Band Engineering Of Graphitic Carbon Nitride-Based Materials For Visible Light Photocatalysis

Photocatalysis is an important strategy to solve the current environmental and energy problems.The research on traditional photocatalytic technology is mainly focused on ultraviolet-light-induced photocatalysis.Since the energy of ultraviolet light accounts for only 5%of the solar spectrum,the research of visible-light-driven(VLD)photocatalysis is the key to improve utilization of solar light.Graphitic carbon nitride has drawn lots of attention owing to high stability,environmental friendliness,simple preparation,and rich raw material reserves.However,the application of bulk graphitic carbon nitride is limited by low carrier mobility,high recombination rate of photogenerated electron-hole pairs,and small light absorption range.Thus,it is urgent to improve VLD photocatalytic activity of graphitic carbon nitride effectively.In this paper,we modulated the band structure of graphitic carbon nitride to apply to different VLD photocatalysis systems.Three kinds of graphitic carbon nitride-based VLD photocatalysts were synthesized:oxygen-doped porous graphitic carbon nitride(OA-g-C3N4),black phosphorus nanosheets modified graphitic carbon nitride nanosheets(BPCNS),and highly crystallized triazine-modified graphitic carbon nitride with ample(CNV).The VLD photocatalytic activity of these photocatalysts was investigated by degradation of bisphenol A(BPA)and nitrogen fixation.OA-g-C3N4 was synthesized by simple polymerization of cheap oxalic acid and urea.The light absorption ranges of OA-g-C3N4 were extended,the maximum absorption wavelength was up to 750 nm.In addition,the separation rate of photogenerated electron-hole pairs in OA-g-C3N4 was significantly improved.The band structure of OA-g-C3N4 changed compared with that of bulk graphitic carbon nitride.In detail,valence band(VB)shifted down to 2.46 eV,leading to the enhanced oxidation activity of photogenerated holes.The holes on the VB could react with water to form hydroxyl radicals.A metal-free semiconductor-black phosphorus nanosheets was used as cocatalyst to synthesize BPCNS through an ultrasonic-calcination method.The BPCNS photocatalysts were applied to VLD photocatalytic nitrogen fixation.The BPCNS showed a two-dimensional nanosheet structure with a thickness of about 4-5 nm.Stable C-P bonds were formed with the calcination of BPCNS.It was found that the photogenerated carriers could be efficiently separated through C-P bonds.In addition,the formation of C-P bonds reduced the stability of the ?-conjugated system in graphitic carbon nitride,which facilitated the excitation of BPCNS.Moreover,the formation of C-P bonds also changed the band structure of graphitic carbon nitride.Besides,chemical stability of BPCNS was significantly improved.The BPCNS only remained stable after exposed in the air for 30 days.In addition to the high chemical stability,BPCNS also had high catalytic stability.CNV was prepared by the calcination of melamine in vacuum.The results showed that there was a large amount of triazine in CNV.Compared with bulk graphitic carbon nitride,the band gap of CNV was widened.Moreover,the electron distribution in CNV was changed due to the distortion of structure.The CB position up-shifted by 0.63 eV and a new mid gap was formed.CNV can be excited by visible light with long wavelength(?750nm)with the presence of the mid gap.The photo-induced electron hole pairs could separate efficiently due to the triazine/heptazine interfaces.The VLD photocatalytic nitrogen fixation rate of CNV was 16 times as high as that of graphitic carbon nitride in distilled water.CNV showed photocatalytic nitrogen fixation activity under near-infrared light irradiation(700 nm).