DOI: 10.1002/chem.201101410

Mesoporous Au/TiO2 Nanocomposite Microspheres for Visible-LightPhotocatalysis

Guannan Wang, Xiaofei Wang, Junfeng Liu,* and Xiaoming Sun[a]

Introduction

Since the first report on the use of TiO2 as a photocatalystby Fujishima and Honda,[1] numerous studies towards im-proving its photocatalytic efficiency have been reported,such as band-tailoring by doping transition metals or non-metal atoms and the encapsulation of I2 or organic-dye-sen-sitization to obtain a visible-light photoresponse.[2–5] In thisregard, one alternative that is attracting increasing interestto effect photosensitization is the use of plasmonic nanopar-ticles. Plasmonic nanoparticles that are made of noblemetals (such as Au and Ag) are known to serve as sensitiz-ers to enhance the absorption of large-band-gap photocata-lysts over a broad UV/Vis/NIR spectroscopic range, owingto their strong surface plasmon resonance (SPR).[6–12] In ad-dition, part of the interest in employing Au(Ag)/TiO2 inphotocatalysis has arisen from their use as heterogeneouscatalysts. However, studies that deal with the photocatalyticactivity of Au/TiO2 are still scarce. Very recently, Au sup-ported on P25 titania was reported to show visible-light-driven water-splitting activity.[11] Other approaches to en-hance the performance of TiO2 include increasing the sur-face area by nanostructuring, because the physicochemicalproperties of TiO2 nanostructures are size- and morphology-dependent.[13, 14] Mesoporous TiO2 has also attracted interestbecause its microstructure offers several unique features: itsopen porosity in often hierarchical pore structures allowsrapid access for reactive gases and even liquids.[15] Thus, the

design of an ideal photocatalyst microstructure that com-bines well-defined nanocrystals (NCs), a mesoporous struc-ture, and plasmonic noble-metal sensitizing would be ofgreat interest.

Recently, an emulsion-based bottom-up self-assembly(EBS) process was proposed as a superior approach to pre-formed, monodisperse NCs into colloidal microspheres.[16]

An additional merit of this process was that mesoporousstructures could be achieved by subsequent annealing toremove the organic components within the micro-spheres.[17,18] The components and properties of the micro-spheres could be adjusted by employing various well-definedNCs as building blocks; thus, by combining the properties ofeach component would achieve cooperatively enhanced per-formance. This EBS approach made the design of ideal mi-crostructures with combined function for advanced photoca-talysis possible.

Herein, we report the use of the EBS approach to designa Au/TiO2 mesoporous nanocomposite structure that has en-hanced visible-light photocatalytic activity. Au was used asthe sensitizer because it does not undergo corrosion underphotocatalytic conditions, and it exhibits a strong SPR ab-sorption in the visible-light region owing to the collectiveexcitation of electrons in the nanoparticles.[19] The strong re-sponse toward visible light was ascribed to the mesoporousstructure, with high specific surface area (above 270 m2 g�1)and highly open mesopores (about 5 nm), which induced ex-cellent adsorption ability for concentrating organic mole-cules at the vicinity of the microspheres and strong SPR ab-sorption of metallic Au components that were stably con-fined in the hybrid microspheres.

Abstract: Mesoporous Au/TiO2 nano-composite microspheres have been syn-thesized by using a microemulsion-based bottom-up self-assembly (EBS)process starting from monodispersegold and titania nanocrystals as build-ing blocks. The microspheres had largesurface areas (above 270 m2 g�1) andopen mesopores (about 5 nm), which

led to the adsorption-driven concentra-tion of organic molecules in the vicinityof the microspheres. Au nanoparticles,which were stably confined within the

microspheres, enhanced the absorptionover the broad UV/Vis/NIR spectro-scopic range, owing to their strong sur-face plasmon resonance (SPR); asa result, the Au nanoparticles promot-ed the visible-light photo-induced deg-radation of organic compounds.

Keywords: composite materials ·gold · mesoporous materials · pho-tochemistry · titanium

[a] G. Wang, X. Wang, Prof. J. Liu, Prof. X. SunState Key Laboratory of Chemical Resource EngineeringBeijing University of Chemical TechnologyBeijing, 100029 (P. R. China)E-mail : ljf@mail.buct.edu.cn

Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/chem.201101410.

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Results and Discussion

The concept for forming mesoporous Au/TiO2 nanocompo-site microspheres by using the EBS approach was based onan oil-in-water (O/W) microemulsion system (Figure 1).

