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Synthesis of fluorinated TiO2 hollow microspheres and their photocatalytic activity under visible light
Applied Surface Science 257 (2011) 5879–5884
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synthesis of fluorinated TiO2 hollow microspheres and their photocatalytic activity under visible light Li Junqi a,b,∗ , Wang Defang a , Liu Hui a , He Zuoli a , Zhu Zhenfeng a a b
School of Materials Science and Engineering, Shaanxi University of Science & Technology, Xi’an, Shaanxi 710021, China Key Laboratory of Auxiliary Chemistry & Technology for Chemical Industry, Ministry of Education, Shaanxi University of Science and Technology, Xi’an, Shaanxi 710021, China
a r t i c l e
i n f o
Article history: Received 9 November 2010 Received in revised form 18 January 2011 Accepted 31 January 2011 Available online 26 February 2011 Keywords: Fluorinated TiO2 Hollow microspheres Photocatalysis
a b s t r a c t Fluorinated TiO2 hollow microspheres with three-dimensional hierarchical architecture were prepared by solvothermally treatment using solid microspheres as precursor. The obtained solid and hollow TiO2 microspheres were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET), X-ray photoelectron spectroscopy (XPS), UV–Vis diffuse reflectance spectrum (DRS) and photoluminescence (PL) spectra. The photocatalytic activity of as-prepared solid and hollow TiO2 microspheres was determined by degradation of methyl orange (MO) under visible light irradiation. The results showed that the surface fluorination, the existence of accessible mesopores channels, and the increased light harvesting abilities could remarkably improve the photocatalytic activity of TiO2 hollow microspheres. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor photocatalysts have attracted much attention in the past decades owing to their applications to environmental purification and solar energy conversion [1–4]. Among photocatalysts, TiO2 has received the most attention as a photocatalytic material because of its superior photocatalytic activity, chemical stability, low cost and nontoxicity [5–8]. The morphology and microstructure of TiO2 , which are very important to their photocatalytic activity, are significantly influenced by the preparative conditions and methods [9–11]. Fabrication of TiO2 hollow microspheres has recently attracted enormous attention because of their low density, high surface area, good surface permeability as well as large light-harvesting efficiencies [12]. It is also expected that that higher energy conversion efficiency and photocatalytic activity could be achieved using TiO2 hollow microspheres as photocatalysts. On the other hand, due to the wide bandgap of TiO2 (3.2 eV for anatase), the technological application seems limited by several factors, among which the most restrictive one is the need of using an ultraviolet (wavelength < 387 nm) as excitation source [13]. So it can only capture less than 5% of the solar irradiance at Earth’s surface. For the sake of efficient use of sunlight, or use of visible region
∗ Corresponding author at: School of Materials Science and Engineering, Shaanxi University of Science & Technology, Xi’an, Shaanxi 710021, China. Tel.: +86 29 86168688; fax: +86 29 86168688. E-mail addresses: [email protected], [email protected] (L. Junqi). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.01.130
of the spectrum, the technology of enlarging the absorption scope of TiO2 may then appear as an appealing challenge for developing the future generation of photocatalysts. TiO2 -based photocatalysts doped with anions (e.g. C, N, S, F) have been widely investigated in order to shift the optical absorption edge of TiO2 towards lower energy, thereby increasing the photocatalytic activity in visible region [14–18]. Among these, the F-doped or surface fluorination of TiO2 has been found to be a most effective method to promote the photocatalytic activity [19–22]. In this study, for the enhancement of visible-light-driven photocatalytic activity of TiO2 , we prepared visible responsive fluorinated TiO2 hollow microspheres with solvothermally treatment using solid microspheres as precursor. The morphology and microstructure of solid and hollow microspheres were investigated in detail. The obtained fluorinated TiO2 hollow microspheres exhibited unique three-dimensional hierarchical architecture and demonstrated a significantly improved photocatalytic performance. 2. Experimental 2.1. Synthesis TiO2 solid microspheres were prepared by controlled hydrolysis of Tetra-n-butyl Titanate (TBT) in ethanol solution. In a typical preparation process of TiO2 solid microspheres, a given amount of aqueous salt solution was first mixed with 100 mL of anhydrous ethanol, followed by addition of 2 mL Tetra-n-butyl Titanate (TBT) at ambient temperature under magnetic stirring. After 10 h the reaction was finished and the precipitated microspheres were
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collected by centrifugation, washed with ethanol. The final mesoporous TiO2 solid microspheres were produced by calcinations of the dried samples for 1 h at 500 ◦ C. For TiO2 hollow microspheres, 0.8 g of the obtained solid microspheres and 0.18 g NaF were dispersed into a mixture of 20 mL ethanol and 10 mL deionized water with the assistance of strong magnetic stirring. The final mixture was directly transferred into a 100 mL Teflon autoclave, heated by microwave irradiation in the microwave reactor at 180 ◦ C for 1 h. The solvothermally treated powders were obtained by washing with deionized water and ethanol several times and air drying. 2.2. Characterization The crystalline phase of the samples was characterized using X-ray diffraction (XRD-D/max2200pc, Japan) technique with Cu K␣ radiation of wavelength k = 0.15418 nm. Scanning electron micrographs SEM images were obtain on JSM-6700F (JEOL Japan). TEM images were taken on JEM2010 (JEOL Japan). The specific surface area was determined by the Brunauer–Emmett–Teller (BET) method. Nitrogen adsorption/desorption isotherms were measured at −196 ◦ C by using a JW-004A system. X-ray photoelectron spectroscopy (XPS) analyses were carried out in an ultrahigh vacuum (UHV) chamber with a base pressure below 2.66 × 10−7 Pa at room temperature. Photoemission spectra were recorded by a Kratos Axis Ultra spectrometer equipped with a standard monochromatic Al K␣ excitation source (hv = 1486.71 eV). The pass energy and step size of low-resolution XPS scan were adjusted to 40 and 0.1 eV. The binding energy (BE) scale was calibrated by measuring a C1 s peak at 285.0 eV from the surface contamination. The resultant XPS spectra were simulated by assuming the photoelectron peak as Gaussian line shape. The adsorption spectra of TiO2 microspheres were recorded on a PE lamda 950 UV/Vis spectrophotometer equipped with an integration sphere attachment.
