Bi6Fe1.9Co0.1Ti3O18纳米片负载Au尺寸对其可见光光催化性能的影响

葛 文 刘 空

(1云南师范大学能源与环境科学学院，昆明 650500)

(2云南师范大学，可再生能源材料先进技术与制备教育部重点实验室，昆明 650500)

(3昆明学院化学化工学院，昆明 650214)

0 Introduction

Semiconductor photocatalysis is considered to be one of the most promising green and environmentallyfriendly approaches for solving the issues of environmental pollution and increasing energy demand[1-5].In recent years,bismuth-based oxides with Aurivilliuslayered structure such as BiOX,Bi2WO6,Bi2MoO6,BiVO4,BiFeO3and Bi2O2CO3have been considered to be highly promising visible-light-activated photocatalysts due to their unique crystal and electronic structure[6-14].In particular,bismuth-based complex oxides Bin+1Fen-3Ti3O3n+3formed by perovskite-type(Bin-1Fen-3Ti3O3n+1)2-blocks sandwiched between fluorite-type(Bi2O2)2+slabs have received increasing attention due to their photocatalytic activity in the degradation of organic pollutants.By adjusting the layer number(n=3,4,5,6,etc.)or doping ions(Co3+,La3+,Ni2+,Mn2+,etc.),the Bin+1Fen-3Ti3O3n+3displayed prominent visible-light-driven photocatalytic and other unique properties.In 2008,Sun et al.prepared an excellent visible-light driven photocatalyst Bi5FeTi3O15with 4 layer by a facile hydrothermal method[15].In 2013,Hou et al.synthesized the Bi4Ti3O12(n=3)nanofibers with enhanced visible-light photocatalytic activity[16].In 2014,Li et al.prepared the visible light responsive Bi7Fe3Ti3O21(n=6)nanoshelf photocatalysts[17].In 2015,by Co ions doping,Ge et al.synthesized the Bi6Fe1.9Co0.1Ti3O18(n=5)nanocrystal with different morphologys exhibiting visible-light photocatalytic activity[18].However,the performance of the pristine Bin+1Fen-3Ti3O3n+3materials is limited by their low photocatalytic activity.Therefore,the decoration of noble metals on semiconductor photocatalysts is a promising approach for enhancing their photocatalytic activity.

The high activity of the decorated semiconductor photocatalysts is related to the generation and migration of the photogenerated hot electrons arising due to the Schottky barrier formed between the metal and the semiconductor.Moreover,noble metals nanoparticles can improve visible light absorption due to the surface plasmon resonance(SPR)effect[19-22].In 2017,Liu et al.synthesized the Au nanoparticles loaded on Fe-doped Bi4Ti3O12nanosheets with exposed{001}facets.The Au-2% Fe/Bi4Ti3O12sample exhibited the highest photocatalytic activity compared with other samples[23].In 2018,Li et al.prepared a high efficiency photocatalyst Bi4Ti3O12/Au,in which Au nanorods selectively anchored on the active (001) facets ofBi4Ti3O12nanosheets[24].In 2019,Zhao et al.synthesized the Au-Ag@Bi4Ti3O12composite with the aim of synergistically enhancing the photocatalytic performance[25].Gu et al.prepared the Au@Bi6Fe2Ti3O18nanofibers by electrospinning method with improved the photocatalysis performance[26].

Therefore,in this work,we synthesized visible-light-driven BFCTO/Au nanocomposite photocatalysts by a facile assembly method.The photocatalytic activity of the obtained nanocomposites was enhanced relative to that of pristine BFCTO by introducing Au nanoparticles with different sizes into the BFCTO nanoplates.The BFCTO/Au-1 sample exhibited the strongest photocatalytic activity under visible light irradiation.

1 Experimental

1.1 Materials

All the chemicals,namely Ti(OC4H9)4(≥99.7%),Bi(NO3)3·5H2O (≥99%),Fe(NO3)3·9H2O (≥98.5%),Co(NO3)2·6H2O(≥98.5%),HNO3(65.0～68.0%),NaOH(≥96.0%),ethanol(99.7%),3-aminopropyltriethoxysilane (APTES,＞98%),hydrogen tetrachloroaurate(47.8%),trisodium citrate dehydrate(≥99.0%),were purchased from Sinopharm Chemical Reagent Co.,Ltd.without further purification.

