ISSN 19950780, Nanotechnologies in Russia, 2011, Vol. 6, Nos. 11–12, pp. 757–762. © Pleiades Publishing, Ltd., 2011. Original Russian Text © M.E. Balezin, V.V. Bazarnyi, E.A. Karbovnichaya, S.Yu. Sokovnin, 2011, published in Rossiiskie Nanotekhnologii, 2011, Vol. 6, Nos. 11–12.
ARTICLES
Application of Nanosecond Electron Beam for Production of Silver Nanopowders M. E. Balezina, V. V. Bazarnyib, E. A. Karbovnichayac, and S. Yu. Sokovnina a
Institute of Electrophysics, Ural Branch, Russian Academy of Sciences, ul. Amudsena 106, Yekaterinburg, 620016 Russia b Ural State Academy of Medicine, Yekaterinburg, 620219 Russia c Sverdlovsk Regional Clinical Hospital No. 1, Yekaterinburg, 620149 Russia email:
[email protected] Received March 31, 2011
Abstract—Experiments on the irradiation of silver nitrate solutions in various liquids by the nanosecond elec tron beam of a URT0.5 (0.5 MeV, 50 ns, 1 kW) accelerator facilitated the development of technology for producing silver nanopowders. The powder yield and particle size as a function of the irradiation mode has been established. An increase in the adsorbed dose results in an increased powder yield; however, the particle size decreases and the powder agglomeration rate increases. Therefore, the targeted production of silver nan opowders with a mean particle size in the range of 90 ÷ 1100 nm is possible. DOI: 10.1134/S1995078011060036
INTRODUCTION The production of weakly aggregated silver nanop owders is an important problem for the development of nanostructural materials applied in microelectron ics, electrochemistry, the synthesis of optical electron sensors, pigments, etc. [1]. The bactericidal properties of silver ions are very important; it is due to them that nanopowders can serve as a basis for the development of new classes of bactericidal medications and various drugs in medi cine and agriculture [2, 3]. Methods for the synthesis of silver nanopowders can be subdivided according to how they are pro duced. At present, the main methods for the produc tion of silver nanopowders and aqueous suspensions thereof are chemical methods. It is highly important to create conditions which promote the formation of fine silver particles. Among the chemical methods, the most popular is the recovery of silver particles from an aqueous solu tion of silver salts in the presence of various stabilizing agents [4–6]. The reducing agents include hydrogen and hydrogencontaining compounds (tetrahydrobo rates [4, 7] and citrates of alkali metals [8], hypophos phites, alcohols [9], and organometallic compounds [10]). The recovery of silver nanoparticles can occur both on the surface of preliminarily synthesized latex microspheres in the presence of a reducing agent and at the stage of polymerization of monomers [11]. Physical methods include the sputtering or mechanical breakage of bulk material [12, 13], the photoreduction of silver salts [14], the laser ablation of solids in liquids [15], and the application of an elec tron beam [16].
The disadvantages of chemical methods are as fol lows: the influence of the concentration of the initial components on the reaction course and an aftereffect which is exhibited by the coagulation of the particles in solution. When using the physical methods, it possible to suppress coagulation by adding surfactants. Of all physical methods, the most attractive is the application of an electron beam, because it has been established that at a relatively low fluence of electrons (2 × 1013–3 × 1015 particles/cm2) it is possible to obtain nanopowders with particle sizes of 10–60 nm [16]. An increase in the fluence of electrons results in decreased particle sizes, which is different from chemical meth ods, where an increase in the reaction time leads to increased particle sizes. The application of nanosecond pulseperiodic accelerators [17] makes it possible to sufficiently readily control the time of action on the reaction com ponents; therefore, it was decided to study the possi bility of producing nanopowders on based on them, as well as their biological activity. EXPERIMENTAL RESULTS AND DISCUSSION We performed experiments on the irradiation of sil ver nitrate solutions (AgNO3) by a nanosecond elec tron beam of URT0.5 (0.5 MeV, 50 ns, 1kW [18]) in various liquids with the aim of producing silver nanop owders. The following types of solvent of silver nitrate were studied (0.3 g per 10 ml of solvent): Distilled water; Toluene with the addition of ethylene glycol (0.3 ml) and ethyl alcohol (0.3 ml);
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50 μm Fig. 1. SEM image of the foam of an aqueous solution.
