3D Hierarchical Architectures of Sr2Sb2O7: Hydrothermal Syntheses

CRYSTAL GROWTH & DESIGN

3D Hierarchical Architectures of Sr2Sb2O7: Hydrothermal Syntheses, Formation Mechanisms, and Application in Aqueous-Phase Photocatalysis

2008 VOL. 8, NO. 12 4469–4475

Hun Xue, Zhaohui Li,* Hui Dong, Ling Wu, Xuxu Wang, and Xianzhi Fu* Research Institute of Photocatalysis, Fuzhou UniVersity, State Key Laboratory Breeding Base of Photocatalysis, Fuzhou 350002, P. R. China ReceiVed April 20, 2008; ReVised Manuscript ReceiVed August 18, 2008

ABSTRACT: In the presence of the surfactant cetyltrimethyl ammonium bromide (CTAB) or poly (vinyl pyrrolidone) (PVP), different three-dimensional (3D) hierarchical flowerlike architectures of Sr2Sb2O7 (Sr2Sb2O7 (CTAB) and Sr2Sb2O7 (PVP)) were successfully synthesized via a facile hydrothermal process using Sb2O5 as the starting material. The obtained samples were characterized by X-ray diffraction (XRD), N2-sorption BET surface area, UV-vis diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and Fourier transformation infrared spectroscopy (FT-IR). The formation mechanisms for Sr2Sb2O7 (CTAB) and Sr2Sb2O7 (PVP) were proposed. Both Sr2Sb2O7 (CTAB) and Sr2Sb2O7 (PVP) show obvious photocatalytic activity for the decomposition of salicylic acid under UV light illumination. Sr2Sb2O7 (PVP) shows the best photocatalytic activity for the degradation of salicylic acid due to its higher band gap energy and larger BET specific surface area. Compared to samples prepared without the surfactant, both Sr2Sb2O7 (CTAB) and Sr2Sb2O7 (PVP) can be more readily separated from the slurry system by filtration or sedimentation after the photocatalytic reaction and reused. This makes them ideal photocatalysts for aqueous phase photocatalytic reactions.

1. Introduction It is generally believed that the properties of nanomaterials depend not only on their chemical composition, but also on their structures, including phases, sizes, size distributions, shape, and the dimensionality.1-10 How to control anisotropic inorganic materials at the mesoscopic level is one of the most challenging issues presently faced by synthetic chemists. Recently, research on nanostructures is expanding rapidly into the assembly of nanoparticles in two-dimensional (2D) and three-dimensional (3D) ordered superstructures. In particular, the alignment of nanostructure nanotubes and nanorods into three-dimensional (3D) superstructures by bottom-up approaches has been an exciting field in recent years because of their interesting physical properties and potential applications in many areas.11-13 Generally, two strategies have been utilized for the “bottom-up” chemical synthesis of nanostructured materials: one is the use of hard templates,14,15 which physically confine the size and shape of the growing nanoparticles, and the other is the use of capping agents/surfactants during nanoparticle growth to control its dimension, direction, and morphology.14-25 To date, a wide variety of inorganic materials, including metal, metal oxide, sulfide, hydrate, and other minerals, have been successfully prepared with hierarchical shapes.4,26-30 Among various synthetic methods, hydrothermal/solvothermal methods have shown great facility and flexibility. The morphologies and crystal structures of the final products can be tuned by varying reaction conditions, such as reactant source, stoichiometry, pH value, reaction temperature and time, and ambient circumstances, during synthesis.4,23-25,30-32 Various types of surfactants have been widely used in most solution routes in the synthesis of well structured materials with controlled morphology thanks to their efficient self-assembly properties.33-36 Now, the selfassembly of anisotropic nanostructures, such as nanoplates, nanosheets, nanorods, and nanotubes, requires more effort. * Author to whom all correspondence should be addressed. Tel/Fax: 86-59183738608. E-mail: [email protected] (Z.L.); [email protected].

Figure 1. XRD patterns of Sr2Sb2O7

(PVP)

and Sr2Sb2O7

(CTAB).

