Available online at www.sciencedirect.com

www.elsevier.com/locate/micromeso

Microporous and Mesoporous Materials 112 (2008) 641–646

Short Communication

Polymer-induced generation of anatase TiO2 hollow nanostructures

Xiaoxu Li a, Yujie Xiong b, Lufeng Zou a, Mingtai Wang c, Yi Xie a,*

a Hefei National Laboratory of Physical Sciences at Microscale, Hefei, Anhui 230026, PR Chinab Department of Chemistry, University of Washington, Seattle, WA 98915-1700, USA

c Institute of Plasma Physics, Chinese Academy of Science, Hefei, Anhui 230031, PR China

Received 28 May 2007; received in revised form 4 October 2007; accepted 9 October 2007Available online 4 November 2007

Abstract

In this letter, we presented a polymer-induced generation of anatase TiO2 hollow nanostructures. Amphiphilic polymer PEG-400 wasemployed for the first time to slow down the hydrolization reaction of potassium titanium oxalate and control over the crystal growthand organization. Hollow spheres with a diameter of around 100 nm were finally obtained and demonstrated good dye-sensitized solarcell performances. The shape evolution of anatase TiO2 nanostructures indicated that the formation mechanism was based on an Ost-wald ripening process. We believe this work is a facile synthetic approach to hollow spheres. Besides, it can also give a contribution to theunderstanding of Ostwald ripening process.� 2007 Elsevier Inc. All rights reserved.

Keywords: Titanium dioxide; Dye-sensitized solar cell; Potassium titanium oxalate; Ostwald ripening process

1. Introduction

TiO2 has been intensively investigated for their applica-tions in photo-catalysis and solar cells [1,2]. Comparedwith other photovoltaic materials, anatase phase titaniais outstanding for its stability and wide band gap, andthen widely used in solid state devices [3]. To improvethe materials performances as well as understand theimpact of morphology on solar energy conversion effi-ciency, various nanostructures have been prepared [4–6].Since the last decade, hollow spheres are receiving increas-ing attention [7–10]. Micro/nanospheres with hollow inte-riors offer prospects on catalysis, drug delivery, photoniccrystals and nanodevices [11–13]. Their tailored propertiesnot only improve the energy conversion performances,but also contribute to optical and electronic applications[14]. Besides the soft template and hard template methods[15,16], Zeng and co-workers fabricated anatase TiO2 hol-low spheres using TiF4 precursor under hydrothermal

1387-1811/$ - see front matter � 2007 Elsevier Inc. All rights reserved.

doi:10.1016/j.micromeso.2007.10.034

* Corresponding author. Tel./fax: +86 551 3603987.E-mail address: yxie@ustc.edu.cn (Y. Xie).

conditions [17]. Thiourea, urea and HF were introducedas additives and micron sized hollow spheres were finallyobtained. Recently, Liu and co-workers reported the syn-thesis of porous hollow TiO2 aggregates using Ti(SO4)2

and NH4F precursors [18]. This approach is based onchemical etching mechanism and yields nanoparticleaggregates with diameters around 500–800 nm. Althoughvarious methods have been achieved, the spheres are gen-erally micron sized with low photo-activities. Since thesphere sizes and shell thicknesses heavily influence thephoto-energy conversion properties, it is therefore desir-able to develop convenient alternative strategies for smal-ler and thinner hollow spheres with better solar cellperformances.

In this letter, we report the polymer-induced generationof anatase TiO2 hollow nanostructures. Amphiphilic poly-mer PEG-400 was employed for the first time to slow downthe hydrolization reaction of potassium titanium oxalateand control over the crystal growth and organization. Uni-form hollow spheres with a diameter of around 100 nmwere finally obtained and demonstrated good dye-sensi-tized solar cell performances.

642 X. Li et al. / Microporous and Mesoporous Materials 112 (2008) 641–646

2. Experimental

All the chemicals were of analytical grade and were usedas received. In a typical process, 0.177 g (0.5 mmol) ofpotassium titanium oxalate (PTO) was dissolved in 30 mlof deionized water, followed by the addition of 15 ml ofPEG-400 to form a light yellow homogeneous solution.The solution was vigorously stirred for 5 min, and thenmoved to a 50 ml Teflon-lined stainless steel autoclave.The hydrothermal autoclave was then sealed and heatedat 180 �C in an oven. After 6–16 h, the autoclave wasallowed to cool down to room temperature naturally.The white precipitate was collected by centrifuging andwashing with distilled water for several times, and thendried in a vacuum at 50 �C for 4 h. In the control experi-ments, 15 ml vinyl-pyrrolidone or methyl-pyrrolidone wasused respectively to take the place of PEG-400. Otherexperimental conditions and parameters remained thesame. The white precipitate was also centrifuged, washedand dried in a vacuum at 50 �C for 4 h.

