Bundles of self-assembled boehmite nanostrips from a surfactant free hydrothermal route

Article abstract of DOI:10.1016/j.jallcom.2007.07.036

Boehmite nanostrips with average diameter and crystallite sizes of about 19.5 nm and 4.5 nm were prepared through a surfactant free approach and were characterized with XRD, TEM and DTA/TG analyses. Nanostrips self-assembled to form bundles due to lateral surface adsorptions. The effect of Cl^- ions on enhancement of one-dimensional growth of boehmite nanostructures was investigated. The thermal analysis showed that chlorides reduced the formation temperature of α-Al₂O₃ phase.

Full text of DOI:10.1016/j.jallcom.2007.07.036

Journal of Alloys and Compounds 461 (2008) 551–554
Bundles of self-assembled boehmite nanostrips
from a surfactant free hydrothermal route
Hamed Arami, Mahyar Mazloumi,
Razieh Khalifehzadeh, S.K. Sadrnezhaad
∗
Materials and Energy Research Center, P.O. Box 14155-4777, Tehran, Iran
Received 17 May 2007; accepted 14 July 2007
Available online 19 July 2007
Abstract
Boehmite nanostrips with average diameter and crystallite sizes of about 19.5 nm and 4.5 nm were prepared through a surfactant free approach
and were characterized with XRD, TEM and DTA/TG analyses. Nanostrips self-assembled to form bundles due to lateral surface adsorptions. The
−
effect of Cl ions on enhancement of one-dimensional growth of boehmite nanostructures was investigated. The thermal analysis showed that
chlorides reduced the formation temperature of ␣-Al
2 3
O phase.
©
2007 Published by Elsevier B.V.
Keywords: Nanostructures; Chemical synthesis; Thermal analysis
1
. Introduction
tion of the formation temperature of ␣-Al2O3 phase in order
to impede the grain growth occurrence during high tempera-
ture calcination processes [18]. Nucleation aids such as alumina
During the last decade, one-dimensional (1D) nanostructures
have attracted many researchers due to their weird properties and
applications in nanoscience and nanotechnology [1,2]. Among
extensive literature on the formation of 1D nanostructure com-
pounds, alumina nanostructure phases (i.e. boehmite, ␥- and
(␣-Al2O3) seeds [18], ␣-Fe2O3 [19], V2O [20] and fluorides
5
[21,22] were reported to have significant influence on reducing
the ␣-Al2O3 phase formation temperature. Also, it was indicated
that the fluoride ions enhanced the directional growth of the
alumina nanostructures [12]. Although several researchers have
investigated the effect of chlorides on physicochemical [23], fib-
rils particle sizes [24] and hydrothermal preparation of boehmite
nanostructures [25], no distinct investigation has been attributed
to the effect of chloride ions on formation of one-dimensional
alumina nanostructures and the ␣-Al2O3 phase transformation
temperature. Therefore, the following surfactant free hydrother-
malprocesswasdevelopedtoinvestigatetheorientedattachment
[2,26] of nanocrystals which culminated in obtaining the 1D
␣
-Al2O3) have been reported in various morphologies such
as nanotubes [3–5], nanowires [6], nanorods [7,8], nanofibers
[
[
9,10], lace-like nanoribbons [11] and plate-like nanostructures
12]. Although the thermodynamically stable ␣-Al2O3 phase
can be obtained through a sequence of topotactic [13] and recon-
structive [14] transformations (i.e. boehmite → ␥ → ␦ → ␪ → ␣
[
15]), the morphology remains unchanged and the final prod-
ucts have the same shape as the initial phases [16]. Therefore,
boehmite nanostructures with novel morphologies are relevant
precursors for obtaining ␣-Al2O3 nanoarchitectures. Here, bun-
dles of boehmite nanostrips were prepared through a facile
surfactant free hydrothermal route and were shown to transform
to the ␣-phase at a lower temperature than what conventionally
−
boehmite nanostructures under the influence of Cl ions. The
thermal behavior of the obtained nanostructures was also char-
acterized to determine the effect of chloride ions on ␣-Al2O3
formation temperature.
◦
was reported in the literature (i.e. >1050 C [17]).
One of the important challenges in thermal transformations
of alumina ceramics systems has been attributed to the reduc-
2
. Experimental
All the raw materials were purchased from Merck, Darmstadt, Germany and
used as received without further purification. In a typical experiment, 0.36 g
AlCl3·6H2O powder was dissolved into 10 ml ethanol under vigorous stirring
at room temperature. The mixture was fluxed until a homogenous solution
∗
Corresponding author.
E-mail address: arami@merc.ac.ir (S.K. Sadrnezhaad).
0
925-8388/$ – see front matter © 2007 Published by Elsevier B.V.
doi:10.1016/j.jallcom.2007.07.036

