PERSPECTIVE pubs.acs.org/JPCL
On the Unusual Properties of Anatase TiO2 Exposed by Highly Reactive Facets Wen Qi Fang,† Xue-Qing Gong,‡ and Hua Gui Yang*,† †
Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering and ‡Laboratories for Advanced Materials, Research Institute of Industrial Catalysis, East China University of Science & Technology, Shanghai 200237, People's Republic of China ABSTRACT: As an important metal oxide, anatase titanium dioxide has been widely investigated because of its many promising properties. The properties of anatase TiO2 crystals are largely determined by exposed external surfaces. Since the breakthrough in synthesizing anatase TiO2 single crystals with a large percentage of highly reactive {001} facets in 2008, many unusual properties and applications of these {001} facets dominant in anatase TiO2 have been explored theoretically and experimentally, showing the industrial importance of this semiconductor material. This Perspective focuses on the theoretical simulations and application explorations of the unusual properties of anatase TiO2 bound by highly reactive facets. Research opportunities as well as the challenges for future research in this emerging frontier are also highlighted.
exploited to achieve TiO2 with exposed {001} facets. More importantly, as shown in Figure 1, many promising applications such as photocatalytic degradation of organic dye molecules, photocatalytic water splitting, Liþ ion insertion/extraction performance, and dye-sensitized solar cells (DSSCs) of {001} facet dominated anatase TiO2 have also been demonstrated.811 Herein, we highlight the recent progress of this exciting field and mainly focus on the theoretical simulations and application explorations of the unusual properties of anatase TiO2 exposed by highly reactive facets. Some perspectives are also given to illustrate the opportunities as well as challenges in the future. Anatase TiO2 crystals are formed by a basic building block consisting of a titanium atom surrounded by six oxygen atoms in
O
ver the past years, metallic and semiconducting nanocrystals with tailored facets have attracted intense research interests due to their infinite variety of structural motifs and intrinsic shape-dependent properties.15 Being an abundant, low-cost, and environmentally benign material, titanium dioxide (TiO2) has obtained commercial success and been widely used as a white pigment and in sunscreens, paints, ointments, toothpaste, and so forth.1 Since the discovery of photocatalytic splitting of water on a TiO2 electrode in 1972 (Fujishima and Honda), TiO2 has become the most widely used semiconductor in photocatalysis, and research aimed at enhancing its efficiency has intensified over the years.2 Of the three major polymorphs of TiO2, the rutile TiO2 is the most widely explored in fundmental studies, and anatase is the most widely investigated phase in the applied studies, which plays a central role in many industrial applications such as photovoltaic cells, photo/electrochromics, photocatalysis, photonic crystals, smart surface coatings, and sensors.1,3,6,7 For anatase TiO2, it is usually exposed with low index facets such as {001} and {101}. Theoretical studies indicate that the (001) surface of anatase TiO2 is much more reactive than the thermodynamically more stable (101) surface, and the (001) surfaces may in fact be the dominant source of active sites for various applications (e.g., photocatalytic production of H2).4 Unfortunately, most synthesized anatase TiO2 crystals as well as those naturally occurring are dominated by the thermodynamically stable {101} facets (more than 94%, according to the Wulff construction), which greatly limits the applications of anatase crystals in catalysis, photocatalysis, and photocatalytic water splitting. Very recently, anatase single crystals with 47% of the highly reactive {001} facets have been prepared by using hydrofluoric acid (HF) as a capping agent under hydrothermal conditions.5 This breakthrough has attracted great research interests, and various other reaction systems and capping agents have been r 2011 American Chemical Society
The arrangement and kind of constituent atoms on {001} facets of anatase TiO2 determine its unique geometrical and electronic structures. a more or less distorted octahedral configuration. This distortion is more significant in anatase than in rutile, and a sizable deviation from a 90° bond angle was observed.12,13 The TiTi distances in anatase are greater (3.79 and 3.04 Å versus 3.57 and 2.96 Å in rutile), whereas the TiO distances are shorter than those in Received: January 24, 2011 Accepted: March 4, 2011 Published: March 10, 2011 725
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Figure 1. Schematic diagram showing important applications of TiO2 dominated with highly reactive facets. (Insets adapted by permission from Macmillan Publishers Ltd., Nature (ref 5), copyright 2008, and Wiley-VCH Verlag GmbH & Co. KGaA (ref 46), copyright 2010.)
