Electrodeposition of nanocrystalline metals and alloys from ionic liquids

Article abstract of DOI:10.1002/anie.200350912

Better than water: A method for the electrodeposition of nanocrystalline metals and alloys from ionic liquids has been developed (see schematic representation). This method enables the synthesis of aluminum nanocrystals with average grain sizes of about 10 nm, Al-Mn alloys, as well as Fe and Pd nanocrystals.

Full text of DOI:10.1002/anie.200350912

Communications
Electrodeposition of Nanocrystals
gap increases with decreasing particle size.[7] The hardness of
nanocrystalline metals is found to increase with decreasing
particle size, which is explained by the increased fraction of
atoms located at the grain boundaries; furthermore it is found
that the corrosion resistance often gets better for nanocrystal-
line metals.[8] As an example, nanocrystalline Ni[9] as well as
Au, Cu, Ag, and Fe nanomaterials have been made by
electrochemical means. Adjusting the hardness of a material
simply by varying the grain size opens up many interesting
perspectives for technical applications: nanocrystalline Ni is
used as a hard coating material to achieve the high hardness
and strength of steel without the usual corrosion problems,
and nanocrystalline Co can replace the more expensive
titanium.
Electrochemistry is a powerful tool for making nano-
crystalline materials because the grain size can be adjusted by
varying the electrochemical parameters such as overvoltage,
current density, pulse parameters in pulsed electrodeposition,
bath composition, and temperature.[10] Thus, tailored materi-
als are in principle accessible by simple electrochemical
procedures. However, detailed studies on this topic have
hitherto only been performed in aqueous solutions where
such a procedure is limited to elements that are more noble
than hydrogen. Herein we present a novel procedure by which
bulk nanocrystalline metals and alloys can be electrodepos-
ited from ionic liquids. For the first time we report on the
nanocrystalline electrodeposition of Al, Fe, and an Al-Mn
alloy as well as on nanocrystalline Pd. Nanocrystalline noble
metals are interesting as catalysts, for example, in low-
temperature fuel cells, as a result of their high surface to
volume ratio.
Electrodeposition of Nanocrystalline Metals and
Alloys from Ionic Liquids
Frank Endres,* Mirko Bukowski, Rolf Hempelmann,*
and Harald Natter
By definition ionic liquids are molten salts with a melting
point below 1008C. In general, they are composed of organic
cations, such as substituted imidazolium ions or tetraalkyl-
ammonium ions, and of inorganic or organic anions, such as
AlCl4À, PF6À, BF4À, CF3SO3À, and (CF3SO2)2NÀ. Liquids
based on substituted imidazolium ions and anions of the latter
type show an extraordinarily high chemical and thermal
stability as well as wide electrochemical windows. Further-
more they have very low vapor pressures, even at temper-
atures as high as 3008C. These properties have lead to a great
interest in technical and organic chemistry,[1] particularly with
regard to “green chemistry”.[2] Alot of different systems have
been reported in the literature,[3] and the general trend
nowadays is to make hydrophobic ionic liquids that can be
handled under air.
Ionic liquids are also of great interest for electrochemical
purposes because they have wide electrochemical windows
that can reach values of more than 6 volts, depending on the
substrate.[3] Thus, they allow access to elements and com-
pounds, especially to less-noble nanocrystalline materials that
otherwise cannot be electrodeposited from aqueous or
organic solutions. For example, the liquid 1-butyl-3-methyl-
imidazolium hexafluorophosphate has an electrochemical
window of slightly more than 4 volts on Au(111), and this
value is more than three times that of water. It was shown
recently that the semiconductor germanium can be electro-
deposited in intrinsic form with such a liquid and that,
depending on the bath composition and the electrochemical
parameters, Ge(111) bilayers as well as stable Ge nanocrystals
can be obtained.[4,5] Ionic liquids based on AlCl3 and organic
halides, such as substituted imidazolium ions—which were
used for the studies presented here—are ideally suited for the
electrodeposition of Al and its alloys. A general overview on
the electrodeposition of metals and semiconductors from
ionic liquids is reported in Ref. [6].
