Microstructure and photocatalytic activity of electrospun carbon nanofibers decorated by TiO2 nanoparticles from hydrothermal reaction/blended spinning

Recently, carbon nanofibers@TiO₂ (CNFs@TiO₂) composites as photocatalysts for dye degradation have attracted intense attention. However, only few contributions had been made to investigate systematically the differences between the various preparation approaches and the influence of thermal treatment on the photocatalytic activity. In this work, the electrospun CNFs@TiO₂ composites which were prepared by hydrothermal reaction and blended spinning, respectively, have been fabricated via stabilization in air at 280 °C and then carbonization in N₂ at heat treatment temperature between 500 and 1100 °C. The composites which were prepared by hydrothermal reaction and blended spinning showed the outstanding photocatalytic activity at 900 °C and 1100 °C, respectively. And the photocatalytic activity of composites prepared by hydrothermal reaction was higher than that prepared by blended spinning, but reversibility of the composites showed a reverse trend. These results indicated that the effect of heat treatment temperature on the photocatalytic activity depended on the synergistic effect among the adsorptive property of CNFs, TiO₂ loading amount and anatase phase content in composites. Hence, combining the merits of hydrothermal reaction and blended spinning, a novel method for preparing CNFs@TiO₂ composites with high TiO₂ loading amount and strong interfacial interaction could be envisioned.     

C60/graphene/g-C3N4 composite photocatalyst and mutually- reinforcing synergy to improve hydrogen production in splitting water under visible light radiation

g-C₃N₄ as a new metal-free photocatalytic material for water splitting has attracted much attention in recent years, but its photocatalytic efficiency needs further improvement. Here we synthesized novel C₆₀/graphene/g-C₃N₄ composite photocatalytic materials with high hydrogen generation ability for water splitting under visible light radiation (λ>420 nm). These materials take full advantage of the electron conduction expressing of graphene and the superior-strong electron-attracting ability of C₆₀. The mutually-reinforcing synergy between graphene and C₆₀ improves the migration and utilization efficiency of photo-generated electrons and accelerates the separation of photo-generated charges, thus significantly enhancing the hydrogen generation capacity of g-C₃N₄. The hydrogen production amount and rate of C₆₀/graphene/g-C₃N₄ (10 mg/L C₆₀ and graphene) after 10 h are 5449.5 µmol/g and 545 µmol/g/h, which is 539.6 times of pure g-C₃N₄ under the same condition. The values are 50.8 and 4.24 times of graphene/g-C₃N₄ (10 mg/L graphene) and C₆₀/g-C₃N₄ (10 mg/L C₆₀), respectively. The apparent quantum yield of C₆₀/graphene/g-C₃N₄ (10 mg/L C₆₀ and graphene) in 97 h is about 7.2%. The improvement of hydrogen generation activity in 97 h suggests the high long-time stability of C₆₀/graphene/g-C₃N₄ in photocatalytic water spitting. The photocatalytic ability of C₆₀/graphene/g-C₃N₄ can be controlled by regulating the addition of graphene and C₆₀. The mutually-reinforcing synergy between graphene and C₆₀ was proved by X-ray photoelectron spectroscopy, photoluminescence spectrum and organic electron acceptors of MV²⁺. Thus, the joint action of C₆₀ and graphene promotes the migration, separation and utilization of photo-generated electrons, which is responsible for the significant enhancement of photocatalytic performance.     

Novel 3D flowerlike BiOCl0.7Br0.3 microspheres coupled with graphene sheets with enhanced visible-light photocatalytic activity for the degradation of rhodamine B

Highly efficient visible-light-driven 3D flowerlike BiOCl_0.7Br_0.3 microspheres coupled with graphene sheets with different graphene contents have been synthesized by a facile solvothermal process. The as-prepared samples were characterized by power X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), N₂ adsorption–desorption analysis and UV–vis diffuse reflectance spectra (DRS). Characterization results showed BiOCl_0.7Br_0.3 microspheres were composed of numerous nanoplates with a thickness of about 20 nm and dispersed uniformly on the surface of graphene. Moreover, the photocatalytic activities of these BiOCl_0.7Br_0.3/graphene composites under visible-light irradiation (λ>420 nm) were evaluated by the degradation of rhodamine B (RhB). The results indicate all BiOCl_0.7Br_0.3/graphene composites exhibited much higher photocatalytic activities than pristine BiOCl_0.7Br_0.3, pure BiOBr and BiOCl, and the highest activity was reached by the BiOCl_0.7Br_0.3/graphene composite photocatalyst with 10 wt% of graphene. The enhanced photocatalytic activity could be largely ascribed to more effective charge transportations and separations, larger BET surface areas and the increased light absorption. In addition, a possible photocatalytic mechanism of the BiOCl_0.7Br_0.3/graphene composites on basis of the experimental results was also proposed.     

