TiO2 Nanofiber

Jianqi Zhang , Li Li , Zhixin Xiao , Di Liu , Shuang Wang , Jingjing Zhang , Yuting ...... Wei Zhao , Hongxing Wang , Xiangning Feng , Wangyang Jiang ...
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A Facile in Situ Hydrothermal Method to SrTiO3/TiO2 Nanofiber Heterostructures with High Photocatalytic Activity Tieping Cao,†,‡ Yuejun Li,† Changhua Wang,† Changlu Shao,*,† and Yichun Liu† †

Centre for Advanced Optoelectronic Functional Materials Research, Key Laboratory for UV Light-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, Changchun 130024, People’s Republic of China ‡ Department of Chemistry, Baicheng Teacher’s College, Baicheng, Jilin 137000, China ABSTRACT: Heterostructured SrTiO3/TiO2 nanofibers were fabricated by in situ hydrothermal method using TiO2 nanofibers as both template and reactant. The as-fabricated heterostructures composite included SrTiO3 nanocubes or nanoparticles assembled uniformly on the surface of TiO2 nanofibers. Compared with the pure TiO2 nanofibers, SrTiO3/TiO2 nanofibers exhibited enhanced photocatalytic activity in the decomposition of Rhodamine B (RB) under ultraviolet light. The enhanced photocatalytic activity of SrTiO3/TiO2 nanofibers could be attributed to the improvement of charge separation derived from the coupling effect of TiO2 and SrTiO3 nanocomposite.

1. INTRODUCTION One-dimensional TiO2 nanostructures have become of increasing importance in applications of photocatalysis, photoelectrochemical process, and dye-sensitized solar cells due to their superior properties in comparison with other TiO2 nanostructured counterparts.1-6 Hitherto, enormous efforts have been made to synthesize TiO2 nanofibers, nanowires, and nanotubes toward satisfying specific application requirement.7-10 Therein, TiO2 nanofibers prepared by the electrospinning technique have been of great interest.11-13 The as-electrospun continuous TiO2 nanofibers mats exhibit significant advantages such as high surface areas, three-dimensional open structure, and favorable morphology for ease of handling and recycling. When used as photocatalyst, they can provide a large number of surface-active sites to be accessible for reactants more easily and effectively, are easily recycled, and have large light harvesting ability and thereby are considered as promising candidates for photocatalytic application.14-17 Considering their large surface area and unique morphology, the electrospun TiO2 nanofibers will offer new chances to improve their photocatalytic activity by doping, coupling, deposition, and sensitization. Particularly, fabrication of TiO2 nanofiber-based heterostructures (TiO2/Pt,18 TiO2/ Ag,19 TiO2/SnO2,20 TiO2/CeO2,21 TiO2/Bi2WO6,22 etc.) have received increasing attention recently due to their promotion of the separation of photogenerated electron-hole pairs and thus enhancement of the photocatalytic activity of TiO2 nanofibers. Besides, SrTiO3, a well-known cubic-perovskite-type multimetallic oxide with a band gap of 3.2 eV comparable to TiO2, has attracted considerable attention because of its wide applications in storage batteries, oxygen gas sensors, photocatalysts, and photoelectrodes for dye-sensitized solar cells.23-28 More r 2011 American Chemical Society

interestingly, SrTiO3 offers favorable energetic for photocatalysis since its conduction band edge is 200 mV more negative than TiO2. In this regard, under UV light irradiation, a proper combination of SrTiO3 and TiO2 can lead to not only transfer of electron from the conduction band of SrTiO3 to that of TiO2 but also transfer of hole from the valence band of TiO2 to that of SrTiO3. As such, the improved separation between photogenerated electrons and holes is expected to improve the photocatalytic activity of TiO2. For example, physically mixed SrTiO3 and TiO2 hybrid shows improved photocatalytic activity compared with pure TiO2, which is attributed to the interparticles electron transfer process. Moreover, the better the contact between SrTiO3 and TiO2 particles, the more the increase in activity of TiO2. Therefore, close contact between SrTiO3 and TiO2 is necessary when designing and preparing heterostructures with good photocatalytic performance. Nevertheless, the SrTiO3/ TiO2 heteorstructure photocatalysts are relatively less investigated, say nothing of one-dimensional SrTiO3/TiO2 nanofibers heterostructure, partly due to neither regulation over thermodynamic and kinetic process underlying the size and shape evolution of the constituent domains in such multicomponent oxide nor the adjustment of the surface-interface energy balance by carefully selecting the appropriate fabrication method. On the basis of above considerations, we have been intent to develop a simple synthetic strategy to fabricate SrTiO3/TiO2 nanofibers heterostructure. In this study, we propose a wetchemical, in situ hydrothermal synthesis route in Sr(OH)2 aqueous solution, utilizing TiO2 nanofibers as both template Received: October 20, 2010 Revised: January 16, 2011 Published: February 11, 2011 2946

