Glycine-Mediated Syntheses of Pt Concave Nanocubes with High

Letter pubs.acs.org/Langmuir

Glycine-Mediated Syntheses of Pt Concave Nanocubes with HighIndex {hk0} Facets and Their Enhanced Electrocatalytic Activities Zhi-cheng Zhang,†,‡ Jun-feng Hui,‡ Zhi-Chang Liu,*,† Xin Zhang,*,† Jing Zhuang,‡ and Xun Wang*,‡ †

State Key Laboratory of Heavy Oil Processing, Department of Chemical Engineering, China University of Petroleum, Beijing 102249, PR China ‡ Department of Chemistry, Tsinghua University, Beijing 100084, PR China S Supporting Information *

ABSTRACT: Metal nanocrystals with high-index facets (HIFs) have drawn significant attention for their superior catalysis activity compared to that of low-index faces. However, because of the high surface energy of HIFs, it is still challenging to preserve HIFs during the growth of nanocrystals. In this study, highly selective Pt concave nanocubes (CNCs) with high-index {hk0} facets have been successfully prepared in a simple aqueous solution. The vital role of glycine as the surface controller in the formation of CNCs was demonstrated. These Pt CNCs exhibited enhanced specific activities toward the electro-oxidation of methanol and formic acid in comparison to commercial Pt black and Pt/C catalysts.

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INTRODUCTION Recently, the fabrication of metallic nanocrystals bound by high-index facets (HIFs) has been extensively studied.1−6 Because of the high density of atomic steps, ledges, and kinks, HIFs generally exhibit significantly enhanced catalytic activity toward specific reactions as compared to the most common stable planes.7 To date, electrochemical or wet-chemical methods have been developed for the synthesis of metallic nanocrystals with HIFs (e.g., tetrahexahedral (THH) Pt nanocrystals,8,9 trioctahedral (TOH) Au nanocrystals,10−12 Pd concave nanocubes,13 and THH Pd nanocrystals3). Despite this success, the design and synthesis of metallic nanocrystals enclosed with HIFs remain a great challenge. It is known that HIFs usually disappear during the growth of nanocrystals because of their high surface energies.7 Recent studies have shown that HIFs could be stabilized by the selective adsorption of specific chemical species in solution synthesis.14,15 For instance, Zheng and co-workers reported the synthesis of concave polyhedral Pt nanocrystals with high-index {411} facets via a solvothermal method using hexachloroplatinic acid (H2PtCl6) as the Pt source, poly(vinyl pyrrolidone) (PVP) as the surfactant, amine as the surface controller, and N,Ndimethylformamide (DMF) as the solvent. During the growth of Pt nanocrystals, the {411} exposed facets could be preserved thanks to the coordination of amine, which helps to stabilize the low-coordination Pt sites.14 More recently, Xie and coworkers reported the preparation of multipod Pt nanocrystals with dominant high-index {211} surfaces also through a solvothermal method using Pt(II) acetylacetonate as the Pt precursor and 1-octylamine as the solvent and capping agent. With the addition of formaldehyde as an additional surface© 2012 American Chemical Society

structure regulator, concave Pt nanocrystals exposing {411} facets could be obtained.15 The results above inspired us to use a similar strategy to synthesize metallic nanocrystals with HIFs via a wet-chemical route, especially in an environmentally benign solvent such as water. In this work, we report the facile synthesis of Pt CNCs with high-index {hk0} facets in an aqueous solution by the reduction of H2PtCl6 in the presence of PVP (MW = 30 000) and glycine. In this simple synthetic system, PVP acts as a surfactant, stabilizing agent, and reductant, and glycine serves as a surface controller and coreductant.

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EXPERIMENTAL SECTION

Synthesis of Pt Concave Nanocubes. In a typical synthesis, PVP (MW = 30 000, 400 mg), glycine (75 mg), and 2 mL of H2PtCl6 solution (20 mM) were added to 6.0 mL of deionized water and stirred for 5 min at room temperature. The resulting yellow homogeneous solution was transferred to a 12 mL Teflon-lined stainless-steel autoclave. The sealed vessel was then held at 200 °C for 6 h before it was cooled to room temperature. Note that the heating rate of the autoclave was 6 °C min−1. The products were separated via centrifugation at 11 000 rpm (with a centrifugal force of 11 363g) for 15 min and further purified by ethanol three times. Electrochemical Measurements. Electrochemical experiments were performed using a CHI 650D electrochemical analyzer (CHI Instruments, USA). A conventional three-electrode cell was used, including a saturated calomel electrode (SCE) as the reference electrode, a Pt wire as the counter electrode, and a glassy carbon (GC) Received: July 23, 2012 Revised: October 6, 2012 Published: October 9, 2012 14845

