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Synthesis of Small Platinum Cube with Less Than 3 nm by the Control of Growth Kinetics Keiko Miyabayashi, Shingo Nakamura, and Mikio Miyake* School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan
bS Supporting Information ABSTRACT: The preparation of small shape-controlled Pt nanocrystals, which should be an important material such for as catalytic application, is a hot research target pursued by many active groups. We first succeeded in synthesizing a monodispersed Pt cube of 2.8 nm in size with high shape selectivity. Such simultaneous control of both size and shape of a nanocrystal has been successfully achieved by tuning of multiple conditions for the growth kinetics of Pt nuclei during hydrogen reduction of PtCl42 in aqueous N-,N-dimethylformamide (DMF) solution. The key strategy is to produce small Pt nuclei by the consumption of most precursor ions to avoid the excessive growth of Pt nuclei, in conjunction with promoting adsorption of anionic protective and shape-forming agents by the skillful control of the solvent system. We believe that the proposed facile synthetic approach is applicable to prepare various nanocrystals with both controlled size and shape.
T
he remarkable progress of nanotechnology over a decade has realized size and shape control of metal nanocrystals (NCs) to manipulate electronic,1 optical,2 and catalytic3 properties. Among them, special interest has been paid to Pt NCs because Pt is one of the most important elements of catalysts for various reactions, including production of chemicals such as unsaturated alcohols,3a environmental applications,3d and fuel cells.3e Recent reports demonstrate that the size and shape of Pt NCs significantly affect their catalytic activity and selectivity,3a c,e indicating potential to develop high performance Pt catalysts using reduced amounts of expensive Pt. However, it is still challenging to prepare small shape-controlled Pt NCs, such as cubes of less than 5 nm in size, which should be an important material for catalytic application. Preparation of a Pt cube was first reported by El-Sayed et al. in 1996.4 Since then, many research groups have been developing preparation methods of Pt cubes by the reduction of Pt precursor ions in solutions.5,6 To achieve high shape selectivity and monodispersity for a Pt cube, it is essential to control the reduction rate of Pt precursor ion, followed by selective growth of the (111) facet of Pt nuclei. The strategies adopted so far are categorized into three types: (1) controlling the reduction rate of the precursor ion by mixing different valence precursor ions,5a or by addition of NaNO3,5b (2) acceleration of the growth rate of (111) facets by the addition of Co2+ 5c or Ag+,5d and (3) deceleration of the growth rate of (100) facets by halogen5e,6 or carboxylate ion4,6 adsorption. Although these approaches have attained high shape selectivity, sizes of Pt cubes are still larger than 5 nm, and no report has yet appeared for the preparation of a smaller Pt cube with high shape selectivity. The reason is that the size of Pt nuclei with typically cuboctahedron shape already has r 2011 American Chemical Society
reached 2 3 nm, and there is difficulty in growing specific facets while avoiding size increase.5a,7 Thus, preparation of small Pt nuclei is essential to obtain a small Pt cube, in conjunction with controlled growth of the (111) facet of Pt nuclei. We have noticed that the reported preparation methods for small Pt cubes adopted low Pt precursor ion concentration of less than several millimolar to suppress growth of Pt nuclei. Furthermore, hydrogen reduction may be beneficial to realize the slow growth rate of Pt nuclei, due to low solubility of hydrogen gas in water. Although hydrogen reduction was already applied for the preparation of Pt cubes, precise size or shape control has not been established.8 We have reported the highly shape-selective preparation of a monodispersed Pt cube with controlled size in the range 7 10 nm by hydrogen reduction of PtCl42 in aqueous solution in the presence of I (shape forming agent by specific adsorption on the (100) face) and poly(acrylic acid) sodium salt (PAA) or sodium succinate (SA) (organic protective agent).6 It should be noted that our reported method adopted low precursor ion concentration and hydrogen reduction. Here, we demonstrate facile synthesis of a monodispersed Pt cube of less than 3 nm in size with high shape selectivity by tuning multiple conditions of our previously reported preparation method.6 We have paid special attention to suppress growth of small Pt nuclei by keeping high shape selectivity. In practice, we have tuned the timing of addition of NaI and the solubility of inorganic additives in solution by mixing N,N-dimethylformamide (DMF). Such tuning conditions have been decided by monitoring Received: July 21, 2011 Revised: August 12, 2011 Published: August 31, 2011 4292
dx.doi.org/10.1021/cg200937u | Cryst. Growth Des. 2011, 11, 4292–4295
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Figure 1. TEM images of Pt cube: (a) without timing control of addition of NaI, (b) with timing control, (c) prepared in DMF/H2O = 3:1 with timing control, and (d) with timing and pH control. Figure 3. di/(dE periods.
