Atomic oxygen coverages of up to 0.75 ML may be adsorbed cleanly on Pt(111) surfaces under UHV conditions by exposure to NO2 at 400 K. We have studied this adsorbed oxygen layer by using AES, LEED, UPS, HREELS, TPD, and work function (ΔΦ) measurements. The (2 × 2)-O structure formed at θO = 0.25 ML is still apparent at θO = 0.60 ML and a faint (2 × 2) pattern persists even up to θO = 0.75 ML. AES and ΔΦ measurements show no evidence for chemically distinct species in the adlayer as a function of oxygen coverage. HREELS spectra clearly rule out the presence of molecular oxygen and oxide species over the range of oxygen coverage studied. UPS also shows no shift in binding energy of the oxygen-derived peak as the coverage is increased. These spectroscopic probes indicate that all oxygen is present as atomic oxygen with no indication of oxide formation or molecular oxygen at any coverage. Multiple O2 desorption peaks observed in TPD are interpreted as arising largely from kinetic effects rather than a result of multiple, distinctly different chemical species, even though large changes in the Pt-O bond energy are determined from the TPD data. The activation energy for O2 desorption reflects the sum of the heat of dissociative adsorption of O2 and the activation energy for O2 dissociation. The structure in the O2 TPD spectrum is due to large changes in the activation energy for O2 desorption resulting from increases in the barrier to dissociative O2 chemisorption and decreases in the Pt-O bond energy. These barriers arise from strong repulsive interactions between adsorbed oxygen adatoms that cause sharp reductions in the Pt-O bond strength at these coverages. Finally, we note that our spectroscopic probes are quite insensitive to the changes in the Pt-O bond strength over the entire range of oxygen coverage studied.
All Science Journal Classification (ASJC) codes
- Condensed Matter Physics
- Surfaces and Interfaces
- Surfaces, Coatings and Films
- Materials Chemistry