Letter pubs.acs.org/NanoLett
Growth of Au on Pt Icosahedral Nanoparticles Revealed by Low-Dose In Situ TEM Jianbo Wu,†,‡,§ Wenpei Gao,†,§ Jianguo Wen,∥ Dean J. Miller,∥ Ping Lu,⊥ Jian-Min Zuo,*,†,§ and Hong Yang*,‡ †
Department of Materials Science and Engineering, University of Illinois at Urbana−Champaign, 1304 West Green Street, Urbana, Illinois 61801, United States ‡ Department of Chemical & Biomolecular Engineering, University of Illinois at Urbana−Champaign, MC-712, 600 South Mathews Avenue, Urbana, Illinois 61801, United States § Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana−Champaign, 104 South Goodwin Avenue, Urbana, Illinois 61801, United States ∥ Electron Microscopy Center - Center for Nanoscale Materials, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States ⊥ Sandia National Laboratories, Albuquerque, New Mexico 87185, United States S Supporting Information *
ABSTRACT: A growth mode was revealed by an in situ TEM study of nucleation and growth of Au on Pt icosahedral nanoparticles. Quantitative analysis of growth kinetics was carried out based on real-time TEM data, which shows the process involves: (1) deposition of Au on corner sites of Pt icosahedral nanoparticles, (2) diffusion of Au from corners to terraces and edges, and (3) subsequent layer-by-layer growth of Au on Au surfaces to form Pt@Au core−shell nanoparticles. The in situ TEM results indicate diffusion of Au from corner islands to terraces and edges is a kinetically controlled growth, as evidenced by a measurement of diffusion coefficients for these growth processes. We demonstrated that in situ electron microscopy is a valuable tool for quantitative study of nucleation and growth kinetics and can provide new insight into the design and precise control of heterogeneous nanostructures. KEYWORDS: In situ TEM, flow cell, growth, platinum, nanoparticle
D
in conjunction with low dose electron imaging to observe the growth process on individual nanoparticles in solution. Compared to previous studies reported for closed systems,21,23−26,29,30 the use of a liquid flow cell with low-dose in situ TEM has two important advantages. First, fluid flow enables the observation of initial nucleation and growth events in TEM, because seed crystals are separated from the precursor solution, and deposition during the assembly of the liquid cell can now be eliminated. Second, the effect of the electron beam is minimized under low dose and fluid flow conditions.31 By introducing new solution into the cell continuously, a fresh liquid environment is maintained inside the cell, and the effect of solvated electrons on the reaction is minimized.32 Thus, fluid flow resembles more closely the real synthetic condition than a closed system where solution is sealed in a liquid cell and subject to constant irradiation of the electron beam.
evelopment of three-dimensional (3D) functional nanostructures based on heterogeneous nucleation and growth has attracted considerable research interest for a wide range of applications.1−11 In this context, high-level control of 3D heterostructures is often necessary because unique properties depend on the carefully orchestrated structures.12−16 While dendritic, core−shell, and other heterogeneous nanostructures were reported12,17,18 and ex situ TEM data were used to analyze the formation of these heterostructures,6,19,20 the quantitative study of nucleation and growth kinetics in solution is still uncommon. Recently, liquid cell technology was developed for in situ TEM to study the growth of nanoparticles21,22 and chemical reactions in liquid.23−28 Most of the reported studies have so far been carried out in closed systems, which may not be optimized for studying reaction kinetics. The complex interactions between imaging electrons and solution often result in electron beam-induced growth in a liquid cell without flow,20,22,23,26−28 which complicates the kinetic analysis. Thus, a system optimized for studying reaction kinetics is highly desired. Herewith, we report a study based on the fluid flow technique © XXXX American Chemical Society
Received: January 31, 2015 Revised: February 23, 2015
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DOI: 10.1021/acs.nanolett.5b00414 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 1. Pt icosahedral crystals and Au growth on a Pt icosahedron. (a) 3D model of a Pt icosahedron used as a seed in this growth study. (b) Aberration-corrected TEM micrograph showing the atomic structures of a Pt icosahedron. (c) simulated TEM image of b. (d) Sequential images show the growth of Au on the isolated Pt icosahedral nanoparticle. The triangles indicate the surface change on the corner of Pt icosahedron. (e) The measured distances between corners or sides as a function of time. (f) The measured thickness of a representative corner and side of Pt icosahedron as a function of growth time. The inset indicates how the thickness of the corner and side was measured.
