Template synthesis of LaMnO3+δ ordered nanowire arrays by converse diffusion or convection

Article abstract of DOI:10.1016/j.materresbull.2005.11.018

A method, whereby metal ions and precipitation reagents were transported in the nanochannels of anodic alumina membrane (AAM) templates by converse diffusion or convection and reacted in situ to form into one-dimensional nanostructure of the precursor, was developed. A high-aspect ratio nanowire array of the LaMnO_3+δ was prepared by this method. Electron microscope images showed that the LaMnO_3+δ nanowire array was abundant, uniformly distributed and well-ordered in large area. The individual nanowires are about 60 nm in diameter and their length corresponds to the thickness of the applied AAM template. The analysis of X-ray diffractions demonstrated that the LaMnO_3+δ nanowires are pseudocubic with typical perovskite lattice parameter.

Full text of DOI:10.1016/j.materresbull.2005.11.018

Materials Research Bulletin 41 (2006) 1045–1051
www.elsevier.com/locate/matresbu
Template synthesis of LaMnO3+d ordered nanowire arrays by
converse diffusion or convection
a
a
a
b
Zhong-Ai Hu a, , Hongying Wu , Xiuli Shang , Ren-Jiang Lu¨ , Hu-Lin Li
*
a Key Laboratory of Polymer, College of Chemistry and Chemical Engineering, Northwest Normal University,
Lanzhou 730070, People’s Republic of China
b Chemistry Department of Lanzhou University, Lanzhou 730000, People’s Republic of China
Received 19 May 2005; received in revised form 23 November 2005; accepted 30 November 2005
Available online 20 December 2005
Abstract
A method, whereby metal ions and precipitation reagents were transported in the nanochannels of anodic alumina membrane
(AAM) templates by converse diffusion or convection and reacted in situ to form into one-dimensional nanostructure of the
precursor, was developed. A high-aspect ratio nanowire array of the LaMnO3+d was prepared by this method. Electron microscope
images showed that the LaMnO3+d nanowire array was abundant, uniformly distributed and well-ordered in large area. The
individual nanowires are about 60 nm in diameter and their length corresponds to the thickness of the applied AAM template. The
analysis of X-ray diffractions demonstrated that the LaMnO3+d nanowires are pseudocubic with typical perovskite lattice parameter.
# 2005 Elsevier Ltd. All rights reserved.
Keywords: A. Oxides; B. Chemical synthesis; C. Electron microscopy; C. X-ray diffraction
1. Introduction
LaMnO3 belongs to a class of the perovskite oxides that attract the scientific interest by more than 50 years due to
novel physical and chemical properties. More recently, the interest to the manganite perovskite materials has increased
considerably after revealing its colossal magnetoresistance properties [1,2]. Great effort has been made for both their
potential application of the efficient technological devices and the theoretical and experimental understanding of the
magnetism and the electronic structure [3–5]. In addition, manganite perovskite oxides are characterized by great
stability at high temperature, high mobility of oxygen, and stabilization of unusual cation oxidation states in the
structure. Both latter properties lead to oxygen non-stoichiometry that makes them suitable for the catalytic oxidation,
including hydrogenation and hydrogenolysis of hydrocarbons, CO oxidation, ammonia oxidation, and catalytic
combustion [6–8]. Manganite perovskite-type oxides also prove to be promising materials for application as electrode
materials in solid oxide fuel cells, exhaust gas sensors in automobiles, membranes for separation processes and as
catalysts [9–11]. Recently, a lot of attention has been paid to the method of fabricating these materials, as the properties
depend very much on the synthesis. The compounds of the LaMnO3 family prepared by traditional solid-phase
reaction exhibit usually small specific areas. In order to overcome it the new methods such as gel–sol and
coprecipitation were used to obtain nanoparticles of the perovskite oxides [12,13]. In this paper, a new method was
developed to fabricate the LaMnO3+d nanowire array by the anodic alumina membrane (AAM) templates.
* Corresponding author. Tel.: +86 931 7971533; fax: +86 931 7973255.
E-mail address: Zhongai@nwnu.edu.cn (Z.-A. Hu).
0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2005.11.018

