Chemical and Kinetically Controllable Nucleation, Aggregation

Article pubs.acs.org/JPCC

Chemical and Kinetically Controllable Nucleation, AggregationCoalescence, and Ostwald Ripening Processes in the Synthesis of Magnetic Co−B Nanoparticles Ana B. Dávila-Ibáñez and Verónica Salgueiriño* Departamento de Física Aplicada, Universidade de Vigo, 36310, Vigo, Spain S Supporting Information *

ABSTRACT: This article reports a mechanistic study of the formation of magnetic nanoparticles via aggregation-coalescence and Ostwald ripening processes considering the kinetically controllable nucleation of the Co−B alloy. Citric acid or sodium citrate determines the pH of the medium and modifies the kinetics of the precursor conversion by means of the NaBH4 hydrolysis processes involved. The subsequent steps of aggregation-coalescence and Ostwald ripening become consequently altered and even hindered by the final silica coating, and on the basis of these results, we report the important chemical role of the citrate ion precursor in the whole process.

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INTRODUCTION The synthesis of monodisperse nanoparticles in solution has to deal with the important fact that they are inherently unstable due to their high surface area and, therefore, tend to increase the mean particle size to attain a lower-energy state.1,2 The driving force in this process of sintering corresponds to the increased surface free energy if compared to bulk, by which nanoparticles present a metastable solid state and will inevitably tend to aggregate into larger nanostructures.3 Consequently, the synthesis of nanoparticles, which tries to provide precisely controlled sizes and narrow size distributions, always faces potential aggregation or agglomeration processes. Additionally, since the properties of the nanoparticles (for example, magnetic) are size-dependent, a broad nanoparticle size distribution will always hinder not only the characterization of the properties but also the applications related. Accordingly, accurate tuning of the nanoparticle size and control over the size distribution will always depend on understanding the complexity of the mechanism involved. This mechanism generally fits a scheme on which there is a main chemical reaction that supplies monomers of the sought material, reaching a critical level above its equilibrium solubility so that a burst of nucleation occurs. As monomers condense into nuclei and then contribute to the nanoparticle growth, their concentration becomes highly decreased or even depleted, and different processes such as Ostwald ripening can begin. Under these conditions, smaller nanoparticles dissolve, and larger ones grow causing average nanoparticle diameter to grow and the polydispersity to increase, according to the Lifshitz− Slyozov−Wagner (LSW) theory.4−6 This LSW growth model predicts two asymptotic limits for the growth of nanoparticles, © 2013 American Chemical Society

diffusion and reaction limited growth, and therefore sintering of nanoparticles in solution is typically attributed to masstransport mechanisms involving monomer migration.7 In the mentioned Ostwald ripening process, the atomic species are exchanged among nanoparticles driven by a difference in the chemical potential of atoms in the nanoparticles, as described by the Gibbs−Thomson equation, although different energetic contributions may also play a role.8 Nanoparticles with a larger or smaller radius of curvature have a lower or higher chemical potential, respectively, which results in a net atom transport from the smaller to the larger nanoparticles. In addition to the nanoparticle size, the particle morphology can also influence this process which may imply a different radius of curvature or surface-to-volume ratio.3 The general scheme of most of the colloidal chemistry methods designed for the synthesis of nanoparticles counts on inorganic cores stabilized by organic surfactants. Hence, the successful control in terms of size and size distributions also stems directly from the dynamic surface solvation established at this organic−inorganic interface (which also exerts some influence on the material solubility and therefore on the sintering process). Certainly, these organic molecules are chosen regarding their propensity to adhere to the surface of the growing material that additionally has to allow them exchange on and off the growing clusters to be indeed accessible for growth.9 The fact that more than half the atoms of a nanoparticle may be at this surface renders the organic− Received: December 20, 2012 Revised: February 6, 2013 Published: February 7, 2013 4859

dx.doi.org/10.1021/jp312582z | J. Phys. Chem. C 2013, 117, 4859−4865

The Journal of Physical Chemistry C

Article

Figure 1. TEM micrographs of silica-coated cobalt−boron nanoparticles, synthesized using citric acid (with average core diameter of 140.0 ± 15.2 nm) (a) and sodium citrate (with average core diameter of 88.7 ± 10.3 nm) (b) (0.4 × 10−4 M in both cases). HRTEM image and SAED of Co−B nanoparticle reflecting their aggregated and amorphous nature (c and d). M−H hysteresis loop of silica-coated cobalt−boron nanoparticles (with average core diameter of 88.7 ± 10.3 nm) (e).