Therefore, it remains a significant challenge to develop facile, mild, easily controlled and effective methods for the large-scale synthesis of novel hierarchical architectures assembled controllably from independent and discrete nanobuilding blocks. Sr2Sb2O7 prepared via a conventional solid state reaction has been previously reported to be a photocatalyst for water splitting37 and methyl orange (MO) degradation.38 Our previous study showed that nanocrystalline Sr2Sb2O7 with small particle size and large specific surface area can be successfully synthesized via a facile hydrothermal process using Sb2O5 as the starting material.39 The hydrothermal synthesized nanocrystalline Sr2Sb2O7 showed excellent gas-phase photocatalytic activity for benzene degradation. However, as-synthesized Sr2Sb2O7 is unfavorable for the aqueous-phase phtocatalytic reaction since it is difficult to reclaim the photocatalyst with the small particle size from the slurry reaction system. To extend the application of Sr2Sb2O7 in the aqueous phase photocatalytic reaction, it is necessary to build Sr2Sb2O7 nanostructures. Herein, we reported the preparation of three-dimensional (3D) hierarchical flowerlike Sr2Sb2O7 architectures via a hydrothermal method with the assistance of the surfactant cetyltrimethyl ammonium bromide (CTAB) or poly(vinyl pyrrolidone) (PVP).

10.1021/cg800404e CCC: $40.75  2008 American Chemical Society Published on Web 11/06/2008

4470 Crystal Growth & Design, Vol. 8, No. 12, 2008

Figure 2. SEM images of (a) Sr2Sb2O7

(PVP);

(b) Sr2Sb2O7

Xue et al.

(CTAB).

The growth mechanisms of the 3D hierarchical flowerlike Sr2Sb2O7 nanoarchitectures are also proposed. The photocatalytic activities of Sr2Sb2O7 nanostructures were investigated by photocatalytic decomposition of salicylic acid, a typical target molecule in aqueous phase.

2. Experimental Section 2.1. Syntheses. All reagents were analytical grade and used without further purification. Sr2Sb2O7 with different morphologies (denoted as Sr2Sb2O7 (CTAB) and Sr2Sb2O7 (PVP)) were prepared by the hydrothermal method in the presence of the surfactants CTAB or PVP. In a typical procedure, Sb2O5 powder (2.5 mmol) was added to 8 mL of aqueous solution containing 5 mmol of Sr(CH3COO)2 and 0.1 g of CTAB (or PVP) under stirring. Then, 8 mL of NaOH solution (4 mol · L-1) was added to the resulting mixture under vigorous stirring. The resulting suspensions were transferred into a 23 mL Teflon-lined stainless steel autoclave and sealed tightly. Then the autoclaves were kept at 180 °C for 48 h. After cooling to room temperature naturally, the precipitate were collected, washed with distilled water and absolute ethanol for several times, and then dried in air at 80 °C. Bulk Sr2Sb2O7 sample was also prepared by the conventional solidstate reaction. SrCO3 and Sb2O3 were used as raw materials. Mixed powders were ground, pressed into pellets, and calcined at 1100 °C for 16 h to get the sample (Sr2Sb2O7 (SSR)). 2.2. Characterization. X-ray diffraction (XRD) patterns were collected on a Bruker D8 Advance X-ray diffractometer with CuKR radiation. The accelerating voltage and the applied current were 40 kV and 40 mA, respectively. Data were recorded at a scanning rate of 0.004° 2θ s-1 in the 2θ range of 10° to 70°. It was used to identify the phase present and their crystallite size. The crystallite size was calculated from X-ray line broadening analysis by the Scherrer equation: D ) 0.89λ/β cos θ, where D is the crystal size in nm, λ is the CuKR wavelength (0.15406 nm), β is the half-width of the peak in rad, and θ is the corresponding diffraction angle. UV-visible absorption spectra of the powders were obtained for the dry-pressed disk samples using a UV-visible spectrophotometer (Cary 500 Scan Spectrophotometers, Varian, USA). BaSO4 was used as a reflectance standard in the UV-visible diffuse reflectance experiment. FTIR spectra on the pellets of the samples were recorded on a Nicolet Magna 670 FTIR spectrometer at a resolution of 4 cm-1. The transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) images were measured by JEOL model JEM 2010 EX instrument at the accelerating voltage of 200 kV. The powder particles were supported on a carbon film coated on a 3 mm diameter fine-mesh copper grid. A suspension in ethanol was sonicated and a drop was dripped on the support film. Morphology of the sample was characterized by field emission scanning electron microscopy (SEM) (JSM6700F).

