Graphitic carbon nitride nanotubes: a new material for emerging applications

We provide a critical review of the current state of the synthesis and applications of nano- and micro-tubes of layered graphitic carbon nitride. This emerging material has a huge potential for light-harvesting applications, including light sensing, artificial photosynthesis, selective photocatalysis, hydrogen storage, light-induced motion, membrane technologies, and can become a major competitor for such established materials as carbon and titania dioxide nanotubes. Graphitic carbon nitride tubes (GCNTs) combine visible-light sensitivity, high charge carrier mobility, and exceptional chemical/photochemical stability, imparting this material with unrivaled photocatalytic activities in photosynthetic processes, such as water splitting and carbon dioxide reduction. The unique geometric GCNT structure and versatility of possible chemical modifications allow new photocatalytic applications of GCNTs to be envisaged including selective photocatalysts of multi-electron processes as well as light-induced and light-directed motion of GCNT-based microswimmers. Closely-packed arrays of aligned GCNTs show great promise as multifunctional membrane materials for the light energy conversion and storage, light-driven pumping of liquids, selective adsorption, and electrochemical applications. These emerging applications require synthetic routes to GCNTs with highly controlled morphological parameters and composition to be available. We recognize three major strategies for the GCNT synthesis including templating, supramolecular assembling of precursors, and scrolling of nano-/microsheets, and outline promising routes for further progress of these approaches in the light of the most important emerging applications of GCNTs.

Nanostructured Carbon Nitrides for CO2 Capture and Conversion

Carbon nitride (CN), a 2D material composed of only carbon (C) and nitrogen (N), which are linked by strong covalent bonds, has been used as a metal-devoid and visible-light-active photocatalyst owing to its magnificent optoelectronic and physicochemical properties including suitable bandgap, adjustable energy-band positions, tailor-made surface functionalities, low cost, metal-free nature, and high thermal, chemical, and mechanical stabilities. CN-based materials possess a lot of advantages over conventional metal-based inorganic photocatalysts including ease of synthesis and processing, versatile functionalization or doping, flexibility for surface engineering, low cost, sustainability, and recyclability without any leaching of toxic metals from photocorrosion. Carbon nitrides and their hybrid materials have emerged as attractive candidates for CO₂ capture and its reduction into clean and green low-carbon fuels and valuable chemical feedstock by using sustainable and intermittent renewable energy sources of sunlight and electricity through the heterogeneous photo(electro)catalysis. Here, the latest research results in this field are summarized, including implementation of novel functionalized nanostructured CNs and their hybrid heterostructures in meeting the stringent requirements to raise the efficiency of the CO₂ reduction process by using state-of-the-art photocatalysis, electrocatalysis, photoelectrocatalysis, and feedstock reactions. The research in this field is primarily focused on advancement in the synthesis of nanostructured and functionalized CN-based hybrid heterostructured materials. More importantly, the recent past has seen a surge in studies focusing significantly on exploring the mechanism of their application perspectives, which include the behavior of the materials for the absorption of light, charge separation, and pathways for the transport of CO₂ during the reduction process.

Synergistic Effect of Hydrochloric Acid and Phytic Acid Doping on Polyaniline-Coupled g-C3N4 Nanosheets for Photocatalytic Cr(VI) Reduction and Dye Degradation

In this study, graphitic carbon nitride (g-C₃N₄) nanosheets (CNns) were modified using polyaniline (PANI) codoped with an inorganic (hydrochloric acid, HCl) and an organic (phytic acid, PA) acid. Our results revealed that these samples exhibited extended visible-light absorption and a three-dimensional (3D) hierarchical structure with a large specific surface area. They also inhibited photoluminescence emission, reduced electrical resistance, and provided abundant free radicals, resulting in high photocatalytic performance. The PANI/g-C₃N₄ sample demonstrated outstanding photocatalytic activity of a Cr(VI) removal capacity of 4.76 mg·min^-1·g_c^-1, which is the best record for the reduction of a 100 ppm K₂Cr₂O₇ solution. Moreover, g-C₃N₄ coupled with PANI monotonically doped with HCl or PA did not demonstrate increased activity, suggesting that the codoping of HCl and PA plays a significant role in enhancing the performance. The improved photocatalytic activity of PANI/g-C₃N₄ can be attributed to the interchain and intrachain doping of PA and HCl over PANI, respectively, to create a 3D connected network and synergistically increase the electrical conductivity. Therefore, new insights into g-C₃N₄ coupled with PANI and codoped by HCl and PA may have excellent potential for the design of g-C₃N₄-based compounds for efficient photocatalytic reactions.

Carbon nitride nested tubes with graphene as a dual electron mediator in Z-scheme photocatalytic deoxynivalenol degradation

Schematic illustration of the electronic energy level structure and mechanism in the photocatalytic reaction of Z-scheme CNTZG systems under light irradiation.

Syntheses of mono and bimetallic cyamelurate polymers with reversible chromic behaviour

MOPs and MMOPs were synthesized in water and their crystals exhibit switchable chromic behaviour and reversible solid-state structural transformations.

