One-Pot Facile Synthesis of Cerium-Doped TiO2 Mesoporous

Apr 22, 2013 - Natural collagen fiber-enabled facile synthesis of carbon@Fe 3 O 4 core–shell nanofiber bundles and their application as ultrahigh-ra...
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One-Pot Facile Synthesis of Cerium-Doped TiO2 Mesoporous Nanofibers Using Collagen Fiber As the Biotemplate and Its Application in Visible Light Photocatalysis Gao Xiao,†,‡ Xin Huang,† Xuepin Liao,*,† and Bi Shi*,†,‡ †

Department of Biomass Chemistry and Engineering, Sichuan University, Chengdu 610065, P. R. China Key Laboratory of Leather Chemistry and Engineering of Ministry of Education, Sichuan University, Chengdu 610065, P. R. China

‡

S Supporting Information *

ABSTRACT: Cerium-doped TiO2 (Cex/TiO2) mesoporous nanofibers were prepared by one-pot facile synthesis method using collagen fiber as the biotemplate. The physicochemical properties of the as-prepared Cex/TiO2 nanofibers were well characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), N2 adsorption−desorption isotherms, and UV−vis diffuse reflectance spectrum (UV−vis DRS). The visible light absorption ability and the band gap energy of the Cex/TiO2 nanofibers could be adjusted by changing the doping amount of Ce. For example, when the mole ratio of Ce/Ti was fixed at 0.03, the absorbance wavelength of the Ce0.03/TiO2 reached 739 nm, and the corresponding band gap energy was obviously reduced to 1.678 eV. Photodegradation of Rhodamine B (RhB) was used as the probe reaction to evaluate the visible light photocatalytic activity of the Cex/TiO2 nanofibers. Compared with the undoped TiO2 nanofiber and commercial TiO2 catalyst (Degussa P25), the Cex/TiO2 nanofibers showed the excellent photocatalytic activity. Especially, the degradation degree of RhB using Ce0.03/TiO2 nanofiber reached 99.59% in 80 min, with corresponding TOC removal efficiency of 77.59%.

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INTRODUCTION Heterogeneous photocatalysts have shown great potentials for solar energy conversion and environment remediation.1 Among them, TiO2 is one of the most intensively investigated photocatalysts due to its good photocatalytic activity, chemical stability, nontoxicity, and low-cost.2 However, the implementation of common TiO2 in practical applications is still limited due to its disadvantages of broad bandgap (ca. 3.2 eV for anatase TiO2), responding only to UV light (wavelength < 387 nm) and low quantum efficiency.3 Much efforts have been made to develop highly efficient and visible light responsive photocatalytic TiO2, by doping with transitional metal ion, nonmetal element, or noble metals.4 Among these strategies, doping TiO2 with cerium has attracted much attention because the redox shift between CeO2 and Ce2O3 can provide high capacity to store/release oxygen, which plays the role as an oxygen reservoir to exhibit excellent characteristics in transferring electrons and shifting the adsorption band toward to visible light range (300−800 nm).5 However, the morphology and microstructures of TiO2 photocatalysts also significantly influence its photocatalytic activity. Design of TiO2 photocatalyst with well-defined mesoporous nanostructures is an effective method to obtain high photocatalytic activity because the large surface area and high pore volume of the mesoporous TiO2 nanocatalyst can enhance the adsorption for reactant molecules and improve the © XXXX American Chemical Society

light harvesting, which may extend the spectral response of TiO2 toward visible light irradiations.6,7 Furthermore, nanofibrous structure often exhibits the distinct advantages of interconnected open pore structures, high geometrical flexibility, and low mass transfer resistance, which sharply favors the photodegradation.8 Conventional methods used for the synthesis of mesoporous TiO2 nanofibers includes electrospinning,9 hydrothermal methods,10 sol−gel method,11 and chemical vapor deposition.12 Zhang et al.13 reported the synthesis of hollow mesoporous one dimensional TiO2 nanofibers by coaxial electrospinning of a titanium tetraisopropoxide (TTIP) with polyethylene oxide (PEO) and polyvinylpyrrolidone (PVP). This study showed that the mesoporous surface of the TiO 2 nanofibers influenced its interactions with rhodamine dye, which exhibited an increased degradation rate to the rhodamine dye under visible-light irradiation as compared with the commercial photocatalyst Degussa P25. Y. Suzuki et al.14 successfully prepared long titanate nanofibers in high yield via the direct hydrothermal route using natural rutile as a starting material. However, the relative harsh synthetic conditions and complicated process of Received: December 6, 2012 Revised: April 19, 2013

