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Nanomaterials 2020, 10, 632; doi:10.3390/nano10040632 www.mdpi.com/journal/nanomaterials
Article
Ultrasound-Assisted Hydrazine Reduction Method
for the Preparation of Nickel Nanoparticles,
Physicochemical Characterization and Catalytic
Application in Suzuki-Miyaura Cross-Coupling
Reaction
Adél Anna Ádám
1,2
, Márton Szabados
1,2
, Gábor Varga
1,2
, Ádám Papp
2
, Katalin Musza
1,2
, Zoltán
Kónya
3,4
, Ákos Kukovecz
3
, Pál Sipos
2,5
and István Pálinkó
1,2,
*
1
Department of Organic Chemistry, University of Szeged, Dóm tér 8, H-6720 Szeged, Hungary; lee-
daa@hotmail.com (A.A.A.); szabados12m@gmail.com (M.S.); gaborvarga1988@gmail.com (G.V.);
musza.katalin@chem.u-szeged.hu (K.M.)
2
Material and Solution Structure Research Group, and Interdisciplinary Excellence Centre, Institute of
Chemistry, University of Szeged, Aradi Vértanúk tere 1, H-6720 Szeged, Hungary; papy97@gmail.com
(A.P.); sipos@chem.u-szeged.hu (P.S.)
3
Department of Applied and Environmental Chemistry, University of Szeged, Rerrich B. tér 1, H-6720
Szeged, Hungary; konya@chem.u-szeged.hu (Z.K.); kakos@chem.u-szeged.hu (A.K.)
4
MTA-SZTE Reaction Kinetics and Surface Chemistry Research Group, Rerrich B tér 1, H-6720 Szeged,
Hungary
5
Department of Inorganic and Analytical Chemistry, University of Szeged, Dóm tér 7, H-6720 Szeged,
Hungary
* Correspondence: palinko@chem.u-szeged.hu
Received: 08 March 2020; Accepted: 26 March 2020; Published: 28 March 2020
The work is dedicated to Professor Grzegorz Mlostoń on the occasion of his 70th birthday.
Abstract: In the experimental work leading to this contribution, the parameters of the ultrasound
treatment (temperature, output power, emission periodicity) were varied to learn about the effects
of the sonication on the crystallization of Ni nanoparticles during the hydrazine reduction
technique. The solids were studied in detail by X-ray diffractometry, dynamic light scattering,
thermogravimetry, specific surface area, pore size analysis, temperature-programmed CO
2
/NH
3
desorption and scanning electron microscopy. It was found that the thermal behaviour, specific
surface area, total pore volume and the acid-base character of the solids were mainly determined by
the amount of the nickel hydroxide residues. The highest total acidity was recorded over the solid
under low-power (30 W) continuous ultrasonic treatment. The catalytic behaviour of the
nanoparticles was tested in a Suzuki-Miyaura cross-coupling reaction over five samples prepared
in the conventional as well as the ultrasonic ways. The ultrasonically prepared catalysts usually
performed better, and the highest catalytic activity was measured over the nanoparticles prepared
under low-power (30 W) continuous sonication.
Keywords: nickel nanoparticles; sonocrystallization; structural and morphology characterization;
acid-base properties; activities in the Suzuki-Miyaura cross-coupling reaction
Nanomaterials 2020, 10, 632 2 of 18
1. Introduction
Nanoparticles (NPs) are commonly defined as solid materials with at least one dimension in the
1–100 nanometer size range including even polycrystalline systems with nano-sized crystallites [1].
Nanoparticles exhibit several interesting size-dependent properties, which are different from their
bulk counterparts, such as large surface to volume ratio [2], optical [3,4], magnetic [5,6] and electronic
attributes [7]. The dimensions of the nanoparticles are comparable to those of biological moieties like
viruses (20–50 nm), proteins (5–50 nm) or genes (2 nm wide and 10–100 nm long) revealing the
potential to tag or address these units with NPs. Moreover, some nanoparticles are ferromagnetic,
and they can be easily led by an external magnetic field, therefore, their utilization in cancer research
or in healing processes as targeted drug delivery system is a research field in the lime-light [8,9]. Their
special and tuneable physicochemical properties are connected to wide range of industrial uses (e.g.,
nano-carbon: fillers and black pigment, titania nanoparticles: UV protection, whitening pigment and
solar cell) [10]. The relatively easy control of shape and size of nanoparticles, the enhanced specific
surface area compared to the bulk-phase variants make possible to rationally design the materials for
catalytic applications [1,11].
As economically friendly alternatives of Pd and Pt, Ni nanoparticles have received significant
attention in the field of the catalysis in the last decades. They are frequently utilized in gas-phase
reactions [12−14], e.g., Vargas et al. used m-ZrO2 supported Ni nanoparticles as catalyst in CO2
methanation, and the experimental results showed excellent catalytic activity and selectivity to CH4
formation [12]. There are also several examples for their application in the liquid phase: the Suzuki
cross-coupling [15] and the Ullmann coupling [16] reactions are generally used as probe reactions for
Ni NPs.
It is clear from previous studies that the controlled synthesis of monodisperse nanoparticles is
extremely important for the diverse applications. Fundamentally, there are two different ways for
the preparation of nanocrystals: (i) one is the top-down approach, which mainly applies physical
methods to reduce the size of the particles from the bulk phase to the nano-range using mechanical
energy to prepare nanoparticles via processes connected to deformation, defragmentation, cold
welding or even recrystallization [17−20], (ii) the other is the bottom-up approach, which involves
methods usually performing better as far as the synthesis of size- and shape-control of the
nanoparticles are concerned.
In 1857, Faraday reported a preparation way for the colloidal gold sol via the reduction of
HAuCl4 by phosphorous reagent [21]. For chemical reduction, several reducing agents such as
hydrazine [22,23], sodium borohydride [24] and alcohols [25] can be easily and efficiently applied.
