New Bi2WO6 Nanocages with High Visible-Light-Driven

New Bi2WO6Nanocages with High Visible-Light-Driven

Photocatalytic Activities Prepared in Re?uxing EG

Meng Shang,Wenzhong Wang,*and Haolan Xu

State Key Laboratory of High Performance Ceramics and Super?ne Microstructure,Shanghai Institute

of Ceramics,Chinese Academy of Sciences1295DingXi Road,Shanghai200050,P.R.China

Recei V ed July22,2008;Re V ised Manuscript Recei V ed October5,2008

ABSTRACT:New Bi2WO6nanocages were successfully synthesized with colloidal carbon as the template via a facile re?uxing process in ethylene glycol(EG).The as-prepared nanocages consisted of small nanoparticles with a size of ca.50-80nm.By adjusting the concentration of the precursor,Bi2WO6with different morphologies and microstructures could be obtained.The growth mechanism of such special nanocages was investigated,and EG played a key role in the formation of Bi2WO6nanocages.The Bi2WO6nanocages exhibited excellent visible-light-driven photocatalytic ef?ciency,which was increased to nearly10times that of products prepared by traditional solid-state reactions and commercially available TiO2.Close investigation revealed that the surface area and the porous hollow structure of the as-prepared Bi2WO6nanocages could improve the photocatalytic activities.Moreover, the nanocages could settle naturally in15min,which is bene?cial for separation and recycling,considering their future applications in wastewater treatment.

Introduction

Because of the strong correlation between the shape,size, and structure of nanostructured materials and their physical/ chemical properties,much interest has been focused on design-ing and preparing novel nano-and microstructured materials.1-5 Recently,increasing attention has been paid to the preparation of hollow inorganic spheres of de?ned structure and composition with tailored properties which have immense scienti?c and technological interest.These hollow nanostructures have either been found to have,or are proposed to have,diverse and fascinating applications,such as for catalysts,microcapsule reactors,chemical sensors,delivery and controlled release of drugs,building blocks in the fabrication of photonic band gap crystals,and so on.6-11The most-applied method for the synthesis of hollow micro-or nanospheres by far is the templating of larger colloidal particles via surface precipitation of suitable inorganic molecular precursors on template cores, followed by the removal of the cores by calcination or solvent extraction.12-14Among all the templates,colloidal carbon spheres as novel green templates have received considerable attention due to the availability of colloidal spheres with tightly controlled sizes and surface properties.These monodisperse carbon spheres inherit functional groups and have reactive surfaces,which facilitate the precipitation of metal precursors and nanoparticles.15-17However,most of the hollow spheres fabricated by colloidal carbon were simple metal oxide because of the easy connection between the precursors and functional groups on the surface of the carbon particles.18-21Multicom-ponent oxide hollow spheres are relatively dif?cult to obtain via these templates because most of them are produced by direct precipitation in aqueous solution which breaks down the connection.Thus it is challenging to?nd a way to avoid direct precipitation in the preparation of multicomponent oxide when these carbon spheres are used as hard templates.

Bi2WO6,a kind of multicomponent oxide,is one of the simplest Aurivillius oxides possessing a layered structure.22,23 It has been discovered that Bi2WO6possesses photocatalytic activity,24,25besides ferroelectric piezoelectricity and nonlinear dielectric susceptibility,26which makes it an attractive material. Nowadays,photocatalytic reactions occurring under solar il-lumination have attracted worldwide attention.Taking sunlight into account,it is essential to develop highly effective visible-light-driven photocatalysts.27It is well-known that the photo-catalytic activity is closely interrelated to the size,morphology, and structure of the photocatalysts.28Our group has reported the preparation of Bi2WO6with complex morphologies,namely,?ower-,tire-,helix-,and plate-like shapes,by a facile hydro-thermal process and has found out that the photocatalytic activities could be greatly improved by controlling the nano-structures,decreasing the particle sizes,etc.29Though these are effective ways to enhance the photocatalytic activities by minimizing the particle size so as to achieve higher surface area and more active catalytic sites,it brings another negative effect. When the particle size is decreased to nanoscale,the separation and the recycling of the photocatalysts from the treated water are practical obstacles which hinder their application in industrial use even though they have high photocatalytic activity.Thus, some researchers are working on magnetic photocatalysts that can be separated from the treating system by applying an external magnetic?eld.30Unfortunately,such efforts have not achieved satisfying results yet because introducing magnetic particles into the photocatalysts leads to dramatic decreases in the photocatalytic activity.The search for photocatalysts with high photocatalytic activity that can be separated easily,such as by natural settlement,is still a big challenge.Such photo-catalysts would be more promising considering their future applications in remedying water pollutants.

