Bi9P2O18Cl:Phase Transition and Hydrogen Production by Photocatalytic Water-Splitting

CAO Zhen?Yu WU Yun GAO Jian?Hua

(School of Physics,Northwest University,Xi′an 710069,China)

Abstract:Bi9P2O18Cl crystals were prepared by the high?temperature molten salt method.Single?crystal X?ray dif?fraction data analyses demonstrated that Bi9P2O18Cl can undergo a crystal?to?crystal phase transition from room tem?perature to low temperature.At room temperature,this compound(α?phase)crystallizes in monoclinic space group P21/m(11)with unit cell parameters a=1.149 10(7)nm,b=0.540 64(4)nm,c=1.463 69(9)nm,β=93.741(6)°,V=0.907 38(10)nm3.And at 100 K,it(β?phase)crystallizes in monoclinic space group P21/n(14)with unit cell parame?ters a=1.790 56(4)nm,b=0.538 870(10)nm,c=1.915 57(4)nm,β=103.693(2)°,V=1.795 76(6)nm3.Additionally,high pure powder samples of Bi9P2O18Cl were synthesized using solid?state reaction method,which exhibited promis?ing performance in photocatalytic water splitting to produce H2,the H2evolution rates reached 33.69 μmol·g-1·h-1.CCDC:2128785,α?phase Bi9P2O18Cl;2145491,β?phase Bi9P2O18Cl.

Keywords:Aurivillius;crystal structure;phase transition;photocatalytic hydrogen production

0 Introduction

Hydrogen energy is considered one of the most attractive clean energy sources due to its high energy capacity and environmental friendliness[1?3],but the development of stable hydrolysis catalysts for visible?light reactions is currently a huge technical challenge for the scientific community[4?5].To make better use of the solar spectrum,mixed anionic compounds,such as oxynitrides[6],oxysulphides[7],and oxyhalides[8],are promising candidates because their band gaps are narrower than those of typical oxides.Recently,a series of Aurivillius?like layered bismuth oxyhalides with[Bi2O2]2+layers have been found to have a high potential for visible?light water splitting.They usually have a high valence band maximum due to their highly dispersed O2p orbitals,which results in a very narrow band gap and also is structurally stable for photocata?lytic water separation[5,9?10].The structure of the Aurivillius compound is composed of the fluorite?like layers[Bi2O2]2+interspersed between perovskite?like layers[An-1BnO3n+1]2-(A=Na+,K+,Sr2+,Bi3+,Ba2+,etc.and B=Ti4+,Fe3+,Nb5+,W6+,etc.),where n is the layer number of the perovskite?like layers[11?12].In this struc?ture,the[An-1BnO3n+1]2-layer is sandwiched between the[Bi2O2]2+layers,which will allow rapid separation of photoinduced electrons from holes,resulting in highly quantum ?efficient photocatalysis[13?14].And that layered metal oxide semiconductor particles produce hydro?gen more readily than other compounds,due to the effi?cient movement of light?generated electron?hole pairs to the surface[15].Moreover,the Aurivillius compounds have potential applications in photocatalysis and opti?cal properties due to their unique layered crystal struc?ture and ideal visible?light sensitivity[16?19].

In 1997,Mentre and Abraham reported the struc?ture of Bi9V2O18Cl for the first time[20].In Bi9V2O18Cl,the[Bi2O2]2+layers,similar to those of Aurivillius com?pound,also can be found.In 2020,Ji et al.synthesized the phosphate?analogous compounds Bi9P2O18Cl via sol?gel?assisted solid?state reaction(SSR)and found that Bi9P2O18Cl has a band gap of 2.58 eV and its poten?tials of the conduction and valence bands cover the reduction and oxidation potentials of the water.Addi?tionally,Rietveld′s refinement from powder X ?ray diffraction(PXRD)data showed that Bi9P2O18Cl is isostructural with Bi9V2O18Cl and also contains[Bi2O2]2+layers,which will lead to a huge polarization to enhance the separation of the photoinduced charges[19].These indicate that Bi9P2O18Cl is a suitable semicon?ductor for the visible?light?driven photocatalytic split?ting of water.Therefore,the performance of hydrogen production of Bi9P2O18Cl from water?splitting raises concerns for us.However,the crystal structure of Bi9P2O18Cl remains dubious because there are several disordered O and Bi atoms.To figure out the structure,we grew the crystals and then revisited the structure through the single?crystal diffraction method.Interest?ingly,the structural results showed that Bi9P2O18Cl experiences a crystal?to?crystal phase transition from room temperature to 100 K.Hereafter,the room?and low ?temperature phases can be denoted as“α”and“β”,respectively.In addition,results from photocata?lytic water?splitting also showed that Bi9P2O18Cl was indeed active to produce H2.

