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Stable Ti Self-Doped Anatase-Rutile Mixed TiO with Enhanced Visible Light Utilization and Durability Yan Zhou, Changhong Chen, Ningning Wang, Yingying Li, and Hanming Ding J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00655 • Publication Date (Web): 03 Mar 2016 Downloaded from http://pubs.acs.org on March 4, 2016

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The Journal of Physical Chemistry

Stable Ti3+ Self-Doped Anatase-Rutile Mixed TiO2 with Enhanced Visible Light Utilization and Durability†

Yan Zhou, Changhong Chen, Ningning Wang, Yingying Li, Hanming Ding*

School of Chemistry and Molecular Engineering, East China Normal University, 500 Dongchuan Road, Shanghai 200241, China

ABSTRACT Blue Ti3+ self-doped TiO2 nanoparticles with mixed phases of anatase and rutile have been synthesized via a facile solvothermal method. The phase evolution of the blue TiO2 varying from anatase to rutile has been studied by simply controlling the initial volume ratios of TiCl3 to titanium isopropoxide in the reaction solutions. The blue TiO2 has a distinct improvement of visible light harvesting and remarkable enhancement in photocatalytic activity, which could be ascribed to the presence of Ti3+ centers and the synergetic effect between the two phases. In addition, the as-prepared blue TiO2 shows an excellent stability during the photocatalytic reactions. A photocatalytic mechanism of the self-doped blue TiO2 was proposed at last.

INTRODUCTION Semiconductor photocatalysis for environmental remediation has been extensively investigated, because the photocatalysts can be used to degrade the organic pollutants †

Electronic supplementary information (ESI) available.

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by absorbing light corresponding to their bandgaps. Titanium dioxide (TiO 2) as one of the most promising photocatalysts has gained increased interest in recent years within the scientific community due to its high chemical stability and good photoelectric performance.1,2 However, the limited response solely to ultraviolet light due to its wide bandgap and the fast recombination of photo-induced electrons and holes restrict the improvement of solar energy conversion efficiency. To enhance the separation efficiency of photo-induced electrons and holes, the band alignment between anatase and rutile is a feasible way.3 Dual-phase TiO2 photocatalysts with controllable ratio of anatase to rutile have a higher photocatalytic activity as compared to either pure phase of anatase or rutile due to a synergistic effect between them.4 Moreover, because the physical and chemical properties will be affected seriously by its crystal form, morphology, and size, TiO2 has been intensively studied in materials and catalysis focusing on how to effectively control the crystal structure and morphology.3,4 To overcome the inherent weakness of energy utilization efficiency, a large number of efforts have been devoted to modify TiO2 by doping metallic5 and nonmetallic elements,6 surface sensitization7 and deposition of noble metals.8 The doping of TiO2 with Ti3+ dopants for visible light response has now been confirmed in a variety of studies.9,10 Compared with the traditional doping ways, Ti3+ or oxygen vacancy is a special kind of self-doping without introducing any impurity elements, which is conducive to preserve the intrinsic crystal structure of TiO2.11-13 Recently, both experimental results and theoretical predictions have demonstrated that TiO2 with the defects mediated by self-doping has a spectral response in the visible light region, thus


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having a significantly improved photocatalytic activity under visible light compared with pristine TiO2.14,15 In the reduced TiO2 (TiO2-x) that contains Ti3+ or oxygen vacancies, the defects can behave as important adsorptive and active sites for heterogeneous catalysis.16 According to the principle of electric neutrality, the creation of Ti3+ centers in TiO2 is accompanied with the formation of oxygen vacancies, which could form donor levels in the forbidden bandgap of TiO2.9, 17 Various strategies have been applied to reduce TiO2, such as thermal treatment under a reducing atmosphere or vacuum, laser irradiation, and chemical vapor deposition.18,19 Although these methods have their own unique advantages, they are much limited by harsh reaction conditions and not suitable for large-scale production. Worse still, since the reduction occurs mainly on the surface of TiO2, the surface Ti3+ species are highly unstable and can be easily repaired through oxidation by O2 in air or water.10 Obviously, further efforts are still desirable to explore a simple and effective strategy to synthesize the stable selfdoped TiO2 photocatalysts. Considering the salient characteristics of Ti3+ self-doping and phase junction in photocatalysis as discussed above, they prompt us to consider whether we can use a simple, one-pot and economical method to prepare the Ti3+ selfdoped anatase-rutile mixed TiO2. In this work, blue Ti3+ self-doped TiO2 (denoted as BT) has been prepared via a simple one-pot solvothermal method. Meantime, the mixed phases of anatase and rutile have been simultaneously formed during the self-doping process only by varying the precursor ratio. It should be noted that the Ti3+ doping level, phase content and surface morphology can be controlled in the synthesis process. The Ti3+ self-doping and band



