Browsing by Subject "Mesoporous materials"

Now showing 1 - 19 of 19

Open Access
Assembly of molten transition metal salt surfactant in a confined space for the synthesis of mesoporous metal oxide-rich metal oxide silica thin films
(2011) Karakaya, C.; Türker, Y.; Albayrak, C.; Dag, Ö.
Uniform and homogeneous coating of mesoporous materials with an active (catalytically, photonic, electrical) nanostructure can be very useful for a number of applications. Understanding chemical reactions in a confined space is important in order to design new advanced materials. In this work, we demonstrate that an extensive amount (as high as 53 mol percent) of transition metal salts can be confined between silica walls and two surfactant domains (cetyltrimethylammonium bromide, CTAB, and lauryl ether, C12H25(OCH2CH2)10OH, C12EO10) as molten salts and then converted into sponge-like mesoporous silica–metal oxides by thermal annealing. This investigation has been carried out using two different salts, namely, zinc nitrate hexahydrate, [Zn(H2O)6](NO3)2, and cadmium nitrate tetrahydrate, [Cd(H2O)4](NO3)2, in a broad range of salt concentrations. The ZnO (or CdO) layers are as thin as about ∼1.6 nm and are homogenously coated as crystalline nano-islands over the silica pore walls.
Open Access
Continuous mesoporous pd films by electrochemical deposition in nonionic micellar solution
(American Chemical Society, 2017) Iqbal, M.; Li C.; Wood, K.; Jiang B.; Takei, T.; Dag, Ö.; Baba, D.; Nugraha, A. S.; Asahi, T.; Whitten, A. E.; Hossain, M. S. A.; Malgras, V.; Yamauchi, Y.
Mesoporous metals that combine catalytic activity and high surface area can provide more opportunities for electrochemical applications. Various synthetic methods, including hard and soft templating, have been developed to prepare mesoporous/nanoporous metals. Micelle assembly, typically involved in soft-templates, is flexible and convenient for such purposes. It is, however, difficult to control, and the ordering is significantly destroyed during the metal deposition process, which is detrimental when it comes to designing precisely mesostructured materials. In the present work, mesoporous Pd films were uniformly electrodeposited using a nonionic surfactant, triblock copolymer poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide), as a pore-directing agent. The interaction between micelles and metal precursors greatly influences the metal growth and determines the final structure. The water-coordinated species interact with the ethylene oxide moiety of the micelles to effectively drive the Pd(II) species toward the working electrode surface. From small-angle neutron scattering data, it is found that spherical P123 micelles, with an average diameter of ∼14 nm, are formed in the electrolyte, and the addition of Pd ions does not significantly modify their structure, which is the essence of the micelle assembly approach. The uniformly sized mesopores are formed over the entire mesoporous Pd film and have an average pore diameter of 10.9 nm. Cross-sectional observation of the film also shows mesopores spanning continuously from the bottom to the top of the film. The crystallinity, crystal phase, and electronic coordination state of the Pd film are also confirmed. Through this study, it is found that the optimized surfactant concentration and applied deposition potential are the key factors to govern the formation of homogeneous and well-distributed pores over the entire film. Interestingly, the as-prepared mesoporous Pd films exhibit superior electrocatalytic activity toward the ethanol oxidation reaction by fully utilizing the accessible active surface area. Our approach combines electrochemistry with colloidal and coordination chemistry and is widely applicable to other promising metals and alloy electrocatalysts.
Open Access
Effects of some transition metal salts on the synthesis of mesoporous silica
(2005) Demirörs, Ahmet Faik
Open Access
Fabrication of mesoporous CuO/ZrO2-MCM-41 nanocomposites for photocatalytic reduction of Cr(VI)
(Elsevier, 2017) Nanda, B.; Pradhan, A. C.; Parida, K. M.
