Facets for Enhanced Visible Light Photocatalytic Activity - American

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One-pot Fabrication of C-Fe-codoped TiO2 Sheets with Dominant {001} Facets for Enhanced Visible-light Photocatalytic Activity Dong Yang, Yuanbing Li, Zhenwei Tong, Yuanyuan Sun, and Zhongyi Jiang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie503705w • Publication Date (Web): 14 Nov 2014 Downloaded from http://pubs.acs.org on November 20, 2014

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Industrial & Engineering Chemistry Research

One-pot Fabrication of C-Fe-codoped TiO2 Sheets with Dominant {001} Facets for Enhanced Visible-light Photocatalytic Activity Dong Yang,† Yuanbing Li,† Zhenwei Tong,‡ Yuanyuan Sun,† Zhongyi Jiang,*,‡,§ †

Key Laboratory of Systems Bioengineering of Ministry of Education, School of Chemical

Engineering and Technology, Tianjin University, Tianjin 30072, China; ‡

Key Laboratory for Green Technology, School of Chemical Engineering and Technology, Tianjin

University, Tianjin 300072, China; §

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072,

China) *Corresponding author. Tel.: +86 22 2350 0086; Fax: +86 22 2350 0086; E-mail: [email protected]

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ACS Paragon Plus Environment


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ABSTRACT: C-Fe-codoped TiO2 sheets (CFTS) with dominant {001} facets have been synthesized by a one-pot hydrothermal approach using TiC and Fe2O3 as the reactants in a HNO3-HF mixed solution. The obtained CFTS samples appear as the aggregates of highly truncated bipyramidal microsheets with the length of 1-2 µm and the thickness of 100-200 nm. The doped C atoms exist as O-Ti-C structure or interstitial C in TiO2 lattice; while the doped Fe atoms replace some of Ti4+ to form Ti-O-Fe structure. In comparison with the C-doped TiO2 sheets, all CFTS samples exhibit the narrower band gap, stronger visible-light absorption and higher separation efficiency of photo-generated carriers. Moreover, when the atomic ratio of Fe to Ti is 1%, the CFTS sample shows the highest photocatalytic activity by measuring the decomposition rate of RhB under visible light. These CFTS material may become promising visible-light photocatalyst for organic pollutant degradation and efficient H2 evolution. KEYWORDS: TiO2 sheet; C-Fe-codoping; {001} facets; visible light; photocatalysis

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1. INTRODUCTION Morphological design and control of TiO2 with dominant reactive facets have gained broad exploitation in the past few years, because the performances of TiO2, such as photocatalysis and photoelectric conversion, are susceptible to the surface energy and surface atomic structure.1-4 In 2008, a crucial progress was made by Lu and co-workers,1 who first synthesized anatase TiO2 single crystal microsheets with 47% of {001} facets by using hydrofluoric acid (HF) as a morphology controlling agent under hydrothermal conditions. Thereafter, tremendous efforts have been devoted to synthesize anatase TiO2 micro- and nanosheets with high percentage of reactive {001} facets using this surface fluorination strategy.5-12 Recently, nonmetal-doped anatase TiO2 sheets with dominant {001} facets, including N-, C-, S-doped and C-N-codoped TiO2 sheets, have been successfully synthesized by an in situ hydrothermal method,13-17 in order to triggering the photocatalytic reaction in the visible light region. In such synthetic procedure, the crystalline compound of titanium with nonmetals, such as TiN, TiC, TiS2 and TiCN, were employed as both TiO2 precursor and nonmetal resource, so as to realize the nonmetal self-doping in the lattice of TiO2 sheets. In particular, C doping can produce a deeper state (a decrease about 1.00 eV)15 in the band gap of TiO2 than N or S doping, thus leading to a larger red shift of absorption edge and more efficient visible light response.18, 19 These nonmetal-doped TiO2 sheets exhibited high efficiency for the photocatalytic degradation of organic pollutants and splitting water into H2 under visible light irradiation. However, the partially occupied impurity level produced by 3



