Electron Microscope Study of the Pressure-Induced Phase Transformation and Microstructure Change of TiO2 Nanocrystals

Microstructure transformation of materials under compression is crucial to understanding their high-pressure phase transformation. However, direct observation of the microstructure of compressive materials is a considerable challenge, which impedes the understanding of the relations among phase transformation, microstructure, and material properties. In this study, we used transmission Kikuchi diffraction and transmission electron microscopy to intuitively characterize pressure-induced phase transformation and microstructure of TiO2. We observed the changes of twin boundaries with increasing pressure and intermediate phase TiO2-I of anatase transformed into TiO2-II (α-PbO2 phase) for the first time. The following changes occur during this transformation: anatase (diameter of ∼100 nm) → anatase twins 60° along the [110] zone axis → intermediate TiO2-I twins 60° along the [010] zone axis → TiO2-II twins 90° along the [010] zone axis. These results directly reveal the crystallographic relation among these structures, enhancing our understanding of the phase transformation in TiO2 nanocrystals.

Highly Ordered TiO2 Nanotube Arrays with Engineered Electrochemical Energy Storage Performances

Nanoscale engineering of regular structured materials is immensely demanded in various scientific areas. In this work, vertically oriented TiO₂ nanotube arrays were grown by self-organizing electrochemical anodization. The effects of different fluoride ion concentrations (0.2 and 0.5 wt% NH₄F) and different anodization times (2, 5, 10 and 20 h) on the morphology of nanotubes were systematically studied in an organic electrolyte (glycol). The growth mechanisms of amorphous and anatase TiO₂ nanotubes were also studied. Under optimized conditions, we obtained TiO₂ nanotubes with tube diameters of 70–160 nm and tube lengths of 6.5–45 μm. Serving as free-standing and binder-free electrodes, the kinetic, capacity, and stability performances of TiO₂ nanotubes were tested as lithium-ion battery anodes. This work provides a facile strategy for constructing self-organized materials with optimized functionalities for applications.

Comparative study on structural, electronic, optical and mechanical properties of normal and high pressure phases titanium dioxide using DFT

In this paper, a Self-consistent Orthogonalized linear combination of atomic orbitals (OLCAO) technique with a generalized gradient approximation such as Perdew–Burke–Ernzerhof Solid (GGA-PBE SOL) has been used to scrutinize the structural, optical, electronic and mechanical properties of normal pressure phase (Anatase and Rutile) and high pressure phase i.e., cubic (Fluorite and Pyrite) TiO2. Electronic and optical properties of normal pressure phases of TiO2 are also investigated using (Meta) MGGA-Tran and Blaha (TB09) and obtained results are a close approximation of experimental data. It is seen that the virtually synthesized structural parameter for cubic and tetragonal phases of TiO2 are consistent with experimental and theoretical data. From the effective mass of charge carriers (m*), it can be observed that pyrite TiO2 is having lower effective mass than the fluorite and hence shows higher photocatalytic activity than fluorite. Furthermore, it is seen that fluorite is more dense than anatase, rutile and pyrite TiO2. From the theoretical calculations on the optical properties, it can be concluded that optical absorption occursin the near UV region for high and normal pressue phases of TiO2. Again from the reflectivity characteristics R(ω), it can be concluded that TiO2 can be used as a coating material. Elastic constants, elastic compliance constants, mechanical properties are obtained for anatase, rutile, fluorite and pyrite TiO2. A comparison of the results with previously reported theoretical and experimental data shows that the calculated properties are in better agreement with the previously reported experimental and theoretical results.

