•Dyes can be decomposed by the thermo-driven catalysis of g-C3N4.•A synergetic effect of g-C3N4 and H2O2 is demonstrated.•This approach shows the superior applicability and reusability.•The doping level promotes the separation of thermo-generated e−–h+.Herein, graphitic carbon nitride (g-C3N4) as a novel thermo-driven catalyst was used for catalytic decomposition of organic dyes with the assistance of H2O2. In comparison with the experiment with individual catalyst or H2O2, a prominent enhancement of performance was achieved by the synergetic effect of g-C3N4 and H2O2. Experimental parameters like the H2O2 dosage and reaction temperature were optimized, and the catalyst exhibited excellent application universality for various concentration and different dyes. By physicochemical characterizations, the modification or doping with oxygen-containing groups on carbon nitride framework was demonstrated. We proposed a possible mechanism that the doping level trapped thermo-excited electrons to promote the separation of electrons and holes. On the other hand H2O2 was reduced by the excited electron to produce hydroxyl radical. The quenching experiment suggested the holes and hydroxyl radical were the main active species, thus verifying the synergetic effect of g-C3N4 and H2O2. Additionally, the superior stability and reusability were also proved by the successive five cycles, where the activity was even gradually improved with the increase of cycle number.

•Ionic liquid-capped GQDs (IL-GQDs) were easily prepared.•IL-GQDs was used as label-free fluorescent probe for direct detection of Fe(CN)63−.•Sensitive detection of Fe(CN)63− with low detection limit was achieved.Despite complex molecular and atomic doping, efficient post-functionalization strategies for graphene quantum dots (GQDs) are of key importance to control the physicochemical properties and broaden the practical applications. With ionic liquid as specific modification agents, herein, the preparation of ionic liquid-capped GQDs (IL-GQDs) and its application as label-free fluorescent probe for direct detection of anion were reported. Hydroxyl-functionalized GQDs that could be easily gram-scale synthesized and possessed single-crystalline were chosen as the model GQDs. Also, the most commonly used ionic liquids, water-soluble 1-butyl-3-methyl imidazolium tetrafluoroborate (BMIMBF4) was chosen as the model IL. Under the ultrasonic treatment, BMIMBF4 easily composited with GQDs to form IL-GQDs. The synthesized IL-GQDs were characterized by atomic force microscopy (AFM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and fluorescence (FL) spectrum. After successful combination with IL, the excitation-independent photoluminescence behavior of GQDs presented almost no change, whereas, the anion responsiveness of IL-GQDs drastically improved, which afforded the IL-GQDs a sensitive response to Fe(CN)63−. Based on the strong fluorescence quench, a facile and sensitive detection of Fe(CN)63− was achieved. A wide linear range of 1.0×10−7 to 2.5×10−3 mol l−1 with a low detection limit of 40 nmol l−1 was obtained. As the composition and properties of IL and GQDs could be easily tuned by varying the structure, ionic liquids-capped GQDs might present promising potential for their applications in sensing and catalysis.

•One-step synthesis of S-GQDs was developed.•Successful doping of S atom in GQDs was proven.•S-GQDs exhibited monolayer-graphene thickness and high crystallinity.•Facile and direct fluorescence sensor for sensitive detection of Ag+ was achieved.Sulfur-doped graphene quantum dots (S-GQDs) with bright blue emission have been prepared by a facile one-pot hydrothermal treatment. A specific compound, 1,3,6-trinitropyrene, which has a mother nucleus structure similar with graphene, was chosen as the carbon source and 3-mercaptopropionic acid (MPA) was employed for S-doping and carboxyl groups modification. The synthesized S-GQDs were characterized by atomic force microscopy (AFM), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS) and fluoscence (FL) spectrum. Results indicated that S-GQDs possessed single layer graphene structure with mean size of about 2.5 nm and presented an excitation-independent photoluminescence behavior with maximum excitation/emission wavelength at 360/450 nm, respectively. The sulfur-doping of GODs drastically improved their electronic and chemical properties, which afforded the S-GQDs a sensitive response to Ag+ ions. Furthermore, the S-GQDs were successfully explored as a sensing probe for Ag+ detection with high sensitivity and selectivity. A wide linear range of 0.1-130.0 μM with a low detection limit of 30 nM was obtained. The facile preparation method and the high performace of the as-prepared S-GQDs present promising potential for their applications in sensing, biological imaging and catalysis.

Semiconductor photocatalysis currently suffered three main problems, low solar energy utilization, high photo-generated charge recombination rate and the heavy metal ions release by the photo-corrosion. Herein, we developed a visible-light-driven homojunction photocatalyst with the metal-free two-dimensional (2D) graphitic carbon nitride nanosheets (CNNS). By employing liquid exfoliation and chemical blowing approaches, we obtained two kinds of CNNS materials (le-CNNS and cb-CNNS) with different band structures, and subsequently fabricated the homojunction photocatalyst. This 2D/2D nanocomposited homojunction photocatalyst exhibited enhanced photocatalytic performance compared to these individual 2D nanosheets materials. Moreover, its well universality and reusability were also demonstrated by photo-degradation of various organic pollutants and five successive runs. By studying the optical properties and the electrochemical behavior, the band alignment of this homojunction was illustrated and the possible mechanism was proposed, where the transmitted electrons on the conduction band (CB) of le-CNNS would transport to the CB of cb-CNNS, and the holes on the valence band (VB) of cb-CNNS transferred to the VB of le-CNNS, therefore promoting the photo-induced carrier separation. Additionally, the photoluminescence, electrochemical impendence and photocurrent measurements further demonstrated that the recombination of photo-excited electron-hole pairs had been efficiently suppressed in the homojunction and were respectively collected on different CNNS components.Download high-res image (176KB)Download full-size image

