FE-SEM images of TNAs grown on the unmodified glass substrates at a hydrothermal growth temperature of 160 8C and growth time of 3 h... 4a shows XRD patterns of TNAs grown on the unmodifie
Trang 1Hydrothermal synthesis and characterization of TiO 2 nanorod arrays on glass substrates
Yuxiang Lia, Min Guoa, Mei Zhanga, Xidong Wangb,*
a
Department of Physical Chemistry, University of Science and Technology Beijing, Beijing 100083, PR China
b College of Engineering, Peking University, No 5 of Yiheyuan Street, Haidian District, Beijing 100871, PR China
1 Introduction
TiO2-based one-dimensional structures, especially nanorod/
wire/tube arrays, have attracted much attention owing to their
excellent properties and important applications Compared with
TiO2 nanoparticles, TiO2 nanorod/wire/tube arrays have lower
recombination rate for excited electron–hole pair, unique optical
and electric properties Therefore, they can be widely applied in
many fields, such as photocatalyst, photovoltaic devices, solar
energy batteries and gas sensors[1–4]
In recent years, a number of approaches have been reported to
fabricate TiO2 nanorod/wire/tube arrays, including
template-assisted method, electrochemical anodic oxidation method,
chemical vapor deposition (CVD) and hydrothermal method[5–
8] Among these methods, template-assisted technique combined
with sol–gel or electrochemical deposit process is widely used Lei
et al.[9]reported highly ordered TiO2nanowire arrays which were
prepared in anodic alumina membranes by a sol–gel method Chu
et al.[10]synthesized ordered TiO2–Ru and TiO2–RuO2nanorod
arrays in porous alumina films through cathodic electrodeposition
However, when the AAO template is removed, the nanorod arrays
tend to collapse due to the huge surface tension between each
nanorod[11–13] Electrochemical anodic oxidation of titanium can
fabricate vertically oriented TiO2nanotube arrays Paulose et al.[6] fabricated self-aligned TiO2 nanotube arrays by potentiostatic anodization of Ti foil which has 134mm in length, but it needs high-purity polished titanium foil, which results in a high production cost[14] Moreover, the as-fabricated nanotubes are amorphous and need to be annealed, which results in forming polycrystalline TiO2structures and influencing their photoelectric properties Wu and Yu[15]synthesized well-aligned rutile and anatase TiO2nanorods using CVD method However, this method involves the use of metal catalyst, high-temperature and vacuum technique, which makes the preparation process complicated Compared to the above methods, hydrothermal synthesis of TiO2 nanorod arrays (TNAs) is a promising approach due to its simple process, fast reaction velocity and low cost Therefore, it is
fit for large-scale preparation of TNAs
Up to now, preparing TNAs by hydrothermal approach is rarely reported Recently, Feng et al.[16]prepared TiO2nanorod films by
a low-temperature hydrothermal approach on a glass wafer substrate The as-prepared TiO2 nanorod film looked like a pile
of radial papillae Such morphology limits its applications in many fields For practical applications, especially used for solar cells and photocatalyst, it is better, for TNAs, to have a well-aligned orientation and uniform density distribution Therefore, exploring
a new route for fabricating well-ordered TNAs is very essential
As is well known, surface features of a substrate have strong impact on the morphology of one-dimensional material grown on the substrate Therefore, pre-treatment of the substrate is an
A R T I C L E I N F O
Article history:
Received 3 July 2008
Received in revised form 21 November 2008
Accepted 16 January 2009
Available online 22 January 2009
Keywords:
A Nanostructures
B Crystal growth
D Catalytic properties
A B S T R A C T
Large-scale, well-aligned single crystalline TiO2nanorod arrays were prepared on the pre-treated glass substrate by a hydrothermal approach The as-prepared TiO2nanorod arrays were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy X-X-ray diffraction results show that the main phase of TiO2is rutile Scanning electron microscopy and transmission electron microscopy results demonstrate that the large-scale TiO2nanorod arrays grown on the pre-treated glass substrate are well-aligned single crystal and grow along [0 0 1] direction The average diameter and length of the nanorods are approximately 21 and 400 nm, respectively The photocatalytic activity of TiO2nanorod arrays was investigated by measuring the photodegradation rate of methyl blue aqueous solution under UV irradiation (254 nm) And the results indicate that TiO2nanorod arrays exhibit relatively higher photocatalytic activity
ß2009 Elsevier Ltd All rights reserved
* Corresponding author Tel.: +86 10 8252 9083; fax: +86 10 6275 7532.
