Layered Titanate Nanosheets Research Articles

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.

Preparation of Layered Titanate Nanosheets and Study of Cs+ Adsorption Performance

Preparation of Layered Titanate Nanosheets and Study of Cs+ Adsorption Performance

Nitrogen doping-mediated oxygen vacancies enhancing co-catalyst-free solar photocatalytic H2 production activity in anatase TiO2 nanosheet assembly

Introducing oxygen vacancies (Vo) into TiO2 photocatalyst has been considered an effective strategy for improving co-catalyst-free solar photocatalytic activity. However, the methods used to synthesize it require high pressure/temperature and/or hazardous/costly reagents. Here we propose Vo introduction, concomitant with N-doping in TiO2, as an alternative strategy for achieving efficient co-catalyst-free solar photocatalytic activity under less extreme conditions. After calcination at 450 °C of mesoporous spherical assemblies of a layered titanate nanosheet containing N,N-dimethylformamide as its synthesis solvent in the structure, we successfully synthesized mesoporous spherical assemblies of nanosheets composed of anatase TiO2 nanoparticles with Vo mediated by N doping. This material exhibits good co-catalyst-free solar photocatalytic activity for hydrogen evolution via water splitting under irradiation with simulated solar light, which is considerably higher than that of typical co-catalyst-free defective TiO2 materials. We discuss the possible role of the introduced Vo in facilitating charge separation and raising photocatalytic efficiency.

Tailored Hydrothermal Synthesis of Specific Facet-Dominated TiO2 Nanocrystals from Lepidocrocite-Type Layered Titanate Nanosheets: Systematical Investigation and Enhanced Photocatalytic Performance

A series of samples including leaf-like and rod-like rutile TiO2 nanoparticles with various facets exposed on the surface, parallelepiped-shaped anatase nanoparticles with [111] vertical facet exposed on the surface, irregular anatase nanoparticles, microsized six-point star-like anatase aggregates, and almond-like brookite aggregates had been hydrothermally synthesized from lepidocrocite-type layered titanate nanosheets. A systematical investigation was established to uncover the phase transition and morphological evolution from nanosheets to TiO2 polymorphs, and a phase diagram was determined by adjusting the synthesis parameters of the pH value and temperature. Two kinds of mechanisms composed of the dissolution-deposition process following Ostwald's ripening mechanism and the in situ topochemical conversion process following Ostwald's step rule had been proposed based on the time-dependent hydrothermal experiments. Briefly, the formation of the single-crystalline rutile phase appeared only at high temperatures with very low pH values, and similarly, the brookite phase strictly formed at high temperatures with a very high pH value. Nevertheless, the anatase phase could moderately appear in a wide range of temperatures and pH values. In addition, the single-crystalline rutile adopted a leaf-like morphology at low temperatures with high pH values and a rod-like morphology at high temperatures with low pH values, while the morphological evolution of anatase particles proceeded from irregular to parallelepiped-shaped and finally to six-point star-like morphology, and the crystal size was reduced from 1000 to 5 nm with decreasing pH values. Meanwhile, with the prolongation of the hydrothermal time, the layered titanate nanosheets first dissolved into the amorphous state and further converted into small anatase nanoparticles and finally to rutile or anatase nanoparticles based on the dissolution-deposition process, or the {010}-faceted layered titanate structure first converted into the [111]-vertical faceted anatase nanosheets by the topochemical transformation reaction and then split into the [111]-vertical faceted anatase nanoparticles. More importantly, the mesoporous [111]-vertical faceted anatase nanoparticles exhibited enhanced photocatalytic performance compared to that of Degussa P25, which was ascribed to its superior electronic band structure and effective charge separation. The systematical investigation in this work would be significant for consummating the preparation of the TiO2 polymorphs from layered titanate nanosheets and provided some reference values and guide schemes for the preparation of TiO2 nanoparticles with outstanding photocatalytic performance.

Rotation-Assisted Hydrothermal Synthesis of Thermally Stable Multiwalled Titanate Nanotubes and Their Application to Selective Catalytic Reduction of NO with NH3.

