Synthesis of chain-like carbon nano-onions for application in room temperature volatile organic chemical gas sensors

Abstract

This study reports on the use of chain-like carbon nano-onions (CLCNOs) in volatile organic chemical (VOC) sensors. The discoveries made in this work suggests a possible inexpensive route to fabricate gas detectors that can be used to monitor and/or reduce toxic VOCs in the environment. It was found that, the properties of CLCNOs can easily be tuned by functionalization, heteroatom doping and by varying the synthesis parameters. The facile synthesis of quasi-spherical CLCNOs was achieved using a flame pyrolysis method that allowed for the formation of carbon nanoparticles and manganese dioxide nanorods (MONRs) based composites. CLCNOs were easily synthesized using four combustible oils; (1) coconut oil, (2) cod liver oil, (3) olive oil and (4) sweet oil as carbon sources. Functionalization of CLCNOs was achieved by refluxing the materials in H2O2. CLCNOs were successfully doped with nitrogen using ammonia and pyridine as nitrogen sources using a chemical vapour deposition (CVD) and open flame pyrolysis (OFP) processes respectively. Microscopy data showed that the multi-layered CLCNOs exhibited particle sizes less than 80 nm. Additionally, the TGA results confirmed that the OFP produces cleaner (i.e. no traces of metal impurities) and thermally stable CLCNOs in gram scale as compared to other conventional methods (e.g. CVD and arc-discharge). It was observed that the defect densities of doped and functionalized CLCNOs increased as compared to the as-synthesized CLCNOs. Moreover, XPS results confirmed successful doping and functionalization of CLCNOs. Hence, the as-synthesized, doped and fictionalized CNOs portray varying chemical properties depending on their surface characteristics such as the presence of different types of nitrogen atoms and/or carboxylic groups. On the other hand, a hydrothermal method was used to synthesize metal oxide nanorods (MONRs). The synthesized CLCNOs and MONRs were analysed using various analytical techniques (e.g. electron microscopy and Raman spectroscopy) to study their chemical and physical characteristics. CLCNOs-MONRs-based composite were prepared along with polyvinylpyrrolidone (PVP) and the sensors were used at room temperature (i.e. 25 °C). The PVP was used to stabilize the MONRs for low temperature use. Different state-of-the-art basicsensing devices were fabricated to explore their detection ability towards acetone, ammonia, chloroform, ethanol, and toluene. Interestingly, different CLCNOs/MONRs/PVP composites presented variable dynamic and static characteristics when used in chemiresistive gas sensors fabrication. Different gas sensors showed a great significance of CLCNOs in the composite since they allowed for better electron transport in the sensing layer. A synergistic property was realized between the three materials (i.e. CLCNOs, MONRs and PVP) used in the gas sensing composite layer. Higher signals and/or positive responses were observed for most devices used to detect various VOCs at selected concentrations (in ppm) and operating frequencies (≤ 10 kHz). The investigations also revealed that most sensors were capable of detecting different VOCs depending on the chemical interaction between the VOC and the sensing material. As such, this study presents the use of different CLCNOs/MONRs/PVP composites as gas sensing materials at room temperature. Thus, the findings in this project play a role in the development of gas sensors operated at low frequencies and temperature. Furthermore, this study contributes towards the knowledge of using cost effective electronic devices given less electrical power consumption needed by low frequency operated devices presented herein.