Mechanism of Bi?Ni Phase Formation in a Microwave-Assisted Polyol Process

Article abstract of DOI:10.1002/open.202000236

Typically, intermetallic phases are obtained in solid-state reactions or crystallization from melts, which are highly energy and time consuming. The polyol process takes advantage of low temperatures and short reaction times using easily obtainable starting materials. The formation mechanism of these intermetallic particles has received little attention so far, even though a deeper understanding should allow for better synthesis planning. In this study, we therefore investigated the formation of BiNi particles in ethylene glycol in a microwave-assisted polyol process mechanistically. The coordination behavior in solution was analyzed using HPLC-MS and UV-Vis. Tracking the reaction with PXRD measurements, FT-IR spectroscopy and HR-TEM revealed a successive reduction of Bi³⁺ and Ni²⁺, leading to novel spherical core-shell structure in a first reaction step. Bismuth particles are encased in a matrix of nickel nanoparticles of 2 nm to 6 nm in diameter and oxidation products of ethylene glycol. Step-wise diffusion of nickel into the bismuth particle intermediately results in the bismuth-rich compound Bi₃Ni, which consecutively transforms into the BiNi phase as the reaction progresses. The impacts of the anion type, temperature and pH value were also investigated.

Full text of DOI:10.1002/open.202000236

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Mechanism of BiÀ Ni Phase Formation in a Microwave-
Assisted Polyol Process
[a]
[b]
[a, c]
[a]
Matthias Smuda, Christine Damm, Michael Ruck, and Thomas Doert*
Typically, intermetallic phases are obtained in solid-state
reactions or crystallization from melts, which are highly energy
and time consuming. The polyol process takes advantage of
low temperatures and short reaction times using easily
obtainable starting materials. The formation mechanism of
these intermetallic particles has received little attention so far,
even though a deeper understanding should allow for better
synthesis planning. In this study, we therefore investigated the
formation of BiNi particles in ethylene glycol in a microwave-
assisted polyol process mechanistically. The coordination
behavior in solution was analyzed using HPLC-MS and UV-Vis.
Tracking the reaction with PXRD measurements, FT-IR spectro-
+
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scopy and HR-TEM revealed a successive reduction of Bi and
2
+
Ni , leading to novel spherical core-shell structure in a first
reaction step. Bismuth particles are encased in a matrix of nickel
nanoparticles of 2 nm to 6 nm in diameter and oxidation
products of ethylene glycol. Step-wise diffusion of nickel into
the bismuth particle intermediately results in the bismuth-rich
compound Bi Ni, which consecutively transforms into the BiNi
3
phase as the reaction progresses. The impacts of the anion
type, temperature and pH value were also investigated.
1
. Introduction
threefold purpose in the synthesis, as they serve as solvents,
capping agents, and reduction agents. Metal salt precursors
have a similar dissolution behavior in polyols as in water due to
the chelating effect of the solvent. This chelating effect is also
advantageous in controlling the particle growth and morpho-
Over the last two decades, nanoparticles composed of two or
more metal elements have achieved a great scientific and
technological interest due to their special optical and electronic
properties. These intermetallic particles often display superior
[9]
[
10]
logy and it prevents agglomeration of the prepared particles.
[1–3]
properties in catalysis and electronics.
Various methods to
Additionally, the polyol is easily disposed-off due to its non-
toxic and non-volatile nature. Last, the polyol is used to reduce
the metal to its zero-valent state. The reduction power of a
given polyol increases with rising temperature. Therefore,
higher temperatures are necessary to obtain particles of metals
synthesize intermetallic nanomaterials have been developed so
far, each with its own benefits and drawbacks. The most
common preparation techniques are the co-reduction and
thermal annealing. Further preparation methods, like thermal
decomposition, arc discharge and reverse micelle method are
also used in special cases. Most of these synthesis methods
have disadvantages in either control of morphology, yield, or
conformity to green chemistry.
A very useful and facile method to obtain monometallic or
[4]
[11]
with lower redox potentials. Higher temperatures usually also
lead to a higher crystallinity of the particles. In general, the
reaction temperatures are limited by the boiling points of the
polyols. Higher polyols have higher boiling temperatures, but
[5]
[6,7]
[11]
lower reduction potentials. The low energy and time require-
ments to obtain intermetallic phases in a facile way make the
[8]
intermetallic particles is the polyol process. It uses a broad
spectrum of polyalcohols (polyols), which generally have a
[12]
polyol process attractive for industrial application.
In comparison to solvothermal syntheses in steel autoclaves,
the reaction progress in microwave-assisted synthesis, using
glass vessels, can be controlled in situ, and reaction parameters
can be adjusted accordingly. Additionally, the closed system
provides access to temperatures above the boiling point,
granting increased reduction power of the polyol. Reactions can
be performed within minutes, due to very fast heating rates
and rather homogeneous heat distribution inside the vessel
provided by stirring.
