Zn-Assisted TiO2–x Photocatalyst with Efficient Charge Separation for

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Zn-Assisted TiO X Photocatalyst With Efficient Charge Separation for Enhanced Photocatalytic Activities Wenzhang Fang, Frederic Dappozze, Chantal Guillard, Yi Zhou, Mingyang Xing, Shashank Mishra, Stephane Daniele, and Jinlong Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b03724 • Publication Date (Web): 24 Jul 2017 Downloaded from http://pubs.acs.org on July 28, 2017

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The Journal of Physical Chemistry

Zn-assisted TiO2-x Photocatalyst with Efficient Charge Separation for Enhanced Photocatalytic Activities Wenzhang Fanga, b, Frederic Dappozzeb, Chantal Guillardb, Yi Zhoua, Mingyang Xinga, Shashank Mishrab, Stéphane Danieleb* and Jinlong Zhanga* a

Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry

and Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China b

Université Claude Bernard Lyon 1, Institut de Recherches sur la Catalyse et l’Environnement

de Lyon (IRCELYON), CNRS, UMR 5256, 2 avenue Albert Einstein, 69626 Villeurbanne, France

ACS Paragon Plus Environment

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ABSTRACT A new Zn-assisted method has been employed to synthesize reduced TiO2(TiO2-x) photocatalyst via one-step hydrothermal process. In order to prevent the oxidation of TiO2-x in air, hydrofluoric acid is introduced in the preparative process for the stabilization of the Ti3+ and oxygen vacancies, as confirmed by low-temperature electron paramagnetic resonance (EPR). The obtained reduced TiO2 presents a wide-spectrum solar light absorption including the nearinfrared region. In addition, {110}-{111} and {101}-(001) dual-facets exposures are generated by Cl- and F-based surface terminated reagents, respectively. The generation of {001} and {101} facets on reduced TiO2 samples act as holes and electrons collectors, respectively, which contributes to charge separation of the catalyst. Finally, the synergistic effect between Ti3+ doping and dual-facets exposure results in the high photocatalytic performance for degradation of Rhodamine B and formic acid.


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1. Introduction Reduced TiO2, or TiO2-x photocatalysts, have received plenty of attentions recently.1-3 By hydrogen incorporation or oxygen removal, reduced TiO2 could be achieved with extended solar light absorption in the visible and near-infrared regions. Compared to traditional metal and nonmetal doping, the self-modification of TiO2 does not introduce any alien elements, which contributes to the inhibition of photogenerated electron-hole recombination.4 As a result, reduced TiO2 photocatalyst indicates promising applications for photocatalytic hydrogen generation from water splitting, photodegradation of organic pollutions, photocatalytic CO2 reduction, solar cells, lithium cells, etc.5 In general, reduced TiO2 can be prepared either by the reduction of Ti(IV) precursors or the oxidation of low-valent titanium compounds.6 Until now, a variety of methods have been reported on the synthesis of reduced TiO2, including calcination under H2 atmosphere,1, 7 vacuum activation,2, 8-9 metal reduction,3, 10-12 electrochemical reduction,13-16 UV irradiation,17-19 plasma treatment,20-24 and partial oxidation starting from Ti,25-27 Ti(II)28-30 and Ti(III)31-32 precursors, etc. The design for the preparation of reduced TiO2 makes a significant effect on the intrinsic properties of the final catalysts, including the generation of oxygen vacancies, the incorporation of H (Ti-H groups) and the enhancement of Ti-OH bonds.4 Metallic zinc is reported to be an efficient reducing agent during the preparation of reduced TiO2 nanomaterials. Zheng et al. employed Zn powder as the reducing agent in TiCl4 ethanol solution by hydrothermal process.10 Strong Zn 2p3/2 signal was observed in XPS and the authors ascribed the formation and stabilization of Ti3+ and oxygen vacancies to the Zn doping effect. This effect could be achieved by the generation of ZnO on the surface of TiO2.10, 33 Fu et al. also


