Heptazine-based g-C3N4 Research Articles

Alkali metal ion-doped heptazine-based g-C3N4 quantum dots for efficient adsorption of methyl blue: A DFT perspective

Organic effluents disposed of by industrial plants have created a serious impact on the availability of potable water, disturbed environmental sanity, and hampered the natural thriving of aquatic animals. In particular, the concentrations of methyl blue (MB) dye in different water bodies has lately risen to an alarming level. In the present study, the adsorption performance of metal ion-doped heptazine-based graphitic carbon nitride (g-C3N4) quantum dots towards MB was investigated using first-principles calculations. The computed electronic properties associated with the MB∙∙∙g-C3N4 and MB∙∙∙M+−g-C3N4 clusters in the aqueous medium indicated that doping the carbon nitride sheets with lithium ions exhibits the smallest HOMO-LUMO gap and the highest charge potential, as compared to the potassium and sodium counterparts. The interaction in MB∙∙∙Li+-g-C3N4 was shown to become weaker in acidic media. The effective adsorption of the metal-doped nanosheets was predicted to increase in the order K+-g-C3N4 < Na+-g-C3N4 < Li+-g-C3N4. Protonation of the nanoclusters resulted in a stronger attraction of the dye molecule while the same order for the non-protonated analogs is retained, suggesting that electrostatic attraction is the key factor that controls the adsorption behavior in such systems.

3D-crumpled graphitic carbon nitride achieving promoted visible-light-driven molecular oxygen activation for phenol degradation

Boosting optical absorption and charge transfer of g-C3N4 is of great importance but a challenging task for developing metal-free high-performance photocatalyst. Herein, 3D-crumpled g-C3N4 (DCN) is synthesized through a direct top-down thermal etching strategy. The thermal exfoliation of layered bulk g-C3N4 (BCN) in air atmosphere induces partial distortion of heptazine-based g-C3N4 nanosheet, which further self-assemble into 3D-crumpled network structure. Spectroscopic and photoelectrochemical characterization demonstrate that the unique DCN can not only remarkably extend the visible-light response region to 600 nm by awakening the n-π* electron transition, but also significantly promote O2 activation for selective H2O2 generation owing to the intensified electron delocalization and charge transport ability. Thus, DCN catalyst realizes an excellent photocatalytic phenol degradation rate under visible light irradiation (0.690 h−1), far (4.4-fold) out from the BCN counterparts. This work enables synergistic optimization of optical absorption, charge transport and surface-active sites by constructing a 3D-crumpled structure, which expands the engineering toolbox of metal-free skeleton photocatalyst for developing practical and economical catalysts for environmental remediation.

Effects of vacancies on the electronic structures and photocatalytic properties of g-C3N4

The geometric structures, electronic structures and photocatalytic properties of g-C3N4 with different vacancies are studied by the first-principles calculations. These calculations suggest that introducing different vacancies can cause the relaxation of these adjacent atoms, the formation of dangling bonds, and the deformation of these building blocks. And these band gaps of the heptazine-based graphitic C3N4 sheets are narrowed by different vacancies. The energetic preferability of the heptazine-based g-C3N4 sheets with different vacancies is assessed as functions of the environmental factors, including nitrogen partial pressure and temperature. It is indicated that VC2 and VN2 are the respectively stable vacancies over the investigated nitrogen partial pressure. A transition from VN2 to VC2 should occur at ultrahigh vacuum pN2≥∼10−23atm at 1000 K, and move to higher nitrogen partial pressure with the increasing temperature. According to the calculated absorption coefficient, these g-C3N4 sheets with VC2 and VN2 vacancies have larger absorption than the perfect structure over the visible wavelength range, suggesting that vacancies can increase the catalytic efficiency of graphitic C3N4. These results show that introducing defects, especially vacancies, is an effective approach to enhance the sunlight absorption efficiency of graphitic carbon nitride.

