Printable Hierarchical Nickel Nanowires for Soft Magnetic Applications.

Hierarchical nickel nanowires (h-NiNWs) were synthesized by a simple reduction method and their electrical, magnetic, and electromagnetic characteristics were investigated. These nanowires possess a high magnetic saturation (M s) of 51 emu/g and also a coercivity (H c) of 34.5 Oe, which makes them suitable for soft magnetic sensor applications. Hall transport is being reported for the first time for h-NiNWs, and electrical conductivity at room temperature was studied to assess their applicability as a Hall sensor wherein the Hall coefficient R H was found to be -1.39 × 10-12 Ω cm/Oe. Electromagnetic characterization of synthesized h-NiNWs shows excellent microwave shielding effectiveness of above 24 dB for the Ku band (12.4-18 GHz) and a maximum value of 32 dB at 14 GHz for a sample with a thickness of about 1 mm. A room-temperature-curable screen-printable ink was formulated using the synthesized magnetic nanostructures and printed on different flexible substrates. Printed patterns show promising ferromagnetic properties, and they could be potential candidates for soft magnetic sensor applications.

Introduction

Controlled growth of magnetic nanomaterials finds several cutting-edge applications, owing to their size- and shape-dependent properties. These applications mainly spread across the domains of magnetic recording memories,[1] microwave absorption,[2] magnetic field sensing,[3,4] pathogen capture,[5] cancer theranostics,[6,7] fluid pumping,[8−10] magnetic levitation,[11] bioelectrochemical sensing,[12] and actuation.[13] Magnetic field sensors, which convert magnetic or magnetically encoded information into electrical signals, are essentially based on two important principles, viz., magneto resistance and Hall effect. For sensing the position, velocity, or directional movement in automotive systems, soft magnetic materials are extensively used, which demand low coercivity and hysteresis. These kinds of sensors are also elaborately employed in electronic circuits due to their contactless wear-free operation, low maintenance, and robust design.[14−17] Conventional sensor fabrication demands expensive physical deposition techniques such as ion beam deposition or photolithography.[18,19] Unlike the traditional lithography that involves time-consuming and costly procedures, the printed electronics, which has been a fast emerging technology over the last few years, requires simple printing steps for device fabrication.[20,21]

Ferromagnetic metals like iron, cobalt, and nickel are ideal candidates for soft magnetic applications. In these applications, magnetic behavior is characterized mainly by magnetic anisotropy, besides exchange interaction. For example, ferromagnetic two-dimensional nanosheets are responsible for in-plane anisotropy, whereas nanowires have axis-dependent directional anisotropy, However, in magnetic nanoparticles, no specific orientation is favored.[22] Magnetic nanostructures could be fine-tuned to make them suitable in myriad areas of applications. Among the ferromagnetic nanostructures, nickel is a prominent candidate for soft sensor applications, which has less oxidation problems compared with other ferromagnetic counterparts. Earlier, various morphologies of nickel-based structures have been synthesized like nanoparticles,[23] nanoshells,[24] nanowires,[25] nanoflowers,[26] nanocarpets,[27] and nanoparticle chains.[28] Different chemical and physical techniques have been employed; however, electrodeposition was being chosen as the most widely preferred one for fabrication of nickel nanowire (NiNW) arrays.[29−31] Tabasum et al. developed a new method for synthesizing nickel nanowires and nanotubes by electrodeposition using polycarbonate templates.[32] Ferromagnetic nanotubes with customized dimensions can also be fabricated by atomic layer deposition using nanoporous alumina membranes having different pore diameters.[33] However, template-based methods have been discouraged these days because templates need to be removed prior to employ the nanostructures for real applications. In 2009, Liu and co-workers demonstrated an alternate template-free method to synthesize nickel nanowires under the influence of magnetic field.[34] The field-induced nanowire growth mechanism is well understood now based on the principle that a magnetic field applied during the chemical reaction can strengthen the magnetic dipole interaction among the magnetic nanoparticles, which in turn synchronously influences both the nucleation and the parallel growth of nickel nanostructures.[35,36] Besides, it was further revealed that the nanowire chain lengths can be tailored by adjusting the pH value and reaction temperature.[37] Magnetic induction syntheses were also reported for the fabrication of magnetic nanowires at ambient conditions. For example, Li et al. in 2015 reported the synthesis of Ni nanowires through the self-assembly mechanism assisted by an external magnetic field as a shape control agent.[38]

