Huo Guoyan, Song Dayong, Gao Yangai
(College of Chemistry and Environmental Science, Hebei University, Hebei Baoding, 071002 )

2 EXPERIMENTAL
2.1 Apparatus and reagents          
Y-2000 X-ray diffractometer (Aolong X-ray Apparatus Ltd. Corporation, Dandong), UV-Vis8500 spectrometer (Tianmei Scientific Apparatus Ltd. Corporation, Shanghai), pH-3C acidimeter (Leici Analytic Apparatus Factory, Shanghai), DHG-9023A oven (Shanghai Exact Apparatus and Meter Ltd. Coporation, Shanghai)
    Methyl orange (analytical reagent grade), Sodium hydroxy (analytical reagent grade), hydric sulphate (analytical reagent grade)
2.2 Synthesis of BiFeO₃
An amount of Bi(NO₃)₃ (99.9%) and Fe(NO₃)₃ (99.5%) was dissolved in a diluted HNO₃ solution with stirring and following then citric acid was added to the mixture. Under stirring condition the above mixture solution was heated until it formed dry gel. BiFeO₃ was prepared by hand-mixing dry gel in mortar. Then they were pressed into pellets under 10Mpa pressure for 1 min following then preheating at 500°C for 6h. The sample was annealed in air at 750°C for 16h and it was cooled to room temperature in an oven. Phase analysis and characterization were carried out by Y-2000 X-ray diffractometer.
2.3 Experimental processes and data collection
The maximum absorbance of the methyl orange was obtained by using UV-Vis8500 spectrometer from 180 to 1000nm (see Table 1). The maximum absorbance of methyl orange solution is obtained at 507nm when the pH value of initial methyl orange solution is less than 3.1 and the maximum absorbance of initial orange solution is measured at 464nm when the pH value of methyl methyl orange solution is larger than 4.4. An amount of BiFeO₃ was added to a certain concentration methyl orange solution at a given pH value and following then it was exposed to solar light for several hours. Then, the photocatalyst was filtered. The degradation ratio was calculated based on formula: D = [(A₀ – A_t)/A₀]*100%, where A₀ is the absorbance of original methyl orange solution and A_t is the absorbance of methyl orange solution measured after finishing photocatalyzed reaction.

Table 1 Maximum absorbable wavelength of methyl orange under different pH

3.2 Effect of solution acidity on degradation ratio of methyl orange
Different pH values of methyl orange solution, 1mg/ dm³, 100cm³, was obtained by adding a 2.0mol/dm³ NaOH solution or by adding a 2.0mol/ dm³ H₂SO₄ solution and the absorbance of each solution was measured. Then, photocatalyst, BiFeO₃, 50mg, was introduced into the solutions mentioned above. Following they were exposed to sunlight for 4 hours from 11:00 to 15:00 in summer. Subsequently, these solutions were filtered and the absorbances were measured by using UV-Vis8500 spectrometer. Table 2 represents the influence of original solution acidity on the degradation ratio of methyl orange. It can be seen from Table 2 that optimum acidity of original methyl orange is around 2.0 in pH value and in this pH value the decgradation ratio is up to 99.3%. This can be assigned to different structure of methyl orange in different acidity. The methyl orange structure transition with acidity of solution can be showed as the following:

pH ≤ 3.1, quinonoid structure                                         pH ≥ 4.4, azo-type structure

Table 2 Effect of acidity of solution on degradation ratio of methyl orange

It indicates that quinonoid methyl orange can easily be degradated by BiFeO₃ photocatalysis.

3.3 Effect of methyl orange concentration on the degradation ratio of methyl orange
Photocatalyst, BiFeO₃, 4×100mg, was added to various concentrations of 100cm³ methyl orange solution of pH= 2 adjusted by 2mol/dm³ H₂SO₄, 1mg/dm³, 5mg/dm³, 10mg/dm³ and 15mg/dm³, respectively and following then they were exposed to solar light 4 for hours from 11:00 to 15:00 in summer. Denpendence of degradation of methyl orange on the concentration of methyl orange is shown in Fig. 2. It can be seen from Fig. 2 that degradation ratio of methyl orange decrease with the concetration of methyl orange. When the concentration of methyl orange is less than 5mg/dm³ the degradation ratio decreases slowly and is more than 85%. However, when the concentration of methyl orange is higher than 5mg/dm³ the degradation ratio decreases rapidly. This can be assigned to that the degradation products of methyl orange are absorbed on the surfaces of photocatalyst and the absorbate runs off difficultly.

