Synthesis and characterization of colourful aluminates based on nickel and zinc

Article abstract of DOI:10.1016/j.jallcom.2019.152477

Gradual changes in the chemical composition of spinel cause variations in the distribution of cations in sites, which affects their properties and colouration. Thus, pigments with nickel-zinc spinel structures (Ni_xZn_1-xAl₂O₄) were synthesized by combustion and characterized by varying the chromophore ion content (x = 0; 0.1; 0.2; 0.4 and 1) and calcination temperature (600°, 800° and 1000 °C). Characterization of aluminates through thermal analysis (TGA) showed a less than 10% mass loss until approximately 500 °C in not-calcined samples, indicating the combustion reaction could be classified as adequate. The spinel phase was confirmed in all samples by X-ray diffraction (XRD) with segregated phases: ZnO in samples with x = 0 and NiO in samples with x = 0.4 and 1. The post-synthesis thermal treatment at high temperature led to higher crystallinity of the material and the incorporation of secondary phases into the spinel structure. The crystallite size varied between 22.9 and 43.2 nm. Scanning Electron Microscopy (SEM) analysis indicated that the pore size increased with the increasing calcination temperature, and infrared spectroscopy indicated the displacement of the AlO6 band (665 cm^?1) to AlO4 (730 cm^?1) after the addition of Ni²⁺ ions, revealing the inversion of the crystalline structure. Colorimetric analysis by the CIELab method showed the colour of the zinc-nickel pigments ranged between white (x = 0) and cyan tones (0 < x ≤ 1). Furthermore, the significant influence of the chromophore ion content and calcination temperature on the colouration of the synthesized pigments was verified by the Tukey test at 5% in terms of the luminosity (L*) and chromaticity (a* and b*). The difference between dry powder and that dispersed in varnish was considered evident. The colorimetric data were used to develop a mathematical model to satisfactorily describe the values of L*, a* and b* as functions of the calcination temperature and nickel load.

Full text of DOI:10.1016/j.jallcom.2019.152477

Journal of Alloys and Compounds xxx (xxxx) xxx
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Synthesis and characterization of colourful aluminates based on nickel
and zinc
Thais Cristine de Sousa Santos b, Ana Carolina Monte Almeida b,
Darllan do Rosario Pinheiro c, Cristiane Maria Leal Costa a, Diego Cardoso Estumano d,
a, *
~
Nielson Fernando da Paixao Ribeiro
a
ꢀ
Faculty of Chemical Engineering FEQ/ITEC/UFPA, Rua Augusto Correa, 01, CEP 66075-110, Belem, PA, Brazil
Pos-graduation of Chemical Engineering PPGEQ/ITEC/UFPA, Rua Augusto Correa, 01, CEP 66075-110, Belem, PA, Brazil
Natural Resources Engineering PRODERNA/ITEC/UFPA, Rua Augusto Correa, 01, CEP 66075-110, Belem, PA, Brazil
Faculty of Biotechnology e ICB, Federal University of Para - UFPA, Brazil
b
c
ꢀ
ꢀ
d
ꢀ
a r t i c l e i n f o
a b s t r a c t
Article history:
Gradual changes in the chemical composition of spinel cause variations in the distribution of cations in
sites, which affects their properties and colouration. Thus, pigments with nickel-zinc spinel structures
(NixZn1-xAl2O4) were synthesized by combustion and characterized by varying the chromophore ion
content (x ¼ 0; 0.1; 0.2; 0.4 and 1) and calcination temperature (600ꢀ, 800ꢀ and 1000 ꢀC). Character-
ization of aluminates through thermal analysis (TGA) showed a less than 10% mass loss until approxi-
mately 500 ꢀC in not-calcined samples, indicating the combustion reaction could be classified as
adequate. The spinel phase was confirmed in all samples by X-ray diffraction (XRD) with segregated
phases: ZnO in samples with x ¼ 0 and NiO in samples with x ¼ 0.4 and 1. The post-synthesis thermal
treatment at high temperature led to higher crystallinity of the material and the incorporation of sec-
ondary phases into the spinel structure. The crystallite size varied between 22.9 and 43.2 nm. Scanning
Electron Microscopy (SEM) analysis indicated that the pore size increased with the increasing calcination
temperature, and infrared spectroscopy indicated the displacement of the AlO6 band (665 cmꢁ1) to AlO4
(730 cmꢁ1) after the addition of Ni2þ ions, revealing the inversion of the crystalline structure. Colori-
metric analysis by the CIELab method showed the colour of the zinc-nickel pigments ranged between
white (x ¼ 0) and cyan tones (0 < x ꢂ 1). Furthermore, the significant influence of the chromophore ion
content and calcination temperature on the colouration of the synthesized pigments was verified by the
Tukey test at 5% in terms of the luminosity (L*) and chromaticity (a* and b*). The difference between dry
powder and that dispersed in varnish was considered evident. The colorimetric data were used to
develop a mathematical model to satisfactorily describe the values of L*, a* and b* as functions of the
calcination temperature and nickel load.
Received 4 June 2019
Received in revised form
25 September 2019
Accepted 26 September 2019
Available online xxx
Keywords:
Pigments
Spinel
Aluminate
NickeleZinc
Combustion
© 2019 Elsevier B.V. All rights reserved.
1. Introduction
addition to new processing routes [1e3].
A good pigment has particular features, such as a crystalline
Natural and synthetic pigments have important applications as
colouring agents in industries including ceramics, paints, plastics,
varnishes, and others. The main properties of pigments are influ-
ence aesthetic colouring, and users search for new and exclusive
high-quality products. To meet this demand, it is necessary to focus
research on more resistant materials and exclusive colourings in
structure, thermal and chemical stability, adequate granulometry,
stable colour, homogeneity, reproducibility and low toxicity
whenever possible [4]. In this context, spinel appears to be a ma-
terial possessing all of these characteristics [5]. Spinel has a general
chemical formula (A1-xBx)[AxB2-x]O4 with a CFC unit cell, where A is
a bivalent cation and B is a trivalent cation; the parentheses
represent tetrahedral sites, while the brackets represent octahedral
sites, and X is the inversion parameter. In normal spinel, A3þ ions
occupy octahedral positions and B2þ ions occupy tetrahedral po-
sitions in the structure. In inverse spinel, the bivalent cations and
* Corresponding author.
