Browsing by Subject "Mesoporous thin films"

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Open Access
Lyotropic liquid crystalline (LLC) phosphoric acid-10-lauryl ether: mesophases, proton conductivity and synthesis of transparent mesoporous hydroxyapatite thin films
(2014-06) Tunkara, Ebrima
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.
Open Access
Molten salt assisted self-assembly: synthesis of mesoporous LiCoO2 and LiMn2O4 thin films and investigation of electrocatalytic water oxidation performance of lithium cobaltate
(Wiley-VCH Verlag, 2018) Saat, G.; Balci, F. M.; Alsaç, E. P.; Karadas, F.; Dağ, Ömer
Mesoporous thin films of transition metal lithiates (TML) belong to an important group of materials for the advancement of electrochemical systems. This study demonstrates a simple one pot method to synthesize the first examples of mesoporous LiCoO2 and LiMn2O4 thin films. Molten salt assisted self-assembly can be used to establish an easy route to produce mesoporous TML thin films. The salts (LiNO3 and [Co(H2O)6](NO3)2 or [Mn(H2O)4](NO3)2) and two surfactants (10-lauryl ether and cethyltrimethylammonium bromide (CTAB) or cethyltrimethylammonium nitrate (CTAN)) form stable liquid crystalline mesophases. The charged surfactant is needed for the assembly of the necessary amount of salt in the hydrophilic domains of the mesophase, which produces stable metal lithiate pore-walls upon calcination. The films have a large pore size with a high surface area that can be increased up to 82 m2 g−1. The method described can be adopted to synthesize other metal oxides and metal lithiates. The mesoporous thin films of LiCoO2 show promising performance as water oxidation catalysts under pH 7 and 14 conditions. The electrodes, prepared using CTAN as the cosurfactant, display the lowest overpotentials in the literature among other LiCoO2 systems, as low as 376 mV at 10 mA cm-2 and 282 mV at 1 mA cm-2.
Open Access
Synthesis & characterization of modified mesoporous LiMn2-xCoxO4 thin films as water oxidation electrocatalysts
(2019-09) Karakaya, Irmak
The lithiated transition metal oxides (LMO) are important group of materials in energy applications, particularly as water oxidation electrocatalysts. The mesoporous LiMn2-xCoxO4 thin film has been synthesized by using molten-salt assisted self assembly (MASA) method with a high surface area. Homogenous ethanol solution of nitrate salts (lithium, manganese(II) and cobalt(II)) and surfactants (CTAB and P123) in the presence of a small amount of HNO3 is coated over a glass substrate by spin-coating to form lyotropic liquid crystal (LLC) mesophase that is calcined at elevated temperature to synthesize disordered mesoporous LiMn2-xCoxO4 thin film. The mesophases display diffraction line(s) at small angles, indicating an ordered structure. The cobalt amount (x) has been varied from 0 to 2, keeping the same mesoporous and crystal structures. The films were characterized using XRD, SEM, EDX, TEM, N2 adsorption-desorption techniques. The XRD provided that the end products have a spinel structure with very similar unit cell parameters in all compositions. The surface areas of the films vary from 98 to 144 m2/g with increasing cobalt amount in the films. The SEM images showed that the thin films are uniform with a thickness of around 200-500 nm. The LLC mesophases have been also coated over FTO glass to fabricate electrode for oxygen evolution reaction (OER) and also for electrochemical characterizations. The electrodes prepared from all composition performed as good electrocatalysts, however the Tafel slope decreased from 124 to 66 mV dec-1 going from LiMn2O4 to LiMnCoO4. The overpotential also dropped from 491 mV to 294 mV at 1 mA cm-2 in water oxidation reaction. The LiMn2O4 is the worst electrocatalyst tested in this thesis. It has high Tafel slope, which is not desired and also not stable during electrochemical test. The stability improves with increasing cobalt in the films. The LiMnCoO4 has been reported to be one of the most efficient and stable electrocatalyst even if it is used 120 mA cm-2 current densities. Therefore, the electrode with this composition has been investigated in detail in an alkali media. The mesoporous LiMn2O4 thin film is modified by successive ionic layer adsorption and reaction (SILAR) method to improve its activity and stability. This electrode is dipped into a 1 M cobalt (II) solution, then washed several time to ensure a single layer of cobalt species on the surface, the modified electrode is calcined to produce cobalt rich LiMn2-xCoxO4 surface. Eventhough, the amount of cobalt in the modified electrode is smaller than 1 %, the modification decreased the Tafel slope from 127 to 80 mV dec-1, but the electrode was unstable during water oxidation process in alkali media. A range of LiMn2-xCoxO4 (x = 0 to 0.4) compounds were modified by the SILAR method and tested for OER. The mesoporous LiMn1.6Co0.4O4 (20 % cobalt and 80 % manganese) was used as the substrate and the SILAR method was employed 5-times, the Tafel slope of this electrode decreased from 64 to 46 mV dec-1 with an overpotential decrease from 304 to 265 mV at 1mA cm-2 and 826 to 546 mV at 10 mA cm-2 by modification and displayed a robust property in water oxidation process.
