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

Marcin Kobielusz,Wojciech Macyk,Bunsho Ohtani,Akio Nitta

doi:10.1021/acs.jpclett.1c00262

Marcin Kobielusz, Wojciech Macyk + Show 2 more

Open Access

https://doi.org/10.1021/acs.jpclett.1c00262

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Abstract

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.

Highlights

The diffuse reflectance spectroelectrochemistry (SE-DRS) and reversed doublebeam photoacoustic spectroscopy (RDB-PAS) provide unique, complementary information on the density of electronic states (DOS) in the vicinity of the conduction band bottom
The activity of heterogeneous photocatalysts strongly depends on their physicochemical properties, such as band gap energy, potential of the conduction and valence band edges, phase composition,[1−4] surface area,[5,6] particle size,[7−9] and facet exposition.[10,11]
Letter density of electron traps (ETs) in samples is, in general, very low or almost negligible, the actual photoexcitation of VB electrons to ET must occur from the high density-of-states (DOS) part of VB, not VB top, where the DOS is almost zero and the energy of ERDT is overestimated.[19]

Summary

■ EXPERIMENTAL METHODS

Sulfate anions on its surface, which increases surface acidity, it can be presumed that its band diagram is shifted to lower energy.[37] The discussed SE-DRS and RDB-PAS methods give information on the distribution of electronic states, which play a pivotal role in photocatalytic activity of the studied semiconductors. Application of both, complementary methods enable the determination of the potential of the conduction band bottom and recognition of the character of electronic states. What is important, both methods support information on redox properties of the semiconductors both in their ground and excited states.

■ ACKNOWLEDGMENTS

■ REFERENCES

A Fingerprint of Metal-oxide Powders