Fig. 1. XRD patterns of the TiO2 microspheres. The precursor TiO2 (a), calcined at 500 ◦ C for 1 h (b) and solvothermal treatment (c).
peaks, which indicate the presence of an amorphous phase. In the case of calcination and solvothermal treatment, the peaks at scattering angles of 25.26, 36.88, 47.78 and 62.74◦ correspond to the reflections from the (1 0 1), (0 0 4), (2 0 0) and (2 0 4) crystal planes of anatase TiO2 , respectively. All the reflections can be readily indexed as a pure anatase phase of TiO2 (JCPDS card No. 21-1272), and no other peak was observed. On the basis of the XRD results, the crystal size of the calcination and solvothermal treatment TiO2 calculated by the Scherrer formula were 12.8 nm and 16.9 nm, respectively. This result implies that the TiO2 microspheres are composed of nanocrystals (12.8 nm for calcination and 16.9 nm for solvothermal treatment) and the amorphous phase can be successfully converted to an anatase phase by carrying out calcination and solvothermal treatment.
2.3. Photocatalytic activity measurement
3.2. SEM and TEM analysis
The photocatalytic activities of the prepared TiO2 microspheres were determined by measuring the degradation of methyl orange (MO) in an aqueous solution under visible light irradiation. A 500 W xenon lamp ( > 420 nm) was used as light source. The aqueous system containing 80 mL 0.008 g/L methyl orange (MO) and 0.05 g mesoporous TiO2 microspheres was magnetically stirred in the dark for 1 h to reach the adsorption equilibrium of MO with the photocatalysts, and then exposed to visible light. The suspension was vigorously stirred with the photoreactor during the process and temperature of suspension was maintained at 22 ± 2 ◦ C by circulation of water through an external cooling coil. UV/Vis absorption spectra were recorded at different time intervals to monitor the reaction, and the concentration of MO left in the aqueous system was measured by detecting the absorption at 464 nm, the maximum absorption wavelength for MO on a UV–Vis spectrophotometer (UV1700 Shimadzu). The photoluminescence (PL) emission spectra were measured at room temperature with a 10MW and 325-nm He–Cd laser.
The typical SEM and TEM images of the samples are shown in Fig. 2. The overall morphology of the microspheres is shown in Fig. 2a. It is observed that both the precursor TiO2 and calcined TiO2 are composed of a large quantity of relatively uniform microspheres with an average diameter of about 1 m (Fig. 2a and c). This suggests that calcinations, while transformed the TiO2 microspheres from amorphous to anatase phase, did not destroy the framework of the microspheres. The representative TEM images (Fig. 2e) of the calcined TiO2 reveal that the TiO2 microspheres composed of nanocrystals are mesoporous without a long-range order. The mesoporosity is mainly due to the interparticle porosity. The average diameters of the nano-particles, estimated from the TEM images of the samples (about 11.8 nm), are in good agreement with that calculated from XRD patterns using the Scherrer equation. For the solvothermal treatment TiO2 , the magnified SEM images (Fig. 2b) reveal that TiO2 microspheres with a rough surface are self-assembled hierarchical structures that are composed of many loosely packed elongated TiO2 nanometer-scale crystals as building units. The average diameters of the elongated nano-crystals (16.7 ± 0.8 nm), estimated from the SEM and TEM images (Fig. 2b and f) of the samples, are in good agreement with that calculated from XRD patterns using the Scherrer equation. The details of their interior can be clearly observed by the SEM image of a single broken sphere after being unveiled by ultrasonication (see Fig. 2b (inset)). The hollow structure of TiO2 microspheres can also be identified by TEM image (Fig. 2d). Fig. 2f shows the magnified TEM images for one TiO2 microspheres, in which abundant cavities can be seen, indicating the mesoporous structure of TiO2 hollow microspheres. Based on the above results, it is evident that hierarchical structure TiO2
3. Results and discussion 3.1. XRD analysis TiO2 exists mainly in three crystallographic structures, anatase, rutile and brookite. The XRD peak at 2 = 25.26◦ (1 0 1) is often taken as the characteristic peaks of anatase crystallographic structures. Fig. 1 shows the XRD patterns of the TiO2 microspheres, the precursor TiO2 (a), calcined at 500 ◦ C for 1 h (b) and solvothermal treatment (c). For the precursor TiO2 , there were no diffraction
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Fig. 2. SEM and TEM images of the precursor TiO2 microspheres (a), calcined at 500 ◦ C for 1 h (c) and (e), and solvothermal treatment (b), (d) and (f).
