Lepidocrocite-type Titanate Research Articles

Modulating oxygen vacancies of Fe-doped Ti-based materials with ultrathin nanosheets for enhancing H2S-to-S selective oxidation

Developing highly efficient catalysts for the transformation of hydrogen sulfide (H2S) pollutants into value-added products is crucial for both fundamental catalytic research and industrial chemistry. Herein, ferrititanate nanosheets (H0.8Ti1.2Fe0.8O4, denoted as NS-HTFO) exfoliated from lepidocrocite-type titanates are employed for the first time to catalyze the conversion of H2S to elemental sulfur. Collective experimental characterizations reveal that the as-designed NS-HTFO catalyst possesses ultrathin 2D structure and a large specific surface area, featuring abundant oxygen vacancies. Compared with its sandwich-like precursor of K0.8Ti1.2Fe0.8O4 (denoted as L-KTFO), the NS-HTFO catalyst displays notably enhanced desulfurization activity, achieving 100 % H2S conversion and over 93 % sulfur selectivity at temperatures ranging from 90 to 270 °C. Moreover, no significant decline in sulfur yield is observed over the course of a 100-hour evaluation, showing outstanding breakthrough sulfur capacity up to 4163 mg/gcat. This performance exceeds that of most recently reported catalysts. The possible catalytic mechanism for H2S-to-S selective oxidation over the NS-HTFO catalyst has also been investigated.

Designed functions of oxide/hydroxide nanosheets via elemental replacement/doping.

Partial replacement of one structural element in a solid with another of a similar size was conducted to impart functionality to the solids and modify their properties. This phenomenon is found in nature in coloured gemstones and clay minerals and is used in materials chemistry and physics, endowing materials with useful properties that can be controlled by incorporated heteroelements and their amounts. Depending on the area of research (or expected functions), the replacement is referred to as "isomorphous substitution", "doping", etc. Herein, elemental replacement in two-dimensional (2D) oxides and hydroxides (nanosheets or layered materials) is summarised with emphasis on the uniqueness of their preparation, characterisation and application compared with those of the corresponding bulk materials. Among the 2D materials (graphene, metallenes, transition metal chalcogenides, metal phosphate/phosphonates, MXenes, etc.), 2D oxides and hydroxides are characterised by their presence in nature, facile synthesis and storage under ambient conditions, and possible structural variation from atomic-level nanosheets to thicker nanosheets composed of multilayered structures. The heteroelements to be doped were selected depending on the target application objectively; however, there are structural and synthetic limitations in the doping of heteroelements. In the case of layered double hydroxides (single layer) and layered alkali silicates (from single layer to multiple layers), including layered clay minerals (2 : 1 layer), the replacement (commonly called isomorphous substitution) is discussed to understand/design characteristics such as catalytic, adsorptive (including ion exchange), and swelling properties. Due to the variation in their main components, the design of layered transition metal oxide/hydroxide materials via isomorphous substitution is more versatile; in this case, tuning their band structure, doping both holes and electrons, and creating impurity levels are examined by the elemental replacement of the main components. As typical examples, material design for the photocatalytic function of an ion-exchangeable layered titanate (lepidocrocite-type titanate) and a perovskite niobate (KCa2Nb3O10) is discussed, where elemental replacement is effective in designing their multiple functions.

Intercalation-exfoliation processes during ionic exchange reactions from sodium lepidocrocite-type titanate toward a proton-based trititanate structure.

