Prof. Dr. Ömer Dağ danışmanlığındaki tezler
25 tez · İhsan Doğramacı Bilkent University
Mezogözenekli spinel LiMn2-xMxO4 (M = Mn, Fe, Co, Ni, and Cu) ince filmlerin sentezi, karakterizasyonu ve elektrokimyasal özellikleri
Mesoporous LiMn2-xMxO4 electrodes are promising candidates for efficient oxygen evolution (OER) electrocatalysis. In this study, mesoporous LiMn2-xMxO4 (where M is Mn, Fe, Co, Ni, and Cu) thin films have been fabricated by employing molten-salt assisted self-assembly (MASA) method on fluorine doped tin oxide (FTO) surface. The electrodes are characterized according to their structure, morphology, and thicknesses using various characterization techniques. The electrochemical properties of the films are comprehensively investigated under acidic, alkaline, neutral, and non-aqueous solutions. The manganese oxide-based electrodes undergo Mn(III) and Mn(VI) disproportionation reactions. Here, we have extensively investigated these disproportionation reactions by post-characterization techniques after electrochemical experiments using LiMn2-xMxO4 (M is Fe, Co, Ni, and Cu and x is 0, 0.1, 0.3, 0.4, and 0.67) and Mn3O4 electrodes. The LiMn2O4 thin films are found to be more stable in OER compared to Mn3O4. Lithium de-intercalation of the LiMn2O4 films produces a λ-MnO2 phase robust against Mn(VI) disproportionation. The electrochemical degradation rates are investigated using the LiMn2O4 electrodes, fabricated at various spin rates (from 2000 and 10000 rpm). The film thicknesses are between 150 and 500 nm. The LiMn2O4 electrode at 5000 rpm is more resistant to physical degradation during electrochemical tests. Charge capacity values of the thin films are determined by electrochemical experiments in LiNO3 electrolyte and found to be between 136 and 273 mC/cm2 for the films, then these values are used to calculate their approximate weights (between 30 and 60 μg/cm2). The annealing temperature for the LiMn2O4 thin films is also optimized for a stable OER. The LiMn2O4 film, fabricated at 5000 rpm spin rate and annealed at 300 oC, is found to be a more robust and efficient electrode with a 60 mV/dec Tafel slope and 812 mV overpotential at 10 mA/cm2 current density. The same fabrication parameters are used for the other mesoporous LiMn2-xMxO4 thin films. The LiMn2-xMxO4 thin films are used to collect their N2-adsorption-desorption isotherms. The isotherms display type IV hysteresis, characteristic of mesoporous materials. BET surface areas are estimated as 98, 99, 116, 112, and 75 m2/g for the LiMn2O4, LiMn1.7Fe0.3O4, LiMn1.7Co0.3O4, LiMn1.7Ni0.3O4 and LiMn1.7Cu0.3O4, films, respectively. Moreover, the LiMn2-xMxO4 electrodes (fabricated at 5000 rpm spin rate and 300 oC annealing temperature) are investigated for lithium de-intercalation/intercalation behavior in 1 M LiNO3 solution. Then, the same electrodes are used to collect 300 CVs, CAs, and CPs in 1 M KOH solution to evaluate electrochemical behaviors. From these measurements, the origin of phase separation and bearing lower oxidation states of the nickel and copper at higher x values are identified in the spinel structure. The Mn(VI) disproportionation reaction on the LiMn2-xMxO4 electrodes is investigated by CV cycling experiments in 1 M KOH. The LiMn2O4, LiMn2-xFexO4, and LiMn2-xCuxO4 electrodes undergo fast degradation compared to LiMn2-xCoxO4 and LiMn2-xNixO4 through the dispersion of [MnO4]- and [FeO4]2- ions and dissolution of the CuO phase formed in the electrodes during OER. The LiMn1.7M0.3O4 thin films on FTO are used in OER electrocatalysis and the overpotential values at 10 mA/cm2 are evaluated as 645, 686, and 657 mV for the LiMn1.7Fe0.3O4, LiMn1.7Co0.3O4, LiMn1.7Ni0.3O4 electrodes, respectively. The exact compositions are also coated on graphite substrates and their overpotential values are also evaluated as 629, 462, 440, and 532 mV at 10 mA/cm2 for the LiMn2O4, LiMn1.7Fe0.3O4, LiMn1.7Co0.3O4, LiMn1.7Ni0.3O4 electrodes, respectively. The LiMn1.7Co0.3O4 on graphite and LiMn1.7Ni0.3O4 on FTO electrodes are found to be the most robust and efficient electrodes at a 50 mA/cm2 current density.
Mezo-gözenekli nikel oksit yapılı ince film elektrotların üretimi ve elektrokimyasal özellikleri
n this thesis, robust electroactive mesoporous Ni1-xMnxO thin-film electrodes were synthesized on FTO and graphite rod substrates. Molten salt-assisted self- assembly (MASA) synthesis method was employed to produce uniform thin films. The synthesis started with preparing ethanol solutions, containing various molar ratios of [Mn(H2O)4](NO3)2 and [Ni(H2O)6](NO3)2 (between 1.0 to 0.1 Ni(II)/Mn(II) ratio ) and surfactants (C12H25(OCH2CH2)10OH, C12E10 and C16H33N(CH3)3Br, CTAB). Then, these solutions are coated over conductive substrates to obtain the salt-surfactant lyotropic liquid crystalline (LLC) mesophase. The thin mesophase is calcined in order to produce mesoporous Ni1-xMnxO thin-films on the FTO or graphite. The thin-films form solid solutions with the x value of up to 0.7. The Ni1-xMnxO thin-films transform to NiMnO3, Mn3O4, and Mn2O3 phases at increased Mn ratios and annealing temperatures. The films are mesoporous and were confirmed by N2 adsorption- desorption analysis and typical type IV isotherms characteristic for mesoporous materials. Pore sizes varied from 2.8 to 17.6 nm from Ni-rich to Mn-rich oxides. The surface area reaches to 211 m2/g in Ni0.9Mn1O, while the pure NiO has a BET surface area of 164 m2/g at 350 oC calcination temperature. The FTO and graphite-coated electrodes (FTO-Ni1-xMnxO and G-Ni1-xMnxO) display high charge capacities, but the FTO coated electrodes are unstable and undergo to degradation over extended time of usage. In the first few CV cycles of the FTO-based electrodes, they show an increased capacity, however, decline in further cycles. On the other hand, graphite-based electrodes show better stability and high charge capacity. Origin for increasing charge capacity with cycling is attributed to a transformation of the metal oxides to metal hydroxides. Thus, the electrochemical CV cycling of both pure NiO and Ni1-xMnxO electrodes results in a structural change into a NiO(core)/Ni(OH)2(shell) or Ni1-xMnxO(core)/Ni(OH)2(shell) configurations. The shell thickness is ranged from 2.0 nm (pure NiO) to 1.1 nm (Ni0.9Mn0.1O) at 350°C. Moreover, the shell thicknesses and charge capacities are affected by the pore-wall thicknesses, which increases with increasing annealing temperature. Despite these changes, the manganese addition improves the stability of the electrodes, but there is no improvements on the overpotential on oxygen evolution reaction (OER). Moreover, the annealing temperature reduces the charge capacity, whereas the OER performance remains the same. By using the same MASA method, m-NiO-SiO2 electrodes were synthesized using [Ni(H2O)6](NO3)2 and tetramethyl orthosilicate (TMOS) with CTAB and C12E10 surfactants at different Ni to TMOS ratios. Silica acts as hard template support for NiO, and the film is formed in good quality with bimodal pore size distribution. The sample pore size that was observed is 2.6 nm, which originates from the m-SiO2 domains. The second pore system had also mesopores; the average pore size is 15 nm, calcined at 350 oC. That property helps better infiltration of electrolytes, which is advantageous during electrochemistry. During electrochemical analysis, silica is etched out in basic electrolyte. These electrodes, prepared on graphite substrate have specific surface area of around 130 m2/g. The electrodes show an overpotential 381 mV in the CP experiment at 10 mA/cm2 current density.