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alignment between anatase and rutile phases in the interface junction are combined to achieve an efficient visible light-driven photocatalyst. The blue TiO2 shows strong visible and near infrared light absorption as well as presents remarkably enhanced photocatalytic activity in the degradation of Rhodamine B (RhB) under visible light irradiation. Moreover, the blue TiO2 shows good stability in the photocatalytic processes, thus meeting the requirements of future environmental applications.

EXPERIMENTAL SECTION Chemicals Titanium trichloride solution (TiCl3, ~10% in diluted HCl aqueous solution), isopropanol (analytical grade), titanium isopropoxide (TTIP, analytical grade), urea, and hydrofluoric acid (HF, analytical grade, 40%) were purchased from Sinopharm Chemical Reagent Co., Ltd., China and used without further purification. P25 TiO2 was purchased from Degussa (Evonik).

Preparation of the Photocatalysts The blue Ti3+ self-doped anatase-rutile TiO2 photocatalysts were synthesized by the solvothermal reaction with TiCl3 solution, TTIP, HF, and isopropanol as the precursors. Typically, 4 mL of TTIP, 20 mL of isopropanol, and 1.2 mL of HF were mixed in a beaker and stirred for 5 min. Argon from a gas cylinder was then bubbled into the mixed solution for deaeration. After that, a given volume of TiCl3 solution was added into the beaker. The mixed solution was then transferred into a Teflon-lined stainless steel


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autoclave with 100 mL capacity. The autoclave was sealed under argon atmosphere, heated to 180 oC in an oven and maintained at this temperature for 48 h, and then aircooled to room temperature. The resulting precipitate was collected, washed successively with ethanol and deionized water several times, and dried in a vacuum oven at 60 oC for 8 h. These TiO2 samples synthesized with different volume ratios (R) of TiCl3 to TTIP (0:4, 5:4, 10:4, 15:4, and 30:4) were denoted as BT-0, BT-5, BT-10, BT-15, and BT-30, respectively. N-doped TiO2 (denoted as N-TiO2) was synthesized as described previously.20

Characterization X-ray powder diffraction (XRD) analysis was carried out using a D/Max-RB X-ray powder diffractometer with Cu K radiation. The morphology was observed with a Hitachi S4800 scanning electron microscope (SEM) and a JEOL JEM-100C II transmission electron microscope (TEM) with an accelerating voltage of 200 kV. UVvis diffused reflectance spectra were recorded using a LAMBDA 950 UV/Vis/NIR spectrophotometer. X-ray photoelectron spectra (XPS) were carried out using a Kratos AXIS UltraDLD XPS system with an Al K (1486.6 eV) line at 150 W. Raman spectra were obtained on a Thermo Scientific DXR Raman microscope at room temperature with 532 nm laser excitation. The X-band electron paramagnetic resonance (EPR) spectra were recorded on a Bruker EMX spectrometer equipped with a cylindrical quartz tube operating at 100 kHz field modulation.



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Photocatalytic Activity Evaluation The photocatalytic activity was evaluated by photo-degradation of RhB using a 500 W halogen lamp with a 420 nm cut-off filter as the visible light source. In a typical photocatalytic experiment, 0.1 g of the photocatalyst was dispersed into a 50 mL of 10 mg/L RhB aqueous solution in an 80 mL beaker under magnetically stirring. Before illumination, the suspension was stirred in the dark for 1 h to reach an adsorptiondesorption equilibrium. During the photo-degradation process, 2 mL of the suspension was taken out every 20 min and centrifuged at 4000 rpm for 5 min. Then the RhB concentration from the upper clear solution was analyzed according to its maximum absorption measured by using a Cary 100 spectrophotometer. The stability and reusability of the BT nanoparticles were tested under identical conditions. After each cycle of the photocatalytic experiment, the photocatalyst was separated, washed, and dried for next use.

