Ethylene glycol-mediated synthesis of nanoporous anatase TiO2 rods andrutile TiO2 self-assembly chrysanthemums

Abstract

Nanoporous anatase TiO2 rods and rutile TiO2 chrysanthemums were successfully
synthesized via a simple ethylene glycol-mediated synthesis route. Their
morphologies, phase compositions and components were characterized by SEM, TEM,
Raman and IR, respectively. The results show that a self-assembly growth takes
place in the calcination under vacuum, which makes the titanium glycolate rods
transform into rutile TiO2/C chrysanthemums rather than anatase TiO2 rods. It
also indicates that the carbon plays an important role in the phase transition
process which promotes the phase transition to rutile TiO2 at a lower
temperature (400 oC). It provides a new approach to prepare nanoporous rutile
TiO2 nanomaterials under low temperature.

Full text

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Journal of Alloys and Compounds 471 (2009) 477–480
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Ethylene glycol-mediated synthesis of nanoporous anatase TiO2
rods and rutile TiO2self-assembly chrysanthemums
Quanjun Li, Bingbing Liu∗, Yingai Li, Ran Liu, Xianglin Li, Dongmei Li,
Shidan Yu, Dedi Liu, Peng Wang, Bing Li, Bo Zou, Tian Cui, Guangtian Zou
State Key Laboratory of Superhard Materials, Jilin University, Qianwei Road, Changchun 130012, China
a r t i c l e i n f o
Article history:
Received 27 January 2008
Received in revised form 28 March 2008
Accepted 31 March 2008
Available online 19 May 2008
Keywords:
Nanostructured materials
Chemical synthesis
Crystal structure
Scanning electron microscopy (SEM)
a b s t r a c t
NanoporousanataseTiO2rodsandrutileTiO2chrysanthemumsweresuccessfullysynthesizedviaasimple
ethyleneglycol-mediatedsynthesisroute.Theirmorphologies,phasecompositionsandcomponentswere
characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman
and IR, respectively. The results show that a self-assembly growth takes place in the calcination under
vacuum, which makes the titanium glycolate rods transform into rutile TiO2/C chrysanthemums rather
than anatase TiO2rods. It also indicates that the carbon plays an important role in the phase transition
process which promotes the phase transition to rutile TiO2at a lower temperature (400◦C). It provides a
new approach to prepare nanoporous rutile TiO2nanomaterials under low temperature.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
TiO2materials have attracted considerable attention recently
due to their unique physicochemical properties and applications
in the fields of photocatalysts, gas sensors, photovoltaics, and
lithium ion batteries [1–6]. It is not surprising in view of the
importance of TiO2materials that a wide variety of approaches
for the synthesis of TiO2have been reported, and it remains a
particularly active research field. Various morphologies of TiO2
materials have been synthesized by many approaches. Such as,
nanoparticles were generally obtained from sol–gel method [7].
Nanowires, nanotubes and nanoribbons were widely prepared by
a hydrothermal process [8,9]. Nanowires films were synthesized
by thermal evaporation, thermal deposition and anodic oxida-
tive hydrolysis [10–12]. Recently, ethylene glycol (EG) has been
widely used in the so-called polyol synthesis of metal nanopar-
ticles due to its strong reducing powder and relatively high boiling
point (∼197◦C) [13–15]. Jiang and co-workers have synthesized
metal oxide (including TiO2, SnO2, In2O3, and PbO2) nanowires by
a similar method [13]. More recently, submicrometer-sized TiO2
nanorods were synthesized with EG and tetra-isopropyl orthoti-
tanate by a simple microwave irradiation method [15]. According
∗Corresponding author. Tel.: +86 431 85168256; fax: +86 431 85168256.
E-mail address: liubb@jlu.edu.cn (B. Liu).
to previous research, synthesis of various morphologies of metal
oxides is possible by using a suitable metal salt in EG. How-
ever, there is no report on the synthesis of nanoporous rutile
TiO2chrysanthemum which consists of numerous nanoparticles
with porous structure and is different from those previous reports
of flowerlike TiO2nanomaterials with solid structure [16–19]. In
this study, we report the synthesis of titanium glycolate rods,
nanoporousanataseTiO2rods,andnanoporousrutileTiO2chrysan-
themums by the ethylene glycol-mediated synthesis process and
posttreatment.
2. Experimental
Titanium glycolate rods were synthesized by heating a solution of titanium
alkoxide in EG at 170◦C for 2h. In a typical synthesis, 0.1mL titanium butoxide was
added to a 50-mL flask that contained 10mL EG. The solution was stirred for 30min,
andthenheatedto170◦Cinovenwithoutstirringfor2h.Aftercoolingdowntoroom
temperature, the white flocculate was harvested using centrifugation, followed by
washing with deionized water and ethanol several times to remove excess EG from
the sample. The precipitate was finally dried in oven at 80◦C for 8h in air and used
for further characterization. TiO2rods were obtained by heating the titanium glyco-
late rods at 400◦C for 4h in air. TiO2chrysanthemums were obtained by heating the
titaniumglycolaterodsat400◦Cfor4hinvacuumandtheninair.Themorphologies
ofthesampleswereobservedbyscanningelectronmicroscopy(SEM).Transmission
electron microscopy (TEM) images were obtained on a HITACHI H-8100 microscope
using an acceleration voltage of 200KeV. Raman spectra were excited by radiation
of 514.5nm from a Renishaw inVia Raman spectrometer. IR spectra were acquired
under ambient conditions using an infrared spectrometer (NICOLET AVATAR 370
DTGS).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.03.137

