Crumpled graphene oxide for enhanced room temperature gas sensing: understanding the critical roles of surface morphology and functionalization

Abstract

This work fundamentally explores graphene oxide morphology and functionality with regard to room temperature gas sensing performance. Highly-oxidized, crumpled graphene oxide (HO-CGO) was synthesized using a scalable, aerosol-based process. To minimize sample-to-sample synthesis variability, CGO and flat graphene oxide materials were thermally reduced allowing for a serial library of graphene oxide materials with comparable shape and surface chemistries. Room temperature sensitivity to ethanol was then systematically evaluated as a function of curing temperature, time (i.e., degree of thermal reduction), and morphology. HO-CGO showed the strongest response after one hour of reduction at a relatively mild temperature (220 °C), which removed most of the oxygen functionality. In contrast, flat graphene oxide sheets at the same C/O ratios showed no response. Density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations of ethanol interactions with these material surfaces were employed to develop a mechanistic understanding of the observed enhanced (CGO) sensing response. Adsorption energy calculations revealed that point defects have the most favorable adsorption energy for ethanol, followed by oxygen functionalities, and pristine graphene, respectively. In addition, AIMD on a simulated crumpled structure of graphene oxide indicate that ethanol molecules prefer to adsorb at/in the geometrical valleys of the CGO structure. Enhanced gas sensing performance of CGO is proposed to be a function of structural valleys, which act as both stable sites for oxygen defects and preferential binding sites for the ethanol molecules, whose adsorption occurs through physisorption, with a substantial contribution (∼50%) derived from dispersive forces. This work directly demonstrates the benefits of the crumpled structure of CGO, with concave morphological regions, compared to other carbon-based materials, and informs its processing and material incorporation into functional room temperature gas sensing devices.