Article pubs.acs.org/cm
Guiding Principles in the Galvanic Replacement Reaction of an Underpotentially Deposited Metal Layer for Site-Selective Deposition and Shape and Size Control of Satellite Nanocrystals Yue Yu, Qingbo Zhang, Qiaofeng Yao, Jianping Xie,* and Jim Yang Lee* Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Republic of Singapore S Supporting Information *
ABSTRACT: The combination of sustained under-potential deposition (UPD), galvanic replacement reaction, and control of nanocrystalline growth, is a versatile strategy for engineering the architectural diversity of complex heterogeneous metallic nanocrystals (HMNCs). The strategy has the potential to generate a large variety of HMNC structures if the causality between reaction parameters and architectural elements can be identified and sufficiently understood. We have discovered three guiding principles which are useful for the control of siteselective deposition and the shape and size of the satellite nanocrystals (NCs). These principles are illustrated here in this article using Au/AgPd HMNCs consisting of a Au central NC and AgPd satellite NCs as examples. The three principles are (1) full corner- or edge-selective deposition requires an adequate supply of Ag+ in the reaction solution to sustain the presence of a Ag UPD-layer, the galvanic oxidation of which supplies electrons to the corners or edges of a central NC for a full-island growth; (2) in full corner- or edge-selective deposition, the shape of the satellite NCs can be controlled by the dependence of the growth kinetics of Pd and Ag atoms on exposed facets, {100} facets for Pd and {111} facets for Ag; and (3) the size of the satellite NCs can be controlled by varying the total precursor (Ag + Pd) concentration at a fixed Ag/Pd concentration ratio so long as the requirements for site-selective deposition and shape control are satisfied. The findings presented here could rationalize the design and synthesis of architecturally distinct HMNCs based on the newly developed UPD-layer-induced galvanic replacement reaction. KEYWORDS: nanomaterials, noble metal, heterogeneous nanoparticles, shape-controlled synthesis
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INTRODUCTION Architectural engineering of heterogeneous metallic nanocrystals (HMNCs) can generate metallic nanostructures with structural diversity and complexity emulating those of organic molecules by programming the architecture-determining elements (i.e., the shape and size of the component nanocrystals (NCs) and their spatial relationship).1−8 Similar to molecular engineering where structural diversity is used to create more varied properties for application explorations, the architectural engineering of HMNCs can likewise increase the versatility and utility of metallic NCs as the properties of HMNCs are dependent on their architecture.9−20 However, systematic and independent tuning of the architectural determining elements remains a great challenge because variations in one architectural determining element are often accompanied by variations in other elements. The variety of HMNCs is currently rather limited. We have recently reported a versatile strategy capable of engineering the architectural diversity of HMNCs.21 The strategy combines the galvanic replacement reaction of a © 2013 American Chemical Society
metal layer formed by underpotential deposition (UPD) and control of the kinetics of nanocrystal growth to provide independent control of site-selective deposition and the shape and size of satellite NCs. The UPD process lowers the reduction potential of the more active metal, allowing it to deposit on the surface of a central NC. This UPD-layer then undergoes the galvanic replacement reaction with the less active metal. The electrons generated from the galvanic replacement reaction accumulate preferentially on specific sites of the central NC. This geometry-dependent electron distribution subsequently directs the deposition of satellite NCs on electron-rich sites. The strategy has the potential to generate a variety of HMNC architectures if one has sufficient control of the chemical reactions involved. Several chemical reactions work in tandem to enable the selective deposition to occur concurrently with the size and shape evolution of satellite NCs. Therefore, it Received: August 13, 2013 Revised: October 28, 2013 Published: November 7, 2013 4746
dx.doi.org/10.1021/cm402734r | Chem. Mater. 2013, 25, 4746−4756
Chemistry of Materials
Article
60 μM, and 80 μM, respectively. The concentration, size, and shape of the central NCs were kept the same for all syntheses of HMNCs. Synthesis of Edge-Satellite Au/AgPd HMNCs. The deposition of satellite NCs on the edges instead of corners were accomplished by modifying the surface of the Au central NCs with a Pd modification layer as we have demonstrated in our previous study.21 This Pd modification layer would increase the Ag reactivity for galvanic oxidation. The rate of electron generation would increase resulting in more electrons on the edges. The modification was relatively easy, adding the Pd precursor to the Au central NC solution before the Ag precursor and aging for a fixed period of time. For the synthesis of edge-satellite Au/AgPd HMNCs, 0.155 mL of 38.8 mM ascorbic acid, 60 μL of 100 mM HCl, and 42 μL of 5 mM H2PdCl4 were added to 3 mL of