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Fig. 1. SEM images of titanium glycolate rods (a), TiO2rods (b), self-assembly TiO2/C chrysanthemums (c and d), and TiO2chrysanthemums (e and f). The inset of (d) is the
high magnification image for the root of chrysanthemum.
3. Results and discussion
Fig. 1 shows SEM images of the titanium glycolate rods,
TiO2rods, self-assembly TiO2/C chrysanthemums, and pure TiO2
chrysanthemums. Fig. 1(a) reveals the presence of numerous tita-
niumglycolaterodswithlengthsrangingfromseveralmicrometers
toseveraltensofmicrometers,andtheirdiametersareca.1–10?m.
Fig. 1(b) shows the TiO2rods are similar to that shown in Fig. 1(a),
but there are a few of cracks on the rods are observed. This
demonstratesthatthetitaniumglycolatetransformedintotheTiO2
without changing the rod-like morphology after heat treatment at
400◦C in air. As shown in Fig. 1(c), there are a lot of rods aggre-
gate together and assemble into chrysanthemum-like structure.
Fig. 1(d) shows the chrysanthemum-like microstructure is com-
posed of a number of rods. In addition, numerous rods connect
with one root and form chrysanthemum-like morphology that can
be seen in the inset of Fig. 1(d). It is clear that the diameters of the
chrysanthemum are ca. 150–200?m, and the rods are ca. 1–20?m
in diameter and ca. 80–100?m in length which are larger than
that of the titanium glycolate rods (Fig. 1(a)). The cross-sections
of the rods are irregular polygons as depicted in Fig. 1(d). The
incomplete chrysanthemum-like structure clearly shows that the
chrysanthemum is obtained by the titanium glycolate rods self-
assembly growth. Fig. 1(e) and (f) show the similar results with
Fig.1(c)and(d).Itobviouslyindicatesthatthechrysanthemum-like
structure was essentially preserved in the further heating pro-
cess.
Fig. 2 shows TEM images of TiO2rods, TiO2/C chrysanthemums
and TiO2chrysanthemums, respectively. It further reveals that the
micrometer rods of all samples consist of numerous nanoparti-
cles aggregated to nanoporous geometry. As shown in Fig. 2(a), the
TiO2rodsarecomposedofnumerousnanoparticleswithdiameters
about 30–40nm. Fig. 2(b) shows the TiO2/C chrysanthemums con-
sist of ultrafine nanoparticles with diameters less 10nm. The TiO2
chrysanthemums are composed of nanoparticles about 20–30nm
showninFig.2(c).Thisdemonstratesthatthenanoparticlesfurther

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479
Fig. 2. TEM images of TiO2rods (a), TiO2/C chrysanthemums (b), and TiO2chrysanthemums (c), respectively.
grow and the chrysanthemum shape is still retained, when heating
the TiO2/C chrysanthemum in air.
Fig. 3 shows the Raman spectra of the titanium glycolate rods,
TiO2rods, TiO2/C chrysanthemums and TiO2chrysanthemums,
respectively. Fig. 3(a) shows the Raman spectrum of the tita-
nium glycolate rods. All Raman peaks shown in Fig. 3(b) can be
assignedastheEg(143cm−1),B1g(195cm−1),A1g(395cm−1),orB1g
(515cm−1), and Eg(638cm−1) modes of the anatase phase, respec-
tively. As shown in Fig. 3(c), two strong peaks locate at 1356 and
1594cm−1canbeattributedtothecarbonthatderivesfromthecar-
bonization of the organic compound, the other three peaks at 150
(Eg), 424 (E1g) and 609cm−1(A1g) are attributed to the rutile phase.
AsdepictedinFig.3(d),thepeaksat143(B1g),237(twophononpro-
cess), 442 (Eg), and 610cm−1(A1g) are assigned to the rutile phase,
respectively.Theresultsshowthattheas-preparedTiO2/Cchrysan-
Fig. 3. Raman spectra of titanium glycolate rods (a), TiO2rods (b), self-assembled
chrysanthemums (c), and TiO2chrysanthemums (d).
themums in this process tend to transform into rutile phase due to
the existence of carbon in the crystal, in which the carbon source
is introduced from the alkoxide group. These results are similar to
Tseng and co-worker’s research [20]. It is clear that pure anatase
rods and rutile chrysanthemums are obtained by heating the tita-
niumglycolaterodsat400◦Cfor4hinairandvacuum,respectively.
Fig. 4 shows the IR spectra of the samples which be heated in
various atmosphere. The band at 3400cm−1in Fig. 4(a) is assigned
to the stretching of the O H bond corresponding to physically
absorbed water or EG, and the two sharp absorption bands at
2855and2925cm−1areattributedtothealkyl-CH2symmetricand
asymmetric stretching [13,14]. At approximately 1465cm−1a sig-
nal corresponding to the bending of
bands at 1150–1350cm−1are assigned to the scissoring of the C H
bonds of the
CH2groups [13,14]. In Fig. 4(b)–(d), the stretching
CH2groups appears, and the
Fig. 4. IR spectra of titanium glycolate rods (a), TiO2rods (b), self-assembly TiO2/C
chrysanthemums (c), and TiO2chrysanthemums (d).

