Article pubs.acs.org/JPCC
Hydroxylation of Ultrathin Al2O3/NiAl(110) Films at Environmental Humidity A. Shavorskiy,†,‡ K. Müller,§,∥ J. T. Newberg,‡,# D. E. Starr,*,§,⊥ and H. Bluhm*,‡ †
Advanced Light Source and ‡Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States § Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States S Supporting Information *
ABSTRACT: We have investigated the reaction of ultrathin Al2O3/NiAl(110) films with water vapor from ultrahigh vacuum to relative humidities (RH) up to 10%, over the temperature range from −5 to 65 °C, using ambient pressure X-ray photoelectron spectroscopy (APXPS) and scanning tunneling microscopy (STM). The APXPS experiments show a sharp onset of oxide/hydroxide film growth at ∼0.01% RH, coupled with an increase of the oxygen-to-aluminum ratio, most likely because of the adsorption of OH and H2O species at the oxide surface and their reaction with Al from the NiAl substrate, which leads to a thickening of the oxide film at RH > 0.01%. The STM measurements support the results from APXPS and reveal a change in surface morphology when the Al2O3/NiAl(110) sample was exposed to relative humidities greater than 0.01%.
1. INTRODUCTION Many processes in the environment, the atmosphere, heterogeneous catalysis, and electrochemistry are governed by the interaction of water vapor with surfaces.1−4 The presence of hydroxyl groups and/or water molecules at the oxide surface may change its reactivity with other gas-phase species, influence electronic and ionic conductivity along the surface, and modify its frictional and adhesive properties. A long-standing goal has been to understand physical and chemical processes at solid/ water vapor interfaces on the molecular level, yet simple questions such as the degree of hydroxylation and the amount of adsorbed water at specific conditions (e.g., relative humidity) remain largely unanswered. Using vacuum-based surface science techniques, a detailed understanding of the interaction of water with well-ordered surfaces has been achieved at low temperatures ( 1.5 ratios at RH > 0.1% (see Figure 6). Figure 7 also shows the growth of the oxide/hydroxide layer as a function of time at a RH of ∼0.1% at −5 and 65 °C. The OAl-OH/Otot ratio reaches a stable value at −5 °C (open blue markers) after 45 min, whereas at 65 °C
prepared in vacuum (Figure 8a), after 30 min exposure to 0.01 Torr of water at 23 °C (equivalent to a RH = 0.04%, Figure 8b), and after 90 min in 0.01 Torr of H2O (Figure 8c). Images of the freshly prepared sample show a number of antiphase domain boundaries typical for the Al2O3/NiAl(110) surface.60,61 After 30 min ex situ exposure of the Al2O3/NiAl(110) film to a RH of 0.04%, some of the antiphase domain boundaries partially disappear (cf. white arrow in Figure 8b). We assume that the additional oxidation/hydroxylation, which takes place during water exposure, starts at the reflection domain boundaries and the zigzag antiphase domain boundaries because of the higher defect density at these sites. After further exposure at 0.04% RH for a total time of 1.5 h (see Figure 8c), the antiphase domain boundaries completely disappear and only faint dotted lines in the STM image, marked by black arrows, indicate the original location of the domain boundaries. It should be noted that the ex situ exposure of the Al2O3/NiAl(110) sample for the STM experiments was done at a RH similar to that used to monitor the kinetics of the reaction in the APXPS experiments (see Figure 7). According to the
Figure 7. Ratio between the oxide (solid symbols) and the hydroxide/water (BE = 532.4 eV, see Figure 4, open symbols) peak and total oxygen peak area (excluding gas-phase contribution) versus relative humidity as well as time (middle panel, for RH = 0.1%). 29345
dx.doi.org/10.1021/jp505587t | J. Phys. Chem. C 2014, 118, 29340−29349
The Journal of Physical Chemistry C
Article
XPS data, an exposure of 90 min at 0.04% RH is sufficient to reach a saturation Al2O3/Al(OH)x thickness at room temperature. To confirm the validity of the comparison of the ex situ STM measurements with the in situ XPS experiments, we simulated ex situ exposure conditions in the XPS chamber: After the APXPS measurements at RH > 1%, the water vapor was pumped out of the chamber and the Al2O3/NiAl(110) film was kept at
