Spinel cobalt-based binary metal oxides as emerging materials for energy harvesting devices: synthesis, characterization and synchrotron radiation-enabled investigation.

The synthesis and characterization of spinel cobalt-based metal oxides (MCo 2 O 4 ) with varying 3d-transition metal ions (Ni, Fe, Cu, and Zn) were explored using a hydrothermal process (140 °C for two hours) to be used as alternative counter electrodes for Pt-free dye-sensitized solar cells (DSSCs). Scanning electron microscopy (SEM) and atomic force microscopy (AFM) revealed distinct morphologies for each metal oxide, such as NiCo 2 O 4 nanosheets, Cu Co 2 O 4 nanoleaves, Fe Co 2 O 4 diamond-like, and Zn Co 2 O 4 hexagonal-like structures. The X-ray diffraction analysis confirmed the cubic spinel structure for the prepared MCo 2 O 4 films. The functional groups of MCo 2 O 4 materials were recognized in metal oxides throughout Fourier transform infrared (FTIR) analysis. The local structure analysis using X-ray absorption fine structure (XAFS) at Fe and Co K-edge identified octahedral (Oh) Co 3+ and tetrahedral (Td) Co 2+ coordination, with Zn 2+ and Cu 2+ favoring Td sites, while Ni 3+ and Fe 3+ preferred Oh active sites. Further investigations utilizing the Fourier transformation (FT) analysis showed comparable coordination numbers and interatomic distances ranked as Co-Cu > Co-Fe > Zn-Co > Co-Ni. Furthermore, the utilization of MCo 2 O 4 thin films as counter electrodes in DSSC fabrication showed promising results. Notably, solar cells based on CuCo 2 O 4 and ZnCo 2 O 4 counter electrodes showed 1.9% and 1.13% power conversion efficiency, respectively. These findings indicate the potential of employing these binary metal oxides for efficient and cost-effective photovoltaic device production.