Design And Properties Of BiOCl Nanosheets/Redox Dyes Photocatalytic Color Switching System

The reversible photocatalytic color switching systems driven by semiconductor nanoparticles have attracted considerable attention because of their wide applications in information displays,sensors etc.However,the developed semiconductor nanoparticles with photoreductive activity are mainly limited to TiO2-based photocatalysts,which greatly hinder their broad applications.Here we report a co-capping ligand-assisted strategy for the development of photoreductive BiOCl ultrathin nanosheets with abundant oxygen vacancies.Both the co-capping ligands and oxygen vacancies in BiOCl ultrathin nanosheets act as sacrificial electron donors to efficiently scavenge the photogenerated holes,endowing the BiOCl ultrathin nanosheets high photoreductive activity and thus enabling the photocatalytic color switching of redox dyes.The specific research content is as follows:(1)Preparation of photoreductive BiOCl ultrathin nanosheetsHerein,we synthesize colloidal BiOCl ultrathin nanosheets with an average size of 36 nm and thickness of 1.4-2.8 nm by a solvothermal reaction using mannitol and polyvinylpyrrolidone(PVP)as co-capping ligands.The FTIR and ESR results demonstrate that both PVP and mannitol as co-capping ligands are successfully bonded on BiOCl to promote the formation of BiOCl ultrathin nanosheets with abundant oxygen vacancies.Co-capping ligands and oxygen vacancies play a key role in endowing the BiOCl ultrathin nanosheets high photoreductive activity.(2)Preparation and properties of BiOCl ultrathin nanosheets/redox dyes photocatalytic color switching systemBy integrating the BiOCl ultrathin nanosheets with redox dyes to achieve the reversible color switching of redox dyes.Upon ultraviolet(UV)illumination for 40 s,photogenerated electrons rapidly transfer from BiOCl ultrathin nanosheets and reduce methylene blue(MB)to colorless leuco methylene blue(LMB).The blue color of the hybrid aqueous dispersion disappears rapidly.The colorless system can recover the color by oxidizing LMB in ambient air conditions in 40 min enabling reversible color switching.Near-infrared light(?=730 nm)illumination can shorten the recovery time to 15 min.Moreover,the BiOCl ultrathin nanosheets photocatalytic color switching can be extended to other redox dyes.For example,the hybrid BiOCl ultrathin nanosheets/neutral red(NR)aqueous dispersion shows reversible color switching between red and colorless state upon UV illumination and oxidation in ambient air conditions.Considering the solid film is a preferred form for most practical applications,we successfully integrate the BiOCl ultrathin nanosheets/MB color switching system with poly(vinyl alcohol)(PVA)hydrogel to fabricate a twistable gel film.BiOCl ultrathin nanosheets and MB distribute homogeneously in the PVA gel.It should be noted that the recoloration rate of the gel film from colorless to blue color is significantly enhanced in ambient air conditions compared with that of the hybrid aqueous dispersions.The blue color of the gel film disappears upon UV illumination for 80 s,while the color switches back in 18 min in ambient air conditions.(3)Colorimetric oxygen indicatorsThe gel film with a rapid recoloration rate indicates its high sensitivity to oxygen,which has the potential applications in colorimetric oxygen indicators.Therefore,we design a water-protective polymer layer on the top surface of the gel film to solve the MB-leaching issue in a water environment.We further demonstrate its application in a colorimetric oxygen indicator for food packaging,exhibiting high sensitivity to monitor oxygen leakage by the naked eye.We believe the work opens a new avenue for designing photoreductive semiconductor nanomaterials to enrich the photocatalytic color switching systems and their applications.