Review

Controlled Nickel Nanoparticles: A Review on How Parameters

of Synthesis Can Modulate Their Features and Properties
Felipe Anchieta e Silva 1 , Vera Maria Martins Salim 2 and Thenner Silva Rodrigues 1, *

1 Nanotechnology Engineering Program, Alberto Luiz Coimbra Institute for Graduate Studies and Research in
Engineering, Federal University of Rio de Janeiro, Av. Horácio Macedo, 2030,
Rio de Janeiro 21941-972, RJ, Brazil; felipe.anchieta@coppe.ufrj.br
2 Chemical Engineering Program, Alberto Luiz Coimbra Institute for Graduate Studies and Research in
Engineering, Federal University of Rio de Janeiro, Av. Horácio Macedo, 2030,
Rio de Janeiro 21941-972, RJ, Brazil; vera@peq.coppe.ufrj.br
* Correspondence: thenner@pent.coppe.ufrj.br

Abstract: Nickel nanoparticles have wide-ranging applications in diverse fields, including electronics,
catalysis, and biomedicine. The unique properties of these nanoparticles depend on their physical
and chemical attributes. Consequently, there is a growing interest in understanding the performance
relationships through a nuanced comprehension of their controlled synthesis. This review explores
the advancements related to precisely defined nickel nanoparticles, with a specific focus on unraveling
the connections between performance and their physical/chemical characteristics. The emphasis is
on elucidating how manipulating synthetic parameters, such as precursor concentration, reductant
agent properties, temperature, time, and the presence of stabilizing agents, can provide additional
avenues for refining the performance in terms of size and morphology. Through the analysis of
each variable, we illustrate the methodology for synthesizing well-controlled nickel nanoparticles,
showcasing the ability to exert precision over their composition, size, and surface morphology.

Keywords: nickel nanoparticles; synthesis method; purification method; controlled synthesis

Citation: e Silva, F.A.; Salim, V.M.M.; 1. Introduction

Rodrigues, T.S. Controlled Nickel Nickel (Ni), a versatile transition metal in the VIIIB group, has long been revered for
Nanoparticles: A Review on How its exceptional properties and low cost compared to noble metals, spanning a multitude of
Parameters of Synthesis Can applications. In its bulk metallic form, nickel exhibits robust mechanical strength, excel-
Modulate Their Features and
lent corrosion resistance, and notable conductivity, making it indispensable in industries
Properties. AppliedChem 2024, 4,
ranging from metallurgy to electronics. As advancements in materials science and nanotech-
86–106. https://doi.org/10.3390/
nology burgeon, nickel nanoparticles have emerged as a focal point of research, unlocking
appliedchem4010007
unprecedented opportunities for tailored material design and innovative applications.
Received: 29 December 2023 The unique size-dependent properties of nickel nanoparticles, coupled with their
Revised: 18 February 2024 enhanced reactivity, surface area, high surface energy level, strong magnetism, and low
Accepted: 1 March 2024 melting point, hold promise for innovations in catalysis, medicine, and electronics [1,2].
Published: 13 March 2024 This convergence of traditional nickel metal attributes and the novel characteristics of
nickel nanoparticles marks an exciting frontier in materials science, offering a rich tapestry
of possibilities for technological advancements and interdisciplinary exploration. For
Copyright: © 2024 by the authors.
example, Zhu et al. have been exploring the potential of nickel nanoparticles with the
Licensee MDPI, Basel, Switzerland.
morphology of nanorings as memory cells [3]. Alloys of copper and nickel at the nanoscale
This article is an open access article are being investigated for use in controlled magnetic hyperthermia applications [4] due
distributed under the terms and to their bioactivity [5]. Additionally, nickel nanoparticles have been applied to electrode
conditions of the Creative Commons materials [6] in batteries [7] and solar cells [8].
Attribution (CC BY) license (https:// The significant impact of nickel nanoparticles on various applications stems from
creativecommons.org/licenses/by/ their nanoscale dimensions. This transformative shift from macroscale to nanoscale, as
4.0/). illustrated in the schematic representation contrasting numerous nanoparticles with bulk

AppliedChem 2024, 4, 86–106. https://doi.org/10.3390/appliedchem4010007 https://www.mdpi.com/journal/appliedchem

AppliedChem 2024, 4, FOR PEER REVIEW 2

AppliedChem 2024, 4 The significant impact of nickel nanoparticles on various applications stems from 87
their nanoscale dimensions. This transformative shift from macroscale to nanoscale, as
illustrated in the schematic representation contrasting numerous nanoparticles with bulk
nickel in Figure 1A, leads to substantial alterations in its optical, magnetic, and electronic
properties [9–11]. In the macro form, nickel exhibits limited surface atom incorporation incorporation
within the total
total atom
atomcount
count(Figure
(Figure1B).
1B).The
The final
final atomic
atomic layer
layer aligns
aligns at the
at the frontier
frontier of
of the
the nickel macroparticle, with dotted circles denoting the absent atoms
nickel macroparticle, with dotted circles denoting the absent atoms on the surface. Surface on the surface.
Surface
energy, energy,
defineddefined by the
by the sum sum of dangling
of dangling bonds between
bonds between atoms and atoms and missing
missing atoms, isatoms,
com-
is compensated by the surplus of atoms within the bulk particle. This
pensated by the surplus of atoms within the bulk particle. This surplus aids in balancing surplus aids in
balancing
the surfacethe surface
energy energy originating
originating from the
from the deficient deficient
atoms on theatoms on and
surface the surface
reducesandthe
reduces the interatomic
interatomic spacing betweenspacing between
surface andsurface
interiorand interior
atoms. atoms. Contrastingly,
Contrastingly, at the
at the nanoscale
nanoscale
(Figure 1C),(Figure 1C), theofscarcity
the scarcity interiorofatoms
interior atoms
fails fails to counterbalance
to counterbalance the increased
the increased surface
surface energy caused by dangling bonds adequately. This deficiency
energy caused by dangling bonds adequately. This deficiency prompts atom approxima- prompts atom
approximation within the nanoparticles and a reduction in lattice parameters,
tion within the nanoparticles and a reduction in lattice parameters, consequently altering consequently
altering
geometricgeometric and electronic
and electronic structures structures
[11–14]. [11–14]. Modification
Modification of these of these parameters
parameters can,
can, in turn,
in turn, lead to enhanced nanoparticle
lead to enhanced nanoparticle properties. properties.

Figure 1. (A) Reduction scale from bulk to nano, (B) surface of bulk material, and (C) surface of
Figure 1. (A) Reduction scale from bulk to nano, (B) surface of bulk material, and (C) surface of
nanomaterial. Black and red arrows indicate the internal and surface energies, respectively.
nanomaterial. Black and red arrows indicate the internal and surface energies, respectively.

In this
In this context,
context,thetheproperties
propertiesofofnanoparticles—such
nanoparticles—such asas size,
size, shape,
shape, andand structure—
structure—are
are intricately linked [15]. The crucial aspect in achieving monodisperse
intricately linked [15]. The crucial aspect in achieving monodisperse and uniform nanopar- and uniform na-
noparticles lies in effectively segregating the nucleation and growth
ticles lies in effectively segregating the nucleation and growth processes. The LaMer plotprocesses. The LaMer
plot elucidates
elucidates the nanoparticle
the nanoparticle formationformation phenomenon
phenomenon (Figure(Figure 2), encompassing
2), encompassing three
three distinct
distinct(i)
stages: stages: (i) generation
generation of atoms, of(ii)
atoms, (ii) nucleation,
nucleation, and (iii) and (iii) [16–19].
growth growth [16–19].
During Stage
During Stage I, metal ions
I, metal ions undergo
undergo reduction
reduction to to generate
generate metallic
metallic atoms,
atoms, serving
serving asas
growth species (building blocks) for nucleus and nanoparticle formation.
growth species (building blocks) for nucleus and nanoparticle formation. In this initial In this initial
stage, the
stage, atoms lack
the atoms lack sufficient
sufficient energy
energy for for nucleus
nucleus formation
formation duedue to to lower
lower concentration.
concentration.
Once the concentration of atoms reaches the threshold necessary to overcome the energy
forhomogeneous
barrier for homogeneousnucleation
nucleation(Equation
(Equation (1)),
(1)), thethe formation
formation of the
of the nucleus
nucleus ensuesensues
[16].
[16]. This
This critical
critical point, point, marked
marked by theby red
the dot
red dot in the
in the LaMer
LaMer plot,plot, signifies
signifies thethe attainment
attainment of
of the
the minimum
minimum concentration
concentration of of metal
metal atoms
atoms in in solution,
solution, enabling
enabling their
their aggregation
aggregation into
into a
a stable
stable nucleus
nucleus withwith a critical
a critical radius
radius (r), surface
(r), surface free energy
free energy per area
per unit unit(γ),
areaand(γ),free
and free
Gibbs
Gibbs energy
energy (∆Gv ).(ΔGv).
Stage II, the nucleation phase, involves the aggregation aggregation of atoms
atoms to to shape
shape thethenucleus.
nucleus.
This phase must occur rapidly in controlled synthesis synthesis to ensure
ensure uniform
uniform nucleus
nucleus size.
size.
Conversely, an extended duration in this stage leads to varied nucleus sizes and a broad
particle size distribution due to nucleus aggregation. Stage III, the growth phase, entails
the aggregation of atoms on the surface of the nucleus to foster growth and nanoparticle
AppliedChem 2024, 4 88
AppliedChem 2024, 4, FOR PEER REVIEW 3

formation. Similar to the nucleation strategy, rapid growth is essential for achieving a
formation. Similar to the nucleation strategy, rapid growth is essential for achieving a nar-
narrow particle size distribution.
row particle size distribution.
2 44 3
∆G
G==4πr4πrγγ++3 πrπr∆G
∆Gv (1)
(1)
3

Figure
Figure 2.2. LaMer
LaMerplot
plotdescribing
describingthree
threestages
stagesof
ofmetal
metalnanoparticle
nanoparticleformation
formationin
inthe
thesolution
solutionsystem.
system.
Stage I: Generation of atoms, stage II: Nucleation, and stage III: Growth.
Stage I: Generation of atoms, stage II: Nucleation, and stage III: Growth.

