Titanium Oxide Nanosheets: Graphene Analogues with Versatile

Review pubs.acs.org/CR

Titanium Oxide Nanosheets: Graphene Analogues with Versatile Functionalities Lianzhou Wang*,† and Takayoshi Sasaki*,‡ †

Nanomaterials Centre, School of Chemical Engineering and AIBN, The University of Queensland, St. Lucia, Brisbane, QLD, 4072, Australia ‡ International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan 3.3.1. Wet-Chemistry-Induced Phase Transformation 3.3.2. Thermally Induced Phase Transformation 4. Brief Overview of Potential Applications 4.1. Photoinduced Applications 4.1.1. Photocatalysis Applications 4.1.2. Photovoltaic Applications 4.1.3. Photoluminescence Applications 4.1.4. Photoinduced Composite Hydrogels 4.2. Electrochemical Applications 4.2.1. Lithium Ion Batteries 4.2.2. Proton Exchange Membrane Fuel Cells 4.2.3. Electrochemical Sensors 4.3. Dielectric Nanodevices 4.4. Catalytic Applications 4.5. Biomedical Applications 4.6. Other Applications 4.6.1. Gas Barrier Layers 4.6.2. Composites with Mechanical Properties 5. Summary and Outlook Author Information Corresponding Authors Notes Biographies Acknowledgments References

CONTENTS 1. Introduction 2. Synthesis, Characterization, and Properties of Titanium Oxide Nanosheets 2.1. Starting Layered Titanate Compounds 2.2. Exfoliation of Layered Titanates 2.3. Characterization of Titanium Oxide Nanosheets 2.4. Properties of Titanium Oxide Nanosheets 2.4.1. Optical Properties 2.4.2. Electronic Properties 2.4.3. Catalytic Properties 2.5. Theoretical Understanding of Titanium Oxide Nanosheets 2.6. Titanium Oxide Nanosheet Family: Doping and Multicomponent Oxide Systems 2.6.1. Metal and Nonmetal Doping 2.6.2. Binary and Ternary Oxide Nanosheets Containing Titanium Oxide 3. Nanoarchitecture Design Using Exfoliated Nanosheets as Building Blocks 3.1. Two-Dimensional Thin Films 3.1.1. Drop Casting and Electrophoretic Deposition 3.1.2. Layer-by-Layer Deposition 3.1.3. Langmuir−Blodgett Deposition 3.2. Restacked Powdery Nanostructures 3.2.1. Flocculation 3.2.2. Organic−Inorganic Nanocomposites 3.2.3. Spray-Drying and Freeze-Drying 3.2.4. Layer-by-Layer Assembled Core−Shells and Hollow Shells 3.3. Phase Transformation

© 2014 American Chemical Society

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1. INTRODUCTION Titanium oxide (TiO2) has been among the most studied material systems over the past few decades due to its low cost, chemical stability, nontoxicity, and, more importantly, multifunctionalities in catalysis, photocatalysis, electronics, photovoltaic, and biomedical applications.1−14 Structurally, TiO2 has a number of polymorphs, of which anatase, rutile, and brookite are the most commonly investigated structures. Recent studies have also revealed some interesting properties in less studied polymorphs; for example, the cubic cotunnite-type phase is the hardest known oxide and metastable monoclinic TiO2(B)-type phase owes excellent photo- and electrochemical properties.15−19 In addition to these well-documented structures,

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dx.doi.org/10.1021/cr400627u | Chem. Rev. 2014, 114, 9455−9486

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Table 1. A Collection of Exfoliated Titanium Oxide Nanosheets

led to numerous research projects on TiO2 nanosheets , in which the TiO2 nanoparticles have been controlled to have preferential growth along some directions such as the {001} facet to present apparent 2D morphologies.38−41 In some extreme cases, the thickness of facet-controlled TiO2 particles can be only a few nanometers,42,43 whereas fundamentally such types of nanosheets still have a 3D ordering of the crystallographic structure of either the anatase or rutile phase, which are different from the exfoliated 2D nanosheets with atomic or molecular thickness. Because of the increased percentage of exposed reactive (001) surface, {001}-enriched TiO2 nanoparticles normally exhibit superior performance in many energy and environmental-related applications (such as photocatalysis, catalysis, solar cells, and rechargeable batteries) than the ordinary TiO2 nanocrystallites with dominated low index facets such as {101}. Nevertheless, there are still some challenges for facet-enriched TiO2 nanosheets, such as the use of the highly toxic synthesis capping agent HF, and the deactivation tendency of reactive (001) surface. The progress in facet-controlled TiO2 nanosheets is beyond the scope of this review. Readers may refer to some recent reviews for more information.14,44 To avoid any confusion, the term “nanosheets” in this review article will mainly refer to the elementary unilamellar layers derived from the top-down exfoliation of layered host compounds, which normally exhibit high 2D anisotropy with subnano- to nanometer thickness and infinite planar length, single crystallinity, and well-defined composition. It should be noted that because the compositions of the exfoliated 2D nanosheets slightly deviate from the stoichiometry of TiO2 with a general formula of Ti1‑x□xO2δ‑ (where □ represents vacancies) depending on the starting layered titanate compounds, hereafter we will use “titanium oxide nanosheets” to broadly represent this family of 2D materials. For specific examples, we will refer to the surface charge bearing formula of a particular type of nanosheets, for example, Ti0.91O20.36‑ or Ti0.87O20.52‑.45 In the past decades, a large array of exfoliated nanosheets derived from layered metal oxides, metal phosphates, hydroxides, nitrides, and chalcogenides have been developed.35,36,46−63 This has formed a rapidly expanded nanosheet family; in particular, the nanosheets of MoS2, WS2, BN, MoSe2, NbSe2, NiTe2, and Bi2Te3 derived from layered dichalcogenides and nitrides are experiencing exponential growth in the research sector nowadays.59 There are also some excellent review articles covering various types of nanosheets including

titanium oxide can also be derived from the exfoliation of some Ti−O-bearing layered compounds to offer a unique twodimensional (2D) anisotropy, which is the focus of this article. Layered compounds represent a large group of materials.20 Because of the strong in-plane bonds and weak interaction between the neighboring layers, one of the key features of these layered compounds is the possibility of three-dimensional (3D) structure disintegration into single pieces of host layers, socalled delamination or exfoliation. The exfoliation behavior of layered compounds has been studied for over half a century. One of the most classic examples is the spontaneous swelling and exfoliation of smectite clay minerals in aqueous solution; the resultant single layers of the clay mineral contain aluminosilicate layers with large anisotropy.21−23 More recently, physical exfoliation of graphite into merely one carbon atom thick graphene single sheetsa discovery awarded with a Nobel Prizeis a scientific milestone in the layered material family.24 This discovery has triggered exponential growth of research and development into this fascinating material.25−29 Interestingly, the exfoliation of layered graphite into its graphite oxide (GO) single sheet derivatives using Hummer’s method can be traced back to the 1950s.30 A number of studies have reported the use of exfoliated GO or reduced GO for various applications;31−33 however, the most intriguing properties of isolated single graphene sheets were not revealed until the recent “rediscovery” of the material in 2004.24 Titanium oxide single sheets, which share the structural similarity of graphene sheets, have been researched for nearly 2 decades. Since the mid-1990s, Sasaki and co-workers have studied the swelling− exfoliation behavior of layered titanates.34−37 Through ionexchange, intercalation, swelling, and mechanical shaking processes, the layered titanates can be successfully deassembled into single sheets of titanium oxide. The resultant single sheets, so-called nanosheets, can be regarded as a new class of materials, which have extremely small thickness of ∼1 nm and are up to several tens of micrometers in lateral size. As nanosheets are derived from the wet-chemical exfoliation process, the materials maintain a very high single crystallinity within the single sheets and bear surface charges for excellent colloidal dispersion. More importantly, because of its unique 2D structure, there are a host of distinctive properties that are not easy to acquire in its bulk counterparts. In the vast amount of literature, readers can easily find that the definition of “nanosheets” is quite broad. For example, the recent breakthrough in facet-controlled synthesis of TiO2 has 9456


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Figure 1. Schematic illustration of the crystal structure of a typical lepidocrocite-type titanate and its exfoliation into 2D Ti1‑δO24δ‑ nanosheets.

