Structure Elucidation of a Melam-Melem Adduct by a Combined Approach of Synchrotron X-ray Diffraction and DFT Calculations

Melam-melem (1:1), an adduct compound that can be obtained from dicyandiamide in autoclave reactions at 450 °C and elevated ammonia pressure, had previously been described based on mass spectrometry and NMR spectroscopy, but only incompletely characterized. The crystal structure of this compound has now been elucidated by means of synchrotron microfocus diffraction and subsequent quantum-chemical structure optimization applying DFT methods. The structure was refined in triclinic space group P 1 ‾ based on X-ray data. Cell parameters of a=4.56(2), b=19.34(8), c=21.58(11) Å, α=73.34(11)°, β=89.1(2)°, and γ=88.4(2)° were experimentally obtained. The resulting cell volumes agree with the DFT optimized value to within 7 %. Molecular units in the structure form stacks that are interconnected by a vast array of hydrogen bridge interactions. Remarkably large melam dihedral angles of 48.4° were found that allow melam to interact with melem molecules from different stack layers, thus forming a 3D network. π-stacking interactions appear to play no major role in this structure.

New nitrides: from high pressure-high temperature synthesis to layered nanomaterials and energy applications

We describe work carried out within our group to explore new transition metal and main group nitride phases synthesized using high pressure-high temperature techniques using X-ray diffraction and spectroscopy at synchrotron sources in the USA, UK and France to establish their structures and physical properties. Along with previously published data, we also highlight additional results that have not been presented elsewhere and that represent new areas for further exploration. We also describe new work being carried out to explore the properties of carbon nitride materials being developed for energy applications and the nature of few-layered carbon nitride nanomaterials with atomically ordered structures that form solutions in polar liquids via thermodynamically driven exfoliation. This article is part of the theme issue 'Fifty years of synchrotron science: achievements and opportunities'.

Visible light-activated degradation of natural organic matter (NOM) using zinc-bismuth oxides-graphitic carbon nitride (ZBO-CN) photocatalyst: Mechanistic insights from EEM-PARAFAC

In this study, the complex degradation behavior of natural organic matter (NOM) was explored using photocatalytic oxidation systems with a novel catalyst based on a hybrid composite of zinc-bismuth oxides and g-C₃N₄ (ZBO-CN). The photooxidation system demonstrated the effective removal of NOM under low-intensity visible light irradiation, presenting removal rates of 53-74% and 65-88% on the basis of dissolved organic carbon (DOC) and the UV absorption coefficient (UV₂₅₄), respectively, at 1.5 g/L of the catalyst. The NOM removal showed an increasing trend with a higher ZBO-CN dose. Comparative experiments with the hole and OH radical scavengers revealed that the direct oxidation occurring on the catalyst's surface might be the governing photocatalytic mechanism. Fluorescence excitation emission matrix-parallel factor analysis (EEM-PARAFAC) revealed the individual removal behavior of the different constituents in bulk NOM. Different tendencies towards preferential adsorption and subsequent oxidative removal were found among dissimilar fluorescent components within a bulk terrestrial NOM, following the order of terrestrial humic-like (C1) > humic-like (C2) > microbial humic-like (C3) components. The result suggests the dominant operation of π-π and/or hydrophobic interactions between the NOM and the catalyst. The discriminative removal behavior was more pronounced in visible light versus UV-activated systems, probably due to the incapability of visible light to excite è - h⁺ pairs of ZnO and the triplet state of NOM. The high photoactivity and structural stability of ZBO-CN under visible light implies its potential for an effective, low-cost and energy-saving treatment technology to selectively remove large sized humic-like substances from water.

Formation of an ion-free crystalline carbon nitride and its reversible intercalation with ionic species and molecular water

Crystalline layered carbon nitrides can be inter-converted by simple ion exchange process allowing their properties to be tuned.

The development of processes to tune the properties of materials is essential for the progression of next-generation technologies for catalysis, optoelectronics and sustainability including energy harvesting and conversion. Layered carbon nitrides have also been identified as of significant interest within these fields of application. However, most carbon nitride materials studied to date have poor crystallinity and therefore their properties cannot be readily controlled or easily related to their molecular level or nanoscale structures. Here we report a process for forming a range of crystalline layered carbon nitrides with polytriazine imide (PTI) structures that can be interconverted by simple ion exchange processes, permitting the tunability of their optoelectronic and chemical properties. Notable outcomes of our work are (a) the creation of a crystalline, guest-ion-free PTI compound that (b) can be re-intercalated with ions or molecules using “soft chemistry” approaches. This includes the intercalation of HCl, demonstrating a new ambient pressure route to the layered PTI·xHCl material that was previously only available by a high-pressure-high-temperature route (c). Our work also shows (d) that the intercalant-free (IF-) PTI material spontaneously absorbs up to 10 weight% H₂O from the ambient atmosphere and that this process is reversible, leading to potential applications for membranes and water capture in dry environments.