E) plots of reaction mixture for different reaction
Figure 4. Effects of timing control of addition of NaI on the size and size distribution of Pt cube. (a) Mixing of NaI with aqueous solution containing K2PtCl4 from the beginning induces ligand exchange from Cl to I , which forms PtCl(4 x)Ix2 . Formation of PtCl(4 x)Ix2 with a slow reduction rate leads to a decrease in the number of produced nuclei. The size of the Pt cube therefore increases. Furthermore, the presence of different Pt precursors with different reducing rates causes a wide size distribution of Pt nuclei, namely a Pt cube. (b) When NaI is added just before hydrogen introduction, reduction of most of the PtCl42 proceeds prior to the ligand exchange and a large number of nuclei is produced, which brings small nuclei size, resulting in formation of small and monodispersed Pt cubes.
Figure 2. (a) UV vis spectra of K2PtCl4 in aqueous solution (—) and reaction mixture (---). Peaks at 250 nm originate from PtCl42 . New peaks appeared at 278 and 390 nm in a reaction mixture originated from PtCl(4 x)Ix2 . (b) Absorbances originated from Pt precursor are plotted against the course of reaction time. The absorbance of each reaction condition was normalized by the initial absorbance. (9) NaI was initially added in the reaction mixture. (0) NaI was added just before hydrogen reduction, and the reaction was conducted under H2O. (b) Reaction was conducted in DMF/H2O = 3:1, and NaI was added just before hydrogen reduction.
the reaction mixture, e.g., with UV vis spectroscopy and voltammometric measurements. Compared with conventional Pt
nanoparticle with cuboctahedron, Pt cube tends to aggregate because of the flat facet.7 We anticipate that tuning the solution pH and addition of DMF have positive effects on improving the dispersibility of Pt cube due to enhanced adsorption of succinate anion (protective agent) on Pt cube. The typical procedure to prepare Pt cube before tuning of the reaction conditions is as follows: An aqueous solution containing K2PtCl4 (0.5 mM), sodium succinate (protective agent: 1 mM), and NaI (shape forming agent: 5 mM) was stirred while introducing Ar gas for 30 min, followed by introduction of H2 for 5 min, and further stirred under H2 atmosphere up to 15 h. The average size of the Pt cube prepared by the typical procedure was 5.5 nm, with high shape selectivity, but a few percent of large Pt cubes over 10 nm was obtained, as observed by the TEM image shown in Figure 1a and the size distribution shown in Figure S1a (see the Supporting Information). This result may suggest the presence of different precursors from PtCl42 . To prove this hypothesis, we have measured the UV vis spectrum of the reaction mixture during introduction 4293
dx.doi.org/10.1021/cg200937u |Cryst. Growth Des. 2011, 11, 4292–4295
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Table 1. Atomic Concentration of Pt Cube Measured by XPS atomic conc/% Pt 4f
I 3d
O 1s
N 1s
C 1s
binding energy/eV
71
619
531
398
285
Pt cube (with timing control of addition of NaI)
30.9
7.9
24.3
0.0
36.9
7.0
1.4
36.1
1.4
54.1
Pt cube (DMF/H2O = 3:1) (with timing control of addition of NaI)
of Ar for the initial 30 min. New peaks have been observed at 278 and 390 nm, presumably originated from Pt species different from PtCl42 , which shows peaks at 250 and 320 nm (Figure 2a).9 The most plausible species corresponding to the new peaks are PtCl(4 x)Ix2 , ligand exchanged species of Cl from PtCl42 to I from NaI added as shape forming agent, because the new peaks appear in the presence of PtCl42 and NaI, while the peaks do not appear in the presence of PtCl42 and sodium succinate (see Figure S2 of the Supporting Information). To get further evidence, we have carried out voltammometric measurements of the reaction mixture. Two peaks were observed at around 0.22 and 0.08 V vs Ag/AgCl by di/(dE E) plot, whose relative intensities changed at different stages of the reaction (Figure 3). The peak at 0.08 V was assigned to the reduction of PtCl42 to Pt(0), since a peak at the same potential was detected in K2PtCl4 aqueous solution. This peak almost disappeared by H2 reduction for 3 min, while the peak at 0.22 V became strong during Ar introduction and still remained during hydrogen reduction. The