Does Morphology of a Metal Nanoparticle Play a Role in Ostwald

Jan 8, 2013 - This classical theory did not describe whether the morphology of a metal nanoparticle plays a role in controlling the shape evolution in...
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Does Morphology of a Metal Nanoparticle Play a Role in Ostwald Ripening Processes? Chein Lin Kuo and Kuo Chu Hwang* Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan S Supporting Information *

ABSTRACT: The Ostwald ripening theory is commonly adopted to rationalize the growth of large metal nanoparticles that are formed at the expense of small-sized nanoparticles of higher chemical potential energies. This classical theory did not describe whether the morphology of a metal nanoparticle plays a role in controlling the shape evolution in the ripening processes. Here we show the direct observation of shape evolution among Ag nanoparticles of different morphologies in solutions, and experimental measurements of the relative chemical potential energies (or the electrochemical oxidation potentials) of Ag NPs of different morphologies, including, nanocubes, nanospheres, triangular plates, and decahedral nanoparticles. Theoretical calculation shows that when the diameter of a metal NP is beyond 35 nm, the influence of particle sizes on the oxidation potentials becomes very small. Chemical etching of Ag NPs by Fe(NO3)3 results in preferential removal of atoms at sharp edges/ corners, and negative shift in the oxidation potentials. The measured electrochemical oxidation potentials of Ag NPs are in following orders: nanocubes (43 nm in length, 346 mVAg/AgCl) > nanospheres (53 nm in diameter, 337 mVAg/AgCl) > pentatwinned decahedrons (86 nm in edge length, 315 mVAg/AgCl) > triangular nanoplates (127 nm in edge length and 11 nm in thickness, 293 mVAg/AgCl). KEYWORDS: metal nanoparticles, morphology, chemical potential energy, Ostwald ripening

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oxidation potentials (or higher chemical potential energies) than large ones,26−28 which matches with theoretically predicted values.27,28 It was also observed that surfacedeposited large Ag NPs grow even larger at the expense of dissolution of small NPs on an electrically conductive Indium Tin Oxide (ITO) glass surface.10 These theoretical models,28,29 however, did not predict the effects of morphologies of M NPs on their chemical potential energies. Although the electrochemical oxidation potentials of some M NPs with a particular morphology were reported individually by different groups,30,31 it was never addressed whether morphology of a M NP plays a role in determining the size and shape evolution/distribution in the Ostwald ripening process. Here, we demonstrate experimentally the size and shape evolution of morphologies of Ag NPs in solutions in the absence of any silver salt precursors, and determine the relative chemical potential energies of these M NPs of different morphologies.

ize- and shape-controlled synthesis of metal nanoparticles (M NPs) is currently a very important research topic in the science community.1−9 During formation of M NPs in solution, Ostwald ripening theory is widely adopted to rationalize formation of large particles, which grow at the expense of smaller NPs having higher chemical potential energies.10−12 The Ostwald ripening theory, proposed by Wilhelm Ostwald in 1896,13 did not describe whether the morphology of a M NP plays roles in the size and shape evolution of M NPs.10−12 In the synthesis of M NPs, it is often observed that nanoparticles of a particular morphology, such as, nanocubes,1−3 is preferentially formed over other morphologies. It is not clear what factors determine the morphology evolution and distribution of M NPs. The morphology of a M NP is known to play a very important role in determining their optical,14−17 sensing,18,19 electronic,20,21 and catalysis22−24 properties. Chemical potential energy of an M NP is a key factor determining its chemical stability/reactivity. The relative chemical potential energy (to a reference, such as that of 2H+ + 2e− → H2) can be determined indirectly by measuring the electrochemical oxidation potentials, via the equation ΔG = −nFξ, where ΔG is the difference in the chemical potential energy of a system relative to the reference, and ξ is the electrochemical oxidation potential of the system to be measured.25 The more positive the electrochemical oxidation potential is, the lower the chemical potential energy. Previously, it was demonstrated that the electrochemical oxidation potentials of Ag NPs are strongly dependent on the particle sizes, with small particles having a lower electrochemical © 2013 American Chemical Society

