Unique Advantages of Exfoliated 2D Nanosheets for Tailoring the

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Unique Advantages of Exfoliated 2D Nanosheets for Tailoring the Functionalities of Nanocomposites In Young Kim,† Yun Kyung Jo,† Jang Mee Lee,† Lianzhou Wang,*,‡ and Seong-Ju Hwang*,† †

Department of Chemistry and Nanoscience, College of Natural Sciences, Ewha Womans University, Seoul 120-750, Korea Nanomaterials Centre, School of Chemical Engineering and AIBN, The University of Queensland, St Lucia, Brisbane, Queensland 4072, Australia

‡

ABSTRACT: Hybridization with exfoliated two-dimensional (2D) nanosheets provides a very effective and powerful way not only to control the physicochemical properties of hybridized species but also to explore nanocomposites with novel functionalities. Deliberate coupling between the hybridized species is critically important in maximizing the effect of hybridization on the physicochemical properties and functionality of hybridized components. The very small thickness and extremely large surface of exfoliated 2D nanosheets render these materials ideal candidates for achieving a strong coupling with diverse guest species. This Perspective focuses on the unique characteristics of exfoliated 2D nanosheets as building blocks for designing hybrid materials. Several intriguing examples of strong interaction between exfoliated 2D nanosheets and hybridized species are summarized with an emphasis on the effective control of electronic, optical, structural, and morphological characteristics. An outlook on the future research directions is provided along with new strategies to maximize the coupling in the 2D nanosheet-based hybrid materials.

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physicochemical properties via the nanoscale mixing or coupling between more than two kinds of chemical species as atomic orbitals mix to form new atomic orbitals with the same total electron capacity as the old ones. The hybridization using exfoliated 2D nanosheets is fairly effective not only in tailoring the physicochemical properties of hybridized species but also in creating unexpected functionality via a synergistic coupling between the components. The unique advantages of this new class of 2D nanostructured materials as building blocks for hybridization are described below (also summarized in Figure 1).

ver the past decades, a great deal of research efforts have been devoted to the synthesis, characterization, and application of low-dimensional nanostructured materials.1 As a new emerging family of nanostructured materials, twodimensional (2D) nanosheets derived from layered materials including graphene have received exponentially increased attention because of their unique structural, morphological, and physicochemical properties.2,3 These 2D nanostructured materials can be synthesized by soft-chemical exfoliation of the parent layered materials, which make themselves distinct from other low-dimensional nanostructured materials whose synthesis heavily relies on the crystal growth mechanism.4 The thickness of the exfoliated 2D nanosheets corresponds to the crystallographic thickness of the individual layer of parent layered materials, which is often less than 1 nm.4 Conversely, the lateral crystal size of the pristine layered materials along the in-plane direction remains nearly unchanged, leading to the unusually high morphological anisotropy with subnanometersized thickness and up to micrometer-scale lateral dimension. Because the chemical composition and crystal structure of the pristine layered materials remain intact upon the exfoliation process, it is easy to tailor the properties of the exfoliated nanosheets. Such great flexibility in controlling the chemical composition and crystal structure provides 2D inorganic nanosheets with unique tunable physicochemical properties and rich functionalities.5 Of prime importance is that the exfoliated nanosheets can be used as useful building blocks for fabricating novel functional hybrid materials such as thin films, porous membranes, powdery composites, and so on.5−7 In chemistry, hybridization implies the creation of novel multicomponent materials with synergistically combined © XXXX American Chemical Society

The hybridization using exfoliated 2D nanosheets is fairly effective not only in tailoring the physicochemical properties of hybridized species but also in creating unexpected functionality via a synergistic coupling between the components. (1) Due to the very small thickness of exfoliated 2D nanosheets, all of the component elements can be practically exposed to the surfaces and thus induce strong interaction with hybridized species. Note that such coupling between hybridized Received: September 25, 2014 Accepted: November 13, 2014

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Figure 1. Unique advantages of 2D nanostructured materials for hybridization.