3. Results and discussion

3.1. X-ray diffraction

Structural information of the final product could begiven by powder X-ray diffraction (XRD) analysis, whichwas carried out on a Rigaku D/max-cA X-ray diffractom-eter with Cu Ka radiation (k = 1.54178 A). In the XRDpattern of sample (Fig. 1), all the observed peaks can beindexed to a pure tetragonal anatase phase (JCPDS card,21-1272) with lattice constants a = 3.785 A andc = 9.513 A. No peak of other phases was observed, whichindicates that the product is pure and well crystallized.

3.2. Surface area measurements

The surface area of the sample was determined on aMicromeritics ASAP 2020 analyzer. Nitrogen adsorp-

Fig. 1. Powder XRD pattern of the final product prepared under PEGcapping conditions.

tion–desorption isotherms were measured (Fig. 2), indicat-ing the product is porous.

The Brunauer–Emmett–Teller (BET) method was usedto calculate the surface area and the sample’s BET surfacearea is 101 m2/g.

3.3. Electron microscope measurements

The morphology was examined by the field emissionscanning electron microscope (FESEM, JEOL JSM-6700F) and transmission electron microscope (TEM, Hit-achi H-800) at an acceleration voltage of 200 kV.

3.4. Mechanism discussion

The mechanism of this synthesis can be concluded as anOstwald ripening process. It is well known that Ostwaldripening refers to the process in which larger crystals growat the expense of smaller ones. In recent years, this phe-nomenon has been applied to generation of hollow nano-structures. Since the crystallite of different sizes mayrecrystallize to reach equilibrium in the aqueous solutionsand larger particles are relatively immobile, random solidaggregates therefore tend to generate void inside [19]. Atypical Ostwald ripening process includes three stages: col-loid aggregates, homogenous core/shell structures and hol-low spheres [19,20]. Recrystallization begins with particleconglomeration due to surface energy minimization. Thenredistribution happens in the randomly mixed particlesand generates voids inside. Finally, solid cores may be con-sumed and form incompact vesicles. The entire evolutionprocess exclusively depended on reaction time and differentstages can be observed by taking samples at different hours.In our synthesis, aggregation appeared after 8 h of hydro-thermal treatment with an average diameter of 100–150 nm(Fig. 3A and B). The entire sphere size was small, whichfacilitated the inner particle consuming and evolution.Core/shell structures were clearly observed after 12 h(Fig. 3C and D) and gradually transformed into hollow

Fig. 2. Adsorption/desorption isotherms linear plot of the final product.

Fig. 3. TEM images of the products synthesized using potassium titanium oxalate and PEG-400 in a hydrothermal system at 180 �C for different hours:(A, B) 8 h, (C, D) 12 h, and (E, F) 16 h.

X. Li et al. / Microporous and Mesoporous Materials 112 (2008) 641–646 643

spheres (Fig. 3E and F). Fig. 4 shows typical field emissionscanning electron microscope (FESEM) images of theproduct after 16 h, which are large-scale uniform hollowspheres with diameters around 100 nm. The hollow spheresare porous and the shells are around 20 nm thick. It is wor-thy to mention that a 1:3 PEG/water ratio best yields uni-form hollow spheres. XRD patterns confirmed the anatasephase purity and crystallinity. Lower concentrations ofPEG-400 will lead to irregular solid spheres and higher

Fig. 4. FESEM images of the final anatase hollow spheres obtained in the e

concentrations of PEG-400 will lead to poor crystallizedsolid spheres according to parallel experiments results.

It is well known that the key of oxide nanocrystals gen-eration and complex architectures organization is designinga suitable synthetic route with appropriate hydrolizationspeed and capping reagents. By slowing down the reactionspeed or choosing a suitable precursor at aqueous condi-tions, nanostructures with various size and shape can bereadily achieved [21]. As a typical category of capping

xistence of PEG-400. The scale bar of (A) and (B) is 1 lm and 300 nm.

Fig. 6. Powder XRD pattern of the product prepared under methyl-pyrrolidone capping conditions.

