燒結(jié)反應(yīng)對液相法制得FeWO4微米晶光催化性能的改性

杜泉嶺 后 浩 葉同奇 周紅洋

（合肥工業(yè)大學(xué)宣城校區(qū)化工與食品加工系，宣城 242000）

0 Introduction

The expansion of a wide range of chemical contaminants both in air and water is becoming a critical worldwide issue.Photocatalysis,especially photocatalysis employing semiconductor nanomaterials,appears to be the very appealing route than the conventional chemical oxidation methods for decomposition of toxic compounds to non-hazardous product[1].Febased nanomaterials,including iron oxides,FeVO4,Fe2（MoO4）3,FeWO4,FeIn2S4,Fe3O4-TiO2,Fe-CuS-RGO are investigated for their photocatalytic property with theseadvantages:cheap,non-toxic,environmental friendly,abundantin reserves,broad absorption spectral,and so on[2-5].

The fabrication of self-assembled hierarchical microstructures of inorganic materials with hierarchy architectures and well-defined properties is of great interest to material scientists over the past decades[6].Iron tungstate （FeWO4）,along with the name of“ferberite”, is an isostructural compound that crystallizes in a wolframite-type monoclinic structure belonging to the P2/c space group[7].FeWO4nanomaterials,with various morphologies,were fabricated for its photocatalytic,magnetism,and electrochemical property[7a,8],etc.Yu studied the effect of the volume ratios of H2O and EG solvent on the morphology of FeWO4.In particular,the photocatalytic property was studied to explore their potential application[9].Monodisperse spindle-like FeWO4microcrystals were synthesized,which show high active for the photodegradation of methyl orange[10].Ionic liquid was introduced in reaction system for the synthesis of FeWO4microspheres[11].Network-like ZnO-FeWO4mesoporous heterojunctions with tunable band gaps were prepared for their enhanced visible light photocatalytic performance[12a].

Herein,a solution-based route was introduced to the fabrication of FeWO4microcrystals,and Na2EDTA wasemployed ascomplexingagent,CTAB was employed as surfactant in the reacting system;the annealing treatment was studied for its influence on FeWO4microcrystals′photocatalytic activity,and the temperature of 310℃was carefully chosen.As shown in the following parts,after the annealing treatment,the crystallinity and surface composition of FeWO4microcrystals were changed.The FeWO4microcrystals were investigated in the degradation oforganic contaminants,and rhodamine B （RhB）was employed as model reagent.

1 Experimental

1.1 Synthesis of FeWO4Microcrystals

FeWO4microcrystals were synthesized via a solution-based and route.A typical experiment for synthesizing FeWO4was as follows:Firstly,with magnetic stirring,2.0 mmol·L-1FeSO4·7H2O and Na2WO4weredissolved intotwo50 mL beakers,which included 15 mL distilled water.Then,1.0 mmol·L-1Na2EDTA was added into the FeSO4solution,and dissolved completely.The Na2WO4solution was added to the above mixed solution.Finally,0.1 g CTAB was added and dissolved.By employing 1.0 mol·L-1NaOH solution and 1 mol·L-1HNO3solution,the pH value was adjusted to 8.0.The solution was transferred into a 50 mL Teflon-lined stainless steel autoclave and maintained in an oven at 180℃for 12 h.As the autoclave cooled down naturally to room temperature,the product was collected and washed with distilled water and absolute ethanol at least three times to remove impurities,and was then dried at 80℃for 6 h.At last,the product was annealed under the circumstance of air in the muffle furnace,and the temperature was set at 310℃.

