SYNTHESIS OF CORE-SHELL NANOPARTICLES AND STUDIES ON THEIR PROPERTIES AND APPLICATIONS

Abstract

Nanoparticles exhibit size and shape dependent optical, magnetic and other physicochemical properties, which are different from that of bulk materials. Among the nanoparticles, core-shell nanoparticles are receiving immense interest due to their versatile physicochemical properties and various potential applications. The properties of core-shell nanoparticles strongly depend on composition, size of the core, shell and ratio of core to shell. These materials show improved properties compared to their individual components and this has been attributed to synergistic interaction between the core and shell. Due to their improved properties, core-shell nanoparticles have been used in various applications such as photocatalysis, adsorption, data storage, supercapacitors, solar cells, drug delivery, bio-imaging, tumor therapy, etc. Various physical and chemical methods have been reported for the synthesis of core-shell nanoparticles. But, the synthesis of core-shell nanoparticles with uniform and controlled shell thickness is still a challenge. In the present study, different core-shell nanoparticles were synthesized and the nanoparticles that have been investigated are: (i) SiO2@CdS (Type-I) and ZnO@CdS (Type-II), (ii) semiconductor-metal based core-shell nanoparticles (ZnO@Ag and Cu2O@Ag), and (iii) nanorattle type core-shell nanoparticles (SiO2@Co3O4 and SiO2@Ni-Co mixed metal oxides). The core materials with different shapes were first synthesized by StӦber’s process, homogeneous precipitation, and solution route. In the next step, deposition of shell nanoparticles with different thickness was carried out via novel and economical methods such as homogeneous precipitation and thermal decomposition. The synthesized core-shell nanoparticles were characterized by an array of analytical techniques. After thorough characterization, optical properties of the core-shell nanoparticles were studied. Some of the interesting applications of synthesized core-shell nanoparticles have also been demonstrated. The present thesis consists of seven chapters and a brief description of each chapter is as follows. Chapter 1 deals with a brief historical perspective of nanotechnology, an introduction to coreshell nanoparticles, classification of core-shell nanoparticles, and their various synthetic methods. Different examples elucidating the optical, magnetic and electrochemcial properties of core-shell nanoparticles have been discussed. At the end, some of the multi-functional applications of core-shell nanoparticles in different fields have been briefly described. ii Chapter 2 deals with various analytical techniques which were used to characterize the coreshell nanoparticles and the sample preparation methods for the measurements. The analytical techniques include powder X-ray diffraction, Fourier transform infrared spectroscopy, thermal gravimetric analysis, field emission scanning electron microscopy, energy dispersive X-ray analysis, transmission electron microscopy, selected area electron diffraction, X-ray photoelectron spectroscopy, zeta potential and BET surface area analysis. Optical properties of the core-shell nanoparticles were studied using UV-Visible diffuse reflectance spectroscopy and photoluminescence spectroscopy. Chapter 3 deals with the synthesis of SiO2@CdS (Type-I) and ZnO@CdS (Type-II) core-shell nanoparticles via a novel thermal decomposition approach. This chapter contains two sections and each one of them has been discussed separately. In the first section, SiO2@CdS core-shell nanoparticles have been synthesized by a simple thermal decomposition approach. The synthesis involves two steps. In the first step, SiO2 spheres were synthesized using StÖber’s process. Then, cadmium sulfide nanoparticles were deposited on the SiO2 spheres by the thermal decomposition of cadmium acetate and thiourea in ethylene glycol at 180 oC. XRD results indicate the presence of CdS nanocrystallites in all the SiO2@CdS samples. Electron microscopy images show uniform deposition of cadmium sulfide nanoparticles on the surface of SiO2 spheres. Electron diffraction patterns confirm crystalline nature of cadmium sulfide