Effect of acetylacetone on morphology and crystalline structure of nanostructured TiO2 in titanium alkoxide aqueous solution system

Article abstract of DOI:10.1246/cl.2005.736

Network structure of single-crystal-like TiO₂ nanowires with anatase phase was synthesized at low temperature by the "oriented attachment" mechanism. The most important factor for synthesis of network structure of the TiO₂ nanowires

Full text of DOI:10.1246/cl.2005.736

736
Chemistry Letters Vol.34, No.5 (2005)
Effect of Acetylacetone on Morphology and Crystalline Structure of Nanostructured TiO2
in Titanium Alkoxide Aqueous Solution System
Keizo Nakagawa, Fumin Wang, Yusuke Murata, and Motonari AdachiÃ
Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011
(Received February 1, 2005; CL-050143)
Network structure of single-crystal-like TiO2 nanowires
with anatase phase was synthesized at low temperature by the
‘‘oriented attachment’’ mechanism. The most important factor
for synthesis of network structure of the TiO2 nanowires was
elucidated as the effect of acetylacetone (ACA) on the formation
of nanowires. Morphology and crystalline structure of TiO2 were
varied by changing the mole ratio of ACA to Ti.
(a)
(c)
(b)
(d)
Nanocrystalline TiO2 is one of the most attractive oxides,
owing to its widespread applications in photocatalysis,1 and so-
lar energy conversion.2,3
Recently, a new mechanism for the formation of complex
nanostructures has been proposed, called the ‘‘oriented attach-
ment’’ mechanism.3,4 In this mechanism, nanocrystals can join
together with complete crystallographic alignment and eliminate
free surfaces. We have already reported that a network structure
of single-crystal-like TiO2 nanowires formed by the ‘‘oriented
attachment’’ mechanism using surfactant-assisted self-assem-
bling processes.3 Nanowires were composed of fused nanoparti-
cles of 2–5 nm, and these make nano-network structure. The ori-
entations of crystals of fused nanoparticles were completely
aligned because the lattice images aligned perfectly. This finding
is very interesting, because crystal growth by the oriented attach-
ment mechanism is observed below 373 K while usually much
higher temperature is needed, i.e., 433–523 K. This observation
confirmed a remarkable characteristic of this surfactant-assisted
synthesis method using acetylacetone (ACA). Here, we present
the effect of concentration of ACA on morphology and crystal-
line structure of nanostructured TiO2 and elucidate the important
factors for synthesis of TiO2 nanowires.
Typical synthesis of TiO2 nanostructures is as follows: first
laurylamine hydrochloride (LAHC) was dissolved in distilled
water. Tetraisopropyl orthotitanate (TIPT) was then mixed with
ACA in a glass vessel with desired molar ratios and immediately
added to 0.1 M LAHC aqueous solution of pH 4.6. The molar ra-
tio of TIPT:ACA:LAHC in typical synthesis is 4:4:1.3 When the
two solutions were mixed, precipitation occurred immediately.
The precipitates dissolved completely by stirring the solution
for several days at 313 K, and the solution became transparent.
The reaction temperature was then changed to 353 K. After 4
days, the solution became precipitates or white gels with a thin
yellow liquid layer. TiO2 products were separated by centrifuga-
tion. After washing with 2-propanol and successive centrifuga-
tion, TiO2 powders were dried in vacuum and calcined in air
at 573 K for 24 h.
Figure 1. TEM images of TiO2 nanostructures after reaction
at 353 K for 4 days, (a) TIPT:ACA:LAHC = 4:0:0, inset: SAED
patterns. (b) TIPT:ACA:LAHC = 4:0:1, inset: SAED patterns.
(c) TIPT:ACA:LAHC = 4:4:0, inset: SAED pattern. (d)
HRTEM image of (c).
was adjusted to 4.6 with HCl in case of the solution without
LAHC. When TIPT:ACA:LAHC was 4:0:0 and 4:0:1, white col-
loidal suspensions were formed immediately after mixing of two
reactants. Figure 1 shows the transmission electron microscope
(TEM; JEOL JEM-200CX) images and the selected area elec-
tron-diffraction (SAED) patterns of the gel samples at the differ-
ent mole ratio of TIPT:ACA:LAHC after reaction at 353 K for 4
days. When TIPT:ACA:LAHC was 4:0:0 and 4:0:1, white col-
loidal suspension were formed immediately after mixing of reac-
tants, and gelation did not occur. Nanoparticles with diameter of
8–10 nm are observed as shown in Figures 1a and 1b. Both of
SAED patterns show the Debye–Scherrer rings which can be in-
dexed to those of anatase phase and (121) diffraction of brookite
phase of TiO2. On the contrary, when TIPT:ACA:LAHC was
4:4:0, gelation occurred after reaction at 353 K. Nano-network
structure connecting nanowires with diameter 6–9 nm is ob-
served, and SAED pattern shows only anatase phase as shown
in Figure 1c. High resolution TEM (HRTEM) image of this
nano-network structure is shown in Figure 1d. Clear lattice im-
age of (101) plane of anatase phase is aligned over several par-
ticles, which indicates that the ‘‘oriented attachment’’ occurred
under this condition.
In this synthesis, two reagents were added to the TIPT solu-
tion, that is, LAHC and ACA. In order to examine the roles of
these reagents, three samples at TIPT:ACA:LAHC = 4:0:0,