octahedral Au NC solution with thorough mixing after each addition. After aging the growth solution for 10 min, a predetrermined volume of 0.5 mM AgNO3 was added and mixed by shaking. Finally, 18 μL of 5 mM H2PdCl4 was introduced 30 min after the AgNO3 solution addition. The solution was thoroughly mixed and left on the shaker overnight. The volumes of AgNO3 needed for final concentrations of 5 μM, 10 μM, 20 μM, 40 μM, and 60 μM were 30 μL, 60 μL, 120 μL, 240 μL, and 360 μL, respectively. The concentration, size, and shape of the central NCs were kept the same for all syntheses of HMNCs. Synthesis of Au/AgPt HMNCs. The preparation of Au/AgPt HMNCs was similar to the preparation of corner-satellite Au/AgPd HMNCs except that H2PdCl4 in the growth solution was replaced by H2PtCl6, and the reaction time was extended to one week. In particular, a given volume of 5 mM AgNO3 and 0.155 mL of 38.8 mM ascorbic acid was added to 3 mL of octahedral Au NC solution. After thorough mixing, 60 μL of 100 mM HCl and 42 μL of 5 mM H2PtCl6 were added in turn to the octahedral Au NC solution. The solution was mixed well and left on the thermal mixer for one week. Quasitruncated-octahedral and quasi-octahedral AgPt satellite NCs were prepared by adding 15 and 36 μL of 5 mM AgNO3 for final Ag+ concentrations of 25 mM and 60 mM, respectively. For the preparation of small AgPt satellite NCs, the volumes of H2PtCl6 and AgNO3 solutions were reduced to 2/5 of the previously stated volumes while all other experimental variables remained the same. Materials Characterizations. The structures of central NCs and HMNCs were analyzed by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) on JEM-2010 and JEM-2100F (JEOL) microscopes operating at 200 kV accelerating voltage. Field emission scanning electron microscopy (FESEM) (on a JEOL JSM6700F equipped with the scanning TEM (STEM) function operating at 25 kV) was used to evaluate overall particle morphology and product uniformity. EM samples were typically prepared by dispensing a drop of the washed product on a copper grid followed by drying in air at room temperature.
is important to develop a working understanding of the principles of architectural control so that HMNCs can be designed and fabricated with the desired outcomes. This is an article of our discovery of three principles enabling the independent and rational control of site-selective deposition, shape, and size of satellite NCs in the UPD-layerinduced galvanic replacement reaction system. Each principle identifies the synthesis parameter(s) related to an architectural determining element and illustrates how the synthesis parameters may be rationally controlled to build an overall complex heterostructure element by element (i.e., the ability to vary only one architectural element at a time without affecting others). Au/AgPd HMNCs were used to demonstrate these principles. The sustainable regeneration of the Ag UPD-layer was identified as the most important factor in exclusively siteselective deposition. The shape and size of satellite NCs were dependent on the NC growth kinetics and were controllable by the concentration ratio and the total concentration of Ag and Pd precursors, respectively. Such knowledge is valuable to the continuing development of HMNCs, as it allows the architectural complexity and diversity of HMNCs to be systematically enhanced, and to increasing the reliability of architectural engineering of HMNCs.
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EXPERIMENTAL SECTION
Materials. Hydrogen tetrachloroaurate (III) hydrate (HAuCl4.xH2O, Alfa Aesar, 49.87% Au), palladium(II) chloride (PdCl2, Sigma Aldrich, 98%), silver nitrate (AgNO3, Merck, 99.8%), sodium borohydride (NaBH4, Fluka, 98%), cetyltrimethylammonium bromide (CTAB, Sigma Aldrich, ≥98%), and L-ascorbic acid (Merck, 99%) were used as received. Ultrapure Millipore water (18.2 MΩ) was used as the solvent throughout. All glassware was cleaned in Aqua Regia and rinsed with ethanol and ultrapure water. Five millimolar H2PdCl4 solution was prepared by dissolving 22.25 mg of PdCl2 in 25 mL of 10 mM HCl solution. Synthesis of Octahedral Au Central NCs. Au octahedral seed NCs were prepared first and used as seeds for the preparation of larger octahedral NCs. The synthesis of Au octahedral seed NCs was based on a seed-mediated growth method using small Au NCs as seeds. For the preparation of small Au seed NCs, 7 mL of 75 mM CTAB solution was prepared at 30 °C to dissolve the CTAB. Then, 87.5 μL of 20 mM HAuCl4 solution was added to the CTAB solution. An ice-cold NaBH4 solution (0.6 mL, 10 mM) was then injected quickly into the mixture under vigorous mixing to form a brown seed solution. Stirring continued gently at 30 °C for 2 to 5 h to decompose the excess NaBH4. The seed solution was then diluted 100-fold with ultrapure water. A growth solution was separately prepared by adding 25 μL of 20 mM HAuCl4 solution and 0.387 mL of 38.8 mM ascorbic acid (in that order) into 12.1 mL of 16 mM CTAB solution in a clean test tube at 28 °C with thorough mixing after each addition. Then, 0.15 mL of the diluted seed solution was added to the growth solution and thoroughly mixed. The mixture was left unperturbed at 28 °C overnight. The color of the solution changed to pink indicating the formation of Au NCs. Five milliliters of the octahedral Au seed