SYNTHESIS AND CHARACTERIZATION OF METAL OXIDE/METAL SULFIDE NANOCOMPOSITES

Abstract

Nanoscale materials exhibit size dependent physicochemical properties such as optical, magnetic, catalytic, etc, which vary significantly from that of bulk materials. Nanocomposites are multi-phase materials in which at least one of the phases possesses dimension in nanometer range. Nanocomposites are important because of their multifunctional properties and they have potential applications in diverse areas such as environmental remediation, catalysis, sensors, biomedical and photo-electronics. Various physical and chemical methods have been reported for the synthesis of nanocomposites which include chemical vapour deposition, ball milling, spray pyrolysis, laser ablation, sol-gel, thermal decomposition, precipitation, hydrothermal, and sonochemical methods. In the present thesis, synthesis of different nanocomposites, their characterization and studies on their optical and magnetic properties have been carried out. After thorough characterization, some applications of the nanocomposites have been explored. In the present study, three different types of nanocomposites on the basis of matrix used have been synthesized; (i) NiO-Al2O3 and PbS-Al2O3 nanocomposites, (ii) CdS-TiO2 and Ag2S-TiO2 nanocomposites and (iii) CdS--Fe2O3 and ZnO@-Fe2O3 nanocomposites. The nanocomposites were synthesized using a facile sol-gel process, a two-step sol-gel process followed by thermal decomposition approach and also a one-step thermal decomposition approach. The synthesized nanocomposites were characterized using an array of analytical techniques. After thorough characterization, the nanocomposites were explored for a few applications such as oxidation of styrene, photocatalytic degradation of rhodamine B, photo-reduction of Cr(VI) and photocatalytic degradation of congo red in aqueous solutions. The thesis consists of seven chapters and a brief description on each chapter is given below. Chapter 1 deals with a brief historical background on nanotechnology, general introduction of nanoscale materials their types and synthetic methods. A description has been given on the nanocomposites, their types and different preparation methods. Various size and shape dependent properties of nanocomposites have been discussed with the help of suitable examples. At the end, applications of different nanocomposites depending on their optical, magnetic and electrical properties have been discussed. Chapter 2 deals with the various analytical techniques that were employed for the characterization of the synthesized nanocomposites. The techniques that were used include powder X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, ii thermal gravimetric analysis, atomic absorption spectroscopy, field emission-scanning electron microscopy coupled with energy dispersive X-ray analysis, transmission electron microscopy, selected area electron diffraction, diffuse reflectance spectroscopy, UV-visible spectroscopy, photoluminescence spectroscopy and surface area measurements. The magnetic properties of the synthesized nanocomposites were investigated using a superconducting quantum interference device (SQUID) or a vibrating sample magnetometer (VSM) as a function of magnetic field and temperature. The applications of the nanocomposites were studied using gas chromatography coupled with mass spectroscopy and UV-visible spectroscopy. Chapter 3 deals with the synthesis and characterization of nickel oxide-alumina and lead sulfide-alumina nanocomposites. These nanocomposites have been discussed in two separate sections. In the first section, NiO-Al2O3 nanocomposites were prepared by sol-gel method. XRD results indicate that the NiO-Al2O3 nanocomposites consist of small NiO crystallites (mean size ~ 2.6 nm) and FT-IR results on the nanocomposites show characteristic bands due to nickel oxide and alumina. Raman spectroscopy results show characteristic bands due to nickel oxide. EDX analysis shows the presence of oxygen, aluminium and nickel in the nanocomposites. TEM results indicate uniform