In Situ Formation of Disorder-Engineered TiO2(B)-Anatase

Nov 4, 2015 - Hydrogenation of semiconductors is an efficient way to increase their photocatalytic activity by forming disorder-engineered structures...
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In Situ Formation of Disorder-Engineered TiO2(B)-Anatase Heterophase Junction for Enhanced Photocatalytic Hydrogen Evolution Jinmeng Cai,† Yating Wang,† Yingming Zhu,† Moqing Wu,† Hao Zhang,† Xingang Li,*,† Zheng Jiang,‡ and Ming Meng† †

Collaborative Innovation Center for Chemical Science & Engineering (Tianjin), Tianjin Key Laboratory of Applied Catalysis Science & Engineering, School of Chemical Engineering & Technology, Tianjin University, Tianjin 30072, P. R. China ‡ Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, P. R. China S Supporting Information *

ABSTRACT: Hydrogenation of semiconductors is an efficient way to increase their photocatalytic activity by forming disorder-engineered structures. Herein, we report a facile hydrogenation process of TiO2(B) nanobelts to in situ generate TiO2(B)-anatase heterophase junction with a disordered surface shell. The catalyst exhibits an excellent performance for photocatalytic hydrogen evolution under the simulated solar light irradiation (∼580 μmol h−1, 0.02 g photocatalyst). The atomically well-matched heterophase junction, along with the disorderengineered surface shell, promotes the separation of electron−hole and inhibits their recombination. This strategy can be further employed to design other disorder-engineered composite photocatalysts for solar energy utilization.

KEYWORDS: hydrogenation, TiO2(B), heterophase junction, photocatalytic, water splitting

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showed a disordered layer on the surface, and exhibited largely enhanced solar absorption ability and photocatalytic activity.14 The hydrogenation-induced surface disordered shell was highly efficient and important for the separation and migration of charge carriers, and effectively inhibited the fast recombination of e−−h+ pairs to improve the photocatalytic activity. However, if the above-mentioned heterophase junction and surface disorder are separately employed, the improvement of photocatalytic activity or efficiency is still very limited. Herein, we proposed a strategy for the combination of heterophase junction with disorder-engineered surface to increase the photocatalytic activity in a larger scale. To achieve this goal, we designed a facile route for the in situ formation of the TiO2(B)-anatase heterophase junction with a hydrogenationinduced surface disordered shell. The as-synthesized catalyst exhibits remarkably enhanced photocatalytic water splitting activity for hydrogen production. So, it is believed that this strategy is feasible, and can be referenced for designing of other disorder-engineered composite photocatalysts used for solar energy utilization and water purification.

emiconductor photocatalysts have attracted enormous attention in solar energy utilization, environmental protection, and photocatalytic hydrogen evolution because of their potential application.1 Especially, titanium dioxide (TiO2) nanomaterials have been extensively researched for their good chemical stability, low toxicity, and superior photocatalytic activity.2 The main crystal phases of TiO2 in nature are anatase (tetragonal), rutile (tetragonal), and brookite (orthorhombic). Besides, the monoclinic TiO2(B) as a minor phase is first synthesized in 1980 by Marchand et al.,3 and extensively used in lithium-ion batteries for its open channels in the lattice.4 Although the photocatalytic efficiency of TiO2(B) is lower than anatase, the former phase possesses a narrower bandgap and higher band edge potential of both conduction and valence band.5,6 Thus, if the TiO2(B) and anatase are combined with each other and form a heterophase junction, the photogenerated electrons and holes could transfer to anatase phase and TiO2(B) phase, respectively (Figure 1a). It would decrease their recombination. In addition, the TiO2(B) can be transformed to anatase under appropriate annealing conditions to fabricate efficient junctions between the phases. Recently, disorder-engineered black TiO2 nanomaterials have become a research hotspot due to its greatly enhanced photocatalytic activity.7−13 Chen et al. first performed hydrogenation on TiO2 powder in a 20.0 bar hydrogen atmosphere at about 200 °C for 5 days. The obtained black TiO2 powder © XXXX American Chemical Society

