ARTICLE pubs.acs.org/JPCC
Precursor-Driven Nucleation and Growth Kinetics of Gold Nanostars Abhitosh Kedia and Pandian Senthil Kumar* Department of Physics & Astrophysics, University of Delhi, Delhi 11007, India
bS Supporting Information ABSTRACT:
Recently, seed-mediated approach has been widely utilized for synthesizing monodisperse gold nanostars well-known for plasmonic and sensing applications. Herein, we report a single step seedless/templateless organic synthesis procedure for the complex 3D gold nanostars with a high degree of size/shape control, involving the complexation of polyvinylpyrrolidone (PVP) in polar solvent N,Ndimethylformamide (DMF) and their reduction of AuCl4 ions along with hydrochloric acid (HCl) as an effective mediator. On the basis of the kinetic optical absorption and Fourier transform infrared (FTIR) spectroscopy measurements, a convenient ligand exchange mechanism has been proposed for the first time to the best of our knowledge to understand the evolution of these complex shaped gold nanostructures. The coordination interaction among PVP and DMF as well as PVP DMF AuCl4 has been identified as the major driving factors influencing the temporal evolution of the size/shape-controlled gold nanostars.
’ INTRODUCTION The optical properties of metal nanocrystals, particularly gold, with highly branched/star/multipod morphologies are an exciting new class of unique structures with interesting physicochemical properties and enormous applications in catalysis, plasmonics, photonics, sensing, as well as biomedical engineering and building blocks for nanoscale devices.1 9 Shape control of these branched morphologies during the synthesis stage itself has become a fundamental/essential tool not only to maximize their performance but also to exploit fully their wide range of application potentials. Thus, the synthesis of branched metal nanocrystals is of great importance not only from an academic point of view but also for the development of next-generation materials with substantially enhanced performances. However, unlike the rigorous understanding of growth mechanisms along with pertinent stability of the conventional shapes (such as polyhedron,10 cube,11 rods,12 platelets,13 etc.), solution-phase synthesis of multipod/branched/star/flower-like metal nanocrystals mostly involves seed-mediated or one-pot extreme synthesis conditions as reviewed recently,2 all of which essentially proposes twinning/defect-based growth mechanisms without delving much into their long-term kinetic stability aspects. Furthermore, synthesis procedures involving aqueous r 2011 American Chemical Society
cetyl-trimethyl ammonium bromide (CTAB),7,14,15 bis-(p-sulfonatophenyl) phenylphosphine dihydrate dipotassium (BSPP),16 and common Good’s buffer, N-2-hydroxyethylpiperazine-N-2-ethanoesulphonic acid (HEPES),17 19 also incorporate anisotropic agglomeration as the major factor for the formation of complex branched gold nanostructures, thereby effectively questioning the limiting design/generation/stability of more complex shaped nanostructures with far-reaching applications. In this context, gold nanostars (also referred as nanoflowers), with an oriented attachment of sharp tips/edges, acting as nanoantennas for receiving/ transmitting light at nanometer scale, seem to have caught the attention of researchers worldwide in the past few years.20 The fabrication of this unique shape/structured Au nanostars (in comparison with similar star/branched morphologies) involves the addition of HAuCl4 in the presence of 15 nm PVP-coated Au seeds, when the concentration of PVP in DMF is significantly higher, suggesting the evolution of sharp tips on seed surfaces through successive twinning processes.21 23 Notably, the abovementioned synthesis process can also lead to the formation of same Received: July 6, 2011 Revised: November 21, 2011 Published: December 14, 2011 1679
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Figure 1. Low-magnification TEM images (scale bar = 100 nm) along with magnified image (inset, scale bar = 20 nm) for the as-prepared gold nanostars with 0.6 mM HCl addition (right) and without HCl (left). The corresponding selected area electron diffraction (SAED) pattern and Gaussian fit to the assumed core diameter depict the monodisperse nature with the addition of 0.6 mM HCl to the reaction mixture.
