Improving Photocatalytic H2 Evolution of TiO2 via Formation of {001

Article pubs.acs.org/JPCC

Improving Photocatalytic H2 Evolution of TiO2 via Formation of {001}−{010} Quasi-Heterojunctions Danping Wang,†,‡ Pushkar Kanhere,†,‡ Mingjie Li,§ Qiuling Tay,‡ Yuxin Tang,‡ Yizhong Huang,‡ Tze Chien Sum,*,§ Nripan Mathews,† Thirumany Sritharan,‡ and Zhong Chen*,†,‡ †

Energy Research Institute at NTU (ERI@N), 1 CleanTech Loop, #06-04, CleanTech One, 6371412, Singapore School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore § Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, 637371, Singapore ‡

S Supporting Information *

ABSTRACT: The coexistence of low-index facets with a highly photoactive {001} facet in anatase TiO2 nanocrystals has been recently found beneficial to enhance the photocatalytic performance of TiO2 via a synergistic effect. In this paper, this synergistic effect has been further extended from a single crystal to interconnected nanocrystals with dominating {001} or {010} facet in intact hierarchical TiO2 nanostrucutre. The particles synthesized at the optimal condition showed outstanding photocatalytic hydrogen production of 364.2 μmol·g−1·h−1, which is about four times as much as that of commercial P25 (96.5 μmol·g−1·h−1). Femtosecond transient spectroscopy and density functional theory (DFT) study indicates that effective electron−hole separation takes place within these nanostructures. This new prototype of synergy, which we denote as quasi-heterojunctions, shows that enhanced photocatalytic performance could be derived in the same anatase phase by synthesizing appropriate faceted nanostructures. This work provides a new dimension to facet engineering of TiO2 and other semiconductor photocatalysts. followed the sequence {001} < {101} < {010},24 which is attributed to the combined effect from the surface undercoordinated atoms and band gap structure. Some other groups have recently reported that an optimal ratio of {101}/{001} possibly exists, and thus, simply increasing the proportion of {001} is insufficient.23 For example, Zheng, Dai, and coworkers studied the charge separation in {001}−{101} using density functional theory (DFT).25 They found that electrons will migrate from {001} to {101}, making {101} a suitable redox site, while holes will migrate from {101} to {001}, making {001} an oxidation site. Therefore, the coexistence of other facets along with {001} could possibly help to enhance the photocatalytic performance by the charge separation between the different facets. Although research activity on this emerging concept of interfacet charge transfer is limited at present and the charge transfer mechanism still remains to be clarified, it has surfaced as a new challenge to control the morphology/facet of TiO2 nanoparticles. Inspired by the possibility of enhanced photocatalytic activity in morphology-controlled particles with {001} facets and the

1. INTRODUCTION Facet engineering or shape-controlled synthesis of micro/ nanostructures has attracted tremendous research endeavors in recent years, as the physical and chemical properties of metal and semiconductor materials, including their catalytic reactivity, closely correlate to their morphology and surface atomic configuration.1−3 The morphology control of titanium dioxide (TiO2) has aroused particular research interest because of its important role in environment and energy-related applications such as photosplitting of water, photodegradation of organic pollutants, dye-sensitized solar cells, lithium-ion battery, and so on.4−9 Conventional understanding attributes the number of undercoordinated Ti5c atoms on the surface to good photocatalytic activity of anatase {001} facet, which has prompted researchers to increase the proportion of the reactive {001} facets in micro/nanostructures.10−22 However, lately some researchers have resorted to the potential of cooperative or synergistic effect of {001}, {101}, and {010} facets present together in the particle in photocatalysis.23−26 They have pointed out that, instead of maximizing the proportion of {001} facets, the photocatalytic performance of anatase TiO2 can be improved by both the surface atom coordination and the surface electronic structure. For example, Pan, Cheng, and coworkers found that hydrogen evolution activity of clean surfaces © 2013 American Chemical Society

Received: July 28, 2013 Revised: October 8, 2013 Published: October 9, 2013 22894

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Figure 1. FESEM images of hierarchical TiO2 nanoparticles with different CTAB concentrations: (a) [CTAB] = 0; (b) [CTAB] = 18 mM; (c) [CTAB] = 55 mM; (d) [CTAB] = 73 mM; (e) [CTAB] = 91 mM; (f) [CTAB] = 137 mM. All scale bars represent 100 nm.

