Au Bimetallic Nanoparticles by Physical

Ag/Au bimetallic nanoparticles (BNPs) with a size less than 2 nm were ... by the simultaneous reduction of both salts in solution,(8, 9) while the cor...
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Novel Formation of Ag/Au Bimetallic Nanoparticles by Physical Mixture of Monometallic Nanoparticles in Dispersions and Their Application to Catalysts for Aerobic Glucose Oxidation Haijun Zhang,† Minori Haba,‡ Mitsutaka Okumura,§,⊥ Tomoki Akita,∥,⊥ Shinji Hashimoto,‡ and Naoki Toshima*,‡,⊥ †

College of Materials & Metallurgy, Wuhan University of Science and Technology, Wuhan, Hubei Province 430081, China Department of Applied Chemistry, Tokyo University of Science Yamaguchi, SanyoOnoda-shi, Yamaguchi 756-0884, Japan § Department of Chemistry, Graduate School of Science, Osaka University, Machikaneyama, Toyonaka, Osaka 560-0043, Japan ∥ Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan ⊥ CREST, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan ‡

S Supporting Information *

ABSTRACT: Ag/Au bimetallic nanoparticles (BNPs) with a size less than 2 nm were prepared by physical mixture of colloidal dispersions of Ag and Au nanoparticles (NPs). This provides an example of fabrication of BNPs with selforganization by the reaction between metal NPs. Although Ag/Au BNPs having different structures and compositions are one of the most widely studied bimetallic systems in the literature due to their wide range of uses such as in catalysis, electronics, plasmonics, optical sensing, and surface-enhanced Raman scattering, we first prepared such BNPs by physical mixture and characterized them by UV−vis spectroscopy, SERS, XPS, TEM, and EDS in HR-STEM. The present fabrication method has the advantage of avoiding the unfavorable formation of AgCl precipitates in the reaction process which are always produced when Ag+ ions are used as a starting material in combination with a HAuCl4 precursor. These Ag/Au BNPs showed high catalytic activities for aerobic glucose oxidation, and the highest activity of 11 510 mol of glucose·h−1·mol of metal−1 was observed for the BNPs with a Ag/Au atomic ratio of 1/4; the activity value is about 2 times higher than that of Au NPs with nearly the same particle size. XPS and DFT calculation results show that the negatively charged Au atoms due to the electron charge transfer effects from neighboring Ag atoms and poly(N-vinyl-2-pyrrolidone) act as catalytically active sites and play an important role in the aerobic glucose oxidation.

1. INTRODUCTION Uncovering the growth mechanisms responsible for nanocrystals with various shapes and architectures has been a topic of intense study. Theories on the growth mechanisms largely fall into one of two categories:1−3 growth by monomer attachment and growth by nanoparticle (NP) attachment. According to classical theories of crystal growth, the shape of a nanocrystal is controlled by the relative surface energies of the different crystalline facets. The growth is assumed to proceed by monomer attachment to the existing nuclei. Very recently, Zheng’s group reported the growth of colloidal Pt3Fe nanorods by nanoparticle attachments of Fe and Pt NPs reduced by electron beam illumination.4 Real-time transmission electron microscopy (TEM) imaging revealed the growth of winding polycrystalline Pt3Fe nanorods by shape-directed nanoparticle attachment followed by straightening, orientation, and shape corrections to yield final single-crystal nanorods. Moreover, the spontaneous alloying of Au with Sb atoms5 at ambient © XXXX American Chemical Society

temperatures was also reported, and the kinetic considerations indicate that the diffusion coefficients of those metal NPs and Sb atoms may be many orders of magnitude larger than that of the bulk materials for the alloying. These results in understanding that the mechanism of colloidal nanocrystal growths using nanoparticles (NPs) as building blocks can provide a link between the world of single molecules and that of hierarchical nanostructures, and make way for the rational design of nanomaterials with controlled properties. These results also give a possible way of producing binary NPs by the collision and subsequent coalescence of single-component particles. This possibility has been investigated so far only by simulations,6 which have, however, indicated a promising methodology for experimental applications. Received: December 17, 2012 Revised: July 4, 2013