Synthesis and properties of AlN/MAS/Si3N4 ternary glass-ceramic composites with in-situ grown rod-like β-Si3N4 crystals

The AlN/MAS/Si₃N₄ ternary composites with in-situ grown rod-like β-Si₃N₄ were obtained by a two-step sintering process. The microstructure analysis, compositional investigation as well as properties characterization have been systematically performed. The AlN/MAS/Si₃N₄ ternary composites can be densified at 1650 °C in nitrogen atmosphere. The in-situ grown rod-like β-Si₃N₄ grains are beneficial to the improvement of thermal, mechanical, and dielectric properties. The thermal conductivity of the composites was increased from 14.85 to 28.45 W/(m K) by incorporating 25 wt% α-Si₃N₄. The microstructural characterization shows that the in-situ growth of rod-like β-Si₃N₄ crystals leads to high thermal conductivity. The AlN/MAS/Si₃N₄ ternary composite with the highest thermal conductivity shows a low relative dielectric constant of 6.2, a low dielectric loss of 0.0017, a high bending strength of 325 MPa, a high fracture toughness of 4.1 MPa m^1/2, and a low thermal expansion coefficient (α_{25–300 °C}) of 5.11 × 10^?6/K. This ternary composite with excellent comprehensive performance is expected to be used in high-performance electronic packaging materials.     

Microstructures and EMI shielding properties of composite ceramics reinforced with carbon nanowires and nanowires-nanotubes hybrid

Carbon/ceramic composites are promising candidates as electromagnetic interference (EMI) shielding materials used at various harsh environments. The aim of present work is to prepare and investigate two kinds of composite ceramics reinforced with carbon nanowires (CNWs) and nanowires-nanotubes (CNWs-CNTs) hybrid, respectively. Results indicate that CNWs is highly curved and multi-defected, and CNWs-CNTs hybrid shows the best crystal structure at an optimal catalyst concentration of 5 wt%. When CNWs accounts for 5.15 wt%, the total shielding effectiveness (SE) of CNWs/Si₃N₄ reaches 25.0 dB with absorbed SE of 21.3 dB, meaning that 99.7% incident signal can be blocked, while it reaches 25.4 dB for CNWs-CNTs/Si₃N₄ as the carbon loading only increasing to 3.91 wt%. By contrast, CNWs/Si₃N₄ exhibits better electromagnetic attenuation capability with stronger absorption, mainly due to the unique microstructure of CNWs. Both of two composite ceramics have great potential to be designed as structural and multi-functional materials.     

Fabrication of carbon fiber reinforced ceramic matrix composites with improved oxidation resistance using boron as active filler

Boron was introduced into C_f/SiC composites as active filler to shorten the processing time of PIP process and improve the oxidation resistance of composites. When heat-treated at 1800 °C in N₂ for 1 h, the density of composites with boron (C_f/SiC-BN) increased from 1.71 to 1.78 g/cm³, while that of composites without boron (C_f/SiC) decreased from 1.92 to 1.77 g/cm³. So when boron was used, two cycles of polymer impregnation and pyrolysis (PIP) could be reduced. Meanwhile, the oxidation resistance of composites was greatly improved with the incorporation of boron-bearing species. Most carbon fiber reinforcements in C_f/SiC composite were burnt off when they were oxidized at 800 °C for 10 h. By contrast, only a small amount of carbon fibers in C_f/SiC-BN composite were burnt off. Weight losses for C_f/SiC composite and C_f/SiC-BN composite were about 36 and 16 wt%, respectively.     

