Probing the Electronic Structure of Heterogeneous Metal Interfaces by

Apr 18, 2016 - MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry, ... This article is part of the Richard P. Van Duyne Festschrift spe...
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Probing the Electronic Structure of Heterogeneous Metal Interfaces by Transition Metal Shelled Gold Nanoparticle-Enhanced Raman Spectroscopy Yue-Jiao Zhang,† Song-Bo Li,‡ Sai Duan,§ Bang-An Lu,† Ji Yang,† Rajapandiyan Panneerselvam,† Chao-Yu Li,† Ping-Ping Fang,∥ Zhi-You Zhou,† David Lee Phillips,‡ Jian-Feng Li,*,† and Zhong-Qun Tian† †

MOE Key Laboratory of Spectrochemical Analysis and Instrumentation, State Key Laboratory of Physical Chemistry of Solid Surfaces, Innovation Center of Chemistry for Energy Materials, and College of Chemistry, and College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China ‡ Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China § Department of Theoretical Chemistry and Biology, School of Biotechnology, Royal Institute of Technology, S-106 91 Stockholm, Sweden ∥ MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China S Supporting Information *

ABSTRACT: In heterogeneous catalysis, characterization of heterogeneous metal interfaces of bimetallic catalysts is a crucial step to elucidate the catalytic performance and is a key to develop advanced catalysts. However, analytical techniques such as X-ray photoelectron spectroscopy can only work in vacuum conditions and are difficult to use for in situ analysis. Here, we present efficient and convenient core−shell nanoparticle-enhanced Raman spectroscopy to explore the in situ electronic structures of heterogeneous interfaces (Au@Pd and Au@Pt core−shell NPs) by varying the shell thickness. The experimental observations reported here clearly show that Pd donates electrons to Au, while Pt accepts electrons from Au at the heterogeneous interfaces. This conclusion gains further support from ex situ X-ray photoelectron spectroscopy results. The Au core greatly affects the electronic structures of both the Pd and Pt shells as well as catalytic behaviors. Finally, the asprepared core−shell nanoparticles were used to demonstrate their improved catalytic properties in real electrocatalytic systems such as methanol oxidation and oxygen reduction reactions.

1. INTRODUCTION Bimetallic nanoparticles have gained significant attention from both technological as well as scientific perspectives because of their unique optical, electronic, catalytic, and chemical properties which are distinctly different from monometallic nanoparticles.1−3 In catalysis, metal nanoparticles behave drastically different from their bulk counterparts; on the other hand, bimetallic or trimetallic nanoparticles exhibit superior catalytic activity, selectivity, and stability as compared to their corresponding monometallic nanoparticles due to their heterogeneous metal interfaces. In particular, the modification effect is important when the admetal coverage is in the submonolayer to monolayer regime.4−6 For this reason, standard monometallic catalysts have been widely replaced by bimetallic catalysts. For instance, Tedsree et al. have synthesized Ag@Pd core−shell nanoparticles which can © 2016 American Chemical Society

significantly enhance the production of H2 from formic acid at ambient temperature and ascribe their enhanced catalytic activity to charge transfer effect.7 Wong’s group has presented that Pd-on-Au bimetallic nanoparticles exhibit high catalytic activity for aqueous-phase trichloroethene hydrodechlorination.8,9 Adzic’s research group has demonstrated that Pt nanoparticles with Au clusters possess great stability by raising the Pt oxidation potential in the O2 reduction reaction.10 Xu’s research group has elucidated that the enhanced performance of the Pt-on-Au nanostructures correlates with the surface electronic structures of Pt and their underlying Au NPs.11 Special Issue: Richard P. Van Duyne Festschrift Received: February 24, 2016 Revised: April 16, 2016 Published: April 18, 2016 20684