peak at 0.22 V corresponds to reduction of PtCl(4 x)Ix2 to Pt(0), since the standard reduction potential of PtI42 is reported to be negative (0.40 V) compared with that of PtCl42 (0.76 V).10 The ligand exchange of PtCl42 to PtCl(4 x)Ix2 was also confirmed by varying the amount of NaI. When 20 equiv of NaI to K2PtCl4 was added to the reaction solution, the size and size distribution of Pt cubes became larger and wider than those obtained under typical conditions with 10 equiv of NaI (see Figure S3 of the Supporting Information). The above results indicate that the formation of PtCl(4 x)Ix2 leads to a size increase of the Pt cube. As schematically shown in Figure 4, the formation of PtCl(4 x)Ix2 decelerates reduction of Pt precursors and the number of produced nuclei decreased. This may cause extensive growth of the nuclei accompanied by a rather large size distribution of produced Pt cubes. On the other hand, without ligand exchange, reduction of PtCl42 proceeds easily and produces a large number of nuclei, which brings small nucleus sizes, resulting in formation of small and monodispersed Pt cubes. Therefore, precise control of reaction conditions which prevent formation of PtCl(4 x)Ix2 is essential to synthesize small Pt cubes with narrow size distribution. Our strategy is to just tune the timing of addition of NaI in solution, that is, to add it just before hydrogen reduction, in place of addition at the beginning of Ar introduction. It is demonstrated that monodispersed Pt cube with average size of 3.5 nm was obtained by the modified procedures (Figures 1b and S1b of the Supporting Information). The reduction rate of PtCl(4 x)Ix2 was monitored by the change of relative adsorption intensity at 278 nm during the reaction under different conditions (Figure 2b). The estimated relative absorbances normalized before hydrogenation were 0.2 and 0.7 after 5 h of reaction, with and without timing control of addition of NaI, respectively. The result indicates the effective depletion of PtCl(4 x)Ix2 , if any, when NaI was added just before hydrogen
reduction. Therefore, the absence of PtCl(4 x)Ix2 leads to small Pt cube of 3.5 nm. Our second strategy to prepare smaller Pt cube by keeping monodispersity is to carry out the reaction in a less polar solvent system. A decrease in solvent polarity should cause a decrease in solubility of inorganic chemicals, such as PtCl42 and NaI. Thus, we can expect to retard formation of PtCl(4 x)Ix2 during the reaction. We previously reported that tuning solvent polarity by changing the mixing ratio of water/DMF/toluene was effective in preparing Pt nanowire of highly anisotropic shape.11 The values of the relative dielectric constants of DMF and water are 36 and 78, respectively. Thus, we have added DMF to aqueous solution (DMF/H2O = 3:1 volume ratio). As anticipated, the UV vis spectrum of the reaction solution confirms inhibition of ligand exchange in aqueous DMF solution, since the relative peak intensity at 278 nm originating from PtCl(4 x)Ix2 has disappeared already after 2 h of the reaction (Figure 2b). In addition, such a mixed solvent system with low polarity is expected to accelerate adsorption of succinate ion (protective agent) on Pt nuclei, due to low solubility of succinate ion, resulting in prevention of growth of Pt nuclei. The TEM image shown in Figures 1c and S1c of the Supporting Information demonstrates formation of monodispersed Pt cube with average size of 2.8 nm by reaction in aqueous DMF by tuning NaI addition timing just before H2 reduction. X-ray photoelectron spectroscopy (XPS) analyses were applied to investigate species adsorbed on Pt cube. The results indicate a decrease in the Pt 4f peak as well as increases in the C 1s, O 1s, and N 1s peaks for the Pt cube prepared in aqueous DMF compared with that prepared in water (Table 1 and see Figure S4 of the Supporting Information). Accordingly, the enhanced adsorption of succinate ion and DMF on the Pt surface in the aqueous DMF system is supported by the XPS measurements. In order to improve the dispersibility of Pt cube, the pH of the reaction mixture is controlled. By the hydrogen reduction of PtCl42 to Pt(0), HCl is formed, which brings the pH of the reaction system to a low value around 4.5 (see Figure S5 