RESULTS AND DISCUSSION XRD patterns in Figure 1 display the crystal phase evolution of TiO2 prepared with different volume ratios (R) of TiCl3 to TTIP. For BT-0 and BT-5, all the diffraction peaks agree with those of anatase phase (JCPDS no. 21-1272). No other impurity or new phase was observed. When R was increased to 10:4 (BT-10), a weak diffraction peak appears at 27.4o which corresponds to the (110) crystal facets of rutile phase (JCPDS no. 21-1276), indicating that a mixture of anatase and rutile was obtained. The percentage of rutile phase in the mixture increases when further increasing the R values.


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When R was increased to 30:4 (BT-30), only rutile phase was obtained. We can conclude that the strong acidic condition (H+ comes from the TiCl3 solution in our synthesis conditions) is crucial for the formation of rutile TiO2, which is consistent with the previous report.21 The Cl- ions also play an important role during the formation of rutile TiO2.22 The phase content of each polymorph can be determined from integrated intensities of anatase (101) and rutile (110) peaks, using the following equations. Wa = KaAa/(KaAa + Ar)

(1)

Wr = Ar/(KaAa + Ar)

(2)

Where Wa and Wr represent mass fractions of anatase and rutile, respectively. Aa and Ar represent the integrated intensities for each polymorph correspondingly. Ka stand for a correction coefficient, which value is 0.886.23 In case of BT-10, the mass fraction of anatase was 79%, which is similar to that in P25. The phase compositions of the samples obtained under different conditions are shown in Table S1 (in the Supporting Information). It can be seen that the percentage of anatase TiO2 decreases and that of rutile TiO2 increases when increasing TiCl3 in the precursor solutions. The crystallite size has been calculated using Sherrer's formula, Dhkl = Kλ/Bcosθ with instrumental correction, where Dhkl is the crystalline size of the prepared powder in angstroms, K is Scherer's coefficient taken as 0.89, θ is the Bragg diffraction angle, B is the full width at half maximum (FWHM) of the diffraction peak of (hkl), and λ is the wavelength of X-ray. The corresponding values of D101 for anatase phase were about 80, 55, 40, and 20 nm in BT-0, BT-5, BT-10, and BT-15, respectively. Whilst, the values of D110 for rutile phase were about 70, 80, and 120 nm in BT-10, BT-15, and BT-30, respectively.



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It can be found that the size of anatase TiO2 decreases and that of rutile TiO2 increases when increasing TiCl3 in the precursor solutions. Therefore, the phase composition in the BT photocatalysts can be easily tuned just by changing the proportion of TiCl 3 in the reactant precursor solutions.

Figure 1. XRD patterns of (a) BT-0, (b) BT-5, (c) BT-10, (d) BT-15, and (e) BT-30. Triangle and rhombi refer to anatase and rutile phases, respectively.

The color of all the doped products is blue, and the color depth depends the doping level (insets in Figure 2A). The color variation indicates the enhanced light absorption of TiO2 after Ti3+ self-doping. No color fade was observed in the following three months after the products were synthesized, suggesting that they are stable under ambient conditions (Figure S1). The UV-vis diffuse reflectance spectra of the self-doped TiO2


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nanocrystals are shown in Figure 2. The spectra of P25 and N-TiO2 are presented as the references. As shown in Figure 2, when the initial volume of TiCl3 added in the synthetic system was increased, the absorbance in the visible light region of the final product was highly enhanced, and the absorption band edge moved to longer wavelengths, which is consistent with the color change of the samples (insets in Figure 2A). According to the Kubelka-Munk function and the Tauc equation, the relationship between the absorption coefficient of F(R) and the bandgap energy of Eg can be described by the equation: (hνF(R))1/2 = A (hν - Eg), in which F(R), ν, A and Eg are absorption coefficient, light frequency, proportional constant and bandgap, respectively. The bandgap can be evaluated from the intercept of the straight-line portion of the plot of (hνF(R))1/2 vs. hν.24,25 As shown in Figure 2B, the bandgaps of P25, N-TiO2, BT-0, BT-5, BT-10, BT-15, and BT-30 were determined to be 3.16, 3.05, 3.10, 3.08, 2.91, 2.74, and 2.56 eV, respectively. Therefore, with increasing the amount of TiCl3, the bandgaps of the doped products decreases correspondingly. The decrease in the bandgap could be ascribed to the existence of defect states induced by the Ti3+ doping in the TiO2 bandgap,26,27 and the increased percentage of rutile phase which has a smaller bandgap than anatase phase. This result also suggests that the concentration of Ti3+ in the blue TiO2 products increases with increasing the amount of TiCl3 in the precursor solutions. The existence of defects in the self-doped TiO2 samples can be further confirmed by Raman and EPR analyses.



