International Journal of Photoenergy

Volume 2013 (2013), Article ID 234806, 9 pages

http://dx.doi.org/10.1155/2013/234806

Research Article

Synthesis and Characterization of Pyrochlore Bi₂Sn₂O₇ Doping with Praseodymium by Hydrothermal Method and Its Photocatalytic Activity Study

Weicheng Xu,¹ Guangyin Zhou,¹ Jianzhang Fang,¹ Zhang Liu,¹ YunFang Chen,¹ and Chaoping Cen²

¹College of Chemistry and Environment, South China Normal University, Guangzhou 510631, China

²The Key Laboratory of Water and Air Pollution Control of Guangdong Province, South China Institute of Environmental Sciences, Guangzhou 510655, China

Received 30 May 2013; Revised 15 July 2013; Accepted 15 July 2013

Academic Editor: Jiaguo Yu

Copyright © 2013 Weicheng Xu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Praseodymium doped Bi₂Sn₂O₇ (BSO), as a visible-light responsive photocatalyst, was prepared by a hydrothermal method with different dopant contents. The as-prepared photocatalysts were investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), N₂ adsorption-desorption isotherm, X-ray photoelectron spectroscopy analysis (XPS), and UV-Vis diffuse reflectance spectroscopy (DRS). The photocatalytic activity of prepared catalysts was evaluated by the degradation of Rhodamine Bextra (RhB) and 2,4-dichlorophenol (2,4-DCP) in aqueous solution under visible light irradiation. It was found that Pr doping inhibited the growth of crystalline size and the as-prepared materials were small in size (10–20 nm). In our experiments, Pr-doped BSO samples exhibited enhanced visible-light photocatalytic activity compared to the undoped BSO, and the optimal dopant amount of Pr was 1.0 mol% for the best photocatalytic activity. On the basis of the calculated PL spectra, the mechanism of enhanced photocatalytic activity has been discussed.

1. Introduction

Semiconductor photocatalysis has been a very attractive research topic since its potential in the treatment of biorecalcitrant components in wastewaters [1, 2]. Among the semiconductors, TiO₂ has been widely discussed as a photocatalyst to purify air and water polluted with various hazardous chemicals due to its high photocatalytic activity, low cost, nontoxicity, and good stability [3, 4]. However, low quantum yields and the lack of visible-light utilization blocked its practical application. Therefore, much effort has been done in order to enhance the photocatalytic efficiency and visible-light utilization of TiO₂, such as metal ion doping [5–7], nonmetal doping [8, 9], and coupling with other semiconductors [10]. In addition, the development of novel visible-light respond photocatalysts has been put forward and drawn great attention, such as Bi₂Ti₂O₇ [11, 12], La₂Zr₂O₇ [13], and Bi1.5MgNb1.5O₇ [14].

As a kind of pyrochlore-type composite oxides, bismuth pyrostannate (Bi₂Sn₂O₇) has received considerable attention by a number of groups due mainly to its demonstrated applications in catalysis and gas sensing [15–18]. Moens et al. [19] used Bi₂Sn₂O₇ (BSO) mechanically mixed with MoO₃ to evidence a cooperation catalytic effect between BSO and Bi₂Mo₃O₁₂. Kim et al. [20] utilized a novel method to prepare SnO₂ nanowires with BSO nanoparticles and investigated its oxygen sensing properties. Wu et al. [21] explored band engineering to improve visible light photocatalysis of nano-BSO. All previous research studies showed that it was necessary to take steps to promote the photocatalytic efficiency of pure BSO. Doping with rare earth ions also could reduce the recombination rate of electron-hole pairs and enhance the interfacial charge transfer efficiency [22], such as La-TiO₂ [23], Nd-TiO₂ [24], and Ce-Bi₂O₃ [25]. However, rare-earth doped BSO and its corresponding photocatalytic activity have been rarely investigated.

