Patent Publication Number: US-2013230717-A1

Title: Copper nanostructures and methods for their preparation

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application No. 61/530,734, filed Sep. 2, 2011, which is incorporated herein by reference in its entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH &amp; DEVELOPMENT 
     The claimed subject matter was developed with Government support under NSF Grant Nos. 0804088, 1104614 and ECS-0335765, awarded by the National Science Foundation. The Government has certain rights in the claimed subject matter. 
    
    
     BACKGROUND 
     Copper nanostructures have increasingly been found to have significant utility in the microelectronics and catalysis fields. For example, copper nanowires (e.g., polycrystalline wires that are usually fabricated by lithographic techniques) are currently used as interconnects in computer chips. Copper nanostructures hold great promise for use in microelectronics including low-cost flexible displays, light-emitting diodes and thin film solar cells. Copper nanostructures have also been found to exhibit localized surface plasmon resonance (LSPR) peaks in the visible region. Copper nanoparticles have been widely used as catalysts for water-gas shift and gas detoxification reactions. 
     Metal nanostructures in the shape of nanowires are believed to find widespread use in applications such as the fabrication of transparent electrodes for flexible electronic and display devices. They are also useful in formulating conductive coatings for electrostatic discharging and electromagnetic shielding. Research has conventionally focused on use of silver nanowires for use in such applications. Compared to silver, copper is several orders of magnitude more abundant and is significantly less expensive. Copper nanowires with reduced sizes (i.e., reduced diameters) exhibit increased transmittance of visible light making them even more ideal for electronics use. 
     A continuing need exists for copper nanostructures that are suitable for use in various applications such as microelectronics and catalysis and for methods for producing them. A particular need exists for copper nanowires with relatively small diameters and methods for producing such nanowires. 
     SUMMARY 
     One aspect of the present disclosure is directed to a method for producing a copper nanostructure. A reaction mixture is formed in a reaction vessel. The reaction mixture includes a copper-containing compound, a capping agent and a reducing agent. The copper-containing compound is reduced with the reducing agent to cause copper to form a copper nanostructure. The pressure in the reaction vessel is less than about 190 kPa and/or the temperature of the reaction mixture is less than about 115° C. during formation of the nanostructure. 
     A further aspect of the present disclosure is directed to a population of copper nanowire structures. Each structure has a length and a diameter. The average diameter of the copper nanowire structures is less than about 40 nm and the average ratio of length to diameter of the copper nanowire structures is at least about 10:1. 
     Another aspect of the present disclosure is directed to a copper nanowire structure. The structure includes at least about 60 wt % copper and is characterized by a penta-twinned shape. 