~
E-mail address: nielson@ufpa.br (N. Fernando da Paixao Ribeiro).
https://doi.org/10.1016/j.jallcom.2019.152477
0925-8388/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: T. Cristine de Sousa Santos et al., Synthesis and characterization of colourful aluminates based on nickel and zinc,
Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152477

2
T. Cristine de Sousa Santos et al. / Journal of Alloys and Compounds xxx (xxxx) xxx
half of the trivalent cations are in octahedral positions, and the
remaining trivalent cations are in tetrahedral positions. Between
the two extremes, an intermediate stage exists with a random
distribution of cations [6,7]. These materials have refractory, mag-
netic, semiconducting and chromatic properties in addition to
stability even under harsh thermal conditions and redox industry
treatments [8]. The gradual incorporation of transition metal ions
into spinel structures and the synthetic route affect their physical,
optical and chemical properties. The most visible alteration is in
terms of the colouring, and therefore, complex oxides are widely
used to form new ceramic pigments [5,6,9].
2. Experimental
In the synthesis of colourful aluminate pigments, the chemical
reagents were purchased from commercial sources: nickel nitrate
[Ni(NO3)2.6H2O, Vetec-Brazil], zinc nitrate [Zn(NO3)2.6H2O, Synth-
Brazil], aluminium nitrate [Al(NO3)3.9H2O, Synth-Brazil], and
glycine [NH2CH2COOH, SIGMA-Brazil]. All materials were used as
received, and experiments were carried out with distilled water.
2.1. Sample preparation
One subgroup of such materials contains aluminates, an
important materials class, due to their high thermal stability,
ductility, hardness, hydrophobicity, superficial low acidity, low
temperature of synthesis and high cation diffusion. These proper-
ties are particularly interesting when spinel is used as an inorganic
ceramic pigment in high temperature and/or catalytic materials
(support or catalyst) [10,11].
Spinel synthesis is traditionally realized by conventional
methods [12,13] due to their ease of implementation at the in-
dustrial level. By contrast, alternative routes allow for better char-
acterized pigments and/or lower environmental impacts. Most
unconventional techniques use water; these include co-
precipitation [14,15], sol-gel processes [16], polymeric precursor
methods (Pechini) [17] and combustion [18,19]. The last is based on
an exothermic reaction and is self-supporting between the reduc-
tant propellant and oxidant agent within a homogeneous solution
in addition to being an easy, cheap and efficient method to product
homogeneous and pure nanometric materials [18e21]. The gas
released in the synthesis is essential for the nanometric material
size, and the amount of released gas depends on the fuel used
[22,23]. Glycine is one of the most commonly used fuels because it
is a low-cost amino acid and a large molecule that hinders the
contact between metallic ions in solution, resulting in materials
with nanometric particle sizes [24e26].
Currently, blue pigments are mostly produced starting with
cobalt ions, but these ions are toxic, scarce and costly [27]. Souza
et al. [28] synthesized blue pigments based on the cobalt-
containing CoxZn1-xAl2O4 (x ¼ 0; 0.1; 0.3; 0.5; 0.7; 0. 9 and 1) us-
ing the polymeric precursor method, with the aim of reducing the
cost of production and lowering the amount of toxic chromophore
ions; they found a blue colouring in many shades, which became
more intense with the increasing amount of cobalt. Tang et al. [29]
synthesized blue pigments of cobalt in high purity without a
dopant by a sol-gel method in jet printing applications for roofing
tile ceramic decorations.
Currently, nickel is being investigated as an alternative to cobalt
in the production of blue pigments. Visinescu et al. [30] studied the
synthesis of NixZn1-xAl2O4 (x ¼ 0.1, 0.2, 0.4, 0.6, 0.8, 1) by a sol-gel
method for application as the catalyst in oxidative methane
reforming. The colours obtained varied between blue and green,
with the best characterization of colours at x ¼ 0.6. Lorenzi et al. [5]
synthesized nickel-doped gahnite (ZnAl2O4) to measure blue and
green products separately to explain the colouration. It was
concluded that the Ni2þ in the tetrahedral sites determines the blue
colour even in small amounts and that NiO is responsible for the
greenish colouration. Therefore, considering that zinc aluminate
can be used to support ions and nickel is a chromophore ion, this
work describes the synthesis of NixZn1-xAl2O4 spinels by combus-
tion, using glycine as the propellant. The structural, morphological
and colorimetric properties were found to be functions of the
dopant ion concentration and the calcination temperature.
Furthermore, a mathematical model is proposed to describe the
behaviour of the variables L*, a* and b* by the CIELab method for
the evaluation of the final synthesized pigment.
Spinels (NixZn1-xAl2O4) were synthesized by a combustion
method. Metal nitrates and the fuel were dissolved in water and
heated at 80 ꢀC under constant stirring until total water evapora-
tion and full gel formation. When the gel formed, the temperature
of the hot plate was increased to achieve the ignition temperature
of the propellant. The time of the combustion reaction was
approximately 10 s until reaching the high flame temperature.
Therefore, to investigate the influence of the calcination tempera-
ture on the final properties, each material was separated into four
portions; three were calcined in a muffle furnace at three different
temperatures (up to 600ꢀ, 800ꢀ and 1000 ꢀC) for 6 h. The solid
calcined materials were ground with an agate mortar and pestle.
Finally, the powders were stored for further characterization and
application.
