Size-Controlled Synthesis of Colloidal Silver ... - Technische Chemie

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Size-Controlled Synthesis of Colloidal Silver Nanoparticles Based on Mechanistic Understanding Maria Wuithschick,† Benjamin Paul,‡ Ralf Bienert,§ Adnan Sarfraz,∥ Ulla Vainio,⊥ Michael Sztucki,▽ Ralph Kraehnert,‡ Peter Strasser,‡ Klaus Rademann,† Franziska Emmerling,§ and Jörg Polte*,† †

Department of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor-Straße 2, 12489 Berlin, Germany Technische Chemie, Technische Universität Berlin, Straße des 17 Juni 124, 10623 Berlin, Germany § BAM Federal Institute of Materials Research and Testing, Richard-Willstätter-Straße 11, 12489 Berlin, Germany ∥ Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Straße 1, 40237 Düsseldorf, Germany ⊥ FS-DO at Deutsches Elektronen Synchrotron, Notkestraße 85, 22607 Hamburg, Germany ▽ European Synchrotron Radiation Facility, 6 Rue Jules Horowitz, BP 220, 38043 Grenoble Cedex, France ‡

S Supporting Information *

ABSTRACT: Metal nanoparticles have attracted much attention due to their unique properties. Size control provides an effective key to an accurate adjustment of colloidal properties. The common approach to size control is testing different sets of parameters via trial and error. The actual particle growth mechanisms, and in particular the influences of synthesis parameters on the growth process, remain a black box. As a result, precise size control is rarely achieved for most metal nanoparticles. This contribution presents an approach to size control that is based on mechanistic knowledge. It is exemplified for a common silver nanoparticle synthesis, namely, the reduction of AgClO4 with NaBH4. Conducting this approach allowed a well-directed modification of this synthesis that enables, for the first time, the size-controlled production of silver nanoparticles 4−8 nm in radius without addition of any stabilization agent. KEYWORDS: silver nanoparticles, growth mechanism, SAXS, size control, sodium borohydride

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INTRODUCTION Metal nanoparticles are used for a wide range of applications,1 for example, in spectroscopy,2,3 biomedicine,4−6 and catalysis,7−10 which is the result of their unique catalytic, optical, electronic, and magnetic properties. These properties can be adjusted by altering the nanoparticle size, composition, crystal structure, and morphology.11,12 Consequently, size control provides an effective key to an accurate adjustment of colloidal properties. The common synthetic procedure to obtain colloidal metal nanoparticles is the chemical reduction of a precursor salt with a reducing agent such as sodium citrate or sodium borohydride.13 In general, the synthetic procedure itself is relatively simple, whereas size control is often claimed but rarely achieved.14 The exception might be gold nanoparticles, but for silver, copper, and palladium, only a very few reliable synthetic procedures rxost that deliver monodisperse colloids in a size range of 1−20 nm.15−18 Moreover, most synthetic procedures require additional stabilization agents that can change relevant properties, such as biocompatibility or catalytic activity, making them inappropriate for further use.19−21 The most common approach to size control is testing different sets of parameters via simple trial and error, which makes the synthesis of nanoparticles “rather an art than a science”.22 The © 2013 American Chemical Society

number of scientific contributions that investigate actual nanoparticle growth mechanisms, and in particular the influences of parameters on growth, is still very limited.14,23−26 As a result, nanoparticle growth processes often remain a black box.16,17,27,28 A knowledge-based approach can be more effective to achieve size control. This contribution presents such an approach to size control, which comprises three steps as depicted in Scheme 1: (A) investigation of the growth mechanism in principle, including all relevant physicochemical processes for one set of parameters; (B) investigation of influences of synthesis parameters on the growth mechanism, which leads to identification of size-determining parameters; and (C) deliberate adjustment of the decisive reaction parameters to obtain a desired final particle size distribution. This approach can be applied to different nanoparticles (e.g., metallic, oxidic, or bimetallic particles), matrices (e.g., water, organic solvents, glass),29 and preparation methods (e.g., chemical reduction, photochemical reduction). It requires monitoring particle size distribution and concentration in situ Received: June 6, 2013 Revised: October 8, 2013 Published: November 5, 2013 4679

