Atomic-scale investigation of a new phase transformation process in TiO2 nanofibers

Phase transition behaviour and mechanism of 2D TiO2(B) nanosheets through water-mediated removal of surface ligands

Phase engineering is a central subject in materials research. The recent research interest in the phase transition behavior of atomically thin 2D materials reveals the important role of their surface chemistry. In this study, we investigated the phase transformation of ultrathin TiO2(B) nanosheets to anatase under different conditions. We found that the convenient transformation in water under ambient conditions is driven by the hydrolysis of surface 1,2-ethylenedioxy groups and departure of ethylene glycol. A transformation pathway through the formation of protonic titanate is proposed. The ultrathin structure and the metastable nature of the precursor facilitate the phase conversion to anatase. Our finding offers a new insight into the mechanism of TiO2(B) phase transition from the viewpoint of surface chemistry and may contribute to the potential application of ultrathin TiO2(B) nanosheets in aqueous environments.

Construction of TiO2(B)/Anatase Heterophase Junctions via a Water-Induced Phase Transformation Strategy for Enhanced Photocatalytic Hydrogen Production

The development of facile and green solution-phase routes toward the fabrication of TiO₂-based heterophase junctions with a delicate control of phase and structure is a challenging task. Herein, we report a simple and convenient method to controllably fabricate TiO₂(B)/anatase heterophase junctions, which was successfully realized by utilizing the ideal great solvent of water to treat the presynthesized TiO₂(B) nanosheet precursor at a low temperature of 80 °C. On the basis of phase structure transformation and morphology evolution data, the formation of these TiO₂(B)/anatase heterophase junctions was reasonably explained by a novel water-induced TiO₂(B) → anatase phase transformation mechanism. Benefiting from the desirable structural and photoelectronic advantages of more exposed active sites, enhanced light absorbance, and promoted separation of photogenerated electron-hole pairs, the thus-transformed TiO₂(B)/anatase heterophase junctions exhibit fascinating photocatalytic performance in water splitting. Specifically, with the help of Pt as a cocatalyst and methanol as a sacrificial agent, the H₂ production rate of optimized TiO₂(B)/anatase heterophase junction reaches 6.92 mmol·g^-1·h^-1, which is almost 7.1 and 2.1 times higher than those of the pristine TiO₂(B) nanosheets and the final anatase nanocrystals. More interestingly, the TiO₂(B)/anatase heterophase junction also delivers prominent activity toward pure water splitting to simultaneously produce H₂ and H₂O₂, with evolution rates of up to 1.10 and 0.55 mmol·g^-1·h^-1, respectively. Our work may advance the facile green solvent-mediated synthesis of metal oxide-based heterophase junctions for applications in energy- and environmental-related areas.

Enhancing Lithium and Sodium Storage Properties of TiO2(B) Nanobelts by Doping with Nickel and Zinc

Nickel- and zinc-doped TiO2(B) nanobelts were synthesized using a hydrothermal technique. It was found that the incorporation of 5 at.% Ni into bronze TiO2 expanded the unit cell by 4%. Furthermore, Ni dopant induced the 3d energy levels within TiO2(B) band structure and oxygen defects, narrowing the band gap from 3.28 eV (undoped) to 2.70 eV. Oppositely, Zn entered restrictedly into TiO2(B), but nonetheless, improves its electronic properties (Eg is narrowed to 3.21 eV). The conductivity of nickel- (2.24 × 10-8 S·cm-1) and zinc-containing (3.29 × 10-9 S·cm-1) TiO2(B) exceeds that of unmodified TiO2(B) (1.05 × 10-10 S·cm-1). When tested for electrochemical storage, nickel-doped mesoporous TiO2(B) nanobelts exhibited improved electrochemical performance. For lithium batteries, a reversible capacity of 173 mAh·g-1 was reached after 100 cycles at the current load of 50 mA·g-1, whereas, for unmodified and Zn-doped samples, around 140 and 151 mAh·g-1 was obtained. Moreover, Ni doping enhanced the rate capability of TiO2(B) nanobelts (104 mAh·g-1 at a current density of 1.8 A·g-1). In terms of sodium storage, nickel-doped TiO2(B) nanobelts exhibited improved cycling with a stabilized reversible capacity of 97 mAh·g-1 over 50 cycles at the current load of 35 mA·g-1.

Hydrolytic Modification of SiO2 Microspheres with Na2SiO3 and the Performance of Supported Nano-TiO2 Composite Photocatalyst

Modified microspheres (SiO₂-M) were obtained by the hydrolytic modification of silicon dioxide (SiO₂) microspheres with Na₂SiO₃, and then, SiO₂-M was used as a carrier to prepare a composite photocatalyst (SiO₂-M/TiO₂) using the sol-gel method; i.e., nano-TiO₂ was loaded on the surface of SiO₂-M. The structure, morphology, and photocatalytic properties of SiO₂-M/TiO₂ were investigated. Besides, the mechanism of the effect of SiO₂-M was also explored. The results show that the hydrolytic modification of Na₂SiO₃ coated the surface of SiO₂ microspheres with an amorphous SiO₂ shell layer and increased the quantity of hydroxyl groups. The photocatalytic performance of the composite photocatalyst was slightly better than that of pure nano-TiO₂ and significantly better than that of the composite photocatalyst supported by unmodified SiO₂. Thus, increasing the loading capacity of nano-TiO₂, improving the dispersion of TiO_2, and increasing the active surface sites are essential factors for improving the functional efficiency of nano-TiO₂. This work provides a new concept for the design of composite photocatalysts by optimizing the performance of the carrier.

