Magnetic, structural and chemical properties of cobalt nanoparticles

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Magnetic, structural and chemical properties of cobalt nanoparticles synthesized in ionic liquids Bishoy Morcos, Pierre Lecante, Robert Morel, Paul-Henri Haumesser, and Catherine C Santini Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00271 • Publication Date (Web): 28 May 2018 Downloaded from http://pubs.acs.org on May 28, 2018

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Langmuir

Magnetic, structural and chemical properties of cobalt nanoparticles synthesized in ionic liquids †,‡

Bishoy Morcos,

Pierre Lecante,

¶

§

Robert Morel,

Catherine C. Santini

Paul-Henri Haumesser,

∗,k

and

‡

†Univ.

Grenoble Alpes, F-38000 Grenoble, France ; CEA, LETI, MINATEC Campus, F-38054 Grenoble, France. ‡Univ. Lyon, CNRS-UMR 5265, 43 Bd du 11 Novembre 1918, F-69616, Villeurbanne Cedex, France. ¶Centre d'Elaboration de Matériaux et d'Etudes Structurales, CEMES, CNRS, 29 rue Jeanne Marvig, F-31055 Toulouse, France §Univ. Grenoble Alpes, CEA, CNRS, Grenoble INPa, INAC, SPINTEC, F-38000 Grenoble, France. kUniv. Grenoble Alpes, F-38000 Grenoble, France ; CEA, LETI, MINATEC Campus, F-38054 Grenoble France. E-mail: [email protected] Phone: +33 4 38 78 57 59. Fax: +33 4 38 78 30 34

a Institute of Engineering, Univ. Grenoble Alpes

Abstract Cobalt nanoparticles (CoNPs) exhibit quite unique magnetic, catalytic and optical properties. In this work, imidazolium-based ionic liquids (ILs) are successfully used to 1

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elaborate magnetically responsive suspensions of quite monodisperse CoNPs with diameters below

5 nm.

The as-synthesized CoNPs adopt the non-compact and metastable

structure of -Co that progressively evolves at room temperature towards the stable hcp allotrope of Co. Accordingly, magnetization curves are consistent with zero-valent Co. As expected in this size range, the CoNPs are superparamagnetic at room temperature. Their blocking temperature is found to depend on the size of the IL cation. The CoNPs produced in an IL with a large cation exhibit a very high anisotropy, attributed to an enhanced dipolar coupling of the NPs, even though a larger interparticle distance is observed in this IL. Finally, the presence of surface hydrides on the CoNPs is assessed and paves the way towards the synthesis for Co-based bimetallic NPs.

Introduction Monodisperse metallic nanoparticles (NPs) represent a very interesting area of research owing to their unusual magnetic, electrical, optical and thermal properties. These remarkable characteristics originate from quantum eects associated with a very high surface to volume ratio.

For this reason, they have attracted much interest in a range of applications that

require small size NPs (below 10 nm) with precise control of size, composition and morphology in chemically signicant quantities.

1

Among them, cobalt nanoparticles (CoNPs) exhibit

quite unique magnetic, catalytic and optical properties.

2

As catalysts, colloidal CoNPs show a great potential.

3

For example, they can be used

in the FischerTropsch process to convert CO and H2 into liquid hydrocarbons.

4

In this

particular example, CoNPs showed enhanced selectivity compared with their conventional competitors. Another indirect use of CoNPs in catalysis is as a magnetic carrier for the actual catalyst without taking part in the catalyzed reaction itself. They facilitate the recovery of the catalyst from the reaction medium upon using an external magnetic eld to collect the NPs. For example, a Mn(III) complex supported on magnetic CoNPs has been used as a retrievable heterogeneous catalyst to oxidize alcohol and sulde compounds.

2


5

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CoNPs are also used in the medical eld, particularly in diagnostic tools. ecient contrasting agents in magnetic resonance imaging for cancer diagnosis.

They are

6

They can

also be used for treating cancer cells either by thermally destroying the tumor upon excitation by an alternating magnetic eld of suitable amplitude and frequency, delivery into the tumor cells.

7

or by targeted drug

8

CoNPs can also be used in various technological applications such as data storage, spintronics

10

or magnetic uids.

11

9

Ferrouids are colloidal suspensions of very ne (about

10 nm) magnetic NPs whose viscosity can be modulated upon application of a magnetic eld. They nd applications in dynamic sealing devices, heat dissipation, dampers and many other technological uses.

12

In most of these applications, monodisperse CoNPs are needed with sizes ranging from a few to a few tens of nanometers.

As reported for other metals, metallic CoNPs can be

produced through either physical or chemical methods. Physical methods include sputtering deposition, laser evaporation and then condensation in a gas ow, in a liquid or on a solid surface. However, the limitation of such techniques is the lack of size control for particles smaller than

100 nm.

For this reason, liquid-phase syntheses are usually preferred.

