Browsing by Subject "Liquid Crystal Templating"

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Open Access
Synthesis and characterization of mesoporous metal sulfide and metal selenide thin films using liquid crystalline mesophases
(2012) Türker, Yurdanur
In this thesis, synthesis of the mesoporous CdS and CdSe by using of liquid crystalline templating (LCT) approach has been investigated. In the first part of the thesis, the thermal and structural behavior of the [Cd(H2O)4](NO3)2/surfactant (P85 = ((PEO)26(PPO)40(PEO)26)) binary lyotropic liquid crystalline (LLC) systems have been investigated towards synthesis of the mesoporous cadmium sulfide, CdS, or cadmium selenide (CdSe) directly from the mesostructured CdS (or CdSe) thin films. However, the mesostructured CdS/P85 films (at low salt concentrations), which were obtained by reacting [Cd(H2O)4](NO3)2/P85 LLC thin films under H2S atmosphere, are not stable to calcination process and always produced bulk CdO and CdS domains over the thin films. More metal ion containing [Cd(H2O)4](NO3)2-C12EO10-CTAB mesostructured films produced vast amount of HNO3 under the H2S atmosphere and caused decomposition of CdS back to their nitrates. To overcome above problems, a polymerizing agent, such as titania or silica precursors have been added to salt/surfactant LLC mesophase. Both titania and silica overcame the collapse of the mesophase by rigidifying the structure into mesostructured solid and also by providing stability for a thermal removal of nitrates from the medium. For this investigation, both [Cd(H2O)4](NO3)2 and [Zn(H2O)6](NO3)2 salts and P123 ((PEO)20(PPO)70(PEO)20) and C12EO10-CTAB couple have been used. Well-ordered mesostructured Cd(II) titania films have been obtained up to 15.0 Cd(II)/P123 mole ratio for a 60 mole ratio of Ti(IV)/P123 by spin or dip coating of a mixture of 1-butanol-[Cd(H2O)4](NO3)2-P123-HNO3-Ti(OC4H9)4. Exposing the mesostructured Cd(II)-TiO2 films to H2Se under a N2 atmosphere gave stable CdSe nanoparticles in the channels of the mesostructured rigid titania walls up to 25 mole % Cd(II)/Ti(IV). To further increase the metal ion (Cd(II) and Zn(II)) content in the structure, the C12EO10-CTAB-salt mesophase has been employed. The two surfactant-salt systems, in the presence of a titania precursor, produced sponge like mesoporous CdTiO3 and Zn2TiO4 films up to a mole percent of 57 and 86, respectively, upon calcination. Exposing the mesoporous CdTiO3 to H2S or H2Se atmosphere at RT produced homogeneously distributed CdS or CdSe nanocrystallites on the nanocrystalline TiO2 pore walls, respectively. The reaction of mesoporous Zn2TiO4 with H2Se produced stable ZnSe nanocrystallites on the nanocrystalline TiO2 pore walls. The conversion of titania from CdTiO3 to an anatase and brookite phase under H2S and H2Se atmosphere, respectively, and from Zn2TiO4 to a rutile phase under H2Se were observed for the first time. Adding a silica precursor to the two surfactants (C12EO10-CTAB)-salt mesophase produced mesostructured salted-silica, and its calcination produced sponge-like mesoporous silica-metal oxide (dumped meso-SiO2-CdO and mesoSiO2-ZnO) thin films. Up to ~100 % and ~50 % surface coverage could be achieved by CdO and ZnO as nano-islands over the SiO2 pore walls. Exposing the mesoporous SiO2-CdO and SiO2-ZnO thin film precursors to H2S and H2Se at RT enabled the synthesis of mesoporous SiO2-CdS, SiO2-CdSe, SiO2-ZnS, and SiO2- ZnSe thin films. The MS or MSe nanoflakes could homogenously cover the pore walls of mesoporous silica by retaining the pore morphology of the MO precursors. The H2S and H2Se reactions are slow and can be monitored using UV-Vis absorption spectroscopy and EDS to elucidate the reaction mechanism and kinetics. These data showed that the reaction starts from the top surface of the MO domains and proceeds until Si-O-M bond break. Finally, the SiO2 walls were removed from the meso-SiO2-CdS and meso-SiO2-CdSe films through etching in a dilute HF solution to produce mesoporous CdS (meso-CdS) and mesoporous CdSe (meso-CdSe). Surface of the meso-CdS has been modified using PEI (polyethyleneimine) and photoluminescent meso-CdS were obtained.
Open Access
Synthesis of mesostructured metal sulfides using transition metal salts : pluronic liquid crystalline mesophases
(2007) Türker, Yurdanur
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.