Mezo-gözenekli M2P2O7 (M: Co(II), Ni(II), ve Mn(II)) ve Mn2-xMxP2O7 (M: Co(II) And Ni(II)) ince filmlerinin sentezinde çözücünün rolü ve oksijen evrim reaksiyonu performansları
Mesoporous transition metal pyrophosphates (M2P2O7) have attracted considerable attention due to their diverse structural features and wide range of potential applications in catalysis, ion exchange, and energy storage. This study presents a novel synthesis route for high surface area and porous M2P2O7 materials, focusing on the influence of solvent selection on their structural and electrochemical properties. Co2P2O7, Ni2P2O7, Mn2P2O7, and binary metal pyrophosphates have been synthesized via a molten salt-assisted self-assembly (MASA) method using pyrophosphoric acid (PPA, H4P2O7), salt [Mn(H2O)4](NO3)2, [Co(H2O)6](NO3)2, and [Ni(H2O)6](NO3)2), surfactant (pluronic, P123), and solvents (ethanol, butanol, and methanol). Sol-gel process takes place both in ethanol and butanol, while methanol-based solutions remained cloudy. After centrifugation, the solution, supernatant, and precipitate parts are coated onto substrates to form lyotropic liquid crystalline (LLC) mesophases, followed by calcination at different temperatures to obtain mesoporous metal pyrophosphates. Structural evolution was monitored by small-angle XRD and ATR-FTIR techniques. Aging of the supernatants at room temperature produce semi-solid mesostructures via polymerization between metal species and PPA. Calcination between 300 and 700 ⁰C yields highly porous M2P2O7 materials with BET surface areas of 145, 410, and 83 m2/g for Co(II), Ni(II) and Mn(II) pyrophosphates, respectively. XRD analysis shows that Ni2P2O7 crystallizes at 700⁰C, regardless of the solvent, while the Co2P2O7 and Mn2P2O7 samples crystallize at lower temperatures. Supernatant-derived samples mainly form M3(PO4)2, metal oxides, and some undefined impurities. High crystallization temperatures results in the pores collapse and surface area loss. Electrodes are fabricated on FTO and graphite rod (GR) substrates; between these two electrodes, the GR coated electrodes have a better electrochemical stability. Electrochemical measurements (cyclic voltammetry (CV), multi-step chronoamperometry (MCA), and multi-step chronopotentiometry (MCP)) were performed in 1 M KOH using a three-electrode system. Post electrochemical analysis reveales that M2P2O7 transforms into metal hydroxide in alkaline media. To evaluate the intrinsic activity, CVs were also conducted in a neutralized electrolyte (2.4 M KOH + 0.6 M H4P2O7) solution, where the metal hydroxides exhibite superior OER performance. The Mn2P2O7 electrodes are unstable due to disproportionation of Mn(VI) to MnO4- and MnO2 during OER. To improve performance, Mn2-xMxP2O7 (M=Co(II), Ni(II); x=0.05,0,10, 0,25) binary electrodes are also fabricated, exhibiting higher surface areas and better OER activity. Notably, ethanol- derived MnNi25% (with 25% Ni(II)) and butanol-derived MnCo10% (10% Co(II)) precipitates, calcined at 300⁰C, display high surface areas of 202 and 144 m2/g, respectively. These electrodes display high stability in OER and have overpotentials of 280-300 mV at 1 mA/cm2 current density.
Mezogözenekli CdTiO3 ince filmlerin sentezi ve karakterizasyonu
This thesis work focuses on the synthesis and characterization of mesoporous CdTiO3 thin films by using a salt-surfactant assembly, which is defined as molten salt assisted self-assembly (MASA) method. The MASA method is a proper process to fabricate mesoporous transparent thin films. The characterization of the calcined powder and fresh gel samples was made by using XRD, SEM, TEM, ATR-IR, N2 sorption and POM techniques. The preparation of a clear solution containing ethanol as solvent, [Cd(H2O)4](NO3)2, two different surfactants ((C16H33N(CH3)3)Br and EO20PO70EO20), titanium(IV)butoxide (Ti(OC4H9)4) as titania source and concentrated HNO3 as acid to prevent quick polymerization of titanium alkoxide is the first step for the synthesis of the desired material. The prepared clear solution is coated on the glass substrates to form a liquid crystalline mesophase by the hydrolyzed titania species and the molten salt by the guidance of surfactant domains. Upon the calcination of the fresh gel samples, mesoporous CdTiO3 material is formed. In scope of this thesis work, several parameters were changed to determine the optimum the salt uptake, acid amount, surfactant ratio, coating method and calcination path for the synthesis. Among dip coating, spin coating and drop casting methods, dip coating method gave better results to produce a material with less side products, which was mainly CdO. The amount of HNO3 added to the solution affects the homogeneity and stability of the prepared fresh films. Although changing the amount of HNO3 did not affect the surface area significantly, it was observed that addition of excess amount of acid resulted in an uneven surface on calcined films. CdTiO3 is nanocrystalline at 350oC and stable up to 550oC. The initial calcination temperature is an important parameter to synthesize a material with less side product. Mesoporous CdTiO3 displays 44 to 79 m2/g surface area and pore volume of 0.11 to 0.18 cm3/g depending on the synthesis conditions.
Mezogözenekli LiMPO4'lerin (Mn(II), Fe(II), Co(II), Ni(II)) sentezleri ve karakterizasyonları
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 non-uniform 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.
Gümüş fosfatın liyotropik sıvı kristallerin mezofaz bazlı bir fotokatalizör olarak sentezi ve karakterizasyonu
Increasing energy demands and environmental problems are the driving forces of the current literature. Over the years, many new compounds have been synthesized and also morphological control of the well-known compounds have been the major topics to improve/contribute to the solutions of energy demand and environmental issues. One of these issues is finding an efficient and stable photocatalyst for some of the environmental problems. Ag3PO4 has been a target material for dye degradation and water splitting processes. Silver phosphate has a suitable band gap for photo-oxidation process under visible light irradiation. However, it has stability and reusability problems that needs to be resolved to effectively use as an efficient photo-catalyst. Because of that, many research worked on the synthesis and stability issues of this material. In this thesis, the work focuses on surfactant:Ag(I):H3PO4 lyotropic liquid crystalline mesophase to synthesize mesoporous Ag3PO4. Two different surfactants (small, 10-lauryl ether, C12EO10 and large pluronic, triblock copolymer, P123, HO(CH2CH2O)20-(CH(CH3)CH2O)70-(CH2CH2O)20H), two different silver salts (AgNO3, SN and AgCF3SO3, AgOTf) and two different phosphate precursors (H3PO4 and LiH2PO4) have been used throughout this investigation. Solutions were prepared in water or ethanol by first dissolving surfactant, then adding stoichiometric ratio of AgNO3, and H3PO4. To achieve clear and homogenous solution, a small amount of HNO3 is added to the above solution. Without HNO3, some yellow precipitation occurs that needs to be filtrated out. According to XRD patterns, SEM images, and N2 adsorptiondesorption isotherms, the yellow precipitate is bulk Ag3PO4. Decanted solution and normal acidified solution compares well with each other and the results are similar in ii further steps of the synthesis. Therefore, adding small amount of HNO3 to the solution overcomes the precipitation of bulk Ag3PO4 and used in further steps of the synthesis. Then, the solutions can be spin or drop-cast coated over glass slides to form the mesophases and thin/thick films. The films diffract at small angles, indicating the formation of the mesophase. However, the mesophases are not stable and gradually transform into soft mesocrystals that diffract at small and high angles. Later step is to determine a desired calcination temperature for mesoporosity. Therefore, first a high temperature (over 300˚C) treatments have been applied to burn all surfactant in the films. This ensures mesoporosity, but it also results some bulk formations; silver metal forms at high temperatures. Therefore, the calcination or heat treatment temperature has been gradually reduced down to room temperature (RT). At RT, soft mesocrystal forms that can be heat treated at various low temperatures (70-150˚C) to form Ag3PO4 in many different morphologies; these samples have no silver metal. All Ag3PO4 samples, obtained under different conditions, were tested in Rhodamine-B (Rh-B) dye degradation by visible light irradiation with a good activity. But the catalyst is not stable under catalytic conditions. To solve this problem, some samples were prepared under vacuum to convert surfactants carbons to coat the surface of the catalyst by carbon that stabilized the catalyst. In the last section of the thesis, cation exchange method has been developed to convert pre-formed mesoporous LiMPO4 (M = Mn, Co, and Ni) to Ag3PO4. Mesoporous Ag3PO4 has been obtained from all precursors but the ones obtained from LiCoPO4 performed the best in photo-degradation of dye under visible light and the ones obtained from LiMnPO4 is almost inactive. Therefore, this part needs further studies to understand details of these observations. Introducing carbon and cation exchange methods seem to be effective solutions for the stability problem of this photocatalyst. All synthesis products are tested in the photodegradation experiment and compared with each other. This thesis is partially clarified; how to synthesize mesoporous Ag3PO4, what the behavior of silver in system is, and how to stabilize the catalyst. Furthermore, the cation exchange process opens a new horizon for the Ag3PO4 synthesis.