Herein,for the?rst time we report the fabrication of Bi2WO6 nanocages constructed by oriented nanoparticles via a simple re?ux method in ethylene glycol(EG)with colloidal carbon spheres as the template.A possible formation mechanism of Bi2WO6nanocages is proposed.Bi2WO6with different mor-phologies could be obtained by adjusting the concentration of the precursor.The photodegradation of rhodamine B(RhB)was employed to evaluate the photocatalytic activities of the Bi2WO6 nanocages under visible light(λ>420nm)illumination.It is demonstrated that the Bi2WO6nanocages exhibit excellent visible-light-driven photocatalytic performance.Moreover,these

*Corresponding author.Tel:+862152415295;fax:+862152413122; e-mail:wzwang@https://www.wendangku.net/doc/e212400775.html,.

CRYSTAL GROWTH &DESIGN

2009 VOL.9,NO.2 991–996

10.1021/cg800799a CCC:$40.75 2009American Chemical Society

Published on Web12/11/2008

nanocages could settle naturally in15min,which would be advantageous to their future applications.

Experimental Procedures

All the reagents were of analytical purity and were used as received from Shanghai Chemical Company.In a typical synthesis of colloidal carbon spheres,glucose(～4-8g)was dissolved in deionized water (40mL)to form a clear solution.The solution was then added into a 50mL Te?on-lined autoclave with a stainless steel tank up to80%of the total volume.Then the autoclave was heated at160°C for10h. Subsequently,the autoclave was cooled to room temperature naturally. The resulting precipitates were collected and washed with deionized water and absolute ethanol,then dried at80°C for6h.

The synthesis of Bi2WO6was carried out by using EG as the solvent and carbon sphere as the template.In a typical procedure, Bi(NO3)3·5H2O and Na2WO4·2H2O,in a molar ratio of2:1,were mixed together in100mL of EG so as to form the solution.Then the desired amount of carbon spheres and the EG solution were added into a three-necked?ask equipped with a condenser and re?uxed at160°C for 20h.A rinsing process involving3-5cycles of centrifugation-washing-redispersion was performed with either water or ethanol.The puce samples obtained were oven-dried at80°C for4h and used as the precursors of the nanocages.Carbon-free Bi2WO6nanocages were obtained by calcining the composites at450°C for1h in air to remove the carbon core.Meanwhile,bulk Bi2WO6powder was prepared by a traditional solid-state reaction(named SSR-Bi2WO6)according to a previous study25for comparison with the Bi2WO6nanocages.P25(TiO2 nanoparticles,surface area50m2/g)was purchased from Degussa AG (Germany).

The X-ray diffraction(XRD)patterns of the samples were measured on a D/MAX2250V diffractometer(Rigaku,Japan)using monochro-matized Cu K R(λ)0.15418nm)radiation under40kV and100mA and scanning over the range of10°e2θe70°.The morphologies and microstructures of as-prepared samples were analyzed by scanning electron microscopy(SEM)(JEOL JSM-6700F)and transmission electron microscopy(TEM)(JEOL JEM-2100F,accelerating voltage 200kV).UV-vis diffuse re?ectance spectra of the samples were obtained on an UV-vis spectrophotometer(Hitachi U-3010)using BaSO4as the reference.Nitrogen adsorption-desorption measurements were conducted at77.35K on a Micromeritics Tristar3000analyzer after the samples were degassed at200°C for6h.The Brunauer-Emmett-Teller(BET)surface areas of the products were estimated using the adsorption data.