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nonmetal doping can act as the recombination center, which lowers the quantum efficiency and reactivity of TiO2 photocatalysts.20-24 It is recognized that the metal and nonmetal codoping can reduce the recombination center, because metal ions can play as the mediator for the interfacial charge transfer to prevent the recombination of electron–hole pairs.24-26 Among numerous transition metal dopants, Fe3+ has been considered as a promising candidate, since the Fe3+ dopant can form the shallow charge trap to reduce the electron-hole recombination, and enhance the photocatalytic activity.27-29 Moreover, the ionic radius of Fe3+ (0.785 Å) is similar to that of Ti4+ (0.745 Å), making it easily incorporate into the crystal lattice of TiO2.30 For example, C-Fe-codoped TiO2 fine particles were immobilized on the cotton material by heating in argon at high temperature, which demonstrated higher photocatalytic activity than pure TiO2 for decomposing phenol under UV irradiation.31 Wu et al.32 prepared C-Fe-codoped TiO2 nanoparticles by a sol–gel solvothermal method, which showed enhanced visible photocatalytic performance for degrading acid orange 7 under visible light irradiation. However, to the best of our knowledge, the uniform C-Fe-codoped TiO2 sheets with dominant {001} facets have never been reported until now. In this study, a facile one-pot hydrothermal method is developed to synthesize C-Fe-codoped TiO2 sheets with dominant {001} facets using TiC and Fe2O3 as the precursors in a HF-HNO3 mixed solution system. TiC is chosen as the precursor of TiO2, because it can also be used as the source of C dopant at the same time. Fe2O3 is selected as the Fe3+ source, since it can reversibly react with HNO3 to slowly generate 4


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Fe3+, resulting in the doping of Fe3+into anatase TiO2 sheets without apparently affecting their morphology.33 It is expected that the photocatalytic activity of TiO2 sheets with dominant {001} facets can be effectively improved via the codoping of C and Fe under visible light.

2. EXPERIMENTAL 2.1 Materials. TiC was purchased from Tianjin HEOWNS Company and Fe2O3 was purchased from Tianjin Guangfu Company. All chemicals were of analytical grade, and used without further purification. Doubly deionized water was used in all experiments. 2.2 Preparation of C-Fe-codoped TiO2 Sheets. C-Fe-codoped TiO2 sheets (CFTS) with dominant {001} facets were synthesized by a hydrothermal method. In a typical synthesis procedure, 0.9 g of TiC powder and a certain amount of Fe2O3 were added in 45 mL of HF (1 mol L-1) and HNO3 (1.2 mol L-1) aqueous solution. The suspension was sealed in a 100 mL Teflon-lined stainless steel autoclave at room temperature, followed by keeping at 180 oC for 16 h. Subsequently, the grey precipitate was collected by centrifugation, and washed with ethanol and deionized water for three times to remove dissoluble ionic impurities. After dried at 100 oC in an oven for 6 h, the CFTS samples were acquired at last. Three CFTS samples were prepared through changing the molar ratios of Fe to Ti (0.5%, 1.0% and 1.5%), which are indicated as CFTS-0.5, CFTS-1.0 and CFTS-1.5, respectively. For the purpose of comparison, the C-doped TiO2 sheets (CTS) with dominant {001} facets were synthesized by the same 5



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process without adding Fe2O3 in the reaction system. The TiO2 nanosheets (TNS) were fabricated by a hydrothermal method using tetrabutyl titanate as the precursor and HF (40 wt%) as the capping agent at 180 oC for 24 h.8-10 2.3 Characterization. X-ray diffraction patterns of the samples were obtained by a Rigaku D/max 2500 X-ray diffractometer using Co-Kα irradiation at a scan rate of 4 o min-1 with an accelerating voltage of 40 kV and a current of 40 mA. Scanning electron microscopy (SEM) was performed on a Nanosem 430 instrument under vacuum. Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) were performed on a JEM-2100F electron microscope at an acceleration voltage of 200 kV. The BET surface area of TiO2 samples were evaluated by N2 adsorption–desorption isotherm measurements performed on a Tri-Star 3000 gas adsorption analyzer. Prior to the measurement, the samples were pretreated at 180 oC for 2 h. Chemical compositions of the samples were analyzed by using X-ray photoelectron spectroscopy (XPS, Perkin-Elmer PHI 1600 ESCA) with a monochromatic Mg Kα radiation (1253.6 eV). All binding energies were referenced to the C1s peak (284.6 eV) arising from adventitious carbon. The Raman spectra were performed in a Raman spectrometer (Thermal DXR Microscope) using an Ar+ laser with a wavelength of 532 nm. The UV-vis diffuse reflectance spectroscopy (DRS) were recorded in the range of 200−800 nm with a UV-vis spectrophotometer (U-3010, Hitachi), and BaSO4 was used as the reference. Photoluminescence (PL) fluorescence spectra were obtained at the room temperature using a Fluorolog3-21 spectrometer (Jobin Yvon), and the excitation wavelength is 320 nm. 6