•The synthetic approach offers the merits of low-cost and high efficiency.•The PCNF materials are porous and ultrathin.•The PCNF materials exhibit enhanced photocatalytic performance.•This work provides a method to synthesize other porous two-dimensional materials.A facile one-step approach was employed to synthesize porous g-C3N4 flake (PCNF) via thermally polymerizing melamine with the presence of MgCO3 template. CO2 gas released from the decomposition of MgCO3 prohibited the further polymerization of CN framework and separated the CN layers to produce a flake-like morphology, and meanwhile the removal of MgO nanoparticles resulted in a porous structure. The resultant PCNF exhibited enhanced visible-light photocatalytic activity for degrading organic dyes, and its rate constant is ∼4 times higher than that of bulky CN. What is more, this material presented an excellent stability and superior universality for various pollutants.Download high-res image (206KB)Download full-size image

Well-controlled mesoporosity is of importance for porous carbons as electrochemical electrode materials. However, the ordered mesoporous carbons prepared from the template approaches face the fact of relative low specific surface area in comparison to activated carbons. Herein, we employed a hard-template route associated with the chemical activation to prepare N-doped mesoporous carbon by co-casting of carbon and nitrogen precursors into the pore channels of mesoporous silica. The obtained activated N-doped mesoporous carbon (ANMC) material preserved the morphology and mesoporous structure of template, and meanwhile a secondary mesoporosity was introduced by the KOH activation. It was demonstrated that the dominant porosity in ANMC sample was from mesopore, and it possessed a high mesopore surface area (2505.6 m2 g−1) and mesopore volume (1.74 cm3 g−1). The N dopant was determined to be pyridinic-N, pyrrolic-N, quaternary-N and pyridine-N-oxide, which did not only contribute the pseudocapaticance but also facilitated the electron transfer in the carbon skeleton. The developed mesoporosity and N doping made this material exhibit superior electrochemical performance that was much higher than those of ordered mesoporous carbon, N-doped ordered mesoporous carbon and activated N-doped carbon samples. Also, the specific capacitance 336.9 F g−1 (0.5 A g−1) and the rate capability were higher than those of other reported mesoporous carbons. In addition, the assembled symmetrical supercapaictor simultaneously showed the high energy density and power density, as well as presented the superb cycling ability (∼98.5% capacitance retaining after 5000 runs).Download high-res image (184KB)Download full-size image

•N-doped activated carbon sheets (NACSs) are prepared by a chemical blowing method.•This synthetic method is novel and convenient than other traditional routes.•NACS material possesses hierarchical porous structure and high nitrogen content.•It shows a high specific capacitance of 312 F g−1 and good rate performance.•The energy density of NACS supercapacitor reaches 20.2 Wh kg−1 at 448 W kg−1.N-doped activated carbon sheets (NACS) have been successfully synthesized using glucose as carbon source via melamine-assisted chemical blowing and sequent KOH-activation method. The obtained carbon material possesses a sheet-like morphology with ultrathin thickness, hierarchical micro/mesoporous structure, high specific surface area (up to 1997.5 m2 g−1) and high pore volume (0.94 cm3 g−1). Besides, NACS material with a nitrogen content of 3.06 wt% presents a maximum specific capacitance of 312 F g−1 at a current density of 0.5 A g−1 in 6 M KOH aqueous electrolyte due to the cocontribution of double layer capacitance and pseudocapacitance. It also displays good rate performance (246 F g−1 at 30 A g−1) and cycle stability (∼91.3% retention after 4000 galvanostatic charge-discharge cycles). The assembled NACS-based symmetric capacitor exhibits a maximum energy density of 20.2 Wh kg−1 at a power density of 448 W kg−1 within a voltage range of 0–1.8 V in 0.5 M Na2SO4 aqueous electrolyte. Thus, the unique porous sheet structure and nitrogen-doping characteristic endue the electrode material a potential application for high-performance supercapacitors.N-doped activated carbon sheets have been successfully synthesized by a melamine-assisted chemical blowing route, which exhibit high specific capacitance, excellent rate performance and high energy/power densities.

The colloid of graphitic carbon nitride (g-C3N4) was of great importance for practical application. Herein we introduced an alkali treatment route to efficiently colloidize g-C3N4 under mild conditions by destroying the hydrogen bonds between linearly polymeric melon chains and hydrolyzing partial CNHC bonds linked two tri-s-triazine units. The obtained colloidal suspension was extremely stable due to its negative charges on surface, and the particle size of several hundred nanometers and the nanobelt-like morphology were revealed by electron microscopy and dynamic light scattering technologies. The structural, optical and functional group analysis demonstrated that the structure of CN heterocycles was preserved after the alkali treatment, and the produced colloidal g-C3N4 can be re-assembled by an electrostatic interaction. Moreover, contributing to the reduced electron–hole recombination, the photocatalytic performance of restacked carbon nitride colloids had more enhanced photocatalytic performance than bulk g-C3N4.The graphitic carbon nitride was firstly colloidized by a mild alkali treating route. The resulted g-C3N4 colloid shows an enhanced photocatalytic performance for decomposition of RhB dye under visible light irradiation.

A direct solid state Z-scheme photocatalytic system was fabricated by assembling two-dimensional (2D) g-C3N4 nanosheets (CNNS) and titania nanosheets (TNS), which were obtained from the delamination of their corresponding layered precursors. By introducing TNS, the interlayer restacking of CNNS was effectively prohibited, forming uniform CNNS/TNS composites. The tightly contacted CNNS/TNS interface promoted the charge transfer and therefore improved the separation ratio of photogenerated electron-hole pairs. The photocatalytic performance of CNNS/TNS in various mass ratios was investigated for dye degradation, and the degradation rate of optimal sample 0.7CNNS/0.3TNS was 2.34 and 48.5 times higher than those of proton flocculated pure CNNS and TNS, respectively. Superoxide radicals and hydroxyl radicals were determined as the main active species by the quenching experiment. Moreover, the enhanced generation of superoxide radicals and hydroxyl radicals was confirmed by the absorption spectra of nitroblue tetrazolium and the photoluminescence spectra of 2-hydroxy terephthalic acid, respectively. Finally, we proposed a possible Z-scheme mechanism based on the theoretical calculation and the experimental results.