by Ti atoms leads to the formation of a stoichiometric TiO2 composition, while the substitution of Ti atoms by other metal ions or simple lack of Ti sites results in negative charges in the host Ti−O layers. To balance such charges, alkali metal ions such as K+ and Cs+ are often accommodated into the interlayer galleries. The layered titanates are normally synthesized via the conventional solid-state calcination of a mixture containing TiO2 and alkali metal salts such as carbonates at elevated temperatures.34,35 The solid-state reaction leads to reasonably large sized layered titanate crystallites in the range of submicrometers to micrometers. To a larger extent, the size of such titanate compounds affects the size and crystallinity of the resultant nanosheets. For some specific purposes, researchers do expect to attain nanosheets with larger or smaller lateral sizes; the modification of synthesis conditions is thus important and necessary. For instance, for the application of dielectric devices, large-sized nanosheets are preferred to avoid the possible leak paths in the nanosheet thin films. Large single crystals of layered titanate up to dozens of micrometers (K0.8Ti1.73Li0.27O4) can be intentionally synthesized by a combined melt−recrystallization process in a flux melt, which can be subsequently exfoliated to prepare Ti0.87O20.52‑ nanosheets tens of micrometers in lateral size.70 Instead of using large layered compounds, very tiny nanosheets with a lateral size of merely 2−5 nm can also be obtained by exfoliating the nanosized layered parent compounds including titanate (Na0.8Ti1.8□0.2O4·H2O), manganate (Na0.44MnO2·H2O), and tungstate (Cs4W11O35·H2O).71 Such types of nanosheets with small size in every dimension can be considered as monolayer nanodots, which exhibit noticeably increased band gap energies compared to their bulk counterparts. In contrast to the top-down exfoliation method, elementary single nanosheets of fundamental lepidocrocite structure can also be prepared by a bottom-up approach through either the vapor deposition method72 or wet-chemical one-pot synthesis.73 The former one involves a costly e-beam deposition on Pt substrates under ultrahigh vacuum, while the latter one is

metal oxides, layered double hydroxides (LDHs), and nonmetal oxides.64−69 In this review, we focus solely on titanium oxide based nanosheets, attempting to give a snapshot on the state-ofthe-art research status of these particular types of multifunctional materials. Note that the understanding, rationale, and methodologies on titanium oxide nanosheets introduced herein could also be applicable to a range of other 2D nanostructures. In section 2 of this review, we introduce, in more detail, the synthesis, characterization, and unique properties of the exfoliated titanium oxide nanosheets. Section 3 focuses on new nanoarchitectural designs using 2D nanosheets as building blocks. A rich host of nanostructures including restacked powdery and nanocomposites, thin films prepared with a variety of methods, and phase transformation of nanosheets will be described. Section 4 summarizes the major attempts of the 2D nanosheet-based systems for potential applications in a range of fields including photoinduced applications, electrochemical and dielectric devices, sensors, and biomedical applications. The review concludes with section 5, a brief summary and outlook for future research directions.

2. SYNTHESIS, CHARACTERIZATION, AND PROPERTIES OF TITANIUM OXIDE NANOSHEETS 2.1. Starting Layered Titanate Compounds

The intercalation−exfoliation process provides a generic approach for preparing titanium oxide nanosheets. In this method, the starting layered titanates or, in other words, the parent materials significantly affect the “offspring” nanosheets in terms of crystallinity, composition, size, and functionality. A number of titanium oxide based nanosheets have been obtained by exfoliating various layered compounds, as summarized in Table 1. Figure 1 shows the crystal structure of a typical titanate compound in an orthorhombic system with unit cell dimensions of a ∼ 0.38 nm, b ∼1.7 nm, and c ∼ 0.3 nm. The host layer in the lepidocrocite (γ-FeOOH) type consists of Ti−O6 octahedra, which are connected via edge sharing to form a 2D feature. Theoretically, full occupation of octahedral sites 9457


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very small surface charge density. As a result, the ions have superior stabilization energy, which can be of advantage in adjusting the electric double layers of various nanoparticles in suspensions. To exfoliate the protonated titanates, the materials are normally in contact with tetrabutylammonium hydroxide (TBAOH) aqueous solution (Figure 1). Because of the solid acid nature, the dispersion of protonated titanate in TBA+containing solution, with the assistance of external forces such as mechanical shaking or ultrasonication, allows the intercalation of TBA+ bulky ions into the interlayer species simply driven by the acid−base equilibrium. The intercalation of TBA+ bulky ions eventually expands the interlayer spacing of the titanates, accompanied by the introduction of a large volume of water. This expansion greatly reduces the electrostatic attraction between host layers and counterions. Because of the important role of TBA+ in adjusting the intercalation degree of protonated titanate, the ratio of TBA+ to H+ is often modified to control the exfoliation process. X-ray diffraction (XRD) investigation is a powerful tool to monitor the various stages of exfoliation. As Figure 2A indicates, comparing the

much more cost-effective in the presence of tetrabutylammonium (TBA+) ions, (C4H9)4N+. Uniform diamond-shaped nanosheets with controllable 2D sizes in the range of 7.73 × 5.45 and 27.3 × 19.1 nm2 can be obtained at large scale (>2.6 g per synthesis). It is worth noting that the wet-chemical bottomup method for nanosheet preparation has the merits of low cost and procedure simplicity, but the crystallinity of the resultant nanosheets is still a concern. In addition to size-controlled synthesis, the Ti−O ratio in nanosheets can also be modified. For example, a simple electron beam irradiation on the Ti0.87O20.52‑ nanosheets can reduce it into Ti2O3 nanosheets.74 Interestingly, the knock-on loss of some Ti atoms also induces the structural transformation from edge-shared TiO6 octahedra to a face-shared octahedra structure. Gao et al. used another method to control titanium oxide nanosheets composition by first doping the layered titanate with Mg2+.75 Even though Mg2+ ions were incorporated into the host titanate layer having a composition of CsxTi2‑x/2Mgx/2O4 (x = 0.7), Mg2+ together with Cs+ in the titanate interlayer galleries could be extracted during the ionexchange reaction to produce a protonated titanate HxTi2‑x/2O4‑x/2·H2O. The subsequent exfoliation led to the nanosheets with a slightly changed composition and vacancies ratio, Ti1‑δO2‑δ2δ‑ (δ = 0.175). Likewise, the nanosheets of Ti3O72‑, Ti4O92‑, and Ti5O112‑ can also be prepared by modifying the starting layered titanate compositions.76,77 More composition modification strategies such as metal doping, nonmetal doping, and binary/ternary compound design will be introduced in section 2.6. Because of the ion-exchangeable nature of the alkali metal ions in the interlayer galleries of the layered titanates, these ions can be replaced by protons when treated in acid solutions, while the original layered structures remain unchanged. Such ion-exchange capability of the layered titanates opens up opportunities for rational chemical modification in the materials. Upon protonation, the resultant protonated layered structures often exhibit distinctive solid acidity, in which the interlayer spacing and charge density are important parameters that impact on the following intercalation and exfoliation processes.34 Because of the acid−base intercalation feature of the protonated layered compounds, the materials are able to induce further structural modification for subsequent exfoliation process.

Figure 2. (A) XRD patterns of pristine protonated titanate, H0.7Ti1.825□0.175O4·H2O (a), and colloidal mixtures with different TBA+/H+ molar ratios: 25 (b), 15 (c), 5 (d), and 0.1 (e). (B) XRD patterns for colloidal aggregates centrifuged from suspensions of part A . Here TBA+/H+ = 25 (a), 15 (b), 10 (c), and 2 (d). The numbers indexed to the peaks represent the order of basal reflections in lamellar aggregates. Reprinted with permission from ref 36. Copyright 1998 American Chemical Society.

2.2. Exfoliation of Layered Titanates

The exfoliation or delamination of layered compounds refers to the processes involving various manners to weaken the electrostatic interaction between the layers and disassemble the ordered layered structure into single individual sheets. Owing to the weak interlayer electrostatic interaction due to its low layer charge density, some clay minerals can be spontaneously exfoliated into single sheets upon the uptake of sufficient water. Unfortunately, layered titanates have strong electrostatic interaction between the host layers and balancing counterions. The exfoliation of such compounds thus relies on the deliberately selected bulky species to replace the interlayer protons to reduce host−guest interaction.35,36 Organic quaternary ammonium ions have been selected as effective guest species to expand the interlayer galleries of the titanates because of their relatively large ion size and low surface charge density.78 Taking one of the most effective ions, TBA+, as an example, the butyl chains of the TBA+ ion stretch to four directions, making the ions quite large in size (∼1.2 nm) with

XRD patterns of parent protonated titanate H0.7Ti1.825O4·H2O with the body-centered orthorhombic layered structure, a series of new diffraction peaks at the low 2θ region appear together with a large water halo centered at around 20°−25° for the mixtures of H0.7Ti1.825O4·H2O treated with various doses of TBAOH solutions, while the sharp peaks at higher angular region vanish.36 In particular, the XRD pattern of the sample with 5:1 (TBA+/H+) ratio presents no any sharp XRD peaks apart from the broad halo, suggesting the loss of the 3D crystallographic characteristic of titanate. This information reveals the intercalation and exfoliation behavior of the TBA− titanate system. More evidence can be found from the XRD study on the colloidal aggregates obtained from centrifuging the 9458


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4.80 This new finding gives implications that the intercalation chemistry of well-studied material systems could be highly complex and have unpredicted new phenomena.