Direct functionalization of methane into ethanol over copper modified polymeric carbon nitride via photocatalysis

Direct valorization of methane to its alcohol derivative remains a great challenge. Photocatalysis arises as a promising green strategy which could exploit hydroxyl radical (·OH) to accomplish methane activation. However, both the excessive ·OH from direct H2O oxidation and the neglect of methane activation on the material would cause deep mineralization. Here we introduce Cu species into polymeric carbon nitride (PCN), accomplishing photocatalytic anaerobic methane conversion for the first time with an ethanol productivity of 106 μmol gcat-1 h-1. Cu modified PCN could manage generation and in situ decomposition of H2O2 to produce ·OH, of which Cu species are also active sites for methane adsorption and activation. These features avoid excess ·OH for overoxidation and facilitate methane conversion. Moreover, a hypothetic mechanism through a methane-methanol-ethanol pathway is proposed, emphasizing the synergy of Cu species and the adjacent C atom in PCN for obtaining C2 product.

Electrically-Transduced Chemical Sensors Based on Two-Dimensional Nanomaterials

Electrically-transduced sensors, with their simplicity and compatibility with standard electronic technologies, produce signals that can be efficiently acquired, processed, stored, and analyzed. Two dimensional (2D) nanomaterials, including graphene, phosphorene (BP), transition metal dichalcogenides (TMDCs), and others, have proven to be attractive for the fabrication of high-performance electrically-transduced chemical sensors due to their remarkable electronic and physical properties originating from their 2D structure. This review highlights the advances in electrically-transduced chemical sensing that rely on 2D materials. The structural components of such sensors are described, and the underlying operating principles for different types of architectures are discussed. The structural features, electronic properties, and surface chemistry of 2D nanostructures that dictate their sensing performance are reviewed. Key advances in the application of 2D materials, from both a historical and analytical perspective, are summarized for four different groups of analytes: gases, volatile compounds, ions, and biomolecules. The sensing performance is discussed in the context of the molecular design, structure-property relationships, and device fabrication technology. The outlook of challenges and opportunities for 2D nanomaterials for the future development of electrically-transduced sensors is also presented.

Two-dimensional polyimide heterojunctions for the efficient removal of environmental pollutants under visible-light irradiation

Synergism of enhanced light absorption and well intimate interfacial charge transition providing excellent photocatalytic performance of polymide heterojunctions PI–TD_x.

Effect of calcination temperature, pH and catalyst loading on photodegradation efficiency of urea derived graphitic carbon nitride towards methylene blue dye solution

The appropriate synthesis temperature and optimized photodegradation reaction conditions result in an appreciable enhancement of the photocatalytic activity of urea derived innate g-C₃N₄ towards MB dye degradation.

Structure and reactivity of 2,4,6-tricyano-1,3,5-triazine under high-pressure conditions

2,4,6-Tricyano-1,3,5-triazine is an attractive precursor for the synthesis of extended layered and nanoporous carbon nitrides.

Novel design of hollow g-C3N4 nanofibers decorated with MoS2 and S, N-doped graphene for ternary heterostructures

Herein, we newly design 1-dimensional ternary structure of HGCNF/MoS₂/SNG via a one-pot hydrothermal treatment at relatively low temperature and showed a higher double layer capacitance with HER activity.

Surface-Group-Oriented, Condensation Cyclization-Driven, Nitrogen-Doping Strategy for the Preparation of a Nitrogen-Species-Tunable, Carbon-Material-Supported Pd Catalyst

A nitrogen-carbon framework with the thickness of several molecules was fabricated through a straightforward nitrogen-doping strategy, in which specially designed surface-oxygen-containing groups (SOGs) first introduced onto the porous carbon support were used to guide the generation of a surface-nitrogen-containing structure through condensation reactions between SOGs and the amidogen group of organic amines under hydrothermal conditions. The results indicate that different kinds of SOGs generate different types and abundances of N species. The CO-releasing groups are apt to form a high proportion of amino groups, whereas the CO2-releasing groups, especially carboxyl and lactones, are mainly transformed into pyrrolic-type nitrogen. In the framework with dominant pyrrolic-type nitrogen, an electron-rich Pd activated site composed of Pd, pyrrolic-type N and C is built, in which electron transfer occurs from N to C and Pd atoms. This activated site contributes to the formation of electron-rich activated hydrogen and desorption of p-chloroaniline, which work together to achieve the superior selectivity about 99.90 % of p-chloroaniline and the excellent reusable performance. This strategy not only provides low-cost, nitrogen-doped carbon materials, but also develops a new method for the fabrication of different kinds of nitrogen species structures.

