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Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Procedures
  5. Results and Discussion
  6. Conclusion
  7. Acknowledgements
  8. References

Photocatalytic degradation of methyl orange (MO) as a model of an organic pollution was accomplished with magnetic and porous TiO2/ZnO/Fe3O4/PANI and ZnO/Fe3O4/PANI nanocomposites under visible light irradiation. The structures of nanocomposites were characterized by various techniques including UV–Vis absorption spectroscopy, XRD, SEM, EDS, BET and TGA. Optical absorption investigations show two λmax at 450 and 590 nm for TiO2/ZnO/Fe3O4/PANI nanocomposites respectively possessing optical band gaps about 2.75 and 2.1 eV smaller than that of the neat TiO2 and ZnO nanoparticles. Due to these optical absorptions, the nanocomposites can be considered promising candidates as visible light photocatalysts to produce more electron-hole pairs. The degradation of MO, extremely increased using polymeric photocatalysts and decolorization in the presence of visible light achieved up to 90% in less than 20 min in comparison with the neat nanoparticles (about 10%). All these advantages promise a bright future for these composites as useful photocatalysts. The degradation efficiency of MO using stable nanocomposites was still over 70% after ten times reusing. The highest decolorizing efficiencies were achieved with 0.75 g L−1 of catalyst and 10 mg L−1 of MO at natural pH under visible light irradiation in less than 20 min.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Procedures
  5. Results and Discussion
  6. Conclusion
  7. Acknowledgements
  8. References

Photocatalytic degradation is an environment-friendly and economic method for decomposition of organic pollution in our environment. This method has been used for mineralization of the organics via a series of intermediates into inorganic materials, such as H2O, CO2 and so on. Therefore, semiconductor photocatalysis seems to be a promising technology and has numerous applications in environmental treatment such as purification of water and air as well as hazardous waste remediation [1, 2]. Photocatalysts that are active under visible light are extremely important and desirable for the degradation of organic pollutants. The advantages of photodegradation are: (1) a simple reaction device, (2) a mild reaction condition and (3) no remains of secondary pollution. The semiconductors as photocatalyst are becoming more attractive in the past 10 years due to their potential to solve environmental problems [3-5]. Among semiconductors, TiO2 and ZnO have been used as dominant photocatalysts and they have the ability to detoxificate water from various organic pollutants [6-8]. The wide band gap of TiO2 and ZnO reduces their photocatalytic efficiency and hinders their further application in industry. It is extremely important to improve the photocatalytic efficiency of TiO2 and ZnO by shifting their optical response from the UV to the visible range [9]. Another problem in the usage of TiO2 and ZnO particles is the short lifetime because of the high recombination rate of the photogenerated electron/hole pairs that reduces photocatalytic efficiency. Numerous efforts have been made to enhance the photocatalytic activity of these particles by surface or bulk modifications such as metals deposition, doping with nonmetal atoms, surface chelation, and combination with other metal oxides such as SnO2, Fe2O3 and Cu2O [10-17]. To enhance the photodegradation efficiency of TiO2,photocatalytic activities of coupled TiO2/ZnO composites with different particle size and shape were investigated by some researchers. [18, 19].

Conducting polymers have numerous applications in various electronic devices. Among the conducting polymers, polyaniline (PANI) has been widely studied due to its environmental stability, redox properties and the reversible nature of its electrical conductivity [20, 21]. In our previous work, PANI was synthesized on the surface of TiO2 nanoparticles and a PANI/TiO2 nanocomposite was prepared [22]. The combination of electrical conductivity of PANI and UV-sensitivity of anatase-TiO2 in PANI/TiO2 nanocomposites are expected to find applications in electrochromic devices, nonlinear optical systems and photo electrochemical devices [23, 24]. The PANI/TiO2 nanocomposites were prepared by Wang et al. and the catalysts photocatalytic activities in phenol degradation were investigated [25].

Recently, magnetic catalysts have emerged as new materials for environmental decontamination removal and extracted in reaction medium using an external magnetic field [26-28]. Compared with traditional methods, such as filtration and centrifugation, separating magnetic catalysts requires less energy.

