Effects Of Inorganic Anion/Cation On The Structure And VIS-NIR Spectral Characteristics Of Layered Double Hydroxide Material

Layered double hydroxides（LDHs）are spectrally tunable materials whose spectral features in the visible-near-infrared band change with the type of doping ions and have promising applications in the fields of spectral stealth and visible-light photocatalytic degradation.In this paper,layered double hydroxides with different inorganic anion/cation types were prepared by co-precipitation-hydrothermal method,and the effects of inorganic anions/cations on the structure and spectra of LDHs were systematically investigated,and their modulation mechanisms on the reflectance spectra of LDHs in the visible-near-infrared band were explored.Finally,the spectral performance of LDHs was optimized and designed for controllable regulation according to the practical application requirements.（1）For the application scenario of simulated vegetation spectra,the spectral properties are optimally tuned for the visible（380-750 nm）and near-infrared（750-2500 nm）bands,respectively.In the visible band（VIS）,the transition metal ions are introduced into the LDHs laminate based on the structure-spectral properties of the cation-doped modified LDHs laminate to tune its visible band spectrum.Firstly,different M^II/M^III transition metal ions（M=Ni,Co,Fe,Cr）were selected to replace the LDHs laminate cations in this paper.Due to the existence of a certain degree of deformation in the octahedra corresponding to different transition metal ions,the laminate stability of the layered double hydroxide is destroyed,which leads to a decrease in the overall crystallinity of LDHs and can have an effect on the values of the cell parameters a and c.Secondly,in[M（OH）₆]^4-,the outermost d orbitals of the transition metal ions M have different electron arrangements,and when they absorb specific wavelength photons,some of the d orbitals electrons can be excited from the t₂ d orbitals to the e d orbitals.The corresponding photon absorption effect can have the effect of changing the spectral reflection characteristic curve,making the sample appear different colors in the visible band.In order to simulate the"green peak"in the visible band of vegetation spectrum,different types of transition metal ions can be introduced to provide specific absorption spectral features.（2）In the near infrared（NIR）band,based on the method of adjusting the structure spectral properties of LDHs intercalated with anions,the intensity of water absorption characteristic peak of LDHs was adjusted by changing the type of anions.Firstly,inorganic anions with different particle sizes and valence states were selected to intercalate the interlayer anions.The number,orientation and arrangement of interlayer anions are affected by the charge density and water content,which can be discussed by the change of interlayer spacing of LDHs.When the anion valence state is the same,the interlayer spacing increases with the increase of halogen ion radius;when the anion radius is too large,the interlayer spacing corresponding to Si(W₃O₁₀)₄^4-and PO₄·12WO₃^3-will increase significantly,but the layered structure of LDHs will be destroyed.Secondly,the water absorption characteristic peak intensity of the sample changes with the change of anions.When transition metal anions such as Cr O₄^2-,Fe（CN）₆^3-,Fe（CN）₆^4-are intercalated into the LDHs,the visible spectrum characteristics of LDHs will also change.In order to improve the characteristic peak intensity of water absorption spectrum in near-infrared band of vegetation spectrum,inorganic anions with different valence states and sizes can be introduced to enhance the interlayer crystal water content.（3）For the application of visible light responsive catalyst,the improvement of its photon absorption rate and absorption bandwidth can be achieved by the spectral control of layered double hydroxides by inorganic anions/cations.Firstly,the transition metal anions/cations of Cr O₄^2-and Cr³⁺were selected to regulate the structure and spectrum of LDHs.The samples were characterized by XRD,HR-TEM,FT-IR,XPS and Vis-NIR.The Cr Based LDHs photocatalyst material was successfully prepared.Under the synergistic effect of Cr³⁺and Cr O₄^2-,the average reflectivity of the LDHs in the visible light band was reduced by 57%,and the spectral absorption range of the prepared samples is extended from 380 nm to 880 nm,which significantly improved its absorption capacity for low-energy photons and near-infrared photons.Secondly,the photocatalytic degradation of methylene blue（MB）was carried out under visible light by testing the photodegradation efficiency of different samples.The apparent rate constant(K_app)is 0.0174 min^-1,and the half-life(t_1/2)is 40 min.Compared with traditional LDHs,Mg Al Cr-Cr O₄^2--LDHs system has higher photocatalytic degradation efficiency.