One Step Synthesis of NiO Nanoparticles via Solid-State Thermal Decomposition at Low-Temperature of Novel Aqua(2,9-dimethyl-1,10-phenanthroline)NiCl2 Complex

[NiCl2(C14H12N2)(H2O)] complex has been synthesized from nickel chloride hexahydrate (NiCl2·6H2O) and 2,9-dimethyl-1,10-phenanthroline (dmphen) as N,N-bidentate ligand. The synthesized complex was characterized by elemental analysis, infrared (IR) spectroscopy, ultraviolet-visible (UV-vis) spectroscopy and differential thermal/thermogravimetric analysis (TG/DTA). The complex was further confirmed by single crystal X-ray diffraction (XRD) as triclinic with space group P-1. The desired complex, subjected to thermal decomposition at low temperature of 400 ºC in an open atmosphere, revealed a novel and facile synthesis of pure NiO nanoparticles with uniform spherical particle; the structure of the NiO nanoparticles product was elucidated on the basis of Fourier transform infrared (FT-IR), UV-vis spectroscopy, TG/DTA, XRD, scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDXS) and transmission electron microscopy (TEM).

The starting complex and final NiO nanoparticles product before and after thermal decomposition were characterized on the basis of Fourier transform infrared (FT-IR), ultraviolet-visible (UV-vis) spectroscopy, differential thermal/thermogravimetric analysis (TG/DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDXS) and transmission electron microscopy (TEM); additionally the complex was characterized by elemental analysis and X-ray single crystal structure.
In this work, we wish to report the synthesis and characterization of mononuclear aqua-dichloro (2,9-dimethyl-1,10-phenanthroline-κ 2 N,N)nickel(II) complex. Subsequently, direct thermal decomposition process of the desired complex precursor is one of the most important and straightforward strategies to access structurally elaborated and pure NiO nanoparticles with regular spherical particle.

Synthesis of the Desired Complex and NiO Nanoparticles
The mononuclear 2,9-dimethyl-1,10-phenanthroline-nickel(II) complex was isolated in excellent yield by stirring equivalent amounts of 2,9-dimethyl-1,10-phenanthroline in distilled water with NiCl 2 ·6H 2 O in ethanol [33,34]. NiO nanoparticles were successfully synthesized through thermal decomposition of the (2,9-dimethyl-1,10-phenanthroline)NiCl 2 complex precursor at 400 °C. NiO is formed via decomposition of 2,9-dimethyl-1,10-phenanthroline organic and chloride ligands in open atmosphere to NiO powder product and CO x , NO x , ClO x as expected gases bi-products. Uniform and spherical NiO nanoparticles with weak agglomeration were collected, as seen in Scheme 1. The structures of the desired complex before and after thermal decomposition to prepare the NiO nanoparticles product were subjected to several available physical measurements.

Thermal Decomposition Analysis of NiCl 2 (2,9-Dimethyl-1,10-phenanthroline)·H 2 O Complex to NiO Nanoparticle
The thermal properties TG/DTA of the desired complex was investigated under open atmosphere in the 0-800 °C temperature range and heating rate of 10 °C/min. Typical thermal TG/DTA curve is given in Figure 1. As seen from thermal curves, no uncoordinated water molecules were detected in the complex structure in the range 50-100 °C. One coordinated-water molecule was recorded with weight loss ~5% in the range 130-160 °C and sharp DTA endothermic signal at ~140 °C. Thermal decomposition study of residue complex showed no intermediate decomposition steps of the coordinated chlorides and 2,9-dimethyl-1,10-phenanthroline ligands, both ligands de-structured away from the complex to CO x , NO x and ClO x as gases by-products with one broad step decomposition at 300-500 °C and an exothermic DTA signal at ~405 °C; the final main product was confirmed by IR to be NiO, then subjected to several available physical measurements [16,[24][25][26][27]34].