In this work, for the purpose of preparing photocatalysts which can respond to visible light and also be separated easily by external magnetic fields, we modified the TiO2 and ZnO surfaces with the preparation of the porous TiO2/ZnO/Fe3O4/PANI and ZnO/Fe3O4/PANI nanocomposites as photocatalysts. Using these nanocomposites the photocatalytic efficiency of neat TiO2 and ZnO nanoparticles was enhanced under visible light irradiation in short time. Some experimental parameters such as photocatalysts and MO concentration, and pH of sample were studied and then optimized. The structures of nanocomposites were characterized by various techniques including UV–vis absorption spectroscopy, XRD, SEM, EDS, BET and TGA.

Experimental Procedures

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Procedures
  5. Results and Discussion
  6. Conclusion
  7. Acknowledgements
  8. References

Reagents and materials

Aniline (>99%), titanium (IV) chloride (>99%), zinc nitrate (>99%), sodium dodecyl sulfate (SDS, >85%), FeCl3·6H2O (>99%), FeCl2·4H2O (>99%), ammonium hydroxide (25–30%) and all other reagents were obtained from Merck Company (Whitehouse Station, NJ). Aniline was purified by vacuum distillation in the presence of Zn powder prior to use. The other reagents were used without further purification.

Synthesis of TiO2/ZnO/Fe3O4/PANI and ZnO/Fe3O4/PANI nanocomposites

Preparation of TiO2 and ZnO nanoparticles: For the preparation of TiO2 nanoparticles, a 1.5 m ammonium sulfate solution which consisted of 0.75 m titanium (IV) chloride was used. The aqueous solution was heated at 75°C and maintained at this temperature for 90 min. Then, 2.5 m ammonium hydroxide solution was added drop-wise under high speed stirring until the pH reached 7.0. The precipitated titanium hydroxide was collected and washed with distilled water and ethanol and then dried at 60°C. After being calcined at 350°C for 4 h, the sample was slowly cooled at room temperature.

The ZnO nanoparticles were prepared as described by Tang et al. [29]. Aqueous solutions of anhydrous Zn(NO3)2 (3 g) and urea (3.8 g) were prepared under vigorous stirring. The addition of a surfactant (SDS) inhibits the growth of nanoparticles during the course of precipitation. The reaction system was heated to 95°C for 2 h and Zn(OH)2 colloid was obtained. The precipitates were then filtered and washed until the surfactant was removed from the product. After drying at 80°C, the obtained white powder was calcined at 500°C for 2 h to achieve ZnO nanoparticles.

Preparation of magnetic TiO2/ZnO/Fe3O4 nanocomposites: The TiO2/ZnO nanoparticles were prepared as described by Tang et al. for the synthesis of ZnO nanoparticles in the presence of dispersed nano TiO2 particles [29]. Aqueous solution of Zn(NO3)2 (1 g) and urea was added into a solution of dispersed TiO2 nanoparticles (1 g) under vigorous stirring. The molar ratio of Zn+2 to urea was 1:4. The addition of a surfactant (SDS) inhibits the growth of TiO2/ZnO particles during the course of precipitation. The reaction system was heated to 95°C and after 2 h Zn(OH)2 colloid was obtained. The precipitates were then washed until the surfactant was removed completely. After drying at 75°C, the obtained white powder was calcined at 500°C for 2 h to achieve the TiO2/ZnO nanocomposite.

The magnetite nanoparticles were prepared by the conventional coprecipitation method with some modifications to the Guan et al. method (1:2 ratio of TiO2/ZnO nanocomposite to FeCl3 [30]). Two grams of FeCl3·6H2O and 0.73 g FeCl2·4H2O were dissolved in 35 mL deionized water containing 1 g of TiO2/ZnO under nitrogen gas with vigorous stirring at 85°C. 2.55 mL of 25% NH3 H2O was then added to the reaction medium. The color of the bulk solution changed to black immediately. The TiO2/ZnO/Fe3O4 nanocomposite was washed twice with deionized water and once with 0.02 m NaCl by magnetic decantation.

