SYNTHESIS OF IRON OXIDE-REDUCED GRAPHENE OXIDEBASED NANOHYBRIDS FOR EFFICIENT HUMIDITY AND NO2 GAS SENSING

Abstract

Humidity and gas sensors have become integral to various fields, including agriculture, food processing and storage, pharmaceuticals and medical devices, environmental control, automobiles, industries, and home safety. An ideal humidity and gas sensor should be highly sensitive, selective, portable, cost-effective, and operate at ambient conditions, such as room temperature and low voltage. To cater to these requirements, extensive research and development have been conducted on metal-oxide-semiconductor based sensors, such as iron oxide. Iron oxide sensors are preferred due to their low cost, chemical and thermal stability, compatibility with electronic devices, biocompatibility, and ease of fabrication. γ-Fe2O3 and Fe3O4 phases of iron oxide are strong candidates for humidity and gas-sensing applications. Its benefits include high catalytic activity, narrow direct band gap, good thermal stability, tuneable surface structure, low toxicity, affordability, and Food and Drug Administration (FDA) United States approved iron oxide phases. However, enhancing the sensitivity, fast response/recovery time, stability, flexibility, and low energy consumption of iron oxide based humidity and gas sensors at room temperature remains an area for further development. Great efforts have been made to enhance the sensitivity of metal-oxide semiconductors by using metal-oxide semiconductors doped with metals like Fe, Cu, Cr, Co, Ag, Au, Pt, etc. Doping an element having a strong chemical affinity for a particular water and gas molecule can increase the adsorption of molecules. Doping can also restrict the growth of crystallite size resulting in the increased surface of the doped metal-oxide semiconductor. In addition, defects play a role as preferential adsorption sites for the water and gas molecules, which can be created by doping. Oxygen species and the target gas react faster when metals are catalysts. Moreover, chemical and electronic sensitizations with metals on metal-oxide semiconductors also enhance sensing properties. All the above effects can be very advantageous for improving or modifying the humidity and gas-sensing characteristics of metal-oxide semiconductors. Another approach for enhancing the humidity and gas sensing performance of metal-oxide semiconductors is making their composite with other metal-oxide semiconductors. The heterostructure 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 single composite material. Also, heterostructures can effectively manipulate the local charge carrier concentration by modulating the potential barrier height at the interface and x increasing the material's surface area. This could lead to enhanced gas accessibility. Furthermore, the sensing properties of a 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 crucial in achieving the sensor's high sensitivity. The metal-oxide semiconductors with different morphologies have different natures, e.g., exposed crystallographic planes and increased surface active sites. Thus, high humidity and gas sensing response in iron oxide can be achieved by doping/adding metals, making its heterostructure with other metal-oxide semiconductors, and employing its different morphologies. The high working temperature of metal-oxide semiconductor-based sensors, typically more than 160 °C, is another disadvantage since sufficient thermal energy is needed to go beyond the potential barrier and obtain the necessary electron mobility. Low ignition temperatures for explosive gases make them capable of exploding at greater temperatures. High operating temperatures also cause sensors to use more power and have a shorter lifespan and stability. The construction of humidity and gas sensors at room temperature is more straightforward and less expensive since they use less electricity and don't need a heating element. 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 with sp2 hybridized carbon atoms arranged in a honeycomb hexagonal lattice. Apart from a 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 exhibit the properties of metal-oxide semiconductor and graphene but also show some additional synergistic properties, beneficial for improving humidity and gas sensing properties. Applications for humidity sensors in industrial processes and environmental control are growing in semiconductors, humidity or moisture concentrations continuously monitored during wafer processing to produce highly complex integrated circuits. Numerous home applications exist, including intelligent management of the living space in homes, microwave oven cooking control, and innovative laundry management. Humidity sensors are utilized in motor manufacturing lines and rear window defoggers in the automotive industry. Breathing devices, sterilizers, incubators, manufacturing of drugs, and biological materials all employ humidity sensors in the medical industry. Humidity sensors are xi employed in agriculture for soil moisture surveillance, plantation preservation (dew avoidance), greenhouse air cooling, and grain storage. Humidity sensors are generally used for humidity control in paper and textile manufacturing, food processing, dryers, ovens, film desiccation, and chemical gas purification. Gas sensors also have a wide range of applications in various fields due to their ability to detect the presence or absence of certain gases. Environmental monitoring is one of the most common applications of gas sensors. They monitor air quality and detect pollutants like carbon monoxide, nitrogen oxides, and sulfur dioxide. Gas sensors also play a vital role in industrial safety, especially in industries that deal with hazardous gases, such as mining, oil and gas, and chemical manufacturing. In the medical field, gas sensors are used in equipment like anaesthesia machines and ventilators to monitor the concentration of gases like oxygen and carbon dioxide. Gas sensors are also used in agriculture, fire safety, automotive, food and beverage, and consumer electronics like smartphones and wearable devices. Gas sensors have proven