Synthesis, Characterization, and Photocatalytic ... - ACS Publications

Tiekun Jia*, Weimin Wang, Fei Long, Zhengyi Fu, Hao Wang and Qingjie Zhang. State Key Lab of Advanced Technology for Material Synthesis and Processing...
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J. Phys. Chem. C 2009, 113, 9071–9077

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Synthesis, Characterization, and Photocatalytic Activity of Zn-Doped SnO2 Hierarchical Architectures Assembled by Nanocones Tiekun Jia,* Weimin Wang,* Fei Long, Zhengyi Fu, Hao Wang, and Qingjie Zhang State Key Lab of AdVanced Technology for Material Synthesis and Processing, Wuhan UniVersity of Technology, Wuhan 430070, People’s Republic of China ReceiVed: March 9, 2009; ReVised Manuscript ReceiVed: April 9, 2009

Zn-doped SnO2 hierarchical architectures with nanoflower, nanourchin, and nanotrepang morphologies have been synthesized with a surfactant-free solvothermal synthesis route in the mixed solvents of ethylenediamine (En), ethanol, and deionized water. The observations of field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) showed that Zn-doped SnO2 hierarchical architectures with different morphologies were assembled by the nanocones with diameters ranging from 20 to 60 nm. We found that the alkaline dosage (NaOH) of the solution had a remarkable effect on the morphology of assynthesized products. The UV-visible spectra of Zn-doped SnO2 hierarchical architectures exhibited a blue shift with the decrease of the size of nanocones. The photocatalytic activity of Zn-doped SnO2 samples were evaluated by the degradation of RhB aqueous solution, and the results revealed that the sample with urchin morphology had high photocatalytic activity efficiency. Moreover, the degradation process of RhB induced by Zn-doped SnO2catalyst was investigated in detail as well. Introduction Nanostructured metallic oxides have been considered to be important semiconductors and have attracted much attention due to their exceptional properties in optics, electronics, magnetics, and catalysis.1 The properties and applications of such semiconductors are determined by the morphology, structure, and organization of nanostructured architectures to a great extent. Considerable efforts have been focused on the synthesis of a novel nanostructure with tailored morphology.2-4 However, it is also one of the most important tasks to assemble nanocrystalline nanostructures into the desired hierarchical architectures, for such structures are able to be against aggregation and are promising for wide applications in technical fields due to their excellent properties.5-7 SnO2, as an n-type semiconductor with a wide band gap (3.6 eV), has numerous applications in optoelectronic devices, dyebased solar cells, secondary lithium batteries, electrode materials, transistors, gas sensors, and catalyst supports.8-11 ZnO is considered to be another important functional semiconductor material and has characteristics similar to tin oxide. The mixtures of the two oxides with special nanostructures have been confirmed to be provided with outstanding properties. Zhang et al. reported that SnO2-ZnO nanocomposite sensors showed high selectivity, high sensitivity, strong stability, and prompt response/recovery in detecting trimethylamine (TMA).12 Wang et al. reported that SnO2-ZnO hierarchical nanosheet composites showed higher photocatalytic activity in the degradation of methyl orange.13 However, the investigation of Zn-doped SnO2 has been limited. Zn2+ ions (ion radius, 0.073 nm) can be incorporated into the SnO2 lattice for the modification of tin oxide because Zn2+ has an ion radius similar to that of Sn4+ (ion radius, 0.071 nm). Zn-doped SnO2 nanostructures with flowerlike morphology were synthesized via a hydrothermal * To whom correspondence should be addressed. Phone: +86-2787215421. Fax: +86-27-87215421. E-mail: [email protected] (T.J.); [email protected] (W.W)

