Thermal decomposition of metal nitrates

PVA–TEOS gels for obtaining M(II) ferrite/silica nanocomposites

Marcela Stoia • Paul Barvinschi •

Lucian Barbu-Tudoran

Received: 20 September 2012 / Accepted: 16 October 2012

� Akademiai Kiado, Budapest, Hungary 2012

Abstract The paper presents a study regarding the

preparation of 40 %MIIFe2O4/60 %SiO2 nanocomposites

(M = Ni, Zn, Cu) by thermal decomposition of metal

nitrates—poly(vinyl alcohol)–tetraethyl orthosilicate gels.

Thermal analysis and FT-IR spectroscopy have evidenced

that a redox reaction takes place between PVA and NO3-

ions in the pores of the formed hybrid gels. The result of

this redox reaction is the formation of carboxylate-type

coordination compounds that have the role of a precursor

of the ferrite nanoparticles. By thermal decomposition of

these precursors inside the silica matrix, the corresponding

MFe2O4/SiO2 nanocomposites are obtained starting with

600 �C, as resulting from XRD analysis. Elemental maps

of the corresponding involved elements M (Ni, Zn, Cu), Fe,

and Si have confirmed the homogenous distribution of the

ferrite nanoparticles within the silica matrix. TEM images

have shown that the nanocomposites were obtained as fine

nanoparticles, with diameter up to 20 nm. All nanocom-

posites 40 %MIIFe2O4/60 %SiO2 obtained at 1000 �C

presented magnetic properties characteristic to this type of

nanocomposite.

Keywords Ferrite � Nanocomposites �Poly(vinyl alcohol) � Sol–gel � Silica

Abbreviations

PVA polyvinyl alcohol

TEOS tetraethyl orthosilicate

FTIR Fourier transform infrared spectroscopy

TEM Transmission electron microscopy

XRD X-ray diffraction

Introduction

Nanosized particles exhibit unique chemical and physical

properties compared to the corresponding bulk materials.

Modern applications of different technologies ask for

materials with better control on their size and on their

properties. Composite materials are ideal from this point

of view, combining among crystalline, amorphous, and

polymer phases to enrich and enhance the properties. [1].

Nanocomposite materials composed from nanometric sin-

gle or mixed metal oxide nanoparticles embedded in

amorphous matrices reveal in particular more interesting

magnetic, electric, and catalytic properties [2].

Nanocomposites constituted by particles dispersed in an

inert matrix present unique properties, which significantly

depend not only on the size and distribution of the parti-

cles, but also on the matrices’ morphology and porosity.

Sol–gel synthesis is a successful technique in nanocom-

posite synthesis due to the advanced control on the com-

position, purity, homogeneity, size, and properties of the

dispersed nanoparticles [3]. There are though possibilities,

due to the high surface of the interface between the

M. Stoia

University Politehnica Timisoara, Piata Victoriei no.2,

300006 Timisoara, Romania

P. Barvinschi (&)

West University of Timisoara, Bv. V. Parvan no. 4,

300223 Timisoara, Romania

e-mail: pc_barvi@yahoo.fr

L. Barbu-Tudoran

Babes-Bolyai University, 5-7 Clinicilor Street,

400006 Cluj-Napoca, Romania

123

J Therm Anal Calorim

DOI 10.1007/s10973-012-2786-4

dispersed particles and the silica matrix, that during the

thermal treatment, interaction between the two components

takes place, leading to the formation at high temperature,

but in small amounts, of a corresponding MSiO4 secondary

phase [4].

There have been reports in the literature regarding dif-

ferent variants of sol–gel synthesis methods which involve

different precursors including different polyols as pro-

panediol [5]. The presence of these organics, especially

those with big molecules as polymer, influences the mor-

phology and porosity of the matrix, as well as the forma-

tion of the dispersed nanoparticles, due to the residual

carbon which creates a reducing environment [6, 7].