The
The synthesis
synthesis of of nickel
nickel nanoparticles
nanoparticles can can be
beachieved
achievedthrough
through diverse
diversemethodologies
methodologies
depicted
depicted in in the
the literature, including
including template-assisted,
template-assisted, chemicalchemical reduction,
reduction, and and polyol
polyol
methods,
methods,with withpossible
possibleassistance
assistance from
from intensification
intensification techniques
techniques suchsuch
as microwaves
as microwaves and
hydrothermal
and hydrothermal processes. Each Each
processes. methodology
methodology and theandchosen
the chosen operational conditions
operational conditionscan
can influence
influence distinct distinct
reactionreaction mechanisms
mechanisms in the in the nucleation
nucleation and growth and growth steps, leading
steps, leading to var-
to variations
iations in the morphological,
in the morphological, size, and size, and structural
structural aspects ofaspects of the nanoparticles
the nanoparticles [17]. Moreo- [17].
Moreover, precise control over the nucleation and growth steps,
ver, precise control over the nucleation and growth steps, and consequently over the size, and consequently over the
size, morphology,
morphology, and structure
and structure of theofnanoparticles,
the nanoparticles, can becanattained
be attained by manipulating
by manipulating the
the op-
operational
erational conditions
conditions of of
thethe synthesis
synthesis methodology.Factors
methodology. Factorssuchsuchasasthetheconcentration
concentration and and
nature of
nature of the
the precursor,
precursor, reducing
reducing agent,
agent, stabilizing
stabilizing agent,
agent, solvent,
solvent, reaction
reaction temperature,
temperature,
and time
and time play
play crucial
crucial roles
roles inin achieving
achieving this this control
control [18].
[18]. For
For instance,
instance, thethe choice
choice of of solvent
solvent
can impact ion diffusion, thereby modifying the size and shape
can impact ion diffusion, thereby modifying the size and shape of the nanoparticles. Some of the nanoparticles. Some
solvents, like ethylene glycol, can serve multiple functions, acting
solvents, like ethylene glycol, can serve multiple functions, acting as both a stabilizing and as both a stabilizing
and reducing
reducing agent. agent. Additionally,
Additionally, stabilizing,
stabilizing, surfactant,
surfactant, and and cappingcapping agents
agents influence
influence the
the morphology,
morphology, size,size,
and and structure
structure of theofnanoparticles.
the nanoparticles. DuringDuring nucleation
nucleation and growth,
and growth, pro-
protective
tective agentsagents
fixatefixate on specific
on specific crystalcrystal
faces tofaces to inhibit
inhibit growthgrowthin that in that direction
direction and
and priori-
prioritize other crystal faces. The reduction agent, dependent on
tize other crystal faces. The reduction agent, dependent on its reduction potential, can its reduction potential, can
control nucleation and growth rates, exerting a significant influence
control nucleation and growth rates, exerting a significant influence on the final nanopar- on the final nanopar-
ticles. The
ticles. The reaction
reaction temperature
temperatureaffects affectssolvent
solventviscosity,
viscosity,impacting
impactingthe thediffusion
diffusionof ofions,
ions,
nuclei, and particles [19]. In summary, this review focuses on
nuclei, and particles [19]. In summary, this review focuses on discussing the parameters discussing the parameters
such as precursor, reduction agent, stabilizing agent, solvent, and reaction temperature,
AppliedChem 2024, 4 89
AppliedChem 2024, 4, FOR PEER REVIEW

such as precursor, reduction agent, stabilizing agent, solvent, and reaction temperature,
and their influence
andon theinfluence
their morphological, size, and structural
on the morphological, size,aspects of nickelaspects
and structural nanoparticle
of nickel nanop
synthesis, aiming
cleto achieve well-controlled
synthesis, properties.
aiming to achieve well-controlled properties.

2. Nickel Nanoparticle
2. NickelSynthesis
Nanoparticle Synthesis
The nickel saltThe is an important
nickel salt is anparameter
importantbecauseparameter thebecause
counterion determines
the counterion the
determines th
solubility of theubility
salt in the solvent
of the salt inand
theinfluences
solvent and electrostatic
influencesstabilization, the pH modifier,
electrostatic stabilization, the pH mod
and the complexing and the agent. Nickel nanoparticles
complexing agent. Nickel can be synthesized
nanoparticles can bethrough
synthesizeda variety
throughof a varie
nickel salts, such as nickel
nickel salts,(II)
suchacetate (Ni(CH
as nickel 3 CO2 )2 )(Ni(CH
(II) acetate [20,21],3CO
nickel (II) bis(acetylacetonate)
2)2) [20,21], nickel (II) bis(acetylaceto
(Ni(C5 H7 O2 )2 ) [22,23],
(Ni(C5Hnickel
7O2)2) (II) nitrate
[22,23], (Ni(NO
nickel 3 )2 ) [24,25],
(II) nitrate (Ni(NOnickel (II) oxalate
3)2) [24,25], nickel(NiC
(II)2 O 4 ) [26],(NiC2O4)
oxalate
nickel chloride (NiCl ) [27,28], nickel (II) sulfate
nickel2 chloride (NiCl2) [27,28], nickel (II) (NiSO ) [29], and nickel (II) dodecyl sulfate
4 sulfate (NiSO4) [29], and nickel (II) dodecy
(Ni(DS)2 ) [30]. fate (Ni(DS)2) [30].
The concentration Theofconcentration
nickel salt is a ofcritical
nickelpoint
saltthat
is ainfluences
critical pointnucleation and growth
that influences nucleation
ratio due to thegrowth
chemical kinetic
ratio due to theory [31]. The
the chemical precursor
kinetic theory concentration
[31]. The precursor is determinant
concentration is d
to controlling the minantsize toand morphology
controlling the size andandcanmorphology
be manipulatedand can tobepromote
manipulatedcomplex to promote
morphologies, such plex as nanoflowers, such
morphologies, spikyas nanospheres,
nanoflowers, and spiky
spiky nanowires, and
nanospheres, andincreases
spiky nanowires
the nanoparticleincreases
size [32–34]. Figure 3 shows
the nanoparticle the
size influence
[32–34]. of precursor
Figure 3 shows the concentration
influence of onprecursor
the con
chain-like nanostructured materials; it is possible to see in these chain-like nanostructured
tration on the chain-like nanostructured materials; it is possible to see in these chain
materials that the diameters are materials
70 nm, 100that nm,the180diameters
nm, and 380 2+ concentrations
nanostructured are nm
70 nm,for Ni
100 nm, 180 nm, and 380 nm
of 5 mM, 10 mM, Ni20 mM, and 50 mM,
2+ concentrations of 5respectively
mM, 10 mM,[33]. 20 mM, and 50 mM, respectively [33].

Figure
Figure 3. Influence 3. Influence
of precursor of precursor
concentration onconcentration
the chain-likeon the chain-like nanostructured
nanostructured material size: 5 material
mM size:
(A), 10 mM (B), 20 mM (C), and 50 mM (D). Scale bars in inset images represent
(A), 10 mM (B), 20 mM (C), and 50 mM (D). Scale bars in inset images represent 500 nm. Reproduced 500 nm. Reprod
with permission from [33], Royal Society of Chemistry, 2012.
with permission from [33], Royal Society of Chemistry, 2012.

