Two-Dimensional Power Couple: Carbon Nitride Meets MXene in the Race for Solar Hydrogen

Hydrogen has long been heralded as the fuel of the future: it packs enormous energy per kilogram, emits nothing but water when consumed in a fuel cell, and could, in principle, decarbonize everything from steel mills to long-haul trucking. The catch is that almost none of it exists in its pure form on Earth. It must be pried loose from water or hydrocarbons, and today most of it comes from steam methane reforming and electrolysis, processes that carry heavy economic and environmental costs. A review published in Advances in Industrial and Engineering Chemistry by Kotesh Kumar Mandari and Misook Kang of Yeungnam University now surveys one of the most promising routes around this bottleneck: photocatalytic water splitting using hybrid materials that pair graphitic carbon nitride with a family of two-dimensional conductors known as MXenes.

The logic of photocatalysis is elegantly simple. A semiconductor absorbs a photon, kicking an electron from its valence band into its conduction band and leaving behind a positively charged hole. If the electron survives long enough to reach the surface, it can reduce protons to hydrogen gas; the holes, meanwhile, oxidize water to oxygen. The trouble is that in most materials, the electron and hole recombine almost instantly, releasing their energy as heat and accomplishing nothing. For a photocatalyst to work well, it must absorb a broad swath of the solar spectrum, separate its charge carriers efficiently, and present abundant active sites for the redox reactions. Ideal band gaps fall between roughly 1.23 and 3 electron volts, a window that balances light harvesting against redox power.

Graphitic carbon nitride, or g-C3N4, checks many of these boxes. It is a metal-free polymeric semiconductor built from triazine and heptazine rings of carbon and nitrogen, arranged in extended sp2-hybridized sheets. Its band gap of approximately 2.7 electron volts lets it absorb visible light, it is remarkably stable in acids, bases, and a wide range of organic solvents, and it can be synthesized simply by heating cheap, nitrogen-rich precursors such as melamine, urea, or thiourea to temperatures between roughly 450 and 650 degrees Celsius. Since the landmark 2009 demonstration that g-C3N4 could evolve hydrogen under visible light, the material has become one of the most studied photocatalysts in the world. Yet its Achilles heel is well known: charge carriers in pristine g-C3N4 are mobile only with difficulty, so photogenerated electron-hole pairs recombine rapidly, and the material exploits only a modest fraction of the sunlight that strikes it.

Enter MXenes, the two-dimensional transition metal carbides, nitrides, and carbonitrides first derived from layered MAX phase precursors by selectively etching away their aluminum layers, typically with hydrofluoric acid, lithium fluoride in hydrochloric acid, or molten salts. The resulting sheets, with the general formula Mn+1XnTx, carry surface terminations of hydroxyl, oxygen, or fluorine that tune their electronic structure and hydrophilicity. Crucially, MXenes behave electrically like metals: they conduct electrons with ease, offer enormous surface areas, and can be functionalized in countless ways. In a photocatalytic composite, that metallic conductivity makes them ideal electron sinks, whisking photogenerated electrons away from a semiconductor before recombination can occur. Mandari and Kang argue that this is precisely the missing ingredient for g-C3N4, and their review systematically catalogs how the two materials have been married in type-II, Z-scheme, S-scheme, and Schottky junction architectures.

The numbers emerging from laboratory studies are striking. In a type-II heterojunction, electrons flow to the semiconductor with the lower conduction band while holes migrate to the higher valence band counterpart, spatially separating the charges. Tahir and colleagues anchored g-C3N4 nanosheets onto titanium carbide MXene layers embedded with in situ grown TiO2, and found that the etching time of the MXene, varied from 24 to 96 hours, critically influenced performance. The optimized 48-hour sample delivered 310 micromoles of hydrogen per gram per hour, nearly triple the yield of bulk carbon nitride. Even more dramatic results came from a 0D/2D design in which bimetallic Mo2Ti2C3 MXene quantum dots were electrostatically self-assembled onto g-C3N4 nanosheets. Acting as electron reservoirs and cocatalysts, the quantum dots pushed the hydrogen evolution rate to 2809 micromoles per gram per hour, an order of magnitude above the bare polymer.