dx.doi.org/10.1021/cm401851g | Chem. Mater. 2013, 25, 4679−4689

Chemistry of Materials

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evolution of the size distribution during the second coalescent step (fourth step of the growth mechanism). Detailed Investigation of Growth Processes during Metastable State and Final Coalescence. Particle growth of the standard synthesis (refers to simple 1:1 mixing of 0.5 mM AgClO4 solution and 3 mM NaBH4 solution at ambient conditions) was investigated with time-resolved SAXS at the ID02 beamline of the European Synchrotron Radiation Facility (ESRF) synchrotron storage ring. A free-liquid jet setup was used for containerless measurement. The basic concept of this setup is to extract a small flow of liquid sample continuously from the reaction vessel. The solution is pumped to a nozzle, resulting in a liquid jet at which the SAXS measurements are conducted. Thus, the synthesis can be performed in its undisturbed conventional environment (stirred batch reactor). Furthermore, X-ray-induced effects are minimized and a high time resolution at a low signal-to-noise ratio can be achieved when a synchrotron light source is used. A detailed description of the setup can be found elsewhere.34 To avoid agglomeration of the silver nanoparticles inside the tubing, the distance between reaction vessel and X-ray beam was minimized and only Teflon tubing and connectors were used. From each scattering curve, the particle mean radius, polydispersity, relative volume fraction, and relative number of particles were determined. The size distribution is assumed to be a Schulz− Zimm distribution, which has been shown to be a suitable approximation.39 Selected scattering curves and their corresponding theoretical fits are shown in Supporting Information (section SI-1). Figure 1a shows particle mean radius and volume fraction versus time. The normalized volume fraction embodies the whole volume of all particles. Polydispersity and number of particles versus time are displayed in Figure 1b. The first available scattering curve (t = 5 s) is already assigned to the metastable state. The size distribution of the colloidal solution remains constant during the entire metastable state (t = 5−520 s) with a particle mean radius of 1.5 nm and a polydispersity of 40%. The volume fraction is approximately 100% and remains almost constant. These experimental findings reveal that colloidal stability during the metastable state is sufficient to prevent the nanoparticles from any further growth. The stability decreases after approximately 520 s. The result is a particle growth process with an increase in the mean radius to 6.3 nm, accompanied by a successive decrease of the polydispersity to 25% (see Figure 1a,b). The volume fraction of the final colloidal solution is the same as during the metastable state, which confirms that particle growth is a process of coalescence. During the coalescent step, the volume fraction decreases down to 65% (t = 530 s). This indicates that the mathematical model used to fit the scattering curves cannot describe the colloidal solution accurately at that point. Obviously, the coalescing particles initially form irregular objects that finally reorganize to spheres, whereas the model assumes spherical morphology. On average, one final silver nanoparticle is formed by the coalescence of approximately 50 smaller particles (see Figure 1b). Figure 1c illustrates the shift of size distribution. The overlap of the size distribution before and after coalescence is low, which indicates that all particles participate in the growth process. In conclusion, the high data quality of the synchrotron SAXS investigations show that colloidal stability during the metastable state is sufficient to prevent any particle aggregation and further growth.. The chemical process of BH4− conversion, which is

Scheme 1. Approach to Size Control Based on Mechanistic Knowledge

and time-resolved during the entire growth process. Such experimental information can be obtained by applying several lab-scale and synchrotron small-angle X-ray scattering (SAXS) setups.24,30−37 The knowledge-based approach is exemplified for the synthesis of silver nanoparticles which was adapted from Van Hyning and Zukoski.38 It comprises the wet chemical reduction of silver perchlorate (AgClO4) with sodium borohydride (NaBH4) without any additional stabilizing agents.

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RESULTS AND DISCUSSION Step A: Growth Mechanism in Principle. In our recent paper,39 we were able to deduce the nanoparticle growth mechanism in principle of the herein-investigated silver nanoparticle synthesis. However, important details are still missing to gain a profound understanding of the growth process. Thus, completing step A of the presented approach constitutes the first part of this contribution. Previously, it has been shown that the growth mechanism comprises four steps: (1) rapid reduction of ionic silver to silver atoms, which immediately form dimers, trimers etc.; (2) coalescence of these preliminary formed clusters, resulting in particles 2−3 nm in radius; (3) an intermediate phase of stability, during which the particle mean radius remains constant (referred to as metastable state); and (4) a second coalescence that leads to the final colloids.39 The particle mean radius ranges from 4 to 10 nm (at 20−30% polydispersity) and is poorly reproducible. In addition, the duration of the metastable state can vary between 5 and 20 min. The final colloidal solutions showed no changes after several days. A long-term stability test, in terms of months, was not performed. It was assumed that the metastable state, and thus the growth mechanism, is strongly influenced by the conversion of residual BH4− to B(OH)4−.39 Experimental results indicate that particle growth and chemical conversion of BH4− to B(OH)4− occur in parallel (for details, see the supporting information of Polte et al.).39 Further investigations are required to correlate particle growth with associated physicochemical processes in the colloidal solution. In particular, previous experimental results were insufficient to exclude with certainty any growth of particles during the metastable state. The limited data quality of lab-scale SAXS experiments complicates the detection of very small particles below 1 nm in diameter. In addition, lab-scale SAXS experiments cannot provide detailed information on 4680