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Figure 2. (A) UV-vis diffuse reflectance spectra of (a) P25, (b) N-TiO2, (c) BT-0, (d) BT-5, (e) BT10, (f) BT-15, and (g) BT-30 photocatalysts and the corresponding photos in the insets. (B) The plot of (F(R)hν)1/2 vs. hν using the Kubelka-Munk function and the Tauc equation.


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In order to further investigate the crystalline phase and the effect of Ti3+ selfdoping on the structure of BT-10, Raman spectra have been measured and shown in Figure 3. The Raman scattering peaks in the range of 150-1000 cm-1 can be assigned to the stretching modes of Ti-O bands in TiO2 crystal. As for P25, five typical Raman bands at 142, 197, 398, 515, and 639 cm-1 are ascribed to the anatase phase. A weak peak at 446 cm-1 is ascribed to the rutile phase. Raman spectrum of BT-10 is much similar to that of P25. However, compared with P25, the principal peak at 142 cm-1 of BT-10 has a hypsochromic shift of 5 cm-1 accompanied with peak broadening. This change in Raman spectrum of BT-10 is associated with the disorder in TiO2 crystal, which origins from the defects of oxygen vacancies and Ti3+ centers.27,28

Figure 3. Raman spectra of (a) BT-10 and (b) commercial P25 TiO2.

EPR spectra was recorded to detect the generation of Ti3+ in the doped samples.



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As indicated in Figure 4, BT-0 sample does not show any paramagnetic signal, whereas the other blue TiO2 samples present a strong EPR signal at g = 1.94, which can be assigned to the paramagnetic Ti3+ species.29 Moreover, the EPR signal of Ti3+ species rise progressively in intensity with the increase of the amount of TiCl3, suggesting that more Ti3+ centers are created in the doped samples by increasing the initial volume ratios of TiCl3 to titanium isopropoxide in the reaction solutions. This result is in good accordance with the data of UV-vis diffuse reflectance spectra. The formation of Ti3+ centers in TiO2 is accompanied with the creation of oxygen vacancies. However, no EPR signal at g=2.004 for oxygen vacancies was observed, suggesting that there is no electron trapped on oxygen vacancies associated with Ti3+ centers. This result is in good accordance with the previous report.29 It is accepted that the surface Ti3+ centers can trap O2 molecules in air to generate O2- with an EPR signal around g = 2.02.30 The absence of such EPR signal in BT-10 suggests that there should be no Ti3+ centers present on the surface. Thus it can draw a conclusion that Ti3+ centers exist in the bulk rather than on the surface of the blue TiO2. Having Ti3+ in the bulk is crucial for the stability of the blue Ti3+ self-doped TiO2 in the photocatalytic processes, because the Ti3+ on the surface are easy to be oxidized when exposure to oxygen in air or in the solution. According to the previous report, the introduction of Ti3+ centers can enhance the visible light response of TiO2 and improve the charge-separation efficiency, which subsequently results in an enhanced photocatalytic activity.11


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Figure 4. The X-band EPR spectra of the Ti3+ self-doped TiO2 samples.

SEM has been used to study the morphology of the products. The results are shown in Figure 5. For BT-5 which synthesized under weakly acidic conditions (5 mL of TiCl3), nanoparticles and sometimes nanocrystals with sizes of approximately 50 nm are observed in Figure 5A. When the volume of TiCl3 aqueous solution increases to 10 mL, for the product of BT-10, nanocrystals attached on the nanocuboids are observed in Figure 5B. Compared with BT-5, the size of the nanoparticles or nanocrystals becomes smaller. The nanocuboids are roughly 20 nm in diameter and 100 nm in length. The TEM image further proved the structure and constituent details of the products. As observed in the HRTEM image of BT-10 (Figure 5D), the nanoparticles are attached on the edges of TiO2 nanocuboids. The lattice fringes with a lattice spacing of about 0.35 nm and 0.32 nm can be indexed to the (101) plane of anatase phase and the (110) plane



