Nanoparticles synthesis by electron beam radiolysis +

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1 Cent. Eur. J. Chem. 12(7) DOI: 1478/s x Central European Journal of Chemistry Nanoparticles synthesis by electron beam radiolysis + RICCCE 18 Ioan Călinescu 1, Diana Martin 2, Daniel Ighigeanu 2*, Adina I. Gavrila 1, Adrian Trifan 1, Mariana Patrascu 3, Cornel Munteanu 4, Aurel Diacon 1, Elena Manaila 2, Gabriela Craciun 2 1 University Politehnica of Bucharest, Department of Bioresources and Polymer Science, Bucharest, Romania 2 National Institute for Lasers, Plasma and Radiation Physics, Electron Accelerators Laboratory, Magurele, Ilfov, Romania 3 S.C. CHEMSPEED S.R.L. member of PRIMOSAL Group, Bucharest, Romania 4 Institute of Physical Chemistry Ilie Murgulescu, Romanian Academy, Bucharest, Romania Received 30 September 2013; Accepted 10 December 2013 Abstract: Electron beam (EB) irradiation is a useful method to generate stable silver nanoparticles without the interference of inherent impurities generated from chemical reactions. Our experiments were carried out using linear electron beam accelerators with two different EB absorbed dose rates: 2 kgy min -1 and 7-8 kgy s -1, and with different absorbed dose levels. The optimum conditions for silver nanoparticles (AgNPs) generation by radiolysis, or by radiolysis combined with chemical reduction, were established. In order to obtain a good yield for AgNPs synthesized by radiolysis, a high dose rate is required, resulting in a rapid production process. At low absorbed dose rates, the utilization of a stabilization agent is advisable. By modifying the experimental conditions, the ratio between the chemical and radiolytic reduction process can be adjusted, thus it is possible to obtain nanoparticles with tailored characteristics, depending on the desired application. Keywords: Silver nanoparticles Electron beam Radiolysis Versita Sp. z o.o. 1. Introduction The powerful tools of ionizing radiation, electron accelerators, have been used for radiation processing of materials for more than half a century [1]. Since 1968, important pioneering work concerning silver clusters formed by γ-irradiation in solution has been realized [2-12]. The most important advantages of the EB irradiation process as compared to the chemical reduction method for NP synthesis are [13,14] as follows: (a) it eliminates the need for reducing agents, the reduction process is realized by the radical species generated after irradiation of the solvent; (b) the side reactions caused by the oxidation of the reducing agents are avoided; (c) the reduction process can be controlled by adjusting of the absorbed radiation dose; (d) the surface of the obtained nanoparticles is free of anionic species resulting from the transformation of the reducing agents; (e) the process 774 * ighigeanu@yahoo.com + The article has been presented at the 18th Romanian International Conference on Chemistry and Chemical Engineering - RICCCE18 - held in New Montana, Sinaia, Romania on 4-7

2 I. Călinescu et al. can be efficiently carried out at room temperature. Another important advantage is that the EB can be used for large scale preparation of silver nanoparticles [15]. The metal clusters and nanoparticles formation are the result of several reactions steps as presented in previous studies related to this subject [14,16-18]. The interaction of high-energy radiation with a solution of metal ions induces ionization and excitation of the solvent and leads to the formation of radiolytic molecular and radical species throughout the solution. For example, in aqueous solutions the main species obtained are described in Reaction 1: EB H 2 O e - aq, H 3 O +, H., H 2, H 2 O 2 - Both e aq (E 0 = -2.9 V) and. H (E 0 = -2.4 V) have reduction potentials capable of reducing metal ions to metal atoms (E 0 (Ag + /Ag 0 ) = -1.8 V): (aq) Ag + + e - aq Ag 0 (2) Ag + + H. Ag 0 + H + (3) Metal oxidation by oxidant. OH radicals (E 0 = +1.9 V) was avoided by adding 2-propanol (i-proh) to the reaction mixture before irradiation.. OH and. H radicals are scavenged by i-proh: (CH 3 ) 2 CHOH +. OH (CH 3 ) 2 C. OH + H 