2.1. Reducing Agent

2.1. Reducing Agent
The reducing agent has the function
The reducing agent hasofthe
reducing
function theofnickel ion the
reducing (Ni2+ ) to the
nickel ionnickel
(Ni2+) to the n
◦
atom (Ni ) by the
atomglobal
(Ni°)redox
by thereaction (Equation
global redox (2)),(Equation
reaction in which the(2)),nickel ion the
in which is reduced
nickel ion is red
to a nickel atom,
to areceiving tworeceiving
nickel atom, electronstwofrom the reducing
electrons from the agent
reducing(Rredagent
) (Equation (3)).
(Rred) (Equation (3))
The reducing agent is converted
reducing to oxidizedtospecies
agent is converted (Rspecies
oxidized oxi ) (Equation
(R oxi) (4)). Each
(Equation reductant
(4)). Each reductant
molecule exhibits
eculeunique behavior
exhibits unique to promote
behavior differentdifferent
to promote reduction ratios inratios
reduction the number
in the number of
of electrons donated (n) and the
trons donated reduction
(n) and potential,
the reduction influencing
potential, the finalthe
influencing nanoparticle.
final nanoparticle. S
AppliedChem 2024, 4 90

Some examples of reducing agents are hydrazine (N2 H4 ), polyalcohols, sodium borohy-
dride (NaBH4 ), sodium formaldehyde sulfoxylate, benzyl diethylenetriamine, sodium
hypophosphite (NaH2 PO2 ), borane tributylamine, and oleylamine [30,32,35–41].

Ni2+ + Rred → Nio + Roxi (2)

Ni2+ + 2e− → Nio (3)

Rred → Roxi + ne− (4)
Hydrazine (N2 H4 ) is a widely used reducing agent due to its strong reducing potential,
which is potentialized in the presence of alkaline medium to synthesis of nickel nanopar-
ticles. However, the mechanism to form nickel nanoparticles can be more complex than
the global reaction (Equation (5)) [42]. It occurs through a complexation reaction, where,
depending on the ratio of [N2 H4 ]/[Ni2+ ], it can form tris(hydrazine) nickel (II) chloride
([Ni(N2 H4 )3 ]Cl2 ), bis(hydrazine) nickel(II) chloride ([Ni(N2 H4 )2 ]Cl2 ), and hexa-ammine
nickel (II) chloride ([Ni(NH3 )6 ]Cl2 ), and many pathways.
The ratio of [N2 H4 ]/[Ni2+ ] is determinant to synthesizing well-defined nickel nanopar-
ticles. The ratio [N2 H4 ]/[Ni2+ ] = 1 is the minimum ratio to form metallic nickel, exhibiting
a wide size distribution and agglomeration due to Van der Waals forces and magnetic
attractions [33,43]. When [N2 H4 ]/[Ni2+ ] ≤ 3, the amount of N2 H4 is not enough to reduce
all the Ni2+ to Ni◦ . N2 H4 is catalytically decomposed on the nickel surface to produce N2
and H2 (Equation (6)) and N2 and NH3 (Equation (7)) [10,43,44]. Additionally, a light violet
complex ([Ni(NH3 )6 ]Cl2 ) is formed (Equation (8)) [10,45]. The reaction between NiCl2 and
N2 H4 produces [Ni(N2 H4 )n ]Cl2 , n = 2 or 3 (Equation (9)). When [N2 H4 ]/[Ni2+ ] > 4.5, a
sky-blue-colored complex ([Ni(N2 H4 )2 ]Cl2 ) is formed [46], and when [N2 H4 ]/[Ni2+ ] < 3, a
pink-colored complex ([Ni(N2 H4 )3 ]Cl2 ) is formed [10,43,45,47,48]. Until the solution has
both complexes, the nanoparticles will be uncontrolled in size and morphology due to the
difference between the reaction kinetics of the complexes. The nanoparticles will exhibit
controlled size and morphology when [N2 H4 ]/[Ni2+ ] < 4.5.
However, the reaction can be based on the absence of OH− to reduce the nickel
complex to metallic nickel by direct hydrazine reduction (Equation (10)) [28], and the
presence of OH− in the reaction medium produces a color change to a gray color, indi-
cating the ligand exchange of Cl− ion by OH− to form the nickel hydroxide (Ni(OH)2 )
(Equation (11)) [10,43,45,48]. Subsequently, the color changes from gray to black due to the
subsequent reduction by hydrazine (Equation (12)) [43]. However, the remaining hydrazine
reacts with OH− in solution to generate electrons (Equation (13)) and water to produce
adsorbed hydrogen atoms (H*). The nuclei formed can act as an active center and site for
the adsorption of hydrogen atoms, capturing Ni2+ ions from the solution and reacting to
form Ni◦ (Equation (14)) [10,49].

2Ni2+ (aq) + N2 H4 (aq) + 4OH− (aq) → 2Nio (s) + N2 (g) + 4H2 O(l) (5)

N2 H4 (aq) → N2 (g) + 2H2 (g) (6)

3N2 H4 (aq) → N2 (g) + 4NH3 (aq) (7)
2[Ni(N2 H4 )2 ]Cl2 (aq) + 5N2 H4 (aq) → 2[Ni(NH3 )6 ]Cl2 (s) + 3N2 (g) (8)

2NiCl2 (aq) + nN2 H4 (aq) → 2[Ni(N2 H4 )n ]Cl2 , n = 2, 3 (9)

(aq)

[Ni(N2 H4 )3 ]2+ (aq) + 2N2 H4 (aq) → Nio (s) + 4NH3 + 3N2 (g) + 3H2 (g) + 2H+ (aq) (10)
(g)

[Ni(N2 H4 )n ]Cl2 (aq) + 2NaOH(aq) → Ni(OH)2 (s) + nN2 H4 (aq) + 2NaCl (11)

Ni(OH)2 (s) + N2 H4 (aq) → Nio (s) + N2 (g) + 2H2 O(l) (12)

N2 H4 (aq) + 4OH− (aq) → N2 (g) + 4H2 O(l) + 4e− (13)

AppliedChem 2024, 4 91

Ni2+ (aq) + 2H* (ads) → Nio (s) + 2H+

(aq)
(14)

Alcohols and polyalcohols (polyols) are attractive compounds in synthesized metal

nanoparticles due to their multiple roles in a reaction, acting as a solvent, stabilizer, and/or
reducing agent [50,51]. The polyol reduction mechanism strongly depends on the temper-
ature and generates different products that reflect on nickel synthesis with well-defined
morphology nanoparticles [51,52]. Alcohols and polyols are generally used to reduce metal
ions, and an aldehyde is generated as a by-product of H+ ions (Equation (15)). When
ethylene glycol (EG) is used as a reducing agent, the reaction is conducted at a temperature
above 160 ◦ C, and EG is dehydrated to generate acetaldehyde (Equation (16)). In sequence,
acetaldehyde is responsible for reducing metal ions, accompanied by its oxidation to di-
acetyl (Equation (17)) [53]. In the range from 140 ◦ C to 160 ◦ C, the EG heating in air
generates glycolaldehyde (Equation (18)) and then serves as a reductant for metal ions
while it is oxidized to glyoxal (Equation (19)) [32,50,54]. When the reaction temperature
is below 140 ◦ C, EG acts as a reductant by itself, forming glycolaldehyde as the oxidized
product (Equation (20)) [50].

2R − CH2 OH(eg) + 2Ni2+ (sol) → 2Ni0 (s) + 2R − CHO(sol) + 4H+ (sol) (15)

HOCH2 CH2 OH(sol) → CH3 CHO(sol) + H2 O(sol) (16)

4CH3 CHO(sol) + 2Ni2+ (sol) → 2Ni0 (s) + 2CH3 COCOCH3 (sol) + 4H+ (sol) (17)
2HOCH2 CH2 OH(sol) + O2 (g) → 2HOCH2 CHO(sol) + H2 O(sol) (18)

2HOCH2 CHO(sol) + 2Ni2+ (sol) → 2Ni0 (s) + 2HOCCHO(sol) + 4H+ (sol) (19)

2HOCH2 CH2 OH(sol) + 2Ni2+ (sol) → 2Ni0 (s) + 2HOCH2 CHO(sol) + 4H+ (sol) (20)
Figure 4 shows the ethylene glycol reduction mechanism of nickel [51]. Kyler et al.
propose the reaction mechanism by theoretical modeling, testing different complexes
between nickel and EG [51]. In Figure 4A, the graphical representation shows the change
in energy ∆E (kcal·mol− 1 ) during the reduction reaction between EG and NiCl2 . In this
process, the metal center forms an intermediate phase with EG, and the reduction proceeds
by forming the C−O−Ni bond. In the mechanism, when nickel dissolves in glycol, EG
acts as ligands to form a Ni-glycolate complex. The solution contains OH− ions that take
the H+ from the carbon center to form water, leaving two electrons to create a double
bond with oxygen and two electrons to reduce the metal center. The reaction results
in the production of 2-hydroxyacetaldehyde and 2-hydroxyethanolate (Figure 4B) [51].
However, using only polyol is insufficient to completely reduce all nickel ions and achieve
a well-defined shape and dispersity [46,51,55]. In an effort to address this, researchers
have explored alternative approaches in the literature to achieve the complete reduction of
nickel. These approaches involve introducing additional reducing agents or providing more
energy, employing methods such as hydrazine and NaBH4 , and/or utilizing microwave
irradiation [10,22,28,29,32,38,52,56–62].
The polyol method employs various alcohols and polyols, including ethanol, benzyl
alcohol, 1,2-propanediol, 1,2-butanediol, ethylene glycol, diethylene glycol, triethylene
glycol, tetraethylene glycol, and butylene glycol [32,38,51,61,62]. The choice of alcohols and
polyols introduces particle size and shape variation attributed to the reduction potentials
of these compounds serving as the reductant agent [50,63,64]. As reported by Biacchi and
Schaak, the applied potential required for the initiation of the polyol, and thus the oxidation
potential of the polyols at room temperature and 60 ◦ C, decreases in the order of ethylene
glycol > diethylene glycol > triethylene glycol > tetraethylene glycol. This trend is likely
due to the enhanced electronic stability of alcohols provided by the higher intermolecular
bonding facilitated by the ether functionalities [63].
AppliedChem 2024,44, FOR PEER REVIEW
AppliedChem2024, 927

Figure4.4.The
Figure Theplot
plotof
ofthe
thecalculated
calculatedchange
changeininenergy
energy(A)
(A)and
andreaction
reactionmechanism
mechanismofofNi
Ni reduction
2+2+reduction
by the ethylene glycol (B). Reproduced with permission from [51], American Chemical Society
by the ethylene glycol (B). Reproduced with permission from [51], American Chemical Society Pub-
lications, 2011.
Publications, 2011.

