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On the Use of Amine-Borane Complexes To Synthesize Iron Nanoparticles - Supplementary Information

Authors:
  • Centre National de la Recherche Scientifique, France(CNRS)
Supporting Information
 Copyright Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, 2013
On the Use of Amine–Borane Complexes To Synthesize Iron Nanoparticles
Frdric Pelletier,[a] Diana Ciuculescu,[a] Jean-Gabriel Mattei,[b, c] Pierre Lecante,[c]
Marie-Jos Casanove,[c] Nader Yaacoub,[d] Jean-Marc Greneche,[d]
Carolin Schmitz-Antoniak,[e] and Catherine Amiens*[a]
chem_201204574_sm_miscellaneous_information.pdf
On the use of amine-borane complexes to synthesize Fe nanoparticles
Frédéric Pelletier, Diana Ciuculescu, Jean-Gabriel Mattei, Pierre Lecante, Marie-José Casanove, Nader Yaacoub, Jean-Marc Greneche,
Carolin Schmitz-Antoniak, Catherine Amiens,*
Supporting information
Experimental details:
General methods:
All operations (material preparation, sampling and packaging) were carried out using standard Fischer-Porter
bottle techniques and glove-box under argon. Fe[N(SiMe3)2]2 (>99%) was purchased from Nanomeps, and
mesitylene from Sigma-Aldrich. Mesitylene was purified by distillation under argon (over sodium) and further
degassed by the freeze-pump-thaw technique. Toluene was obtained from a MBRAUN SPS-800 solvent purifier
and also degassed by the freeze-pump-thaw technique. iPr2NH-BH3[1] and iPr2NBH2[2] were prepared and
purified according to published procedures. Dihydrogen was purchased from Air Liquide and contained less
than 3 ppm of H2O and 2 ppm of O2.
Synthesis:
1: Fe[N(SiMe3)2]2 (100 mg , 0.266 mmol) was dissolved in toluene (10 mL, H2O < 3 ppm) in a 200 mL Fischer-
Porter bottle in a glove box. The homogenous green solution was frozen under liquid nitrogen and 2.2
equivalents of iPr2NHBH3 in toluene (67 mg, 0.583 mmol in 4 mL toluene) were canula-transferred into the
Fischer-Porter bottle. A slow warm up of the reaction medium to room temperature afforded a dark solution. The
mixture was further allowed to stir overnight. Then toluene was evaporated to afford a black sticky solid.
2, 4, 5: Fe[N(SiMe3)2]2 (200 mg, 0.53 mmol) was dissolved in toluene (18 mL, H2O < 3 ppm) in a 200 mL
Fischer-Porter bottle in a glove box. The homogenous green solution was frozen under liquid nitrogen and 0.16
(9.8 mg, 0.085 mmol) , 1.1 (67 mg, 0.58 mmol) or 1.6 (98 mg, 0.85 mmol) equivalents of iPr2NHBH3 in 4 mL
toluene were canula-transferred in the Fischer-Porter bottle. The mixture was allowed to warm up to room
temperature to afford a dark brown solution. The solution was then pressurized under 3 bar of H2 and stirred at
110 °C overnight. Then toluene was evaporated to afford a black sticky solid.
3: Fe[N(SiMe3)2]2 (80 mg, 0.21 mmol) was dissolved in toluene (10 mL, H2O < 3 ppm) in a 200 mL Fischer-
Porter bottle in a glove box. The homogenous green solution was frozen under liquid nitrogen and 0.5 (12 mg,
0.105 mmol) equivalents of iPr2NHBH3 in 5 mL toluene were canula-transferred in the Fischer-Porter bottle. The
mixture was allowed to warm up to room temperature to afford a dark brown solution. The solution was then
pressurized under 3 bar of H2 and stirred at 110 °C overnight. Then toluene was evaporated to afford a black
sticky solid.
