Temperature-dependent mechanical properties of Tin+1CnO2 (n = 1, 2) MXene monolayers: a first-principles study

Two-dimensional (2D) transition metal carbides, carbonitrides, and nitrides (named as MXenes) have become of the fastest growing family of 2D materials in terms of compositions and their applications in different areas. One of the least explored properties of MXenes is their mechanical properties. While the basic elastic properties of MXenes have been studied by first-principles, the effects of temperature on the elastic properties have never been explored. In this study, we investigate temperature-dependent structural and mechanical properties of the titanium-containing MXenes (Tin+1CnO2 (n = 1, 2)) based on the first-principles calculations combined with quasi-harmonic approximation. The effective Young's modulus of a single layer of Ti2CO2 and Ti3C2O2 is calculated to be 565 and 482 GPa, respectively, at 0 K. By increasing temperature to 1000 K, Young's moduli of Ti2CO2 and Ti3C2O2 decrease to 469 GPa and 442 GPa, respectively, which indicates a larger reduction in stiffness in thinner MXenes at higher temperatures. Our calculations of the temperature-dependent bond strengths within MXenes showed that titanium and carbon atoms in Ti3C2O2 form stronger bonds than Ti2CO2 and atomic bonds in Ti2CO2 lose their stiffness more than Ti3C2O2 with increasing temperatures. The Debye temperature of these monolayers is also calculated to provide a comparison of the thermal conductivity between these monolayers, in which the results show that the Ti3C2O2 has a higher thermal conductivity than Ti2CO2. Our calculated electronic properties results of the monolayers are also shown that the electrical conductivity of the monolayers would not change with temperature. Our study extends MXenes applications to high-temperature applications, such as structural composite components and aerospace coatings

Improved Long‐Term Stability and Reduced Humidity Effect in Gas Sensing: SiO 2 Ultra‐Thin Layered ZnO Columnar Films

The undoped and metal-doped zinc oxide columnar films (ZnO:Sn, ZnO:Fe, ZnO:Ag, and ZnO:Cu) are covered with an ultra-thin layer of SiO2 (10–20 nm). The electrical, UV, and volatile organic compounds (VOCs) sensing properties are evaluated under different ambient conditions for ≈7 months to investigate the impact of the top SiO2-layer on the long-term stability of samples. The obtained results show a high immunity of sensing properties of SiO2-coated samples to humidity. Furthermore, gas sensing measurements show that the loss in response after 203 days is significantly lower for coated samples indicating higher stability of sensing performance. For ZnO:Fe the gas response is reduced by about 90% after 203 days, but for SiO2-coated ZnO:Fe columnar films the gas response is slightly reduced by only 38%. The density functional theory (DFT) calculations show that water species bind strongly with the surface SiO2 layer atoms with a −0.129 e− charge transfer, which is, much higher compared to the interaction with ethanol and acetone. Calculations show strong binding of water species on the SiO2 layer indicating preferential absorption of water molecules on SiO2. The obtained results demonstrate an important role of the top SiO2 ultra-thin layer in order to produce humidity-tolerant sensitive devices

High-Entropy 2D Carbide MXenes: TiVNbMoC3 and TiVCrMoC3

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 M4C3Tx MXenes. Specifically, we introduce two high-entropy MXenes, TiVNbMoC3Tx and TiVCrMoC3Tx, 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

Evaluation of surfactant flooding using interwell tracer analysis

Surfactant flooding is an important enhanced oil recovery (EOR) method, especially in carbonate oil reservoirs where water flooding may not have an effect on oil recovery as much as for sandstone reservoirs. This is because of the initial wettability of most carbonate reservoirs that is mixed- or oil-wet. Since surfactant flooding has a great impact on both fluid–fluid and rock–fluid interactions, it can be an efficient EOR method for these kinds of reservoirs. Surfactants affect fluid–fluid interactions by reducing interfacial tension (IFT) between water and oil phases and rock–fluid interactions by wettability alteration. The objective of this paper is the evaluation of these two surfactant mechanisms in non-fractured carbonate reservoirs using UTCHEM, the University of Texas chemical compositional simulator. In this paper, first, the laboratory data of two surfactant spontaneous imbibition tests for carbonate cores are successfully matched with modeled data to evaluate the mechanisms of surfactant flooding. Second, the field-scale surfactant flooding is simulated using the experimental data from spontaneous imbibition tests. Several cases are modeled in order to study the effect of surfactant flooding in terms of decreasing IFT and wettability alteration. Since the formation brine salinity in most reservoirs is more than the optimum salinity of surfactant phase behavior, the benefit of combining surfactant and low-salinity water is also investigated. Finally, tracer test simulation is performed to estimate the average oil saturation within the swept pore volume at the end of each recovery mode

Rheological properties of super critical CO2 with Al2O3: Material type, size and temperature effect

CO2 liquid is applied as an Enhanced Oil Recovery (EOR) method in oil reservoirs to increase the displacement efficiency. Because of the high temperature and pressure in the reservoirs, the viscosity of the CO2 decreases leading to poor macroscopic sweep efficiency. We study the effect of morphology of aluminum oxide nanoparticle (Al2O3 NP) on the rheological properties of super critical (SC)-CO2 such as viscosity and self-diffusion coefficient using molecular dynamic (MD). We investigate these properties in detail for relevant temperature (350, 380, and 410) K, pressure, 200 bar, and spherical diameter (1.0, 2.0, and 3.0 nm) at 1% volume fraction. Molecular dynamic (MD) simulates Al2O3 SC-CO2 nanofluid by using two force fields such as condensed-phase molecular potentials for atomistic simulation studies (COMPASS) and Charge optimization many body (COMB). The results show that the viscosity of the nanofluid has a direct proportional to temperature and reversely proportional to NP size. Moreover, NP-based material and NP shape exhibit significant effect of enhancement in the nanofluid viscosity in comparison with the cylindrical CuO NP in our previous study. The relative viscosity is enhanced almost 3.6 times for smallest NP at 380 K. Also, implementing 1.0 nm Al2O3 SC-CO2 nanofluid improves the relative viscosity from 1.94 to 3.59 and then to 3.67 by increasing temperature from 350 to 380 to 410 K