solution was added to 12.5 mL of a growth solution containing 16 mM CTAB, 0.04 mM HAuCl4, and 1.2 mM ascorbic acid to enlarge the octahedral Au NCs (to edge length of 45 nm). The mixture was thoroughly mixed and left undisturbed overnight. Synthesis of Corner-Satellite Au/AgPd HMNCs. For the synthesis of corner-satellite Au/AgPd HMNCs, a given volume of 0.5 mM AgNO3 and 0.155 mL of 38.8 mM ascorbic acid were added to 3 mL of octahedral Au central NC solution. After thorough mixing, 60 μL of 100 mM HCl and 42 μL of 5 mM H2PdCl4 were added to the Au central NC solution. The solutions were mixed well and left on the shaker overnight. Six microliters, 18 μL, 36 μL, 60 μL, 120 μL, 180 μL, 240 μL, 360 μL, and 480 μL of 0.5 mM AgNO3 were used for final concentrations of 1 μM, 3 μM, 6 μM, 10 μM, 20 μM, 30 μM, 40 μM,
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RESULTS AND DISCUSSION
In a typical synthesis of Au/AgPd HMNCs, two metal precursors (Ag and Pd precursors) were added to a colloidal solution of polyhedral Au central NCs for the deposition of AgPd satellite NCs. Because of UPD, the Ag precursor was quickly reduced to form a layer on the Au central NCs. This insitu-formed Ag UPD-layer then underwent galvanic replacement with the Pd(II) ions in the solution. The electrons in the galvanic replacement reaction accumulated preferentially at high curvature sites of the central NC (corners or edges depending on the kinetics of electron generation21). As a result, a spatially heterogeneous distribution of electrons was established and directed the deposition of AgPd satellite NCs on electron-rich sites. Concurrent with the galvanic replacement reaction, the coreduction of the two metal precursors (Ag and Pd precursors) propelled the crystal growth of the AgPd satellite NCs. The programmability of HMNC architecture requires the independent control of three architectural 4747
dx.doi.org/10.1021/cm402734r | Chem. Mater. 2013, 25, 4746−4756
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Scheme 1. Schematic Illustration of the Three Guiding Principles for Site-Selective Deposition, and Shape and Size Evolution of Satellite NCs in the UPD-Induced Galvanic Replacement Reaction Strategya
a
The deposition of corner-satellite HMNCs are used as an example. (A) Principle 1: The sustainability of the Ag UPD-layer determines the deposition behaviour of the satellite NCs. Semi-corner-selective deposition occurs at relatively low Ag+ concentration as a result of the island-to-layer growth (Path I); full-corner-selective deposition occurs at high Ag+ concentration with the continual regeneration of the Ag UPD-layer to support full-island growth (Path II). (B) Principle 2: The shape of the satellite NCs is determined by the combined growth kinetics of Ag and Pd atoms in full-corner-selective deposition. Varying the weights of the growth kinetics of Pd or Ag atoms (which have different preference for exposure facets, {111} for Ag and {100} for Pd) in the combined growth kinetics changes the proportion of the {100} and {111} facets. The geometric models of I.A-C and II.A-F correspond to the HMNCs shown in Figure 1A−C and 2A−F. (C) Principle 3: The size of satellite NCs may be tuned by varying the precursor concentrations at a fixed Ag/Pd concentration ratio if the conditions for site-selective deposition and shape control as defined by Principle 1 and Principle 2 are satisfied.
of the Ag UPD-Layer Are Essential for a Full-CornerSelective Deposition. The site-selective deposition of satellite NCs was made possible by the galvanic replacement reaction of the Ag UPD-layer. It therefore depended strongly on the presence of the Ag UPD-layer and consequently the Ag+ concentration in the growth solution. The corner-satellite HMNCs formed with different AgNO3 concentrations are shown in Figures 1 and 2. The H2PdCl4 concentration in the growth solution was held constant at 70 μM. The deposition of the satellite NCs could follow one of the two major pathways dependent on the Ag+ concentration (Path I and Path II,
determining elements in the satellite NC deposition process, namely, the deposition sites on the central NC, the development of the shape of the satellite NCs, and their size evolution. We have discovered three main principles for the control of these elements, which are illustrated in Scheme 1 using cornersatellite Au/AgPd HMNCs as the example. The concentration, size, and shape of the Au central NCs were kept constant in the synthesis of HMNCs. Principle 1 (Scheme 1A): Sustainability of the Ag UPD-Layer Determines the Deposition Behavior of the Satellite NCs: Regeneration and the Constant Presence 4748
dx.doi.org/10.1021/cm402734r | Chem. Mater. 2013, 25, 4746−4756
Chemistry of Materials
Article
Figure 1. HMNCs in semicorner-selective deposition (Path I, Scheme 1A). The AgNO3 concentrations were (A) 1 μM, (B) 3 μM, and (C) 6 μM. The H2PdCl4 concentration was 70 μM. (Column 1) SEM and (column 2) TEM images of HMNCs over large sampling areas. (Column 3) 2D cross-sectional views of the geometric models, SEM images, and TEM images of individual HMNCs viewed from the ⟨110⟩ directions with insets in TEM images showing the corresponding FFT patterns. The arrows point to the V-shape grooves projected from the gaps and depressions.
Scheme 1A). The first deposition type occurred at relatively low Ag+ concentrations (
