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Optical and structural studies of silver nanoparticles
ARTICLE IN PRESS
Radiation Physics and Chemistry 71 (2004) 1039–1044
Optical and structural studies of silver nanoparticles M.K. Temgire, S.S. Joshi* Department of Chemistry, University of Pune, Pune 411 007, India Received 26 May 2003; accepted 22 October 2003
Abstract Gamma radiolysis method was used to prepare polyvinyl alcohol (PVA) capped silver nanoparticles by optimizing various conditions like metal ion concentration and polymer (PVA) of different molecular weights. The role of different scavengers was also studied. The decrease in particle size was observed with increase in the molecular weight of capping agent. g-radiolytic method provides silver nanoparticles in fully reduced and highly pure state. XRD (X-ray diffraction) technique confirmed the zero valent state of silver. Optical studies were done using UV-visible spectrophotometer to see the variation of electronic structure of the metal sol. Transmission Electron Microscopic (TEM) studies reveal the fcc geometry. The TEM show clearly split Debye-Scherrer rings. The d values calculated from the diffraction ring pattern are in perfect agreement with the ASTM data. Ag particles less than 10 nm are spherical in shape, whereas the particles above 30 nm have structure of pentagonal biprisms or decahedra, referred to as multiply twinned particles. r 2003 Elsevier Ltd. All rights reserved. Keywords: Ag nanoparticles; g-radiolysis; Spherical and multiply twinned particles; Scavenger role
1. Introduction Synthesis of nanoparticle clusters by g-radiation induced method has proved to be effective (Henglein, 1989; Belloni et al., 1994, 1998; Doudna et al., 2002; Belloni and Mostafavi, 2001). In this method, the deareated aqueous solution of metal salt is exposed to g-rays, the species hydrated electron and hydrogen atoms arising from radiolysis of water are strong reducing reagents and they reduce the metal ion to zero valent state (Marignier et al., 1985). H2 OBB-e aq; H3 Oþ ; Hd ; OHd ; H2 ; H2 O2 ; y
ð1Þ
To prevent the oxidation of particles a OH radical scavenger i.e. propan-2-ol is added in situ prior irradiation. Colloidal Ag particles have been extensively studied due to their photosensitivity (Henglein et al., 1991a, b; Heard et al., 1983; Li et al., 1999; Rogach et al., 1997 and references there in) in the work of Henglein’s group it was shown that not only the size, the shape and *Corresponding author. E-mail address: [email protected] (S.S. Joshi).
chemical surface modification (Henglein, 1993), but also the electron density increase in silver particles significantly alters the optical properties of Ag sols (Kapoor, 1998). A blue shift of the silver plasmon absorption band resulting from the electronic polarization of Ag particles was observed for aqueous sols in the case of electron transfer from the free radicals generated radiolytically (Mulvaney and Henglein, 1990; Henglein et al., 1991a, b; Henglein, 1995; Strelow, 1995) or photolytically (Henglein, 1993; Kapoor, 1998) due to adsorption of ions or molecules on metal clusters. PVA capped Cu nanoparticles were also synthesized by girradiation, optimizing metal ion concentration, amount of capping agent and proper control of pH (Joshi et al., 1998). In this work, the results of the investigation of gamma irradiation effect (low and high doses) on the morphology and optical absorption of the nanosize colloidal Ag particles formed in polyvinyl alcohol matrix as a function of molecular weight are presented. The effect of metal salt concentration and various scavengers on optical spectra has been also checked. XRD and TEM studies have been done in order to reveal the structural properties. Significant change in the shape
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M.K. Temgire, S.S. Joshi / Radiation Physics and Chemistry 71 (2004) 1039–1044
and structure from spherical to decahedra is observed with the increase in size.
