X-ray diffraction and IR spectrum for activated surface hydrolysis of Al metal into AlO(OH)·αH2O nanocrystals in a new monoclinic crystal structure

Article abstract of DOI:10.1006/jssc.2000.9034

Activated spontaneous surface hydrolysis of nascent Al metal in distilled water at room temperature results in AlO(OH)·αH₂O (boehmite) nanocrystals of an average 35 nm diameter. The nanocrystals are separated by 2 to 5 nm with a fairly sharp size distribution in the electron micrograph. Their X-ray diffraction is analyzed by assuming a monoclinic crystal structure with lattice parameters a=0.866 nm, b=0.506 nm, c=0.983 nm, and β=94.56°. It is a new structure of boehmite in comparison to the orthorhombic bulk structure. It involves 3.3 times the lattice volume in an orthorhombic lattice enclosed in a smaller surface area, S=3.7×10³ m²/g, as compared to 4.5×10³ m²/g in it. This structure forms in a fast Al surface hydrolysis reaction in this example because it undergoes a fast release of its total surface energy as a function of its fast growth. The molecular vibrational structure of AlO(OH)·αH₂O nanocrystals is analyzed from their IR spectrum which has three characteristic bandgroups at 200-1200 cm^-1, 1300-2500 cm^-1, and 2800-4000 cm^-1. A considerably improved value of the O-H group stretching frequency by as much as 360 cm^-1 relative to the bulk value appears for a proton localized structure in the nanocrystals.

Full text of DOI:10.1006/jssc.2000.9034

Journal of Solid State Chemistry 157, 40}49 (2001)
doi:10.1006/jssc.2000.9034, available online at http://www.idealibrary.com on
X-Ray Diffraction and IR Spectrum for Activated Surface Hydrolysis
of Al Metal into AlO(OH) ' aH2O Nanocrystals in
a New Monoclinic Crystal Structure
S. Rana and S. Ramꢀ
Materials Science Centre, Indian Institute of Technology, Kharagpur-721 302, India
Received July 5, 2000; in revised form October 27, 2000; accepted December 1, 2000
pure transition aluminas or sapphire and Al O composites
ꢁ ꢂ
Activated spontaneous surface hydrolysis of nascent Al metal
in distilled water at room temperature results in AlO(OH) '
aH2O (boehmite) nanocrystals of an average 35 nm diameter.
The nanocrystals are separated by 2 to 5 nm with a fairly sharp
size distribution in the electron micrograph. Their X-ray di4rac-
tion is analyzed by assuming a monoclinic crystal structure with
lattice parameters a ؍ 0.866 nm, b ؍ 0.506 nm, c ؍ 0.983 nm,
and b ؍ 94.563. It is a new structure of boehmite in comparison
to the orthorhombic bulk structure. It involves 3.3 times the
lattice volume in an orthorhombic lattice enclosed in a smaller
(1}5). Its microstructure, i.e., particle morphology, size, and
separation between particles, plays a major role in monitor-
ing the microstructure and other properties of the product
according to experimental conditions. An AlO(OH) ' aH O
ꢁ
precursor of con"ned particle size on a nanometer scale is
useful for deriving a nanocrystalline or mesoporous Al O
ꢁ ꢂ
powder by its controlled thermal decomposition at moder-
ate temperature. Today, the two classes of materials are
used extensively as heterogeneous catalysts, as absorption
surface area, S ؍ 3.7 ؋ 103 m2/g, as compared to 4.5 ؋ 103 m2/g media, and in several other optical and electronic devices
in it. This structure forms in a fast Al surface hydrolysis reaction
in this example because it undergoes a fast release of its total
surface energy as a function of its fast growth. The molecular
vibrational structure of AlO(OH) ' aH2O nanocrystals is ana-
lyzed from their IR spectrum which has three characteristic
bandgroups at 200+1200 cm؊1, 1300+2500 cm؊1, and 2800+
4000 cm؊1. A considerably improved value of the O+H group
stretching frequency by as much as 360 cm؊1 relative to the bulk
value appears for a proton localized structure in the nanocrystals.
( 2001 Academic Press
and components (1}8). Their utility is manifested in their
re"ned microstructures which allow molecules access to
large internal surface areas and cavities that enhance
catalytic activity and adsorption properties. Mesoporous
materials are typically amorphous or nanocrystalline as
per their con"ned dimension of particles and pores on
a nanometer scale.
Several methods, e.g., spray pyrolysis (1), gas condensa-
ꢂ>
tion (2), adiabatic combustion of Al salts (3), sol}gel or
alkoxide method (4, 9), and surface hydrolysis of Al metal
Key Words: X-ray di4raction analysis; IR analysis; AlO(OH) '
aH2O nanocrystals; surface hydrolysis of Al metal; monoclinic (5), are available for preparing a precursor of AlO(OH) '
boehmite crystal structure.
aH O. A special advantage of the last method, which is used
here, over the other methods is that it easily provides separ-
ꢁ
ated AlO(OH) ' aH O particles of a controlled size as small
ꢁ
as 20 to 50 nm. They are in a new monoclinic crystal
ꢀꢃ
INTRODUCTION
structure in comparison to the D orthorhombic bulk
crystal structure (9}12).
ꢁF
AlO(OH) ' aH O (boehmite) is one of the most important
In this article, we report a structural analysis of
raw materials of aluminum and its oxides. It is the main AlO(OH) ' aH O nanocrystals in monoclinic crystal struc-
component of many kinds of bauxites, and regarding the ture using X-ray di!raction and infrared spectrum. The
ꢁ
ꢁ
utility of the latter, the structure and properties of the results are discussed in correlation with X-ray di!raction
boehmite component play a determining role. Moreover, (9}12) and IR or Raman spectroscopy (11}13) of the bulk
synthetic boehmite is used in synthesizing electronically sample. A con"ned AlO(OH) ' aH O structure in nanocrys-
ꢁ
tals results in a modi"ed IR spectrum of the bulk sample. It
presents a proton localized structure with an improved
value of O}H group stretching frequency by as much as
\ꢀ
ꢀ To whom correspondence should be addressed. Fax: (091) 3222-55303.
