Structure, Properties, Preparation, and Application of Layered Titanates

As a typical cation-exchangeable layered compound, layered titanate has a unique open layered structure. Its excellent physical and chemical properties allow its wide use in the energy, environmental protection, electronics, biology, and other fields. This paper reviews the recent progress in the research on the structure, synthesis, properties, and application of layered titanates. Various reactivities, as well as the advantages and disadvantages, of different synthetic methods are discussed. The reaction mechanism and influencing factors of the ion exchange reaction, intercalation reaction, and exfoliation reaction are analyzed. The latest research progress on layered titanates and their modified products in the fields of photocatalysis, adsorption, electrochemistry, and other applications is summarized. Finally, the future development of layered titanate and its exfoliated product two-dimensional nanosheets is proposed.

Combination of High pH and an Antioxidant Improves Chemical Stability of Two-Dimensional Transition-Metal Carbides and Carbonitrides (MXenes) in Aqueous Colloidal Solutions

MXenes, a large family of two-dimensional (2D) transition-metal carbides/nitrides, have attracted increased attention in recent years because of their excellent electronic, mechanical, thermal, and optical properties. Studying chemical properties of MXenes is important to prolong the shelf life of their colloids and provide robust performance of MXenes in devices and applications. While the role of MXene reactivity with the environment, including water and components of air, is becoming more recognized, less is known about the role of parameters influencing the reactivity. In this work, we investigate the individual and combined effects of the pH and antioxidant on chemical stability of Ti₂CT_x, Ti₃CNT_x, and Ti₃C₂T_x MXenes using GC, XPS, UV-vis, and Raman spectroscopy. In contrast to indirect indicators of MXene degradation, such as film conductivity or performance in electrochemical energy storage systems, we focus on detection of reaction products as the most sensitive and direct way of monitoring the chemical transformations of MXenes. Based on our knowledge of MXene chemistry and interactions with the environment, we propose a combination of sodium hydroxide and sodium l-ascorbate to effectively slow down degradation of MXenes in colloidal solutions by suppressing their hydrolysis and oxidation reactions, respectively.

Moss-like Hierarchical Architecture Self-Assembled by Ultrathin Na2Ti3O7 Nanotubes: Synthesis, Electrical Conductivity, and Electrochemical Performance in Sodium-Ion Batteries

Nanocrystalline layer-structured monoclinic Na₂Ti₃O₇ is currently under consideration for usage in solid state electrolyte applications or electrochemical devices, including sodium-ion batteries, fuel cells, and sensors. Herein, a facile one-pot hydrothermal synthetic procedure is developed to prepare self-assembled moss-like hierarchical porous structure constructed by ultrathin Na₂Ti₃O₇ nanotubes with an outer diameter of 6–9 nm, a wall thickness of 2–3 nm, and a length of several hundred nanometers. The phase and chemical transformations, optoelectronic, conductive, and electrochemical properties of as-prepared hierarchically-organized Na₂Ti₃O₇ nanotubes have been studied. It is established that the obtained substance possesses an electrical conductivity of 3.34 × 10⁻⁴ S/cm at room temperature allowing faster motion of charge carriers. Besides, the unique hierarchical Na₂Ti₃O₇ architecture exhibits promising cycling and rate performance as an anode material for sodium-ion batteries. In particular, after 50 charge/discharge cycles at the current loads of 50, 150, 350, and 800 mA/g, the reversible capacities of about 145, 120, 100, and 80 mA∙h/g, respectively, were achieved. Upon prolonged cycling at 350 mA/g, the capacity of approximately 95 mA∙h/g at the 200th cycle was observed with a Coulombic efficiency of almost 100% showing the retention as high as 95.0% initial storage. At last, it is found that residual water in the un-annealed nanotubular Na₂Ti₃O₇ affects its electrochemical properties.

Comparative Study of Proton Exchange in Tri- and Hexatitanates: Correlations between Stability and Electronic Properties

