Mesoporous Crystalline Research Articles

Mesoporus Silicon through Magnesium Reduction of Polymer Templated Silica for High Power Li-Ion Batteries

This work describes the synthesis and electrochemical characterization of mesoporous silicon for high power Li-ion battery applications. Silicon is an attractive Li-ion battery anode material for its high energy density, high elemental abundance and non-toxicity. However, full lithiation of silicon is accompanied by a 300% volume expansion which pulverizes the particles leading to rapid material failure. We have addressed these issues with silicon by developing a highly tunable synthesis method to produce mesoporous silicon from ordered mesoporous silica. This synthesis method leads to micron sized grains consisting of a 3-D interconnected silicon network that should accommodate the electrochemical induced volume expansion while the nanocystalline pore walls provide short ion diffusion lengths for high rate applications.The ordered mesoporous silica starting material is synthesized by evaporation induced self-assembly of sol-gel type precursors and a di-block copolymer. The mesoporous silicon is produced through reduction of the silica precursor with magnesium metal at 650 °C followed by selective chemical etching. The particle morphology and the pore size of both the silica and the silicon can be tuned over a broad range, and have been characterized by small angle scattering, high angle X-ray diffraction, nitrogen porosimetry, and SEM imaging. This synthesis technique produces 1-10 µm silica particles with a cubic lattice of 15 – 20 nm pores and 10 - 15 nm thick pore walls. After magnesium reduction, the porous polycrystalline silicon retains the microscale morphology and the porous nanostructure of the parent material. The silicon pore walls consist of ~30 nm crystallites which are optimal for high power density electrochemical applications.Magnesium reduction of silica is an effective method to reduce silica while preserving the nanostructured architecture, but little is understood about reaction mechanism. In an effort to control the nanoscale architecture of silicon we have investigated the reaction mechanism. We have performed in-situ X-ray diffraction during the reduction process combined with Raman spectroscopy to reveal a multi-step reaction pathway which leads to the mesoporous crystalline silicon final product. This study demonstrates that the reaction of mesoporous silica and magnesium metal first forms amorphous silicon that further reacts to form crystalline magnesium silicide. The remaining mesoporous silica is reduced by both the magnesium metal and the magnesium silicide to produce mesoporous crystalline silicon.The electrochemical properties of mesoporous silicon have been characterized by galvanostatic cycling and cyclic voltammetry. Galvanostatic measurements show capacities on the order of 1700 mAh/g while the nanocrystalline silicon pore walls and interconnected 3-D framework led to an extremely high rate capability. Over 40% of the capacity, or 720 mAh/g, can be accessed in 150 seconds. In light of the high capacity and highest reported power density for mesoporous silicon, this material is an exciting candidate for the next generation of high performance Li-ion batteries.

Bottom-up synthesis of high surface area mesoporous crystalline silicon and evaluation of its hydrogen evolution performance

As an important material for many practical and research applications, porous silicon has attracted interest for decades. Conventional preparations suffer from high mass loss because of their etching nature. A few alternative routes have been reported, including magnesiothermic reduction; however, pre-formed porous precursors are still necessary, leading to complicated syntheses. Here we demonstrate a bottom-up synthesis of mesoporous crystalline silicon materials with high surface area and tunable primary particle/pore size via a self-templating pore formation process. The chemical synthesis utilizes salt by-products as internal self-forming templates that can be easily removed without any etchants. The advantages of these materials, such as their nanosized crystalline primary particles and high surface areas, enable increased photocatalytic hydrogen evolution rate and extended working life. These also make the mesoporous silicon a potential candidate for other applications, such as optoelectronics, drug delivery systems and even lithium-ion batteries.

Crystalline Mesoporous Films of Titanium Oxides with Various Degree of Substoichiometry for Fuel Cells

The first results of the synthesis of mechanically stable, crystalline mesoporous films of titanium oxides with various stoichiometry, which will be applied either as an anode or a cathode for fuel cell electrodes, will be presented. The composition of TiO2 and substoichiomteric TinO2n-1 Magneli phases which has been obtained in a high temperature and in reductive atmosphere is expected to have a potential to serve as an anode for enhanced oxidation of organic fuel and to be employed as a cathode when modified with the co-catalyst (NiO/Ni) to allow an effective oxygen reduction. These applications require high degrees of conductivity and a large contact area with the reaction media. In addition, highly crystalline oxides are desired in order to optimize the properties such as electroactivity or electronic conductivity. The synthesis approach of substoichiometric TinO2n-1 oxides and the structural characterization in view of their potential application in fuel cell systems will be the objective of the presentation.