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example, Weaver and co-workers prepared uniform platinum group overlayers by redox replacement of underpotentialdeposited (UPD) copper on roughened gold electrodes.32 This method can obtain uniform and pinhole-free monolayers of Pd or Pt which have strong SERS enhancement. Our group has proposed a more convenient chemical synthetic method by coating a thin layer of transition metal on SERS active core nanoparticles.33 Interestingly, transition metals can be used for SERS by utilizing the strategy of “borrowing” SERS activity from a Au nanocore through the long-range effect of electromagnetic enhancement. The average SERS enhancement can reach up to 104−105.34,35 Thus, SERS can be used to characterize core−shell nanostructures, in particular, gold based bimetallic catalysts which are widely used for a plethora of important chemical reactions and applications such as CO oxidation,36 alcohol oxidation,37 hydrogen fuel cells,38 and wastewater treatment.39 Additionally, the Raman frequency in the triple bond region is ultrasensitive to the surface chemical bond of probe molecules under different environments, and this property can be exploited to explore the electronic influence of Au on the performance of bimetallic catalysts. Serendipitously, the combination of Au nanoparticles with Pd or Pt monolayers offers a sensitive platform for label-free in situ monitoring of Pd or Pt catalyzed reactions by SERS. In this work, we explore the electronic structure of Au@Pt and Au@Pd core−shell nanoparticles by surface-enhanced Raman spectroscopy. Strikingly, the detailed information on the surface bonding between the transition metals (Pd, Pt) and CO molecules is used to probe the electronic structures of these bimetallic catalysts (Figure 1). The Pd or Pt shell thickness is

The enhanced performance of bimetallic nanostructures can be explained by three fundamental effects, namely, the ensemble, geometric, and ligand effects.12 First, the ensemble effect refers to the assembly of specific surface atoms as a group or individual, which decisively affects the reaction mechanism or byproducts.12,13 Second, geometric effects include expanded or compressed surface atoms which result in surface strain because the surface bond lengths are typically different from those of the parent metals.14,15 This effect gives rise to strain effects that modify the electronic structure of the metal through changes in orbital overlap. Finally, the term “ligand effect” relates to the electronic charge transfer between the core and the surface metal,13,16−20 giving rise to a modification of its electronic structure and consequently its chemical properties. In some cases, all three of these effects can contribute to the enhanced performance of the bimetallic catalysts. However, the performance of a catalyst can be tuned by changing the size, shape, and composition of its metal components. Hence, major efforts have been dedicated to the development of bimetallic catalysts. Therefore, a clearer understanding of the fundamental factors that influence the properties and catalytic ability of bimetallic catalysts will be a key to developing efficient catalysts for future applications. To understand the fundamental factors, bimetallic nanoparticles can be characterized by a range of analytical techniques; for instance, imaging techniques like scanning electron microscope (SEM) can provide details about the morphology of the bimetallic nanoparticles, whereas transmission electron microscope (TEM) can render additional information about the core−shell nanostructures. X-ray photoelectron spectroscopy (XPS) can provide information about the structure, the oxidation state, and some additional information about the bimetallic catalysts. Though the above-mentioned techniques can offer information about some of the key factors of catalytic performance, they are not convenient for in situ studies, and some techniques need to be done in a vacuum environment which is actually different from the normal reaction conditions. Therefore, a detailed investigation about the electronic influence of the core−shell nanoparticles under the real reaction conditions is still necessary. Vibrational spectroscopic techniques such as infrared (IR) spectroscopy and Raman spectroscopy can be used for in situ studies.21,22 IR spectroscopy is a common technique to probe the geometric and electronic structures of bimetallic catalysts. For instance, Zhang and co-workers prepared tunable Au/Pd alloy catalysts and examined the surface structure of the Au−Pd bimetallic nanocatalysts by IR spectroscopy.20 However, IR spectroscopy suffers from CO2 and H2O absorption interference and cannot characterize the low wavenumber region, which directly reflects the molecule−metal bonding information. On the contrary, Raman spectroscopy has a great advantage for avoiding this interference by having a much narrow peak width as compared to that for IR. It can also provide direct vibrational information on the molecule−metal bonding in the low wavenumber region. Surface-enhanced Raman scattering (SERS) is a powerful fingerprint spectroscopic technique that can provide structural information about the adsorbed molecules on noble metal nanostructures.23−31 However, SERS enhancement is restricted to a few noble metals such as Ag, Au, or Cu which can exhibit high SERS activity due to the electromagnetic and chemical enhancement mechanism. To extend the versatility of the SERS technique, various SERS substrates have been developed. For

