Enhancement of hydrogen physisorption on graphene and carbon nanotubes by Li doping

Recent advances in the functionalization, substitutional doping and applications of graphene/graphene composite nanomaterials

Recently, graphene and graphene-based nanomaterials have emerged as advanced carbon functional materials with specialized unique electronic, optical, mechanical, and chemical properties. These properties have made graphene an exceptional material for a wide range of promising applications in biological and non-biological fields. The present review illustrates the structural modifications of pristine graphene resulting in a wide variety of derivatives. The significance of substitutional doping with alkali-metals, alkaline earth metals, and III-VII group elements apart from the transition metals of the periodic table is discussed. The paper reviews various chemical and physical preparation routes of graphene, its derivatives and graphene-based nanocomposites at room and elevated temperatures in various solvents. The difficulty in dispersing it in water and organic solvents make it essential to functionalize graphene and its derivatives. Recent trends and advances are discussed at length. Controlled reduction reactions in the presence of various dopants leading to nanocomposites along with suitable surfactants essential to enhance its potential applications in the semiconductor industry and biological fields are discussed in detail.

Improvement of the hydrogen storage performance of t-graphene-like two-dimensional boron nitride upon selected lithium decoration

In recent years, search for applicable bidimensional (2D) hydrogen storage materials with high capacity and excellent H₂ physisorption properties has attracted considerable attention from scientists and researchers. According to the rational design, and using first-principles calculations, we propose a t-graphene-like boron nitride monolayer (t-B₄N₄) for hydrogen storage application by replacing C atoms in t-graphene with B and N atoms. The thermal stability and polarization mechanisms of lithium atoms adsorbed at the center of octagons on the t-B₄N₄ system were evaluated at 300 K using ab initio molecular dynamics (AIMD) calculations. Moreover, Li-decorated double-sided t-B₄N₄ can store up to 32H₂ molecules with an average hydrogen adsorption energy of 0.217 eV per H₂ and a maximum hydrogen storage capacity of 12.47 wt%. The reversibility of adsorbed hydrogen was checked and the calculated desorption temperature was 161 K, much higher than the critical point for hydrogen. Based on diffusion barriers, the H₂ molecule diffusion kinetics is faster on the t-B₄N₄ surface than that on t-graphene and graphene.

Ultrahigh hydrogen storage capacity of holey graphyne

Holey graphyne (HGY), a novel two-dimensional (2D) single-crystalline carbon allotrope, was recently synthesized by Castro-Stephens coupling reaction. The naturally existing uniform periodic holes in the 2D carbon-carbon network demonstrate its promising potential in energy storage. Herein, we conduct density functional theory (DFT) calculation and ab initio molecular dynamics simulations (AIMD) to predict the H storage properties of a single-layer HGY sheet modified by Li metal atoms. The DFT calculations demonstrate that Li atoms can bind strongly to the HGY sheet without forming clusters, and each Li atom can anchor four H2 molecules with an average adsorption energy of about -0.22 eV/H2. The largest H storage capacity of the doped HGY sheet can reach as high as 12.8 wt%, showing that the Li/HGY complex is an ideal H storage material at ambient conditions. In addition, we investigate the polarization mechanism of the storage media and find that the polarization originates from the electric field induced by both the ionic Li atoms and the weak polarized H2 molecules. Finally, the desorption mechanism of the adsorbed H2 molecules is thoroughly investigated using a kinetic AIMD method.

Complexes of Alkali Metal Cations and Molecular Hydrogen: Potential Energy Surfaces and Bound States

Complexes between metal cations and molecular hydrogen are systems quite amenable for precise spectroscopic and theoretical studies, and at the same time, they are relevant for applications in hydrogen storage and astrochemistry. In this work, we report new intermolecular potential energy surfaces and rovibrational states calculations for complexes involving molecular hydrogen and alkaline metal cations, M⁺-H₂ (M⁺ = Na⁺, K⁺, Rb⁺, Cs⁺). The intermolecular potentials, formulated in an internally consistent way to emphasize differences in the properties of the systems, are represented by simple analytical expressions whose parameters have been optimized from comparison with accurate ab initio calculations. Properties of the low-lying bound states-binding energies, frequencies, and rotational constants-are compared with previous measurements or computations and an overall good agreement is achieved, supporting the reliability of the present formulation. Variations of these properties as a function of the cation size and isotopic substitution, with a proper sequence of ortho and para rotational levels, are also discussed.