1

In par-

ticular, the chemical routes, based on the reduction or decomposition of a suitable precursor of the desired metal, usually provide better size control for nanoparticles smaller than

10 nm.

Several methods can be applied to decompose Co precursors like thermal decomposition, reduction (H2 gas, hydrazine,. . . ) ultrasonic activation, photolysis and hydrolysis reactions. The reaction medium can be water,

13

an organic solvent

14,15

or an ionic liquid.

16

An important limitation for the preparations in aqueous media is the oxidation of CoNPs. For this reason, the chemical route is most widely developed in organic solvents. However, the generated small NPs are only kinetically stable and their agglomeration into the more thermodynamically stable bulk form must be prevented by the addition of organic surfactants or ligands. This allows for excellent size control, but modies the surface chemistry of CoNPs, and may also deteriorate their magnetic properties.

3


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By contrast, similar processes carried out in ionic liquids (ILs) readily produce monodisperse suspensions of metallic NPs without the addition of surfactants or ligands. are salts with very low melting point (typically below unique properties.

19

100 ◦C).

17,18

ILs

These liquids exhibit very

They are solvents that possess high electrical conductivity, they are non-

volatile, chemically and thermally stable. The latter properties are benecial for practical purposes, because they make ILs safer than most conventional solvents. Most importantly, ILs can also act as self-stabilizing media for the synthesis of naked metallic NPs with enhanced size and structure control.

17,20,21

In our Laboratory, a general and versatile synthesis

of metallic NPs with controlled size has been developed in the recent years based on the decomposition of organometallic (OM) precursors under H2 atmosphere.

2224

In ILs, besides the preparation of CoNPs by thermal decomposition of Co2 (CO)8 ,

16,21,25

the decomposition under H2 of OM cobalt precursors under mild conditions is scarcely reported. The main disadvantage of Co2 (CO)8 is the possibility to leave CO groups coordinated to the NP surface. This contamination is responsible for a decrease in the magnetization of the CoNPs.

25

Also, a dissociative adsorption of CO can occur leading to oxide species and

carbon residues on the NP surface.

26

4 3 27 In organic solvents, (η -1,5-cyclooctadiene)(η -cyclooctadienyl) Cobalt (I) Co(COD)(COE) was reduced, under H2 in mild conditions (20 to

150 ◦C, 0.4 MPa

H2 ), in the presence of dif-

ferent stabilizers aording monodispersed metallic CoNPs, with simultaneous formation of volatile hydrocarbons, mainly cyclooctane.

14

In this work, we report the successful synthesis of CoNPs by decomposition of Co(COD)(COE) under H2 in two imidazolium-based ILs, C1 C4 ImNTf2 (m.p. (m.p.

307.45 K). 28

258.15 K)

and C1 C14 ImNTf2

Both ILs only dier by the length of the alkyl chain linked to the im-

idazolium cation, which is longer in the latter IL. The inuence of this alkyl chain on the characteristics of the suspensions and the morphology of the CoNPs is examined.

To as-

sess the metallic nature of the CoNPs, their crystalline structure is studied in details by wide-angle X-ray scattering (WAXS) and high-resolution transmission electron microscopy

4


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(HRTEM). Also, their magnetic properties are investigated by performing superconducting quantum interference device (SQUID) measurements on the suspensions. Finally, their surface chemistry is characterized to assess the presence of surface hydrides.

Experimental Preparation of ILs, solutions and suspensions All operations were performed in the strict absence of oxygen and water under a puried argon atmosphere using glove box (MBraun) and vacuum-line techniques. 1-methylimidazole (>99 %) and 1 chloroalkanes were purchased from Sigma Aldrich and distilled prior to use. Distilled water, freshly distilled toluene (>99 %, Sigma Aldrich) and freshly distilled dichloromethane (>99 %, Sigma Aldrich) were used for the purication of ILs. Bis(triuoromethanesulphonyl)imide lithium salt (>99 %, Solvionic) and (1,5-cyclooctadiene)(1,3-cyclooctadienyl) Cobalt (I) Co(COD)(COE) were kept in a glove box and used as received. The synthesis of the 1-alkyl-3-methylimidazolium bis(triuoromethylsulphonyl)imide ILs, C1 Cn ImNTf2 , was performed as reported in the literature.

29

Their purity was checked by NMR spectra, recorded

on a Bruker Advance spectrometer at 300 MHz for

1

H and at

75.43 MHz

rication, the halide and water contents were found to be below mass spectrometry) and

∼12 ppm

for

100 ppm

13

C. After pu-

(high resolution

(limit of Karl Fischer titration), respectively.

