Photocatalytic properties of libethenite Cu2PO4OH - olivenite Cu2PO4OH solid solution series

The wide range of colored wastewater from industries in the aquatic environment poses a great threat to human and animal health and poses a great obstacle to the ecological ecosystem. Therefore, an effective, efficient, and environment-friendly treatment methods are being sought. Hydroxyl- phosphate and arsenate compounds have recently attracted attention for magnetic and photocatalytic applications in photoreactions with visible light.  They can be a promising alternative to TiO₂, in which the photoabsorption spectrum is in the range of ultraviolet (UV) light owing to its large bandgap, which accounts for only 5% of the sunlight.

Here, we present a detailed analysis of the properties of libethenite Cu₂PO₄OH - olivenite Cu₂AsO₄OH solid solution series, and their photocatalytic activities under visible light. Seven compounds of the solid solution series were successively synthesized by the wet chemical method at 70°C according to the results of Fourier-transform infrared (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) tests. The photocatalytic performance of the samples was thoroughly investigated for the degradation of methylene blue MB solutions under visible light and measurements by UV-vis spectroscopy.

We demonstrated the useful photocatalytic activity of these complex structures for the degradation of methylene blue (MB) dye under visible-light irradiation. The substitution effect of [PO₄]³⁻ anions by [AsO₄]³⁻ results in changes in the bonds of the OH group, which are the origin of the photocatalytic properties of this material, altering the bond length and geometry.  In addition, these substitutions affected the morphology of the precipitating solids, which changed the surface area of the material. This way the substitution of As with P in the solid solution series affected the photocatalytic properties The MB degradation efficiency after 6h declines from ~ 84 % for the Cu₂AsO₄OH and Cu₂PO₄OH down to ~81% for intermediate member.  The present work provides insights leading to a better understanding of the photocatalytic performance of Cu₂PO₄OH and Cu₂AsO₄OH. Thanks to these results it may be beneficial to prepare more efficient photocatalysts based on this material for sunlight photocatalysis, which will also be helpful in designing and preparing novel technologies.

This research was funded by NCN research grant no. 2021/41/N/ST10/03566