journal of materials chemistry c · conditions influence iron crystallinity and oxygen levels,...

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Manipulation of carbon nanotube magnetism withmetal-rich iron nanoparticles†

N. Brack,a P. Kappen,b M. J. S. Spencer,c A. I. R. Herriesd and A. N. Rider*e

Metallic Fe nanoparticles (NPs) were electrodeposited onto ozone functionalized carbon nanotubes

(CNTs) to produce ferromagnetic carbon nanomaterials. Chemical and structural characterisation

of these nanomaterials as a function of iron deposition time was undertaken and related to their

magnetic properties. Density functional theory calculations of the Fe and CNT system were also

undertaken and showed that in the initial stages of electroplating CNT oxygen sites were favoured.

At short deposition times, individual NPs were observed. As electrodeposition continues, individual

crystalline NPs began to overlap and fine grain films were formed. It is evident that the plating

conditions influence iron crystallinity and oxygen levels, producing Fe–CNT materials with wide

ranging coercivity.

1. Introduction

The integration of iron and carbon nanomaterials is highlytopical given the new and enhanced functionalities that may beachieved by their combination. Ferromagnetic carbon nano-materials offer significant interest for many applications such ashigh-capacity hydrogen storage,1 drug delivery,2 electron transferapplications3 and electromagnetic shielding.4 In addition, theymay be suitable materials for electrocatalysts for fuel cell applica-tions, where expensive noble metals such as platinum have beentraditionally employed. Carbon nanotubes (CNTs) represent anideal template for the attachment of metallic species such asmetallic Fe as they provide nanometer dimensions, high surfacearea, high stability and control over particle size distribution.5

The metallic particles may decorate the external walls or beencapsulated within the walls of the CNTs. One of the keychallenges is to obtain an effective attachment of uniformlydispersed metallic nanoparticles (NPs) via surface modificationof the CNTs. In addition, the metallic Fe NPs can be manipu-lated by externally applied magnetic fields.

Several experimental pathways including filling, decorationand electrochemical treatments have been investigated in thedevelopment of carbon based metallic nanomaterials. Transitionmetals such as iron, nickel and cobalt have been encapsulated inCNTs using arc-discharge6–11 high temperature heat treatment,12

ion beam sputtering13 and chemical vapour deposition (CVD).14,15

The limitations of these methods include complicated experi-mental procedures, low yields, low encapsulation efficiency16,17

and poor growth control of the metal filled CNTs.5 Therefore,the successful development of these materials requires alter-native, straight-forward, high-efficiency and well-controlledpreparation methods. Wet decoration of CNTs with magneticNPs18–25 has provided sufficient saturation magnetisation formagnetic separation; however, the NPs are not strongly attachedand hence easily removed, thereby limiting their practical appli-cation. The ability to directly coat the CNT surface provides theopportunity to manipulate the CNT surface chemistry and createa strong chemical linkage with the magnetic NP.

Magnetic nanomaterials have significantly enhanced pro-perties from the bulk material due in part to the large surface tovolume ratios. Consequently, this attribute enables their useat significantly lower concentrations compared to traditionalmaterials, whilst providing similar or improved functionality.However, in the specific case of the interaction of metallic ironwith carbon nanotubes, there is limited experimental charac-terization which correlates the nanoscale structure and chemistryto the magnetic properties and, therefore, limited ability to designthe materials for specific applications. Studies have indicated thatas the dimensions of the magnetic materials reach the nanometerscale, enhanced coercivity and magnetization are observed. It hasbeen suggested that the macroscopic physical characteristicsof the magnetic nanoparticle are dependent on the size, shape,

a Department of Chemistry and Physics, La Trobe Institute for Molecular Science,

La Trobe University, Melbourne, Victoria 3086, Australiab Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3168,

Australiac School of Applied Sciences, RMIT University, GPO Box 2476,

Melbourne VIC 3001, Australiad The Australian Archaeomagnetism Laboratory, Department of Archaeology and

History, La Trobe University, Melbourne, Victoria 3086, Australiae Defence Science and Technology Group, 506 Lorimer St, Fisherman’s Bend,

VIC 3207, Australia. E-mail: [email protected]

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5tc03704b

Received 7th November 2015,Accepted 4th January 2016

DOI: 10.1039/c5tc03704b

www.rsc.org/MaterialsC

Journal ofMaterials Chemistry C

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1216 | J. Mater. Chem. C, 2016, 4, 1215--1227 This journal is©The Royal Society of Chemistry 2016

and morphology of the constituents dispersed in a non-magnetic medium such as carbon.26 The non-magnetic matrixis suggested to improve the magnetic stability of the magneticNPs by reducing the random flipping of the magnetic momentdue to thermal variations.26 Theoretical studies into the inter-action and magnetic properties of Fe atoms with a single-walled CNT have shown that the magnetic properties aredependent on the location of the Fe atom relative to theCNT surface27 and that the outside adsorption sites werethe most favourable.28 Experimental studies investigating theincorporation of ferromagnetic NPs with CNTs have reportedenhanced magnetic properties compared to those of the bulkmaterial.14,16,17,25,29 One explanation for such an improvementwas attributed to the injection of spin polarized electrons fromthe ferromagnetic material into the surrounding delocalizedcarbon bonds.26

In the current study, CNTs were functionalized using anultrasonicated ozone based method (USO)5,30,31 followed by Feelectrodeposition. Fe was selected as the preferred transitionmetal due to its superior magnetic properties over a wide rangeof electromagnetic frequencies compared to iron oxides andcarbides or nickel and cobalt and their associated alloys andalso due to the lower toxicity of Fe. The environmentally friendlyand scalable ozone treatment is performed in an aqueousenvironment and removes the use of harsh chemicals, whilstcausing minimal damage to the CNT structure (cf. strongoxidants). Ozone functionalization introduces oxygen groupswhich facilitates preparation of stable aqueous dispersions andprovision of nucleation sites that are suitable for the controlledattachment of the Fe nanoparticle onto the CNT.32–38 In thecurrent study the iron will be deposited using a novel electro-dynamic pulsed method in a low oxygen environment. It isanticipated that this approach will enable a more uniformcoating of the CNTs across the electrode surface and increasethe metallic character of the Fe NPs. The chemical and structuralproperties of the Fe-coated CNTs will be characterized by X-rayabsorption spectroscopy (XAS), scanning electron microscopy(SEM) and X-ray photoelectron spectroscopy (XPS). The results willbe correlated to the magnetic behaviour of the materials. Densityfunctional theory and ab initio calculations of the Fe and CNTsystem were also undertaken to understand likely adsorption sitesin the initial stages of electroplating and the influence of Fe–CNTinterfacial chemistry on the magnetic properties.