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Table 1. Experimental Conditions and SrTiO3 Nanostructures Characteristics [Sr(OH)2]

temp

sample

(mM)

(°C)

morphology crystal size (nm)

SrTiO3/TiO2 (1) SrTiO3/TiO2 (2)

1.25 12.5

150 150

nanocubes nanocubes

SrTiO3/TiO2 (3)

12.5

120

nanoparticles

150  150  150 250  250  250 40-60

and initial reactant in order to generate TiO2 nanofibers-SrTiO3 nanostructure heterostructures in a rational manner. The adopted synthesis route ensures not only the successful growth of SrTiO3 nanostructures on TiO2 nanofibers substrate but also the high dispersion of SrTiO3 nanostructures on TiO2 nanofibers without aggregation. Notably, by simply tuning the precursor Sr(OH)2 concentration or reaction temperature, the density as well as the morphology of SrTiO3 nanostructures can be further controlled easily in order to improve the photocatalytic property of TiO2 by a greater degree. The SrTiO3/TiO2 heteostructures exhibit much better efficiency on degradation of RB comparing with pure TiO2 nanofibers. Moreover, such a simple and versatile strategy can provide a general way by employing TiO2 nanofibers as precursor material and template to fabricate other TiO2/ ternary complex oxide nanofibers heterostructure, such as TiO2/ BaTiO3 and TiO2/CaTiO3.

Figure 1. XRD patterns of typical SrTiO3/TiO2 (1) nanofibers heterostructure and pure TiO2 nanofibers. catalyst (0.01 g) was filled in a photoreactor designed with an internal light source (50 W high-pressure mercury lamp with main emission wavelength 313 nm and an average light intensity of 2.85 mW cm-2) surrounded by a water-cooling quartz jacket to cool the lamp. The solution was stirred in the dark for 30 min to obtain a good dispersion and establish adsorption-desorption equilibrium between the organic molecules and the catalyst surface. Decrease in the concentration of dye solution was measured with a spectrophotometer at λ = 554 nm at given reaction intervals. At given intervals of illumination, the samples of the reaction solution were taken out and analyzed.

