Shell-Dependent Evolution of Optical and Magnetic Properties of Co

Apr 30, 2012 - Pipsa Mattila , Hanna Heinonen , Kalle Loimula , Johanna Forsman , Leena-Sisko Johansson , Unto Tapper , Riitta Mahlberg , Hans-Peter ...
0 downloads 0 Views 4MB Size
Download PDF
Article pubs.acs.org/JPCC

Shell-Dependent Evolution of Optical and Magnetic Properties of Co@Au Core−Shell Nanoparticles Yujun Song,* Jie Ding, and Yinghui Wang School of Materials Science and Engineering, Beihang University, Beijing 100191, China ABSTRACT: Co@Au core shell nanoparticles (NPs) of different shell thicknesses were fabricated by a combination of the displacement process and the reduction− deposition process in a microfluidic reactor. Changes in the core sizes and the whole sizes of these Co@Au NPs with the shell formation were investigated. Effects of the shell thickness and the size change on the Co−Au interface pinning effects and the interparticle interaction were analyzed and correlated to their magnetic properties and surface plasmon resonance (SPR). Increasing the shell thickness causes an increase of the coercivity at 10 K due to the enhanced interfacial pining effect and decease of the coercivity at 300 K due to the reduced interparticle interaction. The increased core sizes and Co−Au interface pinning effects with the shell formation, and the higher interparticle interaction than that of well-dispersed species, result in significantly enhanced blocking temperature (Tb) for these Co@Au NPs. But the Tb's for these Co@Au NPs slightly decrease with an increase of the shell thickness due to the reduced interparticle interaction. The SPR absorbance shows a line width broadening and an enhanced line shape variation with an increase of the shell thickness and a broad size distribution. Tuning of the optical and magnetic properties of the core−shell nanoparticles via the shell thickness provides an efficient and flexible method to obtain desired magnetic and optical properties for multimode sensing technology and high efficiency solar cell.

1. INTRODUCTION Progress in nanotechnology has led us to pay much attention to nanomaterials with hybrid structures.1−7 These kinds of hybrid nanostructures not only endow individual nanoparticles (NPs) multifunctions often greater than the simple sum of each part but also produce novel physicochemical properties and phenomena through the inner-coupling and the interfacial proximity effects from their constructing parts.1−10 These nanomaterials can be engineered for desired applications in the control and transduction of electromagnetic, magnetic, electronic, thermal, and/or acoustic signals, or for overcoming of some conflicts with size decrease.1−11 The combinations of these tunable properties are fundamentally and practically crucial in the preparation of high efficiency photovoltaic cells,3,12 solar-fuel reactors,12,13 plasmon lasers,14 wide-band stealth materials,15,16 nanoantenna,1,17,18 photonic chips,16,18 and ultrasensitive multimode sensing probes.2,5,6,8,9 Among them, hybrid structures with magnetic components (e.g., Co, Fe, Ni, iron oxides) as cores and optical components (e.g., noble metals) as shells are of much interest since these kinds of structures can endow nanomaterials with unique magnetic, optical, electronic, catalytic, and other physiochemical properties in individual nanoparticles (NPs).2,7,9,19−22 These kind of hybrid structures will be useful for diverse applications, such as magnetic-optical manipulation, enhanced light absorption at wide band, and label-free diagnosis and therapy.2,7,9,19−21 Recently, much progress has been made in the synthesis methods for these kinds of NPs with a uniform shell or core and the investigation of their related magnetic and optical properties.23−28 The optical and magnetic properties of © 2012 American Chemical Society

core−shell nanomaterials significantly depend on core sizes and shapes, shell thickness, and crystal structures of cores and shells, and the interfacial junctions between cores and shells.7,19−32 Effects of the core size and shell thickness and the interaction between cores and shells on their structural and morphological transformations and the related physicochemical properties of these core−shell NPs remain open fundamental issues for their applications.23−28,33,34 Therefore, investigations on the interactions among the size and size distribution, the core and shell microstructures, and the physicochemical properties of these hybrid NPs are critical to facilitate their future application. Recently, we developed a sequential microfluidic process for the synthesis of core−shell Co@Au NPs. Co@Au NPs with different shell thicknesses have been fabricated via a combination of the displacement method and the reduction− deposition process.34 In this article, the shell thickness driven evolution of optical and magnetic properties will be analyzed and discussed, together with the combined effect of their size and size distribution on these properties.

2. EXPERIMENTAL SECTION 2.1. Preparation of Co Nanoparticles. Cobalt nanoparticles were prepared by the reduction of CoCl2 (99.9%, anhydrous) in tetrahydrofuran (THF, 99.90% pure packaged under nitrogen) using lithium hydrotriethylborate (LiBHReceived: January 4, 2012 Revised: April 29, 2012 Published: April 30, 2012 11343

