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pubs.acs.org/crystal Published on Web 07/09/2009 r 2009 American Chemical Society

DOI: 10.1021/cg9004888

2009, Vol. 9

38403843

Growth Kinetics of Nickel Crystals in Nanopores

Cuiyan Yu, Yanwu Xie,*, TaoXu, Yan Chen, Xiaohong Li, WeiLi, Baoting Liu, andXiangyi Zhang*,

State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, 066004

Qinhuangdao, P. R. China, andCollege of Physics Science and Technology, Hebei University, 071002Baoding, P. R. China

Received May 4, 2009; Revised Manuscript Received June 21, 2009

ABSTRACT: The well-controlled synthesis of perfect and homogeneous nanowires requires a fundamental understanding of theirgrowth kinetics. In the present study, we succeeded in studying for the first time the growth kinetics of electrodeposited Ni crystals innanopores by employing the temperature-dependent rate constants yielded from deposition current-time curves. A small growthactivation energy Ea = 0.25( 0.01eV andprefactor tE,0

-1 = 4.5 ( 0.3s-1 aredetermined for theNi crystals in 25 nm diameter pores, andthey increase with the pore diameter following the Meyer-Neldel compensation rule. On the basis of these studies, the growthmechanism of electrodeposited Ni nanowires has been revealed.

Crystal growth is one of the most intensively studied topics inmaterials science and solid-state physics because many technolo-

gicalsystems in the fields of information, communication, energy,transportation,etc., depend on the availability of suitable crystalswith tailoredproperties.1 A substantial progresshas beenmade inthe science and technology of crystal growth, and the basicmechanisms of crystal nucleation and growth are now under-stood at atomic/molecular levels for bulk and thin film systems,which enables a well-controlled synthesis of these materials.1-4

Recently, with the rapid development of nanotechnology, a greatdeal of interesthas beengenerated in the well-controlledsynthesisof nanocrystals at the nanoscale level in order to producenanostructured materials with excellent and tunable functionalproperties.5-11 A profound understanding of the growth kinetics(e.g., growth activation energy and prefactor) of crystals innanoscale spaces is crucial for yielding well-controlled perfectand homogeneous nanowires, which are of particular importance

for fabricating nanodevices with excellent functional properties.Previous studies on the growth mechanism of nanowires havebeen dominantly focused on the thermodynamic processes andthe kinetic studies arelimitedand mostly focusedon the growth ofnanowires yielded by the vapor-liquid-solid route, in particularthe growth of Si nanowires.8-10 Although the electrodepositiontechnique has been proven to be one of the most successfulapproaches to produce various nanowires with controlled length,diameter, and growth orientiation,12,13 thegrowth kineticsand inparticular the microscopic growthmechanism of electrodepositedcrystals in nanopores is far less well understood. This is probablydue to the lack of an experimental technique in obtaining reliabledata on crystal growth in nanopores. More recently, the super-lattice structured nanowires have been significantly employed tostudy the growth mechanism of electrodeposited nanowires.14

Although in principle the growth rate of electrodeposited crystalsis proportional to the current because the total amount ofsubstance being converted is proportional to the amount ofcharge, hydrogen evolution also consumes charge and makesit difficult to identify the growth rate of deposited crystalsexclusively.15 Here, in the exemplary case of Ni crystals, wedemonstrate for the first time that the growth kinetics of electro-deposited crystalsin nanopores can be experimentally determinedby employing the temperature-dependent rate constants yieldedfrom deposition current-time curves. This methodology can befurther extended to the study of growth kinetics of other metal

and semiconductor systems in nanopores and thus is of wideinterest.

In the present study, the Ni crystal was used as model systembecause it is a simple and typical crystal with well-obtainedknowledge of growth processes at the macroscopic scale.

