Long-range ordering of two-dimensional wide bandgap tantalum oxide nanosheets in printed films

Two-dimensional oxide materials are a well-studied, interesting class of materials, enabled by the fact that their bulk layered metal oxides, such as titanates and niobates, can be easily exfoliated within minutes into 2D nanosheets. However, some promising oxide materials, such tantalum oxide, are much more difficult to delaminate, taking several weeks, due to the higher charge density resulting in stronger Coulombic interactions between the layers. This intrinsic constraint has limited detailed studies for exploiting the promising properties of tantalum oxide 2D nanosheets towards enhanced catalysis and energy storage. Here, we have studied in detail the exfoliation mechanism of high charge density 2D materials, specifically tantalum oxide (TaO₃) nanosheets. Optimization of tetrabutylphosphonium hydroxide (TBPOH) as the exfoliation agent in a 2 : 1 ratio to HTaO₃ has resulted in a dramatic reduction of the exfoliation time down to only 36 hours at 80 °C. Furthermore, single monolayers of TaO₃ nanosheets with >95% coverage have been achieved by Langmuir-Blodgett deposition, while thicker layers (ranging from several tens of nanometers up to microns) exhibiting long-range ordering of the present nanosheets have been realized through inkjet printing. Interestingly, scanning tunneling microscopy analysis indicated a wide bandgap of ∼5 eV for the single TaO₃ nanosheets. This value is significantly higher than the reported values between 3.5 and 4.3 eV for the layered RbTaO₃ parent compound, and opens up new opportunities for 2D oxide materials.

Two-Dimensional Molecular Sheets of Transition Metal Oxides toward Wearable Energy Storage

Flexible and wearable electronics have recently sparked intense interest in both academia and industry because they can greatly revolutionize human lives by impacting every aspect of our daily routine. Therefore, developing compatible energy storage devices has become one of the most important research frontiers in this field. Particularly, the development of flexible electrodes is of great significance when considering their essential role in the performance of these devices. Although there is no doubt that transition metal oxide nanomaterials are suitable for providing electrochemical energy storage, individual oxides generally cannot be developed into freestanding electrodes because of their intrinsically low mechanical strength.Two-dimensional sheets with genuine unilamellar thickness are perfect units for the assembly of freestanding and mechanically flexible devices, as they have the advantages of low thickness and good flexibility. Therefore, the development of metal oxide materials into a two-dimensional sheet morphology analogous to graphene is expected to solve the above-mentioned problems. In this Account, we summarize the recent progress on two-dimensional molecular sheets of transition metal oxides for wearable energy storage applications. We start with our understanding of the principle of producing two-dimensional metal oxides from their bulk-layered counterparts. The unique layered structure of the precursors inspired the exploration of their interlayer chemistry, which helps us to understand the processes of swelling and delamination. Rational methods for tuning the chemical composition, size/thickness, and surface chemistry of the obtained nanosheets and how physicochemical properties of the nanosheets can be modulated are then briefly introduced. Subsequently, the orientational alignment of the anisotropic sheets and the origins of their liquid-crystalline characteristics are discussed, which are of vital importance for their subsequent macroscopic assembly. Finally, macroscopic electrodes with geometric diversity ranging from one-dimensional macroscopic fibers to two-dimensional films/papers and three-dimensional monolithic foams are summarized. The intrinsically low mechanical stiffness of metal oxide sheets can be effectively overcome by wisely designing the assembly mode and sheet interfaces to obtain decent mechanical properties integrated with superior electrochemical performance, thereby providing critical advantages for the fabrication of wearable energy storage devices.We expect that this Account will stimulate further efforts toward fundamental research on interface engineering in metal oxide sheet assembly and facilitate wide applications of their designed assemblies in future new-concept energy conversion devices and beyond. In the foreseeable future, we believe that there will be a big explosion in the application of transition metal oxide sheets in flexible electronics.

Introduction of Fe2+ in Fe0.8Ti1.2O40.8− nanosheets via photo reduction and their enhanced electrochemical performance as a lithium ion battery anode

Fe²⁺ doped Fe_0.8Ti_1.2O₄^0.8− nanosheets were prepared via delaminating H_0.8Fe_0.8Ti_1.2O₄ precursor and further photo reduction. It shows improved electrochemical performance due to the enhanced electrical conductivity by the introduction of Fe²⁺.

Exfoliation of crystals

The review is concerned with progress in methods for exfoliation of crystals, from mechanical exfoliation using sticky tape to modern techniques involving sonication-assisted exfoliation, shear exfoliation in liquids using intercalating agents and stabilizers, direct liquid exfoliation and cosolvent exfoliation. The potential of methods of osmotic swelling in water and in organic dispersion media with constant and variable chemical composition of nanosheets, chemical and electrochemical intercalation, exfoliation by hydrazine (including versions resulting in changes in the chemical composition of nanosheets), ionic liquids and supercritical fluids is discussed. Methods for size sorting of nanosheets by density-gradient and cascade centrifugation and the possibility of nanosheet size control are analyzed.

The bibliography includes 136 references.

Soft chemistry of ion-exchangeable layered metal oxides

Disassembly and re-assembly of layered metal oxides by soft chemical approaches can be used to tailor functionalities in artificial photosynthesis, energy storage, optics, and piezoelectrics.

Toward Tunable Photonic Nanosheet Sensors: Strong Influence of the Interlayer Cation on the Sensing Characteristics

An approach toward intercalant tunable nanosheet-based Fabry-Pérot sensors is presented. The intercalant tetrabutylammonium significantly increases the sensitivity of the photonic nose sensor to volatile organic compounds with increasing polarity, enabling polarity-driven color-coded vapor differentiation. Paired with the improved millisecond response times for polar vapors, vapor imaging with spatio-temporal resolution is within reach.

Research on the self-assembly of exfoliated perovskite nanosheets (LaNb2O7−) and cobalt porphyrin utilized for the electrocatalytic oxidation of ascorbic acid

A sandwich-structured nanocomposite of LaNb₂O₇/CoTMPyP was fabricated via electrostatic interactions between LaNb₂O₇⁻ nanosheets and cobalt porphyrin cations, and the obtained hybrid film exhibited excellent electrocatalytic activities toward AA.

Ultrathin HNbWO6 nanosheets: facile synthesis and enhanced hydrogen evolution performance from photocatalytic water splitting

Ultrathin monolayer HNbWO₆ nanosheets were rapidly synthesised with triethanolamine as a stripping agent in just a few minutes. These obtained highly dispersed nanosheets suspensions exhibit an efficient photocatalytic H₂ evolution performance under simulated sunlight irradiation.