NH4TiOF3 Mesocrystals Research Articles

(NH4)3TiOF5 mesocrystals grown by electric-field-assisted oriented-particle aggregation and their conversion to a TiO2 superstructure

This work reports an electrochemical synthesis of (NH4)3TiOF5 mesocrystals consisting of nanocrystalline building units, via oriented particle attachment driven by an external electric field. The obtained (NH4)3TiOF5 mesocrystals are octahedral in shape and can be converted to porous anatase TiO2, retaining their octahedral morphology after thermal annealing treatment. A distinctive evolution process for the (NH4)3TiOF5 mesocrystals with octahedral morphology is revealed, which includes the nucleation and growth of primary small octahedral particles, oriented aggregation of six primary octahedral particles into one large secondary particle, formation of mesocrystalline octahedrons by crystallographic fusion, dissolution of the octahedrons into small nanoparticles, followed by oriented aggregation of the dissolved nanoparticles to form micrometer-sized octahedrons. The alignment and subsequent oriented attachment of nanoparticles driven by an external electric field enable the rapid growth of (NH4)3TiOF5 mesocrystals within 40 s, compared with the tens of hours usually required for the growth of NH4TiOF3 mesocrystals using a hydrothermal method.

In-situ evolution of anodic TiO2 nanotubes to ammonium oxofluorotitanate mesocrystals and their conversion to mesocrystalline TiO2

3D ordered superstructures of mesocrystalline NH4TiOF3 and (NH4)2TiOF4 are synthesized via in-situ evolution from anodic TiO2 nanotube arrays grown on the Ti metal substrate under high current density. As revealed by X-ray diffraction, scanning electron microscopy and transmission electron microscopy characterizations, the obtained NH4TiOF3 and (NH4)2TiOF4 mesocrystals consist of well aligned nanoparticle subunits at atomic level and can be converted to TiO2 (anatase) mesocrystals with their original exterior shapes retained upon thermal annealing. Close examination suggests that NH4TiOF3 and (NH4)2TiOF4 mesocrystals form via a nonclassical crystal growth process involving fast oriented aggregation of particles assisted by electric field-dipole interaction.

Ultrafast and Stable Lithium Storage Enabled by the Electric Field Effect in Layer-Structured Tablet-Like NH4TiOF3 Mesocrystals.

Design and synthesis of advanced electrode materials with fast and stable ion storage are of importance for energy storage applications. Herein, we propose that introducing the heterogeneous interface in layer-structured mesocrystals is an efficient way to greatly improve the rate capability and cycle stability of lithium-ion battery (LIB) devices. NH4TiOF3 mesocrystals were employed as a typical model system to demonstrate the idea. The NH4TiOF3 mesocrystals were obtained via the hydrothermal reaction, and the NH4TiOF3/TiO2 interfaces were generated through calcining at different temperatures under an argon atmosphere. Phase composition, microstructure, and chemical analyses show that the as-prepared NH4TiOF3 mesocrystals possess "tablet-like" morphology, and the formation of the NH4TiOF3/TiO2 interface can be controlled by the calcination temperature. When evaluated as the anode for LIBs, the optimized sample (NH4TiOF3 calcined at 250 °C, NTF-250) shows excellent, fast, and stable lithium storage properties. Specifically, the NTF-250 electrode holds a reversible capacity of 159.5 mA h g-1 after 200 cycles at 0.2 A g-1. At a high current density of 20 A g-1, the electrode still maintains a reversible capacity of 89.6 mA h g-1 and reaches a reversible capacity of 128.6 mA h g-1 at a current density of 1 A g-1 after 2000 cycles. Theoretical and experimental studies show that the synergistic effects of the heterogeneous NH4TiOF3/anatase TiO2 interface in the layer-structured NH4TiOF3 mesocrystals lead to the upgraded electrochemical properties. Especially, the local build-in electric field induced by the nonuniform distribution of charge across the NH4TiOF3/anatase TiO2 interface facilitates the charge transport during the charging and discharging cycling. The current electrode design strategy paves a new way in boosting stable ion storage and thus is of great interest in energy storage and conversion.

In situ formation of defect-engineered N-doped TiO2 porous mesocrystals for enhanced photo-degradation and PEC performance.

N-Doped oxygen defective N/TiO2−x mesocrystal nanocubes were successfully prepared by a facile strategy in our system. Crystal topotactic transformation from NH4TiOF3 mesocrystals facilitated the formation of a porous structure of TiO2. Meanwhile, the introduction of N dopants and oxygen vacancies (OVs) was also achieved during this process. The as-prepared products exhibit much higher photoelectrochemical (PEC) and photocatalytic degradation performance under visible light illumination. It is suggested that the promising catalytic properties result from the synergistic effect of doping, OVs and the amazing porous mesocrystal structure of N/TiO2−x.

