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S-doped Na2Ti6O13@TiO2 core-shell nanorods with an enhanced visible light photocatalytic performance Chao Liua, Ji-yuan Lianga, Rui-rui Hana, Yong-zheng Wanga, Jin Zhaoa, Qian-jin Huangb, Jing Chen*b, Wen-hua Hou*a 5
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x S-doped Na2Ti6O13@TiO2 (S-TTO) core-shell nanorods, with exposed anatase TiO2 {101} facets, were synthesized by a facile calcination method. It was found that the addition of thiourea as the sulfur precursor was beneficial for the formation of anatase TiO2 with a better crystallinity and the doped sulfur atoms favorably stabilized the anatase structure. The substitution of Ti4+ by S6+ in the lattice of S-TTO gave rise to the visible light response and increased the amount of active groups typically as hydroxyl radical adsorbed on the catalyst surface. With the formation of Ti−O−S bond, partial electrons could be transferred from S to O atoms. The electron-deficient S atoms might capture e− and thus inhibited the recombination of photogenerated electron-hole pairs. Meanwhile, the closely-contacted interface was formed between Na2Ti6O13 and anatase TiO2, resulting in a nanoscale heterojunction structure to speed up the separation rate of photogenerated charge carriers. The exposed anatase {101} facets could be acted as a possible reservoir of the photogenerated electrons, yielding a highly reactive surface for the reduction of O2 to O2•− and thus the decrease of recombination probability of electron-hole pairs. In addition, the anisotropically shaped titanate nanorods provided a pathway for the quick transport of charge carriers throughout the longitudinal direction. The combined effects of S doping, nanoheterojunction formation and morphology engineering led to an obviously enhanced photocatalytic performance for the degradation of methylene blue (MB) solution under visible light irradiation. The corresponding photocatalytic mechanism was investigated and discussed in detail. The present work may provide an insight for the fabrication of delicate composite photocatalysts with an excellent performance.
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Introduction
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Semiconductor−based photocatalysis technology has been attracting considerable interest for environmental remediation and clean energy production.1-3 Among various semiconductors, TiO2 has been investigated extensively and intensively for its superior chemical stability, excellent photocatalytic performance, commercial availability, non-toxicity and so on.4-7 However, the narrow light-response range (UV region due to its large band gap of 3.0-3.2 eV) and the fast recombination rate of photogenerated electron−hole pairs greatly reduce the photonic efficiency and represent a major bottleneck for the photocatalytic application of TiO2. Thus, much research has been dedicated to the construction of novel TiO2−based visible light-driven photocatalysts.8 The stoichiometry-dependent structure and physicochemical properties of alkali metal titanates endow them with a broad range of applications, such as photocatalyst,9-11 lithium and sodium ion battery,12-14 gas sensing and antibacterial agent,15 etc. However, as photocatalysts, Na2Ti6O13 itself has a relatively wide bandgap and is thus active only under UV light. In addition, it also shows a high rate of electron-hole recombination. Through This journal is © The Royal Society of Chemistry [year]
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morphology engineering, Na2Ti6O13 nanorods were prepared and found to be suitable both for combination with catalytic active phase (e.g., metal oxides) to raise the separation efficiency of photoexcited charge carriers and for quick transport of charge carriers throughout the longitudinal direction to different reaction sites for subsequent reactions.10 During the past decade, nonmetal doping, morphology control and heterojunction coupling have all been reported to improve the photocatalytic activity, respectively.8, 16-18 The nonmetal doping was an effective modification strategy for the improvement of visible-light-driven photocatalytic activity due to the band gap narrowing.19-21 Recently, Umebayashi et al. have succeeded in preparing visible light-activated S-doped TiO2,22-24 in which sulfur was doped as S2－ and replaced the lattice oxygen in TiO2. On the contrary, it was found that S atoms were incorporated as cations and replaced Ti ions in S-doped TiO2 photocatalysts.25, 26 Undeniably, S doping has attracted extensive attention in order to enhance visible-light photocatalytic activity. On the other hand, by integrating different individual materials within the same structure, heterojunction photocatalysts can incorporate multiple [journal], [year], [vol], 00–00 | 1
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functionalities and effectively speed up the separation rate of photogenerated charge carriers, giving rise to an improved photocatalytic activity.27 In addition, morphology control, particularly in order to obtain the specifically exposed crystal facets, has been developed to enhance the photocatalytic performance.28-31 Thus, it is reasonable to expect that novel efficient visible-light photocatalysts can be designed through the integration of nonmetal doping, heterojunction formation and morphology engineering strategies. For example, it has been reported that the introduction of nitrogen dopant localized in the crystal lattice of both anatase and titanate of titanate-anatase coreshell nanobelts presents a high photocatalytic activity under visible light irradiation, which is largely attributed to N doping, the preferentially grown TiO2 nanorods shell structures and titanate-anatase hetero-interfaces.31 Our group also reported that a novel photocatalyst of N-doped Na2Ti6O13@TiO2 core-shell nanobelts showed an enhanced visible-light photocatalytic performance due to the combination effects of N doping, heterojunction formation and morphology control.32 Herein, we further report that S-doped Na2Ti6O13@TiO2 coreshell nanorods exhibit an excellent photocatalytic activity in the degradation of MB under visible-light irradiation. The structure, surface morphology and photocatalytic performance of the resultant samples have been studied and discussed in detail. Moreover, a possible photocatalytic mechanism is proposed based on the experimental results.
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Photocatalytic reaction tests
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Experimental section Synthesis of titanate and S-doped Na2Ti6O13@TiO2 core-shell nanorods 30
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Na2Ti3O7 was synthesized by calcinating a stoichiometric mixture of Na2CO3 and TiO2 (anatase) in a molar ratio of 1.1:3 at 900 °C for 24 h in air with a heating rate of 10 °C min−1.33 The resultant power was reground, and the same temperature program sequence was repeated once again. To prepare S-doped Na2Ti6O13@TiO2 core-shell nanorods (denoted as S-TTO), 2.0 g of Na2Ti3O7 was first dispersed in ethanol (40 g mL−1) and stirred for 12 h. Then, 6.0 mL of titanium isopropoxide, Ti(O-i-Pr)4, was added dropwise into the above suspension under stirring. After 12 h, the resultant suspension was transferred to a petri-dish in order to allow ethanol evaporation at room temperature. Finally, the resultant dried white solid sample (1.0 g) was finely milled with thiourea (2.0 g) and calcined in air at 500 °C for 10 h to get the target sample.34 For comparison, undoped sample (denoted as TTO) was also prepared through a similar process in which no thiourea was added.
at a scanning rate of 0.2° s−1 in a 2θ range of 5~60°. UV–vis diffuse reflectance spectra were obtained on a UV–vis spectrophotometer (Shimadzu, UV-2401) using BaSO4 as a reference. X-ray photoelectron spectroscopic (XPS) analysis was carried out on an X-ray photoelectron spectrometer (Thermo Fisher Scientific, K-Alpha) equipped with a hemispherical electron analyzer (pass energy of 20 eV) and an Al Kα (hν = 1486.6 eV) X-ray source. The binding energies (BE) were referenced to the adventitious C 1s peak (284.6 eV) which was used as an internal standard to take into account charging effects. A combination of Gaussian and Lorentzian functions was used to fit the curves. Photoluminescence (PL) spectra were recorded on a 48000DSCF luminescence spectrometer at room temperature by using a continuous-wave 325 nm He-Cd laser as the excitation source. The samples were pressed into a thin disk and fixed in a homemade quartz cell. Photocurrent was measured on a CHI 660D electrochemical workstation (Shanghai Chenhua Apparatus Corporation, China) with a three-electrode system in which a 0.25 cm2 modified ITO as working electrode, a Pt wire as counter-electrode and a saturated Ag/AgCl electrode as reference electrode.
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The visible light photocatalytic activity was evaluated by the degradation of MB in aqueous solution. Firstly, 50 mg of catalyst was suspended in an MB aqueous solution (1.0 × 10−5 mol L－1) in the dark for more than 1 h, in order to gain the adsorptiondesorption equilibrium. After the adsorption equilibrium of MB, the catalyst was separated and re-dispersed in an aqueous solution of MB (100 mL, 1.0 × 10−5 mol L － 1) in a beaker with a circulating water system to remove the thermal effect of light. Light from a 300 W Xe lamp, passed through a UV light filter film (to remove radiation with λ< 420 nm), was focused onto the reaction cell. When the light was turned on, at given time intervals, approximately 4 mL of the reaction suspension was sampled and separated by means of high-speed centrifugation. The filtrates were analyzed by recording the maximum absorbance at 664 nm in the UV-visible spectrum of MB. The degradation efficiency at time t was determined from the value of Ct/C0, where C0 is the initial concentration and Ct is the concentration of MB at time t. The active species capture experiments were employed to study the photocatalytic mechanism. Silver nitrate (AgNO3, 1 mmol L－1), sodium oxalate (Na2C2O4, 0.5 mmol L－1), tert-butyl alcohol (t-BuOH, 5 mmol L－1) and p-benzoquinone (BQ, 1 mmol L－1) were added to the MB aqueous solution, respectively. Then, the remaining experimental processes were similar to the above photocatalytic test.
