Paper & Patents

The Hall scattering factor of Sc2CF2, Sc2CO2 and Sc2C(OH)2 is calculated using Rode’s iterative approach by solving the Boltzmann transport equation. This is carried out in conjunction with calculations based on density functional theory. The electrical transport in Sc2CF2, Sc2CO2 and Sc2C(OH)2 is modelled by accounting for both elastic (acoustic and piezoelectric) and inelastic (polar optical phonon) scattering. Polar optical phonon (POP) scattering is the most significant mechanism in these MXenes. We observe that there is a window of carrier concentration where Hall factor acts dramatically; Sc2CF2 obtains an incredibly high value of 2.49 while Sc2CO2 achieves a very small value of approximately 0.5, and Sc2C(OH)2 achieves the so called ideal value of 1. We propose in this paper that such Hall factor behaviour has significant promise in the field of surface group identification in MXenes, an issue that has long baffled researchers.

The record-high responsivity of 0.34 A W−1 is achieved by flexible and transparent Ti3C2Tx-platformed photodetectors (PDs) in self-powered operation. Transparent technology is suitable for future human electronic interface. The Ti3C2Tx/3D semiconductor (Tx = –F, –OH, –O, –Cl) structure has gained considerable attention in the design of high-performance optoelectronic devices. In particular, the unique mechanical and hydrophilic properties of Ti3C2Tx are favored for large-scale flexible photodetectors. However, the self-powered operation and transparency have remained unexplored for Ti3C2Tx/3D semiconductor-based flexible PDs. To access portable and implantable electronic devices, Ti3C2Tx was comprehensively utilized as an active electrical window and charge-transporting layer for transparent PDs. Density functional theory revealed the better charge-transporting channel due to the Ti3C2Tx-functional layer, resulting in the elevated pyro-phototronic current of Ti3C2Tx/Al2O3/ZnO/Ti3C2Tx/ITO/polyethylene terephthalate (PET) PDs. Based on the ultrafast photo-response of the PD (8 μs), an optically transparent (> 68 %) communication system was developed. The flexible and transparent PD (TPD) can process the incident Morse codes of encrypted optical signals to text (“MXENE TPD”) information effectively. This study combines the transparency and self-reliant characteristics into a wearable MXene/bulk semiconductor-based PD, exhibiting great potential for applications in energy-efficient imaging, communication, and health monitoring systems.

Hydrogen-based fuels demand high-density storage that can operate at ambient temperatures. Pd and its alloys are the most studied metal hydrides for hydrogen fuel cell applications. This study presented an alternative Pd alloy for hydrogen storage that can store and release hydrogen at room temperature. The surface of the most commonly studied Pd (110) was modified with Au and Rh such that the hydrogen adsorption energy was 0.49 eV and the release temperature was 365 K. Both values are quite close to the optimal values for the adsorption energy and release temperature of a hydrogen fuel cell in real use. We further show that the modified Pd (110) surface has significantly stronger oxygen evolution reaction (OER) catalytic properties than the pure Pd (110) surface.

Finding a suitable material for hydrogen storage at ambient atmospheric conditions is challenging for material scientists and chemists. In this work, using a first principles based cluster expansion approach, the hydrogen storage capacity of Ti2AC (A = Al, Ti, Cr, Mn, Fe, Co, Ni, Cu, and Zn) MAX phase and its alloys were studied. We found that hydrogen is energetically stable in Ti-A layers in which the tetrahedral site consisting of one A atom and three Ti atoms is energetically more favorable for hydrogen adsorption than other sites in the Ti-A layer. Ti2CuC has the highest hydrogen adsorption energy than other Ti2AC phases. We find that 83.33% Cu doped Ti2AlxCu1-xC alloy structure is both energetically and dynamically stable and can store 3.66 wt% hydrogen at ambient atmospheric conditions, which is higher than both Ti2AlC and Ti2CuC phase. These findings indicate that the hydrogen capacity of the MAX phase can be significantly improved by doping an appropriate atom species.

Spin gapless semiconductors exhibit a finite band gap for one spin channel and a closed gap for another spin channel, and they have emerged as a new state of magnetic materials with a great potential for spintronic applications. The first experimental evidence for spin gapless semiconducting behavior was observed in an inverse Heusler compound Mn2CoAl. Here, we report a detailed investigation of the crystal structure and anomalous Hall effect in Mn2CoAl using experimental and theoretical studies. The analysis of the high-resolution synchrotron x-ray diffraction data shows antisite disorder between Mn and Al atoms within the inverse Heusler structure. The temperature-dependent resistivity shows semiconducting behavior and follows Mooij's criteria for disordered metal. The scaling behavior of the anomalous Hall resistivity suggests that the anomalous Hall effect in Mn2CoAl is primarily governed by an intrinsic mechanism due to the Berry curvature in momentum space. The experimental intrinsic anomalous Hall conductivity (AHC) is found to be ∼35 S/cm, which is considerably larger than the theoretically predicted value for ordered Mn2CoAl. Our first-principles calculations conclude that the antisite disorder between Mn and Al atoms enhances the Berry curvature and hence the value of intrinsic AHC, which is in very good agreement with the experiment.