synthesis route and the products exhibited different optical properties.14,15 Additionally, the size of doped SnO2 nanocrystals decreases due to the incorporation of dopant ions as well.16 Compared with other dopant ions, such as Fe3+ and La3+,16,17 Zn2+ ions have their own advantages as follows. First, ZnO has a similar crystal growth habit in solvothermal surroundings. It is promising to synthesize highly crystalline rutile Zn-doped SnO2 nanostructures with novel morphology and large surface area via a solvothermal technique. Second, the substitution of Zn2+ ions for Sn4+ ions would result in surface modification and the formation of more oxygen vacancies in order to compensate positive charge. Accordingly, it is expected that Zndoped tin oxide hierarchical nanostructures are provided with excellent electric properties and photocatalytic activity. Herein, we report a simple solvothermal synthesis route for the synthesis of Zn-doped SnO2 hierarchical architectures assembled with nanocones. The optical properties and photocatalytic activity of the products were studied as well. Experimental Section Preparation of Zn-Doped SnO2 Hierarchical Architectures. All the reagents (Shanghai Chemical Industrial Company, China) in the experiment were of analytical grade and used without further purification. SnCl4 · 5H2O and Zn(NO3)2 · 6H2O were used as tin and zinc sources, respectively. In a typical procedure, an appropriate amount of NaOH was dissolved in 80 mL of mixed solvents containing 60% water, 20% ethanol, and 20% ethylenediamine (En). The dosage of NaOH was determined by the molar ratio of NaOH to the sum of SnCl4 · 5H2O and Zn(NO3)2 · 6H2O. Then 1.2 g of SnCl4 · 5H2O was added to the mixed solution under rigorous stirring. After constant stirring for 30 min, 0.168 g of Zn(NO3)2 · 6H2O was introduced into the mixed solution. The molar ratio of Sn4+/ Zn2+ was 6. After another 30 min of stirring, the obtained precursor was transferred into a 100 mL Teflon-lined stainless steel autoclave. After the solvothermal treatment process was undertaken at 160 °C for 20 h, the obtained precipitates were

10.1021/jp9021272 CCC: $40.75  2009 American Chemical Society Published on Web 05/05/2009

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Figure 1. XRD patterns of Zn-doped SnO2 samples synthesized under different molar ratios (NaOH/Sn4+ + Zn2+): (a) 10; (b) 8; (c) 6.

centrifuged and washed with distilled water and ethanol five times, followed by drying in a vacuum at 60 °C for 15 h. Characterization. The products were characterized by X-ray diffraction (XRD) on a Rigaku ultimaIII diffractometer equipped with Cu KR radiation (λ ) 0.154 06 nm). The scanning electron microscopy (SEM) images were taken using a Hitachi S-4800 field emission scanning electron microscope (FESEM, 20 kV) equipped with an energy dispersion X-ray spectrometer (EDS). Transmission electron microscopy (TEM) images were performed on a FEI Tecnai G2 20 transmission electron microscope at an acceleration voltage of 200 kV. High resolution transmission electron microscopy (HRTEM) images were carried out on a JEM 2010FEF transmission electron microscope. The surface compositions and chemical state were examined by X-ray photoelectron spectroscopy (XPS, VG Multilab 2000X). UV-visible absorption spectra were recorded on a Shimadzu UV-2550 UV-visible spectrometer. Photocatalytic Measurements. The photocatalytic activity was evaluated by the decolorization of Rhodamine B (RhB) aqueous solution. Consulting ref 18, the measurement was performed at ambient temperature in air and the details are as follows. A certain amount (0.2 g) of Zn-doped SnO2 powder was dissolved in 30 mL of deionized water, and then an aqueous Zn-doped SnO2 suspension was formed under ultrasonic treatment. The suspension was transferred into a dish with a diameter of 9 mm. After drying at 80 °C in an oven for 5 h, the catalyst was prepared by coating a thin film on the surface of the dish. Twenty milliliters of RhB aqueous solution with a concentration of 1.0 × 10-5 M was added to the dish with the prepared catalyst. An adsorption-desorption equilibrium was required to reach among the catalyst, RhB, and water before UV light irradiation. A UV lamp (15 W, 365 nm) was used as light source to trigger the photocatalytic reaction. After irradiating for a certain time, the reacted solution was filtrated to measure the concentration variation of RhB by recording the variation of the intensity of the absorption peak centered at 553 nm using a UV-visible spectrophotometer (UV-2550, Shimadzu, Japan). Results and Discussion Phase Study. Figure 1 shows the XRD patterns of the products synthesized under different conditions. All the diffraction peaks can be indexed to the rutile SnO2 (JCPDS 770451) with tetragonal lattice parameters a ) 4.763 Å and c ) 3.171 Å. The relative intensities of the peaks deviate from those of the bulk material, suggesting the anisotropic growth of the Zn-doped SnO2 hierarchical architectures. No other phase is detected, which reveals the purity of the products. This result can also signify that Zn2+ ions are able to substitute for the

