In addition to spintronics another motivation for exploring ferromagnetic two-dimensional materials is for biomedical applications such as magnetic labeling and hyperthermia treatment of tumors. ...Unfortunately, the widely studied Mn-containing monolayer is not biocompatible, although it is ferromagnetic. Here using first principles calculations combined with Monte Carlo simulations based on the Ising model, we systematically study a class of 2D ferromagnetic monolayers CrX3 (X = Cl, Br, I). The feasibility of exfoliation from their layered bulk phase is confirmed by the small cleavage energy and high in-plane stiffness. Spin-polarized calculations, combined with self-consistently determined Hubbard U that accounts for strong correlation energy, demonstrate that CrX3 (X = Cl, Br, I) monolayers are ferromagnetic and that Cr is trivalent and carries a magnetic moment of 3 μ(B); the resulting Cr(3+) ions are biocompatible. The corresponding Curie temperatures for CrCl3, CrBr3 and CrI3 are found to be 66, 86, and 107 K, respectively, which can be increased to 323, 314, and 293 K by hole doping. The biocompatibility and ferromagnetism render these Cr-containing trihalide monolayers unique for applications.
Penta-graphene: A new carbon allotrope Zhang, Shunhong; Zhou, Jian; Wang, Qian ...
Proceedings of the National Academy of Sciences - PNAS,
02/2015, Volume:
112, Issue:
8
Journal Article
Peer reviewed
Open access
A 2D metastable carbon allotrope, penta-graphene, composed entirely of carbon pentagons and resembling the Cairo pentagonal tiling, is proposed. State-of-the-art theoretical calculations confirm that ...the new carbon polymorph is not only dynamically and mechanically stable, but also can withstand temperatures as high as 1000 K. Due to its unique atomic configuration, penta-graphene has an unusual negative Poisson’s ratio and ultrahigh ideal strength that can even outperform graphene. Furthermore, unlike graphene that needs to be functionalized for opening a band gap, penta-graphene possesses an intrinsic quasi-direct band gap as large as 3.25 eV, close to that of ZnO and GaN. Equally important, penta-graphene can be exfoliated from T12-carbon. When rolled up, it can form pentagon-based nanotubes which are semiconducting, regardless of their chirality. When stacked in different patterns, stable 3D twin structures of T12-carbon are generated with band gaps even larger than that of T12-carbon. The versatility of penta-graphene and its derivatives are expected to have broad applications in nanoelectronics and nanomechanics.
Significance Carbon has many faces––from diamond and graphite to graphene, nanotube, and fullerenes. Whereas hexagons are the primary building blocks of many of these materials, except for C ₂₀ fullerene, carbon structures made exclusively of pentagons are not known. Because many of the exotic properties of carbon are associated with their unique structures, some fundamental questions arise: Is it possible to have materials made exclusively of carbon pentagons and if so will they be stable and have unusual properties? Based on extensive analyses and simulations we show that penta-graphene, composed of only carbon pentagons and resembling Cairo pentagonal tiling, is dynamically, thermally, and mechanically stable. It exhibits negative Poisson's ratio, a large band gap, and an ultrahigh mechanical strength.
Abstract
Single-atom-catalysts (SACs) afford a fascinating activity with respect to other nanomaterials for hydrogen evolution reaction (HER), yet the simplicity of single-atom center limits its ...further modification and utilization. Obtaining bimetallic single-atom-dimer (SAD) structures can reform the electronic structure of SACs with added atomic-level synergistic effect, further improving HER kinetics beyond SACs. However, the synthesis and identification of such SAD structure remains conceptually challenging. Herein, systematic first-principle screening reveals that the synergistic interaction at the NiCo-SAD atomic interface can upshift the d-band center, thereby, facilitate rapid water-dissociation and optimal proton adsorption, accelerating alkaline/acidic HER kinetics. Inspired by theoretical predictions, we develop a facile strategy to obtain NiCo-SAD on N-doped carbon (NiCo-SAD-NC) via in-situ trapping of metal ions followed by pyrolysis with precisely controlled N-moieties. X-ray absorption spectroscopy indicates the emergence of Ni-Co coordination at the atomic-level. The obtained NiCo-SAD-NC exhibits exceptional pH-universal HER-activity, demanding only 54.7 and 61 mV overpotentials at −10 mA cm
−2
in acidic and alkaline media, respectively. This work provides a facile synthetic strategy for SAD catalysts and sheds light on the fundamentals of structure-activity relationships for future applications.
Recent efforts in finding materials suitable for storing hydrogen with large gravimetric density have focused attention on carbon-based nanostructures. Unfortunately, pure carbon nanotubes and ...fullerenes are unsuitable as hydrogen storage materials because of the weak bonding of the hydrogen molecules to the carbon frame. It has been shown very recently that coating of carbon nanostructures with isolated transition metal atoms such as Sc and Ti can increase the binding energy of hydrogen and lead to high storage capacity (up to 8 wt % hydrogen, which is 1.6 times the U.S. Department of Energy target set for 2005). This prediction has led to a great deal of excitement in the fuel cell community see The Fuel Cell Review, http://fcr.iop.org/articles/features/2/7/4. However, this prediction depends on the assumption that the metal atoms coated on the fullerene surface will remain isolated. Using first-principles calculations based on density functional theory, we show that Ti atoms would prefer to cluster on the C60 surface, which can significantly alter the nature of hydrogen bonding, thus affecting not only the amount of stored hydrogen but also their thermodynamics and kinetics.
Lattice thermal conductivity of penta-graphene Wang, Fancy Qian; Yu, Jiabing; Wang, Qian ...
