Metastability and Photoelectrochemical Properties of Cu2SnO3 and Cu2-XLiXTiO3: Two 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 similar to 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 similar to 475 degrees C. Both phases crystallize in the monoclinic crystal system in the 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 red shift with the Cu(I) content, down to similar to 2.31 eV for I and similar to 1.46 eV for II. Electronic structure calculations show that 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 similar to 0.51 me for I and similar to 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 similar to 50 to 250 mu A 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.

Automated summary

Metastable p-type Cu(I)-based semiconductors, Cu2SnO3 (I) and Cu2-xLixTiO3 (II, xmin similar to 0.4), were synthesized via cation-exchange reactions. These crystalline semiconductors, not predicted in any materials databases, have a low reaction temperature of similar to 475 degrees C. Both phases crystallize in the monoclinic crystal system and exhibit edge-shared hexagonal layers. The optical band gaps decrease with Cu(I) content, and the semiconductor films exhibit p-type photocurrents.