Synthesis of mesostructured metal sulfides using transition metal salts : pluronic liquid crystalline mesophases

Abstract

The Liquid Crystalline Templating (LCT) approach has been extensively used to produce mesostructured Metal Sulfides (MS) powders by using nonionic surfactants (CnEOm). The aim in this work is to synthesize larger pore size mesostructured MS at high salt concentrations by mixing Pluronics (PEOxPPOyPEOx, EO = -OCH2CH2-, PO = -OCH(CH3)CH2-) with transition metal salts (TMS) M(H2O)42 in a dilute media. This enables to synthesize thin films of mesostructured MS. In this thesis, the MS (M= Cd, Zn, Cd1-xZnx, Cd1-xCox and Cd1-xMnx) were synthesized by the LCT approach using Pluronic P85 ((PEO)26(PPO)40(PEO)26) and TMS. The P85 and salts can be dissolved in various solvents to obtain clear solution that enables one to increase the salt to pluronic mole ratio up to 30:1. However, the LC mesophases form in the Cd(H2O)42:P85 mole ratio range of 3:1 to 11:1 with a 3D hexagonal structure and P63/mmc space group having unit cell parameters of a = 99.5 Å and c = 162.5 Å with a c/a ratio of 1.633. The CdS thin film samples, obtained by exposing the Cd(H2O)42:P85 LC phase to H2S gas, could retain the mesostructure of the LC mesophase in the mole ratio range of 3:1 to 11:1. The film samples that consist of 50-100 nm mesostructured CdS and free surfactant molecules are uniform and soft in early stages of the H2S reaction. However, in time, the free surfactant molecules diffuse out of the mesostructured CdS and form dendritic structures, producing CdS thin films with huge domains. The CdS thin film samples consist of 4.3 nm CdS nanoparticles that emit orange light under UV irradiation. Well homogenized LC mesophases produce cracked well structured film samples upon H2S reaction. This method can be used to fine tune both the composition (between x=0.0 and 1.0) and the optical band-gap of Cd1-xZnxS nanocrystallites between 2.60 eV and 4.00 eV. The Zn(II) and Cd(II) ions are homogenously doped throughout the mesostructure and nanocrystallites synthesized by this approach are slightly larger in every composition compared to the ones synthesized in the mesostructured silica channels. Also both Co(II) and Mn(II) ions could be incorporated into the CdS lattice with x ≤ 0.15 for stable Cd1-xCoxS and Cd1-xMnxS film samples, respectively. The Co(II) ions occupy the isolated tetrahedral holes in the CdS lattice until x = 0.15 for stable samples. In this thesis, the structure and structural changes in the LC mesophase during the synthesis of MS and particle size analysis of the nanocrystallites were investigated using diffraction (XRD), spectroscopy (FT-IR, micro-Raman and UVVis absorption) and microscopy (OM and SEM) techniques.