Sol-gel method is a generally used technique to synthesise nanoparticles with desired size and
stoichiometric control [26]. Another commonly applied pathway is thermal decomposition, where
the solution of the mixed precursors and additives (like surfactants) is heated, and ultimately, the
solid residue is calcined and nanoparticles are formed [27]. Finally, it is worthwhile to mention an
alternative and green biological synthetic method, where the reducing agents are extracted from
plants, for instance from the leaf of Aloe vera [28], Cinnamomum camphora [29] or Coriandrum sativum
[30].
Nowadays, much attention is paid to sonochemistry due to the unique effects of the extreme
reaction conditions (high temperature and pressure) occurring inside the continuously generated and
collapsed acoustic cavities. These voids can efficiently accumulate the energy of ultrasound waves
and transform into mechanical, thermal, compressional and even light forms. In addition, part of the
sonication energy results in the formation of reactive H2 and O2 gases, H2O2 side-product and
hydroxyl radicals in water and aqueous solutions [31]. Similarly to the mechanochemical processes,
the transient and inhomogeneously distributed hot spots with unusually fast heating and cooling
rates (>109 K/s) can induce unique transformations and the formation of crystal defects not seen
otherwise [32,33]. The utility of sonochemical synthesis as a synthetic tool resides in its versatility. By
varying the reaction conditions, various forms of nanostructured materials can be prepared,
including metals [34–37], bimetals [38,39], oxides [40,41], sulphides [42] and nanostructured
supported [43,44] catalysts. To the best of our knowledge, in the literature only few examples for the
Nanomaterials 2020, 10, 632 3 of 18
synthesis of nickel nanoparticles via ultrasound treatment can be found. They all apply surfactants
to avoid the formation of large aggregates and to tailor the morphology of the obtained particles.
Vargas et al. synthesized nickel nanoparticles with ultrasound assistance from nickel chloride by
chemical reduction investigating the effect of the quality of the reducing agents in detail [12]. In
another work, the nickel nanoparticles were sonically prepared on polyester fabric. Field emission
scanning electron microscopy observation indicated hedgehog-like nickel particles [45]. The
formation of similar aristate spherical Ni NPs were also observed, and the influence of the sonication
time was extensively scrutinized regarding the average grain size of the crystals. The authors
reported about size growth from 5 nm to 65 nm as the ultrasonic-assisted reduction time increased
from 10 min to 150 min [46].
Interestingly, the effect of the ultrasound treatment parameters on the generation of Ni NPs like
the temperature of the applied medium, the intensity and pulse character of ultrasound emission
remained out of scope. Hence, our aim was to map the influence of the ultrasound treatments on the
formation of nickel nanoparticles supplementing the hydrazine reduction method in the absence of
surfactant, which are generally the source of the inconveniences for the application of NPs owing to
their complicated removal from the outer surface. The sonochemically obtained nanoparticles were
compared to those prepared in the conventional way, mechanically or without stirring, in order to
reveal the direct effect of the ultrasound waves. The NPs were characterized using a wide-range of
instrumental techniques, and their catalytic activities were tested in a Suzuki-Miyaura cross-coupling
reaction.
2. Materials and Methods
2.1. Materials
Anhydrous nickel iodide (NiI2), anhydrous potassium hydroxide (KOH) pellets, phenylboronic
acid, biphenyl, N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), toluene and absolute
ethanol were purchased from VWR International (EU). Iodobenzene, potassium carbonate (K2CO3)
were acquired from Sigma-Aldrich company (Budapest, Hungary) and the hydrazine monohydrate
(N2H4 × H2O) (98+%) was ordered form Alfa Aesar (Haverhill, MA, USA). All chemicals were of 99+%
purity and no further purification was required.
2.2. The Ways of Ni Nanoparticle Syntheses
The NPs were synthesized via the hydrazine reduction method assisted by ultrasound treatment
as well. As the first step, the anhydrous nickel iodide (0.17 g) was dissolved directly in absolute
ethanol (4 cm3) while, in another vessel, the mixture of the potassium hydroxide (0.31 g) and
hydrazine monohydrate (0.35 g) were prepared in 1 cm3 of absolute ethanol. The nickel iodide
solution and the base/reducing solution were mixed, placed into glass centrifuge tubes (5 cm3) and
irradiated with ultrasound under ambient atmosphere and pressure at varying temperatures and
time durations. For comparison, samples were prepared with mechanical stirring (magnetic stirrer at
1000 rpm and power of 20 W) or without stirring. The sonication was performed with a Hielscher
ultrasonic homogenizer (UP200Ht, 26 kHz, max. 200 W) (Hielscher Ultrasonics GmbH, Teltow,
Germany) in a thermostated double-walled glass vessel. In this set-up, the target temperatures could
be maintained within ±2.0 °C. The applied output power and the emission periodicity of the
ultrasound application were altered within the range of 30–120 W and 20–100%, respectively. At
100%, the ultrasonic homogenizer worked continuously, otherwise, the sonication time was a fraction
of 100%. For instance, at 20%, the sonication period was one fifth of the active time, and the apparatus
was shut off in four-fifth of the pulse by using the pulse on/off mode. The sonotrode of the device
was 14 mm in diameter, and the average immersion depth was 25–30 mm. The four tubes were fully
dipped into the inner volume of the thermostated glass vessel filled with distilled water, and the
tubes were placed 20 mm away from the sonotrode. In this system, indirect ultrasonic treatment was
performed; the water medium (in the glass vessel) transferred the ultrasound waves from the
sonotrode to the centrifuge tubes. The power calibration of the ultrasonic set-up was performed by
Nanomaterials 2020, 10, 632 4 of 18
the generally used calorimetric method (Table 1) in order to follow and compare the energy
dissipation of the various modes of sonication (the inner volume of the glass vessel was 550 cm3 and
the time profiles in function of time were recorded for 30 min of ultrasound treatment) [47,48]. At the
end of the syntheses, the nanoparticles were collected on 0.22 micron filters (Sigma-Aldrich,
Budapest, Hungary), and were washed with distilled water and ethanol, dried (at 60 °C) and stored
under N2 atmosphere.
