Nanoparticle SnO 2 films as gas sensitive membranes
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0900-O08-06.1
Nanoparticle SnO2 films as gas sensitive membranes B. Schumacher1,2, D. V. Szabó2, S. Schlabach2, R. Ochs2, H. Müller3, M. Bruns3 1 Albert-Ludwigs-Universität, IMTEK, 79085 Freiburg, Germany 2 Forschungszentrum Karlsruhe, IMF III, 76344 Eggenstein-Leopoldshafen, Germany 3 Forschungszentrum Karlsruhe, IFIA, 76344 Eggenstein-Leopoldshafen, Germany ABSTRACT The Karlsruhe Microwave Plasma Process (KMPP), a versatile gas-phase process is applied to produce SnO2 and core shell SnO2/SiO2 nanoparticles which are, respectively, deposited in-situ on preheated Si-Substrates. These substrates are already equipped with an electrode microarray. The proof of sensor concept shows, that mechanically stable, nanoscaled and nanogranular gas sensing layers can be produced. In a first step synthesis and deposition parameters of SnO2 are elaborated, and gas-sensitivity tests are performed. Additionally, annealing experiments are done. The morphology and structure of nanoparticles is characterized by X-ray diffraction and TEM-methods. The layers are investigated by SEM techniques and by XPS. The sensitivity of the nanogranular layer is determined in comparison with a standard microarray equipped with sputtered layers. Particles crystallize in the tetragonal cassiterite structure. It is found that a precursor concentration of 3x10-6mol/l leads to particles with crystallite size in the region of 2nm, whereas a concentration of 5.5x10-4mol/l results in approximately 5nm particles. With the precursor concentration, columnar porous layers of 200nm thickness are obtained after a deposition time of 1min. This thickness is comparable to the one of sputtered layers. First sensor tests show 10 times higher sensitivity to isopropanol, compared to the standard array. The time of response is equivalent. The grain growth observed for bare and core/shell nanoparticles at 300°C is marginal. INTRODUCTION Tin dioxide, a wide-band gap n-type semiconductor oxide is commonly used as gas sensitive material. Usually, sensor layers are produced by sputtering [1,2,3], leading to grain sizes around 30 to 300nm. Due to their high surface/volume ratio, nanoparticles are very promising for an improvement of sensor performance. Many concepts therefore apply nanoparticles via spincoating [4], drop-coating [5,6], or screen-printing [6,7]. Kennedy et al. [8] apply a gas phase process and deposit nanoparticle films. In most cases, initial particle sizes are around 10nm. Deposition processes using colloidal solutions of nanoparticles poses the problem of particle growth, as one has to eliminate the binding phase by a temperature treatment. Additionally, the operation temperature of the gas-sensors usually is in the range of 300°C and higher. Therefore, one expects strong improvements in thermal stability by applying gas permeable layers of nanoparticles as sensing layers. These nanoparticles may be e.g. bare SnO2 or, alternatively, coated nanoparticles, e.g. SnO2/SiO2 nanocomposites. It is known that such a coating reduces the sintering activity of the kerne
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