Particles of metal oxides that are in the nanometer size regime have attracted significant interests because of their atom-like size dependent properties. Recently, synthesis of metal oxide nanoparticles with new inorganic precursor has been interested, taking profit of the tools of organometallic chemistry [1-4]. A major interest at the moment is in the development of organometallic or inorganic compounds for preparation of nanoparticles. Using of the novel compound can be useful and open a new way for preparing nanomaterials to control nanocrystal size, shape and distribution size . A reduction in particle size to nanometer scale results in various interesting properties compared with the bulk properties.
ZnO is a polar inorganic crystalline material with many applications due to its unique combination of interesting properties such as non-toxicity, good electrical, optical and piezoelectric behavior, stability in a hydrogen plasma atmosphere and low price [6,7]. ZnO is a well-known semiconductor with a wide direct band gap (3.37 eV) and a large exciton binding energy of 60 meV at room temperature [8,9]. In addition, it has a wide range of applications as solar cells, luminescent, electrical and acoustic devices, in gas and chemical sensors, as coatings, catalysts, micro-lasers, memory arrays and in biomedical applications [7,10]. Many methods have been developed to synthesize ZnO nanoparticles including thermal decomposition of a metal-organic frameworks (MOF) , thermolysis of Zn-oleate complex , sonochemical method by aqueous-alcoholic-ethylendiamin solutions of zinc nitrate and sodium hydroxide , electrophoretic deposition , sol-gel process , etc. Thermal decomposition method has some advantages such as simple process ability, and low cost to obtain high purity products; hence it is quite promising and facile route for industrial applications. So, thermal decomposition is a suitable rout for preparing of ZnO.
The use of coordination supramolecular compounds as precursors for the preparation of inorganic nanomaterials such as zinc(II) oxide has not been investigated thoroughly. Coordination supramolecular compounds represent an important interface between synthetic chemistry and materials science. We have investigated the synthesis of single-phase hexagonal quasi-spherical ZnO nanoparticles with the average particles size of about 30 nm from thermal decomposition of the Zn(II) supramolecule, bis(2,6 diaminopyridinium)bis(pyridine-2,6-dicarboxylato)zincate(II), as the new precursor in the presence of dry methanol as solvent . Morsali et al.  have reported the synthesis of hexagonal nano-rods ZnO from nanorods of a three-dimensional Zn(II) metal–organic framework, [Zn2(btec)(DMF)2]n, as the new precursor. This group also worked on the synthesis of spherical ZnO nanoparticles from a new coordination polymer, [Zn3(tza)3(H2O)3]n, by thermal decomposition at 500 °C for 4 h . This article focuses on the simple synthetic preparation of nanostructures and single crystals of a Zinc(II) coordination complex, [Zn(dmph)I2] (1), (dmph= 2,9-dimethyl-1,10-phenanthroline), using sonochemical and heat gradient methods, respectively. 1,10-Phenanthroline is the basic chemical moiety leading to an important class of chelating agent . The choice of phenanthroline is mainly due to two factors. This hermetic moiety can provide a further binding site for metal ions. Therefore, polyamine macrocyclics containing this moiety are expected to form more stable complexes than saturated polyamine ligands containing the same number of nitrogen donors. The insertion of phenanthroline in a macrocyclic framework may allow the resulting metal complexes to be solubilized in a polar solvent due to the hydrophobic characteristics displayed by this aromatic unit. It has therefore been increasingly used in both analytical and preparative coordination chemistry . Metal complexes containing ligands such as 1,10-phenathroline and bipyridine have gained importance because of their versatile roles as building blocks for the synthesis of metallodendrimers for supramolecular assemblies in analytical chemistry, catalysis, electrochemistry, ring-opening metathesis polymerization, and biochemistry .
