Perparation And Performance Research Of Novel Bi₄Ti₃O₁₂ And G-C₃N₄ Visible-light Photocatalysts

Semiconductor photocatalysis has been extensively researched for the applications in degradation,transformation and mineralization of environmental pollutants,as well as the conversion of solar energy.It is widely considered by the scientific community to be an effective solution to the environmental pollution and energy shortage problems.Two novel visible responsive photocatalysts were studied in this paper,namely ferroelectric material barium titanate(Bi₄Ti₃O₁₂)and non-metal organic semiconductor graphite carbon nitride?g-C₃N₄?.Due to the inherent limitations of barium titanate and graphite phase carbon nitride,the limited absorption capacity of solar energy and the high recombination rate of photogenerated carriers,so the catalytic performance under visible light is poor.From the aspects of improving the photoexcitation of semiconductor and suppressing photogenerated electron-hole pair recombi-nation,a series of simple modification methods are adopted to prepare high-efficiency photocatalyst.The main research contents are as follows:?1?Single-crystalline Bi₄Ti₃O₁₂ nanosheets with the exposed{001}facets have been successfully prepared using a novel self-catalyzed reaction route,which involves a fast reaction of Bi₂O₃ and TiO₂ at 280°C with the critical assistance of tartaric acid.The as-obtained optimum nanosheet photocatalyst exhibits high visible light photodegradation of methyl orange,indicat-ed by the 3.92 times higher efficiency than that of the Bi₄Ti₃O₁₂ nanosheets prepared by a widely acknowledged molten salt method.It is therefore concluded that the enhanced photo-catalytic activity of the asprepared catalyst is the synergistic effects of oxygen vacancies that increases the separation efficiency of the photo-generated carriers and the band gap narrowing that broadens the absorption edge of light.?2?Bi₄Ti₃O_12-x nanosheet photocatalysts with abundant oxygen vacancies are fabricated by a facile solid-state chemical reduction method for the first time.This method is simple in operation,has short reaction time,and can be conducted at mild temperatures?300⁴00??.oxygen vacancies can be adjusted by tuning the reduction reaction conditions.The optimal Bi₄Ti₃O_12-x is the sample undergoing the reduction treatment at 350?C for 60 min and it affords a hydrogen evolution rate of 129?mol·g^-1·h^-1 under visible-light irradiation,which is about 3.4times that of the pristine Bi₄Ti₃O₁₂.The surface oxygen vacancies states result in the up-shift of the valence band and the narrowing of the band gap.Such energy level structure variation helps promote the separation of photo-generated electron-hole pairs thus leading to enhancement in the visible-light photocatalytic hydrogen evolution.?3?Graphitic carbon nitride with nitrogen defects(g-C₃N_4-x)is prepared by a facile and effective solid-state chemical reduction technique at mild temperature conditions.The cyano groups and nitrogen vacancies,as evidenced by electron paramagnetic resonance,X-ray photoelectron spectrometer,Fourier transform infrared spectra and Solid-state ¹³C MAS NMR spectra,are controllable via adjusting chemical reduction temperature.The maximum H₂evolution rate of 3068?mol·g^-1·h^-1 is achieved with g-C₃N_4-x after chemical reduction treatment at 400?for 1 h,which is 4.85 times that of the pristine g-C₃N₄.It is discovered that nitrogen defects can result in both the up-shift of the valance band and the down-shift of the conduction band,which benefit the absorption of longer wavelength photons and trapping of the photoinduced electrons,therefore reducing the recombination losses of the generated carriers.?4?The oxygen-doped g-C₃N₄ nanotubes were successfully prepared by the hydrothermal-calcination two-step method with the aid of citric acid.The morphology of carbon nitride is affected by the amount of citric acid and hydrothermal time.We propose a synthetic mechanism of citric acid-induced formation of rod-like supramolecular intermediates and carbon nanotubes.The hydrogen evolution rate of the oxygen-doped g-C₃N₄ nanotube photocatalyst reached 3692?mol·g^-1·h^-1 under visible light irradiation.A good tubular structure increases the specific surface area,shortens the migration distance of photogenerated carriers,and promotes the transfer and separation efficiency of photogenerated electron-hole pairs.The oxygen-doping optimizes the electronic band structure of carbon nitride,causing the up-shift of the valance band and the down-shift of the conduction band.The band gap is reduced to broaden the photoresponse range,thereby improving the photocatalytic activity.