2. EXPERIMENTAL SECTION 2.1. Preparation of TiO2 Nanofibers. 1 mL of titanium butyloxide was mixed with 1.5 mL of acetic acid and 5 mL of ethanol. Then the above homogeneous sol was added to 10 mL of poly(vinylpyrrolidone) (PVP) ethanolic solution (8 wt %) with vigorous stirring at room temperature for about 4 h. TiO2 nanofiber mats were prepared by electrospinning the presursor solution from a syringe under an applied electric voltage of 10 kV, followed with calcination in air at 500 °C for 12 h. 2.2. Fabrication of SrTiO3-TiO2 Heterostructure. In a typical procedure, 5 mg of the electrospun TiO2 nanofibers was put into an autoclave containing 20 mL of Sr(OH)2 solution. The concentration of the Sr(OH)2 solution was 1.25 mmol L-1. The pH value of the solution was adjusted to 12 by 0.1 M NaOH solution. The autoclave was heated at 150 °C for 24 h. The as-fabricated product was collected, washed with dilute hydrochloric acid, and deionized water, respectively, and then dried in an oven at 60 °C for 12 h. By this method, the sample of SrTiO3/ TiO2 heterostructures was fabricated, denoted as SrTiO3/TiO2 (1). The second sample, referred as SrTiO3/TiO2 (2), was prepared in the case that the concentration of Sr(OH)2 solution was 10 times higher than that of SrTiO3/TiO2 (1). The third sample, referred as SrTiO3/TiO2 (3), was prepared by the procedure similar to SrTiO3/TiO2 (1), while the reaction temperature was kept at 120 °C. Detailed experimental conditions are listed in Table 1. 2.3. Characterization. The morphology of the as-prepared samples was characterized with a field emission scanning electron microscope (FESEM, Philips XL-30) operated at an accelerating voltage of 20 kV. The high-resolution transmission electron microscope (HRTEM) images were acquired using a JEOL JEM-2100 (acceleration voltage of 200 kV). The compositions of the samples were analyzed with and X-ray diffractometer (Rigaku D/MAX2500, Cu KR line, λ = 0.150 41 nm). X-ray photoelectron spectroscopy (XPS) was performed on a VG ESCALAB LKII instrument with Mg KR ADES (hν = 1253.6 eV) source at a residual gas pressure of below 10-8 Pa. 2.4. Photocatalytic Test. A 100 mL rhodamine B (RB) solution with an initial concentration of 10 mg L-1 in the presence of solid

3. RESULTS AND DISCUSSION The crystal structures of SrTiO3/TiO2(1) composite as well as bare TiO2 nanofibers were revealed by XRD (Figure 1) analysis. The curve a in Figure 1 revealed that the crystal phase of TiO2 nanofibers was anatase with the diffraction peaks at about 2θ = 25.5°, 37.9°, 48.2°, 54.1°, and 55.0°, which could be perfectly indexed to the (101), (004), (200), (105), and (211) crystal faces of anatase TiO2 (PDF card 21-1272, JCPDS). After hydrothermal treatment in Sr(OH)2 solution at 150 °C for 24 h, as shown in curve b, additional diffraction peaks with 2θ values of 32.2°, 39.75°, 46.43°, 57.69°, and 67.87° appeared, corresponding to (110), (111), (200), (211), and (220) crystal planes of cubic SrTiO3, respectively (PDF card 35-734, JCPDS), indicating that part of TiO2 was successfully converted into SrTiO3. Additionally, the XRD peaks belonging to TiO2 in the SrTiO3/ TiO2 composites did not shift compared with the pure TiO2 nanofibers, which could be deduced that the Sr did not substitute Ti and enter into the TiO2 lattices. So, it was obvious that the synthesis route is favor for obtaining multicomponent oxide composite integrating anatase phase TiO2 with the cubic phase SrTiO3. The morphology of samples was observed by FESEM. In the experiments, before hydrothermal treatment, it could be clearly seen that the TiO2 nanofibers with diameters about 200-300 nm were of relatively smooth surface without secondary nanostructures (not shown). After hydrothermal treatment, the composite sample still remained the nonwoven nanofibers’ morphology, as shown in Figure 2. Figure 2a,b presents the typical SEM images of the resulting products after hydrothermal treatment in Sr(OH)2 solution at 150 °C for 24 h. From the panoramic FESEM image (Figure 2a), it is clearly showed that the composites displayed heteroarchitectures, where secondary SrTiO3 nanocubes with high uniform and regular shape were implanted on the primary TiO2 fibers. A SEM image with high magnification 2947

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Figure 2. (a, b) SEM images of SrTiO3/TiO2 (1) nanofibers heterostructure at different magnification. (c) TEM image of SrTiO3/TiO2 (1) nanofibers heterostructure. (d) HRTEM image of the region of SrTiO3/TiO2 (1) heterojunction. (e) HRTEM image of a single SrTiO3 nanocube. (f) SAED pattern from a single SrTiO3 nanocube. (g) EDS spectrum from a single SrTiO3 nanocube. (g) EDS spectrum from the exposed surface of nanofibers.

