Hydrothermal Synthesis and Photocatalytic Properties of Pyrochlore

J. Phys. Chem. C 2007, 111, 11879-11887

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Hydrothermal Synthesis and Photocatalytic Properties of Pyrochlore La2Sn2O7 Nanocubes Jia Zeng,† Hao Wang,*,† YongCai Zhang,‡ Man Kang Zhu,† and Hui Yan† The College of Materials Science and Engineering, Beijing UniVersity of Technology, Beijing 100022, China, and Department of Chemistry and Chemical Engineering, Yangzhou UniVersity, Yangzhou City 225002, China ReceiVed: December 8, 2006; In Final Form: March 12, 2007

Nanocubic La2Sn2O7 photocatalysts with pyrochlore structure have been successfully synthesized by a onepot hydrothermal method. The effects of alkaline concentration, reaction time, and hydrothermal temperature on the structures and morphologies of the resultant products were investigated. On the basis of characterization results from X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and selected area electron diffraction (SAED), a possible growth mechanism of the nanocubes under the hydrothermal conditions was proposed. The absorption spectra of as-prepared cubic La2Sn2O7 photocatalyst were determined by UV-vis spectrometer. Photocatalytic experiments showed that the La2Sn2O7 samples not only had a high activity for degradation of methyl orange, but also had the activity for generating H2 with a rate of 39 µmol/h under ultraviolet light irradiation.

1. Introduction In the past three decades, the photocatalytic properties by semiconductor-based catalysts for producing clean H2 fuel from water and the degradation of organic pollutants have received considerable attention, owing to solar-to-chemical conversion with respect to energy and environmental concerns.1-14 However, most of the previous investigations have focused on the TiO2 photocatalyst because of its high activity and chemical stability. Unfortunately, the shortcomings such as poor solar efficiency hinder their extensive applications. Therefore, in an attempt to eliminate these drawbacks, many strategies on modifying surface or bulk properties of TiO2 have been developed, including doping,5-7 loading the metals as cocatalysts,8 surface chelation,9 and mixing of two semiconductors.10 On the other hand, many researchers have diverted their attention to exploit novel photocatalysts. However, only a limited number of photocatalysts have been identified so far, and most of the active materials are perovskite-type compounds.11-14 Pyrochlore-type oxides are adopted by many binary metal elements having the general formula A2B2O7. They act as potential panacea for the material research fields,15 and show a wide range of interesting properties, that is, they are of particular technological interest for numerous applications including high efficiency catalysis,16 conduction of electricity,17 photoluminescence,18 resistance to radiation damage,19 and as a modal for paramagnetic compounds in the magic angle spinning nuclear magnetic resonance (MAS NMR) studies,20 etc. Especially, owing to their high-temperature stabilities and tunable structure, the catalytic properties of pyrochlore oxides have received much attention.21-23 La2Sn2O7 is a typical cubic pyrochlore oxide,24,25 and several synthetic methodologies for La2Sn2O7 pyrochlore have been developed, such as conventional solid-state reaction,25 coprecipitation,26 hydrothermal synthesis,27,28 etc. In contrast to the * Address correspondence to this author. Phone: 86-10-67392733. Fax: 86-10-67392445. E-mail: [email protected]. † Beijing University of Technology. ‡ Yangzhou University.