6: Fe[N(SiMe3)2]2 (100 mg, 0.266 mmol) was dissolved in toluene (18 mL, H2O < 3 ppm) in a 200 mL Fischer-
Porter bottle in a glove box. The homogenous green solution was frozen under liquid nitrogen and 1 equivalent
of iPr2NBH2 (30 mg, 0.266 mmol) in 4 mL toluene were canula-transferred in the Fischer-Porter bottle. The
mixture was allowed to warm up to room temperature to afford a dark brown solution. The solution was then
stirred at 110 °C overnight. Evaporation of toluene afforded a black sticky solid.
7, 8, 9: in a 500 mL Fischer-Porter bottle, Fe[N(SiMe3)2]3 (1.128 g, 3 mmol) was dissolved in mesitylene (60
mL, H2O < 1 ppm). The green solution was pressurized under 3 bar H2 and allowed to stir in a 150 °C oil bath
overnight. Then, the black solution was equally distributed into four 200 mL Fischer-Porter bottles. The first
one, sample 7, was directly evaporated and recovered with few spatulas of PPO to afford a black solid as a
reference. The other two samples (respectively samples 8 and 9), were charged respectively with 1 equivalent of
iPr2NHBH3 (115 mg, 1 mmol), or 1 equivalent iPr2NBH2 (113 mg, 1 mmol) with respect to the iron content.
After reaction they were treated as sample 7 to afford a black solid.
TEM studies:
Samples for TEM experiments were dispersed in solvent used for the synthesis previous to deposition on
ultrathin carbon films supported by a lacey carbon film on a 400 mesh copper grid. They were exposed to air just
before their introduction into the microscope. TEM images were recorded at the TEMSCAN facility of
University Paul-Sabatier (spot size 2.5nm) on a Jeol JEM 2100 F equipped with a field emission electron source
and operating at 200 kV.
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SI 1: TEM images and size histograms of samples 1-9(from top to bottom). Scale bar: 100nm.
As explained in the paper, the nature of the chemicals present at the surface of the nanoparticles remains
unknown. In this context it is not possible to explain the difference in stabilization (hence in dispersion) from
one sample to the other.
Mossbauer measurements:
The powder was dispersed in vacuum grease, sealed in sample holder and stored under controlled atmosphere in
order to minimize air exposure before and during the measurements. The Mössbauer spectra were obtained using
a spectrometer running in the triangular symmetric mode for the velocity and a radioactive source of 25mCi Co57
diffused into a Rh matrix. The temperature was regulated at 10K in a cryomagnetic OXFORD system out of field
and in presence of external magnetic field of 8T oriented parallel to the -beam. The source is located in a
compensating magnetic coil to prevent from polarisation of the radiation and consequently a splitting of the
monochromatic emission line. The Mössbauer spectra were described by means of a discrete distribution of
hyperfine field linearly correlated to a discrete distribution of isomer shift to well reproduce the asymmetrical
shape of the sextets, with lorentzian lines. The isomer shift values are quoted to that of -Fe at 300K.
WAXS studies:
For the WAXS experiments, small amounts of the pure powders were sealed in 1 mm Lindemann glass
capillaries. Measurements were performed at CEMES using a dedicated two-axis diffractometer equipped with a
graphite flat monochromator in order to select the Molybdenum Kα wavelength (0.07107 nm). Data sets
typically included 457 measurements in the range 0° ≤ θ ≤ 65° for equidistant S values [S=4πsin θ/λ)].
Measurement time was typically 15 hours for each sample. Treatment of the data has been carried out according
to a previously published procedure[3] to allow the analysis of the radial distribution function (RDF) of the
particles.
Comparison of a typical sample (sample 4) to various iron boride structures shows that the closest agreement is
obtained for Fe2B (Fig S2); however, for small and/or disordered objects, inspection in direct space provides a
better view of the system (Fig S3). A RDF was computed from a model defined as a small Fe2B crystal (1.5 nm
in size, including 165 atoms). This simulated RDF is indeed in global agreement with experimental functions,
especially for Samples 4, 5 and 7. However this model small but highly regular still generates very well defined
distances which don’t perfectly match the broad peaks observed on the experimental RDFs, clearly indicating a
strongly disordered structure.
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S2: Comparison of a typical sample (sample 4) to various iron borides in reciprocal space; red line:
experimental data for sample 4; green: diffraction peaks for the respective borides.
