Metal oxides nanoparticles are mainly attractive candidates among all functional materials to be synthesized at the nanometer scale. The distinctive characteristics of metal oxides make them the most varied class of materials, with properties covering almost all aspects of solid-state, physics and materials science . Among all metal oxides nanoparticles, copper oxide (CuO) has gained the best interest because of wide applications, such as solar cell , field emission , catalysis [4, 5] and gas sensing [6, 7]. Many different physical and chemical methods have been proposed to synthesize CuO (NPs), such as quick precipitation , electrospinning method , hydrothermal , sol-gel , thin-film deposition , electrochemical deposition  and self-catalytic growth using template . Among these methods, quick-precipitation and hydrothermal are the most significant for their safety and environmentally friendly , and quick-precipitation is particularly more attractive due to its cost-effectiveness and simple operation. Cupric oxide (CuO) is a transition metal oxide, a p-type semiconducting material with a narrow band gap of 1.2 eV . CuO has monoclinic crystalline structure and cell parameters a = 0.4684 nm, b = 0.3423 nm, c = 0.5128 nm and β = 99.54° . The current study examines well-dispersed CuO nanosheets, which were synthesized by the simple quick-precipitation method. This method is a template free route without using high temperature. In the preparation process, Copper nitrate three hydrate Cu(NO3)2.3H2O, Polyvinylpyrrolidone (PVP) and sodium hydroxide (NaOH) were used as copper precursor, stabilizer and accelerator, respectively. Distilled water was used during the reaction.
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In a typical experimental procedure, 2 g of PVP, poured into a volumetric flask and then added distilled water to bring the volume up to the final 100 ml (PVP 2 wt%). Then 1.45 g Cu(NO3)2.3H2O was dissolved in 60 mL of PVP 2wt%. The solution was added into a round-bottom flask with stirring, the color of mixture was bright blue, and then 15 mL NaOH (1M) was rapidly added into the mixture, nanopowder suspension was formed, and then kept at 60°C for 1 hour. A large amount of black precipitate was produced. After being cooled to room temperature the particles were separated by centrifugation and washed with distilled water 3 times to remove the impurities, and dried in oven at 60°C.
The volume ratio of NaOH has an important influence on the final products. In order to study the effect of the volume ratio of NaOH, the products were prepared at six volume ratios of NaOH including, 1, 2, 5, 10, 15 and 20 mL NaOH (1M). At the end of the reaction, the pH values of all samples were measured (Table. 1). The pictures of all samples after reaction were shown in Fig.1.
Table 1 presents the variants of pH, maximum absorption wavelengths and colors of products. The pH was increased from pH 4 to 12.8 by adding NaOH (1M). After addition of 1, 2, 5 and 10 mL NaOH the color of the solutions became pale blue and remained unchanged until the end of the reaction, (Fig. 1) but at higher pH values which were equivalent to adding 15 and 20 mL NaOH (1M), the color of solutions became intense blue then, it changed in situ into black (Fig. 1), which was the sign of CuO nanostructures formation. The pH values increased moderately adding of 1, 2, 5 and 10 mL NaOH and increased considerably after adding 15 mL NaOH. In column absorption wavelength (λmax), no peak was observed at low pH values but at higher pH values two peaks were observed at 295 and 356 nm in 15 mL and 280 and 340 nm in 20 mL, respectively. The UV visible spectra of all samples shown in Table 1 are shown in Fig.2.
Fig. 2 presents the optical absorption characteristics of PVP 2wt % at different volume ratios of NaOH (1M). This figure illustrates that no peak was observed at lower pH values, whereas two peaks at 295 and 356 nm for the addition of 15 mL and 280 and 340 nm for the addition of 20 mL, were observed, which proved the formation of CuO nanosheets. The observed two peaks in each curve might the existence of two different shapes or sizes of particles.
