納米片自組裝的(BiO)2CO3單分散微米絨球的綠色可控合成及其光催化性能

阮毛毛 宋樂新,* 王青山,* 夏 娟 楊 尊 滕 越許哲遠

(1中國科學技術大學化學系，合肥230026；2阜陽師范學院化學與化工學院，安徽阜陽236037)

納米片自組裝的(BiO)2CO3單分散微米絨球的綠色可控合成及其光催化性能

阮毛毛1宋樂新1,*王青山1,*夏 娟2楊 尊1滕 越1許哲遠1

(1中國科學技術大學化學系，合肥230026；2阜陽師范學院化學與化工學院，安徽阜陽236037)

采用水為溶劑，Bi(NO3)3·5H2O為Bi源，C6H5Na3O7·2H2O(TCD)為配體構筑了前驅(qū)配合物Bi-TCD，通過配合物分解實現(xiàn)了由納米片自組裝的碳酸氧鉍(BS)微米絨球的綠色可控合成，例如，BS的結(jié)構和形貌可經(jīng)由改變反應物濃度和反應時間來調(diào)控。我們發(fā)現(xiàn)，一方面，TCD的配位作用可致BiO+離子緩慢釋出從而調(diào)控BS的形成速率；另一方面，尿素在BS材料的形成過程中起碳源、堿源、形貌調(diào)控劑和晶體成長控制劑的多重作用，通過調(diào)控尿素的濃度制備了三種分別沿著[001]、[110]和[013]優(yōu)勢生長方向的BS晶體。這種合成方法成本低，不需要有機溶劑、模板、表面活性劑、高溫和很長的反應時間；產(chǎn)物分散性好；產(chǎn)率高；且擁有可控的形貌和優(yōu)勢生長方向。特別是由納米片自組裝的BS微米絨球?qū)α_丹明B展現(xiàn)出優(yōu)異的光催化性能。我們相信當前工作將是綠色可控合成和無機微納材料應用方面的一個重要進展。

碳酸氧鉍；綠色可控合成；光催化性能；微米絨球；二水合檸檬酸三鈉

Scheme 1 Objective one is achieved by the coordination of Bi(III) ions with Cit3－and the slow hydrolysis ofurea;objective two is met very satisfactorily due to very large specific surface area and exposed{001}facets of the as-obtained BS material

1 Introduction

Photocatalytic technology has attracted wide attention toward designing novel photocatalysts with excellent activity to mitigate the globalenergy crisis and environmentalpollution1,2.Among the photocatalysts investigated,micro-and nano-materials with layered structures have been a subjectof increasing interest,because their anisotropic crystalstructure and internal static electric field effectcan efficiently improve photoinduced charge separation and transfer3.Ithas been reported thatthe 5d106s2valence orbits of Bi (III)ions can hybridize with the O-2p valence orbits to form a hybridized valence band,thereby allowing oxide semiconductors of the ions to exhibita specific absorption in visible lightrange4, for example,high photocatalytic performance of bismuth oxyhalides5.Bismuth subcarbonate[(BiO)2CO3,BS],a newcomer of Sillen-type semiconductors,with alternating layers ofandions,bears close resemblance to those seen in bismuth oxyhalides5.

In the lastyears,BS has drawn considerable attention because of its strong antibacterial activity,good photoelectric property,and high photocatalytic performance,becoming a very active research area6－12.Several groups have an interest in designing and synthesizing BS micro-and nano-structures with different morphologies8－19.For example,Dong and co-workers14bsuccessfully synthesized monodisperse BS microspheres using ammonia bismuth citrate and urea as reactants through a hydrothermal process at453 K for 12 h.The as-obtained BS microspheres exhibited efficientphotocatalytic removal of NO in indoor air under UV light or visible irradiation.Chen and his coagents8,17obtained nanotube,nanoplate,and nanocube-like structures of BS by a simple reflux or solvothermalsynthesis using ethylene glycol or mannitolas a solvent.They found thatthe solventethylene glycol or mannitol played an important role in controlling the morphology of BS.Cao and his colleagues9successfully prepared persimmon-like BS microstructures using polyacrylamide as a template via a simple hydrothermalprocess at453 K.Moreover, Liu′s group14areported the synthesis of novelhierarchicalrose-like BS microstructures using ammonia bismuth citrate as bismuth source in the presence of polyvinylpyrrolidone through a hydrothermal treatmentat 453 K for 12 h.The BS microstructures assembled by 2D single-crystalline nanosheets with dominant {001}facets exhibited good photocatalytic performance.Recently, Qian′s group15synthesized sponge-like BS structures at453 K for 18 h,and found thatthe morphology and Brunauer-Emmett-Teller (BET)specific surface area of the obtained BS materials could be easily adjusted by changing the concentration of reactants.

