Synthesis of mesostructured metal sulfides using transition metal salts : pluronic liquid crystalline mesophases
Author(s)
Advisor
Dağ, ÖmerDate
2007Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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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)4](NO3)2 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)4](NO3)2: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)4](NO3)2: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.
Keywords
Liquid Crystal TemplatingTransition Metal Complexes
Mesostructured Metal Sulfides
Doping CdS
Mesostructured Thin Films
Pluronics
Nanocrystallites