SYNTHESIS AND CHARACTERIZATION OF CuO-rGO BASED NANOCOMPOSITES FOR NO2 SENSORS

Abstract

Gas sensors are helpful in numerous fields like environment control, automobiles, industries and home safety. An ideal gas sensor is expected to be an economical portable device, highly sensitive, selective and operated at ambient conditions like room temperature and intermediate humidity level. To fulfill these requirements, metal-oxide semiconductor gas sensors have been the subject of immense research and development due to their reasonable cost, good chemical and thermal stabilities, compatibility with electronic devices and ease of fabrication. Generally, the metal oxides for gas sensors are selected on the basis of their electronic structure and the one which possess either d0 (TiO2, WO3, V2O5, CrO3) or d10 (ZnO, SnO2, Cu2O, In2O3) electronic configuration, exhibit feasible gas sensing properties. Among the metal-oxide semiconductors, p-type CuO is very promising candidate for gas sensing application due to its several benefits, such as low cost, low toxicity, high catalytic activity, narrow direct band gap, good thermal stability and tunable surface structure. In spite of these benefits, very less work has been carried out where CuO is employed in sensing oxidizing gases. However, like most of the metal-oxide semiconductors, CuO also has low sensitivity at room temperature. Great efforts have been made to enhance the sensitivity of metal-oxide semiconductors by using metal-oxide semiconductors doped/added with different metals like Fe, Cu, Cr, Co, Ag, Au, Pt etc. Doping/adding an element having strong chemical affinity for a particular gas molecule can increase the adsorption of gas molecules. Doping can also restrict the growth of crystallite size resulting in increased surface of the doped metal-oxide semiconductor. In addition, defects play a role as preferential adsorption sites for gas molecule which can be created by doping. Metals act as a catalyst for accelerating the reaction between oxygen species and target gas. Moreover, chemical and electronic sensitizations with metals on metal-oxide semiconductor also play important roles for enhancing sensing properties. All the above effects can be very advantageous for enhancing or modifying the gas sensing characteristics of metal-oxide semiconductors. Another approach for enhancing the gas sensing performance of metal-oxide semiconductors is making their composite with other metal-oxide semiconductor. The heterostructured material thus formed, contains two or more phases and can result in some synergistic effects by combining different properties of the individual metal-oxide semiconductor into one ii single composite material. Also, heterostructures can effectively manipulate the local charge carrier concentration by modulating the height of potential barrier at the interface and can also increase the surface area of the materials. This could lead to enhanced gas accessibility. Furthermore, sensing properties of metal-oxide semiconductor are closely related to the number of oxygen species and the reactions occurring at the surface upon exposure to different gas molecules, therefore, their microstructure is a key factor to achieve high sensitivity of the sensor. Metal-oxide semiconductor with different morphologies have different natures, e.g. different exposed crystallographic planes and increased surface active sites. Thus, high gas sensing response in CuO can be achieved by doping/adding metals, making its heterostructure with other metal-oxide semiconductor and employing its different morphologies. Another drawback of metal-oxide semiconductor based sensors is high operating temperature which is usually greater than 160 °C, as sufficient thermal energy is required to overcome the potential barrier and achieve the required electron mobility. Explosive gases have low ignition temperature and can result in explosion at higher temperature. Moreover, high operating temperature sensors not only consume more power but also reduces their lifetime and stability. In contrast, room temperature gas sensors are more likable as they consume less power and require no heating element, thus simplifying the fabrication and reduces its cost as well. The high operating temperature of metal-oxide semiconductors can be minimized by making their composite with graphene or its derivatives like graphene oxide (GO) or reduced graphene oxide (rGO). Graphene is an atomic thin 2D sheet where sp2 hybridized carbon atoms are arranged