Cu Bimetallic Nanoparticles via Double-Target ... - ACS Publications

Article Cite This: Langmuir 2017, 33, 12389-12397

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Au/Cu Bimetallic Nanoparticles via Double-Target Sputtering onto a Liquid Polymer Mai Thanh Nguyen,† Hong Zhang,† Lianlian Deng,† Tomoharu Tokunaga,‡ and Tetsu Yonezawa*,† †

Division of Materials Science and Engineering, Faculty of Engineering, Hokkaido University, Kita 13 Nishi 8, Kita-ku, Sapporo 060-8628, Japan ‡ Department of Quantum Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan S Supporting Information *

ABSTRACT: Alloy nanoparticles (NPs) of a bimetal system, Au/Cu, that form intermetallic compounds in a bulk state have been successfully produced using a double-target sputtering technique onto a low-cost and biocompatible liquid polymer (polyethylene glycol, PEG). The formation of an Au/Cu solid solution alloy in individual NPs was revealed by scanning transmission electron microscopy−energy-dispersive X-ray elemental mapping analysis. Altering the sputter currents for Au and Cu targets resulted in a tailored NP composition, but the particle sizes did not significantly vary. We found similar structures, sizes, and optical properties of Au/Cu NPs obtained by doublehead sputtering on carbon-coated transmission electron microscopy grids or PEG and by Au/Cu alloy target sputtering. Random alloy formation occurred in matrix sputtering using double-target heads. This method is advantageous for manipulating the alloy composition through highly independent control of sputter parameters for each metal target.

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INTRODUCTION Synthesis of nanoparticles (NPs) has become an active research field because of various advanced applications of NPs.1−14 Researchers continue to develop more efficient, highly controllable, greener, and simpler preparation methods.15−23 Chemical reduction is an effective procedure to obtain controlsized metal NPs, but hazardous reducing reagents such as sodium borohydride or hydrazine are used to reduce metal ions.24−26 The introduction of a low-vapor-pressure liquid to a sputtering chamber has recently been used as a new approach to directly produce NPs dispersed in liquid media by a topdown vacuum technique.27−31 This method has the advantage of producing pure materials without using toxic reductants. In addition, it can serve as a tool to manipulate particle characteristics by varying the liquid medium, its composition, and functionality.28−31 Wagener and co-workers first reported sputtering onto silicone oil to produce metal NPs, such as magnetic Fe NPs in 1999.28 In 2006, Torimoto et al. introduced an ionic liquid as a substrate for sputtering of Au to obtain more stable NP dispersions.29 NPs of different noble metals (e.g., Pt and Pd) have been prepared in ionic liquid by sputtering.32,33 Molten salt,30 liquid polymers,31 vegetable oils,34 biomolecules,35 and various stabilizing agents36−38 have been used in sputtering to offer functionality, dispersing ability, and versatility in composition and size control of metal NPs. Considerable progress has been made in understanding the formation mechanism of the NPs formed in this method over the last 2 decades.31−40 © 2017 American Chemical Society

Alloy NPs, a mixture of two or more metal elements in single NPs, offer novel and tunable properties that emerge from the coexistence or interaction of two or more elements in single NPs.2,4,5,10,41,42 In colloidal synthesis, the formation of alloy NPs often requires the coreduction of metal cations followed by alloying. However, the formation of heterogeneous structured (core/shell, cluster segregation) NPs can be induced by the reduction order difference of metal sources in solution or homogenous nucleation according to the redox potentials and growth of each metal. Alloying of two different metals was observed even in immiscible bimetallic systems upon reducing the particle size to the nanoscale regime.43 The development of various metal system alloys by reducing the particle size is an alternative approach for the production of alloy NPs from bulk alloys because sputtering of a metal target creates metal atoms/ clusters directly. The creation of alloy particles was evidenced via sputtering of a bimetallic target with an alternative configuration of two metals or an alloy target.44−49 The Torimoto group utilized sputtering of fan-shaped bimetallic targets to ionic liquids to prepare Au/Ag,44 Au/Pt,45 Au/Pd,46 and Au/Cu47 alloy NPs. Wang et al. sputtered alloy targets to obtain Au/Pd48 and Pt/ Ni49 NPs on carbon nanomaterials. The fan-shaped targets are the round targets composed of fan-shaped pieces of each Received: September 11, 2017 Revised: September 30, 2017 Published: October 3, 2017 12389