Asit-tuz-yüzey aktif madde karışımlarından elde edilen liyotropik sıvı kristal arafazlar: mezogözenekli LimpO4' lerin (M=Mn(II),Fe(II),Co(II) ve Ni(II)) sentezi ve karakterizasyonu
This study presents the synthesis and characterization of mesoporous lithium metal phosphates (LMPs) of Mn(II), Fe(II), Co(II), and Ni(II). The LMPs were synthesized using a modified molten salt self-assembly (MASA) method. Clear and homogeneous solutions of lithium nitrate (LiNO3), transition metal nitrate ([M(H2O)6](NO3)2, phosphoric acid (H3PO4, PA), and surfactant (pluronic P123, EO20PO70EO20, where EO is ethylene oxide and PO is propylene oxide) in water were spread on a microscope slide by drop-cast coating method to from a lyotropic liquid crystalline (LLC) mesophase. The mesophases were characterized using polarized optic microscope (POM) and x-ray diffractrometer (XRD) techniques. In the mesophase, the mole ratio of the inorganic components was kept constant (1:1:1, Li(I):M(II):PA) but the inorganic ingredient (lithium salt, transition metal salt, and PA) to surfactant mole ratios were varied from 10 to 90. The mesophases are ordered and diffract at small angles in all compositions. However, the mesophases slowly undergo transformation from LLC mesophase to semisolid mesostructured particles by the hydrolysis of PA and LMP formation over time. The drop-cast coated samples were calcined to produce mesoporous LMPs. The samples were characterized using N2 adsorption-desorption, XRD, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Attenuated total reflectance - fourier-transform infrared spectroscopy (ATR-FTIR) techniques. The LMPs are amorphous up to 400 oC but become crystalline above this temperature. The amorphous mesoporous LMPs have large Brunauer, Emmett and Teller (BET) surface area, around 30-100 m2/g but drops down to a few m2/g upon annealing at 500 oC. The SEM images show that the particle morphology depends on the inorganic/surfactant ratio in the initial mesophase. Both Mn(II) and Co(II) produce the olivine phase of LiMnPO4 (LMnP) and LiCoPO4 (LCoP), respectively, under our reaction conditions. However, Ni(II) samples need either excess lithium source or adjustment of pH of the clear solutions to form olivine phase of LiNiPO4 (LNiP). This adjustment can be done by using LiH2PO4 as the Li(I) and phosphate source in place of LiNO3 and PA. Unlike iron compound, the olivine phases of LMPs of Mn(II), Co(II) and Ni(II) were successfully obtained. In the iron case, it is difficult to keep iron in 2+ oxidation state under our reaction conditions. It undergoes an oxidation to form Fe3+ species. Therefore, mesoporous FePO4 and Li3Fe2(PO4)3 materials were synthesized, where the iron has 3+ oxidation state. Most synthesis has been carried out over glass slides that simply contain 16% of sodium. We found that our samples undergo Na+ ion-exchange reaction with the glass substrates above 300 oC. Therefore, the samples were first calcined at 300 oC over glass substrates and further annealed at higher temperatures in alumina sample holder to produce mesoporous forms. However, if the annealing step is carried over the glass slides, sodium metal phosphates (NaMPs) form in maricite phase. These samples were also characterized by XRD, SEM, TEM, and ATR-FTIR techniques. To eliminate the ion-exchange reactions, other substrates like quartz, pyrex or fluorine doped tin oxide (FTO) were used. However, notice that ion-exchange can also be performed to synthesize mesoporous maricite NaMPs as another synthesis method.
Mezogözenekli nikel oksit ve nikel kobalt oksitince filmlerin sentezi ve karakterizasyonu
In this thesis work, molten-salt assisted self-assembly (MASA) approach was adopted to synthesize mesoporous nickel oxide (m-NiO) and nickel cobaltite (m-NiCo2O4) thin films. The m-NiO and m-NiCo2O4 films were obtained by coating clear ethanol solutions of nickel salt and two surfactants (charged, CTAB and neutral, 10-lauryl ether), and nickel and cobalt salts with the same surfactants, respectively, followed by calcination at different temperatures (between 250 and 500 oC). The method has been established in a very broad range of salt concentrations in the lyotropic liquid crystalline (LLC) mesophase that can be calcined to produce mesoporous thin films. Both Ni(II) and Ni(II)/Co(II) systems form stable and oriented LLC mesophases in a broad range of salt concentrations (salt surfactant mole ratio of 2-8) upon evaporation of ethanol from the media. This can be achieved by either spin coating of the clear solutions (this ensure immediate evaporation of ethanol, leaving the LLC gel phase as thin film) or drop casting and evaporation of ethanol (the gelation process takes more time). At higher salt concentrations (10-30 salt/surfactant mole ratios), the mesophase is disordered and leach out salt crystals. However, those compositions can still be used for the synthesis of mesoporous metal oxides, if the samples are calcined immediately after the gelation step. The mesophase is 2D hexagonal at low salt concentrations and disordered or cubic at higher salt concentrations. The calcined films were characterized by recording x-ray diffraction (XRD), N2-adsorption desorption measurements, imaging (SEM, TEM, and POM) and spectroscopic (UV-Vis, XPS, EDX, and ATR-FTIR) techniques. The N2 adsorption-desorption isotherms are type IV and characteristic for mesoporous materials. The XRD data show that the crystalline m-NiO and m-NiCo2O4 form at around 300 and 250 oC, respectively, with a pore-wall thickness of around 3-4 nm. The pore-walls grow with increasing the calcination/annealing temperature up to 20 nm at around 500 oC. It accords well with the BET surface area that decreases with increasing calcinations temperature; it is 223 m2/g at 300 oC and drops to 20 m2/g at 500oC in mesoporous nickel oxide, and 223 m2/g at 250 oC and drops to 31 m2/g at 500 oC in mesoporous nickel cobaltite. The observed diffraction patterns can be indexed to rock salt cubic structure of NiO and cubic spinel structure of NiCo2O4. The diffraction lines gradually become sharper indicating crystallization and growth of the pore-walls that accord well with the reduction on the surface area. The m-NiO and m-NiCo2O4 films can be coated over FTO glass to use as an electrochromic electrode (oxidation dark-reduction clear) and electrode for water oxidation reactions (WOR) and WOR, respectively. In nickel oxide case, during cyclic voltammograms cycling, water oxidation process, and electrochromic switching, a few atomic layer of nanocrystalline NiO pore-wall is converted to NiOOH in oxidation and Ni(OH)2 upon reduction processes; initially formed nanocrystalline NiO (after calcination) pore-walls become NiO coated Ni(OH)2 (core-shell structure) upon electrochemical treatments. Both NiO and NiCo2O4 having high surface area and electrochemical stability show promising capacitive properties and can be used as electrocatalysts. From the Tafel slope analysis, it has been shown that nickel cobaltite can oxidize water at low overpotentials and therefore can be used as a promising water splitting catalyst.
