RESEARCH PAPER

Investigation of nanostructured Fe3O4 polypyrrolecore-shell composites by X-ray absorbtion spectroscopyand X-ray diffraction using synchrotron radiation

Nicolae Aldea Æ Rodica Turcu Æ Alexandrina Nan ÆIzabella Craciunescu Æ Ovidiu Pana Æ Xie Yaning ÆZhonghua Wu Æ Doina Bica Æ Ladislau Vekas Æ Florica Matei

Received: 4 May 2008 / Accepted: 12 October 2008 / Published online: 22 November 2008

� Springer Science+Business Media B.V. 2008

Abstract In this article, we focus on the structural

peculiarities of nanosized Fe3O4 in the core-shell

nanocomposites obtained by polymerization of con-

ducting polypyrrole shell around Fe3O4 nanoparticles.

The local structure of Fe atoms was determined from

the Extended X-ray Absorption Fine Structure analysis

using our own package computer programs. An X-ray

diffraction method that is capable to determine average

particle size, microstrains, as the particle size distribu-

tion of Fe3O4 nanoparticles is presented. The method is

based on the Fourier analysis of a single X-ray

diffraction profile using a new fitting method based

on the generalized Fermi function facilities. The

crystallites size obtained by X-ray diffraction spectra

analysis was estimated between 3.2 and 10.3 nm.

Significant changes in the first and the second Fe

coordination shell in comparison with standard bulk

were observed. The global and local structure of the

nanosized Fe3O4 are correlated with the synthesis

conditions of the core-shell polypyrrole

nanocomposites.

Keywords X-ray spectroscopy � Synchrotron

radiation � Local and global structure �Magnetite

Nomenclature

k Wave vector

Aj(k) Amplitude function

Ri The radial distance

Ni Number of atoms

Fi(k,r,p) Backscattering amplitude

s Scattering parameter

WF(k) Apodization windows

h Experimental X-ray line profile

g Instrumental X-ray line profile

f True sample function

H(L) Fourier transform of h profile

G(L) Fourier transform of g profile

F(L) Fourier transform of true sample function

F(s)(L) Fourier transform contribution about

crystallite size and stocking fault

probability

Fð�ÞðLÞ Fourier transform contribution about

microstrain of the lattice

N. Aldea (&) � R. Turcu � A. Nan � I. Craciunescu �O. Pana

National Institute for Research and Development

of Isotopic and Molecular Technologies, Cluj-Napoca,

Romania

e-mail: naldea@itim-cj.ro

X. Yaning � Z. Wu

Beijing Synchrotron Radiation Facilities of Beijing,

Electron Positron Collider, National Laboratory, Beijing,

People’s Republic of China

D. Bica � L. Vekas

Romanian Academy, Timisoara Branch, Magnetic Fluids

Laboratory, Timisoara, Romania

F. Matei

Agriculture Sciences and Medicine Veterinary University,

Cluj-Napoca, Romania

123

J Nanopart Res (2009) 11:1429–1439

DOI 10.1007/s11051-008-9536-3

Deff(hkl) Effective crystallite size

h�2ihkl Microstrain of the lattice

A,a,b,c Parameters of generalized Fermi function

DN Uncertainties of atom numbers

DR Uncertainties of coordination shell

DE0 Uncertainties of K edge position

FWHM Full width at half maximum of true

sample function

DSch Crystallite size from Scherrer relation

Greek symbols

l Absorption coefficient

v EXAFS function

r Root means squares

k Mean free path function for inelastic scattering

U Radial structure function

dh Integral width of experimental profile

df Integral width of true sample function

Subscript

j Coordination shell

Introduction

The study of X-ray absorption spectroscopy (XAS)

can yield electronic and structural information about

the local environment around a specific atomic

constituent in the amorphous materials (Lytle et al.

1989; Stern 1988), the location and chemical state of

any catalytic atom on any support (Sinfelt et al. 1984)

and nanoparticle of transition metal oxides (Chen

et al. 2002; Turcu et al. 2004).