Charge transfer effect on Raman shifts of aromatic hydrocarbons with three phenyl rings from ab initio study

To clarify the charge transfer effect on Raman spectra of aromatic hydrocarbons, we investigate the Raman shifts of phenanthrene, p-terphenyl, and anthracene and their negatively charged counterparts by using density functional theory. For the three molecules, upon charge increasing, the computed Raman peaks generally shift down with the exception of a few shifting up. The characteristic Raman modes in the 0-1000 cm^-1 region persist up, while some high-frequency ones change dramatically with three charges transferred. The calculated Raman shifts for one- and two-electron transfer are in agreement with the measured Raman spectra, and in accordance to the stoichiometric ratios 1:1 and 2:1 of the metal atom and aromatic hydrocarbon molecule in recent experimental and theoretical studies. Our theoretical results provide the fundamental information to elucidate the Raman shifts and the stoichiometric ratios for alkali-metal-doped aromatic hydrocarbons.

Tailoring the capability of carbon nitride (C3N) nanosheets toward hydrogen storage upon light transition metal decoration

To nurture the full potential of hydrogen (H₂) as a clean energy carrier, its efficient storage under ambient conditions is of great importance. Owing to the potential of material-based H₂ storage as a promising option, we have employed here first principles density functional theory calculations to study the H₂ storage properties of recently synthesized C₃N monolayers. Despite possessing fascinating structural and mechanical properties C₃N monolayers weakly bind H₂ molecules. However, our van der Waals corrected simulations revealed that the binding properties of H₂ on C₃N could be enhanced considerably by suitable Sc and Ti doping. The stabilities of Sc and Ti dopants on a C₃N surface has been verified by means of reaction barrier calculations and ab initio molecular dynamics simulations. Upon doping with C₃N, the existence of partial positive charges on both Sc and Ti causes multiple H₂ molecules to bind to the dopants through electrostatic interactions with adsorption energies that are within an ideal range. A drastically high H₂ storage capacity of 9.0 wt% could be achieved with two-sided Sc/Ti doping that ensures the promise of C₃N as a high-capacity H₂ storage material.

Snowball formation for Cs+ solvation in molecular hydrogen and deuterium

Interactions of atomic cations with molecular hydrogen are of interest for a wide range of applications in hydrogen technologies. These interactions are fairly strong despite being non-covalent, hence one can ask whether hydrogen molecules would form dense, solid-like, solvation shells around the ion (snowballs) or rather a more weakly bound compound. In this work, the interactions between Cs+ and H2 are studied both experimentally and computationally. Isotopic substitution of H2 by D2 is also investigated. On the one hand, helium nanodroplets doped with cesium and hydrogen or deuterium are ionized by electron impact and the (H2/D2)nCs+ (up to n = 30) clusters formed are identified via mass spectrometry. On the other hand, a new analytical potential energy surface, based on ab initio calculations, is developed and used to study cluster energies and structures by means of classical and quantum-mechanical Monte Carlo methods. The most salient features of the measured ion abundances are remarkably mimicked by the computed evaporation energies, particularly for the clusters composed of deuterium. This result supports the reliability of the present potential energy surface and allows us to recommend its use in related systems. Clusters with either twelve H2 or D2 molecules stand out for their stability and quasi-rigid icosahedral structures. However, the first solvation shell involves thirteen or fourteen molecules for hydrogenated or deuterated clusters, respectively. This shell retains its internal structure when extra molecules are added to the second shell and is nearly solid-like, especially for the deuterated clusters. The role played by three-body induction interactions as well as the rotational degrees of freedom is analyzed and they are found to be significant (up to 15% and 18%, respectively) for the molecules belonging to the first solvation shell.

On the H2 interactions with transition metal adatoms supported on graphene: a systematic density functional study

The attachment of H₂ to the full set of transition metal (TM) adatoms supported on graphene is studied by using density functional theory including dispersion, identifying physisorbed, Kubas, and dissociated states.

Li-decorated carbon ene–yne as a potential high-capacity hydrogen storage medium

Based on comprehensive first-principles calculations, we predict that Li-decorated carbon ene–yne (CEY) can serve as a reversible and high density hydrogen storage medium.

Lithium adsorption and migration in group IV–VI compounds and GeS/graphene heterostructures: a comparative study

The migration energy barriers of lithium migration on monolayer GeS along armchair and zigzag directions, and the distortion degree comparison for bulk GeS and GeS/graphene heterostructures during lithiation.

