Nanoporous anatase TiO2 mesocrystals: additive-free synthesis, remarkable crystalline-phase stability, and improved lithium insertion behavior

Unique spindle-shaped nanoporous anatase TiO(2) mesocrystals with a single-crystal-like structure and tunable sizes were successfully fabricated on a large scale through mesoscale assembly in the tetrabutyl titanate-acetic acid system without any additives under solvothermal conditions. A complex mesoscale assembly process involving slow release of soluble species from metastable solid precursors for the continuous formation of nascent anatase nanocrystals, oriented aggregation of tiny anatase nanocrystals, and entrapment of in situ produced butyl acetate as a porogen was put forward for the formation of the anatase mesocrystals. It was revealed that the acetic acid molecules played multiple key roles during the nonhydrolytic processing of the [001]-oriented, single-crystal-like anatase mesocrystals. The obtained nanoporous anatase mesocrystals exhibited remarkable crystalline-phase stability (i.e., the pure phase of anatase can be retained after being annealed at 900 °C) and improved performance as anode materials for lithium ion batteries, which could be largely attributed to the intrinsic single-crystal-like nature as well as high porosity of the nanoporous mesocrystals.     

Graphene-supported anatase TiO2 nanosheets for fast lithium storage

We have designed a unique hybrid structure by directly growing ultrathin anatase TiO(2) nanosheets onto graphene support for fast lithium storage. With exposed (001) high-energy facets, these TiO(2) nanosheets serve as ideal hosts for fast and efficient lithium storage. On the other hand, the graphene support serves as a highly conductive substrate that is beneficial to the high-rate performance.     

Novel porous anatase TiO2 nanorods and their high lithium electroactivity

We demonstrated a simple approach for the synthesis of a kind of novel porous anatase TiO₂ nanorods. The method is based on a reaction in composite-hydroxide eutectic system and normal atmosphere without using an organic dispersant or capping agent. The synthesis technique is cost effective, easy to control and is adaptable to mass production. This is the first time TiO₂ nanorods with a porous structure are fabricated by using this method. The as-prepared material was characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), nitrogen adsorption and desorption experiments and electrochemical measurements. The results showed that the anatase TiO₂ nanorods obtained in our experiment have a large specific surface area with a porous structure which makes it have a potential application in catalysts and battery materials, especially in lithium ion batteries. In this study, we mainly tested their electrochemical performance as negative materials for lithium ion batteries. Further research to optimize synthesis conditions, particularly to develop their application in the field of catalysis is currently in progress.     

High purity anatase TiO(2) nanocrystals: near room-temperature synthesis, grain growth kinetics, and surface hydration chemistry

High purity, spherical anatase nanocrystals were prepared by a modified sol-gel method. Mixing of anhydrous TiCl(4) with ethanol at about 0 degrees C yielded a yellowish sol that was transformed into phase-pure anatase of 7.7 nm in size after baking at 87 degrees C for 3 days. This synthesis route eliminates the presence of fine seeds of the nanoscale brookite phase that frequently occurs in low-temperature formation reactions and also significantly retards the phase transformation to rutile at high temperatures. Heating the as-is 7.7 nm anatase for 2 h at temperatures up to 600 degrees C leads to an increase in grain size of the anatase nanoparticles to 32 nm. By varying the calcination time from 2 to 48 h at 300 degrees C, the particle size could be controlled between 12 and 15.3 nm. The grain growth kinetics of anatase nanoparticles was found to follow the equation, D(2) - D(0)(2) = k(0)t(m)e((-)(E)(a)/(RT)) with a time exponent m = 0.286(+/-9) and an activation energy of E(a) = 32 +/- 2 kJ x mol(-)(1). Thermogravimetric analysis in combination with infrared and X-ray photoemission spectroscopies has shown the anatase nanocrystals at different sizes to be composed of an interior anatase lattice with surfaces that are hydrogen-bonded to a wide set of energetically nonequivalent groups. With a decrease in particle size, the anatase lattice volume contracts, while the surface hydration increases. The removal of the surface hydration layers causes coarsening of the nanoparticles.     