Received: August 8, 2015 Accepted: November 4, 2015

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DOI: 10.1021/acsami.5b07318 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) Schematic diagram illustrating the charge transfer across the TiO2(B)-anatase heterophase junction. (b) TEM and SEM images of pristine TiO2(B) nanobelts. (c) XRD patterns, and (d) FT-IR spectra of TiO2(B) and different samples (H/N-x, H/N represents hydrogen or nitrogen atmosphere, x is the annealing temperature). The inset in c is the regional enlarged pattern of N-650.

be expected to form simultaneously at the interface between TiO2(B) and anatase. To further demonstrate the existence of heterophase junction, transmission electron microscopy was used to investigate the nanoscale structures of the phases. As shown in Figure 2a, the insets display the selected area electron diffraction (SAED) patterns of H-650. The pattern in the left bottom inset is corresponding to TiO2(B) phase, while the right top inset is corresponding to anatase phase, revealing the TiO2(B)-anatase heterophase junction structure of this sample. The HRTEM image in Figure 2b of a selected area of H-650 further confirms that the sample possesses a heterophase junction with well-matched lattice fringes between the (110) plane of TiO2(B) and the (101) plane of anatase. In addition, Figure 2d−i show that the surface disordered shell can be formed at certain conditions. For the hydrogenated samples of H-575, H-650, and H-750, a disordered shell with a thickness of ∼2 nm on their surface can be clearly observed. The presence of surface disorder suggests the modification or breaking of the symmetry for TiO2(B) and anatase during hydrogenation, as the simplified schematic diagram depicts in Figure 2c.16 Therefore, based on the above analysis, we can demonstrate that the TiO2(B)-anatase heterophase junction with the disorder-engineered surface has been successfully obtained by the in situ hydrogenation process at the tailored temperature region. The photocatalytic performance of the catalysts were evaluated by measuring the photocatalytic hydrogen evolution

The results of the X-ray diffraction (XRD) and the Raman spectra in Figure S1 suggest that the TiO2(B) phase (JCPDS No. 46−1238) is successfully synthesized by proton exchange and thermolysis reactions of alkali metal titanate.5,15 In Figure 1b, the inset SEM image shows that the morphology of the asprepared TiO2(B) is mainly in the form of nanobelt, and the TEM image reveals the coarse surface of TiO2(B). The fabrication of disorder-engineered TiO2(B)-anatase heterophase junction is depicted in detail in the Experimental Section in the Supporting Information. As shown in the SEM images (Figure S2), there is little change between the samples with different hydrogenation temperatures, except that the coarse surface gradually becomes smooth as revealed in the HAADFSTEM images (Figure S3). The XRD patterns (Figure 1c) indicate that the pristine TiO2(B) has a gradual transformation from the TiO2(B) phase to the anatase phase (JCPDS No. 21− 1272) with the hydrogenation temperature increasing from 400 to 750 °C. The two phases coexist in the samples hydrogenated at 575 and 650 °C. Figure 1d shows the Fourier transform infrared (FT-IR) spectra of the samples. A characteristic band at ∼967 cm−1 for TiO2(B) is observed. Furthermore, this band appears in the spectra of H-575, H-650 and N-650, indicating the existence of the TiO2(B) phase in these samples. Combined with the XRD results, it is confirmed that the samples of H-575, H-650 and N-650 consist of the TiO2(B) and anatase phases. Upon above analysis, it is concluded that the anatase phase can be in situ formed by a facile phase transformation of TiO2(B), and an atomically well-matched heterophase junction can also B

DOI: 10.1021/acsami.5b07318 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 2. (a, b) TEM and HRTEM images of H-650. The insets in a are the corresponding selected area electron diffraction (SAED) pattern of H650. (c) Simplified schematic diagram depicts defects and TiO2(B)-anatase heterophase junction. HRTEM images of each sample: (d) TiO2(B), (e) H-400, (f) H-575, (g) H-650, (h) H-750, and (i) N-650.

rate under the simulated solar light (AM 1.5G) irradiation. As shown in Figure 3a, the sample H-650 exhibits the highest hydrogen evolution rate of ∼580 μmol h−1 with 0.02 g of photocatalyst. Both the pure TiO2(B) phase (H-400, ∼ 107 μmol h−1) and the anatase phase (H-750, ∼452 μmol h−1) show much lower activity than H-650. It suggests that the heterophase junction formed between the two phases is rather important for the enhanced catalytic performance. By comparing the activity of the H-650 and N-650, we find that the photocatalytic activity of the H-650 is about 2.4 times higher than that of the N-650 (∼241 μmol h−1), although the latter also has the TiO2(B)-anatase phase junction. The absence of hydrogenation-induced surface disordered shell is responsible for the lower activity of the N-650. Therefore, the superior performance of the H-650 can be attributable to the synergistic effect of the TiO2(B)-anatase heterophase junction, as well as the hydrogenation-induced surface disordered shell. Their cooperating function enhances the generation and migration of e−−h+ pairs, and inhibits the recombination of e−−h+ as discussed below.17

