Materials Letters 64 (2010) 1077–1080

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Materials Letters

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Rapid growth of Fe3O4 nanowires with oxide assisted vapor–solid process

Ming Chang a,⁎, Nai-Feng Hsu a,b, Chin-Chun Chung c, Juti R. Deka a

a Department of Mechanical Engineering, Chung Yuan Christian University, Chung Li, 32023, Taiwanb Department of General Education, Army Academy, Chung-Li, 32023, Taiwanc Department of Chemical Engineering, Army Academy, Chung-Li, 32023. Taiwan

⁎ Corresponding author. Tel.: +886 3 2654323; fax:E-mail address: ming@cycu.edu.tw (M. Chang).

0167-577X/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.matlet.2010.02.018

a b s t r a c t

a r t i c l e i n f o

Article history:Received 8 December 2009Accepted 5 February 2010Available online 12 February 2010

Keywords:Magnetic materialsCrystal growthCapillarity modelResident time theorem

Oxide assisted vapor–solid (VS) process has been used for rapid crystal growth of Fe3O4 nanowires (NWs) onFe:Ni (1:1) alloy substrate. Oxide layers have been created initially on the substrate by heating it inside thequartz tube of a single tube furnace at 600 °C in air. On rising the temperature of the chamber to 700 °C invacuum and flowing argon (Ar) at a flux 10 sccm for 20 min NWs of lengths up to 10 μm and average width120 nm can be grown on the oxidized substrate. Capillarity model and resident time theorem have beencombined to analyze the nanowire growth process. Energy dispersive X-ray (EDX), X-ray diffraction (XRD)and vibrating sample magnetometry (VSM) studies show that the product formed are Fe3O4 NWs which aremagnetic materials.

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1. Introduction

One-dimensional (1D) nanostructure materials such as NWs,nanobelts and nanorods have attracted tremendous attention inrecent years due to their exceptional properties and novel potentialapplications. These structures serve as electrical interconnects andbuilding blocks in microelectromechanical or nanoelectromechanicalsystems (MEMS and NEMS) [1]. Magnetic 1D nanomaterials havereceived much attention because of their possible applications inperpendicular data recording and spintronic devices. Among mag-netic materials, iron oxides, such as ∞-Fe2O3 and Fe3O4 possess manytechnological advantages due to its peculiar electromagnetic proper-ties. Stimulated by the interesting properties and wide applicationpossibilities, researchers have made great effort in synthesizing 1DFe3O4 nanostructures using various techniques [2–13]. The reactionmechanisms with these methods are complicated and hence verytime consuming. Moreover, some quasi-physical methods, such aspulsed laser deposition (PLD) and micro plasma chemical vapordeposition (MPCVD) are very expensive. Short and wide Fe3O4

nanorods have been grown recently by thermal oxidation of Fesubstrate in air [14]. The present investigation reports on the rapidgrowth of Fe3O4 NWs with an oxide assisted VS process. Oxide layerswhich are pre-requisite for synthesis of NWs with this two stepprocess are formed by thermal oxidation of the (Fe:Ni) substrate inair. VS process is used in the second step to grow Fe3O4 nanowire onthe oxidized substrate in the Ar environment. Capillarity model of

thermodynamics and resident time theory are used to analyze thenanowire growth mechanism. The temperature and gas flux havebeen varied during the VS process to obtain nanowire of optimumlength.

2. Synthesis of Fe3O4 NWs

Several Fe:Ni (1:1) alloy pieces are dipped into 0.75 M solution ofoxalic acid for 30 min to use as substrates for growth of NWs. Eachprepared substrate is kept inside the quartz tube of a single tubefurnace and separately heated from 400 °C to 600 °C in air at aconstant ramping rate 10 °C/min. The quartz tube is vacuumed onreaching each temperature and Ar is allowed to flow inside the tube at10 sccm for 20 min (Ar-10 s–20 m). To optimize the growth process,each thermally oxidized substrate is further heated to 700 °C invacuum and Ar is allowed to flow for same time and rate. The gas fluxhas also been varied from 10 to 100 sccm in each case to optimize thegrowth process. The substrate has been naturally cooled down toroom temperature on completion of each experiment.

