Influence of La3+ Substitution on Electrical and Photocatalytic

Article pubs.acs.org/JPCC

Influence of La3+ Substitution on Electrical and Photocatalytic Behavior of Complex Bi2Sn2O7 Oxides Farheen N. Sayed, V. Grover,* B. P. Mandal, and A. K. Tyagi* Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India S Supporting Information *

ABSTRACT: Pyrochlore-type Bi2−xLaxSn2O7 (0.0 ≤ x ≤ 0.20) system has been explored for its facile synthesis and electrical and photocatalytic functionalities. The highly distorted α-polymorph of Bi2Sn2O7 was synthesized, and successive La3+ substitutions were found to increase the lattice symmetry. Electric field dependent polarization measurements showed the ferroelectric hysteresis loop for pure Bi2Sn2O7 for the first time, and La3+ substitution was found to have a bearing on the ferroelectric properties with concomitant increase in leakage current. Temperature-dependent polarization studies were also performed on various nominal compositions. Diffuse reflectance spectroscopy established the tenability of the band gap as the function of La3+ content (2.5−3.0 eV). In order to explore the multifunctionality of this unique bismuth-containing system, the photocatalytic dye degradation of rhodamine B was investigated with Bi2−xLaxSn2O7 (0.0 ≤ x ≤ 0.20) system in both UV and visible regions. The variation in band gap introduced La3+ substitution significantly enhanced the photocatalytic behavior of bismuth stannate for rhodamine B degradation. The rate constant for the nominal composition Bi1.85La0.15Sn2O7 (17.6 × 10−2 min−1) is 6-fold the value for the pure Bi2Sn2O7 (2.75 × 10−2 min−1). The degradation profiles of rhodamine B in the UV and visible regions showed that dye degradation proceeds through different mechanisms which are discussed. The present study attempts to give an insight into the variation in electrical and photocatalytic properties and relate them to changes in structure. Bi2−xLaxSn2O7 (0.0 ≤ x ≤ 0.20) system is being proposed here as multifunctional candidates for lead-free electrical materials and efficient photocatalyst in the UV and visible regions.

1. INTRODUCTION Oxide materials with the pyrochlore structure are known to have many useful properties eventually leading to technological applications which vary from their use in catalysis,1 gas sensing, frustrated magnetism, superconductivity,2 and also ferroelectric behavior.3 Of late, bismuth-containing pyrochlores have gained a lot of interest as important dielectric materials. Typically, pyrochlore structure is adopted by compounds given by the general formula A2B2O7. A detailed and comprehensive structural account of pyrochlores is given by Subramanian et al.4 Pyrochlore structures are stable when the cation radius ratios (rA/rB) lies between 1.46 and 1.78. The ideal pyrochlore structure belongs to the space group Fd3̅m wherein the metal cation A lies at 16d site and B lies at 16c site and both these sites in Fd3m ̅ are at the center of symmetry. The radius ratio of bismuth stannate is 1.70 and hence it is expected to crystallize in pyrochlore structure. The problem, however, arises when the metal ion possesses stereochemically active lone pair which is not comfortable in centrosymmetric site and hence tends to move the ion from this site to a lower symmetry site thereby resulting in lowering of symmetry of the concerned space group. Bi3+-containing compounds are the best examples of such materials. Bismuth in +3 oxidation state is therefore © XXXX American Chemical Society

associated with low-symmetry structures due to the presence of stereochemically active Bi3+ lone pair formed by hybridization of 6s and 6p states. In the case of Bi2Sn2O7, it is structurally manifested as a distorted pyrochlore structure. The predominance of distortion brought about by lone pair of Bi3+ over the radius ratio effect is obvious by the fact that, despite similarity in ionic radii of lanthanides and bismuth, lanthanide stannates are cubic in nature whereas bismuth stannate is not. In an earlier study by Brisse and Knopp,5 Bi2Sn2O7 was described as a pyrochlore-related structure which had certain extra diffraction lines which could not be indexed on cubic pattern. Later, the detailed temperature-dependent studies performed by Shannon et al.6 showed that Bi2Sn2O7 exists in three different polymorphs α (monoclinic), β (face-centered cubic), and γ (cubic). Recently, Evans et al.7 studied the structure of αBi2Sn2O7 by a systematic and exhaustive simulated annealing approach of X-ray and neutron powder diffraction data. The resulting structural model is consistent with the stereochemically active lone pair on Bi3+ leading to shifts of Bi atoms in the Received: January 9, 2013 Revised: April 18, 2013

