Crystallization Pathways of Multicomponent Oxide Nanocrystals

Oct 27, 2010 - Crystallization Pathways of Multicomponent Oxide Nanocrystals: Critical Role of the Metal Cations Distribution in the Case Study of Met...
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DOI: 10.1021/cg100959j

2010, Vol. 10 5176–5181

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Crystallization Pathways of Multicomponent Oxide Nanocrystals: Critical Role of the Metal Cations Distribution in the Case Study of Metal Ferrites Mauro Epifani,*,† Jordi Arbiol,‡ Teresa Andreu,§, and Joan R. Morante§, †

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Consiglio Nazionale delle Ricerche - Istituto per la Microelettronica ed i Microsistemi (C.N.R.-I.M.M.), o Catalana de Recerca i Estudis Avanc-ats (ICREA) and via Monteroni, I-73100 Lecce, Italy, ‡Instituci Institut de Ci encia de Materials de Barcelona, CSIC, Campus de la UAB, 08193 Bellaterra, CAT, Spain, § M2E-IN2UB-XaRMAE, Departament d’Electr onica, Universitat de Barcelona, C. Martı´ i Franqu es 1, 08028 Barcelona, CAT, Spain, and Institut de Recerca en Energia de Catalunya (IREC), C/Josep Pla 2, B3, E-08019 Barcelona, Spain Received July 20, 2010; Revised Manuscript Received October 1, 2010

ABSTRACT: Metal ferrite (MFe2O4, with M = Fe, Mn, Co, Ni, Cu, Zn) nanoparticles were synthesized by processing metal oxide sols in a coordinating environment. The sols were prepared by forced hydrolysis of the starting metal nitrates, in the presence of acetylacetone for avoiding precipitation. Two different processing routes were investigated. In the first, the sol was injected into a hot (160 °C) solution of dodecylamine in tetradecene. In the second route the injection environment was constituted by pure dodecylamine heated at the same temperature. The precipitate from the first route was heat-treated in air at various temperatures, from 200 to 500 °C. The redispersible nanoparticles from the second route were annealed in oleylamine at temperatures up to 220 °C. In the first case, crystallization was obtained only after heat-treatment at 500 °C, while 220 °C was sufficient for crystallizing the nanoparticles dispersed in oleylamine. The samples from the two routes were investigated by X-ray diffraction and transmission electron microscopy/electron energy loss spectroscopy in the case system of NiFe2O4. The product from the first route, after heating at 200 °C, was a disordered material, with a broad size distribution of aggregates and Ni depletion regions. The product from the second route was constituted by discrete nanoparticles with the correct cation stoichiometry. The interpretation of the results allowed concluding that obtaining simple structural reorganization in nanosized volumes is a key factor for crystallization under mild conditions.

Introduction The synthesis of ternary oxide nanocrystals is not as developed as that of binary systems, despite intensive efforts devoted to their preparation. Many relevant examples are already available.1 The main difficulty in their synthesis is most probably the different reactivities of the metal cation precursors. Phase separations must be avoided, and the different metal cations must reach the proper position in the final lattice, influencing the process kinetics. We have been developing in recent works a synthetic route for binary metal oxide nanocrystals.2 It is based on a sol-gel process in a hot coordinating amine environment. Two synthetic modifications were developed, depending on the use of amine solution in tetradecene or of pure amine. While the latter option results in redispersible nanocrystals, the former produces more aggregated species. After investigating several binary systems, we decided to test both synthetic modifications on ternary systems, choosing ferrites as a case study. The synthesis of ferrite nanocrystals is generally accomplished by reverse micelles3 or high temperature decomposition techniques.4 Our sol-gel route was peculiar, since it started from the same precursor, but it was possible to largely differentiate the nanoparticle synthesis and the crystallization treatment. In this way, we obtained two model systems, exemplifying solid state syntheses and colloidal systems, respectively. Our investigation allowed evidencing that nanocrystals can be prepared *To whom correspondence should be addressed. E-mail: mauro.epifani@ le.imm.cnr.it. pubs.acs.org/crystal