The oxygen deficient site on the catalyst has a strong impact on the activation of CO2 for the synthesis of dimethyl carbonate (DMC). The Co3O4/CeO2 catalyst exhibits multiple reduction behavior as cobalt metal species differ in the strength of their interaction with CeO2. This causes the surface reduction from Ce4+ to Ce3+ in solid solution Co-O-Ce. The dispersion of Co3O4 enhanced the formation of oxygen deficient site as revealed by XPS, CO2-chemisorption and TPR. The non-precious Co3O4/CeO2 nanorod was recognized as a potential catalyst for promoting Ce4+ to Ce3+ for CO2 activation and dimethyl carbonate synthesis (81.5% of yield). Energetics of oxygen vacancy formation of low index surfaces of CeO2 was determined with first-principles calculations based on DFT. Results disclosed the Ce4+ to Ce3+ formation energy of CeO2 due to Co substitution and corroborated the experimental results. Further, calculations provide the details of the effect of Co substitution on the electronic structure of reduced CeO2 surfaces. Estimated CO2 adsorption energy indicates (110) as the most active surface for activation of CO2.

Ligands can control the surface chemistry, physicochemical properties, processing, and applications of nanomaterials. MXenes are the fastest growing family of two-dimensional (2D) nanomaterials, showing promise for energy, electronic, and environmental applications. However, complex oxidation states, surface terminal groups, and interaction with the environment have hindered the development of organic ligands suitable for MXenes. Here, we demonstrate a simple, fast, scalable, and universally applicable ligand chemistry for MXenes using alkylated 3,4-dihydroxy-l-phenylalanine (ADOPA). Due to the strong hydrogen-bonding and π-electron interactions between the catechol head and surface terminal groups of MXenes and the presence of a hydrophobic fluorinated alkyl tail compatible with organic solvents, the ADOPA ligands functionalize MXene surfaces under mild reaction conditions without sacrificing their properties. Stable colloidal solutions and highly concentrated liquid crystals of various MXenes, including Ti2CTx, Nb2CTx, V2CTx, Mo2CTx, Ti3C2Tx, Ti3CNTx, Mo2TiC2Tx, Mo2Ti2C3Tx, and Ti4N3Tx, have been produced in various organic solvents. Such products offer excellent electrical conductivity, improved oxidation stability, and excellent processability, enabling applications in flexible electrodes and electromagnetic interference shielding.

Identifying the existence of specific functional groups in MXenes is a difficult topic that has perplexed researchers for a long time. At the same time, the realization of intrinsic ferromagnetism, which is important for two-dimensional (2D) materials’ families, is one of the fascinating properties that MXenes offer. Here, using first-principle calculations, we have examined the actual magneto-transport phenomena in transition metals and nitride-based-functionalized MXenes. We demonstrate that intrinsic anomalous Hall conductivity (AHC) can be used to identify the presence of more probable functional groups. The maximum AHC at Fermi energy, EF, is found in the case of Mn2NO2 (470 S/cm), which is attributed to the presence of avoided band crossing and a larger density of states. Together, when considering all the studied systems, the AHC can be above 2500 S/cm within EF± 0.25 eV. Our findings could be useful not only in guiding the experimentalists by considering AHC as a possible tool in determining the most probable functional groups in 2D ferro(i)magnets but also in designing memory devices with negligible stray fields.

Finding the active center in a bimetallic zeolite imidazolate framework (ZIF) is highly crucial for the electrocatalytic oxygen evolution reaction (OER). In the present study, we constructed a bimetallic ZIF system with cobalt and manganese metal ions and subjected it to an electrospinning technique for feasible fiber formation. The obtained nanofibers delivered a lower overpotential value of 302 mV at a benchmarking current density of 10 mA cm–2 in an electrocatalytic OER study under alkaline conditions. The obtained Tafel slope and charge-transfer resistance values were 125 mV dec–1 and 4 Ω, respectively. The kinetics of the reaction is mainly attributed from the ratio of metals (Co and Mn) present in the catalyst. Jahn–Teller distortion reveals that the electrocatalytic active center on the Mn-incorporated ZIF-67 nanofibers (Mn-ZIF-67-NFs) was found to be Mn3+ along with the Mn2+ and Co2+ ions on the octahedral and tetrahedral sites, respectively, where Co2+ ions tend to suppress the distortion, which is well supported by density functional theory analysis, molecular orbital study, and magnetic studies.

We propose a general rule for estimating the magnetic moments of Co2(cobalt)-based Heusler alloys, especially when doped with late transition metals. We come up with a descriptor that can characterise both pure Co2YZ compounds and the doped ones with the chemical formula Co2Y1−xMxZ (M is the dopant) using online data for magnetic moments of Heusler alloys with Co2YZ structure and compressive sensing approach. The newly proposed descriptor not only depends on the number of valence electrons of the compound also it depends on the number of unoccupied d-electrons in the doping site. A comparison of the performance of the proposed descriptor and the Slater-Pauling rule is made. Unlike the Slater-Pauling rule, which is only effective for half-metallic Heusler compounds, our machine-learning approach is more generic since it applies to any Co2YZ Heusler compounds, regardless of whether they are half-metals or not. We use this new rule to estimate the magnetic moments of a few yet-to-be-discovered Heusler compounds and compare the results to density functional theory (DFT) based calculations. Finally, we use DFT and machine learning investigations to prove their stability.

https://www.sciencedirect.com/science/article/abs/pii/S0304885322007119