Synthesis of Monodisperse, Quasi-Spherical Silver Nanoparticles with

Feb 17, 2014 - Mujeeb Khan , Mohammed Rafi Shaik , Syed Farooq Adil , Shams Tabrez Khan , Abdulrahman Al-Warthan , Mohammed Rafiq H. Siddiqui ...
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Synthesis of Monodisperse, Quasi-Spherical Silver Nanoparticles with Sizes Defined by the Nature of Silver Precursors Houshen Li,† Haibing Xia,*,† Wenchao Ding,† Yijing Li,† Qiurong Shi,† Dayang Wang,‡ and Xutang Tao† †

State Key Laboratory of Crystal Materials, Shandong University, Jinan, 250100, PR China Ian Wark Research Institute, University of South Australia, Adelaide, SA 5095, Australia

‡

S Supporting Information *

ABSTRACT: Monodisperse, quasi-spherical silver nanoparticles (Ag NPs) with controlled sizes have been produced directly in water via adding the aqueous solutions of the mixtures of AgNO3 and sodium citrate to boiling aqueous solutions of ascorbic acid (AA). Different compounds, including NaCl, NaBr, KI, Na2SO4, Na2CO3, Na2S, and Na3PO4, are added to the AgNO3/citrate mixture solutions to form new silver compounds with fairly low solubility in water, which are used as precursors instead of soluble Ag+ ions to synthesize Ag NPs via AA/citrate reduction. This enables us not only to produce monodisperse, quasi-spherical Ag NPs but also to tune the sizes of the resulting NPs from 16 to 30 nm according to the potential of new silver precursors as well as the concentrations of anions.

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investigation of the size-dependent properties of Ag NPs.20−22 We recently demonstrated that monodisperse, quasi-spherical Ag NPs may be produced directly in water via the reduction of AgNO3 with both sodium citrate and ascorbic acid (AA), especially when I− ions are used to tailor the growth of Ag NPs into a quasi-spherical shape via their preferential adsorption on the NP {111} facets.23 Steinigeweg and Schulcker recently used more reactive silver precursorsdiamine silver complexes instead of AgNO3to produce Ag NPs via AA reduction and successfully produced large, monodisperse, quasi-spherical Ag NPs directly in water.24 Encouraged by these results, herein the present work was planned to interrogate how the reactivity of silver compounds formed upon addition of different anions including Cl−, Br−, I−, SO42−, CO32−, PO43−, and S2− ions affects the sizes of Ag NPs obtained via citrate/AA reduction. Cl−, Br−, I−, SO42−, CO32−, PO43−, and S2− ions are well known to form silver compounds with rather low solubility in water. After carefully adjusting the concentration of these anions on the basis of the calculation of the solubility product constants (Ksp) of the corresponding insoluble silver compounds, we can synthesize monodisperse, quasi-spherical Ag NPs with sizes finely tuned from 16 to 30 nm, depending on the nature of additional anions under optimal conditions. In the case of using SO42−, CO32−, and PO43− as well as Cl− ions, which can form silver compounds but have a weak adsorption affinity for Ag NP surfaces, the sizes of as-prepared Ag NPs can be tuned from 23 to 30 nm with 2 nm precision by the redox potential of the