A

dx.doi.org/10.1021/jp400248j | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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plane of the puckered ring of six surrounding oxygen atoms.7 It has been postulated that correlated Bi3+ site displacements lead to complex order structures and phase transitions as the function of temperature. It is commonly observed that the distortion from the normal centrosymmetric structures may lead to interesting electrical behavior. For instance, in the Aurivillius phase ferroelectric SrBi2Ta2O9,8,9 the transition from a paraelectric to a ferroelectric phase at 335 °C is driven in large part by the offcentering tendency of the Bi3+ ions. On similar lines, it is reasonable to expect the existence of interesting electrical properties in Bi2Sn2O7. It has been mentioned by Shannon et al. that this distortion introduced in the pyrochlore structure due to the presence of the lone pair should lead to observation of ferroelectricity specifically in the α-polymorph.6 Unlike ABO 3 perovskite oxides, where ferroelectricity is not uncommon and is a well-studied phenomenon,10 there exist very few ferroelectric pyrochlores because of their centrosymmetric space group. There are a few examples based on Cd2Nb2O711 and its variants12,13 and these compounds still remain the subject of study.14 “High-k” Bi-containing pyrochlore oxides with large dielectric constants exemplified by (Bi1.5Zn0.5)(Nb1.5Zn0.5)O7 (BZN) exist and are attracting considerable attention. In view of all this, Bi2Sn2O7 seems to be a good candidate to explore the existence of ferroelectricity. However, earlier attempts to find a ferroelectric hysteresis loop for α-Bi2Sn2O7 crystals at room temperature were apparently unsuccessful.6 Further, due to the contribution from the lone pair, the band gap of bismuth-containing compounds are found to lie in visible and near-UV regions and there has been intense research for exploring these materials for UV and visible light based photocatalysts. Photocatalytic dye degradation is a very important and common form of advanced oxidation process (AOP) to degrade a large number of various toxic pollutants like synthetic textile dyes and other industrial dyestuffs, which are harmful to the environment, hazardous to human health, and difficult to degrade by natural means. There has been some recent advancement in compounds based on d10 systems such as Sn4+, Bi3+, In3+, etc.,15 which have given encouraging results. Bismuth-based pyrochlores, in particular bismuth−titanium compounds, have been explored as potential photocatalysts. It has been shown that the valence band maximum and the conduction band minimum of Bi2Sn2O7 pyrochlore should consist mainly of hybridized 6s26p0 (on A), 5s05p0 (on B), and 2p on oxygen. A suitable band gap along with the band structure conducive for high mobility of electrons and holes makes this system worth exploring as potential photocatalysts. Very recently, Wu et al.16 have studied the photocatalytic degradation of methyl orange in the presence of nano-Bi2Sn2O7 (γ-modification). The present study intends to explore the synthesis and characterization of the Bi2−xLaxSn2O7 (0.0 ≤ x ≤ 0.20) system in order to study the structural effects introduced in the Bi2Sn2O7 system by La3+ substitution. The electrical properties of this La3+-substituted bismuth stannate system have also been investigated. Further, both UV- and visible-light-mediated photocatalytic behavior of the Bi2−xLaxSn2O7 system for the photodegradation of rhodamine B as case study has been explored.

