Core and grain boundary sensitivity of tungsten-oxide sensor devices by molecular beam assisted particle deposition

In this study, we investigate the synthesis of WO3 and WOx (2.6 >= x <= 2.8) by adding different concentrations of tungsten hexafluoride (WF6) into a H-2/O-2/Ar premixed flame within a low-pressure reactor equipped with a particle-mass spectrometer (PMS). The PMS results show that mean particle diameters d(p) between 5 and 9 nm of the as-synthesized metal-oxides can be obtained by varying the residence time and precursor concentration in the reactor. This result is further validated by N-2 adsorption measurements on the particle surface, which yielded a 91 m(2)/g surface area, corresponding to a spherical particle diameter of 9 nm (Brunauer-Emmett-Teller technique). H-2/O-2 ratios of 1.6 and 0.63 are selected to influence the stoichiometry of the powders, resulting in blue-colored WOx and white WO3 respectively. X-ray diffraction (XRD) analysis of the as-synthesized materials indicates that the powders are mostly amorphous, and the observed broad reflexes can be attributed to the orthorhombic structure of beta-WO3. Thermal annealing at 973 K for 3 h in air resulted in crystalline WO3 comprised of both monoclinic and orthorhombic phases. The transmission electron microscope micrograph analysis shows that the particles exhibit spherical morphology with some degree of agglomeration. Impedance spectroscopy is used for the electrical characterization of tungsten-oxide thin films with a thickness of 50 nm. Furthermore, the temperature-dependent gas-sensing properties of the material deposited on interdigital capacitors are investigated. Sensitivity experiments reveal two contributions to the overall sensitivity, which result from the surface and the core of each particle.