructures at different cathodic current densities: (A) 0.14 mA cm⁻², (B) 0.7 mA cm⁻², (C) 3.5 mA cm⁻², and (D) 7.0 mA cm⁻². The electrodeposition time was 40 min. Grey insets are SEM images with higher magnification; colorful inset in (B) is the digital image of a Hydrangea macrophylla flower.

HRTEM images (Fig. 2A and C) reveal that the inter-fringe distances of Pt nano-petals prepared at 0.7 mA cm⁻² and 7.0 mA cm⁻² were ∼0.224 nm and ∼0.199 nm, respectively, which were ascribed to the lattice spacing of the Pt{111} and Pt{200} planes. However, as for Pt nano-petals obtained at 3.5 mA cm⁻² (Fig. 2B), the inter-fringe distance of the whole Pt nano-petal was ∼0.224 nm, indicating that they grew along the Pt{111} plane direction. All the EDS patterns of Hydrangea macrophylla flower-like Pt nanostructures were the same, as shown in Fig. 2D, which exhibits the characteristic peaks of Cu and Pt. Regardless of the copper and carbon coming from the substrate, Cu grid and the carbon film, only Pt peaks remained, i.e. the resultant 3DPHNs were pure Pt.

Fig. 2 Representative HRTEM images of 3DPHNs obtained at different current densities: (A) 0.7 mA cm⁻², (B) 3.5 mA cm⁻², and (C) 7.0 mA cm⁻². (D) EDS pattern of 3DPHNs.

Current density is crucial for the crystallization of Pt in the process of electrodeposition. At the current density of 3.5 mA cm⁻², only Pt{111} facets were detected, indicating that Pt nano-petals grew exclusively along the {111} direction, which was attributed to the effect of the surface energy of crystal faces.⁴¹ In general, the facets with low surface energy are easily formed during the growth of a crystal. Although the surface energy of the crystal facets depends on the intrinsic properties of materials, it can be remarkably lowered by surface adsorption.⁴² In our case, the growth of Pt nano-petals exclusively along the {111} direction may be closely linked with the surface adsorption of H ions on other facets of Pt. By adjusting the current density of deposition to control surface adsorption of H ions, single-crystal Ni, ZnS and CdS nanowires have been successfully produced.^41,43 On the other hand, a suitable current density was able to ensure PtCl₆²⁻ ions arrive at the electrode from the electrolyte to find thermodynamically favorable locations to attach and grow along {111} orientation. Shen et al.⁴⁰ found that Pt single crystal nanoparticles were obtained only at a medium current density. Either too high or too low current densities produced only multi-crystalline Pt nanoparticles.

To understand the formation mechanism of Pt hierarchical spheres, we tracked the growth process of 3DPHN obtained at 3.5 mA cm⁻² as shown in Fig. 3. When the deposition time (t) increased from 5 min to 40 min (Fig. 1 and 3), we observed the number density of Pt hierarchical spheres rose. At deposition time t = 5 min, sparse Pt hierarchical spheres (diameter, ∼100 nm) were distributed over the entire GC surface (Fig. 3A). On close inspection of the SEM image, ∼14 nm Pt nanoparticles were observed in the gaps among Pt hierarchical spheres, suggesting that Pt hierarchical spheres were formed with these small Pt nanoparticles as nucleation centers. With the increase of the deposition time to 10 min, each Pt hierarchical sphere further grew along the three dimensional directions and had a diameter of ∼140 nm. The gaps between Pt hierarchical spheres decreased. As the electrodeposition time continued to increase from 10 min to 15 min, the size of 3DPHNs increased from ∼140 nm to ∼240 nm and the gaps between Pt hierarchical spheres further decreased. At the same time, the diameter of 3DPHNs increased from ∼500 nm to ∼690 nm. When the deposition time reached 30 min, the morphology of Pt hierarchical structure (Fig. 3D) was similar to that produced after 40 min electrodeposition (Fig. 1D), but the difference is that 40 min electrodeposition continued to create a second-layer Pt hierarchical spheres as observed in Fig. 1D.

Fig. 3 SEM images of the 3D Pt hierarchical nanostructures obtained with the cathodic current density of 3.5 mA cm⁻² at different growth stages: (A) 5 min, (B) 10 min, (C) 15 min, and (D) 30 min. Insets are SEM images with higher magnification.