In 2004, Liu et al. reported the synthesis of hierarchical NiNWs (h-NiNWs) through a self-assembled network of pure Ni nanochains prepared by a simple solution-phase method.[39] In 2007, Ni and co-authors asserted that the formation mechanism of the unique hierarchical structure of NiNWs could be ascribed to the combined effect of precursor reagents, reaction rate, and inherent magnetic interactions.[40] Sarkar et al. described the synthesis of prickly nickel nanowires using a stabilizer-assisted method.[41] In 2011, Pradeep and his co-workers introduced a unique surfactant-free wet chemical synthesis to produce high-purity hierarchical nickel nanowires (h-NiNWs) with various aspect ratios.[42] They even demonstrated the surface-enhanced Raman spectra activity of these pristine hierarchical nanowires. Sooner, Tang et al., while synthesizing NiNWs through a microwave-assisted polyol route, observed that nanowires have rough surfaces at low precursor concentrations but become smoother at high concentrations.[43] In 2016, Xia and Wen optimized the large-scale synthesis conditions of h-NiNWs and found that the size and morphology of NiNWs can be tailored by adjusting the reaction temperature, time length, and surfactant concentration.[44] Logutenko et al. also made a similar observation wherein a decrease in the nickel precursor concentration resulted in a decrease in diameters of the nanowires.[45]

Despite these reports, there have been only very few isolated works aimed to employ these hierarchical nanowires for practical applications. For example, in 2016, the electrical conductivity of h-NiNWs was explored by Kim et al., who fabricated high-performance transparent electrodes based on nickel nanowires.[46] The microwave absorbing properties of solvothermally derived hierarchical nickel nanochains were also investigated.[47] In short, for NiNWs, several of the all-important soft magnetic sensing applications including Hall effect sensing were largely unexplored, which was undertaken in the present work. Herein, we developed a large-scale synthesis protocol for hierarchical nickel nanowires and the electrical, thermal, magnetic, and electromagnetic properties of the synthesized nanostructures were investigated. The screen printing of hierarchical nickel nanowires using a room-temperature-curable screen-printable h-NiNW ink on flexible substrates like (biaxially oriented poly(ethylene terephthalate) (BoPET)) and cellulose has also been demonstrated, and their magnetic response was discussed.

Results and Discussion

Mechanism of Hierarchical Nanowire Formation

The present work describes a one-step process for the reduction of nickel salt by a strong reducing agent.[48] We synthesized hierarchical nickel nanostructures by reducing NiCl2·H2O with hydrazine hydrate. Large-scale synthesis of the nanostructures has been optimized. The reaction mechanism can be explained by the following equationFigure shows the growth mechanism of hierarchical NiNWs. The mechanism of formation of the nanowire structure can be explained as a consequence of the alignment of nickel nanoparticles in a specific crystallographic direction. Ethylene glycol functions as a dispersant as well as a solvent in the current preparation method. The occurrence of small magnetic nuclei is favored by the dispersing action of ethylene glycol. Each small nucleus grows into high-surface-area structures, which are not thermodynamically favorable. To reduce the number of interfaces, the nanoparticles get attached to each other and grow like the stems of plants. Because the small nanoparticles formed are also magnetic, they provide an inherent magnetic field that allows magnetic particles to get self-assembled with each other like tiny magnets, forming a chainlike structure. The magnetic dipolar interaction also plays a role in the growth of nanowires along a particular direction. The formation of acicular structures occurs at positions where two particles join each other, which may be explained by a Cayley tree model.[39] In Cayley trees, the adjoining of two branches leads to the formation of acicular structures.[2] During the growth process, the surface area is reduced, which makes the nanowires thermodynamically more stable.

Figure 1

Growth mechanism of h-NiNWs synthesized by the chemical reduction method.

Structural and Morphological Analysis of Hierarchical NiNWs

Figure a shows the powder X-ray diffraction (XRD) pattern of as-synthesized nanostructures. XRD could be well indexed with JCPDS file no: 00-004-085. The nickel nanowires exhibit a face-centered cubic (fcc) structure with good phase purity. No trace of nickel oxide was observed within the detection limits of XRD. The texture coefficient of each plane can be calculated from XRD using Harris’s analysis as given by the equation[49]where I(h) represents the peak intensity corresponding to the ith plane in the powder diffraction data and Io(h) represents the peak intensity corresponding to the same plane in the JCPDS file. For fcc nickel, three planes are considered here, which are at 2θ = 44.50, 51.85, and 76.38°, and hence the value of n is equal to 3. For a polycrystalline sample, the orientation of individual crystallites plays a major role in determining the physical properties. The texture coefficient defines the preferred direction of orientation of individual crystallites in a polycrystalline sample. If there exists a plane whose texture coefficient is greater than 1.0, the crystal is said to be preferentially oriented in that direction. In the present case, the value of texture coefficient for the (111) plane is 1.18, whereas that of the (200) plane is 0.96 and the (220) plane has a value of 0.85. Hence, we believe that h-NiNWs have a preferred orientation along the (111) plane, which is consistent with earlier studies as well.[50] In template-grown nanowires, the texture plays a significant role in controlling the magnetic properties. The crystallite size also plays a major role in determining the magnetic properties like saturation magnetization and coercivity. The crystallite size can be estimated from the Debye–Scherrer equationwhere λ is the wavelength of Cu Kα radiation used for diffraction, β is the full width at half-maximum, and K defines the shape factor. The average crystallite size is obtained as 39 nm, computed using the major X-ray diffraction peaks.