Topochemical reactions involving ionic exchange have been used to assess a large number of metastable compositions, particularly in layered metal oxides. This method encompasses complex reactions that are poorly explored, yet are of prime importance to understand and control the materials' properties. In this work, we embark on investigating the reactions involved during the ionic exchange between a layered Na-titanate (lepidocrocite-type structure) and an acidic solution (HCl), leading to a protonic (H3O+) titanate (trititanate structure). The reactions involve an ionic exchange provoking a structural change from the lepidocrocite-type to the trititanate structure as shown by real-space refinements of ex situ pair distribution function data. Mobile Na+ ions are exchanged by hydronium ions inducing high proton mobility in the final structure. Moreover, the reaction was followed by ex situ23Na and 1H solid-state MAS NMR which allowed, among other things, confirming that the Na+ ions are in the interlayer space and specifying their local environment. Strikingly, the ionic exchange reaction induces progressive exfoliation of the Na-titanate particles leading to 2-5 nm thin elongated crystallites. To further understand the different steps associated with the ionic exchange, the evolution of the electrolytic conductivity, using conductimetric titration, has been monitored upon HCl addition, enabling characterization of the intercalation(H+)/de-intercalation(Na+) reactions and assessing kinetic parameters. Accordingly, it is hypothesized that the exfoliation of the particles is due to the accumulation of charges at the particle level in relation to the rapid intercalation of protons. This work provides novel insights into ionic exchange reactions involved in layered oxide compounds.

Enhancing Visible Light Absorption of Yellow-Colored Peroxo-Titanate Nanotubes Prepared Using Peroxo Titanium Complex Ions.

Visible light-activated yellow titanate nanotubes (TNTs) modified by peroxo groups were directly synthesized via a facile chemical reaction route using peroxo titanium complex ions as a precursor. Obtained peroxo-modified TNTs (PTNTs) possessed a cylindrical-shaped tubular morphology with an outer diameter of approximately 10 nm. The peroxo titanium functional group (Ti–O–O) was formed between the interlayers of the lepidocrocite-type titanate crystal that was the base structure of TNTs, with the interlayer distance estimated at approximately 10.02 Å. The formation of the peroxo functional groups reduced the electron density adjacent to the titanium atom, raising the valence band to 1.35 eV and forming a band gap of 2.50 eV, which is lower than that of TNTs (3.19 eV). In addition, the peroxo titanium functional group had a negative potential, which increased the chemical adsorption performances with positively charged rhodamine B molecules in water. Meanwhile, the photocatalytic investigation indicated that the PTNTs have enhanced the photocatalytic performance for RhB decolorization under visible light irradiating in comparison with TNTs. These findings show not only the improvement in the photocatalytic performance but also the potential of processing design by selecting the precursor with arbitrary characteristics.

Hydrothermal synthesis of KTi2(PO4)3, α-Ti(HPO4)2·H2O and γ-Ti(PO4)(H2PO4)·2H2O from a lepidocrocite-type titanate

ABSTRACTA potassium titanium phosphate, KTi2(PO4)3, and two types of hydrated titanium phosphates, α-Ti(HPO4)2·H2O and γ-Ti(PO4)(H2PO4)·2H2O, were prepared by hydrothermal reaction using a layered lepidocrocite-type titanate as a starting compound. When layered lepidocrocite-type K0.7(Li,Ti)2O4 (Lss; Ohtsuka Chemical Co., Ltd.) was used at a reaction temperature of 180°C, KTi2(PO4)3 was obtained. The crystal structure of KTi2(PO4)3 was a NASICON-like rhombohedral one with a low thermal expansion coefficient. The thermal expansion coefficient of KTi2(PO4)3 was determined using synchrotron X-ray powder diffraction data (SRXD), and the thermal expansion coefficients for the a-axis, c-axis and unit cell volume were −2.82 x 10−6, 7.16 × 10−6 and 1.49 × 10−6 K−1, respectively. Two types of layered titanium phosphates, α-Ti(HPO4)2·H2O and γ-Ti(PO4)(H2PO4)·2H2O, were prepared by hydrothermal reaction using protonated titanate (Lss-H) derived from lepidocrocite-type potassium titanate (Lss) by the ion-exchange reaction.

Lepidocrocite-Type Titanate Formation from Isostructural Prestructures under Hydrothermal Reactions: Observation by Synchrotron X-ray Total Scattering Analyses.

