**Electrodeposition of WO3 Nanoparticles for Sensing Applications**

L. Santos, J. P. Neto, A. Crespo, P. Baião, P. Barquinha, L. Pereira, R. Martins and E. Fortunato

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/61216

#### **Abstract**

The motivation of using metal oxides is mainly due to its charge storage capabilities, and electrocatalytic, electrochromic and photoelectrochemical properties. But comparing with bulk, nanostructured materials present several advantages related with the spatial confinement, large fraction of surface atoms, high surface energy, strong surface adsorp‐ tion and increased surface to volume ratio, which greatly improves the performances of these materials. The deposition of this materials can be accomplished by a variety of physical and chemical techniques but nowadays, electrodeposited metal oxides are gen‐ erally used in both laboratories and industries due to the flexibility to control structure and morphology of the oxide electrodes combined with a reduced cost. Tungsten oxide (WO3) is a well-studied semiconductor and is used for several applications as chromo‐ genic material, sensor and catalyst. The major important features is its low cost and avail‐ ability, improved stability, easy morphologic and structural control of the nanostructures, reversible change of conductivity, high sensitivity, selectivity and biocompatibility. For the electrodeposition of WO3, more than one method can be adopted: electrodeposition from a precursor solution, anodic oxidation, and electrodeposition of already produced nanoparticles; however, in this case the mechanism of the electrodeposition is not fully understood. In this chapter, a review of the latest published work of electrodeposited nanostructured metal oxides is provided to the reader, with a more detailed explanation of WO3 material applied in sensing devices.

**Keywords:** tungsten oxide, pH sensor, neural recordings, impedance

## **1. Introduction**

Over the past two decades, the revolution in materials science has driven great advances in all areas of science and engineering. Nanoscience and nanotechnology are leading this revolution

© 2015 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

fueled by the industrial progress, the scientific ability to fabricate, model, and manipulate objects with a small number of atoms, and the continuous discovery of new phenomena at the nanoscale [1, 2]. Nanomaterials present unique properties, which are not found in the respec‐ tive bulk materials [3]. Surface and quantum effects arise in nanostructures due to the large surface-to-volume ratio and to the dimensions that are comparable to the electron wavelength, respectively [4, 5].

In the metal oxides field, the discovery of superconductivity [6] and large magnetoresistance [7] has raised researchers' attention, especially to those with transition metals. Moreover, in traditional electronics, oxides are widely used as semiconductors, dielectrics, and conductive electrodes [8]. In the last years, nanostructured metal oxides for sensing applications have achieved significant advances, mainly due to their better thermal and environmental stability compared with organic materials. These devices, based on nanomaterials, can operate with low power consumption and can be easily integrated with nanoelectronics. Furthermore, the construction of sensors in "low-cost" substrates, such as plastic, paper, or textile, is also in demand for application in portable consumer devices [9–13]. Electrodeposition, in this case, is of great interest due to its flexibility to control the structure and morphology of the oxide electrodes combined with the reduced cost [14, 15].

## **2. Electrodeposition**

The term electrodeposition is often used unclearly, referring either to electroplating or to electrophoretic deposition (EPD) [16]. The electroplating process is based on a solution of ionic species, usually in water, while EPD occurs in a suspension of particles. In electroplating, there is a charge transfer during the deposition to produce the metal or oxide layer in the electrode, while in EPD the deposition occurs without any reaction involved (Fig. 1). In fact, the principal driving force for EPD is the charge and the electrophoretic mobility of the particles in the solvent under the influence of an applied electric field, with the drawback that the solvent should be organic in order to avoid water electrolysis [16, 17].

**Figure 1.** Schematic representation of the two types of cathodic electrodeposition processes: (a) electroplating and (b) electrophoretic deposition (EPD).

Another variation of an electrochemical deposition is the electroless (autocatalytic) deposition in which a reducing agent, dissolved in the electrolyte, is the electron source for the redox reaction, and no external power supply is needed [18]. Nevertheless, the electroless deposition will not be discussed in this chapter.

fueled by the industrial progress, the scientific ability to fabricate, model, and manipulate objects with a small number of atoms, and the continuous discovery of new phenomena at the nanoscale [1, 2]. Nanomaterials present unique properties, which are not found in the respec‐ tive bulk materials [3]. Surface and quantum effects arise in nanostructures due to the large surface-to-volume ratio and to the dimensions that are comparable to the electron wavelength,

In the metal oxides field, the discovery of superconductivity [6] and large magnetoresistance [7] has raised researchers' attention, especially to those with transition metals. Moreover, in traditional electronics, oxides are widely used as semiconductors, dielectrics, and conductive electrodes [8]. In the last years, nanostructured metal oxides for sensing applications have achieved significant advances, mainly due to their better thermal and environmental stability compared with organic materials. These devices, based on nanomaterials, can operate with low power consumption and can be easily integrated with nanoelectronics. Furthermore, the construction of sensors in "low-cost" substrates, such as plastic, paper, or textile, is also in demand for application in portable consumer devices [9–13]. Electrodeposition, in this case, is of great interest due to its flexibility to control the structure and morphology of the oxide

The term electrodeposition is often used unclearly, referring either to electroplating or to electrophoretic deposition (EPD) [16]. The electroplating process is based on a solution of ionic species, usually in water, while EPD occurs in a suspension of particles. In electroplating, there is a charge transfer during the deposition to produce the metal or oxide layer in the electrode, while in EPD the deposition occurs without any reaction involved (Fig. 1). In fact, the principal driving force for EPD is the charge and the electrophoretic mobility of the particles in the solvent under the influence of an applied electric field, with the drawback that the solvent

**Figure 1.** Schematic representation of the two types of cathodic electrodeposition processes: (a) electroplating and (b)

respectively [4, 5].

28 Electroplating of Nanostructures

**2. Electrodeposition**

electrophoretic deposition (EPD).

electrodes combined with the reduced cost [14, 15].

should be organic in order to avoid water electrolysis [16, 17].

The first reports on the electrodeposition technique date back to the 19th century; however, the understanding of the process and the electrochemistry involved was only developed in the 20th century and it is believed that further research is still needed to optimize the process [16].

In electroplating, the relation between the current and the overpotential of electrodeposition is given by the Tafel equation (Equation 1), which describes the exponential dependence between the two parameters. Worth mentioning is that with the increase of the overpotential, the ionic current that the electrolyte can supply is limited either by material transport or electrical conductivity [15, 19].

$$\dot{\mathbf{u}} = -\mathbf{FkC} \exp\left(\frac{\alpha \mathbf{F} \eta}{\mathbf{RT}}\right) \tag{1}$$

where *i* is the current, *F* is Faraday's constant, *k* a constant, *C* the concentration of metal ions in solution (which can be initially dissolved in the electrolyte or originated from the dissolution of the metallic anode), *α* the coefficient of symmetry (∼ 0.5), *η* the overpotential, *R* the ideal gas constant, and *T* the absolute temperature (K).

The first attempt to correlate the amount of particles deposited by EPD with the different parameters influencing electrophoresis was first described by Hamaker for electrophoretic cells with a planar geometry. Over the years, Hamaker's law has been adapted and more recently Equation 2 was derived, relating the weight (*W*) of the charged particles deposited per unit area of electrode in the initial period, with different parameters, and disregarding the charge of the free ions [20].

$$\mathbf{W} = \frac{2}{3} \mathbf{C} \varepsilon\_0 \varepsilon\_r \xi \mu^{-1} \mathbf{E} \mathbf{L}^{-1} \mathbf{t} \tag{2}$$

Here, *C* is the concentration of the particles;*ε* 0 and *ε*r the permittivity of vacuum and solvent, respectively; *ξ* the zeta potential of the particles;*μ* the viscosity of the solvent; *E* the applied potential; *L* the distance between the electrodes; and *t* the deposition time. Equation 2 demonstrates that the deposition weight of the charged particles under ideal EPD depends on all the previous parameters. However, if the solvent, the particles, and the apparatus for EPD are not changed, the weight of the deposited particles (*W*) is a function of *C*, *E*, and *t*. Therefore, the mass of the deposited particles, namely the thickness of the films, can be easily controlled by the concentration of the suspension, applied potential, and deposition time [17].

Electrodeposition of conventional metals for coatings has a very long history, with more than 200 years for some metals and alloys. Today, electrodeposition is much more than just a technique for coatings fabrication. In addition to applications such as decorative, wear, and corrosion-protective coatings, electrodeposition is also used for the manufacture of molds, functional coatings for magnetic and electronic applications, and microelectromechanical system components production [5]. In the future, though traditional applications will continue, new ones will rapidly develop, especially in the fields of nanoelectronics, biotechnology, and energy engineering. The electrodeposition of non-metallic materials will become more important and the combination of electrodeposition with other processes will lead to nano‐ structured materials with new and improved properties [21, 22]. Electrodeposition is extreme‐ ly versatile and different applications will keep being explored [23].

## **3. Metal oxide electrodeposition**

Metal oxides are an important class of materials, which benefit from the large electronegativity of oxygen to induce strong bonding with nearby atoms [22]. At the same time, when compared with bulk materials, nanostructured metal oxides benefit from the spatial confinement, the large fraction of surface atoms, high surface energy, strong surface adsorption, and increased surface-to-volume ratio that greatly improves the performance of these materials [24].

The deposition of nanostructured metal oxides has been already reported by both physical and chemical methods [8, 5]. The advantages of electrodeposition include its speed, low cost, high purity, industrial applicability, use of different types of substrates, and production of films with different morphologies and compositions, as multilayers and alloys [21, 22].

In the electroplating of metal oxides, the reaction involved is usually defined by two consec‐ utive steps (Equation 3). First, the hydroxide will precipitate in the surface of the electrode due to the reaction of the metal ion (Mn+) in an alkaline solution, and secondly, the oxide is formed through a condensation/dehydration process. This last step can occur either during electro‐ deposition or by a subsequent annealing procedure [15].

$$\text{M}^{n+}\underset{\text{(aq)}}{\text{+}} + \text{nOH}^{\cdot}\_{\text{(aq)}} \rightarrow \text{M(OH)}\_{\text{n(ads)}} \rightarrow \text{MO} + \text{nH}\_{2}\text{O} \tag{3}$$

Another alternative is the formation of metal oxides by anodic oxidation [15]. In this case, the source of the metal ions is the metallic anode and the metal oxide film will be deposited on top of the metal electrode. The general equation can be described as (Equation 4):

$$\text{M } + m\text{H}\_2\text{O} \leftrightarrow \text{MO}\_{\text{m(s)}} + 2m\text{H}^+\_{\text{(aq)}} + 2m\text{e}^\cdot \tag{4}$$

In EPD, the metal oxide nanoparticles are generally synthesized by different solution based techniques, e.g., sol-gel, precipitation, and hydrothermal synthesis, prior to deposition. The main challenge of this technique is the preparation of a stable dispersion that originates a film with good properties, uniformity, and appropriate thickness. The use of dispersants, binders, or other additives that influences the agglomeration and charge of the particles contributes to the tuning of the properties of the deposited film and need to be considered in defining the EPD parameters [16].

#### **3.1. Applications**

technique for coatings fabrication. In addition to applications such as decorative, wear, and corrosion-protective coatings, electrodeposition is also used for the manufacture of molds, functional coatings for magnetic and electronic applications, and microelectromechanical system components production [5]. In the future, though traditional applications will continue, new ones will rapidly develop, especially in the fields of nanoelectronics, biotechnology, and energy engineering. The electrodeposition of non-metallic materials will become more important and the combination of electrodeposition with other processes will lead to nano‐ structured materials with new and improved properties [21, 22]. Electrodeposition is extreme‐

Metal oxides are an important class of materials, which benefit from the large electronegativity of oxygen to induce strong bonding with nearby atoms [22]. At the same time, when compared with bulk materials, nanostructured metal oxides benefit from the spatial confinement, the large fraction of surface atoms, high surface energy, strong surface adsorption, and increased surface-to-volume ratio that greatly improves the performance of these materials [24].

The deposition of nanostructured metal oxides has been already reported by both physical and chemical methods [8, 5]. The advantages of electrodeposition include its speed, low cost, high purity, industrial applicability, use of different types of substrates, and production of films with different morphologies and compositions, as multilayers and alloys [21, 22].

In the electroplating of metal oxides, the reaction involved is usually defined by two consec‐ utive steps (Equation 3). First, the hydroxide will precipitate in the surface of the electrode due to the reaction of the metal ion (Mn+) in an alkaline solution, and secondly, the oxide is formed through a condensation/dehydration process. This last step can occur either during electro‐

ly versatile and different applications will keep being explored [23].

deposition or by a subsequent annealing procedure [15].

( ) ( ) ( ) ( )

n+ - M OH M OH MO H O

top of the metal electrode. The general equation can be described as (Equation 4):

( ) ( ) + - M H O MO 2 H + 2 e *m mm* <sup>2</sup> m s aq

Another alternative is the formation of metal oxides by anodic oxidation [15]. In this case, the source of the metal ions is the metallic anode and the metal oxide film will be deposited on

In EPD, the metal oxide nanoparticles are generally synthesized by different solution based techniques, e.g., sol-gel, precipitation, and hydrothermal synthesis, prior to deposition. The main challenge of this technique is the preparation of a stable dispersion that originates a film with good properties, uniformity, and appropriate thickness. The use of dispersants, binders,

aq aq n ads <sup>2</sup> + ® ®+ *n n* (3)

+« + (4)

**3. Metal oxide electrodeposition**

30 Electroplating of Nanostructures

Nowadays, electrodeposited nanostructured metal oxides are generally used for different applications in laboratories and industry [15]. The latest published reports on the field, listed in Table 1, evidentiate the diversity of areas where these materials can be applied, as presented below.

