Substitutional Cu doping at Ca and Nb Sites in Ba₃CaNb₂O₉ toward improved photoactivity-a first-principles HSE06 study

Seeking tenable and sustainable alternatives for clean energy harvesting, artificial photosynthesis has emerged as a realm of research. We have investigated the effects of Cu doping at Ca and Nb sites in Ba₃CaNb₂O₉ (BCN) on magneto-optoelectronic properties through Density Functional Theory studies, employing the HSE06 hybrid functional to guide rational syntheses. Our calculations show that while Cu replacement of a Ca atom requires lower formation energy than replacing an Nb atom, oxygen vacancies (O_V) gradually reduce this to a large extent within the thermodynamic formability window when placed at the nearest O site, promoting Nb replacement. Cu doping at the Nb site substantially narrows the band gap, inducing the absorption of visible photons as the dopant concentration increases, while the placement of O_V at CuO₆ causes gradual blue shifts. Cu doping induces weak electron polaron formation in the CuO₆ unit due to net spin polarization in Cu d states, while the placement of the O_V annihilates it, albeit without reducing the oxidation state of Cu due to strong charge delocalization. Cu doping at the Nb site significantly improves the optical activity vis-à-vis pure BCN in the visible light region, especially ∼2-3 eV, resulting in promising candidates for photoelectrochemical catalysts.