Preparation Of MXene-based Composites And Their Photocatalytic Properties

The problem of environmental pollution has been widely concerned with the development of the society,among which the problem of water pollution has seriously endangered the survival of human beings.Photocatalysis is considered to be a clean and efficient technology for the treatment of water pollutants,which can effectively convert light energy into chemical energy.Wide-band gap semiconductors,such as titanium dioxide(TiO₂),have become one of the most popular photocatalysts due to low price,low toxicity,and stable chemical properties.However,rapid recombination of electron-hole pairs in one-component wide-band gap semiconductors results in low photocatalytic activity of the materials,which limits their development in the field of photocatalysis.Among many cocatalysts,carbon materials have attracted extensive attention due to their unique physical and chemical properties.Therefore,the preparation of a kind of composite photocatalyst with carbon material-modified wide-band gap semiconductor has important research significance and good development prospects.In this paper,three new composite materials,Ti₃C₂/TiO₂ composites,mixed-phase Ti₃C₂/TiO₂ composites and Ag/Ti₃C₂/TiO₂composites,were prepared by simple hydrothermal method and calcination method with two-dimensional lamellar Ti₃C₂ as the substrate.The photocatalytic activity of the novel composite catalysts were tested under visible light with RhB as the active probe.The microstructure,elemental composition,internal structure and chemical states of the composites were characterized by scanning electron microscopy(FESEM),transmission electron microscopy(TEM),X-ray diffraction(XRD),Raman spectroscopy(Raman),X-ray photoelectron spectroscopy(XPS),N₂ adsorption-desorption(BET),UV-visible diffuse reflection(UV-Vis DRS),electrochemical tests and fluorescence spectroscopy(PL).In addition,the flow direction of electrons in the composite catalyst was investigated by density functional theory(DFT)calculation,which provides some theoretical guidance for the research and development of more efficient composite catalyst.The specific content is as follows:(1)Preparation of Ti₃C₂/TiO₂ composites:highly active Ti₃C₂/TiO₂ composite photocatalyst was synthesized by solvothermal method using tetra-butyl titanate(Ti(OBu)₄)as titanium source and HF as assistant.The photocatalytic performance of the Ti₃C₂/TiO₂ composites was explored for the degradation of rhodamine B(RhB)under visible light irradiation.The results showed that more highly-active(001)crystal surfaces of TiO₂ were exposed in the Ti₃C₂/TiO₂ composites assisted by HF.The composite consisted of TiO₂ microspheres and multilayer Ti₃C₂exhibited optimal photocatalytic performance with relatively large photocurrent and much lower impedances as compared to commercial TiO₂(P25)and pure TiO₂.The optimized sample exhibited a degradation rate of 93.7%in 60 min,which was 20.5-fold higher than that of P25.Pure Ti₃C₂ and adsorbed terminated Ti₃C₂ were modeled and calculated using Density Functional Theory(DFT).The results showed that the O-terminated and F-terminated on the surface of Ti₃C₂ had an effect on the conductivity of Ti₃C₂,and the presence of F-terminated inhibited the electrical conductivity of Ti₃C₂.At the same time,the work functions of pure Ti₃C₂ and adsorbed Ti₃C₂ with O-terminated and F-terminated were obtained.Since TiO₂ has a larger work function than Ti₃C₂,the photogenerated electrons of TiO₂ will flow to Ti₃C₂,thus enhancing the photocatalytic performance of Ti₃C₂/TiO₂ composite photocatalyst.(2)Preparation of mixed-phase TiO₂/Ti₃C₂ composites:High activity TiO₂(anatase/rutile)/Ti₃C₂(mixed-phase TiO₂/Ti₃C₂)composite catalyst was prepared by calcination reaction,with Ti₃C₂ as Ti source and NaBF₄ as morphology control agent.The microstructure of mixed-phase TiO₂/Ti₃C₂ composites can be adjusted by calcination temperature and assistant content.The results showed that the presence of additive NaBF₄ can promote the conversion of anatase TiO₂ to rutile TiO₂.Since the work function of anatase TiO₂ is greater than that of rutile TiO₂,Z-scheme heterojunction will be formed between anatase TiO₂ and rutile TiO₂,and finally the photogenerated electrons of mixed-phase TiO₂ flow to Ti₃C₂.Appropriate amount of rutile TiO₂ can high-efficiency charge separation and transfer and strong light absorption capacity at the composite interface.The photocatalytic degradation of the samples were measured in visible light.When the sample is prepared with 1.5m M NaBF₄ as additive and calcined at 550?for 4h,it has stronger photocatalytic activity than the samples prepared under other conditions,and the degradation rate is 93.7%within 180 min.(3)Preparation of Ag/Ti₃C₂/TiO₂ composites:Ag nanoparticles were loaded on the surface of Ti₃C₂,and then the products were hydrothermal prepared by Ag/Ti₃C₂/TiO₂ composite photocatalyst with NaBF₄ as morphology control agent.The morphology,structure and chemical states of Ag/Ti₃C₂/TiO₂ composites with different Ag contents were investigated.Among them,Ag/Ti₃C₂/TiO₂ composite photocatalyst containing 5wt%Ag had the strongest visible light photocatalytic activity compared with P25,Ti₃C₂/TiO₂ and other Ag/Ti₃C₂/TiO₂ composite materials.The degradation rate of the optimized Ag/Ti₃C₂/TiO₂ was as high as 96.6%within 180min,which was 2-fold higher than that of P25.In the Ag/Ti₃C₂/TiO₂ system,Ag and Ti₃C₂ as bimetallic systems are used to suppress the photogenerated charge recombination of TiO₂.Therefore,Ag/Ti₃C₂/TiO₂ composites have longer charge life and lower impedance compared with commercial TiO₂(P25).