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ZnO nanoparticles synthesised by mechanochemical processing
Scripta mater. 44 (2001) 1731–1734 www.elsevier.com/locate/scriptamat
ZnO NANOPARTICLES SYNTHESISED BY MECHANOCHEMICAL PROCESSING Takuya Tsuzuki and Paul G. McCormick Research Centre for Advanced Mineral and Materials Processing, The University of Western Australia, Nedlands, Perth, WA 6907, Australia (Received August 21, 2000) (Accepted in revised form December 18, 2000) Keywords: Mechanochemical reaction; Powder processing; Binary oxides; Compound semiconductors
1. Introduction Zinc oxide has a wide range of applications including functional devices, catalysts, pigments, optical materials, cosmetics, UV-absorbers and additives in many industrial products [1–3]. Many methods for the synthesis of ZnO nanoparticles have been investigated [1–5]. Recently, mechanochemical processing has been applied to the synthesis of a wide range of nanoparticulate materials [6 –15]. Mechanochemical processing involves the mechanical activation of solid-state displacement reactions at low temperatures in a ball mill. Milling of precursor powders leads to the formation of a nanoscale composite structure of the starting materials which react during milling or subsequent heat treatment to form separated nanocrystals of the desired phase within a solid matrix. Such mechanochemically formed nanocomposite particles can be further processed into dispersed nano powders simply by selective removal of the matrix phase. In this paper, we report a study of the synthesis of ZnO nanoparticles via the mechanochemical reaction ZnCl2 ⫹ Na2CO3 3 ZnCO3 ⫹ 2NaCl (⌬G ⫽ ⫺80 kJ [16]) and subsequent thermal decomposition of ZnCO3.
2. Experimental Procedure The starting materials were anhydrous ZnCl2 granules (Cerac, 99.5%, ⫺8 mesh), Na2CO3 powder (Aldrich, 99⫹%, ⫺20 mesh) and NaCl (Aldrich, 99.8%, ⬃0.5 mm beads). All the starting materials were dried in air at 150°C overnight prior to use. The NaCl was used as an inert diluent and added to the starting powders. The mixture of starting powders was sealed in a hardened steel vial (AISI 44°C stainless steel) with hardened steel balls of 6.4 mm in diameter, under a high purity Ar-gas atmosphere. Milling was performed with a Spex 8000 mixer/mill using a ball to powder mass ratio of 10: 1. Heat treatment of the as-milled powder was carried out in air in a porcelain crucible for 0.5 hour. Removal of the salt by-product was carried out by washing the powder with de-ionised water, using an ultrasonic bath and a centrifuge. The washed powder was dried in an oven (60°C) for several hours. Powder characterisation was carried out using X-ray diffraction (XRD) (Cu-K␣ radiation), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET) specific surface area analysis, and simultaneous differential thermal analysis (DTA) and thermogravimetric analysis (TGA) measurements. 1359-6462/01/$–see front matter. © 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(01)00793-X
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SYNTHESIS OF ZnO NANOPARTICLES
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Figure 1. XRD spectra of the ZnCl2 ⫹ Na2CO3 ⫹ 8.6NaCl powder mixture milled for up to 4 hours. Milling times are indicated in the figure.
3. Results and Discussion A stoichiometric mixture of the starting powders, corresponding to the reaction equation of ZnCl2 ⫹ Na2CO3 ⫹ 8.6NaCl 3 ZnCO3 ⫹ 10.6NaCl was milled. NaCl was added to the reactants so that the volume ratio of the ZnCO3:NaCl in the product phase was 1:10. Figure 1 shows XRD patterns of the powders milled for up to 4 hours. As the milling time increased, the peaks associated with ZnCl2 and Na2CO3 decreased. With longer milling times, only peaks associated with NaCl were present, indicating that amorphization of the other phases occurred during milling. Figure 2 shows TG/DTA curves of the powder milled for 4 hours. The TGA curve showed that
Figure 2. TG/DTA curves of the ZnCl2 ⫹ Na2CO3 ⫹ 8.6NaCl powder mixture milled for 4 hours.