Table 1. Acoustic energy, particle and dispersion dimensions of the nickel nanoparticles prepared at
25 °C.
Ni NPs samples
Ultrasound
power density
(W/cm3)a
Primary
particle
size (nm)
Predominant
solvodynamic
diameter (nm)
Poly-
dispersity
index
Type of size
distribution
non-stirred − 12 1262 0.147 Unimodal
mechanically
stirred − 14 202 0.368 Bimodal
ultrasonic treatment:
30 W – 20%b 0.007 7 255 0.277 Unimodal
30 W – 40% 0.01 7 396 0.199 Unimodal
30 W – 60% 0.013 7 342 0.288 Unimodal
30 W – 80% 0.017 8 396 0.230 Unimodal
30 W – 100% 0.023 8 712 0.170 Unimodal
60 W – 100% 0.041 8 825 0.198 Unimodal
90 W – 100% 0.057 9 530 0.226 Unimodal
120 W – 100% 0.085 10 190 0.404 Bimodal
a Calculated to 5 cm3 sample volume.
b The percentages mean the emission periodicity of the sonication, for instance, at 40%, the device
was inactive in the three fifth of the pulse and the sonication period was two fifth of the active time,
while at 100%, the ultrasonic homogenizer operated continuously.
2.3. Procedure of the Suzuki-Miyaura Cross-Coupling Reactions
The catalytic test reactions were carried out in a 10 cm3 glass reactor immersed into a preheated
oil bath. Iodobenzene (1.0 mmol), phenylboronic acid (1.2 mmol), potassium carbonate (2.5 mmol)
and Ni NPs (0.15 mmol) were mixed in 4 cm3 of various solvents (DMF, DMSO, toluene), and 1 cm3
of distilled water was added to increase the solubility of the K2CO3 base. The reaction was conducted
under reflux, and the reaction time was varied from 1 to 24 h. The reaction was followed primarily
by thin-layer chromatography (hexane:ethylacetate 19:1), and at the end of the reaction, the solid
catalysts were filtered, and the clear liquids were analysed on a Hewlett-Packard 5890 gas
chromatograph (Hewlett-Paclard, Budapest, Hungary) equipped with 50 m × 0.2 mm × 0.33 µm HP-
FFAP (nitroterephthalic acid modified polyethylene glycol) column and flame ionization detector.
The chromatographic peaks were identified using commercial calibration standards.
2.4. Structural Characterization by Instrumental Methods
The powder X-ray diffractograms (XRD) in the θ = 4–80º range were recorded on a Rigaku
Miniflex II instrument (Rigaku, Tokyo, Japan) with 4°/min scan speed using CuKα (λ = 1.5418 Å)
radiation. The assignation of the reflections in the normalized diffractograms were done with the
help of the Joint Commiee of Powder Diffraction Standards−International Centre for Diffraction
Data (JCPDS-ICDD) database. The primary particle sizes of the NPs were estimated from the most
intense 111 reflections using the Scherrer equation with 0.9 shape factor.
The Fourier-transform infrared (FT-IR) spectra were recorded on a JASCO FT/IR-4700
spectrophotometer (Jasco, Easton, MD, USA) with 4 cm−1 resolution accumulating 256 scans. The
Nanomaterials 2020, 10, 632 5 of 18
spectrometer was equipped with a DTGS detector and a ZnSe ATR accessory. On the normalized
curves, the structural features of the solids were evaluated in the 4000–500 cm−1 wavenumber range.
A Malvern NanoZS dynamic light scattering (DLS) instrument (Malvern Pananalytical, Marvel,
UK) operating with a 4 mW helium-neon laser light source (λ = 633 nm) was used to map the
heterogeneity in the solvodynamic diameters of the aggregated nanoparticles at room temperature.
The measurements were performed in back-scattering mode at 173°, and the nanoparticles were
predispersed in ethylene glycol for 1 h using ultrasonic treatment. The concentration of the light-grey
semi-transparent dispersion was 0.1 mg/cm3.
In order to gain information about the thermal behaviour of the nanoparticles between 25 and
1000 °C, the dried samples were analysed by a Setaram Labsys derivatograph (Setaram
Instrumentation, Caluire, France) applying constant air flow at 3 °C/min heating rate. For the
measurements, 15–25 mg of the samples were placed into high-purity alpha alumina crucibles.
The N2 adsorption-desorption isotherms were registered on a Quantachrome NOVA 3000e
instrument (Quantachrome Instruments, Munich, Germany). The nanoparticles were degassed at 200
°C for 3 h in vacuum to remove the surface-adsorbed species. The specific surface areas were
calculated by the Brunauer–Emmett–Teller equation from the adsorption branches, and the
determination of the average pore sizes and total pore volumes were estimated from the desorption
branches by the Barett–Joyner–Halenda equation.
The basic and acid sites of the solids were characterized by temperature-programmed
desorption (TPD) using 99.9% CO2/He and 99.3% NH3/He (50 cm3/min flow), respectively. TPDs were
performed on a BELCAT-A catalyst analyser (Microtrac MRB, Munich, Germany) equipped with
thermal conductivity detector. Before the measurements, about 20 mg of the samples were
purified/degassed in He atmosphere and quartz cell at 300 °C for 1 h, then the CO2 and NH3 saturation
were conducted at 40 °C and 90 °C, respectively. The TPD profiles were registered up to 600 °C with
10°C/min heating rate.
The morphology and size of the nanocrystals were examined by scanning electron microscopy
(SEM-Hitachi S-4700 instrument) (Hitachi, Tokyo, Japan). For these measurements, the solids were
placed onto conductive carbon adhesive tapes, and a few nm of gold-palladium alloy films were
sublimed onto the surface of the samples in order to avoid charging. Elemental analysis was
performed by energy dispersive X-ray (EDX) measurements (Röntec QX2 spectrometer, Röntec
GmbH, Berlin, Germany), equipped with Be window and coupled to the SEM.
