A Comparative Room Temperature NO2 Gas Sensing Study of CuNi-BTC MOF derived CuO/NiO Binary Metal Oxide and Direct Synthesized CuO/NiO Binary Metal Oxide with rGO Support

Abstract

One of the main gaseous pollutants that endanger the environment and human health is nitrogen dioxide (NO2) gas, which is mostly emitted through combustion sources including cars, industries, and power plants1. Acid rain is caused by NO2, which is also a precursor of particulate matters. Even at quantities of tens of ppb, prolonged exposure to NO2 can cause asthma symptoms or reduced respiratory function2. It is difficult to detect NO2 because of the low levels of the gas in the environment, which normally range from a few to a few tens of ppb. Therefore, for the protection of the environment and of human health, NO2 sensors with high sensitivity, more accuracy and low detection limits (down to ppb level) are extremely desirable. Solid-state metal oxide (MO) based gas sensors have been developed and used for more than five decades because they are compact, extremely sensitive (detection of several target gases in the ppm to ppb range), and less expensive than traditional instrumental analysis. The electrical conduction of the sensing material, which is influenced by the quantity of charge carriers and active sites present on the surface, plays a significant role in the sensing performance of MO-based gas sensors3.The majority of the research on gas sensors to far has focused on employing n-type MO semiconductors like CeO2, SnO2, WO3, or ZnO as gas sensors. In more recent times, p-type semiconductors and MO nanocomposite gas sensors have come into attention. Some p-type gas-sensitive materials have been shown to have considerable surface reactivity to oxidizing and reducing gases at lower working temperatures than n-type materials, which may pave the way for future developments in gas-sensing devices with low energy consumption4.Metal-organic frameworks (MOFs), a novel class of porous hybrid materials, have attracted a lot of attention for gas storage, membrane separation catalysis, and chemical sensors due to their intriguing characteristics, including spatial structure diversity, a large specific surface area, regular porous morphology, and tunable pore structures. There are several benefits of using MOFs as templates to build gas sensors, including homogeneous distributions of various components, high density of metal sites, programmable composition, high porosity, and controllable morphology in MOF-derived hybrid structures. Furthermore, mixed metal oxides formed from MOFs are easily obtained by calcining these MOFs with pre-made components. Another successful method for enhancing the gas sensing characteristics of oxide semiconductors is the formation of nanocomposites, which are composed of two distinct catalytic oxides and also formation of heterojunction5. Towards this context, we report a detailed comparative gas sensing study between MOF derived CuO/NiO (MCN) binary metal oxide and direct synthesized CuO/NiO (CN) binary metal oxide on rGO support. Powerful characterization techniques such as XRD, FTIR, Raman, TGA, FESEM, HRTEM, XPS and BET were carried out for structural and morphological investigation. The combined effect of higher surface area (N2 adsorption desorption isotherm) and defect generated on the surface of MOF derived MCN binary metal oxides (O2 vacancy from XPS) was the reason behind higher response than direct metal oxides. Both MCN and CN binary metal oxides show p-type behaviour when exposed NO2 gas. The MCN metal oxides showed higher responses of 7.4% with low response and recovery time. Merging rGO with MCN oxides enhances room temperature sensing activity, higher response, along with high charge carrier mobility.