distribution of NiO nanoparticles in the Al2O3 matrix. Increase in the band gap of NiO in the nanocomposites compared to pure NiO nanoparticles is observed and the nanocomposites show superparamagnetic behaviour at room temperature. In the second section, PbS-Al2O3 nanocomposites were prepared by the sol-gel the method. X-ray diffraction results indicate that the PbS-Al2O3 nanocomposites consist of PbS nanocrystallites (18.2 to 40.5 nm). FT-IR results on the nanocomposites show characteristic bands due to lead sulfide and alumina and Raman spectroscopy results show characteristic bands due to lead sulfide. EDX analysis shows the presence of oxygen, aluminium, lead and sulfur in the nanocomposites. Transmission electron microscopy images show cube-like and flake-like morphology for pure PbS and Al2O3, respectively and TEM images of the nanocomposites indicate uniform distribution of PbS nanoparticles on the Al2O3 matrix. Diffuse reflectance spectral studies on the nanocomposites indicate near infrared (NIR) absorption and they exhibit blue shift of the band gap of PbS compared to pure PbS nanoparticles. The photoluminescence studies show characteristic peaks due to excitonic emission of PbS nanoparticles in the PbS-Al2O3 nanocomposites. iii Chapter 4 deals with the synthesis and characterization of cadmium sulfide-titanium dioxide and silver sulfide-titanium dioxide nanocomposites. These nanocomposites have been discussed in two separate sections. In the first section, CdS-TiO2 nanocomposites were prepared by a two-step method. In the first step, the TiO2 nanoparticles were prepared by sol-gel method. In the second step, CdS-TiO2 nanocomposites were prepared by the thermal decomposition of cadmium acetate, thiourea and TiO2 nanoparticles in diphenyl ether at 150 oC. The effect of using four different types of TiO2 (as-prepared sol–gel TiO2 nanoparticles, sol–gel TiO2 after calcination at 500 oC, sol–gel TiO2 after calcination at 700 oC and macro-crystalline TiO2) on the structure and optical properties of the nanocomposites was investigated. The CdS–TiO2 nanocomposites consist of nanocrystallites of cubic CdS and anatase TiO2 when as-prepared sol-gel TiO2 nanoparticles, sol-gel TiO2 calcined at 500 oC and macro-crystalline TiO2 were used. On the other hand, when sol-gel TiO2 after calcination at 700 oC was used, the CdS-TiO2 nanocomposites were found to consist of rutile TiO2. In the nanocomposites prepared using as-prepared sol-gel TiO2 nanoparticles and sol-gel TiO2 after calcination at 500 oC, FE-SEM and TEM results indicate uniform distribution of CdS nanoparticles (10.5 ± 1.6 nm) on the TiO2 matrix. In the CdS-TiO2 nanocomposites, a blue shift of the band gap of CdS compared to bulk CdS is observed. In the second section, Ag2S-TiO2 nanocomposites were prepared by a two-step method. In the first step, TiO2 nanoparticles were prepared by sol-gel method. In the second step, Ag2S-TiO2 nanocomposites were prepared by the thermal decomposition of silver acetate, thiourea and TiO2 nanoparticles (calcined at 500 oC) in diphenyl ether at 220 oC. The XRD results indicate that the Ag2S-TiO2 nanocomposites consist of nanocrystallites of Ag2S and TiO2. The FE-SEM images showed that the Ag2S-TiO2 nanocomposites consist of more or less uniform particles with close to spherical morphology while the energy dispersive X-ray analysis studies indicate the presence of silver, sulfur, titanium and oxygen in the nanocomposites. TEM results indicate uniform distribution of Ag2S nanoparticles (8.8 ± 1.9 nm) in the TiO2 matrix. A blue shift of band gap of Ag2S in the Ag2S-TiO2 nanocomposites compared to bulk Ag2S is observed. Chapter 5 deals with the synthesis and characterization of cadmium sulfide-iron oxide nanocomposites and zinc oxide@iron oxide core-shell nanocomposites prepared by the thermal decomposition approach. These systems have been discussed in two separate sections. In the first section, CdS--Fe2O3 nanocomposites have been prepared by a facile single step thermal decomposition approach which involves thermal decomposition of iron iv