Potocatalytic Hydrogen Evolution Based On Chlorophyll-derivative/Ti₃C₂T_x MXene Composites

With the rapid development of the global economy and increasing industrialization,the search for sustainable energy will become one of the most important issues affecting human survival and development in the near future.As an abundant,clean and renewable energy source,hydrogen energy has been considered as one of the most important potential candidates to meet energy consumption needs.The solar-driven photocatalytic water splitting for hydrogen production is one of the most promising strategies for converting solar energy into chemical energy.However,after decades of efforts,the practical application of photocatalytic hydrogen production is still hampered by insufficient visible light absorption,low hydrogen production capacity and quantum efficiency due to rapid recombination of photogenerated electron-hole pairs,and sluggish kinetics of hydrogen precipitation reaction（HER）on the catalyst surface.Compared with other synthetic semiconductor photocatalysts,natural chlorophyll has excellent photochemical and photophysical properties.For example,excellent solar light absorption,adjustable molecular structure,abundant raw material sources,and environmental friendly properties.Compared with other traditional noble metal co-catalysts,Ti₃C₂T_x MXene,an emerged two-dimensional layered material,has been widely reported to show efficient photocatalytic activity,which is mainly attributed to the hydrophilic groups（-OH and-O）on the surface of Ti₃C₂T_x MXene that can easily establish strong connections with various semiconductors.In addition,Ti₃C₂T_x Mxene has a tunable work function and excellent electronic conductivity,which facilitates the light-induced charge transfer from the semiconductor to Ti₃C₂T_x MXene.In this study,we constructed the chlorophyll-based noble metal-free photocatalytic system Chls/Ti₃C₂T_x using chlorophyll derivative（Chls）as a photocatalyst for hydrogen production by photocatalytic water splitting and the non noble metal Ti₃C₂T_x MXene as a co-catalyst for electron capture and carried out a series of investigation.The main details of the work are as follows:First,we fabricated Chl/Ti₃C₂T_x organic-inorganic composites with a two-dimensional accordion-like morphology by depositing chlorophyll derivatives,i.e.,zinc methyl 3-devinyl-3-hydroxymethyl-pyropheophorbide a（Chl）supramolecular aggregates,on the surface of Ti₃C₂T_x MXene,which was inspired by the chlorosome antenna system containing self-aggregated chlorophyll in natural photosynthesis.The composite was constructed and a series of investigations were carried out on chlorophyll-based noble metal-free photocatalytic system Chl/Ti₃C₂T_x by using chlorophyll aggregates as organic semiconductors for photocatalytic splitting of water for hydrogen production and noble metal-free Ti₃C₂T_x MXene as a co-catalyst for electron capture.First,we characterized the structural and morphological features of the Chl/Ti₃C₂T_xcomposites by XRD,FTIR spectra,and SEM.At the same time,we also investigated the light absorption capacity and aggregation state of Chl/Ti₃C₂T_x composites by electron absorption spectroscopy.Moreover,we also tested and analyzed the photocatalytic hydrogen production of Chl/Ti₃C₂T_x composites under visible light irradiation.The results showed that different Chl and Ti₃C₂T_x MXene composite ratios had a large effect on chlorophyll hydrogen production,in which the highest photocatalytic hydrogen production performance of 52±5μmol/h/g_cat was achieved at2%Chl in the Chl/Ti₃C₂T_x composite.Then,in order to analyze the effect of different Chl and Ti₃C₂T_x MXene composite ratios to the electron-hole pair separation efficiency of Chl/Ti₃C₂T_xcomposites,we examined the Chl/Ti₃C₂T_x composite by photocurrent and impedance tests.The results indicate that such high HER activity is mainly attributed to the high-efficiency light-harvesting ability of the Chl aggregates and the remarkably efficient exciton transfer,and the resulting charge separation at the Chl/Ti₃C₂T_x interface.In the previous section,we successfully combined Chl aggregates with Ti₃C₂T_x MXene to fabricate the chlorophyll-based noble metal-free photocatalytic system Chl/Ti₃C₂T_x and used it for photocatalytic hydrogen production,which demonstrated the feasibility of hydrogen generation by using the exciton transfer in Chl aggregates to transfer charge to the Ti₃C₂T_x surface to combine with H⁺in water.However,the photocatalytic performance of this system is still insufficient and needs further improvement.In this work,inspired by the Z-scheme process of natural oxygen-producing photosynthesis,we employ two Chls with different physical characteristics,i.e.,zinc methyl 3-devinyl-3-hydroxymethyl-pyropheophorbide a（Chl-1）acts as a PSI simulator for accepting electrons and methyl 13¹-deoxo-13¹-dicyanomethylene-pyropheophorbide a（Chl-2）acts as a PSII simulator for donating electrons.Furthermore,we prepared chlorophyll-based organic heterojunction noble metal-free photocatalyst Chl-1@Chl-2@Ti₃C₂T_x through a simple alternating deposition process as well as used it for photocatalytic hydrogen production.Compared with normal photocatalysts,the organic heterojunction photocatalyst Chl-1@Chl-2@Ti₃C₂T_xdisplayed a much higher photocatalytic performance than Chl-1@Ti₃C₂T_x or Chl-2@Ti₃C₂T_xphotocatalysts.Meanwhile,to further investigate the reasons for the improved photocatalytic performance,we characterized the structure,morphology,light absorption capacity and carrier separation efficiency.