Photocatalytic activities of the samples were evaluated by the photocatalytic decolorization of rhodamine-B(RhB)under visible light. A500W Xe lamp was used as the light source with a420nm cutoff ?lter to provide visible light irradiation.In every experiment,0.1g of

the photocatalyst was added into100mL of RhB solution(10-5mol/ L).Before illumination,the suspensions were magnetically stirred in the dark for1h to ensure the establishment of an adsorption-desorption equilibrium between the photocatalyst and RhB.Then the solution was exposed to visible light irradiation under magnetic stirring.At given time intervals,a3mL suspension was sampled and centrifuged to remove the photocatalyst particles.Then,the UV-vis adsorption spectrum of the centrifugated solution was recorded using a Hitachi U-3010UV-vis spectrophotometer.

Results and Discussion

The phase and composition of the calcined sample as well as the uncalcined sample were characterized by XRD,as shown in Figure 1.The pattern of Figure1a indicates that the uncalcined Bi2WO6with carbon spheres prepared in the re?ux-ing EG is poorly crystallized.The calcination favors the formation of well-crystallized Bi2WO6comparatively,and the diffraction peaks of this carbon-free Bi2WO6nanocages sample agree well with those of the pure orthorhombic Bi2WO6 according to the JCPDS card no.39-0256,as revealed in Figure 1b.The cell constants of Bi2WO6were calculated to be a) 5.457?,b)16.435?,and c)5.438?.Obviously,the calcined Bi2WO6nanocages sample exhibits higher intensity and narrower diffraction peaks in the XRD pattern compared to the uncalcined Bi2WO6sample,which is due to the enhancement of crystallization.

The carbon spheres were prepared from glucose under hydrothermal conditions at160°C without additives.The narrow size distributions of the?nal products were demonstrated by TEM observation.Figure2a,b shows typical TEM images of such colloidal carbon nanospheres with diameters of500-700 nm.

The morphology and microstructure of the Bi2WO6nanocages prepared by re?uxing EG after calcination were revealed by the SEM images.As shown in Figure3a,all the Bi2WO6 products have a spherical morphology,with diameters ranging from200to400nm.The close-up view of an individual sphere (Figure3b)indicates that the Bi2WO6sample possesses a nanocage-like appearance with a thickness of about50nm. Interestingly,the nanocages are in fact built by nanoparticles with an average size of about70nm,which can be vividly demonstrated by the SEM images(Figure3b).These nanopar-ticles are aligned parallel to the surface of the nanocages with clearly oriented layers like shingles packed togother,pointing toward a common center(Figure3b).More importantly,

the Figure 1.XRD patterns of the calcined and uncalcined Bi2WO6 samples.(a)uncalcined Bi2WO6;(b)calcined Bi2WO6

.

Figure 2.(a)Low-magni?cation TEM image of colloidal carbon nanospheres;(b)high-magni?cation TEM image of colloidal carbon

nanospheres.

Figure3.Typical SEM images of Bi2WO6nanocages.(a)Overall morphology;(b)the close-up view of the nanocages.

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unique nanocage structure with many pores and different diameter sizes,which may serve as the transport paths for small molecules or allow the transmission and multi?ections of visible light within their interior cavities so as to endow these cages with greatly enhanced photocatalytic activities,31are formed in the Bi 2WO 6nanocages as shown in Figure 3.

