Nanocomposites of SnO2 and g-C3N4: Preparation,characterization and photocatalysis under visible LED irradiation

Tin dioxide nanoparticles were prepared in the presence of graphitized carbon nitride (g-C₃N₄) forming nanocomposites with different contents of SnO₂ up to 40 %. G-C₃N₄ was synthetized by heating of melamine at 550 °C in the open air and Sn²⁺ ions were precipitated by sodium hydroxide in g-C₃N₄ aqueous dispersions. Resulting mixtures were dried by freezing at ?20 °C and calcined at 450 °C to obtain SnO₂/g-C₃N₄ nanocomposites.The nanocomposites were characterized by common characterization methods in solid state and in their aqueous dispersions using dynamic light scattering (DLS) analysis and photocatalysis. SnO₂ nanoparticles in the nanocomposites were found to have an average size of 4 nm, however, those precipitated without g-C₃N₄ had an average size of 14 nm. Separation of photoinduced electron and holes via heterojunction between SnO₂ and g-C₃N₄ was demonstrated by photocatalytic decomposition of Rhodamine B (RhB) under LED visible irradiation (416 nm) and photocurrent measurements. The most photocatalytically active nanocomposite contained 10 % of SnO₂. Graphitized carbon nitride was assumed to serve as a template structure for the preparation of SnO₂ nanoparticles with a narrow size distribution without using any stabilizing additives.     

Si3N4/graphene nanocomposites for tribological application in aqueous environments prepared by attritor milling and hot pressing

Advanced ceramic materials have proved their superior wear resistance as well as mechanical and chemical properties in a wide range of industrial applications. Today there are standard materials for components and tools that are exposed to severe tribological, thermal or corrosive conditions. The main aim of this work is to develop novel, highly efficient tribological systems on the basis of ceramic/graphene nanocomposites as well as to prove their superior quality and to demonstrate their suitability for technical applications e.g. for slide bearings and face seals in aqueous media. Current research in the field of ceramic nanocomposites shows that is possible to make ceramic materials with improved mechanical and tribological properties by incorporating graphene into the Si₃N₄ structure. Multilayered graphene (MLG) was prepared by attritor milling at 10 h intensive milling of few micrometer sized graphite powders. The large quantity, very cheap and quick preparation process are a main strengths of our MLG. Si₃N₄/MLG nanocomposites were prepared by attritor milling and sintered by hot pressing (HP). The Si₃N₄ ceramics were produced with 1 wt%, 3 wt%, 5 wt% and 10 wt% content of MLG. Their structure was examined by transmission electron microscopy (TEM). The tribological behavior of composites in aqueous environment was investigated and showed the decreasing character of wear at increased MLG content. This new approach is very promising, since ceramic microstructures can be designed with high toughness and provide improved wear resistance at low friction.     

The influence of lignosulphonate on the properties of single and mixed Si3N4 and ZrO2 suspensions

Lignosulphonate (LS), was evaluated as a dispersing agent for single and mixed powder slips, containing Si₃N₄ and ZrO₂ powders, respectively. The surface charge was investigated by measuring the electrophoretic mobility in dilute powder dispersions. The slip stability was determined by visual observations of the settling of the powder and the fluidity of the slip was monitored by measuring the viscosity. It was evident that the different powder dispersions were most successfully stabilized by a high charge induced at high pH-values. However, in order to produce a sufficient fluidity of the slip, a dispersing agent had to be introduced. The optimal concentration range of LS at pH = 10 was found to be 0·2 wt%. The stabilizing mechanism in alkaline dispersions seems to be electrostatic depletion stabilization. The flow properties were finally tested by recording the viscosity for a mixed (composite) powder containing 68·4 wt% Si₃N₄ and 31·6 wt% ZrO₂ at a total solids content of 29–37 vol%.     

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.     

Synthesis and characterization of graphitic carbon nitride sub-microspheres using microwave method under mild condition

Graphitic carbon nitride(g-C₃N₄) sub-microspheres was first prepared via a facile microwave synthesis through polymerization reaction between cyanuric chloride(C₃N₃Cl₃) and sodium azide (NaN₃) using acetonitrile (ACN) as solvent, and the prepared samples were investigated by XRD, FTIR, XPS, SEM, TEM, UV–Vis, PL, TGA and BET, respectively. The results show that g-C₃N₄ are insoluble to conventional solvents except DMSO, and it exhibits a good chemical stability, thermal stability(< 650 °C), particle size with 0.076–0.137 μm in diameter, surface area of 89.1 m²/g and a band gap of 2.41 eV. Additionally, g-C₃N₄ prepared by microwave method also displays higher thermal stability, smaller particle radius, larger surface area, lower band gap and stronger emission intensity than traditional solvothermal method. Finally, the effect of microwave on the behavior of C₃N₄ sub-microsphere is proposed as well.     