     Various refinements exist of the features noted in relation to the above-mentioned aspects of the present disclosure. Further features may also be incorporated in the above-mentioned aspects of the present disclosure as well. These refinements and additional features may exist individually or in any combination. For instance, various features discussed below in relation to any of the illustrated embodiments of the present disclosure may be incorporated into any of the above-described aspects of the present disclosure, alone or in any combination. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an XRD pattern of a copper nanowire produced according to Example 1; 
         FIGS. 2-3  are SEM images of copper nanowire structures produced according to Example 1; 
         FIG. 4  is a TEM image of copper nanowire structures produced according to Example 1; 
         FIG. 5  is a graph showing the distribution of diameters of copper nanowires produced according to Example 1; 
         FIG. 6  is a TEM image of a portion of a copper nanowire produced according to Example 1; 
         FIG. 7  is a high-resolution TEM image of the region marked by the box in  FIG. 6 ; 
         FIG. 8  is a TEM image of a second portion of a copper nanowire produced according to Example 1; 
         FIG. 9  is a high-resolution TEM image of the region marked by the box in  FIG. 8 ; 
         FIG. 10  is a UV-vis spectra of an aqueous suspension of copper nanowires having an average diameter of about 24 nm and of silver nanowires having an average diameter of about 80 nm; 
         FIG. 11  is a SEM image of copper bi-pyramids that formed after 30 minutes of reaction as produced according to Example 3 with an inset showing the SEM image of a tilted sample showing the pentagonal cross-section of the nanocrystals; 
         FIG. 12  is a SEM image of copper bi-pyramids that formed after 1 hour of reaction as produced according to Example 3; 
         FIG. 13  is a SEM image of copper bi-pyramids that formed after 3 hours of reaction as produced according to Example 3; 
         FIG. 14  is a SEM image of copper bi-pyramids that formed after 6 hours of reaction as produced according to Example 3 with an inset showing the SEM image of a tilted sample showing the pentagonal cross-section of the nanocrystals; 
         FIG. 15  is a TEM image of the copper nanowire of  FIG. 14 ; 
         FIG. 16  is a high-resolution TEM image of the region marked by the box in  FIG. 15 ; 
         FIG. 17  is a UV-vis spectra of the aqueous suspension of copper nanostructures of  FIG. 11 ; 
         FIG. 18  is a SEM image showing one type of pentagonal bi-pyramid; 
         FIG. 19  is a geometric model of the bi-pyramid of  FIG. 18 ; 
         FIG. 20  is a SEM image showing a second type of pentagonal bi-pyramid; 
         FIG. 21  is a geometric model of the bi-pyramid of  FIG. 20 ; 
         FIG. 22  is a SEM image showing a third type of pentagonal bi-pyramid; 
         FIG. 23  is a geometric model of the bi-pyramid of  FIG. 22 ; 
         FIG. 24  is a SEM image of copper nanocubes that formed after 30 minutes of reaction as produced according to Example 4; 
         FIG. 25  is a SEM image of copper nanocubes that formed after 1 hour of reaction as produced according to Example 4; 
         FIG. 26  is a SEM image of copper nanocubes that formed after 6 hours of reaction as produced according to Example 4; 
         FIG. 27  is a XRD pattern of the copper nanocubes produced according to Example 4; 
         FIG. 28  is a TEM image of a copper nanocube produced according to Example 4; 
         FIG. 29  is high-resolution TEM image of the region marked by the box in  FIG. 28 ; 
         FIG. 30  is the UV-vis spectra of three separate aqueous suspensions of 50 nm, 100 nm and 200 nm copper nanocubes; and 
         FIG. 31  is a schematic of the reaction pathways used to produce various copper nanostructures according to Examples 1-4. 
     
