Large-scale, surfactant-free, hydrothermal synthesis of lithium aluminate nanorods: Optimization of parameters and investigation of growth mechanism

Article abstract of DOI:10.1021/ic062227m

Lithium aluminate nanorods were successfully synthesized from Al ₂O₃ nanoparticles and lithium hydroxide by a simple, large-scale hydrothermal process without any surfactant or template. The various reaction parameters were optimized to achieve the maximum yield. The as-obtained nanorods had orthorhombic β-lithium aluminate structure with edges in the range of 40-200 nm and lengths of 1-2 μm confirmed by SEM, TEM, XRD, and NMR. Upon calcination at 1273 K for 12 h it transformed to γ-lithium aluminate, yet maintained the initial morphology, demonstrating the thermal stability. The ratio of lithium hydroxide to aluminum oxide showed a significant effect on the morphology as Li/Al = 1 gives microroses , whereas Li/Al = 3 and Li/Al = 15 gave microbricks and nanorods , respectively. Investigation of the mechanism showed that the nanorods were formed via a rolling-up mechanism. As we used all-inorganic raw materials and a simple synthetic procedure under mild conditions, the scale-up of this process for large-scale production should be very easy.

Full text of DOI:10.1021/ic062227m

Inorg. Chem. 2007, 46, 3176−3184
Large-Scale, Surfactant-Free, Hydrothermal Synthesis of Lithium
Aluminate Nanorods: Optimization of Parameters and Investigation of
Growth Mechanism
Upendra A. Joshi and Jae Sung Lee*
Eco-friendly Catalysis and Energy Laboratory (NRL), Pohang UniVersity of Science and
Technology (POSTECH), San 31 Hyoja-dong, Pohang 790-784, Korea
Received November 22, 2006
Lithium aluminate nanorods were successfully synthesized from Al2O3 nanoparticles and lithium hydroxide by a
simple, large-scale hydrothermal process without any surfactant or template. The various reaction parameters were
optimized to achieve the maximum yield. The as-obtained nanorods had orthorhombic
with edges in the range of 40 200 nm and lengths of 1 m confirmed by SEM, TEM, XRD, and NMR. Upon
calcination at 1273 K for 12 h it transformed to -lithium aluminate, yet maintained the initial morphology,
demonstrating the thermal stability. The ratio of lithium hydroxide to aluminum oxide showed a significant effect on
the morphology as Li/Al 1 gives “microroses”, whereas Li/Al 3 and Li/Al 15 gave “microbricks” and “nanorods”,
â-lithium aluminate structure
−
−2 µ
γ
)
)
)
respectively. Investigation of the mechanism showed that the nanorods were formed via a “rolling-up” mechanism.
As we used all-inorganic raw materials and a simple synthetic procedure under mild conditions, the scale-up of this
process for large-scale production should be very easy.
1. Introduction
Forty years ago, Lehmann and Hesselbarth5 found that
lithium aluminate exists in the two polymorphic forms of R
and γ. Lejus and Collongues6 have determined the crystal-
lographic structure of these phases. The R-form is trigonal
with a NaCl-like arrangement elongated along a 3-fold axis,
where both cations have octahedral coordination (the coor-
dination number of both cations is 6 with equidistant oxygen
neighbors). The structure consists of alternating sheets of
LiO6 and AlO6 octahedra. The structural group is R3m, and
the crystallographic density is dR ) 3.401 g/cm3. Poep-
pelmeier et al.7 have reported the recent values of the lattice
parameters of this phase: a ) b ) 2.7993 Å, c ) 14.180 Å.
The γ-phase is tetragonal with an arrangement similar to
that of crystobalite, where all atoms have tetrahedral
coordination. Both cations have coordination number 4. The
Al atoms are in place of the silicon, and the lithium atoms
occupy the tetrahedral holes in the array of the aluminum
tetrahedral. The crystallographic parameters of this form are
a ) 5.169 Å and c ) 6.268 Å, and the crystallographic
density is dγ ) 2.615 g/cm3. These parameters are very
Lithium aluminate, LiAlO2, is a potential candidate mate-
rial for the tritium breeder of a fusion reactor1 or the
electrolyte matrix of a molten carbonate fuel cell2 (MCFC)
due to its chemical and thermal stability as well as low
radiation damage susceptibility. The capture of a neutron by
6Li is known to occur as follows:
6Li + 1n f 4He + 3T
In preparing ceramic materials, it is important to be able to
have precise control over the microstructure, since the
microstructure is considered to play an important role in
controlling tritium release kinetics from the blanket.3 γ-Li-
AlO2 doped with Fe emits in the deep-red region rendering
it suitable for artificial plant illumination.4
* To whom correspondence should be addressed. E-mail: jlee@
postech.ac.kr. Phone: 82-54-279-2266. Fax: 82-54-279-5528.
(1) Loya, A. D.; Carrera, L. M.; Uren˜a-Nu´n˜ez, F.; Palacios, O.; Bosch,
P. Appl. Radiat. Isot. 2003, 58, 509.
(2) Terada, S.; Higaki, K.; Nagashim, I.; Ito, Y. J. Power Sources 1999,
83, 227.