PDA nanoaggregates can be formed at ultra-high concentrations of dopamine (DA, 8 mg mL−1), promoting application of DA/PDA chemistry in material modification; however, polydopamine (PDA) film also is formed at such concentrations. To achieve a high density of PDA nanoaggregates on the material surface at low concentrations of DA, here, a simple strategy was developed and a novel superhydrophobic/superoleophilic sponge with outstanding adsorbency and flame-retardancy was fabricated. Two-step DA oxidative self-polymerization in different media was applied. After the raw melamine sponge (ME) was first coated with PDA film at 0.5 mg mL−1 of DA in mild buffer (Tris–HCl, pH 8.5), the resulting PDA film was then applied as “solid reagent” and contributed to formation of a high density of PDA nanoaggregates at 3.0 mg mL−1 of DA in a harsh medium (water/ethanol/ammonium). Covalent grafting of n-dodecylthiol (DT, low surface energy molecular) onto PDA nanoaggregates was combined in the second step using the reactivity of PDA. As investigated by scanning electron microscopy (SEM), the as-prepared ME/PDA/DT sponge has a 3D hierarchical structure with a high density of nanoaggregates on an interconnected porous sponge network. The size and density of PDA nanoaggregates on the sponge could be easily controlled by adjusting the concentration of ammonia in the second step. The ME/PDA/DT sponge was superhydrophobic (water contact angle of 161.5°) and superoleophilic (oil contact angle of 0°). Owing to the high loading of nanoaggregates, highly porous structure, and superhydrophobic property, the ME/PDA/DT sponge possessed outstanding adsorption properties towards oil and organic solvents, including high adsorption capacity (weight gains ranged from 6632% to 15112%), fast adsorption kinetics (reaching adsorption capacity in 30 seconds), and good reusability (no obvious decrease of adsorption capacity after ten adsorption/distillation cycles). The ME/PDA/DT sponge exhibited improved flame-retardancy compared with the raw melamine sponge, indicating significant potential for oil adsorption and organic solvent clean-up with low risk of fire and explosion.

A novel one-pot synthesis of sulfur-doped graphene quantum dots (S-GQDs) was proposed based on water-phase molecular fusion with 1,3,6-trinitropyrene, Na2S, and NaOH in a hydrothermal process. A 75% yield was obtained and mass production of S-GQDs with high crystallinity was possible. The prepared S-GQDs gave a stable yellow-green emission within a wide pH range of 2.0–11.0 and exhibited excitation-independent photoluminescence behaviors. As investigated by atomic force microscopy (AFM), the synthesized S-GQDs possessed monolayer-graphene thickness. As illustrated by transmission electron microscopy (TEM), the synthesized S-GQDs exhibited high crystallinity and uniform size (∼3 nm). Successful doping of S atoms in graphene quantum dot lattices was proven by X-ray photoelectron spectroscopy (XPS) characterization. Compared with GQDs, the S-GQDs had drastically changed surface chemistry and showed a selective and sensitive response to Pb2+. Ions such as Na+, K+, Cu2+, Ca2+, Mg2+, Zn2+, Fe3+, Ni2+, Co2+, Cd2+ have no effect on the fluorescence of S-GQDs. Based on the fluorescence quenching of S-GQDs by Pb2+ in water, a facile and direct fluorescence sensor for Pb2+ detection was developed. Under the optimized conditions, the linear response ranged from 0.1 to 140.0 μM with a detection limit of 0.03 μM.

•NRCNNS were synthesized by chemical blowing method.•NRCNNS materials exhibited typical 2D nanosheet morphology.•NRCNNS presented the prominent performance for degradation of organic pollutants.•NRCNNS possessed superior stability and reusability.Herein, we reported the synthesis of nitrogen-rich graphitic carbon nitride nanosheets (NRCNNS) material by an NH4Cl assisted chemical blowing approach, where the released gas from the decomposition of NH4Cl prohibited the further polymerization of CN framework and separated the CN layers. A typical two-dimensional morphology with high aspect-ratio was demonstrated by scanning electron microscopy and transmission electron microscopy observation, and the excess amino group on the skeleton was proved by Fourier transform infrared and X-ray photoelectron spectra technologies. Different from the spectral blue-shift in other carbon nitride nanosheets, an enhanced absorption in visible light region was achieved in our NRCNNS due to the N,O-doping level in the band gap. The optimal NRCNNS material with an NH4Cl/melamine mass ratio of 4.0 exhibited a promoted activity (5 times higher than that of the bulky carbon nitride) and reusability for degradation of organic dye under visible light irradiation. Moreover, a possible mechanism was proposed based on the results of valence band spectra and the quenching experiments of active species.

In this review, the relatively systematic description of g-C3N4 nanosheets for photocatalysis is presented for the first time. We firstly briefly introduce the structure and properties of bulk g-C3N4, and then put emphasis on the comparison of various exfoliation approaches. This review also summarizes the recent development in the fabrication of novel photocatalysts based on nanosheets and their potential applications in energy conversion and environmental treatments. At the end of this article, a summary and perspective are presented for highlighting some challenges and opportunities in the future.

A novel graphitized porous carbon nanosphere (GPCNS) material obtained by a convenient simultaneous activation and graphitization route, which was realized by heating resorcinol–formaldehyde (RF) resin nanospheres immersed with ZnCl2 and FeCl3 in an inert atmosphere, has been reported. A high graphitization level was achieved through the catalytic graphitization by reduced Fe metal, and the hierarchically micro/mesoporous structure was produced in a controlled fashion by tuning the mass ratio of the activating agent ZnCl2 and a carbon precursor. An optimal sample of GPCNS-2 was prepared with a FeCl3/ZnCl2/RF mass ratio of 0.5:2:1, exhibiting a highly graphitized framework and uniform spherical morphology with an average diameter of ∼500 nm, as well as well-interconnected micro/mesoporous structure with a large surface area of 1664.8 m2 g−1. Acting as an electrode material for a supercapacitor application in 6 M KOH electrolyte, GPCNS-2 displayed excellent electrochemical performance with a high specific capacitance of 402.5 F g−1 at a current density of 1 A g−1. Moreover, an electrochemical impedance spectroscopy test demonstrated that the low internal electrical resistance of GPCNS-2 contributed a superior rate capability of above 75% retention rate even at 50 A g−1. Furthermore, the GPCNS-2 electrode possessed an outstanding cycling stability, and about 96% of its initial specific capacitance at 5 A g−1 was maintained after 5000 cycles.

