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

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Published 4 October 2016 © 2016 Vietnam Academy of Science & Technology
, , Citation Ngoc Nhiem Dao et al 2016 Adv. Nat. Sci: Nanosci. Nanotechnol. 7 045003 DOI 10.1088/2043-6262/7/4/045003

2043-6262/7/4/045003

Abstract

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

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1. 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 TiO2 which is cost-effective and less hazardous [1, 2]. However, TiO2 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 TiO2 for absorbing visible light, there has been little research into the photocatalytic activities of doped TiO2. Recently, in refences [57] bismuth ferrite BiFeO3 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 BiFeO3 nanoparticles was difficult to prepare due to the volatility of Bi2O3. Therefore, in the recent studies the authors followed the strategy to synthesize single phase perovskite BiFeO3 with low temperature [1214] and to study the regeneration of the photocatalytic activity on organic degradation [5, 1517].

In the present work we synthesize single phase perovskit BiFeO3 nanoparticles via polymeric precursors prepared by using polyvinyl alcohol (PVA) at relatively low temperature 500 °C. Photocatalytic activity and regeneration of BiFeO3 nanoparticles on the degradation of methyl blue, methyl orange, phenol and diazinon with various conditions also were studied.

2. Experimental

2.1. Chemicals

All agents were analytical grade and as received without further purification. Fe(NO3)3.6H2O, Bi(NO3)3.9H2O, HNO3, KOH, PVA, methylene blue, methylene orange, phenol, diazinon, a solution of H2O2 5% were purchased from Sigma-Aldrich and Merck.

2.2. Synthesis of BiFeO3

PVA used in gel combustion synthesis of BiFeO3 is water soluble and has hydroxyl ligands as side group which provides complexing sites to metal ions [18, 19].

Fe(NO3)3 and Bi(NO3)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 BiFeO3.

2.3. 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 BiFeO3 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).

2.4. 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 A0 measured after stirring for 1 h in the dark was taken as the quantity proportional to the initial concentration C0, and the absorbance At measured after variable periods was taken as the quantity proportional to the residual concentration Ct. The degradation efficiency of the material was calculated by the formula

3. Results and discussion

3.1. BiFeO3 synthesis

In this PVA gel combustion synthesis of BiFeO3, some process conditions such as calcination temperature and Bi/Fe molar ratio of the phase of perovskite BiFeO3 were investigated. The other conditions like pH, gel formation temperature were indicated in the previous works [1214].

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 BiFeO3 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 Bi2O3 and Fe2O3, and the products are pure perovskite oxide with the orthorhombic single phase of perovskite type BiFeO3, all the diffraction peaks coincided with those of standard pattern (JCPDS card No. 86-1518). There was a peak attributed to the β-Bi2O3 phase in x-ray diffraction diagram at a calcination temperature of 550 °C. This means that the perovskite structure of BiFeO3 was destroyed in air to form a single phase of metal oxide.

Figure 1.

Figure 1. X-ray diffraction diagrams of BiFeO3 powder samples prepared by calcination of the precursor at different temoeratures: (a) 250 °C, (b) 450 °C, (c) 500 °C and (d) 550 °C for 2 h.

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The metal molar ratio of the Bi/Fe has a strong influence on the perovskite formation of BiFeO3. 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-Bi2O3, Bi36Fe2O57, and α-Bi2O3 [20]. At the Bi/Fe ratio of 1/1, only orthorhombic single phase of BiFeO3 perovskite type was observed.

Figure 2.

Figure 2. X-ray diffraction diagrams of the BiFeO3 powder samples with Bi/Fe molar ratios of (a) 1/5, (b) 1/3, (c) 1/1, (d) 3/1, and (e) 5/1 calcined at 500 °C for 2 h.

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3.2. Characterization of synthesized BiFeO3

According to the representative results for the factors influenced on phase perovskite formation of BiFeO3, 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 130oC 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 BiFeO3 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 BiFeO3. That means the TG-DTA curves have a strong agreement with the XRD spectra in figure 1.

Figure 3.

Figure 3. TG and DTA diagrams curves of the as-prepared gel.

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The BiFeO3 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 BiFeO3 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].

3.2.2. BET surface area, SEM, TEM images, XRD and EDS

BET surface area of calcined powder was found to be 50 m2 g−1. The data obtains of the nanoparticle BiFeO3 that could be applied in catalysis and adsorption.

TEM and SEM micrograph images of the BiFeO3 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 BiFeO3 nanoparticles in 100 nm scale.

Figure 4.

Figure 4. FE-SEM images of the BiFeO3 powder sample with Bi/Fe molar ratio of 1/1 calcined at 500 °C for 2 h.

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Figure 5.

Figure 5. X-ray diffraction of the BiFeO3 powder sample with Bi/Fe molar ratio of 1/1 calcined at 500 °C for 2 h.

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Figure 6.

Figure 6. TEM images of the BiFeO3 powder sample with Bi/Fe molar ratio of 1/1 calcined at 500 °C for 2 h.

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EDS has again confirmed the composition of the products. Figure 7 shows the EDS spectra of BiFeO3 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).

Figure 7.

Figure 7. EDS analysis of the BiFeO3 powder sample with Bi/Fe molar ratio of 1/1 calcined at 500 °C for 2 h.

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Table 1.  The chemical composition of the BiFeO3.