A hydrothermal method is considered to be convenient and is extensively used in preparing titanate architectures, but the intermediate and final products are complicated and variable. To date, it is accepted that intermediates are tri- and hexatitanates. Here, atomic structures, energetics, and correlations between stability and electronic properties of proton exchange in tri- and hexatitanates, i.e., Na_2-xH_xTi₃O₇ and Na_2-xH_xTi₆O₁₃, are investigated by first-principles calculations. We found that the bond length of Na-O bonds plays a significant role in determining the activity of tunnel oxygen atoms, while the proton substitution sites are closely related to the activity of tunnel O atom in titanates. As H⁺ concentration increases, the formation energy of Na_2-xH_xTi₃O₇ and Na_2-xH_xTi₆O₁₃ decreases first and then increases, suggesting that completely protonated titanates, i.e., H₂Ti₃O₇ and H₂Ti₆O₁₃, are unstable. However, we found that H⁺ substitution would take place even in an alkaline solution both for Na₂Ti₃O₇ and Na₂Ti₆O₁₃. With a decrease in the pH, the process of H⁺ exchange becomes more energetically favorable. Compared to Na₂Ti₃O₇, Na⁺ ions are more easily exchanged by H⁺ ions in Na₂Ti₆O₁₃ at the same pH value. We found that there is a strong correlation between stability and electronic properties during the Na⁺-H⁺ exchange process. Finally, hydrogen bonds are observed in H₂Ti₃O₇ and Na_2-xH_xTi₆O₁₃ complexes, which make them more stable than Na_2-xH_xTi₃O₇ complexes without H-bonds. All of these findings provide insight into understanding the geometry of possible intermediates in the preparation of titanates and suitable conditions for the synthesis of titanates.

Nature-Inspired Nanoflower Structures on Titanium Surface via Alkali Treatment for Biomedical Applications

Titanium based metallic biomaterials for orthopedic implant applications are often associated with biocompatibility problems which can be ameliorated via proper surface modification strategies. Improving the hydrophilic nature of the titanium surface offers an effective strategy to sort out such limitations by intensifying the cellular activity. Development of titania as well as titanate layers on the titanium surface via alkali treatment represents an effective strategy to improve the hydrophilicity of native titanium surface. Inspired from nature, in the present work, we report the formation of three-dimensional (3D) hierarchical nanoflowers resembling Gomphrena globosa flowers developed on commercially pure titanium (cp-Ti) surface via a facile alkali treatment technique. X-ray diffraction studies evidenced anatase and rutile phases of TiO2 confirming the development of titania on the surface. In addition to the TiO2 phase, presence of titanate (Na2Ti3O7) has also been observed as alkali treatment was conducted in NaOH solution. The hydrophilicity of the Ti surface has been enhanced after the alkali treatment as evidenced from wettability studies using static contact angle measurements. This increase in hydrophilicity is due to the enrichment of the surface by TiO2 and titanate and increased roughness of nanoflower surface based on classical Wenzel law. In addition, the alkali-treated surface demonstrated an increased polar surface energy beneficial for biocompatible surfaces.

Assessment of Titanate Nanolayers in Terms of Their Physicochemical and Biological Properties

The surface modification of titanium substrates and its alloys in order to improve their osseointegration properties is one of widely studied issues related to the design and production of modern orthopedic and dental implants. In this paper, we discuss the results concerning Ti6Al4V substrate surface modification by (a) alkaline treatment with a 7 M NaOH solution, and (b) production of a porous coating (anodic oxidation with the use of potential U = 5 V) and then treating its surface in the abovementioned alkaline solution. We compared the apatite-forming ability of unmodified and surface-modified titanium alloy in simulated body fluid (SBF) for 1–4 weeks. Analysis of the X-ray diffraction patterns of synthesized coatings allowed their structure characterization before and after immersing in SBF. The obtained nanolayers were studied using Raman spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), and scanning electron microscopy (SEM) images. Elemental analysis was carried out using X-ray energy dispersion spectroscopy (SEM EDX). Wettability and biointegration activity (on the basis of the degree of integration of MG-63 osteoblast-like cells, L929 fibroblasts, and adipose-derived mesenchymal stem cells cultured in vitro on the sample surface) were also evaluated. The obtained results proved that the surfaces of Ti6Al4V and Ti6Al4V covered by TiO₂ nanoporous coatings, which were modified by titanate layers, promote apatite formation in the environment of body fluids and possess optimal biointegration properties for fibroblasts and osteoblasts.

Developing highly nanoporous titanate structures via wet chemical conversion of DC magnetron sputtered titanium thin films

