Effect of Surfactant Concentration and Aggregation on the Growth

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The Effect of Surfactant Concentration and Aggregation on the Growth Kinetics of Nickel Nanoparticles Alec P. LaGrow, Bridget Ingham, Michael F. Toney, and Richard D. Tilley J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp405314g • Publication Date (Web): 24 Jul 2013 Downloaded from http://pubs.acs.org on August 6, 2013

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The Journal of Physical Chemistry

The Effect of Surfactant Concentration and Aggregation on the Growth Kinetics of Nickel Nanoparticles Alec P. LaGrow1, Bridget Ingham1,2*, Michael F. Toney3, Richard D. Tilley1 1

The MacDiarmid Institute for Advanced Materials and Nanotechnology, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New Zealand 2

3

Callaghan Innovation Research Ltd., P.O. Box 31-310, Lower Hutt 5040, New Zealand

Stanford Synchrotron Radiation Lightsource, 2575 Sand Hill Road, Menlo Park, CA 94025

* [email protected] RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to)

Abstract The effect of trioctylphosphine (TOP) concentration on the growth of nickel nanoparticles is studied using in situ synchrotron small-angle X-ray scattering. The growth kinetics are fitted using a two-step nucleation and autocatalytic growth model. TOP acts as a nucleating agent and 1 ACS Paragon Plus Environment


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then acts as an inhibitor against rapid particle growth. Increasing the TOP concentration results in smaller nanoparticles. Once there is a critical concentration of nickel particles above a certain size, they start to aggregate. This results in a broadening of the particle size distribution at later times due to particles on the outside of the aggregates continuing to grow, while those on the inside cease to grow as the nickel precursor is locally depleted.

Keywords: In situ, small-angle X-ray scattering, nanoparticles, growth kinetics, magnetism, aggregation Introduction The formation of nanoparticles with controlled size is a key goal in materials science, because the nanoparticles can exhibit striking size dependent functional properties.1,2 In inorganic nanoparticle solution synthesis the reaction conditions can be tailored to control the nanoparticle growth, and importantly, the use of surfactants has led to the growth of nanoparticles with controlled sizes3-5 and shapes.5-7

The effect of the surfactant on nanoparticle growth is

dependent on the surfactant's molar equivalent to precursor,8,9 the binding strength of the hydrophilic head group,10,11 and the size of the hydrophobic tail group.4,12 Surfaces dominate the chemical properties of nanoparticles and thus surfactants have a significant effect on particle growth due to their interaction with the particle surface.3 Fundamental studies of the effect of surfactants on the growth kinetics of these reaction systems have investigated how the type of surfactant affects nanoparticle growth.10,11,13,14 From these in situ growth studies, three effects that a surfactant could have on the nucleation and growth were postulated as well as how these affect the size and dispersity.10,11,14 The first is as a ripening agent that stabilizes the monomers in solution. This forms fewer nuclei, thus limiting

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the number of particles formed and leading to the formation of larger particles.14 The second type of surfactant is the nucleating agent, which has a stronger binding constant for the particle surface than the monomer, favoring the formation of nuclei. This also decreases the solubility of the monomers in solution which inhibits the metal atoms in the nanoparticles from dissolving back into solution (e.g., reduce Oswald ripening), resulting in small sized particles with low polydispersity.10

The third type of behavior is the strong binding of surfactants to the

nanoparticles' surfaces, limiting their growth, also resulting in small sized particles with low polydispersity, but this growth inhibition is independent of the nucleation step (e.g., the second surfactant type).11 These studies primarily investigated nanoparticles formed by hot-injection methods, with CdSe,10,14 or gold.11 Herein we examine nanoparticle growth by hydrogen reduction, which has been shown to favor the growth of shape-controlled nanoparticles.5,7,13,15 In particular we investigate how the surfactant and inter-particle interaction affects the growth in this system. Of interest here are nickel nanoparticles, which can be grown via solution methods with good control over the size15 and shape16,17 by varying the concentration of trioctylphosphine (TOP) as a surfactant. Nickel nanoparticles are more reactive, cheaper, and more abundant than noble metals, and have applications in catalysis.18-20 They also display interesting magnetism depending on their size, being ferromagnetic for sizes above 15 nm in diameter,21

and

superparamagnetic for sizes below this.4,16,21-23 Likewise the catalytic properties of nanoparticles are also strongly size-dependent.24-26 Understanding the nanoparticle formation mechanism and how the surfactant concentration affects the particle growth will enable us to better control the size and dispersity of the resulting nanoparticles, thus tailoring their functional properties. Small angle X-ray scattering (SAXS) is a valuable in situ probe of nanoparticle growth, since it gives a