To obtain a better understanding of the formation mechanism of the Bi 2WO 6nanocages,the products formed at different growth stages were collected for TEM measurements.Figure 4a shows the Bi 2WO 6loaded carbon sphere in re?uxing EG at 160°C for 20h.Owing to the poor crystallinity of this uncalcined Bi 2WO 6with carbon spheres (Figure 1a),the Bi 2WO 6nanoparticles were usually no larger than 20nm decorating the surface of carbon nanospheres.Energy dispersive X-ray analysis (EDX)of the spherical core/shell structure revealed the elemen-tal constitution of the Bi 2WO 6loaded carbon spheres (Figure 4b).The four major peaks corresponded to carbon,bismuth,tungsten,and oxygen,respectively.The obtained nanocages were further investigated by means of TEM.Figure 4c displays a panoramic view of the samples after calcination at 450°C for 1h.The contrast between the dark edge and the pale center provides convincing evidence of the hollow structure.The samples exhibited very similar hollow sphere structures with uniform shape and size when the same carbonaceous spheres were used as the templates.The size of the Bi 2WO 6nanocages

(200-400nm)was about 40%of that of the original templates.In the calcination process,the shells become denser,and the spheres contract and cross-link to form the nanocages,which are smaller replicas of the carbon spheres.8,11,15,16There are also some nanocages with connected centers,which may result from the softening and fusion of the carbonaceous spheres during the re?uxing EG process (Figure 4d,e).The close-up view of an individual nanocage (Figure 4f)indicates that the Bi 2WO 6sample possesses a hollow structure appearance with a shell thickness of around 50nm.The nanocages are built from nanoparticles with a size of 50-80nm,which agrees well with that revealed by the SEM images (Figure 3b).

The concentration of the Bi 2WO 6in the re?uxing EG was tuned to a lower value (0.5M),while the other experimental conditions were kept constant.The TEM image of the product before calcination shown in Figure 5a indicates that only some Bi 2WO 6nanoparticles loaded on the surface of the carbon sphere.This is different from that of Bi 2WO 6samples in Figure 4a.The interesting thing is the formation of the Bi 2WO 6with two-dimensional disk-like morphology (Figure 5b)but not the three-dimensional nanocages after the carbon spheres were removed.The size of the disk-like nanoplate (200-400nm)was also about 40%of that of the original templates,which is similar to that of the nanocages.The high-magni?cation TEM image of this Bi 2WO 6sample was shown in Figure 5c.Clearly,the as-synthesized Bi 2WO 6is also composed of small nanopar-ticles with the size between 50and 80nm,and they complect with each other to form the porous structure.The thickness of the nanoplates could be measured by the SEM image (Figure 5d),which was about 50nm.When the concentration was tuned,the two-dimensional structure was formed.This concentration effect is different from that of the simple metal oxide,which is only the variety of the thickness of the shell.20

So far as we know,most of the hollow spheres fabricated by the colloidal carbon are simple metal oxide.Since the surface of the carbon spheres is hydrophilic and possesses a

distribution

Figure 4.(a)Bi 2WO 6(2M)loaded carbon sphere in re?uxing EG at 160°C for 20h;(b)EDX of the Bi 2WO 6loaded carbon spheres;(c)the panoramic view of Bi 2WO 6nanocages;(d)and (e)The Bi 2WO 6nanocages with connected center;(f)the close-up view of an individual Bi 2WO 6

nanocage.

Figure 5.The TEM images of (a)Bi 2WO 6(0.5M)loaded carbon sphere in re?uxing EG at 160°C for 20h and (b)two-dimensional disk-like porous nanoplates of Bi 2WO 6after the carbon spheres were removed;(c)the high-magni?cation TEM image of the porous disk-like nano-plates of Bi 2WO 6;(d)SEM image of the porous disk-like nanoplates of Bi 2WO 6.

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of OH and C d O groups(Scheme1A),the metal ions could be easily adsorbed onto the surface layer of the carbon spheres to form a composite shell.Then,in the following treatment of sonication,hydrolysis,and calcination,the metal oxide hollow spheres formed.However,the multiple-metal oxide functional materials were rarely synthesized in the form of hollow spheres via this template method.This could be ascribed to the different synthetic routes for the multiple-metal oxides,direct precipitation from the aqueous solution,which will lead to the formation of large particles.These large particles might be dif?cult to connect with the templates of carbon spheres,29which favors the formation of dispersed particles rather than hollow spheres.In the aqueous solution,Bi2WO6particles with a relative large size formed when the two reactants were mixed(formulas a-c), which could not bind the surface layer of the carbon spheres to form the hollow structure.However,on the basis of our experiments,it is found that the utilization of solvent EG favored