(3) Carrera, L. M.; Jime´nez-Becerril, J.; Basurto, R.; Arenas, J.; Lo´pez,
B. E.; Bulbulian, S.; Bosch, P. J. Nucl. Mater. 2001, 299, 242.
(4) Kutty, T. R. N.; Nayak, M. Mater. Res. Bull. 1999, 34, 249.
(5) Lehmann, H. A.; Hesselbarth, H. Z. Anorg. Allg. Chem. 1961, 313,
117.
(6) Lejus, A.-M.; Collongues, R. C. R. Acad. Sci. 1962, 254, 2005.
(7) Poeppelmeier, K. R.; Chiang, C. K.; Kipp, D. O. Inorg. Chem. 1988,
27, 4524.
3176 Inorganic Chemistry, Vol. 46, No. 8, 2007
10.1021/ic062227m CCC: $37.00
© 2007 American Chemical Society
Published on Web 03/20/2007

Hydrothermal Synthesis of Lithium Aluminate Nanorods
similar to those of sodium aluminate γ-NaAlO2. Chang and
Margrave8 have documented the existence of the third
allotropic form of LiAlO2, the â phase, where Al shows a
mixed, both octahedral (R-phase) and tetrahedral (γ-phase),
coordination. The lattice parameters they reported for the
monoclinic â-phase with HBO2 arrangement are a ) 8.147
Å, b ) 7.941 Å, c ) 6.303 Å. Mu¨ller et al.9 studied the 27Al
NMR spectra of all three polymorphs of LiAlO2 and
confirmed that the â-LiAlO2 had the orthorhombic structure
reported by West10 but not a monoclinic structure. The
stability limit of these phases is crucial for fabrication of
the MCFC electrolytic matrix or tritium breeding blanket
for fusion reactors. The R-form (low-temperature phase)
transforms to the γ-form at about 1273 K, while the
metastable â-form is assumed to transform to the γ-form at
about 1073 K.
The morphology control of ceramic particle is important
as it affects the characteristic of the final product. Only a
few methods of preparing fibrous lithium aluminate are
proposed. A method of filling a micromold fiber with a
mixture of lithium and aluminum precursors followed by
burning out the micromold was reported.11 But impurities
such as binder and plasticizers were added during mixing
of the precursors, and a high temperature was needed for
burning the micromold. In the method of drawing fibrous
LiAlO2 from a melt, it is difficult to maintain the lithium
and aluminum ratio during heating at high temperatures.
Needle-shaped â-lithium aluminate was obtained by a solid-
state reaction between lithium hydroxide and alumina in
alkali hydroxide.12,13 Li et. al.14 and Lin et al.15 produced
fine γ-lithium aluminate particles from combustion synthesis
in the presence of various fuels. There are a few examples
of synthesis of lithium aluminate by the sol-gel process,16,17
which uses costly metal alkoxide as a precursor. Renoult et
al.18 and Hirano et al.19 prepared LiAlO2 by hydrolysis of
alkoxides of both metals. These routes have potential
drawbacks as they use alkoxide precursors, which are costly
and very difficult to handle for large-scale synthesis as they
hydrolyzed quickly. Jimenez-Becerril et al.20 and Kinoshita
et al.21 reported the synthesis of lithium aluminate by solid-
state reaction using all-inorganic precursors at 973-1073 K.
Several attempts have been made to produce rod-shaped
particles, but the morphology obtained was in the micrometer
scale.22
Hydrothermal synthesis has been widely used for the
preparation of nanostructured materials and has been shown
to be effective in particular for the synthesis of nanowires,
nanorods, and nanotubes23,24 as it has advantages in reducing
processing temperature and yielding a nearly site-ordered
phase while producing fine crystalline materials. Kwon and
Park25 reported the synthesis of lithium aluminate from the
sol-gel process followed by hydrothermal treatment.
Our objective is to develop a simple, large-scale process
for the synthesis of lithium aluminate nanorods from all-
inorganic precursors without using any organic precursor or
surfactant, to make the synthesis process more economic.
Recently, we reported the surfactant-free hydrothermal
synthesis of lithium aluminate microbricks and nanorods as
a preliminary communication.26 In continuation of the work,
we describe in this paper the effect of various synthesis
parameters such as lithium precursor, hydrothermal temper-
ature, time, and calcination temperature on morphology.
Furthermore, we investigated the mechanism for the forma-
tion of nanorods.
2. Experimental Section
2.1. Synthesis of Materials. All chemicals were obtained from
Aldrich and used as is without further purification. In the typical
synthesis of lithium aluminate nanorods, 0.15 mol (3.59 g) of LiOH
and 0.0098 mol (1 g) of Al2O3 nanopowder were stirred together
with 36 mL of distilled water for 1 h. Then the mixture was
transferred to a Teflon-lined reactor (100 mL) and was put into
hydrothermal reaction at a constant temperature (423 K) without
disturbing it for 3 days. The as-obtained white product was separated
by centrifugation, washed with distilled water to remove excess of
LiOH, and then dried in an oven at 373 K for 12 h. A similar
procedure was employed to investigate the effect on morphology
of various lithium precursors such as lithium acetate, lithium nitrate,
lithium carbonate, and lithium chloride. To investigate the effect
of Li/Al molar ratio and hydrothermal temperature, we varied the
concentration (Li/Al ) 3, 5, 12, and 15) and temperature (353,
373, 393, 423, and 453 K). The calcination of as-obtained product
was carried out in a quartz reactor under an air flow of 136 µmol/s
at various temperatures of 773, 973, 1073, 1173, and 1273 K.