Figure 2

(a) XRD of h-NiNWs, which confirms the fcc nickel structure; (b) scanning electron microscopy (SEM) of synthesized structures drop-casted on carbon film; (c) bright field transmission electron microscopy (TEM) of long h-NiNWs; (d) TEM of h-NiNWs showing nanothorns grown radially outward on the surface of wires; (e) energy-dispersive X-ray pattern of h-NiNWs; and (f) selected-area diffraction (SAED) pattern for the polycrystalline sample.

Figure b represents the SEM of the h-NiNW suspensions drop-casted over a microslide and dried in a vacuum oven at 60 °C, whereas Figure c represents the transmission electron micrograph of h-NiNW drop-casted on a carbon-coated copper grid. Evidently, the h-NiNW surfaces are packed with spiky thorns that resemble the stems of a rose plant. These thorns appear standing along the radial direction on the nanowire surfaces. The average length of the thorns appears to be less than 100 nm, whereas the wire diameters lie in the range of 150–300 nm (see Figure d), with lengths on the order of a few micrometers. Figure e represents the energy-dispersive X-ray spectroscopy of the prepared nanostructures. The Ni L and Ni K energy peaks are believed to be signatures for the formation of phase-pure nickel nanowires. A small peak for O K may be due to surface oxidation, and C and Cu peaks are arising out of the TEM grid. Figure f represents the selected-area diffraction (SAED) pattern of hierarchical NiNWs. The bright spots with ringlike structures appear in SAED, as for any polycrystalline sample. All of the major peaks in the diffraction pattern were indexed, which represent the fcc-structured nickel. Figure S1 shows the analysis of the chemical state of h-NiNWs by X-ray photoelectron spectroscopy (XPS) measurement, which was recorded after a period of 4 weeks since its synthesis. The binding energies of Ni 2p1/2 and Ni 2p3/2 of the Ni 2p doublet, shown at 870.4 and 853.3 eV, respectively, indicate the surface oxidation of nickel (Ni2+). Satellite lines are observed at 873.6 and 857.3 eV for Ni 2p1/2 and Ni 2p3/2 components, respectively, which characterize Ni2+ ions. It is evident that the surface of h-NiNWs may get oxidized in an oxygen atmosphere when kept for a period of 4 weeks, which is understandable because nickel exhibits one of the highest parabolic oxidation rate constants among the corrosion resistant metals.

Magnetic Properties of h-NiNWs

Figure a,b represents the hysteresis loops at 10 and 300 K. Magnetocrystalline anisotropy is less for fcc-structured Ni compared to that for magnetic counterparts like rare-earth-based materials because strong spin–orbit coupling is exhibited by rare-earth elements, which is attributed to their heavy nature.[51] Magnetic measurements are carried out for field sweep from −5 to 5 T. The magnetic properties of the nickel nanowires were found to be better than those of spherical Ni nanoparticles because of the shape anisotropy effect.[52] Nonspherical samples have different amounts of magnetization for different directions, and the magnetization would be larger along the long axis. In general, the shape anisotropy constant can be defined as c/a, wherein c denotes the length of the nanostructure and a denotes the radius of the nanostructures.[51] The excellent magnetic sensing of the as-synthesized nanowires in the presence of a laboratory bar magnet is showcased in Video SV1. The saturation magnetization at 300 K is lower than that obtained at 10 K due to the contribution from local surface anisotropy at low temperatures arising from the frozen surface spins.[53] The thermal influence on magnetism is less evident at low temperatures where the applied magnetic field could be utilized completely for aligning the spins without any thermally induced random motions. Figure c represents the M–T curve of the synthesized nickel nanowires from 300 to 700 K at 200 Oe. The Curie temperature (Tc) of the nanostructures is found to be 614 K, as seen in Figure d, which is lower than that of the bulk Ni (627 K).[54] The reduction in Curie temperature can be attributed to an increase in the disorder of magnetic moments as well as the increase in the number of interfaces with size reduction. Hence, the thermal energy may overcome the magnetic ordering at a still lower temperature than for the bulk nickel (∼629 K). The low temperature behavior of ferromagnetic materials could be specified by Bloch’s equation[55]where M(T) and M(0) denote the saturation magnetization at temperatures T and 0 K and Bloch’s constant λ = 3/2. As the temperature increases, there is a reduction in saturation, and after the Curie temperature is reached, the nanostructures undergo transition to a paramagnetic state. Figure e depicts the digital photograph taken for h-NiNWs dispersed in distilled water. As shown, when a magnet is introduced, the sample gets attracted to the magnet due to high magnetic saturation (see Figure f).

