size-controlled synthesis of colloidal silver ... · size-controlled synthesis of colloidal silver...

Size-Controlled Synthesis of Colloidal Silver Nanoparticles Based onMechanistic UnderstandingMaria Wuithschick,† Benjamin Paul,‡ Ralf Bienert,§ Adnan Sarfraz,∥ Ulla Vainio,⊥ Michael Sztucki,▽

Ralph Kraehnert,‡ Peter Strasser,‡ Klaus Rademann,† Franziska Emmerling,§ and Jorg Polte*,†

†Department of Chemistry, Humboldt-Universitat zu Berlin, Brook-Taylor-Straße 2, 12489 Berlin, Germany‡Technische Chemie, Technische Universitat Berlin, Straße des 17 Juni 124, 10623 Berlin, Germany§BAM Federal Institute of Materials Research and Testing, Richard-Willstatter-Straße 11, 12489 Berlin, Germany∥Max-Planck-Institut fur Eisenforschung GmbH, Max-Planck-Straße 1, 40237 Dusseldorf, 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 uniqueproperties. Size control provides an effective key to an accurate adjustment of colloidalproperties. The common approach to size control is testing different sets ofparameters via trial and error. The actual particle growth mechanisms, and inparticular the influences of synthesis parameters on the growth process, remain a blackbox. 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 mechanisticknowledge. It is exemplified for a common silver nanoparticle synthesis, namely, thereduction of AgClO4 with NaBH4. Conducting this approach allowed a well-directedmodification of this synthesis that enables, for the first time, the size-controlledproduction of silver nanoparticles 4−8 nm in radius without addition of anystabilization agent.

KEYWORDS: silver nanoparticles, growth mechanism, SAXS, size control, sodium borohydride

■ INTRODUCTION

Metal nanoparticles are used for a wide range of applications,1

for example, in spectroscopy,2,3 biomedicine,4−6 and cataly-sis,7−10 which is the result of their unique catalytic, optical,electronic, and magnetic properties. These properties can beadjusted by altering the nanoparticle size, composition, crystalstructure, and morphology.11,12 Consequently, size controlprovides an effective key to an accurate adjustment of colloidalproperties.The common synthetic procedure to obtain colloidal metal

nanoparticles is the chemical reduction of a precursor salt witha reducing agent such as sodium citrate or sodiumborohydride.13 In general, the synthetic procedure itself isrelatively simple, whereas size control is often claimed butrarely achieved.14 The exception might be gold nanoparticles,but for silver, copper, and palladium, only a very few reliablesynthetic procedures rxost that deliver monodisperse colloids ina size range of 1−20 nm.15−18 Moreover, most syntheticprocedures require additional stabilization agents that canchange relevant properties, such as biocompatibility or catalyticactivity, making them inappropriate for further use.19−21 Themost common approach to size control is testing different setsof parameters via simple trial and error, which makes thesynthesis of nanoparticles “rather an art than a science”.22 The

number of scientific contributions that investigate actualnanoparticle growth mechanisms, and in particular theinfluences of parameters on growth, is still very limited.14,23−26

As a result, nanoparticle growth processes often remain a blackbox.16,17,27,28

A knowledge-based approach can be more effective toachieve size control. This contribution presents such anapproach to size control, which comprises three steps asdepicted in Scheme 1: (A) investigation of the growthmechanism in principle, including all relevant physicochemicalprocesses for one set of parameters; (B) investigation ofinfluences of synthesis parameters on the growth mechanism,which leads to identification of size-determining parameters;and (C) deliberate adjustment of the decisive reactionparameters 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 requiresmonitoring particle size distribution and concentration in situ

Received: June 6, 2013Revised: October 8, 2013Published: November 5, 2013

Article

pubs.acs.org/cm

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

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and time-resolved during the entire growth process. Suchexperimental information can be obtained by applying severallab-scale and synchrotron small-angle X-ray scattering (SAXS)setups.24,30−37

The knowledge-based approach is exemplified for thesynthesis of silver nanoparticles which was adapted from VanHyning and Zukoski.38 It comprises the wet chemical reductionof silver perchlorate (AgClO4) with sodium borohydride(NaBH4) without any additional stabilizing agents.

■ RESULTS AND DISCUSSIONStep A: Growth Mechanism in Principle. In our recent

paper,39 we were able to deduce the nanoparticle growthmechanism in principle of the herein-investigated silvernanoparticle synthesis. However, important details are stillmissing to gain a profound understanding of the growthprocess. Thus, completing step A of the presented approachconstitutes 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 silveratoms, which immediately form dimers, trimers etc.; (2)coalescence of these preliminary formed clusters, resulting inparticles 2−3 nm in radius; (3) an intermediate phase ofstability, during which the particle mean radius remainsconstant (referred to as metastable state); and (4) a secondcoalescence that leads to the final colloids.39 The particle meanradius ranges from 4 to 10 nm (at 20−30% polydispersity) andis poorly reproducible. In addition, the duration of themetastable state can vary between 5 and 20 min. The finalcolloidal solutions showed no changes after several days. Along-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 residualBH4

− to B(OH)4−.39 Experimental results indicate that particle

growth and chemical conversion of BH4− to B(OH)4

− occur inparallel (for details, see the supporting information of Polte etal.).39 Further investigations are required to correlate particlegrowth with associated physicochemical processes in thecolloidal solution. In particular, previous experimental resultswere insufficient to exclude with certainty any growth ofparticles during the metastable state. The limited data quality oflab-scale SAXS experiments complicates the detection of verysmall particles below 1 nm in diameter. In addition, lab-scaleSAXS experiments cannot provide detailed information on

evolution of the size distribution during the second coalescentstep (fourth step of the growth mechanism).

