The SnSe₂/SnO₂ heterojunctions fabricated through in situ thermal oxidation display high response and excellent selectivity to NO₂ at low working temperature.

In this work, MXene/NiO-composite-based formaldehyde (HCHO) sensing materials were successfully synthesized by an in situ precipitation method. The heterostructures between the MXene and NiO nanoparticles were verified by transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The HCHO sensing performance of the MXene/NiO-based chemiresistive-type sensors was investigated. Compared to pure MXene and NiO materials, the sensing performance of the MXene/NiO-P2-based sensor to HCHO gas at room temperature was significantly enhanced by the formation of MXene/NiO heterojunctions. The response of the MXene/NiO-P2 sensor to 50 ppm HCHO gas was 8.8, which was much higher than that of the pure MXene and NiO. At room temperature, the detectable HCHO concentration of the MXene/NiO-P2-based sensor was 1 ppm, and the response and recovery time to 2 ppm HCHO was 279 s and 346 s, respectively. The MXene/NiO-P2 sensor also exhibited a good selectivity and a long-term stability to HCHO gas for 56 days. The in situ Fourier transform infrared (FTIR) spectra of the MXene/NiO-P2 sensor, when exposed to HCHO gas at different times, were investigated to verify the adsorption reaction products of HCHO molecules.

Layered Au/SnS₂ nanosheet based chemiresistive-type sensors were successfully prepared by using an in situ chemical reduction method followed by the hydrothermal treatment. SEM and XRD were used to study the microscopic morphology and crystal lattice structure of the synthesis of Au/SnS₂ nanomaterials. TEM and XPS characterization were further carried out to verify the formation of the Schottky barrier between SnS₂ nanosheets and Au nanoparticles. The as-fabricated Au/SnS₂ nanosheet based sensor demonstrated excellent sensing properties to low-concentrations of NO₂, and the response of the sensor to 4 ppm NO₂ at 120 °C was approximately 3.94, which was 65% higher than that of the pristine SnS₂ (2.39)-based sensor. Moreover, compared to that (220 s/520 s) of the pristine SnS₂-based sensor, the response/recovery time of the Au/SnS₂-based one was significantly improved, reducing to 42 s/127 s, respectively. The sensor presents a favorable long-term stability with a deviation in the response of less than 4% for 40 days, and a brilliant selectivity to several possible interferents such as NH₃, acetone, toluene, benzene, methanol, ethanol, and formaldehyde. The Schottky barrier that formed at the interface between the SnS₂ nanosheets and Au nanoparticles modulated the conducting channel of the nanocomposites. The “catalysis effect” and “spillover effect” of noble metals jointly improved the sensitivity of the sensor and effectively decreased the response/recovery time.

Gold nanoparticles decorated WS${}_2$ microflakes (Au/WS${}_2$) have been synthesized by an in situ chemical reducing process. A chemiresistive-type sensor using as-synthesized Au/WS${}_2$ heterostructures as sensing materials shows an improved response to different concentrations of ammonia compared to pure WS${}_2$ at room temperature. As the concentrations of gold nanoparticles increased in heterostructures, response/recovery speeds of the sensors became faster although the sensitivity of the sensor was compromised compared to the sensitivity of the sensor with lower concentrations of Au. In addition, the Au/WS${}_2$-based sensor indicated excellent selectivity to formaldehyde, ethanol, benzene and acetone at room temperature. The improved performance of the sensors was attributed to the synergistic effect of electronic sensitization and chemical sensitization between WS${}_2$ and Au.

Tungsten sulfide decorated with indium oxide nanoparticles (In₂O₃/WS₂) was studied for a chemiresistive-type NH₃ sensor at room temperature. It was found that the responses of the developed In₂O₃/WS₂ heterostructure nanocomposite-based sensors are significantly improved to 3.81 from 1.45 for WS₂. The response and recovery time of the heterostructure-based sensor was found to significantly decrease to 88 s/116 s (10 ppm) from 112 s/192 s for the WS₂-based one. The sensor also exhibits excellent selectivity and signal reproducibility. In comparison to WS₂ decorated with both ZnO and SnO₂ in similar ways, the In₂O₃-decorated WS₂ has overall better sensing performance in terms of sensitivity, selectivity and response/recovery speeds for NH₃ from 1 ppm to 10 ppm at room temperature. The improved sensing properties of WS₂ incorporating In₂O₃ could be attributed to the joint enhancement mechanisms of the “electronic and catalytic” sensitizations.