# Synthesis and Characterization of Boehmite Nanofibers

Authors: Jing Yang; Ray L. Frost
Journal: Research Letters in Inorganic Chemistry
 (2008)

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## Abstract

Boehmite nanofibers of high quality were synthesized through a wet-gel conversion process without the use of a surfactant. The long nanofibers of boehmite with clear-cut edges were obtained by steaming the wet-gel precipitate at170∘C for 2 days. Analytical techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), infrared emission spectroscopy (IES), as well as Raman spectroscopy were used to characterize the products.


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## Body

## 1. Introduction

Theγ- and α-Al2O3 polymorphs have found numerous applications in technical ceramics, thin solid
films, catalysis, and many others. In many cases, boehmite γ-AlO(OH) is  the starting material in the applications of alumina
phases. As a typical oxyhydroxy compound, boehmite is also extensively used as an absorbent [1, 2], a catalyst [3], and an
optical material [4].  Nanosized
materials are well
known for their quantum size effects [5] and are expected
to gain novel physical and chemical properties, with many more potential applications
in a wide range of areas. In recent years, boehmite with nanoscale dimensions
and morphological specificity has attracted enormous interest from both
fundamental and practical viewpoints.Boehmite nanofibers were reported to be
assembled with the assistance of poly(ethylene oxide) (PEO) surfactant [6], and tubular γ-Al2O3 was fabricated via
soft solution route using N-cetyl-N,N,N-trimethylammonium
bromide surfactant [7]. A solution-based chemical synthesis of boehmite nanofibers and alumina
nanorods by a modified sol-gel process in the presence of organic solvents has been reported [8]. Shen et al. reported
a steam-assisted solid-phase conversion of amorphous aluminium hydroxide wet gel to well-crystallized
1D nanostructured boehmite nanorods without using surfactants and organic solvents [9]. The process is unique in the simplicity of preparation and the high
efficiency of crystal growth, which can be operated on a large scale.In this work, long, large boehmite nano/microfibers
with high crystallinity and purity were synthesized based on Shen’s methodology [9], and the characterization of these nano/microfibers is reported.


## 2. Experimental

## 2.1. Preparation Procedures

15 g ofAl(NO3)3⋅9H2O
was dissolved in 25 mL ultrapure water, and 28% ammonia aqueous solution was diluted into 10%
solution. At room temperature, 10% ammonia solution was added dropwise into
Al(NO3)3 solution at a constant rate of 5 mL/min while
stirring vigorously.  Ammonia was ceased
to be added when the pH value of the reaction mixture reached 5. The
reaction mixture was stirred constantly in air at room temperature for 1 hour.
The obtained white gel was filtered to obtain the wet gel-cake, which was then
transferred into a glass beaker (25 mL). Before transferring the beaker with wet gel-cake into a Teflon vessel (125 mL), 2 mL ultrapure water was added into the bottom of the vessel. The Teflon vessel was sealed and heated at 170C° for 2 days. The resulting white material was
washed with ultrapure water, centrifuged and dried at 35C° for 2 days.


## 2.2. Characterization

X-ray diffraction analyses were performed on a PANalytical X’Pert PRO X-ray diffractometer, with a Cu X-ray
tube (λ=1.540596Å), operating at
40 kV and 40 mA. The scanning electron microscopy (SEM) images were
taken with a FEI Quanta 200 operating at 25 kV. The specimens were mounted on
SEM mounts with carbon tape and sputter-coated
with a thin layer of gold. The infrared emission spectroscopy was carried out
on a Nicolet Nexus 870 FTIR spectrometer. The emission spectra were collected
at an interval of 50C°, over
the range 100C°–850C°. Further details have been published  [10–13]. Raman spectra were
obtained using a Renishaw 1000 Raman microscope system, which also includes a
monochromator, a filter system, and a charge-coupled device (CCD).  64 Raman spectra were collected using 5 mW of
power at the sample using 20-second scans.
Further details of the Raman technique have been published [14–19].


## 3. Results and Discussion

## 3.1. X-Ray Diffraction

X-ray diffraction
is normally used to determine the phase and phase purity of the synthesized
boehmite.  Figure1 displays well-defined
XRD pattern observed, and all diffraction peaks were perfectly indexed to the
XRD pattern of pure boehmite (JCPDS card 01-083-2384). No XRD peaks representing
other crystalline phases were detected, indicating that the nanofibers of the
synthetic boehmite exhibited excellent crystallinity and a high purity.Figure 1
XRD pattern of the synthetic boehmite nanofibers, after
hydrothermal treatment at170C° for 2 days.


