Thermal and spectroscopic analysis of nickel(II) polyoxalate obtained through the reaction of ethylene glycol with Ni(NO3)26H2O [310489]

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

Mircea NICULESCUa*, Mihai-Cosmin PASCARIUb,c,d*, [anonimizat]

a [anonimizat], 6 Vasile Pârvan Blvd., RO-300223, Timișoara, Romania

b “Vasile Goldiș” [anonimizat], 86 [anonimizat]-310414, Arad, Romania

c “Victor Babeș” [anonimizat], 2 Eftimie Murgu Sq., RO-300041, Timișoara, Romania

d [anonimizat], [anonimizat], 144 Dr. [anonimizat], RO-300569, Timișoara, Romania

* Corresponding authors. Tel.: +[anonimizat]; fax: +[anonimizat].

E-mail addresses: [anonimizat] (M. Niculescu), [anonimizat] (M.C. Pascariu)

ABSTRACT

This paper analyzes the thermal decomposition of nickel(II) polyoxalate hydrate, a homopolynuclear coordination compound having the formula [NiL(H2O)2]n·0.1nH2O, where L is the oxalate anion (C2O42-). [anonimizat]. The proposed decomposition mechanism was confirmed by the Fourier transform infrared spectroscopy (FTIR) analysis of the evolved gases. Solid-[anonimizat] X-ray diffraction (XRD). [anonimizat], FTIR, energy dispersive X-ray spectroscopy (EDX) and transmission electron microscopy (TEM). [anonimizat] a various crystalline morphology and a microporous structure with a [anonimizat] a mixture of Ni and NiO in a 3:2 molar ratio.

Keywords

Ethylene glycol

Nickel(II) nitrate

Nickel(II) polyoxalate

Homopolynuclear coordination compound

Evolved gas analysis

FTIR

1. [anonimizat], are nowadays required in increasing quantities because of the development of modern technologies in multiple fields: [anonimizat], electronics, [anonimizat]. [anonimizat] a [anonimizat]- [anonimizat] [1-11].

In our previous papers [12-21] [anonimizat], 1,2-propanediol and 1,3-propanediol, and certain metal nitrates. All the coordination compounds obtained through this method contain carboxylate or hydroxycarboxylate anions as ligands, i.e. glyoxylate, oxalate, lactate or 3-hydroxypropionate. [anonimizat], have one main advantage over other coordination compounds: [anonimizat], with the release of gaseous species such as carbon oxides (CO, CO2), hydrocarbons (e.g., CH4) and water. The composition of the obtained powders depends on both the structure of the coordination compound and the thermal treatment that was applied.

The use of different preparative methods allows for the production of nickel oxides with specific properties, which include the degree of crystallinity, particle size (from nanophases to millimeters) [22], morphology and surface area [23].

Several papers describe the decomposition of nickel nitrate hexahydrate to nickel oxide [24-26]. The study of the samples obtained at several temperatures and residual pressure conditions suggests the presence of intermediate compounds with different characteristics, which finally convert to NiO with different surface properties.

Recently, nanosized materials have attracted many researchers because of their unusual properties based on the size-quantization effect and the large surface area [27-31]. Nanosized nickel oxide is of great interest because it exhibits anomalous electronic [32-34 and magnetic [35-38] properties. The characteristic properties of nanosized NiO particles also enable one to tailor materials for a variety of applications including catalysis [39-41], electrochromic windows [42] and sensors [43]. These properties can be enhanced by decreasing the particle size, being highly dependent on this property. This is why a precise control of the size and distribution in the nanometer regime is required. In addition, a facile preparation process that allows convenient production of these particles is necessary for miscellaneous new applications. So far, many different methods have been attempted to synthesize nanosized NiO, such as thermal decomposition [44, 45], microemulsion [46], precipitation [47, 48], electrochemical deposition [49], sol-gel technique [50, 51] and surfactant-mediated method [52].