644 X. Li et al. / Microporous and Mesoporous Materials 112 (2008) 641–646

reagents, PEG-400 is favored for its stronger stabilizingabilities resulted from its long chain. To prove the necessityof polymer and make clear the impacts of different cappingreagents, comparative experiments were conducted usingsmall molecular cyclic amides such as vinyl-pyrrolidoneor methyl-pyrrolidone. According to TEM observation,typical colloid aggregates can be found after 6 h and thengradually turned to core/shell structures, as indicated inFig. 5A and B. Representative FESEM image of these sam-ples with different magnification were shown in Fig. 5Dand E, in which the spheres have diameters around500 nm and rough surface. It is also observed that rutilenanorods may grow epitaxially at the surface, confirmedby its XRD pattern (Fig. 6). The XRD pattern of the prod-uct prepared using methyl-pyrrolidone can be indexed to amixture of anatase and rutile TiO2. Its composition ishighly dependent on concentration of amides. This phe-nomenon can be explained by titania crystal growth habi-tation since vinyl-pyrrolidone is more basic than methyl-pyrrolidone and rutile phase prefer an acidic environment.Although homogenous core/shell structures can be success-fully prepared using these cyclic amides, the relatively bigsize hindered their evolution. When we prolonged the reac-tion time, inner particles failed to dissolve and ended upwith collapsed morphology (Fig. 5C). Schematic illustra-tion of the process is described in Scheme 1. This phenom-

Fig. 5. A–C are TEM images of the anatase TiO2 sample obtained at diffehydrothermal system at 180 �C. (A) Typical colloid aggregates after 6 h reacti12 h; (D, E) FESEM images of the core/shell structured spheres.

enon confirmed that long chain polymer capping reagentswere critical to shape evolution and particle size.

3.5. Dye-sensitized solar cell (DSSC) performance

evaluation

TiO2 is the one of the most important photo-electrodematerials in solar cells. To evaluate the solar cell perfor-

rent stages using potassium titanium oxalate and vinyl-pyrrolidone in aon; (B) core/shell structured spheres after 8 h; (C) collapsed spheres after

Scheme 1. Schematic of the Ostwald ripening process involved in the formation of TiO2 hollow spheres. Process A is a low speed evolution induced bypolymer capping reagents. Process B is a fast hydrolization which led to larger size spheres and incomplete evolution.

Table 1DSSC performance measured on the electrodes made of P-25 TiO2 and theas-prepared anatase hollow spheres using PEG-400 as capping reagent

Cell name Voc (V) Isc (mA/cm2) Fill factor (FF) Efficiency (g)

P25-1 0.83 1.56 0.64 0.82P25-2 0.83 2.19 0.67 1.21P25-3 0.82 1.88 0.65 1.00Anatase 1 0.73 2.50 0.63 1.14Anatase 2 0.77 2.50 0.65 1.26Anatase 3 0.73 3.44 0.66 1.64

X. Li et al. / Microporous and Mesoporous Materials 112 (2008) 641–646 645

mances of the anatase hollow spheres, we compared theproduct prepared using polymer capping reagents withcommercial Degussa P-25 titania. The assembly of DSSCwas fabricated by the following method in the literature[22]. TiO2 colloidal suspensions were spin-coated ontoTCO glass (transparent conducting oxide glass, F:SnO2,sheet resistance 8 X/square, TEC-8, LOF, 3 mm, USA),dried at 100 �C for 30 min and heated in the air at 450 �Cfor 30 min. The obtained photo-electrode with porousand nanostructured TiO2 layer was cooled naturally andthen immersed in an ethanol solution (5.0 · 10�4 M) ofbis (tetrabutylammonium) cis-dithiocyanatobis (2,2 0-bipyr-idine-4 04 0-dicarboxylate) ruthenium(II) (dye N719) at roomtemperature for 24 h. H2PtCl6 was sprayed on TCO glassand heated in the air at 410 �C for 20 min to prepare thecounter electrode. Finally, electrolyte (LiI, I2, tri-n-butylphosphate, 2-dimethyl-3-propylimidazolium iodide, 3-methoxypropionitrile) was injected and the cell was sealedfor DSSC characterizations. The characteristic wasrepeated 3 times (cell name: Anatase 1–3 and P25 1–3)and measured under a solar simulator (Xe Light, 300SQ,Changchun Institute of Optics Fine Mechanics and Phys-ics, Chinese Academy of Science, spot size 30 cm · 30 cm)with an illumination of AM 1.5 (1 sun, 100 mW cm�2)and automatically outputted by Testpoint characteristictesting software. Primary performance evaluation parame-ters were determined and listed in Table 1. Isc is the maxi-mum current through the load under short-circuitconditions and Voc the maximum voltage obtainable atthe load under open-circuit conditions. Fill factor is definedas FF = gPAc/(VocIsc). Since Ac (surface area of the solarcell) and P (input light irradiance) are constants duringthe tests, higher fill factor, Isc and Voc lead to betterphoto-energy conversion performances. As shown in Table1, the cell has an average short-circuit current of 2.81 mA/cm2, which is consistent with the relatively small crystalline

