Preparation and Properties of Multiphase Composite Photocatalytic Materials Based on Spindle α-Fe2O3 Nanoparticles

GAO Fang, LI Baoliu, ZHENG Yuanjie, XU Ping

(Key Laboratory of Hubei Province for Coal Conversion and New Carbon Materials, Wuhan University of Science and Technology, Wuhan 430081, China)

Abstract: The spindle-shaped α-Fe2O3 was prepared by hydrothermal method, Fe2O3/ZnO and Fe2O3/ZnO/PANI photocatalytic composite materials were successfully prepared, and their structure and photocatalytic properties were studied. The results show that different amount of PANI has different photocatalytic performance, and the addition of PANI improves the photocatalytic properties of the composite. In addition,Fe2O3/ZnO/PANI(10%) exhibits the best photocatalytic performance. After two hours of UV irradiation, the degradation rates of methyl orange and orange II reach 68.1% and 75.4%, respectively.

Key words: polyaniline; nanotube; ZnO; composites; photocatalysis

1 Introduction

With the rapid development of social science and technology and the gradual improvement of living standards, the whole world has been confronted with the challenges of energy shortage and environmental pollution[1-3]. Industrial wastewater pollution is one of the most common risks to the environment, which has attracted widespread attention. There are still many disadvantages for the conventional physical or chemical methods for water pollution treatment, needing further improvement[4-6]. The photocatalytic treatment for water pollution has been developed rapidly and become research hot sport for recent years because of its distinct advantages comparing with conventional methods,such as low power consumption, stable and nontoxic,lower cost, and no secondary pollution,etc[7,8].

Among iron oxides, α-Fe2O3is the stablest with advantages of large reserves, low cost, low environmental hazard, and narrow band gap[9,10]. The band gap of ZnO is 3.2 eV[11], which has the advantages of simple synthesis, low cost, and high photocatalytic activity. Sudheer[12]and others have researched the effects of ZnO/Fe2O3degrading phenolic wastewater exposed under the fluorescent lamp and proved that this composite semiconductor catalyst can degrade organic wastewater under simulated sunlight, but it still needs intensive research because of its disadvantages,like big feed ratios, low catalytic efficiency,etc. In recent years, the research of inorganic materials and conductive polymers has been more and more remarkable[13,14]. Polyaniline as an environmentally friendly material has a semiconductor energy band structure of the visible light and good absorption properties[15]. Several studies have shown that the photocatalytic activity and light response range of TiO2and ZnO modified by PANI have been significantly improved, indicating the significant application prospect of PANI as photosensitizer of semiconductor photocatalysts. In this paper,spindle-shaped α-Fe2O3was prepared via hydrothermal method, Fe2O3/ZnO/PANI composite photocatalyst was synthetized, and the effect and mechanism analysis for degrading methylene orange and Orange II under UV light irradiation was studied.

2 Experimental

2.1 Material

All chemicals were of analytical grade and used as received without purification. The Orange II(OII)and methyl orange(MO) were prepared by mixing 0.2 g OII and MO powder in 1 000 mL distilled water relatively(200 ppm).

2.2 Synthesis of spindle α-Fe2O3 with hydrothermal method

spindle α-Fe2O3were prepared by the hydrothermal method as follows[16]: 4.8 g of NaOH and 16.21 g of FeCl3·6H2O were dissolved in 100 mL deionized water, respectively. Under vigorous mechanical stirring, the NaOH solution was added dropwise to the FeCl3·6H2O solution at 60 ℃, and the Fe(OH)3sol was obtained after stirring for 1 h. Then the Fe(OH)3sol was diluted to 450 mL. 6.3 g of SDBS was added to obtain a yellow Fe(OH)3-SDBS mixed precipitate and stirred for 1 h. The reddish brown precipitate was obtained by filtering the solution and dried at 60 ℃ for 6 h. In a typical synthesis, 7.5 g of dried complex was dissolved into 60.0 mL of dimethylbenzene, then the solution was poured into Teflonlined autoclaves and heated at 200 ℃ for 6 h. The product was collected by centrifugation, washed several times with deionized water and ethanol, and dried at 60 ℃ for 6 h.

