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Solid State Sciences 12 (2010) 952–955

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Solid State Sciences

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In situ growth of LiFePO4 nanorod arrays under hydrothermal condition

Fei Teng, Sunand Santhanagopalan, Ryan Lemmens, Xiaobao Geng, Pragneshkumar Patel,Dennis Desheng Meng*

Department of Mechanical Engineering–Engineering Mechanics, Michigan Technological University, Houghton, MI 49931, USA

a r t i c l e i n f o

Article history:Received 17 November 2009Received in revised form5 January 2010Accepted 3 February 2010Available online 11 February 2010

Keywords:LiFePO4

Nanorod arraysIn situHydrothermalAnodized alumina oxides

* Corresponding author. Tel.: þ1 906 487 2817; faxE-mail address: dmeng@mtu.edu (D.D. Meng).

1293-2558/$ – see front matter � 2010 Elsevier Massdoi:10.1016/j.solidstatesciences.2010.02.017

a b s t r a c t

A novel in situ combinatorial method has been developed to fabricate LiFePO4 nanorod arrays, duringwhich anodized alumina oxide (AAO) was employed as the template and ethylene glycol/water mediumis used to ensure mass transportation rates of different chemicals to match each other. The samples werethen characterized by X-ray diffractometer (XRD), field emission scanning electron microscopy (FE-SEM),high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED),and energy dispersive X-ray spectroscopy (EDX). After being hydrothermally processed at 160 �C, thehighly-crystallized LiFePO4 arrays were directly obtained, which are composed of single crystal nanorodswith a diameter of 200 nm and a length of 3 mm. The reported synthesis is simple, mild and energy-efficient. A noteworthy advantage over conventional sol–gel–template methods is the elimination ofhigh-temperature annealing.

� 2010 Elsevier Masson SAS. All rights reserved.

1. Introduction

Nanoarrays have attracted significant attention for their appli-cations in energy storage/conversion devices due to large surfaceareas, short distances for charge and mass transport, reducedinternal resistance and high tolerance for volume change [1–4]. Forexample, the V2O5 nanorod arrays have achieved a capacity 4 timesas high as that of thin-film electrodes at high discharge rates [4]. Todate, various techniques have been used to fabricate nanoarraysincluding template synthesis [5,6], vapor deposition [7], chemicalsolution growth [8], electrochemical deposition [9] andsub-micrometer lithography [10]. Among these methods, AAO-templated synthesis is one of the most widely used methods tofabricate highly-ordered nanoarrays [5,6], which is promising toacquire nanoarrays with high purity and large surface areas [11].Use of an AAO template has been proven to be a low-cost andhigh-yield technique for nanoarray synthesis [5,6]. To date, thefabrication of nanoarrays with simple composition, i.e. single ordi-element, by AAO templates has been a fairly common practice,which has provided a variety of metal and semiconductor nano-arrays through electrochemical processes [4–6]. However, thefabrication of nanoarrays with complex, multi-element composi-tions cannot be easily achieved by this method, with only a few

: þ1 906 487 3551.

on SAS. All rights reserved.

examples demonstrated [12–19]. This can be attributed to thedifficulty in preparing homogeneous and stable precursor. Asa result, the compositions of infiltrated precursor sols within thetemplate channels are generally non-stoichiometric, which makesit difficult to obtain single-phase products [12–17,20,21]. Moreover,the sol–gel based methods commonly require post-synthesisannealing at high temperatures to obtain crystallized materials,which consumes large amount of energy. Hence, there are signifi-cant challenges on the synthesis and practical applications ofnanoarrays of multi-element materials.

As one of the most promising cathode materials, LiFePO4 hasbeen intensively investigated for large lithium-ion batteries inelectrical vehicles (EVs) and hybrid electric vehicles (HEVs)[22–24]. To date, various methods have been used to synthesizeLiFePO4 [25–27]. Sides et al. [28] synthesized LiFePO4 nanofibers viaa solution impregnation polycarbonate membrane method. In theirexperiment, however, annealing was used to obtain the crystallizedmaterials. It is desirable to develop a simpler route to synthesizehighly-crystallized LiFePO4 nanorod arrays. In this communication,we have overcome the above-mentioned limitations by combiningthe concepts of mild hydrothermal synthesis with templatemethod to provide a new in situ route for fabricating LiFePO4

nanoarrays, during which ethylene glycol/water (EG/W) was usedas medium. Compared with sol–gel–template methods [12–17,20,21], our approach shows significant advantages on its mildsynthesis environment, low energy-consumption and simplicity,e.g., elimination of additional high-temperature annealing step.