The aim of this paper is to clarify the mechanism involved in the thermal decomposition of nickel(II) polyoxalate [53], in both oxidative and inert atmospheres. Macklen [54] analyzed the thermal behavior of the simple nickel(II) oxalate dihydrate obtained through the classical methods. On the other hand, our study refers to a polynuclear compound, nickel(II) polyoxalate hydrate, obtained by an original method, starting from nickel(II) nitrate and ethylene glycol in the presence of nitric acid. The in situ generation of the ligand, i.e. the oxalate anion (C2O42-), simultaneously with its coordination to the complex generator, i.e. the Ni2+ cation, leads to a polynuclear structure, which possess high stability: it is virtually insoluble in water and in common organic solvents and it can only be decomposed in a strongly acidic medium [53]. This study also shows that, following the thermal decomposition at relatively low temperatures of this complex compound, nickel oxide is obtained in oxidative atmosphere while a mixture of nickel oxide and nickel is produced in inert atmosphere.

This paper is part of a series of studies concerned with the development of new methods for obtaining complex compounds through the oxidation of diols with metal nitrates [53, 55-58]. A large variety of coordination compounds was prepared by using this original synthesis. The main objective of such research is to highlight the importance of the precursor’s nature in the synthesis of simple and mixed metal oxides with various properties and applications.

2. Experimental

The nickel(II) polyoxalate hydrate was prepared starting from nickel(II) nitrate and ethylene glycol, in the presence of nitric acid, by using an original method, as described in a previous paper [53]. The compound was purified by refluxing in an acetone/water (5:1 v/v) mixture in an ultrasonic bath before being used.

For the thermal analysis, a Netzsch STA 409 PC coupled with a Bruker 27 FTIR instrument (for evolved gas detection) were used, with the following measuring conditions: 10 °C min-1 heating speed, 100 mL min-1 synthetic air flow and 20.58 mg sample weight for the oxidative decomposition, and, respectively, a gas flow rate of 100 mL min-1 and a 30.0 mg sample weight in dynamic atmosphere of argon.

The phase composition of the powders obtained through the complex’s thermolysis at 400 and 1000 °C was investigated by X-ray diffractometry (XRD) using a Rigaku Ultima IV diffractometer with CuKα radiation (λ = 1.5406 Å). The lattice parameters (a, b, c) and the average crystallite size (d) were calculated by using the whole pattern profile fitting (WPPF) method. The instrument influence over the lines’ broadening was subtracted by using the diffraction pattern of a Si standard recorded in the same conditions.

The FTIR spectra of the solid products were acquired on a Vertex 70 (Bruker, Germany) FT-IR spectrometer in the 400-4000 cm-1 domain, using KBr pellets.

The Raman spectra were measured at room temperature using a MultiView-1000 system (Nanonics Imaging, Israel) which incorporates the Shamrock 500i Spectrograph (Andor, UK). A laser wavelength of 514.5 nm was used as the excitation source, with a 20 s exposure time and a 300 L mm-1 grating.

For the TEM analyses, the material was deposited from ethanol on a 200 mesh TEM copper grid covered with lacey carbon film. We used a Titan G2 80-200 TEM/STEM (FEI Company, Netherlands) instrument with image correction. The images were registered at 200 kV accelerating voltage. A Digital Micrograph v. 2.12.1579.0 software was used for images recorded in TEM mode, while a TEM Imaging & Analysis v. 4.7 software was used for recording the EDX spectrum. The STEM-EDX elemental distribution maps were recorded with the Esprit v.1.9 software.

3. Results and discussion

3.1. Thermal decomposition in air

In order to clarify the mechanism involved in the thermal decomposition of nickel(II) polyoxalate hydrate, in both air and argon, and the decomposition products formed during its heating, the thermoanalytical methods have been used.

The TG, DTG and DTA curves, corresponding to the thermal decomposition in flowing air of nickel(II) polyoxalate hydrate, are shown in Fig. 1a, while the FTIR curves of the gases evolved during the same process are shown in Fig. 1b [59].

Fig. 1. Thermoanalytical curves for the thermolysis of nickel(II) polyoxalate hydrate in flowing air:

(a) TG, DTG and DTA results; (b) FTIR analysis of generated gaseous products (not to scale): CO2 2361 cm-1, CO 2108 cm-1, H2O 1508 cm-1.

The thermal decomposition reactions were monitored by chemical analysis, XRD, FTIR, energy dispersive X-ray spectroscopy (EDX) and transmission electron microscopy (TEM).

Figs. 2 and 3 comparatively show the FTIR and Raman spectra of the coordination compound and its decomposition products.

Fig. 2. FTIR (a) and Raman (b) spectra of the studied coordination compound.

Fig. 3. FTIR (top) and Raman (bottom) spectra of the air decomposition products at 400 °C (a, c) and 1000 °C (b, d); the FTIR band at 1636 is owned to traces of water.

