(42c) New Applications of Solid-State Synchronous Luminescence Spectroscopy to Study Surface/Interfacial Charge Transfer in Titanium Dioxide and Metal Titanates | AIChE

(42c) New Applications of Solid-State Synchronous Luminescence Spectroscopy to Study Surface/Interfacial Charge Transfer in Titanium Dioxide and Metal Titanates

Authors 

Samokhvalov, A. - Presenter, Rutgers University

The relaxation
and transfer of photoexcited charge at the interface of nanocrystalline
semiconductor with air or water proceeds through the surface, subsurface and
“bulk” electronic midgap states. Charge relaxation pathways significantly
affect the efficiency of operation of electronic, optoelectronic and
magneto-electronic devices and kinetic rates of photocatalytic, catalytic and
other surface chemical reactions. Nanocrystalline titanium oxide and metal
titanates are of interest to photocatalysis, electronics, catalysis, sensing, and
environmental remediation. The pathways and rates of relaxation of excited
charge in nanocrystalline semiconductors are routinely determined by “conventional”
photoluminescence (PL) spectroscopy at the constant excitation λexc or emission λemiss
wavelength. However, the “conventional” PL emission spectra of many
nanocrystalline semiconductors are quite broad and featureless at 25 °C, which
prevents a reliable assignment of charge-trapping states and pathways of
surface and interfacial charge transfer. The exact energies and spatial
localization of charge-trapping midgap states, e.g. oxygen vacancies, remain
elusive. We report a newly developed technique of high-resolution solid-state synchronous
luminescence spectroscopy of nanocrystalline inorganic semiconductors under
ambient conditions. Using this novel method, we determined the energy level diagrams
and the schemes of pathways of photoexcitation and relaxation of excited charge
at the surface and interface “semiconductor/air” for nanocrystalline titanium
dioxide (anatase and rutile), strontium titanate and calcium titanate. In the solid-state
synchronous luminescence spectroscopy, free exciton
or the strongly trapped exciton (STE) were
successfully used as “probe quasi-particles” in combination with water as “probe
molecule”. Compared to “conventional” PL spectra, the solid-state synchronous
luminescence spectra at 25 °C feature the much better spectral resolution. This
allows an accurate determination of the energies of charge-trapping states and their
spatial localization (surface, the sub-surface region, or the “bulk”). In
nanocrystalline rutile, we used free exciton under
the subbandgap photoexcitation at 430 nm in combination
with water adsorbate to assign the well-resolved, narrow luminescence bands of
“green light” (ca. 520 nm) and “green−yellow light” (ca. 560 nm) at 25 °C
to emission from the surface and subsurface oxygen vacancies. In addition, the solid-state
synchronous luminescence spectra are significantly more convenient than
“conventional” PL spectra in the accurate determination of the energies of the subbandgap optical transitions. In nanocrystalline anatase,
the strongly trapped exciton (STE) was used in
combination with water vapor for identification of energy and assignment of
charge-trapping midgap states. In nanocrystalline strontium titanate, the
solid-state continuous-wave (CW) synchronous luminescence spectroscopy at 25 °C
allows the detection of charge-trapping midgap states, which are normally observed
only by the laser-based PL spectroscopy at cryogenic temperatures. Finally, we summarize
our recent work and the perspectives of novel solid-state synchronous
luminescence spectroscopy for characterization of surface and interfacial
charge transfer, and advanced characterization of electronic properties of composite
nanocrystalline solids beyond pure titanium dioxide and metal titanates.