Figure 2. XPS high resolution spectra of Sn, O, and Zn elements of Zn-doped SnO2 sample: (a) Sn 3d; (b) O 1s; (c) Zn 2p.

Sn4+ ions in the lattice. Additionally, the diffraction peaks of the products prepared under different conditions tended to becoming slightly broader from Figure 1a to 1c, which could be attributed to the size of effect of the crystals. X-ray Photoelectron Spectroscopy Analysis. XPS was used to study the surface element composition and the chemical state of the sample prepared under the ratio of 8 (the dosage of NaOH). The high resolution XPS spectra of Sn, Zn, and O elements are shown in Figure 2. From Figure 2a, it can be found that the binding energies of Sn 3d5/2 and Sn 3d3/2 correspond to 486.3 and 494.8 eV, respectively. The value of the Sn 3d5/2 binding energy was lower than that of tin oxide (>486.4 eV). This result was attributed to the oxygen deficiency which decreased the binding energy of Sn.19 The oxygen vacancy deficiency perhaps resulted from the employment of En and the incorporation of Zn2+ into SnO2. Figure 2b presents the XPS high resolution spectrum of oxygen. As shown in Figure 2b, the spectrum of O 1s with an additional shoulder was observable. Thus, this region of O 1s could be fitted into two peaks: one peak centered about 530.2 eV and the other centered about 531.65 eV. The main peak (centered about 530.2 eV) was perhaps attributed to the coordination of oxygen in Sn-O-Sn,

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Figure 3. FESEM images of Zn-doped SnO2 urchins composed of nanocones: (a) low magnification image; (b) zoomed-in image; (c) magnified FESEM image of a single urchin; (d) magnified FESEM image of a single urchin hollow structure.

whereas the binding energy 531.65 eV corresponding to the other peak was higher than that of Zn, which was perhaps ascribed to the coordination of oxygen in Sn-O-Zn. The Zn 2p spectrum is visible in Figure 2c, which confirms the presence of Zn element in the products. Morphology and Structure of Zn-Doped SnO2 Hierarchical Architectures. The typical morphologies of the products were observed by field emission scanning electron microscopy (FESEM). Figure 3a is a low magnification FESEM image of the product, which exhibits microsphere-like morphology. A zoomed-in SEM image (Figure 3b) reveals that the microspheres look like urchins with an average diameter of 1-2 µm. Figure 3c presents the magnified FESEM image of the urchin structure. It is clearly observed from the appearance that the urchins are composed of numerous nanocones with diameters of 30-40 nm and a length about 200 nm. In addition, we also find a hollow urchin with special structure, as shown in Figure 3d. A midlayer exists between the nanocones, and the nanocones adhering to the midlayer grow along the two respective directions: one is inward, and the other is outward. Additionally, the content of zinc of the samples was determined to be 14.57 atom % by EDS (Supporting Information), which was approximate to the theoretical dosage of zinc.15 High resolution transmission electron microscopy (HRTEM) was used to investigate the structure of the nanocones. Figure 4 shows the HRTEM images of the nanocones as demonstrated in Figure 3d. The HRTEM image and its corresponding Fourier transform patterns (FFT) indicate the single crystalline nature of the nanocone (Figure 4b). The space between adjacent lattice planes parallel to the growth direction is 0.33 nm, whereas the space between adjacent lattice planes along the other direction is 0.26 nm. Combined with the result of FFT, the preferential growth direction is determined to be [11j2], which is different from [001]. The reason for the transformation of the preferential growth direction is explained as follows. It is well-known that different crystal faces have different surface energies. For rutile SnO2, the order is as follows: (110) < (100) < (101) < (001).20 In addition, rutile SnO2 has a low axial ratio (c/a ) 0.67), which reduces the possibility of crystal growth along the [001] direction. Furthermore, the role of ethylenediamine (En) has