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electrode (3 mm diameter) as the working electrode. All of the electrode potentials in this letter are quoted versus the SCE. A GC electrode was carefully polished with Al2O3 paste and washed with deionized water before each experiment. After the electrode was dried, the water dispersion of Pt nanocrystals was dropped onto the GC electrode with the same Pt loading of 6 μg. It is noted that the concentration of Pt nanocrystals dispersed in aqueous solution was determined by ICP-OES measurements. After solvent evaporation under an IR lamp, the electrode was illuminated with a UV lamp (10 W with 185 and 254 nm emissions) at a distance of about 5 mm for 12 h to remove the organic capping agents before electrochemical measurements. Then the electrode was covered with 1.5 μL of 0.5 wt % Nafion (Alfa Aesar) in ethanol and dried in air for 0.5 h. For Pt black or Pt/C catalysts, 8 μL of the aqueous dispersion of Pt black (Alfa Aesar, 0.75 mg mL−1) or Pt/C (20 wt % Pt nanoparticles supported on carbon black, Alfa Aesar, 0.75 mg mL−1) was transferred onto the GC electrode, and no UV/ozone treatment was used. The electrolyte was a freshly made 0.5 M H2SO4 + 2 M CH3OH (or 0.25 M HCOOH) solution and was bubbled with N2 for 30 min before electrochemical measurements. Cyclic voltammetry (CV) measurements were performed under N2 flow at room temperature, and the potential was scanned from −0.2 to 1.0 V (vs SCE) at a sweep rate of 50 mV s−1. The electrochemically active surface area (ECSA) of each sample was estimated by CV measurements carried out in a fresh nitrogen-saturated 0.5 M H2SO4 solution, and the potential was scanned from −0.24 to 1.0 V (vs SCE) at a sweep rate of 50 mV s−1. CVs were obtained after 50 cycles.

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RESULTS AND DISCUSSION Figure 1a shows the representative TEM images of as-prepared Pt CNCs. (See also Figure S1a,c,d in the Supporting Information). The resulting product consists of Pt CNCs with a shape selectivity above 90% and an average apex-to-apex diameter of 44.4 ± 3.2 nm (Figure 1a, inset). The image obtained by high-angle annular dark-field scanning TEM (HAADF-STEM) further clearly revealed the concave feature of the nanostructures (Figures 1b and S1b). To visualize the three-dimensional structure of the CNCs better, as shown in Figure 1c, a Pt CNC was tilted 0, 20, and 40° to illustrate the concave faces. Then the concave features became more obvious, and the nanoparticle displayed a darker contrast in the center region as compared to the edges. Figure 1d shows the high-resolution TEM (HRTEM) image of a single Pt CNC. As confirmed by the corresponding fast Fourier transform (FFT) pattern (Figure 1e, top-right inset), the HRTEM image was viewed along the [100] zone axis. On the basis of several previous reports,1,4,5,13 the angles between the facets of the projected concave nanocube and the {100} facets of an ideal cube were determined to be 16, 14, 18, and 10°, thus indicating the presence of some high-index {hk0} facets or steps. The high-magnification HRTEM images further exhibited the concave high-index steps (Figure 1e, bottom-left inset). To reveal the possible growth mechanism involved in the formation of Pt CNCs, a series of control experiments were conducted using the standard procedure. As depicted in Figure 2, no CNCs could be obtained without the addition of glycine. The resultant product consists of the coexistence of small nanoparticles and nanowires assembled from these nanoparticles with a diameter of 3.7 ± 0.5 nm. The length of the nanowires can reach to dozens of nanometers. This also indicates that PVP serves as a reducing agent. Interestingly, when the amount of glycine is 30 mg, the product consists of mixed morphologies including cubes, icosahedra, and other polyhedra with larger sizes. When we increase the amount of glycine to 50 mg, the morphology of the product makes no

Figure 1. (a, c) TEM, (b) HAADF-STEM, and (d, e) HRTEM images of as-synthesized Pt CNCs. The inset in a is the size distribution of Pt CNCs. Panel e is the magnified HRTEM image taken from the selected area marked by the red dashed line in panel d. The top-right inset in e is the corresponding FFT pattern of the HRTEM image in d or e. The bottom-left inset in e is the magnified HRTEM image of the selected area marked by the white dashed line in e. In c, a Pt CNC was tilted 0, 20, and 40° to illustrate the concave faces. Scale bars: 20 nm.

Figure 2. TEM images of Pt nanocrystals that were prepared using the standard procedure, except for the variation in the amount of glycine: (a) 0, (b) 30, (c) 50, and (d) 150 mg.