The present study deals with the preparation and char-

acterization of spinel ferrites MIIFe2O4, where MII = ZnII,

NiII, and CuII inside a silica matrix. In order to obtain

40 %MIIFe2O4/60 %SiO2 nanocomposites, we have used a

versatile route based on the thermal decomposition inside

the SiO2 matrix, of some particular precursors, coordina-

tion compounds of the involved MII and MIII cations with

carboxylate-type ligands obtained in the redox reaction

between metal nitrates and PVA. The redox reaction takes

place similarly when the reacting mixture is embedded in a

silica gel. The thermal decomposition of the as-formed

precursors leads on the formation of an intimate mixture of

the two oxides in a highly reactive state, which further

reacts to form the spinel system embedded in silica matrix.

The thermal evolution of the obtained gels was studied by

thermal and FT-IR analysis. The decomposition products at

400 �C of the precursor/silica system have been annealed

at different temperatures up to 1000 �C in order to study

the evolution of the two oxidic systems inside the silica

matrix. The annealed powders have been characterized by

X-ray diffraction and TEM microscopy.

Experimental

Materials and methods

The amount of metal nitrates (M(NO3)2�6H2O; M = Ni,

Zn, Cu) necessary to obtain 3 g of nanocomposites

40 %MFe2O4/60 %SiO2 was dissolved in the correspond-

ing volume of 4 % PVA solution (PVA Merck,

M = 60,000 g mol-1) in order to have a molar ratio PVA

(monomer):NO3- = 2:1. To the as-obtained clear solutions,

ethanolic solution of TEOS (1:1) was added dropwise and

kept under stirring conditions for 30 min until they became

clear. The clear solutions were allowed to gel at room

temperature. The obtained clear gels were gradually heated

up to 150 �C, when the reaction PVA–NO3- started, with

emission of brown nitrogen oxides. The mass reaction was

kept at this temperature for 5 h. Solid products were

obtained in all cases. These products were ground, ther-

mally decomposed at 350 �C when the primary oxidic

system is formed, and were further annealed at 600, 800,

and 1000 �C.

In order to establish the evolution of the oxidic system

with the annealing temperature, we also synthesized in

the same manner the corresponding simple oxides/silica

nanocomposites for all metals involved (Ni(II), Zn(II),

Cu(II), and Fe(III)) using the same procedure.

The obtained precursors were characterized by FT-IR

spectrometry and thermal analysis. The calcinations prod-

ucts were characterized by FT-IR spectrometry, X-ray

diffraction, EDX analysis, and TEM microscopy.

Characterization techniques

Thermal analysis was performed on a 1500D MOM

Budapest Derivatograph. The heating was achieved in

static air up to 500 �C with a heating rate of 5 �C min-1 on

Pt plates using a-Al2O3 as the inert material, using a mass

sample of 0.100 g. The synthesized powders were char-

acterized by FT-IR spectrometry with a Shimadzu Prestige

FT-IR spectrometer, in KBr pellets, in the range of

400–4,000 cm-1. Phase analysis was achieved with a D8

Advance-Bruker AXS diffractometer, using the Mo-Karadiation (kMo = 0.7093 A). EDX elemental mapping was

recorded on a Quanta 3D FEG (FEI) microscope. TEM

images were acquired using a Jeol-Jem 1010 Microscope.

Results and discussions

Polyvinyl alcohol-based synthesis methods proved to be

performing well for the obtaining of simple and mixed oxides

as fine powders [8]. Due to the multiple role of the PVA in

these synthesis methods, especially for the chelating effect on

the metallic cations [9], stabilizing the ions and preventing

their hydrolysis and precipitation, and the effect on the final

oxide particle size, we have introduced PVA as a reactant in

the sol–gel synthesis of nanocomposites. Thus, in order to

obtain finer oxide particles inside the silica matrix, we have

synthesized gels starting from TEOS, PVA, and the corre-

sponding metal nitrates for MFe2O4/SiO2 nanocomposites.