1.2 Synthesis of BFCTO nanoparticles

The BFCTO nanoparticles were synthesized by the hydrothermal method according to our previous report[18].The concentration of NaOH added to the above solutions was adjusted to 0.75 mol·L-1.

1.3 Synthesis of Au nanoparticles

Au nanoparticles were prepared using the method introduced by Frens[27].Au nanoparticles with various diameters of～23 nm,～36 nm,～55 nm and ～80 nm were prepared by rapidly injecting a 38.8 mmol·L-1sodium citrate solution with different volumes of 2.0,1.5,1.0 and 0.5 mL into a boiling HAuCl4aqueous solution under vigorous stirring,respectively.

1.4 Synthesis of BFCTO/Au nanoparticels

The prepared 0.2 g BFCTO nanoparticles were added into the APTES solution,and stirred for 6 h at room temperature.Then the prepared Au nanoparticles solution was added.The obtained nanoparticles were collected by centrifugation and washed for several times with deionized water,and dried at 60℃in vacuum for 12 h.In this work,Au nanoparticles with various diameters(～23 nm,～36 nm,～55 nm and ～80 nm)were loaded on the surface of BFCTO nanoplates.The obtained products were denoted as BFCTO/Au-1,BFCTO/Au-2,BFCTO/Au-3 and BFCTO/Au-4,respectively.

1.5 Characterization

Powder X-ray diffraction(XRD)patterns were characterized by Philips X′pert diffractometer operating at 40 kV and 30 mA with a scan step width of 0.02°in a 2θrange of from 10°to 70°employing CuKαradiation(λ=0.154 05 nm).Sizes and morphologies of the samples were observed by scanning electron microscopy(SEM,JEOL,JSM-6700F,10 kV)and transmission electron microscopy(TEM,Tecnai G2 TF30,200 kV).The X-ray photoelectron spectroscopy(XPS)measurements were carried out on an ESCALAB 250 system with a monochromatic AlKαX-ray source(Thermo-VG Scientific). Ultraviolet-visible-near infrared (UVVisible-NIR)diffuse reflectance spectra were recorded with a Shimadzu SolidSpec-3700 equipped with an integrating sphere,and BaSO4was used as the reference.The specific surface area was measured by BET(Brunauer-Emmett-Teller)method using the N2adsorption isotherm(Quantachrome QuadraWin).The photoluminescence(PL)spectra(lex=325 nm)were recorded by a fluorescence spectrophotometer(FLS 980).

1.6 Photocatalytic activity test

The photocatalytic performances of the as-prepared samples were evaluated by the decomposition of rhodamine B(RhB)/methyl orange(MO)aqueous solution under visible-light irradiation.The light source was a 20 W fluorescent lamp with wavelength of 400～720 nm.The irradiation distance between the light source and the liqud level of RhB aqueous solution was 15 cm.Prior to irradiation,the RhB solution(50 mL,5 mg·L-1)with 50 mg photocatalyst was stirred for 30 min in the dark to ensure the establishment of an adsorption-desorption equilibrium.At a certain time interval,3 mL of the reaction solution was taken and centrifuged.The absorbances of filtrates were measured on a UV-Visible spectrometer at a maximum absorption wavelength of 554 or 463 nm.

2 Results and discussion

The synthetic route used to obtain the BFCTO/Au samples is illustrated in Fig.1a.The BFCTO nanoplates′surfaces were positively charged after their modification with APTES.This enabled the uniform assembly of the negatively charged Au nanoparticles on the BFCTO nanoplate surfaces[28].As show in Fig.1b,the XRD patterns of the BFCTO nanoplates can be indexed by a single-phase orthorhombic lattice(B2cbspace group,PDF No.21-0101)with no detectable secondary phases[14].All of the BFCTO/Au samples showed XRD patterns similar to that of BFCTO,suggesting that the decoration with Au nanoparticles has no obvious influence on the BFCTO crystal structure.Additionally,a weak diffraction peak appeared at 38.2°corresponding to the(111)reflection plane of Au(PDF No.04-0784).The intensity of this peak increased with increasing Au nanoparticle size[29].

