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.     

Synthesis and properties of nanocomposites of WO3 and exfoliated g-C3N4

The nanocomposites of WO₃ nanoparticles and exfoliated graphitized C₃N₄ (g-C₃N₄) particles were prepared and their properties were studied. For this purpose, common methods used for characterization of solid samples were completed with dynamic light scattering (DLS) method and photocatalysis, which are suitable for study of aqueous dispersions.The WO₃ nanoparticles of monoclinic structures were prepared by a hydrothermal method from sodium tungstate and g-C₃N₄ particles were prepared by calcination of melamine forming bulk g-C₃N₄, which was further thermally exfoliated. Its specific surface area (SSA) was 115 m² g⁻¹.The nanocomposites were prepared by mixing of WO₃ nanoparticles and g-C₃N₄ structures in aqueous dispersions acidified by hydrochloric acid at pH = 2 followed by their separation and calcination at 450 °C. The real content of WO₃ was determined at 19 wt%, 52 wt% and 63 wt%. It was found by the DLS analysis that the g-C₃N₄ particles were covered by the WO₃ nanoparticles or their agglomerates creating the nanocomposites that were stable in aqueous dispersions even under intensive ultrasonic field. Using transmission electron microscopy (TEM) the average size of the pure WO₃ nanoparticles and those in the nanocomposites was 73 nm and 72 nm, respectively.The formation of heterojunction between both components was investigated by UV–Vis diffuse reflectance (DRS) and photoluminescence (PL) spectroscopy, high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), photocatalysis and photocurrent measurements. The photocatalytic decomposition of phenol under the LED source of 416 nm identified the formation of Z-scheme heterojunction, which was confirmed by the photocurrents measurements. The photocatalytic activity of the nanocomposites decreased with the increasing content of WO₃, which was explained by shielding of the g-C₃N₄ surface by bigger WO₃ agglomerates. This study also demonstrates a unique combination of various characterization techniques working in solid and liquid phase.     

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.     

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.     

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.     

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.     

Carbon nitride nanowires/nanofibers: A novel template-free synthesis from a cyanuric chloride–melamine precursor towards enhanced adsorption and visible-light photocatalytic performance

The development of a graphitic carbon nitride (g-C₃N₄) photocatalyst is of great importance to a variety of visible utilization application fields. The desired high efficiency can be achieved by employing well-controlled g-C₃N₄ nanostructures. In this study, we successfully synthesized high surface area g-C₃N₄ nanowires and nanofibers using a cyanuric chloride and melamine precursor dispersed in a solvothermal reaction and with a subsequent calcination step. The obtained novel nanowire product had a diameter of 10–20 nm and a length of several hundreds of nanometers, while the nanofibers revealed fibrous nanostructures of randomly dispersed fibers with an average diameter of ~15 nm. The adsorption and photocatalytic experimental results indicated that the as-prepared nanowires and nanofibers showed enhanced activities compared with bulk g-C₃N₄. Based on our experimental results, a possible photocatalytic mechanism with hydroxyl and superoxide radical species as the main active species in photocatalysis was proposed. Moreover, our strategy may provide progress toward the design and practical application of 1D g-C₃N₄ nanostructures in the adsorption and photocatalytic degradation of pollutants.     

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.     

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.     

Micro/nano-structured graphitic carbon nitride–Ag nanoparticle hybrids as surface-enhanced Raman scattering substrates with much improved long-term stability

Surface-enhanced Raman scattering (SERS) substrates with high SERS activity and stability are important for SERS sensors. A facile method was developed to fabricate efficient and stable SERS substrates by combining Ag nanoparticles (NPs) and micro-scale sheeted graphitic carbon nitride (g-C₃N₄). The g-C₃N₄/Ag NPs hybrid could provide a great number of hot spots and concentrated the analyte by the π–π stacking interaction between analyte molecules and g-C₃N₄, making a dramatic Raman enhancement. Moreover, the g-C₃N₄/Ag NPs hybrid uniformly immobilized Ag NPs on the surface and edges of g-C₃N₄ sheets by an interaction between Ag NPs and g-C₃N₄, leading to much improved long-term stability. This could be explained in terms of the electron–donor effect of g-C₃N₄, which was further confirmed by density functional theory calculations. The inherent Raman enhancing effect of g-C₃N₄ itself also contributed to the total SERS responses. Due to multiple enhancement contributions, the g-C₃N₄/Ag NPs hybrid exhibited a strong Raman enhancement effect for with an enhancement factor of 4.6 × 10⁸ (evaluated by using crystal violet as a probe), and possessed wide adaptability from dyes, pesticides to bio-molecules.     

