Dr. Viorel Chihaia (brainmap code U-1700-028N-9263) graduated the Physics Faculty of Bucharest University in 1989 and received the Ph.D. degree in chemistry in 1999 from the Institute of Physical Chemistry “Ilie Murgulescu” of Romanian Academy, Bucharest, Romania (IPC). Since October 1990 he has been employed at IPC, where he is currently a senior researcher 2. During this period he worked also abroad at several institutions: Keimyung University, Taegu, Korea (2000), Göttingen University, Germany (2001-2004), Institute of Atomic and Molecular Sciences, Academia Sinica, Taiwan (2010), Jülich Supercomputing Centre, Forschungszentrum Jülich, Germany (2011-2015). He has conducted (six) and participated to (eight) in national and international scientific projects.
Science & Technology Development Journal - Engineering and Technology
Solving the problem of the environment related to CO2 emission is an urgent and challenging task ... more Solving the problem of the environment related to CO2 emission is an urgent and challenging task to prevent the escalation of climate change, which is due to increasing energy demand from fossil fuels and industrial activities. Using graphene to capture CO2 gas is of great interest to mitigate global warming, where CO2 capture by doped graphene has been shown to significantly improve the CO2 adsorption capacity compared to that of pure graphene. In particular, N-doped graphene is a unique structure that was suggested for CO2 capture by the adsorption phenomenon. However, there is no research available to clarify nitrogen doping concentration in graphene on CO2 adsorption. Therefore, this work has been devoted to elucidating the problem using the density functional theory method considering van der Waals interaction. We showed that increasing the doping content of nitrogen by 3.1, 6.3, 9.4, and 12.5% will increase the CO2 adsorption energy, monotonically for N content below 9.4% and ...
Improving the slow kinetics of the oxygen reduction reaction (ORR) on the cathode of the proton e... more Improving the slow kinetics of the oxygen reduction reaction (ORR) on the cathode of the proton exchange membrane fuel cells to achieve the performance at a practical level is an important task. PdCo alloys appeared as a promising electrocatalyst. Much attention has been devoted to the study of the effects of the Co content on the ORR activity of PdCo films and PdCo/C nanoparticles where the Co atoms can be at the topmost surface layer. While Pd-skin/PdCo alloys with the topmost layer formed only by Pd have been proved to provide a very high ORR activity and high durability, no researches are available in the literature for the effects of the Co content on the ORR activity of Pd-skin/PdCo alloys. Hence, the effects of the Co content on the ORR activity of Pd-skin/PdCo alloys are clarified in this work by using the density functional theory calculations and Norskov’s thermodynamic model. Our results predicted that the ORR activity increases monotonically with the increase of the Co content. This behavior i...
Unsaturated metal centers in metal-organic framework MIL-88A are able to significantly enhance th... more Unsaturated metal centers in metal-organic framework MIL-88A are able to significantly enhance the amount of gas adsorbed at ambient temperatures and low pressures. This material has been investigated for various applications; however, it has not yet been tested for hydrogen storage. In this research, we examined the interaction of hydrogen gas (H 2) with Co-MIL-88A by using the van der Waals dispersioncorrected density functional theory calculations. The H 2 molecule was found to adsorb most favorably at the hollow site of the metal trimers in Co-MIL-88A because of the maximum overlap between the bonding state of the H 2 molecule and the total density of state of the Co-MIL-88A. In addition, the hydrogen adsorption isotherms were also assessed by grand canonical Monte Carlo simulations. The results showed that Co-MIL-88A is one of the most effective H 2 storage materials.
In this work we design some atomic scale simulation methods as investigative tools in the study o... more In this work we design some atomic scale simulation methods as investigative tools in the study of the formation of compounds for the reversible storage of hydrogen in bulk materials. It was verified that the reaction between the LiBH 4 and MgH 2 is energetically favored for temperatures above 280 K and that this system can be used in the hydrogen storage and the fuel cell application. To identify the reaction mechanism at the interface of LiBH 4 , MgH 2 and carbon layers we performed some Molecular Dynamics simulations and QM/MM calculations. The results show that the layers of ions formed at the interface with the graphite may assure the right arrangement of the atoms to start the formation of the crystals. Moreover, the presence of the hexagonal layers of graphite may play a role as a pattern template for the layers of boron atoms in the MgB 2 lattice.
We present a review of our work on the dynamics of clathrate hydrates (gas hydrates). The experim... more We present a review of our work on the dynamics of clathrate hydrates (gas hydrates). The experimental results obtained with inelastic neutron scattering are compared with molecular-dynamics calculations. The vibrations of the guest molecules and their coupling to the cages is found to depend critically on the size, shape and electrostatic properties of the encaged guest. Atoms like xenon, that are large enough to fill the cages, show close-to-harmonic behaviour and couple strongly to the cage vibrations. Small atoms and molecules fully explore the anharmonicities of the potential within the cage, in particular at low frequencies and low temperatures. Their dynamic response is broad in energy and they couple weakly to the cage vibrations. The relevance of the microscopic dynamics for cage stability and the glasslike thermal conductivity is discussed. We equally place the observed dynamic peculiarities into the broader context of vibrations in disordered systems. Raman spectroscopic results on internal guest vibrations at high frequencies reflect also the influence of guest-host interactions and are discussed in the framework of the loose-cage tight-cage model.
