Enhanced light-driven hydrogen generation on carbon quantum dots with TiO2 nanoparticles

Solar to hydrogen (H-2) conversion systems based on carbon nanomaterials have shown great potentials in the clean energy field recently. However, for most systems, energy level alignments and light-induced redox processes are still unclear, which hinder artificial designing for higher efficiency of solar energy conversion and further applications. Here we report 77% enhancement in the light-driven H-2 generation efficiency of N,S co-doped carbon quantum dot (N,S-CQD) aqueous system by adding TiO2 nanoparticles. Using steady-state and transient spectroscopy, four specific energy levels of CQDs are confirmed with the band gaps of 3.55 eV (X-4), 2.99 eV (X-3), 2.76 eV (X-2) and 1.75 eV (X-1), respectively. The X-2 energy band is highly active for H+ reduction with a longer lifetime of 13.38 ns. Moreover, the observed low efficiency of intrinsic transition from X-3 to X-2 band of N,S-CQDs accounts for the poor performance of solar to H-2 conversion for pure N,S-CQDs based on H-2 generation and detailed time-resolved spectroscopic results. The mechanism of H-2 generation enhancement can be explained by multiple electron transfer processes between N,S-CQDs and TiO2 NPs where TiO2 NPs act as electron intermediates that efficiently transfer electrons from the inert band (X-3) to the active band (X-2) for H-2 generation. This study enriches the fundamental understanding of N,S-CQDs and provides a new pathway toward high-performance N,S-CQD-based solar to H-2 conversion systems.

Automated summary

Solar to hydrogen conversion systems based on carbon nanomaterials have shown great potentials in the clean energy field. This study enhances the understanding of N,S-CQDs and provides a new pathway towards high-performance solar to H-2 conversion systems by adding TiO2 nanoparticles to improve H-2 generation efficiency.