E-mail: sram@matsc.iitkgp.ernet.in.
360 cm over the bulk value.
40
0022-4596/01 $35.00
Copyright ( 2001 by Academic Press
All rights of reproduction in any form reserved.

XRD AND IR OF BOEHMITE NANOCRYSTALS FORMATION
41
EXPERIMENTAL
monotonically only over higher temperatures to 800 K.
A value of a+0.70 is calculated from a total of 29.8% loss
Synthesis
in mass of the sample in (2a#1)H O. A reasonably larger
ꢁ
AlO(OH) ' aH O nanocrystals are synthesized by surface a51 appears in the sample derived from sol}gel or other
ꢁ
hydrolysis of a nascent Al metal in water at room temper- wet methods (5, 9). No Al(OH) ) ' xH O forms in this reac-
ꢂ
ꢁ
ature (RT). In this process, #at surfaces of a thin Al metal tion. It has a larger mass loss as small as 34.6% even at x"0.
plate (99.5% pure) of thickness &1.5 mm were degreased, Usually, x51, and it re#ects a 47% or larger mass loss.
washed in distilled water, and treated with 0.1 M HCl for
The X-ray di!ractogram of the sample is analyzed with
10 min. Excess acid, if any, was carefully washed away along a PW 1710 X-ray di!ractometer using "ltered CoKa radia-
with oxidized Al metal at the surface in distilled water. The tion of wavelength j"0.17902 nm. The average particle
refreshed Al metal surface still has a thin surface-oxidized size and microstructural characteristics are studied with
layer. As such it does not permit hydrolysis in air or water. a JEM 200CX transmission electron microscope in conjunc-
A reactive Al surface is obtained on rubbing the refreshed tion with an analyzer for in situ studies of electron di!rac-
\ꢀ
surface with a small amount (10 mg) of HgCl . In this tion. The infrared spectrum (from 200 to 4000 cm ) is
ꢁ
ꢁ>
process, Hg cations, which come in intimate contact with measured for a sample dispersed in a KBr pellet in a 10:90
Al metal at the surface, are immediately reduced to Hg ratio with a JASCO FTIR-5300 infrared spectrophotometer.
\ꢀ
metal by an instantaneous coreduction reaction with Al The reported frequency values are accurate to $2 cm for
\ꢀ
metal through a thinned surface oxide layer, 2Al#3HgCl
sharp bands and $10 cm for the broad bands.
ꢁ
P2AlCl #3Hg. The obtained Hg metal, in this well-
ꢂ
known reaction (5), forms a thin amalgam with the nascent
RESULTS AND DISCUSSION
Reaction Process
Al surface, and the surface oxides, if any, segregate and #oat
over it. Those are easily washed along with excess HgCl , if
ꢁ
any, and the AlCl by-product in distilled water, leaving
ꢂ
A fast surface hydrolysis of Al atoms occurs at the nascent
Al surface in a fast reaction with H O molecules as soon as
the sample is immersed in distilled water at RT. It is a highly
exothermic reaction which occurs by chemical dissociation
of H O at the Al metal surface, H O 0 H #OH . The
OH anions instantaneously react with Al atoms and hy-
drolyze them into AlO(OH) ' aH O while the H cations
behind a desirably reactive sample of the nascent Al surface.
ꢁ
As soon as it is immersed in distilled water at RT, it rapidly
hydrolyzes into AlO(OH) ' aH O nanocrystals with a vigor-
ꢁ
ous exothermic reaction, Al#(a#2)H OPAlO(OH) '
>
\
ꢁ
ꢁ
ꢁ
ꢁ
ꢂ
aH O# H . Finally, the AlO(OH) ' aH O sample is "l-
\
ꢁ
ꢁ
ꢁ
tered and dried at RT. It is a "nely divided loose powder.
The synthesis of such a re"ned powder is not so easy by wet
methods (1}5), which often result in a wet gel with several
by-product impurities.
>
ꢁ
are released as H gas.
ꢁ
As proposed earlier (5), AlO(OH) ' aH O forms in this
example through intermediate reactions in a self-controlled
ꢁ
manner in successive steps,
Measurements and Analysis
-&\
-&\
ꢂ>
ꢁ>
>
Al!3e (Al ) &" [Al(OH)] &" [Al(OH) ] P
The nanocrystalline AlO(OH) ' aH O powder is stable at
ꢁ
ꢁ
room temperature. On heating, it desorbs part of the aH O
ꢁ
and dissociates to Al O with two primary thermal signals
ꢁ ꢂ
ꢁ
?& -
ꢀ
AlO(OH)# H &" AlO(OH) ' aH O.
