Layered Titanate Research Articles

Photocatalytic Activity and Stability of Organically Modified Layered Perovskite-like Titanates HLnTiO4 (Ln = La, Nd) in the Reaction of Hydrogen Evolution from Aqueous Methanol

Two series of hybrid inorganic–organic materials, prepared via interlayer organic modification of protonated Ruddlesden–Popper phases HLnTiO4 (Ln = La, Nd) with n-alkylamines and n-alkoxy groups of various lengths, have been systematically studied with respect to photocatalytic hydrogen evolution from aqueous methanol under near-ultraviolet irradiation for the first time. Photocatalytic measurements were organized in such a way as to control a wide range of parameters, including the hydrogen generation rate, quantum efficiency of the reaction, potential dark activity of the sample, its actual volume concentration in the suspension, pH of the medium and stability of the photocatalytic material under the operating conditions. The insertion of the organic modifiers into the interlayer space of the titanates allowed obtaining new, more efficient photocatalytic materials, being up to 68 and 29 times superior in the activity in comparison with the initial unmodified compounds HLnTiO4 and a reference photocatalyst TiO2 P25 Degussa, respectively. The hydrogen evolution rate over the samples correlates with the extent of their interlayer hydration, as in the case of the inorganic–organic derivatives of other layered perovskites reported earlier. However, the HLnTiO4-based samples demonstrate increased stability with regard to the photodegradation of the interlayer organic components as compared with related H2Ln2Ti3O10-based hybrid materials.

DFT-guided flux synthesis of a family of layered titanates crystallizing in the lepidocrocite structure type

K0.72Fe0.72Ti1.28O4, a layered mixed-metal oxide belonging to the lepidocrocite structure type, was grown out of a molten KCl-KF mixture at 800 °C. The compound crystallizes in orthorhombic space group Cmcm and exhibits two-dimensional layers, composed of mixed Fe/Ti octahedra, separated by K ions that occupy the interlayer layer space and maintain charge balance. The targeted flux synthesis of the Ni, Cu, and Zn analogs was carried out after density functional theory (DFT) calculations predicted their formation to be energetically favorable, yielding K0.77Ni0.38Ti1.62O4, K0.77Cu0.38Ti1.62O4, and K0.73Zn0.36Ti1.64O4. All four compositions can be prepared via conventional solid-state technique by mixing and heating stoichiometric amounts of KNO3, appropriate transition metal oxide precursors, and TiO2.

KOH-Based Hydrothermal Synthesis of Iron-Rich Titanate Nanosheets Assembled into 3D Hierarchical Architectures from Natural Ilmenite Mineral Sands

The synthesis of titanate nanostructures from low-cost mineral precursors is a topic of continuous interest, considering not only their fundamental aspects but also the benefits of incorporating such nanomaterials in a wide variety of applications. In this work, iron-rich titanate nanosheets were synthesized from Ecuadorian ilmenite sands (ilmenite–hematite solid solution-IHSS) through an alkaline hydrothermal treatment (AHT) using potassium hydroxide (KOH). The effect of the duration of the KOH-AHT was assessed at 180 °C for 24, 48, 72, and 96 h. The morphology evolution over time and the plausible formation mechanisms of titanate nanostructures were discussed. The most significant morphological transformation was observed after 72 h. At this time interval, the titanate nanostructures were assembled into well-defined 3D hierarchical architectures such as book-block-like arrangements with open channels. Based on X-ray powder diffraction (XRPD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS) analyses, it was determined that these nanostructures correspond to iron-rich layered titanates (Fe/Ti mass ratio of 7.1). Moreover, it was evidenced that the conversion of the precursor into layered nanostructures was not complete, since for all the tested reaction times the presence of remaining IHSS was identified. Our experiments demonstrated that the Ecuadorian ilmenite sands are relatively stable in KOH medium.

Single-Atom Sn-Loaded Exfoliated Layered Titanate Revealing Enhanced Photocatalytic Activity in Hydrogen Generation.

