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

Abstract

Macroporous materials 1,2 based on metal oxides have become strategic for various applications like photonic crystals, 3À5 catalysts, 6À8 and sensor devices, 9À13 mainly due to their lightweight, their high surface area, and the well-defined porous architecture with a limited agglomerated morphology. 2 In sensing research the improvement in selectivity, reproducibility, and stability expected for the gas sensors based on macroporous metal oxides is still a challenge for pollutant detection. 14 Unfortunately, the delicate method needed to prepare macroporous materials still limits their wide employment in effective devices. 1,2 Macroporous metal oxides are typically synthesized by alkoxidebased solÀgel processes, employing two steps: 11,15À20 (i) the selfassembling of an ordered array of colloidal microspheres of polystyrene, polymethyl methacrylate, or silica, acting as template; (ii) the filling of interstices of the structure by metal alkoxide solution, followed by solÀgel hydrolysis and densification.The subsequent thermal treatment which removes the template leads to the macroporous structure called inverted opal, where the macropores are interconnected through holes resulting from the contact between the template spheres. 2 The application of this synthetic method becomes particularly tricky wherein highly reactive metal alkoxides are used as oxide precursors, 21,22 or if the oxide needs to be doped by sensitizing centers, 11 for example, by transition metal atoms. Moreover, when the preparation of films is required, such as in the case of semiconductor oxide-based gas sensors, the two-step approach appears unsuitable to produce metal oxide thin films having high reproducible morphology, homogeneously embedding dopants, like transition metal centers.To easily obtain macroporous metal oxides with high surface area and homogeneous dopant distribution, our group recently developed a novel one-step preparation of SnO 2 and Pt-doped SnO 2 as inverse opal thin films by dip-coating deposition of solÀgel precursors. 23 The procedure enables the self-assembled formation of the closely packed polystyrene (PS) microsphere array and the simultaneous infiltration of the precursors into the voids of the structure. Thus, the oxide solid skeleton around the spheres was obtained in one-step, unlike the conventional Received: October 22, 2010 ABSTRACT: Macroporous WO 3 films with inverted opal structure were synthesized by one-step procedure, which involves the self-assembly of the spherical templating agents and the simultaneous solÀgel condensation of the semiconductor alkoxide precursor. Transition metal doping, aimed to enhance the WO 3 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 WO 3 host, while Pt undergoes reduction and aggregation to form nanoclusters located at the oxide surface. Upon interaction with NH 3 , the electrical conductivity of transition metal doped-WO 3 increase...

Surface interaction of WO3 nanocrystals with NH3. Role of the exposed crystal surfaces and porous structure in enhancing the electrical response

“…18,19 Moreover high surface area favors the gas-oxide interaction 18,20 and, in particular, meso and macroporous structures signicantly improve gas diffusion throughout the layer and consequently the sensing performance. 20,21 The sensor response can signicantly be enhanced by noble metals centres, e.g. Cr and Pt, homogeneously dispersed into the oxide matrix, which catalyze the electron transfer between gas molecules and the semiconductor.…”

“…In parallel, to study the inuence of the porous structure and of the surface area, we prepared macroporous WO 3 inverted opal lms according to the above mentioned one-step procedure 21 and hierarchical tungsten oxide layers by an innovative one-step dual-templating (block copolymer Pluronic P123 and colloidal PMMA microspheres) strategy which guarantees the simultaneous presence of macropores and mesopores.…”

“…The gure inset evidences that the lattice fringes of the nanocrystal have a spacing of 3.74Å, which reasonably matches the interplanar distance of the {020} surface of the monoclinic tungsten trioxide (ICDD 71-0131). 21 TEM images of WO 3 -DP lms reveal that WO 3 aggregates are constituted of irregularly shaped interconnected nanocrystals ( Fig. 6e and f) with a broad size distribution ranging from 15 to 50 nm, when the statistics is applied to Paperabout 100 particles.…”

“…As the sensing properties are presumably not only affected by the structure of the exposed crystal surfaces [18][19][20][21][22][23][24] but also by their surface area, the electrical responses of shape controlled nanocrystals under NH 3 (74 ppm)-air at 225…”

Effect of solution concentration on physicochemical and gas sensing properties of sprayed WO3 thin films

“…[2]. Among them WO 3 exhibits excellent sensing properties towards various oxidizing and reducing gases, such as NOx, H 2 S, ethanol, NH 3 , CO, and O 3 in the midst of other gases [3][4][5][6][7][8]. MOS based gas sensors are chemiresistive sensors in which resistance of the sensor changes due to the chemisorption of the gas on to the sensor surface [9].…”