Adsorption characteristics of citric acid on Fe3O4 (001), (011), and (111) surfaces.

Magnetite (001), (011), and (111) surfaces were the focus of our study. Magnetite (001) surface has two different terminations, that is, Fe and 2Fe4O. Magnetite (011) surface has two different terminations, that is, 2Fe4O and 2Fe2Fe4O. Magnetite (111) surface has six different terminations, that is, Fe, Fe, Fe, 3Fe, 4O, and 4O. Comparing surface energies of (001), (011), and (111) surfaces, (001) has the smallest surface energy, and (111) has the largest surface energy except for Fe termination, which means that (001) surface is the easiest to be cleaved, followed by (011) and (111) surfaces. Comparing adsorption energies of citric acid onto (001), (011), and (111) surfaces, (111) has the largest adsorption energies except for Fe termination, and (001) has the smallest adsorption energies, 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 of magnetite surface does not 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.