The amount of fuel and nitrates used in the preparation of the
aluminates was based on a stoichiometric balance by the following
P
equation: nivi ¼ 0, where ni is the number of moles of each
reactant and vi is the oxidation number, so that the fuel/oxidizer
ratio corresponds to the stoichiometric ratio. The combustion re-
actions can be described as follows:
xNi(NO3)2.6H2O þ (1-x)
Zn(NO3)2.6H2O þ 2Al(NO3)3.9H2O þ 4.4NH2CH2COOH / NixZn1-
xAl2O4 þ 35.1H2O þ 8.9CO2 þ 6.2N2
The powder materials were prepared with nominal composi-
tions of x ¼ 0; 0.1; 0.2; 0.4 and 1. According to the preparation, they
were identified by the composition of nickel (x) and the calcination
temperature (y). For example, Ni0.1Zn600 refers to a sample con-
taining 0.1 nickel and calcined at 600 ꢀC. Furthermore, samples
without thermal treatment are designated as “SC”.
2.2. Thermodynamic calculation of flame temperature
To calculate the adiabatic flame temperature of combustion, Eqs.
(1)e(3) were used [31]. The flame temperature is the highest
temperature achieved during the combustion reaction.
X
X
D
Ho ¼
nD
HoP ꢁ
n
D
HRo
(1)
T
ð
jD
Hoj ¼
nCPdT
(2)
To
CP
R
¼ A þ BT þ CT2 þ DTꢁ2
(3)
Where n is the number of moles;
D
HoR and
D
HoP are the standard
enthalpies of formation of the reactants and products, respectively;
and T is the adiabatic flame temperature and starting temperature
(298 K). Cp is the heat capacity of the products at constant pressure.
The thermodynamic data of the reactants and products [31,32] are
available from the literature and listed in Tables 1 and 2. Due to a
Please cite this article as: T. Cristine de Sousa Santos et al., Synthesis and characterization of colourful aluminates based on nickel and zinc,
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T. Cristine de Sousa Santos et al. / Journal of Alloys and Compounds xxx (xxxx) xxx
3
Table 1
space.
Thermodynamic data required for the calculation of the adiabatic flame temperature
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
[32].
D
E ¼
D
L2 þ
D
a2 þ
D
b2
(4)
Compound
Entalphy of formation (kcal.molꢁ1
)
In the CIELab system, L* is the degree of lightness or darkness,
Ni(NO3)2.6H20(c)
Zn(NO3)2.6H20(c)
Al(NO3)3.9H20(c)
NH2CH2COOH(c)
ZnAl2O4(c)
ꢁ534.83
ꢁ551.30
ꢁ897.57
ꢁ126.31
ꢁ493.26
ꢁ457.60
ꢁ489.69
ꢁ486.13
ꢁ479.0
ꢁ57.80
ꢁ94.05
0
ranging from white (L* ¼ 100) to black (L* ¼ 0). Positive values of a*
correspond to red, and negative values correspond to green. Posi-
tive values of b* correspond to yellow, while negative values
correspond to blue.
NiAl2O4(c)
Ni0,1Zn0,9Al2O4(c)a
Ni0,2Zn0,8Al2O4(c)a
Ni0,4Zn0,6Al2O4(c)a
H2O(g)
3. Results and discussions
CO2(g)
N2(g)
3.1. Pigment synthesis
(c): crystalline; (g): gas.
the entalphy of formation was calculated in the appropriate stoichiometric
proportions of pure spinel: NiAl2O4 and ZnAl2O4.
In this study, synthesis of the pigments resulted in materials
with a spongy, extremely porous and fragmentary appearance,
which is characteristic of this experimental route; the results are
shown in Fig. 1. As seen in the literature, the combustion method
involves the self-ignition of an aqueous solution containing an
oxidizer (the corresponding metal nitrates) and an organic fuel,
such as glycine. The rapid evolution and large volume of gases
released during the process immediately cool the product, limiting
agglomeration and leading to the formation of nanocrystalline
powders [18,19]. Furthermore, an evident bluish colouration is
observed, provided by the nickel ions, and the dark sections
correspond to carbon deposited from an incomplete combustion
reaction.
Table 3 presents the adiabatic flame temperature data calculated
according to Eq. (2). The values vary slightly but are consistent with
the data from the literature [35]. The values slightly varied because
the variables that can significantly influence the flame temperature,
such as the type of fuel and fuel-oxidizing ratio, were kept constant
[22]. In addition, the total number of moles of gases were un-
changed when replacing zinc with nickel, as both the nickel nitrate
and zinc nitrate used were in their hexahydrate forms.
a
Table 2
Constants of equation (5) [31].
Chemical Species
A
103
B
106
C
10ꢁ5
D
CO2
N2
H2O
5.457
3.280
3.470
1.045
0.593
1.450
e
e
e
ꢁ1.157
0.04
0.121
lack of information on the thermodynamic properties of spinels,
the heat capacity of MgAl2O4 (1.90613 cal molꢁ1 Kꢁ1) [33,34] is used
because the compounds are isostructural and their divalent
metallic radii [Ni2þ, Zn2þ and Mg2þ] are nearly the same [35].
2.3. Characterization
The thermal behaviour of aluminate was determined using
thermogravimetric analysis (NETZSCH-STA 449 F3 Jupiter model)
with a heating rate of 10 ꢀC/min from 25 ꢀC to 700 ꢀC in an alumina
crucible (sample holder) with a nitrogen atmosphere at a flow rate
in 50 mL/min.
3.2. Thermal analysis characterization
To evaluate the thermal behaviour of the material and confirm
the presence of carbonaceous compounds deposited on the surface
of pigments, thermogravimetric analysis was conducted. The re-
sults for Ni0Zn, Ni0.4Zn and Ni1Zn are presented in Fig. 2. All
samples exhibit similar behaviours, with the mass loss profile
following a standard pattern: the first event of mass loss occurred
The crystalline phases were determined by X-ray diffraction
(XRD) using the powder method. The diffraction patterns were
determined with a D8 ADVANCE Bruker diffractometer equipped
with a goniometer (theta/theta) and Cu anode ceramic X-ray tube
(Ka1 1,54058A), model 10190376 (2 kW/60 kV). The diffractograms
were based on the detection range of 10ꢀꢂ2
q
ꢂ 80ꢀ with a step size
of 0.02ꢀ and a speed of 0.01ꢀsꢁ1. The crystallite sizes were calcu-
lated using the Scherrer formula. The quantified phases and lattice
parameters were calculated by the Rietveld methodology.