[a] M. Smuda, Prof. Dr. M. Ruck, Prof. Dr. T. Doert
Faculty of Chemistry and Food Chemistry
Technische Universität Dresden
01062 Dresden (Germany)
E-mail: thomas.doert@tu-dresden.de
b] C. Damm
[
Leibniz Institute for Solid State and Materials Research
Helmholtzstr. 20
01069 Dresden (Germany)
c] Prof. Dr. M. Ruck
[
Max Planck Institute for Chemical Physics of Solids
Nöthnizer Str 40
01187 Dresden (Germany)
With this setup, several intermetallic compounds have been
synthesized in quantitative yields. Heise et al. synthesized a
plethora of intermetallic nanoparticles using various element
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/open.202000236
[13–15]
combinations.
Even though the polyol process itself was
©
2020 The Authors. Published by Wiley-VCH GmbH. This is an open access
studied and developed intensively over the last 30 years, there
are still many details uncovered. Especially for intermetallic
particles, little is known about the reaction and formation
mechanism. Schaak et al. performed a number of syntheses of
article under the terms of the Creative Commons Attribution Non-Com-
mercial NoDerivs License, which permits use and distribution in any med-
ium, provided the original work is properly cited, the use is non-commercial
and no modifications or adaptations are made.
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[
16–18]
binary and ternary intermetallic compounds.
They system-
At around 180°C, a greyish-white X-ray amorphous powder
precipitates, which probably consist of a bismuth-glycolate
coordination polymer. Raising the reaction temperature to
210°C changes the color of the suspension to black, indicating
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atically studied the formation of selected particles to elucidate
the reaction mechanism and the influence of the solvents.
In the present study, the formation of bismuth-nickel
particles, especially due to the interesting superconducting
ferromagnetic behavior of micro- and nanostructured Bi Ni
has been investigated in ethylene glycol (EG) as a solvent.
Systematic experiments have been performed, changing reac-
tion times, temperatures, pH values and metal salts. The
products and intermediates were synthesized in a microwave
assisted polyol process, isolated and characterized by powder
X-ray diffraction (PXRD), scanning electron microscopy (SEM),
high resolution transmission electron microscopy (HR-TEM),
scanning electron microscopy (SEM), energy-dispersive X-ray
spectroscopy (EDS), high-pressure liquid chromatography
coupled with mass spectrometry, UV-Vis spectroscopy, and
Fourier transformed infrared spectroscopy (FT-IR).
[18,19]
3
+
0
the reduction of Bi
to Bi . According to powder X-ray
[13,18]
diffraction, the resulting spherical particles have the crystal
structure of the rhombohedral standard modification of
bismuth. SEM images reveal a broad size distribution with
diameter ranging from 50 nm to 250 nm. Size and shape of the
particles depend on the reaction time and temperature. With-
out the addition of surfactants, reaction times up to 2 h lead to
agglomeration of the particles. Considering the low melting
point of bismuth of 271°C, these reaction conditions may
facilitate intra-particle sintering and partial melting of the
3
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[
23]
particles, resulting in an almost perfect spherical shape.
2
.2. Reduction of Ni salts in EG
2
. Results and Discussion
In general, nickel acetate, is reducible in polyols between 200°C
[
24]
and 250°C depending on additional promoting agents.
2
.1. Reduction of Bi(NO ) in EG
Ni(OAc) slowly dissolves in EG resulting in a light green
3
3
2
solution. UV-Vis spectroscopy of the solution shows a typical
absorbance spectrum for octahedral complexes with a distinct
3
+
2+
Considering the standard potentials of Bi and Ni in aqueous
solutions (Equations 1 and 2), bismuth is generally more noble
and therefore its cation is easier to reduce. Although these
values are in a strict sense only valid for aqueous systems, it is
reasonable to assume, that the relative reduction behavior in
polyols is comparable.
3
3
band at 395 nm, attributed to the A (F)! T (P) transition,
2
g
1g
[21]
indicating a threefold complexation by EG, which is also in
[
22,25]
agreement with reported crystal structures.
bands at 662 nm and 727 nm have been assigned to the A
(F)! T (F) and A (F)! T (F) transitions respectively (Fig-
Additional
3
2g
3
3
3
1
g
2g
2g
ure S2†). We observed reduction of nickel acetate in EG only at
temperatures exceeding 250° C without addition of auxiliaries.
Nickel nitrate is easily soluble in EG yielding a light green
solution. Analogously to nickel acetate, the solution contains
octahedral complexes with identical UV-Vis bands. Additionally,
the spectrum shows a characteristic band at 303 nm, which can
3
þ
À
E� ¼ þ0:31 V
E� ¼ À 0:28 V
Bi þ 3e ! Bi
Ni þ 2eÀ ! Ni
(1)
(2)
2þ
Additionally, reactions containing nitrate anions are prone
to produce nitrous gases upon heating, which are then capable
of reacting with the organic and inorganic intermediates
[
26]
be attributed to the nitrate ion (Figure S2†). To identify the
predominant species in solution, HPLC-MS measurements were
performed. Due to an increased affinity of the eluent
acetonitrile towards nickel, some fractions, namely at 223 m/z,
indicate a partial ligand exchange EG vs. acetonitrile during
elution (Figure S3†). Nevertheless, the overall tendency clearly
leans toward mononuclear species in solution, being in agree-
ment with the postulated and experimentally determined
(Equation 3):
À
þ
À
E� ¼ þ0:957 V
NO3 þ 4H þ 3e ! NO þ 2H O
(3)
2
Bismuth nitrate easily dissolves in EG resulting in a clear
solution. As reported before, the centrosymmetric dinuclear
complex [Bi(C H O )(EG)(NO ) ] can be crystallized from such
solutions at ambient temperature, which means that EG is
deprotonated at ambient temperature without addition of a
base.
