5850
J. Phys. Chem. C 2008, 112, 5850-5855
Nanocrystalline Ternary Wide Band Gap p-Block Metal Semiconductor Sr2Sb2O7: Hydrothermal Syntheses and Photocatalytic Benzene Degradation Hun Xue, Zhaohui Li,* Ling Wu, Zhengxin Ding, Xuxu Wang, and Xianzhi Fu* Research Institute of Photocatalysis, Fuzhou UniVersity, State Key Laboratory Breeding Base of Photocatalysis, Fuzhou 350002, P. R. China ReceiVed: December 31, 2007; In Final Form: February 17, 2008
Nanocrystalline Sr2Sb2O7 with small particles and a large BET specific area was prepared successfully via a facile hydrothermal method from Sb2O5. The obtained sample was 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 as-prepared Sr2Sb2O7 showed high photocatalytic activity for benzene degradation under 254 nm UV irradiations, and no deactivation was observed during the prolonged photocatalytic reaction. The photocatalytic performance of the hydrothermal synthesized Sr2Sb2O7 was superior to both P25 and Sr2Sb2O7 prepared by the conventional solid-state reaction. A possible mechanism of the photocatalysis over Sr2Sb2O7 was also proposed.
1. Introduction Benzene is a widespread environmental pollutant. Because of its high toxicity, confirmed carcinogenicity, and environmental persistence, it is regarded as the priority hazardous substance.1 The development of an efficient and mild technology to removing benzene from the ambient environment is absolutely necessarily. TiO2-based photocatalytic oxidation (PCO) has been established to be one of the most promising technologies for environmental remediation.2 The PCO technology is environmentally friendly, capable of performing at room temperature, and it can treat organic pollutants at extremely low concentrations. Although numerous VOCs can be readily photocatalyzed to CO2 and H2O on TiO2, PCO meets with limited success in the treatment of aromatic compounds like benzene because of the deactivation of TiO2 resulted from the accumulation of the stable reaction intermediates on the surface.3,4 Loading of a noble metal (Pt, Pd, or Rh) has been used to enhance the performance of TiO2 for photocatalytic oxidation of benzene in the gas phase.5-7 Rh/TiO2 was shown to be the best one, but there is concern about its stability because of the oxidation of Rh nanoparticles on the surface of TiO2.7 Sitkiewitz et al.8 and Einaga et al.9 have improved the efficiency of the TiO2 photocatalyst toward complete oxidization of benzene by adding a sufficient amount of H2O ([H2O] ) 22 000 ppm) in the reaction feed gas to suppress the formation of reaction intermediates on the surface of TiO2. Recently, it has been demonstrated that the introduction of magnetic field10 or H211,12 into the photochemical reaction system can greatly improve the efficiency of Pt/TiO2 for the photodecomposition of benzene at room temperature. However, it is not easy to realize such a complicated hybrid system for photocatalytic air purification. Therefore, the development of the photocatalyst with high performance for benzene degradation is indispensable in view * Authors to whom all correspondence should be addressed. E-mail:
[email protected] (Z. Li);
[email protected] (X. Fu). Tel (Fax): 86-591-83738608.