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reported that the formed TiO2/ZnO2 heterojunctions would efficiently prohibit the recombination of photogenerated elections and holes.33-34 During the hydrothermal process, Ti4+ was reduced into Ti3+ by zinc and further stabilized by Zn2+ species. However, in our experiment, the Ti3+ introduced by Zn reduction is not stable and is likely to be oxidized in air. More effect should be made for the stabilization of the obtained Ti3+ species and oxygen vacancies in TiO2 catalyst. Special facets exposure on TiO2 samples has been proven to an efficient way for the separation of photogenerated electrons and holes. Yang et al. reported that the introduction of hydrofluoric acid contributes to surface energy minimization of TiO2, acting as surface terminated reagent.35 Yu et al. and Xing et al. also reported the electron collection on {101} facets and hole enrichment on {001} facets, respectively, which finally contribute to the enhanced photocatalytic CO2 reduction and photodegradation of organic compounds.36-37 Herein, we have synthesized a variety of reduced TiO2 samples via a simple one-pot hydrothermal process. Zn powder is used as the reducing agent and HF is employed for the stabilization of the formed Ti3+ species and oxygen vacancies. The reduced TiO2 samples present enhanced solar light absorption compared to the pristine TiO2. F-based surface terminated reagent introduces the formation of {001} and {101} facets, which promotes the separation of photogenerated electrons and holes. As the result, reduced TiO2 samples show improved photocatalytic degradation of formic acid under UVA/Vis irradiation. 2. Methods and Characterization 2.1 Synthesis of reduced TiO2 catalysts with different Zn/Ti molar ratio


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In a typical 50 mL Teflon-lined stainless-steel autoclave, 1 mL TiCl4 (Sigma-Aldrich) was added into 30 mL 1M HCl aqueous solution under magnetic stirring. Then different amount of Zn powder (Sigma-Aldrich, < 10 µm, 0.1482 g, Zn/Ti molar ratio = 1:4) was added into the mixture. The autoclave was placed in the oven at 180 oC for 24 h and then cooled down naturally to room temperature. Finally, the samples were washed by water for 3 times, dried under vacuum at 60 o

C for 12 h, collected and denoted as r:s-TiO2-x (r:s refers to Zn/Ti molar ratio). The blank

sample prepared without Zn powder was denoted as TiO2. 2.2 Synthesis of reduced TiO2 catalysts with the adding of HF In a typical 50 mL Teflon-lined stainless-steel autoclave, 1 mL TiCl4 was added into 30 mL 1 M HCl aqueous solution under magnetic stirring. Then certain amount (m mL) of HF solution (40%, Merck KGaA) was added into the mixture carefully. 0.1482 g of Zn powder (SigmaAldrich) was added into the solution (Zn/Ti molar ratio = 1:4) and the autoclave was placed in the oven at 180 oC for 24 h. After cooling to room temperature, the samples were collected and washed with water 3 times (4000 rpm, 10 min). Finally, the catalysts were dried in the oven and named as Zn-TiO2-x-m (m refers the adding amount of HF). For comparison, TiO2-1.00 was prepared in the absence of Zn powder. 2.3 Characterization techniques Powder X-ray diffraction (XRD) was performed on Bruker D8 machine using Kα radiation (λ = 1.5406 nm). The XRD results were analyzed by EVA software platform. BET surface area measurements were carried out using Micromeritics ASAP 2020 surface area and porosity analyzer by N2 adsorption-desorption at 77 K. The samples were pre-dried at 300 oC in N2. Fieldemission scanning electron microscopy (FE-SEM) were performed on Hitachi S4800. UV-Vis


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diffuse reflectance spectra were obtained by referencing the white BaSO4 sample disk as the standard on a Shimadzu UV-2550 Scan UV-Vis spectrophotometer, the data was recorded from 200 – 800 nm. UV-Vis-NIR diffuse reflectance spectra were recorded on a PerkinElmer UV-VisNIR spectrometer Lambda 950, the data was collected at the range of 200 – 2500 nm. X-ray photoelectron spectroscopy (XPS) was conducted on Thermo Scientific ESCALAB 250 equipped with Al Kα radiation. Electron paramagnetic resonance (EPR) was recorded at 100 K on a Bruker EMX-8/2.7 EPR spectrometer. Transmission electron microscopy (TEM) and highresolution transmission electron microscopy (HR-TEM) spectra were recorded on JEOL JEM1400 and JEM-2100, respectively, to investigate the morphologies of these samples. 2.4 Photocatalytic performance Photodegradation of Rhodamine B (RhB, 20 mg/L) was selected for testing the photocatalytic performance of r:s-TiO2-x samples. 0.07 g of the sample was added into a 100-mL quartz photoreactor containing 70 mL of Rhodamine B (20 mg/L) under magnetic stirring. After stirring the solution in dark for 30 min to achieve the adsorption-desorption equilibrium, the solution was placed under visible light irradiation. Visible light was simulated with a 500 W tungsten halogen lamp equipped with a UV cut-off filter (λ > 420 nm). The lamp was cooled with flowing water around it in a quartz cylindrical jacket. During the photodegradation process, a certain amount of solution was taken out of the mixture at the given time intervals and immediately centrifuged to remove the solid (12000 rpm for 10 min). The supernatant was analyzed on Shimadzu UV-2450 ultraviolet visible spectrometer by recording variations of absorption in UV-vis spectra of RhB. Photodegradation of formic acid by the synthesized reduced TiO2 was carried out to investigate the photocatalytic performance of Zn-TiO2-x-m samples. In a 100 mL quartz


