Article pubs.acs.org/JPCC
Evolution of Magnetic and Structural Properties during Iron Plating of Carbon Nanotubes Narelle Brack,*,† Peter Kappen,†,∥ Andy I. R. Herries,‡,∥ Antony Trueman,§,∥ and Andrew N. Rider§,∥ †
Department of Physics and ‡Australian Archaeomagnetism Laboratory, Department of Archaeology, Environment and Community Planning, La Trobe University, Melbourne, Victoria 3086, Australia § DSTO, 506 Lorimer Street, Fisherman’s Bend, Melbourne, Victoria 3207, Australia S Supporting Information *
ABSTRACT: Iron nanoparticles have been electrochemically deposited onto carbon nanotube (CNT) films. The CNT films were prepared by electrophoretic deposition using CNTs that were functionalized using a novel ozone-based process. Chemical characterization of the iron films was undertaken as a function of deposition time and related to their magnetic properties. In the initial stage of film growth, 20 nm cubic iron-rich crystals nucleate on individual CNTs. As the film grows, the iron crystals coalesce into a more continuous film and the elemental iron concentrations increase above 70 atomic percent. Changes in the chemical composition of the films during growth are reflected in the ferromagnetic properties, which show much higher coercivity values for thicker films relative to bulk iron. The film coercivity is related to the nanosized-cubic iron particles which form on the CNTs and is significantly enhanced when compared to that of iron films that form on planar graphite substrates, where the cubic crystal structure is not observed.
1. INTRODUCTION Traditionally, carbon nanotubes (CNTs) are integrated into larger material systems but often only have a simple function, such as to modify mechanical or electrical properties. To extend their application from simple to complex functions, it would be beneficial to manipulate the magnetic properties of CNTs. CNTs are the ideal template material for attaching and aligning the desired magnetic species, providing nanometer physical dimensions, thermal stability, and control over magnetic particle size distribution.1 Carbon nanomaterials, either filled or coated with a magnetic species such as Fe, can then be manipulated or controlled with external magnetic fields. Some studies have exploited the superparamagnetic properties of Fe2O3 and Fe3O4 to produce nanomaterials with a range of applications.2−5 However, there are a number of important applications where iron in its metallic, ferromagnetic phase (Fe0) is required. For example, in electromagnetic shielding applications, Fe0 provides the best absorption properties at high frequency.6 There is also some evidence to suggest that FeC phases can further enhance the absorption properties relative to Fe0.7 Theoretical studies also indicate that Fe0 in a metal organic framework can offer a suitable environment for high capacity hydrogen storage.8 There have also been a number of theoretical studies examining the interaction of Fe0 with carbon nanostructures for drug delivery9 and electron transfer applications.10 In the current study, the development of magnetic properties with chemical and structural changes occurring during electrodeposition of iron onto carbon nanotubes will be examined due to the benefits the metallic phase offers in the applications described. © 2014 American Chemical Society
The development of carbon-based magnetic nanomaterials has received considerable interest and followed several experimental pathways. Carbon nanotubes have been filled with transition metals such as iron, nickel, and cobalt using arccharge,11−16 high temperature heat treatment,17 ion beam sputtering,18 and chemical vapor deposition.15,19,20 Each of these methods has suffered various drawbacks such as complicated experimental procedures, low yields, and poor growth control of the metal-filled CNTs. In addition, the encapsulation efficiency of the metal is low, as demonstrated by the variability in the metal particle size and chemical state and their random distribution within the CNTs.21,22 An alternate approach is referred to as wet decoration with magnetic nanoparticles.23−30 CNTs were decorated with magnetic nanoparticles of several nanometers in diameter, which provided sufficient saturation magnetization for magnetic separation. However, little attention was given to relating the chemical structure of the materials to the magnetic properties. The disadvantage of the wet decoration approach is that the nanoparticles are not strongly attached to CNT surface, which limits the practical application. Therefore, a strong chemical linkage between the CNT and the magnetic nanoparticle is required. The strong iron to CNT bond can in part be provided via electrochemical deposition, which can be applied to large areas and provide controlled and uniform coatings and microstructures with relatively mild chemicals.31 Electrodeposition Received: February 27, 2014 Revised: May 7, 2014 Published: May 27, 2014 13218
dx.doi.org/10.1021/jp502078t | J. Phys. Chem. C 2014, 118, 13218−13227
The Journal of Physical Chemistry C
Article
between the film growth mechanisms of a readily applied process and the material’s magnetic properties that develop during the growth. The chemical and structural properties of Fe-coated functionalized CNTs will be characterized using Xray absorption spectroscopy (XAS), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). This suite of methods enables both surface and bulk properties of the iron phases to be probed. These findings will then be correlated to the magnetic behavior of the materials, as determined by magnetometry. The study will establish how the magnetic properties of the Fe film evolve from the initial nucleation of 20 nm cubic crystals to more continuous 200 nm thick films.