branched nanostructures in the absence of preformed seeds, but with significantly higher polydispersity and larger particle sizes, strongly correlating the polar character of the solvent DMF as well as the universally soluble homogeneous polymer PVP along with pH sensitive HAuCl4. Encouraged by this idea of seedless reduction, the yield, perfection of the star shape, and monodispersity are all greatly improved by employing highly effective mediator, hydrochloric acid, HCl, which would make the entire process more efficient and effortless as was previously done.11 More importantly, the robustness of this reaction and the dependence of morphology on the concentration of HCl were also investigated. Thus, the main theme of our present work involves a methodical molecular kinetic study (which has not yet been carefully looked upon) on the interaction of PVP in DMF and the corresponding metal precursors in comparison with HCl addition, which would yield crucial information in evaluating the nucleation/monodisperse growth of these intriguingly branched metal nanostructures. The knowledge on this basic interaction not only benefits the researchers on the preparation/stability of the polymer stabilized metal colloids but also on their effective applications. The unclear notion on the reduction effect of PVP24 26 with respect to the dispersed solvent subsequently leading to the formation of variety of gold nanostructures in general is also being delineated. There is an urgent need to pursue this work in terms of utilizing PVP in designing exotic morphologies of Au nanostructures, further extending the same procedure for the formation of various other metal, semiconductor, and metal oxide nanoparticles.
’ MATERIALS AND METHODS In a typical synthesis, a freshly prepared solution of 10 mM PVP (MW = 10 000, Aldrich) in DMF (Merck) is homogeneously mixed with an adequate amount of tetracholoroauric acid
(HAuCl4 3 3H2O, Aldrich) so as to make the molar ratio of PVP to metal (calculated in terms of polymer repeating unit or monomer chain length) as ∼3250, and the whole solution is continuously stirred under normal room temperature conditions. The color of the solution starts changing from pale yellow to colorless and then finally becomes blue, indicating the formation of gold nanostars. Concentrated HCl (Merck) solution (1 M) is added in aliquots to DMF and mixed well just before the addition of PVP, and the rest of the reaction conditions are the same as mentioned above. Optical absorption measurements were carried out in all of our as-prepared nanoparticle solution samples in the wavelength range of 200 1100 nm using a Thermo Scientific absorption spectrophotometer. TEM samples were prepared by drying the 5-fold centrifuged samples in ethanol at around 3500 rpm (to remove excess PVP) on carbon Formvar-coated copper grids, and the images were acquired using a FE-Technai G2 system operated at an accelerating voltage of 300 kV. Aliquots of solution samples were drop coated on the well-dried KBr pellets, and immediately their infrared measurements were carried out (to avoid the influence of moisture) over the wavenumber range of 400 4000 cm 1 using a Thermo Scientific FTIR spectrophotometer. All measurements were done at room temperature, unless otherwise specified.
’ RESULTS AND DISCUSSION The key factor in the present synthetic procedure is based on the complexing ability of the polar aprotic solvent DMF, in conjunction with the structurally similar homogeneous polymer, PVP, which additionally controls the interfacial/intermediate nanostructures facilitating a kinetically controlled growth, resulting in mesostructural organization of spike/tips even in the absence of external energy sources. 1680
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Figure 2. Low-magnification TEM images (scale bar = 100 nm) of the Au nanostructures formed in the reaction mixture (DMF+PVP+HAuCl4) quenched in the presence of excess acetone at different intervals of time, clearly illustrating the sequential growth of 3-D branched nanostructures.
Interestingly, the TEM images of the as-prepared, exceedingly reproducible, and extremely stable gold nanostars (Figure 1) reveal a complex multilayered core with a number of (non)planar single crystalline spikes radially branching out from the center, resembling prominently a flower-like structure. Remarkably, similar star-like morphologies with a high degree of size/shape control were obtained utilizing the seed-mediated growth as previously reported,21 23 which has been normalized in the present case with controlled addition of 0.6 mM HCl, as significantly evidenced from the Gaussian nature of the core diameter distributions plot. The irregular clustered bright spots obtained in the selected area electron diffraction (SAED) pattern (Figure 1) not only confirm the interlaced, nonplanar 3-D nature of the core, but also the strong Æ011æ direction oriented crystallinity of the individual spike/tips in the as-formed gold nanostars (Figure 1), thereby ruling out the intrusion of twinning or any such defects, right from the nucleation stage itself. Surprisingly, none of the other reported star/multipod/branched nanogold14 19 morphologies show such strong/long-term stable crystalline nature, evincing great interest in our present size/ shape tunable gold nanostars for their promising plasmonic/ sensing applications.1 9 Because of the presence of a high polymer to metal ratio, in situ kinetic microscopic measurements would be extremely difficult under the present experimental conditions so as to understand the nucleation and growth of the mesostructural organization necessary for the formation of these complex 3D oriented gold nanostars. Alternatively, the abundant acetone quenched reaction mixture (containing DMF, PVP, and HAuCl4) at various time intervals sequentially identifies the growth of branched nanostructures as a result of selective attachment and simultaneous ripening of dynamically nucleated anisotropic gold seed particles (as shown in Figures 2 and S1) strongly mediated by the excess crystal habit modifier, PVP, which could further be quantified from the kinetic optical and FTIR studies as discussed below. Therefore, room-temperature kinetic disproportionation of Au3+ into Au+ and subsequently to Au nanoparticles was followed by UV visible spectroscopy to understand the formation mechanism of these complex shaped gold nanostars. When the molar ratio of PVP to metal ions was kept at ∼3250 in DMF solvent, the Au3+ CTTS (charge-transfer-to-solvent) absorption band (strongly dependent on the solvent character) at 325 nm disappears in the first minute itself, clearly revealing that the absorbance value at any instant of time cannot be
Figure 3. Kinetic optical absorption spectra (normalized at 400 nm) recorded for every 4 min interval for the growth of gold nanostars (a) without HCl and (b) with 0.6 mM HCl. The spectra are manually stacked one above the other so as to clearly depict the formation and growth of transverse as well as longitudinal surface plasmon resonance peaks as indicated by the arrows.