amount of surfactant to control the particle size. In a typical reaction, 0.1 g of titanium tetrafluoride (TiF4, Aldrich) was dissolved in 25 mL of triethylene glycol (TEG, Reagentplus 99%, Sigma-Aldrich) followed by addition of cetyltrimethylammonium bromide (CTAB, ≥98%, Sigma) as a surfactant with 3 h of vigorous magnetic stirring at room temperature. After thorough mixing, 5 mL of glacial acetic acid (100%, Merck) was added into the solution and vigorously stirred for another 1 h. Then the solution was transferred to a 40 mL Teflon liner and sealed in an autoclave for hydrothermal reaction at 180 °C for 24 h. The product was harvested by centrifuging and washed thoroughly with ethanol before ovendrying. The dried TiO2 nanoflower powder was transferred to a 4 mL glass vial and heat-treated in a muffle oven at 350, 450, and 550 °C at a ramping rate of 5 °C/min and duration of 4 h. After heat treatment the sample was oven-cooled to ambient temperature. To investigate the effect of CTAB concentration on the formation of the TiO2 nanoflower structures, its concentration was varied from 0 (no CTAB) to 18, 55, 73, 91, and 137 mM, which corresponds to molar ratio of [CTAB]/ [TiF4] from 0 to 0.67, 2.03, 2.70, 3.37, and 5.07. 2.2. Photocatalytic H2 Evolution Test. The as-synthesized and calcined TiO2 powders were used in the photocatalytic H2 evolution test. P25 (Degussa) was also used as a benchmark. The photocatalytic reaction was carried out in a closed-gas circulation system with a reaction cell made up of

possible synergistic effect with other facets, we embarked on the synthesis of particles designed at nanoscale to have intrinsically active facets. In this paper, we report the synthesis of such particles, the control of their size, the improvement of their structural stability and crystallinity, and the exploitation of their synergistic effect as a photocatalyst. We started from the preparation and optimization of 3D flower-like anatase TiO2 nanospheres, which consist of {001} faceted nanosheets via a surfactant-mediated solvothermal method. After that, we utilized the “unfavorable” crystallographic phenomena, which is that nanosized {001} sheets are likely to evolve into particles with more thermodynamically stable facets during heat treatment,27,28 to let some of the {001} nanosheets transform to other facets (e.g., {010} nanorods in this work) by controlling the heating process. The calcined nanostructure remained intact and showed outstanding photocatalytic H2 evolution compared to P25. With femtosecond transient spectroscopy measurement of charge lifetime and theoretical support from DFT calculations on the surface states of {001} and {010} facets, the concept of {001}−{010} “quasiheterojunctions” nanostructure is thus proposed.

2. EXPERIMENTAL DETAILS 2.1. Synthesis and Size Control of Hierarchical TiO2 Nanoflowers. The hierarchical TiO2 nanoflower was synthesized via a modified method,29 with the addition of different 22895

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Figure 2. TEM images of time-evolved investigation on the growth mechanism of hierarchical nanoflowers: (a) t = 1.5 h; (b, c) t = 3 h; (d) t = 6 h; (e) t = 12 h; (f) t = 24 h; (g) schematic illustration of growth mechanism.

that with pump excitation). The 350 nm pump pulses with pump fluence of 80 μJ/cm2 were focused onto a 200 μm spot and overlapped with 800 nm probe light (focused by a parabolic mirror to a spot of ∼20 μm diameter). The probe pulse transmitted through the solution sample in the quartz cell was monitored using a monochromator/photomultiplier tube (PMT) configuration coupled to a lock-in amplifier. The pump beam was chopped at 83 Hz and used as the reference frequency for the lock-in. 2.4. Electronic Structure Calculations. The electronic structures of {001} and {010} surfaces of TiO2 were studied by quantum chemical calculation, within density functional theory formalism. All calculations were performed using CASTEP30 as implemented in Materials Studio 5.0. An energy cutoff of 400 eV and K-points mesh of 2 × 2 × 1 were used for geometry optimization of the surfaces. The optimization was carried out until the force on each atom became 550 °C > 350 °C > P25 > uncalcined. By comparing the τavg of the surface-trapped electrons with the photocatalytic results in Figure 4a, the transient transmission spectroscopy results could satisfactorily account for the trend: the longer the lifetime of the surfacetrapped electrons, the higher is the photocatalytic activity. It is expected that the uncalcined sample will show the lowest reactivity and the shortest photoexcited electron lifetime (τavg) due to the surfactant capping on {001} surface and the defects existing in the nanosheets. However, for the 350 °C calcined sample, which has quite similar morphology and surface area to the as-prepared sample, it showed a longer photoexcited electron lifetime as well as higher hydrogen production than P25. This could be attributed to the presence of interconnected facets, the improved crystallinity of {001} nanosheets after heating, and the partial removal of organic fragments from CTAB (see earlier FTIR results). The presence of such organic residues on the crystal surface is expected to shorten the PIA lifetimes. A significant increase in photoexcited electron lifetime and hydrogen production was observed for the 450 °C calcined sample. This could be attributed to a few factors: the greater presence of interconnected facets, improved crystallinity, and elimination of the capping effect. However, when the calcination temperature was further increased to 550 °C, even though there were further improvements to the crystallinity of the anatase phase and elimination of the capping effect, the lifetime of the photoexcited carriers shortened and its photoreactivity reduced as compared to the 450 °C calcined sample. These findings clearly show that the photoexcited

