Photoinduced Terminal Fluorine and Ti 3+ in TiOF 2 /TiO 2 Heterostructure for Enhanced Charge Transfer

Open AccessCCS ChemistryCOMMUNICATION1 Dec 2020Photoinduced Terminal Fluorine and Ti3+ in TiOF2/TiO2 Heterostructure for Enhanced Charge Transfer Jie Hu†, Yi Lu†, Xiao-Long Liu, Christoph Janiak, Wei Geng, Si-Ming Wu, Xiao-Fang Zhao, Li-Ying Wang, Ge Tian, Yuexing Zhang, Bao-Lian Su and Xiao-Yu Yang Jie Hu† State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070 School of Materials Science and Engineering, Chongqing Jiaotong University, Chongqing 400074 †J. Hu and Y. Lu contributed equally to this work.Google Scholar More articles by this author , Yi Lu† State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070 School of Chemical Engineering and Technology, School of Materials, Sun Yat-sen University,Zhuhai 519000 †J. Hu and Y. Lu contributed equally to this work.Google Scholar More articles by this author , Xiao-Long Liu School of Chemical Engineering and Technology, School of Materials, Sun Yat-sen University,Zhuhai 519000 Google Scholar More articles by this author , Christoph Janiak Institut für Anorganische Chemie und Strukturchemie, Heinrich-Heine-Universität Düsseldorf, Düsseldorf 40204 Google Scholar More articles by this author , Wei Geng State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070 School of Chemical Engineering and Technology, School of Materials, Sun Yat-sen University,Zhuhai 519000 Google Scholar More articles by this author , Si-Ming Wu State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070 Google Scholar More articles by this author , Xiao-Fang Zhao State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070 Google Scholar More articles by this author , Li-Ying Wang State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071 Google Scholar More articles by this author , Ge Tian *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070 School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138 Google Scholar More articles by this author , Yuexing Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Ministry of Education Key Laboratory for the Synthesis and Application of Organic Functional Molecules, College of Chemistry and Chemical Engineering, Hubei University, Wuhan 430062 Google Scholar More articles by this author , Bao-Lian Su *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070 Laboratory of Inorganic Materials Chemistry (CMI), University of Namur, Namur B-5000, Google Scholar More articles by this author and Xiao-Yu Yang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070 School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.020.202000305 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail As an effective way to enhance the photo/electro-catalytic performance of titanium dioxide (TiO2) is to explore the positive roles of doped fluorine sites in the fluorinated TiO2 systems, which, currently still lacks the direct experimental evidence due to the complexity of the species involved. Herein, we have fabricated TiOF2/TiO2 with interfacial bridging fluorine (Ti2–F) via a coherent phase transition through hydrothermal synthesis. Nuclear magnetic resonance and electron paramagnetic resonance characterization have provided strong evidence of the transformation of the doped fluorine from Ti2–F to Ti1–F and the subsequent generation of Ti3+ at the interface of the TiOF2 and TiO2 under UV–visible (UV–vis) light irradiation. Density functional theory (DFT) calculations and photo/electrochemical measurements further confirmed the electron donor behavior of the Ti3+. The benefit is a significantly enhanced charge transfer efficiency in TiOF2/TiO2, which not only resulted in improved performances for the photodegradation of acetone being 5.5 times higher than the commercial TiO2 but also supported high capacity for sodium-ion storage. Thus, the TiOF2/TiO2 with Ti2–F provided a perfect structure to investigate the roles of fluorine sites in fluorinated TiO2 systems and their interaction with material properties. Download figure Download PowerPoint Introduction Titanium dioxide (TiO2) is one of the most attractive semiconductors,1–3 and the heteroatom substitution in the oxygen or interstitial lattice sites has been considered as a very promising means of improving its photo/electro performances.4,5 The impurity states greatly enhance the absorption of visible light, overlap sufficiently with the band states of TiO2 to transfer carriers to reactive sites, and significantly increased the conduction