Colloidal dispersions of Ag-Pt composite nanoparticles were prepared by gamma radiolysis technique in the presence of nonionic surfactant Brij'97. Simultaneous as well as sequential reduction methods were employed in order to study the structural formation of Ag-Pt bimetallic clusters. Similar shape and trend was observed in optical spectra for both methods. Radiolysis yielded nearly spherical Ag-Pt bimetallic clusters by use of AgNO3 instead of AgClO4. The disappearance of the silver resonance and the simultaneous growth of the 260 nm resonance are independent of cluster structure and degree of alloying. To understand formation of Ag-Pt aggregate, the optical studies were also done as a function of amount of dose absorbed, concentration of surfactant, that is, Brij'97. The shape of the absorption spectrum did not change with increase in gamma radiation dose. TEM analysis exhibited fine dispersions of Ag-Pt clusters surrounded by a mantle when capped with Brij'97. The particle size obtained was in the range of 5–9 nm. On the basis of optical, XRD, and TEM analysis, alloy formation is discussed.
Since 1973 [
While reviewing the work done for the past decade by Mulvaney’s and Henglein’s group [
In Au-Ag system, it has been shown how segregation or alloying of metals can be controlled by the reduction rate as a function of dose absorbed [
The aim of the present work is to check and confirm the formation of intermetallic character of Ag-Pt by systematically studying the evolution of optical spectra with increasing dose using surfactant Brij’97 by simultaneous and sequential reduction methods. Brij’97 is nonionic surfactant generally called poly (10) oxyethylene oleyl ether (POE). In general, increasing alkyl group length will decrease water solubility; increasing the length of POE will increase water solubility. Due to their amphiphilic character, detergent molecules aggregate in solution to form micelles. They can also align at aqueous/nonaqueous interfaces, reducing surface tension, increasing miscibility, and stabilizing emulsions. The surfactants are known to influence the coalescence and also the kinetics of bimetallic cluster growth alloyed or segregated. To understand the formation of alloy nanoparticles, several experiments were carried out by varying total dose absorbed, concentration of surfactant, concentration of precursor salts, and type of counter ion added. The alloy formation and morphology of these bimetallic particles are investigated by UV-visible spectroscopy, X-ray diffractometry (XRD), and transmission electron microscopy (TEM) analyses.
All reagents were pure grade chemicals. AgNO3, AgClO4, and H2PtCl6 were from Fluka, Switzerland, propan-2-ol was from Qualigens, India, and the commercial nonionic surfactant used was Brij’97 (poly-(10)-oxyethylene oleyl ether) purchased from Altas, USA.
The silver-platinum nanosize colloid was prepared by
The radiolytic reduction mechanism is well known [
Second carbon atom is attacked and a secondary radical CH3
Complete reduction of Ag ions was monitored by optical spectra on a Hitachi 220A spectrophotometer, which gave no change in absorption spectra at 410 nm after delivering a total dose up to 13.8 kGy. The optical absorption spectra were also taken for various amounts of dose absorbed until no more changes occurred. Individual component spectra were taken, and it was checked that Brij’97 is not reducing the precursor AgNO3 and H2PtCl6 salts before irradiation.
In both methods of reduction employed for Ag-Pt system, the complete disappearance of the Ag plasmon peak was observed from the very beginning. After irradiation, the solutions remained colloidal and stable for a couple of days.
TEM analyses for these bimetallic particles were carried out with Philips (model CM200) operated at 200 kV. Specimens for TEM were prepared by drying under nitrogen atmosphere droplets of colloidal dispersion on a carbon-supported formvar-coated copper grid. Sizes of more than 200 particles were measured on micrographs to obtain size histograms. Selected area electron diffraction (SAED) was employed for structural characterization. The electron diffraction pattern shows fcc structure from polycrystalline sample.
The X-ray diffraction patterns were taken on a Philips powder X-Ray Diffractometry (PW 1840). These patterns were compared with ASTM data (ASTM card no. 783 & 802) of bulk Ag and Pt. The size of the nanoclusters was approximately estimated from Scherer formula.