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RESULTS AND DISCUSSION To investigate the morphology effects on the electrochemical oxidation potentials, we tuned the morphologies of as-prepared Ag NPs by chemical etching using different molar percentages of ferric nitrate,32,33 so that the decrease of particle volumes (or sizes) can be well-controlled. To make it possible to compare our results with literature data, we set the condition of our Received: September 27, 2012 Revised: December 16, 2012 Published: January 8, 2013 365

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electrochemical measurements to be the same as that reported in the literature..25 Because it was shown that aggregation of Ag NPs will change their electrochemical oxidation potentials, the distribution of Ag NPs on ITO glass was examined by scanning electron microscope (SEM) to make sure no aggregation of Ag NPs (see the Supporting Information, Figure S1). To avoid the interference of surface-chelated dispersion reagent on the electrochemical oxidation potentials, we removed the dispersion reagent, such as PVP or citrate anion, by repeated washing of Ag NPs by DI water as well as by scanning the Ag NPs-ITO electrode to +0.15 VAg/AgCl and back to 0 V twice (see the Supporting Information, Figure S2, for details). As shown in the inset of Figure 1, chemical etching of Ag nanocubes (∼83

Scheme 1. Possible Exposure of New Crystalline Facets after Early Stages of Chemical Etching of Ag NPs with Different Morphologies

Figure 1. Linear sweep voltametry scanning curves (vs a Ag/AgCl reference electrode, in a 0.1 M H2SO4 aqueous solution) of Ag nanocubes (average length ≈ 83 nm) under chemical etching by different molar percentages (relative to Ag atomic concentration) of Fe(NO3)3. Inset: TEM images of Ag nanocubes under 0, 2, 4, and 40 mol % of etching by Fe3+. The scale bar is 50 nm.

value (i.e., toward a higher chemical potential energies, see Figures S3−S6 in the Supporting Information). Such a large shift in the electrochemical oxidation potential upon small (2− 4%) volume change clearly indicate the important contribution of morphology to the chemical potential energies of M NPs. To further compare the chemical potential energies among Ag NPs of different morpholiges, we prepare Ag nanocubes, nanospheres, penta-twinned decahedrons and triangular nanoplates with nearly the same size/volume. The dimensions of the Ag NPs are the followings: Ag nanospheres (53 nm in average diameter), nanocubes (43 nm in average length), pentatwinned decadehrons (51 nm in edge length), and triangular nanoplates (127 nm in the edge length, and 11 nm in thickness) (see also Figures S7−S10 in the Supporting Information for TEM images and electron diffraction patterns). The corresponding volumes of these Ag NPs are 77952, 79507, 76430, and 76824 nm3 (or 1.0:1.02:0.98:0.985) for nanospheres, nanocubes, penta-twinned decahedrons, and triangular nanoplates, respectively. The variation in the volumes is ±2%. From the volumes, one can calculate both the corresponding diameters of the equivalent nanospheres for these Ag NPs, and the expected electrochemical oxidation potentials according to the equation in the reference 26. The calculated differences in the electrochemical oxidation potentials of Ag nanocube, pentatwinned decahedron, and triangular nanoplate, relative to that of nanosphere, are +0.13, −0.04, and −0.02 mV, respectively. As shown in Figure 2, the electrochemical oxidation potentials were determined to be +346, +337, +315, and +293 mVAg/AgCl for Ag nanocubes, nanospheres, penta-twinned decahedrons, and triangular nanoplates, respectively. The differences in the electrochemical oxidation potentials of nanocubes, pentatwinned decahedrons, and triangular nanoplates, relative to that of the nanospheres, are +9, −22, and −44 mV, respectively. Since the size effect has been eliminated (nearly completely), the difference in the electrochemical oxidation potentials is