hollow microspheres composed of nano-crystals with mesoporous structure have been successfully prepared by a facile combination of sol-gel and solvothermal processes. Such novel hollow structure with three-dimensionally interconnected but aperiodic pore channels would facilitate the accessibility of reactants to the active sites within the mesoarchitecture. This is an attractive feature for heterogeneous catalysis. The formation of mesoporous TiO2 hollow microspheres is a complicated process involving a dissolution (inside TiO2 microspheres)-redeposition (on the surface of TiO2 microspheres) procedure during solvothermal treatment. The aggregated TiO2 solid microspheres are chemically etched by corrosive F− , followed
by the recrystallization of their surfaces. The accessible mesopores channels and corrosive nanoparticles are hence simultaneously generated. Moreover, the intruded F− may enrich at the center of the spheres, creating a hollow interior. The building units of aggregated nanoparticles, which comprised additional hollow structure, are 16.7 ± 0.8 nm. 3.3. BET analysis Fig. 3 shows the N2 -sorption isotherms (inset) of calcined and solvothermal treatment TiO2 microspheres with their corresponding pore size distribution curve calculated from the
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Fig. 3. BJH (Barrett–Joyner–Halenda) pore size distribution and Nitrogen adsorption/desorption isotherms of calcined and solvothermal treatment TiO2 microspheres.
desorption branch of the N2 -sorption isotherms by the BJH (Barrett–Joyner–Halenda) method. The isotherms are typical type IV-like with a type H2 hysteresis loop, which indicates the presence of mesoporous materials according to IUPAC classification. The specific surface area of calcined TiO2 is 60.8 m2 /g using the BET (Brunauer–Emmett–Teller) method, and the pore diameter is about 7.9 nm. While, the mesoporous TiO2 hollow microspheres prepared using solvothermal processes have higher surface area (90.1 m2 /g) and larger pore diameter (16.5 nm), which provides more surface active sites and pore-channels for the chemisorptions and diffusion of reactants. 3.4. XPS analysis The elemental composition of TiO2 hollow microspheres with solvothermal treatment was further determined by XPS (Fig. 4). XPS survey spectra (Fig. 4a) indicate that the samples synthesized in NaF solution during solvothermal treatment contain Ti, O, F, and C. The C element was mainly ascribed to the adventitious hydrocarbon from XPS itself. Fig. 4b shows the high-resolution XPS spectra of F 1s region. The only peak at 684.3 eV was assigned to the F− anions that are physically adsorbed on the surface of mesoporous TiO2 microspheres (≡Ti–F). No signal for F− in the lattice of TiO2 (BE = 688.5 eV) is found [23–25]. Furthermore, the binding energies of Ti 2p3/2 and Ti 2p1/2 are equal to 458.9 and 464.4 eV, respectively (see Fig. 4c), which are identical to that of Ti4+ of bulk TiO2 . Therefore, it can be easily concluded from the above XPS results that the oxygen in TiO2 lattice is not substituted by F and F element only exists in surface fluoride ((≡Ti–F) [25,26]. This is not difficult to understand because, on the one hand, the solvothermal environment can accelerate crystallization of TiO2 due to the in situ dissolution-recrystallization process, resulting in the reduction of the number of defects and impurity in TiO2 crystals, on the other hand, surface fluorination can be easily carried out by a simple ligand exchange reaction between surface hydroxyl groups on TiO2 and fluoride anions (F− ) due to the high F–Ti bonding energy. Therefore, it is not surprising that the surface fluorination modification of TiO2 readily takes place under a solvothermal environment. The actual amount of F in TiO2 microspheres is 6.89%, which is lower than that of the precursor solution. This is easy to understand because F− is only adsorbed on the surface of TiO2 microspheres. It is well known that the fluorination on the surface of TiO2 can accelerate the photocatalytic degradation of a wide range of organic pollutants since the • OH radicals generated on the surface of TiO2 microspheres are more mobile than those generated on pure TiO2
Fig. 4. XPS survey spectra (a) and high-resolution XPS spectra of the F 1s (b) and Ti 2p (c) region taken on the TiO2 hollow microspheres with solvothermal treatment.
under light irradiation [27]: •+
≡ Ti–OH + h+ ↔≡ Ti–OH vb
(1)
Ti–F + H2 O (or OH− ) + h+ →≡ Ti–F + • OHfree + H+ vb
(2)
Also, the redox potential of free OH radicals in solution (ca. 2.3 V vs NHE at pH 7) is larger than that of surface-adsorbed OH radicals on TiO2 (about 1.5–1.7 V vs NHE at pH 7) [24].