Results and discussion
Characterization
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The morphology was investigated by scanning electron microscopy (SEM, JEOL JEM-6300F) and transmission electron microscopy (TEM, JEOL JEM-200CX, operating at an accelerating voltage of 200 kV). For TEM observation, the samples were dispersed in ethanol by ultrasonic treatment and dropped onto carbon-coated copper grids. XRD patterns of the obtained samples were taken on a Philip-X’Pert X-ray diffractometer with a Cu Kα radiation (λ = 1.5418 Å) and Ni filter 2 | Journal Name, [year], [vol], 00–00
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Morphology Fig. 1 shows EM images of Na2Ti3O7 and S-TTO. The pure Na2Ti3O7 (Fig. 1a) consists of nanorods of ~200-300 nm in thickness and ~1 µm in length. Compared with pure titanate, the hierarchical core-shell structure of S-TTO is clearly visible and anatase TiO2 nanoparticles (NPs) are densely assembled on the external surface of titanate nanorods (Fig. 1b). Moreover, the surfaces and edges of S-TTO become out-of-flatness, and the This journal is © The Royal Society of Chemistry [year]
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XRD analysis
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electron-hole pairs and the further transfer of charge carriers to different reaction sites for subsequent photocatalytic reactions.
As shown in Fig. 2, the XRD pattern of the as-prepared layered Na2Ti3O7 is in good accordance with that in the literature, indicating the formation of pure Na2Ti3O7 phase.33 After first mixing Na2Ti3O7 with Ti(O-i-Pr)4 and then calcinating with thiourea, the resulted S-TTO shows a XRD pattern with significantly decreased intensities and slightly broadened diffraction peaks. This may be due to the coverage of anatase TiO2 NPs on the external surface of titanate. In addition, It was also observed that the phase of Na2Ti3O7 was fully disappeared and the crystal phase of Na2Ti6O13 appeared, indicating that a phase transition from Na2Ti3O7 to Na2Ti6O13 was fully accomplished due to the higher thermodynamic and structure stability of Na2Ti6O13 than Na2Ti3O7 at high temperature.32, 40 By comparison, after calcinating without thiourea, the resulted TTO showed the similar XRD pattern with S-TTO and only a slight decrease in intensities, indicating that the addition of thiourea is not responsible for the structural evolution from the phase Na2Ti3O7 to Na2Ti6O13. However, compared with TTO, S-TTO shows a strong (101) diffraction peak at 25.4°, indicating that the addition of thiourea is beneficial for the formation of anatase TiO2 with a better crystallinity and significantly exposed anatase {101} facets.
Fig. 1 EM images of Na2Ti3O7 and S-TTO. (a) SEM of Na2Ti3O7, (b) SEM of S-TTO, (c-e) HRTEM of S-TTO.
It has been reported that the anatase nanocrystals with the reductive {101} facets could be acted as a possible reservoir of the photogenerated electrons, yielding a highly reactive surface for the reduction of O2 to O2•−.35, 36 Furthermore, octahedral anatase crystals with {101} facets exhibited a relatively high photocatalytic activity for the oxidative decomposition of organic compounds due to the unique structure characteristic of the (101) surface.37 Thus, S-TTO composite with specifically exposed anatase {101} facets is expected to decrease the recombination probability of electron-hole pairs, leading to an enhanced photocatalytic activity. On the other hand, it has been found that anisotropically shaped particles, such as nanobelts,29 nanorods38 and nanotubes39, generally have a lower recombination rate of electron-hole pairs due to the fact that those structures possess a higher charge carrier mobility and provide a pathway for the quick transport of charge carriers throughout the longitudinal direction. Thus, the long anisotropically shaped titanate nanorods in S-TTO are beneficial both for the efficient separation of This journal is © The Royal Society of Chemistry [year]
Fig. 2 XRD patterns of Na2Ti3O7, TTO, S-TTO and commercial anatase TiO2 (The standard data for monoclinic Na2Ti6O13 (PDF # 14-0277) is also presented at the bottom for comparison.) 70
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In the crystal structure of Na2Ti6O13, TiO6 octahedra share edges at one level in line group and each group is joined above and below to similar groups by further edge sharing, resulting in a zigzag ribbon structure.32, 40 It has been reported that TiO6 octahedra sharing four edges and the zigzag ribbon structure are also observed in the anatase lattice.41 Thus, anatase TiO2 can be deposited on the external surface of Na2Ti6O13 to form a heterojunction, due to the common structural features of TiO6 octahedra in two components.32 X-ray photoelectron spectroscopic (XPS) studies
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overall crystallinity is decreased to a certain extent. EDS and EDS mapping of S-TTO (see Fig. S1 and Fig.S2) confirm that the sulfur is homogeneously distributed in the resulted composite. As shown in Fig. 1c, the HRTEM image of S-TTO also confirms that the resulted composite material possesses a hierarchical coreshell nano-heterostructure with the anatase phase forming the shell around the titanate core. The shell thickness of TiO2 is around 15~25 nm. For TiO2 NPs (Fig. 1d), the well-resolved interplanar spacing of the adjacent lattice fringes has the value of ~0.35 nm. Considering the crystallographic symmetry of anatase TiO2 NPs, the dominant exposed facets can be identified as {101} planes, which are the most thermodynamically stable facets of anatase TiO2.35 For the Na2Ti6O13 core (Fig. 1e), the clear lattice fringe confirms the presence of {20-1} plane with a lattice spacing of 0.63 nm. In addition, the HRTEM image of S-TTO also demonstrates the highly crystalline nature of Na2Ti6O13 and anatase TiO2, and the formation of a closely contacted interface between Na2Ti6O13 and anatase TiO2.
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environments and chemical states of S, Ti and O in S-TTO. It can be seen from Fig. 3a that the sample consists of S, C, Ti and O elements. The carbon peak is attributed to adventitious hydrocarbon from the XPS instrument. It should be noted that the nitrogen peak does not appear in the XPS spectrum. As shown in Fig. 3b, the S 2p state in S-TTO has a broadened peak due to the overlap of the split sublevels, 2p3/2 and 2p1/2 states. After fitting, two strong peaks, with a separation of 1.2 eV by spin-orbit coupling,42 are clearly observed at 168.1 and 169.3 eV, respectively. These two peaks suggest that the sulfur, in the form of S6+, partially substituted for Ti4+ in the lattice, leading to the formation of Ti-O-S bonds in S-TTO sample.25 The formation of cationic S-TTO could create a charge imbalance in the lattice of catalyst, and the extra positive charge was probably neutralized by the hydroxide ions.25 The adsorbed hydroxide ions (OH−ads) could capture the photoinduced holes to form active species typically as hydroxyl radicals. These hydroxyl radicals are the main species responsible for the degradation of organic pollutants. Furthermore, SO42− groups were formed on the surface of S-TTO due to the heat treatment under atmospheric conditions (two possible coordination models between SO42− and TiO2 are shown in inset of Fig. 3b).43, 44 The surface-adsorbed SO42− could act as the efficient electron trapping center.44 Combined with EM results, it is reasonable to assume that S6+ is mainly incorporated into the lattice of TiO2 as titanate core is covered with TiO2 shell and S doping proceeds from the exterior to the interior.32 Furthermore, no peaks are observed within 160~163 eV in the S 2p XPS spectrum. These peaks are corresponding to the formation of Ti-S bond when the oxygen atoms in S-TTO lattice are replaced by S atoms.45 This suggests that the substitution of Ti4+ by S6+ is chemically more favorable than replacing O2− with S2− under our synthetic conditions, which was also confirmed by the previous studies of S-doped TiO2.25
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Na2Ti3O7, and thus can be assigned to the octahedrally coordinated Ti. In addition, there shows no shoulder at a lower binding energy which is classically attributed to Ti3+ species due to oxygen vacancies, indicating that the Ti element mainly exists as the chemical state of Ti4+ in S-TTO.46 As shown in Fig. 3d, the O 1s spectrum can be evaluated by two peak fitting. The main peak at a lower binding energy of 529.9 eV can be ascribed to lattice oxygen in composite, while the signal at 531.4 eV is associated with oxygen in sulfate, as well as surface hydroxyl groups.47 UV-visible diffuse reflectance spectra The optical absorptions of the as-prepared Na2Ti3O7, TTO and S-TTO were investigated using UV-visible diffuse reflectance spectra, and the results are given in Fig. 4. Only an absorption band, which is attributed to the band-to-band transition, can be observed in the UV region for Na2Ti3O7. By comparison, upon hybridization with TiO2, the resulted TTO shows a much stronger absorption in UV region. In addition, it can also be observed that the bandgap energy of TTO is noticeably red-shifted to ~385 nm (ca. 3.22 eV) according to the position of the absorption edge. Apart from the possible quantum size effect,48 the main reason might be the formation of nanoheterojunction between Na2Ti6O13 (ca.3.53 eV) and anatase TiO2 (ca. 3.20 eV) as a result of the electronic coupling between two components within the closely contacted interface. Furthermore, after S doping, the resulted S-TTO shows not only a relatively stronger absorption in the UV region than the corresponding undoped TTO, but also a significant absorption tail in the visible region. Combined with XPS results, the enhanced visible-light absorption is attributed to the formation of Ti−O−S bonds due to the fact that the S6+ ions are mainly incorporated into the lattice of anatase TiO2 and thus the alteration of its crystal and electronic structure.25 The substitution of Ti4+ by S6+ in S-TTO would be similar to the situation of transition-metal ion doping and hence intra-band-gap states close to the CB edges were created.49
Fig. 3 XPS spectra of S-TTO (a) a total spectrum, (b) S 2p spectrum (inset shows two possible structures of the surface-adsorbed sulfate groups in S-TTO.), (c) Ti 2p spectrum, and (d) O 1s spectrum. 80
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It can be seen in Fig. 3c that Ti 2p spectrum of S-TTO consists of two peaks at ~457.9 (Ti 2p3/2) and 463.6 eV (Ti 2p1/2). Compared with TTO, a slight shift of Ti 2p to a lower binding energy was noticed after S doping, which is likely attributed to the interaction between titanium atoms and SO42− anions, resulting in the increase of electron density around the Ti atoms. However, these two peaks are quite similar to those in TTO and 4 | Journal Name, [year], [vol], 00–00
Fig. 4 Diffuse reflectance UV-visible spectra of Na2Ti3O7, TTO, and STTO (inset is the photograph of S-TTO).
