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2D carbides and nitrides of transition metals, also known as MXenes, are an emerging class of 2D nanomaterials that have shown excellent performances and broad application prospects in the fields of energy storage, catalysis, sensing, electromagnetic shielding, electronics and photonics, and life sciences. This unusual diversity of applications is due to their superior hydrophilicity and conductivity, high carrier concentration, ultra-high volumetric capacitance, rich surface chemistry, and large specific surface area. However, it is difficult to make MXenes with the desired surface functional groups that deliver high reactivity and high stability, because most MXenes are extracted from ceramics (MAX phase) by an etching process, where a large number of metal atoms are inevitably exposed on the surface, with other anions and cations embedded uncontrollably. The exposed metal atoms and implanted ions are thermodynamically unstable and readily react with trace oxygen or oxygen-containing groups to form the corresponding metal oxides or degrade chemically, resulting in a sharp decline in activity and loss of excellent physicochemical properties. The addition of certain synergistic additives during the intercalation and chemical modification of surface functional groups under non-hazardous conditions can result in stable and efficient MXene-based materials with exceptional optical, electrical, and magnetic properties. This review discusses several such methods, mainly additive-mediated intercalation and chemical modification of the surface functional groups of MXene-based materials, followed by their potential applications. Finally, perspectives are given to discuss the future challenges and promising opportunities of this exciting field. [ABSTRACT FROM AUTHOR]

Hydrogels have recently garnered tremendous interest due to their potential application in soft electronics, human–machine interfaces, sensors, actuators, and flexible energy storage. Benefiting from their impressive combination of hydrophilicity, metallic conductivity, high aspect ratio morphology, and widely tuneable properties, when two-dimensional (2D) transition metal carbides/nitrides (MXenes) are incorporated into hydrogel systems, they offer exciting and versatile platforms for the design of MXene-based soft materials with tunable application-specific properties. The intriguing and, in some cases, unique properties of MXene hydrogels are governed by complex gel structures and gelation mechanisms, which require in-depth investigation and engineering at the nanoscale. On the other hand, the formulation of MXenes into hydrogels can significantly increase the stability of MXenes, which is often the limiting factor for many MXene-based applications. Moreover, through simple treatments, derivatives of MXene hydrogels, such as aerogels, can be obtained, further expanding their versatility. This tutorial review intends to show the enormous potential of MXene hydrogels in expanding the application range of both hydrogels and MXenes, as well as increasing the performance of MXene-based devices. We elucidate the existing structures of various MXene-containing hydrogel systems along with their gelation mechanisms and the interconnecting driving forces. We then discuss their distinctive properties stemming from the integration of MXenes into hydrogels, which have revealed an enhanced performance, compared to either MXenes or hydrogels alone, in many applications (energy storage/harvesting, biomedicine, catalysis, electromagnetic interference shielding, and sensing). [ABSTRACT FROM AUTHOR]

Two-dimensional transition metal carbides and/or nitrides (MXenes) have shown promise in developing electrochemical storage of metal ions within conductive galleries due to redox reactions with transition metal atoms. Here, effect of surface chemistry on electrochemical storage of sodium ions within MXene interlayers is investigated by etching Ti3AlC2 MAX using different etchants – HF, HF/HCl, and HF/H2SO4. [ABSTRACT FROM AUTHOR]

MXenes have demonstrated high performance as negative electrodes in supercapacitors with aqueous electrolytes due to their high redox capacitance. However, oxidation limits their use under positive potential, requiring asymmetric devices with positive electrodes made of other materials which are usually less capacitive compared to MXenes and therefore limit the device performances. Here, we report the synthesis of two-dimensional molybdenum vanadium carbides (MoxV4−xC3), previously unexplored double transition metal MXenes, by selective etching of aluminum from MoxV4−xAlC3 MAX phase precursors. Unlike the ordered double transition metal MXenes reported previously, MoxV4−xC3 exhibits a Mo–V solid solution in the transition metal layers. We have synthesized and characterized four different compositions of MoxV4−xC3 with x = 1, 1.5, 2, and 2.7. We showed that by changing the Mo : V ratio, the surface terminations (O : F ratio), and electrical and electrochemical properties of the resulting MXenes can be tuned. The Mo2.7V1.3C3 composition showed a remarkable volumetric capacitance (up to 860 F cm−3) and high electrical conductivity (830 S cm−1) at room temperature. Moreover, these solid solution MXenes have demonstrated the ability to operate in a wider range of positive potentials compared to other MXenes. Following this discovery, we coupled a Mo2.7V1.3C3 positive electrode with a well-studied Ti3C2 MXene negative electrode to create an all-MXene supercapacitor, as a proof of concept. [ABSTRACT FROM AUTHOR]