Detailed Investigation of Growth Processes duringMetastable State and Final Coalescence. Particle growth ofthe standard synthesis (refers to simple 1:1 mixing of 0.5 mMAgClO4 solution and 3 mM NaBH4 solution at ambientconditions) was investigated with time-resolved SAXS at theID02 beamline of the European Synchrotron Radiation Facility(ESRF) synchrotron storage ring. A free-liquid jet setup wasused for containerless measurement. The basic concept of thissetup is to extract a small flow of liquid sample continuouslyfrom the reaction vessel. The solution is pumped to a nozzle,resulting in a liquid jet at which the SAXS measurements areconducted. Thus, the synthesis can be performed in itsundisturbed conventional environment (stirred batch reactor).Furthermore, X-ray-induced effects are minimized and a hightime resolution at a low signal-to-noise ratio can be achievedwhen a synchrotron light source is used. A detailed descriptionof the setup can be found elsewhere.34 To avoid agglomerationof the silver nanoparticles inside the tubing, the distancebetween reaction vessel and X-ray beam was minimized andonly Teflon tubing and connectors were used. From eachscattering curve, the particle mean radius, polydispersity,relative volume fraction, and relative number of particles weredetermined. The size distribution is assumed to be a Schulz−Zimm distribution, which has been shown to be a suitableapproximation.39 Selected scattering curves and their corre-sponding 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 thewhole volume of all particles. Polydispersity and number ofparticles versus time are displayed in Figure 1b. The firstavailable scattering curve (t = 5 s) is already assigned to themetastable state. The size distribution of the colloidal solutionremains constant during the entire metastable state (t = 5−520s) with a particle mean radius of 1.5 nm and a polydispersity of40%. The volume fraction is approximately 100% and remainsalmost constant. These experimental findings reveal thatcolloidal stability during the metastable state is sufficient toprevent 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 radiusto 6.3 nm, accompanied by a successive decrease of thepolydispersity to 25% (see Figure 1a,b). The volume fraction ofthe final colloidal solution is the same as during the metastablestate, which confirms that particle growth is a process ofcoalescence. During the coalescent step, the volume fractiondecreases down to 65% (t = 530 s). This indicates that themathematical model used to fit the scattering curves cannotdescribe the colloidal solution accurately at that point.Obviously, the coalescing particles initially form irregularobjects that finally reorganize to spheres, whereas the modelassumes spherical morphology. On average, one final silvernanoparticle is formed by the coalescence of approximately 50smaller particles (see Figure 1b). Figure 1c illustrates the shiftof size distribution. The overlap of the size distribution beforeand after coalescence is low, which indicates that all particlesparticipate in the growth process.In conclusion, the high data quality of the synchrotron SAXS

investigations show that colloidal stability during the metastablestate is sufficient to prevent any particle aggregation and furthergrowth.. The chemical process of BH4

− conversion, which is

Scheme 1. Approach to Size Control Based on MechanisticKnowledge

Chemistry of Materials Article

dx.doi.org/10.1021/cm401851g | Chem. Mater. 2013, 25, 4679−46894680

assumed to cause the rapid loss of stability and initiates thesecond coalescence, was investigated nextChemical Conversion of BH4

− during Particle Growth.Two types of reaction can occur in an aqueous solution ofsodium borohydride: (i) BH4

− can act as a source ofnucleophilic hydride H−, which can reduce a variety of metalions Mz+;40 and (ii) the H− ligands can be replaced by watermolecules (hydrolysis). The reactions can be described by thefollowing simplified equations (for details, see section SI-2 inSupporting Information):

+ +

→ + + +

+ −

− +

z z

z z z

M BH 4H O

M B(OH) 3.5H H

z4 2

04 2 (i)

+ → +− −BH 4H O B(OH) 4H4 2 4 2 (ii)

Although NaBH4 is a moderate reducing agent compared toother metal hydrides, the reducing species H− is provided fastenough to reduce metal ions such as Au3+ or Ag+ withinmilliseconds.33 In comparison, hydrolysis is a much slowerprocess.41,42

According to eq i, 1 mol of Ag+ is reduced by the equivalentamount of BH4

−. To ensure complete reduction of the metalprecursor, the reducing agent is always used in excess (for thestandard synthesis, in 6-fold excess). Therefore, BH4

− remainsin the colloidal solution after the ionic silver is reduced. Theremaining BH4

− is converted during the nanoparticle growthprocess. Previous experiments indicated that the metastablestate ends at the point of total BH4

− consumption (see sectionSI-6 of our previous work).39 It was shown that the addition ofsmall amounts of HAuCl4 during the metastable state leads tothe reduction of ionic gold by residual BH4

−, which generatesgold nanoparticles besides the existent silver nanoparticles.After the second coalescence, HAuCl4 is no longer reducedwhen added to the colloidal silver solution. Another simpleexperiment supports the correlation between amount ofresidual BH4