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2.4 Photocatalytic Experiment. Photocatalytic activity of TiO2 samples was evaluated by measuring the decomposition rate of Rhodamine B (RhB) at room temperature under visible light. In a typical experiment, 60 mg photocatalysts were added in a 30 mL of 15 mg L-1 RhB solution. The suspension was magnetically stirred for 1 h in the dark to reach the adsorption-desorption equilibrium before visible light irradiation. A 500 W xenon lamp, as the simulated visible light source, was put in a cylindrical glass vessel with a circulating water glass jacket, and a 420 nm cutoff filter was positioned outside the cylindrical glass vessel to remove any ultraviolet radiation wavelengths below 420 nm. After visible light irradiation, 2 mL of reaction suspension was extracted and centrifuged to measure the residual concentration of RhB by an UV-vis spectroscopy (U-3900, Hitachi) at 553 nm every 20 min.

3. RESULTS AND DISCUSSION 3.1 Crystal Structure and Morphology. The XRD technique was used to analyze the crystallographic information of TiO2 products. The XRD patterns of TiC, TNS, CTS, and CFTS are presented in Fig. 1a. The cubic phase (JCPDS file No. 65-5408) can be identified from the XRD curve of TiC. No diffraction peak of TiC is observed in the patterns of CTS and CFTS, indicating that the TiC precursor is oxidized completely by HNO3 in the hydrothermal process. It was reported that TiC was oxidized into TiO2 by HNO3 as the following equation expressed:15, 19 (1) All the XRD patterns of CTS and CFTS match the anatase phase (JCPDS file No. 7



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21-1272). No diffraction peak that is ascribed to crystalline phase containing iron (Fe2O3 and Fe2TiO5), emerges in the curve of CFTS samples, suggesting that Fe3+ inserts into the TiO2 lattice, and replaces some of Ti4+ sites.34 The relative crystallinity of CTS and CFTS samples are calculated using the (101) diffraction peak of anatase, and the results are listed in Table 1. It is observed that the crystallinity of CFTS is lower than that of CTS, and decreases gradually with increasing the doping amount of Fe3+, indicating that the Fe3+ incorporation prevents the crystallization of TiO2 in the hydrothermal process. The enlarged (101) diffraction peaks of CTS and CFTS are shown in Fig. 1b. Compared with CTS, the (101) diffraction peak of CFTS slightly shifts toward the small angle, and reduces little by little along with the increase of Fe doping amount, indicating the expansion of the crystal lattice. The anatase lattice parameters are calculated based on the (101) and (200) diffraction peaks,35 and listed in Table 1. It can be seen that the parameter c enhances a little with the Fe doping, while the parameter a and b are almost unchanged. These results prove that the doping Fe atoms substitute the Ti atoms in the lattice of TiO2. Figure 2 shows typical SEM images of TiC, CTS and CFTS-1.0. The precursor TiC appears as a large, irregular and compact monolith (Fig. 2a). After the hydrothermal reaction in HNO3-HF solution, both CTS (Fig. 2b) and CFTS-1.0 (Fig. 2c) display as the aggregates of highly truncated bipyramidal microsheets with the length of 1-2 µm and thickness of 100-200 nm, which are larger than those in the previous work.15, 17 Their similar morphology indicates that the incorporation of Fe into anatase TiO2 sheets does not affect the growth of TiO2 microsheets. According to the equilibrium 8


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shape predicted by Wulff construction and the highly truncated octahedral bipyramidal morphology of anatase TiO2 crystal,36 the flat and square surfaces on the sheet can be ascribed to {001} facets, and other isosceles trapezoidal surfaces are {101} facets. The percentage of {001} facets in the CTS and CTFS is calculated according to the following formula:1, 33 S 001 = 2 A2

