Monovalent Sulfur Oxoanions Enable Millimeter-Long Single Crystalline h-WO3 Nanowire Synthesis
Here we discuss a chemical capping misunderstanding, which has intrinsically hindered to extend the length of hexagonal(h)-WO3 nanowires in previous studies. Although divalent sulfate ions (SO42-) have been strongly believed as efficient capping ions to direct an anisotropic h-WO3 nanowire growth, we found that the presence of SO42- is highly detrimental for the anisotropic crystal growth of h-WO3 nanowires, and a monovalent sulfur oxoanion (HSO4-) rather than the SO42- only substantially promotes the anisotropic h-WO3 nanowire growths. Ab initio electronic structure simulations reveal that the monovalent sulfur oxoanions are more preferentially able to cap the sidewall plane (100) of h-WO3 nanowires due to the lower hydration energy when compared with SO42-. Based on this capping strategy, using monovalent sulfur oxoanion (CH3SO3-), which cannot generate divalent sulfur oxoanions, we successfully fabricate ultra-long h-WO3 nanowires up to millimeter range (1.2 mm) for the wider range of precursor concentrations. We demonstrate the feasibility of these millimeter-long h-WO3 nanowires for electrical sensing of molecules (lung cancer biomarker: nonanal) on the flexible substrates, which can be operated at room temperature with the mechanical flexibility even bending cycles up to 104 times due to the enhanced textile effect.