DOI: 10.1021/acs.langmuir.7b03194 Langmuir 2017, 33, 12389−12397

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Langmuir Table 1. Sputter Currents, Composition, and Size for Cu, Au, and Au/Cu Alloy NPs sputter current

Cu content

sample no.

ICua/mA

IAua/mA

products

CCu,ICPb/mol %

1 2 3 4 5 6 7 8 9 10 11g 12g

0 0 40 50 30 40 50 30 40 50 20 40

10 20 0 0 10 10 10 20 20 20

Au Au Cu Cu Au/Cu Au/Cu Au/Cu Au/Cu Au/Cu Au/Cu Au/Cu Au/Cu

0 0 100 100 57 65 71 34 42 48 43 43

CCu/gridc/mol %

particle size CCu/PEGd/mol %

0 0 100 100

0 0 100 100

58 ± 11

41 ± 11

43 ± 15 44 ± 8 37 ± 11

24 ± 9

dgride/nm 1.7 1.8 1.2 1.3 1.3 2.1 1.7 1.7 2.2 2.6 1.7 2.5

± ± ± ± ± ± ± ± ± ± ± ±

0.7 0.5 0.2 0.3 0.4 0.5 0.5 0.5 0.6 0.8 0.3 1.7

dPEGf/nm 2.6 2.7 1.0 1.5 1.7 1.9 2.0 2.1 2.5 2.2 2.0 2.5

± ± ± ± ± ± ± ± ± ± ± ±

0.8 0.9 0.3 0.4 0.4 0.5 0.5 0.6 1.0 0.6 0.5 0.9

a Sputter current for Cu and Au targets or Au/Cu target. bCu content measured by ICP-OES. cCu content measured by EDX mapping in samples sputtered on the TEM grid. dCu content measured by EDX mapping in samples sputtered on PEG. eSize of the particles sputtered on the TEM grid. f Size of the particles sputtered on PEG. gSputter using an Au/Cu alloy target.

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element, the composition of which can be controlled by the contents of the elements. Alloy targets should be prepared with each composition. Furthermore, the composition of a single alloy target may vary during sputtering because the sputtering rates strongly depend on the elements. Alternatively, the cosputtering of two metal targets was introduced for various alloy compositions that were obtained in a cavity array and inserted into the chamber by single sputtering.50 We designed a double-target head system for sputtering onto a polymer liquid to separately control many sputtering parameters to varying the composition, size, and structure of the resulting NPs. We previously reported the successful preparation of Au/Ag solid solution NPs, in which Au and Ag are miscible uniformly in single NPs, in liquid polyethylene glycol (PEG) using a double-head system for sputtering of Au and Ag targets.51,52 We demonstrated that varying the sputter currents of each target could be used to tailor the compositions and tune the surface plasmon absorbance or photoluminescence properties of the resulting Au/Ag NPs.51,52 The Au/Cu system created intermetallic compounds in the bulk state, which was different from the Au/Ag system that formed a complete solid solution for the entire composition.53 Recently, Torimoto et al. found that random Au/Cu alloy NPs were formed by sputtering of a bimetallic Au/Cu target (1/1 mol/ mol) onto an ionic liquid and transformed to an L10-ordered structure by subsequent heating at elevated temperatures.47 We investigated the random alloy formation of sputtered Au/Cu NPs near room temperature by the double-head sputtering of Au and Cu. Random alloys were formed in the resulting Au/Cu NPs, which were sputtered onto PEG and directly on a transmission electron microscopy (TEM) grid. We found that the Au/Cu NPs produced using two separated monometallic targets have similar structures, sizes, and optical properties with those produced using an Au/Cu alloy target. This confirms the random alloy formation by double-head sputtering. A larger particle size with broader size distribution was obtained for the sample prepared in PEG compared to the sample prepared on a TEM grid using similar sputter currents, which suggested that the particle aggregation and alloying occurred in the liquid phase.

