Structural Water Accommodation in Co3O4: A Combined Neutron and Synchrotron Radiation Diffraction and DFT Study

Spinels like Co3O4 have acquired relevance because of their photocatalytic, electrocatalytic, optical and magnetic properties. In this context, we investigated the defect structure evolution of compounds synthetized using the nitrate precursor method and after annealing cycles at temperatures ranging from 260 to 650 °C by means of thermogravimetric analysis (TGA), neutron powder diffraction (NPD), X-ray powder diffraction (XRPD) coupled to Pair Distribution Function (PDF) analysis, and Density Functional Theory (DFT) calculations. Deuterated and hydrogenated precursors were adopted to produce the samples for NPD and XRPD experiments, respectively. TGA measurements displayed weight losses, the extent of which increased on lowering the preparation annealing temperature, suggesting that the adopted wet synthesis introduces structural water in the sample. Both XRPD and NPD revealed the presence of vacancies in tetrahedral cobalt sites (𝑉″𝐶𝑜1) whose concentration at RT decreases on raising the annealing temperatures, while octahedral cobalt and oxygen sites were fully occupied in all the samples. In addition, the 𝑉″𝐶𝑜1 presence induces a shrinking of the volume of the CoO4 tetrahedra. The combination of DFT calculation and diffraction revealed that deuterium/hydrogen ions (𝐷•𝑖/𝐻•𝑖), introduced during the synthesis by the nitrate precursor balanced the 𝑉″𝐶𝑜1. Finally, DFT calculations revealed that (𝐷•𝑖/𝐻•𝑖) in Co3O4 forms hydroxyl groups.