Molecular Weight of Photoactive Macromolecules Governs the Interfacial Photochemistry of Few-Layer Black Phosphorus

The photochemical behavior of two-dimensional (2D) semiconductors in aqueous environments is conventionally framed primarily by reactive oxygen species (ROS) generation and consumption balance, yet the molecular weight (MW) heterogeneity of ubiquitous dissolved organic matter and its interfacial modulation remain poorly understood. Using few-layer black phosphorus (FLBP) and MW-fractionated fulvic acid (FA) as a model system, we employ an integrated analytical platform, hollow fiber flow field-flow fractionation–inductively coupled plasma mass spectrometry (HF5-ICP-MS) for thickness-based fractionation and element-specific quantification of FLBP, coupled with fluorescence spectroscopy and ultrahigh-resolution mass spectrometry for molecular-level characterization of FA. We find that FA accelerates FLBP photodegradation while suppressing its photocatalytic activity across all MW fractions. Three distinct MW-dependent interfacial interaction modes account for this behavior: low-MW FA (<3 kDa) acts as a freely diffusing, sustained 1O2 photosensitizer with negligible compositional degradation, promoting FLBP degradation without substantially occupying surface sites; intermediate-MW FA (3–10 kDa) exhibits irradiation-enhanced surface adsorption, simultaneously delivering ROS to the FLBP interface and progressively blocking active sites; high-MW FA (>100 kDa) undergoes FLBP-mediated photoinduced disaggregation, selectively releasing aliphatic fragments while retaining condensed aromatic moieties on the nanosheet surface, maintaining interfacial ROS generation and surface occupancy. Photocatalytic tests show that surface site blockage by adsorbed FA outweighs supplementary ROS generation. These findings demonstrate that interfacial macromolecular organization, not radical yield alone, plays a critical role in the aqueous photochemistry of FLBP in macromolecule-rich media, and provide a mechanistic basis for understanding the behavior of 2D nanomaterials in complex aquatic systems.