Two-dimensional (2D) layered materials with a high intercalation pseudocapacitance have long been investigated for Li+-ion-based electrochemical energy storage. By contrast, the exploration of guest ions other than Li+ has been limited, although promising. The present study investigates intercalation/deintercalation behaviors of various metal ions in 2D layered MnO2 with various interlayer distances, K-birnessite nanobelt (K-MnO2), its protonated form (H-MnO2), and a freeze-dried sample of exfoliated nanosheets. Series of metal ions, such as monovalent Li+, Na+, and K+ and divalent Mg2+, exhibit reversible intercalation during charge/discharge cycling, delivering high-rate pseudocapacitances. In particular, the freeze-dried MnO2 of exfoliated nanosheets restacked with the largest interlayer spacing and a less compact 3D network exhibits the best rate capability and a stable cyclability over 5000 cycles. Both theoretical calculation and kinetic analysis reveal that the increased interlayer distance facilitates the fast diffusion of cations in layered MnO2 hosts. The results presented herein provide a basis for the controllable synthesis of layered nanostructures for high-rate electrochemical energy storage using various single- and multivalent ions.Keywords: cation intercalation/deintercalation; interlayer spacing; metal ion-based energy storage; rate capability; two-dimensional layered nanostructures;

Multiferroic materials, in which the electronic polarization can be switched by a magnetic field and vice versa, are of fundamental importance for new electronic technologies. However, there exist very few single-phase materials that exhibit such cross-coupling properties at room temperature, and heterostructures with a strong magnetoelectric coupling have only been made with complex techniques. Here, we present a rational design for multiferroic materials by use of a layer-by-layer engineering of 2D nanosheets. Our approach to new multiferroic materials is the artificial construction of high-quality superlattices by interleaving ferromagnetic Ti0.8Co0.2O2 nanosheets with dielectric perovskite-structured Ca2Nb3O10 nanosheets. Such an artificial structuring allows us to engineer the interlayer coupling, and the (Ti0.8Co0.2O2/Ca2Nb3O10/Ti0.8Co0.2O2) superlattices induce room-temperature ferroelectricity in the presence of the ferromagnetic order. Our technique provides a new route for tailoring artificial multiferroic materials in a highly controllable manner.

The heteroassembly of two-dimensional (2D) nanosheets has attracted rapidly increasing attention for designing new materials and nanodevices, in which the properties of the individual components can be modulated through the concerted interaction between the different 2D nanosheets. Here, we report on the layer-by-layer integration of photofunctional titania nanosheets and conductive reduced graphene oxide (rGO) to enhance the photochemical activity of the titania nanosheets. Heteroassembled films were fabricated by sequentially assembling graphene oxide (GO) and titania nanosheets with a cationic polymer and subsequently exposing to UV light to reduce the GO. The films showed an accelerated photoinduced hydrophilic conversion, the rate of which was 2.8 times higher than that of a film solely of the titania nanosheets. This behavior indicates that the rGO worked as an electron transfer mediator and improved the photoinduced charge separation efficiency. The intimate contact between two different 2D nanosheets promotes the efficient utilization of photogenerated carriers.

Two-dimensional (2D) materials, represented by graphene, have attracted tremendous interest due to their ultimate structural anisotropy and fascinating resultant properties. The search for 2D material alternatives to graphene, molecularly thin with diverse composition, structure, and functionality, has become a hot research topic. A wide variety of layered metal oxides and hydroxides have been exfoliated into the form of individual host layers, that is, 2D nanosheets. This Account presents an overview of 2D oxide and hydroxide nanosheets on the following subtopics: (1) controllable preparation of high-quality nanosheets and (2) molecular assembly and the exploration of functionality of the nanosheets.High-quality exfoliation is generally achieved via a multistep soft chemical process, comprised of ion-exchange, osmotic swelling, and exfoliation. A high degree of hydration-induced swelling, typically triggered by intercalation of organo-ammonium ions, is a critical stage leading to the high-yield production of molecularly thin nanosheets. Recent studies reveal that massive swelling, an astounding ∼100 times the original size, can be induced by a range of amine solutions. The degree of swelling is controlled by the balance of osmotic pressure between the inner gallery and the outer electrolyte solution, which is strongly influenced by amine molarity. Conversely, the stability of the resultant swollen structure is dependent on the chemical nature of the amine/ammonium ions. Particular species of lower polarity and bulky size, for example, quaternary ammonium ions, are beneficial in promoting exfoliation.Rational design and tuning of the lateral dimension, chemical composition, and structure of nanosheets are vital in exploring diverse functionalities. The lateral dimension of the nanosheets can be tuned by controlling the crystal size of the parent layered compounds, as well as the kinetics of the exfoliating reaction, for example, the type of amine/ammonium ions, their concentration, and the mode of exfoliation (manual versus mechanical shaking, etc.). Employing optimum conditions enables the production of high-quality nanosheets with a lateral size as large as several tens of micrometers. A couple of examples tailoring the nanosheets have been demonstrated with a highlight on a novel class of 2D perovskite-type oxide nanosheets with a finely tuned composition and a progressively increasing thickness at a step of 0.4–0.5 nm (corresponding to the height of the MO6 octahedron).The charge-bearing nanosheets can be organized through solution-based molecular assembly techniques (e.g., electrostatic layer-by-layer deposition, Langmuir–Blodgett method) to produce highly organized nanofilms, superlattices, etc., the exploration of which holds great potential for the development of various electronic and optical applications, among others.

The surface charge of various anionic unilamellar nanosheets, such as graphene oxide (GO), Ti0.87O20.52–, and Ca2Nb3O10– nanosheets, has been successfully modified to be positive by interaction with polycations while maintaining a monodispersed state. A dilute anionic nanosheet suspension was slowly added dropwise into an aqueous solution of high molecular weight polycations, which attach on the surface of the anionic nanosheets via electrostatic interaction. Surface modification and transformation to positively charged nanosheets were confirmed by various characterizations including atomic force microscopy and zeta potential measurements. Because the sizes of the polycations used are much larger than the nanosheets, the polymer chains may run off the nanosheet edges and fold to the fronts of the nanosheets, which could be a reason for the continued dispersion of the modified nanosheets in the suspension. By slowly adding a suspension of polycation-modified nanosheets and pristine anionic nanosheet dropwise into water under suitable conditions, a superlatticelike heteroassembly can be readily produced. Characterizations including transmission electron microscopy and X-ray diffraction measurements provide evidence for the formation of the alternately stacked structures. This approach enables the combination of various pairs of anionic nanosheets with different functionalities, providing a new opportunity for the creation of unique bulk-scale functional materials and their applications.