2.3 Preparation of nano-α-Fe2O3/ZnO composite

375 mL of zinc acetate ethanol solution (0.01 mol/L) was prepared and injected into a 1 L three-necked flask at 60 ℃. Under continuous stirring, 195 mL of sodium hydroxide ethanol solution (0.03 mol/L) was added dropwise. The mixture was kept at 60 ℃ and stirred for 2 h to form a sol containing ZnO nanoparticles.1.22 g of Fe2O3powder was weighed, dispersed in 50 mL of ethanol, and ultrasonically dispersed for 30 min.The mixture of Fe2O3and ethanol was transferred to the sol of ZnO, stirred for 40 min at 60 ℃ to obtain the precipitate[17]. The precipitate was washed with ethanol and deionized water and dried at 60 ℃ for 6 h to obtain Fe2O3/ZnO composite material.

2.4 Preparation of nano-α-Fe2O3/ZnO/PANI composite

3.1 g of Fe2O3/ZnO was dispersed in 60 mL of distilled water with ultrasonic for 30 min. 0.027 mL,0.14 mL, 0.30 mL, and 0.48 mL of aniline monomer was dropped into 100 mL of distilled water. The mixture was stirred for 0.5 h with magnetic force to obtain a dispersion of aniline. Further, 0.134 g, 0.70 g, 1.50 g and 2.40 g of ammonium persulfate were separately weighed into 50 mL of water to prepare an ammonium persulfate solution. The dispersed Fe2O3/ZnO was mixed with aniline, and an ammonium persulfate solution was slowly added dropwise thereto, and stirred at normal temperature for 16 hours. The precipitate was washed with ethanol and deionized water and dried at 60 ℃ for 6 h, thereby obtaining Fe2O3/ZnO/PANI composite with a polyaniline content of 1%, 5%, 10% and 15%, respectively.

2.5 Photocatalysis experiment

In a typical process, 0.15 g freeze-dried nanoparticle sample was put into 150 mL OII or MO aqueous solution and the concentration was 10 ppm. Keep them in the dark for 30 min to get well dispersed and reach an adsorption/desorption equilibrium. Then turn on the UV lamp, sample at regular intervals, and centrifuge to measure the absorbance of the supernatant. The OII and MO degradation(D) were calculated according to the following equations:D(%)=C/C0×100=A/A0, whereC0is the initial Orange II and methyl orange concentration,Cis the instantaneous Orange II and methyl orange concentration,A0shows initial absorbance, and acorresponds to variable absorbance.

2.6 Characterization

Morphology maps were examined by field emission scanning electron microscopy(SEM, Nova 400,FEI, Netherlands, where the accelerating voltage used was 80 kV). FTIR spectra of the samples were made with a KBr pellet, scannings from 4 000 to 400 cm-1were measured on a FTIR spectrometer(Vertex70,Bruker, Germany). XRD analysis were recorded using Bruker AXS D8 Advanced instrument equipped with CuKα radiation(λ=1.541 8 Å). Besides, the UV-vis adsorption spectroscopy measurements were obtained using a UV-vis spectrophotometer(MAPADA UV-1800)at room temperature.

3 Results and discussion

3.1 Morphology and component

Fig.1(a)-(f) are the SEM images of α-Fe2O3and different PANI content of Fe2O3/ZnO/PANI composites.It can be seen from Fig.1(a), α-Fe2O3is spindle-shaped and uniform relatively. The longitudinal dimension of a single particle is about 380 nm, and the lateral dimension is about 70 nm. In Fig.1(b), all spindle-shaped α-Fe2O3surfaces become rough compared with pure α-Fe2O3particles, and there are tiny particles on the surface, which can be preliminarily judged as ZnO generated during the preparation process.

In Fig.1(c)-1(f), as the content of PANI in the Fe2O3/ZnO/PANI composites increases gradually, the morphology of the composite materials changes and emerges agglomeration gradually. Although all the graphs have a certain agglomerate phenomenon, the size of individual particles cannot be accurately calculated, but it can still be seen that individual particles are nano-sized. This will result in a nanosized effect of the composite catalysts and may have a strong photocatalytic activity.

