Electrochemically Shape-Controlled Synthesis in Deep Eutectic

Dec 13, 2011 - ... to Prepare Pt Nanocrystals Enclosed by High-Index Facets with High Catalytic Activity ... {910} and vicinal high-index facets, whic...
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Electrochemically Shape-Controlled Synthesis in Deep Eutectic Solvents—A New Route to Prepare Pt Nanocrystals Enclosed by High-Index Facets with High Catalytic Activity Lu Wei,†,‡ You-Jun Fan,*,† Na Tian,‡ Zhi-You Zhou,‡ Xue-Qin Zhao,‡ Bing-Wei Mao,‡ and Shi-Gang Sun*,‡ †

Key Laboratory for the Chemistry and Molecular Engineering of Medicinal Resources (Ministry of Education of China), College of Chemistry and Chemical Engineering, Guangxi Normal University, Guilin 541004, China ‡ State Key Laboratory for Physical Chemistry of Solid Surfaces, and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China

bS Supporting Information ABSTRACT: In the present paper, we have developed for the first time the electrochemically shape-controlled synthesis in deep eutectic solvents (DESs) for the preparation of Pt nanocrystals enclosed by high-index facets. Monodispersed concave tetrahexahedral Pt nanocrystals (THH Pt NCs) have been prepared through this new route. The concave THH Pt NCs were characterized by SEM, TEM, and AFM. The as-prepared concave Pt NCs are bounded with {910} and vicinal high-index facets, which exhibit superior catalytic activity and stability to those of the commercial Pt black catalyst for ethanol electrooxidation. We have demonstrated also that the electrochemically shape-controlled synthesis in DESs proves advantageous in controlling the size and shape of Pt NCs without the addition of seeds, surfactants, or other chemicals and could be applied in the synthesis of other noble metal NCs with high surface energy and high catalytic activity.

1. INTRODUCTION The fcc (face-centered cubic) noble metal (e.g., Pt, Pd) nanocrystals (NCs) enclosed by high-index facets have attracted intensive interest due to their high catalytic activity and potential applications in important fields such as fuel cells and (petro) chemical industry.1 6 High-index facets always expose a high density of low-coordinated atoms on steps, ledges, and kinks, which constitute active sites of catalyst,6 and have generally a high surface energy. As a result, the fcc metal NCs bounded with highindex facets exhibit essentially higher catalytic activity than those of fcc metal NCs enclosed by low-index facets, on which the surface atoms are the most compactly arranged resulting in a low surface energy. As the NCs of high surface energy are eliminated during their growth in a conventional shapecontrolled synthesis process due to the thermodynamics that drives the NCs minimizing their total surface energy, the synthesis of NCs of high surface energy is always a big challenge. The breakthrough of synthesis of metal NCs of high surface energy has been made first by the development of an electrochemical route.1 3 Recently, wet-chemistry methods have been also developed. Various metal NCs of high surface energy including rods,7,8 dendritic structures,9 11 and polyhedra12 22 were obtained. It is worthwhile mentioning that in the wetchemistry methods, all metal NCs of high surface energy mentioned above were synthesized in aqueous solution with the addition of seeds, surfactants, additives, and stabilizers. As a consequence, the wet-chemistry route presents disadvantages r 2011 American Chemical Society

of applying directly the synthesized NCs in (electro)catalysis due to the inertness of the NCs when they are covered with stabilizers. In this paper, we report for the first time the development of electrochemically shape-controlled synthesis of metal NCs of high surface energy in deep eutectic solvents (DESs) without the addition of seeds, surfactants, or other chemicals. The DESs are promising solvents to be used in the synthesis of nanomaterials because of their remarkable physicochemical properties, such as high conductivity, viscosity, surface tensions, polarity, thermal stability, and negligible vapor pressure.23 28 To the best of our knowledge, only Sun and co-workers reported the synthesis of star-shaped Au NCs enclosed with {331} and vicinal high-index facets and thus high surface energy in DESs by chemical reduction route.29 The combination of the DESs and electrochemical method produced concave tetrahexahedral (THH) Pt NCs enclosed by {910} and vicinal high-index facets in DESs. The growth of concave THH Pt NCs by this new route is straightforward and controllable in NCs’ size and shape. In comparison with commercial Pt black catalyst, the as-synthesized concave THH Pt NCs display a much higher catalytic activity and stability toward ethanol electrooxidation in acidic media.

