The Enhanced Electrons and Holes Separation for Bi2MoO6/TiO2 Z-scheme Heterojunction by Ag Loading

RUAN Chenliang ,WANG Wei ,DAI Zhenxiang ,ZHENG Ganhong＊

(1. School of Physics and Materials Science, Anhui University, Hefei 230039, China; 2. School of Physics and Optoeletronics Engineering,Anhui University, Hefei 230039, China)

Abstract: The Bi2MoO6/TiO2 and Bi2MoO6/Ag/TiO2 composites were solvothermally synthesized and characterized by X-ray diffraction (XRD),X-ray photoelectron spectroscopy (XPS),and (high resolution)transmission electron microscopy ((HR)TEM).The Bi2MoO6/TiO2 and Bi2MoO6/Ag/TiO2 composites exhibited higher photocatalytic activity than pure Bi2MoO6.100% of the RhB dye molecules could be decomposed over Bi2MoO6/Ag/TiO2 composite in 120 min.The enhanced photocatalytic activity of Bi2MoO6/TiO2 and Bi2MoO6/Ag/TiO2 composite was attributed to the efficient separation of photoinduced electrons and holes.The mechanism for the enhanced photocatalytic activity is discussed.

Key words: photocatalysis;Ag;heterojunction;Bi2MoO6/TiO2;photocatalytic activity

1 Introduction

Semiconductor photocatalytic technology has been widely accepted as the most promising method for addressing environmental pollution and energy shortage.The photocatalyst is crucial for the photocatalytic technology and great efforts have been devoted to develop efficient photocatalysts.TiO2,as an attractive photocatalyst,has been widely studied and applied in the photocatalysis field because of its perfect photocatalytic activity[1].However,it exhibits high photocatalytic activities only under UV light and its application is greatly limited because UV light only accounts 3%-5% of the solar light spectrum.Therefore,visible light active photocatalysts have been developed,such as Bi-based[2],and Ag-based[3].

Bismuth-containing semiconductors,such as Bi2MoO6[4],Bi2O3[5],BiVO4[6],and BiOX[7]have been extensively investigated as efficient photocatalysts due to their good chemical stability and large response toward visible light.Among the Bi-based compounds,Bi2MoO6with a narrow band gap(2.5-2.8 eV) shows good photocatalytic activity in presence of visible light.Moreover,Bi2MoO6is characterized by alternative perovskite (MoO4)2+slaps and stacking of (Bi2O2)2+layers,which facilitates effective separation of photoinduced electron-hole pairs,resulting in high photocatalytic activity[8].However,the rapid recombination of electrons and holes after photoexcitation leads to low efficiency of Bi2MoO6.In order to overcome these drawbacks,some efforts have been made,such as rare earth elements doping[9],morphology control[10],and loading metal[11].In addition,a very useful approach to delay the recombination of the photo-excited electron-hole pairs of Bi2MoO6is the formation of Bi2MoO6-based heterojunctions with wide bandgap semiconductor[12-15].

In the current study,TiO2and Bi2MoO6photocatalyst were used to solvothermally prepare Bi2MoO6/TiO2and Bi2MoO6/Ag/TiO2composites in order to reduce the recombination of photoinduced electrons and holes and enhance the photocatatlytic performance of Bi2MoO6.The photocatalytic measurements suggests that this couple between Bi2MoO6and TiO2can enhance the photocatalytic activity,especially for Bi2MoO6/Ag/TiO2.The degradation ratio of RhB was up to 100% after 120 minutes in the Bi2MoO6/Ag/TiO2composite.Details of the photocatalytic mechanism are explored in this work.

