Article pubs.acs.org/crystal
Simultaneous Reduction of Metal Ions by Multiple Reducing Agents Initiates the Asymmetric Growth of Metallic Nanocrystals Mahmoud A. Mahmoud* Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States S Supporting Information *
ABSTRACT: Thermodynamically unfavorable metallic nanocrystals can be prepared only by the growth of the nanocrystals under kinetically controlled experimental conditions. The common technique to drive the growth of metallic nanocrystals under kinetic control is to adjust the rate of the generation of metal atoms to be slower than the rate of deposition of such atoms onto the surface of nanocrystal nuclei, which form in the first step of the nanoparticle synthesis. The kinetically controlled growth leads to the formation of seeds with crystal defects, which are needed for the growth of anisotropic nanocrystals such as silver nanodisks (AgNDs). The simultaneous multiple asymmetric reduction technique (SMART) is introduced here to successfully prepare AgNDs of controllable sizes and on a large scale within a few seconds. The SMART is simply based on the simultaneous reduction of silver ions with a strong reducing agent such as borohydride (redox potential of 1.24 V) and a weak reducing agent such as L-ascorbic acid (redox potential of 0.35 V) in the presence of a polyvinylpyrrolidone capping agent. The random formation and deposition of silver atoms by the two different reducing agents generated stacking faults in the growing nanocrystal. The hexagonal closepacked {111} layers of silver atoms were then deposited on the surface of the growing nanocrystal containing stacked faults along the [111] plane. This initiated asymmetric growth necessary for the formation of platelike seeds with planar twin defects, which is required for the formation of anisotropic AgNDs.
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good yields, different “contaminant” shapes were also observed.19 The yield of the AgNPts was improved by using different capping materials and adding different chemical additives that initiate the formation of platelike seeds with the stacking faults required for the formation of AgNDs and AgNPts.20−22 These seeds would not be formed during thermodynamically controlled growth, and citrate ions were suggested to be responsible for the formation of platelike seeds undergoing kinetically controlled growth. Recently, AgNPts were prepared without the assistance of citrate ions but via the Ostwald ripening technique.23,24 In all the earlier approaches that reported the synthesis of AgNPts and AgNDs, slowing the growth of the silver nanocrystal was mandatory for the formation of the platelike seeds under kinetic control.13,16,25 The reaction rate was slowed by adding a ligand, such as citric acid18 or polyacrylamide,16 that forms a complex with the silver ions and ultimately slows their reduction. Hydrogen peroxide (H2O2) was used as an etching agent during the synthesis of silver nanoplates.25,26 However, the borohydride (BH4−) was used to reduce the silver ions into silver atoms, resulting in the nucleation and growth of the silver nanocrystal. H2O2 inhibits
INTRODUCTION Plasmonic nanoparticles are characterized by their exciting optical properties that are attributed to their strong interaction with light, leading to localized surface plasmon resonance (LSPR) spectra and the generation of strong plasmonic fields.1−4 For many applications, a maximization of the plasmon field intensity and distribution as well as strict control over the LSPR peak position is highly desirable.5−7 Plasmonic nanoparticles with isotropic shapes introduce relatively weaker plasmon fields, and tuning their LSPR peak is very limited compared with that of anisotropic nanoparticles.8−11 Homogeneous growth of metallic nanocrystals always produces isotropic nanoparticles.8,10,12,13 Producing anisotropic shapes requires breaking the symmetry of the growth of the nanocrystal, which has been successfully achieved by chemical,14 photochemical,15 physical,16 and mechanical techniques.17 Silver nanoplates (AgNPts) and silver nanodisks (AgNDs) are two examples of anisotropic two-dimensional plasmonic nanoparticles, and their LSPR spectrum can be easily tuned in the visible and IR regions.18 Many efforts have focused on the synthesis of AgNDs and AgNPts, beginning with the photochemical transformation of silver nanospheres into AgNPts.15 This first approach reported the synthesis of AgNPts based on the seedmediated technique introduced by Chen and Carroll.19 Although this technique successfully produced AgNPts with © XXXX American Chemical Society
Received: April 29, 2015 Revised: July 15, 2015
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DOI: 10.1021/acs.cgd.5b00592 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 1. TEM images of silver nanodisks prepared by the simultaneous reduction of 0.60 mL of silver ions (60 mM) mixed with 200 mL of a 0.145 mM aqueous solution of PVP, 0.12 mL of sodium borohydride (5 mM), and 78 mM ascorbic acid with volumes of (A) 5, (B) 4, (C) 3, (D) 2, (E) 1, and (F) 0.5 mL. The diameter of the nanodisks was increased with a decrease in the amount of added ascorbic acid. The scale bars are 100 nm.
