Variable work function of semiconducting thin-film oxide electrodes: a case study of SnO 2 and TiO 2

By atomic layer deposition, we prepared TiO 2 thin films, which do not crack upon thermal treatment at 450–500 °C. The calcination changes the film’s work function by tens of meV, as evidenced by electrochemical impedance (Mott-Schottky) and Kelvin probe analyses. In contrast, the work function of ALD-SnO 2 is enhanced by hundreds of meV after this heat treatment. The work function of calcined ALD-SnO 2 films is by ca. 0.3–0.4 eV larger than that of the cassiterite single-crystal electrode. The as-prepared ALD-SnO 2 film exhibits significant anodic photocurrent at potentials, when the calcined film is photoelectrochemically inactive. The ALD growth of SnO 2 on the Au(111) substrate occurs preferentially at the Au grain boundaries. In spite of its non-conformal morphology, the Au-supported SnO 2 film still blocks perfectly the anodic oxidation of ferrocyanide. Electrochemical doping of ALD-SnO 2 by lithium causes a decrease of the work functions by 0.1–0.2 eV in a broad range of film thicknesses.