Study On Synthesis, Characterization And Photocatalytic Properties Of FeWO₄Powders By Hydrothermal Process

Monoclinic ferrous tungstate（FeWO₄） as an important functional inorganicmaterials in Tungstate family, have highly potential and technologicalapplications in many areas, such as light, electricity, magnetism, conductors/superconductors, catalytic, and biological fields. Thus, the synthesis ofwell-crystallized FeWO₄crystallites with complex and super structure, especiallyFeWO₄nano-particles, has recently drawn increasing attention. Diversetechniques have been developed to synthesize FeWO₄powders. So far,researchers have been successfully prepared FeWO₄micro-structure materials ofhigher photocatalytic activity under visible light irradiation having hexagonalsymmetry flower-like structures, and with a flower-like ferromagnetic FeWO₄nanostructured materials, and so on.In this paper, we selected the Hydrothermal Method to papare FeWO₄nanomaterials, desired to study its relationship between structure andphotocatalytic properties. The main contents of this paper included:1. Using ferrous ammonium sulfate[（NH₄）₂Fe（SO₄）2·6H₂O] and sodiumtungstate[Na₂WO₄·2H₂O] of analytical grade as the starting material, changed thepH of precursor solution and successfully prepared pure phase FeWO₄microcrystallines under the conditions of the hydrothermal reaction temperatureof160℃for24h. It was easy to obtain pure FeWO₄microcrystallines withdifferent morphology in a wide range of the pH=2¹1. The results showed that:The powders displayed a hexagonal flakes morphology under the acidicconditions of pH=2. With the increase of pH in the neutral conditions （pH=7）,the morphology changed from hexagonal flakes into tiny nanoparticles with auniform grain size of100nm. Increasing the pH to9, the morphology changed from tiny nanoparticles into thin rods，and the rod-like structure FeWO₄microcrystalline have a clear preferential orientation along the （002） planes. ByUV-Vis analyses, the optical band gap of the FeWO₄powders was calculated tobe about2.40eV, the absorption edge of the sample extended nearly to the wholespectra of visible light. The photocatalytic properties of the samples wereresearched through the photodegradation experiment in visible light, in order todiscuss the relationship between the morphology and photocatalytic activity．Theresults showed that the catalytic ability FeWO₄was not excellent enough, and itneeded to be further improved.2. The rod-like structure FeWO₄microcrystalline was successfully preparedunder the pH=9with uniform size distribution. The optimal reaction conditionsto prepare well-crystallized FeWO₄was showed as followed: hydrothermaltemperature=160℃, hydrothermal time=24h, filling ratiio=60%, the precursorconcentration was0.1mol/L, Fe/W=1:1. Under these conditions, we investigatethe impact on the morphology of the changes of the iron source and addingtemplate agent, then analysed the reasons from the point of the crystal’s internalstructure.3. We successfully prepared pure phase FeWO₄crystallites under theconditions of the hydrothermal reaction temperature of190℃for24h, pH=7. Onthis basis, we studied the growth kinetics of the FeWO₄nanocrystallites and itwas activation energy Ea=17.36kJ/mol. The influence factors on the crystallinityof FeWO₄nanocrystallites were investigated with the experiment of orthogonaldesign in this system. The influence factors on the crystallinity of FeWO₄nanocrystallites were investigated, the results showed as followed: reactiontemperature> pH> reaction time. The best crystallinity of sample was A₂B₂B₂bythe study of R, corresponding to the reaction temperature was160℃, reactiontime was24h, and the pH=7.4. Using Ferrous ammonium sulfate[（NH₄）₂Fe（SO₄）2·6H₂O] and Sodiumtungstate[Na₂WO₄·2H₂O] of analytical grade as the starting material, at the sametime, we added in small biomolecules L-cystein as reducing agent and thetemplate agent, prepared well-crystallized FeWO₄microcrystalline in differentpH. We studied the relationship among morphology, structure and photocatalyticproperties. We come to a conclusion from photocatalytic performance: Synthesis of FeWO₄nanocrystals by L-cysteine-assisted via a HydrothermalMethod under pH=9exhibit effective photocatalytic activity for the degradationof rhodamine B （RhB） dyes under visible light conditions, it reached to81.0%.Under this conditions, we also discussed the photocatalytic performance withdifferent amount of FeWO₄or L-cysteine, we found that: the amount of catalystor L-cysteine that excessive or too little are not benefit for degradation, theappropriate content of the catalyst should be20mg, the appropriate content ofthe L-cysteine should be3mmol.