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the growth of the silver nanocrystal by etching some silver atoms away from the surface of the growing nanocrystal.26 The dynamic equilibrium between the reduction of silver ions and the oxidation of the silver atoms induced the formation of small silver nanoparticles containing different kinds of defects.26 Citrate ions were used to stabilize only the twinned defected nanocrystal that was required for the formation of the nanoplates, while hydrogen peroxide dissolves the other defected nanocrystals.26 In this study, the simultaneous multiple asymmetric reduction technique (SMART) is introduced; it allows metallic nanocrystals to grow into anisotropic shapes such as AgNDs. This new technique is based on the simultaneous reduction of silver ions by two reducing agents, one of which is strong, such as borohydride (BH4−), and the other of which is weak, such as L-ascorbic acid (AA), in an aqueous solution of polyvinylpyrrolidone. Interestingly, unlike the case in previous reports,20−22 the AgNDs were formed within seconds with large volumes, controllable sizes, and tunable LSPR spectra using nontoxic reagents (see the movie in the Supporting Information). The SMART mechanism is discussed on the basis of high-resolution electron imaging and by studying the role of each reducing agent during the reduction of silver ions.
EXPERIMENTAL SECTION
Silver nanodisks (AgNDs) were prepared by a single-pot experiment. In a 500 mL glass flat bottom bottle, 0.60 mL of a 60 mM AgNO3 aqueous solution was added to 200 mL of a 0.145 mM aqueous solution of polyvinylpyrrolidone (PVP; MW = 55000). To prepare AgNDs of different sizes, different volumes (0.5, 1, 2, 3, 4, and 5 mL) of 78 mM L-ascorbic acid (AA) were added to the reaction bottle. With gentle swirling of the reaction mixture, 0.12 mL of 5 mM NaBH4 was injected, which initiated the reduction reaction, and the swirling was continued for 10 s. Although the reduction of the silver ions into AgNDs finished within 10 s, the solution was left in the dark for 10 min before the byproducts were removed for increased stability of the AgNDs. Table S1 reports the final concentration of PVP, AgNO3, AA, and BH4− used for the synthesis of the AgNDs after mixing. The AgNDs were cleaned by centrifugation in 50 mL plastic tubes at 12000 rpm for 30 min, and the precipitated particles were then dispersed in 50 mL of deionized (DI) water. The cleaned AgNDs prepared by this method are stable when stored in the dark for several weeks. For longterm storage, the precipitated AgNDs were dispersed in 20 mL of ethanol. In this case, the disks were stable for more than three months. The overgrowth of AgNDs was conducted as follows. AgNDs prepared with 4 mL of AA reduction were precipitated by centrifugation; the precipitate was dispersed in a 0.145 mM aqueous solution of PVP until the LSPR peak at ∼530 nm had an optical density of 1.2. Then, 3 mL of hydrazine hydrate (HH) (20 mM) or 1 B
DOI: 10.1021/acs.cgd.5b00592 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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nanocrystal.31,36 Finally, the nanoparticle seed grows into the final characteristic nanoparticle. Studying the mechanism of the formation of the nanocrystals involves characterizing the seed that should be isolated during the formation of the nanocrystal.35 The seed provides useful information about the mechanism of the formation and growth of the nanocrystals.22 It is not easy to isolate the seed formed during the synthesis of AgNDs by the SMART for investigation, because the whole reduction process was accomplished in just a few seconds. Studying the mechanism in this case was based on the characterization of the final nanodisk by high-resolution TEM imaging. Panels A and B of Figure 2 show the HR-TEM images of AgNDs from the top
mL of AA (78 mM) was added under slow stirring to 50 mL of the resulting AgND solution (OD = 1.2), followed by 0.1 mL of AgNO3 (60 mM). The red color of the AgND solution became yellow after the addition of AA or blue in the case of the addition of HH. The resulting AgNDs were then cleaned by centrifugation in 15 mL centrifugation tubes for 20 min at 8000 rpm in the case of HH reduction and at 12000 rpm for AA reduction. The effect of changing the concentration of BH4− during the SMART experiment was examined by repeating the same procedure for AgND synthesis but adding different volumes of BH4− (0.06, 0.12, and 0.24 mL) and fixing the volume of AA at 3.5 mL.