A scanning electron microscope (SEM), VEGA3 System Shi-
madzu model, was used to investigate the microstructure of the
powders. The secondary electron beam was used, and the samples
were previously metalized by the deposition of gold film.
Infrared spectra were recorded in the 400e4000 cmꢁ1 spectral
region by means of a Shimadzu (IRPrestige-21). Analyses were
performed on self-supporting pellets diluted with KBr.
The CIELab chromaticity coordinates were taken directly from
the calcined powders by means of a Konica Minolta Color Reader
CR-10 portable colourimeter using D65 illumination and the 10ꢀ
standard observer. To verify the paint formulation and properties,
the powders were sifted through a 200-mesh sieve and dispersed
in a colourless acrylic varnish (ACRILEX) at a proportion of 10%. The
CIELab parameters were taken after 48 h drying at room tempera-
ture. Finally, the differences in colour between the dry and
dispersed powder were calculated using Eq. (4), which is the simple
distance between two points in the three-dimensional colorimetric
Fig. 1. Material synthesized by the combustion method.
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T. Cristine de Sousa Santos et al. / Journal of Alloys and Compounds xxx (xxxx) xxx
Table 3
Micrographs of samples Ni0.4ZnSC (Fig. 3a), Ni0.4Zn600 (Fig. 3b)
and Ni0.4Zn1000 (Fig. 3c) are shown in Fig. 3. The material, in
general, is spongy in appearance and extremely porous. No definite
structure can be seen, and isolated particles are not identified, only
aggregates. The large aggregation involves nanostructured particles
and results from the attractive van der Waals force [36]. It is
possible to verify that the calcination process led to an increased
number of pores and changes in pore size when comparing the
results of fresh samples and those calcined at 600 ꢀC (Fig. 3a and b).
This fact is attributed to the superficial cleaning of the sample as a
result of the clearance of pores caused by carbon output in the
forms of CO and CO2 [37]. Evaluating the results obtained by ther-
mogravimetric analysis (Fig. 2), it is verified that the mass loss at
approximately 500 ꢀC corresponded to the loss of residual carbon
from the synthesis process; thus, calcination at 600 ꢀC promotes
mass loss and consequently cleans the surface via combustion, as
previously observed. SEM images of the materials calcined at
1000 ꢀC (Fig. 3c) show the structural differences between the not
calcined and calcined at 600 ꢀC samples, Fig. 3aeb, respectively; the
disappearance of small pores followed by increases in the number
and size of macropores is noted. In addition, some breaking points
are observed in the structure resulting from the high-temperature
synthesis. The main consequences of this are a low pore volume
and low surface area, which affect the dispersion of pigments in the
matrix.
Theoretical adiabatic flame temperature of the nickel series.
x
Theoretical adiabatic
Total Number of
Gaseous Moles
flame temperature (ꢀC)
0
967.0
963.4
959.8
952.7
931.7
50.2
50.2
50.2
50.2
50.2
0.1
0.2
0.4
1
3.4. Crystallographic phase formation
The diffractograms used to identify the crystalline phases
formed through synthesis and the subsequent thermal treatment of
nickel-zinc pigments are shown in two forms: as a function of
temperature for each composition (Fig. 4) and as a function of
composition for each temperature (Fig. 5).
Fig. 2. Thermal analysis of the prepared powders.
due to the loss of water at approximately 100 ꢀC, and the second
event was seen at 500 ꢀC, which corresponds to residual carbon
burning from CO and CO2 left after the incomplete combustion
reaction, as confirmed by visual data analysis. From 500 ꢀC on-
wards, a significant variance of mass was not found, which implies
the full cleaning of the surface of the produced pigment. The small
lost mass (<10%) presented by the samples can be explained the
values of the maximum flame temperatures (Table 3); all were
above 900 ꢀC, which makes the combustion reaction practically
complete.
3.4.1. Effects of the calcination temperature
The diffractograms presented in Fig. 4 confirm the development
of the cubic spinel structure at all of the study temperatures (ICSD-
9559, ICSD-69520 and AMCSD-000147 for ZnAl2O4, Ni0.4Zn0.6Al2O4
and NiAl2O4, respectively), including the not-calcined samples,
which indicates that the combustion method is appropriate to
produce aluminates with nickel and zinc. In the samples without
nickel in their compositions (Fig. 4a), not-calcined (Ni0ZnSC) and
calcined at 600 ꢀC (Ni0Zn600) displayed a segregated ZnO (ICSD-
26170) phase, indicating that the energy of the synthesis reaction
(combustion energy) and a calcination process up to 600 ꢀC are
insufficient to complete the incorporation of Zn2þ ions into the
spinel structure. As the maximum flame temperature reached
approximately 900 ꢀC, it was expected that the pure structure
3.3. Microstructure and porosity characterization
The effect of the calcination temperature on the morphology of
the materials was determined by XRD and SEM analyses.
Fig. 3. SEM of the Ni0.4Zn samples: a) not-calcined; b) calcined at 600 ꢀC; c) calcined at 1000 ꢀC.
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5
Fig. 4. X-ray diffractograms with varying calcination temperatures: a) Ni0Zn; b) Ni0.2Zn; c) Ni0.4Zn; d) Ni1Zn.
Fig. 5. X-ray diffractograms with varying compositions: a) not-calcined; b) 600 ꢀC; c) 800 ꢀC; d) 1000 ꢀC.
would be obtained; however, the temperature only remained that
high for a few seconds, which was insufficient to promote the
complete incorporation of Zn2þ ions and obtain the pure spinel
phase. As seen in the literature, the ZnO and ZnAl2O4 phases are
obtained with thermal treatments of the precursor powder at
temperatures of up to 700 ꢀC [11].