2.3. Photocatalytic Activity Measurements. Photocatalytic reactions were performed in a quartz tube with 4.7 cm inner diameter and 16.5 cm length. Four 4W UV lamps with a wavelength centered at 254 nm (Philips, TUV 4W/G4 T5) were used as illuminating source. 150 mg of powdered photocatalysts were suspended in 150 mL of salicylic acid aqueous solution (5 × 10-4 mol/L) and stirred for 1 h before irradiation to ensure the reach of the adsorption/desorption equilibrium. A 4 mL suspension was taken at 30 min intervals during the experiment and centrifuged (TDL-5-A). The resulting solution was analyzed on a Varian UV-vis-NIR spectrophotometer (model: Cary500). The percentage of degradation is reported as C/C0. C is the absorption of methyl orange at each irradiated time interval of the main peak of the absorption spectrum at wavelength 295 nm. C0 is the absorption of the starting concentration when adsorption/desorption equilibrium was achieved.

3. Results and Discussion 3.1. Phase and Morphology. Our previous study has revealed that nanocrystalline Sr2Sb2O7 with a large specific area and small particle size can be obtained from a 180 °C hydrothermal treatment of Sb2O5 and Sr(COOCH3)2 for 48 h under strong basic conditions.21 Under similar reaction conditions, the addition of the surfactants of PVP or CTAB did not influence the phase formation of Sr2Sb2O7, as elucidated in the X-ray diffraction (XRD) of the as-prepared samples (Figure 1). Only the pure phase of Sr2Sb2O7 (JCPDS card 781774) was obtained in the presence of PVP, CTAB or without the surfactant. However, compared to the standard card, the (220) peak for both the surfactant-assisted hydrothermal synthesized products (Sr2Sb2O7 (CTAB) and Sr2Sb2O7 (PVP)) is stronger, implying the highly preferential growth of the resultant Sr2Sb2O7. The average crystallite sizes calculated from the Scherrer equation for the surfactant-assisted synthesized Sr2Sb2O7 are all about 6nm, comparable to the sample prepared without the surfactant. The additions of the surfactants of PVP or CTAB significant influence the morphology of the resultant nanocrystalline Sr2Sb2O7. The panoramic SEM images of the PVP-assisted synthesized Sr2Sb2O7 (Sr2Sb2O7 (PVP)) and CTAB-assisted synthesized Sr2Sb2O7 (Sr2Sb2O7 (CTAB)) (Figure 2a,b) show that both consist almost entirely of 3D flowerlike architectures and indicate that the flowerlike architectures can be successfully synthesized on a large scale. This is obviously different from the previous report that small particles of Sr2Sb2O7 with a size

3D Hierarchical Architectures of Sr2Sb2O7

Crystal Growth & Design, Vol. 8, No. 12, 2008 4471

Figure 3. TEM images of (a)Sr2Sb2O7 (PVP); (b) Sr2Sb2O7 (CTAB); HRTEM images of (c) Sr2Sb2O7 (PVP); (d) Sr2Sb2O7 (CTAB); (e) HRTEM image showing the crystallographic-orientated growth of Sr2Sb2O7 nanorod, (inset) the FFT pattern of the squared area on the Sr2Sb2O7 rod.

of 6 nm were obtained without the surfactant. At first glance, Sr2Sb2O7 (pvp) and Sr2Sb2O7 (CTAB) have almost similar 3D flowerlike architectures with diameters of about 200 nm. However, a careful observation of the TEM images of Sr2Sb2O7 (CTAB) and Sr2Sb2O7 (PVP) reveal that the flowerlike patterns in these two architectures are different (Figure 3a,b). The TEM

image reveals that the flowerlike Sr2Sb2O7 (pvp) architecture consists of many nanorods with a diameter of 5-10 nm and a length up to 50-100 nm in a seemingly randomly arranged assembly. However, the uniform morphology and the capability of sustaining a long ultrasonic treatment suggests that such a flowerlike architecture of Sr2Sb2O7 (pvp) is not a random


Figure 4. TEM images of Sr2Sb2O7

(PVP)

Xue et al.

