Black Anatase Formation by Annealing of Amorphous Nanoparticles and the Role of the Ti2O3 Shell in Self-Organized Crystallization by Particle Attachment

The Mystery of Black TiO2: Insights from Combined Surface Science and In Situ Electrochemical Methods

Titanium dioxide (TiO₂) is often employed as a light absorber, electron-transporting material and catalyst in different energy and environmental applications. Heat treatment in a hydrogen atmosphere generates black TiO₂ (b-TiO₂), allowing better absorption of visible light, which placed this material in the forefront of research. At the same time, hydrogen treatment also introduces trap states, and the question of whether these states are beneficial or harmful is rather controversial and depends strongly on the application. We employed combined surface science and in situ electrochemical methods to scrutinize the effect of these states on the photoelectrochemical (PEC), electrocatalytic (EC), and charge storage properties of b-TiO₂. Lower photocurrents were recorded with the increasing number of defect sites, but the EC and charge storage properties improved. We also found that the PEC properties can be enhanced by trap state passivation through Li⁺ ion intercalation in a two-step process. This passivation can only be achieved by utilizing small size cations in the electrolyte (Li⁺) but not with bulky ones (Bu₄N⁺). The presented insights will help to resolve some of the controversies in the literature and also provide rational trap state engineering strategies.

Defect Engineering of Pt/TiO2-x Photocatalysts via Reduction Treatment Assisted by Hydrogen Spillover

Defect engineering of metal oxides is a facile and promising strategy to improve their photocatalytic activity. In the present study, Pt/TiO_2-x was prepared by a reduction treatment assisted by hydrogen spillover to pure rutile, anatase, and brookite and was subsequently used for hydrogen production from an aqueous methanol solution. With increasing reduction temperature, the photocatalytic activity of the rutile Pt/TiO_2-x increased substantially, whereas the activity of anatase Pt/TiO_2-x decreased and that of brookite Pt/TiO_2-x was independent of the treatment temperature. Electron-spin resonance analysis revealed that rutile and brookite possess similar defect sites (Ti³⁺ and concomitant oxygen vacancy) after the reduction at 600 °C, whereas different resonance signals were observed for anatase after the reduction at 600 °C. During the reduction process, electrons donated from spillover hydrogen migrate between the conduction band and the inherent midgap states. This research demonstrates that the depth of the inherent midgap states, depending on the crystal phases, influences the generation of defects, which play a key role in the photocatalytic performance of Pt/TiO_2-x.

Architecture and Performance of the Novel Sulfur Host Material Based on Ti2O3 Microspheres for Lithium-Sulfur Batteries

Lithium-sulfur batteries are considered as promising next-generation green secondary batteries. Irrespective of the enhancement of the cycling stability or the suppression of polysulfide species shuttle, although much progress has recently been achieved, improving the conductivity of host materials and capturing the sulfide species as far as possible are still hot topics in the research of lithium-sulfur batteries nowadays. Here, we put forward a novel sulfur host architecture based on Ti₂O₃ microspheres fabricated by magnesiothermic reduction. The Ti₂O₃ microspheres possess both high electronic conductivity and excellent ability of anchoring lithium polysulfide species. The high electronic conductivity endowed by a narrow band gap can adequately activate insulative sulfur and reduce the battery resistance so that high specific capacity and excellent rate capability can be achieved, while the polar Ti₂O₃ could afford abundant polar active sites for the absorption of polysulfides for high capacity retention. As a result, Ti₂O₃ microspheres are applied in the research of lithium-sulfur batteries; excellent electrochemical performance has been revealed. The initial specific capacity is 1245 mAh g^-1 at 0.2C, with 91.57% capacity retention after 180 cycles. Even with a high areal loading of 3.6 mg cm^-2, an initial capacity of 665 mAh g^-1 at 0.5C and a good capacity retention of 70.98% after 300 cycles could be achieved. Apparently, the preparation and application of Ti₂O₃ microspheres can not only further extend the application field of the Ti-based compound but also boost the electrochemical performance of lithium-sulfur batteries.

Measuring the areal density of nanomaterials by electron energy-loss spectroscopy

Thickness measurements of nanomaterials are usually performed using transmission electron microscopy (TEM) techniques such as convergent beam electron diffraction (CBED) patterns analysis and the log-ratio method based on electron energy-loss spectroscopy (EELS) spectrum. However, it is challenging to obtain both the thickness and elemental information, especially in non-crystalline materials or for very thin samples. In this work, we establish a series of procedures to calculate the areal density of the material by directly measuring the inelastic scattering probability in a thin sample. Core-loss EELS are fit with a quantitative model to extract atomic areal density. Knowledge of one of the parameters (volume density or sample thickness) allows a measurement of the other. The absolute error between the known thicknesses and those measured was less than 4% using two-dimensional materials with a well-defined thickness as test samples, which is much better than the log-ratio method for very thin samples. One promising advantage of this method is the thickness/areal density determination in mixed phase/element systems. We use Ag-Co bimetallic triangles and black rutile as examples to calculate the thickness map in mixture systems in different cases. We also demonstrate this technique can be applied to measure the argon gas density in spherical cavities. This allows a temperature vs pressure curve to be obtained and illustrates the unique capability of this technique.

Surface Mechanoengineering of a Zr-Based Bulk Metallic Glass via Ar-Nanobubble Doping To Probe Cell Sensitivity to Rigid Materials

In this study, a new materials platform, utilizing the amorphous microstructure of bulk metallic glasses (BMGs) and the versatility of ion implantation, was developed for the fundamental investigation of cell responses to substrate-rigidity variations in the gigapascal modulus range, which was previously unattainable with polymeric materials. The surface rigidity of a Zr-Al-Ni-Cu-Y BMG was modulated with low-energy Ar-ion implantation because of the impartment of Ar nanobubbles into the amorphous matrix. Surface softening was achieved due to the formation of nanobubble-doped transitional zones in the Zr-based BMG substrate. Bone-forming cell studies on this newly designed platform demonstrated that mechanical cues, accompanied by the potential effects of other surface properties (i.e., roughness, morphology, and chemistry), contributed to modulating cell behaviors. Cell adhesion and actin filaments were found to be less established on less stiff surfaces, especially on the surface with an elastic modulus of 51 GPa. Cell growth appeared to be affected by surface-mechanical properties. A lower stiffness was generally related to a higher growth rate. Findings in this study broadened our fundamental understanding concerning the mechanosensing of bone cells on stiff substrates. It also suggests that surface mechanoengineering of metallic materials could be a potential strategy to promote osseointegration of such materials for bone-implant applications. Further investigations are proposed to fine-tune the ion implantation variables in order to further distinguish the surface-mechanical effect on bone-forming cell activities from the contributions of other surface properties.