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In our research, adsorption characteristics of citric acid on Fe3O4 (001), (011), and (111) surfaces was studied by using first-principles study.
The space group of magnetite crystal is FD-3M, in which a=b=c, and, and α=β=γ=90o.
Lattice parameter a was determined from equation of state, in which a was changed from 8.28Å to 8.79Å. Total energy of bulk magnetite has a minimum value at a=8.54Å, which is in good agreement with experimental value (a=8.40Å).
To compare surface stabilities, (001), (011) and (111) surfaces with different terminated atoms were created, in which vacuum width was set to 30Å to remove interaction between top and bottom layers of surface slab.
(001), (011), and (111) surfaces slabs include 4, 8 and 12 layers respectively. Each layer consists of 28 atoms, that is, 12 Fe atoms and 16 O atoms.
During surface relaxation, 14 top atoms (4 layers for (001) surface, 2 layers for (011) surface and 6 layers for (111) surface) are allowed to relax, and 14 bottom atoms are fixed.
Comparing surface energies of (001), (011) and (111) surfaces with different terminations, (001) with Fe termination has the smallest surface energy, and (111) with Fe termination has the largest surface energy, which means that (001) surface with Fe termination is the easiest to be cleaved, and (111) surface with Fe termination is the most difficult to be cleaved thermodynamically.
Comparing (001), (011), and (111) surfaces, (001) surface is the easiest to be formed, while (111) surface is the most difficult to be formed.
However, comparing adsorption energies of citric acid onto (001), (011), and (111) surfaces, (111) surface has the largest adsorption energy except for Fe termination, while (001) surface has the smallest adsorption energy, which means that (111) surface is the most active for citric acid adsorption, followed by (011) and (001) surfaces.
PDOS (partial density of states) of citric acid adsorbed onto (001), (011) and (111) surfaces with different terminations shows that 3d orbital of Fe in magnetite surface doesn't contribute to the adsorption, and 4s orbital of Fe of magnetite surface and 2s and 2p orbitals of O of citric acid contribute to the adsorption.
Our results of this study were published in Journal "Journal of Molecular Modeling" under the title of "Adsorption characteristics of citric acid on Fe3O4 (001), (011), and (111) surfaces" (https://doi.org/10.1007/s00894-021-04950-7).