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
The paper describes the studies started by an inter-disciplinary team in finding a disinfection m... more The paper describes the studies started by an inter-disciplinary team in finding a disinfection method using a coating with photocatalytically activated antimicrobial properties by visible spectrum radiation. An own method of preparation led to the obtaining of a photocatalytic pigment based on copper-doped titanium dioxide to move the activation spectrum to the visible range. The preparation method was designed so as to allow transposition into industrial production. The demonstration and measurement of the photocatalytic effect for certification as an industrial product was done based on an adapted method, starting from two existing standards. A washable paint containing the photocatalytic pigment was tested "in situ", the results demonstrating the reduction of microbial load. The paper is based on current knowledge about light-activated antimicrobiological agents (LAAs) in an attempt to further the study of visible spectrum radiation, which in combination with a series ...
Adsorption-journal of The International Adsorption Society, Mar 10, 2020
MIL-88A metal-organic framework with the unsaturated Fe metal coordination sites has demonstrated... more MIL-88A metal-organic framework with the unsaturated Fe metal coordination sites has demonstrated to be a promising material for gas storage and capture. However, the hydrogen storage capacity of MIL-88A has to be improved to meet the practical level at the ambient conditions. In this research, we elucidated the effects of transition metal substitution on the hydrogen storage capability of MIL-88A. The trivalent transition metals including Sc, Ti, V, Cr, Mn, and Ni have been selected to substitute for Fe in MIL-88A. Using the van der Waals dispersion-corrected density functional theory calculations, we explored the most favorable adsorption configurations of the hydrogen molecule in M-MIL-88A (M = Sc, Ti, V, Cr, Mn, Ni). We found that the V-MIL-88A has the strongest binding energy of 17 kJ mol −1 with the hydrogen molecule in the side-on configuration on the metal site. Besides, the grand canonical Monte Carlo simulations showed that the metal substitution greatly influences not only the favorable adsorption configuration and energy but also the hydrogen uptake due to the modification of the H 2 @MIL-88A interaction. Sc-MIL-88A was found to offer the highest gravimetric H 2 uptake compared to the other M-MIL-88A. The value is 5.13 wt% at (cryogenic temperature 77 K, 50 bar) and 0.72 wt% at (room temperature 298 K, 100 bar) for the absolute; 4.63 wt% at (77 K, 10 bar) and 0.29 wt% at (298 K, 100 bar) for the excess capacity. Furthermore, Sc-MIL-88A also exhibited the highest volumetric uptake up to 52 g L −
Journal of Theoretical and Computational Chemistry, Jun 1, 2004
This work signifies the next step in our way in the magnetic properties simulation of spin cluste... more This work signifies the next step in our way in the magnetic properties simulation of spin clusters and extended networks containing quantum spins, by original FORTRAN codes based on Heisenberg–Dirac–VanVleck (HDVV) or Ising approaches, using Full Diagonalization Heisenberg Matrix (FDHM) or Monte Carlo–Metropolis (MCM) procedure, respectively. We present the results of magnetic Monte Carlo studies on a magnetite type lattice, Ising model ferrimagnet that provide insight into the exchange interactions involved in Cubic Ferrospinels. We have demonstrated that a comparatively simple model can reproduce ferrimagnetic behavior of ferrospinels, particularly for magnetite.
Journal of Physics: Condensed Matter, Jun 19, 2012
With first-principles DFT calculations, the interaction between graphene and SiO 2 surface has be... more With first-principles DFT calculations, the interaction between graphene and SiO 2 surface has been analyzed by constructing the different configurations based on α-quartz and cristobalite structures. The single layer graphene can stay stably on SiO 2 surface is explained based on the general consideration of configuration structures of SiO 2 surface. It is also found that the oxygen defect in SiO 2 surface can shift the Fermi level of graphene down which opens out the mechanism of hole-doping effect of graphene absorbed on SiO 2 surface observed in experiments.
Calphad-computer Coupling of Phase Diagrams and Thermochemistry, Mar 1, 2018
This paper focuses primarily on the P-T phase diagram determination by considering six polymorphs... more This paper focuses primarily on the P-T phase diagram determination by considering six polymorphs of magnesium hydride (α-rutile TiO 2 , P4 2 /mnm, β-cubic modified CaF 2 , Pa 3, γ-orthorhombic PbO 2 , Pbcn, δ'orthorhombic, Pbca and cubic-Fm 3 m). The Gibbs free energy and other thermodynamic properties were evaluated by DFT-based thermodynamic calculations, within the frame of the quasi-harmonic approximation, for the pressure range 0-10 GPa and temperatures between 0 and 1200 K. Furthermore, the structural, energetic, and electronic properties of the investigated structures are conversed.
