Reaction of water with the (100) and (111) surfaces of Fe3O4

We have examined changes in the electronic structure of magnetite (100) and (111) surfaces after reaction with water vapor (p(H2O) ranging from 10−9 to 9 Torr) and liquid water at 298 K using chemical shifts in the O 1s core level photoelectron spectra obtained with a synchrotron radiation source. The surfaces were prepared in ultra-high vacuum (UHV) from natural magnetite crystals. We found that the vapor pressure, p(H2O), at which water first reacts with magnetite is similar for the two surfaces (≤10−5 Torr for 3 min exposures, corresponding to doses of ≤1.8×103 Langmuirs) and is consistent with a small sticking coefficient. This reaction is manifested in the O 1s spectra by the growth of a shoulder at about 1.5 eV lower kinetic energy than the main lattice oxygen feature. We attributed this new feature to hydroxyl groups resulting from dissociative chemisorption of water on the magnetite surfaces, initially on defect sites. p(H2O) for the onset of an extensive hydroxylation reaction is ≈10−3 Torr (3 min dose or ≈1.8×105 Langmuirs). Magnetite (100) and (111) surfaces exposed to higher p(H2O) react more extensively, with hydroxylation extending several layers (≈8 Å) deep into the bulk. A comparison of O KVV Auger K-edge absorption spectra of water vapor-exposed magnetite (100) and (111) surfaces with the corresponding total yield spectra of goethite (α-FeOOH), limonite (FeOOH·nH2O), and hematite (α-Fe2O3) clearly shows that the reaction product on the magnetite surfaces is not goethite, limonite or hematite. In addition, similarity of the Fe L3M23M23 Auger yield L-edge absorption spectra before and after exposure of the magnetite (111) surface to liquid water indicates that the oxidation state of iron is unchanged. We also measured O 1s chemical shifts on magnetite (111) surfaces which had been immersed in liquid water. Surprisingly, these immersion experiments resulted in smaller chemical shifts and lower intensities of hydroxyl features relative to magnetite samples exposed to the highest water vapor pressures (10 Torr for 3 min or 1.8×109 Langmuirs) in our dosing experiments. To assess the thermal stability of the hydroxylated surfaces, we conducted a series of stepwise annealing experiments to 700°C. These annealing experiments indicate that once the magnetite surface is hydroxylated it is extremely difficult to thermally clean without Ar+ sputtering.