Aggregate structure and crystallite size of platinum nanoparticles synthesized by ethanol reduction
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- Wang, S. & Tseng, W.J. J Nanopart Res (2009) 11: 947. doi:10.1007/s11051-008-9482-0
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Monodispersed platinum (Pt) nanoparticles were synthesized from reducing hydrated hydrogen hexachloroplatinic acid (H2PtCl6·nH2O) with ethanol in the presence of polyvinylpyrrolidone (PVP) as a steric stabilizer. Concentration of both PVP and ethanol influenced the aggregate structure and crystallite size of the nanoparticles. When the molar ratio of monomeric unit of PVP to Pt, i.e., [PVP]/[Pt], was one, the synthesized Pt particles coagulated pronouncedly into an inter-connected particulate network or self-organized into spherical superstructures with an apparent diameter ranging from 60 to 80 nm, depending on the ethanol concentration. The geometry and structure of these complex aggregates were characterized by fractal analysis. Fractal dimensions of 2.13–2.23 in three dimensions were determined from the Richardson’s plot, which suggests that a reaction-limited cluster–cluster aggregation model (RCLA) was operative. The Pt colloids became apparently more stable when the [PVP]/[Pt] ratio was increased greater than 20. Crystallite size of the Pt nanoparticles was found to increase linearly with the ethanol concentration as the [PVP]/[Pt] was held at one. This suggests that the reduction rate of PtCl62− ions in solution is critically important to the synthesized crystallite size.
Nanoscale metal particles present novel physical and chemical properties that are substantially different from their bulk counterpart, and are of fundamental interest to both homogeneous and heterogeneous catalysis applications (Paulus et al. 2000; Liu et al. 2004; Kim et al. 2004). Monodispersed platinum colloids with tailored particle shape, size, and particle-size distribution have been prepared by different methods such as sol process, micelle, sol gel, chemical precipitation, hydrothermal synthesis, pyrolysis, electrodeposition, and vapor deposition (Tian et al. 2007; Ahmadi et al. 1996; Narayanan and El-Sayed 2004; Burda et al. 2005). The morphology and structure of the synthesized platinum nanoparticles appear to play an important role in their catalytic activity (Tian et al. 2007), owning mainly to the various number densities of coordinatively unsaturated atoms situated at the corners and edges of the particle surface that are potential binding sites for catalysis (Feldheim 2007).
The synthesis of particulate noble metals by aqueous alcohol reduction of metal salts in the presence of polymeric stabilizer (or protector), in particular, has been reported to be an enabling technique toward a better control of the synthesized particle morphology and the aggregated structure. Hirai and co-workers (Hirai et al. 1978, 1979; Hirai 1979) were the first to examine the catalytic activity and the formation mechanism of metal particles protected by polyvinyl alcohol or polyvinylpyrrolidone (PVP) in methanol. Duff et al. (1995) further confirmed that the aggregation of platinum particles was suppressed by a high [PVP]/[Pt] ratio which in turn facilitated the dispersion of platinum sols so that the particles with a more uniform morphology become attainable. Chen and Akashi (1997) synthesized colloidal platinum nanoparticles that were protected by poly (N-isopropylacrylamide) in ethanol/water mixtures by the reduction of PtCl62−. They reported that the protective polymer serves not only as a stabilizer, but also as a functional component conferring catalytic activity and selectivity. Teranishi et al. (1999) further revealed that the mean diameter of monodispersed Pt nanoparticles can be controlled from 1.9 to 3.3 nm by adjusting the kind of alcohol and the PVP concentration used in the sol process. The size of Pt particles with an ascending order, i.e., 1-propanol < ethanol < methanol, was found when various alcohols were used. This suggests that the reduction rate of PtCl62− ions in solution is critically important to the synthesized Pt particles. In addition, the synthesized particle size was found to decrease linearly with the alcohol concentration over the [PVP]/[Pt] ratio range from 10 to 40.
Even though the size dependence of alcohol-reduced platinum sols at relatively high PVP concentrations have been examined extensively in the literature, characterizations of the aggregate structure and the crystallite-size dependence of the platinum nanoparticles over a broad range of alcohol fractions with a particular emphasis at low [PVP]/[Pt] ratios are yet rare. In this regard, Shiraishi et al. (2000) have reported the use of transmission electron microscopy (TEM) and the Taylor dispersion method to examine the PVP-protected platinum nanoclusters. They found that the Pt particles tend to form a hierarchical superstructure of spherical shape in ethanol–water mixtures at low PVP concentrations. Size of the superstructure decreased with the increasing PVP concentration, and the reduced superstructure size gave rise to an enhanced catalytic activity, in parallel with those reported by others (Teranishi et al. 1999; Hirai et al. 1979). Nonetheless, the effect of ethanol concentration to the aggregated structure of Pt particles was never mentioned. In this article, platinum nanoparticles were synthesized from hydrated hydrogen hexachloroplatinic acid in ethanol with a broad range of ethanol–water fractions (0.2–0.8) and [PVP]/[Pt] molar ratios (1–60). Structure and morphology of the coagulated nanoparticle clusters were examined by TEM, and their fractal dimensions determined. Crystallite size of the synthesized Pt particles was then compared with that reported by Teranishi et al. (1999).
