Hydrothermal routes to various controllable morphologies of nanostructural lithium aluminate

Article abstract of DOI:10.1016/j.materresbull.2006.11.018

The α-LiAlO₂ powders have been successfully prepared by a hydrothermal route based on using the surfactant of heax-adecyltrimethyl ammonium bromide (CTAB) as the template. One-dimensional (1D) nanorods with higher and lower aspect ratio, 2D mesoporous microsheets were respectively observed with different concentration of the surfactants. A high specific surface area up to 151 m²/g was obtained by this method. The formation mechanism of the nanostructural lithium aluminate was proposed.

Full text of DOI:10.1016/j.materresbull.2006.11.018

Materials Research Bulletin 42 (2007) 1407–1413
www.elsevier.com/locate/matresbu
Hydrothermal routes to various controllable morphologies of
nanostructural lithium aluminate
*
Linfeng Hu , Bin Qiao, Zilong Tang, Zhongtai Zhang
State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering,
Tsinghua University, Beijing 100084, PR China
Received 3 May 2006; received in revised form 5 November 2006; accepted 7 November 2006
Available online 18 December 2006
Abstract
The a-LiAlO
2
powders have been successfully prepared by a hydrothermal route based on using the surfactant of heax-
adecyltrimethyl ammonium bromide (CTAB) as the template. One-dimensional (1D) nanorods with higher and lower aspect ratio,
D mesoporous microsheets were respectively observed with different concentration of the surfactants. A high specific surface area
2
2
up to 151 m /g was obtained by this method. The formation mechanism of the nanostructural lithium aluminate was proposed.
2006 Elsevier Ltd. All rights reserved.
#
Keywords: A. Nanostructures; B. Chemical synthesis; C. Electron microscopy; C. X-ray diffraction
1
. Introduction
The development of nanoscience and nanotechnology requires precise control over the crystal structures, sizes and
dimensionalities of materials in nanoscale. Up to now, many novel nanostructures have been successfully synthesized,
such as nanotubes/nanowires/nanorods [1–3], and nanobelts [4]. Usually, a specific method might be applied to the
synthesis of a certain class of materials that share similar crystal structures in nature; so it seems particularly important
to study the formation mechanism of such materials under certain conditions.
Lithium aluminate (LiAlO ) exhibits excellent thermo-chemical stability and favorable irradiation behavior. It is
2
widely used as a matrix material in molten carbonate fuel cells (MCFC) and as a tritum-breeding material in nuclear
fusion reactor [5,6]. Furthermore, the addition of 3 wt% 10 mm LiAlO particles into polymeric electrolyte of lithium
2
secondary battery can enhance the ionic conductivity [7]. LiAlO presents three different crystalline phases: a-, b- and
2
g-LiAlO , which are hexagonal, monoclinic and tetragonal, respectively. Wherein, g-LiAlO is the stable phase at high
2
2
temperature, and a- or b- LiAlO phases transform to g-LiAlO at an elevated temperature [8]. Previously, LiAlO was
2
2
2
prepared by conventional solid state methods, combustion synthesis or sol–gel methods [9–11]. High temperature and
long time reaction were required in the solid state reactions, and the sol–gel method had a disadvantage in controlling the
2
morphology of particles. Recently, LiAlO particles with a high surface area of 45 m /g, were prepared by a ultrasonic
2
spray pyrolysis from an aqueous solution of aluminum nitrate and lithium salt [12]. Sokolov and Stein [13] synthesized
LiAlO with three dimensional periodic arrays of interconnected macropores by a colloidal crystal templating (PMMA)
2
*
Corresponding author. Tel.: +86 10 62772623; fax: +86 10 62772623.
E-mail address: hlf05@mails.tsinghua.edu.cn (L. Hu).
0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2006.11.018