Mesoporous nanocomposites of CuO/ZrO2–MCM-41 (CuO@ZM-41) was designed by incorporating mesoporous ZrO2 (Z) into the high surface area MCM-41 (M-41) framework followed by loading CuO by wetness impregnation method keeping Si/Zr ratio 10. The nanocomposites were studied under PXRD, N2 sorption, DRS spectra, FTIR, XPS, NMR, HRTEM and PL to evaluate structural, morphological, optical properties and also the mesoporosity nature of the samples. The photo-reduction of Cr6+ was performed over CuO@ZM-41 by varying pH, substrate concentration, and irradiation time and catalyst dose. Among all the catalysts, 2 CuO@ZM-41 was found to be efficient photocatalyst for the photo-reduction of Cr6+. Nearly 100% reduction of Cr6+ has been achieved by 2 CuO@ZM-41 within 30 min. Intra-particle mesoporosity, high surface area, presence of CuO nanorods and electron transfer properties are the key factors for enhancing the photo-reduction activity of 2CuO@ZM-41.
Open Access
Lyotropic liquid crystalline mesophases from acid-salt-surfactant systems: synthesis and characterization of mesoporous LiMPO4 (M=Mn(II),Fe(II),Co(II) AND Ni(II))
(2019-09) Uzunok, Işıl
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.
Open Access
Mesoporous MnCo2O4 NiCo2O4 and ZnCo2O4 thin-film electrodes as electrocatalysts for the oxygen evolution reaction in alkaline solutions
(American Chemical Society, 2021-03-22) Amirzhanova, Assel; Akmanşen, Nesibe; Karakaya, Irmak; Dağ, Ömer
The oxygen evolution reaction (OER) is the bottleneck of the electrochemical water-splitting process, where the use of porous metal oxide electrodes is beneficial. In this work, we introduce a one-pot synthesis method to fabricate a series of mesoporous metal cobaltite (m-MCo2O4, M = Mn, Ni, and Zn) electrodes for the OER. The method involves preparation and coating of a homogeneous clear solution of all ingredients (metal salts and surfactants) over a fluorine-doped tin oxide surface as a thin lyotropic liquid crystalline film and calcination (as low as 250 °C) to obtain a 400 nm thick crystalline m-MCo2O4 electrode with a spinel structure. Mesophases and m-MCo2O4 films are characterized using structural and electrochemical techniques. All electrodes are stable during the electrochemical test in 1 M KOH aqueous solution and perform at as low as 204 mV overpotential at 1 mA/cm2 current density; the m-MnCo2O4 electrode works at current densities of 1, 10, and 100 mA/cm2 at 227, 300, and 383 mV overpotentials after compensating the IR drop, respectively. The Tafel slope is 60 mV/dec for the m-NiCo2O4 and m-ZnCo2O4 electrodes, but it gradually increases to 85 mV/dec in the m-MnCo2O4 electrode by thermal treatment, indicating a change in the OER mechanism.
Unknown
Morphological control of mesoporosity and nanoparticles within Co3O4-CuO electrospun nanofibers: quantum confinement and visible light photocatalysis performance
(American Chemical Society, 2017-09) Pradhan, A. C.; Uyar, Tamer
The one-dimensional (1D) mesoporous and interconnected nanoparticles (NPs) enriched composite Co3O4-CuO nanofibers (NFs) in the ratio Co:Cu = 1/4 (Co3O4-CuO NFs) composite have been synthesized by electrospinning and calcination of mixed polymeric template. Not merely the mesoporous composite Co3O4-CuO NFs but also single mesoporous Co3O4 NFs and CuO NFs have been produced for comparison. The choice of mixed polymer templates such as polyvinylpyrrolidone (PVP) and polyethylene glycol (PEG) for electrospinning is responsible for the formation of 1D mesoporous NFs. The HR-TEM result showed evolution of interconnected nanoparticles (NPs) and creation of mesoporosity in all electrospun NFs. The quantum confinement is due to NPs within NFs and has been proved by the surface-enhanced Raman scattering (SERS) study and the UV-vis-NRI diffuse reflectance spectra (DRS). The high intense photoluminescence (PL) spectra showing blue shift of all NFs also confirmed the quantum confinement phenomena. The lowering of PL spectrum after mixing of CuO in Co3O4 nanofibers framework (Co3O4-CuO NFs) proved CuO as an efficient visible light response low cost cocatalyst/charge separator. The red shifting of the band gap in composite Co3O4-CuO NFs is due to the internal charge transfer between Co2+ to Co3+ and Cu2+, proved by UV-vis absorption spectroscopy. Creation of oxygen vacancies by mixing of CuO and Co3O4 also prevents the electron-hole recombination and enhances the photocatalytic activity in composite Co3O4-CuO NFs. The photocurrent density, Mott-Schottky (MS), and electrochemical impedance spectroscopy (EIS) studies of all NFs favor the high photocatalytic performance. The mesoporous composite Co3O4-CuO NFs exhibits high photocatalytic activity toward phenolic compounds degradation as compared to the other two NFs (Co3O4 NFs and CuO NFs). The kinetic study of phenolic compounds followed first order rate equation. The high photocatalytic activity of composite Co3O4-CuO NFs is attributed to the formation of mesoporosity and interconnected NPs within NFs framework, quantum confinement, extended light absorption property, internal charge transfer, and effective photogenerated charge separations.