the formation of the Bi2WO6nanocages(formulas d and e). When Bi(NO3)3·5H2O and Na2WO4·2H2O were mixed together in EG,no precipitation was observed and the mixture was colorless and transparent under stirring,which was different from that in the aqueous system.This might be attributed to the unique coordination of EG(formula d).The employ of EG produced the complex of M(Bi3+or WO42-)with OH via the coordination(Scheme1B),which could be adsorbed onto the surface layer of the carbon spheres.In the re?uxing EG solution with the template of the carbon spheres,the complex gradually decomposed to release the metal ions,followed by the reaction for Bi2WO6crystals(formula e).As a result,Bi2WO6grains coated carbon spheres were formed(Scheme1C,Figure4a). In the later calcination process(Step2in Scheme1),Bi2WO6 nanoparticles,the building blocks of the nanocages,grew from these small grains.Then the nanocages were formed as replicas of the carbon sphere when the concentration of the precursor was higher(Scheme1D,as shown in Figure4c).When the concentration of the Bi2WO6was tuned to a lower value,though the nanoparticles connected to each other,they may not support the structure as cages after calcination.Therefore,the structure collapses to form the2D disk-like porous nanoplates(Scheme 1E,as show in Figure5c).The Bi2WO6nanocages are desirable due to their low density,small size,and high surface areas.Such porous structure provides ef?cient transport pathways to their interior voids,which is critical for catalyst,delivery,and other applications.Moreover,it is different from the general hollow sphere structure of the simple metal oxide which had only tiny pores and was suspended in the solution.This causes dif?culties for the separation and recycling of these hollow spheres for industrial use.However,the large pores of the nanocages could allow the water or the dye solution get inside of the hollow spheres rapidly.Thus these nanocages not only possess enhanced photocatalytic activities,but also could settle naturally in15 min,as shown in Figure6.

The UV-vis diffuse re?ectance spectrum of the Bi2WO6 nanocages sample is shown in Figure7.According to the spectrum,the Bi2WO6nanocages present the photoabsorption properties from UV light to visible light shorter than470nm due to the band gap transition.The color of the sample is yellow, which is in accordance with its absorption spectrum.The steep shape of the spectrum indicates that the visible light absorption is not due to the transition from the impurity level but due to the band gap transition.32After the calculation,33the band gap (E g)of the Bi2WO6is estimated to be about2.69eV from the onset of the absorption edge(inset of Figure7).This indicates that the Bi2WO6nanocages have a suitable band gap for photocatalytic decomposition of organic contaminants under visible light irradiation.

Rhodamine-B(RhB),a widely used dye,was selected as the model pollutant to evaluate the photocatalytic activity of the

Scheme1.Formation Mechanism of the Bi2WO6

Nanocages

Figure6.The experiments of natural settlement.(a)Bi2WO6nanocages

suspended in water;(b)settled naturally after15

min.

Figure7.The UV-vis diffuse re?ectance spectrum of the Bi2WO6

nanocages sample.

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Bi 2WO 6nanocages.Its characteristic absorption at about 553nm has been used to monitor the photocatalytic degradation process.29Figure 8displays the temporal evolution of the spectra during the photodegradation of RhB mediated by the Bi 2WO 6nanocages sample under visible light illumination (λ>420nm).A rapid decrease of RhB absorption at the wavelength of 553nm is observed,and the spectral maximum shifted from 553to 500nm.The color of the suspension changed gradually from pink to light green,which is in agreement with the shift of the major absorption.It was well-reported that the RhB photodeg-radation occurred via two competitive processes:N-demethy-lation and the destruction of the conjugated structure.34The sharp decrease and shift of the major absorption band within 50min indicate that the Bi 2WO 6sample exhibits excellent photocatalytic activity in the degradation of RhB due to its novel structure of nanocages.