500 nm Fig. 2. SEM image of the sediment of an aqueous solution.
200 nm Fig. 3. SEM image of nanopowder in an ammonia solu tion.
Ethyl alcohol; Isopropyl alcohol; 10% ammonia solution; Isopropyl alcohol with the addition of 2 ml 10% ammonia solution. The solutions were poured in a Petri dish in such a way that the liquid layer was no higher than 1 mm. Irradiation was performed at an accelerating fre
quency of 10 Hz; the adsorbed dose on the liquid sur face was 0.36 MGy per one minute. The dose level was based on the data in [16], because the calculated value of the adsorbed dose for the fluence lower threshold equals about 0.6 MGy. It should be noted that in all solvents the reaction of silver recovery was violent, especially in aqueous solu tions. When aqueous solutions were irradiated, part of the reaction products floated as foam and the other part precipitated. The foam was of a laminated structure (Fig. 1), and the sediment contained sufficiently agglomerated nanopowder (Fig. 2) with particles from 30 to 200 nm in size. Irradiation in ammonia solution makes it possible to eliminate foam formation and obtain less agglomerated powders (Fig. 3). However, the best results were obtained upon irra diation in hydrocarbon solvents. During the irradiation of the solution in toluene [19], weakly agglomerated silver powders with particle sizes of 3−5 nm were produced (Fig. 4). It has been established that an increase in irradiation time from 1 to 10 min does not lead to a decrease in particle sizes like in [16]; to increase in the reaction yield; or, after reaching an optimal dose at an irradiation time of 5 min (Fig. 5), to an increase in the agglomeration rate of the produced powders and the formation of jointing between the particles (Fig. 6). Therefore, it is possible to conclude that, after their reduction to atoms by the products of alcohol radioly sis, silver ions appearing in the course of the electron pulse (the mechanism is described elsewhere [16]) are combined into clusters in the pauses between the pulses; their sizes then increase to nanoparticles in the course of subsequent irradiation and are then com bined into agglomerates. The main difference of this process from the process described in [16] is that, although the irradiation doses are close (~MGy/s), in our case the energy is evolved in a pulse of about 50 ns, that is, the times of the processes of irradiation by elec trons differ by several orders of magnitude. Therefore, no inhibition of nanoparticle growth occurs in the pauses between the pulses (~100 ms) by OH* radicals as was described in [16]: the lifetime of the radicals (~100 ns) is obviously not sufficient. It should be mentioned that the existence of ethyl ene glycol in toluene solution significantly compli cates the preparation of specimens for TEM analysis. Therefore, the solution was irradiated in isopropyl alcohol and weakly agglomerated powders were obtained (Fig. 7); however, in order to achieve good solubility of silver nitrate, it is necessary to add ammo nia solution. The specific surface area of the powder equals 3.97 m2/g, dBET = 140 nm. According to the data of Xray phase analysis, the powder contains two crystal line phases:
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50 nm Fig. 4. TEM image, irradiation in toluene (1 min).
Fig. 5. TEM image, irradiation in toluene (5 min).
200 nm Fig. 7. SEM image of nanopowder in an isopropyl alcohol solution with ammonia.
50 nm Fig. 6. TEM image, irradiation in toluene (10 min).
(i) Ag with a facecentered cubic structure (90– 95%). The size of the coherentscattering region is 56 nm; period a = 4.086 Å; (ii) AgNO3 in orthorhombic modification (5– 10%); the size of the coherentscattering region is 130 ± 30 nm. The results of Xray phase analysis (Fig. 8) are con firmed by SÅÌ images of nanopowder obtained by irra diation in isopropyl alcohol solution with ammonia; two different fractions are well observed (Fig. 7). This served as a basis for the development of foun dations for the radiative–chemical technology of pow der production. The technological flowchart (Fig. 9) makes it possible to irradiate the solution in the selected environment (carbon dioxide or nitrogen), NANOTECHNOLOGIES IN RUSSIA
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which reduces the influence of air radiolysis products on the produced powders. From service tank 5 (1.9 l), the solution under the pressure of gas from reservoir 1 (0.2 kgf/cm2) is fed to the reaction chamber (RC), where it is irradiated. The irradiated solution with the obtained powder is col lected in receiving tank 7. After the precipitation of the powder for one day, the liquid is discharged and used for irradiation, whereas the concentration of silver nitrate is adjusted to the initial value in the solution. The sediment with concentrated suspension is dried, the powder is collected, and alcohol vapors are cap tured for further usage. The reaction chamber consists of outlet foil and stainless steel flange; the gap between them is no higher than 1 mm. The flange ends are equipped with two fittings for the solution inlet and outlet pipelines. The flange is positioned in such a way so as to locate the stiffening ribs of the outlet flange on the axis of the
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Counts 6
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Fig. 8. Diffraction pattern of silver nanopowder.