nm in length) by Fe3+ ions leads to passivation of corners and edges, indicating that metal atoms at edges and corners have less coordiantion numbers, and are less stable, resulting in higher chemical reactivity than those on planes. The crystalline surface on the 6 cubic faces is the {100} facet. Corrosion of the edges and corners will lead to decrease in the surface area of the {100} facets and simultaneously new exposure of the {110} and {111} facets3−5,34 at early stage of chemical etching (see also the Scheme 1). The oxidation potential of as-prepared Ag nanocubes appears at ∼354 mV (vs an Ag/AgCl reference electrode). Upon chemical etching, the oxidation potential of Ag nanocubes shifts to a more negative value (or a higher chemical potential energy) in accompany with quick etching of atoms at corners and edges. Theoretical calculation26,29 predicts slightly negative shifts of 0.055 and 0.108 mV, respectively, upon chemical etching by 2 and 4 mol % (relative to the Ag atomic concentration) of Fe3+ ions. The observed much larger negative shifts of 29.3 and 37.3 mV, for 2 and 4 mol % etching, respectively, of Ag nanocubes cannot be explained by the theoretical model.26,28,29 Clearly, the change in the morphology of an Ag NP plays an important role in determining the redox potential of an Ag NP. Similar to nanocubes, we also prepare Ag penta-twinned decahedrons (∼41 nm in edge length), silver triangular nanoplates (∼60 nm in length and 10 nm in thickness), silver nanospheres (∼55 nm in diameter), and measure their electrochemical oxidation potentials. The results are similar to what was observed in the Ag nanocubes, that is, upon slight morphology change induced by Fe3+ chemical etching, the electrochemical oxidation potentials shifts to a more negative 366

