This journal is c The Royal Society of Chemistry 2012 Chem. Commun.

Cite this: DOI: 10.1039/c2cc30391d

Synthesis of monodisperse mesoporous TiO2 spheres with tunable sizes

between 0.6 and 3.1 lm and effects of reaction temperature, Ti source

purity, and type of alkylamine on size and monodispersityw

Myun Pyo Hong, Jang Yong Kim, Koteswararao Vemula, Hyun Sung Kim and

Kyung Byung Yoon*

Received 17th January 2012, Accepted 6th March 2012

DOI: 10.1039/c2cc30391d

We report a novel method for synthesizing monodisperse mesoporous

TiO2 spheres (sizes = 0.6–3.1 lm) by hydrolysis of titanium

isopropoxide (TIP) in a mixture of C8–C16 n-alkylamine, water,

and ethanol. The size increases with decreasing temperature,

TIP concentration, and water concentration, and upon purifying

TIP. n-Dodecylamine gives the highest monodispersity.

Mesoporous TiO2 spheres1–23 have been used as photocatalysts,12,13

working electrodes and scattering layer materials for dye sensitized

solar cells,14–17 materials for lithium ion batteries,18 luminescence

enhancing matrices,19 and building blocks for photonic band gap

materials,20 and others.23 In these applications, best results and

reproducibility of functions are expected if the mesoporous TiO2

spheres are uniform in size. Accordingly, a variety of methods

(ESIw 1) have been developed for the syntheses of monodisperse

mesoporous TiO2 spheres (MMTSs).

The methods for producing MMTSs can be divided into

two; template assisted8–11 and autogenesis.1–7,12–23 In the case

of the former, MMTSs are prepared by incorporation of a

Ti source into the templates followed by hydrolysis of the

Ti source or calcination. The size and monodispersity of the

MMTSs are controlled by those of templates. The sizes of

MMTSs obtained by these methods were 500–600 nm11 and

4–27 mm.8–10

In the case of the latter (autogenesis), MMTSs are prepared by

a two-step procedure; preparation of monodisperse amorphous

precursor spheres (MAPSs) and their crystallization into

MMTSs by a hydrothermal reaction or calcination. The size

and monodispersity of the obtained MMTSs are determined

by those of the produced MAPSs. Efforts have therefore been

directed at the development of the methods for preparing

MAPSs with high monodispersity.1–7,12–22

MAPSs have been obtained through a careful control of

the rates of hydrolysis of the Ti sources and the subsequent

condensation of the produced reactive Ti species in various

reaction media. The Ti sources for the formation of MAPSs

usually include tetraalkoxy titanium [Ti(OR)4],2–8,10,11,13–18,20,21,23

TiCl4,1,9,12 tetraalkyl titanium (TiR4).

19,22 The reaction media are

usually consisted of solvent (typically alcohol), water, catalysts,

and the additives that affect the mesopore sizes. The catalysts are

divided into forward catalysts such as acids,1,6,9,12,21 bases

(alkylamines),7,13–17 and salts5,6 and the reverse catalysts such

as hydroxyl propyl cellulose4 and poly alcohols.19 The salts

give rise to increases in rates of hydrolysis and condensation

by increasing the ionic strength in the media.6 In this respect,

salts can be classified as the forward catalysts. Polymer

adsorbents decrease the reaction rate by decreasing the

concentration of the reactive hydrolysed Ti species in the bulk

solution by adsorbing them onto the polymers. In this respect

polymers can be classified as reverse catalysts.

It has been observed that the size of MAPS increases with

decreasing concentration of the Ti source, water, and forward

catalysts, and with increasing concentration of the reverse

catalysts, thus, with decreasing reaction rates. This phenomenon

indicates that the decrease of reaction rate also gives rise to the

decrease of the number of nuclei for MAPS, during the initial

stage of reaction. As a result, if no new nuclei are additionally

formed during the growth stage, the size of MAPS increases

upon decreasing the number of nuclei.

