Distinguishing nanowire and nanotube formation by the deposition current transients
© Proenca et al.; licensee Springer. 2012
Received: 14 March 2012
Accepted: 12 May 2012
Published: 31 May 2012
High aspect ratio Ni nanowires (NWs) and nanotubes (NTs) were electrodeposited inside ordered arrays of self-assembled pores (approximately 50 nm in diameter and approximately 50 μm in length) in anodic alumina templates by a potentiostatic method. The current transients monitored during each process allowed us to distinguish between NW and NT formation. The depositions were long enough for the deposited metal to reach the top of the template and form a continuous Ni film. The overfilling process was found to occur in two steps when depositing NWs and in a single step in the case of NTs. A comparative study of the morphological, structural, and magnetic properties of the Ni NWs and NTs was performed using scanning electron microscopy, X-ray diffraction, and vibrating sample magnetometry, respectively.
One-dimensional metallic nanostructured materials have attracted extensive attention in recent years because of their technological importance in nanometer-scale devices and information storage systems [1–6]. By combining deposition techniques with nanoporous templates, one can easily tune the growth of these nanostructures and form concentric and multisegmented nanotubes (NTs) and nanowires (NWs) [7–10].
A large number of works have focused on magnetic NWs embedded in ordered arrays of self-assembled pores in alumina membranes [11–17]. However, only a few works have been reported on ordered arrays of magnetic NTs [18–22]. These novel nanostructures have recently attracted much interest due to their inner functionalizable surfaces that can be used for drug delivery [23–25] and also owing to their interesting magnetic behavior [20–22]. In particular, Ni NTs have been prepared in nanoporous membranes by triblock copolymer-assisted hard-template method , electroplating , a sequential electrochemical synthetic method inside conducting polypyrrole NTs , and using chemically modified templates .
In recent reports, the magnetic behavior of arrays of magnetic NTs was investigated [8, 20–22, 26–28]. Nevertheless, there are a number of different parameters that seem to lead to diverse results. For example, Wang et al. found higher remanence values for Ni NT arrays with different diameters (25 to 220 nm) when applying the field parallel to the nanotube's axis, evidencing the important role played by shape anisotropy in the magnetic hysteresis loops. However, other works showed that the easy axis of Ni NT arrays with diameters of approximately 200 nm is perpendicular to the tube axis [21, 26, 28]. Such differences are related to the distinct preparation methods, geometrical characteristics, ordering degree of hexagonal symmetry, or the final crystalline structure of the magnetic elements. Polycrystalline Ni NWs and NTs, with 45-nm diameters, fabricated using a direct-current electrodeposition method also showed similar magnetic hysteresis loops for both NW and NT arrays . In the present work, we used a potentiostatic electrodeposition method to grow Ni NWs and NTs strongly textured along the  direction, showing different coercivity values along the parallel direction. One should also note that few reports can be found on the preparation of small-diameter (<60 nm) Ni NT arrays [22, 27]. Reducing the diameter of these tube-like nanostructures, one expects to increase their coercivity and remanence values along the parallel direction.
The objective of the present work is to address a comparative analysis of the careful preparation of Ni NW and NT arrays with well-controlled ordering and their structural and magnetic response. These nanostructures were prepared by potentiostatic electrodeposition of Ni into suitably modified nanoporous alumina templates (NpATs) opened from both top and bottom sides . Long-range ordering of hexagonal symmetry of the alumina membranes with 105-nm interpore distance and 50-nm pore diameter was achieved [29, 30]. The final NW/NT diameter was approximately 50 nm, while the NTs' wall thickness was approximately 5 nm, and their length corresponded to the membrane thickness (50 μm). The time of deposition was controlled by the overfilling process in which a Ni continuous film was formed on top of the membrane. In this work, we found this process to be considerably different when depositing NWs or NTs inside the pores. A thorough comparative study of the current transients monitored during the electrodeposition process showed a clear distinction between NW and NT formation. The presence of Ni NWs and NTs inside the nanopores was also confirmed by scanning electron microscopy (SEM).
For the deposition of NWs, a 100-nm-thick Au film was sputtered at the opened pores' bottom of the NpAT to serve as the working electrode. In the case of NT formation, a thinner (approximately 40 nm thick) Au layer was sputtered so as not to completely close the bottom of the pores . To avoid leaking of the electrolyte at the opened membrane bottom, a non-conductive varnish was coated on top of the 40-nm-thick Au contact (Figure 1). A Pt mesh was used as the counter electrode and Ag/AgCl (in 4 M KCl) as the reference electrode (0.197 V vs. standard hydrogen electrode). The depositions were performed in a Watts bath (1.14 M NiSO4·6H2O, 0.19 M NiCl2·6H2O, and 0.73 M H3BO3) at −1.5 V vs. Ag/AgCl, using a Solartron 1480 MultiStat (Solartron Analytical, Farnborough, Hampshire, UK). During the electrodeposition, the electrolyte was magnetically stirred at 250 rpm and kept at a constant temperature of 35°C within a pH range between 3.5 and 4 .
To explore the differences between NW and NT formation, the depositions were performed until a continuous Ni film was formed on top of the NpAT. Different stages were then found in the overfilling process, and to better understand them, several experiments were made, stopping the deposition at each stage (Figure 1).