acetylacetonate, cadmium acetate and thiourea in diphenyl ether at 200 oC. The effect of varying the concentration of iron acetylacetonate, cadmium acetate and thiourea during synthesis of the nanocomposites on the crystal phases, microstructure and properties of the CdS--Fe2O3 nanocomposites was investigated. The XRD results confirm the presence of CdS nanocrystals (1.2 to 2.9 nm) in the CdS--Fe2O3 nanocomposites. FT-IR results on the nanocomposites show characteristic bands due to -Fe2O3 and CdS. FE-SEM results indicate more or less uniform particles with close to spherical morphology while the energy dispersive X-ray analysis studies indicate the presence of cadmium, sulfur, iron and oxygen in the nanocomposites. TEM results indicate formation of agglomerated sphere-like particles in the nanocomposites. The CdS--Fe2O3 nanocomposites show band gap absorption (1.8 to 2.5 eV) and characteristics photoluminescence in the visible region, and exhibit superparamagnetic behaviour at room temperature. In the second section, ZnO@-Fe2O3 core-shell nanocomposites were prepared by a thermal decomposition approach. ZnO nanorods were first synthesized by the calcination of zinc acetate at 300 oC. -Fe2O3 nanoparticles were then deposited on the surface of ZnO nanorods by the thermal decomposition of iron acetylacetonate at 200 oC in diphenyl ether. XRD studies on as prepared and calcined samples suggest the phase of as prepared iron oxide nanoparticles as -Fe2O3. FT-IR results on the as prepared iron oxide nanoparticles and ZnO@-Fe2O3 core-shell nanocomposites show IR bands due to -Fe2O3. FE-SEM images indicate the formation of shell of iron oxide on the ZnO nanorods while the energy dispersive X-ray analysis studies indicate the presence of zinc, iron and oxygen in the nanocomposites. Transmission electron microscopy studies clearly show that the ZnO possesses rod morphology (length = 1.1 ± 0.1 μm, diameter = 40 ± 7 nm) and TEM images of the ZnO@-Fe2O3 nanocomposites show uniform shell formation of -Fe2O3 on the surface of ZnO nanorods and thickness of the -Fe2O3 shell varies from 10 to 20 nm. The diffuse reflectance spectra of ZnO@-Fe2O3 nanocomposites reveal extended optical absorption in the visible range (400–600 nm) and the photoluminescence spectra indicate that the ZnO@-Fe2O3 nanocomposites exhibit enhanced defect emission. The magnetic measurements on the ZnO@-Fe2O3 nanocomposites reveal superparamagnetic behaviour at room temperature with characteristic blocking temperature which indicates the presence of iron oxide nanoparticles shell on the ZnO nanorods. v Chapter 6 deals with the various applications that were explored using the synthesized nanocomposites in the present study. NiO-Al2O3 nanocomposites were used as the catalyst in the oxidation of styrene. The nanocomposites show better catalytic activity for the oxidation of styrene using tert-butyl hydroperoxide as the oxidant and also show higher selectivity for styrene oxide with higher total conversion compared to pure NiO nanoparticles. The CdS-TiO2 nanocomposites were explored as photocatalyst for the photodegradation of rhodamine B and reduction of Cr(VI) in aqueous solutions in the presence of sunlight. The CdS–TiO2 nanocomposites act as good catalyst for the photodegradation of rhodamine B and reduction of Cr(VI) compared to pure CdS and TiO2 nanoparticles. The Ag2S-TiO2 nanocomposites were found to act as better photocatalyst for the photodegradation of rhodamine B in aqueous solution compared to pure Ag2S and TiO2 nanoparticles. The CdS--Fe2O3 nanocomposites and ZnO@-Fe2O3 core-shell nanocomposites were explored for the photocatalytic degradation of congo red in aqueous solutions in the presence of sunlight. The CdS--Fe2O3 nanocomposites show better photocatalytic activity compared to pure -Fe2O3 and CdS nanoparticles. Also, the ZnO@-Fe2O3 core-shell nanocomposites show better photocatalytic performance compared to pure ZnO nanorods and -Fe2O3 nanoparticles. Chapter 7 deals with the overall summary of the work done in the thesis and also discusses the future prospects.