Titanium-Doped Cu2O Thin Films: Oxide Alloying Versus Phase Separation

Reactive Ti/Cu co-deposition onto Pt(111) in 50 mbar O 2 is investigated with X-ray photoelectron spectroscopy (XPS), electron diffraction (LEED) and low-temperature scanning tunneling microscopy (STM). While as-grown films develop homogenous Ti/Cu/O mixtures without crystalline order, already moderate oxygen annealing at 560 K initiates a massive phase separation into surface TiO 2 and interface Cu 2 O layers. At low Ti content, Ti decorates the nano-pyramidal reconstruction of the Cu 2 O(111) surface, replacing the regular Cu 4 O pyramids with TiO x units. At higher concentration, raft-like TiO 2 crystallites, exposing a (4 × 2) added-row reconstruction, grow on top of the Cu 2 O films. A nearly equal Ti/Cu ratio triggers the formation of a pn -junction, made of n-type TiO 2 at the surface and p-type Cu 2 O at the interface. Our work rationalizes the absence of Ti/Cu mixed oxide phases and discusses the driving forces for phase separation into the binary oxide parents.