Synthesis and Characterization of Pyrochlore Bi 2 Sn 2 O 7 Doping with Praseodymium by Hydrothermal Method and Its Photocatalytic Activity Study

Praseodymium doped Bi 2 Sn 2 O 7 (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), N 2 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-preparedmaterials 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 1.0mol% for the best photocatalytic activity. On the basis of the calculated PL spectra, the mechanism of enhanced photocatalytic activity has been discussed.


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, TiO 2 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 TiO 2 , such as metal ion doping [5][6][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 Bi 2 Ti 2 O 7 [11,12], La 2 Zr 2 O 7 [13], and Bi 1.5 MgNb 1.5 O 7 [14].
As a kind of pyrochlore-type composite oxides, bismuth pyrostannate (Bi 2 Sn 2 O 7 ) has received considerable attention by a number of groups due mainly to its demonstrated applications in catalysis and gas sensing [15][16][17][18].Moens et al. [19] used Bi 2 Sn 2 O 7 (BSO) mechanically mixed with MoO 3 to evidence a cooperation catalytic effect between BSO and Bi 2 Mo 3 O 12 .Kim et al. [20] utilized a novel method to prepare SnO 2 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-TiO 2 [23], Nd-TiO 2 [24], and Ce-Bi 2 O 3 [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.

Experimental
2.1.Synthesis.All chemicals were obtained commercially and used without further purification.Bi(NO 3 ) 3 ⋅5H 2 O and K 2 SnO 3 ⋅3H 2 O 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(NO 3 ) 3 ⋅5H 2 O (3 mmol) and 0.9 g of K 2 SnO 3 ⋅3H 2 O (3 mmol) was added to 80.0 mL of deionized water and then mixed with Pr(NO 3 ) 3 ⋅6H 2 O 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.

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 K X-ray beam (1486.6 eV).UV-Vis diffuse reflectance spectrum (DRS) was determined on a Hitachi U-3010 UV-Vis spectrophotometer with BaSO 4 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.

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.

Phase Structure and Morphology of the As-Prepared
Samples.BSO has three polymorphs (monoclinic , facecentered cubic , and cubic pyrochlore ) [17], and the thermodynamically stable phase is -BSO.In this paper, -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 2 = 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].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.
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).

N 2
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 m 2 /g, but after Pr coating, the BET surface area of 1.0%-Pr-BSO is increased to 61.6 m 2 /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.

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 Bi4 f 5/2 and Bi4 f 7/2 , correspond to Bi 4+ and Bi 3+ , respectively [27].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.

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.

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. 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.

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 H 2 O or OH − to form hydroxyl radicals ( • OH) and the electronhole 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 O 2 to form O 2 •− .• OH and O 2 •− 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.

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.

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

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