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Apparently, S doping could greatly promote the light harvesting ability. As the energy potentials of CB and VB are – 0.53 and 2.67 eV in anatase TiO2,50 and S-TTO has a strong absorption below 446 nm (ca. 2.78 eV), the energy potentials of CB and VB in S-doped anatase are –0.11 and 2.67 eV. It This journal is © The Royal Society of Chemistry [year]
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indicates that only anatase TiO2 can be excited by visible light to generate electron-hole pairs in S-TTO. Moreover, the visible light response behavior after S doping can be also clearly evidenced from the apparent color change of the resulting sample from white to yellow (see inset of Fig. 4).
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Visible light photocatalytic performance
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The visible light photocatalytic activities of the obtained samples were evaluated by the degradation of MB aqueous solution and are shown in Fig. 5. The photocatalytic degradation of MB was monitored by the blank test and the result indicates that the degradation of MB is neglectable in the absence of photocatalyst under visible light irradiation. There is a nonnegligible reduction of MB concentration on Na2Ti3O7 and P25, which is essentially due to the dye-sensitized photocatalysis, as both Na2Ti3O7 and P25 have a large energy band-gap and cannot be activated by visible light.
Fig. 5 (a) Visible light photocatalytic degradation rate of MB over different samples, (b) cycling experiments of S-TTO for MB degradation under visible light irradiation.
After hybridization with TiO2, the resulted TTO shows an enhanced photocatalytic activity than Na2Ti3O7 and P25, which is essentially due to the formation of heterojunction and thus the effective spatial separation of photo-generated electron-hole pairs. Furthermore, after S doping, a significantly enhanced photocatalytic performance was observed in S-TTO, showing the highest activity with 98.6% MB photodegraded in 100 min. It indicates that S doping essentially tunes the electric structure of TTO and thus further effectively enhances the photocatalytic performance. Combined with the results from XPS and UV-vis spectra, the observed photobleaching over S-TTO under visible light irradiation should be mainly due to the oxidative photodegradation of the dye molecules rather than selfphotosensitized oxidation or other reasons. More importantly, compared with the commercial P25, S-TTO shows an obviously enhanced photocatalytic performance. To evaluate the stability of S-TTO photocatalyst, multiple photodegradation tests of MB under visible light illumination were carried out (Fig. 5b). After five times recycling experiments, a high photoactivity with 95.9% MB photodegraded in 100 min still can be reached. It indicates that S-TTO has an excellent photocatalytic stability. The PL technique is useful to reveal the migration, transfer, and recombination process of photogenerated electron-hole pairs in semiconductors. A lower fluorescence emission intensity implies a lower electron-hole recombination rate and thus corresponds to a higher photocatalytic activity. In order to understand the role of TiO2 hybridization and S doping in enhancing visible light photocatalytic activity for MB degradation, Na2Ti3O7, TTO and S-TTO were characterized by This journal is © The Royal Society of Chemistry [year]
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PL with an excitation wavelength of 325 nm and the results are shown in Fig 6a. Na2Ti3O7 shows a strong PL emission peak at ~467 nm which is ascribed to the band gap recombination of electron-hole pairs.51 It indicates that electrons and holes recombine rapidly. The peak positions in TTO and S-TTO are similar to those in pure Na2Ti3O7. It can be observed that TTO has an obviously reduced PL intensity than Na2Ti3O7, indicating that TTO has a much lower recombination rate of photogenerated electron-hole pairs. It confirms the importance of heterojunction in hindering the recombination of electrons and holes. On the other hand, compared with TTO, the PL of S-TTO is further suppressed and shows a diminished intensity in the visible region, indicating that both TiO2 hybridization and S doping can effectively accelerate the separation of charge carriers.
Fig. 6 (a) PL spectra of Na2Ti3O7, TTO and S-TTO with an excitation wavelength of 325 nm, and (b) photocurrent responses of Na2Ti3O7, TTO and S-TTO. 70
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It is well-known that the photocatalytic redox reactions are intimately relevant to the separation efficiency of photoinduced electron-hole pairs arisen from the excited semiconductor materials. To qualitatively investigate the separation efficiency of photoinduced charges during the photoreactions, the photocurrent response was determined for Na2Ti3O7, TTO and S-TTO (Fig. 6b). It can be obviously seen that fast and stable photocurrent responses are observed in all electrodes, and the photoresponsive phenomenon is entirely reversible. Under visible light illumination, pure Na2Ti3O7 electrode shows the weakest response due to its large band gap. On the contrary, the photocurrent of TTO electrode is about 3.2 times higher than that of Na2Ti3O7 electrode. The remarkable photocurrent enhancement of TTO photocatalyst reveals an enhanced separation efficiency of the photogenerated electrons and holes, which can be attributed to the formation of a heterojunction structure between Na2Ti6O13 and anatase TiO2. Furthermore, after S doping, the photocurrent of the resulted S-TTO is much higher than that of Na2Ti3O7 and TTO, being attributed not only to the formation of a nano-heterojunction interface structure between Na2Ti6O13 and anatase TiO2 but also to the enhancement of the visible light response arisen from S doping. Active species and possible photocatalytic mechanism
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Based on previously reported studies,52, 53 in order to understand which reactive species played an important role in photocatalysis under visible light irradiation, the main active species were detected by adding four different chemicals, i.e., Na2C2O4 (holes scavenger), AgNO3 (electrons scavenger), tBuOH (•OH scavenger) and BQ (O2•− scavenger), into the photocatalytic reaction systems. It was noted that the MB Journal Name, [year], [vol], 00–00 | 5
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photodegradation was suppressed when the scavengers were added in the photocatalytic reaction system, indicating the importance of the corresponding active species.
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potential of O2/O2•− (−0.33 V vs. NHE).52 Meanwhile, as the VB potential of S-doped anatase TiO2 of (2.67 eV) is much more positive than the standard redox potential of •OH/OH− (1.99 eV vs. NHE) 53, 54, the formed holes can react with OH− groups or H2O molecules to produce •OH radicals. The formed •OH radicals are able to oxidize the organic pollutant due to their high oxidative capacity. Therefore, the efficiently separation of photogenerated electrons and holes can be achieved, leading to the improved utilization of charge carriers and ultimately the enhanced visiblelight photodegradation rate of MB.
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Fig. 7 Effects of different scavengers on the degradation efficiency of MB.
As shown in Fig. 7, different scavengers have different effects on the degradation of MB over S-TTO sample. The degradation efficiency was greatly decreased to 64.3%, 74.9%, 78.2% and 89.6% upon addition of t-BuOH, AgNO3, BQ and Na2C2O4, respectively. It indicates that •OH, e−, O2•− and h+ all participate in the photocatalytic process. Moreover, •OH is the most crucial species and h+ contributes to a lesser extent in MB degradation.
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Fig. 8 (a) Energy levels of Na2Ti6O13 and anatase TiO2 using normal hydrogen electrode (NHE) as reference at pH 7. The potential of CB band in TiO2 was calculated from ECB = –0.12–0.059 pH. (b) A schematic illustration of S-TTO composite catalyst for the photodegradation of MB.
Based on the above results and discussion, a possible mechanism for the photodegradation reaction over S-TTO is proposed and schematically shown in Fig. 8. Under visible light irradiation, S-doped anatase in S-TTO can be excited to generate electron-hole pairs. As the energy potentials of CB and VB in Na2Ti6O13 are −0.50 and 3.03 eV while those in S-doped anatase are −0.11 and 2.67 eV, electrons are excited from VB to intraband-gap states created by the substitution of Ti4+ by S6+, and then to CB. The photogenerated electrons accumulated on TiO2 will migrate from CB of TiO2 to that of Na2Ti6O13 due to the potential difference. In such a way, the photogenerated electrons can be effectively collected by Na2Ti6O13 while holes by TiO2. The anisotropically shaped titanate nanorods provide a pathway for the quick transport of photogenerated electrons throughout the longitudinal direction to the catalyst surface and thus be collected by exposed anatase {101} facets and surface-adsorbed SO42−. These electrons then react with dissolved oxygen to generate O2•− and further •OH because the CB edge potential of Na2Ti6O13 (−0.50 V vs. NHE) was more negative than the standard redox
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A facile calcination method has been developed to prepare Sdoped Na2Ti6O13@TiO2 core-shell nanorods, in which anatase TiO2 NPs with exposed {101} facets are deposited on the external surface of titanate core, leading to the formation of nanoheterojunction structure between two components due to the common structural features of TiO6 octahedra in anatase TiO2 and Na2Ti6O13. The resulted composite displayed an excellent visible-light photocatalytic performance in degrading MB solution due to the following reasons. Firstly, the nanoheterojunction formed between Na2Ti6O13 and anatase TiO2 speeds up the separation rate of photogenerated charge carriers. Secondly, the partial substitution of Ti4+ by S6+ in the lattice of anatase TiO2 enhances the visible-light absorption and leads to the formation of considerable adsorbed hydroxide ions which could capture the photoinduced holes to form active hydroxyl radicals. In addition, with the formation of Ti−O−S bond, partial electrons can be transferred from S to O atoms. The electrondeficient S atoms might capture e− and thus inhibits the recombination of photogenerated electron-hole pairs. Thirdly, the exposed anatase {101} facets could be acted as a possible reservoir of the photogenerated electrons, yielding a highly reactive surface for the reduction of O2 to O2•− and thus the decrease of recombination probability of electron-hole pairs. Fourthly, the anisotropically shaped titanate nanorods provide a pathway for the quick transport of charge carriers throughout the longitudinal direction to different reaction sites for subsequent photocatalytic reactions. Finally, the doped sulfur atoms favorably stabilize the anatase structure, leading to a high photocatalytic stability. The present work may provide an important indication of how to construct novel visible-light photocatalysts through an integration of hybridization, doping and morphology engineering.
