Aqueous Phase Synthesis of Au–Cu Core–Shell Nanocubes and

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Aqueous Phase Synthesis of Au−Cu Core−Shell Nanocubes and Octahedra with Tunable Sizes and Noncentrally Located Cores Chi-Fu Hsia, Mahesh Madasu, and Michael H. Huang* Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan S Supporting Information *

ABSTRACT: Copper nanocubes with tunable edge lengths over the range from 49 to 136 nm and ultrasmall octahedra with opposite corner distances of 45, 51, and 58 nm have been synthesized in aqueous solutions by reducing CuCl2 or copper acetate with ascorbic acid in the presence of octahedral gold nanocrystal cores and hexadecylamine (HDA) at 100 °C for 45 min to 1.5 h. Addition of HDA increases the solution pH and acts as a coordinating ligand to the copper ions to facilitate controlled copper shell growth. Due to ultralarge lattice mismatch between Au and Cu, nonuniform copper deposition yields cubes and octahedra with noncentrally located gold cores. The Au−Cu octahedra show little shift in the plasmonic band with increasing particle size. For Au−Cu nanocubes, the degree of absorption band red-shift gets smaller as cube size increases. The Au−Cu nanocubes have shown reasonable reactivity toward 4-nitrophenol reduction at 40 °C.

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INTRODUCTION Plasmonic metals such as Au, Ag, and Pd have been synthesized in aqueous solution with cubic, octahedral, rhombic dodecahedral, and other intermediate nanostructures.1−9 If singlecomponent nanocrystals with shape evolution are more difficult to obtain, use of metal cores may be a solution to forming shell materials with desirable particle morphologies. Facet-dependent catalytic properties have been observed using these polyhedral nanocrystals, showing particle shape control can bring benefits to organocatalysis performance.5,10−14 Naturally, the next synthetic challenge to overcome is the growth of shapecontrolled copper nanocrystals, because Cu is an important and abundant plasmonic material with unexplored catalytic activities. Literature reports on the growth of Cu nanocrystals are rare because reduction of copper(II) sources tends to yield Cu2O. Preparation of polyhedral Cu nanoparticles with shape tunability is therefore very challenging, especially when synthesis is carried out in an aqueous solution. Tuan and coworkers prepared 76 nm copper nanocubes by injecting CuCl in an organic mixture of squalane, octadecylamine, oleylamine (OLA), and trioctylphosphine (TOP) heated to 330 °C.15 Large copper octahedra with an edge length of 145 nm were synthesized similarly by heating a mixture of CuCl, TOP, and OLA to 335 °C.16 In another study, a mixture of Ni(acac)2, CuCl2, and TOP dissolved in OLA was heated to 180 °C for 4 h to generate copper nanocubes and nanowires.17 To grow Cu nanocubes in an aqueous solution, CuCl2, glucose, and hexadecylamine (HDA) dissolved in water was heated to 100 °C for 6 h.18 By doubling the concentration of HDA used, Cu nanowires were produced. Studies exploring the formation of Cu shells using Au and Pd cores yielded only Au−Cu particles © XXXX American Chemical Society

exposing {111} faces and Pd−Cu cubes bound by {100} surfaces, and particle size control was not demonstrated.19,20 Anisotropic Au−Cu rods have been synthesized from small polyhedral Au cores.21 Clearly, direct synthesis of Cu polyhedra in an aqueous solution with shape variation remains difficult. We would like to address this synthetic challenge by using Au cores for the aqueous-phase formation of Au−Cu nanocubes and octahedra with tunable sizes. In the present study, we have prepared Au−Cu core−shell nanocubes and octahedra in aqueous solutions with tunable sizes by reducing CuCl2 with ascorbic acid in the presence of octahedral gold nanocrystal cores and HDA at 100 °C for 45 min to 1.5 h. The Au−Cu octahedra have ultrasmall sizes. The function of HDA has been examined, suggesting its role as a coordinating ligand and a pH tuning agent. Optical properties of the Au−Cu nanocrystals were also examined. Anisotropic core−shell particles were obtained because the Au cores are not centrally positioned. Au−Cu nanocubes have been used to catalyze 4-nitrophenol reduction. These small Cu nanocubes and octahedra have potential as efficient catalysts for various organic reactions.