The deposition of metal/metal oxide nanoparticles composites allowed advances on the protective coatings field. Sajjadnejad et al. [25] improved the corrosion resistance of zinc by co-depositing TiO2 nanoparticles, while Zeng et al. [26] incorporated CeO2 nanoparticles to improve the corrosion behavior of nickel coatings. Charlot et al. [27] opened the discussion of the kinetics and mechanism of the anodic EPD of SiO2 nanoparticles to improve the control of the thickness and properties of these coatings (Fig. 2).

**Figure 2.** Scanning electron microscopy (SEM) images of a film cross-section obtained from a suspension with a mass fraction of 3% of nanoparticles under an applied electric field of (a) 6 V cm−1 and of (b) 60 V cm−1. Reprinted from [27], with permission from Elsevier.

Metal oxide nanostructures are already known to show good catalytic properties. Tu et al. [28] produced Cu2O-Cu nanoparticles in carbon paper via electroplating. This procedure is an easy, one-step technique that can be an attractive candidate as a visible-light-driven photocatalyst. At the same time, Yoon et al. [29] studied the influence of 2D and 3D structures on electrode‐ posited Cu2O films by controlling the electrolyte pH and by using polystyrene (PS) beads as template, respectively. This techniques allowed the production of electrodes with increased surface area. Battaglia et al. [30] also improved the catalytic performance of different Ni electrodes by electrodepositing IrO2 nanostructures through different electrochemical meth‐ ods. The composites obtained by galvanostatic deposition of the oxide catalyst presented the best activity for water splitting applications.

Solid oxide fuel cells (SOFC) have shown to be a good alternative for electric power generation systems. SOFC show high energy conversion efficiency, clean power generation, reliability, modularity, fuel adaptability, noise-free, excellent long-term stability, and versatility for direct conversion of chemical energy to electrical energy. In this field, Das and Basu [31] applied the EPD technique to deposit yttria-stabilized zirconia (YSZ) nanoparticles on a NiO-YSZ sub‐ strate, which after sintering was suitable for application in SOFC (Fig. 3).

**Figure 3.** Field-emission SEM images of top view and cross-section of yttria-stabilized zirconia (YSZ) electrophoretic deposition coating (a) (c) as-deposited and (b) (d) sintered at 1400 °C for 6 h, directly deposited onto the conducting polymers such as polypyrrole-coated NiO-YSZ substrate at a constant applied voltage of 15 V. Reprinted from [31], with permission from John Wiley and Sons.

EPD was also the technique used to deposit TiO2 nanoparticles for dye-sensitized solar cells (DSSC) [32] and Li-ion micro-batteries applications [33]. For DSSC, the thickness of the TiO2 films was controlled by changing the deposition time and the I2 dosage that electrically charge the nanoparticles, while for batteries, the EPD was performed with different TiO2 structures and different 3D aluminum collectors configurations (Fig. 4). The effect of the substrate was also tested in the EPD of ZnO nanoparticles for conductive fabrics applications [34]. Liu et al. [35] studied the EDP of metal oxides using celestine blue as charging and dispersing agent. The nanostructured MnO2 films were applied for energy storage in electrochemical superca‐ pacitors with high capacitance and excellent capacitance retention at high charge-discharge rates.

Solid oxide fuel cells (SOFC) have shown to be a good alternative for electric power generation systems. SOFC show high energy conversion efficiency, clean power generation, reliability, modularity, fuel adaptability, noise-free, excellent long-term stability, and versatility for direct conversion of chemical energy to electrical energy. In this field, Das and Basu [31] applied the EPD technique to deposit yttria-stabilized zirconia (YSZ) nanoparticles on a NiO-YSZ sub‐

**Figure 3.** Field-emission SEM images of top view and cross-section of yttria-stabilized zirconia (YSZ) electrophoretic deposition coating (a) (c) as-deposited and (b) (d) sintered at 1400 °C for 6 h, directly deposited onto the conducting polymers such as polypyrrole-coated NiO-YSZ substrate at a constant applied voltage of 15 V. Reprinted from [31],

EPD was also the technique used to deposit TiO2 nanoparticles for dye-sensitized solar cells (DSSC) [32] and Li-ion micro-batteries applications [33]. For DSSC, the thickness of the TiO2 films was controlled by changing the deposition time and the I2 dosage that electrically charge the nanoparticles, while for batteries, the EPD was performed with different TiO2 structures and different 3D aluminum collectors configurations (Fig. 4). The effect of the substrate was also tested in the EPD of ZnO nanoparticles for conductive fabrics applications [34]. Liu et al. [35] studied the EDP of metal oxides using celestine blue as charging and dispersing agent. The nanostructured MnO2 films were applied for energy storage in electrochemical superca‐ pacitors with high capacitance and excellent capacitance retention at high charge-discharge

with permission from John Wiley and Sons.

32 Electroplating of Nanostructures

rates.

strate, which after sintering was suitable for application in SOFC (Fig. 3).

**Figure 4.** SEM micrographs of aluminium rods obtained by pulsed galvanostatic deposition using a PC membrane with (a) 2 μm-pore size and (c) 1 μm-pore size with electrophoretically deposited P25 particles attached to the respec‐ tive 3D Al substrates (b) and (d). Reprinted from [33], with the permission from Elsevier.

The use of metal oxides offers functionalities that vary from electrically conducting to insu‐ lating and from highly catalytic to inert, which are useful for sensing applications. Different types of metal oxide sensors have been investigated for several decades, and it has been proved that the reduction of crystallite size provided a significant increase in the sensing performan‐ ces. Even if less established, these type of sensors are very promising and new developments are being accomplished every day [36].

Recently, Cu2O nanostructures were electroplated to produce a facile and economic photo‐ electrochemical sensor [37], while Ir2O3 was deposited in stretchable and multiplexed pH sensors [38]. This sensor combined electrochemical, microfabrication, and printing techniques and was successfully applied in beating explanted cardiac tissue, with accurate spatiotemporal monitoring of changes in pH (Fig. 5).

Monitoring analgesic drugs with the use of biosensors allows a rapid, reliable, and sensitive method without the requirement of a sample pre-treatment. For that, alloys deposition allows the combination of different materials properties without compromising thickness or surface area available. The biosensors developed by Narang et al. [39] were produced by EPD of an Fe2O3 magnetic nanoparticle coated with ZrO suspension containing chitosan, prior to enzyme (horseradish peroxidase) immobilization. Also the combination of Fe2O3 with carbon nano‐ tubes and chitosan was earlier used by Batra et al. [40] to immobilize hemoglobin and were applied as an amperometric biosensor.

**Figure 5.** (a) Picture of the produced pH sensors with the magnified images of the gold electrodes before (lower left) and after (lower right) IrOx electroplating. The scale bars correspond to 5 and 0.5 mm for the upper and the lower im‐ ages, respectively. (b) Schematic illustration of the chemical reactions during IrOx electroplating. Reprinted from [38], with permission from John Wiley and Sons.


**Table 1.** List of the latest published research on electrodeposited metal oxide nanostructures/nanomaterials.

## **4. Nanostructured WO3**

Tungsten oxide (WO3) is a well-studied semiconductor used for several applications such as chromogenic material, sensor, and catalyst [41]. The major advantages is its low cost and availability, improved stability, reversible change of conductivity and optical properties, high sensitivity, selectivity, and biocompatibility [42].

Transition-metal oxides, especially those with d0 and d10 electronic configurations, as WO3, TiO2, or ZnO show interesting properties and stability that are important for sensing applica‐ tions [43]. The energy band gap of WO3 corresponds to the difference between the energy levels of the valence band formed by the filled O 2p orbitals and the conduction band formed by empty W 5d orbitals, ranging from 2.6 to 3.25 eV [44]. In nanostructured WO3, the bandgap generally increases with the reduction of the grain size, which is attributed to the quantum confinement effect [45]. Tungsten oxide is also well known for its properties in a non-stoichio‐ metric form, since its lattice can support a significant concentration of oxygen vacancies [44].

## **4.1. WO3 electrodeposition**

**Figure 5.** (a) Picture of the produced pH sensors with the magnified images of the gold electrodes before (lower left) and after (lower right) IrOx electroplating. The scale bars correspond to 5 and 0.5 mm for the upper and the lower im‐ ages, respectively. (b) Schematic illustration of the chemical reactions during IrOx electroplating. Reprinted from [38],

**Corrosion and wear resistive coatings** Zn-TiO2, Ni-CeO2, SiO2 [25][26][27] **Photocatalyst** Cu2O-Cu, Cu2O [28][29] **Water splitting** Ni-IrO2 [30] **Solid oxide fuel cell** Y2O3-ZrO2 (YSZ) [31] **Dye-sensitized solar cell** TiO2 [32] **Li-ion micro-battery** TiO2 [33] **Conductive fabric** ZnO [34] **Supercapacitor** MnO2 [35] **Photoelectrochemical sensor** Cu2O [37] **pH sensor** Ir2O3 [38] **Biosensor** ZrO@Fe3O4, cMWCNT-Fe3O4 [39][40]

**Table 1.** List of the latest published research on electrodeposited metal oxide nanostructures/nanomaterials.

**Application Nanomaterials/Composites References**

with permission from John Wiley and Sons.

34 Electroplating of Nanostructures

Many liquid and vapor phase synthesis methods have been used to synthesize WO3 [45]. Nevertheless, for the electrodeposition of nanostructured WO3 films, more than one method can be adopted: electroplating from a precursor solution [46, 47], anodic oxidation from a metal layer [47–49], and electrodeposition from a WO3 nanoparticles dispersion [50, 51]. A list of the latest reports is presented in Table 2.



**Table 2.** Resume of the latest published research on electrodeposited nanostructured WO3 with the respective precursors and final applications.

Cathodic electroplating is usually based on the local increase of the pH near the electrode surface due to the reduction of O2 or H2O, which induces precipitation of metal ions present in the solution as metal oxide or hydroxide. For the deposition of WO3, the reactions involved in the formation of the oxide are usually based on the formation of the peroxytungstate (W2O112-) intermediate from a tungstate salt (or from the reaction of metallic tungsten with hydrogen peroxide), as described in Equations 5 and 6 [46, 52].

$$2\text{NO}\_4^{\cdot 2} + 4\text{H}\_2\text{O}\_2 + 2\text{H}^+ \rightarrow \text{W}\_2\text{O}\_{11}^{\cdot 2} + 5\text{H}\_2\text{O} \tag{5}$$

$$\text{W}\_2\text{O}\_{11}^{2+} + 2\text{H}^+ \rightarrow 2\text{WO}\_3 + 2\text{O}\_2 + \text{H}\_2\text{O} \tag{6}$$

Depending on the electrochemical potential and solution pH, the WO3 phase may also be involved in other reactions, as the formation of sub-stoichiometric oxide and tungsten bronze (Equations 7 and 8) or even re-dissolution of the oxide phase (Equation 9). The reduced phases formed by these reactions have higher conductivity and hydrophilicity than WO3 and should be considered during characterization of the deposited films [53].

$$2\text{ WO}\_3 + 2y\text{H}^+ + 2y\text{e}^\cdot \leftrightarrow \text{WO}\_{3-y} + y\text{H}\_2\text{O} \tag{7}$$

$$\text{a }\text{WO}\_3 + \text{xH}^+ + \text{xe}^\cdot \leftrightarrow \text{H}\_x\text{WO}\_3 \tag{8}$$

$$\text{WO}\_3 + \text{H}\_2\text{O} \rightarrow \text{WO}\_4^{2-} + 2\text{H}^+ \tag{9}$$

For the anodic oxidation procedure, the general equation can be expressed as Equation 10 [54] and the full mechanism is explained by the occurrence of different reactions simultaneously, as the synthesis of surface oxide films (e.g., W2O5, WO2) and tungstate ions (WO4 2-) [55, 56]. This oxidation is usually followed by the slow dissolution of the oxide phase, as in Equations 11 or 12 depending on the solution pH [55, 57].

$$\text{CH} + 3\text{H}\_2\text{O} \leftrightarrow \text{WO}\_3 + 6\text{H}^+ + 6\text{e}^- \tag{10}$$

$$\text{H}\_3\text{PO}\_3 + 2\text{H}^+ \rightarrow \text{WO}\_2^{2+} + \text{H}\_2\text{O} \tag{11}$$

$$\text{WO}\_3 + 2\text{OH}^- \rightarrow \text{WO}\_4^{2-} + \text{H}\_2\text{O} \tag{12}$$

In the case of the deposition from WO3 nanoparticles dispersions (EPD), the mechanism is not yet fully understood. The majority of the authors agree that the deposition occurs through an electrophoretic mechanism driven by the surface charge of the particles [51, 58], but in fact, the potential (or current) applied during deposition can also promote tungsten reduction from W6+ to W5+ that is counterbalanced by the cation intercalation into the oxide structure, as described in Equation 8, thus forming tungsten bronze (H*x*WO3) [50]. In the work of Liu et al. [50], XRD and optical characterization showed that HWO3 was obtained as the main phase of the deposited films, which supports the hypothesis of the mechanism via electrochemical reduction. Furthermore, since the reduced WO3 is significantly more conductive than the oxidized form, it allowed continuous film growth. In the future, further analysis of the deposited films should be conducted to confirm the electrochemical deposition mechanism.

