Visible light induced photocatalytic removal of Cr(VI) over TiO2-based nanosheets loaded with surface-enriched CoOx nanoparticles and its synergism with phenol oxidation

• Co-TNSs with high stability and regenerate usability were prepared by one-pot method.
• CoOx loading results in the changes in the structure of the TNSs and red shift.
• CoOx loading leads to obvious increase of the Ti3+ ions content.
• CoOx loading can effectively trap electrons to enhance the separation of charges.
• Co-TNSs exhibit excellent synergistic photocatalytic activity for Cr(VI) and phenol.

Visible-light-driven (VLD) CoOx loaded TiO2-based nanosheets (Co-TNSs) with surface-enrich CoOx nanoparticles were successfully synthesized by using a facile one-pot hydrothermal method. The photocatalysts were characterized by SEM, AFM, HRTEM, XRD, Raman, XPS, FL and the surface photocurrent (SPC) technique, etc. The as-synthesized samples exhibit sheet-like structure with large specific surface area (260–350 cm2/g) and small thickness (3–4 nm). The CoOx nanoparticles (1–2 nm) are highly dispersed on the surface of TNSs. Loaded CoOx nanoparticles not only have influenced the crystal structure, crystallinity and surface area of the TNSs, but also resulted in considerable enhancement of visible-light (VL) absorption and a red-shift of the band gap of the TNSs. XPS results reveal that cobalt mainly exists in the form of Co2+ and Co3+, and the binding energies of TiO bonds are changing with the loaded amount of CoOx nanoparticles. FL and SPC results indicate appropriate amount of CoOx (2.5 at.%) loading can effectively inhibit the recombination of photo-generated electron-hole pairs, thus improve the separation efficiency of charge carriers.The photocatalytic activity of the samples was evaluated by the dichromate (Cr(VI)) solution under VL irradiation. It can be found that the Co-TNSs photocatalysts showed excellent VL photocatalytic activity for the removal of Cr(VI). Initially the photocatalytic activity increased with the content of CoOx, and then decreased after attaining a maximum value at an optimal content (2.5 at.%) for degradation of aqueous Cr(VI) solution. The addition of Ca2+ promotes photocatalysis owing to its ionic bridging function in the form of TiOH+Cr(VI)–Ca2+Cr(VI) linkages, while SO42− slightly inhibits the photo-reduction of Cr(VI), indicating good synergy of photocatalysis and adsorption even at higher ionic strength of electrolyte. The photocatalytic reduction of Cr(VI) is also significantly promoted by phenol and synergism between Cr(VI) reduction and degradation of phenol which is demonstrated by measuring the effect of multiple usage of Co-TNSs on its photocatalytic efficiency. Desorbed Co-TNSs (Co-TNSs-Des) are easily regenerated and reused for Cr(VI) removal with excellent performance. A possible alternate mechanism for the enhancement of photocatalytic activity under visible light irradiation is also proposed.

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