The corresponding assignments of the FTIR and Raman spectra of nickel(II) polyoxalate hydrate are given in Table 1 [44, 53, 54, 60-63].

Table 1. Assignment of the FTIR and Raman spectra of nickel(II) polyoxalate hydrate (band positions in cm-1).

vs – very strong; s – strong; m – medium; w – weak; br – broad; sh – shoulder; H2O*- coordinated water.

The intense FTIR band at 1639 cm-1 (Fig. 2a) is attributed to the asymmetrical vibration of the carboxylate ion and the value shows that the resonance from the carboxylate group is maintained during complex formation, the metal-carboxylate bond being preponderantly ionic. The band with maximum at 1385 cm-1 is attributed to the sy(OCO) symmetric vibration. The absence of the bands from the 1720-1660 cm-1 range, attributed to the asy(C=O) vibration in the case of coordination compounds in which C2O42- acts as a bidentate ligand, shows that, in the synthesized complex compound, the resonance of the carboxylate groups is achieved and that the four oxygen atoms are equivalent, the oxalate anion being a bridging ligand [63]. 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 positions of the corresponding absorptions in oxalate-bridged complexes [54]. The very sharp and strong band at 488 cm-1 is attributed to the (Ni-O) vibration and/or ring deformation.

The FTIR spectra of the thermal decomposition products show the characteristic bands of nickel oxide. In the FTIR spectrum of the thermal decomposition product at 400° C (Fig. 3a), besides the two absorption bands of NiO at 438 and 625 cm-1, other bands do appear which indicate the presence of traces of the partially decomposed coordination compound. At the same time, the FTIR spectrum of the thermal decomposition product at 1000 °C (Fig. 3b) shows the absorption bands at 457, 560 (shoulder) and 669 cm-1, slightly shifted with respect to the characteristic bands of nickel(II) oxide mentioned in the literature [64, 65]. The more relevant bands of nickel(II) polyoxalate, found at 1639, 1385, 1360, 1317, 827 and 488 cm-1, all disappear, leaving place for the two characteristic bands of nickel oxide. The other weak bands are not relevant for the decomposition of nickel(II) polyoxalate. Also, the broadness of the 457 cm-1 band indicates that the NiO powder consists of nanocrystals [66, 67].

After analyzing the Raman spectra of the thermal decomposition products (Figs. 3c, 3d), we can observe the disappearance of the characteristic bands for the coordination compound, confirming its degradation. The more relevant bands of nickel(II) polyoxalate hydrate (Fig. 2b), found at 1477, 1454, 1048, 918 and 536 cm-1, all disappear, being replaced by the characteristic bands of nickel oxide. The Raman spectrum of the thermal decomposition product at 1000 °C (Fig. 3d) shows the characteristic bands of nickel oxide (351, 542, 729, 1075 cm-1), in accordance with literature data [60, 62]. Also, the Raman spectrum of the thermal decomposition product at 400 °C (Fig. 3c) reveals the presence of slightly shifted bands, confirming that it is somewhat contaminated with traces of the partially decomposed precursor.

In conclusion, based on the FTIR and Raman spectra, we can say that the nickel(II) polyoxalate has totally decomposed, forming nickel oxide as the final solid-state product.

The XRD patterns of the powders obtained by coordination compound’s annealing at 400 and 1000 °C are presented in Fig. 4. Both patterns record the diffraction lines of the single phase NiO (rhombohedral, ICDD file 01-078-4374). The powder 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 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 the values found in the ICDD file (a = b = 2.9633 Å, c = 7.2553 Å).

Fig. 4. XRD patterns of the powders obtained by coordination compound’s annealing at 400 and 1000 °C.

These results allow some conclusions to be drawn regarding the thermoanalytical curves presented in Fig. 1.

The TG profile shows, in the first two steps, the removal of the lattice water and of the coordinated water molecules (mass loss: calculated 22.88%; experimental: 23.00%). Two endothermic DTA peaks located between 25 and 160 °C (weak maximum at 134 °C) and, respectively, between 160 and 286 °C (maximum at 260 °C), and a change on the DTG curve in the same temperature ranges correspond to the removal of water. A completely dehydrated compound is produced around 300 °C, as confirmed in Fig. 1b. These results are in good agreement with those from the literature for nickel(II) oxalate dihydrate obtained through classical methods [54, 68].