Emerging trends in sensors based on carbon nitride materials

A new class of functional materials, carbon nitrides, has recently attracted the attention of researchers.

Chemical Simultaneous Synthesis Strategy of Two Nitrogen-Rich Carbon Nanomaterials for All-Solid-State Symmetric Supercapacitor

Present work demonstrates a single step process for simultaneous synthesis of metal-nanoparticle-encapsulated nitrogen-doped bamboo-shaped carbon nanotubes (M/N-BCNTs) and graphitic carbon nitride (G-C₃N₃). The synthesis of two different carbon nanostructures in a single step is recognized for the first time. This process involves the use of inexpensive and nontoxic precursors such as melamine as carbon and nitrogen sources for the growth of G-C₃N₃ and M/N-BCNTs. In this technique, the utilization of unwanted gases such as ammonia and hydrocarbons released during the decomposition of melamine is the key to grow M/N-BCNTs over the catalyst along with the formation of G-C₃N₄. The implementation of M/N-BCNTs as the electrode material for all-solid-state symmetric supercapacitor results in a maximum specific capacitance of ∼368 F g^-1 with excellent electrochemical stability with 97% capacity retention after 10 000 cycles. Furthermore, fabricated symmetric supercapacitor shows maximum high energy and power density up to 10.88 W h kg^-1 and 2.06 kW kg^-1, respectively. The superior electrochemical activity of M/N-BCNTs can be attributed to its high surface to area volume ratio, unique structural characteristics, ultrahigh electrical conductivity, and carrier mobility.

Carbon nitrides and metal nanoparticles: from controlled synthesis to design principles for improved photocatalysis

The use of sunlight to drive chemical reactions via photocatalysis is of paramount importance towards a sustainable future. Among several photocatalysts, earth-abundant polymeric carbon nitride (PCN, often wrongly named g-C3N4) has emerged as an attractive candidate due to its ability to absorb light efficiently in the visible and near-infrared ranges, chemical stability, non-toxicity, straightforward synthesis, and versatility as a platform for constructing hybrid materials. Especially, hybrids with metal nanoparticles offer the unique possibility of combining the catalytic, electronic, and optical properties of metal nanoparticles with PCN. Here, we provide a comprehensive overview of PCN materials and their hybrids, emphasizing heterostructures with metal nanoparticles. We focus on recent advances encompassing synthetic strategies, design principles, photocatalytic applications, and charge-transfer mechanisms. We also discuss how the localized surface plasmon resonance (LSPR) effect of some noble metals NPs (e.g. Au, Ag, and Cu), bimetallic compositions, and even non-noble metals NPs (e.g., Bi) synergistically contribute with PCN in light-driven transformations. Finally, we provide a perspective on the field, in which the understanding of the enhancement mechanisms combined with truly controlled synthesis can act as a powerful tool to the establishment of the design principles needed to take the field of photocatalysis with PCN to a new level, where the desired properties and performances can be planned in advance, and the target material synthesized accordingly.

Spectroscopic Fingerprints of Intermolecular H-Bonding Interactions in Carbon Nitride Model Compounds

The effect of intermolecular H-bonding interactions on the local electronic structure of N-containing functional groups (amino group and pyridine-like N) that are characteristic of polymeric carbon nitride materials p-CN(H), a new class of metal-free organophotocatalysts, was investigated. Specifically, the melamine molecule, a building block of p-CN(H), was characterized by X-ray photoelectron (XPS) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The molecule was studied as a noninteracting system in the gas phase and in the solid state within a H-bonded network. With the support of DFT simulations of the spectra, it was found that the H-bonds mainly affect the N 1s level of the amino group, leaving the N 1s level of the pyridine-like N mostly unperturbed. This is responsible for a reduction of the chemical shift between the two XPS N 1s levels relative to free melamine. Consequently, N K-edge NEXAFS resonances involving the amino N 1s level also shift to lower photon energies. Moreover, the solid-state absorption spectra showed significant modification/quenching of resonances related to transitions from the amino N 1s level to σ* orbitals involving the NH₂ termini.

