GENERATION AND CHARACTERIZATION OF SINTERED ZnO NANOPARTICLES

Transcription

1 Sustain. Environ. Res., (6), (1) 397 (Formerly, J. Environ. Eng. Manage.) GENERATION AND CHARACTERIZATION OF SINTERED ZnO NANOPARTICLES Yao-Chuan Lee, 1 Chen-Ting Lien, 2 Chun-Wan Chen, 3 Sheng-Hsiu Huang, 4 Chih-Chieh Chen, 5 Pei-Chen Kuo, 2 Jin-Yuan Syu, 2 Yuan-Yi Chang, 2 Yi-Mim Huang 2 and Wen-Yinn Lin 2, * 1 Department of Safety, Health and Environmental Engineering Tungnan University Taipei 222, Taiwan 2 Institute of Environmental Engineering and Management National Taipei University of Technology Taipei 16, Taiwan 3 Institute of Occupational Safety and Health Council of Labor Affairs Taipei 13, Taiwan 4 Institute of Environmental Health National Taiwan University Taipei 16, Taiwan 5 Occupational Medicine and Industrial Hygiene National Taiwan University Taipei 16, Taiwan Key Words: Nanoparticles, evaporation/condensation, zinc oxide, sintering ABSTRACT Zinc oxide (ZnO) nano-powders are widely applied as photocatalytic and luminiferous materials for deodorization and as antibacterial treatment component. A nanoparticle generation system has been assembled and tested in the work. A high temperature furnace was applied to generate ZnO nanoparticles by evaporation-condensation method. A quartz tube through the primary furnace served as a reactor, and commercialized zinc powders were put in a quartz boat. The zinc powders were evaporated and carried by nitrogen gas and subsequently reacted with oxygen supplied by compressor air. The gas stream with ZnO nanoparticles was then introduced into the secondary furnace for sintering the particles. This method provided a simple and stable way to generate high concentration ZnO nanoparticles. The morphology of synthesized ZnO nanoparticles was observed through transmission electron microscopy. The size distribution showed that the mode of ZnO nanoparticles in the size range -1 nm could be generated under different conditions by the system monitored by a scanning mobility particle sizer. Furthermore, the mode variation of sintering effect showed that smaller ZnO nanoparticles were more easily sintered and then the mode reduced, i.e., from 45 to 17 nm (62%) compared with 75 to 6 nm (%). However, the sintering effect of ZnO particles was not apparent for size larger than 1 nm judged by the almost unchanged mode. The morphology of ZnO nanoparticles tended to be more spherical when the sintering temperature increased. Therefore, various morphologies of ZnO nanoparticle could be produced by this system. ZnO nanoparticles should be further researched for the toxicity of inhalation and cytotoxicity of nanomaterials. INTRODUCTION Zinc oxide (ZnO) is a highly degenerate semiconductor with a wide band gap of 3.22 ev [1]. ZnO, due to its specific chemical, surface and microstructural characteristics, can be widely applied to chemical, medical, optical, electronic and textile products for its photocatalytic or UV light absorption characteristics to take advantage of its excellent luminiferous, deodorization and antibacterial capability [2]. The size *Corresponding author wylin@ntut.edu.tw