[22,27,28]
crystal structures.
that of nickel acetate, in that the necessary temperature to
The reduction behavior is different to
2
5
2
3 2 2
[22]
0
induce a quick reduction to Ni exceeds 280°C, which is in
agreement with the slow formation of intermetallic compounds
explained below.
To obtain information about the species present in solution,
HPLC-MS measurements of dilute solutions of Bi(NO ) in EG
Earlier studies reported, that NiCl ·6H O is not reduced in
3
3
2
2
were performed at room temperature. A distinct signal can be
observed for a dinuclear complex with two neutral EG ligands
and one deprotonated ligand. The primary species, however,
seems to be a mononuclear complex, with two deprotonated
EG ligands. Apparently, the nitrate anions react with EG to
EG unless strong reducing agents or NaOH were added to the
solution.
anions tend to bind directly to Ni . Antti et al. reported the
crystal structure of a compound that crystallized from a solution
of NiCl2 in EG, which contained the dinuclear complex
[Ni Cl (EG) ] with bridging chlorido and chelating EG ligands
2 2 4
in octahedral geometry as determined from UV-Vis spectra.
To determine the nickel-containing complexes in solution,
[29,30]
Due to their strong Lewis base character, chloride
2
+
2
+
produce HNO (Figure S1†), which is counterintuitive regarding
the acid constants.
3
[
31]
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HPLC-MS measurements were performed (Figure S4†). The
mass spectra show a clear signal for a species that has exactly
the mass of this dinuclear complex, which suggests
ation, the sole solid product obtained was bismuth according
to PXRD, confirming that the reduction power of EG is not high
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enough to reduce Ni to Ni (Figure S5†). At approx. 250°C,
the reaction slowly generated intermetallic phases (Figure 1).
After 30 min, subtle and broad reflections in the range 30°ꢀ
2
+
[
Ni Cl (EG) ]
to be also the dominant species in solution.
2
2
4
2
+
Apparently, the affinity of chloride towards Ni causes the
observed inertness against reduction by EG.
2θꢀ35° (CuÀ Kα ) emerge alongside the dominating reflections
1
of bismuth, which can be attributed to Bi Ni. After 45 min, the
3
reflections of the Bi Ni phase are clearly visible. Additional
3
2
.3. Reaction of Bi(NO ) with Ni(OAc)
2
distinct reflections, most obviously at 2θ=30.3°, are visible,
which can be assigned to BiNi, while the reflections of
elemental bismuth have completely vanished. The final product
after approx. 60 min is an almost phase-pure BiNi powder, with
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Upon heating a mixture of Bi(NO3)3 and Ni(OAc)2 in EG to
temperatures between 210°C and 240°C by microwave irradi-
minor Bi Ni impurities probably due to stoichiometric errors.
3
Evidently, no traces of elemental nickel can be found in all
samples.
To monitor the incorporation of nickel into the particles,
HR-TEM images were recorded. The images revealed a shell
around the spherical bismuth particles. Fixating the particles in
epoxy resin and slicing them with a focused ion beam,
uncovered the core-shell structure of the particles (Figure 2a),
which, to the best of our knowledge, is the first reported
spherical self-organized bismuth-nickel core-shell nanostruc-
ture. Previously reported core-shell structures were obtained by
[32]
electrodeposition in a matrix. The bismuth cores have a broad
size distribution with diameters between 30 nm and 250 nm,
whereas the nickel shell has a thickness between 50 nm and
2
50 nm. The shell consists of densely packed crystalline nano-
particles of 2 nm to 10 nm in diameter (Figure 2b). The
composition of these particles was investigated by EDS, SAED,
and STEM-mapping. EDS point analysis clearly identified nickel
in the shell around the bismuth core. EDS line-scans across the
particles exhibit an increased signal of nickel in the shell and a
drop of intensity in the central part, which we interpret as a full
encasement with Ni (Figure 2c). To determine the structure of
the shell SAED measurements were performed (Figure 2d). The
observed diffraction rings can clearly be assigned to the
diffraction pattern of elemental nickel, especially for the lattice
planes (111), (200), (220) and (311). Some non-indexed single
reflections can probably be attributed to Bi or the newly formed
Figure 1. PXRD patterns of solid reaction products obtained from the
reaction of Ni(OAc) and Bi(NO ) in EG at 250°C after different reaction
2
3
3
times. Bi (ref.), BiNi (ref.) and Bi
3
Ni (ref.) are the calculated powder diagrams
3
based on the crystal structure of Bi, BiNi and Bi Ni according to ICSD No.
6
4703, No. 410878 and No. 391336 respectively.
Bi Ni in the interphase region. Apparently, the nickel nano-
3
particles did not provide any reflections in the PXRD measure-
ments due to finite size effects. STEM mapping of sliced
particles further corroborated the core-shell structure showing
the clear distinction of a nickel shell and a bismuth core,
displaying no mixing between the individual structures (Fig-
ure S6†).
