J. Phys. Chem. C 2007, 111, 1123-1130
1123
Shape-Controlled Syntheses of Gold Nanoprisms and Nanorods Influenced by Specific Adsorption of Halide Ions Tai Hwan Ha,* Hee-Joon Koo, and Bong Hyun Chung* BioNanotechnology Research Center, Korea Research Institute of Bioscience and Biotechnolgoy (KRIBB), Daejeon 305-806, Korea ReceiVed: October 2, 2006; In Final Form: NoVember 14, 2006
This paper describes the effect of halide ions during the seed-mediated growth of gold nanoparticles employing cetyltrimethylammonium bromide (CTAB) as a cationic surfactant system. With the addition of a small amount of iodide ion (∼20 µM) in a growth solution, the major product of the gold nanostructures formed were notably changed into triangular nanoprisms in the presence of excessive bromide ion (∼0.1 M); otherwise, in its absence, nanorods with an aspect ratio of ∼11 were the main products. The major role of the iodide ion was in retarding the overall rate of crystal growth, and the iodide adsorption appeared to repress the crystal growth along Au(111) direction, resulting in Au(111)-faced triangular nanoprisms. When the counteranions of the surfactant were replaced with chloride ions, a novel nanostructure (i.e., nanorice) was manufactured, which demonstrates the effectiveness of the adsorption of halide ions. However, this finding is quite contrasted with the work of the Sastry group (J. Nanosci. Nanotechnol. 2005, 5, 1721-1727), wherein iodide ions strongly suppress the formation of nanoprism. The distinctive results are attributed to different experimental conditions for reducing gold precursors. Nonetheless, overall these observations suggest that the specific adsorption of halide ions is an important factor for a complete control over the shape developments in the seed-mediated growth of gold crystals.
1. Introduction The variety of the optoelectronic properties of metal nanomaterials mainly originates from the diversity of their shapes and sizes in addition to their inherent electronic properties. For a decade, the main stream of research efforts has focused on novel routes for isotropic nanoparticles or on the deliberate control of a high production yield and improved homogeneity.1-3 Isotropic alloy nanoparticles composed of two or more metal ingredients were also exploited to tune their optical properties.4-7 In addition to ramifying the composition, shape-controlled (thus anisotropic) metal nanostructures have been highlighted recently, mainly due to their unusual optical properties, i.e., Raman scattering at the surface of the nanostructures or diverse apparent colors that stem from their distinct sizes and shapes.8-16 The manufacture of these nanomaterials has been a starting point for the development of innovative biosensors,17-20 optoelectronic devices,21 bioimaging,22 and biochip applications.23 Concomitantly, more advanced metal nanostructures that were composited with other inorganic nanoparticles or aligned superstructures of different nanoparticles have been attempted to tackle future challenges in nanoelectronics and in biosensor applications.24-27 In an effort to control the shape of nanocrystals, diverse synthetic routes has been explored for nanocubes,13,14 hollow spheres,20 nanorods (or nanowires),15,16 and triangular prisms9-11 from inorganic precursors of silver or gold. Their manufacturing protocols have been continually improved by elaborate controls over reaction parameters related to the production yield, homogeneity, cost, and environmental friendliness. For instance, a gold nanorod (or nanowire) can be prepared by employing conventional wet chemistry,15,16,28 photochemical,29 and elec* Corresponding authors. E-mail:
[email protected] (T.H.H.);
[email protected] (B.H.C.).
trochemical methods.30 Murphy et al. and other groups have developed a seed-mediated growth procedure to create gold nanorods in which cetyltrimethylammonium bromide (CTAB) is used as a major shape-directing organic template, with ascorbic acid used as a major reductant.15,16 Judicious control over the composition of the surfactant,16,31 the preparation of the seed solution,32 and the ratios of additives33 in the growth solution rendered the creation of the gold nanorods with diverse aspect ratios, higher production yields, and specific morphologies. However, the manufacture of gold nanomaterials with different shapes appears more of an art than a science, as batchto-batch experiment yielded the desired nanocrystal only with varying degrees of reproducibility. These previous investigations largely suggest that the reaction conditions or mechanisms for the modulation of crystal shapes are not well understood to the extent necessary for the mass production of nanocrystals with desired morphologies. Herein, in the seed-mediated crystal growth, a description of the manufacture of mainly two types of gold nanostructures such as triangular nanoprism or linear nanorod is given, along with that of spherical faceted nanoparticles as a ubiquitously present byproduct. The presence of distinct halide ions and their molar ratios in this reaction condition resulted in the formation of diverse morphologies, i.e., spheroid, triangular (or irregularly circular) nanoplates, or nanorods. In particular, a minuscule amount of iodide ion was crucial for the formation of a triangular disk (and a circular disk at higher concentration) instead of a nanorod. In spite of the minimal amount of iodide ion, the dramatic change of the nanocrystal morphology is largely attributed to the adsorption of iodide on low-indexed gold surfaces, allowing for a distinct growth rate of the present redox pair (HAuCl4-CTAB/ascorbic acid) on each facet.