Fig.1 (a)Schematic diagram of synthesis procedure used to obtain BFCTO/Au nanocomposites；(b)XRD patterns of the samples

According to Fig.2a and 2g,the BFCTO displayed well-defined shape of nanoplate.The edge length ranged from 80 to 350 nm with an average thickness of 25 nm.Moreover,the elemental mapping images of Bi,Fe,Co,Ti and O are shown in Fig.2b～2f from SEM image in Fig.2a.The results indicate that all the elements have quite uniform distribution over the whole imaging area.As seen from the HRTEM image in Fig.2h,the lattice fringes with spacings of 0.273,0.273 and 0.382 nm can be attributed to(200),(020)and(110)facets of BFCTO orthorhombic phase,respectively[12,14].Besides,the HRTEM image of laterally-viewed of nanosheet in Fig.2i displayed five pseudo-perovskite layers between two bismuth oxide layers.And the inserted FFT patternsconfirm the formation ofwell-developed single-crystalline of BFCTO.

Fig.2 (a)SEM image for BFCTO；(b～f)EDS mapping images of Bi,Fe,Co,Ti,O elements,respectively；(g)TEM image for BFCTO and(h,i)HRTEM images taken from(g)indicated by red and green rectangle,respectively

As showed in Fig.3,the morphology of all BFCTO/Au samples was investigated by SEM,TEM and HRTEM characterizations.Fig.3a and 3b display the Au nanoparticles with the size of～23 nm assembled on the BFCTO nanoplate surface.In Fig.3a,the Au nanoparticles are marked by green rectangles,and the corresponding EDS spectrum that detected the Au element content is shown in the inset of Fig.3a.An examination of the TEM image presented in Fig.3b further proved that the Au nanoparticles with the size of～23 nm were deposited on the BFCTO nanoplates.The EDS spectrum presented in Fig.3c provided the information about the elemental compositions of the BFCTO/Au(～23 nm)nanocomposites marked by the yellow rectangle in Fig.3b,demonstrating the presence of Au in the nanocomposites.The HRTEM image presented in the inset in Fig.3c shows the lattice spacings of 0.23 nm for the(111)plane of Au nanoparticles[30-31].Additionally,the obtained TEM images(Fig.3d～3f)revealed the morphology of other Au nanoparticles with various sizes(～36 nm,～55 nm and ～80 nm)deposited on the BFCTO nanoplates.The SEM,TEM and HRTEM results presented in Fig.3 confirm that Au nanoparticles with various sizes were deposited on the surfaces of the BFCTO nanoplates.

Fig.3 (a)SEM and(b)TEM images of BFCTO/Au-1 nanocomposites；(c)EDS spectra of Au nanoparticle marked by the yellow rectangle in(b)and corresponding HRTEM image of Au nanoparticle(Inset)；(d)TEM images of BFCTO/Au-2(e)BFCTO/Au-3 and(f)BFCTO/Au-4 nanocomposites

To determine the chemical state of elements and the surface defects,XPS analysis was carried out on all samples and the results are shown in Fig.4.The obtained binding energies in XPS analysis were corrected by specimen charging which was executed by referencing the C1sline to 284.5 eV.Comparison of the XPS spectra presented in Fig.4a to the full survey spectrum of BFCTO shows that Au,Bi,Fe,Co,Ti and O are present in the BFCTO/Au samples.For the Au XPS spectrum presented in Fig.4b,two significant binding energy peaks at 83.7 and 87.2eV were observed corresponding to the Au4f7/2and Au4f5/2electronic states,respectively,indicating that the Au NPs exist in the metallic state[32].Additionally,it was observed in XPS spectrum of O(Fig.4g)that the O1speak can be divided into three different peaks at about 533.2,531.0 and 529.4 eV,indicating the co-existence of three different chemical states of the oxygen atoms on this surface.The main peak at 531.0 eV is attributed to the Ti-O bonds,the small peak at 529.4 eV is assigned to the Bi-O bonds,and the peak at 533.2 eV may be due to the oxygen adsorbed on the surface[33-35].Besides,as shown in(Fig.4c～4f),the peaks of Bi4f5/2and Bi4f7/2,Fe2p1/2and Fe2p3/2,Co2p1/2,and Co2p3/2and Ti2p1/2and Ti2p3/2suggest that Bi,Fe,Co and Ti are+3,+3,+3 and+4valencestates,respectively[36-39].