revealed the more detailed structural characteristics of the heteroarchitectures (Figure 2b), from which we could see SrTiO3 nanocubes closely biting into the TiO2 nanofibers. The observed edge length of SrTiO3 nanocubes was about 120-180 nm. The typical TEM image of an individual SrTiO3/TiO2(1) heteroarchical fiber is clearly shown in Figure 2c. As it can be seen, the cubic SrTiO3 nanoparticles had a narrow size distribution. They possessed the sizes length about 150 nm, coinciding with the results from the FESEM observations. Additionally, the closer observation at the junction of nanocubes and nanofibers showed that SrTiO3 nanocubes had their roots inside the TiO2 nanofibers, suggesting that the SrTiO3 nanocubes were not just loosely attached to the nanofiber surface. The HRTEM image

from the junction displayed two types of clear lattice fringes as shown in Figure 2d,e. One set of the fringes spacing was ca. 0.35 nm, corresponding to the (101) plane of anatase crystal structure of TiO2. Another set of the fringes spacing measures ca. 0.27 nm, which corresponded to the (110) lattice spacing of the cubic phase of SrTiO3. Selected-area electron diffraction (SAED) pattern (Figure 2f) from single nanocube clearly demonstrated the single-crystal nature of the nanocube and were indexed in accordance with the cubic SrTiO3 unit cell. The energy dispersive X-ray spectroscopy (EDS) analysis results from different positions along the nanofibers are shown in Figure 2g,h. EDS data for the nanocube (Figure 2g) confirmed that the nanocube was composed of Sr, Ti, and O, and the molar ratio of Sr/Ti was 2948

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Figure 3. (a) XPS fully scanned spectra of the typical SrTiO3/TiO2 (1) nanofibers heterostructure and pure TiO2 nanofibers. (b) XPS spectra of Ti 2p for the typical SrTiO3/TiO2 (1) nanofibers heterostructure and pure TiO2 nanofibers.

calculated to be about 1:1. Also, EDS data for the adjacent bare surface exposed of the nanofiber displayed that the surface was composed of only Ti and O (Figure 2h). Clearly, the nanocube was pure SrTiO3, and the bare surface exposed of nanofibers was pure TiO2. C and Cu peaks in Figure 2g,h were attributed to the TEM grid used to support the nanofibers. The chemical composition and purity of the SrTiO3/TiO2(1) heterostructure sample were studied by XPS analysis and compared with those of the pure TiO2 nanofibers. The fully scanned spectra (Figure 3a) showed that elements Ti, Sr, O, and C existed in SrTiO3/TiO2 heterostructures, while the C element could be ascribed to the adventitious carbon-based contaminant, and the binding energy for C 1s peak at 284.6 eV was used as the reference for calibration. The high-resolution XPS spectra with scanning over the area corresponding to the binding energies for the Ti 2p region around 460 eV were analyzed (Figure 3b). For pure TiO2 nanofibers, the peak located at 464.2 eV corresponded to the Ti 2p1/2 and another one located at 458.5 eV was assigned to Ti 2p3/2. For SrTiO3/TiO2 composite, the shape of a wide and asymmetric peak of Ti 2p spectrum indicated that there could be more than one chemical state according to the binding energy. Using the XPS Peak fitting program, version 4.1, the Ti 2p XPS spectrum could be fitted to two kinds of chemical states. Specifically, the spectrum showed two sets of doublet peaks ascribed to Ti4þ/TiO2 (Ti 2p1/2, 464.4 eV; Ti 2p3/2, 458.6 eV) and Ti4þ/SrTiO3 (Ti 2p1/2, 463.5 eV; Ti 2p3/2, 457.8 eV).29 Moreover, the atomic ratio of Sr to Ti was determined to be 1:4 from the experimental XPS peak areas and their relative sensitivity factors. Accordingly, the molar ratio of SrTiO3 to TiO2 could be calculated to be ca. 1:3.