Bimetallic Ag-Cu Supported on Graphitic Carbon Nitride Nanotubes for Improved Visible-Light Photocatalytic Hydrogen Production

This work, for the first time, reports visible-light active bare graphitic carbon nitride nanotubes (C₃N₄ NTs) for photocatalytic hydrogen generation, even in the absence of any cocatalyst. Upon uniform dispersion of the cocatalysts, Ag-Cu nanoparticles, on the well-ordered bare C₃N₄ NTs, they exhibit twice the H₂ evolution rate of the bare C₃N₄ NTs. The improved activity is attributed to their unique tubular nanostructure, strong metal-support interaction, and efficient photoinduced electron-hole separation compared to their bare and monometallic counterparts, evidenced by complementary characterization techniques. This work reveals that the H₂ production rates correlate well with the oxidation potentials of the sacrificial reagents used. Triethylamine (TEA) outperforms other sacrificial reagents, including triethanolamine (TEOA) and methanol. Mechanistic studies on the role of various sacrificial reagents in photocatalytic H₂ generation demonstrate that irreversible photodegradation of TEA into diethylamine and acetaldehyde via monoelectronic oxidation contributes to the improved H₂ yield. Similarly, TEOA is oxidized to diethanolamine and glycolaldehyde, whereas methanol is unable to quickly capture the photoinduced holes and remains intact due to the low oxidation potential.

Two isomeric solid carbon nitrides with 1 : 1 stoichiometry which exhibit strong mechanical anisotropy

Two isomeric layered carbon nitride polymorphs are characterized using reliable theoretical methods. The NCNC phase, which is predicted for the first time, has a number of differences with the isomeric NCCN polymorph in its electronic, spectral and mechanical properties.

Structural, electronic and optical properties of a hybrid triazine-based graphitic carbon nitride and graphene nanocomposite

The interfacial effect on the structural, electronic and optical properties of a hybrid triazine-based graphitic carbon nitride and graphene nanocomposite is calculated using the first-principles method.

Stability and electronic structures of triazine-based carbon nitride nanotubes

In contrast to the case of carbon nanotubes, the negative strain energy indicates that single-walled and double-walled triazine-based carbon nitride nanotubes (TACNNTs) are appreciably more stable than the single layer and the bilayer of g^t-C₃N₄, respectively. Boron-doping turns the material into a magnetic semiconductor.

Transparent half metallic g-C4N3 nanotubes: potential multifunctional applications for spintronics and optical devices

Multifunctional material brings many interesting issues because of various potential device applications. Using first principles calculations, we predict that the graphitic carbon nitride (g-C4N3) nanotubes can display multifunctional properties for both spintronics and optical device applications. Very interestingly, armchair tubes (n, n) with n = 2, 3, 4, 5, 6 and (5, 0) zigzag tubes are found to be half metallic, while zigzag tubes (n, 0) with n = 4, 6 show an antiferromagnetic ground state with band gaps. However, larger zigzag tubes of (7, 0), (8, 0), and (10, 0) are turned out to be half metallic. Along with the half metallic behavior of the tubes, those tubes seem to be optically transparent in the visible range. Due to these magnetic and optical properties, we suggest that the g-C4N3 nanotubes (CNNTs) can be used for both ideal spintronics and transparent electrode materials. We also explored the stability of magnetic state and nanotube structure using ab initio molecular dynamics. The CNNTs were found to be thermally stable and the magnetic moment was robust against the structural deformation at 300 K. Overall, our theoretical prediction in one dimensional CNNTs may provide a new physics in spintronics and also in other device applications because of potential multifunctional properties.

Metal-functionalized single-walled graphitic carbon nitride nanotubes: a first-principles study on magnetic property

The magnetic properties of metal-functionalized graphitic carbon nitride nanotubes were investigated based on first-principles calculations. The graphitic carbon nitride nanotube can be either ferromagnetic or antiferromagnetic by functionalizing with different metal atoms. The W- and Ti-functionalized nanotubes are ferromagnetic, which are attributed to carrier-mediated interactions because of the coupling between the spin-polarized d and p electrons and the formation of the impurity bands close to the band edges. However, Cr-, Mn-, Co-, and Ni-functionalized nanotubes are antiferromagnetic because of the anti-alignment of the magnetic moments between neighboring metal atoms. The functionalized nanotubes may be used in spintronics and hydrogen storage.

First-principles study of CN carbon nitride nanotubes

The structural, electronic and optical properties of carbon nitride (CN) nanotubes (CNNTs) built from triazine units were investigated by using density-functional theory (DFT) within the generalized gradient approximation. The total energies per CN unit (relative values) of armchair CNNTs are smaller than those of zigzag CNNTs for the same tube index. This leads to unsmooth tubular surfaces for nitrogen-nitrogen lone pair repulsions and porous structures. The armchair CNNTs appear as polygons while zigzag CNNTs are circles. Both the armchair and zigzag CNNTs are direct band gap semiconductors and their band gaps are dependent on tube size and chirality. The calculated dielectric functions of CNNTs are dependent on tube size and the directions of polarization. Moreover, they may be used as photocatalysts to split water to produce hydrogen.