Nanoparticle/Microsphere,TiO2/Bi2WO6,Z-scheme,Heterojunction,with,Excellent,Visiblelight,Photocatalytic,Performance

ZHAO Mengxue,LIU Yulin,LIAO Guosheng,HE Jinyun＊,QIN Ningbo

(1.Key Laboratory of New Processing Technology for Nonferrous Metal＆Materials,Ministry of Education/Guangxi Key Laboratory of Optical and Electronic Materials and Devices,Guilin University of Technology,Guilin 541004,China;2.School of Urban Construction,Wuhan University of Science and Technology,Wuhan 541004,China;3.School of Chemistry and Chemical Engineering,Guangxi University,Nanning 530004,China)

Abstract: Flower-like BiOBr/Bi2WO6 Z-scheme heterojunction was prepared by a twostep solvothermal method.BiOBr microspheres were firstly synthesized through solvothermal method.Then the out part of the BiOBr microspheres was designed to react with Na2WO4 forming Bi2WO6 nanosheets through ion exchange process.The BiOBr/Bi2WO6 heterojunction has a larger BET surface area,smaller energy band gap,faster transfer of charge carriers and a much better visible-light photocatalytic performance than that of BiOBr and Bi2WO6.It also has a better cycling stability than that of the BiOBr.Possible photocatalytic mechanism of the BiOBr/Bi2WO6 heterojunction is proposed.

Key words: TiO2/Bi2WO6;Z-scheme;visible-light photocatalytic performance;hydrothermal method

Nowadays,environmental contamination is a serious problem with the development of industry and agriculture.To solve this problem,photocatalytic technique is popularly applied due to its merits of energy-saving and high efficiency.TiO2is the most widely used photocatalyst due to its high photocatalytic activity,chemical stable,non-toxic,cheap raw material and easy to obtain.However,its large band gap (Eg≈3.2 eV) makes it can only absorb ultraviolet light (λ<420 nm) which is only 8.7% part of sun light.Researchers have tried many ways,such as coating,ion doping,noble metal deposition and heterojunction construction[1],to make TiO2visible-light active.Among them,one of the most effective method is to constructZ-scheme heterojunction of TiO2[2].

As an excellent visible-light photocatalyst,Bi2WO6(Eg≈2.75 eV) has been deeply studied and many achievements have been got[3].Bi2WO6and TiO2have matched band structure and they can form TiO2/Bi2WO6heterojunction with excellent photocatalytic performance[4].For example,Zhaoet alprepared a TiO2/Bi2WO6sol precursor,then TiO2/Bi2WO6nanofiber was prepared through electrospinning method[5].Luet alfirst synthesized TiO2nanotube arrays on a titanium mesh,then Bi2WO6nanosheets were composed with TiO2through hydrothermal method[6].Wanget alsynthesized flower-like Bi2WO6microspheres through hydrothermal method,then Bi2WO6/TiO2heterojunction was obtained by a self-deposition method[7].Songet alsynthesized nano Bi2WO6through solvothermal method,then hollow TiO2@Bi2WO6microspheres were prepared by precipitation-calcination method[8].These reported TiO2/Bi2WO6heterojunctions all had an improved visible-light photocatalytic activity than single TiO2or Bi2WO6.But they all had been testified as type II heterojunction,which would has lower redox ability thanZ-scheme heterojunction[9].

Herein,we reported a new nanoparticle/microsphere TiO2/Bi2WO6Z-scheme heterojunction for the first time.A two-step hydrothermal method was applied: Flower-like Bi2WO6microspheres were firstly synthesized through hydrothermal method.Then TiO2nanoparticles were integrated with the out part of Bi2WO6microsphere through a second hydrothermal method.The TiO2/Bi2WO6heterojunction had a greatly enhanced visible-light photocatalytic performance compared with single TiO2and Bi2WO6.

2.1 Materials

All the reagents used in this work were analytically pure.Na2WO4·2H2O,Bi(NO3)3·5H2O,Ti(SO4)2,glucose,Rhodamine B (RhB),tetracycline hydrochloride(TC),absolute ethyl alcohol,acetic acid were obtained from Xilong Scientific Co.,Ltd.of China.