Titanate nanotubes are widely applied in various fields, including photocatalysts and electronic devices, but their weak thermal stability limits their application for catalyst support. Here, we found that titanate nanotubes with a thick multiwalled structure of 15 layers or more can be prepared by using rotation-assisted hydrothermal synthesis. The porous structure of conventional nanotubes synthesized without rotation collapsed easily after thermal treatment, whereas the nanotubes having a thick multiwalled structure retained their pore structure and the specific surface area (∼300 m2/g) even after calcination at 400 °C in air. Systematic variation of rotation speed suggested that rotation in the synthesis process accelerated the stacking of layered titanate nanosheets, which are known to be intermediates of nanotubes. Thus, the rapid assembly of titanate nanosheets facilitated by rotation led to the formation of nanotubes with a multiwalled structure. Overly fast rotation, however, caused excessive stacking and created a thicker structure that cannot be easily wrapped into nanotubes. Therefore, it is essential to maintain the optimum rotation speed to obtain both the nanotube morphology and the thick multiwalled structure. Vanadium-tungsten-oxide catalyst supported on the multiwalled titanate nanotubes was used in NH3-selective catalytic reduction, which showed stable NO x reduction performance with high selectivity to N2, which may originate from the suppressed sintering of VO x on multiwalled nanotubes. This study demonstrates that the morphology of nanotubes can be tuned by controlling the degree of interaction supplied by external forces.

Intercalation of conjugated polyelectrolytes in layered titanate nanosheets for enhancement in photocatalytic activity

Cationic π-conjugated polyelectrolytes (CPEs) were successfully intercalated into layered titanate (TiO2), while exfoliated layered titanates were reassembled into lamellar structures. The resulting composites showed significantly enhanced visible light absorbance and enhanced photocatalytic activity with respect to layered titanate, which was proven by the photodegradation of methylene blue in aqueous media under irradiation by visible light. Notably, some amount of tetrabutylammonium (TBA) used for the delamination of a layered titanate could be co-intercalated with CPEs, and the residual TBA could be removed via continuously repeating the intercalation process, maintaining the spatial arrangement and content of CPEs in the confined area of layered titanates. However, with the presence of TBA, the hybrid demonstrated greater performance in the photodegradation of methylene blue at any given period of time and almost complete degradation of such dye within 8 min. The significant difference in photocatalytic efficiency with or without TBA suggested that the TBA effectively reduced the surface energy at the interface between the organic components and aqueous medium, which was evidenced by the dispersity assessment of hybrids with or without TBA.

Polymorphic Evolution of TiO2 in Hydrothermal Reaction System of Layered Titanate Nanosheets

AbstractThe hydrothermal formation mechanisms of TiO2 polymorphs of brookite and anatase, and their nanocomposites were investigated systematically in a mixed solution reaction system of titanium isopropoxide (TTIP) and tetramethylammonium hydroxide (TMAOH) by using XRD, FE‐SEM and TEM observations, and SAED. The titanate nanosheets with lepidocrocite‐type layered structure are formed firstly, and then transformed to the TiO2 polymorphs by topotactic structural transformation reactions in the reaction system. The pH value and the concentration of TMA+ play the key roles in the formation of TiO2 polymorphs. The small‐sized layered titanate nanosheets are formed and transformed to anatase phase under low pH and low TMA+ concentration conditions, while the large‐sized layered titanate nanosheets are formed and transferred to brookite phase under high pH and high TMA+ concentration conditions. The anatase‐brookite nanocomposites can be obtained from the mixture of small‐ and large‐sized nanosheets under middle pH conditions, in which the anatase nanocrystals are formed on the surface of brookite nanocrystal.