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Scheme 1. Schematic Illustration for the Preparation of the Cex/TiO2 Nanofibers

Rhodamine B (RhB) as probe reaction, and their photocatalytic activities were compared with the commercial photocatalyst of Degussa P25.

the above methods make them difficult to be widely used in industry. More recently, biotemplate synthesis has been emerging as a convenient method for fabricating advanced materials with sophisticated hierarchical structures. The hollow cobalt oxide nanoparticles were prepared using the protein-regulated sitespecific reconstitution process in aqueous solution.15 It was reported that the nanocelluloses, such as cellulose nanocrystals (CNC) and microfibrillated cellulose (MFC), have been used as building blocks for the construction of hierarchical functional nanomaterials.16 Zhang and his co-workers17 reported the synthesis of TiO2 nanocatalyst by using bacterial cellulose membranes as nature templates. At present, it is highly attractive to find new biotemplates for the synthesis of nanomaterials with specific structures. Collagen fiber (CF), one of the most abundant biomasses in natural world, is generally produced from animal skins as a byproduct of the food industry. CF is formed by self-assembled collagen molecules with multiple levels of hierarchical interwoven structure. The collagen molecule has a rodlike shape with 1.5 nm in diameter and 300 nm in length, which is composed of three polypeptide chains with a right-handed triple helical structure, and these collagen molecules are packed together longitudinally in a quarter stagger arrangement with a gap region length of 67 nm.18 The self-assembly of collagen molecules leads to the formation of microfibrils, which further organize into fibrils and even larger fiber bundles.19 However, CF has abundant functional groups such as −OH, −COOH, −CONH2, and−NH2, which are able to react with many metal ions, such as Ti4+, Cr3+, Fe3+, and Al3+. All these unique properties of CF make it an ideal biotemplate for the synthesis of inorganic nanofibers. In the present investigation, we successfully synthesized Cedoped TiO2 (Cex/TiO2) nanofibers with mesoporous structure by using collagen fiber as the template. Herein, we tried to improve the photocatalytic activity of TiO2 by incorporating Ce together with the construction of mesoporous nanofibers. The influences of Ce content on the physicochemical properties of the Cex/TiO2 nanofibers were systematically investigated, including phase composition, crystal size, specific surface area, pore size distribution, and band gap energy. Subsequently, the photocatalytic activity of the Cex/TiO2 nanofibers under visible light was evaluated using the photocatalytic degradation of