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triangular nanoplates. By taking into account the factor of structural defects, the relative sequence of chemical potential energies of M NPs will shift from the above S/V sequence to the observed sequence in the electrochemical oxidation potentials, except that the relative order of nanocubes and nanospheres is still reversed. The nanosphere is also known to be formed by aggregation of many small nanoclusters and thus is polycrystalline.41 The structural defects in nanospheres can elevate the chemical potential energy so that its chemical potential energy surpasses that of the nanocubes. Experimentally, we also confirm that nanocubes have single crystalline structure, whereras nanospheres have polycrystalline structures, as evidenced by the selective area electron diffraction (SAED) patterns (see also the Supporting Information, Figures S7 and S8, for the SAED pattern and TEM images of Ag nanocubes and nanospheres). Our SAED and XRD patterns also show that both decahedron and nanoplates contain the (111) stacking faults, as evidenced by the existence of the forbidden 1/3 {422} reflection spots, and the XRD band at 14.25° which is corresponding to a d spacing of 2.5 Å (see the Supporting Information, Figure S11). The existence of the forbidden 1/3 {422} reflection spots, and the XRD band at 14.25° is known to be the characteristic fingerprint for the presence of the (111) stacking faults in the Ag penta-twinned decahedrons and triangular nanoplates.32,34,42−44 The above argument regarding the structural defects seems to be able to explain our observed relative sequence in the chemical potential energies of Ag NPs of different morphologies. However, the amount of structural defects existing in metal NPs is strongly dependent on the experimental processes used to prepare metal NPs. When the amount of structural defects changes, the observed relative sequence of chemical potential energies (as well as the relative stabilities) of M NPs might also vary accordingly. This is probably why in some cases the most stable morphology obtained experimentally is a morphology other than nanocubes.1−9 Nevertheless, our experimental results indicate that the structural defect is one of important factors controlling the relative stabilities and shape evolution of M NPs. In addition to the S/V ratio and structural defects, the factor of different surface energies for different facets can also play a role in determining the total chemical potential energy of an M NP (see results in Figure 1). The surface energies (γ) of face centered cubic (fcc) crystals, including Au, Ag, and Cu, usually decreases in the following order: γ(111) < γ(100) < γ(110).40,45,46 Upon chemical etching, the morphology of Ag NPs changes in accompany with partial disappearance of original surface(s) and appearance of new crystalline surfaces. The results in Figure 1 and Figures S3 and S4 in the Supporting Information clearly show that new exposure of high energy facets upon chemical etching sensitively leads to negative shift in the electrochemical oxidation potentials, for example, newly formed high energy (110) surface vs original (100) surface in the case of nanocubes (see Scheme 1 and results in Figure 1); and newly formed high energy (110) surface vs original (111) surface in the cases of penta-twinned decahedrons and triangular nanoplates. Overall, the results in Figure 1 and Figures S3 and S4 in the Supporting Information clearly indicate that the factor of different surface energies does play a role in determining the total chemical potential energy of M NPs. The factor of different surface energy alone does not favor the higher chemical potential energies of penta-twinned decahedron (111) and triangular plate (111+ 110) over those of nanocube (100) and nanosphere (111 + 100 + 110), since γ(111) < γ(100)
80% with some penta-twinned decahedrons and tetrahedrons as side products. The average length of Ag triangular nanoplates is ∼60 ± 5 nm and average thickness ∼10 nm. Ag triangular nanoplates (∼127 nm in edge length and∼11 nm in thickness) was prepared by a two-steps process. The first step is to prepare Ag seeds by following the procedure reported by Mirkin, C. A. [Adv. Mater. 2005, 17, 412]. The growth of large triangular nanoplates was done by a similar process reported by Xia and co-workers [Angew. Chem. Int. Ed. 2011, 50, 244−249]. An aqueous solution of silver nitrate (0.1 mM, 30 mL), trisodium citrate(50 mM, 0.9 mL), PVP (Mw ≈ 29 000 g mol−1, 0.7 mM, 1.5 mL), and hydrogen peroxide (30 wt %, 80 μL) were mixed and vigorously stirred at room temperature. To this mixture, sodium borohydride (0.1 mM, 250 μL) was rapidly injected and vigorously stirred for 2 h. After 2 h, the colloidal aqueous solution turned to blue color. The colloid was stored in dark overnight as seed solution to grow different sizes of nanoplates. In the growth process, 10 mL of seed solution was mixed with 10 mL of DDI-water and vigorously stirred at room temperature. An aqueous solution of PVP (50 mM, 1.1 mL) and ascorbic acid (50 mM, 0.5 mL) was injected into the seed solution. After 5 min, 1 mL of silver nitrate aqueous solution (5 mM) was added into the solution and stirred for another 2 h. The reaction steps were repeated to grow different sizes of triangular nanoplates. The final product was collected by centrifugation and washed by acetone and water. The collected large triangular nanoplates were then redispersed in water. The yield of Ag triangular plates is >95% with some spheres as minor products. The average length of Ag triangular nanoplates is ∼127 nm and average thickness ∼11 nm, as determined by TEM. Synthesis of Ag Nanospheres. In a typical experiment, an ethylene glycol solution containing AgNO3 (47 mM), PVP (147 mM) and NaBr (0.055 mM) was rapidly heated by microwave irradiation via 3 cycles of a “30 sec on-30 sec off” process. Then the solution was washed by acetone and water a few times to remove EG and free PVP. The yield of Ag nanospheres is >95% with some penta-twined decahedrons as minor products. The average diameter of as produced Ag nanospheres is ∼55 ± 5 nm. Chemical Etching Process..32,33 In a typical experiment, 1.35 mg of Ag nanoparticles of different morphologies was dispersed in an aqueous solution (1.5 mL) containing 1 mg PVP in a centrifugation tube. Then, different amounts (molar percentage relative to the Ag atomic concentraton) of Fe(NO3)3 (0.1 mM in the stock solution) were added into a solution in a centrifugation tube to allow chemical etching of Ag nanoparticles. After 10 min chemical etching by Fe3+ ion, the remaining Ag nanoparticles were collected by centrifugation (10 000 rpm), and examined by TEM. Electrochemical Oxidation of Ag Nanoparticles. An ITOcoated glass with a surface area of 1 × 4 cm2 and resistance, R, of 8−12 Ω was used as a working electrode for the measurements of electrochemical oxidation potentials of Ag NPs. The ITO electrode was cleaned in acetone, ethanol and 2-propanol (20 min in each solvent) by sonication, and dried under N2. To make the electrode surface hydrophilic, the surface of ITO electrode was modified to have amine terminal groups by reaction with 3-aminopropyltriethoxysilane (APTES) for 30 min the same way as reported in the literature.26 Amino-terminal groups will form complex with Ag NPs via electrostatic interactions.26 The ITO electrode was then dipped into an Ag NPs containing aqueous solution for 5 s to have Ag NPs coated on the electrode surface (via electrostatic interaction with the amino surface groups on ITO electrode). The electrode was then dried by N2 gas purging. To avoid aggregation-induced distortion of electrochemical oxidation potentials (i.e., so-called the coverage effect)26 was examined the Ag NPs on ITO glass by scanning electron microscope (SEM) to ensure good distribution of individual Ag NP without formation of aggregates (see SEM images shown in Figure S1 in the