X-ray absorption near edge structure (XANES) is

sensitive to local geometries and electronic structure

of atoms that constitute the nanoparticles. The

changes of the coordination geometry and the oxida-

tion state on decreasing the crystallite size and the

interaction with molecules absorbed on nanoparticles

surface can be extracted from XANES spectrum.

Extended X-ray absorption fine structure (EXAFS)

is a specific element of the scattering technique in

which a core electron ejected by an X-ray photon

probes the local environment of the absorbing atom.

The ejected photoelectron backscattered by the

neighbouring atoms around the absorbing atom

interferes constructively with the outgoing electron

wave, depending on the energy of the photoelectron.

The energy of the photoelectron is equal to the

difference between the X-ray energy photon energy

and a threshold energy associated with the ejection of

the electron.

X-ray diffraction (XRD) line broadening investi-

gations of nanostructured materials have been limited

to find the average crystallite size from the integral

breadth or the full width at half maximum (FWHM)

of a diffraction profile. In the case of nanostructured

materials due to the difficulty of performing satisfac-

tory intensity measurements on the higher order

reflections, it is impossible to obtain two orders of

(hkl) profile. Consequently, it is not possible to apply

the classical method of Warren and Averbach

(Warren 1969).

In this study, we report the structural investigation

of novel core-shell hybrid nanostructures based on

magnetite covered with polypyrrole (PPY).

Conducting polymers such as PPY and composites

based on PPY represent advanced materials with

great fundamental and applicative interest due to their

special properties: tailoring of electronic properties

by the molecular structure; controllable electrical

conductivity by doping with different ions; synthesis

in a variety of micro and nanostructured shapes

(films, nanowires, nanoparticles), low cost, high-yield

synthesis for low-cost commercial production.

During the past years, the nanocomposites poly-

pyrrole/magnetic nanoparticles possessing both

conducting properties and magnetic response are

considered to be of interest because of their potential

applications in sensors, electrical–magnetic shields,

microwave absorbing materials, and magnetic sepa-

ration. Moreover, the nanocomposites based on PPY

and magnetites are biocompatible and offer great

promise for applications in biotechnology.

The development of magnetic core-shell nanopar-

ticles with a magnetic core and a conducting

polymeric shell offers the advantage of tailoring the

magnetic, electrical properties and functionalizing the

magnetic particles. Structural investigation of these

novel materials is a key issue to understand their

magnetic and electrical properties.

We developed a rigorous analysis of the X-ray line

profile (XRLP) in terms of Fourier transform where

zero strains assumption is not required. The apparatus

employed in a measurement generally affects the

obtained data and a considerable amount of work has

been done to make resolution corrections. In the case

of XRLP, the convolution of true data function by the

instrumental function produced by a well-annealed

1430 J Nanopart Res (2009) 11:1429–1439

123

sample is described by Fredholm integral equation of

the first kind (Aldea et al. 2005). A rigorous way for

solving this equation is Stokes method based on

Fourier transform technique. The local and global

structure of nanosized Fe3O4 in the nanocomposites

based on PPY was determined from EXAFS and

XRD analysis.

Theoretical background

EXAFS analysis

The interference between the outgoing and backscat-

tered electron waves has the effect of modulating the

X-ray absorption coefficient. The EXAFS function

v(k) is defined in terms of the atomic absorption

coefficient by

vðkÞ ¼ lðkÞ � l0ðkÞl0ðkÞ

; ð1Þ

where k is the electron wave vector, l(k) refers to the

absorption by an atom from the material of interest,

and l0(k) refers to the atom in the free state. Theories

of the EXAFS based on the single scattering approx-

imation of the ejected photoelectron by atoms in

immediate vicinity of the absorbing atom gives an

expression for v(k) of the form: (Aldea et al. 2000)