There is no doubt that the results of of these authors were qualitatively representative,but it is well to note that in most instances the synthesis techniques either required a high temperature and/or a long reaction time to be applied15,18,19,or tended to use organic solvents or templates8,15,17,which do notsatisfy the requirements of green synthesis due to the creation of organic waste and pollutants.Therefore,a green,mild and controllable synthesis method needs to be developed.This is one objective of the presentwork(Scheme 1).

In this study,a simple one-pot hydrothermal synthesis was carried out in a sealed Teflon-lined stainless steel autoclave at 413 K for 10 h using deionized water,bismuth nitrate pentahydrate[Bi(NO3)3·5H2O,BNP],and urea as solvent,bismuth source and carbon source respectively,when trisodium citrate dihydrate [Na3Cit·2H2O,TCD]was used as a coordination agentto fabricate the citrate complex Bi-Citas a precursor20,21.White crystals of BS with spherical-shape were successfully obtained.The main advantages of this method,when compared to previously available synthesis methods2,3,9,10,are its simplicity(comes from simple inorganic salts rather than expensive Bi-Cit)and the absence of polymer ligands.

The other objective of the present work is to improve photo-catalytic performance of BS by the analysis of the relation between the structure and photocatalytic performance of BS (Scheme 1).The as-prepared BS materials were characterized by various techniques.Our results provide new insights in understanding the catalytic capabilities of BS and how they mightbe enhanced by controlled crystal-facet synthesis.We believe that these results are significantmilestones towards the controllable fabrication of Bi-based photocatalysts based on the concept of green synthesis.

2 Experimental

2.1 Materials

Citric acid,BNP,TCD,sodium tartrate dehydrate,urea,sodium hydroxide and sodium sulfate were purchased from Shanghai Chemical Reagent Company.Rhodamine B(RhB)was obtained from Aladdin Chemistry Co.Ltd.Lead.Allother reagents were the best available commercial products,used without further purification.Water used in the preparation of solutions for measurements was distilled and deionized carefully before use.

2.2 Preparation of the BS materials

The BS-1 was synthesized by a facile hydrothermal process. First,TCD of 0.882 g(3 mmol)and BNP of 0.484 g(1 mmol) were dissolved in 40 mL of deionized water at room temperature under vigorous stirring for 0.5 h,and then urea of 0.301 g(5 mmol)was added to the solution.This solution was stirred for another 0.5 h.Subsequently,the solution was removed to a Teflonlined stainless autoclave(50 mL).The autoclave was maintained at 413 K for 10 h,and then cooled gradually to ambient temperature.Finally,a white precipitate was collected via centrifugation, and washed with deionized water and absolute ethanol several times and dried in vacuum.Allof the other samples including the BS-2 and BS-3 were synthesized in a similar fashion.

2.3 Materialcharacterization

The X-ray diffraction(XRD)measurements were recorded on a Philips X′Pert Pro X-ray diffractometer equipped with monochromatized Cu Kα(λ=0.15418 nm)radiation operated at 40 kV and 40 mA in the range:10°≤2θ≤65°.The field emission scanning electron microscope(FE-SEM)images were performed by using a Supra 40 FE-SEM.The transmission electron microscopy(TEM),high-resolution(HR-TEM)images,and selectedarea electron diffraction(SAED)patterns were taken on JEF 2100F microscope performing at 200 kV.The UV-Vis diffusereflectance spectrum(DRS)of the BS-1 was recorded employing a Shimadzu DUV-3700 spectrophotometer in the wavelength between 220 and 1000 nm.Barium sulfate powder was used as the reflectance standard materialto adjustbaseline parameters.UVVis spectra were done with a Shimadzu UV 3600 spectrometer in the range of 200－800 nm.X-ray photoelectron spectroscopy (XPS)measurements were done using an ESCALAB 250 spectrometer with Al Kαradiation(1486.6 eV)in ultra-high vacuum (2.67×10－7Pa).And all of the values of binding energy were referenced to C 1s peak(284.8 eV)with an energy resolution of 0.16 eV.Nitrogen adsorption/desorption isotherms were acquired using Micromeritics ASAP-2000 at77 K.The photoluminescence (PL)measurements were performed on a Perkin Elmer Luminescence spectrometer L550B atroom temperature(excited at280 nm).

