Macroporous WO3 Thin Films Active in NH3 Sensing: Role of the Hosted Cr Isolated Centers and Pt Nanoclusters

Macroporous WO3 films with inverted opal structure were synthesized by one-step procedure, which involves the self-assembly of the spherical templating agents and the simultaneous sol gel condensation of the semiconductor alkoxide precursor. Transition metal doping, aimed to enhance the WO3 electrical response, was carried out by including Cr(III) and Pt(IV) centers in the oxide matrix. It turned out that Cr remains as homogeneously dispersed Cr(III) centers inside the WO3 host, while Pt undergoes reduction and aggregation to form nanoclusters located at the oxide surface. Upon interaction with NH3, the electrical conductivity of transition metal doped-WO3 increases, especially in the presence of Pt dopant, resulting in outstanding sensing properties (S = 110 +/- 15 at T = 225 degrees C and [NH3] = 74 ppm). A mechanism was suggested to explain the excellent electrical response of Pt-doped films with respect to the Cr-doped ones. This associates the easy chemisorption of ammonia on the WO3 nanocrystals, promoted by the inverted opal structure, with the catalytic action exerted by the surface Pt nanoclusters on the N-H bond dissociation. The overall results indicate that in Pt-doped WO3 films the effects of the macroporosity positively combine with the electrical sensitization promoted by the metal nanoclusters, thus providing very lightweight materials which display high functionality even at relatively low temperatures. We expect that this synergistic effect can be exploited to realize other functional hierarchical metal oxide structures to be used as gas sensors or catalysts.