Journal of Sol-Gel Science and Technology 35, 175181, 2005c 2005 Springer Science + Business Media, Inc. Manufactured in The Netherlands.

Sol-Gel Synthesis of Nanoscaled Spinels Using Propylene Oxideas a Gelation Agent

HONGTAO CUI, MARCOS ZAYAT AND DAVID LEVYInstituto de Ciencia de Materiales de Madrid, C.S.I.C, 28049 Cantoblanco, Madrid, Spain

htcui@icmm.csic.esmarcos.Zayat@icmm.csic.es

d.levy@icmm.csic.es

Received October 14, 2004; Accepted March 31, 2005

Abstract. Spinel nanoparticles of CoAl2O4 (blue), CoCr2O4 (bluish green), ZnFe2O4 (brown) and CuCr2O4(black) were synthesized by a sol-gel route with propylene oxide as a gelation agent. This method has proven to bean effective route to synthesize mixed oxide nanoparticles, especially for that with one of metal ion having a formalcharge of less than +3. Transmission electron micrographs show the small particle size (less than 60 nm) of thefour pigments and their narrow particle size distributions.

Keywords: sol-gel, nanoparticles, mixed oxide, epoxide

1. Introduction

Mixed oxides with spinel structure (AB2O4) are im-portant inorganic metalloid materials, widely used ascatalysts [1], cathode materials [2], heat-resistant pig-ments [3], etc. Spinels are usually synthesized bythe conventional high temperature method of solid-state chemistry [4], which results in spinel parti-cles with low surface areas. In order to synthesizespinels with high surface area, different wet chemistrytechniques have been attempted such as coprecipita-tion [5], polymeric gel [6], hydrothermal method [7],microemulsion [8], heterogeneous precipitation [9],sonochemical method [10], combustion [11], sol-gel[12], etc. These methods allow a substantial reductionof the temperature of processing, minimizing there-fore, the undesired aggregation of the particles duringcalcination.

Among these methods, the sol-gel process showspromising potential for the synthesis of mixed oxides,owing to its high purity, good chemical homogeneity,

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low calcination temperatures, etc. The major disadvan-tages of using the metal alkoxide based sol-gel pro-cess are its moisture sensitivity and the unavailabil-ity of suitable commercial precursors especially formixed-metal oxides. The sol-gel synthesis of mixedoxides from alkoxide mixture usually suffers from thedifferent hydrolysis susceptibilities of the individualcomponents and the benefits of improved homogene-ity can be lost during the hydrolysis of the alkoxides,which may ultimately lead to component segregationand mixed phases in the final materials. To achievehomogeneous mixed oxides with predetermined com-positions, the difference in reactivity has been mini-mized by controlled prehydrolysis of the less reactiveprecursor [13], by chemical modification of the pre-cursors [14], by using single-source heterobimetallicalkoxide precursors [15], or by non-hydrolytic sol-gelprocesses [16]. Non-alkoxide sol-gel process, involv-ing hydrolysis and condensation of metal salts, avoidsthe disadvantage of alkoxide sol-gel process (high sen-sitivity to moist environment), however, has still thedisadvantage of different hydrolysis susceptibilities ofthe individual components.

176 Cui, Zayat and Levy

(1)

Another non-alkoxide sol-gel route involves the us-age of an epoxide as a gelation agent. This procedurerequires relatively few steps to obtain metal oxides,and is low-temperature and low-cost. The epoxide actsas an acid scavenger through protonation of the epox-ide oxygen and subsequent ring opening by the nucle-ophilic anionic conjugate base. According to the reac-tion Eq. (1), epoxide consumes protons from the aquacomplexes [M(H2O)x ]n+, which promotes the hydrol-ysis and condensation of the complexes resulting in theformation of a gel [17]. Some metal oxide aerogels [1720] and nanoparticles [2123] have been prepared bythis process. For mixed oxides, only lanthanide-silicategels were prepared at Lawrence Livermore NationalLaboratory [24], silicate-aluminate by Itoh et al. [25]and yttrium doped zirconia by Xie et al. [21]. However,no work has been reported on the synthesis of bimetal-lic mixed oxides by the epoxide method in which oneof metal ions has a formal charge of less than +3. Asstated by Gash et al. [17, 18], metal oxide gels can-not be prepared from aqueous metal ions that have aformal charge of less than +3 by the epoxide method.Gash claimed that the acidities of the aquo complexes[M(H2O)x ]n+ of M2+ ions are much lower than thoseof the Mn+ ions with (n = 3, 4, 5, 6). This would slowdown the protonation of the added epoxide with a sub-sequent rise in the pH. The slow rate of the process mayallow alternative side reactions to occur to a significantdegree and cause precipitation to take place.