Molecular dynamics simulation has been performed to study the structure of water-methanol mixture... more Molecular dynamics simulation has been performed to study the structure of water-methanol mixtures. Besides the evaluation of partial radial distribution functions describing the hydrogen-bonded structure of the mixtures with different composition, the statistical analysis of configurations was introduced resulting in a new insight in the clustering properties and topology of hydrogen-bonded network. The results have shown that mixtures of methanol and water exhibit extended structures in solution. At low methanol concentration the water molecules form a percolated network, the methanol molecules are incorporated as monomers or short chains and together form a percolated system. In methanol-rich mixtures short water chains and longer methanol chains build up the hydrogen-bonded clusters in the system. On the basis of the statistical analysis of configurations obtained from molecular dynamics simulation it has been found that more methanol molecules are connected to non-cyclic entities, while more water molecules form rings that might have been predicted on the basis of the stoichiometry of the mixtures. This finding can be explained by the presence of microscopic configurational inhomogeneity in water-methanol mixtures.
Understanding the interaction of water and graphene is crucial for various applications such as w... more Understanding the interaction of water and graphene is crucial for various applications such as water purification, desalination, and electrocatalysis. Experimental and theoretical studies have already investigated water adsorption on N-and B-doped graphene. However, there are no reports available that elucidate the influences of the N and B doping content in graphene on the microscopic geometrical structure and the electronic properties of the adsorbed water. Thus, this work is devoted to solving this problem using self-consistent van der Waals density functional theory calculations. The N and B doping contents of 0.0, 3.1, 6.3, and 9.4% were considered. The results showed that the binding energy of water increases almost linearly as a function of doping content at all concentrations for N-doped graphene but below 6.3% for B-doped graphene. In the linear range, the binding energy increases by approximately 30 meV for each increment of the doping ratio. Analyses of the geometric and electronic structures explained the enhancement of the water-graphene interaction with the variation in doping percentage.
Monolayer MoS 2 has attracted much attention due to its high on/off current ratio, transparency, ... more Monolayer MoS 2 has attracted much attention due to its high on/off current ratio, transparency, and suitability for optoelectronic devices. Surface doping by molecular adsorption has proven to be an effective method to facilitate the usage of MoS 2. However, there are no works available to systematically clarify the effects of the adsorption of F 4 TCNQ, PTCDA, and tetracene on the electronic and optical properties of the material. Therefore, this work elucidated the problem by using density functional theory calculations. We found that the adsorption of F 4 TCNQ and PTCDA turns MoS 2 into a p-type semiconductor, while the tetracene converts MoS 2 into an n-type semiconductor. The occurrence of a new energy level in the conduction band for F 4 TCNQ and PTCDA and the valence band for tetracene reduces the bandgap of the monolayer MoS 2. Besides, the MoS 2 /F 4 TCNQ and MoS 2 /PTCDA systems exhibit an auxiliary optical peak at the long wavelengths of 950 and 850 nm, respectively. Contrastingly, the MoS 2 /tetracene modifies the optical spectrum of the monolayer MoS 2 only in the ultraviolet region. The findings are in good agreement with the experiments.
Journal of Science: Advanced Materials and Devices, Jun 1, 2018
The Pd-Co alloy is a good candidate for the perpendicular magnetic recording and related applicat... more The Pd-Co alloy is a good candidate for the perpendicular magnetic recording and related applications. However, no research is available to clarify the influences of local structures on the magnetic anisotropy of the Pd-Co alloy. Therefore, in this work, we studied the effects of Co arrangement on the magnetic anisotropy of ultrathin Pd 4 Co(111) film with 20% Co content by using the density functional theory calculations. We found that a Co monolayer in the surface layer of the ultrathin film offers a large in-plane magnetic anisotropy while the Co atoms mixed inside the Pd matrix exhibit the perpendicular magnetic anisotropy. Notably, a Co monolayer beneath the surface layer of the Pd matrix maximizes the perpendicular magnetic anisotropy up to 1.85 erg/cm 2. Electronic properties were also analyzed to clarify the magnetic anisotropy of the ultrathin film.