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Although NaBH4 is a moderate reducing agent compared to other metal hydrides, the reducing species H− is provided fast enough to reduce metal ions such as Au3+ or Ag+ within milliseconds.33 In comparison, hydrolysis is a much slower process.41,42 According to eq i, 1 mol of Ag+ is reduced by the equivalent amount of BH4−. To ensure complete reduction of the metal precursor, the reducing agent is always used in excess (for the standard synthesis, in 6-fold excess). Therefore, BH4− remains in the colloidal solution after the ionic silver is reduced. The remaining BH4− is converted during the nanoparticle growth process. Previous experiments indicated that the metastable state ends at the point of total BH4− consumption (see section SI-6 of our previous work).39 It was shown that the addition of small amounts of HAuCl4 during the metastable state leads to the reduction of ionic gold by residual BH4−, which generates gold nanoparticles besides the existent silver nanoparticles. After the second coalescence, HAuCl4 is no longer reduced when added to the colloidal silver solution. Another simple experiment supports the correlation between amount of residual BH4− and duration of the metastable state. Adding fresh NaBH4 solution to the colloidal solution during the metastable state extends the duration of this phase (for details, see section SI-3 in Supporting Information). The kinetics of BH4− consumption can be investigated by time-resolved monitoring of H2 evolution from the reaction vessel, since BH4− conversion is accompanied by the release of hydrogen (see eqs i and ii). The released hydrogen can be detected quantitatively by mass spectrometry. An according setup is described in the Experimental Section and in section SI-4 in Supporting Information. Two samples were investigated: (i) colloidal silver synthesis and (ii) a comparison sample for which the NaBH4 solution is mixed with an equal volume of water instead of silver precursor solution. Figure 2a depicts the volume flow of released hydrogen versus time of the colloidal solution (red line) and the corresponding NaBH4 solution (black line). Integration of the curves gives the total volume of released hydrogen and is shown in Figure 2b. For both samples, the maximum volume flow of hydrogen is detected at the beginning. The maximum flow is 0.06 mL/min for the colloidal solution and 0.017 mL/min for the NaBH4 solution. The flow decreases with increasing reaction time and is zero after approximately 20 min for the colloidal solution and after approximately 12 h (see section SI-4 in Supporting Information) for the NaBH4 solution. The duration of the metastable state can vary between 5 and 20 min for repeated experiments. In this particular experiment, the second coalescent step of the colloidal system was observed after approximately 20 min; thus it coincides with the end of H2 detection. For the NaBH4 solution, the total volume of detected hydrogen is 1.4 mL, which is in agreement with calculations for a full BH4− conversion according to reaction ii and the ideal gas law. The detected total volume of H2 for the colloidal solution is only 0.7 mL, which is a result of the experimental setup: the procedure of mixing the reactants and sealing the reaction vessel takes 5 s. Since reaction i is a very fast process, H2 evolved from the reduction of Ag+ was not detected. The mass spectrometric experiments reveal that H 2 evolution, and thus BH4− conversion, is highly accelerated in the presence of silver nanoparticles (see Figure 2). This is not surprising since catalytic activity of metal nanoparticles toward the conversion of BH4− was observed in a variety of previous studies.43−45 Tetraborohydride is converted during the entire

Figure 1. Results of synchrotron SAXS investigations of standard colloidal synthesis: (a) particle mean radius and normalized volume fraction (last data point set as 100%) vs reaction time; (b) polydispersity and relative number of particles (last data point set as 1) vs reaction time; and (c) calculated particle size distributions for selected reaction times.

assumed to cause the rapid loss of stability and initiates the second coalescence, was investigated next Chemical Conversion of BH4− during Particle Growth. Two types of reaction can occur in an aqueous solution of sodium borohydride: (i) BH4− can act as a source of nucleophilic hydride H−, which can reduce a variety of metal ions Mz+;40 and (ii) the H− ligands can be replaced by water molecules (hydrolysis). The reactions can be described by the following simplified equations (for details, see section SI-2 in Supporting Information): Mz + + z BH4 − + z 4H 2O → M 0 + z B(OH)4 − + z 3.5H 2 + z H+ BH4 − + 4H 2O → B(OH)4 − + 4H 2