Unknown
Organization of bridging organics in periodic mesoporous organosilicas (PMOs)-polarization micro-raman spectroscopy
(Wiley, 2001) Dag, Ö.; Ozin, G. A.
The organization of bridging organics in oriented periodic mesoporous organosilica film (OPMOF) was demonstrated using the polarization micro-Raman spectroscopy (PMRS) in conjunction with powder x-ray diffraction (PXRD) and polarization optical microscopy (POM). The synthesis and the structural characterization of hexagonal symmetry OPMOF containing bridge-bonded ethane, ethene inside the silica channel walls were described. The mesoscale channels were found to run parallel to the surface of the underlying glass substrates as demonstrated by the PXRD measurements. A hexagonal array of channels with glassy silica organosilica walls was the best description of the structure shown by the PMRS measurements of OPMOF.
Open Access
A porosity difference based selective dissolution strategy to prepare shape-tailored hollow mesoporous silica nanoparticles
(Royal Society of Chemistry, 2015) Yildirim, A.; Bayındır, Mehmet
This article reports a general method to prepare hollow mesoporous silica nanoparticles with tailored morphology. The method is based on selective dissolution of porous cores of solid silica shell/porous silica core nanoparticles under mild conditions without the need for corrosive or toxic etchants. First, core-shell nanospheres or nanorods are prepared in a one-pot reaction. Then, mesoporous cores of the nanoparticles are selectively dissolved by incubating them in phosphate buffered saline (PBS) at 65 °C for one day. Surprisingly, shells of the resulting hollow particles contain both small and large mesopores which makes the particles very suitable for adsorption and desorption of a wide range of molecules. In addition, we proposed a mechanism for selective dissolution of porous cores of the core-shell nanoparticles.
Open Access
Role of organic and inorganic additives on the assembly of CTAB-P123 and the morphology of mesoporous silica particles
(2009) Poyraz, A. S.; Dag, Ö.
Mesoporous silica particles with various morphologies and structures have been synthesized by controlling the solubility, micellization, and assembly of a charged surfactant (cethyltrimethylammonium bromide, CTAB) and a pluronic (PEO20PPO70PEO20, P123) couple using an organic (benzene) or an inorganic (SO4 2-, NO3 -, or Cl-) additive. The effect of CTAB, with or without one of the Hofmeister ions or benzene in various concentrations, on the morphology, pore-size, pore-structure and the nature of the silica particles has been investigated. Increasing the lyotropic anion (SO4 2-) or benzene concentration of the synthesis media creates wormlike particles with enlarged pores and reduced wall thickness. However, the hydrotropic anion (NO3 -) influenced the solubility of the charged surfactant and increased the CTAB concentration in the CTAB-P123 micelles, and as a result, in the mesoporous silica particles. The surface area, unit cell, and pore size of the silica particles are diminished by increasing the nitrate ion centration. The effects of the Cl- ion are between the SO4 2- and NO3 -ions. It influenced the P123 at low and CTAB at high concentrations. At low CTAB/ P123 mol ratios, the Cl- ion affects mainly the P123, but at high CTAB/P123 it affects both the CTAB and P123. By carefully adjusting these ingredients (CTAB, SO4 2-, Cl-, NO3 - and benzene), not only the morphology of the particles, but also the pore-size and pore-structure of the mesoporous silica particles could be adjusted. The investigations were carried out by preparing a series of powder samples and, by varying the CTAB/P123 mol ratio (between 3.0 and 6.0) and the concentration of the organic (0.17 to 0.90 M) or inorganic (at 0.25, 0.50, or 1.00 M) additive in the synthesis media. The powder samples were analyzed using microscopy (SEM, TEM, and POM), diffraction (PXRD), and spectroscopy (FTIR, Raman, UV-vis, and EDS) techniques toward above goals. © 2009 American Chemical Society.