2. Experimental2.1 Materials and preparation

Multi-walled carbon nanotubes (CNTs) (CM-95, Hanwha Nano-tech, Korea) were oxidized by ultrasonicated ozonolysis (USO). TheCNTs have a diameter of 10–15 nm and length of 10–20 mm.39

Moisture-free oxygen with a flow rate of 500 mL min�1 was passedthrough an ozone generator (TG-20, Ozone Solutions, USA) andinto the aqueous-CNTs solution that was cooled at 5 1C. High-powered sonication used a 12.7 mm diameter horn operating at60 W. The total treatment times exceeded 30 h.

USO-treated CNTs were electrophoretically deposited (EPD)onto gold coated (B200 nm Au film) and polished graphitediscs, or gold coated (B40 nm Au film) aluminium foil (5 mm),to produce thin uniform films around 1 mm in thickness.The films were deposited at the anode using a field of5 V cm�1 and dried overnight and then at 110 1C for 60 min.The CNT film was subsequently electrodeposited with iron overa 1 cm2 area in a bath consisting of FeSO4�7H2O (17 g), H3BO4(35 g) and glycerol (204 g) in deionized water (255 g) using astandard three-electrode cell with graphite counter electrodesand a standard calomel reference electrode (SCE). The appliedpotential to the sample was pulsed at �1.6 V versus the SCEfor 0.5 s and then reduced to �0.95 V for 1 s. Solutions werenitrogen-purged for 1 h prior to plating and during the electro-deposition process. Plating times between 1 and 60 minwere used.

2.2 Spectroscopic analyses

The iron plated samples were characterized by X-ray photo-electron spectroscopy (XPS) using a Kratos Nova spectrometerwith a monochromatized Al Ka1 (1486.6 eV) X-ray sourceoperated at 150 W with a 160 eV and 20 eV pass energiesfor survey and region spectra, respectively. All spectra wereacquired using a 901 take-off angle with respect to the samplesurface. The spectrometer energy scale was calibrated using theAu 4f7/2 photoelectron peak at a binding energy of 83.98 eV.The analysis area was 700 mm � 300 mm. Spectra were quanti-fied using Kratos XPS elemental sensitivity data after Shirleybackground subtraction. Atomic concentration uncertaintiesfor all fitted spectra were estimated to be �10% of the measuredvalue. Depth profiles were performed using an argon-ion gunoperated at 4 kV and 5 � 10�8 Torr argon base pressure and acurrent density of 44 mA cm�2, which led to a calibrated sputterrate on Ta2O5 of 6 nm min

�1.40

2.3 XAS measurements

X-ray Absorption Spectroscopy (XAS) experiments were performedat the wiggler XAS Beamline at the Australian Synchrotron.Spectra were acquired at room temperature at the Fe–Kabsorption edge using a Si(111) double-crystal monochromator.The monochromator was operated at the peak of the rockingcurve (‘‘fully tuned’’), and higher harmonics were rejected usinga Si (vertically focusing) and a Rh coated (toroidal re-focussing)mirror. The beam size at the sample was about 2 � 0.5 mm2(H � V). Data were acquired in fluorescence mode using a PIPSdetector (Canberra; 5000 mm2 active area), or in transmissionmode using standard ion chambers (Oken; U = 250 V; He flow0.3 L min�1). Per scan, the energy step width was 3 eV below theedge, 0.25 eV around the edge (7100–7160 eV), and constant ink-space thereafter (dk = 0.035 Å�1) to k = 12 Å�1. An iron foilplaced between the second and third ion chambers served as areference and for energy calibration purposes (E0 = 7110.8 eV).Iron-plated CNT samples prepared on the gold coated alumi-nium foil were encapsulated in Kapton tape and loaded ontoclean PMMA sample holders to minimize Bragg diffraction.Data were analyzed using the freeware XANDA Dactyloscope.

Paper Journal of Materials Chemistry C


Standard polynomials were used for background subtraction,and spectra were normalized to an edge jump of 1.

2.4 SEM analysis

SEM of the iron-plated CNTs was performed on a LEO 1530VPusing a 5 kV accelerating voltage with a 1.5 nm sputter-deposited iridium layer to prevent sample charging.

2.5 Magnetic measurements

The magnetic properties of the samples detailed in Section 2.1were determined using an Arico JR-6 dual speed spinner magneto-meter and Rema6W software program. The sample was demag-netized at 800 mT using a Molspin alternating field demagnetizerand the remaining remanence was measured. To measure thesaturation isothermal remanent magnetization (Ms) and coercivityof remanence (HCr) of the samples, stepwise isothermal remanentmagnetization and backfields (IRM; 20 mT, 40 mT, 60 mT, 80 mT,100 mT, 125 mT, 150 mT, 200 mT, 250 mT, 300 mT, 400 mT,600 mT, 800 mT, �20 mT, �40 mT, �60 mT, �80 mT, �100 mT,�200 mT and �300 mT) were applied to the samples using aMagnetic Measurements MMPM10 Pulse Magnetizer. Ms valueswere determined at 1 T and all measurements were volumecorrected. The volume was determined by measuring the thicknessof the deposited iron coating using synchrotron X-ray fluorescence(XRF) measurements for the 1 cm2 coated surface area.

