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Journal of Hazardous Materials 181 (2010) 11021108

Contents lists available at ScienceDirect

Journal of Hazardous Materials

journa l homepage: www.e lsev ier .com/ locate / jhazmat

ynthesis of monoclinic structured BiVO4 spindly microtubes in deep eutecticolvent and their application for dye degradation

ei Liu , Yaqin Yu, Lixin Cao, Ge Su, Xiaoyun Liu, Lan Zhang, Yonggang Wangnstitute of Materials Science and Engineering, Ocean University of China, Qingdao 266100, PR China

r t i c l e i n f o

rticle history:eceived 15 January 2010eceived in revised form 11 May 2010ccepted 27 May 2010vailable online 4 June 2010

a b s t r a c t

Monoclinic structured spindly bismuth vanadate microtubes were fabricated on a large scale by a sim-ple ionothermal treatment in the environment-friendly green solvent of urea/choline chloride. Theas-prepared samples were characterized by XRD, SEM, TEM, IR and their photocatalytic activity wasevaluated by photocatalytic decolorization of rhodamine B aqueous solution under visible-light irradia-tion. As-obtained BiVO4 microtubes exhibit the spindly shape with a side length of ca. 800nm and a walleywords:ismuth vanadateeep eutectic solventicrotubehotocatalyst

thickness of ca. 100nm. The opening of these microtubes presents a saw-toothed structure, which is sel-dom in other tube-shaped materials. The formation mechanism of the spindly microtubes is ascribed tothe complex cooperation of the reactioncrystallization process controlled by BiOCl and the nucleation-growth process of nanosheets induced by solvent molecules attached on the surface of microtubes. Suchspindly microtubes exhibit much higher visible-light photocatalytic activity than that of bulk BiVO4 pre-pared by solid-state reaction, possibly resulting from their large surface area and improved crystallinity.. Introduction

Muchattentionhasbeen focusedon roomtemperature ionic liq-ids in recent years due to their successful applications as benignolvents for organic chemical reactions, separations, electrochem-cal chemistry and materials chemistry [1,2]. In contrast to theuccess of ionic liquids, the deep eutectic solvent (DES), whichxhibits very similar solvent properties to ionic liquids, is still in itsnfancy [3,4]. DES is themixture of twoormore compounds andhaslower melting point than that of either of its constituents. Exam-les of DES include the mixtures of quaternary ammonium saltse.g. choline chloride) with neutral organic hydrogen-bond donorssuch as amides, amines, and carboxylic acids) [59]. Unlike ioniciquids, these eutectic mixtures are considered as the desirablenvironment-friendly green solvents because they are nonreac-ive with water, biodegradable, and excellent solvents for a wideariety of solutes. Furthermore, another advantage of DES overonic liquids is easy to prepare with low-cost rawmaterials, whichakes DES to be particularly desirable for applications in the large-

cale synthesis of new functional materials. In spite of the greatnterest in incorporating inorganic self-assemblies into DES, theumber of works reporting on the use of DES in the synthesis oficro/nanomaterials is still limited [1012].

Corresponding author. Tel.: +86 532 66781690; fax: +86 532 66781332.E-mail address: weiliu@ouc.edu.cn (W. Liu).

304-3894/$ see front matter 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.jhazmat.2010.05.128 2010 Elsevier B.V. All rights reserved.

Recently, the decomposition of harmful organic and inor-ganic pollutants using photosensitized semiconductors as catalystshas attracted increasing attention [1315]. Of them, bismuthvanadate (BiVO4) has been recognized as a strong photocatalyst(bandgap: 2.4 eV) for pollutant decomposing due to its photocat-alytic response to the visible-light region and relatively strongoxidation properties for water splitting [1625]. Three crystallinephases of BiVO4 materials have been reported, named as mon-oclinic scheelite, tetragonal zircon and tetragonal scheelite. Ofthem, monoclinic BiVO4 exhibits much higher visible-light photo-catalytic activityover theother forms, giving rise tomoreattentionsand wider researches. Besides crystalline form, the photocatalyticproperty of BiVO4 also depends strongly on the microstructure ofparticles. To further improve the visible-light photocatalytic activ-ity, a few submicron- or nanometer-sized BiVO4 particles withsheet, tube, rod or sphere shape have been prepared [2629].However, surfactants or organic templates are generally added inthe reactions in order to control their microstructures, causingthe production of lots of acid-alkali wastewater containing refrac-tory organics. Therefore, a new template-free and environmentallybenign green route is expected in thepreparation of photocatalysts.

