METAL SULFIDE NANOPARTICLES AND NANOCOMPOSITES: SYNTHESIS, PROPERTIES AND APPLICATIONS

Abstract

Nanotechnology deals with research related to materials falling in the nano size range. On reducing the dimensions to nanoscale, materials often show unique physicochemical properties different from their bulk counterparts. Metal sulfide nanoparticles possess fascinating optical, electrical, structural, electric and magnetic properties which can be controlled by tuning their size, shape, and composition. Metal sulfide nanocomposites are flexible by design and their properties can be tuned and designed as per the demand. They exhibit new and enhanced properties compared to their constituents that make them promising candidates for different applications in the field of biotechnology, catalysis, sensing, energy storage and conservation. Various methods have been reported in the literature for the synthesis of metal sulfide nanoparticles and their nanocomposites. Recently, researchers are interested in the preparation of such materials using novel synthetic methods which are easy, economical and environment friendly. In the current thesis, metal sulfide nanoparticles and metal sulfide nanocomposites were synthesized using a thermal decomposition approach. The metal sulfide nanoparticles synthesized include copper sulfide (CuS and Cu1.96S) and bornite (Cu5FeS4). ZnO@CuS core-shell nanocomposites were synthesized using the thermal decomposition approach. The synthesized metal sulfide nanoparticles and the nanocomposites were characterized using different sophisticated analytical techniques. The synthesized nanoparticles and nanocomposites were explored for applications such as peroxidase-like activity and photocatalysis. More details on the contents of the thesis are given below. Chapter 1 begins with an introduction to nanotechnology. It further discusses classification of metal sulfide nanoparticles and metal sulfide nanocomposites. The chapter elucidates various synthetic techniques that have been used for the synthesis of metal sulfide nanoparticles and their nanocomposites. This is followed by a short description of interesting physicochemical properties and applications of metal sulfide nanoparticles. The enhanced properties and applications of metal sulfide nanocomposites have also been discussed. Chapter 2 deals with a concise description of various characterization techniques that were used to analyze the metal sulfide nanoparticles and nanocomposites synthesized in the present study. Phase analysis was carried out using powder X-ray diffraction (XRD). Spectroscopic methods such as Fourier transform infrared (FT-IR) and Raman spectroscopy were employed to identify the organic content present in the samples and also check their purity. Thermogravimetric analysis (TGA) and II differential scanning calorimetry (DSC) were used to understand the thermal stability of the samples and phase transitions. Field emission scanning electron microscopy (FE-SEM) coupled with energy dispersive X-ray analysis (EDXA) and TEM were used for size, morphology and elemental analysis of the samples. X-ray photoelectron spectroscopy (XPS) was employed to understand the oxidation states and surface chemistry of the synthesized materials. Optical properties of the metal sulfide nanoparticles and nanocomposites were investigated using diffuse reflectance spectroscopy (DRS) and UV-Vis spectroscopy. Magnetic properties of the samples were studied using a vibrating sample magnetometer (VSM). Chapter 3 deals with the synthesis of copper sulfide nanoparticles with four different morphologies using a novel thermal decomposition method and their characterization. The copper sulfide nanoparticles were prepared by thermal decomposition of a single molecular precursor, copper thioacetamide complex ([Cu3TAA3Cl3]) in the presence and absence of solvent at 200 °C. The solvents used for the thermal decomposition of the single molecular precursor include diphenyl ether (DPE), ethylene glycol (EG) and 1-octadecene (ODE). The formation of [Cu3TAA3Cl3] complex and copper sulfide nanoparticles was confirmed by techniques such as XRD, FE-SEM, TGA, CHNS, TEM and DRS. Copper sulfide nanoparticles were formed in pure covellite (CuS) phase when the thermal decomposition was carried out in different solvents and a mixture of covellite and djurleite (Cu1.96S) is formed during the solventless (solid state) thermal decomposition. FE-SEM and TEM analyses results indicate formation of copper sulfide nanoparticles with hierarchical morphologies when the synthesis was carried out using DPE and ODE as solvents and also solventless condition. When EG was used as the solvent, agglomerated copper sulfide nanoparticles with no specific morphology were formed. It was observed that different facets are exposed in the morphologically different copper sulfide nanoparticles. Chapter 4 outlines the synthesis and characterization