DOI: 10.1021/cg9004888
Growth Kinetics of Nickel Crystals in Nanopores Cuiyan Yu,† Yanwu Xie,*,† Tao Xu,† Yan Chen,† Xiaohong Li,‡ Wei Li,† Baoting Liu,‡ and Xiangyi Zhang*,†
2009, Vol. 9 3840–3843
†
State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, 066004 Qinhuangdao, P. R. China, and ‡College of Physics Science and Technology, Hebei University, 071002 Baoding, 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 their growth kinetics. In the present study, we succeeded in studying for the first time the growth kinetics of electrodeposited Ni crystals in nanopores by employing the temperature-dependent rate constants yielded from deposition current-time curves. A small growth -1 activation energy Ea = 0.25 ( 0.01 eV and prefactor t-1 are determined for the Ni crystals in 25 nm diameter pores, and E,0 = 4.5 ( 0.3 s they increase with the pore diameter following the Meyer-Neldel compensation rule. On the basis of these studies, the growth mechanism of electrodeposited Ni nanowires has been revealed. Crystal growth is one of the most intensively studied topics in materials science and solid-state physics because many technological systems in the fields of information, communication, energy, transportation, etc., depend on the availability of suitable crystals with tailored properties.1 A substantial progress has been made in the science and technology of crystal growth, and the basic mechanisms of crystal nucleation and growth are now understood 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 great deal of interest has been generated in the well-controlled synthesis of nanocrystals at the nanoscale level in order to produce nanostructured materials with excellent and tunable functional properties.5-11 A profound understanding of the growth kinetics (e.g., growth activation energy and prefactor) of crystals in nanoscale spaces is crucial for yielding well-controlled perfect and homogeneous nanowires, which are of particular importance for fabricating nanodevices with excellent functional properties. Previous studies on the growth mechanism of nanowires have been dominantly focused on the thermodynamic processes and the kinetic studies are limited and mostly focused on the growth of nanowires yielded by the vapor-liquid-solid route, in particular the growth of Si nanowires.8-10 Although the electrodeposition technique has been proven to be one of the most successful approaches to produce various nanowires with controlled length, diameter, and growth orientiation,12,13 the growth kinetics and in particular the microscopic growth mechanism of electrodeposited crystals in nanopores is far less well understood. This is probably due to the lack of an experimental technique in obtaining reliable data on crystal growth in nanopores. More recently, the superlattice structured nanowires have been significantly employed to study the growth mechanism of electrodeposited nanowires.14 Although in principle the growth rate of electrodeposited crystals is proportional to the current because the total amount of substance being converted is proportional to the amount of charge, hydrogen evolution also consumes charge and makes it difficult to identify the growth rate of deposited crystals exclusively.15 Here, in the exemplary case of Ni crystals, we demonstrate for the first time that the growth kinetics of electrodeposited crystals in nanopores can be experimentally determined by employing the temperature-dependent rate constants yielded from deposition current-time curves. This methodology can be further extended to the study of growth kinetics of other metal *Corresponding author. E-mail:
[email protected] (X.Z.); xieyanwu@ ysu.edu.cn (Y.X.). pubs.acs.org/crystal
Published on Web 07/09/2009
and semiconductor systems in nanopores and thus is of wide interest. In the present study, the Ni crystal was used as model system because it is a simple and typical crystal with well-obtained knowledge of growth processes at the macroscopic scale. The deposition of Ni was performed in a homemade threeelectrode system that was placed in a temperature-controlled chamber with a precision of ( 0.1 °C, where the pure Ni plank and a standard calomel electrode (SCE) were used as the counter electrode and the reference electrode, respectively. The two-step anodization technique was employed to prepare porous alumina templates (PATs) with different pore diameters d ≈ 25, 40, and 160 nm and a thickness of ∼700, 600, and 500 nm, respectively. A Cu film with a thickness of ∼100 nm was deposited on one side of the PATs and served as the cathode electrode by employing a thermal evaporation approach. The electrolyte was made of 1.3 M NiSO4 3 6H2O and 0.6 M H3BO3. Deposition experiments were performed at a constant potential of -1.0 V (versus SCE) in the temperature range from 273 to 288 K, and the current-time curves were measured using a computer-controlled recording system with a time resolution of 1 10-1 s. All deposition parameters used in the present study were experimentally determined by considering the application of the present methodology and the time needed to perform deposition experiments. The pore diameter and thickness of PATs were determined using a XL S-FEG scanning electron microscope (SEM). The microstructure of deposited nanowires was studied by employing a Rigaku D/ max-2500 X-ray diffractometer (XRD) with Cu KR radiation and a JEM-2010 transmission electron microscope (TEM). For TEM studies, the PAT was fully dissolved using NaOH (0.5M) solution. We want to point out here that the quality, uniformity, and pretreatments of the pure Al sheets used in the present studies as well as the two-step anodization process of PATs were strictly controlled in order to eliminate the uncertainties affecting the pore diameter and pore length of PATs. Moreover, the deposition of Cu film, electrodeposition processes (in particular the deposition temperature), and the recording of current-time curves were also strictly controlled so as to eliminate the effect of uncertainties on the determination of characteristic time tE. For each deposition temperature, more than three repeatable measurements have been performed to reduce the effects of experimental uncertainties. At a given potential, under the pseudosteady-state conditions, the deposition current is directly proportional to the area of the electrodeposit.16 Therefore, the deposition process of crystals in nanopores can be directly monitored from the current response. As shown in Figure 1 (for T = 275.5 K), after the initial transient r 2009 American Chemical Society
Communication
Figure 1. Deposition current-time curves measured for electrodeposition of Ni into a porous alumina template (PAT) with 40 nm diameter pores at a constant potential of -1.0 V at different deposition temperatures. The insets indicate three stages of the electrodeposition process (see text). The tE is the characteristic time indicating the end of the growth process of Ni crystals in the nanopores (stage I) and can be determined from the current-time curves (see this figure, T = 275.5 K).
