Effect of arsenate and phosphate substitution on hydroxyl group in libethenite Cu2PO4OH - olivenite Cu2AsO4OH solid solution series

Abstract

Libethenite and olivenite have been regarded as intriguing photocatalysts because of their singular structures. Copper hydroxy-phosphates and -arsenates exhibit distinctive properties: they incorporate bridging hydroxyl (OH) groups shared by neighboring copper atoms. In materials employed for photoelectrochemical applications, the significance of surface OH groups and OH-related defects frequently varies and is contingent upon the specific material system and the reaction under consideration. Hydroxyl groups can enhance photocatalytic performance by generating OH radicals or serving as crucial agents in catalytic reactions. Therefore, the presence of an OH group within the crystal structure of the material can give rise to potentially interesting behaviors. Seven compounds within the solid solution series of libethenite Cu2PO4OH and olivenite Cu2AsO4OH with stoichiometric ratio (Cu2(Px,As1- xO4)OH, x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1 were synthesized at 70 °C via aqueous solutions, and their properties were examined using X-ray diffraction (XRD), Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), Fourier-transform infrared spectroscopy (FTIR) and Raman spectroscopy. The effect of substituting (PO4)3– ions with (AsO4)3– ions on the alteration of the lattice parameters and spectral characteristics was elucidated by analyzing the correlation between the chemical composition and X-Ray diffraction peak positions. The substitution process leads to modifications in the unit cell parameters and volume. These isomorphic substitutions can be also discerned through alterations in the infrared and Raman spectra, which exhibit variations in the position and intensity of the bands corresponding to phosphates, arsenates, and hydroxyl ions. The incorporation of As into the solid solution resulted in altered bond lengths and geometries, which is consistent with the symmetry transition from orthorhombic libethenite to monoclinic olivenite. The variations in the unit cell parameters and the shifts in the peak position exhibited non-linear trends, and a discernible step was observed between the Cu2(P0·3As0.7)O4OH and Cu2(P0·5As0.5)O4OH intermediate phases. The findings of this study will prove beneficial in evaluating the feasibility of the libethenite-olivenite isometric series as a photocatalysts.