The optical properties of all samples were investigated using UV-Vis-NIR absorption spectra.As shown in Fig.5a,the BFCTO sample absorbed light in the visible range due to the Fe and Co dopant ions.Additionally,the color photos of all samples were shown in the inset in Fig.5a.The BFCTO sample was yellow.As the Au nanoparticle size decreased,the color of BFCTO/Au samples gradually became darker.More importantly,compared to BFCTO,all of the BFCTO/Au samples showed an enhanced,red-shifted and broadened absorption in the visible light region that generally can be ascribed to the following reasons:（ⅰ） as the Au nanoparticles size is increased,the SPR peak has a red-shift[40-42]；（ⅱ） the strong plasmon coupling effect among the neighboring Au nanoparticles can result in broaden SPR peak[43-45]；（ⅲ） the strong interaction between BFCTO and Au nanoparticels may alter the SPR feature due to the change in the dielectric property of the surrounding microenvironment[46].Meanwhile,after the decoration with Au nanoparticles,the band gap(Eg)energies of all BFCTO/Au samples decreased compared with BFCTO nanoparticles(Eg=2.410 eV).Inter-estingly,with increase of Au nanoparticles size,theEgvalues gradually increased.The obtainedEgwere 2.316,2.333,2.357 and 2.369 eV for BFCTO/Au-1,BFCTO/Au-2,BFCTO/Au-3 and BFCTO/Au-4,respectively(Fig.5b).

Fig.4 (a)Full scan XPS spectra of all samples；(b)Au4f,(c)Bi4f,(d)Fe2p,(e)Co2p,(f)Ti2p and(g)O1s XPS spectra of BFCTO/Au-4 nanocomposite

The N2adsorption-desorption isotherms and BET specific surface areas(SBET)of different samples are showed in Fig.6.The isotherms of samples showed that the samples had stronger interaction with N2at the lower pressure region.And the hysteresis loops were presented at higher pressure region for all catalysts.Furthermore,the calculatedSBETwere 13,12,12,11 and 10 m2·g-1for BFCTO/Au-1,BFCTO/Au-2,BFCTO/Au-3,BFCTO/Au-4 and BFCTO samples,respectively(Fig.6f).Apparently,after combining the Au nanoparticles with BFCTO,theSBETof BFCTO/Au system increased.However,with increase of Au nanoparticles size,theSBETof BFCTO/Au samples decreased gradually,which was related to its morphological characters as depicted in TEM image(Fig.3).

Fig.5 (a)UV-Vis diffuse reflectance spectra of the samples；(b)Relationships between(αhν)2 and photon energy(hν)for the samples

Fig.6 (a～e)N2adsorption-desorption isotherms of various samples；(f)BET specific surface areas for various samples