directly correlated with the metal ion/nanoparticle concentration. Hence, the measured kinetic absorption spectra were plotted (in Figure 3) upon normalization at 400 nm, where the absorbance is mainly due to interband transitions,27 hence independent of size/ shape, facilitating direct comparison. A broad hump-like feature starts emerging from 500 nm onward right from the first minute of the reaction (Figure 3a), indicating the dynamic/instantaneous nucleation of random anisotropic/elongated gold seed particles due to the composite reduction ability of DMF PVP complex as well as the presence of a relatively high monomer to metal ratio (Figure 2a). These primary gold seed particles selectively agglomerate themselves to reduce the overall surface energy of the reaction mixture, which specifically changes the color of the solution from colorless to a purple tint during the first 40 min of the reaction (figure 2b), thereby gradually enhancing the absorption hump into two visibly prominent peaks, a small one centered around 560 nm 1681
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Figure 4. (a) Nucleation and growth curve for the formation of Au nanostars as derived from the size/shape-independent Au0 concentration at 400 nm in Figure 3. The arrow marks show the time scale for quenching the reaction as shown in Figure 2. (b) Comparison of UV visible optical absorbance spectra of as-prepared gold nanostars with (0.6 mM) and without HCl, illustrating monodispersity from the Gaussian nature of longitudinal surface plasmon resonance peaks.
and a broader one centered around 800 nm. Thereafter, suppressed smoothening of the agglomerates takes place by intraparticle ripening arbitrated by the surrounding interactively networked DMF PVP complex, signaling the stabilization of the absorption peaks at 580 and 940 nm after 70 min of the reaction time (Figure 2c), which further remains stable for more than a year when stored at room temperature. With optimal addition of HCl (up to 0.6 mM), the reaction essentially proceeds in a similar fashion, except for the delayed induction time (50 min), with peaks at around 590 and 905 nm (Figures 3b and S1), implying that both the proton and the chloride ion of HCl play a significant role. The proton is preferentially solvated by DMF,28 31 whereas the chloride ions maintain the equilibrium concentration of AuCl4 ions, thus decreasing the net nucleation rate. To separate the roles played by proton and chloride, we replace HCl with equimolar amounts of HNO3 and KCl (Figures S2 and S3). However, with HNO3, due to the absence of Cl ions, uneven nucleation of seed particles dominates, while with KCl, only weak solvation by DMF takes place due to the electronegative character of K+ ions, thereby leading to polydispersity. These observations suggest that HCl is an excellent mediator candidate for the synthesis of monodisperse Au nanostars. The temporal evolution curve (Figure 4a) plotted for the isosbestic point at 400 nm (indicating the size/shape-independent Au0 concentration) shows that the reaction follows an autocatalytic course, possibly due to rapid dynamic nucleation of anisotropic gold seed particles followed by controlled growth and stabilization. This is so because the DMF solvent molecules with their characteristic large dipole moment and the excess vinyl pyrrolidone monomers with their strong crystal habit modifying
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ability both are coordinating reagents to the sequentially formed Au nanocrystals, thereby maintaining the necessary balance between nucleation and growth, which is the key toward nanocrystals with a controlled size and shape. Thus, PVP in DMF not only promotes the nucleation of Au seed particles but also prohibits/ regulates their anisotropic aggregation. In summary, homogeneous nucleation leading to monodisperse size/shaped nanocrystals is often characterized by sufficiently high nucleation but reduced/controlled growth rate. Under pristine conditions (PVP in DMF along with HAuCl4), kinetic effects dominate throughout the reaction process, resulting in wide size distribution (see Figure 1a) of gold nanostars as was also observed from the broad longitudinal surface plasmon resonance (SPR) peak centered at around 940 nm (Figure 4b). Simple addition of 0.6 mM HCl to DMF not only standardizes the kinetics, but also intuitively blue shifts the longitudinal SPR peak to 905 nm, ensuring its Gaussian nature, indicative of the narrow size distribution, as depicted from the TEM images (Figure 1b) as well, whereas the size/shape tuning using the