optimizing the hybridization effect via the enhancement of the matching of electronic structures of the hybridized components. (5) The exfoliated 2D nanosheets of inorganic solids form stable colloidal suspensions in polar solvents. Except the cationic nanosheets of layered double hydroxides (LDHs), most of inorganic nanosheets are negatively charged and hydrophilic, which is quite similar to the chemical nature of chemically prepared graphene, that is, reduced graphene oxide (rG-O). Thus, the inorganic nanosheets can form a stable colloidal mixture with other inorganic nanosheets and rG-O. The resulting mixture suspensions can be used as an effective precursor for designing multicomponent inorganic−carbon hybrid materials in the form of powdery composites, multilayer films, freestanding membranes, and so on.11 (6) The exfoliated nanosheets have wide surface area readily available for the anchoring of diverse functional groups as well as for the immobilization of many nanostructured materials. The surface modification of nanosheets via anchoring of diverse functional groups makes it possible to tailor important surface properties such as hydrophilicity, hydrophobicity, surface charge, and surface structure. Such a flexible control of surface nature provides high possibilities to tune the functionality of hybrid materials. The above-listed unique properties of exfoliated 2D nanosheets offer rich opportunities for the synthesis of diverse types of hybrid materials with tailorable properties and functionalities. A strong coupling with nanosheets is fairly powerful in achieving unprecedented effects of hybridization. This Perspective is not intended for the simple survey on the current research activities for 2D nanosheets and their hybrid materials but for the provision of insights for the unique capability of 2D nanosheet-based hybridization to create unprecedented properties and unexpected functionalities. Among many classes of inorganic materials that can form exfoliated nanosheets, this Perspective deals with the nanosheets of layered metal oxides and LDHs as well as graphene because these materials are

species and 2D nanosheets is more drastically increased than that in other low-dimensional nanostructures including 0D quantum dots (QDs), 1D nanowires, and nanotubes because the subnanometer-level thickness of 2D nanosheets is much smaller than the nanometer-sized dimensions of other 0D and 1D nanomaterials. There is a great opportunity to control the electronic structure and physicochemical properties of nanostructured species and also to optimize their functionality through the hybridization with 2D nanosheets. (2) The exfoliated 2D nanosheets of inorganic solids are nearly identical to graphene nanosheets in terms of anisotropic morphology. Such a morphological similarity is beneficial in achieving a strong interaction between inorganic nanosheets and graphene, which is quite useful in improving the functionality of inorganic solids through the coupling with graphene. (3) The self-assembly between exfoliated 2D nanosheets and other nanostructured species leads to the house-of-cards-type stacking structure of sheet-like crystallites.8 In this class of resultant hybrid materials, there are mesopores formed by restacking of 2D crystallites and micropores formed by the intercalation structure of hybrid materials. Such a hierarchical porous structure is quite effective in improving many functionalities of hybrid materials such as adsorptivity, catalytic activity, electrochemical activity, and so forth.6,8,9 (4) While most of nanostructured materials are synthesized by a crystal growth mechanism, 2D nanosheets are obtained by the soft-chemical exfoliation process of the pristine layered materials. Because the host materials can be prepared by conventional solid-state reactions at elevated temperatures, the chemical compositions of these materials are readily controllable by cation and anion substitutions.10 In the course of the exfoliation process, the chemical composition of the host layer remains intact. Thus, the chemically substituted derivatives of 2D nanosheets can be easily obtained by the exfoliation of modified layered materials. Such a high flexibility in tuning their chemical composition and crystal structure is very crucial in 4150

