Revealing the morphology regulated crystal facet effect in thin-film photocatalysts: a case study on gas-liquid interface derived BiOI thin-films.

Preparing photocatalysts in the form of thin films represents an efficient strategy for their recovery from target solutions. However, the origin of the low apparent photocatalytic performance observed in thin-film photocatalysts compared to their powder counterparts remains puzzling. Here, we develop a unique gas-liquid film growth technique to fabricate Janus-structured BiOI films on a liquid surface with excellent transferability. This method allows selective exposure of the desired film surface morphology without interfering with other film properties, thereby offering an opportunity to uncover the intrinsic role of micro/nanoscale geometric arrangements in determining the photocatalytic performance of BiOI films. The variation in photocatalytic performance as a function of film thickness is found to be highly surface-dependent. A flat or volcano-shaped curve is observed depending on whether the dense gas-phase surface (compact and smooth morphology) or the porous liquid-phase surface (composed of stacked nanosheets) is selectively exposed. Systematic morphological analysis of the micro/nanoscale geometric arrangements establishes a qualitative correlation with the photon utilization efficiency of the film, elucidating how morphology-regulated crystal facet effects influence the charge separation process in thin-film photocatalysts.