Enhanced thermal stability of phosphate capped magnetite nanoparticlesT. Muthukumaran and John Philip

Citation: Journal of Applied Physics 115, 224304 (2014); doi: 10.1063/1.4882737 View online: http://dx.doi.org/10.1063/1.4882737 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/115/22?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Investigating thermal stability of structural defects and its effect on d0 ferromagnetism in undoped SnO2 J. Appl. Phys. 113, 244307 (2013); 10.1063/1.4812382 Nanomagnetic chelators for removal of toxic metal ions AIP Conf. Proc. 1512, 440 (2013); 10.1063/1.4791100 A facile approach to enhance the high temperature stability of magnetite nanoparticles with improved magneticproperty J. Appl. Phys. 113, 044314 (2013); 10.1063/1.4789610 High temperature phase transformation studies in magnetite nanoparticles doped with Co2+ ion J. Appl. Phys. 112, 054320 (2012); 10.1063/1.4748318 Rapid mixing: A route to synthesize magnetite nanoparticles with high moment Appl. Phys. Lett. 99, 222501 (2011); 10.1063/1.3662965

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Enhanced thermal stability of phosphate capped magnetite nanoparticles

T. Muthukumaran and John Philipa)

SMARTS, Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, Kalpakkam,Tamil Nadu-603 102, India

(Received 21 March 2014; accepted 29 May 2014; published online 11 June 2014)

We have studied the effect of phosphate capping on the high temperature thermal stability and

magnetic properties of magnetite (Fe3O4) nanoparticles synthesized through a single-step

co-precipitation method. The prepared magnetic nanoparticles are characterized using various

techniques. When annealed in air, the phosphate capped nanoparticle undergoes a magnetic to

non-magnetic phase transition at a temperature of 689 �C as compared to 580 �C in the uncoated

nanoparticle of similar size. The observed high temperature phase stability of phosphate capped

nanoparticle is attributed to the formation of a phosphocarbonaceous shell over the nanoparticles,

which acts as a covalently attached protective layer and improves the thermal stability of the core

material by increasing the activation energy. The phosphocarbonaceous shell prevents the intrusion of

heat, oxygen, volatiles, and mass into the magnetic core. At higher temperatures, the coalescence of

nanoparticles occurs along with the restructuring of the phosphocarbonaceous shell into a vitreous

semisolid layer on the nanoparticles, which is confirmed from the small angle X-ray scattering,

Fourier transform infra red spectroscopy, and transmission electron microscopy measurements. The

probable mechanism for the enhancement of thermal stability of phosphocarbonaceous capped

nanoparticles is discussed. VC 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4882737]

I. INTRODUCTION

Nanoparticles are at the helm of advanced materials

because of their superior physiochemical properties compared

to their bulk counterparts due to the large surface area to vol-

ume ratio and quantum size effects. Bare nanoparticles are

known for their instability towards oxidation and agglomera-

tion owing to their excess reactivity due to unsaturated sur-

face bonds.1 Therefore, new strategies are being employed to

improve the stability of nanoparticles against agglomeration

and oxidation. Capping of nanoparticles with suitable mole-

cules or providing a protective shell on nanoparticles are

some of the approaches used in overcoming the oxidation,

agglomeration, and low thermal stability. Upon functionaliza-

tion of inorganic nanomaterials, the inorganic-organic inter-

actions can affect the structural and magnetic properties of

magnetic nanoparticles.2,3 The selection of a particular sur-

factant or organic or inorganic material as a coating agent

depends on its end use.4,5 Recently, it has been established

that the bonding with an oxygen atom of the capping agent

on iron oxide nanoparticle results in a surface O�Fe atomic

configuration, providing a bulk-like environment in the sur-

face layer and an enhanced magnetism.6 The “electron mag-

netic chiral dichroism” studies combined with density

functional calculations provided conclusive evidence for a

strong covalent bond between the organic acid and the nano-

particle surface that contributes to a chemical passivation of

the surface and enhanced magnetization, compared to bare

nanoparticles, due to changes in the crystal environment and

the occupancies of d orbitals in certain surface iron sites close

to the bulk Fe3O4.6 It has also been shown that the nature of

linking of surface layer on nanoparticles affects the spin cant-

ing. For example, carboxylate coupling induces a spin canting

on iron oxide nanoparticles while phosphonated molecules do

not.7

Among the magnetic nanoparticles, magnetite and

maghemite are widely used materials because of their inter-

esting magnetic properties.8 Oleic acid is widely used as a

capping agent for magnetite nanoparticles. The phosphate

ions have strong affinity to bind on metal oxides and such

phosphate coating acts as a corrosion/oxidation inhibitor.7,9–11

The flame retarding nature of phosphate groups has been

exploited to enhance the thermal stability of TiO2, where the

phosphate coating enables preservation of the useful anatase

phase that has a better photocatalytic activity at higher

temperature.12–18 The flame retardancy of phosphate coating

has also been utilized in the textile industries.19,20 The

“oxidized shell” can lead to a decrease in the net magnetiza-

tion of oxide nanoparticle due to the surface spin disorder that

arises from the reduced coordination of surface cations and

broken exchange bonds between the surface spins. In general,

the magnetite nanoparticles are converted to a nonmagnetic

phase at �500 �C.21 Therefore, “new strategies are required

to produce magnetite nanoparticles” with enhanced thermal

stability.22 Here, we investigate the high temperature thermal

stability of technologically important magnetite phase after

capping the nanoparticles with a phosphate layer coating, for

the first time.