2. EXPERIMENTAL SECTION 2.1. Synthesis. All the reactants were obtained commercially and used without further purification. Bi(NO3)3·5H2O, SnCl4·5H2O, and La(NO3)3·6H2O were used as the starting reactants. In a typical synthesis of Bi2−xLaxSn2O7 (0.0 ≤ x ≤ 0.20) compounds, stoichiometric amounts of all the three reactants were dissolved in minimum amount of dilute HNO3. The mixed hydroxides were then precipitated from this solution by adding liquid NH3 till the solution was alkaline. The precipitates were then filtered, washed thoroughly with water, and dried at 400−500 °C. The dried powders were pressed into pellets and heated at 750 °C for 10 h. 2.2. Characterization. The products so obtained were characterized by room temperature powder X-ray diffraction. Xray diffraction (XRD) measurements (10−90°) were carried out on the synthesized samples for characterization as well as to check the phase purity, using monochromatized Cu Kα radiation on a Philips X-ray diffractometer, model PW 1927. The scan rate of 0.02°/s was used for recording the X-ray diffraction patterns. Silicon was used as an external standard for correction due to instrumental broadening. Electric field dependent polarization was measured using Aixacct TF Analyzer 2000. The analysis was done by performing measurements at a range of frequencies and voltages. The P− E behavior was also investigated at different temperatures using the same setup. 2.3. Optical and Photocatalytic Studies. Diffuse reflectance UV−visible spectra were recorded in the 200−900 nm region, employing a JASCO Model V-670 spectrometer. BaSO4 was used as the reference. Rhodamine B was taken up as the model pollutant to test the efficiency of the Bi2−xLaxSn2O7 (0.0 ≤ x ≤ 0.20) system as photocatalyst. For the photocatalytic experiments, a 100 mL batch cylindrical Pyrex glass reactor was used. The irradiation source was a 400 W medium-pressure Hg lamp (SAIC) located inside a quartz tube, situated perpendicularly in the reactor with 170 mW/cm2 flux for UV-based studies and for the visible region. The initial concentration of the dye solution was taken to be 10−5 M. In a typical experiment, the reaction mixture consisted of 50 mL of 10−5 M dye solution containing catalyst powder at the concentration of 1 mg/mL. A blank containing 50 mL of dye solution without any catalyst was also taken. The dye−catalyst suspension was magnetically stirred for 30−40 min in dark to attain the adsorption−desorption equilibrium of dye on catalyst. Subsequently, the dye−catalyst suspension and the blank were irradiated with UV light and small aliquots were withdrawn at regular intervals of time (keeping the volume of reaction mixture almost constant) and UV−visible spectra were recorded. The extent of rhodamine B decomposition was determined by monitoring the decrease in absorbance value at 552 nm. The studies were carried out under neutral pH and ambient conditions. 3. RESULTS AND DISCUSSION 3.1. Structural Studies. Bi2Sn2O7 was synthesized through a coprecipitation reaction of Bi(NO3)3·5H2O and SnCl4·H2O in basic aqueous solution followed by heating at higher temperature (750 °C). The process of synthesis of the product can be proposed as follows. Addition of NH4OH leads to the formation of Sn(OH)417 which further gets converted to amorphous SnO2 due to the hydrolysis. Simultaneously, the hydroxide species reacts with Bi3+ to form Bi(OH)3. The asB



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which forces Bi3+ atoms to move away from their ideal positions leading to the noncubic α- and β-polymorphs in addition to the cubic γ-polymorph. The XRD patterns of all the nominal compositions with increasing amounts of La3+ in Bi2Sn2O7 are shown in Figure 2a.

formed Bi(OH)3 further reacts with SnO2 at high temperatures to obtain Bi2Sn2O7 product. The suggested reaction process can be formulated as Sn 4 + + 4OH− → Sn(OH)4 → SnO2

Bi(NO3)3 + 3OH− → Bi(OH)3 750 ° C

2Bi(OH)3 + 2SnO2 ⎯⎯⎯⎯⎯→ Bi 2Sn2O7 + 3H 2O

The XRD patterns of all the products in Bi2−xLaxSn2O7 (0.0 ≤ x ≤ 0.2) were recorded and carefully analyzed. Figure 1

Figure 1. X-ray diffraction pattern of Bi2Sn2O7 formed at 750 °C.

shows the XRD pattern for Bi2Sn2O7 as observed. It was observed that the powder XRD patterns of various nominal compositions synthesized at 750 °C consisted of main peaks belonging to the pyrochlore family along with some small superstructure peaks. The presence of extra small peaks at 24.5° and 31.9° and many superstructure peaks between 35° and 45° indicates the presence of less symmetric α- or β-polymorphs of Bi2Sn2O7. There is a very small difference in the XRD patterns of α- and β-polymorphs of Bi2Sn2O76,7 which is manisfested by the splitting of certain peaks in α-form. The XRD pattern belonging to Bi2Sn2O7 synthesized in the present study was matched with that reported in the literature6,7 and it was established to be α-Bi2Sn2O7. The inset in Figure 1 shows the splitting of peak at 33.5° which is expected to be present in the α-form. Hence, it can be concluded that under present synthesis conditions α-Bi2Sn2O7 is obtained. This is also the thermodynamically stable polymorph of Bi2Sn2O7 at room temperature as reported in literature.7 It may be noted that Bi2Sn2O7 has been one of the most complex structures to be solved in recent years. As reported,7 this is due in part to the complexity of the deviation of this phase from its structural aristotype rendering it intractable using conventional refinement methods. According to them, it contains 176 atoms and, in the present study, the introduction of La3+ will introduce further greater number of variables. Hence, the attempts to refine the structures have not been made in the present study and, rather, the structures of the solid solutions have been concluded by carefully matching the X-ray diffraction patterns of the various nominal compositions with those reported in literature for Bi2Sn2O7. The difference in structure of stannates for Bi3+ and Ln3+ (lanthanides) which crystallize as cubic pyrochlores (space group Fd3̅m), even though the ionic sizes of Ln3+ and Bi3+ are similar, has been attributed to the presence of lone pair on Bi3+,

Figure 2. (a) X-ray diffraction patterns of all the nominal compositions in Bi2−xLaxSn2O7, where x is (i) 0.0, (ii) 0.05, (iii) 0.10, (iv) 0.15, and (v) 0.20. (b) Splitting of peak at 33.3° with increase in La3+ content in Bi2−xLaxSn2O7, where x is (i) 0.0, (ii) 0.05, (iii) 0.10, (iv) 0.15, and (v) 0.20 .