2.2. Characterization of Materials. X-ray diffraction (XRD)
patterns were obtained on a MAC-Science M18XHF diffractometer
with Cu KR radiation (40 kV, 200 mA). The scanning electron
microscopy (SEM) analyses were carried out to see the overall
morphology of the samples on FE-SEM, FEI XL30S with an
accelerating voltage of 3 kV. The transmission electron microscope
(TEM) samples were prepared by dispersing the material in ethanol
using ultrasonication and dropping it onto a holey carbon film
supported on a copper grid and drying in an oven at 353 K. The
low-magnification TEM images were taken on a Philips CM-30
(8) Chang, C. H.; Margrave, J. L. J. Am. Chem. Soc. 1968, 90, 2020.
(9) Mu¨ller, D.; Gessner, W.; Scheler, G. Polyhedron 1983, 2, 1195.
(10) West, A. R. Z. Kristallogr. 1975, 141, 422.
(11) Fain, C. C.; Lowe, G. M.; Frianeza-kullberg, T. N.; McDonald, D. P.
Patent WO 94/03409, February 17, 1994.
(12) Watnabe, A.; Takeuchi, Y.; Saeki, G. J. Am. Ceram. Soc. 1987, 70,
C268.
(13) Saeki, G.; Watanabe, A. J. Ceram. Soc. Jpn. 1992, 100, 1421.
(14) Li, F.; Hu, K.; Li, J.; Zhang, D.; Chen, G. J. Nucl. Mater. 2002, 300,
82.
(15) Lin, W.; Bai, X.; Ling, Y.; Yang, J.; Ma, W. J. Mater. Sci. 2003, 38,
3883.
(16) Oksuzomer, F.; Koc, S. N.; Boz, I.; Gurkaynak, M. A. Mater. Res.
Bull. 2004, 39, 715.
(17) Valenzuela, M. A.; Te´llez, L.; Bosch, P.; Balmori, H. Mater. Lett.
2001, 47, 252.
(18) Renoult, O.; Boilot, J.-P.; Boncoeur, M. J. Am. Ceram. Soc. 1994,
77, 249.
(19) Hirano, S.-I.; Hayashi, T.; Kageyama, T. J. Am. Ceram. Soc. 1987,
70, 171.
(20) Jimenez-Becerril, J.; Bosch, P.; Bulbulian, S. J. Nucl. Mater. 1991,
185, 304.
(22) Kim, S.-D.; Hyun, S.-H.; Lim, T. H.; Hong, S. A. J. Power Sources
2004, 137, 24.
(23) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.;
Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353.
(24) Joshi, U. A.; Lee, J. S. Small 2005, 1, 1172.
(21) Kinoshita, K.; Sim, J. W.; Ackerman, J. P. Mater. Res. Bull. 1978,
13, 445.
(25) Kwon, S. W.; Park, S. B. J. Mater. Sci. 2000, 35, 1973.
(26) Joshi, U. A.; Chung, S. H.; Lee, J. S. Chem. Commun. 2005, 35, 4471.
Inorganic Chemistry, Vol. 46, No. 8, 2007 3177

Joshi and Lee
Figure 2. XRD pattern of as-obtained nanorods. The highly crystalline
peaks of orthorhombic â-lithium aluminate are shown by an /.
while nanowires are analogous structures with an aspect ratio
greater than 20.27 As the product obtained here has an aspect
ratio of ∼10 and cross sections of these nanorods are
rectangular, we could call them “rectangular nanorods”.
Typically by this process we can produce 100 g of nanorods
as shown in Figure 1e. Hence, this proves that our process
is easy to scale-up for large-scale production.
Figure 1. (a) Low-magnification SEM image of as-obtained nanorods (bar
scale 10 µm). (b) A high-magnification SEM image further confirms that
nanorods have edges in the range of 40-200 nm and lengths of 1-2 µm
(bar scale 2 µm). (c and d) Typical cross sections of nanorods showing a
rectangular shape. (e) LiAlO2 nanorods produced in a large scale. A typical
batch size is 100 g. The AA battery cell is shown for comparison of
dimension.
An X-ray diffraction study was carried out to find out the
structure of the as-obtained lithium aluminate nanorods.
Figure 2 shows the typical X-ray diffraction pattern of the
lithium aluminate nanorods, which confirms that the as-
obtained nanorods are crystalline, phase pure, and have the
orthorhombic â-polymorph. The unit cell parameters for
these orthorhombic â-lithium aluminate nanorods were
determined to be a ) 5.3 Å, b ) 6.3 Å, and c ) 4.9 Å,
which are in agreement with the corresponding bulk materials
(space group symmetry Pna21 (33), JCPDS 33-0785).