The formation of titanium dioxides, such as rutile and anatase, is known to proceed through the formation of a lepidocrocite-type layered structure under hydrothermal conditions, but the nucleation of this intermediate is still not understood well. Here, the nucleation of lepidocrocite-type layered titanates under hydrothermal conditions is observed by tracking the structural changes by in situ time-resolved pair distribution function analyses. We found that titanate clusters or corrugated layered prestructures having <1 nm domains with lepidocrocite-type connectivity were formed even before thermal treatment in alkaline aqueous solution. Upon thermal treatment, a two-dimensional layered structure grew directly from the prestructure, not from the amorphous polymeric hydroxide dissolved in the solution. Thus, we conclude that the formation of the lepidocrocite-like prestructure is the key for forming a layered titanate under hydrothermal conditions.

Layered titanate hierarchical spheres as a promising pseudocapacitive electrode material for high rate lithium ion batteries

Layered titanates with large interlayer spacing (ca. 0.8–0.9 nm) facilitate Li+ movement with less resistance, and are considered suitable anode materials for high power lithium ion batteries (LIBs). Herein, lepidocrocite-type titanate hierarchical spheres (W-THSs) assembled by nanowires are synthesized by supercritical hydrothermal treatment of sodium peroxotitanate complex solution and subsequent acid washing. Nanowire components in the W-THSs interpenetrate each other to form a mesoporous network with high specific surface area (183.6 m2 g−1), which provides electrolyte penetration channels, enlarged electrode/electrolyte contact area, and short charge transport path, favoring fast Li+ insertion/extraction. Consequently, W-THSs exhibits an excellent electrochemical performance, giving a high discharge capacity of 123.6 mAh g−1 even at 5.0 A g−1, and still retaining 180.8 mAh g−1 after 500 cycles at 1.0 A g−1. The rate capability of W-THSs is much superior to the compared anatase TiO2 hierarchical spheres (AHSs) obtained by calcining W-THSs in the air. Moreover, cyclic voltammetry analyses reveal that a pseudocapacitive process occurs during the Li+ storage in W-THSs, significantly contributing to the excellent rate capability.

Insights into Li+, Na+, and K+ Intercalation in Lepidocrocite-Type Layered TiO2 Structures

A lamellar lepidocrocite-type titanate structure with ∼25% Ti4+ vacancies was recently synthesized, and it showed potential for use as an electrode in rechargeable lithium-ion batteries. In addition to lithium, we explore this material’s ability to accommodate other monovalent ions with greater natural abundance (e.g., sodium and potassium) in order to develop lower-cost alternatives to lithium-ion batteries constructed from more widely available elements. Galvanostatic discharge/charge curves for the lepidocrocite material indicate that increasing the ionic radius of the monovalent ion results in a deteriorating performance of the electrode. Using first-principles electronic structure calculations, we identify the relaxed geometries of the structure while varying the placement of the ion in the structure. We then use these geometries to compute the energy of formations. Additionally, we determine that all ions are favorable in the structure, but interlayer positions are preferred compared to vacancy posi...

Electrochemical Performance of C-Centered Lepidocrocite-Type Titanate for Na-Ion Battery