Since urea is one of the normal metabolites of mammals and

choline chloride is one kind of common food additives withoutany environmental side-effects, theDES of urea/choline chloride, asone pure environmentally compatible green solvent, absorbs moreattention. In this study, we report firstly the production of visible-light-driven BiVO4 spindly microtubes in the deep eutectic solvent

dx.doi.org/10.1016/j.jhazmat.2010.05.128http://www.sciencedirect.com/science/journal/03043894http://www.elsevier.com/locate/jhazmatmailto:weiliu@ouc.edu.cndx.doi.org/10.1016/j.jhazmat.2010.05.128

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f urea/choline chloride. The composition andmicrostructure of as-reparedproducts are investigatedand theirphotocatalytic activitys also evaluated.

. Experimental

.1. Synthesis of spindly BiVO4 microtubes

All the chemicals in this study were used as received fromhanghai Chemical Regent Factory of China without further purifi-ation. Distilled water was used in all experiments. The spindlyicrotubes were prepared through a simple ionothermal method.

n the typical process, the deep eutectic solvents (2 g) were synthe-ized by heating a mixture of choline chloride and urea in the 1:2olar ratio at 70 C for 30min to a colorless liquid. Then 1g wateras added into the solution to adjust the viscosity of the DES. Afteragnetic stirring to homogenous liquid, Bi(NO3)35H2O (48.5mg)nd NH4VO3 (11.7mg) were added into the above solution underagnetic stirring. The light yellow aqueous solution obtained afterontinuous vigorous stirring for 30min was transferred into a 15-L Teflonlined autoclave, which was filled with 80% of the totalolume. The autoclave was sealed and kept at 150 C for 15h.he final light yellowish products were centrifuged, washed witheionized water and absolute ethanol for several times, and thenried at 80 C in air. For comparison, bulk BiVO4 particles werelso prepared by the traditional solid-state reaction (SSR-BiVO4)ccording to the previous literature [30].

.2. Characterization

The powder X-ray diffraction (XRD) patterns of as-synthesizedamples were measured on a X-ray diffractometer (Bruker D8DVANCE) using monochromatized Cu K (=0.15418nm) radi-tion under 40kV and 100mA. The particles were spread on alass slide specimen holder and the scattered intensity was mea-ured between 10 and 70 at a scanning rate of 2 =5/min. Theorphologies and microstructures of as-prepared samples werexamined with scanning electron microscopy (SEM, JSM-6700F)nd equippedwith an energy-dispersive X-ray spectroscope (EDS).ransmission electron microscopy (TEM) observations were car-ied out on a JEOL JEM-2100 instrument with accelerating voltage00kV in bright-field and selected-area electron diffraction (SAED)odes. The specimens used for TEM studies were dispersed inbsolute ethanol by ultrasonic treatment. The sample was thenropped onto a copper grid coated with a holey carbon film andried in air. UVvis diffuse reflectance spectroscopy (UVvis-DRS)n the spectral range from 200 to 700nm was performed with aitachi U-3010 UVvis spectrometer. The baseline correction waserformed using a calibrated reference sample of barium sulfate.he BrunauerEmmettTeller (BET) surface areas were estimatedith theuse of aMicromeritics ASAP2010Analyzer. IR spectrawereollected using a Digilab-FTS-80 spectrophotometer using pressedBr pellets of the samples. Photoluminescence spectra were mea-ured by Fluorolog-3-P with 450W Xe lamp.

