MXene's surface functionalization patterns and their impacts on magnetism.

Two-dimensional transition metal carbides and nitrides (MXenes) are a promising group of materials with a broad palette of applications. Surface terminations are a result of MXene preparation, and post-processing can also lead to partial coverage. Despite applicability and fundamental properties being driven by termination patterns, it is not fully clear how they behave on MXene surfaces with various degrees of surface coverage. Here, as the first step, we used density functional theory to predict possible patterns in prototypic Ti2C MXene, demonstrating the different behavior of the two most frequent terminal atoms, oxygen, and fluorine. Oxygen (with formal charge -2e) prefers a zigzag line both-side adsorption pattern on bare Ti2C, attracting the next adsorbent at a minimal distance. Oxygen defects in fully O-terminated MXene tend to form similar zigzag line vacancy patterns. On the other hand, fluorine (with a formal charge of -1e) prefers one-side flake (island) adsorption on bare Ti2C and a similar desorption style from fully fluorinated Ti2C. The magnetic behavior of the MXene is subsequently driven by the patterns, either compensating locally and holding the global magnetic state of the MXene until some limit (oxygen case) or gradually increasing the total magnetism through summation of local effects (fluorine case). The systematic combinatoric study of Ti2CTx with various coverages (0 ≤ x ≤ 2) of distinct terminal atoms T = O or F brings encouraging possibilities of tunable behavior of MXenes and provides useful guidance for its modeling towards electronic nanodevices.

Correction: α-Fe2O3/TiO2 3D hierarchical nanostructures for enhanced photoelectrochemical water splitting.

Correction for 'α-Fe2O3/TiO2 3D hierarchical nanostructures for enhanced photoelectrochemical water splitting' by Hyungkyu Han et al., Nanoscale, 2017, 9, 134-142, https://doi.org/10.1039/C6NR06908H.

Prediction of induced magnetism in 2D Ti2C based MXenes by manipulating the mixed surface functionalization and metal substitution computed by xTB model Hamiltonian of the DFTB method.

We employed the recently developed density functional tight binding (DFTB) method's Hamiltonian, GFN1-xTB, for modeling the mixed termination in Ti2C MXenes, namely three types of termination by combining -O and -OH, -O and -F, and -F and -OH. We demonstrated that the approach yields reliable predictions for the electronic and magnetic properties of such MXenes. The first highlighted result is that the mixed surface functionalization in Ti2CAxBy MXenes induces spin polarization with diverse magnetic alignments, including ferromagnetism and two types of antiferromagnetism. We further identified the magnetic alignment for the investigated MXene in terms of the compositions of the terminal groups. Moreover, the effect of the transition metal (Ti) substituted by the Sc atom on the electronic and magnetic properties was also investigated. We found that the studied systems maintain the magnetism and the metallic characteristics. A magnetic transition from antiferromagnetic (AFM) to ferrimagnetic (FiM) ordering was found for ScTi15C8F8(OH)8 and ScTi15C8F12(OH)4 compounds. Finally, we proved that incorporating the Sc atom into the lattice of Ti2CO2 and the mixed surface termination in Ti2CAxBy is an effective strategy to induce magnetism. Our study may provide a new potential application for designing MXene-based spintronics.

Excitons, optical spectra, and electronic properties of semiconducting Hf-based MXenes.

Semiconducting MXenes are an intriguing two-dimensional (2D) material class with promising electronic and optoelectronic properties. Here, we focused on recently prepared Hf-based MXenes, namely, Hf3C2O2 and Hf2CO2. Using the first-principles calculation and excited state corrections, we proved their dynamical stability, reconciled their semiconducting behavior, and obtained fundamental gaps by using the many-body GW method (indirect 1.1 and 2.2 eV; direct 1.4 and 3.5 eV). Using the Bethe-Salpeter equation, we subsequently provided optical gaps (0.9 and 2.7 eV, respectively), exciton binding energies, absorption spectra, and other properties of excitons in both Hf-based MXenes. The indirect character of both 2D materials further allowed for a significant decrease of excitation energies by considering indirect excitons with exciton momentum along the Γ-M path in the Brillouin zone. The first bright excitons are strongly delocalized in real space while contributed by only a limited number of electron-hole pairs around the M point in the k-space from the valence and conduction band. A diverse range of excitonic states in Hf3C2O2 MXene lead to a 4% and 13% absorptance for the first and second peaks in the infrared region of absorption spectra, respectively. In contrast, a prominent 28% absorptance peak in the visible region appears in Hf2CO2 MXene. Results from radiative lifetime calculations indicate the promising potential of these materials in optoelectric devices requiring sustained and efficient exciton behavior.