10.1021/jp066454l CCC: $37.00 © 2007 American Chemical Society Published on Web 12/29/2006
1124 J. Phys. Chem. C, Vol. 111, No. 3, 2007 2. Experimental Section 2.1 Chemicals. Hydrogen tetrachloroaurate(III) trihydrate (HAuCl4‚3H2O, 99.9%), cetyltrimethylammounium bromide (CTAB, 99%), sodium borohydride (NaBH4, 99%), L-ascorbic acid (99%), potassium bromide (KBr, 99%), potassium iodide (KI, 99%), potassium cyanide (KCN, 97%), potassium thiocyanide (KSCN, 99+%), and cetyltrimethylammounium chloride (CTAC, 25 wt % solution in water) were purchased from Aldrich (Milwaukee, WI) and used as received. Potassium chloride (KCl, 99.5%) and trisodium citrate dehydrate (99%) were purchased from Junsei (Japan) and Kanto (Japan). Deionized water was used throughout during the preparations of aqueous solutions. 2.2. Methods. 2.2.1. Preparation of the Gold Seeds. A 20 mL volume of aqueous solution containing 2.5 × 10-4 M HAuCl4 and 2.5 × 10-4 M trisodium citrate was prepared in a conical flask. To this solution, 0.6 mL of ice-cold 0.1 M NaBH4 solution was quickly added with vigorous stirring. The solution became pink in color immediately after the addition of NaBH4 solution, and the solution was sustained for 2 min with vigorous stirring. The seed particles in this solution were used as seeds within 2-5 h after preparation. The average particle size measured from the transmission electron micrograph was 2-4 nm.32 2.2.2. Growth of Gold Nanocrystals from the Seed Solutions. For the growth of nanorods, 10 mL of growth solution containing 2.5 × 10-4 M HAuCl4 and 0.1 M cetyltrimethylammonium bromide (CTAB) was mixed with 0.2 mL of freshly prepared ascorbic acid solution (0.1M) in a clean test tube, which resulted in a colorless solution; the concentration of ascorbic acid is four times as large as the value usually applied in the formation of nanorod and is adopted as an optimum for the growth of nanoprisms. To this solution, 0.025 mL of the seed solution was added and gently mixed. No further stirring or agitation was done during crystal growth. Within 5-10 min, the solution color became reddish in color. For the growth of nanoprisms, the growth solutions were prepared by adding 10 mL of a mixture of a solution identical with that used to synthesize gold nanorods in addition to small concentrations of KI. To this solution, 0.2 mL of 0.1 M ascorbic acid was added, which resulted in a colorless solution. Then, 0.025 mL of seed was mixed with the solution. As a control experiment, a comparable amount of KSCN or KCN (100 µM) was added into the growth solution instead of KI. For the synthesis of nanorices, the same solution as that used to synthesize gold nanorods was prepared containing cetyltrimethylammonium chloride (CTAC) instead of CTAB. 2.3. Instrumentation. Absorption spectra of the solutions were taken with a DU800 UV/vis spectrophotometer (Beckman Coulter) in a wavelength range from 400 to 1100 nm. Transmission electron microscopy (TEM) images were acquired with a CM20 (Philips) electron microscope at 120 kV using a carboncoated copper grid. The high-resolution image samples were examined under a Tecnai F30 super-twin (FEI) transmission electron microscope at 300 kV. 3. Results and Discussion Murphy et al. developed a three-step seed-mediated growth procedure to manufacture gold nanorods with controllable aspect ratios.15,31-33 In this protocol, it was presumed that a cationic surfactant, CTAB, acts as a soft template to direct the nanostructure. The crystal growth is controlled by the interplay of many kinetic and thermodynamic parameters to yield nanocrystals with a desired morphology. For instance, employing a
Ha et al. surfactant with a longer aliphatic surfactant and smaller seed solution yielded longer nanorods. The presence of silver ions dramatically improved the yield of the formation of a short gold nanorod to an extent greater than 95%,16,34 and a further modulation of the relative ratio yielded nanoprism, hexagon, cube, and even star-shaped branched nanoparticles at a lower concentration of CTAB.33 Among the chemicals present in the seed-mediated growth of gold nanocrystals, the influences of ascorbic acid, CTAB, chloroaurate ion, and their ratios have been extensively studied.15,16,31-34 Since several reports also treated the content of halide ions (mainly chloride, bromide, and iodide ions) as a systematic variable for the shape control of metal nanostructures, an investigation on the halide ion effect during the seed-mediated growth attracted our attention. These specific adsorptions are well-documented in the field of surface chemistry and electrochemistry;35 all the halide ions are specifically adsorbed onto low-indexed gold surfaces (i.e., Au(111), Au(110), and Au(100)) except for the fluoride ion. Moreover, it is well-known in colloidal chemistry that a counterion affects the process of micellar adsorption and/or the surface morphology of cationic surfactants residing on a solid surface.36,37 Assuming that the growth from a seed crystal would be governed by halide adlayers and/or CTAB bilayer structures, a halide ion in a reaction vessel may be a more effective variable in the area of anisotropic crystal growth. Inspired by this background information, the influences of various halide ions upon the formation of anisotropic gold nanostructures were systematically investigated. In this study, only the first step of the seed-mediated