E-mail address: xidong@coe.pku.edu.cn (X Wang).
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Trang 2PT-substrate on the morphology of the prepared TNAs have been
discussed Furthermore, the photocatalytic activity of the prepared
TNAs was also investigated
2 Experimental
2.1 Materials
All chemicals were of analytical reagent grade and used
without further purification And all the aqueous solutions
were prepared using de-ionized water The microscope slides
(1–1.2 mm thick), used as substrates, were firstly ultrasonic
cleaned by acetone, ethanol and de-ionized water, respectively
prior to use
2.2 Preparation of the colloid solution for coating substrate
Firstly, 17 ml tetrabutyl titanate was dissolved in the mixed
solution of 12 ml ethanol and 5 ml diethanolamine, and then
stirred for 1 h to obtain a precursor solution Secondly, the mixed
solution of 1.7 ml de-ionized water, 34 ml ethanol and 0.13 ml
hydrochloric acid (37% HCl) were added to the above precursor
solution and further violently stirred for 15 min Finally, the
as-prepared mixed solution was aged for 24 h at room temperature in
order to form a homogeneous and stable colloid solution, which
served as the coating solution
2.3 Pre-treatment of the substrate
The coating colloid solution was coated onto the cleaned glass
substrate using spin coater (KW-4A, made by the Chinese Academy
of Sciences) at the rate of 3000 rpm for 30 s Subsequently, the
substrate was dried in air and annealed at 700 8C for 30 min in
muffle furnace The process was repeated for two times to produce
TiO2nanocrystal seeds layer with appropriate thickness, and thus
the pre-treated substrate (PT-substrate) were prepared
2.4 Hydrothermal growth of TNAs
The hydrothermal precursor solution was prepared by mixing
0.05 M titanium trichloride (TiCl3) aqueous solution saturated
with sodium chloride After the precursor solution was stirred for
5 min, it was transferred into a polytetrafluoroethylene vessel
which was placed in a sealed kettle, and then PT-substrate was
and high-resolution TEM (HRTEM, JEM-2010) were used to further elucidate the microstructure and phase characterization X-ray diffraction (XRD) analysis was performed with a Rigaku DMAX-RB diffractometer using Cu Karadiation
2.6 Photocatalytic activity measurement The photocatalytic activity of TNAs was investigated by measuring the photodegradation rate of methyl blue aqueous solution Photocatalytic reaction was carried out in a
200 ml quartz glass vessel under which there was an 18 W
UV lamp (254 nm wavelength) The 0.1 g/l methyl blue aqueous solution (150 ml) and the as-prepared TNAs were placed into the reaction vessel, keeping the face of TNAs upward At the given intervals of UV illumination, the specimen of methyl blue solution was collected and analyzed using an UV spectro-photometer (UNIC, UV-2100, absorption at lmax= 600 nm for methyl blue)
3 Results and discussion 3.1 Morphology and crystal structure of TNAs Modification of the substrate has strong impact on the morphology and alignment ordering of TNAs Fig 1shows the FE-SEM photographs of TNAs grown on the unmodified glass substrate It can be seen fromFig 1a that the unmodified substrate
is clean and nothing is observed on its surface, and TiO2nanorods grow radially on the unmodified substrate and form many papillae
in a random pattern, as illustrated inFig 1b and c The FE-SEM photographs of TNAs grown on the pre-coated substrate without heat treatment are shown inFig 2 It can be seen that the pre-coated substrate (Fig 2a) is different from the unmodified substrate (Fig 1a), which covered with a layer of film composed
of fine particles Top view SEM images (Fig 2b and c) show that TNAs grow more vertically on the substrate and have a better orientation after the substrate is coated with TiO2nanoparticles This phenomenon can be explained that the glass substrate is amorphous and the distribution of nucleation sites is not uniform After TiO2colloid solution is coated onto the glass substrate, the surface is covered with a layer of compact TiO2 nanoparticles Although TiO2 nanoparticles are amorphous, they can provide uniform heterogeneous nucleation sites effectively Therefore, the as-prepared TiO2nanorods have relatively better growth
