UDK 539.2:537.622 ISSN 1580-2949 Izvirni znanstveni članek MTAEC9, 37(3-4)129(2003) V. USKOKOVI], M. DROFENIK: SYNTHESIS OF NANOCRYSTALLINE NICKEL-ZINC FERRITES ... SYNTHESIS OF NANOCRYSTALLINE NICKEL-ZINC FERRITES WITHIN REVERSE MICELLES SINTEZA NANOKRISTALINIČNIH NIKELJ-CINKOVIH FERITOV V REVERZNIH MICELAH Vuk Uskokovi}1, Miha Drofenik2 1 »Jožef Stefan« Institute, Jamova 39, 1000 Ljubljana, Slovenija 2 Faculty of Chemistry and Chemical Engineering, Smetanova 17, 2000 Maribor, Slovenija vuk.uskokovicŽijs.si Prejem rokopisa - received: 2003-04-23; sprejem za objavo - accepted for publication: 2003-06-16 Nanocrystalline nickel-zinc ferrites were synthesized via a reverse micelle microemulsion route. The precursor cations were precipitated in the microemulsion system CTAB/1-hexanol/water. A subsequent oxidizing reaction was used to synthesize the nickel-zinc ferrite. The obtained nanoparticles were less than 20 nm in size. Keywords: nanomaterials, ferrite, powders synthesis, microemulsion, reverse micelle Nanokristalinične nikelj-cinkove ferite smo sintetizirali z reverzno mikroemulzijsko metodo. Prekurzorski kationi so bili precipitirani v mikroemulzijskem sistemu CTAB/1-heksanol/voda, z naslednjo reakcijo oksidacije pa je bil dobljen nikelj-cinkov ferit. Velikost sintetiziranih nanodelcev je bila manjša od 20 nm. Ključne besede: nanomateriali, ferit, sinteza prahov, mikroemulzija, reverzne micele 1 INTRODUCTION The reverse micelle synthesis of ferrite nanocrystal-lites has recently been shown to be a very promising method for obtaining ultrafine (superpara)magnetic particles with a number of actual and potential fields of application1-3. The microemulsion technique is an excellent method for the preparation of nanosized magnetic powders since themorphology of theparticles, uniform in terms of size and shape, can be effectively designed by changing the composition of the water-in-oil microemulsions in the applied phase diagram. The presence of water is important sinceit is ableto vary thesizeof thesurfactant aggregates, or so-called »water pool«, characterized by water-to-surfactant molar ratio (parameter w). In the process of synthesizing ultrafine ferrite materials within reverse micelles there are many parameters that influence the synthesized-powder morphology and stoichiometry, and hence its properties and applicability. In this study we limited our investigation to the influence of different proportions of the components of the system on the morphology and magnetization of the prepared ferrite nano powders. 2 EXPERIMENTAL PROCEDURE For the microemulsion system we used a three-component system consisting of hexadecyl trimethyl ammonium bromide(CTAB) as a surfactant, 1-hexanol as an oil phase and water as an aqueous phase4. In order to obtain theright composition, which would form reverse micelles, the composition of the water-in-oil microemulsions was modified. Microemulsions were formed at temperatures around 45 °C with well-defined amounts of solutions of ferro- (1 M), nickel- (0,25 M) and zinc- (0,25 M) sulphates in deionized water encapsulated in water pools of the microemulsion in proportions to match the desired ferrite stoichiometry, which was Ni0,5Zn0,5Fe2O4. A solution of NaOH was used as a precipitating agent. The precipitated samples were left to age for half an hour. After that time, hydrogen peroxide was added to themixture, and at this stagetheprocess of forming the spinel crystal structure of nickel-zinc ferrite was initiated. The co-precipitation synthesis of nickel-zinc ferrite, which takes place within the reverse micellar structures, can be described with the following chemical reaction: xNi2+ + yZn2+ + (3-x-y)Fe2+ + 6OH + O2 -> 2 -> NixZnyFe3-x-yO4 + 3H2O and in our experiments, x + y was set to be 1. Analytical meaurements were performed in order to detect possible leaching of Zn2+ ions. Themagnetization of the samples was determined using the Manics DSM10 suscepto-magnetometer. Specific surface-area measurements were performed with a Micromeritics Gemini II surface-area analyzer by using the multi-point BET method. With the Debye-Scherrer equation and the (311) X-ray diffraction line broadening, we obtained average particle sizes. TEM observations were performed in MATERIALI IN TEHNOLOGIJE 37 (2003) 3-4 129 V. USKOKOVI], M. DROFENIK: SYNTHESIS OF NANOCRYSTALLINE NICKEL-ZINC FERRITES Figure 1: Composition diagram of CTAB/1-hexanol/water microemulsion system with the synthesized samples labelled on the diagram Slika 1: Fazni diagram mikroemulzijskega sistema CTAB/1-heksanol/ voda z označenimi sintetiziranimi vzorci order to examinethemorphology of thepowders at the nanometer level. 3RESULTS AND DISCUSSION Figure 1 shows thepositions of thesynthesized samples (denoted as L, S...) in the phase diagram, and according to thereports of Ekwall et al.5, all thesamples fall into the region of reverse micelles. Both theCTAB/1-hexanol and H2O/CTAB ratios (the latter described by the parameter w = ŠH2O] / ŠS], where brackets indicate concentrations) were varied, which caused theaveragesizeof thegrains formed with the assistance of the reverse micelles to change as well. A characteristic XRD pattern of a representative sampleis shown in Figure 2. The powder is crystalline, with an averagegrain sizeof theferriteparticles of around 10 nm. Table 1 shows thecalculated averageparticlesizes from specific surface-area measurements as well as from the X-ray diffraction broadening of line (311), together with thesaturation magnetization of thesamples at room temperature, parameter w and the CTAB/hexanol weight ratio. Our results indicate that the synthesis of the nanosized nickel-zinc ferrite at different points in the phasediagram, shown in Figure1, yields products which differ in terms of their average grain size and magnetization. Table 1: Molar ratio of water to CTAB (w), CTAB/hexanol weight ratio (?), specific surfacearea of thesample(AS), averageparticlesize estimated from the samples' specific surface areas (dA), average particlesizeestimated from thebroadening of the(311) X-ray diffraction peak (dx) and room-temperature saturation magnetization (?s) Tabela 1: Molsko razmerje voda/CTAB (w), masno razmerje CTAB/ heksanol (?), specifična površina vzorcev (AS), povprečna velikost delcev, ocenjena iz specifične površine vzorcev (dA), povprečna velikost delcev, ocenjena iz širine (311) difrakcijskega vrha (dx) in nasičena magnetizacija pri sobni temperaturi (?s) Sample w a AS (m2/g) dA (nm) dx (nm) (emu/g) L 36,44 0,833 65 19,4 9,9 13.8 A2 29,57 0,527 90 13,9 12,9 21,1 A 24,63 0,552 172 7,3 10,1 27,3 E2 19,66 0,525 119 10,5 8,5 17,0 S 14,91 0,829 106 11,8 8,2 20,4 M 9,20 0,131 130 9,6 5,8 8,3 B 8,28 0,617 / / / 5,0 As can be seen from Table 1, theaverageparticle sizeis in therangeof 5.8 to 12.9 nm, whilethe saturation magnetization at room temperature is between 5 and 27.3 emu/g. The average particle sizes obtained from the specific surface-area measurements have, in most cases, greater values than the same quantity obtained from the X-ray diffraction measurements, which suggests that the final powders are agglomerated. Ferrites with such a small average particle size are expected to exhibit superparamagnetic behaviour. Nevertheless, the measured coercivities of samples A and E2 at room temperature gave us values of 25,4 Oe and 24,8 Oe, respectively, which is an obvious sign that Figure2: X-ray diffractogram of thesampleS. Thepeaks denoted with S stand for spinel crystal structure of nickel-zinc ferrite Slika 2: Rentgenski difraktogram vzorca S. Vrhovi, označeni z S, pomenijo spinelno kristalno strukturo nikelj-cinkovih feritov Figure 3: A TEM image of a typical nano powder, synthesized using a reverse