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EXPERIMENTAL SECTION

Chemicals. Anhydrous copper(II) chloride (CuCl2; 97%), ascorbic acid (AA, 99.7%), and ammonia borane (NH3BH3, 97%) were purchased from Sigma-Aldrich. Cupric acetate monohydrate (Cu(CH3COO)2·H2O) was obtained from J.T. Baker. Hydrogen tetrachloroaurate trihydrate (HAuCl4·3H2O, 99.99%) and hexadecylReceived: January 28, 2016 Revised: March 23, 2016

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DOI: 10.1021/acs.chemmater.6b00377 Chem. Mater. XXXX, XXX, XXX−XXX

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Figure 1. SEM images of Au−Cu core−shell nanocubes with average edge lengths of (a) 49, (b) 55, (c) 65, (d) 78, (e) 97, (f) 128, and (g) 136 nm. All scale bars are equal to 100 nm.

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amine (CH3(CH2)15NH2, HDA) were acquired from Alfa Aesar. 4Nitrophenol (99%) was obtained from Acros Organics. All above chemicals were used as received without further purification. Ultrapure distilled and deionized water (18.3 MΩ) was used. Synthesis of Au−Cu Core−Shell Nanocubes and Octahedra with Tunable Sizes. Octahedral gold nanocrystals with an average opposite corner distance of 35 nm were synthesized following our previously reported procedure.22 To make Au−Cu core−shell nanocubes with increasing average sizes from 47 to 138 nm, 0.024 g of HDA was first dissolved in deionized water in a glass vial, followed by the addition of 0.1 M CuCl2 solution and octahedral Au core solution. Next, different amounts of ascorbic acid solution were introduced. The total solution volume is 10 mL, and the final HDA concentration is 0.01 M. For exact reagent amounts, please see Table S1. The glass vial was then heated for 45 min to 1 h in an oven with temperature set at 100 °C. After the reaction, the glass vial was cooled to room temperature. To remove HDA, the solution was centrifuged at 8500 rpm for 10 min. The supernatant was removed, and the precipitate was redispersed in deionized water for two more cycles of centrifugation. The particles were finally dispersed in 2 mL of water. Au−Cu core−shell octahedra were prepared following the same procedure. However, Cu(CH3COO)2 was chosen as the copper source and less amounts of ascorbic acid were used. The heating time was 1.5 h. See Table S2 for the exact reagent amounts used in the preparation of Au−Cu octahedra. Use of Au−Cu Nanocubes for 4-Nitrophenol Reduction. For a typical catalysis reaction, 0.025 mmol of 4-nitrophenol solution was added to 3 mL of 2:1 volume ratio of water to ethanol. Then, 78 nm Au−Cu nanocubes weighing 3 mg were introduced to the above solution. Finally, 0.75 mmol of NH3BH3 was added as a reducing agent to the reaction mixture and stirred at 40 °C. At selected time points, 30 μL of the solution was withdrawn and diluted with 1 mL of deionized water. The solution was then centrifuged at 8500 rpm for 2 min to remove the nanocubes. UV−vis spectrum of the solution was recorded immediately. After the completion of the reduction reaction, the light greenish yellow starting solution turned to colorless due to full conversion to the 4-aminophenol product. Instrumentation. Scanning electron microscopy (SEM) images of the nanocrystals were obtained using a JEOL JSM-7000F electron microscope. Transmission electron microscope (TEM) characterization was performed on a JEOL JEM-2100 microscope with an operating voltage of 200 kV. Elemental line scanning images were acquired by energy-dispersive X-ray spectroscopy (EDS) using the same JEOL JEM-2100 microscope equipped with a STEM unit and an Inca Energy 250 detector from Oxford Instrument. X-ray diffraction (XRD) patterns were recorded on a Shimadzu XRD-6000 diffractometer with Cu Kα radiation. UV−vis absorption spectra were taken using a JASCO V-670 spectrophotometer. X-ray photoelectron spectroscopy (XPS) spectra were collected using an ULVAC-PHI Quantera SXM spectrometer.

