Article pubs.acs.org/JPCC
Serial Morphological Transformations of Au Nanocrystals via PostSynthetic Galvanic Dissolution and Recursive Growth Chih-Wen Yang,†,∥ Shih-Cheng Hsu,†,§,∥ Mei-Ying Chung,† Mei-Chun Tseng,† Te-Wei Chiu,§ and Chun-Hong Kuo*,† †
Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, Taipei 10608, Taiwan
§
S Supporting Information *
ABSTRACT: Geometric modification of Au nanostructures is typically achieved in multistep reactions, where synthesis parameters need to be well-controlled. In this work, we report a facile method using IrCl3 to refine morphologically diverse Au nanostructures and trigger their morphological transformations. The synthesis is accomplished at room temperature by an iterative process of galvanic dissolution and recursive growth. Seeds retrieved after the dissolution of different Au nanostructure archetypes served in the structural recovery and morphological transformation via rapid and slow regrowth, respectively. The rapid regrowth was accomplished by adding ascorbic acid (AA), while the slow regrowth occurred spontaneously. In the structural recovery, the nanostructures regrew back to their original morphologies. Improvements in the shape quality and size distributions were observed for the rapid regrowth case. In the spontaneous slow regrowth transformation, the resulting nanostructures were encased by {111} facets, minimizing total surface energy through the more closely packed planes. Transformation of the four nanostructure archetypes showed correlation, trending toward these lower indexed facets and to twinned structures (from RDs to OCTs, OCTs to TPs, and TPs to PSs). Surveying all observations, our work of the metal cation-mediated geometric modulation of Au nanostructures delivers important clues in understanding nanoparticle synthesis and provides a new path for the fabrication of nanocrystals with high-quality size and shape distribution.
■
Typically, the reductions of AuCl4− to AuCl2−, and AuCl2− to Au(0) are both electropositive reactions (E0 = 0.926 and 1.154 V vs. NHE) and the disproportionation of AuCl2− should be unfavorable. However, the existence of CTAB drastically alters the stable equilibrium position and enables gold nanoparticles to be quantitatively dissolved by the gold salt, which is attributed to the binding of AuCl4− and CTAB micelles. This makes the ellipticities of anisotropic Au nanoparticles decrease. In the same principle, O’Brien et al. further discovered that the changed ellipticities of retrieved Au nanoseeds led to the formation of isomeric Au morphologies.25 In their work, single-crystalline Au nanorods were quantitatively dissolved to generate pseudospherical nanoseeds via iterative dissolution and regrowth. The ellipticities of pseudospherical nanoseeds change with varying the amount of Au(3+) salt. When these nanoseeds were utilized, various platonic nanocrystals formed in extremely high uniformity through rapid reduction by adding ascorbic acid (AA). Park et al. found that geometric information on the original nanostructures was conserved and retrievable.26 They
INTRODUCTION A wide range of applications for metallic nanocrystals are dependent on size and shape effects. This reflects the importance of their structural geometries.1−5 Fine-tailoring of metallic nanostructure brings improvements to their reactivity, polarity, scattering, magnetism, and biological activity.6−13 Geometric transformation of metallic nanostructures can be achieved via rational design in the growth kinetics, and synthesis strategies have been proposed based on the theory.14−21 In these strategies, evolved structures are obtained one by one via individual operations, which usually take several steps and undesirable time frames. Therefore, it is important to develop facile strategies for the direct modification of metallic nanostructures postsynthesis, where changing sizes, surface facets, and other transformations (polycrystallinity, anisotropy) after reactions can be used for sensing and catalysis.22,23 Because the initial structures of nanosized seeds are believed to be the geometric origins of resulting nanocrystals, engineering by structural tuning postsynthesis is promising if the nanoseeds of postsynthesized nanocrystals are retrievable and tailorable. ́ Rodriguez-Ferná ndez et al. first demonstrated this possibility via spatially directed oxidation when they found tips of Au nanorods were selectively etched by Au3+−CTAB complexes.24 © XXXX American Chemical Society
Received: October 21, 2015 Revised: December 3, 2015
A
DOI: 10.1021/acs.jpcc.5b10305 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 1. SEM images of (a−d) original, (e−h) rapidly regrown, and (i−l) slowly regrown Au nanostructures. The recovery of nanostructures is favorable after rapid reductive growth adding ascorbic acid; whereas the transformation is favorable after slow reductive growth without ascorbic acid. All the resulting morphologies after slow regrowth form with their surfaces encased by {111} facets. The transformation shows a serial in which the nanostructures are converted from RDs to OCTs, TPs, and PSs. The scale bars in all inset panels are 50 nm.