Figure 1. Schematic illustration of probing the electronic structures of core−shell nanoparticles by SERS method.

varied to examine the electronic influence of the Au core on the transition metal monolayers, which is also proven by the XPS technique. Furthermore, this effect is exemplified in different electrocatalytic systems to demonstrate their potential catalyst applications. Overall, our study leads to a better understanding about the fundamental interactions between the Au core and the transition metal shells which influence the catalytic activity of the Pd or Pt monolayers.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Chloroauric acid (HAuCl4·4H2O), chloroplatinic acid (H2PtCl6·6H2O), palladium chloride (PdCl2), sodium citrate, and ascorbic acid were purchased from Alfa Aesar. Pyridine, HCl, and HNO3 were purchased from Sinopharm Chemicals Reagent Co. Ltd. All of these chemicals 20685

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2.7. Theoretical Section. All of the density functional theory (DFT) calculations were performed by the Vienna ab initio simulation package (VASP)41 with the Perdew−Burke− Ernzerhof (PBE) functional.42 The projector augmented-wave pseudopotentials,43 which took the scalar relativistic corrections into account, were used to represent the core electrons. Meanwhile, the wave functions were expanded by plane-wave basis sets with the energy cutoff of 400 eV. The Methfessel and Paxton method44 with a broadening factor of 0.1 eV was adopted for improving convergence of the electronic structure calculations, and the total energies were extrapolated to 0 K. The calculated lattice constant of Au at the PBE functional level was 4.174 Å, which agrees well with the experimental value of 4.078 Å.45 With the calculated lattice constant, six (1 × 1) supercells of the Au(111), Au(100), and Au(110) slabs were used to simulate the Au core, and one layer of Pd and Pt was used for the shell. For the k-point sampling of the supercell, a 16 × 16 × 1 mesh of the Monkhorst−Pack grid46 was used for the Brillouin zone integration. Enough vacuum layer (>35 Å) was added along the z-axis in the supercell, and the dipole correction47,48 was used to avoid artificial interactions. During the optimization, the positions of the three bottom layers of the substrate were fixed, while all of the atoms were allowed to relax along the z-axis. The force criterion was 0.02 eV/Å for optimizations.