Platelet crystals of a layered perovskite showed massive accordion-like swelling in a tetrabutylammonium hydroxide solution. The permeation of the solution induced the huge expansion of the interlayer spacing as well as the crystal thickness up to 50-fold, leading to a very high water content of >90 wt%.

Tuning of the electrical properties of graphene via photoexcitation of a heteroassembled material has started to attract attention for electronic and optoelectronic applications. Actually photoinduced carrier doping from the hexagonal boron nitride (h-BN) substrate greatly modulated the transport property of the top layer graphene, showing promising potential for this approach. However, for practical applications, the large scale production of this two-dimensional heterostructure is needed. Here, a superlattice film constructed from reduced graphene oxide (rGO) and photoactive titania nanosheets (Ti0.87O20.52–) was employed as a channel to construct a field effect transistor (FET) device, and its UV light response on the electrical transport property was examined. The UV light illumination induced significant improvement of the electrical conductance by ∼7 times on the basis of simultaneous enhancements of the electron carrier concentration and its mobility in rGO. Furthermore, the polarity of the FET response changed from ambipolar to n-type unipolar. Such modulated properties persisted in vacuum even after the UV light was turned off. These interesting behaviors may be explained in terms of photomodulation effects from Ti0.87O20.52– nanosheets. The photoexcited electrons in Ti0.87O20.52– are injected into rGO to increase the electron carrier concentration as high as 7.6 × 1013 cm–2. On the other hand, the holes are likely trapped in the Ti0.87O20.52– nanosheets. These photocarriers undergo reduction and oxidation of oxygen and water molecules adsorbed in the film, respectively, which act as carrier scattering centers, contributing to the enhancement of the carrier mobility. Since the film likely contains more water molecules than oxygen, upon extinction of UV light, a major portion of electrons (∼80% of the concentration at the UV off) survives in rGO, showing the highly enhanced conductance for days. This surpassing photomodulated FET response and its persistency observed in the present superlattice system of rGO/Ti0.87O20.52– are noteworthy compared with previous studies such as the device with a heteroassembly of graphene/h-BN.Keywords: charge accumulation; photomodulation; reduced graphene oxide; superlattice; titania nanosheet;

Platy microcrystals of a typical layered material, protonated titanate, have been shown to undergo an enormous degree of swelling in aqueous solutions of various amines, including tertiary amines, quaternary ammonium hydroxides, and primary amines. Introducing these solutions expanded the crystal gallery height by up to ∼100-fold. Through systematic analysis, we determined that ammonium ion intercalation is predominantly affected by the acid–base equilibrium and that the degree of swelling or inflow of H2O is controlled by the osmotic pressure balance between the gallery and the solution environment, both of which are relatively independent of electrolyte identity but substantially dependent on molarity. In solutions of tertiary amines and quaternary ammonium hydroxides, the uptake of ammonium ions increases nearly linearly with increasing external concentration before reaching a saturation plateau, i.e., ∼40% relative to the cation-exchange capacity of the crystals used. The only exception is tetrabutylammonium ions, which yield a lower saturation value, ∼30%, owing to steric effects. The swelling behaviors in some primary amine solutions differ as a result of the effect of attractive forces between amine solute molecules on the solution osmotic pressure. Although the swelling is essentially colligative in nature, the stability of the resultant swollen structure is heavily dependent on the chemical nature of the guest ions. Intercalated ions of higher polarity and smaller size help stabilize the swollen structure, whereas ions of lower polarity and larger size lead readily to exfoliation. The insight gained from this study sheds new light on both the incorporation of guest molecules into a gallery of layered structures in general and the exfoliation of materials into elementary single-layer nanosheets.

Two different kinds of two-dimensional (2D) materials, graphene oxide (GO) and titanium oxide nanosheets (Ti0.87O20.52−), were self-assembled layer-by-layer using a polycation as a linker into a superlattice film. Successful construction of an alternate molecular assembly was confirmed by atomic force microscopy and UV-visible absorption spectroscopy as well as X-ray diffraction analysis. Exposure of the resulting film to UV light effectively promoted photocatalytic reduction of GO as well as decomposition of the polycation, which are due to their intimate molecular-level contact. The reduction completed within 3 hours, bringing about a decrease of the sheet resistance by ∼106. This process provides a clean and mild route to reduced graphene oxide (rGO), showing advantages over other chemical and thermal reduction processes. A field-effect-transistor device was fabricated using the resulting superlattice assembly of rGO/Ti0.87O20.52− as a channel material. The rGO in the film was found to work as a unipolar n-type conductor, which is in contrast to ambipolar or unipolar p-type behavior mostly reported for rGO films. This unique property may be associated with the electron doping effect from Ti0.87O20.52− nanosheets. A significant improvement in the conductance and electron carrier mobility by more than one order of magnitude was revealed, which may be accounted for by the heteroassembly with Ti0.87O20.52− nanosheets with a high dielectric constant as well as the better 2D structure of rGO produced via the soft photocatalytic reduction.

One of the basic requirements for attaining a good epitaxy is a close structural matching between a substrate and a growing crystal epilayer. This restrictive requirement causes a major obstacle for its wide application to a range of functional crystal films in electronic, magnetic or optical devices. One approach for overcoming this problem is the so-called van der Waals epitaxy (VDWE) method, which can effectively implement the epitaxy of various crystals on cleaved faces of layered materials having no dangling bonds. The weak adatom–substrate interaction without directional covalent bonding plays a crucial role in the initial stage of VDWE, which drastically relaxes the lattice matching limitation. However, the method requires special materials for use as a substrate, thereby meaning that its applicability is limited. In this study, the concept is extended to the two-dimensional (2D) lattice of inorganic nanosheets, which are molecularly thin 2D crystals produced via artificial exfoliation of layered metal oxides. The nanosheets can neatly cover the surface of conventional substrates such as glass via a facile solution-based process. Similar to the above-mentioned cleaved faces of layered materials, such substrates can promote VDWE-like crystal growth because of their dangling bond-free nature. Based on this principle, we have demonstrated a selective deposition of highly textured (100), (110) and (111) SrTiO3 films, a fundamentally important archetype of functional crystals, on glass substrates covered with single-layer nanosheets with suitable 2D periodicities as a trigger for VDWE-like film growth. The rich varieties of nanosheet structures and their facile deposition onto almost any kinds of substrates provide a significant advantage, expanding potential applications for a range of devices based on functional crystal films.