Received: October 11, 2011 Revised: November 28, 2011 Published: December 13, 2011 2040

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2. EXPERIMENTAL SECTION Synthesis of DESs. Choline chloride [HOC2H4N(CH3)3Cl] (Shanghai Chemical Reagent Ltd., China 99%) was recrystallized from absolute ethanol, filtered, and dried under vacuum. Urea (Shanghai Chemical Reagent Ltd., China >99%) was recrystallized from Millipore water (18.0 MΩ cm) provided by a Milli-Q Lab apparatus (Nihon Millipore Ltd.), filtered, and dried under vacuum prior to use. DESs were formed by stirring the above two components together, in the stated proportion (molar ratio of choline chloride/urea = 1:2), at 80 °C until a homogeneous, colorless liquid formed. The prepared DESs, once formulated, were kept in a vacuum at 80 °C prior to use. Concave THH Pt NCs Synthesis. Electrochemical preparation of concave THH Pt NCs in DESs was carried out in a standard three-electrode cell connected to a 263A potentiostat/galvanostat (EG&G), with a platinum wire counter electrode and a Pt quasi-reference electrode. The working electrode was a glassy carbon disk (GC, Φ = 3 mm), which was polished with 5.0, 1.0, 0.3 μm Al2O3 powder and washed ultrasonically in ultrapure water before each experiment. In a typical procedure, the monodisperse concave THH Pt NCs were electrodeposited directly on GC substrate in 19.3 mM H2PtCl6/DESs solution at 80 °C using a programmed electrodeposition method reported previously.3 The GC electrode was first subjected to a potential step from 1.20 V (vs Pt) to 1.50 V (EN), and this potential was maintained for 1 s to generate Pt nuclei. The growth of the Pt nuclei into concave THH Pt NCs was achieved by applying a square-wave potential (f = 10 Hz) with the lower (EL) and upper (EU) potential limits of 1.30 and 0.30 V, respectively. It is noted that the concentration of the precursor used in DESs is significantly larger than those used for the synthesis of Pt and Pd THH NCs in aqueous solutions.1,3 This may be a consequence of the high viscosity of DESs,23,24 which decreases the mass transportation of reactive species in DESs. AFM Measurements. AFM experiments were performed using an Agilent Technologies 5500 atomic force microscope at room temperature of 20 °C. The concave THH Pt NCs were imaged by tapping mode. Cantilevers were 100 μm long, with nominal spring constant and resonant frequency of 11.8 N/m and 240 kHz, respectively (tip radius ∼ 10 nm) (NSG 10, NT-MDT Co., Moscow, Russia). Electrocatalytic Performance Measurements. The electrocatalytic performances of the concave THH Pt NCs were measured in 0.1 M ethanol + 0.1 M HClO4 solution at room temperature (25 °C). The solutions were deaerated by purging with pure N2 gas before experiment, and a flux of N2 was kept over the solution during measurements to prevent the interference of atmospheric oxygen. A saturated calomel electrode (SCE) was used as reference electrode, and all potentials in the electrochemical performance tests are quoted versus the SCE scale.

3. RESULTS AND DISCUSSION The as-prepared concave THH Pt NCs were carefully characterized by scanning electron microscopy (SEM, LEO-1530) and transmission electron microscopy (TEM, FEI Tecnai-F30), as illustrated in Figure 1. It can be clearly seen that the Pt NCs appear to have a concave cubic morphology, and the faces of each cube have contrast lines in the shape of an “X” (Figure 1a and b). The SEM images (Figure 1c) of concave THH Pt NCs tilted 0°,

Figure 1. (a) Large-area, (b) enlarged SEM images of concave THH Pt NCs electrodeposited on GC in 19.3 mM H2PtCl6/DESs solution at 80 °C by a programmed electrodeposition method: EL = 1.3 V, EU = 0.3 V, growth time = 60 min. (c) SEM, (d) TEM images of concave THH Pt NCs tilted 0°, 45°, and 90° to illustrate the concave faces. Scale bar of the inset is 100 nm. (e) SAED patterns of concave THH Pt NCs oriented along the [100], [111], and [110] directions.

Figure 2. (a) Typical AFM images of concave THH Pt NCs in Figure 1a. (b) Magnified AFM image of concave THH Pt NCs indicated by the white arrow in (a). (c) SEM and (d) model images of a concave THH Pt NCs. (e) Cross-sectional profiles through white lines XX and YY in image b, showing the XX and YY cross-sectional surfaces and the angles between concave facets and (001) plane.