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SYNTHESIS OF ZnO NANOPARTICLES
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Figure 3. XRD spectra of the ZnCl2 ⫹ Na2CO3 ⫹ 8.6NaCl powder mixture before and after heat treatment and subsequent washing.
weight loss of 6% occurred in the temperature range of 170°C - 380°C. This value agrees well with the theoretical mass loss of 5.9% associated with the reaction ZnCO3 224 ZnO ⫹ CO2(g). No exothermic or endothermic peak was evident in the DTA curve. Figure 3 shows XRD spectra of the powders milled for 4 hours (a) before heat treatment, (b) after heat treatment at 250°C for 0.5 hour, (c) at 400°C for 0.5 hour, and (d) after subsequent washing. After heat treatment at 300°C, peaks corresponding to ZnCO3 were evident in the spectrum along with ZnO peaks. The XRD pattern for the sample after heat treated at 400°C showed peaks corresponding to ZnO and NaCl, indicative of the occurrence of thermal decomposition of ZnCO3. Removal of the NaCl yielded only ZnO diffraction peaks. Figure 4 shows a typical TEM micrograph of the ZnO powder heat treated and subsequently washed.
Figure 4. TEM micrograph of the ZnO nanoparticles obtained from a ZnCl2 ⫹ Na2CO3 ⫹ 8.6NaCl powder mixture.
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SYNTHESIS OF ZnO NANOPARTICLES
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The powder consisted of 10 – 40 nm size particles having equiaxed morphology. The particles appeared to be well separated from each other. Dark field imaging showed that each particle was a single crystal. No Na, Cl or Fe was detected by EDS. ICP-AES measurements on the ZnO nanopowder gave 0.041 wt% Fe, 0.06 wt% Na and 0.076 wt% Cl. Particle size distribution obtained from TEM study had the mean particle size of 26.2 nm and standard deviation of 8.6 nm, indicative of a very narrow size distribution which is a typical feature of mechanochemically synthesised nanoparticles [15]. BET surface area of the ZnO powder was 47.3 m2/g, which corresponds to a spherical particle size of 27 nm. The mean crystallite size estimated from the XRD peak width at 2 ⫽ 36o using the Scherrer equation [17] was 28.7 nm. The particle sizes from TEM examination were in good agreement with both XRD and BET surface area measurements. Heat treatment of the powders milled for less than 4 hours led to the formation of 100 –500 nm size aggregates. Milling without NaCl for 4 hours and subsequent heat treatment at 400°C resulted in the formation of l00 –1000 nm size aggregates. These results indicate that, in order to form separated nanoparticles, it is necessary for a nanostructure to be developed during milling and for the volume fraction of the nanoparticle phase to be sufficiently low (⬍0.2) to prevent the crystallites of the desired phase being interconnected through the composite as-milled particles [8,12,18]. 4. Conclusions A solid-state displacement reaction between ZnCl2 and Na2CO3 was induced by mechanochemical processing in a steady state manner, forming ZnCO3 in a NaCl matrix. Heat treatment of the as-milled powders at 400°C led to the thermal decomposition of ZnCO3, resulting in ZnO nanoparticles embedded in the NaCl matrix. The mean particle size was ⬃27 nm. Since the mechanochemically formed ZnCO3 nanoparticles were isolated in the NaCl matrix, sintering of the ZnO powder did not occur during heat treatment. The particles had nearly uniform equiaxed shapes, and a narrow size distribution. Mechanochemical processing is particularly suitable for a large scale production of ZnO. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