Titanate structures have been widely investigated as biomedical component surfaces due to their bioactive, osteoinductive and antibacterial properties. However, these surfaces are limited to Ti and its alloys, due to the nature of the chemical conversion employed. The authors present a new method for generating nanoporous titanate structures on alternative biomaterial surfaces, such as other metals/alloys, ceramics and polymers, to produce bioactive and/or antibacterial properties in a simple yet effective way. Wet chemical (NaOH; 5 M; 60 °C; 24 h) conversion of DC magnetron sputtered Ti surfaces on 316L stainless steel were investigated to explore effects of microstructure on sodium titanate conversion. It was found that the more equiaxed thin films (B/300) generated the thickest titanate structures (ca. 1.6 μm), which disagreed with the proposed hypothesis of columnar structures allowing greater NaOH ingress. All film parameters tested ultimately generated titanate structures, as confirmed via EDX, SEM, XPS, XRD, FTIR and Raman analyses. Additionally, the more columnar structures (NB/NH & B/NH) had a greater quantity of Na (ca. 26 at.%) in the top portion of the films, as confirmed via XPS, however, on average the Na content was consistent across the films (ca. 5-9 at.%). Film adhesion for the more columnar structures (ca. 42 MPa), even on polished substrates, were close to that of the FDA requirement for plasma-sprayed HA coatings (ca. 50 MPa). This study demonstrates the potential of these surfaces to be applied onto a wide variety of material types, even polymeric materials, due to the lower processing temperatures utilised, with the vision to generate bioactive and/or antibacterial properties on a plethora of bioinert materials.

Synthesis of Na2Ti3O7 nanorods by a V-assisted route and investigation of their battery performance

We report the V-assisted synthesis of Na₂Ti₃O₇ nanorods via a conventional solid state reaction technique.

New Insight on the Formation of Sodium Titanates 1D Nanostructures and Its Application on CO2 Hydrogenation

The aim of this work is focused on the study of a series of non-traditional catalytic nanomaterials to transform greenhouse CO₂ gas into added-value products. We found encouraging results of CO₂ hydrogenation activity over sodium titanates with different morphologies. The yield to methanol increases with the increase in the Na incorporated in the nanostructures confirming the proposed mechanism. Samples were prepared at different times of hydrothermal treatment (HTT) with NaOH solutions, and these materials were labeled as Ti-nR-x with x as the hours on the HTT. HRTEM initially showed sphere-shaped nanoparticles in the TiO₂ commercial nanopowder, increasing the HTT resulted in morphological changes in which the structures passed from nanosheets and finally to nanorods after 30 h. The X-ray diffraction and Raman spectroscopy results indicated the formation of sodium titanates such as Na₂Ti₃O₇ with short Ti-O bonds and that Na begins to be incorporated into the distorted TiO₆ crystalline structure after 5 h of HTT (until 12 wt%). The crystalline and shape transformation resulted in a significant modification on the textural properties passing from 51 m².g⁻¹ to 150 m².g⁻¹ and from a pore volume of 0.12 cm³.g⁻¹ to 1.03 cm³.g⁻¹ for Ti-ref and Ti-nR-30 respectively. We also observed differences in the electronic properties as the bandgap presented a blue shift from 3.16 eV on the TiO₂ reference nano-powder to 3.44 eV for the Ti-nR-30 calcined sample. This fact coincides with the presence of a more electron-rich state of the Ti⁴⁺ and the formation of negative charge layer induced by the presence of Na⁺ interlayer cations detected by XPS analysis, at the same this helped us to explain the catalytic activity results.

Synthesis, characterization and enhanced visible-light photocatalytic activity of novel NiO/HTi2NbO7nanocomposite

The decreased electron–hole recombination rate of the NiO/HTi₂NbO₇nanocomposite plays a crucial role in improving visible-light photocatalytic activity.

Continuous Hydrothermal Synthesis of Inorganic Nanoparticles: Applications and Future Directions

Nanomaterials are at the leading edge of the emerging field of nanotechnology. Their unique and tunable size-dependent properties (in the range 1-100 nm) make these materials indispensable in many modern technological applications. In this Review, we summarize the state-of-art in the manufacture and applications of inorganic nanoparticles made using continuous hydrothermal flow synthesis (CHFS) processes. First, we introduce ideal requirements of any flow process for nanoceramics production, outline different approaches to CHFS, and introduce the pertinent properties of supercritical water and issues around mixing in flow, to generate nanoparticles. This Review then gives comprehensive coverage of the current application space for CHFS-made nanomaterials including optical, healthcare, electronics (including sensors, information, and communication technologies), catalysis, devices (including energy harvesting/conversion/fuels), and energy storage applications. Thereafter, topics of precursor chemistry and products, as well as materials or structures, are discussed (surface-functionalized hybrids, nanocomposites, nanograined coatings and monoliths, and metal-organic frameworks). Later, this Review focuses on some of the key apparatus innovations in the field, such as in situ flow/rapid heating systems (to investigate kinetics and mechanisms), approaches to high throughput flow syntheses (for nanomaterials discovery), as well as recent developments in scale-up of hydrothermal flow processes. Finally, this Review covers environmental considerations, future directions and capabilities, along with the conclusions and outlook.