of the application of photocatalysis for benzene treatment, yet it remains a great challenge until now. Recently, three wide band gap p-block metal oxides or hydroxides β-Ga2O3,13 In(OH)3,14 and InOOH15 have been reported to show high activity toward benzene degradation under UV irradiation. In addition to their high activity for the benzene oxidation, they are extremely stable and no deactivation has been observed during the prolonged reaction. A detailed study on InOOH has revealed that the high photocatalytic performance is related to its peculiar electronic structure as a wide band gap p-block semiconductor. The dispersive conduction band of InOOH, which is composed of the hybridization of In 5s and In 5p orbitals, can promote the mobility of the photogenerated electrons and enhance the charge separation.15 In addition to this, the wide band gap endows the photogenerated charge carriers in InOOH with strong enough redox ability to react with surface-adsorbed H2O to produce ‚OH radicals. The degradation of benzene via the ‚OH radical’s way makes InOOH capable of maintaining a clean catalyst surface and maintains a long stability during the photocatalytic reaction. β-Ga2O3, In(OH)3, and InOOH are binary p-block semiconductors.13-15 Because the wide band gap p-block semiconductors share some common characteristic in terms of their electronic structure, we hope that the superior photocatalytic performance observed on these binary p-block semiconductors can also be found in the more versatile ternary p-block metal semiconductors. The introduction of the second metal would probably make a more dispersive conduction band of the ternary semiconductor by hybridizations of the orbitals of the second metal with those of the p-block metal and therefore is helpful for the photocatalytic reaction. Sr2Sb2O7 is a ternary wide band gap p-block semiconductor. It has been studied previously as a photocatalyst for water splitting16 and photocatalytic decomposition of methyl orange.17 However, its application for benzene degradation has never been explored. Besides this, the only reported synthetic method for Sr2Sb2O7 is the solid-state reaction method.16,17 This process usually suffers from problems such as excessive crystal growth,
10.1021/jp712186r CCC: $40.75 © 2008 American Chemical Society Published on Web 03/27/2008
Wide Band Gap p-Block Metal Semiconductor Sr2Sb2O7 small BET specific surface area, and so forth. All of these would lead to a lower photocatalytic activity. The development of an inexpensive, facile, and fast method in the preparation of Sr2Sb2O7 with large specific surface area and small particle size is intriguing and indispensable in view of the application of Sr2Sb2O7 for photocatalytic benzene degradation. Recently a generic, facile, and effective hydrothermal method in the preparation of multiplex metal oxides, like CuGeO3,18 ZnGa2O4,19 KNbO3,20,21 LiMn2O4,22 and BaTe3O723 directly from the respective metal oxides has been developed. However, to the best of our knowledge, this method has not been used in the synthesis of antimonates. So in this manuscript we report the facile synthesis of nanocrystalline Sr2Sb2O7 with small particle size and large BET specific surface area from Sb2O5 via the hydrothermal method. The photocatalytic performance for benzene degradation over the as-prepared Sr2Sb2O7 and the photocatalytic mechanism are also studied. Sr2Sb2O7 represents the first ternary wide band gap p-block metal semiconductor that shows superior performance in the degradation of gaseous benzene. 2. Experimental Section 2.1. Syntheses. All of the reagents were analytical-grade and used without further purification. The Sr2Sb2O7 nanocrystalline samples were prepared by the hydrothermal method using Sr(CH3COO)2‚0.5H2O and Sb2O5 as starting materials. In a typical procedure, Sb2O5 powder (0.81 g, 2.5 mmol) was added to 8 mL of aqueous solution containing Sr(CH3COO)2‚0.5H2O (1.07 g, 5.0 mmol) under stirring. The pH of the resulting mixture was adjusted by nitric acid solution or sodium hydrate solution under vigorous stirring. The mixture was loaded into a 23 mL Teflon-lined autoclave, filled with deionized water up to 70% of the total volume and sealed tightly. Then the autoclaves were kept at 180 °C for different reaction times. After cooling to room temperature, the precipitates were collected, washed with distilled water and absolute ethanol several times, and then dried in air at 80 °C (0.8 g, 60%). For comparison, the bulk Sr2Sb2O7 sample was prepared from SrCO3 and Sb2O3 via the conventional solid-state reaction according to literature.16 Mixed powders were grinded, pressed into pellets, and calcined at 1100 °C for 16 h to get the sample (Sr2Sb2O7(SSR)). 2.2. Characterizations. X-ray diffraction (XRD) patterns were collected on a Bruker D8 Advance X-ray diffractometer with Cu KR 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 Scherer equation: D ) 0.89λ /β cos θ, where D is the crystal size in nm, λ is the Cu Ka 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 (UV-DRS) of the powders were obtained for the dry-pressed disk samples using a UV-visible spectrophotometer (Cary 500 Scan Spectrophotometers, Varian). BaSO4 was used as a reflectance standard in the UV-visible diffuse reflectance experiment. The transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were measured by a JEOL model JEM 2010 EX instrument at an accelerating voltage of 200 kV. The powder particles were supported on a carbon film coated on a 3-mmdiameter fine-mesh copper grid. A suspension in ethanol was sonicated, and a drop was dripped on the support film. The
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Figure 1. XRD patterns of the samples prepared at 180 °C for 48 h with different pH values, (a) pH ) 1; (b) pH ) 5; (c) pH ) 7; (d) pH ) 13; (e) [OH-] ) 1 mol‚L-1; (f) [OH-] ) 2 mol‚L-1. (b) Sr1.36Sb2O6; (*) SrSb2O6; (1) Sr2Sb2O7.
