Two-dimensional Covalent Organic Frameworks: Intrinsic Synergy Promoting Photocatalytic Hydrogen Evolution

Covalent organic frameworks(COFs) are emerging photocatalysts for hydrogen evolution in water splitting in recent years. They offer a pre-designable platform to design tailor-made structures and chemically adjustable functionality in terms of photocatalysis. In this review, we summarize the recent striking progress of COF-based photocatalysts in design and synthesis. Firstly, different approaches to functionalizing building blocks, diversifying linkages, extending π-conjugation and establishing D-A conjugation are illustrated for enhancing photocatalytic activity. Next, post-modification of backbones and pores is detailed for emphasizing the synergistic catalytic uniqueness of COFs. Besides, the strategy of preparing COF-related composites with various semiconductors is outlined for optimizing the electronic properties. Finally, we conclude with the current challenges and promising opportunities for the exploration of new COF-based photocatalysts.     

Cascaded electron transition in CuWO_4/CdS/CDs heterostructure accelerating charge separation towards enhanced photocatalytic activity

CuWO_4,as an n-type oxide semiconductor with a bandgap of 2.2 eV,has stimulated enormous interest as a potential broad-spectrum-active photocatalyst for environmental pollution remediations.However,rapid charge recombination greatly hinders its practical applications.Herein,we present a cascaded electron transition pathway in a ternary heterostructure consisting of CdS quantum dots,carbon dots(CDs) and CuWO_4 hollow spheres,which proves to greatly facilitate the photogenerated electron-hole separation,and eventually boosts the degradation efficiency of phenol and congo red by 100% and 46%compared to bare CuWO_4.The enhanced performance of the CuWO_4/CdS/CDs heterostructure mainly originates from the unidirectional electron migration from CdS to CuWO_4 and then to the organics through CDs.This work elucidates the electron transfer kinetics in multi-phase system and provides a new design paradigm for optimizing the properties of CuWO_4 based photocatalysts.     

Nanostructured BiVO4 Derived from Bi-MOF for Enhanced Visible-light Photodegradation

BiVO4,a promising visible-light responding photocatalyst,has aroused extensive research interest because of inexpensiveness and excellent chemical stability.However,its main drawback is the poor photoinduced charge-transfer dynamics.Building nanostructures is an effective way to tackle this problem.Herein,we put forward a new method to prepare nanostructured BiVO4 from Bi-based metal-organic frameworks[Bi-MOF(CAU-17)]precursor.The as-prepared material has a rod-like morphology inherited from the Bi-MOF sacrificial template and consists of small nanoparticle as building blocks.Compared with its counterparts prepared by conventional methods,MOF-derived nanostructured BiVO4 shows better light absorption ability,narrower bandgap,and improved electrical conductivity as well as reduced recombination.Consequently,BiVO4 nanostructure demonstrates high photocatalytic activity under visible light towards the degradation of methylene blue.Methylene blue can be degraded up to 90%within 30 min with a reaction rate constant of 0.058 min-1.Moreover,the cycling stability of the catalyst is excellent to withstand unchanged degradation efficiency for at least 5 cycles.     

Recent advances in round-the-clock photocatalytic system: Mechanisms,characterization techniques and applications

Solar energy-driven semiconductor photocatalysis has gathered increasing interest in the field of energy and environmental applications. However, a vital problem that limits its application is that photocatalysis requires a continuous light source to perform redox reaction. The ability of keeping catalytic activity in the dark has been the ultimate goal for the wide application of photocatalysis. More and more efforts have been paid to develop photocatalysts to perform photocatalytic reactions under both light and dark conditions, which is so called “round-the-clock photocatalytic system” (RTCPS). RTCPS with an ability of energy storage can work well under both daytime and nighttime, which widely used in the removal of heavy metal ion, the degradation of organic pollutant, disinfection and hydrogen generation. The important potential of RTCPS necessitate timely reviews of the recent advances to streamline efforts. Thus, this review aimed to summarize the recent advances in RTCPS, including the mechanism, characterization techniques and applications. Moreover, future challenge and research direction on the mechanistic study, material design and potential applications are also discussed.     

Thin film photocatalysis for environmental remediation: A status review

This review accounts, various metal oxide and metal sulfide thin films available for photodegradation of several organic compounds. Due to difficulties in recycling and to avoid rigorous recollection of powder catalysts, the thin film catalyst are gaining rapid attention for photocatalytic applications. The semiconducting thin films are growing as promising photocatalyst for water treatment. This review focuses mainly on the photocatalytic activity of metal oxide thin films in terms of its stability, charge transport and absorption properties. Thin film photocatalyst provides the increased efficiency and cost reduction of device. Furthermore, this review summarizes some key factors regarding the enhancement in photocatalytic performance of thin films.     

