Near-Infrared-Triggered Nitrogen Fixation over Upconversion Nanoparticles Assembled Carbon Nitride Nanotubes with Nitrogen Vacancies.

Photocatalytic artificial fixation of N2 to NH3 occurs over NaYF4:Yb,Tm (NYF) upconversion nanoparticles (NPs) decorated carbon nitride nanotubes with nitrogen vacancies (NYF/NV-CNNTs) in water under near-infrared (NIR) light irradiation. NYF NPs with a particle size of ca. 20 nm were uniformly distributed on the surface of NV-CNNTs. The NYF/NV-CNNTs with 15 wt % NYF exhibited the highest NH3 production yield of 1.72 mmol L-1 gcat-1, corresponding to an apparent quantum efficiency of 0.50% under NIR light illumination, and about three times higher the activity of the bare CNNTs under UV-filtered solar light. 15N isotope-labeling NMR results confirm that the N source of ammonia originates from the photochemical N2 reduction. The spectroelectrochemical measurements reveal that NVs can greatly facilitate the photogenerated electron transfer without energy loss, while the presence of NYF NPs shifts both the deep trap state and the edge of conduction band toward a lower potential. Moreover, NYF NPs endow the photocatalyst with a NIR light absorption via the fluorescence resonance energy transfer process, and NVs have the ability to enhance the active sites for a stronger adsorption of N2 and decrease the surface quenching effect of NYF NPs, which thus can promote the energy migration within the heterojunctions. This work opens the way toward full solar spectrum photocatalysis for sustainable ammonia synthesis under aqueous system.

Efficient upconverting carbon nitride nanotubes for near-infrared-driven photocatalytic hydrogen production.

We report a facile chemical technique for synthesizing nanotube-based hybrid materials for near-infrared-driven photocatalytic hydrogen (H2) production. Upconversion nanoparticles (UCNPs), NaYF4:Yb,Tm,Gd (NYFG) and NaYF4:Yb,Tm (NYF), were engineered on C3N4 nanotubes (C3N4 NTs) separately to construct heterojunction structures. With a UCNP loading content of 15 wt%, the NYFG/C3N4 NT heterojunction exhibits the highest H2 generation rate of 311.6 µmol g-1 with an apparent quantum efficiency of 0.80 ‰, about 1.4 times higher than that of the NYF/C3N4 NT nanocomposite under 980 nm laser irradiation. Comprehensive characterization reveals that the enhanced photocatalytic performance of the Gd doped nanostructure is attributed to the synergistic effect, stronger interaction, higher emission intensities, and faster charge transfer between the UCNPs and C3N4 NTs. Moreover, the steady-state and dynamic fluorescence spectra indicate that the energy from NYFG NPs was transferred to C3N4 NTs via a fluorescence-resonance energy-transfer process. Our work demonstrates the potential of developing near-infrared-responsive photocatalysts for energy and environmental applications.