A facile photoassisted route to synthesis N, F-codoped oxygen-deficient TiO2 with enhanced photocatalytic performance under visible light irradiation

Herein, we report a facile and economical photoassisted strategy for synthesizing the highly active N, F-codoped oxygen-deficient TiO2 with coexposed {001} and {101} facets. NH4TiOF3 mesocrystals were used to act as the resource of dopants and the intermediate to fabricate TiO2 with highly active {001} facets. Comprehensive analysis based on X-ray photoelectron spectroscopy, transmission electron microscopy and electron spin resonances manifested that F, N and oxygen vacancies were simultaneously introduced to TiO2 through the photoassisted process. The test of phenol and Rhodamine B (RhB) degradation under visible light demonstrates that the as-prepared N, F codoped oxygen-deficient TiO2 exhibits higher photocatalytic activity than its references. The increased photocatalytic performances results from the synergetic effect of the induced Vo’s and N, F codoping in TiO2 with co-exposed {001} and {101} facets, favoring the visible light utilization as well as the separation of photogenerated carriers. This strategy is expected to provide a new insight into the design of high performance photocatalysts.

Hollow nanocubes constructed from <001> oriented anatase TiO2nanoarrays: topotactic conversion and fast lithium-ion storage

Mechanically stable titanium dioxide (TiO2) with the abilities of rapidly storing and releasing Li+ can be potentially applied in electric and hybrid electric vehicles, due to its ability to enhance the stability and safety, as well as the high current performance, of lithium ion batteries (LIBs). Herein, we rationally and facilely synthesized oriented anatase TiO2 nanoarrays (OATNs) from the NH4TiOF3 mesocrystal precursor through topotactic conversion and in situ epitaxial growth under moderate conditions. This study proves that the crystallization, porous structure, and orientation of OATNs are controllable, which affect the electronic and electrochemical properties and the Li+ diffusion coefficient. The optimal OATNs formed by hydrothermally treating NH4TiOF3 mesocrystals with an H3BO3 aqueous solution for 10 h (OATNs-10) delivered a high capacity of ca. 115 mA h g−1 at a current density of 50 C (170 mA g−1 of 1 C) even after continuous 2000 cycles with a Coulombic efficiency of ca. 100%. This indicates a high current rate performance and excellent stability. The unique properties of OATNs-10 make them a promising candidate for practical applications in LIBs.

New insights into polymer mediated formation of anatase mesocrystals

The reaction between (NH4)2TiF6 and H3BO3 in the presence of varying quantities of PEG-6000 was used to form NH4TiOF3 mesocrystals (MCs). The amount of PEG-6000, employed as a template, is crucial to the formation of defect free, non-agglomerated NH4TiOF3 MCs; high concentrations lead to MC agglomeration, lower ones result in centralized defects. This polymer-mediated formation process may be understood using an analogy with known polymerization reactions. The oxofluorotitanate MCs readily undergo a thermal topotactic transformation to give anatase MCs with photocatalytic activity. The TiO2 MCs are porous, with highly orientated lamellar crystallites that form part of the larger mesocrystal structure.

Selective synthesis of TiO2 single nanocrystals and titanate nanotubes: a controllable atomic arrangement approach via NH4TiOF3 mesocrystals.

Nanostructured titania and titanate have been considered as very important materials used in photocatalysis, photovoltaics, gas sensing and other electronic industries. In principle, their common structural feature is that the precursor phase involving TiO6 octahedra or similar building units may be converted to any of the nanostructured titania and titanate forms in a controllable way. Based on the atomic arrangement of ionic liquid-mediated NH4TiOF3 mesocrystals, TiO2 nanocrystals and titanate nanotubes were selectively obtained in H3BO3 and NaOH media, respectively, by using a simple hydrothermal method. Interestingly, the titanate nanotubes were successfully formed by extraction of NH4(+) and F from NH4TiOF3 in a milder alkaline environment as low as 1 M NaOH, rather than conventional treatment of TiO2 in 10 M NaOH. The as-prepared TiO2 nanocrystals with exposed {001} facets exhibit a high photocatalytic activity and sedimentation rate as compared to commercial TiO2. Upon further doping or ion-exchange, the newly prepared TiO2 nanocrystals will show potential applications in the environment.

Synthesis of octagonal microdisks assembled from anatase TiO2 nanosheets with exposed {001} facets

Octagonal TiO2 microdisks constructed orderly from anatase nanosheet building blocks (NSBBs) with exposed {001} facets were synthesized using a facile liquid phase precipitation method combined with subsequent heat treatment. The in-situ generated BF4− and F− adsorbed onto {001} facets of NH4TiOF3 (as precursor of TiO2) decreased the surface energy instead of extremely poisonous and corrosive HF. Polymer surfactant is likely to further stabilize the {001} facets and it may induce NH4TiOF3 nanocrystals as a linkage to align high-orderly by lateral expansion for the formation of NH4TiOF3 mesocrystals. Sintering temperature for the heat treatment of NH4TiOF3 microdisks has a considerable effect on TiO2 microdisks for the extent of exposure of the {001} surface. Symmetrically growths on [100] and [110] directions parallel to the {001} surface are favored which lead to the well-defined octagonal flat-shape. TiO2 microdisks show an excellent adsorption capacity in dark and enhanced reactive activity under irradiation of UV-light for the degradation of methylene blue, owing to the high-ordered organization of nanosheets and large exposure of high-energy facets.