Ti3C2 MXene-Derived Sodium/Potassium Titanate Nanoribbons for High-Performance Sodium/Potassium Ion Batteries with Enhanced Capacities

Sodium and potassium ion batteries hold promise for next-generation energy storage systems due to their rich abundance and low cost, but are facing great challenges in optimum electrode materials for actual applications. Here, ultrathin nanoribbons of sodium titanate (M-NTO, NaTi_1.5O_8.3) and potassium titanate (M-KTO, K₂Ti₄O₉) were successfully synthesized by a simultaneous oxidation and alkalization process of Ti₃C₂ MXene. Benefiting from the suitable interlayer spacing (0.90 nm for M-NTO, 0.93 nm for M-KTO), ultrathin thickness (<11 nm), narrow widths of nanoribbons (<60 nm), and open macroporous structures for enhanced ion insertion/extraction kinetics, the resulting M-NTO exhibited a large reversible capacity of 191 mAh g^-1 at 200 mA g^-1 for sodium storage, higher than those of pristine Ti₃C₂ (178 mAh g^-1) and commercial TiC derivatives (86 mAh g^-1). Notably, M-KTO displayed a superior reversible capacity of 151 mAh g^-1 at 50 mA g^-1 and 88 mAh g^-1 at a high rate of 300 mA g^-1 and long-term stable cyclability over 900 times, which outperforms other Ti-based layered materials reported to date. Moreover, this strategy is facile and highly flexible and can be extended for preparing a large number of MXene-derived materials, from the 60+ group of MAX phases, for various applications such as supercapacitors, batteries, and electrocatalysts.

Na2Ti3O7 Nanoplatelets and Nanosheets Derived from a Modified Exfoliation Process for Use as a High-Capacity Sodium-Ion Negative Electrode

The increasing interest in Na-ion batteries (NIBs) can be traced to sodium abundance, its low cost compared to lithium, and its intercalation chemistry being similar to that of lithium. We report that the electrochemical properties of a promising negative electrode material, Na₂Ti₃O₇, are improved by exfoliating its layered structure and forming 2D nanoscale morphologies, nanoplatelets, and nanosheets. Exfoliation of Na₂Ti₃O₇ was carried out by controlling the amount of proton exchange for Na⁺ and then proceeding with the intercalation of larger cations such as methylammonium and propylammonium. An optimized mixture of nanoplatelets and nanosheets exhibited the best electrochemical performance in terms of high capacities in the range of 100-150 mA h g^-1 at high rates with stable cycling over several hundred cycles. These properties far exceed those of the corresponding bulk material, which is characterized by slow charge-storage kinetics and poor long-term stability. The results reported in this study demonstrate that charge-storage processes directed at 2D morphologies of surfaces and few layers of sheets are an exciting direction for improving the energy and power density of electrode materials for NIBs.

Novel fabrication of a nitrogen-doped mesoporous TiO2-nanorod titanate heterojunction to enhance the photocatalytic degradation of dyes under visible light

One-pot synthesis of a mesoporous nitrogen-doped TiO₂-nanorod titanate heterojunction was developed for effective photocatalytic degradation of dyes.

Titanate and titania nanostructured materials for environmental and energy applications: a review

The state-of-the-art development of fabrication strategies of multi-dimensional titanate and titania nanostructures is reviewed first. This is followed by an overview of their potential applications in environmental, energy, and biomedical sectors.

Mildly alkaline preparation and methylene blue adsorption capacity of hierarchical flower-like sodium titanate

The hydrothermal preparation of flower-like layered sodium titanate architectures in a weakly alkaline medium is reported. NaCl was used as a morphology-directing agent, and a growth mechanism was proposed. The hierarchical structure is assembled from one-dimensional nanoribbons and exhibits an excellent removal capacity toward methylene blue (MB). A pseudo-second-order kinetic model was found to well describe the adsorption kinetics. Kinetic studies demonstrated that the overall rate of MB adsorption was controlled by surface adsorption and intraparticle diffusion. Results of this work are of great significance for environmental applications of flower-like layered sodium titanate architectures as a promising adsorbent material used for water purification.