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of rutile phase, respectively. The result is in accordance with the XRD result in Figure 1c. In this way, a well-defined phase junction was formed between anatase and rutile phases. As the (110) lattice fringe of rutile is parallel to the long axis of the nanocuboid, the growth direction of TiO2 nanocuboid is along the [001] direction.31 These nanocuboids continue to grow along the [001] direction forming nanorods when the solution acidity increases. For BT-30 (Figure 5C), more nanorods are observed, but very less nanocrystals can be seen. Some of nanorods are assembled together with T-shaped or cross-shaped morphologies. At the same time, TiO2 exists as pure rutile phase. Both the two forms of titania polymorphs consist of the same fundamental [TiO6] octahedral units with different spatial arrangements sharing the edges and corners in a different way. Anatase consists of edge-shared octahedra to form zigzag ribbons along [221], while rutile is composed of corner-shared octahedra to form linear chains parallel to [001], in which the [TiO6] chains are linked together via corner connections along [110] and [1-10].32 The connectivity of the [TiO6] octahedra is strongly dependent by the pH values and added ions.22, 33 When the acidity and Cl- concentration were low, the complex [Ti(OH)n(Cl)m]2+ (n+m=6) carried more OH ligands, increasing the probability of edge sharing, thus promoting the formation of anatase phase. While at high acidity and concentration of Cl- ions, the complex had less OH ligands, thus favoring corner-sharing and the formation of rutile phase. During the crystallization, anatase-type crystallite seems to form prior to rutile-type. However, once rutile crystallizes, anatase nucleation will be either inhibited or transformed to rutile.34 Therefore, TiO2 prefers to homogeneously nucleate in the anatase form at higher pH


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values in the case of BT-5. When the particle size of anatase nanoparticles increased largely, they become unstable at strongly acidic conditions. Rutile thus heterogeneously nucleates via the aggregation of anatase nanoparticles at lower pH values.35 Thus the TiO2 crystalline polymorphs change from anatase to mixed phases, and then to rutile when the volume of TiCl3 aqueous solution increased from 5 mL to 30 mL. Meantime, the morphology changes from nanoparticles to nanoparticles/nanocuboids, and finally to nanorods. The morphology evolution follows a dissolution-precipitation mechanism under hydrothermal conditions in which the phase transformation takes place via dissolution of anatase nanoparticles and precipitation of rutile.36,37 The chloride ions play an important role in the morphological control of rutile phase. The presence of chloride ions is known to suppress the growth of the (110) planes and thus enhance the growth along the [001] direction, resulting in the formation of TiO2 nanocuboids and nanorods.31 During the formation of titania, Ti3+ ions were introduced in the crystal lattice, thus Ti3+ self-doped titania products were gotten. The hydrolysis and condensation processes result in the formation of Ti(IV)-O-Ti(III) bonds from isopropoxides through the following reactions (Eqs.1-4).38,39

Ti(IV)-OR + H2O  Ti(IV)-OH + ROH

(1)

Ti(III)-Cl + H2O  Ti(III)-OH + H+ + Cl-

(2)

Ti(IV)-OH + Ti(III)-OH  Ti(IV)-O-Ti(III)-□-Ti(III)  TiO2-x

(3)

Ti(III)-Cl + Ti(IV)-OR  Ti(IV)-O-Ti(III)-□-Ti(III)  TiO2-x

(4)



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In the hydrolyzation process of TTIP, the metal hydroxides were formed by the nucleophilic substitution of the corresponding terminal isopropoxide groups (-OR) with water molecules (Eq. 1). The hydrolysis of TiCl3 is similar to that of TTIP (Eq. 2). The condensation between two hydrolyzed titanium species (Eq. 3) or titanium isopropoxides and halides (Eq. 4) leads to Ti(IV)-O-Ti(III) bonds by release of water and alkyl halide, respectively. As the rate of hydrolysis of titanium isopropoxide is much faster than that of TiCl3, no Ti2O3 can be formed in the acidic conditions. Accompanied with the formation of Ti3+ sites, oxygen vacancies should be generated nearest these Ti3+ ions in order to keep the charge balance.26,40 Thus, the dominant intrinsic defects in TiO2 are Ti3+ centers and oxygen vacancies. In this way, the Ti3+ self-doped TiO2 was obtained. Compared with the previous methods, the incorporation of Ti3+ ions into the TiO2 matrix is directly derived from the precursor of TiCl3, which is expected to increase the doping concentration of Ti3+ ions and oxygen vacancies in TiO2. By varying the acidity and chloride ions of the precursor solutions, the Ti3+ selfdoped anatase-rutile TiO2 with different morphologies were produced.