Analysis of the surface chemistry by FT-IR measurements
also revealed the existence of organic materials in the early
stages of the reaction (Figure 3). Absorption bands were
À 1
assigned to characteristic OH stretching at 3360 cm
to
À 1
À 1
À 1
3
200 cm , CH stretching at 2930 cm to 2860 cm and C-OH
À 1
À 1
deformation modes at 1300 cm to 1450 cm . In comparison
to pristine EG, the OH stretching band is slightly shifted towards
lower energies and significantly reduced in intensity, which
implies a partial deprotonation of the polyol and assumingly a
coordination to the particle surfaces. The absorption band
Figure 2. a) Images of Bi particle encased in a Ni nanoparticle shell (FIB slice)
after 10 min reaction time. b) HR-TEM image of the shell showing individual
crystalline particles. c) EDX line-scan indicating the elemental distribution of
Bi (green) and Ni (red) across a random core-shell particle. d) Representative
SAED pattern of the particle shell with Miller indices of Ni: red arrows
pointing towards non-indexed reflections (For the SAED area see Fig-
ure S7†).
À 1
À 1
between 1580 cm and 1640 cm , which does not occur with
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the reaction temperature to the melting point of bismuth
fosters the diffusion and formation of the intermetallic phases.
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2
.4. Verification of the Solid-State Diffusion Mechanism
To obtain additional information on the proposed diffusion
mechanism in the solid, the individual steps in the reaction
were emulated (Figure 4). In a first step, the reduction of
bismuth was separated from the process. Bismuth particles
were prepared (for details, see experimental section) and
immediately re-suspended in EG. After adding Ni(OAc)2 the
suspension was heated to 250°C for 60 min. This reaction
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yielded almost pure BiNi powder with small residues of Bi Ni.
3
This proves that simultaneous reduction of the two metals is
not essential and is in line with the idea of bismuth particles
acting as nucleation sites for nickel nanoparticles. As reaction
temperatures below 250°C yield no intermetallic compound a
Figure 3. FT-IR spectra of powders obtained from the reaction of Ni(OAc)
2
3 3
and Bi(NO ) in EG after different reaction times. The interval for the
absorption band of a COO group (stretching mode) is highlighted.
direct reaction of Ni(OAc) with Bi is unlikely, see above.
2
The solid-state reaction of Bi Ni with addition nickel to BiNi
3
was reproduced starting from freshly produced Bi Ni particles
3
pristine EG, can be attributed to COO stretching modes. These
bands can be caused by acetate anions of the starting materials
or by carboxylate anions resulting from the oxidation of EG. As
this band also occurs in the reaction without acetate anions in
the starting materials, oxidation products seem to be reason-
able. We assume, that carboxylate anions are attached to the
positively charged surface of the nickel particles and hold the
and Ni(OAc) . Despite the considerable difference between the
2
melting point of Bi Ni (467°C) and the applied reaction temper-
3
ature of 250°C, phase-pure BiNi was formed, confirming the
diffusion of in-situ formed nickel into the Bi Ni particles.
3
In order to assess the importance of direct contact between
bismuth and locally formed nickel particles, particles of both
elements were produced separately and then dispersed togeth-
er in EG. After 60 min at 250°C, the solid product was a mixture
[
33]
shell together.
1
The absorption bands located at around
À 1
315 cm are attributed to residues of nitrate anions. Towards
of bismuth and Bi Ni with small amounts of BiNi and nickel.
3
the end of the reaction, the characteristic bands of the organic
products almost disappear as the nickel shell slowly depletes
There are two aspects that make a difference. On the one hand,
nickel particles obtained from a standard polyol process have
diameters between 100 nm and 200 nm, which is 10 to 20 times
larger than the particles observed in the nickel shell of our
bismuth-nickel core-shell particles. The time to consume such
(Figure S8a–c†). The respective IR spectra only show a wide-
range absorption, which is typical for metallic samples.
2
+
Following these findings, we suggest, that, Ni is reduced
on the surface of previously formed bismuth particles, which
act as nucleation sites for heterogeneous reduction. According
to the nucleation theory, heterogeneous nucleation separates
nucleation from particle growth, resulting in more or less
[34]
monodisperse particles. Furthermore, the higher nucleation
rate increases the number of particles, which leads to a smaller
[35]
diameter of the final particles. Several TEM images corrobo-
rate this hypothesis, as only a few small nickel particles were
found separated from the bismuth particles. After particle
growth, a diffusion process begins at the interfaces. Since the
concentration of elemental nickel in the bismuth particle is low
at the beginning, the bismuth-rich phase Bi Ni is formed first, as
3
has been shown by PXRD measurements. As the reaction
proceeds, more nickel is incorporated, which leads to a mixture
of Bi Ni in the inner regions and BiNi in the outer region of the
3
particles. With respect to the observed sequence of product
formation, it is reasonable to assume, that a nickel flux into the
bismuth core is the predominant diffusion process. It might be
based on the lower atomic mass and the smaller atomic radius
Figure 4. PXRD patterns of products obtained after reacting a) bismuth
particles with Ni(OAc) , b) Bi Ni particles with Ni(OAc) and c) bismuth
2 3 2
particles with nickel particles in EG at 250°C for 1 h. Bi (ref.), BiNi (ref.) and
[36]
of nickel (1.38 Å) compared to bismuth (1.70 Å). We also do
not observe hollow product particles that would be expected if
bismuth was the mobile partner. Nonetheless, the vicinity of
Bi Ni (ref.) are the calculated powder diagrams based on the crystal structure
3
of Bi, BiNi and Bi Ni according to ICSD No. 64703, No. 410878 and No.
