Controllable Synthesis And Properties Study Of CeO₂-based Micro-nano Composites

In this study,a series of CeO₂-based micro/nanocomposites with various microstructures have been constructed through the electrospinning and hydrothermal techniques.Specifically,ZIFs-induced CeO₂-based micro/nano photocatalysts,including double-tubular CeO₂/Co₃O₄composite fibers,CeO₂/Zn O composite fibers with curled edges,and ridge-like CeO₂/Zn O composite fibers,were synthesized via the electrospinning method.The hydrothermal approach was utilized to prepare CeO₂/WO_2.9 and CeO₂/In₂O₃ gas-sensing composites with oxygen-rich vacancies.Additionally,Cu-MOFs-induced spiral tubular CeO₂/Cu₂O gas-sensing composites were fabricated by using the electrospinning process.The influencing mechanisms of phase composition,microstructure,surface/interface transport characteristics,and energy band structures different material systems on photocatalytic and gas-sensing performances have been systematically investigated,and the corresponding enhanced photocatalytic and enhanced gas-sensing mechanisms have also been presented in the current work.The specific research contents are listed as follows:1.The synthesis,photocatalytic performances,and the enhanced mechanisms of ZIFs-induced CeO₂-based micro/nanofibers have been investigated.The findings indicate that the addition of suitable ZIFs particles into the precursor electrospinning solution can effectively regulate the crystal nucleation and growth process during the reactions.Consequently,it significantly changes the microstructures of CeO₂-based composites,which increases their specific surface area,and results in the excellent photocatalytic properties.The morphologies of CeO₂-based micro/nanomaterials can be changed from the belt-like structures composed of porous particles to the smooth and coiled ones,and then formed the double-tubular structures,as the adding amount of ZIF-67 increasing.Furthermore,the increased wall thickness of the double-tubular structures can be observed as the content of ZIF-67 increasing continuously.It is clear that the highest photocatalytic degradation efficiency of methylene blue（MB）can be found for Ce/Co-2（70 min,88.15%）.Compared with other samples,both the specific surface area and effective heterojunction interface of Ce/Co-2 are markedly increased,leading to the improved adsorption capacity of the organic dye molecules and the large electron/hole separation efficiency.In addition,the microstructures of CeO₂/Zn O composites can be precisely controlled by introducing ZIF-8 particles with approximately 140 nm in diameter into the precursor electrospinning solution.For example,the diameters of Ce/Zn-2 with the obviously curled edges increase to 3.9-4.0μm,along with the highest photocatalytic degradation efficiency of MB（70 min,96.59%）.It is found that the curled microstructures can generate larger surface activities and unique electron transport process,promoting the Ce³⁺/Ce⁴⁺cycling efficiency on the surface.Meanwhile,the formation of CeO₂/Zn O heterostructures contributes to the reduced recombination probability of electron/hole pairs during photocatalytic degradation.Additionally,ZIF-8 particles with a size of about 90 nm,prepared by using a room temperature precipitation method,have been introduced into the precursor electrospinning solution to induce the formation of the ridge-like structures on the surface and adjust the surface oxygen vacancy content by changing the adding amounts of ZIF-8.The photocatalytic activity of Ce/Zn-5 for MB（60 min,95.12%）is 3.54 times higher than that of pure CeO₂.The enhanced photocatalytic mechanism is mainly ascribed from the new surface/interface electron transport mechanism,which is induced by the synergistic effect of the formation of n-n heterojunctions and the unique ridge-like structures of CeO₂/Zn O composites.2.Surface oxygen vacancy CeO₂-based heterostructures have been fabricated via a hydrothermal method,and the gas-sensing performances and the enhanced mechanism of various composites have been studied.A series of CeO₂/WO_2.9 composites with tunable structure and thickness can be obtained by introducing different amounts of H₂WO₄ into the hydrothermal process.It is clear that Ce/W-0.25 exhibits a response value of 23.68 to 100ppm n-butanol at room temperature.By combining n-type CeO₂ with p-type WO_2.9,the gas-sensing response can compensate for the resistance changes of n-type CeO₂ to several targeted gases,thereby compensating for the n-type characteristics of the composites and enabling p-type response.The unique surface double oxygen vacancy engineering between CeO₂ and WO_2.9 plays a significant role in enhancing gas-sensing behavior,forming CeO₂-WO_2.9heterojunctions and the effective surface/interface transport mechanism.The oxygen vacancy on the CeO₂ surface can drive the electron transfer between WO_2.9 and CeO₂,promoting the interconversion between Ce³⁺and Ce⁴⁺.This work provides the new idea and approach for constructing a novel n-butanol room temperature sensors based on CeO₂-based heterostructures with double oxygen vacancies.Using the above method,In N₃O₉·x H₂O is employed for the substitution of W sources during the hydrothermal process,and the gas-sensing properties of CeO₂/In₂O₃ composites with oxygen-rich vacancies are investigated.It is shown that the hollow gear-like Ce/In-0.15has the highest response value of 11.1 to 100 ppm NH₃ at 45℃,obviously better than other samples.The free migration of electrons can be promoted due to the larger number of surface reactive oxygen species of Ce/In-0.15 can be obtained,as well as the coexistence of multiple valence states of CeO₂,consequently leading to the relatively high gas-sensing response and the observably accelerated response time.The coexistence and valency of Ce³⁺and Ce⁴⁺in the Ce/In-0.15 matrix can effectively increase the content of oxygen vacancy on the surface,and oxygen vacancy acts as the electron trapping center,which allows the resistance to quickly reach equilibrium and significantly reduces the recovery time of gear-like CeO₂/In₂O₃composites.3.The electrospinning method is used to prepare a spiral tubular CeO₂/Cu₂O composites,and the corresponding gas-sensing performance and the enhanced mechanism under UV light excitation are investigated.The energy band structure,state density（PDOS）,and effective mass of holes and electrons（m_e and m_h）of CeO₂ and Cu₂O are separately calculated by using density functional theory（DFT）for verifying the magnitude of carrier mobility,confirming the feasibility of improving the utilization rate of UV light by superimposing a certain amount of Cu₂O on the CeO₂ matrix.The amount of Cu component added in the precursor electrospinning solution can be used for tuning the morphology evolution of CeO₂-based composites.For example,Cu-MOFs-induced Ce/Cu-0.10 displays a typical helical tubular structure with the diameters of 210-240 nm.The formation of CeO₂-Cu₂O heterojunctions can significantly reduce the optimal operating temperatures of the gas sensors for detecting n-pentanol,irrespective of whether the material surface is irradiated by UV light.For example,the optimal operating temperature of Ce/Cu-0.10 for detecting n-pentanol is 200℃.While the response value of Ce/Cu-0.10 to n-pentanol under UV light excitation can be 2.87 times higher than that without UV excitation,indicating the better stability.Moreover,the gas selectivity of Ce/Cu-0.10 for n-pentanol is significantly enhanced under UV light excitation.The enhanced gas-sensing mechanism of CeO₂/Cu₂O composites is mainly attributed to the formation of an internal electric field between the p-n junctions of Cu₂O-CeO₂,which can reduce the probability of electron/hole recombination.Under UV light excitation,the photogenerated charge carriers on the surface of the material further increase,therefore improving the migration rate.Additionally,the unique helical tubular structure of Ce/Cu-0.10and the large specific surface area can prominently improve the gas adsorption and reaction processes.