Thermal behavior of nickel( II) polyoxalate obtained through the reaction of ethylene glycol with nickel(II) nitrate 1 [610246]

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Thermal behavior of nickel( II) polyoxalate obtained through the reaction of ethylene glycol with nickel(II) nitrate 1
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Mircea NICULESCUa,*, Mihai -Cosmin PASCARIUb,c,d,*, Andrei RACUd, Bogdan ȚĂRANUd 3
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a University Politehnica Timișoara, Faculty of Industrial Chemistry and Environmental Engineering, 6 Vasile Pârvan Blvd., 5
RO-300223, Timișoara, Romania 6
b “Vasile Goldiș” Western University of Arad, Faculty of Pharmacy, 86 Liviu Rebreanu, RO -310414, Arad, Romania 7
c “Victor Babeș” University of Medicine and Pharmac y of Timișoara, Faculty of Medicine, 2 Eftimie Murgu Sq., RO -300041, 8
Timișoara, Romania 9
d National Institute of Research & Development for Electrochemistry and Condensed Matter – INCEMC, Renewable Energies 10
– Photovoltaic Laboratory, 144 Dr. Aurel Păunescu -Podeanu, RO -300569, Timișoara, Romania 11
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* Corresponding author s. Tel.: +[anonimizat]; fax: +[anonimizat] . 13
E-mail addresses: [anonimizat] (M. Niculescu), [anonimizat] (M. -C. Pascariu) 14
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ABSTRACT 16
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This paper analyzes the thermal d ecomposition of nickel( II) polyoxalate hydrate, a homopo lynuclear coordination compound 18
having the formula [Ni (C2O4)(H2O)2]n·0.1nH2O. The thermolysis was conducted in both dynamic oxidative and inert 19
atmospheres by simulta neously applying the TG, DTG and DTA analytical techniques. The proposed decomposition 20
mechanism was confirmed by the Fourier transform infrared spectroscopy (FTIR) analysis of the evolved gases. Solid -state 21
decomposition products formed during heating wer e investigated by chemical analysis, FTIR and Raman spectroscopy and X – 22
ray diffraction (XRD). The structure, morphology and properties of the final decomposition product s were characterized by 23
XRD, FTIR, energy dispersive X -ray spectroscopy and transmissio n electron microscopy. The se analyse s show that the final 24
decomposition product in oxidative atmosphere is nickel oxide , with a various crystalline morphology and a microporous 25
structure with a large specific area , whereas in inert atmosphere is a mixture of Ni and NiO in a 3:2 molar ratio . 26
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Keywords 28
Nickel(II) polyoxalate 29
Homopoly nuclear coordination compound 30
Ethylene glycol 31
Nickel oxide 32
Evolved gas analysis 33
Fourier transform infrared spectroscopy 34
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1. Introduction 44
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Oxidic systems , generated through various methods , are nowadays required in increasing quantities b ecause of the 46
development of modern technologies in multiple fields : catalysis and electrocatalysis, ceramic pigments, electronics, physical 47
supports and carriers in device s intended f or data processing , and also pharmaceutical industry. One of the synthetic pathways 48
used for their preparation , which has recently known a rapid development , is represented by the thermal conversion of homo – 49
and heteropolynuclear metal complexes , which con tain anions of carboxylic acids as ligands [1-11]. 50
In our previous papers [12-21] we have presented the products of the redox reactions between several diols , such as 51
ethylene glycol , 1,2-propanediol and 1,3 -propanediol, and certain metal nitrates. All the coordination compounds obtained 52
through this method contain carboxylate or hydroxycarbox ylate anions as ligands , i.e. glyoxylate, oxalate, lactate or 3 – 53
hydroxypropionate. These complexes, which contain relatively simple organic ligands , have one main adva ntage over other 54
coordination compounds : they undergo thermal induced degrada tion to metals, alloys and oxidic structures at considerably low 55
