This document summarizes a study on synthesizing monodisperse mesoporous TiO2 spheres with tunable sizes between 0.6 and 3.1 μm. The key findings are: 1) Increasing the reaction temperature or Ti source (titanium isopropoxide) concentration results in smaller sphere sizes with lower monodispersity. 2) Using purified titanium isopropoxide and n-dodecylamine as a surfactant leads to the largest spheres sizes with good monodispersity. 3) Decreasing the water or titanium isopropoxide concentration in the reaction increases the sphere size but can decrease monodispersity.

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Cite this: DOI: 10.1039/c2cc30391d
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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
Published on 06 March 2012 on http://pubs.rsc.org | doi:10.1039/C2CC30391D
Kyung Byung Yoon*
Received 17th January 2012, Accepted 6th March 2012
DOI: 10.1039/c2cc30391d
Downloaded by Sogang University on 24 March 2012
We report a novel method for synthesizing monodisperse mesoporous condensation of the produced reactive Ti species in various
TiO2 spheres (sizes = 0.6–3.1 lm) by hydrolysis of titanium reaction media. The Ti sources for the formation of MAPSs
isopropoxide (TIP) in a mixture of C8–C16 n-alkylamine, water, usually include tetraalkoxy titanium [Ti(OR)4],2–8,10,11,13–18,20,21,23
and ethanol. The size increases with decreasing temperature, TiCl4,1,9,12 tetraalkyl titanium (TiR4).19,22 The reaction media are
TIP concentration, and water concentration, and upon purifying usually consisted of solvent (typically alcohol), water, catalysts,
TIP. n-Dodecylamine gives the highest monodispersity. and the additives that affect the mesopore sizes. The catalysts are
divided into forward catalysts such as acids,1,6,9,12,21 bases
Mesoporous TiO2 spheres1–23 have been used as photocatalysts,12,13 (alkylamines),7,13–17 and salts5,6 and the reverse catalysts such
working electrodes and scattering layer materials for dye sensitized as hydroxyl propyl cellulose4 and poly alcohols.19 The salts
solar cells,14–17 materials for lithium ion batteries,18 luminescence give rise to increases in rates of hydrolysis and condensation
enhancing matrices,19 and building blocks for photonic band gap by increasing the ionic strength in the media.6 In this respect,
materials,20 and others.23 In these applications, best results and salts can be classified as the forward catalysts. Polymer
reproducibility of functions are expected if the mesoporous TiO2 adsorbents decrease the reaction rate by decreasing the
spheres are uniform in size. Accordingly, a variety of methods concentration of the reactive hydrolysed Ti species in the bulk
(ESIw 1) have been developed for the syntheses of monodisperse solution by adsorbing them onto the polymers. In this respect
mesoporous TiO2 spheres (MMTSs). polymers can be classified as reverse catalysts.
The methods for producing MMTSs can be divided into It has been observed that the size of MAPS increases with
two; template assisted8–11 and autogenesis.1–7,12–23 In the case decreasing concentration of the Ti source, water, and forward
of the former, MMTSs are prepared by incorporation of a catalysts, and with increasing concentration of the reverse
Ti source into the templates followed by hydrolysis of the catalysts, thus, with decreasing reaction rates. This phenomenon
Ti source or calcination. The size and monodispersity of the indicates that the decrease of reaction rate also gives rise to the
MMTSs are controlled by those of templates. The sizes of decrease of the number of nuclei for MAPS, during the initial
MMTSs obtained by these methods were 500–600 nm11 and stage of reaction. As a result, if no new nuclei are additionally
4–27 mm.8–10 formed during the growth stage, the size of MAPS increases
In the case of the latter (autogenesis), MMTSs are prepared by upon decreasing the number of nuclei.
a two-step procedure; preparation of monodisperse amorphous An apparent exception to the above general trend was the
precursor spheres (MAPSs) and their crystallization into synthesis of MAPS by adding a Ti source [Ti(OBu)4] dissolved
MMTSs by a hydrothermal reaction or calcination. The size in ethylene glycol (EG) into acetone, where the size of MAPS
and monodispersity of the obtained MMTSs are determined increases upon increasing the concentration of the glycolated
by those of the produced MAPSs. Efforts have therefore been Ti [the Ti species chelated by EG {Ti(EG)2}].20 Because of the
directed at the development of the methods for preparing fact that this reaction takes place only when the amount of
MAPSs with high monodispersity.1–7,12–22 acetone is large, it is concluded that Ti(EG)2 undergoes
MAPSs have been obtained through a careful control of hydrolysis only after replacing the Ti-chelating EG with
the rates of hydrolysis of the Ti sources and the subsequent 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
Korea Centre for Artificial Photosynthesis, Centre for Microcrystal EG solution of Ti(EG)2, and Ti(EG)2 only acts as the nutrient to
Assembly, Centre for Nanomaterials, Department of Chemistry, the initially formed nuclei after removal of EG by acetone.
Sogang University, Seoul, Korea. E-mail: yoonkb@sogang.ac.kr; Regardless of the methods, the sizes of MAPSs obtained by
Fax: +82-2-706-4269
w Electronic supplementary information (ESI) available. See DOI: autogenesis methods were usually submicron (200–1000 nm)
10.1039/c2cc30391d with few exceptions in which the size reached 1.2 mm.
This journal is c The Royal Society of Chemistry 2012 Chem. Commun.