Modeling size and edge functionalization of MXene-based quantum dots and their effect on electronic and magnetic properties.

In the last six years, the synthesis of MXene-based quantum dots (MXQDs) has gained widespread attention. Due to the quantum confinement effect, it is possible to significantly improve their properties compared to 2D counterparts, such as higher chemical stability and better electronic and optical properties. However, despite the growing interest in their properties, much remains unexplored. One of the biggest challenges is to study in more detail the structure of quantum dots, in particular, their edge functionalization and its effect on their properties. In this paper, the structural stability and electronic and magnetic properties of Ti2CO2 QDs based on different lateral dimensions and edge functionalization (-O, -F, and -OH) are investigated using density functional theory. The study shows that the energy gap of Ti2CO2-O QDs decreases with increasing lateral size for both nonmagnetic (spin-unpolarized, close shell) and magnetic (spin-polarized, open shell) cases. Furthermore, the magnetic behavior of quantum dots was revealed by shrinking from 2D Ti2CO2 to 0D Ti2CO2 QDs with lateral dimensions below 1.4 nm. The binding energy confirms the stability of all three types of edge functionalization, while the most stable structure was observed under fully saturated edge oxygenation. Moreover, it was also found that the spin density distribution and the energy gap of Ti2CO2-X QDs (X = O, F, and OH) are both dependent on the type of atom saturation. Size and edge confinement modeling has been demonstrated to be an effective tool for tuning the electronic and magnetic properties of MXQDs. Moreover, the observed enhanced spin polarization together with tunable magnetic properties makes the ultrafine Ti2CO2-X QDs promising candidates for spintronic applications.

Benchmarking fundamental gap of Sc2C(OH)2 MXene by many-body methods.

Sc2C(OH)2 is a prototypical non-magnetic member of MXenes, a promising transition-metal-based 2D material family, with a direct bandgap. We provide here a benchmark of its fundamental gap Δ obtained from many-body GW and fixed-node diffusion Monte Carlo methods. Both approaches independently arrive at a similar value of Δ âˆ¼ 1.3 eV, suggesting the validity of both methods. Such a bandgap makes Sc2C(OH)2 a 2D semiconductor suitable for optoelectronic applications. The absorbance spectra and the first exciton binding energy (0.63 eV), based on the Bethe-Salpeter equation, are presented as well. The reported results may serve to delineate experimental uncertainties and enable selection of reasonable approximations such as density functional theory functionals, for use in modeling of related MXenes.

The electronic and optical properties of III-V binary 2D semiconductors: how to achieve high precision from accurate many-body methods.

Seven hexagonal 2D materials consisting of elements of the IIIA and VA groups (BN, BP, BAs, AlN, GaN, GaP, and GaAs) were theoretically studied using first-principles methods. Simultaneous convergence in all principal parameters of the accurate many-body perturbational GW approach and the subsequent Bethe-Salpeter equation (BSE) was necessary to achieve precise fundamental and optical gaps, exciton binding energies, and absorbance spectra. Various convergence rates of studied properties in the case of different materials were visualized and explained. Benchmark calculations show several 2D materials from this set that strongly absorb in the visible and ultraviolet parts of the spectra, and therefore can be promising materials for (opto)electronic applications.

Electronic Nature Transition and Magnetism Creation in Vacancy-Defected Ti2CO2 MXene under Biaxial Strain: A DFTB + U Study.