The solutions were freshly prepared by dissolving Co(COD)(COE) in C1 Cn ImNTf2 to the desired concentration (5

× 10−2 mol · L−1 ) in a Schlenk tube under stirring at room tem-

perature. Suspensions of CoNPs were prepared as needed by decomposing these solutions under H2 . For this purpose,

2 mL

of the organometallic solution was canuled into an auto-

clave under argon. At this stage, the Ar atmosphere was replaced by H2 (0.3 MPa dynamic pressure). Then, the reaction medium was warmed to

10 min

100 ◦C

without stirring and kept for

to ensure thermal equilibrium was reached. Finally, the H2 pressure in the autoclave

was raised to

0.4 MPa,

the autoclave was sealed and kept at

5


100 ◦C

during typically

4 h.

At

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the end of the reaction, the resulting dark suspension was placed under vacuum to eliminate the volatile by-products and stored under argon in a glove box.

Characterizations Whenever required, the gaseous products evolved during syntheses in autoclave were quantitatively analyzed by gas chromatography (GC) on a P6890 chromatograph equipped with a ame ionization detector (FID) and an Al2 O3 /KCl column (L=50 m, thickness

5 µm).

Both the injector and detector temperatures were xed at

injection volume was surements, a

φint =0.32 mm,

20 m

1 µL.

The temperature was xed at

190 ◦C.

230 ◦C,

lm

and the

For inline GC-mass mea-

CP-PoraBOND Q column was used to separate the products. They were

then analyzed using an Agilent GC 6850 MS 5975C mass spectrometer. Thermogravimetric Analysis (TGA) was performed using a Mettler Toledo TGA DSC1. For diuse reectance infrared Fourier transform spectroscopy (DRIFT) analysis, an exact weight of the precursor (10 to

15 mg)

was rst weighed in an aluminum crucible and trans-

ferred in a Harrick high temperature cell that was tightly closed under argon inside the glove box and then xed into a Thermo Scientic Nicolet 6700 FT-IR equipment. was initially purged for of H2 at

20 ◦C.

20 min under 20 mL · min−1

and kept at this temperature for

The sample

of argon. It was then put under

5 h.

0.4 MPa

An IR spectrum was recorded every

minute to monitor the decomposition of the precursor. All the suspensions were analyzed by transmission electron microscopy (TEM) using a JEOL 2100FEF (eld eect gun energy ltering) microscope. The analysis was performed directly with the liquid samples, as ILs do not evaporate, even under high vacuum.

The

liquid lms were observed using an acceleration voltage of 200 kV. Size histograms were determined from a minimum of 500 particles per sample and tted by a lognormal law. Whenever required, high resolution transmission electron microscopy (HRTEM) analysis was performed using the same microscope. Small-Angle X-ray Scattering (SAXS) experiments were performed on the CRG-BM02

6


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beam line at the European Synchrotron Radiation Facilities (ESRF) at

21.6 keV.

pensions of CoNPs in C1 C4 ImNTf2 were contained in quartz capillaries of

1.5 mm in diameter

with wall thickness of

10 µm

lled and sealed under argon.

The sus-

The capillaries were mounted

vertically on a multiple sample holder. The scattered intensities were detected with a FOC Roper Scientic CCD camera (1340×1300 pixels, distance of

1360 mm.

50 µm

pixel size) at a sample-to-detector

The direct beam was stopped by a Rh disk of

3 mm

in diameter. The

SAXS set-up was under a primary vacuum except around the sample holder. A photomultiplier (PM) with a removable kapton foil placed just after the samples was used for both sample alignment and transmission measurements. The scattering curves were corrected for instrumental and IL contributions and tted with conventional models implemented in the pySAXS python module.

30

For wide-angle X-ray scattering (WAXS) experiments, the suspensions were poured in

1.5 mm

quartz capillaries in the glove box and sealed. The capillaries were irradiated with

graphite monochromated Mo-Kα-radiation (0.710 69 Å) in a two-axis diractometer. data collection time was tant S values.

64 h

for a set of 457 measurements in the

060◦ θ

The

range for equidis-

Because of the very strong scattering from IL, accurate correction using a

measurement of a capillary lled with the pure IL was mandatory. After correction, the signal from the NPs in C1 C4 ImNTf2 was however quite weak, but could as expected be increased upon concentrating the suspension by magnetic decantation. In the case of C1 C14 ImNTf2 , however, the NP response remained too weak to yield reliable results. Magnetic properties of the suspensions were measured using a Quantum Design MPMS SQUID magnetometer. Magnetization was measured with applied eld

µ0 H

up to

2T ,

near room temperature but below the ILs' melting points (300 K for C1 C14 ImNTf2 and for C1 C4 ImNTf2 ), then at

10 K for both suspensions.

the samples were cooled down under

2 T.

rst

250 K

For the low-temperature measurement

The blocking temperature for the superparam-

agnetic clusters was measured using the standard eld-cooledzero eld-cooled procedure with

5 mT.