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times with water and then one time with ethanol under nitrogen to remove extra agents and byproducts. The residual ethanol of the PVPprotected Au(A) colloid was removed using a rotary evaporator at 40 °C. The PVP-protected Au(A) NPs were finally obtained by vacuum drying at 40 °C for 48 h. 2. In the other series (designated as “R” series), Au(R) and Ag(R) NPs stabilized by PVP were prepared by rapid injection of NaBH4; i.e., a NaBH4 solution was rapidly injected into an aqueous solution of AuCl4−/PVP or Ag+/PVP.12 For example, Au(R) NPs were prepared as follows: An aqueous solution of HAuCl4·4H2O (50 mL, 0.44 mM) was added to an aqueous PVP solution (50 mL, 44 mM) under vigorous stirring. The mixtures were stirred for 30 min in an ice−water bath of 0 °C. Then, an aqueous solution of NaBH4 (6.7 mL, 16.5 mM, 0 °C) was rapidly injected into the mixture under vigorous stirring. The addition time of rapid injection of NaBH4 into the AuCl4−/PVP solution was within 5 s. The molar ratio of PVP monomer units to the total metal ions (RPVP) was 100, and the molar ratio of NaBH4 to the total metal ions (RNaBH4) was 5. The color of the reaction mixtures immediately turned from pale yellow to dark brown, suggesting the formation of small Au(R) NPs. After additional stirring at 0 °C for 1 h, PVP-protected Au(R) NPs as transparent and brownish colloidal dispersions were washed using the same above-mentioned procedures. 2.3. Preparation of Ag/Au BNPs by Physical Mixture of MNPs. Two series of Ag/Au BNPs stabilized by PVP were prepared by the physical mixture as follows: In one series (designated as “R” series), the PVP-protected Ag/Au(R) BNPs with designed compositions were prepared by mixing the colloidal dispersions of PVPprotected Au(R) NPs with the colloidal dispersion of PVP-protected Ag(R) NPs in the required atomic ratios at room temperature for 2 h. In another series (designated as “A” series), the Ag/Au(A) BNPs were prepared by mixing the colloidal dispersions of Ag(A) and Au(A) NPs fabricated by the alcohol reduction at RT for 1 week. 2.4. Characterization of NPs. UV−vis (ultraviolet and visible light) absorption spectra were measured over a range of 200−800 nm with a Shimadzu UV-2500PC recording spectrophotometer using a quartz cell with a 10 mm optical path length. Surface-enhanced Raman scattering (SERS) was measured with a JASCO NR-1800 Raman spectrometer equipped with a Princeton Instrument Spec-10 LN2-cooled CCD detector. Raman excitation was provided by a Spectra Physics Stabilite Model 2017 Ar+ laser operated at 515.5 nm with ca. 10 mW incident power. Transmission electron microscopy (TEM) images were observed with a JEOL TEM 1230 at an accelerated voltage of 80 kV. The specimens were obtained by placing one or two drops of the colloidal dispersions of BNPs in ethanol onto a thin amorphous carbon film covered copper microgrid and evaporating the solvent in air at RT. Prior to specimen preparation, the colloidal dispersions in ethanol were sonicated for 10 min to obtain a better particle dispersion on the copper grid. Image analysis was performed with iTEM software (Olympus Soft Imaging Solution GmbH). For each sample, generally at least 200 particles from different parts of the grid were used to estimate the mean diameter and size distribution of the particles. Images of high resolution TEM (HR-TEM) and bright field scanning TEM (BF-STEM) were observed with a JEOL TEM 2010F microscopy at an accelerated voltage of 200 kV at UBE Scientific Analysis Laboratory. Energy dispersive X-ray spectroscopy (EDS) measurements were carried out with a NORAN UTW type Si(Li) semiconducting detector with about 1-nm beam diameter attached to the HR-TEM equipment. The metal contents of the PVP-protected Ag/Au BNPs were determined by optical emission spectroscopy with inductively coupled plasma (ICP-OES, Varian 720-ES). For this purpose the samples were solubilized in aqua regia (HCl/HNO3). ICP results showed that the metal compositions in final Ag/Au BNPs were almost the same as those in the starting solution. XPS measurement was performed using a Quantum 2000 spectrometer (PHILIPS) under Al Kα radiation (E = 1486.6 eV). Binding energies (BE) are normalized by the C(1s) binding energy of adventitious carbon contamination taken to be 284.6 eV. The analyses of Au and Ag were based on the Au 4f7/2 and Ag 3d5/2 peaks.

Ag/Au bimetallic nanoparticles (BNPs) have unique catalytic, electronic, and optical properties, which are distinct from those of the corresponding monometallic nanoparticles (MNPs).7 These interesting properties are dependent not only on the elemental compositions but also on their geometrical arrangements. The fabricated Ag/Au bimetallic nanoparticles can be generally divided into two types in the structure: the alloyed and the core/shell structure BNPs. The alloyed Ag/Au BNPs are usually synthesized by the simultaneous reduction of both salts in solution,8,9 while the core/shell Ag/Au BNPs have been fabricated by controlled deposition of the shell metal onto a seed of the core metal.10,11 We have already reported the formation of the core/shell structure Ag/Au BNPs using simultaneous reduction of NaBH4 (rapid injection of NaBH4)12 and self-organization of mixing Au colloidal dispersions with Ag+ solutions at room temperature.13 Although the preparation of Ag/Au BNPs having different structures and various Ag/Au ratios is one of the most widely studied bimetallic systems in the literature,14−19 it has been believed that the preparation of Ag/Au BNPs with a tiny size (about 2 nm) on a large scale is difficult because Ag+ ions easily form precipitates with halogen ions, such as AgCl precipitates, in aqueous solutions when Ag+ ions are used as a starting material in combination with a HAuCl4 precursor. Here, we report the synthesis of the Ag/Au BNPs with a size of 2 nm on a large scale by self-organization, i.e., mixing Ag NPs with Au NPs dispersed in an aqueous solution at room temperature (RT) for 2 h. To the best of our knowledge, there are no reports on the synthesis of poly(N-vinyl-2-pyrrolidone) (PVP)-protected Ag/Au BNPs by physical mixtures using colloidal dispersions of Ag and Au NPs. By using this method, moreover, the above-mentioned AgCl precipitate problem can be solved, since the mixing of Ag and Au NPs is performed after the Ag and Au precursors are separately reduced and purified to form the corresponding NPs. The prepared Ag/Au BNPs have been analyzed by UV−vis spectroscopy, SERS, TEM, dark field scanning TEM (DF-STEM), energy dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). The catalytic activities of thus-prepared BNPs with various compositions have also been investigated for aerobic glucose oxidation. Our results also provide evidence that NPs can act as “artificial atoms”2 and form building blocks for the growth of intricate nanocrystals.