Figure 3

(a, b) M–H of h-NiNWs at 10 and 300 K, (c) M–T of h-NiNWs, (d) plot of dM/dT Vs temperature of h-NiNWs, (e) as-synthesized h-NiNW dispersed in distilled water, and (f) magnetic response of h-NiNWs in the presence of a bar magnet.

In the present investigation, we have tested magnetic properties of hierarchical nickel nanowires in two different forms: (a) hierarchical nickel nanowire powder and (b) powder consolidated into a pellet by forming in a uniaxial press under a pressure of 150 MPa. Figure depicts the M–H hysteresis curve of hierarchical NiNWs recorded after 30 days of preparation, and data taken at 300 K clearly demonstrate their ferromagnetic nature with magnetic saturation (Ms) value of 49 emu/g and remnant magnetization Mr of 12.9 emu/g. The bulk Ni exhibits a saturation value of 55 emu/g. The lowering of Ms is suspected to be due to surface oxidation. This magnetization value is found to be better than that reported in the literature for prickly nickel nanowires.[56] From Figure a, one could estimate that the hierarchical nickel nanowires have a squareness value of 0.263 and coercivity of 175 Oe. Ultrathin nanowires are likely to be single crystalline and hence their coercivity is dictated by the wire diameter. When the wire diameter increases, the number of magnetic domains within the particle increases, which results in lower coercivity values. The hierarchical nanowires in the present study could be considered as multidomain systems, with nanowires and nanothorns oriented in different directions. We have done measurements both in powder and consolidated pellet forms. When the magnetic measurements are performed for the powder form where a large number of hierarchical wires are present with diverse orientations, the magnetic interaction between them plays a role in assuming lower coercivity values.[57]

Figure 4

(a) M–H of powder and pellet forms of nickel nanostructures at 300 K and (b) hysteresis loops magnified between −300 and +300 Oe, for the h-NiNW powder and pellet.

On the other hand, when the magnetic properties of pelletized hierarchical nickel nanowires were tested, we found that saturation magnetization increased to 51.5 emu/g, whereas remnant magnetization decreased to 3.55 emu/g. The squareness of consolidated h-NiNWs is found to be 0.068. This phenomenon can be explained by considering the interparticle interaction among magnetic nanostructures. Furthermore, the coercivity of the consolidated h-NiNW is 34.5 Oe, which is considerably lower compared to that of the powder (175 Oe). The coercivity reduction is also believed to be due to the magnetostatic interparticle interaction.[51] This is because a magnetic particle exerts a field around its neighboring particles, which assists them to be magnetized using a lesser field than that is actually required. As the packing density increases, the interparticle interaction increases and hence a lowering of the coercivity results.

Electrical and Thermal Conductivity of the Nickel Nanostructures

The direct current (dc) electrical properties of the nickel nanostructure can be explored by the four-probe method (see Figure S2). Here, the two current probes were connected at the outer ends of the preheated rectangular pellets and the voltage probes were connected at the inner ends of the rectangular block. When current was varied from 0.1 to 1.1 A, the voltage developed across the inner leads was measured. The plotted graph shown in Figure a exhibited a linear nature, typical of any conducting metallic structure. The conductivity of the nanostructures is obtained using eqs and 15 in the Experimental Section.

Figure 5

(a) I–V plot of Ni at 300 K and (b) thermal conductivity vs temperature for 180–330 K.

The dc conductivity value of h-NiNWs was found to be 2.19 × 106 S/m, whereas the conductivity value reported for bulk nickel was 1.43 × 107 S/m.[54] The reduction in electrical conductivity can be attributed to the surface oxidation in the h-NiNWs and the increased surface scattering due to the decrease in size. In spite of this, the h-NiNWs exhibit better conductivity than the nanoparticles due to the connectivity of the one-dimensional structures. The electrons can travel with less scattering for a long distance in a nanowire compared with a nanoparticle where they need to transit from one nanoparticle to another and hence the increase in the number of interfaces contributes to the reduced conductivity.

