CoAu

Article pubs.acs.org/JPCC

Investigation of Structural and Magnetic Properties of CoPt/CoAu Bimetallic Nanochains by X‑ray Absorption Spectroscopy Wei-Feng Huang,†,‡,# Qian Zhang,§,# Dong-Feng Zhang,§ Jing Zhou,∥ Cheng Si,‡ Lin Guo,*,‡ Wang-Sheng Chu,*,† and Zi-Yu Wu*,†,‡ †

National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, P. R., China Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, P. R. China § School of Chemistry & Environment, Beihang University, Beijing 100191, P. R. China ∥ Chinese Academy of Sciences, Shanghai Institute Applied Physics, Shanghai 201204, P. R. China ‡

S Supporting Information *

ABSTRACT: Cobalt-based nanohybrid chains with diameters ranging from 20 to 50 nm have been prepared by a mild solution method. Structural, morphological, and magnetic characterization of the as-prepared products have been performed using X-ray diffraction, transmission electron microscopy, and superconducting quantum interference device apparatus. In addition, the electronic structure and the local atomic distribution were both investigated by X-ray absorption spectroscopy. Results show that different kinds of noble metals play a key role in electronic structure, atomic distribution, and magnetic properties even though they have similar morphology and long-range order structure. The changes of the electronic structure in the CoPt sample occur during the large alloying process while the CoAu sample maintains its lattice structure with negligible electronic changes. The difference between the electronic structure and the atomic distribution of these two different Co-based nanohybrid chains leads to different magnetic behaviors versus temperature.

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INTRODUCTION It is of great interest to exploit properties of novel advanced bimetallic nanoparticles (NPs) with controlled magnetic and optoelectronic properties due to their potential applications in optics, data storage, nanoelectronics, and biology.1−5 In fact, because of synergic effects,1−3 nanohybrids are always characterized by new or enhanced properties compared with a single nanostructure. Among bimetallic NPs, alloys and intermetallic compounds of a magnetic element such Co with noble metal elements, for example, Pt and Au based systems, show magneto-optic bifunctionalities attracting a lot of attention.6−13 However, several factors may affect the performance of nanohybrids: (a) the bimetallic component; (b) the structure and morphology; and (c) the alloying extent and the atomic distribution of bimetallics.12−16 As an example, Park and coworkers investigated the phase transitions of Cocore-Ptshell structures toward a c-axis-compressed face-centered tetragonal (fct) solid solution alloy of CoPt NPs that changes its magnetic response from superparamagnetism to ferromagnetism.8 Bigot et al. also revealed a clear correlation between the magnetization dynamics and the crystalline structure of NPs for the same Cocore-Ptshell sample.13 Although high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) are useful methods to probe an alloy structure, information about the atomic distribution and the electronic © 2013 American Chemical Society

transformation are lacking. Actually, the X-ray absorption spectroscopy (XAS) is a technique sensitive to the short-range order and to the local and partial empty density of states.17,18 X-ray absorption near-edge structure (XANES) allows a fingerprint of the local geometry identifying the oxidation state, the fractional d-electron density in a 3d transition metal, and the electronic environment around the absorbing atoms.19−21 Meanwhile Extended X-ray absorption fine structure (EXAFS) is able to return bond distances, number, and types of backscattering atoms surrounding the photoabsorber. The latter method has been utilized successfully to characterize the structure of many bimetallic NPs systems.22−25 As an example, to get a precise characterization of bimetallic NPs structure, Hwang et al. proposed a general XAS methodology that describes quantitatively the alloy extent and the atomic distribution. It was successfully applied to reveal the relationship between structure and catalytic activity of Pt−Co/ C bimetallic NPs and other systems.22,26−28 In a previous work, we showed a successful synthesis procedure capable of producing two different Co-based nanohybrid chains, that is, CoPt and CoAu alloys.29 In this Received: January 28, 2013 Revised: March 15, 2013 Published: March 15, 2013 6872

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Figure 1. Typical TEM images showing the nanochain structure (a) and a magnified view (b) of CoPt NCs and of CoAu NCs (c,d), respectively.

