Graphene Chemiresistors for Sensing Applications

Abstract

Graphene has attracted significant attention since its discovery in 2004, as a result of its good mechanical, optical and electrical properties, that renders suitable for numerous applications. The most common method to produce high-quality monolayer graphene is chemical vapour deposition (CVD). However, the cold wall commercial equipment to synthesise it is too expensive, and even the cost of a small piece of graphene on copper could pose as a bottleneck for laboratories to study graphene. An alternative graphene production is the electrochemical exfoliation of graphite foil in inorganic salts; however, its quality is inherently compromised.
On the other hand, gas sensors capable of detecting vapours markers in real-time are desirable. Graphene materials have attracted scientific interest for the fabrication of gas sensors because of their single atom thick two-dimensional structures, high conductivity, and large specific surface area. In addition to this, graphene can be functionalised, opening the door to make highly selective graphene-based gas sensors.
In this work, two different approaches for graphene synthesis were used: low-pressure chemical vapour deposition (LPCVD) and simultaneous electrochemical exfoliation and functionalisation. Graphene films were grown on copper foil using methane as a carbon source. Herein a custom designed set up and an accompanying standard operation procedure is reported as well as the expected batch to batch variations and variation introduced by the position in the reactor. The synthesis of graphene films, comparable in quality and uniformity to those produced by commercial brands, has been achieved through the use of a cost-effective LPCVD setup and easily accessible copper foil without any pre-treatment. Despite the slight variations observed in the reported metrics, any residual strain and unintentional doping can partially explain these. Furthermore, the average spectrum of each sample exhibits a weak D peak signal, suggesting few defects comparable to those found in commercial samples.
Electrochemical exfoliated graphene functionalised with azide groups was synthesised by simultaneous electrochemical exfoliation and functionalisation from graphite foil in sodium sulphate/sodium azide electrolyte. This method can be an alternative to easily produce azidated graphene in a larger scale. The azidated graphene flakes range from monolayer to few-layer and the electrical conductivity was preserved. Here, we used the azide groups on the surface of graphene to covalently attach a sensing molecule to the surface of graphene, demonstrating its convenient application in the development of graphene chemical resistors.
Both materials, graphene produced by CVD and electrochemical exfoliation, were functionalized with a sensing molecule, that has shown selectivity to cyclohexanone, and tested as chemiresistor under cyclohexanone, acetone, hexane, and ethanol. CVD graphene and electrochemical exfoliated graphene-based sensors show a better sensitivity with functionalization than without functionalization when they are exposed to different concentrations of cyclohexanone vapour; however, the latter shown better performance in terms of response, and sensitivity, having a limit of detection of 4.55 ppm. Figure 1 describes the whole process of the work done.