Preparation And Property Tuning Of Photochromic System Based On Doped TiO_2-x Nanoparticles And Dyes

Reversible photochromic materials have broad applications in anti-counterfeiting,molecular switches,detectors,smart windows,and so on,owing to that light has great advantages of clean,non-direct contact,and precise spatiotemporal control.Inorganic and organic materials currently face challenges in practical applications because of their limitations in single color change and low cycle performance.The reversible photochromic systems based on semiconductor nanoparticles/redox dyes have recently attracted great attentions by integrating the advantages of both organic and inorganic materials,showing the advantages of the rapid photoresponse,excellent cycling properties,high color contrast,and so on.However,there are still many problems in reversible photochromic systems based on semiconductor nanoparticles/redox dyes for real application,such as uncontrollable color recovery speed,poor cycle performance,photoresponse mainly to high-energy ultraviolet(UV)light,and limited functions.Therefore,it is still a great challenge to develop reversible photochromic systems based on semiconductor nanoparticles/redox dyes with controllable photochromic properties,rich functions,and even visible light response.Based on the above problems,we are developing a hydrazine-assisted(N2H4·H2O)solvothermal method for the synthesis of alkaline Ti3+self-doped TiO2-x nanoparticles,enabling the TiO2-x nanoparticles/methylene blue(MB)-based photochromic system with tunable recoloration process;a pH/photo-dual responsive color switching system will be constracted by integrating the pH-dependent and redox-driven color switchable neutral red(NR)as color component with TiO2-x nanoparticles;a Sn-doping strategy will is developing for the synthesis of Sn-TiO2-x nanoparticles with visible light responsive photoreductive activity.And visible light responsive color switching systems based on TiO2-x nanoparticles/redox dyes will be further prepared.Moreover,TiO2-x nanoparticle/redox dyes/agarose gel films will be prepared by using agarose as a matrix,and their potential applications in self-erasing rewritable paper,encryption/decryption of information,and dynamic color change of light-printed information will be explored.The main research contents and results are as follows:(1)Preparation and properties of reversible photochromic systems based on Ti3+self-doped TiO2-x nanoparticles/MBThe Ti3+self-doped TiO2-x nanoparticles with high colloidal stability in alkaline solution through a hydrazine-mediated self-doping strategy.The ammonia derived from N2H4·H2O during the reaction process provides an alkaline solution for the growth of TiO2-x nanoparticles,ensuring the colloidal stability of TiO2-x nanoparticles in the alkaline solution even at a high pH value.Ti3+species act as effective internal sacrificial electron donor(SEDs)can dramatically increase hole scavenging capacity of TiO2-x nanoparticles and thus enable fast decoloration rate of TiO2-x nanoparticles/MB/H2O reversible photochromic system under UV illumination(<6 s)and significantly increase the photoreversible color switching cyclability(≥ 100 times).Simultaneously,the redox potential of MB is highly depended on pH value,indicating that the recoloration rate could be regulated on-demand by changing the pH value of the system.The TiO2-x nanoparticles/MB/agarose gel film is prepared by combining TiO2-x nanoparticles,MB solution and biodegradable agarose matrix.The on-demand regulation of the recoloration rate of the gel film is successfully achieved in a wide range by simply treating the film with acidic/basic vapor.We further demonstrate the potential applications of the gel film in self-erasing rewritable paper and transient optical information encryption.(2)Preparation and properties of pH/photo-dual responsive reversible color switching systems based on TiO2-x nanoparticles/NRThe pH/photo-dual responsive color switching systems are prepared by introducing the pH-dependent and redox-driven color switchable NR as color component to integrate with TiO2-x nanoparticles,exhibiting excellent pH responsive color changing performance and reversible photochromic performance.Owing the excellent pH-dependent and redox-driven color switching of NR,the high photoreductive activity and stability of TiO2-x nanoparticles,and porous network structure of agarose matrix,the TiO2-x nanoparticles/NR/agarose gel film exhibits outstanding dual pH-/photo-response color switching properties.Upon applying acidic vapor,the gel film changes its initial orange color to red and purplish red rapidly due to the progressive protonation of NR to NRH+in the gel film by acidic vapor.Upon applying basic vapor,the color could reversibly switch back to orange because of the deprotonation of NRH+to NR in the gel film by basic vapor.The gel film shows a fast color switching in 62 s between yellow and red upon basic and acidic vapor with high reversible cycling performance(>20 times).The gel film at both acidic and basic conditions exhibits outstanding photoreversible color switching properties,including decoloration in 13 and 18 s upon UV illumination and recoloration in 3 and 8 min at both states under ambient air,respectively,and reversible color switching more than 13 times.Furthermore,we demonstrate the application of TiO2-x nanoparticles/NR/agarose gel film in dynamic color changing rewritable media,in which the information created on the film by photo-printing shows interesting dynamic color changes.(3)Preparation and properties of visible light responsive reversible photochromic systems based on Sn-TiO2-x nanoparticles/redox dyesSn-TiO2-x nanoparticles are achieved by a solvothermal reaction using SnCl2·H2O as dopant,N2H4·H2O as reducing agent and EG as solvent.Compared to anatase TiO2,Sn-doping significantly red-shifts the absorption edge of 55 nm of Sn-TiO2-x nanoparticles from UV to visible region with a broad shoulder,and simultaneously produces both Ti3+species and oxygen vacancies,which could act as effective SEDs to scavenge photogenerated holes of Sn-TiO2-x nanoparticles under visible light illumination.The Sn-TiO2-x nanoparticles exhibit excellent visible light responsive photoreductive activity,enabling the photocatalytic reduction of various redox dyes,such as MB,phenosafranine(PS),and methylene green(MG).The Sn-TiO2-x nanoparticles/redox dyes/agarose nanocomposite films are further prepared by integrating Sn-TiO2-x nanoparticles,redox dyes with agarose as matrix,which exhibits outstanding photoreversible color switching properties,including decoloration in 6 or 8 s under blue light illumination,coloration in 190,100,or 185 s in ambient air.Moreover,the Sn-TiO2-x nanoparticles/MB-NR/agarose gel film is prepared by using mixed dyes of NR and MB as color components.Owing to that the decoloration rate of NR is much lower than that of MB,the green gel film could be used in time-sensitive,self-erasing rewritable paper.