other cumbersome techniques, the hydrothermal synthesis, which can be defined as a “bottom-up” soft chemical method simply by treating the aqueous solutions or precursors at a relatively low reaction temperature in the sealed vessels, is particularly promising in the preparation of complex oxides with high-quality and high surface areas, or in some cases, reduction in sizes of the materials.29 In particular, with regard to the particle size, nanostructured materials are usually better for photocatalysis due to their high specific surface area and quantum effects.30 In the present work, we report the synthesis of La2Sn2O7 nanocube photocatalysts through a facile hydrothermal pathway. The optimum synthesis condition, structure, and morphology of the nanocubes are investigated, and a possible growth mechanism of the nanocubes is presented here. Finally, we also research the photocatalytic activities of the products under the irradiation of ultraviolet light. 2. Experimental Section 2.1. Preparation of Materials. All the reagents were analytically pure, commercially available, and used without further purification. Equal molar amounts of La(NO3)3‚6H2O and SnCl4‚5H2O were put into a beaker and dissolved by deionized water. Then NaOH solutions with different concentrations acting as the mineralizer were dropped into the above solutions to form white precipitation mixtures. After several minutes of magnetic stirring, the mixture was transferred into a Teflon-lined stainless steel autoclave (50 mL capacity) that was filled with the mixture to 80% of the total volume. The autoclave was kept at a temperature between 120 and 200 °C for a designed period of time under autogenerous pressure, so that we could examine the influences of the reaction temperature and time on the structures and morphologies of the resultant products. Then the autoclave was cooled to ambient temperature naturally, and the resulting precipitates were collected by centrifugation at 3000 rpm several times, washed with deionized water and ethanol thoroughly, and dried at 80 °C in an oven for 5 h before further characterization.

10.1021/jp0684628 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/21/2007

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Zeng et al. 12 h in the presence of different alkaline concentrations. It could be seen that the contents of NaOH played a crucial role in determining the phases of the resultant products. When 0-0.5 mol/L NaOH solutions were used, tetragonal phase SnO2 (JCPDS 72-1147) was obtained (Figure 1a,b), while a good crystalline La2Sn2O7 specimen with a cubic pyrochlore structure (JCPDS 13-0082) could be obtained with a 0.8 mol/L NaOH solution (Figure 1c). However, when the concentration of NaOH solutions was further increased to 1 mol/L, a mixture of La(OH)3 and La2Sn2O7 was obtained (Figure 1d), while hexagonal phase La(OH)3 (JCPDS 83-2034) was obtained with a 2 mol/L NaOH solution (Figure 1e). According to these results, the possible reaction processes were proposed as follows:

Figure 1. XRD patterns of the as-prepared samples at 200 °C for 12 h, using NaOH concentrations of (a) 0, (b) 0.5, (c) 0.8, (d) 1, and (e) 2 M.

2.2. Characterization of Materials. Crystallographic phases of the prepared samples were investigated by X-ray diffraction (XRD, Brucker D8-advance diffractometer with Cu KR radiation, λ ) 1.5406 Å, 40 kV, 40 mA). The morphology and composition of the samples were examined with scanning electron microscopy (SEM, Hitachi Model S-3500N), transmission electron microscopy (TEM, JEM-2010F, 200 kV), and selected area electron diffraction (SAED, JEOL-2010F, 200 kV). Sample grids were prepared by sonicating powdered samples in ethanol for 20 min and evaporating one drop of the suspension onto a carbon-coated, holey film supported on a copper grid for the above measurements. The optical absorption studies were carried out with an ultraviolet-visible-near-infrared (UV-vis-NIR) spectrophotometer (Shimadzu, UV-3101PC). The specific surface areas of the powders were determined by using a Micromeritics ASAP 2020 specific surface area and porosity analyzer in the method of Brunauer-Emmett-Teller (BET) nitrogen adsorption and desorption. 2.3. Measurement of Photocatalytic Properties. The photodegradation property of the products was tested in our homemade instruments. In this case, 0.1 g of product was dispersed into a beaker, which was filled with 100 mL of 10 mg/L methyl orange (MO) solution. Prior to irradiation, the suspensions were ultrasonically sonicated for 10 min and then magnetically stirred in a dark condition for 30 min to establish adsorption/desorption equilibrium. The suspensions were then irradiated under the UV-light. A 400 W high-pressure Hg lamp (Institute of Electric Light Source, Beijing) with a maximum emission at about 280 nm was used as the light source. The concentrations of MO solution were determined by measuring the absorbance at 464 nm with the UV-vis spectrophotometer. Photocatalytic activity of water-spilting was determined at room temperature in a closed gas circulation system with use of a high-pressure 400 W Hg lamp with a maximum emission at about 280 nm placed in an outer irradiation-type quartz reaction cell. The catalyst (0.5 g) was suspended in distilled water (total volume: 150 mL), which consisted of 25 mL of methanol, by magnetic stirring. The rates of H2 evolution were analyzed by on-line gas chromatography (Shimadzu, GC-14C, TCD, molecular sieve 5 Å column and Ar carrier). 3. Results and Discussion 3.1. Preparation of La2Sn2O7 Nanocrystals. Effects of Alkaline Concentrations. Figure 1 shows the representative XRD patterns of the hydrothermally derived samples at 200 °C for

