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 Is cor e level photoelectron spectra obtained with a synchrotron radiation source. The surfaces were prepared in ultra-high vacuum (UHV) from natural magneti te crystals. We found that the vapor pressure, p(H2O), at which water first reacts with magnetite is similar for the two surfaces (less than or equal to 10(-5) Torr for 3 min exposures, corresponding to doses of less than or equal to 1.8 x 10(3) Langmuirs) and is consistent with a small sticking coe fficient. This reaction is manifested in the O Is spectra by the growth of a shoulder at about 1.5 eV tower kinetic energy than the main lattice oxyge n feature. We attributed this new feature to hydroxyl groups resulting from dissociative chemisorption of water on the magnetite surfaces, initially o n defect sites. p(H2O) for the onset of an extensive hydroxylation reaction is approximate to 10(-3) Torr (3 min dose or approximate to 1.8 x 10(5) La ngmuirs). Magnetite (100) and (Ill)surfaces exposed to higher p(H2O) react more extensively, with hydroxylation extending several layers (approximate to 8 Angstrom) deep into the bulk. A comparison of O KVV Auger K-edge absor ption spectra of water vapor-exposed magnetite (100) and (111) surfaces wit h the corresponding total yield spectra of goethite (alpha-FeOOH), limonite (FeOOH . nH(2)O), and hematite (alpha-Fe2O3) clearly shows that the reacti on product on the magnetite surfaces is not goethite, limonite or hematite. In addition, similarity of the Fe L3M23M23 Auger yield L-edge absorption s pectra before and after exposure of the magnetite (111) surface to liquid w ater indicates that the oxidation state of iron is unchanged, We also measu red O Is chemical shifts on magnetite (111) surfaces which had been immerse d in liquid water. Surprisingly, these immersion experiments resulted in sm aller chemical shifts and lower intensities of hydroxyl features relative t o magnetite samples exposed to the highest water vapor pressures (10 Torr f or 3 min or 1.8 x 10(9) Langmuirs) in our dosing experiments. To assess the thermal stability of the hydroxylated surfaces. we conducted a series of s tepwise annealing experiments to 700 degrees C. These annealing experiments indicate that once the magnetite surface is hydroxylated it is extremely d ifficult to thermally clean without Ar+ sputtering. O 2000 Published by Els evier Science B.V. All rights reserved.