Li Guoping, Luo Yunjun, Yao Weishang, Zhu Liang ,Tan Huimin
(School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081)

Received on Oct. 22, 2004; Supported by the Trans-century Training Programme Foundational for Talents by the State Education Commission.

Abstract Preparation of dendritic silver particles by direct chemical reduction of silver ions under the protection of PVP was studied in this work. Effects of molar ratio of PVP to silver ions and the reaction time on the morphology and structure of silver particles were also described. When PVP/AgNO₃=1 and the reaction time was about 1h, the well-defined dendritic silver particles were achieved. The mechanism of dendritic silver nanoparticle has been proposed. Firstly, the complex of PVP-silver ions is constructed. Secondly, the complex promotes silver nucleations. Thirdly, aggregation and limited dispersion leads to formation of dendritic silver nanoparticle.
Keywords Dendritic silver nanoparticles, PVP, Direct chemical reducation

1 INTRODUCTION   
In recent years, noble metal particles have been widely studied for their excellent properties ^[1,2], and their potential applications in microelectronics, optical, electronic, magnetic devices, and catalyst ^[2-4]. As we know, the intrinsic properties of a noble metal nanoparticle strongly depended on its morphology and structure. Silver nanoparticles with different shapes has attracted a lot of interest because silver exhibits the highest electrical and thermal conductivities among all metals. Since dendritic structure has many special characteristics, such as its large surface area, well conductivity etc. and the ability to provide a natural framework for the study of disordered system, dendritic nanostructure of noble metal is an attractive supramolecular structure. Recently, various methods have been developed for preparing dendritic noble metal nanoparticles, such as template-based method ^[5], gas evaporation method ^[6], and electrochemical deposition ^[7]. Dendritic silver nanoparticles were observed by ultraviolet irradiation photoreduction technique, or by sonoelectrochemical methods, or under microwave irradiation ^[8]. Zhu et al. used nickel nanotubes as reducing agent through a hydrothermal method to prepare dendritic silver, and discussed the important role of nickel nanotubes for dendritic growth ^[9].
    Herein, the study of formation of dendritic silver with PVP (poly (N-vinly-2-pyrrolidione)) as the stabilizer and ethanol as reducing agent has been reported. The condition and apparatus of this method are mild. It is focused on the effect of PVP/ AgNO₃ mole ration on the morphology and structure of the dendritic silver. Based on TEM and XRD, the mechanism for forming the dendritic silver will be proposed.

2 EXPERIMENTAL
All chemicals were of analytical grade and were used without further purification. Silver nitrate, ethanol were purchased from Chemical Reagent Company, Beijing, China. PVP (K30) was imported from Fluka Chemie. The above materials were dissolved in deionized water to form aqueous solution.
    In a typical process, 10 ml aqueous solution of PVP (0.1 mol/L) (pH=5.18) was added into a 100ml three-necked flask which was equipped with a condenser, magnetic stirring bar, and thermcontroller, then 5 ml aqueous solution of AgNO₃ (0.1 mol/L) was added by syringe when the solution was being stirred. After 10 minutes, 5ml ethanol was added drop by drop by syringe and the mixed solution was stirred .The reaction solution was kept under nitrogen and at 80℃ until the reduction was complete.
    UV-Vis absorption was recorded with a Unico UV2102PC UV-Vis spectrophotometer. Transmission electron microscopy and electron diffraction image of product were taken with JEM-2010 apparatus. Samples for them were prepared by dropping product onto carbon-covered cooper net, and the solvent was evaporated naturally. FT-IR spectra were observed with NEXUS-470 FTIR (Nicolet) and X-ray powder (XRD) analysis was performed on a Japan Rigaku diffractometer with Cu as target.

3 RESULTS AND DISCUSSION
3.1 Influence of PVP/ AgNO₃ mole ratio on the size, morphology of product                     
Varying amounts of the protective agent PVP were used for investigating the effect of the polymer on silver particles size and shape. Fig.1 was the UV-Vis absorption spectrum of the prepared silver nanoparticles. The absorption peak centered at about 420 nm is the characteristic peak for silver particles and is attributed to the plasmon absorption band of silver particles. With an increase in the ratio of PVP/AgNO₃ (one mole of PVP means one unit of PVP), the intensity and the maximum wavelength of the peak increased. The large change in the optical spectra implied that the dendritic silver transform to the sphere-like silver nanoparticles.