Fig. 3 shows the XRD pattern of the obtained samples at different volume ratios of NaOH (1, 2, 5 10, 15 and 20 mL). X-ray diffraction (XRD) confirmed the existence of gerhardtite Cu2(OH)3NO3 (Ref Cod# 98-001-7168) at lower pH values (see Fig. 3 (a-d)). This showed that in these volume ratios pure CuO was not formed whereas at higher pH values the formation of pure CuO was observed (see Figs. 3 (e) and 3 (f)), all diffraction peaks can be indexed as the monoclinic phase of CuO (Ref Cod# 98-004-8595). The peaks at 2θ values of 32.53, 35.50, 38.67, 46.12, 48.85, 53.43, 58.31, 61.53, 66.33, 67.99, 72.41 and 75.20 correspond to the crystal planes of copper oxide (110), (002), (111), (112), (202), (020), (202), (113), (311), (113), (311) and (004), respectively, which are in good agreement with recent studies . This result demonstrated that the reaction at lower pH values could not be completed; however, the excess of NaOH complete the reaction.
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In order to study the size and shape of final products TEM was carried out. Figs. 4 (a) and 4 (b) illustrate the TEM images of products for the addition of 5 and 10 mL NaOH, respectively. It can be clearly seen that amorphous structures were formed. It is worthy to note that this reaction under acidic condition (low pH values) leads to the formation of amorphous Cu2(OH)3NO3.
Figs. 5 (a) and 6 (a) show the results for the addition of 15 and 20 mL NaOH. These figures show that sheets-like CuO were formed with a width of 177 ± 50 nm (Fig. 5 (b)) and length of 903 ± 300 nm (Fig. 5 (c)) for 15 mL NaOH and a width of 103 ± 31 nm (Fig. 6 (b)) and length of 610 ± 263 nm (Fig. 6 (c)) for 20 mL NaOH. In contrast for the Fig. 5 (a) with Fig. 6 (a), the results depicted that due to higher pH value in addition of 20 mL NaOH agglomeration was happened. It can be explained by the effect of adsorption of polymer. PVP can retard the agglomeration and growth of nanoparticles by steric effect . At high pH values there was small steric repulsive force. As a result, the surface charge of particles decrease and agglomeration increased. The relationships between pH of suspension, dissociation ratio of polymer and molecular weight of polymer are vital to the degree of steric repulsive force and the adsorption ratio of polymer . The high magnification image of CuO nanosheets prepared in 15 mL is shown in Fig. 7. This image revealed that each sheet-like particle is composed of rod-like particles (rod- like is indicated by arrows). The reason for the formation of sheet-like CuO from CuO nanorods can be described based on orientated attachment (OA) mechanism. In this process, a larger crystal structure is formed from small ones by direct jointing of suitable crystal planes  because the formation of larger crystals can significantly reduce the interfacial energy of some primary nanoparticle. The CuO nanosheets are formed throughout orientated attachment of small particles along the  direction [22, 23].
The morphology of products prepared in addition of 15 and 20 mL NaOH were shown in Figs
8 (a) and 8 (b). Fig. 8 (a) indicates that layered sheet-like CuO were formed. Fig. 8 (a) also revealed that, among the products, a proportion of the rod-like CuO (rod-like indicated by arrows). This figure confirmed that oriented attachment (OA) mechanism is liable for the formation of sheet-like CuO, which is same to the synthesis of CuO hierarchical nanostructures . Fig. 8 (b) shows the morphology of the products obtained in addition of 20 mL NaOH, indicated that rug-like structures were formed.
Energy Dispersive Analysis of X-ray (EDAX) was carried out on the obtained CuO Nanosheets at15 KeV using the Field Emission Scanning Electron Microscopy (FE-SEM). EDAX spectrum (Figs. 9 (a) and (b)) related to the of CuO nanosheets obtained in PVP 1 wt% in addition of 15 and 20 mL NaOH, obviously confirms the presence of Cu and O peaks. The result of EDAX also confirmed, the atomic ratio of Cu to O is 1:1. The specimens were coated by gold (Au) before FE-SEM observation.
In order to confirm that the CuO nanosheets were modified by PVP, the FT-IR technique was carried out. Fig. 10 shows FT-IR spectra for pure PVP (Fig. 10 (a)), CuO nanosheets produced in 15 mL NaOH (Fig. 10 (b)) and CuO nanosheets produced in 20 mL NaOH (Fig. 10 (c)). In FT-IR spectrum of pure PVP, the peak at 1662 cm-1 corresponds to the peaks of C=O. In contrast for the curve (Fig. 10 (a)) with another cure, the results showed that the peak of C=O shifted to 1645cm-1 (Fig. 10 (b)) and 1640 cm-1 (Fig. 10 (c)) in the FTIR of CuO nanosheets. These results indicated the existence of weak chemical bonds between the band of C=O and CuO nanosheets . Metal oxide commonly gives absorption bands below 1000 cm-1 . Absorption bands observed at 603 and 483 cm-1 in (Fig. 10 (b)) and absorption bands at 607 and 482 cm-1 in (Fig. 10 (c)) were characteristics of Cu -O.