Catalytic properties in N2O decomposition of mixed cobalt–iron spinels

The effect of introduction of iron in the Co_3 − xFe_xO₄ on catalytic activity in N₂O decomposition was investigated. The spinel catalysts were characterized by XRD, SEM, RS, BET methods, work function measurements and Mössbauer spectroscopy. Introduction of iron in the Co_3 − xFe_xO₄ spinel catalysts at the level of x < 1 leads to preferential substitution of Fe³⁺ in tetrahedral sites, whereas for x > 1 also octahedral ones are substituted. A strong correlation between deN₂O activity (T_50%) and work function was observed showing that electronic factor controls the catalytic reactivity of Co–Fe spinels. The results revealed that the active centers for N₂O decomposition are cobalt ions and thus even a low level of their substitution by iron leads to substantial decrease of the deN₂O activity of the cobalt spinel.     

Highly active SO2-resistant ex-framework FeMFI catalysts for direct N2O decomposition

Ex-framework FeMFI catalysts, prepared by isomorphous substitution of iron in the aluminosilicate or gallosilicate MFI-type framework and activation by calcination at 823 K and steaming (300 mbar H₂O in N₂) at 873 K, show high activity and stability in N₂O decomposition in the presence of O₂, CO₂, H₂O, and SO₂. The N₂O conversion of the ex-framework catalysts in simulated tail-gas mixtures was >80% at 800 K and 75,000 h⁻¹. The specific activity per mole of Fe (turnover frequency, TOF) of the ex-framework catalysts in N₂O–He is four to nine times higher than observed for catalysts prepared by conventional solid and liquid-ion exchange, and sublimation methods. The stability of ex-framework catalysts for the direct N₂O decomposition, in the absence of any reductant, is remarkable, showing no significant deactivation (at N₂O conversion levels ranging from 20 to 65%) after 600 h on stream. Sublimed and especially ion-exchanged FeZSM-5 catalysts show a strong irreversible deactivation in feed mixtures containing H₂O and SO₂. The effect of SO₂ on the catalytic performance of FeMFI catalysts is discussed, as well as the applicability of the ex-framework FeMFI catalysts in fluid-bed combustion facilities.     

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.     

Effect of support synthesis methods on structure and performance of VOx/CeO2 catalysts in low-temperature NH3-SCR of NO

CeO₂ supports were prepared by a citrate (C) or a precipitation method (P) before deposition of vanadia by wet impregnation to obtain supported V/CeO₂ catalysts. The V/CeO₂-P catalyst is more active, reaching ≈ 100% NO conversion and N₂ selectivity already below 225 °C at a space velocity of GHSV = 70,000 h^− 1. XRD, UV-vis-DRS, Raman, pseudo-in-situ-XPS and operando-EPR spectroscopy revealed that this is due to higher surface area and a more effective incorporation of V sites into the support surface, which keeps them in their active valence states + 5 and + 4 and prevents reduction to inactive V^3 + as observed on V/CeO₂-C.     

Novel spindle-shaped nanoporous TiO2 coupled graphitic g-C3N4 nanosheets with enhanced visible-light photocatalytic activity

Highly efficient visible-light-driven heterojunction photocatalysts, spindle-shaped nanoporous TiO₂ coupled with graphitic g-C₃N₄ nanosheets have been synthesized by a facile one-step solvothermal method. The as-prepared photocatalysts were characterized by X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N₂ adsorption-desorption analysis and UV–vis diffuse reflectance spectrometry (DRS), proving a successful modification of TiO₂ with g-C₃N₄. The results showed spindle-shaped nanoporous TiO₂ microspheres with a uniform diameter of about 200 nm dispersed uniformly on the surface of graphitic g-C₃N₄ nanosheets. The g-C₃N₄/TiO₂ hybrid materials exhibited higher photocatalytic activity than either pure g-C₃N₄ or nanoporous TiO₂ towards degradation of typical rhodamine B (RhB), methyl blue (MB) and methyl orange (MO) dyes under visible light (>420 nm), which can be largely ascribed to the increased light absorption, larger BET surface area and higher efficient separation of photogenerated electron–hole pairs due to the formation of heterostructure. In addition, the possible transferred and separated behavior of electron–hole pairs and photocatalytic mechanisms on basis of the experimental results are also proposed in detail.     