After the discovery of electrochemical Na+ insertion into non-graphitizable carbon[1], the hard carbon has been recognized as the most promising candidate of a negative electrode material for sodium ion batteries. On another research front, various titanium oxides and titanates including TiO2[2], spinel-type Li[Li1/3Ti5/3]O4[3], NASICON-type NaTi2(PO4)3[4] and Na2Ti3O7[5] have also attracted much attention owing to its average redox potential where metal dendrite formation would not occur. Among them, Na2Ti3O7 delivers 177 mAh g-1 at first deinsertion with approximately 50% in coulombic efficiency, and its insertion potential of 0.3 V possibly realizes a safer practical Na-ion battery system compared to hard carbon. However, the specific capacity of titanium-based electrode is generally much lower than that of hard carbon, which causes consistent drawback to bring them into first choice. Recently, as a new titanium-based negative electrode candidate, Doeff and her colleagues have reported lepidocrocite-type titanate with a primitive lattice (P-type) and proposed Na0.8[Ti1.73Li0.27]O2･nH2O with a bilayer arrangement of water[6]. The material has a large theoretical capacity of 192 mAh g-1 and delivers more than 140 mAh g-1 of reversible capacity. In this study, since there is a possibility that the presence of interlayer water disturbs insertion of Na+ ions, and P-type Na0.8[Ti1.73Li0.27]O2･nH2O transforms into Na0.8[Ti1.73Li0.27]O2 with a C-centered lattice (C-type) as removal of interlayer water by drying, we synthesize C-type Na0.8[Ti1.73Li0.27]O2 to investigate the electrochemical performance and reaction mechanism for the first time. First, P-type Na0.9[Ti1.7Li0.3]O2･nH2O was prepared by ionic-exchange from K0.9[Ti1.7Li0.3]O2 with 5 M NaCl solution, and then the obtained white powder was dried at 80 to 500 ºC. The as-prepared materials were characterized by XRD, SEM, TG-DTA and their electrochemical property was measured in Na system. The working electrodes, consisting of Na0.9[Ti1.7Li0.3]O2, acetylene black and binder with the mass ratio of 80:10:5, were prepared with N-methylprrolidone and the obtained slurry was casted on a Al foil current collector. After drying the electrodes at 80 ºC in vacuum for overnight, 2023-type coin cells were assembled in a Ar-filled glove-box with 1 mol m-3 NaPF6 in PC against sodium metal. The charge and discharge measurements were carried out at room temperature at a current density of 10 mA g-1 in the voltage range between 2.0 and 0.1 V vs. Na/Na+. From XRD pattern of the synthesized sample, C-type Na0.9[Ti1.7Li0.3]O2 with lattice parameters of a = 3.7867(2), b = 14.4096(8) and c = 2.96935(19) Å was obtained. As shown in Fig.1(a) and (b), C-type Na0.9[Ti1.7Li0.3]O2 shows first reversible capacity of 145 mAh g-1 with nearly 80% in coulombic efficiency, and demonstrates excellent capacity retention at the 40th cycle. In addition, the reversible capacity even at 10C rate reached to 81% of that with C/17.5 (= 10 mA g-1) (Fig. 2). Detailed results of electrochemical performances and its charge and discharge mechanism will be discussed in the conference.

First Principles Investigation of Sodium Intercalation Mechanisms into Corrugated Titanate Structures for Sodium-Ion Battery Anodes

Sodium titanates have emerged as attractive anode materials for sodium ion batteries due to their ability to reversibly intercalate sodium ions at low voltages, while exhibiting a rich compositional range with varying crystal structures. In this work we investigate two types of titanates that have been recently synthesized and tested exhibiting similar sloping voltage profiles. The first set of materials is based on Na1+xTi3O6(OH)×2H2O, also known as sodium nonatitanate1,2 (NNT), and the second is based on a family of lepidocrocite structures such as Na0.8Ti1.73Li0.27O4 and Na0.8Ti1.4Mg0.6O4 3. Lepidocrocite titanates consist of corrugated metal oxide layers between which large cations such as potassium or sodium and, optionally, water are inserted. The layers can be stacked in-phase (P-type) or anti-phase (C-type) with one another. NNT shares some structural features with the lepidocrocite type titanates, having a stepped layered structure consisting of edge-sharing Ti6O14 units linked by corners to form steps, with sodium ions and water between the layers. A dehydration process irreversibly removes the water while maintaining the gross structural features of NNT. Using density functional theory (DFT) we investigated the structural changes occurring during sodium intercalation, and calculated voltage profiles for the different materials. We also calculated changes to the sodium diffusion energy barriers at different sodium concentrations. Structural results indicate that sodium intercalation is a site-limited process, with the NNT having higher capacity for sodium insertion compared to the lepidocrocite structures. Our results also indicate that the layers in the lepidocrocite Na0.8Ti1.73Li0.27O4 structure in space group Cmmm are able to accommodate more sodium cations by shifting layers, compared to the same composition in space group Pmmm. This is corroborated by the experimental evidence on samples with the different stacking arrangements, prepared by different heating regimes. The ability for layers to shift is also attributed to the presence of the lower valence ions Li or Mg in the metal oxide layers, which have different abilities to transition from the titanium layer to sodium sites, depending on charge density. The energy landscape for the lepidocrocite structure indicates strong M-M interactions with secondary ions such as Li or Mg preferring to distribute among different titanium layers. The presence of lower valence cations also reduces electrostatic repulsion forces leading to sodium binding energies between 0.2-0.3 eV, which dominate sodium diffusion pathways in the lepidocrocite structure. Sodium diffusion is also found to have preferential orientation perpendicular to the corrugated titanium layers. Energy barriers are also examined at different sodium concentrations in order to determine changes at different states of charge. References Shirpour M., Cabana J., Doeff M. “New materials based on a layered sodium titanate for dual electrochemical Na and Li intercalation systems”, Energy Environ. Sci., 6, 2538 (2013).Seshadri D., Shirpour M., Doeff M. “Electrochemical Properties of Electrodes Derived from NaTi3O6OH·2H2O in Sodium and Lithium Cells” J. Electrochem. Soc. 2015Vol 162, issue 1, A52-A59.Shirpour M., Cabana J., Doeff M. “Lepidocrocite-type Layered Titanate Structures: New Lithium and Sodium Ion Intercalation Anode Materials.” Chemistry of Materials 2014 26 (8), 2502-2512.

