In situ thermal decomposition route: Preparation and characterization of nano nickel, cobalt, and copper oxides using an aromatic amine complexes as a low-cost simple precursor

The main interest now is the development of metallic or inorganic-organic compounds to prepare nanoparticle materials. The use of new compounds could be bene ﬁ cial and open a new method for preparing nanomaterials to control the size, shape, and size of the nanocrystals. In this article, the thermal decomposition of [M 2 (o-tol) 2 (H 2 O) 8 ] Cl 4 (where o-tol is ortho -tolidine compound, M = Ni 2+ , Co 2+ , Cu 2+ ) new precursor complex was discussed in solid-state conditions. The thermal decomposition route showed that the synthesized three complexes were easily decomposed into NiO, Co 3 O 4 and CuO nanoparticles. This decomposition was performed at low temperatures (~600 o C) in atmospheric air without using any expensive and toxic solvent or complicated equipment. The obtained product was identi ﬁ ed by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX). FT-IR, XRD and EDX analyses revealed that the NiO nanoparticles exhibit a face-centered-cubic lattice structure with a crystallite size of 9–12 nm. The formation of a highly pure spinel-type Co 3 O 4 phase with cubic structure showed that the Co 3 O 4 nanoparticles have a sphere-like morphology with an average size of 8–10 nm. The XRD patterns of the CuO con ﬁ rmed that the monoclinic phase with the average diameter of the spherical nanoparticles was approximately 9–15 nm.


INTRODUCTION
Ortho-tolidine is an organic compound with the molecular formula (CH 3 -C 6 H 4 -NH 2 ) 2 . It is a colorless organic compound, slightly soluble in water, forms salts with acids, used mainly for dye production 1 , as intermediate to produce soluble azo dyes and insoluble textile pigments, leather, paper industries, and to produce certain elastomers. o-Tolidine was widely used as a reagent/indicator in analytical, clinical, and forensic chemistry, such as in the analytical determination of gold, or determination of the chlorine level in swimming pool water 2 .
Transition metal oxide nanoparticles represent an important class of inorganic nanomaterials that have been investigated extensively due to their interesting catalytic, electronic, and magnetic properties relative to those of the bulk counterparts, and the wide scope of their potential applications 3 . The nickel oxide nanoparticles (NiO) are one of the supreme transition metal oxides, it has been a p-type semiconductor behavior. The different features of the NiO nanoparticles have prompted different research topics. The quantum size, high specifi c surface area, volume, and macroscopic quantum tunneling effects revealed the unique magnetic, electronic, catalytic, chemical, and optical properties of the NiO nanoparticles. These properties encouraged their extensive application in ultra-magnetic devices, photoelectric smart windows, photocatalytic applications, electrochemical supercapacitors, photoelectric devices, and gas sensors 4-9 . NiO nanoparticles are also magnetic nanoparticles with good electrochemical activity, which motivate their use in electrochemical biosensors 8, 9 . Cobalt oxide (Co 3 O 4 ) with spinel-type as a semiconductor material having realizable applications in gas sensors 10 , heterogeneous catalysts 11 , electrochemical devices 12 , lithium-ion batteries 13 , materials magnetism 14, 15 and photocatalysts 16 . In literature, increasing interest has been focused on the synthesis of Co 3 O 4 nanostructures due to the infl uence of particle size on their properties and applications 17 . Various wet chemical methods such as hydrothermal/thermal solvent method 18 , combustion method 19 , microwave heating 20 , gel solution process 21 , spray pyrolysis 22 , sonochemical method 23 , co-sedimentation 24 , ionic liquid-assisted method 25 , a polyol method 26 and a non-aqueous method 27 are reported for the assembly of Co 3 O 4 nanostructures. However, most of these methods involve complex processes, high sintering temperatures, and expensive and toxic precursors. Additionally, they are either time-consuming or require expensive tools. Solid-state thermolysis of molecular precursors is the simplest and least expensive to preparing metal oxide nanostructures. This promising technology offers many unique advantages and signifi cant advantages over other methods including easy work, relatively short reaction time, and preparation of numerous inorganic nanomaterials with unique sizes, specifi c shapes, and narrow size distribution 28 . The copper oxide (CuO) has been studied as a p-type semiconductor material with a narrow band gap of 1.2 eV, because of its natural abundance of raw materials, low-cost production processing, non-toxic nature, and reasonably good electrical and optical properties. CuO nanoparticles have been of great interest due to their potential applications in a wide range of fi elds including electronic and optoelectronic devices, such as microelectromechanical systems, fi eld-effect transistors, electrochemical cells, gas sensors, magnetic storage media, solar cells, and fi eld emitters, and nanocatalysis devices. It has also been emphasized recently that regardless of size, the shape of the nanostructure is equally important for controlling various properties 29 . The main task of this work is to synthesis nanoparticles of NiO, Co 3 O 4 , and CuO using the thermal decomposition method and its physicochemical characterizations.
This article is aimed to synthesis of three transition metal oxide nanoparticles from a simple precursor in a short time and with an easy, low temperature, low-cost method without using any special instrument. To the best of our knowledge, this is the fi rst report on the synthesis of NiO, Co 3 O 4 , and CuO nanoparticles from aromatic amine with [M 2 (o-tol) 2 (H 2 O) 8 ]Cl 4 precursor complex.

