Sonocrystallization of ZIF-8 on Electrostatic Spinning TiO2 Nanofibers

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Sonocrystallization of ZIF-8 on electrostatic spinning TiO2 nanofibers surface with enhanced photocatalysis property through synergistic effect Xue Zeng, Liuqing Huang, Chaonan Wang, Jianshu Wang, Jintang Li, and Xuetao Luo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05746 • Publication Date (Web): 19 Jul 2016 Downloaded from http://pubs.acs.org on July 20, 2016

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ACS Applied Materials & Interfaces

Sonocrystallization of ZIF-8 on Electrostatic Spinning TiO2 Nanofibers Surface with Enhanced Photocatalysis Property through Synergistic Effect

Xue Zeng, Liuqing Huang, Chaonan Wang, Jianshu Wang, Jintang Li and Xuetao Luo*

Fujian Key Laboratory of Advanced Materials, College of Materials, Xiamen University, Xiamen, Fujian, P. R. China. *E-mail: [email protected]; Fax: 86-0592-2188503; Tel: 86-0592-2188503.

ABSTRACT: Semiconductor–MOFs hybrid photocatalysts have attracted increasing attention because of their enhanced photocatalytic activity. However, the effect of the interface reaction between semiconductor and MOFs is rarely studied. In this work, we studied on the synthesis and photocatalytic activity of ZIF-8 decorated electrostatic spinning TiO2 nanofibers (TiO2 ESNFs). TiO2/ZIF-8 hybrid photocatalysts were prepared via a facile sonochemical route. It was crucial that the ZIF-8 was assembled homogenously on the surface of TiO2 ESNFs and formed a N−Ti−O bond under sonochemical treatment, which may result in reducing recombination of the electron−hole pairs. The chemically bonded TiO2/ZIF-8 nanocomposites displayed excellent performance of thermal stability, controllable crystallinity and great enhancement of photocatalytic activity in Rhodamine B (Rh B) photodegradation. Furthermore, the UV-vis light adsorption spectra of TiO2/ZIF-8 nanocomposites showed that the ZIF-8 photosensitizer extended the spectral response of TiO2 to the visible region. The new strategy reported here can enrich the method for designing new Semiconductor-MOFs hybrid photocatalysts.

KEYWORDS: hybrid photocatalysts, electrostatic spinning TiO2 nanofibers, ZIF-8, TiO2/ZIF8 nanocomposites, photodegradation. 1

ACS Paragon Plus Environment


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1. INTRODUCTION Metal–organic frameworks (MOFs) are an intriguing family of well-ordered crystalline solids consisting of inorganic metal ions (or clusters) and multitopic organic ligand linkers.1 Because of their large surface areas, well-defined porous structures and high 2

photocatalytic activity, MOFs have been considered as photocatalysis materials for water-splitting,3-4 CO2 reduction,5-6 and environmental remediation.7-9 However, catalytic efficiencies of most MOFs are lower than that of the conventional semiconductive photocatalysts (e.g., ZnO, TiO2, and CdS). In order to improve the catalytic efficiencies, MOFs were designed to combine with other photocatalysts and considered as hybrid structures of Semiconductor-MOFs.10 Some unique physical and chemical properties are likely to be derived from synergistic effects caused by semiconductor-MOF hybrid structures. In this case, the photocatalytic efficiency of porous MOFs and semiconductors are both improved. ) ions and 2Zeolitic imidazolate frameworks-8 (ZIF-8), which is constructed by Zn(Ⅱ methylimidazole ligands, has become the most widely studied catalytic MOFs owing to its high thermal and chemical stabilities in aqueous solution.11 ZIF-8 was usually selected as photocatalyst to decompose organic pollutants under UV light irradiation. 13

12-

To date, several illustrations of semiconductor-MOFs hybrid structures such as

Zn2GeO4/ZIF-8,

14

15

ZnO/ZIF-8,

16

and CdTe/ZIF-8,

have shown potential improvement

of photocatalytic performance. Among the metal oxide semiconductive nanomaterials, TiO2 nanofibers prepared by electrospinning approach (TiO2 ESNFs) have attracted a lot of attention due to their advantages of non-toxicity, thermal and chemical stability.1720

The TiO2 ESNFs exhibits higher catalytic efficiency compared to TiO2 nanoparticles

(such as P25). Recently, ZIF-8/TiO2 semiconductor–MOF hybrid structures were synthesized by loading ZIF-8 on TiO2 (nanotube arrays or nanoparticles structures). Isimjan et al.21 prepared Pt/ZIF-8 loaded TiO2 nanotube arrays (TiO2 NTAs) by adding the microporous ZIF-8 on the surface of TiO2 nanotubes. The resulting Pt/ZIF-8 loaded TiO2 NTAs were proven to be more sensitive to visible light than Pt loaded TiO2 NTAs 2


