The hexagonal WO3 polymorph, h-WO3, has attracted attention due to its interatomic channels, allowing for a greater degree of intercalation compared to other WO3 polymorphs. Our research group has previously demonstrated h-WO3 to be a highly sensitive gas sensing material for a flu biomarker, isoprene. In this work, the gas sensing performance of this polymorph has been further investigated in two distinct configurations of the material produced by different processing routes. The first sample was synthesized using Na2WO4∙2H2O and showed (100) faceting. The second sample was synthesized using WCl6 and showed (001) faceting. The gas sensing response of the nanostructured films deposited using the (100) textured h-WO3 sample 1 had a higher response to acetone at 350 °C. The (001) textured h-WO3 sample 2 favored isoprene at 350 °C. The selectivity of the latter to isoprene is explained in terms of the dangling bonds present on the (001) facets. The tungsten and oxygen dangling bonds present on the (001) plane favor the adsorption of the isoprene molecule over that of the acetone molecule due to the oxygen containing dipole present in the acetone molecule.

In binary metal oxides (BMO), polymorphic transitions can result in various crystallographic structures which have been shown to exhibit very different and distinct physical and chemical properties. Thus, exact structural determination is essential as these changes in their crystal structures offer fine control over a wide variety of different properties and, therefore, open up a wide field of applications. However, distinguishing between different BMO polymorphs is not trivial. A combination of high-resolution X-ray diffraction (XRD), Raman and infrared-ray (IR) spectroscopy might be perform to identify phases. However, this is not a universal approach because strategies for phase identification vary with materials system, and, in the case of BMO polymorphs, might not work at all because of limitations of standard Raman/IR spectrum data of inorganics crystal. Conventional electron-microscopy-based characterization techniques, such as selected area electron diffraction (SAED), nanobeam diffraction (NBE), highresolution TEM (HRTEM), high-resolution STEM (HRSTEM), provide information that is not always sufficient for polymorph determination, especially in nanocrystalline BMO. A general and universal approach to overcome this and to confirm a specific crystal structure is performing TEM tilt experiments obtaining data from at least 4-6 zone axis orientation (e.g., using SAED patterns, NBE patterns, HRTEM or HRSTEM images). However, this is a complicate experiment and, in case of beam-sensitive materials, this time-consuming approach might not work.

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Hexagonal tungsten trioxide (h-WO3) has shown great potential for application in electrochromic devices, gas sensors, battery electrodes, and as photo-catalysts. The h-WO3 structure features a unique large network of open hexagonal channels that allow for intercalation. The hydrothermal synthesis of h-WO3 using sodium tungstate dihydrate as a precursor is widely explored, however, the residual alkaline ions are difficult to eliminate during the synthesis. The solvothermal synthesis using tungsten hexachloride as starting materials largely avoids the use of alkaline ions, but the effect of various synthesis parameters is not well-understood yet. To resolve these ambiguities, this study provides a reliable route to obtain h-WO3 via solvothermal synthesis and dehydration annealing. The effects of precursor concentration, water content, synthesis temperature, and synthesis time, as well as dehydration temperature, on the as-synthesized crystal structure and crystal morphology are studied.

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