(2) 2

2

2( B − A ) cos θ S 001 P001 = S 001 + S101

S101 =

(3) (4)

where A and B are the average lengths of square {001} and bipyramid facet sides, respectively, which can be measured from SEM images; S001 and S101 are the total areas of {001} and {101} facets in a TiO2 single sheet, respectively; θ is the theoretical angle between the {001} and {101} facets of anatase; P001 is the percentage of dominant {001} facets. The percentage of {001} facets in the CTFS-1.0 is calculated to be about 62%，which is slightly smaller than that of CTS (65%). Theoretical prediction and experimental studies indicated that the {001} facet of anatase TiO2 was highly reactive, on which 100% Ti atoms are 5-coordinated Ti (Ti5c) and the Ti-O-Ti bond angle is very large, because of its high surface energy and unique surface structure.37,

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Although the percentage of {001} facets can be

regulated as high as 98.7%,39 the photocatalytic activity of anatase TiO2 microsheets is not completely proportional to it. Usually, TiO2 sheets with 50~80% of {001} facets exhibit high photocatalytic activity,40 therefore, the as-prepared TiO2 sheets with about 60% of {001} facets are suitable for the photocatalytic application. 9



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Figure 3a illustrates the TEM image of a single TiO2 sheet of CFTS-1.0, clearly confirming

the

highly

truncated

octahedral

bipyramidal morphology.

The

corresponding selected area electron diffraction (SAED) pattern (inset) is indexed into the diffraction spots of [001] zone, demonstrating that the flat and square surface belongs to the (001) plane of anatase TiO2. The HRTEM image (Fig. 3b) recorded on the marked area in Fig. 3a exhibits the (020) and (200) atomic planes with a lattice spacing of 0.19 nm and a vertical angle. It was reported that the surface-adsorbed fluorine atoms could stabilize the {001} facets of anatase TiO2 via reducing the surface energy, and hence enlarging its high percentage.1 Irrespective of the etching effect of HF on the {001} facets under the high HF concentration, more atomic ratio of F/Ti led to higher percentage of dominant {001} facets.5, 7, 9 However, the addition of HNO3 and Fe2O3 only has a little influence on the percentage of {001} facets, which can be attributed to the competitive adsorption of NO3- on the {001} facet with F-. Figure 4 illustrates the nitrogen adsorption–desorption isotherms and corresponding pore-size distribution curves (inset) of CTS and CFTS-1.0. It is observed that both of them exhibit type IV adsorption isotherms with a H3 hysteresis loop at high relative pressure 0.45 < P/P0 < 0.99, which indicates the presence of mesopores in the products.10, 41, 42 The pore-size distribution curves reveal that both of them contain two mesopores with the diameter about 3.7 and 9.2 nm. As we all know, the single crystal TiO2 microsheets are nonporous, therefore, the formation of these slit-like mesopores in CTS and CFTS is ascribed to the aggregation of TiO2 sheets. Such organized 10


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porous structures would provide efficient transport pathways for the reactants to enhance the photocatalytic performance. As listed in Table 1, the specific surface area of CFTS samples is a little lower than that of CTS. This result can be assigned to the fact that the Fe doping inhibits the growth of anatase TiO2 crystallites, and accelerates the aggregation of TiO2 sheets. The surface elemental composition and binding state of CFTS samples was investigated by XPS using CFTS-1.0 as a representative (Fig. 5). The survey XPS spectrum in Fig. 5a reveals that the CFTS contains Ti, O, C, F and Fe elements, affirming the successful doping of C and Fe. The high-resolution Ti2p XPS spectrum consists of two peaks at 458.4 and 464.1 eV (Fig. 5b), which are assigned to the binding energy of Ti2p3/2 and Ti2p1/2, respectively, indicating that the titanium element exists in the form of Ti4+ in CFTS. The typical Ti2p peak of TiC at about 455.2 eV is not observed, further demonstrating that TiC is completely transformed into TiO2 in the hydrothermal treatment, in consistence with the XRD results. The binding energy of F1s core electrons at 684.7 eV (Fig. 5c) can be attributed to residual surface Ti–F species.2, 10, 13 The carbon species display three bonding states at 282.1, 284.6 and 288.3 eV, respectively (Fig. 5d). Generally, the C1s peak at 284.6 eV is assigned to the adventitious carbon; the peak around 282.1 eV is ascribed to the carbon replacing the oxygen in the lattice of TiO2 to form O-Ti-C bonds;15, 19, 43 the peak at 288.3 eV indicates the presence of C-O bonds, which is assigned to the carbon substituting for some of the lattice titanium atoms to form the Ti-O-C structure.19, 44, 45 In the Fe2p spectrum (Fig. 5e), two peaks at 713.7 and 709.9 eV should be assigned to 11