2. EXPERIMENTAL SECTION 2.1. Materials. Hydrogen tetrachloroaurate(III) tetrahydrate (HAuCl4·4H2O, 99.9%), purchased from Tokyo Kasei Kogyo, Ltd., and silver perchlorate (AgClO4, 99.99%), sodium tetrahydroborate (NaBH4, 99.0%), and PVP (poly(N-vinyl-2-pyrrolidone), K30, molecular weight about 40 000), purchased from Wako Pure Chemical Industries Ltd., were used without further purification. All glassware and Teflon-coated magnetic stirring bars were cleaned with aqua regia, followed by copious rinsing with purified water. Water was purified with a Millipore Milli-RX 12 plus water system. 2.2. Preparation of PVP-Protected Au and Ag NPs. Two series of Ag and Au NPs stabilized by PVP were prepared as follows: 1. In one series (designated as “A” series), Ag(A) and Au(A) NPs were prepared by an alcohol reduction method. For example, Au(A) NPs were prepared as follows: A solution of HAuCl4·4H2O (50 mL, 1.32 mM, ethanol/water = 1/4 (v/v)) was added into a PVP (50 mL, 132 mM in monomer unit, ethanol/water = 1/4 (v/v)) solution and then stirred at RT for 15 min. The mixed solutions were stirred and heated to reflux at 100 °C for 2 h. In the last, the colloidal dispersions obtained were filtered by an ultrafilter membrane with a cutoff molecular weight of 10 000 (Toyo Roshi Kaisha Ltd.) and washed two B

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2.5. Glucose Oxidation at Controlled pH. The performances of all catalysts were evaluated by aerobic glucose oxidation as a model reaction. The reactions were carried out at 60 °C in a 50-mL glass beaker settled in a thermostat (about 2000 mL). During the experiment, the pH of the reaction solution was kept constant at 9.4 by addition of 1 mol L−1 NaOH using an automatic potentiometric titrator (Kyoto Electronics Mfg. Co. Ltd., Japan). Oxygen was bubbled into the solution with a flow rate of 100 mL min−1 at an atmospheric pressure. The solution was vigorously stirred with a magnetic stirrer. The starting concentration and volume of the glucose solution were 0.264 mol·L−1 and 30 mL, respectively, and the starting weight of the catalyst was about 2 mg. The catalytic tests were automatically carried out for 2 h. Activity (mol of glucose·h−1·mol of metal−1) values were calculated from the slope of a fitted line of a NaOH amount−time curve as shown in Figure S1 in the Supporting Information. The catalytic activities of all the samples were measured at least twice at the same conditions, and the mean values of the measured results were used for comparison. 2.6. Density Functional Theory (DFT) Calculation. The structure and properties of the M55 clusters are calculated using the DMol3 DFT package. In these calculations, an all-electron relativistic core treatment and a doubled numerical basis set with polarization functions were employed. The rPBE functional is used for the DFT calculations. For all calculations, the spin restricted SCF calculations were carried out with a convergence criterion of 10−5 au for the total energy and the electron density. We used the convergence criteria of 0.004 hartree/Å on the force parameters, 0.005 Å on the displacement parameter, and 2 × 10−5 hartree on the total energy in the geometry optimization. The Mulliken population analysis was used for the investigation of the atomic charges of the investigated clusters. For these calculations, the rPBE functional and DNP basis sets are used.

and Au(R) NPs (Au/Ag = 60/40). The results show that only one surface plasmon peak is observed in all the UV−vis spectra of the Ag/Au(R) colloidal dispersions prepared by stirring the mixtures in the period from 0 to 2 h. Moreover, the peak intensity at 380 nm was dramatically increased after the mixture of Ag(R) and Au(R) NPs without the appearance of the surface plasmon resonance peak due to Au NP dispersions expected at ca. 520 nm during the initial mixing stage of 0−20 min. With the mixing time increase from 20 to 120 min, the plasmon resonance peak around 380 nm gradually shifts to low energy; i.e., a red shift of plasmon resonance occurs. The increasing absorbance and the red shift in resonance peak suggest that some kind of conformation change of Au(R) and Ag(R) NPs occurs. UV−vis spectra of colloidal dispersions of the Ag/ Au(R) BNPs at various Ag/Au ratios are shown as a function of mixing time in Figures S3−S5 in the Supporting Information, revealing similar phenomena. These results suggest the formation of the Ag/Au alloy BNPs using the present physical mixture. By comparing the spectra of Figure 1 and Figures S3− S5 in the Supporting Information, it can be concluded that the alloying time of Ag/Au BNPs varies with the Ag contents. When the Ag content is low, for example, for the BNPs with Ag content of ≤40 atom %, the total mixing time of 2 h is enough for the complete formation of Ag/Au BNPs in the present physical mixture process (Figure 1). For the BNPs with higher content of Ag, however, it takes a long mixing time to form the alloyed Ag/Au BNPs. To examine the influence of the Ag content on the formation of the Ag/Au BNPs, we recorded the UV−visible spectra of the mixed dispersions of the Ag(R) and Au(R) NPs with compositions of Au/Ag = 20/80 and Au/Ag = 70/30 in the period from 0 to 72 h under conditions otherwise identical to those used for preparing samples in section 2.3. The relationship between the wavelength of the plasmon peak around 400 nm and the mixing time shown in Figure S6 in the Supporting Information clearly illustrates that, as the mixing time increases from 5 to 360 min, a red shift of plasmon resonance occurs. After that there are almost no changes of the plasmon peak as the mixing time increases, which suggests that the alloying formation process is nearly finished after the mixing time of 360 min. These results indicate that the reaction time required for the BNPs with high content of Ag is about 3 times longer than that for BNPs with lower content of Ag. When a colloidal dispersion of PVP-protected Au(R) MNPs was mixed with various amounts of the colloidal dispersions of Ag(R) MNPs at room temperature for 2 h, only one plasmon absorption was observed for all the UV−vis spectra of the final BNPs (Figure 2). The plasmon peak has various relative intensities and changeable peak positions, too, depending on

3. RESULTS AND DISCUSSION 3.1. UV−Vis and Raman Spectra of the Dispersions of Ag/Au BNPs. The colloidal dispersions of Ag(R) and Au(R) NPs were prepared by rapid injection of NaBH4 from the corresponding ions. The UV−vis spectra of the dispersions of the Ag and Au NPs are shown in Figure S2a in the Supporting Information. The spectrum of the starting Au(R) NPs exhibits a very weak plasmon absorbance at 520 nm, which is consistent with previously reported results,12 indicating the formation of enough small NPs. The spectrum of original Ag(R) NPs consists of a single and narrow surface plasmon resonance (SPR) band at about 380 nm. The TEM micrographs and size distribution histograms of the Ag(R) and Au(R) NPs are shown in Figure S2b in the Supporting Information. The average diameters based on the size distributions are 1.4 ± 0.5 nm for Au NPs and 4.5 ± 2.2 nm for Ag NPs, respectively. The colloidal dispersions of Ag/Au(R) BNPs were prepared by mixing the colloidal dispersions of Ag(R) NPs and those of Au(R) NPs. Figure 1 shows the UV−vis absorbance spectra as a function of mixing time of the mixed dispersions of the Ag(R)

Figure 2. UV−vis spectra of colloidal dispersions of Ag/Au(R) BNPs as a function of composition prepared by mixing Ag(R) and Au(R) NPs.