rutile (1.937 and 1.966 Å in anatase versus 1.946 and 1.983 Å in rutile).14 These differences in lattice structures are responsible for the different mass densities and electronic band structures between the two polymorphs of TiO2. For anatase TiO2, each octahedron is in contact with eight neighbors (four sharing an edge and four sharing a corner). The (001) surface of anatase TiO2 is formed by the corner-sharing octahedra and the stacking of the octahedra leads to three-fold-coordinated (3c) oxygen atoms. The ideal (001) surface exhibits exclusively five-fold-coordinated (5c) Ti atoms as well as two raised 2c and two lowered 3c oxygens bonding in the [100] and [010] directions, respectively. Upon relaxation, the surface corrugation increases slightly, from 0.82 to 0.95 Å.4 Thus, the (001)-(1 1) surface is not very stable and can undergo a two-domain (1 4) reconstruction. Herman et al. reported this reconstruction process after sputtering and annealing the (1 1) surface of an anatase TiO2 film grown on SrTiO3 under ultrahigh vacuum (UHV) conditions.15 Then, Lazzeri and Selloni suggested a so-called added molecule (ADM) structure, which is very consistent with the appearance of the (1 4) reconstruction in scanning tunneling microscope (STM) studies. In this model, rows of surface bridging 2c oxygens of the (1 1) anatase TiO2 surface are periodically replaced by rows of TiO3 species, which leads to a relief of the large surface tensile stress.16 There is a wide agreement that the surface electronic structure relatively resembles that of the bulk, except for some nonstoichiometric surfaces.1,12 However, the densities of states (DOS) of surface atoms are dramatically affected by the surface functional groups like OH and H.17,22 Hengerer et al. found that water reduction and photo-oxidation take place at more negative potentials for the (001) surface than those for the (101) surface.17 This could be attributed to the different dissociative chemisorptions of water molecules on these two surfaces. The basic understandings of the electronic structures of clean TiO2 surfaces and bulk were also given by Henrich et al. and Chen et al.1,18
Both theoretical calculations and experimental results indicate that the minority (001) surfaces are more reactive than the prominently exposed stable (101) surfaces and play a key role in the reactivity of anatase TiO2 crystals. In particular, the interaction of water and other typical probe molecules such as methanol and formic acid with (001) surfaces has been intensively investigated by theoretical calculations, which was motivated by its central role in many applications in clean energy and environmental remediation areas such as photocatalytic water splitting and photodegradation of pollutants.1925 Vittadini et al. studied the adsorption of H2O on the (101) and (001)-1 1 surfaces of anatase TiO2 by first-principles density functional theory (DFT) calculations.4 Their calculations were based on various H2O coverages θ (θ was defined as the ratio between the number of adsorbed H2O molecules and the number of surface 5c Ti sites). For H2O absorbed on the anatase (101) surface, nondissociative molecular adsorption at 5c Ti sites was predicted, irrespective of low or full monolayer coverage. By contrast, at the anatase (001)1 1 surface, H2O molecules were found to dissociate spontaneously for θ e 1. Similar results were also concluded by Arrouvel and co-workers (see Figure 2);21 they combined DFT simulations and thermodynamic analysis to determine the surface hydroxylation states as a function of temperature and water pressure. According to their calculations, chemisorbed water molecules could no longer exist above 450 K either on the (101) or on the (100) surfaces, while the OH species which formed on the (001)-1 1 surface after water dissociation could not be completely removed until the temperature was higher than 840 K. Similar to what was found in Vittadini’s study, Barnard et al. reported the DFT results of the structure and energetics of the (001)-1 1 surface of the anatase TiO2 polymorphs passivated with complete monolayers of adsorbates chosen to represent acidic and basic conditions.24 The bridging oxygens (O2c) of the anatase (001)-1 1 surface were found to undergo an outward to inward displacement from acidic to basic conditions. Within their model, the results indicated that the 726
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Figure 2. Adsorption energy of H2O molecules on the (001)-1 1 surface with various coverages. (Reprinted from ref 21, copyright 2004, with permission from Elsevier.)