Figure 4. (a) Side view TEM image of Zn-doped SnO2 urchins; (b) HRTEM image of an individual nanocone on the urchin.

been reported for the synthesis of SnO2 and Fe2O3 nanostructures in previous studies.21,22 Herein, it could be inferred that En was likely to serve as a structure-directing agent for controlling the formation of Zn-doped SnO2 hierarchical architectures. In the solvothermal process, the crystal growth habit would be affected remarkably by the formed complex composed of cations and En, which also led to the preferential growth direction transforming to [11j2]. The dosage of NaOH had a strong influence on the morphologies and structures of the products. Here, we use the molar ratio of OH- to the sum of Sn4+ and Zn2+ to express the NaOH dosage. When the ratio was 10, Zn-doped SnO2 hierarchical architecture composed of nanoflowers was formed (Figure 5a). The nanoflower structures are also constituted by nanocones, but the diameter was about 50 nm and the length was about 200-400 nm. When the ratio decreased to 8, Zn-doped SnO2 hierarchical architecture composed of nanourchins was formed. Typical morphology is represented in Figure 3. When the ratio was 6, the morphology of Zn-doped SnO2 hierarchical architecture was changed significantly. The nanotrepang structures were dominant in the product, as shown in Figure 5c. These novel nanotrepang structures have lengths of 0.4-1 µm and diameters of 100-200 nm. On each nanotrepang, there are a number of uniform Zn-doped SnO2 crystallites radially oriented along the main trunk. From the high magnification SEM images shown in Figure 5d, it can be seen that each nanotrepang is

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Figure 5. (a) FESEM image of Zn-doped SnO2 nanoflowers; (b) TEM image of Zn-doped SnO2 nanoflowers; (c) low magnification FESEM image of Zn-doped SnO2 nanotrepangs; (d) high magnification FESEM image of Zn-doped SnO2 nanotrepangs; (e) TEM image of Zn-doped SnO2 nanotrepangs; (f) TEM image of Zn-doped SnO2 nanorings.

composed of nanocones grown approximately perpendicular along the main trunk. Additionally, a novel morphology ring structure can also be found in the product, as shown in Figure 5f. The ring structures are also assembled by nanocones, but the diameter becomes smaller and the length is much shorter compared to those of nanoflower and nanourchin structures. This result might be attributed to the weak alkalinity, in which the rate of nuclei formation as well as crystal growth decreases. Based on the above experimental results, a possible mechanism could be proposed. In the initial stage of the solvothermal process, the complex precursor of Sn(OH)62- was certain to exist in the mixed solution due to the introduction of the excess dosage of sodium hydroxide. ZnSn(OH)6 would be produced when Zn2+ was introduced into the mixed solution containing Sn4+ and alkali. At the same time, the products of Sn(OH)62and Zn(OH)42- were produced by the reaction of OH- and ZnSn(OH)6, which would be accelerated with the increase of solvothermal temperature and pressure. In the next step, Sn(OH)62- and Zn(OH)42- reacted with En, which resulted in the formation of the coordinated ions of [Sn(En)n](OH)62-and [Zn(En)m](OH)42-, respectively, where m and n are positive integers. Then, the precursors of [Sn(En)m](OH)62- and [Zn(En)m](OH)42-were decomposed in the latter solvothermal process and the nuclei of SnO2 and ZnO were produced. The formed nuclei would grow into appropriate morphologies assisted by the soft template of En. Afterward, OH- ions and En were released from the precursors, which would take

part in the next cycle. The proposed reactions can be expressed as follows.

