Investigation Of The Micro Scale Regulation,Assembly,Crystal Growth And Properties Of TiO₂ In High Pressure Solvothermal System

With the rapid development of nanomaterials,hydrothermal/solvothermal synthesis has become a very important method to control the synthesis,self-assembly and crystal growth of nanocrystals.The final morphology of the products mainly depend on the species of the solvent,reaction duration,reaction temperature,reaction pressure,concentration of the reactants,addictive and so on.As one of the same important factor,the research of the nanomaterial growth process under high pressure is rarely reported due to the high requirements for the experimental devices.In the current hydrothermal/solvothermal synthesis process,the pressure is closed relates to the reaction pressure and filling factor,which further limit the study for the pressure.As it is known to all,the pressure plays an important role in the synthesis of nanomaterials and its growth process.High pressure can significantly reduce the reaction activation energy and chemical reaction barrier,so that the reaction can happen at a relatively low temperature;High pressure can reduce the critical nucleus size,which is beneficial for the control of the final size of the nanomaterials;High pressure can influence the adsorption state of the addictive,leading to the controllable growth process of the nanomaterials;High pressure can limit the thickness of ion diffusion layer,which provide another potential way for the synthesis of the relatively large nanocrystals;High pressure can reduce the degree of disorder of the nanomaterial system,result in the oriented assembly and second growth of the nanocrystals;High pressure can significantly improve the crystal quality and reduce the defect concentration.With the assistance of the newly-developed high pressure solvethermal device,we investigated how the high reaction pressure?10 MPa-250 MPa?influence the phase,morphology,structure of the final products.The main results are listed as follows:1,TiO₂ mesocrystals synthesis via a low-temperature-high-energy/pressure method that exhibit excellent Li-ion battery performanceAnatase TiO₂ mesocrystals with uniformly distributed nanopores with diameters of ca.-200 nm was successfully obtained by a newly-developed low-temperature-high-energy method.The anatase TiO₂ mesocrystals was successfully obtained from the Ti precursor at relatively low temperature?100 ??.The introduction of high pressure into the reaction system significantly reduce the activation energy for the reaction,which further break the balance between the precursor and TiO₂ embryos,resulting in the smaller nanocrystals with higher surface energy.The oriented assembly of the nanocrystals along the[001]direction finally decided the structure and morphology of the mesocrystals.Theoretical investigation of the high pressure in the reaction system proved its important role.The as-obtained anatase TiO₂ mesocrystals exhibit excellent cycling stability with capacity of 221.8 m Ah g-1 under the current rate of 100 mA g-1.2,The pressure induced oriented attachment for preparation of the TiO₂ 3D superstructure and the performance of CH3NH3PbI3/TiO₂ hybrid photodetectorWe successfully prepared the TiO₂ 3D superstructure in the high pressure solvothermal method based on the pressure induced oriented attachment.Only dispersive TiO₂ nanocrystals can be got with the conventional solvothermal method.The theoretical results indicated the preferential adsorption of the oleic acid molecules on the {101} facets of the TiO₂ nanocrystals,leading to the oriented attachment along the[001]direction,thus the formation of the nanorods.With the assistance of the high pressure,the nanorods tend to further assemble into 3D superstructure.The high pressure played an important role in the oriented growth of the nanocrystals and the oriented attachment process induced by the oleic acid molecules.The CH3NH3PbI3/TiO₂ hybrid photodetector shows relatively excellent detectivity and good linearity for the better carrier transport properties of the ordered structure.3,The controllable growth of the antase TiO₂ ultrathin nanosheet under high pressure and enhanced photoresponse performance of CH3NH3PbI3/TiO₂ composite filmsThe high-pressure hydrothermal method was developed to synthesize high-crystalline ultrathin?2-3 nm?TiO₂ nanosheets bounded with high-percentage {001}facets.The better adsorption behavior of the F.under high pressure helped the control of the growth of the {001} facet,leading to the synthesis of the ultrathin TiO₂ nanosheets.On the other hand,the high pressure employed during the reaction process improved the crystallinity of the product.When these TiO₂ nanosheets were utilized in the fabrication of perovskite-TiO₂ composite photodetector,strikingly increased responsivity and detectivity were achieved.Furthermore,when the films were conducted by a high-pressure thermal treatment,both the particle size and uniformity of the composite films greatly increased,resulting in more excellent performance of the hybrid photodetectors.A sonication assisted synthesis of the ultralong anatase TiO₂ nanobelt was proposed with the as-obtained ultrathin nanosheets as presusor.Compared to the reported method for the synthesis of TiO₂ nanobelt,the sonication assisted method was more environmental friendly and facial to operate.4,The controllable synthesis of Sn doped TiO₂ nanocrystals and its electrochemical properties.The influence of the high pressure in the preparation process of the Sn doped TiO₂ nanocrystals was investigated with the high pressure solvothermal method.Under low reaction pressure,the doping of Sn was easily achieved with the crystalline transformation from anatase to rutile.The topotactic transformation mechanism of solid TOF2 precursor was illustrated as the main reason.As mentioned before,the high pressure can significantly improve the crystal quality and reduce the defect concentration.The doping of Sn thus introduced the lattice distortion and defects into the TiO₂ nanocrystals.The high pressure during the reaction process induced the crystalline transformation of rutile TiO₂ to anatase TiO₂.Then the Sn-doped rutile TiO₂ hollow nanocrystals are calcined and tested as anode in the lithium-ion battery.They deliver a high reversible specific capacity of 251.3 mAhg-1 at 0.1 Ag-1 and retain-110 mA h g-1 after 500 cycles at a high current rate 5 A g-1?30 C?,which is much higher than most of the reported work.