− and duration of the metastable state. Addingfresh NaBH4 solution to the colloidal solution during themetastable state extends the duration of this phase (for details,see section SI-3 in Supporting Information).The kinetics of BH4

− consumption can be investigated bytime-resolved monitoring of H2 evolution from the reactionvessel, since BH4

− conversion is accompanied by the release ofhydrogen (see eqs i and ii). The released hydrogen can bedetected quantitatively by mass spectrometry. An accordingsetup is described in the Experimental Section and in sectionSI-4 in Supporting Information. Two samples were inves-tigated: (i) colloidal silver synthesis and (ii) a comparisonsample for which the NaBH4 solution is mixed with an equalvolume of water instead of silver precursor solution. Figure 2adepicts the volume flow of released hydrogen versus time of thecolloidal solution (red line) and the corresponding NaBH4solution (black line). Integration of the curves gives the totalvolume 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/minfor the colloidal solution and 0.017 mL/min for the NaBH4solution. The flow decreases with increasing reaction time andis zero after approximately 20 min for the colloidal solution andafter approximately 12 h (see section SI-4 in SupportingInformation) for the NaBH4 solution. The duration of themetastable state can vary between 5 and 20 min for repeatedexperiments. In this particular experiment, the secondcoalescent step of the colloidal system was observed afterapproximately 20 min; thus it coincides with the end of H2detection. For the NaBH4 solution, the total volume of detectedhydrogen is 1.4 mL, which is in agreement with calculations fora full BH4

− conversion according to reaction ii and the ideal gaslaw. The detected total volume of H2 for the colloidal solutionis only 0.7 mL, which is a result of the experimental setup: theprocedure of mixing the reactants and sealing the reactionvessel takes 5 s. Since reaction i is a very fast process, H2evolved from the reduction of Ag+ was not detected.The mass spectrometric experiments reveal that H2

evolution, and thus BH4− conversion, is highly accelerated in

the presence of silver nanoparticles (see Figure 2). This is notsurprising since catalytic activity of metal nanoparticles towardthe conversion of BH4

− was observed in a variety of previousstudies.43−45 Tetraborohydride is converted during the entire

Figure 1. Results of synchrotron SAXS investigations of standardcolloidal synthesis: (a) particle mean radius and normalized volumefraction (last data point set as 100%) vs reaction time; (b)polydispersity and relative number of particles (last data point set as1) vs reaction time; and (c) calculated particle size distributions forselected reaction times.



metastable state. The end of H2 release indicates the completeconversion of BH4

− and coincides with the second coalescentstep. Coalescent processes result from a decrease of colloidalstability. Therefore, the destabilization of the primary formednanoparticles is most likely associated with the progressingconversion of BH4

− to B(OH)4−. The chemical conversion

could influence the particle stability by the specific adsorptionof 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, theamount of ions [BH4

− and/or B(OH)4−] that could adsorb

at the silver particle surface changes gradually, whereas thestability of the colloidal silver solution decreases relativelyabruptly.It was shown for many silver nanoparticle systems that a

silver oxide layer is formed upon storage in ozone but also inaqueous solution at ambient conditions.46−50Therefore, thedecrease of colloidal stability could result from a suddenformation of a silver oxide layer at the nanoparticle surface.Residual BH4

− might continuously reverse any surfaceoxidation of the particles during the metastable state. Itscomplete consumption at the end of the metastable state couldinitiate a collective surface modification of all particles andconsequently a “simultaneous” decrease of colloidal stability ofall particles. As a result, the particles undergo further growthdue 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 themetastable state. The decrease of particle stability that initiatesthe second coalescent step correlates with full conversion of

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 growthmechanism was studied for one combination of synthesisparameters ([AgClO4] = 0.5 mM, [NaBH4] = 3 mM, 1:1mixing, stirring speed = 300 rpm, room temperature). In step B,influences of different reaction parameters on the growthmechanism, and thus on the final size distributions, areinvestigated. A strategy to achieve a reproducible synthesis isdeduced, which is the basic requirement to develop size-controllable nanoparticle syntheses.

Influence of Reactant Concentrations on Growth Mech-anism. Already in the 1990s, Glavee et al.51 pointed out thatthe reduction of metal ions with BH4

− is a complex interplay ofseveral chemical processes (reduction, hydrolysis, catalysis) thatis influenced by numerous parameters such as reactantconcentrations and pH value. Thus, it is not surprising thatthe final particle size of the investigated synthesis is verysensitive to concentrations of AgClO4 and NaBH4, temper-ature, ionic strength, and even parameters that are often notconsidered to influence the synthesis, such as the type ofreaction vessel, stirring speed, and mixing procedure (fordetails, see section SI-5 in Supporting Information). Therefore,the same type of reaction vessel was used for all lab-scaleexperiments, the stirring speed was kept constant (300 rpm),and the same mixing procedure (1:1 mixing of 2 × 5 mL) wasused. The temperature was kept constant at 23 °C (±1 °C). Inaddition, the reducing agent solution was always preparedfreshly and used within 1 min. Nevertheless, the exactpreparation of NaBH4 solution is difficult since NaBH4 ishygroscopic and easily absorbs moisture.52 In section SI-6(Supporting Information), it is demonstrated that the mass ofNaBH4 powder increases drastically (by up to 300%) if thesubstance is not stored under water-free conditions. Thus, theas-received NaBH4, powder was partitioned in small units andstored under argon. However, even if all these precautions areconsidered and samples are prepared simultaneously fromidentical reactant solutions, the final size distribution is stillpoorly reproducible. As a result, the final particle mean radiusobtained from the standard silver nanoparticle synthesis canvary between 4 and 10 nm. A low level of reproducibility is alsoapparent for other reactant concentrations, as illustrated by aparameter variation study shown in Supporting Information(section SI-7). The syntheses were carried out simultaneouslythree times with identical reactants. However, standarddeviations of the mean radii are relatively high (between 0.3and 1 nm). As a principal tendency, the average particle meanradius increases with increasing AgClO4 and decreasing NaBH4concentration.Nevertheless, the insufficient reproducibility of the final

