Z-scheme ZnO/Bi₂WO₆ heterojunction for efficient photocatalytic mineralization of gaseous formaldehyde under simulated sunlight

Gaseous formaldehyde, a prevalent indoor air pollutant, poses significant risks to environmental quality and human health. To address the pressing need for efficient indoor air purification, a series of xZnO/Bi₂WO₆ (x = 0.5, 1, 1.5, and 3 wt%) heterojunction photocatalysts were synthesized via a hydrothermal method and evaluated for gaseous formaldehyde degradation in a continuous flow-type batch reactor under simulated sunlight. Among them, the 1 % ZnO-loaded Bi₂WO₆ composite (denoted as 2-ZBW) exhibited the highest activity, removing 81 % of 200 ppm formaldehyde at 40 % relative humidity within 120 min, with a pseudo-first-order rate constant approximately 3.5 times greater than that of pristine Bi₂WO₆. This superior performance is primarily ascribed to the efficient charge separation and transfer enabled by the Z-scheme heterojunction structure. The 2-ZBW composite also achieved a high mineralization efficiency, converting 95 % of degraded formaldehyde into CO₂ and maintaining excellent stability and reusability over successive cycles. Radical scavenging and EPR experiments confirmed that superoxide (^•O₂⁻) and hydroxyl (^•OH) radicals are the primary reactive species driving the degradation process. These findings highlight the potential of Z-scheme ZnO/Bi₂WO₆ heterojunction photocatalysis as robust and efficient candidates for indoor air purification applications.