SnO2 nanoparticles prepared by mechanochemical processing

SnO2 nanoparticles prepared by mechanochemical processing

Scripta mater. 44 (2001) 1787–1790 www.elsevier.com/locate/scriptamat SnO2 NANOPARTICLES PREPARED BY MECHANOCHEMICAL PROCESSING L.M. Cukrov, T. Tsuzu...

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Scripta mater. 44 (2001) 1787–1790 www.elsevier.com/locate/scriptamat

SnO2 NANOPARTICLES PREPARED BY MECHANOCHEMICAL PROCESSING L.M. Cukrov, T. Tsuzuki and P.G. McCormick Research Centre for Advanced Mineral and Materials Processing, The University of Western Australia, Nedlands, WA 6907, Australia (Received August 21, 2000) (Accepted December 16, 2000) Keywords: Mechanochemical processing; X-ray diffraction; Semiconductors (SnO2); Microstructure

Introduction SnO2 is an n-type semiconductor that has been extensively utilised for its unique electrical and catalytic properties. One area of primary importance is the field of solid state gas sensors for environmental monitoring, where SnO2 has been established as the predominant sensing material. Many factors affect the sensing properties of semiconductor gas sensors, but one such important factor is the microstructure of the sensing layer. It has been recognised that when the crystallite size of SnO2 is controlled to less than approximately 10nm, the sensitivity to a number of reducing gases is significantly increased (1). As a result, the production of nanosized SnO2 for gas sensing applications has been widely investigated. Nanosized SnO2 for sensing applications has been produced by several methods, including sol-gel processing (2), spray pyrolysis (3), pulsed laser ablation (4), chemical vapour deposition (5), sputtering (6) and numerous others. Nanocrystalline structures have been achieved by these techniques, but often with a very high degree of agglomeration. Mechanochemical processing is an alternative method for the production of nanosized materials, where separated nanoparticles can be produced. The mechanochemical process uses a conventional ball mill within which suitable precursor powders are milled to form a nanocomposite mixture of the starting materials. The repeated ball-powder collisions induce structural changes and continually regenerate the reacting interfaces, allowing chemical reactions to occur (7). A wide range of materials have been synthesised by mechanochemical processing, including Fe, ZrO2 and ZnS (8 –10). In this paper, the synthesis of SnO2 nanoparticles by mechanochemical processing for gas sensing applications is reported. The effect of milling conditions, including milling time, heat treatment temperature and ball size is investigated.

Experimental The starting materials were anhydrous SnCl2 (99.9%), granular anhydrous Na2CO3 (99.5⫹%) and NaCl (99.9%) as a diluent. The powders were sealed under a high purity argon atmosphere in a hardened steel vial with chromium-steel balls of 6.4 mm in diameter, unless otherwise stated. Milling was performed in a Spex 8000 mixer/mill using a ball-to-powder mass ratio of 10:1. The as-milled powder was subsequently heat treated in an air atmosphere to produce SnO2. Removal of the NaCl by-product was carried out with deionised water using an ultrasonic bath and a centrifuge. 1359-6462/01/$–see front matter. © 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(01)00736-9

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Figure 1. X-ray diffraction patterns of as-milled powder, milled for (a) 0 hours, (b) 2 hours and (c) 8 hours; (d) 8 hours, heat treated at 700°C and washed.

The structure of the powder was examined using a Siemens D5000 X-ray diffractometer with Cu-K␣ radiation. The SnO2 crystallite size was determined using Scherrer’s equation (11). The microstructure and particle size were examined using a Philips 430 transmission electron microscope (TEM). The surface area of the powder was measured using a Micromeritics Gemini 2360 BET surface area analyser. Results and Discussion Figure 1 shows x-ray diffraction patterns for the unmilled powder, after milling for 2 hours and for 8 hours. The x-ray diffraction pattern for the powder milled for 8 hours, heat treated at 700°C and washed is also shown. For the unmilled powder, the peaks for SnCl2, Na2CO3 and NaCl can be clearly seen. After milling for 2 hours, the peaks associated with SnCl2 and Na2CO3 disappeared while the NaCl peaks remained, indicating the amorphisation of SnCl2 and Na2CO3. Small broad peaks associated with SnO can just be distinguished. After milling for 8 hours, the broad peaks associated with SnO have increased in intensity. The x-ray diffraction pattern of the milled, heat treated and washed powder shows the presence of two phases of SnO2. The stable tetragonal phase is present in the greatest proportion while the high pressure orthorhombic phase is also present. Previous studies have reported the presence of orthorhombic SnO2, together with tetragonal SnO2, in ultrafine 5–10 nm SnO2 powders (12). The stabilisation of high pressure phases in ultrafine particles may be explained by an increased effective internal pressure resulting from the surface energy, according to the Laplace equation (13). Figure 2 shows the XRD crystallite size and BET particle size of SnO2 as a function of milling time after heat treatment at 700°C and washing. An estimate of the XRD crystallite size was obtained using the Scherrer formula on the (110) tetragonal peak. The most intense peak was chosen to minimise the error introduced from the overlapping of peaks from the tetragonal and orthorhombic phases. The BET particle size was obtained by assuming spherical particles. Both the XRD crystallite size and BET particle size decrease for milling times up to 8 hours and then remain approximately constant for longer milling times. The BET size follows the same trend as the XRD size but is consistently higher. This may indicate that some agglomeration is present. Figure 3 shows the SnO2 XRD crystallite size as a function of heat treatment temperature for a milling time of 8 hours. The crystallite size slowly increases from approximately 7nm at 300°C and then

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Figure 2. Average XRD crystallite size and BET particle size as a function of milling time after heat treatment at 700°C and washing.

increases more rapidly for higher temperatures. The colour of the SnO2 powder was dark grey for the 300°C heat treatment, which lightened progressively until a white powder was produced at 600°C. The greyness of the powders may be attributed to the incomplete oxidation of SnO to SnO2. The effect of changing the ball size from 12.7mm progressively down to 2.4mm was also investigated. However, the changes in both XRD crystallite sizes and BET particle sizes were almost negligible. Figure 4 shows a TEM micrograph of the SnO2 powder milled for 4 hours, heat treated at 700°C, and washed. The particle sizes range from 5 to 30 nm, which is in good agreement with both the XRD and BET sizes. Both separated and moderately agglomerated particles appear to be present. Conclusions Mechanochemical processing of SnCl2 and Na2CO3 resulted in the formation of nanocrystalline SnO during milling, which was then oxidised to SnO2 after heat treatment in an air atmosphere. Both tetragonal and orthorhombic SnO2 were present. The XRD crystallite size and the BET particle size

Figure 3. Average XRD crystallite size as a function of heat treatment temperature after milling for 8 hours.

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Figure 4. TEM micrograph of SnO2 powder milled for 4 hours, heat treated at 700°C and washed.

decreased until approximately 8 hours milling and then remained constant. The XRD crystallite size increased with increasing heat treatment temperature. TEM analysis found both separated and agglomerated particles ranging in size from 5 to 30 nm. Future work will involve testing the response of mechanochemically processed SnO2 to various reducing gases. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

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