1408
L. Hu et al. / Materials Research Bulletin 42 (2007) 1407–1413
2
wet chemical process, of which the surface area was 56 m /g. However, these products contained some subsidiary
substances and it was not easy to have high purity.
Presently, hydrothermal synthesis is normally applied for the preparation of different inorganic materials in a
nanocrystalline state, such as mesoporous silicon [14], MgO nanorods [15], carbon nanotubes [16]. Here we reported a
hydrothermal method for preparation a series of nanostructural a-LiAlO in controllable morphologies using a single-
2
chain cationic surfactant (CTAB) as template. 1D nanorods with higher and lower aspect ratio, 2D mesoporous
microsheets were respectively obtained by the method of changing the concentration of the surfactant. The specific
surface area of the hydrothermal product was much higher than the reported results. The synthesis procedure provided
a simple method to prepare nanostructure lithium aluminate.
2
. Experiment
The starting materials selected were LiOHÁH O (A.R.), aluminum iso-propoxide (AIP) (A.R.), CTAB (A.R.). The
2
details of the preparation were indicated as follows: distilled water was added slowly to the mixture of LiOHÁH O, AIP
2
and CTAB. The molar ratio of initial reagents for each sample was shown in Table 1. (The critical micelle
À4
concentration (CMC) value of CTAB at room temperature is 9.9 Â 10 M [17].) The mixture was stirred 30 min with
ultrasonic irradiation to obtain the homogeneous gel. Then the well-mixed gel was put into a Teflon autoclave vessel.
The hydrothermal reaction was followed at 463 K under autogenous pressure for 10 h. The precursor was washed
several times by absolute ethanol, dried at 353 K for 10 h, and then calcined at 773 K for 4 h in air.
A JEOL JEM-2011 transmission electron microscopy (TEM) with acceleration voltage of 200 kVand a JEOL JSM-
6460LV scanning electron microscopy (SEM) were used to observe the morphology of the samples. The phase
structure of the as-produced sample was characterized by a Bruker D8 advance X-ray diffractometer with
˚
monochromatized Cu Ka radiation (l = 1.5418 A). The special-surface area was estimated by the Brunauer–Emmett–
Teller (BET) four points method, on the basis of the nitrogen gas adsorption isotherm (77.4 K) with a NOVA4000 high
gas sorption analyzer. The pore size distribution was also analyzed with the same apparatus.
3
. Results and discussion
3
.1. Phase structure
The LiAlO phase was not found from the precursor that obtained after hydrothermal reaction in Fig. 1a. The
2
precursor, composed of CTAB (JCPDS No. 48-2454), AIP (JCPDS No. 30-1508) and LiOH (JCPDS No. 01-1021),
showed sharp and intense diffraction peaks, which indicated that precursor was well crystalline. The as-prepared
¯
samples after calcination at 500 8C can be characterized to be a hexagonal (R3m) a-LiAlO phase (Fig. 1b), with
2
˚
lattice constants a = 2.800 A and c = 14.18 A (JCPDS No. 44-0024). This result demonstrated that starting materials
˚
did not change during the hydrothermal process and a-LiAlO phase was generated from the precursor during the heat-
2
treatment stage. There were no significant differences in the wide-angle patterns between the samples of A–C.
Compared with the precursor, the weak and broad diffraction peaks for the products suggested that the products might
be composed of very small crystals with the size in nanoscale.
The insert of Fig. 1b was the small angle diffraction patterns for the precursor and the as-prepared sample C. The clear
diffractions peaks at around 3.58 and 78 could be observed from the XRD patterns of the precursor, showing the uniform
ordering of the pore structure. However, the XRD patterns of the sample C after calcination did not show any resolved
diffraction peak, which implied absence of meso-structure order in the pore arrangement. This results showed that the
uniformly meso-structured precursor has been obtained under the hydrothermal conditions because of the existence of
the surfactant template. However, the ordering meso-structure was completely lost during the calcination process.
Table 1
The molar ratio of initial reagents for preparation of each sample
Sample
LiOHÁH
2
O (mmol)
AIP (mmol)
CTAB (mmol)
Distilled water (mmol)
A
B
C
600
600
600
600
600
600
0.050 (30 CMC)
0.860 (500 CMC)
5.180 (3000 CMC)
100
100
100