The polyol
Figure methodexamples
5 presents employs of various
nickelalcohols and polyols,
nanoparticles including
synthesized ethanol,
using benzyl
the polyol
alcohol,with
method 1,2-propanediol,
different polyols. 1,2-butanediol, ethylenewere
The upper images glycol, diethylene
produced glycol,
with the same triethylene
quantity
glycol,
of NiCl2tetraethylene
as the precursor, glycol, and butylene oxide
trioctylphosphine glycol(TOPO),
[32,38,51,61,62].
as the cappingThe choice
agent, of alcohols
hydrazine
and
as thepolyols
reducing introduces
agent, and particle size andirradiation
microwave shape variation ◦ C (left and
at 160attributed to middle)
the reduction
and 200 ◦C
poten-
tials ofutilizing
(right), these compounds serving
either ethylene as the
glycol reductant
(Figure 5C) or agent [50,63,64].
diethylene As(Figure
glycol reported by Biacchi
5D,E) as the
solvent.
and Schaak,When theethylene
appliedglycol
potentialis present,
required nanowires are formed
for the initiation withpolyol,
of the a length andof thus
several
the
micrometers and a width of 120 nm. The rough surface is a
oxidation potential of the polyols at room temperature and 60 °C, decreases in the order result of small nanoparticle
agglomeration
of ethylene glycol through the oriented
> diethylene attachment
glycol > triethylenemechanism associated with
glycol > tetraethylene Ostwald
glycol. Thisripen-
trend
ing in an due
is likely anisotropic growth [64].
to the enhanced On thestability
electronic other hand, in the presence
of alcohols providedofby diethylene
the higherglycol,
inter-
agglomerated
molecular bonding spherical nanoparticles
facilitated are observed.
by the ether This suggests
functionalities [63]. that the glycol structure
influences
Figure the5 synthesis, a correlation
presents examples of that becomes
nickel evident when
nanoparticles analyzing
synthesized the the
using molecular
polyol
structures of both ethylene glycol and diethylene glycol (Figure
method with different polyols. The upper images were produced with the same quantity 5A,B).
Figure
of NiCl 2 as 4 thedemonstrates that the hydroxyloxide
precursor, trioctylphosphine group connects
(TOPO), ethylene
as the capping glycol to nickel,
agent, hydra-
creating
zine as the reducing agent, and microwave irradiation at 160 °C (left and middle) formed
a cyclic molecule with the metal. In diethylene glycol, these connections are and 200
by
°Cthe terminal
(right), hydroxyl
utilizing eithergroups
ethylene dueglycol
to stress in the
(Figure structure
5C) of the molecule,
or diethylene leading
glycol (Figure to
5D,E)
the detachment
as the solvent. of the ether
When group
ethylene in theismiddle
glycol present, of nanowires
the molecule. areThis
formedetherwith
group interacts
a length of
with other nickel complexes, inducing agglomeration. In
several micrometers and a width of 120 nm. The rough surface is a result of small nano- the experiments conducted
at 200 ◦ C,
particle the nanoparticles
agglomeration through stillthe
agglomerate, but with mechanism
oriented attachment less intensity and a noticeable
associated with Ost-
chain morphology (Figure 5E). The interpretation of this
wald ripening in an anisotropic growth [64]. On the other hand, in the presence phenomenon is that theofhigher
dieth-
temperature
ylene glycol,weakens the interaction
agglomerated sphericalofnanoparticles
the ether group arewith the nickel
observed. Thiscomplex,
suggestsallowing
that the
for greater
glycol freedom
structure and, consequently,
influences the synthesis, less agglomeration
a correlation thatof the nanoparticles
becomes evident when [22].analyz-
The flower-like (Figure 5F) and urchin-like (Figure 5G) nanostructured
ing the molecular structures of both ethylene glycol and diethylene glycol (Figure 5A,B). materials were
synthesized
Figure using a surfactant-free
4 demonstrates that thesolvothermal
hydroxyl group polyol method.
connects The process
ethylene glycol involved
to nickel,
NiCl2 as the precursor, NaOH as the precipitant agent, a small amount of water (or none),
creating a cyclic molecule with the metal. In diethylene glycol, these connections are
and 1,2-propanediol as the reducing agent and solvent [32]. The nanostructures are formed
formed by the terminal hydroxyl groups due to stress in the structure of the molecule,
by reducing Ni2+ by 1,2-propanediol according to Equations (18) and (19), resulting in the
leading to the detachment of the ether group in the middle of the molecule. This ether
generation of nuclei and random agglomeration, as illustrated in Figure 2. This process
group interacts with other nickel complexes, inducing agglomeration. In the experiments
produces nanospheres with a rough surface. The concentration of water in the reaction
conducted at 200 °C, the nanoparticles still agglomerate, but with less intensity and a no-
alters the growth kinetics, shifting Equation (18) to the left and reducing CH3 CH2 CHO.
ticeable chain morphology (Figure 5E). The interpretation of this phenomenon is that the
Propanal serves as the actual reducing agent for nickel, and an increase in water content
higher temperature weakens the interaction of the ether group with the nickel complex,
effectively slows down the reduction kinetics. Once the nanosphere is formed, the reaction
allowing for greater freedom and, consequently, less agglomeration of the nanoparticles
becomes thermodynamically dominated rather than kinetic. This leads to an anisotropic
[22].
growth direction of a magnetic crystalline, minimizing anisotropic magnetic energy and
The flower-like (Figure 5F) and urchin-like (Figure 5G) nanostructured materials
forming urchin-like nanopagodas with a diameter of 1 µm [32,65–67]. In contrast, during
were synthesized growth,
quasi-equilibrium using a thesurfactant-free
nanoplates grow solvothermal polyol method.
on high-energy facets {111} Thetoprocess
minimize in-
volved NiCl 2 as the precursor, NaOH as the precipitant agent, a small amount of water
total surface free energy according to Ostwald ripening and the Gibbs–Thomson law. This
(or none),
results and
in the 1,2-propanediol
formation as the reducing
of flower-like agent and
nanostructured solvent
material [32].
with The nanostructures
a diameter of 1.5 µm
are formed
and a thickness by reducing
of 10 nm Ni
2+ by 1,2-propanediol according to Equations (18) and (19), re-
[32,68].
sulting in the generation of nuclei and random agglomeration, as illustrated in Figure 2.
 AppliedChem
AppliedChem 2024,4 4, FOR PEER REVIEW
2024, 9
93