size indicated by TEM images and large surface area ofproduct. The data of fill factors and photo-energy convert-ing efficiency suggest that the as-prepared anatase hollowspheres product outperformed the commercial P-25, indi-cating its potential use in industry [23].

4. Conclusion

In conclusion, we present a polymer-induced generationof anatase TiO2 hollow nanostructures. With the strongcapping effect, uniform hollow spheres with relatively smallsize and high surface area were obtained via a modifiedOstwald ripening process. As a potential application, theseobtained hollow spheres exhibited excellent DSSC perfor-mance. We believe that this simple approach provided afacile alternative to the synthesis of anatase TiO2 hollowspheres at a large-scale, and may also serve as a directand convincing proof on the Ostwald mechanism for theformation of hollow structures.

Acknowledgments

This work was financially supported by the NationalNatural Science Foundation of China (No. 20621061)

646 X. Li et al. / Microporous and Mesoporous Materials 112 (2008) 641–646

and the state key project of fundamental research fornanomaterials and nanostructures (2005CB623601).

References

[1] B. O’Regan, M. Graetzel, Nature 353 (1991) 737.[2] A. Hagfeldt, M. Graetzel, Acc. Chem. Res 33 (2002) 269.[3] M. Graetzel, Nature 414 (2001) 338.[4] Z.Y. Yuan, W.Z. Zhou, B.L. Su, Chem. Commun. (2002) 1202.[5] D. Li, Y.N. Xia, Nano. Lett. 4 (2004) 933.[6] M. Niederberger, M.H. Bartl, G.D. Stuck, Chem. Mater. 14 (2002)

4364.[7] D.B. Zhang, L.M. Qi, J.M. Ma, H.M. Cheng, Adv. Mater. 14 (2002)

1499.[8] Y.D. Yin, Y. Lu, B. Gates, Y.N. Xia, Chem. Mater. 13 (2001) 1146.[9] X.M. Sun, J.F. Liu, Y.D. Li, Chem. Eur. J. 12 (2006) 2039.

[10] X.X. Li, Y.J. Xiong, Z.Q. Li, Y. Xie, Inorg. Chem. 45 (2006) 3493.[11] X.J. Zhang, D. Li, Angew. Chem. Int. Ed. 45 (2006) 5971.[12] X.C. Jiang, T. Herricks, Y.N. Xia, Adv. Mater. 15 (2003) 1205.

[13] J. Li, H.C. Zeng, Angew. Chem. Int. Ed. 44 (2005) 4342.[14] Y.F. Zhu, J.J. Shi, Z.Y. Zhang, C. Zhang, X.R. Zhang, Anal. Chem.

74 (2002) 120.[15] Y.Z. Li, T. Kunitake, S. Fujikawa, J. Phys. Chem. B 110 (2006)

13000.[16] Z.Y. Zhong, Y.D. Yin, B. Gates, Y.N. Xia, Adv. Mater. 12 (2000)

206.[17] H.G. Yang, H.C. Zeng, J. Phys. Chem. B 108 (2004) 3492.[18] Z.W. Liu, D.D. Sun, P. Guo, J.O. Leckie, Chem. Eur. J. 13 (2007)

1851.[19] B. Liu, H.C. Zeng, Small 5 (2005) 566.[20] X.W. Lou, Y. Wang, C.L. Yuan, J.Y. Lee, L.A. Archer, Adv. Mater.

18 (2006) 2325.[21] Z.H. Zhang, X.H. Zhong, S.H. Liu, D.F. Li, M.Y. Han, Angew.

Chem. Int. Ed. 44 (2005) 3466.[22] C.N. Zhang, M.T. Wang, F. Li, M.G. Kong, L. Guo, W.W. Xu, X.G.

Zhu, K.J. Wang, Plasma Sci. Technol. 7 (2005) 2962.[23] S. Nakade, Y. Saito, W. Kubo, T. Kitamura, Y. Wada, S. Yanagida,

J. Phys. Chem. B 107 (2003) 8607.