To be selected as sorbents for gas storage, metal-organic frameworks (MOFs) must be stable to avo... more To be selected as sorbents for gas storage, metal-organic frameworks (MOFs) must be stable to avoid collapsed in humid media. MIL-88 series (abbreviated as MIL-88s) including MIL-88A, B, C, D satisfies high flexibility and stability; it, therefore, may become a suitable candidate for hydrogen storage based on the adsorption. In this work, the grand canonical Monte Carlo simulations for the pressures below 100 bar showed that in MIL-88 series MIL-88D exhibits the highest absolute and excess gravimetric H 2 capacities of 5.15 wt% and 4.03 wt% at 77 K, and 0.69 wt% and 0.23 wt% at 298 K, respectively. Meanwhile, MIL-88A has the highest absolute and excess volumetric H 2 uptakes of 50.69 g/L and 44.32 g/L at 77 K, and 6.97 g/L and 2.49 g/L at 298 K. These results are comparable to the best MOFs for hydrogen storage to date. It was shown that the hydrogen uptakes depend on the special surface area and the pore volume of the MIL-88s, apart from depending on the type of the ligand. By utilizing the van der Waals dispersion-corrected density functional theory (DFT) calculations, we elucidated the interaction between the H 2 molecule and the MIL-88 series. The adsorption energy, as well as the isosteric heat of adsorption, revealed that the H 2-MIL-88C interaction is strongest despite its lowest storage capacity. This observation implies an implicit role of electronic structure on the H 2 adsorption capacities at the considered conditions. However, at the low temperature, the DFT calculations could elucidate the preferred adsorption sites of hydrogen molecule on the surface of MIL-88s. Besides, we also found that the interaction is dominated by the bonding state of the H 2 molecule and the p orbitals of the O and C atoms of the MIL-88s. The most substantial overlap between the electronic density of states (DOS) of the MIL-88C and the DOS of the H 2 molecule leads to the most robust interaction between the H 2 molecule and the MIL-88C.
Korean Journal of Chemical Engineering, May 1, 2000
Molecular dynamics simulations have been carried out to investigate nearest-neighbor distribution... more Molecular dynamics simulations have been carried out to investigate nearest-neighbor distribution fimctions and closely related quantifies for the system of hard-spheres. The nearest-neighbor distribution fimction and the exclusion probability function were computed to examine the density dependence on the structul-al ~void' and 'particle' properties. Simulation results were used to access the applicabilities of various theoretical predictions based on the scaled-particle theory, the Percus-Yevick equation, and the Camahan-Starling approximation. For lower density systems the three different approximations give the nearest-neighbor distribution fimctions which are very close to one another and also to the resulting simulation data. Among those theoretical predictions, the Canlallan-Starling approximarion gives remarkably good agreement with the simulation data even for higher density systems. Also calculated is the nth moment of the nearest-neighbor distribution functions, in which the corresponding length scale is directly related to the measurement of the characteristic pore-size distribution.
The activity of the oxygen reduction reaction (ORR) on the cathode is one of the dominant factors... more The activity of the oxygen reduction reaction (ORR) on the cathode is one of the dominant factors in the performance of proton exchange membrane fuel cells. Iron porphyrin has low cost, environmental benignity, and maximum efficiency of metal usage. Therefore, this material can be a promising singleatomic metal dispersion catalyst for fuel cell cathodes. The variation of functional groups was proven to effectively modify the activity of the ORR on the iron porphyrin. However, the influences of functional groups on the mechanisms of the ORR remain ambiguous. This work paid attention to the substitution of carboxyl (-COOH), methyl (-CH 3), and amino (-NH 2) functional groups at the meso positions of the porphyrin ring. By using van der Waals density functional theory (vdW-DF) calculations, we found that the ORR mechanisms can follow the associative and dissociative pathways, respectively. The Gibbs free energy diagrams revealed that the rate-limiting step occurs at the second hydrogenation step for the first pathway and the O 2 dissociation step for the second pathway for all considered functional groups. The thermodynamic energy barrier at the rate-limiting step was found to be in the following order: porphyrin-(CH 3) 4 < porphyrin-(NH 2) 4 < original porphyrin < porphyrin-(COOH) 4 for the associative mechanism and porphyrin-(NH 2) 4 < porphyrin-(CH 3) 4 < porphyrin-(COOH) 4 < original porphyrin for the dissociative pathway. The findings suggested that porphyrin-(CH 3) 4 and porphyrin-(NH 2) 4 should be the best choices among the considered substrates for the oxygen reduction reaction. Furthermore, the interaction between the ORR intermediates and the substrates was attributed to the resonance of the d z 2 , d xz , and d yz components of the Fe d orbital and the C and N p orbitals of the substrates with the p orbitals of the oxygen atoms in the intermediates. Finally, the nature of the interaction between the adsorbent and adsorbate was charge transfer.
Molecular dynamics simulations have been performed for liquid formamide using two different types... more Molecular dynamics simulations have been performed for liquid formamide using two different types of potential model (OPLS, Cordeiro). The structural results obtained from simulation were compared to experimental (x-ray and neutron diffraction measurements) outcomes. A generally good agreement for both models examined has been found, but in the hydrogen bonded region (2.9 Å) the Cordeiro model shows a slightly better fit. Besides the evaluation of partial radial distribution functions, orientational correlation functions and energy distribution functions, describing the hydrogen bonded structure, have been calculated based on the statistical analysis of configurations, resulting into a new insight in the clustering properties and topology of hydrogen bonded network. It has been shown that in liquid formamide exists a continuous hydrogen bonded network and from the analysis of the distribution of small rings revealed the ring size distribution in liquid formamide. Our study resulted that the ring size distribution of the hydrogen bonded liquid formamide shows a broad distribution with a maximum around 11. It has been found that the topology in formamide is significantly different than in water.