Synthesis of Ti4O7/Ti3O5 Dual-Phase Nanofibers with Coherent Interface for Oxygen Reduction Reaction Electrocatalysts

Electrocatalysts play an important role in oxygen reduction reaction (ORR) in promoting the reaction process. Although commercial Pt/C exhibits excellent performance in ORR, the low duration, high cost, and poor methanol tolerance seriously restrict its sustainable development and application. Ti_nO_2n-1 (3 ≤ n ≤ 10) is a series of titanium sub-oxide materials with excellent electrical conductivity, electrochemical activity, and stability, which have been widely applied in the field of energy storage and catalysis. Herein, we design and synthesize Ti₄O₇/Ti₃O₅ (T4/T3) dual-phase nanofibers with excellent ORR catalytic performance through hydrothermal growth, which is followed by a precisely controlled calcination process. The H₂Ti₃O₇ precursor with uniform size can be first obtained by optimizing the hydrothermal growth parameters. By precisely controlling the amount of reducing agent, calcination temperature, and holding time, the T4/T3 dual-phase nanofibers with uniform morphology and coherent interfaces can be obtained. The orientation relationships between T4 and T3 are confirmed to be [ 001 ] T 3 / / [ 031 ] T 4 , ( 100 ) T 3 / / ( 92 6 ¯ ) T 4 , and ( 010 ) T 3 / / ( 1 2 ¯ 6 ) T 4 , respectively, based on comprehensive transmission electron microscopy (TEM) investigations. Furthermore, such dual-phase nanofibers exhibit the onset potential and half-wave potential of 0.90 V and 0.75 V as the ORR electrocatalysts in alkaline media, respectively, which illustrates the excellent ORR catalytic performance. The rotating ring-disk electrode (RRDE) experiment confirmed the electron transfer number of 3.0 for such catalysts, which indicates a mixture of two electron and four electron transfer reaction pathways. Moreover, the methanol tolerance and cycling stability of the catalysts are also investigated accordingly.

Ti2O3/TiO2 heterophase junctions with enhanced charge separation and spatially separated active sites for photocatalytic CO2 reduction

Ti₂O₃/TiO₂ heterophase junctions with enhanced charge separation and spatially separated active sites for photocatalytic CO₂ reduction.

On the interface crystallography of heat induced self-welded TiO2 nanofibers grown by oriented attachment

The TiO₂ (B)–TiO₂ (B), TiO₂ (B)–anatase and anatase–anatase self-welded nanofibers have been investigated by TEM. The different exposed facets lead to the formation of different interface structures during the oriented attachment growth process.

Heterostructure TiO2 polymorphs design and structure adjustment for photocatalysis

Atomic composite-structure materials play an important role in energy generation and storage application fields for their advanced performance. Constructing heterostructured semiconductors is a promising strategy to devise photocatalytic systems with high activity. However, most studied hererostructures are those semiconductors with different materials formed by multi-steps, researches on in-situ formed hererostructure originated from the same precursor are few reported, and the effects of different structure ratios on photocatalytic performance are ambiguous. Here, according to in-situ temperature X-ray diffraction and transmission electron microscope techniques, a nano-sized in-situ formed heterostructure of TiO₂ semiconductors with anatase and TiO₂-B crystalline structures were designed, their structure ratios were adjusted, the heterostructure interface and photocatalytic reaction mechanism were also detected. Results show that high-quality heterojunction and optimum structure ratios have vital influence on photocatalytic performance, there is an obvious synergetic effect between anatase and TiO₂-B structure, degradation reactions on methyl orange (MO) under ultraviolet light irradiation prove that the highest activity toward MO removal can be obtained for material with 82.5% anatase structure.

Evidence of anatase intergrowths formed during slow cooling of reduced ilmenite

Controlling the parameters during synthetic rutile production is essential to minimize production costs and ensure final product quality. Powder X-ray diffraction (PXRD) is typically used within the industry to guide process control. This work investigated the source of unusual features observed in the PXRD pattern of a slow-cooled reduced ilmenite (RI), which were not observed for a rapid-cooled RI. For the slow-cooled RI, the 002 peak ofM3O5(anosovite) had disappeared and the intensity of the \bar 203, 203, 204 and 402 peaks had decreased significantly compared to the pattern for the rapid-cooled RI. Using transmission electron microscopy, selected area electron diffraction (SAED) and pair distribution function (PDF) analysis, the authors attribute these features toM3O5–anatase intergrowth formation, which causes a loss in long-range order along theM3O5caxis. Strong diffuse streaking in the SAED patterns was also evident and supported the presence of disordered intergrowths from the oxidation ofM3O5. PDF analysis showed a significant improvement in the fit to the data for the slow-cooled RI, primarily in the <17 Å region, when anatase was added to the PDF model. The results presented here highlight the importance of the reduction and cooling stages during the formation of these industrially relevant RI minerals, which may be used to direct the production process and final TiO2product quality.