particle size demands further elucidation of which steps of thegrowth mechanism are sensitive to synthesis parameters. It ispossible that small changes in the synthetic procedure (e.g.,mixing conditions) already influence the outcome of the firstcoalescent step considerably. Even small differences of the sizedistribution after the first coalescent step might affect thesecond coalescent step and therefore the final particle size.Therefore, it is necessary to extend the parameter study bymechanistic investigations. For five selected points of theparameter study (highlighted in section SI-7 in SupportingInformation), the particle growth process was investigatedtime-resolved with lab-scale SAXS.

Figure 2. Results of mass spectrometric investigations on hydrogenrelease during silver nanoparticle synthesis and corresponding pureNaBH4 solution: (a) volume flow of hydrogen vs reaction time and(b) total volume of hydrogen vs reaction time.



The results of the mathematical modeling are displayed inFigure 3 (note that the standard synthesis is displayed twice).Selected scattering curves and their corresponding theoreticalfits are shown in Supporting Information (section SI-8).The SAXS investigations reveal that differences between final

particle sizes of repeated syntheses and between syntheses withconcentration variations are caused by the second coalescentstep. Particle size after the first coalescent step (highlighted bylight gray bars) is about the same for all investigated systems(mean radius approximately 2 nm at 50% polydispersity). Afterthe second coalescent step, the mean radii vary considerably(highlighted by dark gray bars). For example, mean radii afterthe first coalescent step of the standard synthesis (diagram inthe middle panels) are nearly identical for all three experimentalruns, but the final mean radii deviate between approximately 7and 10 nm. As a consequence, reproducibility and particle sizecontrol can be achieved only by controlling the second step ofcoalescence.The second coalescent step is a very complex process. It

comprises aggregation of spherical nanoparticles with a broadsize distribution (polydispersity approximately 50%) that mustreorganize to give again spherical-shaped colloids. Theaggregation is caused by a loss of colloidal stability, whichcorrelates with the full conversion of BH4

−. The driving forcefor reorganization of the aggregated nanoparticles is a gain ofenergy (surface energy vs bulk energy).53 The process isstrongly affected by particle surface chemistry,54 such as thepresumed surface oxidation. Thus, both processesaggrega-tion and reorganizationand consequently the secondcoalescent step are dependent on the kinetics of the BH4

− →B(OH)4

− conversion and the associated oxidation of theparticle surface. The conversion rate of BH4

− depends on manyparameters, such as reactant concentrations, reaction temper-ature, stirring speed, and catalytic properties of the nano-particles formed after the first coalescent step.41,42 Therefore,these parameters have to be precisely controlled to adjust theparticle size distribution. However, absolutely accurate controlof these parameters under normal lab conditions is almost

impossible: (i) the hygroscopy of solid NaBH4 and its fastchange of chemical composition upon dissolving in waterimpede an exact adjustment of the reducing agent concen-tration; (ii) identical mixing and stirring conditions are hard toachieve; (iii) small temperature variations can hardly beeliminated; and (iv) the exact amount of dissolved oxygencan hardly be controlled.55

Instead of attempting to control BH4− consumption, a much

more practical approach to achieve a reproducible nanoparticlesynthesis could be elimination of the complex secondcoalescent step. For this purpose, the synthesis needs to bemodified so that the two separated coalescent steps merge toone single step, thus eliminating the metastable state. Theresults displayed in Figure 3 clearly show that the duration ofthe metastable state (highlighted by blue dotted lines)correlates with the amount of residual BH4

−. The durationdecreases with decreasing amount of residual BH4

−. From theresults of the mechanistic investigations, it can be expected thatthe metastable state will vanish if the NaBH4 concentration isreduced to the concentration of AgClO4. This can be achieved(i) by reducing the quantity of dissolved NaBH4 or (ii) by agingthe 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 reducedquantity of NaBH4 was examined for an AgClO4 concentrationof 0.5 mM. The concentration of NaBH4 was successivelyreduced from a ratio R = [BH4

−]/[Ag+] of 2 to 1. The finalcolloidal solutions were investigated with lab-scale SAXS andUV−vis spectroscopy. The results are shown in SupportingInformation (section SI-9). The duration of the metastablestate decreases to zero exactly for R = 1. However, the colloidalstability at this ratio is low, which leads to precipitation within 2min.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 ofnanoparticles proceeds within hours. In the following, the

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



influence of the NaBH4 aging process on the nanoparticlegrowth mechanism is investigated.NaBH4 Aging: Key to Reproducibility. The chemical

composition of the reducing agent solution changes withtime, due to the conversion of BH4

− to B(OH)4−. The

conversion starts immediately after NaBH4 is dissolved inwater. To investigate how the hydrolysis progress of thereducing agent solution (referred as aging) influences thenanoparticle growth process, a standard NaBH4 solution (3mM) was prepared and stored at ambient conditions (T = 23°C). For various times between 0 and 1020 min after dissolvingthe NaBH4 powder (referred as aging time ta), the solution wasused to prepare simultaneously three colloidal silver solutionsby reduction of a standard 0.5 mM AgClO4 solution. Sizedistributions of the final nanoparticless were determined withlab-scale SAXS. Selected scattering curves and correspondingfits are shown in Supporting Information (section SI-10).Results of the mathematical modeling are displayed in Figure 4.