According to our previous studies [6], the presence of

PVA beside TEOS in the gelation stage leads to the for-

mation of hybrid PVA–SiO2 gels, influencing matrices’

morphology during the thermal treatment. Also, we have

established [8] that during heating of the aqueous solu-

tions’ PVA–metal nitrates around 100 �C, a redox reaction

takes place between PVA and the nitrate ions, leading to

carboxylate ions that coordinate with the metal cations

leading to carboxylate compounds that represent excellent

precursors for the corresponding metal oxides.

M. Stoia et al.

123

In order to study the behavior of PVA and the nitrate

ions inside the silica gels, we have first studied the gels

synthesized with each metal nitrate involved separately:

Fe(NO3)3, Ni(NO3)2, Zn(NO3)2, and Cu(NO3)2. The

obtained gels have been dried at 60 �C for 5 h and then

characterized by DTA analysis.

Figure 1 presents the DTA curves for the obtained gels.

All DTA curves evidenced, in the range of 100–150 �C, an

exothermic effect assigned to the redox reaction between

the PVA and nitrate ions. As a result of this reaction, some

metal carboxylate probably form, which decompose at a

temperature higher than 300 �C, leading to the second

largest exothermic effects.

In order to verify the assignment of the exothermic

process that takes place in the synthesized gels, we have

thermally treated these gels at 150 �C for 5 h, when no

emission of reddish-brown nitrogen oxides was observed

anymore. All gels (dried at 60 �C and thermally treated at

150 �C) have been characterized by FT-IR spectroscopy.

The obtained FT-IR spectra are shown in Fig. 2a for the

gels heated at 60 �C and for the gels heated at 150 �C in

Fig. 2b.

All spectra of the gels dried at 60 �C (Fig. 2a) present a

strong band at *1,384 cm-1, characteristic to the vibra-

tions of N–O bonds in NO3- species [10], evidencing the

presence of these ions in the gels. A similarity can be

observed between the spectra of the gels FePVSi and

CuPVSi, where the presence of the strong bands in the

range of 1,600–1,700 cm-1 characteristic to –COOH

groups evidences the beginning of the redox reaction PVA–

NO3- at low temperatures (60 �C). Also, in these cases,

the bands characteristic to silica gels (*460, *800, and

1,000–1,300 cm-1 [11]) are stronger, suggesting a higher

degree of condensation.

In the case of the gels heated at 150 �C, in all FT-IR

spectra (Fig. 2b), the band at*1,384 cm-1 is missing due to

the complete oxidation of NO3- ions in the reaction with

PVA. Instead, a medium band at*1,440 cm-1 appears in all

spectra, which can be assigned to the symmetric vibrations

of the –COO- groups [12]. In this case, all spectra present

the strong band located in the range of 1,600–1,700 cm-1

characteristic to the asymmetrical stretching vibrations

of –COO- groups coordinated with the cations present

in the system. Also, all FT-IR spectra exhibit strong

bands characteristic to silica matrices (*460, *800, and

1,000–1,300 cm-1) to the hydrogen bonded water molecule

or –OH groups (3,000–3,600 cm-1) and to the vibrations of

C–H bond from the PVA chains (2,800–3,000 cm-1).

From the FT-IR spectra, it can be seen that PVA

behaves in silica gels similar to the case of the aqueous

solutions, interacting with the nitrates’ ions to form car-

boxylate species that coordinate with the positive ions

contained in the pores of the gels. Thus, the carboxylate

precursors are formed in the pores of the silica gels in all

cases.

The performed thermal and FT-IR study on the gels

containing a single metal nitrate proved the interaction

between the PVA and each metal nitrate involved in our

synthesis, with the formation of metal carboxylate within

the pores of the silica gel.