.3. Photocatalytic test

Rhodamine B (RhB), one of the N-containing dyes, which areesistant biodegradation and direct photolysis, is a popular probeolecule in the heterogeneous photocatalysis reaction. The eval-ation of photocatalytic activity of the prepared samples for the

hotocatalytic decolorization of RhB aqueous solution was per-ormed at ambient temperature, as reported in our previous studies31]. Experimental procedures were as follows: the weight of cata-ysts used in each experiment was kept 0.2 g. A 250mL beaker wasried in an oven at 80 C for about 2h to evaporate the water andFig. 1. XRDpatternof spindlyBiVO4 microtubespreparedby ionothermal treatmentat 150 C for 15h in the solvent of urea/choline chloride.

then cooled to room temperature before used. 100mL RhB aque-ous solution with a concentration of 1105 M was added intothe beaker in each experiment, and then the photocatalyst powderput into the RhB solution to form the homogenerous suspension byultrasonicbath for5min.A500WXe lamp(10 cmabove thebeaker)was used as the light source, and the beaker was covered with UVcutoff filter to completely remove any radiation below 400nm andto ensure illumination by visible-light only. Prior to irradiation, thebeaker containing catalysts and RhB aqueous solution was keptin the dark for 30min to ensure the establishment of an adsorp-tion/desorption equilibrium between catalysts and RhBmolecules.After irradiation for adesignated time, thedispersionwasfiltered toseparate the photocatalyst particles, and the RhB concentration ofthe filtrate were monitored by checking the absorbance at 553nmduring thephotodegradationprocessusing theUVvis spectropho-tometer.

3. Results and discussion

3.1. Characterization of spindly BiVO4 microtubes

The phase and purity of the products obtained under 150 Cfor 15h was investigated by the XRD measurement. As shown inFig. 1, all the diffraction peaks can be readily indexed to a puremonoclinic scheelitephaseBiVO4 of lattice constantsa=0.7331nm,b=0.7331nm and c=0.6462nm, which agrees with the reportedvalues of a=0.7300nm, b=0.7300nm and c=0.6457nm (JCPDSNo. 48-0744). No peaks for any other phases or impurities weredetected.

The morphology and microstructure of as-prepared BiVO4 par-ticles were measured by the electron microscopy. The SEM image(Fig. 2a) shows that the product is well-defined microtubes witha length of 810m, in which particles with other morphologiescannot be observed. Close observation of the samples (Fig. 2b)demonstrates that the individual tube exhibits a spindly shapewitha side length of ca. 800nm and a wall thickness of ca. 100nm.Interestingly, the opening of these microtubes presents the novelsaw-toothedstructure,which is seldominother tube-shapedmate-rials reported in previous literatures. Connected with tooth tip oftwoopenings, thedistinct grooves on the rough surface of tubes canbe identified clearly anddivide the tube into four long columnswithsharp heads. In this way, the hollow interior structure of products

can also be depicted as the encapsulation of such four long columnsin parallel. The corresponding TEM image (Fig. 2c) demonstratesthat the BiVO4 particles are spindle in shape with dentate open-ings, but limited information about the hollow interior structure ofas-prepared samples can be obtained due to the large particle size

1104 W. Liu et al. / Journal of Hazardous Materials 181 (2010) 11021108

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products are nearly the same as that of the 10h one except for aig. 2. (a) Low magnification SEM image of as-obtained product; (b) Close observatingle BiVO4 microtube; (d) the corresponding SAED pattern taken from the tooth t

nd the strong electron absorption of heavy metal atom of Bi. Theelected-area electron diffraction (SAED) pattern (Fig. 2d) revealshe single-crystal nature of the BiVO4 spindly tubes, and the EDattern can be indexed to the diffraction pattern of [0 10] zone axisndicating a preferred (010) surface orientation of the tubes.