    
    
     DETAILED DESCRIPTION 
     The field of the disclosure relates to copper nanostructures and, more particularly, to copper nanostructures with relatively small dimensions and methods for producing such structures. The ratios of the various reaction products may be adjusted to produce other structures such as tad-pole shaped nanowires, nanocubes or pentagonal bi-pyramids. 
     Provisions of the present disclosure are directed to copper nanostructures (e.g., nanowires) and methods for producing copper nanostructures. Without being held to any particular theory, it has been found that copper nanostructures formed at relatively low pressures (e.g., atmospheric pressure) and/or low temperatures (e.g., 100° C. or less) have a relatively small diameter. Further it has been found that by adjusting the concentration of the components of the reaction mixture and/or adjusting the respective ratios of the components, the shape of the resulting nanostructure may be changed. 
     Methods for Producing Copper Nanostructures 
     Generally the copper nanostructures of the present disclosure are produced by forming a reaction mixture that contains a copper-containing compound, a capping agent and a reducing agent. The copper-containing compound is reduced by the reducing agent to produce elemental copper that forms the nanostructure. During reduction, the pressure and/or temperature of the reaction vessel may be maintained relatively low (e.g., a pressure of less than about 190 kPa and/or a temperature of less than about 115° C.) such that nanowires with a relatively small diameter may be produced. 
     Suitable copper-containing compounds that may be included in the reaction mixture include any compounds from which elemental copper)(Cu 0  is formed upon contact with a reducing agent or during electrolysis or an electroless deposition method, or upon decomposition. Exemplary copper-containing compounds include copper (II) nitrate (Cu(NO 3 ) 2 , anhydrous or hydrated), copper (II) sulfate (CuSO 4 , anhydrous or hydrated), copper (II) chloride (CuCl 2 , anhydrous or hydrated), copper (II) hydroxide (Cu(OH) 2 , anhydrous or hydrated), copper (II) acetate (Cu(CH 3 COO) 2 , anhydrous or hydrated), and copper (II) trifluoroacetate (Cu(CF 3 COO) 2 , anhydrous or hydrated). Suitable copper-containing compounds may also include various ligands and/or chelates that contain copper without limitation. 
     The reducing agent that is combined with the copper-containing compound is any compound (or ligand or chelate) that reduces copper ions into elemental copper to deposit as a nanostructure seed or as part of the growing copper nanostructure. Suitable reducing agents include glucose (a or (3 form) and ascorbic acid. 
     In addition to the copper-containing compound and the reducing agent, a capping agent is included in the reaction mixture. The capping agent stabilizes the resulting nanostructure (e.g., by changing the surface energies of different facets) and prevents aggregation between the structures. The capping agent becomes incorporated into the matrix during formation of the copper nanostructure-based composites. Suitable capping agents include alkylamines. Alkylamines have the general structure of Formula (I) shown below 
     