Ultrasonic irradiation is considered a green energy source because of shorter reaction times and higher yields in comparison with thermal energy sources. Due to these advantages, recently chemists have focused their attention on this type of energy source . Sonochemistry is the research area in which molecules undergo a reaction as a result of employing powerful ultrasound radiation (20 kHz – 10 MHz) . The results reveal that compared with traditional synthetic techniques, such as solvent diffusion technique, hydrothermal and solvothermal methods, ultrasonic synthesis is a simple, efficient, low cost, and environmentally friendly approach to nano-scale coordination supramolecular compound [24, 25]. These extreme conditions permit access to a range of chemical reaction space normally not accessible, for example many coordination supramolecular compounds have been synthesized via sonochemical method, such as: La(III)-LH2 , Zr(IV)-Neocuproine , Cu(I)-Neocuproine , Pb(II)-LH2 , Zr(IV)-Isophthalic acid , Cd(II)-Thioacetamide , Zr(IV)-LH2 , and Zn(II)-LH2 . To proceed, we report the facile preparation of hexagonal wurtzite structure of ZnO nanoparticles by thermal decomposition of nano-sized compound 1 at 600 °C under air atmosphere and without any surfactant or capping molecules.
Materials and physical techniques
All reagents and solvents used in this work were commercially available and employed without further purification. A multiwave ultrasonic generator (Sonicator-4000; Misonix, USA), equipped with a converter/transducer and titanium oscillator (horn), 12.5 mm in diameter, operating at 20 kHz with a maximum power output of 600 W, was used for ultrasonic irradiation. The ultrasonic generator automatically adjusts the power level. Melting points were measured with a Thermo Scientific 9200 apparatus and are uncorrected. Elemental analyses (carbon, hydrogen, and nitrogen) were performed using a Heraeus CHN–O– Rapid analyzer. FT-IR spectra were recorded on a Bruker tensor 27 spectrophotometer in the range 400–3500 cm-1 using the KBr disk technique. X-ray powder diffraction (XRD) measurements were performed using a Philips X’pert diffractometer with monochromated Cu-Kα (λ= 1.5418 Å) radiation at room temperature in the 2θ range of 10-70°. The crystallite sizes of selected samples were estimated using the Scherrer formula. The simulated XRD powder pattern based on single crystal data were prepared using Mercury software . Thermogravimetric analysis (TGA) and differential thermal analyses (DTA) of the title compound were performed on a computer-controlled PL-STA 1500 apparatus. Powder sample of 1 was loaded into alumina pans and heated with a ramp rate of 10 °C/min from room temperature to 600 °C under argon atmosphere. The samples were characterized by a scanning electron microscope (SEM) (Philips XL 30) equipped with an energy dispersive X-ray (EDAX) microanalysis with gold coating. Crystallographic measurements of compound 1 were carried out at 296(2) K using a Bruker APEX-II CCD area-detector diffractometer. The intensity data were collected using graphite monochromated Mo-Kα radiation (λ= 0.71073 Å). Absorption corrections were applied with the program SADABS . The structure was solved by direct methods SHELXS-97 , and refined by full-matrix least-squares techniques on F2 using SHELXL-97  with refinement of F2 against all reflections. All non-hydrogen atoms were refined anistropically and hydrogen atoms were located and included in their calculated positions. The final full-matrix, least-squares refinement on F2 was applied for all observed reflections [I> 2σ(I)]. Crystallographic data and details of data collections and structure refinements of compound 1 are listed in Table 2.
Synthesis of [Zn(dmph)I2] (1) as single crystal
To isolate single crystals of [Zn(dmph)I2] (1), 2,9-dimethyl-1,10-phenanthroline (0.5 mmol, 0.104 g), zinc(II) acetate (0.5 mmol, 0.109 g) and potassium iodide (1 mmol, 0.116 g) were placed in the main arm of a branched tube to be heated. Methanol was carefully added to fill both arms, and then the arm heated was placed in oil bath at 60 °C. After 10 days, yellow crystals were deposited in the cooler arm which was filtered off, washed with water and air dried (0.241 g, yield: 73%), m.p. 300 °C. Anal. Calc. for C14H12N2ZnI2 (%): C, 31.86; H, 2.27; N, 5.31. Found (%): C, 32.11; H, 2.07; N, 5.35. FT-IR (cm-1) selected bands observed were: 3427(b), 3052(w), 1621(w), 1589(w), 1503(s), 1425(w), 1371(w), 1150(w), 1028(w), 861(w), 780(w), 726(w), 548(w), and 433(w).