Sn4+ + 6OH T Sn(OH)62Zn2+ + Sn(OH)62- T ZnSn(OH)6 ZnSn(OH)6 + 4OH- T Sn(OH)62- + ZnSn(OH)42Sn(OH)62- + nEn T [Sn(En)n] + (OH)62Zn(OH)42- + mEn T [Zn(En)m] + (OH)42(1 - x)[Sn(En)n](OH)62- + x[Zn(En)](OH)42- f Sn1-xZnxO2 + 2H2O + 2(1 - x)OH- + (n - nx + mx)En (0 < x < 1) As is well-known, the structure of nanocrystals is mostly dependent on the nuclei formation rate and concentration. In the synthetic process, the concentration of NaOH controlled the decomposition process of ZnSn(OH)6 and release rates of Sn(OH)62- and Zn(OH)42-, and further determined the nuclei formation process. As analyzed above, it can be inferred that the variation of sodium hydroxide dosage would induce different formation rates and concentrations of the nuclei, which could be critical for the formation of various Zn-doped SnO2 hierarchical architectures assisted by En. Therefore, the sodium hydroxide dosage determined the

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Figure 6. UV-visible spectra of Zn-doped SnO2 samples synthesized under different molar ratios (NaOH/Sn4+ + Zn2+): (a) 10; (b) 8; (c) 6.

structure of Zn-doped SnO2 hierarchical architecture by controlling the formation rate and concentration of the nuclei. Furthermore, it is noteworthy that Zn2+ ions play an important role in forming Zn-doped SnO2 hierarchical architecture. In the absence of Zn2+ ions, a similar morphology was not obtained. Thus, it is rational to infer that Zn2+ ions could also act as a structure-directing agent in the growth of Zndoped SnO2 nanocrystals. UV-Visible Spectra. Figure 6 shows the UV-visible spectra of three Zn-doped SnO2 samples composed of different nanostructures synthesized under different dosages of NaOH. The variations of (Rpν)2 versus the photon energy pν for sample c are plotted in the inset of Figure 6. For a semiconductor sample, the optical absorption near the band edge is determined by the equation (Rpν)2 ) A(pν - Eg)n/2, where R, p, ν, Eg, and A are the adsorption coefficient, Planck’s constant over 2π, radiation frequency, band gap, and a constant, respectively.23 Among them, n decides the characteristics of the transition in a semiconductor. In our experiment, the value of n for Zn-doped SnO2 is 1. After the determination of these parameters, the relationship of (Rpν)2 vs pν is plotted to obtain the band gap. Eg, the energy of the band gap, can be estimated from the intercept of the linear portion of the plot. The energy of the band gap of sample c is 3.7 eV, as shown in the inset of Figure 6. Furthermore, the energy of the band gap can be estimated to be 3.53 eV for sample a and 3.66 eV for sample b by using the same method. Moreover, from the figure, it can be clearly seen that the near band absorption of the three samples exhibits a blue shift with the decrease of sodium hydroxide dosage, which results in the decrease of nanocrystal size, as analyzed above. Thus, this shift in UV spectra could be ascribed to the size effect of crystals.24 As demonstrated in Figure 4a, the diameters of the bottoms of nanocones are about 20-30 nm, whereas the diameters of the tops of nanocones are about to 2-3 nm, which are close to the exciton Bohr radius (2.7 nm) of SnO2. Therefore, the size effect of crystals should be taken into account for the variation in band gap energies. Herein, consulting the previous study,25 the effective mass approximation can be appropriate for a quantitative description of the particle size dependence of the band gap energies:

Egeff ) Eg +

p2π2 1.8e2 + ... 2 εR 2µR

where Egeff is the effective band gap energy, Eg is the bulk band gap energy, Ris the particle radius, p is Planck’s constant over 2π, µ is the electron-hole effective mass, and ε is the static