2 O (4) (CH 3 ) 2 CHOH +. H (CH 3 ) 2 C. OH + H 2 (5) Metal atoms (Ag 0 ), obtained in solution from the metal ions reduction tend to associate with other ions and can go into oligomers and larger clusters: Ag 0 + Ag + Ag 2 +. (1) Ag m+x x+ (6) The metal atoms (Ag 0 ) can also condensate, forming neutral nuclei which can grow by adding more metal ions on the surface of the nanoparticles. These ions are reduced by radicals obtained from the. OH attack on i-proh. By these reactions, large metal nanoparticles are obtained ((Ag) L ): Ag 0 + Ag 0 Ag 2 Ag n (7) Ag n + Ag + Ag n+1 + Ag n (CH 3 ) 2 C. OH (Ag) L + (CH 3 ) 2 CO + H + (9) (8) The growth of the nanoparticles is limited by a stabilizer to obtain narrow size distributions. An interaction between the initially formed aggregates with radiation that limits the formation of the larger aggregates is also possible giving rise to smaller nanoparticles. A similar process has been suggested to occur in laser ablation of surface silver atoms [19]: (Ag) L (Ag) S + yag 0 (10) Among metal nanoparticles, silver nanoparticles (AgNPs) constitute a model system due to the possible monoelectronic reduction of the ions down to zerovalent state, the stability of noncomplexed cations and the easy complexation with various ligands and the intense optical absorption band of the AgNPs [20]. The present paper reports the synthesis of AgNPs by EB irradiation in the presence or absence of reducing or stabilizing agents. The effects of absorbed dose (expressed in Gy, 1 Gy = 1 J kg -1 ), absorbed dose rate (expressed in Gy s -1 ) and chemical composition of reaction mixture on the formation yield are investigated as are the size and stability of silver clusters and nanoparticles. We have determined the reaction conditions which conduct the process of AgNPs formation by a radiolytic or chemical reduction mechanism. 2. Experimental procedure 2.1. Reagents AgNO 3 (ACS, Sigma Aldrich), tannic acid (TA (C 76 H 52 O 46 ) M w = 170 g mol -1 (Merck), polyvinyl alcohol (PVA, M w =27000 g mol -1 and hydrolysis degree 95-99%) and 2-propanol (IPA, Fluka) were used as supplied. AgNO 3 solutions were freshly prepared in ultrapure water (conductivity 6 MΩ). The solutions used were purged of air by bubbling argon and then irradiated at room temperature. The formation of AgNPs by EBI at various doses was studied in different reaction compositions: IPA - [AgNO 3 ]= 0.1 mm; [IPA] = 0.5 M; IPA-PVA - [AgNO 3 ]= 0.1 mm; [IPA] = 0.5 M; PVA, weight ratio PVA:AgNO 3 = 500:1 IPA-TA - [AgNO 3 ]= 0.1 mm; [IPA] = 0.5 M;TA, molar ratio TA:AgNO 3 = 5:1 IPA-TA-PVA - [AgNO 3 ]= 0.1 mm; [IPA] = 0.5 M;TA, molar ratio TA:AgNO 3 = 5:1; PVA, weight ratio PVA:AgNO 3 = 500:1 775

3 Nanoparticles synthesis by electron beam radiolysis 2.2. Irradiation facilities Fig. 1 shows the innovative experimental installation for AgNPs preparation with EB. It is based on the use of the electron linear accelerators built in INFLPR- Accelerators Laboratory. Two electron accelerators have been employed for an optimum dose rate control: ALIN 10 for a dose rate of 2 kgy min -1 and ALID 7 for a dose rate of 5-10 kgy s -1 to ascertain the effect of the absorbed dose rate. The ALIN-10 accelerator is a laboratory installation designed for fundamental research. It is located in a horizontal position inside the irradiation room and is equipped with a post-acceleration beam focusing and bending to project accelerated EB either on the accelerating structure axis (through a horizontal vacuum window exit port) or at right angles to the accelerator structure (through a vertical vacuum window exit port). It is currently used to develop new technologies in the field of environment depollution, material processing, biomedicine and medical studies. The accelerator ALID-7 is an industrial equipment, initially designed and optimized to produce maximum X-ray output for nondestructive testing. Now, this accelerator can be used for applied radiation research with semi-pilot installation for monomer mixture polymerization, rubber mixture vulcanization, acid gases and volatile organic compounds removal, residual water and sludge decontamination, and