Recently, much interest has been placed on wearable electronics, for example wearable gas sensor. With wearability, gas sensor will be more pragmatic to be utilized for everyman to monitor health conditions by checking their breath and/or by protecting themselves ubiquitously from hazardous gases. [1] To improve the performance of wearable gas sensor, essential functions of gas sensor such as sensitivity, selectivity, and stability should be more developed as adapting the conditions to be wearable. Among many approaches to enhance these functions, incorporating two nanomaterials have been adapted in metal oxide sensor (MOS) due to their synergistic effect on gas performance. i.e. SnO₂/ZnO composite. [2] When contacting two different types of materials, heterojunction is created at the interface by transferring charge carriers, resulting in enhanced sensing performance.

For low temperature operation, two-dimensional (2D) materials such as graphene oxide (GO) has been introduced as a sensing material. [3] Due to its unique characteristics, GO has successfully demonstrated room temperature gas sensing to ammonia and NO₂. In a similar manner of MOS, hybrid GO/metal oxide composites have been attempted to enhance sensing performance. By hybridizing two different materials, synergistic effect on heterojunction increased gas selectivity, and tailored morphology of the composite enhanced gas response. [4] To increase conductivity of the composite at room temperature, chemical or thermal reduction process typically accompanies. However, chemical reduction using toxic chemical such as hydrazine can have the issue to apply on wearable sensor due to their potential danger to human. Also, thermal reduction would not be compatible to wearable fabrics having low thermal resistance. [5]

In this study, graphene oxide (GO) nanosheets in ethanol were photo-reduced with an aid of UV irradiation in which TiO₂ or ZnO was added as typical photocatalyst. Room temperature gas sensing was conducted toward various VOC gases before and after photoreduction, and comparative results depending on photocatalyst werer studied. Due to the effect of heterostructure of GO and TiO₂ (ZnO), the composite can identify ethanol, methanol, and acetone at room temperature. After photo-reduction, the resistance of the composite was much reduced as showing stable initial resistance, and gas sensing behavior was reversed to p-type owing to the reduction of GO. Relying on the peculiarity of the photocatalysts, initial resistance and gas response were changed, which suggests the worth of comparative study of photocatalyst. The color change of the composite was clearly observed after UV irradiation. The morphologies and structure of the composites were examined by SEM and XRD, and the comparative study on photocatalyst were implemented by UV-vis, XPS, and gas sensing results.

References

[1] Bandodkar, Amay J., Itthipon Jeerapan, and Joseph Wang. "Wearable chemical sensors: Present challenges and future prospects." ACS Sensors 1.5 (2016): 464-482.

[2] Miller, Derek R., Sheikh A. Akbar, and Patricia A. Morris. "Nanoscale metal oxide-based heterojunctions for gas sensing: a review." Sensors and Actuators B: Chemical 204 (2014): 250-272.

[3] Yang, Wei, et al. "Two-dimensional layered nanomaterials for gas-sensing applications." Inorganic Chemistry Frontiers 3.4 (2016): 433-451.

[4] Zhang, Jun, et al. "Nanostructured Materials for Room‐Temperature Gas Sensors." Advanced Materials 28.5 (2016): 795-831.

[5] Pei, Songfeng, and Hui-Ming Cheng. "The reduction of graphene oxide." Carbon 50.9 (2012): 3210-3228.

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).