Modified-graphene and carbon nanodots for nitrogen dioxide sensing: Effects of interfacial electronic modulation and dimensionality on gas interaction
Currently, there is an enormous demand for the development of high performance nitrogen dioxide (NO2) gas sensors for environmental pollution monitoring. Hence, there is a constant quest to replace traditional metal oxide nanostructures as gas sensing materials due to the challenges associated with high temperature working condition. Carbon nanomaterials, particularly graphene because of its unique properties have recently attracted a great deal of interests for gas sensing applications. Abundant defects and functional groups on reduced graphene oxide (rGO), a derivative of graphene, not only facilitate gas adsorption but also provide the ease of selective functionalization with specific organic and inorganic groups for achieving selectivity. The interfacial interactions at the junctions of rGO and nanostructures support the modulation of electronic properties, making the graphene hybrid highly responsive to external chemical perturbations. A chemiresistor device based on Sr nanoparticles (NPs) decorated rGO (rGO-Sr) is presented for detecting NO2 gas over a wide concentration range of 500 ppb to 104 ppm. It is a unique study using hybrid of rGO with a low work function alkaline earth metal. At a concentration of 1 ppm, rGO-Sr exhibited an approximate 222% increase in response as compared to rGO. The calculated detection limit (DL) of the sensor was 478 ppb that is close to the experimentally observed limit. The hybrid sensor was also highly selective to NO2 amongst other gases like CO, SO2, CH4, NH3 and exhibited good sensing responses at different humidity conditions at room temperature, thus presenting it as a promising candidate for selective NO2 sensing at room temperature. Electron transfer from Sr to rGO is induced by work function difference due to which more electrons populate the rGO, thus facilitating rapid charge transfer to electrophilic NO2. Also, Sr NPs, possess high adsorption energy for NO2 which plays an important role in fast and selective adsorption of NO2 at room temperature. Thus, engineering the work function of rGO triggered by the work function differences in graphene hybrid can efficiently create a highly sensitive and selective NO2 sensor. The study was further extended to develop interfaces of rGO with semiconductor, molybdenum disulphide (MoS2) and a noble metal, silver (Ag). Thus, chemiresistive devices based on rGO-MoS2 hybrid (GM) and rGO-MoS2-Ag (GMA) were fabricated for NO2 interaction over a wide concentration range. Both the devices showed much higher sensor responses than rGO alone. Also, both the devices revealed an approximate 500% increase in response to NO2 exposure than rGO device at a concentration of 1 ppm. Moreover, the calculated DL of the GM and GMA sensors were 147 ppb and 70 ppb, respectively. The increased sensor performance of GM compared to rGO is attributed to the defect-dominated adsorption of NO2 molecules on rGO and MoS2. Also, modulation of fermi level at the p-p interface of rGO and MoS2 on NO2 exposure provides enhanced sensitivity. Further, on integration of Ag NPs onto rGO-MoS2 matrix, an interfacial electron transfer occurs from Ag to rGO and MoS2 induced by the work function differences, thus facilitating electron withdrawal by NO2. Moreover, the Ag NPs provide catalytic effect due to which more active intermediate species like NO- and O- are formed that are adsorbed on both rGO and MoS2 thus leading to enhanced sensitivity. Again, such an approach effectively pave the way for engineering nanostructures for tailoring sensitivity and selectivity. Various allotropes of carbon facilitate a unique study on the effect of size on the gas interaction. Thus, the interaction of NO2 gas with zero dimensional carbon nanodots (CNDs) has been further exploited. It is observed that the reduction in dimension of carbon structure, from two-dimensional rGO to zero-dimensional CND has a great impact on the electronic interaction process with NO2. The usual charge transfer sensing mechanism observed in rGO was found hindered in CNDs on exposure to NO2, because charge traps induced by water molecules screen any charge transfer due to NO2 molecules. The functional groups on the surface of CNDs attract ambient water molecules, which in turn act as charge traps and thus, result in the hysteresis in the current–voltage response, where area of hysteresis revealed a strong dependence on gas interaction time. Thus, this study leads to not only develop a deep understanding on the effect of size but also a novel functionality is presented on the gas interaction at small scale interfaces.