II. EXPERIMENTAL METHOD

A. Chemicals

All chemicals used were GR grade and used without fur-

ther purifications. The salts FeSO4.7H2O, FeCl3.6H2O, and

30% aqueous NH3 were procured from E-Merck. Phosphoric

a)Author to whom correspondence should be addressed. E-mail:

philip@igcar.gov.in. Fax: 00 91-44-27450356. Tel.: 00 91 44 27480232.

0021-8979/2014/115(22)/224304/9/$30.00 VC 2014 AIP Publishing LLC115, 224304-1

JOURNAL OF APPLIED PHYSICS 115, 224304 (2014)

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acid, Ethanol, Isopropanol, HCl, and NaOH were procured

from S.D. Fine Chem. Ltd. Ultrapure water (resistivity �15

MX) obtained from Elga, filtered through 0.22 lm filter, was

used for all the experiments.

B. Synthesis of phosphate coated Fe3O4 (FePh)nanoparticles

The FePh nanoparticles were synthesized by co-precipi-

tation.23 The 1:1 ratio fresh aqueous solutions of 0.2M

FeCl3.6H2O and 0. 1M FeSO4.7H2O were mixed and stirred

at 800 rpm for 5 min. at a temperature of 60 �C. Then, the

stirring rate was raised to 1500 rpm, followed by the rapid

addition of aqueous ammonia. The solution was digested for

10 min. at 80 �C and then 25 ml of orthophosphoric acid

(88%) was added into the mixture. After adjusting the pH to

�9.5, the solution was incubated for 1 h. The settled par-

ticles were separated by decanting. The above prepared FePh

particles were vacuum dried. Before coating, a small fraction

of uncoated magnetite particles was separated from the reac-

tion vessel and washed with excess of water. It was then

used to compare the properties with the phosphate capped

nanoparticles and was named as FeUc.

A Carbolite (MTF12/38/400, UK) horizontal tube fur-

nace with a ceramic work tube of length 450 mm that has a

uniform heat zone length of 140 mm was used for the air

annealing of nanoparticles. The FePh samples annealed at

different temperatures of 300, 560, 575, 590, 600, 650, and

900 �C are hereafter called as FeP300, FeP560, FeP575, FeP590,

FeP600, FeP650, and FeP900, respectively. Annealing was done

at a heating rate of 10 �C/min with a dwell time of 1 h.

C. Characterization techniques

The powder XRD pattern was recorded by using Rigaku,

Ultima IV system that employs a Cu-Ka (1.5418 A) radiation

source. The 2h data were measured for a range of 20�–80�

with a scan rate of 2� per min. and step size of 0.02. The crys-

tallite size (d) was determined using the Scherrer formula,

d ¼ kkb cos h where, k¼ 0.89, k¼ 1.5418 A, b is the peak width

at half maximum of the highest peak (FWHM); h is the half

of diffraction angle. The Small angle X-ray scattering (SAXS)

analysis was carried out using Rigaku ultima IV instrument in

the transmission geometry to study the morphology, shape,

and average size of the particles. The scattering intensity I(q)

was measured as a function of scattering vector (q ¼ 4psinhk ).

The scattering intensity plot was fitted with spherical and core

shell models to find the size distribution of the particles and

the shell thickness. Transmission Electron Microscopy (TEM)

investigation was performed using a JEOL 2011 TEM at an

acceleration voltage of 200 kV. A drop of magnetite dispersed

in isopropanol was placed on a carbon coated copper grid

(0.3 cm diameter, mesh size of 200 holes/cm) and dried over-

night at room temperature. The morphology of the nanopar-

ticle was obtained from the images using the Image Pro Plus

software. The infrared spectra of the air annealed samples

were obtained for the wavenumber range of 480–3600 cm�1

using ABB Bomem MB 3000 instrument. The samples were

mixed thoroughly with KBr powder (1.5 wt. %) in an agate

mortar and pressed to form a transparent pellet to carry out

the spectral scan. The thermal stability of FePh and FeUc was

also analyzed using a Simultaneous Thermo Gravimetric

Analysis—Differential Scanning Calorimetric system,

TGA-DSC-1, 1100LF (Mettler Toledo) for the temperature

range of 30 �C to 1000 �C with a heating rate of 10 �C/min. in

the oxygen atmosphere. Magnetic measurements of the air

annealed samples were carried out using a Cryogen free

Vibrating sample magnetometer of Mini VSM (Cryogenics

Ltd. UK) at room temperature (300 K) under the applied mag-

netic field range of �2 to þ2T. The hydrodynamic size distri-

bution and zeta potential were obtained for all the air

annealed samples dispersed in water by a Zeta nanosizer

(Malvern ZEN3600, UK), which employs the non-invasive

back scatter technology based Dynamic light scattering (DLS)