2.4 Photocatalytic measurements

The photocatalytic activities of the BS materials were evaluated by the degradation of RhB in aqueous solution under visible light irradiation.Metal halide lamp(Shanghai Yaming Lighting Co. Ltd.,XY73,220 V,150 W)was used as the light source.In a typical experiment,25 mg of the BS-1 sample was added to a solution of RhB(50 mL,10 mg·L－1).Before being irradiated,the solution was stirred in the dark for 60 min atroom temperature to establish the adsorption equilibrium between the solution and the photocatalyst.Subsequently,the solution was irradiated under metal halide lamp for 70 min.Finally,the BS catalyst was separated by centrifugation and the supernatant solution was analyzed using an UV-Vis spectrophotometer.

2.5 Photoelectrochemicalmeasurements

The BS-1 material was coated on the indium-tin oxide electrode.The electrode was immerged in Na2SO4solution(0.5 mol· L－1).Current-time curves were obtained by an electrochemical analyzer system,CHI760(Chenhua,Shanghai,China)in a threecompartment cell with a working electrode,a platinum plate counter electrode and a Ag/AgClreference electrode under a bias voltage of 0.5 V using the excitation light of Xe lamp(PLSSXE300,300 W)as the lightsource.Electrochemical impedance spectroscopy(EIS)experiments were carried out under visible lightirradiation in 0.1 mol·L－1KClsolution containing 5.0 mmol· L－1K3[Fe(CN)6]/K4[Fe(CN)6](1:1,molar ratio)mixture as a redox probe in the frequency range of 10－2to 105Hz with a perturbation signalof 10 mV.

3 Results and discussion

3.1 Structure and morphology of the BS-1 and BS-2

Fig.1a shows the XRD pattern of the BS-1.Allof the diffraction peaks are in perfect agreement with the tetragonal phase of BS [JCPDS 41-1488;space group I4/mmm#139;a=0.3865 nm and c=1.367 nm]22,23.No impurity phase was detected.The(002), (004)and(006)diffraction peaks are much sharper and stronger in intensity than those of the others,suggesting that the crystals preferentially grow along the[001]crystalllographic direction13,24,25.

The FE-SEM of the BS-1 in Fig.1b indicates a large scale feature over the entire image,stressing high monodispersity and uniform size of about 1.6μm.The magnified images in Figs.1c and 1d exhibitthat the BS-1 particles have a pompon-like morphology assembled by a side-by-side arrangementof nanosheets (side length,200 nm;thickness,40 nm).The TEMimage(Fig.1e) ofa single micropompon reveals a regular sphericalparticle shape. The HR-TEM image(Fig.1f)from an edge of the micropompon illustrates that the interplanar spacing of lattice fringes is 0.275 nm,which is indexed with the(110)plane in XRD pattern.This is demonstrated by the factthatthe intersection angle between the (110)planes is 90°.The SAED pattern(the insetof Fig.1f)takenfrom the edge of the same micropompon as the HR-TEM pattern shown in Fig.1f confirms the single-crystallinity ofthe nanosheet.

3.2 Formation process of the monodisperse BS-1 micropompons

Initially,BNPwas easily hydrolyzed into insoluble BiONO3(Eq. (1))26,resulting in the release of H+ions into the solution(pH～2.8),and then the BiONO3was reacted with the ligand Cit3–to produce the Bi-Cit precursor complex20,21(pH～6.0,Eq.(2)). Clearly,the transfer between the two forms of Bi(III)is markedly dependent on the acidity of the medium.Thereafter,urea in the solution was hydrolyzed to OH－(pH～9.5)andions9,27,and the Bi-Citprecursor in the alkaline solution was hydrolyzed into BiO+9,14b,when heated(Eqs.(3－6)).Finally,theions were reacted with the BiO+ions to form the BS-1(Eq.

In order to further understand the formation mechanism of BS, we examined the effect of reaction parameters including the reactantconcentration,reaction time and temperature.There were severalinteresting findings.

