Tailoring the structural, electronic, mechanical, and optical properties of SrTiO3 via 7% Cu doping at the Ti site: A comprehensive DFT study

Strontium titanate (SrTiO3) is a wide-band-gap (~3.2 eV) perovskite oxide semiconductor with a high dielectric constant and excellent thermal and chemical stability, properties that have made it attractive for photocatalysis, photovoltaics, sensors, and electronic devices. Its large band gap, however, restricts light absorption to the ultraviolet region and limits its efficiency in visible-light-driven applications. Doping with transition metals is a widely used strategy to narrow the band gap of SrTiO3 and extend its optical response into the visible range. In this work, the structural, electronic, mechanical, and optical properties of 7% Cu-doped SrTiO3 (SrTi0.93Cu0.07O3) were investigated using first-principles density functional theory (DFT) calculations performed with the CASTEP module of BIOVIA Materials Studio. A 3×3×3 supercell containing 135 atoms was constructed, and a single Ti atom was substituted with Cu to obtain a doping concentration of ~7.4%, close to the targeted 7%. Calculations were carried out primarily with the GGA–PBE and GGA–RPBE exchange correlation functionals, with additional functionals (GGA-PW91, GGA-WC, and LDA) used to benchmark band-gap predictions. The results show that Cu substitution introduces localized Cu 3d states near the valence band maximum, narrowing the band gap of pristine SrTiO3 (~3.2 eV) to below 2 eV in the doped system, while producing a slight expansion of the lattice parameters consistent with the larger ionic radius of Cu2+ relative to Ti4+. The narrowed gap is accompanied by a red shift of the optical absorption edge into the visible region, an increase in the static dielectric constant and refractive index, and enhanced optical conductivity in the visible range. Elastic-constant calculations further indicate that the doped structure remains mechanically stable under the Born–Huang criteria, with moderate directional anisotropy in its elastic response. These results indicate that 7% Cu doping is an effective route for enhancing the visible-light-harvesting and charge-transport properties of SrTiO3, supporting its potential use in photocatalytic and photovoltaic applications.

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