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Aniline as a stabilizer for metal nanoparticles Article in Materials Letters · August 2003 DOI: 10.1016/S0167-577X(03)00235-0
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Materials Letters 57 (2003) 3889 – 3894 www.elsevier.com/locate/matlet
Aniline as a stabilizer for metal nanoparticles Anjali A. Athawale a,*, S.V. Bhagwat a, Prachi P. Katre a, Asha J. Chandwadkar b, P. Karandikar b a
Department of Chemistry, University of Pune, Ganeshkhind Road, Pune 411 007, India National Chemical Laboratory, Dr. Homi Bhabha Marg, Pashan, Pune 411 008, India
b
Received 28 October 2002; received in revised form 27 February 2003; accepted 11 March 2003
Abstract Palladium nanoparticles were synthesized by two different methods, i.e. reflux and g-radiolysis in the presence of various monomers like aniline, N-ethyl aniline, N-methyl aniline, o-anisidine and o-toluidine as the stabilizing agent for the Pd nanoparticles. UV – Visible spectral analysis reveals that the aniline renders best stability to the Pd nanoparticles up to a period of 96 h. Nanocomposites were synthesized by polymerizing aniline stabilized Pdj nanoparticle solution by using ammonium persulphate as an oxidizing agent. The average particle size of the nanoparticles calculated from X-ray diffraction patterns were f 24 nm (reflux method) and f 28 nm (g-irradiation method). The above results are supported by TEM analysis. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Aniline; Stabilizer; Reflux; g-Irradiation; Nanomaterials; Nanocomposites
1. Introduction Transition metal nanoparticles dispersed within a polymeric matrix offer attractive routes for combining properties stemming from inorganic nanoparticles (such as certain magnetic, electronic, optical or catalytic properties) and the polymers (such as the processibility and film forming properties) [1]. Nanostructured transition metal particles are of great interest from both fundamental and practical viewpoints because of the quantum size effect which is derived
* Corresponding author. Tel.: +91-20-560-1229; fax: +91-20569-1728. E-mail address:
[email protected] (A.A. Athawale).
from the dramatic reduction of number of free electrons [2]. Nanoparticles have a large surface to volume ratio and consequently exhibit increased surface activity as compared to bulk materials [3]. A number of reports available on the synthesis of nanoparticles by different methods like sol – gel method [4], sonochemical method [5], reflux method [1], girradiation method [6], chemical reduction method [7], and pulse sonoelectrochemical method [8] show that bare metal nanoparticles are very unstable. Therefore, stabilizers are added to improve their stability for further applications. External stabilizers like tetra alkyl ammonium salts [9,10] have been used or the nanoparticles formed were supported by a polymer matrix [11]. In the present paper, we report the synthesis of Pd nanoparticles by two different methods viz. reflux and
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00235-0
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g-irradiation along with the studies related to the stability of the synthesized nanoparticles. Among the various stabilizers added, aniline was found to be the most effective. We have chosen the Pd metal for our work because Pd is an attractive element from the viewpoint of magnetism [12] and catalytic activity [13]. For the application purpose and to maintain the zero valent state of the metal permanently, polymerization of the bare nanoparticle solution containing aniline has been carried out to obtain the ‘‘MetalPolymer nanocomposites.’’
3. Results and discussion Various monomers have been utilized to serve as a stabilizer for the formation of Pd-nanoparticles by both reflux and g-irradiation methods. The different monomers used were N-ethyl aniline, N-methyl aniline, o-anisidine and o-toluidine. UV –Visible spectra are given in Fig. 1. From this figure, it is observed that the nanoparticles formed have a wide distribution of particles with the kmax in between f 320 and 450 nm. Moreover, the nanoparticles formed show a loss of stability after a period of 24 h. Hence, the further studies with reference to the stability of the nano-
2. Materials and methods All chemicals used were of A.R. grade. The monomers aniline, N-ethyl aniline, N-methyl aniline, oanisidine and o-toluidine were distilled doubly prior to use. Methanol –water (ratio 1.5:1) solvent system was used throughout the work. The nanoparticles were synthesized by two methods: namely, reflux and girradiation. A mixture of the monomer (0.1 M) and PdCl25H2O (10 4 M) was diluted to a volume of 100 ml with methanol – water and bubbled with nitrogen gas for 15 min to remove the oxygen content followed by refluxing at a fixed temperature for few hours. Similarly, the 100 ml nitrogen-bubbled solution was exposed to a total dose of 198 kGy by using a 60 Co g-ray source . UV – Visible spectra of the bare nanoparticles as well as the nanocomposites were recorded on a Shimadzu UV-1601 having a spectral resolution of 2 nm and Hitachi 2A UV – Visible spectrophotometers between 300– 800 and 300 –600 nm, respectively. Pdj state nanoparticles were polymerized to form Pdj/polyaniline metal nanocomposites. The Pd nanoparticle solution was polymerized by dropwise addition of the oxidizing agent (ammonium persulphate 0.1 M) under constant stirring with the help of the soxhlet funnel at a fixed temperature of 4– 5 jC for 2 h, followed by the filtration and drying of the nanocomposite powders in an oven at 50 jC for 24 h. The dried nanocomposites were used for X-ray measurements. The X-ray diffractograms were obtained on a Philips PW 1710 model with Cu – Ka ˚ ) ranging from 0j to 90j. TEM radiation (k = 1.540 A micrographs were taken on the instrument JEOL, JEM 1200EX at an operating voltage of 80 kV using copper grids of 400 mesh size.