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Q. Li et al. / Journal of Alloys and Compounds 471 (2009) 477–480
of the C H bonds in the TiO2rods is not observed, only the Ti O
bonds are remained. This indicates that the organic components
were removed from the final products by the heat treatment.
It has been found that titanium alkoxide was added to EG
and heated to 170◦C for 2h under rigorous stirring, the alkoxide
was transformed into a chain-like, glycolate complex that sub-
sequently crystallized into uniform nanowires or nanorods, then,
titanium glycolate converts into TiO2without change its morphol-
ogy[13–15].However,tooursurprise,weobtainedtherutileTiO2/C
chrysanthemum-like structure by heating the titanium glycolate
rods at 400◦C in vacuum. In this process, titanium glycolate rods
were aggregated and self-assembled grown into the chrysanthe-
mums with the dehydration and carbonization of organic groups
of titanium glycolate rods in vacuum. As a result, the rods of
chrysanthemums are larger than the titanium glycolate rods both
in diameter and in length (as shown in Fig. 1). In addition, the rods
of the TiO2/C chrysanthemums are composed of numerous ultra-
finenanoparticles(asshowninFig.2),whichalsodemonstratethat
the presence of carbon inhibits the growth of TiO2nanoparticles. It
was known that the ethylene glycolate titanium was coordinated
with Ti4+to form TiO6octahedra, and the weakened O H bond
resulted in the O H bond tend to break and react with the other
O H bond to produce H2O [14]. A self-assembly process was found
in Wei’s report, in which the titanium glycolate rods are used to
prepare tubular titanate [14]. In this study, we suggest that the
chrysanthemum-like TiO2/C architecture is formed from the reac-
tionsbetweentitaniumglycolate,whichincludingdehydrationand
carbonization that take place during the heat treatment under vac-
uum. The TiO2rods obtained directly from heating the titanium
glycolate rods in air, which is possible due to the oxygen atmo-
sphere that affects the dehydration and oxidation of organic group
in titanium glycolate rods. Therefore, the titanium glycolate rods
self-assemblygrowintoTiO2/Cchrysanthemumsbyheattreatment
in vacuum rather than retain their original rod-like shape in air.
TiO2rods, TiO2/C chrysanthemums and TiO2chrysanthemums all
present nanoporosity that are attributed to the empty spaces that
formed from the elimination of organic substance or carbon.
4. Conclusions
In summary, nanoporous anatase TiO2 rods and rutile TiO2
chrysanthemums were synthesized via an ethylene glycol-
mediated synthesis route. The self-assembly growth process was
found during the heat treatment under vacuum, in which the
titanium glycolate rods convert into rutile TiO2/C chrysanthe-
mums.Thepossiblemechanismwasproposedthatthedehydration
and carbonization of organic groups of titanium glycolate rods
in vacuum result in the titanium glycolate rods aggregated and
self-assembled grown into the rutile TiO2/C chrysanthemum-
like microstructure. The pure rutile TiO2chrysanthemums were
obtained by heating the TiO2/C chrysanthemums at 400◦C in air. It
also indicates that the carbon plays an important role in the phase
transitionprocess.Webelievethatsuchaself-assemblyprocesshas
large potential to prepare nanostructure metal oxides with novel
morphologies.
Acknowledgements
This work was supported financially by NSFC (10674053,
20773043, 10574053), the Cultivation Fund of the Key Scientific
and Technical Innovation Project (2004-295) of MOE of China,
the National Basic Research Program of China (2005 CB724400,
2001CB711201), the Program for Changjiang Scholar and Innova-
tive Research Team in Unversity (IRT0625), the 2007 Cheung Kong
Scholars Programme of China, the National Found for Fostering Tal-
ents of basic science (J0730311), and the Project for Scientific and
Technical Development of Jilin Province.
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