Table 1The phase composition and particle morphologies of the samples.

Sample ID Phase composition Particle morphologyc

Sample 1a LiFePO4 Nanorod arraysSample 2b LiFePO4 Irregular particles

a Prepared by in situ hydrothermal treatment in the presence of AAO templates.b Recovered from the remaining solution after the AAO templates were taken out.c Observed by FE-SEM.

F. Teng et al. / Solid State Sciences 12 (2010) 952–955 953

2. Experimental

2.1. Synthesis of LiFePO4 samples

All the chemicals are analytic grade, and are used as received.The LiFePO4 nanorod arrays were prepared by an in situ hydro-thermal AAO template-based process, during which L-ascorbic acidwas added as a mild reducing agent to prevent oxidation of Fe(II).The preparation procedures are shown in Fig. 1. Typically, themeasured amounts of LiOOCCH3, FeCl2 $7H2O, H3PO4, and L-ascor-bic acid with the molar ratios of 3:1:1:1 were added to 35 mL ofethylene glycol/deionized water mixture medium (EG/W¼ 1/1volumetric ratio). After intensive magnetic stirring for 1 h at roomtemperature, the homogeneous solution was transferred intoa 50 mL Teflon�-lined stainless steel autoclave. The AAO templateswere then placed in the autoclave cell with the backside leaningagainst the cell wall. The autoclave was heated to and maintained at160 �C for 12 h. After being cooled naturally to room temperature,the impregnated AAO templates were taken out and washed withDI water to completely remove any residual substance on theexternal surface. Next, the AAO sample was placed in 1 M NaOHsolution and stirred for 30 min so as to remove AAO. The samplewas recovered by centrifuging and washed with DI water until thefinal pH value was 7. The sample was dried at room temperature for24 h and was designated as Sample 1. For comparative study,another sample was recovered from the residual colloid solutionafter the AAO template was taken out, following the similarprocedures above. The as-obtained sample was designated asSample 2 (Table 1).

2.2. Characterization

Scanning electron microscopy (SEM) images were taken witha Hitachi S-4700 field emission scanning electron microscope (FE-SEM). Before FE-SEM inspection, the samples were coated with 5-nm-thick platinum/palladium layer by direct current (DC) sput-tering. The acceleration voltage was 15 keV, and the accelerationcurrent was 1.2 nA. The morphology, crystalline properties, surfacestructure, and element composition of the samples were deter-mined by using high-resolution transmission electron microscopy(HRTEM). We employed a JEOL JEM-4000FX system equipped withelectron diffraction and energy dispersive X-ray spectroscopy (EDX)attachments with an acceleration voltage of 200 kV. The powderswere first ultrasonically dispersed in ethanol, and then depositedon a thin amorphous carbon film supported by a copper grid. Thecrystal structures of the samples were characterized by X-raypowder diffractometer (XRD, Rigaku D/MAX-RB), using graphitemonochromatized Cu Ka radiation (l¼ 0.154 nm), operating at

Hydrothermal treatment

EG/WAutoclav

L-Ascorbic acid

LiOOCCH

H3PO4

AAO

FeCl2

Separation

Fig. 1. Schematic preparation procedures of the samples by the in situ tem

40 kV and 50 mA. The XRD patterns were obtained in the range of15–70� (2q) at a scanning rate of 5� min�1.

3. Results and discussion

3.1. Morphologies and crystal structures of the LiFePO4 nanorodarrays

Under hydrothermal conditions, it was found that nucleationand growth of LiFePO4 was effectively restrained by the nanoporesof the AAO templates. Fig. 2 gives the typical XRD patterns of the as-synthesized samples. As shown in Fig. 2(a), all the diffraction peaksof Sample 1 can be well indexed to single-phase LiFePO4 with anorthorhombic olivine structure (JCPDS card no. 81-1173). Thediffraction profiles are identified to the ordered olivine structureand indexed by the space group of orthorhombic Pnma, in whichthe Li ions occupy the octahedral sites (4a); Fe atoms occupy theoctahedral sites (4c); and P atoms occupy tetrahedral sites (4c). It isclear that highly-crystallized LiFePO4 crystals can be formed underthe mild hydrothermal conditions without post-synthesis anneal-ing at high temperatures. We also investigated the phase compo-sitions of Sample 2, which was obtained from the residual colloidsolution after the AAO template was taken out. The highly-crystallized LiFePO4 crystals were again found, as shown inFig. 2(b).