dx.doi.org/10.1021/jp300118z | J. Phys. Chem. C 2012, 116, 11343−11350

The Journal of Physical Chemistry C

Article

Scheme 1. Co@Au Nanoparticle Formation by Displacement Process

Scheme 2. Shell Thickness Increase by the Reduction−Deposition Process

2.2. Preparation of Co@Au Nanoparticle with Different Shell Thickness. The first batch of Co@Au nanoparticles was prepared using the displacement process in the microfluidic reactor, as shown in Scheme 1. 0.12 g of KAuCl4 was dissolved into 50 mL of THF solution and then mixed with the preformed 50 mL Co nanoparticles THF solution in the microfluidic reactor at room temperature at a flow rate of 0.08 mL/min. Then the mixture was left quietly for two more hours to fulfill the displacement reaction. After that, the nanoparticles gradually precipitated down to the bottle and the top solution became blue on leaving a magnet under the receiver. This blue color indicated the formation of some Co2+ ions by displacing part of surface Co atoms of NPs with KAuCl4. After all of the formed Co@Au nanoparticles settled down, the top blue solution was decanted and the particles were washed using THF two or three times until no blue color appeared in the top solution. Then the NPs were redispersed in 50 mL of THF by intensive shaking. A 10 mL NPs solution was taken out and the NPs settled down by a magnet and dried in inert atmosphere for further characterization. The following reduction−deposition process was used to increase the thickness and coating integrity of the shells. As shown in Scheme 2, gold atoms by the reduction of gold salts will attach on the surface of the preformed Co@Au NPs fabricated by the displacement method and grow to form a continuous surface layer. Typically, a certain amount of Li[B(Et)3H]-SB12 complex THF solution was formed by adding 1.4 g of SB12 and 5 mL of Li[B(Et)3H] THF solution to the 40 mL Co@Au nanoparticle solution formed by the displacement process. The KAuCl4/THF solution was prepared by disolving 1.4 g of KAuCl4 in 45 mL of THF. The KAuCl4THF solution and the mixture of Li[B(Et)3H]-SB12 complex and the first batch of Co@Au nanoparticle solution were delivered into the micromixer at room temperature at a flow rate of 0.08 mL/min. After the reaction was finished, the mixture was left quietly for two more hours to ripen the shells. After that, the nanoparticles settled down to the bottle using a magnet and the top solution became light pink. This pink color indicated the formation of a little bit of Au NPs during the reduction−deposition process. After all the formed Co@Au NPs settled down, the top solution was decanted and the particles were washed using THF two or three times until no pink color appeared in the top solution. Then the NPs were dispersed in 50 mL of THF again by intensive shaking. The 10 mL NPs suspension was taken out and the NPs inside were separated using a magnet and dried for further characterization.

(C2H5)3, 1 M solution in THF) as a reducing agent and 3(N,N-dimethyldodecylammonia)-propanesulfonate (SB12, 98%) as a stabilizer according to the chemical reaction given below (eq 1).35 C12H 25N(CH3)2 (C3H6‐SO3) + LiBEt3H THF

⎯⎯⎯⎯→ C12H 25N(CH3)2 (C3H6‐SO3) ·LiBEt3H C12H 25N(CH3)2 (C3H6‐SO3) ·LiBEt3H + CoCl 2 THF

⎯⎯⎯⎯→ COcolloid · C12H 25N(CH3)2 (C3H6‐SO3) + LiCl 1 + BEt3 + H 2 2 (1)

The design and fabrication of the microfluidic reactor had been discussed extensively in our previous publications, which had been successfully applied to the fabrication of Co, Cu, Pd, Fe, Ru, Pt, and CoSm alloy nanoparticles and to the control of their size, shape, crystal structure, and growth.36−44 In this article, we extended this process to sequentially synthesize Co@Au NPs using presynthesized Co NPs as cores. The saturated cobalt salt solution was prepared by dissolving CoCl2 (Sigma, 99.9%, anhydrous) of 0.2 g into 50 mL of tetrahydrofuran (THF, water free) by untrasonicating under nitrogen protection. The reducing agent solution was prepared by adding 5 mL of 1 mM Li[B(Et)3H]/THF solution into 45 mL of THF with 0.1 g of SB12 to form Li[B(Et)3H]-SB12 complex solution. The 50 mL saturated CoCl2/THF solution and the 50 mL reducing agent solution with SB12 were delivered into the microfluidic reactor using self-priming pumps (120SPI-30, Bio-Chem Valve, Inc.) at a flow rate of 0.08 mL/ min at 50 °C. The produced nanoparticles were collected in the receiver flask during reaction. Different from our previous approach in the fabrication of amorphous nanoparticles,38 the as-synthesized nanoparticles were collected at room temperature (∼20 °C) for convenience. After the reaction was finished, the 20 mL nanoparticle solution was taken out and about 1 mL of ethanol was added into the 20 mL solution to break down the equilibrium of the colloidal solution to let the particle settle down. Then the supernatant solution was decanted and the particles were washed using a mixture of ethanol and THF (20 V% ethanol) twice and then dried to obtain a fine black powder for further characterization. NPs in the left 80 mL solution were settled down using a magnet and the NPs were washed twice only using THF and then dispersed into 50 mL of THF again for the core−shell nanoparticle synthesis. 11344

dx.doi.org/10.1021/jp300118z | J. Phys. Chem. C 2012, 116, 11343−11350

The Journal of Physical Chemistry C

Article

The above reduction−deposition procedure was repeated once to further increase the shell thickness. This time, the KAuCl4 solution was formed by dissolving 0.6 g of KAuCl4 in 43 mL of THF and the mixture of Li[B(Et)3H]-SB12 complex and Co@Au nanoparticle solution was formed by adding 0.6 g of SB12 and 3 mL of Li[B(Et)3H] THF solution into the 40 mL of Co@Au nanoparticle solution formed after the first reduction−deposition process. When the reaction was completed in the microfluidic reactor, the same procedure as the first reduction−deposition process was followed to obtain the Co@Au NPs with thicker shells. The diluted Co NPs and Co@Au NPs in wax for magnetic property measurement was prepared by dissolving 10−20 times of weights of the NPs in the THF solution into the NPs THF solution, and then the mixtures were dried under a vacuum. 2.3. Characterization of Co and Co@Au Core−Shell Nanoparticles. The particle size and shape were characterized using transmission electron microscopy (TEM, JEOL 2010, 200 kV, 0.23 nm) by placing a drop of well dispersed Co NPs in oxygen free water or ethanol on a carbon-coated copper TEM grid at room temperature and allowed to dry. Magnetic data were obtained using dried powder sample with a superconducting quantum interference device (MPMS-5S). UV−vis spectroscopy was used to characterize the surface plasmon resonance absorbance of the prepared Co NPs, Au NPs, and Co@Au core−shell NPs.