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

the final size distribution is reproducible and the mean radiusincreases from 6 to 9.3 nm. Within a period of more than 90min (ta approximately 650−740 min), the mean radius evenremains constant at 9.3 nm. This period is referred to as sizeplateau. For ta > 800 min (phase III), the residual amount ofBH4

− is insufficient to reduce Ag+ completely, which isillustrated by a decreasing volume fraction (reflects the totalvolume of all silver nanoparticles). In phase III the particlemean 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 tais displayed in Figure 4b. During phase I, the duration of themetastable state decreases with ta. The color change, whichindicates the second coalescence, disappears with the beginningof phase II. Exemplarily, section SI-11 (Supporting Informa-tion) shows two time-resolved UV−vis investigations forcolloidal syntheses of phase I (ta = 90 min) and phase II (ta= 500 min). For ta = 90 min, a significant change of themaximum 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 observedby time-resolved SAXS by applying the free-liquid jet setup atthe ID02 beamline of the synchrotron light source ESRF.Exemplarily, Figure 5 depicts the size distribution versus time

for two colloidal syntheses during phase I (ta = 240 min) andphase II (ta = 660 min). Selected scattering curves andcorresponding fits are displayed in section SI-12 of SupportingInformation.For ta = 240 min, the particle mean radius at the first

available measuring point (5 s) is approximately 3 nm

Figure 4. Results of lab-scale SAXS investigations on the influence ofthe NaBH4 aging process on nanoparticle synthesis. (a) Final particlemean radius (polydispersity = 30% and for the last three aging times25%) and normalized volume fraction (first data point set as 100%) vsaging time. (b) Duration of the metastable state vs aging time. Thediagram can be divided into three parts: a phase with poorreproducibility of the final size distribution (I), a phase with goodreproducibility (II), and a phase of incomplete precursor reduction(III).

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



(polydispersity of 50%) and increases to approximately 5 nm(polydispersity of 30%) at a reaction time of around 150 s. Incontrast, for synthesis with the 660-min-old NaBH4 , thesecond coalescent step vanishes. The particle mean radiusremains constant at 8.5 nm (polydispersity of 35%) from thefirst measurement point (5 s). These experiments show that theparticle growth mechanism changes due to aging of thereducing agent solution. For ta > 400 min, only one fastcoalescent step is observed. Actually, this is a surprising resultsince the silver precursor is still reduced completely between400 and 800 min. This means the molar ratio between theNaBH4 solution used and Ag+ is at least equimolar, and mostlikely above 1, during phase II. In contrast, the results presentedin Figure 2 suggest that the second coalescence coincides with acomplete depletion of BH4

−. However, in this experiment noB(OH)4

− is present when the reactants are mixed. Therefore,the vanishing of the second coalescent step during phase IImight 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 twoseparated coalescent steps as expected. In addition, theobtained final size distribution remains almost constant withina size plateau. This period of time is ideal for conducting aparameter variation study that aims to identify size-determiningparameters.Parameter Variation within Size Plateau. It was shown that

reproducibility of the synthesis can be improved significantly byaging the NaBH4 solution. This observation can be used toconduct a reliable parametric study. An aged NaBH4 solution(ta within the size plateau) was used to reduce a standard 0.5mM AgClO4 solution. This parameter study includes variationsof AgClO4 concentration, ionic strength [Na+, ClO4

−,B(OH)4

−], temperature, and pH. Size distributions of thefinal colloids were investigated with lab-scale SAXS. Selectedscattering curves and their corresponding fits can be found inSupporting Information (section SI-13).Figure 6a displays the results of AgClO4 concentration

variation. For decreasing silver precursor concentration, thefinal 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 radiusincreases only slightly with increasing ionic strength. Figure 6ddisplays the results of temperature variation. The final particlemean radius decreases for decreasing temperature. For 0.5 and10 °C, a particle mean radius of 8.3 and 8.6 nm, respectively, isobtained. Figure 6e depicts the final mean radius obtained uponaddition of perchloric acid (HClO4). The mean radius remainsalmost constant even for the addition of 250 μL of acid (pH =3). Figure 6f illustrates the influence of additional B(OH)4

− onthe final particle size. Unfortunately, it is not possible todetermine the total concentration of B(OH)4

− present duringthe Ag+ reduction, since the exact chemical composition of thereducing agent (exact ratio [BH4

−]/[B(OH)4−]) is unknown.

