Adsorption of NO<sub>2</sub> by hydrazine hydrate-reduced graphene oxide

Abstract

Reduced graphene oxide, as a candidate for gas detection due to its unique atomic structure, is arousing the wide interest of researchers. In this paper, hydrazine hydrate is used to reduce graphene oxide prepared by the modified Hummers method. A chemical resistance gas sensor is fabricated. The prepared reduced graphene oxide is used as a gas sensitive layer of Au planar interdigital electrode. The gas sensing characteristics such as responsivity, recovery and repeatability of NO<sub>2</sub> gas are studied. The results show that the graphene oxide reduced by hydrazine hydrate can detect the NO<sub>2</sub> gas at a concentration of 1−40 ppm under room temperature. It has good responsivity and repeatability. The recovery rate can reach more than 71%. However, the sensitivity is only 0.00201 ppm<sup>–1</sup>, and there is much room for improvement. In addition, the response time and recovery time for NO<sub>2</sub> at 5 ppm concentration are 319 s and 776 s, respectively. The sensing mechanism of the hydrazine hydrate-reduced graphene oxide gas sensor can be attributed to charge transfer between the NO<sub>2</sub> molecule and the sensing material. The outstanding electrical properties of the reduced graphene oxide promote the electron transfer process. This allows the sensor to exhibit excellent gas sensing performance at room temperature. The reduced graphene oxide appears as a typical p-type semiconductor and the oxidizing gas NO<sub>2</sub> acts as an electron acceptor. Therefore, the adsorption of NO<sub>2</sub> gas leads to the enhancement of the hole density and conductivity of the reduced graphene oxide. Another reason is the presence of defects and oxygen-containing functional groups on graphene sheets. Some oxygen-containing groups remain on the graphene surface after an incomplete reduction reaction. Compared with pure graphene, the reduced graphene oxide has hydroxyl groups and epoxy groups remaining on the surface. These functional groups will functionalize the material and promote the adsorption of gases. At the same time, the reduction reaction will further produce vacancies and structural defects. This will provide more reaction sites and thus conduce to the material further adsorbing the gas. In summary, the experimental research in this paper is of significance for studying the mechanism and characteristics of the reduced graphene oxide by using hydrazine hydrate as a reducing agent, and it can provide reference and lay a foundation for the applications of future graphene sensors.