The sample results for Ni0.1Zn (supplementary material) and
Ni0.2Zn (Fig. 4b) show similar diffractograms with the presence of
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T. Cristine de Sousa Santos et al. / Journal of Alloys and Compounds xxx (xxxx) xxx
the pure spinel phase at all of the temperatures studied. The non-
appearance of segregated zinc oxide may be caused by the
decrease of the zinc content in the material with the addition of a
small amount of nickel in the structure, as a smaller ionic radius is
advantageous for the formation of the pure spinel phase. Another
explanation may be that the amount of ZnO may have been too
small to be observed by the XRD technique.
right. This displacement results from the ion exchange from Zn2þ to
Ni2þ since they have different ionic radii. The nickel radius (0.69 Å)
is lower than that of zinc (0.74 Å), so it causes a shift to the right
[38]. Similar results were observed by Pandit et al. [39] in studies of
the substitution of Fe3þ (0.645 Å) to Al3þ (0.535 Å).
The different occupations of the sites between the cations in
nickel-zinc aluminates can cause changes in the planar densities
and thus changes the planar directions, causing the appearance and
disappearance of certain peaks. In the diffractograms shown in
In the samples with an increased nickel content at x ¼ 0.4
(Fig. 4c) and x ¼ 1(Fig. 4d), the pure spinel phase was obtained for
samples calcined at 800 ꢀC and 1000 ꢀC. In the samples Ni0.4ZnSC
and Ni0.4Zn600, along with all materials where x ¼ 1 (Fig. 4c and
d), a nickel oxide phase (ICSD-9866) was identified, indicating the
segregation of ions in the surface of the material. Specifically, for
samples Ni1ZnSC and Ni1Zn600, there was also segregated metallic
nickel (AMCSD-0011153). Therefore, it was revealed that adding
nickel ions into the spinel structure requires more energy to com-
plete, a fact which is adequately explained by the affinity of the
divalent nickel for the octahedral sites, causing an inversion of
cations (Al3þ and Ni2þ) in the sites. In normal spinel, the octahedral
sites are only occupied by trivalent cations while the tetrahedral
sites contain bivalent cations. Octahedral sites occupied by Ni2þ
have been described by Lorenzi et al. [5] and Gaudon et al. [17], and
network enthalpy calculations involving simple ionic methods
indicate that the normal spinel structure is most stable [7], and
thus, the inverse spinel requires more energy.
For all samples, the post-synthesis thermal treatment led to an
elongation and definition of the characteristic peaks of spinel,
revealing increased crystallinity; the increase of temperature
caused a decrease of the peaks of the unwanted phases until they
disappeared. This behaviour suggests that the crystallization pro-
cess eliminates oxygen and that metal ions in segregated oxides are
incorporated into the spinel structure at high temperatures,
proving the essential nature of calcination to the crystallization of
the material.
Fig. 5, a crystalline peak at the 2
q
¼ 19ꢀ position appeared for the
samples where x ¼ 1, indexed according to Miller (I111). For samples
starting with x ¼ 0.4, peaks at the position of 2
q
¼ 49ꢀ, referenced
to Miller indices (I331), disappeared. gahnite (Ni0Zn) is a normal
spinel with Zn2þ cations located at the tetrahedral sites, and it has
an energetically unaffordable inversion between zinc and
aluminium cations [30]. The peak intensity I(311) may be assumed to
originate from the contributions of Al3þ located at octahedral po-
sitions, and thus, the greater its intensity, the smaller the degree of
inversion of its structure. Conversely, I(111) is related to the presence
of nickel, which promotes the inversion process.
Quantitative phase analysis and cation inversion rate was con-
ducted using the Rietveld method, and the results are shown in
Table 4. The reliability factors of refinement varied of CRp ¼ 5e9%,
CRwp ¼ 7e9% and Rbragg ¼ 1.0e2.1. By evaluating the effect of the
calcination temperature on the quantification of phases for samples
in which the nickel content x ¼ 1 (Table 4), it can be seen that
incorporation of metal nickel into the structure does not direct
occur. First, metal nickel is transformed into nickel oxide and then
incorporated in the spinel structure. This process is confirmed by
the quantification of phases between the Ni1Zn series samples.
When the calcination temperature increased to 800 ꢀC, an
increasing amount of NiO and a decreasing amount of Ni0 were
found. Combined with the small variation in the amount of the
spinel phase, it is found that metallic nickel transforms in nickel
oxide before incorporation into the spinel structure. Otherwise,
consider the sample Ni1Zn1000: all nickel oxide was inserted into
the structure, with a proportional decrease of the amount of NiO.
The work of Visinescu et al. [30] explains that the carbon residue
oxidation in the burning powder process of the precursor implies
the reduction of nickel oxide and degradation under an air atmo-
sphere. The redox sequence Ni2þ/Ni0/Ni2þ implies
3.4.2. Effects of load of nickel
Fig. 5 shows diffractograms of the samples at each temperature
with varied contents of chromophore ions. To improve visualiza-
tion, the most intense peak at the Miller index position (I311) was
highlighted. At all temperatures, a similar behaviour was observed:
at increased nickel concentrations, peaks were displaced to the
Table 4
Phase quantification, lattice parameter and crystallite size.