EXPERIMENTAL SECTION

Double-Target Sputter Head System. In our designed doubletarget system, each target was separately connected to the power supply and voltage/current control unit. The distances between the target, the liquid substrate, and the applied current/voltage were adjusted independently for each target. The target heads were cooled by a chilled solvent. The metal target size was 50 mmϕ. The Au (99.99%) target and Au/Cu alloy target (1/1 mol/mol) were purchased from Tanaka Precious Metals (Tokyo), and the Cu (99.96%) target was purchased from Nilaco (Tokyo). Preparation of NPs. PEG (Junsei, Japan) was degassed and dried at 80 °C for 2 h under vacuum. Then, 10 cm3 of PEG was added to a glass Petri dish (63 mmϕ) and inserted into the sputter chamber. The distance between the center of each target and the center of the liquid PEG substrate was 110 mm. After the chamber was purged with Ar 10 times to eliminate oxygen molecules and prevent oxidation of Cu nanoclusters and NPs, the pressure inside the sputter chamber was adjusted to 2 Pa with Ar. The sputtering was carried out for 30 min at 30 °C at a stirring speed of 80 rpm. The capturing medium (PEG) was stirred to obtain homogenous NPs. The applied current on each metal target is given in Table 1. An Au/Cu alloy target was also used in one of the double heads at the sputtering currents of 10, 20, and 40 mA. For TEM observation and X-ray photoelectron spectroscopy (XPS) analyses, a carbon-coated Cu or Mo TEM grid or a silicon wafer is placed at the center of the empty Petri dish and used to collect the sputtered particles for 30 s at various sputter currents of Au/Cu alloy target, single metallic target, and double targets. TEM samples of NPs dispersed in PEG were prepared as follows: a TEM grid was dipped in PEG-containing NPs and was carefully dipped in ethanol to dissolve PEG and finally dried under vacuum before TEM observation. Characterization. Ultraviolet−visible (UV−vis) spectra of the obtained NP dispersions were measured by a Shimadzu UV-1800 spectrophotometer with a quartz cell of 1 cm optical path length. The size, surface morphology, composition, and elemental mapping of NPs were analyzed using a transmission electron microscope (JEOL JEM2100F, 200 kV) and a scanning transmission electron microscope (STEM) with energy-dispersive X-ray spectroscopy (EDX) (JEOL JEM-ARM200F, 200 kV and FEI Titan Cube, 200 kV). Inductively coupled plasma-optical emission spectroscopy (ICP-OES, ICPE-9000) was used to determine the elemental composition. The Au/Cu samples were prepared via direct sputtering of Au and Cu double targets and Au/Cu alloy target on a glass substrate for 30 min at various sputtering currents to eliminate polymer matrix contamination from the average composition measurement. XPS with X-ray radiation from an Al Kα anode (1486.6 eV) was used to analyze the chemical state of the sputtered particles. The sputtered Au/Cu NPs were deposited on cleaned Si wafers for the measurement. Si 2p3/2 (99.2 12390