The fluid flow cell was constructed using two 50 nm thick silicon nitride windows separated by a 250 nm thick spacer (Figure S1). The reaction chamber was connected to a liquid pump (see the TEM characterization section in Supporting Information for detail). An aqueous solution containing chloroauric acid (HAuCl4) and citric acid was introduced into the cell to trigger the deposition of Au on icosahedral Pt seed nanoparticles. The in situ TEM study was carried out under fluid flow condition and at a dose of 30 e/Å2·s, which was optimized by the dose control experiments that also examined the combined effects of low dose and liquid flow. At the commonly used imaging dose of 1500 e/Å2·s, Au grew in a dendritic fashion (Figure S2 and Movie S1) which was also observed previously for electron beam-induced growth of nanostructures.26 Since the amount of solvated electrons in solution is expected to be proportional to electron dose, the observed dendritic growth at high dose levels suggests the metal precursors are reduced in a solution by high-level of solvated electrons.21,27,33,34 At a dose level of 300 e/Å2·s and with no flow, beam-induced homogeneous growth of Au nanoparticles occurred in solution, followed by the deposition on Pt icosahedra (Movie S2). When the dose level was kept at 300 e/Å2·s, but with fluid flow on, no visible change on particle surfaces was observed (Movie S3). At a dose level of 30 e/Å2·s, no growth was observed in solution under flow conditions. These control experiments indicate no obvious beam-induced growth occurs at a dose level of 30 e/Å2·s under the flow condition and that fluid flow helps further reduce the effect of electron beam. Pt icosahedral seeds were transferred into the liquid flow cell to study the growth of Au on Pt seed crystals (Figure S3). Figure 1a−c show experimental and simulated TEM images of
a Pt icosahedral nanocrystal. The TEM micrograph shows these nanoparticles had an average edge length of ∼15 nm. They had the overall shape and characteristic twin boundaries matching those in the computer-simulated TEM image viewed along the 2/3-fold rotational symmetry axis. Figure 1d shows a series of TEM micrographs obtained in situ after the aqueous mixture of Au precursor (HAuCl4) and citric acid was introduced at a flow rate of 5 μL/min. The presence of liquid in the fluid cell was verified by electron diffraction (Figure S4) and further confirmed by the observation of bubbles generated intentionally under intense irradiation of the electron beam (Figures S4 and S5). Deposition of Au was monitored by in situ TEM through analyzing the surface changes of an isolated Pt icosahedron along the ⟨111⟩ viewing axis of a tetrahedron subunit (Figure 1d). Distances along the six labeled directions were used to quantitatively analyze the growth of Au on the Pt icosahedron (Figure S6). These six distances are categorized into two groups: three distances between the two opposite corners (Dc) and three distances between the two parallel opposite sides (Ds). In this sequence, recording started 5 min after the moment the stock solution was introduced into the liquid cell. All TEM micrographs were cross-correlated to correct the small, if any, movement of the nanoparticle to get an accurate measurement of distances. The in situ TEM data show conformal coating of Au on all corners and edges, followed by a characteristic oscillation in Dc with continuous growth on the sides (Movie S4). Both Dc and Ds began to increase around 600 s after the fluid flow was introduced (Figure 1e). Dc grew by 2 nm within the first 1000 s, followed by a drop in thickness by about 1 nm between 1000 and 1200 s. During the same time period, all three Ds values B
DOI: 10.1021/acs.nanolett.5b00414 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters increased continuously. After the initial stage, Dc oscillated up to 2 nm on all corners (Figure 1e). This growth pattern was observed for the deposition of Au on other isolated icosahedra as well (Figure S7), indicating the growth of Au on corners and edges follows a characteristic oscillation in D. The in situ TEM data show Au deposited on selected corners after the first 727 s (the region indicated by white arrow tips in Figure 1d). Instead of a uniform deposition on the entire icosahedral surface, large protrusions resulting from the nucleation of Au on the Pt nanoparticle were observed at the corner regions up to ∼894 s, as indicated by the white arrow tips. Au deposited on all six corners up to 1032 s, while all Ds values increased at a relatively slow growth rate compared to Dc. The surface grew in thickness near the corners as labeled on the bottom of the image taken at 1100 s (Figure 1d). This lateral growth was also observed near the bottom corner at 1000, 1274 and 1460 s (labeled with black arrows), while Dc decreased slightly, as indicated by the blue arrow tips. The same growth pattern was observed for other seed particles as well (Figure S7). This