Sn4+ + 3H2O T H2SnO3 + 4H+

(1)

H+ + OH- f H2O

(2)

∆

H2SnO3 798 SnO2V + H2O

(3)

La3+ + 3OH- f La(OH)3V

(4)

2La(OH)3 + 2H2SnO3 f La2Sn2O7V + 5H2O

(5)

H2SnO3 + 2OH- + H2O f Sn(OH)62-

(6)

It was well-known that mineralizers such as MOH (M ) K, Na, etc.) played a fundamental function in the hydrothermal process.31,32 At the beginning of the experimental stage, the La(NO3)3 and SnCl4 were dissolved and ionized in water. When there was no NaOH in the solution, owing to the stronger hydrolysis effect of Sn4+ ions, eq 1 occurred (pH ∼1) and resulted in the formation of colloidal deposition H2SnO3, then, these reactants were sealed in the autoclave and heated, SnO2 deposits (Figure 1a and 1b) were formed according to the eq 3, and a small amount content of OH- was counteracted thoroughly by H+ via eq 2. However, La3+ ions were still kept in the solution and washed away after reaction. When the content of NaOH reached 0.8 M, the H+ ions formed from the strong hydrolysis effect of Sn4+ ions were counteracted by OH-. The neutralization of H+ led to the break of the equilibrium of eq 1 and the formation of colloidal H2SnO3. At the same time, La(OH)3 precipitates were formed through eq 4. Then, during the hydrothermal process under an appropriate condition, La2Sn2O7 precipitates were formed from the neutralization reaction, eq 5, between La(OH)3 and H2SnO3, because the solubility of La2Sn2O7 was lower than that of La(OH)3 under this condition. With the increase of the NaOH contents, La(OH)3 precipitates (formed by eq 4) increased gradually and could not be dissolved, as was shown in Figure 1d,e. In the meantime, Sn species were washed away after hydrothermal reaction according to eq 6. Effects of Temperature and Reaction Time. Temperature effects on the formation of crystalline La2Sn2O7 are demonstrated through the changes of the XRD patterns in Figure 2. Only the diffraction peaks of La(OH)3 were found when the temperature was increased from 120 to 160 °C, and the dasheddotted line indicated the special peak of La(OH)3 (2θ ) 27.97°). When the reactants were treated at 180 °C for 12 h, all the diffraction peaks demonstrated that La2Sn2O7 nanocrystals formed. By further increasing the temperature to 200 °C, the diffraction patterns became more clear-cut and intense, indicating better crystallization of the La2Sn2O7 specimens. It was reasonable to deduce that the appropriate reaction temperature

Pyrochlore La2Sn2O7 Nanocubes

Figure 2. XRD patterns of the temperature series treated for 12 h.

Figure 3. XRD patterns of the time series treated at 200 °C.

