Nanoparticles assembly of boehmite nanofibers without a surfactant

Jun Zhang a, Fengjun Shi a, Jing Lin a, Si Yi Wei a, Dongfeng Chen b,Jian Min Gao a, Zhixin Huang a, Xiao Xia Ding a, Chengcun Tang a,*

a College of Physical Science and Technology, Central China Normal University, Wuhan 430079, PR Chinab China Institute of Atomic Energy, Beijing 102413, PR China

Received 12 May 2007; received in revised form 12 July 2007; accepted 13 July 2007

Available online 20 July 2007

Abstract

Self-assembly of aluminum hydrate particles into larger boehmite (g-AlOOH) nanofibers was illustrated by a facile

hydrothermal method without using any surfactants. The size and morphology of boehmite nanofibers could be controlled by

adjusting the pH value of the reaction mixture. The resulting products were characterized by XRD (X-ray diffraction), FTIR

(Fourier transform infrared spectra), SEM (scanning electron microscopy), and TEM (transmission electron microscopy). The

specific surface area and pore-size distribution of the obtained product as determined by gas-sorption measurements show that the

boehmite nanofibers possess high-surface area and porosity properties. A possible formation mechanism of nanofibers via a

nanoparticle assembly procedure is proposed based on the experiments under the different conditions.

# 2007 Elsevier Ltd. All rights reserved.

Keywords: A. Nanostructures; B. Chemical synthesis; C. Electron microscopy; D. Crystal structure; D. Surface properties

1. Introduction

In the past decade, design and fabrication of nanostructures based on metal oxides and hydroxides have attracted

much attention because of their peculiar electronic and optical properties and their potential applications in industry

and technology. g-Alumina is one of the most important oxides and has been studied intensively over a long period of

time because of their potential for broad applications in adsorbents, catalysts, and catalyst supports [1]. The synthesis

of nanostructured alumina, especially one-dimensional nanostructures, has received considerable interest due to their

novel properties, such as high-elastic modulus, thermal and chemical stability, and optical characteristics [2]. The g-

alumina can be obtained through the dehydration of the boehmite form of g-AlOOH at temperatures in the range of

400–700 8C. During heating, the boehmite nanostructures undergo an isomorphous transformation to nanocrystalline

g-alumina, and the products can retain the morphology of the parent boehmite nanostructures [3,4].

Therefore, various morphologies of boehmite nanostructures have been synthesized, such as nanoparticles [5],

nanofibers [6,7], aligned nanowires [8], nanobelts [9], nanotubes [10,11], and flowerlike three-dimensional

nanoarchitectures [12]. In addition, recent results show that different morphologies of boehmite such as nanofibers,

nanotubes, and nanosheets could be controlled by metal-ion doping [13,14]. Due to its random three-dimensional

network structure, fibrillar boehmite has the advantage of larger pore size, higher specific surface area, and higher pore

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Materials Research Bulletin 43 (2008) 1709–1715

* Corresponding author. Tel.: +86 27 67861185; fax: +86 27 67861185.

E-mail address: cctang@phy.ccnu.edu.cn (C. Tang).

0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2007.07.022

volume compared with spherical or corpuscular one. Fibrillar form of boehmite was originally studied by Bugosh [15]

via hydrothermal treatment of basic aluminum solution and further developed by a number of researchers [16,17].

Alkoxide-derived boehmite is normally produced by the Yoldas process [18]. The process involves hydrolysis of

aluminum iso-propoxide (or sec-butoxide) in excess of water, followed by peptization via acid addition. The alkoxide

route was also used to produce boehmite fibers by hydrothermal treatment [19]. In addition, boehmite nanofibers have

been successfully synthesized by surfactant-assisted sol–gel or hydrothermal methods [3,6]. However, the removal of

surfactant with retention of the boehmite phase was proved to be difficult, inevitably introducing heterogeneous

impurities and making their application and investigations in catalysis difficult due to impurity surfactant absorption.

Thus, exploring the preparation of high-purity 1D nanofibers without using expensive alkoxide or bothersome

surfactant has been of interest. Recently, Tang et al. obtained boehmite nanofibers by a sol-hydrothermal method

without using any surfactant [7], whereas the adopted reaction temperature was 250 8C, therefore, the high

temperature may obstruct its application.

However, there are less reports about boehmite nanofibers assembled from nanoparticles, in which surfactant is

adopted as a hierarchical structure director [20,21]. Herein, we report a facile solution-based hydrothermal synthetic

pathway to the large-scale synthesis of boehmite nanofibers assembled by nanoparticles in the absence of surfactants,

and at a lower reaction temperature of 200 8C. The porosity properties of the obtained nanofibers may be utilized as

high-performanced catalysts or catalyst supports.