       
         
         
             
             
         
       
     
     wherein R 1  is an alkyl group (or substituted alkyl group) and R 2  and R 3  are either hydrogen or an alkyl group (or substituted alkyl group). In some embodiments, the alkyl group of R 1  has 25 carbon atoms or less. One particularly preferred alkylamine is hexadecylamine (“HDA”). HDA has been found to be an effective capping agent for copper and has a strong selectivity toward the {100} facets of the nanostructure. In some particular embodiments, HDA is used as a capping agent and glucose is used as a reducing agent. In such embodiments, copper nanostructures may be produced in relatively large quantities with high purity and good uniformity. Other alkylamines of Formula (I) that may be used include octadecylamine and oleylamine. 
     Generally, the components that form the reaction mixture are dissolved in water; however in some embodiments an organic solvent may be used or even a two-solvent system may be used. The copper-containing compound, the reducing agent and capping agent may be added to any suitable reaction vessel in any manner suitable to those of skill in the art (e.g., as solids or in solution form and in any order of addition). Suitable vessels may be lab scale (e.g., reaction vials) or may be commercial-scale (e.g., steel vessels which may be polymer-lined). Preferably the reaction vessel is agitated during formation of the copper nanostructures. The nanostructures may be produced batch-wise or in a continuous manner (e.g., a continuous-stirred tank reactor (CSTR)). 
     Upon formation of the reaction mixture, the reaction contents are heated. Generally, the reaction mixture is heated to a temperature less than about 115° C. In some embodiments, the reaction mixture is heated to a temperature less than about 110° C. or less than about 105° C. Preferably, the reaction mixture is heated to a temperature of 100° C. or less to prevent the reaction mixture from boiling causing the pressure of the reaction contents to increase as in pressurized vessel systems. It is preferred that the reaction mixture be maintained at about ambient pressure (101 kPa) or less. However in some embodiments, the pressure is maintained to be below about 190 kPa, less than about 150 kPa, less than about 125 kPa or less than about 105 kPa. 
     In this regard, it has been found that by utilizing a reduced temperature (e.g., less than about 115° C. and preferably less than about 100° C.) and/or a reduced pressure (e.g., less than about 190 kPa and preferably 101 kPa or less) copper nanostructures and, in particular, copper nanowires are produced with a relatively small diameter (e.g., less than about 40 nm, less than about 30 nm or even less than about 25 nm). Without being bound to any particular theory, it is believed the reduced temperature and/or pressure influences nucleation of the copper nanostructure. It is believed that the seeds that are produced at such reduced temperatures and pressures have a decahedral shape which allows nanowires having a penta-twinned structure to be produced. Such penta-twinned copper structures have a relatively small diameter compared to conventional copper nanostructures. 
     Generally the reaction is substantially complete after 6 hours or less. Other reaction times may be used depending on the concentration of components added, the desired structure of the nanomaterial and the desired conversion. Reaction times may be at least about 30 minutes, at least about 1 hour, at least about 3 hours, at least about 5 hours, from about 30 minutes to about 6 hours or from about 30 minutes to about 3 hours. 
     The nanostructure that forms as a result of the process of embodiments of the present disclosure depends on the relative reaction rates and, in particular, the amount of reducing agent and/or capping agent present in the reaction mixture. At relatively low reaction rates, decahedral seeds are nucleated and form penta-twinned nanowires with relatively uniform diameter due to anisotropic growth ( FIGS. 2-4 ). At greater reaction rates, isotropic growth is promoted during early stage of growth. As the reaction continues, the reaction rate becomes smaller and the structure narrows to form a pentagonal bi-pyramid ( FIG. 11 ). As the reaction proceeds, an even smaller reaction rate results and the pentagonal bi-pyramid further grows into tadpole-shaped nanowires ( FIGS. 12-15 ). 
     In some embodiments, the reaction conditions are controlled such that single crystal seeds are nucleated rather than decahedral seeds. This allows nanocubes ( FIGS. 24-26 ) to form rather than nanowires and/or bi-pyramids. 
     In this regard, copper nanowires have been found to be produced without formation of bi-pyramids ( FIGS. 2-4 ) at relatively low concentrations of reducing agent and relatively high concentrations of capping agent. If the concentration of reducing agent is increased, pentagonal bi-pyramids form and the bi-pyramids taper off to form nanowires as the reaction proceeds. In contrast, if the concentration of capping agent is lowered, nanocubes form. Without being bound to any particular theory, it is believed that nanocubes may form due to oxidative etching. The oxidative etching causes single crystal seeds to form which results in growth of nanocubes. Such oxidative etching is blocked by the capping and protective effect of the capping agent (e.g., HDA) at higher concentrations of capping agent allowing multiply twinned copper seeds to form. 
     The relative molar concentrations between copper, reducing agent and capping agent that may result in formation of the various structures are shown in Table 1 below. Generally these ratios were used in Examples 1-4 described below. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Relative amounts of components used to 
               