3. Results and Discussion
The influence of the ultrasonic treatment on the formation of the nanoparticles were studied in
detail varying the intensity and the emission character of the ultrasound treatment.
3.1. Effects of the Sonication on the Physicochemical Properties of the Nickel Nanoparticles
In the X-ray diffractograms (Figures S1 and S2, Supporting Information), the three reflections
with Bragg indices 111, 200 and 220 can be observed indicating the successive evolution of face-
centred cubic structure of Ni crystallites (JCPDS#04-0850) in the ultrasonically-aided syntheses as
well as those performed with mechanical stirring or without stirring (Figure S2). However, intense
baseline rise is seen between 10° and 20° 2θ values, a sign for the presence of amorphous phase, for
the samples prepared without stirring and low intensity sonication (30 W power and 20% ultrasound
emission periodicity). Moreover, significant reduction in the primary crystallite sizes can be observed
due to ultrasonic treatments compared to the samples prepared in the non-stirred or the mechanically
stirred ways (Table 1). Nevertheless, the estimated sizes only changed slightly on varying the
ultrasound operation parameters. On increasing the ultrasound power density, the ultrasound waves
with enhanced amplitudes and accelerated emission periodicity resulted in only slight growth of the
crystallites, presumably, due to the more vigorous liquid motion and mass transfer induced. In
ultrasound treatments, the influence of solvent temperature is generally twofold: the higher
temperature means increased vapour pressure inside the cavitation voids and results in less intense
collapses, while the formation of cavities got more facile due to the weakening matrix interactions
Nanomaterials 2020, 10, 632 6 of 18
(dipole, van der Waals, hydrogen bonding) between the solvent molecules. To map the exact effect
of the ultrasound treatment at various temperature on the formation of nickel nanoparticles detailed
investigations were performed between 5 °C and 75 °C (Figure 1). Interestingly, the X-ray powder
diffractograms show similar primary crystallite sizes (7–8 nm); this was a remarkably different
observation compared to the mechanically stirred cases reported in our previous study [49], where
the increasing temperature resulted in the gradual growth in crystallite sizes from 6 nm (at 5 °C) to
14 nm (at 75 °C). On ultrasonic treatment at 25 °C, 50 °C and 75 °C, the diffractograms indicated the
presence of metallic nickel, while at 5 °C, the reflections of the β-Ni(OH)2 intermediate (JCPDS#74-
2075) could solely be observed. However, at longer ultrasonic stirring time (6 and 8 h) at this
temperature, the most intense 111 reflection of the metallic nickel phase appeared next to the signals
of nickel(II) hydroxide (Figure S3).
10 20 30 40 50 60 70 80
7 nm
8 nm
♠ 001
♠ 110
♠ 011
100 ♠
220 ♣
♣ 111
75°C
50°C
25°C
Intensity (a.u.)
2θ (°)
5°C
♣ metallic nickel
♠ nickel(II) hydroxide
♣ 200
8 nm
Figure 1. XRD patterns of the nickel nanoparticles prepared by 4 h continuous ultrasound treatment
at 30 W output power and varied reaction temperatures.
To understand the effect of the ultrasound treatment on the nucleation and growth steps of the
nickel crystals and to find out the reason for the generation of the crystallites in close to constant sizes
and for the delayed synthesis of the metallic nickel at 5 °C, detailed knowledge about the formation
mechanism of the Ni nanoparticles is indispensable. Nevertheless, the formation mechanism is still
not clear in every detail, in spite of the intense research [23,50–52]. In the most accepted mechanistic
model, the nickel-hydrazine complexes with different hydrazine contents are formed first, then the
thermodynamically more stable nickel(II) hydroxide is evolved from the reaction of the hydrazine
complex and the alkali additive. The final step is the dissolution of the nanosized nickel hydroxide
particles and the reduction of the dissolved Ni(II) ions by the hydrazine molecules. However, the
transformation of the solid Ni(OH)2 into metallic nickel cannot be excluded, because of the standard
redox potential of hydrazine in basic environment (−1.16 eV, N2H4 + 4 OH− = N2 + 4 H2O + 4 e−) is
suitable not only for the reduction of dissolved Ni2+ cations (−0.25 eV, Ni2+ + 2 e− = Ni0), but for the
solid Ni(OH)2 (−0.72 eV, Ni(OH)2 + 2 e− = Ni0 + 2 OH−) as well. In addition, there are evidences that
the alkali additive acts as catalyst [22], and the formation of nickel(III) oxide hydroxide intermediate
phase is the precursor of the Ni NPs [53], especially in a reactive environment provided by the
propagation of ultrasound in the liquid medium [31].
Owing to the numerous steps in nanoparticle formation, sonication can affect the formation of
Ni NPs in many ways. The hot spots are able to assist in both the formation of unstable and stable
nuclei, and the free radicals can further enhance the reduction potential of the synthesis environment
[45,54]. Moreover, the rapid (100–200 m/s) liquid microjets penetrate into the voids and their impact
Nanomaterials 2020, 10, 632 7 of 18
on the solid substrates causes their disintegration and turbulent flows in the fluid, hence the solubility
of the base, hydrazine complexes/nickel hydroxide/nickel oxide hydroxide can be intensified locally.