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measure of the volume-averaged particle size, without requiring the particles to be crystalline. SAXS also allows particle-particle interactions in solution to be studied. Here we have used in situ synchrotron small-angle X-ray scattering to monitor the growth and inter-particle interaction of nickel nanoparticles in solution as a function of time and for different TOP/nickel precursor ratios, to understand the effect of TOP concentration on the growth kinetics.

Experimental Nickel (II) acetylacetonate [Ni(acac)2] (Aldrich, 95%), oleylamine (Sigma-Aldrich, 70%), and trioctylphosphine (Sigma-Aldrich, 90%) were used as received. Ni(acac)2 is hygroscopic and is rapidly hydrated on contact with air to form Ni(acac)2.2H2O. For this reason the Ni(acac)2 was allowed to fully hydrate, and the resulting Ni(acac)2.2H2O was used as the precursor in all of the experiments. The in situ growth synthesis reactions were carried out by dissolving 0.0073 g of Ni(acac)2.2H2O (0.03 mmol), in 0.5 mL of oleylamine, and then flushed with flowing nitrogen to remove excess oxygen. To this solution 0 – 14 µL of TOP was injected to give 0 - 2 molar equivalents of TOP to Ni(acac)2.2H2O. The final solution was mixed in a sonicator to ensure homogeneity. The Ni concentration was 0.06 mol/L. Then 50 µL of the solution was drawn up and injected into the synchrotron reaction cell. An in situ synthesis cell and heating unit were used, as described in previous publications.13,27 Briefly, the cell consists of a stainless steel body with a small reservoir, 5 mm diameter and 1 mm thick, with mica windows, into which the precursor solution is injected. The top of the cell is then sealed, evacuated and backfilled with hydrogen gas several times, and finally sealed with 100 kPa hydrogen for the experiment. SAXS patterns were recorded in transmission. The heating unit is capable of heating at 40K/min. The heating unit was mounted on the beam line, and the


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sealed cell inserted at a low temperature (between 20-50°C). The cell was then heated to 200°C at the maximum rate. t = 0 was taken as the time at which the cell reached the target temperature. In situ synchrotron SAXS experiments were performed at beam line 1-4 of the Stanford Synchrotron Radiation Lightsource, using an X-ray wavelength of 1.488 Å and a beam size of 1 × 1 mm. The sample-detector distance was calibrated using a silver behenate standard and was 1.16 m. The detector used was a PI-SCX CCD (2084 x 2084 pixels, apparent pixel size 60 µm). Exposure times were 30 seconds. After the experiment the cell was cooled to room temperature and then dismantled. The nanoparticles were extracted with a 100 µL syringe. The sample was cleaned via centrifugation at 14,000 rpm with a 50:50 mixture of the reaction solution and ethanol. The nanoparticles were then further purified via centrifugation with a 50:50 mixture of toluene and ethanol four times before they were dried for TEM imaging. The samples were prepared for TEM by suspending the nanoparticles in solution and then drop-casting them onto a carbon coated copper grid. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were taken on a JEOL 2010 microscope operating at an acceleration voltage of 200 kV. The SAXS raw data were radially averaged using Fit2D to obtain one-dimensional plots of intensity versus the scattering vector, q (q = (4π/λ)sinθ, where θ is half of the scattering angle). Background subtraction was performed by subtracting the scattering pattern recorded of the cell with toluene. The resulting I(q) data were fitted using the Irena 'Modeling II' macros.28

Results



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Figure 1 displays background-subtracted data for six experiments performed at different TOP/Ni ratios from 0 to 2. For TOP/Ni ratios at or below 0.25, large polydisperse particles are observed to form, evidenced by the rapid rise in the intensity at low q and lack of prominent 'shoulders' which indicate characteristic particle sizes. This is reflected in the TEM images of the particles collected at the end of the reaction (Figure 2). For TOP/Ni ratios at or above 0.5, the initial scans show evidence of formation of particles with low polydispersity, evidenced by a pronounced shoulder with associated oscillations at high q (~0.06 – 0.2 Å-1). This is particularly evident for the two samples with TOP/Ni = 1 (D and E). With the exception of one of the TOP/Ni = 1 samples (sample E), at later times a correlation peak is observed to form, which indicates aggregation of the particles. At much later times the overall intensity starts to decrease. This indicates a decrease in the total volume of scatterers, which is attributed to large aggregates of particles settling out of solution.