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The authors greatly appreciate the financial support of the Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP, 20130091110010), Natural Science Foundation of Jiangsu Province (BK2011438), National Science Fund for Talent Training in Basic Science (No. J1103310), the National Basic Research Program (973 Project) (No. 2009CB623504) and the Modern Analysis Center of Nanjing University.
Notes and references This journal is © The Royal Society of Chemistry [year]
Physical Chemistry Chemical Physics Accepted Manuscript
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Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210093, P. R. China. E-mail: [email protected] 75 5
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Department of Applied Chemistry, College of Science, Nanjing Tech University, Nanjing, 211816, P. R. China. E-mail: [email protected] 1.
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20
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25
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30
14. 15. 35
16. 17. 40
18. 19.
45
20. 21. 22.
50
23. 24. 55
25. 26.
60
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30. 31. 32.
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J. S. Jang, H. G. Kim and J. S. Lee, Catal. Today, 2012, 185, 270277. C. Chen, W. Ma and J. Zhao, Chem. Soc. Rev., 2010, 39, 4206-4219. X. Chen, S. Shen, L. Guo and S. Mao, Chem. Rev., 2010, 110, 65036570. A. L. Linsebigler, G. Lu and J. T. Yates, Chem. Rev., 1995, 95, 735758. J. Tian, Z. Zhao, A. Kumar, R. I. Boughton and H. Liu, Chem. Soc. Rev., 2014, 43, 6920-6937. L. Sang, Y. Zhao and C. Burda, Chem. Rev., 2014, 114, 9283-9318. Z. Zhai, C. H. Hu, X. Yang, L. Zhang, C. Liu, Y. Fan and W. Hou, J. Mater. Chem., 2012, 22, 19122-19131. M. Dahl, Y. Liu and Y. Yin, Chem. Rev., 2014, 114, 9853-9889. S. Ogura, M. Kohno, K. Sato and Y. Inoue, J. Mater. Chem., 1998, 8, 2335-2337. Y. Inoue, T. Kubokawa and K. Sato, J. Phys. Chem., 1991, 95, 40594063. H. Yoshida, M. Takeuchi, M. Sato, L. Zhang, T. Teshima and M. G. Chaskar, Catal. Today, 2014, 232, 158-164. A. Rudola, K. Saravanan, S. Devaraj, H. Gong and P. Balaya, Chem. Commun., 2013, 49, 7451-7453. J. C. Perez-Flores, C. Baehtz, M. Hoelzel, A. Kuhn and F. GarciaAlvarado, Phys. Chem. Chem. Phys., 2012, 14, 2892-2899. P. Senguttuvan, G. L. Rousse, V. Seznec, J. M. Tarascon and M. R. Palacín, Chem. Mater., 2011, 23, 4109-4111. Y. Wang, G. Du, H. Liu, D. Liu, S. Qin, N. Wang, C. Hu, X. Tao, J. Jiao, J. Wang and Z. L. Wang, Adv. Funct. Mater., 2008, 18, 11311137. Z. Zhai, Y. Huang, L. Xu, X. Yang, C. Hu, L. Zhang, Y. Fan and W. Hou, Nano Res., 2011, 4, 635-647. Z. Zhai, X. Yang, L. Xu, C. Hu, L. Zhang, W. Hou and Y. Fan, Nanoscale, 2012, 4, 547-556. L. Zhang, C. Hu, L. Cheng, W. Ding, W. Hou and J. Chen, Chin. J. Catal., 2013, 34, 2089-2097. G. Zhang, Y. Zhang, M. Nadagouda, C. Han, K. O'Shea, S. M. ElSheikh, A. A. Ismail and D. D. Dionysiou, Appl. Catal. B: Environ., 2014, 144, 614-621. S. Ida, Y. Okamoto, M. Matsuka, H. Hagiwara and T. Ishihara, J. Am. Chem. Soc., 2012, 134, 15773-15782. R. Asahi, T. Morikawa, H. Irie and T. Ohwaki, Chem. Rev., 2014, 114, 9824-9852. T. Umebayashi, T. Yamaki, H. Itoh and K. Asai, Appl. Phys. Lett., 2002, 81, 454-456. T. Umebayashi, T. Yamaki, S. Tanaka and K. Asai, Chem. Letts, 2003, 32, 330-331. T. Umebayashi, T. Yamaki, S. Yamamoto, A. Miyashita, S. Tanaka, T. Sumita and K. Asai, J. Appl. Phys, 2003, 93, 5156-5160. J. Yu, W. Ho, J. Yu, H. Yi, P. Wong and J. Zhao, Environ. Sci. Technol., 2005, 39, 1175-1179. T. Ohno, M. Akiyoshi, T. Umebayashi, K. Asai, T. Mitsui and M. Matsumura, Appl. Catal. A: Gen., 2004, 265, 115-121. A. Kudo and Y. Miseki, Chem. Soc. Rev., 2009, 38, 253-278. L. Ye, J. Liu, Z. Jiang, T. Peng and L. Zan, Nanoscale, 2013, 5, 9391-9396. N. Wu, J. Wang, D. N. Tafen, H. Wang, J. Zheng, J. Lewis, X. Liu, S. S. Leonard and A. Manivannan, J. Am. Chem. Soc., 2010, 132, 6679-6685. J. Pan, G. Liu, G. Q. Lu and H. Cheng, Angew. Chem., Int. Ed., 2011, 50, 2133-2137. Z. Xiong and X. S. Zhao, J. Am. Chem. Soc., 2012, 134, 5754-5757. C. Liu, T. Sun, L. Wu, J. Liang, Q. Huang, J. Chen and W. Hou, Appl. Catal. B: Environ., 2015, 170, 17-24. N. Miyamoto, K. Kuroda and M. Ogawa, J. Mater. Chem., 2004, 14, 165-170.
This journal is © The Royal Society of Chemistry [year]
80
85
90
95
100
105
110
34. L. Zhang, C. Hu, J. Zhang, L. Cheng, Z. Zhai, J. Chen, W. Ding and W. Hou, Chem. Commun., 2013, 49, 7507-7509. 35. G. Liu, H. Yang, J. Pan, Y. Yang, G. Lu and H. Cheng, Chem. Rev., 2014, 114, 9559-9612. 36. M. D’Arienzo, J. Carbajo, A. Bahamonde, M. Crippa, S. Polizzi, R. Scotti, L. Wahba and F. Morazzoni, J. Am. Chem. Soc., 2011, 133, 17652-17661. 37. F. Amano, T. Yasumoto, O. O. Prieto-Mahaney, S. Uchida, T. Shibayama and B. Ohtani, Chem. Commun., 2009, 2311-2313. 38. P. D. Cozzoli, A. Kornowski and H. Weller, J. Am. Chem. Soc., 2003, 125, 14539-14548. 39. A. Riss, M. J. Elser, J. Bernardi and O. Diwald, J. Am. Chem. Soc., 2009, 131, 6198-6206. 40. H. Liu, D. Yang, Z. Zheng, X. Ke, E. Waclawik, H. Zhu and R. L. Frost, J. Raman Spectrosc., 2010, 41, 1331-1337. 41. J. K. Burdett, T. Hughbanks, G. J. Miller, J. W. Richardson and J. V. Smith, J. Am. Chem. Soc., 1987, 109, 3639-3646. 42. B. J. Lindberg and K. Hamrin, Acta Chem. Scand., 1970, 24, 36613671. 43. D. Ma, Y. Xin, M. Gao and J. Wu, Appl. Catal. B: Environ., 2014, 147, 49-57. 44. P. V. R. K. Ramacharyulu, J. Praveen Kumar, G. K. Prasad and B. Sreedhar, Mater. Chem. Phys., 2014, 148, 692-698. 45. S. Yin, K. Ihara, Y. Aita, M. Komatsu and T. Sato, J. Photoch. Photobio. A: Chem., 2006, 179, 105-114. 46. D. J. V. Pulsipher, I. T. Martin and E. R. Fisher, ACS Appl. Mater. Inter., 2010, 2, 1743-1753. 47. G. Colón, M. C. Hidalgo, G. Munuera, I. Ferino, M. G. Cutrufello and J. A. Navío, Appl. Catal. B: Environ., 2006, 63, 45-59. 48. Z. Chen, B. Lin, Y. Chen, K. Zhang, B. Li and H. Zhu, J. Phys. Chem. Solids, 2010, 71, 841-847. 49. Y. Zhao, C. Li, X. Liu, F. Gu, H. L. Du and L. Shi, Appl. Catal. B: Environ., 2008, 79, 208-215. 50. T. Tachikawa, M. Fujitsuka and T. Majima, J. Phy. Chem. C, 2007, 111, 5259-5275. 51. C. Liu, L. Wu, J. Chen, J. Liang, C. Li, H. Ji and W. Hou, Phys. Chem. Chem. Phys., 2014, 16, 13409-13417. 52. D. Zhang, Q. Wang, L. Wang and L. Zhang, J. Mater. Chem. A, 2015, 3, 3576-3585. 53. K. Li, S. Gao, Q. Wang, H. Xu, Z. Wang, B. Huang, Y. Dai and J. Lu, ACS Appl. Mater. Inter., 2015, 7, 9023-9030. 54. J. Yu, S. Wang, J. Low and W. Xiao, Phys. Chem. Chem. Phys., 2013, 15, 16883-16890.