SI 3: RDF of samples 1, 4-6; comparison to signatures expected for Fe2B and polytetrahedral Fe NPs (sample 7)
EXAFS measurements and analysis:
Absorption spectra at Fe K edge were measured on the C spectrometer at Hasylab in Hamburg, Germany.[4] The
samples, diluted with poly-3,5-dimethylphenylene-oxide (Aldrich), were prepared as 5 mm large pellets sealed
between Kapton foils to protect them from air oxidation. The dilution was adjusted to ensure optimal absorption.
The measurements were done in transmission mode at room temperature using a double crystal monochromator
set for diffraction from silicon (111) planes. Before presenting the results and starting the discussion, we would
like to make a few general remarks on the possibility to distinguish different elemental contributions to the
EXAFS signal and its analysis. The amplitude of oscillations as a function of wave number k depends not only
on the distance between absorber and backscattering atoms, it follows the k-dependent effective backscattering
amplitude of the backscattering element in vicinity to the absorber. As a rule of thumb one may realise that
lighter elements have their maximum backscattering amplitude at low wave numbers k, while with increasing
atomic number the maximum is shifted towards higher k values. As an example, we show in Fig. SI4 the k-
dependent backscattering amplitudes for the light elements B, N, O, that may be present in our samples, and the
heavier element Fe. Weighting the backscattering amplitudes with higher orders of k, e.g. k3, increases the
amplitude at higher wave numbers much more and suppresses the light elements’ contributions, consequently.
For all the light elements mentioned above, a local maximum around 20 nm-1 (denoted 1) and the main peak
around 30−35 nm-1 (denoted 2) appears. The main peak (denoted 2) is shifting with increasing atomic number
from kmax = 30 nm-1 for B with an atomic number of z = 5 to about kmax = 62 nm-1 for Fe with z = 26. Although B
exhibits an additional local maximum around 60 nm-1 (denoted 3), the similar k-dependence of the
backscattering amplitudes makes it difficult or almost impossible to distinguish between the light elements B, N,
and O by having a look at the amplitude of EXAFS oscillations without further data treatment. Since the
standard EXAFS analysis based on Fourier transform does not offer the resolution in k needed here, we used a
wavelet transform to analyse the EXAFS with a resolution both in k and real space. We start the presentation of
the analysis with the standard analysis method before coming to the wavelet analysis. At the end of this section,
the results from both methods are compared.
A standard Fourier-based treatment of the data was carried out via the Athena and Artemis software[5][6]
based on the algorithms of the FEFFIT[7] and FEFF[8, 9] programs. The Fourier transform (FT) was performed
using a Kaiser-Bessel window with dk = 1 nm-1 in the range of about 18 nm-1 k 112.5 nm-1.We would like to
emphasise that no phase correction was applied. Thus, the radial distance obtained by FT of the k-dependent
EXAFS data is not the geometric distance.
The k1- and k3-weighted EXAFS data for the samples that may contain boron are shown in Fig. SI5
(upper panel) together with the data of an Fe2B alloy sample. While the k1-weighted EXAFS (left) is sensitive to
the light elements as stated above but also on the x-ray absorption near-edge structure the k3-weighting of
EXAFS data (right) shifts the focus to higher wave numbers, i.e. heavier elements.
Fig SI4 : Backscattering amplitudes weighted by wave number k for iron, boron, nitrogen, and oxygen. Note
the different scaling factors.
Fig. SI5: EXAFS oscillations (upper panel) weighted either by k1 (left) or k3 (right) as well as corresponding
magnitude of Fourier transform (lower panel) for samples 1, 4, 6-9 (but 7, cf. Fig. SI6) and a Fe2B reference
sample.
Fig. SI6: EXAFS oscillations (upper panel) weighted either by k1 (left) or k3 (right) as well as magnitude and
imaginary part of Fourier transform (respectively middle and lower panel) for samples 7-9 and oxidised
sample 7 after exposure to air. Note the different scaling for the oxidised sample.
At a first glance, the EXAFS signals are quite similar for all the nanoparticles samples, but there are
significant differences to the Fe2B sample excluding the formation of an Fe2B alloy as further discussed below.