2 Experimental

2.1 Materials synthesis

2.1.1 Synthesis of Bi2MoO6photocatalyst

Pure Bi2MoO6sample was prepared via hydrothermal route.Stoichiometric amount of Bi(NO3)3·5H2O was dissolved in ethylene glycol.Na2MoO4·2H2O and NaOH were separately dissolved in deionized water by stirring for 30 min using a magnetic stirrer.The NaOH solution was added to adjust the pH value to 7.A certain amount of sodium dodecyl sulfate (SDS)dissolved in deionized water was added drop wise to the above mixture under vigorous stirring for 30 min.Next,the mixture was placed in a 1 000 mL hastelloy autoclave (Parr 4577) and reacted at 160℃ for 12 h.After the autoclave was cooled to room temperature naturally,the products were separated by centrifugation,washed sequentially with ethanol and deionized-water several times,and finally dried at 80℃ to obtain the Bi2MoO6sample.

2.1.2 Synthesis of Bi2MoO6/TiO2composite photocatalyst

The Bi2MoO6sample and an amount of TiO2(Commercial,high purity) were dissolved in a mixture solution of water and alcohol at 60 ℃ thermostatic water bath with mechanical stirring of 400 rpm for 6 h.After cooling to room temperature naturally,the product was washed several times with deionized water and ethanol followed by drying at 80 ℃ to obtain the final samples.

2.1.3 Synthesis of Bi2MoO6/Ag/TiO2composite photocatalyst

An amount of Bi2MoO6was dissolved in ethylene glycol under magnetic stirring for 10 min.Then,different levels of AgNO3were added to the mixed solution.The mixture was sonicated for 30 min.The obtained mixture was then sealed in Teflonlined stainless autoclave and heated at 150 ℃ for 120 min.After cooling to room temperature naturally,the product was washed with deionized water and ethanol repeatedly,and finally dried at 80 ℃ to obtain the Bi2MoO6/Ag sample.The Bi2MoO6/Ag sample and an amount of TiO2were dissolved in a mixture solution of water and alcohol at 60 ℃ thermostatic water bath with mechanical stirring of 400 rpm for 6 h.After cooling to room temperature naturally,the product was washed several times with deionized water and ethanol followed by drying at 80 ℃ to obtain the final samples.

2.2 Characterization

The crystal structure of the product was characterized by XRD using an X-ray diffractometer(XRD dx-2000 SSC) with Cu Kα radiation (λ=1.541 8 Å)over a scanning range of 10°-80° with a step of 0.02°.Using X-ray photoelectron spectroscopy (XPS,Elcalab 250 Xi,American),the valence states of ions were also tested.Surface morphologies and lattice fringes were observed through transmission electron microscopy (TEM) and high resolution transmission electron microscopy ((HR)TEM,JEOL JEM-2100).Optical diffuse reflectance spectra were recorded on a Japan Shimazdu UV-3600 (Japan) using BaSO4as a reference.

2.3 Photocatalytic reaction

The photocatalytic activities of Bi2MoO6samples were evaluated by investigating the degradation of methyl blue under a 350 W Xe lamp light.In each experimental,200 mg of the photocatalyst was added to 80 mL of Rhodamine B (RhB) solution (10 mg/L) in a test tube.Before illumination,the suspensions were magnetically stirred in dark for 60 min to ensure establishment of adsorption-desorption equilibrium between the photocatalysts and RhB.The solution under magnetic stirring was then exposed to Xe lamp light irradiation.At a given interval,the test tubes were sampled and then centrifuged to remove the photocatalyst particles.The ratio of remaining dye concentration to its initial concentration,C/C0,was obtained by calculating the ratio of the corresponding absorbance.

3 Results and discussion

3.1 XRD measurement

Fig.1 presents the crystalline phases of Bi2MoO6,Bi2MoO6/TiO2,and Bi2MoO6/Ag/TiO2hybrids.It can be seen that all diffraction peaks of pure Bi2MoO6are in good agreement with those of the (Bi2MoO6,JCPDS card No.21-0102).The diffraction peak of both Bi2MoO6and TiO2phases can be observed in both Bi2MoO6/TiO2and Bi2MoO6/Ag/TiO2composites.However,Ag and TiO2can not be detected in the Bi2MoO6/TiO2and Bi2MoO6/Ag/TiO2composites due to the small amount.