of the Supporting Information), to change succinate to succinic acid according to known pKa values (succinate/succinic acid is 4.0 5.3). We used NaOH to keep pH at around 8, as proved experimentally (see Figure S5 of the Supporting Information). The succinate anion present at high pH contributes a preferable protective agent, because the surface of the Pt cube is known to be positive.12 More importantly, the second COO group of succinate anion may play a role in electrostatic repulsion of Pt cube adsorbed by succinate. The TEM image of Pt cube prepared under high solution pH in Figure 1d shows a highly dispersed state of Pt cube. It should be noted that the Pt cubes obtained under such pH conditions have a larger size and size distribution (Figure 1d) compared with the others shown in Figures 1a c. Presumably, these results may be related to the presence of PtCl(4 x)Ix2-, since the relative absorbance at 278 nm, originated from PtCl(4 x)Ix2 , was 0.8 compared with the initial absorbance 4294
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Crystal Growth & Design even after 5 h of reaction. It should be noted that the dispersibility of Pt cube was also improved by the aqueous DMF system (see Figure 1c). Although the decisive mechanism for improving the dispersibility by DMF is not clear at present, the low dielectric constant of aqueous DMF may facilitate adsorption of succinate anion on Pt cube. In conclusion, we have tuned multiple conditions to easily synthesize small Pt cubes with high shape selectivity together with monodispersivness by hydrogen reduction of K2PtCl4 in aqueous solution containing disodium succinate and NaI. We can obtain Pt cubes of 3.5 nm by tuning the timing of addition of NaI just before hydrogen reduction. Such modification of reaction conditions prevents formation of PtCl(4 x)Ix2 with a slow reduction rate compared with that of PtCl42 . Use of aqueous DMF with a low relative dielectric constant as the reaction solution accelerates adsorption of succinate and NaI on Pt nuclei, resulting in formation of Pt cube of 2.8 nm. Thus, to realize simultaneous control of the shape and size of Pt cube, a key strategy is to produce small Pt nuclei by the consumption of most precursor ions to avoid the excessive growth of Pt nuclei, in conjunction with promoting adsorption of anionic protective and shape-forming agents by the skilful control of the solvent system. A demonstrated strategy to control multiple conditions will be applicable for various systems, to open the way to prepare a wide variety of NCs with simultaneously controlled sizes and shapes.
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bS
Supporting Information. Details of the synthesis of Pt cubes and additional characterization; size distribution of Pt cubes, TEM of a Pt cube prepared by an excess amount of NaI, UV vis spectra of K2PtCl4 with disodium succinate or NaI for clarifying the peak of PtCl(4 x)Ix2 , XPS spectra of a Pt cube, and plot of pH change of reaction mixtures during reduction. This material is available free of charge via the Internet at http://pubs. acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: +81-761-51-1540. Fax: +81-761-51-1149. E-mail: miyake@ jaist.ac.jp. Web address: http://www.jaist.ac.jp/profiles/info_e. php?profile_id=98.
’ ACKNOWLEDGMENT This research was supported by the program for Development of PEFC Technologies Aiming for Practical Application/Base Technology/Analysis of Morphology, Electrochemical Reaction and Mass Transfer for MEA Materials (No. 10000806-0) from NEDO, Japan. ’ REFERENCES (1) (a) Jeong, S.; Woo, K.; Kim, D.; Lim, S.; Kim, J. S.; Shin, H.; Xia, Y.; Moon, J. Adv. Funct. Mater. 2008, 18, 679–686. (b) Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Angew. Chem., Int. Ed. 2009, 48, 60–103. (2) (a) Eychmuller, A. J. Phys. Chem. B 2000, 104, 6514–6528. (b) Skrabalak, S. E.; Chen, J.; Au, L.; Lu, X.; Li, X.; Xia, Y. Adv. Mater. 2007, 19, 3177–3184. (3) (a) Grass, M. E.; Somorjai, G. A. Catal. Lett. 2009, 128, 1–8. (b) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 12663–12676. (c) Bratlie, K. M.; Lee, H.; Komvopoulos, K.; Yang, P. D.; Somorjai, G. A. Nano Lett. 2007, 7, 3097–3101. (d) Schulz, J.; 4295
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