Colloidal dispersions of metals exhibit absorption bands or broad regions of absorption in UV-visible region. These are due to the excitation of plasmon resonances or interband transitions and are a characteristic property of the metallic nature of the particles. The optical properties of silver were studied using PVA [
The absorption band of silver particles is strongly influenced by chemisorbed molecules. The surface plasmon spectrum of metal nanoclusters is highly dependent on the surrounding environment (i.e., metal ion concentration, amount of polymer or surfactant, pH, etc.) and also on the amount of dose absorbed in case of radiolytic preparation.
Figure
Absorption spectra of Ag-Pt particles prepared by both sequential (b–f) and simultaneous methods. (a) depicts Ag plasmon peak obtained after irradiation for a dose of 13.8 kGy. Dotted line depicts the simultaneous reduction of AgNO3 and H2PtCl6 with Brij’97 for a total dose of 7.9 kGy. AgNO3 = [1 × 10−3 M], Brij’97 = [1 × 10−2 M], H2PtCl6 = [1 × 10−3 M] in an inert atmosphere; optical path: 1 cm.
Henglein estimated reduction potentials of Ag+ to Ag° in acidic solution is
The reduction potential being lower, silver gets reduced preferentially and it transfers electrons to Pt and thereby reducing Pt stepwise. Irrespective of the procedure, Ag plasmon is disappearing which can be explained on the basis of reduction potentials of silver and platinum.
In case of simultaneous reduction method as indicated by the dotted line in Figure
The spectra exhibit the same shape at all doses confirming alloy formation which is in agreement with Remita’s group.
In case of bimetallic system, the evolution of the optical absorption spectra with the dose of irradiation is very informative. In order to check the formation of intermetallic character of Ag-Pt, optical spectra were studied with increasing amount of dose.
Experiments were performed to check the interaction stepwise between irradiated silver colloid and precursor Pt4+ ions in the form of H2PtCl6.
Figure
(a) Absorption spectra of Ag nanoparticles initially synthesized by gamma-radiolysis at a dose 2.0 kGy using Brij’97. AgNO3 = [1 × 10−3 M], Brij’97 = [1 × 10−2 M], 2-propanol = [2 × 10−1 M], and H2PtCl6 = [1 × 10−3 M]. Absorption spectra recorded immediately after addition of Pt4+ ions. (b) Pt4+ ions in presence of 2-propanol were added to irradiated silver colloid shown by the dotted line. (c) Addition of AgNO3 and 2-propanol to irradiated Pt colloid shown by the dotted line in an inert atmosphere. Optical path: 1 cm.
Figures
(a) TEM image and electron diffraction spot pattern obtained from Brij’97 capped Ag nanoparticles by gamma radiolysis. AgNO3 = [1 × 10−3 M], Brij’97 = [1 × 10−2 M], and 2-propanol = [2 × 10−1 M] irradiated with a dose of 1.9 kGy in an inert atmosphere. Corresponding size histogram inset of Ag particles. The solid line is a Gaussian fit to the PSD data. (b) TEM image and electron diffraction spot pattern obtained from Brij’97 capped Pt nanoparticles by gamma radiolysis. H2PtCl6 = [1 × 10−3 M], Brij’97 = [1 × 10−2 M], and 2-propanol = [2 × 10−1 M] in an inert atmosphere with a dose of 1.9 kGy corresponding size histogram inset for Pt particles. The solid line is a Gaussian fit to the PSD data. (c) TEM images and electron diffraction spot pattern obtained from Brij’97 capped of Ag-Pt particles by simultaneous reduction method. AgNO3 = [1 × 10−3 M], H2PtCl6 = [1 × 10−3 M], Brij’97 = [1 × 10−2 M], and 2-propanol = [2 × 10−1 M] irradiated with a dose of 7.9 kGy in an inert atmosphere, corresponding size histogram inset for Ag-Pt bimetallic nanoparticles. The solid line is a Gaussian fit to the PSD data.
