SYNTHESIS OF DOPED METAL OXIDE AND METAL SULFIDE NANOPARTICLES AND THEIR APPLICATIONS

Abstract

Nanomaterials are materials which have at least one dimension between 1 and 100 nm. They possess interesting optical, electrical, magnetic and mechanical properties. Owing to these properties, they are potential candidates in various applications. The properties of nanomaterials can be enhanced by doping. Vacancies and surface defects are introduced in nanomaterials due to doping which lead to enhanced reactivity and tunable optical, morphological, magnetic and electrical properties. Doped metal oxide and metal sulfide nanoparticles possess interesting physicochemical properties which make them suitable candidates for various applications such as environmental remediation, catalysis, sensors, supercapacitors, Li/Na ion batteries, water splitting, and drug delivery. The present thesis deals with synthesis of various doped metal oxide and metal sulfide nanoparticles using thermal decomposition, homogeneous precipitation and sol-gel methods. The doped metal oxide and metal sulfide nanoparticles investigated in the present study are: (i) Zn2+ doped SnS2 nanoparticles, (ii) Fe3+ doped SnS2 nanoparticles, (iii) Fe3+ doped NiCo2O4 nanoparticles, (iv) Zn2+ doped NiCo2O4 nanoparticles, and (v) Cu2+ doped MgO nanoparticles. The synthesized doped metal oxide and metal sulfide nanoparticles were characterized using various analytical techniques. The effect of dopant concentration on the optical and magnetic properties of the doped metal oxide and metal sulfide nanoparticles has been studied. After characterization, different applications such as adsorption, peroxidase-like activity, styrene oxidation and catalytic reduction of 4-nitrophenol have been explored using the doped metal oxide and metal sulfide nanoparticles. The present thesis consists of six Chapters and a brief description of each Chapter is given below. Chapter 1 deals with an introduction to nanotechnology, classification of nanomaterials on the basis of dimensionality and composition, and different types of doping in nanomaterials. The synthesis of various doped metal oxide and metal sulfide nanoparticles using different methods has been briefly discussed. The changes in electrical, morphological, optical and magnetic properties upon incorporation of dopant ions in the metal oxide and metal sulfide nanoparticles have been briefly discussed. At the end, different applications such as environmental remediation, catalysis, sensors, supercapacitors, dye sensitized solar cells and antibacterial activity using the doped metal oxide and metal sulfide nanoparticles have been elucidated. Chapter 2 deals with a brief discussion on the principle of various characterization techniques employed in the present study and also procedures for sample preparation for the characterization of the doped metal oxide and metal sulfide nanoparticles. An array of analytical techniques including powder XRD, TGA, FT-IR, CHNS analysis, FE-SEM, EDXA, TEM, ICP-MS, BET, PL spectroscopy, zeta potential analyzer and XPS were employed for the characterization of doped metal oxide and metal sulfide nanoparticles. The studies on optical and magnetic properties of the doped metal oxide and metal sulfide nanoparticles were done using diffuse reflectance spectroscopy (DRS) and physical property measurement system (PPMS), respectively. The different applications of doped metal oxide and metal sulfide nanoparticles were studied using UV-Vis spectroscopy and gas chromatography. Chapter 3 deals with the synthesis of metal doped SnS2 nanoparticles via thermal decomposition approach and their applications. This Chapter is divided into two sections. In the first section, Zn2+ doped SnS2 nanoparticles (Sn1−xZnxS2) were prepared at 200 ℃ using thermal decomposition method. XRD results of SnS2 nanoparticles after incorporation of Zn2+, indicate formation of single phase up to x ≤ 0.5 and phase separation occurs when x ≥ 0.6. A change in morphology of SnS2 from nanoflakes to nanoflowers upon doping with Zn2+ is observed from FE-SEM and TEM studies. The presence of Sn4+, Zn2+ and S2− in Sn1−xZnxS2 nanoparticles is confirmed from XPS studies. Blue shift of band gap is observed in Zn2+ doped SnS2 nanoparticles with respect to pure SnS2 nanoparticles and the band gap varies from 2.24 eV to 2.48 eV after the incorporation of Zn2+ in SnS2 nanoparticles. The Zn2+ doped SnS2 nanoparticles were explored as adsorbent for the removal of rhodamine B (RhB) from aqueous solutions. Owing to higher surface area and negative surface charge, the Zn2+ doped SnS2 nanoparticles perform as better adsorbent for the adsorption of RhB from aqueous solutions with faster kinetics of adsorption as compared to pure SnS2 nanoparticles. In the second section, Fe3+ doped SnS2 (Sn1−xFexS2) nanoparticles were synthesized at 200 ℃ using thermal decomposition method. From XRD results, formation of single phase is observed up to x = 0.2 while phase separation is observed when x > 0.2 in Sn1−xFexS2 nanoparticles. Flake-like morphology in pure SnS2 and