IR Spectral Investigation
IR spectrum in particular showed five main sets of characteristic absorptions in the range 3410, 3090, 2940, 530 and 390 cm −1 , which can be assigned to, coordinated-H 2 O, H-Ph, H-CH 2 , Ni-N and Ni-Cl stretching vibrations, respectively as in Figure 2a, all bands of the of (2,9-dimethyl-1,10-phenanthroline)NiCl 2 complex disappeared after thermal decomposition at 400 °C and a strong band at around 440 cm −1 is observed, as seen in Figure 2b, which was assigned to the Ni-O stretching of the octahedral NiO 6 groups in the face center cubic NiO structure [30][31][32][33].

Electronic Absorption Spectral Study
The optical properties of the NiCl 2 (2,9-dimethyl-1,10-phenanthroline)·H 2 O complex was investigated by UV-vis spectroscopy (Figure 3a). For comparison, UV-vis spectra of the prepared NiCl 2 (2,9-dimethyl-1,10-phenanthroline)·H 2 O complex and NiO nanoparticles using water as solvent are also presented in Figure 3b. As expected, the aqueous solution of the starting complex in Figure 3a exhibits multiple absorptions in the UV-visible regions. The ligand displayed typical ligand-centered π→π* transitions at 240, 280 and 304 nm. Upon coordination with nickel ions, there are minor changes of these bands. The visible spectra of the desired complex was obtained at higher concentration (10 −4 M) with the maximum absorption at 350, 530 and 560 nm can be assigned to d to d electron transition or Metal to Ligand Charge Transfer (MLCT) [32][33][34][35][36][37]. It was clearly evident that the UV-vis spectrum of the NiO nanoparticles is quite different from that starting complex, confirming the strong band that appeared at 355 nm is due to NiO nanoparticle, not Ni(II) complex. This strong absorption band is attributed to the electronic transition from the valence band to the conduction band in the NiO semiconductor [36,37]. In addition, the UV-vis spectrum of a commercial bulk NiO powder does not show any observable absorption band [36]. The optical band gap of NiO nanoparticles has been calculated from the absorption spectrum using the Tauc relation [38].
Where C is a constant, ε is molar extinction coefficient, E g is the average band gap of the material and n depends on the type of transition. For n = ½, E g in Equation (1); is direct allowed band gap. The average band gap was estimated from the intercept of linear portion of the (εhν) 2 vs. hν plots, as shown in Figure 4.
The strongest absorption peak of the NiO prepared sample appears at around 355 nm, which is fairly blue shifted from the absorption edge of bulk NiO nanoparticle [35,36]. The band gap energy calculated from UV-absorption is 3.96 eV. This value is higher than bulk NiO i.e., E g = 3.74 eV. So it is highly agreed that the synthesized NiO is in nano scale [14,26].

EDX Analysis
EDX analysis of the NiCl 2 (2,9-dimethyl-1,10-phenanthroline)·H 2 O complex and NiO nanoparticles product were represented in Figure 5. EDX of complex revealed several signals come from Ni, Cl, N, C and O, Figure 5a. After thermal decomposition process only Ni and O signals come from the NiO nanoparticles formation as seen in Figure 5b. 2.6. X-ray Single Crystal of NiCl 2 (2,9-Dimethyl-1,10-phenanthroline)·H 2

O Complex and XRD Powder of NiO
The molecular structure is shown in Figure 6 and selected bond distances and angles are given in the Table 1. The complex was crystallized in triclinic P-1 space group. The Ni(II) ion is five-coordinated to two N atoms of 2,9-dimethyl-1,10-phenanthroline and two Cl ions and one O atom of water. The overall geometry around each nickel center atom is in a slightly distorted triangular bipyramid configuration. Several H-O and H-Cl hydrogen-bonds were detected which may stabilized the structure of mononuclear nickel(II). Figure 7 shows powder XRD patterns of the decomposition NiCl 2 (2,9-dimethyl-1,10-phenanthroline)·H 2 O complex product at 400 °C reveals only the diffraction peaks attributable to NiO with face-centered cubic phase at 2θ = 37.  The average size of the NiO nanoparticles was estimated using the relative intensity peak (220) by the Debye-Scherrer equation [39], was found to be 16 nm and increase in sharpness of XRD peaks indicates that particles are in crystalline nature: Where λ is the wavelength (λ = 1.542 Å) (Cu-K α ), β is the full width at half maximum (FWHM) of the line, and θ is the diffraction angle.