3. RESULTS AND DISCUSSION The seed Co nanoparticles for the shell formation were prepared by modifying the previously well-developed microfluidic process performed at a flow rate of 0.08 mL/min and an enhanced reaction temperature (50 °C).37,38,40 These NPs show a broad size distribution from 1.9 nm to 6.5 nm (Figure 1i-c) with an average size of 4.0 ± 1.1 nm according to their TEM images (Figure 1i-a). Clearly, these NPs have better crystallization than those previous NPs synthesized at room temperature, as evidenced by one example (Figure 1i-b) showing a lattice parameter of 2.19 Å that is not observed in those previous NPs.37 The TEM image (Figure 1ii-a) and the histogram of sizes (Figure 1ii-c) suggest that these Co@Au NPs almost have the same size and size distribution, with an averaged diameter of 3.9 ± 1.0 nm for the whole NPs. Although the magnified images for several NPs show a layer of slightly ordered atoms on the surface or edge of the NPs (Figure 1ii-b), no obvious Au crystal fringe or a continuous edge rim can be observed as previous reports.1,45 The calculated shell thickness from the displacement of the 4.0 nm Co NPs formed from 0.16 g of CoCl2 with the 0.12 g of KAuCl4 is only 0.3 nm, indicating that the shell is averaged only about one layer of Au atom thick (radius of Au: 0.135 nm) on the average and the leftover Co core is about 3.4 nm after the displacement of some surface Co atoms by Au atoms. This calculation indicates that this kind of layer is too thin to form a continuous Au shell. As a result, it only increases the roughness of surfaces of NPs after the Au atoms take part of some surface sites of Co atoms to form local tiny Au clusters. This process is very similar to the surface etching or doping effect. This kind of surface will provide lots of active anchoring sites for further addition of Au atoms to form a thick and continuous Au shell. As seen from Figure 1iii-a-b, the distinct variation in the contrast between the dark cores and the light edge rims in the NPs can be clearly observed after the first reduction−

Figure 1. The wide view of the TEM image of the Co nanoparticles (ia), the TEM image of the specific Co nanoparticles with lattice fringe of 2.19 Å (i-b), and the histogram of the size distribution of Co nanoparticles (i-c). The wide view of the TEM image of the Co@Au nanoparticles (ii-a), the TEM image of the specific Co@Au nanoparticles (ii-b), and the histogram of the size distribution of Co@Au nanoparticles (ii-c) formed by the displacement process. The wide view of the TEM image of the Co@Au nanoparticles (iii-a), the TEM image of the specific Co@Au nanoparticles (iii-b), and the histogram of the size distribution of the cores and the whole Co@Au nanoparticles (iii-c) formed by the displacement and the first reduction−deposition process. The wide view of the TEM image of the Co@Au nanoparticles (iv-a), the TEM image of the specific Co@ Au nanoparticles (iv-b) and the histogram of the size distribution of the cores and the whole Co@Au nanoparticles (iv-c) formed by the second reduce-deposition process.

deposition of Au atoms on the Co@Au NPs formed by the displacement method. These light edge rims suggest the formation of continuous shells on the surface of the cores, as the previous observation for these kinds of NPs.1,24,26,45 The bright edge rims are made of Au shells since the mass contrast dominates over diffraction contrast in the TEM image, rending the part in NPs formed by a larger atomic number element (i.e., Au) lighter than that formed by a smaller atomic number element (i.e., Co or Fe).28,45 The formed Co@Au NPs have an average size of 7.3 ± 2.7 nm, ranging from 3.6 to 15.0 nm (Figure 1iii-c), broader than the Co@Au NPs of ∼0.3 nm thick shell. The Au shells become thick enough to be observed after the first reduction−deposition of additional Au on the surface of Co@Au NPs formed only by the displacement process. However, the averaged core size increases to 4.9 ± 2.5 nm, larger than the theoretical core size (3.4 nm) after the displacement process. Their core size distribution becomes 11345

dx.doi.org/10.1021/jp300118z | J. Phys. Chem. C 2012, 116, 11343−11350

The Journal of Physical Chemistry C

Article

broad, ranging from 1.2 to 12.6 nm (Figure 1iii-c). This result suggests that some NPs may merge together at some local surface areas of NPs with thin or no coating of shells to form larger core sized NPs during the growth of the shell by the reduction−deposition process. 37 The shell thickness is measured as 1.2 ± 0.9 nm, smaller than the theoretical value (2.1 nm) possibly due to the increased core size and broad core size distribution (large sized NPs will consume more Au to form shells of the same thickness). The reduction−deposition process was repeated again to increase the shell coverage and thickness. As shown in Figure 1iv-a-b, some NPs indeed show a relatively small core and large shell (particularly for those NPs marked by arrows). However, some small NPs without clear core−shell structure features also appear. This result indicates that some Au atoms do not deposit on the surface the Co@Au NPs but self-grow to pure Au NPs, as often observed in the reaction system with a strong reducing agent.24 Simultaneously, clustering or aggregation of NPs becomes intensive to connect several Co@Au NPs together due to the bridging effect of these small Au NPs (Figure 1iv-b). If the NPs with clear core−shell structures are involved, the size distributions of the cores and the whole NPs are very broad (Figure 1iv-c). The average core size is about 5.4 ± 1.8 nm, ranging from 3.3 to 8.3 nm; and the average size of the whole NPs is about 9.1 ± 2.7 nm, ranging from 4.6 to 13.7 nm; These NPs give an average shell thickness of 1.9 ± 0.6 nm, smaller than the calculated shell thickness (∼2.5 nm) after the second reduction−deposition. The reduced shell thickness mainly resulted from the formation of pure Au NPs as observed from their TEM images (Figure 1iv-a). Comparing with the shell thickness of the 7.3 ± 2.7 nm Co@Au NPs after the first reduction−deposition process, an additional 0.7 nm thick Au layer is coated on the Co@Au NPs. The formation of shells affects the fine structure transition of Co@Au core−shell nanoparticles that will be discussed in detail in another article.34 These shell-driven fine structure transitions, the core size, and the shell thickness changes will affect the magnetic properties and surface plasmon resonance of Co@Au NPs since they are sensitive to the size and size distribution, the local atom arrangements, and the physicochemical properties of the core−shell interface,8,34,37,38 which will be discussed below. Figure 2A and 2B shows the magnetic hysteresis loops for these NPs at 10 K and 300 K, respectively. Their saturated magnetization, coercivity, and blocking temperatures are summarized in Table 1. It is not surprising that the magnetic saturation decreases because of the decreased content of the magnetic element (Co) in the NPs with larger shell thickness. No biased coercivities in their hysteresis loops suggest that they are free of oxidation or only ultrathin oxidation layers are formed.23,37,38,42,46 At 10 K, the coercivity (Hc) slightly increases from 600 Oe for the bare Co NPs to 750 Oe for the Co@Au NPs with a thin Au layer (0.3 nm), then to 960 Oe for the Co@Au NPs after further coating of a thick layer of Au (1.2 nm), and then significantly increases to 3450 Oe for the Co@Au NPs with a Au layer of 1.9 nm. The coercivity of the Co@Au NPs should be expected to decrease only if the reduced Co core size (∼3.4 nm) in the Co@Au NPs is considered.45,47 This reversed experimental result can be explained by the pinning effect of cobalt spins at the Co−Au interfaces as proposed by Krishnan et al.23 The Co−Au interface has a great deal of disordered lattice mismatch between Co and Au due to multiple nucleation sites during heterogeneous nucleation to form the Au shell.33 This disorder