The almost complete breakdown of the anhydrous compound, in the third step, takes place within a very short temperature range (286-345 °C) as shown by a steep slope on the TG curve with the inflection point at 313 °C. The total experimental mass loss of 58.80% suggests that the product of the conversion is NiO. The formation of this product is accompanied by a very sharp exothermic DTA peak located at 322 °C.

After analyzing the FTIR curves from Fig. 1b we can confirm the water release, which presents two peaks: a weak one around 145 °C and the main one around 263 °C. The carbon dioxide shows a peak around 265 °C and a very intense peak around 323 °C.

An EDX quantitative elemental analysis of very small areas revealed that, on the surface, the thermal decomposition 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 should be noted that the area analyzed by EDX was smaller than 50 nm in diameter.

Fig. 5. EDX profile in a surface area (C and Cu peaks belong to the grid).

Table 2 shows the stoichiometry and composition of the nickel oxide analyzed by EDX in a surface area.

Table 2. Composition and stoichiometry of NiO obtained from EDX.

This indicates that, on the surface, the product of thermal decomposition of the coordination compound in air is an oxygen-deficient non-stoichiometric nickel oxide.

In order to obtain useful information about the surface morphology and particle size of the nickel oxide obtained by thermal decomposition of the complex, the TEM analysis was performed. The TEM images (Fig. 6) at different magnification values show that the thermal decomposition product has a microporous structure with a large specific area and that there are no well-defined particles. The particles seem to be parallelepipeds with some defined edges but with faces which seem to be formed by small and conglomerated particles (Fig. 6a) that remind of rhombohedral crystallites. Other faces can be seen as a group of well-differentiated particles but without defined forms. The increase in the magnification (Figs. 6b – 6d) did not allow for a better differentiation of the particles. The formation of NiO aggregates, comprising very tiny three-dimensional disordered primary nanoparticles, was clearly visible.

Fig. 6. TEM images of the 1000°C oxidative decomposition product (NiO).

The nickel oxide obtained by oxidative thermal decomposition of the [NiL(H2O)2]n·0.1nH2O polynuclear coordination compound is a product with various crystalline morphology. The particles exhibit irregular forms and their size is widely distributed between 4 nm and 1 μm.

3.2. Thermal decomposition in argon

The TG, DTG and DTA curves, corresponding to the argon thermal decomposition of nickel(II) polyoxalate hydrate, are shown in Fig. 7a, while the FTIR curves of the gases evolved during the same process are shown in Fig. 7b.

Fig. 7. Thermoanalytical curves for the thermolysis of nickel(II) polyoxalate hydrate in argon: (a) TG, DTG and DTA results; (b) FTIR analysis of generated gaseous products (not to scale): CO2 2361 cm-1, CO 2108 cm-1, H2O 1508 cm-1.

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 coordinated water molecules (mass loss: calculated 22.87; experimental: 23.32). The first mass loss, up until around 165°C, has a maximum intensity recorded at 108°C and a corresponding endothermic DTA peak located at 138°C, as confirmed by the FTIR spectrum (maximum at 150°C), while the second mass loss, up until around 291°C, has a process maximum intensity at 264°C and a corresponding endothermic DTA peak at 268°C. The latter is also confirmed by the FTIR spectrum (maximum at 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 400°C), with CO2 release. A weak trace of CO was recorded in the third step around 343°C. The total mass loss of 64.69% suggests that the product of the conversion is a mixture of Ni and NiO in a 3:2 molar ratio. The following analyses confirm this result.

The FTIR spectrum of the final argon decomposition product is shown in Fig. 8. The spectrum shows a few absorption bands at 476, 555 (shoulder) and 663 cm-1, slightly shifted compared with those from Fig. 3b, probably due to the presence of metallic nickel.

Fig. 8. FTIR spectrum of the 1000 °C argon decomposition product; the FTIR band at 1630 cm-1 is owned to traces of water.

The XRD pattern of the powder obtained by coordination compound’s annealing in argon at 1000 °C is presented in Fig. 9. The XRD pattern records the diffraction lines of the NiO (rhombohedral, ICDD 01-089-3080) and metallic nickel (cubic, ICDD 00-004-0850).

Fig. 9. XRD patterns of the powder obtained by nickel(II) polyoxalate hydrate’s annealing in argon at 1000 °C.

