Metal-organic frameworks (MOFs) provide an intriguing way to design hybrid materials from organic linkers and metal ions and have attracted considerable attention because of their glamorous structures and potential applications in materials science and industrial technologies [1–4] including gas storage [5–7], separation [8–10], catalysis [11–13], magnetic resonance imaging (MRI) [14–17] and drug delivery [18–21]. Meanwhile, powders constituted by metal oxides are suitable products for the inorganic chemical industries. They find application in the fields of adsorption technology [22,23], heterogeneous catalysis , pigments technology  and as precursors for sintered ceramics . Cupric oxide (CuO) is one of the most popular p-type semiconductor oxides, with a narrow band gap of 1.2 eV, that has been widely studied for a number of remarkable properties that can be used as heterogeneous catalysts in many significant chemical processes [24,27], as gas sensors [28,29], as a cathode material for electrochemical applications [30,31] and dye-sensitized solar cells [32,33]. So far, various nanostructured CuO crystals have been successfully synthesized through various methods such as sonochemical methods [34,35], double-jet precipitation methods [36,37], templating methods , precipitation , and wet-chemical methods [40,41]. The present work describes a facile route for preparation of CuO nanostructures by direct pyrolysis of two different Cu(II) metal-organic frameworks, [Cu2(btec)(2,2’-bipy)2]∞ (1), and [Cu(BDC)(bipy)](BDCH2) (2), as precursors under air atmosphere. The resulting nanomaterials were characterized by means of powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), and energy dispersive X-ray microanalysis (EDX).
MATERIALS AND METHODS
All the ligands and transition metal salts were obtained commercially and used as received. X-ray powder diffraction patterns were measured using a Philips PW1800 powder diffractometer for Cu-Kα (λ = 0.17887 Å) with a scan speed of 1 s/step and a step size of 0.04°. The samples were characterized with a scanning electron microscope (SEM) (Hitachi S4160 and Philips XL30) with gold coating. Energy-dispersive X-ray analysis (EDX) was used to fulfill elemental microanalysis with a Philips XL30 operating at 17.0 kV.
Synthesis of [Cu2(btec)(2,2’-bipy)2]∞ (1)
[Cu2(btec)(2,2’-bipy)2]∞ (1) was synthesized by the method according to the literature . In a typical synthesis of 1, a mixture of Cu(OAc)2·2H2O, H4btec, NaOH, 2,2’-bipy, and H2O with the molar ratio of 0.6:0.3:1.2:0.6:399.6 was stirred for 30 min, sealed in a Teflon-lined stainless steel autoclave and heated at 120 °C for 2 days. After cooling to room temperature, the dark blue powder product of 1 was collected.
Synthesis of [Cu(BDC)(bipy)](BDCH2) (2)
Compound [Cu(BDC)(bipy)](BDCH2) (2) was prepared using the reported method . A mixture of Cu(NO3) 2. 2.5H2O (0.725 g, 0.3 mmol), BDCH2 (0.05 g, 0.3 mmol), bipy (0.047 g, 0.3 mmol), and H2O (3 ml) was heated in a Teflon-lined autoclave at 150 °C for 2 days and then cooled to room temperature. The blue powder product was filtered, washed with H2O and ethanol, and air-dried to give [Cu(BDC)(bipy)](BDCH2).
Synthesis of copper oxide nanoparticles
The precursors 1 and 2 were placed in a ceramic boat separately and calcinated in the furnace at 500 °C for 4 h under air atmosphere. After cooling at room temperature, black products were collected. Calcination at other temperatures (400 and 600 °C) followed the same process above.
RESULTS AND DISCUSSION
The structure of compounds [Cu2(btec)(2,2’-bipy)2]∞ (1) and [Cu(BDC)(bipy)](BDCH2) (2) were previously analyzed and reported [42,43]. The simulated diffraction pattern of the reflections of the single crystal diffraction agrees with the experimental PXRD pattern of the formed powder MOFs, as shown in Figs. 1 and 2, respectively. In this study, CuO nanostructures were simply synthesized by direct calcination of the Cu-MOFs at 500 °C under air atmosphere without using any additional reducing agent or template. The crystalline structures and phase purity of as-synthesized products were first examined by PXRD with the results shown in Figs. 3 and 4, respectively. Fig. 3 shows the crystal structure of product resulted from 1 is corresponding to copper(II) oxide (CuO) (Tenorite; S.G.: C2/c; cell parameters: a = 4.685; b = 3.23; c = 5.132; β = 99.52; JCPDS file no. 41-0254). No peaks of other phases can be found, indicating that a pure product was obtained. Also, the peak positions appeared from 2 corresponding to a mixture of Cu2O (Cuprite; S.G.: Pn3m; with lattice constant a = 4.269; JCPDS file no. 05-0667) and CuO (Tenorite; JCPDS file no. 41-0254) in major phase, as shown in Fig. 4. Furthermore, the EDX spectra at 600 °C in Figs. 5 and 6 demonstrate that copper oxide is generated. The morphology and sizes of as-synthesized copper oxide products were further characterized by SEM. Reaction conditions have a large effect on the final products. To investigate the role of reaction temperature on the final products during calcination of the Cu-MOFs under air atmosphere, we further studied the morphology and particle size of the final products by calcination of the Cu-MOFs at different reaction temperatures varying from 400 to 600 °C for 4 h, (Figs. 7-8). So, we can see that with the variation of reaction temperature, copper oxide nanoparticles can be generated, but with different morphologies. As shown from histogram in Fig. 9 (a-c), the particles size was obtained 65, 140, and 65 nm, respectively, at 400, 500, and 600 °C for 1, and the formed nanoparticles from 2 have a diameter about 100, 45 and 120 nm at 400, 500 and 600 °C, respectively (Fig. 10 (a-c)).
In summary, we have explained the successful preparation of copper oxide nanoparticles by direct calcination of the 1D ladder-like [Cu2(btec)(2,2’-bipy)2]∞ metal-organic framework and porous coordination polymer [Cu(BDC)(bipy)](BDCH2) under air atmosphere. Calcination temperature has a great effect on the size and shape of the final products. The variation of reaction temperature from 400 to 600 °C led to the different particle sizes and morphologies of copper oxide nanoparticles. This process builds a direct relation between metal-carboxylate MOF crystals and metal oxide nanostructures and further opens a new application field for MOFs.
This work was supported by the Lorestan University.
CONFLICT OF INTEREST
The author declared to no conflict of interest.