[2]
ꢁ
ꢁ
ꢁ
A and B at &350 and 575 K (5) with 14.0 and 13.1% mass
loss, respectively, as follows:
ꢁ>
>
The metastable [Al(OH)] and [Al(OH) ] intermedi-
ate reaction species appear one after the other. The latter is
ꢁ
1*'ꢀ!ꢁ ꢄ
\
relatively stabilized and does not react with OH anion so
2 AlO(OH) ' aH O &&" 2AlO(OH)' aꢀH O#2(a!aꢀ) H O
ꢁ
ꢁ
ꢁ
rapidly to form a stable Al(OH) molecule. As a result, by
the time another OH group recombines with it and forms
ꢂ
\
1*'ꢀ!ꢁ
Al O ' cH O#(1#2aꢀ!c) H O
a stable product,
ꢁ
ꢂ
ꢁ
ꢁ
ꢁ
V& -
>
\
[Al(OH) ] #OH &" Al(OH) ' xH O,
[3]
ꢁ
ꢂ
ꢁ
Al O #cH O
ꢁ
ꢂ
ꢁ
2 AlO(OH) ' aH OPAl O #(2a#1) H O
[1]
it transforms to an AlO(OH) molecule just by releasing
ꢁ
ꢁ
ꢂ
ꢁ
>
a proton H . The reaction involves only a minor redistribu-
A trace c (equivalent to 2.7% mass of the total sample) of tion of its local properties of internal energy and electronic
ꢀ
H O remains in the "nal specimen at 675 K. It is released charges so it occurs fast within a time interval t ꢀt . Here,
ꢁ
ꢁ
ꢁ

42
RANA AND RAM
ꢀ
>
t is the time interval within which the [Al(OH) ] reaction As per Eq. [4], a value of r"R,3p/*G appears at
ꢁ
ꢁ
ꢂ
species converts into Al(OH) by a recombination reaction *G"0. It is 1.5 times larger than the r* value. Here, we can
with OH in Eq. [3] in a con"ned surface reaction as in this de"ne the saturation time *t that it takes in growing to
example.
ꢂ
\
ꢃ
R through r*. The value of *t can be estimated by using the
ꢃ
The nascent AlO(OH) molecules arrange in groups in average growth velocity of particle (r'r*) u with the condi-
a speci"c manner according to the experimental conditions. tion R"u*t . We studied the equilibrium growth of
In principle, a group of molecules nucleate and grow into AlO(OH) in "bers at the nascent Al metal surface in a hu-
ꢃ
a stable crystal particle if it succeeds in attaining a critical mid air (16). Fibers as long as 5 mm (30 lm in diameter (16))
ꢇ
size d* (diameter). Formation of a surface of stable particles, grow with an average value of u"0.4;10 nm/s within the
d4d*, of surface area A, adds a surface energy )"Ap to "rst 2 min of the reaction. An activated reaction with an
the equilibrium Gibb's free energy of the system. In a spheri- order of improved u value occurs in water. Using this
cal shaped particle of radius r, it involves a change in the average u value together with the numerical value of
Gibb's free energy,
R"1.5r*+2.9 nm in the above relation we get a value of
*t "0.7 ls, which as such is an adequately small value,
ꢃ
ꢆ
ꢂ
ꢁ
*G"! ( )nr *G #4nr p,
[4] ascribing a pretty fast chemical reaction to this example. As
ꢂ
ꢂ
described elsewhere (14), the value of u and in turn the value
where p is the interfacial energy and *G ('0) is the change of R depends on the driving force of the reaction. This is
ꢂ
\
ꢂ>
in the Gibb's free energy on its formation from a randomly reasonably large in the Al!3e PAl , with 1.662 V elec-
dispersed assembly of molecules (amorphous structure) tromotive force (17), surface hydrolysis in this example.
ꢂ
ꢁ
(14, 15). It has a maximum *G*"16np /3(*G ) value at
In this fast reaction, AlO(OH) ' aH O molecules order in
ꢂ
ꢁ
a critical r"r*"2p/*G . Using the numerical p"0.70 a speci"c crystal structure in a speci"c morphology of space
ꢂ
ꢁ
ꢂ
J/m and *G "740 MJ/m values (15) gives r*"1.9 nm. It lattice of average size R to assist the fast release of ) in the
ꢂ
is the minimal r value of the particle which can grow into grain growth through R. This is possible with a feasibly
a stable particle, r'r*. In principle, a structure grows at the large building structure (crystal unit cell) of the particle. This
expense of its excess ) energy. A large value of ) in extreme- is the case in this example of AlO(OH) ' aH O in a mono-
ꢁ
ly small particles inputs a retarding potential in the forma- clinic crystal structure which has a lattice volume < that is
tion of their ordered structure at *G'0. In this case, it is 3.3 times the orthorhombic bulk structure.
only the volume Gibb's free energy in the "rst term in
Eq. [4] that drives the growth of particle.
X-Ray Diwraction and Microstructure
Since at r"r* the sample exists in a nonequilibrium state
far above the equilibrium value, the growth of particle
The X-ray di!ractogram (Fig. 1) of AlO(OH) ' aH O
ꢁ
occurs very fast to attain a reasonably stable size at *G40. powder consists of a total of 14 distinct peaks between 10
FIG. 1. X-ray di!ractogram for AlO(OH) ' aH O nanocrystals of monoclinic crystal structure (Cꢅ space group). The di!ractogram for the bulk
ꢁ
ꢁF
crystals is included in the inset.

XRD AND IR OF BOEHMITE NANOCRYSTALS FORMATION
43
orthorhombic crystal structure (10). It has a total of 20
di!erent peaks, I53, in this region with lattice parameters
a"0.370 nm, b"1.222 nm, and c"0.286 nm (10). For
example, part of the di!ractogram is included in the inset to
Fig. 1. The most intense peak in this case occurs in the (020)
re#ection at 0.6110 nm with the second and third most
intense peaks in the (120) and (140) re#ections at 0.3160 and
0.2340 nm, respectively.
TABLE 1
Interplanar Spacing dhkl and Relative Intensities (I ) of
Characteristic Peaks in the X-Ray Powder Di4ractogram of
AlO(OH) ' aH2O Nanocrystals
d
(nm)
FIJ
Observed
Calculated?
