Huayang Zhang

Recent advances in non-metal modification of graphitic carbon nitride for photocatalysis: a historic review

Photocatalysis is a green, feasible and versatile technology that has been widely used for energy conversion and environmental applications. As photocatalysis bears a great potential for solar energy utilization, enormous investigations have been implemented in the past decades. The fundamental mechanism and some applications were well addressed in the last century. Currently, the major focus in photocatalysis research is the design and development of photocatalyst materials. This review firstly introduces the historic milestones in photocatalysis studies and then a comprehensive survey is conducted on the metal-based photocatalysts, including TiO2-based photocatalysts, ZnO and other metal oxides, metal sulfides, metal nitrides, and plasmon photocatalysts. From a historical viewpoint, particular attention is paid to metal-free graphitic carbon nitride (g-C3N4), a novel visible-light photocatalyst. Various modification techniques for g-C3N4 are summarized and analyzed. In terms of its metal-free nature, the fabrication of a porous structure, shape-control synthesis and non-metal doping are discussed in detail. Photocatalytic studies on g-C3N4-based catalysts are introduced. Some emerging elemental photocatalysts are also introduced. Finally, perspectives on non-metal photocatalyst design and development are provided.

Nitrogen- and Sulfur-Codoped Hierarchically Porous Carbon for Adsorptive and Oxidative Removal of Pharmaceutical Contaminants

Heteroatom (nitrogen and sulfur)-codoped porous carbons (N-S-PCs) with high surface areas and hierarchically porous structures were successfully synthesized via direct pyrolysis of a mixture of glucose, sodium bicarbonate, and thiourea. The resulting N-S-PCs exhibit excellent adsorption abilities and are highly efficient for potassium persulfate activation when employed as catalysts for the oxidative degradation of sulfachloropyridazine (SCP) solutions. The adsorption capacities of N-S-PC-2 (which contains 4.51 atom % nitrogen and 0.22 atom % sulfur and exhibits SBET of 1608 m(2) g(-1)) are 73, 7, and 3 times higher than those of graphene oxide, reduced graphene oxide, and commercial single-walled carbon nanotube, respectively. For oxidation, the reaction rate constant of N-S-PC-2 is 0.28 min(-1). This approach not only contributes to the large-scale production and application of high-quality catalysts in water remediation but also provides an innovative strategy for the production of heteroatom-doped PCs for energy applications.

Flower-like Cobalt Hydroxide/Oxide on Graphitic Carbon Nitride for Visible-Light-Driven Water Oxidation

Direct water oxidation via photocatalysis is a four-electron and multiple-proton process which requires high extra energy input to produce free dioxygen gas, making it exacting, especially under visible light irradiation. To improve the oxygen evolution reaction rates (OERs) and utilize more visible light, flower-like cobalt hydroxide/oxide (Fw-Co(OH)2/Fw-Co3O4) photocatalysts were prepared and loaded onto graphitic carbon nitride (g-C3N4) by a facile coating method in this work. Influenced by the unique three-dimensional morphologies, the synthesized Fw-Co(OH)2 or Fw-Co3O4/g-C3N4 hybrids reveal favorable combination and synergism reflected by the modified photoelectric activities and the improved OER performances. Attributed to its prominent hydrotalcite structure, Fw-Co(OH)2 shows better cocatalytic activity for g-C3N4 modification compared with that of Fw-Co3O4. Specifically, 7 wt % Fw-Co(OH)2/g-C3N4 photocatalyst exhibits photocurrent density 4 times higher and OER performance 5 times better than pristine g-C3N4. This work unambiguously promotes the application of sustainable g-C3N4 in water oxidation.