5
52
H. Arami et al. / Journal of Alloys and Compounds 461 (2008) 551–554
was obtained. Then 40 ml deionized water was added slowly to the solution
and the mixture was refluxed for 2 h. The resultant colorless solution was put
the adjacent single nanostrips through the following suggested
mechanism.
into a Teflon-lined stainless steel autoclave with autogenous pressure control at
The initial dissolution of the raw material in the alcoholic
solution led to the reduction of the hydroxylation rate of pre-
cursor and inhibition of crystallites growth. Therefore, in this
investigation the AlCl3·6H2O powder was dissolved into ethanol
toinhibitthefastgrowthofthecrystallitesaftersubsequentwater
addition. The following reaction is expected for the formation
of Al-alcoholic complex clusters:
◦
2
00 C for 24 h. Then the autoclave was cooled down to the room temperature,
naturally. A gelatinous precipitate was obtained which was then centrifuged,
◦
washed with deionized water and dried in an oven at 70 C for 24 h. The crys-
talline phase of the obtained powder was characterized with X-ray diffraction
analysis (XRD, Philips X’Pert diffractometer) between the Bragg’s diffraction
◦
angle range of 5–70 and the pattern was plotted after ␣2-stripping. The mor-
phologies and nanostructures were determined with a 200 keV transmission
electron microscope (TEM) equipped with a field emission gun (FEG-STEM,
Philips CM200). Also, differential thermal analysis (DTA/TG, STA1640) was
AlCl3·6H2O + 148C2H OH
◦
◦
5
performed with a heating rate of 5 C/min from room temperature up to 1200 C
to evaluate the transformation temperatures and thermal behavior of the obtained
product.
→
Al(OC2H )3·145C2H OH + 6H2O + 3HCl
(1)
5
5
Subsequent water addition culminated in occurrence of hydrox-
ylation process on the surface of the obtained complex clusters
and formation of Al(OH)3 colloidal crystallites:
3
. Results and discussions
X-ray diffraction results illustrated that the obtained prod-
uct was composed of pure boehmite phase with orthorhombic
crystal structure, Amam space group (63) and cell constants
Al(OC2H )3·145C2H OH + 3H2O
5
5
→
Al(OH)3 + 148C2H OH
(2)
5
˚
˚
˚
of about a = 3.7 A, b = 12.22 A and c = 2.66 A. The XRD peaks
were well matched with ␥-AlOOH reported in the JCPDS
File No. 21-1307. The average crystallite size of the obtained
boehmite powder was calculated through Scherrer formula
Because of the presence of hydroxyl groups on the surface
of aluminum trihydroxide colloids, an ion exchange would
−
occur between surface OH groups and Cl ions in the solu-
−
tion. These Cl -capped Al(OH)3 clusters form directionally due
(
d = 0.9λ/B cos θ, where d, λ, B and θ are average crystallite
to the oriented attachment mechanism [2,26] and the influence
size, Cu Kα wavelength (0.1541 nm), full width at half maxi-
mum intensity (FWHM) of (0 2 0) peak in radians and Bragg’s
diffraction angle, respectively) to be about 4.5 nm (Fig. 1).
The transmission electron microscopy (TEM) observations
reveal the boehmite nanostrips with average diameter of
19.5 nm and average length of about 300 nm (Fig. 2a–c). The
selected area electron diffraction (SAED) pattern of the nanos-
trips shown inset of Fig. 2a, illustrated the dotted reflections of
−
of Cl ions in promoting the directional growth (Fig. 3). In
hydrothermal conditions these one-dimensional Al(OH)3 clus-
ters, transformed to boehmite (AlOOH) nanostrips.
It has been shown previously that fluoride ions can enhance
the directional growth and form 1D nanostructures [12], due
to the formation of an AlOF intermediate compound [12,22].
In order to further assess the above mechanism and demonstrate
∼
−
the effect of Cl ions in promoting the one-dimensional growth,
(
1 3 1) and (0 2 2) crystallographic planes and a pale ring pattern
a same experiment was carried out utilizing Al(NO3)3·9H2O as
the starting material. In this case the following reactions can
occur:
attributed to the (2 0 2) plane of boehmite orthorhombic crys-
tal structure, indicating the polycrystalline and nanocrystalline
nature of the obtained boehmite nanostrips. The strip-like mor-
phologyofthenanostructuresisclearlyobviousin Fig. 2b, which
indicates two individual nanostrips with mesoporous structures.
As Fig. 2c exhibits, bundles of nanostrips have formed due to
the lateral self-assembly of several individual nanostrips, pos-
sibly as the result of the presence of hydrogen bonds between
Al(NO3)3·9H2O + 148C2H5OH
→
Al(OC2H )3·145C2H OH + 9H2O + 3HNO3
(3)
5
5
In contrast to the previously mentioned experiment, it is clear
−
that the ionic species in this case is NO3 . TEM investigations
(
Fig. 4) elucidated the formation of nanoparticles, indicating that
−
Cl ions have an effective influence on 1D growth of boehmite
in the previous case.
In the boehmite lattice, oxygen ions are arranged in a dis-
torted octahedral configuration around aluminum and organized
in parallel layers linked by hydrogen bonds [27]. Therefore, the
individual boehmite nanostrips self-assemble laterally due to the
surface hydrogen bonds and form bundles of boehmite nanos-
trips as indicated in Figs. 2c and 3. Ma et al. [7] have indicated
that the formation of arrays of boehmite nanorods was due to the
effect of sodium dodecyl benzene sulfonate (SDBS) surfactant.
In this investigation, such bundles of nanostrips were formed
without the influence of any surfactant.
The thermal behavior of the obtained boehmite nanostruc-
tures were characterized through DTA/TG investigations. The
differential thermal analysis (DTA) and thermogravimetric (TG)
Fig. 1. XRD pattern of the obtained product.