The photocatalytic activities of BFCTO and all BFCTO/Au samples were investigated by the photodegradation of RhB under visible light,which was a typical organic azo-dye pollutant in the textile industry.Fig.7a shows the RhB degradation efficiency curves as a function of the irradiation time for all samples.It is clear that the photocatalytic activity increases with the increasing amount of the Au nanoparticles introduced into the BFCTO nanoplates；thus,for all samples,the photocatalytic activity was enhanced relative to pristine BFCTO.The BFCTO/Au-1 sample exhibited the strongest photocatalytic activity under visible light irradiation for 150 min,and the photocatalytic activities of all of the samples followed the order of BFCTO/Au-1(80.1%)＞BFCTO/Au-3(71.3%)＞BFCTO/Au-4(67.8%)＞BFCTO/Au-2(56.5%)＞BFCTO(38.9%).Meanwhile,the calculated values of the apparent rate constants(kapp)for all of the samples are displayed in Fig.7b.The order of the calculatedkappvalues for all samples was:BFCTO/Au-1(0.012 18 min-1)＞BFCTO/Au-3(0.008 55 min-1)＞BFCTO/Au-4(0.007 34 min-1)＞BFCTO/Au-2(0.00521min-1)＞BFCTO(0.00309min-1).It is clear that the BFCTO/Au-1 nanocomposite shows the highest photodegradation rate.This phenomenon may be attributed to the following reasons.Firstly,as shown in Fig.6,the BFCTO/Au-1 sample has the largest specific surface area.This provides sufficient interfacial area for the adsorption of RhB molecules.Moreover,the large specific surface area increases the number of active sites and promotes the separation efficiency of the electron-hole pairs in photocatalytic reactions.Secondly,the SPR effect of Au nanoparticles enhances visible light absorption,improving the photocatalytic performance.Thirdly,the decorated Au nanoparticles promote the generation and migration of the photogenerated hot electrons due to the formation of a Schottky barrier between the Au nanoparticles and BFCTO[2,5,23,47-50].As depicted in Fig.7c,no obvious decay for photocatalytic decomposition of RhB after five cycles can be found,indicating the good durability and stability of BFCTO/Au-1.Besides,as shown in Fig.7d,the photocatalytic activities of all samples were also tested by the photodegradation of MO under visible light.Obviously,the photodegradation efficiency of MO was not as good as RhB.

Fig.7 (a)Photocatalytic RhB degradation curves in the presence of BFCTO and all of the BFCTO/Au samples under visible light irradiation；(b)Corresponding kinetic linear simulation curves of RhB under visible irradiation for the samples；(c)Cyclic photocatalytic degradation experiments of RhB by BFCTO/Au-1 under the visible light irradiation；(d)Photocatalytic methyl orange(MO)degradation curves for the samples

Based on the above characterization and analysis results,a possible mechanism was proposed for the enhancement in photocatalytic activity of BFCTO/Au(Fig.8).Because of SPR effect,the visible light absorption of BFCTO/Au samples enhanced.Therefore,under visible light illumination,both of BFCTO and Au nanoparticles can be photoexcited to generate photoinduced carriers[19-22].Besides,the PL spectrum showed in Fig.S1(Supporting information)reveals the separation and recombination ofphoto-generated electron-hole pairs of photocatalysts.Clearly,when Au nanoparticles are on the surface of the BFCTO,the PL intensity decreases significantly,indicating lower recombination rate ofphoto-electrons and holes pairs[23-24].By overcoming Schottky barrier,plasmonic excited hot electron of Au nanoparticle is injected to the conduction band(CB)of BFCTO.Thus,the more electrons in the CB of BFCTO can react with O2to produce·O2-.Simultaneously,the photogenerated holes in the valence band(VB)of BFCTO can directly oxidize H2O to yield·OH.As a result,the photocatalytic performance can be effectively enhanced by loading Au nanoparticles on BFCTO[51-52].

3 Conclusions

In summary,we have successfully fabricated visible-light-absorbing BFCTO/Au nanocomposites for the first time.The introduction of Au nanoparticles with different sizes into BFCTO nanoplates enhances the photocatalytic activity with the BFCTO/Au-1 sample exhibiting the highest photocatalytic activity.

Acknowledgements:This research was funded by the National Natural Science Foundation of China (Grants No.21701140,51262032),the Guiding Program of Scientific Research Fund of Yunnan Education Department (Grant No.2017ZDX049),the Program for Innovative Research Team(in Science and Technology)in University of Yunnan Province and the Doctor Start-up Foundation of Yunnan Normal University(No.2016zb001).

Supporting information is available at http://www.wjhxxb.cn