In the present work, in order to expand its absorption in visible light, the Pr-doping within the BSO photocatalyst was carried out by hydrothermal method. Furthermore, the effects of different Pr-doping amounts and the reasons accounting for the photocatalytic results were discussed. Meanwhile, the model pollutants of RhB and 2,4-CP were chosen to evaluate the photocatalytic activity of as-prepared photocatalysts under visible light irradiation.

2. Experimental

2.1. Synthesis

All chemicals were obtained commercially and used without further purification. Bi(NO₃)₃·5H₂O and K₂SnO₃·3H₂O were used as the starting materials for the syntheses. Pr-doped BSO was synthesized by hydrothermal method, and in every aqueous phase, dosage of 1.46 g of Bi(NO₃)₃·5H₂O (3 mmol) and 0.9 g of K₂SnO₃·3H₂O (3 mmol) was added to 80.0 mL of deionized water and then mixed with Pr(NO₃)₃·6H₂O according to the doping ratio of Pr/BSO of 0, 0.5, 1.0, 2.0, and 3.0 mol %, respectively. Under vigorous stirring, the pH value of the mixture was adjusted to 12 by using 2 mol/L KOH. After the solids were completely dissolved in the solution at room temperature, the autoclave was sealed, heated under autogenerous pressure at 180°C for 24 h, and then cooled down to room temperature naturally. The resultant precipitates were filtered, washed with deionized water and anhydrous ethanol, and then dried in a vacuum oven at 60°C for 12 h. The final products were milled using an agate mortar before characterization. The samples were labeled as x%-Pr-BSO, where x% was the molar ratio of Pr to BSO.

2.2. Characterization

The as-prepared samples were characterized by powder X-ray diffraction (XRD) instrument (Bruker D8 ADVANCE). The scanning electron microscopy (SEM) images were obtained on a JEOL JSM 6700F instrument with an accelerating voltage of 20 kV. Transmission electron micrographs (TEM) images were recorded using a JEM-2100HR (JEOL, Japan) microscope at an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) analysis was performed on an ESCALAB 250 photoelectron spectroscopy (Thermo-VG Scientific) at 2.0 × 10⁻¹⁰ mbar using Al Kα X-ray beam (1486.6 eV). UV-Vis diffuse reflectance spectrum (DRS) was determined on a Hitachi U-3010 UV-Vis spectrophotometer with BaSO₄ as the background between 250 and 800 nm. The BET surface area and pore volume of samples were measured by a Micromeritics ASAP 2010 nitrogen adsorption apparatus. The photoluminescence (PL) spectra of photocatalysts were recorded by a fluorescence spectrophotometer (Shimadzu Rf-540, Japan) using a 150 W xenon lamp as light source.

2.3. Photocatalytic Activity Measurement

RhB and 2,4-DCP were adopted as a representative organic pollutant to evaluate the photocatalytic activity of as-prepared samples under visible light irradiation. All photocatalytic experiments were conducted at room temperature. The catalyst (0.1 g) was immersed in a 250 mL Pyrex beaker with 100 mL of RhB solution (5 mg/L) or 2,4-DCP solution (20 mg/L). The suspensions were magnetically stirred in the dark for 40 min prior to irradiation to achieve adsorption-desorption equilibrium on the catalysts surface at room temperature. As for the photocatalytic reactions, a 300 W Xe arc lamp (LTIC 300BF, BoYi Ltd., China), with a 420 nm cut-off filter, was provided about 10 cm from the top surface of the suspensions, and the concentration of pollutants during photocatalysis was monitored by pulling out about 5.0 mL suspension periodically from the reactor and centrifuging for analysis. A UV-vis 8500 spectrophotometer (Shanghai Tianmei Science Apparatus Ltd., Co., China) was used to analyse the RhB and the high performance liquid chromatography (HPLC, Shimadzu) was assisted to determine the degradation of 2,4-DCP.