1.2 Characterization

The obtained FeWO4samples were characterized on a Philips X′Pert Pro Super X-ray powder diffractometer with Cu Kα radiation （λ=0.154 187 4 nm）,the working voltage was set to 40 kV,and the working current was set to 30 mA,and the scanning area was from 10°to 80°.The morphology was examined with a JEOL JSM-6490F scanning electron microscope（SEM）,a Hitachi （Tokyo,Japan）H-800 TEM at an accelerating voltage of 200 kV,and a high-resolution transmission electron microscope （HRTEM）（JEOL-2010）operated at an acceleration voltage of 200 kV.Energy dispersive spectrometer （EDS）analysis was obtained with an EDAX detector installed on the same SEM.The X-ray photoelectron spectra （XPS）were collected on a Thermo Scientific ESCALAB 250Xi X-ray photoelectron spectrometer.FT-IR spectra were recorded on a Nicolet Model 759 Fourier Transmission Infrared spectrometer with wavenumbers in the range of 500～4 000 cm-1.Brunauer-Emmett-Teller（BET）specific surface area was determined from nitrogen adsorption at 77 K on a TristarⅡ3020 M surface area and pore size analyzer.The optical properties of FeWO4samples were studied by UV-Vis diffuse reflectance spectra on a Lambda 950 spectrophotometer.Thermogravimetric analysis （TGA）was carried out on a TGA-50 thermal analyzer（Shimadzu Corporation）at a heating rate of 10 ℃·min-1in air.

1.3 Photocatalytic test

The photocatalytic activities ofthe FeWO4microcrystals before and after annealing treatment were evaluated through the degradation experiment of RhB in an aqueous solution under UV and visible light from a 500 W Xe lamp （modeling sunlight）.30 mg FeWO4microcrystals photocatalyst was poured in 30 mL of RhB aqueous solution （with the concentration of 30 μmol·L-1）at room temperature.Before light was turned on,the solution was continuously stirred for 30 min in the dark to ensure the establishment of an adsorption-desorption equilibrium.The concentration of RhB during the degradation was monitored by colorimetry using a SolidSpec-3700 spectrophotometer at room temperature.

2 Results and discussion

2.1 Phase and morphology analysis of FeWO4 before and after the annealing process

Fig.1 （a）XRD patterns of FeWO4samples;（b）SEM images of crossed FeWO4nanoflakes;（c）EDS spectrum of FeWO4nanoflakes

The crystallite and phase of as-prepared FeWO4samples were characterized by X-ray powder diffraction patterns.The diffraction patterns of FeWO4powders are shown in Fig.1（a）（unannealed line）.Comparing with the standard card （PDF No.85-1354）,the product could be indexed as a monoclinic-phase of FeWO4（space group of P2/c）,with the calculated lattice constants of a=0.472 nm,b=0.57 nm,c=0.495 4 nm,and β=90°.No other peaks were detected,indicating the purity of the sample.As shown in the SEM images of Fig.1（b）,FeWO4microcrystal is composed of crossed nanoflakes;and the width of these nanoflakes are from 50 to 200 nm.The element and composition of the products were studied via EDS analysis in Fig.1 （c）,labeled to be the peaks of Fe,W and O elements in the EDS spectrum.Peaks of Au element are originated from the Au grid.The peaks of C element probably result from organic residue.The TEM image of FeWO4microcrystal is shown in Fig.2（a）,and the HR-TEM image （Fig.2（b）taken from a part of a typical FeWO4nanoflake,displays the clearly resolved lattice fringes,indicating the integrality of crystallinity.The interplanar spacing was measured as 0.295 and 0.38 nm,corresponding to the （111）and （110）plane of FeWO4monoclinic-phase respectively,with the angle of 98.8°,which is consistent with the literature result[12b].

XRD patternsofannealed and unannealed FeWO4powders are provided for comparing.As shown in Fig.1（a）,all of the peaks are labeled to pure FeWO4monoclinic-single crystals,in good agreement with the literature results （PDF No.85-1354）,no any impurity formed during the annealing process.The result indicated that the crystallinity of FeWO4nanoflakes is improved during the annealing treatment.Furthermore,the SEM images in Fig.3 indicated that the FeWO4nanoflakes morphology unchanged during the annealing treatment,with only a little of cracking.