1 Experimental

1.1 Syntheses

NH4H2PO4(98%,Aladdin),BiCl3(99%,Aladdin),Bi2O3(99%,Aladdin)were purchased from a commer?cial provider and used without further purification.The powder sample of the title compound was synthesized by the conventional SSR method.In the reaction,Bi2O3,BiCl3,and NH4H2PO4were mixed in a molar ratio of 13∶1∶6 and packed into a Platinum crucible.After that,the mixture was heated in a muffle furnace at 300℃for 6 h,500℃for 6 h,and 700℃for 10 h with several mid ?grinding.Finally,a pale?yellow powder sample was obtained.

Single crystals were obtained by the high?temperature molten salt method.0.3 g Bi9P2O18Cl and 1.0 g CsCl were ground sufficiently in the Ar?filled glovebox and then encapsulated into a quartz tube using a vacuum sealing device.After that,the mixture was melted at 850℃,held for 5 h,and then slowly cooled down at a rate of 3℃·h-1to 550℃.Finally,the colorless transparent crystals were successfully obtained.

1.2 PXRD

The PXRD data were collected by a Bruker D8 advance diffractometer(tube voltage was 40 kV;tube current was 40 mA)using Cu Kα radiation(λ=0.154 18 nm)with a step size of 0.02°at room temperature.The PXRD data were collected for phase purity checking and Rietveld refinement(5°≤2θ≤95°,1.0 s·step-1).

1.3 Single-crystal X-ray diffraction

Single crystals were selected under an optical microscope and mounted on glass fibers for X?ray diffraction studies.The diffraction data of single?crystal samples were respectively collected at room tempera?ture and low temperature on a Bruker Smart diffractom?eter equipped with a CCD area detector and MoKα(λ=0.071 073 nm)radiation source.Intensities were corrected for Lorenz and polarization effects.The structure was solved directly using the SHELEX?97 program[21]and refined onF2by least?squares,full?matrix techniques.The final results were tested using PLATON[22]and no additional symmetry elements were found.The main related crystallographic data are listed in Table 1(The residual peaks show a little big because there are nine heavy?atoms Bi in one?unit cell and the quality of single crystals is not so good).The atomic coordinates and equivalent isotropic displacement parameters of theα?andβ?phases are summarized in Table S1 and Table S2,and the selected bond lengths are given in Table 2 and Table 3.

Table 1 Crystal data and structure refinement for Bi9P2O18Cl

Table 2 Selected bond lengths(nm)and bond angles(°)of α-phase Bi9P2O18Cl

Table 3 Selected bond lengths(nm)and bond angles(°)of β-phase Bi9P2O18Cl

CCDC:2128785,α?phase Bi9P2O18Cl;2145491,β?phase Bi9P2O18Cl.

Continued Table 2

1.4 UV-Vis-NIR diffuse reflectance

A PerkinElmer Lambda 950 UV?Vis?NIR spectro?photometer was performed to collect the optical diffuse reflectance data in the range of 200?1 200 nm.

1.5 Calculations of electronic structures

The DFT(density functional theory)calculation was given by using the Vienna ab initio Simulation Package (VASP)[23]with the projector?augmented wave(PAW)pseudopotentials[24].The Perdew?Burke?Ernzerhof(PBE)of the generalized gradient approxima?tion(GGA)[25]was applied in the exchange and correla?tion potentials.Gamma grids of 2×5×2 and 5×1×1 were applied for the room?temperature and low?temperature bulk cell,respectively.