3
391336 respectively.
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large nickel particles is much longer. On the other hand,
effective collisions must occur between the two types of freely
dispersed particles to allow reaction. Additionally, the more
nickel is used up, the longer it takes for a free nickel particle to
successfully collide with a suitable reaction partner. Hence, for a
fast and quantitative formation of the intermetallic phases, the
heterogeneous reduction of nickel on the surface of bismuth
particles is crucial.
formation of small particles is favored in the presence of
hydroxide ions, which increases the diffusion rate in subsequent
1
2
3
4
5
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7
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[24,39]
reactions.
Hence, a high pH improves the reaction kinetics,
leading to shorter reaction times.
The reaction of Bi(NO ) with Ni(OAc) using NaOH to tune
3
3
2
the pH value was monitored by PXRD measurements of the
intermediate products (Figure 6). Overall, the reaction follows
the same scheme as under neutral conditions. The first
observable solid phase is elemental Bi. In contrast to the
reaction without base, the intermetallic compound Bi Ni
3
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5
[
40]
emerges quicker, confirming promoted nickel reduction. The
retention time of newly formed Bi Ni is noticeably long.
2
.5. Effects of Temperature and pH in the Reaction of Bi(NO3)3
and Ni(OAc)2
3
Seemingly, the faster production of nickel leads to a quicker
Diffusion can be actively promoted by raising the temperature
of the system leading to an increased mobility in the solid,
which facilitates the incorporation of nickel in the bismuth
onset of Bi Ni formation, but the diffusion rate in the solid is
3
unchanged, making it the rate limiting factor. Nonetheless, the
final phase BiNi is achieved faster than in neutral media (45 min
vs. 60 min), which confirms the promoting effect of the added
base. One can conclude that for the reaction of bismuth nitrate
with nickel acetate, the temperature has a higher impact on the
reaction kinetics than the addition of NaOH.
[37]
core. Hence, the reaction temperature was raised to 260°C
and 270°C respectively, which accelerated the reaction im-
mensely (Figure 5). At 260°C, the BiNi phase is almost
completely formed within 30 min, yet residues of Bi Ni are
3
detectable. Raising the reaction temperature to 270°C yields
phase-pure BiNi powder in probably less than 30 min. In
addition to the accelerated diffusion, elevated temperatures
increase the reduction power of EG, which leads to higher
nucleation rates. As mentioned before, higher nucleation rates
produce smaller particles, which directly affects the time for
particle to be consumed in the follow-up diffusion-controlled
reaction.
The influence of the basicity of the solution is also seen if
Bi(OAc) and Ni(OAc) are reacted in EG. This reaction generally
3
2
shows a slightly shorter reaction time compared to the Bi(NO ) /
3
3
Ni(OAc) mixture due to the increased number of acetate anions
2
in solution. FT-IR measurements display a full deprotonation of
EG, which leads to a higher affinity towards nickel and results in
a higher reduction power. (Figure S9 and S10†)
The pH has a substantial effect on the reduction power of
the polyol. A base added to the reaction solution is able to
deprotonate the alcohol group to generate alkoxides, which
have a higher electron density and a higher affinity to
coordinate the metal ions than the alcohol. Theoretical studies
postulated that hydroxide anions further deprotonate the α-
carbon of the alkoxide ligand, which leaves two electrons for
2.6. Reaction of Bi(NO ) with Ni(NO )
3 2
3
3
To compare the reaction behavior of different nickel salts and
to investigate the impact of the anion on the reaction kinetics,
we also analyzed the reaction of Bi(NO ) with Ni(NO ) instead
3
3
3 2
of Ni(OAc) The overall reaction conditions remained the same.
2
.
[38]
the reduction of the metal cation. It was observed that the
Figure 5. PXRD patterns of solid reaction products obtained from the
Figure 6. PXRD patterns of solid reaction products obtained from the
reaction of Ni(OAc) and Bi(NO ) in EG at 250°C in alkaline solution after
3
different reaction times. Bi (ref.), BiNi (ref.) and Bi Ni (ref.) are the calculated
reaction of Ni(OAc) and Bi(NO ) in EG after 30 min at higher reaction
2
3
3
2
3 3
temperatures. BiNi (ref.) and Bi
3
Ni (ref.) are the calculated diagrams based on
the crystal structure of BiNi and Bi
91336 respectively.
3
Ni according to ICSD No. 410878 and No.
3
diagrams based on the crystal structure of Bi, BiNi and Bi Ni according to
3
ICSD No. 64703, No. 410878 and No. 391336 respectively.
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The obtained intermediates were collected and analyzed by
PXRD measurements (Figure 7).
almost undetectable for all collected intermediates at 250°C.
