SYNTHESIS, CHARACTERIZATION AND APPLICATIONS OF MIXED METAL OXIDE NANOPARTICLES

Abstract

Nanotechnology is considered as the most attractive field now a days. Nanoscale materials exhibit physicochemical properties that are distinctively different from that of their bulk counterparts. Metal oxide nanoparticles play a very important role in various fields and they exhibit interesting electronic and magnetic properties. The combination of two or more metal oxide nanoparticles can lead to materials with multi-functional properties. The chemical reactivity and the physical properties (e.g. electrical, magnetic and optical) of mixed metal oxide nanoparticles can be different from that of constituent metal oxide nanoparticles. The present thesis deals with the synthesis, characterization and applications of mixed metal oxide nanoparticles. The thesis consists of seven chapters. Chapter 1 consists of a general introduction to nanoscale materials, metal oxides, mixed metal oxide nanoparticles and a summary of their optical, magnetic and electrical properties. Some of the reported chemical methods for the synthesis of mixed metal oxide nanoparticles have been discussed. The effect of introduction of a second metal oxide on the optical, electrical and magnetic properties of the parent metal oxides has been discussed. Various applications of mixed metal oxide nanoparticles such as heterogeneous catalysis, sensors, photovoltaic cells and drug delivery systems have also been discussed. Chapter 2 discusses the experimental details and analytical techniques that have been used for the characterization of the mixed metal oxide nanoparticles synthesized in the present study. The various techniques that have been employed include powder X-ray diffraction, thermal gravimetric analysis, CHN analysis, Fourier transform infrared spectroscopy, field emission scanning electron microscopy, energy dispersive X-ray analysis, transmission electron microscopy, surface area analysis, UV-Vis diffuse reflectance spectroscopy (DRS) and magnetic measurements using a superconducting quantum interference device (SQUID). Chapter 3 deals with the synthesis and characterization of CuO@NiO and SiO2@NiO coreshell nanoparticles by homogeneous precipitation method. Precipitation of nickel and copper ions with the help of urea as the homogeneous precipitating agent followed by calcination leads to the formation of CuO@NiO core-shell nanoparticles. The CuO@NiO core-shell nanoparticles were characterized by various analytical techniques. The FE-SEM images and TEM results of CuO@NiO indicated that the core is composed of platelet-like CuO and the shell consists of petal-like nickel oxide nanoparticles. The pH of the ii solution was found to play an important role in the formation of CuO@NiO core-shell nanoparticles. First, Cu2+ ions precipitate as Cu2(OH)3NO3 in the pH range 5-6. With further increase in the pH, precipitation of Ni2+ starts as α-Ni(OH)2. The precursor (which consists of Cu2(OH)3NO3 and α-Ni(OH)2) on calcination at 350 oC lead to the core-shell nanoparticles. The CuO@NiO core-shell nanoparticles show two band gap adsorptions; one at 1.9 eV due to CuO and the second at 4.17 eV due to NiO. The blue shift of band gap of CuO and NiO nanoparticles in CuO@NiO compared to the bulk is attributed to the quantum size effect. For the synthesis of SiO2@NiO nanoparticles, silica spheres were first prepared by the Stober’s process. Then, nickel ions were precipitated using urea over the SiO2 spheres in an aqueous medium to form a shell of α-Ni(OH)2. The as prepared samples on calcination at 500 oC led to the formation of SiO2@NiO core-shell nanoparticles. The shell thickness of NiO in the core-shell nanoparticles was controlled using different concentrations of Ni2+ salt during the homogeneous precipitation. X-ray analysis showed the formation of cubic nickel oxide in the core-shell nanoparticles. FE-SEM and TEM analyses demonstrated the formation of SiO2@NiO core-shell nanoparticles. The magnetic behaviour of the core-shell nanoparticles was investigated using SQUID. The M-H curves for pure NiO nanoparticles and SiO2@NiO core-shell nanoparticles at 300 K indicated antiferromagnetic nature. The M-H curves for SiO2@NiO core-shell nanoparticles at 5 K showed close to superparamagnetic