INTRODUCTION Silver nanoparticles (Ag NPs) are expected to exhibit peculiar size-dependent physicochemical properties,1 surface plasmon resonance (SPR) in particular,2−4 which is of paramount importance in a number of technical applications such as surface-enhanced Raman scattering (SERS),5−9 single-molecule labeling and recognition,10 and antimicrobial coatings.11 Among the myriad synthesis methods developed so far, the reduction of Ag+ ions directly in water with a reducing agent such as NaBH4,12 ascorbic acid,13 or citrate14 is the most easily accessible technique for producing Ag NPs with controlled sizes and relatively round shapes. However, this aqueous synthesis strategy usually leads to Ag NPs with a fairly broad size distribution and especially to different shapes varying from quasi-spheres to rods and to prisms. For instance, citrate is extensively used as a reducing agent for the synthesis of noble metal NPs. It is well known for its simple and safe operation, its negligible toxicity, and its ease of ligand exchange. Great success in producing monodisperse gold NPs with an excellent quasispherical shape has been demonstrated.15,16 However, citrate reduction usually results in Ag NPs with fairly broad size and shape distributions.14,17 Although monodisperse Ag NPs can be prepared in an organic solvent,18 the organic ligand surface coating has largely limited their technical applicability in biomedicine. To date, no reliable techniques are available to produce Ag NPs directly with monodisperse sizes and truly quasi-spherical shapes in water. Consequently, all of the Ag NPs used in the current studies have a broad size distribution; the toxicity investigations of Ag NPs are not accurate.19 Thus, it is imperative to solve the critical challenge of preparing monodisperse Ag NPs in water to achieve a quantitative © 2014 American Chemical Society

Received: December 9, 2013 Revised: January 17, 2014 Published: February 17, 2014 2498

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Table 1. Summary of the Ksp Values of Different Silver Compounds at 25 °C and at 100 °C in Water, the Maximum Concentrations of Anions Calculated to Form the Precipitates of the Corresponding Silver Compounds in Water at a AgNO3 Concentration of 0.297 mM at 100 °C, and the Optimal Concentrations of the Anions Used in Aqueous Solutions of AgNO3/ Citrate Mixtures for the Formation of Monodisperse, Quasi-Spherical Ag NPs silver compounds AgCl AgBr AgI Ag2S Ag2SO4 Ag2CO3 Ag3PO4

Ksp values at 25 °C 1.77 5.35 8.52 6.3 1.2 8.46 8.89

× × × × × × ×

10−10 10−13 10−17 10−50 10−5 10−12 10−17

Ksp values at 100 °C 3.57 5.05 7.67 3.62 5.09 2.16 1.49

× × × × × × ×

maximum concentrations of anions (μM)

optimal concentrations of anions used (μM)

91.8 1.7 2.58 × 10−3 4.1 × 10−27 2.33 × 104 286 202

80 80 0.06 0.15 60 0.040 0.20

10−8 10−10 10−13 10−40 10−5 10−10 10−13

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RESULTS AND DISCUSSION In most studies, reducing agents with different reducing abilities were chosen to adjust the nucleation rate during NP synthesis. For instance, in our previous work, the stronger AA in boiling water rapidly consumes a large number of Ag+ ions and leads to fast nucleation. Thus, monodisperse, quasi-spherical Ag NPs with a size of about 31 nm were obtained via AA/citrate reduction. However, it is difficult to finely adjust the nucleation rate by just selecting the reducing agents. In the present work, the potential of silver precursors was adjusted by adding different anions (such as Cl−, Br−, I−, SO42−, CO32−, PO43−, and S2− ions) because they can strongly associate with Ag+ ions to form new silver precursors in the reaction media. Thus, both the concentration of new silver precursors and their redox potential are expected to affect the nucleation rate during Ag NP growth and in turn alter the NP size provided the concentrations of AA and silver ions are kept the same. According to their preferential absorption on the facets of Ag NPs, these anions were grouped for investigation. Synthesis of Ag NPs via AA/Citrate Reduction in the Presence of Cl−, Br−, or I− Ions. It is known that Cl− and I− ions tend to adsorb preferentially on the {111} facets of Ag NPs.25 These two halide anions have been harnessed to control the growth of Ag NPs in a quasi-spherical shape during AA/ citrate reduction. The addition of these anions (such as Cl−, Br−, I−, SO42−, CO32−, PO43−, and S2− ions) to the aqueous solution of Ag+ ions is known to yield precipitates with a fairly low solubility in water, which may act as heterogeneous nuclei during the growth of Ag NPs via AA/citrate reduction. According to the literature,26−28 here we calculated the solubility product constants (Ksp) of different silver compounds and the maximum concentrations of the corresponding anions in water at the AgNO3 concentration used in the current work (Table 1). All of the data for the following calculations were abstracted from refs 26−28. In general, the enthalpy of the reaction and Ksp are calculated as follows