2. Experimental section 2.1. Materials Silver nitrate was obtained from Merck. polyvinyl alcohol (PVA) of molecular weights (14,000, 30,000 and 125,000) was from S.D. fine Chemicals, India. Scavengers used were methanol from S.D. Fine Chemicals, ethanol from Farco Chemical supplies, Beijing, China, 2-propanol was from Qualigens, India. Iso-butanol and tert. butyl alcohols were of BDH (India) Pvt. Ltd., cyclohexanol was of J.T. Baker Chemical Co., Philipsburg. All chemicals, including solvents were used as received. Deionized water was prepared with a Milli-Q water purification system. 2.2. Preparation of silver nanoparticles To the deareated aqueous solution of AgNO3, 2propanol was added as hydroxyl radical scavenger in situ with polyvinyl alcohol as encapsulating agent. Then it was irradiated with g-rays using 60Co source at a dose rate of 1.0 or 2.0 kGy. Silver nanoparticles capped with various molecular weights of PVA were prepared under similar conditions. Also, the effect of scavengers was studied from lower alcohols like CH3OH to higher alcohols like cyclohexanol. Gamma radiolytic method is advantageous for its ease, reproducibility; no chemical contaminations added and can be carried out at ambient temperature. 2.3. Mechanism The mechanism of radiolytic reduction of aqueous solution is carried out by the hydrated electrons and organic radicals formed. Isopropanol plays an important role in scavenging the OH radicals, which hinders the oxidation process of zerovalent silver. The electrons and radicals reduce the Ag+ ions to yield the Ag atoms, which are forming colloidal silver particles via various growth processes. The solvated electron formed in the radiolysis reduces the dissolved silver ions to silver metal according to the following reactions. Agþ þe aq -Ag :
ð2Þ
The surface plasmon peak observed for Ag in presence of propan-2-ol and obtained at a low dose. 2-propanol scavenges OH and H radicals as shown in the following reaction: OHd þCH3 CHðOHÞCH3 -H2 O þ H3 CCd ðOHÞCH3 ; ð3Þ
Hd þCH3 CHðOHÞCH3 -H2 þH3 CCd ðOHÞCH3 :
ð4Þ
A secondary radical H3CC (OH) CH3 is formed which efficiently reduces the precursor metal ions Ag+ to Ag when associated with an atom as in the charged dimer Ag+ 2 or when adsorbed on clusters. Agþ 2 þCH3 2C2ðOHÞ2CH3 -Ag2 þH3 C2CO2CH3 þHþ ;
ð5Þ
Agþ n þCH3 2C2ðOHÞ2CH3 -Agn þH3 C2CO2CH3 þHþ :
ð6Þ
2.4. Characterization Optical spectra were taken on an UV-Visible Spectrophotometer model 220A, Hitachi. TEM micrographs were obtained on a Technai 12 and CM200 Philips Electron Microscope operated at an accelerating voltage of 120 and 200 kV, respectively. Characterization was accomplished by TEM micrographs of Ag nanoparticles. The samples were placed after drying on a formvar coated copper grids. The X-ray diffraction patterns were taken on a Philips powder X-ray diffractometer (PW 1840). These patterns were compared with the ASTM data (ASTM card no. 783) for bulk. Ag nanocluster size was estimated by Scherrer formula. Characterization was accomplished by TEM micrographs of Ag nanoparticles.
3. Results and discussion The reduction of silver ions in aqueous solutions generally yields colloidal silver with particle diameter of several nanometers. These particles have specific optical properties indicated by the presence of intense absorption band at 380 nm caused by collective excitation of all the free electrons in the particles (Henglein, 1993). The optical properties of silver were studied using PVA of different molecular weights 125,000, 30,000 and 14,000 at 104 M concentration. Metal atoms formed by irradiation or any other method tend to coalesce into oligomers which themselves progressively grow into larger clusters. However, the coalescence must be limited by adding a polymeric molecule acting as a cluster stabilizer. Functional groups with high affinity for the metal ensure the anchoring of the molecule at the cluster surface while the polymeric chain protects the cluster from coalescing with the next one through electrostatic repulsion or steric hindrance. PVA was selected as a capping agent as it only stabilizes but does not reduce ions before irradiation. The studies of silver clusters were done using different molecular weights of PVA. Before irradiation the UV-visible sprectrum shows no absorption in the range 260–800 nm. The samples were
ARTICLE IN PRESS M.K. Temgire, S.S. Joshi / Radiation Physics and Chemistry 71 (2004) 1039–1044
irradiated for a total dose of 1.0 and 2.0 kGy forming a dark yellow colloidal solution. For lowest molecular weight of PVA, slightly turbid solution was obtained. The spectra a, b in Fig. 1A and B exhibit, intense peaks centered around 410 nm whereas the peak is damped and much broader with a slight red shift changing the peak position at 440 nm in the spectrum c. Mostafavi et al. also report the broadness of the band in their studies of Agn clusters with polyacrylate ions (Mostafavi et al., 1990). The OH groups of PVA molecule anchor the Ag molecule at the cluster surface, while the polymeric chain protects the cluster from fusion with the next silver molecule. The final size of silver clusters stabilized by the
polymer is of few tens of nanometers and the sol presents the classical surface absorption band at 410 nm. The dose rate being low, the slow irradiation favors adsorption of ions which are not yet reduced onto atoms or small clusters formed at the start of irradiation and this process is not prevented by the polymer nor is the reduction in situ of the ions by electron transfer from reducing radicals. The electrons from small clusters can be transferred to larger ones coated with adsorbed ions and polymers. The final size of the cluster may thus be higher than the limit imposed by the polymer when the coalescence just occurs as at high dose rate (Belloni and Mostafavi, 2001) and the size is thus larger with the irradiation time reflected in the broadness of the band in the range 410–440 nm. The radiolytic reduction method allows a control of the progressive extent of reduction due to accurate knowledge of the dose used. The silver atoms are produced in deareated solution by radiation induced Ag+ ion precursors. The solvated e aq and H atoms arising from radiolysis of water are strong reducing agents: (E H2O/e (H+/ aq)=2.87 V(NHE) and E H)=2.3 V(NHE). They easily reduce metal ions into the zerovalent state as shown by Eqs. (2) and (3). The atoms formed dimerize when encountering or associate with excess Ag+ ions by a cascade of coalescence processes, these species progressively coalesce into larger clusters. Ag þAg -Ag2 ;
ð7Þ
Ag þ Agþ -Agþ 2;
ð8Þ
Agn þAgþ -Agþ nþ1 ;
ð9Þ
Agþ nþ1 þeaq -Agnþ1 :
Fig. 1. UV-visible spectra of silver nanoparticles synthesized by gamma radiolysis varying PVA of different molecular weights (a) 125,000, (b) 30,000, (c) 14,000, AgNO3=5 103 M with a dose of 1.0 kGy (A) and 2.0 kGy (B). Concentration of 2propanol: 0.2 M in an inert atmosphere. Optical path: 1 cm.
