Design And Performance Study Of Photo/Thermal Responsive Materials And VO₂ Composites

Vanadium dioxide（VO₂）is a widely studied thermochromic material,which has a reversible phase transition from insulating phase（monoclinic M,P21/c）to metallic phase（rutile R,P42/mnm）at a critical temperature（T_C）of 68 ^oC.The abrupt decrease of infrared transmittance in the metallic phase makes VO₂ a potential candidate for thermochromic energy efficient windows to cut down building energy consumption.However,there are some long-standing issues that hinder its application in energy efficient windows,such as high T_C,low luminous transmittance(T_lum),and undesirable solar modulation ability(ΔT_sol).In addition,the synthesis protocols of VO₂（M）are complicated somehow,the phase state of VO₂（M）is difficult to control and the stability of VO₂（M）is poor while exposes in air for a long period.To seek more reasonable solutions to the above issues,this Ph D thesis aims at developing and designing novel approaches to achieve the targets,as demonstrated below:（1）Developed a simple but efficient hydrothermal method for one-step synthesis of VO₂（M）nanoparticles with lower phase transition temperature（T_C）and the variables related to reaction temperature,time,and the molar ratio of reactants will be systematically investigated.Among them,the hydrazine（N₂H₄）concentrations in hydrothermal processing are found to greatly affect the crystal structure and morphology,in which the mechanism can be understood by Density Functional Theory（DFT）calculations.Besides,experimental results combine with DFT calculations reveal that OVs in VO₂（M）nanoparticles can effectively reduce the T_C（from 68 ^oC to～50 ^oC）,and the composite film prepared by dispersing OVs-VO₂ nanoparticles into polyvinyl butyral（PVB）exhibits excellent optical properties(T_lum=43.4%andΔT_sol=17.3%).（2）Developed a thermochromic[（C₂H₅）₂NH₂]₂NiX₄（X=Cl,Br）complex,and the Gibbs free energy,binding energy and formation energy of the complex are investigated by Density Functional Theory（DFT）.In addition,the phase transition process of the complex has been studied by in situ FT-IR spectroscopy.It can be found that the complex is easy to absorb moisture in air and lose thermochromic properties,so[（C₂H₅）₂NH₂]₂NiX₄@Si O₂ core-shell structured nanosphere is designed and synthesized,and the protective mechanism of Si O₂shell is studied.Through a layer-by-layer coating technique,a[（C₂H₅）₂NH₂]₂NiX₄@Si O₂/PS thin layer can be coated onto a VO₂（M）/PVB thin layer as an energy-saving coating.This composite film is capable of modulating visible region and the near-infrared region,achievingΔT_sol up to 25.7%.Besides,the intrinsic yellow-brown color of VO₂（M）/PVB film also changed（from yellow to green）.（3）A VO₂（M）/spiropyran hybrid composite film with ptoto/thermal response is designed and prepared for the first time.The effects of light source(visible light,sunlight,UV_365nm,UV_254nm)and spiropyran content on the color and optical properties(T_lumandΔT_sol)of spiropyran films are systematically studied.Among them,the spiropyran film with the spiropyran content of 1.0 wt%exhibits excellent solar modulation ability(T_{lum,visible light}=97.9%,T_lum,sunlight=69.2%,ΔT_sol=12.0%).And when the film is transferred to visible light,T_lum can recover more than 90%of the initial state in 15 minutes.In addition,FT-IR characterization reveals that there is no obvious interaction between spiropyran and VO₂（M）nanoparticles in the hybrid composite film.Elemental image mapping proves that spiropyran molecules are uniformly dispersed in the film without obvious agglomeration.This composite film is capable of modulating UV-Vis region and the near-infrared region,achievingΔT_sol up to 23.6%.The film color also changs from yellow to pink as the light sources changed.（4）A―grafting to‖method is used to synthesize core-shell structured VO₂@PMMA-b-PHFBMA nanoparticles innovatively.The PMMA,PMMA-b-PHFBMA and PMMA-b-PHFBMA-b-PSPA macromolecules with narrow molecular weight distribution are prepared by RAFT polymerization.VO₂@PMMA-b-PHFBMA nanoparticles are prepared by thiol-ene click reaction on the surface of vinyl-functionalized VO₂ nanoparticles,and the dispersibility,durability,compatibility and antacid properties are investigated.It is worth mentioning that the photochromic spiropyran acrylic（SPA）is introduced into the PMMA-b-PHFBMA polymer chains via RAFT polymerization creatively.Through a layer-by-layer coating technique,a PMMA-b-PHFBMA-b-PSPA thin layer can be coated onto a VO₂@PMMA-b-PHFBMA/PVB thin layer as an energy-saving coating.This composite film is capable of modulating visible region and the near-infrared region,achievingΔT_sol up to20.3%.Besides,the intrinsic yellow-brown color of VO₂ film also changed.