Influence of nitrogen doping in sumanene framework toward hydrogen storage: A computational study

Two conditions are important to obtain appropriate substances for hydrogen storage; high surface area and fitting binding energy (BE). Doping is a key strategy that improves BE. We investigated hydrogen adsorption onto twenty six nitrogen disubstituted isomers of sumanene (C₁₉N₂H₁₂) by MP2/6-311++G(d,p)//B3LYP/6-31+G(d) and M06-2X/6-31+G(d) levels of theory. Effect of nitrogen doping in different positions of sumanene was checked. To obtain better BE, basis set superposition error (BSSE) and zero point energy (ZPE) corrections were used. Anticipating of adsorption sites and extra details about adsorption process was done by molecular electrostatic potential (MEP) surfaces. Various types of density of state (DOS) diagrams such as total DOS (TDOS), projected DOS (PDOS) and overlap population DOS (OPDOS) and natural bond orbital (NBO) analysis were used to find better insight on the adsorption properties. In addition of temperature depending of the BE, HOMO-LUMO gap (HLG), dipole moment, reactivity and stability, bowl depth and natural population analysis (NPA) of the isomers were studied. A physisorption mechanism for adsorption was proposed and a trivial change was seen. Place of nitrogen atoms in sumanene frame causes to binding energy increases or decreases compared with pristine sumanene. The best and the worst isomers and category of isomers were suggested.

A First Principles study on Boron-doped Graphene decorated by Ni-Ti-Mg atoms for Enhanced Hydrogen Storage Performance

We proposed a new solid state material for hydrogen storage, which consists of a combination of both transition and alkaline earth metal atoms decorating a boron-doped graphene surface. Hydrogen adsorption and desorption on this material was investigated using density functional theory calculations. We find that the diffusion barriers for H atom migration and desorption energies are lower than for the previously designed mediums and the proposed medium can reach the gravimetric capacity of ~6.5 wt % hydrogen, which is much higher than the DOE target for the year 2015. Molecular Dynamics simulations show that metal atoms are stably adsorbed on the B doped graphene surface without clustering, which will enhance the hydrogen storage capacity.

Universal roles of hydrogen in electrochemical performance of graphene: high rate capacity and atomistic origins

Atomic hydrogen exists ubiquitously in graphene materials made by chemical methods. Yet determining the effect of hydrogen on the electrochemical performance of graphene remains a significant challenge. Here we report the experimental observations of high rate capacity in hydrogen-treated 3-dimensional (3D) graphene nanofoam electrodes for lithium ion batteries. Structural and electronic characterization suggests that defect sites and hydrogen play synergistic roles in disrupting sp(2) graphene to facilitate fast lithium transport and reversible surface binding, as evidenced by the fast charge-transfer kinetics and increased capacitive contribution in hydrogen-treated 3D graphene. In concert with experiments, multiscale calculations reveal that defect complexes in graphene are prerequisite for low-temperature hydrogenation, and that the hydrogenation of defective or functionalized sites at strained domain boundaries plays a beneficial role in improving rate capacity by opening gaps to facilitate easier Li penetration. Additional reversible capacity is provided by enhanced lithium binding near hydrogen-terminated edge sites. These findings provide qualitative insights in helping the design of graphene-based materials for high-power electrodes.

Hydrogen storage properties of light metal adatoms (Li, Na) decorated fluorographene monolayer

Owing to its high energy density, the potential of hydrogen (H2) as an energy carrier has been immense, however its storage remains a big obstacle and calls for an efficient storage medium. By means of density functional theory (DFT) in spin polarized generalized gradient approximation (GGA), we have investigated the structural, electronic and hydrogen storage properties of a light alkali metal (Li, Na) functionalized fluorographene monolayer (FG). Metal adatoms bind to the FG with significantly high binding energy, much higher than their cohesive energies, which helps to achieve a uniform distribution of metal adatoms on the monolayer and consequently ensure reversibility. Due to a difference of electronegativities, each metal adatom transfers a substantial amount of its charge to the FG monolayer and attains a partial positive state, which facilitates the adsorption of multiple H2 molecules around the adatoms by electrostatic as well as van der Waals interactions. To get a better description of H2 adsorption energies with metal-doped systems, we have also performed calculations using van der Waals corrections. For both the functionalized systems, the results indicate a reasonably high H2 storage capacity with H2 adsorption energies falling into the range for the practical applications.

Choosing a density functional for modeling adsorptive hydrogen storage: reference quantum mechanical calculations and a comparison of dispersion-corrected density functionals

Various dispersion-corrected density functionals are compared with high level QM data for several model complexes for adsorptive hydrogen storage.

A low-surface energy carbon allotrope: the case for bcc-C6

Graphite may be viewed as a low-surface-energy carbon allotrope with little layer–layer interaction.