small-scale commercial irradiation services: polyelectrolyte s preparation for waste and potable water treatment, sterilization of plastic plates with agarose gel used for electrophoresis and others. Both accelerators are of a traveling wave type, driven by 2 MW peak power tunable EEV M5125 type magnetrons (English Electric Valve Company Ltd.) operating in S-band (3 GHz). Several important physical characteristics of the EB are of great interest. The first is the available output power of the electron beam (P EB ), which represents the basic processing capability of a given electron source for a required dose. The second characteristic is related to the energy of the accelerated EB (E EB ), which determines the depth to which EB will penetrate the product. In the accelerator output arrangement with horizontal and vertical exit ports, the electron beam passes a ring-shaped EB collection monitor shown in Fig. 1. This monitor intercepts only a fraction of the scattered electron beam and gives together with its associated electronics instrumentation, a relative value of the absorbed dose rate. It has been initially calibrated by several dosimeter systems (such as Calorimetric Dosimeter and Chemical Ceric Dosimeter for high dose rates, Chemical Fricke Dosimeter for medium and small dose rates and Advanced Markus Chamber type 3404 Figure 1. Experimental installation for AgNPs preparation by EB irradiation. 1- Exit head of electron accelerators (6 MeV, 2 kgy min -1 or 5 MeV, 5-10 kgy s -1 ); 2 - Ringshaped EB collection monitor which intercepts only a fraction from EB field (together with its associated electronic instrumentation is used for monitoring the dose rate and accumulated dose during irradiation process); 3 - EB field; 4 - Al foil; 5 - Reaction vessel; 6 - The irradiated samples (the thickness of this is established in concordance with the sample densities and EB energy used in experiments - useful penetration); 7 - Magnetic stirrer; 8 - Metering pump for adding the precursor solution in reaction vessel, before irradiation; 9 - Argon bubbling system. used with UNIDOS Universal Dosimeter for very small dose rates) placed at the position where the irradiation is performed. The measured optimum values of the EB peak current I EB and EB energy E EB to produce maximum output power P EB for a fixed pulse duration τ EB and repetition frequency f EB are as follows: ALIN-10: E EB = 6.23 MeV; I EB =75 ma; P EB = 164 W (f EB = 100 Hz, τ EB = 3.5 µs); ALID-7: E EB = 5.5 MeV; I EB =130 ma; P EB = 134 W (f EB = 50 Hz, τ EB = 3.75 µs). The thickness of the irradiated samples is established in concordance with the sample densities and EB energy used in experiments (useful penetration). Before irradiation with EB, the precursor solution is added with a metering pump by a remote system from the accelerator control room, in the reaction vessel. Irradiation time, dose rate and integrated dose are also controlled and displayed on the control desk from the accelerator control room Analysis UV-VIS absorption spectra were recorded with an Able Jasco V-550 spectrophotometer. The AgNPs morphology was investigated by TEM and HRTEM analysis by using a high resolution transmission electron FEI Tecnai F30 S-Twin microscope. A drop of the AgNPs suspension was mounted on a holey carbon film copper grid allowing the solvent to evaporate at room temperature. 776

4 I. Călinescu et al. Abs (a.u.) 3. Results and Discussion kgy 2 kgy 10 kgy 1 kgy 3.1. EB radiolysis at low dose rates 2 kgy min The influence of irradiation dose in obtaining AgNPs without stabilizing or reducing agents The experiments were designed to explore the effect of absorbed dose on the generation of AgNPs, using different absorbed dose levels: 1 kgy, 2 kgy, 5kGy and 10 kgy. The synthesis was carried out using AgNO 3 (0.1 mm) and isopropyl alcohol (0.5 M), without any other stabilizing or reducing agents. The results (Fig. 2) show that the nanoparticles generation process is efficient only at low irradiation doses (1 and 2 kgy) which afford a suitable UV-VIS response with well-defined peaks. The increase of the dose (in the range 2 to 5 