Browsing by Subject "Liquid crystals."

Now showing 1 - 9 of 9

Open Access
Effects of some transition metal salts on the synthesis of mesoporous silica
(2005) Demirörs, Ahmet Faik
Open Access
Investigation of lithium salt-nonionic surfactant mesophases and their applications in solar cells as gel electrolyte
(2013) Barım, Gözde
Some salts and some nonionic surfactants self-assemble together into lyotropic liquid crystalline (LLC) mesophases. The salt can be either in aqueous solution phase or in its molten phase in the self-assembly process. Concentrated aqueous solutions of lithium salts (LiCl, LiBr, or LiI) and pluronics (triblock copolymers, such as P65, P85, P103, or P123) or 10-lauryl ether (C12H25(CH2CH2O)10OH, denoted as C12EO10) type nonionic surfactant mesophases were investigated in this thesis work. The LLC mesophases are well ordered between 5.0 and 25.0 salt/pluronics and 2.0 and 10.0 salt/C12EO10 mole ratios, and remain stable for months under the ambient conditions. The water molecules remain as the hydrates under open atmosphere in the LLC mesophases of lithium salts-nonionic surfactants. The lithium salt-pluronic LLC mesophases are birefringent and have a hexagonal mesophase in a broad range of salt concentrations. The unit cell of the mesophases increases and a transition from the hexagonal to a cubic mesophases occurs upon increasing the salt content of the media. Moreover the LLC mesophases are ordered and stable up to 25.0 salt to pluronic mole ratio. At higher salt content, one can observe either a disordered phase or co-existence of salt crystals and mesophase. There is a big demand on the gel electrolytes for dye sensitized solar cells (DSSC) in order to overcome solvent problems caused by liquid electrolytes. The LLC mesophases of LiI, LiCl and LiBr salts with 10-lauryl ether (C12EO10) has been considered as gel-electrolyte for the DSSC. We demonstrate that the LiI/I2 couple can be incorporated into above LLC mesophases of various lithium salt-nonionic surfactant systems. Those LLC phases, with LiI/I2 couple have been characterized by means of diffraction, microscopy, spectroscopy and conductivity measurements. The LLC mesophases diffract at small angles and do not show any phase segregation upon incorporating the LiI/I2 redox couple. The LLC mesophases of these systems are 2D hexagonal, and they remain stable under ambient conditions for months. In the LLC media, the iodide ion and iodine molecule react to produce triodide ion in the media. The iodide/triodide (I- /I3 - ) redox couples containing gel electrolytes were formed and their solar performance was investigated by using a solar simulator and a cell consisting of a dye sensitized anode (FTO-dye modified TiO2), gel-electrolyte, and a cathode (FTO-Pt nanoparticles). The LLC mesophases of various lithium saltnonionic surfactant systems with the I- /I3 - redox couple were characterized using POM (Polarized Optical Microscope), XRD (X-ray Diffraction), FT-IR (Fourier Transform Infrared Spectroscopy) and Raman techniques. These new LLC mesophases can be used as gel electrolytes in solar cells after incorporation of redox couple into the media and display responses as good as commonly used liquid electrolytes.