Figure 7. (a) Variations of adsorption spectra of aqueous RhB solution in the presence of catalyst Zn-doped SnO2 urchin sample; (b) variations of adsorption spectra of aqueous RhB solution in the presence of catalyst Zn-doped SnO2 flower sample.

dielectric constant. It can be inferred that the band gap energies of Zn-SnO2 products could be enlarged with the decrease of the size of crystals from the above analysis. Photocatalytic Activity. In order to evaluate the photocatalytic activity of Zn-doped SnO2 samples as a catalyst, we chose two kinds of samples with different structures: one sample with urchin morphology and the other sample with nanoflower morphology, labeled as catalysts ZSu and ZSf, respectively. It was noteworthy that the photocatalytic decolorization of RhB was not achieved under ultraviolet illumination in the absence of Zn-doped SnO2 samples. Figure 7a and 7b present the variations of adsorption spectra of aqueous RhB solution in the presence of the catalysts ZSu and ZSf, respectively. It was found that the intensity of the characteristic adsorption peak (λ ) 553 nm) decreased dramatically with the illumination time in the degradation process, as shown in Figure 7a. At the same time, the characteristic adsorption began to shift slightly to low wavelength when the irradiation time was 30 min. A newly shaped peak in the photocatalytic process occurred which deviated from the characteristic adsorption with the time prolonging. The characteristic adsorption peak disappeared during irradiation time up to 90 min; afterward, the shaped peak disappeared gradually and the RhB aqueous solution was degraded completely when the irradiation time reached 210 min. As concerned the catalyst ZSf, the photocatalytic process was similar. However, it took a longer time (300 min) for disappearance of the characteristic adsorption peak and the complete degradation of RhB. It is well-known that the RhB photodegradation of Zndoped SnO2 catalyst progressed via two competitive processes: N-deethylated and the destruction of the conjugated structure.26,27 In our experiment, the adsorption maximum of

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the degraded RhB solution exhibited concomitant and hypsochromic shifts. The hypsochromic shift was proposed to be resulted from the formation of a series of N-deethylated intermediates in a stepwise manner.26 In the case of the degradation process of the catalyst ZSu, RhB was almost degraded at the irradiation time of 90 min and the adsorption maximum shifted from 553 to 498 nm after 30 min of irradiation. However, it is notable to point out that the produced N-deethylated substance was difficult to degrade compared to the previous substance corresponding to 553 nm. At the same time, we also observed that the color of the suspension changed gradually from pink to light green. Further irradiation caused the decrease of the adsorption band at 498 nm, and the suspension became sequentially colorless. The degradation process caused by Zn-doped SnO2 catalyst altered not only the photodegradation rate of the pollutant but also the mechanistic pathways of the pollutant degradation, since the degradation of RhB induced by pure SnO2 exhibited no hypsochromic shift in the previous study.28 The Zn-doped SnO2 catalyst promoted the ratio of the deethylation process to the cleavage of chromophore structure. The enhancement of photocatalytic activity was mainly ascribed to the formation of oxygen vacancies and the increase of effective electron mobility. When Sn4+ ions were substituted by Zn2+, oxygen vacancies would be formed in SnO2. Oxygen vacancies at surfaces could promote the O2 adsorption and serve as the centers to capture the photoinduced electrons during the photocatalytic reaction processes. Consequently, the recombination of photoinduced charge carriers could be effectively inhibited. This result was well in agreement with the previous report.29 Furthermore, the nanostructure of Zndoped SnO2 hierarchical architecture has a remarkable effect on the photocatalytic activity. Compared with the nanoflower structure sample, the sample with urchin structures has larger surface area and band gap energy. The augment of the surface area of the sample with urchin structures was caused by two aspects: one was the decrease of the size of nanocones, and the other was a special structure with the midlayer between nanocones. The surface area has a remarkable effect on the photocatalytic activity of semiconductors, which has been confirmed in previous studies.30,31 It could be expected that the ratio of the surface charge carrier transfer rate to the electron-hole recombination rate could be promoted due to the increase of the available surface active site concentration and the higher photonic efficiency.32 Additionally, the augment of the band gap energy could enhance the redox potentials of photogenerated electrons and holes; consequently, the photocatalytic activity could be improved.33 Therefore, the urchin structures composed of nanocones were in favor of the transfer of electrons and holes generated inside the crystal to the surface, and facilitated the degradation of RhB. Conclusions Zn-doped SnO2 hierarchical architectures assembled by nanocones were successfully synthesized via a solvothermal approach in the mixed solvents of ethylenediamine, ethanol, and deionized water. The proposed growth mechanism of Zndoped SnO2 hierarchical architectures was studied. The dosage of NaOH influenced the morphologies of Zn-doped SnO2 hierarchical architectures significantly. With the augment of the NaOH dosage, the morphology was transformed from nanotrepang structure to nanourchin structure, and it evolved into nanoflower structure when the molar ratio of

