07) 4283–4286www.elsevier.com/locate/matlet

Materials Letters 61 (20

Synthesis and characterization of Ce-doped mesoporous anatasewith long-range ordered mesostructure

Shuai Yuan a,⁎, Yi Chen a, Liyi Shi a,⁎, Jianhui Fang a, Jianping Zhang a,Jinlong Zhang b, Hiromi Yamashita c

a Research Center of Nanoscience and Nanotechnology, Shanghai University, 99 Shangda Road, Shanghai 200444, PR Chinab Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology,

130 Meilong Road, Shanghai 200237, PR Chinac Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan

Received 22 December 2006; accepted 27 January 2007Available online 6 February 2007

Abstract

The effect of cerium cation on the formation of titania crystallites using H+ as catalyst under mild condition has been investigated. The resultsindicated that the presence of cerium cation can improve the selectivity of producing anatase crystallites and inhibit the growth of crystallites. TheCe-doped mesoporous titania with highly crystallized pore walls consisting of anatase nanoparticles was synthesized by anatase crystallitesassembly. The long-range ordered mesostructure was characterized by low angle and wide angle X-ray diffraction (XRD), N2 adsorption–desorption, transmission electron microscopy (TEM) and selected area electron diffraction (SAED).© 2007 Elsevier B.V. All rights reserved.

Keywords: Mesoporous; Nanomaterials; Anatase; Crystal structure; Cerium cation

1. Introduction

According to the pore diameter (d) of porous solids, they canbe divided into three categories, i.e., microporous (db2 nm),mesoporous (2 nmbdb50 nm) and macroporous (dN50 nm)materials [1]. Zeolites, the members of a great family of micro-porous materials, have been applied in many fields for a longtime [2]. However, the blossom of mesoporous materials isconsidered to start from the successful synthesis of MCM-41 byMobil in 1992 [3]. The difference between zeolites and meso-porous materials is not only in the pore size but also in thecrystal structure of the pore walls. Zeolites have crystallineframeworks. On the contrary, mesoporous materials are usuallyconstituted of amorphous pore walls and their applications arelimited by poor thermostability and hydrothermostability. Re-cently, more and more efforts have been done on the preparationof mesoporous materials with crystallized walls to overcome theshortages of mesoporous materials with amorphous walls [4–7].

⁎ Corresponding authors. Tel./fax: +86 21 66135215.E-mail addresses: s.yuan@shu.edu.cn (S. Yuan), sly@snpc.org.cn (L. Shi).

0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2007.01.086

Mesoporous titania with highly crystallized walls is animportant kind of mesoporous material and has moreadvantages than titania nanocrystals because of its high surfacearea, long-range ordered porous structure, facile recovery andlow risk to human health [8]. For example, the rare earth-dopedmesoporous titania combining the advantages of mesoporousmaterials and nanomaterials exhibited more applications incatalysis and optics than rare earth-doped titania nanocrystals[9,10].

Based on previous researches, the cerium cation-doped me-soporous titania with long-range order and highly crystallizedpore walls was synthesized by using crystalline nanoparticles asassembly units [11,12].

2. Experimental section

2.1. Chemicals

Tetrabutyl titanate (Ti(OBu)4) was CP grade. Nitric acid (65–68%) and cerium nitrate (Ce(NO3)3·6H2O) were all AR grade.EO20PO70EO20 (P-123) was a commercial product from Aldrich.

Table 1The XRD results of samples

Samples Anatase Rutile

Size (nm) Fraction (%) Size (nm) Fraction (%)

H1Ce0 2.7 74.3 1.6 25.7H2Ce0 9.0 26.3 25.5 73.7H2Ce5a 8.8 50.6 21.9 49.4H2Ce5b 7.7 60.4 20.1 39.6

4284 S. Yuan et al. / Materials Letters 61 (2007) 4283–4286

2.2. Synthesis

The typical synthesis of crystalline nanoparticles was asfollows: Ti(OC4H9)4 (5 ml, 14.7 mmol) was added to 15 mlaqueous solution of HNO3 (29.4 mmol HNO3) dropwise underviolent stirring at 313 K in 20 min. After stirring for additional60 min, the mixture was allowed to stand until two layersseparated. The upper layer of butyl alcohol was removed to geta transparent sol. The transparent sol was kept at 313 K for1 day to get a white precipitate. Then, the precipitate wasfiltrated and dried at 313 K. The product was denoted as H2Ce0,which meant the mole ratio of H+:Ti was equal to 2:1 and themole ratio of Ce:Ti was equal to 0:1. The 5 at.% Ce-dopedtitania samples were prepared by adding 0.3200 g Ce(NO3)3·6H2O (0.74 mmol Ce(NO3)3) into the solution afterthe hydrolysis of Ti(OC4H9)4 for 30 min or before the additionof Ti(OC4H9)4, and the products were denoted as H2Ce5a andH2Ce5b, respectively.

