http://localhost:8081/jspui/handle/123456789/4965 2025-07-01T16:25:57Z http://localhost:8081/jspui/handle/123456789/17176 Title: FABRICATION AND STUDY OF ZNO NANOWIRES, RGO AND RGO-ZNO NANOCOMPOSITE BASED GAS SENSORS Authors: Kumar, Nagesh 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 d° (TiO2, W03, Sc103, V20, Cr03 and perovskites such as ScTiO3, LiNb03) or d'° (Zn0, Sn02. Cu20, ln,03) electronic configuration of cations, exhibit feasible gas sensing properties. Although there exist a few metal oxides with d (0<n<10) configuration of cations (NiO, V02, Cr203, 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, Zn0 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 Zn 2i and 02 ions. Zn0 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. 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 OD, I D, 2D and 3D nanostructure of Zn0 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 I D Zn0 nanostructures (nanotubes, nanowires and nanorods) makes them suitable to connect with the macroscopic world for electrical and many other physical measurements. Therefore, ID 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 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 I D 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 OD fullerenes and I D 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 m/s) exhibits very little Johnson's noise which makes it a potential material for gas sensing applications. 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. iv 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 1Wdifferent 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 40 like X-ray photo electron spectroscopy (XPS), Fourier transform infrared spectroscopy I (FTIR) and Ranian 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 gas. Here we also discussed the possible mechanism which explains the observed sensing properties of the sensor. 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 11 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' at room V temperature and exhibits good gas sensing properties for various concentrations of Cl, and NO2 gases. Furthermore, the as-synthesized GO and rGO thin films show excellent 40 bacterial toxicity for both Gram +ve (B. cereus) and Gram -ye (E. coli) models of bacteria which implies that GO and rGO can be used as effective antibacterial coatings. 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 NO2 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 concentration. it is observed that O,/( response initially decreases Slighltly but later it become almost constant. The sixth chapter contains a brief summary of the work presented in the thesis, concluding remarks and the scope for future work. 2014-03-01T00:00:00Z http://localhost:8081/jspui/handle/123456789/17104 Title: SYNTHESIS AND CHARACTERIZATION OF HYDROPHOBIC Pd/W03 THIN FILMS FOR HYDROGEN SENSING Authors: Jain, Sonam Abstract: Hydrogen is a clean energy source with a potential to be used as a fuel for industrial applications. 1-lowever, it is explosive at room temperature and therefore its leak detection becomes an important area of research. In the present work the structural, optical and hydrogen sensing properties of nanostructured Pd-capped tungsten oxide (Pd/W03) thin films. The Structural parameters have been tailored at varying substrate temperature (100-400°C) and oxygen partial pressure (P02= 0.5-1.2 Pa) to obtain the optimized parameter. The micro-structural, morphological and structural analyses of the as-deposited films have been done by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM) ,Atomic force Microscopy (AFM). Hydrophobicity was found to be a direct function of surface roughness. High hydrophobicity was observed for the samples deposited at higher substrate temperature. The samples were hydrogenated at 2 bar hydrogen pressure in an operating temperature range 27- 150°C. Optical transmittance spectra was observed transparent for W03 thin films deposited at P02 = 0.5 Pa while it drastically decreases to 50% for hydrogenated Pd/W03 thin films. A transition from hydrophilic to hydrophobic surface was observed with increase in substrate temperature. The hydrophobicity was observed as the main reason for increase in sensor response with increase in substrate temperatures. Using AFM analysis, it was observed that the surface roughness followed the same trend as hydrophobicity. Hydrophobicity played a keen role in limiting the effect of water vapor during recovery. To increase the hydrophobicity further to increase the sensor response of the samples and study the effect of hydrophobicity on hydrogen sensing properties. To study this effect Pd/W03 thin films were prepared at different oxygen partial pressure. The contact angle and roughness were found maximum for the films deposited at P02 0.5 Pa. Recovery time of the sample was observed to improve with hydrophobicity. Increase in hydrophobicity implies limitation in the wettability of the samples. High hydrophobicity gives rise to surface tension which repels water from the surface. Reduced energy barrier combined with the hydrophobicity raised the sensor response of the film substantially making it a suitable choice for hydrogen sensing. Films deposited at P02 = 0.5 Pa showed best response time of I sec and recovery time of 8 min in the presence of 2 bar hydrogen pressure at 500C. 2014-06-01T00:00:00Z http://localhost:8081/jspui/handle/123456789/17103 Title: REMOVAL OF ARSENIC CONTAMINATION FROM WATER BY COMPOSITE NANOFIBRE Authors: Maurya, Sanjay Kumar Abstract: Although arsenic contaminated water is local problem but it affects all over the world especially Bangladesh where largest population is exposed under the arsenic poisoning. Studies from the countries which are exposed under arsenic contaminated water indicate un 10 people who drink arsenic contaminated water for long term may die from cancers. Traditional treatment technology fail or not that much effective to get desired level of arsenic removal. But research shows nZVl particle may be the potential alternate for the removal of arsenic contamination from water. So in this investigation composite nanolibre of nZVl particle and polymer have been synthesized and analysed. This fibrous material can be used as insoluble alternate for conventional powder material adsorbent which are soluble and hard to reverse and can be used as filter and adsorbent material. Results of this experiment suggest that composite nanofibre synthesized be electrospinning process can be used as remediation agent for arsenic contaminated water and out of this fibrous material one can make column filled with these fibres and that column can work as arsenic purification kit 2014-06-01T00:00:00Z http://localhost:8081/jspui/handle/123456789/17102 Title: SYNTHESIS AND CHARACTERIZATION OF Si-CNT BASED Li-ION BATTERY ANODE Authors: Chouksey, Sameer Abstract: Secondary energy storage devices are in high demand owing to the recent developments in portable Electronic devices, Electric vehicles and Electric storage systems. In last two decades, Li-ion batteries have come out as one of the promising energy storage devices. Future applications require much higher energy storage capacity, than a conventional Li-ion battery. In order to improve specific capacity, retention ability and life of the battery, different anode materials like CNT, Sn02, Si etc were proposed. A pure Si anode is known to offer maximum theoretical specific capacity of 4200mAh/g. However, it's poor retention capacity due to the large volume change around 400% during the lithiation-delithiation process, hampers its wide acceptance. This problem could be addressed by incorporating different composite anode materials, Si-CNT composite is one of them. This dissertation deals with an easy route of synthesis and characterization of a Si-CNT composite based Liion battery anode. In this the CNTs were functionalized using HNO3 to make the interaction stronger between Si and CNT. Si was mixed with functionalised CNT by using simple probe ultrasonicator. As prepared Si-FCNT composite was pasted over a thin copper foil (current collector) using PVDF as binder. The prepared anode was characterised with different techniques such as XRD, FTIR and electrochemical characterisations. The prepared anode offered good initial specific capacity of 663mAh/g. 2014-06-01T00:00:00Z