Open Access
Synthesis and characterization of mesoporous cadmium titanate thin films
(2018-01) Özkök, Zeynep
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.
Open Access
Synthesis of mesoporous lithium titanate thin films and monoliths as an anode material for high-rate lithium-ion batteries
(Wiley-VCH Verlag, 2016) Balcı, F. M.; Kudu, Ö. U.; Yılmaz, E.; Dag, Ö.
Mesoporous Li4Ti5O12 (LTO) thin film is an important anode material for lithium-ion batteries (LIBs). Mesoporous films could be prepared by self-assembly processes. A molten-salt-assisted self-assembly (MASA) process is used to prepare mesoporous thin films of LTOs. Clear solutions of CTAB, P123, LiNO3, HNO3, and Ti(OC4H9)4 in ethanol form gel-like meso-ordered films upon either spin or spray coating. In the assembly process, the CTAB/P123 molar ratio of 14 is required to accommodate enough salt species in the mesophase, in which the LiI/P123 ratio can be varied between molar ratios of 28 and 72. Calcination of the meso-ordered films produces transparent mesoporous spinel LTO films that are abbreviated as Cxx-yyy-zzz or CAxx-yyy-zzz (C=calcined, CA=calcined–annealed, xx=LiI/P123 molar ratio, and yyy=calcination and zzz=annealing temperatures in Celsius) herein. All samples were characterized by using XRD, TEM, N2-sorption, and Raman techniques and it was found that, at all compositions, the LTO spinel phase formed with or without an anatase phase as an impurity. Electrochemical characterization of the films shows excellent performance at different current rates. The CA40-350-450 sample performs best among all samples tested, yielding an average discharge capacity of (176±1) mA h g−1 at C/2 and (139±4) mA h g−1 at 50 C and keeping 92 % of its initial discharge capacity upon 50 cycles at C/2.
Open Access
Synthesis, characterization and pore size control of mesoporous li4ti5o12, cotio3 and mntio3 thin films
(2014-07) Avcı, Civan
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…
Open Access
Template-free synthesis of organically modified silica mesoporous thin films for TNT sensing
(American Chemical Society, 2010) Yildirim, A.; Budunoglu, H.; Deniz, H.; Güler, Mustafa O.; Bayındır, Mehmet
In this paper, we present a facile, template-free sol−gel method to produce fluorescent and highly mesoporous organically modified silica (ORMOSIL) thin films for vapor phase sensing of TNT. An alkyltrifunctional, methyltrimethoxysilane MTMS precursor was used to impart hydrophobic behavior to gel network in order to form the spring back effect. In this way, porous films (up to 74% porosity) are obtained at ambient conditions. Fluorescent molecules are physically encapsulated in the ORMOSIL network during gelation. Fluorescence of the films was found to be stable even after 3 months, proving the successful fixing of the dye into the ORMOSIL network. The functional ORMOSIL thin films exhibited high fluorescence quenching upon exposition to TNT and DNT vapor. Fluorescence quenching responses of the films are thickness-dependent and higher fluorescence quenching efficiency was observed for the thinnest film (8.6% in 10 s). The prepared mesoporous ORMOSIL thin films have great potential in new sensor and catalysis applications.