taking into account the morphology (i.e., the S/V ratio and different crystalline facets) as well as structural defects, in addition to the size factor, so that it can predict more accurately the size and shape evolution of M NPs in solutions.

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EXPERIMENTAL SECTION

Chemicals. The sources of chemicals used in this study are listed below: silver nitrate (Aldrich, 99%), L-arginine (Aldrich, 98%), trisodium citrate(Sigma-Aldrich), Hypergen peroxide (H2O2, 30% R.D.H chemical) poly(vinyl pyrrolidone) (PVP, Aldrich, molecular weight, Mw = 29 000, 40 000, 55 000 g/mol), sodium borohydride (Aldrich, 99%), Na2S (Aldrich), ascrobic acid(AA, J. T. Baker), ethylene glycol (EG, J. T. Baker), sodium bromide (Aldrich), and ferric nitrate (Showa). These chemicals were used as received without further purification. Preparation of Ag Nanocubes. Ag nanocubes were prepared by following the literature procedure.1−3 Briefly, 20 mL of ethylene glycol was placed in a 100 mL single neck flask and heated to 160 °C in an oil bath for 1 h. Meanwhile, two other EG solutions (6 mL each) were prepared, one containing 94 mM AgNO3, and the other containing 147 mM PVP and 0.11 mM NaBr in ethylene glycol. 500 μL of NaBr (10 mM) and 100 μL of Fe(NO3)3 (27 μM) were added to the flask, followed by addition of AgNO3-EG solution at 10 min later. These two solutions were injected into the single neck flask by a two channel syringe pump (45 mL/h), and then heated for additional 3 h at 160 °C. Then the solution was cooled to room temperature by immersing the flask in cool water. Ag nanocubes were collected by high speed centrifugation (10 000 rpm), and washed by acetone and water several times in order to remove the EG and free PVP. The yield of Ag nanocubes was >95% with some tetrahedrons and nanospheres as minor products. The size of nanocubes was determined by transmission electron microscope (TEM, Jeol, JEM-2100, 200 KV) to be ∼83 ± 7 nm in length. Preparation of ∼43 nm Ag Nanocubes. Ag nanocubes were prepared by following the literature procedure.33In a typical process, 6 mL ethylene glycol was heated under stirring for 1 h in a 20 mL glass vial. While the EG was heated, two EG solutions containing AgNO3 (48 mg/mL) or PVP(20 mg/mL, MW ≈ 55 000) were prepared. After 1 h, the hot EG solution was blowed with dry N2 gas for 5 min to remove the water absorbed in EG. Then, 80 μL of Na2S (3 mM) in EG solution was injected into hot EG, and 1.5 and 0.5 mL of the PVP and AgNO3 EG solutions were injected . As silver nitrate was added, the solution immediately turned purple-red. After several minutes, the solution changes to an opalescent ruddy-brown and concurrently becomes opaque. Ag nanocubes were collected by high speed centrifugation (9000 rpm), and washed by acetone/water several times in order to remove the EG and free PVP. The yield of Ag nanocubes was >95% with some tetrahedrons and nanospheres as minor products. The average size of nanocubes was determined by TEM to be ∼43 nm in length. Synthesis of ∼41 nm and ∼51 nm Ag Penta-Twinned Decahedrons.6 A precursor solution of silver NPs was prepared. A few solutions, including 0.5 mL of 50 mM sodium citrate, 0.015 mL of PVP (50 mM), 0.05 mL of L-arginine (5 mM), and 0.200 mL of AgNO3 (5 mM), were added to 7.0 mL of deionized water in a vial. Then, silver ions were reduced to form penta-twinned decahedrons by addition of 0.080 mL of NaBH4 (100 mM). The resulting pale yellow solution was stirred slowly until it becomes bright yellow color (it takes several min.). The bright yellow solution was then irradiated with blue LED light. The Ag nanoparticles were collected by centrifugation (10 000 rpm), and washed with water a few times to remove PVP. In the regrowth step, 70 mL of decahedral nanoparticles-containing solution was concentrated to 1 mL and then added into 70 mL of the above bright yellow solution. The solution was then irradiated with blue LED light. After repeating the above process once, the nanoparticles were collected by centrifugation. The final product was then redispersed into water. The yield of Ag penta-twinned decahedrons is larger than 90% with some tetrahedrons and spheres as minor products. The average diameter of Ag penta-twinned 369

