Molecular Struture Modification and Photocatalytic Activiy Enhancement of Polymer Carbon Nitride Semiconductor

Author:Li Ze Hao

Supervisor:zhang zheng guo

Database:Doctor

Degree Year:2019

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Pages:129

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Photocatalysis is a potential solution for effectively alleviating the global energy crisis and environmental pollution.Developing efficient,robust and low-cost photocatalysts is essential to this technology.Although a large variety of semiconductor photocatalysts have been developed,none of them can meet the practical requirements.Polymeric carbon nitride(PCN)is a promising non-toxic,environmentally friendly,low-cost catalyst with good physicochemical stability and visible-light response.However,the photocatalytic activity of PCN is not good,one reason is that the molecular structure is not conducive to the high recombination of the photo-induced electron-hole pairs,the other is that the narrowed visible light response region.Among various strategies for improving the photocatalytic activity of PCN,molecular structure modification can solve the above problems from the molecular structure level,which is an effective way to prepare high performance PCN photocatalyst.This paper aims to enhance its photocatalytic activity by regulating the molecular structure of PCN.On the one hand,the suitable organic moleculars are copolymerized with the precursor to prepare the molecular doped PCN,on the other hand,the transfer characteristics of the charge between the heptazine rings are enhanced by modifying the bridging N in the molecular structure of PCN.Finally,modified PCN photocatalysts with high photocatalytic activities were obtained.We present the following results:1.In order to introduce a pyrimidine ring with a high electronegative of aromatic C=C bonds,2,4-diaminopyrimidine(DAP)was used to combine with urea for preparing doped PCN,and the charge transfer relationship between the doped PCN and the cocatalyst was investigated by DFT calculation.It is found that the optimal doped PCN sample,CN-DAP36,has a narrowed band gap,reduced photoluminescent emission,and longer carrier lifetime,as compared with the undoped PCN.The hydrogen evolution rate of the doped PCN is found to be 2.80 mmol/(h×g),6.09 times that of the undoped PCN(0.46 mmol/(h×g))under visible light irradiation.Furthermore,according to the theoretical calculations,the pyrimidine groups in DAP possess a stronger adsorption capacity for the Pt particles than the tri-s-triazine of PCN does,thus leading to more Pt particles deposited near the pyrimidine rings.The extending in optical absorption,the reduction in charge recombination and the enhancement in charge transport,along with the facilitation in the interfacial charge transfer from the doped PCN sample to Pt,contribute to the enhanced photocatalytic performance of the doped PCN.2.In order to further enhance the photocatalytic activity of PCN,theobromine,a compound composed of an imidazole ring and a pyrimidine ring with high electronegative,was first copolymerized with urea to prepared co-doped PCN with imidazole ring and pyrimidine ring.Experimental investigations and theoretical calculations indicate that,a narrowing in band gap and a positive shift in valence band positon happened to the theobromine doped PCN,owing to the synergistic effect between the pyrimidine ring and the imidazole ring in the theobromine molecule.Moreover,it is shown that the doping with theobromine at a suitable mass fraction makes the obtained sample exhibit decreased photoluminescent emission,enhanced photocurrent density,and reduced charge–transport resistance.Consequently,an enhancement in the photocatalytic activity for water oxidation is found for the sample,which oxygen evolution rate is 4.43 times higher than that of the undoped PCN.3.A new strategy of PCN photocatalyst with methyl-modified bridging N was developed for the key problem that the bridging N limit the transfer of photogenerated charge between different heptazine rings.Namely,the dicyandiamide was hydrothermally treated in triethanolamine(TEOA)aqueous solution,and then bridging N-modified PCN with 2D mesopores was obtained by condensation polymerization.It is found that the optimal bridging N-modified PCN sample,HCN-0.4,could not only produce a large number of lone pair electrons,but also had stronger reduction ability due to the negative shift of the CB position;although the bandgap of HCN-0.4 was increased,because of the presence of midgap state in HCN-0.4 could allow the utilization of visible light with the wavelength up to 680 nm for H2production,due to the photogenerated electrons excited from VB to the midgap state are capable of reducing protons to evolve H2 in thermodynamics.According to the DFT,the bridging N atoms contribute to the LUMO of HCN-0.4,implying the transfer of electrons from one heptazine unit to another unit through the bridging N atoms was available,therefore,the carriers recombination was greatly suppressed.The hydrogen evolution rate of HCN-0.4was found to be 31.38 mmol/(h×g),which was 56.04 times that of BCN(0.56 mmol/(h×g))under visible light irradiation.The hydrogen evolution rate of HCN-0.4/Ni(6.10 mmol/(h×g))was 10.89 times higher than that of BCN/Pt(0.56 mmol/(h×g)).Furthermore,the AQE of HCN-0.4 was measured to be 0.32%at 670 nm due to the presence of the midgap state.