Research Progress on Ammonia Sensors Based on Ti3C2Tx MXene at Room Temperature: A Review

Abstract

Ammonia (NH₃) potentially harms human health, the ecosystem, industrial and agricultural production, and other fields. Therefore, the detection of NH₃ has broad prospects and important significance. Ti₃C₂T_x is a common MXene material that is great for detecting NH₃ at room temperature because it has a two-dimensional layered structure, a large specific surface area, is easy to functionalize on the surface, is sensitive to gases at room temperature, and is very selective for NH₃. This review provides a detailed description of the preparation process as well as recent advances in the development of gas-sensing materials based on Ti₃C₂T_x MXene for room-temperature NH₃ detection. It also analyzes the advantages and disadvantages of various preparation and synthesis methods for Ti₃C₂T_x MXene's performance. Since the gas-sensitive performance of pure Ti₃C₂T_x MXene regarding NH₃ can be further improved, this review discusses additional composite materials, including metal oxides, conductive polymers, and two-dimensional materials that can be used to improve the sensitivity of pure Ti₃C₂T_x MXene to NH₃. Furthermore, the present state of research on the NH₃ sensitivity mechanism of Ti₃C₂T_x MXene-based sensors is summarized in this study. Finally, this paper analyzes the challenges and future prospects of Ti₃C₂T_x MXene-based gas-sensitive materials for room-temperature NH₃ detection.

Keywords: Ti3C2Tx MXene; ammonia; gas sensors; room-temperature; sensitivity mechanism.

Figure 4

(a) Dynamic response curves of TT8 h-, TT12 h-, TT16 h- and TT32 h-based sensors; (b) dynamic response curves with UV illumination on T-T-12 h and without UV illumination on TT12 h; (c) schematic illustration of band diagrams of Ti₃C₂T_x and (001) TiO₂; (d) schematic illustration of ammonia gas-sensing mechanism under UV irradiation [64]. (e) The transient response curves of the composite sensors with different WO₃ contents to 1 ppm NH₃ at room temperature; (f) the dependence of the composite sensor response on the WO₃ content; (g) schematic illustration of the gas-sensing mechanism and the energy band structure diagram of Ti₃C₂T_x/WO₃ before and after exposed NH₃ [70].

Figure 5

(a) SEM image of Ti₃C₂T_x MXene; (b) SEM image of PANI/Ti₃C₂T_x; (c) PANI/Ti₃C₂T_x composite nanofiber sensor for the detection of NH₃ at room temperature; (d) UPS spectra of pure Ti₃C₂T_x sensor and (e) PANI/Ti₃C₂T_x composite sensors; (f) the PANI/Ti₃C₂T_x-based flexible sensor bent at different angles [82]. FESEM images of (g) Ti₃C₂T_x MXene (inset is Ti₃C₂T_x MXene on an AAO membrane) and (h) PEDOT:PSS/Ti₃C₂T_x MXene composites. The cross-sectional FESEM images of (i) Ti₃C₂T_x MXene films and (j) PEDOT:PSS/Ti₃C₂T_x MXene films, (k) effect of the Ti₃C₂T_x MXene content in PEDOT-PSS on the sensor response toward 100 ppm NH₃ at room temperature (27 °C), and (l) gas response of the PEDOT:PSS/Ti₃C₂T_x MXene composite-based sensor against 100 ppm NH₃ bent at different angles [94].

Figure 1

Ti₃AlC₂ is etched as Ti₃C₂T_x. (a) Structural schematic diagram of Ti₃AlC₂, (b) schematic of the process by which -OH replaces Al atoms after HF treatment, and (c) hydrogen bond breaking and nanolayer separation after sonication treatment [14].

Figure 2

(a) SEM image of the Ti₃C₂T_x MXene after HF treatment; (b) by comparing the single-layer band structures of Ti₃C₂(OH)₂, Ti₃C₂F₂, and Ti₃C₂, it can be seen that Ti₃C₂T_x exhibits a change from metal to semiconductor due to changes in surface functional groups [14]; (c) XRD pattern of Ti₃AlC₂ and as-prepared Ti₃C₂T_x; (d) SEM images of Ti₃C₂ nanoflakes after exfoliation by TMAOH [33].

Figure 3

Adsorption process of H₂O and NH₃ molecules on the surface of alkalized Ti₃C₂T_x [40].

Figure 6

The resistance variation curves of the SM-5 sensor to (a) ppm-level NH₃ concentration from 2 to 100 ppm and (b) ppb-level NH₃ concentration from 10 ppb to 1 ppm at 18 °C; (c) the long-term stability of the SM-5 sensor to 1 ppm NH₃ for 20 days [56]. (d,e) Ti₃C₂T_x MXene@TiO₂/MoS₂; (f) dynamic sensing performance of the sensor-based Ti₃C₂T_x MXene to NH₃ at a room temperature of 27 °C and an RH of 43%; (g) energy band diagrams of Ti₃C₂T_x MXene@TiO₂/MoS₂ sensors [30].

Figure 7

(a) Comparison of the gas response of MXene film, rGO fiber, and MXene/rGO hybrid fiber; (b) schematic illustration of the fiber bending test. The “M” stands for multimeter. (c) Cyclic bending fatigue versus resistance difference of the rGO fiber and MXene/rGO hybrid fiber [19]. (d) FE-SEM images of Ti₃C₂T_x MXene/GO/CuO/ZnO nanocomposite and (e) pristine Ti₃C₂T_x Mxene. (f) Schematic diagram of the energy band structure of the Ti₃C₂T_x MXene/GO/CuO/ZnO heterostructure [25].

Figure 8

(a) Schematic diagram of the possible gas-sensing mechanisms of the Ti₃C₂T_x MXene for NH₃ [18]. (b) The gas-sensing mechanism diagram of the Ti₃C₂T_x/In₂O₃ composite materials. (c,d) Schematic diagram of the electron transfer at the interface of Ti₃C₂T_x/In₂O₃ composite materials in the air and NH₃ [57].