Ferric carbide nanocrystals encapsulated in nitrogen-doped carbon nanotubes as an outstanding environmental catalyst

Nitrogen-doped carbon nanotubes encapsulating iron carbide (Fe3C) nanocrystals (Fe3C@NCNT) were fabricated by a simple and direct pyrolysis method using melamine and ferric chloride as the C, N and Fe precursors. The surface morphology, structure and composition of the Fe3C@NCNT materials were thoroughly investigated. The nanomaterials were employed as novel catalysts for peroxymonosulfate (PMS) activation; outstanding efficiency, high stability and excellent reusability were observed in the catalytic oxidation of organics. The encapsulated Fe3C nanoparticles played a key role in the emerging synergetic effects of the carbide and the protective graphitic layers. In addition, the quaternary N and trace amounts of iron on the CNT surface acted as the active sites. Various quenching experiments were carried out to elucidate the catalytic mechanism of Fe3C@NCNT. It was found that singlet oxygen, superoxide, sulfate and hydroxyl radicals worked together to degrade phenol solutions. Due to their simple synthesis method, low-cost precursors, unique structure and excellent catalytic activity and stability, these novel iron-carbide-based composites have great potential as new strategic materials for environmental catalysis.

Preparation of a p-n heterojunction BiFeO3@TiO2 photocatalyst with a core–shell structure for visible-light photocatalytic degradation

Abstract Magnetically separable bismuth ferrite (BiFeO3) nanoparticles were fabricated by a citrate self-combustion method and coated with titanium dioxide (TiO2) by hydrolysis of titanium butoxide (Ti(OBu)4) to form BiFeO3@TiO2 core–shell nanocomposites with different mass ratios of TiO2 to BiFeO3. The photocatalytic performance of the catalysts was comprehensively investigated via photocatalytic oxidation of methyl violet (MV) under both ultraviolet and visible-light irradiation. The BiFeO3@TiO2 samples exhibited better photocatalytic performance than either BiFeO3 or TiO2 alone, and a BiFeO3@TiO2 sample with a mass ratio of 1:1 and TiO2 shell thickness of 50–100 nm showed the highest photo-oxidation activity of the catalysts. The enhanced photocatalytic activity was ascribed to the formation of a p-n junction of BiFeO3 and TiO2 with high charge separation efficiency as well as strong light absorption ability. Photoelectrochemical Mott–Schottky (MS) measurements revealed that both the charge carrier transportation and donor density of BiFeO3 were markedly enhanced after introduction of TiO2. The mechanism of MV degradation is mainly attributed to hydroxyl radicals and photogenerated electrons based on energy band theory and the formation of an internal electrostatic field. In addition, the unique core–shell structure of BiFeO3@TiO2 also promotes charge transfer at the BiFeO3/TiO2 interface by increasing the contact area between BiFeO3 and TiO2. Finally, the photocatalytic activity of BiFeO3@TiO2 was further confirmed by degradation of other industrial dyes under visible-light irradiation.

Three-Dimensional BiOI/BiOX (X = Cl or Br) Nanohybrids for Enhanced Visible-Light Photocatalytic Activity

Three-dimensional flower-like BiOI/BiOX (X = Br or Cl) hybrids were synthesized via a facile one-pot solvothermal approach. With systematic characterizations by X-ray diffraction (XRD), scanning electron microscopy (SEM), Transmission electron microscopy (TEM), the Brunauer-Emmett-Teller (BET)specific surface area, X-ray photoelectron spectroscopy (XPS), and the UV-Vis diffuse reflectance spectra (DRS), the BiOI/BiOCl composites showed a fluffy and porous 3-D architecture with a large specific surface area (SSA) and high capability for light absorption. Among all the BiOX (X = Cl, Br, I) and BiOI/BiOX (X = Cl or Br) composites, BiOI/BiOCl stands out as the most efficient photocatalyst under both visible and UV light irradiations for methyl orange (MO) oxidation. The reaction rate of MO degradation on BiOI/BiOCl was 2.1 times higher than that on pure BiOI under visible light. Moreover, BiOI/BiOCl exhibited enhanced water oxidation efficiency for O2 evolution which was 1.5 times higher than BiOI. The enhancement of photocatalytic activity could be attributed to the formation of a heterojunction between BiOI and BiOCl, with a nanoporous structure, a larger SSA, and a stronger light absorbance capacity especially in the visible-light region. The in situ electron paramagnetic resonance (EPR) revealed that BiOI/BiOCl composites could effectively evolve superoxide radicals and hydroxyl radicals for photodegradation, and the superoxide radicals are the dominant reactive species. The superb photocatalytic activity of BiOI/BiOCl could be utilized for the degradation of various industrial dyes under natural sunlight irradiation which is of high significance for the remediation of industrial wastewater in the future.