(i) (ii) 4681



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residual BH4− and might be caused by oxidation of the nanoparticle surface.. Step B: Influence of Reaction Parameters on Growth Mechanism. In the previous section, the particle growth mechanism was studied for one combination of synthesis parameters ([AgClO4] = 0.5 mM, [NaBH4] = 3 mM, 1:1 mixing, stirring speed = 300 rpm, room temperature). In step B, influences of different reaction parameters on the growth mechanism, and thus on the final size distributions, are investigated. A strategy to achieve a reproducible synthesis is deduced, which is the basic requirement to develop sizecontrollable nanoparticle syntheses. Influence of Reactant Concentrations on Growth Mechanism. Already in the 1990s, Glavee et al.51 pointed out that the reduction of metal ions with BH4− is a complex interplay of several chemical processes (reduction, hydrolysis, catalysis) that is influenced by numerous parameters such as reactant concentrations and pH value. Thus, it is not surprising that the final particle size of the investigated synthesis is very sensitive to concentrations of AgClO4 and NaBH4, temperature, ionic strength, and even parameters that are often not considered to influence the synthesis, such as the type of reaction vessel, stirring speed, and mixing procedure (for details, see section SI-5 in Supporting Information). Therefore, the same type of reaction vessel was used for all lab-scale experiments, the stirring speed was kept constant (300 rpm), and the same mixing procedure (1:1 mixing of 2 × 5 mL) was used. The temperature was kept constant at 23 °C (±1 °C). In addition, the reducing agent solution was always prepared freshly and used within 1 min. Nevertheless, the exact preparation of NaBH4 solution is difficult since NaBH4 is hygroscopic and easily absorbs moisture.52 In section SI-6 (Supporting Information), it is demonstrated that the mass of NaBH4 powder increases drastically (by up to 300%) if the substance is not stored under water-free conditions. Thus, the as-received NaBH4, powder was partitioned in small units and stored under argon. However, even if all these precautions are considered and samples are prepared simultaneously from identical reactant solutions, the final size distribution is still poorly reproducible. As a result, the final particle mean radius obtained from the standard silver nanoparticle synthesis can vary between 4 and 10 nm. A low level of reproducibility is also apparent for other reactant concentrations, as illustrated by a parameter variation study shown in Supporting Information (section SI-7). The syntheses were carried out simultaneously three times with identical reactants. However, standard deviations of the mean radii are relatively high (between 0.3 and 1 nm). As a principal tendency, the average particle mean radius increases with increasing AgClO4 and decreasing NaBH4 concentration. Nevertheless, the insufficient reproducibility of the final particle size demands further elucidation of which steps of the growth mechanism are sensitive to synthesis parameters. It is possible that small changes in the synthetic procedure (e.g., mixing conditions) already influence the outcome of the first coalescent step considerably. Even small differences of the size distribution after the first coalescent step might affect the second coalescent step and therefore the final particle size. Therefore, it is necessary to extend the parameter study by mechanistic investigations. For five selected points of the parameter study (highlighted in section SI-7 in Supporting Information), the particle growth process was investigated time-resolved with lab-scale SAXS.

Figure 2. Results of mass spectrometric investigations on hydrogen release during silver nanoparticle synthesis and corresponding pure NaBH4 solution: (a) volume flow of hydrogen vs reaction time and (b) total volume of hydrogen vs reaction time.

metastable state. The end of H2 release indicates the complete conversion of BH4− and coincides with the second coalescent step. Coalescent processes result from a decrease of colloidal stability. Therefore, the destabilization of the primary formed nanoparticles is most likely associated with the progressing conversion of BH4− to B(OH)4−. The chemical conversion could influence the particle stability by the specific adsorption of ionic species: stability might increase by adsorption of BH4− or decrease by adsorption of B(OH)4− at the particle surface. A paper by Andrieux et al.45 shows that BH4− adsorbs or even dissociates on cobalt nanoparticle surfaces. However, the amount of ions [BH4− and/or B(OH)4−] that could adsorb at the silver particle surface changes gradually, whereas the stability of the colloidal silver solution decreases relatively abruptly. It was shown for many silver nanoparticle systems that a silver oxide layer is formed upon storage in ozone but also in aqueous solution at ambient conditions.46−50Therefore, the decrease of colloidal stability could result from a sudden formation of a silver oxide layer at the nanoparticle surface. Residual BH4− might continuously reverse any surface oxidation of the particles during the metastable state. Its complete consumption at the end of the metastable state could initiate a collective surface modification of all particles and consequently a “simultaneous” decrease of colloidal stability of all particles. As a result, the particles undergo further growth due to coalescence until a stable size is reached. Summary of Step A. It was shown that colloidal stability remains sufficient to inhibit any particle growth during the metastable state. The decrease of particle stability that initiates the second coalescent step correlates with full conversion of 4682



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Figure 3. Results of time-resolved SAXS investigations: mean radius (polydispersity = 50% before and 25% after final coalescence) vs reaction time for varied (a) silver perchlorate concentration and (b) reducing agent concentration. The displayed concentrations refer to solutions used for the syntheses and mixed 1:1. Different symbols refer to different synthesis repetitions. Light and dark gray bars highlight particle sizes after the first and second coalescent steps, respectively. The blue dotted line highlights the duration of the metastable state. Results of the standard synthesis are displayed twice (middle panels).