ga
_
Sample
Phases (%)
Spinel
Lattice Parameter (A)
Crystal Size (nm)
ZnO
NiO
Nio
Ni0ZnSC
Ni0Zn600
Ni0Zn800
97.07
97.27
100
100
100
100
100
100
100
100
100
100
100
98.14
100
100
86.76
87.44
88.07
98.87
2.93
2.73
e
e
e
8.087
8.087
8.087
8.085
8.082
8.077
8.081
8.077
8.083
8.074
8.079
8.076
8.064
8.061
8.068
8.065
8.045
8.041
8.041
8.051
22.808
23.197
28.535
45.278
23.770
24.029
22.874
28.906
27.202
25.114
27.865
31.332
30.466
26.109
31.612
35.239
34.607
42.989
32.035
43.254
e
e
e
e
e
e
e
Ni0Zn1000
Ni0.1ZnSC
Ni0.1Zn600
Ni0.1Zn800
Ni0.1Zn1000
Ni0.2ZnSC
Ni0.2Zn600
Ni0.2Zn800
Ni0.2Zn1000
Ni0.4ZnSC
Ni0.4Zn600
Ni0.4Zn800
Ni0.4Zn1000
Ni1ZnSC
e
e
e
e
e
e
e
0.280
0.271
0.289
0.285
0.695
0.691
0.702
0.708
0.721
0.720
0.725
0.723
0.841
0.845
0.840
0.844
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
1.86
e
e
e
e
e
e
e
e
8.33
10.57
11.93
1.13
4.91
1.99
e
Ni1Zn600
Ni1Zn800
Ni1Zn1000
e
e
e
e
a
Fraction of Ni2þ ions in octahedral sites.
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7
NiO/Ni0/NiO; the reaction is continuous with the elimination of
oxygen and nickel accommodation in the aluminate structure.
The temperature effects on the crystallite size and the unit cell
parameters were investigated. Fig. 6aec shows these values,
respectively. There was no significant change in the crystallite size
between the not-calcined and calcined samples at 600 ꢀC (Fig. 6a),
again indicating that the power generated from the lower tem-
perature is not enough to change the material structure. Analysing
both together with the theoretical adiabatic flame temperature
(Table 3) indicates that synthesis, even for a few seconds, reaches
temperatures above 900 ꢀC, but the large volume of gas released
during the fast expansion of the synthesis reaction dissipates the
heat and limits the temperature increase, which affects crystal
nucleation, substantially reducing growth and stabilizing the
structure. The samples calcined at 800ꢀ and 1000 ꢀC (Fig. 6a)
showed crystallite growth due to the sintering of the material. The
lattice parameter as a function of temperature (Fig. 6c) had little
variation, suggesting the stabilization of the structures formed. All
of the lattice parameters calculated by XRD are solid, with a cubic
spinel structure of value 8.088 [19].
Fig. 6bed shows the chromophore ion content effects on the
crystallite size and the lattice parameter, respectively. In relation
with the crystallite size (Fig. 6b), the not calcined and calcined
samples at 600 ꢀC have a similar behaviour: increases in the crys-
tallite size are caused by the increased nickel content in the
chemical composition of the synthesis materials, implying that the
power generated in the synthesis and calcination at 600 ꢀC is not
enough to change or order the material structures. A different
behaviour was seen for the other samples calcined at 800 ꢀC and
1000 ꢀC: between 0 < x < 0.1, there is a reduction in the particle
size. From x ¼ 0.1 onwards, growth occurs with the increasing
nickel content. This decrease may be a result of the rearrangement
of atoms in the crystalline structure, which reduces the effects and,
consequently, reduces the entropy of the system by changing the
particle size. The nickel content increase promotes the inversion of
the structure, which makes the system unstable, causing crystal
growth from x ¼ 0.1. This result is corroborated by analysing the
results of the series calcined at 1000 ꢀC, which had greater energy
provided by the system and, consequently, a greater stabilization
and percentage decrease in the particle size of the sample Ni0Zn
compared with the sample Ni0.1Zn.
The lattice parameter relation (Fig. 6d) was observed to linearly
decrease with nickel incorporation into the spinel, indicating the
formation of a solid solution, implying isomorphic substitutions of
Zn2þ by Ni2þ ions. The decrease is attributed to the nickel radius
(0.69 Å) being smaller than that of zinc (0.74 Å), confirming the
diffractogram analysis results (Fig. 5), which show a right shift of
the peaks.
3.5. Infrared spectroscopy
The infrared spectra of the synthesized pigments were obtained
and are shown in Fig. 7. Analysing all of the sample results, no
significant variations with the increasing calcination temperature
were observed; only a small displacement of the bands was noted,
which was caused by the better accommodation of ions at their
preferred sites. All of the spectra present bands in the range of
500e900 cmꢁ1, which are associated with the vibrations of M ꢁ O,
AleO and M-O-Al and are characteristic of the spinel structure [28],
confirming the XRD analysis results that identified the major phase
of all samples.
The FT-IR results of all the Ni0Zn and Ni0.1Zn samples (Fig. 7a
and b) present absorption bands close to 497, 553 and 661 cmꢁ1
,
which are attributed to the stretching vibrations of aluminium
located in the octahedral sites (AlO6) [6] and zinc located in
tetrahedral sites [3]. The default band is around 720 cmꢁ1 and is
associated with the vibration of aluminium at tetrahedral sites
(AlO4), showing that the ZnAl2O4 spinel is normal (bivalent
metallic ions in tetrahedral sites and trivalent metallic ions in
octahedral sites of the crystalline structure). This zinc aluminate
Fig. 6. a) Particle size as a function of the calcination temperature; b) particle size as a function of the nickel content; c) lattice parameter as a function of the calcination tem-
perature; d) lattice parameter as a function of the nickel content.
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T. Cristine de Sousa Santos et al. / Journal of Alloys and Compounds xxx (xxxx) xxx
Fig. 7. FT-IR with varying temperatures: a) Ni0Zn; b) Ni0.1Zn; c) Ni0.4Zn; d) Ni1Zn.
behaviour is most common [30].
sites (NiO4) at low amounts. The band absorption at 449 cmꢁ1
represents NiO [11], confirming its identity in the diffractograms
(Fig. 4d).