Fig.2 Denpendence of degradation of methyl orange on original concentration of methyl orange

3.4 Effect of amount of photocatalyst on the degradation ratio of methyl orange
Different amounts of BiFeO₃, 40mg, 60mg, 80mg and 100mg, were added to 100cm³, 5mg/dm³, methyl orange solution of pH » 2 adjusted by 2mol/dm³ H₂SO₄, respectively. Afterwards, they were exposed to sunlight for 4 hours from 11:00 to 15:00 in summer. Dependence of degradation of methyl orange on the amount of photocatalyst, BiFeO₃, is shown in Fig. 3. It can be seen from Fig. 3 that degradation ratio of methyl orange increases rapidly and that degradation ratio is up to 93% when the amount of photocatalyst is more than 80mg in 200cm³ methyl orange solution.

Fig.3 Effect of amount of BiFeO₃ on degradation of methyl orange

3.5 Effect of irradiation time in sunlight on the degradation ratio of methyl orange
100mg photocatalyst, BiFeO₃, was added to 100cm³, 5mg/dm³, methyl orange solution of pH » 2 adjusted by 2mol/dm³ H₂SO₄. Subsequently, the solution was irradiated for different times in solar light. The dependence of degradation ratio of methyl orange on the irradiation time is presented in Fig. 4. It can be seen from Fig. 4 that the degradation ratio of methyl orange increases with irradiation time. From experimental result it can be derived that the degradation products are absorbed on the surfaces of photocatalyst, BiFeO₃, and the absorbate escapes slowly from photocatalyst. Therefore, escaping the products of degradation methyl orange from the surface of photocatalyst, BiFeO₃, is the determining-step in this reaction.

Fig. 4 Dependence of degradation ratio of methyl orange on the irradiation time

3.6 Reaction mechanism
The acidity-dependent degradation can mainly be attributed to the variations of surface charge properties of the photocatalyst. Afterward, this condition changes the absorbing behaviour of methyl orange on the photocatalyst surface. Since methyl orange has an anionic configuration its adsorption is preferred in an acidic solution. One hand, this increases methyl orange in close contact with the catalyst and enhances its oxidative degradation by positive holes or hydroxyl radicals upon photoexcitation. On the other hand, quinonoid structure of methyl orange is unstable. Based on the experimental results and analysis mentioned above, a reaction mechanism was proposed as follows:
Photo-catalyst          (1)
          (2)
    (3)
where MO and MM represent methyl orange and micromolecular substance, respectively.

ACKNOWLEDGEMENTS  This work was supported by Foundation for Doctorate, Education Department of Hebei Province (B2002102), China.

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BiFeO₃光催化甲基橙溶液降解研究
霍国燕 宋大勇 高艳改
（河北大学化学与环境科学学院，河北 保定，071002）
摘要 用溶胶凝胶法合成钙钛矿型BiFeO₃化合物，并用X射线衍射仪（XRD）对产物进行物相分析：样品属于菱方晶系，空间群为R3c. 以此化合物为光催化剂研究了溶液酸度、光催化剂的量、光照时间对甲基橙降解率的影响以及甲基橙浓度对其光催化降解率的影响，结果表明：BiFeO₃光催化降解甲基橙溶液的最佳酸度是pH值在2.0左右；催化剂BiFeO₃的最佳质量浓度为1.00 g/L，并且随着甲基橙浓度的增大其降解率降低；随着光照时间的增加，甲基橙的脱色率增加，并提出了光解的可能机理。
关键词 钙钛矿型化合物BiFeO₃ 甲基橙 光催化 降解率