mixture, as depicted in Figure 2B.36 A couple of distinctive features in the XRD patterns suggest the different stages of TBA/titanate intercalation chemistry; a broad diffraction halo in the samples with TBA+/H+ ratios of 1−5 is the indication of scattering from randomly restacked aggregates of exfoliated nanosheets, a fingerprint feature of the exfoliation phenomenon. A series of sharp diffraction peaks for samples obtained from higher TBA+/H+ ratios suggest that a large amount of TBA+ solution is introduced into the neighboring layers of titanate, which behaves like a blanket of electrolyte solution to neutralize the surface charges of titanate, so-called osmotic swelling. In contrast, a very limited TBA dose (TBA+/H+ < 0.5) results in the TBA-intercalated form of layered titanate. Along with the XRD study, the physical appearance of TBA− H0.7Ti1.825O4·H2O mixture solutions also provides complementary information on the exfoliation stages. Noticeable sedimentation is evident in samples with a TBA+/H+ ratio 95% coverage, can be obtained under an optimized surface pressure. This transfer can be repeated for a desired number of times to produce a multilayer film with a designed thickness. More importantly, the technique can be applied to deposit the nanosheets with different sizes and compositions onto a range of substrates, including quartz, ITO, glass, Au, and silicon substrates.155,156 One of the key advantages of LB assembly is its capability to deposit nearly perfectly packed monolayer films, which can be subsequently used in various applications, for example, as the seed layer for oriented crystal film growth.

applied to LBL assembly of exfoliated graphene nanosheets with Ti0.91O20.36‑ nanosheets. Using PEI or PDDA as a binder, the exfoliated GO or graphene can be LBL assembled with Ti0.91O20.36‑ nanosheets to form multilayer structure.147,148 Thanks to the photocatalytic activity of Ti0.91O20.36‑ nanosheets, the GO can be photoreduced to graphene in situ upon exposure of the films to UV irradiation. Such films were reported to have ultrafast electron transfer behavior in the Ti0.91O20.36‑ and graphene neighboring layers.147 In addition to exfoliated nanosheets of smectite, metal phosphates, metal oxides, and layered perovskites, LDHs are another class of layered compounds with a general chemical formula of [M2+1‑xM3+x(OH)2][An‑x/n·mH2O].67 The host hydroxide layer is a typical brucite structure, in which divalent metal ions are partially substituted by trivalent metal ions to bear positive surface charges in the host layers. The exfoliation of LDHs also leads to a stable suspension containing individual layers of the LDH sheets.54−56 Because of the similarity of the 2D shape but oppositely charged surface of the titanium oxide and LDH nanosheets, it becomes possible to assemble the truly inorganic sandwichlike structures on the molecular level with intimate contact. The first example was the use of Mg2/3Al1/3(OH)20.33+ LDH nanosheets and Ti0.91O20.36‑ nanosheets to form multilayered superlattice-like assemblies.148 XRD patterns of the resultant thin films reveal the densely packed structure with an interlayer spacing of ∼1.2 nm, reflecting the repeating packing of (Mg2/3Al1/3(OH)20.33+/ Ti0.91O20.36‑) bilayers (Figure 16). The expansion of LDH nanosheet composition to other multifunctional transition metal ions, including Co2+ and Ni2+, can offer future opportunities to further tailor the multilayer properties. 3.1.3. Langmuir−Blodgett Deposition. Because of the irregular shape of the exfoliated nanosheets and nonselective strong electrostatic attraction between oppositely charged

3.2. Restacked Powdery Nanostructures

3.2.1. Flocculation. The colloidal suspension of exfoliated nanosheets is stabilized by TBA+ ions, while the addition of appropriate electrolytes can significantly alter the colloidal chemistry of nanosheet suspension, undergoing subsequent restacking of the nanosheets with counterions embedded in the interlayer galleries. The resulting flocculates generally have a lamellar structure but are distinct from their parent layered precursors due to the lack of regular stacking order of the nanosheets in 3D direction. From the viewpoint of nanostructural design, the exfoliation of nanosheets opens up opportunities for new functional material development, because the nanosheets in suspension expose extremely large surface areas upon which the guest species can anchor. On the contrary, intercalation chemistry has the limitation of guest species selection in terms of size, shape, configuration, and charge density. The resultant materials from restacking of exfoliated nanosheets also have the advantage of a nanoporous characteristic, which would be of particular importance for catalytic and photocatalytic applications. The restacking of exfoliated clays, metal oxides, metal phosphates, and perovskites has been studied widely. The first example on the flocculation of exfoliated Ti0.91O20.36‑

Figure 16. (A) Scheme of sandwichlike nanocomposite formation using oppositely charged Ti0.91O20.36‑ and LDH nanosheets as building blocks. Both thin films and flocculated powdery sample can be obtained simply through the electrostatic interaction of different nanosheets. (B) HRTEM images of two types of flocculated powdery samples: (a) Ti0.91O20.36‑/Mg2/3Al1/3(OH)20.33+ and (b) Ca2Nb3O10-/ Mg2/3Al1/3(OH)20.33+. Reprinted with permission from ref 149. Copyright 2007 American Chemical Society. 9468


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Figure 17. Scheme of LB assembly of Ti0.87O20.52‑ nanosheets (left) and AFM image of densely packed exfoliated nanosheet films on a solid substrate (right).

nanosheets was to use Al13 Keggin ions as a pillaring agent. Because of the high surface charge (7+) of Al13 clusters, a strong electrostatic interaction with negatively charged Ti0.91O20.36‑ nanosheets induces the flocculation to occur rapidly to produce a pillared structure having the interlayer distance of 1.6 nm.157,158 By carefully adjusting the ratio of Keggin ions to nanosheets, a series of pillared architectures with large interlayer spacing up to 2.6 nm in the restacked materials can be obtained. Considering the relatively small cluster size of ∼0.86 nm for Al13 ions, this value suggests a unique doublelayered Al13 pillared nanosheet structure. The resultant pillared structure is mesoporous with a high specific surface area of ∼300 m2/g and remarkable acidity active sites. The feature of a double-layered pillared structure was further verified in other exfoliated nanosheet systems,159 indicating the generality of the restacking approach involving Al13 clusters. Considering the stabilization mechanism of colloidal nanosheets in a TBA+-containing solution, the use of smaller ions, such as Li+ and H+, and rare earth metal clusters Eu(phen)2 can also easily induce the restacking of nanosheets.160−162 In addition to small-sized nanoclusters, various large-sized nanoparticles of metal oxides have also been used to induce the restacking of titanium oxide nanosheets. Hwang and co-workers reported a series of Ti0.91O20.36‑ nanosheet porous composites pillared with metal oxides including TiO2, CrOx, and FeOx.163−165 The concept has been subsequently extended to a the nanosheet composites composed of a variety of nanoparticles, including metal oxides, metals, and metal sulfates by various research groups.166−170 Other noteworthy progress is the combination of other exfoliated nanosheets such as Zn− Al-based LDHs and graphene nanosheets with titanium oxide nanosheets to form sandwichlike structures.147,171 One of the key features of all the restacked materials is the nanoporous structure in the composites, along with the new functionality offered by various types of guest species. The new composites often have interesting properties in catalytic, photocatalytic, electrochemical, and sensing areas, which will be discussed in detail in section 3.3.2. The use of positively charge species to induce the flocculation of nanosheets is easily understandable; however, Liu et al. reported an interesting restacking system by incorporating I2 molecule clusters without surface charge. The extremely large surface area of Ti0.91O20.36‑ nanosheets is believed to play a key role for the adsorption of I2 molecules onto the Ti0.91O20.36‑ nanosheets (Figure 18).98 Depending on

Figure 18. (Top) Scheme of the exfoliation−reassembly process of Ti0.91O20.36‑ nanosheets coupled with I2 molecules. (Bottom) Plots of the transformed Kubelka−Munk function as a function of light energy. The samples refer to Ti0.91O20.36‑ sheets coupled with different amounts of I2 molecules: a, X = 0; b, X = 0.4 atom %; c, X = 1.2 atom %. Reprinted with permission from ref 99. Copyright 2011 Royal Chemical Society.

the dose of I2 clusters, the interlayer spacing of the restacked materials can be modified. The resultant materials exhibit a typical slitlike mesoporosity with high surface area. As mentioned in section 2.5, the coupling of I2 molecules leads to an upshift of the VBM and band gap narrowing of the hybrid materials, which features increased visible light harvesting. The restacking of nanosheets has been thought to be a random packing process; however, a recent detailed microscopy study and force field calculations revealed that the restacking of Ti0.87O20.52‑ nanosheets can also follow some trends; i.e., the rotation angle of neighboring restacked layers has a certain correlation with the lattice distortion ratio a/c.172 This is because the 2D nanosheets in suspension are not perfectly flat; the curvature of Ti0.87O20.52‑ nanosheets with a certain degree of 9469


