SYNTHESIS OF METAL OXIDE BASED CORE-SHELL HETERONANOSTRUCTURES AND THEIR APPLICATIONS

Abstract

Nanotechnology deals with understanding and utilization of materials in the nanoscale range i.e., 1-100 nm. Nanomaterials exhibit unique properties that depend on their size, shape, composition, etc with respect to their bulk counterparts. Among the various nanomaterials, core-shell heteronanostructures (nanoparticles) are of immense interest. Core-shell heteronanostructures are multi-functional materials with a unique design in which the inner core material is covered by the outer shell material with one of the phases in the nanometer range. As compared to their individual constituents, the core-shell heteronanostructures possess different physicochemical properties. The properties of core-shell heteronanostructures can be conveniently tuned depending on the desired application by varying the size and shape of the core or shell or by varying the entire core or shell material. The flexibility in designing core-shell heteronanostructures has led to their use in diverse applications such as photocatalysis, adsorption, catalysis, lithium-ion batteries, sensors, water splitting, bio-imaging, drug delivery, etc. Various chemical and physical methods have been reported in the literature for the synthesis of core-shell heteronanostructures. However, still their easy and economical synthesis is challenging. In the present thesis, different core-shell heteronanostructures have been synthesized and characterized and they are: (i) TiO2@α-Fe2O3 and TiO2@Ag core-shell heteronanostructures, (ii) Zn2TiO4@CdS core-shell heteronanostructures and (iii) SiO2@ZnS core-shell nanoparticles. The synthesis of core-shell heteronanostructures was done in two steps. In the first step, the core material was synthesized using either sol-gel method or ethylene glycol mediated route. In the second step, coating of the core with shell nanoparticles was carried out using a thermal decomposition approach. The synthesized core-shell heteronanostructures were characterized using various analytical techniques that prove their formation. The optical properties of coreshell heteronanostructures were studied using photoluminescence spectroscopy and diffuse reflectance spectroscopy. The magnetic properties of core-shell heteronanostructures were investigated using a vibrating sample magnetometer. After thorough characterization, the synthesized core-shell heteronanostructures were explored for different applications. The present thesis consists of seven chapters and a brief description of each chapter is as follows. Chapter 1 starts with a brief introduction to nanotechnology followed by an introduction to coreshell heteronanostructures. Then, classification of different types of core-shell nanomaterials, and different chemical and physical routes for their synthesis have been discussed. Further this chapter deals with various interesting physicochemical properties of core-shell nanomaterials. In the end, various interesting applications of core-shell heteronanostructures in different areas have been discussed. Chapter 2 deals with various analytical techniques that have been used in the present thesis to characterize the synthesized core-shell heteronanostructures and also their sample preparation methods for the measurements. The various analytical techniques used for the characterization include powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), field emission scanning electron microscopy (FESEM), energy dispersive X-ray analysis (EDXA), transmission electron microscopy (TEM), selected area electron diffraction (SAED), X-ray photoelectron spectroscopy (XPS), zeta potential and BET surface area analyses. Optical properties of the core-shell heteronanostructures were carried out using diffuse reflectance spectroscopy (DRS) and photoluminescence spectroscopy (PL). Magnetic properties of the core-shell heteronanostructures were studied using a vibrating sample magnetometer (VSM). Chapter 3 deals with the synthesis of TiO2 based core-shell heteronanostructures that include (i) TiO2@α-Fe2O3 and TiO2@Ag by a novel thermal decomposition