In order to obtain the corresponding MFe2O4/SiO2

nanocomposites, the gels containing the mixture of two

metal nitrates—Fe(NO3)3 and M(NO3)2, with M = Ni (gel

FeNiPVSi), M = Zn (gel FeZnPVSi) or Cu (gel FeCu

PVSi)—have been dried at 60 �C and thermally heated at

150 �C and characterized by thermal analysis and FT-IR

spectroscopy, in comparison with the gel PVSi (without

metal nitrates).

Figure 3 presents TG and DTA curves of the gels PVSi

(a) and FeZnPVSi (b) dried at 60 �C. The two studied gels

present a completely different thermal behavior. Thus,

PVSi gel presents only endothermic effects up to 400 �C,

assigned to the loss of water from the condensation reac-

tion and from the PVA dehydration. The oxidative

decomposition of the PVA chains, probably condensed

within the silica network, takes place after 400 �C, with

formation of carbonaceous residue that burns around

500 �C. The gel FeZnPVSi (Fig. 3b) presents an exother-

mic effect with a maximum at 170 �C, which corresponds

to the redox reaction between PVA and NO3- ions with the

formation of carboxylate coordination compounds embed-

ded in silica gel. These compounds thermally decompose

with a large exothermic effect in the range of 250–400 �C.

Taking into account the results of the thermal analysis,

we have decided to thermally treat the dried gels at 150 �C

0 50 100 150 200 250 300

Temperature/°C

DTA

sig

nal/a

.u.

(exo

up)

350 400 450 500 550

(1)

(2)

(3)

(4)120°

145°

110°

105°

600

Fig. 1 DTA curves of the gels dried at 60 �C (1) FePVSi; (2)

ZnPVSi; (3) NiPVSi; (4) CuPVSi

Thermal decomposition of metal nitrates

123

for 5 h in order to obtain the corresponding Fe(III)-M(II)-

carboxylates in the pores of the silica gels. The thermal

behavior of these gels heated at 150 �C was very similar.

Figure 4 comparatively presents the TG and DTA curves of

the gels PVSi and FeCuPVSi obtained at 150 �C.

The thermal curves of the gel PVSi without metal nitrates

show a thermal decomposition in three steps, the last step being

associated with an exothermic effect, with a maximum at 460 �C,

corresponding to the burning of PVA degradation products. The

gel FeCuPVASi exhibits a small mass loss up to 150 �C, corre-

sponding to the elimination of the volatile products of the con-

densation reaction and of the adsorbed water, a step associated

with a weak endothermic effect. The thermal decomposition of

the carboxylates formed in the pores of the silica gel takes place in

the range of 200–400 �C with a 45 % mass loss and a large

exothermic effect, with a maximum at 320 �C.

According to the thermal analysis, the thermal decom-

position of ferrites’ precursors within the pores of the silica

4000 3500 3000 2500

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.u.

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ittan

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.u.

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(4)

(3)

(2)

(1)

(a) (b)

(4)

(3)

(2)

(1)

4000 3500 3000 2500 2000 1500 1000 500

Fig. 2 FT-IR spectra of the gels dried at 60 �C (a) and thermally treated at 150 �C (b): (1) FePVSi; (2) ZnPVSi; (3) NiPVSi; (4) CuPVSi

0

–10

–20

–30

–40

–50

–60

–70

–80

–900 50 100 150 200 250 300

Temperature/°C

Mas

s lo

ss/%

0

–10

–20

–30

–40

–50

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–70

–80

–90

Mas

s lo

ss/%

Temperature/°C350 400 450 500 550 600 0 50 100 150 200 250 300 350 400 450 500 550

DTA

sig

nal/a

.u.

(exo

up)

DTA

si g

nal/a

.u.

(exo

up)

420°

(a) (b)

510°

340°

170°

TG

DTA

TG

DTA115°

Fig. 3 TG and DTA curves of the gels dried at 60 �C a PVSi and b FeZnPVSi

M. Stoia et al.

123

gel takes place up to 400 �C, so the annealing temperatures

have to be higher than this final temperature.

In order to confirm the thermal evolution of the gels, we

have characterized the gels PVSi and FeMPVSi thermally

treated at different temperatures by FT-IR spectroscopy.