To further confirm the superiority of the H-650, we also prepared a Pt loaded P25 for comparison. The sample of H-650 shows the significantly enhanced performance than the Pt/P25 (Figure S4). In order to understand the essence of the improved activity of the catalyst, the surface chemical bonding and element binding energy were measured by X-ray photoelectron spectroscopy (XPS). As shown in Figure 3b, the Ti 2p XPS spectra are identical, indicating the similar bonding environment of Ti atoms before and after hydrogenation.14 The two peaks of Ti 2p3/2 and 2p1/2, belonging to the characteristic peaks of Ti4+ in TiO2, are centered at the binding energies of 458.3 and 464.0 eV, respectively.18,19 By using the peak-fitting deconvolution, the O 1s spectra are deconvoluted to two peaks of surface lattice oxygen (OL) and surface Ti−OH groups (OOH).9−11 The binding energy of OL (529.6 ± 0.2 eV) of the different samples has little change, while the position of OOH shows much difference. Among these samples, the H-650 exhibits the highest O 1s binding energy of OOH (532.2 eV). In addition, the ratio of OOH/(OOH+OL) changes obviously after the hydrogenation (Table S1).18 C

DOI: 10.1021/acsami.5b07318 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 3. (a) Photocatalytic hydrogen evolution of the as-prepared different photocatalysts under the simulated solar light irradiation. (b) O 1s and Ti 2p XPS spectra of pristine TiO2(B), H-650, H-750, and N-650. (c) Radial structure functions (RSFs) of Ti K-edge of the samples including a reference anatase TiO2.

Figure 4. (a) UV−vis diffuse reflectance spectra of the four typical samples. The inset is the corresponding Kubelka−Munk transformed UV−vis diffuse reflectance spectra. (b) Valence band XPS spectra of the typical samples. (c) HRTEM image of H-650. (d) FFT image of the selected area in c marked with a black line box. (e) IFFT image performed on the red arrow pointed spots in d.

bond between Ti and surface lattice oxygen, thus forming the hydroxyl groups and Vo’s.12 The Vo’s can act as electron donor to improve the electrons transfer at the interface of the catalyst, inhibiting their recombination with holes.11 The ratio of surface OOH has a certain relation to the hydrophilicity of the catalyst surface, which may also enhance the photocatalytic activity.20

Compared with the pristine TiO2(B), the hydrogenated sample of H-650 has the highest surface OOH/(OOH+OL) ratio. The increased surface OOH concentration may be ascribed to the formation of defects, such as oxygen vacancies (Vo’s). It is wellknown that under H2 atmosphere, Pt can readily dissociate H2 molecule into atomic species (H·), which can interrupt the D