In this work, a pre-designed Zr-based-MOF encompassing an organic linker with a redox active core is synthesized and its structure–property relationship as a supercapacitor electrode is investigated. An enhanced performance is revealed by the combination of this MOF's high porosity and redox core incorporation, which alters its double-layer and pseudocapacitance, respectively. An increase in the capacitance performance by a factor of two is achieved via post-synthetic structure rigidification using organic pillars. [ABSTRACT FROM AUTHOR]

SiOx is a promising anode material for lithium-ion batteries (LIBs) due to its relatively high capacity. Nevertheless, the poor conductivity and large volume expansion of SiOx upon Li+ insertion/extraction limit its application. Herein, a novel strategy for preparing ternary Ni/SiOx/nitrogen-doped carbon (NSC) composites has been developed through in situ transformation of a Ni3Si2O5(OH)4/nitrogen-doped carbon precursor derived from dried bamboo leaves. The 3D interconnected SiOx/nitrogen-doped carbon (SC) framework provides sufficient void spaces for relieving the volume change during the lithiation process while facilitating electrolyte infiltration and lithium ion diffusion processes. The growth of uniform Ni nanoparticles (NPs) on the surface of the SC matrix restricts the formation of cracks, reduces volume expansion during the lithiation process, and effectively improves the electrical conductivity of SiOx. The optimized sample delivers a high discharge capacity of 864.6 mA h g−1 after 70 cycles at 200 mA g−1 and a superior rate capability of 289.8 mA h g−1 at 10 A g−1. The electrode delivers a capacity of 427.6 mA h g−1 even at 5 A g−1 after 1000 cycles along with outstanding capacity retention (∼100%). Our method provides insight into the utilization of biomass towards high performance energy storage through simple and low-cost procedures. [ABSTRACT FROM AUTHOR]

Tin-doped indium oxide (ITO) transparent conducting electrodes are widely used across the display industry, and are currently the cornerstone of photovoltaic device developments, taking a substantial share in the manufacturing cost of large-area modules. However, cost and supply considerations are set to limit the extensive use of indium for optoelectronic device applications and, in turn, alternative transparent conducting oxide (TCO) materials are required. In this report, we show that aluminum-doped zinc oxide (AZO) thin films grown by atomic layer deposition (ALD) are sufficiently conductive and transparent to outperform ITO as the cathode in inverted polymer solar cells. Reference polymer solar cells made with atomic-layer-deposited AZO cathodes, PCE10 as the polymer donor and PC71BM as the fullerene acceptor (model systems), reach power conversion efficiencies of ca. 10% (compared to ca. 9% with ITO-coated glass), without compromising other figures of merit. These ALD-grown AZO electrodes are promising for a wide range of optoelectronic device applications relying on TCOs. [ABSTRACT FROM AUTHOR]

Three-dimensional (3D) electrode architectures for conformal deposition and effective use of catalysts are an emerging area with significant interest in electrocatalytic applications. In this study, we report the fabrication of monolithic, self-standing, 3D graphitic carbon scaffolds with conformally deposited Pt by atomic layer deposition (ALD) as a hydrogen evolution reaction catalyst. Laser scribing is employed to transform polyimide into 3D porous graphitic carbon, which possesses good electronic conductivity and numerous edge plane sites. This laser scribed graphene (LSG) architecture makes it possible to fabricate a monolithic electrocatalyst support without any binders or conductive additives. The synergistic effect between the ALD of Pt and the 3D network of LSG provides an avenue for minimal yet effective usage of Pt leading to an enhanced HER activity. This strategy establishes a general approach for inexpensive and large scale HER device fabrication with minimum catalyst cost. [ABSTRACT FROM AUTHOR]