impossible: (i) the hygroscopy of solid NaBH4 and its fast change of chemical composition upon dissolving in water impede an exact adjustment of the reducing agent concentration; (ii) identical mixing and stirring conditions are hard to achieve; (iii) small temperature variations can hardly be eliminated; and (iv) the exact amount of dissolved oxygen can hardly be controlled.55 Instead of attempting to control BH4− consumption, a much more practical approach to achieve a reproducible nanoparticle synthesis could be elimination of the complex second coalescent step. For this purpose, the synthesis needs to be modified so that the two separated coalescent steps merge to one single step, thus eliminating the metastable state. The results displayed in Figure 3 clearly show that the duration of the metastable state (highlighted by blue dotted lines) correlates with the amount of residual BH4−. The duration decreases with decreasing amount of residual BH4−. From the results of the mechanistic investigations, it can be expected that the metastable state will vanish if the NaBH4 concentration is reduced to the concentration of AgClO4. This can be achieved (i) by reducing the quantity of dissolved NaBH4 or (ii) by aging the as-used reducing agent solution, which has a NaBH4 excess (hydrolysis leads to a decrease of BH4− concentration).42 Merging the two coalescent steps by use of a reduced quantity of NaBH4 was examined for an AgClO4 concentration of 0.5 mM. The concentration of NaBH4 was successively reduced from a ratio R = [BH4−]/[Ag+] of 2 to 1. The final colloidal solutions were investigated with lab-scale SAXS and UV−vis spectroscopy. The results are shown in Supporting Information (section SI-9). The duration of the metastable state decreases to zero exactly for R = 1. However, the colloidal stability at this ratio is low, which leads to precipitation within 2 min. The alternative approach is based on aging of the NaBH4 solution. The BH4− concentration decreases exponentially due to hydrolysis,42 and complete conversion in the absence of nanoparticles proceeds within hours. In the following, the

The results of the mathematical modeling are displayed in Figure 3 (note that the standard synthesis is displayed twice). Selected scattering curves and their corresponding theoretical fits are shown in Supporting Information (section SI-8). The SAXS investigations reveal that differences between final particle sizes of repeated syntheses and between syntheses with concentration variations are caused by the second coalescent step. Particle size after the first coalescent step (highlighted by light gray bars) is about the same for all investigated systems (mean radius approximately 2 nm at 50% polydispersity). After the second coalescent step, the mean radii vary considerably (highlighted by dark gray bars). For example, mean radii after the first coalescent step of the standard synthesis (diagram in the middle panels) are nearly identical for all three experimental runs, but the final mean radii deviate between approximately 7 and 10 nm. As a consequence, reproducibility and particle size control can be achieved only by controlling the second step of coalescence. The second coalescent step is a very complex process. It comprises aggregation of spherical nanoparticles with a broad size distribution (polydispersity approximately 50%) that must reorganize to give again spherical-shaped colloids. The aggregation is caused by a loss of colloidal stability, which correlates with the full conversion of BH4−. The driving force for reorganization of the aggregated nanoparticles is a gain of energy (surface energy vs bulk energy).53 The process is strongly affected by particle surface chemistry,54 such as the presumed surface oxidation. Thus, both processesaggregation and reorganizationand consequently the second coalescent step are dependent on the kinetics of the BH4− → B(OH)4− conversion and the associated oxidation of the particle surface. The conversion rate of BH4− depends on many parameters, such as reactant concentrations, reaction temperature, stirring speed, and catalytic properties of the nanoparticles formed after the first coalescent step.41,42 Therefore, these parameters have to be precisely controlled to adjust the particle size distribution. However, absolutely accurate control of these parameters under normal lab conditions is almost 4683



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the final size distribution is reproducible and the mean radius increases from 6 to 9.3 nm. Within a period of more than 90 min (ta approximately 650−740 min), the mean radius even remains constant at 9.3 nm. This period is referred to as size plateau. For ta > 800 min (phase III), the residual amount of BH4− is insufficient to reduce Ag+ completely, which is illustrated by a decreasing volume fraction (reflects the total volume of all silver nanoparticles). In phase III the particle mean radius decreases significantly with increasing ta. The duration of the metastable state (reaction time until a significant color change is observed; see Polte et al.39) versus ta is displayed in Figure 4b. During phase I, the duration of the metastable state decreases with ta. The color change, which indicates the second coalescence, disappears with the beginning of phase II. Exemplarily, section SI-11 (Supporting Information) shows two time-resolved UV−vis investigations for colloidal syntheses of phase I (ta = 90 min) and phase II (ta = 500 min). For ta = 90 min, a significant change of the maximum absorbance and wavelength is observed at 240 s, while the UV−vis spectrum for ta = 500 min remains constant. In addition, the vanishing of the metastable state was observed by time-resolved SAXS by applying the free-liquid jet setup at the ID02 beamline of the synchrotron light source ESRF. Exemplarily, Figure 5 depicts the size distribution versus time