involved in the previous nucleation step and in the subsequent aggregation. Herein we demonstrate that the mechanism of formation of Co−B nanoparticles consists on different steps of nucleation, aggregation, and growth (ruled by the nonmolecular processes of coalescence and ripening). This sintering process, described statistically by the evolution in size, size distribution, and sizedependent morphology of the nanoparticles, is discussed in light of a simple approach based on the very important chemical role exerted by the citrate ion precursor (citric acid or sodium citrate) that permits us to tune reaction conditions and subsequent steps of aggregation.

inorganic interface crucial as well in the whole formation process.10 Most of the mechanistic investigations have focused on the interaction of surfactants with crystal surfaces during nucleation and growth, but less is known about the kinetics of the main reaction and its influence on the nanoparticle formation and even less if the surfactants are involved in the kinetics of the main reaction. In many cases, the precursor reaction is considered an integral part of monodisperse particle formation given that it provides an internal reservoir of solute that is continually released during growth. The balance between solute production and consumption during this growth can maintain the supersaturation at a level that causes size distribution focusing11 and can consequently define the resultant nanoparticle size and size distribution.12 A number of studies have probed the influence of the kinetics of precursor conversion to monomer in nanoparticle synthesis.12−16 If precursors slowly convert to monomers as nanoparticles grow, this leads to a pseudosteady state monomer concentration that is controlled by the kinetics of precursor conversion rather than controlled by the solubility as in Ostwald ripening, and that can influence the narrowing of the size distribution. Clark and co-workers demonstrated quantitatively the relationship between monomer production rates and nanoparticle growth according to a theoretical model by which they were able to show that “size focusing” might be extended throughout a synthesis if monomer production can be sustained at sufficient rates during growth and even more efficiently if the nucleation and monomer production can be decoupled.6 For different synthetic schemes consisting of precursors, organic surfactants, and solvent (sometimes the surfactant also serves as solvent), nucleation, growth, and size focusing and/or Ostwald ripening are well documented. However, fewer are the cases on which the surfactant employed to control the growth process by means of the organic− inorganic interface also exerts control on the chemical reactions

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EXPERIMENTAL SECTION The synthesis of Co−B nanoparticles was performed as follows: a solution of the target compound (cobalt chloride hexahydrate (0.4 M) (Fluka)) was injected (0.1 mL) while being vigorously and mechanically stirred, to an aqueous solution (100 mL) of sodium borohydride (NaBH4) (4.4 mM) (Sigma Aldrich) and citric acid (Sigma Aldrich) (in the range between 0.2 × 10−4 and 1.2 × 10−4 M and prepared 2 min 30 s in advance), in an open air atmosphere. Ten minutes after the cobalt injection, 400 mL of an ethanolic solution containing 25 μL of TEOS (tetraethoxysilane) (Aldrich) was added. Fifteen minutes later the solution was centrifuged and the washed precipitate dispersed in ethanol (20 mL). The process was performed at 20 °C. A second series of synthetic processes to produce Co−B nanoparticles were also carried out using sodium citrate (Sigma-Aldrich) instead of citric acid. The range of concentrations chosen in this case was between 0.1 × 10−4 and 0.6 × 10−4 M although the processes were performed following exactly the same protocol. Samples for TEM were prepared by depositing them upon a carbon-coated copper grid. TEM measurements were performed on a Philips CM20 microscope operating at 100 kV. 4860



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nanoparticles to reach their final size before the silica coating, which may be governed to some extent by the electrostatic stabilization of the particles by means of the citrate ions present in solution23 and/or by an Ostwald ripening. The main reaction governing these experiments corresponds to the cobalt-catalyzed hydrolysis of sodium borohydride. This catalyzed hydrolysis that triggers very important activities in terms of hydrogen storage material24 was adapted to produce magnetic nanoparticles taking advantage of the organic− inorganic interface given by the citrate ions present in solution and the Co−B alloy formed. Similar nanostructures have been previously identified in the formation of a magnetic shell based on the same reactions but formed onto positively charged silica spheres.21 Since these negatively charged magnetic aggregates were reported to be driven to the surface of positively charged silica spheres, using a layer-by-layer self-assembly technique based on polyelectrolytes, we can assume the negatively charged citrate ions as the aggregate capping agents or surfactants. Figure 2 includes the diameter of the magnetic