The images of the AAO template are shown in Fig. S1 (seeSupporting information (SI)). The pore size of the AAO template isconfirmed to be 200 nm, consistent with the specification providedby the vendor. The surface morphologies of the as-preparedsamples were characterized by FE-SEM, as illustrated in Fig. 3.Fig. 3(a) shows the side-view image of the nanoarrays of Sample 1.It can be observed that the nanorods are arranged parallel to eachother to form an orderly array. The length of the nanorods is about3 mm and is fairly uniform. Fig. 3(b) and (c) shows the top-viewimage of this array at different magnifications. It is clear that thenanorods are aligned vertically. The nanorods have a diameter of200 nm which is consistent with the size of the AAO pores. Thisagain confirms the significant spatial limitations induced by theAAO template on the nanorod arrays. These highly-crystallized

Separation Washing & Drying

Removal of AAO with 1M NaOH solution

Washing & Drying

AAO

Colloid Sample 2

Sample 1: Nanorod Arrays

plate-hydrothermal route: EG, ethylene glycol; W, deionized water.

15 20 25 30 35 40 45 50 55 60 65 70

(212)

(321)

(223)

(020)

(201)

(331)

(113)

(430)

(412)(222)

(421)

(022)

(112)

(221)

(121)(311)

(301)(211)

(111)

(011)(210)

(401)

(101)

In

ten

sity/a.u

.

2 Theta/degree

a

b

(200)

Fig. 2. XRD patterns of the as-prepared samples by different preparation methodscorresponding to those in Table 1: (a) Sample 1, prepared by the in situ template-hydrothermal route; (b) Sample 2, prepared from the residual colloid solution aftertaking out AAO templates; the lowest patterns show the theoretical standard JCPDScard No 81-1173.

F. Teng et al. / Solid State Sciences 12 (2010) 952–955954

arrays of LiFePO4 nanorods have not been reported previously. Wehave also investigated the morphology of Sample 2 obtained fromthe remained colloidal solution (Fig. S2, see SI). This sampledisplays irregular morphologies. This shows that under hydro-thermal conditions, the AAO template plays an important role incontrolling not only the size but also the growth direction of theLiFePO4 crystals.

HRTEM was also performed to examine the morphology andcrystal structure of the sample. Fig. 3(d) reveals that the nanorodshave a diameter of 200 nm, which is consistent with the FE-SEMresults. Their SAED patterns are shown in Fig. 3(e). The cleardiffraction spots correspond to the (020), (222) and (202) planes ofLiFePO4 crystals, revealing the single-crystalline nature of thenanorods. It is worth noting that the single-crystalline LiFePO4

nanorod arrays have been directly obtained via this template-

Fig. 3. FE-SEM and HRTEM micrographs of the as-prepared Sample 1 corresponding to thomagnification; (d) HRTEM image; (e) SAED patterns; (f) lattice fringe.

hydrothermal method under mild temperature, while a high-temperature annealing is usually needed to obtain the crystal-lized form for conventional sol–gel–template route [12–17,20,21].Fig. 3(f) displays the clear crystal lattices with the d-spacing of0.285 nm, corresponding to the (020) plane of orthorhombicLiFePO4 crystals. This also confirms the single-crystalline nature ofthe nanorods and suggests that the crystal preferentially growsalong the {020} direction. The EDX spectra of this sample have beenshown in Fig. S3 (see SI). The peaks of Fe and P can be observedclearly, but the peaks of lithium did not appear due to the light massof lithium. Note that the peaks of copper are caused by the coppergrid, which is used as the holder of the HRTEM sample.