Liquid flame spray fabrication of WO3-reduced graphene oxide nanocomposites for enhanced O3-sensing performances

WO₃ is one of the inspiring sensing materials that show high response to O₃; an efficient fabrication of WO₃ film with incorporation of complementary additives is essential for enhanced sensitivity. Here we report film deposition by liquid flame spraying, characterization of nanostructured WO₃-reduced graphene oxide (rGO) composites and their gas-sensing activities to O₃. The starting feedstock was prepared from WC_l6 and rGO for pyrolysis synthesis by flame spraying. Nano-porous WO₃-rGO films were successfully fabricated and characterized by transmission electron microscopy, field emission scanning electron microscopy, Raman spectrometry, thermal analyses and X-ray diffraction. Nanosized WO₃ grains exhibited oriented nucleation on rGO flakes whereas rGO retained intact its nano-structural features after spraying. Constrained grain growth of WO₃ of 60–70 nm in size was realized in the rGO-containing films with as compared to ~220 nm in the pure WO₃ film. The WO₃-rGO film sensors showed quicker response to O₃ and faster recovery than rGO-free WO₃ film sensors. Addition of rGO in 1.0 wt% or 3.0 wt% in the films caused a significantly reduced effective working temperature of the film sensors from ~ 250 °C to ~ 150 °C.     

Inorganic–organic photocatalyst BiPO4/g-C3N4 for efficient removal of gaseous toluene under visible light irradiation

BiPO₄/g-C₃N₄ with different amounts of BiPO₄ was prepared through wet impregnation with calcination method. The BiPO₄/g-C₃N₄ showed large surface area (172.9 m² g^− 1) and the incorporation of BiPO₄ caused a red-shift of g-C₃N₄ in visible light. The photocatalytic degradation of toluene over the samples was investigated. The degradation of toluene could get 82% in BiPO₄/g-C₃N₄ photocatalysts under optimum reaction conditions. The BiPO₄/g-C₃N₄ exhibited a higher photocatalytic activity than pure g-C₃N₄ or BiPO₄. The improved photoactivity of BiPO₄/g-C₃N₄ could be attributed to strong absorption in visible light and effective separation of photo-induced hole-electron pairs between BiPO₄ and g-C₃N₄.     

Polyimide/nano-TiO2 hybrid films having benzoxazole pendent groups: In situ sol–gel preparation and evaluation of properties

Polyimide/titania (PI/TiO₂) nanocomposite films have been successfully fabricated through the in situ formation of TiO₂ within a PI matrix via sol–gel method. Poly(amic acid) (PAA), which is the precursor of PI, was successfully synthesized by mixing pyromellitic dianhydride (PMDA), with equimolar amount of a diamine monomer having a pendent benzoxazole unit and two flexible ether linkages in N,N-dimethylformamide (DMF) solvent. Tetraethyl orthotitanate [Ti(OEt)₄] and acetylacetone were then added to the resulted PAA. After imidization at high temperature, PI/TiO₂ hybrid films were formed. The structure and morphology of the hybrid nanocomposites with different titania contents (0 wt%, 5 wt%, 10 wt%, and 15 wt%) were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, and transmission electron microscopy. The results indicate that the TiO₂ nanoparticles were homogeneously dispersed in the hybrid films. The thermogravimetric analysis of nanocomposites confirms the improvement in the thermal stability with the increase in the percentage of titania nanoparticle. Transmission electron microscopy showed that the nanoparticles with an average diameter of 25–40 nm were dispersed in the polymer matrix.     

Direct microwave synthesis of graphitic C3N4 with improved visible-light photocatalytic activity

The graphitic carbon nitride (g-C₃N₄) was rapidly synthesized via direct high-energy microwave heating approach. During the preparation process, only low-cost melamine and artificial graphite powders were used, without any metal catalysts or inert protective gas. The microstructure was investigated by using X-ray diffraction (XRD), Flourier transformed infrared (FT-IR), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM). The spectra of XRD and HRTEM indicated that the obtained g-C₃N₄ had a high crystallinity. The optical spectra covering Photoluminescence (PL) and Ultraviolet-visible (UV–vis) were also measured at room temperature. PL peak and UV–vis absorption edge of the g-C₃N₄ were shown at 455 nm and 469 nm, respectively, indicating visible-light photocatalytic property. Finally, the photocatalytic activity of g-C₃N₄ was investigated and evaluated as photocatalyst for the photo-degradation of Rhodamine B (RhB) and Methyl Orange (MO) in aqueous solution under visible-light (λ>420 nm) irradiation, respectively. Results indicated that the g-C₃N₄ sample displayed an excellent performance of removing of RhB and MO due to the improved crystallinity and large surface area of 126 m²/g. After the visible-light photocatalytic reaction for 40 min, the decolorization ratios of RhB and MO reached up to 100% and 94.2%, respectively.     

