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Recent Submissions

  • Selective Electrocatalytic Oxidation of Biomass-derived 5-Hydroxymethylfurfural (HMF) to 2,5-Diformylfuran (DFF): From Mechanistic Investigations to Catalyst Recovery

    Kisszekelyi, Peter; Hardian, Rifan; Vovusha, Hakkim; Chen, Binglin; Zeng, Xianhai; Schwingenschlögl, Udo; Kupai, Jozsef; Szekely, Gyorgy (ChemSusChem, Wiley, 2020-04-27) [Article]
    The catalytic transformation of bio-derived compounds, specifically 5-hydroxymethyfurfural (HMF), into value-added chemicals may provide sustainable alternatives to crude oil and natural gas based products. HMF can be obtained from fructose and successfully converted to 2,5-diformylfuran (DFF) by an environmental-friendly organic electrosynthesis performed in an ElectraSyn reactor, using cost-effective and sustainable graphite (anode) and stainless steel (cathode) electrodes in an undivided cell, without the need for conventional precious metal electrodes. In this work, we perform the electrocatalysis of HMF using green solvents such as acetonitrile, γ-valerolactone, as well as PolarClean that is used in electrocatalysis for the first time. The reaction parameters, and the synergistic effects of the TEMPO catalyst and 2,6-lutidine base are explored both experimentally and through computation modelling. The molecular design and synthesis of a size-enlarged C3-symmetric tris-TEMPO catalyst are also performed to facilitate a sustainable reaction work-up via nanofiltration. The obtained performance is then compared with those obtained by heterogeneous TEMPO alternatives recovered via an external magnetic field and microfiltration. Results show that our new method of electrocatalytic oxidation of HMF to DFF can be achieved with excellent selectivity, good yield, and excellent catalyst recovery.
  • Finely Tuned Submicroporous Thin-Film Molecular Sieve Membranes for Highly Efficient Fluid Separations

    Ali, Zain; Ghanem, Bader; Wang, Yingge; Pacheco Oreamuno, Federico; Ogieglo, Wojciech; Vovusha, Hakkim; Genduso, Giuseppe; Schwingenschlögl, Udo; Han, Yu; Pinnau, Ingo (Advanced Materials, Wiley, 2020-04-22) [Article]
    Polymeric membranes with increasingly high permselective performances are gaining a significant role in lowering the energy burden and improving the environmental sustainability of complex chemical separations. However, the commercial deployment of newly designed materials with promising intrinsic properties for fluid separations has been stalled by challenges associated with fabrication and scale up of low-cost, high-performance, defect-free thin-film composite (TFC) membranes. Here, a facile method to fabricate next-generation TFC membranes using a bridged-bicyclic triptycene tetra-acyl chloride (Trip) building block with a large fraction of finely tuned structural submicroporosity (pore size < 4 Å) is demonstrated. The TFCs exhibit superb potential for removal of small (≈200 g mol−1) organic microcontaminants from organic solvent streams by showing both improved rejection and permeance in organic systems compared to current state-of-the-art commercial membranes. The TFCs also display unprecedented properties for desalination applications with performance located far above the current water permeance/sodium chloride rejection trendline. The strategy of using highly contorted triptycene building blocks with well-defined interconnected internal free volume elements establishes a scalable, generalized approach to fabricate highly selective, submicroporous TFC membranes for a wide variety of challenging energy-intensive fluid separations.
  • Gas Sensing Performance of Pristine and Monovacant C6BN Monolayers Evaluated by Density Functional Theory and the Nonequilibrium Green’s Function Formalism

    Babar, Vasudeo Pandurang; Sharma, Sitansh; Schwingenschlögl, Udo (The Journal of Physical Chemistry C, American Chemical Society (ACS), 2020-03-04) [Article]
    The application potential of pristine and monovacant C6BN for sensing gaseous pollutants (CO, CO2, NO, NO2, NH3, H2S, and SO2) is investigated using density functional theory with van der Waals dispersion correction. The adsorption sites and distances are determined. In addition to applying widely used theoretical approaches (adsorption energy, charge transfer, and work function) to evaluate gas sensing properties, the current−voltage characteristics are calculated before and after gas adsorption, using the nonequilibrium Green’s function formalism. The reliability of the approaches is analyzed. From a material point of view, we observe that all molecules under investigation physisorb on pristine C6BN. However, it turns out that pristine C6BN cannot be used for sensitive sensing, which we attribute to tiny charge transfers and band gap changes. On the other hand, we find that monovacancies in C6BN improve the adsorption energy and, in turn, enhance the sensitivity
  • Identification and Resolution of Unphysical Multielectron Excitations in the Real-Time Time-Dependent Kohn-Sham Formulation

