Sensors and Acruators B, 18-19 (1994) 99-102 99 Nanocrystalline metal oxide gas sensors Alan V. Chadwick, Nigel V. Russell, Adam R. Whitham and Alan Wilson* Centre for MaterialsResearch, Chemical Labomtory, Uniwdy of fint, Canterbury Cl2 7NH (UK) Abstract Samples of nanocrystalline ZnO have been prepared and X-ray absorption spectra (EXAFS) have been recorded at the Zn K-edge. Comparison with bulk ZnO shows that the amplitude of the EXAFS is dramatically reduced compared to the bulk polycrystalline sample and this has been attriiuted to a large fraction of the atoms being situated in grain boundary regions. Quantitative analysis of the data suggests that a model in which coordination numbers, radial distances and Debye-Waller factors were allowed to vary produced the best fit to the data. The nanocrystalline particles were found to be asymmetrical, approximately 50 A in the 100 direction and 30 A in the 002 direction, and the Zn-Zn coordination was reduced from 12 to around 8. Initial gas sensing results show an encouraging response to low ppm concentrations of toluene vapour. In the last decade, nanostructured materials with remarkable properties, including improved catalysts, malleable ceramics and ferrofluids have been developed. These properties are believed to be related to the presence of disordered interfaces that may constitute up to 50% by volume of the sample [l]. The physical understanding of the modification of these properties and sample characterisetion considerably lags behind the synthetic chemistry of these materials. Of particular interest is the potentially enhanced gas-sensing capa- bilities of nanocrystalline semiconducting metal oxides; advantages enhanced by the low temperature fabrication techniques and high proportion of grain boundaries. XRD and EXAFS have been successfully used to characterise metal nanoclystals and suggest that struc- turally they consist of a crystalline component, with atomic arrangement the same as the bulk crystal, and a grain boundary component consisting of a disordered arrangement with a broad spectrum of interatomic spacings. Thus nanocrystalline metals can be said to exhibit both short range order and long range disorder. There are two interesting features of nanocrystals that are of significance in gas sensor production; the ability to dope at unusually high concentrations (above the normal solubility limit), and to have carefully controlled doping levels (which is difficult with normal high tem- perature sintering routes). A key to the understanding of the results is a detailed knowledge of the structural environments of the hosts and dopants. In order to *Author to whom correspondence should be addressed. ascertain the r61e of dopants it is necessary to fully character& the undoped nanocrystalline starting ma- terial and this is one objective of the present paper. Experimental Mat er ials Samples of nanocrystalline ZnO were prepared by reacting the basic chlorides with n-butylamine [2, 31. We have found [4] that samples can be doped with other cations by incorporating the appropriate basic chloride with the zinc salt before the reduction reaction with the amine; these results will be reported in full elsewhere. The pure ZnO samples were characterised by transmission electron microscopy (TEM) and X-ray powder diftiaction and crystals had similar dimensions to those reported by Garcia-Ma&e2 et al. [3]. The X-ray di&actograms showed considerable peak broad- ening compared to those for commercial, bulk ZnO and an analysis of the peak widths using the Scherrer formula indicated that the nanocrystals were asymmetric with dimensions -50 A in the 100 direction and -30 A in the 002 direction. The TEM micrographs confirmed the nanocxystalline nature of the samples and revealed a platelet geometry of the crystals. E&IFS EXAFS takes advantage of the fact that the oscil- lations observed on the high energy side of X-ray absorption edges in condensed materials are a function of the distance from, and number and type of atoms undergoing the X-ray transition [5]. For a given incident 09254005194/$07.00 Q 1994 Elscvier Sequoia. All rights reserved SSDI 0925-4005(93)00928-R