First,the effectof the concentration of TCD on the morphology of a group of BS was investigated by changing the molar ratio of TCD and BNP from 3:1 to 0:1,1:1,2:1,and 5:1.As shown in Fig.S1(Supporting Information(SI)),no appreciable difference was detected in their XRD patterns,and allthe diffraction peaks belong to the tetragonalphase of BS.Fig.S2(SI)shows thatthe creation of BS microparticles as well as the uniformity and dispersity of the particles was strongly dependent on the molar ratio. An adequate molar ratio to form regular sphericalparticles seemed to be between 2:1 and 3:1,and a higher or lower ratio led to either the formation of irregular particles or the appearance of clusters.These results provide a significant clue regarding the role of TCD.Although TCD mightnotact as a carbon source,itdid play an importantpartin affecting the morphology of BS,namely, acting as a morphology control agent.This can be understood if we consider thatthe role of TCD is to form a precursor complex (Eq.(2)),thus decreasing the formation rate of BS due to a direct competitive interaction for the BiO+ions between the coordination equilibrium(Eq.(6))and the precipitation equilibrium(Eq.(7)).

Fig.1 XRD patterns ofthe BS-1 and-2(a),FE-SEMimages(b－d),TEM image(e),and HR-TEMimage(f)ofthe BS-1; FE-SEM images(g and h),TEMimage(i),and HR-TEMimage(j)of the BS-2The insetin Fig.1f is the SAEDpattern ofthe same place as shown in Fig.1f.The insetin Fig.1jis the SAED pattern of the same place as shown in Fig.1j.

Second,the effectof the concentration of urea on the formation of BS was studied by a series of similar syntheses with BNP(1 mmol)and urea of 0.420 g(7 mmol),0.180 g(3 mmol),0.060 g (1 mmol),0.030 g(0.5 mmol)and 0 g(0 mmol)to get the materials:BS-2,-3,-4,-5 and-6,respectively.The XRD analysis (Fig.1a and Fig.2)reveals that the particles are pure and homogeneous,having the same tetragonal structure as their sister compound BS-1.The particle size increases,and especially the increase in the degree of crystallinity becomes more pronounced with the increase of the concentration of urea as can be seen from Fig.1(a,c,g),and Fig.2.Moreover,the relative intensity of the (110)diffraction peak in the BS-3 and-4 is significantly higher than thatof the other three,indicating that the two crystals have a preferred orientation along the[110],which differs from the [001]orientation of the BS-1.Upon increase(BS-2)or decrease (BS-5)of the concentration of urea from this level,the preferred growth orientation is along the[013].These findings highly suggest a potential role of urea in the regulation of BS crystal growth direction and thereby in the modulation of the BS structure.The control experiments supported the presence of the structuraldifference between the BS materials.As shown in Fig.1 (g,h),the BS-2 formed a nest-like structure(diameter,～1.3μm), with an internal hollow(Fig.1i),which was self-assembled by single crystalnanosheets(see the insetin Fig.1j).The spacing oflattice fringes observed is 0.372 nm,corresponding to lattice spacing of(011)plane(Fig.1j).Further,we found thatonly Bi2O4(BS-6)was formed in the absence of urea(Fig.S3(SI)).

In lightof the above observations,we think thatthe role of urea merits greater emphasis.On the one hand,it was used as the only carbon source for the formation of.This was verified by the factthat TCD did notdecompose atthis temperature(Fig.S4(SI)), though it could be used as a carbon source at higher temperatures15,19.On the other hand,the presence of OH－ions from the hydrolysis of urea28,29contributed to the hydrolysis reaction of the Bi-Citcomplex to generate BiO+(Eq.(6)).In other words,higher pH in this system was required to promote the hydrolysis of the complex.A simple experimentdemonstrated this.Alower yield (42.0%)of BS was obtained when TCD was replaced by citric acid(pH～6.3 in the system)while sodium tartrate(pH～8.7 in the system)afforded a high yield(80.4%,Fig.S5(SI)).In the other experiment(Fig.S6(SI)),when urea was substituted by NaOH to create the same alkaline conditions(i.e.,the molar ratio of NaOH to urea is 2:1),only Bi2O3was observed.The results not only verified a synergistic effect of the combination of citrate and urea on the formation of BS,butalso highlighted thata slow supply of OH－ions7,8,16was required for decreasing the formation rate of BS, perhaps by the competitive interaction between the two equilibria (Eqs.(6)and(7)).

Itshould be noted thatthe materials(BS-1－5)were constructed at the condition of excessive urea.As seen from Eq.(5),the increase ofions signifies the decrease of OH－ions.Thus,the relative concentrations ofand OH－ions change with changing the initialconcentration of urea.Recent reports showed thatexcessiveions can cause a preferred growth orientation of BS9,10,while a suitable pH may facilitate the oriented growth30,31. Therefore,we have good reason to speculate the difference in the crystal structure of the BS materials may be attributed to the difference in the concentrations of CO2－and OH－

ions induced by

3the initialconcentration of urea.

