Farid
Kameche
ab,
Anh-Tu
Ngo
ab,
Caroline
Salzemann
ab,
Marco
Cordeiro
c,
Eli
Sutter
c and
Christophe
Petit
*ab
aSorbonne Universités, UPMC Univ Paris 06, UMR 8233, MONARIS, 4 place Jussieu, F-75005, Paris, France. E-mail: christophe.petit@upmc.fr
bCNRS, UMR 8233, MONARIS, 4 place Jussieu, F-75005, Paris, France
cCenter for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973, USA
First published on 17th March 2015
CoxPt100−x nanoalloys have been synthesized by two different chemical processes either at high or at low temperature. Their physical properties and the order/disorder phase transition induced by annealing have been investigated depending on the route of synthesis. It is demonstrated that the chemical synthesis at high temperature allows stabilization of the fcc structure of the native nanoalloys while the soft chemical approach yields mainly poly or non crystalline structure. As a result the approach of the order/disorder phase transition is strongly modified as observed by high-resolution transmission electron microscopy (HR-TEM) studies performed during in situ annealing of the different nanoalloys. The control of the nanocrystallinity leads to significant decrease in the chemical ordering temperature as the ordered structure is observed at temperatures as low as 420 °C. This in turn preserves the individual nanocrystals and prevents their coalescence usually observed during the annealing necessary for the transition to an ordered phase.
In a sustainable approach, wet chemistry is well adapted to produce such nanoalloys in large amounts.4 However at the nanometer scale, as the properties are strongly dependent on the size and the surface state (raw or passivated), it is crucial to develop methods where the crystallinity, the polydispersity in size and composition are finely controlled. Furthermore, despite the large amount of work made on the synthesis of nanoalloys by the chemical way,1,2 there are still open questions considering the control of composition and especially the segregation process. For example, in chemical approaches, bimetallic nanoparticles are always passivated by an organic molecule and mainly obtained in a disordered A1 phase where both metals are randomly dispersed in the crystalline lattice (Fig. 1A).
Among the nanoalloys, magnetic nanocrystals (NCs) are promising materials due to their strong potential in the development of applications especially for high-density data storage.5–8 In this last domain, bimetallic alloys such as CoPt, FePt or CoRh represent a particularly interesting class of materials for the improvement of the recording on magnetic storage systems. Indeed, alloys such as CoPt (or FePt) have an ordered crystalline phase (L10) around the equi-atomic composition, which is intrinsic to the tetragonal symmetry (fct) of the crystal structure (Fig. 1C) and an ordered fcc structure (L12) for compositions around Co3Pt or CoPt3 (Fig. 1B).9,10 The ordered L10 phase of the CoPt system is of particular interest thanks to its high coercivity (10 kOe) and the high magnetocrystalline anisotropy (4.9 × 106 J m−3). In fact, CoPt NCs in the L10 phase have large uniaxial magnetic anisotropy energy and thus have the potential to exceed the superparamagnetic limit.11,12 Moreover, these CoPt magnetic NCs are monodomain magnetic particles for a typical size below 10 nm.13 Beyond this typical size, the NCs become polydomain magnetic and this leads to domain wall formation.14 In the case of CoPt3, the L12 phase presents also a very high magnetocrystalline anisotropy (2 × 106 J m−3). Therefore, high control of both the chemical composition and size of the NCs alloy is essential for optimizing the magnetic nanoscale behavior.
Using the colloidal route, the CoPt's synthesis leads to the formation of NCs with the A1 disordered structure. A thermal annealing is required to induce ordering toward L10 or L12 structure, depending on the composition.15 For supported nanoparticles, the thermal assistances at high temperature (>300 °C) can lead to coalescence and sintering effects, which increase the size, modify the shape and destroy their organization.15,16 Indeed, the driving force for coalescence and the change of shape is the minimization of surface energy by elimination of interfaces and appearance of grain boundaries and defects.15 Thus, the elaboration of well-defined nanoparticles in the L10 or L12 ordered phase remains very difficult. We report here a new approach for the synthesis of CoxPt100−x nanoalloys allowing control of the nanocrystallinity and as a consequence of the order/disorder phase transition of the bimetallic CoPt NCs. The enhanced crystallinity allows decrease the transition temperature and so to minimize the coalescence effect usually reported during the annealing process. This is demonstrated by comparing the evolution of same nanoalloys obtained by wet chemistry either at high or at low temperature.