Due to the fact that the FT-IR spectra have not presented

particularities with the M(II) ion, we present in Fig. 5 the

FT-IR spectra for the gels PVSi and FeZnPVSi heated at

different temperatures.

The FT-IR spectra of the gels dried at 60 �C are clearly

different. The spectrum of the gel PVSi (Fig. 5a) presents

bands characteristic to the siloxane network (450, 794,

1,040, 3,332 cm-1), but not very pronounced, revealing a

low degree of condensation, probably due to the interaction

between the siloxane network and PVA, that leads to the

co-condensation of the two polymers. The shifting of the

band located at 1,040 cm-1 to lower wavenumbers than

usual for silica gels (1,080 cm-1) may be due to the

overlapping of the band characteristic to the –OH groups

from PVA. Also, a band located at 1,750 cm-1 character-

istic to the unhydrolyzed acetate groups from PVA [13]

appears in the spectrum together with the bands located in

the range of 2,700–3,000 cm-1 characteristic to –CH– and

–CH2– groups from the PVA chain. The presence of PVA

polymer, intertwined with the siloxane polymer, is con-

firmed by the band located at 1,190 cm-1 assigned by some

authors to the PVA polymer [14].

In the case of the gel FeZnPVA, dried at 60 �C, the

siloxane network has a higher condensation degree prob-

ably due to the coordination of PVA with the cations

present in the gel, and the reduced interaction of the

PVA–OH group with the groups of the siloxane network.

Thus, the characteristic bands (453, 802, 958, 1,060 cm-1)

are stronger in this case. The strong band located at

1,382 cm-1 proves the presence of the nitrate ions in the

gel; still, the redox reaction with the PVA already begins at

this temperature, as it results from the large and intense

band located at 1,680 cm-1 including the band character-

istic to the adsorbed water (1,640 cm-1) and the band

characteristic to asymmetrical stretching vibrations of the

–COO- groups formed by PVA oxidation.

After thermal treatment at 150 �C, both FT-IR spectra

(Fig. 5a, b) show changes. Thus, in both cases, the bands

characteristic to the silica matrix (*450, *800,

*1,070 cm-1) are more pronounced showing an advanced

condensation degree of the matrix. Both FT-IR spectra

show bands characteristic to the vibrations of C–H bonds

from PVA chains, in the region of 2,800–3,000 cm-1 [15].

The difference between the two spectra consists in the very

strong band located at 1,676 cm-1 that appears in the

spectrum of the FeZnPVSi gel corresponding to the

asymmetric stretching vibrations of the –COO- groups

formed by PVA oxidation, that coordinates with the metal

ions present in the system. The redox reaction is complete,

as evidenced by the absence of the band located at

1,384 cm-1.

The gels thermally treated at 300 �C are more similar,

due to the oxidative decomposition of the major part of

both metal carboxylates and condensed PVA chains inside

the silica network. In the case of FeZnPVSi gel, the band

corresponding to the vibrations of O–H bonds from H2O

molecules or –OH groups associated through hydrogen

bonds is very large with various shoulders, showing the

existence of different types of –OH groups in this com-

posite. In the case of the composites obtained at 500 �C,

the FT-IR spectra are very similar, due to the complete

decomposition of the organic part. The band in the range of

3,000–3,700 cm-1 makes the difference between the two

0 50 100 150 200 250 300

Temperature/°C Temperature/°C350 400 450 500 550 0 50 100 150 200 250 300 350 400 450 500 550

0

–10

–20

–30

–40

–50

–60

–70

–80

–90

Mas

s lo

ss/%

0

–10

–20

–30

–40

–50

–60

–70

–80

–90

Mas

s lo

ss/%

DTA

sig

nal/a

.u.

(exo

up)

DTA

sig

nal/a

.u.

