Highly Selective Room Temperature Detection of NH₃ and NO_x Using Oxygen-Deficient W₁₈O₄₉-Supported WS₂ Heterojunctions

In this paper, we reported the controlled synthesis of tungsten disulfide/reduced tungsten oxide (WS₂/W₁₈O₄₉) heterojunctions for highly efficient room temperature NO_x and ammonia (NH₃) sensors. X-ray diffraction analysis revealed the formation of the oxygen-deficient W₁₈O₄₉ phase along with WS₂. Field-emission scanning electron microscopy and transmission electron microscopy displayed the formation of WS₂ flakes over W₁₈O₄₉ nanorods. X-ray photoelectron spectroscopy showed the presence of tungsten in W⁴⁺, W⁵⁺, and W⁶⁺ oxidation states corresponding to WS₂ and W₁₈O₄₉, respectively. The WS₂/W₁₈O₄₉ heterojunction sensor exhibited sub-ppm level sensitivity to NO_x and NH₃ at room temperature. The heterojunction sensor detected 0.6 ppm NO_x and 0.5 ppm NH₃, with a corresponding response of 7.1 and 3.8%, respectively. The limit of detection of the sensor was calculated to be 0.05 and 0.17 ppm for NH₃ and NO_x, respectively. The cyclic stability test showed that the sensor exhibited high stability even after 24 cycles for the detection of NH₃ and 14 cycles for NO_x. Compared to pristine WO₃ and WS₂, the WS₂/W₁₈O₄₉ heterojunction showed high selectivity toward NO_x and NH₃. The results could be useful for the development of room temperature NO_x and NH₃ sensors.