Wearable electronics have evolved from life-supporting devices for solders to fashion accessories such as smart watches. Wearable devices are not limited to on-body devices because they can be further transformed by integrating with other surfaces such building and vehicles. Accordingly, the market demand for wearables has greatly increased, and the research of gas sensor has attempted to adapt wearables with the power of nanotechnology. On the wearable platform, miniature gas sensors will provide real-time information of the atmosphere to protect each personnel from possible hazardous chemical attacks. In addition, wearable gas sensor can be facilitated to monitor human’s breath as medical applications. [1] The main keys for wearable gas sensors are to decrease working temperature and increase sensing performance. Recently, two-dimensional (2D) nanostructured materials have been highlighted as new sensing materials due to their working ability at low temperature. Unlike conventional metal oxides, 2D materials such as graphene are able to work at room temperature with comparable gas response to NO₂ gas. However, insufficient sensing performance, such as sluggish recovery and low gas response, need to be developed. [2] As one attempt to enhance performance, 2D material was hybridized with metal oxide. By incorporating two nanostructured materials, synergistic effect, which comes from benefits of each materials, can improve sensing performance. [3] Another novel innovation is the introduction of newly discovered 2D materials. 2D transition metal carbides and/or carbonitrides (called MXenes) are a new family of 2D materials, and much interest is being paid to them due to their attractive properties for many different applications, particularly in energy storage. Ti₃C₂ is the first discovered and most researched MXene. Considering the surface functional groups on Ti₃C₂, this MXene has the potential to be suitable material for wearable gas sensing applications working at low temperature. [4]

In this study, 2D nanostructured materials and their hybrids were investigated for wearable gas sensing applications. Graphene oxide (GO) was incorporated with TiO2 nanoparticles, and the composite was photo-reduced under UV irradiation. Room temperature gas sensing was carried out against various VOC gases, and sensing performance was evaluated by comparing with pure GO. With the tailored hetero-junction at the interfaces of GO and TiO₂, the composite can identify ethanol, methanol, and acetone, and its gas response was enhanced. After photo-reduction, the gas sensing behavior was converted from n-type to p-type due to reduction of GO. Color change of the composites was also observed. In addition, we introduced new 2D nanostructured materials, MXenes, for gas sensing. Ti₃C₂ (MXene) was synthesized by selectively eliminating Al from Ti₃AlC₂ (MAX) using LiF salt and HCl acid. Ti₃C₂ was deposited as sensing material on a flexible polymer film using facile drop casting. The structural and morphological study of the prepared Ti₃C₂ was conducted by XRD, SEM, and EDS, and the surface bonding was probed by FTIR. The sensing properties of the Ti₃C₂ sensor were investigated with various reducing gases at RT, and the predicted sensing mechanism was proposed. [5]

References

[1] Patel, S., Park, H., Bonato, P., Chan, L., & Rodgers, M. (2012). A review of wearable sensors and systems with application in rehabilitation. Journal of neuroengineering and rehabilitation, 9(1), 21.

[2] Choi, S. J., & Kim, I. D. (2018). Recent Developments in 2D Nanomaterials for Chemiresistive-Type Gas Sensors. Electronic Materials Letters, 1-40.

[3] Meng, F. L., Guo, Z., & Huang, X. J. (2015). Graphene-based hybrids for chemiresistive gas sensors. TrAC Trends in Analytical Chemistry, 68, 37-47.

[4] Yu, X. F., Li, Y. C., Cheng, J. B., Liu, Z. B., Li, Q. Z., Li, W. Z., ... & Xiao, B. (2015). Monolayer Ti2CO2: a promising candidate for NH3 sensor or capturer with high sensitivity and selectivity. ACS applied materials & interfaces, 7(24), 13707-13713.

[5] Lee, E., VahidMohammadi, A., Prorok, B. C., Yoon, Y. S., Beidaghi, M., & Kim, D. J. (2017). Room Temperature Gas Sensing of Two-Dimensional Titanium Carbide (MXene). ACS applied materials & interfaces, 9(42), 37184-37190.

This research was partially supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP), grant funded by the Korea Government Ministry of Trade, Industry and Energy (20158520000210), and Agency for Defense Development (ADD) as global cooperative research for high performance and light weight bio-urine based fuel cell (UD160050BD).