was an important parameter to form the cubic pyrochlore structure La2Sn2O7 during the hydrothermal processing. Figure 3 shows the XRD patterns of the time series samples synthesized at 200 °C. When the reaction time was less than 3 h, the diffraction peaks of the resultant products were mainly attributed to La(OH)3. After 3 h of hydrothermal treatment, the feeble La2Sn2O7 crystalline phase appeared. All the diffraction peaks in the patterns could be indexed to a pure cubic phase La2Sn2O7 after 8 h. It was clear that the phase transformation was developed gradually in the time series condition, and shorter reaction time was in favor of forming the La(OH)3 phase. At this point, the XRD pattern of the 18 h sample was nearly the same as that of the 12 h one, except for a little intensity decrease. In general, the crystals grew larger for longer reaction times, as estimated according to the peak expansion on the basis of the well-known Scherrer equation in the time range. 3.2. Morphologies and Microstructures of the La2Sn2O7 Samples. To further investigate the effects of reaction time and hydrothermal temperature on morphologies and microstructures of the as-prepared samples, the well-defined results are shown in detail in Figures 4 and 5 by TEM. Figure 4a indicates that nearly spherical colloids formed by agglomeration of nanoparticles after 3 h hydrothermal reaction. When the reactants were hydrothermally treated for 5 h, the obtained product was comprised of nanospheres in the range of several tens to 150 nm, as well as rod-like products with a relatively uniform diameter of about 10 nm. XRD patterns and SAED images indicated that the disordered one-dimensional structure was the typically single-crystal La(OH)3 (shown in Figure 4b). When the reactants were treated for 8 h, some square-shaped products appeared in the edge region of the spheres, but it was still very ambiguous. With the time

J. Phys. Chem. C, Vol. 111, No. 32, 2007 11881 increasing, more nanocube nuclei formed and grew further at the cost of the La(OH)3 nanorods. Finally, the nanorods disappeared completely when the reaction time was extended to 12 h, almost all of the samples exhibited the square morphology with the size of 20-60 nm. At the corner of Figure 4d, the SAED pattern confirms that the cubic phase La2Sn2O7 single crystals were formed. The effect of reaction temperatures on the morphology evolution of the products is shown in Figure 5, which is similar to the situation of time series samples. Figure 5a shows the TEM micrograph of the as-prepared sample at 120 °C, from which one could see that the morphology of this sample is basically irregular. With an increase in the reaction temperature to 160 °C, there was still mainly La(OH)3, which was testified by XRD (shown in Figure 2). The magnified image clearly revealed the rod-like morphology among the irregular colloid (see the arrow in Figure 5b). The inset of Figure 5b shows the corresponding SAED pattern, which can be identified as the La(OH)3 polycrystals. La2Sn2O7 nanocubes could be attained at 180 °C for 12 h. Furthermore, in this case, with an increase in the reaction time, La(OH)3 started to show rod-like morphology, but owing to the different stability (or solubility) under the hydrothermal condition, the nanorods disappeared gradually with the growth of La2Sn2O7 nanocubes. Thereby, the La2Sn2O7 particles gradually grew in size and became better faceted at the cost of the nanorods. At 200 °C for 12 h, well square-shaped products formed (see Figure 5d), which was in accord with XRD results (shown in Figure 2) and the following SEM photographs. Figure 6 shows the typical SEM images of La2Sn2O7 nanocubes and indicates that the large quantity of product is achieved by using the hydrothermal approach, and almost all of the samples exhibit the cubic morphology. The crystallinity of the above-synthesized La2Sn2O7 nanocubes was also confirmed with our SAED experiment. A spot pattern from a La2Sn2O7 single-crystal cube is shown in Figure 7. In this case, the electron beam was incident along the [001] direction, and the spot array had a 4-fold axis that can be indexed with hk0 (i.e., [001] zone spots, in accordance with the extinction rule of electron diffraction of space group Fd3m), indicating a cuboid symmetry for the single-crystal nanocube. To further confirm the morphology of this cubic single crystal, tilted TEM investigation was carried out to obtain the transmission images from different viewing angles. The first image (shown in Figure 8b) was that of the particle prior to tilting 0° with a square shape and relatively clear-cut edges. When the copper grid was tilted with an angle of 28° along the Y axis, it was observed that the image of the particle was elongated and there was a transition of contrast along the direction of elongation. The center of the specimen was apparently darker than the tilted edges (see Figure 8c). The edges became fuzzy when the tilting angle was increased because of the shorter travel distance for penetrating electrons or the defects of nonperfect single crystal. All these observations confirmed that the particles were indeed cube-like. It was also interesting to note that during the tilting experiments, the well-defined morphologies of the very tiny nanocrystals might be demonstrated through this method.33 3.3. Growth Mechanism of La2Sn2O7 Nanocubes. It is wellknown that in the hydrothermal process, phase transformation proceeds along a series of increasingly stable intermediates depending on the solubility of the nanocrystals, which are strongly influenced by synthesis conditions such as mineralizers concentration, reaction time, temperature, etc. According to the TEM images, XRD patterns, and SAED results of the products