3. Results and Discussion

3.1. Phase Structure and Morphology of the As-Prepared Samples

BSO has three polymorphs (monoclinic α, face-centered cubic β, and cubic pyrochlore γ) [17], and the thermodynamically stable phase is α-BSO. In this paper, γ-BSO is obtained by a hydrothermal method. The powder XRD patterns of different Pr-doped BSO photocatalysts are presented in Figure 1. The XRD of samples shows several strong peaks at 2θ = 28.8°, 33.4°, 47.9°, and 56.9°, which represent the formation of pyrochlore structure. The diffraction peaks of pure BSO and the doped samples are similar and are found to be in a good agreement with the standard XRD patterns of cubic pyrochlore (JCPDS no. 87-0284). In terms of Pr-doped BSO, the peak at 28.8° corresponding to characteristic peak of the crystal plane (222) becomes broader and the relative intensity decreases with increasing Pr ion dosage. On the basis of the Scherrer equation, the average size of crystallite is calculated from the (222) peak of pyrochlore BSO. The results are summarized in Table 1, and the crystallite size decreased with increasing the amount of praseodymium. It can be attributed to the segregation of the praseodymium cations at the grain boundary, which inhibited the grain growth by restricting direct contact of grains [26].

Table 1: Crystallite size, BET surface areas, and pore size of different photocatalysts.

Sample
Crystallite size (222)/(nm)
(m²/g)
Pore size (nm)

Pure BSO
13.5
45.3
9.4

0.5%-Pr-BSO
12.5
52.4
9.1

1.0%-Pr-BSO
11.3
61.6
8.1

2.0%-Pr-BSO
10.6
59.3
8.5

3.0%-Pr-BSO
9.8
54.7
8.8

Figure 1: XRD analysis of different Pr-doped photocatalysts.

Figure 2 shows the typical SEM images of pure BSO and 1.0%-Pr-BSO. As seen from the SEM micrograph, a large number of small particles are presented in Figure 2(a) (BSO) and Figure 2(b) (1.0%-Pr-BSO), demonstrating that high yield of products can be readily achieved through this approach. The diameters of particles that consisted of spherical clusters are around 8–20 nm.

(a)

(b)

Figure 2: SEM images of as-prepared samples: (a) BSO sample and (b) 1%-Pr-BSO sample.

Figures 3(a) and 3(b) show the TEM photograph of BSO and 1.0%-Pr-BSO, respectively. From Figure 3(a), it can be seen that BSO nanoparticles have irregular lump-like and spherical-like morphology, and the diameter is about 8–20 nm. However, 1.0%-Pr-BSO with a thin layer of about 2 nm in thickness has smaller grain size than BSO, which is consistent well with that calculated from the XRD data (Table 1).

(a)

(b)

Figure 3: TEM images of (a) BSO sample and (b) 1%-Pr-BSO sample.

3.2. N₂ Adsorption/Desorption Isotherm Analysis

The specific surface areas and pore size distributions of the BSO and 1.0%-Pr-BSO are measured using nitrogen gas sorption. The nitrogen adsorption-desorption isotherms of BSO (Figure 4(a)) and 1.0%-Pr-BSO (Figure 4(b)) exhibit stepwise adsorption and desorption (type-IV isotherm), indicating that the samples have mesoporous structure [26]. The specific surface area of BSO is 45.3 m²/g, but after Pr coating, the BET surface area of 1.0%-Pr-BSO is increased to 61.6 m²/g. High specific surface area of 1.0%-Pr-BSO will provide more active site for the photocatalytic reaction and enhance the interfacial reaction process. The pore size distribution plots of as-prepared samples calculated using the BJH equation from the absorption branch of the isotherm are shown in the inset of Figure 4. The result suggests that the spherical BSO sample has pronounced mesoporosity of narrow pore size distribution with average pore diameter around 12 nm. The specific surface area and pore diameter are calculated and also listed in Table 1.

(a)

(b)

Figure 4: The N₂ sorption isotherms and pore size distribution of (a) BSO and (b) 1%-Pr-BSO.

