Two Dimensional Mxene (Ti3C2) Decorated By SnO2 Nanocrystals for Enhanced Chemical Gas Sensors at Room Temperature
Authors: He Tingting , Huang Baoyu , Li Xiaogan 
Published: July 2021

Introduction
A low-cost chemical sensor for ammonia (NH3) detection is highly demanded nowadays for several important applications. Two-dimensional (2D) nanomaterials such as graphene/reduced graphene oxide (rGO), transitional metal dicharcogenides, black phosphor and transition metal carbonitride (MXene) have found a promising application in the fields of gas sensing electronics at room temperature due to their unique microstructure, electrical properties and excellent gas adsorptions ability. Decoration of the secondary phase such as noble metals and metal oxide nanocrystals to form heterojunctions with two-dimensional materials can effectively improve the inadequacy of two-dimensional materials based gas sensors. It takes advantages of the difference in the Fermi level of each phase to make the electrons transfer from one material to the other and thus enriching the electron concentration on the surface of one material that is dominant in the electrical transport. In this way, more ionized oxygen is chemisorbed, leading to the improvement of the gas sensing properties [1].
In this work, 2D MXene heterojunctions incorporated with SnO2 nanoparticles have been synthesized by using hydrothermal method. The as-fabricated MXene/SnO2 heterojunction based chemiresistive-type sensor showed excellent sensitivity to different concentrations of ammonia from 0.5-100 ppm at room temperature. The response has about 20 times higher than those reported in literature to 100 ppm NH3 and an excellent selectivity.
Preparation and characterizations
The Ti3AlC2 was purchased directly from Beijing Forsman Corp. The concentrated hydrochloric acid (HCl) was diluted to 9 M. 2 g of lithium fluoride (LiF) was added into 20 ml diluted hydrochloric acid and stirred with magnetic mixer. 2 g Ti3AlC2 was slowly and evenly dispersed in the solution over a period of several minutes and subsequently stirred at 35 ℃ for 48 h to obtain the MXene. 70 mg of MXene powder was dispersed in 30 ml of deionized water after grinding and sonicated for 30 min. 73.5 mg of stannic chloride pentahydrate (SnCl4·5H2O) was added into the suspension and stirred with magnetic stirrer at room temperature. The suspension was then loaded in a 50 ml Teflon-lined autoclave. The autoclave was sealed and heated in an KSL-1200X hydrothermal system at 180 ℃ for 12 h to obtain the MXene-SnO2 hybirds. The samples were then characterized by XRD, SEM, XPS, Raman and TEM-ED. The MXene/SnO2 based chemirestive-type sensor was measured by a static volumetric method. The resistance of the sensor is directly measured by a digital multimeter (Agilent 34410A). The responses of the sensors were defined as the relative change in the resistances of the sensors in the air and those in the tested gas.

Results and Conclusions

Fig.1a shows the XRD of the prepared samples. Appearance and down-shift of the peak at ~10 oC indicates the successful achievements of MXene and thus the MXene-SnO2 composite heterojunctions [2-3].Fig.1 (b-c) show the micrographs of the surface morphology of the MXene and heterojunctions. The etched MXene material has obvious layered structure with sizes ranging from 2 μm to 4 μm. The thickness of MXene is about 75 nm indicating the sample contains about 100 single layers of MXene. The granular SnO2 nanoparticles are intimately attached to the surface of MXene. Fig.2 (b) shows the Raman spectra of MXene and Mxene/SnO2 heterojunction. The appearance of both D-band and G-band in the MXene/SnO2 heterojunction sample indicated that MXene remained well during the hydrothermal reaction. Fig.2(c) shows the Ti core level (2p) spectra of the MXene and MXene/SnO2. Ti - X corresponds to titanium carbide or titanium oxynitride. Fig.2d indicates that the peaks at 530.1 and 532 eV correspond to oxygen vacancies and O=C-OH, respectively, suggesting that these and surface-adsorbed oxygen tend to form more active sites, thereby enhancing sensing capability.
The sensors using MXene/SnO2 heterojuction shows excellent response and selectivity to low-level NH3 from 0.5-9 ppm at RT (Fig.3). Since the work function of MXene (3.4 eV) was slightly lower than that of SnO2 (4.7 eV), Schottky-type junctions were formed across MXene/SnO2 interfaces (Fig.4). Electrons would transfer from SnO2 to MXene, and the depletion layer was deepened on the surface of MXene. MXene could then chemically adsorb more oxygen, and the response of the MXene/SnO2 heterojunction was thus significantly enhanced.

References

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Figure 1