2 398 Sustain. Environ. Res., (6), (1) and the morphology of ZnO have great effects on the properties. Many different-shaped ZnO structures have been prepared in order to fit different applications in nano- or micro-scaled optoelectronic devices, sensors, and networks [3-7]. It also offers low cost, mild reaction conditions, high photochemical reactivity, and low environmental toxicity, while affording the use of sunlight [8-1]. Since the late 199s, there has been increasing interests in producing specific nanoparticles as a basis for novel nanostructure materials and devices. In order to obtain the desired materials, various methods have been developed for the generation of target particles. Most of these particles were made by hot processes such as flame synthesis, thermal metal spraying and furnace methods, which led to the generation of metal and/or metal oxide particles with small particle sizes [11-15]. In comparison with other high temperature methods, one of the advantages of flame and furnace method to generated particles is agglomerates of particles in nanometer size. Furthermore, flame and furnace are simple and widely used at the laboratory scale, and can be easily modified, allowing a comparison of the effect of different parameter modifications on the final particle size distribution and number concentration. Bulk material evaporating and condensing in an inert gas environment is a widely used approach to produce aerosols which maintain at a relative constant and high concentration over a specified time [16]. Using a furnace, primary non-agglomerated spherical particles below 1 nm in mobility diameter may be produced through homogeneous nucleation by quenching the hot aerosol flow [17]. These particles in turn can be used to generate spherical or fractal-like nanoparticles of sizes smaller than 1 nm through coagulation, depending on generation factors [18,19]. The particle size generated by evaporation/condensation method is primarily determined by different temperature profile, flow rate of carrier gas and location of the boat for bulk material along the reaction tube [15]. Some researchers have used silver particles generated through homogeneous nucleation and coagulation in furnaces to study the mobility diameter of diffusion-grown aggregates in sizes ranging up to 1 nm []. In this research, ZnO nanoparticles were produced by high-temperature furnace using commercialized zinc powder as the raw material and oxygen in the dry filtered air as the reaction gas. The aim of this study was to assemble an aerosol generation system and to demonstrate that it could be capable of producing ZnO nanoparticles from to 1 nm. Moreover, the sintering effect at different temperature could affect the properties of ZnO nanoparticles in the secondary furnace. The generation and characterization of sintered ZnO nanoparticles were investigated. METHODS The target of this study was to characterize an aerosol generator capable of producing classified particles with mobility diameter from to 1 nm, with morphologies ranging from fractal-like to spherical. The experimental setup in this study consisted of three subsystems including the primary ZnO nanoparticle generation furnace, the secondary furnace for particle sintering and particles monitoring system. The experimental scheme is shown in Fig Nanoparticle Generation System The furnace consisted of a 54 cm long, 46 cm width, 68 cm height, and 3 cm heat zone up to 15 C maximum temperature. Fixed-weight commercialized zinc powders were placed in a quartz boat located in the middle of quartz reaction tube and heated to temperature at approximately 5 C in the primary furnace to evaporate the zinc (melting point = 4 C). The outer/inner diameters and length of the quartz tube were 37 mm, 3 mm and 1 m, respectively. Zinc vapor was carried through the reaction tube by inert nitrogen gas supplied by cylinder gas. The reaction air supplied by air compressor was injected after the Zn evaporation zone to form ZnO particles. 2. Sintering of ZnO Nanoparticles The generated ZnO nanoparticles from primary furnace were then introduced into the secondary furnace for sintering. A quartz tube similar to the tube in the primary furnace was served as sintering zone. Furthermore, the residence time of the gas stream in the sintering zone was 7 s. Different sintering temperature from 5 to 1 C was executed. 3. Particles Measurement Systems The size distribution and concentration of ZnO particles generated were monitored at outlet of the secondary furnace by scanning mobility particle sizer, SMPS (TSI Inc., St. Paul, MN). The morphology variation of ZnO nanoparticles after sintering process was examined with transmission electron microscopy (TEM) photograph. RESULTS AND DISCUSSION 1. Generation of ZnO Nanoparticles The particle sizes which were produced by evaporation/condensation method were influenced by many factors, including furnace temperature and flow rate of carrier gas and reaction air in primary furnace. Hence, different factors were used in this research to generate target ZnO nanoparticles.