1041
ð10Þ
The fast reactions (8) and (9) of ion association with atoms or clusters play an important role in the cluster growth mechanism. The competition between the reduction of free silver ions and absorbed ones is controlled by the rate of reducing radical formation therefore, the cluster formation by direct reduction followed by coalescence is predominant at high irradiation dose rate and the final cluster size is smaller (Belloni and Mostafavi, 2001). According to Mie theory (Henglein, 1993 and reference there in) and its extension the interaction of light with the electrons of small metal particles results in an absorption band whose shape and intensity depend on the complex dielectric constant of the metal, the cluster size and the environment. Thus the absorption spectrum shows a broad band at about 410 nm in agreement with the result reported earlier. The concentration of AgNO3 was varied from 101 to 104 M. Variation in concentration of the Ag salt had a direct impact on the absorption spectra. As it can be seen from Fig. 2A, concentration of Ag ions controls the
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M.K. Temgire, S.S. Joshi / Radiation Physics and Chemistry 71 (2004) 1039–1044 Table 1 Effect of particle size of Ag for various molecular weights of PVA Sr.No.
m.w. of PVA
Diameter (nm)
(a) (b) (c)
14,000 30,000 125,000
21.472.0 19.471.2 18.670.8
Synthesis of silver nanoparticles was carried out at high dose (47.5 kGy) of gamma rays using AgNO3 (5 103 M). Concentration of 2-propanol: 0.2 M.
Fig. 2. (A) UV visible spectra of silver nanoparticles synthesized by gamma radiolysis using different AgNO3 concentrations: (a) 1 101 M, (b) 1 102 M, (c) 1 103 M, (d) 5 104 M, PVA=1 104 M. Concentration of 2-propanol: 0.2 M with a saturated dose of 47.5 kGy. Optical path: 1 cm. (B) XRD patterns of silver nanoparticles with different PVA of varying molecular weights 14,000, 30,000 and 125,000 (1 104 M). Peak assignments are indicated in the figure. AgNO3=5 103 M and 2-propanol: 0.2 M with a saturated dose of 47.5 kGy.
metallic character of Ag particles. While observations in the concentration range from 104 to 103 M the maximum of the absorption band was located at 416 and 418 nm, at higher concentrations 102 to 101 M oligomeric silver particles are obtained which do not show the characteristic absorption band at around 400 nm. Under total reduction conditions (dose of about 47.0 kGy) at higher concentrations the small peaks observed at around 290–310 nm are attributed to the formation of Agn clusters (Mostafavi et al., 1990; Kapoor et al., 1994). This is further confirmed by Xray Diffraction patterns shown in Fig. 2B for various
concentrations of Ag salt. Increase in concentrations of AgNO3 shows a definite change in particle size ranging from 16 to 24 nm. Table 1 shows the particle diameter of Ag particles as a function of molecular weight of PVA. Scavengers play a significant role in scavenging the OH radicals as discussed earlier. To check the scavenging ability, a series of alcohols were studied. The alcohols used were methanol, ethanol, 2-propanol, isobutanol, tert-butanol and cyclohexanol (0.2 M). The formation of Ag particles was carried out by a series of alcohols in presence of PVA (1 104 M) and the samples were irradiated for half an hour (1.0 kGy). The optical plasmon resonance absorption peak lies around 420 nm for all the alcohols, which gave dark yellowish brown colored colloidal solution. The solutions are stable for a few days. As it is established from the radiolytic mechanism that primary alcohols are oxidized to aldehydes whereas secondary alcohols are oxidized to mixture of aldehydes and ketones. Tertiary and higher branched alcohols cannot be oxidized to ketones without rupture of C–C bond and so are quite resistant to oxidation in neutral and alkaline media. The use of 2-propanol throughout our studies is due to efficient scavenging and reducing action towards the precursor silver ions to Ag as shown by reactions (5) and (6). Transmission electron micrograph studies (operated at 200 kV) were done for Ag particles formed (with a dose of 1.0 kGy) using AgNO3 (5 103), PVA (1 104) and 2-propanol (0.2 M). The TEM studies (Fig. 3a and b) revealed the particle size distribution ranging from 1.8 to 17.8 nm with average particle diameter of 6.5 nm. The particles are spherical in shape. Examination of other electron micrograph (Fig. 4a and b) shows silver in pentagonal biprisms or Decahedra these are referred to as ‘multiply twinned particles’ (Heard et al., 1983). In a recent feature article (Kelly et al., 2003) have described the progress in the theory of Nanoparticles, Optical properties, particularly methods for solving Maxwell’s equations for light scattering from particles arbitrary shape in a complex environment. It also includes the discussion of analytical and numerical methods for calculating extinction and scattering
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Fig. 3. (a) TEM image and (b) corresponding histogram of PVA capped silver nanoparticles obtained by Gamma radiolysis in an inert atmosphere. Solid line is the Gaussian fit to the PSD data. AgNO3 concentration: 5 103 M, PVA (m.w.125,000): 1 104 M and 2propanol: 0.2 M irradiated with a dose of 1.0 kGy.
