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.     

Facile synthesis of WO3 nanorods/g-C3N4 composites with enhanced photocatalytic activity

In this paper, WO₃ nanorods (NRs)/g-C₃N₄ composite photocatalysts were constructed by assembling WO₃ NRs with sheet-like g-C₃N₄. The as-synthesized photocatalysts were characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, UV–vis diffuse reflectance spectroscopy and photoluminescence. The photocatalytic activity of the photocatalysts was evaluated by degradation of Rhodamine B (RhB) under simulated sunlight irradiation. Compared to pristine WO₃ NRs and g-C₃N₄, WO₃ NRs/g-C₃N₄ composites exhibit greatly enhanced photocatalytic activities. The enhanced performance of WO₃ NRs/g-C₃N₄ composite photocatalysts was mainly ascribed to the synergistic effect between WO₃ NRs and g-C₃N₄, which improved the photogenerated carrier separation. A possible degradation mechanism of RhB over the WO₃ NRs/g-C₃N₄ composite photocatalysts was proposed.     

Facile synthesis of carbon self-doped g-C3N4 for enhanced photocatalytic hydrogen evolution

Graphite carbon nitride (g-C₃N₄) is an appealing metal-free photocatalyst for hydrogen evolution, but the potential has been limited by its poor visible-light absorption and unsatisfactory separation of photo-induced carriers. Herein, a facile one-pot strategy to fabricate carbon self-doped g-C₃N₄ composite through the calcination of dicyanamide and trace amounts of dimethylformamide is presented. The as-obtained carbon self-doped catalyst is investigated by X-ray photoelectron spectroscopy (XPS), confirming the substitution of carbon atoms in original sites of bridging nitrogen. We demonstrate that the as-prepared materials display remarkably improved visible-light absorption and optimized electronic structure under the premise of principally maintaining the tri-s-triazine based crystal framework and surface properties. Furthermore, the carbon doped g-C₃N₄ composite simultaneously weakens the transportation barrier of charge carriers, suppresses charge recombination and raises the separated efficiency of photoinduced holes and electrons on account of the extension of pi conjugated system. As a result, carbon self-doped g-C₃N₄ exhibits 4.3 times greater photocurrent density and 5.2 times higher hydrogen evolution rate compared with its bulk counterpart under visible light irradiation.     

Amino group-rich porous g-C3N4 nanosheet photocatalyst: Facile oxalic acid-induced synthesis and improved H2-evolution activity

The reasonable modulation of tri-s-triazine structure units of g-C₃N₄ is an effective method to optimize its intrinsic electronic and optical properties, thus boosting its photocatalytic hydrogen-evolution activity. Herein, amino groups are successfully introduced into the tri-s-triazine structure units of g-C₃N₄ nanosheets to improve their H₂-evolution activity via a facile oxalic acid-induced supramolecular assembly strategy. In this case, the resulting amino group-rich porous g-C₃N₄ nanosheets display a loose and fluffy structure with a large specific surface area (70.41 m² g^?1) and pore volume (0.50 cm³? g^??1), and enhanced visible-light absorption (450–800 nm). Photocatalytic tests reveal that the amino group-rich porous g-C₃N₄ nanosheets (AP-CN1.0 nanosheets) exhibit a significantly elevated photocatalytic H₂-production activity (130.7 μmol h^?1, AQE = 5.58%), which is much greater than that of bulk g-C₃N₄ by a factor of 4.9 times. The enhanced hydrogen-generation performance of amino group-rich porous g-C₃N₄ nanosheets can be mainly attributed to the introduction of more amino groups, which can reinforce the visible-light absorption and work as the interfacial hydrogen-generation active centers to boost the photocatalytic hydrogen production. The present facile and effective regulation of tri-s-triazine structure units may provide an ideal route for the exploitation of novel and highly efficient g-C₃N₄ photocatalysts.     

Ag/g-C3N4 photocatalysts: Microwave-assisted synthesis and enhanced visible-light photocatalytic activity

Ag/g-C₃N₄ photocatalysts were synthesized by a rapid microwave-assisted polyol process. The characterization results showed monodisperse Ag nanoparticles with diameters of a few nanometers closely attached to the edges of g-C₃N₄. The presence of Ag nanoparticles in Ag/g-C₃N₄ photocatalysts enhanced the visible-light absorption and suppressed the recombination of photogenerated electron/hole pairs. The Ag/g-C₃N₄ photocatalysts exhibited the superior visible-light responsive photocatalytic activity for rhodamine B degradation. The mechanism of visible-light induced photocatalysis over Ag/g-C₃N₄ photocatalysts was also discussed.     