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S-doped Na2Ti6O13@TiO2 core-shell nanorods with an enhanced visible light photocatalytic performance Chao Liua, Ji-yuan Lianga, Rui-rui Hana, Yong-zheng Wanga, Jin Zhaoa, Qian-jin Huangb, Jing Chen*b, Wen-hua Hou*a 5
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x S-doped Na2Ti6O13@TiO2 (S-TTO) core-shell nanorods, with exposed anatase TiO2 {101} facets, were synthesized by a facile calcination method. It was found that the addition of thiourea as the sulfur precursor was beneficial for the formation of anatase TiO2 with a better crystallinity and the doped sulfur atoms favorably stabilized the anatase structure. The substitution of Ti4+ by S6+ in the lattice of S-TTO gave rise to the visible light response and increased the amount of active groups typically as hydroxyl radical adsorbed on the catalyst surface. With the formation of Ti−O−S bond, partial electrons could be transferred from S to O atoms. The electron-deficient S atoms might capture e− and thus inhibited the recombination of photogenerated electron-hole pairs. Meanwhile, the closely-contacted interface was formed between Na2Ti6O13 and anatase TiO2, resulting in a nanoscale heterojunction structure to speed up the separation rate of photogenerated charge carriers. The exposed anatase {101} facets could be acted as a possible reservoir of the photogenerated electrons, yielding a highly reactive surface for the reduction of O2 to O2•− and thus the decrease of recombination probability of electron-hole pairs. In addition, the anisotropically shaped titanate nanorods provided a pathway for the quick transport of charge carriers throughout the longitudinal direction. The combined effects of S doping, nanoheterojunction formation and morphology engineering led to an obviously enhanced photocatalytic performance for the degradation of methylene blue (MB) solution under visible light irradiation. The corresponding photocatalytic mechanism was investigated and discussed in detail. The present work may provide an insight for the fabrication of delicate composite photocatalysts with an excellent performance.
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Introduction
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Semiconductor−based photocatalysis technology has been attracting considerable interest for environmental remediation and clean energy production.1-3 Among various semiconductors, TiO2 has been investigated extensively and intensively for its superior chemical stability, excellent photocatalytic performance, commercial availability, non-toxicity and so on.4-7 However, the narrow light-response range (UV region due to its large band gap of 3.0-3.2 eV) and the fast recombination rate of photogenerated electron−hole pairs greatly reduce the photonic efficiency and represent a major bottleneck for the photocatalytic application of TiO2. Thus, much research has been dedicated to the construction of novel TiO2−based visible light-driven photocatalysts.8 The stoichiometry-dependent structure and physicochemical properties of alkali metal titanates endow them with a broad range of applications, such as photocatalyst,9-11 lithium and sodium ion battery,12-14 gas sensing and antibacterial agent,15 etc. However, as photocatalysts, Na2Ti6O13 itself has a relatively wide bandgap and is thus active only under UV light. In addition, it also shows a high rate of electron-hole recombination. Through This journal is © The Royal Society of Chemistry [year]
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morphology engineering, Na2Ti6O13 nanorods were prepared and found to be suitable both for combination with catalytic active phase (e.g., metal oxides) to raise the separation efficiency of photoexcited charge carriers and for quick transport of charge carriers throughout the longitudinal direction to different reaction sites for subsequent reactions.10 During the past decade, nonmetal doping, morphology control and heterojunction coupling have all been reported to improve the photocatalytic activity, respectively.8, 16-18 The nonmetal doping was an effective modification strategy for the improvement of visible-light-driven photocatalytic activity due to the band gap narrowing.19-21 Recently, Umebayashi et al. have succeeded in preparing visible light-activated S-doped TiO2,22-24 in which sulfur was doped as S2－ and replaced the lattice oxygen in TiO2. On the contrary, it was found that S atoms were incorporated as cations and replaced Ti ions in S-doped TiO2 photocatalysts.25, 26 Undeniably, S doping has attracted extensive attention in order to enhance visible-light photocatalytic activity. On the other hand, by integrating different individual materials within the same structure, heterojunction photocatalysts can incorporate multiple [journal], [year], [vol], 00–00 | 1
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functionalities and effectively speed up the separation rate of photogenerated charge carriers, giving rise to an improved photocatalytic activity.27 In addition, morphology control, particularly in order to obtain the specifically exposed crystal facets, has been developed to enhance the photocatalytic performance.28-31 Thus, it is reasonable to expect that novel efficient visible-light photocatalysts can be designed through the integration of nonmetal doping, heterojunction formation and morphology engineering strategies. For example, it has been reported that the introduction of nitrogen dopant localized in the crystal lattice of both anatase and titanate of titanate-anatase coreshell nanobelts presents a high photocatalytic activity under visible light irradiation, which is largely attributed to N doping, the preferentially grown TiO2 nanorods shell structures and titanate-anatase hetero-interfaces.31 Our group also reported that a novel photocatalyst of N-doped Na2Ti6O13@TiO2 core-shell nanobelts showed an enhanced visible-light photocatalytic performance due to the combination effects of N doping, heterojunction formation and morphology control.32 Herein, we further report that S-doped Na2Ti6O13@TiO2 coreshell nanorods exhibit an excellent photocatalytic activity in the degradation of MB under visible-light irradiation. The structure, surface morphology and photocatalytic performance of the resultant samples have been studied and discussed in detail. Moreover, a possible photocatalytic mechanism is proposed based on the experimental results.
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Photocatalytic reaction tests
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Experimental section Synthesis of titanate and S-doped Na2Ti6O13@TiO2 core-shell nanorods 30
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Na2Ti3O7 was synthesized by calcinating a stoichiometric mixture of Na2CO3 and TiO2 (anatase) in a molar ratio of 1.1:3 at 900 °C for 24 h in air with a heating rate of 10 °C min−1.33 The resultant power was reground, and the same temperature program sequence was repeated once again. To prepare S-doped Na2Ti6O13@TiO2 core-shell nanorods (denoted as S-TTO), 2.0 g of Na2Ti3O7 was first dispersed in ethanol (40 g mL−1) and stirred for 12 h. Then, 6.0 mL of titanium isopropoxide, Ti(O-i-Pr)4, was added dropwise into the above suspension under stirring. After 12 h, the resultant suspension was transferred to a petri-dish in order to allow ethanol evaporation at room temperature. Finally, the resultant dried white solid sample (1.0 g) was finely milled with thiourea (2.0 g) and calcined in air at 500 °C for 10 h to get the target sample.34 For comparison, undoped sample (denoted as TTO) was also prepared through a similar process in which no thiourea was added.
at a scanning rate of 0.2° s−1 in a 2θ range of 5~60°. UV–vis diffuse reflectance spectra were obtained on a UV–vis spectrophotometer (Shimadzu, UV-2401) using BaSO4 as a reference. X-ray photoelectron spectroscopic (XPS) analysis was carried out on an X-ray photoelectron spectrometer (Thermo Fisher Scientific, K-Alpha) equipped with a hemispherical electron analyzer (pass energy of 20 eV) and an Al Kα (hν = 1486.6 eV) X-ray source. The binding energies (BE) were referenced to the adventitious C 1s peak (284.6 eV) which was used as an internal standard to take into account charging effects. A combination of Gaussian and Lorentzian functions was used to fit the curves. Photoluminescence (PL) spectra were recorded on a 48000DSCF luminescence spectrometer at room temperature by using a continuous-wave 325 nm He-Cd laser as the excitation source. The samples were pressed into a thin disk and fixed in a homemade quartz cell. Photocurrent was measured on a CHI 660D electrochemical workstation (Shanghai Chenhua Apparatus Corporation, China) with a three-electrode system in which a 0.25 cm2 modified ITO as working electrode, a Pt wire as counter-electrode and a saturated Ag/AgCl electrode as reference electrode.
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The visible light photocatalytic activity was evaluated by the degradation of MB in aqueous solution. Firstly, 50 mg of catalyst was suspended in an MB aqueous solution (1.0 × 10−5 mol L－1) in the dark for more than 1 h, in order to gain the adsorptiondesorption equilibrium. After the adsorption equilibrium of MB, the catalyst was separated and re-dispersed in an aqueous solution of MB (100 mL, 1.0 × 10−5 mol L － 1) in a beaker with a circulating water system to remove the thermal effect of light. Light from a 300 W Xe lamp, passed through a UV light filter film (to remove radiation with λ< 420 nm), was focused onto the reaction cell. When the light was turned on, at given time intervals, approximately 4 mL of the reaction suspension was sampled and separated by means of high-speed centrifugation. The filtrates were analyzed by recording the maximum absorbance at 664 nm in the UV-visible spectrum of MB. The degradation efficiency at time t was determined from the value of Ct/C0, where C0 is the initial concentration and Ct is the concentration of MB at time t. The active species capture experiments were employed to study the photocatalytic mechanism. Silver nitrate (AgNO3, 1 mmol L－1), sodium oxalate (Na2C2O4, 0.5 mmol L－1), tert-butyl alcohol (t-BuOH, 5 mmol L－1) and p-benzoquinone (BQ, 1 mmol L－1) were added to the MB aqueous solution, respectively. Then, the remaining experimental processes were similar to the above photocatalytic test.