Fig.1 SEM images of Fe2O3 (a) and Fe2O3/ZnO/PANI composite materials(the content of PANI is 0(b), 1%(c), 5%(d), 10%(e), 15%(f) respectively)

3.2 IR analysis

Fig.2 shows the FTIR spectra of α-Fe2O3, Fe2O3/ZnO and Fe2O3/ZnO/PAIN composites.

Fig.2 FTIR spectra of synthesized α-Fe2O3, Fe2O3/ZnO and Fe2O3/ZnO/PAIN composites

The peaks located at 469 cm-1and 544 cm-1represent the Fe-O stretching vibration ofα-Fe2O3. The absorption at 667 cm-1was caused by Zn-O stretching of zinc oxide. The absorption at 1 519 cm-1and 1 464 cm-1corresponded to the quinone ring and benzene ring, respectively. The appearance of bands in FTIR spectra at 1 105 and at 3 441 cm-1assigned to quinone nitrogen atom and N-H stretching vibrations, respectively. The FTIR spectra proved the success of the synthesis of Fe2O3/ZnO/PAIN composites. Moreover, the three samples showed absorption peaks near 1 050 cm-1, which corresponded to the characteristic absorption peaks of the sulfonic acid group. This was due to the addition of sodium dodecylbenzenesulfonate(SDBS)when preparingα-Fe2O3. The stretching vibration absorption peak of the adsorbed water could be clearly observed around 3 420 cm-1, which was due to the small particle size of the composite materials and the large specific surface area, which was easy to adsorb the water.

3.3 X-ray diffraction

The phase purity and structure of the sample were analyzed by XRD and the XRD patterns are shown in Fig.3. In contrast to the standard diffraction peaks(-JCPDS card No.36-1451,a=0.324 9 nm,c=0.520 6 nm), there were eight appreciable characteristic peaks at about 2θ= 24.13°, 33.14°, 35.61°, 40.82°, 49.47°,54.08°, 62.44° and 63.98°, which were corresponding to the crystalline structure of the hematite phase ofα-Fe2O3. No other peaks appeared in pureα-Fe2O3sample, which indicated that there were no impurities generated in Fe2O3.

For the ZnO, all the XRD peaks at 2θvalues of 31.76, 34.42, 36.25, 47.53, 56.60, 67.69 and 69.09 corresponded to (100), (002), (101), (102), (110),(112) and (201) planes of ZnO, indicating that ZnO was present in Fe2O3/ZnO materials. The diffraction peaks at 2θangles of 8.62°、16.34°、20.41°、25.32°and 26.92°were consistent with the crystal structure of PANI, which indicated that the Fe2O3/ZnO and PANI were well combined and the Fe2O3/ZnO/PANI heterostructure was successfully synthesized. In addition,there was no impurity peak in XRD spectrum, which meant the high purity of the samples.

Fig.3 XRD patterns of α-Fe2O3, Fe2O3/ZnO and Fe2O3/ZnO/PANI composites

3.4 Photo-degradation of AZO dyes

The photocatalytic activities were estimated by orange II and methyl orange degradation tests under ultraviolet light irradiation. All the photocatalytic experiments were carried out under the same conditions. In the experiment, two representative azo dyes of orange II and methyl orange were selected as simulated organic pollutants. The initial concentration of both dyes was 5 ppm and the catalyst dosage was 1 g/L.

As shown in Fig.4(a) and (b), after the addition of photocatalyst, the concentration of OII and MO decreased due to the surface absorption before turning on the light. All Fe2O3/ZnO/PANI composites showed better photocatalytic performance than other photocatalysts, and the 10% sample had the best photocatalytic performance. The results indicate that loading PANI onto the surface of Fe2O3/ZnO can expedite the photocatalytic activity. After two hours, the degradation rates of orange II and methyl orange by Fe2O3/ZnO/PANI(10%) composites were 75.4% and 68.1%, respectively.