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RESULTS AND DISCUSSION Figure 1A−F shows the TEM images of AgNDs prepared by the SMART at different concentrations of AA; the amount of AA was decreased from panel A to F of Figure 1. ImageJ was used to determine the diameters of the AgNDs measured from three TEM images for each batch shown in Figure S1. AgNDs with diameters of 25.9 ± 4.8, 30.9 ± 6.5, 33.3 ± 7.7, 37.7 ± 7.1, 42.3 ± 9.4, and 46.8 ± 5.8 nm were obtained when the volume of added AA was decreased to 5, 4, 3, 2, 1, and 0.5 mL, respectively. Although the diameter of the AgNDs was controlled by the concentration of AA, the thickness of all the prepared AgNDs is comparable (∼8 nm). TEM imaging was used to image the sides of aggregated AgNDs aligned vertically. Aggregation of the AgNDs was induced by a highspeed centrifugation of the colloidal solution of AgNDs. Even in a highly aggregated state, it is still rare for the AgNDs to align vertically, and consequently, a simple statistical analysis for 10 particles form each batch of AgNDs of different diameters was conducted to determine their thickness (∼8 nm). An example of a more accurate statistical analysis that was conducted for 48 AgNDs with a diameter of 30.8 ± 6.5 nm is shown in Figure S2. The analysis indicated the thickness of the 30.8 ± 6.5 nm AgNDs to be 7.9 ± 0.2 nm. It is well understood that the first step in the formation of metallic nanocrystals by colloidal chemical approaches via either seed-mediated or seedless techniques is the formation of small metallic clusters (nucleation).9,27,28 The small cluster is obtained from aggregation of the atoms of the metal, which are produced as result of the reduction of the metal ion by a reducing agent or the decomposition of a precursor compound.16 Most metals crystallize into face-center-cubic (fcc) crystal structures composed of {111}, {100}, and {110} facets with surface energies of 0.553, 0.653, and 0.953 eV, respectively.29 The metallic clusters grow into larger single crystals or polycrystalline nanoparticle seeds under either thermodynamic or kinetic control.30,31 Thermodynamically controlled growth produces seeds of minimized total surface energy.30 Single-crystal truncated octahedra (Wulff polyhedrons) terminated by a mixture of the low-energy {111} and {100} facets are the most thermodynamically favored seeds.30,32 Polyhedron seeds containing single and multiple twinned defects can also be formed under thermodynamic control.33,34 Conversely, when the growth of the nuclei produces thermodynamically unfavorable seeds, such as a platelike seed with stacking faults and/or twin defects, the growth is said to be under kinetic control.31 Growth under kinetic control has been reported by slowing the rate of reduction of the metal ions or the rate of decomposition of the metal precursor.16,35 For platelike seeds, slowing the generation of the metal atoms initiates the growth of the fcc nanocrystals through random hexagonal close-packed (rhcp) {111} layers along the [111] plane, causing the formation of stacking faults in the growing
Figure 2. HR-TEM images collected for an individual silver nanodisk from (A) the top face perpendicular to the plane of the nanodisk (the inset shows a Fourier transformation of the image) and (B) the side face parallel to the plane of the nanodisk. Fringes of d-spacing of 2.5 and 1.4 Å were observed in the top view of the nanodisk, corresponding to the forbidden 1/3{422} facet and the {110} facets, respectively. The side view of the nanodisk has a twin plane defect bounded with {111} facets and a mixture of {111} and {100} facets with d-spacings of 2.4 and 2.0 Å, respectively.
