Promoting photocatalytic hydrogen generation by Mn dopants in 1- dimensional nanorods

The growing global energy crisis and the compelling requirement to reduce the overdependence on fossil fuels have increased the need for sustainable green energy. Over the last decade, numerous renewable energy sources have been investigated, including wind, solar, hydropower, and hydrogen fuel. In particular, photocatalytic hydrogen generation via water splitting has drawn significant research attention as a promising strategy for addressing the energy crisis.

Among the strategies for producing hydrogen gas by water splitting, photocatalysis using sunlight is deemed highly suitable for green energy harvesting and sustainability. For instance, photogenerated charge carriers from semiconductor nanocrystals (NCs) can facilitate photocatalytic redox reactions like water splitting. Among the existing semiconductors for generating hydrogen gas by splitting water, CdS NCs are widely used due to their favorable bandgap of ~2.4 eV. However, their photocatalytic activity is limited by insufficient charge carriers and charge separation resulting from fast exciton recombination.

Recently, it has been established that advanced-shaped CdS NCs with improved charge separation can increase the efficiency of different photocatalytic redox reactions. The charge separation can also be increased by adding a metal co-catalyst which can efficiently accept electrons by electron transfer. Pt nanoparticles are the mostly used co-catalysts in CdS photocatalytic water splitting to produce higher hydrogen yields. Unfortunately, altering the dimensionality of the CdS photocatalyst and/or adding Pt co-catalysts is insufficient to address the inherent problem of fast exciton recombination due to their short lifetimes.

On this account, a team of Syracuse University researchers: Walker MacSwain, Hanjie Lin, Dr. Zhi-Jun Li, Dr. Shuya Li, Chun Chu and Professor Weiwei Zheng in collaboration with Lacie Dube, Professor Ou Chen from Brown University, and Professor Gyu Leem from the State University of New York College of Environmental Science and Forestry (SUNY-ESF) developed a new photocatalyst by doping Mn²⁺ ions into one-dimensional (1D) CdS nanorods with Pt tips. Its feasibility for photocatalytic water splitting for hydrogen gas generation was validated. Their work is currently published in the research journal, Journal of Materials Chemistry A.

The authors reported that incorporating Mn²⁺ dopants inside the 1D CdS nanorods enhanced the charge separation as it resulted in a significantly longer lifetime of dopants (~ milliseconds) than that of host excitons (~nanoseconds). This was attributed to the occurrence of two-step charge transport, including host-to-dopant energy transfer and transfer of electrons from Mn²⁺ dopants to Pt nanoparticles. The additional charge transport pathways induced by the long lifetime dopants prevented fast exciton recombination, resulting in improved photocatalytic water splitting than the undoped counterpart.

The water splitting efficiency of Mn:CdS-Pt nanorods increased substantially with the yield of hydrogen gas improved by 448%. Electrochemical impedance spectroscopy analysis revealed lower charge transport resistance after incorporating Mn²⁺ dopants with long lifetimes. It was noted that since Pt has a greater work function than CdS semiconductors, electrons were less likely to leave Pt nanoparticles to recombine on CdS. Due to the electron transfer from nanorods to the metal nanoparticles, a significant impedance decrease after tipping with Pt nanoparticles was reported.

In summary, the study successfully demonstrated the possibility of transferring electrons via Mn dopants in 1D CdS nanorods for improved photocatalytic hydrogen gas generation. The presented photocatalyst took advantage of the long lifetime of Mn²⁺ dopants to enhance the charge separation, electron transfer and hydrogen yield. The increased charge separation arises from the additional relaxation pathway resulting in efficient photocatalysis on the metal co-catalysts. In a statement to Advances in Engineering, the corresponding author Professor Weiwei Zheng said the study findings would provide new opportunities for high-performance photocatalytic redox reactions.