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EXPERIMENTAL SECTION Materials. CF was prepared from bovine skin according to the procedures in our previous work.20 Briefly, the cattle hide was cleaned, unhaired, fleshed, defatted, limed, and delimed according to the procedures of leather processing to remove proteoglycan and other noncollagenous substances. The obtained skin was dehydrated by absolute ethyl alcohol, dried in vacuum, and pulverized into powder. Ti(SO4)2, Ce(SO4)2, and other chemicals were all analytical grade. Preparation of the Cex/TiO2 Nanofibers. CF (15.0 g) was suspended in 400 mL of deionized water at 25 °C for 1.0 h. The pH of the mixture was adjusted to 1.8−2.0 by using H2SO4−HCOOH solution (H2SO4:HCOOH = 10, v/v). After the mixture was stirred at 25 °C for 2 h, 24.0 g of Ti(SO4)2 and a desired amount of Ce(SO4)2 were added in above mixture and kept under constant stirring for another 4.0 h. Subsequently, a proper amount of NaHCO3 solution (15%, w/w) was dropwise added into the reaction system within 2.0 h to increase its pH to 3.8−4.0 and reacted at 40 °C for another 10.0 h. When the reaction was completed, the product was collected by filtration, washed with deionized water, and dried at 45 °C for 4.0 h. These intermediates were further treated by temperature-programmed calcination at 600 °C for 4.0 h to remove the collagen fiber template. Finally, a series of Cex/ TiO2 (x represents the molar ratio of Ce4+ to Ti4+) nanofibers were obtained. Characterizations of the Cex/TiO2 Nanofibers. The surface morphology of the Cex/TiO2 nanofibers was observed by field emission scanning electron microscopy (FESEM, Hitachi 4700, Japan). Transmission electron microscope (TEM) observation of the Ce0.03/TiO2 nanofiber was taken on Tecnai G2F20 (TEM, FEI, The Netherlands) operating at 200 kV. The X-ray diffraction patterns (XRD, Philips X′Pert Pro-MPD, Netherlands) using Cu−Kα radiation (λ = 0.154 nm) of the Cex/TiO2 nanofibers were performed to identify the crystalline structures. The specific surface area and the pore size distribution of the samples were determined by N2 adsoprtion/ desorption at 77 K on a Micrometrics ASAP-2010 adsorption apparatus. Ultraviolet−visible diffuse reflectance (UV−vis DR) B

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Figure 1. FESEM images of CF (a) and the Ce0.03/TiO2 nanofiber (b,c) and the EDX analysis of the Ce0.03/TiO2 nanofiber (d).

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RESULTS AND DISCUSSION Catalyst Preparation and Characterizations. The schematic illustration for preparation of the Cex/TiO2 nanofibers is shown in Scheme 1. The metal precursors first chelated with the functional groups of the collagen molecule (−COOH, −CONH2, and −NH2) via the formation of coordination complexes.22,23 Subsequently, the organic template was completely removed by high temperature calcination, and meanwhile, the metal complexes were accordingly converted to the corresponding oxides. The resultant metal oxides can well keep the fibrous morphology of CF because CF has high reactive capacity toward the metal species, which ensures a uniform dispersion of metal precursor on the CF. Because the dosage of Ti precursor is considerably higher than that of Ce precursor, Ti oxides will form the skeleton structure of the inorganic nanofibers, and the small amount of Ce will embed into the Ti oxide crystals, thus leading to the formation of the Cex/TiO2 nanofibers. Additionally, the vacant spaces containing in the hierarchical structure of CF will lead to the formation of mesoporous structures during the calcination process. As shown in Figure 1b, the Ce0.03/TiO2 nanofiber shows the morphology of well-ordered hierarchical fiber bundles with approximately 450−550 nm in outer diameter. In Figure 1c, it is observed that the fiber bundles exhibit the similar pattern of collagen microfibrils with an average diameter of 20−50 nm. Compared with the FESEM image of nature CF (Figure 1a), it can be concluded that the well-defined hierarchical morphology of CF is faithfully replicated in the Ce0.03/TiO2 nanofiber. In addition, the EDX analysis in Figure 1d further confirms that Ce was successfully incorporated in the TiO2 nanofibers. The content of oxygen % of all samples was determined by the EDX analysis (Supporting Information 2). It can be clearly found that the content of oxygen % was significantly changed with the increase of Ce doped. The oxygen % of Cex/TiO2 nanofibers is

spectra were recorded in the range of 200−800 nm by UV−vis spectrophotometer (UV−vis, UV-3600, Shimadzu, Japan) equipped with an integrating sphere using BaSO4 as a reference. Catalytic Behavior of the Cex/TiO2 Nanofibers for the Photodegradation of RhB under Visible Light. Photoasisted catalytic experiments were implemented in a selfdesigned reactor.21 A 500 mL RhB solution was mixed with catalyst in the outer glass tube of the reactor, and the temperature was maintained at 25 °C. The initial concentrations of RhB and the catalyst were 0.1 mmol/L and 1.0 g/L, respectively. The stirred mixture was irradiated by a 150 W halogen lamp, using optical filter to cut off the short wavelength components (λ < 420 nm). The reactor was well aerated and stirred with a magnetic stirrer to ensure sufficient mixing, and 5 mL of reaction liquid was sampled at an indicated interval for the analyses of UV−vis and TOC. The concentration of RhB was determined using a UV−vis spectrophotometer (UV2501PC, Shimadzu). The total organic carbon (TOC) was performed by a TOC analyzer (Takmar Dohrmamn Apollo 9000). The decoloration degree and mineralization degree of RhB were calculated by the following equations: decoloration degree =