view (perpendicular to the flat surface) and from the side view, respectively. The d-spacing of the top view image of the individual AgND is 2.5 Å, which can be assigned to forbidden 1/3{422} reflections.31,36 Fringes with d-spacings of 1.4 Å were also observed in the top view of the individual AgND corresponding to the {220} facets.31 Figure 2B shows the HR-TEM image of the AgND collected from its side face. It is clear that this side face is composed of a mix of {111} and {100} facets with d-spacings of 2.0 and 2.4 Å, respectively. The planar twin fault was observed parallel to the flat faces of the nanodisk bounded by {111} facets that are responsible for the appearance of the forbidden 1/3{422} reflections.31,37 It is C
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Figure 3. TEM images of silver nanocrystals prepared upon the reduction of a 200 mL solution of silver nitrate (0.18 mM) in 0.145 mM PVP, with 3.5 mL of L-ascorbic acid (78 mM) and different volumes of sodium borohydride (5 mM). (A) When 0.06 mL of borohydride was added, triangular silver nanoplates were obtained in addition to larger silver nanodisks. (B) Silver nanodisks were formed when 0.12 mL of borohydride was added. (C) Small silver nanostructures with a broad size and shape distribution of discs and plates were formed when the volume of BH4− was increased to 0.24 mL. The scale bars are 100 nm.
Figure 4. TEM images of (A) silver nanodisks prepared by the reduction of 0.18 mM silver nitrate in 200 mL of 0.145 mM PVP with 4 mL of Lascorbic acid (78 mM) and 0.12 mL of sodium borohydride (5 mM), (B) silver nanodisks after overgrowth by L-ascorbic acid reduction of silver ions (silver atoms deposited on the top and bottom faces of the nanodisks), and (C) silver nanodisks overgrown by the reduction of silver ions by hydrazine hydrate (the silver atoms tended to deposit specifically on the side of the nanodisks during growth). The scale bar is 100 nm.
SMART, but what bears further investigation is whether BH4− plays a role in the growth of the silver nanocrystals into nanodisks. Although the concentration of BH4− seems low, in principle each BH4− molecule could provide eight electrons, which could be sufficient to reduce eight silver ions. Two experiments were conducted to examine the role of BH4− in the SMART, the first being adding BH4− before AA during the reduction of the silver ions. Interestingly, silver nanospheres were obtained, and no AgNDs or silver nanoplates were observed. Nanospheres resulted from the growth of a single crystalline seed, suggesting that AA has a role in the formation of the platelike twinned seed required for either AgND or silver nanoplate formation. Second, the concentration of BH4− in the SMART was varied. The volume of 5 mM BH4− was decreased to 0.06 mL from the volume of 0.12 mL normally used to produce AgNDs. Triangular silver nanoplates from the TEM image shown in Figure 3A were obtained. These triangular silver nanoplates are larger than the AgNDs prepared with the regular concentration of BH4− (see the TEM image in Figure 3B). Conversely, when the volume of BH4− used was doubled to 0.24 mL, smaller AgNDs were obtained (see the TEM image of the AgNDs in Figure 3C). The silver nanoplates obtained upon lowering the concentration of BH4− have been shown to form as a result of the growth of small hexagonal plate seeds of planar twin faults terminated with low-energy {111} facets under thermodynamic control.38 This suggests that changing the concentration of BH4− controls the number of nuclei formed but not the shape of the seed, which results in the formation of silver nanoplates with larger sizes with a decrease in the amount of BH4− added. Decreasing the number of formed silver nuclei resulted in an increase in the amount of remaining silver ions required for the growth of the formed seed.