A novel non-light-driven catalysis by the delaminated two dimensional titanate nanosheets (TNSs) has been explored for degradation of organic dyes with hydrogen peroxide (H2O2). This catalyst can efficiently remove dyes at high concentration and over a wide pH range, as well as with a long cycle number and superior universality.

A stable colloid of g-C3N4 nanosheets was prepared on a large scale via a H2SO4 exfoliation route. The amphoteric nature due to the simultaneous presence of carboxyl and amino groups provided a strategy to fabricate heterostructures by an electrostatic re-assembly between g-C3N4 nanosheets and various charged guests in different pH systems.

•BiOBr/pg-C3N4 was prepared by an electrostatically-driven in-situ growth method.•BiOBr/pg-C3N4 exhibited a superior visible-light activity and stability.•The efficient separation of charges due to the intimate interface of BiOBr/pg-C3N4.In this work, enhanced photocatalytic activity of BiOBr/graphitic C3N4 heterojunctions for degradation of Rhodamine B (RhB) were obtained by the protonation pretreatment of graphitic C3N4 with hydrochloric acid. A possibly electrostatic interaction between protonated graphitic C3N4 (pg-C3N4) and BiOBr provided a closely intimate interface in the heterojunction, which was demonstrated by the results of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). This tight coupling was favorable for the charge transfer between pg-C3N4 and BiOBr, and therefore promoted the effective separation of photogenerated electron–hole pairs. The effect of composition in heterojunctions on photocatalytic activity was investigated, and the optimal photocatalyst with a BiOBr/pg-C3N4 mass ratio of 7:3 showed a superior activity, which was 35.03 and 33.72 times higher than that over pg-C3N4 and BiOBr, respectively. Radical trap experiments confirmed that the holes and superoxide radical species were the main reactive species in the RhB photodegradation process. Moreover, the stability of BiOBr/pg-C3N4 heterojunction was also tested and the RhB degradation efficiency declined by only 9.6% after seven successive cycles.Graphical abstract

•An activated ordered mesoporous carbon (AOMC) material was prepared via a facile carbonization/activation route.•This carbonization/activation method is simple, timesaving and inexpensive than other approaches.•The AOMC material possesses developed hierarchical micro/mesoporous structure, higher surface area and large pore volume.•It exhibits high specific capacitance, good rate performance and excellent cycling stability.Activated ordered mesoporous carbon (AOMC) material was prepared by a one-step carbonization/activation method using SBA-15 as hard template, furfuryl alchohol (FA) as carbon source and ZnCl2 as activation agent. The as-synthesized AOMC possesses hierarchical micro/mesoporous structure and high BET surface area (up to 1465.7 m2 g− 1), which accelerate the ion-transport and charge-transfer during charging/discharging process for supercapacitors. A maximum specific capacitance value of 270 F g− 1 for AOMC electrode at a current density of 1 A g− 1 in 6 M KOH aqueous solution was obtained from the charge–discharge curves. Meanwhile, it also exhibited outstanding rate performance (~ 80% capacity retention from 0.5 A g− 1 to 20 A g− 1) and excellent cycling stability (∼ 94% retention after 5000 continuous charge–discharge cycles). The assembled AOMC-based symmetric cell delivers a maximum energy density of 11.8 Wh kg− 1 at a power density of 450.1 W kg− 1 under a voltage range of 0–1.8 V in 0.5 M Na2SO4 aqueous electrolyte.

•An exfoliation-restacking strategy was used.•The catalyst possesses a porous structure with large surface area.•The catalyst exhibits a visible light absorption with –OOH modification.•The catalyst shows much higher activity than P25 and the layered precursor.•The catalyst has a high stability for dye degradation.In this work, an extremely high activity for visible light driven photocatalytic degradation of organic dyes was achieved on a restacked titanate nanosheets (rs-TNSs) material with the assistance of hydrogen peroxide (H2O2). The photocatalyst was obtained via exfoliating layered titanate precursor and the following flocculation by acidic solution, and its porous structure was further characterized with these technologies of scan electron microscopy, X-ray diffraction and nitrogen sorption. This rs-TNSs material exhibited prominent activity for photodecomposing Rhodamine B, ~ 90% dyes being removed in the first 5 min, and the photocatalytic efficiency was much higher than those of the layered precursor and commercial Degussa P25 TiO2 under the same condition. The possible mechanism for enhanced photocatalytic activity was proposed: enlargement of the absorption spectrum to visible region by the surface modification of peroxide groups; acceptance of photogenerated electrons by H2O2 to reduce the recombination of electron–hole pairs and produce oxidative species of hydroxyl radicals. Additionally, the influences of H2O2 dosage, dye concentration and the reusability of photocatalyst were further investigated.

This paper describes the preparation of graphitic porous carbon spheres (GPCS) from spherical resorcinol/formaldehyde resin by Fe-catalysis at 900 °C. The GPCS were characterized by their highly graphitized structures, uniform spherical morphology with an average diameter of ∼450 nm, pore size of 1–4 nm and relatively large surface area of ∼1100 m2 g−1. Their electrochemical performance was studied using cyclic voltammetry and galvanostatic charge–discharge measurements, and the results showed an enhanced charge storage capacity, with a specific capacitance of 127.4 F g−1 in 2 M KOH at a current density of 0.2 A g−1 that was nearly 3 times larger than that of amorphous porous carbon spheres. Moreover, electrochemical impedance spectroscopy tests demonstrated the low electrical resistance and ion transfer resistance of the GPCS, which resulted in the high retention of specific capacitance at a 10 A g−1 current density. The recycling experiments indicated their superior stability, and 96% of their initial specific capacitance was maintained after 5000 cycles.