H. Arami et al. / Journal of Alloys and Compounds 461 (2008) 551–554
553
Fig. 2. TEM images of the obtained (a) boehmite nanostrips (inset is the diffraction pattern), (b) two individual nanostrips and (c) bundles of nanostrips.
curves are exhibited in Fig. 5. Four distinguishable areas can
be recognized in the curves due to the thermal transformations
occurred on the boehmite powder. The total weight loss and
transformation temperatures related to each stage are elucidated
in Table 1. Stage A is related to the evaporation of physically
and chemically absorbed water with a slight endothermic peak
sented in the literature. The ␣-Al2O3 formation temperature in
nanoscale systems was reported to occur in the temperatures
◦
more than ∼1000 C [14,17]. Various researchers have investi-
gated the effect of additives in reducing the temperature of the
mentioned phase transformation [18–20]. Among them, fluo-
rides were reported to reduce the transformation temperature to a
high extent [12,21,22]. However, there is no distinct result in the
◦
around 350 C, while stage B is attributed to the endother-
mic transformation of boehmite to transition ␥-Al2O3 phase at
◦
about 470 C with a total weight loss of 11.27%. The exother-
◦
mic peak around 535 C at stage C is due to elimination of
volatiles. Stage D with the least weight loss, is an important part
in determining the thermal behavior of the obtained powders
◦
since the ␣-Al2O3 phase formation occurred at around 945 C
which is a low temperature in comparison with the data pre-
Fig. 3. Schematic representation of the proposed formation mechanism of the
boehmite nanostrips and bundles of nanostrips.
Fig. 4. TEM image of boehmite nanoparticle obtained through utilization of
Al(NO3)3·9H2O as the precursor.

