Regulation Of Energy Band Structure Of Titanium Dioxide And Its Application In NADH Regeneration

Titanium dioxide attracted great attention in many applications including photocatalysis and photoelectric and photochemical conversions due to its excellent photophysical properties,nontoxic,corrosion resistance and photo/chemical stabilities.However,intrinsic defections of TiO₂ including wide band gap and high recombination of photoinduced electron-hole pairs limit its further application.Many efforts such as element doping,dye sensitization and hybridization with other semiconductors or metals had been devoted to solve such problems,however,it is not satisfied in photocatalytic efficiency although prominent progresses had been made in extending the light absorption and suppressing recombination of charges recently.Impurity level can be formed within the band gap by element doping,which plays a critical role in improving the visible light response.As is known,the photocatalytic properties of semiconductors are determined by the position of band position in addition to the band gap.For example,the electrons in conduction band（CB）may initiate the reduction reaction,and the reduction capability is determined by the position of CB;the holes in the VB involve the oxidation reaction,and the oxidation capability is determined by the position of VB.Therefore,it is highly desirable to prepare novel titanium dioxide with controllable band positon and narrowed band gap either in photocatalysis and photoelectrical/photochemical conversions.Till now,it is still a great challenge to modulate the band position especially the upward shift of the conduction band.Nitrogen doped titanium dioxide was prepared via a simple sol-gel process using Pluronic F127 as template and amino acid as nitrogen dopant.The prepared titanium dioxide exhibited enhanced reduction potential,sensitive visible light response and photocatalytic ability,and its band position,band gap and crystal structure of the prepared titanium dioxide can be regulated by the change of nitrogen dopant.In the presence of the N-doped TiO₂,visible-light-driven regeneration of NADH can be fulfilled independent of any electron mediator.This sheds new light for the design and construction of novel semiconductors with tunable energy band structure,and provides new ideas for the photomediated regeneration of NADH via an electron mediator free strategy.1)The nitrogen doped TiO₂ was prepared using glycine as nitrogen dopant,and characterized by high resolution transmission electron microscope（HRTEM）,scanning transmission electron microscope（STEM）,X ray photoelectron energy spectra（XPS）,X ray diffraction（XRD）and other measurements.The results of HRTEM,STEM and XPS indicated that nitrogen atoms entered into the lattice of TiO₂at both substitutional and interstitial positions,while carbon existed as films on the surface of the TiO₂ nanoparticles,and then labeled as N-TiO₂@C.The XRD and Mott–Schottky analysis of the N-TiO₂@C demonstrated that nitrogen doping didn’t change its anatase structure and n-type semiconductor.Interestingly,for the glycine doped samples,upward shift of the conduction band,narrowed band gap and creation of oxygen vocation can be observed.These novel structure endow them with enhanced reduction potential,high visible light response and excellent separation and transfer properties of charges.The N-TiO₂@C exhibited excellent photocatalytic abilities in NADH regeneration,in which the conversion of NADH reached over than 70%in the absence of any electron mediator,much greater than that of undoped TiO₂（11.3%）,P25（15.3%）and previous reports（less than 50%）.The prepared NADH can be used in the enzymatic synthesis of methanol,indicating that it is enzymatically active 1,4-NADH.Additionally,the effect of template,dopant and calcination perameters were also discussed.Concentration of F127 affected the size and surface area of the product,while dosage of glycine plays a key role in the band structure and resulted photocatalytic properties.the crystal of the product increased with the increase of calcination temperature and extension of calcination time but at the expense of the decrease of nitrogen doping content.In one word,the N-TiO₂@C with desired integrated performance can be obtained using 1M of F127 as template and 0.5 M of glycine as the dopant,and calcination at400℃for 4 hours.2)Another kind of nitrogen doped titanium dioxide（N-TiO₂）was prepared using lysine as the dopant via a similar sol-gel process with the glycine doped titanium dioxide（N-TiO₂@C）,and characterized using HRTEM,STEM,XRD,XPS and electrochemical station and other measurements.Nitrogen atom incorporated into the N-TiO₂ lattices at both substitutional and interstitial positions,resulting in elevation of conduction band and narrow of band gap.Different with the N-TiO₂@C,there are short rod-like nanostructures in addition to nanoparticles.More interestingly,there are anatase and rutile phase in the short rod-like structures,and the content of short rod-like structures and the ratio of anatase and rutile can be well regulated by the change of lysine concentration.the N-TiO₂ exhibited extremely high photocatalytic activities attributed to its mixed crystal and special energy band structure.The conversion of NADH reached over than 90%independent of electron mediator.Additionally,the N-TiO₂ also showed great potential in the degradation of organic compounds.Greater than90%of 4-chlorine phenol was removed in the presence of the N-TiO₂ under visible light irradiation.The results in this paper provide new ideas in the design and construction of advanced semiconductors with controllable energy band and crystal structure,and promote the advancement of highly efficient utilization of solar energy,photocatalytic production of organic chemicals and fixation of CO₂ via a coupled photocatalytic and enzymatic strategy.