In a typical synthesis procedure of CoPt nanoalloys, platinum acetylacetonate (1.25 × 10−1 mmol), cobalt oleate (2.5 × 10−1 mmol) and 1, 2 hexadecanediol (5.6 × 10−1 mmol) are dissolved in a mixture of 10 ml of 1-octadecene, oleic acid (1.88 × 10−1 mmol) and oleylamine (1.88 × 10−1 mmol) in a three-necked round bottom flask, with magnetic stirring under nitrogen at room temperature for 1 h. In the reported polyol synthesis of CoPt, dioctyl-ether is often used as solvent but the obtained nanoparticles present crystalline defects.21 In order to control the nanocrystallinity, we use another solvent with a higher boiling point: 1-octadecene (b.p. 317 °C). Hence the mixture is heated to the boiling point of the solvent and is refluxed for 30 min and then cooled to room temperature, giving a black dispersion, indicating the formation of the nanoparticles. The NCs are isolated by centrifuging and washed with a large excess mixture containing hexane (4%) ethanol (43%) and acetone (43%). The NCs, capped by oleic acid, can be dispersed in organic solvents such as chloroform, hexane or toluene.
However, only few reports on the synthesis of CoPt nanoalloys by the polyol route exist, due to the difficulty to synthesize stable cobalt precursor derivate from acetylacetonate. Thus, the synthesis described here presents an original alternative using cobalt oleate and platinum acetylacetonate as precursor. Fig. 2 shows CoxPt100−x nanoalloys obtained by the polyol process with two different compositions as determined by the EDS analysis (Fig. 2G and H). They can be obtained easily by tuning the initial salt composition: Co32Pt68 nanoalloys, in average composition are obtained by mixing 1.04 × 10−1 mmol cobalt oleate to 1.25 × 10−1 mmol platinum acetylacetonate (i.e., molar ratio 1.04–1.25) (Fig. 2A, C, E and G), while Co52Pt48 is obtained when the molar ratio is 2.5–1.25 (Fig. 2B, D, F and H). In both case, TEM images show well-dispersed spherical NCs (Fig. 2A and B). It can be seen that the NCs are homogeneous in size, shape and electronic contrast indicating the absence of segregation or core–shell structure.16 A slight decrease in the average size while preserving the same polydispersity is observed for different composition of the CoPt, i.e. 2.9 nm with a polydispersity of 13% for Co32Pt68 and 2.5 nm with a polydispersity of 13% for Co52Pt48. Furthermore, HRTEM observations reveal crystalline nanoparticles as evidenced by clearly resolved lattice fringes (Fig. 2C and D). The selected area electron diffraction patterns (Fig. 2E and F) are similar to platinum one,26 which indicates the formation of the face-centered-cubic (fcc) CoPt structure, which is on a chemically disordered phase A1 (Fig. 1A) independent of the composition. Diffraction patterns appear more diffuse as the cobalt composition increases, which is consistent with an increase of the chemical disorder. This is characteristic of an alloy by substitution where platinum and cobalt atoms are randomly distributed on the platinum lattice: the higher the cobalt content, the less structural order.19 As demonstrated previously, the combination of the SEM-EDS analysis (same composition for all the point of analysis on different film made with the same NCs) and the results obtained by TEM (homogeneous contrast and no segregation) and HR-TEM (no core–shell structure) confirms the formation of nanoalloys with the specified composition.27 The second method used to form nanoalloys involves transfer of the metal ion from a polar phase to a non-polar phase using a transferring agent. Such an approach has been developed for synthesis of metallic nanoparticles by Brust et al. in the 90's.28 This liquid–liquid phase transfer (LLPT) method, also called two-phase synthesis, has been largely used to synthesize metallic nanoparticles as silver, gold, platinum or palladium. It typically involves the transfer of the metal precursor (metallic ions) from an aqueous solution to an organic solution containing a capping molecule as alkanethiol or amine. The transfer is assisted by a phase transfer agent such as tetrakis(decyl)ammonium bromide (TDAB). Reduction of metallic precursor is then carried out by adding an aqueous solution of reducing agent (mainly NaBH4) under vigorous stirring. We have reported previously the synthesis of CoxPt100−x nanoalloys by using the LLPT method.19 In order to compare with the NCs prepared by the polyol process, nanoalloys with an average composition of the Co34Pt66 and an average size of 2.2 nm have been synthesized by the LLPT method (Fig. 3). TEM and HRTEM images confirm the formation of nanoalloys and the electron diffraction patterns are indexed to the face-centred cubic (fcc) phase (A1 phase, Fig. 1A).