Facile fabrication of ordered assembled TiO2/g-C3N4 nanosheets with enhanced photocatalytic activity

A series of assembled porous TiO₂/g-C₃N₄ (TC) powders composed of spherical nanoparticles were synthesized by controlling the molar ratio of urea to tetrabutyl titanate (TBOT) in a facile hydrothermal process. A nanosheets-constructed hierarchical structure was obtained at the molar ratio of urea to TBOT of 10:1, which possessed uniform mesopores with bimodal distribution (0.5–1.5 nm and 2–20 nm) and interconnected macropores between TC nanosheets. The specific surface area achieved 98.4 m² g^?1. X-ray diffraction (XRD) patterns and high resolution transmission electron microscope (HRTEM) analysis proved that the nanosheets are made of overlapping TC nanocomposite. Photoluminescence (PL) spectra results illustrated that a well-defined hierarchical porous structure is particularly desired for the low recombination rate of carriers. Further, the TC-decorated carbon fiber (CF) cloth was obtained based on the nanosheets assembled hierarchical structure, which showed more outstanding photocatalytic behavior with high degradation capability for Rhodamine B (RhB) (99.9%) and tetracycline hydrochloride (89.8%) at 60 min by 500 W Xe lamp irradiation. After five consecutive cycles, the degradation efficiencies of TC/CF cloth for both RhB and tetracycline hydrochloride all remained above 90% of the initial value.     

NiTiO3/Ag3PO4 composites with improved photocatalytic activity under visible-light irradiation

A series of NiTiO₃/Ag₃PO₄ composites were prepared by a simple ion-exchange deposition method. The composition and morphology of the samples were determined by XRD, EDS and SEM. UV–vis DRS was used to characterize their optical absorption properties. The loading of NiTiO₃ could promote the efficient separation of photoinduced electron-hole pairs and enhance the charge carrier transport to improve the photocatalytic activity. The APO-5 composite exhibited the best photocatalytic activity for the degradation of methyl violet (MV). Kinetics studies indicated that the APO-5 composite showed an apparent rate constant of 0.115 min⁻¹, which was 2.61 times that of pure Ag₃PO₄.     

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.     

CdS nanoparticles decorated hexagonal Fe2O3 nanosheets with a Z-scheme photogenerated electron transfer path for improved visible-light photocatalytic hydrogen production

Photocatalytic water splitting for hydrogen production (H₂) is one of the main potential applications of photocatalytic technology, which can use solar energy as the energy required for chemical reactions to alleviate the energy crisis. In this work, zero-dimensional/two-dimensional (0D/2D) contact surface CdS/α-Fe₂O₃ (CF) heterojunction photocatalyst was synthesized via a simple solvothermal method. Photocatalytic hydrogen production experiments revealed that the CF-15 sample shows the optimal photocatalytic H₂ rate (1806 (μmol·h^-1·g^-1)) and apparent quantum efficiency (AQE = 13.7% at λ = 420 nm). The enhancement of photocatalytic performance is mainly attributed to the contact of 0D/2D interface and the synergistic effect of Z-scheme electron transfer mechanism. This work provides an effective way for modified composite semiconductor photocatalyst by constructing special interface heterojunction to achieve highly efficiently catalysis.     

Sol−gel-based hydrothermal method for the synthesis of 3D flower-like ZnO microstructures composed of nanosheets for photocatalytic applications

Self-assembled 3D flower-like ZnO microstructures composed of nanosheets have been prepared on a large scale through a sol−gel-assisted hydrothermal method using Zn(NO₃)₂·6H₂O, citric acid, and NaOH as raw materials. The product has been characterized by X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The optical properties of the product have been examined by room temperature photoluminescence (PL) measurements. A possible growth mechanism of the 3D flower-like ZnO is proposed based on the results of experiments carried out for different hydrothermal treatment times. Experiments at different hydrothermal treatment temperatures have also been carried out to investigate their effect on the final morphology of the ZnO. The photocatalytic activities of the as-prepared ZnO have been evaluated by photodegradation of Reactive Blue 14 (KGL) under ultraviolet (UV) irradiation. The experimental results demonstrated that self-assembled 3D flower-like ZnO composed of nanosheets could be obtained over a relatively broad temperature range (90−150 °C) after 17 h of hydrothermal treatment. All of the products showed good photocatalytic performance, with the degree of degradation of KGL exceeding 82% after 120 min. In particular, the sample prepared at 120 °C for 17 h exhibited superior photocatalytic activity to other ZnO samples and commercial ZnO, and it almost completely degraded a KGL solution within 40 min. The relationship between photocatalytic activity and the structure, surface defects, and surface areas of the samples is also discussed.     