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fabrication of multilayered hybrid films. A classic example is the alternative deposition of anionic nanosheets and oppositely charged guest species like polyelectrolytes (PEs), biomolecules, proteins, dyes, nanoparticles, and nanosheets to fabricate multilayered films.5−7,15 With this LbL method, the stacking pattern and chemical composition of reassembled layers can be precisely tailored at a nanometer-scale precision. Therefore, the application of the LbL technique provides a valuable opportunity to probe the effects of stacking patterns and interlayer interaction on the physiochemical properties of the resulting multilayer films. Furthermore, the combination of two different nanosheets in the LbL films can create unexpected functionality via synergistic coupling between two existing properties. Also, the swelling property of the layered materials makes possible the crystallization of guest species in their interlayer space, leading to the synthesis of a hybridized superlattice.16 Alternatively, the vacuum-assisted filtration of a colloidal mixture of two kinds of nanosheets makes possible the fabrication of homogeneously mixed hybrid films.17 Similarly, the application of a diverse deposition method for colloidal mixtures of two kinds of nanosheets leads to the fabrication of hybrid films containing two kinds of nanosheets. Also, the nanosheet-based hybrid materials can be synthesized by the anchoring of guest species on the surface of the exfoliated 2D nanosheets.18 Because the exfoliated nanosheets have surface charge, large surface area, and a well-defined crystal plane, precursor ions can be adsorbed on the surface of nanosheets, which is followed by the crystal growth of hybridized species. The presence of a surface defect and significant curvature of the semiconductor particle limits the crystal growth of metallic domains on the surface of the semiconductor. Conversely, the epitaxial growth of metal nanostructures can be successfully achieved for the 2D nanosheets of the semiconductor, which is attributable to their well-defined flat surface.19,20 Such an anchoring−crystal growth of nanoparticles on the 2D nanosheets can be controlled by tuning the electrostatic interaction between precursors and the chemical environment of reaction media. In contrast to graphene nanosheets, the exfoliated 2D nanosheets of inorganic solids possess a broad spectrum of physicochemical properties from insulating to metallic and from diamagnetic to ferromagnetic.5−7 The great flexibility in controlling their chemical composition and crystal structure makes possible free-tuning of the physicochemical properties and functionalities of the inorganic 2D nanosheets. The hybridization using the exfoliated 2D nanosheets of inorganic materials provides valuable opportunity to explore novel hybrid materials with tailorable and diverse physicochemical properties and functionalities. The strong coupling of 2D nanosheets with hybridized species makes it possible to control the diverse aspects of component materials in terms of electronic, optical, structural, morphological, and surface natures. Several examples of the fine-tuning of a material’s properties through hybridization with 2D nanosheets are discussed below. The control of the optical property of nanostructured materials is one of the most important issues because many applications of nanomaterials originate from their unique optical property. Due to the quantum size effect caused by very small sheet thickness, the optical properties of exfoliated nanosheets can be remarkably different from those of their parent layered compounds. One of the typical examples is the exfoliated Ti0.91O2 nanosheets, which exhibit an optical absorption peak at around 265 nm, corresponding to an apparently