2.6 Computational details

2.6.1. Density functional theory calculations. The calcula-tions were carried out using the Vienna ab initio SimulationPackage (VASP)41–43 with the generalized gradient spin approxi-mation (GGSA), using the functional of Perdew, Burke andErnzerhof.44 The projector augmented wave (PAW)45 was usedwith a cut-off energy of 400 eV. K-space sampling was performedusing the scheme of Monkhorst and Pack46 with a k-point meshof 1 � 1 � 11.

A single walled (8,8) armchair CNT was modelled using asupercell with periodic boundary conditions. A vacuum spacerof at least 10 Å was included in the x- and y-directions toprevent interactions between periodic images. Replication ofthe supercell in the z-direction reproduces the length of theCNT. The size of the repeat unit in the z-direction was 7.4085 Å.For ozone adsorbed on the CNT, described as NT(8,8)–ozone,we used the model determined previously.47 Our calculatedadsorption energy value for this structure of �2.55 eV agreeswell with their calculated value of �2.57 eV.

To model the adsorption of Fe onto the modified CNTs, wechose to place 1 Fe atom with a zero valence state in differentsites on the NT(8,8)–ozone structure. We note that this modelsonly the very initial Fe growth process, however, we believe thatthis is an important step and understanding where the Fe willinitially adsorb, and its associated properties, is crucial to helpunderstand the overall process. The Fe atom was initiallylocated at least 3 Å above the CNT surface and during thegeometry optimisation all atoms were allowed to relax until theHelman–Feynman force on each atom was o0.01 eV Å�1 and

the energy had converged to 10�4 eV. The calculations wereperformed as spin polarized. Vibrational frequency calculationswere run by diagonalizing a finite difference construction of theHessian matrix with displacements of 0.015 Å (allowing onlythe Fe atoms to relax). All structures presented here wereconfirmed to be minima. The Bader charges on individualatoms were calculated using the procedure described byHenkelman et al.48

2.6.2. Valence band modelling. Modelling of the XPS valenceband (VB) used Gaussian 09, 64 bit, Revision D.01.49 TheMolecular Orbital (MO) model used density functional theory(DFT) with the Becke three parameter gradient corrected func-tional50 and gradient corrected correlation of Lee51 (B3YLP) andthe Dunning basis set (LanL2DZ)52 on the optimized model.Density of state (DOS) and MO compositions were calculatedusing the AOMix software.53 Calculated VB spectra were adjustedby correcting the DOS levels by Scofield photoelectric cross-sections54 according to the MO compositions at each energylevel.55,56 MO energy levels were also convoluted using aGaussian/Lorentzian peak57 with a 0.9 ratio and a 1.5 eV fullwidth at half maximum (FWHM).

3. Results and discussion3.1 Chemical and structural characterisation of Fe platedCNTs

The surface chemistry of the USO treated CNTs as a function oftreatment time is shown in Fig. 1. The high resolution C 1sspectra show an increase in the ether/alcohol (R–C–O) andcarbonyl bonding (R–CQO), with the ratio of the R–C–O bondingincreasing relative the R–CQO bonding with treatment time(refer Table S1 for compositions, ESI†). The presence of thesefunctional groups enabled improved dispersion of the CNTsunder aqueous conditions and the preparation of thin filmsusing EPD. The oxygen groups may also provide preferredadsorption sites for the Fe2+ during electrodeposition.

Fig. 1 Experimental XPS C 1s spectra of CNTs processed using the USOtreatment for periods up to 31 h, indicating the increase in the C–O andCQO functionalities.

Journal of Materials Chemistry C Paper


Table 1 shows the surface chemical change in the USO treated-CNTs with plating time, measured with XPS. The Fe–CNT sampleswere coated in a thin layer of glycerol, a component of the platingsolution, as supported by the C 1s spectrum, which consisted of asignificant C–O signal, despite extensive rinsing with acetone,alcohol and deionized water. The data shown in Table 1 corre-sponds to the analysis following a 30 s argon etch, which wassuccessful in the removal of the glycerol. Based on the calibratedetch rate, the chemisorbed glycerol layer must be less than 3 nmthick. The USO treated CNTs indicate similar Fe concentrations attreatment times less than 10 min, however, at longer treatmenttimes the relative concentration increases until at 30 min when avalue of B55 atom% was observed. Traces of boron and sulphurwere detected on all samples and this result is attributed toresidues from the electroplating bath. The XPS data is consistentwith the deposition of metallic iron with a thin oxide/hydroxidesurface layer.

3.2 Molecular modelling

3.2.1. Molecular modelling-USO treated CNTs. The USOtreated CNTs were represented by an armchair (8,8) single-walled CNT with the ozone adsorbed in the configurationdetermined previously,47 where ozone (O3) dissociates on theCNT outer-wall to give two adsorbed O atoms, with the other Oatom dissociating on a C atom and desorbing as CO (Fig. 2).This structure provides the two primary surface functionalgroups, namely a surface ether and carbonyl group that wereobserved in the C 1s spectra (Fig. 1) of the USO treated CNTs.

3.2.2. Valence band modelling. Fig. 3 shows the measuredXPS valence band spectra for the USO treated CNTs. As thetreatment time increases, the degree of oxidation and disorder

in the CNTs increases, as indicated by the increase in the O 2speak between 25 and 30 eV and the increase in intensitybetween 3 and 10 eV due to O 2p/C 2p bonding. These changescorrelate with the core-level spectra (Fig. 1). Modelling of the VBspectra, based on the MO model detailed in Fig. 2, is shownin Fig. 4. The energy and intensity of the main C sp2 and etherand carbonyl functional groups correspond well with theexperimental data in which the ratio of C–O and CQO bondingwas estimated from the component fits in Fig. 1. Whilst thepeak broadening and increased intensity between 3 and 10 eVis not captured, the similarity in the model and experimentsuggest that conclusions drawn from the MO modellingprovide a realistic description of the CNT and iron chemistryassociated with the early stage plating processes.