lateral surface diffusion can be identified in the stage 3, 4, 6, and 7 in Figure S7, respectively. Figure S7b summarizes the changes in all three Ds and three Dc as a function of time for the deposition of Au on the Pt icosahedron. A change of thickness of the deposited layers as a function of time shows the corner starts to grow at ∼600 s, while thickness on the sides began to increase at 800 s (Figure 1f). The 200 s delay indicates the nucleation of Au preferentially occurred on corner sites first, and the growth rate of Au on corner sites is slightly faster than that on the side (Figure 1f and Figure S8). The favored deposition at the corners competed with the process of atom diffusion from these regions to other surfaces due to the different growth rates. This diffusion process is usually difficult to observe, because metal could deposit faster than it diffuse along the surfaces, especially at room temperature.2 To better understand the surface diffusion during Au growth on the Pt surface, we changed the growth condition by turning off the flow of stock solution of Au precursor and reductant after 1200 s. Without a fresh supply of nutrient, surface diffusion became more pronounced. Movie S5 and Figure 2 show the process of Au deposition on the surfaces of
an icosahedral Pt nanoparticle in liquid throughout the experiments. The bright field TEM images were taken using an exposure time of 1 s. Small particle movements were seen during observation, while axial particle rotations were ruled out based on the experimental evidence. The time was counted from 5 min after the flow was turned on. When flow was on, Au first formed islands at the corners of the Pt icosahedron, marked with triangles in Figure 2a−f. This preferred deposition could be attributed to the low coordination numbers and strong binding of Au atoms on corner Pt atoms.35 At the rightmost corner, a clearly visible tip was observed at 800 s after the beginning of observation, and this tip continued to grow over the next 400 s (Figure 2a−c and Movie S5). Significant growth was also observed at the other three corners, as shown from the particle shape profiles drawn according to the TEM micrographs (Figure 2g). After the nucleation and initial growth of Au at the corners, growth at the surface became pronounced, especially during the time period when the liquid flow was turned off. Figure 2d−f shows the evolution of the tip at the rightmost corner (red arrow) after the flow was stopped at 1200 s after the reaction (also see Movie S5). The length of this tip started to decrease at ∼1300 s, while the Au layer started to grow on the side of icosahedron as labeled with orange arrows (Figure 2d). At 1400 s (200 s after the flow was stopped), the length of the Au tip decreased dramatically while the thickness on the side of Pt icosahedron increased (Figure 2f). Figure S9 shows the entire growth sequence of a different Pt@Au nanoparticle during the time period of the flow being turned off at 1200 s and back on at 1500 s. The entire Pt icosahedron was coated by Au at 1541 s. Quantitative analysis of nucleation, diffusion, and conformal coating of Au on a Pt particle surface is possible based on the in situ TEM data (Figure 3). Growth of Au on Pt nanoparticles was analyzed by measuring the distance and time, and illustrated in Figure 3a. Dc and Ds were obtained from the TEM micrographs recorded at a rate of 0.1−0.5 frame/s. There are three distinctive regions for the entire sequence. In Region I, Dc increased at a rate of 0.013 nm/s up to 1220 s after the observation started, while Ds increased at a rate of 0.010 nm/s, but the growth was ∼200 s behind that in Dc (Figure 3). In Region II between 1230 to 1500 s with the flow off, Dc decreased at a rate of ∼0.017 nm/s, while Ds continued to increase at the similar rate of ∼0.012 nm/s. In Region III, the growth rates of both Dc and Ds increased significantly to ∼0.07 nm/s after flow was renewed at 1500 s into the observation. Thus, once the Au-coated icosahedron grew into a shape without a clear distinction among the specific facets, the growth rate between Dc and Ds became similar. The growth kinetics were analyzed quantitatively based on the Lifshitz−Slyozov−Wagner (LSW) theory using the data obtained from the in situ TEM study. In the analysis, the average of the measurements from each of the three different directions was used for each data point. The growth rate, dr/dt, was obtained according to the following equation that takes into consideration both reaction and diffusion:36 (1/rb − 1/r ) 2σVm2c∞ dr = dt RT (1/D + 1/kdr ) r
Figure 2. TEM micrographs showing the shape evalution of the Pt@ Au nanparticles during the nucleation and growth stages: (a−c) with a flow of HAuCl4 and citric acid solution at 5 μL/min, and (d−f) at the stage for surface diffusion when the flow was turned off. (g) Illustration of the shape evolution of a Pt@Au nanparticle during the process as measured from the TEM micrographs.