The aim of this work is to apply the sol-gel epoxideroute for the preparation of spinel nanoparticles, us-ing propylene oxide as the gelation agent. In order toshow the capability of this method for the preparationof different mixed oxides, several pigments with spinelstructure, such as CoAl2O4 (blue), CoCr2O4 (bluishgreen), ZnFe2O4 (brown) and CuCr2O4 (black) will beprepared, all of them having one of metal ions with aformal charge of less than +3.

2. Experimental Section

2.1. Materials

Cobalt nitrate hexahydrate (Co(NO3)26H2O), chrom-ium nitrate nonahydrate (Cr(NO3)39H2O), ferric

nitrate nonahydrate (Fe(NO3)39H2O), zinc nitratehexahydrate (Zn(NO3)26H2O) and propylene ox-ide (PPO) were obtained from Aldrich; Aluminiumnonahydrate (Al(NO3)39H2O) and copper nitratehemipentahydrate (Cu(NO3)22.5H2O) were fromFluka; Ethanol absolute was from merck. All thesereagents were used as received.

2.2. Preparation of Pigments Nanoparticles

All syntheses were performed under the same experi-mental conditions except for the different calcinationtemperatures (see Table 1). In a typical synthesis ofCoAl2O4 nanoparticles, a 15 ml clear pink ethanol solu-tion was obtained by dissolving 0.62 g Co(NO3)26H2Oand 2.59 g Al(NO3)39H2O ([M2+Co + M3+Al ] =0.6 mol l1) in ethanol. After the addition of 4.96 gpropylene oxide (propylene oxide/(M2+Co + M3+Al ) = 10)to the ethanol solution, an exothermic reaction occurredwithin several minutes, followed by a gradual gel for-mation from the top to the bottom of the solution. Thegel formation time, which depends on the metal ions,metal ion concentration and mol ratio of propylene ox-ide, varied from a few seconds to several hours. In theabove-mentioned case of CoAl2O4, the gel formationoccurred within 3 minutes and the gel became light pur-ple within several minutes. The gel was aged in a closedvessel at 50C for 3 hours, dried in an open vessel at50C for 24 hours and then treated at 100C for 12 addi-tional hours. The resulting xerogel was ground to pow-der and calcined at the given temperature for 1 hour toobtain nanoparticles. In order to obtain nanoparticles,the calcination temperature of each pigment should be

Table 1. Gel formation time, calcination temperature andparticle size obtained by TEM.

Approximate gel Calcination ParticleSample formation time temperature (C) size (nm)

CoAl2O4 3 minutes 750 1050CoCr2O4 21 minutes 700 1020CuCr2O4 21 minutes 700 2060ZnFe2O4 a few seconds 700 820

Sol-Gel Synthesis of Nanoscaled Spinels Using Propylene Oxide as a Gelation Agent 177

the lowest temperature needed for the formation of thespecific clean colored phases, reducing the aggregationof the particles during calcination.

2.3. Characterization

The thermal behaviour (TG/DTA) of the samples wasstudied by a Seiko SSC/5200 (TG/DTA 320U) in staticair atmosphere from ambient temperature to 1200C ata heating rate of 10C/min. The XRD patterns of thesamples were measured in a Philips PW 1710 diffrac-tometer using Cu K radiation. The morphology of theparticles was observed by a JEOL 2000 transmissionelectron microscope working at 200 kV, and the par-ticle size and size distribution were estimated in theTEM micrograph.