Figure b represents the thermal conductivity of the nickel samples measured by a thermal transport system (TTO) probe from 180 to 350 K. Heat transport in magnetic materials may be mediated by magnons and phonons. Obviously, the thermal conductivity in a metallic conductor has electronic contribution in addition to phonon contribution. The electronic contribution has been dominant for metallic particles like Ni. Magnon transport is influenced by the arrangement of domain walls as well as applied magnetic fields as this may cause magnon scattering. The reason for an increasing trend in thermal conductivity for polycrystalline nanowires in the low-temperature region was already reported in the literature (4–150 K), which was explained to be due to the presence of spare domain walls.[58] In Figure b, for applied temperatures of 180–350 K, the magnon scattering may cause a decrease in thermal conductivity with increasing temperature. A similar trend has been reported earlier for magnetic nanowires also.[58] The thermal conductivity of the structures was found to be 33 W/m K at room temperature, which is very much less than the reported value of bulk Ni (90 W/m K) but comparable with that reported for a nickel nanowire (22 W/m K).[59] The reduction in thermal conductivity occurred due to the increased surface scattering caused by the increased number of interfaces in the h-NiNWs. The presence of porosity in the tested sample can also contribute toward the lowering of thermal conductivity.

Hall Effect Measurement of Nickel Nanostructures

Hall effect measurements of nickel nanowires have been schematically shown in Figure a. A planar Hall effect has been reported earlier for nickel, and this effect was used for detecting magnetic microbeads; the Hall effect plays a major role in determining the magnetic sensor applications.[18,60] This has an important medical application as it can be used to detect magnetic structures attached to various drug delivery systems for targeting specific affected sites. The transverse voltage developed across a semiconductor or conductor by the application of a magnetic field in a direction perpendicular to the current flow is called the Hall voltage. The applied magnetic field deflects the charge carriers, which results in the development of Hall voltage (VH), and the phenomenon is termed as Hall effect.[61] The corresponding resistance is the Hall resistance. The Hall effect measurement setup can be used to calculate the concentration of the charge carriers (n), Hall coefficient (RH), and their mobility (μ). In the present case, Hall resistances for magnetic fields from −9 to +9 T were measured, and Hall resistivity versus magnetic field data is plotted in Figure b. The accurate values can be obtained from the equationswhere t is the thickness of the Hall specimen and ρ is the resistivity. The Hall coefficient can be calculated using the equationThe slope of the ρ–B graph gives the magnitude of the Hall coefficient. A negative slope indicates the majority carrier as electrons, and the concentration of charge carriers n can be obtained byThe value of RH(58) is calculated to be −1.39 × 10–12 Ω cm/Oe, which is comparable to that reported for bulk Ni (−5.1 × 10–12 Ω cm/Oe). The carrier concentration can be found to be 4.49 × 1028/m3.

Figure 6

(a) Schematic representation of the Hall effect measurement setup and (b) Hall effect measurement using the four-probe method at 300 K for fields from −9 to +9 T.

Electromagnetic Interference (EMI) Shielding of h-NiNWs

Interference of electromagnetic waves for communication is considered to be a major road block in the era of information technology, governed by a number of devices arranged in miniaturized chips. Metal nanostructures provide a sustainable solution for this issue, which hamper the effective data transmission between various devices in the telecommunication industry. The hierarchical NiNWs were made in the form of consolidated rectangular blocks and heat-treated at reducing atmospheres, to give them a compact form. A promising electromagnetic shield can be defined as a material with lower ac skin depth. The skin depth of a material is the distance at which the amplitude of the incoming radiation is reduced to 1/e times its original wavelength.[62] The skin depth δ is given bywhere f is the frequency of the radiation, σ is the conductivity of the material, and μ is the permeability of the material.

The permeability of a ferromagnetic material can be calculated by the following equation[63]Here, Ms represents the magnetic saturation, Hc is the coercivity, k is called a proportion constant, a and b are constants that depend on the material, λ is the magnetostriction constant, and ξ denotes the elastic strain value in the crystal structure. As the saturation magnetization increases, permeability also increases. The coercivity is also an important factor that decides the permeability of the material. Thus, it became evident that a better shield should have excellent conductivity and permeability.

Metal nanostructures are good solutions for EMI shielding due to their superior electrical conductivity. The eddy current loss, a, of a conductor in an electromagnetic field is calculated using the equation[64]where d is the thickness, μ0 is the permeability, and σ is the conductivity of the material.

As the thickness d increases, the eddy current loss increases and thus the shielding effectiveness increases. But an ideal EMI shield should have a thickness less than 2 mm for practical purposes.[65] Nickel is a ferromagnetic material at room temperature and possesses good conductivity and permeability, which are evident from the previous measurements.

The shielding effectiveness of a material can be determined experimentally using a transmission waveguide technique.[66] The shielding effectiveness by absorption and reflection can be calculated by equations 16 and 17.