By maintaining the same temperature and varying the nickel
content (Fig. 8), it is easier to observe the displacement of the ab-
sorption band from AlO6 to AlO4, which indicates the inversion of
the structure with the increase in the amount of nickel. This
inversion results from nickel entering the octahedral sites, forcing
For Ni0.2Zn, a decreasing low shift at 500 cmꢁ1 was observed,
which may have been caused by the decrease in the amount of zinc.
The tendency of the displacement of the peaks to the right was
confirmed with the samples of Ni0.4Zn and Ni1Zn (Fig. 7c and d).
The bands near 500-524 cmꢁ1 are assigned to the stretching of
nickel in the octahedral sites (NiO6) [40], and those near 602-
610 cmꢁ1(least intense) are attributed to nickel in the tetrahedral
Fig. 8. FT-IR with varying nickel contents: a) not-calcined; b) 600 ꢀC; c) 800 ꢀC; d) 1000 ꢀC.
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T. Cristine de Sousa Santos et al. / Journal of Alloys and Compounds xxx (xxxx) xxx
9
aluminate to the tetrahedral sites. The change in ion position pro-
motes the final spinel properties with the more perceptible
colouring.
calcination temperature but a variable chromophore ion content.
Observing the visual pigments presented in Table 5, it is noted
that the samples Ni0Zn800 and Ni0Zn1000 tend to be white. White
was expected because zinc has all of its d orbitals filled and thus
was not colourful. The samples of x > 0 have cyan tones. With the
increasing temperature, the tonality becomes easier to visually
identify, and with the increase of the nickel content, the samples
become darker.
The Tukey analysis at 5% of the colorimetric CIELab variables
identified the same statistical values to a* for x ¼ 0 at 600 ꢀC and
800 ꢀC. However, in all samples, at least one of the variables pre-
sented average values that were different from the others, and as
the pigment colour is characterized by three variables in CIELab,
only one pigment being different is enough to distinguish different
colours between pigments. Therefore, changes of both the ion
content and calcination temperature caused significant changes in
the nickel-zinc pigment colour. The Tukey analysis at 5% confirms
3.6. Colourimetry
Table 5 shows the visual aspects and average values of the
colorimetric variables (CIELab) for all of the pigments produced and
dispersed in a colourless varnish, along with the quantitative dif-
ference of the colour between them. Each variable was measured in
triplicate, and the average values were subjected to the Tukey test
at the 5% level of probability to evaluate whether the colours ob-
tained were significantly differentdthat is, if changes in the chro-
mophore ion content and the calcination temperature significantly
influenced the colours formed. The test was conducted on a set of
samples with the same chromophore content and a variable calci-
nation temperature and on a set of samples with a constant
Table 5
Colorimetric properties of powdered pigments e NixZn1-xAl2O4.
Sample
Ni0Zn
Powder dry
Powder dispersed in varnish colorless
Visual Aspect
L*
a*
b*
Visual Aspect
L*
a*
b*
DE
28.83aa
1.07aa
7.17aa
23.23
1.10
5.80
5.76
Ni0Zn
43.70ba
46.23ca
34.90ab
38.43bb
0.77aa
5.50ba
5.77ca
4.80ab
2.97bb
42.03
37.33
23.23
42.03
ꢁ1.00
ꢁ1.23
1.10
4.10
4.93
5.80
4.10
2.80
8.93
4.65
6.91
Ni0Zn
ꢁ0.83ca
ꢁ1.23ab
ꢁ2.67bb
Ni0.1Zn600
Ni0.1Zn800
ꢁ1.00
Ni0.1Zn1000
45.43ca
ꢁ3.17bb
1.20cb
37.33
ꢁ1.23
4.93
1.98
Ni0.2Zn600
Ni0.2Zn800
25.37ac
36.37bc
ꢁ0.40ac
ꢁ3.73bc
4.93ab
2.90bb
30.57
32.63
ꢁ0.13
ꢁ5.57
3.53
0.57
6.24
6.13
Ni0.2Zn1000
Ni0.4Zn600
Ni0.4Zn800
Ni0.4Zn1000
Ni1Zn600
40.57cc
33.07ad
33.63ad
35.17cd
21.40ae
ꢁ4.17cc
ꢁ1.07ab
ꢁ3.73bc
ꢁ3.73bc
ꢁ3.73ae
0.67cc
3.13ad
4.40bd
4.40bd
0.87ae
45.20
19.27
30.47
33.67
22.87
ꢁ2.47
ꢁ0.77
ꢁ5.00
ꢁ4.73
ꢁ2.17
ꢁ0.63
3.67
7.00
10.43
2.57
7.79
1.41
1.83
ꢁ0.40
1.27
Ni1Zn800
29.40be
28.43ce
ꢁ5.23be
ꢁ6.00ce
5.27be
3.33ce
31.20
27.53
ꢁ4.57
ꢁ4.30
4.40
2.97
1.78
Ni1Zn1000
10.94
a, b, c, d, e
mean values following the same set of letters and same column no differ from each other by Tukey 5% test.
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T. Cristine de Sousa Santos et al. / Journal of Alloys and Compounds xxx (xxxx) xxx
the visual analysis of differences in pigments, as shown in Table 5.
Furthermore, Table 5 clearly displays that the colours are visually
different between the dry powder and that dispersed in varnish,
although the dispersed colours continue to give the same cyan
governed by ions present in the structure of NiO in samples, indi-
cating the incomplete incorporation of nickel in the spinel struc-
ture. The movement of tetrahedral ions to the octahedral sites
confirms the observed FT-IR results: changes in the binding field
and the d orbitals also cause colouring changes. To predict the
values of the CIELab parameters and, consequently, the final colour
of the pigments formed under different synthetic conditions, sur-
face maps were built from the CIELab data as functions of both the
calcination temperature and nickel content. The results are shown
in Fig. 10, and the functions obtained are given by equations
(5)e(7). Each equation satisfactorily describes the experimental
data and can be used to predict he material behaviour.
tones. The values of
DE (Equation (4)) shows no trend. Only
Ni1Zn600 exhibited a colour difference between 0 and 1.5, meaning
the materials compared exhibited a similar colour. Some samples
had
DE in the range of 1.5e5, which indicates that different colours
were observed. The majority of samples had
D
E values above 5,
implying a clearly observed difference in colour [41].