I
h
k
l
0.4850
0.4377
0.3198
0.2358
0.2210
0.1872
0.1721
0.1598
0.1560
0.1458
0.1430
0.1392
0.1332
0.1212
0.4899
0.4365
0.3195
0.2388
0.2167
0.1818
0.1681
0.1596
0.1558
0.1432
0.1396
0.1346
0.1325
0.1210
100
59
29
12
58
9
25
4
3
10
4
3
8
3
0
1
1
2
3
3
3
3
4
4
4
4
4
4
0
1
1
1
1
1
2
1
0
2
0
1
3
3
2
0
2
3
2
4
3
5
4
3
5
5
1
3
All of the peaks observed in this di!ractogram are suc-
cessfully assigned assuming a monoclinic crystal structure
ꢅ
(C space group) with lattice parameters a"0.866 nm,
ꢁF
b"0.506 nm, c"0.983 nm, and angle b"94.563. It has
ꢂ
ꢂ
<"0.429 nm and density o"2.25 g/cm . The most in-
tense peak in this di!ractogram is ascribed to the (002)
re#ection while the second and third most intense peaks are
ascribed to the (110) and (312) re#ections, respectively. As-
signment of the individual peaks along with their relative
intensities and (hkl) values is given in Table 1.
The present AlO(OH) ' aH O structure is very similar to
ꢁ
the monoclinic structure for Al(OH) (5, 18). As compared
ꢂ
in Table 2, both structures have z"8 molecules per crystal
? The d values are calculated from the average lattice parameters
FIJ
ꢅ
unit cell in C monoclinic crystal structure. Analysis of
a"0.866, b"0.506, c"0.983 nm and b"94.563 for the monoclinic crys-
ꢁF
a monoclinic structure and its lattice parameters from X-ray
tal structure.
di!raction pattern is not simple, unlike in other structures.
The formulation is scarcely available in the literature.
Therefore, we report a complete analysis of it in the
and 1003 on the 2h scale with peak intensity I53. The most following.
intense peak lies at 0.4850 nm with the second and third
In the conventional notation, the position vector of
most intense peaks at 0.4377 and 0.2210 nm, respectively. a point P in a crystal plane (hkl) in reciprocal lattice space is
An analysis of the widths of these peaks by the Debye} written as
Scherrer relation predicts an average crystallite size
d&30 nm. This di!ractogram di!ers from that in the
r*"ha*#kb*#lc*,
[5]
TABLE 2
Crystal Structure, Lattice Parameters, Density (q), Lattice Volume (V), Lattice Surface Area (A), and
Lattice Surface Energy (X) of Synthetic AlO(OH) and Al(OH)3 Powders
Sample
Nature
Porous
Structure/lattice parameters
< (nmꢂ)
o (g/cmꢂ)?
A (10ꢂ mꢁ/g)
) (kJ/g)
AlO(OH) ' aH O
(boehmite)
Monoclinic
0.429
2.25 (1.85)
3.7
2.59
ꢁ
a"0.866 nm, b"0.506 nm,
c"0.983 nm, b"94.563,
Z"8
AlO(OH)
(boehmite)
Bulk
Bulk
Orthorhombic
0.129
0.426
3.01
2.44
4.5
3.4
3.15
2.38
a"0.286 nm, b"1.222 nm,
c"0.370 nm, Z"4
Monoclinic
Al(OH)
ꢂ
(gibbsite)
a"0.868 nm, b"0.507 nm,
c"0.972 nm, b"94.563,
Z"8
Al(OH)
ꢂ
Bulk
Monoclinic
0.206
2.53
4.2
2.94
(bayerite)
a"0.506 nm, b"0.867 nm,
c"0.471 nm, b"90.263,
Z"8
? The value of o is calculated from the lattice volume. The observed value for porous boehmite is given in parentheses.

44
RANA AND RAM
where a*"1/a, b*"1/b, and c*"1/c are the reciprocal
A TEM micrograph along with the corresponding elec-
lattice vectors. For a monoclinic structure with aObOc, tron di!ractogram for AlO(OH) ' aH O powder is given in
ꢁ
a"c"903, and b'903, we can write from Eq. [5],
Fig. 2. It shows particles in an ellipsoidal shape of an
average D &35 nm diameter and u&1.8 aspect ratio. The
!ꢂ
particles are self-organized in a mesoporous structure
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
r* "h a* #k b* #l c* !2hla*c* cos b,
[6]
through small pores with 2 to 5 nm distance between par-
ticles in accordance with the presumably directional Al
metal surface hydrolysis. The electron di!raction pattern
\ꢀ \ꢀ
\ꢀ
where a*"(a sin b) , b*"b , and c*"(c sin b)
.
Using r*"1/d as the interplanar spacing it reduces to
(Fig. 2b) has four resolved rings at d "0.492, 0.320, 0.237,
FIJ
FIJ
and 0.190 nm in the (002), (112), (213), and (314) re#ections
for the monoclinic structure of the sample. These d values
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
1
h
l
1
k
b
2hlaccos bDsin b
FIJ
"
# !
#
ꢁ
.
[7]
compare well with the positions of peaks at 0.4850, 0.3198,
0.2358, and 0.1872 nm in the X-ray di!raction.
ꢁ
d
ꢁ
c
C
a
FIJ
The AlO(OH) ' aH O powder has ꢀ"18% porosity,
ꢁ
This is the standard working relation, which provides a cal- estimated from the di!erence in observed density o"
ꢂ
ꢂ
culation for the lattice parameters using experimental 1.85 g/ cm from the theoretical o"2.25 g/cm value. As
d
values in (hkl) peaks in X-ray di!ractogram. It describes the monoclinic crystal structure involves < as much as 3.3
FIJ
the di!ractogram of AlO(OH) ' aH O in Fig. 1 well with times that in the orthorhombic bulk structure, it has
a standard deviation of $0.005 nm in a, b, and c values.