Figure 5.
Figure 5. Ethylene
Ethyleneglycol
glycol(A)
(A)and diethylene
and diethyleneglycol (B) (B)
glycol molecules, where
molecules, white,
where red, and
white, red,black
and balls
black
represent the hydrogen, oxygen, and carbon atoms. Nickel nanoparticles synthesized by the polyol
balls represent the hydrogen, oxygen, and carbon atoms. Nickel nanoparticles synthesized by
method in the presence of ethylene glycol (C), diethylene glycol (D,E), 1,2-propanediol (F), 1,2-pro-
the polyoland
panediol method
waterin(G),
theand
presence of ethylene
ethylene glycol
glycol (H). (C), diethylene
The images (C–E) areglycol (D,E),with
reproduced 1,2-propanediol
permission
(F),
from1,2-propanediol
[62], American and waterSociety
Chemical (G), and ethylene glycol
Publications, (H). images
2008. The The images
(F–G)(C–E) are reproduced
are reproduced with
with permission
permission from from [62], American
[32], Royal Society of Chemical
Chemistry,Society Publications,
2011. The image (H) 2008. The images
is reproduced with(F–G) are
permis-
sion from [69],
reproduced withElsevier,
permission2009.
from [32], Royal Society of Chemistry, 2011. The image (H) is reproduced
with permission from [69], Elsevier, 2009.
2Ni ( ) + BH ) + 2H5H
( Figure O( ) + 2nPVP( ) → 2Ni (PVP) ( ) + 2H ( ) + 4H ( ) + BO ( ) (21)
displays nickel nanowires synthesized using a magnetic field-assisted polyol
method. The process involves NiSO4 as the precursor, NaOH as the pH modifier agent,
2Ni as
hydrazine ) + 2H
( the PO ( agent,
reducing )
→ 2Ni and + 2HPO glycol
( )ethylene ( )
+ asH the
O( ) solvent [69]. The magnetic field- (22)
assisted
Onsynthesis
the contrary,method is known
sodium for synthesizing
hypophosphite (NaH2magnetic
PO2) acts materials
as a weaker with ferromagnetic
reducing agent
or
compared to hydrazine. This results in a slower nucleation process, leading to largerWhen
paramagnetic properties, such as iron, nickel, cobalt, copper, and manganese. par-
magnetic
ticle size nanoparticles
and lower particle are exposed to a magnetic
concentration. Being afield, they behave
soft reductant, as permanent
it facilitates magnets
an extended
due to magnetic
nucleation process,interaction
as depicted [70].
in This
Figure method typically produces
6C. A considerable numberone-dimensional
of nuclei are formed (1D)
particles like wires and rods due to the magnetic particles aligning along
during the initial stages of nucleation, resulting in the creation of smaller particles. As the the magnetic line
force
reaction continues, more nickel atoms are reduced and adsorbed onto the primary nuclei.to
[67]. In the synthesis process, the precursor is first added to the solution, applied
the magnetic
These nuclei field,
act asand then
seeds inreduced.
the Ostwald Thisrepeating
leads to the formation
process, of spiky
ultimately nanowires,
producing with
larger
the nucleus dispersed in
and hierarchical particles [74].the solution and fixed in the wire, resulting in spiky protuberances
on theAnother
surface.approach
The magnetic field
utilizing NaHis applied to the solution in the second synthesis, and
2PO4 has been employed to synthesize hollow nano-
the precursor is dropped into it. This slight difference
particles (Figure 6D). Typically, the soft template for micellar in the synthesis
formationprocess
can be results
generated in a
smoother
through an surface
emulsionon the nanowire.
system This is due to
with water-in-oil the droplet-induced
(W/O) [75], polymers [30], process bringing the
or counterions,
nickel nuclei closer to each other, promoting a more even distribution
such as dodecyl sulfate in Ni(DS)2 [76]. Nickel ions are located around the micelle due to [69,71,72].
Other reducing
electrostatic forces at agents haveinterface.
the micelle been employed more of
The reduction discreetly thanupon
nickel occurs thoseadding
discussed
the
above, indicating a significant potential to expand the knowledge
reductant to the solution (Equation (22)) [37], leading to the formation of the shell frontier of synthesizing
nickel nanoparticles.
[30,75,76]. Notably, the Sodium
reducingborohydride
agent must (NaBH 4 ) and
be soft stands out as
reduce thea metal
more potent reducing
at a lower ratio
agent compared to hydrazine
than nickel adsorbed on the micelle (N H ), with reduction potentials of 1.24
2 4surface. Otherwise, it may result in bulk nanoparti-V and 1.16 V, re-
spectively, and the capability to donate double the number of electrons
cles, such as flower- or wire-like nanoparticles [76]. Hollow nickel nanoparticles were (8 electrons). As
demonstrated in Figure 2, when the nucleation and growth processes are well separated,
AppliedChem 2024, 4 94

particle size can be controlled. Fast nucleation produces small particles and a high concen-
tration of particles by consuming all ion precursors simultaneously, while slow nucleation
results in large particles and a low concentration of particles due to gradual reduction over
time [73].
Supporting this argument, Figure 6A illustrates using NaBH4 as a potent reducing
agent in a polyol method with PVP as a capping agent. In this case, a significant number of
nickel ion precursors are simultaneously converted into nuclei (Equation (21)), promoting
a narrow size distribution and small particles with a diameter ranging from 3.4 to 3.8 nm,
along with a high concentration of nanoparticles. Additionally, the presence of PVP and
ethylene glycol stabilizes the nucleus, preventing agglomeration [56]. Similarly, nickel
nanoparticles were synthesized with a narrow size distribution in an aqueous solution with
oleic acid and sodium dodecyl sulfate (SDS) as surfactants (Figure 6B). This outcome is
attributed to the well-controlled nucleation and growth processes facilitated by NaBH4 and
the protective agents that prevent agglomeration [25].

2Ni2+ (eg) + BH4− (eg) + 2H2 O(eg) + 2nPVP(eg) → 2Ni0 (PVP)n (s) + 2H2 (g) + 4H+ (eg) + BO2− (eg) (21)

2Ni2+ (aq) + 2H2 PO2 (aq) → 2Ni0 (s) + 2HPO2− (aq) + H2 O(l) (22)

On the contrary, sodium hypophosphite (NaH2 PO2 ) acts as a weaker reducing agent
compared to hydrazine. This results in a slower nucleation process, leading to larger
particle size and lower particle concentration. Being a soft reductant, it facilitates an
extended nucleation process, as depicted in Figure 6C. A considerable number of nuclei are
formed during the initial stages of nucleation, resulting in the creation of smaller particles.
As the reaction continues, more nickel atoms are reduced and adsorbed onto the primary
nuclei. These nuclei act as seeds in the Ostwald repeating process, ultimately producing
larger and hierarchical particles [74].
Another approach utilizing NaH2 PO4 has been employed to synthesize hollow nanopar-
ticles (Figure 6D). Typically, the soft template for micellar formation can be generated
through an emulsion system with water-in-oil (W/O) [75], polymers [30], or counterions,
such as dodecyl sulfate in Ni(DS)2 [76]. Nickel ions are located around the micelle due to
electrostatic forces at the micelle interface. The reduction of nickel occurs upon adding the
reductant to the solution (Equation (22)) [37], leading to the formation of the shell [30,75,76].
Notably, the reducing agent must be soft and reduce the metal at a lower ratio than nickel
adsorbed on the micelle surface. Otherwise, it may result in bulk nanoparticles, such as
flower- or wire-like nanoparticles [76]. Hollow nickel nanoparticles were synthesized using
citric acid in an alternative method. The use of bis(N-α-amido-glycylglycine)-1,7-heptane
dicarboxylate (HG12) led to the formation of a peptide nanotube. Citric acid, acting as an
intermediate reductant weaker than hydrazine but stronger than NaH2 PO4 , resulted in
nanoparticles with a size of 30 nm on the nanotube surface instead of covering the tube
with a single shell [77].
In nanoparticle synthesis, the choice of reducing agent plays a crucial role in determin-
ing the size, morphology, and properties of the resulting nanoparticles. Reductant agents
can be categorized into three groups based on their reduction potential strength: strong,
medium, and weak. Strong reductants, such as hydrazine and NaBH4 , are known for
inducing burst nucleation, where numerous growth species form simultaneously, leading
to the rapid generation of small and uniform nuclei, smaller than 10 nm. Adjusting the pH
level can enhance the reduction potential of these reductant agents.
Medium-strength reductants, including polyols and citric acid, offer a moderate reduc-
tion potential and reaction rate, resulting in nanoparticles with tailored sizes (approximately
70 nm) and morphologies. Polyols serve multiple roles as reducing agents, solvents, and
stabilizing agents, striking a balance between reduction capability and reaction control.
The polyol reduction mechanism, influenced by factors like temperature and precursor
concentration, yields nanoparticles with diverse morphologies. Additionally, variations in
AppliedChem 2024, 4 95

AppliedChem 2024, 4, FOR PEER REVIEW 10

the reduction potentials of different alcohols and polyols impact their suitability as reducing
agents, thereby influencing the nanoparticle synthesis process. Weak reductant agents, such
as ascorbic acid
synthesized andcitric
using sodiumacidhypophosphite (NaH
in an alternative 2 PO2 ),The
method. possess lower
use of reduction potentials
bis(N-α-amido-glycyl-
and slower reaction rates. These agents are often utilized for the gradual
glycine)-1,7-heptane dicarboxylate (HG12) led to the formation of a peptide reduction of metal
nanotube.
ions, leading to slower nucleation and growth processes. Consequently, weak
Citric acid, acting as an intermediate reductant weaker than hydrazine but stronger thanreductants
require
NaH2PO longer reaction times to convert all precursors, potentially resulting in the formation
4, resulted in nanoparticles with a size of 30 nm on the nanotube surface instead
of
oflarger nanoparticles.
covering the tube with a single shell [77].

