TU Dresden » Fakultäten » Mathematik und Naturwissenschaften » Chemie und Lebensmittelchemie » Physikalische Chemie » Theoretische Chemie

Research

Current Research activities



Proton-transfer reactions in soft matter

Proton-transfer reactions and mechanisms are essential in technological processes, e.g. in proton-exchange membrane fuel cells (PEMFCs). The fuel cell membrane is responsible for the transport of protons between the electrodes. Different membrane materials are used to date, of which Nafion is the most common one. However, the proton transport in Nafion membranes critically depends on the water content, which limits the operating temperature to the boiling point of water.
State of the art research on water-free proton transport favors protogenic groups other than the sulfonic-acid group that is essential in the Nafion membrane. Possible candidates are phosphonic acid and imidazole. We study these materials with respect to their proton-transporting properties using quantum and classical molecular dynamics as well as Monte-Carlo methods.

- -
Fig. 1: Alignment of Imidazole guest molecules in a MIL-53(Al) MOF channel. Fig 2: Sampled NN and NH distances (upper panels) and partial N and H charges (lower panels) during proton transfer in liquid imidazole (left column) and a MIL-53(Al) MOF channel (right column). The statistical errors are highlighted as shaded areas.

References

(1)    E. Eisbein, J.-O. Joswig, G. Seifert, J. Phys. Chem. C 118 (2014) 13035-13041.
(2)    W. L. Cavalcanti, D. F. Portaluppi, J.-O. Joswig, J. Chem. Phys. 133 (2010) 104703/1-6.
(3)    J.-O. Joswig, G. Seifert, J. Phys. Chem. B 113 (2009) 8475-8480.
(4)    J.-O. Joswig, S. Hazebroucq, and G. Seifert, J. Mol. Struc.: THEOCHEM 816 (2007) 119-123.

Contact:
Dr. Jan-Ole Joswig

Hydrogen storage in graphene-based nanostructures

Hydrogen storage by physisorption in graphene-based nanostructures is certainly comparable to the performance of state-of-art "reference" materials such as metal-organic frameworks (MOFs) [their uptake normally does not exceed ~7 wt% at 77 K]. As a result, graphene-based materials represent a practical alternative to MOFs which are (in many cases) humidity-sensitive (i.e., subject to hydrolysis) and very expensive. Our recent focus has been on computational studies (classical Grand canonical Monte Carlo) in multilayered graphene systems with defect-induced additional porosity. We considered model systems which consist of infinite parallel graphene layers with "holes" of different sizes (~6-30 Å). We found that the incorporation of defects (preferably small "holes", up to 10 Å in diameter, cf. Figure 1) results in the increase of theoretically possible surface areas beyond the limits of ideal defect-free graphene (~2700 m2/g) with the values approaching ~5000 m2/g. This leads to promising hydrogen storage capacities of up to 6.5 wt% at 77 K (15 bar). Modelling was supported by an experimental work performed in the group of Dr. Talyzin (Umeå University, Sweden).



Figure 1. An example of a 3D stacking of perforated graphene sheets (a perspective view).

References

(1)    I.A. Baburin, A. Klechikov, G. Mercier, A. Talyzin, G. Seifert, Int. J. Hydrogen Energy. 2015. Vol. 40 (2015) 6594-6599

Contact:
Dr. Igor Baburin

Metal-organic frameworks

In recent years the group of Prof. Seifert has gained a wide experience in simulating the properties of porous solids, mainly metal-organic and covalent organic frameworks as well as porous carbons. This experience involves the calculation of gas adsorption isotherms by classical GCMC methods, identification of adsorption sites by molecular dynamics simulations at the DFTB (density-functional based tight binding) level of theory and the flexibility ("breathing") of metal-organic frameworks based on ab initio molecular dynamics. Furthermore, there is a strong expertise in topological modeling of not-yet-synthesized structures applied to zeolitic imidazolate-based frameworks (ZIFs, cf. Fig. 1).


Figure 1. Most promising LiB(imid)4 frameworks for hydrogen storage applications.

