Effect of reduction temperature on structure and hydrogen sensitivity of graphene oxides at room temperature

Abstract

As precursors exfoliated from graphite oxide gels, graphene oxide thin films are annealed in a temperature range of 100 ℃ to 350 ℃ to obtain a series of reduced graphene oxide samples with different reduction degrees. For the gas sensing experiments, the reduced graphene oxide thin film gas sensing element is prepared by spin coating with Ag-Pd integrated electronic device (Ag-Pd IED). The functional groups, structures, and gas sensing performance of all the samples are investigated by X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, and gas sensing measurement. The results show that the structure of the graphene oxide samples are transformed to the graphitic structure after reduction at different thermal treatment temperatures. When the reduction temperature is lower than 150 ℃, materials exhibit features of graphite oxide. When the reduction temperature reaches about 200 ℃, the samples show characteristics transformed from graphite oxide to reduced graphite oxide gradually. When the temperature is higher than 250 ℃, materials show features of reduced graphite oxide. During the reduction process, the disorder degree increases from 0.85 to 1.59, and then decreases slightly to 1.41 with the rise of temperature. Additionally, the oxygen containing functional groups are removed with the increasing reduction temperature, and these functional groups can be removed at specific temperatures. In the lower temperature stage (100-200 ℃), the first kind of oxygen containing functional group removed is the hydroxyl group (C-OH) and the epoxy group (C-O-C) is the second. In the higher temperature stage (250-350 ℃), the main removed oxygen containing functional groups are the epoxy group (C-O-C) and the carbonyl group (C=O). The materials treated at 150, 200, 350 ℃ exhibit n-type, ambipolar, and p-type behaviors, respectively, while rGO-200 exhibits considerable increase in resistance upon exposure to hydrogen gas. rGO-200 exhibits very small decrease of resistance at room temperature and moderate increase of resistance at elevated temperatures upon exposure to hydrogen gas, while rGO-350 exhibits considerable decrease of resistance at room temperature upon exposure to hydrogen gas. These results indicate that the reduction temperature affects the distribution of density of states (DOS) in the band gap as well as the band gap size. The graphene oxide and the reduced products at low temperature show good sensitivity to hydrogen gas. With the increasing reduction temperature, the sensitivity fades while the response time and recovery time increases. The gas sensor exhibits high sensitivity (88.56%) and short response time (30 s) when exposed to the 10-4 hydrogen gas at room temperature.