Results and discussion
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The morphology was investigated by scanning electron microscopy (SEM, JEOL JEM-6300F) and transmission electron microscopy (TEM, JEOL JEM-200CX, operating at an accelerating voltage of 200 kV). For TEM observation, the samples were dispersed in ethanol by ultrasonic treatment and dropped onto carbon-coated copper grids. XRD patterns of the obtained samples were taken on a Philip-X’Pert X-ray diffractometer with a Cu Kα radiation (λ = 1.5418 Å) and Ni filter 2 | Journal Name, [year], [vol], 00–00
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Morphology Fig. 1 shows EM images of Na2Ti3O7 and S-TTO. The pure Na2Ti3O7 (Fig. 1a) consists of nanorods of ~200-300 nm in thickness and ~1 µm in length. Compared with pure titanate, the hierarchical core-shell structure of S-TTO is clearly visible and anatase TiO2 nanoparticles (NPs) are densely assembled on the external surface of titanate nanorods (Fig. 1b). Moreover, the surfaces and edges of S-TTO become out-of-flatness, and the This journal is © The Royal Society of Chemistry [year]
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XRD analysis
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electron-hole pairs and the further transfer of charge carriers to different reaction sites for subsequent photocatalytic reactions.
As shown in Fig. 2, the XRD pattern of the as-prepared layered Na2Ti3O7 is in good accordance with that in the literature, indicating the formation of pure Na2Ti3O7 phase.33 After first mixing Na2Ti3O7 with Ti(O-i-Pr)4 and then calcinating with thiourea, the resulted S-TTO shows a XRD pattern with significantly decreased intensities and slightly broadened diffraction peaks. This may be due to the coverage of anatase TiO2 NPs on the external surface of titanate. In addition, It was also observed that the phase of Na2Ti3O7 was fully disappeared and the crystal phase of Na2Ti6O13 appeared, indicating that a phase transition from Na2Ti3O7 to Na2Ti6O13 was fully accomplished due to the higher thermodynamic and structure stability of Na2Ti6O13 than Na2Ti3O7 at high temperature.32, 40 By comparison, after calcinating without thiourea, the resulted TTO showed the similar XRD pattern with S-TTO and only a slight decrease in intensities, indicating that the addition of thiourea is not responsible for the structural evolution from the phase Na2Ti3O7 to Na2Ti6O13. However, compared with TTO, S-TTO shows a strong (101) diffraction peak at 25.4°, indicating that the addition of thiourea is beneficial for the formation of anatase TiO2 with a better crystallinity and significantly exposed anatase {101} facets.
Fig. 1 EM images of Na2Ti3O7 and S-TTO. (a) SEM of Na2Ti3O7, (b) SEM of S-TTO, (c-e) HRTEM of S-TTO.
It has been reported that the anatase nanocrystals with the reductive {101} facets could be acted as a possible reservoir of the photogenerated electrons, yielding a highly reactive surface for the reduction of O2 to O2•−.35, 36 Furthermore, octahedral anatase crystals with {101} facets exhibited a relatively high photocatalytic activity for the oxidative decomposition of organic compounds due to the unique structure characteristic of the (101) surface.37 Thus, S-TTO composite with specifically exposed anatase {101} facets is expected to decrease the recombination probability of electron-hole pairs, leading to an enhanced photocatalytic activity. On the other hand, it has been found that anisotropically shaped particles, such as nanobelts,29 nanorods38 and nanotubes39, generally have a lower recombination rate of electron-hole pairs due to the fact that those structures possess a higher charge carrier mobility and provide a pathway for the quick transport of charge carriers throughout the longitudinal direction. Thus, the long anisotropically shaped titanate nanorods in S-TTO are beneficial both for the efficient separation of This journal is © The Royal Society of Chemistry [year]
Fig. 2 XRD patterns of Na2Ti3O7, TTO, S-TTO and commercial anatase TiO2 (The standard data for monoclinic Na2Ti6O13 (PDF # 14-0277) is also presented at the bottom for comparison.) 70
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In the crystal structure of Na2Ti6O13, TiO6 octahedra share edges at one level in line group and each group is joined above and below to similar groups by further edge sharing, resulting in a zigzag ribbon structure.32, 40 It has been reported that TiO6 octahedra sharing four edges and the zigzag ribbon structure are also observed in the anatase lattice.41 Thus, anatase TiO2 can be deposited on the external surface of Na2Ti6O13 to form a heterojunction, due to the common structural features of TiO6 octahedra in two components.32 X-ray photoelectron spectroscopic (XPS) studies
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overall crystallinity is decreased to a certain extent. EDS and EDS mapping of S-TTO (see Fig. S1 and Fig.S2) confirm that the sulfur is homogeneously distributed in the resulted composite. As shown in Fig. 1c, the HRTEM image of S-TTO also confirms that the resulted composite material possesses a hierarchical coreshell nano-heterostructure with the anatase phase forming the shell around the titanate core. The shell thickness of TiO2 is around 15~25 nm. For TiO2 NPs (Fig. 1d), the well-resolved interplanar spacing of the adjacent lattice fringes has the value of ~0.35 nm. Considering the crystallographic symmetry of anatase TiO2 NPs, the dominant exposed facets can be identified as {101} planes, which are the most thermodynamically stable facets of anatase TiO2.35 For the Na2Ti6O13 core (Fig. 1e), the clear lattice fringe confirms the presence of {20-1} plane with a lattice spacing of 0.63 nm. In addition, the HRTEM image of S-TTO also demonstrates the highly crystalline nature of Na2Ti6O13 and anatase TiO2, and the formation of a closely contacted interface between Na2Ti6O13 and anatase TiO2.
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environments and chemical states of S, Ti and O in S-TTO. It can be seen from Fig. 3a that the sample consists of S, C, Ti and O elements. The carbon peak is attributed to adventitious hydrocarbon from the XPS instrument. It should be noted that the nitrogen peak does not appear in the XPS spectrum. As shown in Fig. 3b, the S 2p state in S-TTO has a broadened peak due to the overlap of the split sublevels, 2p3/2 and 2p1/2 states. After fitting, two strong peaks, with a separation of 1.2 eV by spin-orbit coupling,42 are clearly observed at 168.1 and 169.3 eV, respectively. These two peaks suggest that the sulfur, in the form of S6+, partially substituted for Ti4+ in the lattice, leading to the formation of Ti-O-S bonds in S-TTO sample.25 The formation of cationic S-TTO could create a charge imbalance in the lattice of catalyst, and the extra positive charge was probably neutralized by the hydroxide ions.25 The adsorbed hydroxide ions (OH−ads) could capture the photoinduced holes to form active species typically as hydroxyl radicals. These hydroxyl radicals are the main species responsible for the degradation of organic pollutants. Furthermore, SO42− groups were formed on the surface of S-TTO due to the heat treatment under atmospheric conditions (two possible coordination models between SO42− and TiO2 are shown in inset of Fig. 3b).43, 44 The surface-adsorbed SO42− could act as the efficient electron trapping center.44 Combined with EM results, it is reasonable to assume that S6+ is mainly incorporated into the lattice of TiO2 as titanate core is covered with TiO2 shell and S doping proceeds from the exterior to the interior.32 Furthermore, no peaks are observed within 160~163 eV in the S 2p XPS spectrum. These peaks are corresponding to the formation of Ti-S bond when the oxygen atoms in S-TTO lattice are replaced by S atoms.45 This suggests that the substitution of Ti4+ by S6+ is chemically more favorable than replacing O2− with S2− under our synthetic conditions, which was also confirmed by the previous studies of S-doped TiO2.25
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Na2Ti3O7, and thus can be assigned to the octahedrally coordinated Ti. In addition, there shows no shoulder at a lower binding energy which is classically attributed to Ti3+ species due to oxygen vacancies, indicating that the Ti element mainly exists as the chemical state of Ti4+ in S-TTO.46 As shown in Fig. 3d, the O 1s spectrum can be evaluated by two peak fitting. The main peak at a lower binding energy of 529.9 eV can be ascribed to lattice oxygen in composite, while the signal at 531.4 eV is associated with oxygen in sulfate, as well as surface hydroxyl groups.47 UV-visible diffuse reflectance spectra The optical absorptions of the as-prepared Na2Ti3O7, TTO and S-TTO were investigated using UV-visible diffuse reflectance spectra, and the results are given in Fig. 4. Only an absorption band, which is attributed to the band-to-band transition, can be observed in the UV region for Na2Ti3O7. By comparison, upon hybridization with TiO2, the resulted TTO shows a much stronger absorption in UV region. In addition, it can also be observed that the bandgap energy of TTO is noticeably red-shifted to ~385 nm (ca. 3.22 eV) according to the position of the absorption edge. Apart from the possible quantum size effect,48 the main reason might be the formation of nanoheterojunction between Na2Ti6O13 (ca.3.53 eV) and anatase TiO2 (ca. 3.20 eV) as a result of the electronic coupling between two components within the closely contacted interface. Furthermore, after S doping, the resulted S-TTO shows not only a relatively stronger absorption in the UV region than the corresponding undoped TTO, but also a significant absorption tail in the visible region. Combined with XPS results, the enhanced visible-light absorption is attributed to the formation of Ti−O−S bonds due to the fact that the S6+ ions are mainly incorporated into the lattice of anatase TiO2 and thus the alteration of its crystal and electronic structure.25 The substitution of Ti4+ by S6+ in S-TTO would be similar to the situation of transition-metal ion doping and hence intra-band-gap states close to the CB edges were created.49
Fig. 3 XPS spectra of S-TTO (a) a total spectrum, (b) S 2p spectrum (inset shows two possible structures of the surface-adsorbed sulfate groups in S-TTO.), (c) Ti 2p spectrum, and (d) O 1s spectrum. 80
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It can be seen in Fig. 3c that Ti 2p spectrum of S-TTO consists of two peaks at ~457.9 (Ti 2p3/2) and 463.6 eV (Ti 2p1/2). Compared with TTO, a slight shift of Ti 2p to a lower binding energy was noticed after S doping, which is likely attributed to the interaction between titanium atoms and SO42− anions, resulting in the increase of electron density around the Ti atoms. However, these two peaks are quite similar to those in TTO and 4 | Journal Name, [year], [vol], 00–00
Fig. 4 Diffuse reflectance UV-visible spectra of Na2Ti3O7, TTO, and STTO (inset is the photograph of S-TTO).