Open Access
Salt-acid-surfactant lyotropic liquid crystalline mesophases: synthesis of highly transparent mesoporous calcium hydroxyapatite thin films
(Wiley-VCH Verlag, 2016) Tunkara, E.; Dag, Ö.
Even though calcium hydroxyapatite [Ca10(PO4)6(OH)2, HAp] is one of the most investigated materials in the literature, the synthesis of mesoporous transparent thin film of HAp has not yet been reported. We show herein that mixtures of phosphoric acid (H3PO4·H2O, PA), calcium nitrate tetrahydrate [Ca(NO3)2·4H2O, CaN] and non‐ionic surfactant [C12H25(OCH2CH2)10OH, C12E10] can self‐assemble into stable lyotropic liquid crystalline (LLC) mesophases. The clear aqueous solutions of the mixtures can be spin‐coated over any substrate and then calcined to form highly transparent mesoporous HAp (mHAp) thin films. From among the compositions studied, three molar ratios of CaN/PA/C12E10 [3.3:2:1 (low), 5.8:3.5:1 (intermediate) and 8.4:5:1 (high)] were chosen for large‐scale preparation to investigate their structural and thermal properties. The mHAp films form at around 300 °C and fully crystalize at 500 °C, retaining their transparency, uniformity and porosity in all compositions with few differences. The surface area and pore volume decrease, and the pore size and pore size distribution increase with increasing annealing temperature for all compositions.
Open Access
Silver nitrate-oligo (ethylene oxide) surfactant mesoporous nanocomposite films and monoliths
(2000-09) Samarskaya, Ol'ga
The purpose of this work is to improve and simplify the method of synthesis of metal functionalized mesoporous materials. This study has two particular goals. The first goal is to incorporate silver in its ionic form and to achieve its homogeneous distribution within the pores of meso-silicon oxide. The second goal is to establish the influence of concentration of silver present in the system on structure of the porous silica materials. Silver nitrate salt dissolved in hexagonal mesophase of polyoxyethylene 10 lauryl ether (non-ionic PEO-type surfactant) was evenly distributed within silica framework which is tailored through liquid crystalline templating-sol-gel processing. In this approach, lyotropic liquid crystalline mixture containing silver ion and amphiphilic oligo (ethylene oxide) precursor organizes in hexagonal phase in the presence of nitric acid and water at room temperature. This preformed silver containing LC mesophase is utilized as a template for subsequent condensation-polymerization reacting of Si (OCH subscript 3) subscript 4) which results in formation of silicon oxide matrix as a direct cast mesophase formed by the template. The amount of silver nitrate homogeneously mixed in LC hexagonal phase of oligo-ethylene oxide/water system alters the mesophase. The template, lyotropic hexagonal mesophase made up by silver nitrate which is dissolved in PEO-type surfactant/water system in the certain concentration range, can be used to synthesize silver containing silica-based mesoporous materials. It is determined that C subscript 12 E subscript 10:H subscript 2 0 (50 wt%):HNO subscript 3 system preserves its hexagonal LC phase in the presence of Ag ions up to 0.9 silver to surfactant molar ratios. Higher concentrations of silver nitrate in surfactant mesophase induce formation of white soft solid phase, which is assigned to the Ag ion/surfactant/ nitrate ion complex. The template mixtures of 0.1-0.7 silver nitrate to surfactant molar ratios yield silver containing 3D-hexagonal meso-silicon oxide. However, at higher silver nitrate concentration amorphous disordered materials form. Homogeneously distributed Ag ions were successfully reduced to Ag nunoclusters on both internal and external surface of mesoporous silica materials by hydrazine in the gas phase.
