Metastability and Photoelectrochemical Properties of Cu2SnO3 and Cu2-xLixTiO3: Two New Cu(I)-Based Oxides with Delafossite Structures

Metastable, p-type Cu(I)-based semiconductors were synthesized using cation exchange reactions between delafossite-type layered precursors and CuCl flux, yielding Cu2SnO3 (I) and Cu2-xLixTiO3 (II, xmin ~ 0.4). These represent the first reported crystalline semiconductors found in the Cu-Sn-O or Cu-Ti-O chemical systems (and not currently predicted within any materials databases), with their kinetic stabilization requiring a relatively low reaction temperature of ~475°C. Both phases crystallize in the monoclinic crystal system in space group C2/c, exhibiting edge-shared hexagonal ‘MO3’ (M = Sn or Ti) layers that also contain octahedrally-coordinated Li(I)/Cu(I) cations. These layers are bridged by linearly-coordinated Cu(I) cations. Magnetic susceptibility measurements confirm the +1 oxidation state of the copper cations. The optical band gaps were found to be indirect and to significantly redshift with Cu(I) content, down to ~2.31 eV for I and ~1.46 eV for II. Electronic structure calculations show the decreased band gaps can be attributed to a higher-energy valence band derived from the filled 3d10 orbitals of the Cu(I) cations, which most notably arise from the octahedrally-coordinated Cu(I) cations within the layers. Total energy calculations reveal an increasing metastability with respect to decomposition to Cu2O and SnO2 or TiO2 as a result of occupation of the intralayer sites by Cu(I) cations. In both phases, their edge-shared hexagonal layers lead to highly-dispersive conduction bands and small electron effective masses of ~0.51 me for I and ~0.41 me for II. Polycrystalline films of both were deposited onto fluorine-doped tin oxide slides and exhibited p-type photocurrents under 100 mW cm-2 irradiation in the range of ~50 to 250 μA cm-2. This study thus reveals new fundamental relationships between the origin of metastability in Cu(I)-oxide semiconductors, i.e., octahedral coordination, and enhanced optical and photoelectrochemical properties.