Supercritical CO₂-assisted Fabrication Of Two-Dimentional Defect-rich Vanadium Dioxide And Its Functional Research

Non-layered transition metal oxide VO2(M)is a typical strongly correlated material.Temperature-induced structural phase transition along with metal-insulator transition(MIT)at near room temperature(TMIT=68℃)makes its optical properties change from infrared transparency of semiconductor M phase(T<TMIT)to infrared reflection of metal R phase(T>TMIT),so it is widely used in thermochromic materials.However,due to its strong metal-oxygen covalent bond in three-dimensional(3D)space,there are few reports on two-dimensional(2D)nanosheets.in M phase.And we know that when MIT happens,the lattice constants of the R phase are elongated with respect to the M phase by about 0.6%and 0.4%along the aR and bR axis,respectively.In this work,2D micron-sized nanolayered VO2(M)nanosheets are successfully fabricated by utilizing the temperature-dependent structural phase transition of VO2 and strain engineering of CO2.The strong strain field of CO2 can introduce a massive amount of V atomic defects in the 3D crystal structure,and the expanded lattice channels of R phase make it more easily for CO2 molecules to intercalate and diffuse along cR channels.This directional diffusion can further induce the transformation of the 3D defective network into 2D nanosheets.The obtained 2D defective VO2(M)nanosheets exhibit a series of unique magnetic and optical properties.Due to the symmetry breaking of order parameter caused by the local amorphous phase and the 3 D-to-2D transformation,the obtained 2D defective Mphase nanosheets show room temperature ferromagnetism which do not exist in bulk materials;At the same time,the absorption and photoluminescence intensity of 2D defect nanosheets in visible and near infrared region are significantly enhanced compared with bulk materials.In addition,the 2D defective nanosheets solution of 1 mg mL-1 show good photothermal conversion performance when illuminated by 808 nm laser with a power of 1.06 W cm-2.The main contents are as follows:(1)Preparation of VO2(M)bulk sample and the preliminary study of supercritical induced phase transition reactionFirstly,thermodynamically metastable VO2(B)was prepared through hydrothermal treated V2O5 at 230℃ for 48 h.On this basis,VO2(M)was obtained by calcining VO2(B)at 700 ℃ for 2 h in Ar atmosphere.XRD results indicated that the product shows high purity and crystallinity.Meanwhile,the morphology and structure were identified in detail by XPS,SEM,Raman,FTIR and DSC.Then,according to the polarity of solvents and the enthalpy of mixing between them and VO2(M),9 common solvents were screened,and finally NMP/H2O mixed solvent was selected to exfoliate VO2(M).In addition,as the acquisition of micron-sized VO2(M)nanosheets,VNMP:VH2O=9:1 was determined for subsequent experiments.(2)VO2(M)nanosheets prepared by supercritical CO2In supercritical CO2 system,we successfully prepared micron-sized 2D nonlayered VO2(M)nanosheets by using temperature-induced structural phase transition of VO2 and strain engineering of CO2.When the temperature is below TMIT,the shear stress field caused by CO2 diffusion along the aM axis successfully transforms bulk materials to 50 nm nanosheets with the exposed plane of(201).When the temperature is higher than TMIT,the larger channel brought by the lattice expansion of R phase accommodate more CO2 molecules to intercalate and diffuse along cR-channel,thus generating a stronger stress field.When it cooled down to room temperature,micronsized 2D defective M-phase nanosheets are obtained,and the exposed plane is(201).The nanosheets produced a massive number of defects and local amorphous phases due to the lack of vanadium atoms.Meanwhile,by further adjusting the pressure and reaction time,it is confirmed that the strain engineering brought by directional diffusion of CO2 molecules in the VO2 channel is the key to prepare 2D M-phase nanosheets.(3)The application of 2D defective VO2(M)nanosheets.The magnetic and optical applications of the prepared 2D defective VO2(M)nanosheets were preliminarily explored.In the preparation process of M-phase nanosheets,the crystalline to amorphous phase transition and the 3D-to-2D transformation will cause the symmetry breaking of order parameter,and then the magnetic moment is introduced into nonmagnetic M-phase VO2,which endow it a unique at room-temperature ferromagnetism.At the same time,compared with bulk materials,the absorption and photoluminescence intensity of 2D defective nanosheets in visible and near infrared region are significantly enhanced.Furthermore,the obtained 2D nanosheets show good photothermal conversion performance with a photothermal conversion efficiency of 34.09%under the laser irradiation of 808 nm and 1.06 W cm-2.