Based on these results,we conclude thaturea acts notonly as a carbon source and an appropriate alkaline source but also as a morphology control agentand especially a crystal growth control agent.This is an interesting result.The role of urea in many synthetic strategies was well established32－35,butthis is the first example in which it can exert so many functions for inorganic synthesis,i.e.,acting as a multifunction reagent.Therefore,we expectthatthis resultcan be extended to other inorganic materials and more complex inorganic structures.

Fig.2 FE-SEMimages and XRD patterns of the BS-1,-2,-3,-4 and-5

Finally,we performed several time-dependentexperiments to elucidate the crystalformation process of BS.Fig.S7(SI)displays the morphologicalevolution of the BS materials obtained atthe identical pathway used for Fig.1c butat different growth stages: 2,4,6,8 and 12 h.Atan early stage of 2 h,irregular nanoparticles (diameter,～120 nm)were formed.Subsequently(4 h),some of the nanoparticles were changed into nanosheets(thickness,～30 nm) by spontaneous organization,and a few spherical structures constructed by the nanosheets appeared.When the reaction time was prolonged to 6 h,almost allthe nanoparticles were changed into nanosheetstructures,and more self-assembled microspheres were observed.With increasing the reaction time to 8 h,the nanosheets almost disappeared,and the self-assembled microspheres grew larger,buthaving different sizes.The larger particles grew at the cost of the small ones as described by the Gibbs-Thomson law36.After the reaction continued for two more hours (10 h),the uniform pompon-like microstructures were formed (Fig.1c).When the reaction time was further increased to 12 h,the size of the micropompons was increased(Fig.S7).It is worth stressing that the crystallinity of the BS materials increases with increasing reaction time(Fig.S8(SI)).Since this crystallinity increase was accompanied by a steady sharpening of the crystalline diffraction peaks,we consider thatthe crystallinity increase may be related to an improvementin the quality of the crystals. Furthermore,we found that temperatures can influence the size and uniformity of the BS particles formed(Figs.S9 and S10(SI)). The proper temperature is about413 K(Fig.1b),because a lower (393 K)or higher(433 K)led to nonuniform sphericalstructures. These observations convincingly demonstrate that the crystal growth of the microsphere-like BS was controlled by an Ostwald ripening process37.

On the basis of the above results,a possible five-step growth process is presented in Fig.3.Initially,the precursor complex Bi-Citwas formed(Step I)in water.Then,with the aid of hydrolysis of urea to produce OH－andions(Step II),the complex was hydrolyzed to release free BiO+ions(Step III).Subsequently,theions were reacted with the BiO+ions to produce BS crystal nuclei(Step IV).With the increase of reaction time,the crystal nuclei gradually grew up to form nanosheets.Finally,the nanosheets were stacked together,and self-assembled into BS microspheres(Step V).The optimal conditions for the micro-pompon-like BS were determined to be as follows:TCD/BNPmolar ratio,3/1;urea/BNP,5/1;temperature,413 K;and time,10 h.

Although some existing synthetic methods can obtain uniform size and good morphology of BS materials8,9,17,butthere are many problems in these methods such as the use of organic solvents, polymer additives or expensive complexes of bismuth(Table S1 (SI)),which is not conducive to the realization of large-scale production.Especially,the organic solvents and polymer additives are likely to cause pollution to the environment.Recently,some researchers tried to synthesize BS materials at room temperature or in the absence of coordination agents16b,butthey were unable to achieve the controllable synthesis of BS.Moreover,the synthesized materials usually have either large sizes or disordered layer structures16b.Also,there were some attempts to improve the synthesis methods of BS,such as the use of citrate as a coordination agentor using urea as carbon source to controlthe reaction process6b,15,butthe improved methods stillhave some problems, such as long reaction time,high temperature or poor product morphology.To the bestof our knowledge,our work provides the first example in which the formation mechanism of BS has been associated with the synergistic effect of the concentration combination of citrate and urea,thereby not only overcoming the problems of currentconcern in the synthetic field of BS,butalso achieving a substantial progress to the controlled growth of BS nanostructures.We believe thatthe method can be extended to a wide range of carbonate materials.