The coupling between clusters at

10 K

7

was qualitatively assessed by measuring


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the remanent magnetization, as a function of the applied eld, using two dierent procedures. The rst is the isothermal remanent magnetization (MIRM ), that is measured after the application and removal of an increasing eld to a previously de-magnetized sample. As a function of the applied eld,

MIRM H

is expected to grow from zero in the fully demag-

netized state, up to a maximum when H is above the irreversibility eld. The second is the DC demagnetization curve (MDCD ) where the remanent magnetization is measured from the (negative) saturated state by application of increasing (positive) demagnetizing elds. As a function of the applied demagnetization eld,

MDCD H

is expected to grow from negative

value in the initial fully saturated state, up to a positive maximum when H is above the irreversibility eld.

31

Surface chemistry To characterize the surface chemistry of the CoNPs, their activity towards the hydrogenation of ethylene was measured. For this purpose, a freshly synthetized suspension of CoNPs in C1 C4 ImNTf2 was placed under Ar, then exposed to

11.4 kPa

of C2 H4 at room temperature.

An accurate gas manometer was used to monitor the gas pressure evolution. The pressure was stabilized after

24 h.

The gaseous products were then collected and analyzed by gas chro-

matography (GC). To further collect the partially hydrogenated species such as Co−C2 H5 (see Figure 9),

11.6 KPa

of H2 were introduced and the reactor was heated at

stable pressure was reached (after

23 h).

100 ◦C

until a

The collected gaseous species were identied and

quantied as above. A second experiment was conducted, in which a freshly synthetized suspension of CoNPs in C1 C4 ImNTf2 was placed under Ar for was kept at

15 min.

Me4 Sn was then added, and the medium

50 ◦C under stirring and static Ar pressure.

After 24h, all gaseous products were

collected and analyzed by GC.

8


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Results and discussion Thermal decomposition of Co(COD)(COE) The thermal stability of solid Co(COD)(COE) under N2 atmosphere was rst veried. For this purpose, TGA measurements were undertaken. sition deduced from the tangent method was

129 ◦C

The onset temperature of decompo(Figure S1).

started to decompose at a lower temperature with a mass loss of

1%

However, the product at

99 ◦C.

Nevertheless,

this demonstrates that Co(COD)(COE) is stable enough under non reactive atmosphere for practical purposes. By contrast, this compound readily decomposes under H2 at room temperature.

This

reaction was monitored using diuse reectance infrared Fourier transform (DRIFT) and online GC-mass analyses during

5h

at

20 ◦C

under

0.4 MPa

of H2 . The evolution of the IR

spectra with time is shown in Figure S2. Upon applying H2 , all the peaks associated with unsaturated ligands (cyclooctadiene and cyclooctadienyl) gradually disappeared. After less than

60 min

only, the characteristic peaks of cyclooctane only could be observed.

Hence,

the decomposition of the Co(I) solid precursor upon exposure to H2 occurs even at room temperature and leads, as expected, to the hydrogenation of the unsaturated ligands into saturated cyclooctane. Finally, the decomposition of the dissolved precursor in C1 C4 ImNTf2 was monitored using quantitative online GC-mass analysis. For this purpose, a

0.4 MPa

exposed to

of H2 at

100 ◦C.

5 × 10−2 mol/L solution was

As for the solid, GC-mass spectra of the gas phase

showed cyclooctane as the main decomposition product.

However, in this case, traces of

cyclooctene and pentalene were detected as well, possibly due to a series of isomerization reactions. after

8 h.

32

Based on the amount of detected cyclooctane, the decomposition was completed

After

3 h,

only

70 %

of conversion was achieved.

It should be noted that the

outgassing of cyclooctane out of the IL is probably slow, and that the actual decomposition of Co(COD)(COE) is probably faster. Nevertheless, this reaction remains signicantly slower

9


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than for solid Co(COD)(COE). Based on these observations, all subsequent syntheses were performed by decomposing solutions of Co(COD)(COE) under H2 at

100 ◦C.

Synthesis of CoNPs in ILs Under these conditions, solutions of Co(COD)(COE) in C1 C4 ImNTf2 were converted into black suspensions of CoNPs, as revealed by TEM observations (Figure 1a and b). However, unlike other metals, the CoNPs were not uniformly dispersed in the liquid, but packed into agglomerates whose size was typically several hundreds of nanometers. Nevertheless, after

30 min

reaction in C1 C4 ImNTf2 , these NPs were still isolated from each other by the liquid

(Figure 1a). Their size was measured to be

4.6 ± 1.5 nm

in diameter, even if this estimation

is rather inaccurate, due to this agglomeration and considering the limited contrast of the CoNPs with respect to the surrounding liquid (Figure S3). After

4h

reaction, which is the

duration needed to decompose most of the cobalt precursor, the CoNPs were too close to allow for accurate size estimation (Figure 1b). For this reason, this duration was not exceeded in further experiments. It should be noted that upon using C1 C14 ImNTf2 , the separation between individual CoNPs remained large even after

4h

(Figure 1c). This is conrmed by

the radial distribution of the NPs derived by adequate image analysis, that reveals a NP-NP distance of

4.2 nm(Figure

S4d).