The action of a tetrapropylammonium hydroxide solution on lamellar zeolite precursor MCM-22P produced a stable aqueous colloidal suspension which was shown by X-ray diffraction, small angle X-ray scattering and atomic force microscopy to contain ultrathin two-dimensional (2D) crystallites, including one-unit cell thick (i.e., 2.5 nm) monolayers.

The swelling and exfoliation behavior of protonated layered oxides using an organo-phosphonium base, tetrabutylphosphonium hydroxide (TBPOH), was examined for the first time. The action of the aqueous solution induced massive interlayer expansion up to ∼100-fold. The swollen crystals were immediately broken and completely exfoliated into unilamellar nanosheets in 1–2 h by shaking.

Layered rare earth hydroxides (LREHs) represent a new family of layered host compounds that integrate attractive physicochemical properties of rare earth elements with the wide tunability of guest anions. The compounds have attracted significant research attention, and potential applications have been found in various fields such as optics, catalysis, bio-medicine and so on. In this perspective, we describe our recent progress in the synthesis, structure characterization, and development of functionalities of the LREH compounds. A unique homogeneous alkalization method, in which RE ions are precipitated from a solution containing RE salt, concentrated target anions and hexamethylenetetramine, has been employed to effectively produce highly crystalline LREH samples. A range of anionic forms including chloride-, nitrate-, sulfate- and organodisulfonate-series, have been synthesized and structurally characterized. Two types of cationic rare earth hydroxide layers, {[RE2(OH)5(H2O)2]+}∞ for the chloride- and nitrate-series and {[RE(OH)2(H2O)]+}∞ for the sulfate- and organodisulfonate-series, are classified. Unique dehydration/rehydration behaviors or thermal phase evolution of the LREH compounds have been revealed and discussed in relation to the crystal structures. An outlook for potential applications of LREH compounds as anion exchangers, precursors to unique functional oxides, and optical phosphors is described.

All-nanosheet ultrathin capacitors of Ru0.95O20.2–/Ca2Nb3O10–/Ru0.95O20.2– were successfully assembled through facile room-temperature solution-based processes. As a bottom electrode, conductive Ru0.95O20.2– nanosheets were first assembled on a quartz glass substrate through a sequential adsorption process with polycations. On top of the Ru0.95O20.2– nanosheet film, Ca2Nb3O10– nanosheets were deposited by the Langmuir–Blodgett technique to serve as a dielectric layer. Deposition parameters were optimized for each process to construct a densely packed multilayer structure. The multilayer buildup process was monitored by various characterizations such as atomic force microscopy (AFM), ultraviolet–visible absorption spectra, and X-ray diffraction data, which provided compelling evidence for regular growth of Ru0.95O20.2– and Ca2Nb3O10– nanosheet films with the designed multilayer structures. Finally, an array of circular films (50 μm ϕ) of Ru0.95O20.2– nanosheets was fabricated as top electrodes on the as-deposited nanosheet films by combining the standard photolithography and sequential adsorption processes. Microscopic observations by AFM and cross-sectional transmission electron microscopy, as well as nanoscopic elemental analysis, visualized the sandwich metal–insulator–metal structure of Ru0.95O20.2–/Ca2Nb3O10–/Ru0.95O20.2– with a total thickness less than 30 nm. Electrical measurements indicate that the system really works as an ultrathin capacitor, achieving a capacitance density of ∼27.5 μF cm–2, which is far superior to currently available commercial capacitor devices. This work demonstrates the great potential of functional oxide nanosheets as components for nanoelectronics, thus contributing to the development of next-generation high-performance electronic devices.Keywords: capacitance density; dielectric property; oxide nanosheets; solution-based assembly processes; ultrathin capacitors

A systematic study has been conducted to examine the thermal stability of layer-by-layer assembled films of perovskite-type nanosheets, (Ca2Nb3O10–)n (n = 1–10), which exhibit superior dielectric and insulating properties. In-plane and out-of-plane X-ray diffraction data as well as observations by atomic force microscopy and transmission electron microscopy indicated the high thermal robustness of the nanosheet films. In a monolayer film with an extremely small thickness of ∼2 nm, the nanosheet was stable up to 800 °C, the temperature above which segregation into CaNb2O6 and Ca2Nb2O7 began. The critical temperature moderately decreased as the film thickness, or the number of nanosheet layers, increased, and reached 700 °C for seven- and 10-layer films, which is comparable to the phase transformation temperature for a bulk phase of the protonic layered oxide of HCa2Nb3O10·1.5H2O as a precursor of the nanosheet. This thermal stabilization of perovskite-type nanosheets should be associated with restricted nucleation and crystal growth peculiar to such ultrathin 2D bound systems. The stable high-k dielectric response (εr = 210) and highly insulating nature (J < 10–7 A cm–2) remained substantially unchanged even after the nanosheet film was annealed up to 600 °C. This study demonstrates the high thermal stability of 2D perovskite-type niobate nanosheets in terms of structure and dielectric properties, which suggests promising potential for future high-k devices operable over a wide temperature range.Keywords: layer-by-layer assembly; nanodielectrics; perovskite-type nanosheets; thermal stability

Osmotic swelling and exfoliation behaviors in a lepidocrocite-type titanate H1.07Ti1.73O4·H2O were investigated upon reactions with tetramethylammonium (TMA+) and tetrabutylammonium (TBA+) cations. The reaction products in various physical states (suspension, wet aggregate, and deposited nanosheets) were characterized by several techniques, including X-ray diffraction under controlled humidity, small-angle X-ray scattering, particle size analysis, and atomic force microscopy. As the ratio of tetraalkylammonium ion in a solution to exchangeable proton in a solid decreased, the predominant product changed from the osmotically swollen phase, having an interlayer spacing d of several tens of nanometers, to the exfoliated nanosheets. The different behaviors of two cations in the osmotic swelling were evident from the slope and the transition point in the d versus C–1/2 plot, where C is the concentration of the cations. At a short reaction time, crystallites of a few stacks were obtained as a major product in the reaction with TMA+. On the other hand, a mixture of those crystallites and a significant portion of unilamellar nanosheets were obtained in the reaction with TBA+. In both cases, those stacks were ultimately thinned down at long reaction time to unilamellar nanosheets. The lateral size of the nanosheets could be controlled, depending on the type of the cations, the tetraalkylammonium-to-proton ratios, and the mode of the reaction (manual versus mechanical shaking). The nanosheets produced by TMA+ had large lateral sizes up to tens of micrometers, and the suspension showed a distinctive silky appearance based on liquid crystallinity. Our work provides insights into the fundamentals of osmotic swelling and exfoliation, allowing a better understanding of the preparation of nanosheets, which are one of the most important building blocks in nanoarchitectonics.Keywords: exfoliation; lepidocrocite; swelling; tetraalkylammonium cation; titanate nanosheets;