3 Lee et al.: ZnO Nanoparticle Generation 399 Air TEM Sampler HEPA Compressor HEPA Rotameter Quartz tube Exhaust Joint Sintering furnace Coagulation chamber Generating furnace Sampling tube HEPA N 2 Rotameter SMPS EEPS DMA CPC Personal computer Fig. 1. Schematic experimental setup (HEPA: High Efficiency Particulate Air; EEPS: Engine Exhaust Particle Sizer; DMA: Differential Mobility Analyzer; CPC: Condensation Particle Counter) The temperature profile of evaporation zone is important with respect to nanoparticle generation. In particular, in the evaporation/condensation method, a high temperature near the melting point of the source material is required. Figure 2 shows the temperature responses as a function of time for furnace and quartz tube during the heating up stage. The time taken to reach the target temperature was about 15 min. After about 15 min, the temperature of bulk zinc powder in the boat, which was closely related to the evaporation rate of the source material, was maintained at constant. Hence, the method was appropriate for the stable generation of nanoparticles. The flow rate of nitrogen and reaction air in theprimary furnace affects the Zn vapor concentra- Fig. 2. Temperature responses as a function of time for furnace and quartz tube. Fig. 3. Relationship between mode of ZnO particle with carrier gas and reaction air flow rates (Oven temperature: 5 C). tion in the tube, which is important to the vapor condensation and coagulation rate. The particle size is determined by these two factors primarily. Figure 3 shows the relationship between particle size (denoted by mode of the size distribution) and flow rate of nitrogen as well as reaction air at a specified operating condition. The experimental results suggest that both at higher carrier gas and reaction air flow rates result in lower particle mode size. These trends are similar to those reported by others [17,19]. It is likely that the temperature of bulk zinc powder in the boat and particle collision frequency would be decreased at higher flow rate rendering smaller particle size.

4 Sustain. Environ. Res., (6), (1) Total concentration (1 5 # cm -3 ) Total concentration Mode Mode (nm) Temperature ( C) Fig. 4. Relationship between temperature and total number concentration and mode of particle size. (N 2 = 1 L min -1, reaction air = 2 L min -1 ). The temperature in furnace is an important factor in determining the vapor concentration and temperature gradient. Figure 4 plots show the relationship between temperature (48-53 C) with mode and yield of ZnO nanoparticles. The experimental results suggest that higher temperatures result in larger particle size and higher total number concentration. These results are similar to those reported by Scheibel and Porstendorfer [15] and Jung et al. [21]. It is likely that the zinc vapor in the reaction tube and particle collision frequency would be increased in higher temperature resulting in larger particle size. The size distribution of ZnO nanoparticles is influenced by furnace temperature, reaction air flow rate and nitrogen flow rate. It is easily to manipulate these conditions to generate ZnO nanoparticles at anticipated target size in the range of -1 nm. The size distribution curves of ZnO nanoparticles with ten nanometer mode difference are shown in Fig. 5. The corresponding generation parameters of the system are summarized in Table Sintering Characterization of ZnO Nanoparticles The characteristics of the sintering of ZnO nanoparticles were investigated with respect to size change and morphology of ZnO nanoparticles at different Fig. 5. The size distribution of ZnO nanoparticles at difference conditions. sintering temperatures based on SMPS data and TEM photographes. Fig. 6a shows the mode variations of ZnO nanoparticles after sintering at different temperatures. The sintering effect of ZnO nanoparticles is compared for different mode: 45, 75, and 126 nm. The mode of ZnO nanoparticles decreases when the sintering temperature increases. The mode variation shows that smaller ZnO nanoparticles are more easy to be sintered and then the mode is reduced, i.e., from 45 to 17 nm (62%) compared with that from 75 to 6 nm (%). However, the sintering effect of ZnO particles is not apparent for size larger than 1 nm judged by the almost unchanged mode. Figure 6b shows the geometric mean size of ZnO nanoparticles after sintering at different temperatures. The geometric mean diameter of ZnO nanoparticles decreases when the sintering temperature increases. The morphology variation of ZnO nanoparticles after sintering process were examined with TEM photograph. Hence, the particles were filtered on prepared coated carbon copper grids. Figure 7 shows the TEM images of ZnO nanoparticles sintered at temperature of 1-6 C. The morphology of ZnO nanoparticles tends to be more spherical when the sintering temperature increases. CONCLUSIONS A ZnO nanoparticles generation system had been assembled and tested for the operation conditions and characteristics of particles generated by the system. Table 1. The generating parameters for specified size distribution of ZnO nanoparticles N 2 (L min -1 ) Reaction air (L min -1 ) Dilution air (L min -1 ) Oven temp. ( C) Mode (nm)