growth procedure was examined, as the second and third steps are only employed for the elongation of the nanocrystals formed in the previous step. In the “original” crystal growth (as shown in Figure 1a), a gold nanorod was successfully synthesized, as predicted from the previous reports. The average aspect ratio was determined to be 11 ( 3, and a comparable number of faceted nanoparticles and a relatively small number of triangular nanoprisms were simultaneously observed. As counted from TEM images, the yield of nanorod was about 42 ( 3% in number density and the yield of nanoprism was 10 ( 3% (see the Supporting Information). As shown in the HRTEM image (Figure 1c), the observed nanorod was a twinned pentahedron rod along the 011 direction with five or more {100}/{110} faces.9,38 The power spectrum of a corresponding image shows a pattern similar to that of the previously reported results; the diffraction spots in the power spectrum indicated typical two zone axes (i.e., [11h1] and [11h0]) due to the twinning.39-41 In its UV/vis spectra, the corresponding band for the nanorod could not be definitely assigned in the present dynamic range (data not shown), but its presence could be recognized by a floating and rising baseline over ∼1000 nm. In contrast, the addition of a trace of iodide ion (∼20 µM in a final concentration) into the reaction vessel dramatically changed the shape of the gold crystal, as evidenced in UV/vis spectra and the corresponding TEM image (Figure 1b). In UV/ vis spectroscopic studies, as discussed in the later section, a new and broad band near ∼1000 nm can be assigned as the nanoprism band. In Figure 1b, a substantial number of nanoprisms with an edge length of 98 (17 nm were clearly observable with a comparable number of faceted symmetric nanoparticles (i.e., hexagons and pentagons); the production yield of nanoprism was estimated to be 45 ( 5% (see the Supporting Information). The observed faceted pentagons or hexagons all had only Au(111) faces.39,42,43 Intriguingly, a gold
Shape-Controlled Syntheses of Gold Nanoprisms
Figure 1. TEM images of manufactured gold nanostructures (a) in the absence of iodide ions and (b) in the presence of a trace amount of iodide ion (∼10 µM). The as-prepared gold nanocrystals were mildly centrifuged at ∼1000 rpm for 5 min to separate isotropic and faceted nanoparticles and were loaded on TEM grids. Also shown are highresolution TEM images of (c) a single nanorod and (d) a triangular nanoprism obtained from (a) and (b), respectively, along with the corresponding power spectra (insets). The power spectrum of a nanorod (inset of (c)) shows typical diffraction patterns of pentatwinned nanorod (two zone axes: [11h1] and [11h0]), while that of the triangular nanoprism (inset of (d)) shows hexagonal diffraction patterns from the {220} planes (single zone axis: [11h1]).
nanorod, which would have been a major product in the absence of iodide ions, was barely observable in the presence of iodide ion. The normal orientation of the observed flat nanoprism was assigned as the 111 direction, as a hexagonal pattern on the power spectrum was definitely observed, which corresponds to diffractions from {220} planes in the reciprocal space according to the basic fcc structure of metallic gold (a ) 0.4078 nm).39,42,43 To investigate the effect of iodide ion more deeply, the concentration dependence and the growth kinetics were examined. As shown in Figure 2a, upon the increase in the concentration of the iodide ions from 10 µM to 10 mM, the intensity and position of the nanoprism band monotonically decreased and blue-shifted, respectively, indicating that iodide ions over a critical concentration (∼5 µM) relatively suppress the growth of nanoprisms compared to faceted nanoparticles; at concentrations lower than ∼2 µM, the formation of nanorods was again observable. Moreover, the addition of iodide ion retarded the growth rate of all the nanoparticles, as can be seen in the following kinetic experiment (Figure 2b). In the kinetic UV/vis observations recorded at an interval of 150 s, the band at 530 nm was intensified without any movement of the position during the crystal growth. In contrast, the band of nanoprism centered initially at ∼700 nm gradually red-shifted to ∼1000 nm, indicative of the growing dimension of a single nanoprism. When the absorbance at 530 nm versus time evolution was recorded, it was also revealed that an increase in the concentration of iodide ions substantially slowed the kinetics of the faceted nanoparticles, as denoted in the inset of Figure 2b; the growth rate (at the concentration of 200 µM) estimated by the initial
J. Phys. Chem. C, Vol. 111, No. 3, 2007 1125
Figure 2. (a) UV/vis spectra demonstrating the dependence of the iodide concentration on the formation of nanoprism. (b) Kinetic UV/ vis spectra plotted at an interval of 150 s, showing the developments of the nanoprism band in the presence of iodide ion (200 µM). In the inset, the band intensities of faceted nanoparticles (at ∼530 nm) in the presence of iodide ions (200 µM and 10 mM) were plotted versus time and compared with the result in its absence. (c) UV/vis spectra showing the pH dependence of nanoprism formation in a short pH range (2.23.5). (d) TEM images showing typical morphology changes with the addition iodide ion (top images) and with pH variations (bottom images) during the formation of nanoprisms.
