New Synthesis of Ferrite–Silica Nanocomposites by a Sol–Gel Auto-Combustion

New synthesis of ferrite–silica nanocomposites by a sol–gel auto-combustion

C. Cannas, A. Musinu, D. Peddis and G. PiccalugaDipartimento di Scienze Chimiche, Cittadella Universitaria di Monserrato, Bivio per Sestu, 09042 Monser-rato, Cagliari, Italy (Tel.: +39-070-6754380; Fax: +39-070-6754388; E-mail: [email protected])

Received 12 September 2003; accepted in revised form 23 February 2004

Key words: autocombustion, sol–gel, ferrite, nanocomposite, nanoparticle, synthesis

Abstract

A sol–gel autocombustion method was used to synthesize nanometric metal-oxide powders, and was ex-tended for the first time to prepare ferrite–silica nanocomposites. The gels obtained by mixing suitableamounts of citric acid, metal nitrates, ammonia (pure phases) and tetraethylortosilicate (nanocomposites)were converted directly to ferrite (either c-Fe2O3 or CoFe2O4) or ferrite–silica composites through a rapidautocombustion reaction. The combustion involves a thermally induced autocatalytic oxidation–reductionreaction between the nitrate and the citrate ions. The sample characterization by X-ray diffraction,transmission electron microscopy and N2 physisorption measurements revealed nanosized pure phasepowders and nanocomposites in which small spherical nanoparticles (mean size 3.5 and 5.0 nm, respectivelyfor the c-Fe2O3 and CoFe2O4) are homogeneously dispersed over a mesoporous silica matrix.

Introduction

On account of their scientific and technologicalinterest, many preparation methods have beenproposed for the synthesis of nanometric metaloxide particles. Because of their advantages (ver-satility, possible manipulation of matter at themolecular level, better control of the particle size,shape and size distributions), the use of chemicalmethods has been rapidly growing (Edelstein &Cammarata, 1996; Nalwa, 2000). Potential diffi-culties, however, are present in chemical process-ing, arising from its complexity, possible occurringof impurities, economical drawbacks. In the lightof these considerations, great interest has attracteda very promising way proposed for the preparationof multicomponent oxide ceramic powders basedon a sol–gel autocombustion process. Precursorgels are often prepared from an aqueous solutionof the metal nitrates and an organic complexantsuch as citric acid (Roy et al., 1993; Chakrabortyet al., 1994, 1996; Devi & Maiti, 1994; Shafer et al.,

1997; Yue et al., 1999), carboxylate azides (Rav-indranathan, 1986, 1987), urea (Kingaley & Patil,1988), or glycine (Chick et al., 1990; Pedersonet al., 1991) .Due to the good capability of chelating metallic

ions and to low decomposition temperatures, citricacid is suited for obtaining precursors of transitionmetal oxides (Pederson et al., 1991; Roy et al.,1993; Chakraborty et al., 1994; Devi & Maiti,1994; Shafer et al., 1997). Many nitrate citrate gels,when heated on a hot plate, burn in a self-propa-gating process, generating a large amount of heatand nontoxic gases in a short time, thus convertingthe precursor mixture directly to the productwithout any further calcination treatment (Cha-kraborty et al., 1994; Yue et al., 1999). The citricacid plays two important roles: on one hand, it isthe fuel for the combustion reaction; on the otherhand, it forms complexes with metal ions pre-venting the precipitation of hydroxilated com-pounds; the nitrate ion is the burning oxidizer.Moreover, among the various complexants

Journal of Nanoparticle Research 6: 223–232, 2004.� 2004 Kluwer Academic Publishers. Printed in the Netherlands.