important to mention that the HR-TEM results of the AgND prepared by the SMART are similar to that reported for AgNDs prepared by other techniques. Most of these techniques were based on using citrate ions25 that have been proven to have a dual role during the synthesis of silver nanoplates: first as a capping agent, stopping growth on the {111} facet by binding to that surface, and second by forming a complex with the silver ions, slowing the rate of their reduction and supporting the formation of the thermodynamically unfavorable platelike seed with twin defects under kinetic control.18 In contrast, the SMART is much faster than the other reported techniques, and no capping was added to stop the growth on the {111} facet; however, PVP was used as a capping agent that binds to the {100} facet. Changing the molecular weight of the PVP did not affect the formation of the AgNDs by the SMART but did affect the quality. Using PVP with a molecular weight of 29K at the same concentration as that of the 55K form produced different size distributions of AgNDs. PVP can act as a reducing agent when it is heated. As the preparation of the AgNDs is accomplished at room temperature, PVP is not expected to play an active role in the reduction of silver ions in the SMART. To examine if AA has a capping role in the SMART, AA was replaced by another weak reducing agent such as hydroquinone. Interestingly, AgNDs are formed in the case of hydroquinone. For more clarification of the mechanism of AgND formation, further investigation has to be conducted. In the SMART, BH4− (strong reducing agent with a potential of 1.24 V) and AA (weak reducing agent with a potential of 0.39 V) were used to reduce silver ions in the presence of PVP. AA alone does not reduce the silver ions until BH4− is added, which initiates the reduction reaction. The ratio of the concentrations of BH4− to silver ions was 1:56, while the AA:silver ion concentration ratio was tuned from 1.11:1 to 11.1:1. This confirms the role of BH4− in the nucleation in the D
DOI: 10.1021/acs.cgd.5b00592 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 5. Schematic diagram of the mechanism of the growth of silver nanocrystals symmetrically into a nanosphere through the formation of a Wulff polyhedron single-crystal seed and asymmetrically by the SMART through the platelike seed with a twin plane. In the SMART, the liquidlayered boundary layer was disturbed, while in the symmetrical growth, the liquid-layered boundary layer is homogeneous.
Figure 6. (A) Localized surface plasmon resonance spectrum of silver nanodisks with a thickness of 7.9 ± 0.2 nm and diameters of 25.9 ± 4.8 nm (black), 30.9 ± 6.5 nm (red), 33.3 ± 7.7 nm (blue), 37.7 ± 7.1 nm (magenta), 42.3 ± 9.4 nm (brown), and 46.8 ± 5.8 nm (cyan). The diameter of the nanodisks was increased with an increase in the amount of L-ascorbic acid added during the synthesis. (B) Linear relationship between the aspect ratio of the silver nanodisks (the ratio between the diameter and thickness) and their LSPR peak position.
The mechanism of the formation of the silver nanodisks by the SMART can be summarized as follows. BH4− reduces some of the silver ions into atoms aggregated into nuclei, which then grow into small clusters of fcc crystal structure under thermodynamic control. The clusters are surrounded by a thin layer (few nanometers) of liquid-layered bounded (LLB) regions17 at the interface between the crystalline facet and the solution. The LLB is suggested to be the Volmer adsorption region,39 which was recently confirmed by molecular dynamics simulation calculations.17 Inside the LLB, silver ions are reduced to atoms by the reducing agent. The atoms are then desolvated and diffuse to crystallize on the surface of the cluster. Upon introduction of a strong reducing agent such as BH4−, the rate of generation of the silver atoms is faster than their rate of diffusion to the surface of the cluster, supporting the formation of thermodynamically favorable seeds.25 The LLB in this case will be uniform, and the silver atoms crystallize on the surface of the cluster without defect formation. When AA is involved in the reduction of silver ions with BH4− as in the case of the SMART, the LLB will be disturbed, stacking faults in the grown crystal will be initiated, and the growth of the nanocrystal will proceed by rhcp layers. The rhcp layers lead to the formation of platelike seeds of planar twin planes. The resulting seed growth by the deposition of silver atoms onto the defect plane under thermodynamic growth by the dual