3. Results

3.1. Gel Formation

In multimetallic systems, phase segregation is usuallyobserved due to the different hydrolysis rates of the hy-drated metal ions of the different components. In thecase of CoAl2O4, the hydrolysis and condensation rateof Al3+ alone in ethanol solution was rather fast, form-ing gels within a few minutes; on the contrary, thatof Co2+ alone in solution was much slower, giving aprecipitate about 10 minutes after the propylene oxidewas added. However, it is very interesting that a gelwas formed without any precipitation in the ethanolmixture solution of Al3+ and Co2+ ions after the ad-dition of propylene oxide. The colour of the mixedions gel changed from pink to light purple within sev-eral minutes after the gel was formed. After aging atambient temperature for several hours, a layer of pinksolution appeared on the upper part of the gel, showingthat the hydrolysis of the Co2+ was not complete. Af-ter two days of aging at ambient temperature, the pinkcolour of the solution disappeared, accounting for thecomplete hydrolysis of the Co2+. The same effect wasobserved when the gel was aged at 50C for 3 hours.

The different hydrolysis rates of individual precursorions may lead to certain inhomogeneity and formationof zones in the oxide gel that are rich in the ions exhibit-ing faster hydrolysis and condensation rates, resultingin a partial segregation of the phases and the formationof single oxides. This is particularly important for thepreparation of pigments, where even using stoichio-metric ratios, one of the metal oxides can be in excess

deteriorating the coloration of the bulk pigment. In theCoAl2O4 pigment, an excess of the white Al2O3 willreduce the intensity of the blue coloration of the pig-ment, while excess cobalt oxide will form a segregatedCo3O4 black phase, resulting in an important darken-ing of the pigment coloration, which will be discussedin the following part.

The processes of gels formation of CoCr2O4,CuCr2O4 and ZnFe2O4 are very different from thatof CoAl2O4. The formation of CoCr2O4 and CuCr2O4gels took at least 21 minutes, while only a few secondsis required for the formation of ZnFe2O4 gel.

In general, it is very interesting, that mixing M2+ andM3+ ions in the ethanol solution prevented the precip-itation of the M(OH)2 when the propylene oxide wasadded to the mixture.

3.2. TG-DTA Analysis

The four DTA/TG curves (Fig. 1) of the thermaldegradation of the CoAl2O4, CoCr2O4, CuCr2O4 andZnFe2O4 precursor powder with stoichiometric com-position show a progressive weight loss from roomtemperature to around 100C accompanied by a broadendothermic peak, which is attributed to the evap-oration of free water. Above this temperature, twoweak exothermic peak around 177, 244C and a verystrong and sharp exothermic peak around 280C, ac-companied by a steep weight loss, are observed in theDTA/TG curve of CoAl2O4 precursor due to the burn-ing of the organic matter. The thermal behaviour ofCoCr2O4, CuCr2O4 and ZnFe2O4 precursors is verydifferent from that of CoAl2O4 precursor. Only onestrong and sharp exothermic peak, accompanied bya steep weight loss, is found in their DTA curves at188C for CoCr2O4, 210C for CuCr2O4 and 226Cfor ZnFe2O4 due to the burning of organics. Abovethese temperatures only a small weight loss is observed,which is attributed to the release of water arising fromcondensation reactions.

3.3. Coloration and Crystallographic Analysis

3.3.1. CoAl2O4. Samples were synthesized using dif-ferent Co:Al mol ratios (0.034 Co/Al 0.67) andcalcined at 750C which is the lowest temperature forthe formation of the bright blue CoAl2O4 phase. Thiscalcination temperature is lower than that reported forother preparation methods [3].

178 Cui, Zayat and Levy

Figure 1. DTA-TG curves of stoichiometric CoAl2O4, CoCr2O4,CuCr2O4 and ZnFe2O4 precursors.