Figure 2. Hysteresis loops of (i) the Co nanoparticles; (ii) the Co@Au nanoparticles formed via the displacement process; (iii) the Co@Au nanoparticles formed via the displacement process and the first gold salt reduction-deposition process; (iv) the Co@Au nanoparticles formed via the displacement process and the second gold salt reduction−deposition process at 10 K (A) and 300 K (B).

is often associated with large strain with energy given by Es = 2 μVε2(1 + σ)/9(1 − σ), where μ, V, ε, and σ are the shear modulus, volume of core, strain, and Poisson’s ratio, respectively.48,49 Such an interface strain in the magnetic NPs can pin the spin rotation at the interface and thus enhance the coercivity of magnetic NPs.50,51 In our case, this interface pinning effect can be possibly affected by the shell coverage, the shell thickness, the core size change, and the atom arrangement at the Co−Au interface during the formation of the Au shell. When the coating of Au is only 0.3 nm thick, no fully covered Au shell can be formed and the Au atoms would form very tiny clusters doping on the surface of the Co crystals. In this case, the gold atoms and the cobalt atoms have enough space to adjust their positions to match the interfacial crystal lattices (details will be discussed in our future article34). The interface strain is weak and only located in the areas with Au−Co interface, where the pinning effect of cobalt spins is weak, leading to a partial coherent Co− Au interface.33 These kinds of partially coated Co NPs will have a slightly increased Hc (from 600 Oe to 750 Oe) and little anisotropy energy change (evidenced by the slight increase of the magnetic anisotropy field Hk,M from 610 to 625 Oe). The Hk,M is defined from the magnetization curves as the crossing point of linear extrapolations of low field magnetization curve and saturation curve.52 When the coverage and thickness of Au shells can be further increased by the first reduction− deposition of additional Au atoms, an almost continuous thin shell (∼1.2 nm) will be formed. The interface strain will disperse on the whole surface of the Co NPs. Consequently, the pinning effect of cobalt spins occurs almost on the whole surface of the Co NPs and the coherence of the Co−Au interface will be much reduced.33 Their Hc (960 Oe) and 11346

dx.doi.org/10.1021/jp300118z | J. Phys. Chem. C 2012, 116, 11343−11350

The Journal of Physical Chemistry C

Article

Table 1. Magnetic Properties of Co Nanoparticles and Co@Au Nanoparticles with Difference Shell Thickness Ms, (emu/g) sample 4.0 nm Co NPs 3.9 nm Co@Au with 0.3 nm shell 7.3 nm Co@Au with 1.2 nm shell mixture of 9.1 nm Co@Au with 1.9 nm shell and Au NPs diluted 7.3 nm Co@Au with 1.2 nm shell in wax

Co content (wt %) whole NPs 100 53.6 7.4 5.1 7.4

108.3 60.5 11.5 7.3 11.3

anisotropy energy (Hk,M: 750 Oe) will be further increased (Table 1). When more and more Au atoms deposit on the thin Au shell via the second reduction−deposition process, Co cores are completely wrapped by the thick Au layers (∼1.9 nm). The Co−Au interface becomes extremely incoherent with a large interface strain since the fully covered thick Au layer cannot permit each Co atom to rearrange freely. The greatly increased interface strain will result in an extremely high interface energy, which will strongly pin the Co spin rotation at the interface intensively. Thereby, the magnetic anisotropy can be increased significantly (as evidenced by the increased anisotropy field Hk,M: 935 Oe), and thus the coercivity of the NPs will be enhanced distinctly (∼3450 Oe at 10 K). Interestingly, the coercivity at 300 K decreases with an increase in shell thickness, from 80 Oe for the pure Co NPs, to 45 Oe for the Co@Au NPs with 0.3 nm Au layer, and then to 42 Oe for the Co@Au NPs with 1.2 nm Au layer, and then to zero for the Co@Au NPs with 1.9 nm Au layer (Table 1). This result indicates that the thermal fluctuation energy is increased and the core−shell interface strain pinning effect is reduced at the elevated temperature. In this case, the shell separation effect will reduce the interparticle dipole coupling effect, leading to a reduced coercivity with an increase of Au shell thickness.28,53 As observed in Figure 3 (data in Table 1), the blocking temperatures (the temperature from ferromagnetic to paramagnetic transition) for the pure Co NPs, the Co@Au NPs with 0.3 nm Au layer, and the Co@Au NPs with 1.2 nm Au layer, are 310 K, 300 K, and 300 K, respectively. The Tb for the Co@Au NPs with 1.9 nm Au shells and mixed with some pure Au NPs is decreased to 280 K, giving these NPs still paramagnetic properties at 300 K (Hc = 0 Oe, Figure 3-iv). According to the results for the Tb of ∼10 nm Co NPs obtained by Kristhnan et al., the size of Co NPs with the Tb in 280−310 K is about 10.8−11.2 nm estimated by the equation of 25kbTb = KV for single domain magnetic NP assuming a constant K for our Co NPs.27,54 Here kb, K, and V are the Boltzmann’s constant, the magnetic anisotropy, and the volume of the particle.54 There are several possible reasons for the enhanced Tb's. One is the high K (as evidenced by the increased Hk,M) caused by the Co−Au interface strain. Another is the increased particle size during the first and second reduction-deposition process (Figure 1 iii-c: the averaged Co core size of 4.9 nm; Figure 1 iv-c: the averaged Co core size of 5.4 nm). The third reason may be the interparticle interaction. The coupling constant for dipolar interaction can be defined as the ratio of the dipolar and anisotropy energies by the equation: αd = (π/ 12)(Ms2/K)(D/d)3, where D and d are the average diameter and the interparticle distance and Ms is the saturation of the core NPs.55 Since the interparticle distance in the samples for the magnetic measurement (the NPs sealed in a capsule almost in contact with each other) is far less than that of well-separated NPs. In these cases, the αd would be enhanced significantly, leading to enhanced effective K, and thus a higher Tb than the