The structural, electronic, and magnetic properties of vacancy defect in Ti2CO2 MXene and the effect of strain have been investigated using the density functional tight-binding (DFTB) approach including spin-polarization with Hubbard onsite correction (DFTB + U). The band gap of pure Ti2CO2 is ∼1.3 eV, which decreases to ∼0.4 and ∼1.1 eV in the case of C- and O-vacancies, respectively, i.e., the semiconducting behavior is retained. In contrast, Ti2CO2 undergoes semiconductor-to-metal transition by the introduction of a single Ti-vacancy. This transition is the result of introduced localized states in the vicinity of the Fermi level by the vacancy. Both Ti- and O-vacancies have zero net magnetic moments. Interestingly, the nonmagnetic (NM) ground state of semiconducting Ti2CO2 turns into a magnetic semiconductor by introducing a C-vacancy with a magnetization of ∼2 µB/cell. Furthermore, we studied the effect of strain on the electronic structure and magnetic properties of Ti-, C-, and O-vacant Ti2CO2. The nature of the band gap in the presence of single O-vacancy remains indirect in both compression and tensile strain, and the size of the band gap decreases. Compression strain on Ti-vacant Ti2CO2 changes metal into a direct semiconductor, and the metallic character remains under tensile biaxial strain. In opposition, a semiconductor-to-metal transition occurs by applying a compressive biaxial strain on C-vacant Ti2CO2. We also find that the magnetism is preserved under tensile strain and suppressed under compression strain on VC-Ti2CO2. Moreover, we show that double C-vacancies maintain magnetism. Our findings provide important characteristics for the application of the most frequent MXene material and should motivate further investigations because experimentally achieved MXenes always contain point defects.

Iron Nitride Nanoparticles for Enhanced Reductive Dechlorination of Trichloroethylene.

Nitriding has been used for decades to improve the corrosion resistance of iron and steel materials. Moreover, iron nitrides (FexN) have been shown to give an outstanding catalytic performance in a wide range of applications. We demonstrate that nitriding also substantially enhances the reactivity of zerovalent iron nanoparticles (nZVI) used for groundwater remediation, alongside reducing particle corrosion. Two different types of FexN nanoparticles were synthesized by passing gaseous NH3/N2 mixtures over pristine nZVI at elevated temperatures. The resulting particles were composed mostly of face-centered cubic (γ'-Fe4N) and hexagonal close-packed (ε-Fe2-3N) arrangements. Nitriding was found to increase the particles' water contact angle and surface availability of iron in reduced forms. The two types of FexN nanoparticles showed a 20- and 5-fold increase in the trichloroethylene (TCE) dechlorination rate, compared to pristine nZVI, and about a 3-fold reduction in the hydrogen evolution rate. This was related to a low energy barrier of 27.0 kJ mol-1 for the first dechlorination step of TCE on the γ'-Fe4N(001) surface, as revealed by density functional theory calculations with an implicit solvation model. TCE dechlorination experiments with aged particles showed that the γ'-Fe4N nanoparticles retained high reactivity even after three months of aging. This combined theoretical-experimental study shows that FexN nanoparticles represent a new and potentially important tool for TCE dechlorination.

DFTB investigations of the electronic and magnetic properties of fluorographene with vacancies and with adsorbed chemical groups.

The electronic and magnetic properties of fluorographene (CF) in the presence of F-vacancy defects and/or chemical groups (-OH, -CN, or -NH2) were computationally investigated within the framework of the density functional tight-binding (DFTB) method. The current method parameterization allowed us to perform accurate electronic structure calculations (at the ab initio level of many-body methods in the particular case of CF) for hundreds of atoms in the computational cell. We show that the F-vacancy and/or chemical groups influence the magnetic structure, which depends on the number of defects and their distribution between the two sides of the graphene plane. Interestingly, we pointed out a possibility of imprinting local magnetism not only via F-vacancy and -OH combinations, but also using F-vacancies and -CN or -NH2 groups. In such structures, the magnetic ordering and the total magnetic moments depend on their adsorption sites and their presence in the same or on opposite sides of the graphene plane. We devote particular attention to the interacting chemical group with the F-vacancies. The interaction between the adsorbed chemical group and the unpaired spins associated with the F-vacancies in CF gives rise to interesting magnetic structures. Finally, the zigzag-like direction is shown as the most preferred for the defluorination of CF. Stable ferrimagnetic zigzag chains with interesting properties are considered to be basic magnetic features in perturbed CF. Our work provides new guidelines for engineering multifunctional spintronic components using CF as a base material. We believe, in particular, that the magnetism is predominantly controlled by the F-vacancies, and the ferromagnet can ideally be regulated via the adsorption of a chemical group on a defective CF supercell.