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According to the results above, we suggest that the critical role of glycine in the formation of concave nanostructures might originate from the selective binding of glycine on these high-index {hk0} facets of Pt nanocrystals during growth, which could be confirmed by the result of the Fourier transform IR (FT-IR) of Pt CNCs where all of the typical absorption bands of glycine could be observed (Figure S6). Furthermore, to correlate the formation of concave nanostructures with the groups (i.e., amino groups and carboxyl groups), ethylamine or acetic acid was employed to substitute glycine for the synthesis. Unfortunately, no concave but irregular nanoparticles were obtained (Figure S7). This may reveal that the synergetic effect of the amino group and carboxyl group favors the formation of concave nanocrystals, which is further supported by the formation of concave cubic Pt nanocrystals by using ethylenediamine-tetraacetic acid disodium salt (EDTA-2Na) instead of glycine (Figure S8) in the hydrothermal protocol. In addition, as mentioned above, glycine has two ionizable functional groups. Thus, the starting solution pH determined the ionization of the glycine ligand and consequently the formation of different Gly-Pt(IV) complexes to result in different rates of the formation of Pt nanocrystals (Figure S9). To gain insight into the morphological evolution of Pt CNCs, time sequential evolution experiments were carried out during the hydrothermal process. As shown in Figure 3, the size of the Pt nanoparticles increased with the reaction time during 6 h. The average apex-to-apex diameter of the nanoparticles synthesized at 1.5, 3, 9, and 12 h was 24.5 ± 1.4, 36.9 ± 2.7, 44.9 ± 3.1, and 45.2 ± 3.6 nm, respectively. It is seen that after 6 h the size of the nanoparticles did not change further. However, the concave feature could be maintained even after 12 h of reaction. To evaluate the electrochemical properties of as-prepared Pt CNCs, the electro-oxidation of methanol and formic acid was tested. For comparison, the commercial Pt black and Pt/C (Alfa Aesar) catalysts were used as references (Figure S10). Before each electrochemical test, Pt CNCs were irradiated with a UV lamp (185 and 254 nm) for 12 h in air to remove the capping agents.17−20 TEM images show that the shape and size of Pt CNCs could be well maintained after the treatment (Figure S11). Figure 4 shows the comparison of the cyclic voltammogram (CV) curves for the electro-oxidation of methanol and formic acid on the Pt CNCs, commercial Pt black, and Pt/C. The specific current density (J) was normalized to the electrochemically active surface area (ECSA), which was measured by integrating the electric charges on the adsorption/desorption peak of hydrogen regions

Figure 3. TEM images of the Pt CNCs produced after (a) 1.5, (b) 3, (c) 9, and (d) 12 h of reaction.

difference with respect to that shown in Figure 1 for 75 mg of glycine, except the sizes. However, when the amount of glycine is increased to 150 mg and even more, the change in the degree of concavity of the CNCs can be clearly observed, which may lead to the exposure of other different high-index {hk0} facets. However, fewer products could be obtained upon further increasing the amount of glycine. The slower reducing rate may be attributed to the coordination effects of glycine to PtCl62−.16 These results showed that the use of an appropriate amount of glycine would manipulate the reducing kinetics to form Pt CNCs, and we suggest that glycine mainly plays the critical role of shape and size controllers. In the synthesis, we found that the use of PVP is not essential to the formation of concave nanostructures, but without PVP added to the system, the resultant products were heavily aggregated with the shapes of multipods (Figure S2). This shows that PVP acts as a stabilizing agent to protect and disperse the Pt nanocrystals, and glycine also can be used as a coreductant apart from PVP. In addition, we also found that the reaction temperature and the concentration of Pt precusor have no notable effect on the formation of concave nanoparticles but do have an effect on the degree of concavity (Figure S3−5).

Figure 4. CV curves for the electro-oxidation of (a) methanol and (b) formic acid by the Pt CNCs, commercial Pt black, and Pt/C (Alfa Aesar). The methanol oxidation was recorded in 0.5 M H2SO4 + 2 M CH3OH solution at a scan rate of 50 mV s−1. The formic acid oxidation was recorded in 0.5 M H2SO4 + 0.25 M HCOOH solution at a scan rate of 50 mV s−1. 14847

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(Figure S12). The peak current densities of methanol (formic acid) oxidation in the positive potential scan were 1.19 (0.57), 0.68 (0.34), and 0.47 (0.23) mA cm−2 on the Pt CNCs, Pt black, and Pt/C catalyst, respectively. This indicates that Pt CNCs exhibit enhanced specific activity toward methanol and formic acid oxidation compared to commercial Pt black and Pt/ C catalysts. The electrocatalytic activity of Pt CNCs is almost 1.8 (1.7) times that of Pt black and 2.5 (2.5) times that of Pt/C in the electro-oxidation of methanol (formic acid).

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OUTLOOK We have demonstrated the facile synthesis of Pt CNCs with high shape selectivity in an aqueous solution. The use of glycine as the surface controller allows the formation of the CNCs enclosed by high-index {hk0} facets. These Pt CNCs exhibited higher electrocatalytic activity per unit surface area than commercial Pt black and Pt/C catalysts in the electro-oxidation of methanol and formic acid. The current facile synthesis method is being extended to prepare other noble metals or their alloy nanocrystals with HIFs for highly active catalysts.

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ASSOCIATED CONTENT

S Supporting Information *

Materials and characterization methods. Additional TEM and HRTEM images and electrocatalysis results. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(Z.-C.L.) E-mail: [email protected]. (X.Z.) E-mail: zhangxin@ cup.edu.cn. (X.W.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the NSFC (91127040, 20921001, and 20903119), and the State Key Project of Fundamental Research for Nanoscience and Nanotechnology (2011CB932402).

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REFERENCES

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