This Perspective is not intended for the simple survey on the current research activities for 2D nanosheets and their hybrid materials but for the provision of insights for the unique capability of 2D nanosheet-based hybridization to create unprecedented properties and unexpected functionalities. receiving intense research efforts and are expected to boost remarkable changes in properties and functionalities. As described above, the 2D nanosheets of layered solids can be prepared by the soft-chemical exfoliation process of the pristine layered materials into individual monolayers. There are three representative routes of chemical exfoliation to 2D inorganic nanosheets, for example, (1) the intercalation of bulky guest species via ion-exchange method, (2) the interlayer H2 evolution via the hydroxylation of interlayer lithium ions, and (3) the automatic solvation of inorganic layers.6 The intercalation of bulky guest species normally leads to the swelling and exfoliation of layered metal oxides, LDHs, and layered metal carbides/nitrides (MXene). The exfoliation to MXene is achieved by the chemical etching of the A component from Mn+1AXn (M: transition metal; A: III or IV A element; X: C and/or N) and the subsequent intercalation of bulky guest species.12 The second method of interlayer H2 evolution is applicable for the exfoliation of layered metal chalcogenides. The third method of infinite solvation induces the exfoliation of LDHs, aluminosilicate, and metal chalcogenides.13 In the case of graphene nanosheets, the rG-O nanosheets can be chemically synthesized by the oxidation of graphite with strong acid treatment and the following reduction of the graphene oxide to rG-O nanosheets.14 As for chemical synthetic routes to nanosheet-based hybrid materials, several methods such as electrostatically driven reassembling, layer-by-layer (LbL) deposition, and crystal growth on the surface sites of nanosheets have been developed.5−7 Because most of the exfoliated nanosheets including rG-O nanosheets possess sufficient surface charge, many hybridization methods applicable for 2D nanosheets rely on an electrostatic interaction between hybridized components. That is, anionic nanosheets such as layered metal oxides, layered metal chalcogenides, and rG-O can be hybridized with cationic species whereas cationic LDH nanosheets can be used as a host for accommodating anionic species. In contrast to the conventional intercalation process involving interlayer diffusion of guest species, there is almost no limitation in the diffusivity of guest species in the present reassembling process with exfoliated 2D nanosheets. Therefore, a wide spectrum of guest species including bulky nanoparticles and biological macromolecules can be hybridized with the exfoliated 2D nanosheets. The electrostatically driven flocculation between colloidal nanosheets and oppositely charged guest species leads to the formation of powdery nanocomposites with intercalation structure.8,9 On the basis of the same charge neutralization mechanism, the exfoliated 2D inorganic nanosheets with sufficient layer charge are ideal building blocks for the LbL 4151

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Hybridization of the exfoliated nanosheets via electrostatically driven reassembly can also lead to remarkable optical property change. One apparent structure-relating reason is that the restacking of nanosheets leads to the change of its unique 2D feature to a disordered 3D structure. More importantly, due to the extremely large surface area of the nanosheets in suspension, they can offer rich opportunities for many types of functional guest species to anchor, thus leading to the optical property changes in the hybrid composites. In one instance, the surface of the lepidocrocite-type Ti0.91O2 nanosheet consists of coordinatively unsaturated oxygen ions only, which is different from the surface structure of anatase/rutile TiO2. Such a surface characteristic of layered titanate makes possible a strong electric coupling with hybridized species via the chemical bonding with coordinatively unsaturated oxygen ions. To date, a large bunch of guest species including metal ions, metal and metal oxide nanoparticles, dyes, and metal complexes have been used to modify the structures, compositions, and properties of the nanosheet-based hybrids. Another noteworthy example is to use rare earth metal species to interact with the exfoliated nanosheets, which might lead to drastically increased photoluminescence (PL) in the hybridized materials. For example, the incorporation of various lanthanide ions into the framework of perovskite nanosheets (K2Ln2Ti3O10, KLnNb2O7, and RbLnTa2O7) can endow the materials with strong PL emissions,30 whereas the reassembly of Ti0.91O2 nanosheets with lanthanide metal clusters such as Eu(phen)2 and Tb(phen)2 also gives rise to remarkable PL signal in the composite systems.31 As illustrated in Figure 3, the high PL efficiency of the lanthanide ions in nanosheet-based hybrid material can be achieved via an efficient energy transfer from the Ti−O network to lanthanide ions. Note that these strategies in tuning the optical properties, either doping or flocculation, might be extendable from one type of nanosheet to another. Due to the rapid growth of the 2D nanosheet family including the transition-metal dichalcogenide in the past few years, it is anticipated that there will be exponentially growing hybrids and composites to be developed in the years ahead. In relation to the tailoring of optical properties, a great deal of research efforts are devoted to the control of the electronic structure of nanostructured materials because many functionalities of these materials are related to their tunable electronic property.6,7 In one instance, there are intense research activities devoted for the application of nanostructured material for solar energy harvesting.6,7 To optimize the efficiency of nanostructured semiconductors for the conversion of solar energy to chemical/electrical energy, it is crucially important to precisely tailor their electronic structures. Because many of the semiconducting metal oxides such as TiO2 and ZnO are inactive in visible-light-induced photocatalytic reactions because of their wide-band-gap energy and improper band positions, much effort has been made to modify these materials with visible light photocatalytic activity via the hybridization with other nanostructured semiconductors.32 This is due to the fact that the hybridization between two kinds of semiconductors with different electronic structures leads to a formation of new electronic structure by coupling of different materials. Depending on the band arrangements of semiconductors, hybridized electronic structure can be classified into three types, including straddling gap (type I), staggered gap (type II), and broken gap (type III), as shown in Figure 4. Among these three, the hybridization of type II is fairly useful not only in extending the lifetime of photoinduced electrons and holes via an internal