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the rapid evaporation of water in sprayed droplets leads to the formation of spherical aggregates of Ti0.91O20.36‑/TBA composites, and the subsequent calcination at 650 °C results in the formation of unique hollow spheres of tens of micrometers, while the shell thickness is merely less than 100 nm (Figure 19a). Via these processes, shape control into flaky and hollow

lattice distortion induces a slightly different degree of stacking behavior. Note that this finding may be applicable to other nanosheet systems, but more accurate control of the restacking process is still not feasible. Another interesting work describes the use of click chemistry to control the flocculation of exfoliated nanosheets. By deliberately modifying two types of exfoliated nanosheets, Ti0.91O20.36‑ and WO3, using alkene and alkyl thiol groups in organic solvents, a thiol−ene reaction can be triggered upon the addition of radical initiator to the suspension.173 This process leads to the formation of flocculated powders containing hundreds of layers of (Ti 0.91 O 20.36‑/WO3 ) nanosheets packed in an alternate sandwichlike fashion. Similarly, the coflocculation behavior of the mixture containing two types of nanosheets, such as Ti0.91O20.36-/MnO2δ- and Ti0.91O20.36-/Ca2Nb3O10-, with different ratios can also interpreted by detailed XRD studies and calculations.174 These studies can help us to understand the exfoliation−restacking process and shed light on new pathways for rational control of innovative nanoarchitecture design and fabrication, whereas the different flocculation behavior in various nanosheets systems needs to be further elucidated. 3.2.2. Organic−Inorganic Nanocomposites. The use of exfoliated aluminosilicate clay nanosheets as fillers for polymer−inorganic nanocomposites has been studied extensively.175,176 Good reasons for developing such composites are the low cost and easy exfoliation and the mechanical/thermal stability that these nanosheets can bring to the new composites. Typical methods such as in situ polymerization of monomers/ nanosheets mixture or intercalation of clays with polymers are generally used for synthesis of such types of composites. The resultant materials have found applications spanning from food packing and barrier films to fire retardants and structural materials. The selection of inorganic materials also extends to a host of layered materials, including MoS2, MoO3, GO, and graphene.26,59,177,178 Conversely, possibly because of the relatively short history of exfoliated titanium oxide nanosheets, they have received less attention as additives for polymer− nanosheet composite preparation. Earlier examples include a simple exfoliation−restacking strategy by adding aqueous polymer poly(ethylene oxide) (PEO) or poly(vinylpyrrolidone) (PVP) solutions to the colloidal Ti0.91O20.36‑ suspension;179 the addition of the protons can induce the restacking of nanosheets with trapped polymer molecules in their reassembled galleries. An exciting recent composite is the hydrogel design using Ti0.87O20.52‑ nanosheets as both a photocatalytic polymerization initiator and a crosslinker.180 More details will be introduced in section 4.1.4. 3.2.3. Spray-Drying and Freeze-Drying. To some extent, flocculation induced by guest species in a colloidal suspension provides freedom for the nanosheets to reassemble in certain levels of structural arrangements. In addition to this strategy, spray-drying and freeze-drying can be regarded as forced flocculation without much chemistry involved. Nevertheless, these straightforward methods also lead to restacked nanosheet structures with interesting features. For example, the extracted Ti0.91O20.36‑ nanosheet aggregates from freeze-drying have a cottonlike appearance. The self-standing flakes have a typical size of tens of micrometers in lateral size but the thickness is smaller, measuring tens of nanometers.181,182 Calcination at elevated temperature can remove the intergallery TBA+ species, consequently rendering the materials high specific surface area due to the randomly packed architecture. By feeding the colloidal suspension of Ti0.91O20.36‑ nanosheets to a spray-drier,

Figure 19. TEM images of hollow shell structures prepared by spraydrying (a, b) and LBL assembled Ti0.91O20.36‑ nanosheets with Al13 Keggin ions on removable polymer templates (c, d). Reprinted with permission from ref 182 and 138. Copyright 1998 and 2004, respectively, American Chemical Society.

shell microstructures has been attained to TiO2 as a versatile material, which has opened new applications. For example, on the basis of such unique morphologies as well as wide-gap semiconducting nature, these materials have been incorporated into UV-shielding foundation makeups. Instead of spray-drying the colloidal Ti0.91O20.36‑ nanosheets directly, nanosheets can also be first flocculated using H+ to obtain an H0.68Ti1.83□0.17O4·H2O precipitate, which is subsequently redispersed for spray-drying.183 The product obtained with this modified spray-drying process does not contain TBA+ species, thus avoiding the follow-up calcination step. With a specific surface area of ∼120 m2 g−1, the material demonstrates a reasonably high capacity as an anode material for lithium ion battery initially; unfortunately, its cycling capability is not good possibly due to the side reactions. 3.2.4. Layer-by-Layer Assembled Core−Shells and Hollow Shells. The LBL self-assembly technique can be used for not only planar thin film fabrication, but also core− shell structure design. Caruso et al. was among the pioneering researchers reporting the LBL coating of polyelectrolytes on polymer spheres in a colloidal suspension.184,185 Because of the charge bearing nature of exfoliated nanosheets, the LBL technique has also been applied to fabricate a composite of Ti0.91O20.36‑ nanosheet shell @ polymer sphere core in a 3D fashion.186 The flexibility of the ultrathin nanosheets allows them to cover the spherical surface of “substrates” in a good manner. However, the selection of a polymer template with appropriate size is important. Because the Ti0.91O20.36‑ nano9470


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Figure 20. Schematic diagram of hydrothermally induced phase transformation of titanium oxide nanosheets (designated as HTO nanosheet herein). Reprinted with permission from ref 193. Copyright 2007 American Chemical Society.

3.3.1. Wet-Chemistry-Induced Phase Transformation. Similar to the flocculation process, the conversion of 2D nanosheets to other structures can be achieved by simply adjusting the ionic strength of the colloid suspension. The use of a strong base NaOH as the flocculation agent can induce precipitation of various nanosheets. The subsequent washing step has the purpose to extract the Na+ from the interlayer galleries of the nanosheets, which simultaneously leads to the possible peel-off of the nanosheets for aggregates. In some cases, the 2D nanosheets can naturally roll up to form a nanotube-like structure.190 Note that there are some exfoliated nanosheets, such as Nb6O174‑, that have asymmetrical M−O host layers,68,191 providing the driving force to spontaneously form the tubular structure. Regarding nanosheets of titanium oxide and MnO20.4‑ with symmetrical crystallographic structures, the rolling up process is still not very clear and the yield of phase transformation generally is quite low. In addition to the room-temperature process, wet-chemically hydrothermal treatment of titanate nanosheets at mild temperature can also induce the phase transformation. 1D titanate nanotubes have been studied since 1998.17 The hydrothermal treatment of anatase TiO2 nanoparticles in a high concentration of NaOH solution leads to the formation of titanate nanotubes, which consists of a typical scroll-like layered nanosheet structure. In this regard, the rolling-up process of Ti0.91O20.36‑ nanosheets to form nanotubes reported by Ma et al. can be understood to some extent.190 Considering the structural similarity between layered titanate, nanosheets, and protonated nanotubes, Hara and co-workers studied the

sheets usually have an average lateral size of several hundreds of nanometers, the use of a small template with less than 100 nm will inevitably lead to interconnected coverage of multiple template particles. The resultant products thus appear to be wrapped particles rather than individual core−shell structure. A suitable template should have a particle size larger than the average lateral size of nanosheets to allow faithful coating. Upon calcination or UV irradiation of the core−shell structures, the polymer core can be removed and the balloonlike hollow sphere, which faithfully replicates the shape of the template spheres, is obtained. Like LBL thin film fabrication, in addition to polyelectrolytes as glues, other charge-bearing species can also be used to prepare the core−shell and hollow spheres. Al13 Keggin ion is one typical example;138 the pillaring role of the clusters leads to the formation of a highly porous hollow nanostructure (Figure 19b). The use of nanosheets for core− shell and hollow shell fabrications has also been expanded to other exfoliated systems including MnO20.4‑, LDHs, graphene nanosheets, and the composite shells of Ti0.91O20.36‑/graphene.187−189 Such types of new structures have the merits of a precisely controllable shell thickness, large internal voids, and porous shells for guest species to diffuse through, which can be good candidates for nanosized containers and reactors. 3.3. Phase Transformation

As mentioned, the exfoliated nanosheets can be regarded as a new class of nanoscale materials, which not only can be used as building blocks but have the capacity of structural/phase transformation from 2D shape to other nanostructures. 9471


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Figure 21. Scheme of thermally induced phase transformation of monolayer Ti0.87O20.52‑ nanosheets; crystallization temperature as a function of nanosheet layers; and AFM images of nanosheets deposited on Si wafer heated at heated at 700, 800, and 900 °C, respectively: (a) monolayer, (b) bilayer, and (c) trilayer films. Reprinted with permission from ref 195. Copyright 2007 American Chemical Society.