Figure 4. (a) Hydrogen evolution for the as-prepared and 350, 450, and 550 °C calcined samples. (b) Normalized −ΔT/T at 800 nm probe wavelength. The pump wavelength is 350 nm, and the pump fluency is 80 μJ/cm2. The solid lines are triple-exponential decay fitting curves with the system response deconvolved. The fitted lifetimes and their respective weighting factors (in %) are compiled in Table 1.

important to the photocatalytic efficiency,37,38 transient transmission spectroscopy was performed on samples to monitor the lifetimes of photoexcited carriers (see Figure 4b and Table 1). The samples were probed at 800 nm wavelength as previous transient transmission spectroscopy studies revealed that the photoinduced absorption (PIA) at 800 nm wavelength arises from the surface-trapped photoexcited electrons.38 The lifetimes of these photoexcited electrons is an important factor in photocatalytic H2 generation. Figure 4b shows the normalized negative differential transmittance (DT) signatures (i.e., −ΔT/ T or PIA) of the five samples. Each DT trace exhibits a fast increase in absorption (or decrease in transmittance) that is followed by multiexponential decays as the system returns back

Table 1. Decay Time Constants τ and the Weighting Factors A in % for ΔT/T Monitored at 800 nm (The Uncertainties of the Fitted Lifetimes Are Given in Parentheses) −ΔT/T decay lifetimes

weighting factor (%)

average lifetime

sample

τ1 (ps)

τ2 (ps)

τ3 (ps)

A1

A2

A3

τavg (ps)

350 °C 450 °C 550 °C P25 uncalcined

16 (±2) 10 (±1) 12 (±1) 17 (±2) 1.0 (±0.5)

122 (±6) 178 (±9) 106 (±5) 126 (±6) 43 (±2)

3260 (±60) 5100 (±100) 2280 (±40) 2960 (±60) 840 (±20)

23 22 13 29 34

39 37 25 48 43

38 41 62 23 23

1290 (±60) 2200 (±100) 1440 (±70) 710 (±30) 210 (±10)

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exist. This potential difference could promote the separation of electrons and holes across the interface of the two surfaces. As illustrated in Figure 5b, the photoexcited electrons from the {001} surface could be transported to the CB of the {010} surface, making {001} rich in holes for oxidation reactions predominantly. On the other hand, the {010} surface becomes electron-rich and contributes to reduction reactions. 3.6. Quasi-Heterojunctions. The electronic structure calculations have clearly indicated the existence of a potential difference between {010} and {001} surfaces. This, coupled with the carrier lifetime measurements in femtosecond transient spectroscopy, supports the hypothesis of effective electron− hole separation occurring at the junction. We name such interconnected anatase TiO2 facets “quasi-heterojunctions” because a heterojunction is conventionally defined between different crystals (phases). Because of the presence of {010}− {001} quasi-heterojunctions in the 450 and 550 °C calcined samples, efficient charge separation was achieved due to the prolonged lifetime of the charge carriers, eventually leading to the greatly enhanced photocatalytic hydrogen production. Note that the minor presence of {101} facets on {010} rods could also help in the charge separation.23,25,26,40 As more {001} facets undergo reconstruction at 550 °C, the number of {001}− {010} quasi-heterojunctions is reduced. Therefore, the lifetime of the 550 °C calcined sample was shorter than that of the one calcined at 450 °C, as confirmed by the trend in photocatalytic hydrogen production. The scheme of how the quasi-heterojunctions promote efficient charge separation of our photocatalyst is shown in Figure 6. Charge transfer from one facet to the other on a single anatase crystal was reported previously, although more efforts need to be spent to fully understand the band structure of each facet. In contrast to previous reports, the charge separation in our nanoflowers can take place among several quasiheterojunctions originating from the same hierarchical structure. For example, when one {010} facet is connected with a few {001} facets, the {010} facet can serve as an electron acceptor for all adjacent {001} facets under photoexcitation. Hence, our photocatalyst works in a more complicated, but efficient way, due to the hierarchical structure. Thus, photogenerated excitons can easily diffuse among the remarkable number of quasi-heterojunctions. We believe this hypothesis is applicable to nanostructures in other material systems. By combining quasi-heterojunction with other techniques such as cocatalyst loading and/or element doping, we will be able to synthesize photocatalysts with greatly improved performance.

