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[Display omitted] • MXenes synthesized using molten salts (fluorinated and non-fluorinated). • Mechanical, optical, and electronic properties of MXenes are discussed. • Bandgap of MXenes is tuned for improved photoelectrochemical properties. • MXene can be applied as photoelectrochemical sensors (chemical, biological). MXene is a newly discovered family of 2D transition metal carbides, nitrides, or carbonitrides that is predominantly conductive. The most common MXene, titanium carbide (Ti 3 C 2), can be used as a conductive material to enhance the photocurrent generated by photoelectrochemical (PEC) sensors. In order to produce MXene sheets that are of high quality for PEC sensors and other light-based applications, it is imperative that the synthesis methods for MXene production continue to be improved and that an extensive understanding of its properties is gained. Several fluorinated and non-fluorinated MXene synthesis methods from MAX are discussed in this review. Detailed description of the mechanical, optical, topological, electronic, and electrochemical properties of the resultant MXenes are also presented. A critical analysis of the density functional theory (DFT) data for pure MXene, as well as the changes in the optical bandgap of MXene when doped with other heteroatoms to form heterojunctions or ternary nanocomposites, are extensively described. Additionally, a systematic exposition is done on the current MXene-based PEC sensors, highlighting the critical parameters that yield a high performing sensor. The review concludes with an opinion on the future direction of MXenes in PEC sensing and the electrochemical field. [ABSTRACT FROM AUTHOR]

The exfoliation behavior and superior photothermal conversion performance of six different MXenes have systematically revealed by combining experimental and theoretical studies, which will shed lights onto the effective design of diverse MXene materials and promote their applications from wearable devices, biomedical technology to solar water desalination. [Display omitted] • The different mono-M and double-M MXenes have been successfully synthesized. • The exfoliation behavior of MAX phases have been understood by theoretical study. • All the MXenes display fast-response and superior photothermal conversion ability. • The V 2 CT x MXene exhibits best saturation temperature, which outperforms Ti 3 C 2 T x. • The results promote the rapid development of MXenes in photothermal applications. Two dimensional transition metal carbides (MXenes) have attracted significant interest due to their unique layer structure, chemical diversity and superior physiochemical performances. However, most studies have focused on Ti 3 C 2 T x , and the synthesis process and photothermal conversion capability of other MXenes remain unclear. In this work, we have systematically investigated the exfoliation behavior of six different MXenes, including mono-transition-metal (Ti 3 C 2 T x , V 2 CT x , and Nb 2 CT x) and double-transition-metal (TiVCT x , TiNbCT x , and NbVCT x) MXenes derived from MAX phases, integrated with their photothermal conversion performance by combining experimental and theoretical studies. The phase, morphology, composition and surface groups of the different MXenes have been analyzed using XRD, SEM, XPS, and HRTEM techniques. Our results indicate a systematic variation in etching difficulty from difficult to easy, as Nb 2 AlC > NbVAlC > V 2 AlC > TiNbAlC > TiVAlC > Ti 3 AlC 2 , which correlates with the chemical composition and etching duration. Furthermore, the exfoliation factor m from density functional theory calculations reveals that the bond energies of M−A bonds are much weaker than M−X bonds in MAX phases, which agrees well with the experimental results. Interestingly that, all the obtained MXenes exhibit remarkable photothermal conversion performance with fast response, where the MXenes beyond Ti 3 C 2 T x , such as V 2 CT x , NbVCT x , Nb 2 CT x , TiVCT x , and TiNbCT x , display higher saturation temperature of 95, 88, 81, 78, and 71 °C, respectively, than that of Ti 3 C 2 T x (65 °C). These findings could potentially facilitate the efficient design and development of diverse MXene materials with outstanding photothermal conversion performance for applications in wearable devices, biomedical technology and solar water desalination. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ag and Au electrocatalysts supported on transition metal carbides (TMCs) and nitrides (TMNs) were synthesized and evaluated for CO 2 RR. • TaC and TaN were identified as promising supports for Ag and Au catalysts for electroreduction of CO 2 to synthesis gas. • ΔG (*HOCO) was identified as a key descriptor for CO 2 RR based on a linear correlation between ΔG (*HOCO) and J ECSA (CO). • Electron transfer from TMC/TMN substrates to Ag/Au played a dominant role in controlling the CO 2 RR activity. Synthesis gas produced from the electrochemical carbon dioxide reduction reaction (CO 2 RR) using renewable energy represents a promising solution toward carbon neutrality. Both high activity and controlled CO/H 2 ratios are crucial for coupling CO 2 RR with downstream thermochemical reactions. In the current study, transition metal carbides (TMCs) and nitrides (TMNs) of Group VB elements (V, Nb and Ta) were used as supports to tune the CO 2 RR activity and CO/H 2 ratios over Ag- and Au-based electrocatalysts. Combined electrochemical measurements and density functional theory (DFT) calculations identified the Gibbs free energy for adsorption of the *HOCO intermediate (ΔG (*HOCO)) as a potential descriptor for CO 2 RR, which could be adjusted by using different TMC and TMN supports. As opposed to structural strain effects, the electronic modification was found to be the key controlling factor in the interactions between TMC/TMN and supported Ag/Au. TaC and TaN have been identified as promising supports for Ag- and Au-based electrocatalysts for CO 2 RR to synthesis gas production. [ABSTRACT FROM AUTHOR]

• 1 wt.% Pt was loaded on different TMCs and used as the electrocatalysts for MOR. • The specific activity followed the trend of Pt/NbC > Pt/TaC > Pt/WC > Pt/TiC > Pt/VC > Pt/ZrC. • H and CO binding energies were identified as descriptors for Pt/TMCs activity in MOR. • A mechanism was proposed to explain the promoting role of the metal carbides in MOR Some classes of electrocatalysts based on Pt supported early transition metal carbides (TMCs) have shown promise for methanol oxidation reaction (MOR). To bridge the material gap, we studied some of the promising and new electrocatalysts for MOR. We synthesized 1%wt Pt/TMCs (TMCs of group IV (Ti and Zr), group V (V, Nb and Ta) and group VI (W)) via wet impregnation method as low loading electrocatalysts for MOR in alkaline media. The synthesized materials were characterized by X-ray diffraction (XRD), inductively coupled plasma-optical emission spectroscopy (ICP-OES), N 2 physisorption using Brunauer-Emmett-Teller (BET), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The activity of the electrocatalysts were elucidated for MOR in alkaline media via combination of experimental and theoretical methods. Among all the investigated electrocatalysts, Pt/NbC was found to have the highest specific activity (3.58 mA cm Pt −2) while Pt/ZrC had the least (0.410 mA cm Pt −2). Tafel slope measurements for Pt/TMC electrocatalysts with the exception of Pt/TiC varied from region of low potential to region of high potential and were 121.2 ± 11 mV dec−1 and 234 ± 10 mV dec−1 respectively, indicating change in limiting steps from C-H scission to CO poisoning. Density functional theory (DFT) calculations of the binding energies of H and CO on the Pt/TMC surfaces were correlated to their specific activity, and volcano-type relationships were discovered, which indicated that neither too weak nor too strong bond of H and CO on the electrocatalyst surfaces were favorable for high activity during MOR. Finally, a feasible reaction mechanism for Pt/TMC electrocatalysts in MOR was proposed based on the experimental and theoretical results. [ABSTRACT FROM AUTHOR]