All the nanostructures obtained at different current densities showed the characteristic peaks of Pt metal when they were cycled in H₂SO₄ solution from −0.25 V to 1.20 V (Fig. 4A): hydrogen adsorption/desorption peaks at the range between −0.25 V and 0.05 V, oxidation peaks of Pt (∼0.8 V) at the positive-going segments, and reduction peaks of Pt oxides (∼0.48 V) at the negative-going segments. Additionally, the major differences for cyclic voltammograms of the different samples were the magnitude of hydrogen adsorption/desorption peak current, which was well-known to be closely linked with the electrochemical active area (ECA) of the Pt nanostructure. The larger the ECA, the larger was the magnitude of hydrogen adsorption/desorption peak current. This trend was consistent with that of the reduction peak current of platinum oxide at ∼0.48 V. The structural stabilities of 3DPHNs obtained at different current densities were evaluated by considering the retention of ECAs under continuous cycling from −0.25 V to 1.0 V in H₂SO₄ as shown in Fig. 4B. ECAs were calculated by integrating the charge of the hydrogen adsorption/desorption peaks.⁴⁴ Among all the samples, 3DPHNs obtained at a current density of 0.14 mA cm⁻² had the worse stability and the ECA declined rapidly to ∼20% of the initial value after 700 cycles (9.7 h). Other samples were relatively stable, and the retention of ECAs was more than 55% after 700 cycles. The 3DPHNs obtained at a current density of 3.5 mA cm⁻² exhibited the best mechanical stability: ∼75% of ECA was maintained after 700 cycles, which was larger than that of the commercial Pt/C catalyst (68% retention) (Fig. 4B curve e).

Fig. 4 (A) Cyclic voltammograms for 3DPHNs in deoxygenated 0.5 mol L⁻¹ H₂SO₄ solution. The scan rate was 50 mV s⁻¹. (B) The relationship between the retention of ECAs and cycle number, n. The current density of deposition: (a) 0.14 mA cm⁻², (b) 0.7 mA cm⁻², (c) 3.5 mA cm⁻², (d) 7.0 mA cm⁻² and (e) Pt/C.

To better understand the relation between the electrocatalytic activities (EA) and Pt nanostructures, all the as-prepared 3DPHNs and commercial Pt/C were used to electrocatalyze methanol oxidation as shown in Fig. 5A. All catalytic currents were normalized to ECAs. In the positive-going direction, oxidation peaks appeared in the potential range from 0.61 V to 0.68 V due to methanol oxidation.³⁵ The difference of peak potentials may be attributed to the fine difference of Pt nanostructures.⁴⁵ With the increase of current densities of Pt deposition from 0.14 mA cm⁻² to 3.5 mA cm⁻², the current densities of methanol oxidation peaks increased and reached up to a maximum value at 3.5 mA cm⁻², which was slightly higher than that of commercial Pt/C. However, when the deposition current density further increased to 7.0 mA cm⁻², the oxidation peak current density decreased. In the negative-going direction, another oxidation peak appeared in the range from 0.47 V to 0.51 V, which was attributed to the oxidations of adsorbed CO or CO-like species.⁴⁶ The cyclic voltammetric study indicated that 3DPHN prepared at 3.5 mA cm⁻² had the best specific EA and they are better than the Pt/C commercial catalyst, which is similar to the Pt dendritic hierarchical nanostructures reported before.⁴⁷

Fig. 5 (A) Cyclic voltammograms for 3DPHNs in nitrogen-saturated 1.0 mol L⁻¹ CH₃OH + 0.5 mol L⁻¹ H₂SO₄ solution. The scan rate was 50 mV s⁻¹. (B) The relationship between the retention of EAs and cycle number, n. The current densities of deposition: (a) 0.14 mA cm⁻², (b) 0.7 mA cm⁻², (c) 3.5 mA cm⁻², (d) 7.0 mA cm⁻² and (e) Pt/C.

The EAs of all 3DPHNs were further evaluated by continuous cycling from −0.2 V to 1.0 V in 1.0 mol L⁻¹ CH₃OH + 0.5 mol L⁻¹ H₂SO₄ as shown in Fig. 5B. All EAs decreased with the increase of cycle number, n. Among all the samples, 3DPHNs obtained at the current density of 3.5 mA cm⁻² exhibited the best stability after 700 cycles (9.7 h). The EA of the 3DPHNs decreased slowly with the increase of the cycle number, n, and remained at 58% of the initial value. The EAs of the 3DPHNs obtained at the current densities of 0.14 mA cm⁻² and 0.7 mA cm⁻² decreased steeply to 20% and 25% of the initial EAs, respectively, after 350 cycles and remained almost steady from 350 to 700 cycles. The EA of 3DPHN obtained at the current density of 7.0 mA cm⁻² decreased to 30% of the initial value after 450 cycles and remained steady from 350 to 700 cycles. It indicated that 3DPHNs obtained at 3.5 mA cm⁻² had the best EA stability, which was better than the commercial Pt/C catalyst (47% retention of EA, curve e).