Considering these factors, possible modifications are expected in the structural properties of the
intermediates, therefore our attention was focused at studying the influence of the ultrasound
treatment on the solids. For this, X-ray diffractometry and infrared spectroscopy was used. Let us
note that the Ni NPs were prepared without additives, the hydroxides were formed in the reaction
of the nickel iodide and potassium hydroxide, while the complex particles were prepared through
mixing nickel iodide and hydrazine in absolute ethanol under mechanical stirring or ultrasonic
treatment. Even though sonication did not result in significant changes in the crystal structure of the
nickel hydroxides, the nickel−hydrazine−iodide complexes suffered considerable amorphization
(Figure S4). Furthermore, several green dots appeared in the dried solids after sonication, in contrast
to the uniformly violet end product obtained with mechanical stirring. The IR spectra of the
complexes obtained with mechanical stirring or ultrasonic treatment revealed the characteristic
signals of the hydrazine molecules (Figure 2): from above 3000 cm−1 the stretching vibrations of the
NH2 groups and at 1585, 1555 cm−1, their bending vibrations are seen. The strong and overlapping
adsorption peaks are attributed to their twisting modes around 1170 cm−1, while the intense signal at
580 cm−1 can be connected to the M−N stretching vibrations [55,56]. On ultrasound treatment, the
vibration at 3230 cm−1 intensified, the simplified and sharpened twisting mode of the NH2 groups,
the double peaks at 965 and 925 cm−1 connected to the presence of [Ni(N2H4)6] I2 and [Ni(N2H4)4]I2
vanished, and a new signal appeared at 950 cm−1. These observations indicate the sonochemically
induced generation of the [Ni(N2H4)2] I2 form confirmed by the observed green coloured parts as well
[57,58]. Moreover, the appearing peak at 615 cm−1 is also originated from the formation of this
complex [51].
4000 3500 3000 2500 2000 1500 1000 500
3160
3185
3230
mechanically stirred
hydrazine complex
sonicated
hydrazine complex
A
b
sor
b
ance
(
a.u.
)
Wavenumber
(
cm
−1
)
3100
3290
1585
1170
965,
950
925
580
615
1555
Figure 2. Infrared spectra of the nickel−hydrazine−iodide complexes formed on mechanical stirring
or sonication (30 W output power and continuous emission) for 4 h at 75 °C.
3.2. Analysis of the Aggregation Tendency of the Nanoparticles
The polydispersity indices (PdI) (from 0.147 to 0.404) obtained from dynamic light scattering
measurements attested no strict correlation between the character of the synthesis method used and
the heterogeneity of sizes/size distribution of nanoparticle aggregates (Table 1). Nevertheless, the PdI
values were largely lower for the ultrasonically prepared samples compared to the mechanical stirred
one, but always higher than the non-stirred case. Unimodal size distributions were mostly observed
at room temperature, except when mechanical stirring or the highly intense (120 W) continuous
ultrasonic treatment was applied. In these instances, bimodal distributions were observed (Figure 3).
Nanomaterials 2020, 10, 632 8 of 18
At 50 °C and 75 °C, the nanoparticles formed secondary, aggregated particles in the range of 50–600
nm with high polydispersity index values of 0.593 and 0.436, respectively. The ultrasound treatment
had crucial effects on the aggregation tendency of the nanoparticles: the defragmentation influence
of the destroyed cavitation voids with the vigorous mass transfer could overcome the attractive
electrostatic and van der Waals forces between the particles. The investigation of the emission
periodicity and the output power of the ultrasound treatment proved that the both parameters were
essential, by their help, the polydispersity indices could be decreased compared to the mechanically
stirred case. The variation of the ultrasound emission towards shorter sonication periods resulted in
lower average solvodynamic diameters as did the ultrasonic treatments with amplifying the
ultrasound power density; however, the highly intense (120 W) ultrasonic treatment could only result
in the lowest solvodynamic diameter of the aggregates.
100 1000
0
5
10
15
20
25
1550 nm
1260 nm
200 nm
160 nm
210 nm
710 nm
non-stirred mechanical stirring 30 W, 25°C 30 W, 50°C
30 W, 75°C
Number of particles (%)
Diameter (nm)
Figure 3. Number-based size distribution curves of the nickel nanoparticles prepared without stirring
(25 °C), with mechanical stirring (25 °C) and under ultrasonic treatment (30 W output power,
continuous sonication) at various temperatures.
Five samples were selected for further characterization and catalytic studies. Three of them were
prepared by ultrasound treatment and the other two, used as references, by mechanical stirring or
without stirring. We intended to study the effects ultrasound periodicity at constant power and of
intense (power density 0.085 W/cm3) ultrasound treatment on the catalytic properties of
nanoparticles, therefore, samples labelled as 30 W – 20%, 30 W – 100% and 120 W – 100% were chosen.
3.3. Thermal Properties of the Nickel Nanoparticles
First, the thermal behaviour of the selected samples was investigated. Thermal analysis
measurements revealed two separate weight losses under 300 °C and a strong mass gain with
exothermic peak in the 360–380 °C region for every sample (Figure 4 and Figure S5). The latter signal
indicated the oxidation of the nickel particles, the X-ray traces of the residues (Figure S6) displayed
the typical reflections of NiO phase (JCPDS#78-0643). Partial oxidation only took place, since the
gains in weight were between 12 and 17%, always lower than the amount of the weight increase (27%)
corresponding to full oxidation. Moreover, the TG curves indicated gradual mass decrease above 430
°C. These two observations may be originated from the presence of contaminations and/or the
continuous formation and decomposition of nickel oxide phases (both from Ni(OH)2 and Ni NPs)
with significant oxygen deficiency [59,60], and/or the Ni NPs having melting point around 500 °C
started to evaporate [61,62].
Nanomaterials 2020, 10, 632 9 of 18
200 400 600 800 1000
80
82
84
86
88
90
92
94
96
98
100
Heatflow
(
μV
)
DTG (mg/min)
Mass (%)
Furnace temperature (°C)
Endothermic
Exothermic
10
8
6
4
2
30 W − 20% ultrasound treatment
260
90
370
375
-0.12
-0.10
-0.08
-0.06
-0.04
-0.02
0.00
0.02
0.04
0.06
0.08
200 400 600 800 1000
94
96
98
100
102
104
106
108
110
Mass
(%)
Furnace temperature (°C)
Heatflow
(
μV
)
DTG (mg/min)
14
12
10
8
6
4
2
-0.04
-0.02
0.00
0.02
0.04
0.06
0.08
0.10
0.12
30 W − 100% ultrasound treatment
250 365
360
90
Endothermic
Exothermic
Figure 4. Thermal analysis curves for the sonochemically prepared Ni nanoparticles (NPs) treated at
different operating parameters.
