Theoretical investigations of optoelectronic properties, photocatalytic performance as a water splitting photocatalyst and band gap engineering with transition metals (TM = Fe and Co) of K3VO4, Na3VO4 and Zn3V2O8: a first-principles study

First-principles density functional investigations of the structural, electronic, optical and thermodynamic properties of K₃VO₄, Na₃VO₄ and Zn₃V₂O₈ were performed using generalized gradient approximation (GGA) via ultrasoft pseudopotential and density functional theory (DFT). Their electronic structure was analyzed with a focus on the nature of electronic states near band edges. The electronic band structure revealed that between 6% Fe and 6% Co, 6% Co significantly tuned the band gap with the emergence of new states at the gamma point. Notable variations were highlighted in the electronic properties of Na₃V_(1−x)Fe_xO₄, Na₃V_(1−x)Co_xO₄, K₃V_(1−x)Fe_xO₄, K₃V_(1−x)Co_xO₄, Zn_3(1−x)V_2(1−x)Co_xO₈ and Zn_3(1−x)V_2(1−x)Fe_xO₈ (where x = 0.06) due to the different natures of the unoccupied 3d states of Fe and Co. Density of states analysis as well as α (spin up) and β (spin down) magnetic moments showed that cobalt can reduce the band gap by positioning the valence band higher than O 2p orbitals and the conduction band lower than V 3d orbitals. Mulliken charge distribution revealed the presence of the 6s² lone pair on Zn, greater population and short bond length in V–O bonds. Hence, the hardness and covalent character develops owing to the V–O bond. Elastic properties, including bulk modulus, shear modulus, Pugh ratio and Poisson ratio, were computed and showed Zn₃V₂O₈ to be mechanically more stable than Na₃VO₄ and K₃VO₄. Optimal values of optical properties, such as absorption, reflectivity, dielectric function, refractive index and loss functions, demonstrated Zn₃V₂O₈ as an efficient photocatalytic compound. The optimum trend within finite temperature ranges utilizing quasi-harmonic technique is illustrated by calculating thermodynamic parameters. Theoretical investigations presented here will open up a new line of exploration of the photocatalytic characteristics of orthovanadates.