kgy) results in the broadening of the peaks, the increase of absorption and decrease of the wavelength of maximum absorption. This indicates that the AgNPs formation process is not diffusioncontrolled and at high doses and longer irradiation periods the size distribution is large [21]. At high doses (10 kgy) a degradation of the formed AgNPs takes place, explaining the decrease of the absorption peak maximum and area compared to the values obtained at 5 kgy. Together with the AgNPs, clusters of Ag atoms are also obtained. Oligomeric silver ions clusters are generally characterized by peaks within the wavelength domain of nm [22-24]. The cluster concentration increases with the irradiation dose (between 1 and 5 kgy) and then decreases at higher doses. In the absence of stabilizing agents, the clusters decay; after 3 months, the peak between nm has a very low intensity for the sample obtained at 1 kgy. Figure 2. The UV-VIS spectra for the samples obtained by irradiation at low dose rate and IPA composition. Inset: the effect of radiation dose on the absorption peak λ max (nm) In Fig. 3A, the TEM images corresponding to experiments performed at 1 kgy dose level are presented. The AgNPs are almost perfectly spherical with a diameter of only 6-7 nm. They are present in very low concentrations, indicating that not all the precursor has reacted (in some of the TEM images it is possible to observe AgNO 3 crystals obtained during the drying process of the sample). The samples obtained at higher irradiation doses, after storage for a few months, present visible deposits of Ag particles, indicating the low stability of suspension. We conclude that for these conditions, the formation of AgNPs is efficient for irradiation doses of around 2 kgy; at lower dose level, the formation process is not complete, whereas at higher doses, degradation phenomenon occurs. 3.. The influence of irradiation dose in obtaining AgNPs in the presence of stabilizing agent (PVA) The influence of irradiation doses of 1 kgy, 2 kgy, 5 kgy and 10 kgy on samples containing AgNO 3 (0.1 mm), PVA (weight PVA/AgNO 3 = 500:1) and isopropyl alcohol (0.5 M) at a ph level of 5.5 was investigated. The results (Fig. 4) reveal that the 1 kgy dose is not sufficient to realize AgNPs generation, but starting with 2 kgy the quantity of AgNPs is significant. In Fig. 4, it is possible to notice that the increase of the dose over 2 kgy results in the formation of more AgNPs (absorption at nm) and also clusters (absorption at nm). The quantity of clusters that formed is diminished compared to the samples that were obtained without PVA. The yield of AgNPs is largely increased in the presence of PVA as a stabilizing agent (higher absorption values are registered). For these samples, the dose rate influences both the wavelength (size of the particles) as well as the absorption value (particles concentration). The highest influence is in the 1 to 2 kgy domain, where an important increase of the absorption value is registered and which maintained almost at a constant level for higher doses. The degradation of the obtained AgNPs at doses higher than 5 kgy is negligible due to the steric stabilization in the presence of PVA. The samples are stable and after 3 months they maintain their color, and no deposits are observed. These observations are consistent with literature data presenting PVA as a reducing agent [25] but also as a stabilizing agent [26]. The AgNPs obtained at a dose level of 5 kgy were analyzed by TEM (see Fig. 3B). The images show AgNPs with a diameter of nm, but not perfect spherical, anisotropy due to the presence of the polymer which influences the diffusion of the silver atoms in a certain extent during the growth process. 777