corresponding silver compounds when the anion concentration is adjusted to be sufficiently low to ensure no formation of insoluble compounds. In the case of Br−, I−, and S2− ions, which strongly adsorb on the surfaces of Ag NPs or their special crystallographic facets, their concentrations have to be adjusted to be relatively high to form insoluble silver compounds as sufficient heterogeneous nuclei in order to offset the anisotropic crystal growth due to preferential facet adsorption and thus form monodisperse, quasi-spherical NPs. This heterogeneousnucleation-based process leads to smaller Ag NPs in the size range of 16−23 nm.

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EXPERIMENTAL SECTION

Materials. Silver nitrate (AgNO3 99+%) was purchased from Alfa Aesar (Tianjin, China). Trisodium citrate dihydrate (Na3C6H5O7), potassium iodide (KI), sodium chloride (NaCl), sodium bromide (NaBr), sodium sulfide nonahydrate (Na2S·9H2O), anhydrous sodium carbonate (Na2CO3), anhydrous sodium sulfate (Na2SO4), trisodium phosphate dodecahydrate (Na3PO4·12H2O), and ascorbic acid (AA) were purchased from Sinopharm Chemical Reagent Co. Ltd. All chemicals were used as received. All glassware was cleaned with aqua regia (3:1 v/v HCl (37%)/HNO3 (65%)) and then rinsed thoroughly with H2O before use. (Caution! Aqua regia solutions are dangerous and should be used with extreme care; never store these solutions in closed containers.) The water in all experiments was Milli-Q water (18 MΩ cm, Millipore). Synthesis of Ag NPs via the AA/Citrate Reduction Protocol in the Presence of Different Anions. An aqueous solution of AA (50 μL, 0.10 M) was added to 47.5 mL of boiling water, followed by further boiling for 1 min. An aqueous solution of sodium citrate ( 1 mL, 1 wt %), an aqueous solution of AgNO3 (0.25 mL, 1 wt %), and a given number of anions were consecutively added to water with stirring at room temperature. In our work, the concentrations of both AA and silver ions were kept the same and the concentrations of additional anions were optimized to ensure the formation of monodisperse, quasi-spherical Ag NPs (listed in Table 1). The total volume of each solution was adjusted to 2.5 mL. After 5 min of incubation at room temperature, the solutions were injected into the aforementioned boiling aqueous solutions of AA. The color of the reaction solutions quickly changed from colorless to yellow. The transparent and yellow reaction solutions were boiled for another hour with stirring to warrant the formation of uniform, quasi-spherical Ag NPs in all of the experiments. Characterization. The absorption spectra were recorded via UV− vis spectroscopy with a Cary 50 spectrophotometer using 10 mm path length quartz cuvettes. Transmission electron microscopy (TEM) images were obtained with a JEOL JEM-2100F transmission electron microscope operating at an acceleration voltage of 200 kV. Zeta potential measurements were conducted using a Malvern Zetasizer Nano ZS equipped with a 50 mW 633 nm laser. All measurements were performed in quintuplicate.

ΔH ⊖ =

ln

K sp , T2 K sp , T1

⊖ ⊖ − ∑ ΔHreactant ∑ ΔHresultant

=

ΔH ⊖ ⎛ 1 1⎞ ⎜ − ⎟ R ⎝ T1 T2 ⎠

(1)

(2)

where Ksp is the solubility product, T is the temperature, R is the gas constant, and ΔH⊖ is the enthalpy of the reaction, which varies little with temperature. In our case, ΔH⊖ is used as the constant (Table 2) for calculation simplicity (ΔH⊖ ≈ ⊖ ΔH⊖ 298 K). For example, the ΔH and Ksp values of the AgBr compound at 25 °C is 84.4 kJ mol−1 and 5.35 × 10−13, 2499

