Author

Bruce Odekirk

Date

August 1982

Document Type

Dissertation

Degree Name

Ph.D.

Department

Dept. of Applied Physics

Institution

Oregon Graduate Center

Abstract

A study of the characteristics of extrinsic, ceramic SrTiO3 and TiO2 is presented. These materials have been known to assist in the photoelectrochemical dissociation of water without undergoing decomposition, but their wide intrinsic bandgaps (3.0-3.2 eV) make them unsuitable for efficient solar energy conversion. Consequently, a wide variety of samples were studied in the highly extrinsic state, produced both by the introduction of impurities and by strong reduction. Electrical conductivity (throughout the range 10 ≤ T ≤ 500 K), thermopower, and photoconductivity were all measured, and compared to photoelectrochemical (PEC) cell performance. Single crystal samples of strongly reduced TiO2 were similarly examined for comparison with the ceramic materials. It was found that oxygen vacancies produced upon high temperature reduction of undoped SrTiO3 form O[subscript v]-impurity bands, with activation energies in the range 0.03-0.075 eV (for n = 10[superscript 17]-10[superscript 13] cm[superscript -3]). Impurity band formation was also found in La-doped SrTiO3 for [La] ≥ 0.2%, and the spectral response confirmed expectations from dark conductivity measurements that the conduction and impurity bands are fully merged with a substantial band tail when [La] = 1.0%. This resulted in the diminution of the optical bandgap from ~ 3.2 eV to ~ 2.8 eV, and with that, significantly enhanced photoanode performance was obtained. The behavior of σ with reduction for La:SrTiO3 confirmed that the defect structure at room temperature consists of controlled atomic imperfection under oxidizing conditions, which gives way to a controlled valency mechanism upon reduction. This transition produces a semiconducting material. High temperature reduction of undoped TiO2 was found to produce a large number of donor defects, with room temperature conduction-band electron densities in the range n = 10[superscript 17] - 10[superscript 19] cm[superscript -3]. Ta-doped TiO2 ceramics demonstrated activated transport in the temperature range 100-300 K, with minimum activation energies of 0.01-0.04 eV, which increased with [Ta]. The activation energy was found to increase with P[subscript O2] for single crystal TiO2, but not for undoped ceramics, indicating that the extended defect concentration plays a strong role in determining the rate of thermal quenching of defect-related charge carriers. Impurity dominated conduction in the P [subscript O2] range studied (10[superscript -8] - 10[superscript -18] atm) was found only for the highest doping level ([Ta] = 1.0%), with activation energies of 0.04-0.06 eV. Consequently, Ta-doping was not found to substantially alter the photoresponse or the PEC cell performance of TiO2 ceramics.

Identifier

doi:10.6083/M46D5QXH

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