3.2.3. Density functional theory calculations – Fe adsorption.The preferred adsorption sites for Fe on the (8,8)-ozone CNT areshown in Fig. 5 along with the calculated properties in Table 2.

Table 1 XPS analysis of CNTs following treatment using the USO processand pulsed electrodeposition of iron NPs

Relative % atomic concentrations

Fe O C B S

CNTs — 0.7 99.3 — —USO-CNTs — 14.4 83.4 — —USO-CNT+ Fe (min)1 24.6 27.8 41.6 0.8 4.63 20.7 18.8 58.2 2.2 0.15 17.9 14.8 65.4 2.0 —10 21.9 21.0 53.0 3.8 0.415 34.8 24.5 37.9 2.0 0.830 55.3 17.2 25.2 1.9 —

Fig. 2 Optimized structures of the (8,8) armchair CNT with adsorbedozone. (C = grey, O = red; the C atoms defining the adsorption site arecoloured in pink).

Fig. 3 Experimental XPS valence band spectra of CNTs processed usingthe USO treatment for periods up to 31 h, indicating the increase in oxygenand broadening caused by increased disorder.

Fig. 4 Experimental XPS valence band spectra of CNTs processed for 0 hand 31 h USO treatment compared to the theoretical spectra calculatedfrom the 8,8 CNT MO model before and after ozone absorption, with therelative fragment contributions indicated.



The calculations show that Fe prefers to adsorb in more highlycoordinated sites, bonding to the O atom of the carbonyl groupand two C atoms of the CNT (Fig. 5a and b). The Fe–O bondlength is B1.90 Å, which is similar to the Fe–O bond lengths intetrahedral Fe3+ sites in maghemite (g-Fe2O3) of 1.88 Å and slightlyshorter than octahedral Fe3+ sites of 2.03 Å.58 The bond length isalso shorter than Fe2+ sites in wustite (FeO) of 2.15 Å. The CQObond length increases by B0.09 Å while the C–O bond of the ethergroup lengthens by B0.05 Å on the side where the Fe adsorbs.The Fe–C bond lengths are B2 Å, which is comparable to theFe–C bond length in the iron carbide, cementite (Fe3C), of 2.01 Å.

59

Overall, it is clear that the Fe bonds strongly to the CNTsurface, and is likely to lead to the formation of Fe–O bonds atthe iron plated/CNT interface. Fe also adsorbs in a doubly-coordinated adsorption site (Fig. 5c), where Fe bonds to the

carbonyl O atom and one CNT atom. For structure ‘d’ (Fig. 5d)Fe adsorbs to 3 surface atoms, but is primarily bonded to thecarbonyl O atom and one CNT atom as it was in Fig. 5c. Forstructure 5 (Fig. 5e), the Fe is only bonded to the carbonylO atom and is a less favourable adsorption site. Adsorption onthe unmodified, or clean area, of the nanotube (Fig. 5f) wasshown to be least favoured. Hence, in the experiment, it islikely that the growth of Fe NPs on the CNT surface starts on asurface carbonyl group, with the other sites or regions beingless favourable.

For all adsorption sites, the Fe is positively charged (Table 2).Its adsorption on the surface causes a redistribution of electronssuch that the adsorbed O atoms retain a negative charge whilethe nanotube gains some electrons but retains an overall positivecharge. The stability of the system decreases for adsorption siteswhich increase the positive charge on the nanotube. Further, thepresence of Fe induces a magnetic moment on all structures(Table 2). The calculations show that the main contribution tothe magnetic moment comes from the Fe 3d orbitals (Table S2,ESI†), with small contributions from the carbonyl O atom 2porbitals followed by the ether O atom 2p orbitals. When Feadsorbs on the clean regions of the CNT, the calculated magneticmoment is significantly smaller.

The values of the magnetic moments are typically around3.4 mB, which is lower than the theoretical values for Fe

2+ or Fe3+

of 4 mB or 5 mB. The magnetic moments measured for Fe2+ in

FeO range between 3.3–4.2 mB60,61 compared to a theoretical

value of 3.8–4.3 mB calculated using VASP.62 The Fe3+ ions in

octahedral and tetrahedral sites of maghemite (g-Fe2O3) haveexperimental values around 4.4 mB.

58 This suggests that thepresence of the CNT offsets some of the charge transfer thatoccurs between the oxygen and Fe atoms in iron–oxide structuresand decreases the resulting moment. It may be expected that theFe–O magnetic moment could increase as the CNT oxidationstate increases and less charge transfer from the C sp2 bondswould be available.

The magnetic moment values for the Fe–O–CNT bonding arelower than expected for Fe in the Fe2+ and Fe3+ state but higherthan bulk a-Fe, 2.2 mB (also calculated with VASP in the currentwork), and cementite, which has an experimental value of1.8 mB compared to 1.7–2 mB calculated with VASP.

63 Themagnetic moment determined for Fe adsorbed onto the graphiticarea of the CNT of 0.01 mB is significantly lower than the reportedcementite values. The result indicates that the oxygen functionalgroups have a significant effect on the Fe bonding with theC sp2 graphitic region of the CNT, dramatically reducing the dband magnetic moment. The results suggest that the magneticproperties of the CNTs could be controlled by adjusting thelevel of oxygen functionalization, with higher ferromagneticbehaviour corresponding to low functional group concentra-tions and higher functionalization levels leading to propertiesmore typical of iron oxides in ferrous (+2) or ferric (+3) oxida-tion states. It would be expected that, in the current study, withhigher concentrations of oxygen surface groups that the ironnanoparticles would form an oxide layer between the iron-coreand the CNT surface.

Fig. 5 Optimized structures of Fe adsorbed on the NT(8,8)–ozone.(C = grey, O = red, Fe = blue; the C atoms closest to the adsorbedO atoms are coloured pink).