Enhanced visible light photocatalytic performance of crystalline g-C3N4 nanosheets by one-step molten salt method

The crystalline g-C3N4 nanosheets have been successfully prepared by one-step molten salts method. The microstructures and optical properties of as-prepared samples were characterized by different techniques, such as XRD, EDS, XPS, SEM, TEM, BET, PL and Uv–vis DRS. The results showed that the unit of the crystalline g-C3N4 nanosheets was composed of triazine ring, which was different from heptazine-based g-C3N4. The crystalline g-C3N4 nanosheets were thin and transparent, which were similar to graphene. The crystalline g-C3N4 nanosheets exhibits the excellent photocatalytic activity, which can degrade 98% RhB within 120 min under visible light irradiation. However, the degradation efficiency of bulk g-C3N4 is only 40% within 120 min. The enhanced photocatalytic activity of g-C3N4 nanosheets can be attributed to 2D nanosheet morphology, which inhibits the electron-hole recombination. In addition, the high specific surface area of g-C3N4 nanosheets positively increases the contact area with pollutants, making the degradation reaction more efficiently. A plausible photocatalytic mechanism was proposed for the degradation of RhB under visible light irradiation.

Photocatalytic production of H2O2 from water and dioxygen only under visible light using organic polymers: Systematic study of the effects of heteroatoms

The photocatalytic production of H2O2 that needs dioxygen, water, and light only has been proposed and investigated. However, most of the studies employed organic electron donors. In this work, organic photocatalysts that can produce H2O2 without organic electron donors are designed with a bottom-up approach. Although the heptazine-based g-C3N4 cannot induce water oxidation, a different kind of organic polymer (CN) obtained from the condensation of s-triazine and pyrimidine can produce H2O2 via dioxygen reduction coupled with water oxidation. The incorporation of O and P elements in CN structure (CNO, CNP, CNOP) increases the visible light absorption and hinders the photodecomposition of H2O2, respectively. The C-O and P-O bonds act as the charge trapping sites and also the preferred adsorption sites of a key intermediate, •OOH. As a result, the CNOP exhibits the highest production of H2O2 using dioxygen, water, and visible light only.

Highly crystalline carbon nitride hollow spheres with enhanced photocatalytic performance

Graphitic carbon nitride (g-C3N4) has emerged as a remarkably promising photocatalyst for addressing environmental and energy issues; however, it exhibits only moderate photocatalytic activity because of its low specific surface area and high recombination of carriers. Preparation of crystalline g-C3N4 by the molten salt method has proven to be an effective method to improve the photocatalytic activity. However, crystalline g-C3N4 prepared by the conventional molten salt method exhibits a less regular morphology. Herein, highly crystalline g-C3N4 hollow spheres (CCNHS) were successfully prepared by the molten salt method using cyanuric acid-melamine as a precursor. The higher crystallization of the CCNHS samples not only repaired the structural defects at the surface of the CCNHS samples but also established a built-in electric field between heptazine-based g-C3N4 and triazine-based g-C3N4. The hollow structure improved the level of light energy utilization and increased the number of active sites for photocatalytic reactions. Because of the above characteristics, the as-prepared CCNHS samples simultaneously realized photocatalytic hydrogen evolution with the degradation of the plasticizer bisphenol A. This research offers a new perspective on the structural optimization of supramolecular self-assembly.

Crystalline isotype heptazine-/triazine-based carbon nitride heterojunctions for an improved hydrogen evolution

The establishment of a built-in electric field between different phases of the same material is one of the most effective techniques to promote photocatalytic activity. Crystalline graphitic carbon nitride (g-C3N4) based on the heptazine or triazine structure has recently attracted extensive attention because of its excellent photocatalytic activity. However, the framework of the crystalline isotype heptazine-/triazine-based g-C3N4 (HTCN) heterojunctions remains ambiguous. Here, we developed an easy and reliable approach to prepare crystalline HTCN heterojunction by using preheated melamine as precursor and molten salts as the liquid reaction media. We investigated for the first time the interfacial interactions of the HTCN heterojunction by first principles calculations. The energy band structures show that the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of the heptazine-based g-C3N4 (HCN) are more negative than those of the triazine-based g-C3N4 (TCN) after their close contact. Therefore, the HTCN heterojunction displays remarkable charge transfer from HCN to TCN, which is beneficial to enhance photocatalytic activity. Moreover, the phase ratio of TCN and HCN was studied from 77:23 to 31:69 by changing the preheating temperature. The optimal HTCN-500 samples show the highest photocatalytic hydrogen production rate at 890 μmol h−1 g−1 with apparent quantum yield of 26.7 % at 420 nm, which is higher than those of BCN, TCN, and HCN by 15, 8 and 1.6 times, respectively. The phase content of HTCN-500 is 16 % TCN and 84 % HCN. The appropriate phase ratio between HCN and TCN increases the formation of photogenerated electrons and ameliorates the inability of the bridged nitrogen atoms in the heptazine frameworks to transfer the photogenerated electrons. This work presents a novel method to prepare a HTCN heterojunction and provides a new pathway for designing the heterostructure between different phases of the same material.