The role of AA during the growth of the silver nanocrystals into AgNDs was examined by allowing small AgNDs to overgrow using AA (weak reducing agent) and hydrazine hydrate (strong reducing agent with a reduction potential of 1.15 V) to reduce silver ions. Figure 4A shows the TEM image of AgNDs prepared by 4 mL AA reduction via the SMART. When the AgNDs were overgrown in an AA solution, silver atoms tended to deposit on both the top and bottom faces of the nanodisks, making them much thicker (see the TEM image in Figure 4B). Repeating the overgrowth in a HH solution resulted in the depositions of silver atoms on the sides of the AgNDs, increasing their diameters under thermodynamic control, and AgNDs with flat sharp edges were obtained (see the TEM image in Figure 4C). The results of the overgrowth experiments suggest the following: AA favors the kinetic control growth mechanism on the surface of AgNDs, while strong reducing agents such as HH support the thermodynamic control mechanism of AgND growth, PVP does not bind to the top and the bottom faces of the AgNDs but to the {100} facets located on the side of the AgNDs (see the HR-TEM image in Figure 2B), and during the synthesis of AgNDs by the SMART, the growth of the silver nanocrystals into AgNDs is controlled not only by the AA but also by the presence of BH4−. E
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Figure 7. (A) LSPR spectrum of silver nanocrystals prepared by the SMART when 4 mL of ascorbic acid (78 mM) was used and borohydride (5 mM) added in volumes of 0.06 mL (black), 0.12 mL (red), and 0.24 mL (blue). The LSPR spectrum appeared at a longer wavelength when a smaller volume of borohydride was used (0.06 mL) because of the increase in the size of the formed silver nanocrystals. The LSPR spectrum of the silver nanocrystals prepared with a large volume of borohydride was broad, because of size heterogeneity. (B) LSPR spectrum of silver nanodisks prepared with 3.5 mL ascorbic acid (78 mM) and 0.12 mL borohydride (5 mM) reductions (black), overgrown AgNDs by ascorbic acid reduction (red), and silver nanodisks after overgrowth using hydrazine hydrate (blue). The LSPR spectrum was blue-shifted upon overgrowth by ascorbic acid because of the deposition of silver atoms on the top and bottom faces of the silver nanodisks, while the red-shift in the LSPR was observed when the overgrowth was conducted by the hydrazine hydrate reduction because of the deposition of silver atoms on the sides of the nanodisks, increasing their diameter.
BH4− and AA reduction leads to the addition of silver atoms perpendicular to the twin plane. It was observed that the thickness of AgNDs was independent of AA concentration, indicating that seeds grew along the top and bottom by the AA reaction until reaching a critical thickness, and from the sides by BH4−. When the AA concentration was decreased, the rate of the growth of the seed was lowered, allowing BH4− to deposit more silver atoms on the sides before the growth was stopped and increasing the diameter of the AgNDs. The mechanism of the formation and growth of the silver nanocrystal is summarized in Figure 5. Optical Properties of AgNDs. AgNDs40 and AgNPts41,3 are two-dimensional anisotropic plasmonic nanomaterials that have strong interactions with light because of the LSPR phenomena. When these two-dimensional plasmonic nanoparticles interact with resonant light, their conduction band electrons oscillate either along the plane of the nanoparticle (inplane) or perpendicular to the plane of the nanoparticle (outof-plane).3,15,41 Consequently, AgNPts support two dipolar plasmon resonance modes.15 For example, AgNPts (100 nm width, 16 nm thickness) showed two dipolar plasmon resonance modes at 770 (strong) and 410 nm (weak) corresponding to the in-plane and out-of-plane electron oscillations, respectively.3,15 Two low-intensity LSPR peaks located at 370 and 340 nm were observed, assigned to the inplane and out-of-plane quadrupole plasmon resonance modes, respectively.3,15 The in-plane dipole plasmon resonance peak becomes less intense and shifts to a higher energy upon snipping the sharp corners of the prismatic AgNPts, but the out-of-plane dipole plasmon resonance peak is not affected.15 The ratio between the peak intensity of the in-plane and out-ofplane plasmon resonance in the truncated prism is higher than that in the case of the sharp tip one.3,15 Figure 6A shows the LSPR spectra of AgNDs with different diameters. Three LSPR spectral peaks were observed: two lowintensity peaks located at high energies (∼335 and 410 nm) with a sharp peak at a low energy. The sharp LSPR peaks appeared at 503, 535, 567, 599, 626, and 661 nm for the