were of analytical grade and used as received, without further purification. Water was purified with a Milli-Q system (18.2 MΩ) before use in any of the following procedures. 2.2. Preparation of Au@Pd Nanoparticles. First, Au seeds with a diameter of about 55 nm were prepared by reducing AuCl4− using the sodium citrate method.40 To prepare the Au@Pd nanoparticles, 30 mL of Au seed (55 nm) solution was mixed with a calculated amount of H2PdCl4 under stirring and then cooled to 4 °C in an ice bath. To this mixture, 5-fold of ascorbic acid (10 mM) was slowly added with the help of a peristaltic pump under vigorous stirring over a period of time, and the stirring was continued for another 30 min. To achieve complete reduction of H2PdCl4, the concentration of ascorbic acid is usually 5 times greater than the amount of H2PdCl4. In order to obtain 0.35, 0.70, 1.4, 2.8, and 6.8 nm Pd shell thickness, 0.4, 0.8, 1.64, 3.46, and 9.97 mL portions of H2PdCl4 (1 mM) were added to the Au seeds, respectively. The color change from red-brown to dark brown, which indicates the formation of the Au@Pd nanoparticles. In addition, the morphology of the Au@Pd nanoparticles was examined using SEM (Figure S1A) and TEM (Figure S2). Also, we can clearly observe the core−shell structure. The Au@Pt nanoparticles (Figure S1B) were prepared by the same method, while these reactions took place in a 90 °C water bath. 2.3. Preparation of Au@Pt and Au@Pd Attached to Glassy Carbon Electrodes. The freshly prepared core−shell nanoparticles were centrifuged to remove the excess reactants, and the surfactants at 5500 rpm were done twice. Concentrated nanoparticles (20 μL) were dropped onto an electrochemically cleaned glassy carbon electrode surface by a pipet and dried under vacuum. Finally, the nanoparticle-modified electrode showed a uniform yellow color, reflecting the optical properties of the kernel of gold. 2.4. Electrochemical Measurements. Electrochemical measurements were conducted on a CHI 631A electrochemical workstation (CH Instruments, Shanghai, China). Before being used as a support electrode, the glassy carbon (GC) electrode (f = 5 mm) was mechanically polished successively with alumina powder (Buehler Ltd., Lake Bluff, IL) from 3 μm down to 0.05 μm to achieve a mirror finish surface, followed by sonication in Milli-Q water for 3 min after each polishing step. Then, it was cleaned in 0.5 M H2SO4 by scanning the potential between −0.25 and +1.25 V at 500 mV/s for 30 min before being used in the experiments. 2.5. Raman Measurements. Raman spectra were recorded on a LabRam I confocal microprobe Raman system (Dilor, France) in a homemade spectroelectrochemical cell. The excitation line was 632.8 nm from a He−Ne laser with a power of about 10 mW on the sample. A long-working length (8 mm) objective with 50× magnification was used to direct the laser to the sample surface and collect the Raman signal in a backscattering geometry. A spectroelectrochemical cell, with a Pt wire and a saturated calomel electrode (SCE) serving as the counter and the reference electrodes, respectively, was used for both the electrochemical and the in situ electrochemical SERS measurements. 2.6. X-ray Photoelectron Spectroscopy (XPS) and Scanning Electron Microscopy (SEM). XPS experiments were performed by a Multilab 2000 X-ray photoelectron spectroscopy instrument produced by the British Thermo Electron. The morphology of the nanoparticles on the wafer was observed by a LEO1530 scanning electron microscope produced by the Germany LEO company.

3. RESULTS AND DISCUSSION 3.1. SERS Characterization of Au@Pd/Pt Bimetallic Nanoparticles. To examine the electronic structure of Au@ Pd/Pt bimetallic nanoparticles, the shell thickness of the transition metals was precisely varied while the diameter of the Au core (55 nm) was kept constant. The SERS enhancement primarily depends on the nature of the metal nanostructures and the interparticle distance which dramatically affects the plasmonic coupling efficiency.26,31 Figure 2 shows the potential-

Figure 2. SERS spectra of CO adsorbed on Au@Pd (A) and Au@Pt NPs (B) with different shell thicknesses (a−e) (0.35, 0.7, 1.4, 2.8, 7.0 nm) in a solution of 0.1 M HClO4 saturated with CO at 0.0 V.

dependent SERS spectra of CO adsorbed on the Au@Pd (A) and Au@Pt (B) nanoparticles at the GC electrodes. The spectra were obtained after the electrode was held at 0.0 V in a CO saturated 0.1 M HClO4 for 15 min to ensure the formation of a CO adlayer on the nanoparticle surface. Thus, the surface coverages of CO at the nanoparticles surface are consistent. The peaks in high wavenumber region at 1972 and 2081 cm−1 are ascribed to νCO(bridge) and νCO(top) on Pd surface, while the peak at 2084 cm−1 is assigned to the stretching mode of C−O with atop configurations bound to Pt.49 No Raman peaks can 20686