In 150 mL ethanol–water mixtures 0.05 g of H2PtCl6·nH2O (99.9%, Aldrich, USA) was dissolved. The volume fraction of ethanol in the solvent mixtures varied from 0.2 to 0.8. Reagent-grade PVP (molecular weight 10,000, Sigma-Aldrich, USA) was added in the ethanol–water mixtures to form solutions with [PVP]/[Pt] molar ratio of 1, 20, 40, and 60, respectively. The solutions were stirred vigorously and placed in a water bath at a constant temperature of 95 °C for an isothermal holding of 1 h with continuous agitation. The solution mixtures generally changed their color from yellowish to dark brown, indicative of reaching a complete reduction for the PtCl62− ions to Pt(0) crystallites (Duff et al. 1995; Chen and Akashi 1997; Teranishi et al. 1999). Some of the solutions were carefully siphoned out by a pipette, and were dropped onto carbon-coated copper grids for TEM examination (JEM-2010, JEOL, Japan). The digitized TEM micrographs were then treated by an image analyzer program (Image-pro Plus 6.0). The rest of the solutions were centrifuged at 10,000 rpm and the settled powders were rinsed and washed in deionized water ultrasonically. This rinse-and-washing operation was repeated for at least three times to ensure a complete removal of the weakly adsorbed or non-adsorbing PVP molecules on the nanoparticles surface. Phase structure of the dried powders was determined by x-ray diffractometry (XRD, MAC MXTIII, Japan) using CuKα radiation.
The self-similar structure and geometry of the synthesized Pt aggregates can hence be described quantitatively in terms of the experimentally determined fractal dimension. Nonetheless, the D′ value determined from the measurement of perimeter–area relationship of TEM micrographs is indeed not a truly scale-invariant property, and a lower D′ is often resulted when a higher magnification is used (Sánchez-López et al. 2000). Therefore, a fixed magnification of 100,000 × (corresponding to area of 180 nm × 180 nm) was used throughout this study for all the samples examined to avoid such a problem for direct comparison purposes. Several micrographs were measured to give a mean fractal dimension for each sample.
Results and discussion
Aggregate structure of the synthesized Pt nanoparticles
An increasing ethanol fraction in the solvent mixture favors the formation of Pt(0) nuclei. The increased number of nucleus would then lead to an enhanced probability for the successive collision events to occur during their growth process. Formation of complex aggregate structures is resulted unless a sufficient amount of PVP polymers is present to “protect” the synthesized Pt solids from the aggregation. For the case that PVP concentration is insufficient to give a complete protection at the low [PVP]/[Pt] ratio, the formation of aggregated Pt structure becomes inevitable and is in theory favored to form as the ethanol fraction is increased.
Fractal analysis of the aggregate structure
Colloidal Pt nanoparticles were synthesized in an ethanol–water solvent mixtures in the presence of polyvinylpyrrolidone (PVP) molecules by the reduction of PtCl62−. The aggregated structure and crystallite size of the crystalline Pt particles were examined over broad [PVP]/[Pt] ratios and ethanol fractions. The particles coalesce to form disordered aggregates when PVP concentration is insufficient to provide a complete coverage on the particle surface. The aggregated structure can be of spherical superstructure or an interconnected particulate network, depending on the ethanol fraction. Fractal analyses show that the aggregated structure at [PVP]/[Pt] = 1 is belonged to the reaction-limited cluster–cluster aggregation (RCLA) over the ethanol fractions examined. The fractal dimension Df values are found to increase slightly with the increasing ethanol concentration. This slight increase enables particles to rearrange themselves into a more compact structure, so that the spherical superstructure collapses into a continuous particulate network. In addition, the presence of PVP molecules not only alters the aggregated structure, but also affects the crystallite size of the synthesized Pt. The Pt crystallite size varies with the ethanol fraction depending on the [PVP]/[Pt] ratio. This can be explained by the relative reduction rate of PtCl62− to form Pt(0) crystallite when the ethanol concentration was altered and the coverage of PVP molecules on the crystallites for avoiding the particle coalescence.
This work was financially supported by the National Science Council through contract no. 95-2218-E-005-005.