An approach to fabricate Ba0.5Sr0.5TiO3 (BST) films with a preferred orientation on a glass substrate by pulsed laser deposition was developed. To ensure a preferred crystallographic orientation, we utilized a molecularly thin Ca2Nb3O10 perovskite nanosheet as a seed layer and successfully fabricated BST films with a nearly perfect (100)-axis orientation. The 100 nm films after annealing at 450 °C in air showed a good dielectric performance (εr > 400), which was comparable to the εr value of epitaxially grown films with the same thickness. These results indicate that the nanosheet seed layer plays a crucial role in controlled film growth, realizing a nearly intrinsic performance of BST.Keywords: Ba1−xSrxTiO3; dielectrics; nanosheet; seed layer; thin-film growth;

We report the synthesis and structure characterization of a new family of lanthanide-based inorganic–organic hybrid frameworks, Ln2(OH)4[O3S(CH2)nSO3]·2H2O (Ln = La, Ce, Pr, Nd, Sm; n = 3, 4), and their oxide derivatives. Highly crystallized samples were synthesized by homogeneous precipitation of Ln3+ ions from a solution containing α,ω-organodisulfonate salts promoted by slow hydrolysis of hexamethylenetetramine. The crystal structure solved from powder X-ray diffraction data revealed that this material comprises two-dimensional cationic lanthanide hydroxide {[Ln(OH)2(H2O)]+}∞ layers, which are cross-linked by α,ω-organodisulfonate ligands into a three-dimensional pillared framework. This hybrid framework can be regarded as a derivative of UCl3-type Ln(OH)3 involving penetration of organic chains into two {LnO9} polyhedra. Substitutional modification of the lanthanide coordination promotes a 2D arrangement of the {LnO9} polyhedra. A new hybrid oxide, Ln2O2[O3S(CH2)nSO3], which is supposed to consist of alternating {[Ln2O2]2+}∞ layers and α,ω-organodisulfonate ligands, can be derived from the hydroxide form upon dehydration/dehydroxylation. These hybrid frameworks provide new opportunities to engineer the interlayer chemistry of layered structures and achieve advanced functionalities coupled with the advantages of lanthanide elements.

Layered rubidium tungstate, Rb4W11O35, with a two-dimensional (2D) bronze-type tunnel structure was successfully delaminated into colloidal nanosheets via a soft-chemical process involving acid exchange and subsequent intercalation of tetrabutylammonium ions. Characterizations by transmission electron microscopy and atomic force microscopy confirmed the formation of unilamellar 2D nanosheet crystallites with a unique thickness of ∼3 nm and an average lateral size of 400 nm. The obtained nanosheets exhibited reversible color change upon UV-light excitation via an optical band gap of 3.5 eV. The ultimate 2D aspect ratio favorable for an adsorption of charge-compensating cations to trapped electrons working as a color center is presumably responsible for highly efficient photochromic behavior. Its coloration mainly consists of a broad band at a wavelength of 1800 nm and longer, which is much different from that of the common tungstate nanomaterials. Thus, the chromogenic nanosheet obtained in this study features the intense UV absorption and optically switchable visible-to-IR absorption, which may be useful for window applications such as cutoff filters and heat-absorbing films.

Exfoliated two-dimensional (2D) unilamellar nanosheets of Ca2Nb3O10–, TiNbO5–, Ti2NbO7–, and Ti5NbO143– were deposited layer-by-layer to produce multilayer films on indium–tin–oxide (ITO)-coated glass electrodes, and their electrochemical and photoelectrochemical properties were explored. The layer-by-layer assembly process via sequential adsorption with counter polycations was monitored by UV–visible absorption spectra and X-ray diffraction measurements, which confirmed the successful growth of films, where nanosheets and polycations are alternately stacked at a separation of 1.6–2.4 nm. Exposure to UV light totally removed polycations, producing inorganic films. Cyclic voltammetry on Ti and/or Nb oxide nanosheet electrodes thus fabricated showed reduction/oxidation (Ti3+/Ti4+ and Nb4+/Nb5+) peaks associated with insertion/extraction of Li+ ions into/from intersheet galleries of the films. The extent of the redox reaction is found to be governed by the cation density in the nanosheet gallery. Anodic photocurrents of the oxide nanosheet electrodes were observed under UV light irradiation. These action spectra showed close resemblance to optical absorption profiles of the colloidal nanosheets, indicating that the photocurrent was generated from the nanosheets. Their analysis indicates that the nanosheets of Ca2Nb3O10–, TiNbO5–, Ti2NbO7–, and Ti5NbO143– are all indirect transition-type wide-gap semiconductors with bandgap energies of 3.44, 3.68, 3.64, and 3.53 eV, respectively. These values are larger than those for corresponding parent layered oxide compounds before delamination, suggesting confinement effects into 2D nanosheet structure. Furthermore, the value was invariable for the films with a different number of nanosheet layers, indicating that quantized nanosheets were electronically isolated with each other. In addition, photocurrent generation was measured as a function of applied electrode potential, and the flatband potential was estimated from the photocurrent onset values as −1.12, −1.33, −1.30, and −1.29 V vs Ag/Ag+, for Ca2Nb3O10–, TiNbO5–, Ti2NbO7–, and Ti5NbO143– nanosheets, respectively, providing a diagram of electronic band structure for the nanosheets.

Unique photocatalysts based on novel two-dimensional (2D) oxide nanosheets have been synthesized and their photochemical activity has been examined. Monolayer films of titanoniobate and niobate (TiNbO5, Ti2NbO7, Ti5NbO14, and Nb3O8) nanosheets, synthesized by exfoliating layered oxide precursors through a soft-chemical procedure, were fabricated on quartz glass substrate via a sequential adsorption method. All the nanosheet films exhibited good photoinduced hydrophilicity, while their oxidation activity was very low. This behavior can be regarded as inherent features of nanosheet-type photocatalysts having molecularly thin 2D anisotropy. Such ultrathin flexible structures are advantageous for facilitating photo-driven surface wettability change. Especially, the hydrophilic conversion property of Nb3O8 nanosheet was highly efficient, showing activity that was at least one order of magnitude superior to that of the widely used photocatalyst film of polycrystalline anatase TiO2. Moreover, the monolayer film of Nb3O8 nanosheet was found to have enhanced thermal stability and chemical resistance, particularly against diffusion of sodium ions at elevated temperature: Nb3O8 nanosheet film heated on sodium-rich glass (soda-lime glass) substrate maintained excellent hydrophilic conversion activity, whereas Ti0.87O2 nanosheet as well as anatase (TiO2) based photocatalysts was virtually deactivated. These features are a great advantage of Nb3O8 nanosheet photocatalysts for developing the practical super-hydrophilic applications where post-annealing is indispensable.