vðkÞ ¼X

j

AjðkÞsin½2krj þ djðkÞ� ð2Þ

where the summation extends over j coordination

shell, rj is the radial distance from the jth shell, and

dj(k) is the total phase-shift function. The amplitude

function Aj(k) is given by

AjðkÞ ¼Nj

kr2j

!Fðk; rj; pÞexp½�2rj=kjðkÞ � k2r2

j �

ð3Þ

In this expression, Nj is the number of atoms in the jth

shell, rj is the root means squares deviation of distance

about rj, F(k,r,p) is the backscattering amplitude and

kj(k) is the mean free path function for inelastic

scattering. The backscattering factor and the phase shift

depend on the kind of atom responsible for scattering

and its coordination shell (McKale et al. 1988). The

analysis of EXAFS data for obtaining structural infor-

mation [Nj, rj, rj, k(k)] generally proceeds by the use of

the Fourier transform. From v(k), the radial structure

function (RSF) can be derived. The single shell may be

isolated by Fourier transform,

UðrÞ ¼Z1

�1

knvðkÞWFðkÞexpð�2ikrÞdk: ð4Þ

The EXAFS signal is weighted by kn (n = 1, 2, 3)

to get the distribution function of atom distances.

Different apodization windows WF(k) are available

as Kaiser or Hanning. An inverse Fourier transform

of the RSF can be obtained for any coordination shell,

vjðkÞ ¼ ð1=knÞWFðkÞZR2j

R1j

UðrÞexpð2ikrÞdr: ð5Þ

The theoretical equation for vj(k) function is given

by

vjðkÞ ¼ AjðkÞsin½2krj þ djðkÞ�; ð6Þ

where the subscript j refers to the jth coordination

shell. The structural parameters for the first coordi-

nation shell are determined by fitting the theoretical

function vj(k) Eq. 6 to the vj(k) function derived from

Eq. 5. In empirical EXAFS calculation, F(k,r,p) and

dj(k) are conveniently parameterized (Cramer and

Hodgson 1979; Scott 1985; Ellis and Freeman 1995).

Eight coefficients are introduced for each shell:

Fsðk; r; pÞ ¼ c0½expðc1k þ c2k2Þ�=kc3 ð7Þ

dsðkÞ ¼ a�1k�1 þ a0 þ a1k þ a2k2 ð8ÞThe coefficients c0, c1, c2, c3, a-1, a0, a1 and a2 are

derived from the EXAFS spectrum of a compound

whose structure is accurately known. The values Ns

and rs for each coordination shell of the standard

sample are known. The trial values of the eight

coefficients can be calculated by algebraic consider-

ation, and then they are varied until the fit between

the observed and calculated EXAFS is optimized.

XRD analysis

The XRD pattern of a crystal can be described in

terms of scattering intensity as a function of

scattering direction defined by the scattering angle

2h, or by the scattering parameter s ¼ 2 sin hk ; where k

is wavelength of the incident radiation. We shall

discuss the XRD for the mosaic structure model in

J Nanopart Res (2009) 11:1429–1439 1431

123

which the atoms are arranged in blocks, each block

itself being an ideal crystal, but with adjacent blocks

not accurately fitted together. The experimental

XRLP, h, represents the convolution between the

true sample f and the instrumental function g:

hðsÞ ¼Z

gðs� s�Þf ðs�Þds� ð9Þ

The Eq. 9 is equivalent with the following relation

HðLÞ ¼ GðLÞFðLÞ; ð10Þ

where F(L), H(L) and G(L) are Fourier transforms

(FT) of the true sample, experimental XRLP and

instrumental function, respectively. The variable L is

the perpendicular distance to the (hkl) reflection

planes. The normalized F(L) can be described as the

product of two factors, F(s)(L) and Fð�ÞðLÞ. The factor

F(s)(L) describes the contribution of crystallite size

and stocking fault probability while the factor Fð�ÞðLÞgives information about the microstrain of the lattice.

Based on Warren and Averbach theory (Warren

1969), the general form of the Fourier transform of

the true sample for cubic lattices is given by relation

(Aldea et al. 2005),

FðsÞðLÞ ¼ e� jLj

Deff ðhklÞ; Fð�ÞðLÞ ¼ e�

2p2h�2Lihklh2

0L2

a2 ð11Þ

where Deff(hkl) is the effective crystallite size, h�2ihkl

is the microstrain of the lattice and h20 ¼ h2 þ k2 þ l2.