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By simply changing the experimental parameters (i.e., Sr(OH)2 precursor solution concentration, reaction temperature, etc.), the secondary SrTiO3 nanostructures grown on TiO2 nanofibers with different density and shape could be facilely controlled. Figure 4a,b shows the SEM images of the sample SrTiO3/TiO2 (2). It could be observed that the density of the nanocubes was dramatically increased when the precursor concentration Sr(OH)2 was increased by 10 times. In addition, the length of nanocubes also changed from ca. 150 to 250 nm. On the other hand, when the concentration of precursor is 12.5 mmol L-1 and the growth temperature was decreased to 120 °C, the morphology of secondary SrTiO3 nanostructures grown on TiO2 nanofibers changed significantly. Figure 4c,d shows the SEM images of the as-fabricated sample SrTiO3/TiO2 (3). It could be observed that SrTiO3 nanoparticles instead of nanocubes were formed on the nanofiber substrates. The nanoparticles had diameters of 40-60 nm. EDS analysis revealed that the ratio of Sr to Ti in SrTiO3/TiO2 (2) and SrTiO3/TiO2 (3) was both close to 1:2. XRD analysis for the as-fabricated SrTiO3/TiO2 heterostrucures under different conditions was also performed, and the results are shown in Figure 4e. In curve a, enhancement of SrTiO3 diffraction peaks intensities and reduce of anatase TiO2 peaks intensities were observed, indicating that more TiO2 was converted into SrTiO3 when the Sr(OH)2 solution concentration was increased in sample SrTiO3/TiO2 (2). On the contrary, reduce of SrTiO3 peaks intensities (curve b) in sample SrTiO3/TiO2 (3) compared to sample SrTiO3/TiO2 (1) and SrTiO3/TiO2 (2) could be attribute to the relative worse crystallinity of SrTiO3 crystals. For a substantial view of the growth mechanism of the SrTiO3/TiO2 heterostrucure, the time-dependent evolution process was monitored when the Sr(OH)2 solution concentration is set at 12.5 mM. Figure 5 shows the SEM images of the products that were obtained at 150 °C at different growth stages. At the early stage, very few tiny nanoparticles appeared on the surface of TiO2 nanofibers, as shown in Figure 5 a. When the reaction time was prolonged from 3 to 6 h, the surface of TiO2 nanofibers was covered by a great deal of small nanoparticles with a diameter of ca. 50 nm, as shown in Figure 5b. When the reaction time was 12 h, nanocubes appeared (image c). Prolonging the reaction time to 36 h, the surface of nanofibers was densely packed by nanocubes. From our experimental results, we proposed that the formation of SrTiO3/TiO2 heterostructure might be governed by dissolution and precipitation mechanism, followed by the well-known Ostwald ripening process. The evolution process is illustrated in Figure 6. At the first stage, tiny SrTiO3 nanoparticles were produced when the TiO2 nanofibers was immersed in Sr(OH)2 solution. The formation of SrTiO3 nanoparticles by the TiO2 precursor in the alkaline solution is proposed to be due to a dissolution and precipitation mechanism that involves the dissolution of titanium oxide followed by nucleation of the perovskite SrTiO3 crystal. In our experiments, the dissolution of TiO2 and precipitation of SrTiO3 occurred in the vicinity of the nanofibers surfaces due to the absence of free [Ti(OH)6]2- away from the area, and then as-formed [Ti(OH)6]2- reacted with Sr2þ to form SrTiO3 that nucleated onto the surface of TiO2 nanofibers. As the reaction proceeded, the in situ generated SrTiO3 nanoparticles accumulated on the surface of TiO2 nanofibers to form SrTiO3 nanoparticle layer. Meanwhile, the larger nanocubes grew by the cost of the small particles, and the reduction in surface energy is the primary driving force for the crystal growth and morphology evolution, 2949

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Figure 4. (a, b) SEM image of sample SrTiO3/TiO2 (2) at different magnification. (c, d) SEM image of sample SrTiO3/TiO2 (3) at different magnification. (e) EDS spectra of sample SrTiO3/TiO2 (2) and SrTiO3/TiO2 (3). (f) XRD patterns of sample SrTiO3/TiO2 (2) and SrTiO3/TiO2 (3).