The patterns are similar to the parent Bi2Sn2O7 compounds. It is worth noting that the strong subcell peaks remain essentially unchanged while the weaker superstructure peaks change in intensity and splitting. An important difference that can be observed with increase in La3+ substitution is a prominent decrease in intensity of peaks at 27.8°. Other than that, there is also a gradual decrease in intensity of small peaks at 30.5, 36.5, 39.7, 41.8, 43.7, and 45.6°, etc. Further, the splitting in the peak observed at 33.5° decreases with increase in La3+ substitution (Figure 2b). It can be inferred that replacing Bi3+ with La3+ (similar in size but no lone pair of electrons) introduces a more symmetrical ion at A-site of the lattice which consequently eases the distortion in the lattice. It prompts the structure to move toward β-modification. However, in the range of La3+ substitutions investigated in this study, the structure does not get converted completely to β-polymorph. It may be noted that a very small amount of SnO2 (as impurity) was detected in XRD patterns of some of the nominal compositions which C



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could be due to relatively higher volatility of bismuth which consequently leaves SnO2 in excess. Another interesting observation that is worth a mention is that, in spite of the similar ionic sizes of La3+ (1.16 Å) and Bi3+ (1.17 Å), there is a small but noticeable shift to higher angle side on substituting La3+ in the Bi2Sn2O7 lattice. Obviously, average cationic size is not the driving factor here. It has been stated that asymmetric local environments can lead to increase in the volume of coordination polyhedra.6 Due to the lone pair on Bi3+, its local coordination environment is typically highly asymmetrical and contains a mixture of short and long bonds to oxygen. Accordingly, introduction of La3+ in the lattice makes the average coordination polyhedron more symmetrical and hence the volume decreases. The above effect has also been observed by Evans et al.,7 though in their case the symmetry change was brought about by varying the temperature. 3.2. Electrical Studies. It is very well-known that the presence of structural distortions in the lattice is generally the origin of interesting electrical properties like the occurrence of ferroelectricity and relaxor behavior. The system under study, because of distorted structure, can be an interesting case for electrical studies. It has also been proposed by Shannon et al.6 that, due to distortion bestowed on the α-Bi2Sn2O7 lattice by the presence of Bi3+ lone pair, ferroelectric behavior should be observable. As mentioned earlier, however, ferroelectric hysteresis loop for α-Bi2Sn2O7 crystals at room temperature has not been observed.6 In view of this, P−E studies were performed on the Bi2−xLaxSn2O7 (0.0 ≤ x ≤ 0.20) system. The polarization electric field measurements on pure Bi2Sn2O7 showed a wellformed hysteresis loop which is shown in Figure 3a. The subsequent compositions, i.e., Bi 1.95 La 0.05 Sn 2 O 7 and Bi1.90La0.10Sn2O7, also showed similar ferroelectric loops. However, appearance of lossy behavior cannot be ignored in these cases. For the composition Bi1.85La0.15Sn2O7, the lossy behavior increases and the ferroelectric loop becomes extremely lossy in the case of Bi1.80La0.20Sn2O7 (Figure 3b). The saturation polarization of approximately 0.04 μC/cm2 at 100 Hz was observed for pure Bi2Sn2O7. For the similar field gradient (5 kV/cm), the saturation polarization does not decrease drastically till Bi1.90La0.10Sn2O7, but in general decreases with increase in La3+ content. The leakage current analysis was also performed on this series and similar to the trend in saturation polarization, the leakage current also increases with increases in La3+ content. The leakage current densities for the samples x = 0.0 to 0.1 were within 0.9−1 μA/ cm2 at 5 kV/cm whereas the compositions x = 0.15 and 0.2 show very high leakage current (10.2 and 60.4 μA/cm2, respectively). The variation in leakage current with applied field is depicted in Figure 4. Inset in Figure 4 shows the enlarged view of the trends in pure Bi2Sn2O7 and subsequent two compositions. It is clear from Figure 4 that in the compositions Bi2−xLaxSn2O7 (x = 0.15, 0.20) leakage current increases tremendously as compared to the earlier three compositions. Hence, introduction of La3+ in the lattice is probably leading to increased conduction and the leakage increases with increase in La3+ content. The temperature dependence of electrical behavior was also studied for these nominal compositions. As expected, the saturation polarization decreases with increase in temperature. Figure 5 shows the variation in polarization− electric field behavior of Bi1.95La0.05Sn2O7 at different temperatures. As expected, the retentivity (polarization at zero applied

Figure 3. P−E behavior for Bi2−xLaxSn2O7, where (a) x = 0, 0.05, and 0.10 and (b) x = 0.15 and 0.20 under similar conditions (under applied field gradient of 5 kV/cm).