S. Hingorani, V. Pillai, P. Kumar, M. S. Multani, and D. O. Shah, Mater. Res. Bull. 28, 1303 (1993). H. M. Lin, S. J. Tzeng, P. J. Hsiau, and W. L. Tsai, Nanostruct. Mater. 10, 465 (1998). X. Zhao, S. C. Zhang, C. Li, B. Zheng, and H. Gu, J. Mater. Synth. Process. 5, 227 (1997). S. Sakohara, M. Ishida, and M. A. Anderson, J. Phys. Chem. B. 102, 10169 (1998). M. S. El-Shall, D. Gravier, U. Pernisz, and M. I. Baraton, Nanostruct. Mater. 6, 297 (1995). P. G. McCormick, J. Ding, H. Yang, and T. Tsuzuki, Mater. Res. 1, 85 (1996). G. B. Schaffer and P. G. McCormick, Metall. Trans. A21, 2789 (1991). J. Ding, W. F. Miao, P. G. McCormick, and R. Street, Appl. Phys. Lett. 67, 3804 (1995). J. Ding, T. Tsuzuki, P. G. McCormick, and R. Street, J. Phys. D. Appl. Phys. 29, 2365 (1996). J. Ding, T. Tsuzuki, and P. G. McCormick, J. Am. Ceram. Soc. 79, 2956 (1996). J. Ding, T. Tsuzuki, and P. G. McCormick, Nanostruct. Mater. 8, 75 (1997). T. Tsuzuki, J. Ding, and P. G. McCormick, Phys. B. 239, 378 (1997). T. Tsuzuki, E. Pirault, and P. G. McCormick, Nanostruct. Mater. 11, 125 (1999). T. Tsuzuki and P. G. McCormick, Mater. Sci. Forum, 586, 315 (1999). T. Tsuzuki and P. G. McCormick, Appl. Phys. A. 65, 607 (1997). I. Barin, Thermodynamic Data of Pure Substances, 2nd edn., VCH, Weinheim (1993). B. D. Cullity, Elements of X-ray Diffraction, 2nd edn., p. 102, Addison-Wesley, Reading, MA (1978). T. Tsuzuki and P. G. McCormick, Nanostruct. Mater. 12, 75 (1999).
ZnO NANOPARTICLES SYNTHESISED BY MECHANOCHEMICAL PROCESSING Takuya Tsuzuki and Paul G. McCormick Research Centre for Advanced Mineral and Materials Processing, The University of Western Australia, Nedlands, Perth, WA 6907, Australia (Received August 21, 2000) (Accepted in revised form December 18, 2000) Keywords: Mechanochemical reaction; Powder processing; Binary oxides; Compound semiconductors
1. Introduction Zinc oxide has a wide range of applications including functional devices, catalysts, pigments, optical materials, cosmetics, UV-absorbers and additives in many industrial products [1–3]. Many methods for the synthesis of ZnO nanoparticles have been investigated [1–5]. Recently, mechanochemical processing has been applied to the synthesis of a wide range of nanoparticulate materials [6 –15]. Mechanochemical processing involves the mechanical activation of solid-state displacement reactions at low temperatures in a ball mill. Milling of precursor powders leads to the formation of a nanoscale composite structure of the starting materials which react during milling or subsequent heat treatment to form separated nanocrystals of the desired phase within a solid matrix. Such mechanochemically formed nanocomposite particles can be further processed into dispersed nano powders simply by selective removal of the matrix phase. In this paper, we report a study of the synthesis of ZnO nanoparticles via the mechanochemical reaction ZnCl2 ⫹ Na2CO3 3 ZnCO3 ⫹ 2NaCl (⌬G ⫽ ⫺80 kJ [16]) and subsequent thermal decomposition of ZnCO3.