at different growth stages (a) 12 h; (b) 24 h; (c) 36 h; (d) 48 h.

aggregation, but a self-assembly via the surface-to-surface conjunctions between the nanorods. The self-assembly pattern for the flowerlike Sr2Sb2O7 (CTAB) architecture is different. It contains an extremely dense central part with lots of short nanorods protruding radially as evidenced in the TEM image (Figure 3b). The fast Fourier transform (FFT) analysis of the HRTEM image recorded from an individual nanorod of Sr2Sb2O7 (pvp) obtained by violent ultrasonic treatment for a very long time (Figure 3e) clearly demonstrates that the nanorod is a single crystal grown in a direction along the normal of the (224) planes (Figure 3e). The single crystal nature of the nanorod is also supported by the HRTEM observations at the edge of an individual nanorod of Sr2Sb2O7 (pvp) and Sr2Sb2O7 (CTAB) (Figure 3c,d). Both show clear lattice fringes of d ) 0.30 nm, which match that of the (220) plane of Sr2Sb2O7. 3.2. Morphological Evolutions and Formation Mechanisms. Time-dependent experiments were carried out to investigate the morphological evolutions of these two hierarchical flowerlike architectures. During the preparation of the flowerlike Sr2Sb2O7 (PVP) architectures, the TEM image of the products collected after 12 h of hydrothermal treatment shows the coexistence of nanorods and nanoparticles (Figure 4a). When the reaction time was prolonged to 24 h, a large number of nanorods with diameters ranging from 5 to 10 nm and lengths up to 50-100

nm appear (Figure 4b). This indicates that the initial stage is the growth of Sr2Sb2O7 nanoparticles and then followed by an oriented growth of the nanoparticles to give nanorods in the presence of PVP. When reacted for 36 h, pseudoflowerlike nanorod aggregates were obtained (Figure 4c). When the reaction time was increased to 48 h, almost uniform hierarchical flowerlike architectures with diameters about 150 nm were finally obtained (Figure 4d). The TEM images of the samples synthesized with different reaction times using CTAB as a surfactant reveal that the growth process of the hierarchical flowerlike Sr2Sb2O7 (CTAB) architectures is different from that of the Sr2Sb2O7 (PVP) hierarchical nanostructure. The TEM image of the product collected after 12 h hydrothermal treatment shows the existence of only nanoparticles (Figure 5a). When the reaction time was prolonged to 24 h, instead of nanorods observed in the PVP case, nanoparticles with a large particle size were obtained (Figure 5b). This indicates that CTAB induces an isotropic growth of the nuclei while PVP induces an anisotropic growth of the nuclei. Further increasing the reaction time to 36 h, quasiflowerlike nanostructures with a dense spherical core and nanorods attaching to the core were obtained (Figure 5c). After 48 h of reaction, hierarchical flowerlike Sr2Sb2O7 (CTAB) archi-


Figure 5. TEM images of Sr2Sb2O7

(CTAB)


at different growth stages (a) 12 h; (b) 24 h; (c) 36 h; (d) 48 h.

Figure 6. Mechanism for the formation of hierarchical Sr2Sb2O7 flowerlike architectures.

tectures with a central core and lots of short nanorods protruding radially were obtained (Figure 5d). On the basis of these observations, the formation mechanism for the hierarchical flowerlike architectures of Sr2Sb2O7 (PVP) can be depicted in Figure 6a. Since the product obtained without the addition of the surfactants of PVP is small nanoparticles, we can reasonably assume that PVP plays an important role in the formation of the hierarchical flowerlike architectures of Sr2Sb2O7 (PVP). We assume that in the formation process of hierarchical Sr2Sb2O7 (PVP), PVP may have dual functions in controlling the superstructure morphology. One role is to function as potential crystal face inhibitors in the system, which benefited the formation of oriented nucleation, leading to the

Figure 7. The UV-vis diffuse reflectance spectra of Sr2Sb2O7 Sr2Sb2O7 (PVP), Sr2Sb2O7 (Hy), and Sr2Sb2O7 (CTAB).