Efficient solar-light-driven photoelectrochemical water oxidation of one-step in-situ synthesized Co-doped g-C3N4 nanolayers

Cobalt-doped g-C₃N₄ (Co-g-CN) nanolayers were prepared by a single-step thermal treatment with urea and cobalt nitrate. Different amounts of cobalt nitrate were tested to optimize the amount of cobalt dopant in the g-C₃N₄ (g-CN) matrix. Several characterization methods were used to explore the structural and optical properties along with the photoelectrochemical (PEC) performance. X-ray diffraction and Fourier transform infrared studies confirmed that g-CN nanolayers were successfully doped with cobalt without disturbing the basic 2-D structure and tris-triazine units of g-CN. Furthermore, microscopy images demonstrated that the cobalt effectively transformed the short nanosheets into long nanolayers. The cobalt-doping enhanced the visible absorption of g-CN and tuned the bandgap from 2.71 to 2.62 eV. An X-ray photoelectron spectroscopy (XPS) investigation discovered that cobalt entered into the g-CN network as Co²⁺ ions. XPS valence band spectra gave information on the modification in the valence and conduction band edge potentials due to cobalt doping. The photoluminescence intensity from the Co-g-CN samples was lesser than that from g-CN nanosheets, and the PEC activity of the Co-g-CN nanolayers was greater than that of as-prepared g-CN nanosheets. Co-g-CN samples prepared with 15 mg of cobalt nitrate hexahydrate showed a PEC performance of 3.2522 mA/cm², which was greater than that of g-CN nanosheets (1.9246 mA/cm²). The better PEC performance was ascribed to the synergistic consequence of the higher visible absorption obtained by tuning the bandgap and the host–guest interactions between cobalt and g-CN.     

超薄氧掺杂g-C3N4纳米片的制备及其光催化性能

采用H2SO4与KMnO4为氧化剂，通过对热聚合法合成的原始石墨相氮化碳（g-C3N4）超声辅助氧化剥离，制备了超薄氧掺杂g-C3N4纳米片。基于HRTEM、XRD、AFM、XPS表征，考察了超声辅助氧化剥离对原始g-C3N4形貌、结构的影响；通过可见光催化降解刚果红分析了超薄氧掺杂g-C3N4纳米片的光催化性能；通过UV-vis-DRS、EIS、PL分析，探究了超薄氧掺杂g-C3N4纳米片光催化性能的增强机制。结果表明，对原始g-C3N4超声辅助氧化剥离6 h，可获得比表面积为58.45 cm2 g-1、厚度为1.08 nm的超薄氧掺杂g-C3N4纳米片；超薄氧掺杂g-C3N4纳米片（200 mg L-1）对刚果红（20 mg L-1）在120 min内可实现83%的降解，具有良好的光催化性能；与原始g-C3N4相比，超薄氧掺杂g-C3N4纳米片更有利于催化活性位点的暴露与光生载流子的分离传输，从而具有更好的光催化性能。     

Synthesis and photocatalytic activity of g-C3N4/ZnO composite microspheres under visible light exposure

In this paper, a novel g-C₃N₄/ZnO composite microspheres (CZCM) with enhanced photocatalytic activity under visible light exposure were successfully prepared by a self-assembly method followed by calcination in the air. A hierarchical structure in which ZnO microspheres were closely covered with g-C₃N₄ nanosheets was constructed. The microstructure and photocatalytic activities of the CZCM were characterized. The photocatalytic property of CZCM was evaluated by degrading solution Methyl Orange (MO) and Tetracycline (TC). The effects of varied contents of g-C₃N₄ on the photocatalytic capability of CZCM were systematically investigated and the results show that the optimized CZ-15% sample exhibit much higher photocatalytic degradation efficiency than that of bare g-C₃N₄ or ZnO under identical conditions. The analysis of Photoluminescence (PL) and photocurrent (PC) independently conformed that the photo-induced electron-hole (e^?-h⁺) pairs in the CZCM were effectively generated and responsible for the observed photocatalysis. The enhanced adsorption of visible-light and the effective charge separation on the surface of CZCM enabled significant improvement of photocatalytic performance. According to the experimental results and relative energy band levels of the two semiconductors, a possible photocatalysis mechanism for the reaction process is proposed.     

Controlled preparation of P-doped g-C3N4 nanosheets for efficient photocatalytic hydrogen production

Hydrogen production by photolysis of water by sunlight is an environmentally-friendly preparation technology for renewable energy. Graphitic carbon nitride (g-C₃N₄), despite with obvious catalytic effect, is still unsatisfactory for hydrogen production. In this work, phosphorus element is incorporated to tune g-C₃N₄'s property through calcinating the mixture of g-C₃N₄ and NaH₂PO_2, sacrificial agent and co-catalyst also been supplied to help efficient photocatalytic hydrogen production. Phosphorus (P) doped g-C₃N₄ samples (PCN-S) were prepared, and their catalytic properties were studied. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and ultraviolet diffuse reflection (UV-DRS) were used to study their structures and morphologies. The results show that the reaction rate of PCN-S is 318 μmol h⁻¹ g⁻¹, which is 2.98 times as high as pure carbon nitride nanosheets (CN) can do. Our study paves a new avenue, which is simple, environment-friendly and sustainable, to synthesize highly efficient P doping g-C₃N₄ nanosheets for solar energy conversion.     