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Figure 5. SEM images of BT-5 (A), BT-10 (B), BT-30 (C), and HRTEM image of BT-10 (D).

XPS analysis was further performed to investigate the chemical states of Ti and O in BT-10. The full-scale XPS spectrum for BT-10 is shown in Figure S2, in which the Ti, O and C elements could be detected and no other impurities were observed. The C 1s peak is located at 284.7eV, which is referred to the adventitious carbon contaminant. The high-resolution XPS spectra of Ti 2p and O1s in Figure 6 confirmed the chemical compositions of TiO2. As shown in Figure 6A, two primary peaks are attributed to the characteristic Ti 2p1/2 and Ti 2p3/2 peaks of Ti4+. The symmetric curve with a binding energy located at 458.7 eV and 464.3 eV in agreement with that of Ti4+ 2p3/2 and Ti4+ 2p1/2 in TiO2, respectively. The fitting peaks of O 1s at 529.8 eV and 531.2 eV could be ascribed to the lattice oxygen (Ti-O) in TiO2 and the surface hydroxyl group of TiO2 (Ti-OH), respectively. However, the XPS spectra show no evidence of Ti3+ on the surface of TiO2, which should appear 463.6 and 457.9 eV for Ti3+ 2p1/2 and Ti3+ 2p3/2, respectively.41 This result is consistent with the EPR measurement.


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Figure 6. (A) Ti 2p and (B) O 1s XPS spectra of BT-10.

In order to evaluate the photocatalytic performance of the BT samples under visible light irradiation, Rhodamine B (RhB) was chosen as the target pollutant. For



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comparison, the photo-degradation of RhB without photocatalyst (blank) and with P25 as well as N-TiO2 as the reference photocatalysts has been investigated. After the adsorption-desorption equilibrium (Figure S3A), 87.9%, 80.6%, 77.4%, 77.9%, 88.8%, and 94.8% of RhB was left in the reaction system when P25, N-TiO2, BT-5, BT-10, BT15, and BT-30 was used as the photocatalyst, respectively. BT-30 shows the poorest adsorption capacity because its smooth surface has little adhesion to the dye molecules. It is obvious that BT-5 has superior adsorptive ability due to its smaller particle size which is beneficial to the adsorption of dye molecules.16, 42 As observed in Figure 7A, it can be seen that 89.4% of RhB was decomposed in 120 min when BT-10 was used as the photocatalyst. However, for BT-5, BT-15, and BT-30, only 74.2%, 70.8%, and 56.7% of RhB molecules has been degraded, respectively. As can be seen in Figure S3B, the reaction kinetics in all the reaction systems follows the first-order kinetics equation: ln(C/C0) = kt, where k is the apparent rate constant. The rate constants corresponding to BT-5, BT-10, BT-15 and BT-30 were estimated to be 0.0109, 0.0166, 0.00933 and 0.00654 min−1, which is about 2.19, 3.34, 1.86 and 1.31 times as much as that over NTiO2 (0.00499 min−1), respectively. It is generally accepted that anatase TiO2 exhibits a higher photocatalytic activity compared with rutile TiO2. Phase-pure rutile TiO2 of BT30 shows the poorest photocatalytic activity under visible light irradiation due to its rapid recombination rate. The excellent photocatalytic performance of BT-10 can be attributed to its enhanced absorption in the visible light region, which implies that more photogenerated electrons and holes can be created to take part in the photocatalytic reactions process. Moreover, BT-10 contains two phases, in which the interface


