SYNTHESIS, CHARACTERISATION AND STUDIES ON OPTICAL PROPERTIES OF NANOSCALE MATERIALS

Abstract

Nanoscale materials deal with structures, devices, and systems that have novel physicochemical properties and functions due to their small size. Nanoscale materials are generally considered as the materials consisting of particles whose one of the dimensions varies from a few nanometers to about 100 nm. They exhibit interesting optical, electronic and magnetic properties and these properties are very sensitive towards the size and shape of the nanoparticles. Some of the important applications derived from nanoscale materials are controlled drug delivery, threshold lasers, optical filters, spintronic materials, sensors, etc. The interesting optical properties of nanoscale materials such as surface plasmon resonance in metal nanoparticles and quantum size effect in semiconducting nanoparticles are very sensitive towards the size and shape of these materials. For example, silver and gold nanospheres show only one surface plasmon resonance band whereas their nanorods show two surface plasmon bands (longitudinal and transverse modes). Semiconducting nanocrystals exhibit blue shift of band gap absorption with decrease in the crystallite size. The various applications involving optical properties of nanoscale materials include sensing biological molecules, studying DNA interactions, metal enhanced fluorescence detection, surface enhanced Raman scattering, and ultra fast switches. There are many challenges still involved in the synthesis of nanoscale materials and studying their physicochemical properties. In the present thesis, synthesis of nanoscale materials, their characterization and studies on the optical properties of the materials have been carried out. The nanoscale materials that were investigated are: (i) silver nanoparticles synthesized by a novel thermal decomposition approach and the effect of ligand concentration on the self-assembly of silver nanoparticles, (ii) silver, gold and ZnO nanoparticles deposited calcite, (iii) ZnO particles with different shapes synthesized by homogeneous precipitation of suitable precursors and conversion of these precursors to ZnO, and (iv) Au and Ag nanoparticles deposited on ZnO hexagonal plates. The synthesized nanoscale materials were thoroughly characterized using techniques such as XRD, TGA, AAS, FT-IR, CHNS analysis, FE-SEM, TEM, and AFM. After the characterization, the optical properties of the nanoscale materials were investigated by UV-Visible spectroscopy / diffuse reflectance spectroscopy and photoluminescence spectroscopy. Some of the interesting applications of the nanoscale materials, prepared in the present study, have also been explored. A brief description on each chapter in the thesis is given below. Chapter 1 focuses on the general introduction to nanoscale materials especially metals, metal oxides and their optical properties. A brief description has been given on the effect of reduction in particle size on the optical properties of nanoscale materials along with surface and volume effects. The origin of surface plasmon resonance in metal nanoparticles, band gap in semiconducting nanoparticles and the dependence of optical properties on size and shape of the particles have been discussed in detail. Chapter 2 deals with the experimental techniques that have been used for the characterization of the nanoscale materials prepared in the present study. The various techniques that have been employed are powder X- ray diffraction, Fourier transform infra red spectroscopy, CHNS analysis, atomic absorption spectroscopy, thermal gravimetric analysis, field emission scanning electron microscopy, energy dispersive X-ray analysis, transmission electron microscopy, atomic force microscopy, UV-Visible spectroscopy (Diffuse reflectance spectroscopy), and photoluminescence spectroscopy. Chapter 3 deals with the synthesis of monodisperse silver nanoparticles by a novel thermal decomposition approach and the self-assembly of silver nanoparticles on substrates such as glass and quartz. Thermal decomposition of silver acetate in diphenyl ether at 120°C in air in the presence of oleic acid and oleylamine leads to the formation of monodisperse silver nanoparticles. The silver nanoparticles were characterized by powder X-ray diffraction, infra-red spectroscopy, thermal gravimetric and CHN analyses, transmission electron microscopy, selected area electron diffraction, and UV-Visible spectroscopy. The amount of organic content present on the surface of silver nanoparticles affects the self-assembly of nanoparticles, as observed by the TEM measurements. In the presence of oleic acid alone, no self-assembly was observed but the presence of oleylamine along with oleic acid during the synthesis lead to increased organic coating on the surface of silver ii nanoparticles resulting in the self-assembly (cubic or hexagonal) of monodisperse silver nanoparticles. The self-assembly of noble metal nanoparticles on different substrates is very interesting since it can lead to various applications in micro-electronics, spintronics, catalysis, etc. The silver colloids prepared by thermal decomposition using different compositions of ligands (oleic acid and oleylamine) were deposited on surface modified and unmodified quartz and glass substrates. The AFM studies indicate self-assembly of silver nanoparticles on these substrates. The optical properties of the self-assembled silver nanoparticles on the substrates were studied by UV-Visible spectroscopy. The surface plasmon resonance band which is observed at about 410 nm for the silver nanoparticles in the colloidal solution is shifted to about 450 nm on self-assembly of nanoparticles on the substrates. The red shift of the surface plasmon resonance on self-assembly has been attributed to dipolar interactions among the silver nanoparticles. The present method is an