band minimum of TiO2 above the H2/H2O level to ensure their photo/electrochemical activity.6,7 Fluorine (F), as an element with strong bonding to Ti ( D 0 F - T i = 569.0 k J m o l − 1 ) and closest to the atom radius of oxygen, enables the stable doping of F atoms in TiO2 without the distortion of the lattice structure and the retention of high photocatalytic activity without compromising the superior stability of TiO2.5 Thus, F doping in TiO2 provides an effective approach for enhancing the surface acidity, increasing the adsorption of reactant molecules, introducing Ti3+ self-doping, and minimizing the surface energy of the crystal facets.8–10 It has long been assumed that the insertion of F− in the O2− sites of the TiO2 lattice needs one extra electron for charge compensation, which is expected to reduce Ti4+ to Ti3+ with consequent polaronic distortion.11,12 Ti3+ self-doping has been used widely as an effective measure to enhance the photo/electro performance of TiO2 materials. Doping of Ti3+ could improve the electrical conductivity of TiO2 materials and create an intermediate gap between the valence band and conduction band, which enhanced the separation and transportation of photogenerated electron–hole (e−/h+) pairs.13–16 After substituting O atom or hydroxyl groups, the F atoms doped in fluorinated TiO2 system by a variety of chemical bonds, which could be marked as Tix–F, where x is the number of titanium atoms coordinated with the lattice F.17 Owing to the lack of direct experimental evidence, the most controversial point concerns the role of F sites, including terminal F (Ti1–F), bridging F (Ti2–F), and 3-coordinated F (Ti3–F) on the surface or in the lattice, which hindered the identification of the charge-transfer mechanism.17 Especially, for Ti1–F, it seems impossible to clarify the status of the surface lattice F and the adsorbed F species fully, which could both influence the physical and chemical surface properties of TiO2 directly, and thus, might promote the photo/electro performance.18,19 Understanding and characterization of the various F sites in the fluorinated TiO2 systems at the atomic level are, therefore, of fundamental interest and technological importance for the design and applications of high-performance TiO2. The crystal lattice of TiOF2 is constructed entirely by Ti2–F sites (also commonly used to transform to fluorinated TiO2 nanocrystals with Ti3–F sites).20 In the (001) TiOF2-(001) TiO2 interface model, one αTiOF2(001) matches one αTiO2(001) along the α direction, with the lattice mismatch of [(αTiOF2(001) − αTiO2(001))/αTiO2(001) = 0.34%].21,22 The high lattice match of the interface between TiOF2 and TiO2 provides the possibility to direct generation of metastable Ti2–F and Ti3–F sites with the absence of adsorbed F species. This interface offers a perfect structure to experimentally and theoretically discover the roles of Tix–F in TiO2 without any influence of adsorbed F species. Herein, we have fabricated TiOF2/TiO2 nanosheets with a high lattice match via a coherent phase transition (from HTiOF3 to TiOF2/TiO2) by a hydrothermal synthesis process. Nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), and combined photo/electrochemical techniques were utilized in the characterization of the fluorine coordination in the F-doped TiO2. The results showed that the Ti2–F could be converted dynamically into Ti1–F with the generation of Ti3+ at the interface of the TiOF2 and TiO2 phases under UV–visible (UV–vis) irradiation, with significant enhancement of the photo/electro performances. Results and Discussion We utilized high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), transmission electron microscopy (TEM; Figure 1), and scanning electron microscopy (SEM) characterizations shown in the images displayed in the Supporting Information Figure S1, to show that the flower-like self-assembled TiOF2/TiO2 morphology consists of uniform nanosheets (100 nm in size). A powder X-ray diffraction (PXRD) pattern exhibited diffraction peaks of TiOF2/TiO2, attributable to cubic TiOF2 [International Centre for Diffraction Data (ICDD) no. 00-08-0060] and anatase TiO2 (ICDD no. 01-0086-1157) phase ( Supporting Information Figure S2). HAADF-STEM image (Figure 1a) and the corresponding energy-dispersive X-ray spectroscopy (EDX) elemental maps (Figures 1b and 1c) demonstrated that Ti and F were distributed uniformly in the TiOF2/TiO2 nanosheets. Figures 1d and 1e (original image in Supporting Information Figure S3) confirmed further