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The Journal of Physical Chemistry C be found around 2130 cm−1 due to C−O stretching on Au surfaces,50 which means the coated ultrathin Pd or Pt layers on Au NPs are pinhole-free (Figure S3). If Pd or Pt shells are filled with pinholes, the probed molecules or contaminants from the surrounding environment may come into contact with the Au core, which can generate strong interference Raman signals. Therefore, the shell should be compact to prevent the signal from the pinholes. To verify the reproducibility and stability of the substrate, SERS and RSD spectra of the CO molecules in the spectral range 1600−2400 cm−1 were randomly collected from a number of points on the 55 nm [email protected] nm Pd substrate (Figure S4A). SERS mapping also proves its stability and uniformity (Figure S4B). Additionally, in a CV test, no Au reduction peak at around 0.9 V could be found, which further indicated that the Au@Pd and Au@Pt nanoparticles are pinhole-free (Figure S5). In both cases, the SERS spectra clearly depict the relationship between the shell thickness and the SERS intensity: The SERS intensity of the CO stretching mode decreases with increasing shell thickness, which is in good agreement with other results reported in the literature.47 In other words, a higher SERS enhancement will be observed when the shell is thinner. To further probe the surface electronic properties, the C−O stretching mode which is very sensitive to the surface bond was carefully monitored with different shell thicknesses. The C−O is σ− and π* feedback bonded to the surface. The additional free electrons of the metal that fill in the π* feedback orbital will weaken the C−O bond, so that the frequency of the C−O stretching band will be lower than that of gaseous CO. When the Au nanoparticles were coated with ∼1 Pd monolayer, the Raman frequency of the CO was observed at 1972 cm−1 whereas the frequency was shifted to a lower wavenumber 1965 cm−1 for more than 5 Pd monolayers (1.4 nm). This behavior indicates that the surface Pd is in a poor electron state (losing electrons) comparing to the bulk Pd. Thus, the surface properties of the Pd shell differ as a function of shell thickness and are consistent with previously reported results in the literature.51 Interestingly, Kolb et al.51 found that the potential of a zero charge of five or more layers of Pd on Au(111) is identical to the potential of a zero charge of a massive Pd(111) electrode. To observe a similar frequency shift, the C−O stretching mode from the Au@Pt nanoparticles was examined with different shell thicknesses. When the Au nanoparticles were coated with ∼1 Pt monolayer, the Raman frequency of CO was observed at 2084 cm−1 whereas the frequency was shifted to a higher wavenumber 2089 cm−1 for a thicker Pt shell. This behavior indicates that the surface of Pt is in a rich electron state (obtaining electrons) compared to the bulk Pt. Interestingly, the frequency shifts in both cases are opposite and demonstrates the difference in their electronic properties. To gain a clearer understanding about the different behavior between the Au@Pd and Au@Pt nanoparticles, we evaluated the electronic structure between the Au core and the transition metal shells. When Au nanoparticles are coated with 1−2 monolayers of the transition metals, the C−O bond stretch mode is strongly influenced by the Au core, while there is less or no influence when the Au core is coated with multilayers of Pd or Pt shells. The difference in frequency between the monolayer and the multilayers of Pd or Pt may be generated from their different work functions. The work function of a metal can illustrate the ability of the metal to gain or lose electrons. On the basis of information from the CRC manual,45

the work functions of Pd, Au, and Pt are 5.12, 5.10, and 5.65 eV, respectively. The Fermi level of the Au surface is higher than that of Pt, and this means Au may transfer electrons to Pt if Au is in contact with Pt; thus, the Pt surface will be in a rich electronic state in the core−shell nanostructures (for those with 1−2 monolayers). This behavior is consistent with our experimental results which are shown in Figure 2. Nevertheless, on the basis of the values reported in the CRC manual, the work function of Pd is 5.12 eV; thus, the Fermi level of Pd is lower than Au, originally suggesting that Au should transfer electrons to Pd, so that the Pd surface will be in a rich electronic state. On the contrary, the Pd appears to transfer electrons to the Au core, and the results here illustrate that the Pd surface is in a poor electronic state. However, on the basis of the work function values reported in Lange’s handbook of chemistry,52 the work functions of Pd, Au, and Pt are 5, 5.32, and 5.40 eV, respectively. Thus, on the basis of these work function values the electrons can easily transfer from Au to Pt, which is consistent with the work function values reported in the CRC manual and our experimental results. While Pd can transfer electrons to Au on the basis of Lange’s work function values, this is contradictory to the work function values reported in the CRC manual but agrees well with our experimental results. It should be noted that the metal surface work function from the manuals represents the properties of bulk metals. When the metal size is on the nanometer scale, due to the change of the surface structure, the actual work function may differ. 3.2. First-Principles Calculations of Au@Pd/Pt Bimetallic Nanoparticles. To further examine the electronic structures of Au@Pd/Pt NPs, we performed first-principles calculations for monolayer Pd or Pt shells deposited onto the Au surface. Here, the shell materials share the same lattice constant of the Au core, which indicates that the current models contain both electronic and strain effects. The calculated results associated with the (111) surface for Au@ Pd and Au@Pt both exhibit a negative sign for the surface dipole (see Table 1 for details), which clearly shows that Table 1. Calculated Surface Dipole Moment Density in Debye nm−2 for Au@Pd and Au@Pt with Low-Index Surfacesa crystallographic orientation

