Preparation And Photocatalytic Activities Of Mixed Metal Oxide Derived From Layered Double Hydroxides

In recent years,semiconductor photocatalytic technology has shown a great potential as a low-cost,environmentally-friendly and sustainable treatment technology of pollutants,which also aligns with the"zero"waste disposal policy in the water treatment.The ability of this advanced oxidation technology has been widely demonstrated to remove persistent organic compounds and microorganisms in water.Layered Double Hydroxides?LDHs?are a class of clays consisting of brucite-like layers/hydroxide layers and exchangeable interlayer anions.They have attracted considerable interest as photocatalysts owing to the uniform distribution and tunability of metal cations in the brucite-like layers and the exchangeable interlayer anions.Meanwhile,LDHs have great potential as precursors or catalysts supports.In this paper,the zinc-ferric layered double hydroxide?ZnFe-LDH?was synthesized by a co-precipitation method,and the zinc-iron mixed metal oxide?ZnFe-MMO?was prepared by calcinating ZnFe-LDH precursor at high temperature.The phase composition,crystal structure,pore structure,partical morphology and optical properties of the as-prepared samples were characterized by XRD,FTIR,BET,SEM,UV-vis and PL,respectively.Under visible light irradiation,two typical refractory organic pollutants,methylene blue?MB?and tetracycline?TC?were photodegraded to evaluate the photocatalytic ability of all samples.Radical quenching experiments and magnetic resonance spectroscopy?ESR?detection were performed for exploring the possible mechanism of the photodegradation system.XRD,FTIR and SEM results indicated that ZnFe-LDH was successfully transformed into ZnFe-MMO,which is composed of wurtzite zinc oxide?ZnO?and spinel-structured zinc ferrite?ZnFe₂O₄?,at the temperature above 550°C.After the calcination of ZnFe-LDH,the lamellar solid collapsed.The elevation of calcination temperature enhanced the degree of the layered solid collapse and crystallinity.BET results suggested that the specific surface area,pore size,pore volume of ZnFe-MMO reduced after calcination.The decreasing extent increased with the calcination temperature.UV-vis and PL characterization results showed that ZnFe-MMO-700 has broader visible light absorption and higher photoelectron-hole pair separation efficiency.Compared with ZnFe-LDH,the ZnFe-MMO-700 exhibited excellent photocatalytic performance for the degradation of MB and TC with 90%of photodegradation efficiency.In addition,there was TC adsorption on the samples in dark reaction.However,the calcined products have no access to rehydrate and restructure to promote the adsorption,so the adsorption capacity of TC decreases with the increasing calcination temperature.The results of radical quenching experiments and ESR examination revealed that the superoxide radical?·O₂^-?and hydroxyl?·OH?are the main active species in these two photodegradation process,and the possible degradation mechanism was speculated.The enhanced photocatalytic activities was mainly attributed to the formation of heterojunctions between the calcination product ZnO and ZnFe₂O₄,which effectively suppressed the recombination rate of photo-generated electrons and holes and accelerated the formation of·O₂^-and·OH.After four cycle,the ZnFe-MMO-700 composite displayed good stability and reusability in the photodegradation of MB and TC.