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Table 2. Standard Enthalpy (ΔH⊖/kJ mol−1) of Ag+ Ions, Anions, and the Silver Compound and the Calculated Enthalpy of the Reaction ΔH⊖ (kJ mol−1) +

reaction

Ag ions

anions

silver compound

reaction

Ag2SO4 = 2Ag+ + SO42− Ag2CO3 = 2Ag+ + CO32− AgCl = Ag+ + Cl− AgBr = Ag+ + Br− AgI = Ag+ + I− Ag2S = 2Ag+ + S2−

105.6 105.6 105.6 105.6 105.6 105.6

−909.3 −677.1 −167.2 −121.6 −55.2 33.1

−715.9 −505.8 −127.0 −100.4 −61.8 −32.6

17.8 39.9 65.4 84.4 112.2 276.9

Figure 1. TEM images of Ag NPs synthesized by the AA/citrate reduction protocol in the presence of NaBr. The NaBr concentrations are (a) 20 and (b) 80 μM. The concentrations of citrate, AgNO3, and AA are 0.68, 0.297, and 0.10 mM, respectively.

respectively. Herein, the Ksp value of AgBr at 100 °C is calculated to be 5.05 × 10−10 accordingly. The same calculation procedure is applied to obtain the Ksp values of other silver compounds (Table 1) except for Ag3PO4. So far, there is no available data for the standard enthalpy (ΔH⊖/ kJ mol−1) of the Ag3PO4 compound. Thus, the corresponding standard Gibbs free energy (ΔG⊖/kJ mol−1) of Ag+ and PO43− ions and the Ag3PO4 compound were used to obtain the Ksp value (Table S1 and S2). ⊖ ⊖ ⊖ ΔG⊖ = 3ΔG Ag − ΔG Ag + + ΔG PO4 PO 3 − 4

3

concentration calculated from the solubility of AgBr. Ag NPs obtained in the presence of Br− ions are 16 ± 2 nm in size, which are smaller than that obtained in the presence of Cl− or I− ions. Note that the concentration of AA in the range of 0.10−0.20 mM and that of citrate in the range of 0.58−0.85 mM were necessary to the formation of monodisperse, quasispherical Ag NPs via AA/citrate reduction in the presence of halide anions (Figures S3 and S4). Synthesis of Ag NPs via AA/Citrate Reduction in the Presence of SO42−, CO32−, or PO43− Ions. It has been reported that these anions (SO42−, CO32−, and PO43− ions) were expected to adsorb weakly on the facets of Ag NPs with no adsorption preference for one specific type of crystalline facet over others because all of them are absorbed on Ag NPs facets by oxygen species.29−31 Thus, changes in the morphology or size of Ag NPs should not be related to the preferential absorption of these anions (SO42−, CO32−, and PO43− ions) on the facets of Ag NPs. Figure 2 shows that monodisperse, quasi-spherical Ag NPs can be obtained via AA/citrate reduction in the presence of Na 2 SO 4 , Na 2 CO 3 , or Na3 PO 4 under optimal reaction conditions. The presence of SO42− ions seems to have no effect on the growth of Ag NPs. The size of Ag NPs obtained in the presence of SO42− ions is 30 ± 2 nm (Figures 2a and S5), which is very close to that of Ag NPs obtained by the AA/ citrate reduction of AgNO3 (31 ± 2 nm).23 The sizes of Ag NPs obtained in the presence of CO32− and PO43− ions are 27 ± 2 and 25 ± 2 nm, respectively (Figures 2b,c, S6, and S7). This noticeable size decrease suggests that the presence of CO32− and PO43− ions affects the nucleation or the growth process or both during Ag NP formation. The considerably improved size uniformity and shape sphericity can also be evidenced by the UV−vis spectra of the corresponding Ag NPs obtained in the presence of SO42−, CO32−, or PO43− ions (Figure 2d). All of these NPs exhibit fairly narrow and symmetric SPR bands. Their SPR absorption peaks remain a little shifted, being centered at 403, 401, and 400 nm for the NPs obtained in the presence of SO42−, CO32−, and PO43− ions, respectively. The fwhm of Ag NPs obtained in the presence of CO32− and PO43− ions is about 48 nm whereas that in the presence of SO42− ions is about 60 nm. Moreover, for the Ag NPs obtained in the presence of SO42− ions, the SPR absorption at a wavelength above 450 nm is rather obvious (Figure 2d). It is clearly visible that a small number of Ag NPs with other shapes such as spheroidal, short rodlike, and irregular shapes in the resulting Ag NPs are obtained, especially