The generalized Fermi function (GFF) (Aldea

et al. 1996) is a simple function with a minimal

number of parameters, suitable for XRLP global

approximation based on minimization methods and it

is defined by relation:

hðsÞ ¼ A

eaðs�cÞ þ e�bðs�cÞ ; ð12Þ

where A, a, b, c are unknown parameters. The values A, c

describe the amplitude and the position of the XRLP, a, b

control its shape. For our analyses the most important

properties of the GFF can be resumed as

(i) the integral width of experimental XRLP,

dh(a,b) has the following form

dhða; bÞ ¼p

ðaabbÞ1=ðaþbÞcos p

2a�baþb

� � ð13Þ

(ii) by taking into account relations 9–10 the mag-

nitude of F(L) function has the following form

jFðLÞj ¼Ahqg

Agqh

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffifficos2 aþ sinh2 bL

cos2 cþ sinh2 dL

s

ð14Þ

where the arguments of trigonometric and hyperbolic

functions are expressed by

q ¼ a� b

2; q ¼ aþ b

2; a ¼ pqg

2qg

; b ¼ p2

qg

;

c ¼ pqh

2qh

; d ¼ p2

qh

The subscripts g and h refer to the instrumental

and experimental XRLP

(iii) the integral width of the true XRLP sample can

be expressed by the df function

df ðqh; qgÞ ¼p

2qh cospqh

2qg

cospqh

qg

þ 1

!: ð15Þ

Our data processing of the XRLP presented in the

section ‘‘Results and Discussion’’ is consequently

based on the GFF approximation and its remarkable

properties. The effective crystallite size, Deff(hkl) and

the microstrain of the lattice h�2ihkl were calculated

using Eqs. 11–15.

Experimental and data processing

Samples preparation

The magnetic nanocomposites based on PPY were

prepared by the oxidative polymerization of pyrrole

(Py) in aqueous solution containing an oxidant,

ammonium peroxodisulfate (APS) and water-based

magnetic nanofluid (MF). The MF was prepared by

the chemical coprecipitation method to obtain Fe3O4

nanoparticles, which were stably dispersed in water

by double-layer sterical stabilization with different

surfactants combinations: myristic acid (AM) and

dodecylbenzensulphonate (DBS) or lauric acid (AL)

and DBS. The oxidative polymerization of Py in

aqueous solution containing dispersed multi-wall

carbon nanotubes (MWCNTs) and Fe3O4 magnetic

nanofluid results in the attachment of magnetic

nanoparticles on the carbon nanotubes. The reaction

proceeded at room temperature under magnetic

1432 J Nanopart Res (2009) 11:1429–1439

123

stirring for different time intervals between 6 and

20 h. The resulting black precipitate was separated by

centrifugation, washed with water and dried at 60 �C

for 24 h. The preparation conditions for the magnetic

nanocomposites are given in the Table 1.

Measurement methods

The transmission EXAFS and XRD measurements

were carried out in the 4W1B and 4W1C beamlines in

Beijing Synchrotron Radiation Facilities (BSRF)

operating at 50–80 mA and 2.2 GeV at room tem-

perature. The beamline 4W1B is an unfocussed

monochromatic X-ray beam with 4 mrad of horizon-

tal acceptance. The X-rays are monochromatized by a

fixed exit Si double-crystal monochromator. The

features of 4W1B beamline are: energy range of

3.5–22 KeV, energy resolution of DE = 0.5–2 eV at

E = 10 KeV, Bragg angle range of 5–70�, the

crystals Si(111), Si(220) and Si(311) can be alterna-

tively used. A Fe3O4 powder with 99.98% purity was

used as a standard sample. Absorption coefficients of

Fe K edge was determined using a Si(111) double-

crystal monochromator. Ionization chambers moni-

tored the X-ray intensities of incident and transmitted

beams. Harmonics were rejected by detuning of

monochromator. The whole experimental system was

controlled by a personal computer PS/2 for automatic

data acquisition. Special care was taken in sample

preparation, especially for thickness and homogene-

ity of samples to obtain absorption spectra of good

quality. All samples were ground to fine powder and

homogeneously dusted on Scotch tape. We used

energy scanning range from 6,994 to 8,108 eV for

absorption coefficient measurements. The EXAFS

analysis of the absorption coefficient was processed

using computer codes from EXAFS51 to EXAFS56

(Aldea and Indrea 1990a) of our library.