charge carrier lifetimes are indeed prolonged in the nanostructures containing interconnected facetsa strong indication that effective electron−hole separation had taken place. There is a dynamic interplay between contributions from various factors to the lifetimesincluding improved crystallinity, improved charge separation from interconnected facets, and the elimination of surface surfactant. 3.5. Surface Electronic Structures. To gain a clearer understanding of the synergistic effect between the {001} and {010} facets, theoretical band structure calculations were also performed. The partial density of states (PDOS) of {001} and {010} surfaces are plotted in Figure 5. The band-gap values

Figure 5. (a) Partial density of states (PDOS) of {010} and {001} surfaces of anatase TiO2; (b) schematics of the band alignment at the interface between {010} and {001}.

obtained from PDOS plots are 2.3 and 2.0 eV for {010} and {001} surfaces, respectively. These values agree well with the earlier study on TiO2 surface band structure.39 To compare the band-edge potentials and draw conclusions on the band alignment, both PDOS plots were referenced to a common energy value. Deeply located Ti 3s induced energy states (at −56.5 eV) were selected as the reference point, as deep level electronic states would not be significantly affected by the characteristics of the surface. PDOS plots show that the valence band maximum (VBM) of {001} was more positive than that of {010} by ∼0.2 eV. A shoulder-like energy state above the VBM was observed in the case of {001}. On the other hand, the CBM (conduction band minimum) of {001} was more negative than that of {010} by ∼0.1 eV. In this case, extra energy states below the CBM were seen in the case of {010}. Hence, our computation has shown that the valence and conduction band edge energies of the {001} and {010} surfaces of TiO2 are located at different levels. Therefore, at the interface of these two surfaces, a potential difference would

4. CONCLUSION In summary, hierarchical anatase TiO2 nanoflowers consisting of {001} facet dominating nanosheets were synthesized through systematic experimentation of surfactant-assisted solvothermal method. Postannealing the particles at 350, 450, and 550 °C led to outstanding performance of photocatalytic H2 production as compared to that of commercial P25. The 450 °C calcined sample showed the best photocatalytic activity of about three times more H2 produced than that by P25. Femtosecond transient transmission spectroscopy revealed significant differences in the charge carrier lifetimes of calcined and noncalcined samples. The prolonged photoexcited electron lifetime in the former is responsible for the improvement in the photocatalytic performance. The coexistence of crystallites having dominant {010} and {001} facets in the same hierarchical nanosphere was confirmed by TEM of the samples 22900

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Figure 6. (a, b) TEM images of hierarchical antase TiO2 nanospheres after 450 °C calcination, and zoom-in photo at one corner of the particle. (c) Illustration of charge transfer in quasi-heterojunctions in anatase hierarchical nanospheres. Electrons will transfer from {001} to {010}, making {001} a hole-rich zone for oxidation reaction and {010} an electron-rich zone for reduction reaction.

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calcined at 450 and 550 °C. Partial density of states calculations further strengthen the existence of potential difference between {001} and {010} facets. This could promote electron transfer from {001} to {010} when they are photoexcited. Therefore, the lifetime of charge carriers is closely linked to the number of {001}−{010} quasi-heterojunctions, which surpasses the influence of improved crystallinity at high annealing temperatures. On the basis of this study, we suggest that the presence of quasi-heterojunctions within a single-phase material can effectively prolong the lifetime of charge carriers. This finding has opened a new dimension to the strategies for enhancing photocatalytic performance of materials.