Fig. 1 UV-vis spectra of mixed solution containing 5ml (0.1M) AgNO₃ and 5ml(0.1M) PVP after reduction by methanol for 1h



Fig. 2 TEM micrographs of silver particles at the mole ratios of (a) PVP/AgNO₃=1, (b) PVP/AgNO₃=2, (c) PVP/AgNO₃=0.5, (d) PVP/AgNO₃=0, and (f) PVP/AgNO₃=3.

    Fig.2 showed TEM micrographs of the obtained silver particles. When PVP/AgNO₃=0, it revealed irregular shape and sinter-like particles with several micrometers in size (in Fig.2d). The well-defined nanoscale dendritic silver particles with pine-like shape were obtained in the case of PVP/AgNO₃=0.5, 1, and 2 (Fig.2c, a, b). But when PVP/AgNO₃=1, the as-prepared particles showed symmetric branched dendritic structures (Fig.2a). Fig.2c showed that the dendritic silver particles grown from a big cluster in the case of PVP/AgNO₃=0.5, and these some particles with several nanometers in size were seen in Fig.2b (PVP/AgNO₃=2) and the color of the dendritic silver particles were paler. When PVP/AgNO₃=3, the well-dispersed sphere-like particles were obtained (Fig.2f). This means that the particles shape and size distinctly depend on different PVP/AgNO₃ mole ratios and PVP/AgNO₃=1 is a better ratio for preparing the dendritic silver nanoparticles. Hence the PVP/AgNO₃=1 will be used in the next experiment.
    Fig.3 was the typical XRD of as-prepared dendritic silver particles (PVP/AgNO₃=1), in which four strong peaks correspond to the (111), (200), (220) and (311) of face-centered cubic (fcc) Ag (JCPDS No.4-0783).

Fig.3 XRD of the dendritic silver particles (PVP/AgNo₃=1)

Fig.4  The change of the UV-vis absorption spectra of the system as the function of time

3.2 Influence of the reaction time on morphology of product               
The stoichiometric reaction between ethanol and silver ion in aqueous solution can be written as follows:
    4Ag⁺ + CH₃CH₂OH + 5OH^-    ----->   4Ag + CH₃COO^- +4 H₂O
    So, if only ethanol was used, the reduction rate would be slow at room temperature due to low pH (pH=5.18). However the low reduction rate was so necessary to prepare the dendritic silver that had enough time for forming dendritic silver nanoparticles. Therefore the morphology of silver particles was strongly dependent on the reaction time. The intensity of the peaks around 430nm, which is assigned to the plasma resonance of Ag particles, increased and the peak shifted to the longer wavelength (red shift) with an increase of the reaction time (shown in Fig.4), showing an increase in the size of the silver nanoparticles. But when the reaction time exceeded over 70min, the intensity reduced by contraries. The effect may be explained by the changing size and morphology of silver particles. Indeed, in the present method, the well-dispersed sphere-like silver nanoparticles were obtained at early times (Fig.5a), because at this stage, little silver ions were reduced into atoms as a result of lower reducing ability of methanol and correspondingly, the amount of PVP was sufficient for surrounding the silver particles and preventing the obtained particles from agglomeration. The inset SAED pattern of silver nanoparticles indicates that these nanoparticles are single-crystallized with fcc structure. When the reaction time was prolonged to 60min, the well-defined dendritic silver particles were achieved (Fig.5c). But when the reaction time was prolonged, some precipitations appeared in the reaction solution of which the TEM image was shown in Fig.5d owing to the PVP protection effect being not complete. Moreover, Voigt et al. ^[10] reported that the dendritic pattern was formed in non-equilibrium growth, so when the reaction system gradually transformed from non-equilibrium to equilibrium with an increase of the reaction time, the dendritic pattern disappeared.
3.3 Mechanism of PVP protection and formation of dendritic silver nanoparticles
PVP（ ）has a structure of a polyvinyl skeleton with polar groups. The donated lone pairs of both nitrogen and oxygen atoms in the polar groups of one PVP unit may occupy two sp orbital of the silver ions to form a complex compound. We suppose that this is the first step of the dendrtic silver formation, which may be identified by detecting the precipitation while changing the adding order of PVP and methanol, or by comparing the absorption spectrum of the AgNO₃ solution containing PVP or not containing PVP. It is noted that pure PVP has no absorption peak in ultraviolet spectra^[2]. The exhibit of the solution containing AgNO₃ at 323nm resulted from the coordinative bonds of H₂O:Ag:OH₂. At the same Ag ⁺ concentration, the addition of PVP to the AgNO₃ solution produced a new band at 430nm assigned to well-known d-d transition for Ag⁺ of the complex PVP-Ag⁺, which showed that the nitrogen and oxygen of the PVP have a stronger coordinative field than H₂O.
  