The measured structural, optical and electrical properties of Al, Ga and In doped ZnO films deposited using spray pyrolysis are reported over the doping range 0.1–3 at%. Over the entire doping series highly transparent, polycrystalline thin films are prepared. Using AC Hall as a measurement technique we probe the electronic properties of our doped films, deconvoluting the impact of doping on the measured charge carrier concentrations and Hall mobility. We focus on the low doping range i.e. 0.1–1 at%, where we observe unexpected variations in charge carrier concentration and mobility and propose a mechanism to explain our observations. In this doping range highly resistive films are formed hence we highlight AC Hall as a reliable and highly reproducible technique for analysing electrical properties and subsequently elucidate the doping mechanisms. The implementation of a simple, post-deposition heat treatment demonstrated on our AZO results in typical films with charge carrier concentrations exceeding ＞1019 cm−3, and electron mobilities ＞10 cm2 V−1 s−1 and stability exceeding 180 days. We describe in detail the nature of the defect chemistry and the role of intrinsic defects and show, that despite significant variations in dopant species and grain boundary concentrations, that the defect chemistry dominates the electrical characteristics. [ABSTRACT FROM AUTHOR]

By investigating the influence of key growth variables on the measured structural and electrical properties of SnO2 prepared by pulsed laser deposition (PLD) we demonstrate fine control of intrinsic n-type defect formation. Variation of growth temperatures shows oxygen vacancies (VO) as the dominant defect which can be compensated for by thermal oxidation at temperatures ＞500 °C. As a consequence films with carrier concentrations in the range 1016–1019 cm−3 can be prepared by adjusting temperature alone. By altering the background oxygen pressure (PD) we observe a change in the dominant defect – from tin interstitials (Sni) at low PD (＜50 mTorr) to VO at higher PD with similar ranges of carrier concentrations observed. Finally, we demonstrate the importance of controlling the composition target surface used for PLD by exposing a target to ＞100 000 laser pulses. Here carrier concentrations ＞1 × 1020 cm−3 are observed that are associated with high concentrations of Sni which cannot be completely compensated for by modifying the growth parameters. [ABSTRACT FROM AUTHOR]

Controlling the electron transfer process at donor–acceptor interfaces is a research direction that has not yet seen much progress. Here, with careful control of the charge localization on the porphyrin macrocycle using β-cyclodextrin as an external cage, we are able to improve the electron injection efficiency from cationic porphyrin to graphene carboxylate by 120%. The detailed reaction mechanism is also discussed. [ABSTRACT FROM AUTHOR]

Correction for 'Additive-mediated intercalation and surface modification of MXenes' by Jing Zou et al., Chem. Soc. Rev., 2022, DOI: 10.1039/d0cs01487g. [ABSTRACT FROM AUTHOR]

We report the engineering and synthesis of peptide nucleic acid-functionalized Ti 3 C 2 T x MXene nanosheets as a novel transducing material for amplification-free, nanoparticle-free, and isothermal electrochemical detection of microRNA biomarkers. Through bio-orthogonal copper-free click chemistry, azido-modified MXene nanosheets are covalently functionalized with clickable peptide nucleic acid probes targeting prostate cancer biomarker hsa-miR-141. The platform demonstrates a wide dynamic range, single-nucleotide specificity, and 40 aM detection limit outperforming more complex, amplification-based methods. Its versatility, analytical performance, and stability under serum exposure highlight the immense potential of this first example of click-conjugated MXene in the next generation of amplification-free biosensors.

Two-dimensional transition metal carbides and/or nitrides (MXenes) have shown promise in developing electrochemical storage of metal ions within conductive galleries due to redox reactions with transition metal atoms. Here, effect of surface chemistry on electrochemical storage of sodium ions within MXene interlayers is investigated by etching Ti 3 AlC 2 MAX using different etchants - HF, HF/HCl, and HF/H 2 SO 4 .