•A free-standing photocatalytic film was assembled with titanate nanosheets.•This film possesses excellent flexibility and transmittance.•The film exhibits visible light absorption with –OOH modification.•The film shows high activity using an indoor fluorescent lamp.•The catalyst has a high stability for dye degradation.We herein report the reassembly of individual titanate nanosheets (TNSs) that were exfoliated from layered titanates to form a free-standing photocatalytic film by vacuum filtration. This TNSs film was characterized with high orientation, as well as excellent light transparency and flexibility. With the presence of H2O2, an extremely high activity and stability of the TNSs film were achieved for photo-decomposing Rhodamine B (RhB) using a 9 W fluorescent lamp. A possible mechanism was proposed that the modification of –OOH groups enlarged the spectral region to visible light and high separation rate of photogenerated carriers was realized by transferring electrons on conduction band to H2O2. The trapping experiments of active species demonstrated that holes on valence band are the main oxidant species.

Thanks to the unique architectural design, nanosized porous carbon materials exhibit better behavior as electrical double-layer capacitors than conventional carbon-base materials. In this work, porous carbon sphere (PCS) materials with superior porosity and uniform nanospherical morphology were successfully prepared by means of a facile chemical activation route. The analysis of pore structure and morphology of the resultant PCS were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, N2 sorption technology and electron microscope. The results indicated that this PCS material possess remarkable porosity, extremely large surface area (∼2500 m2 g−1), large pore volume (1.37 cm3 g−1) and narrow pore distribution (2.73 nm). The well-developed mesoporous structure and high surface area benefited the PCS to exhibit an excellent charge storage capacity with a specific capacitance of 196 F g−1 in 2 M KOH at a current density of 0.5 A g−1 and long-term cycling stability over 1000 cycles. Compared with ordered mesoporous carbon and other porous carbon materials, PCS present an enhanced electrochemical performance, which could be attributed to its high surface area and well-developed mesoporosity, as well as its nanospherical morphology, favoring the ion accumulation on the electrode surface and facilitating fast electrolyte ion transportation.

Graphitic carbon nitride (C3N4)–BiVO4 heterojunctions with various mass ratios of C3N4 and BiVO4 were synthesized by a simple hydrothermal method. High-resolution transmission electron microscopy (HR-TEM) results show that an interface of intimate contact is formed between C3N4 and BiVO4 in heterojunctions. The UV-vis diffuse reflection spectra reveal that the resulting C3N4–BiVO4 heterojunctions exhibit more intensive absorption within the visible light range in comparison with pure C3N4. The photocatalytic tests demonstrate that the resulting C3N4–BiVO4 heterojunctions possess significantly enhanced photocatalytic activities for methylene blue (MB) degradation under visible light irradiation compared with individual C3N4 and BiVO4. The optimum photocatalytic activity of the 0.7 C3N4–0.3 BiVO4 heterojunction is almost 3.5 and 2.8 times higher than those of individual C3N4 and BiVO4, respectively. On the basis of experimental result, a possible photocatalytic mechanism that has superoxide radical species as the mainly active species in photocatalysis is proposed. Additionally, the present study provides a useful strategy to design heterojunction materials with enhanced photocatalytic performance.

•A solvothermal method was used to prepare C3N4–Bi2MoO6 heterojunction.•The obtained heterojunction has an enhanced visible absorption compared with C3N4.•The material owns a high visible light activity and stability for dye degradation.Novel graphitic carbon nitride (C3N4)–Bi2MoO6 heterojunctions with different contents of Bi2MoO6 nanosheets were in situ synthesized by a simple solvothermal method. The resulting C3N4–Bi2MoO6 heterojunctions possess enhanced absorption within the visible light range compared with pure C3N4. Meanwhile, these C3N4–Bi2MoO6 heterojunctions also exhibit much higher photocatalytic activity for methyl orange (MO) degradation than those of single C3N4 and Bi2MoO6, as well as Degussa P25. Significantly, the optimum photocatalytic activity of the 0.2C3N4–0.8Bi2MoO6 heterojunction is 3.56 and 3.45 times faster than those of either individual C3N4 or Bi2MoO6. Based on the trapping experiment results, we believe that photogenerated holes and superoxide radicals O2− play a major role in C3N4/Bi2MoO6 heterogeneous system under visible light irradiation. What is more, the heterojunction displays excellent stability and reusability. This work provides a promising strategy for constructing more C3N4 based heterojunctions.

•Nanospherical porous NiO was synthesized by a facile hard template method.•The material possesses high surface area and superior mesoporous structure.•The prepared NiO electrodes exhibit prominent specific capacitance.A nanospherical porous NiO electrode material was successfully prepared by a nanocasting method using porous carbon nanospheres as a hard template. The obtained porous NiO materials were characterized with high crystalline degree, larger surface area than 200 m2 g-1, 5–6 nm mesoporous channels and nanospherical morphology with 500–700 nm in diameter, which render them excellent electrochemical performance. The galvanostatic charge–discharge measurements demonstrated that the optimal electrode possessed a high specific capacitance of 1201 F g−1 at a discharge current density of 0.5 A g−1. In addition, specific capacitances at different discharge current densities indicated a good capacitance at high current density and electrochemical impedance spectra suggested low charge transfer resistance of the electrodes. It also exhibited cycling stability of 70% capacity retention after 500 continuous charge/discharge cycles.

•Hollow porous carbon spheres prepared via a facial activation method.•One-step activation treatment to obtain hollow and porous structure simultaneously.•This material possess high surface area, large pore volume and narrow pore size.•It possesses an excellent adsorption property for the removal of phenol.Hollow porous carbon spheres (HPCS) were prepared by a facile activation strategy. During the one-step activation treatment we could obtain the hollow and porous structures simultaneously. The HPCS materials possessed good monodispersity, uniform hollow morphology and very high surface area. Besides, these materials also exhibited excellent adsorption ability for the removal of phenol from the waste water.