Fig.5 Effect of reaction time on TEM images of silver particles

    The subsequent step is PVP-accelerated formation of a large amount of silver nuclei. It was found that the color of the solution changed gradually from pink at the initial reaction to violet at the end of the reaction. TEM observation showed that a large amount of small silver nuclei at the beginning of the reductive reaction were produced (in Fig. 5a).
    The third step is the formation of dendritic sliver particles. There is no doubt that PVP is a kind of polymer to prevent the silver particles from aggregation according to previous works ^[2]. So on the basis of PVP prohibiting particles aggregation, we can explain that well-dispersed sphere-like particles were produced in solution of PVP/AgNO₃=3 (Fig. 2f) and the more strongly aggregated silver particles (Fig. 2d) are due to lack of protective by PVP. Although the prohibiting effect of PVP has been seen in solutions of PVP/AgNO₃=0.5, 1, or 2, partial agglomeration also takes place as a result of incomplete covering of silver particles by PVP. Then new silver particles are produced on the surface of agglomeration and PVP may play a role of making silver particles limited dispersion. The two processes are carried out by turns as reductive reaction goes on, which at last leads to the formation of dendritic silver particles.
    In addition, an aqueous solution of dendritic silver particles stored for over 2 months at room temperature remained stable. The fine stability of the nanocomposite solution was due to PVP as protective agent for Ag particles.

4 CONCLUSION
The well-defined nanoscale dendritic silver particles with pine-like shape has been prepared by reducing silver nitrate with methanol in the presence of PVP as protective agent.
    It was found that the molar ratio between PVP and silver ions and the reaction time could play very important roles in determining the silver morphology, size and its stability.
    The mechanism of dendritic silver nanoparticle has been proposed. Firstly, the complex of PVP-silver ions is constructed. Secondly, the complex promotes silver nucleations. Thirdly, aggregation and limited dispersion leads to formation of dendritic silver nanoparticle.

REFERENCE
[1] Chen S W, Huang K, Stearns J A. Chem. Mater., 2000, 12: 540.
[2] Zhang Z, Zhao B, Hu L. J. Solid. State Chem., 1996, 121: 105.
[3] Chou K S, Ren C Y. Mater. Chem. and Phy., 2000, 64: 241.
[4] Colvin V L, Schlamp M C, Alivisatos A P. Nature, 1994, 370: 354.
[5] Xiao J, Xie Y, Tang R et al. Adv. Mater., 2001,13: 1887.
[6] Gao Y, Jiang P, Liu D et al. Chem. Phys. Lett., 2003, 380: 146.
[7] Wang S, Xin H. J. Phys. Chem. B, 2000, 104: 681.
[8] Liao X, Zhu J, Qiu X et al. J. Nanjin University (Natural Science) (Nanjing Daxue Xuebao), 2002, 38: 119.
[9] Zhu Y, Zheng H, Li Y, et al. Mater. Res. Bull., 2003, 38 : 1829.
[10] Tian Z, Liu J, Voigt J A et al. Nano Lett., 2003, 3: 89.

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