5 Nanoparticles synthesis by electron beam radiolysis B composition type: IPA-PVA, dose = 5 kgy C composition type:ipa TA, dose = 5 kgy D composition type: IPA TA PVA, ph=5, dose =2 kgy TEM images for AgNPs obtained by irradiation at a low dose rate and different compositions. Abs (a.u.) λ max(nm) Figure 3. A composition type: IPA, dose = 1kGy 5 kgy 10 kgy 2 kgy 1 kgy Figure 4. The UV-VIS spectra for the samples obtained by irradiation at low dose rate and IPA-PVA composition. Inset: the effect of radiation dose on the absorption peak The influence of irradiation dose in obtaining AgNPs in the presence of reducing agent (TA) The influence of irradiation doses of 1 kgy, 2 kgy, 3 kgy and 5 kgy on samples containing AgNO3 (0.1 mm), tannic acid (molar ratio AT:AgNO3 = 5:1) and isopropyl alcohol (0.5 M) at a ph level of 5.5 was investigated. In this case, a small influence of the irradiation dose was noticed. Even at low dose levels of 1 kgy, the AgNPs formation process takes place to a certain extent (approximately 70% as indicated by the absorption peaks areas or the absorption value, see Fig. 5. In this case, the absorption at nm is due to Ag clusters, but also due to the presence of tannic acid (TA). The TA has both the capacity of reducing the silver ions as well as providing a steric stabilization for AgNPs; therefore the increase of the dose level does not lead to a decrease of the absorption value and explains the very good stability of the colloidal suspensions. The samples presenting a complete transformation of silver ions (samples obtained at doses higher than 2 kgy) have very stable absorption spectra and after 3 months they are very close to the initial data. The TEM analysis of the sample at 5 kgy presents the AgNPs with an almost spherical shape and a diameter of around nm (Fig. 3C). 778

6 I. Călinescu et al. Abs (a.u.) kgy 3 kgy 2 kgy 1 kgy Figure 5. The UV-VIS spectra for the samples obtained by irradiation at low dose rate and IPA-TA composition. Inset: the effect of radiation dose on the absorption peak Abs (a.u.) kgy 2 kgy 1 kgy Figure 6. The UV-VIS spectra for the samples obtained by irradiation at low dose rate and IPA-TA-PVA composition. Inset: the effect of radiation dose on the absorption peak ph ph=7.5 ph=6 ph=5 Figure 7. The UV-VIS spectra for the samples obtained by irradiation at low dose rate and different ph values, irradiation dose 2 kgy. Composition type IPA-TA-PVA. Inset: the effect of ph on the absorption peak λ max (nm) λ max (nm) λ max (nm) The influence of irradiation dose in obtaining AgNPs in in the the presence presence of reducing of reducing and stabilizing and stabilizing agents (TA+PVA) agents (TA+PVA) The experiments were realized on samples containing AgNO 3 (0.1 mm), tannic acid at a molar ratio TA:AgNO 3 5:1, isopropyl alcohol (0.5 M) and different amounts of NaOH in order to obtain a ph level of 5, 6 and 7.5. The goal has been to determine the influence of dose level on samples with a ph=7.5; 1 kgy, 2 kgy and 5 kgy and the influence of ph (5, 6 and 7.5) for an irradiation dose of 2 kgy. Based on the UV-VIS spectra (Fig. 6), at a ph level of 7.5, it can be noticed that the process takes place mainly due to the reducing character of the tannic acid. This is supported by the lack of variation of the spectra depending on the dose level. It is possible to observe that the increase of the dose level at 2 and 5 kgy allows a small degradation of the AgNPs already formed The influence of ph in obtaining AgNPs in the presence presence of both of both reducing reducing and and stabilizing agents (TA+PVA) (TA+PVA) Analyzing the results of Fig. 7, we can draw the following conclusions: the AgNPs formation process in the presence of TA and PVA takes place due to a radiolytic as well as a chemical mechanism. ph is the parameter which can influence which one of these mechanisms is predominant. At low ph values, the radiolytic mechanism is predominant, whereas at high ph values the chemical mechanism is predominant. Another difference is the type of obtained particles: by radiolysis, both Ag atoms clusters (suggested by the absorption peaks at nm) and AgNPs are formed, while the chemical process allows mainly nanoparticles. The stability of the obtained AgNPs is very good, increasing inverse proportionally with the quantity of cluster formed (at higher ph values). The TEM analysis for the experiments performed at ph=5 (Fig. 3D) shows spherical nanoparticles with a diameter of around 6-8 nm The influence of reaction composition in obtaining AgNPs The experiments were realized at an irradiation dose of 2 kgy while modifying solutions contents beside the AgNO 3 (0.1 mm): only IPA, IPA and TA, IPA and PVA and IPA, PVA and TA. The analysis of the UV-VIS spectra (Fig. 8) shows that the AgNPs generation process is highly dependent on the reaction mixture composition. The purely radiolytic process takes place only in the presence of isopropyl alcohol. In the presence of PVA, the obtained AgNPs present a narrow absorption peak. 779