The beamline 4W1C is a time-shared branch with

beamline 4W1B. The photon beam is deflected by a

bent triangle crystal and led into beamline 4W1C. The

bent triangle crystal with size 40 V 9 120(H)

9 1 mm2 monochromatizes and focuses radiation in

the horizontal direction. A cylindrical mirror focuses

the photons beam in the vertical direction. A focal

spot size of 1.5H 9 1.0V 9 1 mm2 thick was

expected at 18.665 m from light source. The energy

resolution is 0.5 eV at 0.154 nm. A NaI(Tl) detector

was used, signals were amplified and fed to a single

channel analyzer (ORTEC 850) and read out by a

computer. A silicon powder was used as standard

sample for instrumental correction. The scanning

scale, 2h ranges from 28� to 70�. The Fourier

transform of the XRLP (220), (311), (400), (511)

and (440) were processed by computer code SIZE

developed with Maple software. This computer code

is an improved version of XRLINE (Aldea and Indrea

1990b) and XRLINE1 (Aldea et al. 1995) computer

programs. Its purpose is to show that intermediate

processing results in a graphic manner.

Results and discussion

XANES results

The electronic properties of Fe3O4 nanoparticles are

an interesting problem from the XANES perspective

because it involves three types of absorbers: the

tetrahedral Fe3?, the octahedral Fe3? and the octa-

hedral Fe2?. Figure 1 shows XANES spectra for

investigated nanostructured Fe3O4 polypyrrole core-

shell composites as well as magnetite (Fe3O4) as

standard sample. Although the coordination geometry

of the interior atoms in nanoparticles is mostly the

same as that within the bulk sample, the coordination

geometry of the surface atoms could be substantially

different, forming surface defect sites with energy

levels in the mid-gap region. These changes at the

surface could propagate further towards the Fe3O4

nanoparticle core, causing interior lattice disorder.

The XANES spectra of nanoparticles were expected

Table 1 The synthesis

parameters for the magnetic

nanocomposites

a Sample prepared in the

solution containing

dispersed MWCNTs

Ferrofluid Sample MF/Py (v/v) Polymerization

time (h)

Fe3O4: AL ? DBS F10 20 6

Fe3O4: AM ? DBS F12 20 20

Fe3O4: AL ? DBS F14a 2 10

J Nanopart Res (2009) 11:1429–1439 1433

123

to reveal both surface and interior lattice disorder

through the spectral features that directly reflected

changes in the structural and electronic properties of

nanocrystallites as compared to those of bulk metal

oxide. The lower conduction bands of transition

metal oxides were mainly composed of transition

metal 3d orbital, while the upper valence band were

mainly composed of oxygen p orbital (Asahi et al.

2000). Because of the crystal fields of different

lattices, these 3d orbital of the lower conduction

bands were split into sub-bands that gained p

character by mixing with p orbital of the central

metal atom or neighbouring oxygen atoms. Absorp-

tion of the X-ray that resulted from the transition of

1s electron of these sub-band exhibited near edge

features in metal oxide XANES spectra. The (A)

shoulder from Fig. 1 shows that pre-edge features in

XANES spectra of transition-metal K edge are due to

quadrupole transitions from 1s to 3d orbital (Grunes

1983). The (B) area from the same figure represents

the dipole-allowed transitions due to the 3d–4p

mixing of Fe atoms. The (C) area is associated with

d–p mixing between the metal atom and ligands

through bonding and multiple scattering involving the

same atoms with different scattering paths (Modrow

et al. 2003). The features (C) and (D) describe the

width of white line, which gives information about

the cluster size of the metal oxide.

Moreover, other XANES features originated from

multiple scatterings could be used to probe lattice

disorder in Fe3O4 nanoparticles. The threshold energy

of XANES spectrum for standard sample Fe3O4

sample is moved to high energy with about 7 eV.