Monodisperse gold and TiO2 NCs were chosen as buildingblocks. Oil-dispersed Au and TiO2 NCs that were encapsu-lated with ligands (oleylamine and oleic acid, respectively)were pre-mixed and added to a water phase that containedsodium dodecyl sulfate (SDS). After ultrasonic emulsifica-tion, a stable O/W microemulsion system was obtained. TheNCs were confined in microemulsion oil droplets that werestabilized by the surfactants in the aqueous phase. Subse-quently, following the evaporation of cyclohexane from theoil microemulsion droplets, the NCs in the droplets becameconcentrated and spontaneously self-assembled into colloi-dal microspheres that were held together by hydrophobicvan der Waals interactions between the surfactant ligandsabsorbed onto the NCs. Finally, after centrifugation, the col-loidal microspheres were calcined in air to remove the or-ganic capping ligands on the Au and TiO2 NCs and affordthe Au/TiO2 hybrid mesoporous spheres.

This approach had several advantages: 1) TiO2 and AuNCs were pre-synthesized with uniform morphologies andproperties; 2) the Au NCs were well-dispersed in a TiO2

matrix, which enhanced the light absorption; 3) the mesopo-rous structure was created by calcination of the organiccomponents among the NCs; and 4) grain-growth and Au-aggregation were sterically inhibited by the TiO2 matrixduring the calcination process. These advantages endowedthe mesoporous Au/TiO2 nanocomposite microspheres withhigh photocatalytic activity for degrading dyes. In Au cataly-sis, it is well-known that Au loading is an important parame-ter in determining the activity of the resulting material.[20]

Typically, the most-active materials in heterogeneous cataly-sis contain around 1 wt % Au loading. Herein, we preparedfour samples of Au/TiO2 microspheres with differentamounts of gold by using the EBS approach and comparedtheir activities towards visible-light photocatalysis with thatof TiO2, which was devoid of any gold nanoparticles.

The formation of Au/TiO2 nanocomposite microsphereswas clearly demonstrated by TEM, SEM, XRD, elementalanalysis, and pore-size-distribution analysis. Figure 2 A, Bshow TEM images of dispersions of monodisperse titaniananorods (about 3 nm in diameter and 20 nm in length) andgold nanoparticles (about 10 nm in diameter) in cyclohex-ane. After being assembled by using the EBS approach, col-loidal spheres of Au/TiO2 of about 100 nm in diameter wereobtained, in which the gold loading scarcely influenced theirmorphology or size (Figure 2 C–G). Almost all of the NCswere incorporated inside the colloidal spheres, and isolatedparticles were hardly observed outside, which suggested thatthere was no mass-loss during the assembly process; theratio of the Au and TiO2 in the resulting microspheres wasconsistent with that used initially. HRTEM was used to fur-ther analyze the Au/TiO2 samples. The colloidal spheres

Figure 1. Illustration of the concept for forming mesoporous Au/TiO2

nanocomposite microspheres by using the EBS approach.

Figure 2. TEM images of A) as-prepared TiO2 nanocrystals; B) Au nano-particles; C–G) Au/TiO2 samples with different amounts of gold: 0 %(C), 0.5 wt % (D), 1 wt % (E), 2.5 wt % (F), 5 wt % (G); H) HRTEMimages of Au/TiO2 (1 wt % Au).

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were constituted of TiO2 nanorods and Au nanoparticles,which was consistent with the TEM images of the initialNCs (Figure 2 H). The lattice fringes of individual Au nano-particles revealed a lattice spacing of 0.23 �, which was inagreement with the Au (111) plane. Distinct lattice fringesof 3.5 � were observed on the nanorods, which correspond-ed to the (101) planes of the anatase (TiO2) phase.

Mesoporous Au/TiO2 nanocomposite microspheres wereobtained by calcination of the colloidal spheres at 350 8C inair (Figure 3 A). The chemical composition was confirmed

by EDX and ICP analysis. Taking 1 wt % Au/TiO2 sample asan example, strong Ti and O signals and a weak Au signalwere detected in the EDX spectrum, and the relative inten-sity of the Au signal corresponded to the content in thesample (Figure 3 B). Sulfur was also detected, which indicat-ed the doping of titania with sulfur from the surfactant(SDS) during the EBS process. ICP analysis of 1 wt % Au/TiO2 further confirmed the content of Au in the sample,which was about 0.871 %. Elemental analysis showed thatonly trace amounts of organic molecules existed, which indi-cated that the surfactants were effectively removed underthese conditions (see the Supporting Information, Table S1).Moreover, about 2.0 wt % of sulfur was detected, which wasconsistent with the EDX results. The microstructures of as-prepared Au/TiO2 microspheres were examined by XRD.All of the XRD patterns clearly showed the crystallized ana-tase phase of the TiO2 nanocrystals (JCPDS card number84-1285; space group I41/amd (141); a= b=3.784 �, c=

