Role of oxygen functional groups in graphene oxide for reversible room-temperature NO2 sensing
Introduction
The superior electrical, mechanical, thermal, and optical properties of graphene, a two-dimensional carbon monolayer crystal, suggest that it can replace materials currently being used in a wide variety of fields such as electronics, photonics, energy storage and conversion, composites, and bio-applications [1], [2], [3], [4]. In particular, graphene has been regarded as a promising material for chemical sensors and biosensors since the surface without bulk is highly sensitive to the adsorption and desorption of molecules [5]. In addition, irreplaceable advantages of graphene-based sensors, such as easy integration into existing technologies, high transparency, excellent flexibility, and considerable stretchability, make them highly attractive candidates for applications in flexible electronics, the next-generation ubiquitous platform [6], [7], [8], [9], [10].
In graphene-based materials, defects such as oxygen functional groups, Stone–Wales defects, and holes from the basal plane can act as active sites for interaction with molecules [11], [12], [13], [14]. In this regard, graphene oxide (GO) could be considered as an ideal material for gas sensing, but the numerous oxygen functional groups of GO make it too electrically insulating for use as an active material for chemoresistive sensors. Hence, GO is converted into the conducting reduced GO (rGO), which is suitable for chemoresistive sensing, by exposure to reducing chemicals such as hydrazine or to strong alkalis, as well as by thermal annealing, hydrogen plasma treatment, and photocatalytic reduction [15], [16], [17], [18], [19]. Despite the rapidly growing interest and intensive efforts toward developing high-performance rGO-based gas sensors, the sluggish, irreversible recovery and relatively low response of these sensors as compared to existing semiconductor metal oxide gas sensors remain obstacles to be overcome [12], [14], [20], [21], [22], [23]. Potential solutions to the aforesaid problems include the incorporation of noble metal nanoparticles, external heating, nanopatterning, voltage activation, and high-power UV illumination [24], [25], [26], [27]; however, these methods increase the device cost, which negates the gold merit of rGO-based sensors. In particular, we emphasize that none of the previous works on rGO-based gas sensors have demonstrated fully reversible gas sensing at room temperature.
In order to overcome the aforementioned obstacles, it is important to understand the interactions between the graphene-based material and the adsorbed gas molecules. In recent years, theoretical studies have been extensively conducted to investigate the adsorption of gas molecules on graphene-based materials. However, most of these studies have focused on the binding energies of a single gas molecule with perfect graphene, GO, or defective graphene containing missing carbon atoms and oxygen functional groups. No study has been carried out on the role of oxygen functional groups in the recovery to the original state in the removal of the target gas, or on the response upon exposure to the target gas, which is a critical factor affecting the reliable long-term operation of gas sensors. Furthermore, the gas sensing properties of GO with different reduction levels have not yet been studied.
In this work, we report the room-temperature responses of pristine GO, rGO with different reduction levels, and graphene synthesized by chemical vapor deposition (CVD) to NO2, which is one of the most reactant air pollutants at room temperature and has been extensively studied as a model analyte for graphene-based gas sensors. Our experimental results show that GO exhibits almost reversible response to NO2 upon multiple exposures, while rGO and graphene show irreversible responses with lower sensitivities. X-ray photoelectron spectroscopy (XPS) measurements suggest that C–O bonds, rather than CO bonds, are responsible for reversible and high-response NO2 sensing. Density functional theory (DFT) calculations reveal the critical role of the hydroxyl groups in GO for high-performance NO2 sensing.
Section snippets
Synthesis of GO
Graphene oxide (GO) was prepared by the modified Hummers method. To enhance the yield of GO, pre-oxidation of graphite was carried out before conducting its Hummers oxidation. Briefly, 2.0 g of potassium persulfate (K2S2O8, Sigma–Aldrich, US) and 2.0 g of phosphorus pentoxide (P4O10, Sigma–Aldrich, US) were dissolved in 10.0 mL of 98% sulfuric acid (H2SO4, Daejung Chem. & Metals. Co., Korea). Then, 2.0 g of pristine graphite (Flakes, Lot. #MKBL8967V, Sigma–Aldrich, US) was gradually added, and the
Results and discussion
GO was synthesized from natural graphite flakes using the modified Hummers method [28]. The GO aqueous suspensions were obtained after repeated careful separation and purification procedures using an ultracentrifuge. The resultant GO suspensions were stable for several months without aggregation due to the negative charges of the deprotonated carboxylate (–COO–) groups of the GO sheets. Scanning electron microscopy (SEM), and atomic force microscopy (AFM) revealed that the individual GO sheets
Conclusion
We investigated the NO2 sensing properties of chemoresistive GO, rGO, and graphene sensors with different fractions of oxygen functional groups. Response upon the exposure to NO2 and recovery to the original resistance after gas sensing were critically dependent on the amount of oxygen functional groups, such as C−O bonds. The GO sensor exhibited high response and reversible NO2 sensing behavior, while the rGO and graphene sensors showed lower responses and irreversible sensing performance.
Acknowledgements
This work was financially supported by the Center for Integrated Smart Sensors funded by the Ministry of Science, ICT & Future Planning as the Global Frontier Project, the Outstanding Young Researcher Program and the Fusion Research Program for Green Technologies through the National Research Foundation of Korea, and a research program of the Korea Institute of Science and Technology.
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These authors contributed equally to this work.