for particle size measurement. DLS makes use of Brownian

motion of the suspended particles to measure their size and

distribution. The hydrodynamic radius (RH) of the solute par-

ticles was calculated from the diffusion coefficient (D)

involved in the Stokes–Einstein equation RH¼ kBT6pgD. Here, kB

is the Boltzmann constant and g is the viscosity of the solvent.

The microstructure of water dispersed air annealed samples

under magnetic field was studied using an inverted phase con-

trast microscope of DMIRM from Leica.

III. RESULTS AND DISCUSSION

A. XRD, SAXS, and TEM analysis of air annealed FePh

samples

The XRD pattern of air annealed FePh nanoparticles are

shown in Fig. 1. For samples annealed up to 575 �C, the dif-

fraction patterns show peaks corresponding to the (220),

(311), (400), (422), (511), and (440) planes of magnetite. The

(311) peak was the one with the highest intensity. It should be

noted that the c-Fe2O3 (maghemite) phase also shows a simi-

lar XRD pattern with a slightly different lattice parameter.

For stoichiometric magnetite and maghemite, the reported

values of lattice constants are 0.839 and 0.8346 nm (JCPDS

file 39 1346), respectively. Magnetite has an inverse spinel

structure where the Fe2þ ions in octahedral (Oh) sites are sen-

sitive to oxidation. During the process of oxidation of Fe2þ

into Fe3þ (maghemite formation), vacancies are formed,

which can be randomly distributed or partially or totally or-

dered. Maghemite displays superstructure due to cationic and

vacancy ordering. Upon oxidation of magnetite, the bulk is

subjected to an increasing compressive stress leading to a stoi-

chiometric magnetite core surrounded by an oxidized shell

layer of Fe2O3. Because of the identical diffraction patterns, it

is usually difficult to distinguish magnetite and maghemite

from XRD.24 In fact, maghemite has a slightly lower satura-

tion magnetization compared to magnetite. Mossbauer spec-

troscopy and FTIR are being used for distinguishing

magnetite from maghemite. However, both magnetite and

maghemite are suitable for technological applications as both

exhibit superparamagnetism below�15 nm.

For the sample annealed at 590 �C (FeP590), the XRD

signatures of rhombohedral haematite (a-Fe2O3) phase has

emerged along with the cubic maghemite phase (c-Fe2O3),

i.e., the additional diffraction peaks from the planes (104)

and (024) of a-Fe2O3 are observed. For the sample FeP650,

224304-2 T. Muthukumaran and J. Philip J. Appl. Phys. 115, 224304 (2014)

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the intensity ratio of peaks (104) and (110) is �100:87.5,

which indicates that the phase conversion of c-Fe2O3 to

a-Fe2O3 is not complete even at 650 �C. For a-Fe2O3, the

reported ratio is 100:70.25 The incomplete conversion was also

evident from the magnetic nature of FeP650 (Ref. 26) with a re-

sidual magnetization of �7 emu/g. The high temperature

annealing studies in uncoated Fe3O4 nanoparticles (prepared

using ammonia) showed that the maghemite phase is converted

to a-Fe2O3 below 600 �C.24 For the sample FeP900, the inten-

sity ratio from the planes (104) and (110) was 100:71, indicat-

ing a complete conversion from cubic magnetite/maghemite to

rhombohedral hematite phase. The peaks in the diffraction pat-

tern of FeP900 corresponds to the (012), (104), (110), (113),

(024), (116), (018), (214), (300), (208), (010), and (220) planes

of hematite JCPDS card No. 89-2810. The FeUc nanoparticles

prepared also showed a phase transition from cubic to rhombo-

hedral hematite phase at 600 �C (Ref. 25) and for bulk it was

between 500–600 �C.25,27 Therefore, our studies reveal that the

phosphate coating augments the phase transition temperature

of FePh nanoparticles beyond 650 �C.

Upon increasing the annealing temperature, a diffraction

peak arises at 24.3� in FeP650 and FeP900, which corresponds

to the (012) peak of a-Fe2O3. Earlier studies reported that a

peak at 19.6� occurs at room temperature due to the phospho-

carbonaceous char that is shifted to 24.6� as the sample is

annealed to 650 �C. This shift is due to the change in the

interplanar distance of carbon char (shell) from 4.53 to

3.62 A at high temperature. For pure graphite, the interplanar

distance is 3.35 A.28 This confirms the presence of an ordered

phosphocarbonaceous char (phosphate, carboxylic acid, its

derivatives, alkyl, etc.) shell on the nanoparticles, which

gives rise to the observed higher thermal stability of nanopar-

ticles.28 For sample annealed at a higher temperature, few

minor peaks are observed, which are due to the ordering and

modifications of carbon and phosphates in the char shell with

the creation of pores during the decomposition.29,30

The variation of average crystallite size of FePh nanopar-

ticles annealed at different temperatures is shown in Fig. 2.