    Zang, Xiaoning; Schwingenschlögl, Udo; Lusk, Mark T. (Physical Review Letters, American Physical Society (APS), 2020-01-15) [Article]
    We resolve a fundamental issue associated with the conventional Kohn-Sham formulation of real-time time-dependent density functional theory. We show that unphysical multielectron excitations, generated during time propagation of the Kohn-Sham equations due to fixation of the total number of Kohn-Sham orbitals and their occupations, result in incorrect electron density and, therefore, wrong predictions of physical properties. A new formulation is proposed in that the number of Kohn-Sham orbitals and their occupations are updated on the fly, the unphysical multielectron excitations are removed, and the correct electron density is determined. The correctness of the new formulation is demonstrated by simulations of Rabi oscillation, as analytical results are available for comparison in the case of noninteracting electrons.
  • Artificial gauge fields and topological insulators in Moire superlattices

    Shang, Ce; Abbout, Adel; Zang, Xiaoning; Schwingenschlögl, Udo; Manchon, Aurelien (arXiv, 2019-12-01) [Preprint]
    We propose an innovative quantum emulator based on Moire superlattices showing that, by employing periodical modulation on each lattice site, one can create tunable, artificial gauge fields with imprinting Peierls phases on the hopping parameters and realize an analog of novel Haldane-like phase. As an application, we provide a methodology to directly quantify the topological invariant in such a system from a dynamical quench process. This design shows a robustly integrated platform which opens a new door to investigate topological physics.
  • Molecular doping of blue phosphorene: a first-principles investigation.

    Sun, Minglei; Tang, Wencheng; Li, Song; Chou, Jyh Pin; Hu, Alice; Schwingenschlögl, Udo (Journal of physics. Condensed matter : an Institute of Physics journal, IOP Publishing, 2019-10-31) [Article]
    Using first-principles calculations, we show that p-doped blue phosphorene can be obtained by molecular doping with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) and 1,3,4,5,7,8-hexafluorotetracyanonaphthoquinodimethane (F6-TNAP), whereas n-doped blue phosphorene can be realized by doping with tetrathiafulvalene (TTF) and cyclooctadecanonaene (CCO). Moreover, the doping gap can be effectively modulated in each case by applying an external perpendicular electric field. The optical absorption of blue phosphorene can be considerably enhanced in a broad spectral range through the adsorption of CCO, F4-TCNQ, and F6-TNAP molecules, suggesting potential of the doped materials in the field of renewable energy.
  • Methane sorption in a family of qzd-MOFs: A multiscale computational study

    Suetin, Mikhail; Peskov, Maxim; Schwingenschlögl, Udo (Chemical Engineering Journal, Elsevier BV, 2019-10-28) [Article]
    A new family of metal-organic frameworks with qzd topology is proposed and exhaustively studied using multiscale computational analysis (grand canonical Monte Carlo; molecular mechanics; density functional theory) to reveal the structure-property relationships for predicting frameworks with high total methane uptake and working capacity. In our approach we take into account different linkers with triple bonds and/or benzene rings. Grand canonical Monte Carlo simulations demonstrate for several of the designed frameworks excellent methane storage properties, such as a balanced working capacity of 56 wt%, 264 cm3 (STP) cm−3 at 5–80 bar and 240 K.
  • Diffusion equations expressed in molar fractions: Theory and application to ionic diffusion and demixing

    Zhang, Geng; Du, Yong; Schwingenschlögl, Udo (Physical Review E, American Physical Society (APS), 2019-10-17) [Article]
    Molar fractions are used in applied diffusion kinetics for incorporating thermodynamic and kinetic databases. Molar quantities (molar concentration and molar flux) and reduced molar quantities (molar fraction and reduced molar flux) usually are regarded to be equivalent; i.e., molar quantities are replaced with their reduced forms. However, as the fluxes are related to material properties, the diffusion equations expressed in molar fractions are not consistent with the normalization condition of molar fractions. We develop diffusion kinetics consistent with this condition. Our method is applicable to diffusion with total reduced flux, such as diffusion in an external field or flow field. As two case studies, the developed method is used to investigate ionic diffusion in an electrolyte solution and ionic demixing in a semiconductor oxide.
  • 17% Efficient Organic Solar Cells Based on Liquid Exfoliated WS2 as a Replacement for PEDOT:PSS

