Photoelectrochemical and solid-state properties of LuRhO3

Abstract

Rare-earth rhodates of the distorted perovskite structure are semiconductors with a band gap of 2.2 eV. They may be doped either n or p type. The oxides are stable against photodecomposition at the potential for hydrogen evolution. With ceramic p-type LuRhO3 as the cathode and n-type TiO2 as the anode of an electrolytic cell sufficient photopotential is developed both to photoelectrolyze water in sunlight with no externally applied potential and to generate power simultaneously. As synthesized, LuRhO3 is p type as determined by thermoelectric power measurements, but may be doped n type by the introduction of Th+4 during synthesis. Electrical resistivity data show that this oxide passes through compensation as the doping level increases, the activation energy at compensation being one-half the optical band gap. The concentration of donor and acceptor impurities was determined by magnetic susceptibility to be in the vicinity of 1% for all samples. From these data, an estimate of 2.5 cm2/V sec can be made for the mobility of holes and 1 cm2/V sec for the mobility of electrons. Unlike most n-type oxides reported, LuRhO3 possesses deep lying impurity levels and surface capacitance is far from Mott-Schottky behavior. We have developed a theory of surface capacitance for deep levels, and the data agree well with theory. It is interesting to note that the Fermi level of LuRhO3 is pinned to the electrolyte at the same potential independent of whether the oxide is p or n type.