Titanium dioxide TiO2 and alkaline earth metal titanates MTiO3 are inorganic semiconductors of interest to (photo)catalysis, separations, sensing, (opto)electronics, environmental remediation. Photoexcitation, relaxation, and transfer of charge at interface with air or water proceed through surface, subsurface, or “bulk” midgap states. The pathways of charge transfer affect operation of electronic devices and reaction rates. The pathways and rates depend on energy level diagram with valence band (VB), conduction band (CB), intrinsic and extrinsic midgap states. The pathways are routinely determined by “conventional” photoluminescence (PL) spectroscopy at constant excitation λexc or emission λemiss wavelength. However, “conventional” PL spectra of most inorganic solids are broad and featureless at 25 °C. We report newly developed high-resolution solid-state synchronous luminescence spectroscopy (SLS) of inorganic solids under ambient conditions. Solid-state SLS features much narrower peaks than “conventional” PL spectroscopy. We determined energy level diagrams and schemes of charge transfer for TiO2 (anatase and rutile), strontium titanate, and calcium titanate by solid-state SLS. In rutile, free exciton under subbandgap excitation at 430 nm with water adsorbate is used to assign narrow bands of “green light” (520 nm) and “green−yellow light” (560 nm) to surface and subsurface oxygen vacancies. In anatase, strongly trapped exciton (STE) was used to determine energy and localization of charge-trapping midgap state. In SrTiO3, solid-state SLS allows detection of midgap states at 25 °C, which are observed only by laser PL spectroscopy at cryo-temperatures. Finally, we describe our recent work and perspectives of new solid-state SLS for advanced spectroscopic characterization of inorganic composite nanomaterials beyond TiO2 and titanates.

Novel spectroscopic technique for determination of energy levels and charge transfer in water-insoluble inorganic compounds: Solid-state synchronous luminescence spectroscopy

2019 Spring National Meeting

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