Letter pubs.acs.org/NanoLett
Helical Growth of Ultrathin Gold−Copper Nanowires Rubén Mendoza-Cruz,†,‡ Lourdes Bazán-Díaz,†,‡ J. Jesús Velázquez-Salazar,‡ Germán Plascencia-Villa,‡ Daniel Bahena-Uribe,§ José Reyes-Gasga,† David Romeu,† Grégory Guisbiers,*,‡ Raúl Herrera-Becerra,† and Miguel José-Yacamán‡ †
Instituto de Física, Universidad Nacional Autónoma de México, Circuito de la Investigación s/n, Coyoacán 04510, México Distrito Federal, México ‡ Department of Physics and Astronomy, The University of Texas at San Antonio, One UTSA Circle, San Antonio, Texas 78249, United States § Advanced Laboratory of Electron Nanoscopy, Cinvestav, Av. Instituto Politecnico Nacional 2508, Col. San Pedro Zacatenco, Delegación Gustavo A. Madero, Mexico D.F. C.P. 07360, Mexico S Supporting Information *
ABSTRACT: In this work, we report the synthesis and detailed structural characterization of novel helical gold− copper nanowires. The nanowires possess the Boerdijk− Coxeter−Bernal structure, based on the pile up of octahedral, icosahedral, and/or decahedral seeds. They are self-assembled into a coiled manner as individual wires or into a parallelordering way as groups of wires. The helical nanowires are ultrathin with a diameter of less than 10 nm and variable length of several micrometers, presenting a high density of twin boundaries and stacking faults. To the best of our knowledge, such gold−copper nanowires have never been reported previously. KEYWORDS: Nanowires, Boerdijk−Coxeter−Bernal structure, twin boundaries, stacking faults, TEM
U
of the reports on nanowires have focused on the production of Au nanowires using synthesis methods based on the slow reduction of AuCl−oleylamine complexes in organic media.21,28−31 The use of additional compounds in the reaction medium (cosurfactant agents, metallic nanoparticles, and metal ions) has resulted in an enhancement of the yield or the reduction rate. These extra elements served as promoter agents of anisotropic growth, and because there was no reduction of these metal ions they were not incorporated into the wire structure. Bimetallic nanowires represent a special case due to the presence of two different atomic species, and therefore two different nucleation and growth processes. A literature analysis reveals that the limitation of the current synthesis methods for generating nanowires is their inability to reduce the diameter that rarely goes below 10 nm.32 Up to now, only two groups33,34 have been able to synthesize ultrathin helical bimetallic nanowires by wet chemistry methods and both produced Au−Ag nanowires. The goal of this paper is to report the growth of a Boerdijk−Coxeter−Bernal type structure for the Au−Cu system with a self-assembled external coiled shape
nderstanding the growth of nanomaterials is one of the most important and exciting topics in the materials community because it lies at the bottleneck between fundamental research and applications. One-dimensional (1D) nanomaterials1 such as nanotubes and nanowires can adopt two different geometrical configurations, straight and helical. While the first type is the most common, the last one possesses additional opportunities for nanoengineering applications such as periodicity and chirality. The chirality is induced by the elastic strain of the axial screw dislocation.2 Such structures are expected to play a unique role in nanophotonics,3−7 nanocatalysis,8 nanomechanics,9−11 and nanoelectronics.12 Nature already contains a lot of examples involving helical structures with the most well-known being the deoxyribonucleic acid (DNA) but many proteins also adopt a helical structure.13 Helical structures are most often encountered in organic systems whereas they are difficult to produce in inorganic materials. Up to now, different groups have managed to synthesize non-organic helical nanostructures made of SiOx,3,14 SiO2,11 SiC/SiO2,9,15 WO3,4 ZnO,13,16 MoO3,17 MoS2,8 CdS,18 PbS,13 BC,10 CuInSe2,6 AlN,7 GaN,5 InGaN19 and ZnGa2O420 through a template-free method. Several other studies on the synthesis of monometallic nanowires have also been reported,21−23 specifically, Cu,24 Ag,25 Pt,26 and Pd27 nanowires have been successfully synthesized. However, most © XXXX American Chemical Society
Received: October 14, 2015 Revised: January 26, 2016
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DOI: 10.1021/acs.nanolett.5b04184 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 1. Obtained ultrathin nanowires. (A) STEM bright-field micrograph of the ultrathin nanowires with ∼2 nm diameter presenting a coiled arrangement. (B) High-resolution HAADF-STEM image of the ultrathin nanowires showing different structural features. (C−D) Magnified images of the marked regions in B. (E) Atomistic model of the ultrathin nanowires.