influence of the NaBH4 aging process on the nanoparticle growth mechanism is investigated. NaBH4 Aging: Key to Reproducibility. The chemical composition of the reducing agent solution changes with time, due to the conversion of BH4− to B(OH)4−. The conversion starts immediately after NaBH4 is dissolved in water. To investigate how the hydrolysis progress of the reducing agent solution (referred as aging) influences the nanoparticle growth process, a standard NaBH4 solution (3 mM) was prepared and stored at ambient conditions (T = 23 °C). For various times between 0 and 1020 min after dissolving the NaBH4 powder (referred as aging time ta), the solution was used to prepare simultaneously three colloidal silver solutions by reduction of a standard 0.5 mM AgClO4 solution. Size distributions of the final nanoparticless were determined with lab-scale SAXS. Selected scattering curves and corresponding fits are shown in Supporting Information (section SI-10). Results of the mathematical modeling are displayed in Figure 4.

Figure 4. Results of lab-scale SAXS investigations on the influence of the NaBH4 aging process on nanoparticle synthesis. (a) Final particle mean radius (polydispersity = 30% and for the last three aging times 25%) and normalized volume fraction (first data point set as 100%) vs aging time. (b) Duration of the metastable state vs aging time. The diagram can be divided into three parts: a phase with poor reproducibility of the final size distribution (I), a phase with good reproducibility (II), and a phase of incomplete precursor reduction (III).

Figure 5. Results of time-resolved SAXS investigations on the growth mechanism of silver colloids with aged NaBH4 solution as reducing agent. Particle mean radius and polydispersity vs time for aging times of (a) 240 min and (b) 660 min are shown.

Figure 4a depicts the evolution of final particle mean radius and relative volume fraction versus aging time ta. The diagram can be divided into three parts. For 0 min < ta < 400 min (phase I), the final particle mean radius is poorly reproducible but decreases with increasing ta from approximately 8 to 6 nm. The volume fraction remains almost constant, which indicates a full precursor reduction. The volume fraction remains also constant for 400 min < ta < 800 min (phase II). In this phase,

for two colloidal syntheses during phase I (ta = 240 min) and phase II (ta = 660 min). Selected scattering curves and corresponding fits are displayed in section SI-12 of Supporting Information. For ta = 240 min, the particle mean radius at the first available measuring point (5 s) is approximately 3 nm 4684



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(polydispersity of 50%) and increases to approximately 5 nm (polydispersity of 30%) at a reaction time of around 150 s. In contrast, for synthesis with the 660-min-old NaBH4 , the second coalescent step vanishes. The particle mean radius remains constant at 8.5 nm (polydispersity of 35%) from the first measurement point (5 s). These experiments show that the particle growth mechanism changes due to aging of the reducing agent solution. For ta > 400 min, only one fast coalescent step is observed. Actually, this is a surprising result since the silver precursor is still reduced completely between 400 and 800 min. This means the molar ratio between the NaBH4 solution used and Ag+ is at least equimolar, and most likely above 1, during phase II. In contrast, the results presented in Figure 2 suggest that the second coalescence coincides with a complete depletion of BH4−. However, in this experiment no B(OH)4− is present when the reactants are mixed. Therefore, the vanishing of the second coalescent step during phase II might be connected to the B(OH)4− anion or the ratio BH4−/ B(OH)4−. However, the improvement of reproducibility of the final size distribution coincides with the merging of the two separated coalescent steps as expected. In addition, the obtained final size distribution remains almost constant within a size plateau. This period of time is ideal for conducting a parameter variation study that aims to identify size-determining parameters. Parameter Variation within Size Plateau. It was shown that reproducibility of the synthesis can be improved significantly by aging the NaBH4 solution. This observation can be used to conduct a reliable parametric study. An aged NaBH4 solution (ta within the size plateau) was used to reduce a standard 0.5 mM AgClO4 solution. This parameter study includes variations of AgClO4 concentration, ionic strength [Na+, ClO4−, B(OH)4−], temperature, and pH. Size distributions of the final colloids were investigated with lab-scale SAXS. Selected scattering curves and their corresponding fits can be found in Supporting Information (section SI-13). Figure 6a displays the results of AgClO4 concentration variation. For decreasing silver precursor concentration, the final mean radius decreases (for [AgClO4] = 0.25 mM, to r = 7.8 nm). In Figure 6b,c, the results of Na+ and ClO4− concentration variation are displayed. The final mean radius increases only slightly with increasing ionic strength. Figure 6d displays the results of temperature variation. The final particle mean radius decreases for decreasing temperature. For 0.5 and 10 °C, a particle mean radius of 8.3 and 8.6 nm, respectively, is obtained. Figure 6e depicts the final mean radius obtained upon addition of perchloric acid (HClO4). The mean radius remains almost constant even for the addition of 250 μL of acid (pH = 3). Figure 6f illustrates the influence of additional B(OH)4− on the final particle size. Unfortunately, it is not possible to determine the total concentration of B(OH)4− present during the Ag+ reduction, since the exact chemical composition of the reducing agent (exact ratio [BH4−]/[B(OH)4−]) is unknown. However, the absolute amount of ionic silver reduced is 2.5 μmol. The amount of additional B(OH)4− is of the same magnitude. Addition of NaB(OH)4 results in a significant decrease of the particle mean radius. The mean radius decreases to approximately 7.3 nm if the silver precursor solution is adjusted to a NaB(OH)4 concentration of 15 mM. These experimental results show that temperature and ratio between the concentrations of Ag+, BH4−, and B(OH)4− have a major influence on the final size distribution.