Hc, Oe

based on Co wt%

10 K

300 K

Hk,M, Oe at 300 K

Tb K

108.3 112.9 155.4 144.0 152.7

600 750 960 3450 580

80 45 42 0 0

610 625 750 935 640

310 300 300 280 255

Figure 3. Field cooling and no field cooling of (i) the Co nanoparticles; (ii) the Co@Au nanoparticles formed via the displacement process; (iii) the Co@Au nanoparticles formed via the displacement process and the first gold salt reduction−deposition process; (iv) the Co@Au nanoparticles formed via the displacement process and the second gold salt reduction−deposition process.

well-separated NPs.53 In particular, the seed Co particles have been washed intensively by using the mixture of THF and ethanol for the shell formation. This process will not only lead to less surfactant on the surface of Co NPs and some inevitable agglomeration of NPs (see Figure i-a) but also possibly form a very thin oxide layer. All of them favor to increase the Tb.28,37,56,57 In order to reveal the interparticle interaction, the Co NPs THF solution was mixed with some waxes 10−20 times of the weight of Co NPs and dried under a vacuum to obtain wellseparated Co NPs for the magnetic measurement. These diluted Co NPs gave a Tb of only about 200 K, much less than the close-packed Co NPs directly obtained by precipitation from THF solution (310 K). The well-separated Co@Au NPs with the shell thickness of 1.2 nm were also obtained by dissolving wax 10−20 times of the Co NPs weight. As shown in Figure 4A, the diluted Co@Au NPs in wax gave a Tb of 255 K lower than those close contacting species (Figure 3-iii). Their Tb is still higher than the reported Tb for the well-isolated 5−6 nm Co NPs. On the basis of the above experiment, the increased Tb can be reasonably attributed to the Co−Au interfacial pinning effects and some agglomeration of NPs.24 Figure 4B is the hysteresis loops for the Co@Au NPs with 1.2 nm Au shells diluted in wax, whose magnetic properties (Ms, 11347

dx.doi.org/10.1021/jp300118z | J. Phys. Chem. C 2012, 116, 11343−11350

The Journal of Physical Chemistry C

Article

Figure 5. UV−vis spectra of (i) the Co nanoparticles; (ii) the Co@Au nanoparticles formed via the displacement process; (iii) the Co@Au nanoparticles formed via the displacement process and the first gold salt reduction process; (iv) the Co@Au nanoparticles formed via the displacement process and the second gold salt reduction process; (v) the pure Au nanoparticles synthesized at the same conditions without using Co nanoparticles as core.

peak appears at 560 nm due to the strong absorbance from the Au shells in these Co@Au NPs (Figure 5iii). The peak shows red-shift and broader (full-width at half-maximum: fwhm = 150) than the pure Au NPs (peak at 536 nm, fwhm = 75 nm). The red-shift may be caused by the different dielectric constant between the metallic Co and Au, which has been observed in other core−shell NPs with Au shells (e.g., Fe3O4@Au).58 The peak line broadening can be attributed to a combination of effects from the phase retardation, the size distribution of both cores and shells, and the electron scattering at the core−shell interface.8,59 When the Co@Au NPs are further coated to have a thicker shell about 1.9 nm and mixed with some pure Au NPs, the SPR absorbance peak red-shifts to 570 nm and becomes much broader (fwhm = 247 nm, Figure 5iv). If these absorbance spectra are normalized at the same baseline and placed together (Figure 6), it is clear that the Co@Au NPs show much enhanced absorbance at the optical range than both

Figure 4. Hysteresis loops (A) at 10 K and 300 K, and their field cooling and zero field cooling curves (B) of the diluted Co@Au nanoparticles in the wax formed via the displacement process and the first gold salt reduction−deposition process.