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Fig. 7. The photoluminescence (PL) spectra of TiO2 microspheres. Fig. 5. Optical UV–Vis absorption spectra of TiO2 microspheres.
3.5. Absorption spectra and photo degradation Fig. 5 shows the UV–Vis spectra of TiO2 microspheres prepared with calcination and solvothermal treatment. The absorption spectrum of the hollow microspheres with solvothermal treatment exhibits a stronger absorption in the UV–Vis range of 325–800 nm than that of calcination. Clearly, the hollow inner structure associated with accessible mesoporous at the spherical surface allow the light-scattering inside their pore channels as well as their interior hollows, enhancing the light harvesting and thus increase the quantity of photogenerated electrons and hole to participate in the photocatalytic decomposition of the contaminants. Photo degradation of methyl orange was used as a model reaction to compare the photocatalytic activities of calcined and solvothermal treatment TiO2 . Fig. 6 shows the decrease of the concentration of methyl orange with irradiation time in the presence of TiO2 as well as self-degradation of methyl orange in the absence of any photocatalysts. It was found that the self-degradation of methyl orange was negligible under visible light. However, 74% and 21% of methyl orange were degraded in the presence of solvothermal treatment and calcined TiO2 after 120 min under visible light irradiation, respectively. Therefore, solvothermal treatment TiO2 showed much higher photocatalytic activity. The effects of fluorination on the recombination of photogenerated electron/hole pairs were measured by PL emission, as shown in Fig. 7. The intensity of the PL spectra for fluorinated TiO2 hollow microspheres with solvothermally treatment is lower than that of pure calcined TiO2 . Since the observed PL spectrum in
TiO2 can be attributed to the radiative recombination process of self-trapped excitations, or hydroxylated TiO3+ surface complexes from the charge transfer excited state of the highly dispersed TiO2 . The reduction of PL intensity in the fluorinated TiO2 indicates the decrease in the radiative recombination process. As a result, more holes were available for producing more hydroxyl radicals over the fluorinated TiO2 . It is known that the photocatalytic activity of TiO2 is affected by many factors such as surface area, phase structure, crystallinity, surface hydroxyl density, surface acidity, surface defects, and so on [5,27–29]. On the basis of the above characterizations, we attributed the high photocatalytic activity of our mesoporous TiO2 hollow microspheres with solvothermal treatment to several reasons as follows. First, the enhancing light harvesting abilities due to the special structure makes it increase the quantity of photogenerated electrons and hole to participate in the photocatalytic decomposition of the contaminants. Second, the existence of accessible mesopores channel can easily adsorb polar organic molecules for photoactivity enhancement, which is favorable for the photoreaction. Finally, the surface ≡Ti–F group on TiO2 acts as an electron-trapping site but reduces interfacial electron transfer rates by tightly holding trapped electrons due to the strong electronegativity of the fluorine. The generation of free OH radicals is enhanced on TiO2 because direct hole trapping or the generation of surface-bound OH radicals is not allowed on the fluorinated surface. As a result, the photocatalytic decomposition rate of the surface fluorinated TiO2 was superior to that of calcination. 4. Conclusions Fluorinated TiO2 hollow microspheres with three-dimensional hierarchical architecture were prepared by solvothermally treatment using solid microspheres as precursor. The photocatalytic activity of as-prepared solid and hollow TiO2 microspheres was determined by degradation of methyl orange under visible light irradiation. The surface fluorination, the existence of accessible mesopores channels, and the increased light harvesting abilities could remarkably improve the photocatalytic activity of TiO2 hollow microspheres. This approach provides a green, simple and economical method to synthesize uniform fluorinated TiO2 hollow microspheres with high visible light responsive photocatalytic activity. Acknowledgments
Fig. 6. Photo degradation of methyl orange in the presence of TiO2 photocatalysts as well as self-degradation of methyl orange in the absence of any photocatalyst.