A rapid growth in the crystallite size is observed in FeP650

where the crystallite size was almost double than that of

FeP600 (i.e., from 10 to �21 nm). It should be noted that the

cubic phase is partially converted to rhombohedral haematite

phase at 650 �C. Further increase in the annealing tempera-

ture leads to a dramatic growth in the crystallite size due to

the coalescence of particles by solid state diffusion, which

decreases its free energy by reducing the surface area.31 The

observed increase in the intensity and narrowing of diffrac-

tion peaks with annealing temperature is attributed to an

increased crystallinity and size.24 Such a rapid growth of

a-Fe2O3 (�seven fold increase in size) upon annealing was

reported earlier.25,31 The remarkable influence of phosphorus

content over the phase transformation and growth of TiO2

was also reported earlier.12–18

The small angle X-ray scattering measurements per-

formed for the air annealed samples is shown in Fig. 3(a),

where the scattering intensity is plotted as a function of wave

vector. The scattering intensity plot was fitted with a spheri-

cal model equation

IðqÞ ¼ jDqj2 4pq3

sinqD

2

� �� qD

2cos

qD

2

� �� �( )2

; (1)

where D is the diameter of particle and Dq is the difference in

the electron density of particle and the suspended medium.

From the distance distribution function, the most probable size

of the particles was determined. The fit on the scattering data

(IðqÞ a q�4) confirms a nearly spherical shape for the particles.

The probability distribution as a function of size, obtained using

spherical model fit of Eq. (1), is shown in Fig. 3(b).

As the samples are annealed from room temperature to

600 �C (FePh to FeP600), the size of the particles increased

from 12 to 14 nm. On further increasing the annealing tem-

perature to 650 and 900 �C (FeP650 and FeP900), the crystallite

size increased to 16 and 207 nm, respectively. The observed

increase in the size with temperature is consistent with the

XRD results. These results indicate that up to 600 �C, the

phosphocarbonaceous shell is intact that prevents the oxida-

tion of magnetite by restricting the intrusion of heat and oxy-

gen. Above 650 �C, a larger expansion of phosphorus

containing charred crust (intumescence) is reported.26,32–39

FIG. 1. XRD patterns of FePh nanoparticles annealed in air at 575, 590, 650,

and 900 �C.

FIG. 2. Variation of average crystallite size (XRD) with annealing tempera-

ture of FePh nanoparticles.

224304-3 T. Muthukumaran and J. Philip J. Appl. Phys. 115, 224304 (2014)

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Such an expansion of phosphorus containing charred crust

was also reported in P-doped TiO2 and phosphate treated

pinus helepensis.13,40 The average size and maximum proba-

ble size variation of samples annealed at different tempera-

tures obtained from the SAXS data is shown in Fig. 4. The

size measured by SAXS is slightly higher than that from

XRD, due to the contributions of dead layer and the phos-

phocarbonaceous shell present on the particles. In SAXS

technique, which is routinely used for the study of shape and

size of particles, the scattering contrast mainly originates

from the difference in electron density between particles and

the surrounding medium. In the case of core-shell structures,

the contrast can be different, which provides structural infor-

mation in their scattering pattern. Such spatially non uniform

particles have an internal distribution of scattering length

densities. The air annealed FePh particles resemble core-shell

structure, with an overall form factor P(q) due to scattering

from the different density regions of the core and shell.

Usually, the “q” of organic shell will be lesser than the heav-

ier core elements. For a core shell structure,41,42

Fcore�shell q;R;Dð Þ ¼ q2 � q1ð Þ4pq3

sin qRð Þ � qR cos qRð Þ� �

þ q1 � q0ð Þ4pq3

sin q Rþ tð Þ � q Rþ tð Þcos q Rþ tð Þð Þð Þ: (2)

Here, “R” is the radius of the core with scattering length den-

sity “q2,” “t” is the shell thickness with scattering length

density “q1” in a medium of scattering length density “q0.”

This simulation model involves the subtraction of scattering

from the core with a scattering length density equivalent to

that of the shell from the amplitude of the large sphere with

a radius corresponding to the outer radius of the shell (so

that one has the scattering amplitude of the hollow shell

alone). The SAXS data fitted very well with the core-shell

model (Eq. (2)) except for FeUc, FePh, and FeP300 samples,

where there were no shell structure present or the shell thick-

ness was too small.