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due to the reducing charge recombination property of ZIF-8. Nath et al.22 had shown that under UV-visible light illumination, the ZIF-8 loaded commercially TiO2 (P25) composites showed a slight lowering of the band gap of TiO2 nanoparticles from 3.35 eV to 3.25 eV. However, they found no evidence of visible light photocatalytic activity of the resulting TiO2/ZIF-8 nanoparticles. Previous works had well illustrated the photocatalytic

mechanisms

of

semiconductor–MOFs

hybrid

structures.

The

photocatalytic mechanisms can be summarized as three aspects: (i) MOFs with welldefined crystalline structures can act as a photo-sensitizer, leading to effective light harvesting and the enlargement of light contact area; (ii) Photoexcited electron can easily transfer from organic ligands to metal cluster (LCCT); (iii) semiconductor can serve as the main photocatalyst. However, mostly researches were only focused on the unilateral function of semiconductor or MOFs. Little attention had been paid to the interface reaction between MOFs and semiconductors. In our study, a new semiconductor-MOFs hybrid structure was designed by wrapping TiO2 ESNFs in ZIF-8 homogenously via a simple and efficient sonochemical route. Sonochemical processing has been proven to be an effective technique for generating new nanostructure materials. dispersed

under

ultrasonic

before

23

As shown in Scheme 1, TiO2 ESNFs were

immersing

into

ZIF-8

precursor

solution.

Subsequently, ZIF-8 nanoparticles would grow on the surface of these TiO2 ESNFs to yield TiO2/ZIF-8 hybrid nanofibers. This strategy can avoid functionalization of the surface of TiO2 ESNFs with surfactants and shorten the synthesis time. It is known that sonochemical generates radical species such as H and HO due to the acoustic cavitation phenomenon (transient formation, growth, and implosive collapse of bubble acoustic cavitation phenomenon in a liquid medium).24 The Ti3+ on the surface of TiO2 ESNFs could react with the reducing HO to form Ti4+, then the imidazolate linker of ZIF-8 would interact actively with the Ti4+. It is reasonable to assume that TiO2/ZIF-8 nanocomposites possess an N−Ti−O chemically bonded interface. It is known that visible light photocatalytic activity can be easily introduced by doping TiO 2 with S, N, 17, 25-28

and C.

The unique hybrid structure could effectively enhance the photocatalytic

3



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performance of TiO2/ZIF-8 nanocomposite. To the best of our knowledge, the TiO2/ZIF8 NFs hybrid structures chemically bonded with N−Ti−O have not been reported before.

2. EXPERIMENTAL SECTION Synthesis of TiO2 nanofibers The TiO2 nanofibers were synthesized by an electrospinning method. The raw materials for the preparation of TiO2 nanofibers were tetrabutyl titanate (TBOT, purity > 98.0%, Sinopharm), polyvinylpyrrolidone (PVP, Mw ∼1300 000, Aldrich) and Ethylene glycol monomethyl-ether (EGME, purity > 99.0%, Sinopharm). As a typical experiment, a 0.03 mol / L TBOT solution was prepared by dissolving TBOT powders in EGME under vigorous stirring to obtain a transparent solution. 1.4 g PVP were added into 15 mL TOBT solutions and kept stirring for 2 h to form a sol-gel for electrospinning. Afterwards, the precursor gel was fed into a syringe pump (having an internal diameter of 12 mm), which was connected to a 15 kV voltage sources while a collector was grounded. The distance between the needle tip and the collector was 20 cm. The feeding rate of the solution in the syringe was controlled at 1.0 mL / h. After the synthesis, the collected products were calcined at 600 ℃ for 1 h with a heating rate of 1 ℃ / min to remove PVP. A typical SEM image of the resulting nanofibers is shown in Fig. 1a-b. Sonochemical synthesis of TiO2/ZIF-8 nanofiber The strategy used for growing MOF crystals (ZIF-8) on TiO2 electrospinning nanofibers (TiO2 ESNFs) was sonochemical synthesis. Ultrasound equipment used in this study was a KQ-300DE (Kunshan China) model with an adjustable power output (maximum 300 W at 40kHz). As mentioned above, 1 mmol TiO2 ESNFs were dispersed in a methanolic solution (1.3 mmol 2-methylimidazole (2-MI) in 10 mL methanol) under ultrasonic treatment for 5 min, cited as solution A. Then, a solution consisting of 0.65 mmol Zn(NO3) 26H2O and 10 mL methanol were added dropwise into the solution A. The ultrasonic treatment was commended at a power level of 120W 4