In order to investigate the structural environment of Al
atom in our nanorods, we carried out 27Al MAS NMR. Figure
SI-1 in the Supporting Information presents the NMR
spectrum of the lithium aluminate nanorods. The sharp peak
at 79.28 gives clear evidence of the tetrahedral coordination
of the Al in LiAlO2. As we do not observe any chemical
shift due to octahedrally coordinated Al nuclei it is confirmed
that our lithium aluminate nanorods are not in the monoclinic
structure but instead have the orthorhombic structure reported
by Mu¨ller et al.9 and West.10
Representative TEM images are shown in Figure 3. From
Figure 3a, it is evident that the nanorods have fairly uniform
lengths of 1-2 µm and diameters of 40-200 nm. In
particular, no irregular branching or bending is observed,
which implies that the LiAlO2 nanorods grow in 1D with
high crystal perfection. It is important to note that we
obtained nanorods without using any surfactant or template.
The inset of Figure 3a shows a side view of the tip of a
nanorod. These results are consistent with those of SEM. In
order to study the lattice structure and to judge the growth
direction of â-LiAlO2 rectangular nanorods, an HRTEM
instrument operated at 200 kV. Similarly, the field emission, high-
resolution electron microscope (FE-HREM) images and corre-
sponding selected area electron diffraction (SAED) patterns were
obtained on a JEOL JEM 2010F instrument equipped with a Gatan
imaging filter (GIF) system operated at 200 kV. The 27Al solid-
state NMR measurements were performed on a Varian Unity Inova
300 MHz spectrometer (7.4 T) equipped with a 7 mm Chemagnetics
MAS probe head using a sample rotation rate of 5.5 kHz. The 27Al
MAS NMR spectra were obtained at a frequency of 78.156 MHz.
The chemical shifts for 27Al were referred to the position of Al-
3+
(H2O)6 from an AlCl3 aqueous solution.
3. Results and Discussion
3.1. Surfactant-Free Hydrothermal Synthesis of LiAlO2
Nanorods. Hydrothermal treatment (at 423 K for 3 days) of
aluminum oxide nanoparticles with lithium hydroxide in the
absence of any surfactant produces uniform rod-shaped
morphology as shown in Figure 1a. Further confirmation of
the uniformity in morphology and size can be found in a
high-magnification SEM image presented in Figure 1b.
Figure 1c shows the cross section of the as-obtained
nanorods, which clearly shows that the nanorods do not have
a circular cross section but a rectangular one. More evidence
of the rectangular cross section can be found in Figure 1d.
These nanorods are typically 1-2 µm in length, and the
edges are in the range of 40-200 nm. According to
definition, nanorods are structures with a width of 1-100
nm and an aspect ratio (major axis/minor axis) less than 20,
(27) Murphy, C. J.; Jana, N. R. AdV. Mater. 2002, 14, 80.
3178 Inorganic Chemistry, Vol. 46, No. 8, 2007

Hydrothermal Synthesis of Lithium Aluminate Nanorods
Figure 4. SEM images of the products obtained by the hydrothermal
reaction of aluminum oxide with (a) lithium acetate, (b) lithium chloride,
(c) lithium carbonate, (d) lithium nitrate, (e and f) lithium hydroxide from
a different area of the same sample.
Figure 3. (a) Typical TEM image shows that the lengths of nanorods are
in the range of 1-2 µm, whereas the widths are 40-200 nm. The inset
shows the side view of a nanorod. (b) HR-TEM image clearly shows the
lattice fringes of the (110) plane. The inset shows a selected area electron
diffraction pattern.
parts e and f, in spite of exactly the same hydrothermal
conditions. This clearly shows that there must be a vital role
of the anion in the growth of the nanorods. In the literature,
Filankembo et al.28 studied the effect of various salts on the
morphology and growth of the copper nanocrystals. They
reported that, even through they used a template for the
control of the morphology, addition of the chloride ion
remarkably change the morphology of the product. In another
report, Kwon and Park25 did a similar study to find out the
effect of the precursor on the morphology of lithium
aluminate.
They carried out the sol-gel reaction followed by the
hydrothermal process of various lithium and aluminum
precursors and found that the lithium nitrate precursor gave
the platelike morphology, whereas in our study we obtained
the similar morphology with the lithium carbonate precursor.
Hence, it is clear that anions also have an important role in
the crystal growth and morphology. To identify the crystal
phase of all the hydrothermal products, we carried out X-ray
diffraction analysis. Figure 5 shows the typical X-ray
diffraction patterns of the hydrothermal reaction products
with the various lithium precursors. XRD pattern a in Figure
5 shows the â-LiAlO2 phase obtained by the reaction of
lithium hydroxide and aluminum oxide. Hydrothermal reac-
tion of lithium acetate and aluminum oxide gives the mixture
of Al2O3 and lithium acetate (indicated by “o”). It should
be noted that the peak intensity is very low showing that
the phase is not crystalline. Further hydrothermal reaction
of lithium chloride and aluminum oxide forms the LiAl2-
(OH)7·2H2O (JCPDS 40-0710) phase as indicated by pattern
c in Figure 5. A mixture of lithium carbonate (indicated by
an / in Figure 5d) and lithium aluminum oxide carbonate
hydrate (JCPDS 37-0625) is obtained by the hydrothermal
reaction of lithium carbonate and aluminum oxide. Finally,
experiment was carried out. On the basis of the calculation
of the lattice spacing and an analysis of its orientation, it
was found that the nanorods grew perpendicular to [110].
