Scholarly article on topic 'Oriented Growth of Bi2Fe4O9 Crystal and its Photocatalytic Activity'

Oriented Growth of Bi2Fe4O9 Crystal and its Photocatalytic Activity Academic research paper on "Materials engineering"

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Abstract of research paper on Materials engineering, author of scientific article — Dengrong Cai, Dan Du, Shengwen Yu, Jinrong Cheng

Abstract Oriented growth of Bi2Fe4O9 crystals were synthesized by a facile hydrothermal route in the presence of different organic additives. X-ray diffraction was used to characterize the derived powders, indicating that single-phase Bi2Fe4O9 with growth orientation along the (001) and (221) planes were obtained respectively. Extensive structural characterization of the as-prepared samples was observed using Fourier transform infrared spectroscopy, Scanning electron microscope, and UV-vis diffuse reflectance spectrum. The photocatalytic activity of Bi2Fe4O9 powders with different orientation was evaluated on degradation of methyl orange solution under visible light irradiation. The results show that the photocatalytic activity of the (001) plane is much higher than that of the (221) plane.

Academic research paper on topic "Oriented Growth of Bi2Fe4O9 Crystal and its Photocatalytic Activity"

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Procedía Engineering 27 (2012) 577 - 582

Procedía Engineering

www.elsevier.com/Iocate/procedia

2011 Chinese Materials Conference

Oriented growth of Bi2Fe4O9 crystal and its photocatalytic

activity

Dengrong Cai, Dan Du, Shengwen Yu and Jinrong Cheng

School of Materials Science and Engineering, Shanghai University, Shanghai, 200072

ABSTRACT

Oriented growth of Bi2Fe4O9 crystals were synthesized by a facile hydrothermal route in the presence of different organic additives. X-ray diffraction was used to characterize the derived powders, indicating that single-phase Bi2Fe4O9 with growth orientation along the (001) and (221) planes were obtained respectively. Extensive structural characterization of the as-prepared samples was observed using Fourier transform infrared spectroscopy, Scanning electron microscope, and UV-vis diffuse reflectance spectrum. The photocatalytic activity of Bi2Fe4O9 powders with different orientation was evaluated on degradation of methyl orange solution under visible light irradiation. The results show that the photocatalytic activity of the (001) plane is much higher than that of the (221) plane.

© 2011 Publlshed by Elsevier Ltd. Selection and/or peer-review under responsibility of Chinese Materials Research Society

Keywords: Oriented growth; Bi2Fe4O9; photocatalytic activity;

1. INTRODUCTION

Semiconductor photocatalysis using nanocrystalline titanium dioxide (TiO2) has been a promising technology in environmental cleanup [1, 2]. However, the band gap of TiO2 is rather large (3.0-3.2 eV).

Thus, the overall photocatalytic efficiency of TiO2 is limited to UV light region, which accounts for only

4-5% of the incident solar energy. Therefore, many attempts have been made to develop new photocatalysts that are able to respond to visible light irradiation [3, 4]. Recently, the photocatalytic activity of bismuth ferrites, a large family that includes sillenite- and perovskite-type compounds has been studied widely [5-7]. Among these, Bi2Fe4O9 with a band gap of 1.9-2.0 eV[8], probably has a strong absorption from UV light to visible light.

Bi2Fe4O9 has an orthorhombic structure with lattice constants of a=7.950, b=8.428, c=6.005 (space

* Corresponding author.

E-mail address: jrcheag@staff.shu.edu.cn

1877-7058 © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Chinese Materials Research Society doi:10.1016/j.proeng.2011.12.490

group of Pbam). One unit cell of Bi2Fe4O9 contains two formula units with FeO6 octahedral and FeO4 tetrahedral [9]. It is a material of great interest in many fields, such as fuel cells, semiconductor sensors, and scintillators[10-12]. Because of its high catalytic efficiency for catalytic oxidation of NH3 to NO [13, 14], Bi2Fe4O9 has been recommended as an efficient non-platinum catalyst (NPC). Moreover, Bi2Fe4O9 is an interesting magnetic material, which undergoes a transition from paramagnetism to antiferromagnetism at Neel temperature TN ~ 240-265 K [15, 16].

Previous study on tunable morphology of Bi2Fe4O9 crystals by Ruan et al. [17] indicated that Bi2Fe4O9 nanosheets showed higher photocatalytic activity toward methyl orange than that of Bi2Fe4O9 microplates. Sun /t al. [18] also found that flowerlike Bi2Fe4O9 composed of nanoplates along the (100) plane showed high-performance compared with the bulk Bi2Fe4O9. What about the photocatalytic property of Bi2Fe4O9 crystals with oriented growth?