The deposition of Ni was performed in a homemade three-electrode system that was placed in a temperature-controlledchamber with a precision of( 0.1 C, where the pure Ni plankand a standard calomel electrode (SCE) were used as the counterelectrode and the reference electrode, respectively. The two-stepanodization technique was employed to prepare porous aluminatemplates (PATs) with different pore diametersd 25, 40, and160 nmand a thickness of700, 600, and500 nm,respectively. ACu film with a thicknessof100 nmwas deposited onone side ofthe PATs and served as the cathode electrode by employing athermal evaporation approach. The electrolyte was made of1.3 M NiSO4 3 6H2O and 0.6 M H3BO3. Deposition experiments

were performed at a constant potential of-1.0 V (versus SCE) inthe temperature range from 273 to 288 K, and the current-timecurves were measured using a computer-controlled recordingsystem with a time resolution of 1 10-1 s. All deposition para-meters used in the present study were experimentally determinedby considering the application of the present methodology andthe time needed to perform deposition experiments. The porediameter and thickness of PATs were determined using a XLS-FEG scanning electron microscope (SEM). The microstructureof deposited nanowires was studied by employing a Rigaku D/max-2500 X-ray diffractometer (XRD) withCu KR radiationanda JEM-2010 transmission electron microscope (TEM). For TEMstudies,the PAT wasfully dissolved using NaOH(0.5M) solution.

We want to point out here that the quality, uniformity, andpretreatments of the pure Al sheets used in the present studies as

well as the two-step anodization process of PATs were strictlycontrolled in order to eliminate the uncertainties affecting theporediameter andpore length of PATs. Moreover, the depositionof Cu film, electrodeposition processes (in particular the deposi-tion temperature), and the recording of current-timecurves werealsostrictly controlled so as to eliminate the effect of uncertaintieson the determination of characteristic time tE. For each deposi-tion temperature,more thanthree repeatable measurements havebeen performedto reduce the effectsof experimentaluncertainties.

At a given potential, under the pseudosteady-state conditions,the deposition current is directly proportional to the area of theelectrodeposit.16 Therefore, the deposition process of crystals innanopores can be directly monitored from the current response.As shown in Figure1 (for T= 275.5 K), after the initial transient

*Corresponding author. E-mail: xyzh66@ysu.edu.cn (X.Z.); xieyanwu@ysu.edu.cn (Y.X.).

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decrease of deposition current, the current-time curve of thedeposition of Ni into the PATs presents three distinct regions.Region I corresponds to the growth of Ni crystals in nanoporesyielding Ni nanowires. A rapid increase in deposition area occursas the nanopores are completely filled with Ni crystals and theelectrodeposit begins to form hemispherical caps over the end ofeach nanowire(region II), whichleadsto a continuous increaseindeposition current until these caps coalesce into a film, where thedeposition current presents a constant (region III). In region I,afteran initial transient decrease, the current-timecurve presentsa horizontalline, indicating a more uniform growthrate of theNicrystalsin nanopores. Obviously, the transition point between theregions I and II, corresponding to which the deposition time isdefined as the characteristic time tE, indicates the end of thegrowth processof Ni crystalsin nanopores.The variation in the tEwith deposition temperature (see Figure 1) demonstrates a strongtemperature dependence of the growth of electrodeposited Nicrystals in nanopores.

Givena template,the growth rate of Ni crystalsin nanopores isproportional to the reciprocal of the characteristic time tE. The tEdecreaseswith increasing deposition temperature T(seeFigure1),demonstrating a thermal activation process for the growth of Nicrystalsin nanopores. Therefore, the temperature variation of therate constants tE

-1 can yield direct kinetic information on thegrowth of Ni nanowires. According to the Arrhenius equation

t-1E t-1E, 0exp -

Ea

kBT

1

where Ea is the apparent growth activation energy, kB is the

Boltzmann constant,tE,0-1

is the prefactor, andTis the depositiontemperature. Linear fits of eq 1 to the data in Figure 2 yield thevalues ofEa= 0.25 ( 0.01, 0.70 ( 0.02, and 1.26 ( 0.02 eV andtE,0-1 =4.5 ( 0.3, (9.0 ( 0.5) 108, and (1.0 ( 0.1) 1019 s-1 for

the growth of Ni crystals in 25, 40, and 160 nm diameter pores,respectively. The Ea seems to have a linear reduction with theinverse of the pore diameter 1/d(see the inset in Figure 2). Toconfirm this conclusion,Eadata yielded in a wide pore diameterrange are required, which will be performed in the future.