(exo

up)

460°

TG

DTA

340°

TG

DTA

(a) (b)

Fig. 4 TG and DTA curves of the gels thermally treated at 150 �C a PVSi; b FeCuPVSi

Thermal decomposition of metal nitrates

123

powders PVSi and FeZnPVSi; thus, in the later case, a

pronounced shoulder at 3,604 cm-1 indicates the presence

of some inside isolated –OH groups [16].

Taking into account the thermal behavior of the gels, we

have chosen to anneal all the gels at temperatures higher

than 400 �C, which are 600, 800, and 1000 �C.

In order to study the behavior of each metal oxide inside

the silica matrix, we have characterized the single oxide

composite (FePVSi, ZnPVSi, NiPVSi, CuPVSi) annealed

at 600 and 1000 �C by X-ray diffraction. The obtained

XRD patterns are presented in Fig. 6a for 600 �C and

Fig. 6b for 1000 �C.

The XRD pattern (1) of the gel FePVA annealed at

600 �C (Fig. 6a) evidences an amorphous state, with a

beginning of crystallization in the 2-Theta region charac-

teristic to the maghemite phase [17]. The powder ZnPVSi

obtained at 600 �C is completely amorphous, with no dif-

fraction peaks on the corresponding XRD pattern (2). In the

case of composite NiPVSi, the XRD pattern (3) clearly

evidences the crystallization of NiO inside the silica matrix,

while in the case of the sample CuPVSi, the pattern (4)

from Fig. 6a) presents the diffraction peaks corresponding

to CuO.

When the gels are annealed at 1000 �C (Fig. 6b), the

diffraction peaks are quit different. Thus, in the case of the

sample FePVSi, diffraction peaks characteristic to Fe2O3 as

the hematite phase and maghemite phase are present in the

corresponding XRD pattern (1). This confirms the forma-

tion of c-Fe2O3 at a lower temperature, which turns into a-

Fe2O3 at higher temperatures. The presence of c-Fe2O3 in

the composite after calcinations at 1000 �C indicates a

stabilization of this phase inside the silica matrix. This is

favorable for the obtaining of spinel ferrite embedded in

silica matrix, due to the higher reactivity of the spinel

phase c-Fe2O3 compared to a-Fe2O3. In the case of sample

ZnPVSi annealed at 1000 �C, the diffraction pattern evi-

dences, beside the crystallization of ZnO, the formation of

Zn2SiO4 due to the interaction of ZnO with the silica

matrix at high temperatures. In the case of the CuPVSi

composite, there is only one crystalline phase for cupper—

CuO, but additional diffraction peaks appear due to the

crystallization of silica as the cristoballite phase. This

may be due to a possible physical interaction between

cupper ions and the silica matrix that promotes silica

crystallization.

Taking into account the appearance of the secondary

phases after the direct thermal treatment of the gels at

1000 �C, we have decided to apply a controlled thermal

treatment to the gels FeNiPVSi, FeZnPVSi, FeCuPVSi in

order to obtain pure ferrite nanoparticles in silica matrix.

Thus, we have decomposed the carboxylate-type precur-

sors by annealing the samples at 400 �C for 3 h and then

4000 3500 3000 2500 2000

Wavenumber/1/cm Wavenumber/1/cm1500 1000 500 4000 3500 3000 2500 2000 1500 1000 500

Tra

nsm

ittan

ce/a

.u.

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nsm

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.u.

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1080 451

2940 1716

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565

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453

500°

300°

150°

60°

500°

300°

150°

60°(a) (b)

Fig. 5 FT-IR spectra of the gels thermally treated at different temperatures a PVSi; b FeZnPVSi

M. Stoia et al.

123

annealing them at 600 �C for 6 h. The powders obtained at

600 �C were further annealed at 800 and 1000 �C. The as-

resulted powders have been characterized by X-Ray dif-

fraction. Figure 7 presents the XRD patterns registered for

the samples FeNiPVSi (Fig. 7a), FeZnPVSi (Fig. 7b), and

FeCuPVSi (Fig. 7c).

