Production and characterization of titanium and iron oxide nano-sized thin films
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M. Ferroni Dipartimento di Fisica, Universita` di Ferrara and INFM Via Paradiso 12, I-44100 Ferrara, Italy
V. Guidi Dipartimento di Fisica, Universita` di Ferrara and INFM Via Paradiso 12, I-44100 Ferrara, Italy INFN, sezione di Ferrara, Ferrara, Italy
C. Frigeri Istituto CNR-MASPEC Parco Area Delle Scienze, I-43100 Parma, Italy
D. Boscarino Laboratori Nazionali Legnaro, INFN, Via Romea 4, I-35020, Legnaro (Padova), Italy (Received 20 July 2000; accepted 8 March 2001)
We present a simple and reproducible method to obtain TiO2 and Fe2O3 mixed-oxide thin films by reactive radio-frequency sputtering. The influence of iron concentration on the structural properties of the layers has been studied. Structural characterization, carried out by electron microscopy analysis, allowed one to correlate the inhibition of the grain growth of titania to the presence of iron oxide and to its segregation at grain boundaries. This behavior should be ascribed to a superficial-tension phenomenon. As a possible application of these thin films, we have investigated the gas-sensing properties toward CO, with particular focus on the role of Fe. The layers were capable to sense CO down to the level requested for environmental monitoring.
I. INTRODUCTION
Research on gas sensors via semiconductor-based devices seeks new materials. A big effort is made for achievement of sensitive and selective materials and to prove their stability under long-term operation.1 Indeed, novel materials that are seemingly not very attractive for their sensitivity or selectivity can play a significant role for they simply exhibit a response that is remarkably different from that of other known materials. These can profitably be employed as input information for electronic noses to simplify the deconvolution process of outputs.2 Nanostructured highly compact materials are optimal candidates for gas sensing due to a high surface exposed for gas adsorption. An ideal film has to be compact enough to permit a percolating path for the electronic current but not too densified in order to allow the gas to be absorbed over a wide surface on the film. Indeed, it has been demonstrated that the response of a layer versus a gas is bound up to the size of the grains in the film.3 Titania films have met the interest of researchers working in the field. Indeed, thin transparent TiO2 films have been previously made by various vapor-phase methods J. Mater. Res., Vol. 16, No. 6, Jun 2001
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including chemical vapor deposition, reactive evaporation, electron beam evaporation, and sputtering.4 Films made by these processes tend to be coarse-grained crystalline and nonporous. However, it has been demonstrated that titania films can be produced in a nanostructured phase through a relatively easy way via thin-5 and thick-film6 technologies. Such nanostructured layers proved sensitive to gases at temperature3 considerably lower than that coarse-grained titania films used to work.7 That is necessarily to be ascribed to the nanostruc
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