The highly efficient CuO/Co3O4 composite photocatalyst with different morphologies has been synthesized directly on Cu wire mesh by controlling the composition of cobalt-containing solid precursors via a simple hydrothermal method. The structure morphology and composition of the composite photocatalyst have been characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV-visible diffuse reflectance spectra. The photocatalytic result shows that the CuO/Co3O4 coaxial heterostructure is easy to recycle and exhibit enhanced photodegradation activity for methylene blue compared to single CuO nanorod arrays under full spectrum solar light irradiation. The enhanced photocatalytic efficiency of the composite could be ascribed to the synergistic effect of CuO and Co3O4. This study provides a general and effective method in the fabrication of 1D composition NRs with sound heterojunctions that show enhancement of photocatalytic performance and facility of recycling.
Organic dyes, which are widely used in various industrial processes, form an integral part of industrial wastewater. Photocatalytic degradation of organic pollutants by semiconductor photocatalysts is promising for environmental purification and energy conversion [
Nevertheless, most of the widely used photocatalysts have two main limitations: one is the low solar energy conversion efficiency due to their wide band gap and the high recombination ratio of photoinduced electron-hole pairs, and the other is the difficulties of the catalysts’ recycling. Therefore, seeking highly active photocatalysts which are easy to recycle is still an intensifying endeavor worldwide. One-dimensional (1D) nanostructures including nanorods (NRs), nanowires (NWs), nanotubes (NTs), and nanofibers (NFs) have drawn particular attention because of their high specific surface area and facility of electronic transmission [
Therefore, many oxides with 1D nanostructures have been widely used as catalysts in photocatalytic reactions. However, photocatalysis, in general, is blamed for the low quantum yields caused by the electron/hole recombination. For further improvement of photocatalytic activity, the p-n, n-n, and p-p junctions formed in combination with both p-type and n-type semiconductors can effectively reduce the recombination rate of photogenerated electro/hole pairs, which subsequently enhances the photocatalytic activity [
CuO, as a p-type semiconductor with a narrow band gap (1.2 eV in bulk), is one of the most prominent catalysts and is extensively used in environmental catalysis. Spinel cobalt tetroxide (Co3O4) is a compound of CoO and Co2O3 with rich oxygen content and thus also exhibits p-type semiconducting properties. In this study, we presynthesized CuO NRs array grown directly on Cu wire mesh via thermal oxidation and the synthesis of Co3O4 nanostructures with different morphologies by simple and effective hydrothermal method at relatively low temperature on CuO NRs arrays. These hybrid nanostructures were aligned on Cu substrates, which can directly serve as a physical support of these structures. Thus, the photocatalysts can be easily recycled after photocatalytic reactions. Compared with the single CuO NRs and Co3O4 nanoflowers, the composite exhibits a potential synergistic effect with remarkably enhanced photocatalytic performances in terms of MB degradation.
All chemicals (purity of 99.9%) used in this research were of analytical purity and used without further purification. CuO NR arrays were synthesized on copper mesh using a simple and low cost thermal oxidation method as reported before [
CuO/Co3O4 composite was produced by a hydrothermal growth method. CuO/Co3O4 coaxial heterostructures: 0.07 M cobaltous nitrate (Co(NO3)2·6H2O) was dissolved in 30 mL mixture solution of ethylene glycol and water (15 mL : 15 mL). 0.04 M Seignette salt (C4O6H4KNa) and 0.36 M urea (CO(NH2)2) were introduced in the solution above subsequently. CuO/Co3O4 nanosheets: 0.07 M cobaltous nitrate (Co(NO3)2·6H2O) was dissolved in 30 mL mixture solution of ethylene glycol and water (15 mL : 15 mL). 0.14 M ammonium fluoride (NH4F) and 0.36 M urea (CO(NH2)2) were introduced in the solution above subsequently. Co3O4 nanoflowers: 0.07 M cobaltous nitrate (Co(NO3)2·6H2O) and 0.36 M urea (CO(NH2)2) were dissolved in 30 mL deionized water. The solutions were then transferred to Teflon-lined stainless steel autoclave and the copper mesh with CuO NRs was immersed in this solution. The autoclave was maintained at 90°C for 5 hours. After natural cooling, the copper meshes with CuO NRs were removed from the autoclave and washed multiple times with distilled water and then dried in air. Subsequently, the as-synthesized Co3O4/CuO composites were calcined at 350°C for 2 hours. There were three kinds of CuO/Co3O4 composites that were synthesized in the hydrothermal growth process as shown in Figure
Schematic illustration of different morphologies of CuO/Co3O4 composites.
The surface morphology and structure of samples were characterized by field emission scanning electron microscopy (FE-SEM, Hitachi S-4800), X-ray diffraction (XRD, RIGAKU/DMAX), and transmission electron microscopy (TEM, Philips Tecnai G2 F20). Surface chemical analysis of CuO/Co3O4 coaxial heterostructures was performed by X-ray photoelectron spectroscopy (XPS) using a PHL1600ESCA instrument equipped with a monochromatic Mg Ka X-ray source (
Degradation of methylene blue was used to evaluate the photocatalytic activity of Co3O4/CuO composites. The methylene blue without photocatalysts was designed as the blank control experiment. A little piece (9 cm2) of copper mesh with CuO NRs and Co3O4/CuO composites was put in a 50 mL of aqueous solution with methylene blue concentration of 10 mg/L. In order to establish an adsorption/desorption equilibrium, the solution was stirred in the dark for 30 min. A 500 W Xenon lamp was served as the light source. The residual concentration of methylene blue was evaluated by ultraviolet-visible (UV-Vis) spectrometry.