The absence of surrounding the PPY can explain this

behaviour. The investigated samples from Table 1

have about the same value of the threshold energy of

the K edge, but their positions are shifted to lower

energy as compared with the standard Fe3O4. These

features are due to strong electron interaction of Fe3O4

nanocrystallites surrounded by the PPY shell. The

values of the threshold energies were calculated at the

positions of the maximum value for the first derivative

of XANES spectra using ‘‘bell’’ spline function

technique (Aldea et al. 1990a). The threshold energies

and their uncertainties of the investigated samples are

given in the last column of Table 2.

EXAFS results

The extraction of EXAFS signal is based on the

threshold energy of Fe K edge determination

followed by background removal by pre-edge and

after-edge base line fitting with different possible

modelling functions, l0(k) and l(k) evaluation. In

accordance with Eq. 1 EXAFS signals were per-

formed in range 37.5–160 nm-1. In order to obtain

7095 7100 7105 7110 7115 7120 7125 7130 7135 7140 7145 7150

Abs

orpt

ion

coef

ficie

nt

X-ray energy (eV)

standard sample

F10

F12

F14

(A)

(B)

(C)

(D)

Fig. 1 The normalized

absorption coefficients of

Fe K edge for investigated

samples

1434 J Nanopart Res (2009) 11:1429–1439

123

atomic distances distribution, we could carry out the

RSF using Eq. 4. The mean Fe–O distances of the

first and the second coordination shell for the

standard sample at room temperature are close to

values of R1 = 0.189 nm and R2 = 0.209 nm,

respectively. By taking into account this very small

difference between R1, R2 and the discrimination

steps, Dk and Dr of v(k) and U(r) functions and the

relation from fast Fourier transform (FFT) procedure,

it is not possible to obtain a reliable resolution for the

RSF. This means that contributions of the first and

second shells are overlapped. To avoid this disad-

vantage, we used the Filon algorithm for Fourier

transform procedure (Abramowich and Stegun 1968).

Based on this procedure, the Fourier transforms of

k3v(k)WF(k), performed in range 0.02–0.5 nm, are

shown in Fig. 2 for the investigated samples as well

as for standard Fe3O4 powder. The diminution of the

Fourier transform magnitude is a result of the reduced

average coordination number. Each peak from |U(r)|

is shifted from the true distance, due to phase shift

function that is included in EXAFS signal. In the

standard sample, the iron cations [Fe3?] and [Fe2?,

Fe3?] from tetrahedral and octahedral structure are

surrounded by four and six oxygen anions [O2-],

respectively. In the EXAFS measured spectra these

three contributions are averaged.

We proceed by taking the inverse Fourier trans-

form given by Eq. 5 of the first neighbouring peak,

and then extracting the amplitude envelope function

Aj(k) and phase-shift function d(k) in accordance with

Eqs. 7–8 using the standard sample. By Lavenberg-

Marquard fit of Eq. 6 and experimental contribution

for each coordination shell, we then evaluated the

interatomic distances, number of neighbours and edge

position. Figure 3 shows calculated and experimental

EXAFS functions v1(k) of the first shell for the

investigated samples. Table 2 contains the best values

of the local structural parameters. The errors given for

the best-fit parameters have been estimated as DN, DR

and DE0. The average interatomic distances obtained

for the first and second coordination shells have

practically the same values as that of the standard

sample. The average value corresponding to tetrahe-

dral sites for the first shell coordination of each sample

was approximately three atoms. There is a difference

of approximately 25% between the number of atoms

of the investigated samples and the standard (N1 = 4).

For the second coordination shell corresponding to

octahedral sites, the average number of atoms is

diminished only by 10% in the investigated samples

as compared to the standard. Therefore, we inferred

that this diminution of the number of atoms for

tetrahedral and octahedral sites is due to a strong

electron interaction between the magnetite nanoclus-

ters and the surrounding PPY shell.

XRD results

Practically speaking, it is not easy to obtain accurate

values of the crystallite size and microstrain without

extreme care in experimental measurements and

analysis of XRD data. The Fourier analysis of XRLP

validity depends strongly on the magnitude and

nature of the errors propagated in the data analysis.