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Figure 1. Background-subtracted small-angle X-ray scattering data for experiments performed with TOP/Ni ratios of (A) 0, (B) 0.25, (C) 0.5, (D) 1, (E) 1, (F) 2, for selected times as indicated. Arrows show trends in intensity versus time.



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Figure 2. Transmission electron microscope images at the end of the reaction for samples with TOP/Ni ratios of 0 (A), 0.25 (B), 0.5 (C), 1 (D) and 2 (F).

The data were modeled using the Irena 'Modeling II' macros package.28 This allows up to 10 scattering populations to be included, where each population contributes to the intensity according to 2

4  2 I (q ) ∝ V 2 ∫  πr 3  n(r ) f (qr ) S (q )dr 3  where V is the total volume of particles; n(r) is the number distribution (integral normalized to 1); f(qr) is the particle form factor; and S(q) is the structure factor. n(r) was chosen to be a log2   r    1 2   ln  exp − σ   1 2   exp −  r0   where r is the mean size and σ is the normal distribution, n(r ) =   0 2 rσ 2π  2 σ     


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dispersion. f(qr) was the commonly used form factor for spheres. S(q) was chosen to be either 1 (for the 'dilute' system), or in the case where a correlation peak was evident, the structure factor for sticky hard spheres, where the volume fraction (φ) is a key parameter (see the Supplementary Information for the rationale) and describes the effective particle packing density.29,30 It has been shown that the exact form of the size distribution has little effect on I(q).31,32 For low values of dispersion (σ), the log-normal distribution approaches that of a Gaussian function. Two populations were used to describe the aggregation behavior of the Ni particles in solution, corresponding to dilute (non-interacting) particles, and interacting (aggregated) particles; these have the same parameters for their size distributions (r0 and σ), but different S(q). A number of constraints were applied to limit the number of variable parameters in the fits; see the Supplementary Information for more detail. The volume of nanoparticles (Figure 3), the mean particle radius and hard sphere radius (Figure 4), and the dispersion of the size distribution (Figure 5) are extracted for the two Ni particle populations and plotted as functions of time. Uncertainty in the fitted parameters are within the data points. From these variables we investigate how the surfactant ratio affects the decomposition rate of the nickel precursor, the size of the nanoparticles and the dispersity of the particles over time. By combining these three variables the size distributions can be plotted over time to investigate the growth of the system as a whole (Figure 6). The aggregation behavior can be understood by examining the dispersity (Figure 5) and the relative volumes (Figure 3) as a function of time. The growth rate kinetics are obtained by fitting the curves of the total volume of particles as a function of time.



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Figure 3. Volume of particles versus time for experiments with TOP/Ni ratios of 0.5 (C), 1 (D), 1 (E) and 2 (F), obtained from fitting the SAXS data in Figure 1.

Figure 3 shows the development of the volume of each population as a function of time. At the initial times for all samples, the nanoparticles are dilute (isolated) in solution. Aggregation is first observed at 45, 44, and 55 minutes for samples C, D and F respectively. The volume increases steadily, then after approximately 60 minutes the population of aggregated particles becomes dominant for all samples except one of the TOP/Ni = 1 samples which did not show aggregation (sample E).33 The relative fractions of aggregated vs. non-aggregated (dilute) particles also decreases as the TOP/Ni ratio increases from 0.5 to 2 (Figure 3 C, D, F). The total volume


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reaches a plateau as the nickel precursor is depleted and then decreases at later times, which is due to the nanoparticles settling out of solution.