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One-dimensional (1D) nanostructures (nanowires or quantum wires) are ideal systems for investigating the
dependence of electrical transport and mechanical properties on size and dimensionality. They are expected to play an
important role as both interconnects and functional components in the fabrication of nanoscale electronic and
optoelectronic devices. Many unique and fascinating properties have already been proposed or demonstrated for this
class of materials, such as superior mechanic toughness [14], higher luminescence efficiency [15], enhancement of
thermoelectric figure of merit [16], and lowered lasing threshold [17]. Although there is no general guideline that
could be consulted for the design of any kind of desired novel nanowires (or nanorods), there are some major synthetic
methods that are often successfully employed for nanowire formation [18,19]: gas-phase reaction methods,
solvothermal routes, template-directed as well as liquid–crystal assisted syntheses, various solution-based techniques,
and sonochemically driven reactions are widely used. A wealth of templates including step edges present on the
surfaces of a solid substrate [20], channels within a porous material [21], mesoscale structures self-assembled from
organic surfactants [22] or block copolymers [23], biological macromolecules such as DNA strains or rod-shaped
viruses [24], and existing microstructures synthesized using other approaches [25] have been proven to be versatile for
preparing ordered nanostructures. Among these templates, the anodic alumina porous membranes (AAM) have
received considerable attention in synthetic nanostructure materials due to its several unique structure properties, such
as controllable pore diameter, extremely narrow size distribution for pore diameters and their intervals, and ideally
cylindrical shape of pores [26,27], besides they also exhibit good chemical and thermal stability, making them a
suitable material for a variety of deposition method and conditions. Thus, they have been extensively used to fabricate
nanometer-size fibrils, rods, wires, and tubules of different solid materials by a variety of synthetic strategies. Some
metal and alloy nanowire arrays were synthesized by electrochemical deposition with AAM templates [28]. Several
researches used sol–gel method to produce inorganic oxide nanowires inside pores of template [29]. Although it is
relatively advantageous in fabrication of the nanoparticles, this method will encounter serious difficulty in preparing
nanowires with high-aspect ratio because it is hard to pour the viscous sol in the nanochannels with a small pore
diameter. So it is necessary to make an effort to obtain more effective route that the precursor generates in situ and
forms into the nanostructure in the nanochannels of the anodic alumina membrane. In this paper, we report a new
technique to synthesize the long, straight, and uniform LaMnO3+d nanowires. In this technique, the mixed solution
containing La3+ and Mn3+ and precipitation reagents were transported in the nanochannels of AAM templates by
converse convection or diffusion and reacted in situ to form into one-dimensional nanostructure of the precursor.
Subsequently, they were changed into LaMnO3+d nanowires at high temperature.
2. Experimental
2.1. Preparation of anodic alumina membrane
The AAM templates were produced from pieces of high-purity aluminum foil (30 mm  12 mm, 99.99%) via two-
step anodization processes. Before anodization, the aluminum sheets were degreased, etched in alkaline solution,
carefully rinsed, dried, and then annealed under nitrogen ambient at 673 K to remove mechanical stresses and
recrystallize. To obtain the smooth surface the aluminum sheets were electropolished in a mixed solution of
HClO4:CH3CH2OH = 1:4. In the first step of the anodization process, aluminum foils were anodized at constant
voltages of 40 V in 0.3 M H2C2O4 aqueous solution at 273–278 K for 24 h, then the alumina layer obtained was
removed in a mixture of phosphoric acid (6 wt%) and chromic acid (1.5 wt%) at 333 K for 3 h, afterwards, the foils
were re-anodized at the same conditions as these used in the first step for 48 or 72 h. Next, the whole sample was
immersed in saturated Hg2Cl2 solution to separate anodic oxide film from aluminum substrate. A subsequent etching
treatment was carried out in a 6 wt% phosphoric acid solution at 310 K for 1 h to remove the barrier layer on the
bottom side of the AAM, resulting in the freestanding template.
2.2. Preparation of LaMnO3+d nanowires
In the course of experiment, the prepared template was used as membrane to separate a vessel into the two parts.
One of compartments was filled with the stoichiometric mixture solution composed of lanthanum nitrate and
manganese nitrate, and the other was filled with 0.2 M ammonia carbonate solution. The metal ions and carbonate ions
migrated conversely in the nanochannels of AAM templates and reacted in situ to form into one-dimensional