Figure 3. Calculated structures for the dissociated formic acid on clean anatase TiO2(001)-1 1 at low (1/6 ML) coverage: (a) monodentate ester-type (M-Hup), (b) monodentate O-up (M-Oup); (c) bidentate chelating (BC); (d) bidentate bridging (BB1); (e) bidentate bridging above O2c; (f) bidentate bridging (BB2). (Reprinted from ref 30.)
surface free energy was lowest when terminated with water or a higher fraction of H on the surface, but these trends were not correlated with the surface stress. Gong and co-workers also studied methanol adsorption on clean and hydrated anatase TiO2(001)-1 1 using DFT calculations and first-principles molecular dynamics simulations.22 Their results showed that dissociative adsorption of methanol is energetically favored on TiO2(001)-1 1, while it is unfavored on the majority TiO2(101). Furthermore, according to the calculated electronic DOS for the (001)-1 1 and (101) surfaces of anatase TiO2, the high reactivity of the anatase TiO2(001) surface can be associated with its surface O2c atoms. Through the combination of first-principles DFT calculations and a parallel periodic continuum solvation model, the microscopic mechanisms of the oxygen evolution reaction (OER) on differently structured anatase surfaces in aqueous surroundings were systematically studied.26 Although the clean (001) surface has a significant larger population at the top levels of the valence band (VB) than that of the clean (101) surface, this feature is
experimental results reported by Ohno et al.27 On the other hand, OER is not sensitive to the local surface structure of anatase TiO2 because no obvious difference between the (001) and (101) anatase TiO2 surfaces was found at the maximum of the VB or the free-energy change for the first proton removal. Codoping high-valent elements such as Nb þ N or Mo þ C into the anatase subsurface to improve the photocatalysis for OER was also suggested in the same work. A DSSC is composed of two electrodes, the anode and the cathode, and an electrolyte is filled in the space left between the two electrodes to ensure charge transportation through a redox couple. Generally, mesoscopic anatase TiO2 films are involved as a photosensitized anode in DSSCs. The adsorption of a dye molecule on the surfaces of anatase TiO2, particularly on the most frequently exposed (101) surface, has been extensively studied theoretically and experimentally.28,29 The quantum yield of the photogenerated electron-transfer process is closely related to the anchoring groups of the dye molecule. Formic acid that contains a carboxylic group is usually used as a model compound to understand the anchoring nature of the organic dyes on the semiconductor surface because the carboxylic group is one of the most important anchoring groups of the photosensitizers. Gong et al. investigated the formic acid adsorption on anatase TiO2(001) by using DFT calculations and first-principles molecular dynamics simulations.30 Formic acid dissociates spontaneously on both the clean unreconstructed TiO2(001)-1 1 and reconstruction TiO2(001)-1 4 surfaces. Figure 3 shows possible structures of dissociated formic acid on clean 1 1 surfaces. Bidentate bridging (BB) species and bidentate chelating (BC) are the most energetically favorable configurations of dissociated formic acid on 1 1 and 1 4 surfaces, respectively. Interestingly, formic acid still dissociates very easily and adsorbs in different configurations on the partially hydrated 1 1 surfaces. Formation of BB and BC configurations can also be driven by the surface OH groups through H-bonding. Furthermore, inspired by the unique structure of the (001) surface of anatase TiO2 and its potential applications in DSCCs, C-akır et al. systematically investigated the interactions of perylenediimde (PDI)-based dye compounds (BrPDI, BrGly, and BrAsp) with both unreconstructed and reconstructed (001) surfaces of anatase TiO2 through the first-principles plane wave
Theoretical calculations indicate that the surface functional groups on the (001) surface of anatase TiO2 largely affect its stability, adsorptive properties, and catalytic reactivities. almost completely quenched by 1/2 ML of dissociated H2O. The spatial distributions of the lowest-unoccupied wave functions (LUWFs) are indeed distinct between the two anatase TiO2 surfaces. In contrast to the LUWFs of (101), which always have a significant distribution on the surface layer, the LUWFs on (001) are mainly located in the sublayers, which means that the bottom levels of the conduction bands (CBs) on (101) have a larger population than those on (001) surfaces. It is known that the photoelectrons stay on the bottom of the CB, and the surface photoelectrons are essential for the reduction reaction. The {101} facets of anatase TiO2 are thus predicted to have a higher efficiency in photoreduction, which is consistent with the 727
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Figure 5. Schematic image of the insertion of Li ions (purple spheres) in octahedral voids of the anatase lattice. The initial and final states of Liþ are indicated as IS and FS, respectively. (Reprinted from ref 10, copyright 2010, with permission from The Royal Society of Chemistry.)