orienta-Fig 1 FE-SEM images of TNAs grown on the unmodified glass substrates at a hydrothermal growth temperature of 160 8C and growth time of 3 h (a) Unmodified glass
Trang 3tion and density distribution compared with those grown on the
unmodified substrate
Fig 3 shows XRD patterns of the different substrates, which
identify the phase structure of the substrates FromFig 3, it can be
seen that a wide peak in the angle range of 2u= 15–358 appears in
all XRD patterns, which is due to the influence of amorphous phase
of the glass substrate.Fig 3a is XRD patterns of the unmodified
glass substrate, and no other peaks appear.Fig 3b shows XRD
patterns of the substrate with pre-coated TiO2nanoparticles but
not annealed It can be seen that there is still no other peaks of
crystalline TiO2, indicating that TiO2nanoparticles on the substrate
are amorphous When the substrate is coated with TiO2
nanoparticles followed by heat treatment at 700 8C, it can be
seen fromFig 3c, although existing the noise signal from the glass
substrate, the diffraction peaks of TiO2appear and can be indexed
as the rutile phase (JCPDS No 04-0551) and the anatase phase
(JCPDS No 01-0562) of TiO2, suggesting that most of TiO2
nanoparticles transform from amorphous to rutile phase, but
partial TiO2nanoparticles are anatase phase
When the substrate is coated with TiO2nanoparticles followed
by heat treatment at 700 8C, it is clearly observed fromFig 5a that
there is a mass of uniform particles with rutile crystalline phase
(Fig 3c) on the substrate
Fig 4shows XRD patterns of standard rutile phase (JCPDS No
04-0551), standard anatase phase (JCPDS No 01-0562) of TiO2and
TNAs grown on the different substrates.Fig 4a shows XRD patterns
of TNAs grown on the unmodified glass substrate, which shows
that the main phase is rutile phase of TiO2 It can be seen from
Fig 4b that the intensity of 1 1 0 peak of rutile phase increases
when TNAs grew on the substrate with pre-coated TiO2
nanoparticles but not annealed As shown in Fig 4c, after the
pre-coated substrate was annealed, the diffraction intensity of (1 1 0) planes increases even significantly A number of works[19– 22]have reported that the prepared TiO2rods via hydrothermal approach in strongly acidic solution are usually pure rutile phase This is in consistent with the results of XRD in this work However,
a small part of anatase phase of the as-prepared TNAs was detected
by XRD, which might be due to the coated TiO2 nanocrystal particles on the glass substrate FromFig 4, we also see that the diffraction intensity ratio of (1 1 0) to (1 0 1) becomes larger than that of the standard rutile phase of TiO2, indicating that the as-prepared TNAs prefer to grow along the (1 1 0) planes
As is shown inFig 5a, after annealed at 700 8C, there is a mass of uniform particles on the substrate serving as the seeds on which TiO2 nanorods grow Therefore, during hydrothermal reaction process, TiO2crystalline nucleus is apt to form on the coated TiO2 particles and grow along the (1 1 0) planes As a result, TNAs can grow on PT-substrate with well orientation and uniform density distribution, as can be seen fromFig 5b
Fig 5c is the side view SEM image of TNAs grown on PT-substrate It can be seen that there is a layer of TiO2nanocrystal particles film of about 200 nm in thickness (marked by d1) between TiO2nanorod arrays and the glass substrate (marked by d2) The length of TiO2nanorods ranges from 300 to 500 nm From the preliminary experiment, we found the following phenomena When the substrate is coated for one time, the thickness of the seed layer is thinner (about 90 nm) However, the TiO2crystal seeds are not uniformly distributed on the substrate As a result, the orientation of TiO2 nanorod arrays grown on the substrate becomes poor
In addition, the diameter distribution of TiO2nanorods grown
on PT-substrate is illustrated inFig 6 Statistics show that 70% of
Fig 2 FE-SEM images of TNAs grown on the TiO 2 -coated substrate without heat treatment at a hydrothermal growth temperature of 160 8C and growth time of 3 h (a) PT-substrate without heat treatment; (b and c) top view at different magnifications of TNAs.