Fig. 2 TEM images (A and B), HRTEM images (C and D), electronic diffraction (E and F) and EDX analysis (G and H) of (Co30Pt70)Polyol (left images) and (Co50Pt50)Polyol (right images). |
Average diameter | Polydispersity | T B | H C | M R-Normalized | |
---|---|---|---|---|---|
(Co30Pt70)Polyol | 2.9 nm | 11% | 15 K | 4000 Oe | 0.45 |
(Co30Pt70)LLPT | 2.2 nm | 13% | 15 K | 5000 Oe | 0.30 |
These values take into account the volume anisotropy but also the shape and surface anisotropies. Thus the magnetic anisotropy is estimated to Keff = 5.7 × 104 J m−3 for 2.9 nm CoPt obtained by the polyol route and 13 × 104 J m−3 for the 2.2 nm CoPt obtained by the LLPT method. These values are very low compare to that of bulk CoPt3 in the L12 phase (Ka = 2 × 106 J m−3) however, there is a significant difference between the two values obtained for the two sample both in the A1 phase. It is mainly due to the surface effect contribution. This behavior is extremely important around 2 nm, where 70% of atoms are located at the surface. In fact the magnetic energy is due both to Ks, surface anisotropy and to Kv the volume anisotropy. As the size decreases the surface contribution increases. The high influence of surface atoms, which have coordination weaker than the atoms of the core, induces an important modification of the magnetic anisotropy energy.31 It should be noticed that the shape of the ZFC curve depends on the nature of the CoPt nanoalloys. For those synthesized by the LLPT method, CoPtLLPT, the width is significantly larger compare to those obtained by the polyol process, CoPtPolyol. This reflects a larger distribution of the magnetic anisotropy energy (MAE). As the size distributions are similar (Table 1), this effect arises from another source. Let us consider the crystalline structure of the native Co30Pt70 nanoalloys depending on the synthetic route. Table 2 summarizes the distribution of the crystalline structure for both samples as deduced from HRTEM studies (see Fig. 2 and 3). It is clear that the crystalline distribution is larger in the case of CoPtLLPT than for CoPtPolyol.
Monocrystals (%) | Polycrystals (%) | Undetermined (%) | |
---|---|---|---|
(Co30Pt70)Polyol (236 particles) | 33 | 31 | 36 |
(Co30Pt70)LLPT (525 particles) | 21 | 23 | 56 |
This cannot be explained by the size difference as the shape equilibrium calculation does not predict a drastic difference of stability between crystalline fcc structure and the non crystalline one as decahedron in this range of size.32 However, the polyol process occurs at a higher temperature than the liquid–liquid phase transfer method, which is known to favor a better crystallinity. As a matter of fact, it should be noticed that the percentage of undetermined structure and polycrystals is very high for CoPtLLPT, compare to that observed obtained by the polyol method (see Table 2). These amorphous or quasi-amorphous materials have a different magnetic anisotropy compared to the crystalline one, which could explain the larger distribution of MAE observed Fig. 4A. This is confirmed by the hysteresis loop measured in the ferromagnetic regime at 3 K (Fig. 4B and Table 1). The coercivity at 3 K is higher for CoPtLLPT than for CoPtPolyol, which is consistent with a higher magnetic anisotropy. Indeed, the coercivity increases with Keff.33 However, the reduced remanence, Mr/Ms, is lower for CoPtLLPT. This is surprising if we consider only the anisotropy value, but could be expected if we take into account the high level of amorphous or quasi-amorphous materials, which are softer magnets compare to the ones with crystalline structure. It should also be considered that for CoPtLLPT, some NCs are always in the superparamagnetic state even at 3 K due to the larger anisotropy distribution. This explains also the fact that magnetization at saturation is not reached even at 5 T in comparison to the CoPtPolyol.