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photoreactor made by Pyrex with a bottom optical window of 4 cm in diameter, 0.03 g catalyst was added with 30 mL formic acid (50 mg/L), and then stirred in the dark for 30 min to achieve the adsorption-desorption equilibrium. Then the photoreactor was kept under the irradiation of UVA/Vis light irradiation (Philips HPK 125 W, equipped with an optical filter Corning 0.52 to cut off wavelength below 340 nm). The sample was taken out of the solution by 1 mL syringe and filtered using 0.45 µm Millipore filters every given time interval to remove TiO2 particles. The final analyze was performed on VARIAN “Prostar” HPLC apparatus equipped with a single wavelength UV-Vis detector “Prostar325” operating at 210 nm, and a carbohydrate analysis 300x7.8 mm column “ICE COREGEL 87H3” from Transgenomics (USA). Mobile phase was a pH=2 sulfuric acid (Sigma Aldrich) solution 0.005 mol/L, with a 0.7 mL/min flow. 3. Results and Discussion

Figure 1. Photodegradation of Rhodamine B (20 mg/L) of r:s-TiO2-x samples (without HF injection during the preparation) under visible light irradiation (λ > 420 nm). Inserts are the


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photos of the sample 1:4-TiO2-x. However, the blue color (insert, left) of 1:4-TiO2-x fades to white (insert, right) gradually during the washing and collection process. According to several literatures, Zn powder was demonstrated to be an efficient reductant to synthesize reduced TiO2.10, 38 Pei et al. reported that Zn not only serves as the reductant for the generation of reduced TiO2, but also contributes to the stabilization of the obtained Ti3+ and oxygen vacancies on the surface and subsurface of the crystal.39 And this effect was explained as the formation of ZnO on the surface of TiO2.10, 33, 39 However, in our experiment, the Zn-assisted TiO2 samples are not stable and easy to be oxidized in air, with color fading from blue to white (Fig. 1, inserts). It has been reported that Ti3+ or oxygen vacancies on the surface of TiO2 are easy to be oxidized by oxygen in air or water, resulting into the diminution of the defective sites.40-41 In order to investigate the effect of Zn addition, photodegradation of RhB was performed on these samples. Unlike commercial TiO2 photocatalysts, the TiO2 samples synthesized by hydrothermal process in our case present some defects and active sites. Therefore, even the blank TiO2 sample shows photocatalytic activities under visible light irradiation, although its activity is not so high (~40% removal of 20 mg/L RhB after 5 h visible light irradiation, Fig. 1). Among all these samples, the sample with Zn/Ti molar ratio 1:4 presents the highest photocatalytic activity. The enhanced photocatalytic activity of the sample 1:4-TiO2-x should be attributed to the formation of Ti3+ caused by Zn reduction.10 However, the photodegradation rate of the sample 1:4-TiO2-x decreased gradually over time, indicating the diminution of Ti3+ species of TiO2-x sample. Because the photocatalytic performance of TiO2 catalysts under visible light irradiation is mainly attributed to the generation of Ti3+ species,40 the decreased photodegradation rate over time implies that the Ti3+ species introduced by Zn reduction are not stable and easy to be oxidized by oxygen gases while exposing in water.