9.512 �). Characteristic peaks of Au NCs were absent in theXRD patterns, owing to the small amount of gold in themesoporous spheres (Figure 4).

The mesoporous structure of the Au/TiO2 microsphereswas verified by N2-adsorption/desorption isotherms at 77 K.As shown in Figure 5, Au/TiO2 (1 wt %) exhibited a type-IVisotherm and a type-H2 hysteresis loop, which are typical ofmesoporous material structures with ink-bottle pores. TheBarrett–Joyner–Halenda (BJH) pore-size distributioncurves, which were obtained by analysis of the desorptioncurve, indicated that Au/TiO2 mesoporous spheres (Figure 5,inset) possessed pores with sizes of about 5 nm. The Bruna-uer–Emmett–Teller (BET) specific surface areas of Au/TiO2

mesoporous spheres were 277, 324, 303, 268, and 260 m2 g�1

for 0, 0.5, 1, 2.5, and 5 wt % Au/TiO2, respectively, comparedwith 203 m2 g�1 for TiO2 NCs and 50 m2 g�1 for P25.

To investigate the influence of the presence of gold on thephotocatalytic activity, UV/Vis absorption spectra of Au/TiO2 mesoporous microspheres were recorded and com-pared with P25 and TiO2 NCs (Figure 6). P25 showed strong

Figure 3. A) SEM image and B) the corresponding EDS spectrum ofmesoporous Au/TiO2 microspheres (1 wt % Au).

Figure 4. XRD patterns of the as-prepared Au/TiO2 samples with differ-ent amounts of gold: a) 0%, b) 0.5 wt %, c) 1 wt %, d) 2.5 wt %,e) 5 wt %.

Figure 5. N2-adsorption/desorption isotherms of the as-prepared mesopo-rous Au/TiO2 spheres (1 wt % Au). Inset: pore-size distributions calculat-ed by using the BJH method.

Figure 6. UV/Vis absorption spectra plotted as the Kubelka–Munk func-tion of the reflectance (R). Inset: photographs of the photocatalysts:a) P25, b) TiO2 NCs, c) pure TiO2 mesoporous spheres, and d) mesopo-rous Au/TiO2 spheres (1 wt % Au). The dashed vertical line indicates thecutoff wavelength of the filter used in the photocatalytic experiments.

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absorption under UV irradiation (l<380 nm), owing to theband-gap transition of the TiO2 semiconductor. For TiO2

NCs and TiO2 mesoporous spheres, which showed a paleyellow color compared with the white P25 NCs (Figure 6,inset), the optical absorption shifted slightly to higher wave-length in the UV range, owing to S- or N-doping, but almostno absorption was observed in the visible region. The UV/Vis absorption of Au/TiO2 mesoporous spheres exhibited anobvious additional peak in the visible region between 500and 700 nm, which was typical of a surface plasmon Auband. Furthermore, photographs of the Au/TiO2 mesoporousspheres showed a dark red, powdered appearance, therebysuggesting that Au nanoparticles had been introduced intothe mesoporous spheres, which should substantially improveits photoresponse in the visible region.

The photocatalytic properties of the as-prepared sampleswere demonstrated by the degradation of organic com-pounds under visible-light irradiation (Figure 7). Solutions

of AO7 (200 ppm, 100 mL) containing 50 mg of variouspowdered samples were tested for degradation. Under visi-ble-light excitation, titanium dioxide NCs showed particular-ly weak visible-light photoinduced degradation of AO7 mol-ecules, and only a little better than P25. In contrast, TiO2

mesoporous spheres that were obtained by using the EBSassembly of TiO2 NCs exhibited notably higher photocata-lytic activities, despite having the same anatase phase asTiO2 NCs. By loading Au into the NCs, a significant im-provement in AO7 decomposition was achieved. Au/TiO2