The swelling transition of lepidocrocite-type protonated layered titanates into anatase under hydrothermal treatment

The common facets of anatase crystals are the (001) and (101) planes. However, the phase transformation from lepidocrocite-type titanate into anatase by hydrothermal processing yields an anatase microstructure with high concentration of exposed (010) planes. The phase transformation of a lepidocrocite-type protonated layered titanate (HTO) into anatase was studied using XRD, TEM, FTIR, and measurement of pH and zeta potential. It was found that HTO is proton-deficient. The phase transformation process begins after uptake of a sufficient number of protons into the lepidocrocite-type structure. With the uptake of protons new hydroxyl groups form on the internal surfaces of the layered titanate and result in a bilayer state of HTO. The phase transformation reaction is a topotactic dehydration reaction in which anatase forms and water is expelled by syneresis.

Lepidocrocite-type Layered Titanate Structures: New Lithium and Sodium Ion Intercalation Anode Materials

The electrochemical characteristics of lepidocrocite-type titanates derived from K0.8Ti1.73Li0.27O4 are presented for the first time. By exchanging sodium ions for potassium, the practical specific capacity of the titanate in both sodium and lithium half cells is considerably enhanced. Although the gross structural features of the titanate framework are maintained during the ion exchange process, the symmetry changes because sodium occupies different sites from potassium. The smaller size of the sodium ion as compared to potassium and the change in site symmetry allow more alkali metal cations to be inserted reversibly into the structure during discharge in sodium and lithium cells than in the parent compound. Insertion of lithium cations takes place at an average of about 0.8 V vs Li+/Li while sodium intercalation occurs at 0.5 V vs Na+/Na, with sloping voltage profiles exhibited for both cell configurations, implying single-phase processes. Ex situ synchrotron X-ray diffraction measurements show that a lithiated lepidocrocite is formed during discharge in lithium cells, which undergoes further lithium insertion with almost no volume change. In sodium cells, insertion of sodium initially causes an overall expansion of about 12% in the b lattice parameter, but reversible uptake of solvent minimizes changes upon further cycling. In the case of the sodium cells, both the practical capacity and the cyclability are improved when a more compliant binder (polyacrylic acid) that can accommodate volume changes associated with insertion processes is used in place of the more common polyvinylidene fluoride. The ability to tune the electrochemical properties of lepidocrocite titanate structures by varying compositions and utilizing ion exchange processes make them especially versatile anode materials for both lithium and sodium ion battery configurations.

Osmotic Swelling of Layered Compounds as a Route to Producing High-Quality Two-Dimensional Materials. A Comparative Study of Tetramethylammonium versus Tetrabutylammonium Cation in a Lepidocrocite-type Titanate

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.