In the present paper, the Bi2Fe4O9 crystals with (001) or (221) plane orientation were hydrothermally synthesized with the assistance of different organic additives. And the work was mainly focused on the visible light induced degradation of MO solution. The reactivity difference between (001) and (221) plane was also revealed for the first time.

2. EXPERIMENTAL PROCEDURE

5mmol Bi(NO3)3'5H2O and 10 mmol Fe(NO3^9H2O were dissolved in HCl solution (10%). To obtain Bi2Fe4O9 powders with (001) plane orientation, 0.96g polyethylene glycol (PEG 400) was added into the above solution which was stirred until a clear solution formed. And then NaOH aqueous (12 M) was slowly added under vigorous stirring. The final brown red precipitates were transferred into a 40 mL Teflon-lined autoclave up to 80% of the total volume. The hydrothermal reaction was carried out at 200 oC for 24 h, and then cooled to room temperature naturally. The products were washed with distilled water and absolute alcohol for several times, and finally dried at 70 oC for 24 h. For the Bi2Fe4O9 samples with (221) plane orientation, 5 mmol citric acid instead of PEG 400 was added under the same conditions. And the Bi2Fe4O9 samples with random orientation were also synthesized in the absence of organic additives.

The crystal structures of the as-prepared Bi2Fe4O9 samples were determined by X-ray powder diffraction (XRD) using a Rigaku D/Max-2200 diffractometer with Cu Ka radiation (X = 0.1542 nm). The morphology was investigated by Field emission scanning electron microscope (JSM-6700F SEM). UV-vis diffuse reflectance spectrum (UV-DRS) of the powders was measured using a Shimadzu UV-2501PC UV-vis spectrophotometer. Photocatalytic test for methyl orange (MO) degradation was carried out under the irradiation of a 500 W Xe lamp. A 420 nm cut-off filter was employed to provide visible light irradiation. The original concentration of MO solution was 15 mg/L with a Bi2Fe4O9 catalyst loading of 1 g/L. Before irradiation, the suspension with H2O2 of 20mmol was magnetically stirred for 2 h in dark to ensure an adsorption-desorption equilibrium between the photocatalysts and MO solution. The content of the MO solution was evaluated by measuring its absorbance at 464 nm using UV-vis spectrophotometer.

3. RESULTS AND DISCUSSION

The crystal structure of the as-prepared Bi2Fe4O9 samples was determined by X-ray diffraction (XRD), and the XRD patterns are shown in figure 1. All of the peaks in XRD patterns of the three samples could be perfectly indexed to the orthorhombic Bi2Fe4O9 without any impurity (JCPDS card No. 74-1098). It should be noted that the addition of PEG 400 results in a remarkable increase in the relative intensity of the (001) peak (Fig. 1b) compared with the conventional value (Fig. 1a), indicating that a preferred orientation along the (001) plane was formed. Meanwhile, the relative intensity of the (221) peak was

much higher than the conventional value for the Bi2Fe4O9 sample with the addition of citric acid (Fig.1c), suggesting an orientation along the (221) plane.

j* <221)

CI . .... Ill i >

b ffg . m.mi Xs â 9 m mfrn im

a |_ ... U.. . - - ■ » • - » ....

10 20 30 40 50 60 70 28(degree)

Fig.1. XRD patterns of Bi2Fe4O9 powders synthesized with different organic additives. (a) in the absence of additives, (b) PEG 400,

(c) citric acid

Figure 2 shows the FT-IR spectra of the as-prepared Bi2Fe4O9 samples. All the FT-IR spectra of the three samples are similar, and the absorption band at 438, 472, 524, 589, 639, and 813 cm-1 are assigned to Bi2Fe4O9[19]. The absorptive peak at about 1636 cm-1 was attributed to the stretching vibration of the free O-H group of water. No characteristic peaks resulting from PEG 400 or citric acid are observed, indicating that the PEG 400 or citric acid was removed entirely from the Bi2Fe4O9 powders after distilled water and absolute alcohol washing.

