공침법과 졸-겔법에 의한 투명 전기 전도성 코팅용 Indium Tin Oxide(ITO) 나노 입자의 제조

조영상□Shi-Zhong Wang·홍정진↕·장성훈↕·양승만

한국과학기술원 화학공학과

↕LG화학 기술연구원 신소재연구소

Preparation of Indium Tin Oxide(ITO) nanopowder by coprecipitation and sol-gel method for electrically conductive transparent coating

Young-Sang Cho, Shi-Zhong Wang, Jeong-Jin Hong↕, Sung Hoon Jang↕, and Seung-Man Yang

Department of Chemical Engineering, KAIST, Taejon 305-701, Korea

↕Advanced Materials Research Institute, LG Chem. Research Park, Taejon 305-380, Korea

Introduction

Transparent conductive oxides (TCO) coated on glass substrate are important materials in the field of optoelectronics devices such as solar cells, and liquid crystal displays. Among the various TCOs, tin-doped indium oxide (ITO) are widely used because it has the high figure of merit compared with the other n-type TCOs such as antimony-doped tin oxide (ATO) [1]. One of the important application of ITO is electrically conductive coating on CRT to shield electromagnetic radiation. There are several methods to deposit ITO on glass substrate such as CVD, sputtering, vacuum deposition, and sol-gel process. Since these methods are not suitable for mass production of coating, spin coating of redispersed ITO sol is now widely used for industrial purposes. In this study, to deposit redispersed ITO sol on CRT panel, ITO particles are prepared by coprecipitation and sol-gel method. The electrical and optical properties were examined using ITO particles prepared by both coprecipitation and sol-gel method.

Experimental

Materials: Indium chloride tetrahydrate[InCl3·4H2O, Kanto Chemicals, 99.95%] and tin chloride pentahydrate[SnCl4·4H2O, Kanto Chemicals, 98%] as inorganic reactants and ammonium hydroxide[NH4OH, Aldrich, NH3 28∼30%] as a precipitating agent were used as received to prepare ITO particles by coprecipitation method. Indium acetate[In(CH3CO2)3, Aldrich, 99.99%] and tin acetate[Sn(CH3CO2)3, Aldrich] as precursor materials and ammonium hydroxide  as a basic catalyst were used as received to prepare ITO particles by sol-gel method. Regenerated cellulose membrane[MWCO(molecular weight cut-off) 1000, Spectrum Laboratories Inc.] was used to remove organic residues from indium tin hydroxide solution by dialysis. The mixture of ethanol[Merck, 99.9%] and DMF [N,N-dimethylformamide, HCON(CH3)2, Duksan Pharmaceutical Co. Ltd., 99.9%]  was used as a dispersion medium of ITO sol.

Preparation of ITO nanopowder: Both in the coprecipitation and sol-gel method, 0.2 to 0.23 M of indium and 0.021 to 0.022 M of tin precursors were dissolved in distilled water to keep 9:1 atomic ratio of indium/tin, which has been reported to show a maximum electrical conductivity in ITO.[2] Coprecipitation or sol-gel reaction were performed by adding 0.45 ∼0.73 M of aqueous NH4OH solution into precursor solutions slowly. These reactions were performed overnight under the vigorous agitation. In the coprecipitation method, indium-tin hydroxide particles were washed with distilled water to remove residual ions from particle surfaces. During the washing process, centrifugal force(6500∼7500 rpm for 5∼10 min) was applied to the samples. This washing and centrifugation process was repeated 5 times. In the sol-gel method, the aqueous indium tin hydroxide solution were put into dialysis membrane immersed in fresh water to remove residual ions. The distilled water outside the membrane was changed several times and the aqueous indium tin hydroxide solution was washed and centrifuged with the same condition as coprecipitation method.

The washed indium-tin hydroxide particles were collected and dried at 60∼70℃ overnight and crushed by marble mortar. Then, the indium-tin hydroxide particles were calcined at 600℃ for 2 hours in nitrogen atmosphere to yield yellow ITO particles.

Morphology of particles: The morphology of particles was studied by Energy Filtering Transmission Electron Microscopy(EF-TEM, EM912 Omega).

X-ray diffraction measurements: X-ray diffraction spectrums for indium-tin oxide powders were measured with a Rigaku D/MAX-RC. The incident wavelength was Cu K□=0.154056 nm and the detector was moved between 2□=15∼75°. The scan speed was 3。/min.

Preparation of ITO sol for CRT coating: Colloidal suspension of ITO was prepared by dispersing 10 wt. % of dried ITO powders in ethanol and DMF.