2.2 Preparation of TiO2/ Bi2WO6 photocatalists

TiO2/Bi2WO6photocatalists with different amount of TiO2were prepared by a two-step hydrothermal method (Scheme 1).First,flower-like Bi2WO6microspheres were firstly synthesized by hydrothermal method.The typical procedure is as follows: 5 mmol Bi(NO3)3·5H2O was dissolved in 25 mL dilute acetic acid solution (35wt%).Meanwhile,2.5 mmol Na2WO4·2H2O was dissolved in 25 mL deionized water.These two solutions were mixed evenly.Then the mixture was poured into a 100 mL autoclave and kept at 180 ℃ for 15 h in an oven.After the autoclave was cooled to room temperature,the obtained precipitate was washed with deionized water for 5 times and a humid Bi2WO6product was got.Pure Bi2WO6was obtained after freeze dried the humid Bi2WO6.Second,TiO2/Bi2WO6photocatalists with different amount of TiO2were prepared as follows: the as synthesized humid Bi2WO6,Ti(SO4)2and glucose were dispersed evenly in 60 mL deionized water.The molar ratios of Ti(SO4)2and Bi2WO6were 5:1,4:1,3:1,and 2:1,respectively.While the mass ratio of glucose to Ti(SO4)2was 1:24.The mixtures were poured into four 100 mL autoclaves respectively and they were kept at 180 ℃for 12 h in an oven.After the autoclaves were cooled naturally to room temperature,the gained precipitates were washed 3 times with absolute ethanol and deionized water respectively.Then they were freeze-dried for 12 h.The TiO2/Bi2WO6composites were named as TBWO1,TBWO2,TBWO3,and TBWO4respectively with the adding amount of Ti(SO4)2decreasing.Pure TiO2was synthesized with the same procedure without the addition of the humid Bi2WO6.

Scheme 1 Schematic diagram of the preparation of TiO2/Bi2WO6 heterojunction

2.3 Characterizations

Chemical composition and phase structure of the photocatalysts were detected by X-ray diffraction(XRD,X’pert PRO,Panalytical),X-ray photoelectron spectroscopy (XPS,ESCALAB 250Xi,Thermo Scientific) and FT-IR (Nicolet Nexus,Perkin-elmer).The morphology and microstructure of the samples were characterized by field emission scanning electron microscopy (FE-SEM,S-4800,Hitachi),transmission electron microscopy and high resolution transmission electron microscopy (TEM and HRTEM,JEM-2100F,JEOL).An UV-3600 (Shimadzu) spectrophotometer was used to obtain the UV-Vis diffuse reflectance spectra (DRS).The surface area was tested by a BET surface area analyzer (NOVA 1200E,Quantachrome).

2.4 Photocatalytic performances

Two typical contaminants: rhodamine B (RhB),and tetracycline hydrochloride (TC) were used to evaluate photocatalytic activities of the photocatalysts.In the degradation test of TC,50 mL 20 ppm TC solution and 5 mg photocatalyst were ultrasonically mixed in a 80 mL quartz tube.Then the mixture was stirred in dark for 3 hours to achieve the adsorption-desorption equilibrium between the catalyst and TC.Then it was irradiated by a 500 W halogen bulb equipped with a cutoff filter (λ>420 nm).After irradiated for a certain time,5 mL mixture was taken out and it was centrifuged with high speed to separate the catalyst.Then the residual concentration of TC was tested by the UV-3600 spectrophotometer.The degradation tests of RhB was the same as that of TC,except that 5 mg catalyst was used to degrade 10 ppm RhB solution.

2.5 Photoelectrochemical performances

Photoelectrochemical performance of the photocatalysts was tested by an electrochemical workstation(CHI 760E,Chen Hua Instrument Company,China).A three-electrode system was adopted and 1 M Na2SO4solution was used as the electrolyte.The reference electrode was a Pt sheet and the counter electrode was Ag/AgCl (kept in saturated KCl solution).The working electrode was prepared on a ITO glass plate by drop-coating method.A 350 W xenon lamp with a cutoff filter (λ>420 nm) was used as the light source.

3.1 Chemical phase and composition of the photocatalysts

XRD patterns of the samples are shown in Fig.1.The diffraction peaks of the pure TiO2all corresponded to anatase TiO2(JCPDS NO.21-1272).All the peaks of the pure Bi2WO6matched well with orthorhombic Bi2WO6(JCPDS NO.39-0256).While the diffraction peaks of all the TiO2/Bi2WO6composites were composed of typical diffraction peaks of orthorhombic Bi2WO6and anatase TiO2.Furthermore,their peaks’positions were all consistent with that of the pure TiO2and Bi2WO6.So the crystal phase of TiO2and Bi2WO6in TiO2/Bi2WO6composites had not changed.Among the TiO2/Bi2WO6composites,TBWO2had the best photocatalytic performance,so the following characterization was focused on it.

