Introduction
The physical properties such as electric conductivity, magnetic, optical and mechanical characteristics of nanosized metal oxide particles are known to be substantially different from those of bulk materials. The preparation methods of nanosized metal oxide particles have been extensively studied for the precise control of the morphology at the nanometric scale. The studies for nanosized ZnO powders have been conducted due to their size-dependent electronic and optical properties, which offer possibilities for microelectronic devices [1,2]. Recently, ZnO conductive films have been used as transparent electrodes for TFT-LCD and solar cell, etc [3,4]. They give higher durability for the plasma or reduction gas atmosphere than those of ITO films. Lower electric resistivity of ≤ 1×10-3 Ω･cm and higher transparency of ≥ 80% in a range of 380~780 nm for a visible light are requested for the transparent conducting materials [5]. Nanosized ZnO powders are synthesized by using the organic precursor process for the present study. Al2O3 doping is conducted to obtain the higher conducting ZnO powder.
Experimental
Nanosized ZnO Powder Synthesis
Pulp and crystalline cellulose (Asahi Kasei Chemical) were used as precursors. A mixed water solution having a predetermined Zn content was prepared with zinc sulfate heptahydrate (ZnSO4·7H2O) of 99.9% purity and soaked in the precursor for 24 h. The impregnated mixture was dried at 70 to 110ºC. This mixture was passed through several stages of calcinations in the range of 300~600ºC.
The calcination condition was as follows: heating from room temperature to 300ºC for 3 h, maintained at 300ºC for 2 h, heating from 300 to 600ºC at a rate of 50ºC/h, maintained at 500 to 600ºC for 2 h, and cooling in furnace. The color of the resulting powder was light yellow to white. Distilled water was added to the calcined powder and then, the powder was wet milled in a pot mill for 48h, and then dried.
Al2O3 Doping
Aluminum sulfate octahydrate [Al2(SO4)3·8H2O] was added to the zinc sulfate heptahydrate aqueous solution. The calcination procedure was the same as for undoped ZnO powder. Heat treatment under 0.5% hydrogen containing argon gas atmosphere was conducted in an electric furnace at 900 or 1000ºC for 2h. The flow rate of the gas was 10ml/min.
Characterization
The morphology of the powder was examined by Scanning Electron Microscopy (SEM; HITACHI S-4700). The powders were analyzed by X-ray diffractometer (BROKERS axs D5005) using CuKα radiation (40 kV, 150 mA) with a scanning speed of 4o /min and a sampling interval of 0.02o. The particle size was calculated by BET data and also confirmed by direct observation by SEM. The pyrolysis and decomposition behaviors of impregnated precursor were monitored by simultaneous Differential Thermal Analysis and Thermogravimetric analysis (TG-DTA; V2.2A DuPont 9900) up to 1200°C, at a heating rate of 20°C /min in air. The volume resistivities of pure powders were measured by Broadband Dielectric Spectrometer (Concept-40, Novocontrol GmbH) at a frequency of 10MHz.
Result and Discussions
The experiments of the effect of dilute ratio on the raw materials, calcination temperature, precursor, and Al2O3 doping were conducted. The morphology of the particles is granular and the particles are loosely agglomerated because of the shape of micelle or microfibril in the precursor. The sizes of ZnO particles obtained are in a range of 40-290 nm, with a surface area of 3-22 m2/g. The particle size of 40 nm was obtained by selecting the following process conditions: pulp, dilute ratio of 0.2, Al2O3 doping of 2.5 mol%, and calcination temperature of 500ºC.
Three diffraction peaks due to ZnS are observed for the XRD patterns of specimens treated at 900ºC for 2h under the hydrogen containing atmosphere. These diffractions peaks, however, are not observed at 1000ºC. On the other hand, ZnO and ZnAl2SO4 peaks are observed. Small amount of sulfur compounds such as ZnSO4 and ZnS are decomposed to ZnO in the range of 750 to 1000ºC. It was reported that the volume resistivity increased when M2O3(M:Cr,Al) was added to ZnO. [Al2(SO4)3･8H2O] was selected as a doping agent in this experiment. The elements of Al, S, Zn, O, C were analyzed by EDX method as shown in Figure 1. 2.45 mol% Al2O3 in ZnO powder was analyzed by ICP (Induction Coupled Plasma) method after firing at 1000oC.