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ACKNOWLEDGMENTS The authors greatly acknowledge the financial support from Energy Research Institute@NTU (ERI@N) and SingaporeBerkeley Research Initiative for Sustainable Energy (SinBeRISE) project. T. C. Sum also acknowledges the support by the following research grants: NTU start-up grant (M4080514); SPMS Collaborative Research Award (M4080536); Ministry of Education (MOE) Academic Research Fund (AcRF) Tier 2 grant MOE2011-T2-2-051; and the Competitive Research Program (NRF-CRP5-2009-04). Material characterizations were mainly carried out in Material Science and Engineering Department, NTU. Femtosecond transient spectroscopy studies were carried out in the Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, NTU.

ASSOCIATED CONTENT

* Supporting Information

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More characterizations of the nanostructures have been put in Supporting Information, such as crystallographic information (XRD), morphological analysis (FESEM and TEM), organic compound analysis (FTIR and TGA), surface area analysis (BET), and photocatalytic performance results. This information is available free of charge via the Internet at http://pubs. acs.org.

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REFERENCES

(1) Yang, H. G.; Sun, C. H.; Qiao, S. Z.; Zou, J.; Liu, G.; Smith, S. C.; Cheng, H. M.; Lu, G. Q. Anatase TiO2 Single Crystals with a Large Percentage of Reactive Facets. Nature 2008, 453, 638−641. (2) Zhou, K. B.; Li, Y. D. Catalysis Based on Nanocrystals with WellDefined Facets. Angew. Chem., Int. Ed. 2012, 51, 602−613. (3) Liu, G.; Yu, J. C.; Lu, G. Q.; Cheng, H. M. Crystal Facet Engineering of Semiconductor Photocatalysts: Motivations, Advances and Unique Properties. Chem. Commun. 2011, 47, 6763−6783. (4) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37−38. (5) Konstantinou, I. K.; Albanis, T. A. TiO2-Assisted Photocatalytic Degradation of Azo Dyes in Aqueous Solution: Kinetic and Mechanistic InvestigationsA Review. Appl. Catal., B: Environ. 2004, 49, 1−14.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (T.C.S.). *E-mail: [email protected] (Z.C.). Notes