Photocatalytic decolourization of a new water-insoluble organic dye based on phenothiazine by ZnO and TiO2 nanoparticles

A new water-insoluble organic dye, namely, 2-((10-decyl-10H-phenothiazin-3-yl)methylene)malononitrile, was synthesized and fully characterized. It was envisioned that photocatalytic decolourization of a dye-containing long chain would pave the way for the photocatalytic remediation of wastewater containing toxic hydrophobic organic pollutants. Two commercially available nanoparticles, ZnO and TiO₂, were selected, and their photocatalytic decolourization of the dye from aqueous medium were compared. The black UV light irradiation of the colored samples in the presence of TiO₂ (P25) or ZnO resulted in their decolourization and the photocatalytic activity observed for TiO₂ (P25) was better than that of ZnO. The kinetic of decolourization indicated that the process was first-order from which the rate constant was calculated. Also, the effect of pHs on the kinetic of decolourization revealed a negligible effect, indicating that the pH, although it affects the catalysts but has no effect on the organic-based hydrophobic dye and thus no effect on the photocatalytic process.     

Recent advances in application of the graphene-based membrane for water purification

Graphene is an atomic layer thick carbon-based material with unique two-dimensional architecture and extraordinary physiochemical, optical, electrical, and mechanical properties. Graphene and its derivatives show significant promises for the development of nanoporous ultrathin filtration membranes capable of molecular separation properties. Graphene-based nanofiltration membranes featuring distinct laminar structures can offer various novel mass-transport phenomena for purifying water, energy storage and separation, gas separation, and proton conductors. The latest developments in water purification techniques through graphene-based membranes including engineering, design, and fabrication of diverse graphene, graphene-oxide, and graphene-composite membranes are provided here in relation to their application paradigm for purifying water. The critical views on pollutant removal mechanisms for water purification along with optimization measures are specially highlighted. In addition, the challenges, shortcomings, and future prospects are pointed out. The green and large-scale synthesis technology of graphene coupling with advanced membrane fabrication techniques can promote these state-of-the-art nanofiltration membranes for a wide range of applications.     

Preparation of magnetic Fe3O4/TiO2/Ag composite microspheres with enhanced photocatalytic activity

The novel three-component Fe₃O₄/TiO₂/Ag composite mircospheres were prepared via a facile chemical deposition route. The Fe₃O₄/TiO₂ mircospheres were first prepared by the solvothermal method, and then Ag nanoparticles were anchored onto the out-layer of TiO₂ by the tyrosine-reduced method. The as-prepared magnetic Fe₃O₄/TiO₂/Ag composite mircospheres were applied as photocatalysis for the photocatalytic degradation of methylene blue. The results indicate that the photocatalytic activity of Fe₃O₄/TiO₂/Ag composite microspheres is superior to that of Fe₃O₄/TiO₂ due to the dual effects of the enhanced light absorption and reduction of photoelectron–hole pair recombination in TiO₂ with the introduction of Ag NPs. Moreover, these magnetic Fe₃O₄/TiO₂/Ag composite microspheres can be completely removed from the dispersion with the help of magnetic separation and reused with little or no loss of catalytic activity.     

Photochemical synthesis: Using light to build C–C bonds under mild conditions

A review of recent advancements in metal-free arylations via photogenerated triplet aryl cations and decatungstate anion ([W₁₀O₃₂]⁴⁻) photocatalyzed C–C bond formation is reported herein. These approaches are two examples of the great potentialities of photons as green activants in organic synthesis, allowing the functionalization of different chemical substrates under mild conditions (room temperature, aqueous solvents, absence of aggressive and unstable reactants and of expensive transition metal-based catalysts, and chance to use solar light as the energy source).     

Modifications on reduced titanium dioxide photocatalysts: A review

A large variety of reduced titanium dioxide (TiO_2-x) materials have been reported recently. Reduced TiO₂, usually resulting from the removal of oxygen atoms or hydrogen incorporation, is proved to be efficient for achieving highly photocatalytic performance including photodegradation of organic compounds, hydrogen generation from water splitting, CO₂ reduction for CH₄ evolution, solar cells, etc. To further improve the properties and activities of TiO_2-x, a combination of the Ti³⁺ self-doping and other traditional modifications like nonmetals doping has been proposed in the past decades. This paper provides a general and critical review on the further modifications on reduced TiO₂ samples, including non-metal elements (N, B, S, F and I) doping, noble-metal (Au, Pt, Pd and Ag) and iron-group metal (Fe, Co and Ni) grafting, metal oxide compositing, carbon (nanotubes and graphene) and carbon-based-material compositing, special facets exposure (mainly dual {001}-{101} and {111}-{110} facets) of TiO_2-x and ordered structure controlling of TiO_2-x. These modifications enhance the physical and/or chemical properties of the reduced TiO₂, or create new features for the modified TiO_2-x samples, which finally leads to the enhancement of photocatalytic performance. Key examples such as N-doping, Au grafting and graphene-based compositing are discussed carefully, and the mechanisms for solar light enhancement, electron transfer and charge separation are also investigated. Finally, some challenging issues on TiO_2-x catalysts are also proposed to encourage new approaches for preparation of TiO_2-x catalysts with efficiently photocatalytic performance.