Figure 1 shows the XRD pattern of microcrystalline and
nanocrystalline Al. The former was made electrochemically
in a Lewis acidic ionic liquid based on AlCl3 and 1-ethyl-3-
methylimidazolium chloride ([EMIm]+ClÀ) under galvano-
static conditions (55 mol% AlCl3, I = 54 mAcmÀ2, on glassy
carbon), while the latter was made under quite similar
conditions (55 mol% AlCl3, I = 16.67 mAcmÀ2, on glassy
carbon), but with nicotinic acid as an organic additive.[11]
Without this additive, pulsed electrodeposition by galvano-
Nanoscale semiconductors and metals are of great interest
for nanotechnology. The band gap of nanoscale semiconduc-
tors is a function of the particle size, and normally the band
[*] Prof. Dr. F. Endres
FB Physik, Metallurgie und Werkstoffwissenschaften
Technische Universität Clausthal
38678 Clausthal-Zellerfeld (Germany)
Fax : (+49)5323-72-2460
E-mail: frank.endres@tu-clausthal.de
Prof. Dr. R. Hempelmann, Dipl.-Chem. M. Bukowski, Dr. H. Natter
FR 8.13 Physikalische Chemie
Universität des Saarlandes
66123 Saarbrücken (Germany)
Fax : (+49)681-302-4759
E-mail: r.hempelmann@mx.uni-saarland.de
Figure 1. Upper curve: XRD pattern of nanocrystalline Al with a grain
size of 12Æ1 nm prepared from AlCl3/[BMIm]+ClÀ (55/45 mol%).
Lower curve: Microcrystalline reference sample.
3428
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200350912
Angew. Chem. Int. Ed. 2003, 42, 3428 – 3430

Angewandte
Chemie
static or potentiostatic electrodeposition methods gave crystal
sizes above 100 nm. It is evident that the full-width at half-
maximum height of all the peaks in Figure 1 is much wider in
the case of the nanocrystalline sample than in the micro-
crystalline sample. It has been known for a long time that this
peak width can be used in the Scherrer equation to determine
the average grain size.[12] For this sample we obtained an
average grain size of 14.0 Æ 0.3 nm.
of a narrow size distribution of nanocrystals with average sizes
ranging from 10 to 100 nm can be obtained by varying the
bath composition and the electrochemical parameters.[19] In
general, bigger grains are expected to form when lower
current densities and lower overvoltages are used; this
observation is in qualitative accordance with the electro-
chemical version of the Kelvin equation.[20] Preliminary
results on the hardness of our nanocrystalline Al deposits
show that an improvement of between 50 and 150% can be
Whereas Scherrer analysis only gives an average value for
the grain size, the Warren–Averbach procedure also allows achieved relative to the microcrystalline sample.
the logarithmic normal distribution to be determined by
evaluating the complete peak shape.[13,14] It was demonstrated
in the literature that a logarithmic normal distribution
function accurately describes the size distribution of emul-
sions,[15] dust particles,[16] sols,[17] and nanocrystalline metals
such as Pd.[18] Therefore, a logarithmic normal distribution is
also assumed for the samples investigated, and a Warren–
Averbach analysis was performed for the harmonic (111) and
(222) peaks. Avolume-averaged grain size of 12 Æ 1 nm is
obtained for the nanocrystalline Al sample. The respective
logarithmic normal distribution is shown in Figure 2 together
It is also possible to deposit several aluminum alloys from
ionic liquids based on AlCl3.[6] Of particular interest are
electrochemically made Al-Mn alloys,[21] which are widely
used in the automobile industry for lightweight construction.
For the present study we have attempted to make nano-
crystalline AlxMny alloys electrochemically by adding MnCl2
(3.2 mmolLÀ1) and 2 weight% of nicotinic acid to a Lewis
acidic liquid (as described above). The deposition was
performed by using a current density of I = 0.5 mAcmÀ2 on
glassy carbon. Figure 3 shows the XRD pattern of a typical
deposit indicating the dominant Bragg peaks for the single-
Figure 3. XRD pattern of an electrochemically made sample which is
mainly composed of a nanocrystalline Al-Mn alloy with an average
grain size of 26Æ1 nm. Triangles: Al-Mn, circles: Al, squares: Mn.