However, the absolute amount of ionic silver reduced is 2.5μmol. The amount of additional B(OH)4

− is of the samemagnitude. Addition of NaB(OH)4 results in a significantdecrease of the particle mean radius. The mean radius decreasesto approximately 7.3 nm if the silver precursor solution isadjusted to a NaB(OH)4 concentration of 15 mM. Theseexperimental results show that temperature and ratio betweenthe concentrations of Ag+, BH4

−, and B(OH)4− have a major

influence on the final size distribution.

Step C: Size Control. From mechanistic investigations, itwas deduced that the second coalescent step and therefore thefinal particle size can hardly be controlled. It was shown thatthe reproducibility can be improved significantly by mergingthe two coalescent steps. This can be achieved by aging theNaBH4 solution (see Figures 4 and 5). In fact, aging NaBH4represents a decreasing ratio of BH4

− to B(OH)4−.

Furthermore, this decreasing ratio leads to an increasing finalparticle size (see phase II in Figure 4). As a result, NaBH4 agingenables a reproducible and size-controlled synthesis of silvercolloids.However, aging the NaBH4 solution is very laborious since

the reducing agent solution has to be prepared at least 5 h inadvance (begin of phase II; see Figure 4). Furthermore, it isvery difficult for a size control to obtain exactly the demandedaging and thus the demanded BH4

−/B(OH)4− ratio. Chemical

conversion of BH4− to B(OH)4

− can be faster or slower, forexample, due to small temperature variation during storage.

Imitation of NaBH4 Aging. The alternative is an imitation ofthe 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 longerstorage of BH4

− solution due to the hydrolysis of BH4−. The

following synthetic procedure imitates the NaBH4 agingdescribed in step B (see Figure 4): a 3 mM NaBH4 solutionis prepared and stored for at least 1 day. The obtained

Figure 6. Results of SAXS investigations on the influences of reactionparameters on final size distribution. For all syntheses, aged NaBH4solution (within plateau) was used. Final particle mean radius is shownvs (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 at30%. NaClO4 and HClO4 were added to the silver precursor solutionto give the displayed concentrations. The silver precursor solutionswere then mixed 1:1 with the aged reducing agent solution. Results ofstandard precursor solutions are highlighted (●).



B(OH)4− solution is mixed with freshly prepared 3 mM NaBH4

solution in different ratios and immediately used to reduce a 0.5mM AgClO4 solution (1:1 mixing). Figure 7 shows the results

of lab-scale SAXS investigations of final colloidal silver solutionsthat 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 andtheir corresponding fits can be found in SupportingInformation (section SI-14).For a mixture of 35% BH4

− and 65% B(OH)4−, two

separated coalescent steps are observed during the colloidalsynthesis, whereas a mixture that contains only 32.5% BH4

−

leads to a nanoparticle growth mechanism that comprises onlyone single coalescent step. Thus, this mixture corresponds tothe beginning of phase II of the aging experiment (see Figure4). In accordance with the aging experiment, a decreasingpercentage of BH4

− leads to an increasing final mean radius.The mean radius increases from 4 to 8 nm, whereas the meanradius of the corresponding aging experiment is slightly bigger.Nevertheless, this procedure enables a very simple andreproducible access to colloidal silver nanoparticles withaccurate size control between 4 and 8 nm in radius(polydispersity = 30%). To the best of our knowledge, this isthe first size-controlled synthesis of colloidal silver nano-particles that does not require an additional stabilization agent.

■ CONCLUSIONSThis paper presents an approach to size control based onmechanistic knowledge that was exemplified for a commonsilver nanoparticle synthesis (reduction of AgClO4 with anexcess of NaBH4 in aqueous solution). It comprises anunderstanding of the nanoparticle growth mechanism and theinfluences of synthesis parameters on the growth. The growthmechanism consists of four steps and includes two separatedsteps of coalescence. It is shown that the final growth step (thesecond coalescent step) correlates with the conversion ofresidual BH4

− to B(OH)4−. The depletion of BH4

− could causea surface oxidation of the preliminary nanoparticles formed inthe first coalescent step. This would lead to a decrease ofcolloidal stability, which initiates the further growth due tocoalescence. The second coalescence is a complex process thatcan hardly be controlled. As a consequence, the final particlesize is not reproducible. From the mechanistic studies, it wasdeduced that a decreasing ratio of BH4

− to B(OH)4− leads to a

merging of the two coalescent steps. This means that thegrowth mechanism changes from a four-step to a two-stepmechanism similar to the corresponding gold nanoparticlesynthesis.33 The BH4

−/B(OH)4− ratio can be precisely adjusted

by well-defined aging of the reducing agent solution or a simpleimitation of this aging process. Although the exact role of theBH4

−/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 well-

directed modification of the synthesis that allows reproducibleand size-controlled production of silver nanoparticles in therange of 4−8 nm in radius (polydispersity = 30%). Thisrepresents substantial progress for the synthesis of metalcolloids, since syntheses of silver nanoparticles with size controlare rare, especially in aqueous solution and without addition ofstabilizing agents.13,56−60 Our work proves that mechanisticstudies are not only of academic interest but can be the key toimprove the current state of nanoparticle syntheses.