#### **4.2. WO3 sensing applications**

#### *4.2.1. Gas sensors*

**WO3 Precursor Nanostructured film Application References**

**Table 2.** Resume of the latest published research on electrodeposited nanostructured WO3 with the respective

2- + 2-

Depending on the electrochemical potential and solution pH, the WO3 phase may also be involved in other reactions, as the formation of sub-stoichiometric oxide and tungsten bronze (Equations 7 and 8) or even re-dissolution of the oxide phase (Equation 9). The reduced phases formed by these reactions have higher conductivity and hydrophilicity than WO3 and should

4 22 2 11 2 2WO + 4H O + 2H W O + 5H O ® (5)

2- + W O + 2H 2WO + 2O + H O 2 11 ® 3 22 (6)

+ - WO + 2 H + 2 e WO + H O <sup>3</sup> 3- 2 *<sup>y</sup> yy y* « (7)

+ - WO + H e H WO 3 3 *<sup>x</sup> x x* + « (8)

2- + WO + H O WO + 2H 32 4 ® (9)

hydrogen peroxide), as described in Equations 5 and 6 [46, 52].

be considered during characterization of the deposited films [53].

Cathodic electroplating is usually based on the local increase of the pH near the electrode surface due to the reduction of O2 or H2O, which induces precipitation of metal ions present in the solution as metal oxide or hydroxide. For the deposition of WO3, the reactions involved in the formation of the oxide are usually based on the formation of the peroxytungstate (W2O112-) intermediate from a tungstate salt (or from the reaction of metallic tungsten with

precursors and final applications.

36 Electroplating of Nanostructures

**W** WO3 – TiO2 Electrochromic film [54] **W** WO3 H2 sensor [67] **WO3 NPs** WO3/henna Dye sensitized solar cells [68] **WO3 NPs** WO3 Water splitting [51] **WO3 NWs** WO3 Electrochromic film [58] **WO3 NPs** WO3 Electrochromic film [50] **WO3 NPs** WO3 pH sensor [12] **WO3 NPs** WO3 Neural electrodes [69]

> Precise and affordable monitoring of chemical gases is a critical issue for human health, industrial processes, and environmental protection. For that, nanostructured WO3 has been intensively studied due to its excellent sensing capabilities and reproducibility. These charac‐ teristics are mainly ascribed to the increased surface area and complete depletion of carriers within the nanostructure when exposed to the target gas [45]. The gas sensing mechanism is described by the increase or decrease of the conductance of the oxide layer when exposed to reducing (H2, H2S, CO) or oxidizing (NO2, O3, CO2) gases, respectively.

$$\rm H\_2 + \rm O\_{ads} \to H\_2O + e^- \tag{13}$$

In Equation 13, H2 adsorbs and reacts with O formed on the surface of the electrode, increasing the surface conductance and releasing the captured electrons [67].

$$\rm{NO}\_2 + e^- \rightarrow \rm{NO}\_2^- \tag{14}$$

$$\text{2NO}\_2 + \text{2O}\_{\text{ads}} \to \text{2NO}\_2 + \text{O}\_2 \tag{15}$$

When NO2 is targeted on the WO3 surface, it not only reacts with the electrons from the conduction band (Equation 14) but also with the chemisorbed oxygen (Equation 15), thus promoting a depletion on the surface of the electrode and, consequently, the increase on resistance [70, 71].

An example of a hydrogen gas sensor was built by Yang et al. [67] through anodic oxidation of a tungsten layer previously deposited by radio frequency magnetron sputtering on a sapphire substrate (Fig. 6). The nanoporous WO3 film sensor, after annealing at 600°C, exhibited good sensitivity to H2 gas in air.

**Figure 6.** SEM images of tungsten oxide films with different anodic oxidation voltages: (a) 20 V, (b) 30 V, (c) 50 V, and (d) 60 V operating at an electrode distance of 2 cm for 60 min. Reprint from [67], with permission from Cambridge University Press.

#### *4.2.2. Biosensors*

The application of WO3 to other sensing platforms, as in biosensors, is mainly due to the electrical and optical properties mentioned above [72]. In fact, it was already demonstrated that nanoparticles of metal oxides applied to suitable electrode surfaces allow protein immo‐ bilization and biocatalytic processes to be driven electrochemically [73]. However, to the best of the authors' knowledge, only Feng et al. [74] employed electrodeposited nanostructured WO3 films to enhance the hemoglobin protein loadings, accelerate interfacial electron transfer, and improve thermal stability of the adsorbed protein. The influence of the electrodeposition time to the response time and peak current of the electrode is demonstrated in Fig. 7.

**Figure 7.** Influence of electrodeposition time on (a) peak current of the cyclic voltammograms in phosphate buffer sol‐ ution (PBS, pH 6.0) at 100 mV s−1 and (b) typical steady-state response time of Hb/meso-WO3/graphite electrodes. Re‐ printed from [74], with permission from Elsevier.

#### *4.2.3. pH sensors*


2 ads 2 2 2NO + 2O 2NO + O ® (15)


When NO2 is targeted on the WO3 surface, it not only reacts with the electrons from the conduction band (Equation 14) but also with the chemisorbed oxygen (Equation 15), thus promoting a depletion on the surface of the electrode and, consequently, the increase on

An example of a hydrogen gas sensor was built by Yang et al. [67] through anodic oxidation of a tungsten layer previously deposited by radio frequency magnetron sputtering on a sapphire substrate (Fig. 6). The nanoporous WO3 film sensor, after annealing at 600°C,

**Figure 6.** SEM images of tungsten oxide films with different anodic oxidation voltages: (a) 20 V, (b) 30 V, (c) 50 V, and (d) 60 V operating at an electrode distance of 2 cm for 60 min. Reprint from [67], with permission from Cambridge

The application of WO3 to other sensing platforms, as in biosensors, is mainly due to the electrical and optical properties mentioned above [72]. In fact, it was already demonstrated that nanoparticles of metal oxides applied to suitable electrode surfaces allow protein immo‐ bilization and biocatalytic processes to be driven electrochemically [73]. However, to the best of the authors' knowledge, only Feng et al. [74] employed electrodeposited nanostructured

resistance [70, 71].

38 Electroplating of Nanostructures

University Press.

*4.2.2. Biosensors*

exhibited good sensitivity to H2 gas in air.

The pH value can be used as an indicator for disease diagnostics, medical treatment optimi‐ zation, and monitoring of biochemical and biological processes [75]. Nevertheless, the integration of pH sensing systems into the next generation of wearable devices requires a different architecture than currently used in typical glass-type electrodes and a minimal electrode size [76]. In addition, technological and industrial efforts are under way to incorpo‐ rate different sensors into our daily life by assembling these sensors on common substrates such as plastic, textile, and paper [9]. In the work reported earlier [12], flexible pH sensors were based on electrodeposited WO3 sensing layer in a gold/polyimide substrate (Fig. 8). The pH sensing mechanism for this material, even if not fully understood, is believed to be dependent of the redox reaction involving the production of the tungsten bronze with a higher conduc‐ tivity than the tungsten oxide (Equation 8).

#### *4.2.4. Neural electrodes*

Microtechnology allowed the arrangement of multiple microelectrodes on the same substrate over small distances (Fig. 9a). Nevertheless, in order to provide sufficient recording sensitivity to small electrodes for measuring neuron electrical activity, they are often coated with different nanostructured or conducting materials to increase the effective surface area and electrochem‐ ical interface capacitance [77–79]. The interest in utilizing transition metal oxide films is due to its pseudocapacitive character related to chemisorption processes and redox reactions that take place at the surface [80]. Since nanostructured WO3 has already proved to enhance capacitive performances due to its large surface area and low charge transport resistance [52],

**Figure 8.** (a) Voltage response during electrodeposition at 20 μA; (b) topographic and (c) cross-section SEM images of the WO3 electrodeposited layer; and (d) photograph of the prototype WO3 sensor using a flexible Ag/AgCl reference electrode in a non-planar surface made of gelatin-based electrolyte [12].

it was used for neural recordings applications [69]. The optimization of the electrodeposition parameters led to a slight increase on the charge storage capacity (∼10%) and a decrease of the impedance values, of approximately 40% (Fig. 9b and 9c).

**Figure 9.** (a) SEM images of the Neuronexus electrode and a detail of the iridium electrode (lighter area) coated with WO3 nanoparticles, electrodeposited at 30 nA for 15 s; (b) cyclic voltammetry and (c) electrochemical impedance char‐ acterizations of the pristine (black) and coated electrodes (blue) [69].

These preliminary results show the versatility of electrodeposition in different materials and configurations as well as in different sensing mechanisms.

## **5. Conclusions**

Tungsten oxide (WO3) is one of the most studied metal oxide and the sensing performance of this material is of great interest due to the capability of reversible change of both its optical and electrical properties. The evolution in the fields of nanoscience and nanotechnology allowed these materials to replace many organic and metallic materials in a huge range of applications besides creating new areas of development. The increased surface area and the quantum confinement effects in size ranges below 100 nm make nanostructured WO3 a good platform for gas and pH sensors, along with neural electrodes and biosensors.

In the last decade, the use of electrodeposition for nanostructured metal oxide films has been growing due to the versatility of this method in different applications and materials. Just in the last year, applications varied from catalysts and sensors to capacitors. The use of different types of templates and the deposition of composites will contribute to the continued devel‐ opment of this technique.

## **Acknowledgements**

it was used for neural recordings applications [69]. The optimization of the electrodeposition parameters led to a slight increase on the charge storage capacity (∼10%) and a decrease of the

**Figure 8.** (a) Voltage response during electrodeposition at 20 μA; (b) topographic and (c) cross-section SEM images of the WO3 electrodeposited layer; and (d) photograph of the prototype WO3 sensor using a flexible Ag/AgCl reference

**Figure 9.** (a) SEM images of the Neuronexus electrode and a detail of the iridium electrode (lighter area) coated with WO3 nanoparticles, electrodeposited at 30 nA for 15 s; (b) cyclic voltammetry and (c) electrochemical impedance char‐

impedance values, of approximately 40% (Fig. 9b and 9c).

electrode in a non-planar surface made of gelatin-based electrolyte [12].

40 Electroplating of Nanostructures

acterizations of the pristine (black) and coated electrodes (blue) [69].

The authors would like to thank the Portuguese Science Foundation (FCT-MEC) through project EXCL/CTM-NAN/0201/2012, Strategic Project UID/CTM/500025/2013, and doctoral grants SFRH/BD/73810/2010 given to L. Santos and SFRH/BD/76004/2011 given to J. Neto. The authors would like to thank Dr. Adam Kampff from Champalimaud Center of Unknown for the knowledge transfer related with neural electrodes.

## **Author details**

L. Santos1\*, J. P. Neto1,2, A. Crespo1 , P. Baião1,2, P. Barquinha1 , L. Pereira1 , R. Martins1 and E. Fortunato1

\*Address all correspondence to: ls.santos@campus.fct.unl.pt; emf@fct.unl.pt

1 CENIMAT/I3N, Departamento de Ciências dos Materiais, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa and CEMOP/Uninova, Caparica, Portugal

2 Champalimaud Centre for the Unknown, Champalimaud Neuroscience Programme, Lis‐ bon, Portugal

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## **Electrodeposition of Ferromagnetic Nanostructures**

Monika Sharma and Bijoy K. Kuanr

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/61226

#### **Abstract**

The fabrication of one-dimensional ferromagnetic nanostructured materials such as nanowires and nanotubes by the electrodeposition technique is discussed. The size, shape and structural properties of nanostructures are analysed by controlling the dep‐ osition parameters such as precursors used, deposition potential, pH, etc. The growth of nanostructures and various characterization techniques are studied to support their one-dimensionality. A comparative study of ferromagnetic nanowires and nanotubes is made using angular-dependent ferromagnetic resonance technique.

**Keywords:** Electrodeposition, nanowires, nanotubes, ferromagnetic resonance, anodic aluminium oxide

## **1. Introduction**

Recently, one-dimensional ferromagnetic nanostructured materials such as nanodots, antidots, nanowires and nanotubes have attracted intense research interest. These nanostructures have potential applications in fields as diverse as data storage, magnetics, electronics, optical and microwave devices and nanomedicine [1-6]. They often exhibit new and enhanced properties, due to their low-dimensionality and inter-wire interaction as compared to bulk materials. Several methods have been developed for the synthesis of one-dimensional (1D) nanostructures which lead to well-defined dimensions, morphology, crystal structure, and composition. Various methods such as electrochemistry, chemical reduction, vapour-liquidsolid growth, etc., have been used for preparation of magnetic nanostructures [7-9], although template electrodeposition constitutes one of the most general methods to achieve 1D growth [10-11]. Template-based approach allows to systematically vary the size, shape and structural properties of nanostructures through the modification of template and electrodeposition conditions [12]. The template-based growth often allows the growth of polycrystalline nature of nanowires and nanotubes; however, by controlling the various deposition parameters one

© 2015 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

can easily overcome such issues and thus *single crystal nanostructures can be achieved*. Electro‐ deposition technique is also an inexpensive technique which allows the formation of period‐ ically ordered nanostructures in periodic substrates, which enhances their use in potential device applications. The understanding of the growth mechanism would benefit the controlled fabrication of desired metal nanostructures for specific applications. In particular, nanostruc‐ tures have been prepared in nanoporous membranes by triblock copolymer-assisted hardtemplate method [13], electroplating [14], a sequential electrochemical synthetic method [15] and nanoporous templates [16]. Fundamentally, nanowire growth depends upon electrode‐ position parameters and nanotube formation is dependent on the different growth rates of the metal along the wall surface and from the central bottom of the nanochannels. However, systematic studies are still needed to understand the growth mechanism of nanowires and nanotubes.

