Synthesis, Crystal Structure And Photoelectric Properties Of A New Aurigillius Phase Bi_2+nO_2+nSeX_n (X=Cl/Br)

A series of novel Aurivillius phases Bi₃O₃SeX（3311-type）and Bi₄O₄SeX₂（4412-type,X=Cl,Br）were synthesized via traditional solid state reaction in vacuum sealed quartz tubes.These compounds were explored by powder X-ray diffraction,neutron diffraction and electron diffraction.The crystal structures were determined by ab-initio calculation and Rietveld refinement against powder X-ray diffraction data.The Bi₃O₃SeX compounds crystallize in a tetragonal system with space group P 4/nmm.The Bi₃O₃SeCl compound has lattice parameters of a=3.89957（6）?and c=19.6496（4）?.The Br analogue has slightly larger lattice parameters of a=3.92453（17）?and c=20.2468（8）?.Bi₄O₄SeX₂ compounds crystallize in the tetragonal system with space group I 4/mmm.The Bi₄O₄SeCl₂ compound has lattice parameters of a=3.90438（4）?and c=27.06548（28）?.The Br analogue has slightly larger lattice parameters a=3.93642（4）?and c=28.2143（3）?.The crystal structure of Bi_2+nO_2+nSeX_n（X=Cl/Br）is composed of three different layers:monolayer of Se^2-ions,double layers of X^-ions and[Bi₂O₂]²⁺layers.The Se^2-layers and X^-layers are stacked between[Bi₂O₂]²⁺layers to form the structural units of Bi₂O₂Se and BiOX.Structural units pack along c axis via sharing the[Bi₂O₂]²⁺layers to form Bi_2+nO_2+nSeX_n.The Bi₃O₃SeX have an alternative arrangement of a[Bi₂O₂Se_1-?X_?]^?+structural units and one[BiOX_1-?Se_?]^?-units along the c axis,with whole[Bi₂O₂Se_1-?X?]^?+and[BiOX_1-?Se?]^?-blocks,while Bi₄O₄SeX₂ have a 1/2-[Bi₂O₂Se_1-2?X₂?]^2?+structural units and one[BiOX_1-?Se?]^?-units with inversion symmetry.The Bi_2+nO_2+nSeX_n compounds are new layered structure containing three separate anions(O^2-,Cl^-,Se^2-)arranged in a partially ordered manner and only one cation(e.g.Bi³⁺).The interlayer anions of Bi_2+nO_2+n+n SeX_n are locally ordered,resulting in lattice relaxation and interlayer charge coupling,which eliminates lattice strain and maintains system charge-neutrality.XPS analyses indicate the valence state of cation and anions.The bismuth,selenium and halogens take the form of Bi³⁺,Se^2-,Cl^-and Br^-,respectively.Bi_2+nO_2+n+n SeX_n（X=Cl/Br,n=1,2）,indirect transition narrow band-gap semiconductor,has intense absorption for visible light,which was proved by ultraviolet-visible-nearinfrared（UV-vis-NIR）diffuse reflection spectra and first-principle calculation.The observed band gap values as follows:0.66 eV for Bi₃O₃SeCl,0.42 eV for Bi₃O₃SeBr,0.81 eV for Bi₄O₄SeCl₂ and0.68 eV for Bi₄O₄SeBr₂.As counterpart calculated values,0.72 eV for Bi₃O₃SeCl,0.66 eV for Bi₃O₃SeBr,0.8 eV for Bi₄O₄SeCl₂ and 0.71 eV for Bi₄O₄SeBr₂.The calculation indicates that the compound with the same crystal structure has similar band structure.The density of states analyses illuminates that the VBM consists of Se 4p and Bi 6s,while the CBM is mainly constructed by Bi 6p states.The photoelectrochemical measurements show that the photocurrent densities of Bi₃O₃SeX are 0.74（X=Cl）and 0.69（X=Br）?A/cm²,respectively,the photocurrent densities of Bi₄O₄SeX₂ are 0.65（X=Cl）and 0.75（X=Br）?A/cm²,at 1V bias under visible light irradiation.The photo-response properties of Bi_2+nO_2+nSeX_n（X=Cl/Br,n=1,2）compounds are superior than Bi₂O₂Se（0.55?A/cm²）.Markedly,the Nyquist impedance arc of Bi_2+nO_2+nSeX_n（X=Cl/Br,n=1,2）is smaller than Bi₂O₂Se and BiOX,while they have comparable carrier lifetime derived by Bode-phase spectra.Furthermore,Bi₃O₃SeX and Bi₄O₄SeX₂ compounds are all n type semiconductor.These findings are manifest that the new type Aurivillius compounds Bi_2+n+n O_2+nSeX_n（X=Cl/Br,n=1,2）is of promising application prospects for inorganic chemistry and photoelectrochemisty.