(SSR),

construction of anisotropic growth of the nanorods. The other is the PVP stabilizer may be adsorbed onto the surfaces of Sr2Sb2O7 nanorods by coordinating with both nitrogen and oxygen atoms in the polar pyrrolidone groups. This may exhibit steric hindrance and prompt the formation of hierarchical flowerlike architectures from individual nanorods due to its cross-linking ability since PVP has a linear structure and multiple coordinating sites.40,41


Figure 8. FTIR spectra of Sr2Sb2O7

(PVP)

and Sr2Sb2O7

Xue et al.

(CTAB).

Figure 11. Pseudo-first-order plots comparing direct light photolysis in the absence of photocatalyst and photocatalytic degradation in the presence of different photocatalysts. Table 1. The Pseudo-First Order Rate Constants of Salicylic Acid Degradation over Different Photocatalysts

Figure 9. N2-sorption isotherm for Sr2Sb2O7 (PVP), Sr2Sb2O7 (CTAB), and Sr2Sb2O7 (Hy) and the pore size distribution plot for Sr2Sb2O7 (PVP) and Sr2Sb2O7 (CTAB). The pore size distribution was estimated from the desorption branch of the isotherm.

Figure 10. Temporal changes of salicylic acid concentration as monitored by the UV-vis absorption spectra at 297 nm on different samples.

The mechanism for the formation of the hierarchical flowerlike architectures of Sr2Sb2O7 (CTAB) is different from that of Sr2Sb2O7 (PVP). In the beginning of the reaction, a fast supply rate of the reaction precursor like Sb(OH)6- and the electrostatic interactions between CTA+, OH-, and Sb(OH)6- ion-pairs is more likely to lead to an isotropic growth of the nuclei. Therefore, nanoparticles with a larger particle size were obtained. With the reaction proceeding, the concentration of the reaction precursors became lower. A relatively slow supply rate of the reaction precursor induces an anisotropic growth of nanorods attached on the preformed nanoparticle core.16,30,35 A detailed formation mechanism of this hierarchical flowerlike architecture of Sr2Sb2O7 (CTAB) is depicted in Figure 6b. 3.3. UV-vis Diffuse Reflectance Spectra. Figure 7 shows the UV-vis diffuse reflectance spectra of Sr2Sb2O7 (SSR),

photocatalysts

k (h-1)

R

blank Sr2Sb2O7 (SSR) Sr2Sb2O7 (CTAB) Sr2Sb2O7 (Hy) Sr2Sb2O7 (PVP)

0.06353 0.22424 0.35397 0.40353 0.57702

0.99747 0.97186 0.99675 0.99856 0.99748

Sr2Sb2O7 (PVP), Sr2Sb2O7 (CTAB), and the Sr2Sb2O7 (Hy) prepared by the hydrothermal processes without surfactants.39 The wavelength at the absorption edge, λ, is determined as the intercept on the wavelength axis for a tangent line drawn on absorption spectra. Estimated from its UV/vis diffuse reflectance absorption spectrum, the band gap energy for the Sr2Sb2O7 (PVP) is 4.3 eV, which is similar with Sr2Sb2O7 (Hy), but is larger than Sr2Sb2O7 (SSR). The large blue shift in the band gap is probably attributed to the much smaller particles for Sr2Sb2O7 (PVP) and Sr2Sb2O7 (Hy) compared to Sr2Sb2O7 (SSR). For Sr2Sb2O7 (CTAB), the band gap transitions are red-shifted. This is attributed to the remaining CTAB as evidenced from the FTIR spectra of Sr2Sb2O7 (CTAB) (Figure 8). The FTIR spectra of Sr2Sb2O7 (CTAB) shows characteristic methylene peaks at 2920 and 2845 cm-1, suggesting the existence of CTAB in the as-prepared Sr2Sb2O7 42 (CTAB). 3.4. N2-Sorption Isotherm. N2-sorption isotherm (Figure 9) for Sr2Sb2O7 (PVP) and Sr2Sb2O7 (CTAB) both exhibit stepwise adsorption and desorption (type IV isotherm), indicative of porous solids.43 The average pore sizes for the Sr2Sb2O7 (PVP) and Sr2Sb2O7 (CTAB) are 4.3 and 4.6 nm, respectively, with a narrow distribution of pore size. These porosities are originated from internonrods porosities. The BET specific surface areas for Sr2Sb2O7 (PVP) and Sr2Sb2O7 (CTAB) are 29.8 and 19.1 m2 · g-1, respectively, comparable to the sample obtained without the surfactant (24.8 m2 · g-1), and are much higher than that for Sr2Sb2O7 (SSR) (1.2 m2 · g-1). 3.5. Photocatalytic Activity. In our previous study, we found that nanocrystalline Sr2Sb2O7 with small particles prepared via the hydrothermal method showed superior photocatalytic performance for benzene degradation.39 However, photocatalyst with small particles is difficult to be reclaimed from the solution and is not favorable for the aqueous-phase photocatalytic reactions. The capability of self-assembly to form the larger hierarchical flowerlike architecture with the assistance of PVP or CTAB makes Sr2Sb2O7 suitable for the aqueous phase photocatalytic reactions. Herein the photocatalytic activities of Sr2Sb2O7 were evaluated by measuring the degradation of