(3)

ΔG⊖ (4) RT where Ksp is the solubility product, T is the temperature, and R is the gas constant. ΔG⊖ is the Gibbs free energy of the reaction. In our case, ΔG⊖ is used as the constant for ⊖ calculation simplicity (ΔG⊖ ≈ ΔG⊖ 298 K). ΔG and the Ksp value −1 of Ag3PO4 at 25 °C are 91.6 kJ mol and 8.89 × 10−17, respectively. Herein, the Ksp value of the Ag3PO4 compound at 100 °C is calculated to be 1.49 × 10−13 accordingly. The maximum concentration of anions for the formation of the precipitates of the corresponding silver compounds in AgNO3 solution at 100 °C (Supporting Information) was also calculated according to the calculated Ksp values of different silver compounds at 100 °C and is listed in Table 1. As listed in Table 1, the optimal concentrations of Cl− and I− ions are close to and noticeably larger than the maximum concentrations calculated from the solubility of the corresponding silver halide, respectively, so that monodisperse, quasispherical Ag NPs could be reproducibly produced via AA/ citrate reduction. This suggests that the heterogeneous nucleation associated with the formation of silver halide precipitates ought to play a nonnegligible role in the growth of Ag NPs. The Ag NPs obtained in the presence of Cl− and I− ions, respectively, were similar in size (23 ± 2 nm), and the size difference between them was negligible (Figure S1). Distinct from Cl− and I− ions, Br− ions are known to adsorb preferentially onto the {100} rather than {111} facets of Ag NPs.25 When Br− ions were introduced into the aqueous solution of AgNO3/citrate mixtures for AA/citrate reduction, a relatively large number of Ag NPs were obtained in a rodlike shape (Figure 1a). The aspect ratio of rodlike Ag NPs is in the range of 1.2 to 2.3 (about 15% in all of the products). In the current work, as shown in Figure 1b and Figure S2, we can produce monodisperse, quasi-spherical Ag NPs with a negligible number of nonspherical NPs in the presence of Br− ions when the concentration of Br− ions is increased to 80 μM, which is considerably higher than the maximum ln K sp = −

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Figure 2. (a−c) TEM images and (d) UV−vis spectra of the corresponding Ag NPs synthesized via an AA/citrate reduction protocol in the presence of different anions: (a, black curve) Na2SO4, (b, red curve) Na2CO3, and (c, blue curve) Na3PO4. The concentrations of Na2SO4, Na2CO3, and Na3PO4 are 60, 0.040, and 0.20 μM, respectively. The concentrations of citrate, AgNO3, and AA are 0.68, 0.297, and 0.10 mM, respectively. (d) The absorption spectra of Ag NPs obtained via citrate reduction of AgNO3 (dotted black curve) and AA/citrate reduction of AgNO3 (dashed black curve) are also shown for better comparison.

Figure 3. TEM images of the Ag NPs synthesized via AA/citrate reduction in the presence of Na3PO4. The AA concentrations are (a) 0.040, (b) 0.20, (c) 0.30, and (d) 0.40 mM. The concentrations of citrate, Na3PO4, and AgNO3 in the mixture are 0.68 mM, 0.20 μM, and 0.297 mM, respectively.

