Synthesis, FTIR studies and sensor properties of WO3 powders

Several synthetic approaches were used to obtain nano-sized porous and nonporous monoclinic WO3 (m-WO3) powders. All of these methods begin with a standard preparative method where H2WO4 is first generated by passing a Na2WO4 solution through a cation-exchange resin. It is shown that high surface area particles are produced by dripping the H2WO4 exiting from the ion-exchange column into a solution containing oxalate and acetate exchange ligands or alternatively, into a water-in-oil (W/O)-based emulsion. Porous materials are produced using surfactant-templating architectures. The surface properties were investigated by IR spectroscopic studies during thermal evacuation and the use of chemical probes. The nature of the surface depends on the initial evacuation temperature of the WO3 surface as this alters the relative number of the Lewis and Brّnsted acid sites along with the amount of adsorbed water. Infrared studies of the adsorption of various molecules on the powders led to a new size-selective approach to improve selectivity in semiconducting metal oxide (SMO) sensors. The key for achieving high selectivity is based on using a dual sensor configuration where the response on a porous WO3 powder sensor was compared to the response on a nonporous WO3 powder sensor. Detection selectivity between methanol and dimethyl methylphosphonate (DMMP) is obtained because the access of a gas molecule in the interior pore structure of WO3 is size-dependent leading to a size-dependent magnitude change in the conductivity of the SMO sensor.