3.3 Photocatalytic performance of the monodisperse BS micropompons

The specific surface area and porous structure of the BS-1－3 were explored by gas adsorption/desorption measurements in liquid nitrogen(Figs.S11－S13(SI)).The sorption isotherms of the three materials exhibit a similar profile categorized as type IV38with a smallhysteresis loop observed atrelative pressures of0.45－0.97,showing mesoporous characteristics(2－50 nm).Itis worthy of remark that the BS-1 has a much larger BET39specific surface area(36.65 m2·g－1)and a much narrower average pore diameter (8.94 nm)notonly than the BS-2(10.79 m2·g－1,31.95 nm)and BS-3(27.96 m2·g－1,13.27 nm),butalso than those reported by most investigators10,14,16,40,further highlighting the advantage of the presentsynthesis method.There is a decreasing order of specific surface areas:BS-1＞BS-3＞BS-2.Undoubtedly,this difference reflects dissimilar surface features.The factthatthe surface of the BS-1 was loosely covered by numberless interconnecting nanosheets may be a major partof the reason for the increase in BET specific surface area and the decrease in average pore diameter.

Fig.3 Schematic illustration describing the formation process of the BS materials

Such a large difference in specific surface areas and pore diameters allow us to estimate whether there is a similar trend in their photocatalytic activity.Fig.4Adisplays the UV-Vis absorption spectra of RhB(10 mg·L－1)in water afterbeing treated by the BS-1(25 mg)under a metalhalide lamp(0.26 W·cm－2).Clearly, the maximum absorption peak of RhB at 554 nm was gradually decreased with increasing irradiation time.Finally,the peak almost completely disappeared at 70 min of irradiation.The photodegradation degree(ζ,%)of RhB was determined by Eq.(8)41. In this equation,C0and C are the initialconcentration of RhB and its equilibrium concentration after irradiation,respectively.

Our data indicate thattheζvalue for the BS-1 at 70 min is up to 99.6%,butthey are dramatically decreased to 64.7%forthe BS-2(Fig.4B,75.3%at 100 min)and 85.8%for the BS-3(Fig.4B, 98.2%at100 min).Importantly,the values ofζcan stillreach up to 98%for the BS-1 at70 min over the firstthree cycles(Fig.S14 (SI)).Also,no change in crystal structure and surface morphology was observed after the consecutive cycles(Fig.S15(SI)),emphasizing that the BS-1 catalystpossesses a good structuralstability.

Thus,these results give a strong indication thatthe BS-1 has a high photocatalytic activity,stability and sustainability for the degradation of RhB.Furthermore,we noticed that the order of photocatalytic efficiency observed forthe materials(BS-1＞BS-3＞BS-2)agrees with the order ofdecreasing specific surface area established above.Further,the density of O atoms on the{001} facets of the BS-1 is much higher than on the{013}facets for the BS-2 and the{110}facets for the BS-342.Itis known thatthe more oxygen atoms were exposed on the surface,notonly resulting in more photo-induced oxygen vacancies but also enhancing the ability in separating the electron-hole pairs3,36.This comparison suggests thatcontrolof crystalgrowth direction may be important in improving photocatalytic activity,which is in accordance with other studies6a,43.

Fig.4 UV-Vis absorption spectra of the RhB solutions (10 mg·L－1)after being treated by the BS-1 after 0,10,20, 30,40,50,60 and 70 min of visible light irradiation(A), the photodegradation degree of RhB atdifferenttime points after treated by BS-1,-2 and-3(B)

It is worth noting that theζvalue of RhB on the BS-1 is comparative to those previously reported for BS and bismuth oxyhalides,exhibiting an improved photocatalytic performance because the data reported by earlier authors were obtained either atrelatively strong lightlevels,high catalystconcentrations,low dye concentrations or long irradiation times(Table S2(SI)).

To determine whatare the main active species responsible for the degradation of RhB in the photocatalytic process,we carried out a series of trapping experiments to evaluate the effect of radicalscavengers.The photodegradation of RhB on the BS-1 was repeated,butwith addition of ascorbic acid(AC,1 mmol·L－1), isopropyl alcohol(IPA,1 mmol·L－1)and ammonium oxalate (AO,10 mmol·L－1)to quench superoxide radical ions(), hydroxylradicals(·OH)and holes(h+),respectively44,45.As shown in Fig.S16(SI),the degradation of RhB was highly inhibited by AO(10.8%)and AC(20.5%),but no significant decrease was found in the presence of IPA(92.4%).This,of course,gives a strong argumentin favor of the contribution of both·and h+, as the main active ingredients,to the degradation of RhB in aqueous solution.