in the presence of SO42− or CO32− ions (Figure 2a,b). The presence of a small but nonnegligible number of nonspherical Ag NPs was also observed when the Ag NPs were directly synthesized via the AA/citrate reduction of AgNO3 in the absence of any anions. Thus, the present results suggest that the SO42−, CO32−, or PO43− ions indeed have no shaping effect on the growth of NPs during the formation of Ag NPs. The concentrations of SO42−, CO32−, and PO43− ions present in the aqueous solutions of AgNO3/citrate mixtures were adjusted to below the maximum concentration in order to avoid the formation of the precipitates of Ag2SO4, Ag2CO3, or Ag3PO4 and thus minimize the heterogeneous nucleation associated thereafter. To synthesize monodisperse, quasispherical Ag NPs, the concentrations of SO42−, CO32−, and PO43− ions used in AgNO3/citrate mixtures in water have been carefully optimized, as listed in Table 1. The concentration deviation from the optimal values usually led to the formation of more elongated Ag NPs (Figure S8). When SO42−, CO32− or PO43− ions are utilized to define the synthesis of Ag NPs via AA/citrate reduction, the sizes and shapes of the resulting Ag NPs are hardly affected by the AA concentration in the range of 0.10−0.20 mM (Figure 3). Only when the AA concentration is greater than 0.3 mM does the size distribution of Ag NPs become polydisperse and a noticeable number of elongated NPs become visible (Figure 3). In the current work, monodisperse, quasi-spherical Ag NPs can be obtained only when the citrate concentration in the mixture with AgNO3 and different anions is in the range between 0.58 and 0.85 mM (Figure 4). Polydisperse, elongated Ag NPs are observed when the citrate concentration is adjusted to below 0.34 mM (Figure 4a), which should be attributed to

Figure 4. TEM images of the Ag NPs synthesized via AA/citrate reduction in the presence of Na3PO4. The citrate concentrations are (a) 0.34, (b) 0.58, (c) 0.85, and (d) 1.02 mM. The concentrations of AA, Na3PO4, and AgNO3 in the mixture are 0.10 mM, 0.20 μM, and 0.297 mM, respectively.

the fact that there are not sufficient citrate ions to stabilize the growing NPs during AA/citrate reduction. However, polydisperse, elongated Ag NPs are also obtained at citrate concentrations greater than 1.02 mM (Figure 4d). This is possibly because the citrate reduction of Ag+ ions to Ag0 at that high concentration inevitably takes place, thus leading to undesirable nucleation in the aqueous solutions of AgNO3/ 2501

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Figure 5. (a, b) TEM images and (c) UV−vis spectra of the corresponding Ag NPs obtained by AA/citrate reduction in the presence of Na2S. The concentrations of Na2S are (a, black curve) 0.15 μM and (b, red curve) 0.60 μM. The concentrations of citrate, AgNO3, and AA are 1.02, 0.297, and 0.10 mM, respectively.

citrate mixtures bearing additional anions or during the growth of Ag NP especially after the full consumption of AA. To guarantee the formation of monodisperse, quasi-spherical Ag NPs, one ought to strengthen the stabilizing role of citrate and, at the same time, weaken its reducing role during NP growth via AA/citrate reduction in the presence of SO42−, CO32−, or PO43− ions. Synthesis of Ag NPs via AA/Citrate Reduction in the Presence of S2− Ions. Xia and co-workers reported that the presence of Ag2S during the polyol synthesis of Ag nanocubes can catalyze the reduction of Ag+ in a mechanism analogous to the autocatalytic reduction of silver clusters by drastically reducing the reduction potential compared to that of free Ag+.32 In the current work, we also introduced Na2S into the aqueous solutions of AgNO3/citrate mixtures to synthesize Ag NPs via AA/citrate reduction. Because the solubility of Ag2S in water is exceedingly low (on the order of 10−33 M), as indicated in Table 1, the concentration of S2− ions added to the AgNO3/ citrate mixture solution is always far larger than the maximum concentration of anions calculated from the solubility of Ag2S. It is known that S2− ions are able to adsorb strongly on the facets of Ag NPs without preference.33 Figure 5a shows the formation of monodisperse, quasi-spherical Ag NPs with sizes of 21 ± 2 nm obtained in the presence of S2− ions at a concentration of 0.15 μM (Figure S9). The SPR band of the resulting Ag NPs is fairly narrow, with the absorption maximum at 398 nm and a fwhm of 48 nm; the absorption at wavelengths above 450 nm is negligible (Figure 5c). The increase of the concentration of S2− ions to 0.60 μM leads to smaller, quasispherical Ag NPs with a size of 10 nm, which, however, coexists with a noticeable number of Ag NPs with a size of 20 nm (Figure 5b). However, this size distribution broadening causes little change in the SPR spectral profile, which should be due to the fact that as-prepared Ag NPs are fairly quasi-spherical. When different anions are introduced into aqueous AgNO3/ citrate mixtures, they are expected to associate with Ag+ ions to form poorly soluble silver compounds, which will act as precursors instead of Ag+ ions in the growth of Ag NP via AA/ citrate reduction. Table 3 summaries the sizes of Ag NPs obtained in the presence of different anions and the potential values of reducing the corresponding silver compounds to silver. It is abundantly clear that the sizes of as-prepared Ag NPs in general decrease with the decrease in the potential of silver compounds as precursors. Namely, the sizes of as-prepared Ag NPs decrease from 31 to 30, 27, 25, 23, and 21 nm with the following potential decrease in the order of anions: NO3− > SO42− > CO32− > PO43− > Cl− > S2−. A total of 2400 particles from three identical batches (100 particles from each) were counted