2. Experimental

Typically, the 1D g-AlOOH nanofibers were synthesized as follows. Analytical pure AlCl3�6H2O (0.724 g,

0.003 mol) was dissolved into 30 ml of distilled water at room temperature in a beaker, and magnetically stirred to

form a homogeneous solution. 1 M NaOH was then added dropwise until the pH was adjusted to about 4. The formed

clear and colorless solution was transferred into a Teflon-lined stainless autoclave. The autoclave was sealed and

maintained at 200 8C for 24 h, then allowed to cool naturally to room temperature. A white precipitate was recovered.

The product was washed and filtered several times with distilled water, and dried in vacuum at 60 8C for 12 h.

The crystal structure and phase purity of the products were examined by means of X-ray diffraction (XRD) analysis

with Cu Ka radiation. Infrared spectra (IR) were measured on a NICOLET NEXUS470 spectrophotometer. The

spectra were recorded on a KBr pressed disk. The overview of the sample morphology was checked by field emission

scanning electron microscopy (FESEM, JSM-6700F, JEOL), equipped with the system of energy-dispersive

spectroscopy (EDS) analysis. Sample powder was ultrasonically dispersed in acetone and dropped onto a carbon-

coated copper grid for transmission electron microscopy (TEM, JEM-2010F, JEOL) measurement, also equipped with

an energy-dispersive X-ray (EDX) system. The nitrogen adsorption and desorption isotherms at 77 K were measured

using a Micrometrics ASAP 2020 V3.00 H system after the sample had been degassed in a vacuum at 120 8C for

400 min.

3. Results and discussions

XRD analysis was used to determine the structure and purity of the product, as shown in Fig. 1a. All detectable

peaks in this pattern can be assigned by their peak position to an orthorhombic g-AlOOH. Compared with the standard

diffraction peaks from (JCPDS Card No. 21-1307), no other peak was observed associated with the impurities, such as

NaCl or Al2O3. This displays the high purity of the obtained products. More evidence for the formation of g-AlOOH

can also be provided by IR analysis shown in Fig. 1b. The boehmite prepared by the simple hydrothermal conditions

shows absorption bands at 480, 633, 749, 1070, 1160, 1974, 2102, 3096, and 3299 cm�1, which are in good agreement

with those reported in the literature [22,23]. The three strong bands at 480, 633, and 749 cm�1 are ascribed to the

vibration mode of AlO6 while another six bands at 1070, 1160, 1974, 2102, 3096, and 3299 cm�1 can be ascribed to

Al–OH stretching and bending vibrations in the boehmite lattice. The two strong and well-separated absorption bands

at 3096 and 3299 cm�1 indicated that the present boehmite was highly crystalline [24]. In addition, the shoulder at

1640 cm�1 can be ascribed to the bending modes of the adsorbed water [25]. All examinations indicate that the

obtained product is pure g-AlOOH phase.

The morphology of the product dispersed on a carbon-coated copper was examined by FESEM, establishing one-

dimensional fibrous structures, as shown in Fig. 2a. It can be seen that the synthesized nanofibers are of needlelike

J. Zhang et al. / Materials Research Bulletin 43 (2008) 1709–17151710

shape, with a typical length of several micrometers and an average width of about 60 nm. EDX was conducted for

chemical composition of the hierarchical structures, which is shown inset in Fig. 2a. Quantitative analysis gave the

atomic ratio of 1.9 for O/Al. It is close to the ideal value of 2 considering the instrumental error.

The structure characterization of boehmite nanofibers was investigated in detail by SAED and TEM microscopy.

Fig. 2b shows a representative low-magnification TEM image of the synthesized nanofibers, which allows us to

confirm the needlelike shape morphology. It should be noticed that nanofibers surfaces exhibit a zigzag pattern, which

could be seen clearly by the high-resolution TEM as shown in Fig. 2c. Interestingly, the magnified image of a single

boehmite nanofiber indicates that the nanoparticles stack along the fiber axis and form the hierarchical scaffold

structure. This hierarchical nanostructure can be identified clearly by HRTEM, but it is difficult to obtain a clear

J. Zhang et al. / Materials Research Bulletin 43 (2008) 1709–1715 1711

Fig. 1. (a) Typical XRD pattern and (b) infrared spectrum of the products.

Fig. 2. (a) Representative FESEM image and EDX pattern (inset) of the nanofibers, (b) low-magnification and (c) high-magnification TEM image of

the nanofibers, (inset) the SAED result of the products.