Fig.2 （a）TEM image of the as-prepared FeWO4nanoflakes;（b）HR-TEM image from a typical FeWO4nanoflake

Fig.3 SEM images of unannealed （a,c）and annealed （b,d）FeWO4samples

The XPS spectrum of unannealed FeWO4microcrystals is shown in Fig.4.The main peak values at 34.0,283.0,529.0 and 711.0 eV,are labeled to the binding energies of W4f,C1s,O1s,and Fe2p,respectively.The strong intensity of C1s peak in Fig.4 intimates the abundance of carbonaceous adsorbents on the surface of the as-prepared FeWO4nanoparticles.

Fig.4 XPS spectrum of unannealed FeWO4samples

2.2 Photocatalytic activities

Employed as complexing agent,Na2EDTA is crucial for the formation of FeWO4microcrystals,and plays an important role on the photocatalytic property.The degradation experiment of organic contaminants is performed.The RhB solution is chosen as a model reactanttodemonstratetheimpactofannealing treatment on photocatalytic property.

Fig.5 showsthe UV-Visabsorption spectra during the photodegradation of RhB（initial concentration:30 μmol·L-1,30 mL）in the presence of 30 mg FeWO4sample under 500 W Xe lamp（modeling sunlight）.Before the light on,to ensure the adsorptiondesorption equilibrium completely,the suspensions were kept bubbling in darkness for 30 minutes.As shown in Fig.5（a）,the absorption peaks at 556 nm belonging to RhB molecules gradually decreased as the reaction proceeding.The adsorption peaks at 556 nm completely disappeared only after 25 minutes,which reveal the excellent photocatalytic property of the FeWO4sample after annealing.FeWO4without any annealing treatment was also employed as photocatalyst.The result of the RhB degradation of samples is shown in Fig.5 （b）.When the reacting time is set at 25 min,the value of C/C0is about 60%.Then,the degradation experimentbyemploying unannealed FeWO4was carried as long as enough （about 2 hours）until the absorbance values unchanged basically and the degradation rate was found as 77%ultimately.The difference of photocatalytic properties of FeWO4samples before and after annealing is so apparent.For this degradation experiment,full adsorption played a key role in the first twenty minutes.In addition,three photocatalytic cycle experiments were carried out.RhB dyes were completely degraded,and the phase of FeWO4crystallite did not change according to XRD patterns in Fig.5（c）.

Fig.5 （a）UV-Vis absorbance spectra of the RhB solution in the presence of annealed FeWO4;（b）Plots of C/C0vs time;（c）XRD patterns of FeWO4after three photocatalytic cycles

Fig.6 （a）UV-Vis diffuses reflectance spectra of the unannealed FeWO4;（b）Contrast of Eg between the annealed and unannealed samples

The UV-Vis diffuses reflectance spectra of the FeWO4before and after annealing are shown in Fig.6（a）.There is a higher absorption range between 400 and 600 nm,implying the possibility of high photocatalytic activity of the samples under the visible light irradiation.The band gap of the as-prepared FeWO4was calculated via the equation[13]:

Ahν=（hν-Eg）n

A is the absorbance,h is the Planck constant,ν is the photon frequency,Egis the energy gap,and n is the pure numbers associated with the different types of electronic transitions.The value of n for FeWO4equals to 1/2 and its band gap is estimated to be 2.04 eV,which has the same value to the reported values（ca.2.0 eV）of bulk crystals[14].The Egvalues of the annealed and unannealed samples are displayed in Fig.6（b）.Egof the annealed is a little greater,possibly caused by the quantum size effect of the nanoparticles.These Egvalues （2.04 and 2.14 eV）indicate that the FeWO4has a suited band gap for photocatalytic decomposition of organic pollutants under visible light irradiation.