111 (D/nm2)

100 (D/nm2)

110 (D/nm2)

Au@Pd Au@Pt

−0.09 −1.51

0.18 −1.87

0.40 −0.90

a

Here positive and negative signs represent the electron transfer from shell to core and from core to shell, respectively.

electrons transfer from the Au core to both the Pd and the Pt shell in each respective system. These results are consistent with our experimental observation for Au@Pt but unfortunately conflict with that for Au@Pd. Thus, we further considered two other low-index surfaces (110) and (100). For these systems, the opposite (positive) sign of the surface dipole is predicted for Au@Pd, with a much higher value compared to the (111) facet, while for Au@Pt, the negative surface dipole prediction persists (see Table 1 for details). These results indicate that electron transformation from the Pd shell to the Au core is predicted for Au@Pd NPs with (100) and (110) facets, while the opposite electron transformation for the Au@Pt NPs with the same facets. Because of the epitaxial growth of the NPs, we 20687

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Figure 3. XPS spectra of the Au@Pd (A) and Au@Pt (B) NPs with different shell thicknesses are shown. The shell thicknesses are shown in the order of 0.35, 1.4, and 7.0 nm.

Figure 4. Linear voltammograms of CO electrooxidation of Au@Pt NPs (A) and Au@Pd NPs (B) with different shell thicknesses on glassy carbon (GC) electrodes in a solution of 0.1 M HClO4 saturated by CO gas are shown. Scan rate: 0.1 V/s. The shell thicknesses are shown in the order of 0.35, 1.4, and 7.0 nm.

In Figure 3A, the binding energy peaks at 335.4 and 340.6 eV are the characteristic peaks of Pd03d5/2 and Pd03d3/2, respectively. As can be seen in the figure, the surface binding energy of the 0.35 nm Pd shell is higher than the 1.4 or 7.0 nm Pd shells on the Au core which means that the electron cloud of the former (Pd monolayer) is delocalized partly compared to the latter systems (7.0 nm Pd shell ≈ bulk Pd). The thin Pd shell has lost electrons, so its surface valence is slightly more positive than the bulk. In Figure 3B, the binding energy peaks at 71.3 and 74.6 eV are the characteristic peaks of Pt04f7/2 and Pt04f5/2, respectively. The surface binding energy of the 0.35 nm Pt shell was lower than the 1.4 or 7 nm Pt shells coated on the Au core, which means that the thin Pt shell has received electrons from the Au core, so its surface valence is slightly more negative than the latter. Notably, these XPS results provide further evidence to support and confirm our speculations about the reason for the changes observed in the Raman frequencies as the thickness of the shells changes in the bimetallic NP systems. In particular, the electron transfer occurs when the monolayer of the transition metal Pd or Pt are in contact with the Au core. The electronic structural change in the monolayer of the transition metals causes the changes in the Raman frequencies observed in the SERS spectra. Therefore, core−shell nanoparticleenhanced Raman spectroscopy can be conveniently used to characterize the electronic structures of bimetallic catalysts in a precise manner. 3.4. Appropriate Application of the Core−Shell Nanoparticles in Electrocatalysis. By using core−shell nanoparticle-enhanced Raman spectroscopy, we have success-