rate-determining step.10 Hence, the insertion of Liþ is favored along the [001] direction, and the Liþ diffusion coefficient is approximately (2.0 ( 0.8) 1013 cm2/s along the (001) orientation, while it is only (7 ( 2) 1014 in the (101) orientation.36 As a summary for the theoretical investigations on the usual properties of anatase TiO2 exposed by the highly reactive (001) surface, the stability, the electronic ground states, as well as the general adsorptive properties have been intensively studied. However, several challenges still remain, particularly, elucidating mechanisms of these photoinduced surface reactions which take place on the (001) surface and clarifying the differences of catalytic properties between (001) and other surfaces. Furthermore, with the developments of computational science and theoretical methods, approaches such as time-dependent DFT and the many-body Green’s function can be employed to investigate the excited-state-associated behaviors on (001) surfaces and predicate their new properties and promising applications. Photodegradation of Small Organic Molecules. First-principles calculations show that {001} facets of anatase TiO2 may have superior photocatalytic performance to {101} facets, and the first experimental evidence has been obtained through monitoring of the generated •OH radicals, which are typical photocatalytically active species.4,22,26,37 In this work, the anatase TiO2 singlecrystal nanosheets (SCNSs) having 64% {001} facets were synthesized first, and the formation of •OH on the surface of UV-illuminated TiO2 was detected by a photoluminescence (PL) technique with terephthalic acid as a probe molecule. On the basis of the measured BrunauerEmmettTeller (BET) areas, the normalized concentration of •OH generated from TiO2 SCNSs with clean surfaces is more than 5 times higher than that of the benchmarking material, Degussa P25 TiO2 (see Figure 6). The results reveal that TiO2 SCNSs with fluorine-free surfaces show superior photocatalysis activity in forming •OH, which clearly demonstrates that the high density of unsaturated five-fold Ti as well as the unique electronic structure of the {001} facets do substantially enhance the photoreactivity of the anatase TiO2 SCNSs. Also, the stability of anatase TiO2 SCNSs was
Figure 4. Partial densities of states (PDOS) for adsorbed dyes (red) and the total system (gray). The violet dotteddashed line represents the Fermi level. The positions of the HOMO and LUMO levels of the adsorbed dye molecules are marked by cyan and dotteddashed red arrows, respectively. (Reprinted from ref 31, with permission from G€ulseren et al. Copyright (2009) by the American Physical Society.)
pseudopotential method within DFT.31 Electronic structure calculations show that the adsorption of dye molecules may result in a red-shifted spectrum for the titania surface and lowering of the optical threshold to visible light (see Figure 4). Also, the surface structure of anatase TiO2 has an effect on the electronic structure of the dye molecule, and this finding might help to design better photosensitizers as well as the surface structure of anatase TiO2 in the area of photovoltaics and photocatalysis. Importantly, TiO2 in the anatase phase is also considered as one of the most important electrode materials for Li ion batteries because of its promising lithium ion intercalation capacity, cyclability, and rate capability.32,33 Li reaction with the TiO2 polymorphs typically has a volume change of clean {001} facets > TiF-terminated {001} facets. In addition, the tunable photocatalytic
Experimental evidence exhibits superior properties of anatase TiO2 dominated with highly reactive facets in environment- or energy-related applications. selectivity was believed to be due to the adsorption selectivity of HTS; the adsorption of MO was still low after surface modification of HTS by either NaOH washing or calciantion, 729
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Figure 7. Changes in the photocatalysis of hollow TiO2 microspheres (HTSs) before and after surface modification (methyl orange (MO) and methylene blue (MB) are used as probe molecules): (a) as-prepared fluorinated HTS sample; (b) HTS modified by NaOH washing; (c) sample modified by calcination at 600 °C. (Reprinted from ref 8.)
Figure 9. UVvis adsorption spectra of (a) pure anatase TiO2 sheets and (b) nitrogen-doped anatase TiO2 sheet. Insets are the typical SEM image of as-synthesized anatase TiO2 sheets and the plot of the transformed KubelkaMunk function versus the energy of light. (Adapted from ref 9.)