High-throughput continuous hydrothermal synthesis of nanomaterials (part II): unveiling the as-prepared CexZryYzO2-δ phase diagram

High-throughput continuous hydrothermal flow synthesis was used to manufacture 66 unique nanostructured oxide samples in the Ce-Zr-Y-O system. This synthesis approach resulted in a significant increase in throughput compared to that of conventional batch or continuous hydrothermal synthesis methods. The as-prepared library samples were placed into a wellplate for both automated high-throughput powder X-ray diffraction and Raman spectroscopy data collection, which allowed comprehensive structural characterization and phase mapping. The data suggested that a continuous cubic-like phase field connects all three Ce-Zr-O, Ce-Y-O, and Y-Zr-O binary systems together with a smooth and steady transition between the structures of neighboring compositions. The continuous hydrothermal process led to as-prepared crystallite sizes in the range of 2-7 nm (as determined by using the Scherrer equation).

Rapid hydrothermal flow synthesis and characterisation of carbonate- and silicate-substituted calcium phosphates

A range of crystalline and nano-sized carbonate- and silicate-substituted hydroxyapatite has been successfully produced by using continuous hydrothermal flow synthesis technology. Ion-substituted calcium phosphates are better candidates for bone replacement applications (due to improved bioactivity) as compared to phase-pure hydroxyapatite. Urea was used as a carbonate source for synthesising phase pure carbonated hydroxyapatite (CO₃-HA) with ≈5 wt% substituted carbonate content (sample 7.5CO₃-HA) and it was found that a further increase in urea concentration in solution resulted in biphasic mixtures of carbonate-substituted hydroxyapatite and calcium carbonate. Transmission electron microscopy images revealed that the particle size of hydroxyapatite decreased with increasing urea concentration. Energy-dispersive X-ray spectroscopy result revealed a calcium deficient apatite with Ca:P molar ratio of 1.45 (±0.04) in sample 7.5CO₃-HA. For silicate-substituted hydroxyapatite (SiO₄-HA) silicon acetate was used as a silicate ion source. It was observed that a substitution threshold of ∼1.1 wt% exists for synthesis of SiO₄-HA in the continuous hydrothermal flow synthesis system, which could be due to the decreasing yields with progressive increase in silicon acetate concentration. All the as-precipitated powders (without any additional heat treatments) were analysed using techniques including Transmission electron microscopy, X-ray powder diffraction, Differential scanning calorimetry, Thermogravimetric analysis, Raman spectroscopy and Fourier transform infrared spectroscopy.

Phase stability and rapid consolidation of hydroxyapatite–zirconia nano-coprecipitates made using continuous hydrothermal flow synthesis

A rapid and continuous hydrothermal route for the synthesis of nano-sized hydroxyapatite rods co-precipitated with calcium-doped zirconia nanoparticles using a superheated water flow at 450°C and 24.1 MPa as a crystallizing medium is described. Hydroxyapatite and calcium-doped zirconia phases in the powder mixtures could be clearly identified based on particle size and morphology under transmission electron microscopy. Retention of a nanostructure after sintering is crucial to load-bearing applications of hydroxyapatite-based ceramics. Therefore, rapid consolidation of the co-precipitates was investigated using a spark plasma sintering furnace under a range of processing conditions. Samples nominally containing 5 and 10 wt% calcium-doped zirconia and hydroxyapatite made with Ca:P solution molar ratio 2.5 showed excellent thermal stability (investigated using in situ variable temperature X-ray diffraction) and were sintered via spark plasma sintering to >96% sintered densities at 1000°C resulting in hydroxyapatite and calcium-doped zirconia as the only two phases. Mechanical tests of spark plasma sintering sintered samples (containing 10 wt% calcium-doped zirconia) revealed a three-pt flexural strength of 107.7 MPa and Weibull modulus of 9.9. The complementary nature of the spark plasma sintering technique and continuous hydrothermal flow synthesis (which results in retention of a nanostructure even after sintering at elevated temperatures) was hence showcased.

High-strength nanograined and translucent hydroxyapatite monoliths via continuous hydrothermal synthesis and optimized spark plasma sintering

The synthesis of high-strength, completely dense nanograined hydroxyapatite (bioceramic) monoliths is a challenge as high temperatures or long sintering times are often required. In this study, nanorods of hydroxyapatite (HA) and calcium-deficient HA (made using a novel continuous hydrothermal flow synthesis method) were consolidated using spark plasma sintering (SPS) up to full theoretical density in ∼5 min at temperatures up to 1000°C. After significant optimization of the SPS heating and loading cycles, fully dense HA discs were obtained which were translucent, suggesting very high densities. Significantly high three-point flexural strength values for such materials (up to 158 MPa) were measured. Freeze-fracturing of disks followed by scanning electron microscopy investigation revealed selected samples possessed sub-200 nm sized grains and no visible pores, suggesting they were fully dense.