Lithium insertion into TiO2 (anatase): electrochemistry,Raman spectroscopy,and isotope labeling

This article presents a critical discussion of selected structural aspects of electrochemical Li-insertion into TiO₂ (anatase). Recent works are reviewed (almost half of the cited works is from 2010+) with a special attention to the crystal-face-specific phenomena, Raman spectroscopy, and single-crystal and nanocrystalline electrodes. The benefits of isotopic labeling are highlighted for the in-depth understanding of Raman spectra and the Raman spectroelectrochemistry of Li-insertion. The persisting open questions and contradictory issues in the field are discussed too.     

Defect chemistry, redox kinetics, and chemical diffusion of lithium deficient lithium niobate

High-temperature optical in situ spectroscopy was used to investigate the defect absorption, redox kinetics, and chemical diffusion of a lithium deficient (48.4 mol% Li(2)O) congruent melting lithium niobate single crystal (c-LN). Under reducing atmospheres of various oxygen activities, a(O(2)), UV-Vis-NIR spectra measured at 1000 °C are dominated by an absorption band due to free small polarons centered at about 0.93 eV. The polaron band intensity was found to follow a power law of the form a(O(2))(m) with m = -1/4. A chemical reduction model involving electrons localized on niobium ions on regular lattice sites can explain the observed defect absorption and its dependence on oxygen activity. The kinetics of reduction and oxidation processes upon oxygen activity jumps and the associated chemical diffusion coefficients are found in close agreement over a range from -0.70 to -14.70 in log a(O(2)), indicating a reversible redox process. Assuming coupled fluxes of lithium vacancies and free small polarons for the attainment of stoichiometry, the diffusion coefficients of lithium vacancies as well as of lithium ions in the lithium deficient c-LN have been determined at 1000 °C.     

Mesoporous electrode material from alumina-stabilized anatase TiO2 for lithium ion batteries

A mesoporous electrode material whose structure is composed of anatase nanocrystals stabilized by alumina is reported. Powder X-ray diffraction shows the anatase phase only, but micro-Raman spectroscopy shows that the materials have a core-shell morphology with grains of bulk anatase covered by a thin rutile layer on the surface. This structure is unique when compared to analogous materials stabilized by zirconia (PNNL-1). Nitrogen adsorption isotherms demonstrate a monotonous increase in surface area and mesopore volume with increasing Al content. Thin film electrodes from these materials were characterized by lithium insertion electrochemistry. Cyclic voltammograms exhibit significant differences in Li accommodation in Al-free and Al-stabilized materials.     

TiO2 and SnO2@TiO2 hollow spheres assembled from anatase TiO2 nanosheets with enhanced lithium storage properties

TiO(2) and SnO(2)@TiO(2) hollow spheres assembled from anatase TiO(2) nanosheets with exposed (001) high-energy facets are constructed via a templating approach, and the as-prepared samples exhibit enhanced lithium storage properties.     

Dynamics of water confined on a TiO2 (anatase) surface

Inelastic neutron scattering has been employed to probe the vibrational density of states of water confined by an oxide surface, namely, nanoparticles of the anatase polymorph of TiO2. The heat capacity of confined water has been measured by adiabatic calorimetry and compared with values derived from the vibrational density of states. Both inelastic neutron scattering and calorimetry demonstrate restricted mobility and lower heat capacity and entropy of confined water as compared to the bulk.     

Preparation of nitrogen-doped anatase TiO2 nanoworm/nanotube hierarchical structures and its photocatalytic effect

As-anodized amorphous TiO₂ nanotube arrays (TNAs) are immersed in hot ammonia solution (90 °C), which can both spontaneously reconstruct the amorphous TNAs to be anatase nanoworm/nanotube hierarchical structures in situ and simultaneously implant nitrogen into them. These hierarchical structures, having larger surface area, higher electrical conductivity and broader light absorption range than the original TNAs, possess dramatically enhanced photocatalytic activity for degradation of methyl orange (MO) under visible light irradiation. The optimized nitrogen doped hierarchical structures exhibit a best photodegradation rate (K) of 0.722 h⁻¹, which greatly exceeds the degradation rate of the original TNAs annealed in ambient air at 500 °C for 2.5 h. This simple technique would enable us conveniently to design and fabricate highly photoactive one-dimensional TNAs-based functional materials applicable to photocatalysis and solar energy conversion.     