RESULTS AND DISCUSSION Synthesis of Au−Cu Core−Shell Nanocubes and Octahedra and Their Structural Characterization. Figure S1 shows the SEM image of the octahedral Au nanocrystals formed by a hydrothermal synthesis approach. The average opposite corner distance is just 35 nm. They exhibit a surface plasmon resonance (SPR) absorption band centered at 547 nm. Au−Cu nanocubes were synthesized by reducing CuCl2 with ascorbic acid in the presence of HDA and the octahedral Au cores. The solution was heated in a closed glass vial to 100 °C for 45 min to 1 h. By primarily lowering the amount of ascorbic acid introduced, Au−Cu core−shell nanocubes with increasing sizes can be obtained. Figure 1 shows SEM images of the synthesized Au−Cu core−shell nanocubes with average edge lengths of 49, 55, 65, 78, 97, 128, and 136 nm. The particles are highly uniform in size and shape. Figure S2 and Table S3 provide size distribution histograms and standard deviations for these samples. Some right bipyramids bound by the {100} facets were also produced. The Au cores can be seen in the SEM images of smaller nanocubes, and many of them appear not centrally located. Figure S3a displays the XRD pattern of Au−Cu nanocubes. The Cu (200) reflection peak is strongest due to the preferential deposition of the cubes on a substrate with their cubic faces. The Au peaks are observable but weak in intensity, because the Au cores reside inside the cubes. Although hexadecylamine has generally been regarded as a capping agent, the fact that it is an amine suggests that it can act as a base to tune the solution pH.18 The final solution after adding all the reagents for the growth of 78 nm Au−Cu cubes has a pH value of 6.2. In the absence of HDA, the same solution shows a pH at 2.6. Cu(II) ions and ascorbic acid yield an acidic solution. Addition of HDA makes the solution much less acidic, so a significant fraction of ascorbic acid may become ascorbate with a higher reducing power.23 A strongly acidic solution is also not favorable for Cu2+ ion reduction because of possible reduction of dissolved oxygen (O2 + 4H+ + 4e− → 2H2O E° = 1.23 V). HDA can also act as a coordinating ligand to Cu(II) ions forming complex species with its amine group. After adding HDA to a CuCl2 solution, the absorption spectrum changes dramatically (see Figure S4), indicating the formation of Cu−HDA complex. Formation of Cu−HDA complex should lead to a lower reduction potential than that of Cu2+ ion (Cu2+ + 2e− ↔ Cu E° = 0.34 V; [Cu(NH3)4]2+ + e− ↔ [Cu(NH3)2]+ + 2NH3 E° = 0.10 V and [Cu(NH3)2]+ + e− B

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Figure 2. TEM images of Au−Cu core−shell nanocubes with average edge lengths of (a) 55, (b) 65, and (c) 97 nm. The model shows a core−shell right bipyramid.

Figure 3. (a, b) TEM image of a Au−Cu core−shell nanocube taken from the 55 nm cube sample and its corresponding SAED pattern. (c) EDS line scan on a Au−Cu nanocube. The blue and red lines correspond to Au and Cu signals, respectively. (d, e) TEM image of another Au−Cu core−shell nanocube and HR-TEM image of the square region in panel d.