3.3. X-Ray Photoelectron Spectroscopy Analysis

1.0%-Pr-BSO is analyzed by XPS to discuss the elemental composition and chemical state on the BSO surface. The XPS survey spectrum is shown in Figure 5(a). It can be seen that 1.0%-Pr-BSO contains only five elements, namely, Bi, Sn, O, Pr, and C. Carbon probably originates from the calcinations residue of the precursor and adventitious hydrocarbon from the XPS instrument itself. In Bi 4f XPS spectrum (Figure 5(b)), two symmetric peaks at 164.3 eV and 159.1 eV, assigned to Bi 4f5/2 and Bi 4f7/2, correspond to Bi⁴⁺ and Bi³⁺, respectively [27]. From the high resolution XPS spectra, it can be observed that the binding energies of Sn 3d5/2 and O 1s are 486.8 eV and 530.4 eV, respectively. Moreover, the binding energies of Sn 3d5/2 and O 1s are consistent with the literature values for Sn⁴⁺ and O⁻ in BSO [28, 29]. Figure 5(e) shows that the peaks of Pr 3d5/2 and Pr 3d3/2 are located at 933.7 eV and 953.5 eV. The results mean that the Pr oxidation state shows the presence of Pr³⁺ [21].

(a)

(b)

(c)

(d)

(e)

Figure 5: XPS spectra of 1%-Pr-BSO sample: (a) survey spectrum, (b) Bi 4f, (c) Sn 3d, (d) O 1s, and (e) Pr 3d.

3.4. UV-Vis Diffuse Reflectance Spectra

Figure 6 displays UV-Vis diffuse reflection spectrum (DRS) of BSO precursors and Pr-BSO samples with different Pr contents. Figure 6(a) shows a phenomenon that the maximum absorptions of Pr-BSO photocatalysts, except the 1.0%-Pr-BSO sample, are slightly shifted to a shorter wavelength compared to those of pure BSO. The red shift of the absorption edge in 1.0%-Pr-BSO has been attributed to the charge-transfer transition between Pr ion f electrons and the BSO conduction or valence band [24]. As it is shown in Figure 6(b), the band gap achieved by extrapolation in 1.0%-Pr-BSO (460 nm) at the meeting point of the two tangents is about 2.70 eV. The maximum absorption wavelength and band gap of other photocatalysts are displayed in Table 2.

Table 2: Maximum absorption and band gap of as-prepared samples.

Sample
Maximum absorption (nm)
Band gap (eV)

BSO
450
2.76

0.5%-Pr-BSO
436
2.84

1.0%-Pr-BSO
460
2.70

2.0%-Pr-BSO
422
2.94

3.0%-Pr-BSO
436
2.84

(a)

(b)

Figure 6: UV-Vis diffuse reflectance spectra of (a) different Pr-doped BSOs and (b) 1%-Pr-BSO.

3.5. Photocatalytic Activity

To evaluate the photocatalytic activity of BSO and Pr-BSO with different molar ratios, the photodegradation of RhB and 2,4-DCP is carried out in aqueous dispersions under visible light illumination (Figure 7). In order to ensure adsorption/desorption equilibrium, adsorption experiments are carried out on all samples in the dark for 40 min. From Figure 7, it can be observed that the doped samples have a higher absorbtion rate compared with that of the undoped BSO. So it is obvious that Pr ions benefit the adsorption processes. When the BSO nanoparticles are modified by Pr ions, the Pr-BSO photocatalysts show an obvious enhanced photocatalytic activity and model pollutants (RhB and 2,4-DCP) are quickly decomposed with increasing irradiation time. After visible-light irradiation for 4 h, the degradation of RhB and 2,4-DCP over pure BSO reaches 52.0% and 50.2%, respectively. However, 90.8% of RhB and 87.6% of 2,4-DCP are degraded in 4 h irradiation over 1.0%-Pr-BSO. In this experiment, 1.0 mol% praseodymium is the optimal dopant content. It is found that the photocatalytic activity increased with increasing Pr loading to the optimum value and then decreased with further increase of Pr content. This result can be attributed to the influence of the space charge layer thickness [30]. As the doped contents are low, the surface barrier becomes higher and the space charge region becomes narrower, and at last the electron-hole pairs are efficiently separated before recombination. However, when the doped content is above its optimum, the space charge region becomes very narrow and the penetration depth of light into photocatalysts greatly exceeds the space charge layer, so that the electron and hole pairs recombine easily. Consequently, there is an optimum concentration of Pr ions.