The authors declare no competing financial interest. 22901

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(25) Zheng, Z. K.; Huang, B. B.; Lu, J. B.; Qin, X. Y.; Zhang, X. Y.; Dai, Y. Hierarchical TiO2 Microspheres: Synergetic Effect of {001} and {101} Facets for Enhanced Photocatalytic Activity. Chem.Eur. J. 2011, 17, 15032−15038. (26) Chen, Q. F.; Ma, W. H.; Chen, C. C.; Ji, H. W.; Zhao, J. C. Anatase TiO2 Mesocrystals Enclosed by (001) and (101) Facets: Synergistic Effects between Ti3+ and Facets for Their Photocatalytic Performance. Chem.Eur. J. 2012, 18, 12584−12589. (27) Yang, X. H.; Li, Z.; Sun, C. H.; Yang, H. G.; Li, C. Z. Hydrothermal Stability of {001} Faceted Anatase TiO2. Chem. Mater. 2011, 23, 3486−3494. (28) Wang, W.; Lu, C. H.; Ni, Y. R.; Xu, Z. Z. Crystal Facet Growth Behavior and Thermal Stability of {001} Faceted Anatase TiO2: Mechanistic Role of Gaseous HF and Visible-Light Photocatalytic Activity. CrystEngComm 2013, 15, 2537−2543. (29) Yang, W. G.; Li, J. M.; Wang, Y. L.; Zhu, F.; Shi, W. M.; Wan, F. R.; Xu, D. S. A Facile Synthesis of Anatase TiO2 Nanosheets-Based Hierarchical Spheres with over 90% {001} Facets for Dye-Sensitized Solar Cells. Chem. Commun. 2011, 47, 1809−1811. (30) Segall, M. D.; Lindan, P. J. D.; Probert, M. J.; Pickard, C. J.; Hasnip, P. J.; Clark, S. J.; Payne, M. C. First-Principles Simulation: Ideas, Illustrations and the CASTEP Code. J. Phys.: Condens. Matter 2002, 14, 2717−2744. (31) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Atoms, Molecules, Solids, and SurfacesApplications of the Generalized Gradient Approximation for Exchange and Correlation. Phys. Rev. B 1992, 46, 6671−6687. (32) Zhu, L. P.; Zhang, W. D.; Xiao, H. M.; Yang, Y.; Fu, S. Y. Facile Synthesis of Metallic Co Hierarchical Nanostructured Microspheres by a Simple Solvothermal Process. J. Phys. Chem. C 2008, 112, 10073. (33) Xu, Y. M.; Fang, X. M.; Xiong, J. A.; Zhang, Z. G. Hydrothermal Transformation of Titanate Nanotubes into Single-Crystalline TiO2 Nanomaterials with Controlled Phase Composition and Morphology. Mater. Res. Bull. 2010, 45, 799−804. (34) Zeng, H. C. Ostwald Ripening: A Synthetic Approach for Hollow Nanomaterials. Curr. Nanosci. 2007, 3, 177−181. (35) Lazzeri, M.; Vittadini, A.; Selloni, A. Structure and Energetics of Stoichiometric TiO2 Anatase Surfaces. Phys. Rev. B 2001, 63, 155409. (36) Wang, D. P.; Zeng, H. C. Creation of Interior Space, Architecture of Shell Structure, and Encapsulation of Functional Materials for Mesoporous SiO2 Spheres. Chem. Mater. 2011, 23, 4886−4899. (37) Emilio, C. A.; Litter, M. I.; Kunst, M.; Bouchard, M.; ColbeauJustin, C. Phenol Photodegradation on Platinized-TiO2 Photocatalysts Related to Charge-Carrier Dynamics. Langmuir 2006, 22, 3606−3613. (38) Tamaki, Y.; Furube, A.; Murai, M.; Hara, K.; Katoh, R.; Tachiya, M. Dynamics of Efficient Electron-Hole Separation in TiO2 Nanoparticles Revealed by Femtosecond Transient Absorption Spectroscopy under the Weak-Excitation Condition. Phys. Chem. Chem. Phys. 2007, 9, 1453−1460. (39) Zhao, Z. Y.; Li, Z. S.; Zou, Z. G. Surface Properties and Electronic Structure of Low-Index Stoichiometric Anatase TiO2 Surfaces. J. Phys.: Condens. Matter 2010, 22, 175008. (40) Jiang, Z. L.; Tang, Y. X.; Tay, Q. L.; Zhang, Y. Y.; Malyi, O. I.; Wang, D. P.; Deng, J. Y.; Lai, Y. K.; Zhou, H. F.; Chen, X. D.; et al. Understanding the Role of Nanostructures for Efficient Hydrogen Generation on Immobilized Photocatalysts. Adv. Energy Mater. 2013, 3, 1368−1380.

