WO₃ Nanolamella Gas Sensor: Porosity Control Using SnO₂ Nanoparticles for Enhanced NO₂ Sensing

Tungsten trioxide (WO₃) is one of the important multifunctional materials used for photocatalytic, photoelectrochemical, battery, and gas sensor applications. Nanostructured WO₃ holds great potential for enhancing the performance of these applications. Here, we report highly sensitive NO₂ sensors using WO₃ nanolamellae and their sensitivity improvement by morphology control using SnO₂ nanoparticles. WO₃ nanolamellae were synthesized by an acidification method starting from Na₂WO₄ and H₂SO₄ and subsequent calcination at 300 °C. The lamellae were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), which clearly showed the formation of single-crystalline nanolamellae with a <i>c</i>-axis orientation. The stacking of each nanolamella to form larger lamellae that were 50–250 nm in lateral size and 15–25 nm in thickness was also revealed. From pore size distribution measurements, we found that introducing monodisperse SnO₂ nanoparticles (ca. 4 nm) into WO₃ lamella-based films improved their porosity, most likely because of effective insertion of nanoparticles into lamella stacks or in between assemblies of lamella stacks. In contrast, the crystallite size was not significantly changed, even by introducing SnO₂. Because of the improvement in porosity, the composites of WO₃ nanolamellae and SnO₂ nanoparticles displayed enhanced sensitivity (sensor response) to NO₂ at dilute concentrations of 20–1000 ppb in air, demonstrating the effectiveness of microstructure control of WO₃ lamella-based films for highly sensitive NO₂ detection. Electrical sensitization by SnO₂ nanoparticles was also considered