Graphitic carbon nitride (g-C₃N₄) is a polymeric semiconductor material composed primarily of carbon and nitrogen, arranged in a layered structure analogous to graphite. Its unique architecture and bonding configurations make it a versatile material for applications in photocatalysis, energy storage, and sensing. The structural properties of g-C₃N₄ are defined by its layered arrangement, covalent bonding, and polymorphism, all of which are influenced by synthesis conditions. Techniques such as X-ray diffraction (XRD) and transmission electron microscopy (TEM) are critical for probing these structural features.
The layered architecture of g-C₃N₄ consists of stacked two-dimensional sheets held together by van der Waals forces. Each sheet is composed of tri-s-triazine (C₆N₇) or triazine (C₃N₃) units connected via tertiary amines. The tri-s-triazine-based structure is the most thermodynamically stable form, featuring a planar arrangement of heptazine rings with periodic vacancies. These vacancies contribute to the material's porosity and active sites for chemical interactions. The interlayer spacing typically ranges between 0.32 and 0.33 nm, slightly larger than that of graphite due to the presence of nitrogen lone pairs that induce repulsion between layers.
Bonding configurations in g-C₃N₄ are primarily sp²-hybridized, with carbon atoms forming covalent bonds with nitrogen in a hexagonal lattice. The nitrogen atoms exhibit two coordination environments: bridging nitrogen (N-(C)₃) and terminal nitrogen (N-H or C=N). The presence of hydrogen in the terminal nitrogen groups allows for hydrogen bonding between layers, influencing the material's mechanical stability and exfoliation behavior. The electronic structure is characterized by a bandgap between 2.4 and 2.8 eV, depending on the degree of condensation and functional group presence.
Polymorphism in g-C₃N₄ arises from different stacking sequences and functional group arrangements. The two primary phases are the alpha-phase (α-C₃N₄) and beta-phase (β-C₃N₄), distinguished by their atomic arrangements and mechanical properties. The alpha-phase exhibits a pseudo-cubic structure with higher density, while the beta-phase adopts a hexagonal configuration similar to graphite. Additionally, defects such as nitrogen vacancies, carbon dopants, and edge-terminated groups introduce variations in electronic and catalytic properties.
Synthesis conditions play a crucial role in determining the structural features of g-C₃N₄. Thermal polycondensation of nitrogen-rich precursors like melamine, urea, or cyanamide is the most common method. The temperature profile significantly impacts the degree of polymerization and interlayer spacing. For instance, synthesis at 500–600 °C yields a partially condensed structure with abundant terminal amino groups, while temperatures above 600 °C produce a more condensed, defect-free framework with reduced interlayer spacing. The heating rate and atmosphere (air, nitrogen, or argon) further influence crystallinity and defect density.
The choice of precursor also affects the structural properties. Melamine-derived g-C₃N₄ tends to form well-ordered layers with uniform porosity, whereas urea-derived samples exhibit higher defect concentrations due to the release of gaseous byproducts during condensation. Post-synthetic treatments such as protonation, exfoliation, or doping can modify interlayer interactions and introduce functional groups. For example, acid treatment increases interlayer spacing by inserting protons between layers, while thermal exfoliation reduces layer thickness and enhances surface area.
X-ray diffraction is a primary tool for analyzing the crystalline structure of g-C₃N₄. The XRD pattern typically shows a strong peak at around 27.4°, corresponding to the (002) plane reflection from interlayer stacking, and a weaker peak near 13.1°, attributed to in-plane structural packing (100) of the heptazine units. The full width at half maximum (FWHM) of these peaks provides insights into crystallite size and disorder. Broader peaks indicate smaller crystallites or higher defect densities, while sharp peaks suggest well-ordered domains.
Transmission electron microscopy offers direct visualization of the layered morphology and defects. High-resolution TEM (HRTEM) reveals the periodic lattice fringes with spacings consistent with the (002) plane, confirming the graphitic-like stacking. Selected-area electron diffraction (SAED) patterns further validate the hexagonal symmetry of the layers. Additionally, TEM can identify localized defects such as vacancies or amorphous regions that are not detectable by XRD. Electron energy-loss spectroscopy (EELS) complements TEM by providing chemical mapping of carbon and nitrogen distribution, highlighting inhomogeneities in stoichiometry.
In summary, the structural properties of graphitic carbon nitride are governed by its layered architecture, covalent bonding, and polymorphism, all of which are tunable through synthesis parameters. The interlayer spacing, defect concentration, and crystallinity are critical factors influencing its functional performance. Techniques like XRD and TEM provide essential insights into these structural characteristics, enabling tailored design for specific applications. Understanding these relationships is key to optimizing g-C₃N₄ for advanced technological uses.