Formaldehyde as a volatile organic compound (VOCs) is one of the main indoor gas pollutants produced by building materials, furniture and interior decoration industry. The excessive exposure to formaldehyde can affect human-being health, and lead to nerve damage. Inhaling lower concentrations of formaldehyde can cause cough, allergies, headaches and other symptoms. It is thus of great significance to monitor formaldehyde efficiently, control indoor environmental pollution and maintain human health. In recent years, spinel oxides have some potential application prospects in the field of gas sensing due to their stable crystal structure and good gas sensing performance. CdGa₂O₄ is a kind of wide-band gap semiconductor spinel structure oxide, which has a good gas sensitivity to formaldehyde gas, but it has some problems such as complex preparation process, low yield, high working temperature and low response. Some studies show that metal oxides modified with two-dimensional materials can effectively improve the performance of gas sensors. MXene, represented by Ti₃C₂T_x, is a class of two–dimensional transition metal carbides or carbonitrides, which is widely concerned because of its unique layered structure, large specific surface area, rich functional groups and good electrical conductivity. In this paper, CdGa₂O₄ nanomaterials were prepared by a solvothermal method. On this basis, Ti₃C₂T_x–CdGa₂O₄ composites with different proportions were prepared by a hydrothermal method. The morphology, surface chemical composition and valence state of the elements were investigated, and the gas–sensitive properties of the composite were analyzed.

Ti₃C₂T_x was prepared by using the mixed solution of concentrated hydrochloric acid and LiF (AR, Shanghai Aladdin Reagent Co., China) as an etching agent, and the Al atomic layer in Ti₃AlC₂ was removed by HF generated. CdGa₂O₄ was prepared from Cd (NO₃)₂·4H₂O (AR, Shanghai Sarn Chemical Technology Co., China) and Ga (NO₃)₃·xH₂O (AR, Shanghai Maclin Chemical Technology Co., LTD., China). The Ti₃C₂T_x–CdGa₂O₄ composite was prepared by mixing CdGa₂O₄ with Ti₃C₂T_x magnetically in different proportions, and then reacting at 180 ℃ for 12 h by a hydrothermal method.The phase composition of the materials was analyzed by a model D8 Advance X-ray diffractometer (XRD, Bruker Co., Germany, 40 kV, 40 mA, Cu K_α, λ=0.15406 nm, 5°–70°) and analyzed by a model JSM–7800F scanning electron microscope (SEM, accelerated voltage 15 kV). The elemental composition of the material was analyzed by a model ESCALAB 250 XI X–ray photoelectron spectrometer (XPS, Thermo Co., USA, accelerated voltage 15 kV), and the specific surface area and pore size distribution of the material were determined by a model ASAP2010C nitrogen adsorption and desorption instrument (Micromeritics Co., USA, accelerated voltage 15kV).

To determine the gas–sensitive properties of Ti₃C₂T_x–CdGa₂O₄ composite material, the sample was ground into a pulp with turpentine permeation alcohol, uniformly coated on the outer wall of alumina ceramic tube. After drying, Ni–Cr heating wire was installed and welded into para–heated components. The working temperature was controlled via adjusting the power added at the both ends of the heating wire and tested after calcination at 170 ℃ for 4 h. The sensitivity was defined as a ratio of the material stability resistances in air and a target gas, and the relative humidity of the test environment was constant (i.e., 50%).

The SEM images show that granular CdGa₂O₄ accumulates on the surface of Ti₃C₂T_x with a layered structure. The EDS spectra show that there are elements Cd, Ga and O in the composites, and the ratio is 1:2:5:7, which may be caused in the presence of Ga₂O₃ components in the composites. According to the adsorption desorption isotherm analysis of XPS and N₂, the functional groups (—O, —OH, —F) in Ti₃C₂T_x provide additional active sites for the adsorption of gas molecules. The addition of Ti₃C₂T_x builds the mesoporous structure, and also increases the specific surface area of the composite, exposing more gas adsorption active sites, thus improving the gas sensitive performance of the sensor. The maximum response value of sample TC–3 to 0.01% formaldehyde at 110 ℃ is 105, which is 9.4 times greater than that of pure CdGa₂O₄. The selectivity and sensitivity of the sensor to different gases at 110 ℃ are investigated. The results show that the sample TC–3 has good selectivity and detection limit for formaldehyde, and the lowest detection limit is 0.00001%, which proves that the prepared sensor TC–3 has a great practical application potential.

In this paper, CdGa₂O₄ nanomaterial was prepared by a solvothermal method, Ti₃C₂T_x with a layered structure was prepared with LiF and concentrated hydrochloric acid system, and then Ti₃C₂T_x–CdGa₂O₄ composite was prepared by a hydrothermal method. The gas sensitive properties of CdGa₂O₄ and a series of Ti₃C₂T_x–CdGa₂O₄ composites were investigated. The results showed that at 110 ℃, the response value of sample TC–3(7% Ti₃C₂T_x–CdGa₂O₄) composite to 0.01% formaldehyde gas was 105, the lowest detection limit was 0.00001%, compared with pure CdGa₂O₄, thus improving the sensitivity and reducing the operating temperature. The combination of appropriate amount of Ti₃C₂T_x could be conducive to improving the gas sensitivity of CdGa₂O₄ material to formaldehyde gas.