Preparation Of Ag₃PO₄ And G-C₃N₄ Based Visible Light Photocatalysts And Study On The Photocatalytic Degradation Of Typical Drugs

Carbamazepine（CBZ）and sulfadiazine（SDZ）are two typical drugs used for anti-epilepsy and anti-infection,respectively.Because of the high annual consumption and nonbiodegradable nature,they are commonly detected in sewage treatment plant effluents,surface water and groundwater.CBZ can gradually damage the liver of aquatic animals through biological accumulation and it can also change the physiological function of invertebrates.SDZ promotes the formation of resistant bacteria or resistant genes of organisms.Both of the two organic compounds have adverse effects on humans,animals and ecosystems via bioaccumulation and bioamplification.Thus,an environmentally friendly and low-energy required technology is urgently needed to treat wastewater containing CBZ and SDZ.A highly efficient visible-light-driven photocatalyst Ag₃PO₄/AgI-Graphene（Ag₃PO₄/AgI-G）was synthesized through a chemical coprecipitation procedure.Characterization analyses indicated that crumby graphene sheets covered on the surface of Ag₃PO₄/AgI uniformly,and the introduction of graphene into the binary Ag₃PO₄/AgI system led to an increase in binding energy,in favor of the electron transfer at the interface.Ag₃PO₄/AgI was composed of spherical Ag₃PO₄ and laminated AgI,which was conducive to the band matching and reasonable structure configuration.The photocatalytic activity of the samples was examined by the CBZ degradation under artifcial visible light or natural sunlight irradiation.Experimental results indicated that the introduction of low mass content of graphene enhanced the photocatalytic performance of Ag₃PO₄/AgI,and the photocatalytic degradation efficiency of CBZ obtained by Ag₃PO₄/AgI-3%G（mass ratio of graphene:Ag₃PO₄/AgI=3:100）reached 93.06%within 21 min,which was much higher than the pure Ag₃PO₄（26.92%）and Ag₃PO₄/AgI（74.38%）.UV–vis diffuse reflectance spectra,photoluminescence（PL）spectra,transient photocurrent responses and electrochemical impedance spectra（EIS）of the samples were conducted to verify the high photocatalytic performance of Ag₃PO₄/AgI-3%G.In addition,possible photocatalytic degradation pathways of CBZ were proposed based on the analysis of transformation products during the reaction.The reactive species trapping experiments and Electron paramagnetic resonance（EPR）analysis demonstrated that h⁺and·O₂^-were the main active oxidant species responsible for the CBZ degradation.The photocatalytic degradation mechanism was proposed.The photoinduced metallic Ag and graphene in the Ag₃PO₄/AgI-3%G composite could promote the electrons transfer of the internal and the external channels,respectively;on the one hand,accelerating internal transmission would generate more hole（h⁺）from Ag₃PO₄;while accelerating external transmission would improve the conversion from O₂ to·O₂^-.This study not only provides a new technique for the synthesis of Ag₃PO₄-based photocatalysts with high photocatalytic activity,but also demonstrates that the Ag₃PO₄/AgI-3%G composite could be a promising photocatalyst for the treatment of waters containing CBZ.The synthesis of environmental-friendly metal-free photocatalysts has great significance in photocatalytic technology.We firstly reported the successful synthesis of in situ epitaxial growth of g-C₃N₄ on carbon dots through a facile thermal polymerization technique.Characterization and density functional theory（DFT）calculations were conducted to clarify the structure engineering and the electronic/chemical properties of the in-plane interconnected carbon dots/g-C₃N₄（C-CN）heterostructures.The heterostructure was formed by the generation of the pyridine ring structure resulted from the reaction between carbon dots and tri-s-triazine.SDZ degradation experiments indicated that the 3C-CN exhibited 3.2 times higher degradation rate for SDZ than the pristine g-C₃N₄.Besides,the 3C-CN heterostructures displayed excellent stability and reusability in five consecutive cycles.The enhanced photocatalytic activity was related to the narrowed band gap and the local electronic density of valance band and conduction band orbitals of the unique plane heterostructures,corroborated by the spectroscopic characterizations and theoretical calculations.Photogenerated holes dominated the degradation of SDZ,while·OH showed a negligible contribution.Moreover,DFT calculation succeeded to predict that the atoms with high Fukin index（f0）on SDZ molecule were more vulnerable to radicals attack.SDZ degradation pathway mainly included smiles-type rearrangement,SO₂ extrusion,ring hydroxylation and S-N bond cleavage processes.The eco-toxicity assessment revealed the generation of less toxic intermediates after photocatalysis.Our findings afford a new technique for constructing g-C₃N₄-based in-plane heterostructures with high and stable photocatalytic efficiency,and highlight the feasible application of metal-free photocatalysts in environmental remediation.To further improve the photocatalytic activity of 3C-CN,herein,we synthesized a novel g-C₃N₄-carbon dots nanosheets（C-CN-NS）photocatalyst through a facile thermal oxidation etching process.The C-CN-NS500（calcined at 500℃）exhibited 2.4 times higher photodegradation rate toward SDZ than the g-C₃N₄-carnon dots（3C-CN）.Compare to 3C-CN,the band gap of C-CN-NS500 was increased by 0.24 e V under the effect of quantum confinement,and the correspondingly valence band of C-CN-NS500 increased by 0.42 e V.This apparently promoted the oxidation potential,which was in favor of the SDZ degradation.And increased specific surface area,improved separation efficiency and prolonged contact time for C-CN-NS500 could promote the degradation of SDZ.EPR analysis and a series of radical trapping experiments showed that h⁺has the strongest effect in photocatalytic degradation,followed by·O₂^-,and·OH played a minimal role in our system.The toxicity assessment authenticated good biocompatibility and low cytotoxity of C-CN-NS500 at low concentration.The results of this work indicate C-CN-NS500 is a promising photocatalyst for degradation of refractory organic contaminant.