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Figure 4. TEM images of the time series treated at 200 °C: (a) 3, (b) 5, (c) 8, and (d) 12 h.

obtained in the different time and temperature series, we suggested that the formation of La2Sn2O7 nanocubes was not a simple two-step “dissolution-recrystallization” process, instead, the probable growth mechanism was elucidated as three stages. The whole process is displayed in Figure 9. When La(NO3)3 and SnCl4 were mixed with NaOH solution in the initial stage, they formed a highly disordered colloidal precursor such as La(OH)3, Sn(OH)4 (or H2SnO3‚H2O), etc. With the time prolonged and temperature elevated, La(OH)3 could rapidly grow primarily along the c-axis of the hexagonal structure to form nanorods. Here, we refer to this stage as the “La(OH)3 rapid growth stage”. In the second stage, due to the relative higher solubility of La(OH)3 compared with La2Sn2O7 under this certain hydrothermal condition, they would dissolve gently and react with Sn species to form La2Sn2O7 nuclei. Here, we refer to this stage as the “dissolved-nucleation stage”. In the final stage, the normal Ostwald ripening process occurred. As the reaction continued, the La2Sn2O7 nuclei grew gradually, the growth of larger particles at the cost of the smaller ones, due to the tendency of the solid phase in the system to self-adjust to reduce the total surface free energy, according to the well-known Gibbs-Thomson law.34 Finally, La2Sn2O7 nanocubes with a size of about 20-60 nm were formed. Here, we refer to this stage as the “La2Sn2O7 ripening stage”. For the growth mechanism of nanocubes, Murphy35 pointed out that the preferential adsorption of molecules and ions in

solution to different crystal faces directed the growth of nuclei into various shapes by controlling the growth rates along different crystal axes. For the crystal shape with a face-centercubic (fcc) nanocrystal, Wang36 suggested that the shape was mainly determined by the ratio of the growth rate in [100] to that in [111], and cubes bound by the six {100} planes will be formed when the ratio is relatively lower, as the plane with the fastest growth rate will disappear quickly. Cheon37 and coworkers verified the above conclusion. According to our experimental results, the growth process of La2Sn2O7 nanocubes was consistent with the above reports. It was meaningful to note that, in the conventional synthesis strategies, the growth of nuclei was controlled by the short-range diffusion of ions in a limited space. Therefore, it was difficult to control the morphology of the final products. However, under the hydrothermal environment, nuclei could grow freely in aqueous solution (an open space) to form the nanocrystals with their natural habit, i.e., nanorods, nanoflowers, nanocubes, etc. From this point of view, the hydrothermal method exhibited a particular advantage in the synthesis of nanostructured materials. In our experiments, the surfaces of La2Sn2O7 nanocubes are not very slippery, which is shown in Figures 7 and 8. Presumably, owing to being involved in the alkaline solution all the time, the crystallization rate of nanocubes would be enhanced; on the other hand, a large number of strong alkaline hydroxy radicals were attached on the surfaces of nanocubes,


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Figure 5. TEM images of the temperature series treated for 12 h: (a) 120, (b) 160, (c) 180, and (d) 200 °C.

