S, N Co-Doped Carbon Dot-Functionalized WO₃ Nanostructures for NO₂ and H₂S Detection

This paper reports the synthesis of a S, N co-doped carbon dot (C-dot)-functionalized WO3 (C(S, N)-WO3) nanostructure via a hydrothermal method, which exhibits ultrasensitivity for NO2 at the ppb level and H2S at the ppm level along with remarkable selectivity. The WO3 morphology is optimized by varying the pH (1.5-3.5) of the solution. Characterization results reveal that the surface morphology at pH 3 is superior for producing uniform nanoneedle nanostructures with high sensitivity toward NO2 and H2S. To further enhance the sensitivity and selectivity of WO3 nanoneedles, the effect of mixing of C-dots into WO3 is investigated systematically. The optimized weight of C-dots in the C(S, N)-WO3 composite is 10 mg, which exhibits sensitivity values of 2.2 to 250 ppb for NO2 and 29.5 to 100 ppm for H2S. The C(S, N)-WO3 composite material shows the highest sensitivity at an optimum substrate temperature of 150 degrees C, without an obvious influence of humidity up to 50% relative humidity. The limit of detection of the composite sensor is 250 ppb NO2 along with excellent repeatability and good long-term stability.