Materials Research Bulletin 40 (2005) 1551–1557
www.elsevier.com/locate/matresbu
Synthesis of aluminium borate nanowires by sol–gel method
Jun Wang a, Jian Sha b,a, Qing Yang a, Youwen Wang c, Deren Yang a,*
a
State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, PR China
b
Department of Physics, Zhejiang University, Hangzhou 310027, PR China
c
Center of Analysis and Measurement, Zhejiang University, Hangzhou 310027, PR China
Received 3 August 2004; received in revised form 20 February 2005; accepted 18 April 2005
Abstract
A sol–gel process followed by annealing was employed to fabricate single crystal aluminium borate (Al4B2O9
and Al18B4O33) nanowires. The diameter of Al4B2O9 nanowires synthesized at 750 8C annealing is ranging from 7
to 17 nm, and that of Al18B4O33 nanowires synthesized at 1050 8C annealing is about 38 nm. Instead of the wellknown vapor–liquid–solid (VLS) mechanism, self-catalytic mechanism was used to explain the growth of the
nanowires.
# 2005 Elsevier Ltd. All rights reserved.
Keywords: A. Oxides; B. Sol–gel chemistry; C. Electron microscopy; C. X-ray diffraction
1. Introduction
One-dimensional nanoscale materials, such as elemental semiconductors, oxides, carbides, and
nitrides, have been widely studied due to their interesting optical, electrical, and mechanical properties
[1–8]. Aluminium borate is a remarkable ceramic material with high elastic modulus and tensile strength,
excellent resistance to corrosion, and chemical stability [9,10]. Aluminium borate whiskers have greater
potential in oxidation-resistant, whisker-reinforced composites than those observed in corresponding
macroscopic single crystals due to a reduction in the number of defects per unit length (compared with
larger structures) that lead to mechanical failure [11–13]. It is thus reasonable to consider aluminium
borate nanowires will exhibit greater strengths than previously observed in micrometer-diameter
* Corresponding author. Tel.: +86 571 87951667; fax: +86 571 87952322.
E-mail address: [email protected] (D. Yang).
0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2005.04.016
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whiskers. Furthermore, the nanowires potentially have considerable applications, such as high strength
materials, reinforced composites materials and electronic ceramics. So we try to synthesize aluminium
borate nanowires due to their unique features. There are many methods to synthesize quasi-nanowires,
such as template-directed synthesis, vapor–liquid–solid method, solvothermal method, arc discharge
method, laser ablation method, physical evaporation method, and chemical vapor deposition method.
Among those methods, chemical vapor deposition and physical evaporation of metal oxide powders are
the most widely used methods to synthesize binary oxide nanowires. Recently, nanowires of aluminium
borate (Al4B2O9 and Al18B4O33) have been synthesized by thermal evaporation method [14,15] and
catalytic synthesis [16]. Single-crystal Al18B4O33 microtubes have also been synthesized [17]. Here we
synthesized two kinds of aluminium borate nanowires (Al4B2O9 and Al18B4O33) by sol–gel process and
following annealing process.
2. Experimental details
Commercial high purity (>99%) Al(NO3)3 and H3BO3 in mol ratio of 1:3, 1:4 and 1:6 were mixed
together in deionized water, respectively. Citric acid was added to serve as ferment, which plays an
important role on the synthesis of aluminium borate nanowires. The mixture solution was evaporated at
150 8C in an oven for 10 h. Then the gel was got. The above gels were put in crucibles, respectively. The
crucibles were heated in a muffle at 750 8C for 4 h under ambient atmosphere. The gel resultant of the
solution of Al(NO3)3 and H3BO3 in mol ratio of 1:3 was also heated at 1050 8C for 4 h. After heating,
white powders were obtained. All solid powders were centrifuged in distilled water to remove the
impurities possibly remaining in the final products, and finally dried at 60 8C in air.
The structure of the products was examined by X-ray diffraction (XRD, D/max – rA, with Cu Ka
radiation). Some powders were also ultrasonically dispersed in chemical reagent ethanol solutions and
then transferred onto copper grids covered with carbon. The morphology and subtle structure of the
nanowires were characterized by transmission electron microscopy (TEM, CM200/Philips, 200 kV
accelerating voltage) and high-resolution TEM (HRTEM). To understand the function of citric acid, the
mixture of Al(NO3)3 and H3BO3 were directly being heated in mol ratio of 1:1 at 1050 8C for 4 h. The
products were also checked by means of XRD and TEM.