Figure 2. XRD patterns of CoAl2O4 particles prepared with dif-ferent Co/Al values and calcined at 750C.

All samples, except for the amorphous sample(Co/Al = 0.034), show the spinel structure of CoAl2O4(JCPDS 44-0160) even for the cobalt rich sample(Co/Al = 0.67) (Fig. 2). Due to the similar ionic ra-dius of Co3+ and Al3+, Co3O4 and CoAl2O4 have thesame spinel structure and it is very difficult to distin-guish them from the diffraction lines of XRD patterns.

The cobalt rich samples (Co/Al = 0.67 and 0.58)exhibited black coloration. According to the literature,the most stable cobalt oxide at room temperature is themixed-valence black Co3O4 that has the same spinelstructure as the aluminate; above 950C, the oxide isreduced to CoO [3]. Therefore, the black coloration ofthe two cobalt rich samples can only be attributed to thepresence of Co3O4. Then, the dark blue coloration ofthe stoichiometric sample (Co/Al = 0.5) and samples(Co/Al = 0.43 and 0.36) is attributed to the mixture ofblack Co3O4 and blue CoAl2O4. The bright blue col-oration of the aluminium rich samples (0.15 Co/Al 0.30) accounts for the presence of the blue CoAl2O4phase alone.

3.3.2. CoCr2O4, CuCr2O4 and ZnFe2O4. For CoCr2O4 CuCr2O4 and ZnFe2O4, on the contrary to what weobserved in CoAl2O4 samples, the pure spinel struc-ture could only be obtained using the spinel stoichio-metric M2+/M3+ = 0.5 atomic ratio. Figure 3 showsthe XRD pattern of CoCr2O4 prepared with differentCo/Cr values. In addition to the spinel CoCr2O4 phase(JCPDS 22-1084), samples prepared with Co/Cr valuesbelow 0.5 show a second phase that could be assigned toCr2O3 (JCPDS 38-1479). Samples with Co/Cr = 0.67

Sol-Gel Synthesis of Nanoscaled Spinels Using Propylene Oxide as a Gelation Agent 179

Figure 3. XRD patterns of CoCr2O4 prepared with different Co/Crvalues and calcined at 700C.

and 0.5 show the pure spinel CoCr2O4 structure. Dueto the similar ionic radius of Co3+ and Cr3+, it is verydifficult to distinguish between Co3O4 and CoCr2O4from the diffraction lines. Then, sample (Co/Cr = 0.67)is most probably a two phases mixture of CoCr2O4 andCo3O4 phases.

A similar behaviour was observed in CuCr2O4 andZnFe2O4 samples. Stoichiometric (Cu/Cr = 0.5 andZn/Fe = 0.5) samples showed only the spinel phase:CuCr2O4 (JCPDS 34-0424) and ZnFe2O4 (JCPDS22-1012) respectively, while any deviation from sto-ichiometry resulted in the presence, in addition to thespinel phase, of the corresponding oxide in excess: CuO(JCPDS 44-0706) or Cr2O3 (JCPDS 38-1479) in thecase of CuCr2O4 (Fig. 4) and ZnO (JCPDS 36-1451)or Fe2O3 (JCPDS 33-0664) in the case of ZnFe2O4(Fig. 5).

3.4. Transmission Electron Microscope Analysis

Transmission electron micrographs of the four pig-ments are shown in Fig. 6. The Fig. 6(a)(c) shownon-agglomerated and well-crystallised nanoparticlesof CoAl2O4 (Co/Al = 0.30), ZnFe2O4 (Zn/Fe = 0.5)and CuCr2O4 (Cu/Cr = 0.5) respectively having theirnarrow particle size distribution (see Table 1). Al-though Fig. 6(d) shows slightly agglomerated CoCr2O4(Co/Cr = 0.5) particles between 50150 nm, it couldbe clearly seen that the agglomerated particles are com-posed of small particles between 1020 nm.