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and kept working for 10 min. Then, the precipitate was harvested by centrifugation (7000 rpm, 15 min), and washed with methanol (20mL×3). Afterwards, TiO2/ZIF-8-1.3 nanofibers (TiO2/ZIF-8-1 NFs, theoretical molar ratio of 2-MI to TiO2 ESNFs is 1.3 : 1) were obtained by annealing the precipitate in air atmosphere at 80 ℃ for crystal seeds formation, and dried overnight. A typical SEM image of the resulting TiO2/ZIF-8-1 NFs is illustrated in Fig. 1 c-d. Structural Characterization Powder X-ray diffraction (PXRD) patterns were performed on a Bruker-Axs D8 Advance X-ray diffractometer in a wide angle range (2θ = 5−50°) with Cu Kα radiation (λ = 0.15406 nm). The Morphology of the samples were obtained on a Hitachi SU70 field-emission scanning electron microscopy (FE-SEM) instrument that operated at 10 kV. Samples for SEM were gold sputtered before the analyses. Transmission electron microscopy (TEM) and HRTEM images were generated with a Tecnai F30 microscope at an accelerating voltage of 300 kV. Fourier Transform infrared (FTIR) spectra of the samples were recorded on a Thermo Scientific Nicolet iS10 FTIR spectrometer in the spectral range of 400−4000 cm−1 using a potassium bromide disk method. Raman spectra were recorded on an inVia Raman microprobe with 785 nm laser excitation. Thermogravimetry (TG) measurements were performed simultaneously on a Netzsch STA 209 F1 Jupiter thermoanalyzer. Photocatalytic activity evaluation The obtained samples were used as photocatalysts for the degradation of Rh B. A control experiment was performed by using pure Rh B solution without a catalyst. The degradation of Rh B was carried out in a 250 mL glass reactor at room temperature, and 100mL of Rh B solution (10 mgL−1) was mixed with 20 mg of the sample. The solution was illuminated using a wideband pulse Xenon lamp with a wavelength from 200 nm to 400 nm and continuously stirred. Before illumination, the solution was magnetically stirred in dark for 30 min to ensure the adsorption/desorption equilibrium. After definite irradiation, the solutions were centrifuged at 7000 rpm for 3 min to 5



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separate the catalyst from the Rh B solution before the UV-vis adsorption measurement. The UV-vis diffuse reflectance spectrum was performed on a UV-vis spectrophotometer (UV-2550, Shimadzu) at room temperature and transformed to an adsorption spectrum according to the Kubelka-Munk relationship.

3. RESULTS AND DISCUSSION Structure characterization of the TiO2/ZIF-8 nanofibers

The preparation procedure for the TiO2/ZIF-8 nanofibers is illustrated in Scheme 1 and the resulting morphologies are shown in Fig. 1. The TiO2 ESNFs were continuous and had an average diameter in 120nm (Fig. 1a). A detailed image (Fig. 1b) clearly shows that a single nanofiber was compactly packed with numerous nonaparticles that had a primary particle size of about 20 nm in diameter, indicating that electrospinning TiO2 ESNFs contained with a lot of aggregated TiO2 nanoparticles, and some porous were shown in the TEM image (Fig. 2a). After the sonochemical process, spherical ZIF-8 was found to cover on the TiO2 nanofiber uniformly with dimensions ranging from 20−50nm (Fig.1c-d). Details of the nanocomporite shown in HRTEM images demonstrated that ZIF-8 nanoparticles were indeed loaded. In Fig. 2c, it can be seen that the TiO2 and ZIF-8 closely bonded together, forming a heterojunction structure through the ultrasonication treatment, which was very important for interface electron transfer between heterogeneous semiconductors. And the interplanar spacings of TiO2/ZIF-8 NFs were measured to be 0.352 nm and 0.325 nm, which correspond to the d spacing of the (101) planes of the anatase structure TiO2 and the (110) planes of the rutile structure TiO2 (Fig. 2d), respectively. Remarkably, we also utilized the conventional in situ synthesis method to prepare TiO2/ZIF-8 NFs, which involved pretreatment of TiO2 ESNFs by stirring with 2MI in methanol solution and subsequently dropwise adding the above solution to the Zn

2+

solution under magnetic stirring for 1h. Compared with the nanocomposites

synthesized using conventional methods, the morphology of outer ZIF-8 shell formed via sonochemical treatment was highly compact, and overall TiO2/ZIF-8 NFs were 6