Preparation of TiO2/ZnO/Fe3O4/PANI and ZnO/Fe3O4/PANI nanocomposites: TiO2/ZnO/Fe3O4/PANI and ZnO/Fe3O4/PANI nanocomposites were prepared by chemical oxidative polymerization of aniline in the presence of TiO2/ZnO/Fe3O4 and ZnO/Fe3O4 nanocomposites, respectively. One gram of each nanocomposite (TiO2/ZnO/Fe3O4 and ZnO/Fe3O4) was dispersed into 100 mL of 1 m HCl aqueous solutions with ultrasonic vibrations for 15 min to obtain a uniform suspension. These nanocomposites were synthesized in 1:1 mass proportions of monomer (aniline) to TiO2/ZnO/Fe3O4 and ZnO/Fe3O4. For example, in TiO2/ZnO/Fe3O4/PANI, 980 μL of aniline was added to sonicated 1 m HCl solution containing 1 g of the TiO2/ZnO/Fe3O4 nanocomposite and stirred for half an hour to disperse aniline in the reaction media and adsorption on the surface of the TiO2/ZnO/Fe3O4 nanocomposite. Then the solution of 1 m HCl containing 2.4 g of (NH4)2S2O8 was added dropwise into the suspension solution of reaction at 0–5°C. The reaction was initiated by oxidant addition under vigorous stirring of the solution. The reactants were stirred for about 5 h and the final mixture was then filtered and washed thoroughly with acetone and distilled water to remove oligomers and unreacted monomers. After drying under vacuum at 60°C for 24 h, the final black powder was obtained. The same procedures were carried out for the synthesis of the ZnO/Fe3O4/PANI nanocomposite.

Instruments

The morphology of the products was studied with a Philips XL-30 scanning electron microscope which was equipped with an energy dispersive X-ray (EDX) detector. The thermal stability of the nanocomposites was determined using a thermogravimetric analyzer (TGA/DTA BAHR: STA 503) under air and a heating rate of 10°C min−1.

The Fourier transform infrared (FT-IR) measurements were carried out using a BOMEM MB-Series FT-IR spectrometer in the form of KBr pellets. For the dispersion of nanoparticles in aqueous medium, a Jencons ultrasonic system was used. A Methrohm digital pH meter 827 equipped with a glass calomel electrode was employed for the adjustment of pH. Samples were irradiated using visible light (fluorescent circular lamp, 22 W, 230 V, 32 400 lux, intensity determined with light meter [LT lutron modelYK-2500LX] and λ > 350 nm). The X-ray powder diffraction (XRD) data were collected on an XD-3A diffractometer using Cu K radiation.

The Brunauer-Emmett-Teller (BET) analysis of the polymeric nanocomposites was analyzed by nitrogen adsorption in a nitrogen adsorption apparatus (BELSORP-18 Plus; BEL Japan). The BET surface area (SBET) was determined using the adsorption data in the relative pressure (P/P0) range of 0–1 by a multipoint BET method. The samples were degassed at 150°C prior to the nitrogen adsorption measurements.

The chemical oxygen demand (COD) was estimated by using standard methods [31] and the total organic carbon (TOC) was measured using a Shimadzu 5000A-TOC analyzer (Japan).

Photocatalytic activity test

The photocatalytic performance of the prepared TiO2 and ZnO nanoparticles and ZnO/Fe3O4, TiO2/ZnO/Fe3O4, TiO2/ZnO/Fe3O4/PANI and ZnO/Fe3O4/PANI nanocomposite powder was assessed from the MO solution photodegradation using the nanocomposites under visible light.

A solution of MO with a concentration of 10 mg L−1 was prepared as an initial solution. 0.1 g photocatalyst powder was well ground in a glass vessel and then incorporated into a 100 mL initial MO solution. Before turning the lamp on, the suspensions containing MO and photocatalysts were sonicated in the dark for 1 min to achieve a stable suspension and adsorption–desorption equilibrium to be established. During the reaction, the suspension was kept under visible light and the suspension was continuously stirred during the process. Subsequently, the samples were taken out from the reactor and the suspended nanocomposites were separated from the mixture via a permanent hand-held magnet in less than 30 s. The transparent solution was analyzed for determination of MO concentration by measuring the maximum wavelength (465 nm) using a UV–Vis spectrophotometer [7].