can infer that Au@Pd and Au@Pt NPs are probably dominated by (100) and/or (110) surfaces. According to the CO frequency Raman shift observed in our SERS experiments, we speculate that the electronic structure effect exists in both the Au−Pd and the Au−Pt nanoparticles systems, which is well-correlated with the electrochemical behavior (Figure S3). In the Au@Pd system, Pd is likely to transfer electrons to Au, so Pd is in a poor electronic state; thus, the Raman frequency is shifted toward a lower wavenumber. In the Au@Pt system, Au is likely to transfer electrons to Pt, so Pt is in a rich electronic state. Thus, the Raman frequency is shifted to a higher wavenumber. On the basis of this interpretation, we can also extend this strategy to other bimetallic systems as well. This kind of information will be of great advantage for the design and improvement of the next generation of catalysts. In the following section, we find further evidence for our inference with results from X-ray photoelectron spectroscopy data. 3.3. XPS Characterization of Au@Pd/Pt Bimetallic Nanoparticles. In order to detect the electronic structures of Au@Pd and Au@Pt nanoparticles accurately and provide direct experimental evidence for our inference described above, we conducted XPS experiments with different shell thicknesses for the Au@Pd and Au@Pt systems. XPS is one of the most important surface-sensitive analytical techniques, which is widely used for the analysis of surface elemental composition and chemical state, especially to measure the atomic valence, electron cloud, and the energy level structure of the surface atoms. Figure 3 shows the XPS spectra of the Au@Pd (A) and Au@Pt (B) NPs with different shell thicknesses. 20688

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Figure 5. (A) Cyclic voltammograms for methanol oxidation using Au@Pd NPs with different shell thicknesses on glassy carbon (GC) electrodes in a solution of 1 M CH3OH + 0.5 M KOH, scan rate: 20 mV/s. (B) Shown are polarization curves for the O2 reduction reaction (ORR) on Au@Pt NPs catalysts on a rotating disk electrode in an O2-saturated 0.1 M HClO4 solution at room temperature. Sweep direction, negative; sweep rate, 10 mV/s; rotation rate, 1600 rpm. (C) Catalytic activity for the ORR normalized by Pt−CO area. The shell thicknesses are shown in the order of 0.35, 1.4, and 7.0 nm.

fully determined the electronic structure of the Au nanoparticles coated with several monolayers of transition metals in the previous sections. To examine the applicability of the Au@ Pd and Au@Pt NPs, the core−shell nanoparticles with different shell thicknesses were first applied for CO electrocatalytic oxidation reaction. Though CO oxidation is a model chemical reaction, it unravels the mysteries and richness of heterogeneous catalysis. As shown in Figure 4A, the initial peak potential of the CO oxidation varies with the shell thickness of the Au@Pt nanoparticles. When the Au core is coated with a monolayer of Pt, the CO oxidation peak potential is at 0.65 V, compared to the bulk Pt electrode value (0.45 V),53 the potential has varied by 0.2 V which is likely due to the electrons that transfer from the Au to the Pt. This behavior infers that the Au nanoparticles coated with a monolayer of Pt are not suitable for CO oxidation when compared to the bulk Pt electrode. As expected, the Au@Pd nanoparticles exhibit better catalytic activity when compared to the bulk Pd electrode. As we mentioned earlier, the Pd shell gives electrons to the Au core, and therefore, the peak potential of the Au@Pd nanoparticles (∼monolayers) is at 0.57 V, which is lower than the bulk electrode value (0.7 V). Figure 4B clearly shows the peak position of the Au@Pd nanoparticles with different shell thicknesses toward CO oxidation. In this way, the catalytic performance of a catalyst can be tuned by varying the shell thickness by making use of the information regarding its electronic structure. In addition, the core−shell nanoparticles examined here were also applied to the methanol oxidation reaction. In the previous case, the Pd monolayer on the Au core exhibited better catalytic activity than its bulk part, whereas the Au@Pd had almost no catalytic activity in the acid system. Therefore, the reaction was carried out in an alkaline solution. Figure 5A shows the cyclic voltammograms of methanol oxidation using Au@Pd NPs with different shell thicknesses. We can clearly observe that the Au core coated with a monolayer of Pd shell advanced the oxidation peaks about 0.1 V more than the bulk Pd electrode (7.0 nm), which means it has some increased performance for the methanol oxidation reaction as well. We demonstrated that Au@Pd with a monolayer of Pd shell exhibits better catalytic activity compared to Au@Pd with multilayer Pd shells for the oxidation reaction. We infer that the electron transfer from Pd to Au may improve the oxidation reactions which are losing electrons.

