Synthesis and photocatalytic performance of Bi₁₂O₁₇Cl₂/TiO₂ composite catalysts

In this study, the Bi12O17Cl2 photocatalyst was synthesized at room temperature via a facile route using BiCl3·5H2O, NaOH, and CH3CH2OH as the raw materials. Subsequently, a series of Bi12O17Cl2/TiO2 composite photocatalysts was fabricated by mechanically mixing Bi12O17Cl2 and TiO2 at different mass ratios. A set of characterization techniques, including X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, ultraviolet (UV)‒visible absorption spectroscopy, and transient photocurrent response measurements, were used to systematically investigate the crystal structures, morphological features, elemental chemical states, and optical properties of the as-prepared composite materials. The photocatalytic degradation efficiencies of methyl orange (MO) over pure TiO2 and the synthesized Bi12O17Cl2/TiO2 composites were evaluated under both UV and visible light irradiations for Bi12O17Cl2 mass fractions of 1%, 3%, 5%, and 7%. Under UV irradiation produced by a mercury lamp for 1.25 h, the degradation efficiencies of MO were 89% for pure TiO2, 83% for 1% Bi12O17Cl2/TiO2, 100% for 3% Bi12O17Cl2/TiO2, 98% for 5% Bi12O17Cl2/TiO2, and 95% for 7% Bi12O17Cl2/TiO2. The photocatalytic activity of MO was in the following order: 3% Bi12O17Cl2/TiO2 > 5% Bi12O17Cl2/TiO2 > 7% Bi12O17Cl2/TiO2 > TiO2 > 1% Bi12O17Cl2/TiO2. Thus, the highest photocatalytic activity was exhibited by the 3% composite. As the Bi12O17Cl2 mass fraction increased from 1% to 7%, the MO degradation efficiency first increased and then decreased, which can be attributed to the formation of a p-n heterojunction between p-type Bi12O17Cl2 and n-type TiO2. The heterojunction not only accelerated the migration of photogenerated charge carriers but also significantly suppressed the recombination of electron-hole pairs, thereby prolonging the carrier lifetime and enhancing the photocatalytic performance of MO. When MO was irradiated by visible light produced by a 300 W xenon lamp for 2 h, the MO degradation efficiencies were 61% (TiO2), 83% (1% composite), 90% (3% composite), 49% (5% composite), and 42% (7% composite) and were in the following order: 3% Bi12O17Cl2/TiO2 > 1% Bi12O17Cl2/TiO2 > TiO2 > 5% Bi12O17Cl2/TiO2 > 7% Bi12O17Cl2/TiO2. As with UV light irradiation, the degradation efficiency of MO first increased and then decreased with increasing Bi12O17Cl2 content, with the 3% composite exhibiting the highest visible light activity (90% MO degradation). The chemical oxygen demand (COD) removal efficiency was measured to assess the mineralization degree of MO. The COD removal efficiencies of MO were 60% (TiO2), 63% (1% composite), 75% (3% composite), 24% (5% composite), and 11% (7% composite), which matched the order of the visible light degradation activity. The 3% Bi12O17Cl2/TiO2 composite demonstrated the optimal mineralization capability, with a COD removal rate of 75%. Mechanistic studies revealed that compared with Bi12O17Cl2, TiO2 had a more negative conduction band (CB) and a more positive valence band (VB). Under light irradiation, the photogenerated electrons in the CB of TiO2 migrated to the CB of Bi12O17Cl2, while the holes in the VB of TiO2 transferred to the VB of Bi12O17Cl2. The effective charge separation inhibited electron‒hole recombination and promoted the formation of reactive oxygen species, such as ·O2- produced from the reaction of electrons with O2 and h+, which exhibited strong oxidizing capacity that decomposed MO into inorganic products, such as CO2, H2O, and N2. This study therefore provides valuable insights into the development of novel composite photocatalysts with enhanced performance, offering promising applications in the advanced treatment of organic dye pollutants and environmental protection.