This is probably related to the steric eect of the larger

cations in this IL as compared to C1 C4 ImNTf2 . The NP size was found to be similar as in C1 C4 ImNTf2 (4.5

± 1.3 nm,

see Figure S4c).

Nevertheless, C1 C4 ImNTf2 is more interesting for practical purposes because of its lower viscosity and melting temperature. Moreover, this suspension was responsive to a magnet, as reported for similar suspensions in conventional solvents.

vide infra ).

centrate the CoNPs for WAXS measurements (

12

This eect was used to con-

Therefore, it was important to

measure the size of the CoNPs synthesized in this IL. As TEM is not well adapted for this purpose, small angle X-ray scattering (SAXS) experiments were undertaken with a suspension of CoNPs in C1 C4 ImNTf2 . The corresponding

10


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(a)

(b)

(c)

Figure 1: TEM images of CoNPs generated by the decomposition of Co(COD)(COE) in C1 C4 ImNTf2 under

0.4 MPa

of H2 for (a)

30 min

and (b)

4 h,

and (c) in C1 C14 ImNTf2 for

4 h. scattering curve corrected for instrumental and IL contributions is printed in Figure 2. By contrast with similar suspensions of other metals,

33

this curve is signicantly atter. Such

a shape can be associated with the agglomeration of the CoNPs observed in TEM, as suggested by the attening of scattering curves of suspensions of interacting latex particles with increasing particle concentration.

34

Such a situation is usually handled by modulating the

form factor (corresponding to the shape of the NPs) with a structure factor (oscillating around 1) that describes their interaction.

35

The most simple model, which is the interac-

tion of hard spheres, proved not to be able to t the experimental curve (when both the NP and hard sphere radii were allowed to adjust, the NP radius always exceeded the hard sphere radius, which is unphysical). By contrast, the data could be tted using the sticky hard sphere model implemented in pySAXS (red curve in Figure 2, along with the separate contributions of the structure factorin green and the form factorin blue).

30

A monodis-

perse suspension was considered for this t. The hard sphere diameter adjusted to

2.64 nm,

with a stickiness conecient (τ ) of 0.36 and a volume fraction (η ) of 0.335. The diameter of the CoNPs deduced from this procedure is

2.56 nm.

This size is signicantly smaller than

measured by TEM or deduced from magnetic measurements (

vide infra ). This could be due

to the sedimentation of the largest CoNPs in the vertical capillary during measurements,

11


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enhancing the contribution of the smallest NPs.

Figure

2:

X-ray

scattering

curve

of

CoNPs

generated

by

the

4h

decomposition

Co(COD)(COE) in C1 C4 ImNTf2 under H2 and best t with a sticky hard sphere model.

of

30

An important conclusion that can be drawn from this SAXS measurement is that the size of the NPs after

4h

is not signicantly dierent from the size measured after

30 min

reaction. This indicates that the IL is ecient at inhibiting the growth (or agglomeration) of CoNPs and stabilizing small NPs. This conclusion encompasses any uncontrolled nucleation during temperature ramp up (which is unlikely, considering decomposition kinetics measured by GC-mass) or thermal equilibration (10 min, see experimental section).

Structural analysis of CoNPs The as-prepared CoNPs exhibited a clear crystalline structure under high-resolution TEM (HRTEM, Figure 3a).

As a result, fast-Fourier transform (FFT) analysis of the pictures

yielded a pattern of well dened spots in the reciprocal space (Figure 3).

This pattern

was compared to the known crystalline structures of metallic Co (non-metallic structures were discarded, as magnetic measurements unambiguously demonstrate that our CoNPs are

vide infra ).

metallic (

12


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In all cases, spots corresponding to an interplanar distance of about served.

0.35 nm

were ob-

It ts with the (001) distance in the hexagonal close-packed (hcp) structure, the

stable allotrope of Co, or with the (100) distance in the face-centered cubic (fcc) structure. Nevertheless, the other spots could not be indexed using either of these structures. Finally, this distance also corresponds to the (111) planes of a metastable cubic structure of Co usually referred to as

-Co. 36

Indeed, it is shown in Figure 3b that the pattern in the FFT

image can be completely indexed using this phase.

(a)

(b)

Figure 3: (a) Exemplary HRTEM image of CoNPs generated in C1 C4 ImNTf2 and (b) FFT of the particle in the white square indexed by a cubic structure along the (-321) zone axis with a lattice parameter of

6.1 Å

corresponding to

-Co. 36

All CoNPs analyzed by HRTEM exhibited this -Co structure. To conrm this, wide angle X-ray scattering (WAXS) measurements were undertaken. As expected, CoNPs prepared in C1 C4 ImNTf2 did not produce well dened diraction peaks (Figure 4a). The diusion curve compares well with the signal reported for CoNPs with the

-Co

structure.