References

(1)    I.A. Baburin, B. Assfour, G. Seifert, S. Leoni, Dalton Trans. Vol. 40 (2011) 3796-3798
(2)    H. C. Hoffmann, B. Assfour, F. Epperlein, N. Klein, S. Paasch, I. Senkovska, S. Kaskel, G. Seifert and E. Brunner, J. Am. Chem. Soc. 133 (2011) 8681-8690
(3)    B. Assfour, S. Leoni, G. Seifert, J. Phys. Chem. C. 114 (2010) 13381-13384
(4)    E. Eisbein, J.-O. Joswig, G. Seifert, J. Phys. Chem. C. 118 (2014) 13035-13041

Contact:
Dr. Igor Baburin

DFT investigations of magnetic properties in metal-organic frameworks (MOFs)

Metal organic frameworks (MOFs) gained very high interest over the last decade due to their high porosity, low density and absorption behaviour. Some MOFs exhibit transition metal centers, which can lead to a certain magnetic ground state within the system. In order to investigate the magnetic ground state of e.g. the exible MOF DUT-8(Ni) [1,2], ab initio investigations in the framework of density functional theory (DFT) have been carried out. This MOF exists in two crystalline structures (DUT-8(Ni)open /DUT-8(Ni)closed), where both of them exhibit low-spin states, thus an antiparallel alignment of the spins at the magnetic centers.

- - -
(a) Unit cells of DUT-8(Ni)closed (b) Supercell for DUT-8(Ni)open (c) Model system
Structure of unit cells, a supercell and a model system for DUT-8(Ni).

Further investigations included several model systems. Those models consist of the two magnetic centers and their chemical environment. Different alterations of this chemical environment were introduced to see the corresponding change in the magnetic behaviour. This allows an insight into what could happen to the crystalline systems if such changes were implied into the periodic structures. Some alterations to the model systems, especially the ones including a removal of the NH3 groups, show that the ground changes from low-spin to high-spin. This provides a starting point for further investigations which might lead to a stable high-spin MOF.
Another interesting research field is the structural determination of MOFs using NMR. A favourable canditate for NMR is 129Xe as it is mononuclear, chemically inert and has a spin of I = 1/2. Experiments are carried out for the MOFs UiO-66 and UiO-67, which are Zr based structures. The corresponding NMR parameters for the chemical shift of Xe δXe inside such structures can be determined by means of DFT. This can help to understand the experimental spectrum and to predict trends of δXe for different conditions.

References

(1)    V. Bon et al., Phys. Chem. Chem. Phys. 17 (2015) 17471-17479
(2)    K. Trepte, S. Schwalbe, G. Seifert, Phys. Chem. Chem. Phys. 17 (2015) 17122-17129

Contact:
Kai Trepte

MoS2 Nanotubes and Layers

Since their discovery in the early 90s of the 20th century, inorganic nanomaterials are of great interest due to their attractive electronic, mechanical, optical and magnetic properties. For that reason we investigate these properties theoretically using amongst others the DFTB method especially for MoS2 nanomaterials. Bulk MoS2 is an indirect semiconductor whose band gap becomes direct due to quantum confinement, when monolayer MoS2 is fabricated by separating single layers from the bulk material. The semiconducting nature is a big advantage offering the opportunity of, e.g., constructing transistors with low power dissipation. Additionally, due to their robust mechanical behaviour monolayer MoS2 could be suitable for a variety of applications such as reinforcing elements in composites and for fabrication of flexible electronic devices. To get a better understanding how these materials and their properties behave under specific conditions and how they could be influenced and utilised for different applications, we have performed molecular dynamics simulations to investigate the mechanical stability and resistance, as well as the breaking behaviour of MoS2 nanotubes and layers. Furthermore, we analysed the change of the electronic structure during these structural deformations and introduced local structural defects caused by missing atoms to get an idea of the size of their influence on the mentioned properties.

- -

References

(1)   T. Lorenz, D. Teich, J.-O. Joswig, G. Seifert, J. Phys. Chem. C 116 (2012) 11714-21
(2)   T. Lorenz, J.-O. Joswig, G. Seifert, 2D Mater. 1 (2014) 011007
(3)   T. Lorenz, M. Ghorbani-Asl, J.-O. Joswig, T. Heine, G. Seifert, Nanotechnology 25 (2014) 445201
(4)   J.-O. Joswig, T. Lorenz, T. B. Wendumu, S. Gemming, G. Seifert, Acc. Chem. Res. 48 (2015) 48-55

Contact:
Tommy Lorenz

Properties of boron-rich systems

The electron-deficient element boron represents a challenge to both experiment and theory. The list of unknowns and questions is long: How can we understand the chemical bonding? Why are boron-rich crystal structures so complex? How many elemental phases exist? What is the ground state structure? How does the phase diagram look like? Are there non-icosahedral nanostructures? etc.