Hc, Hk,M) are summarized in Table 1. Recalling these data for the close packed Co@Au NPs with the 1.2 nm Au shell, the interparticle interaction has little effect on the Ms but can reduce their Hc and magnetic anisotropy (as evidenced by the reduced anisotropy field Hk,M). In addition, the field cooling (FC)−zero field cooling (ZFC) curves for these NPs do not show relative narrow peaks as those well-dispersed Co NPs or core shell NPs with narrow or uniform size distribution,24,53 which can be attributed to their broad size distribution in cores and the whole NPs (Figure 1: c), the Co−Au interfacial pinning effect and some agglomeration among NPs.23,28,57 As the shell thickness increases, effects from the reduced interparticle interactions on the Tb can be compensated for by the slightly increased Co core sizes, leading to the slightly reduced T b with the shell thickness increase.27,28,54,55 Au shells will endow the Co@Au NPs with surface plasmon resonance (SPR) properties. Because of their unique electronic/dielectric properties, the metallic Co cores will affect the SPR of the Co@Au NPs to make them different from the corresponding pure Au NPs. UV−vis spectra show a slightly decreased intensity for pure Co NPs (Figure 5i) and a distinct peak at 536 nm for the pure Au NPs (Figure 5v) in the recorded wavelength range. When the Co NPs are coated with very thin Au layers (0.3 nm), the NPs give a weak and broad peak starting from 510 nm (Figure 5ii). With the coating becoming uniform and the thickness increase, a pronounced

Figure 6. The surface Plasmon resonance absorbance for (i) pure Co nanoparticles; (ii) Co@Au nanoparticles with 0.3 nm Au shell; (iii) Co@Au nanoparticles with 1.2 nm Au shell; (iv) Co@Au nanoparticles with 1.9 nm shells and mixed with some pure Au nanoparticles; (v) the pure Au nanoparticles. 11348

dx.doi.org/10.1021/jp300118z | J. Phys. Chem. C 2012, 116, 11343−11350

The Journal of Physical Chemistry C

Article

Fundamental Research Funds for the Central Universities (YWF-11-03-Q-002), and some technique suggestions and English proofreading by Dr. Nair Selvakumar in University of Toronto.

bare Co NPs and bare Au NPs when the coating thickness is more than 1.2 nm. In addition, the line shape variations in their UV−vis spectra become intensive. Comparing their line shape variations with the bare Co NPs, we can see that the increased variation in these spectra is possibly caused by the magnetic Co cores and the electromagnetic interaction between the shells and the cores initiated by the electromagnetic field. The enhanced absorbance and the wavelength broadening effects caused by the coating of a noble metal shell on the magnetic cores are very beneficial to enhance the efficiency of the wideband photovoltaic solar cell and high sensitive multifunctional sensing systems.6,8,12,37,41

■

(1) Levin, C. S.; Hofmann, C.; Ali, T. A.; Kelly, A. T.; Morosan, E.; Nordlander, P.; Whitmire, K. H.; Halas, N. J. Magnetic-plasmonic core-shell nanoparticles. ACS Nano 2009, 3 (6), 1379−1388. (2) Garcia-Torres, J.; Vallés, E.; Gómez, E. Synthesis and characterization of Co@Ag core-shell nanoparticles. J. Nanopart. Res. 2010, 12, 2189−2199. (3) Smith, A. M.; Nie, S. Semiconductor nanocrystals: structure, properties and band gap engineering. Acc. Chem. Res. 2010, 43 (2), 190−200. (4) Zhang, J.; Tang, Y.; Lee, K.; Ouyang, M. Tailoring light-matterspin interactions in colloidal hetero-nanostructures. Nature 2010, 466, 91−95. (5) Tang, Z.; Song, Y.; Sun, S.; Zhang, T.; Jiang, Y. Magnetic field driving gradient effects on the microstructure in amorphous-nanocrystalline cobalt alloy ribbons. Nanoscale 2012, 4, 386−393. (6) Jin, Y.; Jia, C.; Huang, S.-w.; O’Donnell, M.; Gao, X. Multifunctional nanoparticles as coupled contrast agents. Nat. Commun. 2010, 1 (41), 1−8. (7) Cozzoli, P. D.; Pellegrino, T.; Manna, L. Synthesis, properties and perspectives of hybrid nanocrystal structures. Chem. Soc. Rev. 2006, 35, 1195−1208. (8) Song, Y.; Zhang, Z.; Elsayled-Ali, H. E.; Wang, H.; Zou, S.; Wang, Q.; Henry, L. L.; Zhang, T. Identification of single nanoparticles. Nanoscale 2011, 3, 31−44. (9) Schotter, J.; Bethge, O.; Maier, T.; Brueckl, H. Recognition of biomolecular interactions by plasmon resonance shifts in single- and multicomponent magnetic nanoparticles. Appl. Phys. Lett. 2008, 93, 144105. (10) Montero, M. I.; Liu, K.; Stoll, O. M.; Hoffmann, A.; Åkerman, J. ́ J. I.; Vicent, J. L.; Baker, S. M.; Russell, T. P.; Leighton, C.; J.; Martın, Nogúes, J.; Schuller, I. K. Nanostructures and the proximity effect. J. Phys. D: Appl. Phys 2002, 35, 2398−2402. (11) Skumryev, V.; Stoyanov, S.; Zhang, Y.; Hadjipanayis; Givord, G. D.; Nogues, J. Beating the superparamagnetic limit with exchange bias. Nature 2003, 423, 850−855. (12) Atwater, H. A.; Polman, A. Plasmonics for improved photovoltaic devices. Nat. Mater. 2010, 9, 205−213. (13) Thimsen, E.; Formal, F. L.; Gratzel, M.; Warren, S. C. Influence of plasmonic Au nanoparticles on the photoactivit of Fe2O3 electrodes for water splitting. Nano Lett. 2011, 11, 35−43. (14) Oulton, R. F.; Sorger, V. J.; Zentgraf, T.; MA, R.-M.; Gladden, C.; Dai, L.; Bartal, G.; Zhang, X. Plasmon lasers at deep subwavelength scale. Nature 2009, 461, 629−632. (15) Soukoulis, C. M.; Wegener, M. Optical metamaterials-more bulky and less lossy. Science 2010, 330, 1633−1634. (16) Atwater, H. A. The promise of plasmonics. Sci. Am. 2007, 56− 63. (17) Ko, K. D.; Kumar, A.; Fung, K. H.; Ambekar, R.; Liu, G. L.; Fang, N. X.; Toussaint, K. C. J. Nonlinear optical response from arrays of Au Bowtie nanoantennas. Nano Lett. 2011, 11, 61−65. (18) Ozbay, E. Plasmonics: Merging photonics and electronics at nanoscale dimensions. Science 2006, 311, 189−193. (19) Kim, H.; Achermann, M.; Balet, L. P.; Hollingsworth, J. A.; Klimov, V. I. Synthesis and characterization of Co/CdSe core/shell nanocomposites: Bifunctional magnetic-optical nanocrystals. J. Am. Chem. Soc. 2005, 127 (2), 544−546. (20) Bai, J.; Wang, J.-P. High-magnetic-moment core-shell-type FeCo-Au/Ag nanopaticles. Appl. Phys. Lett. 2005, 87, 152502. (21) Song, Y. J.; Guo, Z.; O’Brian, H. M.; Hormes, J. F.; Kumar, C. Development of Magnetic Nanoparticles with Improved Properties; The J. Bennett Johnston, Sr. Center for Advanced Microstructures and Devices: Baton Rouge, LA, 2004; pp 209−210.