Science & Technology Development Journal - Engineering and Technology
Solving the problem of the environment related to CO2 emission is an urgent and challenging task ... more Solving the problem of the environment related to CO2 emission is an urgent and challenging task to prevent the escalation of climate change, which is due to increasing energy demand from fossil fuels and industrial activities. Using graphene to capture CO2 gas is of great interest to mitigate global warming, where CO2 capture by doped graphene has been shown to significantly improve the CO2 adsorption capacity compared to that of pure graphene. In particular, N-doped graphene is a unique structure that was suggested for CO2 capture by the adsorption phenomenon. However, there is no research available to clarify nitrogen doping concentration in graphene on CO2 adsorption. Therefore, this work has been devoted to elucidating the problem using the density functional theory method considering van der Waals interaction. We showed that increasing the doping content of nitrogen by 3.1, 6.3, 9.4, and 12.5% will increase the CO2 adsorption energy, monotonically for N content below 9.4% and ...
Improving the slow kinetics of the oxygen reduction reaction (ORR) on the cathode of the proton e... more Improving the slow kinetics of the oxygen reduction reaction (ORR) on the cathode of the proton exchange membrane fuel cells to achieve the performance at a practical level is an important task. PdCo alloys appeared as a promising electrocatalyst. Much attention has been devoted to the study of the effects of the Co content on the ORR activity of PdCo films and PdCo/C nanoparticles where the Co atoms can be at the topmost surface layer. While Pd-skin/PdCo alloys with the topmost layer formed only by Pd have been proved to provide a very high ORR activity and high durability, no researches are available in the literature for the effects of the Co content on the ORR activity of Pd-skin/PdCo alloys. Hence, the effects of the Co content on the ORR activity of Pd-skin/PdCo alloys are clarified in this work by using the density functional theory calculations and Norskov’s thermodynamic model. Our results predicted that the ORR activity increases monotonically with the increase of the Co content. This behavior i...
Unsaturated metal centers in metal-organic framework MIL-88A are able to significantly enhance th... more Unsaturated metal centers in metal-organic framework MIL-88A are able to significantly enhance the amount of gas adsorbed at ambient temperatures and low pressures. This material has been investigated for various applications; however, it has not yet been tested for hydrogen storage. In this research, we examined the interaction of hydrogen gas (H 2) with Co-MIL-88A by using the van der Waals dispersioncorrected density functional theory calculations. The H 2 molecule was found to adsorb most favorably at the hollow site of the metal trimers in Co-MIL-88A because of the maximum overlap between the bonding state of the H 2 molecule and the total density of state of the Co-MIL-88A. In addition, the hydrogen adsorption isotherms were also assessed by grand canonical Monte Carlo simulations. The results showed that Co-MIL-88A is one of the most effective H 2 storage materials.
In this work we design some atomic scale simulation methods as investigative tools in the study o... more In this work we design some atomic scale simulation methods as investigative tools in the study of the formation of compounds for the reversible storage of hydrogen in bulk materials. It was verified that the reaction between the LiBH 4 and MgH 2 is energetically favored for temperatures above 280 K and that this system can be used in the hydrogen storage and the fuel cell application. To identify the reaction mechanism at the interface of LiBH 4 , MgH 2 and carbon layers we performed some Molecular Dynamics simulations and QM/MM calculations. The results show that the layers of ions formed at the interface with the graphite may assure the right arrangement of the atoms to start the formation of the crystals. Moreover, the presence of the hexagonal layers of graphite may play a role as a pattern template for the layers of boron atoms in the MgB 2 lattice.
We present a review of our work on the dynamics of clathrate hydrates (gas hydrates). The experim... more We present a review of our work on the dynamics of clathrate hydrates (gas hydrates). The experimental results obtained with inelastic neutron scattering are compared with molecular-dynamics calculations. The vibrations of the guest molecules and their coupling to the cages is found to depend critically on the size, shape and electrostatic properties of the encaged guest. Atoms like xenon, that are large enough to fill the cages, show close-to-harmonic behaviour and couple strongly to the cage vibrations. Small atoms and molecules fully explore the anharmonicities of the potential within the cage, in particular at low frequencies and low temperatures. Their dynamic response is broad in energy and they couple weakly to the cage vibrations. The relevance of the microscopic dynamics for cage stability and the glasslike thermal conductivity is discussed. We equally place the observed dynamic peculiarities into the broader context of vibrations in disordered systems. Raman spectroscopic results on internal guest vibrations at high frequencies reflect also the influence of guest-host interactions and are discussed in the framework of the loose-cage tight-cage model.