Quantum Design SQUID magnetometer. Co K-edge, Pt L3edge, and Au L3-edge XAS were recorded at the U7C beamline station of the National Synchrotron Radiation Laboratory (NSRL) of China. The NSRL storage ring was operated at 0.8 GeV with a maximum current of 300 mA. Fixed-exit Si (111) double crystals were used as the monochromator. Data were collected in transmission mode with ionization chambers filled with a N2/Ar mixture at room temperature. The energy calibration was performed with a Co foil at 7709 eV and a Se foil at 12 658 eV. Data processing and analysis were made by standard procedures.30,31

contribution, combining XAS with TEM, XRD, and SQUID characterizations, we demonstrate that different noble-metal components of nanohybrids produced with similar synthesis methods have different impacts on the electron properties, the atomic distribution, and the magnetic response.

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EXPERIMENTAL SECTION

A similar solution synthesis route was employed to fabricate the cobalt-based alloys, as we discussed in ref 29. Typically, 0.3032 g of CoCl2·6H2O and 0.4260 g of PVP were dissolved in 30.0 mL of ethylene glycol (EG) under constant stirring. Then, 1.5 mL of N2H4·H2O (50 vol %) was added dropwise to the above solution and kept stirring for more than 3 h, forming a homogeneous light-pink solution. Subsequently, 66.4 mL of EG solution of H2PtCl2 (4 × 10−3 mol·L−1)/53 mL EG solution of HAuCl4 (4 × 10−3 mol·L−1) was transferred to the above mixture and stirred for another 0.5 h. The temperature was raised to the boiling point (197 °C) and kept refluxing for 4 h. After the mixture was cooled to room temperature, the black precipitates were collected by centrifugation, and the products were washed several times by ethanol. Characterization. TEM measurements were carried out with a JEOL 2100F microscope working at 200 kV. XRD patterns were recorded on powder samples with a Rigaku D/ max-2200 diffractometer using the Cu Kα radiation (λ = 1.54056 Å) at a scanning rate of 0.02 deg/s ranging from 20 to 90°. Magnetic properties of samples were measured using a

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RESULTS AND DISCUSSION Figure 1 shows the two Co-based nanohybrids: CoPt and CoAu. They have a similar nanochain morphology with diameters ranging from 20 to 50 nm. In Figure 2, we compare the XRD patterns of these samples. All diffraction peaks of the two alloys exhibit a characteristic face-centered cubic (fcc) crystalline structure according to the XRD standards JCPDS nos. 04-0802 and 04-0784 for Pt and Au, respectively. Results indicate that both samples are mainly a single-phase solid solution. However, we cannot ruled out the presence of Co until at present there is a lack of evidence of both Co or Co oxide diffraction peaks in the respective patterns because of the possible presence of small clusters or amorphous products or other heavy atom effects.32 Compared with standard diffraction patterns of Pt and Au powders, respectively, CoPt shows a 6873

