(Keynote) Experimental and Computational Insights into Electronic Structure and Redox Properties of Bare and Doped Co3O4 Nanocrystals of Euhedral Morphology and Their Heterojunctions with Alien Oxides and Carbon Materials
Authors: Sojka Zbigniew , Gryboś Joanna , Zasada Filip , Wierzbicki Szymon , Piskorz Witold , Kruczala Krzysztof 
Published: February 2024

One of the most attractive oxide materials with remarkable record of widespread applications are cobalt spinels and their derivatives obtained by doping with alien redox (3dn) and nonredox (Li) cations or via hybridization with other oxides (CeO2, TiO2, a-Al2O3) or carbon support. They have recently received a great deal of theoretical and practical attention because of an outstanding activity in many redox processes important for photo/electro/catalysis. Among many potential applications, their relevance as promising substitutes for platinum in oxygen reduction reaction (ORR) or oxygen evolution reaction (OER) can be mentioned, in particular. The properties of nanostructured cobalt spinel materials depend on their size, shape, doping and interfacing features. In this context, euhedral spinel nanocrystals that expose well defined crystallographic planes, allow for sensible investigations into establishment of the surface/bulk structure-redox reactivity relationships in real conditions.
In this presentation application of S/TEM and inverse Wulff hull matching techniques supported by image simulation was discussed for elucidation the mechanism of hydrothermal synthesis of euhedral cobalt spinel nanocrystals the their shape retrieving. Periodic spin unrestricted DFT-PW91+U and hybrid DFT/HSE06 calculations were applied for modeling the morphology, structure and electronic properties (DOS structure) relevant for redox behavior of bare and doped cobalt spinel nanocrystals. The results allowed for thorough characterization of the structure and reconstruction of the exposed low index (100), (110), and (111) planes, and explain the influence of dopants. The effect of the valence state of lithium (Li+ or Li0), and its locus on the valence pining of the cobalt cations was determined. DFT calculations together with the ab initio thermodynamic modeling were used to study the electronic structure of the created defects, and stability of different terminations of cobalt spinel and Li-doped Co3O4 under various redox conditions. The associated electronic and spin relaxation processes were discussed as well. For the most stable stoichiometric (100) and (111) facets, the surface redox state diagrams in a wide stoichiometry range were constructed, and analyzed in detail. Intrinsic charge transfer processes involving electron (n-conductivity) and hole (p-conductivity) transfer between the octahedral and tetrahedral cobalt cations were modeled using a small polaron approximation (Marcus-Dupuis model). Formation of n-p junctions (Co3O4|CeO2, Li-Co3O4|CeO2) and MxCo3-xO4/carbon hybrids were analyzed by S/TEM, XPS spectroscopy (band alignment), contactless electronic conductivity and work function measurements, corroborated by DFT modeling. The redox properties of the interface and spinel faceting, and the role of electric conductivity and the nature of the carbon support functional groups were discussed in the context of the ORR, OER performance, and catalytic redox reactivity (activation of O2).
Acknowledgments
S.W. thanks for the financial support of the project “High-performance electrocatalyst for oxygen reduction reaction with low platinum content” carried on within the Program “Pearls of Science” of the Ministry of Education and Science of Poland.