J. Am. Ceram. Soc., 89 [8] 2660–2663 (2006)
DOI: 10.1111/j.1551-2916.2006.01104.x
r 2006 The American Ceramic Society
Journal
Titania Nanoflowers with High Photocatalytic Activity
Jin-Ming Wuw and Bing Huang
Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China
Min Wang
Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong
Akiyoshi Osaka
Biomaterials Laboratory, Faculty of Engineering, Okayama University, Okayama-shi 700-8530, Japan
X-ray diffraction (XRD) experiments were performed using a
Rigaku D/max-3B diffractometer (Tokyo, Japan) with CuKa
radiation, operating at 40 kV and 36 mA (l 5 0.154056 nm).
The morphology of the titania was examined using a LEO 1530
(Carl Zeiss Smt Ltd., Cambridge, UK) field emission scanning
electron microscope (FE-SEM). Before SEM examination, the
samples were sputter coated with a thin gold coating in order to
improve their electrical conductivity.
The photocatalytic property was evaluated using RhB as a
model organic. Photodegradation of 100 mL RhB aqueous solution with an initial concentration of 0.01 mM, in the presence
of the titania film (5 cm 5 cm), was conducted in a pyrex
reactor with a water jacket. The irradiation was provided by a
450 W high-pressure mercury lamp 6 cm above the solution. The
average intensity of UV irradiance reaching the samples was
measured to be ca. 1.4 mW/cm2, using a UV irradiance meter
(Model: UV-A, Beijing Normal University, China, measured for
the wavelength range of 320–400 nm with a peak wavelength of
365 nm). The solution was stirred continuously by a magnetic
stirrer and exposed to air during the photocatalytic reaction. The
solution temperature was maintained at around 301C using a
water-cooling system. The change in RhB concentration was
monitored using a 752 UV-VIS spectrophotometer (Shanghai
Spectrophotometer Factory, Shanghai, China) at a wavelength
of 555 nm, with a quartz cuvette of 1 cm of the optical path
length. For comparison, nanoparticles of commercial Degussa
P-25 TiO2 (a mixture of anatase and rutile, with an average particle size of 30 nm and a BET surface area of 50 m2/g) were deposited on the pretreated Ti plates by repeatedly dipping them in
the P-25 ethanolic suspension (25 g/L) and drying in air at 1051C
to obtain a film weight identical to that of a titania nanoflower
film, ca. 0.9 mg/cm2, which was estimated by weight changes in
the Ti plates before and after titania nanoflower synthesis, assuming that the titanium mass remained constant. When estimating the film weight, it was ensured that titania powders
remaining in the solution and also those loosely attaching to the
film were collected, weighted, and used for calculations.
Titania with nanostructures has attracted considerable attention
due to its potential use in catalysts, gas sensors, photovoltaic
cells, photonic crystals, etc. This paper reports the synthesis of
titania nanoflowers by simply oxidizing pure titanium with hydrogen peroxide solutions containing hexamethylenetetramine
and nitric acid at a low temperature of 353 K. Titania nanoflowers with the crystal structure of anatase or a mixture of
anatase and rutile were obtained after a subsequent thermal
treatment to crystallize the as-precipitated amorphous structure.
Photocatalytic tests revealed an excellent photocatalytic property of the titania nanoflowers.
I. Introduction
T
ITANIA with nanostructures has attracted considerable attention due to its wide use in fields of photocatalysts, gas
sensors, photovoltaic cells, photonic crystals, etc. To date, titania with one-dimensional nanofeatures of rod,1,2 wire,3 whisker,4 and tube4,5 has been fabricated using various physical,
chemical, or electrochemical techniques. A three-dimensional
(3D) nanostructure, ‘‘nanoflower,’’ has been found to exhibit
very unique properties.6 In this paper, we report the synthesis of
titania nanoflowers, for the first time in the literature (to our
knowledge), through a simple solution approach. The excellent
photocatalytic property of the titania nanoflowers produced was
confirmed by photodegradation of trace rhodamine B (RhB) in
water.
II. Experimental Procedure
Titanium plates of dimensions 5 cm 5 cm 0.01 cm were
etched in a 1:3:6 (in volume) mixture of a 55 mass% HF aqueous solution, a 63 mass% HNO3 aqueous solution, and distilled
water, for 2 min. They were then cleaned in an ultrasonic bath.