Figure 1. UV−vis spectra of mixed colloidal dispersions of Ag(R) and Au(R) NPs as a function of mixing time (Au/Ag = 60/40). C

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the feeding atomic ratios of Ag/Au. In the case of Ag90Au10(R) BNPs, for example, one surface plasmon peak with a higher intensity than that of the original Ag(R) NPs is observed in the UV−vis spectra. As the Ag content decreases from 90 to 10 atom %, the plasmon band around 380 nm slightly shifts to a higher wavelength; i.e., a red shift occurs. The UV−vis spectra of the dispersions of the final Ag/Au(R) BNPs shown in Figure 2 suggest the formation of Ag/Au alloys based on the fact that each absorption spectrum has only one plasmon band and that the wavelength at the maximum absorbance shifts in a linear fashion depending on the compositions.9,14 Raman spectra of Ag, Au, and the mixture of Ag/Au NP dispersions were also measured to confirm the formation of alloy BNPs during the mixing process since noble metallic NPs exhibit a phenomenon known as “surface-enhanced Raman scattering” (SERS) in which the scattering cross sections are dramatically enhanced for molecules adsorbed thereon. Figure S7 in the Supporting Information shows the SERS spectra of pyridine in Ag (10.2 nm) and Au (4.1 nm) NP dispersions, and physical mixtures of Ag and Au NP dispersions (Ag/Au = 20/ 80) mixed for 0 and 24 h, respectively. For clarity, only spectra in a limited region of 900−1150 cm−1 involving strong breathing vibrations are shown in this work. The results show that the SERS intensity depends on the kind of NPs; accordingly, the v1 band (symmetric ring breathing mode) at 1003 cm−1 is observed as the most distinct band in the Au NP dispersion SERS spectrum while the v6 band (due to a trigonal breathing) at 1035 cm−1 is identified as the most prominent in the Ag NP dispersion SERS spectrum. Thus the intensity ratios of the band at 1035 cm−1 to that at 1003 cm−1 can be calculated to characterize the SERS enhancement of the various NP dispersions, and the calculation results are also listed in Figure S7 in the Supporting Information for comparison. The results show that the Ag NP dispersion gives a high value of I1035/I1003 (1.2), Au gives a low value (I1035/I1003 = 0.68), and the fresh mixture of Ag and Au NPs indicates an intermediate one (I1035/ I1003 = 0.86). After 24 h from the mixing, in contrast, the mixture dispersion of Ag and Au NPs shows a lower value (I1035/I1003 = 0.73 and 0.69, respectively, for the samples in which pyridine was added to the Ag/Au mixtures kept for 24 h and pyridine was added to the fresh Ag/Au mixtures and then kept for 24 h), which indicates that some reactions occur between Ag and Au NPs during the mixing process and also suggests the formation of alloy Ag/Au NPs by the mixture. 3.2. Structures of Ag/Au(R) BNPs. Figure 3 shows a representative set of TEM images of the prepared Ag/Au(R) BNPs. All the NPs are spherical and well-isolated. The size distribution analysis (Figure S8 in the Supporting Information) based on the TEM images yields the average diameters of 1.5 ± 0.9 nm for Ag10Au90(R), 1.8 ± 1.0 nm for Ag20Au80(R), 2.3 ± 0.8 nm for Ag30Au70(R), 2.2 ± 0.8 nm for Ag40Au60(R), 2.5 ± 1.0 nm for Ag60Au40(R), 3.2 ± 1.5 nm for Ag70Au30(R), 3.5 ± 2.0 nm for Ag80Au20(R), and 3.9 ± 2.0 nm for Ag90Au10(R) BNPs. These results also indicate that the final average particle sizes of the prepared Ag/Au(R) BNPs increase with increasing Ag content. The TEM photographs and size distribution histograms suggest self-organization, which can be defined by the spontaneous formation of BNPs by mixing Ag and Au NPs in dispersions, because the size distribution histograms of the BNPs are narrower than those expected from the summation of the histograms of the monometallic ones. Similar spontaneous formation of BNPs by the physical mixtures was also reported in the case of Ag/Rh BNPs.20

Figure 3. TEM images of Ag/Au(R) BNPs prepared by mixing Ag(R) and Au(R) NPs.