One-pot solvothermal synthesis of ZnSe·xN2H4/GS and ZnSe/N-GS and enhanced visible-light photocatalysis

Doped-graphene has attracted considerable attention in many fields because doping element can alter the electrical properties of graphene. In this paper, we synthesized ZnSe·xN2H4/graphene (ZnSe·xN2H4/GS) and ZnSe/nitrogen-doped graphene (ZnSe/N-GS) nanocomposites with p-n junctions via one-pot solvothermal process. The structure, morphologies and catalytic performance of the ZnSe·xN2H4/GS and ZnSe/N-GS are characterized by X-ray diffraction pattern (XRD), field emission scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy (RS), X-ray photoelectron spectroscopy (XPS), and cathodoluminescence spectrum (CL), respectively. Our experiments show that the as-prepared nanocomposites ZnSe·xN2H4/GS and ZnSe/N-GS exhibit remarkably enhanced photocatalytic activities for methylene blue (MB) dye under visible light irradiation. Even importantly, ZnSe/N-GS would make this degradation process more effective. Overall, this facile and catalyst-free synthesize method in this work could provide new insights into the fabrication of other composites based on doped graphene with high performance photocatalysts, which show their potential applications in producing of hydrogen through water splitting, environmental protection issues.

Theoretical study on the encapsulation of Pd3-based transition metal clusters inside boron nitride nanotubes

Chemical functionalization of the boron nitride nanotube (BNNT) allows a wider flexibility in engineering its electronic and magnetic properties as well as chemical reactivity, thus making it have potential applications in many fields. In the present work, the encapsulation of 13 different Pd(3)M (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Pd, Pt, and Au) clusters inside the (10, 0) BNNT has been studied by performing comprehensive density functional theory (DFT) calculations. Particular attention is paid to searching for the stable configurations, calculating the corresponding binding energies, and evaluating the effects of the encapsulation of Pd(3)M cluster on the electronic and magnetic properties of BNNT. The results indicate that all the studied Pd(3)M clusters can be stably encapsulated inside the (10, 0) BNNT, with binding energies ranging from -0.96 (for Pd(3)Sc) to -5.31 eV (for Pd(3)V). Moreover, due to a certain amount of charge transfer from Pd(3)M clusters to BNNT, certain impurity states are induced within the band gap of pristine BNNT, leading to the reduction of the band gap in various ways. Most Pd(3)M@BNNT nanocomposites exhibit nonzero magnetic moments, which mainly originate from the contribution of the Pd(3)M clusters. In particular, the adsorption of O(2) molecule on BNNT is greatly enhanced due to Pd(3)M encapsulation. The elongation of O-O bonds of the adsorbed O(2) molecules indicates that Pd(3)M@BNNT could be used to fabricate the oxidative catalysis.

Improving biogas separation and methane storage with multilayer graphene nanostructure via layer spacing optimization and lithium doping: a molecular simulation investigation

Methane is a desirable alternative to conventional fossil fuels, and also a main component of biogas from anaerobic fermentation of organic wastes. However, its relatively lower purity and poor storage by existing adsorbent materials negatively affect its wide application. Thus, efficient, cost-effective, and safe adsorbent materials for methane purification and storage are highly desired. In this study, multilayer graphene nanostructures (MGNs) with optimized structure are investigated as a potential adsorbent for this purpose. The effects of layer distance and Li doping on MGN performance in terms of methane storage and acid gas (H(2)S and CO(2)) separation from biogas are examined by molecular simulations. The mechanisms for the interactions between gas molecules and substrates are elucidated by analyzing the binding energy, geometric structures, and charge distribution from the first-principles calculations. The results show that nonhydrocarbons in biogas can be effectively separated using Li-doped MGNs with the optimal layer distance of 0.68 nm, and then the pure methane gas can be stored in MGNs with capacity satisfying the DOE target. This work offers a molecular-level insight into the interactions between gas molecules and MGNs and might provide useful information for development of new materials for methane purification and storage.

Si-doped graphene: an ideal sensor for NO- or NO2-detection and metal-free catalyst for N2O-reduction

Exploring and evaluating the potential applications of two-dimensional graphene is an increasingly hot topic in graphene research. In this paper, by studying the adsorption of NO, N(2)O, and NO(2) on pristine and silicon (Si)-doped graphene with density functional theory methods, we evaluated the possibility of using Si-doped graphene as a candidate to detect or reduce harmful nitrogen oxides. The results indicate that, while adsorption of the three molecules on pristine graphene is very weak, Si-doping enhances the interaction of these molecules with graphene sheet in various ways: (1) two NO molecules can be adsorbed on Si-doped graphene in a paired arrangement, while up to four NO(2) molecules attach to the doped graphene with an average adsorption energy of -0.329 eV; (2) the N(2)O molecule can be reduced easily to the N(2) molecule, leaving an O-atom on the Si-doped graphene. Moreover, we find that adsorption of NO and NO(2) leads to large changes in the electronic properties of Si-doped graphene. On the basis of these results, Si-doped graphene can be expected to be a good sensor for NO and NO(2) detection, as well as a metal-free catalyst for N(2)O reduction.