Toward an Effective Control of the H2 to CO Ratio of Syngas through CO2 Electroreduction over Immobilized Gold Nanoparticles on Layered Titanate Nanosheets

In recent years, the electroreduction of CO2 to valuable products has emerged as a rational answer to rising CO2 emissions and a strategic approach to incorporate renewable electricity from intermittent sources (e.g., wind and solar) into the global energy supply. The reduction of CO2 to CO has been highlighted in the widely explored industrial conversion of syngas (CO and H2) to fuels. Herein, we report a promising electrocatalyst incorporating well-dispersed gold nanoparticles (Au NPs) on ultrathin titanate nanosheets (TiNS). By tuning the contents of Au (in the ranges of 0 to 93 wt % Au) in the hybrid Au/TiNS architecture, CO product selectivity was effectively controlled (in the range of CO Faradaic efficiency from 3 to over 80%) with the sole additional formation of H2, which is of pronounced industrial interest. Most importantly, a control of both component amounts was suggested to result in a variation of corresponding electronic properties based on the interaction between Au NP and TiNS substrate, dictating the stabilization of formed reaction intermediates and resulting product selectivity. In addition, our Au/TiNS achieved optimally high CO and H2 production current densities, with 73 wt % Au at the low cathodic potential region (−0.6 to −0.9 VRHE). The suggested synergetic effect between both catalytic components underlines the promising character of this hybrid system and is expected to significantly add to the strategic production of syngas for subsequent applications.

Solvothermal synthesis of a layered titanate nanosheets and its photocatalytic activity: Effect of Ag doping

Abstract Photocatalytic activity of titanate nanosheets prepared by continuous hydrothermal flow system has been extensively studied earlier. Herein, for the first time, a thin layered titanate nanosheets were fabricated by one-step solvothermal approach. Descriptive study on solvothermal doping of Ag in titanate nanostructure and crystal growth mechanism of the material prepared by solvothermal synthesis has been discussed. Structure, morphology, crystallinity, electronic properties, optical properties and photo-electrochemical parameters have been studied with different level of silver incorporation. Charge transfer among Ti and Ag cluster and change of intermediate energy level results in an electronic renovation of the band structure. The material prepared by solvothermal doping was found to be visible light sensitive without addition of other minerals. Photodegradation of methyl orange and phenol has been observed under visible light irradiation, it shows excellent photocatalytic behavior.

Single-crystalline anatase TiO2 nanoleaf: Simple topochemical synthesis and light-scattering effect for dye-sensitized solar cells

A topochemical process was used to synthesize {010}-faceted leaflike anatase TiO2 nanocrystals from exfoliated layered titanate nanosheets. A light scattering effect of the leaflike anatase nanocrystals with a crystal size of 300nm in length and 30nm in width was studied as a light scattering layer on a P25 nanocrystalline-TiO2 film for dye-sensitized solar cells (DSSCs). The light scattering layer fabricated using the leaflike nanocrystals exhibits a synergistic effect of light scattering and dye-loading for enhancement of light harvesting, which enhanced 28% of energy conversion efficiency of DSSCs by enhancing 33% of Jsc. The leaflike nanocrystals oriented to the direction vertical to the direction of incident light in the scattering layer improve the scattering effect of the scattering layer, and the dye molecules adsorbed on the {010}-faceted anatase exhibit high performance for photoelectron generation.

A alkalescent route for synthesis of titanate nanosheets modified with reduced graphene oxide for enhanced photocatalytic activity

Layered titanate nanosheets (TNS) were directly synthesized by one step in a special reaction system called weak base hydrothermal system, without organic stabilizer and washing or proton exchange. Reduced graphene oxide (rGO)/TNS nanocomposites were synthesized by improved hydrothermal treatment. The obtained samples were characterized by Scanning electron microscope, Transmission electron microscope, X-ray powder diffraction, Raman spectrum, Fourier Transform Infrared spectra, UV–Vis diffuse reflectance spectrum, Photoluminescence, X-rayphotoelectron spectroscopy, X-band electron paramagnetic resonance. The results confirmed that titanate nanosheets were homogeneously dispersed on rGO sheets, and three factors play an important part in the high photocatalytic performance of the samples, including the formation of a Ti–O–C bond expanding the light wavelength absorption range, the adsorption of contaminant molecules, the high separation rate of electron–hole pairs based on the sufficient contact of TNS and rGO nanosheets. The photocatalytic performance of as-prepared rGO/TNS nanocomposites were evaluated by degradation of methylene blue under visible-light irradiation (λ ≥ 400 nm). The acquired results showed that the photocatalytic performace of rGO/TNS nanocomposites were better than that of pure TNS. It could be attributed to the phenomenon that the photogenerated electrons in TNS were easily transferred to the rGO sheets via Ti–O–C between TNS and rGO. This process decreases the recombination rate of electron–hole pairs and lead to the improvement of photocatalytic activity.