which is due to the difference in solubility between larger particles and small particles, according to the well-known solid-solution-solid process named Ostwald ripening process. Finally, as the continuation of the reaction further, the nanoparticles vanished due to the diffusion into growing nanocubes, and larger nanocubes were finally formed on the surface of nanofibers. The photocatalytic degradation of rhodamine B had been chosen as a model reaction to evaluate the photocatalytic activities of the present SrTiO3/TiO2 heterostructures. Figure 7 shows the degradation curves of RB by pure TiO2 and SrTiO3/ TiO2 heterostructures. The order of photocatalytic activities was SrTiO3/TiO2 (1) > SrTiO3/TiO2 (2) > SrTiO3/TiO2 (3) > pure TiO2 nanofibers. The results indicated that the photocatalytic activity of pure TiO2 nanofibers was improved by coupling with SrTiO3, and there was optimum amount of secondary SrTiO3 nanostructures. The enhanced photocatalytic performance of SrTiO3/TiO2 composite nanofibers was benefited

from the presence of nano-nano heterojunction, which favored the separation of photogenerated electrons/holes pairs in the SrTiO3/TiO2 heterostructure. A proposed mechanism for the enhanced photocatalytic efficiency of the SrTiO3/TiO2 composite was described as follows: Under UV light irradiation, both TiO2 and SrTiO3 could be excited; the generated electrons in SrTiO3 and holes in TiO2 were then immigrated to the conduction band (CB) of TiO2 and valence band of SrTiO3, respectively. This transfer process was thermodynamic favorable due to both the CB and VB of SrTiO3 lied higher than that of TiO2.30,31 The efficient charge separation increased the lifetime of the charge carriers and enhanced the efficiency of the interfacial charge transferred to adsorbed substrates leading to higher activity of the SrTiO3/TiO2 heterostructure photocatalyst. Meanwhile, the generated conduction band electrons (e-) probably reacted with dissolved oxygen molecules to yield superoxide radical anions, O2•-, which on protonation generated the hydroperoxy, HO2•, radicals, producing hydroxyl radical 2950

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Figure 5. SEM images of SrTiO3/TiO2 composite obtained at different reaction time: (a) 3, (b) 6, (c) 12, and (d) 36 h.

e- þ O2 f O2 •O2 •- þ H2 O f HO2 • þ OHHO2 • þ H2 O f H2 O2 þ OH• H2 O2 f 2OH• OH• þ RB f CO2 þ H2 O

Figure 6. Proposed scheme for the growth of SrTiO3/TiO2 nanofibers heterostructure from TiO2 nanofibers by hydrothermal reaction in Sr(OH)2 solution.

Figure 7. Curves of photocatalytic degradation of RB over different catalysts.

OH•, which was a strong oxidizing agent to decompose the organic dye.32,33 ðSrTiO3 =TiO2 Þ þ hν f ðSrTiO3 =TiO2 ÞðeCB - þ hVB þ Þ hþ þ OH- f OH•

Furthermore, the as-adopted fabrication route, i.e., hydrothermal synthesis of SrTiO3 by employing TiO2 nanofibers as one of precursor materials, was successful to realize a close contact of SrTiO3 nanocrystals with TiO2 nanoparticles in asfabricated SrTiO3/TiO2 heterostructure, as evidenced by SEM and TEM observation above. Such close contact was more effective in suppression of the electron-hole recombination. As a result, based on this efficient interfacial charge separation mechanism, the photocatalytic activity of SrTiO3/TiO2 heterostructures was higher than that of pure TiO2 nanofibers. Although the heterojunction favored charge carrier transfer, higher surface coverage of SrTiO3 nanostructures decreased the accessibility of the active sites of the TiO2 nanofibers surface, which reduced the photoactivity. Moreover, SrTiO3 itself was less effective as photocatalyst compared to TiO2. Therefore, the photocatalytic activity of SrTiO3/TiO2 composites would decrease with the increase of SrTiO3 contents when the SrTiO3 content was loaded up to a certain level. This could explain why the photocatalytic activity of SrTiO3/TiO2 (2) was lower than that of SrTiO3/TiO2 (1). As for the composite SrTiO3/TiO2 (3), we suggested that the highest photocatalytic activity resulted from the single crystalline nature of SrTiO 3 nanocubes. The higher crystallinity meaned fewer defects in the as-synthesized sample. It was well-known that lattice defects might act as recombination centers for photoinduced electrons and holes, thus significantly reducing the network photocatalytic activity. 2951