Figure 4. Particle size mean and hard sphere radius versus time for experiments with TOP/Ni ratios of 0.5 (C), 1 (D), 1 (E) and 2 (F), obtained from fitting the SAXS data in Figure 1. The mean nanoparticle radius (r0) increases as a function of time up to a plateau (Figure 4), while the final mean radius decreases with increasing TOP/Ni ratio (except for E). For the different samples showing aggregation at later times the final mean radius of the nanoparticles is 7.0 nm for a TOP/Ni ratio of 0.5, decreasing to 6.1 for a TOP/Ni ratio of 1, and then to 5.5 for a TOP/Ni ratio of 2. The final mean size of the TOP/Ni = 1 sample with no aggregation (sample E)



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is 5.0 nm, and has a much narrower final size distribution (Figures 5 and 6). The hard sphere radius (RHS) is also shown in Figure 4 and corresponds to the separation of the centers of particles in the aggregates. The values obtained are consistent with the radius of the particle combined with the thickness of the surfactant shell (approximately 2 nm).

Figure 5. Dispersion (σ) for various TOP/Ni ratios as a function of time. The change in the dispersion parameter of the size distributions over time is plotted in Figure 5, and the development of the size distributions over time are shown in Figure 6. Initially, the dispersion decreases as a function of time for all TOP/Ni ratios (except for the TOP/Ni = 0.5 sample where the size distributions were too broad for the dispersion parameter to be accurately fitted, so for this sample the dispersion was fixed at a value of 0.4 - see the Supplementary Information for more detail). The dispersion increases after 47 minutes and 55 minutes for the TOP/Ni = 1 and 2 samples respectively. The increase in the dispersion begins shortly after the time when aggregation is first observed in these samples. The increasing dispersion at later times is also manifested in the size distribution plots for these samples (Figure 6) where the curves are initially narrow and then broaden markedly. The maximum in the curve moves steadily to larger 12 ACS Paragon Plus Environment

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sizes with time, then at later times the position of the maximum is largely unchanged, but for TOP/Ni between 0.5 and 2 where there is aggregation (samples C, D and F) the proportion of larger particles increases.

Figure 6. Time series of size distributions obtained from fitting the SAXS data for samples with TOP/Ni ratios of 0.5 (C), 1 (D), 1 (E) and 2 (F), as in Figure 1. (Vtotal(r) = V.n(r) where ∞

∫ n(r )dr = 1 .) Arrows indicate the change with time. 0

The onset of aggregation is evidenced by the rapid increase in the volume of the aggregated particles population (Figure 3). Aggregation of particles is a common feature of nanoparticle synthesis, and can occur because of the amount34 and type of capping agents used,35-37 or magnetic effects.38 Typically in polar solvents the stability of the nanoparticles are due to anion’s



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adsorbed to the particles surface, and can be destabilized by introducing a poor solvent or ligand with a lower polarity such as pyridine.35-37 For syntheses in polar solvents, bound ligands sterically stabilize the particles from aggregation, which is shown to be predominantly kinetic in nature with aggregation occurring over longer time scales due to ligand lability.39 The aggregation in this system is size dependent. Similar results have been reported due to the onset of ferromagnetism in larger magnetic nanoparticles.38 We postulate that the observed particle aggregation in this system occurs through magnetic attraction, once there is a sufficient number of particles with a radius larger than the critical radius for ferromagnetic behavior to occur (~7.5 nm21). For sizes below this, spherical nickel nanoparticles are superparamagnetic with less than 2% of the bulk magnetic saturation.4,22,23 The only experiment that did not show a broadening of the size distribution at longer reaction times, or the emergence of a correlation peak implying aggregation, was sample E (with TOP/Ni = 1). For all times, this had a narrow size distribution with no nanoparticles larger than a radius of 7.5 nm (Figure 6). For the nanoparticles formed with 1 TOP/Ni and 2 TOP/Ni (samples D and F respectively), the size distributions are initially narrow and then broaden at later times, once a portion of the particles are above 7.5 nm in radius. It is therefore hypothesized that the presence of nanoparticles with radii greater than 7.5 nm leads to the aggregation of nanoparticles in the system. Once a small number of ferromagnetic particles with radius greater than 7.5 nm are formed, the smaller superparamagnetic particles also align, and aggregates are formed. The presence or absence of magnetic aggregation (viz. the two samples with TOP/Ni = 1) may be caused by a very subtle difference in the mean and the width of the nanoparticles' size distribution, which results in vastly different behavior during nanoparticle growth.


