Citric acid-assisted hydrothermal synthesis of a self-modified Bi2SiO5/Bi12SiO20 heterojunction for efficient photocatalytic degradation of aqueous pollutants

Abstract

A self-modified Bi₂SiO₅/Bi₁₂SiO₂₀ heterojunction was synthesized for the first time via a citric acid-assisted hydrothermal method. It was found that citric acid (CA) played two roles in the formation of the Bi₂SiO₅/Bi₁₂SiO₂₀ heterojunction: enhancement of the specific surface area due to the release of many volatiles from the decomposition of CA and tuning the weight ratio of Bi₂SiO₅ and Bi₁₂SiO₂₀ in the heterojunction by forming different complexes with Bi³⁺. Thus, a self-modified Bi₂SiO₅/Bi₁₂SiO₂₀ heterojunction prepared at a CA dosage of 11.25 mmol presented an I_27.9 : I_29.2 value of 1.52 and a BET specific surface area of 32.6 m² g⁻¹, 5.7 times that without addition of CA. It showed the highest first-order apparent reaction constant of 0.1694 min⁻¹ in the photocatalytic degradation of acid orange 7 (AO7), which is 1.8, 53.0, and 2.2 times that of Bi₂SiO₅, Bi₁₂SiO₂₀, and TiO₂ (P25), respectively. The capture experiments show that the main active species that degraded AO7 are not O₂˙⁻ and ˙OH but h⁺_vb. During the AO7 degradation, four intermediates, sulfonic acid, oxalic acid, benzenesulfonic acid, and 4-aminobenzenesulfonic acid, were identified by capillary electrophoresis/capacitively coupled contactless conductivity detection. Finally, other pollutants, rhodamine B, p-chlorophenol, and tetracycline, were also found to be efficiently removed by the Bi₂SiO₅/Bi₁₂SiO₂₀-mediated photocatalysis system.