AgNDs with diameters of 25.9 ± 4.8, 30.9 ± 6.5, 33.3 ± 7.7, 37.7 ± 7.1, 42.3 ± 9.4, and 46.8 ± 5.8 nm, respectively. To further study the three LSPR peaks of the AgNDs, discretedipole approximation (DDA) calculations were conducted for 30 nm AgNDs with a thickness of 8 nm (see Figure S3). The measured LSPR spectrum of the 30.9 ± 6.5 nm AgNDs agreed well the calculated LSPR spectrum of the 30 nm AgND. As in the case of AgNPts, the peak at 335 nm in the LSPR spectrum of AgNDs is assigned to the out-of-plane quadrupole plasmon resonance mode while the peak at 410 nm is for the out-ofplane dipole plasmon resonance mode. The sharp LSPR spectral peak located at a low energy is assigned to the in-plane dipole plasmon resonance mode. Because of the equal thickness of the prepared AgNDs, the LSPR spectrum peak position of the out-of-plane dipolar oscillation was located at 410 nm in all instances. A linear relationship between the LSPR peak positions of silver nanorods and their aspect ratio (the ratio between the length of the rod and the diameter) has previously been reported.42 Interestingly, a similar linear relationship was obtained between the peak position of the in-plane dipole plasmon resonance mode of the AgNDs and the ratio between their diameters and their thickness (see Figure 6B). Although this linear relationship was satisfied for the AgNDs as in silver nanorods, the LSPR peak position when the diameter and thickness of the AgNDs are equal was predicted to be 375 nm when, in reality, the LSPR peak corresponding to the out-ofplane oscillation was obtained experimentally and confirmed theoretically at 410 nm. The reason for this deviation is based on the fact that the AgNDs are effectively two-dimensional plasmonic structures and the observable plasmon modes are confined to the two-dimensional planes of the nanodisk.40 Finally, as in case of AgNPts,15 the peak height and position of the out-of-plane plasmon resonance mode at 410 nm did not show an observable change upon truncation of the corners, and increasing the diameter of the AgNDs did not affect the peak at 410 nm. Finally, unlike isotropic silver nanoparticles such as nanocubes43 and nanospheres3 that require a remarkable F
DOI: 10.1021/acs.cgd.5b00592 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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borohydride and a weak reducing agent such as L-ascorbic acid. Interestingly, the diameters of the prepared silver nanodisks were easily tuned from 26 to 47 nm, upon a 10-fold decrease in the concentration of L-ascorbic acid. An increase in the diameter of the AgNDs from 26 to 47 nm was accompanied by a red-shift in the plasmon spectrum of AgNDs from 503 to 661 nm. HR-TEM imaging of an individual silver nanodisk showed that the sides of the nanodisks have a planar twinning defect parallel to its face and the side was bounded by {111} facets mixed with {100}. The face of the nanodisk was bounded by the forbidden 1/3{422} facets. Studying the mechanism of the formation of the silver nanodisks by the SMART was crucial because in contrast to former studies that had reported slow growth of silver nanocrystals under the kinetic control required for the formation of nanodisks, a few seconds was sufficient for the completion of the SMART. For a mechanistic study of the SMART, the roles of BH4− and AA were examined separately. When the concentration of BH4− was decreased, silver nanoplates and AgNDs of larger diameters were obtained, while using higher concentrations of BH4− resulted in smaller AgNDs with broader size distributions. Experiments based on the overgrowth of the prepared AgNDs were conducted to clarify the role of the AA in the SMART. When AgNDs were allowed to overgrow by the AA reduction of silver ions, the silver atoms were found to deposit on the two faces of the AgNDs. However, when AA was replaced by a strong reducing agent such as hydrazine hydrate, the silver atoms tended to deposit on the sides of the AgNDs, increasing their diameters. Although the overgrowth of AgNDs was under kinetic control in the presence of AA and under thermodynamic control in the presence of hydrazine hydrate, the time required to complete the overgrowth reaction was found to be similar in both cases (a few seconds). The role of BH4− and AA in the SMART was understood to be in both the formation of seeds and the growth of the seeds into the final AgNDs. The mechanism of formation of AgNDs by the SMART can be summarized as follows: BH4− reduces some silver ions into atoms in solution (nucleation) that accumulate to form small clusters. The clusters are surrounded by a thin liquid-layered boundary of a few nanometers where the silver ions are simultaneously reduced by both AA and BH4− into atoms, and the atoms diffuse to the surface of the cluster. The random formation of silver atoms in the liquid-layered boundary causes frustration of the crystallization of atoms on the facets of the growing nanocrystal and results in the generation of stacking faults. Finally, the growth can be sustained by adding random hexagonal close-packed {111} layers along the [111] plane, forming the platelike seed with planar twin defects required for the formation of the AgNDs. Ultimately, slowing the rate of generation of metal atoms in solution during the growth of the anisotropic metallic nanocrystals under kinetic control is no longer required as shown by the SMART.