(0.5 wt %) mesoporous spheres showed much-higher photo-catalytic activity, and 1 wt % Au/TiO2 mesoporous sphereswere the most effective as visible-light-driven photocata-lysts: AO7 was effectively degraded within 4 h of visible-light illumination. Further increasing the Au content (2.5and 5 wt %) slightly decreased the catalytic activity. The sta-bility of the Au/TiO2 photocatalyst was also studied. Afterthe reaction had been completed, the Au/TiO2 (1 wt %) mes-oporous spheres were studied again by ICP analysis. Thesemesoporous spheres contained 0.865 % Au in the sample(compared with 0.871 % before the reaction), thereby indi-cating that there was no obvious loss of Au during the reac-

tion. In addition, the size of the Au particles was determinedby HRTEM (see the Supporting Information, Figure S1),which confirmed no size-growth had occurred during the re-action.

The visible-light-driven photocatalytic ability was attribut-ed to the structure of the Au/TiO2 mesoporous nanocompo-site microspheres. These materials showed advanced photo-catalytic-degradation efficiency mainly for two reasons:1) Enhanced absorption in the visible spectrum owing to thestrong surface plasmon resonance of the Au NCs and thedoping of sulfur. The photocatalytic activity of Au/TiO2 un-derwent one of two distinctive mechanisms depending onthe excitation wavelength (UV or visible light). The visible-light photocatalysis arose from the excitation of the goldsurface plasmon.[11] As shown in Figure 8, Au NCs acted as

light harvesters upon photoexcitation, and subsequently in-jected electrons into the TiO2 conduction band. This injec-tion led to the generation of holes in the Au NCs and elec-trons in the TiO2 conduction band. Holes are known toeffect organic degradation as they are a powerful oxidant,and, with electrons, a strong reductant, generate highly reac-tive species, such as superoxide, which is a nonselective oxi-dizing agent for organic pollutants. Moreover, the sensitiza-tion of dyes was not excluded because of the absorbance ofAO7 in the visible region. To further verify the effect of Au,control reactions of the photodegradation of phenol, whichdid not absorb in the visible region, was also performed.The results indicated that the photocatalytic activity was sig-nificantly improved by doping with gold (see the SupportingInformation, Figure S2), which confirmed the mechanismshown in Figure 8. Therefore, Au acted as a sensitizer,through its plasmon resonance, and it acting as catalyst toenhance the dye-sensitization might contribute to the im-provement observed in the visible-light photodegradation ofAO7. This result explained why the photocatalytic activityof Au/TiO2 gradually increased with increasing amounts ofAu, when the amount was less than 1 wt %. The decrease incatalytic activity for Au/TiO2 mesoporous spheres with highAu loading (2.5 and 5 wt%) might be attributed to the pres-ence of excess gold particles, which became recombinationcenters for electrons and holes.[21] In addition, the effectivedoping of sulfur into titania also contributed to the en-hanced photocatalytic activity of TiO2 microspheres. 2) Con-

Figure 7. Photodegradation of AO7 by P25 and by the as-synthesizedmesopores.

Figure 8. Proposed rationalization of the photocatalytic activity of Au/TiO2 upon excitation of the Au surface plasmon band.

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centrating organic molecules through the adsorption of mes-oporous structure. The highly reactive radicals generated inthe photoexcitation process undergo further reactions, suchas initiating the photocatalytic degradation of organic mole-cules. Accordingly, if organic compounds are concentratedat the vicinity of the surface of the photocatalyst, the degra-dation efficiency must be improved. The idea of concentrat-ing organic compounds for degradation has been observedpreviously in the formation of carbon-coated TiO2 nanocrys-tals.[22] We achieved a similar phenomenon with mesoporousmicrospheres, which provided high surface areas and highlyopen mesopores. When the samples reached adsorption sat-uration, about 2.5 % AO7 molecules had been concentratedinto mesoporous structure of TiO2 microspheres; in contrast,little adsorption occurred in unassembled TiO2 NCs (see theSupporting Information, Figure S3). In addition, the Auloading obviously improved the adsorption of the organiccompounds (to 5 %). FTIR analysis of the solid phase ofTiO2 and Au/TiO2 (1 wt %) mesoporous spheres also indicat-ed improved adsorption of AO7 molecules upon Au loading(see the Supporting Information, Figure S4). Reactive radi-cals that were generated from the exposed surface of Au/TiO2 diffused into the bulk solution, and so molecules ofAO7 diffused from the bulk solution into the pores withinthe Au/TiO2 spheres owing to adsorption. Therefore, coun-tercurrent flows of AO7 and reactive radicals presumablyoccurred in the pores during photoexcitation, which im-proved the degradation efficiency.

