Pulsed laser deposition of nanostructured tin oxide films for gas sensing applications

Pulsed laser deposition (PLD) technique has been successfully used to not only deposit nanostructured tin dioxide (SnO2) thin films, on alumina substrates, but also to concomitantly achieve their in-situ doping by Pt metal catalyst. The deposition of PLD SnO2 films was investigated at a substrate deposition temperature of 300°C under both vacuum and an oxygen background pressure of 150 mTorr. While the crystalline structure and the composition of the PLD SnO2 films were systematically determined by means of X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques, respectively, their morphology and nanostructure were investigated by scanning and transmission electron microscopies (SEM and TEM, respectively). The PLD films deposited under oxygen consist of polycrystalline pure SnO2 phase, are stoechiometric and exhibit a columnar growth morphology with an apparent inter-columnar porosity. In contrast, those deposited under vacuum consist of a mixture of both a-SnO and poly-SnO2 phases. TEM observations have revealed that the SnO2 columns (of few 10 s nm diameter) that constitute the films are, in turn, composed of almost spherical nanograins. On the other hand, the newly developed in-situ doping approach has been shown not only to provide uniform Pt-doping all through the film thickness, but also to be effective in fabricating high-performance gas sensors. A Pt-doping level of ∼2 at.% was identified as the optimal concentration that leads to the highest sensitivity at an operating temperature of the sensors as low as 200°C. The sensing performances of the developed SnO2 sensors for detecting CO in synthetic air background, over the (10–500 ppm) CO concentration range, are presented.