Fig 3 XRD patterns of the different substrates (a) Unmodified glass substrate; (b)
PT-substrate without heat treatment; (c) PT-substrate at an annealing temperature
Fig 4 XRD patterns of standard rutile phase (JCPDS No 04-0551), standard anatase phase (JCPDS No 01-0562) of TiO 2 and TNAs grown on: (a) unmodified glass substrate; (b) PT-substrate without heat treatment; (c) PT-substrate at an annealing
Trang 4the nanorods have a diameter between 15 and 25 nm and the
average diameter of TiO2nanorods is about 21 nm Compared with
the TNAs reported previously[11,16], the as-prepared TNAs in this
work have been improved in the growth orientation, the density
distribution and the aspect ratio
It can be seen from XRD patterns and SEM images, that the
crystallinity of the prepared nanorods is low In order to improve
the quality of TiO2 nanorods, TNAs were prepared under
hydrothermal temperature of 190 8C The SEM image (Fig 7)
shows obvious tetragonal structure of rutile, which exhibits a
better crystallinity of TiO2nanorod However, it can be seen from
Fig 7that the growth orientation becomes poor, and the diameter distribution of TiO2nanorods becomes wide
The microstructure of TiO2 nanorods was further analyzed using TEM and HRTEM Two rods with a diameter of about 4 nm were selected for study Fig 8a shows the TEM image and the associated selected area electron diffraction (SAED) pattern of TiO2 nanorods The SAED inserted in the upper left corner ofFig 8a demonstrates that TiO2nanorod is a single crystal High-resolution TEM image of one single TiO2 nanorod is shown inFig 8b The lattice fringes indicate that TiO2nanorod is well crystallized The spacing between two fringes is 0.325 nm, corresponding to the interplanar distance of the (1 1 0) index planes of the rutile TiO2, indicating that the as-prepared TiO2 nanorods via hydrothermal approach are the rutile structure Furthermore, it is observed that the (1 1 0) crystal planes are perpendicular to the c-axis, implying
Fig 5 FE-SEM images of TNAs grown on PT-substrate at an annealing temperature of 700 8C and at a hydrothermal growth temperature of 160 8C and growth time of 3 h (a) PT-substrate; (b) top view of TNAs; (c) side view of TNAs.
Fig 6 Diameter distribution of TiO 2 nanorods grown on PT-substrate at an
annealing temperature of 700 8C and at a hydrothermal growth temperature of
Fig 7 SEM image of TiO 2 nanorod arrays grown on the substrate at an annealing temperature of 700 8C at a hydrothermal growth temperature of 190 8C and growth
Trang 5that TiO2nanorods grow along the (1 1 0) crystal planes, and this is
consistent with the results of XRD
Considering the effect of annealing temperature, the pre-coated
substrates were annealed at 450 and 600 8C, respectively However,
after hydrothermal reaction, TiO2nanorods have not grown on the
substrate with annealed at 450 8C, TiO2nanorods begin to grow but
the morphology is very poor when the annealing temperature
increases to 600 8C Therefore, 700 8C was chosen as the annealing
temperature of the PT-substrates in the present experiment
3.2 Growth mechanism of TNAs
Growth of TiO2nanorods on the substrate is strongly related to
rutile’s inherent growth habit and hydrothermal preparation
conditions The structure of rutile consists of chains of TiO6
octahedra along the c-axis[23] The TiO6octahedra in the chain
connect through one shared edge, while the TiO6 octahedra
between two chains connect through points, which implies that
the chemical band in the chain is stronger than that between the
chains According to PBC theory [24–26], periodic bond chain
constructs the crystal, and the direction of the strongest chemical
bond is usually the direction in which the crystal has the fastest
growth velocity Therefore, for rutile TiO2 crystal, the growth
velocity along the [0 0 1] direction is faster than that of the [1 1 0]
direction However, as for the structure of anatase TiO2, all the TiO6
octahedra connect through shared edges, which results in the same
growth velocity in all directions It is the reason that the
as-prepared TiO2nanorods are rutile phase instead of anatase phase
The formation of rutile crystal nucleus in TiCl3strongly acidic
precursor solution can be described by the following process[27]
First, single polymer [TiO(OH2)5]2+forms by the following reactive