mentioned above, citrate gel appears to be the leastexplosive and, therefore, relatively more safe(Chakraborty & Maiti, 1996).The above procedure has the advantages of

inexpensive precursors, simplicity, short prepara-tion time, good quality of the powders, moderateheating; furthermore, it does not require specia-lized equipment, so large quantities can be easilyprepared. Consequently, a variety of ceramicnanopowders have been produced (Chakraborty &Maiti, 1996; Vaqueiro & Quintela, 1998; Devi etal., 2002; Marinsek et al., 2002; Xu et al., 2002),including many fine-grained ferrites materials(Mishra et al., 1997; Yue et al., 1999, 2000, 2001;Xu et al., 2000; Qi et al., 2003), which are of vitaltechnological interest in many fields (biomedicines,magnetic suspensions, information storage, catal-ysis).Our research group has recently prepared

ferritic nanopowders in the system CoFe2O4, bothby a sol–gel (Ennas, 2003), and by a mechano-chemical route (Ennas, in press). In order to op-pose the instability of the nanopowders and theirtendency to agglomerate, nanocomposites havebeen synthesized, dispersing these ferrite particlesin an amorphous SiO2 matrix. On the other hand,the above mentioned sol–gel autocombustionmethod has been used exclusively to prepare purenanopowders, which are highly reactive.Therefore, we propose a novel method that

combine the traditional sol–gel and the sol–gelnitrate–citrate autocombustion in order to provideferrite–silica nanocomposites. In the present paperwe report the preparation of some ferritic nano-powders (c-Fe2O3, CoFe2O4). In the first step, pureoxides were synthetized, in order to test themethod in the simplest conditions. Afterwards, thepreparation of nanocomposites was attempted.Microstructural characterization, performed by X-ray diffraction (XRD) and trasmission electronmicroscopy (TEM), showed that the desired fer-ritic nanophases are easily formed and the condi-tions have been found to obtain ferrite–silicananocomposites.

Experimental and results

The preparation methods for the pure phases andfor the nanocomposites respectively will be de-scribed in the following subsections.

Powder X-ray diffraction spectra of the sampleswere collected using a D500 Diffractometer Sie-mens with a conventional h–2h Bragg Brentanogeometry, Mo -Ka wavelength, graphite mono-chromator on the diffracted beam and scintillationcounter. As a first approximation, the particle sizewas calculated by the X-ray line broadeningmethod using the Scherrer formula.Transmission electron microscopy micrographs

were obtained using a JEOL 200CX microscopeoperating at 200 kV. Finely ground samples weredispersed in octane and further submitted toultrasonic bath. The suspensions were then drop-ped on conventional copper grids for the obser-vations. Selected area diffraction (SAD)micrographs were obtained with an incidentwavelength of about 2.51 · 10)3 nm and a cameralength of 82 cm.Thermogravimetry (TG) and simultaneous dif-

ferential thermal analysis (DTA) of the sampleswere carried out on a Mettler–Toledo TGA/SDTA851. Thermal analysis data were collected in therange 25–1000�C with an heating rate of10�C min)1 under oxygen flow (flow rate ¼ 50ml min)1).N2-physisorption measurements at 77 K were

carried out on a Sorptomatic 1990 System (FisonInstruments). Before analysis, the samples wereoutgassed at 180�C for 18 h by heating at a rate of1�C min)1 under vacuum.

Pure oxide powders

Nanocrystalline c-Fe2O3 and CoFe2O4 powderswere prepared following a procedure similar to theone proposed by Yue et al. (2000) and Xu et al.(2000) and schematized in form of a flow diagramin Figure 1(a).Fe(NO3)3 Æ 9H2O (Aldrich, 98%), Co(NO3)2

Æ6H2O (Aldrich, 98%) and citric acid (Aldrich,99.5%) were used respectively as sources of metalions and chelating-fuel agent. Their aqueoussolutions were mixed together into a Pyrex beakerwith 1:1 molar ratio of metal to citric acid. Thecorresponding nitrate to citrate ratio value re-sulted 0.33 for the iron nitrate solution and 0.37for the iron–cobalt one. The clear orange brown(iron citrate) or purple (ironcobalt citrate) solu-tions thus obtained had a pH < 1. Aqueousammonia (NH4OH 30%) solution was then addeddrop by drop under constant stirring in order to