slope decreased into a third of the value observed in the absence of iodide ions. Furthermore, the addition of iodide ions at a high concentration altered the shape of the nanoprisms and resulted in rough edge characteristics demonstrated in Figure 2c (top images). The edge of nanoprism became somewhat blunted or irregular, and even circular nanoplates were often observed above an iodide concentration of ∼5 mM. This observation is attributed to a possible multilayered solid iodine or similarly dissolution of metallic gold into an AuI salt formation.35 Overall these observations indicate that even a small amount of iodide ion drastically suppresses the crystal growth of Au(111)-faced nanoprisms as well as faceted penta- or hexatwinned nanoparticles. Although not shown here, addition of other anions such as CN- and SCN- (∼100 µM), which are known to behave as pseudohalide ions, had no effect on the crystal growth when CTAB was employed as a main surfactant. The formation of a nanoprism in the presence of iodide ions was dependent on the solution pH and the temperature. As the pH of solution increased from ∼2.2 to ∼3.5, the nanoprism band became intensified with its position red-shifted (Figure 2d), indicating a facile formation of a nanoprism at a higher pH condition. Unlikely with the concentrated iodide ion case, a larger nanoprism without a notable variation of its shape was synthesized, as shown in the lower images of Figure 2c. However, further increase in the pH to higher than ∼4.0 gradually exacerbated the reproducibility of the crystal growth and even totally hindered nanoprism formation above pH ∼4.5. The feasible formation of a nanoprism with an addition of a small amount of NaOH has been observed in a previous report by Mirkin et al.,9 but the growth mechanism was not presented in their investigation. Presumably, the facilitated formation of a nanoprism at higher pH values should be understood on the grounds that the reduction power of ascorbic acid can be strengthened due to the elimination of protons produced by the reduction.44 In other words, the chemical potential of ascorbic
1126 J. Phys. Chem. C, Vol. 111, No. 3, 2007 acid is maintained at a higher state with the addition of NaOH. However, the suppressing effect upon the further pH increase cannot be easily understood at this stage, though a plausible scenario is presented in a later section (vide infra). Meanwhile, as the reaction temperature increases to 30 °C, spherical nanoparticles became dominant (data not shown). In contrast, the band of nanoprisms became intensified and redshifted as the temperature was lowered to 10 °C, although with some precipitation of CTAB during or before the crystal growth. This observation indicates that a lower temperature is advantageous for the formation of nanoprisms, albeit with a substantial broadening of its size distribution. Thus, a water bath maintained at 15 °C was methodically used during the initial growth stage (approximately for ∼1 h) and, subsequently, the reaction solution kept at an ambient temperature (20-25 °C) for further analyses. Recently, the suppression of crystal growth by iodide ions was observed in the reduction of gold precursors by citric acid and by the leaf extract of lemongrass.45,46 In these investigations, the addition of iodide ions yielded a larger number of spherical nanoparticles while drastically decreasing the proportion of nanoprisms. It also distorted the shapes of nanoprisms into circular nanodisks (with fairly irregular edges). These observations were in part repeated in the present experiment, as seen in the upper images of Figure 2c; the proportion of spherical particles increased and the rate of crystal formation was drastically slowed down upon an increase in the concentration of iodide ions. In these studies, it was concluded that bromide ions in the leaf extract of lemongrass had promoted the formation of nanoprisms; presumably, oligomeric sugar groups present in the leaf extract appear to reduce the gold ions. On the other hand, chloride ions in the reduction via the Turkevich approach much stimulated the formation of nanoprisms. At any rate, the addition of iodide ions strongly suppressed the formation of nanoprisms in both conditions. In contrast, for the growth in this study, bromide ions promoted the formation of nanorods as a major product while a minuscule of iodide ion dramatically promoted the formation of nanoprisms. These controversial observations and subsequently distinct conjectures for the growth mechanism should be attributed to experimental differences on several points from the present reaction parameters. First, the seeds both in the Turkevich approach and in the biogenic reduction are formed intact during the reduction process, where seed formation is not virtually separated from a growth stage. However, in this study, the seed was first reduced with NaBH4 and added to a separated growth solution. Second, in the present work (employing CTAB), the reduction reaction on a gold surface is quite different from that either in the Turkevich method or in the method using the leaf extract. The reducing agent (ascorbic acid vs citric acid or sugar) as well as the oxidation state of gold ion (Au(I) vs Au(III)) might be a factor yielding the contradicting observations. As will be mentioned later, a preferred face in a specific surface reaction is strongly dependent on a reaction condition. Finally, the reaction time for the Turkevich method was typically ∼24 h (the time being ∼12 h for the biogenic reduction), while ascorbic acid in the seed-mediated reaction accomplished its reduction within ∼1 h. In addition, the formation of nanorods was hardly observable in the reductions using citric acid or the leaf extract of lemongrass, suggesting quite different reaction mechanisms and subsequently different results. On these grounds, a picture should be avoided that Au(111) is the most hindered face at any gold reduction in the presence of iodide ions; it would rather be dependent on a specific reaction of