37

The deduced

radial distribution function (RDF) is also in good agreement with this structure, with a rst metal-metal distance close to metal-metal distance near

0.255 nm

0.35 nm

(Figure 4b). A noticeable feature is the absence of a

1

(grey area in Figure 4b) . This is the signature for a non

close-packed structure. All these results are thus consistent with the

-Co phase observed by

HRTEM. However, it should be noted that the coherence length is well below

2 nm,

which

1 This is a distance between neighboring atoms, and it must not be confused with the (111) interplanar distance of the -Co crystal mentioned earlier.

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is signicantly smaller than the size estimated by SAXS.

(a)

(b) Figure 4: (a) Scattering curves (corrected from the IL and capillary contributions) and (b) corresponding radial distribution functions (RDF) of CoNPs as-prepared in C1 C4 ImNTf2 and after decantation over a magnet.

Interestingly enough, a drastic modication of the WAXS response was observed for CoNPs in C1 C4 ImNTf2 after a 2-day decantation over a magnet, as shown in Figure 4a. Better dened peaks appeared at positions corresponding to the diraction angles of hcp Co. The RDF is also in good agreement with this structure, with the development of a metalmetal distance near

0.35 nm

(Figure 4b).

37

Hence, it seems that the metastable

-CoNPs

have evolved during the decantation process into stable hcp CoNPs. In the same time, the coherence length has increased to about

3 nm,

in better agreement with SAXS observations.

This indicates that the CoNPs have recystallized during decantation. It should be mentioned

14


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Langmuir

that this transformation was not necessarily quantitative, and that

-CoNPs

were probably

still present. They are not clearly visible because the contribution of the better-crystallized hcp CoNPs is dominating in the WAXS response. Finally, all these results conrm that the as-formed CoNPs adopt the coherence length well below

2 nm,

-Co

phase with a

signicantly smaller than the size estimated by SAXS or

TEM. This suggests that the native small particles are at least partly agglomerated in the bigger objects observed by SAXS or in the TEM. Because of the metastable character of the structure, these objects evolve into maybe larger but certainly better crystallized NPs with the common hcp structure, a process that could be favored by exposure to a magnetic eld.

Magnetic properties of CoNPs To characterize the magnetic properties of the CoNPs in ILs, SQUID measurements were undertaken using suspensions elaborated in C1 C4 ImNTf2 and C1 C14 ImNTf2 . The two pure ILs were measured rst, prior to measuring the NP suspensions. diamagnetic with a low susceptibility, are chemically pure.

−7.5 × 10−9 m3 · kg−1 ,

They were found to be

a good indication that they

Given this low susceptibility, the IL contribution to the magnetic

measurements with NPs can be neglected. The magnetizations at

10 K

for CoNPs synthesized in C1 C4 ImNTf2 and C1 C14 ImNTf2

are plotted in Figure 5. It is rst noticed that despite the fact that the samples were cooled down under

2 T,

the curves are symetrical with respect to the applied eld.

There is no

exchange bias, which is an indication that no Co oxide shell is present around the NPs.

38

It is also observed that for these NPs the remanence is low (respectively 0.20 and 0.12) and the saturation eld is relatively high (respectively

0.5 T

and

1 T).

Both eects can be

attributed to a coupling between clusters, as will be presented below. The applied eld is sucient to achieve magnetic saturation and the calculated magnetizations are respectively

153 A · m2 · kg−1 and 175 A · m2 · kg−1 .

Given the precision in the mass of the measured NPs,

these values are very close to the bulk magnetization of cobalt (162 A

15


· m2 · kg−1 ).

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 29

(a)

(b)

Figure 5: Magnetization curves at

10 K

for CoNPs synthesized in (a) C1 C4 ImNTf2 and (b)

C1 C14 ImNTf2 .

The magnetizations at high temperature are plotted in Figure 6. The temperature was chosen such that the ILs are in their solid state (250 K for C1 C4 ImNTf2 and

300 K

for

C1 C14 ImNTf2 ). At these temperatures the Co clusters are superparamagnetic. In this regime the magnetic size of the clusters can be obtained by tting the function

L(x): M (H) = Ms L

with

Ms

M (H) curve with the Langevin

mµ0 H kB T

= Ms coth

the magnetization at saturation;

the applied eld in

−

kB T mµ0 H

(1)

m the magnetic moment of the NPs in A · m2 ; µ0 H

T; kB = 1.38 × 10−23 J/K

and

T

clusters was calculated using the obtained values for and mass magnetization,

mµ0 H kB T

the temperature in

K.

The size of the

m, assuming Co bulk values for density

8.9 g · cm−3 and 1.62 A · m2 · kg−1 , respectively.The size for the NPs

in C1 C4 ImNTf2 was found to be

4.1 nm,

in excellent agreement with TEM measurements.

The NPs in C1 C14 ImNTf2 were found to be smaller, with a diameter of smaller than observed with TEM (4.5

3.7 nm,

slightly

± 1.3 nm).