Üretim, karakterizasyon vemezopenör elektrolizikalsiyum demir oksit (CaFe2O4) ince film elektrotlar
Transition metal ferrites have attracted the attention of many scientists because of their low cost, high earth abundance, low band gap, and biocompatibility. They can be prepared in different morphologies, and because of this, they may have a high surface area and excellent electrochemical and photoelectrochemical properties. In this thesis, we have prepared mesoporous calcium iron oxide (CFO) thin films using the molten-salt assisted self assembly (MASA) method and analyzed its electrochemical applications for oxygen evolution reaction (OER). The clear and homogeneous aqueous solution of metal salts (Calcium nitrate tetrahydrate, iron (III) nitrate nonahydrate) and surfactants (cethyltrimethyl ammonium bromid, CTAB, and C12H25(OCH2CH2)10OH, C12EO10) were coated on microscope glass slides by various coating techniques to obtain mesophases. Later on, the mesophases and their aging process were analyzed by the small-angle XRD measurements, ATR-FTIR and POM. Diffraction lines between 1 and 5°, 2θ, indicate the formation of ordered lyotropic liquid crystalline mesophases. These mesophases were subjected to calcination at various temperatures, and the powders obtained were further characterized by wide-angle XRD measurements, SEM, EDX, TEM, XPS, ATR-FTIR, and N2-adsorption and desorption techniques. The calcium iron oxide in highly crystalline form are prepared at 800 °C, having thin film morphology. Interestingly, we are able to retain the porous structure even at such a high temperature. The amorphous phase contains calcium carbonate as a side product that was confirmed by ATR-FTIR, XRD and XPS data. The maximum surface area of mesoporous material is 145 m2/g, while water being used as a solvent. Similarly, we prepared the same materials using different precursors (chlorides) and solvent (ethanol) to see the effect of counter anion and solvent on the porosity, self-assembly, morphology, and electrocatalytic performance of the material in the OER. We observed that while using chloride precursors, the material was quite crystalline even at low calcination temperature, i.e., 300 °C. Iron oxide forms at low temperatures and with the increase in temperature, it finally transforms to calcium iron oxide. But in this case, the materials are not as porous and display a surface area of only 5 m2/g at 300 °C. Similarly, we also characterized these materials using the above-mentioned techniques. While using ethanol as a solvent, keeping nitrate precursors the same, and using two different mole ratios of calcium and iron (2:4, 3:6), we also tried to elucidate the effect of solvent on morphology and catalytic properties of materials. In this case, we observed that the surface area did not drop immediately (as in the case of water) but gradually. The maximum surface area, obtained are almost similar to the material prepared by water as a solvent. All the solutions mentioned above (prepared by using different precursors, solvent, and mole ratios) are coated (by dip-coating) on the graphite rod to determine the catalytic activities by various electrochemical experiments (cyclic Voltammetry (CV), chronopotentiometry (CP), and chronoamperometry (CA)). Electrodes are quite stable in all cases, even in harsh conditions (CP at 100 mA for 2 h). Also, enhanced activity may be because of reduced resistance and increased conductivity with the usage. In all cases, the minimum Tafel slopes are almost similar, and vary between 47 and 83 mV/dec. The overpotentials at various current densities are 260 mV for 1 mA/cm2, about 450 mV for 10 mA/cm2, and about 700 mV for 100 mA/cm2. Additionally, effect of the coated material's thickness on the electrocatalyst's activity is also investigated. It has been found that by decreasing the amount of coated material (by diluting up to 100 times), there is no change in the activity of the material. Finally, our results indicate that these types of energy material's (CFO) performance depends on the surface's characteristics rather than the coating material's thickness or the pores' size. Also, we found it unnecessary to waste a large amount of metal salts to fabricate these materials; OER performance is similar regardless of coating thickness. Therefore, the surface reaction is the primary factor in electrode activity, with pore shape being the critical characteristic.
Asit-tuz-yüzey aktif madde liyotropik sıvı kristal arafazlar: Mezogözenekli M2P2O7 ve M2-XM'XP2o7 (M ve M'= MN(II), CO(II) ve NI(II)) tozlarının ve filmlerinin sentezi, karakterizasyonu ve elektrokimyasal davranışları
The mesoporous metal pyrophosphates (M2P2O7) are considered to be important as energy storage materials. This thesis proposes that a surfactant-assisted approach for the synthesis of the mesoporous M2P2O7 would be a good solution since the high surface area is crucial for energy storage materials. A novel synthesis method for the synthesis of mesoporous metal pyrophosphates (Ni2P2O7, Co2P2O7, Mn2P2O7, and binary metal pyrophosphates) is investigated by using a modified MASA (Molten Salt Assisted Self-Assembly) method using related acid; phosphoric acid (PA, H3PO4) or pyrophosphoric acid (PPA, H4P2O7), salts ([Mn(H2O)4](NO3)2, [Co(H2O)6](NO3)2, [Ni(H2O)6](NO3)2) and surfactant (pluronic P123 (EO20PO70EO20, where EO is ethylene oxide and PO is propylene oxide)). Firstly, homogeneous solutions using a broad range of inorganic ingredients are prepared. These solutions are then coated (spin-coating, drop-cast coating, or dip-coating) over a substrate to form lyotropic liquid crystalline (LLC) mesophases that can be calcined at various temperatures to synthesize the mesoporous metal pyrophosphates. The mesophases are characterized using small-angle XRD, ATR-FTIR, POM, and gravimetric measurements during the water evaporation from the solution phases. Aging the mesophases at room temperature forms an ordered (display diffraction(s) at small angles) mesostructured semi-solid (exhibit some cracks under POM with particle-like morphology) M2HxP2O7(NO3)x∙nH2O materials as a result of a polymerization reaction (followed by ATR-FTIR) between transition metal species and PPA. This process initiates in the solution phase and continues within the mesophase by releasing water and nitrate species and becomes stable in 24 h under ambient conditions. The mesostructured semi-solid M2HxP2O7(NO3)x∙nH2O materials are calcined at 300 oC to produce mesoporous spherical M2P2O7 with surface areas of 60, 111, and 41 m2/g for Ni(II), Co(II), and Mn(II) pyrophosphates, respectively. These mesoporous M2P2O7 materials, calcined at 300 oC and higher temperatures, are further characterized using wide-angle XRD, ATR-FTIR, XPS, SEM-EDX, TEM, N2 adsorption-desorption, and electrochemical characterization techniques. Both Co2P2O7 and Mn2P2O7 are amorphous up to 600 oC, then crystallizing at around 600 oC to their alpha and beta phases, respectively. In contrast, the crystallization temperature of Ni2P2O7 is around 700 oC, and it has mainly alpha and minimal delta phases. Mesoporous NiCoP2O7 and MnCoP2O7 with surface areas of 68 and 70 m2/g, respectively, become crystalline at 600 oC to α-NiCoP2O7 and β-MnCoP2O7 phases, and they form solid-solutions when the mole ratio of the metal species is varied. The clear solutions are spin-coated onto an FTO surface and then calcined to produce FTO-coated electrodes; however, those electrodes are not stable during the electrochemical measurements. Therefore, the diluted solutions from the mother liquor are dip-coated over a pure graphite rod (GR) and subsequently calcined to fabricate electrodes of mesoporous metal pyrophosphates. The GR-electrodes, which remain stable during the measurements, are tested using cyclic voltammetry (CV) and galvanostatic charge-discharge measurements with a 3-electrode system in a 3M KOH electrolyte. It is important to note that the metal pyrophosphates transform to their corresponding hydroxides in an alkaline solution during the electrochemical measurements. As a result, the collected data from the electrochemical measurements originate from the M(OH)2 species rather than M2P2O7. The mesoporous spherical Ni2P2O7 material is converted into a very thin needle-like β-Ni(OH)2 (1.5 nm thick and 7 nm wide) in alkaline media, maintaining its spherical morphology. In contrast, the mesoporous spherical Co2P2O7 and Mn2P2O7 particles transform into much thicker plate-like β-Co(OH)2 and β-Mn(OH)2 particles. The transformation time differs depending on the type of metal; the Co2P2O7 and Mn2P2O7 materials transform rapidly (about 30 sec), whereas the complete transformation of Ni2P2O7 to its hydroxide takes around 1 hour. The transformation time determines the particle size and morphology, consequently influencing the capacitance values. The β-Ni(OH)2 exhibits a high charge capacity and specific capacitance (102 mA.s and 368 mF/cm2 at a current density of 1 mA/cm2). However, these values are nearly 10 times smaller in the βMn(OH)2 and β-Co(OH)2 electrodes. The addition of nickel ions to the cobalt system in the preparation of binary metal pyrophosphates enhances the capacity and specific capacitance values, with the sample having β-Ni0.67Co0.33(OH)2 composition displaying the highest capacity value in alkali media (170 mA.s at a current density of 1 mA/cm2). Nevertheless, other binary systems (Mn1-xCox(OH)2 and Ni1-xMnx(OH)2) display almost similar capacity behavior to pure cobalt and manganese systems. Keywords: Mesoporous metal pyrophosphates, lyotropic liquid crystalline (LLC) mesophases, molten-salt assisted self-assembly, mesostructured semi-solid materials, metal hydroxides, specific capacitance.
