Transition metal carbides and nitrides (MXenes), a family of two-dimensional (2D) inorganic compounds, are materials composed of a few atomic layers of transition metal carbides, nitrides, or carbonitrides. Ti3C2, the first 2D layered MXene, was isolated in 2011. This material, which is a layered bulk material analogous to graphite, was derived from its 3D phase, Ti3AlC2 MAX. Since then, material scientists have either determined or predicted the stable phases of >200 different MXenes based on combinations of various transition metals such as Ti, Mo, V, Cr, and their alloys with C and N. Extensive experimental and theoretical studies have shown their exciting potential for energy conversion and electrochemical storage. To this end, we comprehensively summarize the current advances in MXene research. We begin by reviewing the structure types and morphologies and their fabrication routes. The review then discusses the mechanical, electrical, optical, and electrochemical properties of MXenes. The focus then turns to their exciting potential in energy storage and conversion. Energy storage applications include electrodes in rechargeable lithium- and sodium-ion batteries, lithium-sulfur batteries, and supercapacitors. In terms of energy conversion, photocatalytic fuel production, such as hydrogen evolution from water splitting, and carbon dioxide reduction are presented. The potential of MXenes for the photocatalytic degradation of organic pollutants in water, such as dye waste, is also addressed, along with their promise as catalysts for ammonium synthesis from nitrogen. Finally, their application potential is summarized.

Applications of 2D MXenes in energy conversion and storage systems

2D transition metal carbides, carbonitrides, and nitrides, known as MXenes, are a rapidly growing family of 2D materials with close to 30 members experimentally synthesized, and dozens more studied theoretically. They exhibit outstanding electronic, optical, mechanical, and thermal properties with versatile transition metal and surface chemistries. They have shown promise in many applications, such as energy storage, electromagnetic interference shielding, transparent electrodes, sensors, catalysis, photothermal therapy, etc. The high electronic conductivity and wide range of optical absorption properties of MXenes are the key to their success in the aforementioned applications. However, relatively little is currently known about their fundamental electronic and optical properties, limiting their use to their full potential. Here, MXenes' electronic and optical properties from both theoretical and experimental perspectives, as well as applications related to those properties, are discussed, providing a guide for researchers who are exploring those properties of MXenes.

Electronic and Optical Properties of 2D Transition Metal Carbides and Nitrides (MXenes).

Heterocyclic pyrrole molecules are in situ aligned and polymerized in the -absence of an oxidant between layers of the 2D Ti3C2Tx (MXene), resulting in high volumetric and gravimetric capacitances with capacitance retention of 92% after 25,000 cycles at a 100 mV s(-1) scan rate.

Pseudocapacitive Electrodes Produced By Oxidant-Free Polymerization of Pyrrole Between the Layers of 2D Titanium Carbide (MXene)

Two-dimensional transition metal carbides and nitrides known as MXenes, with a general formula of Mn+1Xn (n = 1–3), integrate the advantages of metallic conductive transition metals with large groups of carbides, nitrides, or carbonitrides. They have led to a burgeoning research interest in biomedical applications due to their ultrathin structure and fascinating physiochemical (electronic, optical, magnetic, etc.) properties. In this review, we summarize recent advances in biomedical applications for MXenes. We first introduce the preparation methods and surface modifications with respect to MXenes. Their unique properties are then elaborated. Thirdly, we highlight their various biomedical applications, such as with biosensors, antibacterial materials, bioimaging probes, therapeutics, and theranostics. In the end, the current challenges and new opportunities for MXenes in regard to their biomedical applications are also discussed.

Two-dimensional transition metal carbides and nitrides (MXenes) for biomedical applications.

Owing to the exceptional properties of graphene, intensive studies have been carried out on novel two-dimensional (2D) materials. In the past several years, an elegant exfoliation approach has been used to successfully create a new family of 2D transition metal carbides, nitrides, and carbonitrides, termed MXene, from layered MAX phases. More recently, some unique properties of MXene have been discovered leading to proposals of potential applications. In this review, we summarize the latest progress in development of MXene from both a theoretical and experimental view, with emphasis on the possible applications.

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Recent advances in MXene: Preparation, properties, and applications

TiO2 nanoparticle modified organ-like Ti3C2 MXene nanocomposite encapsulating hemoglobin for a mediator-free biosensor with excellent performances.

https://iopscience.iop.org/article/10.1149/2.0981605jes/pdf

Composites of TiO2 Nanoparticles Deposited on Ti3C2 MXene Nanosheets with Enhanced Electrochemical Performance

https://iopscience.iop.org/article/10.1149/2.0371501jes/pdf

An Organ-Like Titanium Carbide Material (MXene) with Multilayer Structure Encapsulating Hemoglobin for a Mediator-Free Biosensor

Two kinds of Ti3C2/TiO2 hybrids were synthesized by room temperature oxidation methods, named as Ti3C2/TiO2–nanoparticles and Ti3C2/TiO2–nanowires, respectively. Effects of reaction time and concentration of NaOH solution on the microstructures and electrochemical performances of Ti3C2/TiO2 composites were studied in details. When used as electrodes for supercapacitors, both Ti3C2/TiO2–nanoparticles and Ti3C2/TiO2–nanowires exhibit enhanced supercapacitive performances. The electrochemical measurement results show that the specific capacitances of Ti3C2/TiO2–nanoparticles and Ti3C2/TiO2–nanowires electrodes are 128 and 143 F/g at 2 mV/s, respectively, which are larger than that of Ti3C2 MXene (91 F/g). Furthermore, after 6000 charge–discharge cycles at 5 A/g, 88% and 80% of the initial capacitances are retained for Ti3C2/TiO2–nanoparticles and Ti3C2/TiO2–nanowires, respectively. Therefore, this work not only enhances the understanding of the surface states and morphology structures of Ti3C2 MXene at different reaction condition in the aqueous solutions, but also provides simple and facile ways to fabricate Ti3C2/TiO2 composites with superior supercapacitive performances.

https://iopscience.iop.org/article/10.1149/2.1541714jes/pdf

Jianfeng Zhu

Papers

TiO2 nanoparticle modified organ-like Ti3C2 MXene nanocomposite encapsulating hemoglobin for a mediator-free biosensor with excellent performances.

Composites of TiO2 Nanoparticles Deposited on Ti3C2 MXene Nanosheets with Enhanced Electrochemical Performance

An Organ-Like Titanium Carbide Material (MXene) with Multilayer Structure Encapsulating Hemoglobin for a Mediator-Free Biosensor

Room Temperature Oxidation of Ti3C2 MXene for Supercapacitor Electrodes

Enhanced supercapacitive performance of manganese oxides doped two-dimensional titanium carbide nanocomposite in alkaline electrolyte