Fig.1 XRD patterns of the pure TiO2,Bi2WO6,and Bi2WO6/TiO2 catalysts

Surface chemical state of the pure TiO2,Bi2WO6,and TBWO2were studied by XPS.As shown in the full-scale spectra (Fig.2(a)),TBWO2was mainly composed of C,Bi,O,Ti,and W elements.The C element mainly came from XPS test equipment[10].In Fig.2(b),the peaks at 37.23 and 35.08 eV of TBWO2were correspond to W4f5/2and W4f7/2respectively,which indicated that W element existed as positive hexavalent[11].Furthermore,the W4f binding energy of TBWO2was slightly lower than that of the pure Bi2WO6.In Fig.2(c),the two obvious peaks at 164.08 and 158.83 eV of TBWO2were correspond to Bi4f7/2and Bi4f5/2respectively,which indicated the Bi element was positive trivalent[12].As Fig.2(d) shown,the peaks at 464.08 and 458.38 eV of the pure TiO2belonged to Ti2p1/2and Ti2p3/2respectively,indicating that Ti existed as Ti4+[13].While TBWO2had a little higher binding energies of Ti2p1/2and Ti2p3/2(464.28 and 458.63 eV,respectively)than that of the pure TiO2.As Fig.2(e) shown,the O1s peaks at 530.88,531.28,and 532.58 eV for the pure TiO2,Bi2WO6,and TBWO2,respectively belonged to the adsorbed oxygen[14].The peaks at 530.88 and 531.28 eV of the pure Bi2WO6were attributed to the O1s of [Bi2O2]2-and [WO4]2-,respectively[15].For TiO2,the O1s peak of Ti-O bond was 529.56 eV[16].While the O1s peaks of [Bi2O2]2-and Ti-O for TBWO2was 529.73 and 530.38 eV,respectively[16],which were slightly increased compared with that of the pure Bi2WO6and TiO2.These XPS results indicated that TiO2had been integrated with Bi2WO6in TBWO2[17].

Fig.2 XPS spectra of the pure Bi2WO6,TiO2,and TBWO2: (a) Survey spectra;(b) W4f;(c) Bi4f;(d) Ti2p and (e) O1s spectra,respectively

3.2 Micromorphology analysis

FE-SEM,TEM and HRTEM were used to analyze the morphology and microstructure of the catalysts.As Fig.3(a) shown,pure Bi2WO6was composed of flower-like microspheres with the diameter about 3.5µm.These microspheres were constructed by many nanosheets.The pure TiO2was composed mostly by some agglomerated microspheres with a diameter about 1 μm (Fig.3(f)).As Figs.3(b)-3(e) shown,the morphology and particle size of the Bi2WO6/TiO2catalysts were quite like that of the pure Bi2WO6,except that some nanoparticles adhered on the microspheres.EDS elemental mapping of TBWO2(Fig.3(h)) demonstrated that these nanoparticles were TiO2and the microspheres were Bi2WO6.So TiO2had been constructed on the surface of Bi2WO6.

Fig.3 FE-SEM images of Bi2WO6 (a),TBWO1 (b),TBWO2 (c),TBWO3 (d),TBWO4 (e) and TiO2 (f);EDS elemental mapping of TBWO2(g and h)

Fig.4 presents the TEM and HRTEM images of TBWO2.As Fig.4(a) shown,the diameter of the flower-like Bi2WO6microsphere was about 3.5 µm and many nanosheets could be observed on the edge of microsphere.In Fig.4(b),the interplanar spacing of 0.315 and 0.325 nm were ascribed to (131) plane of orthorhombic Bi2WO6and (101) plane of anatase TiO2respectively.Furthermore,it could also be observed that TiO2particles integrated tightly with Bi2WO6microsphere.