structural and catalytic relationship of the materials.192 This will be discussed in section 4.4. Upon hydrothermal treatment of an exfoliated nanosheet suspension under various pH conditions and temperatures, a variety of nanostructures with different phases (anatase and rutile) and rich morphologies can be obtained, as shown in Figure 20.193,194 Two possible mechanisms, i.e., the in situ topotactic structural transformation of nanosheets for anatase formation and the dissolution−deposition reaction to produce rutile phase under extreme acidic condition, are believed to be the reasons for such structural changes. The authors also explained the possible reasons for the highly exposed (010) plane in the needlelike anatase nanoparticles. While the fundamental understanding of such phase conversion is of interest, such a type of synthesis route from layered titanate via nanosheets to anatase could be time-consuming and costly. 3.3.2. Thermally Induced Phase Transformation. Calcination of flocculated titanium oxide nanosheet aggregates is common to transform the lamellar structure into anatase phase at temperatures as low as 400−500 °C. This is easily understandable due to an adequate amount of three-dimensionally distributed Ti and O atoms in the bulky aggregate. However, when a monolayer of Ti0.91O20.36‑ nanosheets isolated on substrate was treated, the phase transformation behavior can change drastically. Fukuda et al. applied total reflection fluorescence X-ray absorption near-edge structure measurement and in-plane X-ray diffraction analysis to study the structural transformation of Ti0.91O20.36‑ nanosheet films with various layer numbers and found that the monolayer can be stable up to an impressive 800 °C before it converts to the thermodynamically stable anatase phase, which is at least 300 °C higher than the phase transformation temperature of its flocculated counterpart (Figure 21).195 This is because the spatial configuration of the monolayer nanosheets, which does not provide a friendly environment for the three-dimensional nucleus of the anatase phase to grow because the atoms must diffuse extensively to form a new 3D crystalline phase. The gradually reduced phase transformation temperature for multilayer films confirms the important role of the confined

spatial configuration. The bulk behavior happens to the thin films with five layers or more, while the smaller nanosheet layers in the films inhibit the atom diffusion to form 3D Ti−O unit cells for anatase to grow freely. Another interesting phenomenon is that the phase transformation of ultrathin films results in the preferential growth of anatase along the c-axis, which can be ascribed to a “templating effect” of the 2D nanosheet structure.196

4. BRIEF OVERVIEW OF POTENTIAL APPLICATIONS 4.1. Photoinduced Applications

4.1.1. Photocatalysis Applications. Photocatalysis as an accelerated catalytic process on a semiconductor in the presence of light has attracted considerable attention over the past 4 decades.1 Aspired by the great potential of “solar + catalyst → clean energy + better environment”, photocatalysis has been regarded as the “Holy Grail” of science for both organic pollutant remediation and clean solar fuel generation, for instance, hydrogen generation via water splitting. In this regard, the design and development of better photocatalysts from the aspects of crystalline control, electronic modulation, and surface functionality modification are of fundamental importance.10−12,118,197−199 TiO2 is the most studied photocatalyst, and the exfoliated titanium oxide nanosheets have also received considerable attention as potential photocatalysts. Fundamentally, photocatalytic processes rely on the redox power of photoinduced charge carriers, i.e., electrons or holes. The photoexcited electrons in CB have reduction power, while the free holes in VB can oxidize various species. If one can manage to efficiently suppress the recombination process of charge carriers and realize high charge separation, the electrons can be used for water splitting to generate hydrogen or, more challengingly, photoreduction of CO2. Taking advantage of the oxidation power of the other half-reaction, the holes or active radicals produced through secondary reactions can photooxidize a broad range of organic pollutants in water and air for environmental remediation or kill and decompose bacteria for biomedical applications. Considering the key criteria for an efficient photocatalyst, the material should preferably have a 9472


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Figure 22. (A) Nitrogen gas adsorption−desorption isotherms of TiO2 nanoparticles (a) and nanoparticle pillared Ti0.91O20.36‑ nanosheets with different mixing ratios (b and c). The inset presents the pore size distribution curves of the samples. (B) Photocatalytic organic degradation curves of various samples for (a) 4-chlorophenol, (b) methylene orange, and (c) methylene blue. Reprinted with permission from ref 163. Copyright 2006 American Chemical Society.

Figure 23. (A) Photocatalytic water splitting for H2 and O2 production of layered titanate, protonated titanate and exfoliated Ti4O92‑ nanosheets. Measurement conditions: 300 W Xe arc lamp, 100 mg of catalyst, 50 mL of aqueous solution containing methanol (20% vol) or 0.01 M AgNO3 for H2 and O2 evolution, respectively. (B) Band gap values and positions of various layered titaniate and Ti4O92‑ nanosheet samples obtained from optical and photoelectrochemical measurements, which provide useful information for interpreting the photocatalytic performance of the catalysts in part A. Reprinted with permission from ref 201. Copyright 2010 American Chemical Society.

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compounds due to the band gap narrowing by coupling I2 molecules within the Ti0.91O20.36‑ nanosheet galleries.98 4.1.1.2. Water Splitting and CO2 Reduction. In comparison to the photo-oxidation half-reaction, water splitting for hydrogen generation is a more challenging process. Because the CBM position of layered titanate is only slightly higher than the redox couple potential of H+/H2, the material does not offer strong reduction power to efficiently split the water for hydrogen generation. For example, nitrogen-doped titanate exhibits apparent visible light absorption up to 450 nm, whereas its performance in water splitting for hydrogen generation is nearly negligible. With the assistance of cocatalysts such as the noble metal Pt, Osterloh and co-workers reported that the Ti0.91O20.36− exhibits a moderate hydrogen production yield (Figure 23).201 By adding a sacrificial agent to the reaction systems, the photocatalysts not only exhibit improved H2 generation efficiency but also are able to produce O2, which is a four-electron transfer process and difficult to achieve. On the contrary, without using any cocatalyst, the nanocomposite composed of exfoliated Ti0.91O20.36− nanosheets pillared with CdS quantum dots (∼2.5 nm) exhibits an impressive photocatalytic hydrogen production rate, in which CdS plays an important role in harvesting light and facilitating charge separation.167 Another interesting development is to mix exfoliated Ti0.91O20.36− nanosheets with visible light responsive Zn−Cr-based LDHs in colloidal suspensions. This process leads to the formation of an alternately packed sandwich-like structure. Due to the visible light responsive Zn−Cr LDH, the nanohybrids exhibit a high photocatalytic O2 evolution rate of up to 1.18 mmol h−1 g−1. The very good band position alignment between Zn−Cr LDH and titanate nanosheets facilitates electron transfer from the conduction band of Zn−Cr to Ti0.91O20.36‑, thus promoting the charge separation (Figure 24).171 Note that the presence of Ti0.91O20.36‑ nanosheets in the composites also partially suppresses the photocorrosion tendency of Zn-based LDH materials, but the longer term utilization of the photocatalyst is still a concern. Along with the photocatalytic water splitting process for hydrogen and oxygen generation, the photoreduction of greenhouse gas CO2 to produce valuable chemicals remains

high surface area, high crystallinity, and abundant surface functional groups for the interfacial catalytic reactions to occur.118 Because the exfoliated nanosheets in suspension open up nearly infinite spaces for guest species to homogeneously adsorb onto the nanosheet surfaces, nanosheet-based photocatalysts are expected to offer unique characteristics in terms of pillared porous structures, high surface areas, high crystallinity, and abundant paperlike active sites for cocatalysts to anchor. 4.1.1.1. Organic Pollutant Degradation. As discussed in section 2.4.2, titanium oxide nanosheets have an enlarged band gap of ∼3.8 eV, and strong UV light is required to activate the nanosheets for photocatalysis. Some earlier studies on nanosheet photocatalysis reveal that, under UV irradiation, nanosheets do have the capability to decompose organic species such as polyelectrolytes embedded in the LBL thin films, dyes attached onto the thin film surfaces, or the polymer template in the core−shell structures.136,138,200 The nanocomposites prepared by pillaring Ti0.91O20.36‑ nanosheets with TiO2 nanoparticles via the exfoliation−reassembling have high mesoporosity and can efficiently decompose a group of organic pollutants, including colorful dyes such as methylene orange/ blue and colorless organic species including 4-chlorophenol under UV light irradiation (Figure 22).163 As UV light only accounts for 3−5% of the whole solar spectrum that reaches the Earth’s surface, to better use solar energy, it is highly desirable to increase the light harvesting of titanium oxide based photocatalysts. In this respect, a group of visible light responsive species, including CdS, CrOx, FeOx, ZnO, and SnO2, have been incorporated into the flocculated titanium oxide nanosheets to induce visible light photocatalytic activity.164−170 Encouragingly, many of thus-prepared samples exhibit efficient photocatalytic activity in decomposing various organic pollutants under visible light (>420 nm). In particular, the charge carrier transfer from one semiconductor to another in the hybrid system plays a significant role in facilitating the charge separation. Taking the composite of the CdS− Ti0.91O20.36‑ as an example, the restacking process offers the composite three features: as CdS has a small band gap of 2.4 eV and higher CB position than that of Ti0.91O20.36‑ nanosheets, good alignment of band structure in the composite facilitates the efficient transfer of photoexcited electrons from the CB of CdS to the CB of Ti0.91O20.36‑ nanosheets by absorbing visible light, thus suppressing the probability of charge carrier recombination; furthermore, the pillared structure offers a significantly increased nanoporous structure, which can increase the interaction of reagents in solution with photocatalysts; the presence of stable Ti0.91O20.36‑ nanosheets also acts like wrapping layers and can prevent the photocorrosion of CdS nanoparticles, thus offering long-term stability of the photocatalysts. In addition to the pillared structures containing visible light responsive species, doping strategies have also been used to introduce visible light photocatalyic activity in the nanosheets. As mentioned in section 2.6.1, both metal- and nonmetaldoped titanates result in visible light active nanosheets in either powdery forms or thin films. For example, the multilayer thin films of N-doped Ti0.91O20.36‑ nanosheets have enhanced photocurrent and lower current onset than its pristine Ti0.91O20.36‑ counterpart under visible light irradiation, indicating that the doping process is effective in increasing solar light utilization.84 The wrapping of elemental I2 molecules can also lead to visible light activity in decomposing model dye