The residual OH stretching band may emerge from left over EG
or absorbed water on the surface. Additionally, an intensive
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
The progress of the reaction is similar to the reaction of
Bi(NO3)3 with Ni(OAc)2 although with strongly decreased
kinetics. The reducibility of nickel cations in the pure nitrate
system is lowered in comparison to a system containing also
acetate as mentioned before due to the higher reduction
temperature of Ni(NO ) . These findings corroborate our recent
À 1
COO stretching band at 1575 cm occurs, which indicates an
oxidation of EG. Taking both observations into consideration, a
complete oxidation of EG to oxalate is very likely due to a
secondary redox reaction between the second alcohol group
and the nitrate anion, which additionally leads to the formation
of nitrous gases À indicated by the brownish color of the
solution À according to Equation 3. After 60 min, almost no IR
3
3
results on mono-metallic systems, showing a dependency of
the reducibility on the counter anion. A detailed description of
this reaction can be found in the SI.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
À 1
bands are visible any more in range between 4000 cm and
À 1
Analysis of the surface chemistry by FT-IR measurements
revealed major differences in oxidation products compared to
the reaction with Ni(OAc)2 (Figure 8). Strikingly, the OH
1200 cm indicating that no oxidation products are bond to
the surface of the intermediates. The final product exhibits a
broad absorption resulting from the metallic character of the
BiNi particles. Seemingly, no organic molecules are attached to
the particles after the reaction has finished.
À 1
À 1
stretching band at 3400 cm to 3200 cm and the otherwise
À 1
À 1
concise CH stretching bands at 2930 cm to 2860 cm are
2
.7. Effect of Temperature and pH in the Reaction of Ni(NO3)3
and Bi(NO3)3
Using a temperature of 260°C instead of 250°C resulted in a
significant shortening of the reaction time due to increased
reduction and diffusion rates (Figure 9). Yet, the reaction
proceeds noticeably slower than reactions with Ni(OAc) , again
2
confirming the dependency of the reduction rate on the anion
type. Alkaline conditions have the same effects as in reactions
with Ni(OAc) , as after 60 min virtually pure BiNi is obtained
2
(Figure 10). Considering the reaction times to achieve a full
conversion, the alkaline conditions have a slightly stronger
effect on the nitrate system than elevated temperatures.
Figure 7. PXRD patterns of the solid reaction products obtained from the
reaction of Ni(NO
times. Bi (ref.), BiNi (ref.) and Bi
the crystal structure of Bi, BiNi and Bi
3
)
3
and Bi(NO
3
)
3
in EG at 250°C after different reaction
3
Ni (ref.) are the calculated diagrams based on
3
Ni according to ICSD No. 64703, No.
4
10878 and No. 391336 respectively.
Figure 9. PXRD patterns of the solid reaction products obtained from the
reaction of Ni(NO ) and Bi(NO ) after different reaction times at 260°C. BiNi
3
3
3 3
Figure 8. FT-IR spectra of powders obtained from the reaction of Ni(NO
and Bi(NO in EG after different reaction times. The interval for the
absorption band of a COO group (stretching mode) is highlighted.
3
)
3
3
(ref.) and Bi Ni (ref.) are the calculated diagrams based on the crystal
3
)
3
structure of BiNi and Bi
respectively.
3
Ni according to ICSD No. 410878 and No. 391336
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on it (Figure S11b†). PXRD patterns of the isolated powder after
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
2
20°C display strong reflections of elemental bismuth and
some weak BiOCl reflections (Figure 11).
The pH seemingly plays an important role in the reduction
3
+
process and therefore NaOH was added in a molar ratio of Bi
:
2
+
À
Ni : OH =1:1:2. Again, BiOCl formed as an intermediate,
which was then partly reduced to bismuth, but no intermetallic
phase was obtained at 250°C. Increasing the NaOH concen-
tration to 1:1:8 resulted in a mixture of Bi Ni and BiNi after
3
6
0 min at 250°C (Figure 11). This suggests that substantial
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
deprotonation of EG is needed for the reduction of the stable
2
+
[
Ni Cl (EG) ] complex.
2 2 4
In conclusion, we propose the following reaction mecha-
nism:
2
NiCl þ 4EG ! ½Ni Cl ðEGÞ ꢁCl
2
2
2
4
2
(4)
(5)
Figure 10. PXRD patterns of the solid reaction products obtained from the
reaction of Ni(NO
3
)
3
and Bi(NO
3
)
3
in EG at 250°C after different reaction times
½
Ni Cl ðEGÞ ꢁðNO Þ þ 2EG þ 2NaOH !
2
2
4
3 2
3
in alkaline solution. Bi (ref.), BiNi (ref.) and Bi Ni (ref.) are the calculated
diagrams based on the crystal structure of Bi, BiNi and Bi
3
Ni according to
2½NiðC H O ÞðEGÞ ꢁNO þ 2NaCl þ 2H O
2
5
2
2
3
2
ICSD No. 64703, No. 410878 and No. 391336 respectively.
2BiðNO Þ þ 3½Ni Cl ðEGÞ ꢁCl þ 4NaOH !