The average diameter, shell thickness, and maximum

probable sizes obtained from the core-shell model fit are

shown in Table I. The good fit with core-shell model further

confirms the presence of a phosphocarbonaceous shell over the

Fe3O4 core upon annealing. It is worthwhile to note that the

sum of the average diameter of the core and its shell thickness

matches well with the average diameter obtained by the sphere

model. The shell formation reported here resembles the iron

oxide @ SnO2 core-shell nanoparticles43 and microwave fabri-

cated magnetic Fe3O4/phenol-formaldehyde core-shell nano-

particles.44 The increase in the char thickness indicates a

continuous evolution of the shell with temperature.32–34,37,39

The lower shell thickness found for FeP650 and FeP900 could be

due to the higher growth of the inner metallic core with

temperature.

The TEM image of FePh shows that the particles are

nearly spherical in shape with an average particle size of

14 6 2 nm Fig. 5(a). The TEM image of the FePh annealed at

900 �C is shown in Fig. 5(b). The size obtained for air

FIG. 3. SAXS data of FePh nanoparticles annealed in air: (a) Scattering in-

tensity vs Scattering vector plot of annealed, unannealed FePh and FeUc

nanoparticles (b) Probability distribution of particles as a function of size of

annealed in air and unannealed FePh nanoparticles obtained by sphere model

fit.

FIG. 4. Influence of annealing temperature on maximum probable size dis-

tribution obtained from SAXS.

224304-4 T. Muthukumaran and J. Philip J. Appl. Phys. 115, 224304 (2014)

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annealed samples FeP590, FeP650, and FeP900 are found to be

17 6 4.7, 18 6 1.5, and 415 nm, respectively. These results

were in good agreement with the SAXS results. A shell of

phosphocarbonaceous char is clearly visible at the surface of

nanoparticles (FeP650 and FeP900 and marked by arrows in

the case of FeP900). The shell thickness observed for FeP900

was �5 nm, which was much larger than the value obtained

from the SAXS analysis.

B. Fourier transform infrared (FT-IR) spectra

The phosphate interaction with magnetite was studied

using FTIR. Figure 6 shows the FTIR spectra of FePh nano-

particles air annealed at different temperatures—300, 560,

575, 590, 600, 650, and 900 �C. A strong Fe-O characteristic

band at �584 cm�1 and a shoulder at �630 cm�1 were due to

the symmetric stretching (�) vibrations of Fe-O in the octahe-

dral and tetrahedral sites.45 A broad P-O-Fe stretching

absorption between 900–1200 cm�1 with a main peak at

�1108 cm�1 and a shoulder at �1010 cm�1 confirmed the

P-O bond stretching. The stretching and associated bending

modes of hydroxyl groups from the physisorbed water mole-

cules showed a broad absorption band in the region of

3000–3600 cm�1 and a sharp band at 1640 cm�1,46–48 respec-

tively, in the FePh nanoparticles. The band at 1404 cm�1 con-

firmed the C-O stretching of carbonates formed by the

reaction of atmospheric CO2 with alkaline hydrated surface.49

The C¼O stretching vibration observed at 2350 cm�1 was

mainly due to the intervention of atmospheric CO2.

With the increase in air annealing temperature, intensity

of the broad asymmetric stretching vibration band between

3000–3600 cm�1 and its associated bending mode at

1640 cm�1 from the hydroxyl group decreased due to a

decrease of surface OH groups and hydrogen bonds (because

of thermal dehydration)18,38 and its reaction with the surface

adsorbed carbon dioxide or carbonates.28,49 This was further

evident from the shifting of the bending mode of OH groups

from 1640 cm�1 to a lower wave number �1610 cm�1 and the

appearance of a few additional minor absorption peaks

between �1504–1465 cm�1 and 1800–1680 cm�1 in the sam-

ples annealed at temperature >300 �C.50 The intensity of C-O

stretching at �1404 cm�1 was found to decrease with anneal-

ing temperature and disappeared completely at 560 �C due to

the formation of phosphocarbonaceous shell. At 900 �C air

annealing, the main absorption peak at �1610 cm�1 and the

minor peaks between �1504–1465 cm�1 and 1800–1680 cm�1

disappeared completely, probably due to the structural rear-

rangements of char.51

Upon increasing the annealing temperature above 300 �C,

the intensity of P-O-Fe stretching band at 900–1200 cm�1 also

reduced. The main peak at �1108 cm�1 due to the nonproto-

nated phosphate complex with C3� symmetry merged with the

P-O stretching peak at �1010 cm�1 leading to a broader and

less intense peak centered at �1038 cm�1 due to the stretching

vibration of P-O-C bond50 formed from the flame retardant

phosphocarbonaceous char.28,52 Also, this P-O-C absorption

peak (peak at �1055 cm�1) becomes narrower at higher

annealing temperatures for FeP600 and FeP650. The P-O-C

absorption peak was shifted to �1090 cm�1 for FeP900 prob-

ably due to the structural rearrangement of the phosphocarbo-

naceous char51 and its binding with structurally different

phases (i.e., a-Fe2O3 above 600 and 650 �C). Interestingly, the

main bands in the 900–1200 cm�1 (signature of the phos-

phates) was seen in all the annealed samples, which showed

the strong binding of phosphates with various phases of Fe3O4.

