High photocatalytic efficiency of a ZnO nanoplate/Fe2O3 nanospindle hybrid using visible light for methylene blue degradation

Abstract

In this work, ZnO nanoplates and Fe₂O₃ nanospindles were successfully fabricated via a simple hydrothermal method using inorganic salts as precursors. The ZnO/Fe₂O₃ hybrid was fabricated using a mechanical mixture of two different ZnO : Fe₂O₃ weight ratios to investigate the effect of weight ratio on catalytic properties. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images showed that ZnO nanoplates (NPls) are about 20 nm thick with lateral dimensions of 100 × 200 nm, and Fe₂O₃ nanospindles (NSs) are about 500 nm long and 50 nm wide. X-ray diffraction (XRD) patterns revealed the successful formation of the ZnO, Fe₂O₃, and ZnO/Fe₂O₃ samples and indicated that their crystallite sizes varied from 20 to 29 nm depending on the ZnO : Fe₂O₃ weight ratio. Ultraviolet-visible (UV-vis) spectra showed that the bandgap energies of ZnO and Fe₂O₃ were 3.15 eV and 2.1 eV, respectively. Energy dispersive X-ray spectroscopy (EDS) results revealed the successful combination of ZnO and Fe₂O₃. Photocatalytic activity of the materials was evaluated through the degradation of methylene blue (MB) in aqueous solution under green light-emitting diode (GLED) irradiation. The results indicated that the ZnO/Fe₂O₃ composite showed a remarkable enhanced degradation capacity compared to bare ZnO NPls and Fe₂O₃ NSs. The ZnO : Fe₂O₃ = 3 : 2 sample demonstrated the best performance among all samples under identical conditions with a degradation efficiency of 99.3% for MB after 85 min. The optimum photocatalytic activity of the sample with ZnO : Fe₂O₃ = 3 : 2 was nearly 3.6% higher than that of the pure ZnO sample and 1.12 times more than that of the pristine Fe₂O₃ sample. Moreover, the highest photo-degradation was obtained at a photocatalyst dosage of 0.25 g l^-1 in dye solution.

Fig. 1. Schematic of the preparation of (a) ZnO nanoplates, (b) Fe₂O₃ nanospindles, and (c) ZnO/Fe₂O₃ hybrid materials.

Fig. 2. (a) XRD patterns of ZnO nanoplates. (b) Comparison of the XRD patterns of pure ZnO, pristine Fe₂O₃ nanospindles and ZnO/Fe₂O₃ hybrids with a ZnO : Fe₂O₃ weight ratio of 3 : 2. (c) The full width at half maximum of a diffraction peak and Bragg diffraction angle; (d) the dependence of crystallite size on the ZnO : Fe₂O₃ weight ratio.

Fig. 3. (a) SEM image of ZnO, (b) TEM image of ZnO, (c) SEM image of Fe₂O₃, and SEM images of ZnO/Fe₂O₃ with magnifications of (d) 2,000, and (e) 10 000.

Fig. 4. (a) The energy-dispersive X-ray spectrum (EDS) of the ZnO : Fe₂O₃ = 3 : 2 hybrid, and (b) the corresponding area of the SEM image.

Fig. 5. (a) UV-vis absorption spectra of pure Fe₂O₃ nanospindles and ZnO/Fe₂O₃ nanocomposites. (b) Tauc's plots derived from corresponding UV-vis spectra.

Fig. 6. Zeta potential distribution of the ZnO : Fe₂O₃ = 3 : 2 sample.

Fig. 7. UV-vis absorbance spectra of (a) the blank 10 ppm MB solution without a catalyst before and after GLED illumination for 85 min; (b) pure ZnO; (c) pristine Fe₂O₃; and ZnO/Fe₂O₃ with different ZnO : Fe₂O₃ weight ratios of (d) 2 : 5, (e) 2 : 3, (f) 1 : 1, (g) 3 : 2, and (h) 5 : 2 under GLED illumination for 0–85 min.

Fig. 8. (a) The dependence of MB concentration on the irradiation time of different samples, (b) the photocatalytic efficiency of different samples, (c) the dependence of C/C_o on irradiation time, and (d) the photocatalytic degradation rate of different samples.

Fig. 9. (a) The dependence of MB concentration on the irradiation times of different amounts of the ZnO : Fe₂O₃ = 3 : 2 sample; (b) a comparison of the photocatalytic efficiencies of different amounts of the ZnO : Fe₂O₃ = 3 : 2 sample.

Fig. 10. The impact of solution pH on the photocatalytic activity of the ZnO : Fe₂O₃ = 3 : 2 sample at (a) pH 3 and (b) pH 9.54; the dependence of the photocatalytic efficiency (c) and ln(C_o/C) (d) on illumination time with an initial MB concentration of 10 ppm.

Fig. 11. The recyclability of the ZnO : Fe₂O₃ = 3 : 2 sample at pH 7 towards 10 ppm MB after 5 cycles.

Fig. 12. A schematic diagram showing the role of the heterojunction of ZnO/Fe₂O₃.