The formation of solid crystals from a homogeneous solution consists of four main events; precursor formation, nucleation, crystal growth and aging . The fabrication of precursor molecules generally occurs through hydrolysis reactions and hydroxylation, forming a zero-charge molecule. The build-up of precursor molecules results in a supersaturation, which is defined as a state in which the liquid (solvent) contains more dissolved solids (solute). Once the supersaturation occurs nucleation and growth continue to take place simultaneously. Supersaturation acts as a driving force for crystallization. In our case, we proposed the following mechanism for CuO nanosheets growth. When a small amount of NaOH was added to a solution (1, 2, 5, 10 mL) a pale blue Cu2(OH)3NO3 precipitate formed instead of Cu(OH)2 (Eq.1). The reason was the solubility product of Cu2(OH)3NO3 had reached the precipitation limit before that of Cu (OH) 2. It is essential to mention that spherical particles of Cu2(OH)3NO3 with amorphous shapes were formed, but when a higher amount of NaOH was added to the solution (15 or 20 mL) an intense blue Cu(OH)2 formed (Eq.2), indicating that supersaturation happened due to hydroxylation. Nucleation happened immediately after supersaturation. In the nucleation step a growth unit was needed for crystals to grow on. The anionic coordinative polyhedral (ACP) theoretical model is liable to explain the growth mechanism. On the basis of this model, the cations are considered to exist in a form of complex whose ligands are OH- ions in aqueous solution, which also form complexes with coordinative numbers that are equal to that the crystal formed, is identified as a growth unit. In aqueous solutions the coordination number of Cu2+ normally keeps six . Consequently, it is anticipated that, the growth units to be Cu(OH)64- of the coordinating octahedron in the NaOH solution. In Cu(OH)64- complex two OH- ligands are located at its axis and four OH- ligands are located at the square plane. The binding energies of four OH- ligands are higher than the two axial ones because the interplane distances of them are shorter than the two axial OH-. Therefore, the two axial OH- ligands are easily dehydrated to form anisotropic CuO nanoparticles. It is considered that the introduction of selective capping agents, such as PVP could control the growth rates of various faces of metal oxide nanoparticles throughout the adsorption on these surfaces. PVP macromolecules could selectively interact with different faces of CuO nanostructures through Cu-N and Cu-O coordination bonds . In the absence of PVP CuO nuclei would grow randomly into irregular shapes. In this case, the oxygen atoms of PVP bound to the CuO. Another main key in this process was the reaction temperature. At low temperatures, the Cu(OH)64- complex formed hydrogen bonds. These hydrogen bonds could stabilize this complex at low temperatures. By increasing the temperature the hydrogen bonds were demolished and the growth units of Cu(OH)64- were destroyed rapidly and finally black CuO nanostructures were form (Eq.3). After nucleation and growth, the particle morphology, particle size and size distribution of produced nanoparticles might change by aging . The aging process consists of two main events; aggregation and coarsening (also known as Ostwald ripening), and according to the result of TEM the shape of nanorods changed into nanosheets due to Ostwald ripening.
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CuO nanosheets were successfully synthesized in PVP by using quick precipitation method at low reaction temperature (60 °C) in the absence of templates. The results illustrated that pure CuO nanostructures were formed only at higher pH values. FT-IR showed that PVP interacted with CuO nanosheets through Cu-O coordination bond. Electron microscopy showed that sheet-like CuO was obtained at higher pH values and also indicated that CuO nanosheets prepared in 15 mL NaOH were formed through oriented attachment of small particles. TEM also depicted that due to high pH value in addition of 20 mL agglomeration was happened. The mechanism of reaction was studied and concluded that supersaturation did not occur at low pH values; as a result, CuO nanostructures were not formed at low pH values.