Two-Step Preparation of Silver Nanoparticles

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The KBr/AgNO3 ratios were 2, 5, 10, 20 and 50 in the both cases to easy comparison of obtained results. Synthesized silver bromide nanoparticles were.

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Journal of Scientific Conference Proceedings Vol. 3, 1–4, 2011

Two-Step Preparation of Silver Nanoparticles Petr Suchomel1 , Robert Prucek1 2 , Ales Panacek1 2 , and Libor Kvitek1 2 ∗ 1

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Department of Physical Chemistry, Palacky University, 771 46 Olomouc, Czech Republic Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Palacky University, 17. Listopadu 12, 77146 Olomouc, Czech Republic

The silver colloidal dispersions were obtained by two-step preparation method. The first step was based on preparation of the colloidal dispersions of silver bromide nanoparticles (NPs), which were reduced by sodium borohydride in the second step. The effective diameters of silver bromide nanoparticles and silver nanoparticles were determined by using of dynamic light scattering (DLS) and transmission electron microscopy (TEM) techniques. The colloidal dispersions of silver bromide nanoparticles were prepared by the reaction of silver nitrate and potassium bromide solutions mixed in various ratios. The influence of KBr/AgNO3 ratio to particle size was verified by using of two preparation ways. The first preparation way was based on using of the various concentration of potassium bromide and constant concentration of silver nitrate in the reaction systems. In the second case, the various concentration of silver nitrate and constant concentration of potassium bromide in the reaction systems were used. The KBr/AgNO3 ratios were 2, 5, 10, 20 and 50 in the both cases to easy comparison of obtained results. Synthesized silver bromide nanoparticles were reduced by sodium borohydride to silver nanoparticles with size between 29 and 50 nm.

Keywords: Silver, Silver Bromide, Nanoparticles, Preparation, Reduction, Sodium Borohydride, Particle-Size.

The nanomaterials are one of the most discussed branches of the scientific research of the several last decades. Very important group of nanomaterials represent the nanoparticles of silver. The potential applications of silver nanoparticles are connected with their physical, chemical and biological activity. Because of these properties, the silver nanoparticles can be used as catalyst in heterogeneous catalysis, for example to oxidation of ethylene to ethylenoxide,1 2 or to the reduction of aromatic nitro compounds.3 Next field of application of Ag NPs poses the surface enhanced Raman spectroscopy (SERS) and surface enhanced Raman resonance spectroscopy (SERRS), where modified silver nanoparticles causes the enhancement of Raman signal.4 5 Maybe one of the most important applications of the Ag NPs is the biological application because of their antibacterial activity.7 8 Silver nanoparticles can be prepared by many different methods. The most frequently used method of Ag NPs preparation is the wet chemical condensation method based on the Ag(I) compounds reduction. The soluble ∗

Author to whom correspondence should be addressed.

J. Sci. Conf. Proc. 2011, Vol. 3, No. 1

Ag(I) compounds can be reduced by many different reducing agents. The commonly used reductants are sodium borohydride, ascorbic acid or reducing saccharides, such as maltose or glucose.9–13 Additionally, silver NPs can be prepared also via reduction of insoluble silver compounds as is silver oxide, which can be reduced electrochemically14 or silver bromide, which is obviously reduced to Ag NPs with strong reducing agents as is sodium borohydride.15 However, these preparation methods are not so widely used in practice as the formerly mentioned methods. Therefore, this work is focused on the preparation of colloidal dispersions of AgBr nanoparticles and their reduction to colloidal dispersions of Ag nanoparticles and study of size of formed particles depending on concentration of reaction compounds.

2. EXPERIMENTAL DETAILS 2.1. Preparation of Silver Bromide Colloidal Dispersion The silver bromide colloidal dispersions were prepared by mixing of silver nitrate (p.a., Tamda a.s.) and potassium bromide (pure, Lachema) solutions. The potassium bromide solution was added into the silver nitrate solution