Selective reduction of NO by NH3 over Fe-zeolite-promoted V2O5-WO3/TiO2-based catalysts: Great suppression of N2O formation and origin of NO removal activity loss

Great depression of the formation N₂O in the selective catalytic reduction of NO by NH₃ (NH₃-SCR) has been studied by combining a V₂O₅-WO₃/TiO₂ (VWT) catalyst with a Fe-exchanged ZSM-5 zeolite (FeZ). At temperatures > 400 °C, N₂O formation was significant over VWT but < 5 ppm over FeZ/VWT catalysts with the FeZ ≥ 8%. Unfortunately, all these FeZ-promoted catalysts disclosed a decrease in deNO_xing performances, due to an enhanced NH₃ oxidation into NO. At temperatures > 350 °C, the chemically-combined VWT-based FeZ systems could facilitate both N₂O reduction with NH₃ and N₂O decomposition, thereby suppressing N₂O emissions in NH₃-SCR reaction.     

Bi-Co3O4 catalyzing N2O decomposition with strong resistance to CO2

Bi added to Co₃O₄ by coprecipitation method significantly decreased the average crystalline size of Co₃O₄ and increased the surface area and the active sites of the catalyst in population for catalyzing the N₂O decomposition. Over Bi_0.02Co that was the optimized from the Bi_xCo catalysts, 2000 ppm of N₂O in pure Ar was completely decomposed at 400 °C in GHSV of 20,000 h^− 1. Outstandingly, the catalyst exhibited a strong resistance to CO₂, stable N₂O conversion larger than 95% was obtained over the catalyst in the presence of 10% CO₂ at the reaction temperature.     

Effects of Fe addition on the structure and catalytic performance of mesoporous Mn/Al–SBA-15 catalysts for the reduction of NO with ammonia

Mesoporous Al–SBA-15 has been synthesized by a hydrothermal method and used as a support for Mn/Al–SBA-15, Fe/Al–SBA-15, and Mn–Fe/Al–SBA-15 catalysts. XRD, N₂ sorption, XPS, H₂-TPR and activity tests have been used to assess the properties of catalysts. The Mn–Fe/Al–SBA-15 catalyst exhibited a higher SCR activity than Mn/Al–SBA-15 or Fe/Al–SBA-15 due to a synergistic effect between Mn and Fe. After the addition of Fe, the binding energy of Mn 2p_3/2 on Mn–Fe/Al–SBA-15(573) decreased by about 0.4 eV and the Mn^4 +/Mn^3 + ratio decreased to 1.10. The appropriate Mn^4 +/Mn^3 + ratio may have a great effect on the reduction of NO over Mn–Fe/Al–SBA-15(573) catalyst.     

Method for the synthesis of water-soluble oxide of graphite-like carbon nitride

The new route to synthesize the oxygen containing carbon nitride is developed. In contrast to known methods, our approach does not require the preliminary preparation of g-C₃N₄: pyrolysis of melamine is carried out at the presence of oxygen. First alongside with the oxygen-doped (~ 8.1%) carbon nitride (O-g-C₃N₄) an oxide of carbon nitride (g-C₃N₄)O was obtained as a new substance that has a structure similar to the graphite oxide. Nanosized powder of (g-C₃N₄)O is easily dissolved and exfoliated in water with the formation of a flake-like solution, which can contain nanosheets from several heptazine layers.     

Si3N4-ZrB2 ceramics prepared at low temperature with improved mechanical properties

Si₃N₄-ZrB₂ ceramics were hot-pressed at 1500 °C using self-synthesized fine ZrB₂ powders containing 2.0 wt% B₂O₃ together with MgO-Re₂O₃ (Re = Y, Yb) additives. Both Si₃N₄ and ZrB₂ grains in the hot-pressed ceramics were featured with elongated and equiaxed morphology. The presence of elongated Si₃N₄ and ZrB₂ grains led to the partial texture of the ceramics under the applied pressure. Vickers hardness and fracture toughness of Si₃N₄-ZrB₂ ceramics with MgO-Re₂O₃ additives prepared at low temperature were about 19–20 GPa and 9–11 MPa m^1/2, respectively, higher than the reported values of Si₃N₄-based ceramics prepared at high temperature (1800 °C or above) under the same test method.