Exploration of New Two Dimensional Titanium Carbides for Room Temperature Gas Sensors
Authors: Lee Eunji , Mohammadi Armin Vahid , Lee Doohee , Yoon Jaesik , Lincoln Christopher , Beidaghi Majid , Woo Sungpil , Yoon Youngsoo , Kim Dong-Joo 
Published: February 2020

The combination of progressive development in gas sensor with a micro and nano technology have potentialized the detection of various gases in huge application such as environmental monitoring system and medical diagnosis. In particular, practical applications can be widely deployed from condition monitoring of grand plantation to sophisticated physiological analysis via patent’s breath due to the capability of gas sensor to identify and quantify multiple gases [1, 2]. In order to be more pragmatic, emphasis has been placed on more functionalities such as portability and wearability. For portable and wearable gas sensor devices, electronic hardware should be flexible for a comfortable fit, and gas detection should work at low temperatures without a power supply. However, commercialized sensing materials are based on metal oxides such as SnO2, ZnO and NiO with working temperatures over 100 °C, which in principle limits application towards wearable gas sensors [3]. Therefore, it is desirable to investigate a suitable new material that can respond at lower working temperature and can be fabricated on flexible substrates.
Two-dimensional transition metal carbides and/or carbonitrides (called MXenes), which are a new family of 2D materials, have attracted huge attention due to their attractive properties in different applications such as energy storage, water purification, electromagnetic interference shielding, gas sensing, etc [4]. Ti3C2 is the first discovered and most studied MXene material that is produced by selective etching of Al atoms from the Ti3AlC2 MAX phase, a hexagonal structured ternary carbide. The surface of the produced Ti3C2 nanosheets, like any other produced MXene, is terminated with oxygen, hydroxyl, and fluorine groups where the concentration of these surface functional groups is largely dependent on the synthesis method [5]. Consequently its properties are dependent on the type and concentration of these functionality groups, through which MXenes with different properties can be tailored for different applications [6].
Herein, we report for the first time to our knowledge, on gas-sensing capabilities of Ti3C2 MXene to detect various gases. We demonstrate the sensing properties of a Ti3C2 device for 100 ppm ammonia, acetone, methanol and ethanol gas bubbling at room temperature. Ti3C2 MXene was synthesized by selective removal of Al from Ti3AlC2 MAX phase using LiF salt and HCl acid in a safer way, and Ti3C2-based gas sensor was fabricated on a flexible polyimide film via simple solution method. The structure of the synthesized Ti3C2 was analyzed with XRD, SEM and EDS, and the surface condition was examined by FTIR. During the gas sensing test, its initial resistance at room temperature was around 10~20kΩ with 8µm film thickness, and it showed p type gas sensing behavior to all four different types of donor gases. The predicted gas sensing mechanism of Ti3C2 will be suggested with a concept of physisorption and chemisorption, and its gas response mechanism will be discussed.
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Acknowledgement
This research was partially supported by the International Collaborative Energy Technology R&D Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (20158520000210).