Carboxylate-coordinated TiO_2-x in TEMPO-oxidized nanocellulose aerogels for visible-light photocatalysis

Although cellulose/TiO₂ photocatalytic aerogels provide recoverable architectures, nanocellulose is typically regarded as passive support, leaving its role in regulating TiO_2-x defect states and charge dynamics unclear. Here, we demonstrate that cellulose surface chemistry actively controls the interfacial electronic behavior of oxygen-vacancy-rich TiO_2-x in monolithic photocatalytic aerogels. Solvothermally synthesized TiO_2-x was immobilized on carboxylate-rich TEMPO-oxidized cellulose nanofibrils (TOCN) and benchmarked against mechanically fibrillated cellulose nanofibrils (MFCN). Spectroscopic, photoelectrochemical, and scavenger analyses showed that TOCN formed stronger Ti³⁺-carboxylate/Ti-O-C interfacial coordination, stabilizing a larger oxygen-vacancy-associated fraction than MFCN (8.6% vs. 6.4%) and suppressing charge recombination. Accordingly, TiO_2-x/TOCN exhibited a photoluminescence lifetime of 7.01 ns, compared with 4.17 ns for unsupported TiO_2-x, an approximately six-fold higher photocurrent, and lower charge-transfer resistance. This interfacial stabilization prolonged photogenerated-hole availability and promoted a hole-dominated oxidation pathway, whereas the weaker TiO_2-x-MFCN interaction was associated with defect-assisted recombination. The optimized aerogel containing 30 wt% TiO_2-x degraded 98.5% of methylene blue within 100 min under simulated solar irradiation and achieved approximately 95% degradation after 8 h under visible light (λ > 400 nm). Its monolithic architecture facilitated recovery and retained 97.5-100% of the mass-normalized activity over five cycles. These findings establish carboxylate-engineered nanocellulose as an active interfacial regulator, rather than a passive scaffold, for stabilizing defective TiO_2-x and designing recoverable visible-light photocatalysts.