Fig. 1. (a) UV – Visible spectra of nanoparticles synthesized by reflux method for various monomers like (1) N-ethyl aniline, (2) aniline, (3) o-toluidine (4) N-methyl aniline, and (5) o-anisidine. (b) UV – Visible spectra of nanoparticles synthesized by girradiation method for various monomers like (1) N-ethyl aniline, (2) aniline, (3) o-toluidine (4) N-methyl aniline, and (5) oanisidine.
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particles synthesized using these monomers as a stabilizer was restricted, confirming that the aniline is found to be the best stabilizer for Pd nanoparticles obtained by both the methods. Fig. 2a shows the spectra of bare Pd nanoparticles synthesized by reflux as well as g-irradiation method together with the spectra of PdCl2 solution. Pd2 + ions exhibit a kmax at f 465 nm. However, the wavelengths exhibited by the Pdj nanoparticles prepared by reflux and g-irradiation method were found to appear at f 320 and f 317 nm, respectively, with sharp peaks. The sharpness of the peak indicates the narrow size distribution of the particles. The blue shift in wavelengths with respect to the Pd2 + ions confirms the nanoparticle formation. The nanoparticles formed
Fig. 3. (a) UV – Visible spectra of Pd nanoparticles synthesized by reflux method the ratio of Pd2 + ions to aniline 1:1000 showing stability up to 96 h. (b) UV – Visible spectra of Pd nanoparticles synthesized by g-irradiation method the ratio of Pd2 + ions to aniline 1:1000 showing stability up to 96 h.
Fig. 2. (a) UV – Visible spectra of bare Pd nanoparticles synthesized by (1) reflux method (2) g-irradiation method along with the spectrum of (3) PdCl2 solution. (b) UV – Visible spectra of Pd nanoparticles synthesized by reflux method at three different ratios (1) 1:100, (2) 1:1000, (3) 1:10,000. (c) UV – Visible spectra of Pd nanoparticles synthesized by g-irradiation method at three different ratios (1) 1:100, (2) 1:1000, (3) 1:10,000.
by both methods were found to be in solution state. One can visibly observe the formation of nanoparticles in terms of colour change of the solution from pale yellow to brown. Experiments were also carried out by varying the ratio of Pd2 + ions to aniline as 1:100, 1:1000 and 1:10,000. In case of the reflux method, Fig. 2b depicts that the ratio of 1:1000 is the best yielding fine nanoparticles. On the other hand, the ratio of 1:100 results in the formation of a broad distribution of particles together with the unreacted Pd 2 + ions (kmax = f 430 nm). Whereas at higher ratio of 1:10,000, a negligible amount of nanoparticles are seen to be formed. Apart from this, time-evolved spectra (Fig. 3a) also reveal that the most stable nanoparticles are obtained at a ratio of 1:1000. The stability being observed up to 96 h. At other ratios, poor stability was observed. The loss of stability could be visibly seen with the precipitation or settling down of the particles. Similarly, nanoparticles synthesized by g-irradiation method are found to exhibit finer particle size as well as higher stability at Pd2 + ions to aniline ratio of 1:1000 (Fig. 2c). At other ratios, although sharp peaks are observed in the UV – Visible spectra implying
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nanoparticle formation, the kmax are at higher wavelengths as well as the stability is found to be poor up to a period of 24 h. On the other hand, at a ratio of 1:1000, stable nanoparticles are observed up to a period of 96 h (Fig. 3b). Another parameter investigated was the variation in the concentration of aniline (0.05 – 1.0 M) keeping that of metal constant. From Fig. 4a and b, one can observe that 0.1 M aniline gives the best results with a narrow distribution of particles as well as higher stability. On the contrary, at concentrations of 0.05, 0.5, 0.75 and 1.0 M, short-lived nanoparticles are formed with a broad particle size distribution. The synthesized stable nanoparticles at a ratio of 1:1000 (Pd2 + ions to aniline) were chosen to prepare the Pdj/ Fig. 5. UV – Visible spectra of Pdj/polyaniline nanocomposites in NMP solution with the Pd nanoparticles synthesized by (1) reflux and (2) g-irradiation method.
