Preparation of Copper Sulfide and Gold Nanoparticles Dispersed in

Chapter 14

Preparation of Copper Sulfide and Gold Nanoparticles Dispersed in Hydroxypropylcellulose—Silica Film with Gas Diffusion Method

Downloaded by UNIV OF SYDNEY on January 27, 2014 | http://pubs.acs.org Publication Date: March 12, 1996 | doi: 10.1021/bk-1996-0622.ch014

W. Yang, H. Inoue, Y. Nakazono, H. Samura, and T. Saegusa Advanced Materials Laboratory, Kansai Research Institute, Kyoto Research Park 17, Chudoji Minami-machi, Shimogyo-Ku, Kyoto 600, Japan A new method for preparation of silicafilmscontaining dispersed nano particles of CuS and Au is reported. These particles were generated through reaction with H S gas with their precursors which were uniformly dissolved in silica films as promoted by a compatibilizer, hydroxypropyl cellulose(HPC). All films with characteristic colors were transparent. The particle size, size distribution and composite structure of the films depended on the precursor concentration and matrix composition. Electric conductivity was observed for the CuS embeddedfilmwith the value of 2.8X10 Ω/. Au particles dispersed homogeneously in HPC-silicafilmwith narrow radius distribution were prepared from HAuCl by H S gas treatment. Au particles with diameter of as small as 3 nm were prepared with this method. 2

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Nano-composite materials with fine semiconductor particles dispersed in the matrix have attracted considerable interest because the properties of the particles are much different from their bulks when the diameters are less than the Bohr exciton radius. Such particles, which are generally named as nano-particles, are characterized by non-stoichiometric surface structure and quantum size effect * ). These properties would lead to new phenomena, new theoretical insights, and new materials and devices. Due to the surface of nano-particle is very active, it is difficult to prepare stable nano-particles without aggregation. To clear this problem, techniques such as addition of surface active agents, or modification of the particle surface, have been developed. With these methods, stable nano-particle colloids of CdS, PbS, ZnS, Fe C^, ZnO, CdSe, etc. were obtained - ). Another approach to prepare nano-particles dispersed materials is to synthesize nano-particles in solid matrices. In this case, precursors are dissolved in the matrices and converted to nano-particles by chemical reaction. The formation of particles in solid is dominated by the solid matrix network sterically. According to this, it is expected that very small particles with narrow radius distribution could be prepared. The distribution of the particles can also be designed by choosing different matrices. For example, a thin film of silica with layer structure was prepared by converting 1

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0097-6156/96/0622-0205$15.00/0 © 19% American Chemical Society

In Nanotechnology; Chow, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

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tetraethoxysilane (TEOS) to silica in L B membrane ), and hybrid materials with clay as matrix were also reported * ). Using gas diffusion method, CdS particles dispersed in organic polymers such as poly(acrylonitrile-styrene) were synthesized from its precursor. Their nonlinear optical properties were studied ). Homogeneous inorganic matrices such as silica have the advantage of durability and transparency, which are important in the application of electronics or optical devices. To achieve this, sol-gel process is an effective method, in which silica sols containing nano-particle precursors are used as starting materials to form films. Using this method, silica films containing semiconductor nano-particles such as chalcogenides could be fabricated. However, in our preliminary study aimed at preparation of chalcogenide nano-particles ), it was found that when metal salts were used as precursors, they precipitated in the silica films after drying. As a result, chalcogenide particles formed by 62S gas treatment were characterized with large particle size and broad size distribution. The particle size broadening is mainly due to the inhomogenous distribution of the precursors in the silica matrices. One method to narrowing the particle size distribution is to choose a matrix which can uniformly distribute the precursors. Our basic consideration to solve this problem, is to add a third component to compatibilize the precursor with silica matrix. It was found that organic polymers with hydroxyl group such as HPC were effective in preparation of homogenous organic polymer-silica films containing precursors such as CuCl2, Pb(CH COO) , Cd(CH COO) , etc.. Upon treating with H S gas, these precursors could be converted to the corresponding chalcogenide particles in diameter less than 10 nm . In this paper, we report the preparation, structure characterization and properties of CuS and Au nano-particles dispersed in HPC-silica films with gas diffusion method. 7

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Experiment The procedure for preparation of HPC-silica films containing CuS and A u nanoparticles is shown in Figure 1. Silica sol F{PC CuS (or Au) precursor HPC-Silica matrix

