One-Pot Facile Synthesis of Cerium-Doped TiO2 MesoporousNanofibers Using Collagen Fiber As the Biotemplate and ItsApplication in Visible Light PhotocatalysisGao 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 nanofiberswere prepared by one-pot facile synthesis method using collagen fiber asthe biotemplate. The physicochemical properties of the as-preparedCex/TiO2 nanofibers were well characterized by field emission scanningelectron microscopy (FESEM), transmission electron microscopy(TEM), X-ray diffraction (XRD), N2 adsorption−desorption isotherms,and UV−vis diffuse reflectance spectrum (UV−vis DRS). The visiblelight absorption ability and the band gap energy of the Cex/TiO2nanofibers could be adjusted by changing the doping amount of Ce. Forexample, when the mole ratio of Ce/Ti was fixed at 0.03, the absorbancewavelength of the Ce0.03/TiO2 reached 739 nm, and the correspondingband gap energy was obviously reduced to 1.678 eV. Photodegradationof Rhodamine B (RhB) was used as the probe reaction to evaluate thevisible light photocatalytic activity of the Cex/TiO2 nanofibers. Compared with the undoped TiO2 nanofiber and commercialTiO2 catalyst (Degussa P25), the Cex/TiO2 nanofibers showed the excellent photocatalytic activity. Especially, the degradationdegree of RhB using Ce0.03/TiO2 nanofiber reached 99.59% in 80 min, with corresponding TOC removal efficiency of 77.59%.
■ INTRODUCTION
Heterogeneous photocatalysts have shown great potentials forsolar energy conversion and environment remediation.1 Amongthem, TiO2 is one of the most intensively investigatedphotocatalysts due to its good photocatalytic activity, chemicalstability, nontoxicity, and low-cost.2 However, the implementa-tion of common TiO2 in practical applications is still limiteddue to its disadvantages of broad bandgap (ca. 3.2 eV foranatase TiO2), responding only to UV light (wavelength < 387nm) and low quantum efficiency.3
Much efforts have been made to develop highly efficient andvisible light responsive photocatalytic TiO2, by doping withtransitional metal ion, nonmetal element, or noble metals.4
Among these strategies, doping TiO2 with cerium has attractedmuch attention because the redox shift between CeO2 andCe2O3 can provide high capacity to store/release oxygen, whichplays the role as an oxygen reservoir to exhibit excellentcharacteristics in transferring electrons and shifting theadsorption band toward to visible light range (300−800 nm).5
However, the morphology and microstructures of TiO2
photocatalysts also significantly influence its photocatalyticactivity. Design of TiO2 photocatalyst with well-definedmesoporous nanostructures is an effective method to obtainhigh photocatalytic activity because the large surface area andhigh pore volume of the mesoporous TiO2 nanocatalyst canenhance the adsorption for reactant molecules and improve the
light harvesting, which may extend the spectral response ofTiO2 toward visible light irradiations.6,7 Furthermore, nano-fibrous structure often exhibits the distinct advantages ofinterconnected open pore structures, high geometricalflexibility, and low mass transfer resistance, which sharplyfavors the photodegradation.8 Conventional methods used forthe synthesis of mesoporous TiO2 nanofibers includeselectrospinning,9 hydrothermal methods,10 sol−gel method,11
and chemical vapor deposition.12 Zhang et al.13 reported thesynthesis of hollow mesoporous one dimensional TiO2
nanofibers by coaxial electrospinning of a titanium tetraisoprop-oxide (TTIP) with polyethylene oxide (PEO) and poly-vinylpyrrolidone (PVP). This study showed that the meso-porous surface of the TiO2 nanofibers influenced itsinteractions with rhodamine dye, which exhibited an increaseddegradation rate to the rhodamine dye under visible-lightirradiation as compared with the commercial photocatalystDegussa P25. Y. Suzuki et al.14 successfully prepared longtitanate nanofibers in high yield via the direct hydrothermalroute using natural rutile as a starting material. However, therelative harsh synthetic conditions and complicated process of