DOI: 10.1021/acsami.5b07318 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces To further demonstrate the existence of Vo’s in the hydrogenated samples, the spectra of the extended X-ray absorption fine structure (EXAFS) of Ti K-edge were recorded. The radial structure functions (RSFs) of Ti K-edge were obtained by Fourier transforming of k3-weighted EXAFS data. As shown in Figure 3c, the first coordination shell at ∼1.5 Å can be ascribed to the scattering of Ti−O coordination shell. Obviously, the hydrogenated samples (H-575, H-650, and H750) have a higher R positions as compared with the TiO2(B), N-650 and reference anatase TiO2. It suggests an elongation of the Ti−O bond length after hydrogenation. The theoretical calculation in the reported studies also revealed that the appearance of Vo’s in the catalyst could lead to the significantly elongated Ti−O distance.21,22 So, the shift of the R position results from the presence of Vo’s formed by hydrogenation. Moreover, the EXAFS results also consist with the HRTEM results (Figure 2d−i) that only the samples with an amorphous layer on the surface (H-575, H-650, and H-750) have an elongated Ti−O coordination distance. The formation of Vo’s would result in a distortion of the surface lattice to form such a surface disordered shell. The distorted surface benefits the electrons transfer from the trapping sites to the photocatalytic reaction sites by eliminating the original energy barrier, and facilitates the trapped electrons therein to participate in the photocatalytic reactions. Both of them efficiently reduce the recombination of photogenerated carriers on the surface region, leading to the enhanced performance.23 The density of state (DOS) of the valence band of the four samples were investigated by the valence band XPS. The hydrogenated samples of H-650 and H-750 show an obvious band tail blue-shifted further toward the vacuum level at about −0.07 eV, whereas the pristine TiO2(B) and N-650 lack such a band tail at the main absorption edge (Figure 4b). This result demonstrates that the formation of the band tail should be ascribed to hydrogenation.14,24 In the hydrogenation process, many Vo’s or defects can be generated, forming the lattice disorder as can be directly observed in the HRTEM image in Figure 4c. By performing the fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT) on the red pointed spots in Figure 4d, the IFFT image (Figure 4e) was obtained. The IFFT image displays two different areas, namely the distorted lattice area and the ordered lattice area with periodic atomic arrangement.12,25−27 The distortion of crystal lattice can be clearly observed along the red dashed line, indicating the formation of defects or disorder. Large amounts of lattice disorder could yield midgap states, which can form a continuum extending to merge with the valence band, thus generating the valence band tail states as depicted in Figure 4b.14 The highly localized nature of midgap states can decrease the holes overlap with the itinerant electrons in spatial. The separation of photoexcited electrons and holes could also improve the photocatalytic efficiency.28 The UV−vis diffuse reflectance spectra in Figure 4a reveal the light absorbance ability of the four typical samples. The hydrogenated samples (H-650 and H-750) and phase junction sample (N-650) exhibit the increased absorption in the longwavelength region than TiO2(B), due to the hydrogenated disorder and the formed TiO 2 (B)-anatase heterophase junction.29 The band gap energies (Eg) are estimated by using the Tauc eqn.30 By extending the vertical segment to the dashed baseline (the inset in Figure 4a), the Eg of TiO2(B) is obtained, i.e., about 3.09 eV. This value is slightly lower than other samples (∼3.18 eV). Combined with the Eg and valence

band XPS, a simplified energy band diagram is displayed in Figure S5. The TiO2(B) possesses upper positions of both valence and conduction band than the pure anatase phase. It indicates that the formed TiO2(B)-anatase heterophase junction promotes the electron migration to the anatase phase and the hole transfer to the TiO2(B) phase. This separation of the charge carriers to the different phases greatly inhibits the recombination of electrons and holes. In summary, we report that the in situ formation of disorderengineered TiO2(B)-anatase heterophase junction can significantly improve the photocatalytic activity of TiO2-based catalysts used for hydrogen evolution via water splitting. The sample of H-650 exhibits a very high hydrogen production rate of ∼580 μmol h−1, much higher than the hydrogenated single phase and the heterophase junction without surface disorder. The enhanced performance is attributed to the efficient charge separation and transfer across the heterophase junction, as well as preventing the fast recombination of electrons and holes in the disorder-engineered surface shell. This combination strategy for the in situ simultaneous formation of both heterophase junction and hydrogenation-induced surface disorder could be applied to design other composite photocatalysts (e.g., α−β Ga2O3, α−β Bi2O3, etc.) to increase their photocatalytic performance.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b07318. Experiment details and additional figures and table (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel/Fax: +86 22-2789-2275. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (U1232118, U1332102, 21476159), the 973 program (2014CB932403), and the Natural Science Foundation of Tianjin, PR China (15JCZDJC37400). Authors are also grateful to the Program of Introducing Talents of Disciplines to China Universities (B06006), and the Specialized Research Fund for the Doctoral Program of Higher Education of China (20120032110014).