Synthesis of nano-structured [Zn(dmph)I2] (1) by the sonochemical method
To prepare nano-sized [Zn(dmph)I2] (1), a proper amount of the solution of zinc(II) acetate (10 ml, 0.1 M) and potassium iodide (20 ml, 0.1 M) in methanol was positioned in a high-density ultrasonic probe, operating at 20 kHz with a maximum power output of 600 W. Then into this solution, a proper volume of neocuproine ligand solution (10 ml, 0.1 M) was added dropwise. After one hour, a white precipitate was formed. It was isolated by centrifugation (4000 rpm, 15 min), washed with water and acetone to remove residual impurities and finally dried in air (0.473 g, yield: 72%), m.p. 300 °C. Found (%): C, 32.74; H, 2.21; N, 5.05. FT-IR (cm-1) selected bands observed were: 3421(b), 3051(w), 1622(w), 1589(w), 1503(s), 1427(s), 1371(w), 1150(w), 1027(w), 861(s), 780(w), 726(w), 548(w), and 439(w).
To study the effect of supercritical condition on stability, size and morphology of nano-structured compound 1, 0.15 g of compound 1 obtained from sonochemical method along with 20 ml of water was transferred into a 50 ml Teflon-lined stainless autoclave. The autoclave was sealed and maintained at 200 °C for 72 hour (Hydrothermal method). After cooling slowly to room temperature, white block shaped crystals of [Zn(dmph)I2] (1) were collected and washed with water and acetone, and dried in air (yield: 0.114 g, 76%), m.p. 300 °C. Found (%): C, 32.28; H, 2.17; N, 5.31. FT-IR (cm-1) selected bands observed were: 3424(b), 3053(w), 1619(w), 1589(w), 1501(s), 1426(s), 1369(w), 1150(w), 1028(w), 858(s), 780(w), 726(w), 547(w) and 440(w). Based on elemental analysis, FT-IR and melting point results, the nano-sized compound 1 were not decomposed, just the size of particles decreased and also leaded to rod-like nanostructures morphology.
Synthesis of ZnOnanoparticles by thermal decomposition of compound 1
For preparation of zinc(II) oxide nanoparticles, degradation of the compound 1 obtained from sonochemical method was done at 600 °C with heating rate 5 °C/min under air atmosphere. After cooling, white precipitate was obtained. The powder XRD diffraction shows that degradation was completed and the entire organic compound was decomposed.
RESULTS AND DISCUSSION
The reaction of neocuproine as ligand with zinc(II) acetate and potassium iodide leads to formation of a Zn(II) coordination complex [Zn(dmph)I2] (1). Nanostructures of compound 1 were obtained in methanolic solution by ultrasonic irradiation at an ambient temperature and atmospheric pressure, while single crystals of compound 1, suitable for X-ray crystallography, were prepared by a heat gradient applied to a methanolic solution of the reagents (the ‘‘branched tube method’’) (Scheme 1), and the structures were confirmed by FT-IR, elemental analysis and powder X-ray diffraction (XRD) patterns. Scheme 2 gives an overview of the methods used for the synthesis of compound 1 using the two different routes. This compound displayed distinct melting point and elemental analysis consistent with the formula C14H12N2ZnI2. The FT-IR spectra of the free neocuproine ligand, nanostructures produced by the sonochemical and hydrothermal methods, and of the bulk material produced by the branched tube method in the frequency range from 400-3500 cm-1, are compared in Fig. 1. A comparison between FT-IR spectrum of free neocuproine ligand (Fig. 1a) and its Zn(II) nanostructures (Figs. 1b and 1c), obviously revealed that the ligand has been coordinated to Zn(II) ion, and new compound of 1 was formed. The elemental analysis and FT-IR spectra of nano-structured 1 prepared by the sonochemical and hydrothermal methods are similar to the data for the single crystalline material prepared by the branched tube method. The only difference among these characteristic peaks is either the peak intensity or a slight shift in the peak position. All distinct absorption peaks of ligand are exhibited in Table 1, and every absorption peak was assigned to corresponding vibration .