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nanostructure. After the reaction lasted about half an hour, the template was transferred into the vessel filled with
distilled water to completely remove reaction residuals. Finally, the template was dried under vacuum at 333 K for 1 h
and then heated at 923 K for 6 h forming arrays of LaMnO3+d nanowires inside the pores of AAM templates.
2.3. Characterization of nanowires
A TEM (JEM1200EX, Japan) was used to investigate morphology of nanowires. For TEM measurement the
treatment of samples is important. A piece of AAM template embedded with LaMnO3+d nanowires was placed in 3 M
NaOH solution to dissolve away aluminum oxide framework. After centrifugal sedimentation the solution was
removed slowly using a syringe and then an amount of distilled water was carefully added to rinse products. The
process was repeated three times. The residuals were completely transferred into 1 mL of ethanol. A carbon grid was
dipped in this dispersed solution and allowed to dry before TEM observation.
SEM images of the resultant LaMnO3+d nanowires were obtained with a JSM-6500LV electron microscope. The
specimens for SEM were prepared as follows. A piece of AAM template embedded with LaMnO3+d nanowires was
fixed to a SEM stub and then about 0.1 mL of 3 M NaOH solutions was dropped into the sample to partially dissolved
alumina film and uncover nanowires from the template. After carefully rinsing with distilled water and drying in air,
the samples were coated with gold thin layer by evaporating at vacuum to form conducting film.
X-ray diffraction (XRD) measurement was performed by a Rigaku D/MAX2400 diffractometer with Cu Ka
radiation to investigate the phase structure and crystal orientation of the LaMnO3+d nanowire array.
3. Results and discussion
Growth of anodic porous alumina has been investigated in detail in various electrolytes [26,27]. Self-organized
growth leads to a uniform array of hexagonal cells, each containing a cylindrical pore separated from aluminum metal
by a barrier layer. The structural properties of the pores depend intensively on the anodization conditions. The AAM
templates prepared in the oxalic acid solution have the periodic pore arrangement with a pore diameter average of
60 nm and a pore density of about 109–1010 cmÀ2. Almost perfect hexagonal ordered domains can be seen over a wide
range of pore distances. Before used as the templates, anodic alumina membrane needs to be etched in an acid
solution to perforate the barrier layer. It is aware that etching process also induces the pore walls to be thin unevenly.
So the exact control of etching time, concentration, and temperature is essential to preparing the anodic alumina
templates with uniform pore diameter. The TEM image of the LaMnO3+d nanowires prepared by using the AAM
template with these uniform pore structures are shown in Fig. 1, indicating that individual nanowires are around
60 nm in diameter and near to 7.5 mm in length. Seen from Fig. 1b, two nanowires are arranged in a y-type. Fig. 1c
shows that several parallel and uniform nanowires form a nanowire array corresponding to the parallel pore channels
of the templates. This arrangement may be relative to the coagulation of nanowires. As shown in figures the fabricated
nanowires are dense and uniform, and their length and diameter depend on the size of the nannochannel of the used
template.
SEM images of nanowire array prepared by using this method are shown in Fig. 2. Few microscopic defects are
observed in the figures. Fig. 2a reveals a cross-section where the alumina matrix of the AAM template has fully been
dissolved away. As shown, the LaMnO3+d nanowires deposited inside the nanochannel of the AAM template are in
order aligned and uniformly distributed. It is correlative to the AAM template with densely parallel nano-holes
arranged in a hexagonal fashion. Fig. 2b shows a split where the alumina matrix of the AAM template has been partly
dissolved and large quantities of LaMnO3+d nanowires remain. Fig. 2c is a planform of the LaMnO3+d nanowire array,
taken at lower magnification. It is seen from the large visual field that LaMnO3+d nanowires fabricated in our
experiment are abundant, uniform, and well-ordered in large area. As shown in the micrographs, the nanowires are
upstanding in high order, but the top of them is inclined to agglutinate together. When the alumina matrix of the AAM
template was dissolved away, the nanowires embedded in the template were gradually released from top to bottom.
Thus, the topside half of nanowires uncovered from the templates is more freestanding than their bottoms because of
residuals of the alumina matrix. This situation leads to interesting sight in the figures. It is conceivable that these
phenomena may result from the high surface energy of the nanowires. From these it can be found that the LaMnO3+d
nanowires are produced in plenty inside the pore channels of the templates. Therefore, it can be reckoned that the
density of the nanowire array is approximately equal to that of the pore of the templates (about 1010 cmÀ2). In addition,