Figure 2. Comparison between the size distribution f of Al nanoparti-
cles (12Æ1 nm) obtained from the TEM picture (inset), a logarithmic
normal fit to these TEM data (solid line; s=1.3, m=9.5), and the
distribution function obtained from the Warren–Averbach analysis
(dashed line; s=1.3, m=8.5).
phase alloy (Al-Mn 1:1[22]); in some samples we also find weak
signals resulting from Mn and Al. This result implies that a
material was obtained that was mainly composed of the Al-
Mn alloy with small amounts of Al and Mn in the deposit (up
to a maximum of 5%). Further structural studies on these co-
deposits are currently under progress. The average grain size
of 26 Æ 1 nm results from a Scherrer analysis of the Al-Mn
Bragg peaks.
Nanocrystalline Fe was also obtained by galvanostatic
plating (5 mAcmÀ2) from a Lewis acidic ionic liquid of
[BMIm]+ClÀ and AlCl3 (63 mol% AlCl3, 4.3 mmolLÀ1 anhy-
drous FeCl3, 2 weight% of nicotinic acid). Scherrer analysis of
the XRD data gives an average grain size of 64 Æ 1 nm. Fe-Al
alloys as well as Fe-Al co-deposits can also be obtained with
higher overvoltages for the electrodeposition.
with a histogram of the crystal size obtained from TEM
measurements (determined from 400 nanoparticles) and a
logarithmic normal distribution of the TEM data. We see, that
the sizes range from about 5 to 20 nm, with most of them in
the range between 8 and 10 nm. It is not surprising that there
is a slight deviation between the results from XRD (Warren–
Averbach analysis) and TEM (inset in Figure 2). TEM
generally yields larger “particle sizes” for face-centered
cubic (fcc) metals (such as Pd or Al) because stacking faults
readily occur in the (111) direction of a fcc structure. The
stacking faults terminate the coherency of the lattice plane
sequence for which XRD is sensitive.
Our results nicely show that grain sizes below 10 nm can
be obtained by simple electrodeposition under galvanostatic
conditions. Further results show that bulk samples composed
Finally we report on the electrodeposition of nanocrystal-
line palladium deposits from a Lewis basic ionic liquid
(44 mol% AlCl3, 56 mol% [BMIm]+ClÀ, 2 weight% nicotinic
Angew. Chem. Int. Ed. 2003, 42, 3428 – 3430
www.angewandte.org
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3429

Communications
[7] T. Trindade, P. O'Brien, N. L. Pickett, Chem. Mater. 2001, 13,
3843–3858.
[8] a) D. G. Morris, Mater. Sci. 1998, 269–272, 11–13; b) D. G. Morris
in Mechanical Behaviour of Nanostructured Materials, Trans
Tech, Aedermannsdorf, Switzerland, 1998.
[9] C. Cheung, F. Djuanda, U. Erb, Nanostruct. Mater. 1995, 5, 513–
523.
[10] Theory and Practice of Pulse Plating, (Eds.:J.-C. Puippe, F.
Leaman ), American Electroplaters and Surface Finishers
Society, Orlando, 1986.
acid) using a Pd counter electrode and a current density of I =
0.66 mAcmÀ2. A1-mm thick microcrystalline copper plate
was used as the substrate and the liquid was heated up to
1008C during electrodeposition. Under these conditions
we obtained Pd nanocrystals with an average grain size of
13 Æ 1 nm. An advantage over aqueous solutions is that
hydrogen evolution does not occur in ionic liquids. In aqueous
solutions this side reaction is hard to control and leads to
hydrogen adsorption by Pd. Porous and brittle deposits are
usually the result. Defect-free nanocrystalline Pd foils would
be of great interest for Pd diffusion membranes and for
hydrogen purification.
[11] M. Bukowski, F. Endres, R. Hempelmann, H. Natter, Ger. Offen
2002, DE10108893.0.