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eV (feature B) is due to electronic transition from 1s to 4p.21,28 It is easy to observe that the CoPt alloy characteristic feature is completely different from both foil and oxides, whereas the CoAu sample has the same features of the Co nanochain sample, still maintaining a strong Co character. To better correlate the electronic distribution and the changes of these features, experimental spectra have been superimposed in Figure 4b. From the comparison between the CoPt alloy and the Co foil, we can point out that the feature A weakens, indicating that the local environment around the photoabsorber atom probably switches from a tetrahedral to an octahedral symmetry. In fact, we may consider that Co atoms are imbedded in the Pt lattice, forming a CoPt alloy. As a consequence, the Co local environment changes from the hexagonal close-packed (hcp) of the Co foil to the fcc of the Pt foil, and in this configuration the electronic transition from 1s to 3d becomes forbidden. The variation of the feature B in the CoPt sample reflects the characteristic of Co 4p unoccupied state. Most importantly, when taking the comparison of CoPt and CoAu systems, a large bond length of Co−Co in CoPt NCs is observed, leading to an increased long-range order, and this is in accordance with the EXAFS data. In addition, the shift of the absorption edge may be associated with the formation of products with a high oxidation state, as demonstrated by EXAFS data. Several variations in the Pt L3 XANES spectra are monitored in the CoPt samples, such as the relative intensity of the white line (feature A) and the position of the main features (features B−D) shown in Figure 5a. The decrease in the white line in the CoPt sample compared with Pt can be associated with an electron transfer from Co to Pt upon alloying, in agreement with a previous report.6 The shift to high energy of the main features, B−D, can be associated with a decrease in the metal bond length according to the Natoli’s rule.34 This result is in agreement with XRD experimental data. On the contrary, neither Co nor Au in the CoAu system shows little electron or structural changes (see Figures 4b and 5b), maintaining their intrinsic characters again in agreement with previous works that pointed out the presence of an immiscibility gap between Co and Au by means of firstprinciple calculations and experimental data.35−37 Actually, the relative reduced Co−Co bond in CoAu NCs compared with that in CoPt NCs, which is supported by EXAFS data, is also significative to stabilize the magnetic properties. Furthermore, it is also important to point out for the CoAu sample that a noble-metal element such as Au not only enhances the magnetic property of Co but also stabilizes it, preventing its

Figure 2. XRD patterns of CoPt (blue) and CoAu (red) NCs samples. In the bottom, the standard patterns of Pt (blue, JCPDS no. 04-0802) and Au (red, JCPDS no. 04-0784).

slight shift of the peaks position, pointing out a decrease in the crystal lattice, whereas CoAu is almost constant. In addition, because XRD is also sensitive to the grain size, the occurrence of broad diffraction peaks suggests the presence of small grain size. On the contrary, despite their similar morphology and longrange order, the two Co-based metallic nanochains show a completely different magnetic behavior. The M(H) measurements performed at 5, 100, and 300 K are shown in Figure 3. The coercivity of the CoPt sample (360 Oe) is slightly higher than that of the CoAu sample (320 Oe) at 5 K. More interestingly, a clear coercivity dependence by the temperature is observed in CoPt, whereas almost no correlation with temperature occurs in CoAu. In addition to the coercivity, the saturation magnetization of both samples shows a similar trend versus temperature. To understand the relationship between atomic/electronic structure and magnetic properties, we have performed XAS experiments on both systems. The electronic properties of these alloys have been carried out by means of XANES spectroscopy. In Figure 4a, we compare Co K-edge absorption spectra of the two alloys and of reference samples: Co foil, Co nanochain,33 and Co oxides. The absorption edge at 7709 eV (feature A) corresponds to the electronic transition from 1s to 3d states, whereas in the Co foil the absorption hump at 7725

Figure 3. Magnetization curve measured at 5, 100, and 300 K for CoPt (a) and CoAu (b) NPs, respectively. 6874

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Figure 4. Co K-edge XANES spectra of (a) as-prepared CoPt/CoAu NCs and chain-like Co NCs compared with the reference spectra of Co foil, CoO, Co2O3, and Co3O4 (inset: a magnified view of the pre-edge region) and (b) comparison of characteristic features of the as-prepared samples.

Figure 5. Normalized XANES spectra at the (a) Pt L3 edge of a Pt foil (black) and CoPt NCs (blue) and (b) Au L3 edge of Au NPs (black) and CoAu NCs (red).