ꢁ
ꢂꢈ \ꢀ \ꢀ
a smaller A/< ratio, ꢀ8.63;10
m
g
, over A/<ꢀ
ꢂꢈ \ꢀ \ꢀ
for the bulk. In a presumably fast
34.88;10
m
g
reaction, it can be argued that small particles, involving
a large ) value, grow on a building structure which supports
a small A/< value. This is achieved if the basic building
structure, i.e., the crystal unit cell, starts with a large size as
in this example.
Infrared Spectrum
The IR spectrum of AlO(OH) ' aH O nanocrystals
ꢁ
\ꢀ
(Fig. 3) has three distinct bandgroups: (i) 200}1200 cm
,
\ꢀ
\ꢀ
(ii) 2800}4000 cm , and (iii) 1300}2500 cm . Predomi-
nately intense bands in groups (i) and (ii) appear in usual
molecular vibrations (cf. Table 3) in AlO(OH) ' aH O while
ꢁ
weak bands in group (iii) arise in a peculiar polymeric
structure as discussed below.
Including three OH group fundamental modes (FM) of
vibration, i.e. O}H stretching (l), O}H angle bending (b),
and O}H deformation (c), an AlO(OH) molecule, with
N"4 atoms, has a total of 3N!6"6 FM vibrations.
Assuming the OH group has a single point mass (X), an
AlO(OH) molecule can be described by a simple nonlinear
triatomic (N"3) molecule in the C the point group. It
Q
describes the three skeleton vibrations of (i) X}Al"O sym-
metric bond stretching (l ), (ii) X}Al"O asymmetric bond
ꢀ
stretching (l ), and (iii) X}Al"O angle bending (l ). All of
ꢂ
ꢁ
these vibrations are allowed in both IR and Raman spectra
in agreement with the observation.
(i) 200}1200 cm\1 region. The IR spectrum with an ex-
panded x-scale in order to show a resolved structure of its
individual band components is reproduced in Fig. 4. It
\ꢀ
consists of two triplet bandgroups at 650 and 1050 cm
\ꢀ
with a single sharp band at 365 cm . The three compo-
\ꢀ
nents of the 1050 cm bandgroup have more or less the
FIG. 2. (a) TEM micrograph and (b) corresponding electron di!racto-
gram for AlO(OH) ' aH O nanocrystals.
ꢁ

XRD AND IR OF BOEHMITE NANOCRYSTALS FORMATION
45
nanocrystals. No such distinct band appears in
AlO(OH) ' aH O bulk crystals (13). It is mixed with the
ꢁ
X}Al"O symmetric stretching vibration which is very close
in frequency to this band and has the same symmetry. The
mixing of the two vibrations is lifted up in a modi"ed O}H
force "eld, evidenced by a considerably enhanced value of
the O}H stretching band, as will be discussed in the next
section, in a con"ned nanocrystal. As a result, the vibrations
appear in two separate bands. This agrees with the fact that
any change in frequency in a bending vibration occurs in
a way opposite to that in the stretching vibration (19).
\ꢀ
Three components in the 650 cm bandgroup lie at 525,
\ꢀ
600, and 760 cm . The bulk sample has three well-resolved
\ꢀ
bands at 651, 746, and 770 cm (13). On H deuteration,
\ꢀ
they shift to 677, 555, and 760 cm (13). The 525 and 760
\ꢀ
cm bands involve X}Al"O and >}O}H angle bending
vibrations, respectively, with > the Al"O group, while the
\ꢀ
600 cm band represents the O}H deformation vibration.
\ꢀ
Here, the 525 cm band is highly sensitive to local struc-
ture because it involves a direct motion of the OH group
with respect to the Al"O bond which is relatively rigid in
position by its large mass. It has a delocalized character in
H-bonding with neighbors and thus it very sensitively in-
#uences the frequency in this band. A broad band feature
\ꢀ
arises in the 600 cm band on overlapping with one (rock-
ing) of the three liberation vibrations of H O. Another H O
ꢁ
ꢁ
liberation vibration (wagging) possibly appears in a charac-
\ꢀ
teristically sharp band at 365 cm . In hydrated salts
FIG. 3. IR spectrum for AlO(OH)' aH O nanocrystals between 200
and 4000 cm\ꢀ.
ꢁ
TABLE 3
Assignments of Vibrational Bands in the IR Spectrum of
AlO(OH) ' aH2O Nanocrystals
\ꢀ
same intensity at 980, 1025, and 1075 cm . The 980 and
\ꢀ
1075 cm bands are attributed to X}Al"O symmetric and
asymmetric stretching vibrations. Two IR bands occur at
Frequency
(cm\ꢀ)
Intensity?
Assignment
H O wagging
\ꢀ
1068 (strong intensity) and 1157 (weak intensity) cm in
365
525
ms
vs
sh
vs
sh
s
s
ms
vw
vw
vw
w
vw
vw
ms
vs
ꢁ
the orthorhombic bulk crystal (13) and at 1022 and
in amorphous AlO(OH) ' aH O (16). The 600
Raman spectrum of bulk crystals has a single band at
1053 cm (corresponding to the 1068 cm band in IR) in
this region (13). Unusually, a predominantly large intensity
occurs in the IR spectrum for this band (symmetric bond
stretching vibration) in bulk crystals, possibly in a proton 1400
decollated structure. Otherwise, it would have a smaller
intensity than the asymmetric stretching band observed
at higher frequency. A proton delocalized structure in
AlO(OH) ' aH O bulk crystals was proposed earlier by
Fripiat et al. (11). It modi"es the basic intensity distribution 3120
in involved vibrations by imposing additional electric dipole
moments during the transitions. The e!ect is predominant
X}Al"O angle bending (l )
ꢁ
H O rocking
\ꢀ
1125 cm
ꢁ
ꢁ
760
840
980
1025
1075
>}O}H angle bending
H O twisting
\ꢀ
\ꢀ
ꢁ
X}Al"O symmetric stretching (l )
ꢀ
O}H bending
X}Al"O asymmetric stretching (l )
ꢂ
1520
1685
1650
1955
Al}O stretching, AlO(OH) dimer
H O, l band
ꢁ
ꢁ
2l
2l
ꢀ
ꢂ
2135
ꢁ
H O, l band
ꢁ
ꢀ
3460
3550
3660
H O, l band
ꢁ ꢂ
vs
ms
O}H stretching
in symmetric vibrations.
