In2O3 Thin Films Obtained Through a Chemical Complexation Based Sol-Gel Process and Their Application as Gas Sensor Devices

A new approach is presented for preparing In2O3 thin films starting from inorganic precursors, based on a very simple but effective modification of the usual precipitation-peptization process. Indium nitrate was dissolved in methanol and In3+ ions were chelated with acetylacetone before adding concentrated base to the resulting solution. Such a route allowed obtaining long-term stable sols, from which films could be deposited on glass substrates by spin-coating. Furthermore, the films exhibited a very good adhesion and uniformity, without any need for adding additives to the solution. The modification of the In precursor was confirmed by thermal analysis, while XRD studies revealed that the films prepared with the modified route result in smaller In2O3 grains compared to the traditional precipitation-peptization process. Optical reflectance measurements on the films further highlighted the difference between the two processes. Gas-sensing tests carried out on the films deposited onto alumina substrates in the temperature range between 100 and 400°C showed that faster responses are obtained at temperatures higher than 250°C. The response value (R/R0, where R is the electrical resistance of the sensor in the test gas and R0 that in dry air) to 100 ppb ozone is remarkably high: it is equal to 1500 for In2O3 with a response time of about 1 minute. The recovery time is about 10 minutes.