Fig.5 presents a possible explanation regarding the degradation mechanism of RhB.Atfirst,under visible lightirradiation,the BS-1 was excited to generate electrons(e－)in the conduction band (CB)and h+in the valence band(VB,Eq.(9)).Meanwhile,the photosensitization of RhB under visible light may induce the generation of RhB radicals(RhB*,Eq.(10))46,and the RhB*with the excited electrons was adsorbed onto the BS-1 surface.Subsequently,the photo-induced electrons interacted with the O2molecules adsorbed on the exposed active{001}facets of the BS-1 to produce·,a very reactive radicalanion intermediate(Eq. (11)),while the photogenerated electrons in the RhB*were injected into the conduction band of the BS-1,forming radical cations·RhB+(Eq.(12)).Atlast,the·RhB+was reacted with the activeand h+,and finally degraded into inorganic compounds such as CO2and H2O(Eq.(13)).

Fig.5 Possible photocatalytic mechanism of RhB on the BS-1

In order to understand the relation between the structure and properties of the BS materials,XPS,UV-Vis DRS,PL,EIS and photoelectric responses were performed to examine how the structure of the materials(BS-1－3)affects their photoelectric conversion.

The XPS analysis displays thatthere are no other elements in these samples besides C,O and Bi.The peak at 284.8 eV can be assigned to adventitious carbon species from the XPS measurement,while the peak at288.9 eVcan be ascribed to the carbonate ion in the BS materials11(Fig.S17(SI)).The UV-Vis DRS analysis (Fig.6A)shows that all the three materials have a similar absorption profile in the range of UV and visible regions,with maximum absorptions at 300,289 and 281 nm for the BS-1,-2 and-3,respectively.A clear blue shiftwas observed as shown by the green arrow in Fig.6A.It is interesting to note,however,that the BS-1 exhibits higher absorption intensity than the other two in the UV and especially visible region,probably indicating a higher visible light utilization efficiency.The optical bandgaps were determined to be 3.03 eVfor the BS-1,3.31 eV for the BS-2 and 3.13 eVfor the BS-3(Fig.6B),based on the Kubelka-Munk function48,correlating well with the order found in the specific surface area analysis.Such a difference in bandgaps may notonly be a reason to make the difference between the maximum absorption wavelengths49.The narrower band gap of the BS-1 led to a wider absorption range,which may be a directfactor responsible for improving its photocatalytic activity due to its higher visiblelight utilization efficiency.The VBs of BS-1,-2 and-3 were measured based on valence-band XPS spectra(Fig.6C),and the edges of the maximum energy were found at approximately 1.87, 1.87 and 1.71 eV,respectively.According to the optical bandgaps, the CB minima occur atapproximately－1.16,－1.44 and－1.42 eV,respectively.In view thatthe oxidation potentials of photogenerated h+in the BS photocatalysts were negative than the standard redox potentialof·OH/OH－(1.99 eV)50,we suggestthat the h+photogenerated on the surface of the BS materials could not reactwith OH－/H2Oto form·OH.Therefore,itis reasonable that h+orare likely responsible for the oxidation of RhB over the BS catalysts,in agreementwith above trapping experiments.The PL profiles(Fig.7A)show thatthe BS-1 has a significantly lower luminescent intensity,in comparison with the BS-2 and-3, strongly implying a lower recombination rate of eand h+under UV lightirradiation(excitation wavelength,280 nm).In particular, its luminescent band covers a considerably wider spectral range (380－620 nm,withoutlarge intensity gradients).

The time-dependent photocurrent responses(Fig.S18(SI)) indicate that upon illumination the photocurrents of the BS electrodes were abruptly increased to maxima of 77.61 nA for the BS-1,42.36 nAfor the BS-2 and 53.23 nAfor the BS-3,as well as having good reproducibility(three times).This means that the BS-1 electrode exhibited a higher efficiency of photoelectric conversion,i.e.,photo-induced charge separation and transfer, compared to the BS-2 and BS-3 electrodes.The analysis of EIS (Fig.7B)reveals that the separation and transfer efficiency of photogenerated electron-hole pairs of the BS-1 is higher than those ofthe BS-2 and BS-3 since the BS-1 electrode presented a smaller radius ofimpedance arc,thereby having a lower interfacialchargetransfer resistance51.

These results indicate that the BS-1 material exhibits a relatively narrow bandgap,a low recombination rate of eand h+,a high efficiency of photoelectric conversion and a smallinterfacial charge-transfer resistance,thus effectively promoting the separation and transfer of charge carriers,which may be why ithas an enhanced photocatalytic performance.

