Vapor Transport Depositon Of Low-dimensional Bi₂Se₃ Nanomaterials And Its Application In Dye-sensitized Solar Cells As Counter Electrode

Three-dimensional?3D?topological insulators?TIs??Bi2Te3,Bi₂Se₃?as a new state of quantum matter are different from traditional metals and semiconductors.They have an insulating bulk state and a current carrying surface state.In TIs,the strong spin-orbit coupling dictates robust,nontrivial surface states,which are topologically protected against back scattering from time-reversal invariant defects and impurities,high mobility.These properties determined that TIs have various potential applications in optoelectronics,spintronics,etc.However,it is still a basic problem to be solved to realize their controllable fabrication and obtain large size two-dimensional single crystal layered materials,which is also the precondition to put them into application.In spite of their outstanding characteristics,TIs are still under restrictions when it comes to application areas.Therefore,it is of great significance to develop new functions of topological insulators and expand the range of their application in chemistry.Bi₂Se₃ has been recognized as a typical TI material with a large nontrivial bulk gap of ⁰.3 eV and exhibits insulativity at room temperature,which possesses the simplest single-Dirac-cone surface state at the ? point?refer to the origin of Brillouin zone?.The unique properties of the Bi₂Se₃ TI provide a good choice for studies.On account of the above-mentioned problems,the primary contents of the paper are described as follows:1?Fabrication of single crystalline Bi₂Se₃ nanostructures by vapor deposition and the exploration of the mechanisms.Vapor deposition is an efficient method to prepare single crystalline nanostructureswith a rich variety of morphologies.In this paper,single crystalline Bi₂Se₃ nanostructures including nanowires,nanoribbons,and nanoplates have been grown by the VS and the VLS growth modes,taking Bi₂Se₃ as precursor.The morphology evolution law of various Bi₂Se₃ nanostructures by the VS and the VLS growth modes are studied.Both of the growth mechanism and kinetics process of Bi₂Se₃ nanostructures in two growth modes are analyzed.Research shows that the nanostructures in VLS mode depend on the degree of supersaturation of the reactants and the wetting angle between the substrate and the Au droplets.At the high temperature zone,wetting angle between the droplet and the substrate is an obtuse angle due to the weak forces between the subatrate and the Au droplet.The specific surface energy of vapor-solid interface is lower than liquid-solid interface so that the growth of nanowires gets more advantageous.At the low temperature zone,the wetting angle between the droplet and the substrate is an acute angle due to the strong forces between the subatrate and the Au droplet.The specific surface energy of vapor-solid interface is higher than liquid-solid interface so that the growth of nanoplates gets more advantageous.In addition,supersaturation is the driving force for crystallization in the VS growth mode.High degree of supersaturation results in nucleation of nanostructures and the growth rate gets faster,while low supersaturation decreases the nucleation of nanostructures and the growth rate.The morphology of product is determined by the surface energy of the crystal surface and the surface migration rate,the growth mode altering from irregular growth into layer-on-layer growth.By exploring the growth mechanisms,we can confirm the great influence of degree of supersaturation,catalyst and surface energy in crystal growth.And we can change the influencing factors by controlling the gas flow rate and growth time.As our study shows,decreasing the gas flow rate and shortening the growth time can effectively decrease the thickness of Bi₂Se₃ nanoplates.2.A two-step growth process of large size Bi₂Se₃ and the exploration of the mechanisms.The fabrication of large-size Bi₂Se₃ by vapor transport is an important aspect of the present study.On the basis of previous studies,a "two-step growth method" for the preparation of large size Bi₂Se₃ lamella structure has been proposed in this paper.First,one-dimensionalBi₂Se₃ nanowires and nanoribbons with high aspect ratio as the original growth template are obtained by VLS growth mechanism.Subsequently,taking Bi₂Se₃ nanowires and nanoribbons as growth templates,a second vapor transport is performed in temperature zone with low degree of supersaturation by VS growth mechanism.Then,Bi₂Se₃ lamella structure with a measure up to hundreds of micrometers was obtained Based on the analysis upon SEM and AFM characterizations,the growth mechanisms of large size nanoplates can be explained as follows.Since the nanowires and nanoribons obtained in the first growth can provide more nucleation sites,in the second growth,the formation of the bridge-like nanostructures originates from the epitaxial growth of nanoplates on the sidewalls of nanoribbons.And the Bi₂Se₃ nanoplates are formed by layer-by-layer growth.Bending of the nanowires and nanoribbon might induce compressive strain accumulation and dislocation in the Bi₂Se₃ nanoplate,contributing to an increase of local surface energy and the formation of layers with different heights and widths.3.Application of Bi₂Se₃ lamellar structure as counter electrode?CE?in dye-sensitized solar cells?DSSCs?.The electrocatalytic activity of counter electrode?CE?in dye-sensitized solar cells?DSSCs?directly affects the power conversion efficiency?PCE?.At present,Pt is commonly used as electrode material,but it is seriously constrained in application owing to high cost.Thus,searching for the substitute materials for counter electrode is very critical.As is reported,Bi₂Se₃ exhibits excellent surface transport characteristics and electrochemical activity,showing its potential to be applied as counter electrode.Two kinds of Bi₂Se₃ nanoplates parallel and perpendicular to the substrate have been obtained on the FTO glass substrate by VS growth mechanism,respectively.Then electrochemistry and photoelectric conversion efficiency tests were performed to explore theperformance of Bi₂Se₃ nanoplatesas counter electrode.As is shown by electrochemistry characterization,the electrical resistivity of Bi₂Se₃ bulk material is two orders of magnitude higher than that of traditional Pt electrodes.However,Bi₂Se₃ parallel and perpendicular to the substrate leads to a lower series resistance?Rs?in circuit owing to a higher nanoplates surface migration rate and is preponderant in conductivity.In addition,vertically aligned Bi₂Se₃ nanoplates counter electrodes possesslower charge-transfer resistance?Rct?and thus owns better electrocatalytic activity.On account of the results of electrochemistry and SEM characterizations,the excellent catalytic activity of vertically aligned Bi₂Se₃ nanoplates is directly related to the special morphology of Bi₂Se₃ nanoplates.On the one hand,Bi₂Se₃ nanoplates perpendicular to the substrate possess more exposed facets?100?with highsurface energycompared with those parallel to the substrate.On the other hand,Bi₂Se₃ nanoplates perpendicular to the substrate possess more active site because there are more edges,margin and steps.In virtue of excellent surface conductivity and considerable catalytic active sites,Bi₂Se₃ nanoplates perpendicular to the substrate as counter electrodes show higher photoelectric conversion efficiency than Pt,indicating that Bi₂Se₃ has a promising application prospect in the area of electrochemistry.