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Single Nanocrystals of Platinum Prepared by Partial Dissolution of Au-Pt Nanoalloys

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Science  30 Jan 2009:
Vol. 323, Issue 5914, pp. 617-620
DOI: 10.1126/science.1166703

Abstract

Small metal nanoparticles that are also highly crystalline have the potential for showing enhanced catalytic activity. We describe the preparation of single nanocrystals of platinum that are 2 to 3 nanometers in diameter. These particles were generated and immobilized on spherical polyelectrolyte brushes consisting of a polystyrene core (diameter of ∼100 nanometers) onto which long chains of a cationic polyelectrolyte were affixed. In a first step, a nanoalloy of gold and platinum (a solid solution) was generated within the layer of cationic polyelectrolyte chains. In a second step, the gold was slowly and selectively dissolved by cyanide ions in the presence of oxygen. Cryogenic transmission electron microscopy, wide-angle x-ray scattering, and high-resolution transmission electron microscopy showed that the resulting platinum nanoparticles are faceted single crystals that remain embedded in the polyelectrolyte-chain layer. The composite systems of the core particles and the platinum single nanocrystals exhibit an excellent colloidal stability, as well as high catalytic activity in hydrogenation reactions in the aqueous phase.

Metallic nanoparticles (NPs) of controlled size and shape have been of great interest recently for a number of possible applications in electronic or optical materials, as well as in catalysis (18). Particle morphology can play a central role in catalysis. For example, faceted Pt crystals can exhibit higher catalytic activity than spherical particles (9, 10), and the activity of the exposed facets may vary considerably (11, 12). The reactivity and selectivity of NPs can be tuned by controlling their morphology; for example, amorphous Pt NPs have exhibited a much reduced catalytic activity (13).

However, faceted nanocrystals (NCs) with a well-developed shape and a narrow size distribution that have been reported generally were in the size range of 100 nm and more. For instances, Sun and Xia obtained well-defined Au and Ag crystals with sizes on the order of 100 nm (4). Nanoprisms of Ag with dimensions around 100 nm were prepared by Jin et al. by photochemical conversion of Ag spheres (3). Anisotropic Ag NPs of similar size were synthesized by Liz-Marzan and co-workers through careful choice of a suitable surfactant (14), and Pt NPs with high-index facets were obtained recently by Tian and co-workers (6). Again, the typical sizes ranged between 50 and 200 nm. The only reported route to faceted single crystals in the size range of a few nanometers was the synthesis of well-defined clusters, for example, Au55 cluster, and a subsequent heat treatment (15).

Platinum NPs have been of particular interest because of their role in many catalytic reactions and because the growth of specific surfaces can be controlled by amphiphilic polymers or suitable surfactants (9, 1619). However, downsizing the Pt NPs to a few nanometers is often accompanied by a broadening of their size distribution and partial loss of the control of the particle shape (5). The reduction of the metal ions and the generation of the NPs seem to proceed very rapidly and leads to disordered structures and distorted crystal shapes.

We present a simple synthesis of faceted, well-defined Pt single NCs with a typical size of 2 to 3 nm. As shown in Fig. 1, the Pt NCs are obtained by partial dissolution of nanoalloys of Pt and Au. In case of larger nanoalloy particles, this procedure leads to spongelike or hollow structures (20, 21). We found that the dissolution of the Au component of the nanoalloy leads to reorganization of the Pt atoms and the formation of well-defined, faceted NCs. We started from a nanoalloy of Au and Pt generated on the surface of a spherical polyelectrolyte brush (SPB), as described recently (22). These particles consist of a solid polystyrene (PS) core with colloidal dimensions (diameter ∼ 100 nm) onto which long charged polymer chains are grafted. Spherical polyelectrolyte brushes are well suited for the generation and immobilization of metallic NPs in the aqueous phase (23, 24) because anionic complex ions of metals, like Au, Ag, Pd, or Pt, can be immobilized in the surface layer of cationic SPB. The confinement of these ions was attributed to the strong electrostatic interaction of the counterion with the highly charged macro-ion. The complex metal ions like PtCl2–6 are thus localized solely in the layer of polyelectrolyte chains of the SPB, so that virtually no metal salt is released into the aqueous phase. Subsequent reduction with NaBH4 under mild conditions led to the formation of metallic NPs, firmly embedded in a dense layer of polyelectrolyte chains.

Fig. 1.