An apparent exception to the above general trend was the

synthesis of MAPS by adding a Ti source [Ti(OBu)4] dissolved

in ethylene glycol (EG) into acetone, where the size of MAPS

increases upon increasing the concentration of the glycolated

Ti [the Ti species chelated by EG {Ti(EG)2}].20 Because of the

fact that this reaction takes place only when the amount of

acetone is large, it is concluded that Ti(EG)2 undergoes

hydrolysis only after replacing the Ti-chelating EG with

acetone. The phenomenon that the size of MAPS increases

with increasing concentration of Ti(EG)220 can be made to fit

into the aforementioned general trend if nuclei already exist in the

EG solution of Ti(EG)2, and Ti(EG)2 only acts as the nutrient to

the initially formed nuclei after removal of EG by acetone.

Regardless of the methods, the sizes of MAPSs obtained by

autogenesis methods were usually submicron (200–1000 nm)

with few exceptions in which the size reached 1.2 mm.

Korea Centre for Artificial Photosynthesis, Centre for MicrocrystalAssembly, Centre for Nanomaterials, Department of Chemistry,Sogang University, Seoul, Korea. E-mail: yoonkb@sogang.ac.kr;Fax: +82-2-706-4269w Electronic supplementary information (ESI) available. See DOI:10.1039/c2cc30391d

ChemComm Dynamic Article Links

www.rsc.org/chemcomm COMMUNICATION

Dow

nloa

ded

by S

ogan

g U

nive

rsity

on

24 M

arch

201

2Pu

blis

hed

on 0

6 M

arch

201

2 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

2CC

3039

1DView Online / Journal Homepage

Chem. Commun. This journal is c The Royal Society of Chemistry 2012

Thus, many important factors that affect the sizes of

MAPSs have been elucidated. However, other important

factors such as temperature (T), purity of the reagent, and

the nature of alkylamine have not been investigated, and the

largest size of MAPS obtained by this method has been 1.2 mm,

and this size limit has not been broken during the last 30 years.

Considering that Ti sources are highly susceptible to hydrolysis,

it is a big challenge to synthesize MAPSs with the sizes larger

than 1.2 mm with high monodispersity.

We now report that the size of MAPS and monodispersity

increase significantly with decreasing T, using purified TIP

as the Ti source, and decreasing concentrations of reagents.

We also report that, out of tested C8–C16 n-alkylamines,

n-dodecylamine (n-DDA) gives the highest monodispersity.

The reagents used for the syntheses of MAPSs in this study

were titanium isopropoxide (TIP), ethanol (EtOH), distilled

deionized water, with n-alkylamines (n-RNH2), where n-alkyl

groups are n-octyl, n-decyl, n-dodecyl, n-tetradecyl, and

n-hexadecyl, respectively (see ESIw 2 for details). TIP was

purified by vacuum-distillation. EtOH was dried by distillation

from an activated 4 A molecular sieve. They were stored in

a glove box charged with dry Ar. In the glove box, dry EtOH

(20 mL, 342.5 mmol), n-DDA (0.124 g, 0.67 mmol), and a

magnetic stirring bar were introduced into a three-necked

round-bottomed flask and each neck was tightly capped with

a septum. Independently, distilled TIP (0.4 mL, 1.35 mmol)

and dry EtOH (0.6 mL) were placed into a vial capped with a

septum (this solution is designated as solution A). Outside the

glove box, 0.16 mL (8.9 mmol) of distilled deionized water was

added into the three-necked flask (this solution is designated as

solution B). Both three-necked flask and vial were placed in an

isopropanol bath whose temperatures were precisely controlled

between 10 and �30 1C using an external chiller. To obtain

�41 1C, acetonitrile and dry ice were used.When the temperatures

of both solutions (solutions A and B) were equilibrated to that

of the bath, solution A was quickly transferred to solution B

with the help of a cannula while solution B was being

vigorously stirred. The standard molar ratio of the reagents

was TIP : n-RNH2 : H2O : EtOH = 1.0 : 0.5 : 6.6 : 253.7.

The obtained MAPSs were crystallized into MMTSs by a

hydrothermal reaction (160 1C for 16 h) and calcination

(500 1C for 6 h), respectively.

The average size of MAPS increased from 700 to 2830 nm as

T decreased from room temperature to �41 1C (Fig. 1a, and

see ESIw 3 for details). The standard deviation (s) of the size

gradually decreased from 28.6 to 1.5% as T decreased from 25

to �20 1C and increased back to 28.6% as T further decreased

to �41 1C (Fig. 1b). Thus, MAPSs with the sizes between 780

and 2580 nm with reasonably high monodispersity (s o 12%)

can be obtained by merely varying T between 5 and �30 1C as

the scanning electron microscope (SEM) images (Fig. 2 and

ESIw 4) show. In particular, with T between 0 and �20 1C,

MAPSs with the average size between 900 and 2040 nm can be

obtained with very high monodispersity (s o 4%).