Figure 4. XRD patterns of CuCr2O4 prepared with different Cu/Crvalues and calcined at 700C.

Figure 5. XRD patterns of ZnFe2O4 calcined prepared with differ-ent Zn/Fe values and calcined at 700C.

4. Discussion

4.1. CoAl2O4

The process of CoAl2O4 gel formation, together withthe existence of the black Co3O4 phase in the sam-ples with Co/Al between 0.43 and 0.5 and the exis-tence of only a bright blue CoAl2O4 phase in the alu-minium rich samples (0.15 Co/Al 0.30) suggestthe following result: in the process of gel formation, analumina gel was formed first without the formation of

180 Cui, Zayat and Levy

Figure 6. Transmission electron micrographs of CoAl2O4 (Co/Al = 0.30) (a), ZnFe2O4 (Zn/Fe = 0.5) (b), CuCr2O4 (Cu/Cr = 0.5) (c) andCoCr2O4 (Co/Cr = 0.5) (d).

cobalt hydroxide precipitate and the hydrated cobaltions were then hydrolysed and condensed with the alu-minium hydroxide at the surface of the alumina gel sur-face. During the calcination, an excess of cobalt, for the

formation of a layer of stoichiometric CoAl2O4 on thealumina surface, results in the formation of a mixturelayer of Co3O4 and CoAl2O4 in samples with Co/Alvalues between 0.36 and 0.5. In samples with a lower

Sol-Gel Synthesis of Nanoscaled Spinels Using Propylene Oxide as a Gelation Agent 181

amount of cobalt (samples with Co/Al between 0.15and 0.30), only a thin layer of stoichiometric CoAl2O4was formed on the alumina surface. The core-shell-type CoAl2O4 particles obtained here in samples with0.15 Co/Al 0.30 are composed of an alumina coreand a CoAl2O4 shell. However, the XRD patterns ofthese samples showed only the diffraction patterns ofCoAl2O4, suggesting that under the synthetic condi-tions the alumina in the core remains amorphous.

4.2. CoCr2O4, CuCr2O4 and ZnFe2O4

For CoCr2O4 and CuCr2O4, the difference in hydrol-ysis and condensation rates between Co2+, Cu2+ andCr3+ in ethanol is not large, resulting in the formationof homogeneous CoCr2O4 and CuCr2O4 gels. This issupported by the experimental results in which the purespinel structure could only be obtained for M2+/M3+= 0.5 and secondary phases were formed for otherM2+/M3+ values as shown in XRD patterns of CoCr2O4and CuCr2O4.

The hydrolysis and condensation rate of Fe3+ alonein ethanol solution was fast and iron oxide gels wereformed within a few seconds after propylene oxide hadbeen added, while precipitate was observed in Zn2+solution at least 5 minutes after addition of propyleneoxide. According to the explanation given for CoAl2O4gels, the formation of ZnFe2O4 gels should be simi-lar to that of CoAl2O4 resulting on the formation ofcore-shell particles. However, when a mixture of Znan Fe ions (Zn/Fe = 0.5) were treated with propyleneoxide, a homogeneous gel was obtained within a fewseconds, giving pure ZnFe2O4 particles after calcina-tions as shown in the XRD patterns of ZnFe2O4. It isvery clear that the presence of Fe ions in the mixtureenhances the reactivity of Zn ions resulting in simul-taneous hydrolysis and condensation of the two ionsand hence, in the formation of homogeneous ZnFe2O4gels.

5. Conclusions

In summary, spinel nanoparticles with one of the metalions having a formal charge of less than +3 were suc-cessfully synthesized through a sol-gel method thatuses propylene oxide as a gelation agent. One of theadvantages of this method is the important reduction

of the required calcination temperatures, as comparedwith previously reported methods, minimizing the un-desired aggregation of the particles. This method wasfound to be an effective route to synthesize mixed oxidenanoparticles with narrow size distribution.

The route described here can also be applied for thesynthesis of mixed oxides with other structures andmore than two metal components.

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