The first mass loss is connected to the removal of physically adsorbed water molecules from the
outer surface, the second one indicates the existence of untransformed and amorphous β-Ni(OH)2
intermediate residue being largely invisible for the XRD technique. The corresponding mass losses
took place in the 245−265 °C temperature range, similarly to those of the as-prepared nickel hydroxide
intermediate phase (Figure S5). The quantities and the calculated β-Ni(OH)2 contents are shown in
Table 2. Although the formation of NiO(OH) cannot be ruled out entirely according to the references
[63,64], it is presumably minute in amount compared to the extent of nickel hydroxide phase. The
highest Ni(OH)2 amount was detected for the non-stirred and the 30 W – 20% ultrasonically irradiated
samples, where the mass transportations were the lowest delaying the generation of the Ni NPs. The
lowest was observed for the mechanical stirred and the 30 W – 100% sonicated samples. The sample
irradiated with continuous 120 W output power was in midway indicating that not all featured
modifications are induced by intense sonication.
Table 2. The results of the thermogravimetric (TG) analysis and the N2 adsorption-desorption
measurements of the nickel nanoparticles.
Ni NP
samples
TG second
mass loss (%)
β-Ni(OH)2
content (m/m%)
Specific surface
area (m2/g)
Total pore
volume (cm3/g)
Average pore
diameter (Å)
non-stirred 2.2 11.3 28.1 0.040 35.4
mechanical
stirring 0.3 1.5 21.9 0.027 35.8
30 W – 20%a 3.7 19.0 39.5 0.054 38.8
30 W – 100% 0.3 1.5 20.4 0.043 36.2
120 W – 100% 1.0 5.1 28.6 0.036 38.8
a The percentages mean the emission periodicity of the ultrasound treatment, for instance, at 20%, the
ultrasonic homogenizer was inactive in the four fifth of the pulse and the sonication period was one fifth of the
active time, while at 100%, the device worked continuously.
Nanomaterials 2020, 10, 632 10 of 18
3.4. Surface and Porous Properties of the Nickel Nanoparticles
The textural parameters of the nickel nanoparticles were investigated by N2 adsorption-
desorption measurements, the isotherms were classified as Type IV (according to the IUPAC) with
H3 hysteresis (Figure S7). The nanoparticles showed mesoporous structure, their porosity
presumably originated from the evolved gaps between the crystallites. The amounts of the total pore
volumes and the specific surface areas proved to be diversely dependent on the nature of the applied
synthetic method; ultrasound treatment could result in larger average pore diameters expanding the
volume of the gaps (Table 2). In general, the specific surface areas/total pore volumes followed
directly the amount of the residual β-Ni(OH)2 resulting in relatively high surface areas and pore
volumes (generally ≤ 100 m2/g and ≤ 0.2 cm3/g, respectively) [65,66]. The highest values were
calculated for the non-stirred and the 30 W – 20% ultrasonically irradiated, while the lowest ones
belong to the mechanically stirred and 30 W – 100% ultrasonically treated samples. The higher surface
area of the sample prepared with the most intense ultrasound treatment (120 W − 100%) can be
explained by the joint effect of the presence of the Ni(OH)2 particles and the enhanced average pore
diameter.
3.5. CO2/NH3 Temperate Programmed Desorption (TPD) Studies of the Selected Samples
The application of base is generally required in the cross-coupling reactions, therefore, studying
acid-base properties of the nanoparticles was necessary. Table 3 summarizes the calculated values of
the total basicity and acidity of the selected Ni NPs originating from the pure metal, the nickel
hydroxide residues, the external nickel oxide layers and even the crystal surface defects of the
nanoparticles [67], which were potentially multiplied by the ultrasonic cavitation. Note that these
TPD investigations somewhat overestimate the number of the acid/base centres due to the formation
of the NH3–NH3 associations [68] and the penetration of the relative small CO2 and NH3 molecules
into the pores hardly accessible for the larger molecules generally used in most organic reactions.
Table 3. Summary of the temperature-programmed desorption (TPD) results for the selected
materials.
Ni NP samples Total basicity
(mmol CO2/g)
Temperature
of peak
maxima (°C)
Total acidity
(mmol NH3/g)
Temperature
of peak maxima (°C)
non-stirred 0.051 90 and 165 0.021 185 and –
mechanically stirred 0.046 90 and 160 0.025 150 and 330
30 W – 20% 0.054 95 and 155 0.105 175 and –
30 W – 100% 0.032 90 and 175 0.257 150 and 370
120 W – 100% 0.088 90 and 165 0.054 190 and –
The basicities of the solids were close to what were indirectly determined by the amount of
Ni(OH)2 residues. The 30 W – 20% and the 120W – 100% samples proved to be the most basic, while
the mechanically stirred and the 30 W – 100% samples displayed lower basicities. The CO2-TPD
profiles showed two peaks under 300 °C; the first at ~90 °C originating from the weak dative bonds
of the NiO and the CO2 species, while the second, with maxima between 155–175 °C can be assigned
to stronger adsorption interaction [69].
The total acidities of the solids were also dependent on the NiO content. The NH3-TPD traces
revealed that the 30 W − 100% and the mechanically stirred samples had the weakest (150 °C) as well
as the lowest number of acidic centres, while the other samples had well-observable desorption
maxima around 180 °C derived from desorption of hydrogen-bonded NH3 molecules [70]. For the
mechanically stirred and the 30 W − 100% ultrasonically prepared samples, peaks with maximum at
330 °C and 370 °C, respectively, were observed, probably due to the desorption of terminally bound
NH3 [71].