change in their particle size to tune their LSPR, AgNDs demonstrated a 158 nm shift in their LSPR peak position with an increase in their diameter from 26 to 47 nm. The optical properties of the plasmonic nanoparticles are sensitive to any change in the shape, size, and dielectric function of the surrounding environment, leading to their use as sensors. The optical properties of the silver nanocrystals, prepared by the SMART with 4 mL of AA and different amounts of BH4−, were examined to track the change in their shapes and size. Figure 7A shows the LSPR spectrum of silver nanocrystals with peak positions located at 574, 542, and 561 nm prepared when the volume of BH4− was 0.06, 0.12, and 0.24 mL, respectively. Because of the large size distribution and the shape heterogeneity of the nanoparticles prepared at 0.24 mL of BH4−, their LSPR spectrum was broad. The larger size of the AgNDs and silver nanoplates prepared with 0.06 mL of BH4− compared with the AgNDs prepared with 0.12 mL of BH4− caused a red-shift in their LSPR peak and increased their peak width. Another possible reason for the broadening of the LSPR spectrum of the nanoparticles, prepared with 0.06 mL of BH4−, is the presence of large prismatic structures. The prismatic shape exhibits two in-plane plasmon modes resulting from the tip-to-tip oscillations and the oscillations from the center of one edge to the opposite tip.40 Because of the relative symmetry of the AgNDs, only one in-plane plasmon mode of high degeneracy is obtained.40 Therefore, the plasmonic excitation energy was directed to one plasmon mode in the case of AgNDs, and this increases the value of the oscillator strength of the AgNDs and thereby decreases the width of the LSPR spectrum.40 The LSPR spectrum corresponding to the out-of-plane electron dipolar oscillation in the case of all the silver nanocrystals prepared at different concentrations of BH4− was located at 410 nm, suggesting similar thicknesses. Figure 7B shows the LSPR spectrum of the AgNDs, prepared by the SMART with 4 mL of AA and 0.12 mL of BH4−, and after overgrowth in an AA and HH solution. The sharp LSPR spectrum was found to shift to the blue from 532 nm for the asprepared AgNDs to 440 nm after silver atoms had been deposited by AA reduction. The LSPR peak at 410 nm for the AgNDs was also blue-shifted to 398 nm. When the overgrowth was conducted by HH reduction, the LSPR peak was redshifted to 651 nm because of the increase in the diameter of the AgNDs while the peak at 410 nm did not shift, confirming that the overgrown AgNDs have a fixed thickness and the effectively two-dimensional plasmonic structure was retained. After the overgrowth with HH, a new low-intensity LSPR peak was observed at 466 nm. This peak is due to the out-of-plane quadrupole plasmon mode of the AgNPts41 that was not observed in case of the AgNDs. The remarkable change in the optical properties of the AgNDs after overgrowth using AA occurs because the shape is no longer flat and consequently the effectively two-dimensional nature is lost40 (see the TEM image in Figure 4B).
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CONCLUSIONS A new technique was introduced to grow silver nanocrystals anisotropically on the basis of the simultaneous multiple asymmetric reduction technique (SMART). The SMART succeeded in the synthesis of silver nanodisks of different diameters but similar thicknesses, and the reaction was completed within a few seconds. In the SMART, the silver ions were reduced by a strong reducing agent such as sodium
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S Supporting Information *
This material is available free of charge via the Internet at http://pubs.acs.org/. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b00592. Statistical analysis of the diameters of AgNDs prepared by the SMART at different concentrations of L-ascorbic G
DOI: 10.1021/acs.cgd.5b00592 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
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acid (Figure S1), statistical analysis of the thickness of AgNDs with a diameter of 30.8 ± 6.5 nm prepared by the SMART (Figure S2), final concentrations of silver nitrate, polyvinylpyrrolidone (PVP; MW = 55,000), Lascorbic acid, and sodium borohydride after their mixing (Table S1), and the LSPR spectrum of 30 nm AgNDs with a thickness of 8 nm calculated by the DDA technique (Figure S3) (PDF) Movie showing prompt synthesis of AgNDs of different diameters (AVI)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Award DE-FG02 09ER46604. I thank Prof. M. A. El-Sayed for his valuable discussion.
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DOI: 10.1021/acs.cgd.5b00592 Cryst. Growth Des. XXXX, XXX, XXX−XXX
