The transport and photo electrochemical properties of La-doped stannate BaSnO3

The electrical conductivity and thermoelectric power of polycrystalline Ba0.98La0.02SnO3-delta, prepared under O-2-free atmosphere, have been measured between 4.2 and 300 K. The existence of onsite bipolaron is due to the tendency to Sn3+ ions to disproportionate into mixed valences Sn2+ and Sn4+ in equivalent sites. The data indicates electron degeneracy like conduction with polaron conductivity and a finite density of states at the Fermi level. The thermal evolution of the conductivity follows an Arrhenius type law with activation energy of 1 meV and electron mobility thermally activated. The conduction mechanism changes to a variable range hopping at low temperatures. From photo electrochemical measurements, the band gap E-g was found to be 3.26 eV and the transition is directly allowed; further indirect interband transition occurs at 2.82 eV. The valence band edge, located at 7.4 eV below vacuum, is made up from O:2p orbital typical of perovskite oxides. The respective values of the slopes (231 and 1104 mV dec(-1)) and the exchange current density J (0.339 mA cm(-2)) in the semi-logarithmic plot predict long-lived material. As expected, the title oxide exhibits a pH-dependent flat band potential V-fb and at pH 13.60 (KOH 1 M), the current-potential characteristic gave a V-fb-value of -0.03 V-SCE, close to that obtained from the capacitance measurement (+0.05 V-SCE). The C-2-potential plot displays a straight linear part with a positive slope confirming the n-type conductivity and a uniform doping density (N-d = 7.24 x 10(20) cm(-3)). The utilization of BaSnO3 as coating for PEC electrode is suggested. (C) 2007 Elsevier B.V. All rights reserved.