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
The paper describes the studies started by an inter-disciplinary team in finding a disinfection m... more The paper describes the studies started by an inter-disciplinary team in finding a disinfection method using a coating with photocatalytically activated antimicrobial properties by visible spectrum radiation. An own method of preparation led to the obtaining of a photocatalytic pigment based on copper-doped titanium dioxide to move the activation spectrum to the visible range. The preparation method was designed so as to allow transposition into industrial production. The demonstration and measurement of the photocatalytic effect for certification as an industrial product was done based on an adapted method, starting from two existing standards. A washable paint containing the photocatalytic pigment was tested "in situ", the results demonstrating the reduction of microbial load. The paper is based on current knowledge about light-activated antimicrobiological agents (LAAs) in an attempt to further the study of visible spectrum radiation, which in combination with a series ...
Adsorption-journal of The International Adsorption Society, Mar 10, 2020
MIL-88A metal-organic framework with the unsaturated Fe metal coordination sites has demonstrated... more MIL-88A metal-organic framework with the unsaturated Fe metal coordination sites has demonstrated to be a promising material for gas storage and capture. However, the hydrogen storage capacity of MIL-88A has to be improved to meet the practical level at the ambient conditions. In this research, we elucidated the effects of transition metal substitution on the hydrogen storage capability of MIL-88A. The trivalent transition metals including Sc, Ti, V, Cr, Mn, and Ni have been selected to substitute for Fe in MIL-88A. Using the van der Waals dispersion-corrected density functional theory calculations, we explored the most favorable adsorption configurations of the hydrogen molecule in M-MIL-88A (M = Sc, Ti, V, Cr, Mn, Ni). We found that the V-MIL-88A has the strongest binding energy of 17 kJ mol −1 with the hydrogen molecule in the side-on configuration on the metal site. Besides, the grand canonical Monte Carlo simulations showed that the metal substitution greatly influences not only the favorable adsorption configuration and energy but also the hydrogen uptake due to the modification of the H 2 @MIL-88A interaction. Sc-MIL-88A was found to offer the highest gravimetric H 2 uptake compared to the other M-MIL-88A. The value is 5.13 wt% at (cryogenic temperature 77 K, 50 bar) and 0.72 wt% at (room temperature 298 K, 100 bar) for the absolute; 4.63 wt% at (77 K, 10 bar) and 0.29 wt% at (298 K, 100 bar) for the excess capacity. Furthermore, Sc-MIL-88A also exhibited the highest volumetric uptake up to 52 g L −
Journal of Theoretical and Computational Chemistry, Jun 1, 2004
This work signifies the next step in our way in the magnetic properties simulation of spin cluste... more This work signifies the next step in our way in the magnetic properties simulation of spin clusters and extended networks containing quantum spins, by original FORTRAN codes based on Heisenberg–Dirac–VanVleck (HDVV) or Ising approaches, using Full Diagonalization Heisenberg Matrix (FDHM) or Monte Carlo–Metropolis (MCM) procedure, respectively. We present the results of magnetic Monte Carlo studies on a magnetite type lattice, Ising model ferrimagnet that provide insight into the exchange interactions involved in Cubic Ferrospinels. We have demonstrated that a comparatively simple model can reproduce ferrimagnetic behavior of ferrospinels, particularly for magnetite.
Journal of Physics: Condensed Matter, Jun 19, 2012
With first-principles DFT calculations, the interaction between graphene and SiO 2 surface has be... more With first-principles DFT calculations, the interaction between graphene and SiO 2 surface has been analyzed by constructing the different configurations based on α-quartz and cristobalite structures. The single layer graphene can stay stably on SiO 2 surface is explained based on the general consideration of configuration structures of SiO 2 surface. It is also found that the oxygen defect in SiO 2 surface can shift the Fermi level of graphene down which opens out the mechanism of hole-doping effect of graphene absorbed on SiO 2 surface observed in experiments.
Calphad-computer Coupling of Phase Diagrams and Thermochemistry, Mar 1, 2018
This paper focuses primarily on the P-T phase diagram determination by considering six polymorphs... more This paper focuses primarily on the P-T phase diagram determination by considering six polymorphs of magnesium hydride (α-rutile TiO 2 , P4 2 /mnm, β-cubic modified CaF 2 , Pa 3, γ-orthorhombic PbO 2 , Pbcn, δ'orthorhombic, Pbca and cubic-Fm 3 m). The Gibbs free energy and other thermodynamic properties were evaluated by DFT-based thermodynamic calculations, within the frame of the quasi-harmonic approximation, for the pressure range 0-10 GPa and temperatures between 0 and 1200 K. Furthermore, the structural, energetic, and electronic properties of the investigated structures are conversed.