in a honeycomb hexagonal lattice. Apart from high surface area to volume ratio, graphene has high charge carrier mobility at low temperature and high conductivity which lead to very little Johnson’s noise. The composite materials of metal-oxide semiconductor and graphene not only exhibit the properties of metal-oxide semiconductor and graphene but also show some additional synergistic properties, beneficial for the improvement of gas sensing properties. In the present thesis, we have chemically synthesized GO and reduced it by thermal treatment. Furthermore, we have synthesized composite films of different CuO nanostructures (doped/added with different metals) with rGO on glass substrate by drop casting method to study their electrical and gas sensing properties at different temperatures and different humidity levels. An attempt has been made in this thesis to fabricate a highly sensitive and selective CuO gas sensor working at ambient conditions, i.e. room temperature iii and intermediate humidity level by making its composite with rGO. Moreover, doping/adding metals, forming heterostructure of CuO and synthesizing different morphologies of CuO help in achieving the goal of the present thesis. Outline of the thesis The present thesis is divided into seven chapters and the chapter wise descriptions are given below: The first chapter presents a brief introduction to gas sensors and their properties. It includes literature survey on metal-oxide semiconductor based gas sensors and description of the factors which influence their gas sensing performance. The synergistic effects of metal-oxide semiconductor-rGO composites as gas sensor are discussed in this chapter. It also outlines the aim and objectives of the present thesis work. The second chapter gives a detailed description of the synthesis methods used for the preparation of the samples like wet-chemical, reflux, hydrothermal and drop casting methods. This chapter also contains description of the gas sensing set-ups used for electrical and gas sensing measurements. Furthermore, the experimental techniques used for the characterization of the synthesized samples are also briefly described. Structural and surface morphological studies are performed using X-ray diffraction (XRD), Field-Emission Scanning Electron Microscopy (FESEM) and Transmission Electron Microscopy (TEM). Energy dispersive X-ray (EDX) spectroscopy has been used for elemental analysis and mapping. Other spectroscopic techniques like X-ray photo electron spectroscopy (XPS) and Raman spectroscopy have been used to investigate the chemical state and composition of the samples. Brunauer-Emmett-Teller (BET) surface area measurements are performed for pore size distribution/pore volume and surface area analysis. The third chapter describes the synthesis of Zn-doped CuO nanostructure with nominal compositions Cu1-XZnXO (x= 0, 0.03, 0.05, 0.07, 0.10, 0.15) via wet chemical method. The FESEM and TEM results show the formation of 1-D nanochain type morphology in pristine CuO and the same is retained up to Zn doping of 7 at% (x=0.07). However, for higher Zn doping (x>0.07) microflower type morphology is observed. The temperature dependent resistance measurements confirm semiconducting behavior of the pristine and Zn-doped CuO- rGO composite films. The gas sensing performances of all composite films for NO2 gas have been systematically investigated. The results demonstrate that Zn doping in CuO remarkably increases the gas sensing response as compared to pristine CuO. For example, the percentage response increased from ~19.6 for pristine CuO-rGO composite to ~54.5 for 5 at% Zn-doped CuO-rGO composite sensor for 40 ppm NO2 at room temperature (~23 °C). iv Furthermore, sensing performance of composite films initially increases with increasing x up to x= 0.07 and after this it starts decreasing with x. The measurements of sensing response for x=0.05 in the temperature range 23-70 °C for 40 ppm NO2 exhibit maximum response at room temperature. Moreover, the composite sensors exhibit almost negligible response to other gases like CO, NH3, H2S and Cl2 at room temperature, indicating their excellent selectivity towards NO2 gas. The detail correlations between the microstructural characteristics of Zn-doped CuO nanostructures and gas sensing behavior of the corresponding composite films have been discussed and described in this chapter. The fourth chapter describes the synthesis of pure and Ag-CuO nanostructures with different Ag contents (Cu1-xAgxO, x= 0, 