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produced more metal atoms/clusters per Ar+ ion bombardment. Composition, STEM−HAADF, and Elemental Mappings of Sputtered Au/Cu NPs. Average compositions of the sputtered Au/Cu samples were measured using ICP-OES and are given in Figure 5. For double-target sputtered samples, the mean Cu content (34−71 at %) increased with an increase of sputter current for the Cu target. Similar results were obtained by a decrease of the sputter current for the Au target. This indicated that sputter current variation of the double-head sputtering with Au and Cu targets allowed a wide range of tailored composition for the produced samples. Each 10 mA decrease in the sputter current for the Au target increased Cu in the sputtered particles compared to the effect of a 10 mA increase in the sputter current for the Cu target. This was due to a higher sputter rate of Au compared to that of Cu. As can be seen in Figure 5, at the same sputter current (i.e., 10 and 20 mA) for the Au and Cu target, the atomic ratio of sputtered materials is Au/Cu = 3:1. The sputter rate was proportional to the amount of the sputtered materials; therefore, the ICP-OES result suggested that the sputter rate of Au was ∼3 times higher than that of Cu in the range of sputter current used. We performed STEM−HAADF (high-angle annular darkfield) and STEM−EDX (energy-dispersive spectrometry) mapping of NPs produced by double-target sputtered samples compared to the single Au/Cu alloy target samples to confirm the alloy formation and composition of single NPs further. In addition, the results for NP samples obtained by direct sputtering on the carbon-coated TEM grid for 30 s were also collected for comparison. On the basis of the ICP-OES data, EDX mappings were performed for samples 10, 9, and 7 which were nearly equimolar Au and Cu, Au rich, and Cu rich in composition, respectively (Figures 6, S1, and S2). The STEM−HAADF and STEM−EDX elemental mapping images of the Au/Cu NP samples sputtered on a carbon-coated Mo TEM grid and onto PEG using the sputter currents 20 mA for Au and 50 mA for Cu target (sample 10) are shown in Figure 6. The EDX elemental mapping images demonstrated the distribution of both Au and Cu in entire single particles sputtered on the TEM grid (Figure 6, top row) and onto PEG (Figure 6, middle row). This was consistent with the elemental mapping results of the sputtered NPs using an Au/Cu alloy target (Figure 6, bottom row), which suggested the formation of the Au/Cu alloy in each particle. The average compositions of Cu estimated from the mapping images (collected in Table 1) were slightly lower, and the compositions estimated from EDX had larger deviations than those measured using ICP-OES (Figure 5). This indicated that the NPs with different compositions might be present in the sputtered samples or that some Cu atoms/clusters were not incorporated into Au/Cu alloy NPs. The intensity line profiles (Figure 7) of these double-head sputtered NPs showed a random atomic distribution of Au and Cu in single NPs. If only Au or Cu atoms were contained in two columns of atoms with similar heights, the theoretical intensity ratio between the column of Au and Cu atoms is 4.5−7.4. This number was estimated from the relationship between the intensity of highangle scattered electron, I, the atomic column proportional to its thickness, t, and the atomic number, Z, of the element in the column to a power α of 1.5−2, that is, I ≈ tZα. However, as observed in our samples, the ratios were 1−2.5 (Figure 7). Randomly mixed Au and Cu atoms were present in each atomic column. The results exhibited the formation of random Au/Cu

eV) was used as a reference for charging effect correction of the narrow scan XPS spectra.

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RESULTS AND DISCUSSION Preparation of Cu/Au Alloy NPs and TEM Observation of Sputtered Cu/Au NPs. Our experimental double-target sputtering system is schematically illustrated in Figure 1. The

Figure 1. Schematic of the sputtering vacuum chamber. Au/Cu alloy NPs formed in the gas phase by Ar+ plasma bombardment of the double target (Au and Cu), falling onto a liquid matrix (PEG) under stirring condition. Rotary and molecular pumps were used to evacuate the chamber.