morphology of the sample was characterized by field emission scanning electron microscopy (SEM) (JSM-6700F). FT-IR spectra were recorded in transmittance mode with a resolution of 4 cm-1 using a Nicolet Nexus 670 FTIR spectrometer and 25 mg of catalyst. Electron spin resonance (ESR) spectra were obtained using Bruker model ESP 300 E electron paramagnetic resonance spectrometer equipped with a quanta-Ray Nd:YAG laser system as the irradiation light source (λ ) 266 nm). The settings were center field, 3480.00 G; microwave frequency, 9.79 GHz; and power, 5.05 mW. 2.3. Photocatalytic Activity Measurements. The photocatalytic oxidation experiment of benzene was conducted in a tubular quartz microreactor operating in a continuous-flow mode. The reactor was surrounded by four 4W UV lamps (wavelength 254 nm). The catalyst loading was 0.3 g (50-70 mesh). Benzene was diluted with a zero air stream (21% oxygen, 79% nitrogen, H2O < 5 ppm, total hydrocarbons < 1 ppm). The resulting gaseous mixture was used to afford a reactant stream. The initial concentrations of benzene and carbon dioxide in the stream were 220 and 0 ppm, respectively. The flow rate of the reactant mixture was kept at 20 mL‚min-1. Simultaneous determination of the concentrations of benzene and carbon dioxide was performed with an online gas chromatograph (HP6890) equipped with a flame ionization detector, a thermal conductivity detector, and a Porapak R column. The reaction temperature was controlled at 27 ( 1 °C by an air-cooling system. Benzene was found to be stable in the catalyst-loaded reactor without illumination, and no degradation of benzene was observed when it was illuminated in the absence of Sr2Sb2O7. For comparison, the photocatalytic activities of commercial TiO2 (Degussa P25) and Sr2Sb2O7(SSR) were also tested under the same reaction conditions and equal catalyst weight as those employed for Sr2Sb2O7. 3. Results and Discussion The pH value plays an important role in controlling the composition of the final products in the hydrothermal process. The XRD patterns of the resultant products through the hydrothermal process at 180 °C for 48 h under different pH values are shown in Figure 1. For the pH value in the region of 1-5, the resultant product is Sr1.36Sb2O6 (JCPDS card 810735). When the pH value lies between 7 and 13, a mixture of Sr1.36Sb2O6 and SrSb2O6(JCPDS card 820519) can be obtained. Pure Sr2Sb2O7 (JCPDS card 781774) can only be obtained at a very high pH value ([OH-] )1 mol‚L-1 or 2 mol‚L-1). The XRD
5852 J. Phys. Chem. C, Vol. 112, No. 15, 2008
Xue et al. of the diffraction peaks become stronger with the increase of hydrothermal time, indicating the enhanced crystallinity (Figure 2). On the basis of the time-dependent experiments, possible chemical reactions involved in the synthesis of Sr2Sb2O7 particles can be formulated as the following:
Sr2+ + 2OH- f Sr(OH)2 Sb2O5 + 5H2O + 2OH- f 2Sb(OH)62Sr(OH)2 + 2Sb(OH)6- f Sr2Sb2O7 + 7H2O
Figure 2. XRD patterns of the samples prepared with [OH-] ) 2 mol‚L-1 at 180 °C for different times, (a) 6 h; (b) 12 h; (c) 24 h; (d) 36 h; (e) 48 h. (*) Sr(OH)2; (b) Sb2O5.