The total shielding effectiveness is calculated as the sum of both the reflection and absorption shielding effectiveness. The reflection effectiveness is contributed by the electrically conducting nature of nickel nanowires. The mechanism of absorption shielding is due to the magnetic and dielectric losses occurring when the electromagnetic waves traverse through the material. Figure a represents the micrograph of porous rectangular blocks of hierarchical NiNWs used for waveguide measurement, whose results are given in Figure b. Figure c represents the schematic of the EMI shielding mechanism. As shown in Figure c, the thorns on the surface of the h-NiNWs act as antennas, which direct the incoming electromagnetic waves toward the interior of the structure. As seen from Figure a, hierarchical NiNWs form well-connected structures, though highly porous as in the case of foams. When electromagnetic waves enter into this kind of connected structures, they suffer attenuation mainly by absorption. The hierarchical nickel nanowires used in the present study exhibited a total shielding effectiveness (SE) of about 24–32 dB in the Ku band, as depicted in Figure b, which can shield 99% of the radiation incident on them and can be a good choice as an EMI shielding material.

Figure 7

(a) SEM of porous rectangular blocks of hierarchical NiNWs used for waveguide measurement, (b) EMI shielding effectiveness of nickel nanostructures for the Ku band measured using the waveguide method, and (c) schematic of the EMI shielding mechanism in h-NiNWs.

Screen Printing of Hierarchical NiNWs

Rheological Properties of Nickel Ink

Translation of the above said properties of hierarchical NiNWs into any practical flexible electronic devices mainly depends on the success of converting the nanostructures into a printable ink suspension. Besides, it is also important to see how their magnetic properties will get modified when we convert them into a thick film coating using a conventional printing process. In the present study, we have adopted a screen printing technique to realize the hierarchical nanowire coatings.

The screen printing ink can be characterized mainly by its rheological properties such as viscosity that can be classified into static and dynamic.[67] The static properties include viscosity and thixotropic behavior. Dynamic properties like viscoelasticity (i.e., elastic-/solidlike (G′) or viscous-/liquidlike (G″)) determine a good screen printing ink. The viscosity of a printing ink can be described by the equation[68]where η is the apparent viscosity, η0 is the viscosity at zero rate of shear, η∞ is the viscosity at infinite rate of shear, G is the gradient of velocity, and α and n (2/3 > n > 1) are constants. Here, α is the constant that describes the creation or breaking of linkage between particles in the ink that varies the viscosity and is responsible for pseudoplastic nature as well. The viscosities of ink for lower filler concentrations were found to be on the order of few mPa s and were beyond the measurement limit for the cone and plate method, and such inks could not be screen-printed due to their uncontrollable flow characteristics. A viscosity of 1–10 Pa s at lower shear rates is considered to be the optimum value for screen printing. The viscosity, as depicted in Figure a for first scheme of ink formulation with 30 wt % filler loading and ethanol as solvent, follows a trend that matches well with the equation and has viscosity in the desirable range. The viscosity increased to a measureable form due to the increased filler loading. Thus, the present formulation may be viewed as ideal for printing applications. The increase in viscosity is attributed to the increased interparticle connection, which reduces the flow rate. In the first scheme of formulation, the use of poly(vinylpyrrolidone) (PVP) helps in stabilizing the dispersion for printing. Ideal screen printing inks exhibit pseudoplastic behavior, which is required to control the postprinting flow characteristics.[69,70] For elastic fluids, the viscosity increases with increased shear rates as the resistance to flow increases. However, for pseudoplastic fluids, the lower shear rates (<20 s–1) correspond to the region of practical importance because most of the automatic screen printers operate in this range. For the second scheme of ink formulation, at higher shear rates, the increased viscosity can be explained to be due to the room temperature drying of the bisolvent system (see Figure S3). Fish oil comprises a large number of fatty acids, which help in optimally dispersing the filler system. With the increase in filler loading, high packing density is attained; there is no adequate fluid in the ink to facilitate the relative motion of particles, and, as expected, the viscosity rises.[71] The viscosity of an ink depends on the viscosity of the solvent systems, which suspend the filler particles. As the filler loading increases, the volume of the solvent system in the formulated ink becomes insufficient to support the motion of filler particles, thus giving an enhanced viscosity value for the ink.

Figure 8

(a) Viscosity vs shear rate of the ready-to-print h-NiNW ink, (b) printed h-NiNW ink on a paper substrate, (c) printed pattern on a BoPET substrate, and (d) atomic force microscopy (AFM) of printed hierarchical NiNWs on BoPET. The photographic image of the as-prepared h-NiNW ink is shown in the inset of (a).

Morphological Studies of the Printed Pattern

Scanning electron microscopy of the printed samples exhibits uniform packing of nickel nanowires as seen from Figure b,c. The nanowires along with the polymer binder spread uniformly throughout the pattern. SEM images of the printed pattern on a conventional paper and biaxially oriented poly(ethylene terephthalate) (BoPET) are shown in Figure b,c whereas Figure d represents the AFM of the pattern printed on BoPET. The surface roughness of the printed hierarchical NiNW-based ink on the BoPET substrate was calculated using Nanoscope analysis software as 180 nm. Obviously, the surface roughness is high due to the uneven thorny structure of the nanowires.