Figure 6. Results of SAXS investigations on the influences of reaction parameters on final size distribution. For all syntheses, aged NaBH4 solution (within plateau) was used. Final particle mean radius is shown vs (a) silver perchlorate concentration, (b) concentration of Na+, (c) concentration of ClO4−, (d) temperature, (e) added volume of HClO4, and (f) added amount of NaB(OH)4. Polydispersity stayed constant at 30%. NaClO4 and HClO4 were added to the silver precursor solution to give the displayed concentrations. The silver precursor solutions were then mixed 1:1 with the aged reducing agent solution. Results of standard precursor solutions are highlighted (●).

Step C: Size Control. From mechanistic investigations, it was deduced that the second coalescent step and therefore the final particle size can hardly be controlled. It was shown that the reproducibility can be improved significantly by merging the two coalescent steps. This can be achieved by aging the NaBH4 solution (see Figures 4 and 5). In fact, aging NaBH4 represents a decreasing ratio of BH 4 − to B(OH) 4 − . Furthermore, this decreasing ratio leads to an increasing final particle size (see phase II in Figure 4). As a result, NaBH4 aging enables a reproducible and size-controlled synthesis of silver colloids. However, aging the NaBH4 solution is very laborious since the reducing agent solution has to be prepared at least 5 h in advance (begin of phase II; see Figure 4). Furthermore, it is very difficult for a size control to obtain exactly the demanded aging and thus the demanded BH4−/B(OH)4− ratio. Chemical conversion of BH4− to B(OH)4− can be faster or slower, for example, due to small temperature variation during storage. Imitation of NaBH4 Aging. The alternative is an imitation of the NaBH4 aging process. A variation of the BH4−/B(OH)4− ratio can also be achieved by simply mixing B(OH)4− with fresh BH4− solution. B(OH)4− solution can be obtained from longer storage of BH4− solution due to the hydrolysis of BH4−. The following synthetic procedure imitates the NaBH4 aging described in step B (see Figure 4): a 3 mM NaBH4 solution is prepared and stored for at least 1 day. The obtained 4685



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B(OH)4− solution is mixed with freshly prepared 3 mM NaBH4 solution in different ratios and immediately used to reduce a 0.5 mM AgClO4 solution (1:1 mixing). Figure 7 shows the results

merging of the two coalescent steps. This means that the growth mechanism changes from a four-step to a two-step mechanism similar to the corresponding gold nanoparticle synthesis.33 The BH4−/B(OH)4− ratio can be precisely adjusted by well-defined aging of the reducing agent solution or a simple imitation of this aging process. Although the exact role of the BH4−/B(OH)4− ratio remains unclear, modification of the synthesis leads to a reproducible growth process. As a result, the gained mechanistic knowledge enabled a welldirected modification of the synthesis that allows reproducible and size-controlled production of silver nanoparticles in the range of 4−8 nm in radius (polydispersity = 30%). This represents substantial progress for the synthesis of metal colloids, since syntheses of silver nanoparticles with size control are rare, especially in aqueous solution and without addition of stabilizing agents.13,56−60 Our work proves that mechanistic studies are not only of academic interest but can be the key to improve the current state of nanoparticle syntheses.

Figure 7. Results of silver nanoparticle syntheses using a mixture of aged 3 mM and fresh 3 mM NaBH4 solution as reducing agent. Final mean radius (polydispersity = 30%) vs percentage of fresh NaBH4 is plotted. The x-axis is inverted since a decreasing percentage of fresh NaBH4 imitates an increasing aging time.

