The family of 2D transition metal carbides, carbonitrides and nitrides (collectively referred to as MXenes) has expanded rapidly since the discovery of Ti3C2 in 2011. The materials reported so far always have surface terminations, such as hydroxyl, oxygen or fluorine, which impart hydrophilicity to their surfaces. About 20 different MXenes have been synthesized, and the structures and properties of dozens more have been theoretically predicted. The availability of solid solutions, the control of surface terminations and a recent discovery of multi-transition-metal layered MXenes offer the potential for synthesis of many new structures. The versatile chemistry of MXenes allows the tuning of properties for applications including energy storage, electromagnetic interference shielding, reinforcement for composites, water purification, gas- and biosensors, lubrication, and photo-, electro- and chemical catalysis. Attractive electronic, optical, plasmonic and thermoelectric properties have also been shown. In this Review, we present the synthesis, structure and properties of MXenes, as well as their energy storage and related applications, and an outlook for future research. More than twenty 2D carbides, nitrides and carbonitrides of transition metals (MXenes) have been synthesized and studied, and dozens more predicted to exist. Highly electrically conductive MXenes show promise in electrical energy storage, electromagnetic interference shielding, electrocatalysis, plasmonics and other applications.

/pdf/2d-metal-carbides-and-nitrides-mxenes-for-energy-storage-4zg2z3wp7h.pdf

2D metal carbides and nitrides (MXenes) for energy storage

There is still an ongoing effort to search for sustainable, clean and highly efficient energy generation to satisfy the energy needs of modern society. Among various advanced technologies, electrocatalysis for the oxygen evolution reaction (OER) plays a key role and numerous new electrocatalysts have been developed to improve the efficiency of gas evolution. Along the way, enormous effort has been devoted to finding high-performance electrocatalysts, which has also stimulated the invention of new techniques to investigate the properties of materials or the fundamental mechanism of the OER. This accumulated knowledge not only establishes the foundation of the mechanism of the OER, but also points out the important criteria for a good electrocatalyst based on a variety of studies. Even though it may be difficult to include all cases, the aim of this review is to inspect the current progress and offer a comprehensive insight toward the OER. This review begins with examining the theoretical principles of electrode kinetics and some measurement criteria for achieving a fair evaluation among the catalysts. The second part of this review acquaints some materials for performing OER activity, in which the metal oxide materials build the basis of OER mechanism while non-oxide materials exhibit greatly promising performance toward overall water-splitting. Attention of this review is also paid to in situ approaches to electrocatalytic behavior during OER, and this information is crucial and can provide efficient strategies to design perfect electrocatalysts for OER. Finally, the OER mechanism from the perspective of both recent experimental and theoretical investigations is discussed, as well as probable strategies for improving OER performance with regards to future developments.

Electrocatalysis for the oxygen evolution reaction: recent development and future perspectives

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, electrocat...

Recent Advances in Ultrathin Two-Dimensional Nanomaterials

High-performance gas sensors prepared using p-type oxide semiconductors such as NiO, CuO, Cr2O3, Co3O4, and Mn3O4 were reviewed. The ionized adsorption of oxygen on p-type oxide semiconductors leads to the formation of hole-accumulation layers (HALs), and conduction occurs mainly along the near-surface HAL. Thus, the chemoresistive variations of undoped p-type oxide semiconductors are lower than those induced at the electron-depletion layers of n-type oxide semiconductors. However, highly sensitive and selective p-type oxide-semiconductor-based gas sensors can be designed either by controlling the carrier concentration through aliovalent doping or by promoting the sensing reaction of a specific gas through doping/loading the sensor material with oxide or noble metal catalysts. The junction between p- and n-type oxide semiconductors fabricated with different contact configurations can provide new strategies for designing gas sensors. p-Type oxide semiconductors with distinctive surface reactivity and oxygen adsorption are also advantageous for enhancing gas selectivity, decreasing the humidity dependence of sensor signals to negligible levels, and improving recovery speed. Accordingly, p-type oxide semiconductors are excellent materials not only for fabricating highly sensitive and selective gas sensors but also valuable additives that provide new functionality in gas sensors, which will enable the development of high-performance gas sensors.

Highly sensitive and selective gas sensors using p-type oxide semiconductors: Overview

The stability of two-dimensional (2D) layers and membranes is subject of a long standing theoretical debate. According to the so called Mermin-Wagner theorem, long wavelength fluctuations destroy the long-range order for 2D crystals. Similarly, 2D membranes embedded in a 3D space have a tendency to be crumpled. These dangerous fluctuations can, however, be suppressed by anharmonic coupling between bending and stretching modes making that a two-dimensional membrane can exist but should present strong height fluctuations. The discovery of graphene, the first truly 2D crystal and the recent experimental observation of ripples in freely hanging graphene makes these issues especially important. Beside the academic interest, understanding the mechanisms of stability of graphene is crucial for understanding electronic transport in this material that is attracting so much interest for its unusual Dirac spectrum and electronic properties. Here we address the nature of these height fluctuations by means of straightforward atomistic Monte Carlo simulations based on a very accurate many-body interatomic potential for carbon. We find that ripples spontaneously appear due to thermal fluctuations with a size distribution peaked around 70 \AA which is compatible with experimental findings (50-100 \AA) but not with the current understanding of stability of flexible membranes. This unexpected result seems to be due to the multiplicity of chemical bonding in carbon.