Figure 6. SEM photographs of La2Sn2O7 nanocubes synthesized at 200 °C for 12 h.

which might corrade some local parts of the crystals. Therefore, the surface defective nanocubes were produced. 3.4. Photoabsorbance Property and Band Structure. Photoabsorbance of the La2Sn2O7 nanocubes was examined by using a UV-vis spectrometer. Figure 10 shows the typical diffuse reflection spectrum of La2Sn2O7 nanocubes (200 °C, 12 h) compared to those of the solid-state reaction (SSR) samples (1200 °C, 48 h). For a crystalline semiconductor, the

Figure 7. TEM image and electron diffraction pattern of the singlecrystal La2Sn2O7 nanocube obtained at 200 °C for 12 h.

optical absorption near the band edge follows the equation38ahυ ) A(hυ - Eg)n/2, where a, υ, Eg, and A were absorption coefficient, light frequency, band gap, and a constant, respectively. According to the equation, the value of n for La2Sn2O7


Zeng et al.

Figure 8. TEM images of La2Sn2O7 nanocubes (200 °C and 12 h); φ is the angle over which the copper grid was tilted.

Figure 9. The schematic diagram of the growth mechanism of the La2Sn2O7 naoncubes.

was 1. The band gap of the La2Sn2O7 nanocubes could be estimated to be 4.3 eV from the onset of the absorption edge. Therefore, the catalyst only had absorption in the UV region. Compared to the hydrothermal specimen, the SSR sample underwent the obvious red shift in the absorption edge. It might thus be ascribed to an increase in the size of the crystals,39 which might be interpreted by the nanosized effect.40 The color of La2Sn2O7 samples was white, as can be expected from the absorption spectrum. The band structure of the photocatalysts played a crucial role in determining catalytic activity. Equation 7 reported by Scaife41 could be applied for the approximate determination of the flat band potential of the photocatalysts:

Vfb(NHE) ) 2.94 - Eg

(7)

where Vfb and Eg represent a flat band potential and a band gap, respectively. This equation could be applied for oxide semiconductor photocatalysts consisting of a metal cation with a d0 or d10 configuration. According to eq 7, the bottom of the conduction band level and the top of the valence band potential of La2Sn2O7 nanocubes were estimated to be -1.36 and +2.94 eV (vs NHE), respectively. Its band structure was roughly described, as was shown in Figure 11. As can be seen, the bottom level of the conduction band was more negative than the reduction potential of H+/H2 (0 V), while the top level of the valence band was more positive than the oxidation potential of O2/H2O (+1.23 V). As indicated by the photocatalytic theory,42 the potentials of the conduction and valence band of La2Sn2O7 could split water into H2 and O2 by the appropriate excited energy. Besides, by comparison of the band structure


Figure 10. UV-vis spectra of La2Sn2O7: (a) SSR sample and (b) nanocubes prepared at 200 °C for 12 h.

Figure 11. The schematic diagram of the band structure of La2Sn2O7 nanocubes.

between La2Sn2O7 and TiO2, photogenerated electron and hole pairs of La2Sn2O7 nanocubes could reduce water to form H2 and O2 more easily than those of TiO2, whereas the band structure was just the thermodynamical requirement. At the same time, for La2Sn2O7, much shorter ultraviolet radiation was needed to excite the photogenerated electron-hole pairs. 3.5. Photocatalytic Properties of La2Sn2O7 Samples. Photodegradation Properties. The photodegradation of MO was tested as a model to evaluate photocatalytic activities of the hydrothermally prepared samples. In Figure 12, curve (a) shows the degradation curve of absolute MO solution without any La2Sn2O7 sample irradiated under ultraviolet light for 50 min, while the other curves denote the degradation of MO solution in the presence of a 1 g/L La2Sn2O7 sample irradiated for different times (b, 5 min; c, 10 min; d,20 min; e, 30 min; f, 40 min; and g, 50 min). As we can see from Figure 12, the solutions containing the La2Sn2O7 samples decolorized gradually, and were almost decolored ultimately when the irradiation time was elongated to 50 min. The degradation ratio of MO solution with La2Sn2O7 samples synthesized was up to 95%. Subsequently, the high activity of La2Sn2O7 can also be confirmed from Figure 13. In addition to experiments with La2Sn2O7 and irradiation, a dark experiment and blank experiment were investigated in the absence of irradiation with La2Sn2O7 or in the presence of irradiation without La2Sn2O7. Figure 13a demonstrates that almost no MO degradation occurs, while

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Figure 12. Degradation curves of MO solutions irradiated under ultraviolet light: (a) without La2Sn2O7 for 50 min and ×ff(b-g) with La2Sn2O7 for 5 (b), 10 (c), 20 (d), 30 (e), 40 (f), and 50 (g).