Fig. 5 shows a typical structural evolution of the (Co30Pt70)Polyols NCs during the in situ annealing process. It can be seen that, in comparison to the native NCs, there is a slight and continuous increase of the average size with the annealing temperature (Table 3). This has been already observed during annealing process of CoPt and attributed to an Ostwald ripening to the benefit of larger nanoparticles.34 HR-TEM patterns at 300 °C show lattices fringes of the NCs, characteristics of the disordered A1 phase with a lattice spacing equal to 2.14 Å (Fig. 5A). This is confirmed by the power spectrum, where only one pair of reflection corresponding to the (111) plane is observed. The deduced lattice parameter is equal to 3.70 Å, which is consistent with an A1 fcc structure. In fact no variation of the lattice parameters is observed compared to the value obtained at room temperature (Fig. 2C). Upon rising the temperature to 420 °C, (Fig. 5B), a drastic structural evolution is observed: some NCs exhibit different fringe patterns characteristics from the ordered fcc L12 phase with a lattice parameters of 3.85 Å as deduced from power spectrum (Fig. 5B3 and Table 4). Indeed the corresponding power spectrum clearly shows characteristics reflections corresponding to the (001) plane, which confirm the formation at low temperature of ordered CoPt3 nanoalloys. It should be noticed that no sintering effect is observed and the NCs still remain isolated on the graphene support. In fact, on the particles prepared by the polyol process at high temperatures graphitic shells can be observed (Fig. 5). These shells are very thin and might be due to the capping agent transforming during annealing, which actually protects the particles against coalescence.
25 °C | 300 °C | 420 °C | 550 °C | 680 °C | |
---|---|---|---|---|---|
Mean diameter | 2.9 nm | 2.7 nm | 3.0 nm | 3.3 nm | 3.2 nm |
Polydispersity | 11% | 12% | 17% | 15% | 12% |
Temperature | 300 °C | 420 °C | 550 °C | 680 °C |
---|---|---|---|---|
Number of nanoparticles studied with observable atomic planes | 161 | 161 | 191 | 77 |
Lattice parameters (Å) | 3.70 | 3.85 | 3.86 | 3.90 |
% A1 | 100 | 83 | 59 | 43 |
% L12 | 0 | 17 | 41 | 57 |
L12/A1 | 0 | 0.20 | 0.69 | 1.33 |
Further increase of the temperature induces the ordering of more and more NCs (Fig. 6 and Table 3) as well as a continuous increase of the lattice parameters. This isotropic expansion of the lattice parameter is characteristic of the ordering of CoPt3 indicating a homogeneous order–disorder transformation.34 The bulk value is equal to 3.9 Å,35 reached in our case at 680 °C, which indicates that the ordering is total.
Table 4 shows also the statistical evolution of the ordering of the NCs. At 680 °C, 57% of the observed NCs are in the ordered L12 structure. In fact there is a linear evolution of this percentage with the annealing temperature as shown of Fig. 6 in relation with the evolution of the lattice parameters. This indicates a continuous and monotonic transition from the ordered to the disordered fcc structure of CoPt3. Indeed, this low temperature transition is surprising. Even if it has been reported, and calculated, a decrease of the transition temperature in the CoxPt1−x phase diagram in case of NCs compared to bulk materials,32,36 this value is close to 180 °C below the bulk value (around 760 °C29), for 3 nm in size NCs. Here we observe significantly larger decrease of 350 °C of the order/disorder transition.
Fig. 7 and Table 5 show the same experiment with Co30Pt70 NCs prepared by the LLPT method. The behavior of these NCs strongly differs from both structural and ordering point of view.
