Synthesis and Electrochemical Behavior of Single-Crystal Magnetite

J. Phys. Chem. C 2008, 112, 5301-5306

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ARTICLES Synthesis and Electrochemical Behavior of Single-Crystal Magnetite Nanoparticles F. J. Santos,*,† L. C. Varanda,§ L. C. Ferracin,‡ and M. Jafelicci, Jr.† Laborato´ rio de Materials Magne´ ticos e Colo´ ides, Instituto de Quı´mica, UniVersidade Estadual Paulista (Unesp), 14800-900 Araraquara (SP), Brazil, Instituto de Quı´mica de Sa˜ o Carlos (USP), CP 780, 13560-970 Sa˜ o Carlos (SP), Brazil, and Votorantim Cimentos, Diretoria Te´ cnica, Laborato´ rio Central RodoVia PR-092, 1303, Abranches, 82130-570 Curitiba (PR), Brazil ReceiVed: August 24, 2007; In Final Form: January 22, 2008

Water-dispersed magnetite nanoparticle synthesis from iron(II) chloride in dimethyl sulfoxide (DMSO)water solution at different DMSO-water ratios in alkaline medium was reported. TEM and XRD results suggest a single-crystal formation with mean particle size in the range 4-27 nm. Magnetic nanoparticles are formed by the oxidative hydrolysis reaction from green rust species that leads to FeOOH formation, followed by autocatalysis of the adsorbed available Fe(II) on the FeOOH surfaces. The available hydroxyl groups seem to be dependent on the DMSO-water ratio due to strong molecular interactions presented by the solvent mixture. Goethite phase on the magnetite surface was observed by XRD data only for sample synthesized in the absence of DMSO. In addition, cyclic voltammetry with carbon paste electroactive electrode (CV-CPEE) results reveal two reduction peaks near 0 and +400 mV associated with the presence of iron(III) in different chemical environments related to the surface composition of magnetite nanoparticles. The peak near +400 mV is related to a passivate thin layer surface such as goethite on the magnetite nanoparticle, assigned to the intensive hydrolysis reaction due to strong interactions between DMSO-water molecules in the initial solvent mixture that result in a hydroxyl group excess in the medium. Pure magnetite phase was only observed in the samples prepared at 30% (30W) and 80% (80W) water in DMSO in agreement with the structured molecular solvent cluster formation. The goethite phase present on the magnetite nanoparticle surface like a thin passivate layer only was detectable using CV-CPEE, which is a very efficient, cheap, and powerful tool for surface characterization, and it is able to determine the passivate oxyhydroxide or oxide thin layer presence on the nanoparticle surface.

Introduction Magnetic colloids with size, phase, composition, and shapecontrolled nanostructures, have attracted great interest from researchers given the growing attention paid recently to metallic Pt-based and iron oxide nanoparticles1 because of their potential applications in a wide range of areas, including magnetic fluids,2-4 catalysis,5,6 datastorage,7-9 environmentalremediation,10-12 and biotechnology/biomedicine such as cell separation, tissue repair, DNA/RNA analysis, drug delivery, immunoassay, cancer therapy, and contrast enhancers for magnetic resonance imaging (MRI) diagnostics.1,13-20 Impressive progress has been made in a number of suitable syntheses of iron oxide magnetic nanoparticles with well-defined particle size and narrow particle size distribution.1,21,22 Despite its success in synthesizing a wide variety of size-, shape- and composition-controlled particles, the typical approach produces magnetic nanocrystals with hydrophobic surfaces. However, successful application in the areas listed above is highly dependent on the stability of the * Corresponding author. Telephone: +55 16 3301 6776. E-mail: [email protected]. † Unesp. ‡ Votorantim Cimentos. § Instituto de Quı´mica de Sa ˜ o Carlos.