the above methods make them difficult to be widely used inindustry.More recently, biotemplate synthesis has been emerging as a
convenient method for fabricating advanced materials withsophisticated hierarchical structures. The hollow cobalt oxidenanoparticles were prepared using the protein-regulated site-specific reconstitution process in aqueous solution.15 It wasreported that the nanocelluloses, such as cellulose nanocrystals(CNC) and microfibrillated cellulose (MFC), have been usedas building blocks for the construction of hierarchical functionalnanomaterials.16 Zhang and his co-workers17 reported thesynthesis of TiO2 nanocatalyst by using bacterial cellulosemembranes as nature templates. At present, it is highlyattractive to find new biotemplates for the synthesis ofnanomaterials with specific structures.Collagen fiber (CF), one of the most abundant biomasses in
natural world, is generally produced from animal skins as abyproduct of the food industry. CF is formed by self-assembledcollagen molecules with multiple levels of hierarchical inter-woven structure. The collagen molecule has a rodlike shapewith 1.5 nm in diameter and 300 nm in length, which iscomposed of three polypeptide chains with a right-handedtriple helical structure, and these collagen molecules are packedtogether longitudinally in a quarter stagger arrangement with agap region length of 67 nm.18 The self-assembly of collagenmolecules leads to the formation of microfibrils, which furtherorganize 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 metalions, such as Ti4+, Cr3+, Fe3+, and Al3+. All these uniqueproperties of CF make it an ideal biotemplate for the synthesisof inorganic nanofibers.In the present investigation, we successfully synthesized Ce-
doped TiO2 (Cex/TiO2) nanofibers with mesoporous structureby using collagen fiber as the template. Herein, we tried toimprove the photocatalytic activity of TiO2 by incorporating Cetogether with the construction of mesoporous nanofibers. Theinfluences of Ce content on the physicochemical properties ofthe Cex/TiO2 nanofibers were systematically investigated,including phase composition, crystal size, specific surface area,pore size distribution, and band gap energy. Subsequently, thephotocatalytic activity of the Cex/TiO2 nanofibers under visiblelight was evaluated using the photocatalytic degradation of
Rhodamine B (RhB) as probe reaction, and their photocatalyticactivities were compared with the commercial photocatalyst ofDegussa P25.
■ EXPERIMENTAL SECTION
Materials. CF was prepared from bovine skin according tothe procedures in our previous work.20 Briefly, the cattle hidewas cleaned, unhaired, fleshed, defatted, limed, and delimedaccording to the procedures of leather processing to removeproteoglycan and other noncollagenous substances. Theobtained skin was dehydrated by absolute ethyl alcohol, driedin 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 usingH2SO4−HCOOH solution (H2SO4:HCOOH = 10, v/v). Afterthe mixture was stirred at 25 °C for 2 h, 24.0 g of Ti(SO4)2 anda desired amount of Ce(SO4)2 were added in above mixtureand 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 hto increase its pH to 3.8−4.0 and reacted at 40 °C for another10.0 h. When the reaction was completed, the product wascollected by filtration, washed with deionized water, and driedat 45 °C for 4.0 h. These intermediates were further treated bytemperature-programmed calcination at 600 °C for 4.0 h toremove the collagen fiber template. Finally, a series of Cex/TiO2 (x represents the molar ratio of Ce4+ to Ti4+) nanofiberswere obtained.