The precise formation mechanism of BNPs prepared by physical mixture using two kinds of MNPs is not clear yet. Zheng et al.4 declared that interactions between NPs during the formation of Pt3Fe nanorods are complex and likely include van der Waals forces, hydrophobic attractions, and charge−charge interactions. They also suggested that dipolar NP interactions should play a prominent role during the growth of Pt3Fe nanorods from nanoparticle attachments of Fe and Pt NPs reduced by electron beam illumination. Our isothermal titration calorimetry (ITC) results showed that the initial step of the forming process of the Rh/Ag BNPs formed by physical mixture is strongly exothermic.21 When the alcohol dispersion of PVP-protected Rh MNPs with an average diameter of 2.3 nm was titrated into the alcoholic dispersion of PVP-protected Ag MNPs, a strong exothermic enthalpy change, ΔH, was observed; i.e., ΔH = −908 kJ/mol for Ag (small) MNPs with an average diameter of 10.8 nm and ΔH = −963 kJ/mol for Ag (large) MNPs with an average diameter of 22.5 nm. The strength of interaction increases in the order Rh/Ag > Pd/Ag > D

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Pt/Ag. The strong exothermic reaction was considered as a driving force to from low entropy BNPs by the physical mixing of two kinds of MNPs. These previously mentioned results suggest that exothermic interaction may play an important role for this realignment during physical mixture. The surface structures, compositions, and segregation properties of BNPs are very important for determining chemical reactivity, especially the catalytic activity. To investigate the compositions of the prepared Ag/Au(R) BNPs, 27 particles with various sizes ranging from 1.0 to 9.0 nm were randomly chosen for the STEM−EDS measurement (Figure 4). The dot-EDS results (Table 1) of these particles

Table 1. Dot-EDS in Figure 4 of Ag20Au80(R) BNPs Prepared by Mixing Ag(R) and Au(R) NPs composition/atom % particle

size/nm

Ag

Au

12 15 13 20 24 9 14 1 23 4 21 2 16 26 10 6 17 27 11 22 25 5 3 18 7 8 19

8.5 6.0 4.9 4.7 4.2 4.1 4.1 3.9 3.8 3.7 3.3 2.8 2.7 2.7 2.6 2.6 2.6 2.5 2.3 2.2 2.1 2.1 2.1 1.7 1.6 1.2 1.2

30 10 20 12 3 27 19 18 26 4 2 8 16 1 2 7 17 8 0 1 10 0 12 8 0 11 0

70 90 80 88 97 73 81 82 74 96 98 92 84 99 98 93 83 92 100 99 90 100 88 92 100 89 100

particle. On the other hand, complete uniformity is very difficult in the present case. Nevertheless, the reaction between NPs is a very important concept and the reaction does occur between Au and Ag NPs. Since the sizes of Au and Ag NPs are not completely uniform and the reaction rates may depend on the size of the NPs, the produced bimetallic NPs may have various sizes and compositions. (The reaction of small NPs is expected to be faster than that of large NPs. Moreover, we cannot prepare Ag and Au NPs with the same average size at the present time.) It is well-known that only one plasmon peak, having various relative intensities and changeable peak positions depending on the Ag contents, can be observed in the UV−vis spectra of Ag/ Au alloy BNPs.22−24 Moreover, the shift from the peak position of the plasmon band of pure Au to that of pure Ag with increasing Ag content can be clearly observed in the formation of Ag/Au alloy. The UV−vis absorption spectra of Aucore/Agshell BNPs with large mean particle size (usually more than 5 nm) have been also reported.24 Aucore/Agshell BNPs usually have two SPR bands in the UV−vis profile near the Ag and Au SPR band. The appearance of two plasmon bands is usually ascribed to the partial coverage of gold by silver nanoparticle. In the present Ag/Au BNPs, only one plasmon peak formed and the blue shift of the plasmon peak with increasing Ag content was observed in the UV−vis spectra (Figure 2). These results suggest that the prepared BNPs should have a random alloy structure. However, the line-EDS results in Figure S9 in the Supporting Information show that a nonrandom alloy structure with a Au-rich core and a Ag-rich shell is also formed in the present Ag20Au80(R) BNPs. How can the structure difference of the present Ag/Au BNPs

Figure 4. HR-STEM images of the final Ag20Au80(R) BNPs prepared by mixing Ag(R) and Au(R) NPs.

confirm that most of the NPs formed by the physical mixtures are composed of Ag and Au. Line-EDS will be a rather powerful and straight tool for the characterization of the structure of Ag/ Au BNPs if the electron beam is small enough in size. The elemental ratio of Ag20Au80(R) BNPs was measured by EDS at various parts of a particles along a cross line (Figure S9 in the Supporting Information). Since the size of the EDS electron beam is 1 nm diameter, the composition of different parts of a nanoparticle can be examined independently. The line-EDS results (Figure S9 in the Supporting Information) show that there are more Au existing in the center of the nanoparticle than Ag and more Ag existing in the edge of the nanoparticle than Au. This result suggests that a nonrandom alloy structure with a Au-rich core and a Ag-rich shell is formed in present Ag20Au80(R) BNPs. The EDS compositions of individual particle prepared by the present method are not very uniform and are little different from that used in the fed solution (Ag:Au = 20:80). The results of UV−vis spectroscopy cannot guarantee that all the NPs are alloys since small NPs can form alloys rapidly, but large NPs cannot completely finish the process as shown in Figure S6 in the Supporting Information. It should be pointed out that, the HR-TEM−EDS data shown in Figure 4 and Figure S9 in the Supporting Information indicated that most of the NPs are alloys even though the compositions may vary particle by E

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considering that there are so many Ag and Au NPs in the present colloidal dispersion (about 2.4 × 1017 1.4-nm Au NPs and 9.4 × 1015 4.5-nm Ag NPs for the present process), it can be reasonably suggested that the coalescence of the NPs should not be the rate-determining step for the formation of the present Ag/Au alloy NPs. Yasuda, Mori, and co-workers26 found that, at room temperature (and even at 245 K), Cu atom diffusion coefficients (D ≥ 1.1 × 10−19 m2 s−1) are approximately 9 orders of magnitude greater than those measured in bulk crystalline Cu−Au alloys or for the Cu dissolution in bulk Au (D ≥ 2.4 × 10−28 m2 s−1 at 300 K). Considering that the average size of the present Au NPs is about 1.4 nm and the Ag/ Au alloying process is finished within 2 h for the samples with lower content of Ag NPs, one can estimate the diffusion coefficient, D, of Au in Ag using the proper diffusion equation:27