Comparative study on the pressure-induced phase transformation of anatase TiO2 hollow and solid microspheres

Nanostructured anatase TiO₂ undergoes pressure-induced phase transformation, and the transformation sequence is significantly different from the bulk counterpart. The size and the morphology are found both playing a critical role in the phase transformation behavior. In this work, we prepare anatase TiO₂ microspheres using a hydrothermal method. By controlling the reaction time, hollow and solid spheres of similar diameters are prepared. TEM and XRD analysis reveals that these microspheres are aggregates of anatase nanocrystalline of size between 15-16 nm. The phase transformation behaviour under high temperature is examined in situ using both Raman spectroscopy and synchrotron x-ray diffraction. We find that although both solid and hollow spheres are micron-sized, they undergo phase transformation sequence similar to nanomaterials with size of several tens of nanometers. Hollow spheres exhibit a higher compressibility than the solid spheres. A detailed analysis based on the formation mechanism of the spheres is performed to explain the unique phase transformation behavior of these materials.

Structural Phase Transition and Metallization of Nanocrystalline Rutile Investigated by High-Pressure Raman Spectroscopy and Electrical Conductivity

We investigate the structural, vibrational, and electrical transport properties of nanocrystalline rutile and its high-pressure polymorphs by Raman spectroscopy, and AC complex impedance spectroscopy in conjunction with the high-resolution transmission electron microscopy (HRTEM) up to ~25.0 GPa using the diamond anvil cell (DAC). Experimental results indicate that the structural phase transition and metallization for nanocrystalline rutile occurred with increasing pressure up to ~12.3 and ~14.5 GPa, respectively. The structural phase transition of sample at ~12.3 GPa is confirmed as a baddeleyite phase, which is verified by six new Raman characteristic peaks. The metallization of the baddeleyite phase is manifested by the temperature-dependent electrical conductivity measurements at ~14.5 GPa. However, upon decompression, the structural phase transition from the metallic baddeleyite to columbite phases at ~7.2 GPa is characterized by the inflexion point of the pressure coefficient and the pressure-dependent electrical conductivity. The recovered columbite phase is always retained to the atmospheric condition, which belongs to an irreversible phase transformation.

Pressure Induced Nanoparticle Phase Behavior, Property, and Applications

Nanoparticle (NP) high pressure behavior has been extensively studied over the years. In this review, we summarize recent progress on the studies of pressure induced NP phase behavior, property, and applications. This review starts with a brief overview of high pressure characterization techniques, coupled with synchrotron X-ray scattering, Raman, fluorescence, and absorption. Then, we survey the pressure induced phase transition of NP atomic crystal structure including size dependent phase transition, amorphization, and threshold pressures using several typical NP material systems as examples. Next, we discuss the pressure induced phase transition of NP mesoscale structures including topics on pressure induced interparticle separation distance, NP coupling, and NP coalescence. Pressure induced new properties and applications in different NP systems are highlighted. Finally, outlooks with future directions are discussed.

An immutable array of TiO2 nanotubes to pressures over 30 GPa

We report the successful formation of an immutable array of α-PbO₂ phase TiO₂ nanotubes by compression of a TiO₂ nanotube array in an anatase phase. During compression to 31.3 GPa, the TiO₂ nanotubes started to directly transform from an anatase phase to a baddeleyite phase at 14.5 GPa and completed the transition at 30.1 GPa. Under decompression, the baddeleyite phase transformed to an α-PbO₂ phase at 4.6 GPa, which was quenchable to ambient pressure. Notably the tubular array microstructure was retained after the application of ultra high pressure and undergoing a series of phase transformations. Measurements indicated that the nanotubes in the array possessed higher compressibility than in the bulk form. The highly aligned array structure is believed to reinforce the nanotubes themselves, giving exceptional stability. This, as well as the wall thickness, may also account for their different phase transition pathway.

Morphology- and lattice stability-dependent performance of nanostructured Li4Ti5O12 probed by in situ high-pressure Raman spectroscopy and synchrotron X-ray diffraction

In situ high-pressure measurements of two different nanostructured Li₄Ti₅O₁₂ materials revealed important structural origins that influence their electrochemical performance.

Pressure induced structural transformations of anatase TiO2 nanotubes probed by Raman spectroscopy and synchrotron X-ray diffraction

Pressure-induced transformations of anatase TiO₂ nanotubes probed by in situ Raman spectroscopy and synchrotron X-ray diffraction reveal novel compression behaviors.