3. Results and discussion
Fig. 1 shows the XRD pattern and the TEM image of the product synthesized by annealing the gel of
Al(NO3)3 and H3BO3 in mol ratio of 1:3 at 750 8C for 4 h. Curve a shows the XRD pattern of the assynthesized product before being centrifuged in deionized water. As marked in the figure, the spectrum
consists of two sets of peaks. According to the standard JCPDS cards, one set corresponds to the
orthorhombic structure with a nominal composition Al4B2O9, the second to cubic B2O3 that resulted from
the decomposition of H3BO3. After centrifuged in deionized water, the sample became pure orthorhombic aluminium borate Al4B2O9, and the boron oxide was dissolved in deionized water so that no
relative peaks could be found in the spectrum, as curve b in Fig. 1a. The fitting crystalline parameters are
a = 14.74 Å, b = 15.26 Å and c = 5.557 Å (JCPDS, 29-0010). Furthermore, it can be seen in the TEM
image of Fig. 1b that most Al4B2O9 nanowires are about 7 nm in diameter. From the HRTEM image
J. Wang et al. / Materials Research Bulletin 40 (2005) 1551–1557
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Fig. 1. XRD pattern (a), TEM images (b), HRTEM (c) and diameter distribution (d) of the product synthesized by annealing the
gel of Al(NO3)3 and H3BO3 in mol ratio of 1:3 at 750 8C for 4 h.
(Fig. 1c), it can be seen clearly that this nanowire crystallizes well. The distance between the parallel
fringes is about 0.48 nm, corresponding to the [1 1 1] planes of Al4B2O9. By measuring over 50 nanowires from recorded TEM images, the average diameter of the nanowires was determined to be 7.3 nm.
As shown in Fig. 1d, most nanowires are in the range of 6–8 nm, indicating very good diameter
uniformity. A sample exhibiting this diameter uniformity and distribution is presented by the TEM image
in Fig. 1b. The diameter distribution of other samples synthesized at different experimental conditions
was measured by the same method, and it was found that all the nanowires have very good diameter
uniformity.
Fig. 2a shows the XRD pattern of the product synthesized by annealing the gel of Al(NO3)3 and H3BO3
in mol ratio of 1:6 at 750 8C for 4 h. The pattern can be indexed as orthorhombic aluminium borate
Al4B2O9. The fitting crystalline parameters are a = 14.8 Å, b = 15.1 Å and c = 5.6 Å (JCPDS, 09-0158),
which is minor different from the product synthesized by annealing the gel of Al(NO3)3 and H3BO3 in
mol ratio of 1:3 at 750 8C for 4 h, might due to the shift of the peaks as the mol ratio of H3BO3 increased.
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Fig. 2. XRD pattern (a) and TEM images (b and c) of the product synthesized by annealing the gel of Al(NO3)3 and H3BO3 in
mol ratio of 1:6 at 750 8C for 4 h. The inset shows SAED of a nanowire.
Examination by TEM established the presence of one-dimensional nanowire structures, as shown in
Fig. 2b and c. The significant proportion of the TEM specimen is Al4B2O9 nanowires with average
diameter of 17 nm. Some flakes are seen in Fig. 2b. It can also be found that the average diameter of
Al4B2O9 nanowires increases from 7 to 17 nm as the mol ratio of Al(NO3)3 and H3BO3 ranges from 1:3 to
1:6, and the quantity of the flakes also increases. We ascribe this phenomenon to the increase of B2O3,
which resulted in the nanowires growing continuously for the large sticking coefficient. The inset of
Fig. 2c is the selected area electron diffraction (SAED) pattern of an individual nanowire. It shows that
this nanowire crystallizes well.
Fig. 3a shows the XRD pattern of the product when the mol ratio of Al(NO3)3 and H3BO3 was 1:3 and
the heating temperature was 1050 8C. The pattern can be indexed as orthorhombic Al18B4O33. The fitting
crystalline parameters are a = 7.687 Å, b = 15.01 Å and c = 5.663 Å (JCPDS, 32-0003). The TEM
observation shows that the average diameter of Al18B4O33 nanowires is about 38 nm. The inset of Fig. 3b
shows a typical SAED pattern taken from a single nanowire that can be indexed as an orthorhombic
Al18B4O33 phase. The SAED pattern shows that the nanowires are single crystal.