previous seed-mediated approach could only extend the longitudinal SPR to a maximum of 800 nm, illustrating the simplicity/ robustness of our seedless approach. Furthermore, the transverse SPR peak at around 580/590 nm is tilted in comparison with that of gold nanorods12 or any other branched/multipod14 19 nanostructures (Figure S4), visually signifying its microengineered floral core design drastically different from that of routinely faceted nanostructures, vastly relating to the dual role of PVP, both as a reductant25,26 and as a crystal-habit modifier,10,32,33 simultaneously promoting reduction/stabilization onto specific crystal faces while preventing other facets, leading to a plethora of exotic nanostructures robustly depending on the respective reaction conditions.34 Specifically, the irregular morphology of these real gold nanostar systems defies the conventional dimensionality and plasmonic hybridization criteria, which in turn imposes a challenging task for modeling/simulating their integral optical features. On the basis of the present experimental results, it is essentially concluded that 0.6 mM HCl represents an ideal concentration for the synthesis of monodisperse gold nanostars. With increasing addition of HCl, protonated DMF along with conformational changes in PVP as well as the conproportionation of AuCl4 ions fundamentally dictates the reaction mixture, thereby delaying the nucleation to a large extent, further leading to step-by-step dissociation of star like Au nanostructures (Figure S5), condensing toward the formation of spherical gold nanoparticles, strongly suggesting that excess HCl dramatically modifies the course of the reaction, steering toward a more conventional thermodynamic regime. After clearly identifying macroscopic kinetic/thermodynamic regimes, we now focus our attention exclusively on the molecular interactions responsible for addressing the morphology evolution of the as-formed nanostructures. To better understand and elucidate a plausible reaction mechanism, the following aspects have been taken into consideration: (a) polymer (PVP) and solvent (DMF/HCl) interaction (b) interaction of PVP DMF/HCl complex with HAuCl4 solution (c) kinetic study on the size/shape evolution of Au nanostars It is clearly seen from Table 1 that both PVP and DMF have similar vibrational peaks and thus have analogous FTIR spectra, which itself makes its direct data analysis more complicated.40 1682
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Table 1. PVP, DMF, PVP DMF Complex, and Baseline-Corrected PVP DMF Complex IR and Their Corresponding Peak Assignments PVP IR (refs 35 37) 3432
DMF IR (refs 38,39)
DMF PVP IR observed
DMF PVP IR after baseline correction
3498
3234, 3580
3447
OH stretch
2956
asymmetric ring (CH2) stretch asymmetric (CH3) stretch
2932
2925
2857
symmetric (CH) stretch
2877
2872 1695, 1628
2955 2875
assignments
1663
1664
symmetric (CdO) stretch
1664
1495
1495
symmetric (CN) stretch
1495
1495
ring (CH2) scissor + ring (C N)
1463
1445
1440 1413
1435 1425
1388
1383
1257
1280
1098
1086
1064
1063
662
659
1463 1423
stretch + (CH) bending
1439 1439 1411
asymmetric (CH3) bending bending (N CH3)
1388
symmetric (CH3) bending
CH bend + ring (CH2) wag + (NC) stretch
1374
ring (CH2) wag + (CN) stretch
1290 1255
asymmetric (C2N) stretch CH2 twist + NC stretch
1229 1170
weak ring CH2 twist
1071
(CN) stretch rocking (CH3)
1096 1064 934
(C C) ring breathing
846
C C ring
736
(C C) chain
651
(N CdO) bend 661
bending (CO) + asymmetric (NC) stretch
In principle, FTIR spectroscopy neither allows discrimination nor differentiation of (non)bonding carbonyl groups from PVP and DMF; therefore, other sophisticated techniques such as 13C NMR and Raman (being more sensitive to the chemical environment) were already proposed for PVP in DMF solutions under normal conditions.41,42 Moreover, by combining FTIR and UV Raman spectroscopy results, PVP interaction on the surface of size/ shape-controlled Pt and Rh nanoparticles has been well postulated, without taking into account any of the synthesis parameters, which essentially monitor the effective surface functionalization.42,43 Convinced with their subtle nature, we questioned, in fact challenged, ourselves in specifying the effective role of solvent DMF (with(out) HCl) in binding/complexing with PVP, thereby dictating the reaction with different metal ions, transforming them into size/shape-controlled metal nanostructures, through the scholarly data analysis of traditional room-temperature kinetic FTIR spectra (the measurement details of which are illustrated in Figure S6) described as follows: From a chemical thermodynamics viewpoint, for PVP in DMF solution, the carbonyl group of the lactam forms cooperative hydrogen bonding with the C H group of DMF, thus neutralizing the polar character of the polymer as well as maintaining the “apparent” basic pH of the solvent DMF. In addition, there are dipole dipole interactions between the carbonyl groups of DMF and the lactam of PVP molecules, resulting in polar solvent DMF molecules acting as “cross-linking points” between entangled inter-/intramolecular PVP chains (higher the PVP concentration, higher is the entanglement), leading to the formation of a complex solvophoboic interacting network.44 47 Controlled
