MXene-based Functional Nanomaterials:Synthesis And Organic Contaminant Removal From Water

Due to the organics-polluted wastewater is complex and diverse and it is very difficult to remove,the synthesis of new high-efficiency catalysts for degrading organic contaminants in water have always been considered an effective treatment method for water pollution control.MXenes is a new type of two-dimensional transition metal carbide or nitride nanomaterial,which has a graphene-like layered structure,possesses a high carrier migration rate,and shows an excellent mechanics,optoelectronics,photothermal conversion and adsorption properties.MXene(Ti₃C₂)as a member of MXenes family and composed of titanium and carbon,provides a good natural platform for the composite of titanium dioxide and carbon-based materials.Moreover,the combination of MXene and suitable narrow-gap semiconductors can obtain visible-light photocatalytic activity,facilitate the separation of photo-generated carriers,effectively inhibit photo-generated electron-hole recombination,and thus improve the activity of the photocatalyst.In this study,MXene-based catalysts with binary,ternary,and quaternary heterojunction structures were synthesized by using Ti₃C₂ as raw material,and their applications in the field of advanced oxidation process,such as ultraviolet and visible light photocatalysis,and persulfate(PS)activation were investigated.The charge transfer mechanism of MXene-based catalysts in the photocatalytic process and PS activation,and the photocatalytic degradation mechanism of organic contaminants were demonstrated.The degradation performance of the four synthesized materials were compared by degrading SMZ in two real surface water samples and the applicability of the four materials in real wastewater treatment was also discussed.This study could provide new materials and technologies for water pollution control with good application prospects.The main conclusions are as follows:(1)The binary heterojunction structure of i N-Ti₃C₂/TiO₂ composites were synthesized by in situ calcining isopropylamine(i PA)modified Ti₃C₂,which showed a good UV photocatalytic performance for the degradation of methylene blue(MB)from water.During the four cycles of use,the photocatalytic degradation removal rate of MB by i N-Ti₃C₂/TiO₂ kept higher than 90%.The main mechanism was the nitrogen doping enhanced the conductivity of Ti₃C₂,thereby enhanced the photoelectrochemical performance of i N-Ti₃C₂/TiO₂,which was conducive to the separation of photogenerated carriers;the produced·OH was the most important active radical for the photocatalytic degradation of MB.(2)Bi OI/(001)TiO₂/Ti₃C₂(BTT)composites with ternary heterojunction structure were synthesized by in situ growing Bi OI on the surface of(001)TiO₂/Ti₃C₂ synthesized by hydrothermal oxidizing Ti₃C₂.BTT could completely degraded Rhodamine B(Rh B)in water within 80 minutes under visible light irradiation and the photocatalytic degradation efficiency remained stable in the four cycles reaction.The main mechanism was that the formated p-n and Schottky double heterojunction among Bi OI,(001)TiO₂ and Ti₃C₂ enhanced the separation efficiency of photogenerated electron-hole pairs,which promoted the generation of·O₂^-for the degradation of Rh B.(3)Quaternary heterojunction structure of Bi₂O₃/C₃N₄/TiO₂@C(BCTC)composite catalysts were synthesized by calcining Bi(NO₃)₃·5H₂O,urea,and Ti₃C₂.BCTC could completely degraded sulfamethoxazole(SMZ)form water within 100 minutes in the initial p H of the solution is 5 and under visible light irradiation,the photocatalytic degradation efficiency remained stable in the four cycles reaction.The mainly mechanism was that the formed p-n and type-?double heterojunction among Bi₂O₃,C₃N₄,and TiO₂,and the carbon layer generated by the calcination of Ti₃C₂ greatly enhancing the separation efficiency of photogenerated electron-hole pairs and the generation of h⁺for the photocatalytic degradation of SMZ.(4)The N/S-TiO₂@C catalyst was synthesized by using Ti₃C₂ as raw material,cetyltrimethylammonium bromide as nitrogen source,and sulfur powder as sulfur source via in situ oxidation method.N/S-TiO₂@C could efficiently activate PS and completely degraded SMZ from water within 30 min in the initial p H of the solution is 7 and under visible light irradiation.The main mechanism was that the co-doping of nitrogen and sulfur in TiO₂ provided N/S-TiO₂@C visible light photocatalytic activity and extended the lifetime of photo-generated electrons;the photogenerated electrons from N/S-TiO₂@C could generate·OH and·O₂^-;at the same time,photogenerated electrons could activate PS to generate higher active SO₄·^-;In addition,the defects generated by the nitrogen and sulfur co-doped carbon layer could react with S₂O₈^2-to form ¹O₂.These active radicals jointly participated in the degradation of SMZ.(5)Compared with reported relevant materials,the synthesized four materials had stronger capabilities to improve the catalytic performance of active components,i.e.,TiO₂,Bi OI,Bi₂O₃,and PS in the four materials.The two real surface water samples had quite different physicochemical properties and lowered the degradation rate of the target pollutants by the four materials compared with ultrapure water.The pollutant removal rates in the surface water samples were still up to 58-81%,indicating the high applicability of the four materials in wastewater treatment.Among the four materials,N/S-TiO₂@C activated PS had the best degradation performance for SMZ under visible light irradiation.