nanoparticles on the surface of silica and HRTEM images clearly show the lattice fringes due to cubic cadmium sulfide. Diffuse reflectance spectroscopy results show blue shift of band gap absorption of SiO2@CdS core-shell nanoparticles with respect to bulk cadmium sulfide and this is attributed to quantum size effect. Photoluminescence results show enhancement in intensity of band edge emission of CdS and weaker emission due to surface defects in the SiO2@CdS core-shell nanoparticles compared to pure cadmium sulfide nanoparticles. In the second section, ZnO@CdS core-shell heteronanostructures with different shell thickness (20 nm to 45 nm) have been successfully synthesized by a novel thermal decomposition approach and the synthesis involves three steps. In the first step, ZnO nanorods were synthesized by homogeneous precipitation method. Then, the surface of ZnO nanorods was functionalized using citric acid as the surface modifying agent. Finally, cadmium sulfide shell was deposited on the surface modified ZnO nanorods by the thermal decomposition of cadmium acetate and thiourea in ethylene glycol at 180 oC. XRD results indicate the presence iii of ZnO and CdS in all the ZnO@CdS samples. SEM and TEM results prove the deposition of CdS shell on the surface of ZnO nanorods. SAED patterns indicate crystalline nature of the ZnO@CdS core-shell nanoparticles. DRS results show blue shift of CdS band gap absorption in ZnO@CdS with respect to bulk CdS and PL results show evidence for synergistic interaction between ZnO and CdS nanoparticles. Chapter 4 deals with the synthesis of semiconductor-metal based core-shell nanoparticles (ZnO@Ag and Cu2O@Ag) via a novel thermal decomposition approach. This chapter contains two sections and each one of them has been discussed separately. In the first section, ZnO@Ag core-shell heteronanostructures with varying amounts of silver nanoparticles on ZnO nanorods were synthesized via a novel and economical thermal decomposition approach. ZnO nanorods were first synthesized by homogeneous precipitation method and silver nanoparticles were subsequently deposited on the surface of ZnO nanorods by the thermal decomposition of silver acetate in diphenyl ether at 220 oC. The amount of silver nanoparticles on the ZnO nanorods was controlled by varying the concentration of silver acetate during the thermal decomposition. XRD results confirm the presence of silver nanoparticles (size = 24-31 nm) in the ZnO@Ag samples. SEM and TEM images prove the presence of silver nanoparticles on the surface of ZnO nanorods. XPS results indicate the presence of metallic silver in the ZnO@Ag core-shell heteronanostructures. DRS results show characteristic surface plasmon resonance absorption due to silver nanoparticles and PL results indicate an effective separation of photogenerated electron-hole pairs in the ZnO@Ag coreshell heteronanostructures as compared to that in pristine ZnO nanorods. In the second section, Cu2O@Ag polyhedral core-shell nanoparticles with different morphologies (rhombicuboctahedron, cuboctahedron, truncated octahedron, and octahedron) have been successfully synthesized via a novel thermal decomposition approach and the synthesis involves two steps. In the first step, Cu2O polyhedral microcrystals with various morphologies were synthesized via a solution route. In the second step, silver shell was deposited on Cu2O samples by thermal decomposition of silver acetate at 220 oC followed by growth at 150 oC in diphenyl ether. XRD results confirm the presence of Cu2O and silver in the Cu2O@Ag samples. SEM images show rhombicuboctahedron, cuboctahedron, truncated octahedron, and octahedron morphologies for Cu2O samples. SEM and TEM studies prove the formation of silver nanoparticles shell on the Cu2O polyhedral microcrystals. SAED patterns confirm the crystalline nature of Cu2O@Ag samples and diffuse reflectance spectra of iv Cu2O@Ag polyhedral core-shell nanoparticles show band gap absorption due to Cu2O as well as surface plasmon resonance due to silver nanoparticles. Chapter 5 deals with the synthesis of SiO2@Co3O4 and SiO2@Ni-Co mixed metal oxide coreshell nanorattles via homogeneous precipitation method. This chapter contains two sections and each one of them has