1937-6456/2011/3/001/004

doi:10.1166/jcp.2011.1094

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1. INTRODUCTION

Two-Step Preparation of Silver Nanoparticles

with vigorous stirring at magnetic stirrer. The influence of various KBr/AgNO3 ratios on the size of the produced silver bromide particles was studied by using of two different synthetic ways. In the first way, the constant concentration of silver nitrate and various concentration of potassium bromide in the reaction systems were used. The final concentration of AgNO3 in the reaction system was 1 · 10−3 M and concentration of KBr was varied in the range 2 · 10−3 – 5 · 10−2 M. In the second case the solution with constant concentration of potassium bromide (final concentration in the reaction system 2 · 10−3 M) was mixed with various concentration of silver nitrate. Its final concentrations in the reaction system was varied in the range 1 · 10−3 – 5 · 10−5 M. To easy comparison of results, obtained in both types of experiments, the KBr/AgNO3 ratio was adjusted to values of 2, 5, 10, 20 and 50. The size of the prepared AgBr nanoparticles was determined using of dynamic light scattering (Zeta Plus, Brookhaven Instr. Co., USA) and transmission electron microscopy (JEM 2010, Jeol Ltd., Japan) techniques.

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2.2. Reduction of Silver Bromide NPs to Silver NPs Prepared silver bromide colloidal dispersions were reduced by sodium borohydride (pure, Janssen Chimica), which was chosen as the best reducing agent for this purpose. Solution of NaBH4 , prepared immediately before using was added into the AgBr NPs colloidal dispersions with vigorous stirring of the reaction system at magnetic stirrer. The final concentration in the reaction system of NaBH4 was adjusted to 1 · 10−2 M. Ten minutes after addition of NaBH4 solution to silver bromide dispersion, the samples of prepared Ag NP dispersions were studied using of DLS and also by TEM. All reported experiments were realized at laboratory temperature at about 25  C without any preservation of the precise reaction temperature.

3. RESULTS AND DISCUSSION 3.1. AgBr NPs The silver bromide nanoparticles were prepared by two reaction ways to observe an effect of the reaction conditions on the produced silver bromide particles. The results of the realized experiments presented in Table I show, that effective diameter of the prepared silver bromide particles highly depended on KBr/AgNO3 ratio, however the sequence of the reaction component addition has only a little influence on the size of the prepared silver bromide particles. The increasing KBr concentration at the reaction system with constant concentration of AgNO3 , and decreasing 2

Suchomel et al. Table I. The influence of KBr/AgNO3 ratio on the particle size of AgBr particles prepared at the (a) constant concentration of AgNO3 and various concentration of KBr, (b) constant concentration of KBr and various concentration of AgNO3 in the reaction system. KBr/AgNO3 ratio 2 5 10 20 50

d AgBr (nm) constant concentration of AgNO3 53 59 90 121 232

d AgBr (nm) constant concentration of KBr 55 56 84 105 151

AgNO3 concentration in the system with constant concentration of KBr leaded to the particles with bigger effective diameter. The increasing size of AgBr particles in the presence of the excess of KBr can be explained by the formation of AgBrx−1− complexes, whose stability increase x with increasing number of Br− ions in comparison with number of Ag+ ions in the solution. Increasing solubility of silver bromide compounds leads to the formation of fewer amounts of nuclei, which are important for crystallization. And finally, fewer amounts of nuclei cause the formation of fewer amounts of bigger AgBr particles. This hypothesis can explain increase of effective diameter of prepared silver bromide nanoparticles, especially in the case, when high concentration of KBr is used in synthetic step with KBr/AgNO3 ratio equal to values of 20 and 50. From the dependency of effective diameter of the prepared AgBr particles on the KBr/KNO3 ratio, plotted in Figure 1, one can see that the particle sizes of the AgBr particles prepared in both reaction ways corresponded together up to KBr/AgNO3 ratio of 10. Over KBr/AgNO3 ratio of 10, there were small differences between samples prepared in the case of constant concentration of AgNO3 and samples prepared in the case of constant concentration of KBr. These differences can be again explained by the formation of AgBrx−1− complexes. The concentrations of KBr x in the samples prepared with constant concentration of AgNO3 at KBr/AgNO3 ratio 20 and 50 were 2 · 10−2 and 5 · 10−2 M, respectively. However, in the systems, where the AgBr particles were prepared in the presence of various concentration of AgNO3 , concentration of KBr was only 2 · 10−3 M in all cases. Thus the concentration of Br− ions in samples with constant concentration of AgNO3 and various concentrations of KBr were much more higher then concentration of Br− ions in the samples with various concentration of AgNO3 and constant concentration of KBr. This higher concentration of KBr in the reaction system caused the increase of probability of formation of AgBrx−1− complexes. In this way, the increase of silx ver bromide solubility again leaded to formation of bigger AgBr NPs in samples prepared in the presence of constant concentration of AgNO3 and various concentration of KBr. J. Sci. Conf. Proc. 3, 1–4, 2011

Suchomel et al.