micellar microemulsion mixture Slika 3: TEM-mikrograf tipičnega nanoprahu, sintetiziranega z reverzno mikroemulzijsko zmesjo 130 MATERIALI IN TEHNOLOGIJE 37 (2003) 3-4 V. USKOKOVI], M. DROFENIK: SYNTHESIS OF NANOCRYSTALLINE NICKEL-ZINC FERRITES a Figure 4: Saturation magnetization (a) and theaverageparticlesize (b) versus temperature of annealing for 2 hours in air Sliki 4: Nasičena magnetizacija (a) in povprečna velikost delcev (b) kot funkcija temperature segrevanja dve uri na zraku agglomeration, to a certain extent, occurred within those samples during their syntheses. Figure 3 shows us a typical electron micrograph of sample A, which was synthesized within reverse micelles. Wecan seethat thesampleconsists of very fine particles that are about 10 nm in size. These observations areconsistent with theaverageparticlesize obtained from the specific surface measurements (9.6 nm), whiletheaverageparticlesizeobtained from the broadening of the(311) diffraction lineis 5.8 nm. The tendency of the powder to agglomerate is also obvious. Even though the synthesized magnetic particles are singledomain in nature, theexchangeinteraction is not limited to individual particles and so the whole agglomerate in Figure 3 behaves as one giant magnetic particle showing no superparamagnetic behaviour, which was evidenced by the very large values of coercivity, typical for agglomerated magnetic nanosystems6. After annealing sample A in air at different temperatures for two hours, an increase in the values of the saturation magnetization was observed (Figure 4a): from about 27 emu/g, for the as-dried powder, a very slight increase for the sample heated at 400 °C, and then the saturation magnetization jumped to more than 90 emu/g for the sample which was heated at 1200 °C. A rearrangement of the magnetically disordered surface of the particles and a refinement of the crystal structure obviously led to a better superexchange interaction between the magnetic moments of individual ions and thus to a higher saturation magnetization. On theother hand, theheating of sampleA at different temperatures caused a decrease in the specific surface, which was induced by the growth of bigger grains as a result of thedisappearanceof thesmaller ones. The dependence of the average grain size, as calculated from the measured specific surface area, on thetemperatureof theannealing is shown in Figure 4b. 4 CONCLUSION Nickel-zinc ferrites synthesized via a reverse micelle microemulsion were nanosized. The properties of the powders changed significantly, depending on the composition of the microemulsion system. We found that together with varying the size of the reverse micelles described by the parameter w and weight proportion between the three basic components of the microemulsion mixture, various essential properties of the nanoferrite powders including average particle size, morphology, specific surface area, crystallization degree and saturation magnetization change as well. 5 REFERENCES 1 K. Raj, B. Moskowitz, R. Casciary - Advances in ferrofluid technology, J. Magn. Magn. Mater. 149 (1995) 174-180 2 I. Šafarik, M. Šafarikova - Magnetic nanoparticles and bioscience, Mont. Chem. 133 (2002) 737-759 3 D. O. Yener, H. Giesche - Synthesis of pure and manganese-, nickel-, and zinc-doped ferrite particles in water-in-oil micro-emulsions, J. Am. Ceram. Soc. 84 (2001) 9, 1987-95 4 M. P. Pileni - Reverse micelles as microreactors, J. Phys. Chem. 97 (1993) 6961-6973 5 P. Ekwall, L. Mandell, K. Fontell - The cetyltrimethylammonium bromide-hexanol-water system, J. Colloid Interface Sci. 29 (1968) 4, 639-646 6 C. P. Bean, J. D. Livingston - Superparamagnetism, J. Appl. Phys. 30 (1959) 4, 120S - 129S b MATERIALI IN TEHNOLOGIJE 37 (2003) 3-4 131 V. USKOKOVI], M. DROFENIK: SYNTHESIS OF NANOCRYSTALLINE NICKEL-ZINC FERRITES 132 MATERIALI IN TEHNOLOGIJE 37 (2003) 3-4