The inset of Figure 3b shows the corresponding selected area
electron diffraction (SAED) pattern.
To investigate the influence of various parameters such
as lithium precursors, hydrothermal temperature, and calcina-
tion temperature on the morphology, we carried out the same
hydrothermal reaction under different conditions.
3.2. Effect of the Lithium Precursor on the Morphol-
ogy. Hydrothermal treatments (at 423 K for 3 days) of
aluminum oxide nanoparticles with various lithium precursors
were carried out to find out their effect on the morphology
of the lithium aluminate. Figure 4 presents the SEM images
of the as-obtained product of the hydrothermal reaction.
Figure 4a shows the lump-type morphology obtained by the
hydrothermal reaction of lithium acetate and aluminum oxide
nanoparticles. It should be note that there is no particular
morphology observed in this sample. Figure 4b shows the
product obtained by the hydrothermal reaction of lithium
chloride and aluminum oxide nanoparticles. In this case, we
could not find any particular morphology either. Further
hydrothermal treatment of lithium carbonate and aluminum
oxide nanoparticles gives the platelike morphology as shown
in Figure 4c. The same reaction with the lithium nitrate and
aluminum oxide gives very fine particles. It is observed from
Figure 4d, that the particles do not have uniform morphology.
Finally, hydrothermal reaction of lithium hydroxide and
aluminum oxide nanoparticles gives the rod-shaped morphol-
ogy as shown in Figure 4, parts e and f.
It is important to note that only lithium hydroxide gives
the uniform rod-shaped morphology as shown in Figure 4,
(28) Filankembo, A.; Giorgio, S.; Lisiecki, I.; Pileni, M. P. J. Phys. Chem.
B 2003, 107, 7492.
Inorganic Chemistry, Vol. 46, No. 8, 2007 3179

Joshi and Lee
Figure 5. XRD pattern of the products obtained by hydrothermal reaction
of aluminum oxide nanoparticles with various lithium precursors: (a) lithium
hydroxide, (b) lithium acetate, (c) lithium chloride, (d) lithium carbonate,
and (e) lithium nitrate.
reaction between lithium nitrate and aluminum oxide also
gives a mixture of aluminum oxide and lithium nitrate. Thus,
only lithium hydroxide yields lithium aluminate of high phase
purity.
Poeppelmeier and Hwu29 prepared a precursor via salt
imbibition of lithium dialuminate [LiAl2(OH)6]+(OH)-‚2H2O
and LiOH·H2O to give Li/Al ) 1:1. After calcination of this
precursor at 400 °C for 100 h they obtained R-LiAlO2.7 The
main disadvantage of this route is that it required a very long
time. The product obtained in our procedure is â-LiAlO2
nanorods, without any surfactant or high-temperature calci-
nation.
Figure 6. SEM images of products obtained from the hydrothermal
reaction of aluminum oxide nanoparticles with various mole ratios of lithium
hydroxide: (a and b) Li/Al ) 1, (c and d) Li/Al ) 3, (e) Li/Al ) 7, (f)
Li/Al ) 10, (g and h) Li/Al ) 15.
parts e and f. When we change the Li/Al ratio to 15 we
obtained uniform nanorods, as presented in Figure 6, parts
g and h.
