Decolourization of crystal violet using nano-sized novel fluorite structure Ga2Zr2- x W x O7 photocatalyst under visible light irradiation

Abstract

Fluorite-type Zr-based oxides with the composition Ga₂Zr_2-x W _x O₇ (x = 0, 0.05, 0.1, 0.15 and 0.2) were prepared using the citrate technique. Appropriate characterizations of all prepared materials were carried out. X-ray diffraction clarified that the undoped and W-doped Ga₂Zr₂O₇ samples were crystallized in the cubic fluorite phase structure. The average particle size of the samples was in the range of 3-8 nm. The lowest band gap (1.7 eV) and the highest surface area (124.3 m² g^-1) were recorded for Ga₂Zr_0.85W_0.15O_7. The photocatalytic impacts of the prepared systems were studied by removal of crystal violet (CV) dye employing visible light illumination and taking into consideration the initial dye concentrations, duration of visible irradiation treatment, catalysts dose and the dopant concentration. The obtained results showed higher dye removal with the boost of the catalyst dosage. W doping shifted the absorption to the visible light range by decreasing the band gap from 4.95 eV for parent Ga₂Zr₂O₇ to 1.7 eV for 15 mol% tungsten-doped Ga₂Zr₂O₇ enhancing the photocatalytic decolourization of CV from 4.2% to 83.6% for undoped and 15 mol% W-doped Ga₂Zr₂O₇, respectively, at optimum operating conditions (pH 9, 1 g l^-1 catalyst dose and 300 min) while heavily doped W sample containing 20 mol% W showed lower removal than 15 mol% W-doped Ga₂Zr₂O₇. Complete CV degradation using 15 mol% W-doped Ga₂Zr₂O₇ was attained with the assistance of 25 mmol l^-1 hydrogen peroxide. The reaction is aligned to pseudo-first-order kinetics. Different scavengers were introduced to decide the significance of the reactive species in CV degradation. ${\text{O}}_2^{-\bullet }$ and h ⁺ had the major role in the degradation of CV by Ga₂Zr_2-x W _x O₇ system compared with HO^•.

Keywords: Ga2Zr2−xWxO7; crystal violet dye; fluorite phase; nanomaterials; photocatalytic degradation; wastewater treatment.