               
                 grow various copper nanostructures. 
               
            
           
           
               
               
               
               
            
               
                   
                   
                   
                 NANO-BI- 
               
               
                   
                 NANOWIRES 
                 NANOCUBES 
                 PYRAMIDS 
               
               
                   
                   
               
            
           
           
               
            
               
                 Concentration (mol/l) 
               
            
           
           
               
               
               
               
            
               
                 Copper 
                 0.012 
                 0.012 
                 0.012 
               
               
                 Capping Agent 
                 0.075 
                 0.037 
                 0.075 
               
               
                 Reducing Agent 
                 0.028 
                 0.028 
                 0.055 
               
            
           
           
               
            
               
                 Molar Ratios 
               
            
           
           
               
               
               
               
            
               
                 Capping Agent/ 
                 6.1 
                 3.0 
                 6.1 
               
               
                 Copper 
               
               
                 Reducing Agent/ 
                 2.3 
                 2.3 
                 4.5 
               
               
                 Copper 
               
               
                 Capping Agent/ 
                 2.7 
                 1.3 
                 1.3 
               
               
                 Reducing Agent 
               
               
                   
               
            
           
         
       
     
     In this regard, the relative amounts of the components may be adjusted to produce the desired structure as appreciated by those of skill in the art. 
     Copper Nanowires 
     Copper nanowires produced in accordance with the present disclosure are characterized by a relatively small diameter and a high aspect ratio. Generally, the population of copper nanowire structures that are produced according to embodiments of the present disclosure have an average diameter of less than about 40 nm. In some embodiments the population has an average diameter of less than about 30 nm, less than about 25 nm, from about 10 nm to about 40 nm, from about 10 nm to about 30 nm from about 15 nm to about 40 nm, from about 15 nm to about 30 nm, from about 20 nm to about 40 nm or from about 20 nm to about 30 nm. The average length of the copper nanowire structures produced according to embodiments of the present disclosure may be at least about 10 nm, at least about 100 nm or even at least about 1 mm. In some embodiments, the average aspect ratio (i.e., the average ratio of length to diameter of the copper nanowire structures) is at least about 10:1. In other embodiments, the aspect ratio is at least about 50:1, at least about 100:1, at least about 1000:1, at least about 10,000:1 or even at least about 25,000:1. 
     The population of nanowires contains copper and amounts of organic material (e.g., the capping agent). In this regard, the amount of copper in the population of nanowires (and in each nanowire) by at least about 60 wt % copper or, as in other embodiments, at least about 70 wt % copper, at least about 80 wt % copper, from about 60 wt % to about 99 wt % copper or from about 70 wt % to about 95 wt % copper. 
     In this regard, the properties applied above may be an average of the population of copper nanowires that is produced or of individual nanowires. Populations of copper nanowires may include at least about 100 copper nanowires, at least about 1000 copper nanowires, at least about 10,000 copper nanowires, at least about 1×10 6  copper nanowires or even at least about 1×10 9  copper nanowires. 
     The copper nanowires of the present disclosure have been found to have a penta-twinned structure (i.e., five single crystallites bound together). It is believed the penta-twinned structure is bound by ten {111} facets at the two ends and five {100} side faces. It should be noted that the copper nanowires are not constructed on a template or membrane. In contrast, metallic copper atoms themselves give the nanowire its structural characteristics. 
     Other Nanostructures 
     As discussed above, other structures may be produced by varying the reaction conditions. In some embodiments, a tadpole shaped nanostructure may be produced in which a bi-pyramid structure tapers from a base of about 200 nm ( FIG. 11 ). If the reaction is allowed to continue, the reaction slows and a nanowire with a radius less than about 40 nm extends from the point of the bi-pyramid ( FIGS. 12-15 ). In some embodiments, the reaction conditions are controlled such that copper nanocubes are formed. In the initial stage of reaction (e.g., at about 1 hour), the cube sides are about 50 nm in size ( FIG. 25 ). If the reaction is allowed to continue (e.g., for about 6 hours) the edges of the cube grow to about 200 nm in size ( FIG. 26 ). 
