Low-temperature synthesis and investigations on photocatalytic activity of nanoparticles BiFeO3 for methylene blue and methylene orange degradation and some toxic organic compounds

The photocatalytic BiFeO3 perovskite nanoparticles were fabricated by gel combustion method using polyvinyl alcohol and corresponding metal nitrate precursors under the optimum mild conditions such as pH 2, gel formation temperature of 80 °C, metal/polyvinyl alcohol molar ratio of 1/3, metal molar ratio Bi/Fe of 1/1 and calcination temperature at 500 °C for 2 h. The prepared sample was characterized by x-ray diffraction, field scanning electron microscopy, transmission electron microscopy, Brunauer–Emmetl–Teller nitrogen adsorption method at 77 K, energy dispersive x-ray spectroscopy, ultraviolet-visible light spectrophotometry, and thermal analysis. The effects of molar ratios of starting material and calcination temperature on phase formation and morphology were investigated. The degradation of methylene blue, methylene orange and some toxic organic compounds such as phenol and diazinon under visible light irradiation by photocatalytic BiFeO3 nanoparticles were evaluated at different parameters and conditions such as the light intensity determined from the light source to the measured sample, the addition H2O2, reaction time and the regeneration performance. Obtained results showed that the synthesized perovskite BiFeO3 nanoparticles for the optimized sample have a size smaller than 50 nm and the high mean surface area of 50 m2 g−1. Degradation efficiency was almost 90.0% for methylene blue and 80.0% for methylene orange with added H2O2 after 30 min of reaction. After the 3rd time of regeneration, the BiFeO3 nanoparticles still have 92.8% of the degradation performance for removing methylene blue. Phenol and diazinon toxic compound were degraded with the performance of 92.42% and 85.7%, respectively, for 150 min


Introduction
Previously, in the treatment of water for improving its quality one often applied different chemical, physical, biological methods or their combinations, but their efficiencies were low because of the high cost and the low regeneration performance. Recently, there arose a strong interest in the use of photocatalytic TiO 2 which is cost-effective and less hazardous [1,2]. However, TiO 2 has some disadvantages such as high bandgap (approximately 3.2 eV) equivalent to the absorption wavelength in the ultraviolet range, and being difficult to recover and to regenerate [3,4]. Although there have been various studies on TiO 2 for absorbing visible light, there has been little research into the photocatalytic activities of doped TiO 2 . Recently, in refences [5][6][7] bismuth ferrite BiFeO 3 with the bandgap around 2.1 eV [8] was prepared and investigated. It is an easily reusable material and has magnetic properties at room temperature [9,10]. The previous research was focused on solid state reaction at high temperature [11], and the single phase of perovskit BiFeO 3 nanoparticles was difficult to prepare due to the volatility of Bi 2 O 3 . Therefore, in the recent studies the authors followed the strategy to synthesize single phase perovskite BiFeO 3 with low temperature [12][13][14] and to study the regeneration of the photocatalytic activity on organic degradation [5,[15][16][17].
In the present work we synthesize single phase perovskit BiFeO 3 nanoparticles via polymeric precursors prepared by using polyvinyl alcohol (PVA) at relatively low temperature 500°C. Photocatalytic activity and regeneration of BiFeO 3 nanoparticles on the degradation of methyl blue, methyl orange, phenol and diazinon with various conditions also were studied.

Synthesis of BiFeO 3
PVA used in gel combustion synthesis of BiFeO 3 is water soluble and has hydroxyl ligands as side group which provides complexing sites to metal ions [18,19].
Fe(NO 3 ) 3 and Bi(NO 3 ) 3 were mixed together in different proportions in a molar to give a concentrated solution. PVA solution was obtained by dissolution in water at 80°C then the metal solution was added to the PVA solution to have the suitable amount. The ratio of metal and PVA was applied 1/3 under the pH of 2. The solution was continuously stirred with a magnetic stirrer to remove the excess of water and turned into a very viscous and clear transparent brown-red-colour gel. A homogeneous solution was proved by the clearness of the solution. The viscous gel was dried out for 4 h in an air oven at 120°C, then the product with no turbidity or precipitation was calcinated at the suitable temperature in air for 2 h to obtain the perovskite-like single phase BiFeO 3 .

Characterization methods
The products obtained during different stages were characterized by x-ray diffraction (XRD) using Siemens D-5000 diffractometer (Germany) with Cu-Kα radiation (λ=0.154059 Å) in the range of 2θ=10°-95°, and a scanning rate of 0.02°s −1 . The average crystalline size of the BiFeO 3 was calculated from the half-width of the ceria (111) peak according to the Scherrer's equation, where the Scherrer constant was taken as 0.89. The micromorphology of the nanoparticle was evaluated by field emission scanning electron microscopy (FE-SEM) of Hitachi S-4800 microscope (Japan) and transmission electron microscopy (TEM) of JEOL JEM-1010 (USA). The surface chemical composition (EDS) of the sample was determined by Hitachi S 4800 spectrometer. Thermogravimetric analysis and differential thermal analysis (TGA-DTA) diagrams of the gel precursors were carried out on a Setaram Labsys EVO (France) from room temperature to 900°C in the air with a heating rate of 10°min −1 . The specific area was determined by using nitrogen adsorption at 77 K and the linear portion of the Brunauer-Emmett-Teller (BET) model, the average pore size was calculated by using the Barrett-Jovner-Halenda (BJH) formula, and the Quantachrome Autosorb-iQ Station 1 (USA).