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more continuous and integrated (Fig. S1a−b). PXRD analysis showed that the crystalline phases of TiO2/ZIF-8 NFs prepared by sonochemical method were purer (Fig. S1c). TiO2 ESNFs was easily broke up under strong stirring for a long time. On the contrary, ultrasonication not only remarkably reduced the reacting time for preparing TiO2/ZIF-8 NFs, but also dispersed the TiO2 ESNFs. P25, a well-known commercial TiO2 photocatalyst, was used as substrate material instead of TiO2 ESNFs. P25/ZIF-8-1.3 nanoparticles (P25/ZIF-8-1.3 NPs) were synthesized under the same experimental conditions, which involving pretreatment with

the 2MI precursor

solution and subsequent ZIF-8 shell growth under

sonochemical treatment for 10min. Texture properties of the achieved P25/ZIF-8-1.3 nanoparticles and achieved TiO2/ZIF-8-1.3 NFs are shown in Fig. 3. Compared with TiO2/ZIF-8 ESNFs, the synthesized product formed a core-shell structure with a uniformed thickness of the ZIF-8 shell (Fig. 3a). The crystalline structure of the prepared samples was further confirmed by the characteristic PXRD diffraction analysis. As shown in Fig. 3(b), the TiO2 substrate nanofiber and TiO2/ZIF-8 nanofibers were both composed of anatase/rutile hybrid structure: the peaks at 2θ value of 25.3°, 37.9°, and 48.1° can be indexed to (101), (004), and (200) crystal planes with a d spacing of 3.52 nm, 2.37 nm, and 1.89 nm, respectively. This results show that indicated the formation of a well-defined anatase structure (JCPDS card No. 21-1272) has formed. And the peaks at 27.5°, 36.1°, and 44.1°corresponded to the (110), (101), and (210) planes of rutile structure, with d spacings of 3.24, 2.48, and 2.18 Å,18 which were consistent with the HRTEM results. Besides the diffraction peaks of TiO2, several weak diffraction peaks of the ZIF-8 appeared on the PXRD spectrum of TiO2/ZIF-8 NFs and P25/ZIF-8 NPs, respectively, indicating the formation of a thin layer of ZIF-8 on the surface of anatase/rutile TiO2. To evaluate thermal behavior of the prepared TiO2/ZIF-8-1.3 NFs and confirm the practical proportion of ZIF-8, thermal gravimetry analysis (TGA) was utilized under an N2 atmosphere. The TG curves of bare TiO2 ESNFs and TiO2/ZIF-8-1.3 NFs were presented in Fig. 3c. TG curves of bare P25 and P25/ZIF-8-1.3 NPs were presented in Fig. 3d. For the bare TiO2 ESNFs and bare P25, TG analysis showed only ~2.42% 7



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mass loss up to 850 ℃. This change could be attributed to the evaporation of H2O adsorbed in the samples. According to the reported research, TG curve of ZIF-8 exhibited a mass-loss step of 11.8 wt.% up to 200 ℃, corresponding to the evaporation of guest molecules (MeOH) from the cavities and desorption of the unreacted ligand (e.g. 2-methylimidazole) from the nanocrystals.14 TG curves of the synthesized TiO2/ZIF-8-1.3 NFs and TiO2/ZIF-8-1.3 NPs revealed similar mass loss trends to ZIF-8 nanocrystals. The mass loss (~8%) for TiO2/ZIF-8-1.3 NFs in the range up to 850 ℃ indicated that the TiO2/ZIF-8-1.3 NFs contained approximately 11 wt.% ZIF-8 and 89 wt.% TiO2 nanofiber (Fig. 3c); The mass loss (~15%) for TiO2/ZIF-8-1.3 NPs in the range up to 850 ℃ indicated that the TiO2/ZIF-8-1.3 NPs contained nearly 21 wt.% ZIF-8 and 79 wt.% P25 (Fig. 3d); Illustrated that the actual amount of ZIF-8 contained in the TiO2/ZIF-8 composites was less than the theoretical calculated value (the theoretical calculated mass ratio of ZIF-8 to the total weight was 65 wt.%). These results explained that ZIF-8 not only growth on the surface of TiO2 but also consumed some of its precursors and then crystallized in the solution. Formation mechanism of TiO2/ZIF-8 hybrid structure To investigate the effect of ZIF-8 content on the structural morphology of hierarchical TiO2/ZIF-8 nanostructure. A series of TiO2/ZIF-8-x nanocomposites were synthesized by the similar procedure of TiO2/ZIF-8-1.3, TiO2/ZIF-8-1 and TiO2/ZIF-8-2 nanofibers were synthesized when 1 mmol 2-MI and 2 mmol 2-MI were used, respectively. The moral ratio of 2-MI/Zn(NO3)26H2O fixed at 2 : 1 in all samples. The morphologies of TiO2/ZIF-8-x NFs obtained by sonochemical method with the different molar ratio of TiO2 NFs : 2-MI are shown in Fig. 4. It is clear that the fiber structure remained intact after the ultrasonication process. The morphology and average diameter of the outer ZIF-8 sheath of the TiO2 ESNFs can be controlled by changing the TiO2/ZIF-8 mass ratio. With the TiO2 : 2-MI molar ratio of 1 : 1, the smooth fiber surface became grainy ill-defined, covering with ZIF-8 unclei (18 nm in diameter) uniformly (Fig. 4a). When the moral ratio of TiO2 : 2-MI increased to 1 : 1.3, these ZIF-8 nanoparticles gradually became larger in a sphere morphology and their 8


