Study On Preparation,Characterization And Related Properties Of NiO Nanosheets And Its Composites

The 3d transition metal oxides have special photoelectric properties,because of the strong correlation properties between the 3d electrons of transition metal ions.So,the research and application of this kind of materials have always been a hot topic in physics and optics.Since Ni2+ ion has a 3d electronic structure,NiO,as a wide bandgap semiconductor material(3.6 eV-4.0 eV)with the strong correlation property,is used in photocatalytic,magnetism,optics and so on.Associated with strongly electron correlated properties,the study of the photons and electrons interaction process in NiO is of great significance for revealing the dynamics of electron transition under strong optical field and expanding its application.Meanwhile,due to the existence of defects in NiO,it presents the p-type conduction characteristic.According to the design principle of surface heterostructure,it is also an important research topic for constructed p-n heterostructure composite materials to improve the photochemical properties of bulk materials.In this paper,the NiO nanosheets as the research object are focused on exploring the dynamic process of electron transition and developing the laser devices.And p-n heterostructure composite materials are constructed with TiO2 to improve the separation of photogenerated carriers and explore the photocatalytic activity.Firstly,NiO nanosheets are prepared by hydrothermal method.The structure and properties are characterized in detail,the results indicate that NiO nanosheets have broadband light absorption characteristics,including UV absorption caused by the interband transition process,visible absorption and near-infrared absorption caused by the intraband transition processes.At the same time,the growth mechanism is also studied;Second,based on the electron intraband transition arising from ground state 3A2g(F)to excited state 1Eg(D)of Ni2+ 3d spin-orbit splitting,the nonlinear optical response of NiO nanosheet film is studied at the 720 nm wavelength,and the feasibility of the obtained mode-locked visible laser is verified by theoretical calculation.In addition,based on the study of visible waveband nonlinear optical performance of the NiO nanosheets,according to electron interband transition caused by the process from of O2-2p to Ni2+3d and the electron intraband transition arising from ground state 3A2g(F)to excited state 3T2g(F)of Ni2+ 3d spin-orbit splitting,the nonlinear optical properties and kinetic processes generated by different electron transition processes of NiO nanosheet films are studied by a 1064 nm pulsed laser.The application prospect is also discussed in the optical modulator.With a sample optimized for saturable absorption,all-solid-state pulsed lasers with wavelengths of 640.3 nm,1.06 ?m and 1.34 ?m are successfully realized.Finally,based on p-type conductivity of NiO,NiO nanosheet/TiO2 nanorod-constructed p-n heterostructures are successfully fabricated via the hydrothermal route,the photocatalytic performances of the p-n heterostructure composite materials are studied.The specific research work are as follows:1.The NiO nanosheet is prepared by using the hydrothermal route,and a growth mechanism of NiO nanosheets is proposed.By characterization of the morphology and structure of NiO,the NiO has sheet morphology and cubic crystal structure.At the same time,NiO nanosheets have a porosity structure and large specific surface area,which are beneficial to optical absorption.By the characterization of UV-Vis-NIR absorption spectrum,we find that NiO nanosheets have three absorption bands:(1)the interband absorption edge of the nanosheets occurs at about 360 nm,which arises from a transition from the O2-2p to Ni2+ 3d;(2)the absorption band recorded in the range 600-800 nm belongs to the electron intraband transition process of 3A2g(F)? 1Eg(D)in terms of the Ni2+ 3d spin-orbit splitting of NiO in the visible region;(3)the electron intraband transition process caused by the 3A2g(F)?3T2g(F)throughout the NIR region are observed in the range between 1000 and 1400 nm.2.Based on the electron intraband transition process arising from ground state 3A2g(F)to excited state 1Eg(D)of Ni2+ 3d spin-orbit splitting,the fluence-dependent nonlinear optical response of NiO nanosheet film is characterized at the 720 nm wavelength.The results show that NiO has a saturation absorption effect in the visible region,the higher modulation depth(24.59%)and lower saturation fluence(4.41 nJ cm-2)are achieved,and the feasibility of the obtained mode-locked visible laser is verified by theoretical calculation.In addition,according to electron interband transition caused by the process from of O2-2p to Ni2+ 3d and the electron intraband transition arising from ground state 3A2g(F)to excited state 3T2g(F)of Ni2+ 3d spin-orbit splitting,the high-order nonlinear optical properties generated by different electron transition processes of NiO nanosheet films are explored by using the Z-scan technique and a 1064 nm pulsed laser.We find that the optical response and kinetic processes of the different NiO samples are distinctive by the thickness-dependence electron transitions,including saturable absorption of intraband transitions,three-photon absorption of interband transitions and emission,and saturable absorption of three-photon absorption.The application prospect is also discussed in optical switches,optical limiters and up-conversion luminescence.3.Based on the study of the visible waveband and near-infrared waveband nonlinear optical properties of the NiO nanosheets,all-solid-state pulsed lasers with wavelengths of 640.3 nm,1.06 ?m and 1.34 ?m are successfully realized by taking advantage of the saturable absorption properties of the NiO nanosheet film and laser resonator design,the pulse width is 30 ps,85 ps and 34 ps,respectively.At the same time,we also study the passively Q-switched laser at the wavelength of 1.34 ?m,achieving the shortest pulse width of 678 ns and the biggest repetition rate of 166 kHz.These results show that NiO nanosheets can be used as a saturable absorber to realize broadband laser modulation and enrich the types of visible waveband and near-infrared waveband optical modulator.Meanwhile,the study of electron transition process generated by 3d spin-orbit splitting in 3d transition metal oxides should be helpful for the development of ultrafast photonics and related devices design.4.According to the p-type conductivity of NiO and the design principle of surface heterostructure of nanomaterials,NiO nanosheet/TiO2 nanorod-constructed p-n heterostructure composite materials are successfully fabricated via the hydrothermal route.By the morphology characterization of p-n heterostructures,the NiO is not easy to load and grow on the smooth surface of TiO2 nanorods without acid treatment.In order to solve the problem of uneven distribution of NiO,the surface of the TiO2 nanorods becomes rougher through acid treatment,which can provides nucleation sites for the nucleation and growth of NiO nanosheets.The UV-Vis diffuse reflectance spectra show that the spectra of p-n hetero structure nanocomposites are redshifted,which is beneficial to optical absorption and broadened the absorption wavelength range.The photocatalytic performances of the as-prepared samples is evaluated by the photodegradation of methyl orange(MO).The effects of different photocatalyst,photocatalytic time and reutilization on photocatalytic degradation of MO are studied,the kinetic analysis of the MO degradation is also performed.Compared to other samples,the NiO nanosheet/acid-corroded TiO2 nanorod heterostructure exhibits the best photocatalytic activity,degradation rate and reutilization.At the same time,the photocatalytic mechanism of the heterostructures is proposed.A local electric field is built in the interface between the NiO nanosheets and the TiO2 nanorods,which can promote the separation of photogenerated electron-hole pairs and improve the efficiency of photocatalytic degradation of organic pollutants.