Influence of the structure of carbon onions on their electrochemical performance in supercapacitor electrodes

Onion-like carbon (OLC), also known as carbon onions, is an attractive material for electrical energy storage in regards to high rate, high power applications. We report the most up to date, systematic, and extensive study of the electrochemical behavior of carbon onions in aqueous (1 M sulfuric acid, H₂SO₄) and organic (1 M tetraethylammonium tetrafluoroborate, TEA-BF₄, and 1 M tetrabutylammonium tetrafluoroborate, TBA-BF₄, in acetonitrile) electrolytes. The physical and electrical properties of OLC are studied as a function of the synthesis temperature and compared with diamond soot, carbon black, and activated carbon. To obtain a molecular scale picture of the processes at the OLC-electrolyte interface, we supplement the experimental work with molecular dynamics (MD) simulations of carbon onions in organic electrolytes. The capacitive performance of OLC exceeds other carbon materials at high charge/discharge rates (up to 50 V s^?1; time constant τ ～ 10 ms). OLC produced from detonation soot has a performance similar to that of OLC from highly purified nanodiamond. While OLC produced at 1500 °C has the largest specific surface area, OLC produced at 1800 °C has the highest conductivity and shows the best capacitive performance at high rates.     

A novel TiO2 − xNx/BN composite photocatalyst: Synthesis,characterization and enhanced photocatalytic activity for Rhodamine B degradation under visible light

A novel TiO_2 − xN_x/BN composite photocatalyst was prepared via a facile method using melamine–boron acid adducts (M·2B) and tetrabutyl titanate as reactants. The morphological results confirmed that nitrogen-doped TiO₂ nanoparticles were uniformly coated on the surface of porous BN fibers. A red shift of absorption edge from 400 nm (pure TiO₂) to 520 nm (TiO_2 − xN_x/BN composites) was observed in their UV–Vis light absorption spectra. The TiO_2 − xN_x/BN photocatalysts exhibited enhanced photocatalytic activity for the degradation of Rhodamine B (RhB) and the highest photocatalytic degradation efficiency reached 97.8% under visible light irradiation for 40 min. The mechanism of enhanced photocatalytic activity was finally proposed.     

Novel visible light-induced g-C3N4–Sb2S3/Sb4O5Cl2 composite photocatalysts for efficient degradation of methyl orange

A series of g-C₃N₄–Sb₂S₃/Sb₄O₅Cl₂ (SCL-CX) composite photocatalysts were successfully prepared via a hydrothermal method. The as-prepared materials were characterized by TM3000, powder X-ray diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS) and UV–vis diffuse reflectance spectra (UV–vis DRS). The obtained photocatalyst showed higher photocatalytic activity than pure g-C₃N₄, Sb₄O₅Cl₂ and Sb₂S₃/Sb₄O₅Cl₂ (SCL). The optimum photocatalytic of the composite with the mass of 170 mg g-C₃N₄ and a degradation efficiency up to 95% for methyl orange (MO) under visible light was achieved within 60 min. The enhanced photocatalytic performance could be attributed to the stronger absorption in the visible region and the more efficient electron–hole separation.     

Extraordinary toughening enhancement and flexural strength in Si3N4 composites using graphene sheets

Two types of Si₃N₄ composites containing graphene nanostructures using two different graphene sources, pristine graphene nanoplatelets and graphene oxide layers were produced by Spark Plasma Sintering. The maximum toughness of 10.4 MPa m^1/2, measured by flexure testing of pre-cracked bars, was achieved for a composite (∼60β/40α-Si₃N₄, ∼300 nm grain size) with 4 vol.% of reduced graphene oxide, indicating a toughening enhancement of 135% when compared to a similar Si₃N₄. This was also accompanied by a 10% increase in flexure strength (1040 MPa). For the composites with thicker graphene nanoplateletes only a 40% of toughness increase (6.6 MPa m^1/2) without strength improvement was observed for the same filler content. The large difference in the maximum toughness values accomplished for both types of composites was attributed to variations in the graphene/Si₃N₄ interface characteristics and the extent of monolayer graphene exfoliation.     

The Microstructure and thermal diffusivity of carbon nanostructures/Si3N4 composites processed using spark plasma sintering

Graphene nanoribbons (GNRs) were obtained by unzipping multiwall carbon nanotubes (MWCNTs). Three different silicon nitride-carbon nanostructures were prepared by spark plasma sintering (SPS): ceramic composites that contained 1 wt% carbon nanofibers (CNFs), 1 wt% MWCNTs and 1 wt% GNRs respectively. The α to β-Si₃N₄ transformation ratio and thermal diffusivity of GNR/Si₃N₄ composites were higher than both CNF/Si₃N₄ composites and MWCNT/Si₃N₄ composites. Furthermore, the higher thermal diffusivities of GNR/Si₃N₄ composites can primarily be attributed to the higher number of elongate β-Si₃N₄ grains.     