In this work, to understand the function of citric acid, the mixture of Al(NO3)3 and H3BO3 were
directly heated in the mol ratio of 1:1 at 1050 8C for 4 h. For comparison, the gel of Al(NO3)3 and H3BO3
in the same ratio produced by a sol–gel process was also treated at 1050 8C for 4 h. Fig. 4a shows the
XRD pattern of the product when the mol ratio of Al(NO3)3 and H3BO3 was 1:1 and the annealing
temperature was 1050 8C by direct heating. It shows that the product is orthorhombic Al18B4O33. For the
sol–gel process following by annealing, the same product was received. Fig. 4b and c show the TEM
image of the products synthesized by direct heating and sol–gel process, respectively. It can be seen that
the nanowires synthesized by the sol–gel process have the uniform diameter about 100 nm, but those
synthesized by direct heating have the diameter from 50 to 300 nm. Furthermore, the nano-particles can
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Fig. 3. XRD pattern (a) and TEM images (b) of the product synthesized by annealing the gel of Al(NO3)3 and H3BO3 in mol
ratio of 1:3 at 1050 8C for 4 h. The inset shows SAED pattern taken from a single nanowire, and the patterns agree well with
orthorhombic Al18B4O33 phase.
be also observed in Fig. 4b. Thus, it is concluded that citric acid can uniform the diameter of the
nanowires. The same conclusion has been achieved for the products of Al(NO3)3 and H3BO3 with
different ratios at 750 and 1050 8C in the direct heating and sol–gel process.
Moreover, it can be also found that the quantity of H3BO3 should be excessive corresponding to the
stoichiometry of Al4B2O9 and Al18B4O33. No nanowires could be got as the quantity of H3BO3 decreases,
e.g. the mol ratio of Al(NO3)3:H3BO3 is 3:1. This indicates that the mol ratio of Al(NO3)3 and H3BO3
plays a key role on the growth of nanowires.
The possible growth mechanism of the as-synthesized aluminum borate nanowires might be as
follows: in our experiments no tips were found at the end of the nanowires, which is the character of
vapor–liquid–solid mechanism [18,19]. So the VLS mechanism is not suitable for the as-prepared
nanowire growth. Here we use the self-catalytic mechanism to explain the growth of our single crystal
nanowires. In our method, the reaction temperature is 750 and 1050 8C. During the process, H3BO3 will
decompose into B2O3 at about 250 8C, then B2O3 will melt at about 450 8C. With the increase of
temperature, Al(NO3)3 will decompose into Al2O3 grains at about 500 8C. The reactions are as following:
4H3 BO3 ! 2B2 O3 ðsolidÞ þ 6H2 O ðgasÞ
(1)
B2 O3 ðsolidÞ ! B2 O3 ðliquidÞ
(2)
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Fig. 4. XRD pattern (a) and TEM image (b) of the product synthesized by directly heating the mixture of Al(NO3)3 and H3BO3
in mol ratio of 1:1 at 1050 8C for 4 h; TEM image (c) of the product synthesized by annealing the gel of Al(NO3)3 and H3BO3 in
mol ratio of 1:1 at 1050 8C for 4 h.
2AlðNO3 Þ3 ! Al2 O3 ðsolidÞ þ 6NO2 ðgasÞ
(3)
2Al2 O3 þ B2 O3 ! Al4 B2 O9 ðsolidÞ
(4)
9Al2 O3 þ 2B2 O3 ! Al18 B4 O33 ðsolidÞ
(5)
At 750 and 1050 8C, small Al2O3 grains will dissolve into molten droplets of B2O3. All these small Al2O3
grains will serve as nuclei for the growth of nanowires just like Au catalysts used in VLS growth. Then
Al2O3 grains will react with B2O3. The Al2O3 grains absorb the liquid B2O3 and subsequently yield
Al4B2O9 and Al18B4O33 nanowires at 750 and 1050 8C, respectively. According to the principle of the
lowest energy, the mixture will form Al4B2O9 nanowires at 750 8C but Al18B4O33 nanowires at 1050 8C.
And the citric acid can uniform the diameter of the nanowires.
4. Conclusion
In conclusions, the single crystalline aluminium borate nanowires with the diameter about 7–17 and
38 nm (Al4B2O9 and Al18B4O33) were fabricated at different temperature by the sol–gel process
following by annealing at different temperatures. From XRD and TEM results, it is found that the
nanowires are single crystal with uniform diameter. The growth mechanism of the nanowires might be
self-catalytic mechanism.
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Acknowledgement
This work was supported by the National Natural Science Foundation of China (Nos. 50272057 and
60225010) and the Key Project of Chinese Ministry of Education.
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