HRTEM analyses were carried out on a JEOL JEM2010F TEM with a field emission gun working at 200 kV. ICP analysis was performed using a Perkin-Elmer Optima 4300 DV. Magnetic measurements were performed using SQUID Magnetometry. pH values were obtained using a Crison-20+ pH-meter.

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RESULTS AND DISCUSSION Co−B nanoparticles were synthesized combining a Co2+ ion precursor and sodium borohydride (NaBH4) in aqueous solution, offering average sizes ranging from 50 to 250 nm, as the amount of citric acid is decreased, and between 80 and 140 nm, as the amount of sodium citrate is varied. In all the cases, smaller subunits aggregate to form bigger and spherical or quasi-spherical magnetic cores that are subsequently coated with a thin layer of silica (exploiting the hydrolysis and condensation of TEOS in the aqueous/ethanolic solution). Figures 1a and b show TEM images of nanoparticles formed using a citrate ion precursor (citric acid and sodium citrate, respectively) concentration of 0.4 × 10−4 M. These silicacoated Co−B nanoparticles present size distributions (referred to the average diameter of the magnetic core) located around 140.0 ± 15.2 nm (a) and 88.7 ± 10.3 nm (b), respectively. Clearly, the nanoparticles in Figure 1b present a more rounded shape and smoother surface compared to the ones in Figure 1a. Figures 1c and d include an HRTEM image and the selected area electron diffraction (SAED) pattern, respectively, of a silica-coated Co−B nanoparticle, which reflect its aggregated nature, the thin silica coating, and the characteristic amorphous nature of both the core and shell. This chemical procedure provides nanoparticles of an amorphous alloy (Co−B) with different CoxBy chemical composition.17−19 In our case, the ICP analyses confirmed an average Co 90 B 10 chemical composition in the samples analyzed, which is reasonable as the amounts of both CoCl2 and sodium borohydride (in excess) in all the experiments stay constant. This amorphous magnetic alloy corresponds to a material having a noncrystalline structure, i.e., a glassy state stabilized alloying the metallic cobalt with B as the glass former. Figure 1e shows the hysteresis curve of the silica-coated Co−B nanoparticles (with an average magnetic core diameter of 88.7 ± 10.3 nm) collected at 5 K and characterized by the coercive field HC = 180 Oe. Because of the amorphous nature of the magnetic core, wide domain walls and almost no defects characterize this type of nanoparticle, and therefore, HC is expected to be very small (order of millioersteds).20 However, the relatively important coercitivity value encountered considering the amorphous alloy can be justified in terms of the aggregated nature of the magnetic cores that render the nanoparticles pining-type magnets.19,21 The mechanism of synthesis of the Co−B magnetic cores in the presence of citrate ions from citric acid or sodium citrate can be divided into four steps starting from the cobalt-catalyzed hydrolysis of NaBH4 that releases the Co−B monomer. In a second step, the Co−B monomers form dimers, trimers, etc. that aggregate and coalesce (a process of coalescence reduces the total interface area, and accordingly it can be understood as a merging of formed aggregates)22,23 until reaching certain stability that stops the process of coalescence providing nuclei. After these two processes the nuclei have a ∼5 nm average size.19 The third step implies an important and abrupt change in colloidal stability of these nuclei that gives place to aggregates along with a corresponding decrease in the number of particles and a broader size distribution. There is a consequent induced fourth step of the growth regime of the

Figure 2. Dependence of the diameter of the Co−B magnetic core as a function of the citrate ion precursor employed: citric acid (black squares) and sodium citrate (red dots) (solid and dotted lines are just to guide the eye).