decrease of deposition current, the current-time curve of the deposition of Ni into the PATs presents three distinct regions. Region I corresponds to the growth of Ni crystals in nanopores yielding Ni nanowires. A rapid increase in deposition area occurs as the nanopores are completely filled with Ni crystals and the electrodeposit begins to form hemispherical caps over the end of each nanowire (region II), which leads to a continuous increase in deposition current until these caps coalesce into a film, where the deposition current presents a constant (region III). In region I, after an initial transient decrease, the current-time curve presents a horizontal line, indicating a more uniform growth rate of the Ni crystals in nanopores. Obviously, the transition point between the regions I and II, corresponding to which the deposition time is defined as the characteristic time tE, indicates the end of the growth process of Ni crystals in nanopores. The variation in the tE with deposition temperature (see Figure 1) demonstrates a strong temperature dependence of the growth of electrodeposited Ni crystals in nanopores. Given a template, the growth rate of Ni crystals in nanopores is proportional to the reciprocal of the characteristic time tE. The tE decreases with increasing deposition temperature T (see Figure 1), demonstrating a thermal activation process for the growth of Ni crystals in nanopores. Therefore, the temperature variation of the rate constants t-1 E can yield direct kinetic information on the growth of Ni nanowires. According to the Arrhenius equation Ea tE-1 ¼ tE-1 ð1Þ exp ,0 kB T where Ea is the apparent growth activation energy, kB is the Boltzmann constant, t-1 E,0 is the prefactor, and T is the deposition temperature. Linear fits of eq 1 to the data in Figure 2 yield the values of Ea = 0.25 ( 0.01, 0.70 ( 0.02, and 1.26 ( 0.02 eV and 8 19 -1 t-1 for E,0 =4.5 ( 0.3, (9.0 ( 0.5) 10 , and (1.0 ( 0.1) 10 s the growth of Ni crystals in 25, 40, and 160 nm diameter pores, respectively. The Ea seems to have a linear reduction with the inverse of the pore diameter 1/d (see the inset in Figure 2). To confirm this conclusion, Ea data yielded in a wide pore diameter range are required, which will be performed in the future. We want to point out here that the initial transient decrease of deposition current (see Figure 1) is a typical characteristic for electrodepositing crystals into the nanopores of PATs as demonstrated in previous studies,16-19 which has been attributed to the mass transport limitation17 or the creation of the diffusion layer.18 Because this process lasts only a short time as compared
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Figure 2. Deposition temperature dependence of the rate constant t-1 E of Ni crystal growth into the PATs with different pore diameters. Linear fits to these data according to eq 1 yield the activation energy Ea and the prefactor t-1 E,0 for Ni crystal growth in the nanopores. The temperature T0 at the crossover of three Arrhenius plots is the isokinetic temperature.
Figure 3. Prefactor t-1 E,0 versus activation energy Ea for the growth of electrodeposited Ni crystals in nanopores. The variation of Ea with ln(t-1 E,0) follows the Meyer-Neldel compensation rule, from which the excitation energy kBT0 = 23.9 meV for the Ni crystal growth is yielded.
