FABRICATION AND STUDY OF ZnO NANOWIRES, rGO AND rGO-ZnO NANOCOMPOSITE BASED GAS SENSORS

Abstract

When a bulk material is transformed to nanoscale material (at least one dimension between 1 to 100 nm), its electronic structure changes which in turns modifies its various chemical and physical properties. Thus nanoscale material behaves differently from its bulk counterpart and can be considered as a new material. Among all the nanoscale materials, metal oxide nanostructures have been studied extensively because of their numerous technological applications. It has been observed that only a few metal oxides, which possess either d0 (TiO2, WO3, Sc2O3, V2O5, CrO3 and perovskites such as ScTiO3, LiNbO3) or d10 (ZnO, SnO2, Cu2O, In2O3) electronic configuration of cations, exhibit feasible gas sensing properties. Although there exist a few metal oxides with dn (0<n<10) configuration of cations (NiO, VO2, Cr2O3, RuO2 etc.) which are sensitive to the environment in their vicinity, but these are structurally unstable under oxidation or reduction processes. Among the metal oxide nanostructures, ZnO is one of the most studied semiconducting materials. It possesses thermodynamically highly stable wurtzite (hexagonal close packed) crystal structure in which lattice constant ratio c/a ~ 1.60 deviates slightly from the ideal value of hexagonal cell (c/a = 1.633) due to difference in electronegativity values of Zn2+ and O2- It has been reported that when the size of a nanostructure material reduces to less than or equal to the Debye length of the material, the mobile charge carrier density within the whole nanostructure will depend on the surface redox process. This implies that nanostructures with smaller grain size or better aspect ratio will exhibit higher sensitivity. Thus 0D, 1D, 2D and 3D nanostructure of ZnO have been extensively studied worldwide to utilize their excellent gas sensing properties for fabrication of improved gas sensing devices at low cost. Moreover, the longer dimension of 1D ZnO nanostructures (nanotubes, nanowires and nanorods) makes them suitable to connect with the macroscopic world for electrical and many other physical measurements. Therefore, 1D nanostructures are more appropriate for the fabrication of nanoelectronic devices like gas sensors, electron-field emitters and logic devices etc. However, in order to have a control over the material properties and developing functional devices, it is necessary to synthesize nanostructures with high degree of regularity and alignment at low cost. A number of techniques have ions. ZnO exhibits almost all the unique properties required to make it a feasible gas sensor such as moderate direct band gap (3.37 eV), high mobility of conduction electrons, better chemical and thermal stability under ambient conditions and good activity in redox reactions. iv been developed in this regard so far but most of them are very sensitive to precursor composition and decomposition conditions. In addition, some of these techniques are expensive too, thus a reliable and low cost synthesis technique which may be used to synthesize ordered 1D nanostructures is still being sought. Many research groups have reported that gas sensors based on an individual nanostructure exhibit excellent gas sensing properties but the processing of an individual nanostructure is not so easy hence not suitable for the mass production of the sensor. This problem can be minimized by using bunches or bundles of well-aligned nanostructures as the sensing material for gas sensors. Furthermore, ZnO, due to its direct band gap of 3.37 eV at room temperature and much higher exciton binding energy (60 meV) as compared to other semiconductor materials, has potential applications in short wavelength optoelectronic devices such as blue-, violet-, and UV- light emitting diodes (LEDS) and laser diodes (LDs). In the present thesis we report the facile synthesis of highly ordered luminescent ZnO nanowire arrays using low temperature anodic aluminum oxide (AAO) template route which can be economically produced in large scale quantity. The as synthesized ordered ZnO nanowire arrays based gas sensor has been fabricated using simple micromechanical technique. Another important nanosacle material, which we have investigated in the present research work, is a 2D allotrope of carbon known as graphene. Graphene is an atomic thin layer of carbon atoms arranged in a honeycomb hexagonal lattice with sp2 hybrization. The other graphitic nanomaterials like 0D fullerenes and 1D carbon nanotubes can be considered as the different geometrical forms of it. Graphene has become a big hub for numerous applications in diverse research domains that exploit its excellent mechanical, electrical, chemical, biological, thermal and optical properties after its experimental discovery in 2004. Along with other fascinating properties graphene, with high