that the ultrathin TiOF2/TiO2 nanosheets had uniform size and thickness. Figures 1f and 1g (original image in Supporting Information Figure S4a) showed that the lattice fringes of 0.38 nm of the (100) plane of TiOF223 are in very good agreement with twice the (200) surface plane separation of TiO2 (0.19 nm), suggesting a high-level lattice match owing to the direct phase transformation from TiOF2 to TiO2. The amorphous structure of interface might have been caused by the lattice stress, and strong interface interaction ( Supporting Information Figures S4b and S4c).24 Note that this very thick nanofusion domain could interconnect with the ordered lattice fringes of TiOF2 and TiO2, which also enabled the quantized ballistic transport of electrons, and thus, beneficial for the enhancement of charge mobility and the minimization of energy loss.25–28 Figure 1 | (a–d) HAADF-STEM images and corresponding EDX elemental maps of TiOF2/TiO2. (e) TEM image of TiOF2/TiO2. (f) HRTEM image of nanocrystalline domains of anatase TiO2, TiOF2, and interface. (g) Inverse FFT pattern of the region I of interfacial domains of TiO2 and TiOF2. HAADF-STEM, high-angle annular dark-field scanning transmission electron microscopy; EDX, dispersive X-ray spectroscopy; TEM, transmission electron microscopy; HRTEM, high-resolution transmission electron microscopy; FFT, fast Fourier transform. Download figure Download PowerPoint To clarify the structure of the F-doped interface in TiOF2/TiO2, 19F solid-state NMR spectroscopy was performed. As shown in Figure 2a, the NMR signal at ∼15 ppm with multiple spin sidebands assigned to the Ti2–F from the TiOF2 lattice was identified in both TiOF2 and TiOF2/TiO2.29 Besides, a new signal at −84 ppm appeared in TiOF2/TiO2, which was assigned to the Ti3–F from a previous report.29 Note that the sample of TiOF2/TiO2 exhibited an NMR chemical shift of −151 ppm after UV–vis light irradiation. According to previous studies, this signal was attributable to Ti1–F.30 These Ti1–F bonds also influenced the Raman spectrum and led to a slight redshift due to the change in the electronic environment surrounding the lattice Ti atoms ( Supporting Information Figure S5). There was almost no change in chemical shifts in the NMR spectra of TiOF2 before and after irradiation ( Supporting Information Figure S6). Notably, with the prolongation of UV–vis light irradiation, the relative intensity of the NMR line at −151 ppm increased gradually, which revealed the possibility of a higher content of the Ti1–F being formed under irradiation. After the TiOF2/TiO2-I30 stored for 2 months, the signal assigned to Ti1–F (−151 ppm) showed an increased relative intensity, indicating the transformation continued even under ambient light. Furthermore, the Ti1–F signal increased dramatically for an additional 30 min of UV–vis light irradiation, suggesting that the photoinduced Ti1–F was stable and the photoinduced transformation was sustainable ( Supporting Information Figure S7). To gain more insight into the structural feature of the interfacial F, EPR experiments were conducted before and after UV–vis light irradiation, as depicted in Figure 2b and Supporting Information Figure S8. The spectra of TiOF2, TiO2, and TiOF2/TiO2 showed no distinct EPR signal before irradiation. In contrast, under the UV–vis light irradiation for 2 min, a significantly new signal at g = 1.935 appeared in TiOF2/TiO2, and its intensity increased further after 5 min of irradiation. This new signal was assigned to Ti3+, according to previous reports.31–33 Additionally, a small peak was observed at g = 2.001 after light irradiation, which could be attributed to oxygen vacancies.25,28 Together with the 19F NMR results, we inferred that the terminal Ti1–F sites and Ti3+ defects were induced under the UV–vis light irradiation, and the content increased with irradiation time. X-ray photoelectron spectroscopy (XPS) investigations of TiOF2/TiO2, TiOF2, and TiO2 before and after irradiation yielded information about the chemical state of the surface. As shown in Figure 2c, the growth of terminal Ti1–F in TiOF2/TiO2 after light irradiation caused an F 1s core-level shift of ∼−0.33 eV.11,34 Simultaneously, the F 1s core level of TiOF2 showed no significant shift after light irradiation, as no chemical state change occurred around the F atoms ( Supporting Information Figure S9a). Followed by the light irradiation, the corresponding Ti 2p3/2 core level of TiOF2/TiO2 shifted to higher binding energy, which was attributed to