Figure 5. Vibrational peak evolution with time (as indicated by the arrow mark) in the selected wavenumber ranges for the nucleation/growth of Au nanostars with (a) DMF PVP HAuCl4 and (b) DMF/HCl PVP HAuCl4 system demonstrating substantial contributions from carbonyl CdO (1664 cm 1) and nitrogen coupled with other chemical bonds at 1071 and 661 cm 1, respectively. For comparison, individual spectra for DMF/HCl, PVP, and DMF/HCl PVP are also given.
addition of HCl (0.6 mM) weakens the hydrogen bonding between PVP and DMF, thus slightly modifying their dipole dipole interactions and thereby promoting higher entangled state. From the literature survey, it has been clearly understood that DMF acts as a reactive solvent medium only under extreme 1683
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Scheme 1. Cartoon Representation Illustrating the Reaction between DMF PVP and HAuCl4 Leading to the Formation of Au Nanostars through Ligand Exchange Mechanism
conditions such as temperature, microwave/ultrasonic irradiation, etc.19,48,49 Under the present room-temperature reaction conditions, DMF largely remains as a solvent, but cohesive with PVP (higher the PVP concentration, stronger is the cohesiveness), thus impulsively quantifying the PVP DMF complexes as the major driving factor in influencing the effective formation of ensemble of different size/shaped gold nanostructures. In this regard, the predominant solvent DMF background in all of our FTIR spectra has been removed by way of baseline correction with free DMF (so as to remove the excess DMF vibrational signatures),36 which greatly simplifies our problem deemed with excellent agreement from the individual deconvoluted spectra as well (Figure S7), thereby validating the novelty of our approach. Figure 5 illustrates the free DMF baseline-corrected, sliced FTIR spectra (the full range FTIR spectra are given in Figure S6) shown in comparison with DMF and PVP spectra, demonstrating the kinetic changes in its three major constituents CdO coupled with C N and N CdO stretch, so as to discuss with clarity the chronological molecular evolution of the gold nanostructures. This free DMF baseline-corrected PVP DMF complex not only reveals meticulously the fractionation of bound (1624 cm 1) and unbound (1690 cm 1) carbonyl groups (Figure 5), but also enumerates substantial changes in the C N (1094 and 1068 cm 1) as well as OdC N (661 cm 1) stretching as shown. This is corroborated with the general fact that in PVP the carbonyl interaction dominates (as steric hindrance weakens the amine group interaction),24 while in DMF (a small molecule), the amide group interaction predominates,49 thereby distinctly identifying the significance of local PVP DMF molecular structure in governing the overall physicochemical aspects of this complex fluid. Essentially, similar arguments hold good for the 0.6 mM HCl-modified PVP DMF complex, except that the nucleophilicity of PVP decreases. When a trace amount of aqueous HAuCl4 is added to this concentrated PVP in DMF solution, an unstable metal complex such as PVP DMF+ AuCl4 forms through coordination
interaction. This is the foremost reason for the diverse reaction mechanism of other cationic metal salts39,50 52 with PVP, as compared to gold or platinum, mostly existing as anions in their salts, which in turn is responsible for the formation of uncommon nanostructures in a straightforward manner.2,53 Because of the dynamic instability of this coordinated metal complex in solution, both the (un)bound (CdO) as well as the polar (C N) groups in the pyrrolidone ring of intra-/interchain PVP initiate the donation of their lone pair of electrons to the metal centered square planar AuCl4 ion;54 thereby ligand substitution readily occurs, forming intermediate [AuCl3(PVP)] complexes, which sequentially disproportionate to yield Au0 species, as depicted by the corresponding correlated changes in the