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.     

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.     

Preparation and photocatalytic activity of g-C3N4/TiO2 heterojunctions under solar light illumination

The solar light sensitive g-C₃N₄/TiO₂ heterojunction photocatalysts containing 20, 50, 80, and 90 wt% graphitic carbon nitride (g-C₃N₄) were prepared by growing Titania (TiO₂) nanoparticles on the surfaces of g-C₃N₄ particles via one step hydrothermal process. The hydrothermal reactions were allowed to take place at 110 °C at autogenous pressure for 1 h. Raman spectroscopy analyses confirmed that an interface developed between the surfaces of TiO₂ and g-C₃N₄ nanoparticles. The photocatalyst containing 80 wt% g-C₃N₄ was subsequently heat treated 1 h at temperatures between 350 and 500 °C to improve the photocatalytic efficiency. Structural and optical properties of the prepared g-C₃N₄/TiO₂ heterojunction nanocomposites were compared with those of the pristine TiO₂ and pristine g-C₃N₄ powders. Photocatalytic activity of all the nanocomposites and the pristine TiO₂ and g-C₃N₄ powders were assessed by the Methylene Blue (MB) degradation test under solar light illumination. g-C₃N₄/TiO₂ heterojunction photocatalysts exhibited better photocatalytic activity for the degradation of MB than both pristine TiO₂ and g-C₃N₄. The photocatalytic efficiency of the g-C₃N₄/TiO₂ heterojunction photocatalyst heat treated at 400 °C for 1 h is 1.45 times better than that of the pristine TiO₂ powder, 2.20 times better than that of the pristine g-C₃N₄ powder, and 1.24 times better than that of the commercially available TiO₂ powder (Degussa P25). The improvement in photocatalytic efficiency was related to i) the generation of reactive oxidation species induced by photogenerated electrons, ii) the reduced recombination rate for electron-hole pairs, and iii) large specific surface area.     

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.     

Diamond dispersed Si3N4 composites obtained by pulsed electric current sintering

30 vol.% 2 and 30 μm diamond dispersed Si₃N₄ matrix composites were prepared by pulsed electric current sintering (PECS) for 4 min at 100 MPa in the 1550–1750 °C range. The densification behaviour, microstructure, Si₃N₄ phase transformation and stiffness of the composites were assessed, as well as the thermal stability of the dispersed diamond phase. Monolithic Si₃N₄ with 4 wt% Al₂O₃ and 5 wt% Y₂O₃ sintering additives was fully densified at 1550 °C for 4 min and 60 MPa. The densification and α to β-Si₃N₄ transformation were substantially suppressed upon adding 30 vol.% diamond particles. Diamond graphitisation in the Si₃N₄ matrix was closely correlated to the sintering temperature and grit size. The dispersed coarse grained diamonds significantly improved the fracture toughness of the diamond composite, whereas the Vickers hardness was comparable to that of the Si₃N₄ matrix ceramic. The Elastic modulus measurements were found to be an excellent tool to assess diamond graphitisation in a Si₃N₄ matrix.     

Dispersing WO3 in carbon aerogel makes an outstanding supercapacitor electrode material

Tungsten oxide, originally poor in capacitive performance, was made an excellent electrode material for supercapacitors, by dispersing it to carbon aerogels (CA), a conductive and mesoporous hosting template, that drastically improved the utilization of WO₃ for capacitance generation. The WO₃ was introduced to the CA, in a form of well-dispersed single crystalline nanoparticles of 15–40 nm in size, with a simple immersion-calcination process. A one order of magnitude improvement in specific capacitance was achieved with the present composition, from 54 F/g for WO₃ nanoparticles to 700 F/g for WO₃/CA composites (scaned at 25 mV/s in 0.5 M H₂SO₄ over a potential window of −0.3 to 0.5 V). The WO₃/CA composites exhibited an excellent high rate capability with a 60% retention in specific capacitance at 500 mV/s, almost perfect cycle efficiency of 99%, and outstanding cycling stability of only 5% decay in specific capacitance after 4000 cycles.     