Photocatalytic activity investigation
Methylene blue (MB), methylene orange (MO) and some toxic organic compounds were prepared in various concentrations, and the solutions were settled down in dark. Then the prepared solutions were irradiated together with the photocatalytic material under different conditions in the visible light by using Ace photochemical power supplies and mercury vapor lamps (USA) with 450 W (7825-3) lamp in a 50 mm quartz well. The tested solution was maintained constant throughout by a cooling circulating system. The concentrations of MB, MO and toxic organic compounds before and after the reaction were determined by using the photometric colorimetric method and the UV-1800 Shimadzu spectrophotometer (Japan). After the reaction, the catalyst was separated by centrifugation. The absorbance A 0 measured after stirring for 1 h in the dark was taken as the quantity proportional to the initial concentration C 0 , and the absorbance A t measured after variable periods was taken as the quantity proportional to the residual concentration C t . The degradation efficiency of the material was calculated by the formula

BiFeO 3 synthesis
In this PVA gel combustion synthesis of BiFeO 3, some process conditions such as calcination temperature and Bi/Fe molar ratio of the phase of perovskite BiFeO 3 were investigated. The other conditions like pH, gel formation temperature were indicated in the previous works [12][13][14]. Figure 1 shows XRD diagrams of the synthesized samples with metal/PVA molar ratio of 1/3 calcined at 250°C, 450°C, 500°C and 550°C for 2 h. For the case of the samples calcined at 250°C, no crystalline phase was observed which corresponded to the amorphous powder. The typical peak represented for the crystalline BiFeO 3 occurred at a temperature of 450°C. However, the signal was more clearly shown when increasing temperature was kept to reach 500°C. XRD diagrams of samples calcined at 500°C showed no peaks attributable to Bi 2 O 3 and Fe 2 O 3 , and the products are pure perovskite oxide with the orthorhombic single phase of perovskite type BiFeO 3 , all the diffraction peaks coincided with those of standard pattern (JCPDS card No. 86-1518). There was a peak attributed to the β-Bi 2 O 3 phase in x-ray diffraction diagram at a calcination temperature of 550°C. This means that the perovskite structure of BiFeO 3 was destroyed in air to form a single phase of metal oxide.
The metal molar ratio of the Bi/Fe has a strong influence on the perovskite formation of BiFeO 3 . The XRD patterns of the synthesized material with different Bi/Fe of 5/1, 3/1, 1/ 1, 1/3, 1/5 and calcination temperature of 500°C were illustrated in figure 2. It can be clearly seen that if the metal molar ratios of Bi/Fe differ from 1/1, then there exist different phases such as β-Bi 2 O 3 , Bi 36 Fe 2 O 57, and α-Bi 2 O 3 [20]. At the Bi/Fe ratio of 1/1, only orthorhombic single phase of BiFeO 3 perovskite type was observed.

Characterization of synthesized BiFeO 3
According to the representative results for the factors influenced on phase perovskite formation of BiFeO 3 , the optimum process conditions are calcination temperature of 500°C in air, metal molar ratio Bi/Fe of 1/1, gel formation of 80°C, metal/PVA molar ratio of 1/3, and solution pH 2.
3.2.1. TG-DTA analysis. The TG and DTA diagrams of the gel precursor illustrated in figure 3 which have two discrete weight losses were obtained around 122.11°C and 301.9°C. It has been proved by two endothermic peaks in the DTA curve. The first loss of weight (7.64%) was in a range of 70°C to 130 o C accompanied by a peak near 122.11°C in DTA curve caused by the loss of surface absorbed water or residual water in the increasing temperature processes. The major weight loss (25.49%) between 280°C and 450°C with the maximum at 301.9°C was due to the oxidation decomposition of the PVA and the decomposition of the nitrate of the precursor. Besides, the DTA curve has shown an exothermic peak near 325.56°C that might be the perovskite type formation of BiFeO 3 from the amorphous component. At the temperature of 400°C, there was no changing weight of the samples, which corresponded to the stabilized perovskite type of BiFeO 3 . That means the TG-DTA curves have a strong agreement with the XRD spectra in figure 1.
The BiFeO 3 material under the optimum process conditions is in the orthorhombic single phase of the   perovskite type, as reported in some papers using the solvothermal method [16,21]. So, the single phase of the perovskite type BiFeO 3 was synthesized by gel combustion method successfully under a low temperature of 500°C with the easily prepared initial components. This temperature is lower than the temperature used by Hengky et al [11] and the prepared nanomaterials have smaller size than the one reported by Gao et al [5]. BET surface area of calcined powder was found to be 50 m 2 g −1 . The data obtains of the nanoparticle BiFeO 3 that could be applied in catalysis and adsorption.
TEM and SEM micrograph images of the BiFeO 3 particles provide the information about their size and morphology. It can be seen clearly from the figure 4 that the homogeneous morphology of the sample was again proved by x-ray diffraction (figure 5) and the nanosize of the particle in a range smaller than 50 nm was observed. Figure 6 illustrates the TEM micrograph image of the BiFeO 3 nanoparticles in 100 nm scale.
EDS has again confirmed the composition of the products. Figure 7 shows the EDS spectra of BiFeO 3 particles. The EDS spectrum pointed out the presence of bismuth, iron, and oxygen of the prepared sample. The    composition of the elements in the sample obtained by the PVA-gel combustion method was 67.21%; 16.36%; and 15.74% for bismuth; iron and oxygen, respectively, in the agreement with theoretical calculation (table 1).