It is interesting to note that only Li/Al ) 15 gives the
nanorods and ratios of 2 and 3 give the microroses and
microbricks, respectively. Wang and Li31 reported the
synthesis of lanthanide hydroxide single-crystal nanowires
from lanthanide oxide dissolved in concentrated nitric acid
and reprecipitated by KOH. They observed a significant pH
effect, as pH ) 6-7 gave sheets, pH ) 9 gave nanowires,
and pH ) 14 also gave nanowires, but in decreased aspect
ratio. A higher pH value implies a higher OH- ion
concentration and a higher chemical potential in solution. A
higher chemical potential is preferable for the growth of
nanowires or nanorods. Thus, when we use excess LiOH
which produces higher OH- ion concentrations and hence
higher chemical potential reset, the nanorod morphology is
obtained. But in the absence of this excess OH- ion, further
crystal growth yields microbricks. Further decrease in OH-
concentration gives a curved sheetlike morphology, which
happens to look like roses. This shows that the lithium
hydroxide concentration can have a critical effect on the
morphology of the product. X-ray analyses of all the products
were carried out to find out the phases. It is seen from Figure
7 that the product obtained from the hydrothermal reaction
with Li/Al )1 is lithium aluminate hydrate (Li2Al2O4, JCPDS
01-1306, pattern a). Although the peak intensity is different
from the reference, all the peak positions match well with
that of Li2Al2O4, whereas microbricks and nanorods are
â-LiAlO2 (pattern b and e). X-ray diffraction patterns c and
3.3. Effect of Lithium Hydroxide to Aluminum Oxide
Molar Ratio on the Morphology. As we explained in the
section 3.2, only lithium hydroxide gives the rod-shaped
morphology. To study the effect of lithium to aluminum
oxide molar ratio on the morphology, we carried out the
hydrothermal reaction of various amounts of lithium hy-
droxide with aluminum oxide. The results are shown in the
form of SEM images in Figure 6. A Li/Al ratio of 1 gives
the flower-type morphology as shown in Figure 6a. These
flowers are typically 20 µm in size and look like roses; hence,
we call them “microroses”. Further evidence of this type of
morphology can be observed in Figure 6b, where a number
of microroses are aligned on one another like a bouquet. We
believe that these roses are formed due to spiral crystal
growth controlled by screw dislocations, which is a typical
text book example.30 Figure 6, parts c and d, shows SEM
images of the products from synthesis with Li/Al ) 3. From
the SEM it is clear that the morphology of the as-obtained
product looks like bricks. As the typical length is 25-27
µm and the edges are 8-9 µm, we called them “micro-
bricks”. The uniformity of the microbricks is impressive over
a wide area as shown in the Supporting Information (Figure
SI-2). Further synthesis with Li/Al ) 7 and Li/Al ) 10 gives
a mixture of nanorods and particles as shown in Figure 6,
(29) Poeppelmeier, K. R.; Hwu, S.-J. Inorg. Chem. 1987, 26, 3297.
(30) Mullin, J. W. Crystallization, 3rd ed.; Butterworth-Heinemann:
Oxford, 1997.
(31) Wang, X.; Li, Y. Angew. Chem., Int. Ed. 2002, 41, 4790.
3180 Inorganic Chemistry, Vol. 46, No. 8, 2007

Hydrothermal Synthesis of Lithium Aluminate Nanorods
Figure 7. XRD pattern of the products obtained from the hydrothermal
reaction of aluminum oxide nanoparticles with lithium hydroxide at different
Li/Al mole ratios: (a) Li/Al ) 1, (b) Li/Al ) 3, (c) Li/Al ) 7, (d) Li/Al
) 10, and (e) Li/Al ) 15.
Figure 8. SEM images showing the effect of hydrothermal temperature
on the morphology: (a) 353, (b) 373, (c) 393, (d and e) 423, and (f) 453
Table 1. Effects of the Li/Al Ratio on Structure and Morphology
K.
lithium hydroxide
to aluminum oxide
molar ratio (Li/Al)
Table 2. Effects of Hydrothermal Temperature on the Structure and
Morphology
XRD phase
morphology
microroses
hydrothermal
1
lithium aluminate hydrate
(Li2Al2O4‚xH2O)
â-LiAlO2
temp (K)
XRD phase
morphology
3
7
10
15
microbricks
353
373
393
423
453
LiOH + LiAl2(OH)7‚2H2O hexagonal plates + particles
â-LiAlO2
particles + nanorod
particles + nanorod
nanorods
LiAl2(OH)7‚2H2O
â-LiAlO2
plate-type
â-LiAlO2
particles + nanorod
â-LiAlO2
â-LiAlO2
nanorod
â-LiAlO2
agglomerated nanorods
d shows that products obtained by the Li/Al molar ratios of
7 and 10 also represent the â-LiAlO2 phase. Table 1
summarizes the morphology and crystal phases observed for
various Li/Al ratios. Thus, we can tailor the morphology from
microbricks to nanorods just by changing the Li/Al ratio and
still maintain the same phase, â-LiAlO2. The morphology
control could be very important for the various applications.
Furthermore, as we do not use any surfactant or template,
scale-up of this process for large-scale production should
be very easy.
reaction should be 423 K, as this temperature gives only
uniform nanorods.
X-ray analysis results are shown in Figure 9. From the
XRD patterns of Figure 9, parts a and b, it can be concluded
that the phases formed at the hydrothermal temperature 353
and 373 K are a mixture of LiOH and LiAl2(OH)7·2H2O and
pure LiAl2(OH)7·2H2O, respectively. It seems that LiOH
reacts with Al2O3 to form LiAl2(OH)7·2H2O at 353 K, and
as the reaction temperature increases to 393-423 K, it is
converted to the â-LiAlO2 phase as shown in Figure 9, parts
c and d. By further increase in the hydrothermal temperature
from 423 to 453 K we obtained â-LiAlO2 as the major phase
and Li2Al2O4 as the minor phase as shown in Figure 9e.
3.5. Effect of Calcination Temperature on the Mor-
phology and Thermal Stability. As-obtained lithium alu-
minate nanorods are calcined at various temperatures (773,
973, 1073, 1173, and 1273 K) under air flow. The effect of
calcination on the morphology of the nanorods can be clearly
observed in the SEM images in Figure 10. Figure 10a shows
the morphology of the sample calcined at 773 K, which
indicates that there is not much change in the morphology.