Figure6.6. Reducing
Figure Reducing agents: sodium borohydride
borohydride inin polyol
polyolmethod
method(A),
(A),aqueous
aqueoussolution
solution(B),
(B),sodium
sodium
hypophosphite produced
hypophosphite produced in
in bulk
bulk nanoparticles
nanoparticles (C),
(C), and
and hollow
hollow nanoparticles
nanoparticles(D),
(D),where
wherearrow
arrow
points to
points to aa broken
broken hollow
hollow nanoparticle.
nanoparticle. The
The image
image (A)
(A)isisreproduced
reproducedwith
withpermission
permissionfrom
from[56],
[56],
Elsevier, 2007. The image (B) is reproduced with permission from [25], American Chemical Society
Elsevier, 2007. The image (B) is reproduced with permission from [25], American Chemical Society
Publications, 2009. The image (C) is reproduced with permission from [74], Elsevier, 2010. The im-
Publications, 2009. The
age D is reproduced image
with (C) is reproduced
permission from [30], with
John permission from [74],
Wiley and Sons, 2003.Elsevier, 2010. The image
(D) is reproduced with permission from [30], John Wiley and Sons, 2003.
In nanoparticle synthesis, the choice of reducing agent plays a crucial role in deter-
2.2. Stabilizing Agent
mining the size, morphology, and properties of the resulting nanoparticles. Reductant
Stabilizing
agents agents can into
can be categorized takethree
the form of additives,
groups including
based on their organic
reduction molecules,
potential poly-
strength:
mers, or surfactants, as well as ions, such as counterions of precursors
strong, medium, and weak. Strong reductants, such as hydrazine and NaBH4, are known that bind to specific
crystalline
for inducing surfaces.
burst These agentswhere
nucleation, play a crucial role growth
numerous in steric species
stabilization,
form protecting against
simultaneously,
uncontrolled
leading to the rapid generation of small and uniform nuclei, smaller than 10 nm. Adjustingto
growth. However, it is important to note that the exposed plane tends
grow
the pHwhen
levelthe
cansurface
enhanceis stabilized,
the reduction as illustrated
potential ofinthese
Figure 7 [48,49].agents.
reductant This phenomenon is
rootedMedium-strength
in nucleation andreductants,
growth theory, as depicted by the LaMer curve
including polyols and citric acid, offer [50]. In this theory,
a moderate re-
aduction
crystal potential
is an ion and
thatreaction
undergoes reduction to produce metallic atoms, promoting
rate, resulting in nanoparticles with tailored sizes (approx- their
imately 70 nm) and morphologies. Polyols serve multiple roles as reducing agents,
 v (23)
CPP =
a l
As previously mentioned, the head group in the surfactant plays a crucial role in
nanoparticle synthesis, influencing the morphology of micelles and the interaction of the
AppliedChem 2024, 4 96
surfactant with ions, intermediates, and particle surfaces. The charge in the head group in
the surfactant can be cationic (X ), anionic (X ), nonionic (X°), and zwitterionic (X Y ), and
+ − + −

different choices during nickel synthesis can impact the result. Notable examples include
aggregation
cationic surfactants to form
like CTAB, small TEAB,
TBAB, nuclei through
and TC12self-nucleation [51]. As a result,
AB, anionic surfactants the concentration
like SDS,
and nonionic surfactants such as TOP, Tween, and D-sorbitol. Given that the charge ofan increase in
of atoms decreases while the cluster concentration increases, leading to
nickel ions (Ninuclei size, ultimately
2+) is positive forming2 is
and that Ni(OH) tiny
an crystals. The with
intermediate role of the stabilizing
a neutral charge in agent is crucial
the reaction solution, the interaction between surfactants and these entities can influence defined, and
for controlling the shape and/or size because the crystalline surface is well
the stabilizing
the synthesis process. agent can
For instance, CTAB,perform moresurfactant,
a cationic efficiently.is In Figure
known to7a, crystal
reduce massgrowth without
a stabilizing agent is shown, where the nuclei are incorporated
transfer and the reduction process [24,82,83]. In the reaction system, CTAB forms CTA onto all crystal
+ surfaces,
resulting
and faces challenges inin uniform growth.
effectively coating InNiFigure
2+ due 7b, a moderate amount
to electrostatic repulsionof stabilizing
[83]. Underagent is used to
alkaline conditions, CTAB may interact to cover Ni(OH)2 or form CTA Ni(OH)4planes
control the synthesis to form a cube. In this example, the −exposed , whereare
the(111) and (100),
and the stabilizing agent preferentially interacts with
latter is composed of Ni(OH)4 . This interaction with CTAB can pack the Ni(OH)2, pre-
2− the (100) plane, inhibiting its growth.
As a result, the exposed (111) plane grows easily until the desired
venting its reduction to Ni° [24,71,82,83]. In general, stabilizing agents interact with nickel surface is achieved. In
Figure 7c, an excessive amount of stabilizing agent is employed, fixing, and protecting all
ions, Ni(OH)2, and nickel particles through the more energetic groups on molecules, such
exposed surfaces. This inhibits crystal growth, leading to the formation of smaller particles.
as C−O, −OH, N+, S−O, P=O [24,25,34,80,84–87].

Figure 7. Control over nanocrystal shape using capping agents: (a) no capping agent, (b) a moderate
Figure 7. Control over nanocrystal shape using capping agents: (a) no capping agent, (b) a moderate
amount of capping agent, and (c) a large amount of capping agent.
amount of capping agent, and (c) a large amount of capping agent.

As indicated earlier, nanoparticles exhibit high surface energy, resulting in thermo-

dynamic instability at the surface and a propensity to minimize surface energy through
aggregation, driven by the reduction of surface area. Self-aggregation can be enhanced by
van der Waals forces and magnetic dipole–dipole interactions, particularly in magnetic
materials [78–80]. However, stabilizing agents can counteract this tendency by utilizing
π–π interactions, electrostatic forces, and hydrogen bonding to prevent agglomeration.
They play a crucial role in organizing the building blocks into a controlled morphology and
size [72]. Numerous stabilizing agents, including surfactants and other compounds, can
be employed in nickel synthesis, for example, cetyltrimethylammonium bromide (CTAB),
tetraethylammonium bromide (TBAB), tetrabutylammonium bromide (TEAB), tetra dode-
cyl ammonium bromide (TC12 AB), citric acid, sodium dodecyl sulfonate (SDS), Tween 40,
Tween 80, D-sorbitol, PEG 6000, sodium carboxyl methylcellulose (Na-CMC), hydroxyethyl
carboxymethyl cellulose (HECMC), trioctylphosphine oxide (TOPO), trioctylphosphine
(TOP), and poly(vinylpyrrolidone) (PVP) [21,26,34,42,62,73,76,81–88].
The main function of the stabilizing agent is to prevent particle agglomeration and
stabilize the surface charge of the nanoparticles. A critical consideration in selecting the
stabilizing agent is its polarity and electrical charge characteristics, which can be categorized
AppliedChem 2024, 4 97

as cationic, anionic, and nonionic. These characteristics can be expressed by the Critical
Packing Parameter (CPP), representing the minimum interfacial area occupied by the
stabilizer molecule. The CPP equation is defined as depicted in Equation (23), where υ is
the volume of the hydrophobic chain, lc is the length of the hydrophobic chain, and ao is the
interfacial area occupied by the hydrophilic head group [29,81]. Estimating this parameter
can provide insights into molecular packing and the preferred stabilizer structure, whether
spherical or cylindrical. This phenomenon is exemplified using cetyltrimethylammonium
bromide (CTAB) in a polyol method [29]. CTAB exhibits two Critical Micelle Concentrations
(CMC) at 2 mM and 20 mM. At concentrations below 2 mM, nanoparticles form with a
spherical-like shape, while concentrations above 20 mM result in nanoparticles with a
wire-like shape. This behavior can be explained by the tendency of the hydrophobic group
(tail) to minimize contact with polar molecules like water, while the hydrophilic group
(head) maximizes contact [81].
v
CPP = (23)
a0 lc
As previously mentioned, the head group in the surfactant plays a crucial role in
nanoparticle synthesis, influencing the morphology of micelles and the interaction of the
surfactant with ions, intermediates, and particle surfaces. The charge in the head group in
the surfactant can be cationic (X+ ), anionic (X− ), nonionic (X◦ ), and zwitterionic (X+ Y− ),
and different choices during nickel synthesis can impact the result. Notable examples
include cationic surfactants like CTAB, TBAB, TEAB, and TC12 AB, anionic surfactants like
SDS, and nonionic surfactants such as TOP, Tween, and D-sorbitol. Given that the charge
of nickel ions (Ni2+ ) is positive and that Ni(OH)2 is an intermediate with a neutral charge
in the reaction solution, the interaction between surfactants and these entities can influence
the synthesis process. For instance, CTAB, a cationic surfactant, is known to reduce mass
transfer and the reduction process [24,82,83]. In the reaction system, CTAB forms CTA+
and faces challenges in effectively coating Ni2+ due to electrostatic repulsion [83]. Under
alkaline conditions, CTAB may interact to cover Ni(OH)2 or form CTA− Ni(OH)4 , where
the latter is composed of Ni(OH)4 2− . This interaction with CTAB can pack the Ni(OH)2 ,
preventing its reduction to Ni◦ [24,71,82,83]. In general, stabilizing agents interact with
nickel ions, Ni(OH)2 , and nickel particles through the more energetic groups on molecules,
such as C−O, −OH, N+ , S−O, P=O [24,25,34,80,84–87].
The stabilizing agent plays a crucial role in the synthesis of nanoparticles. Figure 8
illustrates examples of stabilizing agents utilized in the synthesis of nickel nanostructured
materials. The impact of stabilizing agents in hydrothermal syntheses is demonstrated
without cetyltrimethylammonium bromide (CTAB) (Figure 8A) and with CTAB (Figure 8B).
In the absence of CTAB, crystals can grow in all directions, forming a rod (secondary
particle) within the primary particle. However, when CTAB is present, the surfactant forms
a layer on the initial growth surface, preventing growth around the thorn side faces and
promoting growth on top where there is no surfactant. This results in the formation of
urchin-like nanobelts due to steric hindrance [24]. Certain organic modifiers can serve a
dual function. For instance, ethylene glycol can act as both a solvent and a stabilizing agent,
while citric acid, as shown earlier, serves as a reducing agent and a stabilizing agent in
this example. In this case, hierarchical nanostructured materials were synthesized using
an ultrasonic alcohol method with hydrazine and citric acid. The combination of these
reductant molecules initially formed small particles measuring 9 nm. With citric acid
preventing agglomeration, the nanoparticles aggregated due to magnetic dipole interaction,
reducing the anisotropic magnetic energy, and forming secondary spherical particles with
a diameter of 254 nm [84].
The number and type of anchors on the polymer stabilizing agent can indicate the
strength of the interaction between the stabilizing agent and the particle surface, while
the size of the polymer can reflect the degree of coverage and protection. In Figure 8, two
PVP chain lengths are depicted, with molecular weights of 10,000 (Figure 8D) [55] and
30,000 (Figure 8E) [62]. PVP has one anchor point (C=O) in each monomer of the molecule
AppliedChem 2024, 4 98