3
3
2
2
4
2
(
6)
2
BiOCl þ 3½Ni Cl ðEGÞ ꢁðNO Þ þ 4NaCl þ 2H O
2
2
4
3
2
2
2
.8. Reaction of Bi(NO ) with NiCl and NiBr
3 3 2 2
Dissolving NiCl in EG leads to the dinuclear complex
2
2
+
As expected, reacting Bi(NO ) and NiCl in EG at 250°C yielded
[Ni Cl (EG) ] (Equation 4). Due to alkaline conditions, EG is
2 2 4
3
3
2
no intermetallic particles but only elemental bismuth. Interest-
deprotonated generating the alkoxide, which breaks the
3
+
2+
ingly, prior to the reduction of Bi a brown solid precipitated.
The precipitate was identified as BiOCl by PXRD (Figure 11),
which crystallized in thin platelets of different size in the micron
and submicron range with smooth surfaces (Figure S11a†). The
brown color of the precipitate (BiOCl is colorless) is probably
caused by finely dispersed nanoparticles of bismuth. The
reduction of BiOCl proceeds at 220°C and occurs directly at the
surface of the platelets, which can be seen by the cleavage and
roughening of the surface and the formation of small spheres
dinuclear complex and abstracts chloride from Ni
(Equa-
tion 5). Upon heating to approximately 180°C, BiOCl precip-
itates (Equation 6). Further heating induces the reduction of the
+
mononuclear [Ni(C H O )(EG) ] complex, resulting in elemental
2
5
2
2
nickel and nitrous gases, as described in Equation 3.
NiBr shows a similar reaction behavior as NiCl , however,
2
2
the addition of NaOH is not a necessity in this case. This can be
rationalized by the weaker nucleophilicity of bromide compared
to chloride, which leads to the formation of a homoleptic nickel
complex with only EG ligands. A crystalline precipitate was
obtained by dissolving NiBr in EG at 70°C and subsequent slow
2
cooling to room temperature. The monoclinic crystals consist of
2
+
a mononuclear species [Ni(EG) ]Br , in which Ni is six-fold
3
2
coordinated by three bidentate EG ligands without any bridging
halide atoms (Figure 12), corroborating the hypothesis formu-
lated in Equation 6.
Upon heating the reaction mixture of NiBr and Bi(NO ) in
2
3 3
EG, BiOBr precipitates at around 180°C. Increasing the reaction
Figure 11. PXRD patterns of solid products obtained from the reaction of
NiCl and Bi(NO ) in EG at various temperatures and NaOH concentrations.
2 3 3
Reactions with declared NaOH concentrations were performed at 250°C for
1
h. Bi (ref.), BiOCl (ref.), BiNi (ref.) and Bi Ni (ref.) are the calculated diagrams
3
2
+
based on the crystal structure of Bi, BiOCl, BiNi and Bi
3
Ni according to ICSD
Figure 12. Molecular structure of the complex [Ni(EG)
3
] . Hydrogen atoms
No. 64703, No. 195115 No. 410878 and No. 391336 respectively.
are omitted for clarity. Ellipsoids comprise 90% of the probability density.
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temperature to 250°C for 30 minutes yielded a mixture of Bi Ni
findings, nickel nitrate is harder to be reduced than nickel
3
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
[41]
and Bi (Figure 13). In the absence of Bi(NO ) , NiBr cannot be
acetate,
but the overall reaction mechanism seems un-
3
3
2
reduced at this temperature. To test whether this effect has to
be attributed to bismuth or nitrate, the previously obtained
BiOBr was used as a bismuth source, with otherwise unchanged
reaction conditions. After 60 min of reaction, the PXRD revealed
reflections of bismuth only, without any traces of elemental
nickel or nickel containing compounds (Figure S12†). Obviously,
the presence of nitrate is essential for the reduction of NiBr2.
changed. Nickel halides, however, introduce a different reaction
mechanism, as they need a suitable anion at the bismuth cation
to perform an anion exchange. The exchange leads to the
formation of bismuth oxide halides and easily reducible nickel
species. The subsequent phase formation then follows the
previous reaction pattern. Nickel chloride poses a special case
due to the formation of a dinuclear complex species in solution.
The reaction needs a very high concentration of NaOH to
produce enough alkoxides in order to break the bridging bonds
in this complex. In general, the reducibility can be ordered as
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
3. Conclusions
À
À
À
À
AcO >NO >Br >Cl .