Having a closer look to the k1-weighted spectra, one may note that the two samples for which AoB took active
part (sample 6 and sample 9) have a larger amplitude at low wave numbers that may indicate a higher amount of
light elements like B, C, N or O. This can also be seen in the FT of the EXAFS data presented in Fig. SI5 on the
lower panel. Especially the FT of the k1-weighted EXAFS data that is very sensitive to light elements show a
large contribution at small radial distances, which is still significant in the FT of the k3-weighted data. For these
two samples, in addition to the main peak slightly above 0.2 nm (denoted C1 for the k1-weighted and C3 for the
k3-weighted data, respectively), a significantly enhanced broad shoulder at smaller radial distance appears
(denoted B1, B3), which is smaller for the samples synthesised by reaction with AeB. However all samples
exhibit a very similar pronounced fine structure at small radial distances between 0.06 and 0.10 nm (denoted A1,
A3). Note that the spike in the spectrum of sample 4 was removed prior to the FT yielding a slightly different
fine structure. The EXAFS data of sample 7, i.e. pure Fe particles obtained by hydrogenation, are presented in
Fig. SI6 together with the ones of sample 8 (same as sample 7, but with 1eq. AeB) and 9 (same as sample 7, but
with 1eq. AoB) and the oxidised sample 7 after exposure to air. In this figure, the differences discussed above
can be seen even more clearly. Additionally it is evidenced that the structure of all NPs investigated here is at
least very similar to the structure of pure Fe NPs of polytetrahedral structure (sample 7) or amorphous Fe[10] but
far from the typical pattern of crystalline Fe2B (cf. Fig. SI5 and Ref.[11]). This rules out the formation of any Fe2B
crystallites upon exposure of the nanoparticles to AeB or AoB either during synthesis or after, but is not
sufficient to rule out any inclusion of boron in particular, since boron might form sub-stoechiometric alloys with
iron. However, comparing the data of samples 8 and 9 to the ones of the oxidised sample 7, may give rise to the
assumption that the two peaks A1 and B1 in the FT of the k1-weighted EXAFS (i.e. A3 and B3 in the FT of the k3-
weighted EXAFS) may be caused by a slight oxidation of the samples. Alternatively, the presence of nitrogen
may be possible. The same holds of course for samples 1, 4, and 6. Since the FT in this region exhibits a larger
magnitude for samples 6 and 9, they may contain slightly more oxygen or nitrogen than the other samples. For
radial distances in the range of 0.2−0.3 nm, an additional contribution seems to be present in samples 8 and 9 (as
well as in 1, 4, 6, cf. Fig. SI5), compared to the pure Fe nanoparticles of sample 7. These differences in the FT
do not exclude the existence of boron in close vicinity to the Fe absorbers, without giving the possibility to
identify the type of backscattering atoms in a straight-forward manner.
Alternatively to the Fourier-based analysis, a wavelet transform (WT) was used to analyse the light
elements’ contributions to the overall EXAFS signal in detail. For this purpose, the AGU-Vallen Wavelet
software was used, which was developed for acoustic emission analyses,[12][13] but can easily be adapted for
EXAFS analyses.[14] In a simple picture, the WT can be understood as a full transform of the EXAFS signal after
cutting it into several intervals in a smart way so that one gets a high resolution both in radial distance and wave
number. Details can be found in Ref.[14]. Since the position of maximum backscattering amplitude depends on
the atomic number, the WT gives the possibility to directly visualize contributions of different atomic species to
the overall EXAFS signal. However, it is still difficult to distinguish between neighbouring elements of the
periodic table. Fortunately in the case of boron, a distinct fine structure in the k-dependent backscattering
amplitude [11]separates it from other light elements like nitrogen, and oxygen as shown in Fig. SI4. The ratio
between this additional maximum and the main peak (i.e. 2 and 3 in Fig. SI4) is about 1.34.
Fig. SI7: Magnitude of wavelet transform of the EXAFS data. From top to down: samples 7 and 7
oxidized, 1, 4, 6, 8, 9, and oxidised sample 1.