CoPt alloy but also that an oxide contribution occurs in agreement with the XANES analysis. As expected, the reduction of both the oscillation frequency and the amplitude of the k2weighted spectrum indicates that Pt mixes with Co forming an effective alloy with a Pt−Co bond of ∼2.6 Å (Figure 6b,d). On the contrary, the local coordination environment of both Co and Au (see Figure 7) is similar to their reference nanomaterials, pointing out a hardly alloying extent of these products, a behavior consistent with theoretical calculations. In fact, both Au and Co share a fcc crystal structure.21,28 Moreover, in the

oxidation in some extent, as will be discussed in the next section. To better understand the local environment of the products, we performed the EXAFS analysis. In the CoPt system, Co samples exhibit completely different oscillations in both phase and amplitude, as shown by the k2-weighted EXAFS signal. (See Figure 6a.) The presence of a Co−O bond at ∼1.9 Å and a Co−Pt bond at ∼2.6 Å in the Fourier transform of the Co K edge of the k2χ(k) spectrum of the CoPt system (Figure 6c) points out not only that Co takes part in the formation of the 6875

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Figure 6. (a,b) k2-weighted EXAFS spectra and (c,d) corresponding k2-weighted FT for the as-prepared CoPt NCs, Co NPs, and a Pt foil at the Co K-edge and Pt L3-edge, respectively.

CoAu system, Co remains stable with no oxidization or alloying, while clearly a partial oxidization occurs in the CoPt system. To gain a better and quantitative structural description in terms of coordination type, number, and interatomic distances of bimetallic nanochains, we fit the first shell coordination. Results are summarized in the Supporting Information, whereas structural parameters of the fit are listed in Table 1. A XAS method based on four parameters, Pobserved, Robserved, Prandom, and Rrandom, has been proposed to evaluate the atomic distribution and the alloy extent of bimetallic NPs.22 The parameter Pobserved is the ratio of the coordination number of the scattering atoms B around the absorbing atom (∑NA‑i) (we may define Pobserved = NPt/Au−Co/∑NPt/Au‑i). Exchanging A with B, we may introduce Robserved (Robserved = NCo−Pt/Au/∑NCo‑i), whereas Prandom and Rrandom are 0.5 for a perfect bimetallic NPs system. The alloying extent of elements A (JA) and B (JB) for an A−B bimetallic NPs system can be calculated using the equations: JPt/Au =

JCo =

Pobserved × 100% Prandom

R observed × 100% R random

With this method, we may estimate both the atomic distribution and the alloying extent of bimetallic nanochains. On the basis of the parameters in Table 2, structural model graphs are shown in Figure 8. In the CoPt model system, the Co atoms are randomly imbedded in the Pt lattice, whereas in the CoAu model system, Co and Au atoms form independent small clusters connected between them like a jigsaw.29 However, in this estimation of the alloying extent, we have to emphasize that we are neglecting contributions from the metal oxides.28 In fact, although bimetallic NPs systems may have similar nanochain morphologies and long-range order structure as TEM and XRD data pointed out, the atomic distribution and the electron transition of these systems are strongly dependent on the type of the noble metal. Furthermore, their different atomic distributions significantly affect the measured magnetic properties. Actually, the above simple analysis allows us to also explain the difference among the magnetic properties of these similar Co-based bimetallic nanochains. It originates from the electronic and atomic distributions induced by alloying. The large enhancement of the coercivity at low temperature in the CoPt sample is mainly due to the large charge transfer, that is, the number of electrons going from Co to Pt,6 which is on the contrary negligible in the CoAu sample. Nevertheless, increasing the temperature, if compared with the anisotropic

(1)

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Figure 7. (a,b) k2-weighted EXAFS spectra and (c,d) corresponding k2-weighted FT for the as-prepared CoPt NCs, Co NPs, and Au NPs at Co Kedge and Au L3-edge, respectively.