Liyotropik sıvı kristalin (lsk) fosforik asit-10-loril eter: arafazlar, proton iletkenlik ve şeffaf mezogözenekli hidroksiapatit ince filmlerin sentezi
Many salts, acids, and bases with low deliquescence relative humidity (DRH) can organize non-ionic surfactants into lyotropic liquid crystalline (LLC) mesophases that form a ready platform for the synthesis of mesoporous materials. In this study, we show that phosphoric acid (H3PO4, PA) with low DRH value can also be used as a solvent in assembling non-ionic surfactant (C12H25(OCH2CH2)10OH, C12EO10) into stable LLC mesophases within a broad range of composition (the concentration can be as high as 20 PA/C12EO10 mole ratio).The PA/C12EO10 mesophase is bi-continuous cubic phase (V1) in extremely low concentrations (2 PA/C12EO10 mole ratio), 2D/3D hexagonal phases (H1) at moderate compositions (3 to 5 PA/C12EO10 mole ratio) and micelle cubic (I1) at high, (more than 5) H3PO4/C12EO10 mole ratios, with a typical unit cell parameter of 127, 55, and 116 Å, respectively. The mesophases of the lower concentrated samples (less than 15 mole ratio) have high thermal stability, with melting points greater than 120 oC. However the melting point drops to less than 50 oC for extremely high concentrations (more than 17 PA/C12EO10 mole ratio). The LLC mesophases were also found to exhibit high proton conductivities (~10-3 S/cm) at room temperature. The proton conductivities were even higher (10-2 S/cm) at some elevated temperatures and reduced to (10-4 S/cm) at temperatures less than 0oC. The conductivity in the cubic phase is slightly higher. Both the temperature and composition-dependent conductivity obey the most accepted proton conductivity mechanisms: Grotthuss and Vehicle. We went further to show that the combination of H3PO4 and another low DRH species, such as Ca(NO3)2·4H2O also form stable mesophases; without precipitating salts, under a wide range of concentration, from 5.3/1 to 13.3/1 precursor to surfactant ratio. High acidity stabilizes both the aqueous solution as well as the LLC phases. The clear solutions obtained from the precursor-surfactant mixtures were spin coated on glass substrates (as thin as a few hundred nanometers) and calcined to form transparent nano-size mesoporous hydroxyapatite (HAp) thin films. The formation of semi-crystalline HAp in our synthetic approach is not a straight forward process; it involves the formation of some intermediate products and also requires a calcination temperature of at least 300 oC. The formation, which starts at 300 oC, is preceded by the evaporation of nitric acid and excess water molecules to the surrounding. The crystallization continues at 400 oC and completes at 500 oC, keeping the uniformity, porosity, and transparency of the films. Films of the 5.3/1 ratio,calcined at 300 oC have high surface area of up to 96 m2/g, which dropped down to 20 m2/g at 500 oC. The mesopores start collapsing at around 600 oC. The pore size, pore walls, and the pore volumes were obtained from the N2 sorption measurements and the values are 22.4 nm, 10 nm, and 0.58 cm3/g, respectively. We also investigated the effect of precursor concentration on both the pore sizes, as well as the thicknesses of the pore walls. The results showed a reduction of surface area, and also narrower pore size distribution with increasing concentration. Temperature was also observed to have the same effect on crystallinity in all the compositions studied. All the investigations on these two systems were carried out using XRD (X-ray diffraction), FT-IR (Fourrier Transform Infrared Spectroscopy), Raman spectroscopy, POM (Polarized Light Optical Microscope), N2-sorption measurements, PEIS (Potentiostatic Electrochemical Impedance Spectroscopy), TEM (Transition Electron Microscopy), SEM (Scanning Electron Microscopy) etc
Mezogözenekli Li4Ti5O12, CoTiO3 ve MnTiO3 ince filmlerin sentezi, karakterizasyonu ve gözenek boyutu kontrolü
Salt-surfactant lyotropic liquid crystalline mesophases can be used to produce mesoporous highly transparent thin films of metal titanates. In this study, the salt-surfactant assembly is described as molten salt assisted self-assembly (MASA) process that was optimized for the synthesis of mesoporous CoTiO3, MnTiO3 and Li4Ti5O12 thin films with high specific surface area and narrow pore size distribution.The materials have been characterized using x-ray diffraction (XRD), Raman and UV-Visible absorption spectroscopy, Transmission Electron Microscopy (TEM), and nitrogen adsorption/desorption techniques. An initial clear solution containing two different surfactants (C12H25(OCH2CH2)10OH, C12EO10, and C16H33N(CH3)3Br, CTAB), nitrate salt ([Co(H2O)6](NO3)2 or [Mn(H2O)6](NO3)2 or LiNO3) of the convenient metal, titanium(IV)butoxide (Ti(OC4H9)4, TTB) as titania source and ethanol as solvent is prepared at an appropriate pH. Spin or spray coating methods was employed to coat the substrates using above solutions. During the coating process, a liquid crystalline mesophase is formed instantaneously upon the evaporation of the solvent. The hydrophilic surfactant domains guide the molten salt and hydrolysis products of TTB to form a three dimensional porous network throughout the film. The synthesis is completed with a fast calcination step (10-20 min) at temperatures ranging between 350 oC to 550 oC. Mesoporous CoTiO3, MnTiO3 and Li4Ti5O12 display uniform pores with a pore size of 25 to 55 Å, surface area of 193 to 445 m2/g and pore volume of 0.17 to 0.43 cm3/g depending on the composition and synthesis conditions. The surface area, pore-size, pore-wall thickness, pore volume and crystallinity of the pore-walls can be controlled by simply controlling the calcination or annealing steps of the process without damaging the mesoporous network. The films, produced by employing the MASA approach, are optically transparent and exhibit good adhesion on commonly used substrates (glass, silicon, aluminum… etc.). Both CoTiO3 and MnTiO3 are semi crystalline at low temperatures and undergo segregation into metal oxide and titania above 500 oC. However, Li4Ti5O12 is nanocrystalline even at 350 oC and stable up to 550 oC. The initial calcination temperature and duration are two important parameters to further control the pore and crystallinity related properties in all three titanates. The counter anion of the salt also plays an important role to adjust the porosity and to further modify. In this investigation, we also used the bromide salt of cobalt(II) and found out that one can incorporate graphitic carbon into mesoporous network. The MASA process, that is further expanded in this work, is not limited to metal titanates, investigated in this work and previous works; it is a general and new synthetic route to produce many other mesoporous metal oxides as powders, as well as thin films, such as LiCoO2, LiMn2O4, etc…
Eriyik tuz yardımlı kendiliğinden oluşma (EYKO): Mezogözenekli SiO2-CdSe ve TiO2-CdSe ince filmlerinin sentezi, karakterizasyonu ve güneş pili performansları
A series of solutions of a salt ([Cd(H2O)4](NO3)2), a polymerizing agent (Si(OCH3)4, TMOS or Ti(OC4H9), TTBO), two surfactants (cetyltrimethylammonium bromide, CTAB and 10-lauryl ether, C12H25(OCH2CH2)10OH, C12EO10) have been prepared and used as the molten salt assisted self-assembly precursors by only changing the Cd(II)/surfactant mole ratios for the preparation of mesoporous films. The thin films were prepared in two steps: in the first step, the titania particles (titania particles are typically 20-25 nm (P25)), dispersed in ethanol, have been spin coated over various substrates and annealed at 450oC and in the second step, the clear solutions given above (salt, CTAB, C12E10, silica or titania source and ethanol) were drop casted or spin coated over the titania (P25) films and calcined at 450oC. Slow calcination of the films (with an increment of 1 oC/min), starting from the melting point of the salt (around 65oC) to 450 oC has produced the mesoporous silica/titania-cadmium oxide (CdO) thin films denoted as meso-CdO-SiO2-P25 and meso-CdTiO3-P25. The clear solutions were also used to make the thin films without P25 and denoted as meso-CdTiO3. The films were then exposed to a H2Se atmosphere at 100oC for 30 min, and the samples were denoted as meso-CdSe-SiO2-P25, meso-CdSe-TiO2-P25 and meso-CdSe-TiO2. The silica sample was further treated with a dilute HF solution (etching process) that results silica free meso-CdSe-P25. The characterization of the materials produced in this thesis was made by using XRD, FT-IR spectroscopy, UV-VIS, Raman, EDX and Solar Measurement techniques. Silica