C0 − Ct × 100% C0

mineralization degree =

TOC0 − TOCt × 100% TOC0

(1)

(2)

where C0 and Ct are the concentration of RhB in initial solution and after light irradiation at time t, respectively; TOC0 and TOCt are the content of total organic carbon of RhB in the initial solution and after light irradiation at time t, respectively. C

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crystallite growth of titania through the formation of Ce−O−Ti bonds, which increase the diffusion barrier at the titania grain junctions.26 In addition, the average crystallite sizes of the Cex/ TiO2 nanofibers are calculated according to the anatase (101) diffraction peaks by using the Scherrer equation:27

higher than that of pure TiO2, which exhibited higher photo catalytic activity. For Cex/TiO2 nanofibers, the content of oxygen % was first increased and then reduced with the increase of Ce doped, which was consistent with the trend of their photocatalytic activity (Figure 7). TEM and HRTEM images of CF and the Ce0.03/TiO2 nanofiber are shown in Figure 2. In Figure 2a, the CF template

D=

Kλ β cos θ

where D is the average crystalline size, the X-ray wavelength (λ) was 0.1542 nm, K was the constant usually taken as 0.89, and β is the full width at half-maximum (fwhm) value at a particular 2θ angle after subtraction of equipment broadening, 2θ = 25.4° for anatase phase. We use Cu Ka as the anode at 40 kV and 100 mA, and all the lattice parameters of the samples were calculated and queried by MDI Jade 5.0 software. The instrumental broadening correction (Supporting Information 1) and all the preferences are determined according to the United States standard. For example, the fwhm values of X-ray diffraction analysis of Ce0.01/TiO2 at 2θ = 25.4° was 0.876 before correction, but it was 0.836 after correction. As shown in Table 1, the Cex/TiO2 nanofibers have relative smaller crystal size (5.1−14.4 nm) than those of pure TiO2 (21.5 nm) and P25 (30 nm), and the crystal size is decreased with the increase of Ce content. Probably, the formation of Ce−O−Ti bonds inhibited the crystallite growth of TiO2 during calcination. Compared with pure anatase TiO2, the shift of Cex/TiO2 is obvious, as shown in the inset of Figure 3. However, the rutile phase was almost disappeared. In general, the phase transformation from anatase to rutile often takes place at higher calcination temperature along with the increase of particle size. Doped cerium may be presented as the so-called second phase on the surface of primary TiO2. This second phase inhibits the crystallite growth of rutile phase, so the phase transformation of TiO2 is greatly inhibited.28 In addition, it was found that the intensity of peaks was slightly decreased and the width of the (101) plane diffraction peak of anatase (2θ = 25.4) becomes broader with the increase of Ce doping amount. These facts suggested that the reduction of the crystallization and the decrease of the crystalline size.29 The content of Ce doped in the Cex/TiO2 nanofibers significantly influences the phase of TiO2. The peak intensity of rutil TiO2 is gradually decreased along with the increase of Ce content, and the TiO2 is completely transferred from rutil phase to anatase phase when the amount of doped Ce reaches the molar ratio of 0.03. These results suggest that the doping of Ce produced some lattice deformations and additional deformation energy, which suppress the transformation of TiO2 from anatase phase to rutile phase during the high temperature calcination process.30 Compared with rutil TiO2, anatase TiO2 is generally considered to be more active due to its higher reduction potential and lower recombination rate of electron− hole pairs.31 Thus, the Cex/TiO2 nanofibers are expected to have high photocatalytic activity. Figure 4a shows the N2 adsorption/desorption isotherms of the Cex/TiO2 nanofibers. All the Cex/TiO2 nanofibers exhibit type IV isotherm curves with clear hysteresis loops, which are associated with the characteristics of mesoporous materials.32 By increasing the amount of doped Ce, the specific surface area and the pore volume of the Cex/TiO2 nanofibers are first increased and then decreased. As shown in Table 1, the Ce0.03/ TiO2 exhibits the highest specific surface area of 81.56 m2 g−1. In Figure 4b, the pore size distribution of all the Cex/TiO2 nanofibers are in the range of 2−10 nm, which confirms the