Conclusion

Au/TiO2 nanocomposite microspheres were synthesizedfrom the self-assembly of Au and TiO2 NCs by using a modi-fied EBS approach. These microspheres adopted a mesopo-rous structure with very high specific surface areas (above270 m2 g�1) after calcination, and thus exhibited excellent ad-sorption ability for concentrating AO7 molecules into theholes of the mesoporous spheres. Gold NCs that were stablyconfined within the spheres enhanced the absorption overthe broad UV/Vis/NIR spectroscopic range and acted aslight harvesters that injected electrons into the semiconduc-tor conduction band. As a result, Au/TiO2 mesoporousspheres promoted the photocatalytic degradation efficiencyof organic molecules under visible light; the optimal amountof Au was about 1 wt %. Considering the unique structureof these hybrid microspheres, these results provide new in-sights in applying this type of nanomaterial in photocataly-sis.

Experimental Section

Synthesis of TiO2 NCs : TiO2 NCs were prepared by solvothermal meth-ods using cyclohexane as the dispersant.[23] In a typical synthesis, oleicacid (7 mL) and cyclohexane (20 mL) were mixed in a dried Teflon auto-clave and stirred for 10 min at room temperature. Tetrabutyltitanate

(1 mL) and triethylamine (5 mL) were added to the mixed solution. Thevessel was then sealed and heated at 180 8C for 24 h. After being cooledto room temperature, the resulting suspension was precipitated by the ad-dition of absolute EtOH and separated via centrifugation. TiO2 NCswere obtained following three successive cycles of washing of the disper-sion in cyclohexane, precipitating with EtOH, and centrifugation.

Synthesis of Au NCs : In a typical reaction,[24] a solution of HAuCl4·3H2Oin oleylamine (2 mL, 20 mg L�1) was added to toluene (6 mL). The mix-ture was then transferred to a 10 mL Teflon autoclave, heated at 80 8C for10 h, and cooled to room temperature. The product was precipitated bythe addition of EtOH and separated by centrifugation. The Au NCs wereredispersed in cyclohexane to give a red dispersion.

Synthesis of Au/TiO2 mesoporous nanocomposite microspheres : To pre-pare mesoporous microspheres, water-dispersed colloidal spheres werefirst synthesized by an EBS assembly of NCs.[16] In a typical reaction,SDS (0.3 g) was added to deionized water (100 mL), and then a solutioncontaining TiO2 NCs and various amount of Au NCs (0, 0.5, 1, 2.5,5 wt %) in cyclohexane (about 10 mL) was added to the aqueous solu-tion. This system was emulsified by ultrasonic treatment under magneticstirring, and heated at 70 8C for 6 h with constant stirring to evaporatethe cyclohexane. After the reaction was cooled to room temperature, theproducts were washed with water and collected, then dried at 70 8C. Mes-oporous microspheres were obtained by calcination of the products in airat 350 8C for 6 h.

Characterization : The morphologies of the photocatalysts were examinedby using TEM (Hitachi H-800), high-resolution TEM (HRTEM; JEOLJEM-2100, operated at 200 kV), field-emission SEM (Zeiss SUPRA 55),FTIR spectroscopy (Nicolet 6700), elemental analysis (vario EL cubeV2.0.1), inductively coupled plasmon (ICP; Shimadzu 7500) and energy-dispersive X-ray spectroscopy (EDX). Powder X-ray diffraction (XRD)patterns of samples were recorded on a Shimadzu XRD-6000 diffractom-eter with Cu Ka radiation (40 kV, 30 mA, l=1.5418 �). BET specificsurface areas and average pore sizes were obtained from the N2-adsorp-tion/desorption isotherms at 77 K (Autosorb-1, Quantachrome). A Hita-chi 3100 UV/Vis system equipped with a diffuse reflectance accessorywas used to record the reflectance spectra of the samples over the range200–800 nm.

Photocatalysis : Photocatalytic experiments were carried out in a home-made glass reactor with recycled water in which the simulated wastewater (200 ppm of Acid Orange 7) was degraded under visible light. Thelight was generated from a Xe lamp (500 W) and filtered with a UV-cutoff filter (Shanghai Lansheng Electronics Co., Ltd. DTB420, l>420 nm).A solution containing the as-prepared catalyst was stirred continuouslywith a magnetic stirrer to ensure uniform mixing. The samples were col-lected at regular intervals and then centrifuged to remove the catalyst.The UV/Vis absorption spectra were obtained by using a UNICO UV-2802PC UV/Visible scanning spectrophotometer.