Figure 13. The degradation rate (C/C0) of MO as a function of irradiation time (C0 and C were the equilibrium concentration of MO before and after UV irradiation, respectively): (a) a dark experiment (without irradiation); (b) a blank experiment (in the absence of La2Sn2O7); (c) La2Sn2O7 under irradiation; and (d) Degussa P25 TiO2 under irradiation.

Figure 13b shows that only a small quantity of MO is degraded (less than 20%, it can be interpreted by the photolysis effect). For the purposes of comparison, the photocatalytic degradation of MO was carried out by using Degussa P25 TiO2 and La2Sn2O7 under the same condition. These experimental results showed that La2Sn2O7 exhibited almost the same photocatalytic activity as Degussa P25 TiO2 (shown in Figure 13c,d), except for elongating the irradiation time. La2Sn2O7 as a kind of heterogeneous photocatalyst can be easily recycled by a simple filtration. After five recycles for the photodegradation of methyl orange, the catalyst did not exhibit an obvious loss of activity, as shown in Figure 14, confirming that La2Sn2O7 was not photocorroded during the photocatalytic oxidation of the pollutant molecules. H2-EVolution Properties. The photocatalyst showed not only the high activity of degradation of MO, but also the activity for photocatalytic H2 evolution under UV irradiation. Figure 15 shows H2 evolution from an aqueous solution containing sacrificial reagents CH3OH over La2Sn2O7 nanocubes under UV irradiation. Herein, no cocatalyst (i.e., Pt, Ru, NiOx) was loaded on the La2Sn2O7 samples. The amount of evolved H2 gases increased linearly with an increase of irradiation time, while no gases were evolved upon turning off the light. This result confirmed that the H2 evolution came from a photocatalytic process. The average evolution rate was about 39 umol/h.


Figure 14. Cycling runs in the photocatalytic degradation of MO in the presence of La2Sn2O7 under UV irradiation.

Zeng et al. of active sites would increase, so the photocatalytic reaction would enhance dramatically. On the other hand, the hydrothermal environment also provided the conditions for producing highly crystalline products at low temperature. Although many reaction steps of photocatalysis took place on the surface, a defect in the structure could provide a site for energy-wasteful electron-hole recombination that reduced the efficiency of photocatalysis. In the end, the photocatalytic activity of La2Sn2O7 should have some connections with the intrinsic structure of the crystal, which impacted the efficiency of the move of the charge carriers.45 In a crystal of La2Sn2O7 that was constructed of octahedral SnO6, these octahedra should be equilateral polyhedra and connect each other by sharing vertexes, and the presence of the network of corner-shared octahedrons was certainly the important factor increasing the mobility of the charged carriers,45 which governed the photocatalytic activity of the materials. Practically all of the active photocatalysts possessed the network of corner-shared octahedrons. 4. Conclusions

Figure 15. H2-evolution properties over the La2Sn2O7 photocatalysts in methanol water. Conditions: catalyst 0.5 g; reactant solution 150 mL (CH3OH: 25 mL); 400 W high-pressure mercury lamp; outer irradiation cell made of quartz.

