Preparation And Room Temperature Modification Of Flexible Black Titanium Dioxide Nanofiber Materials

As a multifunctional material,titanium dioxide（TiO₂）has the advantages of high chemical stability,excellent optical properties,non-toxicity,low cost,and is widely used in glass,ceramics,cosmetics,refractory coatings,medicine,food technology,energy,environment,and other fields.Since Fujishima and Honda pioneered the work of photocatalytic water splitting on TiO₂ electrodes in 1972,the photocatalytic application of TiO₂ has been greatly developed,which can be used for photocatalytic and electrocatalytic water splitting,CO₂ reduction,pollutant degradation,and so on.In these applications,TiO₂ nanoparticles are the most commonly used materials.However,the widely studied TiO₂ particle suspension phase has some shortcomings in its applications.Firstly,the large surface/interfacial area of TiO₂ NPs leads to slow carrier transport,and the wide optical bandgap results in limited solar absorption.Secondly,TiO₂ nanoparticles are easy to agglomerate and inactivate,difficult to recycle,and may cause secondary pollution,which is not economically feasible.To solve these problems,a promising alternative method is to prepare one-dimensional（1D）self-supported nanofibrous TiO₂.First,materials based on TiO₂ nanofibers have desirable mechanical stability and structural integrity so that they can be easily integrated into reaction systems.Second,TiO₂ nanofibers exhibit some new properties and performance improvements in specific regions.On the one hand,TiO₂ NFs have a large aspect ratio and high specific surface areas,which can reduce the charge transport length.When being directly used as a self-supporting carrier,it can overcome the problem of easy aggregation of NPs,thus increasing the active sites.On the other hand,the randomly stacked NFs network facilitates the large-scale transport of reactants and products.Due to these attractive merits,the applications of TiO₂ NFs in the fields of photocatalysis,electrocatalysis,and energy have attracted extensive attention from researchers.At present,researchers have reported a number of preparation methods related to one-dimensional TiO₂ nanofibers,including vapor deposition,template synthesis method,electrochemical anodization,electrospinning technique and so on.In particular,electrospinning is the most commonly used method for fabricating continuous TiO₂nanofibers because of its simple equipment,good continuity,structural adjustability,and easy functional modification.However,the polycrystalline structure of electrospun TiO₂ nanofibers and the pore defects caused by polymer pyrolysis make them generally mechanically fragile and easy to break when bent.In addition,the original TiO₂nanofiber has a wide band gap,showing a low utilization of solar energy.Moreover,the chemical inertia and low electronic conductivity of the original TiO₂nanofiber has greatly hinder their wide applications.Therefore,developing flexible TiO₂-based nanofibrous membranes,improving their optical response,and regulating their electronic structure through defect design is of great significance for their applications in catalysis,energy,and environmental fields.In this paper,we have carried out a series of research on the theme of the flexibility design and room temperature defect modification of electrospun TiO₂ nanofiber materials.Firstly,an electrospinning method based on ball milling and bending draft was proposed to prepare flexible TiO₂ nanofibers with ordered assembled grain.The mechanism of ordered grain assembly of TiO₂ nanofibers was systematically studied,and the internal correlation mechanism between the microstructure of TiO₂ nanofibers and their flexibility was explored.Further,the flexible black TiO_2-δnanofibers were obtained by the room-temperature contact domino-cascade lithium reduction modification method,which has abundant oxygen vacancies and significantly improved conductivity.On this basis,the flexible black Li_xTiO_2-δnanofibers were obtained by the non-contact topological chemical lithium intercalation method.The construction of the intercalation reaction path related to the color change of TiO₂ nanofiber film was investigated,and the internal relationship between the structure of black Li_xTiO_2-δnanofibers,the oxygen vacancy defects,and the improvement of their conductivity was explored.The main research contents and results are summarized as follows:（1）An electrospinning process based on ball milling and bending and drawing was developed to prepare a flexible TiO₂ nanofiber film with ordered grains.First,the spinning solution was treated by ball milling,and then the precursor nanofiber