Abstract
This work demonstrates the first utilization of natural palm sugar to induce the direct formation of hexagonal tungsten oxide (WO3) with a hierarchical three-dimensional (3D) structure assembled by nanorods. The concentration of palm sugar controls the final phase and morphology of the WO3 product. Lower concentrations of palm sugar favor the formation of hexagonal WO3 with a nanorod-assembled hierarchical 3D morphology, while higher concentrations promote the formation of spherical monoclinic WO3 particles. When employed for the sensing of toxic nitrogen dioxide (NO2) gas, the hierarchical 3D hexagonal WO3 particles exhibit a very high response to NO2 gas at an optimum temperature of 250 °C, while also showing outstanding selectivity and stable responses toward NO2 gas over a period of 24 days. The superior NO2 sensing performance of these particles is attributed to the hierarchical 3D structure, which can offer more adsorption sites for NO2 molecules, while the assembling nanorods may provide more effective diffusion and transport for the NO2 molecules. Furthermore, the density functional theory (DFT) calculations reveal that NO2 can gain more charge from hexagonal WO3 than from monoclinic WO3 upon adsorption, leading to a greater change in resistance.