Fig. 4. (a) TEM image and (b) electron diffraction spot pattern of polycrystalline Ag nanoparticles obtained from PVA capping in an inert atmosphere AgNO3: 5 103 M, PVA: 1 104 M and 2-propanol: 0.2 M irradiated with a dose of 1.0 kGy.
cross-section, local fields, and other optical properties for non-spherical particles. Though the experimental conditions were same the micrograph in Fig. 3a is at high magnification, while in Fig. 4 at low magnification. However, the selected area diffraction ring patterns of polycrystalline sample indicate that the structure is fcc as in the bulk metal. The micrograph shows clearly split Debye Scherrer rings. Table 2 shows perfect agreement in the d values calculated from the diffraction ring patterns and ASTM data. It is therefore suggested that the early nucleus giving rise to the particle was of
pentagonal symmetry, but that the twinned fcc crystallites have been growing on the surfaces of the nucleus with the same fcc structure, keeping eventually a pentagonal shape (Belloni et al., 1998).
4. Conclusion Silver nanoparticles have been synthesized by gamma radiolysis method. The optical studies reveal that the surface plasmon band of Ag clusters is indeed affected
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Table 2 Comparison of interplanar spacings calculated from TEM and XRD measurements with ASTM data for silver I=Io
hkl
( ASTM d (A)
( TEM d (A)
( XRD d (A)
100 40 25 26 12 4 15 12
111 200 220 311 222 400 331 420
2.359 2.044 1.445 1.231 1.179 1.027 0.937 0.913
2.210 1.869 1.458 1.235 1.088 0.972 0.867 —
2.350 2.036 1.442 1.229 1.178 — — —
Ag particles were synthesized using PVA (m.w. 125,000) at a concentration: 1 104 M and 2-propanol: 0.2 M with a dose of 1.0 kGy.
by the amount of dose absorbed, the nature and average molecular weight of the polymer used. It is also confirmed from the above studies that not only the size, shape, chemical surface modification but electron density increase also alter the optical properties. XRD and TEM studies reveal the fcc geometry. Ag particles less than 10 nm are spherical in shape, whereas the particles above 30 nm have structure of pentagonal biprisms or decahedra, referred to as multiply twinned particles.
Acknowledgements M.K. Temgire is thankful to DAE, Government of India and Unilever Industries (Pvt.) Ltd. for their financial support and to Sophisticated Analytical Instruments Facility (RSIC), IIT-Bombay for the TEM facility.
References Belloni, J., Mostafavi, M., 2001. In: Jonah, C., Rao, M. (Eds.), Radiation Chemistry, Present Status and Future Trends. Elsevier, Amsterdam, pp. 411–452. Belloni, J., Amblard, J., Marignier, J.L., Mostafavi, M., 1994. Cluster Atoms and Molecules In: Haberland, H. (Ed.), Vol. 2. Springer, Berlin, pp. 290–311. Belloni, J., Mostafavi, M., Remita, H., Marignier, J.L., Delcourt, M.O., 1998. Synthesis, chemistry and some applications of metal nanoparticles. New J. Chem. 22, 1239–1255. Doudna, C.M., Bertino, M.F., Tokuhiro, A.T., 2002. Structural investigation of Ag-Pd clusters synthesized with the radiolysis method. Langmuir 18, 2434–2435.