3
Systematic experiments were carried out to study the reaction
mechanism of the formation of intermetallic phases in the
binary system bismuth-nickel and the dependency on the nickel Experimental Section
precursor, the temperature, pH value and anion type. The
process involves consecutive reduction of bismuth and nickel
cations. Herein, the nickel cations are most likely reduced on
the surface of the bismuth particles, forming a hitherto
unknown self-organized spherical core-shell structure, which is
linked by the oxidation products of EG. Thereupon, a diffusion
process is responsible for the generation of the intermetallic
Chemicals
Ethylene glycol (Fluka 99.5%) was used as received. Hydrated nickel
(
II) and bismuth(III) salts were used for all syntheses: Ni(OAc) ·4H O
2 2
(Merck, 99%), Ni(NO ) ·6H O (Riedel-de Haën, 98%), NiCl ·6H O
3
2
2
2
2
(Grüssing, 98%). NiBr
·3H
O (Alfa Aesar, 98%), Bi(NO ) ·5H O (VWR,
2
2
3 3 2
9
8%). For some experiments, solutions of NaOH (Grüssing, 99%) in
À 1
EG with a concentration of 1 moll was used. For FIB lamella
preparation G1 epoxy resin (Gatan) and carbon nanoparticles
phases. The bismuth-rich phase Bi Ni is formed first, as nickel
3
diffuses into the bismuth core. After prolonged reaction times,
the final BiNi phase is achieved and the nickel shell is lost. In
the case of nickel acetate, EG is oxidized to acetate, whereas
nickel nitrate induces a secondary redox reaction, where nitrate
is reduced to NO, while oxidizing the pre-formed acetate to an
oxalate. The overall kinetics are significantly promoted by either
increasing the reaction temperature or the pH value. Increasing
the temperature leads to a slightly increased reduction rate and
a high increase in diffusion rate. Under alkaline conditions, the
reduction rate is significantly increased, leading to faster onset
of intermetallic phase formation. In accordance with previous
(Vulcan XC 72, 25–75 nm) were used.
Synthesis
All experiments were performed in a microwave-assisted polyol
process in a CEM Discover SP synthesis and were carried out in
35 mL Pyrex® vessels equipped with magnetic stirring bars ensuring
proper mixing of the reagents. All reaction mixtures were heated to
the desired temperature within 4 min by using microwave
radiation. For each synthesis, the respective bismuth and nickel
compounds (0.5 mmol) were mixed with 10 ml of EG. In case of
À 1
addition of 1 moll NaOH solution, less EG was added to keep the
reaction volume of 10 ml. Bismuth particles were obtained by
dissolving 1 mmol of Bi(NO ) ·5H O in 10 ml of EG. The mixture was
3
3
2
heated to 220°C for 10 min. Nickel particles were obtained by
dissolving 1 mmol of Ni(OAc) ·4H O in 10 ml of EG. The mixture
2
2
was heated to 250
mixing 1.5 mmol of Bi(NO ) ·5H O and 0.5 mmol of Ni(OAc) ·4H O
in 9 ml of EG and 1 ml of 1 moll NaOH solution. The mixture was
heated to 250°C for 30 min. After the adjusted reaction time, the
vessel was cooled with a stream of air to 60°C inside the microwave
device and then removed. The black powdery products were
isolated by centrifugation at 4400 rpm, washed several times with
°
C for 10 min. Bi Ni particles were obtained by
3
3 3
2
À 1
2
2
5
–10 mL of ethanol or acetone to remove residual solvent, and
dried overnight under vacuum at room temperature.
Preparation of [Ni(EG )]Br Crystals
3
2
The crystals were prepared by dissolving NiBr ·3H O (1.9 g,
2
2
7
mmol) in 2 ml of EG at 70°C. Afterwards, the dark green solution
was slowly cooled down to room temperature and stored at 4°C.
After one weak plate-like crystals were visible.
Figure 13. PXRD patterns of solid products obtained from the reaction of
NiBr and Bi(NO ) in EG after 30 min at different reaction temperatures. Bi
2
3 3
(
ref.), BiOBr (ref.) and Bi
3
Ni (ref.) are the calculated diagrams based on the
Ni according to ICSD No. 64703, No.
1225, No. 410878 and No. 391336 respectively.
crystal structure of Bi, BiOBr and Bi
6
3
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Preparation of FIB Lamellae
was financially supported by the Deutsche Forschungsgemein-
schaft (project-id 413437826).
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
The epoxy resin was mixed with the carbon nanoparticles and the
reaction products until a pulpy mixture was obtained, which then
was hardened overnight at 80°C at ambient pressure conditions.
The lamellae were achieved with a FIB Helios NanoLab 600i (FEI) Conflict of Interest
using gallium ions at a voltage of max. 30 kV. Afterwards, the
lamellae were transferred onto copper grids and fixed by platinum
ion deposition before they were transferred into the HR-TEM.
The authors declare no conflict of interest.
Keywords: intermetallic phases
·
nanoparticles
·
polyol
Powder Characterization
processes · microwave assisted synthesis · reduction reactions
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
PXRD patterns were recorded at 296(1) K with an X’Pert Pro MPD
diffractometer (PANalytical) equipped with
monochromator by using Cu-Kα1 radiation (λ =1.54056 Å). The
powder diagrams of Bi (deposition number CSD-64703), Bi N (CSD-
a curved Ge(111)
3
[
[
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3
91336), BiNi (CSD-410878), BiOCl (CSD-195115) and BiOBr (CSD-
61225) used as references have been calculated from single crystal
[
42]
structure data deposited in the ICSD Fourier-transform infrared
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filament was used. Ultraviolet-visible light spectroscopy was
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260 Infinity II LC system (Agilent Technologies) coupled with a
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a scan rate of 1 s .
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3
2
[
[
19] A. J. Biacchi, R. E. Schaak, ACS Nano 2011, 5, 8089–8099.
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ChemistryOpen 2020, 9, 1085–1094
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© 2020 The Authors. Published by Wiley-VCH GmbH