Fig.1 XRD patterns of the prepared Bi2MoO6,Bi2MoO6/TiO2,and Bi2MoO6/Ag/TiO2 composites

3.2 XPS measurement

Chemical composition of the as-synthesized materials and different valence states of the components were subsequently studied by means of XPS analysis.The fully typical scanned spectrum of the Ag/Bi2MoO6nanocomposites in the range from 0 to 1 200 eV is shown in Fig.2(a),which clearly indicated the coexistence of Bi,Mo,Ag,Ti,and O.The obvious peaks corresponding to Bi5d,Bi5p,Bi4f,Bi4d,Bi4p,Mo3d,Mo3p,C1s,O1s,Ag3d and Ti2p could be detected,as shown in Fig.2(a).To further identify the chemical state of the energy element,high resolution XPS spectrum of Bi2MoO6/Ag/TiO2composite was recorded and presented in Figs.2(b)-2(d).The peaks centered at 235.4 and 232.3 eV were assigned to Mo 3d3/2and Mo 3d5/2,indicating that the Mo species in our samples is in the form of Mo6+[12].The value of 374.1 and 368.1 eV for Ag 3d3/2and Ag 3d5/2can be assigned to a Ag0[16]as presented in Fig.2(c).The peaks at 468.8 and 465.7 eV were assigned as the Ti 2p3/2and Ti 2p1/2binding energy,respectively,as shown in Fig.2(d)[17].

Fig.2 XPS spectra of Bi2MoO6/Ag/TiO2 composite: (a) Survey of the sample;(b)Mo 3d;(c)Ag 3d;(d) Ti 2p

3.3 Morphology and microstructure

The morphology and nanostructures of Bi2MoO6and Bi2MoO6/Ag/TiO2composite were investigated by TEM and HRTEM.Thin and transparent twodimensional nanoplates are observed in both Bi2MoO6and Bi2MoO6/Ag/TiO2composite as shown in Figs.3(a)and 3(b).The HRTEM image of Bi2MoO6and Bi2MoO6/Ag/TiO2composite was shown in Figs.3(c)and 3(d).The uniform fringe with an interval of 0.311 nm was indexed to the (131) facet of Bi2MoO6as shown in Fig.3(c).It can be seen that the lattice spacings of 0.273 and 0.339 nm correspond to the(002) plane of Bi2MoO6and the (101) plane of TiO2,respectively.Furthermore,the lattice fringe spacing of 0.229 nm matches well with the (111) crystal plane of Ag as presented in Fig.3(d).These three obvious lattice spacings confirmed the coexistence of Bi2MoO6,Ag,and TiO2phases,which is accordance with the XPS result.The heterojunction is formed at the interfaces of those materials among Bi2MoO6,Ag,and TiO2.The kind of heterojunction is beneficial for the transfer of charges during the photocatalytic process through theZ-scheme strategy.

Fig.3 TEM micrographs of (a) Bi2MoO6 and (b) Bi2MoO6/Ag/TiO2;HRTEM image of (c) Bi2MoO6 and (d) Bi2MoO6/Ag/TiO2

3.4 UV-vis diffuse reflectance spectra

To check the optical property of as-prepared samples,UV-visible diffuse reflectance spectra of Bi2MoO6,Bi2MoO6/TiO2,and Bi2MoO6/Ag/TiO2samples were recorded,and the results are presented in Fig.4.For Bi2MoO6,one strong absorption region appeared in the range of 200-400 nm.This strong absorption region can be ascribed to intrinsic band gap transition,which results from electron transition from O 2p orbitals to Mo 4d orbitals[18].The absorption range did not cause much change along with addition of TiO2and Ag,possibly owing to the quite small contents TiO2and Ag involved.