(diameter ∼2 nm) with a high density of twin boundaries and stacking faults. Gold−copper alloys have played an important role in our civilization for many centuries.35 Nowadays, these alloys still continue to attract interest at the nanoscale for its miscibility all over the composition range and their ability to form ordered phases. Gold−copper nanoparticles are important catalysts36 in the oxidation of carbon monoxide and37−40 in the partial oxidation of methanol to produce hydrogen fuels;41 optically, they present a strong surface plasmon resonance (SPR) that can be tuned through the composition.42 In this work, gold− copper nanowires were synthesized through a wet-chemical synthesis route by mixing tetrachloroaurate trihydrate (HAuCl4) and copper dichloride (CuCl2) within a solution of octadecylamine (ODA) and D-(+)-glucose. Detailed protocols of the experimental section can be found in the Supporting Information. The helical morphology was obtained for those ultrathin nanowires. Transmission electron microscopy (TEM) and high-angle annular dark-field-scanning transmission electron microscopy (HAADF-STEM) images of the obtained gold−copper nanowires are shown in Figure 1. Helical ultrathin nanowires exhibited an average diameter of 1.8 ± 0.3 nm and several micrometers in length (Figure 1A,B). Typically, the ultrathin nanowires were arranged on the supporting carbon film as either coils or ribbons of parallel, regularly spaced nanowires (Figures 1A, S1, and S2). The average spacing between nanowires in a ribbon was found to be ∼1.4 nm. The manner the nanowires arranged themselves reveals a high ductility. A high-magnification image is shown in Figure 1B. These ultrathin nanowires presented two different atomic structures along their length revealed by a zigzag contrast due to twin boundaries (Figure 1C) and a fanlike structure (Figure 1D). They also presented a high density of twins and stacking faults along their lengths. An atomistic model of these nanowires at first approximation is shown in Figure 1E. In order to obtain a better understanding of the structure of the nanowires, HAADF-STEM studies were performed with results shown in Figure 2 revealing the presence of twin boundaries (TB) and stacking faults (SF). Figure 2A shows an image of a single nanowire growth nearly parallel to the ⟨111⟩ direction of a faced-centered cubic (fcc) cell. The measured interplanar distances were 0.22 and 0.19 nm corresponding to {111} and {200} planes a crystalline FCC structure, respectively. Different defect-rich areas are shown magnified in Figure 2B. The (11-1) [112] nanotwinned grains revealed a ABCACBABCABC...
Figure 2. (A) HAADF-STEM image of a single ultrathin helical nanowire oriented near to the [-110] axis zone. (B−D) Magnified zones with multiple defects highlighted. The atomic packing variations are marked with colored circles: red, blue, and green corresponding to A, B, C atomic layers, respectively. (E−G) FFT of each region is shown on the right in order to accentuate the observed defects. Scale bar: 2 nm.