HRTEM image in our lab because the boehmite is considerably unstable under TEM electron beam irradiation. A

typical selected area electron diffraction (SAED) pattern is inset in Fig. 2c. Indexing of the pattern indicated that the

nanofibers possess a crystal structure consistent with the orthorhombic form of boehmite. The ring image constructed

by single crystalline spot array of the observed SAED may be a consequence of the assembly of different sized

nanoparticles.

In order to investigate the formation process of g-AlOOH nanofibers, FESEM was used to examine the samples

subjected to the heat treatment for different durations, indeed verifying a typical hydrothermal ripening process with

the assistance of an oriented attachment mechanism. Fig. 3a and b shows FESEM images of the samples after the

hydrothermal reaction for 6 h and 12 h under the present synthetic conditions. These images exhibit the time evolution

of g-AlOOH nanostructures from nanoparticles to nanofibers at 200 8C. Nanoparticle is the dominated morphology for

the samples obtained after 6 h reaction (Fig. 3a), and the nanoparticles closely stack together with a typical diameter of

20 nm. After the reaction for 12 h, short fibers and irregular nanoparticles coexist in the intermediate products

(Fig. 3b). Further increasing the reaction time leads to the formation of pure nanofibers, as shown in Fig. 2a that is the

FESEM image of the final product after treating for 24 h. Almost, all nanoparticles occurred previously disappeared.

According to above investigations, it is reasonable to use the Oswald ripening and oriented attachment mechanism

[26,27] to explain the formation of g-AlOOH nanofibers in the highly supersaturated solution. The formation of g-

AlOOH is based on the reaction of aluminum trichloride with sodium hydroxide under hydrothermal conditions as

follows:

Al3þ þ 3OH� ! AlðOHÞ3ðamorphousÞ (1)

AlðOHÞ3 ! g-AlOOH þ H2O (2)

Firstly, the addition of NaOH to AlCl3�6H2O solution led to the formation of amorphous colloid Al(OH)3. The amount

of the formed Al(OH)3 at this stage is little and no precipitation can be visible because of the pH value is around 4 in the

final solution. With increasing the temperature, the formed amorphous Al(OH)3 were converted to form both small and

large g-AlOOH nanoparticles in the nonequilibrium solution of autoclave at the initial stage of the hydrothermal

reaction (Fig. 3a). Then small nanoparticles have been gradually dissolved to generate free ions in the solution, and

J. Zhang et al. / Materials Research Bulletin 43 (2008) 1709–17151712

Fig. 3. FESEM images of the products under the present synthetic conditions at pH 4 for (a) 6 h; (b) 12 h, and 24 h at (c) pH 5; (d) pH 7.

spontaneously transferred onto the surfaces of some large nanoparticles. Due to the free energy difference for the

particles with the different size, the larger nanoparticles grew at the cost of the smaller particles through Ostwald

ripening, according to the well-known Gibbs–Thomson law [28]. Continuing this ripening process the starting

nanoparticles gradually arranged along the main crystallographic axes to form nanofibers via an oriented attachment

mechanism (Fig. 3b), finally forming the longer nanofibers.

As known, hydrothermal methods have been shown to be an effective way to synthesize of one-dimension

nanostructured materials. In contrast to the synthetic strategies, such as the vapor–liquid–solid (VLS) [29] or solution–

liquid–solid (SLS) [30] growth mechanism, which generally describes the 1D nanostructures growth by catalytic or

self-catalytic technique and requires a liquid droplet located at the top of 1D nanostructures as the catalytic active site

to direct 1D growth [31]. The present synthetic method does not use the catalysts to serve as the energetically favorable

site for the absorption of reactant atoms. Thus, it is reasonable to assume that the driving force for the anisotropic

growth of g-AlOOH possibly results from the inherent crystal structure of boehmite materials and their chemical

potential in solution.

The orthorhombic structures g-AlOOH has a distinctive layered structure [32], which can be found in several

complex hydroxyl compounds, such as FeOOH, and InOOH et al. Fig. 4 depicts a proposed structure of a portion of g-

AlOOH lamellar. One monolayer is one layer of deformed octahedra with an aluminum atom near their center, two

hydroxyls and four oxygen atoms in their vertices. The coordination geometry around the oxygen ions corresponds to a

distorted tetrahedron. The octahedral joined by edges result in AlO(OH) polymeric layers. These layers are held

together by hydrogen bonds between the OH� groups of each octahedron. The OH� groups within the structure could

form zigzag chains between the planes of oxygen ions. With the distinctly layered structures, boehmite possess a

preferential growth direction with the lowest growth energy. This makes the crystal growth along the certain direction.

