Effective activation of peroxymonosulfate by CoCr-LDH for removing organic contaminants in water: from lab-scale to practical applications

In this study, CoCr layered double hydroxide material (CoCr-LDH) was prepared and used as an effective catalyst for peroxymonosulfate (PMS) activation to degrade organics in water. The prepared CoCr-LDH material had a crystalline structure and relatively porous structure, as determined by various surface analyses. In Rhodamine B (RhB) removal, the most outstanding PMS activation ability belongs to the material with a Co:Cr molar ratio of 2:1. The removal of RhB follows pseudo-first-order kinetics (R2 > 0.99) with an activation energy of 38.23 kJ/mol and efficiency of 98% after 7 min of treatment, and the total organic carbon of the solution reduced 47.2% after 10 min. The activation and oxidation mechanisms were proposed and the RhB degradation pathways were suggested with the key contribution of O2•- and 1O2. Notably, CoCr-LDH can activate PMS over a wide pH range of 4 - 9, and apply to a wide range of organic pollutants and aqueous environments. The material has high stability and good recovery, which can be reused for 5 cycles with a stable efficiency of above 88%, suggesting a high potential for practical recalcitrant water treatment via PMS activation by heterogeneous catalysts.

Catalyzing Artificial Photosynthesis with TiO2 Heterostructures and Hybrids: Emerging Trends in a Classical yet Contemporary Photocatalyst

Titanium dioxide (TiO2) stands out as a versatile transition-metal oxide with applications ranging from energy conversion/storage and environmental remediation to sensors and optoelectronics. While extensively researched for these emerging applications, TiO2 has also achieved commercial success in various fields including paints, inks, pharmaceuticals, food additives, and advanced medicine. Thanks to the tunability of their structural, morphological, optical, and electronic characteristics, TiO2 nanomaterials are among the most researched engineering materials. Besides these inherent advantages, the low cost, low toxicity, and biocompatibility of TiO2 nanomaterials position them as a sustainable choice of functional materials for energy conversion. Although TiO2 is a classical photocatalyst well-known for its structural stability and high surface activity, TiO2-based photocatalysis is still an active area of research particularly in the context of catalyzing artificial photosynthesis. This review provides a comprehensive overview of the latest developments and emerging trends in TiO2 heterostructures and hybrids for artificial photosynthesis. It begins by discussing the common synthesis methods for TiO2 nanomaterials, including hydrothermal synthesis and sol-gel synthesis. It then delves into TiO2 nanomaterials and their photocatalytic mechanisms, highlighting the key advancements that have been made in recent years. The strategies to enhance the photocatalytic efficiency of TiO2, including surface modification, doping modulation, heterojunction construction, and synergy of composite materials, with a specific emphasis on their applications in artificial photosynthesis, are discussed. TiO2-based heterostructures and hybrids present exciting opportunities for catalyzing solar fuel production, organic degradation, and CO2 reduction via artificial photosynthesis. This review offers an overview of the latest trends and advancements, while also highlighting the ongoing challenges and prospects for future developments in this classical yet rapidly evolving field.

Microflowery, Microspherical, and Fan-Shaped TiO2 Crystals via Hierarchical Self-Assembly of Nanorods with Exposed Specific Crystal Facets and Enhanced Photocatalytic Performance

In this paper, khaki titanium dioxide (TiO2) crystals via hierarchical self-assembly of nanorods with different morphologies and specific exposed crystal facets were prepared for the first time by using a TiCl3 treatment process in the presence and absence of morphology-controlling agents. The crystal structure, morphology, microstructure, specific surface area, and separation efficiency of photogenerated electron-hole pairs of the synthesized TiO2 crystals were characterized. The photocatalytic and recycled performances of the synthesized TiO2 crystals in the presence of shape-controlling agents, such as ammonium sulfate (AS), ammonium carbonate (AC), and urea, and the absence of shape-controlling agents (the obtained TiO2 crystals were expressed as AS-TiO2, AC-TiO2, urea-TiO2, and No-TiO2, respectively) were evaluated and compared with the commercial TiO2 (CM-TiO2) crystals. The AS-TiO2 microspheres with exposed uncertain facets exhibited enhanced photocatalytic activity for the degradation of methylene blue solution, which can be attributed to the combined effect of the anatase phase structure, relatively larger specific surface area, and the effective separation of the photogenerated electron-holes.