patterns of the as-prepared Sr2Sb2O7 samples show that the peaks are wide, indicating that the average crystallite sizes of the Sr2Sb2O7 samples are small. The average crystallite sizes calculated from the Scherrer equation are about 6 nm for the Sr2Sb2O7 samples prepared through the hydrothermal process. Time-dependent experiments have been carried out for a better understanding of the formation of the Sr2Sb2O7 nanocrystallites. A mixture of Sr2Sb2O7, unreacted Sb2O5, and Sr(OH)2 (JCPDS card 712365) is obtained when the reaction proceeds for only 6 h. Although pure Sr2Sb2O7 can be obtained when the hydrothermal time is longer than 12 h, the intensities
It is possible that in basic conditions the nuclei of Sr(OH)2 form and Sb2O5 are partly hydrolyzed to give Sb(OH)6- species. The as-formed Sr(OH)2 nuclei could serve as the nucleation centers for the subsequent reaction between Sr(OH)2 and Sb(OH)6- to give Sr2Sb2O7 nanoparticles. This reaction continues until all of the Sb2O5 are consumed. The typical SEM image of Sr2Sb2O7 prepared at 180 °C for 48 h with a concentration of OH- of 2 mol‚L-1 is shown in Figure 3a. It shows that the as-prepared Sr2Sb2O7 consists entirely of small particles. The TEM image of Sr2Sb2O7 (Figure 3b) shows that the average values of the nanocrystalline Sr2Sb2O7 are around 6 nm, which is consistent with the result from XRD according to the Scherrer formula. The HRTEM image (Figure 3c) shows clear lattice fringes. The fringes of d ) 0.30 nm match that of the (220) plane of Sr2Sb2O7. The EDS indicates that the atomic ratio of Sr, Sb, and O is about stoichiometry 2:2:7 and no impurity exists. The SEM image of the sample Sr2Sb2O7(SSR) prepared via a conventional solid-state
Figure 3. Images of Sr2Sb2O7 (180 °C, 48 h, [OH-] ) 2 mol‚L-1) (a) SEM image; (b) TEM image; (c) HRTEM image; (d) SEM image of Sr2Sb2O7(SSR).
Wide Band Gap p-Block Metal Semiconductor Sr2Sb2O7
Figure 4. Nitrogen adsorption-desorption isotherm and the pore size distribution plot for Sr2Sb2O7 (180 °C, 48 h, [OH-] ) 2 mol‚L-1). The pore size distribution was estimated from the desorption branch of the isotherm.
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Figure 6. Conversion of C6H6 and (b) the amount of produced CO2 over the Sr2Sb2O7 (180 °C, 48 h, [OH-] ) 2 mol‚L-1) as a function of reaction time, with TiO2 (P25) and Sr2Sb2O7(SSR) as references. (9), (b), and (2) are the conversion of C6H6 over the Sr2Sb2O7, TiO2, and Sr2Sb2O7(SSR), respectively; (0), (O), and (∆) are the amounts of produced CO2 over the Sr2Sb2O7, TiO2, and Sr2Sb2O7(SSR), respectively.
Figure 5. Diffuse reflectance absorption spectra of Sr2Sb2O7 (180 °C, 48 h, [OH-] ) 2 mol‚L-1) and Sr2Sb2O7(SSR).
method exhibits coarse particle agglomerates of irregular morphology with a wide size distribution (Figure 3d). It is clear that the crystallite sizes of the products obtained through the hydrothermal process are much smaller than those of the sample prepared via the conventional solid-state reaction. The difference in crystallite size leads to their significantly different BET surface areas (24.8 m2‚g-1 for hydrothermal prepared Sr2Sb2O7 and only 1.2 m2‚g-1 for Sr2Sb2O7(SSR)). The N2-sorption isotherm (Figure 4) for the hydrothermal prepared Sr2Sb2O7 exhibits stepwise adsorption and desorption (type-IV isotherm), indicative of a mesoporous solid.13 The average pore size for the Sr2Sb2O7 is 4.0 nm with a narrow distribution of pore size. This porosity originates from the interparticle porosity as evidenced in the TEM image. Therefore, compared to the conventional solid-state method, the hydrothermal method is a practical method to prepare nanocrystalline Sr2Sb2O7 samples with small particles and large BET specific surface. The optical absorption of the as-prepared samples was measured by a UV-vis spectrometer. Figure 5 shows the UVvis diffuse reflectance spectra of Sr2Sb2O7 and Sr2Sb2O7(SSR). The wavelength at the absorption edge, λ, is determined as the intercept on the wavelength axis for a tangent line drawn on absorption spectra. The absorption for Sr2Sb2O7 locates at ca. 295 nm, corresponding to a band gap of about 4.2 eV, which is larger than that of Sr2Sb2O7(SSR). The blue shift in the band gap can be attributed to the much smaller particles for hydrothermal synthesized Sr2Sb2O7 compared to Sr2Sb2O7(SSR). The photocatalytic activity for benzene degradation over the hydrothermal prepared Sr2Sb2O7 was carried out under 254 nm UV illumination. Figure 6 shows the conversion of benzene and the amount of the produced CO2 over hydrothermal synthesized Sr2Sb2O7, Sr2Sb2O7(SSR). and TiO2 (Degussa P25) as a function of illumination time. The conversion over P25 is initially 10.5%, but it gradually decreases to 4.2% with time on stream. This is
Figure 7. IR spectra of the used samples of Sr2Sb2O7 (180 °C, 48 h, [OH-] ) 2 mol‚L-1) for benzene photooxidation and the fresh samples of Sr2Sb2O7 as references.