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Apparently, S doping could greatly promote the light harvesting ability. As the energy potentials of CB and VB are – 0.53 and 2.67 eV in anatase TiO2,50 and S-TTO has a strong absorption below 446 nm (ca. 2.78 eV), the energy potentials of CB and VB in S-doped anatase are –0.11 and 2.67 eV. It This journal is © The Royal Society of Chemistry [year]
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indicates that only anatase TiO2 can be excited by visible light to generate electron-hole pairs in S-TTO. Moreover, the visible light response behavior after S doping can be also clearly evidenced from the apparent color change of the resulting sample from white to yellow (see inset of Fig. 4).
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Visible light photocatalytic performance
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The visible light photocatalytic activities of the obtained samples were evaluated by the degradation of MB aqueous solution and are shown in Fig. 5. The photocatalytic degradation of MB was monitored by the blank test and the result indicates that the degradation of MB is neglectable in the absence of photocatalyst under visible light irradiation. There is a nonnegligible reduction of MB concentration on Na2Ti3O7 and P25, which is essentially due to the dye-sensitized photocatalysis, as both Na2Ti3O7 and P25 have a large energy band-gap and cannot be activated by visible light.
Fig. 5 (a) Visible light photocatalytic degradation rate of MB over different samples, (b) cycling experiments of S-TTO for MB degradation under visible light irradiation.
After hybridization with TiO2, the resulted TTO shows an enhanced photocatalytic activity than Na2Ti3O7 and P25, which is essentially due to the formation of heterojunction and thus the effective spatial separation of photo-generated electron-hole pairs. Furthermore, after S doping, a significantly enhanced photocatalytic performance was observed in S-TTO, showing the highest activity with 98.6% MB photodegraded in 100 min. It indicates that S doping essentially tunes the electric structure of TTO and thus further effectively enhances the photocatalytic performance. Combined with the results from XPS and UV-vis spectra, the observed photobleaching over S-TTO under visible light irradiation should be mainly due to the oxidative photodegradation of the dye molecules rather than selfphotosensitized oxidation or other reasons. More importantly, compared with the commercial P25, S-TTO shows an obviously enhanced photocatalytic performance. To evaluate the stability of S-TTO photocatalyst, multiple photodegradation tests of MB under visible light illumination were carried out (Fig. 5b). After five times recycling experiments, a high photoactivity with 95.9% MB photodegraded in 100 min still can be reached. It indicates that S-TTO has an excellent photocatalytic stability. The PL technique is useful to reveal the migration, transfer, and recombination process of photogenerated electron-hole pairs in semiconductors. A lower fluorescence emission intensity implies a lower electron-hole recombination rate and thus corresponds to a higher photocatalytic activity. In order to understand the role of TiO2 hybridization and S doping in enhancing visible light photocatalytic activity for MB degradation, Na2Ti3O7, TTO and S-TTO were characterized by This journal is © The Royal Society of Chemistry [year]
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PL with an excitation wavelength of 325 nm and the results are shown in Fig 6a. Na2Ti3O7 shows a strong PL emission peak at ~467 nm which is ascribed to the band gap recombination of electron-hole pairs.51 It indicates that electrons and holes recombine rapidly. The peak positions in TTO and S-TTO are similar to those in pure Na2Ti3O7. It can be observed that TTO has an obviously reduced PL intensity than Na2Ti3O7, indicating that TTO has a much lower recombination rate of photogenerated electron-hole pairs. It confirms the importance of heterojunction in hindering the recombination of electrons and holes. On the other hand, compared with TTO, the PL of S-TTO is further suppressed and shows a diminished intensity in the visible region, indicating that both TiO2 hybridization and S doping can effectively accelerate the separation of charge carriers.
Fig. 6 (a) PL spectra of Na2Ti3O7, TTO and S-TTO with an excitation wavelength of 325 nm, and (b) photocurrent responses of Na2Ti3O7, TTO and S-TTO. 70
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It is well-known that the photocatalytic redox reactions are intimately relevant to the separation efficiency of photoinduced electron-hole pairs arisen from the excited semiconductor materials. To qualitatively investigate the separation efficiency of photoinduced charges during the photoreactions, the photocurrent response was determined for Na2Ti3O7, TTO and S-TTO (Fig. 6b). It can be obviously seen that fast and stable photocurrent responses are observed in all electrodes, and the photoresponsive phenomenon is entirely reversible. Under visible light illumination, pure Na2Ti3O7 electrode shows the weakest response due to its large band gap. On the contrary, the photocurrent of TTO electrode is about 3.2 times higher than that of Na2Ti3O7 electrode. The remarkable photocurrent enhancement of TTO photocatalyst reveals an enhanced separation efficiency of the photogenerated electrons and holes, which can be attributed to the formation of a heterojunction structure between Na2Ti6O13 and anatase TiO2. Furthermore, after S doping, the photocurrent of the resulted S-TTO is much higher than that of Na2Ti3O7 and TTO, being attributed not only to the formation of a nano-heterojunction interface structure between Na2Ti6O13 and anatase TiO2 but also to the enhancement of the visible light response arisen from S doping. Active species and possible photocatalytic mechanism
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Based on previously reported studies,52, 53 in order to understand which reactive species played an important role in photocatalysis under visible light irradiation, the main active species were detected by adding four different chemicals, i.e., Na2C2O4 (holes scavenger), AgNO3 (electrons scavenger), tBuOH (•OH scavenger) and BQ (O2•− scavenger), into the photocatalytic reaction systems. It was noted that the MB Journal Name, [year], [vol], 00–00 | 5
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photodegradation was suppressed when the scavengers were added in the photocatalytic reaction system, indicating the importance of the corresponding active species.
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potential of O2/O2•− (−0.33 V vs. NHE).52 Meanwhile, as the VB potential of S-doped anatase TiO2 of (2.67 eV) is much more positive than the standard redox potential of •OH/OH− (1.99 eV vs. NHE) 53, 54, the formed holes can react with OH− groups or H2O molecules to produce •OH radicals. The formed •OH radicals are able to oxidize the organic pollutant due to their high oxidative capacity. Therefore, the efficiently separation of photogenerated electrons and holes can be achieved, leading to the improved utilization of charge carriers and ultimately the enhanced visiblelight photodegradation rate of MB.
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Fig. 7 Effects of different scavengers on the degradation efficiency of MB.
As shown in Fig. 7, different scavengers have different effects on the degradation of MB over S-TTO sample. The degradation efficiency was greatly decreased to 64.3%, 74.9%, 78.2% and 89.6% upon addition of t-BuOH, AgNO3, BQ and Na2C2O4, respectively. It indicates that •OH, e−, O2•− and h+ all participate in the photocatalytic process. Moreover, •OH is the most crucial species and h+ contributes to a lesser extent in MB degradation.
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Fig. 8 (a) Energy levels of Na2Ti6O13 and anatase TiO2 using normal hydrogen electrode (NHE) as reference at pH 7. The potential of CB band in TiO2 was calculated from ECB = –0.12–0.059 pH. (b) A schematic illustration of S-TTO composite catalyst for the photodegradation of MB.
Based on the above results and discussion, a possible mechanism for the photodegradation reaction over S-TTO is proposed and schematically shown in Fig. 8. Under visible light irradiation, S-doped anatase in S-TTO can be excited to generate electron-hole pairs. As the energy potentials of CB and VB in Na2Ti6O13 are −0.50 and 3.03 eV while those in S-doped anatase are −0.11 and 2.67 eV, electrons are excited from VB to intraband-gap states created by the substitution of Ti4+ by S6+, and then to CB. The photogenerated electrons accumulated on TiO2 will migrate from CB of TiO2 to that of Na2Ti6O13 due to the potential difference. In such a way, the photogenerated electrons can be effectively collected by Na2Ti6O13 while holes by TiO2. The anisotropically shaped titanate nanorods provide a pathway for the quick transport of photogenerated electrons throughout the longitudinal direction to the catalyst surface and thus be collected by exposed anatase {101} facets and surface-adsorbed SO42−. These electrons then react with dissolved oxygen to generate O2•− and further •OH because the CB edge potential of Na2Ti6O13 (−0.50 V vs. NHE) was more negative than the standard redox
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A facile calcination method has been developed to prepare Sdoped Na2Ti6O13@TiO2 core-shell nanorods, in which anatase TiO2 NPs with exposed {101} facets are deposited on the external surface of titanate core, leading to the formation of nanoheterojunction structure between two components due to the common structural features of TiO6 octahedra in anatase TiO2 and Na2Ti6O13. The resulted composite displayed an excellent visible-light photocatalytic performance in degrading MB solution due to the following reasons. Firstly, the nanoheterojunction formed between Na2Ti6O13 and anatase TiO2 speeds up the separation rate of photogenerated charge carriers. Secondly, the partial substitution of Ti4+ by S6+ in the lattice of anatase TiO2 enhances the visible-light absorption and leads to the formation of considerable adsorbed hydroxide ions which could capture the photoinduced holes to form active hydroxyl radicals. In addition, with the formation of Ti−O−S bond, partial electrons can be transferred from S to O atoms. The electrondeficient S atoms might capture e− and thus inhibits the recombination of photogenerated electron-hole pairs. Thirdly, the exposed anatase {101} facets could be acted as a possible reservoir of the photogenerated electrons, yielding a highly reactive surface for the reduction of O2 to O2•− and thus the decrease of recombination probability of electron-hole pairs. Fourthly, the anisotropically shaped titanate nanorods provide a pathway for the quick transport of charge carriers throughout the longitudinal direction to different reaction sites for subsequent photocatalytic reactions. Finally, the doped sulfur atoms favorably stabilize the anatase structure, leading to a high photocatalytic stability. The present work may provide an important indication of how to construct novel visible-light photocatalysts through an integration of hybridization, doping and morphology engineering.