45°, and 90° illustrated that each concave face of the Pt NCs has four trigonal facets. In the TEM images (Figure 1d), the cubes exhibit darker contrast in the middle region as compared with the edges. The selected-area electron diffraction (SAED) patterns (Figure 1e) of individual Pt NCs oriented along the [100], [111], 2041

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Figure 4. SEM images of Pt nanostructures electrodeposited on GC in 19.3 mM H2PtCl6/DESs solution at 80 °C by the programmed electrodeposition method for the growth time of 60 min: (a) EL = 1.4 V, EU = 0.3 V; (b) EL = 1.3 V, EU = 0.1 V; (c) EL = 1.3 V, EU = 0.4 V. Scale bar of the inset is 100 nm.

Figure 3. SEM images of concave THH Pt NCs for the deposition time of (a) 20, (b) 40, (c) 60, and (d) 80 min. (e) Size histograms of concave THH Pt NCs in (a), (b), (c), and (d), respectively, after counting more than 220 particles for each sample.

and [110] directions demonstrated their single-crystalline structure. To further characterize the surface structure of concave THH Pt NCs, AFM measurements were carried out to monitor the angles of concave facets. Figure 2a is the typical AFM image of concave THH Pt NCs, from which we captured the AFM image of a concave Pt NCs along the [001] orientation, as shown in Figure 2b. The corresponding profile lines along the [010] and [100] orientation (indicated by XX and YY) are shown in Figure 2e. We can clearly observe the angles between the four concave facets and the (001) plane, which is in good agreement with the models of the concave THH NCs and their crosssectional surfaces (Figure 2d and e). In Figure 2e, the profile lines at the edge of the concave THH NCs are not vertical, which is due to the expansion effect of the AFM tip.30 32 However, this effect does not influence the angles between concave facets and the (001) plane. The angles between the concave facets and the (001) plane are measured to be 5.61° and 6.34° in XX profile and 5.76° and 6.23° in YY profile as indicated in Figure 2e. The values

suggest that the surface of the concave THH Pt NCs consists of {10, 1, 0} and {910} facets. By measuring the angles of several concave Pt NCs, we could conclude that the concave THH Pt NCs were mainly enclosed by {910} and {10, 1, 0} facets, together with some other vicinal high-index facets of {11, 1, 0}, and {12, 1, 0} (Supporting Information Table S1). On such highindex facets, a high density of step and kink atoms is presented (Supprting Information Figure S2). The size of the concave THH Pt NCs can be controlled by varying the growth time. The SEM images of concave THH Pt NCs produced with growth times of 20, 40, 60, and 80 min are illustrated in Figure 3a d, respectively. The corresponding sizes are measured, respectively, to be 62.5 ( 10, 107.5 ( 7, 167.5 ( 15, and 370 ( 20 nm (Figure 3e). It can be found that, in all cases, the concave faces of THH Pt NCs are maintained regardless of their size, although the facets are better defined or more easily assignable for larger particles. However, some rough structures on the concave faces of THH Pt NCs can be observed for the growth time of 80 min (Figure 3d), indicating the overgrowth of the Pt NCs. The Pt NCs of various shape and surface structure including cubes and truncated cubes can be also obtained simply by adjusting the EL or EU value. When EL is 1.4 V, the cubic Pt NCs about 200 nm in size were produced (Figure 4a). On the other hand, when EU is 0.1 V, the truncated cubic Pt NCs about 300 nm in size were obtained (Figure 4b). Further varying EU to 0.4 V, the concave cubic Pt NCs about 100 nm in size were generated (Figure 4c). It is worthwhile noting that unlike the concave THH Pt NCs, the as-prepared concave cubic Pt NCs are only bounded by six concave surfaces. Interestingly, the shape of Pt NCs depends on the solvents used in preparation. When the Pt and Pd NCs were prepared in aqueous solution by the electrochemical method, the convex THH shape can be obtained.1,3 The growth mechanism of concave THH Pt NCs in DESs may be associated with the composition and properties of DESs, such as the presence of Cl ions in DESs.16 Systematic studies aimed at understanding the effect of these factors on the growth of concave THH Pt NCs are still underway. The concave THH Pt NCs with an average size of 167.5 nm were used as catalysts for the electrochemical characterization and ethanol electrooxidation. Figure 5 compares the cyclic 2042