Visible light photocatalytic activity of MB and MO
The nanoparticle perovskite type BiFeO 3 after synthesis under optimum conditions was used to investigate the photocatalytic activity for the MB and MO degradation. The nanomaterial was studied at different conditions under the simulated natural light systems.
3.3.1. Photocatalytic performance in the darkness. Both solutions MB and MO were unchanged after 24 h. So, the perovskite nanoparticles BiFeO 3 in the darkness for 24 h has not been able to degrade MO and MB solution.

Photocatalytic activity under the visible irradiation.
With the ratio of materials/solution of 1.0 g L −1 and the initial concentration C o =10 ppm of MB and C o =10 ppm of MO, the prepared sample was stirred 30 min during the experiment. The investigation of the photocatalytic activity of the nanoparticles was described in figure 8. The figure shows that the degradation performance of the material for MB over the first 15 min was only 26.0%, then it increased to reach 90.0% when the reaction time was 3 h. Whereas the MO degradation is very low around 6.0% after 15 min of the illumination and it turns out to reach a peak at 20.0% (table 2). So, to increase the photocatalytic activity we added H 2 O 2 to the reaction solution.
The photodegradation of the MO and MB by the perovskite BiFeO 3 was assemble as follow [22] and figure 9:      figure 10 shows the adsorption of the nanoparticle BiFeO 3 in the range visible irradiation after third-time regeneration. The efficiency of degradation of  the BiFeO 3 powder was slightly decreasing after triple times of using for one sample from over 99.9% to 92.8%.

Toxic organic compounds degradation
To study the further application of the material, two toxic organic compounds were chosen to be decomposed under the visible light: phenol at concentration of 500 ppm and diazinon at concentration of 1 ppm. We assumed that the organic matter was degraded completely to CO 2 and H 2 O form as [23]. The total organic chemical was caculated by Walkley-Black methods. The decomposition performance of the phenol and diazinon in solution by the perovskite BiFeO 3 single phase nanocrystalline powder was showed the figure 11. From the results in figure 11, its can be seen clearly that when the time increases from 15 min to 150 min, the degradation performance of phenol and diazinon increases from 25.5% to 92.42% and from 15% to 85.7%, respectively. It also can be concluded that the reaction of both organic compounds happenned faster in first 60 min, 62.6% of phenol and 43.3% of diazinon were degraded. The photodecomposition process of phenol ocurred with higher efficiency. Approximately 93% of phenol was degraded after 150 min of illumination which is much higher than the CuO/ CeO 2 performance by Massa et al [23], whereas 85.7% of diazinon, which similar to N-doped TiO 2 system by Asadi et al [24] was decomposed at the same condition.

Conclusions
The single phase nanocrystalline powder of the BiFeO 3 with the average nanoparticle size <50 nm, a surface area of 50 m 2 g −1 was successfully synthesized from polymeric precursors made by polyvinyl alcohol as homogenizer under low temperature of 500°C and optimum conditions: pH 2, gel formation temperature of 80°C, metal molar ratio Bi/Fe of 1/ 1 and metal/PVA of 1/3. The photocatalytic activity and the regeneration performance of prepared nanomaterial was also studied for methylene blue and methylene orange degradation under the visible light irradiation. The nanoparticle perovskite BiFeO 3 was showed a high photocatalytic ability to decompose the organic pollutants: over 90.0% of the MB was removed under the visible irradiation for 3 h and 99.0% with additive H 2 O 2 for 30 min while around 66.0% of the MO was cleared from wastewater for 2 h. After the 3rd time of regeneration, the material also able be used by the evidence of 92.8% of removing MB with adding H 2 O 2 . During 150 min of the reaction, 92.42% of phenol and 85.7% of diazinon were decomposed.