Enhanced visible-light photocatalytic properties of g-C3N4 by coupling with ZnAl2O4

A series of g-C₃N₄/ZnAl₂O₄ composites were prepared using a conventional calcination method and the heterostructures were systematically characterized. It was found that the combination of g-C₃N₄ with ZnAl₂O₄ significantly improve their photocatalytic activities. The optimum photocatalyst of composite is at 5% (wt%) of ZnAl₂O₄, whose degradation efficiency for methyl orange (MO) was 96% within 120 min under visible-light irradiation. The formation of heterojunction between g-C₃N₄ and ZnAl₂O₄ can facilitate efficient charge separation of photogenerated electron-hole pairs, which were confirmed by electrochemical impedance spectroscopy (EIS). As a result, the photocatalytic properties of composites were enhanced.     

Synthesis of Mo-doped graphitic carbon nitride catalysts and their photocatalytic activity in the reduction of CO2 with H2O

Molybdenum doped graphitic carbon nitride (g-C₃N₄) catalysts were prepared by a simple pyrolysis method using melamine and ammonium molybdate as precursors. The characterization results indicated that the obtained Mo-doped g-C₃N₄ catalysts had worm-like mesostructures with higher surface area. Introduction of Mo species can effectively extend the spectral response property and reduce the recombination rate of photogenerated electrons and holes. CO₂ photocatalytic reduction tests showed that the Mo-doped g-C₃N₄ catalysts exhibited considerably higher activity (the highest CO and CH₄ yields of 887 and 123 μmol g^− 1-cat., respectively, after 8 h of UV irradiation.) compared with pure g-C₃N₄ from melamine.     

Effect of carbon doping on WO3/TiO2 coupled oxide and its photocatalytic activity on diclofenac degradation

The improved photocatalyst carbon-doped WO₃/TiO₂ mixed oxide was synthesized in this study using the sol–gel method. The catalyst was thoroughly characterized by X-ray diffraction (XRD), diffuse reflectance UV–vis spectroscopy, N₂ adsorption desorption analysis, scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM/EDX), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The photocatalytic efficiency of the prepared materials was evaluated with respect to the degradation of sodium diclofenac (DCF) in a batch reactor irradiated under simulated solar light. The progress of the degradation process of the drug was evaluated by high-performance liquid chromatography (HPLC), whereas mineralization was monitored by total organic carbon analysis (TOC) and ion chromatography (IC). The results of the photocatalytic evaluation indicated that the modified catalyst with tungsten and carbon (TWC) exhibited higher photocatalytic activity than TiO₂ (T) and WO₃/TiO₂ (TW) in the degradation and mineralization of diclofenac (TWC>TW>T). Complete degradation of diclofenac occurred at 250 kJ m⁻² of accumulated energy, whereas 82.4% mineralization at 400 kJ m⁻² was achieved using the photocatalytic system WO₃/TiO₂-C. The improvement in the photocatalytic activity was attributed to the synergistic effect between carbon and WO₃ incorporated into the TiO₂ structure.     

Preparation of Cu2O/exfoliated graphite composites with high visible light photocatalytic performance and stability

Cu₂O/exfoliated graphite composites (Cu₂O/EG (1 wt%), Cu₂O/EG (4 wt%), Cu₂O/EG (7 wt%), Cu₂O/EG (10 wt%), and Cu₂O/EG (15 wt%)) were prepared by the precipitation method. The photocatalytic activity of the material was evaluated using the decolorization of methyl orange (MO) solution as model reaction. Results showed that Cu₂O deposited on the worm-like flakes of EG in the form of nanocrystals. The EG provided a three-dimensional environment for photocatalytic reaction and endowed a high adsorption capacity for the sample. Under optimal conditions, the decolorization efficiencies of MO for 60 min reached 96.7%. Recycling of the catalyst showed Cu₂O/EG composites (10 wt%) to possess high photocatalytic efficiency even when repeatedly used for five times.     