bound CdO peak (starting from 1624 cm 1 for the PVP DMF complex, shifts to 1645 cm 1 with HAuCl4 addition, and then stabilizes at 1636 cm 1 following successive ligand replacement), completely in agreement with the nucleation/growth time derived from optical absorption measurements (Figure 4). The OdC N and C N stretching peaks, sensitive to the formation of metal complexes,42 45 also shift slightly but definitely due to the entangled state overpowering the steric hindrance effect of the monomer chains confining the amine group of the PVP polymer (Figure 5). Besides, this is attributed to the fact that at very high molar polymer to metal ratios, not only the carbonyl groups but also inter-/intrachain amine groups predominantly participate in the charge transfer interaction illustrated through visible changes in the kinetic shape/intensity of carbonyl and amide groups as shown in Figure 5, which is being reported for the first time to the best of our knowledge. In the same manner, for 0.6 mM HCl addition to DMF, the above arguments hold true, except for the increase in induction time, as vindicated by the action of both proton and chloride ions. This simple but effective ligand exchange mechanism is schematically represented as shown in Scheme 1. The in situ formed small gold seeds compete with PVP molecules for DMF, inducing a slight desolvation by destroying the partial network of PVP molecules. These two synergetic effects, desolvation and coordination interaction, simultaneously influence the state of aggregation of the PVP molecules in 1684
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The Journal of Physical Chemistry C DMF,44,45 thus allowing the stabilizer polymer chemisorbed on the surface of the gold nuclei/seeds in an intricate manner, reducing their growth rate (Figure 5) and preventing the flocculation of the gold organosols by steric stabilization. By virtue, this very idea of desolvation (common to HCl addition as well) necessitates the compactness of metal polymer complexes and in turn refutes unwarranted claims of any generalized oxidation products (as shown by the absence of any new vibrational bands in Figure S6), which should otherwise be reaction centric. Specifically, this strong chemisorption due to high monomer to metal ratio ensures that the pyrrolidone rings of PVP bind in a sideon bridging fashion rather than a single coordination with the metal ion/nanoparticle surface, as envisaged from the classical coordination chemistry model50,54 for metal nanocrystals capped by surface ligands. Analogously, in the present case, PVP forms strong coordinating bonds via intra-/interchain amine-N and carbonylO donors, which bridge adjacent metal atoms on the nanoparticle surface. This is in sharp contrast with the recent SERS/DFT studies suggesting that simultaneous binding of both amine and carbonyl groups of PVP on the metal surface leads to unstable surface coverage.55 In effect, PVP polymer breathes around the metal nanoparticle, as a function of the oxidation state of surface metal atoms, concomitantly corroborating the size/shape of the asformed metal nanostructures.
’ CONCLUSIONS In summary, monodisperse size/shape-controlled complex 3D multilayered gold nanostars have been synthesized using a single step seedless/templateless synthesis procedure utilizing HCl as an effective mediator under normal conditions. TEM images reveal the prominent flower-like morphology with crisscrossed (non)planar single crystalline spike/tips branching out from the complex core, making them unique in comparison with an ensemble of reported planar nanostars. The kinetic optical absorption and FTIR measurements clearly illustrate an autocatalytic type reaction, based on which the general ligand exchange mechanism has been proposed for the first time to the best of our knowledge. This simple but effective ligand exchange mechanism further demonstrates the different PVP reduction ability of cationic and anionic metal salts. The coordination interaction among PVP in DMF along with HAuCl4 emerges as the major driving factor in influencing the temporal evolution of size/shape-controlled gold nanostructures. ’ ASSOCIATED CONTENT
bS Supporting Information. TEM images and UV vis and FTIR spectra illustrated in Figures S1 S7. This material is available free of charge via the Internet at http://pubs.acs.org. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT A.K. acknowledges CSIR for JRF. We are grateful to USIC, MTech (Nano), University of Delhi, for materials characterization facilities.
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