TABLE I. SAXS analysis results: Average diameter, shell thickness, and

maximum probable size distribution of FePh with temperature.

SAXS Data

Sample

Average

core

diameter (nm)

Shell thickness

(nm)

Maximum

probable

distribution (nm)

XRD (TEM)

diameter (nm)

FeP560 13.1 1.5 9.6 9.8 6 1 (—)

FeP575 12.9 2.1 9.0 9.3 6 1 (—)

FeP590 13.1 0.9 10.0 10.5 6 1 (17 6 5)

FeP600 12.6 1.3 9.9 10.4 6 1 (—)

FeP650 15.4 0.9 13.6 21 6 2 (18 6 2)

FeP900 208.1 0.14 204.6 —(415)

FIG. 5. TEM images of (a) FePh nanoparticles and (b) FeP900 showing phos-

phocarbonaceous char over the particle (marked by arrows).

FIG. 6. FTIR spectra of FePh nanoparticles annealed in air at different tem-

peratures—300, 560, 575, 590, 600, 650, and 900 �C along with unannealed

sample.

224304-5 T. Muthukumaran and J. Philip J. Appl. Phys. 115, 224304 (2014)

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As XRD cannot distinguish the c-Fe2O3 phase from mag-

netite24 the effect of phosphate coating on the phase conver-

sion of Fe3O4 to c-Fe2O3 could not be discerned from the

XRD. Since the protective phosphocarbonaceous shell was

formed at �270 �C (Ref. 28) (i.e., before the transition of

Fe3O4 to c-Fe2O3), there is a likely possibility that the Fe3O4

phase is not converted to c-Fe2O3 phase. Mossbauer experi-

ments can only throw light on this aspect. The absorption peak

at 2350 cm�1 in all the calcinated Fe3O4 samples was due to

the interference of atmospheric CO2. However, such signatures

disappear completely in FeP900 due to the structural reorganiza-

tion of char shell that prevents the adsorption of CO2.53

Figure 7 shows the DSC curves of FePh and FeUc nano-

particles under air atmosphere. The exothermic peaks

observed at �580 and 689 �C in Feuc and FePh, respectively,

indicates the phase transition of maghemite to hematite. This

shows that the phosphate coating on FePh augments the mag-

netic to non-magnetic phase transition temperature by

109 �C. The enhancement in the thermal stability is attrib-

uted to the presence of a protective phosphocarbonaceous

shell39,49,53,54 made of phosphorus, carbon, hydrogen, and

oxygen.43 The minor endothermic peak of FePh at 858 �Ccould be due to the melting of phosphate polymorphs shell.39

This is in accordance with the earlier report of phosphate

melting at 810 �C. The difference in melting temperature

(�48 �C) in the present and earlier study could be due to the

different heating rates used. These results further corroborate

the fact that the phosphate coating extends the maghemite to

hematite phase conversion by �100 �C. However, in the case

of TGA measurements, no weight loss is observed over this

temperature range (30–1000 �C) due to the higher thermal

stability of phosphate coated magnetite.

C. Magnetic properties

Figure 8 shows the M-H curves of FePh nanoparticles

annealed at different temperatures. The Ms of FePh was

52 emu/g, which was 11.6 emu/g less than that of FeUc

(63.6 emu/g) because of the adsorbed nonmagnetic phosphate

shell in the former case. A single domain with a colossal spin

is formed below a critical particle size where a large fraction

of atoms reside at the surface of the particles with broken

translation symmetry and the magnetic behavior becomes

highly dependent on the magnetic anisotropy energy of the

individual particle and the magnetic dipole-dipole interaction

between the particles. If the magnetic anisotropy is more than

the dipole-dipole interaction, the system follows Neel-Brown

relaxation and exhibits superparamagnetism. For superpara-

magnetic Fe3O4, the magnetization can be described by a

Langevin-type law. MMS¼ LðnÞ ¼ cothðnÞ � 1

n, where n ¼l0mpH

kBT and MS ð¼ emMÞ is the saturation magnetization of

Fe3O4, l0 is the magnetic permeability of vacuum, mp is the

magnetic moment of the particle, H is the applied field, kB is

the Boltzmann constant, T is the temperature, em is the mag-

netic volume concentration or magnetic packing fraction, and

M is the spontaneous magnetization of Fe3O4 nanoparticles.

For the air annealed samples FeP300, FeP560, FeP575, FeP590,

FeP600, FeP650, and FeP900 the saturation magnetization values

are 46.5, 45, 47.8, 46.4, 44, 7.3, and 0.01 emu/g, respectively.