film was formed by electrospinning.Subsequently,the precursor nanofiber membrane was subjected to high-temperature calcination assisted by curved-drafting,and flexible TiO₂nanofibers with orderly assembled TiO₂ crystals in a single fiber were obtained.The effects of ball-milling and curved-drafting on the microstructure and mechanical properties of TiO₂ nanofibers were studied respectively.Ball-milling can reduce the size of colloidal particles in solution,and curved-drafting can induce the orderly assembly of the grains in TiO₂ nanofibers.Compared with the TiO₂ nanofiber film composed of large and uneven grains,the tensile strength of the orderly assembled flexible TiO₂ nanofibers increased from 0.11 MPa to 0.62 MPa,and it had a low bending rigidity of 22 m N,which exhibited a softness comparable non-woven fabrics and paper towels.In addition,a single TiO₂ nanofiber was characterized by the FIB test,and a single flexible TiO₂ nanofiber could bend and knot without any breaking under the action of the FIB probe.In addition,through further characterization of HRTEM and AFM,it is found that a single TiO₂ nanofiber had a low elastic modulus and an ordered amorphous region,which produces a continuous stress dispersion path when the nanofiber is stressed,thus making TiO₂ nanofibers show good flexibility.（2）A design method of domino-cascade reduction method based on lithium reduction was developed to prepare flexible black TiO_2-δnanofibers at room temperature.A soft TiO₂ nanofiber film was first covered on a lithium plate and was dipping some drops of solvents.It was found that the white TiO₂ nanofiber film changed from white to black in 1 min,and the conductivity increased from 0 to 40 S/m.First,the reduction of TiO₂nanofibers by different metals at room temperature was systematically studied.It was found that at room temperature,only lithium metal with strong reducibility could reduce TiO₂.Secondly,the effects of the size of lithium sheet and the type and amount of organic solvent on the reduction reaction were explored.It was found that TiO₂ in contact with lithium metal can be reduced quickly,while the non-contact part needs a longer time to be gradually reduced.With the increase in the amount of organic solvent,the conductivity of the prepared black TiO₂ will gradually increase and then remain stable.In addition,adding a certain concentration of lithium salt to the solvent can accelerate the process of lithium reduction of TiO₂.The research shows that the color,band gap,Ti³⁺defect,and conductivity of TiO₂ nanofiber film can be controlled by lithium reduction.The conductive black anatase TiO_2-δwith a disordered layer of 3 nm on the surface was obtained,which has a conductivity of up to 40 S/m.At the same time,this method is scalable and can prepare flexible conductive Sn O₂,Ba TiO₃,and Li_0.33La_0.56TiO₃ nanofiber films with the same method.（3）Based on the contact lithium reduction,a non-contact topological chemical lithium intercalation method was developed to prepare flexible black Li_xTiO_2-δnanofibers.Five different charge-driven models were designed for the first time.Used a flexible TiO₂ nanofiber film as the main material and intercalate the preset Li⁺-ions into the TiO₂ lattice slowly（～1.3μm/s）,rapidly（～1.6mm/s）or ultrafast（～5mm/s）.The Li⁺-intercalation causes real-time color changes of the TiO₂ films from white to blue and then black,corresponding to the new structures of blue Li_xTiO₂ and black TiO_2-δderived in TiO₂,and the enhanced conductivity from 0 to 1 and 40 S/m.The experiment shows that the synchronous conduction of electrons and ions is a necessary condition for intercalation,and the initial concentration of electrons and Li⁺-ions that reside on the TiO₂ nanofiber first determine the electron transfer paths,which then establish the Li⁺-intercalation paths.Through the real-time color change of the orientation of TiO₂nanofibers,the intercalation reaction based on Li⁺-ion or electronic gating was proposed.And the explosive,single-channel twisty,and multi-channel linear intercalation reaction paths were established.Moreover,the visualization design of topological intercalation reaction is realized for the first time.At the same time,the structural stability of Li_xTiO_2-δnanofibers prepared by different models was explored.And found that the color and conductivity of Li_xTiO_2-δnanofibers are closely related to the intercalation structure and path.Both the color and conductivity are closely related to the intercalated structures.Li_xTiO₂ nanofibers contained a low stable but high conductive black TiO_2-δstructure（>40 S/m）and a stable but low conductive blue Li_xTiO₂ structure（1～40 S/m）.