Atomic Swap in Two-Dimensional Nitrides Unlocks Five New Water-Splitting Photocatalysts

A single-atom substitution could transform one of the most exciting families of two-dimensional materials from a laboratory curiosity into a working engine for solar fuels. In a new first-principles study published in the Journal of Saudi Chemical Society, researchers led by Dan Hong of Chengdu University of Traditional Chinese Medicine report that swapping the silicon atoms in MA2N4 monolayers for other group III, IV, and V elements dramatically reshapes the materials’ electronic structure, and in five cases produces candidates capable of driving the oxygen evolution half of photocatalytic water splitting under illumination.

The MA2N4 family burst onto the scene in 2020, when chemists synthesized MoSi2N4 by chemical vapor deposition, creating a septuple-atomic-layer semiconductor with a band gap of roughly 1.94 electronvolts and remarkable mechanical strength of up to 66 gigapascals. The discovery opened a vast compositional playground: researchers quickly predicted dozens of analogous compounds by varying the transition metal (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, or W) and the group IV element (A = Si or Ge). Yet the flagship material MoSi2N4 carries an intrinsic handicap for photocatalysis. Its relatively large band gap and high exciton binding energy cause photogenerated electron-hole pairs to recombine rapidly, and it absorbs visible light poorly, two defects that severely limit any practical use in splitting water into hydrogen and oxygen.

Doping and substitution are the standard weapons for fixing such flaws, because they can narrow band gaps, widen the visible-light absorption window, suppress electron-hole recombination, and extend carrier lifetimes. But systematic studies of substitution within the MA2Z4 system had been scarce, leaving a significant gap in understanding how different atoms tune these materials for specific applications. Hong and colleagues, including Qi-Jun Liu, Tao Jiang, Hui Liu, Zheng-Tang Liu, and Yu-Lan Ren, set out to close that gap by replacing the A-site silicon with carbon, germanium, tin, lead, and other elements across nine transition-metal variants, then tracking every consequence from lattice constants to catalytic overpotentials.

The calculations were performed with the CASTEP plane-wave code using density functional theory, first with the generalized gradient approximation of Perdew, Burke, and Ernzerhof to screen structures rapidly, and then with the screened Coulomb hybrid functional HSE06 for accurate electronic properties. The authors note that conventional DFT systematically underestimates band gaps because it cannot properly treat the strong on-site Coulomb repulsion among localized d orbitals, whereas hybrid functionals containing a fraction of Hartree-Fock exchange describe semiconductor band structures far more faithfully. A 20-angstrom vacuum layer separated the monolayers, a 6 by 6 by 1 Monkhorst-Pack grid sampled the Brillouin zone, and structural relaxations continued until forces fell below 0.01 electronvolts per angstrom.

After filtering for positive band gaps and confirming dynamical stability with phonon spectra, the team arrived at 17 stable, semiconducting MA2N4 compounds, including CrC2N4, MoC2N4, WC2N4, TiC2N4, ZrGe2N4, ZrSn2N4, ZrPb2N4, and HfGe2N4 among others. The substitution follows clear chemical logic. Moving down a group enlarges the atomic radius and the lattice, while the unusually strong carbon-nitrogen bond produces distinctly contracted structures. Near the Fermi level, the electronic states are dominated by hybridization between transition-metal d orbitals and nitrogen p orbitals, with a modest contribution from the A-site p states. As the A-site element changes from carbon to silicon to germanium, the degree of hybridization shifts, dispersing the bands, weakening orbital localization, and tuning the band gaps in predictable sequences, for example from 2.565 to 0.522 to 0.361 electronvolts across CrC2N4, CrSi2N4, and CrGe2N4.

Band gaps alone, however, do not make a photocatalyst. Charge carriers must survive long enough to reach the surface. Here the study leaned on three interlocking metrics: carrier effective mass, the ratio of hole to electron effective mass, and exciton binding energy. A light effective mass implies high mobility; a large departure of the hole-to-electron mass ratio from unity implies rapid separation and slow recombination of photogenerated pairs; and a small exciton binding energy makes it easier for absorbed photons to free mobile charges. The calculated exciton binding energies, mostly between 0.411 and 1.774 electronvolts, beat the well-known photocatalyst g-C3N4 at 1.2 electronvolts, with several materials falling below 1 electronvolt. Compounds such as MoC2N4 showed the lowest predicted recombination rates, directly addressing the carrier-separation weakness that has plagued MoSi2N4.

Optical absorption provided the third filter. Replacing silicon with germanium narrows the band gap and pushes absorption deeper into the visible spectrum, and the group VI materials CrA2N4, MoA2N4, and WA2N4 with silicon or germanium at the A site absorbed visible light up to five orders of magnitude more efficiently than their carbon counterparts, in line with their smaller gaps and higher dielectric constants. The zirconium, titanium, and hafnium variants absorbed even better overall, with TiC2N4 and the zirconium series standing out. In short, the substitution strategy repaired both of MoSi2N4’s intrinsic photocatalytic defects, poor charge separation and weak visible-light response.

The final and most demanding test was the oxygen evolution reaction itself, a four-step process in which water adsorbs to the surface and passes through hydroxyl, oxo, and hydroperoxyl intermediates before releasing oxygen. The team evaluated adsorption at the metal, A-site, and nitrogen top sites, computed free-energy profiles including zero-point and entropic corrections, and extracted the limiting potential for each material. Photocatalysis works only when the photovoltage supplied by photogenerated holes exceeds that limiting potential. Five compounds cleared the bar: MoC2N4, WC2N4, ZrSi2N4, ZrGe2N4, and HfGe2N4. For the first three the rate-determining step is the initial formation of the hydroxyl intermediate, while for WC2N4 and ZrSi2N4 it is the third step, and in every case the free-energy landscape tilts downhill once the photoexcited holes contribute their potential, meaning the full reaction proceeds spontaneously under illumination.

Notably, ZrSi2N4 and HfGe2N4 achieve this despite unremarkable carrier mobility and recombination figures, because their valence band maxima sit at especially favorable energies. The study thus illustrates that photocatalytic performance is a multi-parameter balancing act, and that atomic-scale engineering can shift each parameter independently. By demonstrating that a deliberate change of one sublattice can convert a family of inert semiconductors into credible solar-fuel catalysts, the work hands experimentalists a concrete, computationally vetted shortlist, and adds momentum to the broader effort to harvest sunlight directly for clean hydrogen and oxygen production.

Subject of Research: A-site substitution engineering of MA2N4 two-dimensional materials for photocatalytic water splitting studied by first-principles calculations

Article Title: Electronic structure engineering and photocatalytic potential of MA2N4 two-dimensional materials: a first-principles perspective

Article References: Hong, D., Liu, Q.-J., Jiang, T., Liu, H., Liu, Z.-T., & Ren, Y.-L. (2026). Electronic structure engineering and photocatalytic potential of MA2N4 two-dimensional materials: a first-principles perspective. Journal of Saudi Chemical Society, 30(4), Article 57. https://doi.org/10.1007/s44442-026-00109-2

Image Credits: AI Generated

DOI: 10.1007/s44442-026-00109-2

Keywords: two-dimensional materials, MA2N4, photocatalysis, water splitting, oxygen evolution reaction, first-principles calculations, density functional theory, band gap engineering, exciton binding energy, MoSi2N4, HSE06, visible light absorption