Potassium polytungstate nanoparticles by combustion aerosol technology for benzene sensing

Polytungstates are oxygen-linked assemblies of highly oxidized tungsten polyhedra, valued for their tunability and stability in diverse applications. Traditional synthesis methods (hydrothermal, solvothermal, solid-state) offer material variety but are limited in scalability and their ability to yield nanostructured materials due to long reaction times and high temperatures. Here, we introduce flame aerosol synthesis as a single-step, rapid and dry method to prepare K$_2$W$_7$O$_{22}$ nanoparticulate powders and coatings. Thereby, monocrystalline and phase-pure K$_2$W$_7$O$_{22}$ with varying crystal-sizes were obtained by controlling flame temperature, residence time and metal ion concentration during particle formation by nucleation, coagulation and sintering. X-ray diffraction and electron microscopy identified the high potassium tolerance of the K$_2$W$_7$O$_{22}$ lattice (K/W ratio up to 0.6) and phase stability up to 400 $^\circ$C, before other polytungstates and WO$_3$ polymorphs were formed, respectively. Porous films of such K$_2$W$_7$O$_{22}$ nanoparticles featured n-type semiconductor behavior that was utilized for the chemoresistive quantification of the air pollutant benzene down to 0.2 parts-per-million at 20% relative humidity. Such sensors were quite selective over other compounds (e.g. alcohols, aldehydes, ketones, CO, NH$_3$ or H$_2$), in particular to chemically similar toluene and xylene (>18).