Ferromagnetic nanowires and nanotubes exhibit unique and tunable magnetic properties due to their inherent shape anisotropy. Current interest in research on ferromagnetic nanowires and nanotubes is stimulated by their potential applications in different fields such as spin‐ tronics, biotechnology, future ultra-high-density magnetic recording media and high-frequen‐ cy devices [17-21]. For applications such as magnetic recording media, the nanowire's diameter and the inter-wire distance should be as small as possible to increase the areal recording density. Thus, for storage devices magnetic studies concentrate on sub-100 nm nanostructures. The high aspect ratio of the nanowires (Length/Diameter) causes high coercivity, which is helpful in suppressing the onset of the 'superparamagnetic limit', which is considered to be very important for preventing the loss of magnetically recorded information among the nanowires. The inter-wire interaction and magnetic dipole coupling can be controlled by suitable separation among the nanowires.

Several groups have investigated elemental ferromagnetic nanowires such as Fe, Co and Ni with different pore diameters in alumina templates [12-21]. Huang et al. reported single crystal Co nanowires with different diameters and observed that the coercivity and squareness decreases with increase of pore diameter [22]. The size of the crystallites within the Fe, Co and Ni nanowires, as well as the crystalline structure of the nanowires, depends upon the deposi‐ tion conditions such as the pH of the solutions and the deposition parameters. Giant-magnetoresistance (GMR) properties were found in Co/Cu multi-layered nanowires electrodeposited in nanoporous polymer template [23]. Recently, these ferromagnetic nanowire embedded templates were used as a substrate for making high-frequency devices [24-26].

In this chapter, we shall describe three different aspects of this topic:


**3.** We will then continue discussing the various characterization techniques and physical properties of these nanostructures. A comparative study of ferromagnetic nanowires and nanotubes has been made using static and dynamic magnetization techniques.

## **2. Template-based approach**

can easily overcome such issues and thus *single crystal nanostructures can be achieved*. Electro‐ deposition technique is also an inexpensive technique which allows the formation of period‐ ically ordered nanostructures in periodic substrates, which enhances their use in potential device applications. The understanding of the growth mechanism would benefit the controlled fabrication of desired metal nanostructures for specific applications. In particular, nanostruc‐ tures have been prepared in nanoporous membranes by triblock copolymer-assisted hardtemplate method [13], electroplating [14], a sequential electrochemical synthetic method [15] and nanoporous templates [16]. Fundamentally, nanowire growth depends upon electrode‐ position parameters and nanotube formation is dependent on the different growth rates of the metal along the wall surface and from the central bottom of the nanochannels. However, systematic studies are still needed to understand the growth mechanism of nanowires and

Ferromagnetic nanowires and nanotubes exhibit unique and tunable magnetic properties due to their inherent shape anisotropy. Current interest in research on ferromagnetic nanowires and nanotubes is stimulated by their potential applications in different fields such as spin‐ tronics, biotechnology, future ultra-high-density magnetic recording media and high-frequen‐ cy devices [17-21]. For applications such as magnetic recording media, the nanowire's diameter and the inter-wire distance should be as small as possible to increase the areal recording density. Thus, for storage devices magnetic studies concentrate on sub-100 nm nanostructures. The high aspect ratio of the nanowires (Length/Diameter) causes high coercivity, which is helpful in suppressing the onset of the 'superparamagnetic limit', which is considered to be very important for preventing the loss of magnetically recorded information among the nanowires. The inter-wire interaction and magnetic dipole coupling can be controlled by

Several groups have investigated elemental ferromagnetic nanowires such as Fe, Co and Ni with different pore diameters in alumina templates [12-21]. Huang et al. reported single crystal Co nanowires with different diameters and observed that the coercivity and squareness decreases with increase of pore diameter [22]. The size of the crystallites within the Fe, Co and Ni nanowires, as well as the crystalline structure of the nanowires, depends upon the deposi‐ tion conditions such as the pH of the solutions and the deposition parameters. Giant-magnetoresistance (GMR) properties were found in Co/Cu multi-layered nanowires electrodeposited in nanoporous polymer template [23]. Recently, these ferromagnetic nanowire embedded

**1.** We first discuss in brief about the template approach for fabricating one-dimensional nanostructures. The most commonly used templates, i.e. anodic alumina membrane

**2.** We will then describe the electrochemical deposition technique used to synthesize ferromagnetic nanowire and nanotube arrays. The size, shape and structural properties of nanostructures are controlled by the type of template used and a number of growth

templates were used as a substrate for making high-frequency devices [24-26].

(AAM) and polycarbonate (PC) membranes, will be discussed in detail.

parameters such as precursors used, deposition potential, pH, etc.

In this chapter, we shall describe three different aspects of this topic:

nanotubes.

50 Electroplating of Nanostructures

suitable separation among the nanowires.

Template electrodeposition is one of the most general techniques to realize one-dimensional growth. In this straightforward approach, the nanostructures are grown electrochemically inside a hard-template material (mould) adopting its shape [27, 28]. Usually, a template (membrane) is a material that has pores of radii varying from a few μm to tens of nm. These porous membranes are generally used in filtration technologies for the separation of different species such as polymers, colloids, molecules, salts, etc. Anodic aluminium oxide (AAO) membrane and polycarbonate (PC) membranes are commonly used membranes for the synthesis of one-dimensional nanostructures [29, 30]. However, PC membranes are disadvan‐ tageous as they have random pores and are very flexible. During the heating process, these membranes can lead to distortion, which is one of the major drawbacks for device applications. Also, removal of the template occurs before complete densification of the nanostructures. These factors result in broken and deformed nanostructures. Anodic aluminium oxide (AAO) template overcomes these difficulties as these membranes have uniform and parallel pores and are therefore commonly used for synthesis of 1D nanostructures. Two-step anodization of aluminium sheet in acidic medium solutions of sulphuric, oxalic, or phosphoric acids is used to synthesize AAO templates [29, 31]. Depending upon the anodization conditions, pore densities as high as 1011 pores/cm2 can be obtained and due to mechanical stress at the aluminium-alumina interface, these pores are arranged in a regular hexagonal array as shown in Fig. 1 [32]. A large array of orderly arranged pores with high aspect ratio makes AAO templates ideal for growing nanostructures. The thickness of the template, which determines the length of nanowires, only depends upon the oxidation time and can lead to formation of 1-60 μm thick oxide layers. AAO templates are chemically and thermally inert, causing pure synthesis of ferromagnetic materials. Also, the solution being deposited must wet the internal pore walls and for growth of nanotubules, deposition should start from the pore wall and should proceed inward.

In the anodization process, an electrical circuit is established between a cathode and a film of aluminium which serves as the anode. Then, anodic oxidation or anodization of the film occurs in accordance to the following reaction [33]:

$$2\text{Al} + 3\text{H}\_2\text{O} = \text{Al}\_2\text{O}\_3 + 3\text{H}\_2, \text{ AlG}^\circ = -864.6\text{ kJ} \tag{1}$$

where ΔG° is the standard Gibbs free energy change. During the anodization, initially a planar barrier film forms followed by pore development leading to the formation of the relatively regular porous anodic film, which thickens in time.

inward.

only depends upon the oxidation time and can lead to formation of 1-60 µm thick oxide

layers. AAO templates are chemically and thermally inert, causing pure synthesis of

and for growth of nanotubules, deposition should start from the pore wall and should proceed

Fig. 1: Schematic diagram of the cross section of the porous anodic aluminium oxide showing the nanopores arranged in hexagonal cells. **Figure 1.** Schematic diagram of the cross section of the porous anodic aluminium oxide showing the nanopores ar‐ ranged in hexagonal cells.

In the anodization process, an electrical circuit is established between a cathode and a film of aluminium which serves as the anode. Then, anodic oxidation or anodization of the film occurs in accordance to the following reaction [33]: In two-step anodization, usually the first and second anodization steps could be conducted in the same conditions. The oxide layer formed in the first step is removed by wet chemical dissolution in a mixture of suitable chemicals for an appropriate time, depending on the anodizing time. Based on the applied anodizing voltage and also the type of electrolyte, pores with diameters ranging from few nm to 200 nm can be produced. Li et al. demonstrated a formula between inter-pore distance (Dnm) and anodizing voltage (V) [34]:

$$\mathbf{D}\_{\rm nn} = \text{-1.7} + \text{2.81 V} \left( \text{volts} \right) \tag{2}$$

Dnm = -1.7 + 2.81 V(volts) (2)

where ∆G° is the standard Gibbs free energy change. During the anodization, initially a planar barrier film forms followed by pore development leading to the formation of the relatively regular porous anodic film, which thickens in time. In two-step anmodization, usually the first and second anodization steps could be conducted in the same conditions. The oxide layer formed in the first step is removed by wet chemical High-purity (99.99%) aluminium (Al) foil was used for anodization. It was ultrasonically degreased at room temperature in trichloroethylene for 5 min, and then etched in 1.0 M NaOH for 3 min. Before anodization, Al foil was electrochemically polished in a mixed solution of HClO4 and ethanol. A two-step anodization was used to obtain highly ordered pores. Al foil was anodized at 40 V in a 0.3 M oxalic acid at 0 °C for about 12 h in the first anodization step. The oxide layer formed in first anodization was chemically removed in a mixture of phosphoric acid and chromic acid which creates a footprint of nanopores on the Al surface. A second anodization was carried out with the same solutions and steps used in first anodization, resulting in the formation of highly ordered pores [29, 31].

dissolution in a mixture of suitable chemicals for an appropriate time, depending on the

anodizing time. Based on the applied anodizing voltage and also the type of electrolyte, pores

with diameters ranging from few nm to 200 nm can be produced. A. P. Li et al. demonstrated

The hHigh-purity (99.99%) Aaluminium (Al) foil was used for anodization. It was

ultrasonically degreased at room temperature in trichloroethylene for 5 min., and then etched

in 1.0 M NaOH for 3 minutes. Before anodization, Al foil was electrochemically polished in

a mixed solution of HClO4 and ethanol. A two-step anodization was used to obtain highly

a formula between inter-pore distance (Dnm) and anodizing voltage (V) [34]:

In this chapter, we show results using the commercially purchased AAO templates from Whatman Ltd. and observe that these AAO templates were branched from one side (O-ring support side) as shown in Fig. 2.

**Figure 2.** Branching observed in anodic alumina templates commercially available from Whatman.

## **3. Electrodeposition**

only depends upon the oxidation time and can lead to formation of 1-60 µm thick oxide

layers. AAO templates are chemically and thermally inert, causing pure synthesis of

ferromagnetic materials. Also, the solution being deposited must wet the internal pore walls

and for growth of nanotubules, deposition should start from the pore wall and should proceed

Fig. 1: Schematic diagram of the cross section of the porous anodic aluminium oxide showing the nanopores

**Figure 1.** Schematic diagram of the cross section of the porous anodic aluminium oxide showing the nanopores ar‐

In the anodization process, an electrical circuit is established between a cathode and a film of

In two-step anodization, usually the first and second anodization steps could be conducted in the same conditions. The oxide layer formed in the first step is removed by wet chemical dissolution in a mixture of suitable chemicals for an appropriate time, depending on the anodizing time. Based on the applied anodizing voltage and also the type of electrolyte, pores with diameters ranging from few nm to 200 nm can be produced. Li et al. demonstrated a

aluminium which serves as the anode. Then, anodic oxidation or anodization of the film

formula between inter-pore distance (Dnm) and anodizing voltage (V) [34]:

where ∆G° is the standard Gibbs free energy change. During the anodization, initially a

High-purity (99.99%) aluminium (Al) foil was used for anodization. It was ultrasonically degreased at room temperature in trichloroethylene for 5 min, and then etched in 1.0 M NaOH for 3 min. Before anodization, Al foil was electrochemically polished in a mixed solution of HClO4 and ethanol. A two-step anodization was used to obtain highly ordered pores. Al foil was anodized at 40 V in a 0.3 M oxalic acid at 0 °C for about 12 h in the first anodization step. The oxide layer formed in first anodization was chemically removed in a mixture of phosphoric acid and chromic acid which creates a footprint of nanopores on the Al surface. A second anodization was carried out with the same solutions and steps used in first anodization,

planar barrier film forms followed by pore development leading to the formation of the

In two-step anmodization, usually the first and second anodization steps could be conducted

in the same conditions. The oxide layer formed in the first step is removed by wet chemical

dissolution in a mixture of suitable chemicals for an appropriate time, depending on the

anodizing time. Based on the applied anodizing voltage and also the type of electrolyte, pores

with diameters ranging from few nm to 200 nm can be produced. A. P. Li et al. demonstrated

The hHigh-purity (99.99%) Aaluminium (Al) foil was used for anodization. It was

ultrasonically degreased at room temperature in trichloroethylene for 5 min., and then etched

in 1.0 M NaOH for 3 minutes. Before anodization, Al foil was electrochemically polished in

a mixed solution of HClO4 and ethanol. A two-step anodization was used to obtain highly

a formula between inter-pore distance (Dnm) and anodizing voltage (V) [34]:

resulting in the formation of highly ordered pores [29, 31].