4:0:1 and 4:4:0 were synthesized. The pH of aqueous solution
Figure 2 shows the X-ray diffraction (XRD; Rigaku RAD-
IIC) patterns of TiO2 nanostructures after reaction at 353 K for
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.5 (2005)
737
TiO2, JCPDS 21-1272
Anatase
(a)
(b)
TIPT : ACA : LAHC
= 4 : 4 : 0
= 4 : 0 :1
= 4 : 0: 0
TiO2, JCPDS 29-1360
Brookite
Figure 3. (a) TEM image and (b) HRTEM image of TiO2 nano-
network structure after reaction at 353 K for 4 days at TIPT:
ACA:LAHC = 4:8:1, inset: SAED pattern.
40
60
20
80
2 theta/degree
0.6
0.5
Figure 2. XRD patterns of TiO2 nanostructures at TIPT:ACA:
LAHC = 4:4:0, 4:0:1 and 4:4:0 after reaction at 353 K for 4
days.
TIPT:ACA:LAHC =4:8:1
0.4
ST-01
0.3
TIPT:ACA:LAHC =4:4:1
4 days. The peaks of nanostructures at TIPT:ACA:LAHC =
4:0:0 and 4:0:1 could be indexed to anatase phase (JCPDS file
No.21-1272) and (121) diffraction of bookite phase of TiO2
(JCPDS file No. 29-1360). However, the peaks at TIPT:ACA:
LAHC = 4:4:0 could be indexed to only anatase phase. These
results coincide with the results of TEM images and SAED pat-
terns. In addition, an increase in the peak height ratio of (004)/
(200) and a sharpening of the (004) peak are observed, which in-
dicates the presence of nanocrystalline anatase with a typical
anisotropic growth pattern along the [001] direction.
0.2
TIPT:ACA:LAHC =4:4:0
*
0.1
0
0
2
4
6
Time/min
Figure 4. Photocatalytic activity of TiO2 nanostructured mate-
rials together with ST-01.
formed by the oriented attachment mechanism from the HRTEM
image as shown in Figure 3b.
ACA has often been used in sol–gel processing as a chemi-
cal additive to reduce the reactivity of metallic alkoxides.5 The
binding of ACA to titanium and slowdown of reactivity lead
to oriented attachment. In this synthesis, the nanostructure of
TiO2 is affected by ACA considerably. First, TIPT without
ACA systems, i.e., TIPT:ACA:LAHC = 4:0:0 and 4:0:1, give
only TiO2 nanoparticles. Whereas TIPT with ACA systems,
i.e., TIPT:ACA:LAHC = 4:4:0 and 4:4:1,3 gives a network
structure of TiO2 nanowires. Thus, big morphology change is in-
duced by ACA. Second, the hydrolysis and condensation reac-
tions are significantly decreased by modification of TIPT with
ACA. This decrease in reaction rates might enable the occur-
rence of the oriented attachment, because nanoparticles pro-
duced by reaction of TIPT has enough time to select the crystal-
line face to fuse with each other. Third, the crystalline structure
of nanosize TiO2 changes, i.e., a pure anatase phase is formed in
the system including ACA, whereas some amounts of brookite
phase are included in the products when ACA is not included
in the reactants. This effect also might be attributed to the de-
crease in reaction rates, because TiO2 can select most stable
structure when they react with nanocrystals. Thus, it is revealed
that ACA plays an important role of not only making hydrolysis
and condensation reactions slow, but also controlling the mor-
phology and crystalline structure of TiO2.
Photocatalytic activity of TiO2 nanostructured materials af-
ter calcinations aÀt 573 K for 24 h was measured through the for-
mation rate of I3 due to the oxidation of IÀ to I2 in excess IÀ
condition. 10 mg of TiO2 samples were suspended by magnetic
stirring in 10 mL of 0.2 M KI aqueous solution, and the solution
was irradiated with 365-nm ray (UV-lamp 15 W). Figure 4
shows the results of photocatalytic activity in the case of com-
parison based on the weight of the TiO2 samples, together with
the standard TiO2 powders: ST-01. The activity at TIPT:ACA:
LAHC = 4:4:1 is higher than that at TIPT:ACA:LAHC =
4:4:0. The activity at TIPT:ACA:LAHC = 4:8:1 shows higher
value among these nano-network structures and is nearly equal
to that of ST-01. The specific surface areas of TiO2 samples at
the TIPT:ACA:LAHC = 4:4:0, 4:4:1, 4:8:1 and ST-01 are
126.0, 133.2, 121.9, and 300.0 m2/g, respectively. These results
including TEM images imply that LAHC and ACA would great-
ly affect the morphology and photocatalytic activity.
The authors gratefully acknowledge Professor S. Isoda
(The Institute for Chemical Research, Kyoto University) for
his assistance with TEM experiments.
References
1
A. Fujishima, T. N. Rao, and D. A. Tryk, J. Photochem. Photo-
biol., C, 1, 1 (2000).
In order to investigate the role of ACA and LAHC in detail,
the TIPT:ACA:LAHC is changed from 4:4:1 to 4:8:1. When
TIPT:ACA:LAHC = 4:8:1, nano-network structure which is
similar to the structure of TIPT:ACA:LAHC = 4:4:13 and
4:4:0, is observed and SAED pattern shows anatase phase as
shown in Figure 3a. This structure is composed of a little longer
and thicker nanowires than that of structures reported in
Figure 1c. It is found that this nano-network structure is also
2
3
B. O’Regan and M. Grazel, Nature, 353, 737 (1991).
M. Adachi, Y. Murata, J. Takao, J. Jiu, M. Sakamoto, and F.
Wang, J. Am. Chem. Soc., 126, 14943 (2004).
G. Oskam, A. Nellore, R. L. Penn, and P. C. Searon, J. Phys.
Chem. B, 107, 1734 (2003).
4
5
C. J. Brinker and G. W. Scherer, ‘‘Sol-Gel Science,’’ Academic
Press, San Diego (1990), pp 55–59, and the references therein.
Published on the web (Advance View) April 22, 2005; DOI 10.1246/cl.2005.736