temperatures, with the release of gaseous species such as carbon oxides (CO, CO 2), hydrocarbons (e.g., CH 4) and wa ter. The 56
composition of the obtained powders depend s on both the structure of the coordination compound and the thermal treatment 57
that was applied . 58
The use of different preparative methods allows for the production of nickel oxides with specific properties , which 59
include the degree of crystallinity, particle size (from nanophases to millimeters) [22], morpholog y and surface area [23]. 60
Several papers describe the decomposition of nickel nitrate hexahydrate to nickel oxide [24-26]. The study of the 61
samples ob tained at several temperatures and residual pressure conditions suggests the presence of intermediate compounds 62
with different characteristics, which finally convert t o NiO with different surface properties. 63
Recently, nanosized materials have attracted man y research ers because of their unusual properties based on the size- 64
quantization effect and the large surface area [27-31]. Nanosized nickel oxide is of great interest because it exhibits anomalous 65
electronic [32-34] and magnetic [35-38] properties. The characteristic properties of nanosized NiO particles also enable one to 66
tailor materials for a variety of applications includi ng catalysis [39-41], electrochromic windows [42] and sensors [43]. These 67
properties can be enhanced by decreasing the particle size , being highly dependent on this property. This is why a precise 68
control of the size and distribution in the nanometer regim e is required. In addition, a facile preparation process that allows 69
convenient production of these particles is necessary for miscellaneous new applications. So far, many different methods have 70
been attempted to synthesize nanosized NiO, such as thermal d ecomposition [44, 45], microemulsion [46], precipitation [47, 71
48], electrochemical deposition [49], sol-gel technique [50, 51] and surfactant -mediated method [52]. 72
The aim of this paper is to clarify the mechanism involved in the thermal decomposi tion of n ickel(II) polyoxalate , 73
[Ni(C2O4)(H2O)2]n·0.1nH2O [53], in both oxidative and inert atmospheres . Macklen [54] analyzed the thermal behavior of the 74
simple nickel(II) oxalate dihydrate obtained through the classical methods. On the other hand, our study refers to a polynucl ear 75
compound , nickel(II) polyoxalate hydrate, obtained by an original method, starting from nickel(II) nitrate and ethylene glycol 76
in the presence of nitric acid. The in situ generation of the ligand, i.e. the oxalate anion (C 2O42-), simultaneously with its 77
coordinati on to the complex generator, i.e. the Ni2+ cation, leads to a polynuclear structure, which possess high stability: it is 78
virtually insoluble in water and in common organic solvents and it can only be decomposed in a strongly acidic medium [53]. 79
This study also shows that, following the thermal decomposition at relatively low temperatures of this complex compound, 80
nickel oxide is obtained in oxidative atmosphere while a mixture of nickel oxide and nickel is produced in inert atmosphere. 81
This paper is part of a series of studies concern ed with the development of new methods f or obtaining co mplex 82
compounds through the oxid ation of diols with metal nitrates [53, 55-58]. A large variety of coordination comp ounds was 83
prepared by using this original synthesis. The main objective of such research is to highlight the importance of the precursor’s 84
nature in the synthesis of simple and mixed metal oxides with various properties and applications. 85
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2. Materials and methods 87
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The nickel(II) polyoxalate hydrate was prepared starting from nickel(II) nitrate and ethylene glycol , in the presence of 89
nitric acid , by using an original method, as described in a previous paper [53]. The compound was purified by refluxing in an 90
acetone/water (5:1 v/v) mixture in an ultrasonic bath bef ore being used. 91
For the thermal analysis, a Netzsch STA 409 PC coupled with a Bruker 27 FTIR instrument (for evolved gas detection) 92
