Molten Salt Assisted Self Assembly (MASA): Synthesis of Mesoporous Metal Titanate (CoTiO₃, MnTiO₃, and Li₄Ti₅O₁₂) Thin Films and Monoliths

Abstract

Mesoporous metal titanates are very important class of materials for clean energy applications, specifically transition metal titanates and lithium titanates. The molten salt assisted self-assembly (MASA) process offers a new synthetic route to produce mesoporous metal titanate thin films. The process is conducted as follows: first a clear solution that contains two solvents (namely the hydrated salt (Co(NO 3 ) 2 · 6H 2 O or Mn(NO 3 ) 2 ·6H 2 O, or LiNO 3 ·xH 2 O, and ethanol), two surfactants (cethyltrimethylammonium bromide, CTAB, and 10-lauryl ether, C 12 EO 10 ), an acid and titanium source (titanium tetrabutoxide, TTB) is prepared and then spin or spray coated over a substrate to form a thin or thick lyotropic liquid crystalline (LLC) film, respectively. Finally, the films are converted into transparent spongy mesoporous metal titanates by a fast calcination step. Three mesoporous metal titanates (namely, CoTiO 3 , MnTiO 3 , and Li 4 Ti 5 O 12 ) have been successfully synthesized and structurally/thermally characterized using microscopy, spectroscopy, diffraction, and thermal techniques. The mesoporous cobalt and manganese titanates are stable up to 500°C and collapse at around 550°C into nanocrystalline Co 3 O 4 − TiO 2 and Mn 2 O 3 −TiO 2 ; however, lithium titanate is stable up to 550°C and crystalline even at 350°C. The crystallinity and pore size of these titanates can be adjusted by simply controlling the annealing and/or calcination temperatures. ■ INTRODUCTIONThe melting point of metal salts can be reduced significantly in a confined space -known as the confinement effect -such that they may act as solvents in a self-assembly process.1 Organizing surfactants into lyotropic liquid crystalline (LLC) mesophases by using molten salts can be beneficial in the production of new materials for clean energy applications.2,3 Recently, we have demonstrated that the molten phase of salts can be used as a solvent in the synthesis of mesoporus metal oxide modified silica 2 as well as titania. 3 This process was introduced as molten salt assisted self-assembly (MASA).2,3 The MASA is a useful process in producing materials that are difficult to produce using known synthesis techniques, and it can be regarded as a new synthetic route. Two solvents and two surfactants are needed in this self-assembly process. The first solvent is volatile and is used to homogenize the mixture of the ingredients to produce a clear solution, 2,3 and the second solvent is a salt, which organizes the mixture into an LLC mesophase upon evaporation of the first one. 3 Simply, a clear solution of a mixture of all the ingredients (salt, surfactants, water (or ethanol), polymerizing agent) can be spin coated over a substrate to form a thin film. Further polymerization of the polymerizing component (silica or titania source) takes place in the self-assembled soft media. The MASA process allows for efficient and homogeneous contact between the polymerizing silica or titania and the salt ions. The spin coated fresh samples are usually ...

Synthesis of Mesoporous Lithium Titanate Thin Films and Monoliths as an Anode Material for High‐Rate Lithium‐Ion Batteries

“…[7,[12][13][14][15][16][17] In particular, producing mesoporousL TO materials results in ar eduction of the polarizatione ffects and an increased performance at high currentr ates because the mesoporous structure provides ah ighs pecific surface area, narrow pore size distribution, and good permeation. [18][19][20][21][22][23] Dag et al developed am olten-salt-assisted self-assembly (MASA) method with ah ighers urfacea rea and thinner pore walls by using smalln onionic surfactants. In addition to limiting effects on the electronic conductivity,sincethey are insulating, binders also slow down Li + ion exchangeb etween the electrode and electrolyte.…”

“…[13] Furthermore, the directd eposition of the synthesizedm aterialo nt he current collectore liminates the necessity of using binders, which are used to enhance mechanical connectionsb etween active powder materials in the slurry-castingt echnique. [23] In this assembly process, the lithium source, as alt, and ap olymerizing titania source can self-assemble nonionic surfactants into mesophases that can be cal-cined at higher temperature to produce mesoporous LTOt hin films and monoliths. [17] There are only af ew methods of producing mesoporous LTOm aterials by using as urfactantt emplate.…”

“…In addition to limiting effects on the electronic conductivity,sincethey are insulating, binders also slow down Li + ion exchangeb etween the electrode and electrolyte. [23] The MASA approach has many advantages over other synthetic methods. [18][19][20][21][22][23] Dag et al developed am olten-salt-assisted self-assembly (MASA) method with ah ighers urfacea rea and thinner pore walls by using smalln onionic surfactants.…”

“…[23][24][25] Notably, nonionic surfactants, such as oligo(ethylene oxides) andp luronics, can form LLC mesophases by using many differents alts as solvents. In this assembly process, the key step is the formation of al yotropic liquid crystalline (LLC) mesophase with the salt and surfactant species.…”

“…[29] The major driving force for the formation of as alt-surfactant mesophasei st he high hygroscopicityo ft he salt. [32] Furthermore, the addition of polymerizinga gents, such as metal alkoxides, into this media does not disturb the mesophase [23][24][25] that is ready for high-temperature calcination for the synthesis of ad esired metal oxide. [28,31] Adding acharged surfactant to the LLC media further enhances salt uptake of the media that mayb en ecessary to design stable mesoporous materials with those salts as precursors.…”