The EDS analysis (the percent content of the elements (wt%): O,2.50%; V, 15.71%; Bi, 71.80%) reveals that the elemental composi-ion of the spindly tubes is vanadium, bismuth, and oxygen with aolar ratio of Bi:V=3.36:3.38, which is very close to the Bi:Vmolar

atio (1:1) in monoclinic BiVO4. The BET surface area of spindlyiVO4 samples is estimated to be ca. 3.46m2/g, which was muchigher than that of the reference SSR-BiVO4 sample of ca. 0.75m2/g.

.2. Formation of spindly BiVO4 microtubesTo investigate the formation process of spindly BiVO4 micro-ubes, a detailed time course experiment was carried out. Fig. 3hows the XRD patterns of time series samples. It can be seen fromig. 3a that only few weak reflections of the precursors without

ig. 3. XRD patterns of as-prepared BiVO4 powders at various treated time (a) pre-ursors; (b) 40min; (c) 1h; (d) 2h; (e) 10h and (f) 15h.microtubes showing the hollow structure and wall thickness; (c) TEM image for at of the microtube.

ionothermal treatment can be identified and are assigned to bis-muth oxychloride BiOCl (PDF-2 No.06-0249). As the time increasesto 40min, the peak intensity of BiOCl decreases obviously while anew broad peak at 2 =28 appears (Fig. 3b). After 1h, the diffrac-tion peaks ofmonoclinic BiVO4 can be clearly observed at 2 =18.6

and 28.9, and the remaining ones at 2 =32.5 and 46.7 areassigned to BiOCl (Fig. 3c). This indicates that the mixture phasesof monoclinic BiVO4 and BiOCl are produced in the sample aftertreated for 1h. With the time is prolonged to 2h, the peaks corre-sponding to monoclinic BiVO4 become more and more dominant,and the diffraction peaks corresponding to BiOCl disappear exceptfor few tiny peaks (Fig. 3d). After treated for 10h, all crystal diffrac-tion peaks can be indexed to be a pure monoclinic phase BiVO4(Fig. 3e). When the time increases to 15h, the XRD pattern of thelittle intensity increasing (Fig. 3f). FT-IR absorption spectra of theprecursor and the samples prepared after ionothermal treatmentfor 40min, 1h, 10h and 15hwere alsomeasured (Fig. 4). The spec-trum of precursor demonstrates a weak absorption of (BiO) at

Fig. 4. (a) IR spectrum of the precursor and IR spectra of samples after ionothermaltreatment at 150 C for (b) 40min, (c) 1h, (d) 10h and (e) 15h in DES solution.

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round 529 cm1, that can be interpreted as the presence of bis-uth oxochloride [32]. The absorptions of 1(VO4) and 3(VO4) (atround 751 cm1) can be attributed to the vanadium amorphoushase and the band at 1637 cm1 may be assigned to the absorbedO2 in the air [29]. The remaining peaks including the broad peakf (OH) at 3430 cm, the weak peaks of (CH2), (CH3) and (CH3)t 2923, 2856, 2958 and 1460 cm1, and the peak of (N+CH2) at400 cm1, mainly correspond to choline chloride molecules [33].herefore, the precursor can be considered as themixture of BiOCl,anadiumamorphous phase and adsorbed organicmolecules.Withrolonged ionothermal treatment, it is observed that the absorp-ions of 1(VO4) and 3(VO4) gradually increase in quantity, whilehe peak for BiCl gradually decrease, which is in agreement withhe aboveXRD results. In addition, thepeaks assigned to the cholinehloridedisappear graduallywith the increase of treated time, indi-ating that most of choline chloride molecules have been removeduring the crystallization process of BiVO4. Whereas the enhancedbsorption bands of (N+CH2) and (CH3) are observed in theamples treated for 10 and 15h, which may results from the ori-nted adsorption of a small amount of organic molecules on theurface of samples.