Sorption of Nuclear Waste Components by Layered Hydrazinium Titanate: a Straightforward Route to Durable Ceramic Forms

ABSTRACTLayered hydrazinium titanate LHT-9, (N2H5)1/2Ti1.87O4 is a new nanohybrid material related to lepidocrocite-type titanates. Unique combination of ion exchange, reductive properties, surface activity due to Brønsted acid sites and occurrence of surface titanyl groups allows exploring LHT-9 for simultaneous uptake of almost all components of liquid nuclear wastes. LHT-9 irreversibly removes technetium, molybdenum, palladium and selenium from their aqueous solutions by specific mechanism of reductive adsorption. For removal of cesium, strontium, transition elements, actinides and lanthanides LHT-9 provides mechanisms of ion exchange and surface complexation. Products of adsorption are nanocrystalline and homogeneous powders loaded with 5 to 15 wt. % of radionuclides and non-radioactive elements. LHT-9 can be applied as ready-to-use precursor for one-step synthesis of durable titanate ceramic waste forms similar to SYNROC. An essential advantage of LHT-9 in comparison with other titanate sorbents (monosodium titanate and peroxo-titanate materials) is the absence of Na in its composition that permits arbitrary tailoring of sorbent properties by simple pre-treatment with the desired elements. Results on sorption of americium, cesium, strontium and lanthanides by LHT-9 are discussed.

Layered Hydrazinium Titanate: Advanced Reductive Adsorbent and Chemical Toolkit for Design of Titanium Dioxide Nanomaterials

LHT-9, a layered hydrazinium titanate with an interlayer spacing of ~9 Å, is a new nanohybrid compound combining the redox functionality of hydrazine, the ion-exchange properties of layered titanate, the large surface area of quasi-two-dimensional crystallites, surface Brønsted acidity, and the occurrence of surface titanyl bonds. LHT-9, ideally formulated as (N(2)H(5))(1/2)Ti(1.87)O(4), relates to a family of lepidocrocite-type titanates. It possesses a high uptake capacity of ~50 elements of the periodic table. Irreversibility of reductive adsorption allows LHT-9 to be used for cumulative extraction of reducible moieties (noble metals, chromate, mercury, etc.) from industrial solutions and wastewaters. Unlike sodium titanates that do not tolerate an acidic environment, LHT-9 is capable of uptake of transition metals and lanthanides at pH > 3. Adsorption products loaded with the desired elements retain their layered structures and can be used as precursors for tailored titanium dioxide nanomaterials. In this respect, the uptake of metal ions by LHT-9 can be considered as a method complementary to electrostatic self-assembly deposition (ESD) and layer-by-layer self-assembly (LBL) techniques. LHT-9 is readily synthesized in one step by a mild fluoride route involving hydrazine-induced hydrolysis of hexafluorotitanic acid under near-ambient conditions.

Formation Efficiency of One-Dimensional Nanostructured Titanium Oxide Affected by the Structure and Composition of Titanate Precursor: A Mechanism Study

The formation mechanism and atomic arrangement of one-dimensional (1D) nanostructured titanates have been systematically investigated with combinative spectroscopic and microscopic analyses. First of all, the influence of the interlayer distances of the lepidocrocite-type titanate precursor on the formation efficiency of 1D nanobelts has been studied, clearly demonstrating that the increase of basal spacing upon the protonation does not promote the formation of 1D nanostructures. This finding did not support the previously proposed exfoliation mechanism of the layered titanate precursor. Of special interest is that the poor efficiency of the protonated titanate as a precursor could be enhanced by Fe substitution, underscoring the importance of precursor dissolution. According to time-dependent electron microscopic analyses, the hydrothermal alkali treatment initially gave rise not only to a decrease in the particle size of the precursor but also to the formation of 1D titanate nanobelts with lepidocrocite...