Therefore, Au@Pt NPs would be an appropriate catalyst for the reduction reactions; thus, we applied the Au@Pt NPs with different shell thicknesses to the oxygen reduction reaction (ORR), which is one of the most important electrochemical reactions.54−56 Figure 5B shows that the reductive potential of Au@Pt with a 0.35 nm Pt shell is advanced compared to that of the Au@Pt system with 1.4 or 7.0 nm Pt shells. As can be seen in Figure 5C, the Au@Pt NPs with a 0.35 nm shell thickness also exhibit the best catalytic activity for ORR, which indicates that the electron transfer from the Au core to the Pt shell facilitates this reduction reaction. Overall, Au@Pd NPs with a thin shell have better catalytic performance toward CO oxidation and methanol oxidation, whereas Au@Pt NPs with thin shells have better catalytic performance toward the oxygen reduction reaction. Thus, the SERS technique can be conveniently used to characterize the bimetallic catalysts because our SERS data are in good agreement with the XPS data and the electrochemical data as well. Therefore, we conclude that, without considering other factors, the catalytic performance of a bimetallic catalyst can be tuned with the shell thickness by understanding the electronic influence of the core material. This kind of SERS investigation will help us to realize suitable applications for different kinds of bimetallic catalysts.

4. CONCLUSIONS In summary, Au@Pd and Au@Pt NPs with different shell thicknesses are characterized by the core−shell nanoparticleenhanced Raman spectroscopy technique. We can conclude that the Fermi level of the Au core and the transition metal shell affect the electronic structure of the bimetallic nanoparticles and eventually govern the catalytic behavior of the nanoparticles. According to the CO frequency shifts observed in the SERS spectra, the electron transfer from the Pd shell to the Au core results in a shift toward lower wavenumber, and the electron transfer from the Au core to the Pt shell leads to a reverse result. This study shows the efficacy of the SERS technique under ambient conditions, which is more convenient than other analytical techniques that are typically carried out in vacuum conditions. In addition, our SERS observations accord well with the X-ray photoelectron spectroscopy data. Further, the as-prepared bimetallic catalysts were used to demonstrate their potential applications in the CO oxidation, methanol oxidation, and oxygen reduction reactions. In closing, core− shell nanoparticle-enhanced Raman spectroscopy can be 20689

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The Journal of Physical Chemistry C

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employed to probe the electronic structural changes in bimetallic nanoparticles. Importantly, we can apply this strategy to evaluate similar kinds of catalysts to better understand and optimize the catalytic performance of bimetallic catalysts for various applications in the future.

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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.jpcc.6b01879. SEM and TEM images, SERS spectra, and the electrochemical behavior of the Au@Pd and Au@Pt core−shell nanoparticles (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-592-2186192. Author Contributions

Y.-J.Z. and S.-B.L. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSFC (21522508 and 21427813), Thousand Youth Talents Plan of China, and Fundamental Research Funds for the Central Universities (Xiamen University, 20720150039).

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REFERENCES

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DOI: 10.1021/acs.jpcc.6b01879 J. Phys. Chem. C 2016, 120, 20684−20691