Preparation of Hierarchical Structure Au/ZnO Composite for Enhanced Photocatalytic Performance: Characterization, Effects of Reaction Parameters, and Oxidizing Agent Investigations

Abstract

Zinc oxide (ZnO) has been shown as a potential photocatalyst under ultraviolet (UV) light but its catalytic activity has a limitation under visible (Vis) light due to the wide bandgap energy and the rapid recombination of electrons and holes. Thus, hierarchical structure Au/ZnO composites were fabricated by the hydrothermal method and chemical reduction method for enhanced photocatalytic performance under visible light. As-prepared composites were characterized by UV-vis diffuse reflectance spectra (DR/UV-Vis), field emission scanning electron microscope (FESEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and electron paramagnetic resonance (EPR). The Au/ZnO-5 composite showed the highest adsorption among as-prepared samples in the range of 250-550 nm, having bandgap energy of 0.13 eV. Au nanoparticles of about 3-5 nm were well dispersed on hierarchical flower ZnO with approximately 10-15 μm. The EPR signal at g = 1.965 on both ZnO and Au/ZnO samples was attributed to oxygen vacancy Vo•, but the presence of Au led to a decrease in signal strength of Au/ZnO composite, showing the degradation efficiency (DE) and reaction rate of 99.2% and 0.109 min-1, respectively; these were larger than those of other samples. The effects of reaction parameters and oxidizing agents on photocatalytic performance were investigated and showed that the presence of H2O2 and O2 could improve the reaction of composite. In addition, the kinetic and photocatalytic mechanism of tartrazine (TA) on catalysts were studied by the first-order kinetic model and characterized analyses.

Preparation of Au/ZnO/Fe3O4 Composite for Degradation of Tartrazine under Visible Light

“…According to our previous study, Au/ZnO composite at 5 wt% of Au had the best photocatalytic performance compared to other Au contents, the degradation efficiency for tartrazine approached 99.2% in 60 min [13]. However, Au/ZnO is very low density, it is difficult to recover Au/ZnO from the solution after the catalysis process.…”

“…The precipitate was filtered, washed with distilled water and ethanol a few times, and dried at 90°C for 24 h. Finally, the ZnO nanoflowers were obtained after calcinating the precipitate at 400 °C for 2 h with a heating rate of 2 °C/min. The formation of ZnO was described by the following Equations ( 1)-( 5) [13]:…”

“…There are several types of research on doping precious metals into ZnO to reduce band gap energy, increase visible light absorption, and lower electron and hole recombination. To solve this problem, the proposed solution is to dope an appropriate amount of precious metals such as Au, Ag, Pt, and Pb, which enhances sunlight absorption and reduces electron-hole recombination, resulting in increased decomposition of organic substances [7][8][9][10][11][12][13][14][15].…”

Preparation of Novel Triangular Prism e -Zn(OH)2 by the Facile Precipitation Route for the Photocatalysis Under Visible Light

“…This can be explained by following the reason: At higher pH, more and more • OH radicals were generated from OHions on the surface of the catalyst, leading to the enhanced decomposition of TCH. However, in an alkaline medium, the Zn(OH) 2 can be dissolved (Zn(OH) (16) . The plausible photocatalytic mechanism of Zn(OH) 2 for the decomposition process of TCH was illustrated in Figure 8.…”

“…The produced • O 2 -and • OH radicals play important role in the photocatalytic process (17) . These powerful radicals can rapidly degrade the TCH into intermediates and entirely decompose the TCH to CO 2 , and H 2 O (16) .…”