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Fig. 1. TEM photographs: (a and b) freestanding LaMnO3+d nanowires and (c) parallel LaMnO3+d nanowire array.
it can be estimated that the length of them are more than 40 mm and the diameter is equivalent to the pore diameter of
the template.
The structural chemistry of perovskite lattices is very complex, and there are a whole series (with hundreds of
individual members) of hexagonal, rhombohedral, orthorhombic, and tetragonal distortions possible. The structural
investigations [30] of the stoichiometric LaMnO3+d showed that oxidation of Mn ions over the level 3+ in
stoichiometric LaMnO3 induces vacancies on La and Mn sites, which are present therein in equal quantities. Thus, the
entire structure skeleton is forced into becoming cation deficient. In other words, LaMnO3+d is not a compound with
precise stoichiometry. Therefore, LaMnO3+d can exist in different structure forms, depending on the La/Mn ratio,

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Fig. 2. SEM images of LaMnO3+d nanowire array: (a) cross-section view, (b) top view in low magnification, and (c) top view in high magnification.
temperature, and oxygen pressure. Within the temperture range 833 2 1473 K, and under the oxidizing conditions
prevalent in air, LaMnO3+d exhibits three polymorphic forms; the rhombohedral existing up to ꢀ1130 K, the nearly
tetragonal appearing between ꢀ1130 and ꢀ1300 K, and the orthorhombic one stable above ꢀ1300 K [4]. In our
experiment, the precursors with molar La/Mn ratio = 1 was shaped in the nanochannels of the AAM template, and then
they were changed into the LaMnO3+d nanowires at 923 K. The XRD spectrum of LaMnO3+d nanowires embedded in
the template is shown in Fig. 3. The major diffraction peaks of LaMnO3+d are observed and their positions are in
agreement with the JCPDS standard (card no.: 50-298). The pattern is pseudocubic with typical perovskite lattice
parameter since different components are not resolved and different distortions are equally possible based these data.
Although the nanowires are prepared at relatively low temperature, which is favorable to rhombohedral-phase,

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Fig. 3. X-ray diffraction spectra of LaMnO3+d nanowire array embedded in the AAM template.
precursors formed within the nanochannels of AAM templates may exhibit abnormal pyrogenation and phase
transition because nanomaterials are more easily melted than the bulk materials. In addition, the specific field of
nanowires could stabilize orthorhombic modifications. It should be pointed out that the observed reflections show
some specific features of nanowire. For example, the major peak broadening may be due to diminutiveness in diameter
of nanowires or size of crystal particles of which nanowires are composed. Furthermore, the anisotropic peak
broadening and intensifying results from the preferred orientation of nanowires.
4. Conclusion
We have developed the new technique that the precursor is generated in situ, formed into one-dimensional
nanostructure in the nanochannels of the AAM template, and then changed into the LaMnO3+d nanowires at 923 K.
The analysis of the X-ray diffraction demonstrated that LaMnO3+d nanowires are pseudocubic with typical perovskite
lattice parameter. Electron microscope images showed that nanowire array was abundant, uniformly distributed, and
well-ordered in large area. It is estimated from the electron microscope photographs that the diameter of the individual
nanowire is about 60 nm and the length was corresponding to the thickness of the applied AAM template.
Acknowledgements
This work was supported by Project of KJCXGC-01 of Northwest Normal University, and major project of Gansu
Science and Technology Committee (2GS035-A52-026). We would like to express our sincere thanks to Chief
Engineer Tao Xu and Jiazheng Zhao of Lanzhou Institute of Physical Chemistry, Chinese Academy of Science,
Engineer Hailong Pang of Northwest Normal University for TEM and SEM measurement and analysis, and to Chief
Engineer Da-Kang Song of the Material Department of Lanzhou University for measurement and analysis of XRD
data.
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