The gradual drop in the magnetization with increase in

annealing temperature (up to 600 �C) is probably due to the

formation of maghemite phase on the surface of particles.

The dramatic reduction in the Ms values in FeP650 and FeP900

samples is attributed to the maghemite to a-Fe2O3 phase con-

version of the inner core. The retention of magnetization in

the air annealed samples up to 600 �C confirms the extended

phase stability of FePh samples. The saturation magnetization

data further confirms that the complete conversion of magne-

tite to hematite occurs at 900 �C as confirmed by other techni-

ques.25 The saturation magnetization of FeP575 and FeP590

were slightly larger than that of the FeP560. This could be due

to the existence of magnetic order,55,56 or the effective coor-

dination of phosphate with the Fe3O4 during annealing57,58 or

due to the reduced surface spin-canting.58 This observation is

consistent with the earlier studies by Daou7 where it was

found that the phosphonate coupling agent enhances the mag-

netic properties by increasing the magnetic order in the oxi-

dized layer through super exchange magnetic interactions.

D. Water based dispersions of FePh nanoparticles

In this part, we discuss the size, zeta potential, and dis-

persion stability of water based air annealed FePh at different

FIG. 7. DSC data of FeUc and FePh nanoparticles.

FIG. 8. M-H curves of FePh nanoparticles annealed at different temperatures.

224304-6 T. Muthukumaran and J. Philip J. Appl. Phys. 115, 224304 (2014)

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temperatures with phosphate layer coating. Such phosphate

capped metal oxides and hydroxides that are stable at biolog-

ical pH have important applications in water treatment, cor-

rosion resistance, fabrics, and biomedical applications, such

as drug delivery, targeting, tumor/cancer therapy, etc.

Figure 9 shows the hydrodynamic size distribution of air

annealed FePh nanoparticles dispersed in water. The inset of

Fig. 9. shows the photograph of the corresponding disper-

sions along with FePh. The hydrodynamic sizes of disper-

sions of FeP300, FeP560, FeP575, FeP590, FeP600, FeP650, and

FeP900 are found to be 105.7, 825, 141.8, 825, 955, 164.2,

and 141.8 nm, respectively. These hydrodynamic sizes are

much larger than that of the size obtained from XRD and

SAXS analysis. This is mainly due to agglomeration of par-

ticles and the adsorbed solvent layer over the particles

because of the higher affinity of phosphate moieties on the

particles towards water molecules through hydrogen

bonds.59 The zeta potential for the samples of FeP300, FeP560,

FeP575, FeP590, FeP600, FeP650, and FeP900 are found to be

�36.6, �23.3, �24.1, �20.6, �20.2, �27, and �26.1 mV,

respectively. The zeta potential values are shown in Fig. 10.

This further confirms the continuous change in the morphol-

ogy and charge density of the phosphate or phosphocarbona-

ceous shell with annealing temperature from 300 to 900 �C.

Figures 11(a)–11(g) show the phase contrast optical mi-

croscopy images of water dispersed FePh nanoparticles air

annealed at different temperatures in the absence of magnetic

field and (h-n) show the corresponding samples in presence

of magnetic field of �300 G. Except randomly distributed

bigger spherical clusters due to annealing effect, no chains

are observed in the absence of magnetic field. However,

aligning of particles to form chains of different aspect ratio

is observed in presence of magnetic field. In the absence of

external magnetic field, the magnetic nanoparticles are dis-

persed randomly with their own moments. However, in pres-

ence of an external magnetic field, individual magnetite

nanoparticles acquire induced dipole moment m given by

m ¼ p6

a3vH0, where a is the diameter of the nanoparticle, vis the effective susceptibility of individual nanoparticles, and

H0 is the magnitude of the external magnetic field. The

effective magnetic interaction between two magnetic nano-

particles can be described by the coupling constant

L ¼ � U a;0ð ÞKBT ¼

pl0a3v2H20

72KBT . L is the ratio of the magnetic inter-

action energy to the thermal energy (kBT) in the system.

Here, kB is the Boltzmann constant and T is the temperature.

The magnetic nanoparticles in the dispersion self assemble

into aligned structures when L� 1. As the magnetic field is

increased, the moments of the particles start to align them-

selves along the field direction. The longitudinal and trans-

verse particle fluctuations of wave vector k in a dipolar chain

create local variations in the concentration of dipoles, which

introduces fluctuations in the lateral field.

Haghgooie and Doyle60 shown that the head-to-tail

aggregation is required to form the single chains which does

not involve any significant energy barrier and hence the sys-

tem is not kinetically limited from reaching the lowest

energy state. Their energy barrier model explains the net

attraction forces between chains due to the surrounding

chains by taking the volume fraction into account. When two

rigid chains of colloids approach laterally, the interaction

energy curve consists of an attractive energy well and a re-

pulsive interaction for parallel chains (of same length).60

When chains are at off registry (different length/shifted with

one another), attractive energy well causes zippering of

chains. Thus, under parallel magnetic field, thick linear

chains are observed due to field induced zippering of chains.