Carbon (New York),
August 2016, 2016-08-00, 2016-08-01, Volume:
105, Issue:
C
Journal Article
Peer reviewed
Open access
Motivated by the unique geometry and novel properties of penta-graphene proposed recently as a new carbon allotrope consisting of pure pentagons Zhang et al. Proc. Natl. Acad. Sci. 2015, 112, 2372, ...we systematically investigated its phonon transport properties by solving exactly the linearized phonon Boltzmann transport equation combined with first principles calculations. The intrinsic lattice thermal conductivity Klat of penta-graphene is found to be about 645 W/mK at room temperature, which is significantly reduced as compared to that of graphene. The underlying reason is the strong anharmonic effect introduced by the buckled pentagonal structure with hybridized sp2 and sp3 bonding. A detailed analysis of the phonons of penta-graphene reveals that the ZA mode is the primary heat carrier (nearly 60%). The Klat is dominated by three-phonon scattering where the scattering rate of the Normal scattering process is comparable to that of the Umklapp scattering process. The phonon mean free path of the collective phonon excitations is in the order of micrometers. Complementing the high thermal conductivity of graphene, the low thermal conductivity of penta-graphene adds additional features to the family of carbon materials for thermal applications.
Two-dimensional (2D) topological insulators (TIs) have been recognized as a new class of quantum state of matter. They are distinguished from normal 2D insulators with their nontrivial band-structure ...topology identified by the Z sub(2) number as protected by time-reversal symmetry (TRS). Two-dimensional TIs have intriguing spin-velocity locked conducting edge states and insulating properties in the bulk. In the edge states, the electrons with opposite spins propagate in opposite directions and the backscattering is fully prohibited when the TRS is conserved. This leads to a quantized dissipationless "two-lane highway" for charge and spin transportation and promises potential applications. Up to now, only very few 2D systems have been discovered to possess this property. The lack of suitable material obstructs further study and application. Here, by using first-principles calculations, we propose that functionalized MXenes with oxygen, M sub(2) CO sub(2) (M=W, Mo, and Cr), are 2D TIs with the largest gap of 0.194 eV in the W case. They are dynamically stable and natively antioxidant. Most importantly, they are very likely to be easily synthesized by recently developed selective chemical etching of transition-metal carbides (the M sub()n1AX sub()nphase). This will pave the way to tremendous applications of 2D TIs, such as "ideal" conducting wire, multifunctional spintronic devices, and the realization of topological superconductivity and Majorana modes for quantum computing.
Layered MAX phases are exfoliated into 2D single layers and multilayers, so‐called MXenes. Using first‐principles calculations, the formation and electronic properties of various MXene systems, M2C ...(M = Sc, Ti, V, Cr, Zr, Nb, Ta) and M2N (M = Ti, Cr, Zr) with surfaces chemically functionalized by F, OH, and O groups, are examined. Upon appropriate surface functionalization, Sc2C, Ti2C, Zr2C, and Hf2C MXenes are expected to become semiconductors. It is also derived theoretically that functionalized Cr2C and Cr2N MXenes are magnetic. Thermoelectric calculations based on the Boltzmann theory imply that semiconducting MXenes attain very large Seebeck coefficients at low temperatures.
2D, single‐layer nanostructures of transition metal carbides and nitrides, so‐called MXene, here M2C (M = Sc, Ti, V, Cr, Zr, Nb, Ta) and M2N (M = Ti, Cr, Zr), which are chemically functionalized by F, OH, and O groups, are shown to have potential applications in optoelectronic, spintronic, and thermoelectric nanodevices based on first‐principles calculations.
The capability of AlN nanostructures (nanocages, nanocones, nanotubes, and nanowires) to store hydrogen has been studied using gradient-corrected density functional theory. In contrast to bulk AlN, ...which has the wurtzite structure and four-fold coordination, the Al sites in AlN nanostructures are unsaturated and have two- and three-fold coordination. Each Al atom is capable of binding one H2 molecule in quasi-molecular form, leading to 4.7 wt % hydrogen, irrespective of the topology of the nanostructures. With the exception of AlN nanotubes, energetics does not support the adsorption of additional hydrogen. The binding energies of hydrogen to these unsaturated metal sites lie in the range of 0.1−0.2 eV/H2 and are ideal for applications under ambient thermodynamic conditions. Furthermore, these materials do not suffer from the clustering problem that often plagues metal-coated carbon nanostructures.
Earth-abundant, nonprecious, and efficient electrocatalysts for effective hydrogen evolution reaction (HER) are crucial for future large-scale green energy production. Low-cost two-dimensional MXenes ...have been widely studied in energy-storage devices owing to their unique chemical and physical properties and have recently attracted scientists in the electrocatalysis field. Nevertheless, their electrocatalytic activity still remains unsatisfactory. Herein, we present a facile and general strategy using ammonia heat treatment to enhance the hydrogen evolution catalysis of Ti3C2T x MXenes by modification with a nitrogen heteroatom. Importantly, our approach is focused on revealing: (1) the contribution of all possible incorporated N species including Ti–N, N–H, and N in O–Ti–N, rather than considering only that of Ti–N x motifs as previously reported for N-doped MXene electrocatalysts, and their role in inducing a change in the electronic configuration of the as-prepared catalysts, which then leads to increased electrical conductivity and improved intrinsic catalytic reactivity; and (2) the importance of controlling the proper amount of N obtained at a suitable calcined temperature to assist the shift of the Gibbs free energy for hydrogen adsorption (ΔH ad*) approaching 0 eV (ideal value), as proved by the density functional theory. Moreover, experimental findings indicate that nitrogen-doped Ti3C2T x annealed at 600 °C shows superior improved HER electrocatalytic performance compared to pristine Ti3C2T x , with an onset potential of −30 mV and an overpotential as low as 198 at 10 mA cm–2, as well as a much smaller Tafel slope of 92 mV dec–1.
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