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size increased to about 50 nm in diameter (Fig. 4b). Intergrown ZIF-8 NPs with rhombic dodecahedra morphology formed when the TiO2 NFs : 2-MI molar ratio raised up to 1 : 2

(Fig. 4c). Besides the above results, it was found that the resulting

composite nanofibers appeared to have increased surface roughness with the moral ratio of TiO2 NFs/2MI increased, probably caused by growth of dodecahedra structured ZIF-8. It was concluded that the content of ZIF-8 precursor in the synthesis solution had a significant effect on the microstructural evolution of ZIF-8 nanocrystals. The shape of the ZIF-8 nanocrystals was changed from grainy to spherical and formed rhombic dodecahedron when the content of ZIF-8 was excessive. Furthermore, the existence of elements of Ti, O, C, N, and Zn (green, yellow, blue, red, and brown) was detected by elemental mapping analysis (Fig.S2b). The phase transformation of hierarchical TiO2/ZIF-8 nanostructure was also inferred from the PXRD patterns (Fig. 5a). As the concentration of ZIF-8 precursor solution increased, the ZIF-8-related peaks became stronger, indicating the increasing content of ZIF-8 in the prepared samples. Fig. 5b shows the FTIR spectrum of TiO2/ZIF-8-x (x = 1, 1.3, and 2). The formation of the N−Ti−O bond was further confirmed by FTIR spectroscopy. Fig.5b shows a broad peak at 3400 cm−1, which corresponds to the O−H stretch region. Compared with the reference samples of ZIF-8 and pristine TiO2 ESNFs, FTIR analysis of the TiO2/ZIF-8 composite revealed the presence of an adsorption band at 421 cm−1 (Zn−N stretch), another adsorption band at 1574 cm−1 (C=N stretch) and also two bands at 1421 cm−1 and 996 cm−1 (C−N stretch), which were typically characteristic for ZIF-8.

29-30

The

broad absorption below 1000 cm−1 can be found in all composites. The absorption peak around 456 cm−1 was the typical vibration of the Ti−O−Ti bond in TiO 2.31-33 It was worth noting that a significant adsorption appeared at 508 cm−1, which is assigned to the formation of

34-35

typical N−Ti−O bonds.

Raman spectrum can also provide

evidence of the structure of the TiO2/ZIF-8 composite. The Raman spectra of the TiO2 ESNFs and the composites prepared with different TiO2:ZIF-8 ratio was showed by Fig. −1

S3. The Raman active fundamental modes correspondingly at 144 cm

(Eg), 399 cm

−1

(B1g), 513 cm−1 (A1g), and 638 cm−1 (Eg) for the samples match with the anatase 9



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structure of the TiO2.36 The vibration modes at 686 cm−1 corresponding to imidazole ring puckering, indicating the presence of ZIF-8.

37

It is worth noting that the peaks

corresponding to the Eg mode for the TiO2/ZIF-8 composites are slightly red-shifted compared with bare TiO2 ESNFs. (Fig. S3b). The red-shift of the Eg band can be attributed to the strain on the TiO2/ZIF-8 surface caused by crystal lattice defects of TiO2 and the introduction of N atoms. The presence of the N−Ti−O bonds may be caused by a weak binding between the Ti4+ and the organic ligand (2-MI) of ZIF-8.38 According to the literature,24, valence of Ti atoms increases from TiO

3+

to TiO

4+

39

the

when radical hydroxyl groups are

induced under sonochemical treatment. During the sonochemical pretreatment of TiO2/ZIF-8 precursor solutions, TiO