Ha et al. interest. As such, a plausible rationale is given for the present observation of mainly two different crystal growths under the current reaction condition, thereby providing a more systematic picture for the growth of gold nanocrystals. At this stage, the gold reduction by iodide ions (3I- f I3+ 2e-) is excluded since no noticeable change in color was observed before the addition of the seed solution as noted in the Supporting Information; red pink from gold nanocrystals or deep purple from iodine is expected from the iodide oxidation. It is also noteworthy that chloride ions of AuCl4- appear to be replaced by bromide ions excessively present in a CTAB solution (0.1 M). Similarly, although the ligand exchange was also observed in the iodide concentration higher than 0.2 mM, the growth solution demonstrated very similar spectral features upon the addition of ascorbic acid; the solution turned transparent and exhibited no further color changes at least for 24 h as observed in the growth solution without iodide ions. The ligand exchange appears not to play a major role in the nanoprism formation as the nanocrystals were successfully formed in a very low concentration of iodide ions (10-200 µM), in which range minimal spectral changes (i.e., the ligand exchanges) were observed (see the Supporting Informtion). Upon the addition of a small amount of iodide ion, it is less plausible to assume that the microstructures of the surfactant in a bulk solution will change, since the added chemical was of negligible amount compared to the concentrations of CTAB and HAuCl4. In general, the cationic surfactant forms a globular micelle structure in a concentration over its critical micelle concentration (cmc, ∼1 mM for CTAB).36,37 As the concentration of the surfactant increases in aqueous solution, the microstructure experiences several phase transitions, becoming, for instance, hexagonal, cubic, or lamellar structures depending on the aliphatic chain length, cationic head group, salt concentrations, and/or added chemicals.47-49 Considering the working concentration of the present system (0.10 M), spherical or short rod-shaped micelle structures are dominant and appear to be invariant with the addition of a minute amount of iodide ion.48,49 Additionally, it is notable that the reactive gold complexes (Au(III) or Au(I)) are in a tightly bounded form with the CTAB micelle as revealed in a previous investigation.50 Regardless of the bulk structure of CTAB, the bilayer formation (or micellar adsorption) on growing gold nanostructures is an additional possible explanation for the morphology change, as the influx of chemicals into the gold surface appears to be more tightly controlled through the bilayer structure.51 This picture can be further enforced by the observation that, irrespective of the presence of iodide ions, neither a well-defined nanostructure was found nor were once-formed gold crystals of random shapes stabilized in the absence of CTAB. It has been reported that the cationic surfactant (CTAB) was adsorbed onto negatively charged glass or mica surfaces, wherein the surface morphology of CTAB deviated from spherical micelle to rodlike (or wormlike tubular) micelles or planar bilayer structures.36,51,52 Their adsorption kinetics and surface morphologies were dependent on the bulk concentration, the ionic strength, the counterion, and the chain length of a surfactant in question. At present, it is noteworthy that the adsorption of CTAB onto a gold surface was mediated by the precedent specific adsorption of halide ions, rendering the surface to bear net negative charges. In other words, under the CTAB template, an adsorption of bromide ions is followed by a bilayer formation on gold nanostructures; the adsorption of a CTA+ ion onto gold surface as expressed in a small number of papers appears to describe this picture implicitly.12,33 A series of adsorption
Shape-Controlled Syntheses of Gold Nanoprisms experiments of CTAB with different counterions on a silicon surface showed that a bromide ion has 5-fold greater binding tendency for a CTA+ cation than a chloride ion on the micelle surface.36,37 Simultaneously, a lower cmc in bulk solutions and less curved micelle formation were facilitated in CTAB compared to cetyltrimethylammonium chloride (CTAC); this observation was attributed to the fact that the bromide ion is much more effective in screening accumulated charges on the micelle surface.36,37,47 In these studies, an iodide ion was reported to bind even stronger with the cationic micelle surface compared to a bromide ion.36,47 On these grounds, in the presence of an iodide ion, a nanoprism seems to be favored over the formation of a nanorod, which requires more of a curved surface geometry. However, the stabilized bilayer formation appeared to make rather a limited contribution to the morphology change, as the concentration of the iodide ions was ∼5000 times lower than that of bromide ion (10 µM vs 0.1 M) and the iodide ions were equally distributed not only to the adsorbed micelle (or bilayer) structure on the gold nanostructures but to globular micelles present in the bulk solution. Finally considered is whether the specific adsorption layer underlying the CTAB bilayer structures was indeed affected by the addition of iodide ions. The strong adsorption of halide ions on low-indexed gold surfaces has been reported by a myriad of studies.35 Many electrochemical investigations combined with STM or surface X-ray scattering have revealed that the adsorption power on an Au(111) surface is in the order of iodide, bromide, and chloride ions.35,53,54 For instance, the Gibbs free energy change during the specific adsorption of iodide ions on an Au(111) surface was estimated to be much larger than that of bromide ion by 30-70 kJ/mol, yet the precise values were dependent on the surface potentials and on specific crystallinity.35,54 As such, the added iodide ions appear to displace bromide ions preadsorbed on the low-indexed facets of gold seeds or as-grown gold crystals. The interaction of iodide ions with metal atoms was unique compared to chloride ions in that metal-halide bonding is dominated by covalent characteristics, as evidenced in a SERS investigation showing potentialindependent vibrational frequencies of the metal-halide bond.53,55 This is further manifested in potential excursion experiments demonstrating the onset of iodide ion adsorption at extremely negative voltages (e.g., -1.0 V vs SCE reference). Although the stronger adsorption tendency of iodide ions over other halide ions has been firmly supported through various experimental evidence, it has remained elusive which facet is more reactive toward a gold reduction by ascorbic acid in the presence of these halide ions. If one infers from facets observed in the nanoprism formation, Au(111) faces associated with an iodide adlayer inhibit gold reduction more severely than other facets (i.e., Au(110) or Au(100)); in the presence of a bromide adlayer, by contrast, Au(100) and/or Au(110) facets are less reactive to the same reaction. According to investigations of several surface reactions, distinct surface activities for different facets have been reported. For instance, it was demonstrated that an Au(100) face in an alkaline solution had the highest activity in terms of an oxygen/hydrogen peroxide reduction compared to other low-indexed surfaces, by nearly 4 orders of magnitude.56 Conversely, the Au(111) face was more effective than Au(100) or Au(110) faces for the reduction of hexaaminecobalt ions.57,58 In spite of these examples, it is thus far impracticable to surmise which facet is less preferred during the growth of a gold crystal, as the detailed adlayer structure and/or surface reconstruction by the adlayer are largely unknown;59 a preferred facet in a reduction seems to vary reaction