Temperature | 300 °C | 400 °C | 550 °C | 680 °C |
---|---|---|---|---|
Number of nanoparticles studied with observable atomic planes | 51 | 101 | 99 | 30 |
Lattices parameters (Å) | 3.91 | 3.60 | 3.84 | 3.63 |
% A1 | 100 | 97 | 79 | 87 |
% L12 | 0 | 3 | 21 | 13 |
L12/A1 | 0 | 0.03 | 0.27 | 0.15 |
Similarly to the previous case of NCs prepared by the polyol process, no ordering is observed for the LLPT prepared NCs, upon increasing the temperature to 300 °C. For the NCs prepared by the LLPT process, however, significant coalescence occurs, even if isolated NCs are still present on the graphene support film. This effect probably arises from the nature of the passivating agent on the CoPt surfaces, depending on the chemical route. Indeed, alkylamines are used in this soft synthetic method where as (CoxPt100−x)Polyol are coated by carboxylate derivatives (see Experimental methods). In case of (CoxPt100−x)LLPT, this is due to the growth process taking place at room temperature, which is totally inhibited if chemically bond agents are used as a capping agent.37 As a result, their protection is not strong enough to avoid a coalescence process. However, operating in situ on deposited NCs on the TEM grid limit the progression of this process during the annealing and allows us to study the ordering of (Co30Pt70)LLPT. From a crystalline point of view, no transition is observed at 300 °C independent of the size of the NCs. The lattice parameter also corresponds to the (111) plane of the A1 disordered fcc structure (Table 5). At 400 °C only a very small part of NCs undergo a structural evolution towards the L12 fcc ordered phase: 3% of the observed NCs when it was 17% for the (Co30Pt70)Polyols. This finding cannot be explained only by a size effect as the corresponding transition is not observed for the smallest NCs. Further increase of the annealing temperature yields an increase of the number of NCs in the L12 phase, but their proportion is always significantly lower than for the (Co30Pt70)Polyols. (Fig. 6 and Table 5). At 680 °C only 15% of the observed NCs present a L12 structure.
The established strong difference of chemical ordering depending on the chemical route of preparation of the NCs can be understood if we take into account the difference of crystallinity reported above. It has been reported in similar annealing process of CoPt nanoalloys that the transformation of A1 to L10 (or L12) is a multi-step process.34 There is first a transition from polycrystalline, or non-crystalline phase as decahedron, to a fcc truncated octahedron structure. Simultaneously, the NCs size increases and their shape becomes more and more isotropic. Further increase of the temperature induces the chemical ordering of the NCs keeping a fcc octahedron structure. Indeed it has been reported that the non-crystallinity of the NCs is a limiting factor to achieve the chemical order. Furthermore coalescence of the NCs during the annealing process often yield to high angle grain boundaries, which also prevent or slow down the chemical ordering and the homogeneous crystalline transition as numerous energetic barriers should be overcome to achieve the correct crystalline structure.15 Thus chemical ordering can only be reached at higher temperature. In our case, on one hand (CoPt)LLPT present an important part of non crystalline or amorphous structure (Table 2), on the other hand the passivating agent is not strong enough to prevent the coalescence conversely to the case of (CoPt)Polyol. As a result the chemical ordering appears at lower temperature and on larger scale for CoPt nanoalloys made by the polyol process compared to similar NCs synthesized by the LLPT process. This is probably reinforced by the fact that the polyol process is a high temperature synthetic method, which favors a better crystallinity of the NCs. Thus the first step of the order transition considered above is limited or inexistent. Moreover, this assumption is further confirmed if we consider the chemical ordering of Co50Pt50 synthesized by the polyol process (Fig. 8). Again Co50Pt50 NCs present mainly a disordered fcc A1 crystalline structure, with a truncated octahedron shape, and only few NCs with non-crystalline structure are observed. As a consequence, similar behavior as reported above for Co30Pt70, is observed during the in situ annealing in the TEM. For a 50–50 composition the ordered structure, as deduced from the phase diagram, is the tetragonal fct L10 phase (see Fig. 1). It can be obtained by annealing because chemical synthesis only produce the disordered A1 structure. The in situ TEM experiments with these NCs show slight coarsening due to the Ostwald ripening of the NCs as their size increase from 2.5 to 3.0 nm but without coalescence. Furthermore, ordering occurs also at low temperature (420 °C) and the nanoparticles retain their ordered L10 structure after cooling down to low temperature as observed on Fig. 8E and F. Our previous study on NCs synthesized by the LLPT method with same size, but passivated also by alkylamine chains showed a huge coalescence and a chemical ordering only above 500 °C.16 Again, we observe the beginning of chemical ordering at lower temperature for (CoPt)Polyol than for (CoPt)LLPT independent of the composition.
This journal is © the Owner Societies 2015 |