Figure 4. Variation of leakage current with applied field for the Bi2−xLaxSn2O7 (0.0 ≤ x ≤ 0.20) system. Inset depicts the difference in behavior of leakage current for compositions with x = 0, 0.05, and 0.10.

field) decreases with increase in temperature and the P−E loops narrow down with increase in temperature. From the electrical analysis of these samples, it is quite obvious that bismuth stannate system consists of lossy ferroelectric materials. However, this is not highly unexpected, since the band gap measurements (discussed later) show that D



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Figure 5. Variation of P−E behavior of Bi1.95La0.05Sn2O7 with temperature.

Figure 6. Diffuse reflectance spectra for Bi2−xLaxSn2O7 (0.0 ≤ x ≤ 0.20) system: (a) x = 0.0, (b) 0.05, (c) 0.10, (d) 0.15, and (e) 0.20.

Table 1. Leakage Current Recorded for Various Compositions in Bi2−xLaxSn2O7 at the Applied Field of 4 kV/ cm for Comparison no.

nominal composition

leakage current (at 4 kV/cm) (μA/cm2)

1 2 3 4 5

Bi2Sn2O7 Bi1.95La0.05Sn2O7 Bi1.90La0.10Sn2O7 Bi1.85La0.15Sn2O7 Bi1.80La0.20Sn2O7

0.4 0.9 1.2 11 60

pyrochlores as compared to regular cubic pyrochlores. With increase in La3+ content, the effect of lone pair of electrons decreases which accounts for the blue shift observed in band gap with increase in La3+ content. The “indirect band gaps” are calculated using the α1/2 (indirect transition allowed), where α is α∝

(hν − Eg + Ep)2 Ep

( )−1

exp

kT

+

(hν − Eg − Ep)2 Ep

( )

1 − exp − kT

Here, Ep is the energy of the phonon that assists in the transition, k is Boltzmann’s constant, T is the thermodynamic temperature, α is the absorption coefficient, ν is light frequency, and h is Planck’s constant. The system is found to possess tunable band gap which varies from ∼2.5 to 3.0 eV on gradually varying the La3+ content in the Bi2−xLaxSn2O7 (0 ≤ x ≤ 0.20) system. The band gaps for the system are listed in Table 2.

these materials are semiconductors. The semiconductor ferroelectrics will have higher probability of undergoing leakage losses as compared to insulator ferroelectric materials. Further, since more symmetric structure would provide less hindrance to the movement of ions, this may increase the probability of enhanced conduction in a higher La3+-containing system (more symmetric), thus increasing the magnitude of leakage current. 3.3. Diffused Reflectance UV−Visible and Photocatalytic Studies. The compounds involving bismuth and tin have held researchers’ attention for their applicability as robust catalysts. Bi2Sn2O7 has the potential to be explored as high-performance photocatalyst because of the s- orbital contribution to both valence band and conduction band which endows it with high electron and hole mobility thereby increasing its catalytic efficiency.16 For a material to act as a photocatalyst, it should have appropriate band gap to enable it to absorb effectively, thus creating charge carriers which further aid in catalysis by producing radicals to degrade the dye molecules. Diffuse-reflectance UV studies (DR-UV) were performed on pure and La-substituted Bi2Sn2O7 to determine the band gaps as well as to explore the variation of band gap with La substitution in the present series. Figure 6 represents the diffuse reflectance UV−visible (DR-UV) spectra of Bi2−xLaxSn2O7 (0.0 ≤ x ≤ 0.20) system over different wavelength ranges. The figure clearly shows that, with increase in La3+ substitution in Bi2Sn2O7 (Figure 6, curves b−e), the absorption edge exhibits blue shift as compared to the undoped Bi2Sn2O7 (curve a) and this shift is found to increase with the increase in La3+ content. Thus, the band gap increases with increase in La3+ content. It has, in fact, been reported that band gap of pure La2Sn2O7 is 4.3 eV.18 This could be explained based on the fact that Bi3+ consists of the lone pair (nonbonding) which contributes to the decrease in band gap in bismuth

Table 2. Band Gaps for Various Nominal Compositions in Bi2−xLaxSn2O7 (0.0≤ x ≤ 0.20) As Determined from Diffuse Reflectance Studies no.

nominal composition

band gap (eV)