were used, with the following m easuring conditions: 10 °C min-1 heating speed, 100 mL min-1 synthetic air flow and 20.58 mg 93
sample weight for the oxidative decomposition, and, respectively, a gas flow rate of 100 mL min-1 and a 30.0 mg sample 94
weight in dynamic atmosphere of argon . 95
The phase composition of the powders obtained through the complex’s thermolysis at 400 and 1000 °C was 96
investigated by X -ray diffractometry (XRD) using a Rigaku Ultima IV diffractometer with CuK α radiation ( λ = 1.5406 Å). The 97
lattice parameters ( a, b, c) and the average crystallite size ( d) were calculated by using the whole pattern profile fitting (WPPF) 98
method. The instrument influence over the lines’ broadening was subtracted by using the diff raction pattern of a Si standard 99
recorded in the same conditions. 100
The FTIR spectra of the solid products were acquired on a Vertex 70 (Bruker, Germany) FT -IR spectrometer in the 101
400-4000 cm-1 domain, using KBr pellets. 102
The Raman spectra were measured at ro om temperature using a MultiView -1000 system (Nanonics Ima ging, Israel) 103
which incorporates the Shamrock 500i Spectrograph (Andor, UK). A laser wavelength of 514.5 nm was used as the excitation 104
source, with a 20 s exposure time and a 300 L mm-1 grating. 105
For the TEM analyses, the material was deposited from ethanol on a 200 mesh TEM copper grid covered with lacey 106
carbon film. We used a Titan G2 80 -200 TEM/STEM (FEI Company, Netherlands) instrument with image correction. The 107
images were registered at 200 kV ac celerating voltage. A Digital Micrograph v. 2.12.1579.0 software was used for images 108
recorded in TEM mode, while a TEM Imaging & Analysis v. 4.7 software was used for recording the EDX spectrum. The 109
STEM -EDX elemental distribution maps were recorded with t he Esprit v.1.9 software. 110
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3. Results and discussion 112
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3.1. Thermal decomposition in air 114
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In order to clarify the mechanism involved in the thermal decomposition of nickel(II) polyoxalate hydrate , in both air 116
and argon , and the decomposition products formed during its heating, the thermoana lytical methods have been used. 117
The TG, DTG and DTA curves, corresponding to the thermal decomposition in flowing air of nickel(II) polyoxalate 118
hydrate, are shown in Fig. 1 a, while the FTIR curves of the gases evolved duri ng the same process are shown in Fig. 1b [59]. 119
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Fig. 1 . Thermoanalytical curves for the thermolysis of nickel(II) polyoxalate hydrate in flowing air : 124
(a) TG, DTG and DTA results ; (b) FTIR analysis of generated gaseous products (not to scale): CO 2 2361 cm-1, CO 2108 cm-1, 125
H2O 1508 cm-1. 126
127
The thermal decomposition reactions were monitored by chemical analysis, XRD, FTIR, energy dispersive X -ray 128
spectroscopy (EDX) and transmission electron microscopy (TEM). 129
Figs. 2 and 3 comparatively show the FTIR and Raman spectra of the coordination compound and its decomposition 130
products. 131
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133
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Fig. 2 . FTIR (a) and R aman (b) spectra of the studied coordination compound . 135
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138
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Fig. 3. FTIR (top) and R aman (bottom) spectra of the air decomposition products at 400 °C (a, c) and 1000 °C (b, d); the FTIR 140
band at 1636 is owned to traces of water . 141
142
The corresponding assignments of the FTIR and R aman spectra of nickel(II) polyoxalate hydrate are given in Table 1 143
[44, 53, 54, 60 -63]. 144
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Table 1 . Assignment of the FTIR and Rama n spectra of nickel(II) polyoxalate hydrate (band positions in cm-1). 147
148
FTIR Raman Assignments
3398 /3389 vs, br ν(OH) (H 2O), hydrogen bonding
1707 w asy(C-O)
1639 vs , br 1625 w asy(OCO) + δ(H2O*)
1474 w 1477 vs sy(C-O) + (C-C)
1454 s, sh sy(C-O) + δ(OCO)
1385 vs sy(OCO )