recording the shift of Au surface plasmon resonance (SPR) absorption peaks and their structural evolution. Isomeric morphologies of Au nanostructures encased by {111} facets, including octahedra, triangular nanoplates, and polyspheres, were eventually obtained after the slow regrowth. Importantly, the slow transformation of Au nanostructures proceeded from RDs to OCTs, OCTs to TPs, and TPs to PSs. The PSs were not changed to any other morphology. The evolution in the nanostructures moves toward minimizing the total surface energy according to the order of γ{111} < γ{100} < γ{110}. Other factors controlling the structural recovery and transformation were also discussed. The capping by CTAC was found critical to achieve the structural recovery. Both the Ir3+ and Cl− ions assisted further structural transformation when higher amounts of them were used. For example, the RDs could be converted to TPs or PSs directly. To our best knowledge, we are the first to look into the transformation between multiple Au nanostructures and explore their correlation. Our findings deliver critical clues to the field of noble metal nanofabrication that inspires a continuing extensive investigation.
noted that rounded nanodisks were obtained after iterative dissolution of the triangular nanoplates and these were finally transformed to hexagonal shapes after adding AA. The regrowth was anisotropic along the plate edges. In addition to using an Au(3+) salt, ferric ions, cupric ions, oxygen, and different acids all brought about the same results.25,27−30 Although the oxidation and reductive growth by AA over the Au nanospheres, nanorods, and nanoplates were studied, the literature lacks a way to investigate the proceedings of spontaneous regrowth after dissolution. In this work, we present a facile strategy using IrCl3 to trigger the process of dissolution and regrowth in the aqueous solutions of Au nanostructures. Rhombic dodecahedra (RDs), octahedra (OCTs), triangular plates (TPs), and polycrystalline spheres (PSs) were used as the nanostructure archetypes. All were prewashed and redispersed in 0.1 M CTAC aqueous solutions to exclude differences in their reaction environments from having an effect. Their corresponding nanoseeds in pseudospherical/Wulff shapes were retrieved after dissolution for a short while at room temperature followed by concomitant rapid or slow regrowth. The rapid regrowth was carried out by adding AA. Indeed, structural recovery occurred, demonstrating reversibility in the dissolution of Au nanostructures. The slow regrowth was performed by leaving the reaction solutions undisturbed at room temperature without the addition of AA. This induced spontaneous transformation in the nanostructures. The spontaneous process was studied by
■
EXPERIMENTAL SECTION
Materials. Hydrogen tetrachloroaurate trihydrate (HAuCl4· 3H2O, 99.99%, Alfa Aesar), ascorbic acid (C6H8O6, AA, 99.7%, Sigma-Aldrich), iridium chloride hydrate (IrCl3·xH2O, Ir content 53%, UniRegion biotech), sodium borohydride (NaBH4, 98%, B
DOI: 10.1021/acs.jpcc.5b10305 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 2. UV−vis spectra and size-distribution histograms of Au archetype and their recovered nanostructures after rapid regrowth.