approach. This chapter consists of two sections and each of them has been discussed separately. In the first section, TiO2@α-Fe2O3 core-shell heteronanostructures with different shell thickness were synthesized by a thermal decomposition approach in two steps. In the first step, TiO2 spheres were synthesized using sol-gel method and an iron-urea complex (precursor for α-Fe2O3) was synthesized using precipitation method. In the second step, TiO2@α-Fe2O3 core-shell heteronanostructures were synthesized by refluxing the iron-urea complex with TiO2 spheres in a mixture of solvents at 200 oC. The as-prepared samples were then calcined at 500 oC to obtain TiO2@α-Fe2O3 core-shell heteronanostructures. The thickness of shell (-Fe2O3) could be controlled by varying the amount of iron-urea complex, which is the precursor for iron oxide nanoparticles. XRD analysis proves the formation of anatase and hematite in the TiO2@-Fe2O3 core-shell heteronanostructures. SEM studies confirm deposition of α-Fe2O3 shell on the surface of TiO2 spheres and TEM analysis proves the formation of uniform -Fe2O3 shell on the TiO2 spheres. XPS measurements prove the presence of Ti4+, Fe3+, and O2- and surface hydroxyls in the core-shell heteronanostructures. From UV-visible DRS spectroscopy, it was observed that the TiO2@-Fe2O3 core-shell heteronanostructures absorb in both UV and visible regions. The band gap of -Fe2O3 in TiO2@-Fe2O3 can be tuned by varying the thickness of the shell (-Fe2O3). Field dependent (M-H) magnetic measurements indicate weak ferromagnetic behavior of TiO2@-Fe2O3 core-shell nanoparticles at 300 K and superparamagnetic behavior at 5 K. Temperature dependent magnetic studies (M-T) show characteristic Morin transition of - Fe2O3 in the core-shell nanoparticles. In the second section, TiO2@Ag core-shell heteronanostructures were synthesized in two steps. At first, TiO2 spheres were synthesized using the sol-gel method. In the second step, silver nanoparticles were coated on the surface of TiO2 spheres by thermal decomposition approach to obtain TiO2@Ag core-shell heteronanostructures. The size of Ag nanoparticles in the TiO2@Ag heteronanostructures could be controlled by using different amounts of silver acetate (precursor for Ag nanoparticles) during the synthesis. Characterization of the synthesized TiO2@Ag coreshell heteronanostructures were done using different analytical techniques. XRD analysis confirms the formation of anatase and metallic silver in the core-shell heteronanostructures and FT-IR studies indicate purity of the TiO2@Ag core-shell heteronanostructures. XPS studies proves the presence of Ti4+ and metallic silver in the core-shell heteronanostructures and FESEM and TEM studies prove the coating of Ag nanoparticles on the surface of TiO2 spheres. EDXA results confirm the presence of Ti, O, and Ag in the core-shell samples. SAED analysis indicates polycrystallinity of the TiO2@Ag samples with rings attributed to both anatase and metallic silver. DRS results of the TiO2@Ag core-shell heteronanostructures indicate two bands that can be attributed to surface plasmon resonance of metallic silver nanoparticles and band gap absorption of TiO2. Chapter 4 deals with the synthesis of Zn2TiO4@CdS core-shell heteronanostructures using the thermal decomposition approach. The Zn2TiO4@CdS core-shell heteronanostructures were synthesized in two steps. In the first step, synthesis of Zn-Ti glycolate spheres was carried out using an ethylene glycol mediated route. This is followed by calcination at 500 oC to obtain the Zn2TiO4 spheres. Bis(thiourea) cadmium acetate complex ([Cd(CS(NH2)2)2]·(CH3COO)2)) (precursor for CdS nanoparticles) was also synthesized using precipitation method. In the second step, Zn2TiO4@CdS core-shell heteronanostructures with Zn2TiO4 as core and CdS nanoparticles as shell were synthesized by thermal decomposition approach. The size of CdS nanoparticles in the shell could be easily varied by varying the amount of single molecular CdS precursor (bis(thiourea)cadmium acetate). Various analytical techniques prove the formation of Zn2TiO4@CdS core-shell heteronanostructures. Powder XRD results