2Al + 3H2O = Al2O3 + 3H2, ∆G° = -864.6 kJ (1)

D = -1.7 + 2.81 V volts nm ( ) (2)

Dnm = -1.7 + 2.81 V(volts) (2)

inward.

arranged in hexagonal cells.

occurs in accordance to the following reaction [33]:

ranged in hexagonal cells.

52 Electroplating of Nanostructures

relatively regular porous anodic film, which thickens in time.

Electrodeposition, also known as electrochemical deposition, is a process which involves the oriented diffusion of charged reactive species through a solution when an external electric field is applied, and the reduction of the charged growth species at the deposition surface. Surface charge will develop when a solid is immersed in a polar solvent or an electrolyte solution. The Nernst equation which described the electrode potential is given by

$$V = V\_0 + \frac{RT}{n\_iF} \ln\left(a\_i\right) \tag{3}$$

where *V*<sup>0</sup> is the potential difference between the electrode and the solution for unity activity *ai* of the ions, *F* is the Faraday's constant, *R* is the gas constant and *T* is the temperature.

Electrons will transfer from the electrode to the solution and the electrolyte will be reduced when the electrode potential is higher than the energy level of a vacant molecular orbital in the electrolyte, as shown in Fig. 3. On the other hand, electrolyte oxidation will occur, i.e. the electrons will transfer from the electrolyte to the electrode, if the electrode potential is lower than the energy level of an occupied molecular orbital in the electrolyte, as shown in Fig. 3. When equilibrium is achieved, these reactions will stop. Electrolysis is a process that converts electrical energy to chemical potential in an electrolytic cell where charged species flow from cathode to anode [35]. than the energy level of an occupied molecular orbital in the electrolyte, as shown in Fig. 3. When equilibrium is achieved, these reactions will stop. Electrolysis is a process thatwhich converts electrical energy to chemical potential in an electrolytic cell where charged species flow from cathode to anode [35].

Electrons will transfer from the electrode to the solution and the electrolyte will be reduced,

when the electrode potential is higher than the energy level of a vacant molecular orbital in

the electrolyte, as shown in Fig. 3. On the other hand, electrolyte oxidation will occur, i.e. the

Formatted: Font: Not Italic

Fig. 3: Representation of (A) the reduction and (B) oxidation of a species A in solution. The molecular orbitals (MO) shown for species A are the highest occupied MO and lowest vacant MO [46]. **Figure 3.** Representation of (A) the reduction and (B) oxidation of a species A in solution. The molecular orbitals (MO) shown for species A are the highest occupied MO and lowest vacant MO [46].

In electrolysis, by selecting the over-potential as means of adjusting the driving force for the In electrolysis, by selecting the over-potential as means of adjusting the driving force for the reaction, the reaction rate of a system can be easily controlled. With increasing cathodic potential, the concentration of the electrochemical active species, reacting at the cathode, is

reaction, the reaction rate of a system can be easily controlled. With increasing cathodic

potential, the concentration of the electrochemical active species, reacting at the cathode, is

increasingly reduced in the immediate vicinity of the electrode until every incoming ion is

directly reduced. The reaction becomes limited by mass transport processes arising from the

depletion of cations in the diffusion layer. The current stays constant even if the over-

potential is further increased. This current, limited by diffusion, is denoted diffusion

threshold current Id. In the case of convection, the diffusion layer is of constant thickness �

(stationary case). Mass transport can be described by Fick's first law of diffusion, and the

limiting current � (at an electrode with the area A) only depends on the bulk electrolyte

concentration �:

than the energy level of an occupied molecular orbital in the electrolyte, as shown in Fig. 3. When equilibrium is achieved, these reactions will stop. Electrolysis is a process thatwhich converts electrical energy to chemical potential in an electrolytic cell where charged species increasingly reduced in the immediate vicinity of the electrode until every incoming ion is directly reduced. The reaction becomes limited by mass transport processes arising from the depletion of cations in the diffusion layer. The current stays constant even if the over-potential is further increased. This current, limited by diffusion, is denoted diffusion threshold current *Id*. In the case of convection, the diffusion layer is of constant thickness *δ* (stationary case). Mass transport can be described by Fick's first law of diffusion, and the limiting current *Id* (at an electrode with the area A) only depends on the bulk electrolyte concentration *c*0 :

Formatted: Font: Not Italic

Electrons will transfer from the electrode to the solution and the electrolyte will be reduced when the electrode potential is higher than the energy level of a vacant molecular orbital in the electrolyte, as shown in Fig. 3. On the other hand, electrolyte oxidation will occur, i.e. the electrons will transfer from the electrolyte to the electrode, if the electrode potential is lower than the energy level of an occupied molecular orbital in the electrolyte, as shown in Fig. 3. When equilibrium is achieved, these reactions will stop. Electrolysis is a process that converts electrical energy to chemical potential in an electrolytic cell where charged species flow from

Electrons will transfer from the electrode to the solution and the electrolyte will be reduced,

when the electrode potential is higher than the energy level of a vacant molecular orbital in

the electrolyte, as shown in Fig. 3. On the other hand, electrolyte oxidation will occur, i.e. the

electrons will transfer from the electrolyte to the electrode, if the electrode potential is lower

Vacant MO

Occupied MO

Vacant MO

Occupied MO

**Figure 3.** Representation of (A) the reduction and (B) oxidation of a species A in solution. The molecular orbitals (MO)

In electrolysis, by selecting the over-potential as means of adjusting the driving force for the reaction, the reaction rate of a system can be easily controlled. With increasing cathodic potential, the concentration of the electrochemical active species, reacting at the cathode, is

Electrode Solution Electrode Solution

A + e A-

e

A - e A<sup>+</sup>

e

cathode to anode [35].

54 Electroplating of Nanostructures

(A)

(B)

Potential Energy level

Potential Energy level

of electrons

(MO) shown for species A are the highest occupied MO and lowest vacant MO [46].

shown for species A are the highest occupied MO and lowest vacant MO [46].

of electrons

reaction, the reaction rate of a system can be easily controlled. With increasing cathodic

potential, the concentration of the electrochemical active species, reacting at the cathode, is

increasingly reduced in the immediate vicinity of the electrode until every incoming ion is

directly reduced. The reaction becomes limited by mass transport processes arising from the

depletion of cations in the diffusion layer. The current stays constant even if the over-

potential is further increased. This current, limited by diffusion, is denoted diffusion

threshold current Id. In the case of convection, the diffusion layer is of constant thickness �

(stationary case). Mass transport can be described by Fick's first law of diffusion, and the

limiting current � (at an electrode with the area A) only depends on the bulk electrolyte

flow from cathode to anode [35].

concentration �:

$$I\_d = nFAD \frac{c\_0}{\delta} \tag{4}$$

The advantages of electrodeposition technique over physical deposition methods are as follows: no need of vacuum equipment, easier handling, higher deposition rates and easier to prepare thick and continuous films. The metal deposition operation depends on a great number of chemical and operational parameters such as local current density, electrolyte concentrations, complexing agents, buffer capacity, pH, levelling agents, brighteners, surfac‐ tants, contaminants, temperature, agitation, substrate properties, cleaning procedure. All these parameters act on the structure of the deposit and also on its composition, in terms of alloy and its properties. Accordingly, the determination of these parameters is very important.

Generally, a given metal is electrodeposited from a cell consisting two conductive electrodes, a reference electrode to maintain the potential between the conductive electrodes, an electro‐ lyte and a power supply for conducting an electrical current through the cell or applying an external electric field in the electrolyte. As the current flows into the cell, an oxidation reaction occurs on one electrode (called anode) by charging growth species (typically positively charged metal ions) into the electrolyte and consequently a reduction reaction takes place on the other electrode (called cathode), reducing the charged growth species at the growth or deposition surface as a metallic layer. The process can be carried out using potentiostatic or galvanostatic deposition, depending upon whether applied potential or current is adjusted precisely for the given material to be deposited.

Fig. 3: Representation of (A) the reduction and (B) oxidation of a species A in solution. The molecular orbitals In order to get a qualitative interpretation of the different processes that occur at an electrode, a cyclic voltammogram can be a very helpful tool. The basic idea is that a potential sweep is applied from one to another potential and vice versa with a certain constant speed (mV/s), resulting in a potential triangle (sawtooth) in time (Fig. 4(A)). Because the sign of the potential is changed, oxidative and reductive processes can be distinguished. In addition, varying the scan rate, i.e. triangle slope, different processes at the electrode can be measured. Figure 4 shows the triangle slope and the current flowing through the cathode as function of applied potential.

In electrolysis, by selecting the over-potential as means of adjusting the driving force for the A similar method has been frequently used for electrodeposition of magnetic nanowires into nanoporous membranes. A metal layer like Au is deposited on one side of the template to provide electrical contact which serves as working electrode, a platinum strip as counter electrode and a saturated calomel electrode (SCE) as a reference electrode. Figure 5(A) **E**

**I (A)**

**F**

**A**

**E**

**D**

**B C**

electrodeposition. Chronoamperometry of nanowire deposition is shown in Fig. 5(B). A pre-Fig. 4: A triangular-shaped potential scan and the resulting current response at a stagnant electrode. **Figure 4.** A triangular-shaped potential scan and the resulting current response at a stagnant electrode.

illustrates the common set-up for the template-based growth of nanowires using electrode‐ position. Chronoamperometry of nanowire deposition is shown in Fig. 5(B). A pre-growth stage occurs when a sudden drop of current takes place once the potential is applied through the cell during which nucleation starts at the pore bottoms. Consequently, a slightly increased current is observed at which the metal is growing in the pores. As pores are filled, current decreases with a large gradient versus time. At the final step, hemispherical caps, originating from each nanowire, form a coherent planar layer that expanded to cover the whole surface of the template. Thus, the effective cathode area increases and a rapidly increasing deposition current can be observed. growth stage occurs when a sudden drop of current takes place once the potential is applied through the cell during which nucleation starts at the pore bottoms. Consequently, a slightly increased current is observed at which the metal is growing in the pores. As pores are filled, current decreases with a large gradient versus time. At the final step, hemispherical caps, originating from each nanowire, form a coherent planar layer that expanded to cover the whole surface of the template. Thus, the effective cathode area increases and a rapidly increasing deposition current can be observed. A similar method has been frequently used for electrodeposition of magnetic nanowires into nanoporous membranes. A metal layer like Au is deposited on one side of the template to provide electrical contact which serves as working electrode, a platinum strip as counter electrode and a saturated calomel electrode (SCE) as a reference electrode. Figure. 5(A) illustrates the common set-up for the template-based growth of nanowires using electrodeposition. Chronoamperometry of nanowire deposition is shown in Fig. 5(B). A pre-

growth stage occurs when a sudden drop of current takes place once the potential is applieds

Fig. 5: (A) Schematic of electrochemical cell used for nanostructure synthesis. (B) Typlical chronoamperommetry plot during electrodeposition. **Figure 5.** (A) Schematic of electrochemical cell used for nanostructure synthesis. (B) Typlical chronoamperommetry plot during electrodeposition.

For nanotube growth in AAO, instead of a thick Au layer, a very thin layer of Au <30 nm was sputtered on one side of AAO template, so as not to completely block bottom of the pores as

Fig. 5: (A) Schematic of electrochemical cell used for nanostructure synthesis. (B) Typlical chrono-

amperommetry plot during electrodeposition.

**Figure 6.** Steps followed for depositing (A) nanowires and (B) nanotubes in AAO template.

illustrates the common set-up for the template-based growth of nanowires using electrode‐ position. Chronoamperometry of nanowire deposition is shown in Fig. 5(B). A pre-growth stage occurs when a sudden drop of current takes place once the potential is applied through the cell during which nucleation starts at the pore bottoms. Consequently, a slightly increased current is observed at which the metal is growing in the pores. As pores are filled, current decreases with a large gradient versus time. At the final step, hemispherical caps, originating from each nanowire, form a coherent planar layer that expanded to cover the whole surface of the template. Thus, the effective cathode area increases and a rapidly increasing deposition

A similar method has been frequently used for electrodeposition of magnetic nanowires into nanoporous membranes. A metal layer like Au is deposited on one side of the template to provide electrical contact which serves as working electrode, a platinum strip as counter electrode and a saturated calomel electrode (SCE) as a reference electrode. Figure. 5(A) illustrates the common set-up for the template-based growth of nanowires using electrodeposition. Chronoamperometry of nanowire deposition is shown in Fig. 5(B). A pregrowth stage occurs when a sudden drop of current takes place once the potential is applieds through the cell during which nucleation starts at the pore bottoms. Consequently, a slightly increased current is observed at which the metal is growing in the pores. As pores are filled, current decreases with a large gradient versus time. At the final step, hemispherical caps, originating from each nanowire, form a coherent planar layer that expanded to cover the whole surface of the template. Thus, the effective cathode area increases and a rapidly

Fig. 4: A triangular-shaped potential scan and the resulting current response at a stagnant electrode.