Due to the quantum size effect caused by very small sheet thickness, the optical properties of exfoliated nanosheets can be remarkably different from those of their parent layered compounds. larger band gap of ∼3.8 eV than its counterparts with similar composition such as anatase TiO2.7,21 The optical properties of the nanosheets can be deliberately tuned by changing the compositions of the pristine parent layered compounds. For the LDH nanosheet family, because of the rich options of transition-metal ions M2+ and M3+ in the general LDH formula [M2+1−xM3+x(OH)2][An−x/n·mH2O], the optical properties of the LDHs can be easily tuned in a broad range of spectrum,22,23 while for other classes of nanosheets, due to the crystallographic restriction of the layered structure, there is not much space to change the composition to a large extent. Nevertheless, the optical properties of the nanosheets can still be tailored by doping strategies. Still taking exfoliated Ti0.91O2 nanosheets as the example, the layered titanate compounds can be doped with a variety of metal ions including Fe, Ni, Co, and Mn and nonmetal species such as nitrogen.24−27 The incorporation of cationic/anionic dopants into the metal−oxygen framework can induce the band gap narrowing in the layered compounds, which subsequently leads to altered optical properties and a rich collection of colorful nanosheets upon the exfoliation procedure. One of the interesting features is that for a single ion doping, such as nitrogen doping, different parent materials or doping processes can result in drastically different optical properties in the exfoliated nanosheets. As illustrated in Figure 2, compared to the milky suspension of pristine Ti0.91O2 nanosheets, the suspension of N-doped

Figure 2. Photoimages of the colloidal suspensions of exfoliated nanosheets. From left: pristine Ti0.91O2, N-doped Ti0.91O2−xNx, Ca2Ta3O9.7N0.2, and Ca2Nb3O10−xNy. (Reprinted with permission from refs 27−29.) Copyright 2009 and 2013 RSC Publication and 2012 ACS Publications.

Ti0.91O2−xNx nanosheets shows a vivid yellowish color, while the nitrogen-doped Ca2Ta3O9.7N0.2 and Ca2Nb3O10−xNy nanosheets exhibit bright orange and blackish colors, respectively.27−29 Clearly, such color difference is related to the electronic structure and band gap modulation. Some possible reasons for such apparent variation could be associated with the N-doping concentration and the intrinsic nature of the parent transition-metal oxide compounds, while fundamental understanding on such an optical property difference is worth further investigation. 4152

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Figure 3. A model for the energy transfer leading to PL (left) in the Gd1.4Eu0.6Ti3O10−nanosheet system and (right) in the Ln(phen)2 complex. S1 and T1: excited singlet and triplet states of the ligand in free Ln(phen)2 complex; S1′ and T1′: excited singlet and triplet states of the ligand in the Ti0.91O2/Ln(phen)2 composite; Ln: Eu or Tb. (Reprinted with permission from refs 30 and31. Copyright 2006 and 2008 ACS Publications.)