Published on Web 10/27/2010

under much milder conditions (short heating at 220 °C) if the two different metal cations are homogeneously distributed in nanosized regions. If extensive cross-linking occurs, depletion regions of one of the two cations may be present. We finally note that we could successfully prepare a whole series of metal ferrites: MnFe2O4, Fe3O4, CoFe2O4, NiFe2O4, ZnFe2O4, and CuFe2O4. Experimental Section 1. Chemicals. Metal nitrates (Fe(NO3)3 3 9H2O, Ni(NO3)2 3 6H2O, Cu(NO3)2 3 2.5H2O, Co(NO3)2 3 6H2O, Zn(NO3)2 3 6H2O, Mn(NO3)2 3 xH2O (x = 4-6)) were provided by Sigma-Aldrich. All the solvents, together with acetylacetone (acacH), were purchased from SigmaAldrich in analytical grade and used without further purification. We used n-dodecylamine (DA, 98%) in the solution processing, and oleylamine (OA, technical grade) in the nanoparticles crystallization step, all provided by Sigma-Aldrich. 2. Preparation of Precursor Solutions. In a glass beaker, 2.56 mmol (1.033 g) of Fe(NO3)3 3 9H2O was mixed with 1.28 mmol of the prescribed metal nitrate, followed by dissolution in 10 mL of methanol. Then, 0.78 mL of acacH was added, followed, after 15 min, by 0.35 mL of ammonia solution (30 wt % solution in water). No precipitation occurred even after many months. The precursor solution for pure iron oxide was identically prepared, only skipping the addition of the second metal nitrate. 3. Processing of Precursor Solutions. In a 500 mL three-neck flask, equipped with a condenser, a thermometer, and a septum, a solution of 1 mL of DA in 10 mL of tetradecene or 10 mL of pure DA was thoroughly degassed with nitrogen and heated up to 160 °C. The two synthesis typologies will be denoted with HT (high temperature) and SC (soft chemistry), respectively. When the temperature reached 160 °C, 2 mL of the starting solution was rapidly injected r 2010 American Chemical Society

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Scheme 1. Pictorial Illustration of (1) Homocondensation of Metal Oxide Species and of (2) the Desired Formation of Intimately Mixed Metal Oxide Species in the Precursor Solution

Figure 1. XRD patterns measured on the indicated as-prepared nanoparticles from the HT route.

through the septum, with evolution of vapors. The temperature dropped to about 80 °C. After 1 h, the flask was removed from the heater. An opaque slurry or a clear, deep red suspension was obtained in HT and SC syntheses, respectively. Methanol and toluene or only methanol was added for HT and SC syntheses, respectively, resulting in flocculation, after which the synthesis product was recovered by centrifugation and further washed in methanol. The deep red precipitate will be referred to as the “as-prepared nanoparticles” in the text. Then it was processed as follows. 4. Processing of Synthesis Products. The product from the HT synthesis was dried at 90 °C and heat-treated at temperatures from 200 to 500 °C for 1 h in a muffle furnace in air atmosphere, with a heating rate of 5 °C/min. The precipitate from the SC synthesis was dispersed in 5 mL of oleylamine, giving a clear, deep red suspension, which was poured into a 25 mL Teflon bottle and further sealed with a silicone tape. Then, the bottle was heated for 2 h at 190 or 220 °C in a muffle furnace, with a heating rate of 5 °C/min. After cooling, the product was recovered by addition of methanol and centrifugation, washed in acetone, and dried in air at 90 °C. 5. Characterization of the Materials. X-ray diffraction (XRD) measurements were performed on a Panalytical diffractometer working with Cu K radiation (λ = 1.5406 A˚) using a Bragg-Brentano geometry. High-resolution transmission electron microscopy (HRTEM) observations were carried out with a field emission gun microscope Jeol 2010F, which works at 200 kV and has a point-topoint resolution of 0.19 nm. Electron energy loss spectroscopy (EELS) spectra were obtained in a Gatan Image Filter (GIF 2001) coupled to the Jeol 2010F microscope. Spectra achieved an energy resolution of 1.2 eV.