Green H2 generation through layered materials plays a significant role among a wide variety of materials owing to their high theoretical surface area and distinctive features in (photo)catalysis. Layered titanates (LTs) are a class of these materials, but they suffer from large bandgaps and a layers' stacked form. We first address the successful exfoliation of bulk LT to exfoliated few-layer sheets via long-term dilute HCl treatment at room temperature without any organic exfoliating agents. Then, we demonstrate a substantial photocatalytic activity enhancement through the loading of Sn single atoms on exfoliated LTs (K0.8Ti1.73Li0.27O4). Comprehensive analysis, including time-resolved photoluminescence spectroscopy, revealed the modification of electronic and physical properties of the exfoliated layered titanate for better solar photocatalysis. Upon treating the exfoliated titanate in SnCl2 solution, a Sn single atom was successfully loaded on the exfoliated titanate, which was characterized by spectroscopic and microscopic techniques, including aberration-corrected transmission electron microscopy. The exfoliated titanate with an optimal Sn loading exhibited a good photocatalytic H2 evolution from water containing methanol and from ammonia borane (AB) dehydrogenation, which was not only enhanced from the pristine LT, but higher than conventional TiO2-based photocatalysts like Au-loaded P25.

Altering the Alkaline Metal Ions in Lepidocrocite-Type Layered Titanate for Sodium-Ion Batteries

The relatively large ionic radius of the Na ion is one of the primary reasons for the slow diffusion of Na ions compared to that of Li ions in de/intercalation processes in sodium-ion batteries (SIBs). Interlayer expansion of intercalation hosts is one of the effective techniques for facilitating Na-ion diffusion. For most ionic layered compounds, interlayer expansion relies on intercalation of guest ions. It is important to investigate the role of these ions for material development of SIBs. In this study, alkali-metal ions (Li+, Na+, K+, and Cs+) with different sizes were intercalated into lepidocrocite-type layered titanate by a simple ion-exchange technique to achieve interlayer modulation and those were then evaluated as anode materials for SIBs. By controlling the intercalated alkaline ion species, basal spacings of layered titanates (LTs) in the range of 0.68 to 0.85 nm were obtained. Interestingly, the largest interlayer spacing induced by the large size of Cs did not yield the best performance, while the Na intercalated layered titanate (Na-ILT) demonstrated a superior performance with a specific capacity of 153 mAh g-1 at a current density of 0.1 A g-1. We found that the phenomena can be explained by the high alkaline metal ion concentration and the efficient utilization of the active sites in Na-ILT. The detailed analysis indicates that large intercalating ions like Cs can hamper sodium-ion diffusion although the interlayer spacing is large. Our work suggests that adopting an appropriate interlayer ion species is key to developing highly efficient layered electrode materials for SIBs.

Layered sodium titanate with a matched lattice: a single ion conductor in a solid-state sodium metal battery

The layered Na0.98Ti1.3O3 (NTO) sheet is matched with Na in the lattice, which behaves like a single ion conductor with a high ion transference number due to the weak interactions between the lamellar Na+ ions and unmoved anionic Ti–O–Ti layers in NTO.

Defect formation and ambivalent effects on electrochemical performance in layered sodium titanate Na2Ti3O7.

Point defects can be formed readily in layered transition metal oxides used as electrode materials for alkali-ion batteries but their influence on the electrode performance is yet obscure. In this work, we report a systematic first-principles study of intrinsic point defects and defect complexes in sodium titanate Na2Ti3O7, a low-voltage anode material for sodium-ion batteries. Within the density functional theory framework, we calculate the defect formation energies with a set of atomic chemical potentials, which define the synthesis conditions for the stable Na2Ti3O7 compound. Given the atomic chemical potential landscape and defect formation energies, we find that Na interstitials (Nai+), Na antisites (NaTi3-), and Na vacancies (VNa-) are dominant defects depending on the synthesis conditions. Furthermore, our calculations reveal that O vacancies (VO) and Ti antisites (TiNa) lower the electrode potential compared with the perfect system, whereas Ti vacancies (VTi) and NaTi increase the voltage. Finally, we evaluate the activation barriers for vacancy-mediated Na diffusion in the defective systems, finding that the intrinsic point defects improve the Na ion conduction. Our results provide a profound understanding of defect formation and influences on electrode performance, paving a way to designing high-performance anode materials.

Efficient p-n Heterojunction Photocatalyst Composed of Bismuth Oxyiodide and Layered Titanate.

