Title : Composition-driven bandgap engineering of X(1−x)Bi(x)TO₂ (X = Ga, Al, In) for efficient energy conversion applications
Abstract:
Bandgap engineering in functional oxide alloys is a key strategy for designing next-generation energy conversion materials with tailored optical absorption and optimized electronic structure. In this work, we investigate the compositional tuning of the electronic and structural properties of X(1−x)Bi(x)TO₂ alloys (X = Ga, Al, In) across a broad substitution range (x = 0, 0.25, 0.50, 0.75, 1.00) using first-principles calculations within VASP. Hybrid-functional calculations were performed using the HSE06 approach to obtain reliable bandgap trends relevant to solar-driven energy conversion.The results reveal a clear composition-dependent modification of the band structure, where Bi substitution induces a systematic bandgap reduction in all investigated systems. For example, the calculated HSE06 bandgap decreases from Eg = [Eg₍x=0₎] eV (XTO₂) to Eg = [Eg₍x=1₎] eV (BiTO₂), with intermediate compositions exhibiting non-linear behavior indicative of strong alloying effects. This non-linearity can be quantitatively described through a bandgap bowing parameter, yielding b₍bow₎ = [b(Ga)] eV, [b(Al)] eV, and [b(In)] eV for the Ga-, Al-, and In-based alloys, respectively. Structurally, full geometry optimizations show that Bi incorporation leads to noticeable lattice expansion and symmetry lowering, reflected in the optimized lattice constants a = [a] Å, b = [b] Å, and c = [c] Å for each composition. Notably, none of the relaxed structures retain cubic symmetry, suggesting that local strain and coordination changes contribute to the observed electronic modulation. Based on the resulting bandgap windows, the most promising compositions fall within energy conversion–relevant ranges, where Eg ≈ 1.6–2.2 eV supports visible-light photoelectrochemical processes and Eg ≈ 0.8–1.6 eV is favorable for photovoltaic or optoelectronic absorber applications. The outcomes provide a rational compositional roadmap for optimizing X(1−x)Bi(x)TO₂ alloys toward high-performance energy conversion and sustainable nanomaterials design.
Keywords: bandgap engineering; HSE06; VASP; bismuth substitution; oxide alloys; energy conversion; photocatalysis; photovoltaics