J. Phys. Chem. C, Vol. 111, No. 3, 2007 1127
Figure 3. UV/vis spectra and corresponding typical TEM images of diverse gold nanostructures formed under the CTAC template (a) with dilute concentrations of bromide ions (1 mM), and (c) in the presence of iodide ions (50-100 µM) instead of bromide ions.
by reaction. Nevertheless, it seems quite plausible that the exclusive observation of Au(111) as a planar face in the nanoprism is attributed to its markedly reduced reactivity caused by iodide adlayer compared to other facets. However, this conjecture should be confined in the present reaction species (i.e., ascorbic acid and Au(I)-CTA+ system) since an adverse trend was observed in the gold reduction by citric acid without a noticeable surfactant; the gold nanoprism is one of the preferred nanostructures with the addition of chloride ions.46 To fortify the plausible scenario for the effect of halide ions, further control experiments were carried out by monitoring the crystal shapes formed under various reaction constraints. As bromide ions remain a stronger adsorbate on low-indexed gold surfaces compared to chloride ions,35 a new template system employing CTAC appeared attractive as an alternative directing agent; the length of aliphatic chain is identical for CTAC but has a different counterion (i.e., chloride ion). This control system was expected to be fruitful for outlining the overall role of halide ions on crystal growth under a cationic surfactant system and was expected to provide additional evidence, albeit circumstantial, for the mechanistic picture described above. Currently, CTAC is assumed to have a micelle structure nearly identical with that of CTAB due to the very similar chemical structures of these two surfactants.36,37 As shown in Figure 3a, only isotropic and nanorice-shaped nanoparticles, instead of nanorods or nanoprisms, were exclu-
1128 J. Phys. Chem. C, Vol. 111, No. 3, 2007 sively observed under the CTAC template. Two corresponding bands at ∼540 nm and ∼700 nm in the UV/vis spectra were observed, respectively (Figure 3a). Upon the further increase in the concentration of bromide ions up to ∼1 mM (with the addition of KBr), the nanorice band at ∼700 nm become more intensified, as seen in both the UV/vis spectra and TEM image (Figure 3a and the inset). Although the nanorice seemingly resembles bipyramidal structures formed recently in the presence of silver ions, the band in fact was an ensemble of diverse nanostructures. Intriguingly, the nanorice band abruptly diminished on the further increase in the bromide concentration (Figure 3b); short nanorods were instead manifested at higher concentrations over ∼5 mM. At even higher concentrations over ∼20 mM of bromide ions, the formation of nanorods was again pronounced with a similar aspect ratio; concomitantly, some nanoprisms were observable as in the CTAB system without iodide ion. Over a concentration of ∼100 mM, the nanorod formation was nearly identical with the pure CTAB system in terms of the aspect ratio and relative yield (see the TEM image of Figure 3b). The manufacture of nanoparticles with few anisotropic characteristics in the presence of chloride ions as a major counterion is largely consistent with the picture mentioned above; the facet-specific crystal growth seems less important under a chloride adlayer considering the lower adsorption tendency of chloride ion onto gold faces. The improved production of the nanorice band (the band of ∼700 nm in Figure 3a) in the presence of bromide ions (∼1 mM) appears to have been caused by a cooperative adlayer formation of both halide ions, but the detailed mechanism of the morphology development is left for a future study. Moreover, through these observations, it becomes clear that bromide ions are essential for a nanorod formation under the CTA+-halide ion surfactant system. At a glance, this observation appears to be contradicted with a recent study reporting the formation of shorter nanorods, instead of the nanorice particles, with the addition of CTAC.50 However, recalling that CTAB (8 mM) was maintained in the growth solution as an invariant even in the addition of CTAC (up to 4 mM), the report appears not to be inconsonant with the present result but to be rather revived here. As a small amount of iodide ion (50 µM) was added to the reaction vessel containing CTAC, nanoprisms were again observable with a complete absence of nanorod. Judging from the corresponding UV/vis spectra, the yield of nanoprisms at a concentration of ∼100 µM was reasonably comparable with the value obtainable in the CTAB template with a trace of iodide ion but had a relatively broader size distribution (Figure 3c). As the iodide concentration increased to over ∼200 µM, the nanoprism band reduced in intensity and blue-shifted as observed in the CTAB template. Considering the relatively higher concentration of iodide ions for the first appearance of the nanoprism compared to the CTAB system (50 µM vs 5 µM), it is implicated that bromide ions also played some role in the nanoprism formation with the addition of iodide ion under the CTAB template. Overall, these observations stress (1) that a trace amount of iodide ion is essential in the formation of nanoprisms regardless of the concentration of bromide or chloride ions in the solution, (2) that chloride ions solely or in combination with bromide ions under a critical concentration (∼1 mM), conversely, stimulate the formation of nanoparticles of irregular shapes or ill-defined nanorice crystals, and (3) that bromide ions over a set concentration (∼20 mM) facilitated the formation of nanorods.