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Fig. 9. Technological flowchart of silver nanopowder production: (1) gas reservoir, (2) gas redactor, (3) electromagnetic valve, (4) electrocontact gauge, (5) service tank, (6) URT0.5 accelerator, and (7) receiving tank.
solution flow; these ribs support the outlet foil. The foil inflexions, created upon depressurization, form the flow dividers and, therefore, the efficient agitation of the solution, which reduces the irradiation hetero geneity across the depth. A solution with the following composition was used: 3 g AgNO3, 10 ml distilled water, and 500 ml iso propyl alcohol. The produced powders are characterized by a spe cific surface area in the range of 0.9–2.6 m2/g (Table 1); the particle size range is 219–572 nm, respectively. The specific surface area Ssp of the powders was measured by the BET method using a TriStar 3000 V6.03 analyzer. The yield of the produced powder increases with an increase in the adsorbed dose (Table 1, Fig. 10). How ever, the specific surface area of the powder decreases (Table 1, Fig. 11); i.e., the particles are coarsened and the formation of agglomerates occurs.
After ultrasonic treatment, the specific surface area increases 2–3 times. It should be noted that the silver particles obtained from the solutions with multiatom alcohol (ethylene glycol) are of finer sizes than the particles obtained from the solutions with monoatom (isopropyl) alcohol. Silver compounds were tested on thrombocytes of nearly healthy donors (14 persons in group 1) and patients with various kinds of thrombocytopathy who had a reduced aggregative ability of thrombocytes regard less of the nosological form (16 persons in group 2). The suspension of thrombocytes for analysis was obtained from venous blood stabilized by 0.11 M sodium citrate using the standard procedure. With this aim, the blood was centrifuged at 1000 rpm for 7 min and further processing was performed with the obtained plasma layer enriched with thrombocytes (thrombocyte concentration: 180–280 × 109/l). A 0.05% solution (0.1 ml) of the studied matter was
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Table 1. Experimental results on the production of silver nanopowders at various modes of irradiation Accelerator frequency, Hz
Irradiation time, s
3.3 10 20 50
45 45 45 45
Adsorbed dose dur Normalized value ing the residence of powder weight time of solution in per solution vol chamber D, kGy ume, mg/cm3 2.5 7.4 15 37.4
0.03 0.09 0.10 0.29
Productivity, mg/kGy
Specific surface area Ssp, m2/g
0.22 0.15 0.11 0.12
2.6 – 0.99 0.83
Table 2. Values of optical aggregation of thrombocytes ADP
Collagen
Ristomycin
Group 1 with initially normal values of optical aggregation of thrombocytes (n = 16) Initial specimen 70.8 ± 1.7 62.5 ± 4.6 78.8 ± 1.3 Specimen with Ag nanoparticles 70.6 ± 2.6 62.8 ± 6.2 80.3 ± 1.2 Variation of aggregation extent, % ↓0.3 ↑0.5 ↑2 Confidence index (p < 0.05) 0.948 0.97 0.398 Group 2 with initially low values of optical aggregation of thrombocytes (n = 14) Initial specimen 33.6 ± 3.5 44.7 ± 6.3 61.9 ± 4.3 Specimen with Ag nanoparticles 55.5 ± 4.7 50.4 ± 7.5 77.6 ± 2.1 Variation of aggregation extent, % ↑65.2 ↑12.8 ↑25.4 Confidence index (p < 0.05)