1360 vs , 1317 vs sy(C-O) + δ(OCO)
1048 m sy(C-O)
920 w 918 m (C-C)
857 w (C-C)
827 s sy(C-C) + δ(OCO)
754 m , 669 m, 648 m ρ(H2O)
592 m, 536 s δring and/or lattice water
488 vs, ~460 sh (Ni-O) and/or δring
vs – very strong; s – strong; m – medium; w – weak; br – broad; sh – shoulder ; H2O*- coordinated water . 149
150
The intense FTIR band at 16 39 cm-1 (Fig. 2 a) is attributed to the asymmetrical vibration of the carboxylate ion and the 151
value shows that the resonance from the carboxylate group is maintained during complex formation, the metal -carboxylate 152
bond being preponderantly ionic . The band with maximum at 1385 cm-1 is attributed to the sy(OCO ) symmetric vibration. The 153
absence of the bands from the 1720 -1660 cm-1 range , attributed to the asy(C=O) vibration in the case of coordination 154
compounds in which C 2O42- acts as a bidentate ligand , shows that , in the synthesized complex compound , the resonance of the 155
carboxylate groups is achieved and that the four oxygen a toms are equivalent, the oxalate anion being a bridging ligand [63]. 156
At the same time, the value for sy(OCO), i.e. 1385 cm-1, along with the δ(OCO) found at 1317 cm-1, are in agreement with the 157
position s of the corresponding absorption s in oxalate -bridged complex es [54]. The very sharp and strong band at 488 cm-1 is 158
attributed to the (Ni-O) vibration and/or ring deformation. 159
The FTIR spectra o f the thermal decomposition products show the characteristic bands of nickel oxide. In the FTIR 160
spectrum of the thermal decomposition product at 400° C (Fig. 3a ), beside s the two absorption bands of NiO at 438 and 625 161
cm-1, other bands do appear which indicate the presence of traces of the partially decomposed coordination compound . At the 162
same time, the FTIR spectrum of the thermal decomposition product at 1000 °C (Fig. 3b ) shows the absorption bands at 457, 163
560 (shoulder) and 669 cm-1, slightly shifted wi th respect to the characteristic bands of nickel(II) oxide mentioned in the 164
literature [64, 65]. The more relevant bands of nickel(II) polyoxalate, found at 1639, 1385, 1360, 13 17, 827 and 488 cm-1, all 165
disappear, leaving place for the two characteristic bands of nickel oxide . The other weak bands are not relevant for the 166
decomposition of nickel(II) polyoxalate. Also, t he broadness of the 457 cm-1 band indicates that the NiO powder consists of 167
nanocrystals [66, 67]. 168
After a nalyzing the Raman spectra of the thermal decomposition products (Figs. 3c, 3d), we can observe the 169
disappearance of the characteristic bands for the coordination compound, confirming its degradation. The more relevant bands 170
of nickel(II) polyoxalate hydrate ( Fig. 2 b), found at 1477, 1454, 1048, 918 and 536 cm-1, all disappear, being replaced by the 171
characteristic bands of nickel oxide. The Raman spectrum of the thermal decomposition product at 1000 °C (Fig. 3d ) shows 172

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the characteristic bands of nickel oxide ( 351, 542, 729 , 1075 cm-1), in a ccordance with literature data [60, 62]. Also, the Raman 173
spectrum of the thermal decomposition product at 400 °C (Fig. 3 c) reveals the presence of slightly shifted bands , confirming 174
that it is somewhat contaminated with traces of the partially decomposed precursor . 175
In conclus ion, based on the FTIR and Raman spectra, we can say that the nickel(II) polyoxalate has totally decomposed, 176
forming nickel oxide as the final solid -state product. 177
The XRD patterns of the powders obtained by coordination compound’s anne aling at 400 and 1000 °C are presented in 178
Fig. 4 . Both patterns record the diffraction lines of the single phase NiO (rhombohedral, ICDD file 01 -078-4374). The powder 179
annealed at 400 °C is composed of much smaller crystallites (4.4 nm) compared to the one annealed at 1000 °C (37.4 nm). For 180
the powder annealed at 1000 °C, the calculated values of the lattice parameters , a = b = 2.9575 Å, c = 7.2464 Å , are close to 181
the values found in the ICDD file ( a = b = 2.9633 Å, c = 7.2553 Å). 182
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184