The difference in the spin up and down components of thecalculated partial density of states (PDOS) (Fig. 6) of the moststable Fe-adsorbed structure (Fig. 5a) is also consistent with theexistence of a magnetic moment on this system. In the valenceband region around the Fermi level, it is the Fe orbitals thatcontribute most to the down states, while the up states aremainly composed of C 2p followed by O and Fe states. Theoverlapping of states from the C, Fe and O are consistent withthe formation of the strong Fe–O bond on the CNT.

3.3 Morphological characterisation of Fe nanomaterials andcoating growth

Fig. 7 shows the morphology of the Fe electrodeposited ontothe USO-treated CNTs as a function of deposition time. Withinthe first 5 min crystal growth is initiated in the formation of20 to 45 nm cubic particles spread across the CNT surface. Asthe growth proceeds, the coverage increases and the size ofthe particles increase to approximately 70 nm. After 15 mindeposition time, there is evidence of the formation of new seedcrystals amongst the larger crystals. By 30 min, the iron crystalsbegin to overlap, however, there is still evidence of seedformation within the crevices. Higher magnification imagesare shown in Fig. S1 and S2 (ESI†). Fig. S1 (ESI†) shows highmagnification images of the cubic shaped iron crystals and

their growth interaction with the CNT at 15 min growth.The cubic crystals are observed on the surface of the CNTsand within the CNT film. In our previous studies using

Table 2 Calculated properties for Fe adsorbed on the NT(8,8)–ozone

Site Relative energy (eV)

Bond length, d (Å) q (e)

mBCQO C–O–C Fe–O Fe–C Fe OCQO OC–O–C CNT

No Fe — 1.24 1.38, 1.38 — — — �1.04 �1.08 2.11 0.00a 0.00 1.33 1.38, 1.44 1.89 2.00 0.86 �1.11 �1.04 1.29 3.36

2.09b 0.01 1.32 1.37, 1.42 1.90 1.98 0.87 �1.10 �1.07 1.31 2.16

2.11c 0.07 1.32 1.38, 1.38 1.94 1.99 0.77 �1.10 �1.09 1.43 3.40d 0.09 1.32 1.38, 1.39 1.95 2.03 0.75 �1.09 �1.09 1.43 3.38

2.28e 0.62 1.30 1.39, 1.39 1.87 — 0.51 �1.14 �1.10 1.73 3.48f 1.56 1.24 1.38, 1.38 — 2.08 � 4 0.68 �1.07 �1.10 1.49 0.01

2.18 � 2

Relative energy (eV) compared to most stable structure; calculated bond lengths (d); partial charges (q); magnetic moment (mB).

Fig. 6 Partial density of states (PDOS) of the most stable structure of Feadsorbed on the NT(8,8)–ozone showing up-spin (positive) and down-spin (negative) states.

Fig. 7 Images for CNTs with iron–nanoparticle coatings produced bypulsed electrodeposition for (a) 5 min, (b) 15 min and (c) 30 min.



potentiostatic plating without glycerol,5 the rate of iron deposi-tion was similar, however, the coatings deposited by pulsedplating show a smaller and more uniform distribution of crystalsizes (Fig. S2, ESI†). This suggests that limiting the diffusion ofthe Fe2+ ions with the glycerol has reduced the growth rateduring the potential pulse at the preferred nucleation sites andassists development of new nucleation sites in subsequentpulses.64 The pulsed deposition also provided a more uniformcoating over the 1 cm2 area of the electrode, compared topotentiostatic deposition where the edges had thicker coatingthan the centre of the electrode.5

The thickness of the coatings was estimated by synchrotronXRF measurements (Fig. 8). The evolution of the Fe coating followsa linear growth pattern if it is assumed the measured areal densityof the iron corresponds to a continuous iron layer with a bulkdensity of 7.874 g cm�3. The crystal sizes appear to reach limitinglength around 70 nm, estimated from the SEM images (Fig. 7).Based on the growth rate of approximately 9 nm min�1 (Fig. 8),then it would take approximately 8 min for a single crystal of 70 nmto grow. For growth times beyond 8 min, it is possible for newcrystals to begin to grow on existing crystals, leading to multiplecrystal layers. Clearly, this is an approximation as the SEM imagesshow the iron deposits as discreet crystals and growth is nothomogeneous, despite improvements provided by the pulsingtechnique. This contrasts with the potentiostatically grown coat-ings, where growth continues linearly from the initial nucleationsite (Fig. S1, ESI†) and a coating at 30 min comprises single-overlapping crystals around 200 nm in length and thickness.

The elemental composition of the iron coating was furtherinterrogated and Fig. 9 shows XPS depth profiles of the 5 minand 30 min Fe–CNT samples. The 5 min sample (Fig. 9a) showsan initial rapid decrease in the relative oxygen concentrationand increase in the relative iron concentration in the first 5 nmof etching, indicating the removal of surface oxides and theelectrolyte residue. The iron concentration begins to decline at45 nm, which would correspond with the removal of the Fecrystals seen in Fig. 7a and the average thickness estimated

in Fig. 8. The O concentration is between 5 and 10% from5 to 45 nm and is associated with the residual oxygen withinthe Fe coating, oxygen from the functionalized CNTs and theinterfacial iron–oxide layer that is predicted to form from themolecular modelling (Table 2).

The 30 min Fe–CNT sample (Fig. 9b) shows similar initialtrends to the 5 min sample, with the rapid removal of the outeroxide layer and the higher iron concentration corresponding tothe higher surface coverage observed in Fig. 7c. The oxygenconcentration is lower than the 5 min sample throughout theprofile and is consistent with the lower surface area of the ironcrystals as they begin to overlap, reducing the total oxidecoating coverage. The lower oxygen concentration is alsoassociated with the reduced quantity of CNTs in the analysisvolume, due to the greater coverage from the thicker ironcoating. The rapid reduction in oxygen concentration alsosuggests that an oxide layer forms on the crystal surfaces postgrowth, indicating that the iron crystals could be composed ofa metallic core with a thin oxide sheath less than 5 nm thick.The decrease in the iron signal at 55 nm would correspondto the removal of the fresh crystal layer that had nucleatedfrom the CNT surface between the thicker coating areas.Support for this conclusion is provided by the high resolutionFe 2p spectra (Fig. 10) for the 30 min sample. Prior to etching,Fe3+ iron species dominate (B711.2 eV) with a small con-tribution from Fe0 species (B707.0 eV). At 6 nm depth, thepeak at 711.2 eV is removed, indicating the removal of theoxide layer and exposure of the underlying elemental Fespecies. It should be noted that some oxide reduction causedby the Argon ion sputtering process will also occur during thedepth profiling.