1.6 Photocatalytic H2production

To investigate the photocatalytic hydrogen produc?tion performance of Bi9P2O18Cl,a 300 W xenon lamp was used as a light source to imitate sunlight,and photocatalytic hydrogen production experiments were carried out at ambient temperature and pressure.The production of H2was achieved by dispersing 20 mg of catalyst into an aqueous solution containing Na2S/Na2SO3(Na2S 0.1 mol·L-1,Na2SO30.1 mol·L-1).The reaction vessel was evacuated and purged by Ar for about 20 min to completely remove air before irradia?tion.The photocatalytic reaction was typically per?formed for 4 h.To detect the amount of H2,1 mL gas component was extracted from the vessel and then in?jected into a gas chromatograph(GC 7900,Tianmei,TCD,detector,Shanghai,nitrogen carrier gas)equipped with a TCD(thermal conductivity detector)detector.

2 Results and discussion

2.1 Syntheses

The title compound can be synthesized using the SSR method.And its single crystals can be obtained in the CsCl molten media.To refine the structure and check the phase purity of Bi9P2O18Cl,the Rietveld method was performed to the PXRD data by using the TOPAS software.The starting structural model was con?structed using the crystallographic data obtained from our single?crystal diffraction.The final refinement con?verged to small reliability factors(Rwp=15.55%,Rp=9.61%,GOF=6.58).As shown in Fig.1,the experimen?tal PXRD pattern is in good agreement with the pattern calculated on the base of single?crystal diffraction data of Bi9P2O18Cl,indicating that the high pure samples were synthesized.

Fig.1 Rietveld refinement XRD pattern of Bi9P2O18Cl

2.2 Crystal structure description

2.2.1 Room?temperature structure(α?phase)

Bi9P2O18Cl at room temperature crystallizes in the monoclinic space group P21/m (11)with unit cell parameters a=1.149 10(7)nm,b=0.540 64(4)nm,c=1.463 69(9)nm,β=93.741(6)°,and V=0.907 38(10)nm3.The α?phase structure from our single?crystal diffrac?tion data is almost the same as that of Bi9V2O18Cl[20]and Bi9P2O18Cl from PXRD data[19].Therefore,herein we don′t discuss the detailed α?phase structure anymore,but the atomic coordinates,selected bond lengths,and bond angles are still given in Table S1(Supporting in?formation)and Table 2.It is worth mentioning that in the process of solving the α ?phase structure from our single?crystal diffraction data,many attempts to remove the disordered O and Bi atoms were made,but failed,indicating the disordered O and Bi atoms exist indeed in the α?phase structure of Bi9P2O18Cl.

2.2.2 Low?temperature structure(β?phase)

When the temperature decreases,the single?crystal X?ray diffraction analyses reveal that Bi9P2O18Cl under?goes a crystal?to?crystal phase transition.At 100 K,Bi9P2O18Cl crystallizes in the monoclinic space group P21/n(14)with unit cell parameters a=1.790 56(4)nm,b=0.538 870(10)nm,c=1.915 57(4)nm,β=103.693(2)°,and V=1.795 76(6)nm3.The asymmetric unit includes nine independent Bi atoms,two P atoms,eighteen O atoms,and one Cl atom,which all occupy fully at 4e positions and no disorder appears(Table S2).As listed in Table 3,the bond length of Bi—O ranges from 0.207 3(10)to 0.274 4(9)nm,and the bond length of Bi—Cl ranges from 0.320 44(30)to 0.352 02(30)nm,which don′t show a big difference from the Bi—O and B—Cl bond lengths of theα?phase structure and Bi9V2O18Cl.But the P—O bond lengths and coordina?tion environment of P are different betweenα?andβ?phase structures.In theα?phase,P1 is four?coordinated to O atoms with reasonable P—O band distances(Fig.2a).While P2 atom is linked with the disordered O11,O12,and O13 to form a configuration as shown in Fig.2b.Because the site occupancy of O11,O12,and O13 is 0.5,the configuration is still considered as a PO4tetrahedron.A long(0.162(2)nm)and short(0.148(2)nm)P—O bond lengths appear in the PO4tetrahedron.By contrast,in theβ?phase,P1 and P2 atoms are respectively coordinated with four oxygen atoms to form normal PO4tetrahedra with reasonable P—O bond lengths and angles(Fig.2c,2d).Fig.3 shows the projected views ofα?andβ?phases alongb?direction.As can be seen,the two frameworks,constructed by[Bi2O2]2+layers stacking along[101]direction,look very similar.The main difference lies in the change of unit cells.The unit cell of theβ?phase is almost twice that of theα?phase.The reason may be that when the temperature decrease,the atom vibrations become not drastic so that the disorder of O and Bi atoms disappear and the symmetry reduces,which makes the unit cell of α?phase enlarge.