224

adjust the pH value to about 7. The dark brownsolutions were allowed to evaporate on a hot platemaintaining the solution temperature at 80–90�C.As a result of the increasing concentration, theviscosity rised due to the crosslinking of carboxy-lato–metal complexes into a three dimensionalstructure (Narendar & Messine, 1997), with ionicmetal carboxyl and ammonium–carboxyl bonds(Pierre et al., 1990), or hydrogen bonds (Jang etal., 1995), and, on further dehydratation, a gelstarted to form.On the basis of thermal analysis described be-

low, the gel was submitted to a sudden raising of

the temperature up to 250–300�C, that broughtabout the boiling and the swelling of the gel withevolution of a large amount of nontoxic gases(CO2, H2O, N2) (Jain et al., 1981). The flamelesscombustion started in the hottest zones of thebeaker and propagated from the bottom to the toplike the eruption of a volcano. The autocombus-tion was complete within few seconds giving rise togrey–blue (maghemite) or dark grey (cobaltferrite)branched voluminous powders that fill almostcompletely the beaker.Alternatively, in order to better control the

ignition temperature and to standardize the prep-aration procedure, the dried gel was put in a pre-heated oven at 300�C instead of in a hot plate asdescribed in literature.To investigate the decomposition mechanism of

the precursors and thereby the formation of themetal oxides, the procedure was controlled bythermal analysis.In Figure 2(a, b) TG and DTA curves of the

citrate gels prepared at pH ¼ 7 are compared withthe ones of solid citric acid. The citric acid exhibitstwo endothermic peaks at about 162�C and 207�Cdue to the conversion to aconitic acid C6H6O6 (–H2O) and itaconic acid C5H6O4 (–CO2), and anexothermic peak at about 460�C associated withthe complete decomposition of the polymerizeditaconic acid (Hon et al., 2001). TG and DTAcurves of the two dried gels are very similar and donot show the presence of traces of free citric acid.In fact, besides a weight loss of about 2–3% in the120–180�C temperature region attributable to thewater vaporization, only a vertical step at about200�C accompanied by a drastic weight loss (about80%) appeared. It corresponds to a sharp and in-tense exothermic peak in the DTA curves centeredat 193�C for the iron citrate gel and at 205�C forthe cobalt iron one. This behaviour indicates thatthe decomposition of the gel occurs suddenly as asingle step, as observed in other systems (Chakr-aborty et al., 1994; Devi & Maiti, 1994; Shafer etal., 1997; Mishra et al., 1997; Yue et al., 1999;2000; Xu et al., 2000). The absence of the purecitric acid exothermic peak at about 460�C in thegel curves indicate that the combustion reactionwas already completed. Moreover, an experimentcarried out on citrate gels containing Cl) ionsinstead of NO3

) did not exhibit autocombustion;this corroborates the hypothesis that the reactionis attributed to the presence of nitrate ions, that

Figure 1. Schematic representation of powder preparation

technique for the pure phases (a) and for the nanocomposites

(b).

225

induced an autocatalytic strongly exothermic oxi-dation–reduction reaction.The DTA measurements of the as-burnt pow-

ders indicated the presence of a unique exothermicpeak in the temperature region 280–400�C due tothe combustion of the carbonaceous residues thatare still present after the autocombustion. Thecorrespondent weight loss is about 3–4%. Thesevalues were confirmed by elemental analysis, thatalso evidenced the presence of 1.5–1.6% in weightof nitrogen, due to the slight stoichiometric excessof nitrate compared with citrate for the completeoxidation–reduction reaction (Roy et al., 1993).The XRD patterns of the citrate gels reveal the

amorphous nature of the powder. The patterns ofthe as-burnt powders (Figure 3) indicate thepresence of an ordered single ferrite phase with a

spinel structure. Moreover, in the iron oxidespectrum, besides the main cell reflections of thecubic maghemite, further less intense peaks arevisible, that can be attributed to the formation of atetragonal superstructure deriving from someordering of the vacancies in B-sites of the spinelstructure (Haneda & Morrish, 1977). The meanparticle sizes calculated by applying the Scherrerequation to the [2 0 6] (maghemite) and [2 2 0](cobaltferrite) reflections, are reported in Table 1.The error was estimated by repeating the mea-surements on three different samples.Further thermal treatments of the as-burnt pow-

ders showed that maghemite and CoFe2O4 phasesare stable respectively up to 350�C and 400�C,temperatures over which hematite starts to form.TEM investigation (Figures 4 (a, b) and 5(a, b))

of the as-burnt powders indicates for both samplesthe nanocrystalline nature of the combustion-de-rived powders with a rounded but irregular parti-cle shape. The particle size distribution calculatedover about 2000 particles on several dark fieldimages shows a quite large particle size distribu-tion (5–95 nm) and a mean particle size of 17 nm

0

20

40

60

80

100

Fe citrate gelCoFeCitric Acid

(a)

WeightLoss %

100 200 300 400 500T(°C)

EXO

ENDO

193°C

205°C

162°C207°C

460°C

(b)

citrate gel

Figure 2. TG (a) and DTA (b) plots of the pure phase gels

prepared at pH ¼ 7.