5 Lee et al.: ZnO Nanoparticle Generation 1 Mode (nm) (a) (b) Mode 45 nm Mode 75 nm Mode 126 nm Mode 45 nm Mode 75 nm Mode 126 nm to be sintered and then the mode is significantly reduced, i.e., from 45 to 17 nm (62%) compared with 75 to 6 nm (%). The morphology of ZnO nanoparticles tends to be more spherical when the sintering temperature increases The sintering effect is apparent at temperature above 5 C, which is much lower than the melting point of ZnO (1975 C). Therefore, various morphologies of ZnO nanoparticle could be produced by this system. Different morphologies of ZnO nanoparticle should be further studied for the toxicity of inhalation and cytotoxicity of nanomaterials. ACKNOWLEDGMENTS Geometric mean diameter (nm) 8 6 The authors would like to thank Institute of Occupational Safety and Health, Council of Labor Affairs, Taiwan for financially supporting this research under Contract No. IOSH-97-H322. REFERENCES Sintering temperature ( C) Fig. 6. Size variations of ZnO nanoparticles as function of sintering temperatures at residence time of 7 s. (a) Mode; (b) Geometric mean diameter. (a) (b) (c) (d) (e) (f) Fig. 7. TEM image for observation of morphologies of ZnO nanoparticles at various sintering temperatures: (a) 1 C; (b) C; (c) 3 C; (d) C; (e) 5 C; (f) 6 C. The trends show that at higher carrier gas and reaction air flow rates result in smaller particle mode size. The experimental results suggest that higher furnace temperatures result in high particle mode size and larger total number concentration. It is easy to generate ZnO nanoparticles at anticipated target size in the range of -1 nm. The mode variation of sintering effect shows that smaller ZnO nanoparticles are more easy 1. Lee, M.K. and H.F. Tu, Ultraviolet emission blueshift of ZnO related to Zn. J. Appl. Phys., 11(12), (7). 2. Look, D.C., Recent advances in ZnO materials and devices. Mat. Sci. Eng. B-Solid, 8(1-3), (1). 3. Ding, Y., P.X. Gao and Z.L. Wang, Catalystnanostructure interfacial lattice mismatch in determining the shape of VLS grown nanowires and nanobelts: A case of Sn/ZnO. J. Am. Chem. Soc., 126(7), (4). 4. Huang, M.H., S. Mao, H. Feick, H.Q. Yan, Y.Y. Wu, H. Kind, E. Weber, R. Russo and P..D Yang, Room-temperature ultraviolet nanowire nanolasers. Science, 292(5523), (1). 5. Li, C., G.J. Fang, F.H. Su, G.H. Li, X.G. Wu and X.Z. Zhao, Self-organized ZnO microcombs with cuboid nanobranches by simple thermal evaporation. Cryst. Growth Des., 6(11), (6). 6. Lucas, M., W.J. Mai, R.S. Yang, Z.L. Wang and E. Riedo, Aspect ratio dependence of the elastic properties of ZnO nanobelts. Nano Lett., 7(5), (7). 7. Pan, Z.W., S.M. Mahurin, S. Dai and D.H. Lowndes, Nanowire array gratings with ZnO combs. Nano Lett., 5(4), (5). 8. Kandavelu, V., H. Kastien and K.R. Thampi, Photocatalytic degradation of isothiazolin-3-ones in water and emulsion paints containing nanocrystalline TiO 2 and ZnO catalysts. Appl. Catal. B-Environ., 48(2), (4).

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