A Sustainable Template for Mesoporous Zeolite Synthesis

A generalized synthesis of high-quality, mesoporous zeolite (e.g., MFI-type) nanocrystals is presented, based on a biomass-derived, monolithic N-doped carbonaceous template. As an example, ZSM-5 single crystals with desirable large-diameter (12-16 nm) intracrystalline mesopores are synthesized. The platform provides scope to optimize template dimensions and chemistry for the synthesis of a range of micro-/mesoporous crystalline zeolites in a cost-effective and highly flexible manner.

Oxidation of 4-Chlorophenol by Mesoporous Titania: Effect of Surface Morphological Characteristics

Mesoporous nanocrystalline anatase was prepared via EISA employing CTAB as structure directing agent. The drying rate was used as a key synthesis parameter to increase the average pore diameter. The resultant mesoporous crystalline phases exhibited specific surface areas between 55 and 150 m2 g−1, average unimodal pore sizes of about 3.4 to 5.6 nm, and average crystallite size of around 7 to 13 nm. These mesophases were used as photocatalysts for the degradation of 4-chlorophenol (4CP) with UV light. Under the studied conditions, the mesoporous anatase degraded 100% 4CP. This was twice faster than Degussa P-25. 57% reduction of chemical oxygen demand (COD) value was achieved.

Aspirin degradation in surface-charged TEMPO-oxidized mesoporous crystalline nanocellulose

TEMPO-mediated surface oxidation of mesoporous highly crystalline Cladophora cellulose was used to introduce negative surface charges onto cellulose nanofibrils without significantly altering other structural characteristics. This enabled the investigation of the influence of mesoporous nanocellulose surface charges on aspirin chemical stability to be conducted. The negative surface charges (carboxylate content 0.44±0.01 mmol/g) introduced on the mesoporous crystalline nanocellulose significantly accelerated aspirin degradation, compared to the starting material which had significantly less surface charge (0.06±0.01 mmol/g). This effect followed from an increased aspirin amorphisation ability in mesopores of the oxidized nanocellulose. These results highlight the importance of surface charges in formulating nanocellulose for drug delivery.

One-Pot Soft-Templating Method to Synthesize Crystalline Mesoporous Tantalum Oxide and Its Photocatalytic Activity for Overall Water Splitting

Crystalline mesoporous Ta2O5 has been successfully synthesized by a one-pot route using P-123 as the structure directing agent (SDA). A series of crystalline mesoporous Ta2O5 samples has been prepared by changing the calcination temperature. The surface area decreased and the pore size increased with the increasing calcination temperature, which were the results of crystallite growth. At the same time, the pore volume was well maintained, which means limited shrinkage during the calcination of elevated temperature. The porous structure and crystal structure of as-synthesized mesoporous Ta2O5 were characterized by XRD, TG-DTA, SEM, TEM, and N2 sorption techniques. The photocatalytic activity of the as-synthesized mesoporous Ta2O5 with the cocatalyst NiOx for overall water splitting under ultraviolet (UV) light irradiation was systematically evaluated. The photocatalytic activity of crystalline mesoporous Ta2O5 showed about 3 times that of commercial Ta2O5 powder and 22 times that of amorphous mesoporous Ta2O5.

One-step nanocasting synthesis of crystalline mesoporous CoO without using reducing agent

Ordered mesoporous crystalline CoO material was synthesized by a simple one-step nanocasting synthesis method using mesoporous silica as a hard template and cobalt nitrate Co(NO3)2·6H2O as the metal precursor. The cobalt nitrate precursor was impregnated inside the mesopore space of the hard template and was in-situ converted to Co3O4 in the first stage. Then it was further converted to CoO by directly heating up the intermediate to 740–780°C for 2h in an tube furnace under a constant nitrogen gas flow. No hydrogen or organic reducing agent was used. The obtained mesoporous CoO possesses ordered mesostructure and crystalline framework. This method provides a new facile and green strategy for the synthesis of mesoporous metal oxides with lower oxidation states.