Nanoscale Probing of Local Hydrogen Heterogeneity in Disordered Carbon Nitrides with Vibrational Electron Energy-Loss Spectroscopy

In graphitic carbon nitrides, (photo)catalytic functionality is underpinned by the effect that residual hydrogen content, manifesting in amine (N-H _x) defects, has on its optoelectronic properties. Therefore, a detailed understanding of the variation in the local structure of graphitic carbon nitrides is key for understanding structure-activity relationships. Here, we apply aloof-beam vibrational electron energy-loss spectroscopy in the scanning transmission electron microscope (STEM) to locally detect variations in hydrogen content in two different layered carbon nitrides with nanometer resolution. Through low dose rate TEM, we obtain atomically resolved images from crystalline and disordered carbon nitrides. By employing an aloof-beam configuration in a monochromated STEM, radiation damage can be dramatically reduced, yielding vibrational spectra from carbon nitrides to be assessed on 10's of nanometer length scales. We find that in disordered graphitic carbon nitrides the relative amine content can vary locally up to 27%. Cyano (C≡N) defects originating from uncondensed precursor are also revealed by probing small volumes, which cannot be detected by infrared absorption or Raman scattering spectroscopies. The utility of this technique is realized for heterogeneous soft materials, such as disordered graphitic carbon nitrides, in which methods to probe catalytically active sites remain elusive.

Carbon Nitride Materials as Efficient Catalyst Supports for Proton Exchange Membrane Water Electrolyzers

Carbon nitride materials with graphitic to polymeric structures (gCNH) were investigated as catalyst supports for the proton exchange membrane (PEM) water electrolyzers using IrO₂ nanoparticles as oxygen evolution electrocatalyst. Here, the performance of IrO₂ nanoparticles formed and deposited in situ onto carbon nitride support for PEM water electrolysis was explored based on previous preliminary studies conducted in related systems. The results revealed that this preparation route catalyzed the decomposition of the carbon nitride to form a material with much lower N content. This resulted in a significant enhancement of the performance of the gCNH-IrO₂ (or N-doped C-IrO₂) electrocatalyst that was likely attributed to higher electrical conductivity of the N-doped carbon support.

Carbon Nitride Nanothread Crystals Derived from Pyridine

Carbon nanothreads are a new one-dimensional sp³ carbon nanomaterial. They assemble into hexagonal crystals in a room temperature, nontopochemical solid-state reaction induced by slow compression of benzene to 23 GPa. Here we show that pyridine also reacts under compression to form a well-ordered sp³ product: C₅NH₅ carbon nitride nanothreads. Solid pyridine has a different crystal structure from solid benzene, so the nontopochemical formation of low-dimensional crystalline solids by slow compression of small aromatics may be a general phenomenon that enables chemical design of properties. The nitrogen in the carbon nitride nanothreads may improve processability, alters photoluminescence, and is predicted to reduce the bandgap.

Photophysics and Photocatalysis of Melem: A Spectroscopic Reinvestigation

Graphitic carbon nitride (g-CN) is one potential metal-free photocatalyst. The photocatalytic mechanism of g-CN is related to the heptazine ring building unit. Melem is the simplest heptazine-based compound and g-CN is its polymeric product. Thus, studies on the photophysical properties of melem will help to understand the photocatalytic mechanism of heptazine-based materials. Herein, the spectroscopic features of melem were systematically explored through measuring its absorption spectrum, fluorescence spectrum, and fluorescence decay. Both fluorescence spectroscopy and fluorescence decay measurements show that the condensation of melamine to melem causes stronger photoluminescence, whereas the condensation of melem to g-CN causes weaker photoluminescence. In addition, all observations reveal that a mixture of monomer melem and its higher condensates is more easily obtained during the preparation of melem, and that the higher condensates of melem affect the photophysical properties of melem dominantly. The photocatalytic hydrogen evolution of melem has also been measured and the monomer melem has negligible photoinduced water-splitting activity.