Fig. 2 shows the simulated XRD pattern from single crystal X-ray data (see below) of compound 1 (Fig. 2a) in comparison with the XRD pattern of the typical sample of compound 1 prepared by the sonochemical method (Fig. 2b) in the range 10° < 2θ < 50°. The XRD powder pattern of compound 1 could not be matched with the JCPDS cards due to the encapsulation of Zn(II) atoms with organic moiety. Acceptable matches, with slight differences in 2θ, were observed between the simulated and experimental powder X-ray diffraction patterns. This indicates that the compound obtained by the sonochemical method as nanoparticles is identical to that obtained by single crystal diffraction. The significant broadening of the peaks indicates that the particles are of nanometer dimensions. To estimate the particle size from the broadening of the XRD peaks the Scherrer formula is utilized.
In Eq. (1), λ, and θB are the wavelength of X-ray radiation, (1.5418 Å), the full-width at half-maximum, and the diffraction angle, respectively . Breference (0.08) is the influence of the equipment to produce an incident X-ray which has to be subtracted to get the broadening effects due to the sample. The average particle size of the product was found to be around 85 nm, which was in agreement with the value obtained from the SEM image (Fig. 3a). The morphology of compound 1 prepared by the sonochemical and hydrothermal methods were characterized by scanning electron microscopy (SEM) and is shown in Fig. 3. The SEM image (Fig. 3a) shows that the morphology of the compound 1 contains quasi-cubic shaped particles with a broad distribution. After hydrothermal treatment in an autoclave the compound 1 was not decomposed, just the size of particles decreased and also leaded to rod-like nanostructures morphology (Fig. 3b). In our previous work, we could obtain nanostructures of PbOHBr by thermal treatment method .
The structure of compound 1 was characterized by the single-crystal X-ray diffraction technique (Table 2). The molecular structure of the fundamental building unit for 1 is shown in Fig. 4. In 1, the Zn(II) ion is four-coordinated in a distorted tetrahedral configuration by two N atoms from a neocuproine ligand and two terminal I atoms. The structure of the bromine derivative 1 was reported previously . The corresponding bond lengths (Å) and angles (o) around the Zn(II) center are listed in Table 3. The Zn—N bonds [average 2.109 Å] are somewhat shorter than the Zn—I distances [average 2.536 Å] and they are closed to such bond lengths found in other discrete 1,10-phenanthroline derivatives of zinc complexes [37-40]. In similar study on complex 1, the Zn—N and Zn—I bond lengths are 2.058 and 2.55 Å, respectively . The angle of two N atoms from phen ligand and Zn atom, N1—Zn1—N2, significantly is smaller than I1—Zn1—N2 and I2—Zn1—N1. The angles in title complex is also similar to that of found in other zinc complexes of 1,10-phenanthroline, regardless of geometry of the complex [35,41]. The packing of compound 1, viewed approximately along b and c axes, is shown in Fig. 5. As shown in Fig. 5, the neocuproine acts as a bi-dentate ligand to join neighbouring zinc(II) centers to form a 3D neutral framework. This may be also an issue which needs to be investigated. The title complex crystallizes in the monoclinic system with P21/c space group and four molecules per unit cell.
To examine the thermal stability of the nano-sized compound 1 obtained from sonochemical method, thermogravimetric(TG) and differential thermal analyses (DTA) were carried out between room temperature and 600 °C under an argon flow (Fig. 6). During the heating process, TGA revealed that compound 1 is stable up to 140 °C at which temperature decomposition begins. The TG curve exhibits five distinct decomposition stages between 140 °C to 290 °C may be ascribed to the removal of neocuproine ligand with a mass loss of 37.40 % (theoretically calculated as 39.4 %) and the mass loss between 290 °C to 550 °C is ascribed to removal of iodine atoms (observed: 49.40 %, calcd: 48.10 %). At higher temperatures, the decomposition of the compound occurs to ultimately give solid that appears to be ZnO (observed: 14.4 %, calcd: 15.5 %). Mass loss calculations of the end residue and the XRD pattern of the final decomposition product (Fig. 7) show the formation of ZnO. The DTA curve of compound 1 indicates the decomposition of the compound takes place with three endothermic events at 331 °C, 467 °C, and 520 °C (Fig. 6).