chain. In PVP-10,000, the particle surface appears rough with protrusions, indicating a
less protective layer that allows nuclei diffusion and surface particle growth. Conversely,
PVP-30,000 exhibits a smooth particle surface, suggesting that a longer polymer chain
better prevents nucleus diffusion to the surface and promotes growth. Hydroxyethyl
cellulose (HEC) (Figure 8F) [85] and Tween 80 (Figure 8G) [21] possess multiple anchors
(C–O–C) or/and (C=O), enabling these polymers to be anchored at multiple points. This
allows the polymers to interconnect on various faces in each HEC molecule, forming a
sponge nanoflower.
Certain stabilizing agents exhibit a preference for binding to the {111} crystallographic
plane due to the lower activation energy associated with this plane. By passivating the {111}
plane, growth occurs anisotropically in the <110> plane, which has higher energy [72,89–91].
Figure 8H illustrates triangular nanoplates synthesized through a thermal decomposi-
tion method utilizing oleic acid, oleylamine, and octadecene. Initially, small triangular
nanoplates nucleate within the {111} plane and then grow along the <110> direction, re-
sulting in the formation of larger triangular nanoplates. These small particles align along
the <110> lateral planes, connecting and growing into large triangular and/or hexagonal
nanoplates [90].
Another example is Figure 8I, which shows the flower-like nanostructured materials
with hexagonal columnar petals synthesized by the alcohol method using NaOH, hydrazine,
and dimethylglyoxime (DMG) as the stabilizing agent. This hierarchical structure is formed
by a spherical particle as the primary structure, followed by the confinement of DMG in
hexagonal protrusions on the surface of the sphere to form the initial hexagonal nanoplates.
The DMG is absorbed on the (111) planes and passivates to form the tops and bottoms
of the hexagonal nanoplates, unaffected on the six side (110) planes. The columns are
formed by well-aligning the tops and bottoms of the hexagonal plates (inset Figure 8I) due
to magnetic interactions, resulting in a (111) column growth [72].
In summary, the stabilizing agent is a relevant variable in the synthesis of nickel
nanoparticles by preventing agglomeration and controlling surface charge, size, and mor-
phology. These agents, ranging from organic molecules to polymers and ions, employ
various mechanisms such as steric hindrance and electrostatic forces to maintain particle
dispersion and stability. The choice of stabilizing agent is crucial, considering factors like
polarity and charge characteristics, which influence their interaction with particle surfaces
and intermediates during synthesis. For instance, cationic surfactants like CTAB can hinder
mass transfer and the reduction process, impacting the morphology and size of nanoparti-
cles. Additionally, the number and type of anchors on polymer stabilizing agents dictate
the strength of interaction and degree of surface coverage, thus influencing particle growth
and morphology. Certain stabilizing agents exhibit a preference for binding to specific
crystallographic planes, directing anisotropic growth and resulting in unique nanostruc-
tures. Overall, understanding the interplay between stabilizing agents and nanoparticles
is essential for tailoring synthesis processes and achieving desired material properties
and structures.

2.3. Reaction Temperature

The temperature represents a significant influence on the size, morphology, and
properties of the synthesis of nickel nanoparticles. Temperature impacts reaction kinetics,
nucleation, and growth rates, thereby influencing the particle size distribution, morphology,
and properties of the nickel nanoparticles [92–95]. Elevated temperatures often accelerate
reaction rates, leading to faster nucleation and subsequent particle growth, which can result
in larger nanoparticles with distinct morphologies [94]. Conversely, lower temperatures
tend to impede reaction kinetics, promoting controlled nucleation and growth, ultimately
yielding smaller and more uniform nanoparticles [94].
Additionally, temperature exerts a profound influence on the thermodynamics of
nanoparticle formation. The energy landscape of the synthesis process is intricately linked
to temperature, affecting the stability and crystallinity of the resulting nanoparticles [93].
AppliedChem 2024, 4, FOR PEER REVIEW 14

and intermediates during synthesis. For instance, cationic surfactants like CTAB can hin-
AppliedChem 2024, 4 99
der mass transfer and the reduction process, impacting the morphology and size of nano-
particles. Additionally, the number and type of anchors on polymer stabilizing agents dic-
tate the strength of interaction and degree of surface coverage, thus influencing particle
Higher
growth temperatures
and morphology. may Certain
facilitatestabilizing
the overcoming of energy
agents exhibit barriers, leading
a preference to the
for binding to
formation of more thermodynamically stable structures. On the other hand, lower
specific crystallographic planes, directing anisotropic growth and resulting in unique tempera-
tures can promote
nanostructures. the retention
Overall, of metastable
understanding phases. This
the interplay thermodynamic
between interplay
stabilizing agents andnot
na-
only governs the structural properties of the nanoparticles but also plays a crucial
noparticles is essential for tailoring synthesis processes and achieving desired material role in
determining
properties andtheir long-term stability and performance in various applications [96].
structures.

Figure 8.
Figure 8. Stabilizing
Stabilizingagent
agentemployed
employed ininsynthesis
synthesis of nickel nanoparticles.
of nickel Nickel
nanoparticles. nanoparticles
Nickel with-
nanoparticles
out (A) CTBA, and with CTAB (B), hydroxyethyl cellulose (C), PVP (MW 10,000)
without (A) CTBA, and with CTAB (B), hydroxyethyl cellulose (C), PVP (MW 10,000) (D), PVP (MW (D), PVP (MW
30,000) (E), citric acid (F), Tween 80 (G), octadecene (H), and dimethylglyoxime (I). The images (A)
30,000) (E), citric acid (F), Tween 80 (G), octadecene (H), and dimethylglyoxime (I). The images (A)
and (B) are reproduced with permission from [24], Elsevier, 2006. The image (C) is reproduced with
and (B) are reproduced with permission from [24], Elsevier, 2006. The image (C) is reproduced with
permission from [84], American Chemical Society, 2009. The image (D) is reproduced with permis-
permission
sion from from [84], American
[55], Elsevier, 2005. Chemical
The image Society,
(E) is 2009. The image
reproduced with(D) is reproduced
permission fromwith
[62],permission
American
from [55], Elsevier,
Chemical Society, 2005.
2008. The
The image
image (E)
(F) is
is reproduced
reproducedwith withpermission
permissionfrom
from[62],
[85],American
Elsevier, Chemical
2014. The
Society, 2008.is The
image (G) image (F)with
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from from [85],
[21], Elsevier, 2018.Elsevier,
The image2014.(H)
Theisimage (G) is
reproduced
with permission
reproduced from [90], American
with permission from [21],Chemical Society,
Elsevier, 2018. The2007. The
image image
(H) (I) is reproduced
is reproduced with per-
with permission
mission
from [90],from [72], American
American ChemicalChemical Society,
Society, 2007. The2007.
image (I) is reproduced with permission from [72],
American Chemical Society, 2007.
2.3. Reaction Temperature
The
The temperature
temperature also influences
represents the degree
a significant of freedom
influence in size,
on the atommorphology,
mobility within
and
nanoparticles, playing a decisive role in shaping their properties [97]. At elevated
properties of the synthesis of nickel nanoparticles. Temperature impacts reaction tem-
kinetics,
peratures, the increased kinetic energy of atoms promotes greater mobility, facilitating
dynamic processes such as nucleation, growth, and rearrangement. This heightened degree
of freedom allows atoms the exploration of a broader range of positions and orientations,
influencing the size, morphology, and crystal structure of nanoparticles [98].
AppliedChem 2024, 4 100