↔ Cu + 2NH3 E° = −0.10 V; Cu(ethylenediamine)2+ + e− ↔ Cu + 2 ethylenediamine E° = −0.119 V).24,25 This may modulate the reduction rate for a more controlled Cu deposition process. With a lower reduction potential, heating the solution becomes necessary to promote Cu ion reduction; solution color would not change if heating is not applied after mixing the reagents. Without adding HDA, a messy product can form (data not shown). Further analysis of the core−shell nanocubes requires TEM characterization. Figure 2 shows TEM images of Au−Cu core− shell nanocubes with average edge lengths of 55, 65, and 97 nm. For all these samples, the octahedral gold cores are clearly visible inside cubic and right bipyramid particles. As expected, the same lattice planes of the core and shell components are parallel to each other, such that the six {100} corners of the octahedral Au core point at the six {100} faces of the cubic Cu shell.26 A close inspection reveals that the octahedral Au cores do not reside in the centers of the cubes and bipyramids in over 90% of the particles. Figure 3 presents detailed TEM examination of single Au−Cu nanocubes. Evidently, the octahedral Au cores are not centrally positioned but are found near one face of a cube. Moiré fringes can be seen over the Au core because of overlapping lattice fringes of Au and Cu.

The selected-area electron diffraction (SAED) pattern recorded along the [100] direction gives a square pattern of spots from Cu. Interestingly, many additional spots surrounding each bright Cu spot can be seen due to the double diffraction effect.19 Two diffraction spots from the Au core are marked. Because the unit cell of Au (4.079 Å) is substantially larger than that of Cu (3.615 Å), the Au spots should appear closer to the center spot. Notice that the corresponding Au and Cu spots are aligned on the same line, showing that the (200) planes of the Au core and the (200) planes of the Cu shell are parallel to each other. The high-resolution TEM (HR-TEM) image shown in Figure 3e confirms this exact lattice orientation relationship. The Au (200) planes with a lattice spacing of 2.02 Å run parallel to the Cu (200) planes with a spacing of 1.83 Å. Despite the ultralarge lattice mismatch between Au and Cu at 11.4%, epitaxial growth is still possible to give cubic Cu particles. By comparison, growth of cubic Cu shells on polyhedral Pd cores should not be surprising given their lattice mismatch of just 7.1%.20 It is interesting to note that the nanocube is coated with a thin layer of CuO, since lattice fringes corresponding to those of CuO have been measured (also see Figure S5). CuO coating has also been observed in Pd−Cu nanocubes. Energy-dispersive X-ray spectroscopy C

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Figure 4. TEM images of intermediate particles sampled at different points in the course of 55 nm Au−Cu core−shell nanocube synthesis: (a) 20, (b) 30, (c) 35, (d) 40, (e) 45, and (f) 60 min.

Figure 5. TEM images of Au−Cu core−shell octahedra with average opposite corner distances of (a) 45, (b) 51, and (c) 58 nm and their schematic drawings.

(EDS) line scan on a single nanocube confirms the Au core and Cu shell composition. To further evaluate the surface composition of Au−Cu cubes and octahedra (see discussion below on octahedra), XPS spectra were taken and the results are shown in Figure S6. Interestingly, Cu 2p3/2 and 2p1/2 peaks at, respectively, 932.1 and 952.1 eV for Au−Cu cubes (932.3 and 952.3 eV for Au−Cu octahedra) corresponding to those of Cu2O have been recorded.27 For CuO, the 2p3/2 peak should appear at 933.6 eV with strong satellite peaks.28 However, Cu and Cu2O have the same Cu 2p peak positions (Thermal Scientific XPS data). Cu2O has weak satellite peaks centered at 945 eV, but these peaks are absent or even weaker in Cu metal.27 Judging from the absence of the satellite peaks, surface copper atoms in Au−Cu particles should mostly have a zerovalence of metallic copper. In fact, the oxide coating is not visible in most Au−Cu nanocrystals as seen from the TEM images. To understand how Au−Cu core−shell nanocubes are formed, intermediate products present in the solution during shell growth were examined. Figure 4 gives TEM images of intermediate particles formed after 20, 30, 35, 40, 45, and 60 min of reaction. Although the solution color does not change within 35 min of reaction, a slight extent of shell growth is observable in particles collected after 20 min of reaction. However, some particles still appear without shell formation. Shell growth becomes apparent at 30 min. Interestingly, shell growth occurs only on one side of the Au core. Even though cubic Cu shells have largely been formed after 35−40 min of reaction, many particles still have not developed into a cubic