samples have greater amount of CdSe than titania samples according to the Raman and EDX data which implies that silica samples are more reactive. Also in both silica and titania, the samples with a Cd/surfactant mole ratio of 6 have the greatest amount of CdSe. Fluorine doped SnO2 (FTO) has been used as a transparent conductive substrate for the preparation of the anode electrode for the solar measurements. According to the solar measurements, the silica and titania samples on P25 show greater efficiency than the titania samples without P25 and generally have similar efficiencies (the efficiency is the ratio of the electrical output of a solar cell to the incident energy in the form of sunlight). The most efficient samples are generally the samples prepared using Cd/surfactant mole ratios equal to 6 and 8. In order to increase the efficiency, 3rd row transition metal cations such as Mn(II), Fe(III), Cu(II) and Co(II) are doped into above samples. The efficiency has been increased by 10 % in the silica and 30 % in the titania samples upon doping with 15% Mn(II). The solar cell characteristics of the electrodes were tested using the following parameters; by changing the Cd(II)/surfactant mole ratio (many samples were prepared and used for this purpose), by changing the aging of the electrolyte (Na2S/S8), by using multiple coating the MASA solution in the preparation stage of the electrodes, by doping the samples with various transition metal ions, and finally by modifying the surface of the electrode by coating with ZnS or CTAB. Each of these parameters has an effect on the cell performance. The effect of each parameter is monitored by measuring the I-V characteristics of the cells and found out that the best results were obtained from the electrodes made using 6 or 8 Cd/surfactant mole ratio in MASA system, freshly prepared Na2S/S8 electrolyte, coating of the MASA solution twice on electrode, doping Mn(II) cations with 0.15 Mn(II)/Cd(II) ratio and using CTAB for coating anode electrode.
Li̇tyum tuzlari ve i̇yoni̇k olmayan yüzeyakti̇f li̇yotropi̇k sivi kri̇stal fazlarin boya duyarli güneş hücreleri̇nde jel elektroli̇t olarak kullanilmasi
Liquid crystals are one of the most widely studied and used materials in chemistry. The properties of liquid crystals make them interesting to research also for various electrochemical applications. In this context, lithium salts such as LiI, LiCl, LiBr, and LiNO3 can be assembled using non-ionic surfactants into lyotropic liquid crystalline (LLC) mesophases[1], [2] and used as gel electrolytes in various applications. In this work, the LLC mesophases of LiI with and without other lithium salts (such as LiCl, LiBr, and LiNO3) were prepared using 10-lauryl ether (C12H25(CH2CH2O)10OH, denoted as C12EO10) and characterized using the FT-IR (Fourier Transform Infrared Spectroscopy), Raman spectroscopy, XRD (X-Ray Diffraction), POM (Polarized Optical Microscopy), and AC conductivity measurements. Beside from single salt-surfactant mesophases, we also prepared LiI/I2 redox couple in an LLC phase with the help of a non-ionic surfactant and they were also characterized using the same techniques. We found out that the mesophases can be prepared as gels by directly mixing salt and surfactant with certain amounts of water or as solutions using excess solvent (such as water, ethanol, or acetonitrile) that can be evaporated to form the LLC mesophases. The water content of both sets of samples is the same upon exposing to the atmosphere for a certain time and it only depends on the salt amount and humidity under the ambient conditions (around room temperature and 20-25 % RH). The required water/salt ratio for a stable mesophase is around 3.0 which, it is much lower than the water needed to dissolve those salts in an aqueous media. The water/salt mole ratio closely follows the Hofmeister series of anions, where the water amount order is as follows; LiCl>LiBr>LiI>LiNO3, however, the AC ionic conductivity follows a different order; LiNO3>LiCl>LiBr>LiI. Adding I2 by 1/10 mole ratio of the LiI into the media does not change the properties of the mesophases. The AC conductivity increases with increasing salt and water content of the mesophases with a typical conductivity of around 0.1 to 1.0 mS/cm-1. The mesophases are also stable in a very broad temperature (below 0 °C to 60-130 °C) and salt concentration (2-10 salt/surfactant mole ratio) ranges. Finally, the LiI/I2 mesophases were used as gel-electrolytes in dye sensitized solar cells (DSSCs) as gel-electrolytes and redox couples. A set of samples were prepared with different ratios of the LiI:I2 redox couple (such as, 1:0.1, 1:0.2, 2:0.2, 2:0.3, 3:0.2, 3:0.3, 4:0.4, and 5:0.5) and the solar performances were tested in a DSSC, which contains N719 dye sensitized TiO2 anode and Pt cathode using a solar simulator. However, the LLC phases have gel like structure and it is hard to infiltrate the gel into the pores of dye modified nano-TiO2 films. To overcome the diffusion problem, the gel-electrolytes were also prepared as a solution in excess water, ethanol or acetonitrile that evaporates upon infiltration over time. In addition to this, by changing the procedure of preparing the TiO2 paste, improvement on results was also obtained. The DSSC provides 0.2 % efficiencies with 0.50 fill factors when gel-electrolytes are used. Since water is used for preparing the LLC phases, we had always lower Voc values. However, when it is prepared as a solution with excess ethanol, it provides up to 3.33 % efficiencies with 9.58 mA/cm2 short circuit current and 0.6 V open circuit voltage. Also, new procedure for preparing the TiO2 paste provides us even higher Voc values such as 0.76 V, which is unusual for the water based LLC electrolytes in this area. Key Words: Lyotropic Liquid Crystal, Mesophase, Solar Cell, Gel Electrolyte, Redox Couple, Lithium Salts
Mezogözenekli LiCoO2 ve LiMn2O4 ince filmlerin sentezi ve karakterizasyonu
This work focuses on the adaptation of molten salt assisted self-assembly (MASA) method for the synthesis and characterization of mesoporous LiCoO2 and LiMn2O4 materials without a polymerizing agent in the media. The MASA process is a new method to synthesize mesoporous thin films and monoliths. Fresh gel and calcined solid samples were characterized by using XRD, Raman, N2 sorption, FTIR POM, SEM, TEM and UV-Vis measurements. The first step of the process involves the preparation of clear solutions that contain two hydrated salts, LiNO3.xH2O and Co(NO3)2.6H2O (or Mn(NO3)2.6H2O), CTAB (cethyltrimethylammonium bromide, ionic surfactant) (or CTAN), 10- lauryl ether (C12EO10, ionic surfactant) and ethanol (or H2O) as volatile solvents. Additionally, HNO3 is needed in the case of LiMn2O4 to prevent formation of [Mn(OH)2(H2O)4](s) complex during and/or before the assembly process. The clear solutions are spin coated over various substrates to produce the salt-surfactant LLC mesophase that resists to high temperature treatments (300 550 oC) in air to form first examples of mesoporous metal lithiates. Thesis is designed to give the insights on how to change several parameters that control the features like purity, pore size, surface area and crystallinity of the LiCoO2 and LiMn2O4 mesostructures by determining solvent type, thickness of the LLC film, acid amount, salt uptake, calcination temperature, calcination time and surfactant used for the synthesis. Ethanol was determined to be a good solvent to prepare the clear solutions. The samples, prepared by spin coating gave better results than the ones prepared by drop casting to obtain uniform pores with less side products. Addition of HNO3 has no effect on side product formation in the synthesis of LiCoO2 but it is vital to obtain a stable, homogenous solution in the case of LiMn2O4 preparation. LiCoO2 samples, prepared using CTAB are semicrystalline at lower temperatures and stable up to 550 oC but it undergoes decomposition to Co3O4 and Li2O at higher temperatures. Presence of CTAB in the reaction media has a positive effect on the uniformity pores. However; due to the role of Br- ion on formation of side products, CTAN has been used for further experimentation. Thin films of the calcined samples showed small angle diffraction corresponds to preserved order of the mesophase during calcination. Mesoporous pure HT- LiCoO2 synthesized with MASA is the first example in the literature, having a 65 m2/g specific surface area and 15 nm pore size. Moreover; mesoporous pure LiMn2O4 could be obtained both using CTAB and CTAN with 82 m2/g specific surface area and 11 nm pore size.