We report structure analysis of a new family of rare-earth hydroxides Ln2(OH)4SO4·2H2O (Ln = Pr, Nd, Sm, Eu, Gd, Tb) from synchrotron X-ray and electron diffraction data. Rietveld profile analysis revealed that all members were isostructural and crystallized in a face-centered monoclinic system A2/m (No. 12), in which the monoclinic angles were approximately equal to the right angle, varying from 90.387(1)° for Pr sample to 90.0718(3)° for Tb sample. The structure consisted of LnO9 polyhedra connected by μ3-hydroxyl groups and μ2-water molecules, forming a corrugated two-dimensional layer, which was pillared by bidentated sulfate ions. This series of compounds had a supercell a′ = 2a, b′ = 2b because of the local orientation ordering of SO42–. Structural features along the series, such as unit-cell parameters and average Ln–O distances, represented a progressive contraction associated with the shrinking radius of the lanthanide cations from Pr to Tb.

Two-component colloidal nanosheets were prepared in Ti0.91O2−MnO2, Ca2Nb3O10−Ti0.91O2, and Ca2Nb3O10−MnO2 systems with various compositions, and were flocculated by adding K ions. X-ray powder diffraction patterns of the products showed remarkably broad reflections, and powder pattern simulation was conducted based on the matrix method for diffuse scattering from stacking disorder. An approach similar to that used for analyzing layered composite crystals with one-directional disorder was adopted for the obtained mixed-layer compounds. The structural characteristics of the restacked nanosheets in the three binary nanosheet systems were found to be dependent on the combined pairs. In the Ti0.91O2−MnO2 system, the one-dimensional solid-solution state appeared in the fairly wide composition ranges of x ≤ 0.3 and x ≥ 0.7 for xTi0.91O2−(1 − x)MnO2, and the one-dimensional two-phase stacking state appeared in the intermediate range of 0.3 < x < 0.7. In the Ca2Nb3O10−Ti0.91O2 system, the one-dimensional solid-solution state appeared in the composition ranges x ≤ 0.2 and x ≥ 0.7 for xCa2Nb3O10−(1 − x)Ti0.91O2, and the one-dimensional two-phase coexistence state appeared in the intermediate range. In the Ca2Nb3O10−MnO2 system, flocculation led to phase separation into individually restacked nanosheets, restacked K−Ca2Nb3O10 and restacked K−MnO2, throughout the composition range from 0.9Nb−0.1Mn to 0.1Nb−0.9Mn. Similarities and differences in flocculation behavior in the three binary systems, Ti0.91O2−MnO2, Ca2Nb3O10−Ti0.91O2, and Ca2Nb3O10−MnO2, were examined.

Layered materials, three-dimensional crystals built from stacking two-dimensional components, are attracting intense interest because of their structural anisotropy and the fascinating properties that result. However, the range of such layered materials that can exchange anions is quite small. Continuing efforts have been underway to identify a new class of anion-exchangeable materials. One major goal is the incorporation of rare-earth elements within the host because researchers expect that the marriage of rare-earth skeleton host and the exchangeable species within the interlayer will open up new avenues both for the assembly of layered materials and for the understanding of rare-earth element chemistry. Such lanthanide layered solids have industrial potential. These materials are also of academic importance, serving as an ideal model for studying the cationic size effect on structure stability associated with lanthanide contraction. In this Account, we present the work done by ourselves and others on this novel class of materials. We examine the following four subtopics regarding these layered anionic materials: (1) synthesis strategy and composition diversity, (2) structural features, (3) structure stability with relative humidity, and (4) applications. These materials can be synthesized either by hydrothermal reactions or by homogeneous precipitation, and a variety of anions can be intercalated into the gallery. Although only cations with a suitable size can form the layered structure, the possible range is wide, from early to late lanthanides. We illustrate the effect of lanthanide contraction on properties including morphology, lattice dimensions, and coordination numbers. Because each lanthanide metal ion coordinates water molecules, and the water molecules point directly into the gallery space, this feature plays a critical role in stabilizing the layered structure. In the 9-fold monocapped square antiprism structure, the humidity-triggered transition between high- and low-hydrated phases corresponds to the uptake of H2O molecules at the capping site, which provides further evidence of the importance of water coordination. Applications using this unique combination of rare-earth element chemistry and layered materials include ion-exchange, photoluminescence, catalysis, and biomedical devices. Further exploration of the compounds and new methods for functional modification would dramatically enrich the junction of these two fields, leading to a new generation of layered materials with desirable properties.

This paper describes the topochemical synthesis of Co−Ni layered double hydroxides (LDHs) from brucite-like Co−Ni hydroxides through a novel oxidative intercalation process employing bromine as an oxidizing agent, and their exfoliation into positively charged unilamellar nanosheets in formamide after anion-exchange treatment. In this protocol, hexagonal microplatelets of brucite-like Co−Ni hydroxides in variable composition were prepared by homogeneous precipitation of a mixed solution of divalent cobalt and nickel ions via hexamethylenetetramine hydrolysis. Subsequent treatment of the brucite-like Co−Ni hydroxides with excessive bromine in acetonitrile promoted partial oxidation of Co2+ into Co3+, producing Br−-intercalated Co−Ni LDHs inheriting the hexagonal morphology. This rational topochemical approach was applicable for realizing a pure phase of Co−Ni LDHs with nickel content up to 50% (metal content). Chemical analyses indicated that as-prepared Co−Ni−Br LDHs were unexceptionally characterized by a general chemical formula as [(Co2+1−3x/2Ni2+3x/2)2/3Co3+1/3(OH−)2][Br−1/3·0.5H2O] (x ≤ 0.5), a thermodynamically stable LDH structure with a M2+/M3+ ratio of 2:1. We developed an ethanol-assisted anion-exchange approach, which was effective in preventing carbonate contamination in preparing a variety of inorganic and organic anionic forms of Co−Ni LDHs. As-prepared NO3− intercalated Co−Ni LDHs without substantial carbonate contamination were successfully exfoliated into unilamellar nanosheets bearing positive charges upon contact with formamide. The translucent nanosheet suspensions exhibited characteristic colors depending on the variable Co/Ni ratio.