Under the given synthesis conditions, the average size of pure Ag clusters and pure Pt clusters is 14.9, 11.5 nm whereas that of Ag-Pt composite clusters is 12.5 nm, smaller than silver particles. The pure Ag and Pt particles are spherical in shape and Ag-Pt composite particles are slightly elongated in shape. The experimental results also support the theoretical calculations for ellipsoids in the quasistatic regime [
It should be noticed that in this concentration range, the size distribution is unimodal. If alloy particles are not formed, it would lead to a broad or bimodal size distribution due to different growth rates for the two metal colloids, which may be in turn correlated to surface energy [
Figures
X-ray diffractograms of Ag-Pt bimetallic particles by sequential reduction method for different amounts of Brij’97 (a) [1 × 10−3 M] (b) [2 × 10−2 M] (c) [5 × 10−2 M] concentration of AgNO3 and H2PtCl6 being [1 × 10−3 M].
X-ray diffractograms of Ag-Pt bimetallic particles by simultaneous reduction method for different amounts of Brij’97 (a) [1 × 10−3 M] (b) [2 × 10−2 M] concentration of AgNO3 and H2PtCl6 being [1 × 10−3 M].
The
Table
TEM and XRD characterization of bimetallic Ag/Pt clusters synthesized at high-dose rate at various Brij’97 concentrations with sequential reduction method (Dose: 16.0 kGy).
Sample | [M] | [Brij’97] | Cluster size | Cluster size |
(M) | (M) | by TEM (nm) | by XRD (nm) | |
AgPt-1 | 10−3 | 10−2 | 9 | 12-13 |
AgPt-2 | 10−3 | 2 × 10−2 | 7-8 | 8-9 |
AgPt-3 | 10−3 | 5 × 10−3 | 5-6 | 8-9 |
Ag-Pt-4 | 10−3 | 5 × 10−3 | — | 9-10 |
Table
TEM and XRD characterization of bimetallic Ag/Pt clusters synthesized at high-dose rate at various Brij’97 concentrations with simultaneous reduction method (Dose: 8.0 kGy).
Sample | [M] | [Brij’97] | Cluster size | Cluster size |
(M) | (M) | by TEM (nm) | by XRD (nm) | |
AgPt-5 | 10−3 | 10−2 | 7-8 | 12-13 |
AgPt-6 | 10−3 | 2 × 10−2 | 6-7 | 10-11 |
AgPt-7 | 10−3 | 1 × 10−3 | 7-8 | 10-11 |
It was observed that not only surfactant concentration and type of surfactant that affect the formation and distribution of particles, but the type of irradiation also changes the morphology [
Colloidal particles are subjected to a number of attractive and repulsive forces and the stability of dispersion depends on the interplay of these various forces. These forces and hence the stability of dispersions can be altered or controlled by the adsorption of ions, surfactants, or polymers at the solid-liquid interface. Adsorption of surfactants and polymers depends on the nature of surfactants and polymers. Nonionic surfactants adsorb primarily through hydration or hydrogen bond interactions [
Our present results on optical, XRD, and TEM indicate bimetallic aggregates with alloy formation to some extent. Initially, silver must be getting reduced due to its higher reduction potential and reducing platinum. Tetravalent Pt ions also get reduced into divalent Pt ions by hydrated electron and isopropyl radical. Then the divalent platinum ions disproportionate and are also reduced. Depending on the dose absorbed, reduced atoms, clusters, and excess ions may get associated with forming mixed bimetallic clusters. Further coalescence of the primary bimetallic complex species keeps the metal alloyed.
Gamma radiolysis is a powerful method to synthesize mono- and bimetallic nanoparticles. It gives homodispersed, ultrafine particles without disturbing chemical impurities. By changing the amount of dose absorbed, the particle size can be controlled. Radiolysis yielded nearly spherical Ag-Pt bimetallic clusters by use of AgNO3. Ag-Pt alloyed particles and bimetallic aggregates were prepared by gamma radiolysis using Brij’97. Simultaneous and sequential reduction methods were followed. In both methods, the disappearance of Ag plasmon peak was observed when Pt is added to Ag nanoparticles, independent of cluster structure and degree of alloying from the very beginning. TEM analysis exhibited dispersion of fine particles surrounded by a mantle when capped with Brij’97.
The authors gratefully acknowledge valuable suggestions and discussion with Professor J. Belloni, financial support from the Department of Atomic Energy, Government of India, and partial financial support from Unilever Industries (Pvt.) Ltd. M. K. Temgire and Dr. S. S. Joshi Sophisticated Analytical Instruments Facility (RSIC), IIT-Bombay for the TEM facility.