Methodology-directed synthesis of Bi _{₂ ₅} GaO _{₃ ₉} –Bi _₂ Ga _₄ O _₉ –Bi _₂ O _₃ heterojunctions: unraveling the interplay between fabrication route, interfacial engineering, and photocatalytic efficiency

Abstract The rational design of synthesis pathways plays a decisive role in regulating the phase composition, interfacial architecture, and charge-transfer behavior of heterojunction photocatalysts, thereby directly governing their photocatalytic performance. In this work, a systematic methodology-directed investigation is conducted to construct bismuth–gallium oxide heterojunctions within the Bi _{₂ ₅} GaO _{₃ ₉} –Bi _₂ Ga _₄ O _₉ –Bi _₂ O _₃ system using three representative fabrication routes: hydrothermal-annealing, precipitation-pyrolysis, and impregnation. By deliberately tailoring the synthesis strategy, pure-phase Bi _{₂ ₅} GaO _{₃ ₉} and Bi _₂ Ga _₄ O _₉ , binary heterojunctions (Bi _{₂ ₅} GaO _{₃ ₉} /Bi _₂ O _₃ and Bi _₂ Ga _₄ O _₉ /Bi _₂ O _₃ ), and a ternary Bi _₂ Ga _₄ O _₉ /Bi _{₂ ₅} GaO _{₃ ₉} /Bi _₂ O _₃ heterojunction are successfully obtained. Comprehensive structural, spectroscopic, and photoelectrochemical characterizations reveal that the hydrothermal-annealing route facilitates the formation of well-crystallized nanostructures with intimate interfacial contact, which is highly favorable for interfacial charge transfer. Among all samples, the ternary heterojunction exhibits the most efficient charge-separation behavior, as evidenced by the lowest charge-transfer resistance and the highest photocurrent response, and achieves a high methylene blue degradation efficiency of 96.58% under visible-light irradiation, outperforming the corresponding binary and single-phase counterparts. Radical trapping experiments combined with density functional theory calculations indicate that the enhanced activity originates from a staggered band alignment consistent with a type-II-like charge-transfer mechanism, which promotes directional spatial separation of photogenerated electrons and holes. Furthermore, an auxiliary intelligent photocatalytic evaluation platform based on RGB color analysis is developed to enable rapid and non-destructive monitoring of dye degradation. This study highlights the critical role of synthesis methodology in heterojunction interface engineering and provides a rational strategy for designing high-performance multicomponent photocatalysts through methodology-guided optimization.