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Figure 2. XRD patterns of the as-synthesized TiO2-x samples added with different amount of HF In order to stabilize the obtained Ti3+ and oxygen vacancies, HF was introduced during the preparation process. A rutile-to-anatase transformation is found in XRD patterns (Fig. 2), and it depends on the gradual increase in the amount of HF. A plenty of works has been reported for the anatase-to-rutile transformation and it is widely accepted that transformation from rutile to anatase is difficult to achieve.42-43 According to the best of our knowledge, the rutle-to-anatase transformation was observed in our previous work44 and has never been reported by other researchers before. And we proposed that the transformation may be result from the inhibition of the anatase-rutile transformation by F atoms because of the size and charge effects.43-44


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Figure 3. TEM images of the synthesized TiO2-x samples. The volume of HF added during the preparation is shown at the bottom of each image. It is reported that TiO2 surface terminated with F atoms leads to the decrease of surface energy and the generation of {001} facets.35 Fluoride anion is efficient for the stabilization of the TiO2 surface because of its tendency to form a strong bond with Ti atoms.35 Besides, HF is an efficient structure-directing agent during the preparation of TiO2 catalysts. Therefore, TEM was employed to study the morphology of TiO2 samples, especially the generation of special facets exposure. Fig. 3 shows that the sample without any HF added during the preparation presents a morphology of nanorods with pointed ends. With continuous adding of HF solution, some


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square-plates started to generate except nanorods, and more of them were observed with increasing volume of HF injection. When m reached to 0.15, no nanorods were observed in TEM images any more. Fig. 3 shows that the sample Zn-TiO2-x-0.20 presents the most uniform morphology with some facets formed on the surface of TiO2. It is reported that as a halogen element, chloride serves as surface capping and structural shaping reagent, however, the surface terminated effect of Cl- is weaker than F-,39 resulting in the structure transformation of TiO2 with HF addition. The structure transformation should be caused by the introduction of F atoms. However, with the addition of extra HF, the sharp edges and corners of TiO2 started to be etched and finally these particles assembled to form big particles (Zn-TiO2-x-1.00 in Fig. 3).

Figure 4. HR-TEM of the samples Zn-TiO2-x-0 (a, c) and Zn-TiO2-x-0.20 (b, d) The TEM images show that the nanoparticles of the sample Zn-TiO2-x-0.20 are well dispersed (Fig. S1), and the sample Zn-TiO2-x-0 (Fig. S2a, b) and sample Zn-TiO2-x-0.20 (Fig. S2c, d) are optically displayed by FE-SEM. All these images indicate the morphology transformation of TiO2-x samples owing to HF injection effect. In order to confirm the exact exposed facets, we


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performed HRTEM on the samples Zn-TiO2-x-0 (Fig. 4a, c) and Zn-TiO2-x-0.20 (Fig. 4b, d), respectively. HRTEM images show a nanorod structure of the sample Zn-TiO2-x-0 consists of {110} facets and {111} facets, with lattice finger spacing of 0.325 nm and 0.219 nm, respectively (Fig. 4a, c). Pei et al.45 and Zuo et al.27 reported that the co-exposure of {110} and {111} facets acts as electron and hole collectors, respectively, which contributes the separation of photogenerated charge carriers. However, it is also reported that the high-energy {111} facets would produce more electrons in the photocatalytic reaction, due to the large percentage of undercoordinated Ti and O atoms on the {111} facets.46 As a result, the enriched electrons promoted by undercoordinated atoms and collected holes transferred from {110} facets are likely to recombine on the {111} facets. The sample Zn-TiO2-x-0.20 shows a dual {101}-{001} facets exposure, with lattice fringe spacing of 0.352 nm and 0.237 nm, respectively (Fig. 4b, d). The angle of 68.3o measured in the figure matches well with the theoretically calculated values of {101} and {001} facets.36 It has been reported that in a dual {101}-{001} system, the electrons prefer to gather in the {101} facets while the holes are likely to migrate to the {001} facets, resulting into the separation of electron-hole pairs.36-37, 47 In general, unregularly shaped or single large facet exposed TiO2 samples are easy to form adjacent trapping sites for carriers and present high electron-hole recombination rate, which leads to low photocatalytic performance of TiO2 samples. The dual-exposure of {101}-{001} facets could inhibit the recombination and promote the charge transfer of TiO2 sample under solar light irradiation.


