Construction of Spindle-Shaped Ti3+ Self-Doped TiO2 Photocatalysts Using Triethanolamine-Aqueous as the Medium and Its Photoelectrochemical Properties

To enhance the utilization efficiency of visible light and reduce the recombination of photogenerated electrons and holes, spindle-shaped TiO2 photocatalysts with different Ti3+ concentrations were fabricated by a simple solvothermal strategy using low-cost, environmentally friendly TiH2 and H2O2 as raw materials and triethanolamine-aqueous as the medium. The photocatalytic activities of the obtained photocatalysts were investigated in the presence of visible light. X-ray diffraction (XRD), Raman spectra, transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FT-IR) spectra were applied to characterize the structure, morphologies, and chemical compositions of as-fabricated Ti3+ self-doped TiO2. The concentration of triethanolamine in the mixed solvent plays a significant role on the crystallinity, morphologies, and photocatalytic activities. The electron–hole separation efficiency was found to increase with the increase in the aspect ratio of as-fabricated Ti3+ self-doped TiO2, which was proved by transient photocurrent response and electrochemical impedance spectroscopy.

On-Demand Regulation of Photoreversible Color Switching for Rewritable Paper and Transient Information Encryption

The achievement of photoreversible color switching systems (PCSS) has offered great opportunities for fundamental studies and practical applications. However, the development of PCSS that possessing highly reversible cyclability and on-demand regulation of recoloration process remains a grand challenge. Herein, we report a hydrazine-mediated self-doping strategy for the synthesis of alkaline Ti³⁺ self-doped TiO_2-x nanoparticles, enabling the TiO_2-x nanoparticles/methylene blue based PCSS with long photoreversible cyclability and rapid color switching rate. The Ti³⁺ species as internal sacrificial electron donors significantly improve the photoreductive activity of TiO_2-x nanoparticles, which results in fast decoloration rate and long cycling number of the PCSS. Simultaneously, the alkaline property of TiO_2-x nanoparticles enhances the oxidation kinetics of the PCSS to dramatically accelerate the recoloration rate. Moreover, the PCSS can be integrated elaborately with biodegradable agarose to form flexible color switching films, which exhibit long-waited on-demand regulation of recoloration rate in a wide range. By taking advantage of photoreversible color switching and time-resolved color changing process, we demonstrate their potential application in self-erasing rewritable paper and transient optical information encryption. This work represents a new strategy for the future development of PCSS and their advanced applications.

Organic Moiety-Regulated Photocatalytic Performance of Phosphomolybdate Hybrids for Hexavalent Chromium Reduction

Visible-light-driven photocatalytic Cr(VI) reduction is a promising pathway to moderate environmental pollution, in which the development of photocatalysts is pivotal. Herein, three hourglass-type phosphomolybdate-based hybrids with the formula of: (H₂ bpe)₃ [Zn(H₂ PO₄ )][Zn(bpe)(H₂ O)₂ ]H{Zn[P₄ Mo₆ O₃₁ H₆ ]₂ } ⋅ 6H₂ O (1) Na₆ [H₂ bz]₂ [ZnNa₄ (H₂ O)₅ ]{Zn [P₄ Mo₆ O₃₁ H₃ ]₂ } ⋅ 2H₂ O (2) and (H₂ mbpy) {[Zn(mbpy)(H₂ O)]₂ [Zn(H₂ O)]₂ }{Zn[P₄ Mo₆ O₃₁ H₆ ]₂ } ⋅ 10H₂ O (3) (bpe=trans-1,2-bi(4-pyridyl)-ethylene; bz=4,4'-diaminobiphenyl; mbpy=4,4'-dimethyl-2,2'bipyridine) were synthesized under the guidance of the functional organic moiety modification strategy. Structural analysis showed that hybrids 1-3 have similar 2D layer-like spatial arrangements constructed by {Zn[P₄ Mo₆ ]₂ } clusters and organic components with different conjugated degree. With excellent redox properties and wide visible-light absorption capacities, hybrids 1-3 display favourable photocatalytic activity for Cr(VI) reduction with 79%, 70% and 64% reduction rates, which are superior to that of only inorganic {Zn[P₄ Mo₆ ]₂ } itself (21%). The investigation of organic components on photocatalytic performance of hybrids 1-3 suggested that the organic counter cations (bpe, bz and mbpy) can effectively affect the visible-light absorption, as well as the recombination of photogenerated carriers stemmed from {Zn[P₄ Mo₆ ]₂ } clusters, further promoting their photocatalytic performances towards Cr(VI) reduction. This work provides an experimental basis for the design of functionalized photocatalysts via the modification of organic species.

Liquid-Plasma Hydrogenated Synthesis of Gray Titania with Engineered Surface Defects and Superior Photocatalytic Activity

Defect engineering in photocatalysts recently exhibits promising performances in solar-energy-driven reactions. However, defect engineering techniques developed so far rely on complicated synthesis processes and harsh experimental conditions, which seriously hinder its practical applications. In this work, we demonstrated a facile mass-production approach to synthesize gray titania with engineered surface defects. This technique just requires a simple liquid-plasma treatment under low temperature and atmospheric pressure. The in situ generation of hydrogen atoms caused by liquid plasma is responsible for hydrogenation of TiO₂. Electron paramagnetic resonance (EPR) measurements confirm the existence of surface oxygen vacancies and Ti³⁺ species in gray TiO_2−x. Both kinds of defects concentrations are well controllable and increase with the output plasma power. UV–Vis diffused reflectance spectra show that the bandgap of gray TiO_2−x is 2.9 eV. Due to its extended visible-light absorption and engineered surface defects, gray TiO_2−x exhibits superior visible-light photoactivity. Rhodamine B was used to evaluate the visible-light photodegradation performance, which shows that the removal rate constant of gray TiO_2−x reaches 0.126 min⁻¹ and is 6.5 times of P25 TiO₂. The surface defects produced by liquid-plasma hydrogenation are proved stable in air and water and could be a candidate hydrogenation strategy for other photocatalysts.

Insights into the TiO2-Based Photocatalytic Systems and Their Mechanisms

Photocatalysis is a multifunctional phenomenon that can be employed for energy applications such as H2 production, CO2 reduction into fuels, and environmental applications such as pollutant degradations, antibacterial disinfection, etc. In this direction, it is not an exaggerated fact that TiO2 is blooming in the field of photocatalysis, which is largely explored for various photocatalytic applications. The deeper understanding of TiO2 photocatalysis has led to the design of new photocatalytic materials with multiple functionalities. Accordingly, this paper exclusively reviews the recent developments in the modification of TiO2 photocatalyst towards the understanding of its photocatalytic mechanisms. These modifications generally involve the physical and chemical changes in TiO2 such as anisotropic structuring and integration with other metal oxides, plasmonic materials, carbon-based materials, etc. Such modifications essentially lead to the changes in the energy structure of TiO2 that largely boosts up the photocatalytic process via enhancing the band structure alignments, visible light absorption, carrier separation, and transportation in the system. For instance, the ability to align the band structure in TiO2 makes it suitable for multiple photocatalytic processes such as degradation of various pollutants, H2 production, CO2 conversion, etc. For these reasons, TiO2 can be realized as a prototypical photocatalyst, which paves ways to develop new photocatalytic materials in the field. In this context, this review paper sheds light into the emerging trends in TiO2 in terms of its modifications towards multifunctional photocatalytic applications.