From NH4TiOF3nanoparticles to NH4TiOF3mesocrystals: steric hindrance versus hydrophobic attraction of F127 molecules

The formation of NH4TiOF3 mesocrystals can be largely manipulated by controlling the colloidal and chemical stabilities of NH4TiOF3 nanoparticles as the building blocks. The factors studied in the present work are temperature and the concentration of F127 (a triblock copolymer). At 23 °C and 35 °C, the presence of 10% F127 gives rise to the formation of submicron particles, whereas, a lower concentration of F127 results in submicron particles and aggregates, and a higher concentration produces a combination of NH4TiOF3 mesocrystals and submicron particles. The process is dominated by the formation of NH4TiOF3 mesocrystals at 4 °C, where the chemical conversion from NH4TiOF3 nanoparticles to TiO2 nanoparticles is significantly slowed down. A pancake-like conformation of F127 molecules on the hydrophilic particle surfaces is adopted to explain the concentration dependent steric effect and hydrophobic attraction imparted by their PPO chains. The TiO2 nanocrystals derived from calcination show superior photocatalytic performance, in comparison to TiO2 mesocrystals, due to the much higher specific surface area.

Manipulating the Formation of NH4TiOF3 Mesocrystals: Effects of Temperature, Surfactant, and pH

There are two main competitive processes involved in the formation of NH4TiOF3 mesocrystals. One leads to the assembly of individual NH4TiOF3 nanocrystals and the subsequent formation of NH4TiOF3 m...

The poly(ethylene glycol) assisted preparation of NH4TiOF3 mesocrystals and their topotactic conversion to TiO2

Poly(ethylene glycols) with different molecular weights were used to prepare NH4TiOF3 mesocrystals. The use of PEG-1000, -10 000, or -20 000 produced NH4TiOF3 mesocrystals whereas, PEG-4500 produced TiO2 (anatase) in nanoparticulate form. The shapes and the crystal structures of NH4TiOF3 mesocrystals formed with PEGs are similar to those obtained with the Brij family of nonionic surfactants. The conversion of NH4TiOF3 mesocrystals prepared with PEG-20 000 to TiO2 (anatase) mesocrystals by heat treatment or washing with boric acid was also studied. The TiO2 mesocrystals retained the crystallographical orientation of the parent NH4TiOF3 mesocrystals and the apparent average sizes of the nanoparticles are between several and tens of nanometers.

Formation and Anisotropic Dissolution Behavior of NH4TiOF3 Mesocrystals

We have conducted an investigation into the growth and anisotropic dissolution of NH4TiOF3 mesocrystals, which are converted into anatase TiO2 in an aqueous environment. Growth and dissolution are two competing processes when NH4TiOF3 mesocrystals are formed in an aqueous solution of (NH4)2TiF6, in the presence of fluoride scavenger H3BO3 and surfactant Brij 58 at 35 °C. At the initial stage of fast mesocrystal growth, surfactant molecules are occluded in the central region of the final mesocrystal, which destabilize the mesocrystal. There is a driving force for dissolution of the mesocrystal, whereby the metastable NH4TiOF3 is converted to stable TiO2 by losing N and F elements. An anisotropic propagation of dissolution is observed, as there is a variation in activation energy for dissolution in association with the planar densities of N and F elements on the corresponding planes of NH4TiOF3 mesocrystals. By controlling the competing growth and dissolution processes, NH4TiOF3 mesocrystals of varying morphology and dimension can be obtained.

Ammonium oxotrifluorotitanate: morphology control and conversion to anatase TiO2

AbstractUniform ammonium oxotrifluorotitanate (NH4TiOF3) mesocrystals were synthesized from an aqueous solution containing (NH4)2TiF6 and H3BO3 in presence of nonionic surfactant. Effects of various reaction conditions on the morphology of the NH4TiOF3 mesocrystals were examined by electron microscopy techniques. Results indicate that static solution, high surfactant concentration and low reaction temperature enhance the formation of uniform NH4TiOF3 mesocrystals with regular shapes. By sintering in atmosphere, the NH4TiOF3 mesocrystals were converted to anatase TiO2, which retain the original shapes of the NH4TiOF3 precursor due to similarities on their atom structures. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A Facile Synthesis of Uniform NH4TiOF3Mesocrystals and Their Conversion to TiO2Mesocrystals

Uniform mesocrystals of TiO2 (anatase) have been prepared from mesocrystals of NH4TiOF3. NH4TiOF3 was synthesized from an aqueous solution containing (NH4)2TiF6 and H3BO3 in the presence of a nonionic surfactant Brij 56, Brij 58, or Brij 700, at low temperatures. The exterior shapes of NH4TiOF3 mesocrystals can be tuned by adjusting the reagent concentration, reaction time, reaction temperature, and rate of stirring. The formation of the NH4TiOF3 mesocrystals proceeds via a self-assembly process involving nonclassical crystal growth. By sintering in air at 450 degrees C, or washing with H3BO3 solution at ambient temperatures, the NH4TiOF3 mesocrystals can be converted to mesocrystals of TiO2 (anatase), and the original architecture is retained.