Chlorophyll-sensitized Titania Nanoparticles For Photocatalytic Hydrogen Evaluation

Hydrogen energy is an ideal sustainable new energy source because of its high efficiency,environmental protection,storability,and transportability and other advantages.Semiconductor photocatalytic hydrogen production system is an important system that uses solar energy to split water for hydrogen production,and dye-sensitized semiconductor photocatalytic hydrogen production is one of the important ways of semiconductor photocatalytic hydrogen production.Although there have been many reports on the dye-sensitized semiconductor photocatalytic hydrogen production,the dyes that are the core of the system still suffer from problems such as inefficient use of sunlight,high production costs,and environmentally unfriendly.Chlorophyll dyes have the advantages of rich sources,high extinction coefficient,environmentally friendly,easy extraction and modification,etc.They are potential photosensitizer for hydrogen production.Ti O₂has the characteristics of high stability,high catalytic activity,non-toxicity,low economic cost,etc.It has been widely used in the semiconductor photocatalytic hydrogen production system.However,Ti O₂ has a wide band gap,so it can only show photocatalytic activity in the ultraviolet region.Dye-sensitized TiO₂ is an effective way to make it show catalytic activity in visible light.In this paper,we use the designed chlorophyll dyes and panchromatic chlorophyll-indoline-dyad dyes as photosensitizers to sensitize TiO₂ for photocatalytic hydrogen production.The main contents of the study are as follows:1.We used three kinds of chlorophyll-a derivatives?Chl-1,Chl-2,and Chl-3?located on carboxyl anchoring groups of C3 or C7 position band on tetrapyrrol macrocycle of chlorophyll molecules to sensitize TiO2 for hydrogen production by photocatalytic water splitting in visible light.The anchoring group of the dye is an important"bridge"linking semiconductors and plays an important role in the injection of electrons from dyes to semiconductors.We characterized the light-absorbing properties of the dyes and calculated the energy levels of the dyes by UV-visible absorption spectroscopy and electrochemical cyclic voltammetry curve.And we characterized the charge transfer between the dye and TiO₂ through steady-state and transient fluorescence.In the experiment,ascorbic acid was used as sacrificial agent,and Pt as the cocatalyst,under the visible light of?>400nm,the Ch-2 dye exhibited the highest performance of photosensitized hydrogen production:1577?mol/g,while the sensitized hydrogen production activities of Chl-1and Chl-3were 1074?mol/g and 1149?mol/g,respectively.It is worth noting that these Chls dyes still have photosensitized hydrogen production activities under visible light of?>600nm.We have found that both the carboxyl position and quantity of dyes affect the charge transport between the dye and Ti O₂,and then affect the activity of hydrogen production.The photocatalytic hydrogen production activity of photocatalysts under the panchromatic spectrum is always full of challenges.Therefore,we designed a new organic Dyad,which is composed of an indoline with strong absorption in the green light area and a chlorophyll dye with strong absorption in blue-violet light and red light areas.The Dyad is also used to sensitize TiO₂ for hydrogen production.The separate Chl and Ind dyes are used as a comparative test.Under the light of?>400nm,Dyad exhibited the highest photosensitized hydrogen production activity of4176?mol/g.Under the same conditions,the photosensitized hydrogen production activity of separate Chl or Ind was 1067 or 1204?mol/g,respectively.The interface charge transfer and recombination processes between Dyad,Chl,Ind and Ti O₂ were studied through photocurrent response and electrochemical impedance and other tests.It was found that the high hydrogen production activity of Dyad is not only due to excellent light absorption capability,but also due to the effective charge transfer between dyes and TiO₂.