1 2 3 4 5


2.55 2.76 2.81 2.92 2.95

Rhodamine B was taken as the model pollutant in order to explore the potential of this system as photocatalyst in both UV- and visible-based dye degradation of rhodamine B. Rhodamine B is widely used in textile industries as a dyeing agent. It is generally toxic and the effluent water has to be treated before it is discharged in natural water stream. Since the structure of the stannate compound would vary on substituting La3+, it would throw some light on the variation of photocatalytic property with variation in lanthanum content and in turn with change in structure. Chart 1 shows the structure of rhodamine B at normal pH. The UV−visible spectrum was recorded on rhodamine B and it shows the major absorption peak at 552 nm which is in agreement with that reported in the literature.19 E



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suspension was observed with increased irradiation, which indicates that the dye is getting degraded due to the presence of catalyst. The control experiment employing blank showed that in an otherwise identical condition but in the absence of catalyst, there is no change in absorption peak intensity, thus implying that the dye degradation solely by photolysis is negligible. It is highly significant to notice that, for all the catalysts, 99− 100% dye degradation was achieved during the time intervals studied as observed from Figure 7. An explanation for the high activity of bismuth-containing photocatalysts has been given on the basis of asymmetric electron density of Bi3+ which distorts the crystal lattice. A mixture of Bi 6s and O 2p states is found to dominate the top of the valence band while Sn 5s, O 2p, and Bi 6p states dominate the bottom of the conduction band. On absorbing the incident energy, the electron goes to conduction band which is akin to Sn4+ getting reduced. Sn in its +2 oxidation state, much like Bi3+, is known to form an asymmetric electron density and have a strong preference for distorted structures20 and hence will be stabilized if the structure has some distortion. The Bi−O layers in the distorted Bi2Sn2O7 pyrochlore create a distorted structural backbone for Sn−O layers which will make reduction of the Sn atoms more feasible and energetically favorable.21 This increases the possibility of formation of charge pair necessary to carry on photocatalysis. Further, as is mentioned above, both the valence and

Chart 1

Temporal changes in the concentration of rhodamine B were monitored by examining the variations in maximum absorption at 552 nm. The blank (only dye) as well as the dye-catalyst suspension (catalyst concentration: 1 mg/mL) were irradiated with UV light for different intervals of times and their absorption spectra were recorded as mentioned in the Experimental Section. Figure 7a−c gives the degradation profile of rhodamine B in the presence of the three representative catalyst compositions Bi 2 Sn 2 O 7 , Bi 1.90 La 0.10 Sn 2 O 7 , and Bi1.85La0.15Sn2O7, after different intervals of irradiation. The insets show the absorprtion spectra of blank solutions irradiated in the similar way. The zero minute reading (which was taken after magnetically stirring both blank solution and the dye− catalyst suspension in dark for 30−40 min) showed that in the presence of the catalyst the absorbance of the dye decreases by ∼6−10% in all the cases. This indicates the extent of adsorption of dye on the catalyst particles. A decrease in intensity of the absorption peak of rhodamine B (at 552 nm) in the dye catalyst

Figure 7. UV−visible spectra of rhodamine B at different irradiation times (UV irradiation) in the presence of (a) Bi2Sn2O7, (b) Bi1.90La0.10Sn2O7, and (c) Bi1.85La0.15Sn2O7. F



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making available better charge carrier densities and thus enhancing the photocatalytic activity. An interesting observation was the gradual shift in λmax of the dye solution attributed to the de-ethylation of rhodamine B to give rhodamine which absorbs at shorter wavelength and hence the blue shift increase with increase in duration of irradiation (Figure 7). Its major absorbance band at 552 nm gets slightly blue-shifted during the irradiation process, the significance of which is discussed later in this article. To test the viability of these stannate pyrochlores as visiblelight-based photocatalysts, similar photocatalytic experiments were performed upon irradiation with visible light. As in the case of UV-based catalysis, blank as well as the dye−catalyst suspension was irradiated with visible light and at regular intervals of time, small aliquots were drawn and centrifuged, and the absorbance of the dye solution was read using the spectrophotometer. Temporal changes of the control and dye− catalyst suspension for different La3+ substitutions were monitored by studying variation in the absorption of dye at 552 nm. Figure 8 shows a typical temporal degradation profile