Fig. 8 Thickness of the iron nanoparticle coating deposited onto CNTs asa function of pulsed electrodeposition time as determined from SynchrotronXRF measurements.

Fig. 9 XPS depth profiles through CNT films coated with iron NPs usingpulsed electrodeposition for (a) 5 min and (b) 30 min.



Further chemical characterisation using the XANES part ofthe spectrum was undertaken to determine the bulk chemicalcharacteristics of the Fe electrodeposited coating and provide across-reference with the XPS data. Fig. 11 shows XANES data ofthe Fe electrodeposited on CNTs as a function of depositiontime, Fe electrodeposited onto a thin Au film and a referencespectrum of Fe foil. The spectrum of the sample obtained after5 min deposition indicates the presence of a mixture of metallicFe0 and Fe3+ species with about 50% Fe0 content.

The spectrum consists of a broad white-line with somestructure above the absorption edge. The presence of the peakat 7220 eV photon energy is consistent with a prominent featurepresent in the spectrum of metallic iron (see top spectrum inFig. 11). As the deposition time was increased, the XAS pre-edgeand post-edge structures evolved to become increasingly similarto those of metallic iron (see also the extracted EXAFS functions,w(k), in Fig. 12). By 15 min deposition, the XAS data is consistentwith a predominantly metallic iron coating. By applying a linear

combination fit to the XANES data, using reference spectra(Fe metal and FeOOH), the relative ratio Fe0/Fe3+ as a functionof electrodeposition time was estimated (Fig. 13). The data showthat after 30 min deposition time approximately 80 mol% of theiron is present as Fe0. This result is similar to that observed for30 min potentiostatic Fe electrodeposition.5 Previously, however,the authors reported o20% Fe0 for a 10 min potentiostaticdeposition, whereas in this study for 5 and 15 min plating,values of 50% and 70% are noted. The data suggests that theelectrodynamic plating technique adopted produced Fe crystalswhich were more metallic in nature for shorter deposition times.It is concluded that the pulsed electrodeposition, combined withnitrogen purging, produces coatings which are more uniform incoverage than iron crystals potentiostatically deposited withoutN2 purging.

In Fig. 12 it can also be observed that the EXAFS signalamplitude increased with increasing plating time, indicatingthat either the degree of short-range atomic order or the particle

Fig. 10 High resolution Fe 2p spectra for 30 min Fe electrodepositedonto functionalized CNTs as a function of etch depth calibrated to a rateof 6 nm min�1 on Ta2O5.

Fig. 11 X-ray absorption spectroscopy data for ozone-treated CNTs afterpulsed electrodeposition of iron NPs for times between 5 and 120 mincompared to Fe foil and Fe electrodeposited onto gold.

Fig. 12 Chi plots for ozone-treated CNTs after pulsed electrodepositionof iron NPs for times between 5 and 120 min compared to Fe foil andFe electrodeposited onto gold.

Fig. 13 Elemental Fe (Fe0) and Fe3+ film composition as a functionof electrodeposition time as determined by a linear combination fit ofXAS data.



size increased. Fig. 7 shows that the average crystal size reached amaximum around 70 nm after 15 min and continued growth sawnew crystals nucleating, therefore, it is reasonable to suggest thatbeyond 10–15 min of growth the increase in EXAFS amplitudewas primarily caused by increased short-range order of the ironcrystals.

For example, after 5 min plating time, Fe particles in theorder of 20 to 45 nm in size were observed. The absorption edgestep height of the corresponding XAS spectrum was found to beconsistent with this, indicating an average thickness of Fe inthe beam of about 30 nm. Approximately 50% of the iron wasfound to be in the metallic state, the other half was present asFe3+, modelled using the spectrum of FeOOH in the linearcombination fit (Fig. 13). The XAS amplitude is proportional tothe number of neighbors in the respective coordinationspheres. For the sample after 5 min plating, the XAS amplitudeis about 25% of that for bulk Fe, which is broadly consistentwith mixed Fe0/Fe3+ particles of 30 nm average size as observedusing SEM. With increasing plating time, the XAS amplitudesalso increased until, after 60 min, the amplitude nearlymatched that of bulk iron, consistent with a thick iron coatingand an average thickness of B560 nm as derived from the edgestep height of the XAS spectrum.

The combination of EXAFS, XPS depth-profiling, SEM charac-terization and molecular modelling provide insight into the ironNP growth and chemistry. The modelling suggests that the ironwill preferentially adsorb onto the carbonyl groups, followed byether and graphitic carbon. This is reflected in the growth ofthe iron coating nucleating from different locations on the CNTsurfaces with time and the new crystals forming even after30 min of growth. At the short plating times the iron forms anoxide interfacial region with the functionalized CNT and as theparticles grow the metallic contribution becomes more significant.When plating is terminated the iron NPs form an oxy-hydroxidelayer which is less than 5 nm thick. As the coatings grow and the70 nm crystals begin to overlap, the relative contribution to themetallic iron coating from the interfacial and surface oxy-hydroxidelayer decreases and the metallic character dominates.

3.4 Magnetic properties

The magnetic properties measured at room temperature for the Fetreated CNTs or gold-coated graphite are shown in Table 3. As thedeposition time increases, there is a general decrease in thecoercivity of remanence (HCr), whereas the saturation magnetisa-tion (Ms) values are similar for the CNT samples. The Ms value forthe 5 min treatment was not adjusted for Fe0 content due to thelarge error associated with the calculation for the smaller diameterFe particles, which was approximately 35% if the film thickness,chemical composition and surface homogeneity are considered.Errors for the thicker films are estimated at 10–15%.