Intrinsic ripples in graphene

Free-standing flexible films, constructed from two-dimensional graphitic carbon nitride and titanium carbide (with MXene phase) nanosheets, display outstanding activity and stability in catalyzing the oxygen-evolution reaction in alkaline aqueous system, which originates from the Ti-N(x) motifs acting as electroactive sites, and the hierarchically porous structure with highly hydrophilic surface. With this excellent electrocatalytic ability, comparable to that of the state-of-the-art precious-/transition-metal catalysts and superior to that of most free-standing films reported to date, they are directly used as efficient cathodes in rechargeable zinc-air batteries. Our findings reveal that the rational interaction between different two-dimensional materials can remarkably promote the oxygen electrochemistry, thus boosting the entire clean energy system.

Interacting Carbon Nitride and Titanium Carbide Nanosheets for High-Performance Oxygen Evolution.

A series of mesoporous CuO–Fe 2 O 3 composite oxide catalysts with different CuO contents were prepared by a surfactant-assisted method of nanoparticle assembly. The prepared composite oxides were characterized by X-ray diffraction, N 2 adsorption, transmission electron microscopy, hydrogen temperature-programmed reduction, thermogravimetry–differential thermal analysis and X-ray photoelectron spectroscopy. Their catalytic behavior for low-temperature CO oxidation was studied by using a microreactor-GC system. These mesoporous CuO–Fe 2 O 3 catalysts possess a wormhole-like mesostructure with a narrow pore size distribution and high surface area, exhibiting high catalytic activity and stability for low-temperature CO oxidation. The catalytic behavior depended on the CuO content, the precalcination temperature, the surface area and the particle size of the catalysts. The catalyst with 15 mol% CuO content and calcined at 300 °C exhibited the highest catalytic activity and stability.

Mesoporous CuO–Fe2O3 composite catalysts for low-temperature carbon monoxide oxidation

High-surface area mesoporous CuO/Ce 0.8 Zr 0.2 O 2 catalysts were prepared by a surfactant-assisted method of nanocrystalline particle assembly, and characterized by XRD, N 2 adsorption, TEM, H 2 -TPR, TG-DTA and XPS techniques. The catalytic properties of the CuO/Ce 0.8 Zr 0.2 O 2 nanocatalysts were evaluated by low-temperature carbon monoxide oxidation using a microreactor-GC system. XRD and TEM analysis indicated that the catalyst particles were nanoscaled with cubic, fluorite structure. N 2 adsorption–desorption isotherms revealed a mesoporous nanocatalyst system with high-surface area and uniform pore-size distribution. The results of catalytic activity measurements showed that these mesoporous nanostructured CuO/Ce 0.8 Zr 0.2 O 2 catalysts were very active for low-temperature CO oxidation. The catalytic behavior depended on the CuO loading amount, the calcination temperature, the surface area and the particle size of the catalyst. The catalyst with 25 mol% CuO loading and calcined at 400 °C exhibited the highest catalytic activity.

Preparation, characterization and catalytic behavior of nanostructured mesoporous CuO/Ce0.8Zr0.2O2 catalysts for low-temperature CO oxidation

Ag-doped α-Fe 2 O 3 nanoparticles were synthesized by a chemical coprecipitation method and characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM), thermogravimetry-differential thermal analysis (TG-DTA), X-ray photoelectron spectroscopy (XPS) and Brunauer–Emmett–Teller specific surface area analysis (BET) techniques. Obtained results indicated that spherical Ag grains with size of about 5 nm are highly dispersed on the surface of α-Fe 2 O 3 nanoparticles. The surface area of the Ag/Fe 2 O 3 nanoparticles is several times as large as that of pure α-Fe 2 O 3 . The H 2 S sensing properties of these Ag/Fe 2 O 3 sensors were systematically investigated. In comparison with pure α-Fe 2 O 3 , all of the Ag-doped sensors showed better sensing performance in respect of response, selectivity and optimum operating temperature. The effects of Ag content, calcination and operation temperature on the sensing characteristics of the Ag/α-Fe 2 O 3 sensors were also investigated. The sensor containing 2 wt% Ag and calcined at 400 °C exhibited the maximum response to H 2 S at 160 °C. A possible mechanism for the influence of Ag on the H 2 S-sensing properties of Ag/α-Fe 2 O 3 sensors was proposed.

Jianliang Cao

Papers

Interacting Carbon Nitride and Titanium Carbide Nanosheets for High-Performance Oxygen Evolution.

Mesoporous CuO–Fe2O3 composite catalysts for low-temperature carbon monoxide oxidation

Preparation, characterization and catalytic behavior of nanostructured mesoporous CuO/Ce0.8Zr0.2O2 catalysts for low-temperature CO oxidation

Low-temperature H2S sensors based on Ag-doped α-Fe2O3 nanoparticles

CuO catalysts supported on attapulgite clay for low-temperature CO oxidation