Monovalent Sulfur Oxoanions Enable Millimeter-Long Single Crystalline h-WO3 Nanowire Synthesis

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