Monovalent sulfur oxoanions enable millimeter-long single-crystalline h-WO3 nanowire synthesis

Abstract

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.