The morphology of Co3O4/CuO composites was studied by SEM as shown in Figure
SEM images of (a) CuO NR arrays and different morphology of CuO/Co3O4 composite, (b) CuO/Co3O4 coaxial heterostructures, (c) CuO/Co3O4 nanosheets, and (d) Co3O4 nanoflowers. The insets show the high-magnification images of the corresponding samples. (e) Schematic diagram of the growth mechanism of the CuO/Co3O4 composite photocatalyst.
The formation mechanism of Co3O4 nanostructures could be proposed as follows (Figure
TEM image of a single CuO NR is shown in Figure
TEM/HRTEM images of CuO NR (a) and (b), (c–e) for CuO/Co3O4 coaxial heterostructures, (f–h) for CuO/Co3O4 nanosheets, and (i–k) for Co3O4 nanoflowers.
The TEM image shown in Figures
The panoramic morphology of the Co3O4 nanoflowers is shown in Figure
The XRD pattern (Figure
XRD patterns of as-prepared CuO NRs and CuO/Co3O4 coaxial heterostructures.
In addition, the formation of Co3O4 crystal was also confirmed by Raman spectra, as depicted in Figure
Raman spectra of CuO NRs (a) and CuO/Co3O4 coaxial heterostructures (b).
Figure
XPS spectra of (a) Cu 2p, (b) O 1s for CuO NRs, (c) Cu 2p, and (d) O 1s for CuO/Co3O4 coaxial heterostructures. (e) 7 XPS spectra of Co 2p for CuO/Co3O4 coaxial heterostructures.
The Co 2p3/2 and Co 2p1/2 main peaks are located at 780.8 and 797.0 eV (shown in Figure
The photocatalytic degradation of MB dye on CuO/Co3O4 composites was carried out under full spectrum solar light irradiation. The blank test was carried out to determine the contribution of photolysis of MB. Under the full spectrum solar light irradiation in a period of 180 minutes, self-degradation of MB is about 15% of the original organic MB dye under irradiation. Both CuO NRs and Co3O4 nanoflowers show apparent photocatalytic activity for the MB degradation, while limited improvements were observed by combining the two semiconductors in the sample of CuO/Co3O4 nanosheets. It is clear to see that large amount of Co3O4 grains is accumulated on CuO NRs, and the grain boundaries of Co3O4 particles would impede electronic transfers in CuO/Co3O4 nanosheets. Therefore, the photocatalytic performance of CuO/Co3O4 nanosheets was not improved dramatically. However, CuO/Co3O4 coaxial heterostructures exhibit highest photocatalytic activity among the four samples. After irradiation for 180 minutes, the CuO/Co3O4 coaxial heterostructures are able to degrade about 56% of the original organic MB dye, while the degradation rate of CuO NRs is only about 40% (Figure
(a) Comparison of photocatalytic activities of bare CuO NRs and CuO/Co3O4 composites for the photocatalytic decolorization of MB in water. (b) UV-Vis absorption spectra of CuO NRs and CuO/Co3O4 coaxial heterostructures. (c) Proposed charge separation for CuO/Co3O4 coaxial heterostructures under light irradiation. (d) Cycling degradation curves for CuO/Co3O4 coaxial heterostructures.
The enhanced photocatalytic performance of CuO/Co3O4 composite can mainly be explained by the electron-hole (e-h) charge separation which results in the synergism and coupling effect between the two semiconductors. The conduction band (CB) and valence band (VB) potentials of Co3O4 and CuO can be calculated based on the previous literatures [
The recycle experiments were conducted to evaluate the photostability of CuO/Co3O4 coaxial heterostructures. Compared with powder, copper substrate can make the catalyst easy to recycle. After each cycle, we washed the CuO/Co3O4 coaxial heterostructures with DI water and dried them in air and refreshed MB dye was added in the reactor. The degradation efficiencies at 180 minutes of CuO/Co3O4 coaxial heterostructures photocatalyst were in sequence of 44.15%, 44.59%, 46.79%, and 49.2% (Figure
In order to examine the efficiency of charge carrier trapping, immigration, and transfer, as well as understanding the fate of e−/h+ pairs in semiconductor particles, the CuO NRs and Co3O4/CuO composites were subjected to photoluminescence (PL) measurements. The PL spectra of the samples are shown in Figure
(a) The photoluminescence (PL) spectra of the CuO NRs, CuO/Co3O4 coaxial heterostructures, and CuO/Co3O4 nanosheets. Nitrogen adsorption-desorption isotherm for mesoporosity and BJH pore size distribution plot (inset): (b) CuO NRs; (c) CuO/Co3O4 coaxial heterostructures; (d) CuO/Co3O4 nanosheets.
In summary, novel CuO/Co3O4 coaxial heterostructures were prepared by hydrothermal methods. The morphology of composite strongly affects the efficiency of photodegradation. Compared with CuO NRs, CuO/Co3O4 nanosheets, and Co3O4 nanoflowers, the synthesized CuO/Co3O4 coaxial heterostructures catalysts showed high photocatalytic efficiency for the degradation of MB under full spectrum solar light irradiation. The overlapping of band structure plays an important role in charge transfer and separation for high photocatalytic activity. Because of its stability and being easy to recycle, the CuO/Co3O4 coaxial heterostructures are promising for practical application for water purification.
The authors declare that there is no conflict of interests regarding the publication of this paper.
Financial supports by National Natural Science Foundation of China (51402211) and Natural Science Foundation of Tianjin (15JCQNJC03600) are gratefully acknowledged.