Fig.4 (a) TEM and (b) HRTEM images of TBWO2

3.3 FT-IR analysis

FT-IR (Fig.5) was used to study chemical bond of the catalysts.For all the spectra,the absorption peak located at 2922 cm−1was ascribed to O-H[18].While the peak at 1625 cm−1was attributed to the adsorbed water[19].In the spectrum of pure Bi2WO6,400-1700 cm−1was caused by Bi-O and W-O-W stretching vibration[20,21].While in the spectrum of TiO2,the wide absorption at 400-800 cm-1was caused by Ti-O-Ti and Ti-O stretching[22].In the spectra of TiO2/Bi2WO6catalysts,the peak strength of W-O bond decreased gradually with the loading amount of TiO2,which was consistent with the decreasing of Bi2WO6percentage in TiO2/Bi2WO6catalysts.

Fig.5 FT-IR spectra of the catalysts

3.4 UV-Vis diffuse reflection spectra

Fig.6(a) is the UV-Vis diffuse reflection spectra(DRS) of the catalysts.All TiO2/Bi2WO6composites and Bi2WO6had strong absorption in visible-light region (λ>420 nm).Furthermore,the absorption edges of TiO2/Bi2WO6composites were all red shifted compared with that of pure Bi2WO6.While the pure TiO2almost had no absorption of visible light.Band gap estimation method was adopted to evaluateEgof the catalysts[23].As Fig.6(b) shown,Egvalues of all the TiO2/Bi2WO6composites were lower than that of the TiO2(2.66 eV)and Bi2WO6(2.53 eV).Reduction ofEgvalue could improve the separation efficiency of photogenerated electrons and holes[24].

Fig.6 (a) UV-Vis DRS and (b) evaluation diagram of Eg for the catalysts

3.5 BET surface area and pore size distribution

Large specific surface area is favorable for the adsorption of pollutant and active molecules.While large mesopore volume is beneficial to the diffusion of the photogenerated electrons and holes[25].Nitrogen adsorption-desorption isotherms of the pure TiO2,Bi2WO6,and TBWO2are shown in Fig.7(a).They all showed typical IV type isotherms with H3 hysteresis loops,indicating the presence of slit-like pores.As Table 1 shown,the pure TiO2had the largest BET surface area(142.083 m2/g) and pore volume (0.2289 m3/g).While pure Bi2WO6had the smallest pore volume (0.0302 m3/g) and specific surface area (19.853 m2/g).The BET surface area and pore volume of TBWO2were between that of TiO2and Bi2WO6(56.204 and 0.0911 m3/g,respectively).Furthermore,the everage pore width of TBWO2was bigger than that of the pure Bi2WO6and TiO2.

Fig.7 (a) Nitrogen adsorption-desorption isotherms and (b) pore size distribution of the pure Bi2WO6,TiO2,and TBWO2

Table 1 BET surface area (SBET) and pore analysis of the pure TiO2,Bi2WO6,and TBWO2

3.6 Photocatalytic activity and photoelectric performance

Two typical contaminants: RhB and TC were used to evaluate the photocatalytic activity of catalysts.As Figs.8(a)-8(c) shown,8.7% of RhB and 16.1% of TC were degraded in the blank tests,indicating that these contaminants were relatively stable under visible-light.As Fig.8(a) shown,TBWO2had the best photocatalytic activity in degradation of RhB (99.1%) and TC(92.7%).Its degradation rate of TC was 1.36 times and 1.35 times than those of the pure Bi2WO6and TiO2,respectively.Comparing with recent work of TiO2based heterojunction (Table 2),the performance of our TiO2/Bi2WO6catalysts was equal to them.

Fig.8 Photocatalytic performance of the catalysts in degradation of RhB (a) and TC (c);Temporal evolution of the UV-Vis specra of remnant RhB (b);Kinetic analysis in degradation of RhB (d) and TC (e);Cyclic stability (f) and capture experiments (g) of the catalysts;XRD patterns of TBWO2 before and after cycling test (h) and FE-SEM images after cycling test (i)

Table 2 Recent work of TiO2 based heterojunction in degradation of TC

Table 3 The reaction rate constant (k) and correlation coefficient(R2) of the catalysts

Rate constant (k) of the photocatalytic reaction can be got through linear fitting of ln(C0/Ct)vsirradiation time.C0andCtare the initial and residual concentrations of pollutants[22].As shown in Figs.8(d)-8(e),TBWO2shows the highest degradation rate of RhB(0.0478 min-1) and TC (0.0407 min-1).The reaction rate constant (k) and correlation coefficient (R2) of the catalysts is shown in Table 3.