core in the nanoparticles produced, as a function of the citrate ion precursor (citric acid or sodium citrate). If citrate ions from citric acid are employed, the size of the Co−B magnetic cores ranges from 250 to 50 nm as the citric acid concentration is increased. This approach coincides with the fact that experimental systems with a lower concentration of stabilizing agent (less inhibited particle dissolution) are those in which larger magnetic core size is obtained. If citrate ions from sodium citrate were used, the range of sizes available appears tightened, and there is no clear dependence of the diameter on the concentration of the citrate ion precursor since it decreases from 110 to 80 nm and increases again to around 140 nm as the sodium citrate concentration is increased. The citrate ions from citric acid or sodium citrate can therefore be labeled as ligands or stabilizing agents since it is common to use ions that enhance the electrostatic double layer in colloidal particle systems to counteract the attractive van der Waals forces and prevent or control aggregation processes. Given the magnetic nature of the Co−B material, dipolar interactions (which if strong enough can consequently lead to attaining a specific magnetostatic energy configuration) take place and can influence the further coalescence process. Furthermore, in systems with larger ripening contribution that gives rise to 4861



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Figure 3. (a) Variation of pH as advancing on the reactions to produce Co−B nanoparticles, when using citric acid (0.2 and 0.4 × 10−4 M) (hollow and solid black squares) and sodium citrate (0.2 and 0.4 × 10−4 M) (hollow and solid red dots). Variation of pH after injection of Co2+ ions fitted to a sum of exponentials (b and c).

Table 1. Results from Fitting the Experimental Variation of pH to the Sum of Two Exponentialss pH = pHf + C1e−k1t + C2e−k2t

equation −4

0.2 × 10

[CIP] CIP

citric acid

adj. R-square C1 k1 × 10−3 (s−1) C2 k2 × 10−3 (s−1) pHf

0.4 × 10−4 M

M sodium citrate

0.99947

citric acid

0.99944

sodium citrate

0.99918

0.99859

value

error

value

error

value

error

value

error

−0.45 20.6 −0.57 3.04 8.55

0.032 1.34 0.012 0.527 0.038

−0.42 33.4 −0.61 4.41 8.52

0.016 1.72 0.008 0.29 0.013

−0.269 20.96 −0.504 3.13 8.44

0.044 2.98 0.019 0.884 0.055

−0.272 30.53 −0.464 9.11 8.96

0.063 5.31 0.061 0.97 0.0048

s

CIP: citrate ion precursor.

independently two different concentrations of citric acid or sodium citrate (0.2 and 0.4 × 10−4 M) (hollow and solid black squares and red dots, respectively) to produce Co−B nanoparticles. This plot included in Figure 3a sets the initial moment (t = 0 s) when NaBH4 and citric acid or sodium citrate are dissolved in the solution registering pH values close to neutrality. With the advancement of the direct hydrolysis of the borohydride ions, the pH increases with a similar tendency (although not in absolute values) until a drop is registered in all the cases, coincident with the injection of Co2+ ions (t ∼ 150 s). From that event on, the pH starts increasing again, as expected if considering the two different hydrolysis processes.25 Indeed, if we consider the variation of the pH just after injection of the Co2+ ions, we can fit it to a sum of exponentials, according to the simplistic idea of having two reactions: the direct and the catalyzed hydrolysis of the NaBH4. Figure 3b and c includes the experimental and fitted data comparing the tendencies when using citric acid and sodium citrate at the two mentioned concentrations (0.2 and 0.4 × 10−4 M), establishing in this case the injection of CoCl2 as t0 = 0 s. Table 1 summarizes the parameters obtained from fitting the experimental results, which permits us to evaluate the two processes in terms of k1 an k2 as rate constants (see Supporting Information). As mentioned, the direct hydrolysis is expected to be very much favored compared to the cobalt-catalyzed one and varies depending on the citric acid or sodium citrate concentration employed (which establishes the initial pH at t0 (pH0) (injection of CoCl2)), and therefore we can then consider k1