Bismuth oxyhalides and layered alkali titanates are promising components to design high-performance hybrid photocatalysts. In this work, a hybrid photocatalyst composed of lepidocrocite-type layered cesium titanate (Cs0.7Ti1.77Li0.23O4, CsTLO) and bismuth oxyiodide (BiOI) was designed rationally based on lattice matching. BiOI formed on the layered titanate by ion exchange of CsTLO with Bi cations and subsequent growth of BiOI nanodisks (6 nm in the thickness and 125 nm in the lateral size) in an aqueous solution of cesium iodide, resulting in the hybrid where BiOI nanodisks were lying flat on the layered titanate and exposed the (001) facet predominantly. The present hybrid exhibited efficient photodegradation of methylene blue (4, 10, and 14 times higher than that of CsTLO, Bi-TLO, and BiOI, respectively), which was ascribed to the efficient charge transfer in the bulk and at the interface assisted by the built-in internal electric fields and the high activity of (001) BiOI for direct oxidation of the pollutant.

Photocatalytic Hydrogen Generation from Aqueous Methanol Solution over n-Butylamine-Intercalated Layered Titanate H2La2Ti3O10: Activity and Stability of the Hybrid Photocatalyst

The stability of platinized n-butylamine-intercalated layered titanate H2La2Ti3O10 during the process of photocatalytic hydrogen production from aqueous methanol under UV irradiation has been thoroughly investigated by means of XRD, CHN, TG, 13C NMR, BET, SEM and GC-MS analysis. It was revealed that n-butylamine completely abandons the interlayer space and transforms into n-butyraldehyde within 3 h of the reaction, while the particle morphology and specific surface area of the photocatalyst are preserved. The resulting solid phase contains carbon in at least two different oxidation states, which are attributed to the intermediate products of methanol oxidation bound to the perovskite matrix. The activity of the photocatalyst formed in this way is stable in time and strongly depends on the medium pH, which is not typical of either the parent H2La2Ti3O10 or TiO2. An approximate linear equation φ ≈ 29−2∙pH holds for the apparent quantum efficiency of hydrogen production in the 220–340 nm range at 1 mol. % methanol concentration. In the acidic medium, the photocatalyst under study outperforms the platinized H2La2Ti3O10 by more than one order of magnitude. The variation in methanol concentration allowed a maximum quantum efficiency of hydrogen production of 44% at 10 mol. % to be reached.

Preparation of layered titanate nanosheets and the application for Cs+ adsorption from wastewater, effluents and a simulated brine

Developing a stable and efficient adsorbent to recover Cs+ from liquid resources or to remove radioactive Cs+ from wastewater is challenging. In this study, the precursor Cs2Ti6O13 (CTO) was synthesized by the solvothermal and solid-phase sintering methods for the first time, then it was acid-modified to synthesize a novel H-type titanium-based material (H-CTO nanosheets). A series of batch adsorption experiments was carried out. The adsorption of Cs+ on H-CTO was evaluated under different experimental conditions, including contact time, temperature, initial Cs+ concentration, pH, and interfering ions. Experiments show that H-CTO exhibits a larger adsorption capacity (329 mg/g) and better cycle performance than the currently reported titanium-based adsorbents. The cesium ions were easily eluted by the 0.2 M HCl solution; the adsorption capacity did not decrease significantly after five cycles. Furthermore, the adsorption behavior of Cs+ on H-CTO can be described by the Langmuir isotherm and pseudo-second-order kinetic models. Characterization using IR and Raman spectroscopic analyses indicates that the adsorption mechanism of Cs+ on H-CTO involves OH bond cleavage and OCs bond formation. Thermodynamic studies showed that low temperatures are favorable for Cs+ adsorption and are spontaneous. Overall, the results suggest that H-CTO nanosheets are promising for the recovery or removal of Cs+ ions from Cs-containing liquid ores or radioactive wastewater.

Stabilization of Dinuclear Rhodium and Iridium Clusters on Layered Titanate and Niobate Supports.