behaviour. The saturation magnetization (Ms) and remanent magnetization (Mr) values for SiO2@NiO core-shell nanoparticles were higher compared to that of pure NiO and this is attributed to the uncompensated surface spins on NiO. Chapter 4 deals with the synthesis of SnO2-MgO and TiO2-MgO nanoparticles via sol-gel method with variable magnesium oxide concentrations and studies on their optical properties. The crystallite size of SnO2, in the mixed metal oxide nanoparticles, was found to be dependent on the calcination temperature as well as on the concentration of magnesium methoxide used during the synthesis. After thorough characterization using XRD, FE-SEM and TEM analysis, the optical properties of SnO2-MgO nanoparticles were investigated with the help of DRS. The band gap (Eg) of SnO2 in the SnO2-MgO mixed metal oxide nanoparticles was found to increase as the magnesium content in the mixed metal oxide increased. In the case of TiO2-MgO mixed metal oxide nanoparticles, the phase transformation of TiO2 from anatase to rutile could be controlled by varying the amount of MgO. The optical iii properties of TiO2-MgO mixed metal oxide nanoparticles were investigated using DRS. The band gap of TiO2 in the TiO2-MgO mixed metal oxide nanoparticles calcined at 500 oC varied from 3.65 eV to 3.31 eV with decreasing magnesium oxide concentration. After calcination at 900 oC, the TiO2-MgO nanoparticles showed two band gap absorptions due to the formation of a mixture of rutile and anatase phases of TiO2. Chapter 5 deals with the preparation of metal aluminate nanoparticles (CoAl2O4, NiAl2O4 and CuAl2O4) and nickel oxide based binary mixed metal oxide nanoparticles (NiO-ZnO, NiOCuO and NiO-MgO) by sol-gel method and their characterization. The xerogel powders, obtained by sol-gel process were subjected to calcination in the temperature range of 500 oC – 900 oC to obtain the nanocrystalline metal aluminates. The TEM images of the metal aluminate nanoparticles indicated that the particles were in the nanometer range. The metal aluminate nanoparticles were characterized using an array of analytical techniques. For the synthesis of NiO-ZnO, NiO-CuO and NiO-MgO nanoparticles, different molar ratio of Cu, Zn and Mg precursors (copper acetate/ zinc acetate/ magnesium methoxide) and nickel acetate were hydrolysed with a small amount of water in a mixture of toluene and ethanol, followed by calcination at 500 oC. The TEM images of NiO-ZnO (1:1) and NiO-CuO (1:1) showed particles close to spherical morphology and the average particle size were ~ 19.7 ± 4.2 nm and 33.2 ± 7.9 nm, respectively. The TEM image of NiO-MgO (1:1) nanoparticles showed irregular morphology and the average particles size estimated was 33.8 ± 7.3 nm. The DRS spectra of NiO-ZnO ([Ni2+:Zn2+] = 1:0.25) indicated one band gap absorption at 3.4 eV, whereas NiO-ZnO ([Ni2+:Zn2+]= 1:1) showed two band gap absorptions; one at 3.7 eV due to ZnO and a second at 4.3 eV due to NiO. In NiO-CuO nanoparticles, the band gap of NiO varied from 4.24 eV to 4.18 eV with increasing copper content. The NiO-MgO mixed metal oxide nanoparticles indicated a blue shift in the band gap from 4.98 eV to 5.75 eV with increasing magnesium concentration. One can thus tune the band gap of NiO nanoparticles by mixing an appropriate amount of a second metal oxide. Chapter 6 discusses the applications that were explored using the mixed metal oxide nanoparticles synthesized in the present study. The applications include catalytic reduction of 4-nitrophenol (4-NP), photocatalytic degradation of methylene blue (MB), adsorption of MB, and destructive adsorption of paraoxon. The CuO@NiO core-shell nanoparticles (Chapter 3) show better catalytic reduction of 4-NP compared to that of pure NiO and CuO nanoparticles. The NiO@SiO2 core-shell nanoparticles (Chapter 3) act as a better adsorbent for methylene iv blue compared to pure NiO nanoparticles and silica spheres. The SnO2-MgO nanoparticles (Chapter 4) act as an efficient photocatalyst towards the photodegradation of MB. The reactivity of metal aluminate nanoparticles (Chapter 5) was investigated using the destructive adsorption of paraoxon and catalytic reduction of p-nitrophenol. Chapter 7 summarises the work done in the present study and discusses the future prospects.