Graphitic Carbon Nitride Nanoparticles: Structure, Photocatalytic Properties, Synthesis Methods, and Emerging Applications

Introduction

Solar energy is the most abundant renewable resource, capable of meeting the world’s energy demands for millennia [1]. Its intermittent nature, however, poses challenges for efficient harvesting, storage, and utilization [4]. Technologies that convert solar photons directly into chemical energy—most notably photocatalysis pioneered by Becquerel in 1839 and later advanced by Fujishima and Honda in 1972—offer a promising route for sustainable energy and environmental remediation [3,5]. In particular, photocatalytic degradation of wastewater, a major source of organic contaminants from industrial processes, remains a critical application [3,5].

Polymeric g‑C₃N₄ is a visible‑light responsive semiconductor with a band gap of 2.7 eV and conduction/valence band positions of –1.1 eV and +1.6 eV vs. NHE, respectively [6]. Its robust thermal, acidic, and alkaline stability [7] and simple synthesis from inexpensive N‑rich precursors such as melamine or urea distinguish it from metal‑based photocatalysts that require costly salts [6,8]. Various synthetic strategies—including thermal condensation, solvothermal, CVD, microwave‑assisted, and hydrothermal methods—enable tailored nanostructures with enhanced photocatalytic activity [9].

Given its versatility in water splitting, CO₂ reduction, organic pollutant degradation, and catalytic synthesis, g‑C₃N₄ has attracted extensive research attention in recent years. This review focuses on the latest developments in its structure, synthesis, and applications, with an emphasis on nanostructuring and composite formation.

Review

Graphitic Carbon Nitride and Photocatalysis

Photocatalysis refers to the acceleration of oxidation–reduction reactions by a semiconductor under light irradiation. The process generates electron–hole pairs that transfer to adsorbed species, producing reactive radicals such as •OH and O₂^•– [11]. In g‑C₃N₄, visible‑light excitation creates these charge carriers, enabling water oxidation, hydrogen evolution, and CO₂ reduction when coupled with suitable co‑catalysts or sacrificial agents [12‑16].

Schematic diagram of the basic mechanisms of the photocatalytic activity of water splitting. Reproduced with permission [113, 114]. Copyright 2015 & 2018. The Royal Society of Chemistry

Unlike conventional oxides such as TiO₂ or ZnO, g‑C₃N₄ features a graphitic‑like framework of sp² C–N bonds, enabling efficient charge transport and a tunable band structure [26‑30]. The material’s structure can be manipulated via polymorph selection (α, β, or melon‑based networks) and nanostructuring to improve photocatalytic performance [18,32].

Graphitic Carbon Nitride Nano‑Based Particle

One‑dimensional nanostructures of g‑C₃N₄ exhibit anisotropic electron transport and enhanced surface area, leading to superior photocatalytic activity [17]. 2D nanosheets derived from bulk g‑C₃N₄ via exfoliation (thermal, ultrasonic, or chemical) provide high dispersion and additional reactive sites [46‑48].

a Tri‑s‑triazine and b tri‑s‑triazine as unit structures of g‑C₃N₄. Reproduced with permission [25, 31]. Copyright 2008 Royal Society of Chemistry

Structure of Graphitic Carbon Nitride Nano‑Based Particle

g‑C₃N₄ is a two‑dimensional polymeric network of sp²‑hybridized C and N atoms. Its crystalline structure can be probed by XRD, XPS, and Raman spectroscopy, revealing characteristic peaks corresponding to C–N–C and C=N bonds [19‑23]. The α‑C₃N₄ phase, identified by Yu et al. [24], is a graphite‑like lattice with interlayer spacing slightly larger than that of graphite, reflecting nitrogen substitution in the graphitic framework [25].

Electronic Structure and Properties of g‑C₃N₄

First‑principles studies have identified the most stable heptazine‑based polymorphs (phase 1 > phase 2 > phase 3) and highlighted the role of nitrogen lone pairs in shaping the valence band [26,32]. These insights explain the material’s visible‑light responsiveness and its superior photocatalytic performance compared to traditional oxides.

Preparation of Graphitic Carbon Nitride Nano‑Based Particle

Synthesis

Thermal polycondensation of N‑rich precursors (melamine, urea, cyanamide, dicyandiamide) yields bulk g‑C₃N₄ with low surface area (<10 m² g⁻¹). Mesoporous and nanoscale variants can be achieved via hard or soft templating, sol–gel, or exfoliation, dramatically increasing surface area (up to 830 m² g⁻¹) and porosity [35,49,50].

TEM images of TCN (a & b) and MCN (c & d) using a hard templating approach. Reproduced with permission from [120]. Copyright 2015. Elsevier

Techniques Used in Preparing Graphitic Carbon Nitride Nano‑Based Particle

Key synthesis routes include:

• Thermal condensation (high‑temperature calcination of precursors)
• Solvothermal and hydrothermal methods (providing crystalline products without post‑annealing)
• Chemical vapor deposition (CVD) and plasma‑enhanced CVD for thin films
• Microwave‑assisted synthesis (rapid heating, high surface area)
• Sol–gel and physical vapor deposition (PVD) for controlled morphologies

Applications of Graphitic Carbon Nitride

g‑C₃N₄ demonstrates broad applicability across several domains:

Solar Energy Utilization

By tuning its electronic structure through doping, mesoporosity, or heterojunction formation, g‑C₃N₄ can drive visible‑light photocatalytic water splitting, CO₂ reduction, and solar fuel generation with high efficiency [77,78]. Recent strategies include sacrificial templating to produce mesoporous spheres and rods for enhanced charge separation [79].

Schematic drawing illustrating synthetic route (templating method) and the mechanism of charge separation and photocatalytic process over C3N4 and Ag@C3N4 photocatalysts under light irradiation. Reproduced with permission [124]. Copyright 2014 Elsevier.

Wastewater and Environmental Treatment

Semiconductor photocatalysts such as g‑C₃N₄ effectively degrade a wide range of organic pollutants—including dyes, pesticides, and pharmaceuticals—without generating secondary waste [86‑90]. Enhancements via metal (Cu, Fe) or non‑metal (B, C, O, S) doping, heterojunctions, and noble‑metal loading further boost degradation rates [93‑112]. Ultrafine g‑C₃N₄ nanosheets exhibit superior activity for methylene blue and methyl orange degradation due to their high surface area and reactive edge sites [92,125].

Biomedical and Sensing Applications

g‑C₃N₄ nanoparticles are attractive for bioimaging, drug delivery, and sensing because of their photoluminescence, biocompatibility, and pH‑responsive behavior. Ultrathin nanosheets can label cell membranes and serve as photosensitizers for cancer therapy [53,53].

Future Perspectives

Advancing g‑C₃N₄ research requires:

• Developing scalable synthesis routes that yield high‑surface‑area, defect‑controlled nanostructures.
• Systematic evaluation of photocatalytic efficiency, cost, energy input, and recyclability for real‑world applications.
• Exploring multifunctional composites for integrated solar‑fuel production, environmental remediation, and biomedical devices.

Conclusions

g‑C₃N₄ remains a promising, metal‑free photocatalyst with tunable structure and high stability. Continued innovation in synthesis and nanostructuring will unlock its full potential across energy, environmental, and biomedical fields.

Abbreviations

g‑C₃N₄:

Graphite carbon nitride

TiO₂:

Titanium oxide

ZnO:

Zinc oxide