Synthesis scheme of Pt NCs by de-alloying of a Au-Pt nanoalloy. The carrier particles are SPBs that consist of a solid PS core (RH = 50 nm) onto which cationic polyelectrolyte chains of 2-aminoethylmethacrylate (2-AEMH) are attached. In a first step, the chloride counterions were exchanged against AuCl4 ions; in a second step the rest of Cl ions were exchanged against PtCl42– ions. Bimetallic Au45Pt55 nanoalloy particles were generated by reduction of the mixture of these ions by NaBH4 (22). The composition of the resulting nanoalloy can be adjusted by the ratio of the metal ions in the brush layer. In the final step, cyanide ions and oxygen were used to leach out the Au atoms from the nanoalloy under very mild conditions. This procedure leads to faceted Pt NCs with a few nanometers in diameter embedded in the surface layer of polyelectrolyte chains.

In the same way, particles of nanoalloys (25) can be synthesized and immobilized on the surface layer of SPB particles. We demonstrated this approach by reducing a mixture of the AuCl4 and PtCl2–6 ions in the surface layer of a cationic SPB (22). High-resolution transmission electron microscopy (HR-TEM) has shown that the nanoalloy particles do not exhibit a core-shell structure [compare with the discussion of this point in (25)] but form homogeneous solid solutions (22). The composition of the metals can be varied continuously without disturbing the structure of the particles that consist of a random mixture of Au and Pt (22). Moreover, the final composition is within narrow limits, determined by the ratio of the metal ions introduced into the brush layer earlier (22). The composite particles that consist of the carriers and the metallic NPs exhibit excellent colloidal stability in aqueous solution, which arises from the strong interaction of the alloy NPs with the polyelectrolyte chains affixed to their surface. As depicted in Fig. 1, the cationic polymer chains are interwoven with the NPs and form a dense mesh on the surface of the core particles. This model is supported by careful measurements of the hydrodynamic radius, RH, by dynamic light scattering [see the discussion of this point in (26)]. The thickness, L, of the surface layer is given by L = RHR, where R denotes the radius of the cores. As will be shown below, the colloidal stability of the composites allows us to transform the NPs while keeping them on the surface of the core particles.

We prepared composite particles containing Au-Pt NPs with the composition Au45Pt55, as shown by elemental analysis. Partial dissolution of these alloy NPs then leads to pure Pt NCs. The Au atoms were slowly dissolved by a treatment of the alloy NPs with cyanide ions in presence of oxygen (Fig. 1). In a typical experiment, 4 ml of a 1.9 × 10–5 M NaCN solution was added dropwise within 25 min to 13 ml of a AuPt-SPB suspension [0.04 weight % (wt. %)] at room temperature under air with vigorous stirring. The high dilution of the cyanide ions is crucial for avoiding coagulation or complete dissolution of the NPs. Air was bubbled slowly through the solution to achieve complete removal of the Au atoms. After 3 hours, the dispersion turned blue, indicating the formation of pure Pt NCs (26).

The de-alloying of the Au-Pt-nanoalloy proceeded surprisingly smoothly. Micrographs of the composite particles before and after the leaching process (Fig. 2) were obtained by cryogenic transmission electron microscopy (cryo-TEM) that allow us to analyze the NPs in their native state (27). Thus, a small drop of the suspension of the composite particles is shock-frozen and analyzed by TEM (see supporting online material for details). The Pt NCs can be seen in Fig. 2B as black dots distributed over the SPB. Both the alloy NPs as well as the Pt NCs are distributed more or less uniformly over the carrier particles. The alloy NPs have a rather narrow size distribution that is preserved during the leaching process (fig. S3 histogram).

Fig. 2.

(A) Cryo-TEM micrographs of the Au-Pt-nanoalloy particles (composition: Au45Pt55) generated on the surface of the spherical polyelectrolyte brushes. (B) Composite particles after complete removal of the gold atoms from the Au-Pt-nanoalloy by a mixture of CN ions and O2.

The formation of the cyanide complex is selective for Au (28). Energy-dispersive x-ray spectroscopy (EDX) (figs. S1 and S2) shows that the NPs resulting from de-alloying consist only of Pt. The Pt NPs are still embedded on the surface of the SPB-carrier particles. The colloidal stability of the composite particles was not lost during the reaction with the cyanide ions, nor is any coagulation of the NPs on the surface of the carrier particles observed during this process. We reiterate that rapid addition or high concentrations of the cyanide ions lead to coagulation and coarsening of the metal NPs on the surface.