stacking with a length of three lattice spacings. Figure 2C highlights the ABCACBCBACBCABC... atomic packing, which showed (111)[11-2] twin boundaries and stacking faults perpendicular to the growth direction. Figure 2D corresponds to a single crystal region oriented such that the {200} planes were perpendicular to the growth direction. The fast Fourier transforms (FFT) of each region are also shown on the right indicating that the viewing direction was nearly parallel to the [110] axis zone. The reflection arrangements presented in these FFT resulted from the twins and stacking faults observed in the images.43,44 Figure 3 shows HAADF-STEM images of several helical gold−copper nanowires. Each image corresponds to a different family. Figure 3A shows a nanowire with a singlecrystal structure along most of its length. The viewing direction being nearly parallel to the [1-12] crystallographic axis. The atomic contrast observed indicates that the nanowire is twisted along the growth direction. The contrast reveals that the B
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helicoidal structure is generated by the oriented attachment of small octahedral (or truncated octahedral) seeds sharing {111} facets. Other nanowires with a polycrystalline structure were observed as exemplified in Figure 3B−D. The images show an atomic contrast consistent with the packing of icosahedral and/ or decahedral seeds. The fanlike contrast of Figure 3B and multiply twinned arrangement of Figure 3D have been previously observed for bimetallic nanowires with icosahedral and decahedral units stacking along the wire growth direction.33 In the insets at the bottom are the corresponding FFT of each image, shown to accentuate the observed features. In our case, the atomic contrast is consistent with icosahedra, decahedra and single nanocrystals, which coexist into the synthesized solution that eventually coalesce into a single nanowire. With assistance of image processing, a short-range of stacking ordering was detected in some sections of the ultrathin nanowires. It is well established that in the HAADF technique the image contrast is directly related to the atomic mass (or Znumber) of the elements present in the sample and its thickness. A single nanowire image and its corresponding FFT is shown in Figure 4A,B, respectively. Figure 4C corresponds to a processed image taken from the region marked in Figure 4A. Figure 4E,F shows intensity profiles analysis along and across atomic layers of one nanowire. FFT in Figure 4B shows the crystalline nature of this nanowire. The line profiles shown in Figure 4E and 4F were done across the alternated atomic layers (black line in Figure 4C) and along the {111} planes. The intensities in the profile can be attributed, therefore, to Au-rich or Cu-rich atomic layers as shown in the figures. An atomistic model of the region of interest is presented in Figure 4D Two general mechanisms have been proposed for the adsorbate directed synthesis of metallic nanocrystals, namely directed growth and oriented aggregation.45,46 On the basis of the microscopy results, we propose that the ∼1.8 nm gold−
Figure 3. HAADF-STEM images of helical ultrathin nanowires. (A) Single crystal nanowire showing contrast suggesting a helical structure. (B,C) Nanowires with polycrystalline structure arising through the attachment of icosahedral and/or decahedral seeds. Bottom insets: FFT of the corresponding image highlighting the structural characteristics.
Figure 4. (A) Single nanowire. (B) The [1-12] axis zone FFT from A. (C) Processed image of the region marked in (A) in which an alternated atomic planes are observed. (D) Atomistic model of the nanowire in (C). (E) Intensity line profile of the black line marked in panel C. (F) Intensity profiles of the red and yellow lines in panel C. C
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presence of other Au rich particles in the solution causing the atomic composition to vary along the length of the wire. Twins and stacking faults are known to form during the crystal growth or during phase transformations and are two of the most common defects in face-centered cubic crystals. Their formation may give rise to anisotropic growth and enhanced properties of crystals.59,60 A high density of microtwins in bulk metals with low stacking fault energy, such as gold and copper, affect directly their properties because such defects impede the movement of dislocations, improving the plastic behavior and strength electrical properties, without losing their high electrical conductivity.61 Particularly, at the nanoscale range metal wires with nanotwinned structure present enhanced mechanical properties compared with their bulk counterparts or other defect-growth systems.62−65 The formation of Au−Cu alloyed ultrathin nanowires could generate not only enhanced mechanical properties but also electrical and optical properties compared with monometallic wires. Therefore, to characterize the optical properties of the helical Au−Cu nanowires, the visible-near-infrared (vis-NIR) spectrum has been acquired by a double beam spectrophotometer from Thermo Scientific (Evolution 220), which is shown on Figure 5. The peaks
