Supercritical CO₂-Assisted Nanoconfinement Effect For The Fabrication Of 2D Functional Materials

Facing the global climate change and the increasingly severe shortage of resources,accelerating the layout of the cross-disciplinary science of green technology,new materials and advanced energy has a huge and profound impact on human production and life.The development of two-dimensional（2D）nanomaterials play a key role in this field.Because the extreme aspect ratio of 2D nanomaterials enables them to exhibit unique electronic properties,ultra-high specific surface area and abundant active sites,especially,the presence of coordination-unsaturated surface atoms provide infinite possibilities for the design of 2D functional nanomaterials through structural control strategies such as defects,stress and phase engineering,featuring with optical,electronic and magnetic properties.Supercritical CO₂（SC CO₂）is a well-known green solvent system.CO₂ in the critical state has the characteristics of low interfacial tension,excellent wettability,adjustable solvation energy and charge separation,etc.Moreover,the linear characteristic of CO₂ molecule（3.3?）allows it to enter any confined space that larger than its molecular diameter and,show unique physicochemical effects.It is of great significance to build a green and low-carbon recycling economic system,by using of SC CO₂ technology to construct new nanomaterials that can effectively convert and store sustainable energy,such as light,electricity and solar energy.In this work,four kinds of compounds with well-defined nanostructures are chosen as experimental models.Making use of the solvent properties of SC CO₂ and the unique physical and chemical effect of CO₂ molecules in different confinement spaces,four kinds of 2D functional nanomaterials are obtained.Combining the experimental and theoretical calculation results,the nanoconfinement effect of CO₂ dependent of crystal structures is explained in depth.This work provides new insight and approaches for the construction of novel 2D functional nanomaterials.The main research contents are as follows:（1）Nanoconfinement effect of SC CO₂ in binary layered VS₂VS₂ is a typical layered van der Waals solid,and the interlayer spacing of 5.8?can provide enough space for CO₂ molecules diffusion.We confirmed that CO₂ has temperature and pressure-dependent strain engineering in the vd W gap of layered VS₂,which can controllably convert bulk VS₂ into 2D amorphous VS₂ and in-situ S-doped2D VO₂（D）nanosheets.The as-prepared 2D amorphous structure exhibits unique optical properties,including UV-Vis-NIR full-band absorption and significantly enhanced photoluminescence performance.Meanwhile,owing to the strong absorption and weak reflection in the NIR region,2D amorphous VS₂ and 2D VO₂（D）show superior photothermal conversion performance than the crystal counterpart.The disordered structure of the amorphous nanosheets,tail state absorption,electron and hole recombination trapped by deep level defect states and high crystalline quantum dots obtained by CO₂ shear stress corporately contribute to the excellent photothermal performance of the two-dimensional amorphous phase.In addition,the coexistence of photoluminescence and photothermal conversion performance in 2D amorphous phase provides a new experimental and theoretical insight into the interaction between light and amorphous structure.（2）Nanoconfinement effect of SC CO₂ in ternary layered Bi OClElectrocatalytic CO₂ reduction（CO₂RR）is a technology that converts CO₂ into chemicals with high added value through renewable electricity.Layered bismuth oxychloride（Bi OCl）is formed by stacking[Bi₂O₂]²⁺and bilayer Cl ions alternately.Bismuth（Bi）and oxygen（O）within[Bi₂O₂]²⁺layers are connected via strong covalent bonding,while both of the interactions between[Bi₂O₂]²⁺monolayers and between[Bi₂O₂]²⁺and[Cl]^-layers are van der Waals forces.A vd W gap（6.84?）with Cl ion is formed between two[Bi₂O₂]²⁺monolayers.The strain engineering of SC CO₂ in the vd W gap of layered materials can generate intralayer[Bi₂O₂]²⁺structural distortion in it,enabling Bi OCl to be a highly active CO₂RR electrocatalyst.The DFT calculation and experimental results show that SC CO₂ can destroy the weak van der Waals interaction between the layers and remove the interlayer Cl atoms.The loss of Cl atoms has a positive correlation with the degree of distortion of[Bi₂O₂]²⁺.When the loss of Cl atoms reaches a certain degree,CO₂ can be adsorbed and activated within layers.Further electrocatalytic CO₂RR experiment and DFT calculation show that,compared with the ultrathin nanosheets,the distorted Bi OCl nanoplates can significantly improve the activity and selectivity of the HCOOH pathway while inhibiting the competing reactions of CO and H₂.This strategy greatly promotes the application of green technology in the cross-cutting fields of new material preparation and energy storage conversion.（3）Nanoconfinement effect of SC CO₂ in non-van der Waals layered VO₂2D ferromagnets have unique advantages in the fields of spintronics and ultra-high speed and ultra-high capacity information storage.From the perspective of device application,it is urgent to find and design more 2D ferromagnets with high Curie temperature.In this chapter,the strain engineering of CO₂ is introduced into the non-van der Waals VO₂,converting it into room temperature 2D ferromagnet.The structure of VO₂（B）is a dense packed V-O layer in plane,which are stacked by strong covalent bond.The vertex and edge in VO₆ octahedra can form 3D tunnel-like frameworks.We confirm that the diffusion of CO₂ in the tunnels of VO₂ crystals can generate a strong internal stress field up to GPa.Meanwhile,the pressure can control the diffusion orientation of CO₂ in the tunnels.The selective cleavage of covalent bond can generate two metastable phases,i.e.local amorphous phase and high-index lattice plane,thus,converting the 3D bulk VO₂ into 2D nanosheets.Metastable phases and dimensional change from 3D to 2D can induce symmetry breaking in 2D VO₂,making it exhibit significant enhanced room temperature ferromagnetic response.The DFT theory further proves that the strong internal strain field of CO₂ in the crystal tunnels dominates the geometric transformation and the formation of metastable phase.（4）Nanoconfinement effect of SC CO₂ in non-layered MOF2D metal-organic frameworks（MOFs）combine the advantages of 2D nanomaterials and the modular structure of MOFs.In this work,the confinement effect of SC CO₂ in 3D non-layered Cu-BTC is utilized to reconstruct the secondary structural unit（SBUs）of Cu-BTC,and thus designing a 2D non-layered MOF at the molecular level.Cu-BTC forms face-centered cubic crystals containing an intersecting 3D framework of three distinct guest-accessible pores.A series of experimental and theoretical results show that,the unique solvent properties of SC CO₂ can induce the formation of hydrogen-bond complexes（THF-HOAc）encapsulated by CO₂ clusters,which can provide an ingenious microenvironment for the selective dissociation of BTC ligands and CO₂ coordination on SBUs,thus converting 3D Cu-BTC into 2D non-layered Cu-based MOF.The rational design of CO₂-reconstructed SBUs can develop more multifunctional 2D non-layered MOF for potential gas adsorption/separation,catalysis and sensing applications.More importantly,making use of the interaction between fluid molecules and atomic crystals in confined channels,the crystal microstructure can be reconstructed across scales.