larger particles as in this case, it is reasonable to consider a growth regime that defines the final core of the nanoparticles, and therefore and based on the size tendencies summarized in Figure 2, we can expect the citrate ions to play their role. Although simplistic given the complexity of the reactions on which NaBH4 can be involved,25 we can just focus on the direct or the catalyzed hydrolysis (by the latter one the Co2+ ion reduction takes place and makes possible the recombination of metallic cobalt to produce the amorphous Co−B alloy) processes of borohydride ions. The direct hydrolysis of the NaBH4 is very much favored as performing the whole process in aqueous solution and therefore stays almost invariable independently of the citric acid or sodium citrate concentrations employed. The cobalt-catalyzed hydrolysis on the other hand becomes favored with shifting from citric acid to sodium citrate since in the presence of sodium citrate the concentration of BH4− when injecting Co2+ ions is larger if compared to the case of using citric acid that acidifies the reaction medium and consequently and continuously favors subsequent direct hydrolysis of more NaBH4.26−28 Obtaining precise kinetics data with magnetic materials is pretty challenging, as examples of an optical property such as the surface plasmon band in metals29 or fluorescence in semiconductors to monitor12 are lacking. In our system, citric acid or sodium citrate change the pH of the solution and can consequently influence the balance between the two direct and cobalt-catalyzed hydrolysis processes.25 Figure 3a includes the variation of pH as advancing both reactions, considering 4862



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Figure 4. Flower-like (a) and spherical (b) silica-coated Co−B nanoparticles obtained using citric acid or sodium citrate (0.6 × 10−4 M in both cases), respectively.

citric acid or sodium citrate of 0.4 × 10−4 M, we should expect again a faster nucleation rate which would lead to a bigger number of nuclei and therefore to a smaller final size. In addition, the citrate ions play their role in terms of determining the final size through the organic−inorganic interface they are able to establish since, independently of the precursor, they control the surface of the growing nanoparticles and, accordingly, a general decrease in size with increasing concentration of citrate ions present in solution should be expected. Polte and co-workers have reported the formation mechanism of colloidal silver and gold nanoparticles using sodium borohydride as the reducing agent.23 According to the experimental evidence, they also established the mechanism in four steps and justified the second aggregation-coalescence process from a decrease of electrostatic stabilization of the nanoparticles, impossible to avoid even in the presence of PVP (polyvinylpyrrolidone) and likely correlated to the hydrolysis of the borohydride. In our case, however, to explain the quite large sizes of the nanoparticles produced with this method, the two different size dependencies, and the very spherical instead of aggregate-like nanoparticles attained, we can postulate a final growth regime, governed by an Ostwald ripening and/or a size distribution focusing (besides the final silica coating). There are different mechanisms by which particles increase their final size, such as sintering, coalescence, or Ostwald ripening, and evolve to lessen the free energy of the system at the expense of a reduction of the total available surface area. Basically, the sintering or coalescence process consists of the aggregation of two or more particles at random, but the stabilizing molecules attached to the nanoparticle are able to avoid or control this process by means of the forces of repulsion they can originate. This is indeed our case: the more citrate ions present, the more surface to control and stabilize, leading to smaller nanoparticles (built from smaller subunits as reflected by TEM analysis of the magnetic Co−B core and characteristic of a coalescence process). However, besides coalescence, the so-called Ostwald ripening is also consistent with the synthesis of particles in the presence of a stabilizing agent because they do not need to come into contact for the growth to happen. Accordingly, citrate ions attached to the surface of Co−B aggregates can be considered a flexible film dependent on their curvature. Extra monomer units of Co−B formed (from the chemical reactions or from the smallest