This research was financially supported by Special Fund from Shaanxi Provincial Department of Education (09JK352) and the
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Graduate Innovation Found of Shaanxi University of Science and Technology. References [1] M. Gratzel, Nature 414 (2001) 338–344. [2] M.A. Fox, M.T. Dulay, Chem. Rev. 93 (1993) 341–357. [3] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Nature 388 (1997) 431–432. [4] W.K. Ho, J.C. Yu, S.C. Lee, J. Solid State Chem. 179 (2006) 1171–1776. [5] J. Zhang, Q. Xu, Z.C. Feng, M.J. Li, C. Li, Angew. Chem. Int. Ed. 47 (2008) 1766–1769. [6] X.C. Wang, J.C. Yu, C.M. Ho, A.C. Mak, Chem. Commun. 17 (2005) 2262–2264. [7] S. Indris, R. Amade, P. Heitjans, M. Finger, A. Haeger, D. Hesse, W. Grunert, A. Borger, K.D. Becker, J. Phys. Chem. B 109 (2005) 23274–23278. [8] S.H. Oh, R.R. Finones, C. Daraio, L.H. Chen, S.H. Jin, Biomaterials 26 (2005) 4938–4943. [9] J. Yu, L. Zhang, B. Cheng, Y. Su, J. Phys. Chem. C 111 (2007) 10582–10589. [10] A. Syoufian, O.H. Satriya, K. Nakashima, Catal. Commun. 8 (2007) 755–759. [11] J.G. Yu, G.H. Wang, B. Cheng, M.H. Zhou, Appl. Catal. B 69 (2007) 171–180.
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synthesis of fluorinated TiO2 hollow microspheres and their photocatalytic activity under visible light Li Junqi a,b,∗ , Wang Defang a , Liu Hui a , He Zuoli a , Zhu Zhenfeng a a b
School of Materials Science and Engineering, Shaanxi University of Science & Technology, Xi’an, Shaanxi 710021, China Key Laboratory of Auxiliary Chemistry & Technology for Chemical Industry, Ministry of Education, Shaanxi University of Science and Technology, Xi’an, Shaanxi 710021, China
a r t i c l e
i n f o
Article history: Received 9 November 2010 Received in revised form 18 January 2011 Accepted 31 January 2011 Available online 26 February 2011 Keywords: Fluorinated TiO2 Hollow microspheres Photocatalysis
a b s t r a c t Fluorinated TiO2 hollow microspheres with three-dimensional hierarchical architecture were prepared by solvothermally treatment using solid microspheres as precursor. The obtained solid and hollow TiO2 microspheres were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET), X-ray photoelectron spectroscopy (XPS), UV–Vis diffuse reflectance spectrum (DRS) and photoluminescence (PL) spectra. The photocatalytic activity of as-prepared solid and hollow TiO2 microspheres was determined by degradation of methyl orange (MO) under visible light irradiation. The results showed that the surface fluorination, the existence of accessible mesopores channels, and the increased light harvesting abilities could remarkably improve the photocatalytic activity of TiO2 hollow microspheres. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor photocatalysts have attracted much attention in the past decades owing to their applications to environmental purification and solar energy conversion [1–4]. Among photocatalysts, TiO2 has received the most attention as a photocatalytic material because of its superior photocatalytic activity, chemical stability, low cost and nontoxicity [5–8]. The morphology and microstructure of TiO2 , which are very important to their photocatalytic activity, are significantly influenced by the preparative conditions and methods [9–11]. Fabrication of TiO2 hollow microspheres has recently attracted enormous attention because of their low density, high surface area, good surface permeability as well as large light-harvesting efficiencies [12]. It is also expected that that higher energy conversion efficiency and photocatalytic activity could be achieved using TiO2 hollow microspheres as photocatalysts. On the other hand, due to the wide bandgap of TiO2 (3.2 eV for anatase), the technological application seems limited by several factors, among which the most restrictive one is the need of using an ultraviolet (wavelength < 387 nm) as excitation source [13]. So it can only capture less than 5% of the solar irradiance at Earth’s surface. For the sake of efficient use of sunlight, or use of visible region
∗ Corresponding author at: School of Materials Science and Engineering, Shaanxi University of Science & Technology, Xi’an, Shaanxi 710021, China. Tel.: +86 29 86168688; fax: +86 29 86168688. E-mail addresses: [email protected], [email protected] (L. Junqi). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.01.130