of ZnO@CuS core-shell nanocomposites (type-II). The synthesis of the core-shell nanocomposites was done as follows. First, syntheses of ZnO rods and copper thioacetamide complex, [Cu3TAA3Cl3] were carried out. Then, ZnO@CuS core-shell nanocomposites were synthesized by the thermal decomposition of copper complex ([Cu3TAA3Cl3]) in the presence of ZnO rods in DPE at 200 °C. The ZnO@CuS core-shell nanocomposites were characterized using different techniques such as XRD, FE-SEM, FT-IR, Raman, XPS, and DRS. XRD results confirmed the presence of mixture of CuS and Cu1.96S in the nanocomposites. The effect of reaction time and concentration of copper complex on the extent of coating of copper sulfide nanoparticles on ZnO rods was investigated. FE-SEM and TEM results III indicated that uniform coating of copper sulfide nanoflakes on ZnO rods is achieved at shorter reaction time and lower concentration of the copper complex. SAED results indicated the presence of rings attributed to both CuS and Cu1.96S and set of spots attributed to ZnO in the nanocomposites. XPS studies confirmed the presence of Cu, S, Zn and O on the surface of ZnO rods in the ZnO@CuS nanocomposites. DRS results indicated electronic absorption bands ascribed to the presence of copper sulfide nanoparticles in all the ZnO@CuS nanocomposites. Chapter 5 deals with the synthesis and characterization of bornite (Cu5FeS4) nanoparticles with a novel morphology. The synthesis of bornite nanoparticles was done using a novel thermal decomposition method at 200 °C using iron acetyl acetonate and copper thioacetamide complex. XRD analysis was used to confirm the formation of high bornite nanoparticles. The effect of reaction time and temperature on the formation of bornite nanoparticles was investigated. FE-SEM and TEM analyses indicate hierarchical morphology of bornite nanoparticles; nanoflakes stacked together in the form of tubular structures were obtained. EDXA results indicated uniform elemental distribution of Cu, Fe and S in the bornite sample prepared at 30 min. XPS analysis confirmed the presence of CuS and oxidized iron species on the surface of bornite nanoparticles. The presence of Fe(III) in the bornite nanoparticles was also confirmed from the XPS results. A mixture of copper sulfide and high bornite was obtained if the synthesis was carried out at 160 °C. SAED results on the bornite samples indicated polycrystalline nature of the samples with rings attributed to the planes corresponding to that of high bornite. DRS and UV-Vis spectroscopy were used to investigate optical properties of the bornite samples. Magnetic measurements of the bornite samples (M-H curves) were done at 5 K and 300 K. The field dependent magnetic measurements indicated ferromagnetism at 5 K and paramagnetism at 300 K. Temperature dependent magnetic measurements (M-T) indicated 15 K as the blocking temperature for the bornite nanoparticles. Chapter 6 discusses the applications that were explored using the synthesized metal sulfide nanoparticles and metal sulfide nanocomposites in the present study. This chapter is divided into three sections. In the first section, peroxidase-like activity of copper sulfide nanoparticles with four different morphologies using 3,3ʹ,5,5ʹ-tetramethylbenzidine (TMB) as the substrate has been described. In the presence of H2O2, copper sulfide nanoparticles catalyze one electron oxidation of TMB that results in the formation of a blue color charge transfer complex. The changes in the concentration of the charge transfer complex were monitored by UV-Vis spectroscopy. The effect of concentration of substrate on the peroxidase-like activity was investigated to determine the IV Michaelis constant (Km) and the Km values were compared with that of horseradish peroxidase (HRP) enzyme. The copper sulfide nanoparticles demonstrate better affinity towards H2O2 and TMB compared to HRP. On the basis of peroxidase-like activity, copper sulfide nanoparticles were explored for H2O2 sensing. Section two discusses the peroxidase-like activity of bornite nanoparticles. In the last section, photocatalytic activity of ZnO@CuS nanocomposites under sunlight towards the degradation of congo red in an aqueous solution is discussed. The photocatalytic degradation of congo red by pure ZnO, CuS nanoparticles and ZnO@CuS nanocomposites has been compared. The ZnO@CuS nanocomposites demonstrated enhanced photocatalytic activity as compared to pure ZnO. Recyclability of the catalyst was studied followed by the scavenger tests which suggested the role of O2•⁻and h+ as the active species in the photodegradation of congo red. The ZnO@CuS nanocomposites also demonstrated better photocatalytic activity under sun light towards photo degradation of rhodamine B and crystal violet in aqueous solutions as compared to pure ZnO. Chapter 7 outlines the overall summary of the work presented in the thesis and discusses the future prospects.