The synthesis of Ce-doped mesoporous titania was asfollows: 14.7 mmol Ti(OBu)4 was added dropwise into 15 mlaqueous solution containing 14.7 mmol HNO3, 0.74 mmol Ce(NO3)3·6H2O and 0.34 mmol P-123, followed by stirring at313 K for 60 min. The sol was transferred from a reactor to alarge open Petri dish. The sol extended sufficiently and formed auniform thin layer. The layer was maintained at 313 K for 48 hand at 413 K for 2 h. At last, the thin layer was calcined at 673 Kfor 1 h in airflow. The heating-up rate was 1 K min− 1, and thecooling rate was 5 K min− 1.

2.3. Characterization

X-ray diffraction (XRD) patterns of all samples werecollected in θ–2θ mode using a Rigaku D/MAX-2550diffractometer (CuKα1 radiation, λ=1.5406 Å). The crystallitesize was estimated by applying the Scherrer equation.The weight fraction of rutile was calculated from the inte-

Fig. 1. (A) Wide angle XRD patterns of samples (a) H1Ce0 (H+:Ti=1:1, Ce:Ti=0:1),(NO3)3 was added after the addition of Ti(OBu)4) and (d) H2Ce5b (H+:Ti=2:1, Cdiffraction peak of H1Ce0 in the range from 15° to 35°.

grated intensities of anatase (101), rutile (110) with the equa-tion Wrutile=1 / (0.886AA/AR+1) [13].

The porous texture of Ce-doped mesoporous titania wasanalyzed from nitrogen adsorption–desorption isotherms at77 K by using a Micromeritics ASAP 2000 system. The samplemorphology was observed under transmission electron micros-copy (TEM) on a 2100 JEOL microscope (200 kV) usingcopper grids.

3. Results and discussion

The wide angle XRD patterns of titania crystallites prepared in acidcondition are shown in Fig. 1(A). The diffraction peaks in the range of15–35° indicate that the samples are mixtures of anatase and rutile. Forexample, from the enlarged view of sample H1Ce0, it can be observedthat the measured curve is fitted by one peak at 25.2° and another peakat 27.6° which are the (101) peak of anatase and the (110) peak ofrutile, respectively. According to the formulas mentioned in charac-terization section, the diameters of anatase and rutile crystallites andtheir weight fractions are summarized in Table 1.

From these data, it can be concluded that high H+ concentration canaccelerate the growth of crystallites of both anatase and rutile,especially the formation of rutile [14,15]. However, the addition ofcerium cations can inhibit the growth of crystallites and the formationof rutile. Compared with sample H2Ce5a, sample H2Ce5b containsmore anatase phase, which indicates that the selectivity of producinganatase crystallites is higher when cerium cation was added before thehydrolysis of Ti(OBu)4. The XRD results reveal that it is possible to

(b) H2Ce0 (H+:Ti = 2:1, Ce:Ti=0:1), (c) H2Ce5a (H+:Ti=2:1, Ce:Ti=0.05:1, Cee:Ti=0.05:1, Ce(NO3)3 was added before the addition of Ti(OBu)4). (B) The

Fig. 2. (A) The XRD patterns and (B) N2 adsorption–desorption patterns of Ce-doped mesoporous titania after calcination at 673 K for 1 h.

4285S. Yuan et al. / Materials Letters 61 (2007) 4283–4286

produce nanosized anatase crystallites as the assembly units for theformation of long-range ordered mesoporous titania.

By the similar process described in previous researches [11], the Ce-doped mesoporous titania with crystallized pore walls was prepared.The XRD patterns of Ce-doped mesoporous titania after calcination areshown in Fig. 2(A). The single peak at 1.25° in the low angle XRDpattern indicates that the long-range order of mesoporous titania is stillreserved, and the d100 value due to (100) reflection from hexagonalmesoporous structure is about 7.0 nm. The wide angle XRD shows thatthe mesoporous titania has a crystallized framework. The average sizeof anatase nanocrystals calculated from the Scherrer formula is about6.7 nm.