Fig.5(a) and (b) shows the reaction kinetics curves of the degradation of orange II and methyl orange by different catalysts under ultraviolet light. Under the condition of ultraviolet light, the degradation of dyes by various catalysts conformed to the first-order reaction kinetics. According to the Langmuir-hinshelwood model, the apparent first-order rate constantk, the correlation coefficientRand other parameters of the catalyst in the process of photocatalytic degradation of simulated wastewater can be calculated.

The photocatalyst efficiency, represented as kinetic rate constantk, was decreasing in the following sequence: Fe2O3/ZnO/PANI(10%)＞ Fe2O3/ZnO/PANI(15%)＞ Fe2O3/ZnO/PANI(5%)＞ Fe2O3/ZnO/PANI(1%)＞ Fe2O3/ZnO ＞ Fe2O3＞ Blank. The reaction rate of Fe2O3/ZnO/PANI(10%) composite catalyst was the highest, and the degradation rate constants of OII and methyl orange could reach 0.010 4 min-1and 0.007 7 min-1, respectively.

Fig.4 The experiments of different materials degradate OII(a) and MO(b)

3.5 Photocatalytic mechanism

Based on the above analysis results, a possible mechanism for photocatalytic activity of Fe2O3/ZnO/PANI under UV illumination is proposed in Fig.6. The highest occupied molecular orbital(HOMO) and lowest unoccupied molecular orbital(LUMO) potentials of PANI are 0.8 and -1.9 eV, respectively. The valence band(VB) and conduction band(CB) edges for Fe2O3are estimated to be 2.48 and 0.28 eV, respectively. The VB and CB of ZnO are 2.89 and -0.31 eV, respectively.When irradiated under UV light, the π-π* transition can be induced to generate holes and electrons. The generated h+can transfer to the VB of Fe2O3and PANI from the VB of ZnO and take part in the direct oxidation of dyes to CO2and H2O. The generated e-will migrate to the CB of Fe2O3as the CB edge of ZnO is more negative than that of Fe2O3. At the same time, the generated electrons of PANI can be directly injected into the CB of ZnO. The electrons on the CB of Fe2O3and ZnO can reduce O2to form ·O2-. Thus a series of active h+and ·O2-were successively produced in photocatalytic experiment, increasing the photocatalytic activity of the Fe2O3/ZnO/PANI photocatalyst. Compared with other photocatalysts, the higher photocatalytic ability of Fe2O3/ZnO/PANI composite can be the result of high separation efficiency of photoexcited holes and electrons.

Fig.5 The reaction kinetics of different materials degradate OII(a) and MO(b)

Fig.6 Schematic diagram of photoexcited electron-hole separation process of Fe2O3/ZnO/PANI under UV-Visible irradiation

Fig.7 Degradation curves of MO obtained for different times of cycling photocatalyitic tests by using Fe2O3/ZnO/PANI(10%) heterostructure as photocatalyst.

The reusability of Fe2O3/ZnO/PANI(10%) composite was evaluated by five-times recycling measurements for the degradation of MO, as shown in Fig.7.The composite was obtained by centrifugation and washed with anhydrous ethanol and deionized water for the next test after each cycle of photocatalytic experiment. The discoloration rate of the same Fe2O3/ZnO/PANI(10%) composite was only slightly reduced in the photocatalytic reaction after five recoveries, indicating that the Fe2O3/ZnO/PANI heterostructure has a good repeatability in photodegradate MO aqueous solution.

4 Conclusions

Fe2O3/ZnO composites were synthesized, and Fe2O3/ZnO/PANI composites with different contents of PANI were successfully prepared. Fe2O3/ZnO/PANI composites showed better photocatalytic performance for OII and MO than the pure Fe2O3/ZnO and ZnO materials. The Fe2O3/ZnO/PANI photocatalyst with 10%quality ratio of PANI exhibits the highest photocatalytic activity. Furthermore, this study demonstrates that the as-prepared Fe2O3/ZnO/PANI photocatalyst has great application prospect in the environmental cleanup.