Characterizations of the Cex/TiO2 Nanofibers. Thesurface morphology of the Cex/TiO2 nanofibers was observedby field emission scanning electron microscopy (FESEM,Hitachi 4700, Japan). Transmission electron microscope(TEM) observation of the Ce0.03/TiO2 nanofiber was takenon Tecnai G2F20 (TEM, FEI, The Netherlands) operating at200 kV. The X-ray diffraction patterns (XRD, Philips X′PertPro-MPD, Netherlands) using Cu−Kα radiation (λ = 0.154nm) of the Cex/TiO2 nanofibers were performed to identify thecrystalline structures. The specific surface area and the pore sizedistribution of the samples were determined by N2 adsoprtion/desorption at 77 K on a Micrometrics ASAP-2010 adsorptionapparatus. Ultraviolet−visible diffuse reflectance (UV−vis DR)
Scheme 1. Schematic Illustration for the Preparation of the Cex/TiO2 Nanofibers
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spectra were recorded in the range of 200−800 nm by UV−visspectrophotometer (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. Photo-asisted catalytic experiments were implemented in a self-designed reactor.21 A 500 mL RhB solution was mixed withcatalyst in the outer glass tube of the reactor, and thetemperature was maintained at 25 °C. The initial concen-trations of RhB and the catalyst were 0.1 mmol/L and 1.0 g/L,respectively. The stirred mixture was irradiated by a 150 Whalogen lamp, using optical filter to cut off the short wavelengthcomponents (λ < 420 nm). The reactor was well aerated andstirred with a magnetic stirrer to ensure sufficient mixing, and 5mL of reaction liquid was sampled at an indicated interval forthe analyses of UV−vis and TOC. The concentration of RhBwas determined using a UV−vis spectrophotometer (UV-2501PC, Shimadzu). The total organic carbon (TOC) wasperformed by a TOC analyzer (Takmar Dohrmamn Apollo9000). The decoloration degree and mineralization degree ofRhB were calculated by the following equations:
=−
×C C
Cdecoloration degree 100%t0
0 (1)
=−
×mineralization degreeTOC TOC
TOC100%t0
0 (2)
where C0 and Ct are the concentration of RhB in initial solutionand after light irradiation at time t, respectively; TOC0 andTOCt are the content of total organic carbon of RhB in theinitial solution and after light irradiation at time t, respectively.
■ RESULTS AND DISCUSSION
Catalyst Preparation and Characterizations. Theschematic illustration for preparation of the Cex/TiO2 nano-fibers is shown in Scheme 1. The metal precursors first chelatedwith the functional groups of the collagen molecule (−COOH,−CONH2, and −NH2) via the formation of coordinationcomplexes.22,23 Subsequently, the organic template wascompletely removed by high temperature calcination, andmeanwhile, the metal complexes were accordingly converted tothe corresponding oxides. The resultant metal oxides can wellkeep the fibrous morphology of CF because CF has highreactive capacity toward the metal species, which ensures auniform dispersion of metal precursor on the CF. Because thedosage of Ti precursor is considerably higher than that of Ceprecursor, Ti oxides will form the skeleton structure of theinorganic nanofibers, and the small amount of Ce will embedinto the Ti oxide crystals, thus leading to the formation of theCex/TiO2 nanofibers. Additionally, the vacant spaces contain-ing in the hierarchical structure of CF will lead to the formationof 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 withapproximately 450−550 nm in outer diameter. In Figure 1c, itis observed that the fiber bundles exhibit the similar pattern ofcollagen microfibrils with an average diameter of 20−50 nm.Compared with the FESEM image of nature CF (Figure 1a), itcan be concluded that the well-defined hierarchical morphologyof CF is faithfully replicated in the Ce0.03/TiO2 nanofiber. Inaddition, the EDX analysis in Figure 1d further confirms thatCe was successfully incorporated in the TiO2 nanofibers. Thecontent of oxygen % of all samples was determined by the EDXanalysis (Supporting Information 2). It can be clearly foundthat the content of oxygen % was significantly changed with theincrease of Ce doped. The oxygen % of Cex/TiO2 nanofibers is
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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higher than that of pure TiO2, which exhibited higher photocatalytic activity. For Cex/TiO2 nanofibers, the content ofoxygen % was first increased and then reduced with the increaseof Ce doped, which was consistent with the trend of theirphotocatalytic 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