4. CONCLUSIONS The shell thickness driven evolution of the magnetic properties and surface plasmon resonance of Co@Au core shell nanoparticles were investigated. To the magnetic property evolution, the coercivity measured at 10 K in Co@Au NPs increases first slightly with increase in shell thickness and then dramatically when the shell thickness is increased up to 1.9 nm. This phenomenon can be elucidated by the enhanced Co−Au interfacial pinning effects with the shell evolution from the partial coverage to full coverage and the potential atom rearrangement in the interface. The coercivities of Co@Au NPs at 300 K decrease with the Au shell thickness increase due to the reduced interparticle interaction. The different coercivity change with the shell thickness also suggests a temperaturedependent interaction between the thermal fluctuation energy and the core−shell interface strain pinning effect in Co@Au NPs. The FC−ZFC curves for these NPs do not show distinct narrow peaks, which can be attributed to the combined effects from their broad size distribution in cores and the whole NPs, the Co−Au interfacial pinning effect, and some agglomeration among NPs. The significantly enhanced Tb's for these Co@Au NPs result from the increased core sizes and effective magnetic anisotropy, and the increased interparticle interaction by comparing with those of well-dispersed species. The fact that the blocking temperatures of these Co@Au NPs show a slight decrease with the formation of the full coverage and thick Au shell indicates that the thick Au shell can indeed reduce the interparticle interaction. Comparing to the pure Au NPs, the SPR peak intensity of the core−shell structures can be significantly enhanced with a line width broadening and an increased line shape variation with the shell thickness increase due to the core−shell electronic and magnetic interaction when irradiated by electromagnetic fields. We envision that this kind of core shell NPs with flexibly tunable magnetic and optical properties will provide highly integrated electronic, magnetic, optical, and structural properties uniquely different from monocomponent/structure nanomaterials. They have great potential for applications in multimode sensing technology high efficiency solar cell.

■

REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors appreciate the support from National Science Foundation of China (NSFC, Grant No. 50971010), the 11349

dx.doi.org/10.1021/jp300118z | J. Phys. Chem. C 2012, 116, 11343−11350

The Journal of Physical Chemistry C

Article

(22) Lee, W.-r.; Kim, M. G.; Choi, J.-r.; Park, J.-I.; Ko, S. J.; Oh, S. J.; Cheon, J. Redox-transmetalation process as a generalized synthetic strategy for core-shell magnetic nanoparticles. J. Am. Chem. Soc. 2005, 127, 16090−16097. (23) Wen, T.; Krishnan, K. M. Magnetic properties of Aucore-Coshell nanoparticles. J. Appl. Phys. 2011, 109, 07B515. (24) Bao, Y.; Calderon, H.; Krishnan, K. M. Synthesis and characterization of magnetic-optical Co-Au core-shell nanoparticles. J . Phys. Chem. C 2007, 111, 1941−1944. (25) Wang, L.; Kang, K.; Clavero, C.; Nelson, A. J.; Carroll, K. J.; Carpenter, E. E.; Lukaszew, R. A. Localized surface plasmon resonance enhanced magneto-optical activity in core-shell Fe-Ag nanoparticles. J. Appl. Phys. 2010, 107, 09B303. (26) Carroll, K. J.; Hudgins, D. M.; Spurgeon, S.; Kemner, K. M.; Mishra, B.; Boyanov, M. I.; Brown, L. W., III; Taheri, M. L.; Carpenter, E. E. One-pot aqueous synthesis of Fe and Ag core/shell nanoparticles. Chem. Mater. 2010, 22, 6291−6296. (27) Bao, Y.; Krishnan, K. M. Preparation of functionalized and gold coated cobalt nanocrystals for bio medical applications. J. Mag. Mag. Mater. 2005, 293, 15−19. (28) Cho, S.-J.; Idrobo, J.-C.; Olamit, J.; Liu, K.; Browning, N. D.; Kauzlarich, S. M. Growth mechanisms and oxidation resistance of gold-coated iron nanoparticles. Chem. Mater. 2005, 17, 3181−3186. (29) Zeng, H.; Sun, S. Syntheses, properties, and potential applications of multicomponent magnetic nanoparticles. Adv. Funct. Mater. 2007, 18 (3), 391−400. (30) Jun, Y.-w.; Choi, J.-s.; Cheon, J. Heterostructured magnetic nanoparticles: their versatility and high performance capabilities. Chem. Commun. 2007, 1203−1214. (31) Song, Y.; Nallathamby, P. D.; Huang1, T.; Elsayed-Ali, H. E.; Xu, X.-H. N. Correlation and characterization of 3D morphological dependent localized surface plasmon resonance spectra of single silver nanoparticles using dark-field optical microscopy and AFM. J. Phys. Chem. C 2009, 114, 74−81. (32) Chen, C.; Bobisch, C. A.; Ho, W. Visualization of Fermi’s Golden Rule through imaging of light emission from atomic silver chains. Science 2009, 325 (5943), 981−985. (33) Wen, T.; Krishnan, K. M. Thermal stability and morphological transformations of Aucore-Coshell nanocrucibles. J. Phys. Chem. C 2010, 114, 14838−14842. (34) Song, Y.; Wang, Y.; Ding, J. Shell-driven fine structure transition in Co@Au core-shell nanoparticles. 2012, unpublished manuscript. (35) Bonnemann, H.; Braun, G.; Brijoux, W.; Brinkmann, R.; Schulze, A. T.; Seevogel, K.; Siepen, K. Nanoscale colloidal metals and alloys stabilized by solvents and surfactants: Preparation and use as catalyst precursors. J. Organomet. Chem. 1996, 520, 143−162. (36) Song, Y.; Kumar, C. S. S. R.; Hormes, J. Fabrication of SU-8 based microfluidic reactor on a PEEK substrate sealed by a “flexible semi-solid transfer” (FST) process. J. Micromech. Microeng. 2004, 14, 932−940. (37) Song, Y.; Modrow, H.; Henry, L. L.; Saw, C. K.; Doomes, E. E.; Palshin, V.; Hormes, J.; Kumar, C. S. S. R. Microfluidic synthesis of cobalt nanoparticles. Chem. Mater. 2006, 18 (12), 2817−2827. (38) Song, Y.; Henry, L. L.; Yang, W. T. Stable cobalt amorphous nanoparticles formed by an in-situ rapid cooling microfluidic process. Langmuir 2009, 25 (17), 10209−10217. (39) Song, Y.; Domes, E. E.; Prindle, J.; Tittsworth, R.; Hormes, J.; Kumar, C. S. S. R. Investigations into sulfobetaine-stabilized Cu nanoparticle formation: toward development of a microfluidic synthesis. J. Phys. Chem. B 2005, 109 (19), 9330−9338. (40) Song, Y.; Zhang, T.; Yang, W. T.; Albin, S.; Henry, L. L. Fine crystal structure transition of cobalt nanoparticles formed in a microfluidic reactor. Cryst. Growth Des. 2008, 8 (10), 3766−3772. (41) Song, Y.; Jin, P.; Zhang, T. Microfluidic synthesis of Fe nanoparticles. Mater. Lett. 2010, 64, 1789−1792. (42) Song, Y.; Henry, L. L. Nearly monodispersion CoSm nanoparticles synthesized by a microfluidic reactor. Nanoscale Res. Lett. 2009, 4, 1130−1134.