\ꢀ
The remaining 1025 cm band in this group is assigned
? vs, very strong; s, strong; ms, medium strong; w, weak; vw, very weak;
to an in-plane O}H bending vibration in AlO(OH) ' aH O and sh, shoulder.
ꢁ

46
RANA AND RAM
\ꢀ
\ꢀ
dgroup appears at &3400 cm , *l &450 cm , in
ꢀꢉꢁ
a proton delocalized structure (18).
Two O}H stretching bands also occur in bulk AlO(OH)
\ꢀ
at 3090 and 3297 cm (11, 13). They have been interpreted
as symmetric and asymmetric stretching vibrations in the
O}H group. In principle, a functional OH group as such has
a single O}H bond stretching vibration. Moreover, (11), the
bands shift to higher frequencies with a progressive decrease
in peak intensity on heating the sample at temperature as
\ꢀ
high as 652 K. They appear at 3215 and 3375 cm
at
\ꢀ
652 K, which are still 285 to 335 cm smaller than those in
the nanocrystals. This demonstrates that the nanocrystals
separated through small pores have a decoupled proton
delocalized structure with an enhanced O}H bond strength
over the bulk value. A bulk crystal or a strongly H-bonded
sample has a proton delocalized structure. In it, discrete
vibrational energy levels disappear above a speci"ed level
where the probability of the proton tunneling through the
potential energy barrier is close to the absorption frequency
(11, 13).
(iii) 1300}2500 cm\1 region. The IR spectrum in this
region (Fig. 3 or 6) has three resolved bands at 1400, 1520,
FIG. 4. Expanded IR spectrum for AlO(OH)' aH O nanocrystals on
the x-scale in the 200}1200 cm\ꢀ region.
ꢁ
\ꢀ
and 1650 cm
\ꢀ
with a shoulder at 1685 cm . The 1650
\ꢀ
\ꢀ
\ꢀ
(20, 21), three H O liberation bands lie at 400 to 550 cm
cm band, which is the most intense band in this group, is
ꢁ
\ꢀ
(wagging), 550 to 750 cm
(rocking), and &850 cm
the well-known l band of H O. The other bands do not
ꢁ
ꢁ
(twisting). The H O twisting vibration in AlO(OH) ' aH O occur in bulk AlO(OH) ' aH O in the usual molecular struc-
ꢁ
ꢁ
ꢁ
\ꢀ
appears as a shoulder of the 760 cm band at 840 cm
\ꢀ
.
ture. None of them can be interpreted as an overtone or
combination band. In this case, it is logical to argue that
(ii) 2800}4000 cm\1 region. This part of the AlO(OH) ' they arise due to a peculiar amorphous surface structure in
aH O spectrum describes O}H stretching vibrations in the
ꢁ
OH functional group and H O. As given on an expanded
ꢁ
x-scale in Fig. 5, it has resolved components at 3120, 3460,
\ꢀ
3550, and 3660 cm . The "rst two bands are broad with
\ꢀ
bandwidths *l &220 and 280 cm , while the others are
ꢀꢉꢁ
\ꢀ
rather sharp with *l &90 and 80 cm , respectively. On
the basis of relatively small frequencies and large *l
ꢀꢉꢁ
ꢀꢉꢁ
\ꢀ
values, the 3120 and 3460 cm
bands are assigned to
l and l vibrations of H O. In pure ice, the two bands
ꢀ
ꢂ
ꢁ
\ꢀ
appear at 3150 and 3380 cm with a shoulder in a l over-
ꢁ
\ꢀ
tone at 3220 cm . They shift to 3450 and 3615 cm
\ꢀ
in
\ꢀ
liquid H O and at 3652 and to 3756 cm in gas H O (19).
ꢁ
ꢁ
The signature l overtone in the spectrum in Fig. 5 is very
ꢁ
\ꢀ
much re#ected in the asymmetric shape of the 3460 cm
band at its lower frequency side.
\ꢀ
The sharp bands at 3550 and 3660 cm represent two
localized AlO(OH) molecular vibrations of O}H bond
stretching. This is possible if AlO(OH) exists in two con-
formers with two di!erent orientations of the OH group
stabilized by H-bonding. As described by Farmer (22), it
forms a layer structure linked by chains of H-bonds through
H O. A reasonably large internal water content, a&1,
ꢁ
conducts a strong H-bonding, obscuring the resolved fea-
FIG. 5. Expanded IR spectrum for AlO(OH)' aH O nanocrystals on
ꢁ
tures of the two bands. A single unresolved broad ban- the x-scale in the 2800}4000 cm\ꢀ region.

XRD AND IR OF BOEHMITE NANOCRYSTALS FORMATION
47
In this model AlO(OH) dimer ring structure, the 1400,
\ꢀ
1520, and 1685 cm bands can be ascribed to three Al}O
ring stretching vibrations of A, B, and C in Fig. 7 (curves
b to d). Displacements of Al and O atoms occur along Al}
O}H bonds in vibration A as well as in B. A represents
a symmetric vibration while B represents an asymmetric
\ꢀ
vibration. It is therefore assigned to the 1400 cm
which has a lower value in comparison to 1520 cm
band
\ꢀ
in
vibration B. In vibration C, the displacements of two Al
atoms occur in opposite directions and perpendicular to
those in the O atoms along the Al}O}H bond. It therefore
\ꢀ
refers to the highest frequency band, 1685 cm , among these
three vibrations. It is also an asymmetric mode of vibration.