     EXAMPLES 
     The reaction conditions were varied in Examples 1-4 to produce various structures as shown in  FIG. 31 . It should be noted that other reaction conditions (e.g., component concentrations) may be used to produce the desired nanostructures and the recited conditions are exemplary and should not be considered in a limiting sense. 
     Example 1 
     Production of Copper Nanowires and Images Collected from Same 
     To produce copper nanowires, CuCl 2.2 H 2 O (0.021 g), HDA (0.18 g) and glucose (0.05 g) were dissolved in water (10 ml) in a vial (22.2 ml, borosilicate glass vial, with a black phenolic molded screw cap and polyvinyl-faced pulp liner, VWR International (Radnor, Pa.)) at room temperature. After the vial had been capped, the solution was magnetically stirred at room temperature overnight. The capped vial was then transferred into an oil bath and heated at 100° C. for 6 hours under magnetic stirring. As the reaction proceeded, the solution changed its color from blue to brown and finally red-brown. All the chemicals were obtained from Sigma-Aldrich (St. Louis, Mo.) and used as received. 
     To prepare samples for electron microscopy characterizations, the as-prepared aqueous suspensions were directly dropped onto silicon substrates (for SEM) or carbon-coated copper grids (for TEM and high-resolution TEM) and then dried under the ambient conditions of a chemical laboratory. The silicon substrates or copper grids were then rinsed with hot water (about 60° C.) to remove the excess HDA and glucose, followed by another round of drying. The products could have alternatively been collected as powders by use of centrifugation processes. 
     Scanning electron microscope (SEM) images were captured of the copper nanowires dried on silicon substrates. All SEM images were captured with a field-emission microscope (Nova NanoSEM 230, FEI (Hilsboro, Oreg.)) operated at 15 kV. All transmission electron microscope (TEM) images were conducted with a microscope (Tecnai G2 Spirit, FEI (Hilsboro, Oreg.)) operated at 120 kV. High-resolution TEM imaging was performed using a microscope (2100F, JEOL (Tokyo, Japan)) operated at 200 kV. Powder x-ray diffraction (XRD) patterns were recorded using a diffractometer (DMAX/A, Rigaku (The Woodlands, Tex.)) operated at 35 kV and 35 mA. The concentrations of Cu (II)/Cu (I) left behind in the reaction solutions were determined using an inductively-coupled plasma mass spectrometer (ICP-MS, PerkinElmer (Waltham, Mass.)). 
       FIG. 1  shows the X-ray diffraction (XRD) pattern of a copper nanowire. The three peaks at 20=43.5, 50.7, and 74.4° correspond to diffractions from {111}, {200}, and {220} planes, respectively, of face-centered cubic copper (JCPDS #03-1018). No other phases such as Cu 2 O and CuO were detected. The concentrations of Cu 2+ /Cu +  ions left behind in the reaction solution was measured using inductively-coupled plasma mass spectrometry (ICP-MS). It was determined that the precursor had been converted into atomic copper at a percentage of 93%. 
     The scanning electron microscopy (SEM) image shown in  FIG. 2  demonstrates that copper nanowires could be prepared in high purity, typically approaching 95%, without any post-synthesis separation. Only a very small amount of copper nanocubes was found to co-exist with the nanowires. In addition, the nanowires were found to be highly flexible and some of them showed bending more than 360 degrees without being broken. Both the SEM image at a higher magnification ( FIG. 3 ) and TEM image ( FIG. 4 ) reveal that the nanowires were uniform in diameter and tended to be aligned in parallel to form bundles during sample preparation. The nanowires had an average diameter of 24±4 nm as calculated from 100 nanowires randomly selected from a number of TEM images ( FIG. 5 ). The lengths of the copper nanowires varied in the range of several tens to hundreds of micrometers; some of them were as long as several millimeters. The band-like contrast (see the box in  FIG. 4 ) observed on the TEM images can be attributed to strains caused by bending or twisting. 