Heard, S.M., Grieser, F., Barraclough, C.G., 1983. The characterization of Ag sols by electron microscopy, optical absorption and electrophoresis. J. Coll. Int. Sci. 93 (2), 545–555. Henglein, A., 1989. Small-particle research: physicochemical properties of extremely small colloidal metal and semiconductor particles. Chem. Rev. 89, 1861–1873. Henglein, A., 1993. Physicochemical properties of small metal particles in solutions: microelectrode reactions, chemisoption, composite metal particles, and the atom-to-metal transition. J. Phys. Chem. 97, 5457–5471. Henglein, A., 1995. Electronics of colloidal nanometer particles. Ber. Bunsen-Ges. Phys. Chem. 99, 903–913. Henglein, A., Mulvaney, P., Linnert, T., 1991a. Chemistry of Agn aggregates in aqueous solution: non-metallic oligomeric clusters and metallic particles. Faraday Discussions 92, 31–34. Henglein, A., Mulvaney, P., Linnert, P., 1991b. Chemistry of silver aggregates in aqueous solution: non-metallic oligomers and metallic particles. Electrochim. Acta. 36 11–12, 1735–1741. Joshi, S.S., Patil, S.F., Iyer, V., Mahamuni, S., 1998. Radiation induced synthesis and characterization of copper nanoparticles. Nanostruct. Mater. 10 (7), 1135. Kapoor, S., 1998. Preparation, characterization, and surface modification of silver nanoparticles. Langmuir 14, 1021–1025. Kapoor, S., Lawless, D., Kennepohl, P., Meisel, D., Serpone, N., 1994. Reduction and aggregation of silver ions in aqueous gelatin solutions. Langmuir 10, 3018–3022. Kelly, L.K., Coronado, E., Zhao, L.L., Schatz, G.C., 2003. The optical properties of metal nanoparticles: the influence of size, shape, and dielectric environment. J. Phys. Chem. B 107 (3), 668–677. Li, T., Moon, J., Morrone, A.A., Mecholsky, J.J., Talham, D.R., Adair, J.H., 1999. Preparation of Ag/SiO2 nanosize composites by a reverse micelle and sol-gel technique. Langmuir 15, 4328–4334. Marignier, J.L., Belloni, J., Delcourt, M., Chevalier, J.P., 1985. New microaggregates of non noble metals and alloys prepared by radiation induced reduction. Nature 317, 344–345. Mostafavi, M., Keghouche, Delcourt, M., Belloni, J., 1990. Ultra-slow aggregation process for silver clusters of a few atoms in solution. J. Chem. Phys Lett. 167 (3), 193–197. Mulvaney, P., Henglein, A., 1990. Long-lived nonmetallic silver clusters in aqueous solution: a pulse radiolysis study of their formation. J. Phys. Chem. 94, 4182–4188. Rogach, A.L., Shevchenko, G.P., Afanas’eva, Z.M., Sviridov, V.V., 1997. Changes in the morphology and optical absorption of colloidal silver reduced with formic acid in the polymer matrix under UV irradiation. J. Phys. Chem. B. 101, 8129–8132 (and the references there in). Strelow, F., 1995. Time resolved chemisorptions of I and SH on colloidal silver particles (a stopped flow study). J. Phys. Chem. 99, 11834–11838.
Radiation Physics and Chemistry 71 (2004) 1039–1044
Optical and structural studies of silver nanoparticles M.K. Temgire, S.S. Joshi* Department of Chemistry, University of Pune, Pune 411 007, India Received 26 May 2003; accepted 22 October 2003
Abstract Gamma radiolysis method was used to prepare polyvinyl alcohol (PVA) capped silver nanoparticles by optimizing various conditions like metal ion concentration and polymer (PVA) of different molecular weights. The role of different scavengers was also studied. The decrease in particle size was observed with increase in the molecular weight of capping agent. g-radiolytic method provides silver nanoparticles in fully reduced and highly pure state. XRD (X-ray diffraction) technique confirmed the zero valent state of silver. Optical studies were done using UV-visible spectrophotometer to see the variation of electronic structure of the metal sol. Transmission Electron Microscopic (TEM) studies reveal the fcc geometry. The TEM show clearly split Debye-Scherrer rings. The d values calculated from the diffraction ring pattern are in perfect agreement with the ASTM data. Ag particles less than 10 nm are spherical in shape, whereas the particles above 30 nm have structure of pentagonal biprisms or decahedra, referred to as multiply twinned particles. r 2003 Elsevier Ltd. All rights reserved. Keywords: Ag nanoparticles; g-radiolysis; Spherical and multiply twinned particles; Scavenger role
1. Introduction Synthesis of nanoparticle clusters by g-radiation induced method has proved to be effective (Henglein, 1989; Belloni et al., 1994, 1998; Doudna et al., 2002; Belloni and Mostafavi, 2001). In this method, the deareated aqueous solution of metal salt is exposed to g-rays, the species hydrated electron and hydrogen atoms arising from radiolysis of water are strong reducing reagents and they reduce the metal ion to zero valent state (Marignier et al., 1985). H2 OBB-e aq; H3 Oþ ; Hd ; OHd ; H2 ; H2 O2 ; y
ð1Þ
To prevent the oxidation of particles a OH radical scavenger i.e. propan-2-ol is added in situ prior irradiation. Colloidal Ag particles have been extensively studied due to their photosensitivity (Henglein et al., 1991a, b; Heard et al., 1983; Li et al., 1999; Rogach et al., 1997 and references there in) in the work of Henglein’s group it was shown that not only the size, the shape and *Corresponding author. E-mail address: [email protected] (S.S. Joshi).
chemical surface modification (Henglein, 1993), but also the electron density increase in silver particles significantly alters the optical properties of Ag sols (Kapoor, 1998). A blue shift of the silver plasmon absorption band resulting from the electronic polarization of Ag particles was observed for aqueous sols in the case of electron transfer from the free radicals generated radiolytically (Mulvaney and Henglein, 1990; Henglein et al., 1991a, b; Henglein, 1995; Strelow, 1995) or photolytically (Henglein, 1993; Kapoor, 1998) due to adsorption of ions or molecules on metal clusters. PVA capped Cu nanoparticles were also synthesized by girradiation, optimizing metal ion concentration, amount of capping agent and proper control of pH (Joshi et al., 1998). In this work, the results of the investigation of gamma irradiation effect (low and high doses) on the morphology and optical absorption of the nanosize colloidal Ag particles formed in polyvinyl alcohol matrix as a function of molecular weight are presented. The effect of metal salt concentration and various scavengers on optical spectra has been also checked. XRD and TEM studies have been done in order to reveal the structural properties. Significant change in the shape
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and structure from spherical to decahedra is observed with the increase in size.