been discussed separately. In the first section, SiO2@Co3O4 core-shell nanorattles with different Co3O4 shell thickness have been synthesized by calcination of SiO2@-Co(OH)2 at 500 oC and the synthesis involves two steps. In the first step, SiO2 microspheres were synthesized using StÖber’s process. In the second step, -Co(OH)2 was deposited on SiO2 microspheres via homogeneous precipitation and the obtained samples were calcined at 500 oC to get SiO2@Co3O4 core-shell nanorattles. The shell thickness was controlled by varying the concentration of cobaltous nitrate ([Co2+] = 5, 10 and 15 mM) used during the synthesis. XRD results indicate the presence of Co3O4 in all the SiO2@Co3O4 samples. SEM analysis indicates hierarchical core-shell morphology for SiO2@Co3O4 and TEM results indicate core-shell nanorattle morphology for the particles. SAED patterns demonstrate polycrystalline nature of Co3O4 shell on the SiO2. BET surface area measurements show higher surface area for SiO2@Co3O4 samples as compared to that for pure SiO2 and Co3O4 nanoparticles which is attributed to the nanorattle morphology of SiO2@Co3O4. Diffuse reflectance spectroscopy studies indicate that SiO2@Co3O4 core-shell nanorattles exhibit two absorption bands in the range 420-450 nm and 700-750 nm attributed to two ligand to metal charge transfer transitions (O2− → Co2+ and O2− → Co3+). In the second section, SiO2@Ni-Co mixed metal oxide core-shell nanorattles with different Ni2+:Co2+ molar ratios have been synthesized through a facile, inexpensive and self-template route by the calcination of SiO2@Ni-Co layered double hydroxides at 500 oC. The synthesis involves two steps. In the first step, SiO2 microspheres were synthesized using StÖber’s process. In the second step, Ni-Co layer double hydroxide shell was deposited on the SiO2 microspheres via homogeneous precipitation and the obtained samples were calcined in air at 500 oC to get SiO2@Ni-Co mixed metal oxide core-shell nanorattles. The shell composition was controlled by varying the molar ratio of cobaltous nitrate and nickel nitrate ([Ni2+]: [Co2+] = 7:3, 5:5, 3:7; total concentration = 10 mmol) during the synthesis. XRD results confirm the formation of Ni-Co mixed metal oxides (NiO, Co3O4, NiCo2O4) in all the SiO2@Ni-Co mixed metal oxide core-shell nanorattles. Field emission scanning electron microscopy analysis indicates hierarchical flower-like morphology for the SiO2@Ni-Co mixed metal oxide corev shell nanorattles and transmission electron microscopy analysis confirms the formation of coreshell nanorattles. BET surface area analysis indicates higher surface area for SiO2@Ni-Co mixed metal oxide core-shell nanorattles compared to their counter parts and diffuse reflectance spectra show two band gap absorptions in the mixed metal oxide core-shell nanorattles attributed to metal to ligand charge transfer transitions (Mn+ → O2-). Chapter 6 deals with the applications of core-shell nanoparticles/nanorattles, synthesized in the present study. ZnO@CdS and ZnO@Ag core-shell heteronanostructures were explored as photocatalysts for the degradation of methylene blue in aqueous solutions under sunlight. The ZnO@CdS and ZnO@Ag core-shell heteronanostructures exhibit better photocatalytic activity compared to their individual counter parts. Cu2O@Ag polyhedral core-shell nanoparticles were explored as catalysts for the reduction of 4-nitrophenol and methylene blue in aqueous solutions. The Cu2O@Ag polyhedral core-shell nanoparticles were better catalysts compared to their individual counter parts as well as previously reported catalysts. SiO2@Co3O4 core-shell nanorattles were explored as an artificial peroxidase-like enzyme mimic. The SiO2@Co3O4 core-shell nanorattles exhibit enhanced peroxidase-like activity compared to pure Co3O4 nanoparticles. The SiO2@Ni-Co mixed metal oxide core-shell nanorattles were explored as adsorbents for the removal of rhodamine B and methylene blue and their mixture in aqueous solutions. The SiO2@Ni-Co mixed metal oxide core-shell nanorattles exhibit higher adsorption capacity as compared to pure components as well as physical mixture of NiO and Co3O4. Chapter 7 deals with an overall summary of the work done in the present study and future prospects.