Hollow Square RodLike Microtubes Composed of Anatase Nanocuboids with Coexposed {100}, {010}, and {001} Facets for Improved Photocatalytic Performance

In this study, hollow square rodlike microtubes composed of anatase nanocuboids with coexposed {100}, {010}, and {001} facets were successfully synthesized via a mild hydrothermal treatment method in the presence of NH4F by using layered H2Ti3O7 ribbons as the precursor. The precursor H2Ti3O7 ribbons were prepared from H+/Na+ ion-exchanged Na2Ti3O7. The suspension solution of protonated H2Ti3O7 ribbons was adjusted to desired pH values (0.5-13.0) prior to hydrothermal treatment. The elongated direction of the microtubes is along the b axis, according to the profile of the H2Ti3O7 ribbons. The transformation from staggered [Ti3O7]2- sheets to hollow square rodlike microtubes contained the formation and recombination of the dispersed octahedral [Ti(OH)2(OH2)4]2+ monomers, the formation and growth of the initial anatase nuclei, and the reassembly of the anatase nanocuboids along the b-axis direction during the continuous hydrothermal process. The degradation rate of pH 0.5-TiO2 was the highest at 1.66 × 10-2 min-1, which was 1.3, 1.5, 2.0, 2.3, and 18.4 folds higher than that of pH 3.0-TiO2 (1.27 × 10-2 min-1), pH 7.0-TiO2 (1.11 × 10-2 min-1), pH 5.0-TiO2 (0.83 × 10-2 min-1), P25-TiO2 (0.73 × 10-2 min-1), and the blank sample (0.09 × 10-2 min-1), respectively. Compared with P25-TiO2 and the other anatase TiO2 samples, pH 0.5-TiO2 exhibited the best photocatalytic activity, which was mainly attributed to its larger proportion of {010} (or {100}) facets, smaller crystalline size, higher band gap, and larger specific surface area.

Microwave-Assisted Synthesis of High-Energy Faceted TiO2 Nanocrystals Derived from Exfoliated Porous Metatitanic Acid Nanosheets with Improved Photocatalytic and Photovoltaic Performance

A facile one-pot microwave-assisted hydrothermal synthesis of rutile TiO₂ quadrangular prisms with dominant {110} facets, anatase TiO₂ nanorods and square nanoprisms with co-exposed {101}/[111] facets, anatase TiO₂ nanorhombuses with co-exposed {101}/{010} facets, and anatase TiO₂ nanospindles with dominant {010} facets were reported through the use of exfoliated porous metatitanic acid nanosheets as a precursor. The nanostructures and the formation reaction mechanism of the obtained rutile and anatase TiO₂ nanocrystals from the delaminated nanosheets were investigated. The transformation from the exfoliated metatitanic nanosheets with distorted hexagonal cavities to TiO₂ nanocrystals involved a dissolution reaction of the nanosheets, nucleation of the primary [TiO₆]⁸⁻ monomers, and the growth of rutile-type and anatase-type TiO₂ nuclei during the microwave-assisted hydrothermal reaction. In addition, the photocatalytic activities of the as-prepared anatase nanocrystals were evaluated through the photocatalytic degradation of typical carcinogenic and mutagenic methyl orange (MO) under UV-light irradiation at a normal temperature and pressure. Furthermore, the dye-sensitized solar cell (DSSC) performance of the synthesized anatase TiO₂ nanocrystals with various morphologies and crystal facets was also characterized. The {101}/[111]-faceted pH2.5-T175 nanocrystal showed the highest photocatalytic and photovoltaic performance compared to the other TiO₂ samples, which could be attributed mainly to its minimum particle size and maximum specific surface area.

Facile Formation of Anatase/Rutile TiO2 Nanocomposites with Enhanced Photocatalytic Activity