2. Experimental Procedure The starting materials were anhydrous ZnCl2 granules (Cerac, 99.5%, ⫺8 mesh), Na2CO3 powder (Aldrich, 99⫹%, ⫺20 mesh) and NaCl (Aldrich, 99.8%, ⬃0.5 mm beads). All the starting materials were dried in air at 150°C overnight prior to use. The NaCl was used as an inert diluent and added to the starting powders. The mixture of starting powders was sealed in a hardened steel vial (AISI 44°C stainless steel) with hardened steel balls of 6.4 mm in diameter, under a high purity Ar-gas atmosphere. Milling was performed with a Spex 8000 mixer/mill using a ball to powder mass ratio of 10: 1. Heat treatment of the as-milled powder was carried out in air in a porcelain crucible for 0.5 hour. Removal of the salt by-product was carried out by washing the powder with de-ionised water, using an ultrasonic bath and a centrifuge. The washed powder was dried in an oven (60°C) for several hours. Powder characterisation was carried out using X-ray diffraction (XRD) (Cu-K␣ radiation), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET) specific surface area analysis, and simultaneous differential thermal analysis (DTA) and thermogravimetric analysis (TGA) measurements. 1359-6462/01/$–see front matter. © 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(01)00793-X
1732
SYNTHESIS OF ZnO NANOPARTICLES
Vol. 44, Nos. 8/9
Figure 1. XRD spectra of the ZnCl2 ⫹ Na2CO3 ⫹ 8.6NaCl powder mixture milled for up to 4 hours. Milling times are indicated in the figure.
3. Results and Discussion A stoichiometric mixture of the starting powders, corresponding to the reaction equation of ZnCl2 ⫹ Na2CO3 ⫹ 8.6NaCl 3 ZnCO3 ⫹ 10.6NaCl was milled. NaCl was added to the reactants so that the volume ratio of the ZnCO3:NaCl in the product phase was 1:10. Figure 1 shows XRD patterns of the powders milled for up to 4 hours. As the milling time increased, the peaks associated with ZnCl2 and Na2CO3 decreased. With longer milling times, only peaks associated with NaCl were present, indicating that amorphization of the other phases occurred during milling. Figure 2 shows TG/DTA curves of the powder milled for 4 hours. The TGA curve showed that
Figure 2. TG/DTA curves of the ZnCl2 ⫹ Na2CO3 ⫹ 8.6NaCl powder mixture milled for 4 hours.
Vol. 44, Nos. 8/9
SYNTHESIS OF ZnO NANOPARTICLES
1733
Figure 3. XRD spectra of the ZnCl2 ⫹ Na2CO3 ⫹ 8.6NaCl powder mixture before and after heat treatment and subsequent washing.
weight loss of 6% occurred in the temperature range of 170°C - 380°C. This value agrees well with the theoretical mass loss of 5.9% associated with the reaction ZnCO3 224 ZnO ⫹ CO2(g). No exothermic or endothermic peak was evident in the DTA curve. Figure 3 shows XRD spectra of the powders milled for 4 hours (a) before heat treatment, (b) after heat treatment at 250°C for 0.5 hour, (c) at 400°C for 0.5 hour, and (d) after subsequent washing. After heat treatment at 300°C, peaks corresponding to ZnCO3 were evident in the spectrum along with ZnO peaks. The XRD pattern for the sample after heat treated at 400°C showed peaks corresponding to ZnO and NaCl, indicative of the occurrence of thermal decomposition of ZnCO3. Removal of the NaCl yielded only ZnO diffraction peaks. Figure 4 shows a typical TEM micrograph of the ZnO powder heat treated and subsequently washed.
Figure 4. TEM micrograph of the ZnO nanoparticles obtained from a ZnCl2 ⫹ Na2CO3 ⫹ 8.6NaCl powder mixture.