Hybridization of a Metal–Organic Framework with a Two-Dimensional Metal Oxide Nanosheet: Optimization of Functionality and Stability

An effective way to improve the functionalities and stabilities of metal–organic frameworks (MOFs) is developed by employing exfoliated metal oxide 2D nanosheets as matrix for immobilization. Crystal growth of zeolitic imidazolate framework-8 (ZIF-8) nanocrystals on the surface of layered titanate nanosheets yields intimately coupled nanohybrids of ZIF-8-layered titanate. The resulting nanohybrids show much greater surface areas and larger pore volumes than do the pristine ZIF-8, leading to the remarkable improvement of the CO2 adsorption ability of MOF upon hybridization. Of prime importance is that the thermal- and hydrostabilities of ZIF-8 are significantly enhanced by a strong chemical interaction with the robust titanate nanosheet. A strong interfacial interaction between ZIF-8 and the layered titanate is verified by molecular mechanics simulations and spectroscopic analysis. The universal applicability of the present strategy for the coupling of MOFs and metal oxide nanosheets is substantiated by the stabilization of Ti-MOF-NH2 via the immobilization on exfoliated V2O5 nanosheets. The present study underscores that hybridization with metal oxide 2D nanosheets provides an efficient and universal synthetic route to novel MOF-based hybrid materials with enhanced gas adsorptivity and stability.

Stabilization of Layered Double Oxide in Hybrid Matrix of Graphene and Layered Metal Oxide Nanosheets: An Effective Way To Explore Efficient CO2 Adsorbent

A novel methodology to explore efficient CO2 adsorbent is developed by the stabilization of layered double oxide (LDO) in the hybrid matrix of reduced graphene oxide (rG-O) and layered titanate nanosheets. The electrostatically derived self-assembly between cationic Mg–Al–layered double hydroxide (LDH) nanosheet and anionic graphene oxide (G-O)/layered titanate nanosheets followed by heat treatment at high temperature leads to the cohybridization of LDO (MgO/MgAl2O4) nanocrystals with exfoliated rG-O and layered titanate nanosheets. The incorporation of LDO into the hybrid matrix of rG-O and layered titanate nanosheets is highly effective in increasing its surface area through the formation of mesoporous stacking structure. Of prime importance is that even at very low concentration of titanate (0.3 wt %), an addition of layered titanate nanosheet induces a remarkable surface area expansion of LDO–rG-O nanocomposite from 178 to 330 m2 g–1. This result is attributable to the depression of the self-aggregati...

Exfoliated Metal Oxide Nanosheets as Effective and Applicable Substrates for Atomically Dispersed Metal Nanoparticles with Tailorable Functionalities

An effective methodology to stabilize highly dispersed metal nanoparticles is developed by employing the exfoliated 2D metal oxide nanosheets with variable surface structures as substrates. The selection of appropriate crystal structure of titanate nanosheet is very crucial in stabilizing atomically dispersed Pt nanoparticles through the tuning of chemical interaction between Pt and titanate substrate. A theoretical study using density functional theory calculations confirms the significant influence of the crystal structure of layered titanate nanosheet on the crystal growth behavior of immobilized metal nanoparticle. As a consequence of the good dispersion of Pt nanoparticles, the Pt–trititanate nanohybrids with stronger interaction show much higher content of atomically dispersed Pt and better catalyst performances than do the Pt–lepidocrocite titanate ones. The applicability of the present method for other metal species is evidenced by the successful tuning of the crystal size and functionality of Au nanoparticles via immobilization on layered titanate nanosheets. The functionality of Au for surface‐enhanced Raman spectroscopy becomes improved by the anchoring on the lepidocrocite‐type titanate nanosheet. The present study underscores that the use of the metal oxide 2D nanosheets with appropriate surface structure as substrates is effective in tailoring the crystal growth and the functionalities of immobilized metal nanoparticles.