the generation of lattice Ti3+–F.33 Therefore, these Ti3+ sites could act as potential electron donor sites, availing to the carrier transfer, which could be the reason for the enhanced photo/electro performance.35 No core-level shifts were observed for TiO2 and TiOF2 after light irradiation due to the stabilized chemical environment around the Ti atoms (Figure 2d and Supporting Information Figure S9b). Next, we simulated the geometric structures and calculated the deformation density of the Ti2–F, Ti3–F, and Ti1–F sites (Figures 2e–2j), respectively. The neighboring Ti atoms of Ti1–F sites got more electrons, compared with those on the Ti2–F or Ti3–F sites (Figures 2f, 2h, and 2j). The generation of terminal Ti1–F in TiOF2/TiO2 moved more electrons toward the terminal F atom resulting in the acceleration of the interfacial charge transfer. Figure 2 | (a) 19F NMR spectra, (b) EPR spectra, (c) relative F 1s, and (d) relative Ti 2p3/2 XPS binding energies of TiOF2/TiO2 before and after UV–vis light irradiation. (e–j) Simulated geometric structures (key: blue, F; red, O; gray, Ti) and deformation density of (e, f) Ti2–F, (g, h) Ti3–F, and (i, j) Ti1–F, respectively. TiOF2/TiO2 after UV–vis light irradiation for x minutes is denoted as TiOF2/TiO2–Ix. NMR, nuclear magnetic resonance; EPR, electron paramagnetic resonance; XPS, X-ray photoelectron spectroscopy. Download figure Download PowerPoint Further, we employed electrochemical impedance spectroscopy (EIS), transient photocurrent density, photoluminescence (PL) spectra, and photoactivity investigations to elucidate the charge separation and transfer in the different samples (TiOF2/TiO2, TiO2, and TiOF2) (Figure 3 and Supporting Information Figure S10). In the EIS Nyquist plots, the calculated diameter of the arc radius of the TiOF2/TiO2 was smaller than that of the TiOF2 and TiO2, both in the dark and under irradiation, suggesting a higher charge transfer efficiency of TiOF2/TiO2 (Figure 3a). The charge separation and transfer were also reflected in the transient photocurrent response (Figure 3b).The composite TiO2/TiOF2 exhibited enhanced photocurrent intensity (PI) of 2.1 μA, which is 2.6-fold and 21.2-fold than that of TiO2 and TiOF2, respectively.. Following the UV–vis light being switched off, TiOF2/TiO2 showed an apparent persistent photocurrent tail, indicating the advanced inhibition of interface charge recombination.36 The evident PL quenching of TiOF2/TiO2 implied an antirecombination of photogenerated charges from the excitonic state, which stemmed from the highly effective electron transfer from the TiO2 to the TiOF2 partly via the interface (Figure 3c). As shown in Figure 3d, the photocatalytic degradation of acetone by TiOF2/TiO2 is >11.9-, 7.2-, and 5.5-fold higher than TiO2, TiOF2/TiO2 mixture, and P25, respectively. TiOF2 showed only trace photocatalytic activity. For photocatalytic stability, the crystal structure and morphology of TiOF2/TiO2 after the experiment showed no significant change by observation of the PXRD pattern and SEM image ( Supporting Information Figure S11). These results confirmed that TiOF2/TiO2 was a stable and reusable photocatalyst. As shown in Figure 3e, TiOF2/TiO2, the highest capacity of 235, 211, 195, 180, 162, 148, and 138 mA h g−1 cycled at 0.1, 0.25, 0.5, 1, 2.5, 5, and 10 C, respectively. The reversible capacity of TiOF2/TiO2 (Figure 3f) reached ∼180 mAh g−1 at a current density of 336 mA g−1, which was higher than that of TiOF2 (21 mAh g−1) and TiO2 (150 mAh g−1). Furthermore, the capacity retention of 91.6% of TiOF2/TiO2 attained after 800 cycles demonstrated an excellent cycling performance due to better electronic conductivity.37,38 The photochemical and electrochemical experiments demonstrated that TiOF2/TiO2 demonstrated significant enhancement of photo/electrochemical activity due to its higher charge transfer efficiency. Figure 3 | (a) EIS Nyquist plots, (b) photocurrent density, and (c) PL emission spectra for TiOF2/TiO2, TiO2, and TiOF2. (d) Comparison of the apparent rate constants upon acetone degradation of P25, TiOF2/TiO2, TiO2, TiOF2, and TiOF2–TiO2 (with the same proportion of TiOF2/TiO2). (e) Rate capability and (f) cycling performance at 1 C of TiOF2/TiO2, TiO2, and TiOF2 upon Na insertion/deinsertion densities. EIS, electrochemical impedance spectroscopy; PL, photoluminescence. Download figure Download PowerPoint A schematic illustration of the synthesis of TiOF2/TiO2 nanosheets is shown in Figure 4a. After mixing the titanium source, toluene