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Graphical abstract Highlights • A recrystallization-annealing strategy was developed to fabricate alkalinized g-C 3 N 4 and TiO 2 composite. • The C N bond was dissociated while C OH was formed. • Synergetic effect of adsorption and photocatalysis enhanced the degradation of methylene blue. Abstract This work reports the fabrication of alkalinized g-C 3 N 4 and its composite with TiO 2 via recrystallization-annealing strategy. The increased surface basicity boosted the fast adsorptive removal of methylene blue (MB) while the cooperated TiO 2 enhanced the degradation of adsorbed MB via photocatalysis. The synthesized materials were characterized by XPS, SEM, TEM, PXRD, BET surface area, FT-IR, EDX and TG analyses. TiO 2 nanoparticles were uniformly distributed in the framework of alkalinized g-C 3 N 4 which increased the surface area of g-C 3 N 4. Na was incorporated into g-C 3 N 4 via thermal treatment, and the bond between C and N was dissociated while that between C OH was formed. The alkaline and negatively charged surface of g-C 3 N 4 upon Na addition established a strong electrostatic interaction adsorption scenario with MB leading to higher adsorption efficiency of TiO 2 /Na-g-C 3 N 4 (90%) than pristine g-C 3 N 4 (80%). MB removal efficiency was further intensified (from 90% to 100% within 120 min) upon the sequential operation of adsorption followed by photocatalytic degradation over TiO 2 /Na-g-C 3 N 4 under visible light irradiation. This study can be deemed of potential applications for the removal of MB and similar dyes on industrial level using the newly synthesized TiO 2 /Na-g-C 3 N 4 and synergistic adsorption-photocatalytic oxidation approach. [ABSTRACT FROM AUTHOR]

[Display omitted] • Titania oxide with a shrubby morphology was prepared on carbon cloth to trigger SERS "hotspots" with a high-density. • The structure-dependent SERS and photocatalysis performance were investigated for as-prepared CC/TiO 2 /Ag hybrid substrates. • An actual detection of quercetin in teas was achieved by the self-cleaning SERS substrate. Constructing of plasmonic substrate with the capability of detecting nutritional food quality will soon be emerged as the frontier research directions with broad prospects in surface-enhanced Raman scattering (SERS) technology ascribed to an improvement of living standards. Herein, a novel TiO 2 framework with a shrubby morphology was successfully constructed on carbon cloth (CC) matrix to trigger significantly enhanced electromagnetic "hotspots" after dense Ag network with numerous sharp corners and edges was formed by a photo-induced reduction. Particularly, the structure-dependent optical performance was investigated for these as-prepared CC/TiO 2 /Ag hybrid substrates, screening out an optimal one with an enhancement factor of 6.38 × 107. Regarding a prominent photocatalytic efficiency ascribed to the high electron-hole separation efficiency assisted by the CC matrix and Ag decoration, a recyclable SERS-based monitoring of quercetin (QR) was achieved in teas, demonstrating recovery values distributed from 88.9 % to 106.4 %. The strategy proposed here opened a new way to create high-density "hotspots" for multifunctional SERS substrates, which might play a crucial role in food quality monitoring applications. [ABSTRACT FROM AUTHOR]

• The low-electron-density defect is detected by positron annihilation spectra. • A quantitative correlation between defects and performance is demonstrated. • The correlation is pervasive in various materials and photochemical processes. • Decreasing the defect concentration would promote electron transfer rate. • The sample with lower defect concentration shows higher photochemical activity. Establishing definite correlations between specific defects and physicochemical processes is the prerequisite for applying defect-engineering to optimize material properties. However, systematic investigations are required to fulfill this task. In this work, for the first time, a kind of defect with low electron density is quantitatively demonstrated to be correlative with photocatalysis efficiency by using perovskite oxide Na 0.5 Bi 2.5 Ta 2 O 9 nanosheets as the model material, in which the defect was systematically introduced and engineered. More importantly, the correlation is found to be not only valid for photocatalysis in Na 0.5 Bi 2.5 Ta 2 O 9 , but also is pervasive in various materials and different photochemical processes, though it has been overlooked in previous reports. The defect with low electron density acts as the recombination center for photogenerated carriers, therefore it is detrimental to the photo(electro)chemical performance, and the elimination of this kind of defect will be critical for improving photochemical efficiency. [ABSTRACT FROM AUTHOR]