(a)

(b)

Figure 7: Degradation curves of (a) RhB and (b) 2,4-DCP.

3.6. Visible Light Photodegradation Mechanism

According to many previous investigations on the mechanism of photocatalytic degradation, it is widely confirmed that hydroxyl radicals (•OH) are a key active species in the photocatalytic process [31, 32]. It can attack the adsorbed toxic organic molecules to produce oxidized species and/or decomposed products [33]. The analysis of •OH radical is performed by fluorescence technique using terephthalic acid, which can react with •OH radical in basic solution and generate 2-hydroxy terephthalic acid (TAOH) [34]. TAOH emits a strong fluorescence signal at around 426 nm on the excitation of its own 312 nm absorption band. Therefore, the formation of •OH radicals at the photocatalytic system was detected by PL technique using terephthalic acid as a probe molecule. Figure 8(a) shows the fluorescence spectra of 1%-Pr-BSO catalyst recorded at the same interval of irradiation time. It was obvious that the photoluminescence emission peak at about 426 nm was continuously enhanced. However, no fluorescence emission was observed in the absence of visible light. Figure 8(b) shows the plot of increase in fluorescence intensity against illumination time at 426 nm. The fluorescence intensity by visible light illumination in terephthalic acid solutions increased linearly against time. Based on the results, we could conclude that the large amount of •OH was generated in the 1%-Pr-BSO photocatalytic system under visible light irradiation. The generation of •OH radicals was the main active specials in the photocatalytic system.

(a)

(b)

Figure 8: (a) •OH-trapping photoluminescence spectra of 1%-Pr-BSO in solution of terephthalic acid at room temperature (ex, 312 nm; em, 426 nm) and (b) plots of the induced PL intensities of 1%-Pr-BSO at 426 nm on the irradiation time.

3.7. Discussion on Photocatalysis Process of Pr-BSO

Generally, the photoexcited state of Pr cations is generated by the absorption of light, corresponding to the transition of the electrons situated in the inner 4f orbital to the 4s orbital [35]. The excited state ions have the capability of transferring their excited energy to other adsorbed molecules (f-f transition) [36]. The transitions of 4f electrons can lead to the enforcement of the optical adsorption of catalysts [24]. Reddy et al. [37] reported that rare earth elements could be used as good electron acceptors, and it could capture excited electron and facilitate the separation of electron-hole pairs. As we all know, photons from visible irradiation are used to generate electrons and holes. The holes react with H₂O or OH⁻ to form hydroxyl radicals (•OH) and the electron-hole separation process is improved because BSO sample transfers photoelectrons to Pr when praseodymium appears on BSO surface [22, 30]. Subsequently, the electrons migrate from Pr to the adsorbed O₂ to form . •OH and are important species which posses strong capability of oxidizing and breaking RhB and 2,4-DCP in aqueous solution [25]. The photocatalytic procedure is presented in Figure 9.

Figure 9: Photocatalysis process of Pr-doped BSO photocatalyst under visible light irradiation.

4. Conclusions

Pr-doped BSO materials prepared by a facile hydrothermal method were successfully used in the photocatalytic conversion of RhB and 2,4-DCP under visible light. The nanostructured Pr-BSO samples possessed high photocatalytic activity, fine-grained distribution and large surface area. The praseodymium doping effectively shifted the BSO absorption edge and retarded the growth of crystallite. The as-prepared Pr-BSO photocatalysts displayed approximately the spherical shape and owned the mesoporous surface. An optimal Pr doping concentration existed and was 1.0 mol %, and superfluous Pr would make negative influence on photodegradation efficiency.

Acknowledgment

This work was supported by Guangdong Water and Air Pollution Control Key Laboratory (2011 A060901002) open funded project.

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