**Figure 4.** A triangular-shaped potential scan and the resulting current response at a stagnant electrode.

<sup>0</sup> Potential (V)

Fig. 4: A triangular-shaped potential scan and the resulting current response at a stagnant electrode.

**<sup>0</sup> Potential (V)**

**Time**

**E**

56 Electroplating of Nanostructures

E

A similar method has been frequently used for electrodeposition of magnetic nanowires into

**I (A)**

**0**

**F**

F

A

E

D

B C

**A**

**E**

**D**

**B C**

nanoporous membranes. A metal layer like Au is deposited on one side of the template to

I (µA)

provide electrical contact which serves as working electrode, a platinum strip as counter

electrode and a saturated calomel electrode (SCE) as a reference electrode. Figure 5(A)

illustrates the common set-up for the template-based growth of nanowires using

0

electrodeposition. Chronoamperometry of nanowire deposition is shown in Fig. 5(B). A pre-

growth stage occurs when a sudden drop of current takes place once the potential is applied

through the cell during which nucleation starts at the pore bottoms. Consequently, a slightly

increased current is observed at which the metal is growing in the pores. As pores are filled,

current decreases with a large gradient versus time. At the final step, hemispherical caps,

originating from each nanowire, form a coherent planar layer that expanded to cover the

whole surface of the template. Thus, the effective cathode area increases and a rapidly

Fig. 5: (A) Schematic of electrochemical cell used for nanostructure synthesis. (B) Typlical

**Figure 5.** (A) Schematic of electrochemical cell used for nanostructure synthesis. (B) Typlical chronoamperommetry

For nanotube growth in AAO, instead of a thick Au layer, a very thin layer of Au <30 nm was sputtered on one side of AAO template, so as not to completely block bottom of the pores as

Fig. 5: (A) Schematic of electrochemical cell used for nanostructure synthesis. (B) Typlical chrono-

current can be observed.

increasing deposition current can be observed.

Time

**(A) (B)**

(A) (B)

chronoamperommetry plot during electrodeposition.

increasing deposition current can be observed.

plot during electrodeposition.

amperommetry plot during electrodeposition.

shown in Fig. 6(B). Nanotubes (NTs) wall thickness depends upon back-side Au layer as when Ni ion moves to the pores of the template under the drive of electric field, Ni ion firstly reaches the conducting Au layer of the pore-mouth and is deoxidized Ni atom, and thus forms a tubular structure. The wall thickness of nanotubes also depends upon other factors like the current density; therefore, we choose deposition potential in such a manner to achieve low current density. A special attention is paid to address a comparative analysis of the careful preparation of Ni NW (nanowire) and NT (nanotube) arrays with well-controlled ordering and their structural and magnetic response. Table 1 shows the previous research work on ferromagnetic nanowires and nanotubes for various deposition parameters.


**Table 1.** Electrolytic deposition parameters of various ferromagnetic nanowires and nanotubes.

Electrolytic bath for the deposition of specific ferromagnetic material was prepared using their corresponding sulphate salts; boric acid was added in each bath solution to prevent hydroxide formation and promoting deposition rate. Table 2 describes the electrolytic bath concentrations and electrodeposition parameters used for various nanowires and nanotubes deposited.


**Table 2.** Electrolytic concentration and deposition parameters of various ferromagnetic nanowires.

The deposition potential of the electrolyte bath with respect to the reference electrode SCE is determined by cyclic voltammetry. Figure 7 shows the typical voltammogram for the Ni, Co, NiFe alloy and CoFeB amorphous nanowires used in this chapter. Typically, the potential is chosen at a reducing potential where current starts decreasing. For the growth of the NWs/NTs chrono-amperommetry is used at various deposition times which controls the length of the nanostructures. The wall thickness of nanotubes also depends upon the current density; therefore, we choose deposition potential in such a manner so as to achieve low current density [37].

**Figure 7.** Cyclic voltammogram of various electrolyte baths for determination of deposition potential taken at a sweep of 2 mV/sec.

## **4. Characterization**

**Composition Electrolytes Used Molar Weight/100 ml Deposition Potential**

CoSO4, FeSO4, H3BO3, DMAB

Co75Fe15B10 NWs

58 Electroplating of Nanostructures

density [37].

of 2 mV/sec.

Ni NWs NiSO4, NiCl2, H3BO3 33 g, 4.5 g, 3.8 g -1.0 2.5 Co NWs CoSO4, H3BO3 12 g, 4.5 g -1.3 3.5 Ni60Fe40 NWs NiSO4, FeSO4, H3BO3 12 g, 0.6 g, 4 g -1.2 3

Ni NTs NiSO4, PEG, H3BO3 1.5 g, 3.7 g, 3.5 g -0.7 2

The deposition potential of the electrolyte bath with respect to the reference electrode SCE is determined by cyclic voltammetry. Figure 7 shows the typical voltammogram for the Ni, Co, NiFe alloy and CoFeB amorphous nanowires used in this chapter. Typically, the potential is chosen at a reducing potential where current starts decreasing. For the growth of the NWs/NTs chrono-amperommetry is used at various deposition times which controls the length of the nanostructures. The wall thickness of nanotubes also depends upon the current density; therefore, we choose deposition potential in such a manner so as to achieve low current

**Figure 7.** Cyclic voltammogram of various electrolyte baths for determination of deposition potential taken at a sweep

**Table 2.** Electrolytic concentration and deposition parameters of various ferromagnetic nanowires.

4.95 g, 0.834 g, 4.33 g, 0.049 g

**(V) pH**


In this section, various characterization techniques are discussed to determine the physical and magnetic properties of electrodeposited nanostructures. The morphology and size of nanostructures were characterized by scanning electron microscope (SEM). The qualitative and quantitative elemental composition of nanostructures were done using energy dispersive X-ray analysis (EDX). The structural analysis of ferromagnetic nanowires and nanotubes was done by transmission electron microscope (TEM) and X-ray diffraction (XRD) spectroscopy. Magnetic properties of the samples were tested by superconducting quantum interference device (SQUID).

The morphology of the NWs/NTs was observed by dissolving the template in 3M NaOH solution for 1 h. The separated nanostructures were cleaned several times with de-ionized water and mounted on holder for SEM. Figure 8 (A-D) shows the SEM images of nanowires which revealed that most nanochannels of AAO template are highly filled and parallel to each other. The nanowires have continuous structure without visible defects replicating the pore shapes. It means that ferromagnetic material fills the pores uniformly during electrodeposition under controlled conditions used in this work. Cross-sectional SEM images were taken simply by cleaving of alumina templates. Figure 9 (A-B) shows the SEM images of Ni nanotubes revealing that the wall thickness is around 40 nm and it strongly depends upon the back-side coating of metallic layer.

For EDX analysis, we mounted the nanowire's deposited sample on SEM holder and coated them with carbon since the absorption of X-ray signal by carbon coating is negligible on account of its low atomic number and is taken into account while performing quantitative EDX analysis. Figure 10(A-D) shows the EDX spectrum for various nanowires of Ni, Co, CoFeB and NiFe alloy, confirming the presence of respective elements in the AAO template. The quanti‐ tative analysis for CoFeB nanowires determines the stoichiometry of composite material such as Co75Fe15B10 and for NiFe alloy such as Ni60Fe40.

For TEM characterization, the specimen was obtained by completely dissolving the AAO template in 3M NaOH solution for 24 h and then washing with de-ionized water several times. The free NWs/NTs were then collected from the suspension by applying a magnetic field using a permanent magnet and then rinsed with ethanol and immersed in an ultrasonic bath for 10 min. When operated in the diffraction mode, TEM images also yield information regarding the crystal structure of the nanostructure axis by selected area electron diffraction pattern (SAED).

Figure 11(A-B) shows the TEM image of Co and CoFeB NWs. Figure 12 shows diffraction pattern of the nanowires which revealed the single crystal phase for Co nanowires while the SAED pattern for CoFeB nanowires shows the amorphous phase. TEM images of Ni nanotubes are shown in Fig. 13(A) with inner and outer diameters of about 120 nm and 200 nm. It also confirms that the wall thickness of nanotubes is ~40 nm as observed in the SEM images. The branching due to AAO templates causes the growth of nanotubes in Y-shape nanochannels as observed in Fig. 13(B).

Fig. 8: (A-D) shows the top and cross nanowires. The images confirmed that the diameter of the nanowires are ~200 nm and the interwire spacing between the pores is ~300 nm. D) shows the top and cross-sectional view of AAO templates before and after the nanowires. The images confirmed that the diameter of the nanowires are ~200 nm and the interwire spacing sectional view of AAO templates before and after the deposition of **Figure 8.** (A-D) shows the top and cross-sectional view of AAO templates before and after the deposition of nanowires. The images confirmed that the diameter of the nanowires are ~200 nm and the interwire spacing between the pores is ~300 nm.

Fig. 9: (A-B) SEM images of Ni nanotubes embedded in AAO template with wall thickness of 40 nm. **Figure 9.** (A-B) SEM images of Ni nanotubes embedded in AAO template with wall thickness of 40 nm.

For determining the crystallographic information of the materials, XRD is one of the most important technique. It can provide information about the crystallographic phases, crystallite size, lattice constant, presence of impurities, etc. X-ray diffraction was performed using Philip's

**Figure 10.** EDX spectra of various nanowires deposited in AAO template (A) for Ni, (B) for Co, (C) for CoFeB, and (D) NiFe alloy.

X'pert PRO (Model PW 3040, X-ray wavelength of Cu Kα line λ = 1.54060 Å) diffractometer. The phase was identified using standard diffraction files of JCPDS. The size of the crystalline material can be estimated from the width of the reflection plane using Scherer equation [41]:

$$D\_{crystallic} = \frac{\kappa \mathcal{X}}{\mathcal{B} \cos \theta} \tag{5}$$

where *κ* is a particle shape factor (for spherical particles, *κ* =0.9, *β* is the full width at half maximum (radians) and *Dcrystalline* is diameter of the crystallites (Å). It is useful to eliminate the instrumental line width from the observed one to get a correct broadening value due to small particle size.

For determining the crystallographic information of the materials, XRD is one of the most important technique. It can provide information about the crystallographic phases, crystallite size, lattice constant, presence of impurities, etc. X-ray diffraction was performed using Philip's

Fig. 9: (A-B) SEM images of Ni nanotubes embedded in AAO template with wall thickness of 40 nm.

**Figure 9.** (A-B) SEM images of Ni nanotubes embedded in AAO template with wall thickness of 40 nm.

nanowires. The images confirmed that the diameter of the nanowires are ~200 nm and the interwire spacing

**Figure 8.** (A-D) shows the top and cross-sectional view of AAO templates before and after the deposition of nanowires. The images confirmed that the diameter of the nanowires are ~200 nm and the interwire spacing between the pores is

nanowires. The images confirmed that the diameter of the nanowires are ~200 nm and the interwire spacing B) SEM images of Ni nanotubes embedded in AAO template with wall thickness of 40 nm.3 µm

sectional view of AAO templates before and after the deposition of

200 nm

B

D

D) shows the top and cross-sectional view of AAO templates before and after the nanowires. The images confirmed that the diameter of the nanowires are ~200 nm and the interwire spacing B) SEM images of Ni nanotubes embedded in AAO template with wall thickness of 40 nm.2 µm

200 nm

Fig. 8: (A-D) shows the top and cross

between the pores is ~300 nm.

~300 nm.

A

60 Electroplating of Nanostructures

C

**Figure 11.** (A-B) TEM morphologies of Co and Co75Fe15B10 Fig. 11: (A-B) TEM morphologies of Co and Co75Fe15B nanowires with pore diameter of 200 nm. <sup>10</sup> nanowires with pore diameter of 200 nm.

Fig. 11: (A-B) TEM morphologies of Co and Co75Fe15B10 nanowires with pore diameter of 200 nm.

Figure. 11(A-B) shows the TEM image of Co and CoFeB NWs. Figure. 12 shows diffraction pattern of the nanowires which revealed the single crystal phase for Co nanowires while the Fig. 12: SAED pattern of (A) a single crystal Co nanowires having hcp structure, and (B) amorphous phase of Co75Fe15B10 nanowires. Dotted lines are guidelines to eye. **Figure 12.** SAED pattern of (A) a single crystal Co nanowire having hcp structure and (B) amorphous phase of Co75Fe15B10 nanowires. Dotted lines are guidelines to eye.