This four-membered ring of AlO(OH) dimers can be
compared with the six-membered boroxol ring of l-B O
ꢁ ꢂ
(26). Both B and Al belong to group IIIB of the periodic
table and have the common feature of forming a coordina-
ꢁ\
\
tion polyhedron with O or OH anions. In the boroxol
ring, three B}O ring stretching vibrations appear at 1260,
\ꢀ
1325, and 1475 cm (26, 28). Their values are very sensitive
\ꢀ
to the local structure (28). An average 150 to 200 cm
larger value in the AlO(OH) dimer is feasible in an Al}O
force "eld marginally larger than that over B}O bonds in
boroxol rings.
FIG. 6. Expanded IR spectrum for AlO(OH)' aH O nanocrystals on
the x-scale in the 1300}2500 cm\ꢀ region.
ꢁ
Another relatively weak bandgroup of two bands occurs
AlO(OH) ' aH O nanocrystals. The surface layer is stable in this region (Fig. 5) in 2l and 2l overtone bands at 1955
ꢁ
ꢀ
ꢂ
\ꢀ
and this is why the crystalline core remains embedded in its and 2135 cm . Assignments of all observed bands in the IR
amorphous external shell. The amorphous phase acts as spectrum of AlO(OH) ' aH O nanocrystals are given in
ꢁ
cement to bond nanocrystals which in term form macro- Table 3. A comparison of FM vibrations in crystalline and
scopic particles. The interstices left among the cemented amorphous samples is made in Table 4.
nanocrystals constitute mesopores with active Lewis acid
sites exposed inside. The coexistence of crystalline and
amorphous components in nanocrystals is consistent with
the characteristic di!raction pattern (Fig. 1) with sharp and
di!use lines.
CONCLUSIONS
AlO(OH) ' aH O nanocrystals (of an average 35 nm
diameter) separated by small pores of 2 to 5 nm diameter are
ꢁ
Lewis acid sites in mesoporous solids are of immense synthesized through a self-induced spontaneous surface
interest today owing to their potential applications as sur- hydrolysis of nascent Al metal in pure water at room tem-
face catalysts, catalyst supports, "ller, and other applica- perature. Their crystal structure is analyzed by X-ray dif-
tions (23}25). From CO adsorbed IR studies, Knozinger (23) fraction. It has a new monoclinic structure with lattice
reported the existence of two kinds of Lewis acid sites in parameters a"0.866 nm, b"0.506 nm, c"0.983 nm, and
c-Al O , which has a similar amorphous surface layer. They angle b"94.563 in comparison with the orthorhombic bulk
ꢁ ꢂ
originate from uncoordinated tetrahedral and octahedral structure. It has 3.3 times the volume of the orthorhombic
(or pentahedral) surface Al atoms.
lattice with surface energy )"2.59 kJ/g (3.15 kJ/g in or-
ꢂ>
ꢁ\
The unsaturated Al and O ions on the amorphous thorhombic). AlO(OH) ' aH O forms in this peculiar
ꢁ
super"cial surface in AlO(OH) ' aH O nanocrystals rear- structure because the structure facilitates a fast release of
ꢁ
range in an interconnected network structure by minimizing structural energy as a function of its fast growth in this
the structure's total internal energy. As in conventional reaction. In this condition, AlO(OH) ' aH O readily attains
ꢁ
vitreous v-B O or SiO (26}30), the surface can be gener- a thermodynamic equilibrium value of its surface-to-volume
ꢁ ꢂ
ꢁ
ated in a number of ways. In a simple case, it can be ratio.
described assuming a structural unit of an AlO(OH) The IR spectrum of AlO(OH) ' aH O nanocrystals has
dimer as shown in Fig. 7a. Each structural unit can accom- three distinct band groups: (i) 200}1200 cm , (ii) 2800}
ꢁ
\ꢀ
\ꢀ
\ꢀ
modate one H O molecule. That lies over its planar struc- 4000 cm , and (iii) 1300}2500 cm . The band positions
ꢁ
ture and stabilizes it by H-bonding through two corner O and relative intensities in these bands are modi"ed signi"-
atoms.
cantly in comparison to those in the bulk sample. The bands

48
RANA AND RAM
FIG. 7. Schematic representations of (a) AlO(OH) dimer and (b}d) its A, B, and C three Al}O ring stretching vibrations.
in (i) and (ii) groups are assigned to six AlO(OH) funda- a layer structure linked by H-bonds. The bands shift con-
\ꢀ
mental vibrations and three fundamental and three lattice siderably from 3090 and 3297 cm
\ꢀ
in bulk to 3550 and
\ꢀ
vibrations of aH O. Two O}H stretching vibrations appear 3660 cm , i.e., by as much as 360 cm in the nanocrystals.
ꢁ
in the coexistence of AlO(OH) in two conformers in two This demonstrates
a proton localized structure in
di!erent orientations of the OH group stabilized by H- AlO(OH) ' aH O nanocrystals. Otherwise, a bulk crystal
ꢁ
bonding through H O. AlO(OH) ' aH O molecules form has a proton delocalized structure in which movement of
ꢁ
ꢁ
TABLE 4
Characteristics IR Bands (in cm؊1) for Fundamental Modes of Vibration in Amorphous and Crystalline AlO(OH) ' aH2O Powders
Amorphous
Crystalline
EC method?