has been used to modify the properties of carbon nanostructures as it is a low temperature, cost-effective, scalable, high throughput processing method suitable for industrial environments.31 The room temperature synthesis of magnetic nickel coatings on CNTs using electrodeposition has been reported, showing that the magnetic responses of the material could be tuned within large ranges in terms of saturation magnetic field and coercivity.31 It should be noted, however, that very little work has examined the electrodeposition of metallic iron onto CNTs. Also there has been very limited study in which the magnetic properties have been directly related to the chemical structure of the metallic iron phase. We are also unaware of any work where the CNTs have been chemically functionalized with ozone prior to electrodeposition for the purpose of enhancing the magnetic properties of the CNT films and providing a chemical bond between the carbon and iron. To develop technologies and applications based on the Fe0 magnetic materials, an understanding of structural and magnetic properties of such materials is needed. As the dimension of the magnetic materials reaches nanometer lengths, studies have shown interesting size-dependent properties such as enhanced coercivity and magnetization of the nanomaterial.7 It is suggested that the macroscopic physical characteristics of the magnetic nanoparticle are dependent on the size, shape, and morphology of the constituents dispersed in a nonmagnetic medium such as carbon.7 The existing research is divided into theoretical studies, which examine atomisticscale interactions, and the bulk-scale characterization of the magnetic materials. Theoretical studies have reported that the interaction and magnetic properties of Fe atoms with a singlewalled CNT were dependent on the location of the Fe atom relative to the CNT surface32 and that the outside wall adsorption sites were most favorable.33 In addition, an examination of the interaction of Fe atoms, dimers and nanowires with a single-walled armchair CNT found that the bonding between the outside wall of the CNT was stronger than the inner wall.34 Experimental studies have shown that nanoscale-ferromagnetic particles incorporated with CNTs exhibited enhanced magnetic properties19,21,22,30,35 compared to those of the bulk material. Recently, it was shown that the graphene size and nanoparticle loading could be controlled to enhance electrical and magnetic properties of Fe/graphene nanocomposites.6 One explanation for the enhanced magnetism suggests that the spin polarized electrons from the ferromagnetic material are injected into the surrounding delocalized carbon bonds.7 However, there is limited scientific insight into the influence of the local chemical structure at the interface between the iron and carbon nanomaterial on the overall magnetism and, therefore, limited ability to design the materials for specific applications. In the current study, CNTs were functionalized using an ozone-based method, which serves a 2-fold purpose. First the ozone introduces functional groups onto the carbon nanomaterials, enabling dispersion in aqueous media and facilitating electrophoretic deposition. Second, the functional groups provide an environment for the development of a metal organic framework in which the Fe atoms are firmly anchored to the nanotube structure8 during electrodeposition.36−42 The iron will then be electrochemically deposited onto the functionalized CNTs providing a simple, economic and scalable process for the fabrication of multifunctional materials. The electrochemical process will be characterized to establish links
2. EXPERIMENTAL SECTION 2.1. Materials and Preparation. Multiwalled carbon nanotubes (MWCNTs) (CM-95, Hanwha Nanotech) were oxidized by ultrasonicated ozonolysis. Moisture-free oxygen with a flow rate of 500 mL/min was passed through an ozone generator (TG-20, Ozone Solutions) and into the aqueousMWCNT solution that was cooled at 5 °C.43 High-powered sonication used a 12.7 mm diameter horn operating at 60 W. The total treatment time was 16 h. Ozone-treated MWCNTs were deposited onto graphite discs using electrophoretic deposition (EPD) at 30 V for 5 min with a 1 g/L aqueous solution. The CNT film was subsequently electrodeposited with iron in a bath consisting of FeSO4·7H2O (15 g/L), H3BO4 (35 