copper nanowires were formed by a combination of the seed formation and the seed attachment mechanisms. First, at the initial stages of the reaction small Au−Cu seeds are formed with exposed facets where the atomic arrangement is governed by the binding of primary amine groups. Once these seeds are formed, further growth occurs by their oriented coalescence, preferentially along {111} planes, the 3-fold axis of an octahedra. The matching {111} planes can sometimes be rotated through a 60° angle giving rise to low energy coherent twin boundaries as shown in Figure 2 and to the observed twisted-like contrast. To further support the oriented attachment growth hypothesis of our ultrathin nanowires, aliquots of the colloidal solution were taken and analyzed through TEM at the first stages of the synthesis (Figure S3). From the image, it was clearly noticed that in the first stages of the synthesis small Au− Cu seeds were formed that evolved to nanowires with time. EDS analysis confirmed the presence of the two metals in the seeds. Generally, small metal particles adopt three main shapes, namely icosahedron, decahedron, and cuboctahedron (fcc-type group).47 For Au−Cu bimetallic nanoparticles, the proportion of the conforming atoms influences their final structure, and the most stable morphologies observed correspond to icosahedron, dodecahedron, and truncated octahedron shapes.35,48−50 In our case, glucose directly reduced both gold and copper ions much faster than the typical slow-reduction of AuCl−oleylamine complex used, so that a wider shape distribution could be expected for the produced seeds. Therefore, the obtained small particles with icosahedral, decahedral, and fcc-like structures, together with the subsequent atomic deposition, were driven by the metal−surfactant interactions and produced the different crystallographic orientations in the nanowires once attachment and wire formation was completed. Alkylamine chains, such as ODA and glucose as surfactant and reducing agents, respectively, represent an excellent alternative to promote the growth of anisotropic nanocrystals.51,52 It has been pointed out that magnetic stirring and ultrasonic treatment during the reaction process affects the final structure of nanowires.53 Indeed, in our synthesis method, helical nanowires could be efficiently produced by carefully adjusting the ultrasonic treatment time (Supporting Information, Figure S4). For shorter ultrasonic treatment time, the synthesis become contaminated by straight nanowires. Therefore, the coiled and bending characteristics of our nanowires as well as the presence of high density of twins and stacking faults is directly related to the duration of ultrasonic treatment. Few reports have been made concerning bimetallic ultrafine nanowires,33,54−58 where oriented attachment (such as particle−particle, wire−particle, or wire−wire attachments) was the main formation mechanism, and to the best of our knowledge there are no reports on gold−copper ultrathin nanowires. Although copper ions have been employed to promote the growth of metal wires, copper was not found to be incorporated into the wire structure21 and the obtained nanowires were significantly thicker than the ones reported here. The EDS analysis of both types of nanowires confirmed the presence of gold and copper in the nanowires (Figure S5). The atomic composition was close to Au23Cu77, although it differed slightly from one wire to another (about to 10%). The deviation in atomic composition with respect to the initial molar ratio between the two metals was attributed to the difference in reduction rates of both metals and to the likely
Figure 5. The vis-NIR absorbance spectrum of the ultrathin helical Au−Cu nanowires. The visible and NIR regions are indicated by two different colors. Inset: Photo of the colloidal solution containing the helical Au−Cu nanowires.
centered at 546 and 611 nm represent the surface plasmon resonance (SPR) of Au and Cu, respectively. Moreover, two other peaks appearing in the NIR region are due to the helical morphology of the wires. In conclusion, ultrathin helical gold−copper nanowires have been efficiently synthesized through a wet chemistry route and characterized by advanced transmission electron microscopy and vis-NIR spectroscopy. This opens up possibilities for designing nanomaterials with new physical, chemical, and biological properties that will be difficult to achieve without the helical structure. Furthermore, the important characteristic of ultrathin inorganic nanowires is that they can outperform carbon nanotubes in different applications that involve dispersibility in solution. D
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b04184. Chemical synthesis, characterization, and Figures S1−S5. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This project was supported by grants from the National Center for Research Resources (G12RR013646-12) and the National Institute on Minority Health and Health Disparities (G12MD007591) from the National Institutes of Health. The authors would also like to acknowledge the Mexican Council for Science and Technology, CONACYT (Mexico) through the national scholarship and the DGAPA-UNAM IN108915 project.
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