Fig. 9 shows the colourimetry results obtained by CIELab as
functions of the calcination temperature and nickel content in the
spinel structure, separately. Focusing on luminosity (L*), the same
increase with the calcination temperature was observed in all of the
studied samples. The addition of nickel caused a decrease of the
value of L*, except for the nickel series at 600 ꢀC, which presented
an oscillation. This behaviour confirms the visual assessment result
(Table 5). As ZnAl2O4 is white, when a chromophore dopant is
added to it, the sample tends to present a different tone from white
and darkens (L* decrease). Nickel has an incomplete d orbital, and
the electronic transitions in this orbital leads to the observed
colour. A similar behaviour was also found by Queiroz et al. [6].
For the other CIELab parameters, a* and b*, in general, the values
decreased with the increasing temperature. An increasing nickel
concentration caused a decrease in the value of a*, which indicates
a propensity toward green. b* remained stable with only slight
decreases, tending towards blue. In Table 5, the samples doped with
nickel have a cyan colouring. Nickel aluminates belong to the class
of cyan pigments, whose colour results from a mix of two colours:
blue and green [29,42]. The pigments’ colours confirm the diffrac-
tion analysis results of Ni0.4Zn and Ni1Zn (Fig. 4), in which nickel
oxide was identified as a secondary phase. The observed bluish
colouring is characteristic of Ni2þ ions located in tetrahedral sites,
even in low amounts, while the greenish colouring results from a
mix of nickel-zinc aluminate (blue) with a small amount of NiO
(yellow-green) [5]. Thus, the cyan colouring of aluminates is
L ¼ ꢁ 23:655 ꢁ 0:6078LNi þ 0:1286CT þ 5:629LNi2
ꢁ 0:023LNiCT ꢁ 5:916x10ꢁ5CT2
(5)
(6)
(7)
a * ¼ 13:5853 ꢁ 9:6714LNi ꢁ 0; 03CT þ 5; 329LNi2
ꢁ 7; 384x10ꢁ5LNiCT þ 1; 4833x10ꢁ5CT2
b * ¼ 2:1743 ꢁ 18:818LNi þ 0:0155CT þ 5:6357LNi2
þ 0; 0144LNiCT ꢁ 1:45x10ꢁ5CT2
CT represents the calcination temperature, and LNi represents
the nickel content in the spinel structure.
4. Conclusion
The variable tones obtained through nickel doping in zinc
aluminate are advantageous to industries that use pigments, as
they afford different colours to meet increasingly specific market
demands, possibly replacing cobalt ions in the production of cyan
pigments. The material was synthesized by combustion using
glycine as a propellant at a stoichiometric ratio, and the results
indicated that this route of synthesis is suitable for the production
Fig. 9. Effect of the calcination temperature and chromophore ion content on the colorimetric variables of the pigment powder.
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T. Cristine de Sousa Santos et al. / Journal of Alloys and Compounds xxx (xxxx) xxx
11
Fig. 10. a) Luminosity variable (L*); b) chromaticity variable a*; c) chromaticity variable b* as a function of the calcination temperature and nickel content.
^
raw material, Ceramica 51 (2005) 107e110, https://doi.org/10.1590/S0366-
69132005000200006.
of nanometric spinel powders with high purity. The chromophore
ion does not affect the morphological properties but does affect the
spinel structure. Zinc spinel has a normal structure, and increasing
the value of x (nickel content) causes an inversion of its structure.
Aluminates doped with nickel presented colours varying be-
tween blue and green, similar to dark cyan, and a higher nickel
content led to a more intense cyan colour. The colourimetry data of
[2] G. Buvaneswari, V. Aswathy, R. Rajakumari, Comparison of color and optical
absorbance properties of divalent ion substituted Cu and Zn aluminate spinel
oxides synthesized by combustion method towards pigment application, Dyes
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DE indicates that the dilution of a pigment powder in a colourless
matrix acrylic varnish causes significant changes to the colours of
all samples, although they still have cyan tones. The model pro-
posed to calculate L*, a* and b* at different calcination temperatures
and with varying nickel contents is sufficient to reproduce the
presented experimental data.
pigments: project, production, industrial applying, Ind. Ceram.
3 (1998)
13e17.
[5] G. Lorenzi, G. Baldi, F. Benedetto, V. Faso, P. Lattanzi, M. Romanelli, Spectro-
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[6] R.M. Queiroz, T.L. Coelho, I.M. Queiroz, L.H.O. Pires, C.E.F. Costa, Structural and
thermal characterization of NixZn1-xAl2O4 synthesized by the polymeric pre-
cursor method, J. Therm. Anal. Calorim. 124 (2016) 1023e1028, https://
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Acknowledgments
~
[7] D.F. Shriver, P.W. Atkins, Inorganic Chemistry, third ed., Bookman, Sao Paulo,
2005.
The authors acknowledge to LEVAP, FINEP, LABNANO-AMAZON/
[8] S.A. Hassanzadeh-tabrizi, R. Pournajaf, A. Moradi-faradonbeh, Nanostructured
CuAl2O4: Co-precipitation synthesis, optical and photocatalytic properties,
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5408(02)01004-8.
^
UFPA network and Laboratory of Nanotecnologia Farmaceutica e
Nanofarm-UFPA for the support to the facilities used in this work.
This work has been mainly supported by CNPq, FAPESPA, CAPES in
Procad Amazonia 2018 process number 88881.200618/2018-01.
^
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for synthesis of colorful aluminates, Dyes Pigments 125 (2016) 124e131,
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Appendix A. Supplementary data
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Please cite this article as: T. Cristine de Sousa Santos et al., Synthesis and characterization of colourful aluminates based on nickel and zinc,
Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152477