Anodic formation of low-aspect-ratio porous alumina films for metal-oxide sensor application

Thin nanoporous anodic alumina films, of low aspect ratio (1:1), with two distinctive pore sizes and morphologies were prepared by two-step constant-current anodising of aluminium layers on SiO2/Si substrates in 0.4 mol dm−3 tartaric (TA) and malonic acid (MA) electrolytes and then modified by open-circuit dissolution. The anodic films were employed as a support material for sputtering-deposition of thin WO3 layers in view of exploiting their gas sensing properties. The films and deposits were characterized by scanning electron microscopy, X-ray diffraction and electric resistance measurements at fixed temperatures in the range of 100–300 °C upon NH3 and CO gas exposures. Test sensors prepared from the annealed and stabilized alumina-supported WO3 active layers were insensitive to CO but showed considerably enhanced responses to NH3 at 300 °C, the sensitivity depending upon the anodic film nature, the pore size and the surface morphology. The increased sensor sensitivity is due to the substantially enlarged film surface area of the TA-supported WO3 films and the nanostructured, camomile-like morphology of the MA-supported WO3 films. Sensing mechanisms in the alumina-supported WO3 active layers are discussed.