Fig.4 Diffuse reflectance spectra of Bi2MoO6,Bi2MoO6/Ag,and Bi2MoO6/Ag/TiO

The energy gap in crystalline semiconductors can be estimated using the following equationαhv=A(hv-Eg)n/2,whereα,h,v,EgandAare absorption coefficient,Planck constant,photon frequency,photonic energy gap and one constant,respectively[19].Parameterndepends on the characteristics of the transitions in the semiconductor.For direct transitions,n=1,whereas for indirect transitions,n=4.[20]Forn=1,the energy gap for the absorption edge can be determined by extrapolating the linear portion of the plot (αhv)2vshvto zero absorbance.Based on the equation,the band gap energy of samples were estimated to be about 2.86,3.2 and 3.0 eV for Bi2MoO6,Bi2MoO6/TiO2,and Bi2MoO6/Ag/TiO2samples.Furthermore,the valence band (VB)and conduction band (CB) positions can be obtained based on the following formula:

A KING1 was once hunting2 in a great wood,3 and he hunted the game so eagerly that none of his courtiers4 could follow him. When evening came on he stood still and looked round him, and he saw that he had quite lost himself. He sought a way out, but could find none. Then he saw an old woman with a shaking head coming towards him; but she was a witch.5

whereEVBis the VB potential andECBis the CB potential.Xis the electronegativity of the semiconductor,which is the geometric mean of the electronegativity of the constituent atoms.Eeis the energy of free electrons on the hydrogen scale(4.50 eV),andEgis the bandgap energy of the semiconductor.Xfor Bi2MoO6and TiO2is 5.55 and 5.81,respectively.The valence bandsEVBof Bi2MoO6and TiO2were 2.43 and 3.005 eV,respectively.The conducting bandECBpositions of Bi2MoO6and TiO2were calculated to be-0.43 and -0.385 eV。

3.5 Photocatalytic activity and stability evaluation

The photocatalytic activity and stability of the Bi2MoO6,Bi2MoO6/TiO2,and Bi2MoO6/Ag/TiO2samples were evaluated by photodegradation of a typical water pollutant,rhodamine B (RhB) under visible light irradiation.

Fig.5(a) provides the photocatalytic activity of pure Bi2MoO6,Bi2MoO6/TiO2and Bi2MoO6/Ag/TiO2composite.When the solution is irradiated for 120 min in the absence of any catalyst,little change in RhB concentration is observed.This indicated that the self-photodegradation of RhB is negligible.60%,70% and 100% of the RhB dye molecules could be decomposed over Bi2MoO6,Bi2MoO6/TiO2and Bi2MoO6/Ag/TiO2composite in 120 min,respectively.Compared to pure Bi2MoO6,Bi2MoO6/TiO2and Bi2MoO6/Ag/TiO2composites behave higher photocatalytic activity,especially for Bi2MoO6/Ag/TiO2composite.It suggests the synergistic effect among Bi2MoO6,Ag and TiO2.Fig.5(b) displays the UVVis spectral absorption variation of the RhB solution photodegraded over the Bi2MoO6/Ag/TiO2sample as a function of time.Generally speaking,there are two pathways for the degradation of RhB.One is the cleavage of the all-conjugated structure,with the main absorption peak(about 554 nm) shrinking,whereas the location does not shift.The other is the step-bystep de-ethylation of RhB,which features the blueshift of the absorption peak to 498 nm[21,22].Obviously,in our Bi2MoO6/Ag/TiO2system,the intensity of the absorbance at 554 nm significantly declined with a concomitant blue-shift during the decoloration process.This suggested that both the de-ethylation process and the cleavage of conjugated chromophore structure occurred simultaneously on Bi2MoO6/Ag/TiO2composite.