2. Experimental section 2.1. Materials Silver nitrate was obtained from Merck. polyvinyl alcohol (PVA) of molecular weights (14,000, 30,000 and 125,000) was from S.D. fine Chemicals, India. Scavengers used were methanol from S.D. Fine Chemicals, ethanol from Farco Chemical supplies, Beijing, China, 2-propanol was from Qualigens, India. Iso-butanol and tert. butyl alcohols were of BDH (India) Pvt. Ltd., cyclohexanol was of J.T. Baker Chemical Co., Philipsburg. All chemicals, including solvents were used as received. Deionized water was prepared with a Milli-Q water purification system. 2.2. Preparation of silver nanoparticles To the deareated aqueous solution of AgNO3, 2propanol was added as hydroxyl radical scavenger in situ with polyvinyl alcohol as encapsulating agent. Then it was irradiated with g-rays using 60Co source at a dose rate of 1.0 or 2.0 kGy. Silver nanoparticles capped with various molecular weights of PVA were prepared under similar conditions. Also, the effect of scavengers was studied from lower alcohols like CH3OH to higher alcohols like cyclohexanol. Gamma radiolytic method is advantageous for its ease, reproducibility; no chemical contaminations added and can be carried out at ambient temperature. 2.3. Mechanism The mechanism of radiolytic reduction of aqueous solution is carried out by the hydrated electrons and organic radicals formed. Isopropanol plays an important role in scavenging the OH radicals, which hinders the oxidation process of zerovalent silver. The electrons and radicals reduce the Ag+ ions to yield the Ag atoms, which are forming colloidal silver particles via various growth processes. The solvated electron formed in the radiolysis reduces the dissolved silver ions to silver metal according to the following reactions. Agþ þe aq -Ag :
ð2Þ
The surface plasmon peak observed for Ag in presence of propan-2-ol and obtained at a low dose. 2-propanol scavenges OH and H radicals as shown in the following reaction: OHd þCH3 CHðOHÞCH3 -H2 O þ H3 CCd ðOHÞCH3 ; ð3Þ
Hd þCH3 CHðOHÞCH3 -H2 þH3 CCd ðOHÞCH3 :
ð4Þ
A secondary radical H3CC (OH) CH3 is formed which efficiently reduces the precursor metal ions Ag+ to Ag when associated with an atom as in the charged dimer Ag+ 2 or when adsorbed on clusters. Agþ 2 þCH3 2C2ðOHÞ2CH3 -Ag2 þH3 C2CO2CH3 þHþ ;
ð5Þ
Agþ n þCH3 2C2ðOHÞ2CH3 -Agn þH3 C2CO2CH3 þHþ :
ð6Þ
2.4. Characterization Optical spectra were taken on an UV-Visible Spectrophotometer model 220A, Hitachi. TEM micrographs were obtained on a Technai 12 and CM200 Philips Electron Microscope operated at an accelerating voltage of 120 and 200 kV, respectively. Characterization was accomplished by TEM micrographs of Ag nanoparticles. The samples were placed after drying on a formvar coated copper grids. The X-ray diffraction patterns were taken on a Philips powder X-ray diffractometer (PW 1840). These patterns were compared with the ASTM data (ASTM card no. 783) for bulk. Ag nanocluster size was estimated by Scherrer formula. Characterization was accomplished by TEM micrographs of Ag nanoparticles.