0.02, 0.05, 0.08 and 0.12) via hydrothermal method. Different techniques like XRD, FESEM, TEM, BET, Raman and XPS have been used for the characterization of the samples. FESEM and TEM results depict the formation of nanobrick like morphology for pure and Ag-CuO samples. Moreover, FESEM elemental mapping of Ag-CuO samples shows the homogeneous distribution of Cu, Ag and O in all the samples. The experimental results of the NO2 gas sensing at different operating temperatures ranging from 22-100 °C reveals that Ag-CuO/rGO sensor with 5 at% Ag exhibits maximum response. At room temperature of ~22 °C, Ag-CuO/rGO (5 at% Ag) shows 67.2% response to 20 ppm NO2 which is 1.9 times of the CuO/rGO sensor. Thus, the amount of Ag content in the CuO/rGO sensor influences the NO2 gas sensing response. In addition, Ag-CuO/rGO (5 at% Ag) possesses excellent response time of 35 sec for 20 ppm NO2, good repeatability, stability and selectivity for NO2 gas. Interestingly, Ag-CuO/rGO (5 at% Ag) sensor shows maximum response at an intermediate relative humidity (~52%RH). Moreover, the sensing mechanism for improved sensing performances of the Ag-CuO/rGO sensor has been discussed in this chapter. The fifth chapter describes the synthesis of CuO 1-D nanochains and ZnO nanoseeds via wet chemical and refluxing methods, respectively. Ternary composite films of different compositions of CuO and ZnO with rGO have been synthesized to study their electrical and gas sensing behavior. FESEM and TEM results of CuO–ZnO/rGO (CuO:ZnO=1:1) ternary composite sample display the dispersion of nanochains and nanoseeds on the surface of rGO sheet and there are some regions where ZnO nanoseeds, CuO 1-D nanochains and rGO sheet co-exist, indicating that CuO and ZnO form p-n junctions on rGO sheets. From the gas sensing measurements, we have observed variation in percentage response with changing composition of CuO and ZnO in the ternary composite films of CuO–ZnO/rGO and the best percentage response has been observed at room temperature (~ 23 °C) for all compositions. v At room temperature, CuO–ZnO/rGO (CuO:ZnO=1:1) ternary composite sensor shows superior gas sensing performance. The observed percentage response for 40 ppm NO2 for this sensor is ~ 62.9 at room temperature which is almost 3.1 and 1.3 times higher compared to CuO/rGO and ZnO/rGO sensors, respectively. Moreover, this sensor shows best gas sensing response at intermediate humidity level, good stability for a test period of 5 weeks and also good selectivity for NO2 gas. The likely sensing mechanisms of the ternary composite sensors have also been proposed in this chapter. The sixth chapter describes the synthesis of three CuO nanostructures, namely CuO nanobrick, CuO hierarchical flower-like and CuO nanochain, to study the effect of different morphologies on the gas sensing performance. The XRD patterns of composite samples of CuO nanostructures/rGO indicate their successful formation. BET surface area analysis indicates that CuO hierarchical flower-like has smallest surface area among all the samples but have largest pore size. In contrast, CuO nanochain have largest surface area with smallest pore size. The effect of operating temperature and humidity on the gas sensing behaviour of all sensors have been studied. The study reveals that all sensors show maximum response for NO2 gas at room temperature (~25 °C) and intermediate humidity level (~50%RH). Furthermore, among all studied sensors, the CuO hierarchical flower-like/rGO sensor exhibits maximum NO2 gas sensing response. At room temperature, the CuO hierarchical flower-like/rGO sensor shows response of ~58.1% for 20 ppm NO2 which is ~1.7 times of CuO nanobrick/rGO sensor and almost twice of CuO nanochain/rGO composite sensor. Moreover, the gas sensing behaviours of composite samples synthesized with different weight (wt) ratios of rGO to CuO hierarchical flower-like for NO2 gas have also been studied to find optimum weight ratio of rGO and CuO hierarchical flower-like for which best sensing performance is observed. Consequently, CuO hierarchical flower-like with 25 wt% of rGO exhibits the highest response. All the sensors show good reproducibility when tested for five successive cycles of 20 ppm NO2 and excellent selectivity for NO2 gas. Furthermore, the sensing mechanism has been discussed in this chapter by considering the role of morphology of CuO. The seventh chapter contains a brief summary of the work presented in the thesis, concluding remarks and the scope for future work.