Fig.5 Plots of C/C0 versus the irradiation time for methyl blue aqueous solution for Bi2MoO6,Bi2MoO6/Ag,and Bi2MoO6/Ag/TiO2 samples under visible light (a);Absorption spectra of methyl blue with irradiation time over Bi2MoO6/Ag/TiO2 sample(b);Pseudo first order kinetics fitting data for the photodegradation of methyl blue over Bi2MoO6,Bi2MoO6/Ag,and Bi2MoO6/Ag/TiO2 samples (c);Photodegradation of methyl blue over Bi2MoO6/Ag/TiO2 sample alone,and with the addition of BQ,IPA,and AD(d)

The kinetic behaviors of all samples for photodegradation of RhB are plotted according to the pesudo-first-order kinetic model: ln(C/C0)=-κt,whereC0andCare initial concentration and instant concentration reaction timet,andκis the rate constant.The plot of ln(C/C0)-texhibits a good linearity,as depicted in Fig.5(c).Obviously,here the photocatalytic degradation of RhB is accordance with the firstorder kinetic model.Theκvalue is 0.006 5,0.009 0,and 0.058 2 min-1for Bi2MoO6,Bi2MoO6/TiO2,and Bi2MoO6/Ag/TiO2,respectively.Theκvalue over Bi2MoO6/Ag/TiO2sample (0.058 2 min-1) is about 50 times higher than that of bare Bi2MoO6(0.006 5 min-1).The result suggests that the synergistic effect between Bi2MoO6and Ag,Ag and TiO2plays an important role in improving the photocatalytic activity of Bi2MoO6composite.

In general,photocatalytic degradation of dyes is an oxidative process in which several active radical species may be involved,such as hole (h+),superoxide anion radical (·O2-) and hydroxyl radicals (·OH).To evaluate the role of these active species,scavengers for h+,·O2-and ·OH were added into Bi2MoO6/Ag/TiO2sample.The corresponding result for Bi2MoO6/Ag/TiO2are shown in Fig.5(d).The scavengers used were isopropanol (IPA) for ·OH,benzoquinone (BQ)for ·O2-,and oxaminic acid (OA) for h+.The result indicates that h+is the predominant active specie.Furthermore,a little inhibition are shown by IPA and BQ,indicating that ·OH and ·O2-also contribute to the degradation of RhB for Bi2MoO6/Ag/TiO2sample.

Scheme 1 Schematic diagram of charge carrier transfer process and possible photocatalytic mechanism of Bi2MoO6/TiO2 composite

Scheme 2 Schematic diagram of charge carrier transfer process and possible photocatalytic mechanism of Bi2MoO6/Ag/TiO2 composite

4 Conclusions

The reconstruction of conventional Bi2MoO6with the support of TiO2and Ag through a simple route,and the Bi2MoO6/TiO2and Bi2MoO6/Ag/TiO2were obtained.The photocatalysis showed a significant improvement in the degradation efficiency than pure Bi2MoO6.For Bi2MoO6/TiO2composite,the electrons diffuse from Bi2MoO6to TiO2until an equilibrium state is formed.At the same time,the inner electric field is formed,which can prohibit the transfer of electrons from the CB of Bi2MoO6to that of TiO2.However,the holes transfer from the VB of Bi2MoO6to the VB of TiO2.As a consequence,the electrons and holes in the TiO2and the VB of Bi2MoO6are well isolated,resulting in enhanced photodegradation efficiency of Bi2MoO6.For Bi2MoO6/Ag/TiO2composite,a part of the photoelectrons in the conduction band of Bi2MoO6and TiO2would transfer into metallic Ag through the schottky barriers.In the meanwhile,some photoinduced holes generated by TiO2can also quickly move to Ag,thus inducing the annihilation of the photogenerated electrons.A possibleZ-scheme mechanism over Bi2MoO6/Ag/TiO2composite was formed and metallic Ag may be functioned as an electron-conduction bridge in theZ-scheme structure.

Conflict of interest

All authors declare that there are no competing interests.