4+

on the TiO2 surface could be regard as the

anchoring sites and interact actively with the imidazolate linker of ZIF-8. Afterwards, for ZIF-8 layer colud grow on the surface of the TiO2 directly. Changes in FTIR spectrum indicated that the addition of ZIF-8 precursor affected the surface of TiO2, and N−Ti−O bonds might have been formed by the sonochemical treatment. Such specific structure was potential to facilitate the photocatalytic efficiency. Photocatalytic activity of the samples To evaluate the optical properties, the optical adsorption spectra of the ZIF-8, P25, TiO2 ESNFs, P25/ZIF-8-1.3 (with the P25 : 2-MI moral ratio of 1 : 1.3), and TiO2/ZIF-8 nanocomposites samples were examined and the results are shown in Fig. 6. The ZIF8 absorbs at wavelengths less than 240 nm (which correspond to a band-gap energy of 5.17 eV) (Fig. 6a). The curves of bare P25 and bare TiO2 ESNFs absorb at wavelengths show the absorption thresholds at 385 nm and 388 nm, corresponded to the band gap energy of 3.22 eV and 3.19 eV (Fig.6b), respectively. This phenomenon can be ascribed to lower photo-generated electron–hole pairs recombines capability of TiO2 ESNFs comparing with TiO2 NPs.40 It was interesting to note that the TiO2/ZIF-8-x (x = 1.0, 1.3, and 2.0) composites presented a significant absorption in the visible region between 400 and 500nm compared with P25 and bare TiO2 nanofiber (Fig. 6b), which has not been found in previous reports. This can be attributed to the presence of 10


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a weak binding established between the organic ligand (2-MI) and Ti4+ and resulted in inhibition of interfacial charge recombination,

41

which narrows the band gap and

generating sub-energy levels. Among these semiconductor−MOF hybrid structure nanocomposites, the TiO2/ZIF-8-1 sample showed the strongest absorption under irradiation, and its band gap energy was 3.13, whereas reference P25/ZIF-8-1 NPs shows band gap energy of 3.17 (Fig. 6b). Thus, ZIF-8 can consider to be a narrow band

gap

semiconductor

that

can

be

couple

with

TiO2 to

form

a

heterostructure. Additionally, it was noteworthy that the band gap energy increased when the ZIF-8 content increased. This indicated that excessive ZIF-8 in composite probably increased opportunity for the collision of the excited electrons and holes (e+

-

+

/h ), and promoting the recombination of the photo-generated e /h pairs. According to the above results, we tested the photocatalytic activity of fibrous TiO2/ZIF-8-1 nanocomposite for the degradation of Rh B in aqueous solution under UV irradiation. Before UV light irradiation, the sample suspension was dispersed for 30 min uder magnetic stirrer in the dark to achieve the adsorption–desorption equilibrium. As reference materials, ZIF-8, P25, and P25/ZIF-8-1 NPs were also tested under the same experimental conditions. The result showed that pure ZIF-8 showed rather poor photocatalytic activity under UV light irradiation due to its large band gap (5.17 eV) (Fig. S5). Rh B was slightly degraded in the presence of bare P25 NPs (Fig. 7a), and the complete degradation of RhB was 50 min. Interestingly, the Rh B degraded within 24 min when TiO2/ZIF-8-1 NPs was used as the photocatalyst (Fig. 7c), which remarkably decreased the reaction time of complete Rh B degradation. The plot of C/C0 versus time is represented in Fig. 7(d). P25/ZIF-8-1 NPs and TiO2/ZIF-8-1 NFs displayed higher photocatalytic activity in Rh B degradation compared to bare P25 and ZIF-8 due to their smaller band gap. Moreover, The photocatalytic efficiency, based on the Rh B degradation rate, follows the order TiO2/ZIF-8-1 > TiO2/ZIF-8-1.3 > TiO2/ZIF8-2 > ZIF-8 (Fig. S4, S5). The photocatalytic activities of TiO2 ESNFs and TiO2 /ZIF-8-1 were also measured by the photodegradation of Rh B under visible light (λ > 400 nm), and the results are shown in Fig. S6. Under visible light irradiation, TiO2 ESNFs showed rather poor photocatalytic activity due to its large band gap and only 7.9% of 11



