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 _x O₄, Na₃V_(1-x)Co _x O₄, K₃V_(1-x)Fe _x O₄, K₃V_(1-x)Co _x O₄, Zn_3(1-x)V_2(1-x)Co _x O₈ and Zn_3(1-x)V_2(1-x)Fe _x O₈ (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.