One-step synthesis of novel K+ and cyano groups decorated triazine-/heptazine-based g-C3N4 tubular homojunctions for boosting photocatalytic H2 evolution

Constructing homojunctions with different morphologies, exposing facets, crystal phases or semiconductor types in a photocatalyst is an essential approach to boost the photoactivity. Herein, the g-C3N4 phase homojunctions decorated with cyano groups and K+ were designed and successfully synthesized via only adjusting the reaction temperatures within a facile one-step molten salt route. Such unique junctions were constructed through the overgrowth of triazine-based g-C3N4 nanoparticles embedded in the surface and inner wall of heptazine-based g-C3N4 hollow tubes. Benefited from the multiple advantages of high specific surface area, unique tubular structure, enhanced visible light absorption, and fast charge transfer and separation, the triazine-/heptazine-based g-C3N4 homojunctions drastically enhanced the photocatalytic hydrogen production performance, achieving a 2 and 12-fold improvement than the pristine g-C3N4 microtube and bulk g-C3N4, respectively. This study provides an in-depth insight into the design and fabrication of other g-C3N4-based photocatalysts for more efficient solar energy conversion applications.

Thermal conductivities of two-dimensional graphitic carbon nitrides by molecule dynamics simulation

Two-dimensional (2D) graphitic carbon nitrides (2D GCNs) are a rising class of 2D polymeric materials. Compared with graphene, they have many attractive merits, such as their great semi-conductivity and photo-catalyticity. Despite much progress in studying their various properties, their thermal properties have been given little attention. In this work, thermal conductivities of three kinds of 2D GCNs, heptazine-based g-C3N4 (HEP), triazine-based g-C3N4 (TRI), and 2D polyaniline g-C3N (PANI), were computationally investigated by non-equilibrium molecule dynamics (NEMD) simulations based on both Tersoff and ReaxFF potentials, which offer comprehensive understanding of their thermal properties. It was found that the PANI was predicted to show extrapolated bulk thermal conductivities of 810 W/(m⋅K) and 461.9 W/(m⋅K) by the Tersoff and ReaxFF potentials, respectively. The HEP and TRI have much lower bulk thermal conductivities, ranging from 14.1 to 119 W/(m⋅K). Nevertheless, they are still much higher than those of traditional polymers. The ReaxFF potential also shows that carbon–nitrogen bonds are stiffer than those modeled by the Tersoff potential, resulting in higher phonon harmonicity, and longer phonon mean free paths.

A facile method for fabricating TiO2/g-C3N4 hollow nanotube heterojunction and its visible light photocatalytic performance

Novel TiO2/g-C3N4 hollow nanotube heterojunction was prepared via a molten salts method. The X-ray diffraction (XRD) result showed the crystalline phase of g-C3N4 hollow nanotube was changed from triazine-based g-C3N4 phase transition to heptazine-based g-C3N4 phase with the addition of TiO2 nanoparticles. The square hollow morphology, about 400 nm width and 60–70 nm wall thickness were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The UV-vis diffuse reflectance spectra (DRS) implied that the heterojunction can capture more visible light. The photoluminescence (PL) spectra showed that the heterojunction had efficient separation of photo generated electrons and holes. The heterojunction showed a rapid RhB degradation with the degradation rate of 95% in 60 min.

Microwave-assisted molten-salt rapid synthesis of isotype triazine-/heptazine based g-C3N4 heterojunctions with highly enhanced photocatalytic hydrogen evolution performance