Figure 4. Schematic energy band diagram of three types of semiconductor hybrids.

in Figure 5A, in the obtained CdS−Ti0.91O2 nanohybrids, the CdS QDs exist on the surface and/or grain boundaries of the reassembled titanate nanosheets. Due to the very limited crystal dimension of the components in hybrid materials, the electronic structure of CdS QDs can be strongly coupled with that of the layered titanate nanosheet, leading to an efficient electron transfer between them. As plotted in Figure 5B, UV−vis absorption spectroscopic analysis reveals that the protonated titanate and the tetrabutylammonium (TBA)-intercalated layered titanate are commonly wide-band-gap semiconductors with Eg = 3.2 and 3.4 eV, respectively. The hybridization of the layered titanate with the visible-light-absorbing CdS species gives rise to a remarkable red shift of the absorption edge to the lower-energy region of 2.5 eV. Surprisingly, this value is even smaller than that band gap energy of CdS QD (2.9 eV). The observed remarkable decrease of band gap energy in the nanohybrids can be regarded as clear evidence for the strong electronic coupling between the two components, allowing an efficient electron transfer from the VB of the CdS to the CB and/or indirect bands of the titanate. The observation of a single absorption edge of the CdTi nanohybrids is sharply contrasted with the case of other CdS−TiO2 hybrid systems composed of TiO2 nanoparticles,34 in which two distinct absorption edges corresponding to the band gap energies of each component are discernible. This can be regarded as clear evidence for the unprecedented strong coupling between Ti0.91O2 nanosheets and CdS QDs. As can be seen clearly from Figure 5C, the observed H2-generation rates of the CdS−Ti0.91O2 nanohybrids materials without Pt cocatalyst are much greater than previously reported values for Pt-free CdS-0D TiO2 and CdS1D TiO2 nanotubes, including the Pt-loaded CdS-TiO2 system.33 The present results underscore the usefulness of

charge transfer but also in improving the visible light-harvesting ability. The photoexcited electron and hole are effectively separated in the staggered electronic configuration (type II) and utilized for the photocatalytic reaction. In this case, at least one component of the hybrid material should have a narrowband-gap to absorb visible light. Taking into account the fact that 2D nanosheet forms a strong coupling with the guest species, a direct excitation of electron from the valence band (VB) of semiconductor 2 to the conduction band (CB) of semiconductor 1 can occur, thus leading to the expansion of the absorption range for visible light. For the development of breakthrough technology into advance new material systems for efficient solar energy conversion, the use of 2D nanosheets of semiconductors as building blocks for the design of hybrid-type photocatalysts is an effective approach because the hybridization with 2D nanosheets can induce unusually strong electronic coupling between semiconducting species and tailor their electronic structure to maximize the absorption of visible light. One of the most investigated photocatalysts is titanium oxide (TiO2), and the hybridization of TiO2 with a narrow-band-gap semiconductor can provide a very powerful tool to explore new efficient visible-light-active photocatalysts. The hybridization of lepidocrocite-type Ti0.91O2 nanosheets with diverse narrowband-gap semiconducting nanostructures such as the CdS QD, Ag3PO4 nanoparticle, Zn−Cr−LDH nanosheets, and graphene can provide a powerful tool for designing visible-light-active photocatalysts with tailorable electronic and optical properties. The hybridization between Ti0.91O2 nanosheets and CdS QD can be achieved by the reassembling of anionic Ti0.91O2 nanosheets and cationic CdS QDs.33 The CdS−Ti0.91O2 nanohybrids are synthesized with two different reactant ratios of Cd/Ti = 5 and 2.5 (i.e., CdTi-1 and CdTi-2). As illustrated 4153