conduction band of Bi2Sn2O7 have contributions from s-orbitals which are better dispersed in space. It is known that large band dispersions near valence band and conduction band ensure higher mobility of photoinduced charge carriers, thus encouraging better photocatalysis.22−24 In other words, less localized (more dispersed) nature of s-orbitals is conducive for photogenerated charge carrier migration.15 Also, among all the s, p, d orbitals, s−s transitions might have the least barrier.16 All these factors lead to increased production of photoinduced charge carriers and enhanced mobility, thus increasing the photocatalytic capacity of the material. The percentage product formation as a function of time during the course of the reaction for various compositions was also determined and is shown in Figure S1 in the Supporting Information. The reaction kinetics analysis was performed based on the ‘‘pseudo-first-order’’ model23,25,26 of ln(C0/C) = kt, where C0 and C are the dye concentrations at times 0 and t, and k is the reaction rate constant. The reaction constants, k, were determined from the slope of plots of ln(C0/C) versus t for various compositions (Figure S2, Supporting Information) and are tabulated in Table 3. It was observed that the slope of Table 3. Pseudo-First-Order Rate Constants Determined for Different Nominal Compositions for the Photocatalysis of Rhodamine B in Both UV and Visible Regions k (×10−2) (min−1) no.

nominal composition

in UV region

in visible region

1 2 3 4 5


2.75 8.14 15.06 17.6 16.7

0.39 0.60 0.59 0.61 0.62

the plot and hence the pseudo-first-order rate constant increases with increase in La3+ content doped in Bi2Sn2O7. In fact, the rate constant for Bi1.85La0.15Sn2O7 is approximately 6 times that for pure Bi2Sn2O7 (Table 3). It has been observed by several groups that substituting the semiconductor materials with lanthanide ions enhances the photocatalytic efficiency of the material. This has been attributed to higher physical or chemical adsorption of organic substrate (in this case dye) on the catalyst surface due to the presence of lanthanide ions. Certain studies on the most popular photocatalyst TiO2 have revealed that photocatalytic activity of TiO2 can be significantly enhanced by substituting with lanthanide ions/oxides with 4f electron configurations, as lanthanide ions form complexes with various Lewis bases including organic acids, amines, aldehydes, alcohols, and thiols by the interaction of the functional groups with their f-orbital.27−29 Thus, substitution with lanthanide ions could provide a means to concentrate the organic pollutant at the semiconductor surface and therefore enhance the photoactivity of semiconductor.30,31 An increase in the activity of lanthanide-substituted Bi2Sn2O7 can also be attributed to similar factors. Another explanation that can be invoked for enhanced activity of La-substituted compositions could be the increasing band gap with increase in La3+ content which prevents recombination of the charge carriers (electron and hole) generated by photoirradiation. As explained previously, the favorable composition of the valence band and conduction band in bismuth stannate is conducive for carrier generation and further lanthanum substituting (by virtue of increased band gap) might be contributing to preventing recombination, thus

Figure 8. Representative degradation profile of rhodamine B in the presence of Bi1.85La0.15Sn2O7 under visible light irradiation.

for Bi1.85La0.15Sn2O7 in the visible region. The inset shows the temporal behavior for the blank subjected to similar conditions at the same time intervals. As in the case of UV light, the blank does not show any appreciable decrease in the absorbance of rhodamine B while dye catalyst suspension shows a continuous decrease in absorbance. The percentage product formation in the presence of different nominal compositions as photocatalysts was determined and is depicted in Figure S3 of the Supporting Information. Interestingly, careful comparison of the temporal degradation profile of different catalysts in this series under UV and visible irradiation hint toward different mechanisms operating under UV and visible irradiation. For example, in the case of the catalyst, Bi1.85La0.15Sn2O7 under UV and visible light irradiation (Figures 7 and 8), the λmax undergoes higher blue shift in case of degradation under visible light irradiation. Rhodamine B is the tetraethylated N,N,N′,N′-rhodamine (λmax = 552 nm).32 It has been proposed that, during the degradation process, there is a competition between de-ethylation and degradation. The deethylations will lead to successive formation of N,N,N′triethylated rhodamine (λmax = 539 nm), N,N′-diethylated rhodamine (λmax = 523 nm), N-ethylated rhodamine (λmax = 510 nm), and rhodamine (λmax = 498 nm).30 During the initial G



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of the semiconductor to generate electrons and holes in the conduction and valence bands, respectively. In this case both photogenerated electrons and holes will react to form OH radicals which can attack the dye and degrade it. Hence, probably the higher concentration of OH radicals generated during UV irradiation favors the predominance of degradation of rhodamine B as compared to the de-ethylation reaction, thus the difference. This, in fact, is also supported by the much faster rate of rhodamine degradation under UV irradiation (compare the slopes of UV and visible plots). The pseudo-first-order rate constants were determined for dye degradation under visible light irradiation in the manner similar to that under UV irradiation and the pseudo-first-order rate constants are listed in Table 3. It was observed that initially the rate increases as a small amount of Bi3+ is substituted by La3+ but then the rate constants were essentially the same for all the compositions. It might be taken as the signature of different mechanisms in the visible region wherein the self-photosensitization pathway contributes majorly as opposed to light absorption by semiconductor in the UV region. Hence, small structural and compositional changes brought about in the catalyst by Bi3+ substitution may not be that significant in the visible region as that in the UV region. The catalysts were subjected to several cycles of the dye degradation reaction. The X-ray diffraction patterns (Figure S4, Supporting Information) recorded on the catalysts tested for several dye degradation cycles were found to be similar to those recorded before subjecting them to the dye degradation reaction and hence it can be concluded that they remain unchanged during the process and are stable.