Lattice dynamics in the conformational environment of multilayered hexagonal boron nitride (h-BN) results in peculiar infrared optical responses.

Stacking mismatches in hexagonal boron nitride (h-BN) nanostructures affect their photonic, mechanical, and thermal properties. To access information about the stacked configuration of layered ensembles, highly sophisticated techniques like X-ray photoemission spectroscopy or electron microscopy are necessary. Here, instead, by taking advantage of the geometrical and chemical nature of h-BN, we show how simple structural models, based on shortened interplanar distances, can produce effective charge densities. Accounting these in the non-analytical part of the lattice dynamical description makes it possible to access information about the composition of differently stacked variants in experimental samples characterized by infrared spectroscopy. The results are obtained by density functional theory and confirmed by various functionals and pseudopotential approximations. Even though the method is shown using h-BN, the conclusions are more general and show how effective dielectric models can be considered as valuable theoretical pathways for the vibrational structure of any layered material.

Fundamental gap of fluorographene by many-body GW and fixed-node diffusion Monte Carlo methods.

Fluorographene (FG) is a promising graphene-derived material with a large bandgap. Currently existing predictions of its fundamental gap (Δf) and optical gap (Δopt) significantly vary when compared with experiment. We provide here an ultimate benchmark of Δf for FG by many-body GW and fixed-node diffusion Monte Carlo (FNDMC) methods. Both approaches independently arrive at Δf ≈ 7.1 ± 0.1 eV. In addition, the Bethe-Salpeter equation enabled us to determine the first exciton binding energy, Eb = 1.92 eV. We also point to the possible misinterpretation problem of the results obtained for gaps of solids by FNDMC with single-reference trial wave functions of Bloch orbitals. We argue why instead of Δopt, in the thermodynamic limit, such an approach results in energy differences that rather correspond to Δf, and we also outline conditions when this case actually applies.

A theoretical study of adsorption on iron sulfides towards nanoparticle modeling.

Surface modification of zero-valent iron (nZVI) nanoparticles, which are frequently used in the removal of chlorinated hydrocarbons from contaminated groundwater, can increase their surface stability without significant loss of reactivity. Sulfidation is a process during which thin iron sulfide phases are formed on nZVI particles. In this work, the adsorption capability of two iron sulfide minerals (mackinawite and pyrite) and ZVI with respect to two small polar molecules (H2O and H2S) and trichloroethylene (TCE) was modeled by using the quantum mechanics (QM) approach. High-level QM methods used on cluster models helped in benchmarking and validation of density functional theory methods used on periodic slab models of the (001) surfaces of iron sulfides and the (111) surface of ZVI. This careful computational treatment was necessary for achieving reliable results because modeled iron containing compounds represent computationally demanding systems. The results showed that adsorption was strongly affected by surface topology, accessibility of surface sites, and the shape of adsorbed molecular species. The mackinawite surface is practically hydrophobic having weak interactions with polar molecules (about -5/-6 kcal mol-1), in contrast to the surfaces of pyrite and ZVI (adsorption energies are about three times larger). On the other hand, the adsorption of weakly polar planar TCE molecule is relatively strong and similar for all three surfaces (in the range of -11 to -17 kcal mol-1). Moreover, it was shown that the dominant component of the adsorption energy of TCE had originated from dispersion interactions, which were less important for small polar molecules.

Optical Gaps and Excitonic Properties of 2D Materials by Hybrid Time-Dependent Density Functional Theory: Evidences for Monolayers and Prospects for van der Waals Heterostructures.

