Revisiting the Band Gap problem in bulk Co3O4 and its isostructural Zn and Al derivatives through the lens of theoretical spectroscopy

Abstract

In this work, a systematic computational investigation of the optical band gap (BG) problem of Co3O4 is carried out on the basis of the embedded cluster approach in combination with a series of particle/hole and wavefunction-based approaches. A total number of three experimental Band Gap energies for the bulk Co3O4 have been reported in the literature, the nature of which have remained controversial. It is shown in this work that in an effort to accurately describe the excited states and predict the origin of the experimental BGs in Co3O4, it is of paramount importance to understand the nature the low-lying excited state manifold. In particular, the analysis shows that strong electron correlation effects within and across the A and B sites, containing high-spin tetrahedral Co(II) and low-spin octahedral Co(III) centers respectively, stabilizes a large manifold of 'ionic' excited states that consequently fall into the low-lying optical spectral region. Tackling such a complex excited state problem requires going beyond density functional theory (DFT) particle/hole approaches and employing a range of single and multi-reference wavefunction based methods. In particular, complete active space configuration interaction (CASCI) in conjunction with 2nd order N-electron valence perturbation theory (NEVPT2) provide access to an accurate prediction of all the three experimentally observed BG energies in Co3O4. Our calculations are consistent with the notion that the lowest energy band gap corresponds to ligand field (LF) type of transitions within the local tetrahedral Co(II) centers. Furthermore, the calculations predict that the middle energy band gap is a mixture of LF transitions at site A and metal-to-metal charge transfer (MMCT) transition across A-A' and A-B/B-A' sites involving locally interacting Co(II) and Co(III) centers. This corresponds to the actual optical band gap of Co3O4. Finally, the highest energy band gap is again a mixture of LF transitions at site A and ligand-to-metal charge transfer (LMCT), involving O 2p→Co(II)-3d transitions with, according to our calculations, also some contributions of other MMCT states. Hence, this later energy band corresponds to the actual semiconducting band gap that defines the semiconductor properties of Co3O4.