The products collected at different reaction timewere observedy SEM. Fig. 5a shows that the precursors are well-defined BiOCl

anosheets with a thickness of ca. 10nm, part of which intersectach other into a flower-like morphology (Fig. 5a inset). Whenhe reactive time is prolonged to 40min, the nanosheet aggregatesvolve into ball-like compact microspheres with a diameter of ca.

ig. 5. Morphology evolution of as-prepared BiVO4 samples with the reaction time (a) prhow the close observation of BiOCl aggregations.terials 181 (2010) 11021108 1105

800nm (Fig. 5b). A large amount of small protuberances appeardistinctly on the surface of these spheres (Fig. 5b inset), possiblyderiving from the overlapped nanosheets.When the time increasesto 1h, a few small square-shapedmicrotubes growing out from theconglomeration of microspheres can be observed clearly in Fig. 5c.Close observation (Fig. 5d) of the corresponding crystal nuclei withone concave surface (indicated by the arrow) suggests the initialnucleation and growth process of as-prepared microtubes. As seenfrom Fig. 5e, the product treated for 10h consists almost of whollyspindly microtubes with saw-toothed opening. No microspherescan be found except for a quantity of irregular fragments adsorbedon the surface of the tubes. Then, the microtubes begin to growlonger. Finally, as the time is extended to 15h, BiVO4 microtubeswith a length up to ca. 10m are formed (Fig. 5f).

Based on above results, it is noticeable that BiOCl plays the cru-cial roles in forming the monoclinic BiVO4 microtubes. First, theformation of BiOCl at the initial stage results in the heterogeneousnucleation points for monoclinic phase BiVO4. Second, BiOCl, as abuffer, avoids the direct reaction between Bi3+ and VO3 in thesolvents, thereby controlling efficiently the BiVO4 concentrationwithin the bulk solution. Generally, the simultaneously nucleationand growth of large number of monoclinic phase BiVO4 seeds willgreatly reduce the concentration of BiVO4 in the bulk solution. Due

to the mediation effect of BiOCl, it could not provide enough reac-tants for the growth of BiVO4 crystals. Mass transportation to thegrowing regions would lead to undersaturation in the central partof the growing faces of each seeds, which causes the formation of

ecursors; (b) 40min, (c, d) 1h, (e) 10h and (f) 15h. The insets images of (a) and (b)

1106 W. Liu et al. / Journal of Hazardous Materials 181 (2010) 11021108

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the plots of (h) versus photon energy (h), as shown in inset ofFig. 7. The values estimated from the intercept of the tangents to theplotswere 2.48 and 2.44 eV for spindly BiVO4 tubes and SSR-BiVO4,respectively. The increase in the bandgap of the as-prepared BiVO4spindly microtubes could be indicative of the possible quantumig. 6. (a) Low magnification SEM image of samples treated for 10h; (b) High magf as-prepared samples treated for 10h; (d) TEM photo of the nanosheets and the c

quare-shaped crystal nuclei with hollow interior structure. Thisan be confirmed by the concave surfaces observed in the initialrystal nuclei of BiVO4 (Fig. 5d). Unlike BiVO4 microtubes reportedn previous literatures [28,29], however, the spindly shape of theicrotubes is obtained in our case, which possibly results from

he inductive effect of organic molecules in the solution. In therowth process of microtubes, it is worthy of noting the presenta-ion of the irregular fragments on the external/internal surfaces ofiVO4 microtubes (Fig. 6a and b). The sheet nature of these frag-ents with the thick length of ca. 10nm is observed clearly in