Results and Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Procedures
  5. Results and Discussion
  6. Conclusion
  7. Acknowledgements
  8. References

FT-IR analysis

FT-IR spectra of the nanocomposites are shown in Fig. 1; the FT-IR spectra of neat TiO2, TiO2/ZnO, TiO2/ZnO/Fe3O4, TiO2/ZnO/Fe3O4/PANI, neat ZnO, ZnO/Fe3O4 and ZnO/Fe3O4/PANI nanocomposites are compared. The broad band below 800 cm−1 belongs to the Ti–O bond vibrations of anatase TiO2 nanoparticles [32]. In the spectrum of the pure ZnO nanoparticles, the appearance of a peak at 3475 cm−1 indicates the presence of −OH groups. The peak at 1621 cm−1 is attributed to the vibration of the Zn–O bond [29]. In the TiO2/ZnO/Fe3O4 spectrum the additional peaks around 1420 and 1550 cm−1 are assigned to the presence of Fe3O4 in the synthesized nanocomposites [33].

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Figure 1. FT-IR spectra of neat TiO2 and ZnO, and TiO2/ZnO, TiO2/ZnO/Fe3O4, TiO2/ZnO/Fe3O4/PANI, ZnO/Fe3O4 and ZnO/Fe3O4/PANI nanocomposites.

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A comparison of the polymeric nanocomposite FT-IR spectra (TiO2/ZnO/Fe3O4/PANI and ZnO/Fe3O4/PANI) with TiO2/ZnO/Fe3O4 and ZnO/Fe3O4 nanocomposites demonstrates that the new bands appear around 1460 and 1560 cm−1. These are assigned to C=C stretching vibrations of the quinoid and benzenoid rings, respectively, and indicate the successful preparation of polyaniline [34]. The polyaniline strong band at about 1108 cm−1 is attributed to the delocalization of electrons in a polymer backbone [35].

UV–Vis diffuse reflectance spectra

The UV–Vis diffuse reflectance spectra of ZnO nanoparticles, TiO2/ZnO nanocomposite, TiO2 nanoparticles, TiO2/ZnO/Fe3O4 and TiO2/ZnO/Fe3O4/PANI nanocomposites were also studied to compare their photoabsorption properties (Fig. 2). It can be seen that all the samples had a strong absorption at a wavelength range from 200 to 370 nm (UV region), except a relatively strong absorption in the visible region for TiO2/ZnO/Fe3O4/PANI. It is obvious that the UV–Vis absorption of the TiO2/ZnO/Fe3O4/PANI nanocomposite is redshifted compared to that of neat ZnO and TiO2 nanoparticles because of strong interaction between polymer and nanoparticles. It can be observed that PANI has high absorption in both the UV and visible light regions. Similar results were obtained for the ZnO/Fe3O4/PANI nanocomposite. These results confirm that the synthesized nanocomposites are effectively to extend the absorption of TiO2 and ZnO nanoparticles to visible light range. The band gap energies (E) obtained from the wavelength values (λmax) using equation: E = hc/λ for neat nanoparticles is >3.2 eV whereas for the TiO2/ZnO/Fe3O4/PANI nanocomposite (there are two λmax at 450 and 590 nm), they are about 2.75 and 2.1 eV, respectively. These indicate that the band gap energies of the final polymeric nanocomposites are smaller than that of the neat TiO2 and ZnO nanoparticles and should show a higher photocatalytic activity under visible light irradiation. Hence, the nanocomposites can be excited under visible light illumination to produce more electron-hole pairs, expectingly, resulting in higher photocatalytic activities.

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Figure 2. UV–Vis diffuse reflectance spectra of ZnO and TiO2 nanoparticles, and TiO2/ZnO, TiO2/ZnO/Fe3O4 and TiO2/ZnO/Fe3O4/PANI nanocomposites.

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XRD patterns

For the determination of the crystallographic structure of the neat nanoparticles and nanocomposites, the XRD technique was used. The XRD results of the TiO2 nanoparticles are shown in Fig. 3 demonstrating that the characteristic peaks corresponding to the anatase phase of TiO2, at about 2θ = 24.8, 38, 48, 54, 62, 69 and 75 corresponding to 101, 004, 200, 211, 204 and 220 tetragonal crystal planes of anatase-TiO2.