Novel organic–inorganic composites composed of two visible light responsive semiconductors of graphitic carbon nitride (C3N4) and CdS were successfully synthesized via an “in situ” precipitation–deposition method. The C3N4–CdS heterostructures were fabricated by depositing CdS nanoparticles onto the surface of C3N4. The morphology and optical property of compsoites can be tuned by adjusting the mass ratio of C3N4–CdS, which determines the enhanced level of photocatalytic activity. The optimum activity of 0.7C3N4–0.3CdS photocatalyst is almost 20.5 and 3.1 times higher than those of individual C3N4 and CdS for the degradation of methyl orange, and 41.6 and 2.7 fold higher for the degradation of 4-aminobenzoic acid, respectively. Moreover, its activity is also much higher than those of C3N4–TiO2 and CdS–TiO2 composites, as well as N-modified TiO2. Of special significance is that the present C3N4–CdS composites exhibit high stabilities under illumination, in contrast with CdS. The enhancement in both performance and stability should be assigned to the effective separation and transfer of photogenerated charges originating from the well-matched overlapping band-structures and closely contacted interfaces. Our work highlights that coupling semiconductors with well-matched band energies provides a flexible route to improve the activity and stability of photocatalysts, and gives ideas for the design and synthesis of other highly active and stable materials.

Graphitic carbon nitride (C3N4) was hybridized by Bi2WO6 via a hydrothermal method. The high-resolution transmission electron microscopy (HR-TEM) results reveal that an intimate interface between C3N4 and Bi2WO6 forms in the heterojunctions. The UV–vis diffuse reflection spectra show that the resulting C3N4–Bi2WO6 heterojunctions possess more intensive absorption within the visible light range in comparison with pure Bi2WO6. These excellent structural and spectral properties endowed the C3N4–Bi2WO6 heterojunctions with enhanced photocatalytic activities. Significantly, the optimum photocatalytic activity of the 0.5C3N4–0.5Bi2WO6 heterojunction for the degradation of methyl orange (MO) was almost 3 and 155 times higher than those of either individual C3N4 or Bi2WO6. The possible photocatalytic mechanism with superoxide radical species as the main active species in photocatalysis is proposed on the basis of experimental results. Moreover, the heterojunction depicted high stability and durability during six successive cycles.Keywords: Bi2WO6; C3N4; heterojunction; photocatalysis;

•An exfoliation–restacking strategy and following assembly were used.•The composite possesses a mesoporous structure.•The obtained material exhibits an absorption in the visible region.•The photocatalyst owns a high visible light activity and stability for dye degradation.Yellow–colored pure titania with a mesoporous structure was prepared by the aggregate of titania nanocrystals, which were stabilized by exfoliated titanate nanosheets via an electrostatic interaction. X–ray diffraction patterns and images of transmission electron microscope confirm that titanate sheets are randomly dispersed into the assembled titania nanocrystals without forming any self–restacked phase. This nanocrystals–nanosheets composite exhibits a mesoporous structure with pore size of ~ 6.5 nm and surface area of 236.3 m2 g− 1. Greatly different from the UV–responded properties of titania nanocrystals and titanate nanosheets, the absorption edge of nanocomposite red–shifts to visible light region. The visible light photocatalytic tests demonstrate that this nanocomposited titania shows excellent activity for the degradation of organic dyes, as well as a colorless organic pollutant of 2, 4–dichlorophenol. The possible photocatalytic mechanism that photogenerated holes as the mainly oxidant species in photocatalysis is proposed based on the trapping experiments of hydroxyl radicals or photogenerated holes. Moreover, as the nanocomposite depicts an extreme stability, no obvious deactivation occurs after five cycles.Figure optionsDownload full-size imageDownload as PowerPoint slide

Here, we reported an excellent carbon-based solid acid catalyst for the catalytic synthesis of biodiesel, which was prepared by carbonizing a mesoporous phenolic resin and then followed by sulfonation with concentrated sulfuric acid. The influence of carbonization temperature on the pore structure and acidity was studied. Fourier transform infrared spectroscopy, energy dispersive spectrum analysis and an indirect titration method were used to demonstrate the successful modification of –SO3H groups on the carbon surface and determine the acidities of catalysts. N2 adsorption–desorption and transmission electron microscopy were used to characterize the mesoporous structure and pore structure parameters. These results indicated that the sulfonated catalyst carbonized at a low temperature (400 °C) showed the highest acidity of 2.21 mmol H+ g−1, meanwhile, retaining a mesoporous structure and relatively large surface area. The esterification reaction of oleic acid with methanol was employed to evaluate the performance of catalysts. The sulfonated mesoporous carbon catalyst exhibited a highly efficient activity, above 95% conversion of oleic acid with a 30:1 methanol/oleic acid at 70 °C for 3 h. Experimental parameters, including the molar ratio of reactants, reaction time and reaction temperature, were optimized and a superior recycling property was presented after five consecutive cycles.

The hydrophobic surface of ordered mesoporous carbon (OMC) via a hard-template route is difficult to sulfonate, which limits its practical applications in solid acid catalysis. Here, we reported an OMC solid acid catalyst with large surface area and ordered mesostructure, as well as high acidity. The pretreatment of hard-template prepared OMC with H2O2 brought numerous hydrophilic groups onto the carbon surface, which are favorable for the modification of −SO3H groups by a followed sulfonation treating with sulfuric acid. The results of X-ray diffraction, N2 adsorption–desorption, and transmission electron microscopy demonstrated the preservation of large surface area and ordered mesostructure. Infrared spectra indicated the successful modification of −SO3H groups and in the meantime suggested the existence of hydrophilic groups (−COOH and −OH). The acidity of catalyst estimated by an indirect titration method and the modified amount of −SO3H groups examined by energy dispersive spectra were 2.09 mmol H+ g–1 and 1.86 mmol −SO3H g–1, respectively. Such multifunctionalized OMC material with hydrophilic groups (−SO3H, −COOH, −OH) and the hydrophobic framework (polycyclic aromatic carbon) showed evidently improved catalytic activities for biodiesel production. Furthermore, its excellent stability and recycling property were demonstrated by five consecutive cycles.

Mesoporous carbon/silica composites functionalized with –SO3H groups were prepared via polymerization and carbonization of glucose into mesoporous silica SBA-15 and a followed sulfonation by sulphuric acid. These composites were characterized by powder X-ray diffraction, N2 adsorption–desorption and transmission electron microscopy, which suggested the preservation of ordered mesoporous structure, as well as a novel spherical morphology. The result of fourier transform infrared spectroscopy indicated the successful modification of –SO3H groups and the acidity of catalysts was determined by an indirect titration method. The composite with 40 % carbon loading possessing the highest acidity in synthesized catalysts and the ordered mesoporous structure without pore blocking exhibited a remarkable catalytic activity for biodiesel production. Experimental parameters including the carbon loading, molar ratio of reactants, reaction time and reaction temperature were optimized. In addition, a superior recycling property was exhibited after five consecutive cycles.