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Fe2+2p3/2 and Fe3+2p3/2, respectively.46, 47 Compared to that in Fe2O3 (710.7 eV), the Fe3+2p3/2 peak in CFTS shifts to the low energy, which is attributed to the Fe3+ substituting for some Ti atoms in the lattice to form the Fe–O–Ti structure.48 The formation of Fe2+ may be caused by the photocatalytic reduction of Fe3+ to Fe2+ by the conduction band electron produced during the XPS measurement in vacuum.46 In order to further confirm the crystal phase and surface chemical composition of CFTS samples, their Raman spectra were conducted. The Raman spectrum of anatase TiO2 single crystal often has six typical active modes with 144 (Eg), 197 (Eg), 399 (B1g), 515 (A1g), 519 (B1g, overlapped with the 515 cm−1 band), and 639 cm-1 (Eg).9 As shown in Fig. 6, CFTS has the similar vibration mode to TNS and CTS, which exhibits five peaks at 151, 199, 399, 519, and 639 cm−1, respectively. Compared with TNS, the Eg mode of CTS at 149 cm-1 obviously shifts to the higher wavenumber, which is caused by the C doping into the lattice of TiO2.49, 50 With the additional incorporation of Fe3+ in the TiO2 lattice, this Eg mode of CFTS shifts to the higher frequency of 151 cm-1, again. This phenomenon can be assigned to the formation of Ti–O–Fe, which changes some force constants or results in the production of oxygen deficiency.51 It was reported that the higher the intensity of the B1g and A1g peaks in the Raman spectra was, the higher the percentage of exposed {001} facets was.52 Therefore, the percentage of exposed {001} facets is in the order of TNS > CTS > CFTS-1.0, in accordance with the calculated results (Table 1).

3.2 Optical Properties The changed band structure of TiO2 samples after metal or nonmetal doping is usually investigated by UV-Vis diffuse reflectance spectra. As 12


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displayed in Fig. 7a, all CTS and CFTS samples illustrate much higher visible light absorbance between 400 and 730 nm in comparison to TNS. Furthermore, an obvious long-tail absorption at about 420 nm in the overall visible region is observed, indicating the existence of carbonate species that are formed by the carbon incorporation into the interstitial position of TiO2 lattice.15,

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The band gap of

as-prepared TiO2 samples is estimated from the intercept of the tangents to the plot of the Kubelka-Munk function versus photon energy (hv) since TiO2 is an indirect semiconductor (Fig. 7b). The band gaps of TNS, CTS, CFTS-0.5, CFTS-1.0 and CFTS-1.5 are 3.19, 3.04, 3.01, 2.98 and 2.98 eV, respectively. It was known that the carbon substituting O sites formed localized impurity states above the valance band edge of TiO2, which led to the absorption threshold shift to the visible region. However, the band gap of CTS is much higher than the theoretical value of 2.20 eV based on the calculation of Yu et al.,15 which may be caused by much lower experimental C-doping concentration than that in the theoretical model. The additional Fe doping makes the band gap narrow further, which is assigned to the formation of the Fe3+ impurity level below the conduction band minimum.20, 25 The band gap of CFTS decreases along with increasing the Fe doping ratio, however, it is almost unchanged as the Fe/Ti molar ratio exceeds 1%. The separation efficiency of photo-induced carriers of TiO2 samples was studied by PL spectra, and demonstrated in Fig. 8. It is observed that all TiO2 samples present obvious PL signal with the similar curve shape under the excitation at 320 nm. However, the emission positions of TNS are different from those of the CTS and 13



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CFTS samples, confirming that the electronic state within the band gaps are modified by C and/or Fe doping.53 The PL intensity of CTS is the highest among all the samples, indicating the highest recombination probability of photo-induced electrons and holes.54,

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This result suggests that the sole C doping cannot overcome the

recombination of photo-induced electrons and holes, even though it can improve the conductivity of TiO2 to facilitate the charge diffusion from the bulk to the surface where the heterogeneous photocatalytic reaction takes place. The CFTS samples exhibit the lower PL intensities than CTS and TNS, indicating the lower recombination probability of photo-generated electrons and holes. This result should be attributed to the doping of Fe3+, which can act as the electron-enriched center, and thus effectively improving the separation of photo-generated carriers. In three CFTS samples, the CFTS-1.0 shows the lowest PL intensity, implying that there is a suitable doping amount of Fe3+, because the exceeded Fe3+ ion can also become the recombination center of photo-generated electrons and holes.