he corresponding TEM image (Fig. 6d) and the selected-area elec-ron diffraction pattern (Fig. 6d inset) reveals that these nanosheetsrystallize in the formofmonoclinic BiVO4. Interestingly, highmag-ification SEM photo (Fig. 6c) demonstrates that these nanosheetsot only adsorb but also grow closely together with the wall oficrotubes, thereby giving rise to the rough surfaces and raggeddges of BiVO4 microtubes. This nucleation and further growth ofanosheets derives possibly from the inductive effect of solventolecules attached on the surface of microtubes. Since the polarroups of the solvents such as hydroxyl of choline chloride andarbonyl of urea can easily hydrogen bondwith unsaturated bondsnd hydroxyl groups of crystal nuclei [5,6], part of solvent molec-lar can absorb on the surface of BiVO4 microtubes and therebynduce the nucleation and growth of the nanosheets. This is alsoupported by the above results of IR. With the rise of time, theontinuous and uneven growth of abundant nanosheets on variousarts of the microtubes causes possibly the formation of spindlehape of BiVO4 microtubes eventually. The growth mechanism ofpindly BiVO4, therefore, can be ascribed to the coexist effect ofhe reactioncrystallization process controlled by BiOCl concentra-ion and the nucleation-growth process of nanosheets induced byolvent molecules adsorbed on the surface of microtubes..3. Band gap of the spindly BiVO4 microtubes

It iswell known that the electronic structure of the semiconduc-or usually plays a crucial role in its photocatalytic activity [34,35].ince the valence band (VB) of BiVO4 is composed of hybridizedion SEM image of the corresponding samples; (c) Close observation of the surfaceonding SAED pattern.

Bi 6s and O 2p orbitals, whereas the conduction band is com-posed of V 3d orbitals. The charge transfer upon photo-excitationis thus supposed to occur from Bi 6s and O 2p hybrid orbitals toV 3d orbitals [36]. In order to investigate the electronic structureof as-obtained spindly structure, UVvis absorption spectra weremeasured for the spindly microtubes and SSR-BiVO4 samples. Asshown in Fig. 7, the spindly BiVO4 tubes have the strong absorp-tion in UV and visible-light regions with the steep shape, whichindicates that the visible-light adsorption is due to the bandgaptransition [37].According to theequationEp =K(Ep Eg)1/2 (where is the absorption coefficient, Ep is the discrete photoenergy, Kis a constant, and Eg is the band gap energy) [38]. The energy ofthe band gap of BiVO4 photocatalyst could be thus obtained from

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Fig. 7. UVvis diffuse reflectance spectra of spindly BiVO4 microtubes and SSR-BiVO4 samples. Inset: plots of (h)2 vs. photon energy (h).

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Fig. 9. UVvis spectra takenover timeduring thephotodegradationofRhBmediatedby BiVO4 spindly particles versus SSR-BiVO4. The inset image is the degradationcurves of RhB without photocatalysts and using spindly BiVO4 microtubes and SSR-BiVO4 as photocatalysts.W. Liu et al. / Journal of Hazard

onfinement effects resulting from the nanosized building blocksn the surface of spindly tubes.Fig. 8 shows the comparison of PL spectra of SSR-BVO4 sample

nd spindly BVO4 sample with excitation wavelength of 375nm.he PL emission spectra of two photocatalysts demonstrate theain peaks at similar positions, but the PL intensity of spindlyVO4 sample is lower than that of SSR-BVO4 sample, which clearlyndicates that the recombination between the hole and the elec-ron is greatly inhibited [39]. Hence, it is indicated that the spindlyiVO4 could be used as more efficient photocatalyst compared tohe SSR-BiVO4 catalysts.

.4. Photocatalytic property of spindly BiVO4 microtubes

The visible-light photocatalytic activity of the spindly BiVO4icrotubes was evaluated by photocatalytic decolorization of RhBqueous solution. The blank experiment in the absence of catalystsemonstrates a poor self-degradation of RhB under the visible-ight irradiation. After adsorption/desorption equilibrium betweenhB molecules and the samples, the concentration of RhB do nothangewith increasing reaction time for everymeasurement usingiVO4 samples under dark conditions without light illumination.herefore, the presence of both light illumination and BiVO4 cat-lysts is necessary for the efficient photocatalytic degradation ofhB aqueous solution. These results also suggest that the degra-ation and decolorization of RhB aqueous solution is caused byhotocatalytic reactions on BiVO4 samples under the visible-lightllumination. Fig. 9 shows the UVvis spectra taken over timeuring the photodegradation of RhB mediated by BiVO4 spindlyarticles versus SSR-BiVO4. Accompanying with the decrease ofbsorption of RhB/BiVO4 suspension during the photodegradations the distinct blue-shift of its major absorption band, indicatingemoval of ethyl groups one by one, which is in good accordanceith that in the literature [40]. After irradiation for 300min, thebsorption blue-shifts from 552 to 498nm in the presence ofhe prepared spindly BiVO4, indicating that RhB has being fullyemethylated, while in the same time period no distinct blue-shifts observed for SSR-BiVO4 materials as catalyst. The results demon-trate that the BiVO4 spindly particles have superior photocatalyticctivities comparedwith the SSR-BiVO4 materials. The degradationurves of RhB using spindly BiVO4 microtubes and SSR-BiVO4 as