Fig. 14 shows the magnetic properties of the iron electro-deposited onto CNTs (Table 3) using the pulsed plating method(filled red square). These results are compared to our previousstatic Fe plating study onto CNTs5 (red square symbols). Inaddition, data is included for Fe plated dynamically and staticallyonto a gold substrate (blue unfilled and filled triangles respectively)

to highlight the difference in magnetism for discontinuous(Fe–CNTs) and continuous (Fe–Au) films. The current experi-mental data is shown in the context of magnetic propertiesreported in the literature for NPs prepared using a range ofmethods; including inert gas condensation (IGC)66–72 aerosol-assisted pyrolysis of iron salt73 and ball-milling.74 Additionaldata for iron oxide75–77 and hydroxide NPs78,79 has also beenincluded to provide reference to possible oxide films present onthe iron–CNT samples.

Table 3 Room temperature saturation magnetisation (Ms) and coercivityof remanence (HCr) values for Fe electrodeposited on USO treated CNTs

Fe deposition time, min Msa, emu g�1 Ms

b, emu g�1 of Fe0 HCrc, Oe

5 97.8 — 52715 42.6 96.1 50330 55.3 95.8 31960 81.8 108.5 29030 (Au) 136.4 180.9 107a-Fe 217.365 — 0.9

a emu g�1 = (A m�1)/(7.874 (g cm�3) � film volume (cm3) � 1000).b emu g�1 due to Fe0 using Fe0/Fe3+ from Fig. 13. c Oe = 79.6 A m�1.

Fig. 14 (a) The coercivity (HCr) and (b) magnetic saturation (Ms) for Fe0

dynamically ( ) and statically ( ) plated onto CNTs and planar Fe0 filmsplated dynamically ( ) and statically ( ) onto gold (Au) compared to ironNPs prepared using techniques such as inert gas condensation,66 aerosolpyrolysis of salt solutions73 and ball-milling.74



The HCr results from the literature show the trends for isolatediron NPs66,67 and fine grain films68 or large particles with finegrain structure.74 The maximum HCr in isolated particles or filmsis B23 nm or B35 nm, respectively, which corresponds to singledomain particle size for a-Fe in the different environments. Thedata from the current Fe–CNT work is plotted on the basis of theaverage coating thickness for the 5 min treatment of 44 nm andaverage crystal size of 70 nm for the longer treatments, estimatedfrom coating thickness measurements (Fig. 8) and SEM (Fig. 7).Data from our previous work using Fe potentiostatic-plating ontoCNTs is also included.5

The decrease in HCr with plating time corresponds to theincreasing overlap of the isolated particles leading to a perco-lating network as the particles produce a semi-continuouscoating. The lower HCr for the iron plated onto the planar goldsurface (Au) for 30 min is due to the more compact coating(Fig. S3, ESI†) and is similar to the maximum HCr measured forfine-grain evaporated Fe-NP films68 or ball-milled fine grainFe–FeO powders.74 The HCr for the 30 min Fe coating on the Ausurface (107 Oe) may indicate a finer grain size than estimatedfrom the bulk-crystal sizes for the electroplated iron and couldbe closer to the 35 nm corresponding to the ferromagneticexchange length in fine grain films, which also have a maximumvalue around 100 Oe.

The HCr values for the pulse-plated Fe are all lower than inour previous work using potentiostatic plating.5 In the presentwork the use of nitrogen bubbling has helped to reduce theoxygen content of the iron particles, particularly at the shorterplating times. At the longer plating times the more uniformcoverage of iron particles, facilitated by the pulsed electrode-position, also leads to more percolating networks, which wouldalso reduce HCr. Work examining the electrodeposition ofFe–Co films indicates precipitation of iron hydroxide at thegrain boundaries can provide sites for domain wall pinning,leading to increased HCr.

80 The XPS depth-profiling (Fig. 9)suggests that the electroplated iron contains residual oxygen,which may contribute to the higher HCr values. However, thesimilarity in the planar-iron films deposited on the Au surfaceusing the N2 purged-pulsed or unpurged-potentiostatic plating(Fig. 14), suggest that grain boundary oxides would not makethe major contribution to the significantly larger HCr values foriron-plated CNTs.

The HCr values for the Fe–CNT samples are more likely tobe correlated with the size of the iron particles. The high HCrvalues observed in the potentiostatic Fe–CNT samples (Fig. 14)may indicate a ferromagnetic Fe-core surrounded by an oxideshell. The iron-core diameter would be close to the exchangelength at the highest HCr values. The lower HCr for the pulsedsamples would correspond to a reduced oxide thickness andlarger Fe-core diameter, greater than the iron exchange length.The overall data in Fig. 14 also shows that the nature of theoxide shell does not appear to affect HCr at room temperature.In the current work the oxide appears to be typical of anti-ferromagnetic FeOOH (Fig. S4, ESI†) as determined in ourprevious work.5 In Fig. 14 particles with thick oxide shells offerrimagnetic Fe2O3/Fe3O4

66,70–72 or paramagnetic FeO74 all show

similar results to Fe NPs of similar dimensions with thin oxideshells.66,67,71 Similar trends in HCr are also reported for polymer-coated Fe NPs, with maximum HCr corresponding to an iron coreof 20 nm.81 The explanation for maximum HCr for the polymer-coated Fe NPs was also based on the relationship betweenparticle volume and blocking of free rotation of spins. HCr forthe 30 and 60 min samples are also similar to those reported forcarbon-coated Fe NPs prepared by carbon arc, combustionsynthesis or high pressure CVD82,83 also indicating the ironparticle size is the main factor influencing HCr. The electroplatingtechnique shows that the HCr of the Fe–CNTs can be adjustedover a large range by altering experimental conditions.