The structure of the Pt NPs was analyzed by combining high-angle annular dark-field scanning TEM (HAADF-STEM) and HR-TEM with electron diffraction (ED) and wide-angle x-ray scattering (WAXS). A low-magnification HAADF-STEM micrograph of the PS spheres on the supporting holey carbon film is shown in Fig. 3A, and Fig. 3B (from the same area at higher magnification) shows the uniform distribution of the Pt NPs on the PS spheres. In order to avoid any disturbance of this analysis by the core particles, only NPs sitting on the periphery of the carrier spheres were analyzed by HR-TEM (Fig. 3, C to F). The HR-TEM shows that the Pt NPs contain no grain boundaries and are single crystals. In several cases, the facets can be indexed because the NCs are aligned by chance; in Fig. 3, E and F, the electron diffraction shows directly the hexagonal symmetry of the cubic crystal.

Fig. 3.

(A and B) HAADF-STEM micrographs of the Pt NPs (bright spots) embedded and uniformly dispersed on a surface layer of the spherical polyelectrolyte. (C) HR-TEM micrograph of NPs on the surface of two adjacent carrier particles. (D) HR-TEM micrograph of several NCs. (E and F) HR-TEM micrographs of two different Pt single NCs of sizes 4.6 and 2.8 nm, respectively, showing well-defined facets. All micrographs were acquired at 300 keV. (Insets) The Fourier transforms of the images.

The lattice spacing derived from the (200) reflection as seen in ED (d200 = 0.20 nm) is in good agreement with the bulk value [d200 = 0.196 nm; spacing 0.392 nm (29, 30)]. The overall size of the Pt NCs lies within the range estimated from the volume contraction caused by Au leaching. Our results are consistent with the Au-Pt nanoparticles converted individually into Pt NCs, without coagulation or exchange between different alloy NPs having taken place. Some of the NPs moved under the intense electron beam and rolled on the surface of the PS spheres, but their shape remained stable (no melting was observed).

The high crystallinity observed by HR-TEM might have been induced by the electron beam through local heating and subsequent crystallization. To rule out this possibility, we examined powders of the dried composite particles before and after leaching by WAXS (fig. S4). The average crystallite sizes as derived from the Rietveld analysis were 7.01 ± 0.003 [7.01(3)] nm for the Au45Pt55-NPs and 3.80(4) nm for the resulting Pt NCs. These values agree quite well with crystal sizes as observed by HR-TEM showing that the particles are single-domain NCs. Also, the lattice parameter of 0.401 (± 0.001) nm obtained for the alloy NPs agrees very well with the value expected from Vegard's law, that is, a linear interpolation of the lattice parameters of Au and Pt [see the discussion of this point in (22)], if a solid solution had formed. After the Au leaching step, the lattice parameter decreased to 0.391 (± 0.001) nm, in agreement with the known lattice parameter of pure Pt [0.392 nm (29)]. Thus, full dealloying of the Au-Pt-nanoalloy could be achieved. We note that a partial dealloying by an electrochemical method has been recently reported by Koh and Strasser (31).

As a final point, we discuss the catalytic activity of the Pt NCs affixed to the surface of the SPBs. Previous work (23, 32, 33) showed that the catalytic reduction of p-nitrophenol to p-aminophenol can be used for the analysis of the catalytic activity of Pt NPs. We assumed that reduction rates were independent of the concentration of sodium borohydride because it was in excess compared to p-nitrophenol. Moreover, the apparent rate constant, kapp, was found to be proportional to the surface S of the Pt NPs present in the system (32, 33): Math(1) where ct is the concentration of p-nitrophenol at time t and k1 is the rate constant normalized to S, the surface area normalized to the unit volume of the system. k1 is plotted against the specific surface S of the Pt NCs in the systems in Fig. 4 (32). The Pt NCs exhibit a high catalytic activity and turnover numbers as high as 1580 ± 50, which are among the highest turnover numbers measured so far for this reaction (23, 32, 33). Hence, the composite particles consisting of the SPBs and the Pt NCs present a system with high colloidal stability that may be used for catalysis in an aqueous environment.

Fig. 4.

Catalytic activity of the Pt NCs. k1 (Eq. 1), obtained for the catalytic reduction of p-nitrophenol to p-aminophenol is plotted against the specific surface, S, of the Pt NCs in the solution. S is the total surface of all particles per unit volume.

Supporting Online Material

www.sciencemag.org/cgi/content/full/323/5914/617/DC1

Materials and Methods

Figs. S1 to S4

Table S1

References

References and Notes

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