An understanding of the mechanism of formation of small clusters would help to identify efficient routes to their synthesis.

In this communication, we use density functional theory (DFT) to study the structural (geometry) and electronic properties (vertical detachment energy and electron affinity) of ZnO monomers and dimers that can be used to form ZnO clusters of different sizes, with a view to adapting one or more of them as catalysts or photocatalysts, standing alone or on suitable substrates like graphene, to split water. We also investigate different pairs of exchange functionals and basis sets to optimize their choice in our DFT calculations and to compare the singlet–triplet energy gaps of small ZnO clusters of different sizes to select an optimal cluster size for water splitting. We find that the B3LYP/DGDZVP2 exchange functional/basis set is a reliable combination for use with DFT to calculate the geometry and electronic properties of small ZnO nanoclusters from among several other combinations of exchange functionals and basis sets. Comparisons of the singlet–triplet energy gaps show that the trimer (ZnO)₃ has an energy gap of 58.66 k cal/mol. which is approximately equal to the energy of a visible photon at a wavelength of 500 nm, and a HOMO–LUMO gap of 4.4 eV, making it a suitable choice of photocatalyst for the oxidation of water from among six (ZnO)_n nanoclusters of monomers, with n ranging from 1 to 6. We used this exchange functional/basis set to study the structural and energetic details of hydration and hydrolysis of water absorbed on the (ZnO)₃ nanocatalyst and calculated the corresponding potential energy profiles to identify three sets of singlet–triplet pathways for water splitting. Detailed study of a pathway showed that oxygen is produced after hydrogen, and the rate-determining step is the formation of hydrogen.

Graphene and graphene oxide (GO)-based metal oxides could play an important role in using metal oxide like zinc oxide (ZnO) as photocatalysts to split water. The π conjugation structure of GO shows greater electron mobility and could enhance the photocatalytic performance of the bare ZnO catalyst by increasing the electron-hole separation. In this work, we use density functional theory (DFT) with the B3LYP exchange functional and DGDZVP2 basis set to study the impact of adsorbing (ZnO)₃ nanoparticles on graphene and four different GO models (GO1, GO2, GO4, and GO5) on the hydration and hydrolysis of water that precedes water splitting to produce H₂ and O₂ atoms in the gas phase and compare them with our previous studies on the bare catalyst in the absence of the substrate. The potential energy curves and activation energies are similar, but the triplet states are lower in energy than the singlet states in contrast to the bare (ZnO)₃ catalyst. We extend our calculations to water splitting from the hydrolyzed (ZnO)₃ on GO1 (GO1-(ZnO)₃). The triplet state energy remains lower than the singlet state energy, and hydrogen production precedes the formation of oxygen, but there is no energy inter-crossing during the formation of O₂ that occurs in the absence of a GO1 substrate. Although the hydrolysis reaction pathway follows similar steps in both the bare and GO1-(ZnO)₃, water splitting with (ZnO)₃ absorbed on the GO1 substrate skips two steps as it proceeds toward the production of the second H₂. The production of two hydrogen molecules precedes oxygen formation during water splitting, and the first Zn-H bond formation step is the rate-determining step. The ZnO trimer deposited on GO systems could be potentially attractive nanocatalysts for water splitting.

Per- and polyfluoroalkyl substances (PFAS) are emerging contaminants that persist in soil environments, necessitating reliable models to predict their fate and transport. This study evaluates the performance of three theoretical models in estimating the maximum adsorption capacity (Qmax) of perfluorooctanoic acid (PFOA) on kaolinite and montmorillonite clay minerals. The models assessed include a van der Waals interaction-based approach, a monolayer adsorption capacity model, and a surface site density model emphasizing reactive hydroxyl groups at mineral edges. Benzene, nitrogen, and glyphosate molecules were used as reference compounds for model validation. Results indicated that the van der Waals model significantly underestimated Qmax (0.0007 mg·g⁻¹ for kaolinite), while the monolayer capacity model produced substantial overestimations (17.51 mg·g⁻¹) compared to the experimental range (0.10–10.0 mg·g⁻¹). The surface site density model provided the most accurate predictions (3.39 mg·g⁻¹ for kaolinite), although it slightly underestimated values for montmorillonite (0.20 mg·g⁻¹) by excluding interlayer adsorption. These discrepancies demonstrate that simplified models cannot adequately capture the complex adsorption behavior of PFAS. Accurate prediction requires site-specific approaches incorporating electrostatic forces, hydrogen bonding, and steric effects. As PFAS accumulation in soil directly contributes to groundwater contamination, improving adsorption models is essential for accurate risk assessment and the development of effective remediation strategies.