g/L) and sodium lauryl sulfate (0.5 g/L) using a standard three-electrode cell with graphite counter electrodes and a standard calomel reference electrode (SCE). Plating times between 1 and 90 min were used. 2.2. Spectroscopic Analyses. The iron plated samples were characterized by X-ray photoelectron spectroscopy (XPS) using a Kratos Axis Ultra spectrometer with a monochromatized Al Kα1 (1486.6 eV) X-ray source operated at 150 W with a 160 and 20 eV pass energies for survey and region spectra, respectively. All spectra were acquired using a 90° takeoff angle with respect to the sample surface. The spectrometer energy scale was calibrated using the Au 4f7/2 photoelectron peak at a binding energy of 83.98 eV. The analysis area was 700 μm × 300 μm. Spectra were quantified using Kratos XPS elemental sensitivity data after Shirley background subtraction. Atomic concentration uncertainties for all fitted spectra are estimated to be ±10% of the measured value. Depth profiles were performed using an argon ion gun operated at 4 kV and ∼5 × 10−8 Torr argon base pressure. A sample area of 3 mm × 3 mm was etched with a typical sample current of 3.3 μA. XPS analysis was performed at the center of the crater. X-ray absorption spectroscopy (XAS) experiments were performed at the wiggler XAS Beamline at the Australian Synchrotron. Spectra were acquired at room temperature at the Fe−K absorption edge using a Si(111) double-crystal monochromator. The monochromator was operated at the peak of the rocking curve (“fully tuned”), and higher harmonics were rejected using a Si (vertically focusing) and a Rh-coated (toroidal refocusing) mirror. The beam size at the sample was about 2 × 0.5 mm2 (H × V). Data were acquired either in fluorescence mode using a 100-element HP-Ge solid state detector (Canberra; 25 mm2 per pixel) or in transmission mode using standard ion chambers (Oken; U = 250 V; He flow 0.3 L/ min). Per scan, the energy step width was 3 eV below the edge, 0.3 eV around the edge (7100−7160 eV), and constant in kspace thereafter (δk = 0.035 Å−1). An iron foil mounted 13219
dx.doi.org/10.1021/jp502078t | J. Phys. Chem. C 2014, 118, 13218−13227
The Journal of Physical Chemistry C
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Table 1. Relative Percent Atomic Concentrations for Fe Electrodeposited onto Ozone-Treated MWCNTs as a Function of Deposition Time (Data Averaged over 2 Positions) As Determined by XPS (Error ±10% of the Measured Value) untreated CNTs ozone-treated CNTS Fe deposition time (min) 3 5 10
Fe
O
N
C
0.2
0.6 15.9
0.5
99.3 82.8
7.6 12.0 13.6
25.3 31.4 35.7
0.5 0.3 0.4
64.2 53.2 46.5
S
1.0 0.9 1.5
B
1.2 2.0 2.0
Si
Al
Na
0.3
0.1 0.3
0.1
0.2 0.2 0.3
between the second and third ion chamber served as reference and for energy calibration purposes (E0 = 7110.8 eV44). Ironplated CNT films were isolated from the graphite substrates and loaded onto clean PMMA sample holders to minimize Bragg diffraction from the graphite disks, which interfered with the fluorescence XAS spectra. Data were analyzed using the freeware XANDA Dactyloscope.45 Standard polynomials were used for background subtraction, and spectra were normalized to an edge jump of 1. 2.3. SEM Analysis. SEM and EDS analysis of the ironplated MWCNTS was performed on a LEO 1530VP using a 5 kV accelerating voltage with a 1.5 nm sputter-deposited iridium layer to prevent sample charging. 2.4. Magnetic Measurements. The magnetic properties of the samples were determined using an Aric JR-6 dual speed spinner magnetometer and Rema6W software program. The natural remanent magnetization (NRM; MR) was initially measured in six directions. The sample was then demagnetized at 800 mT using a Molspin alternating field demagnetizer and the remaining remanence was measured. To measure the saturation isothermal remanent magnetization (MS) and coercivity of remanence (HCr) of the samples, stepwise isothermal remanent magnetization and backfields (IRM; +20, +40, +60, +80, +100, +125, +150, +200, +250, +300, +400, +600, +800, −20, −40, −60, −80, −100, and −200 mT) were applied to the samples using a Magnetic Measurements MMPM10 Pulse Magnetizer. MS values were determined at 1T and all measurements were volume corrected. The volume was determined by measuring the mass of the deposited iron film and assuming the density was equivalent to bulk iron (7.95 g/ cm3).
