Two-dimensional (2D) transition metal carbides and nitrides, known as MXenes, are a fast-growing family of 2D materials. MXenes 2D flakes have n + 1 (n = 1–4) atomic layers of transition metals interleaved by carbon/nitrogen layers, but to-date remain limited in composition to one or two transition metals. In this study, by implementing four transition metals, we report the synthesis of multi-principal-element high-entropy M4C3T x MXenes. Specifically, we introduce two high-entropy MXenes, TiVNbMoC3T x and TiVCrMoC3T x , as well as their precursor TiVNbMoAlC3 and TiVCrMoAlC3 high-entropy MAX phases. We used a combination of real and reciprocal space characterization (X-ray diffraction, X-ray photoelectron spectroscopy, energy dispersive X-ray spectroscopy, and scanning transmission electron microscopy) to establish the structure, phase purity, and equimolar distribution of the four transition metals in high-entropy MAX and MXene phases. We use first-principles calculations to compute the formation energies and explore synthesizability of these high-entropy MAX phases. We also show that when three transition metals are used instead of four, under similar synthesis conditions to those of the four-transition-metal MAX phase, two different MAX phases can be formed (i.e., no pure single-phase forms). This finding indicates the importance of configurational entropy in stabilizing the desired single-phase high-entropy MAX over multiphases of MAX, which is essential for the synthesis of phase-pure high-entropy MXenes. The synthesis of high-entropy MXenes significantly expands the compositional variety of the MXene family to further tune their properties, including electronic, magnetic, electrochemical, catalytic, high temperature stability, and mechanical behavior.

The rush to synthesize novel two-dimensional (2D) materials has excited the research community studying ternary-layered carbide and nitride compounds, known as MAX phases, for the past two decades in the...

Electronic and electrochemical properties of Mo2TiC2Tx could be tuned by forming the specific defects and these MXenes could be promising materials for CO2 activation and conversion.

Reducing the operating temperature to room temperature is a serious obstacle on long-life sensitivity with long-term stability performances of gas sensors based on semiconducting oxides and this should be overcome by new nano-technological approaches. In this work, we report the structural, morphological, chemical, optical and gas detection characteristics of Eu-doped ZnO (ZnO:Eu) columnar films as a function of Eu content. The scanning electron microscopy (SEM) investigations showed that columnar films, grown via synthesis from chemical solutions (SCS) approach, are composed of densely packed columnar type grains. The sample sets with a content of ~0.05, 0.1, 0.15 and 0.2 at% of Eu in ZnO:Eu columnar films were studied. The surface functionalization was achieved using PdCl2 aqueous solution with additional thermal annealing in air at 650 ºC. The temperature dependent gas-detection characteristics of Pd-functionalized ZnO:Eu columnar films were measured in detail, showing a good selectivity towards H2 gas at operating OPT temperatures of 200-300 ºC among several test gases and volatile organic compounds (VOCs) vapors; such as methane, ammonia, acetone, ethanol, n-butanol and 2propanol. At an operating temperature OPT of 250 ºC a high gas response Igas/Iair ~ 115 for 100 ppm H2 was obtained. Experimental results indicate that Eu-doping with an optimal content about 0.05-0.1 at% along with Pd-functionalization of ZnO columns leads to a reduction of the operating temperature of the H2 gas sensor. DFT based computations provide mechanistic insights into the gas sensing mechanism by investigating interactions between the Pd-functionalized ZnO:Eu surface and H2 gas molecules supporting the experimentally observed results. The proposed columnar materials and gas sensor structures would provide a special advantage in the fields of fundamental research, applied physics studies, ecological and industrial applications.