Direct synthesis of nanowires with anatase and TiO2-B structures at near ambient conditions

In this study, we present a new approach toward titanium oxide nanowires. In this approach, the growth formation of the wires sets in at a temperature as low as 40 degrees C under ambient pressure. Moreover, we provide evidence that nanowires with distinctive TiO2-anatase and TiO2-B structures can be directly produced without further thermal treatment using controlled reaction conditions.     

Carrier recombination dynamics in anatase TiO2 nanoparticles

We present an experimental study of the radiative recombination dynamics in size-controlled TiO₂ nanoparticles in the range 20–130 nm. Time-integrated photoluminescence spectra clearly show a dominance of self-trapped exciton (STE) emission, with main features not dependent on the nanoparticle size and on its environment. From picosecond time-resolved experiments as a function of the excitation density and the nanoparticle size we address the STE recombination dynamics as the result of two main processes related to the direct STE formation and to the indirect STE formation mediated by non-radiative surface states.     

Light-induced charge separation in anatase TiO2 particles

Ultraviolet light-induced electron-hole pair excitations in anatase TiO(2) powders were studied by a combination of electron paramagnetic resonance and infrared spectroscopy measurements. During continuous UV irradiation in the mW.cm(-2) range, photogenerated electrons are either trapped at localized sites, giving paramagnetic Ti(3+) centers, or remain in the conduction band as EPR silent species which may be observed by their IR absorption. Using low temperatures (90 K) to reduce the rate of the electron-hole recombination processes, trapped electrons and conduction band electrons exhibit lifetimes of hours. The EPR-detected holes produced by photoexcitation are O(-) species, produced from lattice O(2-) ions. It is found that under high vacuum conditions, the major fraction of photoexcited electrons remains in the conduction band. At 298 K, all stable hole and electron states are lost from TiO(2). Defect sites produced by oxygen removal during annealing of anatase TiO(2) are found to produce a Ti(3+) EPR spectrum identical to that of trapped electrons, which originate from photoexcitation of oxidized TiO(2). Efficient electron scavenging by adsorbed O(2) at 140 K is found to produce two long-lived O(2)(-) surface species associated with different cation surface sites. Reduced TiO(2), produced by annealing in vacuum, has been shown to be less efficient in hole trapping than oxidized TiO(2).     

One-pot hydrothermal synthesis of porous anatase TiO2 nanotube formed bundles for lithium ion battery anodes

Anatase phase of TiO₂ nanomaterial has been deemed a potential anode material for lithium ion battery (LIB) applications because of its remarkable electrochemical properties. However, TiO₂ anodes always suffer from intrinsic poor electrical conductivity and slow ion kinetics, which would restrict their practical usage. To address this issue, efficient control and design of the anatase crystal structure of TiO₂ material with desirable morphology is one of the critical approaches. In this work, a good lithium storage capability of 181 mA hr g⁻¹ at rate of 0.2C, high rate performance of 70 mA hr g⁻¹ at 20C, and excellent cyclability of 117 mA hr g⁻¹ with a capacity retention of 93% at 1C after 100 cycles and 84 mA hr g⁻¹ at 10C after 1,000 cycles are observed in an optimized porous anatase TiO₂ one-dimensional nanotube bundle nanomaterial fabricated through a simple hydrothermal process with post calcination treatment. These excellent electrochemical properties of the product can be ascribed to its anatase crystal phase, 1D nanostructure, and porous framework with a large surface area, which provide it with an efficient electrode/electrolyte contact area and a faster ion/electron diffusion pathway.     