Multilayer composite films comprising of TaO3 nanosheet crystallites and poly(diallyldimethylammonium chloride) (PDDA) were assembled via layer-by-layer sequential adsorption. Exposure of the resulting films to UV light promoted photocatalytic decomposition of PDDA in the nanosheet gallery to yield inorganic films. Novel hollow microspheres of Ta2O5 were fabricated by the deposition of a PDDA/TaO3 multilayer nanoshell on polystyrene (PS) beads and the subsequent removal of the PS core and PDDA layers via calcination. The hollow microspheres showed photocatalytic properties effective for hydrogen evolution from an aqueous methanol solution. The shape control into hollow microsphere greatly enhanced the photocatalytic activity. The results highlight the role of the surface area and crystallinity of the catalyst in photocatalytic activity.

The synthesis of a series of new layered rare-earth hydroxide solid solutions and their transformation into (EuxGd1−x)2O3 crystallites are described. Highly crystalline platelets of EuxGd1−x(OH)2.5Cl0.5·0.9H2O solid solutions with various Eu3+/Gd3+ ratios were prepared through a homogeneous precipitation method. The hydroxide solid-solution samples exhibited characteristic Eu3+ photoluminescence properties through the energy transfer from Gd3+ to Eu3+ and the self-excitation of Eu3+. Cubic (EuxGd1−x)2O3 crystallites were obtained via quasi-topotactic transformation of EuxGd1−x(OH)2.5Cl0.5·0.9H2O solid solutions above 800 °C. The as-transformed cubic (EuxGd1−x)2O3 crystallites well retained the original platelet morphology and single crystalline nature, and exhibited greatly enhanced photoluminescence properties with respect to the precursor hydroxides. The Eu3+ content of 0.05 in the cubic (EuxGd1−x)2O3 gave a maximum luminescence intensity, which is comparable with that of a commercial Y2O3:Eu phosphor.

Two-dimensional (2D) nanosheets obtained via exfoliation of layered compounds have attracted intense research in recent years. In particular, the development of exotic 2D systems such as stable graphene and transition-metal oxide nanosheets has sparked new discoveries in condensed matter physics and nanoelectronics. Here, we review the progress made in the synthesis, characterization and properties of oxide nanosheets, highlighting emerging functionalities in electronic and spin-electronic applications. We also present a perspective on the advantages offered by this class of materials for future nanotechnology.

We report the synthesis and characteristics of a rare-earth based layered family, Ln8(OH)20(NO3)4·nH2O with Ln = Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Y, synthesized through homogeneous precipitation of Ln(NO3)3·xH2O with hexamethylenetetramine. The products were uniform and of high crystallinity. Their morphology gradually changed from elongated hexagon (Sm, Eu, Gd) and hexagon (Tb, Dy) to rhombus (Ho, Er, Tm). Selected area electron diffraction revealed that the in-plane structure resembled that of the chloride counterpart, Ln8(OH)20Cl4·mH2O. Unit cell dimensions of the host layer, a and b, decreased with contracting size of lanthanide ions, whereas no such trend was observed for the interlamellar distance, c/2, which is dominated by hydration degree. Stability of the samples with temperature and relative humidity (RH) was examined. At high temperature or low RHs, hydrated water molecules could be removed, which afforded a phase with a basal decrease of ∼0.6 Å. The transition was reversible as revealed by an in situ powder X-ray diffraction study, but a RH hysteresis exists. The reversibility increased with an increase in atomic number or layer charge density. Nitrate anions of both phases could be quantitatively exchanged by other anions under ambient conditions.

Exfoliated unilamellar titania nanosheets of Ti0.87O2 with a lateral size of 10−30 μm were deposited layer-by-layer onto various substrates by Langmuir−Blodgett procedure to produce a highly ordered lamellar nanofilms. The nanosheets dispersed in an aqueous suspension containing quaternary ammonium ions as a supporting electrolyte floated spontaneously at the air/liquid interface, and they were successfully transferred onto the substrate after surface compression. Neat tiling of the nanosheets could be realized at an optimized surface pressure. The film thus obtained was exposed to UV light to turn the substrate surface hydrophilic, which was helpful for stable repetition of monolayer deposition. Layer-by-layer growth was confirmed by UV−visible absorption spectra, which showed progressive enhancement of an absorption band due to the nanosheet. Cross-sectional transmission electron microscopy images visualized the ultrathin film homogeneously deposited on the substrate surface and a lamellar fringe of the layer-by-layer assembled nanosheets was clearly resolved at a higher magnification. X-ray diffraction data on the films showed sharp basal reflections up to the seventh order, and Williamson−Hall analysis of the pattern indicated that the film was coherent across the total thickness with respect to X-ray and that the lattice strain was extremely small. In addition, the first basal reflection was accompanied by small satellite peaks, which are accounted for by the Laue interference function. All these features clearly indicate the formation of a highly ordered lamellar nanostructure of the titania nanosheets comparable to artificial lattice films produced via modern vapor-phase deposition processes. The obtained films showed superior dielectric and insulating properties as a reflection of the highly organized film nanoarchitecture.Keywords: dielectric properties; Langmuir−Blodgett procedure; layer-by-layer deposition; multilayer film; nanosheet

This paper reports the synthesis of alkali-free layered nickel oxides with a highly swollen hydrate structure, derived from NaNiO2 through soft-chemical processes involving oxidation with bromine and subsequent acid treatment. Complete removal of interlayer Na+ ions and subsequent hydration yielded a single phase of HxNiO2·nH2O (x < 1, n ∼1) with an enlarged basal spacing of 9.6 Å. The materials had a monoclinic structure (C2/m); unit cell parameters for a typical composition of H0.66NiO2·0.9H2O were a = 4.8993(8) Å, b = 2.8256(4) Å, c = 9.7598(9) Å, and β = 98.88(2)°. Rietveld refinement revealed that the structure was composed of pseudohexagonal NiO2 sheets accommodating partially occupied three layers of H2O molecules and H3O+ ions in the galleries. The highly expanded layered structure is analogous to Na0.3NiO2·1.3H2O and other layer oxides such as buserite-type manganese oxide and superconducting NaxCoO2·yH2O but differs in alkali-free composition. The 9.6 Å phase underwent partial dehydration to give a basal spacing of 7 Å upon exposure to atmosphere with a relative humidity of <30%.

Layered rare-earth hydroxide crystallites self-assembled at the hexane/water interface were transferred to various substrates to form a monolayer film, which exhibited photoluminescence properties and ion-exchange ability.