Molecular dynamics simulation has been performed to study the structure of water-methanol mixture... more Molecular dynamics simulation has been performed to study the structure of water-methanol mixtures. Besides the evaluation of partial radial distribution functions describing the hydrogen-bonded structure of the mixtures with different composition, the statistical analysis of configurations was introduced resulting in a new insight in the clustering properties and topology of hydrogen-bonded network. The results have shown that mixtures of methanol and water exhibit extended structures in solution. At low methanol concentration the water molecules form a percolated network, the methanol molecules are incorporated as monomers or short chains and together form a percolated system. In methanol-rich mixtures short water chains and longer methanol chains build up the hydrogen-bonded clusters in the system. On the basis of the statistical analysis of configurations obtained from molecular dynamics simulation it has been found that more methanol molecules are connected to non-cyclic entities, while more water molecules form rings that might have been predicted on the basis of the stoichiometry of the mixtures. This finding can be explained by the presence of microscopic configurational inhomogeneity in water-methanol mixtures.
Understanding the interaction of water and graphene is crucial for various applications such as w... more Understanding the interaction of water and graphene is crucial for various applications such as water purification, desalination, and electrocatalysis. Experimental and theoretical studies have already investigated water adsorption on N-and B-doped graphene. However, there are no reports available that elucidate the influences of the N and B doping content in graphene on the microscopic geometrical structure and the electronic properties of the adsorbed water. Thus, this work is devoted to solving this problem using self-consistent van der Waals density functional theory calculations. The N and B doping contents of 0.0, 3.1, 6.3, and 9.4% were considered. The results showed that the binding energy of water increases almost linearly as a function of doping content at all concentrations for N-doped graphene but below 6.3% for B-doped graphene. In the linear range, the binding energy increases by approximately 30 meV for each increment of the doping ratio. Analyses of the geometric and electronic structures explained the enhancement of the water-graphene interaction with the variation in doping percentage.
Monolayer MoS 2 has attracted much attention due to its high on/off current ratio, transparency, ... more Monolayer MoS 2 has attracted much attention due to its high on/off current ratio, transparency, and suitability for optoelectronic devices. Surface doping by molecular adsorption has proven to be an effective method to facilitate the usage of MoS 2. However, there are no works available to systematically clarify the effects of the adsorption of F 4 TCNQ, PTCDA, and tetracene on the electronic and optical properties of the material. Therefore, this work elucidated the problem by using density functional theory calculations. We found that the adsorption of F 4 TCNQ and PTCDA turns MoS 2 into a p-type semiconductor, while the tetracene converts MoS 2 into an n-type semiconductor. The occurrence of a new energy level in the conduction band for F 4 TCNQ and PTCDA and the valence band for tetracene reduces the bandgap of the monolayer MoS 2. Besides, the MoS 2 /F 4 TCNQ and MoS 2 /PTCDA systems exhibit an auxiliary optical peak at the long wavelengths of 950 and 850 nm, respectively. Contrastingly, the MoS 2 /tetracene modifies the optical spectrum of the monolayer MoS 2 only in the ultraviolet region. The findings are in good agreement with the experiments.
Journal of Science: Advanced Materials and Devices, Jun 1, 2018
The Pd-Co alloy is a good candidate for the perpendicular magnetic recording and related applicat... more The Pd-Co alloy is a good candidate for the perpendicular magnetic recording and related applications. However, no research is available to clarify the influences of local structures on the magnetic anisotropy of the Pd-Co alloy. Therefore, in this work, we studied the effects of Co arrangement on the magnetic anisotropy of ultrathin Pd 4 Co(111) film with 20% Co content by using the density functional theory calculations. We found that a Co monolayer in the surface layer of the ultrathin film offers a large in-plane magnetic anisotropy while the Co atoms mixed inside the Pd matrix exhibit the perpendicular magnetic anisotropy. Notably, a Co monolayer beneath the surface layer of the Pd matrix maximizes the perpendicular magnetic anisotropy up to 1.85 erg/cm 2. Electronic properties were also analyzed to clarify the magnetic anisotropy of the ultrathin film.
