In Situ Driven Formation of Anatase/Brookite/Rutile Heterojunction N/TiO₂ Nanocrystals as Sustainable Visible-Light Catalysts

Visible-light active anatase/brookite/rutile (A/B/R) ternary N-doped titania (N/TiO₂) crystals are successfully prepared by a facile sol-gel method using titanium butoxide and benign N-dopant source, guanidinium chloride. Systematically varying the aging time (1, 4, 8, and 12 d), its influence on physicochemical properties of as-obtained spherical heterojunction nanomaterials is studied. Detailed characterizations confirm that a substantial amount of anatase (88% to 50%) is transformed to rutile (2% to 38%) via intermediate brookite phase (9% to 25%) as the function of aging time; not only the A/B/R phase content of the samples is tuned by sol-gel aging time of the precursors solution but also their optical-response and methylene blue photocatalytic properties are profoundly dictated. Notably under visible-light irradiation, the photostable rutile rich mesoporous A/B/R triphasic N/TiO₂ (50% A, 12% B, 38% R) aged for 12 d demonstrates higher degradation activity (97%) with a faster degradation rate (0.033 min^-1) than both lesser aged N/TiO₂ and undoped titania. This enhancement is attributed to the synergistic effect of interstitial-N-doping and optimal A/B/R interfacial charge transfer that leads to higher light absorption, lower bandgap energy and well-separated charge carriers. The current work provides a new perspective for designing highly active visible-light heterostructure nanomaterials with controllable phase composition.