- -
Left: a boron nanotube and its current-voltage relation (calculated). Right: the crystal structure of α-tetragonal boron, consisting of four icosahedral B12 units (grey) and partially occupied interstitial sites (colored).

Our main objective is to tackle these questions by theoretically studying structural, chemical and physical properties of boron bulk and nanostructures. Recent results focused on boron sheets, boron nanotubes and α-tetragonal boron. We are closely collaborating with experimentally working partners and also organize an annual, international boron symposium within the MS&T conference.

References

(1)   J. Kunstmann, V. Bezugly, H. Rabbel, M. H. Rümmeli, G. Cuniberti, Advanced Functional Materials 24 (2014) 4127
(2)   I. G. Gonzalez-Martinez, S. Gorantla, A. Bachmatiuk, V. Bezugly, J. Zhao, T. Gemming, J. Kunstmann, J. Eckert, G. Cuniberti, M. H. Rümmeli, Nano Letters 14 (2014) 799
(3)   V. Bezugly, J. Kunstmann, B. Grundkötter-Stock, T. Frauenheim, T. Niehaus, G. Cuniberti, ACS Nano 5 (2011) 4997 (2011)

Contact:
Dr. Jens Kunstmann

Optical and electronic properties of molybdenum disulfide monolayers

Molybdenum disulfide (MoS2) is a compound among the rich family of layered transition-metal dichalcogenides that has been studied most intensively in the past 40 years. Whereas bulk MoS2 is a semiconductor with an indirect band gap of 1.3 eV, monolayer MoS2 is a direct-gap semiconductor with a band gap of 1.8 eV. Monolayer MoS2 has been used to build transistors with very promising electron mobility. Furthermore, MoS2 has been recently exploited for biosensing and energy-storage applications, and strong absorption and photoluminescence bands were observed in the visible range at 1.9 eV (~650 nm).
Recently, monolayer MoS2 has been in the focus of research because of its exceptional electronic and optical properties. Due to this reason we studied as an example Optical and electronic properties of triangular MoS2 nanoflakes with edge lengths ranging from 1.6 to 10.4 nm. The optical spectra were calculated using the time-dependent extension of the density-functional tight-binding method (TD-DFTB). The size effect in the optical absorption spectra is clearly visible. With decreasing length of the nanoflakes edges, the long-wavelength absorption in the range of visible light is shifted toward short-wavelength absorption, confirming a quantum-confinement-like behavior of these flakes. In contrast, the edges of the nanoflakes exhibit a distinct metallic-like behavior.

- -
Fig. 1: Calculated absorption spectra of a continuous set of MoS2 nanoflakes with different edge lengths. The spectra are shifted in the y direction for a better visibility. Fig 2: (a) Projected Mo-4d density of states of a single Mo153 S342 nanoflake resulting from groups of Mo atoms triangularly. (b) Color code of the groups of Mo atoms (labeled pn, n = 1-6) as used for the PDOS representation in panel a.

References

(1)    T.B. Wendumu, G. Seifert, T. Lorenz, J.-O Joswig and AN Enyashin, J. Phys.Chem. Lett 5(21) (2014) 3636-3640
(2)    J.-O Joswig, T. Lorenz, T.B. Wendumu, S. Gemming, G. Seifert, Acc. Chem. Res. 48 (2015) 48-55

Contact:
T.B. Wendumu


CONTACT


Secretary

Antje Völkel
Tel.: +49 (0) 351 463 34467
Fax: +49 (0) 351 463 35953
E-Mail: theory@chemie.tu-dresden.de


Location
Bergstraße 66c
König-Bau
Raum 107

Postal Address
TU Dresden
Fachrichtung Chemie und Lebensmittelchemie
Professur für Theoretische Chemie
01062 Dresden


Parcels & Deliveries
TU Dresden
Fachrichtung Chemie und Lebensmittelchemie
Professur für Theoretische Chemie
Bergstraße 66c
01062 Dresden