Figure 24. Schematic illustration of band structure alignment of the composite photocatalysts composed of exfoliated Zn−Cr LDH and Ti0.91O20.36‑ nanosheets. Efficient charge transfer from the CB and interband states of Zn−Cr LDH to the CB of Ti0.91O20.36‑ nanosheets is of importance for improving the photocatalytic performance and suppressing photocorrosion of Zn species. Reprinted with permission from ref 171. Copyright 2011 American Chemical Society. 9474


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studied the hydrophilicity of the Ti0.91O20.36‑ nanosheet thin films by investigating the parameters of light intensity and nanosheet layer number.204 It is interesting that the monolayer nanosheets, i.e., around 1 nm film on substrates, can induce remarkable superphilicity ( 160 m2 g−1). These features make the composites not only efficiently adsorb more dye molecules but also have light-scattering effect in DSSCs as photoanodes. More interestingly, detailed dark current potential scan and open circuit voltage decay studies indicate that the presence of Ti0.91O20.36‑ nanosheets in the composites can promote charge separation, possibly due to the unique 2D single-crystalline nature for confined electron transfer, which allows the electrons injected from dyes to

philicity of the nanosheets surface. Unlike this light-relay process, the N-doped Ti0.91O20.36- nanosheets with an extended light absorption shoulder toward the visible light region also exhibit visible-light-induced hydrophilicity. However, the mechanical strength against scratching and long-term selfcleaning functionality of such ultrathin films need further investigation. 4.1.2. Photovoltaic Applications. Mesoporous TiO2 thin films have been widely used in the new generation of dyesensitized solar cells (DSSCs) as photoanodes with the dual function of dye adsorption and as an electron transfer scaffold.2,43 As the thickness of TiO2 photoanode for DSSCs is usually in the range of several to 20 μm, the extremely small thickness of titanium oxide nanosheets renders them unfeasible as photoanode materials directly. However, by using the unique nature of Ti0.91O20.36‑ nanosheets, which is not easily attainable in other materials, researchers have still managed to facilitate the DSSC performance. For instance, using monolayer Ti0.91O20.36‑ nanosheets as the seed layer, 1D rutile TiO2 nanorods can be directly grown on SnO2:F (FTO) glass substrates, which present a more ordered morphology and, importantly, higher overall solar energy conversion efficiency (η) under standard 1.5 AM solar irradiation.208 Itoh et al. used 9476


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Figure 28. (a) Schematic illustration of Ti0.87O20.52‑ nanosheet and molecular structure of vinyl monomers. (b) Photoinduced hydrogelation process triggered by Ti0.87O20.52‑ nanosheets and photos of sol and gel appearance for samples before and after the hydrogelation. (c) Mechanism illustration of three stages: photoexcition (i), photooxidation (ii), and photoinduced hydrogelation (iii). Reprinted with permission from ref 180. Copyright 2013 Nature Publishing Group.

used for photoluminescuence.126 For instance, a group of perovskite-type nanosheets derived from the exfoliation of K2Ln2Ti3O10, KLnNb2O7, and RbLnTa2O7 compounds exhibit intense emissions at different wavelengths, depending on the composition of the nanosheets. Typically, Gd1.4Eu0.6Ti3O102‑ has a two-step energy transfer relay, first from the Ti−O network to Gd3+ within the host slabs and then to Eu3+ for remarkable emissions. Another interesting feature of the nanosheet system is the emission intensity modulation characteristics with the assistance of an external magnetic field, depending on the orientation of the nanosheets. An alternative approach for a photoluminescent-bearing nanosheet system is to combine two types of nanosheets, “Eu(OH)3‑xx+” nanosheets and Ti0.91O20.36‑ nanosheets, in a LBL fashion.213 The sandwichlike structure with in-plane electrostatic interaction between the neighboring nanosheets accounts for a strong photoluminescence signal, while the layer number and deposition sequence play important roles in tuning the emission intensity. All these attempts indicate the diversity of nanosheets for future luminescence device design. 4.1.4. Photoinduced Composite Hydrogels. The hydrogel concept has been applied to exfoliated nanosheet systems for some years. Earlier examples include the polymerization of monomers containing an exfoliated clay base (laponite) to form nanocomposites with excellent mechanical strength.214,215 Recent demonstrations based on graphene derived hydrogels further shed light on the vast potential of such composites for

pass through the Ti0.91O20.36‑ nanosheet network efficiently (Figure 27). Such trifunctional structure, which facilitates dyeloading, light-harvesting, and electron transfer, is difficult to acquire in the photoanodes prepared with other mesoporous TiO2 or facet-controlled nanoparticles. With the recent breakthrough in organometal halide type perovskite-based solid-state solar cells and over 15% of conversion efficiency achieved,211,212 the photoelectrodes can be much thinner than the conventional DSSCs. In this regard, titanium oxide nanosheets and their perovskite analogues are anticipated to find new promising applications in the next generation of low-cost and high-efficiency solar cells. 4.1.3. Photoluminescence Applications. Because exfoliated nanosheets in colloidal suspension have fully opened space for guest species to anchor, the use of photoluminescence agents such as europium (Eu) and terbium (Tb) can induce the restacking of nanosheets to form aggregates.160,162 In such an exfoliation−flocculation process, a remarkably high Eu uptake by the restacked Ti0.91O20.36− nanosheets was attained, which would not be feasible using the conventional intercalation approach with the parent layered titanate. The as-prepared composite exhibited intense emissions from Eu3+. Interestingly, the materials can be excited by either Ti0.91O20.36‑ or Eu3+ ions to have bright red light, indicating the potential use of the materials for opto-electrical devices. Instead of using a rare metal ion to couple with nanosheets to form composites, nanosheet host layers containing lanthanide ions can also be 9477


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the flocculation agent but also as removable template for porous titanium oxide structure.226 The porous restacked nanosheets also demonstrate a good rechargeable capacity of 200 mAh g−1, which is higher than that of unexfoliated parent H1.08Ti1.73O4 materials. This gives an indication on the important role of nanoporosity in the electrode materials for LIB capacity improvement. Considering the limited theoretical capacity of titanate (171 mAh g−1) and graphite materials (372 mAh g−1), a combination of exfoliated Ti0.91O20.36‑ nanosheets with anode candidate SnO2 (theoretical capacity 790 mAh g−1) is a good approach for capacity improvement. Interestingly, the resultant composite with 40% titanate/60% SnO2 delivered a discharge capacity of 860 mAh g−1 in the first cycles.227 Even though the retention of the capability of the hybrid is not good, the capacity of 507 mAh g−1 after 50 cycles is still close to the calculated theoretical capacity of the mixture (Figure 29). It is apparent that the hybridization of layered titanate and SnO2 nanoparticles has the synergetic effect in boosting the capacity.

energy storage and biomedical and mechanical applications.216−218 Taking advantage of the photocatalytic nature of Ti0.87O20.52‑ nanosheets, photolatently tunable hydrogels can also be prepared with the assistance of UV light.180 As Figure 28 depicts, a tiny amount of Ti0.87O20.52‑ nanosheets dispersed in the water-soluble vinyl monomer (N-isopropylacrylamide) can play vital dual roles in triggering the hydrogel generation, i.e., the photoexcited hydroxyl radicals to induce the radical polymerization and cross-linkers to anchor the polymer chains for 3D network formation. The hydrogel thus-prepared not only has excellent thermoresponse behavior but can also be modulated further for other possible applications, including micropatterning and conjugation when in contact with UV light, thanks for the photocatalytic activity of the Ti0.87O20.52‑ nanosheets. Although these findings are only very recent, the Ti0.87O20.52‑ nanosheet based hydrogel concept may lead to an array of new composite materials by simply tuning the nanosheet compositions and polymer matrixes, thus opening a new era for potential application in drug delivery, and mechanical and biomedical devices. 4.2. Electrochemical Applications