Size-controllable Synthesis of Hierarchically Structured Mesoporous Anatase TiO2 Microspheres Covered With {001} Facet

ABSTRACTSynthesis of anatase titanium dioxide (TiO2) with both controllable size and high energy facets is technologically important for its application in photocatalysis, photoelectrochemical cell, and solar cells. Herein, we report a simple and fluorine free hydrothermal method to synthesize hierarchically nanostructured mesoporous anatase TiO2 spheres (MATS) with controllable size, which covered with nearly 100% {001} facet. Mild H2SO4 was used as both a phase-inducer formation of anatase phase and a capping agent to promote oriented growth and formation of {001} facet. PVP acted as morphology control agent to prevent growing larger of the mesoporous anatase TiO2 spheres (MATS) with ∼600 nm in size. Detailed XRD and SEM studies suggested that formation of MTAS is a typical nucleation and growth process. The refining or reconstruction of TiO2 crystal structure during growth resulted in mesoporous crystalline framework that exhibits enhanced photocatalytic degradation of rhodamine B.

Synthesis of ordered mesoporous CuCo2O4 with different textures as anode material for lithium ion battery

A well-ordered mesoporous crystalline CuCo2O4 with coupled or uncoupled sub-structure was synthesized via a nanocasting strategy using mesoporous silica KIT-6 as the hard template. The uncoupled mesoporous CuCo2O4 possesses larger pore size, larger Brunauer–Emmett–Teller (BET) surface area and larger pore volume than the coupled mesoporous CuCo2O4. Cyclic voltammetry (CV) measurement of the electrodes indicates that the average discharge and charge potentials are 1.1 and 2.1V, respectively. The galvanostatic charge–discharge test was conducted at a current of 60mAg−1 in the voltage range of 0.01–3.0V. Uncoupled mesoporous CuCo2O4 exhibits better Li storage performance than the coupled mesoporous CuCo2O4, delivering an initial discharge capacity of 1564mAhg−1 and a reversible capacity of 900mAhg−1 after the 6th cycle. Besides, the rate capability of uncoupled mesoporous CuCo2O4 is higher than that of coupled mesoporous CuCo2O4 due to its more open structure.

Highly Ordered Mesoporous Crystalline MoSe2 Material with Efficient Visible‐Light‐Driven Photocatalytic Activity and Enhanced Lithium Storage Performance

AbstractHighly ordered mesoporous crystalline MoSe2 is synthesized using mesoporous silica SBA‐15 as a hard template via a nanocasting strategy. Selenium powder and phosphomolybdic acid (H3PMo12O40) are used as Se and Mo sources, respectively. The obtained products have a highly ordered hexagonal mesostructure and a rod‐like particle morphology, analogous to the mother template SBA‐15. The UV‐vis‐NIR spectrum of the material shows a strong light absorption throughout the entire visible wavelength region. The direct bandgap is estimated to be 1.37 eV. The high surface area MoSe2 mesostructure shows remarkable photocatalytic activity for the degradation of rhodamine B, a model organic dye, in aqueous solution under visible light irradiation. In addition, the synthesized mesoporous MoSe2 possess a reversible lithium storage capacity of 630 mAh g−1 for at least 35 cycles without any notable decrease. The rate performance of mesoporous MoSe2 is much better than that of analogously synthesized mesoporous MoS2, making it a promising anode for the lithium ion battery.

A template-free synthesis of mesoporous crystalline γ-alumina through partial hydrolysis of aluminum nitrate aqueous solution

The synthesis of mesoporous alumina with large surface area and crystalline nature is still a great challenge because of the susceptibility to hydrolysis of aluminum precursors. Here, we first report a preparation of NH4NO3 and Al species hybrids containing Keggin-Al13 [Al13O4(OH)24(H2O)127+] through a simple condensation of Al(NO3)3 aqueous solution partially hydrolyzed with (NH4)2CO3. Thermal treatment at a low temperature of 200°C could successfully transform the hybrid to γ-alumina, which exhibited a wormhole-like mesoporous structure with high surface area of 380–450m2/g and narrow PSD after completely removing NH4NO3 at 300°C.