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Fig. 4. XRD patterns of the powders obtained by coordination compound ’s annealing at 400 and 1000 °C. 186
187
These results allow some conclusions to be drawn regarding the thermoanalytical curves presented in Fig. 1 . 188
The TG profile shows, in the first two steps, the removal of the lat tice water and of the coordinated water molecules 189
(mass loss: calculated 22.88 %; experimental: 23 .00% ). Two endothermic DTA peaks located between 25 and 160 °C ( weak 190
maximum at 134 °C) and , respectively, between 160 and 286 °C (maximum at 260 °C), and a ch ange on the DTG curve in the 191
same temperature ranges correspond to the removal of water. A completely dehydrated compound is produced around 300 °C, 192
as confirmed in Fig. 1b . These results are in good agreement with those from the literature for nickel(II) oxalate dihydrate 193
obtained through classical methods [54, 68]. 194
The almost complete breakdown of the anhydrous compound, in the third step, takes place within a very short 195
temperature range (286 -345 °C) as shown by a steep slope on the TG curve with the inf lection point at 313 °C. The total 196
experimental mass loss of 58.80% suggests that the product of the conversion is NiO. The formation of this product is 197
accompanied by a very sharp exothermic DTA peak located at 322 °C. 198
After a nalyzing the FTIR curves from Fig. 1b we can confirm the water release, which presents two peaks: a weak one 199
around 14 5 °C and the main one around 26 3 °C. The carbon dioxide shows a peak around 26 5 °C and a very intense peak 200
around 32 3 °C. 201
An EDX quantitative elemental analysis of ver y small areas revealed that, on the surface, the thermal decomposition 202
product at 1000 °C in air is a non -stoichiometric oxide. The EDX profile for an area of the surface is presented in Fig. 5. It 203
should be noted that the area analyzed by EDX was smaller than 50 nm in diameter. 204
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Fig. 5. EDX profile in a surface area (C and Cu peaks belong to the grid) . 208
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Table 2 shows the stoichiometry and composition of the nickel oxide analyzed by EDX in a surface area. 210
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Table 2. Composition and stoichiometry of NiO ob tained from EDX . 212
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Composition Wt. % At. %
Ni
O
Ni:O (a t. ratio) 81.68
18.31
54.87
45.12
1.2
214
This indicates that , on the surface, the product of thermal de composition of the coordination compound in air is an 215
oxygen -deficient non -stoichiometric nickel o xide. 216
In order to obtain useful information about the surface morphology and particle size of the nickel oxide obtained by 217
thermal decomposition of the complex, the TEM analysis was performed . The TEM images ( Fig. 6) at different magnification 218
values show that the thermal decomposition product has a microporous structure with a large specific area and that there are no 219
well-defined particles. The particles seem to be parallelepiped s with some defined edges but with faces which seem to be 220
formed by small and conglomerated particles ( Fig. 6a) that remind of rhombohedral crystallites. Other faces can be seen as a 221
group of well -differentiated particles but without defined forms. The increase in the magnification ( Figs. 6b – 6d) did not allow 222
for a better differe ntiation of the particles. The formation of NiO aggregates, comprising very tiny three -dimensional 223
disordered primary nanoparticles, was clearly visible. 224
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Fig. 6 . TEM images of the 1000°C oxidative decomposition product (NiO) . 229
230
The n ickel oxide ob tained by oxidative thermal decomposition of the [Ni(C2O4)(H2O)2]n·0.1nH 2O polynuclear 231
coordination compound is a product with various crystalline morphology. The particles exhibit irregular forms and their size is 232
widely distr ibuted between 4 nm and 1 μm. 233
234
3.2. Thermal decomposition in argon 235
236
The TG, DTG and DTA curves, corresponding to the argon thermal decomposition of nickel(II) polyoxalate hydrate, 237
