Single-atom catalysts supported on a hybrid structure of boron nitride/graphene for efficient nitrogen fixation via synergistic interfacial interactions.

Hexagonal boron nitride (BN) shows significant chemical stability and promising thermal nitrogen reduction reaction (NRR) activity but suffers from low conductivity in electrolysis with a wide band gap. To overcome this problem, two-dimensional (2D) BN and graphene (G) are designed as a heterostructure, namely BN/G. According to density functional theory (DFT), the higher conductivity of G narrows the band gap of BN by inducing some electronic states near the Fermi energy level (Ef). Once transition metals (TMs) are anchored in the BN/G structure as single atom catalysts (SACs), the NRR activity improves as the inert BN basal layer activates with moderate *NH2 binding energy and further the band gap is reduced to zero. V (vanadium) and W (tungsten) SACs exhibit the best performance with limiting potentials of -0.22 and -0.41 V, respectively. This study helps in understanding the improvement of the NRR activity of BN, providing physical insights into the adsorbate-TM interaction.

Universal Ligands for Dispersion of Two-Dimensional MXene in Organic Solvents.

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.

Unveiling the Role of Charge Transfer in Enhanced Electrochemical Nitrogen Fixation at Single-Atom Catalysts on BX Sheets (X = As, P, Sb).

To tune single-atom catalysts (SACs) for effective nitrogen reduction reaction (NRR), we investigate various transition metals implanted on boron-arsenide (BAs), boron-phosphide (BP), and boron-antimony (BSb) using density functional theory (DFT). Interestingly, W-BAs shows high catalytic activity and excellent selectivity with an insignificant barrier of only 0.05 eV along the distal pathway and a surmountable kinetic barrier of 0.34 eV. The W-BSb and Mo-BSb exhibit high performances with limiting potentials of -0.19 and -0.34 V. The Bader-charge descriptor reveals that the charge transfers from substrate to *NNH in the first protonation step and from *NH3 to substrate in the last protonation step, circumventing a big hurdle in NRR by achieving negative free energy change of *NH2 to *NH3. Furthermore, machine learning (ML) descriptors are introduced to reduce computational cost. Our rational design meets the three critical prerequisites of chemisorbing N2 molecules, stabilizing *NNH, and destabilizing *NH2 adsorbates for high-efficiency NRR.

Lattice Thermal Transport in Monolayer Group 13 Monochalcogenides MX (M = Ga, In; X = S, Se, Te): Interplay of Atomic Mass, Harmonicity, and Lone-Pair-Induced Anharmonicity.

We perform a systematic study of the lattice dynamics and the lattice thermal conductivity, κ, of monolayer group 13 monochalcogenides MX (M = Ga, In; X = S, Se, Te) by combining an iterative solution for linearized phonon Boltzmann transport equation and density functional theory. Among the competing factors influencing κ, harmonic parameters along with the atomic masses dominate over anharmonicity. An increase in atomic mass leads to a decrease in phonon frequencies and phonon group velocities and consequently in κ. At T = 300 K, the calculated κ values are 54.9, 48.1, 44.3, 25.0, 22.3, and 17.3 W m-1 K-1 for GaS, InS, GaSe, InSe, GaTe, and InTe monolayers, respectively. Further analysis of anharmonic scattering rates and average scattering matrix elements evidences that the anharmonicity characterized by the third-order IFCs in GaS and InS are the largest among all monolayer group 13 monochalcogenides despite the largest κ values. This is attributed to a strong interaction between nonbonding lone-pair s electrons around the S atom and adjacent bonding electrons. In addition, the κ of these monolayers further reduces to 50% for sample sizes 300-400 nm. Our findings provide fundamental insights into thermal transport in monolayer group 13 monochalcogenides and should stimulate further experimental exploration of thermal transport in these materials for possible theromoelectric and thermal management applications.

Phonon transport and thermoelectric properties of semiconducting Bi2Te2X (X = S, Se, Te) monolayers.