Results and Discussion 1. Formation Conditions of the Spinel Phases in the HT and SC Routes. The starting solutions were prepared in order to enhance the intimate mixing of the different metal cations, as pictorially represented in Scheme 1. This issue could be particularly critical in our synthesis, due to the different hydrolysis rates of metal ions. For this reason, the metal salts were premixed and acetylacetone was

Figure 2. XRD patterns measured on the NiFe2O4 nanoparticles from the HT route, heat-treated at the indicated temperatures.

added for moderating the hydrolysis rate (see details in the Experimental Section). During the solution processing, DA acts as a catalyst of the cross-linking between the metal oxide species in the precursor solution, finally resulting in nanoparticles. At the same time, it is also a capping agent, limiting the nanoparticle growth. The HT route was designed with a very limited amount of dodecylamine (DA) in the injection stage, so resulting in partially aggregated nanoparticles. In the SC route, injection in pure DA resulted in completely isolated and redispersible nanoparticles. The aim was to obtain different as-prepared materials from the same precursor solution and to evaluate the effect on the nanocrystal formation. We first investigated the as-prepared HT materials, and the related XRD patterns are shown in Figure 1. For each composition, the patterns only show broad bands, indicating very small and, possibly, poorly crystallized nanoparticles. The as-prepared nanoparticles were heat-treated at higher temperatures for obtaining the spinel phases, using the NiFe2O4 system as a case study. The results are shown in Figure 2. The patterns begin to show the reflections of the

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spinel phase only after heat-treatment at 300 °C. The peaks in the 500 °C pattern all belong to the cubic phase of NiFe2O4.5 The heat-treatment at 500 °C was evaluated for the other ferrites, and the results are shown in Figure 3. The patterns clearly show the formation of the expected spinel phase for each composition.6 Only the Fe3O4 phase could not be obtained, since all iron is oxidized to Fe3þ, as expected (see the related XRD pattern in the Supporting Information, Figure S1). The inset in Figure 3 shows the peak shift depending on the ferrite composition. The trend is that expected from the metal cation size in the spinel structure. The XRD patterns of the SC as-prepared nanoparticles displayed similar features to those of Figure 1, again indicating kinetic hindering of crystallization at the low synthesis temperatures. Hence, the as-prepared nanoparticles were dispersed in OA, and the formation conditions of crystalline phases were again investigated in the case of NiFe2O4, with the results shown in Figure 4. A crystallization threshold temperature of 220 °C can be seen. We carried out a similar series of experiments even for CoFe2O4, and the results are also reported in Figure 4. An identical crystallization threshold appears. On this basis, all the systems were subjected to treatment at 220 °C in oleylamine, and the related XRD patterns are shown in Figure 5. The formation of the spinel phase for each composition is evident. The peak broadening masked the shift due to the different metal cations size. The formation of the Fe3O4 phase is a peculiar consequence of the oleylamine environment. Heating in oleic acid

Figure 3. XRD patterns measured on the indicated ferrite nanoparticles from the HT route, heat-treated at 500 °C. The inset shows the enlargement of the 55-65° range.

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provided a waxy, unidentified material. The reason for this behavior is not clear, but we have recently investigated the surface capping influence on the crystallization of CdSe nanoparticles,7 showing that the surface composition crucially determines the crystalline state of the nanoparticles after heating in a bath of capping molecules. 2. TEM/EELS Investigation of the Spinel Formation Pathways in HT and SC Syntheses. a. General Concept of the Different Formation Pathways. We observed that the nanoparticles from the SC route could be more easily crystallized, remaining in a small size regime. Those from the HT route were subjected to a substantial growth, needing 500 °C to be crystallized. For a phenomenological understanding of these results, we started from a qualitative view of the processes occurring during heating. Our concept is pictorially reported in Scheme 2. First of all, in both cases, we start from amorphous materials, as obvious in low temperature sol-gel processes dominated by fast cross-linking reactions. The formation of crystalline structures requires cation rearrangement and/or diffusion to the stable lattice sites. The structure of the regions where rearrangement is needed established the energy need of the process. Now, let us suppose that two kinds of metal cations are present, as in ferrites. They are indicated by the red and blue circles in Scheme 2. In this case, not all the lattice sites are

Figure 5. XRD patterns measured on the indicated nanoparticles heated in oleylamine at 220 °C.

Figure 4. XRD patterns measured on NiFe2O4 and CoFe2O4 nanoparticles from the SC route, heat-treated in oleylamine at the indicated temperatures.

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Figure 6. (A) HRTEM image of the as-prepared HT NiFe2O4 sample; the inset shows the related FFT; (B) HRTEM image of the HT NiFe2O4 sample heat-treated at 200 °C, showing the bigger nanoparticles; (C) same sample, showing the smaller nanoparticles.

