Effects of Annealing Temperature on the Crystal Structure, Morphology, and Optical Properties of Peroxo-Titanate Nanotubes Prepared by Peroxo-Titanium Complex Ion

Stoichiometric Study on Ion Composition of a Precursor in Chemical Bottom-Up Synthesis for Peroxo-Titanate

Layered alkali titanates of the lepidocrocite type are gaining enormous interest in various fields owing to their unique properties. These materials are mainly synthesized through a hydrothermal alkali treatment. However, this method uses a highly concentrated alkali solution, which has high environmental impacts and is therefore unsuitable for mass synthesis. Herein, we propose an efficient method for the large-scale synthesis of layered sodium titanate structures (Na2-x H x Ti2O5) using a recently reported bottom-up chemical process. The effects of the Na:Ti molar ratio in the peroxo-titanium complex ion precursor on the products are investigated through stoichiometric calculations for a molar ratio range of 10:1-1:1. The optimal ratio for the complete ionization of TiH2 (which is the starting material) to form the peroxo-titanium complex ion is found to be 1.1:1. The amount of alkali raw material required is 99.6% lower than that required in the traditional hydrothermal method. The crystal structures and morphologies of the samples are almost identical regardless of the Na:Ti molar ratio. The precursor-derived peroxo bonds narrow the energy band gaps of the layered titanates even when the amount of titanium ions dissolved in the precursor increases. The proposed method is not only an efficient synthetic route for mass production but also has potential applications in the development of photofunctional materials.

Mesoporous Metastable CuTe2 Semiconductor

Binary metastable semiconductor materials offer exciting possibilities in the field of optoelectronics, such as photovoltaics, tunable photosensors, and detectors. However, understanding their properties and translating them into practical applications can sometimes be challenging, owing to their thermodynamic instability. Herein, we report a temperature-controlled crystallization technique involving electrochemical deposition to produce metastable CuTe2 thin films that can reliably function under ambient conditions. A series of in situ heating/cooling cycle tests from room temperature to 200 °C followed by spectral, morphological, and compound analyses (such as ultraviolet-visible light spectroscopy, X-ray diffraction (XRD) analysis, and X-ray photoelectron spectroscopy (XPS)) suggest that the seeding electrodes play a key role in the realization of the metastable phase in CuTe2 films. In particular, CuTe2 films deposited on Al electrodes exhibit superior crystallinity and long-term stability compared with those grown on a Au substrate. The XRD data of thermally annealed CuTe2 thin films deposited on Al show a markedly sharp peak, indicating significantly increased crystal-domain sizes. Our method can be used to achieve the metastable phase of CuTe2 with a bandgap of 1.67 eV and offers outstanding photoresponsivity under different illumination conditions.

A facile bottom-up method for synthesis of peroxo-potassium titanate nanoribbons and visible light photocatalytic activity derived from a peroxo-titanium bond

Low-dimensional titanate nanostructures are gaining attention as a promising material for various photocatalytic applications. However, these conventional titanium oxide-based materials cannot utilize visible light because of their wide bandgap, and their synthesis generally requires high-alkali (10 mol L^-1) and high-temperature (160-200 °C) conditions. Here, we report facile bottom-up synthesis for the visible light-activated peroxo-titanate nanoribbon (PTNR). The use of the peroxo-titanium complex ion containing the potassium ion as a precursor can induce the formation of a layered potassium titanate structure (K_2-x H _x Ti₂O₅) based on the self-organization reaction between titanium complex ions and potassium ions under mild synthetic conditions (0.29-4.39 mol L^-1 KOH, 100 °C). Furthermore, the requirement of potassium ions in the formation of layered potassium titanate was stoichiometrically examined. The layered titanate crystals could be grown anisotropically, which depended on the radius of the cation used. Our results newly revealed that the larger radius of the interlayer cation promotes anisotropic crystal growth. As a result, in the case of the potassium base, a nanoribbon structure with a higher aspect ratio and larger specific surface area than those of lithium and sodium bases was formed. The formed peroxo-titanium functional groups significantly reduced the bandgap of titanate to 2.64 eV. In a photocatalytic decolorization test, the PTNR showed excellent photocatalytic performance based on the large surface area and enhanced light absorption in the visible light range while still performing well under UV light. These findings show not only that the proposed synthetic process has a low environmental impact but also that it contributes to the development of highly functionalized materials for photochemical applications.

Bottom-up method for synthesis of layered lithium titanate nanoplates using ion precursor

A facile bottom-up method for the synthesis of lithium titanate nanoplates using a peroxo titanium complex ion precursor is reported. Instead of employing complicated treatment with high alkali concentration, the self-organization reaction between lithium and titanium ions in the prepared ion precursor can enable the formation of layered lithium titanate crystals (Li_2-xH_xTi₂O₅, where x = 0.1 and 1.52 for as-synthesise and acid-treated samples, respectively) under low alkaline conditions. We demonstrate that layered lithium titanate crystals can be grown anisotropically into individual nanoplates. Our work presents an easy and useful platform for the production of titanate materials with various morphologies based on the interaction with ionic species.

Effect of annealing temperature on structural, optical, and photocatalytic properties of titanium dioxide nanoparticles

Titanium dioxide (TiO₂) nanoparticles (NPs) were produced by simple sol-gel technique and annealed for 2 h in air. The impact of annealing temperature on the physical, morphological, photocatalytic, and optical properties was studied. X-ray diffraction (XRD) measurements proved that a gradual phase change from anatase to rutile occurred with increased annealing temperature. The crystal structures of the NPs were found to change at different temperatures; anatase phase at 450 °C, mixed-phase (anatase/Rutile) at 550–650 °C, and rutile phase at 750 °C. The rise in the annealing temperature improved the crystallinity of the NPs. The crystal and grain size of the NPs increased with the annealing temperature hence reducing the specific surface area due to the condensed boundaries between subunits of the NPs. As per the Kubelka-Munk equation, the bandgap reduced as the temperature elevated. The photocatalytic activity of the NPs was higher in the anatase/rutile mixed-phase than in the single anatase and rutile phase. Experimental results indicated that annealing temperature could effectively change the properties of the TiO₂ NPs.

Graphite Felt Modified by Atomic Layer Deposition with TiO2 Nanocoating Exhibits Super-Hydrophilicity, Low Charge-Transform Resistance, and High Electrochemical Activity

Graphite felt (GF) is a multi-functional material and is widely used as electrodes of electrochemical devices for energy and environmental applications. However, due to the inherent hydrophobicity of graphite felt, it must be hydrophilically pretreated to obtain good electrochemical activity. Metal oxides coating is one of the feasible methods to modify the surface of GF, and in order to ensure that the metal oxides have a better conductivity for obtaining higher electrochemical activity, a subsequent H₂ heat-treatment process is usually adopted. In this study, atomic layer deposition (ALD) is used to deposit TiO₂ nanocoating on graphite felt (GF) for surface modification without any H₂ thermal post-treatment. The results show that the ALD-TiO₂-modified GF (ALD-TiO₂/GF) owns excellent hydrophilicity. Moreover, the ALD-TiO₂/GF exhibits excellent electrochemical properties of low equivalent series resistance (R_s), low charge-transfer resistance (R_ct), and high electrochemical activity. It demonstrates that ALD is an applicable technique for modifying the GF surface. In addition, it can be reasonably imagined that not only TiO₂ film can effectively modify the GF surface, but also other metal oxides grown by ALD with nanoscale-thickness can also obtain the same benefits. We anticipate this work to be a starting point for modifying GF surface by using ALD with metal oxides nanocoating.