Confinement or dimensionality reduction is a novel strategy to reduce the lattice thermal conductivity and, consequently, to improve the thermoelectric conversion performance. Bismuth and tellurium based low-dimensional materials have great potential in this regard. The phonon transport and thermoelectric properties of Bi2Te2X (X = S, Se, Te) monolayers are systematically investigated by employing density functional theory and the Boltzmann transport equation. The calculated lattice thermal conductivity of these 2D systems ranges from ∼1.3 W m-1 K-1 (Bi2Te2Se) to ∼1.5 W m-1 K-1 (Bi2Te3) for a 10 µm system size at room temperature and considering spin-orbit coupling in harmonic force constants. This remarkably low lattice thermal conductivity is attributed to small group velocities and enhanced anharmonic phonon scattering rates. A detailed analysis is presented in terms of mode-level phonon group velocities, anharmonic scattering rates and phonon mean free paths. Our results reveal that the thermal transport in these 2D systems is dominated by in-plane transverse acoustic modes. Additionally, the thermal conductivity can be further reduced by decreasing the sample size due to phonon-boundary scattering. The thermoelectric properties including the Seebeck coefficient, power factor and electrical conductivity are calculated using the semi-classical Boltzmann transport equation within the rigid band approximation. The low thermal conductivities coupled with their high carrier mobilities lead to good thermoelectric power factors. With optimal carrier doping, a figure of merit ∼0.6 can be achieved at room temperature, which increases to ∼0.8 at 700 K, thus making them promising candidates for thermoelectric applications.

Prediction and Characterization of NaGaS2, A High Thermal Conductivity Mid-Infrared Nonlinear Optical Material for High-Power Laser Frequency Conversion.

Infrared nonlinear optical (IR NLO) crystals are the major materials to widen the output range of solid-state lasers to mid- or far-infrared regions. The IR NLO crystals used in the middle IR region are still inadequate for high-power laser applications because of deleterious thermal effects (lensing and expansion), low laser-induced damage threshold, and two-photon absorption. Herein, the unbiased global minimum search method was used for the first time to search for IR NLO optical materials and ultimately found a new IR NLO material NaGaS2. It meets the stringent demands for IR NLO materials pumped by high-power laser with the highest thermal conductivity among common IR NLO materials able to avoid two-photon absorption, a classic nonlinear coefficient, and wide infrared transparency.

Thermal transport in monolayer InSe.

Two-dimensional InSe, a recently synthesized semiconductor having a moderate band gap, has gained attention due to its ultra high mobility and high photo-responsivity. In this work, we calculate the lattice thermal conductivity (κ) of monolayer InSe by solving the phonon Boltzmann transport equation (BTE) with first-principles calculated inter atomic force constants. κ of monolayer InSe is isotropic and found to be around 27.6 W m [Formula: see text] at room temperature along the in-plane direction. The size dependence of κ shows the size effect can persist up to 20 µm. Further, κ can be reduced to half by tuning the sample size to 300 nm. This low value suggests that κ might be a limiting factor for emerging nanoelectronic applications of monolayer InSe.

Diffusive nature of thermal transport in stanene.

Using the phonon Boltzmann transport formalism and density functional theory based calculations, we show that stanene has a low thermal conductivity. For a sample size of 1 × 1 µm(2) (L × W), the lattice thermal conductivities along the zigzag and armchair directions are 10.83 W m(-1) K(-1) and 9.2 W m(-1) K(-1) respectively, at room temperature, indicating anisotropy in thermal transport. The low values of thermal conductivity are due to large anharmonicity in the crystal resulting in high Grüneisen parameters, and low group velocities. The room temperature effective phonon mean free path is found to be around 17 nm indicating that the thermal transport in stanene is completely diffusive in nature. Furthermore, our study reveals the relative importance of the contributing phonon branches and that, at very low temperatures, the contribution to lattice thermal conductivity comes from the flexural acoustic (ZA) branch and at higher temperatures it is dominated by the longitudinal acoustic (LA) branch. We also show that the lattice thermal conductivity of stanene can further be reduced by tuning the sample size and creating rough surfaces at the edges. Such tunability of lattice thermal conductivity in stanene suggests its applications in thermoelectric devices.