Jia et al. introduced NaOH to the sum of Sn4+ and Zn2+ was 10. The HRTEM image revealed that ethylenediamine (En) played an important role in shaping the nanostructures of Zn-doped SnO2 hierarchical architectures and directing the crystal growth along the [11j 2] direction. More oxygen vacancies were formed and the UV-visble spectra exhibited a blue shift due to the size effect of crystals. The photocatalytic activity results of Zn-doped SnO2 catalyst showed that Zn-doped SnO2 hierarchical architecture with urchin structure exhibited a high efficiency of photocatalytic activity, which was ascribed to the oxygen vacancies caused by the incorporation of Zn2+ and the structure of the catalyst. Acknowledgment. The work was financially supported by the Ministry of Education of China (PCSIRT0644) and the National Science Foundation of China (A3 Foresight Project No. 50821140308). We also express our thanks to Mr. Dongshan Zhao from Wuhan University for his assistance with the HRTEM characterizations. Supporting Information Available: EDS spectrum of the obtained Zn-SnO2 products with urchin morphology. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) Morles, A. M.; Lieber, C. M. Science 1998, 279, 208. (b) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2002, 124, 3343. (c) Li, L.; Pan, S. S.; Dou, X. C.; Zhu, Y. G.; Huang, X. H.; Yang, Y. W.; Li, G. H.; Zhang, L. D. J. Phys. Chem. C 2007, 111, 7288. (2) (a) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. D. Nature 2005, 437, 121. (b) Saito, N.; Haneda, H.; Sekiguchi, T.; Ohashi, N.; Sakaguchi, N.; Koumoto, K. AdV. Mater. 2002, 14, 418. (c) Huynh, W.; Peng, X. G.; Alivisatos, A. P. AdV. Mater. 1999, 11, 923. (3) Li, L.; Yang, Y. W.; Li, G. H.; Zhang, L. D. Small 2006, 2, 548. (4) Mattoussi, H.; Radzilowski, L. H.; Dabbousi, B. O.; Thomas, E. L.; Bawendi, M. G.; Rubner, M. F. J. Appl. Phys. 1998, 83, 7965. (5) Yang, P. D. Nature 2003, 425, 243. (6) Shi, T. H.; Qi, M. L.; Ma, J. M.; Cheng, H. M.; Zhu, B. Y. AdV. Mater. 2003, 15, 1647. (7) Lee, S. M.; Jun, Y.; Cho, S. N.; Cheon, J. J. Am. Chem. Soc. 2002, 124, 11244. (8) Lee, J. S.; Sim, S. K.; Min, B.; Cho, K.; Kim, S. W.; Kim, S. J. Cryst. Growth 2004, 267, 145. (9) Xu, G.; Zhang, Y. W.; Sun, X.; Xu, C. L.; Yan, C. H. J. Phys. Chem. B 2005, 109, 3269. (10) Sugimoto, H.; Tsukube, H.; Tanaka, K. Eur. J. Inorg. Chem. 2004, 23, 4550. (11) Jia, N. Q.; Zhou, Q.; Liu, L.; Yan, M. M.; Jiang, Z. Y. J. Electroanal. Chem. 2005, 580, 213. (12) Zhang, W. H.; Zhang, W. D. Sens. Actuators, B: Chem. 2008, 134, 403. (13) Wang, W. W.; Zhu, Y. J.; Yang, L. X. AdV. Funct. Mater. 2007, 17, 59. (14) Li, Z.; Li, X.; Zhang, X.; Qian, Y. J. Cryst. Growth 2006, 291, 258. (15) Cheng, G.; Wu, K.; Zhao, P.; Cheng, Y.; He, X.; Huang, K. J. Cryst. Growth 2007, 309, 53. (16) Rania, S.; Royb, S. C.; Kararc, N.; Bhatnagara, M. C. Solid State Commun. 2007, 141, 214. (17) Hieu, N. V.; Kim, H. R.; Ju, B. K.; Lee, J. H. Sens. Actuators, B: Chem. 2008, 133, 228. (18) Yu, J. G.; Yu, H. G.; Cheng, B. J. Phys. Chem. B 2003, 107, 13871. (19) Peng, X. S.; Meng, G. W.; Wang, X. F.; Wang, Y. W.; Zhang, J.; Liu, X.; Zhang, L. D. Chem. Mater. 2002, 14, 4490. (20) Leite, E. R.; Giraldi, T. R.; Pontes, F. M.; Longo, E.; Beltran, A.; Andres, J. Appl. Phys. Lett. 2003, 83, 1566. (21) Cheng, G.; Wu, K.; Zhao, P.; Cheng, Y.; He, X.; Huang, K. Nanotechnology 2007, 18, 355604. (22) Mitra, S.; Das, S.; Mandal, K.; Chaudhuri, S. Nanotechnology 2007, 18, 275608. (23) Butler, M. A. J. Appl. Phys. 1977, 48, 1914. (24) Anandan, S.; Vinu, A.; Sheeja Lovely, K. L. P.; Gokulakrishnan, N.; Srinivasu, P.; Mori, T.; Murugesan, V.; Sivamurugan, V.; Ariga, K. J. Mol. Catal. A: Chem. 2006, 266, 149.

