Sunlight activated anodic freestanding ZrO₂ nanotube arrays for Cr(VI) photoreduction

Abstract

Visible-light-active freestanding zirconia (ZrO) nanotube (FSZNT) arrays were fabricated by a facile electrochemical anodization method in fluoride containing ethylene glycol electrolyte added to it was 1 vol% of potassium carbonate (KCO) at 60 V for 1 h. Poor adhesion at the metal∣oxide interface was induced by KCO leading to the formation of FSZNT flakes. The effect of the crystal structures of the FSZNTs e.g., amorphous, amorphous/tetragonal, and tetragonal/monoclinic was investigated towards the photocatalytic reduction of 10 ppm hexavalent chromium, Cr(VI) at pH 2 under sunlight. The results demonstrate the amorphous FSZNTs exhibited the highest Cr(VI) removal efficiency than the crystalline FSZNTs (95% versus 33% after 5 h). The high photocatalytic activity of the amorphous FSZNTs can be attributed to enhanced Cr(VI) adsorption, high visible light absorption, and better charge carrier separation. The low photocatalytic activity of the crystalline FSZNTs annealed at 500 °C was mainly attributed to poor Cr(VI) adsorption, low visible light absorption, and less photoactive monoclinic-ZrO.

Comparison of ZrO2, TiO2, and α-Fe2O3 nanotube arrays on Cr(VI) photoreduction fabricated by anodization of Zr, Ti, and Fe foils

“…Meanwhile, the rapid e − -h + recombination in the ZNTs reduces the chances of the electrons to be transferred to the oxide surface for reduction of Cr(VI). In contrast with our previous report [13], the annealed ZNTs can be activated under sunlight due to their high catalyst loading (1 g l −1 ) and free from underneath metal substrate (in the form of freestanding nanotubes). So that, more active sites can be exposed to the Cr(VI) ions.…”

“…This suggests the FNTs and ZNTs can be activated under a visible spectrum while TNTs are more photoactive under UV. The visible light-harvesting capability of the ZNTs can be ascribed to the formation of defect states like oxygen vacancies that narrowing the band gap [13].…”

“…Thus, the trapped electrons can be excited at various energy levels [17] upon irradiation. High intensity of PL emission in the ZNTs is ascribed to rapid charge carriers recombination due to annihilation of oxygen vacancies and impurities at high temperatures that used for charge carriers trapping [13,18]. Conversely, low PL intensity of TNTs and FNTs indicate better charge carriers separation in the oxide.…”

“…Nevertheless, due to the inherent defects that present in the oxides, sunlight activation can be achieved. Based on our previous reports [13,15,16], Cr(VI) photoreduction on ZNTs and TNTs are possible under visible light due to the presence of oxygen vacancies forming trap states within the band gap of the oxides especially in amorphous state (without annealed). This work aims at comparing the properties and photocatalytic activity of the annealed ZrO 2, TiO 2 , and α-Fe 2 O 3 nanotubes under sunlight and UV for the reduction of Cr(VI).…”

“…To create oxide nanotubes, the anodic process can be applied whereby a metal foil is polarized in an electrolyte comprising of fluoride ions (F − ). During anodization, oxidation occurs to produce an oxide film which then will be etched by the Fions to form pores which then will grow into the larger and more stable nanotubular structure [11][12][13].…”