Beyond the extensively studied photoinduced applications, titanium oxide nanosheets can also offer intriguing properties in electrochemical applications due to either the electrochemically active Ti ions or the inherent 2D nanostructure. 4.2.1. Lithium Ion Batteries. On the basis of the lithium intercalation chemistry in host materials, cathode and anode materials are the most important components of rechargeable lithium ion batteries (LIBs).219−221 An enormous research effort has been made to improve the battery performance in terms of safety, cost, higher energy density and power density, and superior cycling capability and charging rate. In addition to the research efforts on cathode materials such as LiCoO2, LiMn2O4, Li−Ni−Co−Mn−O, and LiFePO4, graphite has been studied as the common anode material, which has an Li ion insertion potential range between 0.05 and 0.3 V versus Li+/Li. However, the use of graphite anodes has significant safety concerns due to the formation of lithium dendrites on the graphite surface during the charge−discharge process. In this regard, titanate materials stand out as a promising safer anode candidate, because they are not only cheap, but have a quite safe lithiation potential of ∼1.6 V (versus Li+/Li) among a large number of alternative anode materials.222 Another merit of titanium oxides is the very insignificant volume expansion (120 °C) with lower electrolyte permeability rates have been extensively investigated. The composite design, by introducing a small amount of inorganic fillers, can not only increase the thermal stability of the polymeric membranes but also provide some barriers for blocking the crossover of the reactants. In this regard, 2D nanosheets with a highly anisotropic nature can be excellent fillers. Marani et al. developed a type of composite PEM material by adding a small amount of exfoliated Ti0.91O20.36‑ nanosheets (1.67 wt %) to the sulfonated poly(ether ether ketone) (SPEEK).230 The as-prepared membranes exhibit an improved proton conductivity of 4.14 × 10−2 S cm−1 at a temperature of 140 °C under 100% relative humidity conditions, which is more than 2 times the conductivity of a pristine SPEEK membrane. It is believed that the unique 2D structure of Ti0.91O20.36‑ nanosheet fillers is the key not only to improve the electrochemical property and thermal stability but also to reduce fuel crossover of the PEMFC. In addition to the PEMFC, another important challenge for fuel cells is to develop low-cost, highly efficient electrocatalysts for the oxidation process. In this context, titanium oxide nanosheets can also be used to improve the performance of electrocatalysts in fuel cells. Because of the instability and high cost of Ru catalysts in direct methanol fuel cells (DMFC), Saida et al. investigated the use of Ti0.91O20.36‑ nanosheets to reduce the Ru catalyst loss during operation. Interestingly, not only has the catalyst retention been drastically improved up to 2000 cycles of operation but the Ti0.91O20.36‑/Pt composites also demonstrated enhanced methanol electro-oxidation properties.231 Another new type of composite catalysts composed of Pt/graphene/layered titanate nanosheets not only has abundant nanoporosity but also exhibits electrocatalytic activity for the oxygen reduction reaction, being superior to those of commercial Pt/C catalysts.232 4.2.3. Electrochemical Sensors. Using the ion-exchangeable capacity of the layered titanate, the nanosheets can act as a highly sensitive electrode to be used for stripping voltametric monitoring of trace heavy metal ions Hg2+.233 The Na+ intercalated host materials can efficiently capture the Hg2+ from solution with high sensitivity and selectivity, standing out from a group of other heavy metal ions due to the lower hardness of Hg2+ than other metal ions. The proof-of-concept study of this ultrasensitive detecting system on real mushroom samples confirms the feasibility of the method in toxic metal ion control, which may find practical application in sensing fields.

Figure 30. Figure-of-merit (FOM) values and dielectric constant of a host of oxide nanosheet high-κ thin films. The property of bulk TiO2 (bottom left) is also presented for comparison. Reprinted with permission from ref 66. Copyright 2012 Wiley-VCH.

the TiO2 nanosheets is attributable to such high-κ property. The subsequent investigations on various oxide nanosheet systems in d0 electronic configuration indicate that Ca2Nb3O10-, SrNb3O10-, and Ti2NbO7- systems have even larger dielectric performance, with a ε value up to 320.156,238 It is apparent that application of highly anisotropic crystalline nanosheets in this domain will underpin many important applications including nanocapacitors and gate devices. For more details, please refer to a comprehensive review articles on this topic.69 Titanium oxide nanosheets doped with other transition metal ions such as Co, Mn, and Fe not only lead to visible light responsive activity but more interestingly result in significant property changes. For example, the thin films prepared with exfoliated Ti0.8Co0.2O20.4‑ nanosheets exhibit a gigantic magneto-optical (MO) response due to their unique anisotropic 2D structure (Figure 31).109 Similarly, heavily doped Ti0.6Fe0.4O20.4‑ also exhibits a MO effect on the same order as that of Ti0.8Co0.2O20.4‑ nanosheets of 104 deg cm−1. Very interestingly, the superlattice films with alternately deposited Ti0.8Co0.2O20.4‑/Ti0.6Fe0.4O20.4‑ drastically amplify the MO response up to 3 × 105 deg cm−1 at the wavelength of 400−550 nm.110 It is believed that a synergetic effect arising from the intimate contact of planar nanosheets with different composition and interlayer d−d transition between Co2+ and Fe3+ is responsible for such a huge MO effect. This example sheds light on the potential of nanosheet architecture manipulation, which may lead to innovative magnetic data

4.3. Dielectric Nanodevices

Nanoelectronics represent one of the hottest research topics in the nanotechnology field,234 which features interatomic interactions that are difficult to realize in microelectronic 9479


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Figure 31. (A) (a) Schematic illustration of ferromagnetic nanosheets in a superlattice fashion. (b) LBL-assembled multilayer thin films composed of (Ti0.8Co0.2O20.4‑/Ti0.6Fe0.4O20.4‑)5 bilayers. (c) Cross-sectional HRTEM image of the resultant nanoarchitecture films deposited on a SrTiO3:Nb substrate. Reprinted with permission from ref 98. Copyright 2011 American Chemical Society. (B) Magneto-optical spectra of multilayer thin films using Ti0.8Co0.2O20.4‑ and Ti0.6Fe0.4O20.4‑ as building blocks: (a) (PDDA/Ti0.8Co0.2O20.4‑)10 and (PDDA/Ti0.6Fe0.4O20.4‑)10 and (b) (Ti1.6Co0.4O40.4‑)5(Ti1.2Fe0.8O40.4‑)5 and (Ti0.8Co0.2O20.4‑/Ti0.6Fe0.4O20.4‑)5. Reprinted with permission from ref 65. Copyright 2009 Royal Chemical Society.

stretched flat nanosheets. The distorted lattice may be the key reason for the formation of excessive Brønsted acid sites. Together with the contribution from Lewis acid sites, carbocation reaction was promoted on the nanotube materials.

storage and spin−electronic devices by rational selection of the nanosheet composition and deliberate design of film architecture. Considering trajectory in 2D material research nowadays,69,239 the assembly and combination of different types of nanosheets can provide vast opportunities for new system design, almost like playing with Lego blocks to build a variety of possible new nanodevices.

4.5. Biomedical Applications

Compared to the exploration in energy, environment, and nanoelectronics fields, the application of nanosheets in the biomedical area has attracted much less attention. Nevertheless, the potential use of the nanosheets and their composites in this important area cannot be underestimated. It is known that TiO2 as photocatalysts can kill bacteria due to the strong redox power of the photoexcited charge carriers.11 The use of antibacterial coatings in public sectors such as hospitals and aged care facilities is appealing. Taking advantage of Ti0.91O20.36‑ nanosheets for LBL assembly, exfoliated nanosheets, and positively charged natural antibacterial materials, lysozymes can be assembled into multilayer films.243 Under UV irradiation, the composite thin films can efficiently kill model bacteria Micrococcus lysodeikticus. Even without the use of lysozymes, Ti0.91O20.36‑ nanosheets can also kill the bacteria, although it takes slightly longer. Because of their extremely large surface and charge-bearing nature, exfoliated nanosheets can also be a good host material for immobilizing proteins, such as myoglobin, horseradish peroxidase, hemoglobin, bovine serum albumin, and lipase.244 Some of these immobilized systems exhibit good protein uptake rates and excellent thermal stability. Like many other nanoporous capsules developed recently, the titanium oxide nanosheets should also have potential as drug carrier in pharmaceutical applications, but no report is available so far.