salicylic acid under UV illuminations. Temporal changes in the concentration of salicylic acid were monitored by examining the variations in maximal absorption in UV-vis spectra at 297 nm. Figure 10 shows the results of the degradation of salicylic acid in the presence of different samples. After 3.5 h illumination, the conversion of salicylic acid was 98.4%, 82.2%, 75.5%, and 52.8% for Sr2Sb2O7 (PVP), Sr2Sb2O7 (Hy), Sr2Sb2O7 (CTAB) and Sr2Sb2O7 (SSR), respectively. It is observed that the particles of Sr2Sb2O7 (PVP) and Sr2Sb2O7 (CTAB) were easier to be reclaimed from the slurry system than that of Sr2Sb2O7 (Hy) and are therefore more suitable for the aqueous-phase photocatalytic reactions. To quantitatively understand the reaction kinetics of the salicylic acid degradation in our experiments, we applied the pseudo-first-order model as expressed by eq 1, which is generally used for photocatalytic degradation process if the initial concentration of pollutant is low:38

ln(C0 ⁄ C) ) kt

(1)

where C0 is the initial concentration, C is the concentration after a time (t) of the salicylic acid degradation, and k is the firstorder rate constant. Figure 11 is the photocatalytic reaction kinetics of salicylic acid degradation in solution based on the data plotted in Figure 10. The rate constants obtained from the regression lines in Figure 11 are included in Table 1. It is clear that a fairly good correlation to the pseudo-first-order reaction kinetics was found. The determined reaction rate constant k for salicylic acid degradation was 0.57702, 0.40353, 0.35397, and 0.22424 h-1 for Sr2Sb2O7 (PVP), Sr2Sb2O7 (Hy), Sr2Sb2O7 (CTAB) and Sr2Sb2O7 (SSR), respectively (Table 1). The photocatalytic activity for salicylic acid degradation followed the order of Sr2Sb2O7 (PVP) > Sr2Sb2O7 (Hy) > Sr2Sb2O7 (CTAB) > Sr2Sb2O7 (SSR). Compared to the sample prepared via the conventional solid state method, samples prepared via the hydrothermal process showed much higher photocatalytic activity in the degradation of salicylic acid. This can be attributed to the nanoscale particle size and a relatively large surface area. The highest photocatalytic activity is observed on Sr2Sb2O7 (PVP) among the three samples prepared via the hydrothermal method. This is probably due to its higher band gap, larger specific surface area, and its hierarchical flowerlike nanostructure.

4. Conclusions In summary, 3D hierarchical flowerlike architectures of Sr2Sb2O7 can be successfully prepared via a facile hydrothermal method with the assistance of PVP and CTAB. Possible mechanisms were proposed to elucidate their formations. These hierarchical Sr2Sb2O7 flowerlike architectures show photocatalytic activity for the degradation of salicyclic acid under UV light irradiation. The prepared hierarchical Sr2Sb2O7 flowerlike architectures are of great interest in environmental purification. Acknowledgment. The work was supported by National Natural Science Foundation of China (20537010, 20677009), National Basic Research Program of China (973 Program: 2007CB613306, 2007CB616907), grant from Fujian Province (E0710009). Z.L. thanks program for New Century Excellent Talents in University (NCET-05-0572), State Education Ministry of P. R. China.

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CG800404E

3D Hierarchical Architectures of Sr2Sb2O7: Hydrothermal Syntheses

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