Magnetic Characterization of the Printed Pattern

Figure a shows the magnetic characterization of hierarchical NiNWs printed on a paper and on BoPET for a magnetic field variation from −9 to +9 T at 300 K. When printed on a cellulose-based paper using an ink with a filler loading of 30 wt %, the squareness increased to 0.3666 that may be due to dispersant effect of separating nickel nanowires. The coercivity increased to 156.78 Oe, yet less than the coercivity of the hierarchical NiNW powder, as given in Figure b. When printed on BoPET, squareness increased to 0.398, whereas coercivity diminished to 145 Oe, which is lower than the coercivities of the h-NiNW powder (Figure b) and that printed on a paper. These results indicate that squareness improved on printing. Despite being porous and showing thorny nanowire morphology, the hysteresis loops of the printed patterns exhibit ferromagnetic nature, which qualifies them to be used as an effective magnetic sensor. The performance of the magnetic printed patterns with few micrometer thickness on these most widely used flexible substrates shows promising applications in designing wearable or rollable sensors. The biocompatibility and toxicity data are also to be assessed to make them optimized for wearable sensor applications.

Figure 9

(a) M–H of printed hierarchical NiNWs on a paper and (b) on BoPET.

Conclusions

Hierarchical nickel nanostructures were synthesized using a simple reduction method employing ethylene glycol. The developed structures exhibited Ms of 51 emu/g, better electrical conductivity (2.19 × 106 S/m), and an impressive thermal conductivity of 33 W/m K even for a densification of about 40%. Even with high porosity in the compacted form, the promising properties of h-NiNWs are due to their effectively interconnected networks, similar to a foam structure. Hence, the material in the present study may be proposed as a good candidate for a soft magnetic sensor. EMI SE of about 32 dB at 18 GHz qualifies them as a suitable candidate for developing coatings for microwave shielding applications. Hall effect measurements show that the nickel nanowires could be used for sensing magnetic microbeads for biological applications and improvement of sensitivity could be optimized by tuning the electrical resistivity, the diameter, and aspect ratios of hierarchical structures. The novel formulation of the screen-printable ink using these nanostructures could make printing of magnetic devices for industrial applications. Low-cost flexible magnetic sensors would be a reality with this kind of magnetic printed structures replacing lithographically patterned expensive magnetic thin films.

Experimental Section

Materials

Nickel(II) chloride hexahydrate (98%, Sigma-Aldrich), hydrazine hydrate (Merck), and ethylene glycol (Merck) were utilized for h-NiNW synthesis. Poly(vinylpyrrolidone) (PVP-40 000, Sigma-Aldrich), ethyl cellulose (Sigma-Aldrich), absolute ethanol (Merck), fish oil (Arjuna Natural Extracts, Kerala, India), poly(vinyl butyral) (Butvar-98, Sigma), and xylene (Sigma-Aldrich) were used for ink formulation.

Template-free Synthesis of Hierarchical Nickel Nanowires

For the large-scale synthesis of hierarchical nickel nanowires, 1 M nickel(II) chloride hexahydrate solution in water was prepared by adding 5.9470 g of nickel salt in 25 mL of distilled water. Then, 6 mL of prepared solution was added to a mixture of ethylene glycol and water (in the volumetric ratio 3:1) to obtain a total amount of 400 mL. This precursor was heated for 30 min to reach 120 °C, followed by the addition of 24 mL of hydrazine hydrate solution to obtain a blue solution, which instantly turns into dark blue and then to a black network finally. The reaction was kept for 20 min for complete reduction to occur. The black network was separated using a magnet and washed with ethanol and water using a refrigerated centrifuge at an rpm of 13 500 for 15 min. The resultant black precipitate was dried in a vacuum oven at 60 °C for 24 h.

Ink Formulation and Screen Printing

Ink formulation was done for two types of solvent systems and thus there are two schemes. In the first scheme of formulation, ethanol was taken as a solvent, poly(vinylpyrrolidone) as a dispersant, and ethyl cellulose as a binder. The loadings of the filler have been varied from 15 to 20, 25, and 30 wt % with respect to the solvent. Considering the ideal rheological characteristics, the filler loading has been optimized as 30 wt %. The dispersant loading optimized was 10 wt %, whereas the binder was kept as 10 wt %. In an alternate formulation scheme, the ink formulation consisted of ethanol/xylene as the vehicle, poly(vinyl butyral) was used as the binder, whereas fish oil was used as the dispersant. The binder loading has been fixed as 10 wt % with respect to the filler loading. The dispersant is optimized to be 5 wt % of the filler loading. In this case also, the optimum filler concentration obtained was 30 wt %. Proper mixing of ink’s ingredients in the case of both formulations was ensured by ultrasonication with a pulse frequency of 80 Hz. The formulated ink was printed with the help of a screen printer (EKRA, Asys Group, Germany) using a 325-mesh polyester/nylon screen. The ink was printed using three strokes on rigid as well as flexible substrates. The second formulation delivered better printed patterns on a paper substrate. The printed ink was formulated so as to be dried at room temperature.