    Lin, Yuanbao; Adilbekova, Begimai; Firdaus, Yuliar; Yengel, Emre; Faber, Hendrik; Sajjad, Muhammad; Zheng, Xiaopeng; Yarali, Emre; Seitkhan, Akmaral; Bakr, Osman; El Labban, Abdulrahman; Schwingenschlögl, Udo; Tung, Vincent; McCulloch, Iain; Laquai, Frédéric; Anthopoulos, Thomas D. (Advanced Materials, Wiley, 2019-09-30) [Article]
    The application of liquid-exfoliated 2D transition metal disulfides (TMDs) as the hole transport layers (HTLs) in nonfullerene-based organic solar cells is reported. It is shown that solution processing of few-layer WS2 or MoS2 suspensions directly onto transparent indium tin oxide (ITO) electrodes changes their work function without the need for any further treatment. HTLs comprising WS2 are found to exhibit higher uniformity on ITO than those of MoS2 and consistently yield solar cells with superior power conversion efficiency (PCE), improved fill factor (FF), enhanced short-circuit current (JSC), and lower series resistance than devices based on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and MoS2. Cells based on the ternary bulk-heterojunction PBDB-T-2F:Y6:PC71BM with WS2 as the HTL exhibit the highest PCE of 17%, with an FF of 78%, open-circuit voltage of 0.84 V, and a JSC of 26 mA cm−2. Analysis of the cells' optical and carrier recombination characteristics indicates that the enhanced performance is most likely attributed to a combination of favorable photonic structure and reduced bimolecular recombination losses in WS2-based cells. The achieved PCE is the highest reported to date for organic solar cells comprised of 2D charge transport interlayers and highlights the potential of TMDs as inexpensive HTLs for high-efficiency organic photovoltaics.
  • Point Defects in Blue Phosphorene

    Sun, Minglei; Chou, Jyh Pin; Hu, Alice; Schwingenschlögl, Udo (Chemistry of Materials, American Chemical Society (ACS), 2019-09-19) [Article]
    Using first-principles calculations, we investigate selected defects in blue phosphorene (BlueP). For a single-vacancy (SV) defect, a 5-9 structure is energetically favorable, and for a double-vacancy defect, a 5-8-5 or 555-777 structure is. A P adatom favors the top adsorption site. Scanning tunneling microscopy images are simulated to aid the experimental identification of the defects. Formation of a Stone-Wales defect is found to be most likely, but it can be reverted by thermal annealing. Calculated migration and transformation barriers show that a SV defect can migrate easily. Both a SV defect and a P adatom induce a magnetic moment, thus turning BlueP into a magnetic semiconductor. It turns out that all of the defects under investigation enhance the ability of BlueP to absorb sunlight.
  • Density Functional Theory Analysis of Gas Adsorption on Monolayer and Few Layer Transition Metal Dichalcogenides: Implications for Sensing

    Babar, Vasudeo Pandurang; Vovusha, Hakkim; Schwingenschlögl, Udo (ACS Applied Nano Materials, American Chemical Society (ACS), 2019-09-10) [Article]
    First-principles calculations are performed to compare the adsorption of CO, NH3, NO, and NO2 molecules on monolayer, bilayer, and heterobilayer MoS2 and WS2, using van der Waals corrected density functional theory. Only minor differences are demonstrated for the adsorption behaviors of the monolayer and bilayer systems despite fundamental differences in the electronic structure (direct versus indirect band gap). We also show that NO2 binds stronger to the sensor materials than the other gas molecules, resulting in enhanced charge transfer. Adsorption of paramagnetic NO and NO2 has significant impact on the electronic states, in contrast to adsorption of nonmagnetic CO and NH3.
  • S-wave elastic scattering of o -Ps from H2 at low energy