dipyrromethane (DN-BODIPY) as the fluorescent probe and found that most fluorescence spots were preferentially located on the (101) surface of crystal (Figure 8c).46 This finding may be helpful to interpret some important photocatalytic processes. For example, a large surface area of {101} facets is suitable for decomposition of acetaldehyde, while anatase TiO2 samples with dominant {001} facets have excellent capability to remove NO and toluene through photocatalytic reactions.45,47 Though the detailed mechanism of the face-selective electron and hole separation is still in its infant stage, the selective photocatalysis on anatase TiO2 crystals with specific exposed crystal faces opens a pathway to a deeper understanding of various photocatalysis processes. Photocatalytic Water Splitting. As the photocatalytic properties are strongly affected by the band structure and surface properties of photocatalysts, research interests have recently been focused on octahedral anatase TiO2 dominated with {001} facets because of its applications in photocatalytic hydrogen evolution.9,48 Using visible-light-responsive nitrogen-doped anatase TiO2 sheets with dominant {001} facets, Liu et al. studied their water splitting performance under UVvis illumination.9,49,50 As
Figure 8. (a) Schematic illustration of electrons and positive holes transfer on an anatase TiO2 particle with a specific exposed crystal face. (b) Geometric model of an anatase TiO2 crystal with preferential {001} facets. (c) Fluorescence of DN-BODIPY adsorbed on a single TiO2 crystal. (Adapted from ref 45 and from ref 46, copyright 2010, with permission from Wiley-VCH Verlag GmbH & Co. KGaA.)
while the adsorption of MB was significantly enhanced for both HTS samples. More interestingly, it has been reported that reduction and oxidation sites on the surface of anatase TiO2 single crystals are spatially separated because of selective migration of excited electrons and positive holes (see Figure 8a).27,45 To determine reduction and oxidation sites on the TiO2 crystal precisely, Matsumura and co-workers studied the selective deposition of Pt and PbO2 and found that Pt nanoparticles mainly appeared on the majority (101) surface while PbO2 nanoparticles only can be deposited on the (001) face. Recently, Tachikawa et al. directly evaluated the photocatalytic activity of an individual TiO2 single crystal using as-synthesized redox-responsive boron 730
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Figure 10. Li ion battery cycling performance at different rates of 1 (a) and 10 C (b) by using electrodes containing anatase TiO2 with dominant {001} (blue) and {101} (red) facets. (c and d) Corresponding geometrical models and TEM images of the anatase single crystals. (Adapted from ref 10, copyright 2010, with permission from the Royal Society of Chemistry.)
shown in Figure 9, anatase TiO2 sheets in yellow color exhibit an additional high visible light adsorption band from 400 to 570 nm. The doped anatase TiO2 sheets showed hydrogen evolution rates of 1483 and 81 μmol 3 h1 3 m2 under 220770 and 420 770 nm light irradiation, respectively. However, the corresponding rates for pure anatase TiO2 sheets with a comparable particle size and percentage of {001} facets were only 1333 and 0 μmol 3 h1 3 m2 under the same measurement conditions. Oxygen-deficient anatase TiO2 sheets with dominant {001} facets were synthesized via a facile one-pot hydrothermal route with solid metallic titanium diboride as a precursor and showed an enhancement in hydrogen evolution rate by using anatase TiO2 sheets with stoichiometric surfaces as the benchmark sample (2332 versus 1333 μmol 3 h1 3 m2).49 Recently, we systematically studied the efficiency of hydrogen evolution for ultrathin anatase TiO2 nanosheets with different percentages of exposed {001} facets;51 the hydrogen evolution rates are 4334, 7381, and 6958 μmol 3 h1 3 m2, corresponding to TiO2 nanosheets with 69, 82, and 77% exposed {001} facets, respectively. Even though the photocatalytic H2 production of {001} faceted anatase TiO2 has been investigated theoretically and experimentally, some challenges still exist. For example, the hydrogen evolution rate of such photocatalysts is generally still much lower than that of the state-of-the-art benchmarking photocatalysts such as (Ga1xZnx)(N1xOx), lanthanum-doped NaTaO3, and Pt-PdS/CdS.5254 To further enhance water splitting performance of these unique anatase TiO2 crystals exposed by highly reactive facets, strategies like metalnonmetal codoping, crystal shape engineering, and building heterojunctions can be further applied, which are supposed to alter the intrinsic electronic properties such as indirect band gap and the surface states of the photocatalysts. Anode Materials for Li Ion Batteries. Lithium ion batteries are considered as the most promising energy storage strategy for mobile electronics, electric vehicles, and renewable energy