3.3 Photocatalytic Activity The visible-light photocatalytic activity of all TiO2 samples was evaluated by the decomposition of RhB in aqueous solution, and the results are depicted in Fig. 9a. In the absence of photocatalyst, the RhB concentration remains almost unchanged within 2 h under visible irradiation, indicating that RhB is hardly self-degradable under this condition. After stirring in the dark for 60 min to reach the saturated adsorption, all TiO2 samples show a similar adsorption capacity, although they have different specific surface areas and sizes. The kinetic curves for the RhB degradation over all TiO2 samples are plotted, and illustrated in Fig. 9b. All 14


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of them fit well with the pseudo-first order correlation, ln (C0/C) = kt, where k is the apparent reaction rate constant, C0 and C are the initial and instantaneous concentrations of RhB, respectively. Based on the k values, the photocatalytic activity is in the order of CFTS-1.0 > CFTS-0.5 > CFTS-1.5 > CTS > P25 > TNS. Although pure anatase TiO2 cannot absorb visible light due to the large band gap of 3.20 eV, the degradation efficiency of RhB over TNS is ca. 22% within 2 h, which is assigned to the dye photosensitization. That is, the photo-induced electrons coming from RhB molecules can inject into the conduction band of anatase TiO2, and then are captured by the surface adsorbed O2 to generate active species for the RhB degradation. The CTS has much higher photocatalytic activity than TNS, due to its narrow band gap and corresponding capability of absorbing visible light. After Fe doping, all CFTS samples exhibit enhanced photocatalytic activities in comparison to CTS, which is caused by the fact that the Fe3+ doping effectively improves the separation of photo-generated carriers.

4. CONCLUSIONS In this study, the C-Fe-codoped TiO2 sheets with about 60% of anatase {001} facets have been successfully synthesized by a one-pot hydrothermal method for the first time. In the prepared CFTS samples, the self-doped C atom exists mainly as the O-Ti-C structure and carbonate species, while the doped Fe3+ ion forms the Ti-O-Fe structure. Compared with CTS, the CFTS exhibits the narrower band gap, stronger visible light absorption and higher separation efficiency of photo-generated electrons 15



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and holes, which endow them higher RhB degradation efficiency under visible light. In all the as-prepared TiO2 samples, the CFTS with the Fe/Ti molar ratio of 1% exhibits the highest photocatalytic activity, which can degrade 61.2% RhB molecules under visible light within 2 h. It is underway to prepare other metal and nonmetal codoped TiO2 sheets with dominant anatase {001} facets by this approach, which may become a general technology for synthesizing the high-efficiency visible-light photocatalyst.

ACKNOWLEDGMENTS This work was supported by National Science Fund for Distinguished Young Scholars (21125627), the National Basic Research Program of China (2009CB724705) and the Program of Introducing Talents of Discipline to Universities (No.B06006).

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Figure Captions Figure 1. (a) XRD patterns of TiC, TNS, CTS and CFTS-1.0; (b) the enlarged {101} diffraction peaks of CTS, CFTS-0.5, CFTS-1.0 and CFTS-1.5. Figure 2. Typical SEM images of (a) TiC, (b) CTS and (c) CFTS-1.0. Figure 3. TEM (a) and HRTEM image (b) of CFTS-1.0. The inset in Fig. 3a is the selected area electron diffraction pattern. Figure 4. N2 adsorption–desorption isotherms and the corresponding pore-size distribution curves (inset) of CTS and CFTS-1.0. Figure 5. XPS spectra of CFTS-1.0: (a) survey spectrum, (b) Ti2p, (c) F1s, (d) C1s and (e) Fe2p. Figure 6. Raman spectra of (a) TNS, (b) CTS and (c) CFTS-1.0. Figure 7. UV–vis diffuse reflectance spectra of (a) TNS, CTS, CFTS-0.5, CFTS-1.0 and CFTS-1.5 and (b) their corresponding plots of [F(R∞)hv] 1/2 vs. hv. Figure 8. Photoluminescence spectra of TNS, CTS, CFTS-0.5, CFTS-1.0 and CFTS-1.5. Figure 9. Photocatalytic activities of P25, TNS, CTS, CFTS-0.5, CFTS-1.0 and CFTS-1.5 for the RhB degradation under visible light irradiation (a) and their kinetic curves (b).

