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

Abstract

The synthesis and characterization of spinel cobalt-based metal oxides (MCo₂O₄) 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 NiCo₂O₄ nanosheets, Cu Co₂O₄ nanoleaves, Fe Co₂O₄ diamond-like, and Zn Co₂O₄ hexagonal-like structures. The X-ray diffraction analysis confirmed the cubic spinel structure for the prepared MCo₂O₄ films. The functional groups of MCo₂O₄ 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) Co³⁺ and tetrahedral (Td) Co²⁺ coordination, with Zn²⁺ and Cu²⁺ favoring Td sites, while Ni³⁺ and Fe³⁺ 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 MCo₂O₄ thin films as counter electrodes in DSSC fabrication showed promising results. Notably, solar cells based on CuCo₂O₄ and ZnCo₂O₄ 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.