    Zhang, Junyi; Wu, M. S.; Qian, Y.; Gao, X.; Yang, Y. J.; Varga, K.; Yan, Z. C.; Schwingenschlögl, Udo (Physical Review A, American Physical Society (APS), 2019-09-04) [Article]
    The confined variational method is applied to investigate the low-energy elastic scattering of orthopositronium from H2 by first-principles quantum mechanics. Describing the correlations with explicitly correlated Gaussians, we obtain accurate s-wave phase shifts and pickoff annihilation parameters for different incident momenta. By a least-squares fit of the data to the effective-range theory, we determine the s-wave scattering length AS=2.02a0 and the zero-energy value of the pickoff annihilation parameter, 1Zeff=0.1839. The obtained 1Zeff agrees well with the precise experimental value of 0.186(1) [G. L. Wright, J. Phys. B 16, 4065 (1983)10.1088/0022-3700/16/21/027] and the obtained AS agrees well with the value of 2.1(2)a0 estimated from the average experimental momentum-transfer cross section for positronium energy below 0.3 eV [F. Saito, J. Phys. B 36, 4191 (2003)10.1088/0953-4075/36/20/011].
  • Copper Thiocyanate and Copper Selenocyanate Hole Transport Layers: Determination of Band Offsets with Silicon and Hybrid Perovskites from First Principles

    Sajjad, Muhammad; Singh, Nirpendra; de Bastiani, Michele; De Wolf, Stefaan; Schwingenschlögl, Udo (Physica Status Solidi - Rapid Research Letters, Wiley-VCH, 2019-08-15) [Article]
    Copper thiocyanate (CuSCN) and copper selenocyanate (CuSeCN) combine a high work function with a high optical transparency. To elucidate their potential as transparent hole selective materials, herein, first-principles calculations of the structural and electronic properties are reported, with special attention to the band offsets with crystalline Si and hybrid perovskites (CH3NH3PbI3, CH3NH3PbBr3, and CHN2H4PbBr3). The structural parameters and electronic band structure are obtained using the Perdew–Burke–Ernzerhof functional, resulting in indirect and direct bandgaps of 2.13 and 1.81 eV for CuSCN and CuSeCN, respectively. The (100) surfaces of the two materials do not feature in-gap states, maintaining the semiconducting nature. Band offsets are determined by the electrostatic potential lineup method using slab calculations. Small valence band offsets of 0.10 eV for CuSCN/Si and 0.08 eV for CuSCN/CH3NH3PbI3 are desirably found, i.e., a promising hole transport layer character of CuSCN for Si and CH3NH3PbI3-based solar cells. Type-II band alignment is obtained for all studied heterojunctions.
  • Scalable Synthesis of Amphiphilic Copolymers for CO2- and Water-Selective Membranes: Effect of Copolymer Composition and Chain Length

    Akhtar, Faheem; Kumar, Mahendra; Vovusha, Hakkim; Shevate, Rahul; Villalobos, Luis Francisco; Schwingenschlögl, Udo; Peinemann, Klaus-Viktor (Macromolecules, American Chemical Society (ACS), 2019-08-13) [Article]
    Dehumidification is a critical energy-intensive and crucial process for several industries (e.g., air conditioning and gas dehydration). Polymeric membranes with high water vapor permeability and selectivity are needed to achieve an energy-efficient water vapor removal. Herein, we demonstrate high-performance water vapor transport membranes based on novel amphiphilic tercopolymers. A series of amphiphilic tercopolymers comprising polyacrylonitrile, poly(ethylene glycol) methyl ether methacrylate (PEGMA), and poly(N,N-dimethylamino ethyl methacrylate) (PDMAEMA) segments are synthesized via an economical and facile free radical polymerization. The water vapor permeability increases with the increase in PEGMA chain length and the content of PEGMA segments. The best performing membrane (i.e., PEGMA-9502) achieved a water vapor permeability of 174 kBarrer. By optimizing the content and chain length of the PEGMA segments, the membranes could be tuned for carbon capture applications. The optimized membranes tested for CO2 separation showed a high CO2 permeability of 47 Barrer along with CO2/N2 and CO2/CH4 selectivities of 67 and 23, respectively. This work presents a simple and economic amphiphilic tercopolymer for the fabrication of membranes with excellent gas and water vapor separation performance.
  • Sensitivity enhancement of stanene towards toxic SO2 and H2S

    Vovusha, Hakkim; Hussain, Tanveer; Sajjad, Muhammad; Lee, Hoonkyung; Karton, Amir; Ahuja, Rajeev; Schwingenschlögl, Udo (Applied Surface Science, Elsevier BV, 2019-08-09) [Article]
    Adsorption of S-containing gases on pristine, defective, and heteroatom doped stanene is studied for gas sensing applications by van der Waals corrected density functional theory. SO2 and H2S gas molecules are found to bind to pristine stanene too weakly to alter the electronic properties sufficiently for efficient gas sensing (binding energy of −0.20 and −0.33 eV, respectively). We demonstrate that vacancies and heteroatom doping can enhance the binding energy to −1.67 and −0.74 eV, respectively. It is found that presence of mono-vacancies, tri-vacancies, and In dopants at low concentrations in stanene results in considerable variations of the electronic properties in contact with S-containing gases, thus transforming stanene into an efficient sensing material.
  • Outstanding methane gravimetric working capacity of computationally designed rhr-MOFs