L. Hu et al. / Materials Research Bulletin 42 (2007) 1407–1413
1409
Fig. 1. XRD patterns for the precursor (a) and as-prepared samples (b). Insert is the small-angle diffraction pattern of sample C (precursor and as-
prepared sample).
3
.2. Morphologies
Representative TEM images for each sample were presented in Fig. 2. The morphologies of the products were
different with the concentration of surfactants changed.
It was observed that the product powders of sample A exhibited a morphology of nanorods. The diameter of the
nanorods was about 20–40 nm, with 150–300 nm in length, and aspect ratios (length/width) of about 4–7. The
HRTEM micrograph of the sample showed the d112 interlayer spacing of nanorods was about 0.16 nm (see Fig. 2b),
which was a little bigger than the literature values 0.14 nm (JCPDS No. 44-0024). The morphology of nanorods could
also be found in the TEM images of sample B (Fig. 2c). However, these nanorods showed higher aspect ratio. The
diameter of the nanorods for sample B was about 5–15 nm, and their lengths varied from 150 nm to 500 nm, with the
length/width ratio about 30 (see in Fig. 2d). Moreover, among the nanorods of sample A and B, it seemed that some
nanosheets could be found. Sample C displayed a micrograph of only thin rectangular microsheets, with 2.25 mm and
1
.85 mm in width and length, respectively (Fig. 2e). From the HRTEM image for sample B, many disordered
mesopores (pore size about 5–7 nm) were found on the microsheet (Fig. 2f). The selected-area electron diffraction
SAED) patterns of sample C showed the whole microsheet a single crystal structure. The SEM image for sample C
was shown in Fig. 3, which indicated the thickness of the microsheet was 95 nm, approximately.
(

1410
L. Hu et al. / Materials Research Bulletin 42 (2007) 1407–1413
Fig. 2. Typical TEM micrographs of the samples with the selected area electron diffraction (SAED) pattern inserted.: (a) The nanorods image for
sample A. (b) HRTEM image for sample A. (c) The nanorods with higher aspect ratio micrograph for sample B. (d) TEM image of an individual
nanorod for sample B, with the HRTEM image inserted. (e) The microsheet structure for sample C. (f) HRTEM image of sample C.
3
.3. Specific surface area and pore distribution
Specific surface area of the samples was calculated according to nitrogen adsorption measurements. The specific
2
surface area of samples A–C was 124 m /g, 143 m /g and 151 m /g, respectively, all of which were much higher than
2
2

L. Hu et al. / Materials Research Bulletin 42 (2007) 1407–1413
1411
Fig. 3. SEM image for the sample C.
the reported results [12,13]. Sample C, as a typical example, its isotherm of nitrogen adsorption and desorption (Fig. 4)
demonstrated the typical diffusion bottleneck structure, which exhibited large hysteresis loop of type E with sloping
adsorption branch and steep desorption branch. The hysteresis type was due to the presence of different size of
spheroidal cavities with the same entrance pore diameter, so the pore had a ink-bottle shape. The adsorption on
isotherm at low pressure could be assigned to the monolayer coverage of particle surface, the adsorption at the medium
pressure ( p/p = 0.4–0.85) mainly due to capillary condensation of N in mesopores, while at the high pressure
0
2
( p/p = 0.85–0.99) it was likely contributed by aggregated pores. Pore size distribution was then determined by the
0
Brunauer–Joyner–Halenda (BJH) method. A narrow mesopore size distribution, 4–7 nm was achieved, which was
Fig. 4. Nitrogen adsorption–desorption isotherms and pore size distribution of sample C.

1412
L. Hu et al. / Materials Research Bulletin 42 (2007) 1407–1413
quite consistent with the result estimated from the TEM micrograph in Fig. 2. The whole pore volume of sample C was
0
3
.1769 cm /g.
3
.4. The formation mechanism analysis
It was clear that the starting materials were not changed during the hydrothermal reaction process, so lithium
aluminate phase was produced at the calcination stage. Since the formation mechanism of lithium aluminate
nanostructures is not completely clear, an assumed interpretation for the observed nanostructes can be stated as
follows:
It was known that molecules of CTAB aggregated as templates when the concentration was beyond CMC. For
samples A and B, surfactants formed nanocapsules micelle templates and LiOH and AIP were packaged in the
capsules. During the heat treatment process, the surfactants were decomposed so the nanorods were formed.
Meanwhile, the precursor converted to a-LiAlO2. The length of nanocapsule depended on the surfactant
concentration, that is, low surfactant concentration favored shorter and thicker micelles and high surfactant
concentration favored longer and thinner micelles. In the starting materials, the surfactant concentration for sample B
(nCTAB = 500 CMC) was higher than sample A (nCTAB = 30 CMC). For this reason, sample A showed nanorods
micrograph with lower aspect ratio, and sample B presented nanorods micrograph with higher aspect ratio. This
proposed model was supported by the other observations from the synthesis of MCM-41 silica [18]. The proposed
formation mechanism of the nanorods of lithium aluminate was illustrated schematically in Fig. 5a.
When the surfactant concentration was up to much higher, such as 3000 CMC for sample C, the formation of the
mesoporous microsheets may be suggested in a different way. According to the theory of Jeffrey SB [19], molecules of
CTAB aggregated as lamellar liquid crystal template. The molecule of CTAB was consisted of two tails: one was the
worm-like hydrophobic headgroup, and the other was the ball-like hydrophilic headgroup. All the hydrophilic
headgroups aggregated together at one side of the layer, and all the hydrophobic aggregated at another side, so the
layer had a ball-like side and a worm-like side. Then LiOH and AIP accumulated at the ball-like side of the lamellar
liquid crystal template and also formed a layer shape (called LiOH-AIP layer). Enlargement of one single crystal of
Fig. 5. Proposed formation mechanism of the nanorods (a) and mesoporous microsheets (b) of lithium aluminate.