3. Fe3O4 nanowire growth mechanism

3.1. Thermal oxidation of Fe–Ni alloy substrate and formation of oxidelayers

Three oxide layers, (Fe, Ni)Ox, (Fe, Ni)3O4, and Fe2O3 are generallyformed on thermal oxidation of the Fe:Ni alloy [13]. Fe–Ni alloy iscrystalline innature and showsdistinctpeaks inXRDpatterns (Fig. 1(a))at 43.88°, 51.08° and74.8°.However, nodistinct oxide layer is formedonthermal oxidation of the substrate at 400 °C and 500 °C. On thermaloxidation of the substrate at around550 °C to 600 °C in air, newpeaks at

Fig. 1. XRD patterns (JCPDF card) of Fe–Ni alloy substrate (a)without heating, (b) thermaloxidation at 600 °C in air, (c, d, e) VS at 600 °C, 700 °C and 800 °Cwith Ar-10 s–20 m flow.

1078 M. Chang et al. / Materials Letters 64 (2010) 1077–1080

30.16°, 35.45°, 37.28°, 43.25°, 62.72°, and65.7° (Fig. 1(b)) characteristicsof (Fe, Ni)3O4 layers are evolved. Fe diffuses outwardmore rapidly thanNi and produces iron oxide layer at the outer interface while Nienrichment takes place in the inner interface. Oxygen diffuses inward toform the Ni containing inner oxide layer. Fe2O3 layer forms on thesubstrate surface on heating the substrate at 600 °C in air for fewminutes. This layer dominates the (Fe, Ni)3O4 layer on further heating to800 °C. The substrate has been oxidized below 600 °C in air to avoidunwanted Fe2O3 products. The quartz tube containing the substrate isvacuumed, heated to each operation temperature, and Ar gas is allowedto flow for 20 min. The XRD pattern shows that the intensities of peaksat 30.16°, 35.45°, 43.25°, 53.54°, 56.78°, and 62.72° correspond to Fe3O4

are increasing (Fig. 1(c–e)) with the increase in operation temperaturefrom600 °C to 800 °C. However, a new layer of Fe2O3 is gradually grownon the substrate which can be observed from new peaks in the XRDpattern given as Fig. 1(c). The intensities of thepeaks are increasingwiththe increase in operation temperature (Fig. 1(e)) as it covers the (Fe,Ni)3O4 layer.

Fig. 2. SEM images of Fe3O4 products at different growth temperatures (a) 5

3.2. Influence of temperature on the growth of Fe3O4 NWs

As oxide layers are unstable at high temperature [15,16], oxygenatoms come out and diffuse in the substrate on increasing thetemperature. Introduction of Ar carrier gas in the next step helps inthe rapid growth of Fe3O4 NWs by nucleation [17]. NWs of differentlengths grow on thermal oxidation of substrate from 400 °C to 600 °Cin air and continuous flow of Ar-10 s–20 m at different operatingtemperature ranging from 550 °C to 700 °C. 700 °C is found to be themost favorable operating temperature for growth of nanowire ofoptimum length for all the thermally oxidized substrate. Nanowire ofmaximum length (∼10 μm) can be obtained on substrate whichundergoes thermal oxidation at 600 °C in air and Ar-10 s–20 m at700 °C. Formations of different products on thermal oxidation of thesubstrate at 600 °C in air and VS process at 500 °C to 800 °C are shownin SEM images given as Fig. 2(a–d). Growths of fewNWs (Fig. 2(a)) areobserved on cooling down this substrate to 500 °C in vacuum andcontinuous flow of Ar-10 s–20 m. However, no nanowire is formed onthe substrate thermally oxidized at 500 °C in air and Ar-10 s–20 m atoperating temperature 500 °C. The formation of nanowire at operationtemperature 500 °C can be attributed to the (Fe, Ni)3O4 layer, speciallyformed on thermal oxidation in air at 600 °C. According to capillaritymodel of thermodynamics on effects of deposition rate and substratetemperature onnucleation, the critical radius (r) of the nuclei is relatedto the temperature (T) of the reaction chamber as [16],

∂r=∂T jR N 0 ð1Þ

where R is the deposition rate. Eq. (1) indicates that length and widthof the NWs increase with the increase in temperature. This can clearlydescribe the increase in length and width of NWs from approximately0.5 to 4 μm and 60 to 120 nm (Fig. 2(a–c)), respectively, on increasingthe operation temperature from 500–700 °C. The NWs disappear andbecome nanorods on rising the operation temperature to 800 °C(Fig. 2(d)).