Furthermore, the inset diagram of Figure 15 indicated that no obvious deactivation was observed after three reaction runs, indicating that the as-prepared sample was stable under UV irradiation. The stability of the photocatalyst is important to its applications. Most photocatalysts are not stable in splitting water under UV light illumination, and the doped TiO2 photocatalysts sometimes suffer from this problem.43,44 For La2Sn2O7, after several recycles of degradation MO and water-splitting under UV light, XRD analysis showed that the crystal structure of the photocatalyst was not changed. It was known that an efficient photocatalyst benefited from a good combination of four factors: specific surface area, particle size, crystallinity, and crystal structure. To the best of our knowledge, there was no report about photocatalyic properties of La2Sn2O7 in the previous studies. In our experiments, a facile hydrothermal method was employed to synthesize the sample, and it reduced the size and enhanced the specific surface areas of as-prepared products. Indeed, the size of as-synthesized La2Sn2O7 nanocubes was from 20 to 60 nm, and the specific surface areas of the as-prepared cubic samples were 87.63 m2/ g, while those prepared by the solid-state reaction method were quite low (1.67 m2/g). It could be deduced that the materials with small particle size and high surface areas exhibited the enhanced photoactivity, because the photogenerated electronhole pairs could move effectively to the surface, and the number

Nanocrystalline La2Sn2O7 photocatalysts with the cubic morphology have been successfully synthesized in a simple hydrothermal approach. The synthesis parameters including alkaline concentration, reaction time, and reaction temperature played important roles in the formation of La2Sn2O7 nanocubes. It was proposed that the formation of the nanocubes be via three stages: La(OH)3 rapid growth stage, dissolved-nucleation stage, and La2Sn2O7 ripening stage. For the first time, the La2Sn2O7 was discovered to be a good photocatalyst, which not only exhibited a high activity of degradation of methyl orange, but also had the activity for generating H2 under ultraviolet light irradiation. The results obtained in the present study also indicated that the hydrothermal method would be helpful in exploiting the complex oxide photocatalysts. Acknowledgment. The authors are grateful to the Project of Academic Human Resources Development in Institutions of Higher Learning Under the Jurisdiction of Beijing Municipality (PHR (IHLB)) for financial support. References and Notes (1) Honda, K.; Fujishima, A. Nature 1972, 238, 37. (2) Khan, S. U. M.; Al-Shahry, M.; Ingler, W. B., Jr. Science 2002, 297, 2243. (3) Choi, W.; Kim, S. EnViron. Sci. Technol. 2002, 36, 2019. (4) Maeda, K.; Teramura, K.; Lu, D.; Takata, T.; Saito, N.; Inoue, Y.; Dome, K. Nature 2006, 440, 295. (5) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (6) Irie, H.; Watanabe, Y.; Hashimoto, K. J. Phys. Chem. B 2003, 107, 5483. (7) Mrowetz, M.; Balcerski, W.; Colussi, A. J.; Hoffmann, R. J. Phys. Chem. B 2004, 108, 17269. (8) Kato, H.; Kudo, A. J. Phys. Chem. B 2002, 106, 5029. (9) Reddy, P.; Sun, B.; Smirniotis, G. J. Phys. Chem. B 2004, 108, 17198. (10) Fu, X.; Clark, A.; Yang, Q.; Anderson, A. EnViron. Sci. Technol. 1996, 30, 647. (11) Kudo, A.; Kato, H.; Nakagawa, S. J. Phys. Chem. B 2000, 104, 571. (12) Kato, H.; Asakura, K.; Kudo, A. J. Am. Chem. Soc. 2003, 125, 3082. (13) Kim, H. G.; Hwang, D. W.; Kim, J.; Kim, Y. G.; Lee, J. S. Chem. Commun. 1999, 1077. (14) Hwang, D. W.; Lee, J. S.; Li, W.; Oh, S. H. J. Phys. Chem. B 2003, 107, 4963. (15) Weller, T.; Hughes, W.; Rooke, J.; Knee, S.; Reading, J. Dalton Trans. 2004, 3032. (16) Mims, C. A.; Jacobson, A. J.; Hall, R. B.; Lewandowski. J. T. J. Catal. 1995, 153, 197.

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Hydrothermal Synthesis and Photocatalytic Properties of Pyrochlore

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