as the direct hydrolysis rate constant, according to the fitted values (Table 1). If shifting from citric acid to sodium citrate, both rate constants become increased, which can be related to the fact that a larger amount of BH4− is still present in the solution once the Co2+ ions are injected. It is also important to point out the fact that with increasing the concentration of the citrate ion precursor k1 stays almost constant (in terms of the error range), while k2 increases moderately in the case of citric acid and importantly in the case of sodium citrate. Consequently, the cobalt-catalyzed hydrolysis (the chemical reaction that supplies the Co−B monomer) becomes faster if using sodium citrate instead of citric acid and with the increasing concentration of the citrate ion precursor. The citrate ions may play a critical role in terms of determining the final size through the organic−inorganic interface they are able to establish, but it is the citrate ion precursor which has a major influence on the cobalt-catalyzed hydrolysis rate (kinetics of the whole process) and controls therefore the nucleation and subsequent aggregation, i.e., rules the initial steps of the formation of Co−B magnetic nanoparticles. On the basis of these assumptions, the larger the reaction rate to produce the Co−B monomer, the faster the nucleation, and consequently, an increased number of nuclei will become therefore smaller nanoparticles at the end of the process (considering almost invariant the final amount of magnetic material because an excess of borohydride ions compared to the amount of Co2+ ions injected was used in all the cases). Owen and co-workers have reported a relationship between the precursor reaction rate and the number of nanoparticles formed in such a way that increasing the initial rate led to a greater number of nanocrystals.12 This, overall, agrees with the general tendency of sizes. Figure 2 shows a decrease in the average final magnetic core diameter of the nanoparticles produced with increasing concentration of citrate ions (for sodium citrate only at very low concentrations). At low concentrations, it also shows a general decrease in average size when shifting from citric acid to sodium citrate, which also agrees with the proposed faster nucleation (increased number of nuclei and consequently smaller final sizes) when using sodium citrate. We have to consider however the subsequent steps. In fact, in the case of sodium citrate, the tendency changes for concentrations of citrate ion precursor bigger than 0.3 × 10−4 M, with larger sizes as the concentration of citrate ions is increased. Attending to the rate constants obtained when using a concentration of 4863



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nuclei as dissolving) just have to cross this protective film so that Ostwald ripening may occur to continue the growth process. To guarantee the stability of the Co−B nanoparticles in solution, it is necessary to coat them with an outer layer of silica,30 but accordingly, the silica coating will stop whatever step would be taken after since it implies a solid barrier that hinders any further growth. The silica precipitation has to be fast enough to maintain the magnetic nanoparticles protected and stable in solution, and therefore the sodium silicate method cannot be employed because it requires a much higher pH (pH > 10.5) to induce an additionally very slow silica precipitation.31 The Stöber method implies a much faster precipitation but again compels much higher pH values that would compromise the stability of the nanoparticles.32 A TEOS/EtOH mixture was therefore chosen to be injected after the nanoparticles have formed (10 min after the CoCl2 injection), exploiting therefore a faster acid-catalyzed silica precipitation (the lower the pH value, the faster the precipitation process).33−36 Figure 4 includes TEM images of silica-coated Co−B nanoparticles, formed in the presence of citric acid (a) or sodium citrate (b) (0.6 × 10−4 M in both cases). Ten minutes after the injection of the CoCl2, the solutions reach pH values of 9 and 9.7, respectively, which guarantee an acid-catalyzed silica precipitation once diluted with ethanol (a basic-catalyzed precipitation of silica would require pH > 10.5). However, considering the fact that citric acid or sodium citrate modifies the pH differently, this will also interfere in the silica precipitation. In fact, the different pH attained when using citric acid or sodium citrate will change the kinetics of the whole system, in terms of the hydrolysis of NaBH4 (the direct and the cobaltcatalyzed) and of TEOS (the deposition of the outer silica shell), and accordingly will modify the period of time left for the introduced growth regime. Considering the same concentration of citric acid or sodium citrate, the first reduces the cobalt-catalyzed hydrolysis rate and favors the acidcatalyzed precipitation of silica, while the sodium citrate, instead, increases the cobalt-catalyzed hydrolysis rate but slows down the silica precipitation. Accordingly, different periods of time are left for the growth regime on which the proposed Ostwald ripening of the aggregates would lead them to bigger sizes and a more spherical shape and justify the different morphology of the nanoparticles shown in Figure 4. The magnetic cores of the nanoparticles formed in the presence of citrate ions from the sodium citrate show a circular projected shape (instead of the predominantly flower-like), a smoother and almost defect-free surface, a bigger size, and a narrower size distribution, which is consistent with a surface energy minimization attained in a longer period of time. Figure 5 includes the size distribution (referred to the diameter of the magnetic Co−B core) analysis of both samples. While the nanoparticles synthesized using sodium citrate appear very similar in size (red columns), those synthesized employing citric acid offer a much wider size distribution (black columns). The left size distribution in black represents a transitional bimodal shape characterized by an additional peak toward the larger particle sizes, while the right one instead has reached a shape of the LSW-type, which has a main peak shifted toward the large particle sizes and a tail toward the small sizes. We can postulate therefore that Figure 4a illustrates the aggregation step, while Figure 4b reflects a reaction-controlled growth of the nanoparticles. Kwon and Hyeon have reasoned that after a