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Figure 3. XRD patterns of neat TiO2 and ZnO, and TiO2/ZnO, TiO2/ZnO/Fe3O4, TiO2/ZnO/Fe3O4/PANI, ZnO/Fe3O4 and ZnO/Fe3O4/PANI nanocomposites.

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The diffractogram of ZnO nanoparticles exhibits the peaks at 2θ = 31.8, 34.2, 36.1, 47.2, 56.3, 62.8, 67.9, 69.1 and 70.2 are assigned to the 100, 002, 101, 102, 110, 103, 200, 112 and 201 lattice planes of ZnO hexagonal wurtzite phase. For TiO2/ZnO a broad band located at 34.2 and a weak band at 56.3 are assigned to the hexagonal ZnO. The peaks located at 2θ = 23.7 and 54.6 are probably due to TiO2. Diffraction lines of the TiO2/ZnO/Fe3O4 nanocomposite were observed at 2θ = 30.2, 35.2, 42.6, 53.7, 57.3, 62.8 and 74.9. These diffraction lines can be assigned to the 220, 311, 400, 422, 511, 440 and 533 reflections, respectively, of the pure cubic spinel crystal structure of Fe3O4. The peaks at 2θ = 24, 54.7 and 74.9 are probably due to TiO2 and peaks located at 2θ = 34.2 and 70.3 are related to ZnO.

The ZnO/Fe3O4/PANI nanocomposite diffractogram reveals the doped PANI's partly crystalline structure and the two broad peaks appear at 2θ = 15, 20.6 and 25.8 corresponding to (011), (020) and (200) reflections of PANI in its emeraldine salt form [35]. This result reveals a thin layer of PANI coated on the surface of the nanocomposite; thus, the outer layer of the nanocomposite is the same as that for pure PANI. However, traces of peak at 2θ = 42.7 correspond to Fe3O4 due to 400 reflection, revealing that part of Fe3O4 has not fully coated with PANI. This result may be due to the excess amount of Fe3O4 in the nanocomposites.

Thermogravimetric analysis

Thermogravimetric analysis from room temperature to 800°C was conducted to identify the thermal stability and the amont of polymer in each nanocomposite. The thermograms of the ZnO/Fe3O4/PANI and TiO2/ZnO/Fe3O4/PANI nanocomposites are presented in Fig. 4. The weight loss below 20°°C reflects the loss of humidity and acid.

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Figure 4. Thermogravimetric analysis of ZnO/Fe3O4/PANI and TiO2/ZnO/Fe3O4/PANI nanocomposites.

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According to Fig. 4, the thermogram of the nanocomposites demonstrates that the polymer decomposes below 600°C. The amounts of PANI on the surfaces of TiO2/ZnO/Fe3O4/PANI and ZnO/Fe3O4/PANI nanocomposites are ca 10% and 25%, respectively. Consequently, the above results of the experimental analysis suggest that the nanocatalysts are successfully modified by PANI.

Scanning electron micrograph

Scanning electron micrographs of neat TiO2, TiO2/ZnO, TiO2/ZnO/Fe3O4, TiO2/ZnO/Fe3O4/PANI, neat ZnO, ZnO/Fe3O4 and ZnO/Fe3O4/PANI are presented in Fig. 5a–g respectively. The neat nanoparticles entangled by the van der Waals forces form a dense and robust network structure (Fig. 5a,e). When the neat nanoparticles were modified by the preparation of nanocomposites, the porous and monodispersed morphology was observed.

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Figure 5. SEM of (a) neat TiO2, (b) TiO2/ZnO, (c) TiO2/ZnO/Fe3O4, (d) TiO2/ZnO/Fe3O4/PANI, (e) neat ZnO, (f) ZnO/Fe3O4 and (g) ZnO/Fe3O4/PANI.

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The particle sizes of the modified nanocomposites are about 20–30 nm. In the SEM images of neat nanoparticles, aggregations are observed. The aggregations of nanoparticles are due to high surface energy of nanoparticles but the agglomeration of the modified nanocomposites diminishes obviously. The repulsion of positive charges on the PANI chain (in emeraldine salt form) avoids agglomeration of nanoparticles.