High-temperature hydrothermal synthesis of crystalline mesoporous TiO2 with superior photo catalytic activities

Mesoporous titanium dioxide with crystalline mesopore walls (M-TiO2-ns) have been successfully synthesized through the self-assembly of poly 4-Vinylpyridine template and tetrabutyl titanate precursor based on their complex bond interaction under high temperature (180°C) hydrothermal conditions. X-ray diffraction shows that M-TiO2-ns have highly crystalline mesopore walls with anatase phase characters; N2 sorption–desorption isotherms, SEM and TEM images show that M-TiO2-ns have high BET surface areas (85 and 120m2/g, respectively), large pore volumes (0.32 and 0.34cm3/g, respectively) and crystalline mesopore walls, which exhibit monolithic morphology with crystal sizes around 3–5μm. Interestingly, M-TiO2-ns exhibit much higher catalytic activities and good recyclability in both induced reduction of decabromodiphenyl and oxidation of Rhodamine B under UV light than those of nonporous crystalline TiO2 and M-TiO2 templated by hydrocarbon surfactant of F127, which is even comparable with that of commercial P25. Combination of the unique characters such as crystallinity, stable mesostructure, large BET surface areas and superior photo catalytic activities make M-TiO2-ns a kind of potentially important material for removing of organic pollutions in environment through green photo irradiation processes.

Microwave-assisted Synthesis of Highly Crystalline Mesoporous Hydroxyapatite with a Rod-shaped Morphology

Abstract Mesoporous hydroxyapatite (MHA) with a rod-shaped morphology has been successfully synthesized for the first time employing cetyltrimethylammonium bromide (CTAB) as a template and CaCl2 and K2HPO4 as the precursors for hydroxyapatite under alkaline medium at the reaction temperature of 120 °C via microwave method. The obtained material exhibits a disordered mesoporous structure with a high crystallinity and highly uniform rod-like morphology with an average size of ca. 25 nm in width and 100 nm in length.

Photocatalytic degradation of acridine dyes using anatase and rutile TiO2

The adsorption and photodegradation of acridine orange (AO) and acriflavine (AF) dyes on two mesoporous titania crystalline phases, anatase and rutile, were experimentally studied. Anatase and rutile were characterized by nitrogen adsorption, electron scanning and transmission microscopy, and X-ray diffraction. The adsorption capacity of rutile was higher than that of anatase, while the reverse is observed for photodegradation of both dyes. The adsorption of AF on both adsorbents was higher than that of AO, which was related with the smaller size of AF molecules compared with those of AO, therefore the access of AF to the adsorption sites is favored.

Crystallization induced porosity control and photocatalytic activity of ordered mesoporous TiO2

The evolution of the porous network in mesoporous TiO2 structures during the calcination induced crystallization process has been studied by transmission electron microscopy, small angle X-ray diffraction and N2 adsorption/desorption techniques. It has been observed that on increasing the calcination temperature, the mesoporous structures undergo phase transition from amorphous to crystalline and their porosity changes from the ordered to the disordered state, generating crystalline fragments or aggregated porous particles. The optimum calcination temperature required for fabricating ordered metal oxide meso-structures using a polymer template has been discussed. The ordered mesoporous TiO2 with a crystalline framework showed prolonged photocatalytic activity and repeated usage as an industrial photodegradation catalyst, advantageous to the commercial nanocrystalline TiO2 Degussa P25. Mechanisms of formation and deformation of mesoporous metal oxide crystalline frameworks are proposed.