The TEM images (Fig. 10) at different magnification values reveal that the particles’ shapes remain almost the same with a slight increase in agglomeration.

Fig. 10. TEM images of the 1000°C argon decomposition compound.

4. Conclusions

Three successive decomposition steps for nickel(II) polyoxalate hydrate, the first two endothermic and the last exothermic, were recorded during its thermal decomposition in dynamic air atmosphere.

The thermal conversion of this homopolynuclear coordination compound gave, depending on the oxidative or inert atmosphere, nickel oxide or, respectively, a mixture of Ni and NiO in a 3:2 molar ratio. The XRD results show that the synthesized NiO phase possesses a rhombohedral crystallinity, while metallic nickel phase possesses cubic crystallinity. The TEM images at different magnification values show that the NiO nanoparticles obtained by thermal decomposition of the coordination compound in air were formed by aggregations. These particles exhibit irregular shapes and their size is widely distributed between 4 nm and 1 μm. The TEM images of the mixture of Ni and NiO nanoparticles obtained by thermal decomposition in argon reveal that the particles’ shape is basically the same with a slight increase in agglomeration.

The nonisothermal decomposition kinetics of nickel(II) polyoxalate hydrate will be presented in a future work.

References

[1] Brezeanu M, Safarica E, Segal E, Patron L, Robu T. An analysis of thermal decomposition of coordination polynuclear compounds in Fe(III), Mn(II), Ni(II) systems. Rev. Roum. Chim. 1982;27:137-40.

[2] Brezeanu M, Tatu E, Bocai S, Brezeanu O, Segal E, Patron L. Non-isothermal decomposition kinetics of polynuclear coordination compounds. Thermochim. Acta 1984;78:351-5.

[3] Rădoi I, Bîrzescu M, Golumbioschi F, Ștefănescu M. Obținerea de anozi cu pelicule electrocatalitice active din complecși metalici, folosiți în electroliza apei (Obtaining anodes with electrocatalytically active films from metal complexes, used in water electrolysis). Rev. Chim. (Bucharest) 1985;36:832-6.

[4] Urbanovici E, Segal E, Andruh M, Brezeanu M. Non-isothermal decomposition kinetics of a mononuclear coordination compound of Fe(II). Thermochim. Acta 1987;118:309-12.

[5] Andruh M, Brezeanu M, Paraschivoiu I, Segal E, Thermal behaviour of complex cation-complex anion-type coordination compounds. Part I. Thermochim. Acta 1990;161:247-57.

[6] Dragoe N, Andruh M, Meghea A, Segal E. Thermal behaviour of complex cation-complex anion-type coordination compounds. Part II. Thermochim. Acta 1990;161:259-66.

[7] Birzescu M, Niculescu M, Dumitru R, Budrugeac P, Segal E. Copper(II) oxalate obtained through the reaction of 1,2-ethanediol with Cu(NO3)2·3H2O. Structural investigations and thermal analysis. J. Therm. Anal. Calorim. 2008;94:297-303.

[8] Gao Z, Cui F, Zeng S, Guo L, Shi J. A high surface area superparamagnetic mesoporous spinel ferrite synthesized by a template-free approach and its adsorptive property. Microporous Mesoporous Mater. 2010;132:188-95.

[9] Buta I, Ianasi C, Savii C, Cseh L, Bakardieva S, Linert W, Costisor O. Synthesis and Characterization of New Heterometallic Cobalt-zinc Oxalates Linked by Organic Amines. Rev. Chim. (Bucharest) 2014;65:416-20.

[10] Berbenni V, Milanese C, Bruni G, Girella A, Marini A. Synthesis of calcium metastannate (CaSnO3) by solid state reactions in mechanically activated mixtures calcium citrate tetra hydrate [Ca3(C6H5O7)2·4H2O] – tin(II) oxalate (SnC2O4). Thermochim. Acta 2015;608;59-64.

[11] Baran EJ. Natural iron oxalates and their analogous synthetic counterparts: A review. Chem. Erde – Geochem. 2016;76;449-60.

[12] Niculescu M, Bîrzescu M, Budrugeac P, Segal E. Thermal and structural investigation of the reaction between 1,2-propanediol and Ni(NO3)2⋅6H2O. J. Therm. Anal. Calorim. 2001;63:181-9.