Controlled preparation of P-doped g-C3N4 nanosheets for efficient photocatalytic hydrogen production

Hydrogen production by photolysis of water by sunlight is an environmentally-friendly preparation technology for renewable energy. Graphitic carbon nitride (g-C₃N₄), despite with obvious catalytic effect, is still unsatisfactory for hydrogen production. In this work, phosphorus element is incorporated to tune g-C₃N₄'s property through calcinating the mixture of g-C₃N₄ and NaH₂PO₂, sacrificial agent and co-catalyst also been supplied to help efficient photocatalytic hydrogen production. Phosphorus (P) doped g-C₃N₄ samples (PCN-S) were prepared, and their catalytic properties were studied. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and ultraviolet diffuse reflection (UV-DRS) were used to study their structures and morphologies. The results show that the reaction rate of PCN-S is 318 μmol·h^-1·g^-1, which is 2.98 times as high as pure carbon nitride nanosheets (CN) can do. Our study paves a new avenue, which is simple, environment-friendly and sustainable, to synthesize highly efficient P doping g-C₃N₄ nanosheets for solar energy conversion.     

A simple method to prepare g-C3N4/Ag-polypyrrole composites with enhanced visible-light photocatalytic activity

g-C₃N₄/Ag-polypyrrole (PPy) visible-light photocatalysts were prepared in a simple way, in which g-C₃N₄ was decorated with Ag nanoparticles, and covered by PPy layer. Their photoactivity was evaluated by degradation of rhodamine B. Compared with g-C₃N₄, g-C₃N₄/Ag and g-C₃N₄/PPy, the enhanced photocatalytic activity was obtained and explained according to the improvement of visible-light absorption and reduction of recombination between photogenerated electron-hole pairs with the help of Ag nanoparticles and PPy. Their reusability was evaluated by recycling experiments. The structures and chemical compositions before and after photocatalytic degradation were compared. Moreover, a possible photocatalytic mechanism was discussed in detail.     

Dual Z-scheme heterojunction g-C3N4/Ag3PO4/AgBr photocatalyst with enhanced visible-light photocatalytic activity

Recently, there has been a significant interest in developing high-performance photocatalysts for removing organic pollutants from water environment. Herein, a ternary graphitic C₃N₄ (g-C₃N₄)/Ag₃PO₄/AgBr composite photocatalyst is synthesized using an in-situ precipitation-anion-exchange process and characterized by several spectroscopic and microscopic techniques. During the photocatalytic reaction, X-ray photoelectron spectroscopy clearly illustrated the formation of metallic Ag on the g-C₃N₄/Ag₃PO₄/AgBr composite surface. The ternary composite photocatalyst demonstrated an increased photoactivity under visible light (>420 nm), achieving a complete decolorization of methyl orange (MO) in 5 min. The ternary g-C₃N₄/Ag₃PO₄/AgBr hybrid was also applied to the 2-chlorophenol degradation under visible light, further confirming its excellent photocatalytic activity. In addition, quenching experiments revealed that holes (h⁺) and O₂^?– were the major attack species in the decolorization of MO. The enhanced photoactivity of g-C₃N₄/Ag₃PO₄/AgBr results from the efficient transfer/separation of photoinduced charges with the dual Z-scheme pathway and the charge recombination sites on the formed Ag particles.     

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.     

Enhanced photocatalytic activity of g-C3N4 2D nanosheets through thermal exfoliation using dicyandiamide as precursor

g-C₃N₄ has received extensive attention because of its good chemical stability and environmental friendliness. Since g-C₃N₄ prepared from various precursors had different photocatalytic activities, g-C₃N₄ materials marked as U-gCN, D-gCN and M-gCN were synthesized from various precursors of urea, dicyandiamide and melamine, respectively. The D-gCN and M-gCN with smaller surface area were heated again to obtain exfoliated g-C₃N₄ with 2D nanosheet morphology and larger specific surface area named D-gCN-L and M-gCN-L, respectively. The synthesized bulk g-C₃N₄ and g-C₃N₄ 2D nanosheets were characterized by XRD, SEM, BET, PL, UV–Vis diffuse reflectance spectroscopy, XPS, zeta potential and TG. The photocatalytic degradation of methylene blue (MB) was carried out on U-gCN, D-gCN, M-gCN, D-gCN-L and M-gCN-L, and D-gCN-L shows the highest photocatalytic degradation performance because of its larger specific surface area, lower electron-hole recombination and wide light absorption range.