The EDX spectra are listed in Table 1 demonstrating the percentage of Ti, Zn and Fe in TiO2/ZnO, TiO2/ZnO/Fe3O4 and ZnO/Fe3O4. Because the final nanocomposites contain 10% and 25% of PANI which was grafted from the nanoparticles, the mentioned percentage of nanoparticles is reduced in polymeric photocatalysts (Table 1).

Table 1. EDX data for the composition of nanocomposites
Fe3O4ZnOTiO2PANINanocomposites
58.941.1TiO2/ZnO
35.138.226.7TiO2/ZnO/Fe3O4
59.640.4 ZnO/Fe3O4
31.534.324.210%TiO2/ZnO/Fe3O4/PANI
44.730.3-25%ZnO/Fe3O4/PANI

Photocatalytic activity test

Methyl orange is a dye having azo group and sulphonate (SO−3) showing an absorption peak at 462 nm. Photodegradation experiments were carried out under visible light and in aqueous solution in the presence of different nanoparticles and nanocomposites. Different photocatalysts, viz. TiO2, ZnO, TiO2/ZnO, TiO2/ZnO/Fe3O4, ZnO/Fe3O4, TiO2/Fe3O4/PANI, TiO2/ZnO/Fe3O4/PANI and ZnO/Fe3O4/PANI were investigated for their decolorization efficiency. The decolorization rate was monitored using UV–Vis spectroscopy by changing in the intensity of characteristic peaks. Among various photocatalysts, TiO2/ZnO/Fe3O4/PANI and ZnO/Fe3O4/PANI nanocomposites were easily decolorized aqueous solution in less than 25 min (Fig. 6 shows decolorization of aqueous solution in the presence of visible light using TiO2/ZnO/Fe3O4/PANI nanocomposite). Due to better effect of TiO2/ZnO/Fe3O4/PANI and ZnO/Fe3O4/PANI nanocomposites in the photodegradation of model compounds, all optimization tests were carried out on these two nanocomposites.

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Figure 6. Photocatalytic activity test of TiO2/ZnO/Fe3O4/PANI nanocomposite under visible light irradiation for MO degradation.

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Degradation of MO was carried out in aqueous solution in the presence of nanocomposites (100 mL of MO solution with 10 mg L−1 concentration 0.1 g nanocomposite powder). Furthermore, the optimization of some experimental parameters such as photocatalysts and MO concentrations and pH of sample were studied.

The mechanism of polymeric nanocomposites to enhance photocatalytic activity under visible light irradiation can be explained by PANI action as photosensitizer (Scheme 1). The photocatalysed decolorization of the solution is initiated by the photoexcitation of the doped PANI as semiconductor, followed by the formation of an electron-hole pair on PANI which was coated on the surface of nanoparticles (Eq. (1)). The photogenerated electron transfer into the conducting band of the TiO2/ZnO nanocomposite efficiently. Simultaneously, a positive charged hole might be formed by electrons migrating from the TiO2/ZnO valence band to PANI. With this mechanism PANI can inject electrons into the TiO2/ZnO conducting band under visible light irradiation and trigger the production of very reactive super oxide and hydroxyl radicals, which are responsible for the degradation of organic pollutants.

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Scheme 1. Mechanism of polymeric nanocomposites to enhance photocatalytic activity under visible light irradiation.

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Effect of photocatalyst and MO dosage and solution acidity

To determine the catalyst loading effect, experiments were performed by changing the concentration of the catalyst from 0.25 to 1.5 g L−1 and 10 mg L−1 at neutral pH. The decolorization efficiency of TiO2/ZnO/Fe3O4/PANI and ZnO/Fe3O4/PANI catalysts loading for MO is depicted in Fig. 7. This figure reveals that the initial slopes of the curves increase greatly by increasing catalysts dose from 0.25 to 0.75 g L−1 for MO; thereafter, the rate of decolorization remains almost constant. The maximum decolorization is observed with 0.75 g L−1 of photocatalysts. Thus, it can be concluded that a higher dose of nanocatalysts may not be useful and aggregation reduces irradiation field because of light scattering. Therefore, the catalyst dose of 0.75 g L−1 was fixed for further studies. Also, the optimization of MO concentration (the concentration from 5 to 25 mg L−1) shows 10 mg L−1 as the best concentration of the dye for full degradation (results not shown).