As an example of the influence of temperature on the synthesis of nickel nanoparticles,

Bai et al. reported the synthesis of nickel nanoparticles at different temperatures to produce
nanostars and nanospheres, as depicted in Figure 9 [98]. The nickel nanoparticles were
synthesized using a solvothermal method with ethanol as a solvent, hydrazine as a reducing
agent, and NaOH to increase the reduction potential of the reducing agent. The synthetic
methodology naturally presents high kinetic conditions. At lower temperatures (60 ◦ C),
nanostars with well-defined pronounced arms were produced. At 60 ◦ C (Figure 9A), the
reaction medium exhibits a lower degree of freedom of mobility of growth species, which
are anchored in the first site with higher energy to grow the arms. However, after increasing
the temperature to elevated values, such as 100 ◦ C (Figure 9B) and 140 ◦ C (Figure 9C),
the spikes become smaller and with lower definition, forming a spherical morphology at
180 ◦ C (Figure 9D). This indicates that the degree of freedom of the reaction is increasing,
AppliedChem 2024,and thePEER
4, FOR growth species can have greater mobility on the nanostructured material surface to
REVIEW
minimize the free energy of the system [62,98].

Figure
Figure 9. Influence 9. Influencein
of temperature ofthe
temperature
synthesis in
of the synthesis
nickel of nickelatnanoparticles
nanoparticles (A) 60 ◦ C, (B)at100
(A)◦60
C, °C, (B) 10
(C) 140 °C, and (D) 180 °C. Reproduced with permission from [98], Elsevier, 2008.
(C) 140 ◦ C, and (D) 180 ◦ C. Reproduced with permission from [98], Elsevier, 2008.

In summary,3. Purification
the reaction temperature is a crucial factor in the synthesis of nickel
The purification
nanoparticles by controlling the size,process in the and
morphology, synthesis of nanoparticles
properties. is a crucial step to en
Elevated temperatures
the quality,
typically accelerate uniformity,
reaction and functionality
rates, resulting of the resulting
in faster nucleation nanoparticles
and particle growth,and
ul-should n
timately yielding larger nanoparticles with distinctive morphologies. Conversely, lower above
neglected, and should be performed with careful consideration. As discussed
nickel nanoparticles are synthesized through different synthesis methods that carr
products and unreacted components at the end of the synthesis, which are impurit
the purification step. The presence of these contaminants can significantly impact th
bility, physicochemical properties, and applicability of the nanoparticles. Storing n
particles in the reactional medium can ensure that the nanoparticles continue rea
with the components of the synthesis and growth mechanism, such as Ostwald ripe
AppliedChem 2024, 4 101

temperatures hinder reaction kinetics, promoting controlled nucleation and growth, leading
to the formation of smaller and more uniform nanoparticles.

3. Purification
The purification process in the synthesis of nanoparticles is a crucial step to ensure
the quality, uniformity, and functionality of the resulting nanoparticles and should not
be neglected, and should be performed with careful consideration. As discussed above,
the nickel nanoparticles are synthesized through different synthesis methods that carry
by-products and unreacted components at the end of the synthesis, which are impurities
in the purification step. The presence of these contaminants can significantly impact
the stability, physicochemical properties, and applicability of the nanoparticles. Storing
nanoparticles in the reactional medium can ensure that the nanoparticles continue reacting
with the components of the synthesis and growth mechanism, such as Ostwald ripening and
intraparticle ripening, which can work to minimize the surface energy of the nanoparticle,
potentially changing the size, morphology, and properties. Moreover, the physicochemical
properties of the nanoparticles are strongly dependent on a clear surface to exhibit their
full potential [99].
In general, nanoparticles can be purified using various methodologies, including
centrifugation, magnetic separation, membrane separation (filtration, ultrafiltration, and
dialysis), chromatography methods, and thermal treatment purification [96,99–102]. How-
ever, the specific purification method for each class of nanoparticles may vary. The most
common method for purifying nanoparticles is centrifugation, wherein the nanoparticle
suspension undergoes high-speed rotation to separate the nanoparticles by sedimentation,
based on differences in size, density, and shape, from surrounding unwanted impurities in
the supernatant. Centrifugation can isolate nanoparticles with a specific size distribution,
contributing to the homogeneity of the nanoparticles through parameters such as rotational
speed, centrifugation time, temperature, solvent, viscosity of the medium, and the number
of centrifugation cycles [99,102].
Magnetic separation is a valuable methodology that strategically exploits the magnetic
properties to purify magnetic nanoparticles, such as nickel nanoparticles [102–104]. The
nanoparticle suspension is exposed to an external magnetic field, causing the magnetic
nanoparticles to be selectively separated from the solution while non-magnetic impurities
remain dispersed in the supernatant [92]. Magnetic separation can be controlled through
factors like magnetic field strength, distance from the magnetic source, duration of magnetic
exposure, viscosity of the medium, and temperature [99,102,104].
Another relevant aspect of purification is the affinity and chemical nature of the impu-
rities and solvents used in the separation process. Unwanted components can be classified
into ionic and molecular components, and the separation of molecular components can
be based on solubility, polarity, or dielectric constant [92,105]. Ionic components, which
are soluble in water, can be completely removed using water as a solvent with successive
separation cycles [103,106]. Molecular compounds can be eliminated using molecular sol-
vents such as ethanol, isopropyl alcohol, and acetone, depending on the solubility, polarity,
and dielectric constant of the impurity. In a hypothetical synthesis involving different
components with distinct solubility, polarity, and dielectric constant, it is necessary to use
solvents with ascending or descending dielectric constants in sequence to enable miscibility
between the anterior and posterior solvents [105,106].

4. Conclusions
Significant advances have been made in the synthesis of nickel nanoparticles, with
a focus on understanding and manipulating various variables and parameters. This re-
view emphasizes the exploration of physical and chemical factors, including the precursor
concentration, characteristics of the reducing agent, reaction temperature, and stabilizing
agents, to achieve well-controlled particle size and morphological distribution. The discus-
sion begins with an examination of the nickel salt precursor and its concentration, which
AppliedChem 2024, 4 102

crucially determine the size and morphology of the resulting nanoparticles. Subsequently,
the role of the reducing agent, such as hydrazine, polyalcohols, and sodium borohydride,
is discussed, highlighting its dual function in reducing nickel ions and stabilizing the
reaction. The concentration and nature of the reducing agent play a significant role in
shaping the final characteristics of the nanoparticles, necessitating careful selection and
optimization for desired structures. The reductant agents can be categorized into three
groups based on their reduction potential strength: strong, medium, and weak. Strong
reductants, such as hydrazine and NaBH4 , induce burst nucleation leading to the rapid
generation of small (<10 nm) and uniform nuclei. Medium-strength reductants, including
polyols and citric acid, offer a moderate reduction potential and reaction rate, resulting in
nanoparticles with tailored sizes and morphology. Weak reductant agents, such as ascor-
bic acid and NaH2 PO2 , possess lower reduction potentials, resulting in the formation of
larger nanoparticles. Stabilizing agents are fundamental in preventing uncontrolled growth
and agglomeration through steric or electrostatic stabilization. The choice of stabilizing
agents, such as organic molecules, polymers, surfactants, and ions, is influenced by their
ability to interact with specific crystalline surfaces, ultimately controlling the synthesis
process and resulting in well-defined structures. The impact of reaction temperature on
kinetics, nucleation, and growth rates is discussed, providing insights into its influence
on the size and morphology of nanoparticles. The thermodynamics of nanoparticle for-
mation is elucidated, demonstrating how temperature affects stability, crystallinity, and
long-term performance. Following the synthesis process, the purification step is examined,
emphasizing the importance of removing impurities to ensure the quality and uniformity
of nickel nanoparticles. Considering factors like rotational speed, magnetic field strength,
and solvent characteristics underscores the importance of the purification process. In this
context, this review posits that advancements in nickel nanoparticle synthesis, enabling
precise tuning of multiple physical and chemical parameters, are pivotal for enhancing our
comprehension of, and driving progress in, the fields of chemistry and nanoscience.

Author Contributions: Writing—original draft preparation, F.A.e.S.; writing—review, V.M.M.S.;

writing—review and editing, T.S.R. All authors have read and agreed to the published version of
the manuscript.
Funding: We are grateful to the Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro
(FAPERJ), grant numbers E-26/201.431/2021 and E-26/211.612/2019; to the Conselho Nacional
de Desenvolvimento Científico e Tecnológico—CNPq, grant number 317288/2021-0; and to the
Coordenação de Aperfeiçoamento de Pessoal de Nível Superior—Brasil (CAPES)—Finance Code 001,
grant number 88887.645934/2021-00.
Conflicts of Interest: The authors declare no conflicts of interest.

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