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The Journal of Physical Chemistry C voltammograms (CVs) of the concave THH Pt NCs (red line) and commercial Pt black catalyst (Johnson Matthey, black line) recorded in 0.1 M HClO4 solution. The current between 0.25 and 0.13 V is attributed to hydrogen adsorption/desorption. The electroactive surface area of concave THH Pt NCs and commercial Pt black catalyst was evaluated as, respectively, 0.092 and 0.158 cm2 according to the method described previously.1 In addition, the current between 0.30 and 0.70 V as indicated in Figure 5 is attributed to oxygen adsorption/desorption, which is clearly larger on the concave THH Pt NCs than that on the Pt black catalyst. Previous study has shown that oxygen atoms mainly adsorb on stepped Pt atoms with low coordination numbers at such low potential.33 There is higher density of stepped atoms on concave THH Pt NCs; i.e., more atoms are accessible for oxygen adsorption/desorption at low potential. It is worth noting that the hydrogen adsorption/desorption current on the concave THH Pt NCs near 0.05 V is significantly larger than that on the Pt black catalyst, indicating the formation of high-index facets on the concave THH Pt NCs.34 The electrocatalytic property of the concave THH Pt NCs was evaluated by CV and chronoamperometry methods in acidic solutions at room temperature. Figure 6a displays cyclic voltammograms of the concave THH Pt NCs and commercial Pt black

Figure 5. Cyclic voltammograms of concave THH Pt NCs and Pt black catalyst in 0.1 M HClO4 solution, Scan rate: 50 mV s 1. The TEM images below the CVs are corresponding nanostructures.

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catalyst in 0.1 M ethanol + 0.1 M HClO4 solution. The oxidation current has been normalized to the electroactive surface area. This allowed the current density to be directly used to compare the catalytic activities of different samples. In the positivegoing potential scan, the ethanol electrooxidation on concave THH Pt NCs gives rise to two current peaks near 0.54 and 0.94 V (vs SCE); their peak current densities are 2.32 and 1.86 mA cm 2, respectively. The corresponding values obtained on the Pt black catalyst are only 1.01 and 1.23 mA cm 2. In the negative-going potential scan, the peak current densities of ethanol oxidation on these two samples are 2.0 and 1.13 mA cm 2. These results demonstrate that the electrocatalytic activity of the concave THH Pt NCs for ethanol oxidation is nearly double that of the Pt black catalyst. Figure 6b compares the current time curves of ethanol oxidation at 0.45 V (vs SCE), i.e. the potential at which the current reaches its half height of the peak. It can be seen that the steady current densities (t = 7200 s) of ethanol oxidation on the concave THH Pt NCs and Pt black catalysts are 0.27 and 0.23 mA cm 2, respectively. This further demonstrates that the electrocatalytic activity and stability of the concave THH Pt NCs for ethanol oxidation are superior to those of the Pt black catalyst. The enhancement in electrocatalytic activity and stability of the concave THH Pt NCs could be evidently attributed to their highenergy surfaces with a high density of active sites composed of step and kink atoms, which are beneficial for the C C bond breaking and the oxidation of adsorbed CO fragments in ethanol oxidation.35,36

4. CONCLUSION In summary, we have developed the electrochemically shapecontrolled synthesis in DESs of Pt NCs of high surface energy. The concave THH Pt NCs enclosed by {910} and vicinal highindex facets were obtained through this new route for the first time. The growth of the concave THH Pt NCs in DESs is straightforward and controllable in NCs’ size and shape without the addition of seeds, surfactants, or other chemicals. Thanks to their high density of surface atomic steps, the concave THH Pt NCs exhibit higher electrocatalytic activity and stability than commercial Pt black catalyst toward ethanol oxidation. This study is of significance for shape-controlled synthesis of other noble metal NCs of high surface energy in DESs, as well as in key applications of electrocatalysis, such as in electrochemical sensors and fuel cells.

Figure 6. (a) Cyclic voltammmograms (50 mV s 1) and (b) chronoamperometric curves, measured at 0.45 V (vs SCE), of ethanol oxidation on concave THH Pt NCs (red line) and commercial Pt black catalyst (black line) in 0.1 M ethanol + 0.1 M HClO4 solution. 2043

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’ ASSOCIATED CONTENT

bS

Supporting Information. Cyclic voltammogram of GC substrate in DESs (Figure S1); atomic models of Pt (910), (10,1,0), (11,1,0), (12,1,0) planes (Figure S2); angles of different facets with (001) plane measured from several concave THH Pt NCs (Table S1). This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail [email protected] (Y.-J.F.), sgsun@xmu. edu.cn (S.-G.S.); fax +86-773-2120958 (Y.-J.F.), +86-592-2180181 (S.-G.S.); tel. +86-773-5846279 (Y.-J.F.), +86-592-2180181 (S.-G.S.).

’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (20833005, 21021002), the Opening Foundation Project of the State Key Laboratory of Physical Chemistry of Solid Surfaces (Xiamen University), Guangxi Natural Science Foundation of China (0991093, 2010GXNSFF013001), and the S&T Project of Guangxi Education Department of China (201012MS024). ’ REFERENCES

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