In addition, this weak interaction between two double layers causes the crystal surface to end in the interface,

producing surfaces full of hydroxyls [33]. The driving force of the formation of nanofibers by assembling

nanoparticles possibly originates from the hydrogen bonds on the surface of nanoparticles via oriented attachment

mechanism. So the above surfactant/ligand-free exclusive anisotropic growth habit of nanofibers can be understood

from the view-point of the intrinsic structure of the boehmite.

Previously, Peng and Peng have investigated the effects of chemical potential on the shape evolution of CdSe

nanocrystals with the wurtzite structure, and in the involved one-dimensional nanostructures growth it would be

J. Zhang et al. / Materials Research Bulletin 43 (2008) 1709–1715 1713

Fig. 4. Proposed structure of g-AlOOH.

advantageous to have a higher chemical potential, which is mainly determined by the pH value and solute

concentration of the solutions in the reaction system [34]. Trentler et al. had demonstrated that a faster ionic motion

usually ensures a reversible pathway between the fluid phase and solid phase and allows ions to adopt correct positions

in developing crystal lattices [30]. In our synthetic system, the pH value of the clear, colorless solution was around 4,

there was a higher concentration of Al3+ and OH� in the solution. In this case, their motion speed arrived at the

maximum value, which would be favorable for the growth of crystal. The pH value set at 4 as the optimal condition was

based on many referenced experimental results. When only 0.1 M AlCl3�6H2O solution was used as the hydrothermal

reaction source in the absence of NaOH solution and the pH value was about 3, no white precipitate was obtained.

Shorter nanorods with diameters of 30–40 nm and lengths of 120 nm were obtained when the pH value of the solution

was adjusted at 5 by NaOH (Fig. 3c). Further increasing the pH value of the precursor mixture resulted in the

degradation of nanorods. For example, when the pH was adjusted to 7, shorter nanorods and sheets were observed

(Fig. 3d). From the comparable experimental investigation results, it seems that there exists a value which is optimal

for the preparation of high-aspect-ratio and perfect nanofibers [35,36].

The pore-size distribution and specific surface area of the obtained fibrillar boehmite were studied by nitrogen

sorption. Fig. 5 presents the nitrogen adsorption–desorption isotherms and Barret–Joyner–Halenda (BJH) pore-size

distribution curve (inset) of the boehmite nanoarchitectures. Fig. 5 displays type IV adsorption–desorption isotherms,

which are caused by the weak interaction of adsorbent–adsorbent and the existence of porous structures in the sample.

A hysteresis loop, which is commonly associated with the presence of mesoporosity, is a common feature of type IV

isotherms. In the nitrogen adsorption–desorption isotherms, there are hysteresis loops at 0.55 < P/P0 < 1.0 in the

isotherms of the sample, corresponding to the filling of mesopores produced by the agglomeration of primary particles

in the fibers [37]. BJH calculations for the pore-size distribution, derived from desorption data, reveal a narrow

distribution for the boehmite nanoarchitectures centered at 6–7 nm (Fig. 5, inset). These pores presumably arise from

the spaces among the small nanoparticles within a boehmite nanofiber. The BET specific surface area of the sample is

found to be as much as about 142.2 m2 g�1, is calculated from N2 isotherms at 77 K. The high-BET surface area and

narrow pore-size distribution strongly support the fact that the nanofibers have a nanoporous structure, and indirectly

identify the formation of nanoparticles assembly of boehmite nanofibers.

4. Conclusions

In summary, we have demonstrated a facile and controllable hydrothermal method to synthesize orthorhombic g-

AlOOH nanofibers via a nanoparticle assembly procedure without the presence of any surfactant. The needlelike

nanofibers could be attributed to the inherent anisotropic crystal structure of boehmite materials and their ionic motion

in solution. The Oswald ripening and oriented attachment mechanism has been utilized to expound the formation

process. The influence of pH value in solution on the product morphology was carefully investigated. We found that

the morphologies of product strongly depend on the pH value in the solution. The obtained nanoparticles assembly of

nanofibers may be used as precursors to mesostructured g-alumina. They may also be found other possible

applications that are dependent on the properties of boehmite itself.

J. Zhang et al. / Materials Research Bulletin 43 (2008) 1709–17151714

Fig. 5. N2 absorption and desorption isotherms and pore-size distributions (insets) for the g-AlOOH nanofibers.

Acknowledgments

This work was supported by the Fok Ying Tong Education Foundation (grant no. 91050) and the NNSF of China

(grant no. 50202007).

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