We want to point out here that the initial transient decrease ofdeposition current (see Figure 1) is a typical characteristic forelectrodepositing crystals into the nanopores of PATs as demon-strated in previous studies,16-19 which has been attributed to themass transport limitation17 or the creation of the diffusionlayer.18 Because this process lasts only a short time as compared

with crystal growth in nanopores, it has little influence on thedetermination of the characteristic time tE and thus on thedetermination of both theEaand thetE,0

-1 .Usually, if a process in solids involves an activation energy Ea

that is much larger as compared with both the energies ofexcitations (e.g., infrared vibrations or phonons) and kBT, a largenumber of excitations must be collected for the process to take

place.20,21

By this simple idea, the Meyer-Neldel compensa-tion rule (MNR), which indicates that the prefactor increasesexponentially with the activation energy, has been understoodwell.20-24 In the present study, thevariation oftE,0

-1 with Ea followsthe MNR (see Figure 3), demonstrating a multiexcitation me-chanism for the growth of the Ni crystals in the nanopores.According to the MNR thetE,0

-1 andEaare given20,21,23,24

lnt -1E, 0 C Ea

kBT02

where Cis a constant, the term kBT0 represents the typical energyof excitations, and the T0is the isokinetic temperature at whichvarious Arrhenhius plots cross. A linear fit of eq 2 to the data inFigure 3 yields kBT0 =23.9 meVand thus T0 = 277 K,which isin

Figure 1. Deposition current-time curves measured for electrode-position of Ni into a porous alumina template (PAT) with 40 nmdiameter pores at a constant potential of -1.0 V at differentdeposition temperatures. The insets indicate three stages of theelectrodeposition process (see text). ThetEis the characteristic timeindicating the end of the growth process of Ni crystals in thenanopores (stage I) and can be determined from the current-timecurves (see this figure, T= 275.5 K).

Figure 2. Deposition temperature dependence of the rate constanttE-1 of Ni crystal growthintothe PATs with differentporediameters.

Linearfits to these data accordingto eq 1 yield theactivation energyEa and the prefactor tE,0

-1 forNi crystal growthin thenanopores. Thetemperature T0 at the crossover of three Arrhenius plots is theisokinetic temperature.

Figure 3. PrefactortE,0-1 versus activation energyEafor the growth

of electrodeposited Ni crystals in nanopores. The variation ofEawith ln(tE,0

-1 ) follows the Meyer-Neldel compensation rule, fromwhich the excitation energy kBT0 = 23.9 meV for the Ni crystalgrowth is yielded.

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good agreement with that yielded from the crossover of theArrhenius plots in Figure 2.

Previous study25 show that the frequency distribution functionof the vibrations of Ni crystal has two main peaks, 1= 5.8 THz(h1/kB= 278 K,his the Planck constant) and 2= 8 THz (h2/kB = 384 K). Our experimental temperatures are very close tothe excitation temperature of 1, and the phonon energyhv1 =24 meV is in good agreement with the experimentally determinedexcitation energykBT0= 23.9 meV. Moreover, a surface vibra-

tion with the phonon energy of 24 meV has also been experimen-tally observed on Ni surface.26 These results demonstrate that thephonons of Ni lattice vibrations provide the energy necessary toovercomethe energy barrier Ea for the growth of Ni crystalsin thenanopores. The number n = Ea/h1 of phonons

21,24 necessary forNi crystal growth in 25, 40, and 160 nmpores are calculated to be10, 29, and 53, respectively.

To understand the activation energiesEafor the growth of Nicrystals in nanopores, we need to consider fundamental electro-deposition processes: (1) mass transport of Ni2 in electrolyte

outside nanopores, (2) masstransport of Ni2 throughelectrolyteinside nanopores to the surface of deposited Ni nuclei, (3) chargetransfer of Ni2 at crystal surface, forming Ni adatoms (Ni*), (4)

mass transport of Ni*on crystal surface, and(5) incorporationofNi* into crystal structure. As the processes are sequential, the

rate-limiting stepis the slowest process with the energy barrier Ea.The process (1) would not be the rate-limiting step because theactivation energy for the mass transport of Ni2 in water is 0.2eV,27 much lower than our measured values (see Figure 2).