Table 3. Summary of the Reduction Half-Reactions and Potential Values of Different Silver Compounds at 25 °C, the Sizes of the Ag NPs Obtained Thereof, and the Ag NP Facets onto Which Different Anions Preferentially Adsorb half-reaction Ag + e → Aga Ag2SO4 + 2e → 2Ag + SO42− Ag2CO3 + 2e → 2Ag +CO32− Ag3PO4 + 3e → 3Ag + PO43− AgCl + e → Ag + Cl− AgBr + e → Ag + Br− AgI + e → Ag + I− Ag2S + 2e → 2Ag + S2− +

size (nm)c

preferentially adsorbed facet

zeta potential (mV)g

0.799 0.653

31 ± 3 30 ± 2

noned

−32.6 −35.0

0.477

27 ± 2

noned

−32.0

0.340

25 ± 2

noned

−35.4

0.222

23 ± 2

{111}e

−35.8

0.071

16 ± 2

{100}e

−33.4

−0.152 −0.710

23 ± 2 21 ± 2

{111}e nonef

−33.5 −32.7

potential values (V)b

a Reduction of AgNO3 by AA/citrate. bValues reported in refs 26, 34, and 35. cA total of 2400 particles from three identical batches (100 particles from each) were counted to calculate the average size. The error bars in the sizes correspond to one standard deviation in each case. dAs reported in refs 29−31. eAs reported in refs 23 and 25. fAs reported in ref 33. gZeta potential of Ag NPs obtained solely by citrate was −36.3 mV.

to calculate the average size. The error bars in the sizes correspond to one standard deviation in each case. The deviation of the size distribution of all of the Ag NPs obtained was less than 10%. According to the LaMer model of NP growth,36 the sizes of final NPs ought to be determined dominantly by the numbers of nuclei formed in the nucleation stage. As such, silver compounds with small potentials are easily reduced, allowing fast nucleation and the formation of a large number of nuclei upon nucleation, thus leading to the small size of final Ag NPs. This explanation is definitely rational for Ag NPs obtained in the presence of SO42−, CO32−, PO43−, or Cl− ions because the concentrations of these anions are sufficiently low so as not to precipitate Ag+ ions and thus inhibit potentially heterogeneous nucleation. Compared to other silver compounds, Ag2S has the smallest potential (negative). This cannot be used to rationalize the fact that the use of S2− ions allows the formation of quasi-spherical Ag NPs as small as 10 nm because the concentration of S2− ions used is far larger than the maximum concentration needed to form Ag2S precipitates as a result of the exceedingly low solubility of Ag2S in water, which makes heterogeneous 2502