Fig. 4. (a) UV – Visible spectra of Pd nanoparticles synthesized by reflux method at varying concentrations of aniline (1) 0.05 M, (2) 0.1 M, (3) 0.5 M, (4) 0.75 M, and (5) 1.0 M. (b) UV – Visible spectra of Pd nanoparticles synthesized by g-irradiation method at varying concentrations of aniline (1) 0.05 M, (2) 0.1 M, (3) 0.5 M, (4) 0.75 M, and (5) 1.0 M.
polyaniline nanocomposites. The UV – Visible spectra of Pdj/polyaniline nanocomposites (Fig. 5) show the presence of two peaks in each sample (reflux and girradiation), one observed f 411 and f 393 nm and a broad peak at f 560 nm corresponding to the k –k* transition and pernigraniline phase of the polymer, respectively. However, the presence of Pd nanoparticles cannot be distinguished in the nanocomposite since the peak for Pd nanoparticles is found to overlap with that of the k –k* transition peak of the polymer. The average size of the nanoparticles were determined with the help of XRD as well as TEM measurements. The X-ray diffraction patterns (Fig. 6), which exhibit broad and intense peaks at 2h values between 10j and 35j, can be attributed to polyaniline [14], while the presence of Pd nanoparticles is observed in terms of peaks at 2h f 37.60j, 44.40j, 66.80j and f 39.40j, 46.04j, 68.80j for reflux and g-irradiation methods, respectively. The lower intensity of the peaks results due to the presence of lower concentration of Pdj (10 4 M) in the nanocomposites. These data match well with the ASTM data for the Pd metal. Table 1 quotes the 2h and d values of the Pd nanoparticles obtained by reflux and irradiation method along with the ASTM data. The diffraction angles were found to be shifted towards lower values with a decrease in the particle size. This can be accounted in terms of increase in the interatomic space between Pd – Pd atoms with a decrease in particle size [2].
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Fig. 6. X-ray diffraction patterns of Pdj/polyaniline nanocomposites with the Pd nanoparticles synthesized by (1) reflux and (2) g-irradiation method.
The particle size of the nanoparticles was calculated by applying the Debye Scherrer’s formula: d ¼ 0:9k=2bcosh where d is the mean diameter of particles, k is wavelength and b is full-width at half-maximum (FWHM). The average particle size obtained by the reflux and g-irradiation method were f 24 and f 28 nm, respectively. The transmission electron micrographs and electron diffraction patterns of the Pd/polyaniline nanocomposites synthesized by reflux and g-irradiation method are depicted in Fig. 7a and b. Comparison of the TEM micrographs of the two samples reveals a lower degree of size dispersity of Pd nanoparticles synthesized by reflux method. Average size observed in case of reflux sample is f 24 nm. On the contrary, the particles synthesized by g-irradiation method exhibits a broad distribution of particle size between
Table 1 XRD data of Pd metal nanoparticles synthesized by reflux and g-irradiation method along with the ASTM data
2h
d
ASTM data
Reflux method
g-Irradiation method
40.02j 46.50j 67.82j 2.25 1.95 1.38
37.60j 44.40j 66.80j 2.35 2.03 1.22
39.40j 46.04j 68.80j 2.28 1.94 1.34
Fig. 7. TEM micrographs (magnification 100 K) along with the electron diffraction patterns of Pdj/polyaniline nanocomposite with the Pd nanoparticles synthesized by (a) reflux method and (b) g-irradiation method.
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20 and 50 nm. Similar observations are noted in the electron diffraction patterns of the two samples.
4. Conclusions 1. Aniline serves as the best stabilizer among the various monomers like N-ethyl aniline, N-methyl aniline, o-anisidine and o-toluidine for the highly unstable Pd metal nanoparticles. 2. The ratio of 1:1000 for Pd2 + ions to aniline is observed to be the best for nanoparticles stabilization for both reflux and g-irradiation method. 3. The nanoparticles synthesized by g-irradiation method show polydispersity.
Acknowledgements One of the authors, S.V. Bhagwat, gratefully acknowledges UGC, India for financial support.
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