Fabrication of Si3N4-based composite containing needle-like TiN synthesized using NH3 nitridation of TiO2 nanofiber

A Si₃N₄ composite containing needle-like TiN particles (7 vol%) was fabricated. Needle-like TiN particles several micrometers long were synthesized using NH₃ nitridation of TiO₂ nanofiber, which was obtained using hydrothermal treatment. A mixed powder of α-Si₃N₄ and the needle-like TiN particles with additives was hot pressed at 24 MPa and 1850 °C for 1 h in N₂ atmosphere. Mechanical properties of the composite were compared with those of a composite containing rounded TiN particles and a monolithic β-Si₃N₄ ceramic. The Si₃N₄ matrix of the composites containing TiN was mainly a-phase, suggesting that the α–β phase transformation of Si₃N₄ was inhibited by the presence of TiN. Although fracture strength of the composites was lower, fracture toughness was comparable to that of monolithic β-Si₃N₄ ceramics. Hardness of the composites was about 19 GPa and was greater than that of the monolithic β-Si₃N₄ ceramic.     

Microstructure and fracture toughness of Si3N4 + graphene platelet composites

Silicon nitride + 1 wt% graphene platelet composites were prepared using various graphene platelets (GPLs) as filler. The influence of the addition of GPLs on the microstructure development and on the fracture toughness of Si₃N₄ + GPLs composites was investigated. The GPLs with thickness from 5 nm to 50 nm are relatively homogeneously distributed in the matrix of all composites, however overlapping/bundle formation of GPLs was found, containing 2–4 platelets as well. The single GPLs and overlapped GPLs are located at the boundaries of Si₃N₄, and hinder the grain growth and change the shape of the grains. The fracture toughness was significantly higher for all composites in comparison to the monolithic Si₃N₄ with the highest value of 9.9 MPa m^0.5 for the composite containing the GPLs with smallest dimension. The main toughening mechanisms originated from the presence of graphene platelets, and responsible for the increase in the fracture toughness values are crack deflection, crack branching and crack bridging.     

Synthesis of MoS2/g-C3N4 as a solar light-responsive photocatalyst for organic degradation

A novel molybdenum disulfide (MoS₂) and graphitic carbon nitride (g-C₃N₄) composite photocatalyst was synthesized using a low temperature hydrothermal method. MoS₂ nanoparticles formed on g-C₃N₄ nanosheets greatly enhanced the photocatalytic activity of g-C₃N₄. The photocatalyst was tested for the degradation of methyl orange (MO) under simulated solar light. Composite 3.0 wt.% MoS₂/g-C₃N₄ showed the highest photocatalytic activity for MO decomposition. MoS₂ nanoparticles can increase the interfacial charge transfer and thus prevent the recombination of photo-generated electron–hole pairs. The novel MoS₂/g-C₃N₄ composite is therefore shown as a promising catalyst for photocatalytic degradation of organic pollutants using solar energy.     

Effect of different preparation methods on the microstructure and mechanical properties of Si3N4 ceramic composites

In this study, Si₃N₄ ceramic composites were fabricated by using ball-milling, titration preparation and urea preparation methods, respectively. The effect of different preparation methods on microstructure and mechanical properties of the Si₃N₄ ceramic composites was investigated. Obviously, the Si₃N₄ ceramic composite prepared by the urea preparation method (U-SN sample) showed better sintering behavior and higher mechanical properties than that prepared by the other two methods. Compared with the Si₃N₄ ceramic composite by the titration preparation method (T-SN sample), we could avoid the complex titration process or uncontrollable pH value during the preparation process of the U-SN sample. Meanwhile, the coated Y-Al precursor layer in thickness of nanometers was more homogeneous than that prepared by the traditional titration method. B-SN represented the Si₃N₄ ceramic composite prepared by the ball-milling method. These samples were all sintered from room temperature to 1750 °C via hot-pressing sintering. The U-SN specimen showed the optimal flexural strength and fracture toughness of being 817 MPa and 6.90 MPa/m², respectively, which could be attributed to its smallest grain size (0.46 µm) among these three samples.     

Optimization of spark plasma sintering parameters of Si3N4-SiC composite using response surface methodology (RSM)

The Si₃N₄-SiC micro-nano composites were fabricated via the spark plasma sintering method using MgSiN₂ as an additive. Response surface methodology and central composite design were applied to optimize the spark plasma sintering process for the fabrication of Si₃N₄-SiC/MgSiN₂ with improved density. The relation between the three parameters of sintering including temperature, pressure, and holding time was modeled and the optimized parameters were obtained. The best sintering results obtained for the sintering temperature, holding time, and pressure are 1700 °C, 487 s, and 49 MPa, respectively. The addition of MgSiN₂ as an additive and SiC as a secondary phase were also investigated in the present work. The Si₃N₄−5 vol% SiC composite exhibited high hardness (19 GPa) and fracture toughness values (6.5 MPa m^1/2) at room temperature.