Photoluminescence of anatase and rutile TiO2 particles

Nonaqueous reactions between titanium(IV) chloride and alcohols (benzyl alcohol or n-butanol) were used for the synthesis of anatase TiO2 particles, while rutile TiO2 particles were synthesized in aqueous media by acidic hydrolysis of titanium(IV) chloride. The X-ray diffraction measurements proved the exclusive presence of either the anatase or the rutile phase in prepared samples. The photoluminescence of both kinds of particles (anatase and rutile) with several well-resolved peaks extending in the visible spectral region was observed, and the quantum yield at room temperature was found to be 0.25%. Photon energy up-conversion from colloidal anatase and rutile TiO2 particles was observed at low excitation intensities. The energy of up-converted photoluminescence spans the range of emission of normal photoluminescence. The explanation of photon energy up-conversion involves mid-gap energy levels originating from oxygen vacancies.     

Size-dependent surface activity of rutile and anatase TiO2 nanocrystals: facile surface modification and enhanced photocatalytic performance

The size-dependent surface activity of titania was illustrated through the formation of ultrafine nanocrystals with clean surfaces. It was demonstrated that, when the size of the nanocrystals was small enough, their surface activity could be significantly enhanced, as evidenced by the formation of transparent macroassemblies, their increased dispersity in various solvents, the facile modification of their surface by organic molecules at room temperature, their strong visible-light absorption through coordination with peroxide, and highly enhanced photocatalytic performance.     

Thermal stability of anatase TiO2 aerogels

Titanium dioxide (TiO₂) aerogels were prepared with sol–gel ambient pressure drying method by using titanium tetrachloride (TiCl₄) as precursor and tetraethoxysilane as modifier, calcinated at different temperature and characterized by X‐ray diffraction, transmission electron microscopy and small angle X‐ray scattering. The results showed that the TiO₂ aerogels remained amorphous under 500 °C, changed to anatase from 600 °C and further changed to rutile from 900 °C. Between 60 °C and 500 °C, the primary particles within the samples concentrated mainly upon small sizes, enlarged and diverged remarkably above 600 °C. The crystalline grains grew and agglomerated with the rise of the calcination temperature. The TiO₂ aerogels at a temperature higher than 800 °C have better stability than anatase because of the formation of partial Ti―O―Si bonds. Copyright © 2016 John Wiley & Sons, Ltd.     

Adsorption-site-dependent electronic structure of catechol on the anatase TiO2(101) surface

The adsorption of catechol (1,2-benzendiol) on the anatase TiO(2)(101) surface was studied with synchrotron-based ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy (XPS), and scanning tunneling microscopy (STM). Catechol adsorbs with a unity sticking coefficient and the phenyl ring intact. STM reveals preferred nucleation at step edges and subsurface point defects, followed by 1D growth and the formation of a 2 × 1 superstructure at full coverage. A gap state of ～1 eV above the valence band maximum is observed for dosages in excess of ～0.4 Langmuir, but such a state is absent for lower coverages. The formation of the band gap states thus correlates with the adsorption at regular lattice sites and the onset of self-assembled superstructures.     

Defect surface structures studied by Leed

Low energy electron diffraction has as its primary objective determination of the size, symmetry and contents of the surface unit mesh, but in the case of an adsorbed layer at any coverage less than the maximum the corresponding LEED pattern will contain additional features which are dependent upon the degree of organisation of the adsorbate species. The structure of such a layer must be defective, at very least in the sense that it will not be continuous over the substrate; it may consist, for example, of islands surrounded by clean surface, or the unoccupied sites may be randomly distributed. In addition, other forms of defect will often be present - various adsorbate regions may be displaced relative to one another, a number of different types of unit mesh may occur simultaneously, the adsorbate spacings may not be related to those of the substrate, and so on. The diffraction pattern produced by any of these types of structure will also be defective. Most often this is manifest simply as a high background intensity or a broadening of the spots, but in certain instances split spots, streaks, rings etc. are observed.