Organic compound formaldehyde (HCHO) is a toxic volatile gas with an irritating odour, which is mainly derived from building materials, furniture and various adhesive coatings. Living in an environment containing formaldehyde gas for a long time can cause a series of hazards to human health, such as headaches, allergies, nausea, and even genetic mutations and leukaemia. It is thus necessary to design a gas sensor with a superior sensing performance to detect formaldehyde in real time. ABO₃ type chalcogenide metal oxides have attracted much attention in sensor research, but their application is restricted due to the drawbacks such as poor sensitivity and high operating temperature. Many studies indicate that the modification of metal oxides with two-dimensional transition metal dihalides (2D TMDs) is one of the effective strategies to improve the gas sensing performance, and among the 2D TMDs materials, WS₂ has aroused much interest in sensor research because of its high specific surface area, good thermal stability and excellent electron mobility. The gas sensor performance of metal oxides can be improved via the combination of WS₂ to form a novel nanocomposite material. Hence, in this paper, a series of WS₂-CdSnO₃ composites with different mass ratios were prepared by an ultrasonic mixing method. The microstructure, crystal structure, and valence state structure of WS₂-CdSnO₃ composite material were characterized, and the gas sensing performance of the composite material was analyzed.

CdSnO₃ was prepared with Cd(NO₃)₂·4H₂O (AR, Shanghai Sarn Chemical Technology, China) and SnCl₄·5H₂O (AR) (Shanghai Aladdin Reagent Co., China). CdSnO₃ was ultrasonically compounded with WS₂ (99.9%) to obtain WS₂-CdSnO₃ composites. The physical phase composition, surface morphology, elemental composition, valence and pore size distribution and specific surface area of the composites were characterized by a model SmartLab SE X-ray diffractometer (XRD, Rigaku Co.,), scanning electron microscope (SEM, Tescan Mira Lms), a model K-Alpha X-ray photoelectron spectrometer (XPS, Thermo Scientific Co., USA), and a model ASAP 2460 fully automated specific surface and porosity analyser (BET, Micromeritics Co., USA).

For the determination of gas sensing performance of WS₂-CdSnO₃ composite material, WS₂-CdSnO₃ composite material was put into a mortar with 1-2 drops of terpineol, and thoroughly mixed and ground. The paste was evenly applied to the outer wall of the alumina ceramic tube with a small brush, and an Ni-Cr heating wire put through the ceramic tube and welded firmly to the hexagonal base. The operating temperature was indirectly controlled via adjusting the amount of power added to the two sections of the heating wire. The sensitivity to gases is expressed as a ratio of the stable resistance value of the gas sensing material in air to a stable resistance value in the gas tested. The gas sensing performance of the composites was investigated at a relative humidity of 55% (except for humidity tests).

From the SEM images, the microcube appeared is CdSnO₃, and the cubes structure is agglomerated on the surface of layered WS₂. Based on the EDS analysis, there are five elements Cd, Sn, O, W and S in the composites. It is indicated that WS₂-CdSnO₃ can be prepared. According to the XPS analysis, there are more oxygen vacancies in the sample WSC-2, which can provide more sites for oxygen adsorption, so that more target gases participate in the reaction on the surface of the material. The introduction of WS₂ increases the specific surface area of the sample WSC-2, and the material with a larger specific surface area has more gas adsorption active sites, thus improving the gas sensing performance of the sensor. The sample WSC-2 has the maximum response value of 15.7 for 0.01% formaldehyde at 140 ℃. To check the potential practical application of the prepared WSC-2 sensor, the selectivity and sensitivity of the sensor to different gases were investigated at 140 ℃. The results show that the sample WSC-2 has the optimum selectivity for 0.01% formaldehyde, and the response value is 4.2 times higher than that of CdSnO₃.

A series of WS₂-CdSnO₃ composites with different ratios were prepared via ultrasonic dispersion mixing. WS₂-CdSnO₃ had a large specific surface area, which was beneficial for gas adsorption and diffusion. The gas sensing performance of CdSnO₃ with different ratios of WS₂-CdSnO₃ composites were investigated, and the results showed that 3.5% WS₂-CdSnO₃ had the optimum selectivity for formaldehyde and the superior gas sensing performance at 140 ℃, and the sensitivity for 0.01% formaldehyde was 4.2 times higher than that of CdSnO₃ material, and the detection limit was 0.0001%. Therefore, the 3.5% WS₂-CdSnO₃ composite had potential applications in indoor formaldehyde detection. This study indicated that the modification of metal oxides by two-dimensional TMD could be one of the effective strategies to improve the gas sensing performance.