1 research outputs found
ZnO(101Ì…0) Surface Hydroxylation under Ambient Water Vapor
The
interaction of water vapor with a single crystal ZnO(101Ì…0)
surface was investigated using synchrotron-based ambient pressure
X-ray photoelectron spectroscopy (APXPS). Two isobaric experiments
were performed at 0.3 and 0.07 Torr water vapor pressure at sample
temperatures ranging from 750 to 295 K up to a maximum of 2% relative
humidity (RH). Below 10<sup>–4</sup> % RH the ZnO(101̅0)
interface is covered with ∼0.25 monolayers of OH groups attributed
to dissociation at nonstoichiometric defect sites. At ∼10<sup>–4</sup> % RH there is a sharp onset in increased surface
hydroxylation attributed to reaction at stoichiometric terrace sites.
The surface saturates with an OH monolayer ∼0.26 nm thick and
occurs in the absence of any observable molecularly bound water, suggesting
the formation of a 1 × 1 dissociated monolayer structure. This
is in stark contrast to ultrahigh vacuum experiments and molecular
simulations that show the optimum structure is a 2 × 1 partially
dissociated H<sub>2</sub>O/OH monolayer. The sharp onset to terrace
site hydroxylation at ∼10<sup>–4</sup> % RH for
ZnO(101Ì…0) contrasts with APXPS observations for MgO(100) which
show a sharp onset at 10<sup>–2</sup> % RH. A surface
thermodynamic analysis reveals that this shift to lower RH for ZnO(101Ì…0)
compared to MgO(100) is due to a more favorable Gibbs free energy
for terrace site hydroxylation