■ EXPERIMENTAL SECTIONColloidal Syntheses. In this paper, the standard procedure for

synthesis of colloidal silver nanoparticles in water refers to 1:1 mixingof 0.5 mM AgClO4·H2O and 3 mM NaBH4 solution. The reactantsolutions 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 reducingagent solution was prepared freshly and used within 1 min. The as-received NaBH4 was divided into small portions under inert gas. Anew portion was used for each experiment. The colloidal synthesis wascarried out at ambient conditions (23 °C ± 1 °C). The stirring speedwas 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 wasscaled up to give a total volume of 400 mL of colloidal solution. Thereactant solutions were filled into two glass flasks with a self-manufactured outlet at the bottom and mixed 1:1 within 3 s by a hand-operated pump. A beaker was used as reaction vessel. After use, allglassware was cleaned with concentrated nitric acid and rinsed withgenerous amounts of ultrapure water.

For reactant concentration studies, 1 mM AgClO4 stock solutionwas prepared by dissolving 225.33 mg of AgClO4·H2O in 1 L of waterand diluted to give 0.25, 0.4. 0.5, 0.6, and 0.75 mM precursor solutions.NaBH4 solutions were prepared freshly by dissolving appropriateamounts 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, 0.5mM AgClO4 solution was reduced. NaBH4 solutions were preparedfreshly 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 (0.6mM, 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 mMNaBH4 solution was prepared freshly and stored at ambient conditions(open to atmosphere). After certain aging times, three colloidalsolutions were prepared (standard lab-scale synthetic procedure with atotal volume of 10 mL).

The parameter study within the size plateau (see Figure 4) wascarried out by use of 3 mM NaBH4 solution that was stored 10 h atambient conditions as reducing agent. The silver precursor solutionswere adjusted for different variations:

Figure 7. Results of silver nanoparticle syntheses using a mixture ofaged 3 mM and fresh 3 mM NaBH4 solution as reducing agent. Finalmean radius (polydispersity = 30%) vs percentage of fresh NaBH4 isplotted. The x-axis is inverted since a decreasing percentage of freshNaBH4 imitates an increasing aging time.



(i) AgClO4 stock solution (1 mM) was diluted to give 0.5, 0.45, 0.4and 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 (<98%) waspurchased from Sigma−Aldrich.(iii) Perchloric acid (Sigma−Aldrich, 20%, p.a.) was diluted to

adjust a pH value of 3.0. To samples of 0.5 mM AgClO4 solution (25mL each) were added 50, 100, and 250 μL of HClO4.(iv) AgClO4 stock solution (1 mM) was diluted 1:1 with 0.6, 1.2, 3,

and 30 mM solutions of sodium tetrahydroxyborate. The NaB(OH)4solutions were obtained by diluting 30 mM stock solution. To preparethe stock solution, 1.13 g of NaBH4 were dissolved in 1 L of ultrapurewater and stored at ambient conditions for 1 week. During this time,the borohydride species converts to tetrahydroxyborate. Thecompleteness of this conversion can be proved by adding AgClO4.The solution should remain colorless (no formation of silvernanoparticles), indicating the absence of any BH4

−.(v) For temperature variation, 0.5 mM AgClO4 solution and a

sample of the aged reducing agent solution were cooled to 0.5 and 10°C, respectively. The syntheses were carried out in temperature-controlled water baths.For the aging imitation experiment (see Figure 5), 113.5 mg of

NaBH4 granulate (Sigma−Aldrich, 98%) was dissolved in 1 L of Milli-Q water (3 mM solution). The solution was stored at ambientconditions for 1 day. During this time, the borohydride speciesconverts to tetrahydroxyborate. The completeness of this conversioncan be proved by adding AgClO4 to a sample of the B(OH)4

− solution.The solution should remain colorless (no reduction and thus noformation of silver nanoparticles), indicating the absence of any BH4

−.Portions (85, 82.5, 80, 77.5, 75, 72.5, 70, 67.5, and 65 mL) of theobtained B(OH)4

− solution were filled up to 100 mL with freshlyprepared 3 mM NaBH4 solution. The obtained solutions were used toreduce 0.5 mM AgClO4 solution (standard lab-scale syntheticprocedure with a total volume of 10 mL).Note: The concentrations displayed in the diagrams always refer to

the solutions that are reacted 1:1 with the corresponding reactantsolution.In Situ Small-Angle X-ray Scattering Investigations. Synchro-

tron SAXS Investigations. Synchrotron SAXS investigations wereperformed at the ID02 beamline (ESRF) with a free-liquid jet setup.34

The distance between reaction vessel and jet was minimized to achievea low dead time (approximately 5 s) and to avoid agglomeration ofparticles inside the tubing. The technique offers the possibility tofollow nanoparticle growth in situ with a time resolution that is limitedjust by the photon flux and the acquisition time of the detector. Inaddition, X-ray-induced effects are minimized and contaminationproblems (contamination of capillary walls) are eliminated.Lab-Scale SAXS Investigations of Final Colloidal Solutions.