of the spectrum, the technology of enlarging the absorption scope of TiO2 may then appear as an appealing challenge for developing the future generation of photocatalysts. TiO2 -based photocatalysts doped with anions (e.g. C, N, S, F) have been widely investigated in order to shift the optical absorption edge of TiO2 towards lower energy, thereby increasing the photocatalytic activity in visible region [14–18]. Among these, the F-doped or surface fluorination of TiO2 has been found to be a most effective method to promote the photocatalytic activity [19–22]. In this study, for the enhancement of visible-light-driven photocatalytic activity of TiO2 , we prepared visible responsive fluorinated TiO2 hollow microspheres with solvothermally treatment using solid microspheres as precursor. The morphology and microstructure of solid and hollow microspheres were investigated in detail. The obtained fluorinated TiO2 hollow microspheres exhibited unique three-dimensional hierarchical architecture and demonstrated a significantly improved photocatalytic performance. 2. Experimental 2.1. Synthesis TiO2 solid microspheres were prepared by controlled hydrolysis of Tetra-n-butyl Titanate (TBT) in ethanol solution. In a typical preparation process of TiO2 solid microspheres, a given amount of aqueous salt solution was first mixed with 100 mL of anhydrous ethanol, followed by addition of 2 mL Tetra-n-butyl Titanate (TBT) at ambient temperature under magnetic stirring. After 10 h the reaction was finished and the precipitated microspheres were
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collected by centrifugation, washed with ethanol. The final mesoporous TiO2 solid microspheres were produced by calcinations of the dried samples for 1 h at 500 ◦ C. For TiO2 hollow microspheres, 0.8 g of the obtained solid microspheres and 0.18 g NaF were dispersed into a mixture of 20 mL ethanol and 10 mL deionized water with the assistance of strong magnetic stirring. The final mixture was directly transferred into a 100 mL Teflon autoclave, heated by microwave irradiation in the microwave reactor at 180 ◦ C for 1 h. The solvothermally treated powders were obtained by washing with deionized water and ethanol several times and air drying. 2.2. Characterization The crystalline phase of the samples was characterized using X-ray diffraction (XRD-D/max2200pc, Japan) technique with Cu K␣ radiation of wavelength k = 0.15418 nm. Scanning electron micrographs SEM images were obtain on JSM-6700F (JEOL Japan). TEM images were taken on JEM2010 (JEOL Japan). The specific surface area was determined by the Brunauer–Emmett–Teller (BET) method. Nitrogen adsorption/desorption isotherms were measured at −196 ◦ C by using a JW-004A system. X-ray photoelectron spectroscopy (XPS) analyses were carried out in an ultrahigh vacuum (UHV) chamber with a base pressure below 2.66 × 10−7 Pa at room temperature. Photoemission spectra were recorded by a Kratos Axis Ultra spectrometer equipped with a standard monochromatic Al K␣ excitation source (hv = 1486.71 eV). The pass energy and step size of low-resolution XPS scan were adjusted to 40 and 0.1 eV. The binding energy (BE) scale was calibrated by measuring a C1 s peak at 285.0 eV from the surface contamination. The resultant XPS spectra were simulated by assuming the photoelectron peak as Gaussian line shape. The adsorption spectra of TiO2 microspheres were recorded on a PE lamda 950 UV/Vis spectrophotometer equipped with an integration sphere attachment.
Fig. 1. XRD patterns of the TiO2 microspheres. The precursor TiO2 (a), calcined at 500 ◦ C for 1 h (b) and solvothermal treatment (c).
peaks, which indicate the presence of an amorphous phase. In the case of calcination and solvothermal treatment, the peaks at scattering angles of 25.26, 36.88, 47.78 and 62.74◦ correspond to the reflections from the (1 0 1), (0 0 4), (2 0 0) and (2 0 4) crystal planes of anatase TiO2 , respectively. All the reflections can be readily indexed as a pure anatase phase of TiO2 (JCPDS card No. 21-1272), and no other peak was observed. On the basis of the XRD results, the crystal size of the calcination and solvothermal treatment TiO2 calculated by the Scherrer formula were 12.8 nm and 16.9 nm, respectively. This result implies that the TiO2 microspheres are composed of nanocrystals (12.8 nm for calcination and 16.9 nm for solvothermal treatment) and the amorphous phase can be successfully converted to an anatase phase by carrying out calcination and solvothermal treatment.
2.3. Photocatalytic activity measurement
3.2. SEM and TEM analysis
The photocatalytic activities of the prepared TiO2 microspheres were determined by measuring the degradation of methyl orange (MO) in an aqueous solution under visible light irradiation. A 500 W xenon lamp ( > 420 nm) was used as light source. The aqueous system containing 80 mL 0.008 g/L methyl orange (MO) and 0.05 g mesoporous TiO2 microspheres was magnetically stirred in the dark for 1 h to reach the adsorption equilibrium of MO with the photocatalysts, and then exposed to visible light. The suspension was vigorously stirred with the photoreactor during the process and temperature of suspension was maintained at 22 ± 2 ◦ C by circulation of water through an external cooling coil. UV/Vis absorption spectra were recorded at different time intervals to monitor the reaction, and the concentration of MO left in the aqueous system was measured by detecting the absorption at 464 nm, the maximum absorption wavelength for MO on a UV–Vis spectrophotometer (UV1700 Shimadzu). The photoluminescence (PL) emission spectra were measured at room temperature with a 10MW and 325-nm He–Cd laser.