particles under a range of different conditions. It is also desirable that nanoparticle systems are stable in aqueous medium because a hydrophobic surface greatly limits their applications.1,23 In this context, the development of a new synthetic route is a challenge to chemists and physicists in order to obtain magnetic nanoparticles that are highly dispersed and stable in aqueous medium. In addition, in most of the envisaged applications, the particles best perform when the size of the nanoparticles is below a critical value, which is dependent on the material but is typically around 10-20 nm.1 Then, each nanoparticle becomes a single magnetic domain and shows superparamagnetic behavior when the temperature is above the so-called blocking temperature. Thus, when the particle size decreases to nanometric scale their properties could be sizetunable and their dispersive and stability features in different media are completely dependent on the nanoparticle surface composition. Thus, particle surface properties are also essential concerning the preparation of stable and highly dispersed nanoparticle systems, and their characterization is very important to modulate their applications in wide-ranging areas. Generally, the overall reaction for colloidal magnetic iron oxide formation involves two processes: first it postpones their aqueous medium stabilization, and second it involves the hydrolysis reaction of

10.1021/jp0767922 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/19/2008

5302 J. Phys. Chem. C, Vol. 112, No. 14, 2008 the iron species present on the nanoparticle surfaces. The extension of the hydrolysis reaction cannot be enough to promote a considerable phase segregation, but a slight surface hydroxylation followed by oxidation reaction is expected, which leads to a very thin iron oxyhydroxide formation, such as goethite (R-FeOOH), on the nanoparticle surface.24,25 Although the oxyhydroxide phase is formed in a small amount, the magnetic nanoparticle surface properties are completely changed and the latter influence particle stability behavior. Due to features, such as a small amount in a thin layer on the particle surface, the oxyhydroxide phase is not detected by conventional phase characterization techniques such as X-ray diffraction. Mo¨ssbauer spectroscopy does not distinguish whether this phase is present on the surface or as a segregated phase in the bulk nanoparticle, and it is less expensive than electrokinetic techniques such as photocorrelation spectroscopy. Light scattering or similar techniques are generally needed to determine the surface composition. Then, cyclic voltammetry using a carbon-paste electroactive electrode (CV-CPEE) has been reported to be a helpful technique to characterize inorganic films,26,27 iron oxide,28,29 and superconductor materials.30,31 In this way, CV-CPEE can be a powerful and cheap tool to characterize the presence of different phases and the nanoparticle surface properties. In this work, a suitable method to produce hydrophilic surface and size-controlled water-dispersible magnetite nanoparticles from iron(II) compounds by homogeneous precipitation in a dimethyl sulfoxide-water (DMSO-water) medium is reported. Nanoparticle size, phase purity, and water-dispersible stability control were achieved by the use of DMSO-water (v/v) in appropriate ratios. Additionally, CV-CPEE was used to characterize the nanoparticle surface properties and to determine the presence of the oxyhydroxide phase on nanoparticle surfaces. Also, the electrochemical behavior depends on the particle size distribution of the samples obtained from different DMSOwater ratios. Experimental Section Magnetite Nanoparticle Synthesis. All reagents were of analytical grade from Merck and used without further purification. Magnetite nanoparticles were precipitated by slow addition of 20 mL of concentrated ammonium hydroxide in 100 mL of 0.1 mol L-1 FeCl2‚6H2O in DMSO-water solution previously deaerated with nitrogen flux at 70 °C. Resulting mixtures were vigorously stirred, and the obtained precipitates were washed with acetone and dried at room temperature under vacuum. The DMSO-water ratio was varied from 10% to 100% (v/v). The magnetite samples were identified by the water medium percentage number written before “W” (water) used in each water-DMSO mixture. Physical and Electrochemical Particle Characterization. The X-ray diffraction (XRD) patterns of the powdered samples were performed in a diffractometer (Siemens D5000) operating at 40 kV, 20 mA, and Cu KR (λ ) 1.5418 Å) radiation and a carbon graphite monochromator in the range 10-110° (2θ) and with a scanning rate of 0.020°/10 s in 2θ degrees. The average crystallite size distribution values of the single-crystal nanoparticles were calculated by the Scherrer equation32 using polycrystalline sodium chloride as internal standard in order to consider the instrumental broadening. Transmission electron microscopy (TEM) was performed in a Phillips TEM CM200 microscope by dispersion and deposition of a sample drop onto a carbon-coated copper grid. Magnetite nanoparticles were characterized by cyclic voltammetry at 20 mV s-1. The