Figure 2. (a) TEM image of collagen fiber self-assembled by collagen microfibrils. (b) TEM images of the Ce0.03/TiO2 nanofiber. (c) HRTEM images of partially enlargement of an individual Ce0.03/TiO2 nanofiber. (d) High-magnification TEM images of a selected area in panel c. Inset image shows the corresponding fast Fourier transform (FFT) patterns of the white quadrate area.

is self-assembled by the longitudinally parallel aligned collagen microfibrils (the white dash labeled). In Figure 2b, the Ce0.03/ TiO2 nanofiber also shows the quite similar morphology, and the Ce-doped TiO2 crystals align along the longitudinal axes of CF but not randomly packed together. Indeed, the fine structure of nature CF is completely replicated in the Ce0.03/ TiO2 nanofiber. In Figure 2c, the HRTEM image of the Ce0.03/ TiO2 nanofiber shows clear lattice fringes with an interplanar spacing of 0.352 nm, which corresponds to the {101} plane of TiO2 anatase (shown in Figure 2d), and the upper left inset image in Figure 2d displays the corresponding fast Fourier transform (FFT) pattern of nanocrystal particles, thus suggesting a high crystallization and polycrystalline structure.24 Figure 3 shows the wide-angle XRD patterns of the Cex/ TiO2 (x = 0, 0.01, 0.02, 0.03, 0.04, and 0.05) nanofibers with different content of Ce. For all the samples, the characteristic peaks of anatase TiO2 are observed at 37.8, 48.0, 53.9, 55.0, and 62.7°(JCPDS file No. 21-1272), suggesting the high crystallization of the Cex/TiO2 nanofibers. In addition, the intensive peaks of CeO2 cannot be identified in all samples since the ionic radii of Ce3+/Ce4+ (1.03/1.02 Å) are bigger than that of Ti4+ (0.68 Å); it is thus difficult for Ce3+ and Ce4+ to replace Ti4+ in the crystal lattice. Thus, the Ce atoms are more preferably located at the grain boundaries and grain junctions of the particles.25 However, these relatively large Ce3+/Ce4+ cations at the grain boundaries and grain junctions can inhibit D

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Figure 3. XRD patterns of the Cex/TiO2 nanofibers (A, anatase; R, rutile).

Table 1. Effect of Doped Ce Content on the Textural Properties of Cex/TiO2 Products sample

phase content

crystal size (101) (nm)

surface area BET (m2/g)

BJH (m2/g)

pore diameter (nm)

pore volume (cm3 g−1)

pure TiO2 Ce0.01/TiO2 Ce0.02/TiO2 Ce0.03/TiO2 Ce0.04/TiO2 Ce0.05/TiO2 collage fiber P25

A(51.8%), R(48.2%) A(90.7%) R(9.3%) A A A A

21.5(A) 14.4(A) 10.7 10 9.9 5.1

44.37 66.76 63.95 113.15 57.48 59.32 1.84

A,R

30(A)