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The timing of this onset of aggregation occurs just prior to the increase in the dispersion of the size distributions (Figure 6), which is not observed for the TOP/Ni = 1 experiment (sample E) where no aggregation occurs. This correspondence can be understood by considering the addition of the precursor or monomers to the nanoparticles. The nickel precursor or monomers within an aggregate will be quickly added to the nanoparticles' surfaces, which will cause the inner particles to more or less retain a constant size due to limited diffusion of the nickel monomers or precursor to these inner particles from outside the aggregate. However, particles on the outside of the aggregate are exposed to the nickel monomers or precursor still in solution, and will continue to grow. The increase in polydispersity could be due in part to Ostwald ripening, but in all cases where an increase in dispersion occurs it is preceded by aggregation of nanoparticles. However, the sample with TOP/Ni = 1 which did not exhibit any aggregation retained a narrow size distribution even after all of the precursor was consumed, implying that Oswald ripening is not significant in this case.

Growth kinetics The autocatalytic nanoparticle growth mechanism was applied herein as it has been used to describe nanoparticle growth in systems where a weak reducing agent such as hydrogen, in our case, is present.40-43 The autocatalytic mechanism proceeds by two steps: (1) nucleation by continuous reduction, well below supersaturation (written as A → B where A is precursor and B is particles); (2) rapid autocatalytic decomposition of the metal precursor on the surface of the nanoparticles (written as A + B → 2B). The autocatalytic mechanism is characterized by sigmoidal growth curves.40 From the formalism of Watzky and Finke, it is possible to derive equations to describe the growth as a function of time.40 15 ACS Paragon Plus Environment


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The expression for the concentration of precursor A in solution after time t is given as 40,44

[A](t ) = [A]0

k1 + k 2 [ A]0 k 2 [ A]0 + k1 exp((k1 + k 2 [A]0 )t )

[1]

where [A]0 is the initial precursor concentration, and k1 and k2 are the rate constants for the first and second stages of the growth mechanism respectively (nucleation, and autocatalytic growth). If we assume that the nickel is either present as Ni2+ precursor or as Ni0 in the particles, then because the amount of nickel is conserved, we can write

[A]0 = [A](t ) +

V p (t )ρ

[2]

M wVs

where Vp is the total volume of particles in solution volume (Vs). Mw and ρ are the molecular weight and bulk density of nanocrystalline Ni, respectively. Combining Equations 1 and 2, we obtain an expression for the total volume of particles - one of the parameters obtained from the SAXS fits - as a function of time:

V p (t ) =

M wVs

ρ



[A]0 1 − 

 k1 + k 2 [ A]0  k 2 [ A]0 + k1 exp((k1 + k 2 [A]0 )t ) 

[3]

Equation 3 was fitted to the total volume for each sample (up to a time before the settling of aggregates occurs). These are shown in Figure 7, with the fitted parameters given in Table I.


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Figure 7. Total volume of particles from Figure 3, and fits using equation 3, for samples with TOP/Ni ratios of 0.5 (C), 1 (D), 1 (E) and 2 (F), as in Figure 1.

Table I. Rate constants fitted from curves in Figure 7.

a

TOP/Ni

Curve

k1 (min-1)

k2[A]0 (min-1)

0.5

C

0.0009 ± 0.0002

0.065 ± 0.005

1

D

0.00007 ± 0.00003

0.32 ± 0.02

1a

E

0.0019 ± 0.0001

0.076 ± 0.002

2

F

0.0011 ± 0.0003

0.100 ± 0.007

Sample with no aggregation.

From the fits it can be seen that the growth of the nanoparticles can be described by the autocatalytic growth mechanism. The fit for the TOP/Ni = 1 sample with no aggregation (E) is very good, while the model only holds at time scales shorter than ~50 minutes for the systems which show aggregates of nanoparticles precipitating from solution. This is due to the 17 ACS Paragon Plus Environment


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nanoparticles settling out of solution before Vmax is reached, causing the value of Vmax to be underestimated. For example, the sample with TOP/Ni = 1 with aggregation (D) shows rapid settling of the nanoparticles from solution once aggregation occurs; therefore the actual value of Vmax could be much higher than the observed value. (See the Supplementary Information for additional analysis of the rate constants for this sample.) The values for the rate constants for samples C, E, and F are within a factor of two of each other for each of the two reaction steps and do not show any obvious trends with TOP concentration. k1 relates to the fraction of precursor and monomer that react to form nuclei in a unit of time. k2 relates to the fraction of precursor and monomer that is catalyzed by an existing nucleus to cause particle growth, and is dependent on the initial nickel precursor concentration. k1

Effect of Surfactant Concentration and Aggregation on the Growth

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