Gel method?
3200}3600
Orthorhombic@
Monoclinic
HO}Al"O Group Vibrations
O}H stretching
O}H bending
3460
3090, 3297
3550, 3660
1025
980
X}Al"O stretching (l )
1022
600
1070
632
1068
651
ꢀ
X}Al"O angle bending (l )
525
ꢁ
X}Al"O stretching (l )
ꢂ
1125
600
1150
620
1157
770
1075
760
>}O}H angle bending
H2O Vibrations
1640
O}H stretching (l )
H}O}H angle bending (l )
3120
1650
3460
ꢀ
1650
ꢁ
O}H stretching (l )
ꢂ
? Values are reported from Ref. (16).
@ Values are reported from Ref. (13).

XRD AND IR OF BOEHMITE NANOCRYSTALS FORMATION
49
a proton is extended through the whole structure. In the
proton delocalization process (11, 13), discrete vibrational
energy levels disappear above a speci"ed level where the
probability of proton tunneling through the potential
energy barrier is close to the absorption frequency.
7. H. Yang, N. Coombs, I. Solokov, and G. A. Ozin, Nature (¸ondon) 381,
589 (1996).
8. W. Cai, Y. Zhang, J. Jai, and L. Zhang, Appl. Phys. ¸ett. 73, 2709
(1998).
9. B. E. Yoldas, in &&Ultra structure processing of advanced ceramics''
(J. D. Mackenzie and D. R. Ulrich, Eds.), p. 333. Wiley, New York,
1988.
10. JCPDS "le 21.1307.
11. J. J. Fripiat, H. Bosmans, and P. G. Rouxhet, J. Phys. Chem. 71, 1097
(1967).
12. G. Mariotto, E. Cazzanelli, G. Carturan, R. D. Maggio, and P. Scardi,
J. Solid State Chem. 86, 263 (1990).
The other bandgroup has three distinct components
\ꢀ
(excluding H O bending band at 1650 cm ) at 1400, 1520,
ꢁ
\ꢀ
and 1685 cm , which are ascribed to three Al}O stretching
vibrations in a model ring structure of AlO(OH) ' aH O
ꢁ
dimer. This bandgroup does not appear in the bulk sample.
It arises in a peculiar amorphous surface structure in nanoc- 13. A. B. Kiss, G. Keresztury, and L. Farkas, Spectrochim. Acta A 36, 653
(1980).
rystals. The amorphous phase behaves as a cement to bond
14. A. B. Pevtsov, V. Y. Davydov, N. A. Feoktistov, and V. G. Karpov,
Phys. Rev. B 52, 955 (1995).
15. A. Paul, &&Chemistry of glasses.'' Chapman & Hall, London, 1990.
nanocrystals which in turn form macroscopic particles. The
interstices left among the cemented nanocrystals constitute
mesopores with active Lewis acid sites exposed inside. The
16. S. Ram, T. B. Singh, and L. C. Pathak, Phys. Stat. Sol. (a) 165, 151
(1998).
17. S. Ram and S. Rana, Curr. Sci. 77, 1530 (1999).
18. JCPDS "le 12.460.
19. W. A. Senior and W. K. Thompson, Nature 205, 170 (1965).
20. S. Ram, J. Raman Spectrosc. 18, 537 (1987).
21. V. P. Tayal, B. K. Srivastava, D. P. Khandelwal, and H. D. Bist, Appl.
Spectros. (Rev.) 16, 43 (1980).
coexistence of amorphous and crystalline AlO(OH) ' aH O
ꢁ
components in nanocrystals is consistent with an X-ray
di!ractogram with sharp and di!use lines in re#ections from
di!erent lattice planes.
ACKNOWLEDGMENTS
22. V. C. Farmer, Spectrochim. Acta A 36, 585 (1980).
23. H. Knozinger, in &&Adsorption on Ordered Surfaces of Ionic Solids and
Thin Films'' (H.-J. Freund and E. Umbach, Eds.), Springer Series in
Surface Sciences 33. Springer-Verlag, Berlin, 1993.
24. V. Gruver and J. J. Fripiat, J. Phys. Chem. 98, 8549 (1994).
25. L. J. Alvarez, L. E. Leon, J. F. Sanz, M. J. Capitan, and J. A. Odriozola,
J. Phys. Chem. 99, 17872 (1995).
This work has been "nancially supported by the Council of Scienti"c and
Industrial Research (CSIR), Government of India.
REFERENCES
1. D. S. Albin and S. H. Risbud, Adv. Ceram. Mater. A 23, 242 (1987).
2. R. W. Siegel, Annu. Rev. Mater. Sci. 21, 559 (1991).
26. A. K. Hassan, L. M. Torel, L. Borjesson, and H. Doweidar, Phys. Rev.
B 45, 12797 (1992).
3. V. Hlavacek, Ceram. Bull. 70, 240 (1991).
4. T. Yazawa, K. Kadona, H. Tanaka, T. Sakaguchi, S. Tosubota,
27. M. Fujii, M. Wada, S. Hayashi, and K. Yamamoto, Phys. Rev. B 46,
15930 (1992).
K. Kuaoka, M. Mira, and W. D. Xian, J. Non-Cryst. Solids 170, 105 28. S. Ram, Phys. Rev. B 51, 6280 (1995).
(1994).
29. H. Z. Zhuang, X. W. Zou, Z. Z. Jin, and D. C. Tian, Phys. Rev. B 52,
5. S. Ram and S. Rana, Mater. ¸ett. 42, 52 (2000).
6. C. G. Wu and B. Thomas, Science 264, 1758 (1994).
829 (1995).
30. S. Ram and K. Ram, Infrared Phys. ¹ech. 37, 457 (1996).