## 3.2. Scanning Electron Microscopy

While TEM images
are normally used to show the morphology of the boehmite nanomaterials, if the
nanofibres are large enough in length, then scanning electron microscopy can be
used to prove the nature of the nano/micromaterial. Figure2 presents the SEM image of the synthetic boehmite nanofibers. Nanofibers up to 500 nm long in bundles are clearly seen. Zhu et al. [20]  reported that through a soft-templated process with PEO
surfactant micelles at 100C°, the
nanofibers could be assembled, the maximum length of which was 100 nm. In this
study, the nanofibers were synthesized in the absence of surfactant and in a
supersaturated hydrothermal condition, which was reported by Shen et al. [9]. This steam-assisted self-assembly fabrication of boehmite yielded
high-quality 1D nanostructures with clear-cut edges and high purity. The
structure was confirmed by the XRD patterns.Figure 2
SEM image of the synthetic boehmite nanofibers, after
hydrothermal treatment at170C° for 2 days.


## 3.3. Infrared Emission Spectroscopy

Infrared emission spectroscopy
(IES) is known as a measurement of discrete vibrational frequencies emitted by thermally excited
molecules. The major advantages of IES are that the samples are measured in
situ at the elevated temperature and
IES requires no sample treatment other than making the sample of submicron
particle size. Figure3 clearly shows such a suite of the dehydroxylation of the
synthetic boehmite nanofibers. The dehydroxylation is followed by the decrease
of intensity of the hydroxyl deformation modes and the loss of intensity of the
hydroxyl stretching frequencies. The hydroxyl deformation frequencies are
observed at 840cm−1 and 757cm−1. The spectral changes
in these low-frequency bands show that dehydroxylation commenced at 250C°. The three bands displayed at 3671cm−1, 3360cm−1, and
3140cm−1 are
recognized in the hydroxyl stretching region. Above 450C°, the hydroxyl stretching bands are no longer observed. These spectral changes confirm that dehydroxylation starts at 250C° and is
completed by 450C°. Each of the hydroxyl stretching
bands shows an increase in bandwidth with temperature increase, and this
indicates that the molecular structure of the boehmite nanofibers was becoming
more disordered during the dehydroxylation process.Figure 3
IES spectra of the synthetic boehmite nanofibers, collected
at an interval of50C°, over the range 100C°–850C°.


## 3.4. Raman Spectroscopy

Many minerals,
both natural and synthetic, lend themselves to analysis by Raman spectroscopy.
The great advantage of Raman spectroscopy is that just so long as the materials
are 1 micron in size or larger, then individual nanofibres can be analysed as
is the case for the boehmite fibres shown above.Figure4 depicts
the Raman spectra of the synthetic boehmite nanofibers. The bands at 3216, 3077,
and 2989cm−1 are in the region of the hydroxyl stretching
vibrations. Frost et al. [21] characterized hydroxyl stretching bands in the Raman spectrum of
boehmite and reported bands at 3413, 3283, 3096, and 2997cm−1. The
low-wavenumber region (1100–200cm−1)
of boehmite consists of hydroxyl deformation modes (1050–900cm−1)
and hydroxyl translation modes (800–400cm−1).
The doubly degenerate mode of the AlO6 octahedron caused the observation of the bands at 495 and 675cm−1  [22].  The band at 362cm−1 was attributed to the vibration of fully symmetric Ag mode, in which
all aluminium and oxygen atoms move parallel to the b-axis [23]. The weak bands at 732, 448, and 340cm−1  were attributed
to OH− deformation modes [24].Raman spectra of the synthetic boehmite nanofibers, after
hydrothermal treatment at170C° for 2 days.
(a)(b)


## 4. Conclusions

In this research, long boehmite nanofibers with high crystallinity
and purity were formed under steam-assisted hydrothermal treatment at170C° for 48 hours with pH 5. The structure and morphology of the
nanofibers were detected by XRD and SEM, while the IES spectra illustrated
their thermal properties. Raman spectroscopy was applied to characterize the 1D
nanostructured boehmite.



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*Source: 602198-2008-10-20.xml*