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.

Biomass-derived carbon deposited TiO2 nanotube photocatalysts for enhanced hydrogen production

In this study, titanium oxide nanotubes (TiO₂NTs) were deposited on the surface of activated carbon (AC) by varying the wt% of AC. The physicochemical properties of synthesized TiO₂NTs-AC nanocomposites were analysed by various characterization techniques such as XRD, FT-IR, Raman, DRUV-vis, HR-TEM, XPS, PL, and N₂ physisorption. The FT-IR, EDX, and XPS analyses proved the existence of interaction between AC and TiO₂NTs. This study found that as the content of AC increases, the surface area and pore volume increase while the energy bandgap decreases. The TiO₂NTs-AC nanocomposite with 40% AC exhibited a surface area of 291 m² g^-1, pore volume of 0.045 cm³ g^-1 and half pore width = 8.4 Å and had a wide band gap energy (3.15 eV). In addition, the photocatalytic application of the prepared nanocomposites for photocatalytic H₂ production was investigated. The H₂ was produced via photo-reforming in the presence of a sacrificial agent (methanol) under sunlight irradiation. It was found that the prepared TiO₂NTs-AC nanocomposite with 40% AC acted as an efficient photocatalyst for aqueous-methanol reforming under various optimization conditions. Approximately 18 000 μmol^-1 hydrogen gas was produced via aqueous-methanol reforming under optimized conditions (catalyst dose = 100 mg, temperature = 25 °C, time = 12 hours, vol. of methanol = 20% (v/v), and pH = 7). The reusability of the TiO₂NTs-AC nanocomposite was also investigated for 5 consecutive cycles, and the results suggested only a slight decline in efficiency even after the fifth cycle. This study demonstrates the ability of an activated carbon deposited TiO₂NT catalyst to produce hydrogen effectively under sunlight.

In Situ Raman Investigation of TiO2 Nanotube Array-Based Ultraviolet Photodetectors: Effects of Nanotube Length

TiO₂ nanotube arrays (TNAs) with tube lengths of 4, 6, and 7 μm were prepared via two-step anodization. Thereafter, ultraviolet (UV) photodetectors (PDs) with Au/TiO₂/Au structures were prepared using these TNAs with different tube lengths. The effects of TNA length and device area on the performance of the device were investigated using in situ Raman spectroscopy. The maximum laser/dark current ratio was achieved by using a TNA with a size of 1 × 1 cm² and a length of 7 μm, under a 532 nm laser. In addition, when the device was irradiated with a higher energy laser (325 nm), the UV Raman spectrum was found to be more sensitive than the visible Raman spectrum. At 325 nm, the laser/dark current ratio was nearly 24 times higher than that under a 532 nm laser. Six phonon modes of anatase TNAs were observed, at 144, 199, 395, 514, and 635 cm⁻¹, which were assigned to the E_g(1), E_g(2), B_1g(1), A_1g/B_1g(2), and E_g(3) modes, respectively. The strong low-frequency band at 144 cm⁻¹ was caused by the O-Ti-O bending vibration and is a characteristic band of anatase. The results show that the performance of TNA-based PDs is length-dependent. Surface-enhanced Raman scattering signals of 4-mercaptobenzoic acid (4-MBA) molecules were also observed on the TNA surface. This result indicates that the length-dependent performance may be derived from an increase in the specific surface area of the TNA. In addition, the strong absorption of UV light by the TNAs caused a blueshift of the E_g(1) mode.

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.

Improved photo-induced charge carriers separation through the addition of erbium on TiO2 nanoparticles and its effect on photocatalytic degradation of rhodamine B

Er_xTi_1-xO₂ nanocomposites was prepared by a simple sol-gel method with various proportion of erbium viz., x=0.02, x=0.04, x=0.06, x=0.08 and x=0.10. The prepared nanocomposites were studied using XRD, UV-Vis DRS, Raman spectra, HR-SEM, EDS, TEM, PL and impedance spectroscopy. XRD revealed that modified TiO₂ nanocomposites possessed only the anatase phase with crystallite sizes of about 8.1 to 12.7nm and which is well consistent with TEM analysis. It is seen that erbium ion exist in the nanocomposites based on the analysis of EDS. HR-SEM analysis revealed that the Er_xTi_1-xO₂ nanocomposites are spherical in shape with size between 10 and 20nm. The amount of erbium remarkably affects the structural, optical and electrical properties. Loading erbium could produce 4f energy levels between valence and conduction bands thus narrowing optical band gap and generates visible absorption peaks. It was found that erbium modified TiO₂ nanocomposites induced a shift in Raman. The enhancement of life time of charge carriers was observed on erbium inclusion.