[13] Niculescu M, Bîrzescu M, Budrugeac P, Segal E. Thermal and structural investigation of the reaction between 1,2-propanediol and Co(NO3)2⋅6H2O. J. Therm. Anal. Calorim. 2001;65:881-9.

[14] Niculescu M, Negrea P, Bîrzescu M, Budrugeac P. Structural investigations and thermal analysis of the coordination compound obtained through the reaction of 1,3-propanediol with Co(NO3)2·6H2O. Rev. Roum. Chim. 2003;48:997-1006.

[15] Niculescu M, Dumitru R, Magda A, Bandur G, Sisu E. Noi metode de obținere a unor acizi carboxilici prin reacții de oxidare a poliolilor cu azotați de metal(II). I. Reacții de oxidare a 1,2-propandiolului cu azotați de metal(II) (New methods to obtain carboxylic acids by oxidation reactions of polyols with metal(II) nitrates. I. Oxidation reactions of 1,2-propandiol with metal(II) nitrates). Rev. Chim. (Bucharest) 2007;58:932-6.

[16] Niculescu M, Bîrzescu M, Dumitru R, Sisu E, Budrugeac P. Co(II)–Ni(II) heteropolynuclear coordination compound obtained through the reaction of 1,2-propanediol with metallic nitrates as precursor for mixed oxide of spinel type NiCo2O4. Thermochim. Acta 2009;493:1-5.

[17] Birzescu M, Niculescu M, Dumitru R, Carp O, Segal E. Synthesis, structural characterization and thermal analysis of the cobalt(II) oxalate obtained through the reaction of 1,2-ethanediol with Co(NO3)2·6H2O. J. Therm. Anal. Calorim. 2009;96:979-86.

[18] Niculescu M, Dumitru R, Magda A, Pode V. New methods to obtain carboxylic acids by oxidation reactions of polyols with metal(II) nitrates. II. Oxidation reactions of 1,3-propanediol with metal(II) nitrates. Rev. Chim. (Bucharest) 2013;64:271-4.

[19] Niculescu M, Budrugeac P, Ledeți I, Pode V, Birzescu M. Synthesis and characterization of the polynuclear coordination compound obtained through the reaction of 1,3-propanediol with Cu(NO3)2·3H2O. Rev. Roum. Chim. 2013;58:387-92.

[20] Roșu D, Bîrzescu M, Milea MS, Pascariu MC, Sasca V, Niculescu M. Synthesis-structure relationship in the aqueous ethylene glycol-iron(III) nitrate system. Rev. Roum. Chim. 2014;59:789-96.

[21] Niculescu M, Sasca V, Muntean C, Milea MS, Roșu D, Pascariu MC, Sisu E, Ursoiu I, Pode V, Budrugeac P. Thermal behavior studies of the homopolynuclear coordination compound iron(III) polyoxalate, Thermochim. Acta 2016;623:36–42.

[22] Kang YC, Park SB, Kang YW. Preparation of high surface area nanophase particles by low pressure spray pyrolysis. Nanostruct. Mater. 1995;5:777-91.

[23] Anderson PJ, Horlock RF. Thermal decomposition of magnesium hydroxide. Trans. Faraday Soc. 1962;58:1993-2004.

[24] Paulik F, Paulik J, Arnold M. Investigation of the phase diagram for the system Ni(NO3)2-H2O and examination of the decomposition of Ni(NO3)2 · 6H2O. Thermochim. Acta 1987;121:137-49.

[25] Elmasry MAA, Gaber A, Khater EMH. Thermal Decomposition of Ni(II) and Fe(III) Nitrates and their Mixture. J. Therm. Anal. 1998;52:489-95.

[26] Llewellyn PL, Chevrot V, Ragaï J, Cerclier O, Estienne J, Rouquérol F. Preparation of reactive nickel oxide by the controlled thermolysis of hexahydrated nickel nitrate. Solid State Ionics 1997;101–103:1293-98.

[27] Schmid G. Large clusters and colloids. Metals in the embryonic state. Chem. Rev. 1992;92:1709-27.

[28] Brus L. Capped nanometer silicon electronic materials. Adv. Mater. 1993;5:286-8.

[29] Ershov BG, Janata E, Henglein A. Growth of silver particles in aqueous solution: long-lived "magic" clusters and ionic strength effects. J. Phys. Chem. 1993;97:339-43.

[30] Kamat PV, Photochemistry on nonreactive and reactive (semiconductor) surfaces. Chem. Rev. 1993;93:267-300.

