Interaction of sulfur dioxide and near-ambient pressures of water vapor with cuprous oxide surfaces

The interaction of water vapor and sulfur dioxide (SO₂) with single crystal cuprous oxide (Cu₂O) surfaces of (100) and (111) termination was studied by photoelectron spectroscopy (PES) and density functional theory (DFT). Exposure to near-ambient pressures of water vapor, at 5 × 10-3 %RH and 293 K, hydroxylates both Cu₂O surfaces with OH coverage up to 0.38 copper monolayers (ML) for (100) and 0.25 ML for (111). O 1s surface core level shifts indicate that the hydroxylation lifts the (3,0;1,1) reconstruction of the clean (100) surface. On both clean Cu₂O terminations, SO₂ adsorbs to unsaturated surface oxygen atoms to form SO₃ species with coverage, after a saturating SO₂ dose, corresponding to 0.20 ML on the Cu₂O(100) surface and 0.09 ML for the Cu₂O(111) surface. Our combined DFT and PES results suggest that the SO₂ to SO₃ transformation is largely facilitated by unsaturated copper atoms at the Cu₂O(111) surface. SO₃-terminated surfaces exposed to low doses of water vapor (=100 langmuirs) in ultrahigh vacuum show no adsorption or reaction. However, during exposure to near-ambient pressures of water vapor, the SO₃ species dissociate, and sulfur replaces a Cu₂O lattice oxygen in a reaction that forms Cu₂S. The hydroxylation of the Cu₂O surfaces is believed to play a central role in the reaction.