Ha et al.
Figure 4. (a) Schematic illustration showing the shape-controlled syntheses of Au nanoprisms and nanorods according to the presence of iodide ions under a CTAB template. In the presence of iodide ions (10 µM-10 mM), faceted nanoparticles (separately designated in TEM images) were simultaneously observed with triangular nanoprisms. In the absence of iodide ion, nanorods along with spherical nanoparticles and nanoprisms were dominantly observed. (b) Further schematic cartoon describing the influence of the CTAB template, in conjunction with an iodide adlayer as a passive barrier for crystal growth during the formation of nanoprisms. The white arrows indicate the rate of growth on specific facets.
Under the CTAB template in the absence of iodide ions, the dominant observation of nanorods can be rationalized by the assumption that the bromide adlayer causes both the Au(110) and Au(100) facets to be more hindered than Au(111) facets. Nonetheless, the overall growth rates on the bromide adlayer were much faster than those in the presence of iodide ions, and the apparent preference for a specific facet seems less conspicuous, as both nanostructures (i.e., nanorods and nanoprisms) were simultaneously found along with isotropic nanocrystals. This rationale is fairly understandable with the adsorption tendency of the bromide ion inferior to that of the iodide ion, and this situation is summarized in Figure 4a. In addition, the faceted isotropic nanoparticles considered as a major byproduct in the nanoprism formation (featured at the ∼530 nm band in the UV/vis spectra) also provide some insight into the crystal growth. Although isotropic nanoparticles acquired in the presence of iodide ions were only slightly distinguishable from those obtained in the absence in the UV/ vis spectra, the surface morphologies were slightly different in the HRTEM image, as demonstrated in Figure 4a. The ridges between the 5-fold twinned gold domains in the presence of iodide ions were sharper than those observed in nanocrystals obtained without iodide ions (see the TEM images in Figure 4a). This observation suggests that the crystal growth was more tightly controlled depending on the specific crystal facet exposed. It is also noteworthy that these faceted nanocrystals appear to be species “isomorphous” with a nanoprism, as the
Shape-Controlled Syntheses of Gold Nanoprisms external faces of a typical isotropic nanoparticle are mostly bounded with Au(111) faces (data not shown); a single nanoprism has only two Au(111) facets (see the TEM images in Figure 4a). This isomorphic behavior suggests that the faceted nanoparticles can scarcely be excluded during the growth stage, which is evidenced in the various control experiments described above. When the reduction potential of gold precursors increased by means of pH or temperature elevations, these faceted particles were dominant products compared to triangular nanoprisms, indicating that these isotropic particles are kinetically the more favored species.60 Intriguingly, even upon the suppression of crystal growth through the increase in the concentration of iodide ions (as demonstrated in Figure 2a), the faceted isotropic nanoparticles were again the dominant products compared to the nanoprisms. Overall these observations indicate that the formation of nanoprism in the presence of iodide ion appears to occur in a narrow window of reaction parameters (vide supra). Thus far, the influence of halide ions on the syntheses of gold nanocrystals has been demonstrated through the production of diverse crystal shapes, while the chemical backbone of a cationic surfactant was maintained as an invariant. On the basis of the morphology deviations under several combinations of halide ions, it appears that the specific adsorption of halide ions exerts major exquisite control over the shape evolutions by allowing distinct growth rates dependent on specific gold faces under the current seed-mediated redox pair (Au(I)/ascorbic acid). Alternatively, the CTAB template that was assumed to be a major geometrical constraint, in fact, made a limited contribution to the crystal growth as an agent for the passive protection of the as-grown crystals. The limited role of the surfactant was already suggested in an investigation performed by the Pileni group upon the synthesis of copper nanocrystals; in their study, AOT was used as a soft template and the effects of diverse anions, with the exception of the iodide ion, were investigated.61,62 The authors claimed that a specific adsorption of anions dominated the shape evolution; however, their assertion has been mostly overlooked in the seed-mediated production of gold nanocrystals. Hence, the drastic influence of iodide ions on the formation of nanoprisms may support the crucial role of halide adsorption on the formation of gold nanocrystals. In Figure 4b, the growth of gold nanocrystals