Impact of Composition on the Optical Properties of Colloidal Ternary Spinel Oxide Nanocrystals: Spinel Ferrites versus Spinel Gallates

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Ternary spinel oxides of formula MB2O4 are attractive materials for optoelectronic and photocatalytic applications due to their chemical stability and compositional flexibility. Understanding how the compositions of these materials influence their optical properties is crucial for their further development as photoactive materials. Here, we present a quantitative comparison of the absorption spectra of two families of colloidal ternary spinel oxide nanocrystals:metal ferrites (MFe2O4) and metal gallates (MGa2O4). We found that, despite the similar crystal structure, shape, and surface ligands of the ternary spinel ferrites and gallates, the optical properties of these materials are significantly different. The ferrites exhibit strong absorption throughout the visible region, with spectra and per iron extinction coefficients that do not change significantly upon changing the identity of the M metal. In contrast, the gallates exhibit significantly larger band gaps and much lower extinction coefficients, with spectra that do depend on the identity of the M metal. We assign the visible absorption observed in ferrites to O 2p → Fe 3d charge transfer transitions, whereas the visible range transitions observed in some of the gallates arise from intra-atomic d-d transitions associated with the M2+ cations. We corroborate our findings with band structures calculated by using Hubbard- and Hund-corrected density functional theory computations (DFT+U+J).