The optical properties of two-dimensional (2D) materials are accurately described by many-body methods including specifically pronounced electron-electron and electron-hole effects. Such methods are, however, computationally demanding and applicable on small computational cells only. We provide approximate optical gaps for 2D materials from time-dependent (TD) density functional theory based on a set of specific screened hybrid functionals and show that this approach effectively accounts for all important physical effects including excitons. Optical gap values obtained from the TD-HSE06 approach for a broad gap range 1-6 eV of eight 2D materials are in agreement with both experimental optical gaps and accurate GW+BSE calculations. Further, we show that such an approach is eligible and practicable for van der Waals heterostructures containing incommensurate cells of different monolayers and enables detailed analysis of intra- and interlayer excitonic wave functions. TD-HSE06 is therefore a suitable method for a reliable description of the optical properties of extended periodic 2D systems.

Highly Ordered N-Doped Carbon Dots Photosensitizer on Metal-Organic Framework-Decorated ZnO Nanotubes for Improved Photoelectrochemical Water Splitting.

In spite of having several advantages such as low cost, high chemical stability, and environmentally safe and benign synthetic as well as operational procedures, the full potential of carbon dots (CDs) is yet to be explored as photosensitizers due to the challenges associated with the fabrication of well-arrayed CDs with many other photocatalytic heterostructures. In the present study, a unique combination of metal-organic framework (MOF)-decorated zinc oxide (ZnO) 1D nanostructures as host and CDs as guest species are explored on account of their potential application in photoelectrochemical (PEC) water splitting performance. The synthetic strategy to incorporate well-defined nitrogen-doped carbon dots (N-CDs) arrays onto a zeolitic imidazolate framework-8 (ZIF-8) anchored on ZnO 1D nanostructures allows a facile unification of different components which subsequently plays a decisive role in improving the material's PEC water splitting performance. Simple extension of such strategies is expected to offer significant advantages for the preparation of CD-based heterostructures for photo(electro)catalytics and other related applications.

Accurate many-body calculation of electronic and optical band gap of bulk hexagonal boron nitride.

Many-body perturbational GW approximation in conjunction with the Bethe-Salpeter equation (BSE) has been employed to calculate accurate electronic and optical band gaps of bulk hexagonal boron nitride (h-BN) in the two most important stacking configurations, AA' and AB. The carefully converged results revealed h-BN as an indirect material (indirect gap ≈ 6.1 eV) with a huge excitonic effect (≈0.8 eV) in perfect agreement with recent experiments [Nat. Photonics, 2016, 10, 262; Appl. Phys. Lett., 2016, 109, 122101]. The K-H region of the first Brillouin zone has been shown as the most important for lowest optical excitations in h-BN. Surprisingly, simple scissor corrected DFT has described h-BN band structure at the GW level and subsequent time-dependent DFT with a suitable exchange correlation kernel has provided absorption spectra similar to the full GW+BSE spectra.

Transformations of ferrates(iv,v,vi) in liquids: Mössbauer spectroscopy of frozen solutions.

The kinetics and mechanism of ferrate(iv), (v) and (vi) transformations in water and in polar organic solvents (namely ethanol and tetrahydrofuran) have been investigated by the method of 57Fe Mössbauer spectroscopy of frozen solutions. Ethanol with a very limited amount of water under an inert atmosphere, significantly slows down the transformation reactions of ferrates(iv and v) and provides direct proof of the existence of intermediate states. Simultaneously, ethanol is oxidized to caboxylates in the close vicinity of the surface of ferrate crystallites as proven by X-ray photoelectron spectroscopy. On the contrary, any transformation of ferrate(vi) in pure ethanol (with a very limited amount of water) was not observed. Mössbauer spectroscopy of frozen solutions enabled us to experimentally identify and quantify intermediates of ferrate(iv) and ferrate(v) transformations for the first time. Sodium ferrate(iv) in its tetrahedral form, Na4FeO4, undergoes a two-step charge disproportionation to Fe(iii) and Fe(vi) via a Fe(v) intermediate without any evolution of oxygen in polar protic and aprotic solvents, specifically 2Fe(iv) → Fe(iii) + Fe(v), and Fe(iv) + Fe(v) → Fe(iii) + Fe(vi), i.e. in sum 3Fe(iv) → 2Fe(iii) + Fe(vi). Ferrate(v) (K3FeO4) transforms to Fe(iii) and Fe(vi) without any indication of the Fe(iv) intermediate within the detection limit of the method. In addition to a charge disproportionation reaction proceeding in polar liquids, 3Fe(v) → Fe(iii) + 2Fe(vi), a competitive reduction of Fe(v) directly to Fe(iii) accompanied by oxygen evolution takes place in water. Oxygen evolution was also measured for ferrate(iv and vi) transformations in water, but to a higher and a smaller extent compared to ferrate(v), respectively. The thermodynamics of the suggested ferrate(iv) and ferrate(v) transformation pathways was examined by DFT calculations.