be explained? One possible explanation is based on the small size of Aucore/Agshell BNPs. In the case of large BNPs, two SPR bands will appear, while in the case of small BNPs, only one SPR band may be observed. Another possible explanation is based on the probability that the particle with core/shell structure was detected by TEM just by chance. In fact, the particle we observed by line-EDS analysis with HR-TEM was a rather large particle (ca. 7 nm) because the TEM measurement could be more easily carried out for large particles compared with small particles. Since the measured results of UV−vis spectroscopy can reflect the features of a large amount of NPs, the results of UV−vis spectroscopy should be convincing. In contrast, it is still impossible for us to measure enough numbers of particles by the line-EDS in STEM and to conclude the structure with high statistical accuracy. Thus, according to the results of UV−vis spectroscopy and line-EDS in STEM, we can reasonably estimate that most of the prepared Ag/Au BNPs should have an alloy structure and that only part of them possess the structure with Au-rich core and Ag-rich shell especially in large particles. A possible mechanism for the formation of the alloy nanostructure (Figure S10a in the Supporting Information) and the Au-rich core and Ag-rich shell nanostructures (Figure S10b in the Supporting Information), based on the STEM−EDS and UV−vis results, was speculatively described in the Supporting Information (Figures S10 and S11), even though we have no strict evidence of it. The formation of alloyed Ag/Au NPs by mixture of Au and Ag NPs can proceed in two steps. The first step is coalescence of the NPs, and the second step is interdiffusion of Ag and Au atoms in the coagulate to yield an alloy NP. The coalescence in the first step could be arisen from continuous movement and collision of the NPs (Brownian diffusion) and interactions4 (including van der Waals forces, hydrophobic attractions, and charge−charge interactions) between the colloidal Ag and Au NPs in the dispersion. The characteristic collision time of the NPs in colloidal dispersions can be easily calculated assuming that the NPs diffuse through a continuous medium with viscosity η. Then the collision rate constant is given by the following equation for a typical low viscosity solvent (such as water in the present paper).25 kD = (8/3)kBT /η ≈ 109M −1 s−1

D = (d0/2)2 /6t

(2)

where d0 is the average size of Au NPs and t is the mixing time. This equation yields D ≈ 1.0 × 10−23 m2 s−1. According to Dick et al.,28 the diffusion coefficient of gold in gold NPs of 2 nm is approximately D = 1.0 × 10−28 m2 s−1 at room temperature. While this value is much larger than the bulk value of D = 1.0 × 10−36 m2 s−1, it is still too small to rationalize the present results. However, a relatively high density of defects, particularly vacancies at the bimetallic interface could be considered as the main reason to explain the faster-thanexpected diffusion. A similar explanation was invoked to rationalize fast interface diffusion in Aucore/Agshell BNPs.27 Such defects may be caused by the surface curvature or by the need to replace the stabilizers at the surface of the particle during the alloying process. The diffusion in metals is commonly accepted to proceed via migration of atoms into vacancy defects. Correspondingly, the activation barrier for diffusion is the sum of activation barriers for the creation of the defect and for the migration. Removal of the barrier for creation of the defect at the interface would bring the diffusion coefficient to the level of 10−20−10−19 m2 s−1,27 well within the time scale observed for alloying for present BNPs. 3.3. Catalytic Activities for Aerobic Glucose Oxidation of the Prepared Ag/Au(R) BNPs. In order to get more information on the effect of composition upon the catalytic activity, all the Ag/Au(R) BNPs with various atomic ratios were used as the colloidal dispersion catalysts for aerobic glucose oxidation. The catalytic activity varies with the composition of the BNPs as shown in Figure 5 (left axis). The highest catalytic activity was achieved for the samples of Ag20Au80(R) BNPs,

(1)

where kD is the collision rate constant, kB is Boltzmann’s constant, T is the temperature, η is the viscosity of the aqueous medium, and M is the concentration of the NPs. Because the concentrations of the Ag and Au NPs in the present paper can be approximately estimated to be 8 × 10−6 and 3.1 × 10−7 M, respectively (the concentration of metal ions divided by the average number of atoms in one NP is regarded as the concentration of NPs), then the encounter rate constant of Au and Ag NPs can be approximately calculated to be ken ≈ 8 × 103 and 3.1 × 102 s−1, respectively. (The encounter rate constant between Au and Ag NPs is not considered here since the process is too complex.) Thus, the average collision times (encounter times) for the Ag and Au NPs are τen ≈ 0.125 and 3.2 ms, respectively. Liao et al. studied the growth process of Pt3Fe nanorods from Pt and Fe NPs.4 Their results showed that NP movements are random when the NP is above a critical distance from the receiving NP. As the NP separation decreases to a critical distance of ∼3 nm when it approaches a spherical NP, the NP acquires a drift velocity, which increases with the decrease in distance. All the above-mentioned results show that the collision between the NPs will be very frequent. By

Figure 5. Catalytic activities for glucose oxidation of Ag/Au(R) BNPs by mixing Au(R) and Ag(R) NPs. F