Open Access
Investigation of two new lyotropic liquid crystalline systems : [Zn (formula) and [Zn (formula)
(2008) Albayrak, Cemal
The transition metal aqua complex salts (TMS) can be dissolved in oligo (ethylene oxide) type non-ionic surfactants (CnH2n+1(CH2CH2O)mOH, denoted as CnEOm) with very high salt/surfactant ratios to form lyotropic liquid crystalline (LLC) mesophases. In this study we show that addition of charged surfactants, such as cethyltrimethylammoniumbromide (CTAB) or sodiumdodecylsulfate (SDS) results a new type of LLC in which the solubility of the salts in the LC mesophase of TMS: C12EO10 is enhanced. The LC phase of a [Zn(H2O)6](NO3)2:C12EO10 is hexagonal between 1.2 and 3.2 and cubic (liquid like) above 3.2 salt/ C12EO10 mole ratios. Addition of CTAB or SDS increases the same salt/surfactant mole ratio to 8.0-9.0, which is a record salt amount for a lyotropic liquid crystalline system. The mixed surfactant mesophases have birefringent hexagonal mesophase between 2.0 and 8.0 salt/C12EO10 mole ratios The new mixed surfactant systems can also accomodate high TMSs in the presence of excessive amounts of water (35.0 water:C12EO10 mole ratio). Both systems have similar thermal properties. Izotropisation Temperature (IT) values of the new systems go down with increasing salt and charged surfactant concentrations. The mesophases are stable at high salt concentrations in the presence of high CTAB or SDS concentration in the expense of the stability of the LLC mesophase. The IT values changes from around 80o C down to 32o C with increasing composition of the LLC mesophase. The new mesophase have 2D or 3D hexagonal structure that responds to water content of the phase. A 3D hexagonal phase transforms to 2D hexagonal phase with the evaporation of excess water in both [Zn(H2O)6](NO3)2:C12EO10-CTAB-H2O and [Zn(H2O)6](NO3)2:C12EO10-SDS-H2O systems. The new mesophases were investigated using POM (Polarised optical microscope), and a hot stage under the POM, XRD (X-ray Diffraction), FT-IR (Fourier Transform Infrared Spectroscopy) and Raman techniques. These new LLC systems are good candidates for metal containing mesostructured material synthesis due to their high salt content.
Open Access
Molten Salt Assisted Self-Assembly (MASA) : synthesis of mesoporous silica-ZnO and mesoporous CdO thin films
(2012) Karakaya, Cüneyt
A series of mesostructured salt-silica-two surfactants (salt is [Zn(H2O)6](NO3)2, ZnX or [Cd(H2O)4](NO3)2, CdYand surfactants are cetyltrimethylammonium bromide (CTAB) and 10-lauryl ether, C12H25(OCH2CH2)10OH, C12EO10) thin films were synthesized by changing the Zn(II) or Cd(II)/SiO2 mole ratios. The films were prepared through spin coating of a clear solution of all the ingredients (salt, CTAB, C12E10, silica source (tetramethyl orthosilicate,TMOS, and water) and denoted as meso-silica-ZnX-n and meso-silica-CdY-n, where n is Zn(II) or Cd(II)/SiO2 mole ratios. The synthesis conditions were optimized by using the meso-silica-ZnX-1.14 and meso-silica-CdY-1.14 films and XRD, FT-IR spectroscopy, POM and SEM techniques. The stability of the films, especially in the high salt concentrations, was achieved above the melting point of salts. Slow calcination of the films, starting from the melting point of the salt to 450 oC has produced the mesoporous silica-metal oxide (ZnO and CdO) thin films, and denoted as meso-silica-ZnO-n and meso-silica-CdO-n, with n of 0.29, 0.57, 0.86, 1.14, and 1.43. The calcination process was monitored by measuring the FT-IR spectra and XRD patterns at different temperatures. Structural properties of the mesoporous films have been investigated using FT-IR spectroscopy, XRD, N2 sorption measurements, UV-Vis spectroscopy, SEM, TEM and EDS techniques. It has been found that the meso-silicaZnO-n and meso-silica-CdO-n films consist of nanocrystalline metal oxide nanoplates on the silica pore walls of the mesoporous framework. The formation of nanoplates of metal oxides was confirmed by etching the silica walls using diluted HF solution and by reacting with H2S and H2Se gases. The etching process produced CdO nanoplates without silica framework. The H2S and H2Se reactions with the CdO nanoplates or meso-silica-CdO have converted them to CdS and CdSe nanoplates or meso-silica-CdS and meso-silica-CdSe, respectively. Finally, a hypothetical surface coverage of metal oxide nanoplates has been calculated by combining the data of N2 sorption measurements, UV-Vis spectroscopy and TEM images and found that there is a full coverage of CdO and partial coverage of ZnO over silica walls in the meso-silica-CdO-n and meso-silica-ZnO-n thin films, respectively.