The typical SEM and TEM images of the samples are shown in Fig. 2. The overall morphology of the microspheres is shown in Fig. 2a. It is observed that both the precursor TiO2 and calcined TiO2 are composed of a large quantity of relatively uniform microspheres with an average diameter of about 1 m (Fig. 2a and c). This suggests that calcinations, while transformed the TiO2 microspheres from amorphous to anatase phase, did not destroy the framework of the microspheres. The representative TEM images (Fig. 2e) of the calcined TiO2 reveal that the TiO2 microspheres composed of nanocrystals are mesoporous without a long-range order. The mesoporosity is mainly due to the interparticle porosity. The average diameters of the nano-particles, estimated from the TEM images of the samples (about 11.8 nm), are in good agreement with that calculated from XRD patterns using the Scherrer equation. For the solvothermal treatment TiO2 , the magnified SEM images (Fig. 2b) reveal that TiO2 microspheres with a rough surface are self-assembled hierarchical structures that are composed of many loosely packed elongated TiO2 nanometer-scale crystals as building units. The average diameters of the elongated nano-crystals (16.7 ± 0.8 nm), estimated from the SEM and TEM images (Fig. 2b and f) of the samples, are in good agreement with that calculated from XRD patterns using the Scherrer equation. The details of their interior can be clearly observed by the SEM image of a single broken sphere after being unveiled by ultrasonication (see Fig. 2b (inset)). The hollow structure of TiO2 microspheres can also be identified by TEM image (Fig. 2d). Fig. 2f shows the magnified TEM images for one TiO2 microspheres, in which abundant cavities can be seen, indicating the mesoporous structure of TiO2 hollow microspheres. Based on the above results, it is evident that hierarchical structure TiO2
3. Results and discussion 3.1. XRD analysis TiO2 exists mainly in three crystallographic structures, anatase, rutile and brookite. The XRD peak at 2 = 25.26◦ (1 0 1) is often taken as the characteristic peaks of anatase crystallographic structures. Fig. 1 shows the XRD patterns of the TiO2 microspheres, the precursor TiO2 (a), calcined at 500 ◦ C for 1 h (b) and solvothermal treatment (c). For the precursor TiO2 , there were no diffraction
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Fig. 2. SEM and TEM images of the precursor TiO2 microspheres (a), calcined at 500 ◦ C for 1 h (c) and (e), and solvothermal treatment (b), (d) and (f).
hollow microspheres composed of nano-crystals with mesoporous structure have been successfully prepared by a facile combination of sol-gel and solvothermal processes. Such novel hollow structure with three-dimensionally interconnected but aperiodic pore channels would facilitate the accessibility of reactants to the active sites within the mesoarchitecture. This is an attractive feature for heterogeneous catalysis. The formation of mesoporous TiO2 hollow microspheres is a complicated process involving a dissolution (inside TiO2 microspheres)-redeposition (on the surface of TiO2 microspheres) procedure during solvothermal treatment. The aggregated TiO2 solid microspheres are chemically etched by corrosive F− , followed
by the recrystallization of their surfaces. The accessible mesopores channels and corrosive nanoparticles are hence simultaneously generated. Moreover, the intruded F− may enrich at the center of the spheres, creating a hollow interior. The building units of aggregated nanoparticles, which comprised additional hollow structure, are 16.7 ± 0.8 nm. 3.3. BET analysis Fig. 3 shows the N2 -sorption isotherms (inset) of calcined and solvothermal treatment TiO2 microspheres with their corresponding pore size distribution curve calculated from the
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Fig. 3. BJH (Barrett–Joyner–Halenda) pore size distribution and Nitrogen adsorption/desorption isotherms of calcined and solvothermal treatment TiO2 microspheres.
desorption branch of the N2 -sorption isotherms by the BJH (Barrett–Joyner–Halenda) method. The isotherms are typical type IV-like with a type H2 hysteresis loop, which indicates the presence of mesoporous materials according to IUPAC classification. The specific surface area of calcined TiO2 is 60.8 m2 /g using the BET (Brunauer–Emmett–Teller) method, and the pore diameter is about 7.9 nm. While, the mesoporous TiO2 hollow microspheres prepared using solvothermal processes have higher surface area (90.1 m2 /g) and larger pore diameter (16.5 nm), which provides more surface active sites and pore-channels for the chemisorptions and diffusion of reactants. 3.4. XPS analysis The elemental composition of TiO2 hollow microspheres with solvothermal treatment was further determined by XPS (Fig. 4). XPS survey spectra (Fig. 4a) indicate that the samples synthesized in NaF solution during solvothermal treatment contain Ti, O, F, and C. The C element was mainly ascribed to the adventitious hydrocarbon from XPS itself. Fig. 4b shows the high-resolution XPS spectra of F 1s region. The only peak at 684.3 eV was assigned to the F− anions that are physically adsorbed on the surface of mesoporous TiO2 microspheres (≡Ti–F). No signal for F− in the lattice of TiO2 (BE = 688.5 eV) is found [23–25]. Furthermore, the binding energies of Ti 2p3/2 and Ti 2p1/2 are equal to 458.9 and 464.4 eV, respectively (see Fig. 4c), which are identical to that of Ti4+ of bulk TiO2 . Therefore, it can be easily concluded from the above XPS results that the oxygen in TiO2 lattice is not substituted by F and F element only exists in surface fluoride ((≡Ti–F) [25,26]. This is not difficult to understand because, on the one hand, the solvothermal environment can accelerate crystallization of TiO2 due to the in situ dissolution-recrystallization process, resulting in the reduction of the number of defects and impurity in TiO2 crystals, on the other hand, surface fluorination can be easily carried out by a simple ligand exchange reaction between surface hydroxyl groups on TiO2 and fluoride anions (F− ) due to the high F–Ti bonding energy. Therefore, it is not surprising that the surface fluorination modification of TiO2 readily takes place under a solvothermal environment. The actual amount of F in TiO2 microspheres is 6.89%, which is lower than that of the precursor solution. This is easy to understand because F− is only adsorbed on the surface of TiO2 microspheres. It is well known that the fluorination on the surface of TiO2 can accelerate the photocatalytic degradation of a wide range of organic pollutants since the • OH radicals generated on the surface of TiO2 microspheres are more mobile than those generated on pure TiO2
Fig. 4. XPS survey spectra (a) and high-resolution XPS spectra of the F 1s (b) and Ti 2p (c) region taken on the TiO2 hollow microspheres with solvothermal treatment.