two sputtering Au and Cu targets are positioned facing each other at an angle of 120°. The metal targets were utilized to eject atoms or clusters into the chamber by bombardment of Ar+ ions under high vacuum. The ejected atoms and clusters aggregated in the chamber to form bimetallic nanoclusters and NPs in the gas phase. TEM images of the resulting NPs dispersed in PEG are shown in Figure 2, and the NPs sputtered on the TEM grid are shown in Figure 3. The average sizes of the particles, such as Au, Cu, and Au/Cu NPs, dispersed in PEG were 1.0−2.8 nm (Figure 4 and Table 1), and in particular those of Au/Cu NPs were 1.7−2.8 nm. The present study produced smaller Au/Cu NPs by the double-head sputtering system compared to the Au/Cu particles obtained by mixed Au/Cu single-target sputtering into an ionic liquid at 40 mA, as reported by Torimoto et al. (2.4−3.4 nm).47 Our Au/Cu particles had similar sizes and were smaller than single metallic Au NPs and slightly larger than single metallic Cu NPs in most cases. Compared to the NP sizes obtained by direct sputtering of metal targets to TEM grids for 30 s (Figure 3), the average sizes of NPs obtained in liquid PEG were still slightly larger (within 0.5 nm for Au/Cu NPs), as summarized in Table 1 and plotted in Figure 4. This result indicated that the aggregation or particle growth on the liquid surface or in the liquid phase could occur during sputtering as was reported elsewhere.31,34,40 The aggregation in the liquid was more apparent when sputtering was performed with the Au target only. Among the Au/Cu samples, the NP size increased slightly (within 0.5 nm) as the sputter current increases. The particle sizes of samples 8−10 using a sputter current of 20 mA for the Au target were larger than the particle sizes of samples 5−7 using a sputter current of 10 mA. The higher sputter rate at a higher sputter current 12391


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Figure 2. TEM images of sputtered Au/Cu NPs into PEG: (top) at a constant sputter current for the Au target at 10 mA and the sputter currents for the Cu target at (a) 30, (b) 40, and (c) 50 mA (samples 5, 6, and 7, respectively); (middle) at a constant sputter current for the Au target at 20 mA and the sputter currents for the Cu target at (d) 30, (e) 40, and (f) 50 mA (samples 8, 9, and 10, respectively); (bottom) (g) using only the Au target at a sputter current of 20 mA (sample 2), (h) using only the Cu target at a sputter current of 50 mA (sample 4), and (i) using the Au/Cu alloy target (sample 12).

in PEG (Figure 2g) as was reported elsewhere for sputtered metal NPs onto liquids,30,34,40 which shifted the plasmon absorbance maximum to a longer wavelength than the representative Au NP absorption at 520 nm.54 Similarly, Cu NPs produced at a sputter current of 50 mA (sample 4) showed a weak plasmon resonance at 580 nm, indicating the presence of metallic Cu NPs.55 This plasmon absorbance maximum became negligible in the UV−vis spectrum of sample 3 (Figure S3) produced using a sputter current of 40 mA for Cu. A relatively smaller number of Cu NPs with a size less than 2 nm (Figure 3) were produced at the lower sputtering rate when a lower sputter current was used. UV−vis spectra of Au/Cu NPs (samples 8−10, 2.1 ± 0.6 to 2.5 ± 1.0 nm) produced using double target with the sputter currents of 20 mA for Au and 30−50 mA for Cu exhibited a broad absorbance and without a well-defined peak maximum. Further, these spectra are identical with the UV−vis spectrum of 2.5 ± 0.9 nm Au/Cu alloy NPs produced using a single Au/ Cu alloy target (sample 12, a sputter current of 40 mA). This feature, that is, broad absorbance and nondefined peak maximum, was also observed for Au/Cu alloy NPs obtained via sputtering in ionic liquid using single fan-shaped Au and Cu targets47 or double target.50 Thus, UV−vis results suggest that the double-target sputtered Au/Cu NPs were random alloys as the ones obtained using an Au/Cu alloy target. This is consistent with the EDX mapping results discussed above. We noticed that despite large differences in composition (34−71 at % Cu, Figure 5), these Au/Cu NPs obtained using doubletarget sputtering (samples 8−10) and a single alloy target