SEM Measurement
The SEM micrographs of the NiCl 2 (2,9-dimethyl-1,10-phenanthroline)·H 2 O complex and its decomposition product at 400 °C are presented in Figure 8a. We observed that the starting complex powder was made of very large block crystals in different sizes. The SEM image of the product in Figure 8b clearly shows that the shape and size of particles are quite different from the precursor complex. It can be seen that the product was formed from extremely fine semi-spherical particles that were loosely aggregated. No characteristic morphology of the complex is observed, indicating the complete decomposition into the extremely fine spherical particles.

TEM Measurement
The TEM images of the complex and its decomposition product at 400 °C shown on Figure 9. We observed that the TEM micrograph of the starting complex powder was made of very large block crystals in different sizes Figure 9a. Uniform NiO nanoparticles have sphere shapes with weak agglomeration Figure 9b was collected after thermal decomposition of the complex. The particle sizes possess a narrow distribution in a range from 10 to 20 nm, and the mean particle diameter is about 15 nm. Actually, the mean particle size determined by TEM is very close to the average particle size calculated by Debye-Scherer formula from the XRD pattern.

General Procedure for the Preparation of the Desired Complex
A mixture of nickel chloride hexahydrate NiCl 2 ·6H 2 O (Acros) (100 mg, 4.10 mmol) in distilled water (15 mL) and 2,9-dimethyl-1,10-phenanthroline (Acros) (dmphen) (80 mg, 4.20 mmol) in methanol (4 mL) was stirred for 1 h at room temperature. The solution was concentrated to about 1 mL under reduced pressure and then added to 40 mL of cooled ethanol. This causes the precipitation of (134 mg, ~92% yield) brown powder product that was filtered, and dried, and the crystals were grown by slow diffusion of ethanol into a solution of the complex in water.

General Procedure for the Preparation of NiO Nanoparticles
According to the TG/DTA analysis the 0.5 g of NiCl 2 (2,9-dimethyl-1,10-phenanthroline)·H 2 O complex was decomposed at 400 °C temperatures for 0.5 h in ambient air. The decomposition products were collected for characterization.

Supplementary Material
Crystallographic data has been deposited with the Cambridge Crystallographic Data Centre as supplementary publication number CCDC 910435. Copies of this information may be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44-122-3336-033; E-Mail: deposit@ccdc.cam.ac.uk).

X-ray Structural Analyses for the Complex
The X-ray data for complex was collected ( Table 2) on Xcalibur E goniometer (Agilent Technologies, Oxford Diffraction, Oxford, UK) with enhance X-ray source and Eos CCD detector, graphite-monochromated Mo-K α radiation (λ = 0.71073 Å) using five ω-scans with a total of 350 frames at temperature of 293 K. Data collection, cell parameters evaluation, data reduction and absorption were performed using CrysAlisPro, Agilent Technologies, Version 1.171.35.11 (release 16-05-2011 CrysAlis171 .NET, Oxford, UK). Structure determination was made using SHELXL programs (SHELXL-97, University of Gottingen, Gottingen, Germany) [40].  The structure was solved by direct methods and refined by full-matrix least-squares with anisotropic temperature factor, for the non-hydrogen atoms.

Conclusions
The new [NiCl 2 (C 14 H 12 N 2 )(H 2 O)] complex was subjected to thermal decomposition at low temperature of 400 °C in an open atmosphere in order to prepare uniformed spherical NiO nanoparticles in the range of 10-20 nm. The structures of the complex and the NiO nanoparticles product were elucidated on the basis of FT-IR, UV-vis spectroscope, TG/DTA, XRD, SEM, EDX and TEM. The application of NiO nanoparticles is currently underway in our laboratory.