Facile strategy for synthesis of mesoporous crystalline γ-alumina by partially hydrolyzing aluminum nitrate solution

The facile synthesis of mesoporous γ-alumina is developed through the partial hydrolysis of Al(NO3)3 aqueous solution with (NH4)2CO3 without organic surfactants. In this synthesis, the stable NH4NO3/Al species (AN/Al) hybrid containing Keggin-Al13 polycations is first prepared, which is the key for the successful formation of mesoporous γ-alumina. XRD, 27Al MAS NMR, TEM, and N2 adsorption and desorption results demonstrate that the as-prepared AN/Al hybrid can be transformed to γ-alumina by treatment at 200 °C and exhibit a wormhole-like mesoporous structure with large surface area up to ∼450 m2 g−1, pore volume of ∼0.3 cm3 g−1 and narrow pore size distribution peaked at ∼3.9 nm after completely removing NH4NO3 at 300 °C. The obtained mesoporous γ-aluminas have high thermal stabilities up to 900 °C and excellent hydrothermal stability. The investigation shows that the synergetic effect of NH4NO3 and Al13 species promotes crystallization of Al species to γ-alumina, which may have a unique mechanism distinct from the mesoporous aluminas reported previously. CO2 adsorption measurements indicate that these mesoporous γ-aluminas have a much higher CO2 adsorption capacity than ordered mesoporous alumina synthesized by the surfactant-templating method and conventional γ-alumina derived from aluminum oxyhydroxides. We believe that this research should be useful for providing new insights into the transformation of transition alumina phases and for synthesizing mesoporous γ-alumina with promising properties for various chemical applications.

Phase control of hierarchically structured mesoporous anatase TiO2 microspheres covered with {001} facets

The controlled synthesis of anatase titanium dioxide (TiO2) with both high surface area and high energy facets is technologically important for its application in photocatalysis, photoelectrochemical cells, and solar cells. Here we report a simple and fluorine free hydrothermal method to synthesize hierarchically nanostructured mesoporous anatase TiO2 spheres (MATS), which were covered with {001} facets. Mild H2SO4 was used as both a phase-inducer for the formation of the anatase phase and a capping agent to promote oriented growth and formation of {001} facets. Detailed XRD and SEM studies suggested that formation of MATS follows a typical nucleation and growth process. The refining or reconstruction of TiO2 crystal structure during growth resulted in a mesoporous crystalline framework that exhibits enhanced adsorption and photocatalytic degradation of rhodamine B in comparison with that of commercial Degussa P25 TiO2.

Synthesis of mesoporous MgO single crystals without templates

Single crystals of MgO were synthesized using a simple precipitation method with no templates. The magnesium hydroxide was synthesized at 25 °C or 100 °C by the precipitation of magnesium nitrate solution, aged at room temperature or 100 °C. Precipitation temperature had a great influence on the morphology in the synthesis of magnesium oxide. The N 2 physisorption, XRD and electron microscopic (SEM/TEM, HRTEM) analysis associated with the electron diffraction reveal the different morphological structures and pores of MgO synthesized with different synthesis methods. Precipitation at 25 °C generated macroporous MgO with plate-like crystals. However, the change of precipitation temperature to 100 °C generated mesoporous single crystalline MgO, and the stabilization and growth of MgO(1 1 1) planes (with d-spacing of 0.244 nm) was observed. The formation of mesopores and single crystallinity of MgO synthesized by the precipitation at 100 °C were retained even after calcination at a temperature of 800 °C for 5 h.

New Triblock Copolymer Templates, PEO‐PB‐PEO, for the Synthesis of Titania Films with Controlled Mesopore Size, Wall Thickness, and Bimodal Porosity

The synthesis and properties of a series of new structure-directing triblock copolymers with PEO-PB-PEO structure (PEO = poly(ethylene oxide) and PB = polybutadiene) and their application as superior pore-templates for the preparation of mesoporous titania coatings are reported. Starting from either TiCl4 or from preformed TiO2 nanocrystalline building blocks, mesoporous crystalline titanium oxide films with a significant degree of mesoscopic ordered pores are derived, and the pore size can be controlled by the molecular mass of the template polymer. Moreover, the triblock copolymers form stable micelles already at very low concentration, i.e., prior to solvent evaporation during the evaporation-induced self-assembly process (EISA). Consequently, the thickness of pore walls can be controlled independently of pore size by changing the polymer-to-precursor ratio. Thus, unprecedented control of wall thickness in the structure of mesoporous oxide coatings is achieved. In addition, the micelle formation of the new template polymers is sufficiently distinct from that of typical commercial PPO-PEO-PPO polymers (Pluronics; PPO = poly(propylene oxide)), so that a combination of both polymers facilitates bimodal porosity via dual micelle templating.