The XRD pattern (Figure 11 a) shows the â-phase for this
sample. The intensity of the peak slightly increased as
compared with that of the as-obtained sample. The calcina-
tion at 973 K also shows similar results as represented by
SEM images in Figure 10b and Figure 11b. Figure 10c shows
the SEM images of the sample calcined at 1073 K. The XRD
pattern in Figure 11c for this sample shows a mixture of â-
and γ-phases. However, the peak intensity of the γ-phase is
very small compared to that of the â-phase. It seems that
3.4. Effect of Hydrothermal Temperature on the
Morphology. The SEM images in Figure 8 show the effect
of hydrothermal temperature on morphology of the product
obtained. Figure 8a shows a mixed morphology of plates
and big particles obtained by hydrothermal reaction at 353
K. Figure 8b shows the platelike morphology obtained by
the same reaction at 373 K. It should be noted that the sample
is made of thin hexagonal plates with a typical size of 20
µm. Further SEM images showing the uniformity in the
sample are represented in the Supporting Information (Figure
SI-3). Figure 8c shows a mixture of nanorods and particles
obtained by the hydrothermal reaction at 393 K. The similar
reaction at 423 K gives very uniform nanorods as shown in
Figure 8, parts d and e. Further increase in hydrothermal
temperature to 453 K forms an agglomeration of nanorods
as shown in Figure 8f. It seems that this agglomeration occurs
to achieve the structural stability. Table 2 summarizes the
information about the morphology obtained at various
hydrothermal temperatures. From these results we can
conclude that the optimum temperature for the hydrothermal
Inorganic Chemistry, Vol. 46, No. 8, 2007 3181

Joshi and Lee
Figure 9. XRD pattern of the products obtained from hydrothermal
reaction of lithium hydroxide and aluminum oxide nanoparticles at various
hydrothermal temperatures: (a) 353, (b) 373, (c) 393, (d) 423, and (e) 453
K. The “o” in pattern e shows the minor Li2Al2O4 phase.
Figure 11. XRD pattern of samples calcined at different temperatures:
(a) 773, (b) 973, (c) 1073, (d) 1173, and (e) 1273 K. It should be noted
that pattern c shows the peaks for the â-phase as well as the γ-phase. These
changes are denoted by arrows.
Ribeiro et al.32 studied the influence of heat treatment on
the structural properties of lithium aluminate. They prepared
the γ-lithium aluminate at 1023 K by the sol-gel process.
A mixture of γ-lithium aluminate and LiAl5O8 transformed
to γ-LiAlO2 at 1073 K. Similarly, Hirano et al.19 reported
the transformation of â-LiAlO2 to γ-LiAlO2 at 1023 K. They
observed that the transformation of â-LiAlO2 to γ-LiAlO2
took place at a lower temperature by 473 K than that of
R-LiAlO2 to γ-LiAlO2. The â-to-γ transformation as previ-
ously studied by West9 may takes place by a simple
topotactic mechanism. The reason we did not observe any
morphology change in the thermally treated nanorods may
be the smooth transformation from â-phase to γ-phase
(topotactic) with minimal atomic rearrangement, which
would not disrupt the initial morphology.33 In contrast,
R-phase to γ-phase transformation requires important struc-
tural modifications which may cause instability in the
structure and morphology. Thus, the thermal stability of
LiAlO2 depends on its phase.
Figure 10. SEM images of the nanorods calcined at various temperatures
for 12 h: (a) 773, (b) 973, (c) 1073, (d) 1173, and (e) 1273 K.
the â-phase transforms slowly to the γ-phase. Further
calcination at 1173 K (Figure 11d) shows the majority of
γ-phase peaks, and the â-phase totally vanishes (also
Supporting Information Figure SI-4 clearly shows this
difference). It is important to note that the morphology of
the nanorods is still maintained as shown in Figure 10d
although a few nanorods (<10%) are broken to small
particles. Calcination at 1273 K for 12 h gives the phase-
pure γ-lithium aluminate as shown in the XRD pattern of
Figure 11e. The morphology change in this sample is not
much; most of the nanorods (>90%) still maintain their
morphology. Interestingly, when we calcined our microroses
sample (Li2Al2O4) at 500 °C for 10 h we obtained R-LiAlO2.
In the literature many reports showed the synthesis of
R-LiAlO2 via equimolar mixture of lithium and aluminum
precursor. For example Poeppelmeier et al.7 also reported
the synthesis of high surface area R-LiAlO2 by calcination
of a lithium dialuminate and lithium hydroxide mixture. Here
we obtained the same R-LiAlO2 phase after calcination of a
Li2Al2O4 precursor.
3.6. Investigation of the Formation Mechanism of
Lithium Aluminate Nanorods. To investigate the formation
mechanism of nanorods, a hydrothermal experiment was
carried out while aliquots of reaction mixture were taken at
a regular time intervals of 12 h. The SEM images obtained
by these experiments are shown in Figure 12. As shown in
Figure 12a the agglomeration of particles with fibrous
morphology was observed after the reaction time of 12 h.
After 24 h the fibers grew to form dendrite nanostructures.
