Fabrication And Electrochromic Properties Of Flexible Thin Films Based On Ultrathin V₂O₅ Nanosheets

In the emerging field of flexible displays electronics,there is an ever-growing attention for flexible electrochromic(EC)devices owing to their less energy consumption,excellent stability and superior contrast.The EC layer is the key of an EC device,which is composed of the EC film with excellent electronic and ionic conductivity and good adhesion with the flexible conductive substrate.V₂O₅ not only appears reversible and multicolor changes(yellow-green-blue or more)during the Li⁺ion insertion/extraction processes,but also performs high Li⁺ion storage capacity beneifiting from its exoteric layered structure.Nevetheless,the low electronic conductivity and Li⁺ion diffusion coefficient of V₂O₅ films degrade the response speed and cyclic stability,which restricts their extensive application.Herein,this study aims to fabricate the flexible EC films with high-performance based on ultrathin V₂O₅ nanosheets.Meanwhile,the influence of two modification methods(composite with graphene oxide and Mo⁶⁺doping)on the EC properties of V₂O₅ are investigated.Firstly,the liquid-phase exfoliation method is employed to exfoliate the orthorhombic V₂O₅ powder into ultrathin V₂O₅ nanosheets with high disperision and excellent crystallinity.V₂O₅ nanosheets still remain orthorhombic phase accomplying with the(001)orientation.The lateral dimension and thickness of V₂O₅ nanosheets are about 400 nm and 4 nm,respectively.Flexible V₂O₅ nanosheets films with controllable thicknesses(10?90 nm)are prepared by vacuum filtration and transfered onto the flexible ITO/PET conductive substrate.The influence of filtrate volumes on the thicknesses,morophology and EC properties of V₂O₅ nanosheets films are discussed.The thicknesses,optical contrast(21.3%?47.2%)and coloring/bleaching times(2.3 s/3.5 s?7.2 s/10.5 s)are increased with the incremental filtrate volumes.Secondly,in order to improve the electronic conductivity and thus accelerate the responsion speed of V₂O₅ nanosheets films,V₂O₅ nanosheets/graphene oxide(V₂O₅NSs/GO)films with different bi-layer number are fabricated by layer-by-layer assembly.The hydrogen bonding interactions among V₂O₅ nanosheets,graphene oxide sheets and the ITO/PET substrate avoid the agglomeration of V₂O₅ nanosheets and ensure the strong combination between V₂O₅ NSs/GO films and substrate.Hence,V₂O₅ NSs/GO composite films display the superior optical contrast(?T_max=57.5%)and cyclic stability(?89%).Besides,graphene oxide serves as the flexible conductive framework and forms the interconnected electron transport routes with V₂O₅nanosheets,which reduces the responsion time(1.6 s/2 s)and improves the bending performance(2.7 s/4.3 s).Based on above research results,Mo-V₂O₅ nanosheets are prepared by V₂O₅power doped by Mo⁶⁺ions and liquid-phase exfoliation method.Likewise,flexible Mo-V₂O₅ NSs/GO films are fabricated through layer-by-layer assembly.The doping of Mo⁶⁺(radius slightly larger than V⁵⁺ions)distortes V₂O₅ lattice and increases its interlayer distance,which provides more free spaces for Li⁺ion insertion/extraction processes,thereby enhancing the optical contrast(61.1%).In addition,Mo-V₂O₅NSs/GO films exhibit rapider responsion speed(1.1 s/1.8 s)because the electron conductions are facilitated by the doping of Mo⁶⁺,which provides the extra energy level and decreases the forbidden band width of V₂O₅.