[12] P. Scherrer, Göttinger Nachrichten 1918, 2, 98–100.
[13] C. E. Krill, R. Birringer, Phil. Mag. A 1998, 77, 621–640.
[14] H. Natter, M. Schmelzer, M.-S. Löffler, C. E. Krill, A. Fitch, R.
Hempelmann, J. Phys. Chem. B 2000, 104, 2467–2476.
[15] F. Grum, D. J. Paine, J.L. Simonds, J. Opt. Soc. Am. 1971, 61, 70–
78.
[16] P. Drinker, J. Ind. Hyg. Toxicol. 1925, 7, 305–311.
[17] E. J. Meehan, W. H. Beattie, J. Phys. Chem. 1960, 64, 1006–1016.
[18] C. G. Granqvist, R. A. Buhrman, J. Appl. Phys. 1976, 47, 2200–
2219.
[19] M. Bukowski, F. Endres, R. Hempelmann, H. Natter, unpub-
lished results.
[20] E. Budevski, G. Staikov, W. J. Lorenz in Electrochemical Phase
Formation and Growth, VCH, Weinheim, 1996.
[21] J. C. Li, S. H. Nan, Q. Jiang, Surf. Coat. Technol. 1998, 106, 135–
139.
We have shown for the first time that nanocrystalline
metals and alloys can be made in ionic liquids by electro-
chemical means. As a consequence of their wide electro-
chemical windows, elements can be obtained that otherwise
cannot be electrodeposited from aqueous or organic solu-
tions. The grain sizes can be adjusted by varying the bath
compositions and the electrochemical parameters. Bulk
samples with average grain sizes down to 10 nm can be
made by simple electrodeposition. Of particular interest for
materials science are studies on the hardness and the
corrosion resistance of such nanocrystalline samples.
[22] a) H. Kono, J. Pharm. Belg.J. Phys. Soc. Japan 1958, 13, 1444–
1451; b) JCPDS No. 11–0416.
Experimental Section
The nanocrystalline samples were deposited in a glove box under
nitrogen (H2O and O2 below 2 ppm). The organic salts ([EMIm]+ClÀ
or [BMIm]+ClÀ) were dried for several days under vacuum at about
508C. After careful mixing of the organic component with AlCl3
(Fluka, puriss p. a.), the obtained ionic liquid was put under vacuum
for several hours, with stirring, to remove any residual HCl. The
electrodes were dipped into the melt and fixed. The working
electrodes were either glassy carbon, copper, or aluminum.
To prepare the alloys the appropriate metal halides were added.
The depositions were carried out under galvanostatic conditions with
current densities between 0.25 and 54 mAcmÀ2, with or without
nicotinic acid (as indicated). All experiments were done at room
temperature except those for the palladium depositions, which were
performed at 80–1508C. The deposits generally had thicknesses
between 0.1 and 1 mm.
The X-ray diffraction experiments were carried out with a
Siemens D500 diffractometer. Warren–Averbach analysis was done
by using our own computer-based fitting program.[14]
Received: January 9, 2003
Revised: April 30, 2003 [Z50912]
Keywords: aluminum · electrodeposition · ionic liquids · metals ·
.
nanotechnology
[1] a) P. Wasserscheid, W. Keim, Angew. Chem. 2000, 112, 3927–
3945, Angew. Chem. Int. Ed. 2000, 39, 3773–3789; b) Ionic liquids
in synthesis, (Eds.: P. Wasserscheid, T. Welton), Wiley-VCH,
Weinheim, 2002.
[2] M.J. Earle, K. R. Seddon, Pure Appl. Chem. 2000, 72, 1391–
1398.
[3] R. Hagiwara, Y. Ito, J. Fluorine Chem. 2000, 105, 221–227.
[4] F. Endres, C. Schrodt, Phys. Chem. Chem. Phys. 2000, 2, 5517–
5520.
[5] F. Endres, Sh. Zein El Abedin, Chem. Commun. 2002, 892–893.
[6] F. Endres, ChemPhysChem 2002, 3, 144–154.
3430
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2003, 42, 3428 – 3430