Hydrothermal synthesis and photoluminescence
of TiO
2
nanowires
Y.X. Zhang, G.H. Li
*
, Y.X. Jin, Y. Zhang, J. Zhang, L.D. Zhang
Institute of Solid State Physics, Chinese Academy of Sciences, P.O. Box 1129, Hefei 230031, China
Received 29 July 2002; in final form 20 September 2002
Abstract
Anatase TiO
2
single crystalline nanowires have been successfully synthesized using a simple hydrothermal synthesis
method from TiO
2
nanoparticles. X-ray diffraction, transmission electron microscopy and high-resolution electron
microscopy investigations show the TiO
2
nanowires have high crystallinity with diameter range from 30 to 45 nm and
length in several micrometers. The TiO
2
nanowires can emit blue–green light peaked at 487 nm under excitation at
413 nm.
Ó 2002 Published by Elsevier Science B.V.
1. Introduction
One-dimensional (1D) nanostructured materi-
als have attracted considerable attention due to
their unusual electronic, optical, mechanical
properties and potential applications [1,2]. Over
the past few years, many methods have been suc-
cessfully developed for the fabrication of these

Zhang et al. [14] have fabricated TiO
2
nanowires in
anodic alumina membranes. In this Letter, we
adopt a simple chemical approach to synthesize
single crystalline anatase TiO
2
nanowires.
Chemical Physics Letters 365 (2002) 300–304
www.elsevier.com/locate/cplett
*
Corresponding author. Fax: +86-551-5591434.
E-mail address: [email protected] (G.H. Li).
0009-2614/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V.
PII: S 0 0 09-2614(02)01 4 9 9 - 9
2. Experimental
TiO
2
nanowires were prepared using a chemical
process similar to that described by Kasuga and
co-workers [15]. In a typical preparation proce-
dure, 1 g anatase TiO
2
white powders were placed
into a Teflon-lined autoclave of 50 ml capacity.
Then, the autoclave was filled with 10 M NaOH
aqueous solution up to 80% of the total volume,
sealed into a stainless steel tank and maintained at
200 °C for 24 h without shaking or stirring during
the heating. After the autoclave was naturally

peaks could be indexed as anatase TiO
2
with
crystalline cell constants a ¼ 3:7806, c ¼ 9:4977

AA,
which are basically in agreement with the reported
values (JCPDS No. 21-1272). Although the dif-
fraction peak of brookite (denoted as B in Fig. 1)
can also be found, it is much lower than those of
anatase phase. No characteristic peaks of other
impurities, such as NaCl and Na
2
TiO
3
, were ob-
served, which indicates that the product has high
purity.
Fig. 2 showed a typical SEM image of the as-
prepared TiO
2
nanowires. The SEM image indi-
cated the nanowires are very copious in quantity
and quite clean with no contamination attached to
their surface. On the other hand, some of the
nanowires aggregated into bundles in the solution
Fig. 1. XRD pattern of the as-prepared nanowires (A and B
represent anatase and brookite, respectively).
Fig. 2. A typical SEM image of anatase TiO
2

2
nanowires in
our experiments.
The growth of single-crystalline TiO
2
nanowires
in the hydrothermal condition is achieved for the
first time. Although the exact growth mechanism
of the TiO
2
nanowires is not very clear, we believe
that NaOH plays an important role similar to the
so-called ÔsoftÕ template. In addition, the temper-
ature was definitely also very important for the
growth. At a low temperature, for example at
110 °C [15], because of the limited growth of TiO
2
particles, the layered structures were very thin,
which could easily be rolled up into tubular
structures. While in our experiments, due to the
rapid growth of particles, the layered structures
were very thick, naturally decomposed into wires
after washing with HCl, which could induce a
structural rearrangement, i.e., a morphological
transformation from the layered structures into
the fibrous materials.
Fig. 4 shows the PL spectra of the TiO
2
nanowires at room temperature together with
that for TiO

2
nanowires is higher
than that of TiO
2
nanocrystals under all the cir-
cumstances which indicates that the nanowires
might have higher activity than nanocrystals.
While the PL peaks shape and position of TiO
2
nanowires and nanocrystals are basically identical
under the same excitation wavelength, which
indicates that the PL mechanism of TiO
2
nano-
wires might be the same as that of TiO
2
nanocrystals.
The PL mechanisms of TiO
2
materials have
been intensively studied in the past few years. De
Hart et al. [16] observed a sharp emission line at
412 nm together with two other lines at 419 and
427 nm for TiO
2
anatase single crystalline. They
assigned 412 nm line to free-exciton and the latter
two lines to phonon repetitions of the free-exciton
line. These emission lines were observed to be
superimposed on a broad emission band centered

2
began with trapping of
free electrons by anion vacancies accompanied by
photon emission to yield F-type color centers.
Since zirconia and titania are similar in crystalline
structure and both are wide bandgap metal oxide,
such PL in TiO
2
nanowires may originate from
defect sites, especially from anion vacancies
through the reaction
e þ v
a
! F þ hm;
where v
a
is the anion vacancy and F is the color
center. Of course, the exact photoluminescence
mechanism of TiO
2
nanowires needs further in-
vestigation.
4. Conclusions
In conclusion, we have successfully fabricated
single-crystalline anatase TiO
2
nanowires using a
simple chemical approach. This method produced
a large quantity of single-crystalline nanowires at
relatively high purity and very low cost. The TiO

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