The photocatalytic performances of different photocatalysts were determined by comparing the degradation ef?ciency of RhB with otherwise identical conditions under visible light illumination (λ>420nm)(Figure 9);C was the absorption of RhB at the wavelength of 553nm and C 0was the absorption of RhB after the adsorption equilibrium on Bi 2WO 6nanocages before irradiation.Blank test (RhB without any catalyst)under visible light exhibited little photolysis.The photodegradation ef?ciency was only 5%after 50min,which demonstrated that the degradation of RhB is extremely slow without a photocata-lyst under visible-light illumination.The decrease of RhB with the Bi 2WO 6nanocages in the dark for 1h was similar to that of the blank test,which demonstrated that the absorption of RhB on the as-prepared Bi 2WO 6was negligible after the adsorption -desorption equilibrium was reached.The photo-degradation ef?ciency of RhB by SSR-Bi 2WO 6just reached 10%after 50min of reaction.Similarly,the commercial TiO 2

powders (P25)were obviously inactive under visible light irradiation,as reported by Asahi et al.27b It is interesting to see that in the case of the Bi 2WO 6nanocages the photodegradation ef?ciency of RhB reached nearly 100%after 50min irradiation only,which was increased to nearly 10times that of the SSR-Bi 2WO 6and the commercially available TiO 2.

The photocatalytic activity is affected by many factors which could cooperate with each other and enhance the photocatalytic activity.Among them the particle size and surface area are important factors.35The BET surface area of the Bi 2WO 6nanocages was estimated to be about 14.5m 2/g,which was much higher than that of the reference SSR-Bi 2WO 6(0.6m 2/g).25The high surface area brings not only more surface reached by the visible light and contacted with the RhB but also more active catalytic sites.On the other hand,for randomly generated charge carriers the average diffusion time from the bulk to the surface is given by τ)r 2/π2D ,where r is the grain radius and D is the diffusion coef?cient of the carrier.36If the grain radius decreases,it will reduce the recombination opportunities of the photogenerated electron -hole pairs which could move ef-fectively to the surface to degrade the absorbed RhB molecules.27a The Bi 2WO 6nanocages are built by nanoparticles.The small size of nanoparticles is bene?cial for promoting the photocatalytic ef?ciency because more electron -hole could be separated,transfer to the surface,and act with the organic molecules.Another contribution to the high photocatalytic activity could be the novel nanocages structure.There are plenty of pores in this structure which can be considered as transport paths for the RhB molecules.It bene?ts the reactant molecules to get to the reactive sites on the framework walls of the photocatalysts,whichresultsingoodphotocatalyticperformance.28b,29Furthermore,we believe that the nanocage structure allows transmission and multiple re?ections of visible light within the interior cavity;it thus utilizes the light source more ef?ciently and offers an improved catalytic activity.31

Conclusions

In summary,Bi 2WO 6nanocages were successfully prepared with colloidal carbon spheres as the template via a facile re?uxing process in EG.EG was found to not only act as the reaction medium but also play an important role in the formation of Bi 2WO 6nanocages.Based on the SEM,TEM,and EDX analyses,possible processes for the growth of the Bi 2WO 6nanocages were proposed.It was found that the concentration of the precursor could determine the morphology of the ?nal https://www.wendangku.net/doc/e212400775.html,pared with the corresponding samples prepared by the solid-state reaction and the commercially available TiO 2,the Bi 2WO 6nanocages exhibited higher shape-associated pho-tocatalytic activity (nearly 10times)in the degradation of RhB under visible-light irradiation.This work not only provides an example of shape-dependent photocatalytic properties of the Aurivillius oxides but also opens new possibilities for providing some insight into the design of cage-like hollow sphere of multiple-metal oxide semiconductors,which are desirable due to their low density,high surface area,and properties in natural settlement,separation,and recycling for future applications.Acknowledgment.We acknowledge the ?nancial support from the National Natural Science Foundation of China (No.50672117,No.50732004).

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