essential in ensuring safety, maintaining environmental health, and improving quality of life. The present thesis is divided into six chapters. The First Chapter, entitled "Introduction," briefly introduces humidity and gas sensors and their properties. It includes a literature survey on metaloxide semiconductor based gas sensors and describes the factors influencing their humidity and gas-sensing performance. The synergistic effects of metal-oxide semiconductor-rGO composites as the humidity and gas sensors are discussed in this chapter. It also outlines the aim and objectives of the present thesis work. The Second chapter, entitled "Methodology for synthesis and techniques of characterization," describes the synthesis methods used to prepare the samples, like co-precipitation and wetchemical methods. This chapter also describes the gas sensing set-ups used for electrical and gas sensing measurements. Furthermore, the experimental techniques used to characterize 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 photoelectron 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. xii The Third Chapter, entitled " Ultrafast resistive type γ-Fe2O3-rGO nanohybrids based humidity sensor – a respiratory monitoring tool," discussed the fabrication of a resistive type sensing device by integrating a thin film of iron oxide-rGO nanohybrids on a flexible tempered glass, providing an excellent tool for humidity sensing for different activities. A simple wet method has been reported to integrate γ-Fe2O3-rGO nanohybrid, comprising nanorods of γ-Fe2O3 interfaced to rGO nanosheets, on a hydrophobic surface of FTG. The sensor is fabricated by depositing a thin film of as-synthesized nanohybrids on silver interdigitated electrodes over an FTG substrate. The designed sensor with highly flexible FTG demonstrated an excellent humidity sensing capability with a low detection limit down to 5% RH, an ultrafast response (6 ms) and recovery time (7 ms), high reversibility, repeatability, sensitivity (1.09%) as well as remarkable stability (<180 days). It exhibited multifunctional capabilities to monitor various human activities, such as breathing, speaking, and noncontact sensing and thus making it a smart sensor. These features manifest a tremendous potential of the fabricated sensor for various practical applications such as monitoring personal health and activity, e-skin, and intelligent wearable system. The Fourth Chapter, entitled "Flexible humidity-tolerant γ-Fe2O3-rGO-based nanohybrids for energy efficient selective NO2 gas sensing," reports an energy efficient γ-Fe2O3-rGO-based humidity tolerant nanohybrids fabricated on a flexible tempered glass (FTG), for NO2 gas sensing at room temperature. An interaction between n-type γ-Fe2O3 and p-type rGO semiconductors assisted in forming a p-n heterojunction at their interface, creating oxygen vacancies (Ov) in the γ-Fe2O3 phase, as revealed by XPS analysis. The O2 adsorbed on these sites produced O2- species, facilitating the charge transfer between 𝑂2 − to NO2, eventually reducing resistivity. The bias voltage at 0.3 volt restricted the effect of humidity on NO2 gas sensing in a wide RH range (15– 97%), making it both humidity tolerant and energy efficient. The bending of FTG up to about 130o, resulted in a negligible change in %response (from 56% to 54%) with a minor increase in recovery time, suggesting its potential for usage in flexible electronic devices. Its high selectivity for NO2 gas sensing is manifested by a minor decrease in %response from 56% to 49% in the presence of common air pollutant gases like Cl2, NO2, CO, NH3, C3H6O, H2S, C2H5OH at 50- ppm of each. The role of rGO in nanohybrids towards enhancing the conductivity is corroborated by the significantly lower %response observed (9%) for bare γ-Fe2O3 film on FTG. The efficient sensing of NO2 gas has been correlated based on the comparison of electron affinity (eV) values with other probe gases, following the order: NO2 (2.30) > CO (1.32) > H2O (1.3) > NH3 (0.16) > C3H6O (0.00152) > H2S (-1.16). Thus, the as-designed γ-Fe2O3-rGO-based NO2 gas sensor, xiii operating at 0.3 V at RT, demonstrates selective sensing with a rapid response/recovery time (0.08 min/0.25 min) for 0.5 ppm NO2, having high %response, reproducibility, humidity tolerance in wide RH range and long stability (>52 weeks), suggests it to be a novel NO2 gas sensing device. The Fifth Chapter, entitled "Effect of ultrasonication time on Fe3O4-rGO nanohybrids for energy-efficient, humidity-tolerant, selective NO2 gas sensing on a flexible tempered glass," The manuscript describes the development of Fe3O4-rGO nanohybrids-based energy-efficient, humidity-resistant and capable of sensing NO2 gas at room temperature at ppb level. These nanohybrids are fabricated on a flexible tempered glass (FTG) following a simple wet chemical method. By combining n-type Fe3O4 and p-type rGO semiconductors, a p-n heterojunction is generated at their interface, creating increased oxygen vacancies (Ov) in the Fe3O4 phase, as is revealed by XPS, Raman and BET measurements. The optimum ultrasonication of nanohybrids for 3 h contributed to this aspect. By limiting the operating voltage at 0.1 V, NO2 gas sensing could be performed in a wide humidity range (15% to 97%) without any loss in detection capability and thus making it a truly humidity-tolerant besides being not only energy efficient. The optimized nanohybrids (IOG (2)) demonstrated an excellent %response (190) for 5 ppm NO2 gas at RT. The FTG's flexibility tested by bending it up to 150 deg., resulted in minimal change in response (196 % to 185 % only), with a minor increase in the recovery time, indicating its potential application for the fabrication of flexible electronic devices. Moreover, as-designed sensor shows ultrafast response/recovery time of 8/14 s, long-term stability, a lower detection limit of 100 ppb, and good selectivity. This work thus presents a simple and novel wet chemical approach to design an ultrasensitive hetero-nanostructured NO2 gas sensor. The Sixth chapter summarizes the work presented in the thesis, concluding remarks, and the scope for future work.