After 5 cycles’ reaction,0.93%,0.88%,and 0.98%degrading rate were remained for Bi2WO6,TiO2,and TBWO2,respectively (Fig.8(f)).So TBWO2had the best cycling stability.As Fig.8(h) and Fig.8(i) shown,the composition and morphology of TBWO2had not changed after cycling test,which also testified its stability.In order to determine the active species of TiO2,Bi2WO6and TiO2/Bi2WO6in photocatalytic reaction,capture experiments were carried out.Isopropanol (IPA)and ammonium oxalate (AOM) were added to capture superoxide free radicals (·O2-),hydroxyl free radicals(·OH) and holes (h+) respectively[23,24].As Fig.8(g)shown,the main active specie was ·O2-in degradation of RhB and TC.

The excited PL emission spectra can be used to characterize efficiency of charge carrier trapping,migration and separation.High intensity of PL spectra means a high recombination rate of photogenerated electrons and holes[30].All the catalysts showed a strong emission peaks at 405 nm in their PL spectra excited at 260 nm (Fig.9).TBWO2had the lowest emission peaks.While the pure TiO2had the highest peak intensity.And the pure Bi2WO6had a peak intensity between them.So TBWO2had the highest separation efficiency of charge carriers.

Fig.9 Photoluminescence (PL) spectra of the catalysts excited at 260 nm

Photocurrent response of the catalysts under visible-light was exhibited in Fig.9.Among all the catalysts,TBWO2had the highest photocurrent density,which demonstrated more effective charge separation and transfer in TBWO2.

Fig.10 Photocurrent response of the catalysts

3.7 Mechanism of the enhanced photocatalytic activity

Valence band (VB) of the pure Bi2WO6and TiO2could be acquired from VB-XPS spectra and they were evaluated to be 2.71 and 2.91 eV,respectively (Fig.11).The conduction band (CB) potentials of the pure Bi2WO6and TiO2could be calculated using the empirical formula[31]:

Fig.11 VB edge potential of the pure TiO2 (a) and Bi2WO6 (b) estimated by valance band XPS profile

where,Eg,EVB,andECBare the semiconductor’s band gap,valence band edge potential and conduction band edge potential,respectively.AsEgof the pure Bi2WO6was evaluated to be 2.53 eV (Fig.6(b)),ECBof Bi2WO6was calculated to be 0.18 eV through formula (1).Similarly,ECBof the pure TiO2was calculated to be -0.58 eV.

The photocatalytic mechanism of TiO2/Bi2WO6heterojunction was proposed based on our above experiments and analysis (Fig.12).The oxidation potential of OH-or H2O to form ·OH is about 1.96 eV[32].TheEVBof Bi2WO6and TiO2were both higher than 1.96 eV.So·OH could be produced by both Bi2WO6and TiO2.The redox potential of molecule O2to ·O2-is about -0.33 eV[32].TheECBof TiO2was higher than -0.33 eV,but theECBof Bi2WO6was lower than -0.33 eV.So·O2-could only be produced by TiO2.If TBWO2was a type-II heterojunction,the photogenerated electrons would flow from conduction band edge of TiO2to that of Bi2WO6.Meanwhile,holes would transfer from the edge of Bi2WO6valence band to that of TiO2.So,·O2-would not be generated during the photodegradation reaction[33].However,our capture experiment showed that ·O2-was the main active species in the photocatalytic reaction.Thus,electrons and holes would transfer through anotherZ-scheme route: electrons at the conduction band edge of Bi2WO6would combine with the holes at the valence band edge of TiO2.Therefore,the electrons at the conduction band edge of TiO2remained at a more negative potential,while the holes at the valence band edge of Bi2WO6remained at a higher positive potential than that of the type II heterojunction.SoZ-scheme TiO2/Bi2WO6heterojunction had a strong redox ability and high photocatalytic activity.

Fig.12 Proposed photocatalytic mechanism of Z-scheme TiO2/Bi2WO6 heterojunction

Nanoparticle/microsphereZ-scheme TiO2/Bi2WO6heterojunction was synthesized by a two-step hydrothermal method.The TiO2/Bi2WO6heterojunction had a much higher BET surface area and large pore volume than that of Bi2WO6.A wider visible light absorption and more effecient charge transfer and separation were gained for the TiO2/Bi2WO6heterojunction.It exhibited a superior photocatalytic activity and cycling stability in degradation of RhB,and TC.This work provided a feasible way for preparation ofZ-scheme TiO2/Bi2WO6nanostructures.