Rapid synthesis and construction of graphitic carbon nitride (g-C3N4) based heterojunctions, cost-effective metal-free photocatalysts for hydrogen evolution reaction (HER) under solar irradiation, is of highly practical significance. This work reports a one-pot microwave-assisted molten-salt (mw-ms) process to rapidly synthesize isotype triazine-/heptazine based g-C3N4 heterojunctions with highly enhanced photocatalytic HER performance using melamine as the single-source precursor. The typical sample (mw-ms-g-C3N4) was obtained by thermally polymerizing melamine molecules at 550°C for 30min in the media of eutectic KCl/LiCl salts under microwave irradiation in air. The analyses of phases, chemical compositions and microstructures indicate that the mw-ms-g-C3N4 sample consists of an isotype triazine-/heptazine based g-C3N4 heterojunction, taking on a plate-like morphology with a specific surface area (SBET) of 25.7m2g−1. Comparatively, the g-C3N4 sample synthesized via an electric-resistance molten-salt (er-ms) process at 550°C for 240min is composed of a triazine-based g-C3N4 phase with a SBET of 58.1m2g−1, whereas the samples obtained by electric-resistance heating (er, at 550°C for 240min) and microwave heating (mw, at 550°C for 30min) processes consist of a heptazine-based g-C3N4 phase. The mw-ms-g-C3N4 sample shows a photocatalytic HER rate of 1480μmolg−1h−1, which is 5 times that (300μmolg−1h−1) of the er-ms-g-C3N4 sample, 15 times that (95μmolg−1h−1) of the er-g-C3N4 sample and 23 times that (63μmolg−1h−1) of the mw-g-C3N4 sample, under the similar visible-light (λ≥420nm) irradiation. The typical apparent quantum yield of the mw-ms-g-C3N4 sample at 420nm is up to 10.7%. The UV–vis DR spectra suggest that both the triazine-based g-C3N4 and heptazine-based g-C3N4 phases have a similar bandgap of ∼2.66eV, whereas the Mott-Schottky analysis indicates that the triazine-based g-C3N4 phase has a more positive flat conductive potential (−0.90V) than the triazine-based g-C3N4 phase (−1.22V). Due to the suitable alignment of their energy bandgap structures, the isotype triazine-/heptazine based g-C3N4 hybrids in the mw-ms-g-C3N4 sample form a type II heterojunction of semiconductor/semiconductor, which provides a convenient carrier transfer path and leads to more efficient separation of photo-generated electron-hole pairs than the other samples. The synergistic effects of microwave heating and molten-salt liquid polycondensation provide a robust platform for rapid and large-scale construction of isotype g-C3N4/g-C3N4 heterojunctions as metal-free high-performance HER photocatalysts using a simple single-source precursor.

Divalent Fe Atom Coordination in Two-Dimensional Microporous Graphitic Carbon Nitride

Graphitic carbon nitride (g-C3N4) is a rising two-dimensional material possessing intrinsic semiconducting property with unique geometric configuration featuring superimposed heterocyclic sp(2) carbon and nitrogen network, nonplanar layer chain structure, and alternating buckling. The inherent porous structure of heptazine-based g-C3N4 features electron-rich sp(2) nitrogen, which can be exploited as a stable transition metal coordination site. Multiple metal-functionalized g-C3N4 systems have been reported for versatile applications, but local coordination as well as its electronic structure variation upon incoming metal species is not well understood. Here we present detailed bond coordination of divalent iron (Fe(2+)) through micropore sites of graphitic carbon nitride and provide both experimental and computational evidence supporting the aforementioned proposition. In addition, the utilization of electronic structure variation is demonstrated through comparative photocatalytic activities of pristine and Fe-g-C3N4.

Crystal structure of polymeric carbon nitride and the determination of its process-temperature-induced modifications

Based on the arrangement of two-dimensional ‘melon’, we construct a unit cell for polymeric carbon nitride (PCN) synthesized via thermal polycondensation, whose theoretical diffraction powder pattern includes all major features measured in x-ray diffraction. With the help of this unit cell, we describe the process-temperature-induced crystallographic changes in PCN that occur within a temperature interval between 510 and 610 °C. We also discuss further potential modifications of the unit cell for PCN. It is found that both triazine- and heptazine-based g-C3N4 can only account for minor phases within the investigated synthesis products.

Band gap of C3N4 in the GW approximation

Ab initio calculations based on density functional theory are performed to study the stability of newly proposed C3N4 forms. Heptazine-based g-C3N4 was found to be energetically favored relative to other phases. The quasiparticle band energies of different C3N4 phases are calculated using the GW method. Among the seven phases of C3N4 studied, only the pseudocubic phase and g-h-triazine phase have direct band gaps, and all of the other phases have indirect band gaps. The band gap of α-C3N4, β-C3N4, cubic-C3N4, pseudocubic-C3N4, g-h-triazine, g-o-triazine and g-h-heptazine is 5.49 eV, 4.85 eV, 4.30 eV, 4.13 eV, 2.97 eV, 0.93 eV and 2.88 eV, respectively. From the viewpoint of band gap energies, both the g-h-heptazine and the g-h-triazine phases can be used as suitable photocatalysts for hydrogen production using water.