Table 1. Fit Parameters of the First-Coordination Shell As Obtained from the Least-Squares Fits of Experimental EXAFS Spectra of CoPt NCs and CoAu NCs sample

edge

Co K CoPt Pt L3 CoAu

Co K Au L3

N

bond Co−Co Co−Pt Co−O Co−Co Pt−Co Pt−Pt Co−Co Au−Au

1.1 2.8 0.9 1.3 1.3 6.9 9.7 10.9

± ± ± ± ± ± ± ±

σ2 (10−3 Å)

R (Å) 0.1 0.8 0.2 0.5 0.4 0.7 0.4 0.5

2.60 2.65 1.95 2.5 2.65 2.71 2.48 2.83

± ± ± ± ± ± ± ±

0.06 0.06 0.03 0.12 0.04 0.02 0.02 0.02

10.6 6.9 5.9 7.3 6.5 6.0 6.7 8.8

± ± ± ± ± ± ± ±

2.7 0.3 0.8 0.5 0.7 0.1 0.3 0.3

Table 2. Alloying Extent Values for CoPt/CoAu NCs As Obtained from the Fit of the XAFS Experiment samples

ΣNPt/Au‑i

ΣNCo‑i

Pobserved

Robserved

JPt/Au (%)

JCo (%)

CoPt CoAu

8.2 ± 0.7 10.9 ± 0.5

3.9 ± 0.8 9.7 ± 0.4

0.16 ± 0.02 0

0.72 ± 0.03 0

32 ± 4 0

144 ± 6 0

energy (kuυ, where ku is the Boltzmann constant), the thermal fluctuation energy (kT) may play a key role in the magnetic properties.38 In particular, in the CoPt sample, Co atoms representing a small atomic inset inside the Pt lattice (see Figure 8a) will become unstable around the equilibrium position increasing temperature. On the contrary, Co atoms in the CoAu sample form stable clusters with an fcc lattice structure surrounded by Au clusters, and Co atom is difficult to

move from its equilibrium lattice site. At the same time, no electrons are transferred from Co clusters to Au because of its special electronic structure. In this framework, neither the thermal fluctuation energy (kT) nor the anisotropic energy (kuυ) play a role in the magnetic properties. In other words, because any possible potential dynamics, including electronic transitions and atomic shaking, are limited by the Au clusters properties, it is not difficult to understand why the magnetic 6877

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Figure 8. Atomic distribution (alloying extent) of the as-prepared CoPt (a) and CoAu (b) samples.

properties of the CoAu system do not depend by the temperature. In conclusion, the weak Co−Au electron transfer and the short Co−Co distances associated with the presence of small Co clusters lead to a higher and more stable magnetic response. These results are fully in agreement with the experimental data presented in this contribution.

CONCLUSIONS In this manuscript, we present a simple mild method to synthesize Co-based bimetallic chain like nanohybrids including CoPt and CoAu samples. The relationship between the atomic distribution and the magnetic properties of these two Co-based bimetallic nanohybrids has been investigated. We found that because of homogeneous atomic distribution, CoPt samples exhibit a strong dependence of the coercivity by temperature not observed in CoAu samples. Indeed, the CoPt system shows an alloying behavior, which is a high atomic degree of dispersion, whereas the CoAu system is characterized by a jigsaw network that connect Co and Au clusters together. Finally, we identified by means of the XAS analysis a large charge transfer from Co to Pt in the CoPt sample, whereas a negligible electronic transfer occurs in the CoAu sample. As a consequence, the results achieved allow us to claim that with a proper selection of the second noble metal and an accurate control of the atomic distribution of Co and of the second noble metal we may probably tune the magnetic and electronic properties of transition-metal-based nanostructured bimetallic alloys.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Detailed information on the EXAFS fitting results, TEM and HRTEM images, line-scan elemental mapping data, and atoms ratio for CoAu and CoPt NCs, respectively. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

This work was partially supported by the Knowledge Innovation Program of the Chinese Academy of Sciences (KJCX2-YW-N42), the Key Important Project of the National Natural Science Foundation of China (10734070), the National Natural Science Foundation of China (NSFC 10805055 and 11275227, 11079002), and the National Basic Research Program of China (2009CB930804).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Z.-Y.W.); [email protected] (L.G.); [email protected] (W.-S.C.). Author Contributions #

Wei-Feng Huang and Qian Zhang contributed equally.

Notes

The authors declare no competing financial interest. 6878

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dx.doi.org/10.1021/jp4009674 | J. Phys. Chem. C 2013, 117, 6872−6879