Surface hydrophobicity of boron nitride promotes PFOA photocatalytic degradation

Boron nitride (BN) photodegrades perfluorooctanoic acid (PFOA) in water under 254-nm light illumination more rapidly than TiO₂, which is hypothesized due to its greater surface hydrophobicity. We investigated the role of hydrophobicity on PFOA photocatalysis by comparing BN with anatase TiO₂ under reaction conditions, for which the exposed surface areas were the same. BN exhibited ∼ 3.5 × faster PFOA degradation rate compared to TiO₂ under acidic pH conditions. PFOA adsorption experiments showed that BN had ∼ 2 × higher PFOA surface coverage, consistent with its higher surface hydrophobicity, as corroborated by contact angle measurements. Both materials were comparatively less photocatalytically active at neutral pH, but BN still exhibited ∼ 2.7 × faster PFOA degradation rate, due to less electrostatic attraction between the PFOA headgroup and the catalyst surface. Langmuir-Hinshelwood rate law analysis suggests BN and TiO₂ have comparable photogenerated hole surface concentrations, and density functional theory calculations show that the holes for both photocatalysts can react with surface hydroxyls and with adsorbed PFOA. However, BN has comparatively less surface hydroxyl groups and more adsorbed PFOA, which favors hole reaction with the latter, resulting in a higher PFOA degradation rate. These insights into the role of surface hydrophobicity serve as rationally-guided design principles for improved heterogeneous photocatalysis of persistent surfactants, including the broad suite of per- or poly-fluoroalkyl substances.