Each Ti plate was immersed in 50 mL 30 mass% H2O2 solution
that contained 100 mg hexamethylenetetramine (HMT) and
0.2–4.0 mL 63 mass% HNO3, and kept for 3 days in an oven
maintained at 801C. The surface-oxidized Ti was then rinsed
gently in distilled water and subjected to a thermal treatment at
4001C for 1 h in order to obtain crystallized titania nanoflowers.
III. Results and Discussion
Figure 1 shows the FE-SEM morphology of titania films obtained by oxidizing Ti with 30 mass% H2O2 solutions containing HMT and HNO3 of different concentrations, followed by
heating at 4001C for 1 h. When 0.04M HNO3 was used, titania
films with cracks were found covering the whole Ti plate surface
(Fig. 1(a)). The film consisted of spindle-like particles with sizes
of tens of nanometers (Fig. 1(b)). When the HNO3 concentration increased to a value beyond 0.2M, uniform titania nanoflowers of hundreds of nanometers in diameters were obtained
(Figs. 1(c)–(h)). The nanoflowers appeared similar in general
L. Klein—contributing editor
Manuscript No. 21379. Received January 14, 2006; approved March 17, 2006.
This work was supported by the Natural Science Foundation of China under Grant No.
50502029.
w
Author to whom correspondence should be addressed. e-mail: [email protected]
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Fig. 1. Field emission scanning electron microscope morphology of titania nanoflowers obtained by oxidation of Ti with 30 mass% H2O2 solutions
containing 0.014M hexamethylenetetramine and 0.04M (a, b), 0.2M (c, d), 0.4M (e, f), and 0.8M (g, h) HNO3, followed by heating at 4001C for 1 h.
morphology, that is, several separate dendrites growing outward
in certain directions from the main trunk. The dendrites became
sharper while growing from the trunk to form tips outside, just
like petals of a flower. The width of the dendrites decreased
gradually from ca. 50 to 100 nm on the edge connecting to the
trunk to a few nanometers on the outer edge. The top-center of
the nanoflower was a bundle of several well-aligned nanorods
with morphologies similar to the dendrites (single arrows,
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Fig. 2. Field emission scanning electron microscope image showing the
dendritic growth of titania from a trunk. The titania was obtained by
oxidizing Ti with 30 mass% H2O2 solutions containing 0.014M hexamethylenetetramine and 0.2M HNO3, followed by heating at 4001C
for 1 h.
Figs. 1(d), (f), and (h)). The double-arrowed region in Fig. 1(d)
shows a broken nanorod bundle on the top-center.
The morphology of titania nanoflowers was affected by the
HNO3 concentration. When 0.2M HNO3 was used, the separate
dendrites exhibited mainly fourfold symmetry. The top-center
nanorods bundles were much smaller in diameter. With increasing HNO3 concentration, the titania tended to form well-aligned
nanorods bundles. The diameters of the bundles also increased,
but fewer separate dendrites were found.
Figure 2 shows a high-magnification FE-SEM micrograph
showing the nucleation of nanorods with various sizes on the
primary rod. The nanosized grains covering the nanorods
homogeneously can be ascribed to the ion-sputtered gold.
Figure 3 shows the XRD patterns of titania films obtained by
oxidizing Ti with 30 mass% H2O2 solutions containing HMT
and 0.04, 0.2, and 0.4M HNO3, respectively, followed by heating
at 4001C for 1 h. As a reference, the XRD pattern of titania
obtained by using only H2O2 without any HMT or HNO3 is also
given (Fig. 3(a)). Without HMT or HNO3 in the solution, a
mixture of anatase and rutile, with rutile growing in a preferential (101) plane as evidenced by the XRD pattern, was achieved
directly after soaking the Ti plate in the H2O2 solution, which
remained unchanged after the subsequent thermal treatment.
The surface morphology was identified to be well-aligned nanorods and has been reported previously.2 The addition of HMT
and HNO3 inhibited the low-temperature crystallization of titania and only amorphous titania was identified after the reaction at 801C for 3 days. The subsequent thermal treatment at
4001C for 1 h induced total crystallization of the amorphous titania, while their morphologies remained unchanged. Pure anatase was obtained when the HNO3 concentration was 0.04M
(Fig. 3(b)) and 0.2M (Fig. 3(c)). The titania nanoflowers were
identified to be a mixture of anatase and rutile when 0.4M
HNO3 was used (Fig. 3(d)).