Previous studies show that the ability of mass transport of metalionsin nanpores decreases withdecreasingpore size,19opposite tothe present experimentalresults, suggesting thatprocess(2) is also

not the rate-limiting step. Charge transfer is not believed to bethe rate-limiting step for the growth of the Ni crystals in nano-pores because an exchange current density in the order of 1 10-4 Acm-2 was experimentally determined at 278 K in thepresent work (not shown here), several orders of magnitudelarger than those (1 10-10 to 1 10-6A cm-2)28-30 measuredin electrodeposited Ni in macroscale from the similar Ni/NiSO4system, implying a very fast charge transfer for Ni crystal growthin thenanopores.The energy barrier forprocess(5) is expectedtobe small because no bonds will be broken.10 Therefore, the masstransport of Ni* on crystalsurfaceis the most likely mechanism to

limit the growth of the Ni crystals in the nanopores. Thisconclusion is supported by above results that the phonons of Nilattice vibrations provide the energy necessary to overcome the

energy barrier Ea for the growth of Ni crystals in nanopores,which is strongly relative to the mass transport of Ni* on crystalsurface. This is further strengthened by following discussions.

The Ni nanowires deposited in 25 nm diameter pores have asingle-crystal characteristic with a [110] growth direction (seeFigure 4a), a two-dimensional layer growth mechanism is, there-fore, expected and the activation energy of mass diffusion shouldbe close to that of the intrinsic diffusion on the Ni (110) crystal

face. The activation energies of intrinsic Ni* self-diffusion on(111), (100), and (110) faces are calculated to be 0.06, 0.68, and0.39 eV, respectively.31 A value ofEa 0.31 eV for Ni* self-diffusion on the (110) face was also experimentally determined.32

Taking into account that adatom interactions generally reducethe Ea by tens of millielectronvolts,

33 the measured valueEa =0.25 eV in the present study is in satisfactory agreement withthe intrinsic self-diffusion activation energy on the Ni(110) face.With increasing the pore diameter, the Ni nanowires graduallychange into a polycrystalline structure (see panels b and c inFigure 4), indicating that the two-dimensional layer growthbehavior is violated. This makes the surface of deposited crystalsimperfect with kinks, steps, and terrace vacancies that can act assources or sinks for mobile adatoms, and thus causes the surfacemass diffusion deviate from the intrinsic diffusion and increases

the Ea.34 This qualitatively explains our experimental results in

Figure 2. Moreover, an extrapolation of the Ea-1/dcurve to theunconfined situation (1/d= 0) yields Ea =1.44 eV (see the inset inFigure 2). This value is comparable to that Ea = 0.87-1.69 eVdetermined for the surface mass diffusion of bulk Ni.34,35

In summary, the present studies on the electrodeposited Ninanowires demonstrate that the growth kinetics of depositedcrystals in nanopores can be experimentally determined byemploying the temperature-dependent rate constants yieldedfrom deposition current-time curves. This technique is universaland can be applied to other metal and semiconductor systems for

understanding their growth kinetics in nanopores and thus ofwide interest. Moreover, the present studies provide uniqueinsights into the microscopic growth mechanisms of electrode-posited Ni crystals in nanopores and therefore are of importancefor future studies of high-quality magnetic nanowires.

Acknowledgment. We are indebted to the NSFC (50525102,50671090, 50871095, and 50821001) and the National Basic Re-search Program (2005CB724404) of China for financial support.

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Figure 4. XRD spectra of Ni nanowires electrodeposited into thePATs with different pore diameters at a temperature of 278 K. Thenanowires deposited in 25 nm diameter pores show a single-crystalcharacteristic, which is further confirmed by TEM and the selected-area electron diffraction (ED) studies [see (a)], whereas thosedeposited in 40 and 160 nm diameter pores show a polycrystallinenature [see (b) and (c)]. Some ED points in the inset in (a) areelongated, which is attributed to the existence of defects, e.g.,dislocations and stacking faults in the deposited nanowire, and thusleads to crystal imperfections.36,37

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