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EXPERIMENTAL SECTION

Colloidal Syntheses. In this paper, the standard procedure for synthesis of colloidal silver nanoparticles in water refers to 1:1 mixing of 0.5 mM AgClO4·H2O and 3 mM NaBH4 solution. The reactant solutions were obtained by dissolving 225.33 mg of AgClO4·H2O (Sigma−Aldrich, 99.999%) in 2 L and 113.5 mg of NaBH4 powder (Alfa Aesar, 98%) in 1 L of ultrapure water (18.2 MΩ·cm, Millipore). The silver precursor solution was stored in the dark. The reducing agent solution was prepared freshly and used within 1 min. The asreceived NaBH4 was divided into small portions under inert gas. A new portion was used for each experiment. The colloidal synthesis was carried out at ambient conditions (23 °C ± 1 °C). The stirring speed was kept constant at 300 rpm. For lab-scale syntheses, 5 mL portions of each reactant solution (total volume = 10 mL) were mixed by use of two Eppendorf pipettes. For each synthesis, an unused small glass container (20 mL volume) was used as reaction vessel. For investigations at the synchrotron beamlines, the synthesis was scaled up to give a total volume of 400 mL of colloidal solution. The reactant solutions were filled into two glass flasks with a selfmanufactured outlet at the bottom and mixed 1:1 within 3 s by a handoperated pump. A beaker was used as reaction vessel. After use, all glassware was cleaned with concentrated nitric acid and rinsed with generous amounts of ultrapure water. For reactant concentration studies, 1 mM AgClO4 stock solution was prepared by dissolving 225.33 mg of AgClO4·H2O in 1 L of water and diluted to give 0.25, 0.4. 0.5, 0.6, and 0.75 mM precursor solutions. NaBH4 solutions were prepared freshly by dissolving appropriate amounts of NaBH4 in 1 L of water: 56.75 mg (1.5 mM), 85.13 mg (2.25 mM), 113.5 mg (3 mM), 141.88 mg (3.75 mM), and 170.25 mg (4.5 mM). For investigations on the minimal excess of fresh NaBH4 (R = [NaBH4]/[AgClO4]) required to receive a stable colloidal solution, 0.5 mM AgClO4 solution was reduced. NaBH4 solutions were prepared freshly by dissolving appropriate amounts of NaBH4 in 1 L of water: 18.9 mg (0.5 mM, R = 1), 20.8 mg (0.55 mM, R = 1.1), 22.7 mg (0.6 mM, R = 1.2), 28.4 mg (0.75 mM, R = 1.5), and 37.8 mg (1 mM, R = 2). To study the influence of the NaBH4 aging process, 1 L of 3 mM NaBH4 solution was prepared freshly and stored at ambient conditions (open to atmosphere). After certain aging times, three colloidal solutions were prepared (standard lab-scale synthetic procedure with a total volume of 10 mL). The parameter study within the size plateau (see Figure 4) was carried out by use of 3 mM NaBH4 solution that was stored 10 h at ambient conditions as reducing agent. The silver precursor solutions were adjusted for different variations:

of lab-scale SAXS investigations of final colloidal silver solutions that were synthesized with BH4−/B(OH)4− mixtures containing 15−35% fresh NaBH4. For details of this synthetic procedure, see the Experimental Section. Selected scattering curves and their corresponding fits can be found in Supporting Information (section SI-14). For a mixture of 35% BH4− and 65% B(OH)4−, two separated coalescent steps are observed during the colloidal synthesis, whereas a mixture that contains only 32.5% BH4− leads to a nanoparticle growth mechanism that comprises only one single coalescent step. Thus, this mixture corresponds to the beginning of phase II of the aging experiment (see Figure 4). In accordance with the aging experiment, a decreasing percentage of BH4− leads to an increasing final mean radius. The mean radius increases from 4 to 8 nm, whereas the mean radius of the corresponding aging experiment is slightly bigger. Nevertheless, this procedure enables a very simple and reproducible access to colloidal silver nanoparticles with accurate size control between 4 and 8 nm in radius (polydispersity = 30%). To the best of our knowledge, this is the first size-controlled synthesis of colloidal silver nanoparticles that does not require an additional stabilization agent.

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CONCLUSIONS This paper presents an approach to size control based on mechanistic knowledge that was exemplified for a common silver nanoparticle synthesis (reduction of AgClO4 with an excess of NaBH4 in aqueous solution). It comprises an understanding of the nanoparticle growth mechanism and the influences of synthesis parameters on the growth. The growth mechanism consists of four steps and includes two separated steps of coalescence. It is shown that the final growth step (the second coalescent step) correlates with the conversion of residual BH4− to B(OH)4−. The depletion of BH4− could cause a surface oxidation of the preliminary nanoparticles formed in the first coalescent step. This would lead to a decrease of colloidal stability, which initiates the further growth due to coalescence. The second coalescence is a complex process that can hardly be controlled. As a consequence, the final particle size is not reproducible. From the mechanistic studies, it was deduced that a decreasing ratio of BH4− to B(OH)4− leads to a 4686



Article

(i) AgClO4 stock solution (1 mM) was diluted to give 0.5, 0.45, 0.4 and 0.25 mM silver precursor solutions. (ii) AgClO4 stock solution (1 mM) was diluted 1:1 with 0.6, 1.2, and 3 mM sodium perchlorate solutions. NaClO4·H2O (