[31] Klabunde KJ, Stark J, Koper O, Mohs C, Park DG, Decker S, Jiang Y, Lagadic I, Zhang D. Nanocrystals as Stoichiometric Reagents with Unique Surface Chemistry. J. Phys. Chem. 1996;100:12142-53.

[32] Soriano L., Abbate M, Vogel J, Fuggle JC, Fernández A, González-Elipe AR, Sacchi M, Sanz JM. The electronic structure of mesoscopic NiO particles. Chem. Phys. Lett. 1993;208:460-4.

[33] Alders D, Voogt FC, Hibma T, Sawatzky GA. Nonlocal screening effects in 2p x-ray photoemission spectroscopy of NiO (100). Phys. Rev. B 1996;54:7716-19.

[34] Biju V, Khadar MA. Analysis of AC electrical properties of nanocrystalline nickel oxide. Mater. Sci. Eng. A 2001;304–306:814-817.

[35] Makhlouf SA, Parker FT, Spada FE, Berkowitz AE. Magnetic anomalies in NiO nanoparticles. J. Appl. Phys. 1997;81:5561-3.

[36] Kodama RH, Makhlouf SA, Berkowitz AE. Finite Size Effects in Antiferromagnetic NiO Nanoparticles. Phys. Rev. Lett. 1997;79:1393-6.

[37] Kodama RH. Magnetic nanoparticles. J. Magn. Magn. Mater. 1999;200:359-72.

[38] Bødker F, Hansen MF, Bender Koch CB, Mørup S. Particle interaction effects in antiferromagnetic NiO nanoparticles. J. Magn. Magn. Mater. 2000;221:32-6.

[39] Fievet F, Figlarz M. Preparation and study by electron microscopy of the development of texture with temperature of a porous exhydroxide nickel oxide. J. Catal. 1975;39:350-6.

[40] Alejandre A, Medina F, Salagre P, Fabregat A, Sueiras JE. Characterization and activity of copper and nickel catalysts for the oxidation of phenol aqueous solutions. Appl. Catal. B: Environ. 1998;18:307-15.

[41] Carnes CL, Klabunde KJ. The catalytic methanol synthesis over nanoparticle metal oxide catalysts. J. Mol. Catal. A: Chem. 2003;194:227-36.

[42] Boschloo G, Hagfeldt A. Spectroelectrochemistry of Nanostructured NiO. J. Phys. Chem. B 2001;105:3039-44.

[43] Das D, Pal M, Di Bartolomeo E, Traversa E, Chakravorty D. Synthesis of nanocrystalline nickel oxide by controlled oxidation of nickel nanoparticles and their humidity sensing properties. J. Appl. Phys. 2000;88:6856-60.

[44] Wang Y, Zhu J, Yang X, Lu L, Wang X. Preparation of NiO nanoparticles and their catalytic activity in the thermal decomposition of ammonium perchlorate. Thermochim. Acta 2005;437:106-9.

[45] Li X, Zhang X, Li Z, Qian Y. Synthesis and characteristics of NiO nanoparticles by thermal decomposition of nickel dimethylglyoximate rods. Solid State Commun. 2006;137:581-4.

[46] Han DY, Yang HY, Shen CB, Zhou X, Wang FH. Synthesis and size control of NiO nanoparticles by water-in-oil microemulsion. Powder Tech. 2004;147:113-6.

[47] Deng XY, Chen Z. Preparation of nano-NiO by ammonia precipitation and reaction in solution and competitive balance. Mater. Lett. 2004;58:276-80.

[48] Xin X, Lü Z, Zhou B, Huang X, Zhu R, Sha X, Zhang Y, Su W. Effect of synthesis conditions on the performance of weakly agglomerated nanocrystalline NiO. J. Alloy Compd. 2007;427:251-5.

[49] Dierstein A, Natter H, Meyer F, Stephan HO, Kropf C, Hempelmann R. Electrochemical deposition under oxidizing conditions (EDOC): a new synthesis for nanocrystalline metal oxides. Scr. Mater. 2001;44:2209-12.

[50] Lin C, Al-Muhtaseb SA, Ritter JA. Thermal Treatment of Sol-Gel Derived Nickel Oxide Xerogels. J. Sol-Gel Sci. Tech. 2003;28:133-41.