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Figure 5. (A) Structural model for the CdTi nanohybrid. (B) Diffuse reflectance UV−vis spectra of CdTi-1 (a) (solid lines) and CdTi-2 (b) (dashed lines), CdS QDs (c) (circles), protonated layered titanate (d) (triangles), and TBA-intercalated layered titanate (e) (squares). (C) Visiblelight-induced (λ > 420 nm) production of H2 gas by the as-prepared nanohybrids of CdTi-1 (squares) and CdTi-2 (open circles), the CdS QDs (diamonds), the nonporous composite of CdS and layered titanate (closed circles), protonated titanate (triangles), and TBA-intercalated titanate (inverse triangles). (D) Ag 3d XPS spectra of (a) nanocrystalline Ag3PO4, (b) 0D-TOAP nanohybrid, and (c) 2D-TOAP nanohybrid. (E) Diffuse reflectance UV−vis spectra of 0D-TOAP nanohybrids (red) and 2D-TOAP nanohybrids (blue), as compared with those of P25 TiO2 nanoparticles (green), Ti0.91O2 nanosheets (gray), and nanocrystalline Ag3PO4 (black). (F) Visible-light-induced (λ > 420 nm) O2 generation by nanocrystalline Ag3PO4 (circles), 0D-TOAP nanohybrids (triangles), and 2D-TOAP nanohybrids (squares). (Reprinted with permission from refs 33 and 35. Copyright 2011 and 2014 Wiley-VCH).

0D TiO2−Ag3PO4 nanohybrids clearly demonstrates the unique features of the 2D lepidocrocite-type Ti0.91O2 nanosheet as building blocks for the synthesis of highly efficient hybrid photocatalysts with improved photostability. The observed higher photocatalytic activity of the 2D Ti0.91O2−Ag3PO4 nanohybrids is attributable to the more prominent suppression of electron−hole recombination upon hybridization with 2D Ti0.91O2 nanosheets compared with TiO2 0D nanoparticles (Figure 5F). The creation of novel electronic structure upon the hybridization with 2D nanosheet is also observed in heterolayered nanohybrids composed of two kinds of nanosheets.18,36 In one instance, as shown in high-resolution transmission electron microscopy (HR-TEM) images in Figure 6A, mesoporous LbL ordered nanohybrids highly active for visible-light-induced O2 generation are synthesized by self-assembly between oppositely charged 2D Ti0.91O2 and Zn−Cr−LDH nanosheets.36 To understand the electron-transfer behavior between 2D Ti0.91O2 and Zn−Cr−LDH, the band structure of the Zn−Cr−LDH is estimated from the results of electrochemical cyclovoltammetry (CV) measurement and UV−vis spectroscopy. As illustrated in Figure 6B, the Zn−Cr−LDH compound exhibits higher positions for the upper interband state corresponding to the absorption peak at ∼3.0 eV in the UV−vis spectrum as well as the CB in comparison with the CB of the layered titanate component. The photogenerated electrons in the Zn−Cr−LDH can migrate into the CB of the 2D Ti0.91O2, thus leading to the spatial separation of electrons and holes. The resulting effective suppression of electron−hole recombination is responsible for the high photocatalytic activity of the visiblelight-induced O2 generation after hybridization. Another nanohybrid of 2D Ti0.91O2 nanosheets with rG-O shows very unique electronic structure with the absence of the UV light absorption edge of Ti0.91O2 in UV−vis spectroscopy

2D Ti0.91O2 nanosheets as building blocks for synthesizing a highly efficient photocatalyst hybrid system. The merit of using 2D Ti0.91O2 nanosheets as a component for efficient hybrid-type photocatalysts is further verified by the comparative investigation for the nanohybrid materials composed of Ag3PO4 nanoparticles and 0D/2D nanostructured TiO2.35 As shown in Figure 5D, X-ray photoelectron spectroscopy (XPS) analysis displays that the hybridization with 2D Ti0.91O2 nanosheets induces a noticeable shift of the Ag 3d peak toward the higher-energy side. This result demonstrates the remarkable electron transfer from the Ti0.91O2 nanosheets to Ag3PO4, attributed to the formation of additional chemical bonds between coordinatively unsaturated Ag+ cations and coordinatively unsaturated O2− ions on the surface of 2D layered Ti0.91O2 nanosheets (i.e., a curing of surface defects). Conversely, there is only a negligible difference in the binding energies of the Ag 3d peaks in the spectra of the 0D TiO2−Ag3PO4 nanohybrid (i.e., 0D-TOAP). The UV−vis absorption spectral feature shows that the low-energy shoulder of this peak at