7 Nanoparticles synthesis by electron beam radiolysis IPA-TA-PVA IPA-TA IPA-PVA IPA IPA IPA-TA IPA-PVA IPA-TA- PVA Composition type Figure 8. The UV-VIS spectra for the samples obtained by irradiation at low dose rate and different reaction compositions, irradiation dose 2 kgy, ph=5. Inset: the effect of composition type on the absorption peak IPA - TA - PVA ; ph 7.5 IPA - TA ; ph 5 IPA - PVA ; ph 5 IPA ; ph Irradiation dose (kgy) Figure 9. The absorption maximum value for AgNPs obtained at different reaction compositions and an irradiation dose of 2 kgy. 32 kgy 48 kgy 16 kgy Figure 10. UV-VIS spectra for AgNPs samples obtained at high dose rate, composition type IPA. Inset: the effect of radiation dose on the absorption peak λ max (nm) λ max (nm) This can be explained by the activity of PVA during the AgNPs formation process, which consists of realizing a protective layer on the surface of the AgNPs, thus creating a diffusional barrier for the growth process. In the absence of this barrier, narrow distributions are obtained for the radiolytic process only at very short irradiation periods. The utilization of TA introduces two effects into the system; on one hand the reducing character of TA and on the other hand, the steric stabilization. The reducing effect is demonstrated by the higher absorption and peak area value, compared to the IPA-PVA system. The combined use of PVA and TA allows an increase of the absorption value as well as an increase of the peak area. These conditions allow the narrowest AgNPs distribution, with dimensions of 6-8 nm and extreme stability in time to occur. Fig. 9 presents the influence of the reaction compositions over the absorption maximum of the AgNPs. The data confirms the influence of the irradiation dose at low ph levels. At a ph level of 7.5, in the presence of chemical reducing agents such as TA and stabilizing agents PVA, the AgNPs generation process involves predominantly chemical reduction of silver ions EB radiolysis at high dose rates 8 kgy s -1 The influence of the irradiation dose was investigated for doses: 16 kgy, 32 kgy and 48 kgy using solutions containing only AgNO 3 (0.1 mm) and isopropyl alcohol (0.5 M) in the absence of any stabilizing or reducing agent. The use of high dose rates (8 kgy s -1 ) and short irradiation time requires suitable dose levels to obtain AgNPs in the range of kgy. Based on the obtained UV-VIS spectra (Fig. 10), it can be noticed that, due to the short reaction time of these dose rates, the yield in obtaining AgNPs is slightly higher than at low dose rates. Nevertheless, the irradiation dose must be limited to limit degradation processes. The stability of the NPs obtained at a high dose is limited. The AgNPs should be used soon after synthesis. 4. Conclusions Silver nanoparticles have been obtained through a simple synthesis route involving EB irradiation of a AgNO 3 solution (precursor) in the presence of a compound (IPA), capable of trapping oxidizing radicals (HO ), as well as in the presence of reducing or stabilizing agents (TA and PVA). Linear EB accelerators with two different dose rates of 2 kgy min -1 and 7-8 kgy s -1 were used. 780