Fig.6 UV-Vis diffuse reflectance spectra(A),the plots of(ahν)1/2vs hν(B)and valence-band XPS spectra(C)ofthe BS-1,-2 and-3 a is the opticalabsorption coefficient,h is the Plank′s constant,andνis photon frequency.

Fig.7 PL spectra(λex=280 nm)ofthe BS-1,-2 and-3(A)and the EIS spectra ofthe BS-1,-2 and-3 under visible light irradiation(B)

4 Conclusions

In summary,we have developed a facile and green hydrothermalroute for the controllable synthesis of BS materials with unique microstructures(large specific surface area,ultrafine grain size and high monodispersity).Importantly,the crystal growth directions([001],[013]and[110])of the BS family can be readily tuned by adjusting the concentration combination of citrate and urea.Urea was found to play multiple roles(e.g.,carbon source, appropriate alkaline source,morphology controlagent and crystal growth controlagent)in the formation process of BS crystals.This is the firstreport thatthe formation mechanism of BS was related to the synergistic effectof the concentration combination of citrate and urea.In particular,when compared with those reported by other studies,the BS-1 material shows an improved photocatalytic activity to RhB under visible lightirradiation probably due to an effective separation and transfer of charge carriers on the{001}facets.Overall,this work represents an important contribution to current efforts in understanding the controllable green synthesis and application of inorganic micro-and nano-structures.

Supporting Information:available free of charge via the internetathttp://www.whxb.pku.edu.cn.

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Facile Green Synthesis of Highly Monodisperse Bismuth Subcarbonate Micropompons Self-assembled by Nanosheets: Improved Photocatalytic Performance

RUAN Mao-Mao1SONGLe-Xin1,*WANG Qing-Shan1,*XIAJuan2YANGZun1TENGYue1XU Zhe-Yuan1
(1Department of Chemistry,University of Science and Technology of China,Hefei 230026,P.R.China;2Schoolof Chemistry and Chemical Engineering,Fuyang Normal College,Fuyang 236037,Anhui Province,P.R.China)

This work reports a controlled green synthesis ofhighly monodisperse bismuth subcarbonate(BS) micropompons self-assembled by nanosheets using a simple and facile hydrothermalroute in which deionized water,bismuth nitrate pentahydrate(BNP),and urea were used as the solvent,bismuth source,and carbon source respectively.Trisodium citrate dihydrate(TCD)was used as a coordination agentto fabricate a complex precursor.The structure and morphology ofthe BS materials can be finely modulated by adjusting the initial concentration ratios ofthe reactants or the reaction time.The presence of TCD decreased the formation rate of BS due to a direct competitive interaction for the BiO+ions between a coordination equilibrium and a precipitation equilibrium.Urea played a crucialrole(e.g.,carbon source,alkaline source,morphology control agent,and crystalgrowth controlagent)in the formation ofthe BS microstructures.We obtained three kinds of BS crystals with preferred orientations along[001],[110],and[013]by adjusting the concentration of urea. Our synthesis approach has the advantages oflow cost,high reaction yields,monodisperse particles,controlled morphologies and orientations,and not requiring the use of organic solvents,templates,surfactants,hightemperatures,and long reaction times.Particularly,when compared with those reported by other investigators, the micropompon materialexhibited improved photocatalytic performance for Rhodamine B due to a unique microstructure(large specific surface area,high efficiency ofphotoelectric conversion,smallinterfacialchargetransfer resistance,and active{001}exposed facets).These results indicate a major advance in the controlled green synthesis and the application ofinorganic micro-and nano-materials.

Bismuth subcarbonate;Controlled green synthesis;Photocatalytic performance; Micropompons;Trisodium citrate dihydrate

O643

Voorhees,P.W.J.Stat.Phys.1985,38,231.

10.1007/ BF01017860

doi:10.3866/PKU.WHXB201702101

Received:November 11,2016;Revised:February 10,2017;Published online:February 10,2017.

*Corresponding authors.SONG Le-Xin,Email:solexin@ustc.edu.cn;Tel:+86-551-3492002.WANG Qing-Shan,Email:wqs056@mail.ustc.edu.cn. The projectwas supported by the Natural Science Foundation of Anhui Province,China(1508085MB30)and Fundamental Research Funds for the Central Universities,China(WK2060190052,WK6030000017).

安徽省自然科學基金(1508085MB30)和中央高?；究蒲袑ｍ椯Y金(WK2060190052,WK6030000017)資助項目?Editorialoffice ofActa Physico-Chimica Sinica