30.82 47.33 48.07 81.56 42.72 39.81 2.53 49.5

7.16 11.13 10.94 10.50 10.64 9.65 11.13 8.3

0.06 0.13 0.13 0.21 0.16 0.1 0.01 0.09

The UV−vis DR spectra of the as-prepared mesoporous Cex/TiO2 nanofibers and the Degussa P25 are shown in Figure 6. By increasing the content of doped Ce, the UV−vis DR spectra of the Cex/TiO2 nanofibers are significantly shifted toward visible light as compared with that of P25. The absorbance edge of the Cex/TiO2 nanofibers is shifted from 450 to 739 nm when the mole ratio of Ce/Ti is increased from 0 to 0.03, due to the fact that the Ce doped into the TiO2 crystal grains can greatly increase the visible light absorption ability.35 When the mole ratio of Ce/Ti is higher than 0.03, the visible light absorption ability of the Cex/TiO2 nanofibers is decreased. As we know, doping is usually accompanied by the formation of defects, which can play the role as trap centers to photoelectrons, but excessive doping may lead to some defects to act as the recombination centers for electron hole and thus decrease the photocatalytic activity. The band gap energy (Eg) of the Cex/TiO2 nanofibers was calculated from the UV absorption spectra (Figure 6b) taking into account that α(E) ∝ (E−Eg)m/2, where α(E) is the absorption coefficient for a photon of energy E, and m = 1 for an indirect transition between bands.36 The band gap energies

mesoporous structure of the Cex/TiO2 nanofibers. In general, the mesoporous structure of the Cex/TiO2 will further increase its photocatalytic activity. The ceria in the Cex/TiO2 exist in multiple valence states, as shown in XPS analysis presented in Figure 5. It can be seen that the binding energy of the Ce 3d5/2 peak at 885.086 eV indicated the presence of CeO2 species, and shakeup peaks in the range of 900−910 eV indicated the presence of Ce2O3 in Ce0.03/TiO2 nanofibers.33 Multiple valence states of the redox pair of cerium (Ce3+/Ce4+) is very important for increasing the absorption efficiency. Since cerium could act as an effective electron scavenger to trap the bulk electrons and increase oxygen reserve. The increasing O2 adsorbed on the surface of the Cex/TiO2 nanofibers can easily capture electrons, which hinder the undesirable recombination of electron−hole pair and greatly promote the catalytic oxidation activity.34 It was confirmed by EDX analysis (Supporting Information 2) that the content of oxygen % was first increased and then reduced with the increase of Ce doped, suggesting the content of oxygen % is related to the amount of doped Ce and/or its valence. E

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Figure 4. (a) Nitrogen adsorption/desorption isotherms of the Cex/ TiO2 nanofibers. (b) BJH pore-size distributions of the samples.

Figure 6. (a) UV−vis DR spectra of P25 and the Cex/TiO2 nanofibers. (b) UV−vis relationship between band gap energy and [αhν]1/2 of P25 and Cex/TiO2 nanofibers.

decreased from 2.756 to 1.678 eV. Obviously, the doping of Ce into TiO2 nanofiber significantly reduces the corresponding band gap energy, that is much lower than that of the commercial TiO2 (P25, 2.881 eV). Further increasing the doped amount of Ce from 0.03 to 0.05 (Ce/Ti mole ratio) leads to the increase of band gap energy from 1.678 to 2.442 eV. As shown in the table inserted in Figure 6b, the band gap energy was also increased when the mole ratio of Ce/Ti beyond 0.03, suggesting the increase of recombination centers.34 Hence, there is an optimal doping amount of Ce in the Cex/TiO2 nanofibers, and this value is determined to be 0.03 (Ce/Ti mole ratio) here. Photodegradation of RhB under Visible Light Irradiation Using the Cex/TiO2 Nanofibers. Photodegradation of RhB was used as the probe reaction to evaluate the photocatalytic activity of the Cex/TiO2 nanofibers under visible light irradiation. As shown in Figure 7, the undoped TiO2 nanofiber prepared using collagen fiber as template exhibits relatively higher activity as compared with the commercial Degussa P25. This fact confirms that the mesporous and fibrous structure of TiO2 nanofiber will improve its catalytic activity. Under the same experimental conditions, the decoloration and

Figure 5. XPS patterns of Ce 3d for Ce0.03/TiO2.

estimated by a linear fit of the slope to the abscissa are shown in Figure 6b. Along with the increase of Ce/Ti mole ratio from 0 to 0.03, the band gap energy of the Cex/TiO2 nanofibers is F