Sb-Doped SnO2 Nanorods Underlayer Effect to the α-Fe2 O3 Nanorods Sheathed with TiO2 for Enhanced Photoelectrochemical Water Splitting.

Here, a Sb-doped SnO2 (ATO) nanorod underneath an α-Fe2 O3 nanorod sheathed with TiO2 for photoelectrochemical (PEC) water splitting is reported. The experimental results, corroborated with theoretical analysis, demonstrate that the ATO nanorod underlayer effect on the α-Fe2 O3 nanorod sheathed with TiO2 enhances the PEC water splitting performance. The growth of the well-defined ATO nanorods is reported as a conductive underlayer to improve α-Fe2 O3 PEC water oxidation performance. The α-Fe2 O3 nanorods grown on the ATO nanorods exhibit improved performance for PEC water oxidation compared to α-Fe2 O3 grown on flat fluorine-doped tin oxide glass. Furthermore, a simple and facile TiCl4 chemical treatment further introduces TiO2 passivation layer formation on the α-Fe2 O3 to reduce surface recombination. As a result, these unique nanostructures show dramatically improved photocurrent density (139% higher than that of the pure hematite nanorods).

Adsorption of Organic Molecules to van der Waals Materials: Comparison of Fluorographene and Fluorographite with Graphene and Graphite.

Understanding strength and nature of noncovalent binding to surfaces imposes significant challenge both for computations and experiments. We explored the adsorption of five small nonpolar organic molecules (acetone, acetonitrile, dichloromethane, ethanol, ethyl acetate) to fluorographene and fluorographite using inverse gas chromatography and theoretical calculations, providing new insights into the strength and nature of adsorption of small organic molecules on these surfaces. The measured adsorption enthalpies on fluorographite range from -7 to -13 kcal/mol and are by 1-2 kcal/mol lower than those measured on graphene/graphite, which indicates higher affinity of organic adsorbates to fluorographene than to graphene. The dispersion-corrected functionals performed well, and the nonlocal vdW DFT functionals (particularly optB86b-vdW) achieved the best agreement with the experimental data. Computations show that the adsorption enthalpies are controlled by the interaction energy, which is dominated by London dispersion forces (∼70%). The calculations also show that bonding to structural features, like edges and steps, as well as defects does not significantly increase the adsorption enthalpies, which explains a low sensitivity of measured adsorption enthalpies to coverage. The adopted Langmuir model for fitting experimental data enabled determination of adsorption entropies. The adsorption on the fluorographene/fluorographite surface resulted in an entropy loss equal to approximately 40% of the gas phase entropy.

Room temperature organic magnets derived from sp3 functionalized graphene.

Materials based on metallic elements that have d orbitals and exhibit room temperature magnetism have been known for centuries and applied in a huge range of technologies. Development of room temperature carbon magnets containing exclusively sp orbitals is viewed as great challenge in chemistry, physics, spintronics and materials science. Here we describe a series of room temperature organic magnets prepared by a simple and controllable route based on the substitution of fluorine atoms in fluorographene with hydroxyl groups. Depending on the chemical composition (an F/OH ratio) and sp3 coverage, these new graphene derivatives show room temperature antiferromagnetic ordering, which has never been observed for any sp-based materials. Such 2D magnets undergo a transition to a ferromagnetic state at low temperatures, showing an extraordinarily high magnetic moment. The developed theoretical model addresses the origin of the room temperature magnetism in terms of sp2-conjugated diradical motifs embedded in an sp3 matrix and superexchange interactions via -OH functionalization.