Temperature-dependent mechanical properties of Tin+1CnO2 (n= 1, 2) MXene monolayers: a first-principles study

Two-dimensional (2D) transition metal carbides, carbonitrides, and nitrides (named as MXenes) have become of the fastest growing family of 2D materials in terms of compositions and their applications in different areas. One of the least explored properties of MXenes is their mechanical properties. While the basic elastic properties of MXenes have been studied by first-principles, the effects of temperature on the elastic properties have never been explored. In this study, we investigate temperature-dependent structural and mechanical properties of the titanium-containing MXenes (Tin+1CnO2 (n = 1, 2)) based on the first-principles calculations combined with quasi-harmonic approximation. The effective Young's modulus of a single layer of Ti2CO2 and Ti3C2O2 is calculated to be 565 and 482 GPa, respectively, at 0 K. By increasing temperature to 1000 K, Young's moduli of Ti2CO2 and Ti3C2O2 decrease to 469 GPa and 442 GPa, respectively, which indicates a larger reduction in stiffness in thinner MXenes at higher temperatures. Our calculations of the temperature-dependent bond strengths within MXenes showed that titanium and carbon atoms in Ti3C2O2 form stronger bonds than Ti2CO2 and atomic bonds in Ti2CO2 lose their stiffness more than Ti3C2O2 with increasing temperatures. The Debye temperature of these monolayers is also calculated to provide a comparison of the thermal conductivity between these monolayers, in which the results show that the Ti3C2O2 has a higher thermal conductivity than Ti2CO2. Our calculated electronic properties results of the monolayers are also shown that the electrical conductivity of the monolayers would not change with temperature. Our study extends MXenes applications to high-temperature applications, such as structural composite components and aerospace coatings

Temperature-dependent properties of magnetic CuFeS2 from first-principles calculations: Structure, mechanics, and thermodynamics

Chalcopyrite (CuFeS2) is an antiferromagnetic semiconductor with promising magnetic and electrical properties, although these properties are not yet completely understood. The structural, magnetic, and electronic properties of bulk CuFeS2 were studied via first-principles plane-wave pseudopotential calculations based on density functional theory (DFT) using DFT+U and hybrid functional B3LYP methodology. The temperature-dependent structural, thermal, and mechanical properties of tetragonal CuFeS2 were also investigated via density functional perturbation theory. Furthermore, the structural parameters, elastic constants, bulk and shear moduli, volume expansion, and specific heats as a function of temperature were evaluated. Tetragonal CuFeS2 was found to exhibit negative thermal expansion behavior at temperatures lower than 100 K. A comprehensive comparison of the various calculated parameters with earlier published studies is also presented along with available experimental data and used as a basis to critically discuss the various properties of CuFeS2

Atomic defects in monolayer ordered double transition metal carbide (Mo2TiC2Tx) MXene and CO2 adsorption

Transition metal carbides (MXenes) with formula Mn+1CnTx (n = 2 and 3) have been emerging as a new family of two-dimensional (2D) materials that have great potential in electronic applications and CO2 conversion catalysts. It has been already found that the electronic and electrochemical properties of Ti3C2Tx MXenes can be tuned by replacing the two outer titanium layers with molybdenum layers. Similar to other 2D materials, intrinsic defects can be formed in the synthesized MXene flakes and the formation of defects can influence the performance of these materials. Herein, we systematically study the effect of the different types of structural defects on the structural stability, electronic behavior, and electrochemical properties of ordered Mo2TiC2Tx terminated with the specific surface functional groups of fluorine, oxygen, and hydroxide. The calculated defect formation energies imply that the formation of defects is dependent on the surface terminations, where the O-terminated MXenes demand more energy than the F- and OH-terminated MXenes. We found that defect formation is more feasible in the outer molybdenum layers than in the inner titanium layer. Our results predicted that the CO2 molecule adsorbs on the defective surfaces through a spontaneous and exothermic process that is critical to its capture, while the perfect surface weakly attracts the molecule through a nonspontaneous and endothermic process. Thus, our study predicts that the electronic and electrochemical properties of Mo2TiC2Tx can be tuned by forming specific defects and these MXenes could be promising materials for CO2 adsorption and conversion

MXene-Based Materials for Solar Cell Applications

MXenes are a class of two-dimensional nanomaterials with exceptional tailor-made properties, making them promising candidates for a wide variety of critical applications from energy systems, optics, electromagnetic interference shielding to those advanced sensors, and medical devices. Owing to its mechano-ceramic nature, MXenes have superior thermal, mechanical, and electrical properties. Recently, MXene-based materials are being extensively explored for solar cell applications wherein materials with superior sustainability, performance, and efficiency have been developed in demand to reduce the manufacturing cost of the present solar cell materials as well as enhance the productivity, efficiency, and performance of the MXene-based materials for solar energy harvesting. It is aimed in this review to study those MXenes employed in solar technologies, and in terms of the layout of the current paper, those 2D materials candidates used in solar cell applications are briefly reviewed and discussed, and then the fabrication methods are introduced. The key synthesis methods of MXenes, as well as the electrical, optical, and thermoelectric properties, are explained before those research efforts studying MXenes in solar cell materials are comprehensively discussed. It is believed that the use of MXene in solar technologies is in its infancy stage and many research efforts are yet to be performed on the current pitfalls to fill the existing voids