Type-II junctions, however, pay a hidden price: the staggered band alignment that separates charges also erodes their redox power. Z-scheme systems, modeled on the electron flow of natural photosynthesis, solve this by recombining the low-energy carriers while preserving the high-energy electrons and holes. Parida and coworkers built an all-solid-state Z-scheme from facet-exposed TiO2 derived from MXene layers, boron-doped g-C3N4, and metallic MXene sheets serving as the electron mediator. The optimized composite evolved hydrogen at 408.4 micromoles per hour with an apparent quantum efficiency of 6.7 percent, four and twenty times better than the pristine doped carbon nitride, respectively. Yuan and colleagues went further, combining Mn0.5Cd0.5S, Ti3C2 MXene flakes, and g-C3N4 into a Z-scheme that reached a remarkable 13.7 millimoles of hydrogen per gram per hour under visible light, with the MXene’s polycrystalline lattice, confirmed at a spacing of 0.28 nanometers, enabling rapid electron transport.

Schottky junctions exploit an even more direct trick: when a metallic MXene touches a semiconductor, the difference in their work functions creates a built-in electric field and a Schottky barrier that blocks backflow of electrons. A hollow-sphere photocatalyst assembled from g-C3N4 shells wrapped around Ti3C2Tx MXene achieved 982.8 micromoles per gram per hour, more than 3.5 times the output of protonated g-C3N4 alone, with the three-dimensional hollow architecture shortening electron migration distances and boosting light harvesting. Porous 3D/2D hybrids of the same pairing reached 1948 micromoles per gram per hour, while a vanadium carbide variant, V2C coupled to g-C3N4 by simple physical mixing, delivered a 4.23-fold improvement over the pristine polymer, thanks to a favorable built-in electric field at the intimate 2D/2D interface.

The review also highlights more exotic designs that blur the line between materials science and biology. An S-scheme heterojunction decorated with chlorophyll-a derivatives, whose porphyrin-like magnesium-centered rings act as molecular antennas, achieved 131 micromoles per hour per gram, outperforming the same junction without the pigment. Ternary systems push synergy further still: copolymerized carbon nitride loaded with gold nanoparticles and Ti3C2 MXene showed hydrogen evolution enhanced 99.8-fold under simulated sunlight compared with unmodified g-C3N4, driven by plasmonic light absorption from the gold and MXene-mediated electron transport. A manganese dioxide, boron and oxygen co-doped carbon nitride, and delaminated Ti3C2 hybrid produced both hydrogen at 897.2 micromoles per hour and hydrogen peroxide at 2846.4 micromoles per hour per gram, demonstrating that these platforms can generate valuable chemicals beyond fuel.

None of this means a solar hydrogen economy is around the corner. The authors are candid about the obstacles: MXenes themselves can be unstable under prolonged irradiation, long-term operational durability remains poorly characterized, and translating bench-scale milligrams into industrially relevant reactors demands reproducible, scalable synthesis. Etching MXenes with hydrofluoric acid also raises safety and environmental concerns that molten salt routes only partially resolve. Still, the trajectory is unmistakable. By combining the visible-light responsiveness and robust stability of g-C3N4 with the conductivity, tunability, and surface chemistry of MXenes, and by engineering the junction between them with increasing atomic precision, researchers are systematically dismantling the recombination problem that has plagued photocatalysis for decades. If interface engineering, defect control, and scalable manufacturing can keep pace, these two-dimensional partners may yet turn ordinary water and sunlight into the cleanest fuel on Earth.

Subject of Research: Photocatalytic hydrogen production using g-C3N4/MXene heterostructure catalysts

Article Title: g-C3N4/MXene-based heterostructures: advanced catalysts for efficient and sustainable renewable energy production

Article References: Mandari, K. K., & Kang, M. (2025). g-C3N4/MXene-based heterostructures: advanced catalysts for efficient and sustainable renewable energy production. Advances in Industrial and Engineering Chemistry, 1(1), Article 14. https://doi.org/10.1007/s44405-025-00014-z

Image Credits: AI Generated

DOI: 10.1007/s44405-025-00014-z

Keywords: g-C3N4, MXene, photocatalysis, hydrogen production, water splitting, heterojunctions, Z-scheme, Schottky junction, S-scheme, charge separation, renewable energy, two-dimensional materials