Highly sensitive volatile organic compounds gas sensors based on copper(Ⅱ) oxide

Abstract

Volatile organic compounds (VOCs) include both man-made and naturally occurring organic chemicals which easily evaporate or sublimate from the liquid or solid because of its low boiling point. Since many kinds of VOCs are dangerous to human health or cause harm to the environment, emissions of VOCs and their resulting impact on the air quality was considered as a major environmental concern and lots of studies about VOCs gas sensors are conducted. Recently, VOCs in exhaled breath is also active research field of gas sensors. Exhaled breath contains more than 900 VOCs that are the products of metabolism and these gases such as acetone, NO2, NH3, and H2S have been acknowledged as key biomarker gases to diagnose various diseases including diabetes, asthma, renal disease, halitosis, and lung cancer. Metal oxide materials, which is commonly used sensing materials for VOCs gas sensor, have advantages of high sensitivity, fast response/recovery time and low cost. However, they have critical drawback of poor selectivity which means they also showed good response to other gases as well as target gas. Also, the concentration of VOCs in exhaled breath is very low as ppb level, it is required to develop ultrasensitive gas sensors to detect the VOCs in exhaled breath. In this thesis, we synthesized copper oxide (CuO) based nanomaterials, fabricated sensors and enhanced the gas sensitivity and selectivity of metal oxide semiconductor gas sensors. In the part of HKUST-1 based CuO with platinum nanoparticles (Pt NPs), we proposed the sonochemical synthesis to decorate Pt NPs on HKUST-1 based CuO without strong chemical reductant and CuO from HKUST-1 formed highly porous structures. Since oxygen molecules adsorbed on the Pt NPs can be easily dissociated into oxygen ions due to spill-over effect and activate the HCHO gas selectively to facilitate by its catalytic effect on the Pt NPs surface, gas sensors of CuO with 0.06 wt% Pt NPs showed 82% enhancement compared to gas sensors made of CuO without NPs in detecting HCHO gas of 100 ppb at 225 °C. In the part of ZnO-CuO hollow nanocube, we synthesized p-type CuO hollow nanocube to the surface of n-type ZnO nanosphere and p-n heterojunction was formed. Synthesized ZnO-CuO hollow nanocube had very high surface area which is 336 m$^2$/g and gas response was 11 when sensor was exposed to 1 ppm of acetone. Furthermore, limit of detection was calculated as 8.8 ppb. Also, it showed good selectivity for acetone gas among other VOCs such as ethanol and formaldehyde. From these sensing properties, we concluded that high surface area, small grain size and p-n heterojunction effect can affect to the high sensitivity. Acetone, which is one the VOCs, is a specific breath biomarker for type-1 diabetes and its concentration discriminated diabetes patients and healthy people. Also, it can be monitored to check the effectiveness of ketosis-based dietary programs for healthy people so it is required continuous effort to develop the performance of acetone gas sensors. Through these studies, new materials having high sensitivity to VOCs gases was synthesized and fabricated to sensor array to enhance the selectivity by data analysis for application of diet monitoring systems.