under light irradiation [27]: •+
≡ Ti–OH + h+ ↔≡ Ti–OH vb
(1)
Ti–F + H2 O (or OH− ) + h+ →≡ Ti–F + • OHfree + H+ vb
(2)
Also, the redox potential of free OH radicals in solution (ca. 2.3 V vs NHE at pH 7) is larger than that of surface-adsorbed OH radicals on TiO2 (about 1.5–1.7 V vs NHE at pH 7) [24].
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Fig. 7. The photoluminescence (PL) spectra of TiO2 microspheres. Fig. 5. Optical UV–Vis absorption spectra of TiO2 microspheres.
3.5. Absorption spectra and photo degradation Fig. 5 shows the UV–Vis spectra of TiO2 microspheres prepared with calcination and solvothermal treatment. The absorption spectrum of the hollow microspheres with solvothermal treatment exhibits a stronger absorption in the UV–Vis range of 325–800 nm than that of calcination. Clearly, the hollow inner structure associated with accessible mesoporous at the spherical surface allow the light-scattering inside their pore channels as well as their interior hollows, enhancing the light harvesting and thus increase the quantity of photogenerated electrons and hole to participate in the photocatalytic decomposition of the contaminants. Photo degradation of methyl orange was used as a model reaction to compare the photocatalytic activities of calcined and solvothermal treatment TiO2 . Fig. 6 shows the decrease of the concentration of methyl orange with irradiation time in the presence of TiO2 as well as self-degradation of methyl orange in the absence of any photocatalysts. It was found that the self-degradation of methyl orange was negligible under visible light. However, 74% and 21% of methyl orange were degraded in the presence of solvothermal treatment and calcined TiO2 after 120 min under visible light irradiation, respectively. Therefore, solvothermal treatment TiO2 showed much higher photocatalytic activity. The effects of fluorination on the recombination of photogenerated electron/hole pairs were measured by PL emission, as shown in Fig. 7. The intensity of the PL spectra for fluorinated TiO2 hollow microspheres with solvothermally treatment is lower than that of pure calcined TiO2 . Since the observed PL spectrum in
TiO2 can be attributed to the radiative recombination process of self-trapped excitations, or hydroxylated TiO3+ surface complexes from the charge transfer excited state of the highly dispersed TiO2 . The reduction of PL intensity in the fluorinated TiO2 indicates the decrease in the radiative recombination process. As a result, more holes were available for producing more hydroxyl radicals over the fluorinated TiO2 . It is known that the photocatalytic activity of TiO2 is affected by many factors such as surface area, phase structure, crystallinity, surface hydroxyl density, surface acidity, surface defects, and so on [5,27–29]. On the basis of the above characterizations, we attributed the high photocatalytic activity of our mesoporous TiO2 hollow microspheres with solvothermal treatment to several reasons as follows. First, the enhancing light harvesting abilities due to the special structure makes it increase the quantity of photogenerated electrons and hole to participate in the photocatalytic decomposition of the contaminants. Second, the existence of accessible mesopores channel can easily adsorb polar organic molecules for photoactivity enhancement, which is favorable for the photoreaction. Finally, the surface ≡Ti–F group on TiO2 acts as an electron-trapping site but reduces interfacial electron transfer rates by tightly holding trapped electrons due to the strong electronegativity of the fluorine. The generation of free OH radicals is enhanced on TiO2 because direct hole trapping or the generation of surface-bound OH radicals is not allowed on the fluorinated surface. As a result, the photocatalytic decomposition rate of the surface fluorinated TiO2 was superior to that of calcination. 4. Conclusions Fluorinated TiO2 hollow microspheres with three-dimensional hierarchical architecture were prepared by solvothermally treatment using solid microspheres as precursor. The photocatalytic activity of as-prepared solid and hollow TiO2 microspheres was determined by degradation of methyl orange under visible light irradiation. The surface fluorination, the existence of accessible mesopores channels, and the increased light harvesting abilities could remarkably improve the photocatalytic activity of TiO2 hollow microspheres. This approach provides a green, simple and economical method to synthesize uniform fluorinated TiO2 hollow microspheres with high visible light responsive photocatalytic activity. Acknowledgments
Fig. 6. Photo degradation of methyl orange in the presence of TiO2 photocatalysts as well as self-degradation of methyl orange in the absence of any photocatalyst.
This research was financially supported by Special Fund from Shaanxi Provincial Department of Education (09JK352) and the
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