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Figure 7. Effect of photocatalyst dosage on decolorization efficiency.

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One of the important parameters in photodegradation, taking place on the catalysts, is the pH of the solution, as it indicates different properties of the photocatalyst surface. The degradation of MO under visible light irradiation using polymeric nanocomposites at different pH values (pH = 2–10) and fixed irradiation time (20 min) indicates that both neutral and alkaline conditions (pH >7) are more favorable for degradation than the acidic condition.

At lower pH values, the sulphonate group of MO is neutral and the amine group changes to cation (−NMe2+). It is obvious that there are some =NH+ groups in the conducting polymer chains obtained in acidic solution. However, at lower pH values, MO cations are generally excluded away from the positively charged surface of nanocomposites, and therefore the degradation rate decreases quickly.

Table 3 compares our obtained data (type of the photocatalyst, MO dosage and solution acidity) with some recent studies on MO degradation using various photocatalysts.

Effect of irradiation time and reaction kinetics on photocatalytic degradation of MO

Considering the economical needs, shortening the time of pollutant degradation is a necessary goal. Hence, the effect of visible light irradiation time on the degradation of MO using different nanoparticles and nanocomposites as photocatalysts was reviewed within 100 min irradiation. Both blank experiment (under visible light irradiation without any catalyst and with TiO2/ZnO/Fe3O4/PANI in darkness) results indicate that MO cannot be degraded easily without the photocatalyst or visible irradiation (adsorption of MO dye on catalysts is negligible and less than 10% at the start of the irradiation time of Fig. 8). As shown in Fig. 8a, the degradation of MO dye for all cases increases along irradiation time. It can be seen that the degradation using polymeric photocatalysts (TiO2/ZnO/Fe3O4/PANI and ZnO/Fe3O4/PANI) increases much faster than the others and at less than 20 min, the decolorization ratio of the first two totalled up to 90%. These data support the results found for band gap energies that are smaller for polymeric nanocatalystes in comparison with others. The figure indicates that for the ZnO/Fe3O4 nanocomposite the decrease in activity through the increase in Fe3O4 portion in comparison with TiO2/ZnO/Fe3O4 is negligible.

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Figure 8. (a) Comparison of the photocatalytic degradation of MO in the presence of synthesized photocatalysts under visible light irradiation, and (b) the relationship between −ln(C/C0) and reaction time.

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To determine the photocatalytic degradation kinetics, the relation between −ln Ct/C0 and visible light irradiation time (t) is plotted in Fig. 8b, where Ct is the concentration of MO solution at time t, C0 is the concentration at time 0 and kapp is the slope of plot which yields the photocatalytic activity of the nanocomposites. It is obvious that MO photodegradation using TiO2/ZnO/Fe3O4/PANI and ZnO/Fe3O4/PANI nanocomposites is described by a pseudo first-order kinetic [36]. The derived apparent reaction rate constants (kapp) for the catalysts which have pseudo first-order kinetic are summarized in Table 2. As shown in Fig. 8b and Table 2, the polymeric nanocomposites have higher photocatalytic activity, which is represented by the largest kapp value. For TiO2, ZnO, TiO2/ZnO/Fe3O4 and ZnO/Fe3O4, a nonlinear relation between −ln Ct/C0 and time was observed.

Table 2. Nanocomposite pore size and volume, specific surface area and kapp and R value for polymeric nanocomposites
PhotocatalystMean pore size (nm)Pore volume (m3 g−1)SBET (m2 g−1)kapp (min−1) R
TiO2/ZnO/Fe3O4/PANI8.230.15730.1340.96
ZnO/Fe3O4/PANI4.340.03532.70.0930.96
TiO2/Fe3O4/PANI5.120.08330.30.0830.95

As a large surface area for an appropriate heterogeneous catalyst is necessary, this important character was studied. The photocatalytic behavior of the nanocomposites that is shown in Fig. 8b can be well understood by considering the composition, nanoparticle morphology, powder pore size and particle specific surface area (Table 2). A large specific surface area and small pore size can supply more active sites for nanocomposites. The larger SBET of the TiO2/ZnO/Fe3O4/PANI nanocomposite denotes a better performance.