Chemicals and Instrument Techniques
All Melting points of the compounds were determined in open capillaries in an electrical MPS10-120 melting point apparatus. The magnetic moments were determined on a Guoy balance and the diamagnetic corrections of the complexes were calculated using Pascal's constants. Molar conductivities were measured in DMSO solution at 10 -3 M concentration using a Jenway 4010 conductivity meter. The metal contents were estimated with a gravimetrical method at 800 o C by converted the synthesized metal complexes to metal oxides as a stable form that suitable to calculate the percentage of metal ions. The X-ray diffraction patterns were recorded on X 'Pert PRO PANanalytical X-ray powder diffraction, target copper with secondary monochromate. The transmission electron microscopy images (TEM) were performed using JEOL 100s microscopy.

Analytical and molar conductance data
The [M 2 (o-tol) 2 (H 2 O) 8 ]Cl 4 complexes of nickel, cobalt, and copper (II) metal ions were prepared by heating together methanol/distilled H 2 O solutions of the appropriate ligand and metal chlorides with 1:1 molar ratio. All the prepared complexes are stable at room temperature. The prepared complexes are insoluble in methanol, ethanol, benzene, and acetonitrile, but soluble in DMSO and DMF. The analytical data (carbon, hydrogen, nitrogen, chloride, and metal ions percentages) are presented as mentioned in the experimental section. The molar conductance values of the complexes of (10 -3 M solution in DMSO) are found to be in the range of 180-225 ohm -1 cm 2 mol -1 . These high values indicate that the complexes are ionic with an electrolytic nature 15 . assigned to 4 T 1g (F) → 4 T 2g (F), 4 T 1g (F)→ 4 A 2g (F) and 4 T 1g (F) → 4 T 2g (P) electronic transitions respectively, due to octahedral geometry structure 33 . The absorption spectrum of nickel(II) complex included a three electronic bands at 27397 cm -1 , 16287 cm -1 , and 10288 cm -1 assigned to 3 A 2g (F)→ 3 T 2g (F), 3 A 2g (F)→ 3 T 1g (F), and 3 A 2g (F)→ 3 T 2g (P) transitions respectively, these transitions agreement with octahedral geometry. The magnetic moment of the copper(II) complex (1.81 BM ) is matched with the octahedral geometry. The magnetic moment value for the nickel (II) complex is 3.10 BM due to the octahedral environment around Ni(II) metal ion. Besides this, the magnetic moment of the cobalt (II) complex is 4.64 BM which consistent with octahedral geometry 33 .

Infrared spectra
By comparison between the spectra of both nickel, cobalt, copper(II) complexes (Fig. 1) and the data of o-tol ligand have been studied and assigned in Table 1.
In the case of o-tol free ligand, the distinguish stretching vibration bands of ν(N-H) of NH 2 group and some of stretching vibrations of ν(C-H) aromatic rings are exhibited at 3475, 3412, 3375, 3338, 3213, 3019 cm -1 31 . It is found that the frequencies of -NH 2 groups are shifted to a lower wavenumber at the range 3466-3337 cm -1 , which indicates that the interactions placed among the nitrogen of -NH 2 . To place greater emphasis on the interactions between the metal ions and o-tol ligand, the 2000-1000 cm -1 region was investigated. This region contains the bending vibration motions of -NH 2 group δNH 2 which is infl uenced by complexation and shifted to lower wavenumbers and consequently, the intensity was distorted. Peaks above 3400 cm -1 in the Co(II), Ni(II) and Cu(II) complexes, indicated the presence of coordinated water 31 . The conforming coordination evidence is also displayed by the presence of new bands in the spectra of all the complexes that occur in the range of 585-508 cm -1 and 443-404 cm -1 are the characteristic bands of stretching vibrations ν(M-O) and ν(M-N) respectively 31 that are disappeared in the free ligand spectrum.