alloy NPs by sputtering of an Au/Cu alloy target combined with the EDX elemental mappings. However, we observed that the intensities of the atomic columns (after background subtraction) of single NPs were not uniform (also observed for the sample prepared using an Au/Cu alloy target). A “domain” that comprises about one atomic component can exist in single NPs prepared by double-target sputtering (Figure 7). Unfortunately, the space resolution in the EDX elemental mapping images was not sufficient to distinguish these domains in such small NPs (2 nm). HAADF, elemental mapping images, and intensity line profiles of Au-rich NPs (sample 9, 42 mol % Cu by ICP-OES, sputter currents of 20 and 40 mA for Au and Cu targets, respectively) and Cu-rich NPs (sample 7, 71 mol % Cu by ICP-OES, sputter currents of 10 mA for Au target and 50 mA for Cu) are given in Figures S1 and S2, respectively. These results consistently showed that the double-target system produced random alloy (solid solution) NPs. In addition, EDX and ICP-OES results confirmed that the particle composition could be tailored by controlling the sputter currents for the Au/ Cu system. UV−Vis Spectra of NP Dispersions of the Alloy NPs Obtained by Sputtering. UV−vis spectra of samples 2, 4, 8− 10, and 12 for Au, Cu, and Au/Cu sputtered at 20 mA for Au and 20−50 mA for Cu, and alloy target sputtered Au/Cu NPs are shown in Figure 8. UV−vis spectra of all produced samples 1−12 are given in Figure S3. When only the Au target was used with a sputter current of 20 mA (sample 2), the obtained UV− vis spectrum showed a surface plasmon resonance absorbance maximum at 540 nm. Aggregation of Au particles was observed 12392


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Figure 3. (a−i) TEM images of sputtered Au/Cu NPs onto a TEM grid for samples 5, 6, 7, 8, 9, 10, 2, 4, and 12.

Figure 4. Particle diameter measured using TEM images for samples 1−12 vs sputter current for Cu target. Sputtering with a current of 0 mA for a target indicates that the experiment was performed only on the other target.

Figure 5. Cu content (measured by ICP-OES) was plotted for doublehead sputtered samples at various sputtered currents for the Cu target (10−50 mA) and the sputter currents for the Au target of 10 (empty diamond) and 20 mA (filled diamond). Composition of Cu in the samples prepared by sputtering of the Au/Cu alloy target (doublehead system) is marked with a half-filled diamond. The results were averaged from three measurements, and all deviations were less than 6%.

(sample 12) showed UV−vis spectra with a very similar shape. This can arise from the composition variation among single NPs of the sample as indicated in EDX mappings and the random mixing of Au and Cu.56 It is possible that random mixing of Au and Cu in small-sized sputtered Au/Cu NPs (1.7−2.5 nm) can hinder the appearance of well-defined characteristic absorbance of each metal component. A similar phenomenon in UV−vis spectra was observed for samples 5−7 (sputter currents of 10 mA for Au target and 30−50 mA for Cu target) and 11 (a sputter current of 20 mA for Au/Cu alloy target), as shown in Figure S3. However, these samples produced at a lower sputter current for Au target have a lower absorbance intensity compared with samples 8−10 and 12. On the other words, the absorbance intensity of Au/Cu alloy NPs

is more sensitive to the sputter current of Au target than that of Cu target. On the basis of the ICP-OES results shown above, the sputter rate of Au is approximately three times higher compared with that of Cu. This means that the change in the amount of the produced particles is more sensitive to the change in the sputter current of Au target than that of Cu target. Consequently, a larger number of Au/Cu particles (also slightly larger particle size) was produced for an increase of 10 mA in the sputter current for Au rather than for an increase of 10 mA in the sputter current for Cu target. This resulted in the 12393


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Figure 6. HAADF and EDX mappings [Au L (red), Cu K (green), and overlay of Au and Cu] for nearly equimolar Au/Cu samples (48 at % Cu by ICP-OES) obtained by direct sputtering onto the TEM grid (top row) and onto PEG (middle row, sample 10) with the sputter currents of 50 mA for the Cu target and 20 mA for the Au target. Bottom row shows HAADF and EDX mapping images of Au/Cu particles (sample 11) obtained by direct sputtering of a single Au/Cu alloy target onto the TEM grid at a sputter current of 20 mA. The scale bars are 3, 2, and 5 nm for all images in the top, middle, and bottom rows, respectively.