Atomically dispersed organometallic clusters can provide well-defined nuclearity of active sites for both fundamental studies as well as new regimes of activity and selectivity in chemical transformations. More recently, dinuclear clusters adsorbed onto solid surfaces have shown novel catalytic properties resulting from the synergistic effect of two metal centers to anchor different reactant species. Difficulty in synthesizing, stabilizing, and characterizing isolated atoms and clusters without agglomeration challenges allocating catalytic performance to atomic structure. Here, we explore the stability of dinuclear rhodium and iridium clusters adsorbed onto layered titanate and niobate supports using molecular precursors. Both systems maintain their nuclearity when characterized using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Statistical analysis of HAADF-STEM images revealed that rhodium and iridium dimers had mean cluster-to-cluster distances very similar to what is expected from a random distribution of atoms over a large area, indicating that they are dispersed without aggregation. The stability of dinuclear rhodium clusters supported on titanate nanosheets was also investigated by X-ray absorption fine structure (EXAFS), DRIFTS, and first-principles calculations. Both X-ray absorption spectroscopy and HAADF-STEM simulations, guided by density functional theory (DFT)-optimized structure models, suggested that rhodium dimers adsorb onto the nanosheets in an end-on binding mode that is stable up to 100 °C under reducing conditions. This study highlights that crystalline nanosheets derived from layered metal oxides can be used as model supports to selectively stabilize dinuclear clusters, which could have implications for heterogeneous catalysis.

Rapid exfoliation of H2O2-intercalated layered titanate and visible-photocatalytic hydrogen production performance of exfoliated nanosheet

Usually, it takes 3–7 days to exfoliate the layered titanic acid H1.07Ti1.73O4 (HTO) into Ti1.73O41.07– (TO) nanosheet. The H2O2 molecules can easily enter into the interlayers of the HTO crystal to form the H2O2 intercalated HTO. The H2O2 intercalation can achieve rapid exfoliation of HTO. It takes 1 h to exfoliate into a yellow H2O2-modified TO nanosheet, which has a wide absorption wavelength range and a narrow band gap. Moreover, such 2D nanosheet has a large specific surface area and a large number of reactive sites. And such yellow H2O2-modified TO nanosheet displays some visible photocatalytic hydrogen production performance. When the H2O2-TO nanosheet is loaded with Ni(OH)2, the sample can show a higher photocatalytic hydrogen production performance. The maximum photocatalytic hydrogen production rate of the Ni(OH)2-H2O2-TO catalyst is 32.48 μmol h−1 g−1, which is 4.1 and 1.9 times higher than that of H2O2-TO and Pt-H2O2-TO. This is due to the formation of a type II heterojunction between the H2O2-TO nanosheet and Ni(OH)2. This heterojunction can achieve the effective separation of photogenerated carriers.

Lepidocrocite Titanate–Graphene Composites for Sodium-Ion Batteries

To overcome electronic transport issues of layered titanates in sodium-ion batteries, we have designed and synthesized composites of lepidocrocite titanates with reduced graphene oxide through a solution-based self-assembly approach. The parent lepidocrocite titanate (K0.8[Ti1.73Li0.27]O4) was exfoliated by a soft-chemical approach and mechanical shaking. Exfoliated layered titania sheets (LTO) were then combined with reduced graphene oxide (rGO) layers to assemble into composites through flocculation. Countercations (i.e., Mg2+) were used for the self-assembly of negatively charged titania and rGO nanosheets via flocculation. The carbon content in the composites was tuned from 1 to 17% by changing the ratio of titania and rGO sheets in the mixed colloidal suspensions. Electrodes were processed with as-prepared LTO–rGO composites without any carbon additives and tested in sodium half-cell configurations. Mg+-coagulated LTO–rGO composite electrodes deliver higher capacities than electrodes prepared with coagulated titania sheets and 10% acetylene black in sodium half-cells and display good capacity retention after 50 cycles. Electrochemical impedance spectroscopy results indicate lower charge transfer resistance for LTO–14.5%rGO composites than that of coagulated titania sheets with 10% acetylene black. A power law analysis of cells containing the composites indicate a hybrid mechanism consisting of both surface and diffusional processes. A comparison with a similar system, that of dopamine-derived LTO-C heterostructures, reveal significant differences. While capacities showed a strong dependence on carbon content for the dopamine-derived materials, this was not true for the LTO–rGO composites. Instead, the highest capacity was obtained for the 14.5% rGO sample, with a lower value obtained for the 17% rGO sample. A greater proportion of the redox processes were surface rather than diffusional in nature for the LTO–rGO composites as well.