Fig.2 P—O bond lengths(nm)and coordination environment of P in α?and β?phase structures

Fig.3 Projection view of the crystal structures of α?and β?phase Bi9P2O18Cl along the b?direction

2.3 Experimental band gap

The Kubelka?Munk equation was applied to calcu?late the energy of band gap from the following equa?tion:αhνn=A(hν-Eg),where n=2 for direct transitions and 1/2 for indirect transitions,and α,h,ν,A,and Egare the optical absorption coefficient,Planck constant,light frequency,constant,and band gap respectively.According to the band structure calculated through the first?principle theory,an indirect optical gap was deter?mined.So,the value of 1/2 is adopted to n.As shown in Fig.4,the extrapolation of the linear part intercept at 2.62 eV in the axis of photon energy.This is in general agreement with the experimental results obtained by Ji(2.58 eV)[19].

Fig.4 UV?Vis?NIR absorption spectrum of Bi9P2O18Cl

2.4 Band structures and density of states

The DFT calculation was applied to further under?stand the band structure of Bi9P2O18Cl.As shown in Fig.5,the band gaps of α?and β?phases are 2.64 and 2.62 eV,respectively,which are accordant with the experimental results of 2.62 eV.It can be seen that the electronic structure of α?and β?phases are almost iden?tical.The valance band mostly originates from O2p orbital.The Bi6s,Bi6p,Cl3p,and P3p also make some contribution to the formation of the top of the valance band.The conduction band mostly consists of O2p and Bi6p orbitals.

Fig.5 PDOS(partial density of state)of α?and β?phases Bi9P2O18Cl

2.5 Photocatalytic performance

The photocatalytic hydrogen production activity of Bi9P2O18Cl was investigated under simulated sunlight conditions.As shown in Fig.S1,the results demonstrat?ed that Bi9P2O18Cl had a good photocatalytic perfor?mance with a hydrogen production rate of 33.69 μmol·g-1·h-1.It is well known that heterojunction engineer?ing usually will enhance the photocatalytic perfor?mance,so the junctions of Bi9P2O18Cl with metals or other semiconductors may show amazing photocatalytic performance.

3 Conclusions

In summary,we have prepared Bi9P2O18Cl using SSR and its single crystals were grown through the molten salt method.Single?crystal X?ray diffraction data analyses demonstrated that Bi9P2O18Cl undergoes a crystal?to?crystal phase transition from room tempera?ture to 100 K.The room?temperature phase(α?phase)and low?temperature phase(β?phase)have a similar configuration of atom arrangement.The big difference is that in the α?phase there exist the disorders of O and Bi atoms,but in the β?phase,the disordered O and Bi atoms disappear due to the reduction of symmetry with the decrease of temperature.The DFT calculations reveal that the valance band and conduction band of α?and β?phase Bi9P2O18Cl mostly originates from O2p and Bi2p orbitals,respectively.The calculated band gaps are in good agreement with the experimental value of 2.62 eV.In photocatalytic hydrogen production experi?ments,Bi9P2O18Cl showed excellent photocatalytic performance with an H2production rate of 33.69 μmol·g-1·h-1.

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