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whose value was 11 510 mol of glucose·h−1·mol of metal−1. It is interesting that the high activity does not depend only on the surface area of the catalysts. In fact, even though the activities are normalized by the total surface area, the Ag20Au80(R) BNPs still have the highest catalytic activity (Figure 5, right axis). This suggests that the ratio of Ag/Au = 20/80 is special for the preparation of Ag/Au(R) BNPs with the high activity. The results also show that the maximum activity of the Ag20Au80(R) BNPs was about 2 times higher than that of pure Au(R) NPs (6230 mol of glucose·h−1·mol of metal−1), even though the Ag20Au80(R) BNPs have a larger particle size than that of Au NPs. It is well-known that the catalytic activities of metallic NPs are highly sensitive to their size. A strong dependence of catalytic activities on gold particle sizes has been described mainly for gas-phase reactions.29 For liquid-phase reactions, Comotti et al.30 investigated the relationship between the size of gold colloids and their activities for glucose oxidation and found that the particles must be at least smaller than 10 nm to be active for glucose oxidation. To study the effect of particle sizes on the catalytic activities for aerobic glucose oxidation of the Ag/Au BNPs, another series of Ag/Au(A) BNPs with large particle sizes was also prepared by mixing Au(A) and Ag(A) colloidal dispersions with large particle sizes. (UV−vis spectra and TEM images are shown in Figure S12 in the Supporting Information, and the average diameters are 7.0 ± 1.7 and 18.5 ± 1.7 nm, respectively.) When the Ag(A) and Au(A) NPs were mixed together, two distinct peaks (Figure S13 in the Supporting Information) due to the surface plasmon resonance of individual Ag and Au NPs were still observed in the UV−vis spectra at 420 and 520 nm, respectively, even a day after the mixture. The plasmon peak of Ag finally disappeared within 1 week after the mixture. The results suggest that the particle sizes of the starting Ag and Au NPs have an important effect on the preparation rate of Ag/Au BNPs using present physical mixtures, and that the freshly prepared Ag(R) and Au(R) NPs synthesized by rapid injection of NaBH4 are small enough in size to be highly active and, thus, easily form Ag/Au alloy even by physical mixture at RT for 2 h. Similar results were also obtained by Shibata et al., who used EXAFS to study sizedependent mixing in Aucore/Agshell BNPs (generated radiolytically in solution) having a Au core with diameters of 2.5−20 nm and a Ag shell with variable thickness.27 In the smaller NPs, spontaneous interdiffusion was found to occur at the core−shell boundary. The rate of mixing was found to be size dependent, being greater for smaller particles, and this was attributed to the presence of vacancy defects at the core−shell boundary, rather than simply to the lowering of the particle’s melting point with decreasing size.27 At the present stage, we cannot yet explain the termination mechanism of the Ag/Au alloying process. The termination will occur to minimize the total surface energy of the system. In the case of Ostwald ripening, the growth of large particles occurs. In the present case, in contrast, the number of medium size alloy NPs increases, which may be caused by the balance of the total surface energy of the system. In addition, the alloying process from the present Ag/Au coagulates to alloy NPs requires time. The small coagulates composed of small Ag and Au clusters require a short time, while the large coagulates composed of large clusters require a long time. Shibata et al.27 thought that the presence of vacancies at the bimetallic interface should be responsible for the termination of the alloying process in Aucore/Agshell BNPs. They thought that the

alloying may be viewed as a result of competition between percolation of the defects to the outer surface and migration of the metal atoms into the vacancies. Once a vacancy percolates to the outer interface, its penetration back into the lattice is expected to be extremely slow, and molecular dynamics simulation results show that the presence of the vacancy is very important for the alloying process. The disappearance of the vacancy may lead to a reduced diffusion rate as the atoms penetrate deeper into one another. Figure S14 in the Supporting Information shows a representative set of TEM images of the Ag/Au(A) BNPs prepared by a mixture of Ag(A) and Au(A) NPs with large sizes. The size distribution analysis based on the TEM images yields the average diameters of 7.9 ± 2.1 and 7.4 ± 1.4 nm for Ag50Au50(A) BNPs 1 day and 7 days after the mixture, respectively. The average particle size of the Ag/Au(A) BNPs is much larger than that of corresponding Ag/Au(R) BNPs, and the catalytic activities of the Ag/Au(A) BNPs (Figure S15 in the Supporting Information) are much lower than those of the Ag/Au(R) BNPs. 3.4. Correlation between Negatively Charged Au atoms and Catalytic Activities of Ag/Au(R) BNPs. An obvious question has arisen based on the above-mentioned results. How can the higher catalytic activities of the BNPs than that of Au NPs be explained? One possible mechanism is based on the electronic charge transfer effect between the various kinds of adjacent elements which has been reported as the reason for the high catalytic activities of several kinds of BNPs and TNPs.31−34 This kind of consideration could be applied to the present Ag/Au BNP system, although most of the prepared BNPs can be assumed to have an alloy structure. In order to examine the electronic properties of the catalytically active BNPs, and to confirm the electronegativity of the abovementioned Au atoms, Ag30Au70 BNPs protected by low content of PVP (RPVP = 5) were prepared by simultaneous reduction of Au3+ and Ag+ with rapid injection of NaBH4 (TEM image and size distribution histogram of the BNPs are shown in Figure S16 in the Supporting Information) and investigated by highresolution XPS using monochromated Al Kα electron radiation. As shown in Figure S17 in the Supporting Information, the apparent electron binding energy (BE) of Au 4f7/2 of 82.61 eV in the Ag30Au70 BNPs was about 1.4 eV lower than that of bulk Au (84.0 eV), and about 0.2 eV lower than that of the PVPprotected Au NPs (82.8 eV) with a mean diameter of 1.3 nm.35 The negative shift of the Au 4f BE suggests that the Au atoms in the BNPs are negatively charged. Similar charge transfer effects were also discussed to determine the optimal chemical ordering in Ag/Au BNPs of sizes up to 2 nm by using DFT calculations.36 To further confirm the electron donation from the Ag atoms to Au atoms, DFT calculations were carried out to study the electronic states of the Au atoms in the present Ag/Au NPs. At the present stage, it is still impossible for us to make a huge calculation for the large NPs, such as M147, M309, etc. Hence DFT calculations of a M55 model cluster (Ag31Au24 and Ag43Au12; the subscripts on Ag and Au stand for the number of atoms in the BNPs) were examined for understanding the correlation between the electronic states and the catalytic activity for the aerobic glucose oxidation of the BNPs. The calculation results are shown in Figure 6a,b. The most important result was that Au atoms are indeed negatively charged, while the Ag atoms have positive charges due to the electronic charge transfer from the Ag atom to Au atoms. The G