The columnar chain alignment due to magnetic dipolar inter-

action was evident in the dispersions containing particles

annealed up to 650 �C. However, no such columnar struc-

tures are seen in the sample FeP900 due to the non-magnetic

a-Fe2O3 at 900 �C where dipolar alignment is not possible.

E. Proposed mechanism for enhanced thermalstability

Based on the above results, a mechanism responsible for

the enhanced thermal stability of FePh nanoparticles is pro-

posed. During heating, phosphates at the surface of the par-

ticles dehydrate and react with the adjacent surface

carbonates (formed by the reaction of atmospheric CO2 with

the alkaline hydrated surfaces of Fe3O449,53,61 to form an

ester through polyphosphoric acid intermediate. These esters

dehydrate to form phosphocarbonaceous char. This was

FIG. 9. Hydrodynamic size distribution of air annealed FePh nanoparticles

dispersed in water. Inset: Photographs of the dispersions.

FIG. 10. Zeta potential of air annealed FePh nanoparticles dispersed in water.

224304-7 T. Muthukumaran and J. Philip J. Appl. Phys. 115, 224304 (2014)

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evident in a variety of reactions involving iron in the

presence of carbon and phosphates.26,62 This reactive nature

increases the phosphocarbonaceous char yield,63 which fur-

ther crosslink and structurally order as a solid condensed

phase39,54,64 by interacting with Fe3O4.65 The phosphocarbo-

naceous char acts as a covalently attached protective layer on

the Fe3O4 and retards the decomposition of Fe3O4 particles

by increasing its activation energy.19 The shell prevents the

intrusion of heat, oxygen, volatiles, and mass to and from

the core54,66 and hinders the oxidation of Fe3O4 to c or

a-Fe2O3.67 The efficiency of the flame retardancy depends on

the amount of phosphorus content.20 The SAXS measurement

shows that the char thickness is �1.5 nm at �550 �C. Studies

show that even little quantity of the phosphoric acid is suffi-

cient for the effective flame retardance.19 Presence of phos-

phorus, nitrogen, and carbon causes the expansion of char

leading to a vitreous semisolid layer.32–34,37,39 Our XRD,

SAXS, and TEM results further corroborate the presence of a

vitreous semisolid shell morphology over the Fe3O4 at ele-

vated temperatures. A vapor phase flame retardancy is also

possible because of the presence of phosphorus volatiles like

P2, PO, PO2, HPO2, etc. that can suppress the exothermic

combustion process through a free radical trapping.39,68 Fig.

12 shows the schematic representation of FePh nanoparticles

air annealed in the temperature range of 300–900 �C. The

phosphates on Fe3O4 (shown by furry like structure) are con-

verted to phosphocarbonaceous char shell (shown by petals)

during annealing. At higher temperature, coalescence of

the nanoparticles occurs along with the restructuring of

phosphocarbonaceous char into a vitreous semisolid layer on

the nanoparticles.

IV. CONCLUSIONS

The high temperature thermal stability studies in phos-

phate monolayer capped Fe3O4 nanoparticles shows that the

phosphate capping on the nanoparticle augments the mag-

netic to non-magnetic (hematite) phase transition tempera-

ture by �100 �C. The observed enhanced high temperature

phase stability of phosphate capped nanoparticles is attrib-

uted to the presence of a heat retardant phosphocarbonaceous

shell over the nanoparticles during annealing. At higher tem-

perature, coalescence of the nanoparticles occurs along with

the restructuring of phosphocarbonaceous char into a vitre-

ous semisolid layer on the nanoparticles. The presence of

phosphocarbonaceous char was confirmed from FTIR,

SAXS and TEM measurements. The probable mechanism of

flame retardation by the phosphocarbonaceous shell is dis-

cussed. These findings may have useful applications in tailor-

ing magnetic nanoparticles with enhanced thermal stability.

ACKNOWLEDGMENTS

The authors thank Dr. T. Jayakumar and Dr. P. R.

Vasudeva Rao for fruitful discussions. J.P. thanks the Board

of Research Nuclear Sciences (BRNS) for support through a

research grant for the advanced nanofluid development

program.

FIG. 11. Phase contrast microscopy images of air annealed FePh nanoparticles dispersed in water. (a)–(g) Images of annealed samples FeP300, FeP560,FeP575,

FeP590, FeP600, FeP650, and FeP900 without magnetic field and (h)-(n) their respective images under a magnetic field of �300 G.

FIG. 12. Schematic representation of

FePh nanoparticles during annealing in

air at 300–900 �C. The furry and petals

like structures indicate the phosphates

on Fe3O4 and the phosphocarbona-

ceous char (shell) formed during

annealing.

224304-8 T. Muthukumaran and J. Philip J. Appl. Phys. 115, 224304 (2014)

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