is self-assembled by the longitudinally parallel aligned collagenmicrofibrils (the white dash labeled). In Figure 2b, the Ce0.03/TiO2 nanofiber also shows the quite similar morphology, andthe Ce-doped TiO2 crystals align along the longitudinal axes ofCF but not randomly packed together. Indeed, the finestructure 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 interplanarspacing of 0.352 nm, which corresponds to the {101} plane ofTiO2 anatase (shown in Figure 2d), and the upper left insetimage in Figure 2d displays the corresponding fast Fouriertransform (FFT) pattern of nanocrystal particles, thussuggesting 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 withdifferent content of Ce. For all the samples, the characteristicpeaks of anatase TiO2 are observed at 37.8, 48.0, 53.9, 55.0, and62.7°(JCPDS file No. 21-1272), suggesting the high crystal-lization of the Cex/TiO2 nanofibers. In addition, the intensivepeaks of CeO2 cannot be identified in all samples since theionic radii of Ce3+/Ce4+ (1.03/1.02 Å) are bigger than that ofTi4+ (0.68 Å); it is thus difficult for Ce3+ and Ce4+ to replaceTi4+ in the crystal lattice. Thus, the Ce atoms are morepreferably located at the grain boundaries and grain junctions ofthe particles.25 However, these relatively large Ce3+/Ce4+
cations at the grain boundaries and grain junctions can inhibit
crystallite growth of titania through the formation of Ce−O−Tibonds, which increase the diffusion barrier at the titania grainjunctions.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
λβ θ
=DKcos
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 βis the full width at half-maximum (fwhm) value at a particular2θ angle after subtraction of equipment broadening, 2θ = 25.4°for anatase phase. We use Cu Ka as the anode at 40 kV and 100mA, and all the lattice parameters of the samples werecalculated and queried by MDI Jade 5.0 software. Theinstrumental broadening correction (Supporting Information1) and all the preferences are determined according to theUnited States standard. For example, the fwhm values of X-raydiffraction analysis of Ce0.01/TiO2 at 2θ = 25.4° was 0.876before correction, but it was 0.836 after correction. As shown inTable 1, the Cex/TiO2 nanofibers have relative smaller crystalsize (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 ofCe content. Probably, the formation of Ce−O−Ti bondsinhibited the crystallite growth of TiO2 during calcination.Compared with pure anatase TiO2, the shift of Cex/TiO2 isobvious, as shown in the inset of Figure 3. However, the rutilephase was almost disappeared. In general, the phase trans-formation from anatase to rutile often takes place at highercalcination temperature along with the increase of particle size.Doped cerium may be presented as the so-called second phaseon the surface of primary TiO2. This second phase inhibits thecrystallite growth of rutile phase, so the phase transformation ofTiO2 is greatly inhibited.28 In addition, it was found that theintensity of peaks was slightly decreased and the width of the(101) plane diffraction peak of anatase (2θ = 25.4) becomesbroader with the increase of Ce doping amount. These factssuggested that the reduction of the crystallization and thedecrease of the crystalline size.29
The content of Ce doped in the Cex/TiO2 nanofiberssignificantly influences the phase of TiO2. The peak intensity ofrutil TiO2 is gradually decreased along with the increase of Cecontent, and the TiO2 is completely transferred from rutil phaseto anatase phase when the amount of doped Ce reaches themolar ratio of 0.03. These results suggest that the doping of Ceproduced some lattice deformations and additional deformationenergy, which suppress the transformation of TiO2 fromanatase phase to rutile phase during the high temperaturecalcination process.30 Compared with rutil TiO2, anatase TiO2is generally considered to be more active due to its higherreduction potential and lower recombination rate of electron−hole pairs.31 Thus, the Cex/TiO2 nanofibers are expected tohave high photocatalytic activity.Figure 4a shows the N2 adsorption/desorption isotherms of
the Cex/TiO2 nanofibers. All the Cex/TiO2 nanofibers exhibittype IV isotherm curves with clear hysteresis loops, which areassociated with the characteristics of mesoporous materials.32
By increasing the amount of doped Ce, the specific surface areaand the pore volume of the Cex/TiO2 nanofibers are firstincreased 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 collagenmicrofibrils. (b) TEM images of the Ce0.03/TiO2 nanofiber. (c)HRTEM images of partially enlargement of an individual Ce0.03/TiO2nanofiber. (d) High-magnification TEM images of a selected area inpanel c. Inset image shows the corresponding fast Fourier transform(FFT) patterns of the white quadrate area.
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mesoporous structure of the Cex/TiO2 nanofibers. In general,the mesoporous structure of the Cex/TiO2 will further increaseits 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 thatthe binding energy of the Ce 3d5/2 peak at 885.086 eVindicated the presence of CeO2 species, and shakeup peaks inthe range of 900−910 eV indicated the presence of Ce2O3 inCe0.03/TiO2 nanofibers.