Eriyik tuz yardımlı kendiliğinden oluşma (Eyko) yöntemi: Mezogözenekli geçiş metal oksit ince filmlerin ve cdse duyarlı tio2 foto-anotların sentezi
Fabrication of mesoporous transition metal oxide thin films are important for the development of many energy related technologies. Molten salt-assisted-selfassembly (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 mesoCdO-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 saltsurfactant 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 mesoP25 film, was observed upon using a concentrated MASA solution in the infiltration step. Therefore, multiple loading method has been established to increase the CdSeSiO2 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.
Geçiş metal tuzları:pluronik sıvı kristal mezofazları kullanarak mezoyapılı metal sülfürlerin sentezlenmesi
The Liquid Crystalline Templating (LCT) approach has been extensively usedto produce mesostructured Metal Sulfides (MS) powders by using nonionicsurfactants (CnEOm). The aim in this work is to synthesize larger pore sizemesostructured MS at high salt concentrations by mixing Pluronics(PEOxPPOyPEOx, EO = -OCH2CH2-, PO = -OCH(CH3)CH2-) with transition metalsalts (TMS) [M(H2O)4](NO3)2 in a dilute media. This enables to synthesize thinfilms of mesostructured MS. In this thesis, the MS (M= Cd, Zn, Cd1-xZnx, Cd1-xCoxand 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 varioussolvents to obtain clear solution that enables one to increase the salt to pluronicmole ratio up to 30:1. However, the LC mesophases form in theiv[Cd(H2O)4](NO3)2:P85 mole ratio range of 3:1 to 11:1 with a 3D hexagonalstructure and P63/mmc space group having unit cell parameters of a = 99.5 à andc = 162.5 à with a c/a ratio of 1.633.The CdS thin film samples, obtained by exposing the [Cd(H2O)4](NO3)2:P85LC phase to H2S gas, could retain the mesostructure of the LC mesophase in themole ratio range of 3:1 to 11:1. The film samples that consist of 50-100 nmmesostructured CdS and free surfactant molecules are uniform and soft in earlystages of the H2S reaction. However, in time, the free surfactant molecules diffuseout of the mesostructured CdS and form dendritic structures, producing CdS thinfilms with huge domains. The CdS thin film samples consist of 4.3 nm CdSnanoparticles that emit orange light under UV irradiation. Well homogenized LCmesophases produce cracked well structured film samples upon H2S reaction.This method can be used to fine tune both the composition (between x=0.0and 1.0) and the optical band-gap of Cd1-xZnxS nanocrystallites between 2.60 eVand 4.00 eV. The Zn(II) and Cd(II) ions are homogenously doped throughout themesostructure and nanocrystallites synthesized by this approach are slightly largerin every composition compared to the ones synthesized in the mesostructured silicachannels. Also both Co(II) and Mn(II) ions could be incorporated into the CdSlattice 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 duringthe synthesis of MS and particle size analysis of the nanocrystallites wereinvestigated using diffraction (XRD), spectroscopy (FT-IR, micro-Raman and UV-Vis absorption) and microscopy (OM and SEM) techniques.Keywords: Liquid Crystal Templating, Pluronics, Transition Metal Complexes,Mesostructured Metal Sulfides, Doping CdS, Mesostructured Thin Films,Nanocrystallites.v
İki yeni liyotropik sıvı kristal mezofazın incelenmesi: [Zn(H2O)6(NO3)2-C12EO10-CTAB-H20 and [Zn(H2O)6(NO3)2-C12EO10-SDS-H20
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 80oC down to 32oC 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.
CTABr-pluronic sisteminin kontrolüyle mezogözenekli silika parçacıklarının sentezlenmesi
In the synthesis of mesoporous silica materials, self-assembly of a charged surfactant (cetyltrimethylammoniumbromide, CTABr) and a pluronic (PEOx-PPOy-PEOx where PEO is CH2CH2O and PPO is CH(CH3)CH2O) into micelles have key. By controlling the hydrophilic-hydrophobic character of the CTABr-Pluronic micelles, mesoporous silica particles can be synthesized with different morphologies (sphere, wormlike, crystal-like etc.). The particles generally have 2D hexagonal mesostructure with a high surface area (as high as 800 m2/g). Shape of the micelles as well as the morphology of the particles depend on the hydrophobic nature of the pluronic surfactant and the CTABr amount. The CTABr amount is carefully adjusted to control the morphology and structural order of the particles. The self-assembly of the CTABr-Pluronic micelles and silica species has been achieved by adjusting pH of the synthesismedium to 1.0 in order to produce mesoporous particles with a distinct morphology and mesostructure.Nature of the CTABr-Pluronic micelles can be influenced by adding organic and inorganic additives to the reaction media. The effect of the lyotropic (F-, SO42- and Cl- etc.) and hydrotropic (NO3-, SCN- etc.) anions on the micellization of P85 has been first investigated in the aquoues media using UV-Vis Spectroscopy and ethyl orange dye. Then these inorganics and organic (Benzene) additives, in the synthesis of mesoporous silica, have been used to control the micellization of the CTABr-P123 couples as well as the morphology and the pore structure of the silica particles. Highly ordered particles with larger pores and various pore structures have been synthesized using lyotropic anions in the CTABr-P123 system. Furthermore, the hydrotropic anions control the CTABr content of the CTABr-P123 micelles. Increasing CTABr amount in the CTABr-P123 micelles decreases the wall thickness of the silica particles. The hydrophobic character of the micelles can also be enhanced by adding water insoluble organic additives (benzene). The silica particles, synthesized using CTABr-P123-Benzene system, are well structured, where the higher order X-ray diffraction lines can also be observed.Finally, the catalytic role of F- ions on the polymerization of the silica has been studied in the CTABr-Pluronic system. Addition of F- ion to the reaction medium speeds up the formation process and producing spherical and uniform mesoporous particles less than 20 minutes. The effect of each of the reaction component, F- ion, CTABr and P123 molecules, to the assembly rate has also been investigated by determining the turbidity point (due to the formation of silica particles) of the solutions. A correlation between the particle size and reaction rate has also been brought out.The mesoporous silica particles synthesized in this thesis have been characterized using PXRD, FT-IR and Raman Spectroscopy, SEM, TEM and N2 sorption measurements.
İki yarı iletken içeren mezoyapılı ince filmlerin faz davranışı ve sentezi: Meso-CdS-TİO2
Mesostructured [Cd(H2O)4](NO3)2 - titania - P123((PEO)20(PPO)70(PEO)20, PEO = -OCH2CH2-, PPO = -OCH(CH3)CH2-) materials have been investigated by changing the [Cd(H2O)4](NO3)2 and titania content of the structures. This has been achieved by making thick samples by casting and thin film samples by spin coating of a butanol solution of [Cd(H2O)4](NO3)2, P123, nitric acid and Ti(OC4H9)4. The film samples are named as meso-xCd(II)-yTiO2, where x is theCd(II)/P123 and y is TiO2/P123 mole ratios. Increasing the titania amount in the media has transformed the samples from LC-like to soft and then to rigid mesostructured materials. Changing the amount of [Cd(H2O)4](NO3)2 salt in the media only influenced the mesostructure, such that no change on the mechanical properties is observed. However, the synthesis of rigid mesostructured titania materials required controlled humidity. The rigid film samples were prepared first by spin coating and then by aging under a 50% humidity oven.The mesostructure remains stable upon H2S reaction, in the soft and rigid materials region. However, only rigid samples stand to removal of nitrates from the media that is important to keep the CdS nanoparticles stable in or on the pore walls of mesostructured film samples. The phase behavior of the meso-Cd(II)-TiO2, the structural properties of the meso-xCdS-yTiO2 samples, coordination and elimination of the NO3- ions and the particle size of the CdS nanocrystallites were investigated using diffraction (XRD), spectroscopy (FT-IR, Raman and UV-Vis absorption, EDS)and microscopy (POM, SEM, and TEM) techniques.