The decomposition of H2O2 was accelerated in the presence
of Ti because of its catalytic effect. At the same time, Ti was
attacked by H2O2 to form Ti(OH)4 in the solution, which was
unstable thermodynamically and decomposed to form titania
sol.2,7 Without any additives (i.e., the HMT and HNO3 used in
the current investigation) in the H2O2 solution, crystalline titania nanorods were obtained after 3 days of immersion at 801C
through a dissolution–precipitation mechanism.2 In the current
investigation, titania nanoflowers were obtained through a
similar procedure.
At 801C, HMT decomposed when added in the solution to
give ammonia ions,8 which also increased the pH of the solution,
ðCH2 Þ6 N4 þ 6H2 O þ 4Hþ ¼ 6HCHO " þ4NHþ
4
(1)
The ammonia formed during the reaction would be adsorbed
on specific planes of titania, which we believe contributed predominantly to the formation of nanoflowers. The addition of
HNO3 in the solution decreased the pH value and stabilized the
titania sol and hence decreased the driving force for titania nucleation.9 Thus, it became preferable for titania to nucleate on
the previously precipitated titania rods (Fig. 2). Therefore, with
increasing amounts of HNO3 in the solution, more nanorods
nucleated on the primary rod and the rods nearby tended to
grow to form bundles so as to minimize the surface energy.
It has been reported that an acidic environment favors the
formation of rutile.7,9 Therefore, it is not surprising that the titania nanoflowers obtained using 0.4M HNO3 were mixtures of
anatase and rutile, while those obtained using 0.2M HNO3 were
pure anatase, as can be seen from Fig. 3.
Figure 4 displays the photodegradation curves of RhB in water, assisted by the P-25 film and the titania nanoflower film that
is shown as Fig. 1(c). Without the titania film (consisting of either a P-25 or a titania nanoflower), few RhB molecules decomposed under the UV radiation. For an RhB aqueous solution
with a dilute initial concentration, the photodegradation process
followed roughly the pseudo-first-order reaction10:
c ¼ c0 expðktÞ
Fig. 3. X-ray diffraction patterns of titania nanoflowers obtained by
oxidizing Ti with 30 mass% H2O2 solutions (a) and containing 0.014M
hexamethylenetetramine and 0.04M (b), 0.2M (c), 0.4M (d), and HNO3,
followed by heating at 4001C for 1 h.
Vol. 89, No. 8
(2)
where c0 and c are the initial concentration and the concentration after the reaction duration t, respectively, and k is the pseudo-first-order reaction rate constant. Re-plotting the data
obtained in the ln(c0/c)Bt scale (shown as an inset in Fig. 4)
gave k values of 5.9 103 and 5.6 103 min1 for the titania
nanoflower film and the P-25 film, respectively. The anatase
nanoflower film thus exhibited photocatalytic property comparable to that of the commercial P-25 film, which has been well
recognized as an excellent photocatalyst. This is a general comparison in performance, bearing in mind that the phase composition, microstructure, primary and secondly particle size, and
specific surface area of the two titania films were different, all of
which affect readily the photocatalytic property.11 However,
through the current investigation, it is shown, at least, that the
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IV. Conclusions
1.0
Blank
P-25
Nanoflower film
c/c0
0.8
Titania nanoflowers with the phase composition of pure anatase
or a mixture of anatase and rutile have been synthesized through
a simple solution approach. The nanoflowers are very stable and
can withstand a heat treatment at 4001C. Such titania with 3D
nanostructures possesses excellent photocatalytic property.
0.6
References
0.4
1
0.2
0.0
0
60
120 180 240
Irradiation time /min
Fig. 4. Photodegradation of rhodamine B in water assisted by the P-25
film and the titania nanoflower film shown in Fig. 1(c). The inset shows
the data re-plotted in the ln(c0/c)Bt scale.
titania nanoflower film can possess nearly the same photocatalytic ability to assist photodegradation of RhB in water as that
of P-25 film. We believe that the nanoflower structure contributed positively to the photocatalytic property, due to the large
surface area as a result of the unique nanofeature. It is also
possible that the nanoflowers can constrain the intermediates of
the photocatalytic reactions and hence improve the possibility of
their being further oxidized, as suggested by Walker et al.12
However, the nanoflowers may increase the scattering of the UV
light and hence may have a negative effect on the photocatalytic
activity. Further studies are therefore required to clarify the
structure–property relationship of titania nanoflower and its
photocatalytic performance.
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