Ti₃C₂T_x MXene nanosheets hybridized with bacteriochlorin-carotenoid conjugates for photocatalytic hydrogen evolution

Development of efficient photocatalysts with a wide spectral range for the photocatalytic hydrogen evolution reaction (HER) is a promising way to address the current energy and environmental crises. In particular, utilization of natural photosynthetic pigments in the artificial HER is a rational approach to explore better photocatalysts toward practical applications. In this work, bacteriochlorophyll-a (BChl-a) derivatives combined with a series of carotenoids were synthesized and deposited on the surface of Ti₃C₂T_x MXenes to prepare organic-inorganic composites as noble metal-free photocatalysts for the HER. The optical properties of six kinds of dye-sensitizers, namely, methyl bacterio-pyropheophorbide-a (Dye-1), four bacteriochlorin-carotenoid dyads (Dyes-2-5), and a triad (Dye-6) were investigated both in solutions and in solid thin films, and the composites (Dye(at)Ti₃C₂T_x) were used as photocatalysts for the HER under white light irradiation (lambda > 420 nm) in an aqueous suspension system in the presence of ascorbic acid at pH = 2.8. Among the Dyes-1-6 possessing different side chains at the C17-propionate terminal of bacteriochlorin chromophores, the Ti₃C₂T_x hybrid photocatalyst composed of Dye-4 with a siphonaxanthin analog unit showed the highest hydrogen evolution efficiency, and its best value was 5 times larger compared to that of Dye-1(at)MXene without a conjugated carotenoid chromophore. Other Dye(at)Ti₃C₂T_x composites showed enhanced H₂ evolution abilities over Dye-1(at)MXene, depending on the structural characteristics of dye-sensitizers composed of bacteriochlorin and different kinds of carotenoids. The observed high activity of Dye-4(at)MXene for the HER is attributed to the panchromatic light absorption ability of Dye-4 through the visible to near-infrared range as well as the desirable charge separation at the Dye-4(at)Ti₃C₂T_x interface. This study provides new insights into utilizing photosynthetic dyes to construct MXene hybrid structures suitable for photocatalytic water-splitting H₂ production. (authors)