Browsing by Subject "Lyotropic Liquid Crystals"

Now showing 1 - 3 of 3

Open Access
Molten salt assisted self-assembly process: synthesis of mesoporous transition metal oxide thin films and CdSe sensitized TiO2 photoanodes
(2017-09) Yaman, Muammer Yusuf
Fabrication of mesoporous transition metal oxide thin films are important for the development of many energy related technologies. Molten salt-assisted-self-assembly (MASA) method, that has simple stages such as preparation of a clear solution of the precursors in water or ethanol, coating of this solution on a substrate as a thin film, and calcination of the film at an elevated temperature is a simple and useful method to fabricate mesoporous thin films. In this thesis, mesoporous transition metal oxides thin films and CdSe sensitized TiO2 photoanodes have been fabricated using MASA approach and characterized using multi-analytical techniques. In the first part of the thesis, CdSe/TiO2 thin films were synthesized by reacting mesoporous CdTiO3 thin film under a H2Se gas atmosphere. Many synthesis parameters were optimized in the synthesis of mesoporous CdTiO3 thin films. The film thickness, calcination temperature, and H2Se reaction condition have been changed to determine the optimum synthesis conditions for an efficient photoanode of a quantum dot sensitized solar cell (QDSSC). However, the reactivity of CdTiO3 towards to H2Se gas is low and selenium forms as a side product upon H2Se reaction. For the reactivity problem, mesoporous CdO-SiO2 (denoted as meso-CdO-SiO2) thin films were synthesized to increase the CdSe nanoparticle population, as the sensitizer in the CdSe sensitized TiO2 photoanode. However, The meso-CdSe-SiO2 thin films are not active in solar cells because of an insulating nature of silica. Second part of the thesis involves infiltration of MASA solution (precursors of salt-surfactant and a polymerizing agent, such as Si(OCH3)4 or Ti(OC4H9)4) into the pore of prefabricated films (using P25) of mesoporous titania (denoted as meso-P25). In latter steps, the above films were calcined and then reacted under H2Se to obtain the photoanodes (denoted meso-CdSe-SiO2-P25 and meso-CdSe-TiO2-P25). The synthesis conditions were optimized by changing the synthesis parameters (such as precursor concentrations, calcination, and H2Se reaction temperatures) and using XRD, FTIR, Raman, 29Si-MAS-NMR, EXAFS, XANES, SEM, TEM, N2-sorption techniques. However, formation of a meso-CdO-SiO2 thin film, on top of the meso-P25 film, was observed upon using a concentrated MASA solution in the infiltration step. Therefore, multiple loading method has been established to increase the CdSe-SiO2 layer in the pores of meso-P25 using diluter MASA solutions. Also, the H2Se reaction conditions were optimized by controlling the reaction atmosphere and temperature. Effects of silica amount in the CdO-SiO2 system on the photoanode has been examined by measuring the I-V curves, of the solar cells fabricated using our photoanodes. In the last part of the thesis, the MASA method has been adopted for the synthesis of mesoporous transition metal oxides thin films. Firstly, mesoporous iron oxide film has been synthesized using MASA approach and characterized using above analytical tools. The thermal and structural properties of the [Fe(H2O)6](NO3)3/surfactants (10-lauryl ether and CTAB) mesophases have been investigated for the synthesis of a well-ordered iron oxide films. Effects of calcination temperature, on the crystallinity, and porosity of mesoporous Fe2O3, have been demonstrated by using TEM, SEM, XRD, and N2 sorption techniques. Later, other mesoporous transition metal oxides (such as ZnO, CuO, NiO, Co3O4 and Mn2O3) have been synthesized using the MASA approach. The transition metal salts ([Zn(H2O)6](NO3)2, [Cu(H2O)6](NO3)2, [Ni(H2O)6](NO3)2, [Co(H2O)6](NO3)2, [Mn(H2O)6](NO3)2)-surfactant mesophases have been used as the starting liquid crystalline materials that can be calcined at high temperatures (above 300 °C) to obtain the thin films. The synthesized mesoporous metal oxide thin films were characterized by using above analytical tools.