Open Access
Silver nitrate/oligo(ethylene oxide) surfactant/mesoporous silica nanocomposite films and monoliths
(Academic Press, 2001) Samarskaya, O.; Dag, Ö.
A lyotropic, liquid crystalline (LC) phase of a silver nitrate/oligo(ethylene oxide), water, and acid mixture was used for one-pot synthesis of mesoporous silica materials in which Ag+ ions are uniformly distributed. We established that the AgNO3-to-surfactant mole ratio is very important in a 50 wt% surfactant/water system to preserve the hexagonal LC phase before and after the addition of the silica source. Below a 0.6 AgNO3-to-surfactant mole ratio, the mixture is liquid crystalline and serves as a template for silica polymerization. However, between 0.6 and 0.8 AgNO3-to-surfactant mole ratios, one must control the composition of the mixture during the polymerization processes. Above a 0.8 mole ratio, Ag+ ions undergo phase separation from the reaction mixture by complexing with the surfactant molecules. The resulting silica materials obtained from AgNO3/surfactant ratio above 0.8 have anisotropy but without a hexagonal mesophase. Here, we establish a AgNO3 concentration range in which the LC phase is preserved to template the synthesis of mesoporous silica, and we discuss the structural behavior of the mixtures at AgNO3/surfactant mole ratios of 0.00-2.00, using POM, PXRD, FTIR, and UV-Vis absorption spectroscopy. © 2001 Academic Press.
Open Access
Standing mesochannels: mesoporous PdCu films with vertically aligned mesochannels from nonionic micellar solutions
(American Chemical Society, 2018) Iqbal, M.; Kim, J.; Yuliarto, B.; Jiang B.; Li C.; Dağ, Ömer; Malgras, V.; Yamauchi, Y.
Mesoporous bimetallic palladium (Pd) alloy films with mesochannels perpendicularly aligned to the substrate are expected to show superior electrocatalytic activity and stability. The perpendicular mesochannels allow small molecules to efficiently access the active sites located not only at the surface but also within the film because of low diffusion resistance. When compared to pure Pd films, alloying with a secondary metal such as copper (Cu) is cost-effective and promotes resistance against adventitious poisoning through intermediate reactions known to impair the electrocatalytic performance. Here, we report the synthesis of mesoporous PdCu films by electrochemical deposition in nonionic micellar solutions. The mesoporous structures are vertically aligned on the substrate, and the final content of Pd and Cu can be adjusted by tuning the initial precursor molar ratio in the electrolyte solution.
Open Access
Synthesis and characteization of mesoporous nickel oxide and nickel cobaltite thin films
(2019-09) Amirzhanova, Assel
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.
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.
Embargo
Synthesis, characterization, and electrochemical properties of mesoporous spinel LiMn2-xMxO4 (M = Mn, Fe, Co, Ni, AND Cu) thin films
(2024-05) Durukan, Irmak Karakaya
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.
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.
Open Access
Two-dimensional mesoporous vanadium phosphate nanosheets through liquid crystal templating method toward supercapacitor application
(Elsevier, 2018) Mei, P.; Kaneti, Y. V.; Pramanik, M.; Takei, T.; Dağ, Ömer; Sugahara, Y.; Yamauchi, Y.
Mesoporous vanadium phosphate (VOPO4) nanosheets have been successfully synthesized through an easy and reproducible lyotropic liquid crystals (LLC) templating approach for the first time. Using the triblock copolymer (P123) as a surfactant, VOPO4 precursor with a well-developed 2D hexagonal mesostructure can be obtained. Following complete removal of the template by calcination, crystallized VOPO4 frameworks with less-ordered mesostructure are achieved. The as-prepared mesoporous VOPO4 nanosheets exhibit superior pseudocapacitive performance (767 F g‒1 at 0.5A g‒1) by virtue of the favorable mesostructure that gives rise to abundant easily accessible redox active sites as well as reinforced charge transfer and ion diffusion properties. The charge storage mechanism of the mesoporous VOPO4 nanosheets has been experimentally demonstrated to be based on the reversible two-step redox reactions between V(V) and V(III) in acidic medium. This advantageous LLC templating strategy is expected to open up a new route for designing various mesoporous metal phosphates with superior electrochemical performance for utilization in energy storage devices.