Figure 14 shows the XRD pattern of various ferromagnetic nanowires grown in anodic alumina templates by electrodeposition method. The spectra for the nanowires indicates polycrystalline reflection peaks. The diffraction peaks confirms the *fcc* lattice structure in all deposited nanowires except for CoFeB nanowires. The only intense broad peak of CoFeB nanowires is due to a small bcc CoFe (110) phase, which occurs if the content of Fe is less. The diffractogram indicates that CoFeB nanowires appears in the amorphous phase. We compare the XRD spectra of Ni nanowires and nanotubes as shown in Fig. 15. XRD measurements illustrate a face-centred cubic (*fcc*) Ni pattern for NWs and NTs in AAO templates. Compared to the peak positions of standard Ni (JCPDS, 04-0850), peaks are found to be in agreement with peak positions of Ni ((111), (200), (220)). SAED pattern for CoFeB nanowires shows the amorphous phase. TEM images of Ni nanotubes arewere shown in Fig. 13(A) with inner and outer diameters of about 120 nm and 200 nm. It also confirms that the wall thickness of nanotubes is ~40 nm as observed in the SEM images. The branching due to AAO templates causes the growth of nanotubes in Yshape nano-channels as observed in Fig. 13(B). Figure. 11(A-B) shows the TEM image of Co and CoFeB NWs. Figure. 12 shows diffraction pattern of the nanowires which revealed the single crystal phase for Co nanowires while the SAED pattern for CoFeB nanowires shows the amorphous phase. TEM images of Ni nanotubes arewere shown in Fig. 13(A) with inner and outer diameters of about 120 nm and 200 nm. It also confirms that the wall thickness of nanotubes is ~40 nm as observed in the SEM images. The branching due to AAO templates causes the growth of nanotubes in Yshape nano-channels as observed in Fig. 13(B).

templates. Compared to the peak positions of standard Ni (JCPDS, 04-0850), peaks are found **Figure 13.** (A) TEM image of isolated Ni nanotube, (B) Y-shape growth of nanotube caused by the branching of AAO templates.

to be in agreement with peak positions of Ni ((111), (200), (220)).

Fig. 14: X-ray diffraction pattern of ferromagnetic nanowires electrodeposited in AAO templates. The \* peaks in the XRD pattern of Co NWs are due to the Au layer on back side of the template. **Figure 14.** X-ray diffraction pattern of ferromagnetic nanowires electrodeposited in AAO templates. The \* peaks in the XRD pattern of Co NWs are due to the Au layer on back side of the template.

Figure 14 shows the XRD pattern of various ferromagnetic nanowires grown in anodic alumina templates by electrodeposition method. The spectra for the nanowires indicates polycrystalline reflection peaks. The diffraction peaks confirms the *fcc* lattice structure in all deposited nanowires except for CoFeB nanowires. The only intense broad peak of CoFeB nanowires is due to a small bcc CoFe (110) phase, which occurs if the content of Fe is less. The diffractogram indicates that CoFeB nanowires appears in the amorphous phase. We compare the XRD spectra of Ni nanowires and nanotubes as shown in Fig. 15. XRD measurements illustrate a face-centred cubic (*fcc*) Ni pattern for NWs and NTs in AAO templates. Compared to the peak positions of standard Ni (JCPDS, 04-0850), peaks are found to be in agreement with peak

Figure. 11(A-B) shows the TEM image of Co and CoFeB NWs. Figure. 12 shows diffraction pattern of the nanowires which revealed the single crystal phase for Co nanowires while the SAED pattern for CoFeB nanowires shows the amorphous phase. TEM images of Ni nanotubes arewere shown in Fig. 13(A) with inner and outer diameters of about 120 nm and 200 nm. It also confirms that the wall thickness of nanotubes is ~40 nm as observed in the SEM images. The branching due to AAO templates causes the growth of nanotubes in Y-

Figure. 11(A-B) shows the TEM image of Co and CoFeB NWs. Figure. 12 shows diffraction pattern of the nanowires which revealed the single crystal phase for Co nanowires while the SAED pattern for CoFeB nanowires shows the amorphous phase. TEM images of Ni nanotubes arewere shown in Fig. 13(A) with inner and outer diameters of about 120 nm and 200 nm. It also confirms that the wall thickness of nanotubes is ~40 nm as observed in the SEM images. The branching due to AAO templates causes the growth of nanotubes in Y-

**Figure 12.** SAED pattern of (A) a single crystal Co nanowire having hcp structure and (B) amorphous phase of

Fig. 12: SAED pattern of (A) a single crystal Co nanowires having hcp structure, and (B) amorphous phase of

Fig. 12: SAED pattern of (A) a single crystal Co nanowires having hcp structure, and (B) amorphous phase of

Co NWs CoFeB NWs

Co NWs CoFeB NWs

Co75Fe15B10 nanowires. Dotted lines are guidelines to eye.

Co75Fe15B10 nanowires. Dotted lines are guidelines to eye.

Co75Fe15B10 nanowires. Dotted lines are guidelines to eye.

shape nano-channels as observed in Fig. 13(B).

shape nano-channels as observed in Fig. 13(B).

Fig. 11: (A-B) TEM morphologies of Co and Co75Fe15B10 nanowires with pore diameter of 200 nm.

**Figure 11.** (A-B) TEM morphologies of Co and Co75Fe15B10 Fig. 11: (A-B) TEM morphologies of Co and Co75Fe15B nanowires with pore diameter of 200 nm. <sup>10</sup> nanowires with pore diameter of 200 nm.

200 nm

200 nm

200 nm

200 nm

CoFeB NWs

CoFeB NWs

(A) (B)

(A) (B)

62 Electroplating of Nanostructures

Co NWs

Co NWs

(A) (B)

(A) (B)

positions of Ni ((111), (200), (220)).

The NWs and NTs were found strongly textured along (111) reflection plane. The crystallite size of the deposited nanostructures was determined using Scherrer formula, giving 22 nm and 16 nm for NWs and NTs, respectively. This is due to the fact that their outer diameter is the same, ~200 nm, but NTs have a core cylindrical hole with an estimated diameter of 120 nm. The corresponding d-spacing was observed to be 2.04 nm and 2.03 nm for NWs and The NWs and NTs were found strongly textured along (111) reflection plane. The crystallite size of the deposited nanostructures was determined using Scherrer formula, giving 22 nm and 16 nm for NWs and NTs, respectively. This is due to the fact that their outer diameter is the same, ~200 nm, but NTs have a core cylindrical hole with an estimated diameter of 120 nm. The corresponding d-spacing was observed to be 2.04 nm and 2.03 nm for NWs and NTs, respectively. We also determined the crystallite size of all these nanostructures as given in Table 3.

NTs, respectively. We also determined the crystallite size of all these nanostructures as given

FWHM β

Crystallite Size

d-spacing (nm)

(nm)

(Degree)

Ni NWs 44.33 0.42 22.0 2.04

Ni NTs 44.52 0.56 16.4 2.03

Co NWs 44.35 0.63 14.7 2.04

Table 3: XRD peak positions, FWHM, crystallite size and d-spacing of various nanostructures.

Ni60Fe40 NWs 44.41 0.48 19.3 2.04

in Table 3.

Material Peak Position

2θ (Degree)


**Table 3.** XRD peak positions, FWHM, crystallite size and d-spacing of various nanostructures.

**Figure 15.** XRD spectra of Ni nanowires and nanotubes electrodeposited in AAO templates.

The hysteresis loops were measured in two geometries: when the applied magnetic field *H* is perpendicular to wire's axis and also parallel to wire's axis. The orientation of NWs and NTs embedded in AAO with applied external magnetic field in SQUID was ensured during sample preparation. For parallel and perpendicular arrangement, the templates were fixed in a small piece of folded straw which is further inserted in a new straw in such a way that it is along the easy and hard axis of straw elongation. An analysis of hysteresis loops allowed us to determine the values of coercivity (*Hc*) and normalized remanent magnetization (*Mr*). The saturation magnetization *Ms* value has been taken from reported works [42]. Figure 16 shows the hysteresis loop of typical Ni NWs having length 30 μm with the applied magnetic field parallel and perpendicular to the long axes of the nanowires. The difference between the hysteresis loops for the two orientations suggests the existence of magnetic anisotropy in the sample. The Ni NWs are not exactly uniform from the bottom to the top of the nanowires due to the Yshaped templates being used. Y-shaped portion (Fig. 2) has a different shape anisotropy and dominates in magnetostatic coupling than the long-portion. This can only be seen as an averaged, smoothed curve in the SQUID measurements. Due to averaging, the easy axis loops show a curved shape and a consequent reduction in the squareness. The values of remanent magnetization *Mr*(||) and *Mr*(⊥) for 30 μm sample are 0.26 memu and 0.072 memu, respec‐ tively. This indicates that the magnetic easy axis of the system is along the axis parallel to the nanowires. The squareness (*Mr/Ms*) of the hysteresis curve is greater when the applied field is parallel to the nanowires than perpendicular to it. The values of coercivity *Hc*(||) and *Hc*() for 30 μm sample are 124 Oe and 84 Oe, respectively.

**Material**

64 Electroplating of Nanostructures

**Peak Position 2θ**

**(Degree) FWHM β (Degree) Crystallite Size (nm) d-spacing (nm)**

Ni NWs 44.33 0.42 22.0 2.04 Ni NTs 44.52 0.56 16.4 2.03 Co NWs 44.35 0.63 14.7 2.04 Ni60Fe40 NWs 44.41 0.48 19.3 2.04

**Table 3.** XRD peak positions, FWHM, crystallite size and d-spacing of various nanostructures.

**Figure 15.** XRD spectra of Ni nanowires and nanotubes electrodeposited in AAO templates.

The hysteresis loops were measured in two geometries: when the applied magnetic field *H* is perpendicular to wire's axis and also parallel to wire's axis. The orientation of NWs and NTs embedded in AAO with applied external magnetic field in SQUID was ensured during sample preparation. For parallel and perpendicular arrangement, the templates were fixed in a small piece of folded straw which is further inserted in a new straw in such a way that it is along the easy and hard axis of straw elongation. An analysis of hysteresis loops allowed us to determine the values of coercivity (*Hc*) and normalized remanent magnetization (*Mr*). The saturation magnetization *Ms* value has been taken from reported works [42]. Figure 16 shows the hysteresis loop of typical Ni NWs having length 30 μm with the applied magnetic field parallel and perpendicular to the long axes of the nanowires. The difference between the hysteresis loops for the two orientations suggests the existence of magnetic anisotropy in the sample. The Ni NWs are not exactly uniform from the bottom to the top of the nanowires due to the Yshaped templates being used. Y-shaped portion (Fig. 2) has a different shape anisotropy and

**Figure 16.** Magnetization loops for Ni NWs of length 30 μm at different orientation of the applied magnetic field at room temperature.

Figure 17 shows the magnetic hysteresis loop of Ni nanotube. The nanocylinders geometry, i.e*.* their length, inner and outer radii (Rin and Rout) and wall thickness (tw) strongly affects the magnetization reversal mechanism. In our work also, we observed a magnetization reversal switching in nanotube geometry. One can easily observe that for nanowires (Fig. 16), the easy axis is along the wire's long axis, whereas for nanotubes (Fig. 17), the easy axis is parallel to the tube axis resulting in a magnetization switching. Since the deposited nanostructures have an *fcc* structure, which shows small magneto-crystalline anisotropy, magnetic anisotropy is mainly decided by the competition between the shape anisotropy of the individual NW/NT and the magnetostatic interaction between neighbouring NWs/NTs.

The values of coercivity *Hc*(||) and *Hc*(⊥) for NTs are 32 Oe and 41 Oe, respectively. Both curves are highly sheared, indicating strong inter-tubular interaction. Further, it is clear from the evidence that the array has very low remanence magnetization. The values of remanent

**Figure 17.** Magnetization loops for Ni NTs of length 30 μm at different orientation of the applied magnetic field at room temperature.

magnetization Mr(||) and Mr(⊥) for 30 μm Ni NT sample are 0.006 memu and 0.017 memu, respectively. The remanent magnetization of Ni NTs is very small as compared to NWs, which is due to less volume of magnetic material present in nanotubes (hollow inside).

## **5. High-frequency applications**

Investigations on the magnetization reversal modes of Ni NWs/NTs are done by angulardependent FMR measurements. Here we will discuss the quantitative analysis of resonance frequency (*f*) and frequency line-widths (∆*f*) data. Depending upon the geometry of the nanostructures, basically three main modes of magnetization reversal exist: *coherent mode* (C), where all spins rotate homogenously; *vortex mode* (V), in which vortex-like domain wall nucleates and propagates; *transverse mode* (T), in which spins rotate progressively via propa‐ gation of a transverse domain wall.