Singlet oxygen formation in bio-inspired synthesis of a hollow Ag@AgBr photocatalyst for microbial and chemical decontamination

Hollow Ag@AgBr nanospheres were prepared using a greener version of a microbial process starting with AgCl colloid formation in a yeast extract (YE) and peptone solution. The Kirkendall effect has been exploited to obtain the unique Ag@AgBr heterostructures for excellent solar light-induced oxidizing properties against both microbial and chemical contaminants. Singlet oxygen generation was confirmed by electron spin resonance studies.

Crystal transformation of 2D tungstic acid H 2 WO 4 to WO 3 for enhanced photocatalytic water oxidation

New photocatalytic materials for stable reduction and/or oxidization of water by harvesting a wider range of visible light are indispensable to achieve high practical efficiency in artificial photosynthesis. In this work, we prepared 2D WO3·H2O and WO3 nanosheets by a one-pot hydrothermal method and sequent calcination, focusing on the effects of crystal transformation on band structure and photocatalytic performance for photocatalytic water oxidation in the presence of electron acceptors (Ag+) under simulated solar light irradiation. The as-prepared WO3 nanosheets exhibit enhanced rate of photocatalytic water oxidation, which is 6.3 and 3.6 times higher than that of WO3·H2O nanosheets and commercial WO3, respectively. It is demonstrated that the releasing of water molecules in the crystal phase of tungstic acid results in transformation of the crystal phase from orthorhombic WO3·H2O to monoclinic WO3, significantly improving the activity of photocatalytic water oxidation in the presence of Ag+ because the shift-up of conduction band of WO3 matches well with the electrode potential of Ag+/Ag(s), leading to efficient separation of photoinduced electrons and holes in pure WO3 nanosheets.

Metal-free hybrids of graphitic carbon nitride and nanodiamonds for photoelectrochemical and photocatalytic applications

Graphitic carbon nitride (g-C3N4) has been considered as a metal-free, cost-effective, eco-friendly and efficient catalyst for various photoelectrochemical applications. However, compared to conventional metal-based photocatalysts, its photocatalytic activity is still low because of the low mobility of carriers restricted by the polymer nature. Herein, a series of hybrids of g-C3N4 (GCN) and nanodiamonds (NDs) were synthesized using a solvothermal method. The photoelectrochemical performance and photocatalytic efficiency of the GCN/NDs were investigated by means of the generation of photocurrent and photodegradation of methylene blue (MB) solutions under UV-visible light irradiations. In this study, the sample of GCN/ND-33% derived from 0.1g GCN and 0.05g NDs displayed the highest photocatalytic activity and the strongest photocurrent density. The mechanism of enhanced photoelectrochemical and photocatalytic performances was also discussed.

Heterostructured WO3@CoWO4 bilayer nanosheets for enhanced visible-light photo, electro and photoelectro-chemical oxidation of water

Herein, a facile interface-induced synthesis method is first established to newly fabricate two-dimensional (2D) bilayer nanosheets of WO3@CoWO4 as highly efficient catalysts for enhanced photo, electro and photoelectro-chemical oxygen evolution reactions (OERs). The heterostructure and the interfacial oxygen vacancy of WO3@CoWO4 reduce the energy barriers in the OER. Density functional theory (DFT) calculations and material characterizations reveal that the WO3@CoWO4 p–n heterojunction endows the composite with a narrowed band gap for better visible-light harvesting, rapid charge transfer across the interface and a lower recombination rate of the photo-excited carriers. The interface O-vacancy vests the active Co site with an enhanced density of state (DOS) at the valence band maximum (VBM), which can increase the concentration of the photogenerated holes to improve photocatalytic and photoelectrochemical (PEC) activity. This study presents a proof-of-concept design towards low cost and multi-metal 2D/2D nanosheets for water oxidation applications.