Scheme 2. (A) Sol-Gel Materials with Long-Range Disorder, Induced by Uncontrolled Cross-Linking and (B) Colloidal Sol-Gel System Where the Disorder Is Contained in the Volume of the Isolated Nanoparticles

Figure 7. (A) General TEM view of the HT NiFe2O4 sample heattreated at 500 °C. The red arrows indicate the larger, amorphous nanoparticles, while the green-circled region comprises the nanocrystalline species. (B) HRTEM image of the green-circled region and, in the insets, higher magnification of the squared region and the related power spectra.

chemically equivalent. One cation cannot equivalently occupy any lattice site. It is necessary that a collective redistribution of the two cations to the required lattice sites takes place. This condition further enhances the energy requirement. Hence, we expect favored crystallization if we have smaller reaction volume, with the correct cation stoichiometric ratio. This situation is shown in Scheme 2B with respect to Scheme 2A. Now we see how this scheme represents our ferrites synthesis. Of course, this interpretation was suggested by an a posteriori analysis of the TEM/EELS results, but it was useful to anticipate here as a guideline for the discussion. b. Experimental Results and Discussion. TEM/EELS techniques were used for investigating the metal cation distribution and its dependence on the heat-treatment conditions. We considered again the NiFe2O4 system as a case study and first investigated the HT procedure. Figure 6 shows the TEM images of the as-prepared and 200 °C samples. In the asprepared sample, the mean nanoparticle size was 2.6 ( 0.6 nm. Power spectra obtained on HRTEM images showed that the nanoparticles had a poor crystallinity, in agreement with the XRD data (Figure 1). The rings were wide and not welldefined. The inner ring corresponds to the 0.251 nm spacing of the NiFe2O4 {131} family planes (the most intense for this cubic structure). Instead, the outer ring includes several weak spots placed at around 0.210 nm, which does not correspond to the NiFe2O4 structure. The indexing, together with low resolution images of the 200 °C sample, is shown in the Supporting Information (Figure S2). In the 200 °C sample, again, poor crystallization was observed, in agreement with the XRD pattern (Figure 2). Two kinds of nanoparticles appear: on one hand with sizes up to 20 nm, and a mean size

Figure 8. (left column) EELS spectra measured on the as-prepared (A), 200 °C (B), and 500 °C (C) HT NiFe2O4 samples. (right column) Related concentration profiles reported for Fe, Ni, and O.

of around 10 nm; on the other hand smaller particles with mean size of 2.5 ( 0.6 nm. The power spectra contained features similar to those of the as-prepared sample. Figure 7 shows the TEM images of the NiFe2O4 HT sample heat-treated at 500 °C. In this sample the mean NiFe2O4 nanocrystals size was 4.0 ( 0.8 nm, but some big nanoparticles had diameters up to 20 nm. The 9.3 nm size obtained from the XRD patterns is intermediate between such TEM values, indicating the size

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Figure 9. HRTEM image of the as-prepared pure Fe2O3 sample (A) and related power spectrum (B). The latter corresponds to the 0.251 nm distance in the Fe2O3 hematite structure. In part C, a high resolution image is shown of the same sample heat-treated at 500 °C, showing very large nanocrystals.

Figure 10. (A) HRTEM image of the SC NiFe2O4 sample heated at 190 °C. The inset shows the power spectrum obtained on the whole area. (B) HRTEM image of the SC NiFe2O4 sample heated at 220 °C. (C) Higher magnification of the squared region and related power spectrum.