5
54
H. Arami et al. / Journal of Alloys and Compounds 461 (2008) 551–554
ent numbers of nanostrips due to lateral hydrogen interactions.
The suggested formation mechanism which proposed the for-
−
mation of nanostrips as a result of Cl ions in enhancement of
oriented attachment mechanism was confirmed by relevant data.
Differential thermal analysis of the obtained product showed
a reduction in ␣-Al2O3 phase formation temperature. Totally,
it was shown that chloride ions can enhance the directional
growth and decrease the phase transformation temperature in
the obtained product.
References
[
[
[
1] M. Reches, E. Gazit, Science 300 (2003) 625–627.
2] A.P. Alivisatos, Science 289 (2000) 736–737.
3] Y.F. Mei, X.L. Wu, X.F. Shao, G.S. Huang, G.G. Siu, Phys. Lett. A 309
Fig. 5. DTA/TG curves related to the thermal analysis of the obtained boehmite
nanostrips.
(
2003) 109–113.
4] L. Pu, X. Bao, J. Zou, D. Feng, Angew. Chem. Int. Ed. 40 (2001)
490–1493.
[
[
Table 1
1
Total weight loss and transformation temperature of the obtained nanostrips
during each step of thermal analysis
5] H.C. Lee, H.J. Kim, S.H. Chung, K.H. Lee, H.C. Lee, J.S. Lee, J. Am.
Chem. Soc. 125 (2003) 2882–2883.
◦
[6] Z.L. Xiao, C.Y. Han, U. Welp, H.H. Wang, W.K. Kwok, G.A. Willing,
J.M. Hiller, R.E. Cook, D.J. Miller, G.W. Crabtree, Nano Lett. 2 (2002)
Total weight loss (%)
Transformation temperature ( C)
A
B
C
D
6.44
11.27
13.55
2.35
350
470
535
945
1
293–1297.
[
[
7] M.G. Ma, Y.J. Zhu, Z.L. Xu, Mater. Lett. 61 (2007) 1812–1815.
8] S.C. Kuiry, E. Megen, S.D. Patil, S.A. Deshpande, S. Seal, J. Phys. Chem.
B 109 (2005) 3868–3872.
[
9] Y. Li, J. Liu, Z. Jia, Mater. Lett. 60 (2006) 3586–3590.
[
10] H.Y. Zhu, J.D. Riches, J.C. Barry, Chem. Mater. 14 (2002) 2086–2093.
[11] S.C. Shen, W.K. Ng, Q. Chen, X.T. Zeng, R.B.H. Tan, Mater. Lett. 61
2007) 56–61.
literature concerning the influence of chlorides in decreasing the
-Al2O3 formation temperature. Here, it is proposed that such
reduction in the formation temperature of thermodynamically
(
␣
[
[
12] Y. Wu, Y.F. Zhang, X.X. Huang, J.K. Guo, Ceram. Int. 27 (2001) 265–268.
13] W.D. Kingery, H.K. Bowe, D.R. Uhlman, Introduction to Ceramics, 2nd
ed., Wiley, New York, 1976.
14] F.S. Yen, H.S. Lo, H.L. Wen, R.J. Yang, J. Cryst. Growth 249 (2003)
283–293.
stable ␣-alumina phase is attributed to the effect of preliminary
−
mentioned Cl ions capped on the surface of boehmite nanos-
[
[
[
[
trips and formation of an intermediate AlOCl compound similar
to the previous results of the usage of fluorides as the addi-
tives [21,22]. Probably the same as AlOF [21,28], AlOCl can
accelerate the mass transportation from transition alumina to ␣-
alumina. These results demonstrate that chloride ions have the
same influence as fluorides on enhancement of the directional
growth and reducing the phase transformation temperature of
the alumina phases, however its precise influence needs further
investigations.
15] M. Digne, P. Sautet, P. Raybaud, H. Toulhoat, E. Artacho, J. Phys. Chem.
B 106 (2002) 5155–5162.
16] H.Y. Zhu, X.P. Gao, D.Y. Song, S.P. Ringer, Y.X. Xi, R.L. Frost, Micropor.
Mesopor. Mater. 85 (2005) 226–233.
17] F.S. Yen, M.Y. Wang, J.L. Chang, J. Cryst. Growth 236 (2002) 197–209.
[18] M. Kumagai, G.L. Messing, J. Am. Ceram. Soc. 68 (1985) 500–505.
[
[
[
[
[
19] J.L. McArdle, G.L. Messing, J. Am. Ceram. Soc. 76 (1993) 214–222.
20] G.C. Bye, D.G. Gavin, J. Am. Ceram. Soc. 57 (1974) 55–57.
21] Y. Wu, Y. Zhang, G. Pezzotti, J. Guo, Mater. Lett. 52 (2002) 366–369.
22] J. Li, Y. Wu, Y. Pan, W. Liu, J. Guo, Ceram. Int. 33 (2007) 919–923.
23] M.L.G. Castillo, F.H. Beltr a´ n, J.J. Fripiat, A.R. Hern a´ ndez, R.G. de Le o´ n,
J.N. Bola n˜ os, A.T. Cervantes, X. Bokhimi, Catal. Today 107 (2005)
4
. Conclusion
874–878.
[
[
[
[
24] S. Furuta, H. Katsuki, H. Takagi, J. Mater. Sci. Lett. 13 (1994) 1077–1080.
25] D. Mishra, S. Anand, R.K. Panda, R.P. Das, Mater. Lett. 53 (2002) 133–137.
26] R.L. Penn, J.F. Banfield, Science 281 (1998) 969–971.
Boehmite nanostrips with average diameter of about 19.5 nm
and crystallite size of 4.5 nm were prepared through a surfac-
tant free hydrothermal route from a low cost raw material (i.e.
AlCl3·6H2O). Self-assembled bundles were formed from differ-
27] A.C. Pierre, D.R.J. Uhlmann, Non-Cryst. Solids 82 (1986) 271–276.
[28] K. Daimon, E. Kato, J. Cryst. Growth 75 (1986) 348–352.