Santos et al. electrochemical studies were carried out by conventional cyclic voltammetry with a three-electrode electrochemical cell. The working electrode was a carbon-paste electroactive electrode (CPEE) containing iron oxide (10% w/w) and Nujol as binder, the counter electrode was a platinum wire, and the reference electrode used was a saturated calomel electrode (SCE). All experiments were carried out at room temperature, and potentials in this work are referred to the SCE. The electrochemical cell was filled with a 1.0 mol L-1 HCl solution in deionized water as electrolyte, and the electrodes were connected to a potentiostat/galvanostat, EG&G PAR (Princeton Applied Research) Model 273 (EG&G PAR, Princeton, NJ), PC-interfaced with the cyclic voltammograms registered by M270 software. Results and Discussion The formation of Fe3O4 (magnetite) nanoparticles in solution by air oxidation of ferrous salt under many different conditions, such as anions (Cl-, SO42-, CO32-, NO3-, etc), pH, concentration, temperature, and air flow, among others, has been extensively studied.33-37 Actually, experimental data lead to a more acceptable mechanism proposal, in which the Fe3O4 phase was formed by γ- or R-FeOOH transformation by adsorption of Fe(OH)2 or Fe(OH)+ species onto the oxyhydroxide nanoparticle surface followed by an autocatalytic oxidation reaction.33-35 This mechanism was used to explain the magnetic nanoparticle formation in this work associated with the structure of the DMSO-water mixture used as solvent. When ammonium hydroxide is added to the DMSO-water containing FeCl2‚6H2O solution, a Fe(OH)2 precipitate is formed. In the presence of air, a rapid oxidation takes place that is accompanied by hydrolysis, leading to base consumption. Then, the partial dissolution of the Fe(OH)2 coupled with the oxidation of the released Fe2+ to Fe3+, which precipitates as iron(III) hydroxychloride complex species, reacting with the suspended Fe(OH)2 to yield “green rust” type I (GRI)24,37 with the general formula xFe(OH)2‚Fe(OH)2Cl‚yH2O. At high oxygen concentration, continuous oxidative hydrolysis leads to complete dissolution of both GRI and Fe(OH)2 compounds and also total oxidation of Fe2+, resulting in FeOOH formation. These reactions are accompanied by pH decrease and the oxyhydroxide compounds are precipitated in slightly acid medium at pH ∼3.5-4.0.24,25,34,38 However, in this work, previous solvent deaeration with nitrogen flow decreases the available oxygen and limits the oxidative hydrolysis reaction resulting in a small FeOOH formation and, consequently, Fe(OH)2 species remain in the solution. The dimethyl sulfoxide present in the medium has interesting properties: it can react with water molecules, resulting in DMSO-H+ and OH- species formation,39 maintain constant the solution pH ∼9 due to release of OH- in the solution, favoring dissolution of Fe(OH)2 and formation of Fe3O4, and contribute to the hydrolysis rate control.40 Thus, Fe3O4 formation involves the reaction between FeOOH and iron(II) ion processed in two steps: (a) adsorption step of iron(II) ion on the FeOOH resulting in an intermediate species, and (b) the transformation step of the intermediate to magnetite, according to reaction 1:

2FeOOH + Fe2+ + H2O f [FeOOH]2‚[FeOH]ads+ + H+ f Fe3O4 + H2O + H+ (1) When metal ions are adsorbed on the oxyhydroxide surface, hydrolysis of the metal ions takes place.36 Thus, the magnetite formation by the transformation of FeOOH proceeds via a dissolution-recrystallization process, similar to other iron oxide phases.25 According to Domingo et al., this process has been

Single-Crystal Magnetite Nanoparticles

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Figure 1. Particle size diameters of magnetite powders obtained from XRD samples prepared at different DMSO-water percentages (v/v) in 100 mL of DMSO-water mixture.