Acknowledgements
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The authors greatly appreciate the financial support of the Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP, 20130091110010), Natural Science Foundation of Jiangsu Province (BK2011438), National Science Fund for Talent Training in Basic Science (No. J1103310), the National Basic Research Program (973 Project) (No. 2009CB623504) and the Modern Analysis Center of Nanjing University.
Notes and references This journal is © The Royal Society of Chemistry [year]
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a
Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210093, P. R. China. E-mail: [email protected] 75 5
b
Department of Applied Chemistry, College of Science, Nanjing Tech University, Nanjing, 211816, P. R. China. E-mail: [email protected] 1.
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20
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25
11. 12. 13.
30
14. 15. 35
16. 17. 40
18. 19.
45
20. 21. 22.
50
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J. S. Jang, H. G. Kim and J. S. Lee, Catal. Today, 2012, 185, 270277. C. Chen, W. Ma and J. Zhao, Chem. Soc. Rev., 2010, 39, 4206-4219. X. Chen, S. Shen, L. Guo and S. Mao, Chem. Rev., 2010, 110, 65036570. A. L. Linsebigler, G. Lu and J. T. Yates, Chem. Rev., 1995, 95, 735758. J. Tian, Z. Zhao, A. Kumar, R. I. Boughton and H. Liu, Chem. Soc. Rev., 2014, 43, 6920-6937. L. Sang, Y. Zhao and C. Burda, Chem. Rev., 2014, 114, 9283-9318. Z. Zhai, C. H. Hu, X. Yang, L. Zhang, C. Liu, Y. Fan and W. Hou, J. Mater. Chem., 2012, 22, 19122-19131. M. Dahl, Y. Liu and Y. Yin, Chem. Rev., 2014, 114, 9853-9889. S. Ogura, M. Kohno, K. Sato and Y. Inoue, J. Mater. Chem., 1998, 8, 2335-2337. Y. Inoue, T. Kubokawa and K. Sato, J. Phys. Chem., 1991, 95, 40594063. H. Yoshida, M. Takeuchi, M. Sato, L. Zhang, T. Teshima and M. G. Chaskar, Catal. Today, 2014, 232, 158-164. A. Rudola, K. Saravanan, S. Devaraj, H. Gong and P. Balaya, Chem. Commun., 2013, 49, 7451-7453. J. C. Perez-Flores, C. Baehtz, M. Hoelzel, A. Kuhn and F. GarciaAlvarado, Phys. Chem. Chem. Phys., 2012, 14, 2892-2899. P. Senguttuvan, G. L. Rousse, V. Seznec, J. M. Tarascon and M. R. Palacín, Chem. Mater., 2011, 23, 4109-4111. Y. Wang, G. Du, H. Liu, D. Liu, S. Qin, N. Wang, C. Hu, X. Tao, J. Jiao, J. Wang and Z. L. Wang, Adv. Funct. Mater., 2008, 18, 11311137. Z. Zhai, Y. Huang, L. Xu, X. Yang, C. Hu, L. Zhang, Y. Fan and W. Hou, Nano Res., 2011, 4, 635-647. Z. Zhai, X. Yang, L. Xu, C. Hu, L. Zhang, W. Hou and Y. Fan, Nanoscale, 2012, 4, 547-556. L. Zhang, C. Hu, L. Cheng, W. Ding, W. Hou and J. Chen, Chin. J. Catal., 2013, 34, 2089-2097. G. Zhang, Y. Zhang, M. Nadagouda, C. Han, K. O'Shea, S. M. ElSheikh, A. A. Ismail and D. D. Dionysiou, Appl. Catal. B: Environ., 2014, 144, 614-621. S. Ida, Y. Okamoto, M. Matsuka, H. Hagiwara and T. Ishihara, J. Am. Chem. Soc., 2012, 134, 15773-15782. R. Asahi, T. Morikawa, H. Irie and T. Ohwaki, Chem. Rev., 2014, 114, 9824-9852. T. Umebayashi, T. Yamaki, H. Itoh and K. Asai, Appl. Phys. Lett., 2002, 81, 454-456. T. Umebayashi, T. Yamaki, S. Tanaka and K. Asai, Chem. Letts, 2003, 32, 330-331. T. Umebayashi, T. Yamaki, S. Yamamoto, A. Miyashita, S. Tanaka, T. Sumita and K. Asai, J. Appl. Phys, 2003, 93, 5156-5160. J. Yu, W. Ho, J. Yu, H. Yi, P. Wong and J. Zhao, Environ. Sci. Technol., 2005, 39, 1175-1179. T. Ohno, M. Akiyoshi, T. Umebayashi, K. Asai, T. Mitsui and M. Matsumura, Appl. Catal. A: Gen., 2004, 265, 115-121. A. Kudo and Y. Miseki, Chem. Soc. Rev., 2009, 38, 253-278. L. Ye, J. Liu, Z. Jiang, T. Peng and L. Zan, Nanoscale, 2013, 5, 9391-9396. N. Wu, J. Wang, D. N. Tafen, H. Wang, J. Zheng, J. Lewis, X. Liu, S. S. Leonard and A. Manivannan, J. Am. Chem. Soc., 2010, 132, 6679-6685. J. Pan, G. Liu, G. Q. Lu and H. Cheng, Angew. Chem., Int. Ed., 2011, 50, 2133-2137. Z. Xiong and X. S. Zhao, J. Am. Chem. Soc., 2012, 134, 5754-5757. C. Liu, T. Sun, L. Wu, J. Liang, Q. Huang, J. Chen and W. Hou, Appl. Catal. B: Environ., 2015, 170, 17-24. N. Miyamoto, K. Kuroda and M. Ogawa, J. Mater. Chem., 2004, 14, 165-170.
This journal is © The Royal Society of Chemistry [year]
80
85
90
95
100
105
110
34. L. Zhang, C. Hu, J. Zhang, L. Cheng, Z. Zhai, J. Chen, W. Ding and W. Hou, Chem. Commun., 2013, 49, 7507-7509. 35. G. Liu, H. Yang, J. Pan, Y. Yang, G. Lu and H. Cheng, Chem. Rev., 2014, 114, 9559-9612. 36. M. D’Arienzo, J. Carbajo, A. Bahamonde, M. Crippa, S. Polizzi, R. Scotti, L. Wahba and F. Morazzoni, J. Am. Chem. Soc., 2011, 133, 17652-17661. 37. F. Amano, T. Yasumoto, O. O. Prieto-Mahaney, S. Uchida, T. Shibayama and B. Ohtani, Chem. Commun., 2009, 2311-2313. 38. P. D. Cozzoli, A. Kornowski and H. Weller, J. Am. Chem. Soc., 2003, 125, 14539-14548. 39. A. Riss, M. J. Elser, J. Bernardi and O. Diwald, J. Am. Chem. Soc., 2009, 131, 6198-6206. 40. H. Liu, D. Yang, Z. Zheng, X. Ke, E. Waclawik, H. Zhu and R. L. Frost, J. Raman Spectrosc., 2010, 41, 1331-1337. 41. J. K. Burdett, T. Hughbanks, G. J. Miller, J. W. Richardson and J. V. Smith, J. Am. Chem. Soc., 1987, 109, 3639-3646. 42. B. J. Lindberg and K. Hamrin, Acta Chem. Scand., 1970, 24, 36613671. 43. D. Ma, Y. Xin, M. Gao and J. Wu, Appl. Catal. B: Environ., 2014, 147, 49-57. 44. P. V. R. K. Ramacharyulu, J. Praveen Kumar, G. K. Prasad and B. Sreedhar, Mater. Chem. Phys., 2014, 148, 692-698. 45. S. Yin, K. Ihara, Y. Aita, M. Komatsu and T. Sato, J. Photoch. Photobio. A: Chem., 2006, 179, 105-114. 46. D. J. V. Pulsipher, I. T. Martin and E. R. Fisher, ACS Appl. Mater. Inter., 2010, 2, 1743-1753. 47. G. Colón, M. C. Hidalgo, G. Munuera, I. Ferino, M. G. Cutrufello and J. A. Navío, Appl. Catal. B: Environ., 2006, 63, 45-59. 48. Z. Chen, B. Lin, Y. Chen, K. Zhang, B. Li and H. Zhu, J. Phys. Chem. Solids, 2010, 71, 841-847. 49. Y. Zhao, C. Li, X. Liu, F. Gu, H. L. Du and L. Shi, Appl. Catal. B: Environ., 2008, 79, 208-215. 50. T. Tachikawa, M. Fujitsuka and T. Majima, J. Phy. Chem. C, 2007, 111, 5259-5275. 51. C. Liu, L. Wu, J. Chen, J. Liang, C. Li, H. Ji and W. Hou, Phys. Chem. Chem. Phys., 2014, 16, 13409-13417. 52. D. Zhang, Q. Wang, L. Wang and L. Zhang, J. Mater. Chem. A, 2015, 3, 3576-3585. 53. K. Li, S. Gao, Q. Wang, H. Xu, Z. Wang, B. Huang, Y. Dai and J. Lu, ACS Appl. Mater. Inter., 2015, 7, 9023-9030. 54. J. Yu, S. Wang, J. Low and W. Xiao, Phys. Chem. Chem. Phys., 2013, 15, 16883-16890.
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