structure. Particles with Au cores surrounded by only three {100} faces of Cu shells, and hence give a rectangular contour, have been captured. Particles with rectangular shells are still observed after 45 min of reaction. At 60 min, most particles show the final cubic shell morphology. This observation suggests that anisotropic shell growth occurs initially. Cubic shells form only at a later stage. Because of this unsymmetrical growth mode, possibly resulting from the large lattice mismatch between Au and Cu making initial uniform coating energetically unfavorable, the Au cores are almost never found at the particle center. A similar growth mechanism has been observed for the Pd−Cu case.20 After the Cu shell has reached a certain thickness and lattice strain has sufficiently released, the shell eventually develops into a cubic shape. In contrast to the unsuccessful attempt to form octahedral Cu shells over Pd cores, we show that Au−Cu octahedra can also be prepared.20 Au−Cu core−shell octahedra were synthesized by making an aqueous mixture of HDA, copper acetate, octahedral Au cores, and ascorbic acid and heating the solution to 100 °C for 1.5 h. Particle size can be tuned by adjusting the volume of copper acetate and ascorbic acid solutions added. Figure S7 provides SEM images of Au−Cu core−shell octahedra with sizes of 45, 51, and 58 nm. Their size distribution histograms and standard deviations are shown in Figure S8 and Table S4. These octahedra also have high size and shape uniformity and are considered ultrasmall because of the small Au cores used and thin Cu shell deposited. Figure 5 shows TEM images of Au−Cu core−shell octahedra with average opposite corner distances of 45, 51, and 58 nm. While D

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Figure 6. (a, b) TEM image of a Au−Cu core−shell octahedron taken from the 58 nm octahedron sample and the corresponding HR-TEM image. (c) EDS line scan on a Au−Cu octahedron with red and blue lines corresponding to Au and Cu, respectively.

Figure 7. UV−vis absorption spectra of Au−Cu (a) cubes and (b) octahedra with tunable sizes. The arrows indicate the measured particle sizes.

Figure 8. (a) Time-dependent UV−vis absorption spectra of 4-nitrophenol reduction using 78 nm Au−Cu nanocubes as the catalyst. (b) The corresponding plot of ln(Ct/C0) vs time.

particles with uniform Cu shells are formed, still there are many octahedra with noncentrally positioned Au cores. The formation of particles with seemingly uniform shells can be understood considering the same core and shell morphology and the experimental control to reduce overall particle size by forming thin shells. For particles with thicker shells, such as the 58 nm sample, most octahedra show noncentrally located Au cores. Figure 6 gives TEM characterization on a single Au−Cu octahedron. The Cu shell thickness is only 5 nm or less. The

Au (111) planes and the Cu (111) planes run parallel to each other as expected. EDS line scan also indicates Au core and Cu shell composition. Figure S3b is the XRD pattern of Au−Cu octahedra, showing similarly intense reflection peaks for Au and Cu due to the thin shells. Optical Characterization. Figure 7 displays UV−vis absorption spectra of the synthesized Au−Cu cubes and octahedra with tunable sizes. Because Cu shows a SPR profile at longer wavelengths than that of Au, deposition of a shell of Cu E

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plasmonic band position with increasing particle size. For nanocubes, progressively smaller spectral red-shifts were recorded with increasing cube dimensions. Au−Cu nanocubes have shown moderate catalytic activity toward the 4-nitrophenol reduction reaction. The Cu nanocubes and octahedra can be explored for organocatalysis application.