are shown in Fig. 7a , while the FTIR curves of the gases evolved during the same process are shown in Fig. 7b . 238
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240
241
Fig. 7 . Thermoanalytical curves for the thermolysis of nickel(II) polyoxalate hydrate in argon: (a) TG, DTG and DTA results ; 242
(b) FTIR analysis of generated gaseous products (not to scale): CO 2 2361 cm-1, CO 2108 cm-1, H 2O 1508 cm-1. 243
244
As can be seen in Fig. 7 , the TG profile shows, in the first two steps, the removal of the lattice water and of the 245
coordinated water molecules (mass loss: calculated 22.87; experimental: 23.32). T he first mass loss, up until around 165°C , 246
has a maximum intensity recorded at 108°C and a corresponding endothermic DTA peak located at 138°C, as confirmed by the 247
FTIR spectrum (maximum at 150°C), while the second mass loss, up until around 291°C, has a process maximum intensity at 248
264°C and a c orresponding endothermic DTA peak at 268°C. The latter is also confirmed by the FTIR spectrum (maximum at 249
272°C), which also shows that the breakdown of the C -C bonds begins in this step and continues in step three (up until around 250
400°C), with CO 2 release . A weak trace of CO was recorded in the third step around 343°C. The total mass loss of 64.69% 251
suggests that the product of the conversion is a mixture of Ni and NiO in a 3:2 molar ratio . The following analys es confirm this 252
result . 253
The FTIR spectrum of th e final argon decomposition product is shown in Fig. 8 . The spectrum shows a few absorption 254
bands at 47 6, 555 (shoulder) and 66 3 cm-1, slightly shifted compared with th ose from Fig. 3b , probably due to the presence of 255
metallic nickel. 256
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258
259
Fig. 8 . FTIR spec trum of the 1000 °C argon decomposition product; the FTIR band at 1630 cm-1 is owned to traces of water . 260
261
The XRD pattern of the powder obtained by coordination compound’s annealing in argon at 1000 °C is presented in 262
Fig. 9 . The XRD pattern records the di ffraction lines of the NiO ( rhombohedral, ICDD 01-089-3080) and metallic nickel 263
(cubic, ICDD 00-004-0850). 264
265
266
267
Fig. 9 . XRD patterns of the powder obtained by nickel(II) polyoxalate hydrate ’s annealing in argon at 1000 °C. 268
269
The TEM images ( Fig. 10 ) at diffe rent magnification values reveal that the particles ’ shape s remain almost the same 270
with a slight increase in agglomeration. 271
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Fig. 10 . TEM images of the 1000°C argon decomposition compound . 276
277
4. Conclusions 278
279
Three successive decomposition steps for nickel( II) polyoxalate hydrate, the first two endothermic and the last 280
exothermic , were recorded during its thermal decomposition in dynamic air atmosphere. 281
The thermal conversion of this homopolynuclear coordination compound gave, depending on the oxidati ve or inert 282
atmosphere, nickel oxide or, respectively, a mixture of Ni and NiO in a 3:2 molar ratio. The XRD results show that the 283
synthesized NiO phase possesses a rhombohedral crystallinity , while metallic nickel phase possesses cubic crystallinity. The 284
TEM images at different magnification values show that the NiO nanoparticles obtained by thermal decomposition of the 285
coordination compound in air were formed by aggregations. These particles exhibit irregular shapes and their size is widely 286
distributed be tween 4 nm and 1 μm. The TEM images of the mixture of Ni and NiO nanoparticles obtained by thermal 287
decomposition in argon reveal that the particles ’ shape is basically the same with a slight increase in agglomeration. 288
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Acknowledgement s 293
294
Part of this rese arch was performed at the Center of Genomic Medicine of the “Victor Babeș” University of Medicine 295
and Pharmacy of Timișoara, POSCCE 185/48749, contract 677/09.04.2015. The authors wish to thank prof.dr. Zoltan Szabadai 296
for some of the FTIR spectra. 297
298
Refere nces 299
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