Figure 7. Intensity line profile of atomic columns in the corresponding red dashed lines in HAADF images of Au/Cu NPs obtained by direct sputtering onto the TEM grid (a,b) and by sputtering onto PEG (c) with the sputter currents of 50 mA for the Cu target and 20 mA for the Au target. (d) Intensity profile for Au/Cu NPs prepared by sputtering of an Au/Cu alloy target on the TEM grid.

increase in optical absorbance intensity that is more sensitive to an increase in the sputter current for Au compared with that for Cu target. Moreover, because Cu is prone to oxidation and the oxidation of Cu can hinder its contribution in the UV−vis spectra of Au/Cu NPs, we analyzed the surface properties of Au/Cu NPs. Some oxidation of Cu was observed as shown in XPS fitting and Auger parameters for double-target sputtered Au/Cu NPs (sample 10). A weak satellite peak from 940 to 946 eV in the Cu 2p XPS spectrum (Figure S4) is a typical sign of

CuO, which was in good agreement with the asymmetry of Cu 2p3/2 peaks toward higher binding energy (dotted curves). The Auger parameter of the main component is 1849.0 eV, which confirmed that the main component is metallic Cu (Table S1).38 Judging from the XPS and Auger parameter results, the double-target sputtered samples onto PEG contain metallic Cu and CuO (35 at %). The oxidation of Cu can contribute to the UV−vis absorbance intensity dependence on the sputter current of the Au target rather than that of the Cu target. 12394


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Scientific Research for Young Researcher (B) from JSPS (17K14072). H.Z. thanks CEED, Hokkaido University for the financial support during his stay in Sapporo. Authors thank H. Tsukamoto for his experimental assistance and T. Tanioka and R. Oota for their assistance in TEM observations. We acknowledge M. Watanabe and M. Kiuchi of the Open Facility of Hokkaido University, Sousei Hall for their assistance with the elemental analysis of our samples using ICPE-9000. We thank K. Suzuki for fruitful discussion in XPS measurement.