Figure 3. XRD patterns of the magnetite samples: (() Fe3O4; (#) NH4Cl; (+) R-FeOOH; (/) γ-FeOOH.

Figure 2. Representative transmission electron microscopic picture of magnetite nanoparticles obtained from 80% water in DMSO-water mixture (sample 80W).

Figure 4. Accurate XRD patterns of magnetite samples acquired with large scanning rate: (() Fe3O4; (+) R-FeOOH; (/) γ-FeOOH; (b) NaCl (internal standard); (#) NH4Cl.

described as “autocatalysis”, and rationalized in terms of considering Fe(II) on the surface of the solid as a species of intermediate reactivity between Fe2+ and Fe(OH)2, and occurs only under a pH buffering effect, in this work obtained by the presence of DMSO in the mixture solvent.34,39 Besides the FeOOH nucleation control promoted by the DMSO-water mixture, the nanoparticle size and size distribution can also be related to the singular properties of the DMSOwater mixture used as solvent. A second characteristic of the used solvent is associated with changes in the physicalchemical properties of the mixture, mainly viscosity and density, due to great affinities between water and DMSO. The electric dipole of the DMSO molecules have a strong interaction with the water dipole, resulting in a water molecule rotation and reorientation near the DMSO molecules.41 In addition, the molecular structure and symmetry of both molecules allow a hydrogen bond somewhat stronger than the water-water bonds.41 Thus, this molecular interaction yields molecular aggregates formation, changing the local dielectric constant, viscosity, and density, and resulting in distinct transport properties in different microregions of the mixture. The interactions are expected to be more effective in the small water amount region, and to make difficult the mass transportation, with consequent decreasing of hydrolysis and oxidation reaction rates, allowing large nucleation and small growth, and resulting in a decrease of the particle size as indicated in Figure 1.

The average crystallite sized distributions (Figure 1) of the samples were obtained from XRD data using the Scherrer equation32 and can be associated with the particle size due to single-crystal character indicated by TEM analysis. The results in Figure 1 show that the average particle size is in the range 4-27 nm and that decreases insofar as the water content is decreased in the start solution, in agreement with the DMSOwater aggregation characteristics and more available water molecules to drive the hydrolysis reaction. Also, concerning the results in Figure 1, the average particle size could be divided into three different regions: below 30%, between 30 and 70%, and above 70% of water contents in DMSO. This behavior can be also assigned with the structure of the DMSO-water mixture present, besides the aggregation previously mentioned, a molecular association leading to cluster formation in specific DMSO-water proportions, which are between 25-35% (DMSO-2H2O) and 75-80% (2DMSO-H2O) of DMSO in water, as reported.42,43 The cluster formation induces a new solvent structure and transport properties, and results imply that these new characteristics influence the nanoparticle formation, so that distinct microregions should be established in each case. These factors can induce a chemical transformation process on the particle surface generating additional iron phases as impurities. A representative TEM image of the magnetite nanoparticles obtained from the mixture of 80 mL of water and 20 mL of DMSO denominated by 80W is indicated in Figure 2, where

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Figure 5. Cyclic voltammetry of magnetite samples. The symbol “W” represents the water content (% v/v) in the DMSO-water mixtures: (a) pure paste and reference goethite, (b) 0W, (c) 10W, (d) 20W, (e) 30W, (f) 40W, (g) 50W, (h) 60W, (i) 70W, (j) 80W, (k) 90W, and (l) 100W.

we can observe the strings of particles as a function of magnetic interparticle interaction. The average diameter of particles of this sample is 26.6 ( 0.4 nm. The X-ray powder diffractograms for each sample are shown in Figure 3. In all samples are observed reflections of the magnetite pattern (JCPDF 19-0629). The NH4Cl pattern (JCPDF 07-0007) is also observed in the fresh powders because the particles were only washed with acetone, in which NH4Cl is almost insoluble. Acetone was used because it reduces the surface oxidation and allows a rapid washing and drying of magnetite nanometric samples. In the sample 100W is noted the presence of additional reflections with small intensity attributed to the presence of the goethite