Transition metal oxide nanosheet of Nb3O8 was synthesized by delaminating a layered host compound of KNb3O8. Ultrathin two-dimensional morphology, about 1 nm thickness vs. submicrometer lateral size, was revealed by atomic force microscopy. The colloidal nanosheets were assembled layer-by-layer with polycation onto an indium-tin-oxide (ITO)-coated glass electrode via sequential adsorption technique. UV–vis absorption spectra in the assembly process showed the linear enhancement of absorbance at 270 nm per deposition cycle, indicating the successful formation of multilayer composite films, which can be converted into polymer free films upon exposure to UV light. The cyclic voltammogram of resulting Nb3O8 nanosheet film electrodes exhibited reduction/oxidation (Nb4+/Nb5+) peaks at −1.2 V (vs. Ag/Ag+) attributable to insertion and deintercalation of Li+ ions into and from the nanosheet galleries. The reaction (reduction/oxidation) ratio of the electroactive Nb atom in Nb3O8 nanosheet was calculated to be 6.3% by integrating a charge transfer in a sweep range of +1.0 to −1.9 V. The Nb3O8 nanosheet electrode generated anodic photocurrent in response to UV illumination. Analysis on the photocurrent action spectrum indicated that Nb3O8 nanosheet is an indirect transition-type semiconductor with bandgap energy of 3.58 eV. The flat-band potential was estimated to be −1.32 V vs. Ag/Ag+ from the applied potential dependence.

Layered cesium tungstate, Cs6+xW11O36, with two-dimensional (2D) pyrochlore structure was exfoliated into colloidal unilamellar sheets through a soft-chemical process. Interlayer Cs ions were replaced with protons by acid exchange, and quaternary ammonium ions were subsequently intercalated under optimized conditions. X-ray diffraction (XRD) measurements on gluelike sediment recovered from the colloidal suspension by centrifugation showed a broad pattern of a pronounced wavy profile, which closely matched the square of calculated structure factor for the single host layer. This indicates the total delamination of the layered tungstate into nanosheets of Cs4W11O362−. Microscopic observations by transmission electron microscopy and atomic force microscopy clearly revealed the formation of unilamellar crystallites with a very high 2D anisotropy, a thickness of only ∼2 nm versus lateral size up to several micrometers. In-plane XRD analysis confirmed that the 2D pyrochlore structure was retained. The colloidal cesium tungstate nanosheet showed strong absorption of UV light with sharp onset, suggesting a semiconducting nature. Analysis of the absorption profile provided 3.6 eV as indirect band gap energy, which is 0.8 eV larger than that of the bulk layered precursor, probably due to size quantization. The nanosheet exhibited highly efficient photochromic properties, showing reversible color change upon UV irradiation.Keywords: chromogenic nanomaterial; nanomesh; nanosheet; photochromism; self-assembly; tungsten oxide

This paper describes systematic studies on the synthesis and delamination of manganese oxide nanobelts with the birnessite-type layered structure. K-birnessite nanobelts of K0.33MnO2·0.5H2O, which typically had a length of several tens of micrometers, a width of hundreds of nanometers, and a thickness of ∼15 nm, were synthesized by hydrothermally treating a KMnO4–MnCl2 mixture in a highly concentrated KOH aqueous solution. The nanobelt growth was found to be controlled by the KOH concentration and the molar ratio of Mn2+/MnO4− in the starting reaction mixture. The K-birnessite nanobelts were converted to H-birnessite, H0.08MnO2·0.7H2O, by treatment with a (NH4)2S2O8 aqueous solution, retaining their high crystallinity and beltlike morphology. Swelling and delamination behaviors of the H-birnessite nanobelts in aqueous solutions of quaternary ammonium hydroxides were studied in detail. In tetrabutylammonium hydroxide (TBAOH) solutions, the H-birnessite showed a limited swelling and delamination behavior, whereas the compound underwent osmotic swelling in tetramethylammonium hydroxide (TMAOH) solutions. Water-washing the TMA+-treated samples greatly enhanced the degree of swelling, while maintaining the three-dimensional crystalline order. Upon further contact with a TBAOH solution, the highly swollen phase was dominantly delaminated into unilamellar nanosheets. The nanosheets thus obtained inherited the morphology of the parent nanobelts in their long-axis direction and had lateral sizes of micrometer order. The colloidal suspension of nanosheets showed an optical absorption band around 380 nm, which was drastically changed from the rather constant and featureless absorption of UV to visible light for the birnessite before delamination.

We examined the photochemical properties of well-ordered multilayer films of titania nanosheets prepared on quartz-glass substrate using the layer-by-layer deposition method. The photocatalytic decomposition of gaseous 2-propanol and bleaching of Methylene Blue dye under UV light illumination were measured to evaluate the photocatalytic oxidation ability. Photoinduced hydrophilicity was also studied by measuring the contact angle of water droplets on the film. The results indicated that titania nanosheets had good photoinduced hydrophilicity. The monolayer film of titania nanosheets showed almost identical activity compared with well investigated sol-gel derived anatase TiO2 film, while its photocatalytic oxidation activity was low by more than an order of magnitude. This fact suggests that photoinduced hydrophilicity could not be explained simply in terms of the photocatalytic removal of hydrophobic organic species adsorbed on the surface. The photocatalytic oxidation activity and the photoinduced hydrophilic conversion rate decreased with increasing number of nanosheet layers, suggesting that photogenerated carriers produced in the internal part of the multilayer films can hardly diffuse to the surface layer. Photochemical properties of ultrathin anatase films obtained simply by heating the titania nanosheet films were evaluated as well, and also revealed high photoinduced hydrophilicity.

Exfoliation of layered double hydroxides (LDHs) into single layers provides a new type of nanosheet with ultimate two-dimensional anisotropy and positive charge. In this Highlight article, we briefly review the latest advances in this emerging field. In comparison with the previous studies, we show that micrometer-sized and well-defined LDH nanosheets can be readily attained by synthesizing large crystals of LDH-carbonate via so-called homogeneous precipitation and subsequent exfoliation of the nitrate form in formamide. Some general aspects including the exfoliating process and characterization, a plausible delaminating mechanism, and future challenges, are presented and discussed.

Hollow nanoshells of layered double hydroxide (LDH) have been fabricated using exfoliated LDH nanosheets as a shell building block and polystyrene beads as a sacrificial template.

