INTRODUCTION

Recently nanomaterials due to their unique and symmetric characteristics in comparison with bulk materials are studied on a very large scale. Copper (II) oxide in contrast with other oxide semiconductors doped with 3d-elements has a monoclinic crystal structure. CuO is a P-type semiconductor and it has a very small (1.5 ev) band gap [1, 2] and is mostly used for junction materials such as junction p-n diodes [3, 4]. The catalysts that are enriched by CuO are shown to have a very good activation for the purpose of reduction of contamination caused by NO_X [5, 6]. As a result of different characteristics of CuO and more importantly because of its nanostructure, it has numerous applications. Nano copper oxide is already used in high temperature superconductors [7, 8], optical switches [9, 10], electrode anode batteries [11, 12], heterogeneous catalysis [13-15], and gas sensors [16-18]. Recently, nanostructured CuO is made in different forms such as nanowire, nano film, nanotube and nano sphere [19]. Nano structured CuO has been synthesized by wet chemical techniques, thermal and plasma-based methods methods such as vapor chemical accumulation, electrochemical techniques and hydrothermal method [19]. These methods mostly require very expensive facilities as well as high temperature environment. In this study, sol- gel method is used to produce CuO nanoparticles. The facilities needed by this method are fewer in number and they are not as expensive as those needed for other mentioned methods. More importantly, sol gel method can be carried out at room temperature and there is no need for a high temperature.

MATERIALS AND METHODS

CuO nanoparticles were synthesized through the sol-gel method. For this purpose, aquatic solutions of copper nitride and sodium hydroxide were added simultaneously at room temperature and in this way a gel is formed. The gel was washed for several times by deionized water to withdraw the nitride ions from the gel. The gel were filtered and dried at the 50 degree centigrade to obtain copper hydroxide. Then the copper hydroxide was annealed at different temperatures to achieve the final product, the copper oxide. In order to investigate the effect of the copper nitrite density, we changed the density from 0.1 molar to 0.2 and so forth. However, we keep the density of sodium hydroxide unchanged, which is 1 molar during this investigation. The samples were annealed for 3 hours at 160°C. While, the density of copper nitride was 0.4 molar, the sol was not formed. Also, in this work the effect of the stirring time and the effect of time and temperature on the final product were investigated. The XRD (X- Ray Diffraction) patterns of the samples were achieved through using a diffractometer which was radiated CuK_α. The size of the synthesized nanoparticles was estimated by using Scherer equation.

RESULTS AND DISCUSSION

Effect of Copper Nitride Density

In this part of experiment, the density of copper nitride was changed from 0.1 molar to 0.2 and 0.3 molar. In Fig. 1, the XRD patterns of different samples with different copper nitride densities are shown. The figure shows that by increasing the portion of copper nitride to sodium hydroxide, the size of the particles reduced, however, the Cu₂O phase is appeared as well.  The intensification of picks at 29.5° and 35.5° degrees in figure shows that by the increase in the portion of copper nitride solution over sodium hydroxide solution, leading the crystals to grow in the (111) and (110) plains of copper dioxide. The mean size of different samples is presented in Table 1.

Effect of Stirring time on the Sol

In this part, three samples prepared with different stirring times. One of the samples was not stirred, but other samples were stirred for 10 minutes and 3 hours, respectively. In all cases, the calcination temperature was 160° centigrade and the density of copper oxide was 0.1 molar. It is shown (in Fig. 2) that the stirring time of the sol did not affect the purity of the final product which proves the immediate reactions among precursors. Moreover, it can be seen that the mean size of particles in all samples are 37 nm. So, stirring time does not have any effects on the particles size too.

Effect of Calcination Time

To investigating the effect of calcination time, we changed the time from 3hours to 12 hours. As Fig. 3 shows, when the samples were annealed for a longer time, the intensity of (111) plane increased, too. Other experiment conditions like temperature and density were the same as the former one. The intensification of the (111) plain indicated that the more the calcination time, the more the growth in Cu₂O phase. Also, a rise in calcination time would lead to larger particles which prove the more growth in crystals. The mean particles size is shown in Table 2.

Calcination Temperature Effect

In this part of experiment the density of copper oxide was 0.1 M and all samples were annealed for three hours. However, the experiment is performed with four different calcination temperatures. The XRD patterns for the samples which were synthesized at temperatures of 160, 300, 600 and 900° C are depicted in Fig. 4. It can be inferred from the patterns that when the calcination temperature was higher, the pick of the (111) plain is intensified in comparison to (111) plain. Since the pick for (111) plain of Cu₂O overlaps with the (111) CuO plane, it demonstrates the growth of the Cu₂O phase. By rising temperature to 900°C, one more pick was appeared which is due to the existence of the CuO (202) plain. 

Moreover, at 900°C the intensity of the (111) plain in comparison to the (111) plain was increased which shows that in higher temperatures the crystal are grown in these crystallographic directions and the XRD pattern of nanoparticles is changed to the same pattern of the bulk copper oxide. By risingup the calcination temperature, the synthesized particles became larger and the crystals were grown larger, too. In Table 3, the size of particles is shown and in Fig. 5, the SEM images are for synthesized particles in different temperatures.

Investigation of band gap variation by temperature

For this purpose, from the samples which were synthesized at different temperatures, UV- Visible absorption spectrum was taken and the results are shown in Fig. 6.

It can be inferred from the Fig. 6 that at higher temperatures which means particle mean size is higher too, the absorption edge shifted toward the longer wavelengths. This represents that the larger the particles are, the smaller the band gap is.

CONCLUSION

While the density of copper nitride and sodium hydroxide are 0.1 M and 1M, respectively, pure copper oxide (CuO) is obtained. By raising the portion of the copper nitride density to that of sodium hydroxide, a new phase was appeared (Cu₂O). However, the particles’ size became smaller. Stirring time is lengthened and no noticeable changes in the degree of purity and the particles size occurred. By increasing the calcination time at 160°, the Cu₂O phase in synthesized samples was appeared. Also, the particles were grown larger in size. The XRD pattern of the sample at 900° C is very similar to that of the bulk sample. The leaf shape and nano scale size of particles are shown by the SEM figures. The UV- Visible spectra shows that when the calcination temperature and particles mean size increases, the adsorption edge would move toward the longer wavelengths. While there has been the copper nitride 0.1 M and sodium hydroxide 1 M at 160°C, the best sample was collected. 

CONFLICT OF INTEREST

The authors declare that there is no conflict of interests regarding the publication of this manuscript.