The high recombination rate of photogenerated charges is the main problem that limits the photocatalytic activity of semiconductor photocatalysts. Herein, we have reported novel heterojunctions of BiOBr–carbon nitride (BiOBr–C3N4) fabricated by depositing BiOBr nanoflakes onto the surface of C3N4. These visible light responsive heterojunctions possess intimately contacted interfaces and well-aligned straddling band-structures, which are propitious to the effective separation and transfer of photogenerated charges, bringing an improved performance. Their photocatalytic activities were evaluated by degrading Rhodamine B in aqueous solution induced by visible or indoor light. The optimum photocatalytic activity of the 0.5BiOBr–0.5C3N4 heterojunction was almost 4.9 and 17.2 times as high as those of individual BiOBr and C3N4 under visible light irradiation, and 1.5 and 48.9 times as high under indoor light irradiation, respectively. Moreover, its activity was also much higher than those of TiO2 (P25), BiOBr–TiO2, and C3N4–TiO2 heterojunctions. On the basis of experimental and theoretical results, the photocatalytic mechanism was proposed, which revealed that organic molecules were mainly oxidized by holes concentrated in the valence band of C3N4. Our work highlights that the design of heterojunctions with well-aligned straddling band-structures by combining two visible light responsive semiconductors provides an efficient method to prepare new photocatalysts working with natural light.

A novel Mn-intercalated layered titanate as highly active photocatalyst in visible-light region has been synthesized via a convenient and efficient exfoliation–flocculation approach with divalent Mn ions and monolayer titanate nanosheets. The 0.91 nm interlayer spacing of obtained photocatalyst is in accordance with the sum of the thickness of titanate nanosheet and the diameter of Mn ions. The yellow photocatalyst shows a spectral response in visible-light region and the calculated band gap is 2.59 eV. The photocatalytic performance of this material was evaluated by degradation and mineralization of an aqueous dye methylene blue under visible-light irradiation, and an enhanced photocatalytic activity in comparison with protonated titanate as well as the P25 TiO2 and N-doped TiO2 was obtained. Additionally, the layered structure is retained, no dye ions intercalating occurs during the photocatalysis process, and a ∼90% photocatalytic activity can be remained after reusing 3 cycles.Graphical abstractMn-intercalated layered titanate as a novel and efficient visible-light harvesting photocatalyst was synthesized via a convenient and efficient exfoliation–flocculation approach in a mild condition.Highlights► Mn-intercalated titanate has been prepared by exfoliation–flocculation approach. ► The as-prepared catalyst shows spectral response in the visible-light region. ► Heat treatment at certain temperature enables formation of Mn-doped TiO2. ► Dye can be degradated effectively by the catalyst under visible light irradiation.

A mesoporous CdS/titania heterogeneous structure has been synthesized via the self-assembly of Cd2+ ions and titania nanosheets and the subsequent sulfuration. The composited structure that CdS nanoparticles were encapsulated into titania nanosheets exhibited an enhanced visible light harvesting, as well as a large surface area and two kinds of mesoporous structures derived from the restacking of nanosheets and the pillaring of CdS nanoparticles, respectively. The results of photodegrading rhodamine B under visible light illumination suggested that this CdS/titania composite possessed a high activity compared with its layered titanate precursors and the Cd2+ intercalated form. Moreover, this material was extremely stable and displayed an excellent reusability after 5 cycles.Highlights► An exfoliation-restacking strategy and a subsequent sulfuration were used. ► The composite possesses a large surface area. ► The obtained material exhibits an extended absorption in visible region. ► The catalyst owns a high visible light activity and stability for dye degradation.

A hydrophilic mesoporous carbon (H-MS) has been prepared by a rapid redox reaction between mesoporous carbon (CMK-3) and an acidic potassium permanganate (KMnO4) solution at room temperature. The obtained material has a hydrophilic surface by the modification of oxygen-containing groups, and meanwhile retains the ordered mesoporous structure. No obvious difference of pore size between H-MS and CMK-3, and the slight decrease of surface area and pore volume is due to the modification of oxygen-containing groups on the carbon surface. An improved property for adsorbing dyes in aqueous solution was observed in H-MC, and the adsorption amount at equilibrium is ~ 3 times higher than that of CMK-3.

•Proteins with various isoelectric points were immobilized into titanate layers.•A novel polyelectrolyte-assisted electrostatic self-assembly technique was used.•The native structures of proteins were retained after immobilizing.•The immobilized proteins exhibit excellent thermal stability and reusability.A general strategy was demonstrated here to immobilize proteins with various isoelectric points (IPs) in layered titanates. The immobilization of proteins with relative low IPs, such as bovine serum albumin (BSA) and lipase, in layered titanates was successfully by a novel polyelectrolyte-assisted electrostatic self-assembly technique, which is impossible by a conventional electrostatic self-assembly method. Lysozyme with relative high IP was detractively interacted with negative titanate nanosheets to form a bioinorganic composite. The native structures of proteins were retained after immobilizing although a significant difference in microstructures was observed among these composites. The amounts of immobilized proteins were up to ∼68.3 wt.% for lysozyme, 37.2 wt.% for BSA and 21.5 wt.% for lipase. These composites were stable in the neutral and weakly acidic condition, and only releases <10% proteins in the pH < 4 solution. The immobilized lysozyme and lipase exhibit excellent thermal stability, which retain their initial activities of about 70% at 70 °C for about 40 min. In addition, these composites are reusable, and the residual activities of immobilized enzymes are 68% for lysozyme and 61% for lipase after 10 recycles.Download full-size image