Open Access
New solvents for surfactant self-assembly : molten hydrated salts and concentrated aqueous electrolyte solutions
(2013) Albayrak, Cemal
Lyotropic liquid crystalline (LLC) mesophases are formed by at least two components: a surfactant and a solvent. Common solvents in the surfactant self-assembly include water, organic liquids, and ionic liquids. In this work, we show that molten hydrated salts of the type [M(H2O)m](X)n (where, M is a transiton metal cation and X is a suitable anion such as NO3 - , Cl- , and ClO4 - ), which have melting points close to room temperature (RT), can organize surfactant molecules into LLC mesophases. As an example, we have focused on the [Zn(H2O)6](NO3)2-C12EO10 system (where, C12EO10 is decaethylene monododecyl ether; H3C-(CH2)11-(OCH2CH2)10-OH). A binary phase diagram was constructed between -190oC and 110oC using differential scanning calorimetry (DSC), polarized optical microscopy (POM), X-ray diffractometry (XRD), fourier transform infrared spectroscopy (FT-IR), and raman spectroscopy. The phase diagram closely resembles the phase diagram of H2O-CmEOn systems, exhibiting typical phases such as spherical cubic, hexagonal, and bicontinuous cubic. It is also observed that the phase transitions are dictated by the critical packing parameter (CPP) as the solvent concentration is changed. The mesophases are unusually stable at low temperatures, where a LLC to mesostructured solid transformation has been observed with a glass transiton at - 52oC. The mesostructured solid phase is also stable at -190oC. The confinement of the salt species in the LLC domains prevents the crystallization of the salt at low temperatures. In the second part, from the analogy between [M(H2O)m](X)n type salts and concentrated electrolyte solutions of alkali metal salts, the mixtures of concentrated aqueous solutions of some Li+ salts (LiCl, LiBr, LiI, LiNO3 and LiClO4) with C12EO10 surfactant, were investigated. The mixtures exhibited LLC mesophases in a broad range of compositions. A ternary phase diagram was constructed for the LiNO3-H2O-C12EO10 system at room temperature using XRD and POM tecniques. In the LLC mesophases formed with the Li+ salts, the water remains as hydrated under ambient conditions and open atmosphere. In addition, the effect of anions on the phase behaviour follows a Hofmeister series except for the ClO4 - ion. Ionic conductivty of the LiX-H2O-C12EO10 (where X is Cland NO3 - ) mesophases has been determined in a broad range of the salt concentrations (5 to 7 salt/surfactant mole ratio) and temperature (-13 to 100oC). The LiCl-H2OC12EO10 LLC samples have also been used as a gel-electrolyte to run a polymer electrochromic device. The mesophase shows excellent performance in this device. The investigations were further extended to include some of the Ca2+ salts, namely CaCl2 and Ca(NO3)2. The concentrated aqueous solutions of both salts with C12EO10 and water exhibited LLC mesophases similar to the molten hydrated salts and concentrated solutions of Li+ salts. In the CaCl2.xH2O-C12EO10 system, an LLC to mesocrystalline phase transformation was observed, for the first time, where the salt, water and surfactant species freezes to a mesocrystalline phase at RT. Lastly, many other salt.xH2O-surfactant LLC mesophases were investigated using the following salts: NaCl, NaBr, NaI, CH3COONa, NaSCN, NaClO4, NaNO3, KNO3, KCl, KSCN, KI, MgCl2, Mg(NO3)2 and NaOH. In addition, the LLC mesophases of concentrated H3PO4 acid and C12EO10 were also investigated. Among these compounds, H3PO4 systems exhibited air stable LLC mesophases at RT and 25% relative humdity (RH). The MgCl2 system was found to exhibit air stable LLC mesophases for a couple of hours. The NaI, KSCN and NaClO4 systems were found to be stable at low salt concentrations with little or no mesostructured order. Other salt systems were unstable and leached out salt crystals rapidly. The NaOH system is unstable because of a reaction with CO2 in the air. In summary, we have found a correlation between the deliquescent relative humidity value of the salt and its LLC mesophase formation ability under ambient conditions.