3. Results and discussion The reduction of silver ions in aqueous solutions generally yields colloidal silver with particle diameter of several nanometers. These particles have specific optical properties indicated by the presence of intense absorption band at 380 nm caused by collective excitation of all the free electrons in the particles (Henglein, 1993). The optical properties of silver were studied using PVA of different molecular weights 125,000, 30,000 and 14,000 at 104 M concentration. Metal atoms formed by irradiation or any other method tend to coalesce into oligomers which themselves progressively grow into larger clusters. However, the coalescence must be limited by adding a polymeric molecule acting as a cluster stabilizer. Functional groups with high affinity for the metal ensure the anchoring of the molecule at the cluster surface while the polymeric chain protects the cluster from coalescing with the next one through electrostatic repulsion or steric hindrance. PVA was selected as a capping agent as it only stabilizes but does not reduce ions before irradiation. The studies of silver clusters were done using different molecular weights of PVA. Before irradiation the UV-visible sprectrum shows no absorption in the range 260–800 nm. The samples were
ARTICLE IN PRESS M.K. Temgire, S.S. Joshi / Radiation Physics and Chemistry 71 (2004) 1039–1044
irradiated for a total dose of 1.0 and 2.0 kGy forming a dark yellow colloidal solution. For lowest molecular weight of PVA, slightly turbid solution was obtained. The spectra a, b in Fig. 1A and B exhibit, intense peaks centered around 410 nm whereas the peak is damped and much broader with a slight red shift changing the peak position at 440 nm in the spectrum c. Mostafavi et al. also report the broadness of the band in their studies of Agn clusters with polyacrylate ions (Mostafavi et al., 1990). The OH groups of PVA molecule anchor the Ag molecule at the cluster surface, while the polymeric chain protects the cluster from fusion with the next silver molecule. The final size of silver clusters stabilized by the
polymer is of few tens of nanometers and the sol presents the classical surface absorption band at 410 nm. The dose rate being low, the slow irradiation favors adsorption of ions which are not yet reduced onto atoms or small clusters formed at the start of irradiation and this process is not prevented by the polymer nor is the reduction in situ of the ions by electron transfer from reducing radicals. The electrons from small clusters can be transferred to larger ones coated with adsorbed ions and polymers. The final size of the cluster may thus be higher than the limit imposed by the polymer when the coalescence just occurs as at high dose rate (Belloni and Mostafavi, 2001) and the size is thus larger with the irradiation time reflected in the broadness of the band in the range 410–440 nm. The radiolytic reduction method allows a control of the progressive extent of reduction due to accurate knowledge of the dose used. The silver atoms are produced in deareated solution by radiation induced Ag+ ion precursors. The solvated e aq and H atoms arising from radiolysis of water are strong reducing agents: (E H2O/e (H+/ aq)=2.87 V(NHE) and E H)=2.3 V(NHE). They easily reduce metal ions into the zerovalent state as shown by Eqs. (2) and (3). The atoms formed dimerize when encountering or associate with excess Ag+ ions by a cascade of coalescence processes, these species progressively coalesce into larger clusters. Ag þAg -Ag2 ;
ð7Þ
Ag þ Agþ -Agþ 2;
ð8Þ
Agn þAgþ -Agþ nþ1 ;
ð9Þ
Agþ nþ1 þeaq -Agnþ1 :
Fig. 1. UV-visible spectra of silver nanoparticles synthesized by gamma radiolysis varying PVA of different molecular weights (a) 125,000, (b) 30,000, (c) 14,000, AgNO3=5 103 M with a dose of 1.0 kGy (A) and 2.0 kGy (B). Concentration of 2propanol: 0.2 M in an inert atmosphere. Optical path: 1 cm.
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ð10Þ
The fast reactions (8) and (9) of ion association with atoms or clusters play an important role in the cluster growth mechanism. The competition between the reduction of free silver ions and absorbed ones is controlled by the rate of reducing radical formation therefore, the cluster formation by direct reduction followed by coalescence is predominant at high irradiation dose rate and the final cluster size is smaller (Belloni and Mostafavi, 2001). According to Mie theory (Henglein, 1993 and reference there in) and its extension the interaction of light with the electrons of small metal particles results in an absorption band whose shape and intensity depend on the complex dielectric constant of the metal, the cluster size and the environment. Thus the absorption spectrum shows a broad band at about 410 nm in agreement with the result reported earlier. The concentration of AgNO3 was varied from 101 to 104 M. Variation in concentration of the Ag salt had a direct impact on the absorption spectra. As it can be seen from Fig. 2A, concentration of Ag ions controls the
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M.K. Temgire, S.S. Joshi / Radiation Physics and Chemistry 71 (2004) 1039–1044 Table 1 Effect of particle size of Ag for various molecular weights of PVA Sr.No.
m.w. of PVA
Diameter (nm)
(a) (b) (c)
14,000 30,000 125,000
21.472.0 19.471.2 18.670.8
Synthesis of silver nanoparticles was carried out at high dose (47.5 kGy) of gamma rays using AgNO3 (5 103 M). Concentration of 2-propanol: 0.2 M.
Fig. 2. (A) UV visible spectra of silver nanoparticles synthesized by gamma radiolysis using different AgNO3 concentrations: (a) 1 101 M, (b) 1 102 M, (c) 1 103 M, (d) 5 104 M, PVA=1 104 M. Concentration of 2-propanol: 0.2 M with a saturated dose of 47.5 kGy. Optical path: 1 cm. (B) XRD patterns of silver nanoparticles with different PVA of varying molecular weights 14,000, 30,000 and 125,000 (1 104 M). Peak assignments are indicated in the figure. AgNO3=5 103 M and 2-propanol: 0.2 M with a saturated dose of 47.5 kGy.