In the article (Young et al. 1967) are treated three

systematic errors: uncorrected constant background,

truncation and the effect of the sampling for the

observed profile at a finite number of points that

appear in discrete Fourier analysis. In order to

minimize propagation of these systematic errors, a

global approximation of the XRLP is adopted instead

of the discrete calculus. Therefore, herein the analysis

of the diffraction line broadening in X-ray powder

pattern was analytically calculated using the GFF

facilities. The reason of this choice, as described in

Table 2 The local structural parameters of the investigated samples

Sample The first shell The second shell E0 ± DE0 Shift energy

(eV)N1 ± DN1 Atoms

numbers

R1 ± DR1 Shell radius

(nm)

N2 ± DN2 Atoms

numbers

R2 ± DR2 Shell radius

(nm)

Fe3O4 4 0.189 6 0.209 7123.75 ± 0.15

F10 2.46 ± 0.03 0.189 ± 0.001 5.93 ± 0.03 0.210 ± 0.001 7116.27 ± 0.46

F12 3.11 ± 0.009 0.188 ± 0.001 5.15 ± 0.01 0.209 ± 0.001 7118.35 ± 0.43

F14 3.16 ± 0.03 0.190 ± 0.001 5.25 ± 0.01 0.209 ± 0.002 7114.10 ± 0.51

J Nanopart Res (2009) 11:1429–1439 1435

123

Section ‘‘XRD analysis’’, was simplicity and math-

ematical elegance of the analytical Fourier transform

magnitude and the integral width of the true XRLP

given by Eqs. 14 and 15, respectively. The robustness

of the GFF approximation for the XRLP arises from

the possibility of using the analytical form of Fourier

transform instead of a numerical FFT. It is well

known that the validity of numerical FFT depends on

the filtering technique adopted (Walker 1997). In this

way validity of the microstructural parameters are

closely related to the accuracy of the Fourier

transform magnitude of the true XRLP.

Here we processed only (220), (311), (400), (511)

and (440) profiles. Their experimental relative

0.02 0.06 0.1 0.14 0.18 0.22 0.26 0.3 0.34 0.38 0.42 0.46 0.5

Rad

ial d

istr

ibut

ion

func

tion

Distance (nm)

standard sample

F10

F12

F14

Fig. 2 The Fourier

transforms of the EXAFS

spectra

1601401201008060 40

Module of wave vector (1/nm)

F10

F12

F14

Fig. 3 The experimental

and calculated EXAFS

signals of the first

coordination shell of Fe

atoms for the investigated

samples, F10, F12 and F14

experimental (solid line,

points, dashed line),

calculated (?, 9,*)

1436 J Nanopart Res (2009) 11:1429–1439

123

intensities with respect to 2h values and Si powder as

instrumental broadening effect are shown in Figs. 4

and 5. The next step consist in background correction

of XRLP by polynomial procedures and the determi-

nation of the best parameters of GFF distributions by

nonlinear least squares fit. In order to determine the

nanostructural parameters contained in Eq. 11 we

computed the Fourier transforms of the true XRLP

and integral width using Eqs. 14 and 15. In terms of

the classical Scherrer equation (Turcu et al. 2006) the

crystallite size is directly proportional to wavelength,

inversely proportional to the product from cosines of

gravity centre of the true sample function and its full

width at half the maximum.

In the Section ‘‘EXAFS results’’ we have shown that

the coordination shells radius of the investigated

samples have similar values as the Fe3O4 powder

standard sample. This important result is strongly

correlated with the positions of (220), (311), (400),

(511) and (440) XRLP from the experimental spectra

contained in Fig. 4. Therefore, these results explain

metal oxide features of the investigated clusters despite

the strong deformation of the crystalline structure.

Hydrogen chemisorptions, transmission electron

microscopy, magnetization, electronic paramagnetic

resonance and other methods could also be used to

determine grain size of particles by taking into

account a prior spherical form for the grains. By XRD

method one can be obtain the crystallite size that has

different values for the different crystallographic

planes. There is a large difference between the grain

size and crystallite size due to the physical meaning

of the two concepts. It is possible that the grains of

the magnetite are built up of many Fe3O4 crystallites.