Optical properties of graphitic carbon nitride films prepared by evaporation

Graphitic carbon nitride (g-C₃N₄) consists of two-dimensional sheets of carbon and nitrogen atoms. Films of g-C₃N₄ were prepared by evaporating guanidine carbonate at four different substrate temperatures. The optical absorption band of the films appears at 3.3 eV and the optical energy gaps are calculated to be 2.83–2.90 eV. Band intensity increases with increasing substrate temperature, but the energetic band position does not shift. The photocurrent of g-C₃N₄ films can be observed by irradiation with monochromatic light. While the photosensitivity spectra are in almost complete correspondence with the optical absorption spectra, it is also found that the photocurrent is generated by irradiation at photon energies below the optical energy gap down to 2.5 eV.     

Effects of WO3 on the microstructure and optical transmittance of PLZT ferroelectric ceramics

The influence of the WO₃ addition as sintering aids on the structural, microstructural and optical properties of (Pb,La)(Zr,Ti)O₃—PLZT based ceramics was investigated. Ferroelectric (Pb_0.9La_0.1)(Zr_0.65Ti_0.35)_0.975O₃ + x wt% WO₃ (x = 0.0, 0.5, 1.5 and 2.0) ceramics were densified by oxygen assisted uniaxial hot pressing. From the XRD results it was found that the hot pressed samples displayed single pseudocubic perovskite phase. EDS analysis evidenced the presence of WO₃ rich phase at the grain boundaries. An inhibition of grain growth and an evolution from transgranular to intergranular fracture behavior was observed, as a consequence of the formation of a PbO–WO₃ liquid phase, as the amount of WO₃ addition was increased. The optical transmittance in the visible and infrared range was decreased due to the presence of the liquid phase in grain boundaries, for WO₃ content lower than 2.0 wt%.     

The influence of pulse plating parameters on microstructure and properties of Ni-W-Si3N4 nanocomposite coatings

Nanosized silicon nitride (Si₃N₄) particles reinforced Nickel-tungsten composite coatings were deposited on the surface of C45 steel sheet by pulse electrodeposition. The effect of duty cycle, frequency, current pattern and presence of Si₃N₄ nanoparticles on microstructure, phases and corrosion resistance and mechanical properties of the coatings were investigated. The Si₃N₄ phase was incorporated into Ni-W alloy matrix uniformly and the inclusion content of in the coating was analyzed by energy dispersive x-ray spectrometer (EDS). The structure, microhardness and surface roughness of the coatings was analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), Vickers micro-indenter and atomic force microscopy (AFM). The corrosion protection of steel by the coatings was evaluated by weight loss and electrochemical impedance spectroscopy (EIS). Corrosion rates of the coatings were determined using the Tafel polarization test. The results indicated that the duty cycle of 60%, pulse frequency of 1000 Hz, average current density of 5 A/dm⁻², and Si₃N₄ nanoparticles concentration of 30 g/L were the optimal plating conditions. The amount of Si₃N₄ particles incorporated into the coating that were produced under the optimum plating conditions was 2.1 wt%, and the microhardness was 1031 Hv as well as the crystallite size of this coating was 27 nm.     

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.     

Graphitic carbon nitride nanosheet-assisted preparation of N-enriched mesoporous carbon nanofibers with improved capacitive performance

N-enriched mesoporous carbon nanofibers (NMCNFs) were prepared by an electrospinning technique using graphitic carbon nitride (g-C₃N₄) nanosheets both as sacrificial template and N-doping source. The resultant NMCNF film has a high N-doping level of 8.6 wt% and a high specific surface area of 554 m² g⁻¹. When directly used as the electrode material for supercapacitor, the free-standing NMPCNF film shows a significantly improved capacitive performance including a higher specific capacitance (220 F g⁻¹ at 0.2 A g⁻¹) and a better rate capability (∼70% retention at 20 A g⁻¹) than those of microporous carbon nanofiber film prepared using the same process without using g-C₃N₄ nanosheets (145 F g⁻¹ at 0.2 A g⁻¹ and ∼45% retention at 20 A g⁻¹). Moreover, the NMCNFs show superior stability with only a ∼3% decrease of its initial capacitance after 1000 cycles at a high current density of 10 A g⁻¹. More significantly, the energy density of a symmetrical supercapacitor (SC) based on the NMPCNF film can reach 12.5 Wh kg⁻¹ at a power density of 72 W kg⁻¹.