Box 1129, Hefei 230031, China Received 29 July 2002; in final form 20 September 2002 Abstract Anatase TiO2single crystalline nanowires have been successfully synthesized using a simple hy
Trang 1Hydrothermal synthesis and photoluminescence
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 TiO2single crystalline nanowires have been successfully synthesized using a simple hydrothermal synthesis method from TiO2 nanoparticles X-ray diffraction, transmission electron microscopy and high-resolution electron microscopy investigations show the TiO2nanowires have high crystallinity with diameter range from 30 to 45 nm and length in several micrometers The TiO2 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
properties and potential applications [1,2] Over
the past few years, many methods have been
suc-cessfully developed for the fabrication of these
nanowires, including vapor–liquid–solid (VLS) [2],
solution–liquid–solid (SLS) [3], template-based
synthetic approaches [4], arc discharge [5], and
laser ablation [6], which have all been proved to be
very effective methods However, almost all of the
methods used either catalyst materials or physical
template, which unavoidably brought some
con-tamination to the products Therefore, it is very interesting to explore a new approach to synthesize 1D nanomaterials without using preformed tem-plates or catalyst materials
Among a large amount of nanomaterials, the nanostructured titania materials are of great in-terest for possible application to photovoltaic cells [7], semiconductor photo-catalyst [8], catalyst support [9], and gas and humidity sensor [10] To date, a few methods have been developed to syn-thesize TiO2 nanowires Li et al [11] successfully
were homogeneously mixed Kobayashi et al [12] employed supramolecular assemblies to synthesize
Zhang et al [14] have fabricated TiO2nanowires in anodic alumina membranes In this Letter, we adopt a simple chemical approach to synthesize
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*
Corresponding author Fax: +86-551-5591434.
E-mail address: ghli@mail.issp.ac.cn (G.H Li).
0009-2614/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V.
PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 4 9 9 - 9
Trang 22 Experimental
process similar to that described by Kasuga and
co-workers [15] In a typical preparation
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
the heating After the autoclave was naturally
cooled to room temperature, the obtained sample
was sequentially washed with dilute HCl aqueous
solution, distilled deionized water and absolute
ethanol for several times The samples were dried
white color were obtained
The composition of the sample was examined
by a Japan Rigaku Dmax c A X-ray diffractometer
mor-phologies of the sample were analyzed with
scanning electron microscopy (SEM) (JEOL
JSM-6300), transmission electron microscopy (TEM)
(JEM-200CX) and high-resolution electron
mi-croscopy (HRTEM) (JEM-2010) Samples for
SEM observation were presputtered with a layer of
conducting Pt metal Samples for TEM
observa-tion were prepared by 10 min ultrasonic dispersion
of a small amount of sample in absolute ethanol; a
drop of the solution was then dipped onto a
cop-per microgrid or carbon film and dried in air
be-fore performance Photoluminescence (PL) spectra
were measured in an Edinburgh FLS 920
spec-trophotometer with an Xe lamp as the excitation
light source
3 Results and discussion
The X-ray diffraction (XRD) pattern (Fig 1)
revealed the overall crystalline structure and phase
purity of the nanowires All the relatively sharp
crystalline cell constants a¼ 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
ob-served, which indicates that the product has high purity
Fig 2 showed a typical SEM image of the
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 nanowires.
Trang 3or during the preparation of SEM sample This
might explain why some of the nanowires looked
wider than others
TEM and HRTEM were used to study the fine
structure of the nanowires Fig 3a showed a
electron diffraction (the inset in Fig 3a) recorded
perpendicular to the long axis of this nanowire determines the anatase phase of the obtained sample, which was consistent with the XRD pat-tern The diffraction spots were indexed as (0 0 4), (2 0 0), and (2 0 4) diffraction of anatase TiO2, be-longing to the [0 1 0] zone axis The
HRTEM images Fig 3b shown the corresponding HRTEM image of the nanowire shown in Fig 3a The clear lattice stripes showed that the nanowire has high crystallinity with fewer defects such as microtwins The plane intervals, measured as 0.35
nm, represented the stripe image of the (1 0 1)
forma-tion of single crystalline anatase TiO2nanowires in our experiments
in the hydrothermal condition is achieved for the first time Although the exact growth mechanism
of the TiO2nanowires 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
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
nanowires at room temperature together with
wave-length for curves (1) and (2) is 413 nm, and that for curves (3) and (4) is 473 nm A very strong blue–green PL band can be observed which con-sists of two PL peaks situated at 487 nm (2.55 eV) and 492 nm (2.27 eV), respectively, under excitation at 413 nm The PL peaks position and intensity are obviously different under different excitation wavelengths The main peak is respec-tively located at about 487 and 545 nm under excitation at 413 and 473 nm The PL intensity of
Fig 3 (a) A TEM image of a single anatase TiO 2 nanowire
with a diameter of 40 nm The inset shows a [0 1 0] SAED
re-corded perpendicular to the long axis of the wire (b) The
corresponding HRTEM image of the nanowire showing lattice
planes The space of 0.35 nm corresponds to the distance
be-tween two (1 0 1) planes.
Trang 4weak, while that excited at 473 nm is stable and
strong These results indicate that the optimal
cir-cumstances which indicates that the nanowires
might have higher activity than nanocrystals
nanowires and nanocrystals are basically identical
under the same excitation wavelength, which
nanocrystals
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
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
at 485 nm The broad PL band was ascribed to
bound-exciton emission due to the trapping of
free excitons by titanate groups near defects
nanoparticles at 412 nm They assigned these PL
octahedra Serpone et al [18] reported the PL
wavelength range (465 and 520 nm band) and attributed them to the oxygen vacancies Jin et al [19] observed two peaks at 488 and 510 nm from
and ascribed them to impurities and defects In our case, since the excitation wavelength is far deviated from the absorption edge, the excitons tend to be unstable, and self-trapped excitons as the source of the PL is basically ruled out Stre-kalovsky et al [20] reported that the principal intrinsic effects in the powdered zirconia are an-ion vacancies Emeline et al [21] considered that
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,
defect sites, especially from anion vacancies through the reaction
eþ va! F þ hm;
center Of course, the exact photoluminescence
in-vestigation
4 Conclusions
In conclusion, we have successfully fabricated
simple chemical approach This method produced
a large quantity of single-crystalline nanowires at
nanowires have a very strong PL band at blue– green wavelength range The nanowires might have many potential applications in photocatalysts and photoelectronics
Acknowledgements The authors thank Professor Y Qin for his help
in HRTEM observations The financial support of this work by the Key Project of National Funda-mental Research of China is gratefully acknowl-edged
Fig 4 PL spectra of the TiO 2 samples under different
excita-tion wavelengths at room temperature (1) Nanowires and (2)
nanocrystals under excitation at 413 nm; (3) nanowires and (4)
nanocrystals under excitation at 473 nm.
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