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Table 1. Physicochemical properties of the prepared TiO2 samples. Samples

Crystallinity a

Percentage of exposed {001} facet

SBET (m2 g-1)

Lattice parameters (Å)

Band gap (eV)

TNS

0.41

75%

84

3.19

0.11

CTS

1

65%

13.1

3.04

0.335

CFTS-0.5

0.99

63%

10.5

3.01

0.404

CFTS-1.0

0.97

62%

10

2.98

0.466

CFTS-1.5

0.68

62%

10.3

a=b=3.786 c=9.490 a=b=3.792 c=9.515 a=b=3.792 c =9.527 a=b=3.796 c =9.571 a=b=3.797 c=9.605

2.98

0.352

a

kb (h-1)

The crystallinity of TiO2 is calculated using the {101} diffraction peak of anatase, and CTS is used as a reference sample; b The apparent first order rate constant for the RhB degradation.

18


(204)

(105) (211)

(103) (004) (112)

CFTS-1.0

(200)

(101)

a Relative intensity

CTS

30

(200)

TiC

20

40

(220)

TNS

(111)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50

60

70

2 Theta /degree

b Relative intensity

Page 19 of 35

CFTS-1.5

CFTS-1.0 CFTS-0.5 CTS

24.5

25.0

25.5

26.0

2 Theta /degree

Figure 1.

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. 20


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3.

21


-1

30

CTS CFTS-1.0

3.7 9.2

25

3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Volume adsorbed /cm g , STP


20 15 10

1

10 Pore size /nm

100

5 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure /P/P0

Figure 4.

22


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O1s Ti2s

Ti3s Ti3p

C1s

Ti2p1/2 Ti2p3/2

Intensity /a.u.

Fe2p F1s

a

1000

800

600

400

200

0

Binding energy /eV

458.4

Intensity /a.u.

b

Ti2p

464.1

468

464

460

456

452

Bind energy /eV

c

F1s

684.7

Intensity /a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


690

688

686

684

682

Binding energy /eV

d

C1s

284.6

Intensity /a.u.

Page 23 of 35

288.3

292

290

288

282.1

286

284

282

280

Binding energy /eV

23



720

Fe2p

709.9

e 713.7

Intensity /a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

716

712

708

704

700

Binding energy /eV

Figure 5.

24


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Eg

Intensity /a.u.

Page 25 of 35

c

Eg

B1g

200

400

A1g

Eg

b a

100

150

500 -1

600

700

Wavenumber /cm

Figure 6.

25



1.2

a

TNS CTS CFTS-0.5 CFTS-1.0 CFTS-1.5

Absorbance /a.u.

1.0 0.8 0.6 0.4 0.2 0.0 300

400

500

600

700

Wavelength /nm

5

b

1/2

4

[F(R∞)hv]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3 2


1 0 2.0

2.5

3.0

3.5

4.0

4.5

Band energy /eV

Figure 7.

26


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Intensity /a.u.

Page 27 of 35

400

500

600

700

Wavelength /nm

Figure 8.

27



1.0 a

C/C0

0.8

Blank TNS P25 CTS CFTS-0.5 CFTS-1.0 CFTS-1.5

0.6

0.4

0.2 -60

0

20

40

60

80

100

120

Time /min

0.0 b -0.2

ln(C/C0)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

-0.4

Blank TNS P25 CTS CFTS-0.5 CFTS-1.0

-0.6 -0.8 -1.0

-1

k=0.005 h -1 k=0.11 h -1 k=0.267 h -1 k=0.335 h -1 k=0.404 h -1 k=0.466 h -1

CFTS-1.5 k=0.352 h

0

20

40

60

80

100

120

Time /min

Figure 9.

28


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Facets for Enhanced Visible Light Photocatalytic Activity - American

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