Scattering curves of the final colloidal solutions were recorded byextracting the samples from the batch solution and inserting them in aflow cell of a SAXS instrument (SAXSess, Anton Paar GmbH).Time-Resolved Lab-Scale SAXS Investigations.Mechanistic studies

at lab scale were performed by use of a SAXS instrument (SAXSess,Anton Paar GmbH). The colloidal solution was pumped via Teflontubing into a flow cell that was cooled to 5 °C to suppress particleagglomeration inside the quartz cell.Small-Angle X-ray Scattering Evaluation. Scattering curves of

the colloidal solution were analyzed with the assumptions of sphericalshape, homogeneous electron density, and a Schulz−Zimm sizedistribution. The Schulz−Zimm distribution is given by

= +− +Γ +

+f r z xz x

R z( ) ( 1)

exp[ ( 1) ]( 1)

z 1 2

avg (1)

where Ravg is the mean radius, x = (r/Ravg), z is related to thepolydispersity p (p = σ/Ravg) by z = (1/p2 − 1), and σ2 is the varianceof the distribution. The scattering intensity of nonaggregated particlescan be assumed to be proportional to the form factor of a singleparticle P(q). Thus, the scattering intensity of N monodisperse sphereswith homogeneous electron density with volume Vpart is given by

ρ

= =

= Δ−⎧⎨⎩

⎫⎬⎭

I q NI q NV P q

NVqR qR qR

qR

( ) ( ) ( )

3[sin( ) cos( )]

part part2

part2

2

(2)

In the case of polydisperse spherical particles, one has to sum thescattering intensities over all particle sizes weighted by their frequencyor integrate by use of a size distribution function. It is common to usethe Schulz−Zimm distribution for polydisperse particles. Hence, thescattering intensity is given by

∫=∞

I q N f r V P q r( ) ( ) ( ) d0

part2

(3)

An analytical solution of the integral can be found in Kotlarchyk etal.61 In order to analyze the nucleation and growth mechanism ofnanoparticles, the number of particles is important. This informationcan be obtained from the general relation of I(q = 0) for a singleparticle, which is independent of its shape and size, that is, I =(Δρ)2V2. Thus the scattered intensity I(q = 0) of polydisperse particlescan be written as

ρ= = ⟨ ⟩ ΔI q N V( 0) ( )2 2 (4)

where N is the number of particles and ⟨V2⟩ is the mean value of V2.Due to the overlapping of scattering intensity with the primary beam,I(q = 0) cannot be measured directly, but it is accessible viaextrapolation of I(q) for q → 0.

Hydrogen Monitoring. Mass spectrometry was used to monitorthe hydrogen release from the reaction vessel. A scheme of the setupcan be found in section SI-4a in Supporting Information. The reactantswere mixed in the reaction vessel. Immediately, the vessel was sealedwith a septum and flushed by compressed air. The flow rate wasadjusted to Vin = 10.46 mL/min by use of a mass flow controller, F-201D-FAC-33-P (Bronkhorst Mattig GmbH). A mass spectrometer,OmniStarGSD301C (Pfeiffer Vacuum, Asslar, Germany), was attachedto the reaction vessel. The spectrometer was programmed for multi-ion monitoring in order to record the relative ion intensities ofhydrogen (m/z = 2), nitrogen (m/z = 28), and oxygen (m/z = 32)with a time resolution of 1.7 s. The volume flow of the releasedhydrogen VH2

can be calculated from the percentage of hydrogen PH2:

=

−V

P V

P1HH in

H2

2

2 (5)

UV−Visible Spectroscopy. UV−vis spectra of the colloidalsolutions were recorded on an AvaSpec-2048TEC-2 equipped with adeuterium halogen light source (Avantes, Broomfield, CO), connectedto a 10 mm optical path length cuvette holder via fiber optic cables.The investigations were carried out in a standard 1 mL UV cuvette.Colloidal solutions (200 μL) were extracted and mixed with 300 μL ofpoly(vinylpyrrolidone) solution (MW = 40 000, 6 mg dissolved in 50mL of water) to avoid agglomeration/aggregation of the nanoparticlesinside the cuvette. The time delay between extraction and actualmeasurement was below 5 s.

■ ASSOCIATED CONTENT*S Supporting InformationAdditional text and equations; 14 figures showing selectedscattering curves with linear rather than logarithmic y-scale,selected scattering curves for different reaction times, UV−visspectra showing extended duration of metastable state byaddition of BH4

−, schematic setup for time-resolved hydrogenmonitoring via MS spectrometry and total volume of releasedH2 vs time, selected scattering curves for different mixingconditions, mass vs time for relative humidities of 40% and55%, SAXS data from variation of AgClO4 and NaBH4concentrations, selected scattering curves from time-resolvedinvestigations, UV−vis spectra of selected colloidal solutions,influence of NaBH4 aging time on final size distribution, time-



resolved UV−vis investigations using aged reducing agentsolution, influence of NaBH4 aging time on particle growthmechanism, parameter variation study within the plateau uponvariation of the reducing agent solution (mixing fresh and agedNaBH4), and SAXS data from variation of the reducing agentsolution (mixing fresh and aged NaBH4); one scheme withaverage particle mean radii, ratio of reactant concentrations,and average duration of metastable state for final colloidalsolutions; and two tables listing influences of mixing conditionsand stirring speed on final size distribution and SAXSinvestigations of final colloidal solutions synthesized from 0.5mM AgClO4 solution (PDF). This material is available free ofcharge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail [email protected].

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSJ.P. acknowledges generous funding by the DeutscheForschungsgemeinschaft within Project PO 1744/1-1. M.W.also acknowledges financial support by the Fonds derChemischen Industrie. We acknowledge the EuropeanSynchrotron Radiation Facility for provision of synchrotronradiation facilities, and we thank Dr. T. Narayanan forassistance in using beamline ID02.

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