The magnetic behavior of the CoNPs was further characterized by recording eld-cooled (FC) and zero-eld-cooled (ZFC) curves (Figure 7). It can be noticed that the FC and ZFC curves for the C1 C4 ImNTf2 sample do not overlap above the blocking temperature. FC-ZFC measurement using dierent temperature ranges indicate that the ZFC curves are similar, but

16


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Langmuir

Figure 6: High temperature magnetization curves for CoNPs synthesized in C1 C4 ImNTf2 (at

250 K)

and C1 C14 ImNTf2 (at C1 C14 ImNTf2 are shifted by 50 A ·

300 K). Red curves m2 · kg−1 for clarity.

are Langevin ts.

The curves for

that the FC curves show a higher magnetization when the cooling eld is applied starting from a higher temperature.

This could indicate that some rotation of the clusters takes

place during the eld cooling. For both samples, the ZFC curve exhibits a clear maximum corresponding to the blocking temperature

160 K

Tb ,

which is found to be

7K

in C1 C4 ImNTf2 and

in C1 C14 ImNTf2 . This conrms that in both ILs the CoNPs are superparamagnetic

at room temperature, as expected given their size. However, the large dierence in

Tb

(with

the smaller NPs in C1 C14 ImNTf2 exhibiting a higher blocking temperature) hints at a large dierence in their eective anisotropy

K.

The latter can be deduced using the following

relation:

K= where

VN P

25kB Tb VN P

(2)

is the volume of the CoNPs (derived from the Langevin t). The resulting eective

anisotropy are

6.8 × 105 J · m−3

and

2.0 × 106 J · m−3

for CoNPs synthesized in C1 C4 ImNTf2

and C1 C14 ImNTf2 , respectively. The former value is close to the one reported for fcc Co and

-Co 40 (0.6 × 105 J · m−3 ),

hcp Co (8.2

× 105 J · m−3 ). 41

39

while the latter is even higher than the one reported for

This dierence in anisotropy for CoNPs with similar size and

magnetization and the higher value for CoNPs in C1 C4 ImNTf2 could be explained in part by their dierence in structure, but they most likely arise from a magnetic coupling induced by the clustering of particles (Figure 1).

17

The same inter-particle coupling would


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Page 18 of 29

also be responsible for the weak remanence of the CoNPs observed at

10 K

(Figure 5).

Such coupling can be qualitatively assessed by comparing the remanence magnetization measured with the

IRM

and

DCD

dene the excess magnetization

procedure.

The two measurements are combined to

∆M : 42

∆M = MDCD (H) − [MIRM (∞) − 2MIRM (H)]

(3)

For single-domain clusters that can be described with the Stoner-Wohlfarth model, is zero in the case where there is no magnetic coupling. A positive ing coupling while a negative

∆M

∆M

∆M

indicates a magnetiz-

indicates a demagnetizing coupling. The

∆M

for CoNPs

produced in C1 C14 ImNTf2 , plotted in Figure 8 (red triangles), shows a demagnetizing coupling up to

1 T.

one centered at

The shape of the curve suggests that this coupling has two contributions,

90 mT

and the other centered at about

0.5 T.

A smaller demagnetizing

coupling was found for the CoNPs in C1 C4 ImNTf2 (Figure 8, black circles) with only the low eld contribution at

90 mT.

The lower coupling with respect to CoNPs in C1 C14 ImNTf2

is consistent with the smaller anisotropy constant and remanence. However, it needs to be pointed out that these results are surprising, considering that the CoNPs in C1 C4 ImNTf2 are closer apart than their counterparts in C1 C14 ImNTf2 . For this reason, one would expect more magnetic interactions in C1 C4 ImNTf2 .

Further experiments are certainly needed to

elucidate this question.

Surface chemistry of CoNPs The synthetic route for CoNPs used in this study, which is the decomposition an OM precursor under H2 , is known to produce NPs covered with surface hydrides in the case of the group VIII metals.

This is in particular well established for Ru-NPs synthesized in both

conventional solvents

43

or in ILs.

23

Interestingly enough, these [Ru]s −H species were found

to play a central role in the formation of bimetallic NPs with a Ru core and a Cu shell in

18


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Langmuir

(a)

(b)

Figure 7: FC-ZFC curves of CoNPs synthesized in (a) C1 C4 ImNTf2 and (b) C1 C14 ImNTf2 recorded at 5 mT.

Figure 8: Excess magnetization of CoNPs synthesized in C1 C4 ImNTf2 and C1 C14 ImNTf2 .

19


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C1 C4 ImNTf2 .

33

Page 20 of 29

The association of Co with other metals is of key importance to modulate

their magnetic properties (for instance, ordered 1:1 CoPt alloys are hard magnets).

44

Also,

the formation of a metallic shell around Co is of great interest for (electro)catalytic applications, for instance in fuel cells.

45

Therefore, it is of great interest to determine whether our

CoNPs possess surface hydrides. In a rst experiment, the catalytic activity of freshly prepared CoNPs in C1 C4 ImNTf2 towards the hydrogenation of ethylene was measured.