When unpurified TIP was used as the Ti source, the size of

MAPS also gradually increased as T decreased from room

temperature to �41 1C. However, the average size is smaller

(ranging between 550 and 1710 nm) than the corresponding size

obtained with purified TIP (ranging between 780 and 2580 nm)

and the monodispersity decreased as well (Fig. 1b and ESIw 3).Thus, even when unpurified TIP was used, the size of MAPS

also increased with decreasing T, but less sensitively with respect

to T, and monodispersity also decreased. These phenomena seem

to take place due to pre-existence of nuclei in unpurified TIP,

which were formed by inadvertently introducing moisture during

handling and storage of TIP.

The above results clearly demonstrate the effects of T and

the purity of Ti sources on the size and monodispersity of

MAPS. The increase in size in response to the decrease in T is

also attributed to the formation of a less number of nuclei as T

decreases. The general trend that monodispersity increases

with decreasing T also indicates that the degree of uniformity

of the nuclei growth rates increases as T decreases and as the

purity of Ti sources increases. The decrease in size and the

increase in s upon using unpurified TIP indicate that the total

number of nuclei is higher when unpurified TIP was used due

to the presence of pre-formed nuclei in the unpurified TIP.

Out of C8–C16 n-alkylamines studied in this work, n-DDA

gave the lowest s value (Fig. 1c) at �20 1C. In this respect, the

data shown in Fig. 1 were obtained with n-DDA as n-RNH2,

unless stated otherwise as in Fig. 1c. Upon decreasing the amount

of TIP (Fig. 1d) or water (Fig. 1e) the size of MAPS increased as

shown in the cases of 23 and �20 1C. However, the s value

Fig. 1 The plots of the size (a) and the standard deviation (s) of size(b) ofMAPS vs. temperature (T) and s vs. n-alkyl chain length of n-RNH2

(c), the size of MAPS vs. the amounts of TIP (d), H2O (e), and ethanol (f),

respectively. The vertical bars in a, d, e, and f represent the corresponding

upper and lower size limits.

Dow

nloa

ded

by S

ogan

g U

nive

rsity

on

24 M

arch

201

2Pu

blis

hed

on 0

6 M

arch

201

2 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

2CC

3039

1D

View Online

This journal is c The Royal Society of Chemistry 2012 Chem. Commun.

increased to 20–24%. The gradual increase in the amount of

solvent (EtOH) from 20 to 40, and to 60 mL, respectively, led

to a gradual increase in the average size to 2690 and even to

3060 nm, respectively, although s increased to 10.1 and

16.8%, respectively (Fig. 1f). Although the s value is a bit

high, this is the first case to demonstrate the synthesis of

3.06 mm MAPS.

MAPSs readily transformed intoMMTSs upon hydrothermal

reaction or calcination (ESIw 5 and ESIw 5–7). Their grain

sizes are about 20–30 nm (SI-5). The BET surface areas ranged

between 97.2 and 152.8 m2 g�1. Interestingly, the pore volume

increased from 0.12 to 0.41 cm3 g�1 and the average pore

diameter increased from 5.12 to 12.64 nm (SI-8) upon increasing

the n-alkyl chain length from 8 to 16 (Table 1).

The size of MAPS increases as T decreases and upon

purifying the Ti source. The monodispersity of MAPS increases

as the temperature decreases from room temperature to�20 1C,and decreased upon further decreasing T to �41 1C. The size

also increases as the concentration of TIP or H2O decreases or

upon increasing the amount of EtOH. The reason for the above

phenomenon is attributed to the decrease in the number of

nuclei for the formation of MAPSs and increase in the degree of

uniformity of the growth rate of nuclei. By this way, MMTSs

with the sizes between 900 and 2040 nm can be easily produced

with very high monodispersity (s o 4%).

This work was supported by the Korea Center for

Artificial Photosynthesis (KCAP) located in Sogang University

funded by MEST of the Korean Government through the

National Research Foundation (NRF-2011-C1AAA001-2011-

0030278).