consistent with the previous report that TiO2 can be deactivated completely during the treatment of benzene when the feed gas does not contain sufficient H2O.6 Meanwhile, only a small amount of CO2 (ca. 20 ppm) is produced. For hydrothermal prepared Sr2Sb2O7, the conversion of benzene is maintained at 24% nearly irrespective of reaction time. In addition, more than 160 ppm of CO2 is produced and corresponding to a high mineralization ratio of about 50%. The high conversion and mineralization ratio can be maintained for more than 40 h, during which no obvious deactivation is observed. However for Sr2Sb2O7(SSR), because of its low BET specific surface area, the conversion rate is maintained at 4% and only a small amount of CO2 (ca. 30 ppm) is produced. The hydrothermal method is definitely superior to the conventional solid-state method in the preparation of antimonates with high photocatalytic activities. The hydrothermal prepared Sr2Sb2O7 is a highly efficient photocatalyst in the degradation of benzene. The photocatalytic activity of the hydrothermal synthesized Sr2Sb2O7 based on per unit surface area is much higher than that observed on InOOH and is comparable to that of β-Ga2O3 and In(OH)3.13-15 Like InOOH, β-Ga2O3, and In(OH)3, Sr2Sb2O7 also shows high stability during the photodegradation of benzene. Only a slight color change from white to light yellow has been observed for Sr2Sb2O7 after a prolonged photocatalytic reaction of 40 h. Except a small band at 1582 cm-1 related to the absorbed water and hydroxyl group,14 the FT-IR spectrum of this reacted Sr2Sb2O7 is almost similar to that of the unreacted one (Figure 7). This indicates that Sr2Sb2O7 is also capable of maintaining a clean catalyst surface as InOOH. The clean catalyst surface is
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Figure 8. ESR signals of the DMPO-trapped ‚OH: Sr2Sb2O7 (180 °C, 48 h, [OH-] ) 2 mol‚L-1) system without irradiation and with irradiation for 3, 6, 9, 12, 15, and 18 s respectively. light source: pulsed laser, λ ) 266 nm, 10 pulse s-1.
SCHEME 1: Possible Mechanism of the Photocatalytic Degradation of Benzene over Sr2Sb2O7
a result of the benzene oxidation via the HO‚ radical according to our previous study on InOOH.15 Therefore, the ESR spin-trap with DMPO technique has been carried out on illuminated Sr2Sb2O7 to detect the formation of the HO‚ radicals. Four characteristic peaks of DMPO-‚OH can be observed, and the intensities of the peaks increase with the illumination time as shown in Figure 8. This indicates that the photogenerated holes in illuminated Sr2Sb2O7 also possess strong enough oxidation ability and long enough life to react with the surface-adsorbed H2O. This result is not surprising because Sr2Sb2O7 is a wide band gap metal p-block metal semiconductor. On one hand, the wide band gap endows the photogenerated holes and electrons with strong redox ability. On the other hand, for these wide band gap p-block metal semiconductors, especially the ternary semiconductors, the conduction band is usually highly dispersive because of the hybridizations of the orbitals and can promote the mobility of the photoexcited electrons, leading to enhanced charge separation. It is reported that the conduction band of PbSb2O6 is composed of the hybridization of Pb 6p and Sb 5s orbitals and is highly dispersive.24 It is possible that the orbitals of Sr can hybridize with those of Sb to contribute to a dispersive conduction band of Sr2Sb2O7 as well. On the basis of the above discussions, a possible mechanism for the benzene photodegradation over Sr2Sb2O7 can be proposed (Scheme 1). Sr2Sb2O7 can be efficiently excited to create electron-hole pairs under UV irradiation. The photogenerated electrons and holes are long-lived enough to react with adsorbed O2 and H2O to produce HO‚ radicals. Because the water content in the feed gas is maintained at such a low level (