for the addition of AA. The samples after adding 0.3 mL of 0.01 M IrCl3 solution were kept undisturbed at room temperature for 6 h. The resulting products were collected by centrifuging. Sample Preparations. To prepare samples for characterization, washing and concentrating by centrifugation had to be carried out. Typically, all products were collected at 7000 rpm. for 15 min, followed by removing the supernatant. After adding fresh DI water to reach the original volume, centrifuging was done again to rinse the dispersions. The washing step was repeated twice. Finally, the collected products were diluted to 1 mL and stored in a 1.5-mL centrifuge tube. To prepare samples for scanning electron microscopy (SEM) and X-ray diffractometry, 2−5 μL of concentrated sample solutions was dropped onto silicon wafers of 0.3 × 0.3 cm2 and slowly dried at room temperature. Characterization. SEM images were recorded by Zeiss Ultra Plus operated at accelerating voltage of 10 keV. Powder X-ray diffraction was implemented using Bruker D8 Advance operated at 40 V and 40 mA. UV−Vis absorption spectra were taken on a Hitachi U-3310 spectrophotometer.
Aldrich), sodium citrate dihydrate (HOC(COONa) (CH2COONa)2·2H2O, SC, 99%, J. T. Baker), potassium bromide (KBr, 99%, Sigma-Aldrich), sodium chloride (NaCl, 99%, Sigma-Aldrich), silver nitrate (AgNO3, 99%, SigmaAldrich), hexadecyltrimethylammonium bromide (CH3(CH2)15N(Br) (CH3)3, CTAB, 98%, Sigma-Aldrich), hexadecyltrimethylammonium chloride (CH3(CH2)15N(Cl) (CH3)3, CTAC, 95%, TCI), and potassium iodide (KI, 99%, Sigma-Aldrich) were used as received. Ultrapure deionized water (18.2 MΩ cm−1) was used for all solution preparations. Synthesis of Gold Nanocrystals. All syntheses were referred to previous literatures with modifications.31−36 To make Au nanostructures, strategies of seed-mediated growth at room temperature were used. Details are provided in the Supporting Information. Structural Recovery via Rapid Reduction. Before this experiment, presynthesized nanocrystals were washed, collected, and concentrated from 100 to 1 mL using Eppendorf Centrifuge 5804. In the 1-mL solution, 0.02 mL was taken out and transferred to 10 mL of 0.1 M CTAC solution. Next, 0.3 mL of 0.01 M IrCl3 solution was added to trigger the iterative dissolution at room temperature. After about 8 min, 0.075 mL of 0.04 M AA was added into the mixed solution at room temperature. The solution was collected by centrifuging. Structural Transformation via Slow Reduction. Steps in this experiment were the same as those of rapid reduction except
■
RESULTS AND DISCUSSION Au is chemically durable, as attributed to its unfavorable oxidation potential (from Au0 to AuCl4−, −1.002 V vs NHE) and high bond dissociation energy (Au−Au 226.2 kJ/mol at 298 K). This makes chemical modifications of Au nanostructures C
0
DOI: 10.1021/acs.jpcc.5b10305 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 3. Time-dependent UV−vis spectra of RD archetype nanostructures reacting with IrCl3 (a) in and (b) after 10 min. (c) Relational plot of the Au SPR absorption versus wavelength with the reaction time.
dissolution of Au nanostructures to produce nanoseeds. The dissolved Au0 portions mainly become the AuCl4− complexes instead of the form of AuCl2− because of the unfavorable reduction potential (Au(0) + Ir3+ + 2 Cl− ⇌ AuCl2− + Ir(0), E0 = +0.002 V). The truth is testified in the mass-to-charge patterns of mass spectroscopy (Figure S1). The signal of AuCl2− that ought to show up at 266.9 of the m/z value (Figure S 1d) was not observed in the supernatants taken from the reaction solutions. This implies the absence or extremely low amount of AuCl2− complexes born from dissolution. The rapid reduction can be done adding ascorbic acid to reduce the AuCl4− ions born from reaction 1, initiating regrowth onto the nanoseeds. In the aqueous solution of CTAC, the low pH value (
