Preparation And Photocatalytic Properties Of Fe-containing Layered Double Hydroxides Films

The fast recombination rate of the photogenerated electron-hole pairs,the easily aggregation of LDHs particles and the difficulties in recovery limit layered double hydroxides?LDHs?large-scale practical applications in environmental remediation.To enhance LDHs photocatalytic activity,LDHs films on the surface of conductive cloth with different compositions are synthesized by in situ growth method and coating method,respectively.The effects of laminate compositions,preparation methods and iron ion doping on their structure,morphology and photocatalytic properties of LDHs films have been investigated,which could provide the theoretical and technical basis on photocatalytic degradation of organic dyes.1.LDHs films with different compositions are prepared by the in situ growth method.And the effects of metal ions and additives on the growth habits of LDHs films are investigated.The experimental results show that the solubility product of the hydroxides corresponding to the metal ions affected the precipitation order and deposition amount,resulting in different morphologies of the LDHs films.NiAl-LDHs and MgAl-LDHs are hexagonal sheets.The growth habit of LDHs could be controlled by adding NH₄F.It leads the ab plane of LDHs sheets to grow vertically along the conductive cloth,forming the films with abundant pores.With NH₄F,the morphology of NiFe-LDHs films changes greatly and sheets are stacked to form the spherical aggregates.CoAl-LDHs sheets tend to roll up,forming a rod-like shape2.LDHs films with different compositions are prepared by the coating method,and the effects of metal ions on the growth habit of LDHs films are investigated.The experimental results show that the films composed of NiAl,NiFe and CoAl are all LDHs phases with lamellar structure.MgAl-LDHs tends to generate aluminum fluoride and no LDHs structure is observed.The composition of laminates has an effect on the morphology of LDHs films.NiAl-LDHs and NiFe-LDHs sheets vertically agglomerate into ball,forming uniform porosity with small pore size and high specific surface area.The crystallinity of CoAl-LDHs is poor and the spacing among sheets is small,which results in decrease in specific surface area,pore volume and pore size.3.The LDHs films are modified by iron ion doping using water bath heating.The effects of preparation methods and compositions on the phase,morphology and structure of LDHs films are studied.The location of doped iron ions and their mechanism in photocatalytic degradation of methyl orange are investigated.After modification,the LDHs films prepared by the coating method remain the similar layer phase,but different morphologies.The morphology changes are affected by the composition of LDHs films:after modification,NiAl-LDHs films show unchanged morphology but an increasing specific surface area.For NiFe-LDHs films,the distance among parallel sheets and the specific surface area are smaller.For CoAl-LDHs films,the particles are agglomerated into flower like clusters,which result in an increase of pore size and specific surface area.The doping of iron ions has little effect on the phase and morphology of LDHs films prepared by the in situ method.There are two ways for Fe???located in the LDHs films.Part of Fe?III?replaces Al?III?in the laminate,and the other part adheres to the surface of the particles,providing more redox sites for the photocatalytic reaction.4.Using methyl orange as the target pollutant,the effects of composition?NiAl,MgAl,NiFe,CoAl?,preparation method,Fe?III?doping modification and coexistence Ag ion on the photocatalytic properties of LDHs films are studied.The role of doped iron ions in the photocatalytic process is explained.The experimental results show that:metal ions with reduced positions in the laminate,such as Ni?II?and Co?II?,can be used as photogenerated charge traps to enhance photogenerated electron-hole separation and improve the photocatalytic efficiency of films.Compared with that films prepared by the coating method,LDHs films prepared by the in situ method exhibite better photocatalytic degradation rate.LDHs films prepared by the in situ method are favorited for the transmission of photogenerated charge,which is owing to the tight adhesion of LDHs films and the full use of the conductivity of substrate.Iron ion doping has different effects on the films prepared by different methods.For LDHs films prepared by the in situ method,the doped iron ions pay the role of the photogenerated electron-hole pair trap,thus reducing the photocatalytic degradation rate.For LDHs films prepared by the coating method,the doped iron ions are beneficial to the separation of the photogenerated carrier and effectively improve the photocatalytic degradation rate.The coexistence of Ag⁺and methyl orange has synergistic effect,and the doped iron ions could enhance the synergistic effect.With the increase of Ag⁺concentration,the synergistic effect is more obvious,resulting in the improvement of photocatalytic degradation rate.LDHs films exhibited good stability during photocatalytic process.And no obvious changes in the morphology and structure of LDHs films are observed after photocatalysis.