Propriedades microestruturais e elétricas de nanoestruturas de WO3 puras e dopadas com Mn

Abstract

Tungsten trioxide (WO3) is an n-type wide bandgap semiconductor that has been widely studied due to its properties, being used in the generation of electricity “water splitting”, catalysis, gas sensors, among others. Traditionally, gas sensors require relatively high heating temperatures (>150°C) for surface/gas reactions to be activated. Some studies report that photostimulation can be an efficient strategy to replace thermal activation. Despite promising results, the high recombination rate of photogenerated charge carriers has been one of the disadvantages of photostimulated gas sensors. Therefore, in this work we aimed to synthesize nanostructures of W1−xMnxO3 (0.0 ≤ x ≤ 3.0%; WMO) using the polymeric precursor method to be applied as photostimulated gas sensors. The samples were characterized by different techniques, such as thermogravimetric analysis, differential thermal analysis, X-ray diffraction, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and scanning electron microscopy. Furthermore, electrical measurements were carried out on samples from the WMO system in the absence and presence of gas, and the effect of photostimulation on the sensing properties was also evaluated. Xray diffraction (XRD) measurements revealed the crystallization of the pure WO3 sample for thermal treatments carried out above 450°C, presenting the monoclinic structure of WO3. Furthermore, XRD measurements, Raman spectroscopy and XANES spectroscopy indicated that the addition of Mn ions did not modify the crystalline structure of WO3, however, the presence of a secondary phase, MnWO4, was identified. Scanning electron microscopy analyzes confirmed the nanometric nature of the WMO samples, with particle size increasing as a function of Mn content, i.e., from 102 nm to 370 nm. X-ray photoelectron spectroscopy measurements allowed identifying the presence of Mn2+ in all samples, in addition to indicating the increase in oxygen deficiency in samples with the concentration of Mn. Regarding the electrical characterization of the samples, the measurements showed that the WMO samples were photosensitive to UV, besides they exhibited typical varistor behavior. The evaluation of the sensor performance of the WMO samples showed that they were all sensitive to oxidizing gases (O3 and NO2) at room temperature and under photostimulation. Regarding the influence of Mn ions, the measurements revealed that the samples exhibited a good detection range for NO2 gas (100 to 1000 ppb), and good selectivity for reducing gases (CO and NH3). Moreover, the addition of Mn ions caused an increase in the sensing response to NO2 gas, which was attributed to the good separation of photogenerated charge carriers due to the presence of the MnWO4 secondary phase.