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Figure 5. (a) UV-DRS spectra of the Zn-assisted TiO2-x samples with different HF injections in the preparation. The samples with mainly rutile phase are displayed by line and the samples with mainly anatase phase are displayed by dash. (b) Corresponding plots of transformed KubelkaMunk function against the photon energy of these TiO2-x samples. The calculated band gap of each sample is shown on the right-hand side of this figure. UV-DRS spectrometer was employed to investigate the optical properties of the obtained TiO2x

photocatalysts. Fig. 5a shows that all the reduced TiO2 catalysts with HF added during the

preparation present enhanced solar light absorption compared to the blank sample. The sample Zn-TiO2-x-0 fades to white after being exposed in air, resulting into the low absorption of solar light in UV-DRS spectra. For the samples treated with HF, their blue colors remain stable even after 16 months (Fig. S3). The band gap energy of each sample has been calculated by KubelkaMunk function, as shown in Fig. 5b. It is important to demonstrate that the band gap of TiO2-x increases with the injection of HF solution, both for rutile and anatase TiO2 samples. With continuous adding of HF, the band gap of rutile TiO2-x samples increases from 2.96 eV (Zn-TiO2x-0)

to 2.98 eV (Zn-TiO2-x-0.01), 3.01 eV (Zn-TiO2-x-0.05), and finally 3.02 eV (Zn-TiO2-x-0.08).

For anatase samples, the band gap firstly increases to 3.18 eV for the sample Zn-TiO2-x-0.20, and


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then begins to decrease with extra HF addition during the preparation. This phenomenon should be attributed to upsweep of Ti3+ impurity levels after fluorination, owing to the built-in electric field constructed by F atoms, according to the result of our unpublished work.

Figure 6. UV-Vis-NIR spectra of the obtained TiO2 samples. The photos of the synthesized samples framed with corresponding colors are inserted on the top. The solar light absorption in the near-infrared region was investigated by UV-Vis-NIR spectrometer. All the reduced TiO2 samples treated by fluoride incorporation show enhanced solar light absorption in visible and near-infrared region (Fig. 6). Among all these samples, the sample Zn-TiO2-x-0.08 shows the highest absorption intensity in the UV-Vis-NIR spectra. However, the absorbance of the sample Zn-TiO2-x-0.20 is higher than that of the sample Zn-TiO2x-0.05

in the wavelength over 2060 nm.


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Figure 7. EPR spectra of some selected TiO2-x samples. Inset shows the zoomed view of the selected area. EPR was performed to investigate the unpaired species in the fluoride-treated reduced TiO2 nanomaterials. The g-factors, which are independent of the microwave frequency, are usually used for the identification of a compound instead of field resonance. Fig. 7 shows that all the reduced TiO2 catalysts present increased unpaired electron signals compared to the reference TiO2 sample. The rutile TiO2 sample Zn-TiO2-x-0.08 shows the highest intensity among all these samples, with g⊥ = g// = 1.959 (Fig. 7 and 8), which should be attributed to the formation of Ti3+ species in the bulk of TiO2.38 If additional HF was injected during the preparation process, the concentration of Ti3+ species would decrease. The sample Zn-TiO2-x-0.20 shows relatively lower intensity of EPR signals than the sample Zn-TiO2-x-0.08, with g⊥ = 1.973 and g// = 1.919 in anatase phase. This EPR signal should be ascribed to the active Ti3+ species in the bulk of


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anatase, as reported before.8, 48-49 The results of EPR intensity is consistent with the solar light absorption of TiO2-x samples in Fig. 6. In addition, Fig. 7 shows that extra HF injection does not further increase the Ti3+ concentration in TiO2. Generally speaking, the EPR absorption of solid with isotropic magnetic moment shows a sharp peak, where the peak location is associated with g tensor (gx = gy =gz). For solid with axial magnetic moment, the EPR absorption will show doublet peaks which could be fitting into two peaks corresponding to g⊥ (gx = gy) and g// (gz), as illustrated in Fig. 8.

Figure 8. Schematic illustration of g tensor and the corresponding EPR spectra. Zn-TiO2-x-0.08 and Zn-TiO2-x-0.20 are selected as examples and the EPR spectra are integrated to give the absorption curves. Two common TiO2 solid body models associated with isotropic and axial magnetic moments are shown on the top.