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heterojunction between anatase and rutile phases can accelerate the separation of charge carriers and improve the transfer rate of photogenerated electrons and holes. It is worth noting that a gradual blue shift of the absorption maximum from 553 to 497 nm accompanied with a decreased absorption intensity was observed during the photodegradation process (Figure S3C). Meantime, the absorption intensity decreases quickly as the radiation time was extended. These could be ascribed to the Ndeethylation of RhB.43,44 The absorption peaks located at 497 nm further decrease during the irradiation period, which is assigned to the destruction of the whole chromophore structure of RhB. The mineralization of RhB over BT-10 with both two mechanisms is associated with the surface defects on the doped TiO2.38 The stability of the photocatalyst is critical for its practical application. In order to evaluate the stability of BT-10, the photocatalytic reaction was repeated five times under the same conditions. It can be observed in Figure 7B that the photocatalytic activity does not show any significant deterioration over five consecutive recycling experiments. It is indicates that BT-10 is very stable during the photocatalytic processes under visible light irradiation.



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Figure 7. (A) Photocatalytic degradation curves of RhB over the different photocatalysts under visible light irradiation. (B) Recycling test results using BT-10 as the photocatalyst.

To further investigate if there is change in bulk composition and defect structure during the photocatalytic process, we investigated the crystal structure and Ti3+


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concentration of the fresh and used photocatalyst by the XRD, Raman and EPR techniques. The XRD diffraction pattern for the used BT-10 sample recorded after the photocatalytic reaction is shown in Figure S4A. As comparison with that of the fresh BT-10 sample, there is no apparent change observed in XRD diffraction patterns. The used sample exhibits almost the same Raman characteristics as the fresh BT-10 (Figure S4B), which also confirmed the stability of the defect structure. In addition, the intensity of the EPR spectra did not show obvious decrease after the photocatalytic reaction as shown in Figure 8, suggesting that the Ti3+ centers are stable in the doped TiO2 samples during the photocatalytic processes. All the results hint that the BT-10 sample is very stable during the photocatalytic reaction.

Figure 8. EPR spectra of BT-10 measured before and after a photocatalytic reaction under visible light irradiation.



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On the basis of the above-mentioned experimental facts and analysis, the photocatalytic mechanism of Ti3+ self-doped TiO2 photocatalyst under visible light irradiation is proposed in Figure 9. Isolated states appearing near the conduction band edge in the TiO2 forbidden gap are formed due to the Ti3+ doping and oxygen vacancies.15, 45,46 Under visible light irradiation, the electrons in the valence band could be excited to these localized states, and then further excited from the mid-states to the conduction band, which is responsible for the visible and near infrared light absorption in all the doped BT samples. As the electron affinity of rutile is higher than that of anatase, a type-II staggered band structure forms between anatase and rutile phases when two phases have a close contact. This band structure will efficiently facilitate the smooth flow of photogenerated conduction electrons from anatase to rutile.47,48 These photogenerated electrons can further transfer to the photocatalyst surface and can subsequently be captured by molecular oxygen in solution to form ·O2- and other oxidative species. Meanwhile, the photogenerated holes tend to accumulate on the anatase phase, owing to relative valence band positions of anatase and rutile. The holes on the surface of self-doped TiO2 could directly oxidize the RhB molecules into CO2, H2O, and other intermediates.


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Figure 9. Schematic diagram representing the photocatalytic mechanism of the Ti3+ self-doped blue anatase-rutile TiO2 nanoparticles.

CONCLUSIONS In this work, a series of self-doped anatase-rutile TiO2 nanoparticles photocatalysts were facilely prepared by the hydrothermal method. The enhanced of utilization efficiency for visible light and photocatalytic activities were mainly attributed to the presence of Ti3+ species, which can introduce isolated states in the bandgap of TiO2 and consequently reduce the excitation energy. Moreover, it is worth noting that the synergistic effect of the anatase-rutile mixed phases shows a more positive role in the separation of photogenerated carries. In particular, the self-doped anatase-rutile TiO2 shows superior photocatalytic stability and can be repeatedly used for 5 times without declination in the activity. The work presents a simple and economical method to synthesize efficient and stable visible-light-responsive TiO2 photocatalysts for wide applications in environmental remediation.



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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/xxxxx. Synthesis conditions and phase compositions of the BT samples; additional supporting figures (full-scale XPS spectrum, XRD patterns, and Raman spectra); kinetics of RhB photo-degradation over the different photocatalysts under visible light irradiation and the evolution of UV-vis spectra of RhB as a function of visible light irradiation time over BT-10.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (grant no. 20971044).

REFERENCES


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