systems operating on intermittent energy sources such as wind and solar. Because the performance of Li ion batteries strongly depends on the electrochemical properties of electrodes, the improvement of electrode materials has a substantial effect on the whole battery performance. Anatase TiO2-based nanostructure have been realized as a promising alternative to the carbon-based anode materials because of the advantages such as low cost, environmentally benign, and relatively good battery performance. Using anatase TiO2 with largely exposed {001} facets as electrode materials, the Liþ ion insertion/extraction kinetics and the batteries' performance were systematically investigated by Yang and Lou and their co-workers.10,5557 The corresponding irreversible capacity loss for TiO2 nanosheets with 62% exposed {001} facets is only 10.7%, which is more than 3 times lower than that of anatase-phase TiO2 nanotubes and hollow micro/nanostructures. Moreover, the TiO2 nanosheets dominated with {001} facets also demonstrated excellent capacity retention after prolonged cycling at a 20 C rate.10 In order to further explore possible anisotropy in the Liþ insertion/extraction behavior of anatase TiO2 with different percentages of (001)/(101) surfaces, Sun and co-workers studied the Liþ insertion/extraction kinetics and capacity of the two distinct benchmarking samples mainly exposed by low-index (101) or (001) surfaces (see Figure 10).10 Anatase TiO2 nanosheet hierarchical spheres (TiO2 NSHSs) composed of highly exposed {001} facets also demonstrated the excellent electrochemical energy storage capability and Coulombic efficiency for lithium extraction, excellent capacity retention, and superior rate behavior due to their high surface activity as well as the unique bulk properties.55,56 However, these TiO2 NSHSs might not be stable enough to withstand the high-rate insertion/extraction of lithium ions over extended cycling. Photosensitized Anode Materials in DSSCs. In DSSCs, undoubtedly, the mesoporous titania films are one of the key components for high-energy conversion efficiency. Traditionally, those titania 731
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Figure 11. (a) IV curves for DSSCs fabricated from the ATNHSs and P25 TiO2. (b) IPCE spectra for DSSCs fabricated by ATNHSs and P25 films with the similar thickness (about 14 μm). Low-magnification and high-resolution TEM images of the ATNHSs (c and d). (Adapted from ref 60, copyright 2011, with permission from the Royal Society of Chemistry.)
films are composed of small anatase TiO2 crystals mainly enclosed by {101} facets. However, because half of the Ti atoms on the {101} facets are fully coordinated while all of the Ti atoms on the {001} facets are 5c coordinates, more dye adsorption is expected to occur on the {001} facets, and anatase mesoporous titania films containing {001} facets should give a better cell performance.22,30,5860 Furthermore, the superior light scattering effect as well as excellent light reflecting ability of the mirrorlike 2D {001} facets can further improve the efficiency of photoharvesting and thus enhance the performance of the DSSCs.58,59 In the recent study by Yang et al., anatase TiO2 nanosheet-based hierarchical spheres (ATNHSs) with over 90% of {001} facets were used as photoanodes of DSSCs.60 As shown in Figure 11, the ATNHSs-based DSSC generated an energy conversion efficiency of 7.51%, indicating a 43% increase compared to the standard Degussa P25 photoanode (5.26%). However, there are still much controversy over the crystal face dependence of the dye, and the (001) surface of anatase TiO2 did not show more superior properties than other low-index surfaces, which was confirmed by experiments that utilized N3 and maleic anhydride (MA) to study the surface chemistry of highly pure large natural titania crystals.61,62 Even though the unusual properties of anatase TiO2 exposed by highly reactive facets have been revealed theoretically and experimentally and more applications are expected, some challenges still exist in several aspects. For example, further improving the photocatalytic activities especially in the visible light region will be one key challenge; the chemical/thermal stability of these materials also needs to be systematically investigated. For applications in DSSC, the assembled 3D structures with suitable size are supposed to enhance the light scattering capability and thus improve the efficiency of photoutilization. Furthermore, the hierarchical 3D structures can also help to
avoid the layer-by-layer stacking of anatase TiO2 nanosheets, which can lead to the elimination of the useful highly reactive {001} facets. For the application as anode materials in Li ion batteries, some drawbacks still exist, such as intrinsically low electronic conductivity, large capacity losses in the first cycle, and lattice strains induced by repetitive cycles. Therefore, strategies like metals coating and structural modification can be used to improve the performance of this kind of anode materials in Li ion batteries.
’ AUTHOR INFORMATION Corresponding Author
*Tel/Fax: (þ86) 21 64252127. E-mail:
[email protected].