Prior to NW and NT characterization, the Ni film formed on top of the NpAT was removed by mechanical polishing using alumina powder (particle size of 10 μm). The mechanical polishing was only performed on the upper surface of the nanoporous alumina template, in order to remove the Ni caps that overfilled and are present at the membranes' top, without causing any damage to the electrodeposited nanostructures. The remaining Ni caps on top of the NpATs were then completely etched by ion milling (5 to 10 μm). The milling process was carried out using an ion beam sputter deposition system by the Commonwealth Scientific Corporation . Morphological characterization was performed using SEM (FEI Nova NanoSEM 230 and FEI Quanta 400FEG, FEI Co., Eindhoven, The Netherlands). Structural analysis was performed by X-ray diffraction (XRD; PANalytical X'Pert Pro, PANalytical B.V., Almelo, The Netherlands) using Cu Kα1 radiation (λ = 0.15406 nm) and the Bragg-Brentano θ/2θ geometry . The deposited NWs and NTs were magnetically characterized using a vibrating sample magnetometer (VSM; LOT-Oriel EV7, LOT-Oriel, Leatherhead, Surrey, UK).
Results and discussion
Structural and morphological properties
As for the NTs, their deposition area is approximately 40% of the NWs' area, and their geometry will favor the metal deposition to occur without a preferred orientation (Figure 4). One therefore finds different Ni grain orientations on the film grown on top of the NTs, exhibiting a preferential crystallographic texture along the  direction.
Current transients during deposition
On the other hand, once NTs are present inside the pores, their overfill process occurs in a one-step manner (stage 3 in Figure 5b). During the NT growth inside the pores, the cathode deposition surface is less than 5% of the total NpAT area exposed to the electrolyte. Therefore, as the first NTs emerge and start overfilling, the effective deposition area is drastically enlarged, as confirmed by the sharp increase of the current (stage 3 in Figure 5b). Additionally, the caps formed at the NpAT surface display a spherical shape without well-defined facets (Figure 7). This phenomenon will largely influence the increase in current occurring in only one stage. One should note that as soon as a few spheres are formed on top, the cathode surface area increases to a higher value than the effective area of NT deposition inside the nanopores. Stage 3 of NT deposition should therefore be equivalent to stage 3.2 of NW deposition. Since for the NW growth this only occurs after a small amount of Ni film was already formed on top (during stage 3.1), the overfilling process will exhibit two steps during stage 3 (Figure 1).
When considering the current data from Figure 5 together with the XRD data of Figure 3, one can see that the high current increase, observed when the NTs start overfilling, helps promote the growth orientations along the  and  directions while lowering  texture. However, when the NWs overfill, the current increase is smoother than in the case of the NTs (Figure 5), therefore promoting with less evidence the Ni crystal growth along the  and  directions (Figure 3).
The longitudinal anisotropy of the NW/NT arrays is confirmed by the large remanence in the //-direction loop that nearly reaches the saturation magnetization (MSat) value. The coercive field (HC) is approximately 1,000 Oe for the NW and approximately 600 Oe for the NT arrays, in good agreement with the values reported in previous works [16, 22, 27]. Seemingly, the longitudinal remagnetization involves the presence of a domain wall-like process. Perpendicular hysteresis loops show reduced coercivity (approximately 100 Oe) and the presence of a magnetization rotation reversal mode. Values of the longitudinal anisotropy field can be derived (from extrapolation of initial susceptibility) to be approximately 3,500 and 2,500 Oe for NWs and NTs, respectively, in agreement with the reduced longitudinal coercivity observed for NTs. One should note that the preparation of NTs with small diameters (<60 nm) is a laborious process and very difficult to achieve , so most of the reports on magnetic measurements in NT arrays correspond to NTs with outer diameters of approximately 200 nm. For these nanostructures, one finds very small coercive fields (<100 Oe) and remanence (approximately 0.05) along the //-direction [8, 20, 21, 26, 28]. However, Wang et al.[22, 27] found that coercivity and remanence along the //-direction increase with the decrease of the outer diameter, obtaining an HC//of approximately 610 Oe for outer diameters of approximately 45 nm.
In this work, we fabricated Ni NW and NT arrays in NpATs by a potentiostatic electrodeposition process using a three-electrode cell. The monitorization of the current transients during deposition allowed us to distinguish between NW and NT formation and identify the four main stages of deposition in nanoporous membranes. The overfilling of the deposited metal was found to occur in a two-step process for the NWs and only in one step for the NTs. Morphological and structural characterization of the Ni NWs, NTs, and film overgrown on top of the membrane was performed by SEM and XRD. Both NWs and NTs exhibit a preferential crystallographic growth textured along the  direction. The films overgrown on top illustrated the presence of Ni grains oriented along the , , and  directions. The Ni NWs overgrow with a preferential orientation along the  direction, showing polygonal-shaped caps at the surface of the membrane. On the other hand, the Ni film on top of the NTs exhibits a preferential growth along the  direction and an agglomeration of spherical polycrystalline caps. Magnetic hysteresis loops illustrate a magnetization easy axis parallel to the long axis for both NW and NT arrays, exhibiting stronger anisotropy and higher coercivity values for the NW arrays.
M. P. Proença and C. T. Sousa are thankful to FCT for the doctoral and postdoctoral grants SFRH/BD/43440/2008 and SFRH/BPD/82010/2011, respectively. J. Ventura acknowledges the financial support through FSE/POPH. M Vázquez thanks the Spanish Ministry of Economia y Competitividad, MEC, under project MAT2010-20798-C05-01. J. P. Araújo also thanks the Fundação Gulbenkian for its financial support within the ‘Programa Gulbenkian de Estímulo à Investigação Científica’. The authors acknowledge the funding from FCT through the Associated Laboratory - IN and project PTDC/FIS/105416/2008.
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