Anatase/rutile mixed-phase TiO₂ nanoparticles were synthesized through a simple sol-gel route with further calcination using inexpensive titanium tetrachloride as a titanium source, which effectively reduces the production cost. The structural and optical properties of the prepared materials were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and UV-vis adsorption. The specific surface area was also analyzed by Brunauer-Emmett-Teller (BET) method. The anatase/rutile mixed-phase TiO₂ nanocomposites containing of rod-like, cuboid, and some irregularly shaped anatase nanoparticles (exposed {101} facets) with sizes ranging from tens to more than 100 nanometers, and rod-like rutile nanoparticles (exposed {110} facets) with sizes ranging from tens to more than 100 nanometers. The photocatalytic activities of the obtained anatase/rutile mixed-phase TiO₂ nanoparticles were investigated and compared by evaluating the degradation of hazardous dye methylene blue (MB) under ultraviolet light illumination. Compared to the commercial Degussa P25-TiO₂, the mixed-phase TiO₂ nanocomposites show better photocatalytic activity, which can be attributed to the optimal anatase to rutile ratio and the specific exposed crystal surface on the surface. The anatase/rutile TiO₂ nanocomposites obtained at pH 1.0 (pH1.0-TiO₂) show the best photocatalytic activity, which can be attributed to the optimal heterojunction structure, the smaller average particle size, and the presence of a specific exposed crystal surface. The enhanced photocatalytic activity makes the prepared anatase/rutile TiO₂ photocatalysts a potential candidate in the removal of the organic dyes from colored wastewater.

Synthesis of Au-decorated three-phase-mixed TiO2/phosphate modified active carbon nanocomposites as easily-recycled efficient photocatalysts for degrading high-concentration 2,4-DCP

It is of great significance to fabricate easily-recycled TiO₂ photocatalysts with high activity. Herein, dominant-anatase three-phase (anatase/rutile/brookite)-mixed nanosized TiO₂ with high photocatalytic activity for degrading a high-concentration of 2,4-DCP has been synthesized via a hydrothermal process with HCl as a phase-directing agent, and interestingly the apparent photoactivity could be greatly improved by decorating Au nanoparticles and then coupling phosphate-treated active carbon. The amount-optimized nanocomposite displays ∼12-fold enhancement in degradation rate constant (k) compared to anatase TiO₂. Based on the steady-state surface photovoltage spectra, fluorescence spectra related to the produced ·OH amount, temperature-programmed desorption and O₂ electrochemical reduction curves, it is confirmed that the exceptional photoactivity is mainly attributed to the greatly-enhanced charge separation from the phase-mixed composition, and from the decorated Au as electron acceptors and its promotion effects on O₂ activation. Moreover, the use of phosphate-modified AC as a support is also positive for efficient photocatalytic reactions by accepting electrons and concentrating the pollutants, with recyclable features. This work provides a feasible strategy to fabricate TiO₂-based nano-photocatalysts for degrading high-concentration pollutants to remediate the environment.

Au decorated three-phase-mixed nanosized TiO₂ coupled with phosphate-treated AC as recyclable nanocomposite photocatalysts exhibit excellent photoactivity for degrading high-concentration 2, 4-DCP, mainly due to the improved charge separation and specific surface area.

Doubling the photocatalytic performance of SnO2 by carbon coating mixed-phase particles

Carbon effectively facilitated the charge transfer and separation between the tetragonal and orthorhombic nanoparticles in mixed-phase SnO₂.

A general nonaqueous sol-gel route to g-C3N4-coupling photocatalysts: the case of Z-scheme g-C3N4/TiO2 with enhanced photodegradation toward RhB under visible-light

The g-C₃N₄-coupling TiO₂ photocatalysts with controllable particle size as well as the interface contact were prepared by a general nonaqueous sol-gel method. The structural and morphological features of g-C₃N₄/TiO₂ were investigated through the X-ray diffraction, Fourier transformed infrared spectra, scanning electron microscopy and transmission electron microscopy, respectively. It is found the TiO₂ nanoparticles with a size of 7.3 ± 1.6 nm are uniformly anchored on the surface of the g-C₃N₄ nanosheets in isolation. The photocatalytic properties of as-prepared g-C₃N₄/TiO₂ were tested by degradation of Rhodamine B (RhB) under visible light, and an enhanced activity is observed. The mechanism of the enhanced activity was further investigated through N₂ adsorption-desorption isotherms, UV-vis spectra, photoluminescence spectra, photoelectrochemical measurements, radical trapping experiments and X-ray photoelectron spectroscopy. Furthermore, the photocatalytic performances of obtained g-C₃N₄/TiO₂ under sunlight were also evaluated in aspects of degradation efficiency and stability. The results indicate that the obtained g-C₃N₄/TiO₂ is one promising photocatalyst for practical applications. The study of as-prepared g-C₃N₄/TiO₂ also implies that the present method could be a general route of g-C₃N₄-coupling photocatalysts.