Mezo yapılı silika parçacıklarının SDS-pluronik ikilisi ile sentezi
Controlling the cooperative self assembly and micellization of pluronics and SDS (sodium dodecyl sulfate)are pivotal for the synthesis of mesoporous silica particles. The pH and temperature of the synthesis media, SDS/Pluronic mole ratio, TMOS (tetramethyl orthosilicate)amount, alkali salt amount of the synthesis solution are the parameters, which play significant roles on the micellization and self assembly of surfactants. The synthesis of mesoporous silica particles with distinct morphologies is possible with the precise optimizations of these parameters.In this thesis we have investigated the synthesis of mesoporous silica particles with a well defined morphology and structure using SDS-Pluronic couple as the template. The pore size can be tuned by changing the aggregation number of the surfactant molecules in the micelles, also by changing the pluronic type. The morphological control is achieved mainly by changing the pH and temperature of the synthesis media. At different temperatures and pHs, rods, spheres, muffin and `s? shaped particles have been obtained. The addition of inorganic salts, such as NaNO3, NaCl, and KCl, has also effects on the morphology and meso-structure. Addition of a small amount of NaNO3 changes spherical particles to amorphous silica however, addition of large amount of NaNO3 gives well defined muffin shaped and worm-like particles. The concentration of nitrate ion also affects the pore size and wall thickness of the synthesized particles. The KCl or NaCl salts also have similar effects on the morphology of the silica particles, the morphological transitions have been observed but the role of Cl- ion is minor on the control of pore size.The SDS concentration has important effects on the micellization of pluronics, changing the SDS/Pluronic mole ratio (between 0.05 and 5.0) in the reaction media changes the structure of the mesoporous silica particles. Particularly the SDS concentration has important effects on the surface area of the synthesized particles. The surface area of the samples changes between 100 m2/g and 700 m2/g and the pore size of the particles changes between 3.0 and 6.0 nm by changing the SDS/Pluronic mole ratio. This ratio is also effective on the micropore amount of the samples together with mesopores. The tunable particle size (between 0.2µ to 1000µ) and morphology (spheres, rods, muffin and `s? shaped.) can be achieved by changing the SDS concentration.Furthermore, the low reaction temperature (below RT) is essential for the synthesis of mesoporous silica particles in SDS-Pluronic system. However, the lowtemperature is a problem for micellization. This problem was overcome by using P123, which has low critical micellization concentration (CMC) and critical micellization temperature (CMT) values or by using Hofmeister ions to decrease the pluronic surfactant solubility and the CMC and CMT of the pluronics used. Decreasing solubility of the pluronics causes effective micellization of the surfactants. The well defined micelles are the templates for the synthesis of mesoporous silica particles.Overall , the effects of SDS/Pluronic mole ratio, pH and temperature of the synthesis solution, TMOS concentration, and the additives (alkali salts) have been investigated by synthesis of more than 300 samples that were analyzed using PXRD, SEM, TEM, POM, and N2 sorption techniques.Keywords: Mesoporous silica, SDS, Pluronics, Micellization, Morphology control, Silica rods, Salt effect.
Sıvı kristal mezofazları kullanarak mezogözenekli metal sülfür ve metal selenür ince film sentezi ve karakterizasyonu
In this thesis, synthesis of the mesoporous CdS and CdSe by using of liquidcrystalline templating (LCT) approach has been investigated. In the first part ofthe 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 cadmiumsulfide, CdS, or cadmium selenide (CdSe) directly from the mesostructured CdS(or CdSe) thin films. However, the mesostructured CdS/P85 films (at low saltconcentrations), which were obtained by reacting [Cd(H2O)4](NO3)2/P85 LLCthin films under H2S atmosphere, are not stable to calcination process and alwaysproduced bulk CdO and CdS domains over the thin films. More metal ioncontaining [Cd(H2O)4](NO3)2-C12EO10-CTAB mesostructured films produced vastamount of HNO3 under the H2S atmosphere and caused decomposition of CdSback to their nitrates.To overcome above problems, a polymerizing agent, such as titania or silicaprecursors have been added to salt/surfactant LLC mesophase. Both titania andsilica overcame the collapse of the mesophase by rigidifying the structure intomesostructured solid and also by providing stability for a thermal removal ofnitrates 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-CTABcouple have been used.Well-ordered mesostructured Cd(II) titania films have been obtained up to15.0 Cd(II)/P123 mole ratio for a 60 mole ratio of Ti(IV)/P123 by spin or dipcoating 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 atmospheregave stable CdSe nanoparticles in the channels of the mesostructured rigid titaniawalls up to 25 mole % Cd(II)/Ti(IV). To further increase the metal ion (Cd(II) andZn(II)) content in the structure, the C12EO10-CTAB-salt mesophase has beenemployed. The two surfactant-salt systems, in the presence of a titania precursor,produced sponge like mesoporous CdTiO3 and Zn2TiO4 films up to a mole percentof 57 and 86, respectively, upon calcination. Exposing the mesoporous CdTiO3 toH2S or H2Se atmosphere at RT produced homogeneously distributed CdS or CdSenanocrystallites on the nanocrystalline TiO2 pore walls, respectively. The reactionof mesoporous Zn2TiO4 with H2Se produced stable ZnSe nanocrystallites on thenanocrystalline TiO2 pore walls. The conversion of titania from CdTiO3 to ananatase and brookite phase under H2S and H2Se atmosphere, respectively, andfrom Zn2TiO4 to a rutile phase under H2Se were observed for the first time.Adding a silica precursor to the two surfactants (C12EO10-CTAB)-saltmesophase produced mesostructured salted-silica, and its calcination producedsponge-like mesoporous silica-metal oxide (dumped meso-SiO2-CdO and meso-SiO2-ZnO) thin films. Up to ~100 % and ~50 % surface coverage could beachieved by CdO and ZnO as nano-islands over the SiO2 pore walls. Exposing themesoporous SiO2-CdO and SiO2-ZnO thin film precursors to H2S and H2Se at RTenabled the synthesis of mesoporous SiO2-CdS, SiO2-CdSe, SiO2-ZnS, and SiO2-ZnSe thin films. The MS or MSe nanoflakes could homogenously cover the porewalls of mesoporous silica by retaining the pore morphology of the MOprecursors. The H2S and H2Se reactions are slow and can be monitored using UVviVis absorption spectroscopy and EDS to elucidate the reaction mechanism andkinetics. These data showed that the reaction starts from the top surface of the MOdomains and proceeds until Si-O-M bond break. Finally, the SiO2 walls wereremoved from the meso-SiO2-CdS and meso-SiO2-CdSe films through etching ina dilute HF solution to produce mesoporous CdS (meso-CdS) and mesoporousCdSe (meso-CdSe). Surface of the meso-CdS has been modified using PEI(polyethyleneimine) and photoluminescent meso-CdS were obtained.Keywords: Mesoporous CdS, Mesoporous CdSe, Liquid Crystal Templating,Evaporation Induced Self-Assembly, Mesoporous Titania, Mesoporous Silica
Eriyik tuz yardımlı kendiliğinden oluşma (EYKO): Mezogözenekli silika-ZnO ve mezogözenekli silika-CdO ince filmlerin sentezi
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-silica-ZnO-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.Keywords:Mesoporous ZnO, Mesoporous CdO, Mesoporous silica, Thin Films, Lyotropic liquid crystals.
Yüzey aktifler için yeni çözücüler: Eriyik hidratlı tuzlar ve derişik elektrolit solüsyanları
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.he 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-C12EO10system, 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 saltcrystals 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. Keywords: Lyotropic Liquid Crystals, Molten Salts, Concentrated Aqueous Electrolytes, Self Assembly, Transition Metal Aqua Complex Salts, Alkali Metal Salts, Alkaline Earth Metal Salts.