and aqueous hydrofluoric acid (HF), the layered HTiOF3 forms at the oil/water interface and precipitates [Figures 4a(i) and 4a(ii)],39 which then transforms to generate the TiOF2/TiO2 composite during hydrothermal treatment [Figure 4a(iii)]. Finally, the composite transforms into nanosheets and assembles into a flower-like morphology [Figure 4a(iv)]. The synthesis process is confirmed by PXRD ( Supporting Information Figure S12) and described by the Equations (1–3, Figure 4a).39–41 As shown in Figure 4b, upon absorption of UV–vis light, TiO2 (or TiOF2) could be excited and generates e−/h+ pairs (Equation 4, Figure 4b). These photogenerated electrons might be trapped at metal centers as Ti3+.42 From the density functional theory (DFT) calculation results, and the Ti–F bond distances in interfacial Ti2–F sites are ∼2.065 Å (link with TiOF2 lattice) and 1.783 Å (link with TiO2 lattice), respectively ( Supporting Information Figure S13). This interfacial Ti2–F located at the nanofusion domain is in a metastable status, which is caused by the 0.34% interfacial lattice mismatch ( Supporting Information Figure S5) and shows lower stability in comparison with Ti3–F.12 In Ti2–F bonds, the Ti–F bond linking with the TiOF2 lattice would probably break and cause the formation of terminal Ti1–F and corresponding Ti3+ under the UV–vis light irradiation (Equation 5, Figure 4b).36 The interface with dynamic Ti2–F facilitates the electron transfer and minimizes the energy loss, likely to not only benefit the interfacial conduction and favor the transfer of electrons or Na+ (Figure 4b, left) but also improve the separation of photogenerated carriers in photocatalysis (Figure 4b, right). The nanofusion phase at atomic-scale results in a structural disorder at the interface provides higher capacity and less structural confinement for Na+ in the insertion/extraction reaction. The photoinduced Ti3+ acts as an electron donor and facilitates the photogenerated electrons transfer to the surface/interface. The dynamic Ti2–F is the key to produce Ti3+, and thus, enhances the photocatalytic efficiency. Figure 4 | (a) (i–iv) Schematic procedure for the formation mechanism of TiOF2/TiO2. (b) Proposed mechanism of the photocatalytic properties and electrochemical properties of the TiO2/TiOF2, the holes could oxidize (Ox) the electron donor (D). Download figure Download PowerPoint Conclusion We have fabricated TiOF2/TiO2 nanosheets with dynamic interfacial Ti2–F via a coherent phase transition and show significantly enhanced performances regarding the photogeneration current, photocatalysis, and energy storage. Through the conduction of experiments and theoretical computations we studied the dynamics of the F sites in TiOF2/TiO2 and its specificities thoroughly, relative to (1) the nanofusion domains with minimization of the energy loss, and (2) the generation of Ti3+ during the transformation of Ti1–F from Ti2–F under UV–vis light irradiation. This study gives a new perspective on the role of F sites in fluorinated TiO2 systems, and provides new insight into the investigation of interfacial atoms and the high-performance design of semiconductors. Supporting Information Supporting Information is available. Conflict of Interest There is no conflict of interest to report. Funding Information This research was made possible as a result of a generous grant from the National Key R&D Program of China (no. 2017YFC1103800), the Program for Changjiang Scholars and Innovative Research Team in University (no. IRT_15R52), the joint National Natural Science Foundation of China-Deutsche Forschungsgemeinschaft (NSFC-DFG) project (NSFC grant no. 51861135313 and DFG no. JA466/39-1), the China Postdoctoral Science Foundation (no. 2020M672961), the International Science and Technology Cooperation Program of China (no. 2015DFE52870), the Fundamental Research Funds for the Central Universities (nos. 19lgpy113 and 19lgzd16), the Science and Technology Research Project of Hubei Province Department of Education (no. D20191001), Jilin Province Science and Technology Development Plan (no. 20180101208JC), and the National 111 project (no. B20002), Ministry of Science and Technology and Ministry of Education, China. References 1. Fujishima A.; Honda K.Electrochemical Photolysis of Water at a Semiconductor Electrode.Nature1972, 238, 37–38. Google Scholar 2. Chen X. B.; Mao S. S.Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications.Chem. Rev.2007, 107, 2891–2959. 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