UV-Vis spectra and magnetic susceptibility
The electronic spectrum of copper(II) o-tol complex has three electronic absorption bands at 12422, 17007 and 20747 cm -1 due to 2 B 1g → 2 B 2g , 2 B 1g → 2 A 2g and 2 B 1g → 2 E 1g transitions respectively, which confi rmed that Cu(II) complex has a distorted octahedral geometry 32 . The electronic spectrum of the cobalt(II) complex has three absorption bands at 10965 cm -1 , 17544 cm -1 and 21505 cm -1 Table 1. Infrared frequencies/cm -1 and tentative assignments of o-tol and its complexes proaching or exceeding one, respectively. It is evaluated using the following equation 36 : I crys = D(TEM)⁄D(XRD). The crystallinity index is equal to 0.34, suggesting the mono-crystallinity of NiO nanoparticles. Figure 4 represents the FT IR spectrum of the prepared NiO NPs. The spectrum demonstrates a signifi cant peak at 466 cm -1 which is related to the vibrational mode of Ni-O bond. The weak broad peaks occur at ~1630 cm -1 and 3400 cm -1 are assigned to H-O-H bending vibrations mode and O-H stretching vibrations bond of crystalline water molecules as moisture on the surface of the prepared NiO oxide respectively 31 . Elemental analysis was performed to evaluate the elemental composition of the NiO NPs using EDX spectroscopy. The EDX spectrum of the NiO NPs exhibits distinguish peaks corresponding to Ni and O, as shown in Figure 5.

Characterizations of Co 3 O 4 oxide
The XRD pattern of the prepared Co 3 O 4 in Figure 2, exhibits some diffraction peaks with 2θ values at 18.91°, 31.27°, 36.75°, 38.43°, 44.76°, 55.57°, and 59.22°. These diffraction peaks can be indexed to the crystalline cubic phase Co 3 O 4 with a space group of Fd3m, which agrees with the reported values (JCPDS Card No. 76-1802) 37 . This result confi rms that the Co 3 O 4 phase started to appear at 600°C, as indicated by the FT-IR result (Figure 4). No impurity diffraction peaks were detected in the patterns, indicating that the product is of high purity. Furthermore, the diffraction peaks are markedly sharpness due to the small size effect of the particles. The average sizes of the Co 3 O 4 particles were calculated empirical constant equal to 0.9. Figure 3a reveals the size, shape, and morphology of the prepared NiO NPs checked by TEM image. The average crystallite size is found to be 9-12 nm, moreover, the fi gure confi rms the spherical shape of NiO NPs with slight agglomeration. The spherical shape offers a good contact area, which is recommended for biosensing applications 35 . The crystallinity index (I crys ) assigns the mono-crystallinity or poly-crystallinity of the nanoparticles if its value is ap- It is also can fi nd from this fi gure that the morphology of the particles is almost homogeneous.

Characterizations of CuO oxide
For XRD patterns of the CuO nanoparticles, all peaks can be confi rmed to be the monoclinic phase of CuO (JCPDS no. 48-1548) 35 as shown in Figure 2. The smaller size of CuO nanoparticles can be predicted consistently with the broadening diffraction peaks of XRD patterns. Moreover, the crystallite size based on the broadening diffraction peaks was approximately estimated from the corresponding X-ray spectral peak employing the Debye--Scherrer's formula 34 . The calculated value was estimated to be 15 nm. Figure 3c showed the morphology of highly crystalline CuO nanoparticles from TEM analysis. Nevertheless, the accurate sizes and morphology of the nanoparticles can be estimated from the TEM analysis, TEM images can reveal the internal structure and more accurate measurement of particle size (9-15 nm) and morphology. FTIR spectrum revealed vibration of the CuO band at 524 cm −1 (Fig. 4). The corresponding EDX (Fig. 5) spectrum can also confi rm the presence of Cu and O elements in the sample. The spherical CuO nanoparticles are evaluated by EDX quantitative analysis software (Oxford Instrument) and found to have 55.62