hotocatalysts are shown in Fig. 9 inset. The rate of photodegra-ation by spindly BiVO4 microtubes (98% in 300min) is muchaster than that by SSR-BiVO4 (16%). Notably, the much-enhancedisible-light-responsive photocatalytic property of spindly BiVO4icrotubes could be ascribed to their special tubular structure

ig. 8. The room temperature photoluminescence (PL) spectra of SSR-BVO4 (a), andpindly BVO4 (b) (ex = 375nm).Fig. 10. RhB concentration changes with irradiation time over BiVO4 catalystsionothermally prepared for various time.

morphology. In one respect, the hollow internal structure and therough surface create large surface area, which can usually offermore active adsorption sites and photocatalytic reaction centers;in another respect, lots of surface defects and nanostructure on thesurface of microtubes can effectively promote the separation effi-ciency of the electronhole pairs, thus improving their response tovisible-light.

In addition, the samples with prolonged ionothermal treatmentdemonstrate the improved photocatalytic activity under visible-light irradiation (Fig. 10). For example, the photocatalytic efficiencyfor RhB is only 33% in the presence of the sample ionothermallytreated for 1h and 45% for the sample treated for 10h, while thehighest rate of photodegradation (98%) is observed for the sampleionothermally synthesized for 15h. The possible explanation is dueto the improved crystallization performance of the samples withthe prolonged treated time. Generally, the particles with well crys-tallinity can decrease the defects inside the crystals, which allowsfor the more efficient transfer of electronhole pairs, generatedinside the crystal, to the surface. Therefore, it is not surprising thatthe well-crystallized spindly microtube shows the highest photo-catalytic activity among the prepared samples.4. Conclusions

Monoclinic structured BiVO4 microtubes with a length of ca.10m have been successfully fabricated in the deep eutecticsolvent of urea/choline chloride. As-obtained product exhibits a

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pindly shape with the saw-toothed openings, which is seldom inther tube-shaped materials reported in previous literatures. Theormation mechanism can be ascribed to the coexist effect of theeactioncrystallization process controlled by BiOCl concentrationnd the nucleation-growth process of nanosheets induced by sol-ent molecules attached on the surface of microtubes. Due to theirell crystallization and large surface area, the photodegradationate of as-prepared BiVO4 tubes can be up to 98% under the visible-ight irradiation for 300min, which is much higher than that ofSR-BiVO4. This work might provide an environmentally benignreen method to the design of advanced photocatalytic materi-ls with complex architecture for enhancing visible-light-drivenhotocatalytic activity. It is also extended into design some novelicro/nanostructures of other oxide materials, which is currentlynderway.

cknowledgement

This work is financially supported by the Program for New Cen-ury Excellent Talents in University (NCET-08-0511).

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Synthesis of monoclinic structured BiVO4 spindly microtubes in deep eutectic solvent and their application for dye degrada...IntroductionExperimentalSynthesis of spindly BiVO4 microtubesCharacterizationPhotocatalytic test

Results and discussionCharacterization of spindly BiVO4 microtubesFormation of spindly BiVO4 microtubesBand gap of the spindly BiVO4 microtubesPhotocatalytic property of spindly BiVO4 microtubes

ConclusionsAcknowledgementReferences