L. Hu et al. / Materials Research Bulletin 42 (2007) 1407–1413
1413
LiOH-AIP layer resulted in the products with highly regular shape (rectangular shape). Under the autogenous pressure
of hydrothermal process, the molecules of CTAB could be removed more easily, which led to the LiOH-AIP layer with
long-range ordered porosity. Finally, surfactants were eliminated and the long-range ordering was destroyed by
calcination, many ink-bottle cylindrical pores were formed, which was in great agreement with the result of N2
adsorption analysis. Fig. 5b illustrated schematically the proposed formation mechanism of the mesoporous
microsheets.
4
. Conclusions
In summary, we have succeeded in controllable synthesizing of 1D nanorods/2D mesoporous microsheets
a-LiAlO by controlling the concentrations of the surfactant CTAB. LiAlO nanorods with low and high aspect ratio
2
2
were respectively prepared when the surfactant concentration is 30 CMC, 500 CMC. When the surfactant
concentration was up to much higher such as 3000 CMC, mesoporous microsheet was obtained, which show a high
2
specific surface area of 151 m /g. The value of surface area was much higher than the reported results.
Acknowledgements
Authors gratefully acknowledge the support of National Natural Science Funds of China (Grant Nos. 50472005 and
0372033), as well as the Basic Research Funds of Tsinghua University (Grant No. JC 2003040).
5
References
[
[
[
[
[
[
[
[
[
1] L.H. Qu, C.Q. He, Y. Yang, Mater. Lett. 59 (2005) 4034.
2] J.R. Li, Z.L. Tang, Z.T. Zhang, Electrochem. Commun. 7 (2005) 62.
3] T.Z. Ren, Z.Y. Yuan, B.L. Su, Chem. Phys. Lett. 374 (2003) 170.
4] Y.J. Li, L.P. You, R. Duan, Solid State Commun. 129 (2004) 233.
5] S. Terada, I. Nagashima, K. Higaki, J. Power Sources 75 (1998) 223.
6] M. Nishikawa, T. Kinjyo, S. Beloglazov, J. Nucl. Mater. 335 (2004) 70.
7] M. Morita, T. Fujisaki, M. Yoshimoto, Electrochim. Acta 46 (2001) 1565.
8] R.A. Ribeiro, G.G. Silva, N.D.S. Mohallem, J. Phys. Chem. Solids 62 (2001) 857.
9] G.Y. Hou, J. Zang, Y.L. Wang, Rare Met. Mater. Eng. 33 (2004) 1351.
[
[
[
[
[
[
[
[
[
[
10] F. Li, K. Hu, J.L. Li, J. Nucl. Mater. 300 (2002) 82.
11] F. Oksuzomer, S.N. Koc, I. Boz, Mater. Res. Bull. 39 (2004) 715.
12] Y.C. Kang, S.B. Park, J. Colloid Interface Sci. 182 (1996) 59.
13] S. Sokolov, A. Stein, Mater. Lett. 57 (2003) 3593.
14] W.Z. Zhou, J. Klinowski, Chem. Phys. Lett. 292 (1998) 207.
15] Y. Li, Z. Jing, Y.D. Li, Mater. Chem. Phys. 76 (2002) 119.
16] W.Z. Wang, J.Y. Huang, D.Z. Wang, Carbon 43 (2005) 1317.
17] K. Cui, Q. Cai, X.H. Chen, Micropor. Mesopor. Mater. 68 (2004) 63.
18] Q. Cai, Z.S. Luo, F.Z. Cui, Chem. Mater. 13 (2001) 262.
19] S.V. Jeffey, C.V. James, J. Colloid Interface Sci. 269 (2004) 394.