Physical properties and photoelectrochemical characterization of SrPbO3

SrPbO3-delta is n-type, narrow-bandgap semiconductor crystallizing in a distorted perovskite structure. The electrical conductivity varies with temperature but in a nonmonotonic manner. At high temperatures, the thermal evolution of the conductivity follows an Arrhenius-type law with an activation energy of 0.11 eV attributed to mixed valency states Pb4+/2+ and leading to a dielectric constant of 13.6. The sign of thermopower S is negative, indicating that the mobile carriers are electrons, which is ascribed to oxygen extraction. The electron hopping occurs between similar electronic localized states in the vicinity of the mobility edge with an electron mobility of similar to 10(-2) m(2) V-1 s(-1) that is thermally activated. The variation S(T) suggests a finite density of states at the Fermi level. At low temperatures, the conduction mechanism changes to a variable-range hopping. The weak hybridization of the Pb-O bond, due to the tilting of Pbo(6) octahedra, lowers the energy of the antibonding conduction band resulting in a small optical gap E-g. The E-g value was found to be 1.76 eV and the transition is directly allowed; a further indirect transition occurs at 0.79 eV. The flat potential V-fb measured from the Mott-Schottky plot was equal to -0.38 V-SCE. The upper valence-band edge, located at 6.8 eV below vacuum, is made from mainly the O:2p orbital, whereas the conduction band consists of empty 6s levels. The linear plot log real capacitance-log frequency indicates a capacitive behavior. The relevance of 6s conduction band to the performance of semiconducting photoelectrodes will be discussed. (c) 2007 WILEY-NCH Verlag GmbH & Co. KGaA, Weinheim