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REFERENCES

(1) Chen, X.; Shen, S.; Guo, L.; Mao, S. S. Semiconductor-Based Photocatalytic Hydrogen Generation. Chem. Rev. 2010, 110, 6503− 6570. (2) Wang, X.; Li, Z.; Shi, J.; Yu, Y. One-Dimensional Titanium Dioxide Nanomaterials: Nanowires, Nanorods, and Nanobelts. Chem. Rev. 2014, 114, 9346−9384. (3) Marchand, R.; Brohan, L.; Tournoux, M. TiO2(B) a New Form of Titanium Dioxide and the Potassium Octatitanate K2Ti8O17. Mater. Res. Bull. 1980, 15, 1129−1133. (4) Hu, H.; Yu, L.; Gao, X.; Lin, Z.; Lou, X. W. Hierarchical Tubular Structures Constructed from Ultrathin TiO2(B) Nanosheets for Highly Reversible Lithium Storage. Energy Environ. Sci. 2015, 8, 1480−1483. E

DOI: 10.1021/acsami.5b07318 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Photogenerated Carriers in Anatase TiO2. J. Phys. Chem. C 2013, 117, 24496−24502. (24) Naldoni, A.; Allieta, M.; Santangelo, S.; Marelli, M.; Fabbri, F.; Cappelli, S.; Bianchi, C. L.; Psaro, R.; Dal Santo, V. Effect of Nature and Location of Defects on Bandgap Narrowing in Black TiO2 Nanoparticles. J. Am. Chem. Soc. 2012, 134, 7600−7603. (25) Chen, X.; Liu, L.; Liu, Z.; Marcus, M. A.; Wang, W. C.; Oyler, N. A.; Grass, M. E.; Mao, B.; Glans, P. A.; Yu, P. Y.; Guo, J.; Mao, S. S. Properties of Disorder-Engineered Black Titanium Dioxide Nanoparticles through Hydrogenation. Sci. Rep. 2013, 3, 1510. (26) Xia, T.; Cao, Y.; Oyler, N. A.; Murowchick, J.; Liu, L.; Chen, X. Strong Microwave Absorption of Hydrogenated Wide Bandgap Semiconductor Nanoparticles. ACS Appl. Mater. Interfaces 2015, 7, 10407−10413. (27) Xia, T.; Chen, X. Revealing the Structural Properties of Hydrogenated Black TiO2 Nanocrystals. J. Mater. Chem. A 2013, 1, 2983−2989. (28) Liu, L.; Yu, P. Y.; Chen, X.; Mao, S. S.; Shen, D. Z. Hydrogenation and Disorder in Engineered Black TiO2. Phys. Rev. Lett. 2013, 111, 065505. (29) Xia, T.; Li, N.; Zhang, Y.; Kruger, M. B.; Murowchick, J.; Selloni, A.; Chen, X. Directional Heat Dissipation across the Interface in Anatase-Rutile Nanocomposites. ACS Appl. Mater. Interfaces 2013, 5, 9883−9890. (30) Huxter, V. M.; Mirkovic, T.; Nair, P. S.; Scholes, G. D. Demonstration of Bulk Semiconductor Optical Properties in Processable Ag2S and EuS Nanocrystalline Systems. Adv. Mater. 2008, 20, 2439−2443.