33 Multiple valence states of the redoxpair of cerium (Ce3+/Ce4+) is very important for increasing theabsorption efficiency. Since cerium could act as an effectiveelectron scavenger to trap the bulk electrons and increaseoxygen reserve. The increasing O2 adsorbed on the surface ofthe Cex/TiO2 nanofibers can easily capture electrons, whichhinder the undesirable recombination of electron−hole pairand greatly promote the catalytic oxidation activity.34 It wasconfirmed by EDX analysis (Supporting Information 2) thatthe content of oxygen % was first increased and then reducedwith the increase of Ce doped, suggesting the content ofoxygen % is related to the amount of doped Ce and/or itsvalence.
The UV−vis DR spectra of the as-prepared mesoporousCex/TiO2 nanofibers and the Degussa P25 are shown in Figure6. By increasing the content of doped Ce, the UV−vis DRspectra of the Cex/TiO2 nanofibers are significantly shiftedtoward visible light as compared with that of P25. Theabsorbance edge of the Cex/TiO2 nanofibers is shifted from450 to 739 nm when the mole ratio of Ce/Ti is increased from0 to 0.03, due to the fact that the Ce doped into the TiO2
crystal grains can greatly increase the visible light absorptionability.35 When the mole ratio of Ce/Ti is higher than 0.03, thevisible light absorption ability of the Cex/TiO2 nanofibers isdecreased. As we know, doping is usually accompanied by theformation of defects, which can play the role as trap centers tophotoelectrons, but excessive doping may lead to some defectsto act as the recombination centers for electron hole and thusdecrease the photocatalytic activity.The band gap energy (Eg) of the Cex/TiO2 nanofibers was
calculated from the UV absorption spectra (Figure 6b) takinginto account that α(E) ∝ (E−Eg)m/2, where α(E) is theabsorption coefficient for a photon of energy E, and m = 1 foran indirect transition between bands.36 The band gap energies
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 A(51.8%), R(48.2%) 21.5(A) 30.82 44.37 7.16 0.06Ce0.01/TiO2 A(90.7%) R(9.3%) 14.4(A) 47.33 66.76 11.13 0.13Ce0.02/TiO2 A 10.7 48.07 63.95 10.94 0.13Ce0.03/TiO2 A 10 81.56 113.15 10.50 0.21Ce0.04/TiO2 A 9.9 42.72 57.48 10.64 0.16Ce0.05/TiO2 A 5.1 39.81 59.32 9.65 0.1collage fiber 2.53 1.84 11.13 0.01P25 A,R 30(A) 49.5 8.3 0.09
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estimated by a linear fit of the slope to the abscissa are shown inFigure 6b. Along with the increase of Ce/Ti mole ratio from 0to 0.03, the band gap energy of the Cex/TiO2 nanofibers is
decreased from 2.756 to 1.678 eV. Obviously, the doping of Ceinto TiO2 nanofiber significantly reduces the correspondingband gap energy, that is much lower than that of thecommercial TiO2 (P25, 2.881 eV). Further increasing thedoped 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 2.442eV. As shown in the table inserted in Figure 6b, the band gapenergy was also increased when the mole ratio of Ce/Tibeyond 0.03, suggesting the increase of recombinationcenters.34 Hence, there is an optimal doping amount of Ce inthe Cex/TiO2 nanofibers, and this value is determined to be0.03 (Ce/Ti mole ratio) here.
Photodegradation of RhB under Visible Light Irradi-ation Using the Cex/TiO2 Nanofibers. Photodegradation ofRhB was used as the probe reaction to evaluate thephotocatalytic activity of the Cex/TiO2 nanofibers under visiblelight irradiation. As shown in Figure 7, the undoped TiO2nanofiber prepared using collagen fiber as template exhibitsrelatively higher activity as compared with the commercialDegussa P25. This fact confirms that the mesporous and fibrousstructure of TiO2 nanofiber will improve its catalytic activity.Under the same experimental conditions, the decoloration and
Figure 4. (a) Nitrogen adsorption/desorption isotherms of the Cex/TiO2 nanofibers. (b) BJH pore-size distributions of the samples.
Figure 5. XPS patterns of Ce 3d for Ce0.03/TiO2.