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4. CONCLUSION In summary, the present work indicates that SrTiO3/TiO2 nanofibers heterostructure can be fabricated by means of a relatively straightforward in situ hydrothermal reaction, using TiO2 nanofibers as both template and reactant. By adjusting reaction parameters (i.e., alkaline Sr(OH)2 concentration, reaction time, and temperature), the morphology of SrTiO3 nanostructures (nanocubes or nanoparticles) can be facilely controlled. The formation mechanism of SrTiO3/TiO2 heterostructure is proposed to be governed by the dissolution and precipitation mechanism, followed by the well-known Ostwald ripening process. The enhanced photocatalytic activity of SrTiO3/TiO2 nanofiber heterostructures can be ascribed to the enhanced charge separation derived from the coupling effect the TiO2 and SrTiO3 nanocomposite. More importantly, the method presented here may be extended to synthesize other ternary complex oxides nanofibers as well as TiO2/ternary complex oxide nanofiber heterostructures for various applications, and our research is in progress to address such feasibilities. ’ AUTHOR INFORMATION

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Corresponding Author

*E-mail [email protected]; Tel þ86 431 85099168.

’ ACKNOWLEDGMENT The present work is supported financially by the National Natural Science Foundation of China (No. 50572014, 50972027) and the Program for New Century Excellent Talents in University (NCET-05-0322). ’ REFERENCES (1) Hoffmann, M.; Martin, S.; Choi, W. Chem. Rev. 1995, 95, 69. (2) Chen, X.; Mao, S. Chem. Rev. 2007, 107, 2891. (3) Fujishima, A.; Zhang, X.; Tyrk, D. Surf. Sci. Rep. 2008, 63, 515. (4) Bavykin, D.; Walsh, F. Eur. J. Inorg. Chem. 2009, 8, 977. (5) Ghicov, A.; Schmuki, P. Chem. Commun. 2009, 2791. (6) Shankar, K.; Basham, J.; Allam, N.; Varghese, O.; Mor, G.; Feng, X.; Paulose, M.; Seabold, J.; Choi, K.; Grimes, C. J. Phys. Chem. C 2009, 113, 6327. (7) Li, D.; Xia, Y. Nano Lett. 2003, 3, 555. (8) Liu, B.; Aydil, E. J. Am. Chem. Soc. 2009, 131, 3985. (9) Wang, C.; Shao, C.; Liu, Y.; Li, X. Inorg. Chem. 2009, 48, 1105. (10) Liu, Z.; Misra, M. ACS Nano 2010, 4, 2196. (11) Nair, A. S.; Shengyuan, Y.; Peining, Z.; Ramakrishna, S. Chem. Commun. 2010, 46, 7421. (12) Choi, S. K.; Kim, S.; Lim, S. K.; Park, H. J. Phys. Chem. C 2010, 114, 16475. (13) Chuangchote, S.; Sagawa, T.; Yoshikawa, S. Appl. Phys. Lett. 2008, 93, 033310. (14) Shang, M.; Wang, W.; Yin, W.; Ren, J.; Sun, S.; Zhang, L. Chem. —Eur. J. 2010, 16, 11412. (15) Chuangchote, S.; Jitputti, J.; Sagawa, T.; Yoshikawa, S. ACS Appl. Mater. Interfaces 2009, 1, 1140. (16) Kumar, A.; Jose, R.; Fujihara, K.; Wang, J.; Ramakrishna, S. Chem. Mater. 2007, 19, 6536. (17) Zhan, S.; Chen, D.; Jiao, X.; Song, Y. Chem. Commun. 2007, 20, 2043. (18) Wang, X.; Yu, J. C.; Yip, H. Y.; Wu, L.; Wong, P. K.; Lai, S. Y. Chem.—Eur. J. 2005, 11, 2997. (19) Shan, Z.; Wu, J.; Xu, F.; Huang, F.; Ding, H. J. Phys. Chem. C 2008, 112, 15423. (20) Wang, C.; Shao, C.; Zhang, X.; Liu, Y. Inorg. Chem. 2009, 48, 7261. 2952

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