Ammonia is a toxic and harmful gas that affects the environment and human health. It is thus of great significance to identify and detect ammonia and its concentration rapidly and accurately for human health monitoring and ecological environment management. TiO₂ is widely used in the field of gas sensors due to its advantages of good stability, environmental tolerance, non-pollution, and outstanding contact reaction properties. However, it is gradually found that it has the disadvantages of long response/recovery time and low response value. MXene has a great potential in the field of gas sensing because of its large specific surface area and superior electron mobility rate, but its response value is rather low. In this work, TiO₂/Ti₃C₂ composites were prepared by an one-step hydrothermal method. The synergistic effect of semiconductor properties of TiO₂ and excellent electron transport performance of Ti₃C₂ was analyzed.

TiO₂/Ti₃C₂ composites at different hydrothermal temperatures were prepared by a hydrothermal method with MXene–Ti₃C₂ and butyl titanate as raw materials. The physical phase, surface morphology, specific surface area and pore size distribution, and structure of the samples were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), surface area measurement based on BET, and Raman spectroscopy. In addition, the ammonia sensing properties of TiO₂/Ti₃C₂ composites at room temperature were also analyzed.

TiO₂/Ti₃C₂ composites can be prepared by a hydrothermal method. The XRD patterns indicate that the characteristic peaks of Ti₃C₂ and the diffraction peaks of anatase TiO₂ both appear in the composites. The Raman spectra show the generation of anatase TiO₂. The SEM images indicate that TiO₂ nanoparticles are uniformly loaded on the interlayer and surface of Ti₃C₂. The agglomeration of TiO₂ particles becomes more severe with the increase of hydrothermal temperature. The N₂ adsorption-desorption experiment shows that the specific surface area of prepared TiO₂/Ti₃C₂ is much larger than that of Ti₃C₂. Moreover, the specific surface area of TiO₂/Ti₃C₂-150 is the largest, but it gradually decreases with the increase of hydrothermal temperature possibly due to the agglomeration of the generated TiO₂ particles. The gas sensing test results show that TiO₂/Ti₃C₂ composites and pure TiO₂ have responses to 100 μL/L ammonia at room temperature, while Ti₃C₂ has little response. The response values of TiO₂/Ti₃C₂ composites are significantly better than that of pure TiO₂ obtained at the same temperature. Among all the samples, TiO₂/Ti₃C₂-150 has the maximum response value (i.e., 8.37). Also, the response value of the composites increases with the increase of ammonia concentration, showing a high linear correlation. After the stability test for one month, the response value of TiO₂/Ti₃C₂-150 to 100 μL/L ammonia is only decreased by 14.1%, indicating a long-term stability. Compared to TiO₂-150, the response time of TiO₂/Ti₃C₂-150 decreases from 43 s to 28 s, and the recovery time reduces from 49 s to 37 s, manifesting a high selectivity to ammonia. The stable gas absorption/desorption reaction of TiO₂ and thesuperior electron transport rate of Ti₃C₂ can enhance the response of TiO₂/Ti₃C₂ to ammonia.

The nanoparticles of anatase TiO₂ were supported on the interlayer and surface of Ti₃C₂ by a hydrothermal method. At the hydrothermal temperature of 150 ℃, TiO₂ particles were supported evenly and agglomerated weakly, with a greater specific surface area and smaller particle size. Compared to pure TiO₂ and Ti₃C₂, TiO₂/Ti₃C₂-150 had a good response to ammonia at room temperature. At an ammonia concentration of 300 μL/L, the gas sensing response value of TiO₂/Ti₃C₂-150 was 34.77, which was greater than that of pure TiO₂. In addition, TiO₂/Ti₃C₂-150 also showed a rapid response recovery rate, a good selectivity and a long-term response stability, having a promising application prospect in the field of rapid detection of ammonia at room temperature.

Thermal expansion analysis is a most widely used method for determining the firing temperature of ancient ceramics, which infers the original firing temperature of ancient ceramics via analyzing the inflection point of the thermal expansion curve. However, for the pottery at an original firing temperature that is lower than the formation temperature of glass phase, the inflection point on the thermal expansion curve is actually the formation temperature of glass phase rather than original firing temperature. To solve a problem of inaccurate determination of low-temperature pottery firing temperature by a thermal expansion method, clay samples fired at different temperatures were prepared with clay as a raw material, and analyzed by X-ray diffraction (XRD), synchronous comprehensive thermal analyzer (TG–DTA), and scanning electron microscopy (SEM). In addition, the first derivative curve corresponding to the top rod thermal expansion curve of clay samples fired at different temperatures fitted to the dehydroxylation expansion peak of kaolinite, and the relationship curve between the dehydroxylation expansion peak of kaolinite and the original firing temperature was also obtained.

A clay as a raw material was processed by grinding, screening, drying, pressing, and sintering at different temperatures. Afterwards, the cylindrical samples with a diameter of 5 mm and a length of 25 mm were prepared with clay by polishing. The thermal behavior of clay raw materials was determined by thermogravimetry-differential thermal analysis (TG–DTA). The chemical composition, phase composition, and microstructure of the raw materials and the samples fired at different temperatures were characterized by X-ray fluorescence spectroscopy (XRF), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The linear expansion or contraction process of the samples during the refiring process at different firing temperatures was tested by a thermal expansion instrument.

The XRF spectra show that the chemical composition of raw materials and samples at different firing temperatures is the similar. The XRD patterns and TG–DTA analysis of raw materials indicate that the dehydroxylation of kaolinite occurs at 300–600 ℃. Based on the XRD patterns, the phases in clay samples fired at different temperatures are consistent with those in clay raw materials. At < 900 ℃, the characteristic diffraction peak intensity of kaolinite in clay samples fired at different temperatures changes with the increase of firing temperature. The SEM images show that the typical layered structure of kaolinite in clay raw material begins to lose its structural hydroxyl groups and gradually transforms into metakaolinite, and this transformation gradually deepens as the firing temperature increases. The results of the thermal expansion test of clay samples fired at different temperatures indicate that there is a linear relationship between the dehydroxylation expansion peak of kaolinite on the first derivative curve of thermal expansion and its original firing temperature. This linear relationship can be expressed by a formula, i.e., A= –2.2784817×10^–6T+3.3378709×10^–3. To verify the accuracy of the formula, the thermal expansion test of clay samples is conducted using a push rod, and the calculated firing temperature is not significantly different from its original firing temperature, which are 41℃ and 35 ℃, respectively. The push rod thermal expansion method can be thus used to determine the original firing temperature of low-temperature pottery made from clay.

Before the glass phase formation temperature, kaolinite in the sample could lose the hydroxyl group in its structure and gradually transform into metakaolinite as the firing temperature increases, thus causing the expansion of the thermal expansion curve. The intensity of the thermal expansion peak of kaolinite dehydroxylation gradually decreased as the firing temperature increased. The first-order derivatives of the thermal expansion curves of the samples fired at different temperatures were taken, fiting the kaolinite dehydroxylation peak. This linear relationship with the firing temperature could be expressed by a formula, i.e., A= –2.2784817× 10^–6T+3.3378709× 10^–3. The clay samples fired at 550 ℃ and 650 ℃ were used for the thermal expansion curves, respectively. The clay samples fired at 550 ℃ and 650 ℃ were used to verify the reasonableness of the fitting formula. It was indicated that the firing temperature of low-temperature pottery made of clay could be determined by the top bar thermal expansion method.