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 ofP25 and Cex/TiO2 nanofibers.
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mineralization degree of the TiO2 nanofibers are 40.37% and23.77%, respectively, in 80 min, while they are 35.9% and21.88% for Degussa P25, respectively. The relatively higherphotocatalytic activity of the undoped TiO2 nanofibers shouldbe attributed to the good mass transfer property derived fromthe mesoporous and fibrous structure. The doping of Ce intothe TiO2 nanofibers significantly improves the photocatalyticactivity, and the influence of Ce content (Ce/Ti mole ratio) onthe catalytic activity follows the order 0.03 > 0.02 > 0.01 > 0.04> 0.05 > 0. The most fundamental reason for this phenomenonis the change of band gap energy. Actually, as shown in theinset table in Figure 6b, the band gap energy of the Cex/TiO2nanofibers is decreased from 2.756 to 1.678 eV when the Ce/Timole ratio increased from 0 to 0.03. However, when the dopedamount of Ce kept increasing from 0.03 to 0.05 (Ce/Ti moleratio), the corresponding band gap energy subsequently rosefrom 1.678 to 2.442 eV, the overall results were consistent withthe change trend of the photocatalytic activity. However,doping Ce plays an important role in the interfacial chargetransfer and in the inhibition of electron−hole recombination.However, the cerium species may become the recombinationcenter of electron−hole pairs when excess cerium was doped.37
In addition, it is generally agreement that the thickness of thespace charge layer (W) surrounding the TiO2 particles wasincreased with increasing the doping content, and the Cex/TiO2 nanofibers can not be excited due to the superfluouscerium 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 lowerthan 0.03, the doping of Ce can enhance the electron−holeseparation and prolong the lifetime of the photoelectrons, thusincreasing the photocatalytic activity. However, when the moleratio of Ce/Ti exceeds 0.03, the photocatalytic activity of theCex/TiO2 nanofibers are decreased along with the increase ofCe amount. Obviously, the photocatalytic activities of the Cex/TiO2 nanofibers are closely consistent with the visible lightabsorption ability of the Cex/TiO2 nanofibers. The Ce0.03/TiO2nanofiber shows the highest photocatalytic activity undervisible-light irradiation with the decoloration degree of99.59% and mineralization degree of 77.59% in 80 min. TheTOC removal efficiency of the Ce0.03/TiO2 nanofiber is greatlyhigher than that of the undoped TiO2 nanofiber and theDegussa P25, which therefore confirms the successfulfabrication of visible-light active Cex/TiO2 photocatalyst byusing our strategy.
■ CONCLUSIONS
In this study, we provide a simple and effective method for thesynthesis of Ce-doped TiO2 mesoporous nanofibers by usingcollage fiber as the template. On the basis of FESEM and TEManalyses, the as-synthesized Cex/TiO2 nanofibers well dupli-cated the hierarchical structure and morphology of the naturecollagen fiber. N2 adsorption/desorption analysis revealed thatthe Cex/TiO2 nanofibers belonged to mesoporous materials.The UV−vis DR spectra confirmed that the visible lightabsorption ability and the band gap energy of the Cex/TiO2nanofibers could be adjusted by changing the amount of dopedCe. Because of these outstanding advantages, the Ce0.03/TiO2nanofiber exhibited the highest photocatlytic activity undervisible light irradiation as compared with the undoped TiO2nanofiber and the commercial Degussa P25.
■ ASSOCIATED CONTENT
*S Supporting InformationDetailed information about the correction of the instrumentalbroadening; EDX analysis results for the pure TiO2 and Cex/TiO2 nanofibers. This material is available free of charge via theInternet at http://pubs.acs.org.
NotesThe authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
We gratefully acknowledge the financial supports provided bythe National Natural Science Foundation of China (21176161and 2097611) and the National High Technology R&DProgram of China (863 Program, 2011AA06A108).
Figure 7. Photocatalytic degradation degree (a) and mineralizationdegree (b) of RhB (initial conccentration 0.1 mM, 500 mL, pH =6.53).
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The Journal of Physical Chemistry C Article
dx.doi.org/10.1021/jp312013m | J. Phys. Chem. C XXXX, XXX, XXX−XXXH
