Since the discovery of mechanically exfoliated graphene in 2004, research on ultrathin two-dimensional (2D) nanomaterials has grown exponentially in the fields of condensed matter physics, material science, chemistry, and nanotechnology. Highlighting their compelling physical, chemical, electronic, and optical properties, as well as their various potential applications, in this Review, we summarize the state-of-art progress on the ultrathin 2D nanomaterials with a particular emphasis on their recent advances. First, we introduce the unique advances on ultrathin 2D nanomaterials, followed by the description of their composition and crystal structures. The assortments of their synthetic methods are then summarized, including insights on their advantages and limitations, alongside some recommendations on suitable characterization techniques. We also discuss in detail the utilization of these ultrathin 2D nanomaterials for wide ranges of potential applications among the electronics/optoelectronics, electrocatalysis, batteries, supercapacitors, solar cells, photocatalysis, and sensing platforms. Finally, the challenges and outlooks in this promising field are featured on the basis of its current development.

With the bioinspired design of organic ligands and metallic nodes, novel ultrathin 2D bimetallic metal-organic-framework nanosheets are successfully synthesized, which can serve as advanced 2D biomimetic nanomaterials to mimic heme proteins.

The development of renewable energy storage and conversion devices is one of the most promising ways to address the current energy crisis, along with the global environmental concern. The exploration of suitable active materials is the key factor for the construction of highly efficient, highly stable, low-cost and environmentally friendly energy storage and conversion devices. The ability to prepare two-dimensional (2D) metal dichalcogenide (MDC) nanosheets and their functional composites in high yield and large scale via various solution-based methods in recent years has inspired great research interests in their utilization for renewable energy storage and conversion applications. Here, we will summarize the recent advances of solution-processed 2D MDCs and their hybrid nanomaterials for energy storage and conversion applications, including rechargeable batteries, supercapacitors, electrocatalytic hydrogen generation and solar cells. Moreover, based on the current progress, we will also give some personal insights on the existing challenges and future research directions in this promising field.

Black titania nanotube arrays are prepared for the first time, which exhibit an excellent photoelectrochemical water-splitting performance.

Li-ion batteries, which restrict their wide applications, especially for environments that require mechanical stability and extreme conditions. [8][9][10][11] Alternatively, fl exible supercapacitors, especially quasi-solid state ones, have received considerate attention recently. [12][13][14][15] Although some of the fl exible supercapacitors so far reported can provide high power density and long-term stability, the energy density is relatively low. [16][17][18][19] Thus great challenges still remain in developing the overall high-performance solid/quasi-solid-state fl exible electrochemical energy storage devices with both high energy density and high power density. [20][21][22][23][24] As a typical type of traditional aqueous rechargeable batteries, Ni/Fe battery has been studied for a long period of time, because of its relatively high safety, lowcost, and high energy density. [ 25,26 ] In general, it can provide better safety and lower cost as compared with Li-ion battery; its energy density is higher in contrast to that of common supercapacitors. However, the low power density and poor cycling ability have limited the wide application of Ni/Fe battery. [ 27 ] On the other hand, recent development of nanomaterials and nanotechnology has enabled advanced electrode materials that can greatly enhance the performance of Ni/Fe battery. For example, by proper synergizing of nanostructured active materials (FeO x , NiO, or Ni(OH) 2 ) with carbonaceous materials (such as graphene, carbon nanofi bers, and carbon nanotubes (CNTs)), the power density of Ni/Fe battery can be greatly enhanced. [ 28,29 ] In some of these previous studies, the active materials are controlled in powder forms, such that carbon black and polymer additives have been employed in electrodes, where heavy metal foils or foams are used as the current collectors. The gravimetric/volumetric capacity of the full cell is therefore limited and the cell can be hardly fl exible. Some other works have focused on fl exible electrode materials for Ni/Fe batteries, where the device is assembled using liquid electrolytes. [ 30,31 ] In addition to the overall electrochemical performance, it would be a plus to eliminate liquid electrolytes, such that the safety issue in connection with the potential leakage problem can be solved. In order to promote the application of Ni/Fe batteries as a class of energy storage components for fl exible electronics Aqueous Ni/Fe batteries have great potential as fl exible energy storage devices, owing to their low cost, low toxicity, high safety, and high energy density. However, the poor cycling stability has limited the widely expected application of Ni/Fe batteries, while the use of heavy metal substrates cannot meet the basic requirement for fl exible devices. In this work, a fl exible type of solid-state Ni/Fe batteries with high energy and power densities is rationally developed using needle-like Fe 3 O 4 and fl ake-like NiO directly grown on carbon cloth/carbon nanofi ber (CC-CF) matrix as the anode and cathode, respecti...

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The 2H molybdenum disulfide (MoS ), as a stable hexagonal phase, has been one of the most studied transition metal dichalcogenides over the past decades. In the last five years, the metastable phases of MoS (1T, 1T', 1T'', and 1T''') have seen a revival of interests. Different from the edge-sharing [MoS ] trigonal prisms in the 2H MoS phase, these metastable phases are composed of the edge-sharing [MoS ] octahedra, in which the neighboring Mo-Mo distances differ. Due to the various crystal structures and different electronic configurations of the building [MoS ] motifs, these metastable polytypes are endowed with intriguing physical properties and potential applications in diverse fields. In this Review, the recent research progress on metastable MoS is summarized, especially with an emphasis on the diverse synthetic approaches and the newly uncovered physical properties. The phase structures and electronic band structures are also outlined. In the end, a perspective of the future investigation on metastable MoS is discussed.

Here we report a facile low-temperature solvothermal method by using Li-dissolved ethanediamine to prepare uniform hydrogenated blue H-TiO 2−x with wide spectrum response. H-TiO 2−x possesses a distinct crystalline core−amorphous shell structure (TiO 2 @TiO 2−x ) with numerous oxygen vacancies and doped H in the amorphous shell. Efficient solar to chemical energy conversions, likely photocatalytic reduction of CO 2 , degradation of contaminants, and H 2 generation from water splitting can be achieved over this blue titania. Notably, the optimized H-TiO 2−x (200) shows high activity of CH 4 formation at a rate of 16.2 μmol g −1 h −1 and a selectivity of 79% under full solar irradiation. The kinetic isotope effects measurements reveal that the cleavage of the CO bond from CO 2 rather than the O−H bond from H 2 O is the ratedetermining step in CH 4 formation. Meanwhile, in situ diffuse reflectance infrared Fourier transform spectroscopy shows the existence of the key intermediate CO 2 − species. The formation of intermediate CO 2 − indicates that the defective surface of H-TiO 2−x can efficiently accelerate the adsorption and chemical activation of the extremely stable CO 2 molecule, which makes the single-electron reduction of CO 2 to CO 2 − easier.