metallic character of Ag particles. While observations in the concentration range from 104 to 103 M the maximum of the absorption band was located at 416 and 418 nm, at higher concentrations 102 to 101 M oligomeric silver particles are obtained which do not show the characteristic absorption band at around 400 nm. Under total reduction conditions (dose of about 47.0 kGy) at higher concentrations the small peaks observed at around 290–310 nm are attributed to the formation of Agn clusters (Mostafavi et al., 1990; Kapoor et al., 1994). This is further confirmed by Xray Diffraction patterns shown in Fig. 2B for various
concentrations of Ag salt. Increase in concentrations of AgNO3 shows a definite change in particle size ranging from 16 to 24 nm. Table 1 shows the particle diameter of Ag particles as a function of molecular weight of PVA. Scavengers play a significant role in scavenging the OH radicals as discussed earlier. To check the scavenging ability, a series of alcohols were studied. The alcohols used were methanol, ethanol, 2-propanol, isobutanol, tert-butanol and cyclohexanol (0.2 M). The formation of Ag particles was carried out by a series of alcohols in presence of PVA (1 104 M) and the samples were irradiated for half an hour (1.0 kGy). The optical plasmon resonance absorption peak lies around 420 nm for all the alcohols, which gave dark yellowish brown colored colloidal solution. The solutions are stable for a few days. As it is established from the radiolytic mechanism that primary alcohols are oxidized to aldehydes whereas secondary alcohols are oxidized to mixture of aldehydes and ketones. Tertiary and higher branched alcohols cannot be oxidized to ketones without rupture of C–C bond and so are quite resistant to oxidation in neutral and alkaline media. The use of 2-propanol throughout our studies is due to efficient scavenging and reducing action towards the precursor silver ions to Ag as shown by reactions (5) and (6). Transmission electron micrograph studies (operated at 200 kV) were done for Ag particles formed (with a dose of 1.0 kGy) using AgNO3 (5 103), PVA (1 104) and 2-propanol (0.2 M). The TEM studies (Fig. 3a and b) revealed the particle size distribution ranging from 1.8 to 17.8 nm with average particle diameter of 6.5 nm. The particles are spherical in shape. Examination of other electron micrograph (Fig. 4a and b) shows silver in pentagonal biprisms or Decahedra these are referred to as ‘multiply twinned particles’ (Heard et al., 1983). In a recent feature article (Kelly et al., 2003) have described the progress in the theory of Nanoparticles, Optical properties, particularly methods for solving Maxwell’s equations for light scattering from particles arbitrary shape in a complex environment. It also includes the discussion of analytical and numerical methods for calculating extinction and scattering
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Fig. 3. (a) TEM image and (b) corresponding histogram of PVA capped silver nanoparticles obtained by Gamma radiolysis in an inert atmosphere. Solid line is the Gaussian fit to the PSD data. AgNO3 concentration: 5 103 M, PVA (m.w.125,000): 1 104 M and 2propanol: 0.2 M irradiated with a dose of 1.0 kGy.
Fig. 4. (a) TEM image and (b) electron diffraction spot pattern of polycrystalline Ag nanoparticles obtained from PVA capping in an inert atmosphere AgNO3: 5 103 M, PVA: 1 104 M and 2-propanol: 0.2 M irradiated with a dose of 1.0 kGy.
cross-section, local fields, and other optical properties for non-spherical particles. Though the experimental conditions were same the micrograph in Fig. 3a is at high magnification, while in Fig. 4 at low magnification. However, the selected area diffraction ring patterns of polycrystalline sample indicate that the structure is fcc as in the bulk metal. The micrograph shows clearly split Debye Scherrer rings. Table 2 shows perfect agreement in the d values calculated from the diffraction ring patterns and ASTM data. It is therefore suggested that the early nucleus giving rise to the particle was of
pentagonal symmetry, but that the twinned fcc crystallites have been growing on the surfaces of the nucleus with the same fcc structure, keeping eventually a pentagonal shape (Belloni et al., 1998).
4. Conclusion Silver nanoparticles have been synthesized by gamma radiolysis method. The optical studies reveal that the surface plasmon band of Ag clusters is indeed affected
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Table 2 Comparison of interplanar spacings calculated from TEM and XRD measurements with ASTM data for silver I=Io
hkl
( ASTM d (A)
( TEM d (A)
( XRD d (A)
100 40 25 26 12 4 15 12
111 200 220 311 222 400 331 420
2.359 2.044 1.445 1.231 1.179 1.027 0.937 0.913
2.210 1.869 1.458 1.235 1.088 0.972 0.867 —
2.350 2.036 1.442 1.229 1.178 — — —
Ag particles were synthesized using PVA (m.w. 125,000) at a concentration: 1 104 M and 2-propanol: 0.2 M with a dose of 1.0 kGy.
by the amount of dose absorbed, the nature and average molecular weight of the polymer used. It is also confirmed from the above studies that not only the size, shape, chemical surface modification but electron density increase also alter the optical properties. XRD and TEM studies reveal the fcc geometry. Ag particles less than 10 nm are spherical in shape, whereas the particles above 30 nm have structure of pentagonal biprisms or decahedra, referred to as multiply twinned particles.
Acknowledgements M.K. Temgire is thankful to DAE, Government of India and Unilever Industries (Pvt.) Ltd. for their financial support and to Sophisticated Analytical Instruments Facility (RSIC), IIT-Bombay for the TEM facility.
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