Sn1−xFexS2 nanoparticles is observed from FE-SEM and TEM studies. XPS results confirm the presence of Fe3+ in Sn1−xFexS2 nanoparticles. An increase in band gap of Sn1−xFexS2 nanoparticles from 2.24 eV to 2.68 eV is observed from the DRS results. The Sn1−xFexS2 nanoparticles with low concentration of iron (x = 0.10 and 0.15) show weak ferromagnetic behavior at 300 K and 2 K, while at higher concentration of iron (x = 0.20), the Fe3+ doped SnS2 nanoparticles show paramagnetic behavior at 300 K and superparamagnetic behavior at 2 K. The Sn1−xFexS2 nanoparticles were explored as catalyst for oxidation of 3,3ʹ,5,5ʹ-tetramethyl benzidine (TMB) in the presence of H2O2 (peroxidase-like activity). The Sn1−xFexS2 nanoparticles possess superior catalytic activity for the oxidation of TMB than pure SnS2 nanoparticles and natural enzyme (horseradish peroxidase). Radical scavenger test results suggest that hydroxyl and superoxide radicals are involved in the oxidation of TMB when Sn1−xFexS2 nanoparticles are used as the catalyst. Further, the Sn1−xFexS2 nanoparticles were used for the detection of H2O2. Chapter 4 deals with synthesis of metal doped NiCo2O4 nanoparticles via homogeneous precipitation method and their applications. This Chapter is divided into two sections. In the first section, iron doped NiCo2O4 nanoparticles were successfully synthesized via homogeneous precipitation method followed by calcination. XRD results of iron doped NiCo2O4 nanoparticles demonstrate formation of single phase up to x = 0.5 while phase separation is observed when x ≥ 0.7. TEM results of pure and iron doped NiCo2O4 nanoparticles indicate formation of nanorods composed of nanoparticles. Presence of Fe3+ in Ni1−xFexCo2O4 nanoparticles is confirmed from XPS studies. An increase in band gap of Ni1−xFexCo2O4 nanoparticles as compared to that of pure NiCo2O4 nanoparticles is observed from the DRS results. M-H measurements indicate soft ferromagnetic behavior at 300 K in pure NiCo2O4 and Ni1−xFexCo2O4 nanoparticles. At 5 K, soft ferromagnetic behavior is observed up to x = 0.3, and strong ferromagnetic behavior is observed when x = 0.7, while “wasp-waist” behavior in the M-H curve is observed when x = 0.5. ZFC-FC measurements show only one blocking temperature for pure and iron doped NiCo2O4 nanoparticles up to x = 0.3 while two blocking temperatures are observed when x = 0.5, and the blocking temperature is above room temperature when x = 0.7. The Ni1−xFexCo2O4 nanoparticles were explored as catalyst for the oxidation of 3,3ʹ,5,5ʹ-tetramethyl benzidine in the presence of H2O2 (peroxidase-like activity). The iron doped NiCo2O4 nanoparticles possess better catalytic activity for the oxidation of TMB compared to pure NiCo2O4 nanoparticles and HRP. Radical scavenger test results suggest that hydroxyl and superoxide radicals are involved in the oxidation of TMB when Ni1−xFexCo2O4 nanoparticles are employed as the catalyst.In the second section, Zn2+ doped NiCo2O4 nanoparticles (Ni1−xZnxCo2O4) were synthesized using Zn2+ substituted Ni-Co LDH (layered double hydroxide) precursors prepared via homogeneous precipitation. Phase analysis of precursors and Ni1−xZnxCo2O4 nanoparticles was performed using XRD. TEM results of Ni1−xZnxCo2O4 indicate formation of nanorods consisting of nanoparticles. The presence of Zn2+ in Ni1−xZnxCo2O4 nanoparticles is confirmed from XPS studies. DRS results indicate blue shift of band gap due to incorporation of Zn2+ in NiCo2O4 nanoparticles. After characterization, catalytic activity of Ni1−xZnxCo2O4 nanoparticles was investigated towards the oxidation of styrene. The Ni1−xZnxCo2O4 nanoparticles exhibit better catalytic performance for the oxidation of styrene compared to undoped NiCo2O4 nanoparticles. Chapter 5 deals with synthesis of Cu2+ doped MgO nanoparticles using various concentration of copper acetate (0.5 mmol to 1.5 mmol) via sol-gel method. The Cu2+ doped MgO nanoparticles were synthesized using Cu2+ substituted brucite precursors followed by calcination (at 500 ℃ and 700 ℃). Formation of brucite-like lamellar compounds was observed from XRD results on the precursors. According to the XRD results of Cu2+ substituted brucite precursors calcined at 500 ℃, phase separation is observed when x > 0.5 mmol of copper acetate. However, in the Cu2+ substituted brucite precursors calcined at 700 ℃, single phase is observed up to x = 1.5 mmol of copper acetate. From the electron microscopy studies, flake-like morphology is observed in Cu2+ substituted brucite precursors while agglomerated nanoparticles are observed on calcination at 500 ℃ and 700 ℃. After characterization, catalytic activity of Cu2+ doped MgO nanoparticles was investigated for reduction of 4-nitrophenol to 4-aminophenol. The Cu2+ doped MgO nanoparticles exhibit better catalytic activity for the reduction of 4-nitrophenol at room temperature as compared to pure MgO nanoparticles. The mechanism for the catalytic activity of Cu2+ doped MgO nanoparticles has also been discussed. Chapter 6 deals with an overall summary of the current study and also the future prospects.