In the WT, boron is consequently characterized by two local maxima, the higher one around 30 nm-1,
the lower one around 60 nm-1 with a ratio of 1.34 in the WT magnitudes. The WT was performed on the k1-
weighted EXAFS data in order to emphasize the contributions from light elements. The WT of samples 1, 4, 6−9
are shown in Fig. SI7 including the ones of the boron-free Fe NPs of sample 7 and the oxidised sample 7 which
have been used as references. In agreement to the findings from the FT analysis, for all samples except sample 7,
some light elements (oxygen or nitrogen) contribute to the EXAFS signal at small radial distances. For a better
comparison the radial distances related to local maxima in the FT are marked by A1 A3 as in Figs. SI5 and SI6,
respectively. Although in the total WT the boron contributions can already be seen, a linear combination of the
WT of Fe and oxidised Fe NPs was subtracted. The factors for the linear combination have been chosen in a way
that (i) the dominating oxygen/nitrogen contribution at low radial distances disappears and (ii) the ratio of the
two boron maxima fits roughly 1.34 as discussed above. Position and value of the maximum WT magnitude are
listed in Table SI1. Note that as in the case of FT no phase correction is included. Therefore, the radial
distance obtained is not the geometric distance. However, the positions in both radial distance and wave number
of the WT assigned to boron are the same for all samples within the error bars. The values of the maximum WT
magnitudes reflect the amount of boron. However, an absolute quantification even after comparison to a Fe2B
reference sample (not shown here) seems to be ambiguous since the overall amplitude strongly depends on the
structure which is quite different for Fe2B and the boron containing nanoparticles. A very rough estimation of the
boron content yields (15 5)at% of boron in sample 1, that is the one with the highest amount of boron and has
been synthesised by reacting 2eq. AeB at room temperature, sample 4 (1eq. AeB at room temperature and
subsequent hydrogenation at 100°C) has less boron included. The amount of boron in sample 6 (1eq. AoB) lies
in between the two samples mentioned before. If the AeB or AoB is reacted with NPs preformed by
hydrogenation (sample 8 and 9, respectively), less boron is included.
In addition, we exemplarily present the WT of sample 1 after exposure to air in Fig. SI7 (bottom).
Please note the different scaling for the WT magnitude of the total WT and the subtracted oxide WT: While the
upper limit for the scale was 1.8 arb. Units for the as-prepared samples, for the oxidised samples it is 6.0 arb.
units. However, the difference of WTs to be checked for boron inclusion has the same scale as before. It can
clearly be seen that no boron can be observed in close vicinity to Fe for the oxidised sample 1. Although there is
some difference signal left after subtraction, it does not show the “two peak” finger print of boron, is located at
lower radial distances, i.e. (0.21 0.1) nm and is, with its magnitude of only 3% of the total WT, negligibly
small. Data for oxidised samples were also available for samples 4, 8, and 9. In none of the samples, any traces
of boron could be observed. This may indicate that the oxidation is connected to an extraction of boron from the
NP maybe forming boron oxides like B2O3 at the surface.
sample
1
4
6
8
9
kmax,B (nm-1)
28.0 0.5
27.5 0.5
28.0 0.5
28.0 0.5
27.5 0.5
Rmax,B (nm)
0.245 0.05
0.242 0.05
0.245 0.05
0.245 0.05
0.245 0.05
WTmax,B (arb. units)
0.89 0.02
0.73 0.02
0.81 0.03
0.56 0.02
0.65 0.03
Table SI1: Position in wave number and radial distance, and value of maximum WT magnitude assigned to
boron.
Finally comparing the results obtained from either FT or WT analysis of EXAFS data, one may realise
that the additional contribution in the range of 0.2−0.3 nm seen in the FT magnitude of k1-weighted data (Fig.
SI6, left panel, centre) with respect to the one of pure Fe NPs of sample 7, could be identified by WT as boron
contributions with their maximum between 0.24 and 0.25 nm. The assumption that additional light elements like
oxygen or nitrogen contribute to the EXAFS signal at small radial distances to the FT (denoted A1, A3 and B1, B3
in Figs. SI5 and SI6, respectively), could be verified by WT since the large maximum at small radial distances
correspond to low wave numbers k, which means light elements.
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