Background

Recently, nickel(II) Schiff base complexes have been widely investigated not only for their interesting structures and properties [1, 2], but also for their use as precursor for preparation of nickel oxide nanoparticles [3]. Then, considerable interest has been grown on the preparation and characterization of NiO nanoparticles via thermal decomposition of complexes for their unique applications and properties [4, 5]. Among the transition metal oxides, nickel oxide has attracted much attention due to its properties and applications [6, 7]. Presently several methods, viz. hydrothermal, thermal decomposition, sol–gel, solvothermal and sonochemical are available to prepare NiO nanoparticles [8–10]. However, thermal decomposition method is a better choice as it makes control process conditions, particle size, particle crystal structure and purity possible [3–5]. Salavati-Niasari et al. have reported the synthesis of NiO nanoparticles [3] using four coordinated nickel(II) Schiff base complex Ni(salen) while Farhadi group have synthesized NiO nanoparticles by solid-state thermal decomposition of octahedral nickel(II) complexes, viz. [Ni(en)3](NO3)2 [4] and [Ni(NH3)6](NO3)2 [5].

Recently, our group has synthesized nickel oxides nanoparticles via thermal decomposition method of nickel Schiff base complexes [11–13]. Herein, we have reported the synthesis of NiO nanoparticles by solid-state thermal decomposition of Ni(II) Schiff base complexes, Ni(salbn) (1) and Ni(Me2-salpn) (2) (Scheme 1).

Scheme 1
scheme 1

Chemical structures of Ni(salbn) (1) and Ni(Me2-salpn) (2)

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Experimental

Materials and characterization

All reagents and solvents for synthesis and analysis are commercially available and used as received. Elemental analyses are carried out using a Heraeus CHN-O-Rapid analyzer. X-ray powder diffraction (PXRD) pattern of the nanoparticles is recorded on a Panalytical Empyran with Cu–Kα radiation in the range 2θ = 0°–140°. Fourier transform infrared spectroscopy (FTIR) spectra are recorded as a KBr disk on a Perkin–Elmer FTIR spectrophotometer. Scanning electron microscopy (SEM) images are collected on Philips XL-30ESEM. Transmission electron microscopy (TEM) images are recorded on a JEOL JEM 1,400 instrument with an accelerating voltage of 120 kV.

Preparation of Ni(salbn) (1) and Ni(Me2-salpn) (2) complexes

To obtain a dark red precipitates of complexes, a methanol solution (20 mL) of salbn or Me2-salpn (2 mmol) was added to methanol solution (20 mL) of Ni(NO3)2·6H2O (2 mmol). The resulting solution was stirred for 2 h. The product was removed by filtration, washed with cooled ethanol and dried at room temperature for several days. Anal. calcd. for C18H18NiN2O2 (1): C, 61.24; H, 5.13 and N, 7.93 %; found C, 61.36; H, 5.23 and N, 7.86 %. FTIR (cm−1): 1,624 (C=N). Anal. calcd. for C19H20NiN2O2 (2): C, 62.17; H, 5.49 and N, 7.63 %; found C, 62.26; H, 5.53 and N, 7.76 %. FTIR (cm−1): 1,610 (C=N).

Preparation of NiO nanoparticles

New precursors 1 and 2 are loaded on to a platinum crucible and placed in an oven to be heated at a rate of 10 °C/min in air. Nanoparticles of nickel oxide are synthesized at 450 °C after 3.5 h. To remove any impurities, the final products are washed with ethanol and dried at room temperature for 3 days.

Results and discussion

FTIR spectra

The FTIR spectra of the precursors 1 and 2 are shown in Fig. 1. The characteristic peaks at 1,624 in 1 and 1,610 cm−1 in 2 are attributed to the υ(C=N) stretchings [3–5] which disappeared in Fig. 2 with the appearance of new strong band at 418 cm−1 in nanoparticles prepared from 1 and at 420 cm−1 in nanoparticles prepared from 2, indicating the spinel structure of Ni–O [3–5]. The broad band at around 3,500 cm−1 in the spectrum of NiO nanoparticles is attributed to the adsorbed water on the external surface of the samples. Also, the H-OH bending vibration appears at about 1,640 cm−1 [3–5].

Fig. 1
figure 1

FTIR spectra of Ni(II) complexes 1 (left) and 2 (right)

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Fig. 2
figure 2

FTIR spectra of NiO nanoparticles derived from 1 (left) and 2 (right)

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XRD patterns

Figure 3 shows the XRD patterns (0° < 2θ < 130°) of the nickel oxide nanoparticles prepared via solid-state thermal decomposition from nickel(II) Schiff base complexes 1 and 2. The powders showed the crystalline pattern, and according to standard nickel oxide pattern (JCPDS: 75-0197) [3–5], all diffraction peaks can be well indexed as face-centered cubic phase at about 2θ = 37°, 43°, 62°, 75° and 79°, which can be perfectly related to 111, 200, 220, 311 and 222 crystal planes, respectively. No obvious peaks of impurities were seen in the XRD pattern of NiO obtained by thermal decomposition. Moreover, the observed peaks are sharper and higher in intensity which confirmed the well crystallization of the prepared NiO nanoparticles. These results confirm that at 450 °C the complexes were decomposed completely to nickel oxide.

Fig. 3
figure 3

XRD patterns of NiO nanoparticles prepared from 1 (left) and 2 (right)

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SEM and TEM images

The morphology and microstructure of the NiO nanoparticles have been investigated using SEM and TEM techniques. Figures 4 and 5 are the respective SEM and TEM images of the nanoparticles obtained from 1 and 2 which indicate their particle size around 10–15 nm. The particles are both spherical and cubic. The choice of nickel precursor and method is the main step in the synthesis of nickel oxide nanoparticles (Table 1). Although many different precursors and methods have been used for preparation of nickel oxide [3–5], this report is the scare report on the synthesis of NiO nanoparticles from nickel(II) Schiff base complexes [5–7].

Fig. 4
figure 4

SEM images of NiO nanoparticles derived from 1 (left) and 2 (right)

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Fig. 5
figure 5

TEM images of NiO nanoparticles derived from 1 (left) and 2 (right)

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Table 1 Comparison of particle size of NiO nanoparticles by various methods and precursors
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Conclusions

Pure NiO nanoparticles having average size of 10–15 nm have been successfully prepared by solid-state thermal decomposition (at 450 °C for 3 h) of Ni(II) Schiff base complexes, viz. Ni(salbn) (1) and Ni(Me2-salpn) (2). This facile, inexpensive and nontoxic method may be useful for the preparation of other transition metal oxide nanoparticles.