’ BIOGRAPHIES Wen Qi Fang received his B.Eng in 2009 from East China University of Science & Technology. He is currently a Master degree student (with Prof. H. G. Yang) in Key Laboratory for Ultrafine Materials of Ministry of Education at East China University of Science & Technology. His main research project focuses on the design, synthesis and characterization of electrode materials for different types of optoelectronic devices, specifically dye-sensitized solar cells. Xue-Qing Gong received his B.Eng from Shanghai Jiaotong University (2000) and Ph.D. in chemistry from The Queen’s University of Belfast (2004). From 2004 to 2007, he was a postdoctoral research associate at Princeton University, working with Professor Annabella Selloni. He is currently a Professor at East China University of Science & Technology. His research interests include heterogeneous catalysis, surface science, and materials chemistry, with particular focus on structures of the 732
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surface and interface of (photo)catalytic (nano)materials, their interaction with molecules and other metal/metal oxide materials, and the reaction pathway in industrial catalytic processes. Hua Gui Yang completed his Ph.D. in 2005 at the National University of Singapore. Then, he joined the General Electric (GE) Company as a research scientist and moved to the University of Queensland in 2007 as a Postdoctoral Research Fellow. Now he is a Full Professor of East China University of Science & Technology. His work has been published in top journals such as Nature and featured in the Nature Materials, RSC Chemistry World, and ACS Chemical & Engineering News. Currently, he has interests in design and synthesis of metallic and semiconducting crystals for renewable clean energy and environmental protection applications. Personal homepage: http://clxy.ecust.edu.cn/ szdw/yanghuagui02.htm
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’ ACKNOWLEDGMENT This work was financially supported by the Scientific Research Foundation of East China University of Science & Technology (YD0142125), Pujiang Talents Programme and Major Basic Research Programme of Science and Technology Commission of Shanghai Municipality (09PJ1402800, 10JC1403200), Shuguang Talents Programme of Education Commission of Shanghai Municipality (09SG27), National Natural Science Foundation of China (20973059, 91022023, 21076076, 21073060), Fundamental Research Funds for the Central Universities (WJ0913001), and the Program for New Century Excellent Talents in University (NCET-09-0347). ’ REFERENCES (1) Chen, X. B.; Mao, S. S. Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications. Chem. Rev. 2007, 107, 2891–2959. (2) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37–38. (3) Fujishima, A.; Zhang, X.; Tryk, D. A. TiO2 Photocatalysis and Related Surface Phenomena. Surf. Sci. Rep. 2008, 63, 515–582. (4) Vittadini, A.; Selloni, A.; Rotzinger, F. P.; Gr€atzel, M. Structure and Energetics of Water Adsorbed at TiO2 Anatase (101) and (001) Surfaces. Phys. Rev. Lett. 1998, 81, 2954–2957. (5) 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. (6) Pang, C. L.; Lindsay, R.; Thornton, G. Chemical Reactions on Rutile TiO2(110). Chem. Soc. Rev. 2008, 37, 2328–2353. (7) Dohnalek, Z.; Lyubinetsky, I.; Rousseau, R. Thermally-Driven Processes on Rutile TiO2(110)-(1 1): A Direct View at the Atomic Scale. Prog. Surf. Sci. 2010, 85, 161–205. (8) Liu, S.; Yu, J.; Jaroniec, M. Tunable Photocatalytic Selectivity of Hollow TiO2 Microspheres Composed of Anatase Polyhedra with Exposed {001} Facets. J. Am. Chem. Soc. 2010, 132, 11914–11916. (9) Liu, G.; Yang, H. G.; Wang, X.; Cheng, L.; Pan, J.; Lu, G. Q.; Cheng, H. M. Visible Light Responsive Nitrogen Doped Anatase TiO2 Sheets with Dominant {001} Facets Derived from TiN. J. Am. Chem. Soc. 2009, 131, 12868–12869. (10) Sun, C. H.; Yang, X. H.; Chen, J. S.; Li, Z.; Lou, X. W.; Li, C.; Smith, S. C.; Lu, G. Q.; Yang, H. G. Higher Charge/Discharge Rates of Lithium-Ions across Engineered TiO2 Surfaces Leads to Enhanced Battery Performance. Chem. Commun. 2010, 46, 6129–6131. (11) Yu, J.; Fan, J.; Lv, K. Anatase TiO2 Nanosheets with Exposed (001) Facets: Improved Photoelectric Conversion Efficiency in DyeSensitized Solar Cells. Nanoscale 2010, 2, 2144–2149. 733
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The Journal of Physical Chemistry Letters
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