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The surface property of the reduced TiO2 samples has been studied by XPS. The Ti 2p3/2 and Ti 2p1/2 peaks located at 458.56 and 464.36 eV are assigned as typical Ti-O bonds in TiO2 catalysts. However, a blue shift of Ti 2p3/2 and Ti 2p1/2 peaks was observed for the sample ZnTiO2-x-0 at 458.46 and 464.26 eV, respectively (Fig. 9a). The shift to lower binding energy should be attributed to the doping of Ti3+ species.8 Because XPS can only detect few layers on the surface of TiO2 samples, the presence of Ti3+ signals of Zn-TiO2-x-0 in XPS should be ascribed to the surface Ti3+ species. It means that the introduction of Zn powder is efficient for the reduction of Ti4+ and it promotes the formation of Ti3+. Nevertheless, the generated surface Ti3+ species are low-concentrated (Fig. 7) and unstable in air, making them easy to be oxidized by the oxygen atoms. After fluorination, the surface of TiO2 samples are bonded with fluoride atoms (peaks centered at a binding energy of ~ 684.0 eV should be assigned to a terminal Ti-F bond induced by surface fluorination,50-51 Fig. 9b), and result in the diminution of surface Ti3+ species (the sample Zn-TiO2-x-0.20 in Fig. 9a). The diminution of unstable surface Ti3+ (results from XPS) and increase of total Ti3+ signals (results from EPR) of the obtained TiO2-x samples reveal that the Ti3+ are self-doped into the bulk of the catalyst and F is efficient for the stabilization of reduced TiO2 samples. The process for the stabilization of Ti3+ species induced by surface fluorination is proposed as follows: Without HF injection, the Ti3+ species on the surface layers of TiO2 will be oxidized by the oxygen in air, resulting in the diminution of surface Ti3+. Then the Ti3+ in the deeper layers may migrate from subsurface to the surface of TiO2, leading to the dilution of the obtained Ti3+ species.52 Fluoride anion is efficient for the stabilization of the TiO2 surface because of its tendency to form a strong bond with the surface Ti atoms.35 With HF injection during the preparation, the surface of obtained TiO2-x samples was bonded with F atoms (Ti-F bonds), which prevents the formation of surface oxygen defects, stops the migration of


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subsurface Ti3+ species, and avoids the diminution of the total Ti3+ amount. As the result, the Ti3+ in the bulk of TiO2 is stabilized by the surface fluorination. Similar results of F-induced stabilization of Ti3+ species were also observed in our previous works.44

Figure 9. (a) Ti 2p XPS spectra of the sample TiO2, Zn-TiO2-x-0 and Zn-TiO2-x-0.20. (b) F 1s XPS spectra of the sample Zn-TiO2-x-0 and Zn-TiO2-x-0.20. The peaks located at ~ 684 eV refer to surface Ti-F bond on TiO2-x. Fig. 10 shows the photocatalytic performance of reduced TiO2 for the degradation of colorless formic acid (50 mg/L) under UVA/Vis irradiation. The reduced sample Zn-TiO2-x-0.08 presents lower photodegradation of formic acid than the pristine one. It should be attributed to the lower surface area of Zn-TiO2-x-0.08 (18.0 m2/g, the N2 adsorption-desorption isotherms of the catalysts are displayed in Fig. S4) compared to Zn-TiO2-x-0 (27.2 m2/g). However, sample ZnTiO2-x-0.20 shows much high photocatalytic activity than the pristine one although its surface area is even lower (16.4 m2/g). Considering that a lower surface area usually results in the decrease of specific phtotocatalytic activity, surface area can be excluded from the reason for the enhancement of photocatalytic performance in this case. The enhanced photodegradation


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performance of Zn-TiO2-x-0.20 compared to the reference sample should be ascribed to the generation of Ti3+ species and oxygen defects on reduced TiO2 catalysts.41 In order to investigate the stability of the obtained F-treated TiO2-x photocatalyst, cycling test for the degradation of RhB was carried out on Zn-TiO2-x-0.20 (Fig. S5). The result reveals that the sample Zn-TiO2-x0.20 shows great stability of photodegradation of RhB under simulated solar light irradiation, without any decrease after 5 times tests.