Two-Step Preparation of Silver Nanoparticles

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Effective diameter [nm]

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KBr/AgNO3 ratio Fig. 1. The influence of KBr/AgNO3 ratio on the particle size of AgBr particles prepared in the presence of (a) constant concentration of AgNO3 and various concentration of KBr, (b) constant concentration of KBr and various concentration of AgNO3 .

3.2. Ag NPs The reduction of prepared AgBr nanoparticles was realized by adding 5 ml of 6 · 10−2 M NaBH4 solution into 25 ml of prepared AgBr dispersion. The final concentration of

800 Ag 600

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KBr/AgNO3 ratio (b) 160 AgBr

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NaBH4 in the reaction system was then 1 · 10−2 M. The results presented in graphs in Figure 2 demonstrate the effective diameter of prepared silver nanoparticles from silver bromide nanoparticles obtained by DLS technique. Formed Ag nanoparticles were smaller than AgBr particles, which were reduced. This was caused by destroying of AgBr crystals by strong reducing effect of used NaBH4 . In the presence of constant concentration of KBr were observed approximately constant effective diameters of prepared Ag NPs between 30 and 50 nm (Fig. 3). Otherwise, in the case of samples prepared in the constant concentration of AgNO3 , the increase of Ag particle size with increasing concentration of KBr was observed. TEM images of these samples showed, that there such big nanoparticles were not presented but that aggregates of smaller particles with effective diameter around 40 nm were formed (Fig. 4). This aggregation can be explained by DLVO theory. According to this theory the interactions of particles are defined as the sum of attractive (van der Waals) and repulsive (electrostatic) interactions. The repulsive interaction is caused by existing of electrical double layer which is

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KBr/AgNO3 ratio Fig. 2. The effective diameter of Ag nanoparticles obtained by reduction of AgBr nanoparticles prepared in the presence of: (a) constant concentration of AgNO3 and various concentration of KBr, (b) constant concentration of KBr and various concentration of AgNO3 .

J. Sci. Conf. Proc. 3, 1–4, 2011

Fig. 4. TEM image of aggregated Ag nanoparticles obtained by reduction of AgBr particles prepared in the presence of 1 · 10−3 M AgNO3 and 5 · 10−2 M KBr.

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Effective diameter [nm]

(a) 1000

Fig. 3. TEM image of Ag nanoparticles obtained by reduction of AgBr particles prepared in the presence of 1 · 10−3 M AgNO3 and 1 · 10−2 M KBr.

Two-Step Preparation of Silver Nanoparticles

responsible for surface charge of particles, which stabilize the colloid system.16 Addition of electrolyte causes the increase of ionic strength I, which leads to compression of electrical double layer according Debye–Hückel equation, where the Debye length of the electrical double layer −1 is dependent on the ionic strength of the dispersion:17  r o kT −1 (1)  = 2e2 NA I The compression of electrical double layer enable to move particles closer by van der Waals interactions, system loose his stability and the particles can connect into bigger aggregates.

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4. CONCLUSION The realized study demonstrates facile preparation of silver bromide particles with various effective diameters, which can be effectively transformed into silver nanoparticles. The performed experiments shows, the size of prepared silver bromide nanoparticles highly depends on a reaction compounds ratio and furthermore on concentration of potassium bromide in the reaction system. The particle size of prepared AgBr increases with increasing ratio of KBr/AgNO3 . Following reduction of prepared silver bromide particles enables preparation of silver nanoparticles with effective diameter around 40 nm. The obtained experimental data shows, that stability of prepared silver nanoparticles depend on concentration of Br− ions in reaction system. In the excess of Br− ions nanoparticles quickly aggregate. Acknowledgments: This work has been supported by the Operational Program Research and Development for Innovations—European Social Fund (CZ.1.05/2.1.00/

Suchomel et al.

03.0058), by the Ministry of Education of the Czech Republic (1M6198959201) and by Internal Grant of Palacky University Olomouc (PrF_2011_020).

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Received: 15 May 2011. Accepted: 6 July 2011.

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