Elements Bi Fe O
Theory percentage (%) 66.77 17.90 15.33
Analytical percentage (%) 67.21 16.36 15.74

3.3. Visible light photocatalytic activity of MB and MO

The nanoparticle perovskite type BiFeO3 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 BiFeO3 in the darkness for 24 h has not been able to degrade MO and MB solution.

3.3.2. Photocatalytic activity under the visible irradiation

With the ratio of materials/solution of 1.0 g L−1 and the initial concentration Co = 10 ppm of MB and Co = 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 H2O2 to the reaction solution.

Figure 8.

Figure 8. Adsorbed performance of BiFeO3 powder sample with Bi/Fe molar ratio of 1/1 calcined at 500 °C for 2 h under visible light: (a) the MB and b) the MO solution.

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Table 2.  Photocatalytic performance of the methylene blue and methylene orange degradation.

Time (min) Degradation (%)
  of MB of MO
15 26.0
30 29.42 9.62
45 33.96
60 51.57 12.30
75 56.90
90 63.50 15.38
120 80.0 20.0
150 84.30
180 90.0

The photodegradation of the MO and MB by the perovskite BiFeO3 was assemble as follow [22] and figure 9:

Figure 9.

Figure 9. Proposed mechanism of photocatalytic reaction on BiFeO3.

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3.3.3. Additive H2O2 effect

A small amount of 0.1 ppm of H2O2 was added to a series of solutions prepared in different concentrations from 5 ppm to 30 ppm of MB and from 5 ppm to 10 ppm of MO. The photocatalytic activity of BiFeO3 was illustrated in table 3. There was a difference in degradation efficienciesy of MB and MO. For the MB with the concentrations from 5 ppm to 15 ppm, the degradation efficiency reached 99.0% after 15 min of illumination. In the MB solutions of 20 ppm and 30 ppm concentrations, the remained concentration of MB after 30 min and 40 min of illumination, respectively, was less than 1%. Whereas for the MO solution with concentration of 5 ppm, the degradation efficiency reached 97% after 45 min of illumination. In the case of MO with higher concentration (10 ppm), the degradation efficiency about 64.5% after 120 min under the same condition.

Table 3.  Effect of adding H2O2 on methylene blue and methylene orange degradation.

Solution concentration Degradation performance (%) during time
  15 min 30 min 45 min 60 min 120 min
MB 5 ppm >99
MB 10 ppm >99
MB 15 ppm >99
MB 20 ppm 93.80 >99
MB 30 ppm 78.30 91.40 >99
MO 5 ppm 63.0 81.10 >97
MO 10 ppm 23.80 37.80 43.70 51.20 64.50

3.3.4. Light irradiation distance effect

The other factor that has a strong influence on the photocatalytic activity of the nanomaterial perovskite BiFeO3 is the distance of the illumination. Table 4 revealed the reflectance of the light distance on the MB and MO degradation performance. Despite over 99.0% of MB was decomposed after 45 min with no distance from the solution to the light source, there was only 80.0% of efficiency with the 15 cm of lighting distance after 2 h. The MO decomposition was also reported with higher efficiency around 53.0%, and 66.2% after 60 min and 90 min respectively of the treatment if the distance was zero. However, only 20.0% of MO was degraded correspond to 15 cm light-distance reaction.

Table 4.  Distance effectiveness on the methylene blue and methylene orange degradation.

    MB and MO degradation efficiency (%) during time
Solution concentration Light distance d (cm) 15 min 30 min 45 min 60 min 90 min 120 min
MB 10 ppm 15 26.0 29.42 33.96 51.57 63.50 80.0
  0 47.20 74.80 >99
MO 10 ppm 15 9.62 12.30 15.38 20.0
  0 26.10 36.7 43.5 53.0 66.20

3.4. Regenerative performance

The recovered nanoparticles were dehydrated for 2 h in the air with the temperature of 120 °C, then the product was used again to degrade the MB solution with concentration of 20 ppm. A small amount of H2O2 was added to the solution for the first time of regeneration and the illumination took place for 30 min. The delegration performance after the use of nanomaterial for the first times decreased by only 0.1%. After the second and third times of reusing it decreased to to 99.0% and 92.80%, respectively. These results clearly demonstrated the regeneration ability of BiFeO3 nanoparticles.

The UV-visible curve in figure 10 shows the adsorption of the nanoparticle BiFeO3 in the range visible irradiation after third-time regeneration. The efficiency of degradation of the BiFeO3 powder was slightly decreasing after triple times of using for one sample from over 99.9% to 92.8%.

Figure 10.

Figure 10. UV-visible light absorption of the BiFeO3 powder with Bi/Fe molar ratio of 1/1 calcined at 500 °C for 2 h.

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3.5. 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 CO2 and H2O 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 BiFeO3 single phase nanocrystalline powder was showed the figure 11.

Figure 11.

Figure 11. The degradation efficiency of phenol and diazinon using the perovskite BiFeO3 with optimum condition of the Bi/Fe molar ratio of 1/1 calcined at 500 °C for 2 h under visible light.

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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/CeO2 performance by Massa et al [23], whereas 85.7% of diazinon, which similar to N-doped TiO2 system by Asadi et al [24] was decomposed at the same condition.

4. Conclusions

The single phase nanocrystalline powder of the BiFeO3 with the average nanoparticle size <50 nm, a surface area of 50 m2 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 BiFeO3 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 H2O2 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 H2O2. During 150 min of the reaction, 92.42% of phenol and 85.7% of diazinon were decomposed.

Acknowledgments

This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.02-2013.12.

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