The XRD pattern of the powder obtained by annealing

the precursor FeNiPVSi (Fig. 7a) at 600 �C shows not only

a beginning of crystallization for NiFe2O4, but also a small

diffraction peak that might correspond to metallic nickel,

formed inside the silica matrix and still unreacted. Starting

with 800 �C, well crystallized and pure nickel ferrite is

obtained inside the silica matrix, which presents a slight

tendency to crystallize as quartz [17].

In the case of the precursor FeZnPVSi (Fig. 7b), due to

the controlled thermal treatment, there are no secondary

crystalline phases present beside ZnFe2O4, which begins to

crystallize starting with 600 �C and is very well crystal-

lized at 1000 �C. One can notice that in this case, there is

no sign of any phase resulting from silica matrices’ crys-

tallization. This phenomenon is present in the case of the

composites obtained by annealing the precursor FeCuPVSi

(Fig. 7c) at high temperatures. Thus, the XRD patterns of

the powder annealed at 1000 �C present very pronounced

peaks characteristic to the cristoballite phase of SiO2 [17].

As already stated (Fig. 6a), it seems that copper ions are

responsible for this advanced crystallization of silica

matrix as cristoballite, as the other two composites do not

present this phenomenon. In the literature, this effect was

reported previously for CuFe2O4/SiO2 composites, but no

clear explanations were given.

In order to analyze the distribution of MFe2O4 particles

inside the silica matrix, we have performed elemental

mapping on the composites MFe2O4/SiO2 obtained at

800 �C. Figure 8 presents M, Fe, and Si mapping for the

three nanocomposites: NiFe2O4/SiO2 (Fig. 8a), ZnFe2O4/

SiO2 (Fig. 8b), and CuFe2O4/SiO2 (Fig. 8c). From these

maps, it can be seen that all ferrites are homogenously

dispersed within the silica matrix.

The composites NiFe2O4/SiO2 obtained by annealing of

the precursor gels at 1000 �C have been characterized by

TEM microscopy in order to confirm the nanometric scale

of the embedded MFe2O4 particles. TEM images presented

in Fig. 9a, b show that both NiFe2O4 and ZnFe2O4 are

obtained as fine nanoparticles, with diameters up to 20 nm,

uniformly dispersed within the silica matrix. In the case of

CuFe2O4/SiO2 (Fig. 9c), the ferrite particles are less visi-

ble, probably due to the advanced crystallization of the

silica matrix as cristoballite.

The behavior in an external magnetic field of the

nanocomposites MFe2O4/SiO2 obtained at 1000 �C was

also studied (Fig. 10). From the presented magnetization

curves, it can be seen that they do not reach complete

saturation at 5 kOe. This is often observed in nanosized

materials and can be ascribed to the presence of a spin

disordered surface layer which requires a larger field to be

6 10 15 20

2

Inte

nsity

/a.u

.

Inte

nsity

/a.u

.

θ scale/° 2θ scale/°25

Fe2O3-Maghemite-Q,syn(25-1402)Fe2O3-Hematite,syn(89-0599)

SiO2-Cristoballite beta,syn (89-1403)

Zn2SiO4-Willemite(70-1235)

ZnO-Zincite,syn(36-1451)

CuO-Tenorite,syn(48-1548)

NiO-Bunsenite,syn(47-1049)

Fe2O3-Maghemite-Q,syn(25-1402)

CuO-Tenorite,syn(48-1548)

NiO-Bunsenite,syn(47-1049)

Ni-Nickel,syn(04-0850)

30 35 6 10 15 20 25 30 35

(4)

(3)

(2)

(1)

(4)

(3)

(2)

(1)

(a) (b)

Fig. 6 XRD patterns of the composites obtained at a 600 �C and b 1000 �C from the gels: (1) FePVSi; (2) ZnPVSi; (3) NiPVSi; (4) CuPVSi

Thermal decomposition of metal nitrates

123

8 10 15 20

2θ scale/° 2θ scale/° 2θ scale/°25 30 35

600°

800°

1000°

600°

800°

1000°

600°

800°

1000°

8 10 15 20 25 30 35 8 10 15 20 25 30 35

Inte

nsity

/a.u

.