surface area to volume ratio offers a large exposed area for gas molecules. Graphene with high conductivity and metallic transport properties (Fermi velocity (vF)=106 In the present thesis we have chemically synthesized thin films of graphene oxide (GO) and reduced graphene oxide (rGO) and investigated their electrical, optical, sensing and antibacterial properties. Furthermore, we have also investigated the gas sensing properties of rGO-ZnO nanocomposite at different temperatures. m/s) exhibits very little Johnson’s noise which makes it a potential material for gas sensing applications. v The present thesis is divided into six chapters. The first chapter contains an introductory aspect of the concern research field, summary of previous work carried out by different research groups and motivation of the present work. In the second chapter we have given detailed description of the synthesis techniques like anodization, vacuum injection, spin coating and hydrolysis used to prepare samples. This chapter also contains a brief description of the experimental techniques used for the characterization of the synthesized samples. Structural and surface morphological studies were performed using X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM) in secondary electron (SE) imaging mode, and atomic force microscopy (AFM). Energy dispersive X-ray (EDX) spectroscopy has been used for elemental analysis and mapping. Thermogravimetric analysis (TGA), photoluminescence spectroscopy (PL) and other spectroscopic techniques like X-ray photo electron spectroscopy (XPS), Fourier Transform Infrared Spectroscopy (FTIR) and Raman spectroscopy have been used to investigate the stability, quality and the extent of graphitization of the samples. In the third chapter, we have described the fabrication and successful detaching of AAO template from the Al substrate. In this chapter, empty pores of the template were filled with saturated aqueous solution of Zn(NO3)2 through indigenously developed vacuum injection technique. After sintering in air at 435°C the template was dissolved in NaOH solution and the collected ZnO nanowires were used for further characterization. From the microscopic studies (FE-SEM and TEM), we found that the ZnO nanowires were polycrystalline, dense and uniform throughout the length. In this chapter we have designed a two-Cu electrode set-up to investigate the NH3 gas sensing properties of as synthesized ZnO nanowires arrays at room temperature. Our measurements show that this sensor possesses good % response and fast response- and recovery- times against different concentrations of NH3 The fourth chapter describes the synthesis of GO powder and fabrication of GO and rGO thin films. The successful synthesis of GO and rGO was verified using XRD, TEM, PL, Raman and XPS. TEM micrograph of rGO reveals the presence of a few wrinkles, which is expected on the graphene surfaces. AFM and FESEM images revealed that the synthesized rGO film is almost continuous and homogeneous. The optical, electrical and gas sensing properties of the rGO thin film have been extensively monitored. It is observed that rGO thin film possesses good electrical conductivity ~104 S m gas. Here we also discussed the possible mechanism which explains the observed sensing properties of the sensor. -1 at room vi temperature and exhibits good gas sensing properties for various concentrations of Cl2 and NO2 The fifth chapter describes the synthesis of rGO-ZnO nanocomposite (ZrGO) and rGO powders. The prepared samples were characterized through microscopic techniques (FE-SEM and TEM) to explore the surface morphology and uniformity of the samples. XRD, TEM, EDAX and other spectroscopic techniques (Raman, XPS and FTIR) were employed to verify the quality of the samples and to confirm the presence of ZnO and rGO in the composite. TGA data reveals that ZrGO sample possesses better stability than pristine rGO. The order of stability for the samples is GO < rGO < ZrGO. We have used coil sensors with two Pt terminals and a heating arm to monitor the effect of temperature on electrical and gas sensing properties of the rGO and rGO-ZnO nanocomposite samples. We find that rGO-ZnO nanocomposite possesses better electrical and NO gases. Furthermore, the as-synthesized GO and rGO thin films show excellent bacterial toxicity for both Gram +ve (B. cereus) and Gram -ve (E. coli) models of bacteria which implies that GO and rGO can be used as effective antibacterial coatings. 2 gas sensing properties compared to pristine rGO. It is also observed that rGO-ZnO nanocomposite sensor exhibits highest response (~32%) for 50 ppm NO2 at relatively low temperature (50°C). We have also checked the repeatability of the sensor for five successive cycles for fixed ppm NO2 The sixth chapter contains a brief summary of the work presented in the thesis, concluding remarks and the scope for future work