(6) Oregan, B.; Gratzel, M. High-Efficiency Solar-Cell Based on DyeSensitized Colloidal TiO2 Films. Nature 1991, 353, 737−740. (7) Chen, J. S.; Tan, Y. L.; Li, C. M.; Cheah, Y. L.; Luan, D. Y.; Madhavi, S.; Boey, F. Y. C.; Archer, L. A.; Lou, X. W. Constructing Hierarchical Spheres from Large Ultrathin Anatase TiO2 Nanosheets with Nearly 100% Exposed (001) Facets for Fast Reversible Lithium Storage. J. Am. Chem. Soc. 2010, 132, 6124−6130. (8) Tang, Y. X.; Wee, P. X.; Lai, Y. K.; Wang, X. P.; Gong, D. G.; Kanhere, P. D.; Lim, T. T.; Dong, Z. L.; Chen, Z. Hierarchical TiO2 Nanoflakes and Nanoparticles Hybrid Structure for Improved Photocatalytic Activity. J. Phys. Chem. C 2012, 116, 2772−2780. (9) Crossland, E. J. W.; Noel, N.; Sivaram, V.; Leijtens, T.; Alexander-Webber, J. A.; Snaith, H. J. Mesoporous TiO2 Single Crystals Delivering Enhanced Mobility and Optoelectronic Device Performance. Nature 2013, 495 (7440), 215−219. (10) Yang, H. G.; Liu, G.; Qiao, S. Z.; Sun, C. H.; Jin, Y. G.; Smith, S. C.; Zou, J.; Cheng, H. M.; Lu, G. Q. Solvothermal Synthesis and Photoreactivity of Anatase TiO2 Nanosheets with Dominant {001} Facets. J. Am. Chem. Soc. 2009, 131, 4078−4083. (11) Zhang, D. Q.; Li, G. S.; Yang, X. F.; Yu, J. C. A Micrometer-size TiO2 Single-Crystal Photocatalyst with Remarkable 80% Level of Reactive Facets. Chem. Commun. 2009, 4381−4383. (12) Xiang, Q. J.; Lv, K. L.; Yu, J. G. Pivotal Role of Fluorine in Enhanced Photocatalytic Activity of Anatase TiO2 Nanosheets with Dominant (001) Facets for the Photocatalytic Degradation of Acetone in Air. Appl. Catal., B: Environ. 2010, 96, 557−564. (13) Yu, J. G.; Fan, J. J.; Lv, K. L. Anatase TiO2 Nanosheets with Exposed (001) Facets: Improved Photoelectric Conversion Efficiency in Dye-Sensitized Solar Cells. Nanoscale 2010, 2, 2144−2149. (14) Liu, G.; Sun, C. H.; Yang, H. G.; Smith, S. C.; Wang, L. Z.; Lu, G. Q.; Cheng, H. M. Nanosized Anatase TiO2 Single Crystals for Enhanced Photocatalytic Activity. Chem. Commun. 2010, 46, 755−757. (15) Yang, X. H.; Li, Z.; Liu, G.; Xing, J.; Sun, C. H.; Yang, H. G.; Li, C. Z. Ultra-Thin Anatase TiO2 Nanosheets Dominated with {001} Facets: Thickness-Controlled Synthesis, Growth Mechanism and Water-Splitting Properties. CrystEngComm 2011, 13, 1378−1383. (16) Wu, X.; Chen, Z. G.; Lu, G. Q.; Wang, L. Z. Nanosized Anatase TiO2 Single Crystals with Tunable Exposed (001) Facets for Enhanced Energy Conversion Efficiency of Dye-Sensitized Solar Cells. Adv. Funct. Mater. 2011, 21, 4167−4172. (17) Wen, C. Z.; Zhou, J. Z.; Jiang, H. B.; Hu, Q. H.; Qiao, S. Z.; Yang, H. G. Synthesis of Micro-sized Titanium Dioxide Nanosheets Wholly Exposed with High-Energy {001} and {100} Facets. Chem. Commun. 2011, 47, 4400−4402. (18) Wu, Q.; Liu, M.; Wu, Z. J.; Li, Y. L.; Piao, L. Y. Is Photooxidation Activity of {001} Facets Truly Lower Than That of {101} Facets for Anatase TiO2 Crystals? J. Phys. Chem. C 2012, 116, 26800−26804. (19) Zhu, J. A.; Wang, S. H.; Bian, Z. F.; Xie, S. H.; Cai, C. L.; Wang, J. G.; Yang, H. G.; Li, H. X. Solvothermally Controllable Synthesis of Anatase TiO2 Nanocrystals with Dominant {001} Facets and Enhanced Photocatalytic Activity. CrystEngComm 2010, 12, 2219− 2224. (20) Lai, Z.; Peng, F.; Wang, Y.; Wang, H.; Yu, H.; Liu, P.; Zhao, H. Low Temperature Solvothermal Synthesis of Anatase TiO2 Single Crystals with Wholly {100} and {001} Faceted Surfaces. J. Mater. Chem. 2012, 22 (45), 23906−23912. (21) Wu, B. H.; Guo, C. Y.; Zheng, N. F.; Xie, Z. X.; Stucky, G. D. Nonaqueous Production of Nanostructured Anatase with High-Energy Facets. J. Am. Chem. Soc. 2008, 130, 17563. (22) Dinh, C. T.; Nguyen, T. D.; Kleitz, F.; Do, T. O. ShapeControlled Synthesis of Highly Crystalline Titania Nanocrystals. ACS Nano 2009, 3, 3737−3743. (23) Roy, N.; Sohn, Y.; Pradhan, D. Synergy of Low-Energy {101} and High-Energy {001} TiO2 Crystal Facets for Enhanced Photocatalysis. ACS Nano 2013, 7, 2532−2540. (24) Pan, J.; Liu, G.; Lu, G. M.; Cheng, H. M. On the True Photoreactivity Order of {001}, {010}, and {101} Facets of Anatase TiO2 Crystals. Angew. Chem., Int. Ed. 2011, 50 (9), 2133−2137. 22902

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