under the CTAB template system was schematically illustrated. In the presence of iodide ions, a stable CTAB bilayer on an iodide adlayer preferentially formed due to its overwhelming adsorption tendency compared to other halide ions and controls the supply of added chemicals into gold surface passively. However, at the edge faces of nanoprisms (i.e., Au(110) faces), more curved spherical micelles should be formed as a result of geometric factors, which relatively expedites the influx of the chemicals in the edge directions. In fact, a theoretical consideration revealed that mean free passage times for ions are shortest at the curved surfaces.50 Associated with the differentiated reactivity of Au(110) face with an iodide adlayer, the crystal growth rate in that direction seems to be more facilitated. Nonetheless, the dramatic morphology change from nanorod to nanoprism can be attributed to the iodide adlayer due to the relatively low concentration of iodide ions compared to excessive bromide ions. 3. Conclusions Three distinct gold nanostructures of a nanoprism, nanorod, and nanorice were manufactured by solely controlling the content of halide ions. A miniscule amount of iodide ion in the presence of an excessive amount of other halide ions strongly
J. Phys. Chem. C, Vol. 111, No. 3, 2007 1129 suppressed the overall crystal growth through specific adsorption and stimulated the formation of nanoprisms as a major product. Conversely, bromide ion larger than ∼20 mM preferred to form nanorods with a minor fraction of nanoprisms, while chloride ions solely or in combination with small amount of bromide ion yielded gold nanorice (or distorted bipyramidal nanostructures). With the addition of iodide ions, the Au(111) face was more severely hindered compared to other faces (i.e., Au(110) and Au(100)); as a result, all of the flat faces of the triangular nanoprisms were bounded in that direction. In all cases, faceted isotropic nanoparticles were cosynthesized as a byproduct that could not be excluded under the present synthetic parameters. The most striking feature of this study is that iodide ions at an impurity level dramatically changed the morphologies of gold nanocrystals formed under the same cationic surfactant system (from nanorods to nanoprisms). This study not only elaborates the recent controversial contention that substantial nanoprisms are synthesized under a reaction condition designed to produce nanorods but also provides a deeper insight into the seeded crystal growth.9 However, with recall of a recent report that chloride ions promote the formation of nanoprism while iodide ions strongly suppressing it in the gold reduction with citric acid, it seems obvious that the hindered face with the addition of iodide ion (i.e., Au(111)) is not applicable to all other gold reductions; the determination of a specific face preferred during the growth of nanostructures appears to be governed by the physicochemical nature of the seed preparation, selection of reducing materials, and the presence of relevant surfactant. Additionally, it is also obligatory to understand and manipulate the crystallinity, twinning, surface charges, and size of the seed used, to realize an “on-demand” shape control in the seedmediated growth. Acknowledgment. This work has been supported by grants from the Protein Chip Technology Program (MOST, Korea) and KRIBB Initiative Research Program (KRIBB, Korea). We are grateful to Y. C. Park (NNFC, Korea) for technical comments on the TEM study. Supporting Information Available: Experimental details including TEM figures. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Wilcoxon, J. P.; Provencio, P. P. J. Am. Chem. Soc. 2004, 126, 6402-6408. (2) Kim, P. P.; Oh, S.; Crooks, R. M. Chem. Mater. 2004, 16, 167172. (3) Pastoriza-Santos, I.; Liz-Marzan, L. M. Langmuir 2002, 18, 28882894. (4) Lee, I.; Han, S. W.; Kim, K. Chem. Commun. 2001, 1782-1783. (5) Zhang, J.; Worley, J.; De´nomme´e, S.; Kingston, C.; Jakubek, Z. J.; Deslandes, Y.; Post, M.; Simard, B. J. Phys. Chem. B 2003, 107, 69206923. (6) Mallin, M. P.; Murphy, C. J. Nano Lett. 2002, 2, 1235-1237. (7) Freeman, R. G.; Hommer, M. B.; Grabar, K. C.; Jackson, M. A.; Natan, M. J. J. Phys. Chem. 1996, 100, 718-724. (8) Rex, M.; Hernandez, F. E.; Campiglia, A. D. Anal. Chem. 2006, 78, 445-451. (9) Millstone, J. E.; Park, S.; Shuford, K. L.; Qin, L.; Schatz, G. C.; Mirkin, C. A. J. Am. Chem. Soc. 2005, 5312-5313. (10) Metraux, G. S.; Mirkin, C. A. AdV. Mater. 2005, 17, 412-415. (11) Jin, R.; Cao, Y. C.; Hao, E.; Metraux, G. S.; Schatz, G. C.; Mirkin, C. A. Nature 2003, 425, 487-490. (12) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J.; Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857-13870. (13) Yu, D.; Yam, V. W. -W. J. Am. Chem. Soc. 2004, 126, 1320013201. (14) Sun, Y; Xia, Y. Science 2002, 298, 2176-2179.
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