Zn-Doped SnO2 Hierarchical Architectures (25) Das, S.; Kar, S.; Chaudhuri, S. J. Appl. Phys. 2006, 99, 114303. (26) Lei, P.; Chen, C.; Yang, J.; Ma, W.; Zhao, J.; Zhang, L. EnViron. Sci. Technol. 2005, 39, 8466. (27) Chen, C.; Zhao, W.; Li, J.; Zhao, J. EnViron. Sci. Technol. 2002, 36, 3604. (28) Wang, G.; Lu, W.; Li, J.; Choi, J.; Jeong, Y.; Choi, S. Y.; Park, J. B.; Ryu, M. K.; Lee, K. Small 2006, 2, 1436. (29) Fu, H.; Zhang, S.; Xu, T.; Zhu, Y.; Chen, J. EnViron. Sci. Technol. 2008, 42, 2085.

J. Phys. Chem. C, Vol. 113, No. 21, 2009 9077 (30) Ohko, Y.; Fujishima, A.; Hashimoto, K. J. Phys. Chem. B 1998, 102, 1724. (31) Song, X. F.; Gao, L. J. Phys. Chem. C 2007, 111, 8180. (32) Zhang, Z. B.; Wang, C. C.; Zakaria, R.; Ying, J. Y. J. Phys. Chem. B 1998, 102, 10871. (33) Cao, L. X.; Spiess, F. J.; Huang, A.; Suib, S. L.; Obee, T. N.; Hay, S. O.; Freihaut, J. D. J. Phys. Chem. B 1999, 103, 2912.

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