3.2. The formation process of the LiFePO4 nanorod arrays

The formation process of the nanorod arrays is schematicallyshown in Fig. 4. Both the nanopores of the in situ AAO template andhydrothermal conditions provide a desirable growth environmentfor the array growth. Specifically, the spatially disordered growth ofcrystals is effectively restrained by the nanopores; the latterprovides with a higher crystallization environment than sol–gel–template method does. In our experiment, the EG/W mixturemedium was employed to make sure that the mass transportationrates of different ion species can match each other. Because theviscosity of EG (h¼ 21 mPa s, 20 �C) is much higher than that ofwater (h¼ 1.0087�10�3 mPa s, 20 �C), the diffusive rates of ionswill be reduced significantly in this EG/W medium. Consequently,the mobility rates of different ion species (e.g., Li, Fe) could matcheach other in this high-viscosity mixture medium. As a result, thestoichiometry in the AAO nanopores can be maintained due tosimilar mass transportation rate for different precursors, favoringthe formation of single-phase LiFePO4 crystals. This conclusion hasbeen confirmed by a control experiment in which EG/W mixturesolvent was replaced by deionized water (Fig. S4, see SI). Theproduct obtained with aqueous solution contained a significantamount of impurity crystals (e.g., Li3PO4). Moreover, it has beenreported that the AAO membrane provides suitable heterogeneousnucleation sites [29]. The solid walls of the AAO membrane canpromote the nucleation of crystals in a similar manner as the solid

se in Table 1: (a) side view; (b) top view at a low magnification; (c) top view at a high

a b c d

Fig. 4. Schematic diagram of the nucleation and growth: (a) heteronucleation; (b), crystal growth; (c) separation; (d) removal of AAO.

F. Teng et al. / Solid State Sciences 12 (2010) 952–955 955

impurity in a solution does. Hence, we assumed that the nucleationof the solutes within the AAO nanopores will be faster than those inthe bulk solution. The crystal formation can thus be considereda heterogeneous nucleation process. Under hydrothermal condi-tions, the pH value of the solution slightly increases due to thehydrolysis of CH3COO-, as shown in Eq. (1).

CH3COOL D H2O 5 CH3COOH D OHL (1)

Once the nuclei forms, the newly-generated OH� wouldcombine with the nuclei, resulting in negatively charged nuclei. Onthe other hand, it has been reported that the pore walls of AAOtemplates are positively charged [5]. Due to the presence of elec-trostatic attraction interaction and the availability of heteroge-neous nucleation sites on the walls of AAO nanopores, it isunderstandable that the crystals preferentially nucleated and thengrew gradually into nanorods within the AAO nanopores. As thesolutes are consumed, their concentrations within the AAO nano-pores will become lower than those outside of the AAO nanopores.The concentration difference will drive the bulk solutes continu-ously to transfer into the AAO nanopores to provide nutrition forthe growth of crystals. Further, the LiFePO4 nanoarrays formedthrough the dissolving/re-crystallizing processes. It is alsoconfirmed that the highly-ordered nanoarrays cannot be formedoutside of the AAO template due to the absence of the spatialconfinement effect, although nucleation and growth of irregularparticles were observed thereby. By contrast, the traditionaltemplate-based method relies on sol-filling of nanopores driven bycapillary action [30], which can be significantly undermined if thesol concentration is too high or the sol is not stable under an electricfield [31]. Such restriction doesn’t exist in our proposed method.The reported in situ combinatorial synthesis technology has beenproved to be a promising route to provide ordered nanoarrays withimproved crystallinity by overcoming the drawbacks of sol–gel–template method. The reported approach is simple and economical,and has thus provided an inspiration for the fabrication of othernanoarrays with complex compositions.

4. Conclusions

The highly-crystallized LiFePO4 nanoarrays have been success-fully synthesized by an in situ AAO-templated hydrothermalmethod. The in situ AAO template effectively controls the growth ofthe nanoarrays, the hydrothermal environment favors to formhighly-crystallized structure. The reported method does notrequire post-synthesis annealing at high temperatures. Theprocedure can be considered to be a simple and mild synthesis. Theas-synthesized LiFePO4 nanoarrays are expected to be a promisingcathode material for lithium-ion batteries.

Acknowledgements

This work is financially supported by the Michigan Tech FacultyStartup Fund and National Science Foundation of China(NSFC20944004).

Appendix. Supporting information

Supplementary data associated with this article can be found inthe online version, at: http://ees.elsevier.com and also at doi:10.1016/j.solidstatesciences.2010.02.017.

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