    Suetin, Mikhail; Peskov, Maxim; Schwingenschlögl, Udo (Microporous and Mesoporous Materials, Elsevier BV, 2019-07-29) [Article]
    A multi-scale approach is employed to design metal-organic frameworks (MOFs). The methane sorption properties are studied by grand canonical Monte Carlo simulations to reveal the structure-property relationship with respect to the methane total uptake and working capacity at different temperatures and pressures. We identify rhr-MOFs with outstanding gravimetric working capacity. For example, the BBB MOF (largest studied pore size) achieves a value of 60.7 wt% at 298 K and 5–65 bar.
  • Suppressing X-Migrations and Enhancing the Phase Stability of Cubic FAPbX3 (X = Br, I)

    Oranskaia, Aleksandra; Schwingenschlögl, Udo (Advanced Energy Materials, Wiley, 2019-07-24) [Article]
    Chemical bonding of formamidinium (FA) with the inorganic perovskite skeleton of FAPbX3 (X = Br, I) is studied with emphasis on the differences to methylammonium: stronger hydrogen bonding, the presence of π-anion bonding, and more sterically hindered motion inside the perovskite inorganic cage. Organic cation dopants fitting in the perovskite cubic cell and being capable of hydrogen and halogen bonding with overall doubled bonding strength as compared to FA are proposed. They are shown to suppress not only X-migrations but also the undesirable α–δ phase transition of FAPbI3. In addition, a possible atomistic explanation of the champion solar cell efficiency achieved experimentally is developed.
  • First-principles methodology for determining the angular momentum of excitons

    Zang, Xiaoning; Schwingenschlögl, Udo (Physical Review B, American Physical, 2019-07-15) [Article]
    We develop a methodology for extracting the Kohn-Sham angular momentum of excitons in realistic systems from time-dependent density functional theory. For small systems the exciton populations can be calculated analytically, which allows us to test the methodology for a three-arm H2 molecular ring and a pair of such rings. For larger systems the developed methodology opens a venue to determine the angular momentum of excitons by first principles calculations. A chain of twenty three-arm H2 molecular rings and a triphenylphosphine molecule are investigated as illustrative examples. It is demonstrated that the angular momentum is conserved during the absorption of twisted light.
  • New Paradigm for Gas Sensing by Two-Dimensional Materials

    Babar, Vasudeo Pandurang; Sharma, Sitansh; Schwingenschlögl, Udo (Journal of Physical Chemistry C, American Chemical Society, 2019-05-08) [Article]
    The adsorption behavior and electronic transport properties of CO and NH3 molecules on para-C3Si and meta-C3Si monolayers are studied using first-principles calculations and the non-equilibrium Green's function method. The adsorption sites are determined along with their adsorption energies. It turns out that CO and NH3 molecules physisorb on both monolayers. The current-voltage characteristics show that the para-C3Si monolayer can be used to sense CO and NH3 gases with high sensitivity. In contrast to other two-dimensional materials, the sensing mechanism is not based on charge transfer but on the presence of Dirac states and their susceptibility to symmetry-breaking structural distortions.
  • Ultralow Lattice Thermal Conductivity and Thermoelectric Properties of Monolayer Tl2O

    Sajjad, Muhammad; Singh, Nirpendra; Sattar, Shahid; De Wolf, Stefaan; Schwingenschlögl, Udo (ACS Applied Energy Materials, American Chemical Society (ACS), 2019-04-30) [Article]
    We report first-principles results on the thermal and thermoelectric properties of monolayer Tl2O. The lattice thermal conductivity and electronic transport coefficients are obtained by semiclassical Boltzmann transport theory. Monolayer Tl2O is found to be a semiconductor with a direct band gap of 1.62 eV. The lattice thermal conductivity turns out to be ultralow, for example, 0.17 W/mK at 300 K. Combined with a high power factor, this results in excellent thermoelectric performance. For example, at 500 K the p-type and n-type thermoelectric figures of merit reach peak values of 0.96 and 0.94 at hole and electron concentrations of 1.2 × 1011 and 0.8 × 1011 cm–2, respectively.

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