The global structural parameters obtained for the

investigated samples are summarized in Table 3. The

microstrain parameter of the lattice can also be

correlated with effective crystallite size in the following

way. The value of the effective crystallite size increases

when the microstrain value decreases. Because Scherrer

equation does not take into account the lattice strains,

sometimes nanocrystallites, sizes determined by it are

greater than the results obtained by Eq. 11. Therefore,

the values from the last column of Table 3 are less

reliable than the results from the fifth column.

Conclusions

In this article, it has been shown how, in addition to

EXAFS experiments with their specific advantages,

XRD analysis can add more information for under-

standing the nanostructure of the magnetite

surrounded by the PPY shell. The conclusions that

can be drawn from these studies are

28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70

Rel

ativ

e in

tens

ity

Diffraction angle (2 theta)

(220)

(311)

(400)

(511) (440)

F10

F12

F14

Fig. 4 The relative

intensities of (220), (311),

(400), (511) and (440)

XRLP for the investigated

samples

J Nanopart Res (2009) 11:1429–1439 1437

123

(i) The diminution of the number of atoms from the

first and the second coordination shells of Fe

atoms in the investigated samples point out to

the existence of an electronic interaction

between the magnetite nanoparticles and the

surrounding PPY;

(ii) For XRLP analysis, a global approximation is

applied rather than a numerical Fourier analysis.

The former analysis is better than a numerical

calculation because it can minimize the sys-

tematic errors that can appear in the numerical

Fourier analysis.

0

500

1000

1500

2000

2500

3000

20 25 30 35 40 45 50 55 60 65 70 75 80

Rel

ativ

e in

tens

ity

Diffraction angle (2 theta)

(111)

(220)

(311)

(400)

(331)

Fig. 5 The relative

intensities of (111), (220),

(311), (400) and (331)

experimental XRLP for Si

powder as the instrumental

function

Table 3 The global structural parameters of the investigated samplesa

Sample (hkl) dh (nm-1) df (nm-1) Deff (nm) h�2ihkl 9 104 FWHM (nm-1) DSch (nm)

F10 220 0.2926 0.2828 3.8 6.0395 0.2375 4.2

311 0.2439 0.2412 4.9 3.2178 0.2027 4.9

400 0.1984 0.1971 6.0 1.4724 0.1675 6.0

511 0.2359 0.2352 5.1 1.2523 0.1975 5.1

440 0.2345 0.2323 5.1 1.0379 0.1925 5.2

F12 220 0.2866 0.2830 4.1 6.1159 0.2375 4.2

311 0.2168 0.2149 5.5 2.5463 0.1775 5.6

400 0.2517 0.2472 4.6 2.3213 0.2075 4.8

511 0.1841 0.1818 6.4 0.7383 0.1525 6.6

440 0.2723 0.2271 3.2 0.8652 0.1925 5.2

F14 220 0.1534 0.1508 7.7 1.6685 0.1275 7.8

311 0.1281 0.1227 8.7 0.7882 0.1025 9.8

400 0.1146 0.1142 10.3 0.4799 0.0975 10.3

511 0.1542 0.1539 7.8 0.5286 0.1275 7.8

440 0.1257 0.1113 7.7 0.2218 0.0925 10.8

a dh(a,b) integral width for experimental samples, Eq. 13 df (qh, qg) integral width for true samples, Eq. 15; Deff effective crystallite

size and h�2ihkl mean square of the microstrain Eq. 11; DSch particle size determined by Scherrer equation, DSch = k/(FWHM cos h)

1438 J Nanopart Res (2009) 11:1429–1439

123

(iii) Our numerical results have shown that by using

the GFF distribution we have successfully

obtained reliable global nanostructural

parameters.

Acknowledgments The authors are grateful to BSRF for the

beam time, and to Drs. Hu Tiandou and Liu Tao for their

technical assistance in EXAFS and XRD measurements. The

author (N. A.) is also indebted to Professors Chen Hesheng,

director of The Institute of High Energy Physics, and Fang

Shouxian, director of The BEPC National Lab., respectively,

for their hospitality during his stay. This work is the result of

the Scientific Cooperation Agreement between our institutes.

This work was supported by the research programmers of The

Romanian Ministry of Education and Research (CEEX-

MATNANTECH projects nr. 12/2005 and CNCSIS nr. 1484).

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