broadening of the sample. In Figure 7, the big nanoparticles are indicated by red arrows. They do not correspond to the NiFe2O4 structure and, indeed, are amorphous. The NiFe2O4 nanocrystals are in fact the smaller species in dark contrast which appeared, for example, in the area circled in green. The HRTEM micrograph of the nanocrystals showed that they crystallized in the NiFe2O4 structure. The elemental distribution in the same samples was analyzed by EELS, and the results are shown in Figure 8 (the scanned region is shown in the Supporting Information, Figure S3). In the as-prepared sample, the elements were homogeneously distributed, with a slight Ni substoichiometry. After heat-treatment at 200 °C, the elemental distribution was much less homogeneous and there were regions without the presence of Ni, which were Fe-rich. In particular, in the regions containing the big particles, O, Fe, as well as Ni could be detected. However, when analyzing the space between the big particles, where small particles were placed, no presence of Ni was detected. Finally, after heat-treatment at 500 °C, a more regular elemental distribution was again observed, without areas depleted of Ni. Hence, the nanocrystals were formed by crosslinking of the initially small, amorphous nanoparticles, resulting (200 °C) in larger nanoparticles, up to 20 nm, and segregation of iron-rich species. The latter disappeared after heating at 500 °C, indicating further cross-linking with the other nanoparticles. These results indicate a continuous transformation of the material, where ongoing cross-linking reactions impose further rearrangement for reaching the final crystalline configurations. The complication arising from the presence of two cations could be evidenced as follows. Pure iron oxide samples were prepared and characterized by TEM. The results are reported in Figure 9. The presence of hematite (R-Fe2O3) nanocrystals in the as-prepared sample was clear, while the as-prepared NiFe2O4 sample did not contain them. After heat-treatment at 500 °C, very large hematite nanocrystals, up to 90 nm, were found.

Figure 11. Concentration profile for Fe, Ni, and O as obtained from the analysis of EELS spectra measured on SC NiFe2O4 samples heated in oleylamine at 190 °C (top) and 220 °C (bottom).

In the corresponding NiFe2O4 sample, large residual amorphous regions were present (see TEM results above). The critical role of Ni in the nanoparticle rearrangement was clear. The HT synthesis hence belongs to the situations represented by Scheme 2A and justifies the high temperature crystallization.We then carried out TEM/EELS investigation of the SC materials. In the sample heated at 190 °C (Figure 10), the nanoparticles were mainly amorphous, and the mean size was 2.7 ( 0.4 nm (in agreement with the 2.5 nm value obtained from XRD). Some nanocrystals were also observed, as shown in the Supporting Information (Figure S4). After heating at

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220 °C, crystallization was complete in the NiFe2O4 cubic structure, as shown in Figure 10 and in the Supporting Information (Figure S5 and S6), and in agreement with the XRD results in Figure 4. In this sample, the mean grain size of the nanoparticles was 4.1 ( 0.6 nm (in agreement with the 4.3 nm value obtained from XRD). Crystallization was obtained under much milder conditions with respect to the HT synthesis, and no amorphous species emerged from the analysis of several TEM images. The pressure in the sealed Teflon bottle may help the crystallization process, but the boiling point of OA (about 350 °C) is much higher than the processing temperatures. So, there is a very small difference between the oleylamine vapor pressure at 190 and 220 °C; it would not justify the largely different crystallization extent seen in the related XRD patterns. Above all, it could not justify the difference with the HT synthesis. For this reason, we investigated in detail the cation distribution even in this case. The concentration profile obtained from EELS spectra (the scanned region is shown in the Supporting Information, Figure S7), shown in Figure 11, displayed an oscillating signal, as expected from the presence of not-aggregated nanocrystals. No regions of Ni depletion were observed, but an oxygen substoichiometry in the 190 °C sample with some Ni deficiency with respect to the 220 °C sample. In the SC synthesis, we have discrete and separated nanoparticles already in the as-prepared materials. Moreover, we have just seen that there are no phase separations or Fe-rich regions. This situation resembles Scheme 2B above, and crystallization occurred by heating at only 220 °C. Conclusions The preparation of metal ferrite nanocrystals requires careful control of the initial cation distribution. If segregation of the involved metal cations and continuous cross-linking occur, the nanocrystal formation will require diffusion of the cations until the correct stoichiometry will be reached. Higher temperatures will be needed, resulting in bigger nanocrystals and broader size distribution. It is then essential to achieve intimate mixing of the two cations at the starting solution level and to avoid phase separations in the following processing steps. This result can be achieved by controlling the

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initial precursor chemistry and, in the case of hydrolytic chemistry, by avoiding extensive initial cross-linking. If discrete regions are created where the correct stoichiometry is preserved, nanocrystal formation is obtained by structural reorganization under mild conditions. Acknowledgment. The authors thank the XRD and TEM units of the Serveis Cientificotecnics of the Universitat de Barcelona. Supporting Information Available: XRD data and further TEM images of NiFe2O4 SC nanocrystals (pdf). This material is available free of charge via the Internet at http://pubs.acs.org.

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