To be selected as sorbents for gas storage, metal-organic frameworks (MOFs) must be stable to avo... more To be selected as sorbents for gas storage, metal-organic frameworks (MOFs) must be stable to avoid collapsed in humid media. MIL-88 series (abbreviated as MIL-88s) including MIL-88A, B, C, D satisfies high flexibility and stability; it, therefore, may become a suitable candidate for hydrogen storage based on the adsorption. In this work, the grand canonical Monte Carlo simulations for the pressures below 100 bar showed that in MIL-88 series MIL-88D exhibits the highest absolute and excess gravimetric H 2 capacities of 5.15 wt% and 4.03 wt% at 77 K, and 0.69 wt% and 0.23 wt% at 298 K, respectively. Meanwhile, MIL-88A has the highest absolute and excess volumetric H 2 uptakes of 50.69 g/L and 44.32 g/L at 77 K, and 6.97 g/L and 2.49 g/L at 298 K. These results are comparable to the best MOFs for hydrogen storage to date. It was shown that the hydrogen uptakes depend on the special surface area and the pore volume of the MIL-88s, apart from depending on the type of the ligand. By utilizing the van der Waals dispersion-corrected density functional theory (DFT) calculations, we elucidated the interaction between the H 2 molecule and the MIL-88 series. The adsorption energy, as well as the isosteric heat of adsorption, revealed that the H 2-MIL-88C interaction is strongest despite its lowest storage capacity. This observation implies an implicit role of electronic structure on the H 2 adsorption capacities at the considered conditions. However, at the low temperature, the DFT calculations could elucidate the preferred adsorption sites of hydrogen molecule on the surface of MIL-88s. Besides, we also found that the interaction is dominated by the bonding state of the H 2 molecule and the p orbitals of the O and C atoms of the MIL-88s. The most substantial overlap between the electronic density of states (DOS) of the MIL-88C and the DOS of the H 2 molecule leads to the most robust interaction between the H 2 molecule and the MIL-88C.
Korean Journal of Chemical Engineering, May 1, 2000
Molecular dynamics simulations have been carried out to investigate nearest-neighbor distribution... more Molecular dynamics simulations have been carried out to investigate nearest-neighbor distribution fimctions and closely related quantifies for the system of hard-spheres. The nearest-neighbor distribution fimction and the exclusion probability function were computed to examine the density dependence on the structul-al ~void' and 'particle' properties. Simulation results were used to access the applicabilities of various theoretical predictions based on the scaled-particle theory, the Percus-Yevick equation, and the Camahan-Starling approximation. For lower density systems the three different approximations give the nearest-neighbor distribution fimctions which are very close to one another and also to the resulting simulation data. Among those theoretical predictions, the Canlallan-Starling approximarion gives remarkably good agreement with the simulation data even for higher density systems. Also calculated is the nth moment of the nearest-neighbor distribution functions, in which the corresponding length scale is directly related to the measurement of the characteristic pore-size distribution.
The activity of the oxygen reduction reaction (ORR) on the cathode is one of the dominant factors... more The activity of the oxygen reduction reaction (ORR) on the cathode is one of the dominant factors in the performance of proton exchange membrane fuel cells. Iron porphyrin has low cost, environmental benignity, and maximum efficiency of metal usage. Therefore, this material can be a promising singleatomic metal dispersion catalyst for fuel cell cathodes. The variation of functional groups was proven to effectively modify the activity of the ORR on the iron porphyrin. However, the influences of functional groups on the mechanisms of the ORR remain ambiguous. This work paid attention to the substitution of carboxyl (-COOH), methyl (-CH 3), and amino (-NH 2) functional groups at the meso positions of the porphyrin ring. By using van der Waals density functional theory (vdW-DF) calculations, we found that the ORR mechanisms can follow the associative and dissociative pathways, respectively. The Gibbs free energy diagrams revealed that the rate-limiting step occurs at the second hydrogenation step for the first pathway and the O 2 dissociation step for the second pathway for all considered functional groups. The thermodynamic energy barrier at the rate-limiting step was found to be in the following order: porphyrin-(CH 3) 4 < porphyrin-(NH 2) 4 < original porphyrin < porphyrin-(COOH) 4 for the associative mechanism and porphyrin-(NH 2) 4 < porphyrin-(CH 3) 4 < porphyrin-(COOH) 4 < original porphyrin for the dissociative pathway. The findings suggested that porphyrin-(CH 3) 4 and porphyrin-(NH 2) 4 should be the best choices among the considered substrates for the oxygen reduction reaction. Furthermore, the interaction between the ORR intermediates and the substrates was attributed to the resonance of the d z 2 , d xz , and d yz components of the Fe d orbital and the C and N p orbitals of the substrates with the p orbitals of the oxygen atoms in the intermediates. Finally, the nature of the interaction between the adsorbent and adsorbate was charge transfer.
Molecular dynamics simulations have been performed for liquid formamide using two different types... more Molecular dynamics simulations have been performed for liquid formamide using two different types of potential model (OPLS, Cordeiro). The structural results obtained from simulation were compared to experimental (x-ray and neutron diffraction measurements) outcomes. A generally good agreement for both models examined has been found, but in the hydrogen bonded region (2.9 Å) the Cordeiro model shows a slightly better fit. Besides the evaluation of partial radial distribution functions, orientational correlation functions and energy distribution functions, describing the hydrogen bonded structure, have been calculated based on the statistical analysis of configurations, resulting into a new insight in the clustering properties and topology of hydrogen bonded network. It has been shown that in liquid formamide exists a continuous hydrogen bonded network and from the analysis of the distribution of small rings revealed the ring size distribution in liquid formamide. Our study resulted that the ring size distribution of the hydrogen bonded liquid formamide shows a broad distribution with a maximum around 11. It has been found that the topology in formamide is significantly different than in water.
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