Nanowire at the boundaries of substrate has been used for EDXanalysis to avoid Fe2O3 layers in the interference detection. EDXanalysis shows that a nanowire consist of 65.58 % Fe and 34.42% Oatoms which is in agreement with the commercially available Fe3O4

powders (Nippon Shiyaku Kogyo K.K).

00 °C, (b) 600 °C, (c) 700 °C, and (d) 800 °C and flow of Ar-10 s–20 m.

Fig. 4. (a) SEM image of spreading of NWs on the Si substrate, (b) hysteresis loops of theFe3O4 NWs corresponding to Fig.4 (a) due to application of magnetic field parallel to xand y directions.

1079M. Chang et al. / Materials Letters 64 (2010) 1077–1080

3.3. Different gas flux and its effect on growth morphology of Fe3O4 NWs

Gas flux plays important role in the growth of NWs employing VSapproach. On increasing the gas flux from 10 to 30 sccm at 700 °C,formation of NWs alongwith some particles due to aggregation of fewNWs are observed. The SEM image of formation of different productsuch as NWs, particles and bulk at different gas flux 10, 30, 75 and100 sccm are shown in Fig. 3. The growth of various products atdifferent gas flux can be analyzed very clearly with the resident timetheorem [18]. According to this theorem, interaction time (τ) of thegas with the substrate is related to the pressure (P), volume (V) of thereaction chamber, and the gas flow rate (Q) as,

τ = PV = Q ð2Þ

Eq. (2) indicates that the resident time is inversely proportional tothe flow rate. In this study, the gas molecules instantly heated to700 °C on the substrate at low gas flux (10 sccm) and therefore getlong resident time which helps in the formation of long Fe3O4 NWs(Fig. 3(a)). On increase in gas flux to 30 sccm, the numbers of gasmolecules in contact with substrate increases and as a result densityof NWs increases [13]. Due to increase in density, some of the NWsaggregate and appear as particles. On further increase in gas flux to100 sccm these particles combine and form bulk Fe3O4 which can beobserved in Fig. 3(b–d).

4. Study of magnetic property

NWs are dispersed on Si substrate to avoid the magnetic signalinterfering during the magnetic property measurement. Fig. 4(a)shows SEM image of NWs dispersed on Si substrate. The magnetiza-tion curves for Fe3O4 NWs presented in Fig. 4 (b) indicates that theproduct formed are ferromagnetic in nature. A high coercive field (Hc)of about 694 Oe and squareness (Mr/Ms) about 0.43 can be foundalong x-direction. Coercive field (296 Oe) and squareness (0.31)decrease on application of the magnetic field along y-direction. Thehigh coercive fields and squareness may result from the shapeanisotropy, forcing the magnetic moments to align along the axis ofthe NWs [19].

Fig. 3. SEM images of a vapor–solid process under Ar

5. Conclusions

A simple and rapid experimental technique to grow Fe3O4 NWs onFe–Ni substrate and its boundaries is presented in this investigation.This technique has adopted two-steps, thermal oxidation followed by

gas flux of (a) 10, (b) 30, (c) 75, (d) 100 sccm.

1080 M. Chang et al. / Materials Letters 64 (2010) 1077–1080

VS process to grow the NWs. Heating temperature and gas-flow rateare determined to be two important parameters that play prime rolein the growth of Fe3O4 NWs.

Acknowledgements

The authors gratefully acknowledge the support of SpecificResearch Fields in Chung Yuan Christian University project undergrant CYCU-98-CR-ME and the National Science Council of Taiwanunder project number 96-2221-E-033-042-MY3.

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