As the obtained results demonstrate, the mentioned method using surface modified nanocomposites with conducting polymers as photocatalysts has produced relatively high degradation efficiency under visible light irradiation and natural acidity in a short time. Table 3 demonstrates the comparative data from some recent studies on MO degradation using various photocatalysts.

Table 3. Comparative data from some recent studies on methyl orange (MO) degradation using various photocatalysts
PhotocatalystDegradation time (min)Photocatalyst dosage (g L−1)MO dosage (mg L−1)Solution acidityIrradiation sourceRef.
TiO2/ZnO/Fe3O4/PANI200.7510>7VisibleThis work
ZnO/Fe3O4/PANI200.7510>7VisibleThis work
TiO2/ZnO film3008.2UV(6)
ZnO2401.025>7UV(7)
ZnO–SnO2302.520UV(11)

Photocatalytic stability

The photocatalytic stability of TiO2/ZnO/Fe3O4/PANI and ZnO/Fe3O4/PANI nanocomposites was performed with optimum concentrations of MO and catalyst, and irradiation time (30 min) for each cycling run. The regeneration of the catalyst was ensured by an external magnetic field. The recovered photocatalysts (after drying) were reused in the next cycle. The results shown in Fig. 9a indicate that after 10 successive cycles under visible light, MO degradation is over 70% compared to the first cycling run. These results demonstrate that nanocomposites possess excellent photocatalytic stability and slight decrease in photoactivity after each reaction due to aggregation of nanocomposites during the degradation process.

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Figure 9. (a) Photocatalytic degradation of MO with ZnO/Fe3O4/PANI and TiO2/ZnO/Fe3O4/PANI nanocomposites in different recycling time. (b) FT-IR spectra of TiO2/ZnO/Fe3O4/PANI nanocatalyst before and after reaction.

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The FT-IR spectra of TiO2/ZnO/Fe3O4/PANI nanocatalyst before and after reaction are shown in Fig. 9b, and all of the characteristic peaks of nanocomposites are similar to that of the nanocomposite before the experiment. It indicates that the structure of the catalyst is stable and the polymeric part of the catalyst is not transformed to other organic compounds.

Mineralization studies of MO

As the COD and TOC reflect the extent of degradation or mineralization of organic materials, the percentage change in COD and TOC was studied for MO under degradation optimized conditions as a function of irradiation time using visible light. The results are depicted in Table 4. It can be seen that the speed of COD and TOC removal is slower than MO degradation which may be due to the formation of uncolored smaller species. The degradation of MO takes place in two steps. In the first step, the chromophore −N=N− broken and decolorization was observed. The second step is mineralization, for which a longer irradiation time is required.

Table 4. Comparison of decolorization, chemical oxygen demand (COD) and total organic carbon (TOC) removal percentages under visible light irradiation (after 20 min)
PhotocatalystDecolorization (%)COD (%)TOC (%)
TiO2/ZnO/Fe3O4/PANI969187
ZnO/Fe3O4/PANI918684

Conclusion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Procedures
  5. Results and Discussion
  6. Conclusion
  7. Acknowledgements
  8. References

To the best of our knowledge, this is the first report involving the utilization of three- and four-component nanocomposites for the degradation of MO as an azo dye in the presence of visible light. The degradation efficiency of MO as an organic pollution model was still over 70% after ten times reusing of the nanocomposites. The final results demonstrate that the synthesized nanocomposites are useful for the degradation of water pollutants and may be used several times without appreciable loss of activity. These stable magnetic nanocomposites can be separated easily using an external magnet and reused several times whereas the polymeric part of the catalysts is not transformed to other organic compounds. For polymeric nanocomposites large specific surface area and small pore size can supply more active sites for nanocomposites and larger kapp valuea are observed. All these advantages point to a bright future for these photocatalysts in environmental purification.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Procedures
  5. Results and Discussion
  6. Conclusion
  7. Acknowledgements
  8. References

Special thanks to Dr. Shant Shahbazian and Mani Salarian for their assistance in preparing the manuscript. We gratefully acknowledge financial support from the Research Council of Shahid Beheshti and the Iran National Science Foundation (INSF).

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  6. Conclusion
  7. Acknowledgements
  8. References
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