       FIGS. 6-9  show transmission electron microscopy (TEM) images and the corresponding high-resolution TEM images taken from the middle ( FIGS. 6 and 7 ) and end portions ( FIGS. 8 and 9 ) of two different Cu nanowires, respectively. The insets in  FIG. 7  and  FIG. 9  schematically illustrate the orientations of the copper nanowires relative to the incident electron beam (indicated by arrows). The high-resolution TEM images ( FIGS. 7 and 9 ) show the existence of {111} twin planes parallel to the long axis of the copper nanowire. When the direction of the e-beam was perpendicular to the bottom side of the pentagonal nanowire ( FIG. 7 ), two sets of fringes with lattice spacing of 2.1 nm and 1.3 nm were observed, corresponding to the {111} and {220} planes of copper, respectively.  FIG. 9  shows the high-resolution TEM image taken from a copper nanowire oriented with one of its side faces parallel to the e-beam. The fringes with lattice spacing of 2.1, 1.8, and 1.3 Å could be indexed to the {111}, {200}, and {220} planes of copper, respectively. Based on the analysis of both SEM and high-resolution TEM images, it is evident that the copper nanowires had a penta-twinned structure bound by ten {111} facets at the two ends and five {100} side faces, which are consistent with the results previously reported for other metals (e.g., Ag, Au, and Pd). 
     Example 2 
     Comparison of the UV-Vis Transmission Spectra Between the Copper Nanowires of Example 1 and Silver Nanowires 
     UV-vis spectra were taken with a diode array spectrophotometer (Cary 50, Varian (Palo Alto, Calif.)).  FIG. 10  shows UV-vis transmission spectra recorded from aqueous suspensions of the 24-nm copper nanowires of Example 1 and penta-twinned silver nanowires of 80-nm in diameter (prepared according to the literature) at roughly the same metal concentration (30 μg/ml), suggesting a slightly higher transmittance in the visible region for the copper nanowires. This higher transmittance could be attributed to the smaller diameter of the copper nanowires. 
     Example 3 
     Production of Copper Nanostructures with a Bi-Pyramid Shape and Images Collected from Same 
     The preparation procedure of Example 1 was used to produce copper nanocrystals but the concentration of glucose (i.e., the reducing agent) was increased from 5 to 10 mg/ml. As can be seen from  FIGS. 11-16 , tadpole-like copper nanostructures resulted from the increased amount of reducing agent. In an effort to uncover the growth mechanism, the products obtained at different reaction times were analyzed as detailed in  FIGS. 11-16 . In the initial stage (t=30 min), the solution changed its color from blue to red-brown due to the formation of tapered copper nanocrystals whose diameter gradually changed from 200 to 25 nm over a length of 0.5 to 1 μm ( FIG. 11 ). The tapered cooper nanocrystals exhibited a UV-vis absorption peak around 591 nm ( FIG. 17 ) and are characterized by a pentagonal bi-pyramid structure (see inset of  FIG. 11  and  FIGS. 18-23 ) formed by stretching apart the five-fold apices of a decahedron. 
     After the reaction had proceeded to 1 hour ( FIG. 12 ), thin copper nanowires of about 24 nm in diameter started to appear from the thinner end of a tapered nanocrystal. As the reaction was continued for three hours ( FIGS. 13 and 14 ), the copper nanowires further grew along the long axes with almost no change to their diameters. These results indicate that the tadpole-like copper nanowires originated from the tapered nanocrystals. The SEM image in the inset of  FIG. 14  indicates that the tadpole-like copper nanowires also had a pentagonal cross-section. The TEM and high-resolution TEM images shown in  FIGS. 15 and 16  further confirm a tadpole-like morphology and a penta-twinned structure for the copper nanowires. 
     Example 4 
     Production of Copper Nanostructures with a Cubic Shape and Images Collected from Same 
     The preparation procedure of Example 1 was used to produce copper nanocrystals but the concentration of HDA (i.e., the capping agent) was decreased from 18 mg/ml to 9 mg/ml. Copper nanocubes ( FIGS. 24-26 ) formed rather than copper nanowires.  FIGS. 24-26  are SEM images of the products obtained after 0.5 h, 1 h, and 6 h of reaction, respectively.  FIG. 27  gives an XRD pattern of the nanocubes obtained at 6 h. For bulk copper, the strongest XRD diffraction is the (111) peak, followed by the (200), (220), and (311) peaks. In contrast, the copper nanocubes tend to give (200) diffraction as the strongest peak because of their preferential orientation with {100} planes parallel to the substrate. The high-resolution TEM image of an individual Cu nanocube viewed along the &lt;100&gt; zone axis ( FIGS. 28-29 ) clearly shows well-resolved, continuous fringes with lattice spacing of 1.8 Å, corresponding to the {100} planes, indicating that the nanocube was a single crystal bound by {100} facets. 
       FIG. 30  shows UV-vis absorption spectra taken from the copper nanocubes dispersed in water. The copper nanocubes exhibited a major SPR peak in the visible region, whose position was red-shifted from 565 to 625 nm as the edge length of the nanocubes was increased from 50 to 200 nm. Compared to silver nanocubes with a similar size, the SPR peak of the copper nanocubes was positioned at a much longer wavelength. 
     When introducing elements of the present disclosure or the preferred embodiments(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
     As various changes could be made in the above apparatus and methods without departing from the scope of the disclosure, it is intended that all matter contained in the above description and shown in the accompanying figures shall be interpreted as illustrative and not in a limiting sense.