Figure 3. XRD patterns of the as-burnt pure phase powders.

Table 1. Average particle size obtained by different characterization techniques

Metal oxide BET surface

area (m2/g)

BET average

particle size (nm)

XRD average

particle size (nm)

TEM average

particle size (nm)

c-Fe2O3 31 ± 0.5 37.2 ± 0.4a 19 ± 1b 17 ± 4.0c

CoFe2O4 24 ± 1 48.1 ± 2.1 18 ± 1 15 ± 3.5

aThe error is calculated on two different measurements of the same sample.bThe error is estimated by repeating the measurements on three different samples.cThe error is calculated as standard deviation on about 2000 particles.

226

for the maghemite and 15 nm for the cobalt ferrite.On the top of the Dark field images the SADmicrographs are also reported. They confirm theoccurrence of a cubic cobalt ferrite phase and acubic maghemite with a tetragonal superstructure,suggested by XRD data.

N2 physisorption measurements were also per-formed for the as burnt powders. The surface areavalues obtained by the BET method are summa-rized in Table 1 together with the average particlesize. This was calculated by the formula d ¼ 6/qA,where q is the theoretical density of the mate-rial (5.12 g/cm3 for c-Fe2O3 and 5.20 g/cm3 forCoFe2O4) and A is the specific surface area. Thesmallest size of the particles (XRD, TEM) incomparison with the particle size (BET) is indica-tive of a low degree of agglomeration of the crys-tallites.

Nanocomposites

Two nanocomposites with nominal composition of50% in weight of iron oxide or cobaltferrite wereprepared by a traditional sol–gel technique(hydrolysis and condensation of alkoxide precur-sors) combined with the sol–gel autocombustionused to prepare pure phases (Figure 1(b)).To this end a �0.3 M ethanolic solution of

TEOS was added to an aqueous solution of metalnitrate and citric acid at pH ¼ 4 prepared as de-scribed before. The mixed solutions, having aEtOH/TEOS/H2O molar ratio of 1/0.01/1.90,exhibited a phase separation; ethanol used asmutual solvent for TEOS and water lowers themetal–citrate complex solubility, which is visible asan orange brown phase at the bottom of the Pyrexbeacker. The solutions were poured into a teflonbecker and allowed to gelate in an oven at 40�C.After about 6 h the ethanol evaporation and theevolution of TEOS hydrolysis and condensationreaction led to a homogeneous green–browntransparent solution that after about 24 h trans-formed into a gel. As for the pure phases, on thebasis of the thermal analysis, the gel was submittedto a thermal treatment at 300�C in a preheatedoven. The gel started to eliminate large amounts ofgases and burnt without any flame. The autoigni-tion was complete after about 1–2 min leaving acollapsed branched-shape material.Figure 6 reports the TG and DTA curves of the

iron cobalt citrate nanocomposite gel, comparedwith the ones of pure phase gel prepared atpH ¼ 4. The thermal behaviour of the two gels issimilar; both of them present a single stepdecomposition and therefore an autocombustionmechanism for the formation of the oxides. Somedifferences can be evidenced:

Figure 4. Dark field and SAD images (a) and particle size

distribution (b) of the as-burnt c-Fe2O3 powder.

Figure 5. Dark field and SAD images (a) and particle size

distribution (b) of the as-burnt CoFe2O4 powder.

227

– an increase of the weight loss in the 120–180�Cregion from 2% to 3%, for the pure phase gel, to12–13% for the nanocomposite gel, justified bythe vaporization of ethanol in addition to water;

– a slight shift and a significative broadening of theexothermic peak going from the pure phase gelto the nanocomposite gel. This effect can be dueto the presence of the silica network that delaysand slows down the autocombustion reaction.