Open Access
Synthesis and characterization of mesoporous lithium metal phosphates (LiMPO4) (M= Mn, Fe, Co, Ni)
(2018-03) Çolak, Tuluhan Olcayto
Synthesis of mesoporous lithium metal phosphates have been studied extensively in past after the emerge of lithium iron phosphate as a cathode material in the lithium ion batteries. These materials are proved to be modifiable and useful in lithium ion batteries. This study encompasses synthesis and characterization of the mesoporous LiMPO4 (M= Mn(II), Fe(II), Co(II), and Ni(II)) from lyotropic liquid crystalline (LLC) mesophases, utilizing a method which can be described as a modified molten salt assisted self-assembly (MASA) method. Preparation of clear solutions and LLC mesophases afterwards are quite an effortless process, once optimized, which in its order starts with the clear solution prepared for the synthesis of lithium transition metal phosphate, then the coating of the solution over glass substrate using two methods, the spin coating and drop-casting. The coated films are then calcined to fabricate the mesoporous lithium metal phosphate products. In this thesis, the mesoporous LiMPO4 (M = Mn(II), Fe(II), Co(II), and Ni(II)), are synthesised using the modified MASA method using 10-lauryl ether as the soft template and characterized using multi-analytical techniques (such as FTIR, PXRD, SEM, EDX, and N2 adsorption-desorption). In the initial part of the thesis, the solution stability over time, pH dependence, and concentration of the ingredients were investigated. It was found that through time these solutions precipitate ranging from weeks to hours with an inverse relation with the concentration of used salts, and acid relative to the surfactant. Continued in this part, it was observed that solution stability is also dependent on pH, which was tested using LiOH instead of LiNO3 as the lithium source. It was revealed that, at higher pH values, the solutions are less stable and produce more precipitate. The solutions, prepared using Mn(II), Fe(II), Co(II), and Ni(II), were coated on glass substrates by drop-cast coating and spin coating methods. These two methods were used to determine the best method for a desired amount and morphology of the corresponding products. After testing a broad range of ingredient concentrations, using the Mn(II) system, three concentrations were selected to represent dilute, medium and concentrated ratios of salt and acid versus the surfactant. The aging and temperature dependent changes were monitored using FT-IR spectroscopy; the effect of temperature on both the formation of mesophase and the reactions taking place in the mesophase has been investigated. It appears that the temperature has some profound effects on the mesophase. The mesophase gets disordered by increasing temperature. This trend also correlates well with increasing salt concentration in the media. As the salt concentration increases the temperature required to disrupt the mesophase decreases. The FT-IR spectroscopy study shows that; significant amount of nitrate species and surfactant molecules have been removed from the media at around 160oC. To remove the surfactant completely, minimum temperature of calcination determined to be 250oC. Samples, prepared with low concentration solution of Mn(II) salt coated with both methods, were calcined at 250, 350, and 450oC and characterized using XRD, FT-IR spectroscopy and SEM techniques. It was found that the drop-casting method is favourable over the spin coating method, because the spin coating method failed to produce the desired compound and created metal pyrophosphate instead of lithium metal phosphates. LiMnPO4, LiFePO4, LiCoPO4, and LiNiPO4 were synthesised using drop-cast coating method and characterized by XRD, FT-IR spectroscopy, SEM, EDX, and N2- adsorption-desorption techniques. It was found that these materials are mesoporous and have noticeable surface areas with some by-products. The pores are large and nonuniform in LiMnPO4 and LiCoPO4, but the pores are small (3-6 nm range) in the iron and nickel samples. The surface area also accords with observation and highest (96 m2/g) surface area was recorded from nickel samples. The pores gradually expand with annealing the samples and becomes non-uniform all cases. The undefined crystalline phases require more work to determine their structure and more optimization to obtain the desired material.