[51] Park YR, Kim KJ. Sol–gel preparation and optical characterization of NiO and Ni1−xZnxO thin films. J. Cryst. Growth 2003;258:380-4.

[52] Wang Y, Ma C, Sun X, Li H. Preparation of nanocrystalline metal oxide powders with the surfactant-mediated method. Inorg. Chem. Commun. 2002;5:751-5.

[53] Bîrzescu M, Milea M, Roșu D, Ledeți I, Rafailă M, Sasca V, Niculescu M. Synthesis and thermal analysis of the nickel(II) oxalate obtained through the reaction of ethylene glycol with Ni(NO3)2·6H2O. Rev. Roum. Chim. 2014;59:555-63.

[54] Macklen ED. Influence of atmosphere on the thermal decomposition of some transition metal oxalates. J. Inorg. Nucl. Chem. 1968;30:2689-95.

[55] Niculescu M, Budrugeac P, Structural characterization of nickel oxide obtained by thermal decomposition of polynuclear coordination compound [Ni2(OH)2(H3CCH(OH)COO-)2(H2O)2·0.5H2O]n. Rev. Roum. Chim. 2013;58:381-6.

[56] Niculescu M, Roșu D., Ledeți I, Milea M, Budrugeac P. Thermal and spectroscopic studies of Ni(II)-Fe(III) heteropolynuclear coordination compound obtained through the reaction of 1,2-ethanediol with metallic nitrates. Rev. Roum. Chim. 2013;58:543-52.

[57] Niculescu M, Ledeți I, Bîrzescu M. New methods to obtain carboxylic acids by oxidation reactions of 1,2-ethanediol with metallic nitrates. J. Organomet. Chem. 2014;767:108–111.

[58] Niculescu M, Magda A, Jurca M, Costea L, Pode V. Thermal and kinetic studies of the polynuclear coordination compound obtained through the reaction of 1,3-propanediol with Ni(NO3)2·6H2O. Rev. Chim. (Bucharest) 2015;66:1217-21.

[59] Segal E, Budrugeac P, Carp O, Doca N, Popescu C, Vlase T. Analiza termică. Fundamente și aplicații. Analiza cinetica a transformarilor heterogene (Thermal analysis. Fundamentals and applications. Kinetic analysis of heterogeneous transformations). Bucharest: Publishing House of the Romanian Academy; 2013.

[60] Mironova-Ulmane N, Kuzmin A, Steins I, Grabis J, Sildos I, Pärs M. Raman scattering in nanosized nickel oxide NiO. J. Phys.: Conf. Ser. 2007;93:012039.

[61] Wladimirsky A, Palacios D, D’Antonio MC, González-Baró AC, Baran EJ, Vibrational spectra of the α-MIIC2O4⋅2H2O oxalato complexes, with MII = Co, Ni, Zn. The Journal of the Argentine Chemical Society 2011;98:71-7.

[62] Nowsath Rifaya M, Theivasanthi T, Alagar M. Chemical Capping Synthesis of Nickel Oxide Nanoparticles and their Characterizations Studies. Nanoscience and Nanotechnology 2012;2:134-8.

[63] Dass NN, Sarmah S. Synthesis and Thermal Decomposition of [Ni2(C4H4O6)2]·7H2O. J. Therm. Anal. Calorim. 1999;58:137-45.

[64] Spectral Database for Organic Compounds, SDBS. http://sdbs.db.aist.go.jp/sdbs/cgi-bin/direct_frame_top.cgi. Accessed July 13, 2016.

[65] Bentley FF, Smithson LD, Rozek AL. Infrared Spectra and Characteristic Frequencies ~ 700-300 cm-1. New York – London – Sydney: John Wiley & Sons; 1968.

[66] Qiao H, Wei Z, Yang H, Zhu L, Yan X. Preparation and Characterization of NiO Nanoparticles by Anodic Arc Plasma Method. J. Nanomater. 2009;2009:1-5.

[67] Wang YP, Zhu JW, Zhang LL, Yang XJ, Lu LD, Wang X. Study on the preparation and spectral characteristics of nano-NiO, Guangpuxue Yu Guangpu Fenxi 2006;26:690-3 (article in Chinese).

[68] Fu XM, Yang ZZ. Preparation of spherical NiO nanoparticles by the thermal decomposition of NiC2O4.2H2O precursor in the air. Adv. Mater. Res. (Durnten-Zurich) 2011;228-229:34-7.