easy way to produce monodisperse silver nanoparticles and self-assembled silver nanoparticles on different substrates. Chapter 4 deals with the preparation of silver, gold and ZnO nanoparticles deposited calcite samples and studies on their optical properties. These materials were expected to show interesting optical properties and applications. For the deposition of silver nanoparticles, calcite cubes and aragonite needles were chosen as the substrates. The calcite cubes and aragonite needles were prepared by homogeneous precipitation using urea as the precipitating agent and they were subsequently surface modified with ammonium oxalate. Then, electroless deposition of silver nanoparticles on calcite and aragonite was carried out atroom temperature using formaldehyde as the reducing agent and silver nitrate as the silver source. Surface modification of calcite and aragonite with ammonium oxalate is necessary for the deposition of silver nanoparticles and size of the deposited silver nanoparticles on calcite and aragonite could be controlled by changing the deposition parameters such as concentration of the reagents and deposition time. For example, lower concentration of silver ions (0.01 MAgN03) and shorter deposition times (30 min.) lead to the formation of good quality silver nanoparticles on calcite and aragonite. After thorough characterization by XRD, FT-IR, TGA, CHN analysis and FE-SEM techniques, the optical properties of silver nanoparticles on calcite and aragonite have been investigated. Galvanic displacement and iii electroless deposition were employed for the deposition of gold nanoparticles on calcite. Various deposition parameters were optimised and it was found that electroless deposition is superior compared to galvanic displacement for the uniform deposition of gold nanoparticles on calcite. Optical spectral studies indicate surface plasmon resonance due to the noble metal nanoparticles present on the surface of calcite. ZnO nanoparticles were deposited on calcite by a simple chemical approach and their optical properties were investigated. TheZnOnanoparticles were deposited on the calcite by pre-treatment of surface of the calcite with zinc acetate followed by ZnO deposition. Pretreatment of the calcite's surface was found to be necessary for the uniform deposition of ZnOnanoparticles. Various ZnO deposition parameters such as concentration of the reagents and deposition time were studied. After the characterization of ZnO nanoparticles deposited calcite by XRD, FT-IR, FE-SEM and AFM techniques, band gap measurements and photoluminescence measurements were carried out. The band gap measurements indicate that the ZnO nanoparticles present on calcite exhibit quantum size effect (a blue shift of the band gap compared to macro-crystalline ZnO). In the photoluminescence spectra, the samples showed near band edge emission due to ZnO nanoparticles and also visible emission due to defect centres (oxygen vacancies). Chapter 5 deals with the preparation of ZnO with various shapes, and the deposition of Au and Ag nanoparticles on the ZnO hexagonal plates. The ZnO with different morphologies were prepared by the calcination of precursors which were obtained by the homogeneous precipitation using zinc salts with different anions (CF, S042~, N03~, and CH3COO ) and urea. The precursors and the ZnO particles were characterized by an array of techniques such as XRD, FT-IR, TGA, CHN analysis, AAS, FE-SEM and TEM. The size and shape of the precursor as well as the ZnO particles were found to depend on the anion used. Using ZnCh> led to ZnO particles with hexagonal plate-like morphology, while using Zn(N03)2 and ZnS04 produced ZnO particles with close to spherical morphology. Using Zn(CH3COO)2.2H20 produces irregular shaped agglomerated ZnO particles. In addition, using zinc sulphate leads to smaller ZnO particles (mean size ~ 32 nm) compared to the other zinc salts. The effect of varying the other synthetic conditions such as concentrations ofZn source and urea were also studied. The hexagonal ZnOplates were surface modified iv by passivating the surface with NaOH for the deposition of silver and gold nanoparticles. Surface modification is very important for the uniform deposition of Ag and Au nanoparticles on theZnOhexagonal plates. Without surface modification, there was no good deposition of silver and gold nanoparticles on ZnO. After characterization of Ag and Au nanoparticles deposited ZnO by various techniques, studies on the optical properties were carried out. The results indicate the surface plasmon resonance due to the noble metal nanoparticles present on the surface of ZnO hexagonal plates. Chapter 6 deals with the applications of nanoscale materials that were prepared in the present study. Surface enhanced Raman scattering (SERS), metal enhanced fluorescence (MEF), photocatalytic degradation of Rhodamine-B, and anti-bacterial activity studies were explored as the applications. The silver nanoparticles synthesized by the thermal decomposition method and deposited on a substrate such as quartz (Chapter 3) show metal enhanced fluorescence of Rhodamine-B. The SERS studies using Rhodamine-B as the probe molecule show that the silver nanoparticles deposited on glass (Chapter 3), and silver and gold nanoparticles deposited calcite (Chapter 4) show SERS enhancement of signals due to Rhodamine-B. The signal enhancement has been attributed to the presence of metal nanoparticles on the substrates. The ZnO nanoparticles deposited on calcite act as a good photocatalyst towards the photodegradation of Rhodamine-B in the presence of UV light. Finally, the silver nanoparticles deposited on calcite and aragonite show their efficiency in inhibiting the growth of bacteria such as Staphylococcus aureus, Bacillus subtilis, and Escherichia coli. Chapter 7 summarises the work done in the present study and discusses the future prospects.