Synthesis of Diamond-Shape Titanate Molecular Sheets with Different Sizes and Realization of Quantum Confinement Effect during Dimensionality Reduction from Two to Zero

Synthesis of semiconductor nanoparticles with uniform shapes, sizes, and compositions in series with a gradual size reduction has not been achieved for two-dimensional molecular sheets. We report a large-scale (>2.6 g) synthesis of 0.75-nm-thick diamond-shape lepidocrocite-type titanate molecular sheets with the sizes decreasing from (27.3, 19.1) to (7.7, 5.5), where the numbers in parentheses represent the long and short diagonal lengths, respectively, in nm. This is the first example of synthesizing semiconductor nanoparticles in series with the dimensionality reduction from two to zero, without coating the surfaces with surface-passivating ligands. The titanate molecular sheets showed three exciton-absorption bands in the 4.0-6.5 eV region, the absorption energies of which increased with decreasing the area. Contrary to the common belief, the per-unit cell oscillator strengths gradually increased with increasing area and the per-particle oscillator strengths increased in proportion to the area. The average reduced exciton masses along the two diagonal axes were 0.10 and 0.11 m e, respectively, which were much smaller than those of bulk titanates (by 60-130 times). The estimated average Bohr radii along the two-diagonal axes were 4.8 and 4.3 nm, respectively.

Nanotube Formation from a Sodium Titanate Powder via Low-Temperature Acid Treatment

Through structure-monitoring of nanotube formation from a lamellar sodium titanate, the present work explicitly elucidated the structure of the titanate nanotubes obtained from hydrothermal treatment of TiO(2) with NaOH. A new compound of an orthorhombic lepidocrocite-type sodium titanate was synthesized from calcination of a solid-state mixture of TiO(2) anatase and Na(2)CO(3) powders followed by hydrothermal treatment with NaOH. By treating with acid at 25 degrees C for Na(+) exchange with H(3)O(+), the titanate compound exfoliated and then proceeded with sheet-scrolling to form nanotubes, which had a structure and morphology very close to those of the nanotubes derived from NaOH treatment on TiO(2). During the low-temperature acid treatment, the lepidocrocite-type titanate is transformed from the orthorhombic C-base-centered symmetry to the body-centered symmetry. This transformation, accompanied by a size-contraction of TiO(6)-octahedron units, was critical for the formation of nanotubes. The present work provides direct evidence, for the first time, that the widely reported TiO(2)-derived titanate nanotubes can be obtained at low temperatures by scrolling the sheets exfoliated from the orthorhombic lepidocrocite-type titanate.

Nanodiffraction and Characterization of Titanate Nanotube Prepared by Hydrothermal Method

ABSTRACT Titanate nanotubes were synthesized by simple hydrothermal treatment of TiO2 anatase powders. Their crystal structure was conclusively determined by TEM observation, especially by nanodiffraction mode. As a result, the nanotube has lepidocrocite-type titanate based structure.

Electric Dichroism Studies on an Aqueous Dispersion of Unilamellar Titanium Oxides: Optical Anisotropy near the Absorption Edge

Electric dichroism measurements have been performed on an aqueous dispersion of unilamellar titanium oxides produced by the exfoliation of the following three kinds of titanium oxides: (A) lepidocrocite-type titanate (Cs0.7Ti1.825□0.175O4), (B) tetratitanate (K2Ti4O9), and (C) pentatitanate (Cs2Ti5O11). The AFM images showed that these exfoliated titanium oxides consisted of flakelike particles with an average diameter ranging from 0.1 to 2 μm. Under the applied voltage pulse [(1−20) × 103 V cm-1], an aqueous dispersion of titanium oxides exhibited orientational dichroism in the UV region with a maximum around 300 nm. From the reduced linear dichroism at complete orientation, the orientation of the effective transition moment (μeff) was determined with respect to the oriented axis of a particle. Here μeff was defined as the vector sum of three orthogonal transition moments of a particle. As a result, an angle (α) between μeff and the oriented axis of a particle was determined to be α = 41.6°, 24.6°, and 23.6° at 300 nm for samples A−C, respectively. The results were compared with the recent theoretical calculations of these samples. Electric dichroism was also measured for an aqueous dispersion containing sample A and a cationic molecule, [Ru(phen)3]2+. From the reduced linear dichroism in the visible region, an [Ru(phen)3]2+ ion was concluded to be adsorbed with its 3-fold symmetry axis nearly perpendicular to a surface.