4.4. Catalytic Applications

One of the key characteristics of the parent layered titanates is their ion-exchangeable capability. When the titanates are protonated, there are potentially solid acid catalysts. The acidity properties of the Ti0.91O20.36‑ nanosheets and their flocculated products were explored in earlier years, although not much attention was paid to the catalytic performance.34,240,241 Hara and co-workers found that TiNbO5nanosheets acted as a strong solid catalyst, contributing to the esterification of acetic acid, cracking of cumene, and dehydration of 2-propanol.124 Dias et al. also studied a few types of exfoliated nanosheets, including titanate, niobate, and titanoniobate, and found that the materials were good solid acid catalysts for dehydration of D-xylose to produce furfural.242 Because the restacked materials have higher specific surface areas than the unexfoliated parent materials, they can offer a larger number of assessable acidity sites and consequently higher catalytic performance. Interestingly, the catalytic activity of the titanate nanotubes that are derived from scrolling up of 2D nanosheets can also be modified.193 From the viewpoint of structural distortion, the larger the lattice distortion is, the stronger the acidity that can be obtained in the titanate nanotubes. The nanotubes exhibit improved catalytic performances for the Friedel−Crafts alkylation of toluene at ambient temperature than layered parent titanate and also relatively 9480


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4.6. Other Applications

mentioned that the exfoliation procedure of titanium oxide nanosheets is relatively time-consuming and more complicated compared to many synthesis processes of other TiO 2 counterparts. The enlarged band gap of exfoliated nanosheets can also be a concern for applications involving light harvesting. Nevertheless, the potentials of titanium oxide nanosheets should not be necessarily underestimated by these “disadvantages”. The history of titanium oxide nanosheets is not long, merely less than 2 decades, yet fascinating properties derived from the rational design of nanosheets is blooming, particular in recent years. However, considering the trajectory of titanium oxide nanosheet research, it has fallen behind other types of 2D exfoliated nanosheets, such as graphene. To some extent, this fact is due to the physiochemical properties that different nanosheets could possibly offer, while an equally important point is the understanding of the nanosheets. Historically, graphene oxide was developed in the 1950s, while the research on graphene in recent years reminds us that there is no boundary of scientific findings and many new stories can emerge from old systems. The knowledge acquired from one type of nanosheet could be extended to other types of nanosheets to deliver some unprecedented properties. For instance, the idea of hydrogel was proposed in the reassembly of graphene, and the application of this concept to titanium oxide nanosheets has resulted in very fascinating new properties. Likewise, the accumulated knowledge in one system can also be borrowed to develop other hybrid systems for a variety of applications. Although the practical application of titanium oxide nanosheets has been limited, intensified research has explored the diverse potential uses of the materials. Currently, there are still a number of issues that need to be addressed in future research endeavors. The exfoliation process of titanium oxide nanosheets has been well-established, although it is still a considerably complicated and time-consuming process. Even though to some extent the quality of resultant nanosheets relies on the parent layered materials, the shape and size of the nanosheets are far from uniform. Borrowing the idea from other nanosheets can be a worthwhile, for instance, the PVD method for large and relatively uniform single crystalline nanosheet growth, while the cost is also significant. Considering the aspiration of using nanosheets as “building blocks” for new nanodevices and nanoarchictures, the large variation of size and shape distribution in real-life samples reminds us of the urgent need for new synthesis approaches to be developed. One of the fascinating potential applications of titanium oxide nanosheets and their analogues is the photoinduced applications. However, the visible light utilization of these nanosheets is still very limited. Even though the titanate can be doped with various dopants or coupled with visible light responsive species to offer improved light-harvesting performance, the efficiency and stability of doped materials is still challenging and requires more innovative strategies and deeper understanding of the mechanisms. The hybridization of titanium oxide nanosheets with graphene has provided excellent examples on efficient photocatalyst design due to the synergetic effect of two types of 2D materials. The recent development on a large host of 2D nanomaterials such as WS2, MoS2, BN, and many more certainly create vast research opportunities for novel functional

4.6.1. Gas Barrier Layers. Polymer-based films have been extensively used in the packaging industry and barrier layers.175,245,246 However, the encapsulation of electronic devices such as light-emitting diodes (LED) and solar cells requires restricted water vapor transmission rate (WVTR) and oxygen transmission rates (OTR).247 Like inorganic fillers in PEMs, exfoliated nanosheets can also be used as inorganic additives for barrier layers. Due to their 2D paperlike nature and nonporous single crystalline nature, nanosheets can act as an excellent additive in the polymer matrix to increase the tortuosity for WVTR and OTR suppression. Clay-based nanosheets have been studied quite extensively for such purposes,248,249 while titanium oxide nanosheets derived from hydrothermal process have only recently been used to increase the barrier property of a polyethylene naphthalate (PEN) film. The LBL-assembled PEI/nanosheet multilayer films on PEN substrate can drastically suppress the helium gas permeability by 2 orders of magnitude, i.e., 4.1 × 10−4 g/m2·day for 15 bilayers of PEI/nanosheets in composite PEN vs 3.2 × 10−2g/ m2·day for bare PEN films.250 Note that such types of nanosheets have a lateral size of less than 100 nm, which may not be effective to form densely packed films to provide sufficient tortuosity. The use of exfoliated titanium oxide nanosheets with larger lateral size and high crystallinity is expected to provide a superior gas barrier property, while further study is required. 4.6.2. Composites with Mechanical Properties. The use of exfoliated nanosheets in composites for improved mechanical performance has been studied for decades, including the well-known clay−polymer and recently studied graphene-based nanocomposites. The study on exfoliated titanium oxide nanosheets as an additive for composite design has so far been limited, possibly due to the overwhelming functional properties of the titanium oxide than its mechanical property. Interestingly, a new type of composite can be prepared using a quaternary ammonium cation bearing trimethoxysilyl functional groups to cross-link Ti0.91O20.36‑ nanosheets. The resultant composites exhibit viscoelastic properties, which are very sensitive to a number of parameters, such as humidity, temperature, and light irradiation.180 Particularly, the photoinduced property may be of significant importance for flexible electronics application.

5. SUMMARY AND OUTLOOK As a relatively new member of the TiO2 family, exfoliated nanosheets own many functionalities that are commonly shared by TiO2 materials. However, due to their unique features, titanium oxide nanosheets also exhibit distinctive physicochemical properties that are hardly attained in other bulk TiO2 cousins and facet-controlled nanoparticles. As summarized in the above sections, key characteristics of titanium oxide nanosheets include their extremely high 2D anisotropy, very small thickness of subnanometers to a few nanometers, surfacecharge bearing nature in colloids, and well-defined crystallinity and composition. All these features not only make the nanosheets themselves distinctive from other TiO2 counterparts but also enable them to be used as versatile building blocks for many new nanoarchitectural designs, such as LBL assembly for functional multilayer films and flocculation for nanoporous 3D hybrid composites with high surface areas and sandwiched structures. In the meanwhile, it has to be 9481


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device design. In this respect, the building blocks are ready for more fascinating Lego building. The investigation on titanium oxide nanosheets has so far covered a wide spectrum of potential applications spanning from energy and environment to optical-electronic and biomedical fields. On the other hand, thinking “out of the box” about the paperlike stuff at the nanoscale, what can we possibly do next? Write some information on the sheets, develop mesh structure in the sheets, or assemble the sheets into readable nanobooks? Some of these may sound like fantasy and science fiction, but one should never lose a sense of ambition.

Graduate School of Pure and Applied Sciences, University of Tsukuba (Collateral Office). He received his Doctor degree in Chemistry from the University of Tokyo in 1985. Since 1980, he has worked for the National Institute for Research in Inorganic Materials (NIRIM, now NIMS), in Japan. In 2009, he was appointed as a NIMS fellow. His recent interest has focused on nanosheets obtained by delaminating layered materials.

ACKNOWLEDGMENTS L.W. acknowledges the financial support from Australian Research Council through its Discovery, Linkage and Future Fellowship schemes and is grateful to Prof. G. Lu (UQ), Prof. H. Cheng, G. Liu (IMR, CAS), and a number of postdocs and students at UQ for their contributions to the layered materials/ nanosheet research. T.S. acknowledges the support provided by CREST of the Japan Science and Technology Agency (JST) and the World Premier International Research Center (WPI) Initiative on Materials Nanoarchitectonics, MEXT, Japan. He also thanks staff members, postdocs, and students in the Soft Chemistry Group, MANA, NIMS for their contribution to the nanosheet studies.

AUTHOR INFORMATION Corresponding Authors

*L.W. e-mail: [email protected]. *T.S. e-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies

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Lianzhou Wang is currently a Professor in the School of Chemical Engineering and Research Director of the Nanomaterials Centre, the University of Queensland (UQ), Australia. He received his Ph.D. degree from the Shanghai Institute of Ceramics, Chinese Academy of Sciences in 1999. Before joining UQ in 2004, he has worked at two national institutes (NIMS and AIST) of Japan for 5 years. His research interests include the design and application of semiconducting nanomaterials in renewable energy conversion/storage systems, including photocatalysts and electrode materials for low-cost solar cells and rechargeable batteries.

Takayoshi Sasaki is a Principal Investigator and a Field Coordinator of Nanomaterials field of MANA, NIMS. He is also a Professor of the 9482


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