with crystal growth in nanopores, it has little influence on the determination of the characteristic time tE and thus on the determination of both the Ea and the t-1 E,0. Usually, if a process in solids involves an activation energy Ea that is much larger as compared with both the energies of excitations (e.g., infrared vibrations or phonons) and kBT, a large number of excitations must be collected for the process to take place.20,21 By this simple idea, the Meyer-Neldel compensation rule (MNR), which indicates that the prefactor increases exponentially with the activation energy, has been understood well.20-24 In the present study, the variation of t-1 E,0 with Ea follows the MNR (see Figure 3), demonstrating a multiexcitation mechanism for the growth of the Ni crystals in the nanopores. 20,21,23,24 According to the MNR the t-1 E,0 and Ea are given ln tE-1 ,0 ¼ C þ
Ea kB T0
ð2Þ
where C is a constant, the term kBT0 represents the typical energy of excitations, and the T0 is the isokinetic temperature at which various Arrhenhius plots cross. A linear fit of eq 2 to the data in Figure 3 yields kBT0 =23.9 meV and thus T0 = 277 K, which is in
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good agreement with that yielded from the crossover of the Arrhenius plots in Figure 2. Previous study25 show that the frequency distribution function of the vibrations of Ni crystal has two main peaks, ν1 = 5.8 THz (hν1/kB = 278 K, h is the Planck constant) and ν2 = 8 THz (hν2/ kB = 384 K). Our experimental temperatures are very close to the excitation temperature of ν1, and the phonon energy hv1 = 24 meV is in good agreement with the experimentally determined excitation energy kBT0 = 23.9 meV. Moreover, a surface vibration with the phonon energy of 24 meV has also been experimentally observed on Ni surface.26 These results demonstrate that the phonons of Ni lattice vibrations provide the energy necessary to overcome the energy barrier Ea for the growth of Ni crystals in the nanopores. The number n = Ea/hν1 of phonons21,24 necessary for Ni crystal growth in 25, 40, and 160 nm pores are calculated to be 10, 29, and 53, respectively. To understand the activation energies Ea for the growth of Ni crystals in nanopores, we need to consider fundamental electrodeposition processes: (1) mass transport of Ni2þ in electrolyte outside nanopores, (2) mass transport of Ni2þ through electrolyte inside nanopores to the surface of deposited Ni nuclei, (3) charge transfer of Ni2þ at crystal surface, forming Ni adatoms (Ni*), (4) mass transport of Ni* on crystal surface, and (5) incorporation of Ni* into crystal structure. As the processes are sequential, the rate-limiting step is the slowest process with the energy barrier Ea. The process (1) would not be the rate-limiting step because the activation energy for the mass transport of Ni2þ in water is ∼0.2 eV,27 much lower than our measured values (see Figure 2). Previous studies show that the ability of mass transport of metal ions in nanpores decreases with decreasing pore size,19 opposite to the present experimental results, suggesting that process (2) is also not the rate-limiting step. Charge transfer is not believed to be the rate-limiting step for the growth of the Ni crystals in nanopores because an exchange current density in the order of 1 10-4 Acm-2 was experimentally determined at 278 K in the present work (not shown here), several orders of magnitude larger than those (1 10-10 to 1 10-6A cm-2)28-30 measured in electrodeposited Ni in macroscale from the similar Ni/NiSO4 system, implying a very fast charge transfer for Ni crystal growth in the nanopores. The energy barrier for process (5) is expected to be small because no bonds will be broken.10 Therefore, the mass transport of Ni* on crystal surface is the most likely mechanism to limit the growth of the Ni crystals in the nanopores. This conclusion is supported by above results that the phonons of Ni lattice 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 crystal surface. This is further strengthened by following discussions. The Ni nanowires deposited in 25 nm diameter pores have a single-crystal characteristic with a [110] growth direction (see Figure 4a), a two-dimensional layer growth mechanism is, therefore, expected and the activation energy of mass diffusion should be 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, and 0.39 eV, respectively.31 A value of Ea ≈ 0.31 eV for Ni* selfdiffusion on the (110) face was also experimentally determined.32 Taking into account that adatom interactions generally reduce the Ea by tens of millielectronvolts,33 the measured value Ea =0.25 eV in the present study is in satisfactory agreement with the intrinsic self-diffusion activation energy on the Ni(110) face. With increasing the pore diameter, the Ni nanowires gradually change into a polycrystalline structure (see panels b and c in Figure 4), indicating that the two-dimensional layer growth behavior is violated. This makes the surface of deposited crystals imperfect with kinks, steps, and terrace vacancies that can act as sources or sinks for mobile adatoms, and thus causes the surface mass diffusion deviate from the intrinsic diffusion and increases
Figure 4. XRD spectra of Ni nanowires electrodeposited into the PATs with different pore diameters at a temperature of 278 K. The nanowires deposited in 25 nm diameter pores show a single-crystal characteristic, which is further confirmed by TEM and the selectedarea electron diffraction (ED) studies [see (a)], whereas those deposited in 40 and 160 nm diameter pores show a polycrystalline nature [see (b) and (c)]. Some ED points in the inset in (a) are elongated, which is attributed to the existence of defects, e.g., dislocations and stacking faults in the deposited nanowire, and thus leads to crystal imperfections.36,37
the Ea.34 This qualitatively explains our experimental results in Figure 2. Moreover, an extrapolation of the Ea-1/d curve to the unconfined situation (1/d = 0) yields Ea =1.44 eV (see the inset in Figure 2). This value is comparable to that Ea = 0.87-1.69 eV determined for the surface mass diffusion of bulk Ni.34,35 In summary, the present studies on the electrodeposited Ni nanowires demonstrate that the growth kinetics of deposited crystals in nanopores can be experimentally determined by employing the temperature-dependent rate constants yielded from deposition current-time curves. This technique is universal and can be applied to other metal and semiconductor systems for understanding their growth kinetics in nanopores and thus of wide interest. Moreover, the present studies provide unique insights into the microscopic growth mechanisms of electrodeposited Ni crystals in nanopores and therefore are of importance for future studies of high-quality magnetic nanowires. Acknowledgment. We are indebted to the NSFC (50525102, 50671090, 50871095, and 50821001) and the National Basic Research Program (2005CB724404) of China for financial support.
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