TG analysis was performed in the atmosphere of air.As shown in Fig.7,there is only little of weight loss at 310℃,less than 2.0%,owing to the organic adsorbent burned to CO2,H2O or other residue in air.The weight loss arises 6.75%at 500℃,and eventually runs up to 8.05%from about 550 to 800℃.The latter indicates the possible transformation of FeWO4into other phases[15].Therefore,the annealing treatment was set at 310℃,rather than higher temperature.In fact,the experiment with higher annealing temperature was performed for comparison.The product is better crystalline,probably ascribed to other phase,and poor in photocatalytic degradation experiment of RhB solution.

Fig.7 TG curve of as-prepared FeWO4

Nitrogen adsorption-desorption isotherms of annealed FeWO4are shown in Fig.8.In the region of P/P0=0～0.8,the adsorption isotherm and the desorption isotherm coincide into one line,indicating that there are not mesoporous structures in the FeWO4nanoflakes,and the region of P/P0=0.8～1.0 indicates that accumulation holes exist between material particles.The specific surface area of as-prepared FeWO4samples is about 17 m2·g-1and becomes about 11 m2·g-1after the annealing treatment,which is some different from expectation that the annealing treatment made the specific surface area not raised but decreased.During annealing process FeWO4nanoparticles might agglomerate and grow into larger crystals,which made specific area decreased[16].It is reasonable to conclude that the specific surface area of FeWO4is not the decisive factor for its photocatalytic activity in RhB solution.The recombination of the photo-generated electronsand holesin the FeWO4microcrystals affected phtocatalytic activity in system.It can also be observed from the SEM images in Fig.3 that the grain size of FeWO4nanoparticles increased because of the gradual formation of larger crystals and the consequent coalescing.

Fig.8 Nitrogen adsorption-desorption isotherms of FeWO4 after annealing treatment

Besides,the organic residue was burned up during the annealing process,and the crystal faces,covered by the organic residue,were then exposed.The IR spectrum of the samples is shown in Fig.9,and the typical peaks of FeWO4are labeled in lines.Several peaks （773,860 and 949 cm-1）existing below 1 000 cm-1and the broad band between 3 300 and 3 600 cm-1are in consistence with the O-W-O stretching modes of WO42-,respectively.The peaks below 700 cm-1（including 509 and 657 cm-1）are in consistence with the metal-oxygen （Fe-O）stretching vibrational mode.This result is in accord with the previous report[17].In Fig.9（b）,the peak at 1 468 cm-1is labeled with the C-H in-plane bending vibration,and peaks at 2 850 and 2 922 cm-1are labeled with C-H（saturated hydrocarbon）stretching vibration,indicating the presence of organic residue sticking to products,which accords with the XPS result in Fig.4.As shown in Fig.9（b）,all peaks belonging to hydrocarbon bonds disappear,confirming that the organic residue could be eliminated by annealing treatment without destroying the initial nanostructure.The organic residue was transformed into carbon nanoparticle,enhancing the absorption of RhB,which is helpful to photodegradation of RhB.In conclusion,the introduction of complexing agent and then conversion into carbon nanoparticle are crucial in the reaction system,and Na2EDTA plays an important role on the photocatalytic property.

Fig.9 IR spectra of the annealed （a）and unannealed （b）samples

3 Conclusions

In summary,FeWO4microcrystals were fabricated through a simple solution-based method with Na2EDTA and CTAB employed as complexing agent and surfactant, respectively. The photocatalytic property of FeWO4microcrystals on organic pollutant（RhB）were studied.FeWO4sample,being annealed under the circumstance of air with the temperature of 310℃in the muffle furnace,shows highly active in degradation of RhB experiment.RhB solution with the concentration of 30 μmol·L-1discolored only after 25 min under UV and visible light illumination,which was not reported before.The reason was probably relevant to the burning and carbonization of organic residue bound on FeWO4nanoparticles.This unique FeWO4microcrystal is p otential for application in decontamination field,and the annealing method can be applied in other similar materials to improve their absorption and photocatalytic properties.

Acknowledgments:This work was supported by the Natural Science Foundation of Anhui Province （Grant No.1508085QB46）.

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