(R-FeOOH) phase. More accurate XRD data collected at large scanning rates confirm the presence of goethite phase (JCPDF 29-70713) in the sample 100W (Figure 4); the used start solution is pure water in the magnetite precipitation method. Nevertheless, goethite was not detected by XRD in the other samples. The obtained results for all magnetite nanoparticles were from cyclic voltammetry (CV) performed in the range -350 to +1.100 mV vs SCE at 20 mV s-1. The potentiodynamic profile of the first cycle is different for each sample shown in Figure 5. Two cathodic current peaks were observed near +50 and +400 mV, corresponding to iron(III) species reduction and were assigned to iron(III) species in different chemical environments,

Single-Crystal Magnetite Nanoparticles such as magnetite and goethite, respectively. When compared to the pure goethite (R-FeOOH) potentiodynamic profile (see Figure 5a), the voltammograms of the samples obtained at 30% and 80% (v/v) water in DMSO (Figure 5e,j) do not present cathodic current peaks corresponding to the presence of the goethite phase. However, in contrast to the XRD results that show the goethite phase only present in the 100W sample, CV curves indicate that goethite was only absent in the 30W and 80W magnetite samples (Figure 5). This result suggests that the small amount of goethite phase can be formed by either a noncrystalline phase or by a thin layer on the nanoparticle surface, and for both cases the conventional phase characterization XRD technique is unable to detect the presence of this phase. Thus, CV is a powerful tool to determine the presence of a secondary phase on the nanoparticle surfaces, and it allows the characterization of the surface properties. As previously mentioned, the extension of hydrolysis reaction determines the oxyhydroxide phase formation, and the DMSO molecules have an important effect in the hydrolysis control. Goethite phase formation mainly depends on the available hydroxyl groups, originated from water or DMSO-water reaction molecules, in order to promote the hydrolysis, and excess of this group increases the hydrolysis reaction leading to the oxyhydroxide phase formation.24 As the particle size decreases, a large percentage of all the atoms in a nanoparticle are surface atoms, which implies that surface and interface effects become more important, and the surface more reactive. Owing to this large surface/bulk atoms ratio, a local breaking of the symmetry might lead to changes in the band structure, lattice constant, or/and atom coordination.1 For this reason, additional hydrolysis reactions due to the presence of the OHexcess are expected to occur on the nanoparticle surface, such as a passivation layer, in agreement with the CV results. Moreover, the DMSO-water cluster formation around 30% and 80% water in DMSO seems to avoid generation of this hydroxyl group excess and does not contribute to the additional hydrolysis and to oxyhydroxide phase formation. The molecular structure for DMSO-water in these proportions should be limiting for the hydrolysis process, but it promotes autocatalytic reactions resulting in a pure magnetite phase instead. On the other hand, the sample 0W profile (see Figure 5b) is very similar to that pure goethite (Figure 5a), indicating intensive goethite phase formation when water was also absent from the reaction medium. The DMSO molecules are strong proton acceptors and a minimum water amount is enough to promote the initial hydroxyl group generation, and in the absence of the DMSOwater interactions this hydrolysis seems to be larger than when these interactions are present.42-44 In the DMSO-water medium, hydrogen bond interactions are formed in the aqueous sections separated by microregions containing DMSO, thus directly influencing in the final powder properties,34,39,41-46 and the results in this work suggest that the particle-medium interface may be changing the intrinsic surface properties47,48 and influencing the phase formation. Detailed studies on the preparation of magnetite and goethite phases in pure DMSO and DMSO-water mixtures are currently underway in order to elucidate the influences of the molecular interactions and cluster formations on the oxidative hydrolysis reaction. In addition, CV curves in Figure 5 indicate that the intensity of reduction peaks situated near +50 mV, assigned to the Fe(III) species in the magnetite phase, increases insofar as crystallite size distribution increases, as expected, and alike results were obtained with Mo¨ssbauer data.49 Interparticle interactions, particularly magnetic ones, are relevant in the colloidal stability

