Controllable synthesis of ultrathin WO3 nanotubes and nanowires with excellent gas sensing performance

Tungsten oxides (stoichiometric and non-stoichiometric) have gained extensive research interests because of promising physicochemical properties and diverse applications. Ultrathin walled tungsten oxide (WO3) nanotubes have been fabricated by hydrothermal route in the presence of both K2SO4 and citric acid under mild conditions. WO3 nanocrystals randomly assembled into ultrathin layered structures with highly peaked end which are then interwoven into nanotubes (diameter of 10−15 nm, wall thickness of 1−2 nm, width of 21 nm), because of Ostwald Ripening growth kinetics. Non-stoichiometric WO3-x nanowires (diameter 10−15 nm) were also synthesized using K2SO4 in the absence of organic ligand. The Brunauer–Emmett–Teller (BET) surface area for h-WO3 nanotubes was 71.95 m2 g−1 higher than WO3-x nanowires (51.83 m2 g−1), WO3 nanowires (42.70 m2 g−1) and nanorods (24.60 m2 g−1). Photoluminescence (PL) spectra show that the h-WO3 nanotubes and WO3-x nanowires have more crystal defects and oxygen vacancies. Time-dependent experiments were conducted and the plausible growth mechanism of h-WO3 nanotubes was proposed. The WO3 nanostructures with different morphologies were used as gas sensors that could detect acetone and ethanol with superior sensing performance (Ra/Rg for WO3 nanotubes = 32 & 26, respectively) under mild conditions, notably attributed to the ultrathin wall with high surface areas, crystal defects and oxygen vacancies.