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On the basis of these results, we can conclude that the Au atoms in BNPs are indeed negatively charged, the negatively charged Au atoms are formed due to the charge transfer from the Ag atom as well as that from the PVP molecule, and these negatively charged Au atoms would act as crucial active sites for the aerobic glucose oxidation. The anionic gold atoms could activate molecular oxygen by donating an excess electronic charge to the antibonding orbital, and the resulting superoxoor peroxo-like oxygen promotes glucose oxidation. Similar reasons have been reported to explain the high activity of PVPprotected Au NPs35 and Ag/Au BNPs34 for aerobic oxidation of p-hydroxybenzyl alcohol, and of gas-phase Au clusters for CO oxidation.38,39 Considering the preparation of Ag/Au BNPs aiming at potential use in industrial practice, the methodology described in this paper presents three important technological aspects: (1) Because the reaction batch comprises only the aqueous suspensions of the Ag and Au NPs, the magnitude of the process output can be designed according to various needs (from a few milligrams to several tens of grams). (2) The size of the prepared BNPs can be well-controlled by tailoring the size of the starting NPs. (3) BNPs with sizes of less than 2 nm diameter can be prepared by the present physical mixture. Although the present work was only focused on Ag/Au BNPs, it is reasonable to believe that this strategy is also applicable to other bimetallic systems.

Figure 6. DFT calculations of electronic structures of Ag31Au24 and Ag43Au12 BNPs protected with and without PVP (yellow, Au; blue, Ag; attached substance, PVP molecule). The data shown indicate the Mulliken charges of Au and Ag in the BNPs.

4. CONCLUSION In summary, for the first time, we have developed a novel preparation method for Ag/Au BNPs of size less than 2 nm in diameter using a physical mixture. The colloidal dispersions of Ag/Au BNPs were prepared by mixing the separately prepared Ag and Au NP dispersions at room temperature for 2 h. It was observed that the surface plasmon resonance of Ag/Au BNPs shifts to higher wavelength with Ag content decrease from 90 to 10 atom %, which could be explained by the formation of alloyed Ag/Au NPs. The main parts of BNPs thus prepared may have an alloy structure, which is suggested by EDS and UV−vis spectroscopy. Only part of them possess the structure with a Au-rich core and a Ag-rich shell, especially in large particles, which is suggested by line-EDS analyses. The possible formation mechanisms of the Ag/Au BNPs with the random alloy and core/shell (Au core/Ag shell) structure were proposed. Our results have provided a novel example of the reactions of precious metal NPs to the field of nanochemistry, and can help us understand the mechanism of crystal growth via NP attachment. The Ag/Au BNPs thus prepared were applied as catalysts for the aerobic oxidation of glucose to gluconic acid. The BNPs with the composition of Ag/Au (20/80) had the highest catalytic activity for glucose oxidation among those with various composition ratios and monometallic NPs. The higher catalytic activities of the prepared Ag/Au BNPs than those of the pure Au NPs can be ascribed to the following two factors: (1) the small average diameter, about 2.0 nm, and (2) the negatively charged Au atoms, due to electronic charge transfer from adjacent Ag atoms and PVP, acting as catalytically active sites. The latter factor is supported by DFT calculation of model clusters. We anticipate that our finding will play an important role in the design of complicated nanomaterials and the control of the nanocrystal structure by self-organization for functional catalysts.

DFT calculation results are well consistent with those of XPS. However, it should be emphasized that, for the present Ag/Au BNPs prepared by physical mixture, the explanation of how the atomic distributions in clusters affects the degree of intracluster charge transfer is still a problem for us at the present stage since the DFT calculation cannot be carried out for the model with unsymmetrical structure. Nevertheless, the fact that the symmetrical structure of the Ag43Au12 cluster shown in Figure 6b has the top Au atoms with a larger negative charge than that of the Ag31Au24 cluster (Figure 6a) may suggest that not only the number but also the distribution of the atoms may affect the degree of intracluster charge transfer. Electron transfer from PVP to small Pt clusters has also been reported.37 Can PVP transfer electrons to Au atoms in the present Ag/Au BNPs? In order to address the question, DFT calculation on the electronic structures of the Ag/Au BNPs protected by PVP was also carried out for comparison. Figure 6c,d shows that the calculated Mulliken charges of some Au atoms in Ag31Au24 and Ag43Au12 BNPs protected by PVP are determined to be as low as −0.098 and −0.164, respectively. It should be pointed out that, for the BNPs without PVP as a stabilizer, the Mulliken charges of −0.086 and −0.133 for the Au atoms were observed in Ag31Au24 and Ag43Au12 BNPs, respectively. The more negative Mulliken charges of the Au atoms in PVP-protected Ag/Au BNPs than that of Ag/Au BNPs without PVP indicate that electronic charge is indeed donated to the Au atom from PVP, which provides crucial evidence that PVP not only acts as a stabilizer but also plays a direct role in regulating the electronic structures of the Ag/Au BNPs. The possible electronic charge transfer effects in Ag/Au BNPs are illustrated in Figure S18 in the Supporting Information. H

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ASSOCIATED CONTENT

S Supporting Information *

Example of titration curves, possible mechanism for the formation of the Ag−Au alloy and Ag-rich shell nanostructures by physical mixtures, DF-STEM images and line-EDS, TEM images, UV−vis spectra, SERS spectra, XRD patterns, XPS spectra, schematic illustration of possible electronic charge transfer effect from Ag atoms and PVP molecule in the Ag/Au BNPs, and catalytic activities of Ag/Au BNPs. This material is available free of charge via the Internet at http://pubs.acs.org.

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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 work was financially supported by Grants-in-Aid from the Core Research for Evolutional Science and Technology (CREST) program sponsored by the Japan Science and Technology agency (JST), Japan.

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