(43) Song, Y.; Sun, S.; Zhang, T.; Jin, P.; Han, L. Synthesis of worm and chain-like nanoparticles by a microfluidic reactor process. J. Nanopart. Res. 2010, 12, 2689−2697. (44) Song, Y.; Kumar, C. S. S. R.; Hormes, J. Synthesis of palladium nanoparticles using a continuous flow polymeric micro reactor. J. Nanosci. Nanotech. 2004, 4 (7), 788−793. (45) Mandal, S.; Krishnan, K. M. Co core Au shell nanoparticles: evolution of magnetic properties in the displacement reaction. J. Mater. Chem. 2007, 17, 372−376. (46) Xu, Y.; Wang, J.-P. Magntic properties of heterostructureed CoAu nanoparticles direct-synthesized from gas phase. IEEE Trans. Magn. 2007, 43 (6), 3109−3111. (47) Song, Q.; Zhang, Z. J. Shape control and associated magnetic properties of spinel cobalt ferrite nanocrystals. J. Am. Chem. Soc. 2004, 126, 6164−6168. (48) Eshelby, J. D. The determination of the elastic field of an ellipsoidal inclusion, and related problems. Proc. R. Soc. London, Ser. A 1957, 241 (1226), 376−396. (49) Eshelby, J. D. The elastic field outside an ellipsoidal inclusion. Proc. R. Soc. London, Ser., A 1959, 252 (1271), 561−569. (50) Liu, B. H.; Ding, J. Strain-induced high coercivity in CoFe2O4 powders. App. Phys. Lett. 2006, 88, 042506. (51) Wang, Y. C.; Ding, J.; Yi, J. B.; Liu, B. H.; Yu, T.; Shen, Z. X. High-coercivity Co-ferrite thin films on (100)-SiO2 substrate. Appl. Phys. Lett. 2004, 84 (14), 2596−2598. (52) Zhukov, A.; Blanco, J. M.; Gonzalez, J.; Garcia Prieto, M. J.; Pina, E.; Vazquez, M. Induced magnetic anisotropy in Co-Mn-Si-B amorphous microwires. J. Appl. Phys. 2000, 87 (3), 1402−09. (53) Petit, C.; Wang, Z. L.; Pileni, M. P. Seven-nanometer hexagonal close packed cobalt nanocrystals for high-temperature magnetic applications through a novel annealing process. J . Phys. Chem. B 2005, 109, 15309−15316. (54) Bean, C. P.; Livingston, J. D. Superparamagnetism. J. Appl. Phys. 1959, 30, 120S−129S. (55) Russier, V.; Petit, C.; Legrand, J.; Pileni, M. P. Collective magnetic properties of cobalt nanocrystals self-assembled in a hexagonal network: Theoretical model supported by experiments. Phys. Rev. B 2000, 62, 3910−3916. (56) Spasova, M.; Farle, M., Low-Dimensional Systems: Theory, Preparation, and Some Application; Kluwer Academic Publishers: Dordecht, The Netherlands, 2003; pp 173−192. (57) Sobal, N. S.; Ursula, E.; Mohwald, H.; Giersig, M. Synthesis of core-shell PtCo nanocrystals. J. Phys. Chem. B 2003, 107, 7351−7354. (58) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Berlin, 1995. (59) Westcott, S. L.; Jackson, J. B.; Radloff, C.; Halas, N. J. Relative contributions to the plasmon line shape of metal nanoshells. Phys. Rev. B 2002, 66, 155431.

11350

dx.doi.org/10.1021/jp300118z | J. Phys. Chem. C 2012, 116, 11343−11350