on a Au core red-shifts the SPR absorption, in addition to increasing the particle size.29 After formation of a thin Cu shell, the SPR absorption band for 45 nm Au−Cu octahedra moves to 584 nm from that of octahedral Au cores at 547 nm (a shift of 37 nm). Interestingly, little change in the band maximum was recorded for the 51 and 58 nm Au−Cu octahedra at 586− 587 nm. Plasmonic absorption from the Au cores may contribute to the broad band feature. For Au−Cu cubes with increasing edge lengths from 49 to 97 nm, the Cu SPR absorption band red-shifts from 586 to 614 nm. For comparison, 76 nm Cu nanocubes have been reported to show a SPR band at 588 nm.15 There is also a broad absorption band centered at ∼735 nm for the 49 and 65 nm Au−Cu nanocubes and at longer wavelengths for larger cube samples. This SPR band is attributed to the plasmon excitation from right bipyramids. Their larger dimensions and a more stretched shape shifts the plasmonic band to longer wavelengths. Sensitivity to slight aspect ratio changes in the bipyramids can give rise to this broad band appearance. Strangely, the Cu SPR band becomes essentially unchanged at 614 nm despite a substantial increase in the edge length of Au−Cu cubes from 97 to 136 nm. Figure S9 gives a plot of the absorption band wavelength with respect to the nanocube edge length. It is unclear if the thin CuO layer has any effect on the overall absorption band position. Recently, it has been shown that a thin Cu2O shell can greatly tune the plasmonic band position.30 Another possible explanation is that this decreasing extent of plasmonic band shift with increasing particle size is an intrinsic property of Cu nanostructures. The Au cores have become relatively small and its effect on the plasmonic band is insignificant in large Au−Cu cubes. Catalytic Activity. The 78 nm Au−Cu nanocubes were employed as catalyst for the 4-nitrophenol reduction reaction by NH3BH3 to form 4-aminophenol. Figure 8 gives UV−vis absorption spectra for 4-nitrophenol reduction as a function of reaction time carried out at 40 °C. A ln(Ct/C0) vs time plot is presented, indicating first-order reaction kinetics with a rate constant of 0.0722 min−1. When the reaction was performed at 30 °C, it took 90 min to complete the reaction (data not shown). Compared to the reactivity of Au nanocubes for the same reaction, the Au−Cu cubes show inferior catalytic activity.12 However, Au−Cu nanocubes display better efficiency than Au−Ag cubes for the same reaction.5 These results may be explained by considering the work functions of these metals (ΦAu ∼ 5.3 eV, ΦCu ∼ 4.5 eV, and ΦAg ∼ 4.3 eV). When two metals with different work functions are brought in contact, electrons will redistribute from the metal with a lower work function to one with a higher work function.31 In this case, electrons should flow from the Cu shell to the Au core to reach equilibrium with equal chemical potential. Different surface electron concentrations may cause this catalytic efficiency order.

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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.chemmater.6b00377. Exact reaction conditions used to synthesize Au−Cu nanocubes and octahedra, procedure for the synthesis of octahedral Au cores and their characterization, size distribution histograms, XRD patterns, UV−vis spectra, additional SEM and TEM images, XPS spectra, and a plot of plasmonic band position versus nanocube size. (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 We thank the Ministry of Science and Technology of Taiwan for support of this research (MOST 101-2113-M-007-018-MY3 and 104-2119-M-007-013-MY3). We also thank Shao-Lou Jheng for assistance in the TEM characterization of intermediate products.

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REFERENCES

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CONCLUSIONS Through the use of octahedral Au cores, Cu nanocubes and octahedra have been synthesized in aqueous solution with tunable sizes. The synthetic condition is simple and particle growth is complete in 45 min to 1.5 h. The added hexadecylamine increases the solution pH and acts as a coordinating ligand to the Cu ions. Due to the large lattice mismatch between Au and Cu, nonuniform Cu shell growth yields cubes and octahedra with noncentrally located Au cores. The Au−Cu octahedra are ultrasmall and show little change in F

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Chemistry of Materials

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DOI: 10.1021/acs.chemmater.6b00377 Chem. Mater. XXXX, XXX, XXX−XXX