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Figure 1. EDX mapping analysis of 2.5 mol% Al2O3 doped ZnO powder heat treated at 1000ºC.
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Figure 2 shows the SEM micrograph of reduction treated ZnO powders. It is observed that a particle size of Al2O3 doped ZnO powder increases with the increase of calcination temperature. As shown in Figure 1(b), some particles treated at 900ºC are growing to 500nm in size. On the other hand, the other particles remain at a size of around 40nm.
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Figure 2. SEM micrographs 2.5 mol% Al2O3 doped ZnO powders (a) heat treated at 900ºC and (b) heat treated at 1000ºC.
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Table 1 shows the volume resistivity of Al2O3 doped ZnO powders. The volume resistivities of non-doped ZnO powders give the same order of value of 107Ω･cm regardless of the calcination temperature. The resistivity can drop down to 103Ω･cm after heat treatment at 1000ºC. This value is the almost same value of commercial product. This low resistivity is probably due to the formation of lattice vacancy in the zinc oxide crystal structure. The volume resistivity of Al2O3 doped ZnO powder increased with the increases of the Al content and reduction treatment temperature.
Table 1. Volume resistivities of ZnO powders prepared by precursor and vaporization processes.
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Precursor Process
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40 nm
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Al2O3 doped,
calcined at 600ºC
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2.8×105
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70 nm
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Al2O3 doped,
Reduction treatment at 800ºC
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3.1×104
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70 nm
+ 500 nm
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Al2O3 doped,
Reduction treatment at 1000ºC
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7.0×103
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Vaporization process
(Commercial product)
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0.3 μm
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Al2O3 doped,
Reduction treatment at 1000ºC
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1.8×103
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0.3 μm
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No doping,
calcined at 600ºC
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8.7×107
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0.3 μm
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No doping,
calcined at 800ºC
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6.0×107
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0.3 μm
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No doping,
calcined at 1000ºC
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5.5×107
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Conclusions
Our organo polymeric precursor process enables the size-selective preparation of nanosized ZnO powders over the range of 40 to 200 nm. It is found that the formation of ZnO crystal phase is generated at 370ºC, and the crystallization of ZnO is completed at about 1000ºC through the XRD, SEM and TG-DTA analyses. The crystal sizes of nanosized ZnO powders show an increasing tendency with the increase of calcination temperatures from 600 to 1000ºC. Using Al2O3 doping, ZnO powders give low electric resistivity of the order of 103 Ω･cm and decrease by four orders of magnitude in comparison with that of the corresponding Al2O3 non-doped ZnO powders.
References
1. S. Monticone, R. Tufeu, and A.V. Kanaev, “Complex Nature of the UV and Visible Fluorescence of Colloidal ZnO Nanoparticles” J. Phys. Chem. B, 102 (1990) 2854-2862.
2. E. A. Meulenkamp, “Synthesis and Growth of ZnO Nanoparticles”, J. Phys. Chem. B, 102 (1998) 5566-5572.
3. H. Rensmo, K. keis, H. Lindström, S. Södergren, A. Solbrand, A. Hagfeldt, S.-E. Lindquist, L. N. Wang and M. Muhammed, “High Light-to-Energy Conversion Efficiencies for Solar Cells Based on Nanostructured ZnO Electrodes”, J. Phys. Chem. B, 101 (1997) 2598-2601.
4. S. Chen, R.V. Kumar, A. Gedanken and A. Zaban, “Sonochemical Stnthesis of Crystalline Nanoporous Zinc Oxide Spheres and Their Application in Dye-Sensitized Solar Cells”, Israel J. Chem., 41 (2001) 51-54.
5. T. Minami, “Zinc Oxide Transparent Conducting Thin Films”, Jpn. J. Appl. Phys., 61 (1992) 1255-1258.
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