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Supporting Information). An electrochemical analysis instrument (Autolab, model PGSTAT128N) was used for linear sweep voltametry (LSV) measurements. The electrochemical cell consisted of three electrodes with Ag NPs-ITO glass as the working electrode, a Pt thin film (20 × 20 × 1 mm3) as an auxiliary electrode and an Ag/AgCl reference electrode. All of the electrochemical measurements were performed in 0.1 M H2SO4 electrolyte solution at a scan rate of 1 mV/ s with a potential range of 0.1 to 0.7 V without stirring, the same condition as reported in the literature.26 It was previously reported that magnetic stirring of the solution will lead to complete oxidation of Ag NPs at an earlier time and detached from the electrode surface at a more negative electrode potential (i.e., a negative shift of the peak oxidation potential).26 Therefore, no magnetic stirring was applied during our linear sweep voltametry measurements. The linear sweep voltametry data reported here all are from the first single LSV scan (i.e., not an average of a few oxidation scans or a few oxidation− reduction cycles). Reproducibility of the oxidation potentials was confirmed from at least three fresh ITO-Ag NP samples in fresh electrolyte solutions. When aggregation of Ag NPs on ITO electrode surface occurs, one usually obtains a quite different oxidation potential from those with dilute Ag NP loadings. By increasing the surface area of an ITO glass electrode with the same Ag NP loading density, the electrochemical oxidation current in a single LSV scan can be enhanced to a reasonably good level. To avoid influence of the dispersion reagent, residual dispersion reagents (e.g., citrate anion, or PVP) were removed by repeated washing of Ag NPs by acetone and methanol. Ag NPs were collected by high speed centrifugation (10 000 rpm). The presence of PVP on Ag NPs will lead to appearance of surface enhanced Raman signals[S5] at 1589 and 1360 cm−1. Therefore, the intensity of Raman signal at 1589 cm−1 can be used to examine whether there is residual PVP absorbing on the surface of Ag NPs. After scanning the Ag NPs-ITO electrode to +0.15 VAg/AgCl (at a scanning rate of 100 mV/min) and back to 0.0 V twice, the residual amount of PVP on Ag NPs can be nearly completely removed (see the Supporting Information, Figure S1), as evidenced by disappearance of the surface-enhanced Raman signals. In terms of the purity of Ag NPs, it is still not possible in the literature to prepare a kind of M NPs with 100% purity (or a single morphology). Nevertheless, the electrochemical oxidation potential at the maximum current in the LSV is corresponding to the oxidation potential of the most abundance species (or morphology) present in a sample. Therefore, one still can compare the electrochemical oxidation potentials of Ag NPs with different morphologies in different samples. We compare and discuss the electrochemical oxidation potentials of metal nanoparticles with the most abundant morphology in each sample.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Surface-enhanced Raman spectrum of metal nanoparticles on ITO electrode in the absence of capping agents, linear sweep voltametry scanning curves of Ag penta-twinned decahedrons and triangular plates, and TEM images of ripening products among various Ag nanoparticles in EG at 160 °C for 1 h in the absence of any silver salts. 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]. Fax: (+886) 3571 1082. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support from the National Science Council, Taiwan. 370

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Chemistry of Materials

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dx.doi.org/10.1021/cm3031279 | Chem. Mater. 2013, 25, 365−371