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the initial Rh B were decomposed after 2 h, whereas the TiO2/ZIF-8-1 composite photocatalyst showed improvements in photodegradation rate where 38.8% of the initial Rh B were decomposed after the same time period. Following the above discussion, a tentative mechanism was proposed for the photocatalytic properties of TiO2/ZIF-8 nanocomposites. The synergistic effect between TiO2 and ZIF-8 was the main factor for the enhancement of the photocatalytic active of TiO2 /ZIF-8 nanocomposites. The schematic electron-hole pair generation and transformation between the intersurface of TiO2 ESNFs and ZIF-8 are shown in Scheme 2. On one hand, the inner TiO2 ESNFs was the main photocatalyst and the coating ZIF-8 could act as a co-catalyst. The photocatalysis of ZIF-8 is considered to be achieved by ligand-to-metal charge transfer mechanism (LCCT).42-43 On the other hand, the ZIF-8 coating is beneficial to the charge separation,41 narrowing the band gap narrowing of TiO2 ESNFs(Eg1 to Eg2). In this case, the photoelectrons can easily transfer to the surface of TiO2 ESNFs. The photogenerated electron can be captured by oxygen molecules in the solution to form radical oxide O2-. The O2- generated from TiO2/ZIF-8 nanocomposite can cause the oxidative decomposition of Rh B. Furthermore, chemically bonded structure between ZIF-8 and TiO2 should promote the separation efficiency of the photoinduced e-/h+ pairs, and thus accelerating photodegradation of Rh B.44

4. CONCLUSIONS In summary, we have successfully developed the sonochemical method to fabricate ZIF-8 particles (20-40nm in diameter) on the surface of TiO2 nanofibers without surfactants or additives. The resulting TiO2 /ZIF-8 heterogeneous photocatalyst exhibited much higher photocatalytic activity for the degradation of organic dye Rh B than commercial P25. The highly enhanced photocatalytic activity of the TiO2/ZIF-8 nanocomposites was ascribed to the following factors: (i) the outer ZIF-8 shell with light penetration and scattering properties leads to more efficient light harnessing; (ii) ZIF-8 was considered as co-catalyst together with TiO2, transferring the photoexcitation 12


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electrons from 2Ml organic ligands to metals (LCCT mechanism); (iii) N−Ti−O chemical +

bonds suppressed the recombination of e−/h pairs and thus narrowed the band gap of TiO2 ESNFs substrate and could increase the absorption in the visible light region. The results demonstrated that ZIF-8 is a very promising candidate of co-catalyst for heterogeneous photocatalysis. Further efforts will focus on improving its photocatalytic performance under visible light irradiation. Moreover, the key point of this work is to design new chemically bonded semiconductor-MOF hybrid structural materials in the presence of sonochemical. The developed method opens a new door to a wide range of semiconductor-MOF materials with unusual properties.

ASSOCIATED CONTENT Supporting Information Fig. S1 showing the SEM images of TiO2/ZIF-8 NFs (a-b), and PXRD patterns of TiO2 NFs and TiO2/ZIF-8 NFs prepared by the conventional method (c). Fig. S2 describing the EDS elemental mapping for Ti, O, C, N and Zn in the image of bare TiO2 ESNFs (a) and TiO2/ZIF-8 NFs (b). Fig. S3 Raman spectra of TiO2 ESNFs, TiO2/ZIF-8-1, TiO2/ZIF-8-1.3 and TiO2/ZIF-82. Fig. S4 UV-vis absorbed spectrum for the photocatalytic degradation of Rhodamine B (Rh B) in the presence of ZIF-8. Fig. S5 (a-c) UV-vis absorbed spectrum for the photocatalytic degradation of Rhodamine B (Rh B) in the presence TiO2/ZIF-8-x nanocomposites. (d) Variation of normalized C/C0 of Rh B concentration for different samples (ZTF-1 indicates the TiO2/ZIF-8-1 NFs, ZTF-1.3 indicates the TiO2/ZIF-8-1.3 NFs, ZTF-2 indicates the TiO2/ZIF-8-2 NFs). Fig.S6 UV-vis absorbed spectrum for the photocatalytic degradation of Rhodamine B (Rh B) in the presence of (a) TiO2 ESNFs, (b) TiO2/ZIF-8-1 NFs under visible light irradiation, (c) Variation of normalized C/C0 of Rh B concentration for different samples (C0 indicates the initial concentration of Rh B solution, C indicates the concentration of Rh B at different time period)

This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *X. T. Luo. E-mail: [email protected]. Tel: 0592-2188503. Author Contributions †These authors contributed equally to this work. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We gratefully acknowledge the support of the Scientific and Technological Innovation Platform of Fujian Province (2006L2003), and the assistance of Professor Hao Xue for helpful discussions.

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Scheme 1: Schematic illustration of the preparation of the TiO2/ZIF-8 hybrid nanofibers via sonochemical

Scheme2: Possible Photocatalytic mechanism of TiO2/ZIF-8 nanocomposites. Dotted line: intra-bandgap energy level of TiO2 narrowed by direct interaction with Ti atoms and N atoms during the sonochemical synthesis of TiO2/ZIF-8 nanocomposites.