No other peaks are visible in the curves confirmingthat the reaction was completed.The thermal behaviour and the elemental anal-

ysis of the as-burnt powders confirmed the pres-ence, as for the pure phases, of a 2–3% in weightof carbonaceous residues and a 1.5–1.6% ofnitrogen.The XRD patterns of the two nanocomposites

as-burnt are reported in Figure 7. They exhibitsimilar features, that is broad peaks attributableto a nanosized spinel phase (c-Fe2O3, CoFe2O4)superimposed to the amorphous silica halo.However, a faint peak at about 2Q ¼ 15,corresponding to the most intense d-spacing ofa-Fe2O3, is also present in the Fe2O3–SiO2

nanocomposite. The broadening of the peaks is

indicative of a greater particle size for theCoFe2O4 phase with respect to the maghemiteone.TEM observations (Figures 8(a, b), 9(a, b))

show for both samples the presence of roundednanoparticles homogeneously dispersed overthe silica matrix. A dark field micrographs of thec-Fe2O3–SiO2 composite, reported in Figure 9(a),indicates a narrow particle size distribution with amean particle size of 3.5 nm calculated over 1300

0

20

40

60

80

Co,Fe Composite gel pH4Fe,Co citrate gel pH4

(a)

(b)

Weight Loss %

100 200 300 400 500T(°C)

ENDO

EXO

203°C

210°C

∆T

Figure 6. TG (a) and DTA (b) plots of the pure phase and

composite gels prepared at pH ¼ 4.

Figure 7. XRD patterns of the as-burnt silica-based nano-

composites powders.

Figure 8. Dark Field image (a) and particle size distribution (b)

of the as-burnt c-Fe2O3–SiO2 nanocomposite powder.

228

particles. The mean value of the distribution isslightly shifted to higher values (5 nm) and thedistribution is larger for the CoFe2O4–SiO2

nanocomposite, in agreement with XRD data.Nitrogen physisorption measurements were

performed also on the nanocomposites samples.As an example, in Figure 10 adsorption–desorp-tion isotherms are reported. They show a featureof type IV in the IUPAC classification (Gregg &Sing, 1982), typical of mesoporous powders. Thesurface area calculated from the adsorption curveaccording the Brauner–Emmett–Teller (BET)method is ±190 m2/g. Pore size distribution cal-culated from the desorption branch curve by usingthe Dollimore–Heal method (Dollimore & Heal,1964) indicates a 98% in volume of mesoporousand a 2% of macroporous. The mesoporous natureof the matrix was observable also in the bright fieldTEM micrographs.

Discussion

The results confirm that the sol–gel autocombus-tion is a convenient method to obtain unsupportednanostructured metal oxides. In fact, single phaseoxides with a low content of carbonaceous resi-dues, that can be eliminated by mild calcinationtreatments, are easily achievable. The result isworth of note particularly for the maghemite case.In fact, the majority of synthesis methods (e.g.traditional sol–gel) lead often to mixtures of ma-ghemite and hematite. In this connection, it isprobable that the presence of carbonaceous resi-dues has been profitable. In fact, it has been fre-quently suggested that a reducting ambient (forinstance, due to organic residuals), favour theformation of c-Fe2O3 through the intermedi-ate formation of Fe3O4 (Del Monte et al., 1997;Solinas et al., 2001; Cannas et al., 2002). Becauseof the high speed of the autoignition reaction, thenanoparticles have not the time to form and growin a regular shape. Therefore, although they areroughly spherical, they present quite high values ofsurface area. This makes these products appealingfor catalytic applications, for which many ferriticoxides have been already tested.The preparation conditions described have been

selected as the best in a long series of experiments,by which the effect of the reaction parameters hasbeen explored. The amount of citric acid, pH ofthe gelling solutions and ignition temperature ofthe gels turned out the most critical factors, mainlyfor c-Fe2O3 preparation, for which the possibleformation of hematite imposes a more carefulcontrol of reaction conditions. The increase of thecitric acid concentration improves the gellingbehaviour but decreases the autocombustionreaction rate up to a citrate/nitrate ‡0.5 over which

Figure 9. Dark Field image (a) and particle size distribution (b)

of the as-burnt CoFe2O4–SiO2 nanocomposite powder.

1.0p/p0

0.0 0.2 0.4 0.6 0.8 1

10

n ads

/ m

mol

g-1

0

2

4

6

8

10

Figure 10. Nitrogen adsorption–desorption isotherm of the CoFe2O4–SiO2 nanocomposite powder.