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(1) Nanoparticles From Theory to Application; Schmid, G., Ed.; WileyVCH Verlag GmbH & Co. KGaA, 2004; Chapter 2, pp 32−42. (2) Toshima, N.; Yonezawa, T. Bimetallic NanoparticlesNovel Materials for Chemical and Physical Applications. New J. Chem. 1988, 22, 1179−1201. (3) Gawande, M. B.; Goswami, A.; Felpin, F.-X.; Asefa, T.; Huang, X.; Silva, R.; Zou, X.; Zboril, R.; Varma, R. S. Cu and Cu-Based Nanoparticles: Synthesis and Applications in Catalysis. Chem. Rev. 2016, 116, 3722−3811. (4) Kim, D.; Resasco, J.; Yu, Y.; Asiri, A. M.; Yang, P. Synergistic geometric and electronic effects for electrochemical reduction of carbon dioxide using gold−copper bimetallic nanoparticles. Nat. Commun. 2014, 5, 4948. (5) Holewinski, A.; Idrobo, J.-C.; Linic, S. High-performance Ag−Co alloy catalysts for electrochemical oxygen reduction. Nat. Chem. 2014, 6, 828−834. (6) Park, J.; Zhang, L.; Choi, S.-I.; Roling, L. T.; Lu, N.; Herron, J. A.; Xie, S.; Wang, J.; Kim, M. J.; Mavrikakis, M.; Xia, Y. Atomic Layer-byLayer Deposition of Platinum on Palladium Octahedra for Enhanced Catalysts toward the Oxygen Reduction Reaction. ACS Nano 2015, 9, 2635−2647. (7) Chen, J. E.; Lian, H.-Y.; Dutta, S.; Alshehri, S. M.; Yamauchi, Y.; Nguyen, M. T.; Yonezawa, T.; Wu, K. C.-W. Synthesis of Magnetic Mesoporous Titania Colloidal Crystals Through Evaporation Induced Self-assembly in Emulsion as Effective and Recyclable Photocatalysts. Phys. Chem. Chem. Phys. 2015, 17, 27653−27657. (8) Chen, K.-C.; Dutta, S.; Yamauchi, Y.; Alshehri, S. M.; Nguyen, M. T.; Yonezawa, T.; Shen, K.-H.; Wu, K. C.-W. Mesoporous EuropiumDoped Titania Nanoparticles (Eu-MTNs) for Luminescence-Based Intracellular Bio-Imaging. J. Nanosci. Nanotechnol. 2015, 15, 9802− 9806. (9) Hunt, S. T.; Milina, M.; Alba-Rubio, A. C.; Hendon, C. H.; Dumesic, J. A.; Román-Leshkov, Y. Self-assembly of Noble Metal Monolayers on Transition Metal Carbide Nanoparticle Catalysts. Science 2016, 352, 974−978. (10) Liu, K.; Bai, Y.; Zhang, L.; Yang, Z.; Fan, Q.; Zheng, H.; Yin, Y.; Gao, C. Porous Au−Ag Nanospheres with High-Density and Highly Accessible Hotspots for SERS Analysis. Nano Lett. 2016, 16, 3675− 3681. (11) Liao, Y.-T.; Chen, J. E.; Isida, Y.; Yonezawa, T.; Chang, W.-C.; Alshehri, S. M.; Yamauchi, Y.; Wu, K. C.-W. De Novo Synthesis of Gold-Nanoparticle-Embedded, Nitrogen-Doped Nanoporous Carbon Nanoparticles (Au@NC) with Enhanced Reduction Ability. ChemCatChem 2016, 8, 502−509. (12) De Juan, L. M. Z.; Nguyen, M. T.; Yonezawa, T.; Tokunaga, T.; Tsukamoto, H.; Ishida, Y. Structural Control Parameters for Formation of Single-Crystalline β-Sn Nanorods in Organic Phase. Cryst. Growth Des. 2017, 17, 4554−4562. (13) Wu, L.; Mendoza-Garcia, A.; Li, Q.; Sun, S. Organic Phase Syntheses of Magnetic Nanoparticles and Their Applications. Chem. Rev. 2016, 116, 10473−10512. (14) Shirai, H.; Huang, Y.-Y.; Yonezawa, T.; Tokunaga, T.; Chang, W.-C.; Alshehri, S. M.; Jiang, B.; Yamauchi, Y.; Wu, K. C.-W. Hardtemplating Synthesis of Macroporous Platinum Microballs (MPtM). Mater. Lett. 2016, 164, 488−492. (15) Dahl, J. A.; Maddux, B. L. S.; Hutchison, J. E. Toward Greener Nanosynthesis. Chem. Rev. 2007, 107, 2228−2269.

Figure 8. UV−vis spectra of Au NPs sputtered at 20 mA (sample 2), Cu NPs sputtered at 50 mA (sample 4), and Au/Cu alloy NPs produced using double-target sputtering at the sputter currents of 20 mA for Au and 30−50 mA for Cu (samples 8−10) and using single Au/Cu alloy target sputtering at 40 mA (sample 12). Arrows pointing at the position of the absorbance maxima for single metallic Au and Cu NPs were used for the visual guide.

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CONCLUSIONS Double-head sputtering of Au and Cu targets produced random Au/Cu alloy NPs of 1.7−2.8 nm, as evident from the UV−vis spectra, STEM−HAADF, and EDX mapping images. Varying the sputter current for each target altered the resulting particle composition. The negligible change in UV−vis spectra of the samples with varied Cu target sputter currents was due to the compensation of small particle size, the composition effect of Au/Cu alloy NPs, and the partial oxidation of Cu. Use of protection agents for Cu in alloy samples is under ongoing investigation. Taguchi experimental design can be used for obtaining an optimal condition of alloy formation using this system when considering many other factors involving the sputter process and alloy formation, such as the target-liquid surface distance, pressure of sputter chamber, angle and distance of the targets, and the sputter current/voltage.

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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.langmuir.7b03194. STEM−HAADF images, EDX elemental mappings, and UV−vis and XPS spectra of Au/Cu NPs (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tetsu Yonezawa: 0000-0001-7371-204X Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is partially supported by Hokkaido University. M.T.N. thanks a partial financial support by Grant-in-Aid for 12395


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