J. Phys. Chem. C, Vol. 112, No. 14, 2008 5305 of magnetic dispersed systems. Magnetic measurements for sample 80W show a better magnetic response49 due to enhanced crystallized structure and with a larger particle size distribution than other magnetite samples. Conclusions The method to produce water-disperse magnetite nanoparticles from iron(II) solutions by homogeneous precipitation by oxidative hydrolysis reaction in different proportions of DMSOwater mixtures was successfully achieved. Average particle size diameters in the range 4-27 nm and the particle size distribution were easily tuned by DMSO-water initial volume ratio adjusting. The magnetite formation involves (i) green rust formation, (ii) FeOOH species formation from green rust, (iii) adsorption of available Fe(II) species onto the FeOOH nuclei, and (iv) autocatalysis of Fe(II)-adsorbed FeOOH species to Fe3O4. The overall process was controlled by the hydrolysis ratio, and the available hydroxyl groups seem to be dependent on the DMSOwater ratio due to strong molecular interactions arising from the solvent mixture. Generally, according to the DMSO-water ratio, these interactions lead to hydroxyl species excess in the medium and results in a goethite secondary phase formation on the magnetite nanoparticle surface, changing the nanoparticle dispersion stability. Goethite phase is absent in specific DMSOwater ratios, suggesting that no excess of hydroxyl species is present in the system due to a structured molecular cluster formation (2DMSO-water and DMSO-2water). Concerning the physical characteristics of the goethite phase, a small amount as a thin layer on the nanoparticle surface, the XRD technique is unable to detect it. In contrast, the CV-CPEE technique was very efficient to determine the presence of goethite phase and can be used as a helpful, cheap, and powerful tool in order to analyze the presence of the secondary phase on the magnetic nanoparticle surface. Acknowledgment. The authors thank Dr. V. Sargentelli, Dr. A. E. Mauro, and Dr. A. V. Benedetti for helpful discussions. F.J.S. wishes to thank the CAPES (Brazil) for a scholarship. This research was supported by FAPESP and RHAE/CNPq. References and Notes (1) Lu, A. H.; Salabas, E. L.; Schu¨th, F.; Abadia, A. R. Angew. Chem., Int. Ed. 2007, 46, 1222. (2) Pankhurst, Q. A.; Pollard, R. J. J. Phys.: Condens. Matter 1993, 5, 8487. (3) Pankhurst, Q. A.; Connolly, J.; Jones, S. K.; Dobson, J. J. Phys. D: Appl. Phys. 2003, 36 (13), R167. (4) Pankhurst, Q. A. BT Technol. J. 2006, 24 (3), 33. (5) Tsang, S. C.; Caps, V.; Paraskevas, I.; Chadwick, D.; Thompsett, D. Angew. Chem., Int. Ed. 2004, 43 (42), 5645. (6) Tsang, S. C.; Yu, C. H.; Gao, X. K. Tam J. Phys. Chem. B 2006, 110 (34), 16914. (7) Hyeon, T. Chem. Commun. 2003, 8, 927. (8) Varanda, L. C.; Jafelicci, M. J. Am. Chem. Soc. 2006, 128 (34), 11062. (9) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989. (10) Elliott, D. W.; Zhang, W. X. EnViron. Sci. Technol. 2001, 35 (24), 4922. (11) Li, X. Q.; Elliott, D. W.; Zhang, W. X. Crit. ReV. Solid State Mater. Sci. 2006, 3 (4), 111. (12) Takafuji, M.; Ide, S.; Ihara, H.; Xu, Z. H. Chem. Mater. 2004, 16 (10), 1977. (13) Neuberger, T.; Schopf, B.; Hofmann, H.; Hofmann, M.; von Rechenberg, B. J. Magn. Magn. Mater. 2005, 293, 483. (14) Tartaj, P.; Morales, M. P.; Gonzalez-Carreno, T.; VeintemillasVerdaguer, S.; Serna, C. J. J. Magn. Magn. Mater. 2005, 28, 290. (15) Gupta, A. K.; Gupta, M. Biomaterials 2005, 26 (18), 3995.

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