The mesostructure of calcined Ce-doped mesoporous titania wasalso characterized by N2 adsorption–desorption. Fig. 2(B) shows thetype IV gas adsorption isotherm, which is due to the large mesoporouschannel. The hysteresis loop is an intermediate between type H1 andH2, which indicates that the Ce-doped mesoporous titania has uniformcylindrical mesopores. The average pore diameter calculated by theBJH model is 3.8 nm, and the BET specific surface area is 194 m2 g− 1.The thickness of the pore wall calculated by subtracting the porediameter from 2d100/√3 is about 4.3 nm.

Fig. 3. TEM image and selected area electron diffraction pattern (inset) ofCe-doped mesoporous titania after calcination at 673 K for 1 h.

Assuming anatase nanocrystals as spherical particles, the surfacearea can also be estimated by S=6 /ρd [16]. Here, ρ is the densityof anatase (3.84 g cm− 3), and d is the average diameter calculatedfrom the FWHM of the (101) peak of anatase. The value obtained is233 m2 g− 1 which is close to the BET specific surface area. The resultindicates that the pore walls of Ce-doped mesoporous titania are madeof highly crystallized anatase.

The TEM image of Ce-doped mesoporous titania is shown in Fig. 3.It can be observed that the long-range ordered mesoporous structurewith an area about 100 nm×200 nm is constructed by nanoparticles.The pore diameter and thickness of the pore walls estimated from theTEM image are about 4.0 nm and 4.5 nm, respectively, which are closeto the analysis results from XRD and N2 adsorption–desorption data.The selected area electron diffraction (SAED) pattern shows that thepore walls are made of only anatase nanocrystals. Why the average sizeof anatase nanocrystals obtained from the Scherrer formula can belarger than the thickness of the pore wall has been discussed by otherresearchers [17].

4. Conclusion

In this paper, the effect of cerium cation on the formation oftitania crystallites using H+ as catalyst under mild condition hasbeen investigated. The presence of cerium cation can improvethe selectivity of producing anatase and inhibit the growth ofcrystallites of both rutile and anatase, especially when ceriumcation was added before the hydrolysis of titania precursor.Based on this result, Ce-doped mesoporous anatase with long-range ordered mesostructure was synthesized by titania crystal-lites assembly. The mesostructure of Ce-doped mesoporousanatase was confirmed by various characterizations.

References

[1] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, J. Pierotti, J.Rouquerol, T. Siemieniewska, Pure Appl. Chem. 57 (1985) 603.

[2] G.J.de A.A. Soler-Illia, C. Sanchez, B. Lebeau, J. Patarin, Chem. Rev. 102(2002) 4093.

[3] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature359 (1992) 710.

[4] D. Grosso, G.J.de A.A. Soler-Illia, E.L. Crepaldi, B. Charleux, C. Sanchez,Adv. Funct. Mater. 13 (2003) 37.

4286 S. Yuan et al. / Materials Letters 61 (2007) 4283–4286

[5] I. Kartini, P. Meredith, J.C. Diniz Da Costa, G.Q. Lu, J. Sol-Gel. Sci.Techn. 31 (2004) 185.

[6] M.S. Wong, E.S. Jeng, J.Y. Ying, Nano Lett. 1 (2001) 637.[7] Y. Fang, H. Hu, J. Am. Chem. Soc. 128 (2006) 10636.[8] R.F. Service, Science 300 (2003) 243.[9] T. López, F. Rojas, R. Alexander-Katz, F. Galindo, A. Balankin, A. Buljan,

J. Solid State Chem. 177 (2004) 1873.[10] K.L. Frindell, J. Tang, J.H. Harreld, G.D. Stucky, Chem. Mater. 16 (2004)

3524.[11] S. Yuan, Q. Sheng, J. Zhang, F. Chen, M. Anpo, Mater. Lett. 58 (2004)

2757.

[12] Q. Sheng, S. Yuan, J. Zhang, F. Chen, Microporous Mesoporous Mater. 87(2006) 177.

[13] H. Luo, C. Wang, Y. Yan, Chem. Mater. 15 (2003) 3841.[14] M. Gopal, W.J.M. Chan, L.C. Jonghe, J. Mater. Sci. 32 (1997) 6001.[15] Z. Tang, J. Zhang, Z. Cheng, Z. Zhang, Chem. Phys. 77 (2002) 314.[16] F. Bosc, A. Ayral, P.-A. Albouy, L. Datas, C. Guizard, Chem. Mater. 16

(2004) 2208.[17] S.Y. Choi, M. Mamak, S. Speakman, N. Chopra, G.A. Ozin, Small 1

(2005) 1.