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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,
Published on 06 March 2012 on http://pubs.rsc.org | doi:10.1039/C2CC30391D
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
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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
Fig. 1 The plots of the size (a) and the standard deviation (s) of size
solution B). Both three-necked flask and vial were placed in an
(b) of MAPS vs. temperature (T) and s vs. n-alkyl chain length of n-RNH2
isopropanol bath whose temperatures were precisely controlled
(c), the size of MAPS vs. the amounts of TIP (d), H2O (e), and ethanol (f),
between 10 and À30 1C using an external chiller. To obtain respectively. The vertical bars in a, d, e, and f represent the corresponding
À41 1C, acetonitrile and dry ice were used. When the temperatures upper and lower size limits.
of both solutions (solutions A and B) were equilibrated to that
of the bath, solution A was quickly transferred to solution B and the monodispersity decreased as well (Fig. 1b and ESIw 3).
with the help of a cannula while solution B was being Thus, even when unpurified TIP was used, the size of MAPS
vigorously stirred. The standard molar ratio of the reagents also increased with decreasing T, but less sensitively with respect
was TIP : n-RNH2 : H2O : EtOH = 1.0 : 0.5 : 6.6 : 253.7. to T, and monodispersity also decreased. These phenomena seem
The obtained MAPSs were crystallized into MMTSs by a to take place due to pre-existence of nuclei in unpurified TIP,
hydrothermal reaction (160 1C for 16 h) and calcination which were formed by inadvertently introducing moisture during
(500 1C for 6 h), respectively. handling and storage of TIP.
The average size of MAPS increased from 700 to 2830 nm as The above results clearly demonstrate the effects of T and
T decreased from room temperature to À41 1C (Fig. 1a, and the purity of Ti sources on the size and monodispersity of
see ESIw 3 for details). The standard deviation (s) of the size MAPS. The increase in size in response to the decrease in T is
gradually decreased from 28.6 to 1.5% as T decreased from 25 also attributed to the formation of a less number of nuclei as T
to À20 1C and increased back to 28.6% as T further decreased decreases. The general trend that monodispersity increases
to À41 1C (Fig. 1b). Thus, MAPSs with the sizes between 780 with decreasing T also indicates that the degree of uniformity
and 2580 nm with reasonably high monodispersity (s o 12%) of the nuclei growth rates increases as T decreases and as the
can be obtained by merely varying T between 5 and À30 1C as purity of Ti sources increases. The decrease in size and the
the scanning electron microscope (SEM) images (Fig. 2 and increase in s upon using unpurified TIP indicate that the total
ESIw 4) show. In particular, with T between 0 and À20 1C, number of nuclei is higher when unpurified TIP was used due
MAPSs with the average size between 900 and 2040 nm can be to the presence of pre-formed nuclei in the unpurified TIP.
obtained with very high monodispersity (s o 4%). Out of C8–C16 n-alkylamines studied in this work, n-DDA
When unpurified TIP was used as the Ti source, the size of gave the lowest s value (Fig. 1c) at À20 1C. In this respect, the
MAPS also gradually increased as T decreased from room data shown in Fig. 1 were obtained with n-DDA as n-RNH2,
temperature to À41 1C. However, the average size is smaller unless stated otherwise as in Fig. 1c. Upon decreasing the amount
(ranging between 550 and 1710 nm) than the corresponding size of TIP (Fig. 1d) or water (Fig. 1e) the size of MAPS increased as
obtained with purified TIP (ranging between 780 and 2580 nm) shown in the cases of 23 and À20 1C. However, the s value
Chem. Commun. This journal is c The Royal Society of Chemistry 2012

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Published on 06 March 2012 on http://pubs.rsc.org | doi:10.1039/C2CC30391D
Fig. 2 SEM images of MAPSs synthesized with purified TIP at 5 (a), 0 (b), À5 (c), À10 (d), À15 (e), À20 (f), À25 (g), and À30 1C (h), respectively.
Scale bars = 2 mm. Gel composition: TIP : n-RNH2 : H2O : EtOH = 1.0 : 0.5 : 6.6 : 253.7.
Downloaded by Sogang University on 24 March 2012
Table 1 BET analysis of mesoporous TiO2 beads synthesized by funded by MEST of the Korean Government through the
n-alkyl-amine as surfactant National Research Foundation (NRF-2011-C1AAA001-2011-
n SBET/m2 gÀ1 Vp/cm3 gÀ1 Average pore diameter/nm 0030278).
8 97.24 0.12 5.12 Notes and references
10 149.16 0.28 7.44
12 152.83 0.39 10.08 ´
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between 97.2 and 152.8 m2 gÀ1. Interestingly, the pore volume 12 H. Li, Z. Bian, J. Zhu, D. Zhang, G. Li, Y. Huo, H. Li and Y. Lu,
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This work was supported by the Korea Center for 23 X.-X. Zou, G.-D. Li, Y.-N. Wang, J. Zhao, C. Yan, M.-Y. Guo,
Artificial Photosynthesis (KCAP) located in Sogang University L. Li and J.-S. Chen, Chem. Commun., 2011, 47, 1066.
This journal is c The Royal Society of Chemistry 2012 Chem. Commun.