It is interesting to note that these dendrites are made up of
hexagonal sheets as shown in Figure 12b. These dendrites
become unstable and break down to separate hexagonal
sheets. These sheets start rolling as shown in Figure 13. After
36 h we observe a mixture of nanorods and nanofibers
(Figure 12c). As we provide more time and maintain the
(32) Ribeiro, R. A.; Silva, G. G.; Mohallen, N. D. S. J. Phys. Chem. Solids
2001, 62, 857.
(33) Lee, J. S.; Volpe, L.; Ribeiro, F. H.; Boudart, M. J. Catal. 1988, 112,
44.
3182 Inorganic Chemistry, Vol. 46, No. 8, 2007

Hydrothermal Synthesis of Lithium Aluminate Nanorods
Figure 14. Graphical representation of the events happening during the
nanorods formation.
energy compared to that of nanorods under hydrothermal
conditions during the first 12 h of reaction. At the same time,
the growth units in the solution would diffuse to form the
fibrous structures as observed by SEM in Figure 12a. The
growth of these structural units leads to lower surface energy
forming dendrites having hexagonal sheets as shown in
Figure 12b and Figure 13a (indicated by the dotted line).
Further hydrothermal treatment under the same conditions
makes the dendrites break down, and the formed individual
hexagonal sheets start rolling to achieve thermodynamic
stability. The nanosheets formed during the early stages of
the hydrothermal reaction and the existence of the half-rolled
sheet (Figure 13) provides strong evidence for the “rolling-
up” mechanism. As we provide more time the sheets rolled-
on leading to a small number of nanorods as observed by
SEM after 36 h. After 36 h, the reaction seems to be
kinetically controlled as nanorods grew at the cost of small
fibers and particles. This growth continues until all the fibers
or particles vanish, and fully grown nanorods are obtained
after 72 h. These entire events of nanorod formation via a
roll-on mechanism can be represented schematically on a
time scale as shown in Figure 14. The reason to form
rectangular nanorods as opposed to the more common
cylindrical nanorods is still unclear.35 We simply assume that
as the basic structural unit of our nanorods is orthorhombic
â-lithium aluminate, they grow to form an orthorhombic
nanostructure.
Figure 12. SEM images of the samples taken at regular intervals to find
the formation mechanism: (a) 12, (b) 24, (c) 36, (d) 48, (e) 60, and (f) 72
h.
4. Conclusions
Lithium aluminate nanorods were synthesized by a simple,
surfactant-free, hydrothermal method without using any
template or catalysts from all-inorganic raw materials. The
as-obtained nanorods had orthorhombic â-lithium aluminate
structure with rectangular cross section confirmed by XRD,
NMR, SEM, and TEM. Various parameters such as lithium
precursor, lithium hydroxide to aluminum oxide ratio,
hydrothermal temperature, and calcination temperature have
crucial effects on the growth of nanorods. The presence of
a high concentration of lithium hydroxide (Li/Al ) 15), 423
K hydrothermal reaction temperature, and 3 days of reaction
time are the prerequisites to obtain fully grown lithium
aluminate nanorods with 100% yield. The study on the effect
of calcination temperature showed that the morphology of
the as-obtained nanorods was fairly well preserved even after
calcination at 1273 K for 12 h. Yet, their phase transformed
from â-lithium aluminate to γ-lithium aluminate. The na-
norods appeared to be formed via a “rolling-up” mechanism.
Figure 13. SEM images of a sample treated hydrothermally for 24 h; (a)
shows the dendrite-type morphology formed by the hexagonal sheets. Images
b and c show the top view of the rolling hexagonal nanosheets; arrows
indicate the roll-on direction.
same conditions, these nanofibers disappeared and nanorods
grew. After a further increase in time to 48 h the number of
nanorods grows significantly as shown in Figure 12d. After
60 h of hydrothermal reaction, the nanorods yield reaches
70-80% as confirmed by SEM in Figure 12e. Finally, after
72 h of reaction, 100% nanorods are obtained as seen in
Figure 12f.
We believe that the 1D lithium aluminate nanostructures
are formed by a mechanism very much like the rolling up
of thin sheets reported by Zhang et al.34 for lead telluride
nanowires. Aluminum oxide nanoparticles would spontane-
ously dissolve into solution due to a relatively higher free
(34) Zhang, L.; Yu, J. C.; Mo, M.; Wu, L.; Kwong, K. W.; Li, Q. Small
2005, 1, 349.
(35) Joshi, U. A.; Yoon, S.; Baik, S.; Lee, J. S. J. Phys. Chem. B 2006,
110, 12249.
Inorganic Chemistry, Vol. 46, No. 8, 2007 3183

Joshi and Lee
Typically 100 g of product was obtained on the lab-scale
synthesis. As the method is very simple and we do not use
any expensive organic precursor, the scale-up of this process
should be very easy for large-scale production.
Korea Ministry of Education and Human Resources Devel-
opment through the BK 21 program.
Supporting Information Available: SEM images of micro-
bricks and hexagonal sheets. This material is available free of charge
via the Internet at http://pubs.acs.org.
Acknowledgment. This work has been supported by the
Hydrogen R&D center. We appreciate the support of the
IC062227M
3184 Inorganic Chemistry, Vol. 46, No. 8, 2007