Figure 10. Photocatalytic degradation of formic acid (50 mg/L) for the synthesized reduced TiO2 catalysts under UVA/Vis irradiation According to the EPR results of the reduced TiO2 samples (Fig. 6), Zn-TiO2-x-0.08 presents much higher EPR intensity than the sample Zn-TiO2-x-0.20, which should indicate a better photocatalytic activity of Zn-TiO2-x-0.08. A reverse result in Fig. 10 is displayed, and it should be explained as the formation of special facets on the sample Zn-TiO2-x-0.20. Several works have reported that the exposure of {001} and {101} facets of TiO2 contributes to the separation of


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photogenerated electrons and holes.37, 53 Because the doping of Ti3+ may introduce defective sites which act as recombination centers for the photogenerated electrons and holes, and lead to the decrease of the photocatalytic activities. In our case, the fast electron-hole separation on the dual {001}-{101} facets system should be an efficient solution to this problem, and maintain the charge transfer rate. Compared to unregularly shaped TiO2 photocatalysts, reduced TiO2 samples with dual {101}-{001} facets exposure could achieve extended solar light absorption and improved electron-hole separation rate, which benefit the enhancement of the photocatalytic activities of these catalysts. For the pristine TiO2 sample, electrons and holes are likely to recombine on the {111} facets and result in the decrease of its photocatalytic performance. The photogenerated electron-hole transfer on TiO2 with {111}-{110} and {101}-{001} facets exposure are illustrated in Scheme 1.

Scheme 1. Schematic illustration of charge transfer on {111}-{110} on pristine TiO2 and {101}{001} facets on reduced TiO2 under solar light irradiation. Inserts are the TEM figures of the samples Zn-TiO2-x-0 (left) and Zn-TiO2-x-0.20 (right) with shapes highlighted by dash.


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Finally, the self-doping of Ti3+ and oxygen vacancies generated by Zn reduction contributes to the enhancement of solar light absorption of the obtained TiO2-x samples. And the addition of HF results in the stabilization of Ti3+ and oxygen vacancies, and also the formation of {001} and {101} facets. The synergistic effect of Ti3+ self-doping and special facets exposure improves the photocatalytic activities of the reduced TiO2 samples.

4. Conclusion A new Zn-assisted approach for the synthesis of reduced TiO2 photocatalyst has been reported by hydrothermal process. XPS and EPR confirmed the formation of stable Ti3+ species and oxygen vacancies in the bulk of TiO2. Zn powder is proved to be an efficient reductant for the generation of Ti3+ and oxygen vacancies, while HF aqueous solution contributes to the stabilization of the obtained Ti3+ species through strong Ti-F bonds. The adding of fluoride also promotes the formation of {001} and {101} facets on TiO2 samples. As the result, enhanced solar light absorption can be achieved by Ti3+ doping, and separation of photogenerated electron and hole pairs can be realized on the {001}-{101} dual-facet system. Finally, the synergistic effect of Ti3+ doping and special facts exposure on TiO2-x samples results in the improved photocatalytic performance under UVA/Vis light irradiation.

ASSOCIATED CONTENT Supporting Information.


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The Supporting Information is available free of charge on the ACS Publication website at DOI: TEM images of the sample Zn-TiO2-x-0.20 displayed in different resolutions, FE-SEM images of the samples Zn-TiO2-x-0 and Zn-TiO2-x-0.20, and N2 adsorption-desorption isotherms of several catalysts. (PDF) AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (S. Daniele) *E-mail: [email protected] (J. Zhang) ORCID Wenzhang Fang: 0000-0002-1111-2695 Jinlong Zhang: 0000-0002-1334-6436 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work has been supported by National Nature Science Foundation of China (21577036, 21377038, 21237003 and 21677048), the National Basic Research Program of China (973 Program,

2013CB632403),

State

Key

Research

Development

Program

of

China

(2016YFA0204200), “Chenguang Program” supported by Shanghai Education Development Foundation and Shanghai Municipal Education Commission (14CG30), the Science and Technology Commission of Shanghai Municipality (16JC1401400, 17520711500). W. Fang


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thanks the China Scholarship Council (CSC, file No. 201406740019) for the doctoral fellowship. Authors are thankful to Y. Aizac of IRCELYON for PXRD studies.


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Zn-Assisted TiO2–x Photocatalyst with Efficient Charge Separation for

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