Highly Stable Nanocontainer of APTES-Anchored Layered Titanate Nanosheet for Reliable Protection/Recovery of Nucleic Acid.

A universal technology for the encapsulative protection of unstable anionic species by highly stable layered metal oxide has been developed via the surface modification of a metal oxide nanosheet. The surface anchoring of (3-aminopropyl)triethoxysilane (APTES) on exfoliated titanate nanosheet yields a novel cationic metal oxide nanosheet, which can be universally used for the hybridization with various biological and inorganic anions. The encapsulation of deoxyribonucleic acid (DNA) in the cationic APTES-anchored titanate lattice makes possible the reliable long-term protection of DNA against enzymatic, chemical, and UV−vis light corrosions. The encapsulated DNA can be easily released from the titanate lattice via sonication, underscoring the functionality of the cationic APTES-anchored titanate nanosheet as a stable nanocontainer for DNA. The APTES-anchored titanate nanosheet can be also used as an efficient CO2 adsorbent and a versatile host material for various inorganic anions like polyoxometalates, leading to the synthesis of novel intercalative nanohybrids with unexplored properties and useful functionalities.

High power layered titanate nano-sheets as pseudocapacitive lithium-ion battery anodes

Ultra-thin layered sodium titanate nano-sheets were synthesised using a continuous hydrothermal flow process and the as-prepared materials were investigated as an anode material for lithium-ion batteries. In comparison to previous studies on similar materials, the layered titanates herein showed high electrochemical activity at lower potentials. Cyclic voltammetry measurements in the potential range of 0.05–2.1 V vs. Li/Li+, revealed that charge storage occurred from both lithium-ion intercalation as well as pseudocapacitive surface chemical processes. During electrochemical cycling tests, a high specific current of 0.5 A g−1 was applied and the cells achieved a stable specific capacity of ca. 120 mAh g−1 for over 1200 cycles. Even at an applied current of 10 A g−1, the electrode material delivered a stable specific capacity of 38 mAh g−1, which suggests that this material may be suitable for high power applications.

A Crucial Role of Rh Substituent Ion in Photoinduced Internal Electron Transfer and Enhanced Photocatalytic Activity of CdS-Ti(5.2-x)/6 Rhx /2 O2 Nanohybrids.

The photocatalytic activity and photostability of CdS quantum dot (QD) can be remarkably enhanced by hybridization with Rh-substituted layered titanate nanosheet even at very low Rh substitution rate (<1%). Mesoporous CdS-Ti(5.2-x)/6 Rhx/2O2 nanohybrids are synthesized by a self-assembly of exfoliated Ti(5.2-x)/6 Rhx/2O2 nanosheets with CdS QDs. The partial substitution of Rh(3+)/Rh(4+) ions for Ti(4+) ions in layered titanate is quite effective in enhancing an electronic coupling between hybridized CdS and titanate components via the formation of interband Rh 4d states. A crucial role of Rh substituent ion in the internal electron transfer is obviously evidenced from in situ X-ray absorption spectroscopy showing the elongation of (RhO) bond under visible light irradiation. This is the first spectroscopic evidence for the important role of substituent ion in the photoinduced electron transfer of hybrid-type photocatalyst. The CdS-Ti(5.2-x)/6 Rhx/2O2 nanohybrids show much higher photocatalytic activity for H2 production and better photostability than do CdS and unsubstituted CdS-TiO2 nanohybrid. This result is ascribable to the enhancement of visible light absorptivity, the depression of electron-hole recombination, and the enhanced hole curing of CdS upon Rh substitution. The present study underscores that the hybridization with composition-controlled inorganic nanosheet provides a novel efficient methodology to optimize the photo-related functionalities of semiconductor nanocrystal.

