Investigation of the optical absorbance, electronic, and photocatalytic properties of (Cu_1-xCo_x)₂(OH)PO₄ solid solutions

Transition metal hydroxyl phosphate compounds have attracted recent attention for catalytic and magnetic applications. Here, we present a detailed analysis on the properties of (Cu_1-xCo_x)₂(OH)PO₄ (0 ≤ x ≤ 1) compounds based on the mineral libethenite. Powders were synthesized using hydrothermal methods, and the photocatalytic activity was evaluated with an Fe³⁺/Fe²⁺ redox couple. Introduction of small fractions of Co to Cu₂(OH)PO₄ increased the photocurrent generation, but greater Co substitution caused it to decrease, with Co₂(OH)PO₄ showing the lowest photocurrent. The electronic band structure and density of states (DOS) were investigated using standard density functional theory (DFT) and hybrid functional methods. Hybrid DFT provided a better description of the electronic properties, especially the localized Cu and Co d electrons, in good agreement with the experimentally observed band gaps. The addition of Co to Cu₂(OH)PO₄ led to formation of bands within the band gap arising from Co 3d orbitals, which lowered the band gap <3 eV and changed the band-gap transition from a ligand-to-metal charge transfer (LMCT) to a metal-to-metal charge transfer (MMCT). However, higher concentrations of Co were detrimental to photocurrent generation as a result of the formation of a 3.7 eV MMCT and other electronic factors that could hinder charge separation.