229

the self-combustion is completely inhibited. ThepH value is a critical parameter in order to obtainpure maghemite that is achievable only at pH ¼ 7.A variation of the pH value between 4 and 7 doesnot change the result of the preparation for thecobaltferrite. The maghemite phase is achievableonly by treating the gel in a preheated oven in the290–325�C ignition temperature region; outside ofthis region different mixtures of maghemite andhematite phase form. On the other hand, cobaltferrite can be obtained either in oven or in a hotplate in a wider range of ignition temperatures.As regards ferrite–silica systems, the results

show that the right conditions have been foundalso for the preparation of nanocompositesthrough a fast autocombustion reaction, even if inthis case major difficulties have been encountered.Besides the control of the above mentionedparameters, in fact, the necessity to add ethanol asTEOS solvent lead to phase separation of the ini-tial sol. Moreover, in order to obtain compositesin which the active phase is well dispersed onto thesilica matrix, the gelation time of the silica pre-cursor should be similar to the one of the metalcitrate complex. For example, at the pH conditionsusually used for the pure phase preparation(pH ¼ 7), the TEOS does not react and it iseliminated during the self-combustion reactionleading to pure metal oxides. Interesting resultshave been obtained at pH ¼ 4 and at gelationtemperature of 40�C instead of 80–90�C. In suchconditions it has been possible to obtain the de-sired nanocomposites, even if the maghemite-based one is slightly contaminated by the hematitephase. A good homogeneity in terms of nanopar-ticles dispersion and size distribution has been alsoachieved. The reduced values of particle size andthe more regular shape on respect to the purephases are due to the effect of the matrix thatkeeps the particle separate and prevents theiraggregation. At the same time, the presence of thematrix makes the reaction less fast. Moreover, thematrix is mesoporous rather than microporous asin the traditional sol–gel xerogel product. This isan effect of the foaming action of citric acid, al-ready described in literature (Takahashi et al.,2000), and of the combustion nature of the reac-tion with evolution of large amounts of gases.Preliminary magnetic measurements show that

the nanocomposite samples are superparamagneticwith blocked state at room temperature (Cannas

et al., 2001). Considering the CoFe2O4 bulkanisotropy and the particle volume distribution, asdetermined by TEM analysis, the observed mag-netic behaviour can be mainly ascribed to thestrength of interactions among the nanoparticlesin the matrix, an expected effect of the high con-tent of ferrite in the material.The dispersion of the particles and their inter-

actions obviously strongly depend on the amountof metal oxide incorporated in the silica matrix.The nanoparticles concentration, however, cannotbe changed at will; in fact, in the dilute systemsthe abundance of TEOS and SiO2 could preventthe gel autocombustion to be realized and thefinal products could be obtained only throughusual calcination treatments. Also the possibilityof modulating mean particle size and size distri-bution is still to be ascertained. To this endcareful characterization of the precursors in thesystem metal salt–citric acid — TEOS is impor-tant. In fact, the growth of metal oxide certainlydepends on the interactions of metal ions with thecomplexant agents; on the other hand these de-pend on the interactions of TEOS with citric acid,which under suitable conditions give rise to aso called polymerizable complex. (Leite et al.,2002)

Conclusions

A sol–gel autocombustion method was used tosynthesize nanometric metal-oxide powders, andwas extended for the first time to prepare ferrite–silica nanocomposites. By a suitable choice ofreaction parameters (pH of the starting solutions,concentration of the reactants, ignition tempera-ture) gels obtained by mixing citric acid, metalnitrates, ammonia and tetraethylortosilicate wereconverted in ferrite (either c-Fe2O3 or CoFe2O4) —silica composites through rapid autocombustionreactions. According to X-ray diffraction, trans-mission electron microscopy and N2 physisorptionmeasurements, the composites turned out com-posed of very small nanoparticles (mean size 3.5and 5.0 nm, respectively for the c-Fe2O3 and Co-Fe2O4) homogeneously dispersed over the silicamatrix. This showed a mesoporous type adsorp-tion–desorption isotherm with surface area of190 m2/g.

230

Acknowledgement

This work was supported by MIUR (PRIN 2001)and CNR.

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