Inte

nsity

/a.u

.

Inte

nsity

/a.u

.

NiFe2O4-Trevoite,syn(86-2267)

SiO2-Quartz(88-2302)

Ni-Nickel,syn(04-0850)

ZnFe2O4-Zine iron oxide(82-1049) CuFe2O4-Copper iron oxide(77-0010)

SiO2-Cristoballite beta,syn(82-1403)

CuO-Tenorite,syn(48-1548)

(a) (b) (c)

Fig. 7 XRD patterns of the composites obtained by annealing the precursor a FeNiPVSi, b FeZnPVSi, and c FeCuPVSi at different temperatures

Fig. 8 M, Fe, and Si elemental maps for the nanocomposites obtained at 800 �C a NiFe2O4/SiO2; b ZnFe2O4/SiO2; c CuFe2O4/SiO2

M. Stoia et al.

123

saturated together with the effect of the small size of ferrite

nanoparticles with not very high anisotropy [18]. The

estimated values for the saturation magnetizations of the

three nanocomposites are 18.2 emu g-1 for NiFe2O4/SiO2,

6.5 emu g-1 for ZnFe2O4/SiO2, and 21.8 emu g-1 for

CuFe2O4/SiO2. All estimated values are lower than the

ones reported for the bulk materials due to the small size of

the magnetic nanoparticles dispersed in the nonmagnetic

silica matrix and the surface effect (large influence of the

surface atoms) [19]. The unusual high coercive field (Hc)

for the nanocomposites CuFe2O4/SiO2 may be explained

by the advanced crystallization of the silica matrix, which

creates additional tensions in copper ferrite lattice.

Conclusions

Nanocomposites MFe2O4/SiO2, for M = Ni, Zn, and Cu,

have been successfully prepared by the sol–gel method

using poly(vinyl alcohol), tetra(ethyl)orthosilicate, and the

mixture of metal nitrates. The interaction between the PVA

and nitrate ions inside the silica matrix was studied for each

metal nitrate. Taking into account the interaction of zinc

with the silica matrix, a controlled thermal treatment was

applied to the gels containing the mixture of M(II) and

Fe(III) nitrates. The redox interaction between the PVA

and the mixture of metal nitrates in silica matrices’ pores

was evidenced by thermal analysis.

As a result of this interaction, carboxylates of M(II) and

Fe(III) form embedded in silica matrix. By thermal

decomposition of these precursors and controlled calcina-

tions, in all cases, homogenously dispersed ferrite nano-

particles were obtained inside the silica matrices. In the

case of CuFe2O4/SiO2 nanocomposites, an advanced crys-

tallization of the silica matrix occurred. The magnetic

behavior of the final MFe2O4/SiO2 nanocomposites was

characteristic to this type of nanocomposite, but for

M = Cu, it was influenced by the crystallized silica matrix.

Acknowledgements We are thankful to the University ‘‘Politeh-

nica’’ of Timisoara, Romania, for financing the participation in

CCTA-11 conference. We are also thankful to PhD. Aurel Ercuta,

from the West University of Timisoara, the Faculty of Physics, for the

magnetic measurements performed.

Fig. 9 TEM images of the nanocomposites annealed at 1000 �C

a NiFe2O4/SiO2; b ZnFe2O4/SiO2; c CuFe2O4/SiO2

20(3)

(1)

(2)

10

0

–10

–20–5 –4 –3 –2 –1 0

H [kOe]

σ [e

mu/

g]

1 2 3 4 5

Fig. 10 Magnetization curves of the nanocomposites MFe2O4/SiO2

annealed at 1000 �C (1) M = Ni; (2) M = Zn; (3) M = Cu

Thermal decomposition of metal nitrates

123

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