3500 3000 2500 2000 1 500 1 000 500 Wavenumber (cm ')

Fig.2. FT-IR spectra of Bi2Fe4O9 powders prepared under different conditions. (a) in the absence of additives, (b) PEG 400, (c) citric

The field emission scanning electron microscopy (FESEM) images of Bi2Fe4O9 powders are shown in figure 3. It can be seen that well-defined Bi2Fe4O9 crystals with plate and rectangular morphologies were obtained by changing the organic additives. The Bi2Fe4O9 plates with edge length of 400-600 nm, and a

thickness of ~160 nm were synthesized in the absence of organic additives (Fig. 3 a). As shown in figure 3b, the addition of PEG 400 resulted in a decrease in thickness and an improvement in size uniformity of the plate-like crystals. Based on the flat morphology observed in figure 3b, it is reasonable to conclude that the crystals have their broad surface more or less paralleled to the (001) plane of Bi2Fe4O9. Compared with the morphologies in figure 3a and 3b, it could be found that the (001) facet probably be the main exposed facet [20]. Submicrometer-sized Bi2Fe4O9 crystals with rectangular shape were obtained in the presence of citric acid in figure 3 c, and a greater surface area of (221) plane probably be exhibited according with the analysis from the XRD in figure1c.

Fig.3. FESEM images of Bi2Fe4O9 obtained under different conditions. (a) in the absence of additives, (b) PEG 400, (c) citric acid

Wavelength I cm ' hv ' eV

Fig.4. UV-vis diffuse reflectance spectra of the as-prepared Bi2Fe4O9 powders. (a) The absorption spectra transformed from the diffuse reflectance spectra according to the K-M theory. (b) F(R)1/2 ~ hv curves, where the linear portion is the energy band gap.

Figure 4a shows the UV-vis absorption spectra of the as-prepared Bi2Fe4O9 powders transformed from the diffuse reflectance spectra according to the Kubelka-Munk theory. The "a", "b", and "c" samples were named the Bi2Fe4O9 powders prepared in absence of additives, in presence of PEG 400, and in presence of citric acid, respectively. It can be seen that the absorption of the all the three samples was in the range from the UV light region to visible light. Figure 4b reflects the band gaps which are extrapolated to F(R)12 = 0[17]. Although the samples "a" and "b" were obtained under different conditions, the band gaps were both nearly 2.1 eV. The band gap of the "c" sample was about 2.0 eV which

suggested that the "c" sample showed a little lower oxidation-reduction activity than that of the "a" and

"b" samples.

Fig.5. Degradation curves of MO solution using the Bi2Fe4O9 powders obtained with (a) no additives (b) PEG 400 and (c) citric acid

under visible light irradiation for 3 h.

As shown in figure 5, the photocatalytic activity of the Bi2Fe4O9 samples was evaluated on degradation of MO solution. Under visible light irradiation for 3 h, the degradation efficiency of the Bi2Fe4O9 sample with random orientation toward MO is about 52%. Meanwhile, nearly 93% of MO was removed from the solution by using the Bi2Fe4O9 powders with (001) orientation as catalysts. The Bi2Fe4O9 sample with addition of citric acid showed the lowest degradation efficiency (13%) among the three samples, which was well consistent with the analytical results for the figure 4. Comparison experiment without Bi2Fe4O9 powders was also done. The degradation efficiency of 0.9% indicated that the high-performance of the "b" sample was majorly contributed to the Bi2Fe4O9 powders, and the function of H2O2 was explained in the previous study [18, 20].

On the basis of the above analysis, we can found that different crystal facets have different surface energy levels of the conduction and valence bands, which led to different photocatalytic activity. For Bi2Fe4O9 powders, the (001) facet had higher reactivity than the (221) facet, and the activity of other orientation is also worth studying.

4. CONCLUSIONS

In summary, a hydrothermal method assisted with different organic additives was reported to obtain single-phase Bi2Fe4O9 crystals with growth orientation along the (001) or (221) plane. The photocatalytic test toward MO solution demonstrated that the (001) plane of plate-like Bi2Fe4O9 had much higher activity than the (221) plane of rectangular Bi2Fe4O9. Therefore, it is likely to optimize the photocatalytic activity of such materials by tailoring the exposed high-reactivity facets.

ACKNOWLEDGMENTS

This work was supported by Shanghai Special Foundation of Nanotechnology under Grant No. 1052nm07300, Natural Science Foundation of Shanghai under Grant No. 08ZR1407700, Shanghai education development foundation under grant No. 08SG41, Shanghai Leading Academic Disciplines (S30107) and National Nature Science Foundation of China under Grant No. 50872080.

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