Boron-doped black titanium dioxide for visible-light and sacrificial-agent-free photocatalytic nitrate reduction

The photocatalytic conversion of nitrate (NO₃^-) to ammonia (NH₃) represents a dual-benefit strategy for environmental remediation coupled with the recovery of a valuable chemical feedstock. In this work, we demonstrate that boron-doped black titanium dioxide materials (B_x-TiO₂), synthesized using closo-dodecaborate dianion ([closo-B₁₂H₁₂]^2-) as the boron (B) source, serve as efficient catalysts for photoreduction of NO₃^- to NH₃ in aqueous media under visible-light irradiation. B doping induces the formation of an amorphous phase in TiO₂, which optimizes surface properties, narrows the band gap, and extends the optical absorption spectrum from the ultraviolet to the visible region-resulting in a significant enhancement in light utilization efficiency. Density functional theory (DFT) calculations reveal orbital hybridization between Ti and B, confirming their synergistic catalytic role. Among the series of synthesized materials, B₁-TiO₂ exhibits the highest catalytic activity, achieving an ammonia production rate of 769.0 μmol g_cat^-1 h^-1, which compares favorably with previously reported systems. This work on heterogeneous B doping is expected to serve as a reference for the development of efficient catalysts in follow-up studies.