Herein, we employed the exfoliated two-dimensional (2D) graphitic carbon nitride nanosheets (CNNS) and titania nanosheets (TNS) as building blocks, and these negatively charged nanosheets were flocculated by Cd2+ ions with a followed sulfidation treatment to produce a ternary heterostructure photocatalyst of CNNS/CdS/TNS. This novel nanocomposite exhibited outstanding absorption in visible spectral region, and meanwhile its gradient band structure and the closed interface promoted the separation of photo-generated charge. The relative content of CNNS and TNS in the ternary nanocomposite was optimized, and the optimal photocatalyst with a CNNS/TNS mass ratio of 98:2 could rapidly remove 10 mg L−1 rhodamine B (RhB) in 20 min under visible light irradiation. The calculated rate constant of CNNS/CdS/TNS was 56.87, 12.18, and 6.67 times higher than those of the restacked CNNS and TNS and the individual CdS, as well as 8.31, 6.22 and 2.57 times higher than those of binary CNNS/TNS, CdS/TNS and CdS/CNNS photocatalysts, respectively. Moreover, this nanocomposite possessed a superior durability and universality for degradation of RhB in different concentration and other organic pollutants, including dyes and colorless compounds. Finally, the possible photocatalytic mechanism was proposed based on the theoretical calculation and the active species quenching experiment.

The high recombination rate of photogenerated charges is the main problem that limits the photocatalytic activity of semiconductor photocatalysts. Herein, we have reported novel heterojunctions of BiOBr–carbon nitride (BiOBr–C3N4) fabricated by depositing BiOBr nanoflakes onto the surface of C3N4. These visible light responsive heterojunctions possess intimately contacted interfaces and well-aligned straddling band-structures, which are propitious to the effective separation and transfer of photogenerated charges, bringing an improved performance. Their photocatalytic activities were evaluated by degrading Rhodamine B in aqueous solution induced by visible or indoor light. The optimum photocatalytic activity of the 0.5BiOBr–0.5C3N4 heterojunction was almost 4.9 and 17.2 times as high as those of individual BiOBr and C3N4 under visible light irradiation, and 1.5 and 48.9 times as high under indoor light irradiation, respectively. Moreover, its activity was also much higher than those of TiO2 (P25), BiOBr–TiO2, and C3N4–TiO2 heterojunctions. On the basis of experimental and theoretical results, the photocatalytic mechanism was proposed, which revealed that organic molecules were mainly oxidized by holes concentrated in the valence band of C3N4. Our work highlights that the design of heterojunctions with well-aligned straddling band-structures by combining two visible light responsive semiconductors provides an efficient method to prepare new photocatalysts working with natural light.

Novel organic–inorganic composites composed of two visible light responsive semiconductors of graphitic carbon nitride (C3N4) and CdS were successfully synthesized via an “in situ” precipitation–deposition method. The C3N4–CdS heterostructures were fabricated by depositing CdS nanoparticles onto the surface of C3N4. The morphology and optical property of compsoites can be tuned by adjusting the mass ratio of C3N4–CdS, which determines the enhanced level of photocatalytic activity. The optimum activity of 0.7C3N4–0.3CdS photocatalyst is almost 20.5 and 3.1 times higher than those of individual C3N4 and CdS for the degradation of methyl orange, and 41.6 and 2.7 fold higher for the degradation of 4-aminobenzoic acid, respectively. Moreover, its activity is also much higher than those of C3N4–TiO2 and CdS–TiO2 composites, as well as N-modified TiO2. Of special significance is that the present C3N4–CdS composites exhibit high stabilities under illumination, in contrast with CdS. The enhancement in both performance and stability should be assigned to the effective separation and transfer of photogenerated charges originating from the well-matched overlapping band-structures and closely contacted interfaces. Our work highlights that coupling semiconductors with well-matched band energies provides a flexible route to improve the activity and stability of photocatalysts, and gives ideas for the design and synthesis of other highly active and stable materials.

In this review, the relatively systematic description of g-C3N4 nanosheets for photocatalysis is presented for the first time. We firstly briefly introduce the structure and properties of bulk g-C3N4, and then put emphasis on the comparison of various exfoliation approaches. This review also summarizes the recent development in the fabrication of novel photocatalysts based on nanosheets and their potential applications in energy conversion and environmental treatments. At the end of this article, a summary and perspective are presented for highlighting some challenges and opportunities in the future.

A novel non-light-driven catalysis by the delaminated two dimensional titanate nanosheets (TNSs) has been explored for degradation of organic dyes with hydrogen peroxide (H2O2). This catalyst can efficiently remove dyes at high concentration and over a wide pH range, as well as with a long cycle number and superior universality.

A stable colloid of g-C3N4 nanosheets was prepared on a large scale via a H2SO4 exfoliation route. The amphoteric nature due to the simultaneous presence of carboxyl and amino groups provided a strategy to fabricate heterostructures by an electrostatic re-assembly between g-C3N4 nanosheets and various charged guests in different pH systems.

A novel graphitized porous carbon nanosphere (GPCNS) material obtained by a convenient simultaneous activation and graphitization route, which was realized by heating resorcinol–formaldehyde (RF) resin nanospheres immersed with ZnCl2 and FeCl3 in an inert atmosphere, has been reported. A high graphitization level was achieved through the catalytic graphitization by reduced Fe metal, and the hierarchically micro/mesoporous structure was produced in a controlled fashion by tuning the mass ratio of the activating agent ZnCl2 and a carbon precursor. An optimal sample of GPCNS-2 was prepared with a FeCl3/ZnCl2/RF mass ratio of 0.5:2:1, exhibiting a highly graphitized framework and uniform spherical morphology with an average diameter of ∼500 nm, as well as well-interconnected micro/mesoporous structure with a large surface area of 1664.8 m2 g−1. Acting as an electrode material for a supercapacitor application in 6 M KOH electrolyte, GPCNS-2 displayed excellent electrochemical performance with a high specific capacitance of 402.5 F g−1 at a current density of 1 A g−1. Moreover, an electrochemical impedance spectroscopy test demonstrated that the low internal electrical resistance of GPCNS-2 contributed a superior rate capability of above 75% retention rate even at 50 A g−1. Furthermore, the GPCNS-2 electrode possessed an outstanding cycling stability, and about 96% of its initial specific capacitance at 5 A g−1 was maintained after 5000 cycles.