All 3DPHNs and commercial Pt/C were further polarized at 0.6 V for 4000 s in 1.0 mol L⁻¹ CH₃OH + 0.5 mol L⁻¹ H₂SO₄ solution as shown in Fig. 6. At 4000 s, 3DPHN obtained at 3.5 mA cm⁻² still exhibited the best performance among all the as-prepared samples, having a longer-term stability than Pt/C, which is consistent with the cyclic voltammetric characterization in Fig. 5B, and the catalytic activity was calculated to be ∼2 times better than the commercial Pt/C catalyst according to the catalytic current density, and 4.5 times better than Pt honeycomb structure.³³

Fig. 6 Polarization curves of the as-prepared 3DPHNs in nitrogen-saturated 1.0 mol L⁻¹ CH₃OH + 0.5 mol L⁻¹ H₂SO₄. The current densities of Pt nanostructure deposition: (a) 0.14 mA cm⁻², (b) 0.7 mA cm⁻², (c) 3.5 mA cm⁻², (d) 7.0 mA cm⁻², and (e) Pt/C. The polarization potential was 0.6 V.

The exposed specific facets and the structural morphologies of Pt catalysts are closely related to Pt electrocatalytic activity. The {111} facet is one of the most stable planes among all the facets of Pt. The richness of {111} facets in the surface of 3DPNHs is one main aspect contributing to the enhanced long-term stability as confirmed by Pt nanotubes dominated by {111} facets.²³ Previous study has found that Pt nanoparticles with {111} orientation are generally more CO tolerant than Pt nanoparticles with {100} orientation.⁴⁸ Pt octahedron enclosed by {111} facets were also demonstrated to provide more active sites than the ordinary particle surface, exhibiting an enhanced catalytic activity in methanol electrooxidation.⁴⁹ However, the exposure of the {111} facets cannot be the only reason for the improvement of Pt catalytic activity. Many Pt nanostructures, such as dendritic Pt spheres,⁵⁰ and hollow Pt nanostructures⁵¹ showed lower electrocatalytic activities in cyclic voltammetric measurements than that obtained in our case (∼1.2 mA cm⁻², Fig. 5A) although the spikes of dendritic Pt spheres and the nanoparticles of hollow Pt nanostructures had {111} orientation. The morphology of Pt nanostructures cannot be neglected because it directly affects the mass transport of methanol.^51,52 For example, porous Pt meso-flowers with {111} orientation nanopedals⁵³ and three-dimensional dendritic Pt nanostructures composed of {111} orientation nanoparticles⁴⁰ exhibited excellent electrocatalytic activities that were 2–3 times better than that of the commercial Pt/C due to the improvement of methanol mass transport in catalysts.

The catalytic activities of 3DPHNs obtained at 0.7 and 7.0 mA cm⁻² were different from that of 3DPHN prepared at 3.5 mA cm⁻², the reason of which may be attributed to the difference of the exposed facets as demonstrated in Fig. 2. This satisfactory catalytic activity and stability of 3DPHN obtained at 3.5 mA cm⁻² should be attributed to the exposure of {111} facets and structural morphology. The loss of the catalytic activity may be due to the surface structure rearrangement during the electrochemical cycle and the poisoning of CO-like intermediates.

4. Conclusion

One-step galvanostatic electrodeposition was demonstrated as a simple approach to fabricate three-dimensional Pt hierarchical structures consisting of nano-petals. By controlling the current density of deposition, the morphologies of 3DPHNs were adjusted and transformed from isolated nano-petal microspheres into 3D dense nano-petal arrays. Additionally, 3DPHN obtained at the current density of 3.5 mA cm⁻² has better electrocatalytic activity and stability than the commercial Pt/C catalyst in a long-term run of methanol oxidation, which was attributed to the richness of Pt{111} facets and the special morphology of 3DPHN.

Acknowledgements

This study was financed by NSFC (no. 21176060) and Chemical Science Base (no. J1210040 and J1103312) of Hunan University.

Notes and references

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