Open Access
A new lyotropic liquid crystalline system: Oligo (ethylene oxide) surfactants with transition metal complexes and the synthesis of mesoporous metal sulfides
(2001) Çelik, Özgür
In this study a new templating method, which can be used to synthesise mesporous materials, has been developed. The main objective of this work is to form organic mesophase in the presence of inorganic salts. This is an organic-inorganic hybrid mesophase, which can be used to template the growth of inorganic materials. Here for the first time, a new lyotropic liquid crystalline (LLC) system has been presented from oligo (ethylene oxide) type surfactant and transition metal aqua complexes. The temperature and the metal aqua complex concentration range of the complex/surfactant mixtures have been determined, where the mixtures have a liquid crystalline (LC) phase. Here, the complex refers to Ni(NO3)2·6H2O, Co(NO3)2·6H2O, Zn(NO3)2·6H2O, Cd(NO3)2·4H2O, and CoCl2·6H2O and the surfactant is C12H25(CH2CH2O)10OH, (C12EO10). The addition of the metal aqua complexes directly to the surfactant produces a LC phase. The LC phase obtained from the mixture of these two is more stable than the LC phase obtained from a mixture of free water and surfactant. The FT-IR and UV-Vis absorption, Polarised Optical Microscopy (POM) and Powder X-ray Diffraction measurements show that the coordinated water molecules mediate the formation of the LC phase. Our observations also show that the coordinated water molecules make a stronger interaction with ethylene oxide (EO) chains than free water molecules. The LC templating approach, which is demonstrated as a new system has been used for synthesis of meso-structured metal oxides, metal sulphides and even metal mesh. From all these studies, it is well known that in order to maintain LC phase the metal ion concentration should correspond to metal ion to surfactant mole ratio below 0.8. However, this work shows that the amount of metal aqua complex concentration can be increased up to a 6.5 complex to surfactant mole ratio by maintaining the integrity of the hexagonal and/or cubic structure of the LC phase. This may open a new area for the realisation of new mesostructured materials with better qualities and much higher yields. In the first part of the thesis, the thermal and structural properties of the new LLC phase has been established by using polarized optical microscopy (POM) with an attached hot plate, PXRD, FT-IR and UV-Vis absorption methods. In the second part, the new phase has been used as a template to synthesise mesoporus metal sulfides. The second part of the thesis deals mainly with the structure and synthesis of mesostructured CdS and ZnS. It has been demonstrated that the LC phase of Zn(NO3)2·6H2O, and Cd(NO3)2·4H2O in oligo(ethylene oxide) surfactant survive partially during the reaction with H2S to produce the corresponding metal sulfides.
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
Solid-solution of Cd(formula)Zn(formula)S nanocrystals in the channels of mesostructured silica films
(2006) Akdoğan, Yaşar
Mesostructured silica can be used as a reaction medium to produce solidsolution of Cd1-xZnxS nanocrystals as thin films. These films were synthesized from oligo(ethylene oxide) non-ionic surfactant (CH3(CH2)11(OCH2CH2)10OH, (C12EO10)), cadmium and zinc nitrate salts ([Cd(H2O)4](NO3)2 and [Zn(H2O)6](NO3)2), water, and tetramethylorthosilicate (TMOS, as silica source) mixtures using a liquid crystalline templating (LCT) approach and metal containing liquid crystalline (MLC) mesophase. Metal ion to surfactant mole ratio was 1.0 which determines the stability and structure of the mesostructured silica. The mesostructured silica film has a 3D hexagonal structure with oriented channels. The silica pore size can be controlled by controlling ageing temperature and time. The pore diameter of the silica channels that aged at room temperature (RT) for two days is 4.7 nm and the one aged at 2500 C for 30 minutes is 3.3 nm. Cd(II) and Zn(II) incorporated film samples can be reacted at RT under H2S atmosphere to produce zinc blend, Cd1-xZnxS nanocrystals (nano-Cd1-xZnxS-meso-SiO2) in the channels of the mesostructured silica. The band gaps of the nano-Cd1- xZnxS-meso-SiO2 vary between 2.6 eV for CdS and 4.1 eV for ZnS. The Cd (II) rich nanoparticles are larger (4.4 nm) than Zn (II) rich nanoparticles (3.1 nm). The silica wall thickness that can be controlled by ageing at different temperatures confines the growth of the Cd1-xZnxS nanocrystals in the pores. By controlling the size of the silica channel between 4.7 and 3.3 nm, one can control the band-gap of the CdS nanocrystals between 2.6 and 2.8 eV.
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.