Ultra-hydrophilic layered titanate nanosheet-based nanofiltration membrane with ultrafast water transport for low energy consumption desalination

Nanofiltration (NF) membranes have significant applications to solve global water scarcity issues. An ultra-hydrophilic inorganic NF membrane was fabricated using ultra-hydrophilic layered titanate H1.07Ti1.73O4·nH2O nanosheets (HTO-ns), which exhibits high selectivity for desalination and small molecule separations while maintained an ultrafast water transport at low operating pressure. This membrane would be a low-energy consumption NF membrane. To fabricate the HTO-ns membrane, a straightforward technique, namely, the self-adjusting membrane thickness process (SAMTP), was developed by the restacking process of negatively charged HTO-ns with cations on a porous polytetrafluoroethylene (PTFE) substrate. A large-scaled high-quality HTO-ns membrane can be fabricated on a PTFE substrate surface-modified with a silane coupling agent of N-(2-aminoethyl)-3-aminopropyltrimethoxysilane and Cu2+ adsorption. There are two kinds of nanochannels for water transport through the membrane, namely, interlayer nanochannels and nanosheet edges chink nanochannels. The excellent separation performances were contributed by the interlayer nanochannels and Donnan exclusion effect of the negatively charged HTO-ns. The SAMTP technique developed in this study could also be applied simply to fabricate other metal oxide nanosheet membranes, which would open a new avenue to the high-performance ultra-hydrophilic inorganic nanosheet-based membranes for low energy consumption separation processes.

New layered titanate, NaTi2O3(OH)3, obtained in hydrothermal environments at gigapascal pressures

New layered titanate, NaTi₂O₃(OH)₃, obtained in hydrothermal environments at gigapascal pressures

Corrugated Layered Titanates as High-Voltage Cathode Materials for Potassium-Ion Batteries

Corrugated Layered Titanates as High-Voltage Cathode Materials for Potassium-Ion Batteries

Layered KTO/BiOCl nanostructures for the efficient visible light photocatalytic degradation of harmful dyes

Novel KTO/BiOCl nanostructured photocatalysts with various weight proportions were synthesized using a simple hydrothermal process. The as-prepared nanostructured composite catalysts were characterized by powder X-ray diffraction, Fourier transform infrared spectroscopy, UV–vis diffused reflectance spectroscopy, Raman spectroscopy, field emission scanning electron microscopy, transmission electron microscopy with high resolution, X-ray photoelectron spectroscopy, and photoluminescence (PL). The photocatalytic activity of prepared catalysts was examined using Rhodamine B (RhB) and Congo Red (CR) as the aimed pollutants. BiOCl nanoparticles were distributed uniformly on the surface of the K2Ti4O9 nanobelts. The optical properties showed that the layered titanate with BiOCl nanostructured photocatalyst displayed improved photoresponsivity due to the narrowed bandgap. The PL results showed that the greater inhibition of the electron−hole recombination process and KTO/BiOCl with a mass proportion of 20% revealed the most favorable photocatalytic behavior. The rate constant of RhB and CR degradation was five times as high as that of the bare BiOCl and titanate. The superior photocatalytic performance was attributed to the advancement of heterojunction between the KTO nanobelt and BiOCl. The KTO/BiOCl nanostructure is a promising visible, active photocatalyst, and the photocatalytic mechanism is discussed using the possible band structures of BiOCl and KTO.

Lepidocrocite-Type Layered Titanate Nanoparticles as Photocatalysts for H2 Production

Lepidocrocite-Type Layered Titanate Nanoparticles as Photocatalysts for H₂ Production

Ordering of a Nanoconfined Water Network around Zinc Ions Induces High Proton Conductivity in Layered Titanate

Ordering of a Nanoconfined Water Network around Zinc Ions Induces High Proton Conductivity in Layered Titanate

Durable CuxO/mesoporous TiO2 photocatalyst for stable and efficient hydrogen evolution.

Heterogeneous structures containing highly dispersed semiconductor nanoparticles on a photoactive support are effective for the photocatalytic hydrogen evolution reaction (HER). In this work, the interlayer ion-exchange and space confining nature of layered titanate nanosheets was used to embed copper ions in titanates, which were then transitioned to mesoporous CuxO/TiO2 with highly dispersed CuxO nanostructures. Both experimental and density functional theory (DFT) studies demonstrated that the fine-decoration of CuxO nanostructures and the reducible valence of the copper species enabled stable superior photocatalytic activity. The HER efficiency was enhanced to 12.45 mmol g-1 h-1 for the mesoporous CuxO/TiO2 composites in comparison to an efficiency of 0.38 mmol g-1 h-1 for the non-modified TiO2. Steady HER performances over 10 h, cyclic HER measurement over 60 h, and testing of the composite kept under ambient conditions for over one year, demonstrated excellent stability of the composite against photochemical and wet-chemical erosion.