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In addition, it is generally agreement that the thickness of the space charge layer (W) surrounding the TiO2 particles was increased with increasing the doping content, and the Cex/ TiO2 nanofibers can not be excited due to the superfluous cerium species on their surface to form recombination centers, resulting in lower photocatalytic activity.38 It is clearly shown that when the mole ratio of Ce/Ti is lower than 0.03, the doping of Ce can enhance the electron−hole separation and prolong the lifetime of the photoelectrons, thus increasing the photocatalytic activity. However, when the mole ratio of Ce/Ti exceeds 0.03, the photocatalytic activity of the Cex/TiO2 nanofibers are decreased along with the increase of Ce amount. Obviously, the photocatalytic activities of the Cex/ TiO2 nanofibers are closely consistent with the visible light absorption ability of the Cex/TiO2 nanofibers. The Ce0.03/TiO2 nanofiber shows the highest photocatalytic activity under visible-light irradiation with the decoloration degree of 99.59% and mineralization degree of 77.59% in 80 min. The TOC removal efficiency of the Ce0.03/TiO2 nanofiber is greatly higher than that of the undoped TiO2 nanofiber and the Degussa P25, which therefore confirms the successful fabrication of visible-light active Cex/TiO2 photocatalyst by using our strategy.

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CONCLUSIONS In this study, we provide a simple and effective method for the synthesis of Ce-doped TiO2 mesoporous nanofibers by using collage fiber as the template. On the basis of FESEM and TEM analyses, the as-synthesized Cex/TiO2 nanofibers well duplicated the hierarchical structure and morphology of the nature collagen fiber. N2 adsorption/desorption analysis revealed that the Cex/TiO2 nanofibers belonged to mesoporous materials. The UV−vis DR spectra confirmed that the visible light absorption ability and the band gap energy of the Cex/TiO2 nanofibers could be adjusted by changing the amount of doped Ce. Because of these outstanding advantages, the Ce0.03/TiO2 nanofiber exhibited the highest photocatlytic activity under visible light irradiation as compared with the undoped TiO2 nanofiber and the commercial Degussa P25.

Figure 7. Photocatalytic degradation degree (a) and mineralization degree (b) of RhB (initial conccentration 0.1 mM, 500 mL, pH = 6.53).

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mineralization degree of the TiO2 nanofibers are 40.37% and 23.77%, respectively, in 80 min, while they are 35.9% and 21.88% for Degussa P25, respectively. The relatively higher photocatalytic activity of the undoped TiO2 nanofibers should be attributed to the good mass transfer property derived from the mesoporous and fibrous structure. The doping of Ce into the TiO2 nanofibers significantly improves the photocatalytic activity, and the influence of Ce content (Ce/Ti mole ratio) on the catalytic activity follows the order 0.03 > 0.02 > 0.01 > 0.04 > 0.05 > 0. The most fundamental reason for this phenomenon is the change of band gap energy. Actually, as shown in the inset table in Figure 6b, the band gap energy of the Cex/TiO2 nanofibers is decreased from 2.756 to 1.678 eV when the Ce/Ti mole ratio increased from 0 to 0.03. However, when the doped amount of Ce kept increasing from 0.03 to 0.05 (Ce/Ti mole ratio), the corresponding band gap energy subsequently rose from 1.678 to 2.442 eV, the overall results were consistent with the change trend of the photocatalytic activity. However, doping Ce plays an important role in the interfacial charge transfer and in the inhibition of electron−hole recombination. However, the cerium species may become the recombination center of electron−hole pairs when excess cerium was doped.37

ASSOCIATED CONTENT

S Supporting Information *

Detailed information about the correction of the instrumental broadening; EDX analysis results for the pure TiO2 and Cex/ TiO2 nanofibers. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(X.P.L.) E-mail: [email protected]. Tel: +86-28-85400382. Fax: +86-28-85400356. (B.S.) E-mail: [email protected]. Tel: +86-28-85400356. Fax: +86-28-85400356. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

We gratefully acknowledge the financial supports provided by the National Natural Science Foundation of China (21176161 and 2097611) and the National High Technology R&D Program of China (863 Program, 2011AA06A108). G

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dx.doi.org/10.1021/jp312013m | J. Phys. Chem. C XXXX, XXX, XXX−XXX