Investigation on the Stability of Derivative Melam from Melamine Pyrolysis under High Pressure

Although various kinds of carbon nitride precursors have been proposed, s-triazine-based structures are hardly reported because of their unfavorable energy, higher than that of heptazine-based ones. In this study, we investigate the thermal stability of s-triazine-based melam processed at a high pressure of 5 GPa and a temperature of 400–700 °C and complete the analyses of the composition and structure of the treated samples through X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and elemental analyses (EA). Results show that melam can stably exist up to 600 °C at 5 GPa. XRD and FTIR analyses reveal that residual melamine can be pyrolyzed into melam as temperature increases from 400 °C to 600 °C at a high pressure, suggesting that melam may be purified through high-pressure pyrolysis. Further melam polymerization at a higher pressure is a promising strategy for the preparation of s-triazine-based carbon nitride precursors used for bulk carbon nitride synthesis.

Carbon Nitride Decorated Ball-Flower like Co₃O₄ Hybrid Composite: Hydrothermal Synthesis and Ethanol Gas Sensing Application

Recently, semiconducting metal oxide (SMO) gas sensors have attracted the attention of researchers for high conductivity, labile features by environment, low cost, easy preparation, etc. However, traditional SMOs have some defects such as higher operating temperature and lower response value, which greatly limit their application in the field of gas sensor. In this work, the carbon nitride decorated ball-flower like Co₃O₄ composite was successfully synthesized via a facile hydrothermal method, the composition and morphology of the as-synthesized samples were studied by the techniques of X-ray powder diffraction (XRD), Field-emission scanning electron microscopy (FESEM), Transmission electron microscopy (TEM), Fourier transform infrared spectrometer (FT-IR) and N₂-sorption. As a consequence, the pure Co₃O₄ and the carbon nitride decorated Co₃O₄ both possess ball-flower like structure, and the as-synthesized carbon nitride decorated Co₃O₄ composite exhibits significant sensing properties to ethanol which is 1.6 times higher than that of pure Co₃O₄, furthermore, the composite possesses high selectivity and stability towards ethanol detection.

Two dimensional boron nanosheets: synthesis, properties and applications

As a material generating increasing interest, boron nanosheets have been reviewed from the perspective of their synthesis, properties, application and possible research directions.

Mechanism of photocatalytic water splitting with triazine-based carbon nitrides: insights from ab initio calculations for the triazine–water complex

Valuable theoretical insights into the mechanism of photocatalytic water-splitting using triazine as a model system for carbon-nitride materials.

Nanoscale, conformal films of graphitic carbon nitride deposited at room temperature: a method for construction of heterojunction devices

Graphitic carbon nitrides (GCNs) represent a family of 2D materials composed of carbon and nitrogen with variable amounts of hydrogen, used in a wide variety of applications. We report a method of room temperature thin film deposition which allows ordered GCN layers to be deposited on a very wide variety of substrates, including conductive glass, flexible plastics, nanoparticles and nano-structured surfaces, where they form a highly conformal coating on the nanoscale. Film thicknesses of below 20 nm are achievable. In this way we construct functional nanoscale heterojunctions between TiO₂ nanoparticles and GCN, capable of producing H₂ photocatalytically under visible light irradiation. The films are hydrogen rich, have a band gap around 1.7 eV, display transmission electron microscopy lattice fringes as well as X-ray diffraction peaks despite being deposited at room temperature, and show characteristic Raman and IR bands. We use cluster etching to reveal the chemical environments of C and N in GCN using X-ray photoelectron spectroscopy. We elucidate the mechanism of this deposition, which operates via sequential surface adsorption and reaction analogous to atomic layer deposition. The mechanism may have implications for current models of carbon nitride formation.

Single Crystal, Luminescent Carbon Nitride Nanosheets Formed by Spontaneous Dissolution

A primary method for the production of 2D nanosheets is liquid-phase delamination from their 3D layered bulk analogues. Most strategies currently achieve this objective by significant mechanical energy input or chemical modification but these processes are detrimental to the structure and properties of the resulting 2D nanomaterials. Bulk poly(triazine imide) (PTI)-based carbon nitrides are layered materials with a high degree of crystalline order. Here, we demonstrate that these semiconductors are spontaneously soluble in select polar aprotic solvents, that is, without any chemical or physical intervention. In contrast to more aggressive exfoliation strategies, this thermodynamically driven dissolution process perfectly maintains the crystallographic form of the starting material, yielding solutions of defect-free, hexagonal 2D nanosheets with a well-defined size distribution. This pristine nanosheet structure results in narrow, excitation-wavelength-independent photoluminescence emission spectra. Furthermore, by controlling the aggregation state of the nanosheets, we demonstrate that the emission wavelengths can be tuned from narrow UV to broad-band white. This has potential applicability to a range of optoelectronic devices.