Although these measurements cannot distinguish the Brønsted and Lewis acid sites; however,
because of the lack of mobile protons we can safely state that they are Lewis acid sites. Overall, it is
Nanomaterials 2020, 10, 632 11 of 18
seen that the less acidic solids were the non-stirred and the mechanically stirred samples, while the
30 W − 100% sample had significantly more moderate acidic sites. For these two last samples, a large
difference can be observed; the total acidity of the sonically prepared nanoparticles was ten times
higher, however, their specific surface areas (even if we note the change of the textural parameters
during the dehydration of the Ni(OH)2), nickel hydroxide contents were largely similar. It may be
connected to the smaller total pore volume of the mechanically stirred ones meaning lower accessible
pores/space for the adsorption of NH3 molecules. Another possible explanation can be the ultrasound
induced modification in the number of the Ni step-like and terrace-like sites due to the electron
donation character of the negatively charged terraces and electron accepting capability of the positive
steps [72]. This interpretation could be well applied to translate the lower basicity of the ultrasonically
(30 W – 100%) prepared solids compared to the mechanically stirred ones.
3.6. The selected Materials Studied by Scanning Electron Microscopy
Scanning electron microscopy measurements revealed the largely uniform morphology of the
nanoparticles. Close to spherical particles were observed in the size range of 200–400 nm merged into
larger (>1 µm) aggregates independently from the method of synthesis (Figure 5). However,
continuous ultrasound treatment at 120 W output power could enhance slightly the formation of the
larger and less symmetric spheres, presumably due to the more intense mass transfer and
deforming/frittering effect related to the continuously forming and eliminating cavities. The EDX
probes showed no sign of iodine atoms as direct evidence of the complete transformation of the
starting material; however, oxygen atoms were seen confirming the presence of nickel hydroxide
intermediate. Potassium were found in negligible amount (25−30:1 Ni:K molar ratio) as the residue
of the alkali additive applied (Figure S8).
Nanomaterials 2020, 10, 632 12 of 18
Figure 5. Scanning electron microscopy images of the selected NiNPs (A: non-stirred, B: mechanically
stirred, C: 30 W – 20%, D: 30 W – 100%, E: 120 W – 100% ultrasonically treated samples).
4. Catalytic Application of the Ni NPs in the Suzuki-Miyaura Cross-Coupling Reaction
In recent decades, the impact of the Suzuki-Miyaura cross-coupling (SMC) reaction on academic
and industrial research has become immense. The main advantages of these reactions are the mild
reaction conditions and the high tolerance toward functional groups. Hence, the SMC reaction is
often applied in the large-scale synthesis of fine chemicals and pharmaceuticals [73]. Originally,
expensive palladium- [74,75] and nickel-based complexes [76,77] were used. In this work, our aim
was to use the economically more accessible pure nickel metal in the synthesis of biphenyl product
(Scheme 1) enhancing its activity by ultrasonic treatment. For scouting experiments the 30 W – 100%
sample was used, and the investigations were started with varying the reaction time and the solvents
(Figure 6). The solvents chosen were toluene, dimethyl sulfoxide (DMSO) and dimethyl formamide
(DMF) with water content of 20% v/v in order to aid the dissolution of the K
2
CO
3
base. In DMSO, the
A B
C D
E
Nanomaterials 2020, 10, 632 13 of 18
yield after 24 h was 81%. Interestingly, the biphenyl product only appeared after a 1 h induction
period in toluene; however, the yield after 24 h was 95%. In DMF, the yield after 24 h was only 76%.
+
BOH
OHI
Scheme 1 The Suzuki-Miyaura cross-coupling test reaction between the iodobenzene and
phenylboronic acid.
Figure 6. The biphenyl yield in the reaction of iodobenzene and phenylboronic acid in different
solvents using Ni catalyst prepared with continuous ultrasound treatment (30 W output power) and
at reflux temperature.
Then, for testing the five selected NiNP samples, DMSO and toluene solvents were used, and
sampling was performed after 1 h and 24 h reaction time, respectively (Figure 7). The sampling times
were chosen according to the calculated turn over frequency (TOF = (mol of product/mol of
catalyst)/reaction time) values (Figure S9); in DMSO, the nanoparticles showed the highest activity in
the first hour of the cross-coupling reactions, while it needed more than 20 h in the DMF. The toluene
was selected due to the close 100% yield of biphenyl synthesis, however, the nanoparticles showed
the lowest activity here.
Figure 7. The biphenyl yield in the reaction of iodobenzene and phenylboronic acid in toluene after
24 h (blue) and in DMSO after 1 h (red) with the selected NiNPs samples at reflux temperature.
0
20
40
60
80
100
1246812131416182024
Yield (%)
Reaction time (h)
DMF DMSO
0
20
40
60
80
100
non-stirred mech.
stirring
30 W, 20% 30 W,
100%
120 W,
100%
Yield (%)
Catalyst
24 h, Toluene 1 h, DMSO
73
33
75
43 50
34
95
42
67
38
0.03
0.02 0.02
0.01
0.02
0.22
0.29 0.23
0.28 0.25
TOF (h-1) Yield of biphenyl (%)
Nanomaterials 2020, 10, 632 14 of 18
The nanoparticles prepared with continuous ultrasound treatment at 30 W output power proved
to be the most active catalyst in toluene, interestingly, the other two sonicated samples (30 W – 20%,
120 W – 100%) showed significantly more modest performance, the yields were even lower than those
over the mechanically stirred or the non-stirred materials.
Similarly, in DMSO, the yield and the TOF values were the highest for the mechanically stirred
and the 30 W – 100% ultrasonically synthesised NiNPs.