prove the formation of cubic Zn2TiO4 and hexagonal CdS in the core-shell heteronanostructures. SEM and TEM studies indicate spherical morphology for pure Zn2TiO4 and formation of core-shell morphology in Zn2TiO4@CdS with Zn2TiO4 as the core and CdS nanoparticles as the shell. EDXA results prove the presence of Zn, Ti, O, Cd and S in the Zn2TiO4@CdS core-shell heteronanostructures. SAED analysis indicates polycrystalline nature of the Zn2TiO4@CdS core-shell heteronanostructures with rings attributed to Zn2TiO4 and CdS. TEM elemental mapping of the Zn2TiO4@CdS coreshell heteronanostructures shows the homogeneity of the CdS shell on the surface of Zn2TiO4 spheres. XPS measurements prove the presence of Zn2+, Ti4+, Cd2+ , and S2‾ in the Zn2TiO4@CdS core-shell heteronanostructures. The UV-Vis DRS results indicate that the core-shell nanoparticles absorb in both UV and visible regions. The band gap of CdS in the Zn2TiO4@CdS core-shell heteronanostructures can be varied by varying the amount of cadmium thiourea complex (precursor for CdS nanoparticles) used during the synthesis. Chapter 5 deals with the synthesis of SiO2@ZnS core-shell nanoparticles by a novel thermal decomposition approach. The synthesis of SiO2@ZnS core-shell nanoparticles was done in two steps. First, mesoporous silica spheres were prepared using a modified Stöber’s method. In the second step, SiO2@ZnS core-shell nanoparticles were synthesized by the thermal decomposition method. The characterization of SiO2@ZnS core-shell nanoparticles was done using various analytical techniques. XRD analysis proved the formation of cubic ZnS nanoparticles and mesoporous SiO2 in the core-shell nanoparticles. Morphological studies, using FESEM and TEM, prove the formation of SiO2@ZnS core-shell nanoparticles with SiO2 as the core and ZnS as the shell. EDXA results indicate that Si, O, Zn and S are present in the SiO2@ZnS core-shell nanoparticles with good homogeneity. SAED results indicate polycrystalline nature of the SiO2@ZnS core-shell nanoparticles with rings attributed to ZnS nanoparticles. TEM elemental mapping results indicate uniform deposition of ZnS nanoparticles on the surface of SiO2 spheres. XPS studies prove the presence of Zn2+, S2-, Si4+ and O2- in the SiO2@ZnS core-shell nanoparticles. DRS studies indicate that the SiO2@ZnS core-shell nanoparticles absorb in UV region and the band gap of ZnS nanoparticles in the SiO2@ZnS samples could be tuned by varying the amount of ZnS precursors (zinc acetate and thiourea). PL studies confirm the presence of defect states in the SiO2@ZnS core-shell nanoparticles. Chapter 6 deals with applications of the different core-shell heteronanostructures, synthesized in the present study. The TiO2@α-Fe2O3 core-shell heteronanostructures were explored for photocatalytic degradation of rhodamine B in an aqueous solution under sunlight. It was observed that the TiO2@α-Fe2O3 core-shell heteronanostructures possess better photocatalytic activity in comparison to their individual components. The TiO2@Ag core-shell heteronanostructures were tested for their activity as catalyst for the reduction of 4-nitrophenol in an aqueous solution. It was observed that the TiO2@Ag core-shell heteronanostructures act as better catalyst for the reduction of 4-nitrophenol as compared to their parent materials. The Zn2TiO4@CdS core-shell heteronanostructures were explored for the photocatalytic degradation of crystal violet (CV) in an aqueous solution. It was observed that the Zn2TiO4@CdS core-shell heteronanostructures perform better as photocatalyst for the photodegradation of crystal violet as compared to their parent materials. The scavenger test analysis suggested that the major species responsible for the degradation of CV are hydroxyl and superoxide radicals. The SiO2@ZnS core-shell nanoparticles were tested as adsorbent for the adsorption of congo red. It was found that the SiO2@ZnS core-shell nanoparticles act as better adsorbent as compared to the individual constituents and the kinetics of adsorption is very fast. Chapter 7 presents an overall summary of work done in the present thesis and also discusses future prospects.