To investigate the composition and phase information of the final calcination product, XRD was carried out. Fig. 7 provides the XRD pattern of zinc(II) oxide obtained from thermal decomposition of the compound 1 prepared from sonochemical method at 600 °C with heating rate 5 °C/min under air atmosphere. It can be seen that there are seven peaks corresponding to (100), (002), (101), (102), (110), (103), (200), (112), (201), crystal planes, which matches to the hexagonal phase of ZnO named zincite (JCPDS card no. 36-1451, a= 3.2498 Å, c= 5.2066 Å, S.G.= P63mc (186), and Z= 2) as observed from the corresponding XRD pattern. Compared with the standard diffraction pattern, no peaks of impurities were detected, indicating high purity of the product. In addition, the intense and sharp diffraction peaks suggest the obtained product is crystalline. The broadening of the peaks indicated that the particles were at the nanometer scale, which is in agreement with that observed from SEM images (Figs. 8a and 8b). To estimate the particle size from the broadening of the XRD peaks the Scherrer formula is utilized, average particle size of the product was 31 nm. The evaluated particle sizes were reported in Table 4. These experiments indicate that the size of the coordination complex precursor correlates to the particle size of the formed ZnO nano-structures, and that the nano-sized precursor produces smaller particles of ZnO .
The morphology and size of the zinc(II) oxide nanoparticles were studied by SEM (Figs. 8a and 8b), which shows uniform nanometer scale quasi-spherical ZnO nanoparticles. They exhibited agglomerated particles with a homogeneous distribution. Agglomeration refers to adhesion of particles to each other because of Van der Waals forces of attraction, which is significantly higher in nanoparticles . The size distribution histogram of ZnO nanoparticles (20-30 nm) is shown in Fig. 8c and indicates a relatively uniform distribution. For more characterization, EDAX was performed on the ZnO nanoparticles (Fig.8d). The EDAX spectrum shows the presence of zinc as the only elementary component. This indicates that the product from direct calcination of compound 1 is pure ZnO nanoparticles.
In this study we have successfully demonstrated the synthesis of a nano-sized Zn(II) coordination complex, [Zn(C14H12N2)I2] (1) using a thermal gradient approach and by sonochemical irradiation. Compound 1 has been found to be stable under hydrothermal treatment in an autoclave. Compound 1 was structurally characterized by single crystal X-ray diffraction. The crystal structure of compound 1 reveals that the Zn(II) ion is four-coordinated in a distorted tetrahedral configuration by two N atoms from a neocuproine ligand and two terminal I atoms. To proceed, compound 1 was decomposed at 600 °C under air atmosphere to produce ZnOnanoparticles and characterized by SEM, EDAX, and XRD techniques. The XRD pattern indicates that the well-crystallized zinc(II) oxide nanoparticles can be easily obtained under the current synthetic conditions. However, this method is fast and the product has a good separation with uniform nanometer scale particles. It does not need any additive, and yields smaller particles. In overall, we can concluded that using nano-structured coordination complex can be appropriate precursors for production of nano-sized metal oxide materials with different and interesting morphologies. To the best of our knowledge, this is the first report on the synthesis of ZnO nanoparticles with this precursor.
Crystallographic data for the structure reported in this article has been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-782684. Copies of the data can be obtained on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [Fax: +44-1223/336033; E-mail: email@example.com].
Authors are grateful to Iranian Research Organization for Science and Technology (IROST) and Islamic Azad University, Shahr-e Ray branch and Nanotechnology Initiative Council for their unending effort to provide financial support to undertake this work.
CONFLICT OF INTERESTS
The authors declare that there is no conflict of interests regarding the publication of this paper.