period of photodegradation of rhodamine B, there is a competition between de-ethylation and cleavage of the rhodamine B chromophore ring structure. In the present study, in case of UV degradation profile, λmax of the finally degraded product is 540 nm whereas in the case of visible-lightmediated degradation, λmax = 498 nm. Hence, it clearly indicates that in the case of UV irradiation the degradation of the chromophore ring is the predominant pathway as compared to the de-ethylation steps while the opposite is occurring for the visible light photocatalysis. Of course, the visible light irradiation at the same time causes degradation too. This can be proved by the fact that, if only de-ethylation (and no degradation) occurs, then by comparing the molar extinction coefficients of rhodamine (completely de-ethylated) and rhodamine B it can be shown that the intensity of the peak at 498 nm should be approximately 70% of the initial intensity for the peak at 552 nm.33 However, significant decrease in the intensity of the peak at 498 nm proves further degradation. Also, the temporal variation of λmax shown in Figure 9 indicates the stepwise degradation of rhodamine B, thus providing evidence of the intermediates present in the degradation process during the course of irradiation.

4. CONCLUSION The Bi2−xLaxSn2O7 (0.0 ≤ x ≤ 0.20) system was synthesized by a facile coprecipitation route followed by heating at 750 °C yielding the α-phase of Bi2Sn2O7. The substitution of La3+ tends to decrease the distortion in the bismuth stannate lattice thus tending toward the more symmetrical structure. This has profound bearing on electrical properties of this system. A ferroelectric P−E loop has been observed for the first time for pure Bi2Sn2O7 compound though it had been predicted earlier. The introduction of La3+ deteriorates the ferroelectric behavior thus making it a lossy ferroelectric. The leakage current increases with increase in La3+ substitution. Band gap tunability was observed in this system wherein band gap steadily increases on introduction of La3+ in the lattice. Superior photocatalytic properties for rhodamine B degradation were observed for this system in both the UV and visible regions. The pseudofirst order rate constants were found to increase in lanthanumcontaining compositions. The dye degradation was found to proceed by different routes in the UV and visible region. Bi2−xLaxSn2O7 (0.0 ≤ x ≤ 0.20) system could be potential candidates for lead-free ferroelectrics and efficient photocatalysts.

Figure 9. Temporal variation of λmax in the visible-light-irradiated degradation of rhodamine B in the presence of photocatalyst Bi1.85La0.15Sn2O7.

One plausible explanation for the apparent difference in the degradation pathway could be different mechanisms adopted under UV and visible radiation. The dye molecules are intensely colored conjugate species which absorb strongly in the visible region. Hence, in the visible region, it is predominantly the dye molecule (but not semiconductor particles) that is excited by visible light to appropriated excited states and after that the electron transfer occurs from an excited state of dye to the conduction band (CB) of photocatalyst. This injected CB electron of the semiconductor can then react with the preadsorbed oxygen to form oxidizing species, which can bring about the photooxidiation of dyes.34−36 The process is known as photosensitization, where the dye self-sensitizes its own oxidative transformation by the indirect formation of the oxidizing OH radicals. In the case of UV irradiation, however, even though the self-photosensitization process of the dye still exists to some extent, it is primarily the semiconductor37,38 that mainly absorb the photon with energy larger than the band gap

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ASSOCIATED CONTENT

* Supporting Information S

We present the figures for percentage product formation on rhodamine B degradation under UV and visible light irradiation, the pseudo-first-order rate plot for degradation under UV irradiation along with the XRD spectra of one of the catalysts before and after UV irradiation. This material is available free of charge via the Internet at http://pubs.acs.org. H



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AUTHOR INFORMATION

Corresponding Author

*Phone: 0091-22-2559 5330. Fax: 0091-22-25505151. E-mail: [email protected] (V.G.); [email protected] (A.K.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Department of Atomic Energy’s Science Research Council (DAE-SRC) is acknowledged for supporting this work vide sanction no. 2010/21/9-BRNS/2025 dated 7-12-2010.

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