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nucleation inevitable during the growth of Ag NPs. Monodisperse, quasi-spherical Ag NPs with sizes of 21 ± 2 nm can be obtained in the presence of S2− ions under optimized reaction conditions. The formation of smaller NPs at a higher concentration of S2− ions should be due to the increase in the number of heterogeneous nuclei, Ag2S, with the S2− ion. This inevitable heterogeneous nucleation is also expected to occur during the growth of Ag NPs in the presence of I− and Br− ions because it is difficult to adjust the concentration of I− or Br− ions below the maximum concentration for AgI or AgBr formation (10−9 M). Because the concentrations of Br−, I−, and S2− ions used in our work were still rather low, the presence of silver precipitates (AgBr, AgI, and Ag2S) was hardly detected in the resulting Ag NPs obtained on the basis of our STEM-EDS characterizations. Narrow adsorption bands assigned to AgBr, AgI, or Ag2S were visible in the absorption spectra of the resulting Ag NPs. According to our zeta potential measurements, we found that all of the Ag NPs, obtained in the presence of different anions, exhibited zeta potential values rather close to those stabilized solely by citrate (Table 3). This implies that as-prepared Ag NPs are dominantly stabilized with citrate ions. The size difference between the Ag NPs obtained in the presence of S2− ions and those obtained in the presence of I− or Br− ions should also be correlated with the concentration difference of anions, as indicated in Table 1. The concentration of Br− ions used (8 × 10−5 M) is greater than that of S2− (1.5 × 10−7 M) and I− ions (6 × 10−8 M) by 2 and 3 orders of magnitude, respectively, which should be the reason that the smallest Ag NPs with sizes of 16 ± 2 nm are formed in the presence of Br− ions. Although Br− ions have an adsorption preference of {100} facets over {111} facets during the growth of Ag NPs, it is expected that Br− ions tend to cover these two facets fully at the high concentration,37 thus leading to the formation of quasi-spherical Ag NPs.

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ASSOCIATED CONTENT

S Supporting Information *

Additional TEM images of Ag NPs, calculation of the Ksp value of Ag3PO4 at 100 °C, and maximum concentration of anions for the formation of precipitates of the corresponding silver compounds in AgNO3 solution at 100 °C. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the Natural Science Foundation of China (51172126, 51002086, 51227002, and 51272129), the 973 program (2010CB630702), and the Shandong Provincial Natural Science Foundation (ZR2010EM006). H.X. is grateful to the Program for New Century Excellent Talents in University (NCET-10-0553), the Independent Innovation Foundation of Shandong University (2010JQ013), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry for financial support. D.W. thanks the Australian Research Council for financial support (DP 110104179 and DP 120102959).

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REFERENCES

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CONCLUSIONS We demonstrate a simple method of producing monodisperse, quasi-spherical Ag NPs with controlled sizes via AA/citrate reduction in the presence of additional anions including Cl−, Br−, I−, SO42−, CO32−, PO43−, and S2−. These additional anions are used to transform silver precursors from soluble Ag+ ions to new silver compounds with different potentials for the growth of Ag NPs. Under optimized reaction conditions, the sizes of the resulting monodisperse, quasi-spherical Ag NPs can be finely tuned from 30 to 23 nm according to the potential sequence of new silver precursors, Ag2SO4 > Ag2CO3 > Ag3PO4 > AgCl. This can be readily rationalized according to the LaMer model. When Br−, I−, and S2− ions are used to manipulate the growth of Ag NPs via AA/citrate reduction, the solubility of the resulting AgBr, AgI, and Ag2S are exceedingly low and thus heterogeneous nucleation become inevitable. In this case, the size of the resulting monodisperse, quasi-spherical Ag NPs can be tuned from 23 to 16 nm by adjusting the concentration of anions used. Our work should report the first success in fabricating monodisperse, quasi-spherical Ag NPs with sizes ranging from 16 to 30 nm directly in water and demonstrate the potential for the first time to tune the sizes of Ag NPs by using additional anions to manipulate the redox activity of silver precursors. Thus, the present work should shed new light on the study of the aqueous synthesis of transition-metal nanoparticles for better size and shape control. 2503

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