equations:
½TiðOH2Þ63þ! ½TiðOHÞðOH2Þ52þþ Hþ! ½TiOðOH2Þ5þþ 2Hþ (2)
4½TiOðOH2Þþþ O2þ 4Hþ! 4½TiOðOH2Þ2þþ 2H2O (3)
Then the single polymers [TiO(OH2)5]2+ combine through dehydrating each other to form straight chain polymer by the edge connection Finally, the straight chain polymers connect through points to form rutile crystal nucleus
As is well known, the heterogeneous nucleation of crystalline phase in solution is easier than homogeneous nucleation [28] Therefore, TiO2 nanocrystal particles coated onto the glass substrate followed by heat treatment can be served as the seeds
of heterogeneous nucleation Moreover, the structure of crystalline seeds matches that of the prepared single crystal nanorods, which make well-aligned nanorod arrays be prepared
In addition, Clplays a significant role when TiO2growth units deposit on TiO2crystal seeds, and it can promote TiO2crystal to grow into rods instead of particles As is shown inFig 9, when NaCl
is not added to the hydrothermal precursor solution, TiO2arrays grown on PT-substrate almost do not grow into rods It can be explained that the (1 1 0) plane of rutile is positive polar-face and
Clwill be adsorbed preferentially on its surface, which prevents the contact of the TiO2growth units on the (1 1 0) surface and thus greatly limits the crystal growth along the (0 0 1) planes There-fore, Cl may be the key factor for TiO2 crystal growing along [0 0 1] direction to form rod-like structure
3.3 Photocatalytic property of TNAs Fig 10shows photodegradation rate curves of TNAs grown on different substrates and TiO2 nanoparticles for methyl blue
Fig 8 TEM images of TiO 2 nanorods grown on PT-substrate at an annealing
temperature of 700 8C and at a hydrothermal growth temperature of 160 8C and
with growth time of 3 h (a) TEM image and the associated selected area electron
diffraction of TiO 2 nanorods; (b) lattice fringes image of high-resolution TEM of one
single TiO 2 nanorod at the side surface.
Fig 9 FE-SEM image of the as-prepared TiO 2 arrays grown on PT-substrate when NaCl is not added to the hydrothermal precursor solution at a hydrothermal growth temperature of 160 8C and with growth time of 3 h.
Fig 10 Photodegradation rate curves of TiO 2 nanoparticles (a) and TNAs grown on different substrates for methyl blue solution: (b) PT-substrate without heat treatment; (c) PT-substrate; (d) unmodified substrate Hydrothermal growth
Trang 6can be seen that although the addition quantity of TNAs is much less
than that of TiO2nanoparticles, it has relatively higher
photode-gradation rate In addition, three kinds of TNAs grown on different
substrates have similar photocatalytic activity The higher
photo-catalytic activity of TNAs may result from its crystal structure and
distribution of the arrays TiO2nanoparticles have much more grain
boundary than that of single crystalline TNAs, which will make it
easier for the recombination of electron and hole pair On the other
hand, TiO2nanoparticles may be agglomerated together and reduce
the surface area, while the distribution of the TNAs will protect it
from agglomeration As a result, the photocatalytic activity greatly
improves with the prepared TNAs
4 Conclusions
Large-scale, well-aligned TNAs were fabricated on the modified
glass substrate using hydrothermal approach It has been
demonstrated that TNAs grown on PT-substrate have a better
growth orientation and uniform density compared with those
grown on the unmodified substrates TiO2 nanorod is a single
crystal rutile structure with growth direction along the (1 1 0)
crystal planes About 67% of TiO2 nanorods have a diameter
between 15 and 25 nm and the average diameter is about 21 nm,
and the length of TiO2nanorods ranges from 300 to 500 nm In
addition, it indicates that TiO2 nanoparticles coated on glass
substrate are not completely transformed into rutile phase after
heat treatment at 700 8C, and the (1 1 0) planes are the preferred
orientation The photodegradation experiment implies that single
crystal TNAs have a relatively higher photocatalytic activity
compared with polycrystal TiO2nanoparticles This work
demon-strates that the substrate pre-treatment has a strong impact on the
growth morphology of TNAs and provides a very promising
approach for the nanorod arrays preparation
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