A layered potassium cobalt oxide of KxCoO2 (x = 0.55) was prepared by solid-state reaction between KO2 and Co3O4. We explored soft-chemical reactivities of this material in treatments with acid or bromine solutions by following compositional and structural changes by X-ray diffraction and chemical analysis. The acid treatment promoted K+/H3O+ exchange but concurrently induced the formation of CoOOH as a side product. The compound underwent oxidative deintercalation of K+ upon the action of Br2, which was accompanied by hydration. Reinsertion of alkali metal ions into the resulting Br2-treated sample could be attained by the treatment with corresponding hydroxide solutions.

Novel hollow nanoshells of Mn2O3 with controllable ultrathin shell thickness have been fabricated through layer-by-layer assembly of exfoliated MnO2 nanosheets and polyelectrolytes on polymer bead templates, followed by removal of the polymer cores via calcination.

Fe- or Ni-substituted titania nanosheets have been synthesized by chemically exfoliating precursor layered titanates into colloidal single sheets. Obtained nanosheets were self-assembled via consecutive adsorption with polycations to produce multilayer ultrathin films, which exhibit photocatalytic properties. In the case of Fe system, UV absorption band was widened towards the visible light region in comparison with that of titania nanosheets of Ti0.9O2.

We examined the photochemical properties of well-ordered multilayer films of titania nanosheets prepared on quartz-glass substrate using the layer-by-layer deposition method. The photocatalytic decomposition of gaseous 2-propanol and bleaching of Methylene Blue dye under UV light illumination were measured to evaluate the photocatalytic oxidation ability. Photoinduced hydrophilicity was also studied by measuring the contact angle of water droplets on the film. The results indicated that titania nanosheets had good photoinduced hydrophilicity. The monolayer film of titania nanosheets showed almost identical activity compared with well investigated sol-gel derived anatase TiO2 film, while its photocatalytic oxidation activity was low by more than an order of magnitude. This fact suggests that photoinduced hydrophilicity could not be explained simply in terms of the photocatalytic removal of hydrophobic organic species adsorbed on the surface. The photocatalytic oxidation activity and the photoinduced hydrophilic conversion rate decreased with increasing number of nanosheet layers, suggesting that photogenerated carriers produced in the internal part of the multilayer films can hardly diffuse to the surface layer. Photochemical properties of ultrathin anatase films obtained simply by heating the titania nanosheet films were evaluated as well, and also revealed high photoinduced hydrophilicity.

Two-dimensional (2D) nanosheets obtained via exfoliation of layered compounds have attracted intense research in recent years. In particular, the development of exotic 2D systems such as stable graphene and transition-metal oxide nanosheets has sparked new discoveries in condensed matter physics and nanoelectronics. Here, we review the progress made in the synthesis, characterization and properties of oxide nanosheets, highlighting emerging functionalities in electronic and spin-electronic applications. We also present a perspective on the advantages offered by this class of materials for future nanotechnology.

Layered rare earth hydroxides (LREHs) represent a new family of layered host compounds that integrate attractive physicochemical properties of rare earth elements with the wide tunability of guest anions. The compounds have attracted significant research attention, and potential applications have been found in various fields such as optics, catalysis, bio-medicine and so on. In this perspective, we describe our recent progress in the synthesis, structure characterization, and development of functionalities of the LREH compounds. A unique homogeneous alkalization method, in which RE ions are precipitated from a solution containing RE salt, concentrated target anions and hexamethylenetetramine, has been employed to effectively produce highly crystalline LREH samples. A range of anionic forms including chloride-, nitrate-, sulfate- and organodisulfonate-series, have been synthesized and structurally characterized. Two types of cationic rare earth hydroxide layers, {[RE2(OH)5(H2O)2]+}∞ for the chloride- and nitrate-series and {[RE(OH)2(H2O)]+}∞ for the sulfate- and organodisulfonate-series, are classified. Unique dehydration/rehydration behaviors or thermal phase evolution of the LREH compounds have been revealed and discussed in relation to the crystal structures. An outlook for potential applications of LREH compounds as anion exchangers, precursors to unique functional oxides, and optical phosphors is described.

Platelet crystals of a layered perovskite showed massive accordion-like swelling in a tetrabutylammonium hydroxide solution. The permeation of the solution induced the huge expansion of the interlayer spacing as well as the crystal thickness up to 50-fold, leading to a very high water content of >90 wt%.

The action of a tetrapropylammonium hydroxide solution on lamellar zeolite precursor MCM-22P produced a stable aqueous colloidal suspension which was shown by X-ray diffraction, small angle X-ray scattering and atomic force microscopy to contain ultrathin two-dimensional (2D) crystallites, including one-unit cell thick (i.e., 2.5 nm) monolayers.

The swelling and exfoliation behavior of protonated layered oxides using an organo-phosphonium base, tetrabutylphosphonium hydroxide (TBPOH), was examined for the first time. The action of the aqueous solution induced massive interlayer expansion up to ∼100-fold. The swollen crystals were immediately broken and completely exfoliated into unilamellar nanosheets in 1–2 h by shaking.

One of the basic requirements for attaining a good epitaxy is a close structural matching between a substrate and a growing crystal epilayer. This restrictive requirement causes a major obstacle for its wide application to a range of functional crystal films in electronic, magnetic or optical devices. One approach for overcoming this problem is the so-called van der Waals epitaxy (VDWE) method, which can effectively implement the epitaxy of various crystals on cleaved faces of layered materials having no dangling bonds. The weak adatom–substrate interaction without directional covalent bonding plays a crucial role in the initial stage of VDWE, which drastically relaxes the lattice matching limitation. However, the method requires special materials for use as a substrate, thereby meaning that its applicability is limited. In this study, the concept is extended to the two-dimensional (2D) lattice of inorganic nanosheets, which are molecularly thin 2D crystals produced via artificial exfoliation of layered metal oxides. The nanosheets can neatly cover the surface of conventional substrates such as glass via a facile solution-based process. Similar to the above-mentioned cleaved faces of layered materials, such substrates can promote VDWE-like crystal growth because of their dangling bond-free nature. Based on this principle, we have demonstrated a selective deposition of highly textured (100), (110) and (111) SrTiO3 films, a fundamentally important archetype of functional crystals, on glass substrates covered with single-layer nanosheets with suitable 2D periodicities as a trigger for VDWE-like film growth. The rich varieties of nanosheet structures and their facile deposition onto almost any kinds of substrates provide a significant advantage, expanding potential applications for a range of devices based on functional crystal films.

Layered rare-earth hydroxide crystallites self-assembled at the hexane/water interface were transferred to various substrates to form a monolayer film, which exhibited photoluminescence properties and ion-exchange ability.