Evaluation and comparison of layered titanate nanosheets using TOF‐SIMS and g‐ogram analysis

The evaluation of nanostructure is important to develop the highly controlled nanomaterials. In this study, two kinds of layered titanate nanosheets, which were produced by using hexylamine and laurylamine, respectively, as surfactants were investigated by Gentle Secondary Ion Mass Spectrometry Gentle‐SIMS (G‐SIMS) and g‐ogram, which is the latest Time‐of‐Flight Secondary Ion Mass Spectrometry (TOF‐SIMS) data analysis method for detecting more intact ions and obtaining the information on original chemical structures of samples precisely from complicated TOF‐SIMS spectra. As a result, molecular related ions of the surfactants were detected from each sample, and the structural information of samples was obtained. From both samples, surfactant molecular ions connected with hydrocarbon were detected as more intact ions rather than molecular ions of themselves. It was suggested that hydrophobic domains of their lamellar mesostructure are formed robustly by more than two surfactant molecules connected with each other linearly. After all, important information on the chemical structure of the layered titanate nanosheets, which would be difficult to be found by using typical structural analysis methods such as X‐ray diffraction and transmission electron microscopy, were obtained using G‐SIMS and g‐ogram. Therefore, it was shown that g‐ogram and G‐SIMS are helpful to evaluate the nanostructured materials. And it was also shown that g‐ogram is applicable to organic–inorganic materials which contain long hydrocarbon structures. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Sandwich-structured composite from the direct coassembly of layered titanate nanosheets and Mn porphyrin and its electrocatalytic performance for nitrite oxidation

MnTMPyP–Ti4O9 nanocomposite was prepared by the direct coassembly of oppositely charged building blocks, exfoliated titanate nanosheets and 5,10,15,20-tetrakis(N-methylpyridinium-4-yl) porphyrinato manganese(III), MnTMPyP cations. XRD, IR, SEM, and TEM revealed that Mn porphyrin has been successfully inserted into the lamellar spacing of layered titanate. The metalloporphyrin molecules incorporated within the nanocomposite were found to be tilted approximately 42° against the titanate nanosheets. Further, the obtained MnTMPyP–Ti4O9 nanocomposite has been examined for the electrochemical detection of nitrite using cyclic voltammetry and differential pulse voltammetry techniques. The electrochemical studies demonstrated that MnTMPyP–Ti4O9 nanocomposite detects nitrite linearly over a concentration range of 2.0×10−6–1.57×10−3M with the detection limit of 1.25×10−6M at a signal-to-noise ratio of 3.

Microwave-Assisted Topochemical Conversion of Layered Titanate Nanosheets to {010}-Faceted Anatase Nanocrystals for High Performance Photocatalysts and Dye-Sensitized Solar Cells

The {010}-faceted anatase nanocrystals with controllable crystal size and morphology were synthesized by microwave hydrothermal treatment of layered titanate nanosheet solutions. The nanostructures and formation reaction mechanism of TiO2 nanocrystals were investigated using X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, and selected-area electron diffraction. Their photocatalytic behavior and dye-sensitized solar cell (DSSC) performance were studied and compared with [111]-faceted anatase nanocrystals and anatase nanocrystals without a specific facet on the surface. There are two kinds of reactions in the formation process of the anatase nanocrystals. One is an in situ topochemical conversion reaction of layered titanate structure to anatase structure, and another is the dissolution–deposition reaction on the particle surface. The microwave hydrothermal process is suitable to control the structural conversion reaction for uniform the crystal size and morphology due to its uniform heating mechanism. The UV–visible spectrum results revealed that the bandgap of the TiO2 nanocrystals was enhanced in the order of nanocrystal without specific facet < [111]-faceted nanocrystal < {010}-faceted nanocrystal, which corresponded to their photocatalytic activities. The DSSC performance also was enhanced in the same order, suggesting that the {010}-faceted nanocrystals are promising for the high performance DSSCs.