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SYNTHESIS OF ZnO NANOPARTICLES
Vol. 44, Nos. 8/9
The powder consisted of 10 – 40 nm size particles having equiaxed morphology. The particles appeared to be well separated from each other. Dark field imaging showed that each particle was a single crystal. No Na, Cl or Fe was detected by EDS. ICP-AES measurements on the ZnO nanopowder gave 0.041 wt% Fe, 0.06 wt% Na and 0.076 wt% Cl. Particle size distribution obtained from TEM study had the mean particle size of 26.2 nm and standard deviation of 8.6 nm, indicative of a very narrow size distribution which is a typical feature of mechanochemically synthesised nanoparticles [15]. BET surface area of the ZnO powder was 47.3 m2/g, which corresponds to a spherical particle size of 27 nm. The mean crystallite size estimated from the XRD peak width at 2 ⫽ 36o using the Scherrer equation [17] was 28.7 nm. The particle sizes from TEM examination were in good agreement with both XRD and BET surface area measurements. Heat treatment of the powders milled for less than 4 hours led to the formation of 100 –500 nm size aggregates. Milling without NaCl for 4 hours and subsequent heat treatment at 400°C resulted in the formation of l00 –1000 nm size aggregates. These results indicate that, in order to form separated nanoparticles, it is necessary for a nanostructure to be developed during milling and for the volume fraction of the nanoparticle phase to be sufficiently low (⬍0.2) to prevent the crystallites of the desired phase being interconnected through the composite as-milled particles [8,12,18]. 4. Conclusions A solid-state displacement reaction between ZnCl2 and Na2CO3 was induced by mechanochemical processing in a steady state manner, forming ZnCO3 in a NaCl matrix. Heat treatment of the as-milled powders at 400°C led to the thermal decomposition of ZnCO3, resulting in ZnO nanoparticles embedded in the NaCl matrix. The mean particle size was ⬃27 nm. Since the mechanochemically formed ZnCO3 nanoparticles were isolated in the NaCl matrix, sintering of the ZnO powder did not occur during heat treatment. The particles had nearly uniform equiaxed shapes, and a narrow size distribution. Mechanochemical processing is particularly suitable for a large scale production of ZnO. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
S. Hingorani, V. Pillai, P. Kumar, M. S. Multani, and D. O. Shah, Mater. Res. Bull. 28, 1303 (1993). H. M. Lin, S. J. Tzeng, P. J. Hsiau, and W. L. Tsai, Nanostruct. Mater. 10, 465 (1998). X. Zhao, S. C. Zhang, C. Li, B. Zheng, and H. Gu, J. Mater. Synth. Process. 5, 227 (1997). S. Sakohara, M. Ishida, and M. A. Anderson, J. Phys. Chem. B. 102, 10169 (1998). M. S. El-Shall, D. Gravier, U. Pernisz, and M. I. Baraton, Nanostruct. Mater. 6, 297 (1995). P. G. McCormick, J. Ding, H. Yang, and T. Tsuzuki, Mater. Res. 1, 85 (1996). G. B. Schaffer and P. G. McCormick, Metall. Trans. A21, 2789 (1991). J. Ding, W. F. Miao, P. G. McCormick, and R. Street, Appl. Phys. Lett. 67, 3804 (1995). J. Ding, T. Tsuzuki, P. G. McCormick, and R. Street, J. Phys. D. Appl. Phys. 29, 2365 (1996). J. Ding, T. Tsuzuki, and P. G. McCormick, J. Am. Ceram. Soc. 79, 2956 (1996). J. Ding, T. Tsuzuki, and P. G. McCormick, Nanostruct. Mater. 8, 75 (1997). T. Tsuzuki, J. Ding, and P. G. McCormick, Phys. B. 239, 378 (1997). T. Tsuzuki, E. Pirault, and P. G. McCormick, Nanostruct. Mater. 11, 125 (1999). T. Tsuzuki and P. G. McCormick, Mater. Sci. Forum, 586, 315 (1999). T. Tsuzuki and P. G. McCormick, Appl. Phys. A. 65, 607 (1997). I. Barin, Thermodynamic Data of Pure Substances, 2nd edn., VCH, Weinheim (1993). B. D. Cullity, Elements of X-ray Diffraction, 2nd edn., p. 102, Addison-Wesley, Reading, MA (1978). T. Tsuzuki and P. G. McCormick, Nanostruct. Mater. 12, 75 (1999).