Acknowledgements

This work was supported by the NSFC, the Beijing Natural ScienceFoundation, the Program for New Century Excellent Talents in Universi-ties, and the 973 Program (2011CBA00503, 2011CB932403).

[1] A. Fujishima, K. Honda, Nature 1972, 238, 37.[2] M. Anpo, M. Takeuchi, J. Catal. 2003, 216, 505.[3] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 2001,

293, 269.[4] S. Usseglio, A. Damin, D. Scarano, S. Bordiga, A. Zecchina, C. Lam-

berti, J. Am. Chem. Soc. 2007, 129, 2822.[5] Y. Y. Liang, H. L. Wang, H. Casalongue, Z. Chen, H. J. Dai, Nano

Res. 2010, 3, 701.

Chem. Eur. J. 2012, 18, 5361 – 5366 � 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemeurj.org 5365

FULL PAPERMesoporous Au/TiO2 Nanocomposite Microspheres

[6] P. Wang, B. B. Huang, X. Y. Qin, X. Y. Zhang, Y. Dai, J. Y. Wei,M. H. Whangbo, Angew. Chem. 2008, 120, 8049; Angew. Chem. Int.Ed. 2008, 47, 7931.

[7] R. Georgekutty, M. K. Seery, S. C. Pillai, J. Phys. Chem. C 2008, 112,13563.

[8] J. Yu, G. Dai, B. Huang, J. Phys. Chem. C 2009, 113, 16394.[9] P. Li, Z. Wei, T. Wu, Q. Peng, Y. Li, J. Am. Chem. Soc. 2011, 133,

5660.[10] A. Primo, A. Corma, H. Garcia, Phys. Chem. Chem. Phys. 2011, 13,

886.[11] C. G. Silva, R. Jua�rez, T. Marino, R. Molinari, H. Garcia, J. Am.

Chem. Soc. 2011, 133, 595.[12] S. Navalon, M. Miguel, R. Martin, M. Alvaro, H. Garcia, J. Am.

Chem. Soc. 2011, 133, 2218.[13] C. Liu, S. Yang, ACS Nano 2009, 3, 1025.[14] X. Han, Q. Kuang, M. Jin, Z. Xie, L. Zheng, J. Am. Chem. Soc.

2009, 131, 3152.[15] P. Hartmann, D. K. Lee, B. M. Smarsly, J. Janek, ACS Nano 2010, 4,

3147.[16] F. Bai, D. Wang, Z. Huo, W. Chen, L. Liu, X. Liang, C. Chen, X.

Wang, Q. Peng, Y. Li, Angew. Chem. 2007, 119, 6770; Angew. Chem.Int. Ed. 2007, 46, 6650.

[17] C. Chen, C. Nan, D. Wang, Q. Su, H. Duan, X. Liu, L. Zhang, D.Chu, W. Song, Q. Peng, Y. Li, Angew. Chem. 2011, 123, 3809;Angew. Chem. Int. Ed. 2011, 50, 3725.

[18] Q. Zhang, J. B. Joo, Z. D. Lu, M. Dahl, D. Oliveira, M. M. Ye, Y. D.Yin, Nano Res. 2011, 4, 103.

[19] C. J. Orendorff, T. K. Sau, C. J. Murphy, Small 2006, 2, 636.[20] A. Corma, H. Garcia, Chem. Soc. Rev. 2008, 37, 2096.[21] H. X. Li, Z. F. Bian, J. Zhu, Y. N. Huo, H. Li, Y. F. Lu, J. Am. Chem.

Soc. 2007, 129, 4538.[22] Y. C. Hsu, H. C. Lin, C. W. Lue, Y. T. Liao, C. M. Yang, Appl. Catal.

B 2009, 89, 309.[23] X. L. Li, Q. Peng, J. X. Yi, X. Wang, Y. Li, Chem. Eur. J. 2006, 12,

2383.[24] C. M. Shen, C. Hui, T. Z. Yang, C. W. Xiao, J. F. Tian, L. H. Bao,

S. T. Chen, H. Ding, H. J. Gao, Chem. Mater. 2008, 20, 6939.

Received: May 9, 2011Revised: October 11, 2011

Published online: March 13, 2012

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