(1)
where r is the radius, rb is the critical radius, σ is the interfacial energy between metal and water, c∞ is the concentration of a monomer away from the nanoparticle surface, Vm is the molar volume, D is the diffusion coefficient, kd is the rate constant for C
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Figure 3. (a) Schematic illustration of the processes of Au depositing on a Pt icosahedral nanocrystal. (b) Quantitative analysis of nucleation and growth of Au on Pt nanocrystals.
the deposition, and R is the universal gas constant. When diffusion is the rate limiting step (D ≪ kdr), growth is largely determined by the diffusion of atoms to the surface. The solution of eq 1 gives the relationship between the size of particles and the growth time:36 r 3 − r03 = Kt
K=
8σDVm2c∞ 9RT
the Pt icosahedron is the result of growth of Au on Pt taking away the surface diffusion of Au (∼0.015 nm/s in Region II). A growth rate up to ∼0.07 nm/s was observed experimentally at the late stage of growth, in which period surface diffusion no longer contributed siginificantly to the change in growth rate due to the loss of the surface feature. An additional synthesis experiment was carried out to test whether the insight gained from these in situ studies could be used to guide the preparation of core−shell nanoparticles under typical laboratory conditions (see detail in Supporting Information). In this synthesis, Au began to partially deposit on the Pt icosahedral particles after reaction for 10 min using the same type of reactant mixture as those used in the in situ TEM study. Figure 4 shows the HAADF-STEM and EDS
(2)
where r0 is the radius of particle at the initial stage. If the process is reaction-limited (D ≫ kdr), the growth in thickness based on eq 1 can be rewritten in the following form: r 2 ≈ KR t
(3)
where KR = 2σkdVm2c∞/RT. Based on this equation, the growth rate dr/dt was calculated to be ∼10 nm/s for Au growth on a Pt surface and the value of K to be 9.7 × 103 nm3/s, assuming the gold−water interface σ is 0.187 J/m2,37 Vm is 1.02 cm3/mol, D is 1.4 × 10−5 cm2/s,38 and c∞ is 0.1 × 103 mol/m3. This calculated growth rate is much higher than the experimentally determined value of 0.05 nm/s using the data obtained from the Region III of Figure 3 and Figure S10.36,39 The experimental data, however, could be fitted using the reaction-limited growth model (Figure S11). The KR value was determined to be ∼6.3 nm2/s for Au deposited on the Au surface. The surface diffusion coefficient of Au on the Pt surface was calculated to be ∼1 × 10−3 nm2/s according to the eq S7 (see Supporting Information for detail). This calculated value is in close agreement with that determined by other experimental methods.40 The above results show that there are two competing processes at the initial stage of growth. One is the growth of nanometer-sized Au islands, and the other is surface diffusion or dissolution leading to the subsequent conformal coating. The decreasing rate of Dc is slightly larger than the growth rate of Ds during the period when the fluid is off (Region II). The continuing growth of Ds during the same time period indicates that the growth of Au on the Pt facets is mainly due to the Au atom diffusion from corner islands. The rate of Au surface diffusion was estimated to be 1 × 10−3 nm2/s, using a cylindrical geometry as the initial shape of island of Au in the calculation (Figure S12). The appearance of Au at the corner of
Figure 4. (a) HAADF-STEM image and (b) X-ray EDS elemental maps of an Au layer (red) deposited on a Pt icosahedron (green) obtained at a low concentration of HAuCl4 (0.1 M).
composition analysis obtained by a FEI Titan G2 80-200 STEM with a Cs probe corrector and using the ChemiSTEM technology, operated at 200 kV. The results show Au islands formed at the corners and sides of the Pt icosahedron and the Pt icosahedral shape was largely intact (Figure 4 and Figure S10). These studies demonstrate the feasibility to synthesize Pt@Au core−shell nanoparticles under conditions similar to those observed in the low-dose in situ TEM experiments. In short, our experiemental data points to a new 3D growth model for the formation of core−shell nanostructures involving a hybrid process with nucleation-initial island growth, surface D
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diffusion, and subsequent layer growth. This hybrid model does not fall under any of the three existing growth models; namely, layer-by-layer growth, island growth, and island-on-wetting layer growth.2,6,41−43 This in situ TEM study provides new understanding of the growth process, which can lead to a precise control over chemical reactions and design of synthetic approaches for novel heterogeneous nanostructures.
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ASSOCIATED CONTENT
S Supporting Information *
Details of experimental procedures, additional information on the analysis of growth model. TEM and STEM micrographs (Figure S1−S12) and movies. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported in part by US National Science Foundation (Grant No.: CHE-1213926 to H.Y.) and University of Illinois (H.Y.). J.B.W. was partially supported by Department of Energy (50%, Grant no.: DEFG02-01ER45923 to J.M.Z.) and the Shen Fellowship (50%) from Department of Chemical and Biomolecular Engineering at University of Illinois. W.P.G. is supported by NSF (Grant No.: DMR-1006077 to JMZ). Use of the Center for Nanoscale Materials, including the resources of the Electron Microscopy Center, is supported by the U.S. Department of Energy, Office of Science under Contract No. DE-AC02-06CH11357. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the US Department of Energy’s National Nuclear Security Administration under contract DE-AC0494AL85000. We thank Thao Ngo for helpful discussion.
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