Figure 1. High resolution C 1s spectra of untreated MWCNTs and ozone-treated MWCNTs.
3. RESULTS AND DISCUSSION 3.1. Ozone Functionalization of MWCNTs. Table 1 shows the relative percentage atomic concentrations of the MWCNTS before and after ultrasonicated ozonolysis. Following the treatment, the relative oxygen concentration increased from 0.6 to 15.9 atom % whereas the relative carbon concentration decreased from 99.3 to 82.8 atom % resulting in O/C ratios of 0.006 (before) and 0.19 (after). This result is consistent with previous treatments.43 Traces of iron, nitrogen, silicon, aluminum, and sodium were also detected. Deconvolution of the C 1s peak for the MWCNTS before and after ozonolysis is shown in Figure 1. The peak positions for each component were determined from literature values.43,46 The percentage of oxygen determined from the C 1s peak fitting was cross-referenced with the total oxygen measured from the survey spectrum to ensure analysis consistency. Table 2 provides the C 1s peak positions and the contribution to the total carbon signal. As expected, the predominant changes observed following ozonolysis were an increase in the alcohol, carbonyl and carboxylic acid components. There was also a
reduction in the graphite peak and the energy loss peak associated with the graphite, which is attributed to the combined effects of the sonication and ozonolysis.43 The presence of the additional oxygen containing moieties is important in that they not only improve the dispersive properties of the MWCNTS in aqueous environments but also provide alternative nucleation sites for the electrodeposition of the iron nanoparticles. 3.2. Electrochemical Deposition of Iron. To determine the optimum plating potential, cathodic scans were run using a platinum electrode to determine the electrochemical characteristics of the solution, followed by repeating the experiment on the CNT-coated surface as shown in Figure 2. These results indicated that at a plating potential of −1.4 V versus SCE that the reduction of the ferric ion to metallic iron occurs at an appreciable rate. During the electrochemical deposition of Fe, the current was continually measured as shown in Figure 3. After initial high current levels, the current stabilized near 3.4 mA cm−2 and then increased slowly until the current leveled out and subsequently 13220
dx.doi.org/10.1021/jp502078t | J. Phys. Chem. C 2014, 118, 13218−13227
The Journal of Physical Chemistry C
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Table 2. XPS Peak Fitting Results for the C 1s Spectrum for Untreated and Ozone-Treated MWCNTs C 1s peak fitting component treatment untreated ozonolysis
graphite
C−C
C−O
CO
O−CO
π−π*
284.6 eV 71.1 52.2
285.0 eV 12.0 10.0
286.6 eV 6.1 8.4
288.1 eV 2.0 4.3
289.1 eV 0.0 3.9
290.7 eV 8.8 3.9
the Fe electrodeposited onto the functionalized MWCNTs with deposition time. The growth of the Fe nanoparticles is initiated by the formation of 20 nm cubic crystals, as shown in Figure 5a. Crystal growth was greater at the outer edges of the samples. During the growth process, the small cubic crystals merge into larger grains around 100 nm in length with faceted surfaces. After 15 min deposition time, there is evidence of the formation of new seed crystals as well as continuing growth of the larger crystals as shown in Figure 5b. By 30 min, the Fe crystals have coalesced, forming a more continuous layer across the surface of the MWCNTs. Despite the heterogeneous growth rates observed in localized regions at high magnification, low resolution images indicate a consistent plating pattern over large areas of the surface (Figure S2, Supporting Information). The outer film thickness is equivalent to the size of the individual iron particles (