(5) Yang, D.; Liu, H.; Zheng, Z.; Yuan, Y.; Zhao, J. C.; Waclawik, E. R.; Ke, X.; Zhu, H. An Efficient Photocatalyst Structure: TiO2(B) Nanofibers with a Shell of Anatase Nanocrystals. J. Am. Chem. Soc. 2009, 131, 17885−17893. (6) Lo, H. H.; Gopal, N. O.; Sheu, S. C.; Ke, S. C. Electron Paramagnetic Resonance Investigation of Charge Transfer in TiO2(B)/Anatase and N-TiO2(B)/Anatase Mixed-Phase Nanowires: The Relative Valence and Conduction Band Edges in the Two Phases. J. Phys. Chem. C 2014, 118, 2877−2884. (7) Chen, X.; Liu, L.; Huang, F. Black Titanium Dioxide (TiO2) Nanomaterials. Chem. Soc. Rev. 2015, 44, 1861−1885. (8) Leshuk, T.; Parviz, R.; Everett, P.; Krishnakumar, H.; Varin, R. A.; Gu, F. Photocatalytic Activity of Hydrogenated TiO2. ACS Appl. Mater. Interfaces 2013, 5, 1892−1895. (9) Yan, Y.; Han, M.; Konkin, A.; Koppe, T.; Wang, D.; Andreu, T.; Chen, G.; Vetter, U.; Morante, J. R.; Schaaf, P. Slightly Hydrogenated TiO2 with Enhanced Photocatalytic Performance. J. Mater. Chem. A 2014, 2, 12708−12716. (10) Zhou, W.; Li, W.; Wang, J. Q.; Qu, Y.; Yang, Y.; Xie, Y.; Zhang, K. F.; Wang, L.; Fu, H. G.; Zhao, D. Y. Ordered Mesoporous Black TiO2 as Highly Efficient Hydrogen Evolution Photocatalyst. J. Am. Chem. Soc. 2014, 136, 9280−9283. (11) Wang, G.; Wang, H.; Ling, Y.; Tang, Y.; Yang, X.; Fitzmorris, R. C.; Wang, C.; Zhang, J. Z.; Li, Y. Hydrogen-Treated TiO2 Nanowire Arrays for Photoelectrochemical Water Splitting. Nano Lett. 2011, 11, 3026−3033. (12) Zhu, Y.; Liu, D.; Meng, M. H2 Spillover Enhanced Hydrogenation Capability of TiO2 Used for Photocatalytic Splitting of Water: a Traditional Phenomenon for New Applications. Chem. Commun. 2014, 50, 6049−6051. (13) Wang, Z.; Yang, C.; Lin, T.; Yin, H.; Chen, P.; Wan, D.; Xu, F.; Huang, F.; Lin, J.; Xie, X.; Jiang, M. H-Doped Black Titania with Very High Solar Absorption and Excellent Photocatalysis Enhanced by Localized Surface Plasmon Resonance. Adv. Funct. Mater. 2013, 23, 5444−5450. (14) Chen, X.; Liu, L.; Yu, P. Y.; Mao, S. S. Increasing Solar Absorption for Photocatalysis with Black Hydrogenated Titanium Dioxide Nanocrystals. Science 2011, 331, 746−750. (15) Qin, L.; Pan, X.; Wang, L.; Sun, X.; Zhang, G.; Guo, X. Facile Preparation of Mesoporous TiO2(B) Nanowires with Well-Dispersed Fe2O3 Nanoparticles and Their Photochemical Catalytic Behavior. Appl. Catal., B 2014, 150−151, 544−553. (16) Wang, Z.; Yang, C.; Lin, T.; Yin, H.; Chen, P.; Wan, D.; Xu, F.; Huang, F.; Lin, J.; Xie, X.; Jiang, M. Visible-Light Photocatalytic, Solar Thermal and Photoelectrochemical Properties of Aluminium-Reduced Black Titania. Energy Environ. Sci. 2013, 6, 3007−3014. (17) Xiong, Z.; Zhao, X. S. Nitrogen-Doped Titanate-Anatase CoreShell Nanobelts with Exposed {101} Anatase Facets and Enhanced Visible Light Photocatalytic Activity. J. Am. Chem. Soc. 2012, 134, 5754−5757. (18) Cai, J. M.; Zhu, Y. M.; Liu, D. S.; Meng, M.; Hu, Z. P.; Jiang, Z. Synergistic Effect of Titanate-Anatase Heterostructure and Hydrogenation-Induced Surface Disorder on Photocatalytic Water Splitting. ACS Catal. 2015, 5, 1708−1716. (19) Lian, Z.; Xu, P.; Wang, W.; Zhang, D.; Xiao, S.; Li, X.; Li, G. C60-Decorated CdS/TiO2 Mesoporous Architectures with Enhanced Photostability and Photocatalytic Activity for H2 Evolution. ACS Appl. Mater. Interfaces 2015, 7, 4533−4540. (20) Naldoni, A.; Bianchi, C. L.; Pirola, C.; Suslick, K. S. Porous TiO2 Microspheres with Tunable Properties for Photocatalytic Air Purification. Ultrason. Sonochem. 2013, 20, 445−451. (21) Cheng, H.; Selloni, A. Surface and Subsurface Oxygen Vacancies in Anatase TiO2 and Differences with Rutile. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 092101. (22) Di Valentin, C.; Pacchioni, G.; Selloni, A. Electronic Structure of Defect States in Hydroxylated and Reduced Rutile TiO2(110) Surfaces. Phys. Rev. Lett. 2006, 97, 166803. (23) Ma, X.; Dai, Y.; Guo, M.; Huang, B. Insights into the Role of Surface Distortion in Promoting the Separation and Transfer of F

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