Notes and references

1 E. Matijevic, M. Budnik and L. Meites, J. Colloid Interface Sci.,1977, 61, 302.

2 M. Visca and E. Matijevic, J. Colloid Interface Sci., 1979, 68, 308.3 J. H. Jean and T. A. Ring, Langmuir, 1986, 2, 251.4 J. H. Jean and T. A. Ring, Colloids Surf., 1988, 29, 273.5 S. Eiden-Assmann, J. Widoniak and G. Maret, Chem. Mater.,2004, 16, 6.

6 J.-L. Look and C. F. Zukoski, J. Colloid Interface Sci., 1992,153, 461.

7 D. Chen, L. Cao, F. Huang, P. Imperia, Y.-B. Cheng andR. A. Caruso, J. Am. Chem. Soc., 2010, 132, 4438.

8 U.Meyer, A. Larsson, H.-P. Hentze and R. A. Caruso,Adv. Mater.,2002, 14, 1768.

9 D. G. Shchukin and R. A. Caruso, Chem. Mater., 2004, 16, 2287.10 A. S. Deshpande, D. G. Shchukin, E. Ustinovich, M. Antonietti

and R. A. Caruso, Adv. Funct. Mater., 2005, 15, 239.11 A. Dong, N. Ren, Y. Tang, Y. Wang, Y. Zhang, W. Hua and

Z. Gao, J. Am. Chem. Soc., 2003, 125, 4976.12 H. Li, Z. Bian, J. Zhu, D. Zhang, G. Li, Y. Huo, H. Li and Y. Lu,

J. Am. Chem. Soc., 2007, 129, 8406.13 J. H. Pan, Z. Cai, Y. Yu and X. S. Zhao, J. Mater. Chem., 2011,

21, 11430.14 Y. J. Kim, M. H. Lee, H. J. Kim, G. Lim, Y. S. Choi, N.-G. Park,

K. Kim and W. I. Lee, Adv. Mater., 2009, 21, 3668.15 I. G. Yu, Y. J. Kim, H. J. Kim, C. Lee and W. I. Lee, J. Mater.

Chem., 2011, 21, 532.16 Y. Chen, F. Huang, D. Chen, L. Cao, X. L. Zhang, R. A. Caruso

and Y.-B. Cheng, ChemSusChem, 2011, 4, 1498.17 D. Chen, F. Huang, Y.-B. Cheng and R. A. Caruso, Adv. Mater.,

2009, 21, 2206.18 Y.-G. Guo, Y.-S. Hu and J. Maier, Chem. Commun., 2006, 2783.19 J. Yin, L. Xiang and X. Zhao, Appl. Phys. Lett., 2007, 90, 113112.20 X. Jiang, T. Herricks and Y. Xia, Adv. Mater., 2003, 15, 1205.21 Y. Zhang, G. Li, Y. Wu, Y. Luo and L. Zhang, J. Phys. Chem. B,

2005, 109, 5478.22 G. Yang, P. Hu, Y. Cao, F. Yuan and R. Xu,Nanoscale Res. Lett.,

2010, 5, 1437.23 X.-X. Zou, G.-D. Li, Y.-N. Wang, J. Zhao, C. Yan, M.-Y. Guo,

L. Li and J.-S. Chen, Chem. Commun., 2011, 47, 1066.

Fig. 2 SEM images of MAPSs synthesized with purified TIP at 5 (a), 0 (b), �5 (c),�10 (d),�15 (e),�20 (f),�25 (g), and�30 1C (h), respectively.

Scale bars = 2 mm. Gel composition: TIP : n-RNH2 : H2O : EtOH = 1.0 : 0.5 : 6.6 : 253.7.

Table 1 BET analysis of mesoporous TiO2 beads synthesized byn-alkyl-amine as surfactant

n SBET/m2 g�1 Vp/cm

3 g�1 Average pore diameter/nm

8 97.24 0.12 5.1210 149.16 0.28 7.4412 152.83 0.39 10.0814 130.76 0.36 10.4516 130.29 0.41 12.64

Dow

nloa

ded

by S

ogan

g U

nive

rsity

on

24 M

arch

201

2Pu

blis

hed

on 0

6 M

arch

201

2 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

2CC

3039

1D

View Online