Structures And Photoreaction Mechanism Of TiO₂ Investigated By Solid-state NMR

Driven by the decrease of fossil fuel resources and the environmental concerns,the search for new clean and renewable energy technologies is urgent on the research agendas of many research and development communities.In particular,utilization of solar energy for hydrogen production and water/air decontamination has attracted extensive research interest.Titanium dioxide(TiO2)has been extensively studied and recognized as one of the most promising photocatalysts.But,the wide band gap of pure TiO2 renders it only active in the ultraviolet irradiation range and the rapid combination of photogenerated electrons and holes reduces its photocatalytic activity.In order to improve the solar-light absorption and the photocatalytic efficiency of TiO2,numerous methods have been used to modify the structure of TiO2 photocatalysts.However,because the structure-property relationship of TiO2-based photocatalysts is still not well understood,the structural modification of TiO2 photocatalyst lacks effective guidance and has certain blindness.Herein,the relationship between the structure of photocatalyst and the photocatalytic activity of the TiO2-based photocatalysts B-TiO2-x and C/TiO2-x were investigated by Solid-State NMR,EPR etc.The results obtained would not only facilitate a better understanding of the photocatalytic mechanism at the atomic level but also be helpful for rational design of highly efficient titania-based photocatalysts.(1)The photocatalysts with abundant and stable TI3+ species in the surface layer were obtatined by boron doping for efficient utilization of solar irradiation.The resultant B-TiO2-x photocatalysts exhibit extremely high and stable solar-driven photocatalytic activity toward hydrogen production.The origin of the solar-light activity enhancement in the B-TiO2-x photocatalysts has been thoroughly investigated by various experimental techniques such as XPS,EPR,TEM,Raman,Solid-State NMR and density functional theory(DFT)calculations.The unique structure invoked by presence of sufficient interstitial boron atoms can lead to substantial variations in density of states of B-TiO2-x,which not only significantly narrow the band gap of TiO2 to improve its visible-light absorption,but also promote the photogenerated electron mobility to enhance its solar-light photocatalytic activity.As such,based on our experimental and theorerical results,a schematic illustration of the DOS of B-Ti02-x was proposed and the structure-activity relationship of B-TiO2-x photocatalysts was confirmed.(2)C/TiO2-x photocatalysts were synthesized by vacuum anealing method and HCl treatment.Abundant and stable Ti3+ species were presented in the core of the C/TiO2-x photocatalysts.The detailed structure of C/TiO2-x photocatalysts was investigated by XRD,EPR,XPS and other experimental techniques.It was found that the solar-light absorption and photocatalytic degradation of RhB activity was enhanced significantly by Ti3+ species.Meanwhile,the carbon species in photocatalysts played an important role in the photocatalytic reaction.The kind of surface carbon species on C/TiO2-x photocatalyst investigated by the 13C MAS NMR technique revealed that the carboxylate species on the photocatalysts suppressed the enhancement of the photoreaction activity.(3)The detailed structure-activity relationship of surface hydroxyl groups(Ti-OH)and adsorbed water(H2O)on the TiO2 surface should be the key to clarify the photogenerated hole(h+)transfer mechanism for photocatalytic water splitting,which however is still not well understood.One-and two-dimensional 1H Solid-State NMR techniques were employed to identify surface hydroxyl groups,adsorbed water molecules as well as their spatial proximity/interaction in Ti02 photocatalysts.It was found that although the two different types of Ti-OH(bridging hydroxyl(OHB)and terminal hydroxyl(OHT)groups)were present on the Ti02 surface,only the former is in close spatial proximity to adsorbed H2O,forming hydrated OHB.In-situ 1H and 13C NMR studies of the photocatalytic reaction on TiO2 with different Ti-OH group and different H2O loading illustrated that the enhanced activity was closely correlated to the amount of hydrated OHB groups.To gain insight into the role of hydrated OHB groups in the h+ transfer process,in-situ EPR experiments were performed on TiO2 with variable H2O loading,which revealed that the hydrated OHB groups offer a channel for the transfer of photogenerated hole in the photocatalytic reaction,and the adsorbed H2O could make a synergistic effect with neighboring OHB group to facilitate the formation and evolution of active paramagnetic intermediates.On the basis of experimental observations,the detailed photocatalytic mechanism of water splitting on the surface of TiO2 was proposed.