CuS (or Au) particle

I—H Glass plate Dipping solution

Film formation

H2S gas treatment

Figure 1 Procedure for preparation of HPC-silica film containing CuS and Au nano-particles Generally, a dipping solution for preparation of CuS precursor films was prepared by dissolving C u C l , AICI3 and silica sol in 100 ml of methanol containing 2


14. YANG ET AL.

Preparation of CuS and Au Nanoparticles

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0.5 g of H P C by the amount of corresponding to the composition listed in Table 1 and 2. Dipping solution for Au precursor film was prepared by dissolving 0.5 g of HAuCU* 4 H 0 and 0.29 g of silica sol (nominal S1O2 weight) in 100 ml of methanol containing 0.5 g of HPC. The silica sol was prepared from hydrolysis of tetraethoxysilane (TEOS) with a procedure described as following: To 10 g of TEOS, 2.1 g of water, 1.5 g of methanol and 100 mg of hydrochloric acid(35%) were added. The mixture was stirred at room temperature. After a few minutes the mixture became homogenous, the solution was stirred for further 1 hr to give the silica sol solution. Precursor films were formed on glass plates by dipping method. The films were dried immediately by air flow at room temperature. The number of dip was one and the film thicknesses were about 0.1 μπι. The CuS nano-particles dispersed films were obtained by exposing the precursor films to H S gas for 1 to 2 seconds. C u C l was converted to CuS and the color of the films changed from light yellow to blue. The films were heated at 150 °C under vacuum for 15 to 30 minutes before characterization. The Au nano-particles dispersed films were prepared by both H S gas treatment and thermal decomposition. In the case of H S gas treatment, the precursor film was exposed to H S gas for 1 to 2 seconds and heated at 150 °C under vacuum for 1 hr. The thermal decomposed one was prepared by heating the precursor film at 150 °C under vacuum for 1 hr. Self supporting films for transmission electron microscope(TEM) observation were prepared as following: The nano-particles dispersed films were generated on the glass plates coated with thin polycarbonate (PC) film. The PC film was removed with chloroform to give the self supporting films suspended in chloroform. The particle size, size distribution and composite structure of the films were characterized by T E M (H-7100). Surface electric conductivity of the CuS dispersed films was measured using a MCP-T350 four probes surface resistance meter. UVVisible absorption spectra were recorded with a Ubest-50 UV-Visible spectrophotometer.


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Results and Discussion CuS nano-particles. CuS can generally be synthesized by passing H S gas through a solution of copper(II) salt On the other hand, if H S gas is allowed to diffuse into a solid matrix containing copper(H) ions, precipitated CuS crystal particles yield. The formation of particles can be generally divided into two steps: core formation step and particle growth step. Since the diffusion of copper(II) ions which govern the particle growth is suppressed by the matrix network, this method is expected to be useful for preparing CuS crystals with small radius. The particle size distribution is considered to be mainly attributed to the inhomogeneous distribution of copper(II) ions in the matrix. In order to narrow the particle size distribution, it is necessary to choose a matrix which can uniformly distribute copper(II) salt In this study, we found that HPC is one of the effective matrices which dissolves CuS precursors such as C u C l in solid state. HPC is the derivative of cellulose with chemical structure shown below: 2

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Figure 2

Chemical structure of HPC


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Owing to the hydroxyl and ether groups in the polymer, H PC is also compatible with silica through hydrogen bonds "* ) to form molecular hybrid. This means that H P C can work as a compatibilizer to disperse CuCfe in silica matrix of molecular level, which is the key step in preparation of CuS nano-particles dispersed films containing silica. It is emphasizing that, like other organic-inorganic hybrid materials, H P C silica hybrid is a multifunctional material offering a wide range of interesting properties from its organic and inorganic components, which can be easily controlled by changing the components composition. The effect of H P C to increase the solubility of CuCl2 was examined with polarized microscope observation of the resulting precursor films. Upon reaching the solubility limit, CuCl2 crystals precipitated inside the films were observed. The results are summarized in Table 1. Downloaded by UNIV OF SYDNEY on January 27, 2014 | http://pubs.acs.org Publication Date: March 12, 1996 | doi: 10.1021/bk-1996-0622.ch014

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Table 1 Solubility of C u C l in HPC, silica films Film composition 2

No 1 2 3 4 5

HPC g 0 0.256 0.633 1.0 0.505

SiQ2 g 1.0 0.744 0.376 0 0.293

Solubility limit

A1C1 g

CuCl - 2 H 0 g

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Preparation of Copper Sulfide and Gold Nanoparticles Dispersed in

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