Combined Spectroscopic Methods of Determination of Density of Electronic States: Comparative Analysis of Diffuse Reflectance Spectroelectrochemistry and Reversed Double-Beam Photoacoustic Spectroscopy.

The diffuse reflectance spectroelectrochemistry (SE-DRS) and reversed double-beam photoacoustic spectroscopy (RDB-PAS) provide unique, complementary information on the density of electronic states (DOS) in the vicinity of the conduction band bottom. The measurements are performed under quite different conditions, representing the solid/liquid and solid/gas interfaces in SE-DRS and RDB-PAS, respectively. DOS profiles obtained from both types of measurements can be considered as unique "fingerprints" of the tested materials. The analysis of DOS profiles recorded for 16 different TiO2 samples confirms that both methods similarly describe the shapes of DOS profiles around the conduction band edges. The states characterized by energy higher than VBT (valence-band top) + Eg can be considered as electronic states within the conduction band. Recognition of the potential of the conduction band bottom allows one to classify the electronic states as deep or shallow electron traps or conduction band states, which play different roles in photocatalysis. The comparative analysis shows that both methods provide very useful information which can be used in understanding and predicting the photo(electro)catalytic reactivity of semiconductors.

Noble metal-modified faceted anatase titania photocatalysts: Octahedron versus decahedron.

Octahedral anatase particles (OAP, with eight equivalent {101} facets) and decahedral anatase particles (DAP, with two additional {001} facets) were modified with nanoparticles of noble metals (Au, Ag, Cu). The titania morphology, expressed by the presence of different arrangements of exposed crystal facets, played a key role in the photocatalytic properties of metal-modified faceted titania. In the UV/vis systems, two-faceted configuration of DAP was more favorable for the reaction efficiency than single-faceted OAP because of an efficient charge separation described by the transfer of electrons to {101} facets and holes to {001} facets. Time-resolved microwave conductivity (TRMC) and reversed double-beam photoacoustic spectroscopy (RDB-PAS) confirmed that distribution of electron traps (ET) and mobility of electrons were key-factors of photocatalytic activity. In contrast, metal-modified OAP samples had higher photocatalytic activity than metal-modified DAP and metal-modified commercial titania samples under visible light irradiation. This indicates that the presence of single type of facets ({101}) is favorable for efficient electron transfer via shallow ET, whereas intrinsic properties of DAP result in fast charge carriers' recombination when gold is deposited on {101} facets (migration of "hot" electrons: Au→{101}→Au).

A fingerprint of metal-oxide powders: energy-resolved distribution of electron traps.

Here we propose a method for the identification of metal-oxide powders with the energy-resolved distribution of electron traps and conduction-band bottom position reflecting a surface structure and a bulk structure, respectively, as a fingerprint, based on the degree of coincidence for a given pair of samples, measured using newly developed reversed double-beam photoacoustic spectroscopy.