Characterization

The phase purity of the synthesized h-NiNW was confirmed using X-ray diffraction with Cu Kα radiation (Philips X’Pert PRO diffractometer, PANalytical, Almelo, The Netherlands) of wavelength 1.54 nm. The morphology of the synthesized hierarchical NiNWs was analyzed using a scanning electron microscope (SEM, Zeiss EVO 18 Cryo SEM, Jena, Germany) operating at an accelerating voltage of 15 kV. For this, samples were dispersed in ethanol, drop-casted on a carbon tape, and dried in a vacuum oven at 60 °C. The surface uniformity of the printed patterns was also analyzed using SEM. The high-resolution imaging was done using a transmission electron microscope (TEM, FEI Tecnai G2 30S-TWIN, FEI Co., Hillsboro, OR). For this, samples dispersed in ethanol were drop-casted on a carbon-coated TEM grid and dried in vacuum overnight. Energy-dispersive spectroscopy was performed to analyze the elemental composition. The selected-area diffraction pattern was also recorded for the h-NiNW samples. X-ray photoelectron spectroscopy (XPS) data was measured with an X-ray photoemission spectrometer (Omicron Nanotechnology, Taunusstein, Germany) with Al Kα operating at 1486.7 eV as the X-ray source. Fityk software was utilized for the spectral background (Shirely) deconvolution. Atomic force microscopy (AFM) of the synthesized h-NiNW networks was performed using a Bruker Multimode AFM, Germany, in the tapping mode, employing a silicon tip. The synthesized samples were pelletized using a uniaxial press (Carver), and the dc conductivity of the synthesized samples was measured by the four-probe method using Keithley source meter 2182A and Aplab nanovoltmeter 9710P. Here, the current probes were connected at the outer ends of the sample and the voltage developed was measured using the inner probes connected to a nanovoltmeter. The current was varied from 0.1 to 1 A. The resistivity of the sample can be calculated usingwhere A is the cross-sectional area of the sample in m2 and l is the gap between voltage leads in m and hence the conductivity is found to beThermal conductivity measurement was performed using a physical property measurement system (PPMS DynaCool, Quantum Design Inc., San Diego) with the help of thermal transport option (TTO). Here, a heater shoe is used to supply heat to the sample, and from sample geometry, thermal conductivity is measured. The Hall effect of the synthesized sample was analyzed after pelletizing the powder into a rectangular block (10 mm × 10 mm × 0.9 mm). The sample was measured using the electrical transport option in PPMS. For this, the sample was mounted in a 2.4 cm diameter puck connected by metal wires and the alternating current (ac) transport (ACT) measurement system was used for measuring the Hall voltage. The ACT option includes a high precision current source and voltage detector configured with a 9 T longitudinal magnet at 300 K. The magnetic properties of the samples were measured using a vibrating sample magnetometer probe in PPMS. The magnetic field was varied from −9 to +9 T at 300 K, and the magnetizations of the samples in both bulk and printed forms were studied. M–T measurements were carried out for a temperature range of 300–700 K at a magnetic field of 200 Oe. The complex scattering parameters were measured using a vector network analyzer (E5071C, ENA series, Agilent Technologies, Santa Clara) operating at 300 kHz to 20 GHz. The transmission waveguide technique was used to measure the electromagnetic scattering parameters of h-NiNWs in the Ku band (12.4–8 GHz). For this, the h-NiNWs were compacted and machined to the dimensions 15.8 mm × 7.90 mm to suitably fit in the waveguide. The electromagnetic waves when reach the sample under test, they are either reflected, absorbed, or transmitted. The microwave shielding effectiveness by reflection (SER) and by absorption (SEA) can be calculated using the equations[72]The total shielding effectiveness SET can be represented as the sum of reflection shielding effectiveness and absorption shielding effectiveness.The viscosity of the screen-printed ink was obtained using a modular compact rheometer (MCR 102, Anton Paar, Ashland, VA) employing the cone and plate method (with cone diameter 24.98 mm and cone angle 2.009°). The viscosity measurements were carried out for shear rates from 0 to 100 s–1 at 25 °C.