The Ms values for the Fe-plated CNTs show similar valueswhen corrected for the Fe0 content and are 45% the value forbulk iron (217 emu g�1). The Fe-plated Au surface has a valuearound 80% of the bulk iron and is similar to IGC Fe-NPs witha diameter of 40 nm,67 ball-milled iron with a grain size of20 nm,84 5 nm Fe NPs in silica73 or 30 nm amorphous ironNPs.85 Typically, epitaxially grown Fe films have values of Msclose to bulk-iron once the thickness exceeds 100–200 nm.65,86

Whilst the results in Fig. 14 show that there is a dependence ofMs with particle/crystallite size, it is clear that absolute valuesare very dependent on the system chemistry. Fe NPs o10 nm ina silica matrix73 and 10 nm Fe grains embedded in FeOparticles,74 both have values close to bulk iron, whereas IGCFe-NPs with a diameter of 8 nm67 have an Ms of 96 emu g

�1 andFe-NPs between 10 nm72 and 30 nm70 diameter, with oxide sheaths,have Ms values around 50 emu g

�1. Similarly, 70 emu g�1 wasreported for carbon coated iron NPs with a diameter range of20–80 nm.82,83

Given the 30 min electrodeposited planar-films show goodlong-range order (Fig. 12), it seems reasonable to assume thereduction in the film Ms is affected by oxygen incorporated withinthe coating, at grain boundaries80 and as surface oxy-hydroxidelayers, where the iron spins would be pinned and their netmagnetization would be zero. Support for this is also providedby the lower Ms measured for the Fe-coating potentiostaticallydeposited without nitrogen-purging,5 where Ms was around30% lower than the nitrogen-purged Fe-coating and coatingoxygen levels were higher.

The iron-plated CNTs show a similar trend to the planar-Fefilms, with the nitrogen purged samples showing Ms is approxi-mately 100 emu g�1, which is about 50% higher than the filmsdeposited without purging (Fig. 14). The iron NPs formed onthe CNTs show lower levels of consolidation compared to theplanar films, even after 30 min treatment (Fig. S3, ESI†), whichwould increase the contribution of surface effects on theMs when compared to the planar films. These effects wouldinclude the FeOOH layers and the influence of a residualglycerol layer on the crystallite’s iron core.

The Ms reduction would also include a contribution fromthe Fe–O bonding between the CNT and Fe. The computermodelling (Table 2) shows that the bonding through the Fed orbital is in the direction of the oxygen group, which willeffectively limit the ability of the Fe atom to respond to themagnetic field. The pinned spins where the Fe bonds to the



O atoms of the CNTs would lead to zero magnetization of theinterfacial iron atoms. In a similar way the modelling indicatedthat the iron atoms prefer to bond in higher coordinationenvironments (Fig. 5), which led to a broadening of the densityof states (Fig. 6), reducing the net-moment of the iron atom.A similar effect is created by the formation of the oxy-hydroxidelayer as well as the adsorption of the glycerol. Previous workon the adsorption of alcohol onto Fe NPs showed that the Msvalues were reduced the most by short chain molecules.87

In the present work incorporation of glycerol within the Feparticles could have a similar effect, but greater reduction inMs would be expected for the smaller particles produced atshort treatment times.

The factors leading to lower Ms would be expected to begreater for the shorter treatment times, when the surface areato volume ratios are greatest for the electrodeposited iron NPs.However, the similarity in the corrected Ms values for 15 to60 min CNT treatments, suggest that the exchange couplingenergy develops quickly between the Fe crystallites that affecthigh-field magnetization.

4. Conclusions

Electrodeposition of metallic iron onto ozone-functionalizedCNTs produces ferromagnetic materials with properties rangingbetween isolated NPs and fine-grain films. At short treatmenttimes, iron-crystal growth is favoured at CNT oxygen sites.Subsequently, Fe NPs with iron-core diameters near the magneticexchange length form. Modelling indicates that the interfacebetween the Fe NP and CNT is characteristic of iron oxidebonding with a magnetic moment that would be affected by thelevel of CNT oxygen functionalization. As electrodepositioncontinues, individual crystalline NPs begin to overlap andpercolating networks develop. The coercivity of the Fe–CNTmaterial is mainly affected by particle size at short treatmenttimes and then more by crystallite size as the coating formsnetworks. Plating conditions affect iron crystallinity and oxygenlevels. Consequently, it is possible to produce a wide range ofcoercivities which vary between values found in soft and hardmagnetic materials.

Ms values for the Fe–CNT material show a greater sensitivityto the iron crystallinity and oxygen levels and are only 45%of the bulk Fe value. The similarity in ferromagnetic ironcontribution to Ms values for CNTs electroplated over a widerange of treatment times suggest that weakened magnetism isaffected by oxygen distribution within the iron coating. Theoxygen-rich phase at particle and/or grain boundaries andthe Fe–CNT interface could provide pinning sites for magneticdomain wall motion during magnetization. Further reductionin oxygen levels within the coating will provide improved abilityto increase crystallinity and magnetism of the Fe–CNT material.

The unique properties of the iron–carbon nanomaterialspresented in this work and the ability to tune their magnetismcould lead to applications as electrocatalysts in fuel cellsor for hydrogen storage. The stable bonding and magnetic

permanence may also be beneficial for remediation of waterspolluted with toxic metals.

Acknowledgements

Computational facilities are gratefully acknowledged from thefollowing facilities: the National Computational Infrastructure(NCI) through the National Computational Merit AllocationScheme supported by the Australian Government, the V3Alliance (formerly VPAC – the Victorian Partnership forAdvanced Computing), the Multi-modal Australian ScienceSImaging and Visualisation Environment (MASSIVE), the PawseySupercomputing Centre (formerly iVEC). XAS was undertakenon the wiggler XAS beamline at the Australian Synchrotron,Victoria, Australia. The authors thank Dr Steve Burke (DSTGroup) for valuable discussions on magnetic properties ofmaterials.

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