Figure 18(A-D) shows the transmission response for arrays of Ni NWs and NTs used in the present study. In order to view the effect of interactions between NWs/NTs of FMR spectra, the resonance frequency and frequency line-width as a function of applied field was observed. Resonance frequency increases linearly with the increase of external magnetic field for all four samples. The magnetic bias field was rotated from parallel orientation of nanostructures to that of perpendicular to them for angular variation measurements. The resonance frequency (*fr* = ω/2π) as a function of field orientation can be obtained from Landau–Lifshitz–Gilbert equation as [21, 26]:

Electrodeposition of Ferromagnetic Nanostructures http://dx.doi.org/10.5772/61226 67

$$\left(\frac{\partial}{\partial \boldsymbol{\gamma}}\right)^2 = \left[H\cos\left(\boldsymbol{\theta} - \boldsymbol{\theta}\_H\right) + H\_{\text{eff}}\cos^2\theta\right] \left[H\cos\left(\boldsymbol{\theta} - \boldsymbol{\theta}\_H\right) + H\_{\text{eff}}\cos 2\theta\right] \tag{6}$$

where *ω* = 2*πf* and *γ* = *gμB/ħ* is the gyromagnetic ratio, *θ*<sup>0</sup> is the angle between magnetization equilibrium direction and easy axis, and *θ*<sup>H</sup> is the applied field direction measured from the easy axis. At the resonance frequency *ω* = *ωr*, *θ* = *θH* and the applied field *H* = *Hres* in the saturated case.

magnetization Mr(||) and Mr(⊥) for 30 μm Ni NT sample are 0.006 memu and 0.017 memu, respectively. The remanent magnetization of Ni NTs is very small as compared to NWs, which

**Figure 17.** Magnetization loops for Ni NTs of length 30 μm at different orientation of the applied magnetic field at

Investigations on the magnetization reversal modes of Ni NWs/NTs are done by angulardependent FMR measurements. Here we will discuss the quantitative analysis of resonance frequency (*f*) and frequency line-widths (∆*f*) data. Depending upon the geometry of the nanostructures, basically three main modes of magnetization reversal exist: *coherent mode* (C), where all spins rotate homogenously; *vortex mode* (V), in which vortex-like domain wall nucleates and propagates; *transverse mode* (T), in which spins rotate progressively via propa‐

Figure 18(A-D) shows the transmission response for arrays of Ni NWs and NTs used in the present study. In order to view the effect of interactions between NWs/NTs of FMR spectra, the resonance frequency and frequency line-width as a function of applied field was observed. Resonance frequency increases linearly with the increase of external magnetic field for all four samples. The magnetic bias field was rotated from parallel orientation of nanostructures to that of perpendicular to them for angular variation measurements. The resonance frequency (*fr* = ω/2π) as a function of field orientation can be obtained from Landau–Lifshitz–Gilbert

is due to less volume of magnetic material present in nanotubes (hollow inside).

**5. High-frequency applications**

room temperature.

66 Electroplating of Nanostructures

gation of a transverse domain wall.

equation as [21, 26]:

Fig. 18: (A-D). Experimental transmission response (S21 vs. frequency) at different applied fields along parallel orientation of long axis of ferromagnetic nanowires and nanotubes of diameter 200 nm and various lengths. **Figure 18.** (A-D). Experimental transmission response (S21 vs. frequency) at different applied fields along parallel ori‐ entation of long axis of ferromagnetic nanowires and nanotubes of diameter 200 nm and various lengths. Solid lines are the Lorentzian fits to the experimental data.

Solid lines are the Lorentzian fits to the experimental data. The angular dependence of fr for NWs and NTs is shown in Fig. 19(A). NW array exhibits The angular dependence of *fr* for NWs and NTs is shown in Fig. 19(A). NW array exhibits stronger magnetic interaction than the NT arrays. In nanotubes, there is a transition in magnetization reversal mode at very small tube radius. The curling mode of magnetization reversal in an infinite cylinder predicts that *fr* increases as angle (*θH*) increases from 0° to 90°, whereas coherent rotation mode gives highest and lowest *fr* values for *θ<sup>H</sup>* = 0° and *θ<sup>H</sup>* =90°, respectively.

stronger magnetic interaction than the NT arrays. In nanotubes, there is a transition in

magnetization reversal mode at very small tube radius. The curling mode of magnetization

reversal in an infinite cylinder predicts that fr increases as angle (θH) increases from 0º to 90º,

whereas coherent rotation mode gives highest and lowest fr values forθH = 0º and θH =90º,

Figure. 19(B) shows the theoretical results for angular dependence of fr for NW and NT

arrays. The results were calculated using Eq. (6) for the uniform mode and the corresponding

equilibrium conditions. In a nanowire array, each nanowire is exposed to the field created by

the neighbouring wires in addition to the self-demagnetizing field and hence the

demagnetization field is given as Hd = 2πMs (1-3P), where P is the volume fraction of the

magnetic nanostructure in the matrix. In case of nanotubes, the demagnetization field is no

longer similar to that of the nanowires. Also, it is assumed that the nanotubes do not have the

extreme surfaces as seen in wires to give rise to an additional interaction field that would

oppose the tubes' shape anisotropy. It is also found that the magnetic anisotropy is sensitive

to the demagnetization factor, demonstrating that the magnetostatic interaction between

nanotubes is responsible for the wall-thickness-dependent magnetic anisotropy [43].

respectively.

given as [44]:

needs further investigation.

Figure 19(B) shows the theoretical results for angular dependence of *fr* for NW and NT arrays. The results were calculated using Eq. (6) for the uniform mode and the corresponding equilibrium conditions. In a nanowire array, each nanowire is exposed to the field created by the neighbouring wires in addition to the self-demagnetizing field and hence the demagneti‐ zation field is given as *Hd* = 2πMs (1-3P), where P is the volume fraction of the magnetic nanostructure in the matrix. In case of nanotubes, the demagnetization field is no longer similar to that of the nanowires. Also, it is assumed that the nanotubes do not have the extreme surfaces as seen in wires to give rise to an additional interaction field that would oppose the tubes' shape anisotropy. It is also found that the magnetic anisotropy is sensitive to the demagneti‐ zation factor, demonstrating that the magnetostatic interaction between nanotubes is respon‐ sible for the wall-thickness-dependent magnetic anisotropy [43].

With this consideration, the demagnetization factors used to calculate *Heff* for nanotubes are given as [44]: With this consideration, the demagnetization factors used to calculate Heff for nanotubes are

$$N = \frac{1}{2} \left[ 1 - \mathcal{L}^2 \left( \frac{\mu\_r - 1}{\mu\_r + 1} \right) \right] \tag{7}$$

where *μr* is the relative permeability and λ = Rin/Rout, where Rin is the inner and Rout is the outer radii of the nanotube, respectively. In our case λ is found to be 0.6. where µ<sup>r</sup> is the relative permeability and λ = Rin/Rout, where Rin is the inner and Rout is the outer radii of the nanotube, respectively. In our case λ is found to be 0.6.

Fig. 19: (A) Angular variation of the FMR frequency positions (fr(θH)) measured at an applied field of 9 kOe, (B) theoretical curves fitted from Eq. (6). θH = 0º corresponds to the applied magnetic field parallel to the NWs/NTs axis and θH = 90º corresponds to perpendicular configuration. **Figure 19.** (A) Angular variation of the FMR frequency positions (*fr*(*θH*)) measured at an applied field of 9 kOe, (B) theoretical curves fitted from Eq. (6). *θH* = 0° corresponds to the applied magnetic field parallel to the NWs/NTs axis and *θH* = 90° corresponds to perpendicular configuration.

The bell-shaped fr curves in Fig. 19(A) clearly show a negative effective field for the The bell-shaped *fr* curves in Fig. 19(A) clearly shows a negative effective field for the nanotubes with the easy plane perpendicular to the nanotubes. At the parallel (low and high extreme values of *θH*) and perpendicular conditions there is a good agreement between the theoretical

nanotubes with the easy plane perpendicular to the nanotubes. At the parallel (low and high extreme values of θH) and perpendicular conditions there is a good agreement between the theoretical and experimental fr value for both nanotubes and nanowires, while in the range of θH = 40º-80º and corresponding angles in the next quadrant, there is a difference in the f<sup>r</sup> values. This difference can be attributed to the situation where the spins in the nanostructures under consideration vary spatially and hence the effective field cannot explain the mean magnetization in a particular direction giving rise to the coherent uniform rotation. This reveals the inhomogeneous internal and stray field in the NWs and NTs of small radii, which

Figure. 20(A) shows the FMR line-width (∆f) as a function of θH for all samples. ∆f was

determined from the Lorentzian fit to the transmitted signal from the sample. The line-width

and experimental *fr* value for both nanotubes and nanowires, while in the range of *θH* = 40°-80° and corresponding angles in the next quadrant, there is a difference in the *fr* values. This difference can be attributed to the situation where the spins in the nanostructures under consideration vary spatially and hence the effective field cannot explain the mean magnetiza‐ tion in a particular direction giving rise to the coherent uniform rotation. This reveals the inhomogeneous internal and stray field in the NWs and NTs of small radii, which needs further investigation.

**Figure 20.** (A) Angular variation of FMR line-widths (∆*f*) measured for all samples at an applied magnetic field of 9 kOe, (B) theoretical curves fitted from Eq. (8).

Figure 20(A) shows the FMR line-width (∆*f*) as a function of *θ<sup>H</sup>* for all samples. ∆*f* was determined from the Lorentzian fit to the transmitted signal from the sample. The line-width reflects the distribution of the parameters of individual nanostructures that vary in their exact orientation inside the template as well as in their value of effective anisotropy field. Theoretical curves showing the angular dependence of frequency line-width is shown in Fig. 20(B). The frequency line-width is found using the following relation:

$$
\Delta \left( \frac{\alpha}{\gamma} \right) = \left( \frac{d \left( \alpha / \gamma \right)}{dH} \right) \Delta H \left( \theta\_H \right) \tag{8}
$$

where *ΔH* is given by [45]

Figure 19(B) shows the theoretical results for angular dependence of *fr* for NW and NT arrays. The results were calculated using Eq. (6) for the uniform mode and the corresponding equilibrium conditions. In a nanowire array, each nanowire is exposed to the field created by the neighbouring wires in addition to the self-demagnetizing field and hence the demagneti‐ zation field is given as *Hd* = 2πMs (1-3P), where P is the volume fraction of the magnetic nanostructure in the matrix. In case of nanotubes, the demagnetization field is no longer similar to that of the nanowires. Also, it is assumed that the nanotubes do not have the extreme surfaces as seen in wires to give rise to an additional interaction field that would oppose the tubes' shape anisotropy. It is also found that the magnetic anisotropy is sensitive to the demagneti‐ zation factor, demonstrating that the magnetostatic interaction between nanotubes is respon‐

With this consideration, the demagnetization factors used to calculate *Heff* for nanotubes are

With this consideration, the demagnetization factors used to calculate Heff for nanotubes are

m

<sup>1</sup> 1[ ( <sup>2</sup>

where *μr* is the relative permeability and λ = Rin/Rout, where Rin is the inner and Rout is the outer

Fig. 19: (A) Angular variation of the FMR frequency positions (fr(θH)) measured at an applied field of 9 kOe, (B) theoretical curves fitted from Eq. (6). θH = 0º corresponds to the applied magnetic field parallel to the

**Figure 19.** (A) Angular variation of the FMR frequency positions (*fr*(*θH*)) measured at an applied field of 9 kOe, (B) theoretical curves fitted from Eq. (6). *θH* = 0° corresponds to the applied magnetic field parallel to the NWs/NTs axis

The bell-shaped fr curves in Fig. 19(A) clearly show a negative effective field for the nanotubes with the easy plane perpendicular to the nanotubes. At the parallel (low and high extreme values of θH) and perpendicular conditions there is a good agreement between the theoretical and experimental fr value for both nanotubes and nanowires, while in the range of θH = 40º-80º and corresponding angles in the next quadrant, there is a difference in the f<sup>r</sup> values. This difference can be attributed to the situation where the spins in the nanostructures under consideration vary spatially and hence the effective field cannot explain the mean magnetization in a particular direction giving rise to the coherent uniform rotation. This reveals the inhomogeneous internal and stray field in the NWs and NTs of small radii, which

The bell-shaped *fr* curves in Fig. 19(A) clearly shows a negative effective field for the nanotubes with the easy plane perpendicular to the nanotubes. At the parallel (low and high extreme values of *θH*) and perpendicular conditions there is a good agreement between the theoretical

Figure. 20(A) shows the FMR line-width (∆f) as a function of θH for all samples. ∆f was

determined from the Lorentzian fit to the transmitted signal from the sample. The line-width

where µ<sup>r</sup> is the relative permeability and λ = Rin/Rout, where Rin is the inner and Rout is the

*N* (7)

<sup>µ</sup> <sup>λ</sup> (7)

)] 1

+

r

µ

<sup>1</sup> <sup>2</sup> <sup>1</sup> <sup>1</sup> 2 1 m l

<sup>−</sup> <sup>=</sup> <sup>−</sup>

1 <sup>2</sup>

<sup>r</sup> N

é ù æ ö - = - ê ú ç ÷ ê ú <sup>+</sup> ë û è ø *r r*

sible for the wall-thickness-dependent magnetic anisotropy [43].

radii of the nanotube, respectively. In our case λ is found to be 0.6.

NWs/NTs axis and θH = 90º corresponds to perpendicular configuration.

and *θH* = 90° corresponds to perpendicular configuration.

needs further investigation.

outer radii of the nanotube, respectively. In our case λ is found to be 0.6.

given as [44]:

68 Electroplating of Nanostructures

given as [44]:

$$
\Delta H = \Delta H\_0 + 1.16a \left(\frac{a}{\mathcal{I}}\right) \tag{9}
$$

*ΔH*0 is the frequency independent contribution to the line-width caused by inhomogeneous broadening. It is found that the shape of the line-width curves is similar in both experiment and theory.

These studies show that ferromagnetic nanostructures embedded in AAO templates fabricated by electrodeposition technique are the best candidates for high-frequency application devices.

## **Author details**

Monika Sharma1\* and Bijoy K. Kuanr2

\*Address all correspondence to: monikasharma1604@gmail.com

1 Indian Institute of Technology Delhi, New Delhi, India

2 Special Centre for Nanoscience, Jawaharlal Nehru University, New Delhi, India

## **References**


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These studies show that ferromagnetic nanostructures embedded in AAO templates fabricated by electrodeposition technique are the best candidates for high-frequency application devices.

**Author details**

70 Electroplating of Nanostructures

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