To reduce the secondary particle size of ITO particles, 0.3 mm of zirconia bead was used and ITO particles were ball-milled. After ball milling, the green ITO sol was obtained and it was stable without any precipitation. Dynamic light scattering measurements were performed with a Brookhaven instrument at a wavelength of 674 nm at 90。 of incident angle to measure the secondary particle size of ITO

Preparation of ITO coating on CRT: To deposit coatings on 17-inch of CRT panel, the ITO suspension was diluted and antistatic coating solutions were prepared. After the antistatic coating process was finished, antireflection coating was deposited as second coating layer. Both antistatic and antireflection coatings were prepared by spin coater.

Electrical resistance and reflectance of coating layer: The sheet resistance of coating layer was measured by Loresta HP(MCP-T410, Mitsubishi Chemical). The reflectance spectrum of coating was measured by Darsa Pro-5000 System(Professional Scientific Instrument).

Results and discussion

Powder characterization: Fig. 1 and 2 shows the X-ray diffraction spectrums of particles produced by coprecipitatin and sol-gel method, respectively. XRD spectrum of ITO was found to deviate slightly from that of genuine indium oxide, and was quite different from that of genuine tin oxide. This clearly showed that both coprecipitated and sol-gel ITO have the indium oxide cubic bixbyite crystal structure without any tetragonal tin oxide domain.

Fig. 1 XRD Diffraction Pattern of            Fig. 2 XRD Diffraction Pattern of

Coprecipitated ITO Powder                  Sol-gel Prepared ITO Powder

From the X-ray results, the primary particle sizes were determined by the following Scherrer's equation[3].

[eqn: t= { 0.9 lambda} over {Bcos theta subB }]

t is the primary particle size of crystallite, [eqn: lambda] is the wavelength of incident X-ray, B is the half width of maximum peak intensity, and [eqn: theta] is half of the diffraction angle of peak. According to Scherrer's equation, the calculated primary particle sizes of coprecipitated and sol-gel prepared ITO are 17.45 nm and 16.09 nm, respectively using the (222) peaks.

Fig. 3 and 4 shows TEM images of coprecipitated and sol-gel ITO, respectively. The coprecipitated primary ITO particles are aggregated into rod-shaped secondary particles, but the size of small crystallites constructing the rods are ranging from 10 to 20 nm. The ITO particles prepared by sol-gel method have no defined shape, and the primary particle sizes are ranging from 10 to 20 nm. Primary particle size from TEM image is the same with the XRD result.

Fig. 3 TEM Image of Coprecipitated    Fig. 4 TEM Image of Sol-gel Prepared

ITO Powder                           ITO Powder

The size reduction of ITO particles: Because ITO particles are very easily aggregative in ethanol, it requires 10 to 12 hours of vibratory ball milling to reduce secondary particle size of ITO less than 130 nm. If the secondary particle size is larger than 130 nm, the whitening can be observed on CRT coating layer due to Rayleigh scattering, resulting in the low transmittance. If the secondary particle size is too small, the conductivity of coating layer will decrease due to the increase of contact resistance between ITO particles.

Electrical and optical properties of coating layer: Table 1 shows the electrical properties of coating layer. It was observed that sol-gel ITO coating shows better electrical conductivity compared with coprecipitated ITO coating. Residual ions which were not removed from coprecipitated ITO deteriorate the electrical conductivity of ITO and thus results in higher sheet resistance in coating layer.

Table 1. Electrical and Optical Properties of Coating Layer on 17 inch CRT

	Resistance	Sheet Resistance (×105)	Minimum Reflectance	Wavelength at Minimum Reflectance
Coated with Coprecipitated ITO	229∼233 k□	2.1∼2.3 □/□	1.02∼1.10 %	530∼560 nm
Coated with Sol-gel Prepared ITO	129∼144 k□	1.1∼1.3 □/□	1.08∼1.19 %	530∼540 nm

Although 108□/□ of sheet resistance is satisfactory for preventing the accumulation of charge, it is necessary to form a transparent film having a low resistivity not exceeding 106 □/□, and preferably the order of 102 to 103 □/□ is desired[4]. The surface resistivity in this study is the order of 105□/□, and further study to reduce resistivity is required. About 1% of visible light reflectance in this study is commercially acceptable.

Summary and conclusion

The ultrafine ITO powders were prepared by coprecipitation and sol-gel method to apply antistatic coating on CRT. TEM images show that the primary particle size of ITO powders were ranging from 10 to 20 nm. About 1% of reflectance at the wavelength of 530-560nm was achieved with antireflection coating. The coating layer from sol-gel ITO sol had better electrical property compared to that of coprecipitated ITO sol. Further study to improve the electrical property of ITO is underway.

References

1. R. G. Gordon, MRS bulletin, August 2000, 52-57

2. G. Frank, H. Kstlin, Appl. Phys. A, 1982, 27, 197

3. B. D. Cullity, Elements of X-ray Diffraction, 2nd ed.

4. Toufuku et al., US Patent 5,785,897