46

It was found that the suspension was

indeed catalytically active. This experiment was conducted in two steps to fully recover the ethane gas formed by the reaction according to the reaction scheme in Figure 9. It was found that the total amount of ethane corresponded to a total concentration of

1.65 × 10−4 mol/L

of [Co]s −H.

Figure 9: Reaction scheme for the catalysis of ethylene hydrogenation by CoNPs.

In a second experiment, the hydrogenolysis reaction of Me4 Sn was conducted in a freshly prepared suspension of CoNPs in C1 C4 ImNTf2 . The reaction of Me4 Sn with metal surface hydrides is expected to lead to the evolution of MeH in the gas phase. of methane was observed, and its quantity corresponded to

47

Indeed, the evolution

1.83 × 10−4 mol/L of [Co]s −H, in

good agreement with the previous experiment. Furthermore, the modied NPs were analyzed by STEM and EDX elemental mapping (see Figure S5). The presence of small amounts of Sn was found on the CoNPs colonies, conrming the reaction between [Co]s −H and Me4 Sn. It should be noted that the concentration of [Co]s −H is rather low, as it corresponds to less than

5%

of Co surface coverage. This value should be compared to the previously

reported 1-2 [Ru]s −H per surface metal atom on Ru-NPs.

23

However, even this small amount

of surface hydrides is expected to be sucient to initiate the formation of Co-based bimetallic NPs, which is currently under investigation.

20


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Langmuir

Conclusion Ionic liquids have proven to be smart media for the synthesis of metallic nanoparticles. In this work, imidazolium-based ILs have been successfully used to elaborate magnetically responsive suspensions of CoNPs.

For this purpose, an organometallic precursor of Co,

Co(COD)(COE) was selected because of its reactivity towards H2 that leads to the precipitation of Co with the formation of volatile and harmless organic byproducts. A major benet of using ILs is their ability to stabilize the precipitated metal under the form of small NPs. Remarkably, and in contrast with other metals studied in the same ILs, CoNPs are not homogeneously dispersed in the liquid, but form assemblies with short NP-NP distance slightly exceeding the NP diameter.

For this reason, TEM analysis was completed

with SAXS measurements to estimate the size and size distribution of the NPs, which was always below

5 nm.

Interestingly enough, in an IL with a larger cation (C1 C14 ImNTf2 ), the

interparticle distance was larger, indicating a more ecient stabilization of the NPs. Both HRTEM and WAXS consistently revealed that the as-synthesized CoNPs adopt the non-compact structure of

-Co.

After a 2-day decantation over a magnet, this metastable

structure evolved towards the stable hcp allotrope of Co, as shown by the emergence of well dened diraction peaks in the scattering curves. Accordingly, magnetization curves recorded at

10 K

are consistent with zero-valent Co,

with no evidence of the presence of oxide. The magnetic moment of the CoNPs corresponds to that of the bulk metal. Their size as measured by a Langevin t matches quite well with the TEM observations. As expected in this size range, these NPs are superparamagnetic at room temperature. Interestingly enough, their blocking temperature is much lower in C1 C4 ImNTf2 (7 K) than in C1 C14 ImNTf2 (160 K). Indeed, the CoNPs produced in C1 C14 ImNTf2 exhibit a very high anisotropy. This is attributed to an enhanced dipolar coupling of the NPs in this IL, as shown by excess magnetization measurements. However, this result is in contradiction with the larger interparticle distance in this IL. Finally, the presence of surface hydrides on as-synthesized CoNPs was assessed by mea-

21


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Page 22 of 29

suring their catalytic activity towards hydrogenation or hydrogenolysis reactions. This paves the way towards the synthesis for Co-based bimetallic NPs with controlled size and structure, as already shown for other bimetallic systems.

33

In conclusion, this work demonstrates that ionic liquids are interesting media to synthesize Co nanoparticles with tunable size and magnetic properties.

These suspensions could be

used as ferrouids, especially in applications under vacuum (such as seals), owing to the extremely low volatility of these ILs. Bimetallic Co-based NPs are also ideal candidates in electrocatalytic applications or for the fabrication of advanced magnetic storage devices.

Acknowledgement The authors would like to thank Mireille Maret, Nathalie Boudet and Nils Blanc for their assistance and expertise during SAXS measurements. Allocation of beamtime on the French CRG-BM02 beam-line at the ESRF is gratefully acknowledged. The authors also gratefully acknowledge the French GDR CNRS 3182: "Nanoalliages et nanohybrides à base de métaux" for its scientic support.

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Me2SnR2, MeSnBu3 and SnBu4 (R = methyl, n-butyl, tert-butyl, neopentyl, cyclohexyl) onto metallic rhodium supported on silica.

28

New J. Chem. 2004, 28, 15311537.


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Magnetic, structural and chemical properties of cobalt nanoparticles

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