Figure 5. Particle size distributions (referred to the diameter of the Co−B magnetic core) obtained from the full TEM analysis corresponding to the flower-like and spherical silica-coated Co−B nanoparticles shown in Figure 4 (using citric acid (black columns) or sodium citrate (red columns) 0.6 × 10−4 M, respectively).

reaction-controlled growth mode the nanoparticles appear outside the focusing region because an Ostwald ripening has “defocused” the size distribution, and if the size distribution was initially quite broad, the size uniformity can be improved to a certain degree.37 Accordingly, Figure 5 shows the temporal evolution of the size distributions of Co−B nanoparticles, after aggregation of the nuclei and after coalescence and Ostwald ripening, transforming the transitional size distribution shape of the aggregates, characterized by a shoulder to the large particle side of the main peak, into the characteristic so-called LSW size distribution with a tail to the small particle side of the main peak.3,38 The very big differences in size and size distributions can be explained considering the time window of the whole process. Sodium citrate implies a faster nucleation and a slower precipitation of silica, while citric acid implies a slower nucleation and a faster precipitation of silica, rendering the period for coalescence reshaping and ripening much shorter in the citric acid case. Furthermore, this time window becomes more and more pronounced with increasing concentration of the citrate ion precursor that also justifies the more similar nanoparticles at lower concentrations as those shown in Figure 1.

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CONCLUSION We have postulated a mechanism of formation of silica-coated Co−B nanoparticles on which the citrate ion precursors employed to provide the citrate ions (used as surfactants) simultaneously exert control on the production of the magnetic material and on the precipitation of the outer silica shell and, consequently, on the morphology, average size, and size distribution of the magnetic nanoparticles obtained.

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ASSOCIATED CONTENT

S Supporting Information *

Derivation of equation in Table 1 referred to the variation of pH. This material is available free of charge via the Internet at http://pubs.acs.org. 4864



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(25) Morris, J. H.; Gysling, H. J.; Reed, D. Chem. Rev. 1985, 45, 51− 76. (26) Lu, J.; Dreisenger, D. B.; Cooper, W. C. Hydrometallurgy 1997, 45, 305−322. (27) Dai, H.-B.; Liang, Y.; Ma, L.-P.; Wang, P. J. Phys. Chem. C 2008, 112, 15886−15892. (28) Andrieux, J.; Demirci, U. B.; Hannauer, J.; Gervais, C.; Goutaudier, C.; Miele, P. Int. J. Hydrogen Energy 2011, 36, 224−233. (29) Dávila-Ibáñez, A. B.; Correa-Duarte, M. A.; Salgueiriño, V. J. Mater. Chem. 2010, 20, 326−330. (30) Salgueiriño-Maceira, V.; Correa-Duarte, M. A.; Farle, M.; LópezQuintela, M. A.; Sieradzki, K.; Dı ́az, R. Langmuir 2006, 22, 1455− 1458. (31) Philipse, A. P.; van Brugen, M. P. B.; Pathmamanoharan, C. Langmuir 1994, 10, 92−97. (32) Stober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62−69. (33) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979. (34) Dávila-Ibáñ ez, A. B.; López-Quintela, M. A.; Rivas, J.; Salgueirino, V. J. Phys. Chem. C 2010, 114, 7743−7750. (35) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Harcourt Brace Jovanovich: Boston, MA, 1990. (36) Grzelczak, M.; Correa-Duarte, M. A.; Liz-Marzán, L. M. Small 2006, 2, 1174−1177. (37) Kwon, S. G.; Hyeon, T. Small 2011, 7, 2685−2702. (38) Simonsen, S. B.; Chorkendorff, Ib.; Dahl, S.; Skoglundh, M.; Sehested, J.; Helveg, S. J. Am. Chem. Soc. 2010, 132, 7968−7975.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS V.S. acknowledges the financial support from the Ramón y Cajal (Ministerio de Ciencia e Innovación, Spain) Program. The Xunta de Galicia Regional Government has supported this work under projects 10PXIB312260PR and 2010/78 (Modalidade Emerxentes).

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REFERENCES

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