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Figures

Fig.1 SEM images of (a, b) TiO2 ESNFs and (c, d) TiO2/ZIF-8-1.3 NFs.

Fig.2 TEM images of (a) TiO2 ESNFs and (b) TiO2/ZIF-8-1.3 NFs; (c-d) HRTEM images of the TiO2/ZIF-8-1.3 NFs. 21



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Fig.3 (a) TEM image of P25/ZIF-8-1.3 NPs; (b) PXRD patterns of P25, P25/ZIF-8-1.3 NPs, TiO2 NFs and TiO2/ZIF-8-1.3 NFs; diffraction peaks of anatase phase are marked with A，，diffraction peaks of rutile phase are marked with R (diffraction patterns of simulated ZIF-8 are cited for comparison). TG curve analysis of the samples: (c) bare TiO2 ESNFs and TiO2/ZIF-8-1.3 NFs; (d) bare P25 NPs and P25/ZIF-8-1.3 NPs.

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Fig.4 SEM images of low and high magnifition TiO2/ZIF-8-x nanocomposite obtained by sonochemical (10min) from a methanolic suspensions (20ml) with TiO2 : 2-MI molar ratio of (a) 1:1.0, (b)1:1.3, (c) 1:2.0.

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Fig.5 PXRD patterns (a) and FTIR spectrums (b) of ZIF-8, TiO2 NF and TiO2/ZIF-8 NFs composites with different TiO2:2-MI molar ratio.

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Fig.6 (a) UV-visible spectra ; (b) the magnified image of (a).

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Fig.7 UV-vis absorbed spectrum for the photocatalytic degradation of Rhodamine B (Rh B) in the presence of (a) P25, (b) P25/ZIF-8-1 NPs, (c) TiO2/ZIF-8-1 NFs, (d) Variation of normalized C/C0 of Rh B concentration for different samples (C0 indicates the initial concentration of Rh B solution, C indicates the concentration of Rh B at different time period, ZP25 indicates the P25/ZIF-8-1 NPs, ZTF indicates the TiO2/ZIF-8-1 NFs).

Table of Contents

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Schematic illustration of the preparation of the TiO2/ZIF-8 hybrid nanofibers via sonochemical. 59x31mm (300 x 300 DPI)



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Possible Photocatalytic mechanism of TiO2/ZIF-8 nanocomposites. dotted line: intra-bandgap energy level of TiO2 narrowed by direct interaction with Ti atoms and N atoms during the sonochemical synthesis of TiO2/ZIF-8 nanocomposites. 95x62mm (300 x 300 DPI)


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Fig.1 SEM images of (a, b) TiO2 ESNFs and (c, d) TiO2/ZIF-8-1.3 NFs. 241x181mm (150 x 150 DPI)



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Fig.2 TEM images of (a) TiO2 ESNFs and (b) TiO2/ZIF-8-1.3 NFs; (c-d) HRTEM images of the TiO2/ZIF-8-1.3 NFs. 242x180mm (150 x 150 DPI)


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Fig.3 (a) TEM image of P25/ZIF-8-1.3 NPs; (b) PXRD patterns of P25, P25/ZIF-8-1.3 NPs, TiO2 NFs and TiO2/ZIF-8-1.3 NFs; diffraction peaks of anatase phase are marked with A，diffraction peaks of rutile phase are marked with R (diffraction patterns of simulated ZIF-8 are cited for comparison). TG curve analysis of the samples: (c) bare TiO2 ESNFs and TiO2/ZIF-8-1.3 NFs; (d) bare P25 NPs and P25/ZIF-8-1.3 NPs. 60x45mm (300 x 300 DPI)



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Fig.4 SEM images of low and high magnifition TiO2/ZIF-8-x nanocomposite obtained by sonochemical (10min) from a methanolic suspensions (20ml) with TiO2 : 2-MI molar ratio of (a) 1:1.0, (b)1:1.3, (c) 1:2.0. 89x100mm (300 x 300 DPI)


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Fig.5 PXRD patterns (a) and FTIR spectrums (b) of ZIF-8, TiO2 NF and TiO2/ZIF-8 NFs composites with different TiO2:2-MI molar ratio. 128x234mm (300 x 300 DPI)



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Fig.6 (a) UV-visible spectra ; (b) the magnified image of (a). 122x186mm (300 x 300 DPI)


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89x66mm (300 x 300 DPI)



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Sonocrystallization of ZIF-8 on Electrostatic Spinning TiO2 Nanofibers

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