Although many variables can influence the performance of the catalysts, it is to be noted that the
30 W – 100% catalyst has higher Lewis acidity and it performs significantly better in the 24 h reactions
compared than the other catalysts. It is also known that the oxidative addition steps of the cross-
coupling reactions are successfully promoted by Lewis acid additives [78–81]. Therefore, we propose
that the surface Lewis centres, located in atomic scale close vicinity of the metallic nickel atoms, aid
the formation of the organonickel species through weakening the aryl carbon–iodine bonds [79,82].
As far as recycling properties are concerned, the used 30 W – 100% ultrasonically prepared
catalyst was investigated by X-ray diffractometry after the first 24 h reactions in both toluene and
DMSO. Originally, the NiNPs had the face-centred (fcc) cubic structure identified by the
corresponding reflections. In DMF, this was partially transformed to hexagonal close-packed (hcp)
structure (JCPDS#45-1027) and nickel oxide (JCPDS#78-0643) (Figure S10).
The reusability of the nanoparticles prepared with mechanical stirring and the 30 W – 100%
NiNPs synthesized with ultrasonic treatment were further studied in DMSO and toluene, but the
reaction time was decreased to 1 h when DMSO was the solvent. In toluene, the ultrasonically
synthesized NPs gave 71% yield, while the mechanical stirred ones produced 58% yield in the first
reuse experiment, in DMSO, the yields were 35% and 23%, respectively. The reason of the reduced
yields is probably in direct correlation with the revealed structural modifications (Figure S11). The
NiNPs prepared with mechanical stirring maintained their original crystal framework in toluene, but
with significant amorphization, while unidentified reflections were found beside the signals of the
fcc NiNPs in DMSO. The ultrasonically prepared samples displayed different behaviour; in toluene,
a mixture of NiO, fcc and hcp NiNPs was recorded, while in DMSO, NiS (JCPDS#12-0041) phase was
solely observed. The EDX analysis revealed the presence of sulphur in the both catalysts after the
reactions performed in DMSO solvent indicating chemical reaction between the nickel nanoparticles
and the solvent molecules (Figures S12 and S13).
5. Conclusions
Ultrasound treatment was successfully utilized to influence the hydrazine reduction synthesis
method of the nickel nanoparticles. The sonically induced intermediate complexes for the variously
treated materials could be identified. Sonication was able to maintain an average primary crystallite
size (7–8 nm) independently from the applied temperature, while the use of intense and shorter
sonication periods could reduce the solvodynamic diameters of the secondary, aggregated particles
from 710 nm to 190 nm in the absence of any surfactant. The highest acidity and catalytic activity
were measured for the nanoparticles prepared by mild (30 W output power) and continuous
ultrasound treatment.
Supplementary Materials: The following are available online at www.mdpi.com/2079-4991/10/4/632/s1, Figure
S1: XRD patterns of the nickel nanoparticles prepared under ultrasound treatment with various ultrasound
emission periodicities (duration of treatment: 4 h, temperature of treatment: 25 °C); Figure S2: XRD patterns of
the nickel nanoparticles prepared with mechanical stirring, without stirring or under ultrasonic treatment at
various output power values (duration of treatment: 4 h, temperature of treatment: 25 °C); Figure S3: XRD
patterns of the solid materials formed on ultrasound treatment (30 W − 100%) at 5 °C with 4 h, 6 h or 8 h
treatments or without stirring at 5 °C after 4 h; Figure S4: XRD patterns of the nickel hydroxide and complex
intermediates obtained under mechanical stirring or sonication (30 W − 100%) at 75 °C, after 4 h treatment; Figure
S5: Thermogravimetric curves of the β-Ni(OH)2 and the NiNPs prepared without stirring, with mechanical
stirring or under ultrasound treatment (120 W − 100%); Figure S6: X-ray patterns of the thermogravimetric
residues of the nanoparticles; Figure S7: N2 adsorption-desorption (left) and the pore size distribution (right)
curves of the selected nickel nanoparticles; Figure S8: Energy dispersive X-ray analysis spectrum of NiNPs
prepared at room temperature with ultrasound treatment of 30 W output power and 20% emission periodicity
Nanomaterials 2020, 10, 632 15 of 18
(signals of carbon, aluminum and phosphorous are from the adhesive tape/sample holder); Figure S9: Evolution
of the turn over frequency (TOF) values of the ultrasonically synthesised nanoparticles (30W, continuous
sonication) during the cross-coupling reaction in DMF, DMSO and toluene solvents; Figure S10: XRD patterns
of the used nickel nanoparticle catalyst (30 W − 100%) after the first 24 h run in various media (fcc—face-centered,
hcp—hexagonal close-packed); Figure S11: XRD paerns of NiNPs prepared by 30 W − 100% ultrasound
treatment and mechanical stirring after the repeated run in toluene and DMSO solvents (fcc—face-centered,
hcp—hexagonal close-packed); Figure S12: Energy dispersive X-ray analysis spectrum of NiNPs prepared with
ultrasound treatment (30 W − 100%), after using it as catalyst in DMSO solvent (signals of carbon and oxygen
originate from the adhesive tape/sample holder, the Ni:S molar ratio ~1:1); Figure S13: Energy dispersive X-ray
analysis spectrum of NiNPs synthesized with mechanical stirring, after using it as catalyst in DMSO solvent
(signs of the carbon, aluminium and the oxygen could originate from the adhesive tape/sample holder, the Ni:S
molar ratio ~1:1).
Author Contributions: conceptualization, I.P., P.S. and M.S.; methodology, Z.K., Á.K. and G.V.; investigation,
A.A.A, K.M. and A.P.; resources, I.P. and P.S.; writing—original draft preparation, M.S. and A.A.A.; writing—
review and editing, I.P.; supervision, M.S. and I.P.; project administration, I.P.; funding acquisition, I.P. and Z.K.
All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by the European Union and the Hungarian government through grant
GINOP-2.3.2-5- 2016-00013. The financial help is highly appreciated.
Conflicts of Interest: The authors declare no conflict of interest.
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