8 I. Călinescu et al. The characterization of the AgNPs solutions, obtained by UV-VIS and TEM analysis, allowed the correlation of experimental parameters (such as dose rate, irradiation dose and composition of precursor solution), with the characteristics of the obtained nanoparticles. The experiments realized by radiolysis at a low dose rate allowed the study of the influence of reaction media. Thus, in the presence of PVA, the reduction of silver ions is mainly due to EB irradiation, PVA having the role of stabilizing the AgNPs by steric interaction. The PVA layer acts as a diffusion barrier in the growth process of AgNPs and this is what makes it possible to obtain thin distributions of AgNPs. The use of TA introduces two effects into the system: the reducing feature of TA and the steric stabilization induced by the TA. The reducing effect is demonstrated by the higher absorption and peak area value, compared to the IPA-PVA system. The stabilization effect of TA is lower, compared to PVA. The combined use of PVA and TA affords an increase of the absorption value as well as an increase of the peak area; this aspect indicates that PVA also contributes in the reduction process of the silver ions. The stability of the colloidal system is good and can be increased by realizing a complete reduction of the silver anions which are present in the precursor solution, thus limiting the presence of Ag atoms cluster formations. By modifying the experimental conditions, the ratio between the chemical and radiolytic reduction process can be adjusted, thus, it is possible to obtain nanoparticles with tailored characteristics, depending on the desired application. The generation process by radiolysis allows AgNPs stabilization only by electrostatic interaction (low stability). The process take place in a matter of seconds for radiolysis at high dose rates,, allowing a narrow size distribution of particles with small diameters and a free surface without any stabilizer to be obtained. Thus, the AgNPs can provide numerous applications, given the facile access for modification of their surface, which is practically free. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] A.G. Chmielewski, D.K. Chmielewska, J. Michalik, M.H. Sampa, Nucl. Instrum. Meth. B 265, 339 (2007) J. Pukies, W. Roebke, A. Henglein, Ber. Bunsenges. Phys. Chem. 72, 842 (1968) J. Biswal, N. Misra, L. C. Borde, S. Sabharwal, Radiat. Phys. Chem. 83, 67 (2013) Ž. Jovanović, A. Radosavljević, M. Šiljegović, N. Bibić, V. Mišković-Stanković, Z. Kačarević-Popović, Radiat. Phys. Chem. 81, 1720 (2012) J. Krstić, J. Spasojević, A. Radosavljević, M. Šiljegovć, Z. Kačarević-Popović, Radiat. Phys. Chem. 96, 158 (2014) W.H. Eisa, Y.K. Abdel-Moneam, Y. Shaaban, A.A. Abdel-Fattah, A.M. Abou Zeid, Mater. Chem. Phys. 128, 109 (2011) H. Chu, H.-J. Kim, J. Su Kim, M.-S. Kim, B.-D. Yoon, H.-J. Park, C.Y. Kim, Radiat. Phys. Chem. 81, 180 (2012) Y. Zhao, A. Chen, S. Liang, J. Cryst. Growth 372, 116 (2013) H. Yamamoto, T. Kozawa, S. Tagawa, M. Naito, J. L. Marignier, M. Mostafavi, J. Belloni Radiat. Phys. Chem. 91, 148 (2013) J. Puišo, D. Adlienė, A. Guobiene, I. Prosycevas, R. Plaipaite-Nalivaiko, Mater. Sci. Eng. B 176, 1562 (2011) B. Gupta, D. Gautam, S. Anjum, S. Saxena, A. Kapil, Radiat. Phys. Chem. 92, 54 (2013) Ž. Jovanović, A. Krklješ, J. Stojkovska, S. Tomić, B. Obradović, V. Mišković-Stanković, [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] Z. Kačarević-Popović, Radiat. Phys. Chem. 80, 1208 (2011) Z. Jurasekova, S. Sanchez-Cortes, M. Tamba, A. Torreggiani, Vib. Spectrosc 57, 42 (2011) S.-H. Choi, Y.-P. Zhang, A. Gopalan, K.-P. Lee, H.-D. Kang, Colloids Surf., A 256, 165 (2005) S.-E. Kim, J. Hyun Park, B.C. Lee, J.-C. Lee, Y. Ku Kwon, Radiat. Phys. Chem. 81, 978 (2012) J. Belloni; M. Mostafavi, in P. Braunstein, L.A. Oro, P.R. Raithby (Eds.), Metal Clusters in Chemistry (Wiley-VCH Verlag GmbH, Weinheim, 2008) 1213 G.R. Dey, Radiat. Phys. Chem. 80, 1216 (2011) H. Remita, I. Lampre, M. Mostafavi, E. Balanzat, S. Bouffard, Radiat. Phys. Chem. 72, 575 (2005) S. Eustis, G. Krylova, A. Eremenko, N. Smirnova, A.W. Schill, M. El-Sayed, Photochem. Photobiol. Sci. 4, 154 (2005) V.K. Sharma, R.A. Yngard, Y. Lin, Adv. Colloid Interface Sci. 145, 83 (2009) T. Sugimoto, J. Colloid Interface Sci. 309, 106 (2007) A. Henglein, Chem. Phys. Lett. 154, 473 (1989) M. Treguer, F. Rocco, G. Lelong, A. Le Nestour, T. Cardinal, A. Maali, B. Lounis, Solid State Sci. 7, 812 (2005) H. Xu, K.S. Suslick, Adv. Mater. 22, 1078 (2010) A. Gautam, S. Ram, Mater. Chem. Phys. 119, 266 (2010) F. Zhou, R. Zhou, X. Hao, X. Wu, W. Rao, Y. Chen, D. Gao, Radiat. Phys. Chem. 77, 169 (2008) 781