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Computational materials science for sustainable energy using nanocatalysts from abundant elements

The topic of this proposal is to display recent advances in computational materials science in the area of water splitting. The aim is to highlight synergy opportunities in methodologies with special emphasis on multi-scaling approaches. The ultimate goal is applying the state-of-the-art methods to model earth abundant nanocrystals.


Modern society needs a source of energy generated without harming the environment. The efficiency of devices converting renewable energy by processes such as water splitting relies on a sensible choice of material components. However, larger scale material and device properties such as interface segregation, grain boundary movement, ionic diffusion through porous materials and mechanical loading also strongly impact performance, making the theoretical simulation of realistic devices a challenging multi-scale problem. Although the scientific community has developed expertise in different fields focusing on a range of length and accuracy scales, much less effort has been devoted to integrating and combining these models towards a true multi-scale approach. The ultimate central challenge will be to generate a multiscale modelling platform that will be used world-wide for conducting state-of-the-art multi-scale property prediction of materials. The symposium will broadly cover the current status of multi-scaling approaches, both in method development and application toward water splitting. It intends to focus on bridging the knowledge gaps between different theoretical methods and computer codes in order to facilitate the discovery of novel materials for energy conversion. The objectives of this symposium include building an organized network of scientists working on achieving greater scientific understanding of water splitting and developing approaches for reliable and realistic multi-scale modelling of nano-oxides material architectures. The long-term outcome will be more environmentally friendly energy technologies featuring immeasurably large impact and benefit for society.

Hot topics to be covered by the symposium

  • Computational material science multi-scaling approach and development
  • Integration of quantum mechanics with molecular dynamics
  • Overlap between quantum mechanics and monte carlo simulations
  • Continuum models including microscale models
  • Applications toward design of nano-crystals for water splitting

Keynote speaker:

  • Joachim Sauer, Humboldt University of Berlin, Germany

Invited speakers:

  • Anja Bieberle, Dutch Institute for Fundamental Energy Research, The Netherlands
  • Cristiana Di Valentin, University of Milano-Bicocca, Italy
  • José R. B. Gomes, University of Aivero, Portugal
  • Ricardo Grau-Crespo, University of Reading, United Kingdom
  • Henrik Grönbeck, Chalmers University of Technology, Sweden
  • Eugene Kotomin, Max Planck Institute for Solid State Research, Germany
  • Michael Nolan, University College Cork, Ireland
  • Harald Oberhofer, TU Munich, Germany
  • Mariachiara Pastore, Institute of Molecular Science and Technologies, Italy
  • Patrick Rinke, Aalto University, Finland
  • Bartek Szyja, Wroclaw University of Science and Technology, Poland
  • Tomasz Adam Wesolowski, University of Geneva, Switzerland
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Session 1 : .
Authors : José L. C. Fajín [1], M. Natália D. S. Cordeiro [1], and José R. B. Gomes [2]
Affiliations : [1] LAQV@REQUIMTE, Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto, P-4169-007 Porto, Portugal; [2] CICECO – Instituto de Materiais da Universidade de Aveiro, Departamento de Química, Universidade de Aveiro, Campus Universitário de Santiago, P-3810-193 Aveiro, Portugal.

Resume : The rapid development of computers and of programming in the last few decades has been increasing the potential and accuracy of computational chemistry. It is now possible to combine realistic models and quantum chemical methods for an enhanced treatment of the interactions between the particles in a molecular system. This combination is essential to follow chemical reactions on catalyst model surfaces where bonds are broken and formed in cascades of elementary steps [1]. The catalyst models are based very often on highly regular monoelemental metallic surfaces or nanoparticles, which can be free or deposited on a solid substrate. Nevertheless, it has been found that models with irregular shapes or doped with other elements (multicomponent agent) [2] are much more active and closer to real catalysts. Herewith, we will present results from density functional theory (DFT) calculations on catalytic reactions occurring on multimetallic catalysts [3]. The role of alloying in elementary reactions crucial in several large-scale processes, e.g. in the water gas shift reactions, will be analyzed. Acknowledgments This work was financially supported by the projects projects CICECO-Aveiro Institute of Materials, FCT Ref. UID/CTM/50011/2019, and LAQV@REQUIMTE, FCT Ref. UID/QUI/50006/2019, funded by FEDER, through COMPETE2020 - Programa Operacional Competitividade e Internacionalização (POCI), and by national funds (OE), through FCT/MCTES. References 1. Fajín, J. L. C. et al. J. Catal. 2009, 268, 131 and J. Catal. 2012, 289, 11. 2. Fajín, J. L. C. et al. Chem. Comm. 2011, 47, 8403. 3. Fajín, J. L. C. et al. J. Phys. Chem. C 2012, 116, 10120; Appl. Catal. A: Gen. 2013, 458, 90; J. Phys. Chem. C 2015, 119, 16537; RSC Adv. 2016, 6, 18695 and Appl. Catal. B. 2017, 218, 199.

Authors : E. A. Kotomin, Yu. A. Mastrikov, R. Merkle, M. M. Kuklja, J. Maier
Affiliations : Max Planck Institute for Solid State Research, Stuttgart, Germany; Institute for Solid State Physics, University of Latvia, Riga, Latvia; Materials Science and Engineering Department, University of Maryland, USA

Resume : La1-xSrxMnO3 (LSM) served as one of the first perovskites employed as SOFC cathode and is still used in composites. We analyzed oxygen adsorption, dissociation and migration on the (001) MnO2-terminated surface [1]. Based on first principles calculations, the oxygen reduction reaction (ORR) rate was estimated for different cases. In pure LaMnO3, the MnO2 (001) termination was found to be the energetically most stable, however with an increase of Sr doping, the (La,Sr)O termination becomes more favorable [2]. In this talk, we compare results of first principles calculations on the elementary steps (oxygen vacancy formation, O2 molecule and O atom adsorption) of the ORR on the two alternative polar (La,Sr)O and MnO2 (001) terminations. Slab calculations were performed using VASP computer code with GGA functionals. The results are compared with hybrid calculations using the CRYSTAL code with LCAO basis set. In contrast to the MnO2 termination, the vacancy formation energy on (La,Sr)O is larger, strongly reducing the surface vacancy concentration. Our approach distinguishes two effects - different surface terminations and slab cation stoichiometry (ratio of (La,Sr) ions to Mn affecting the average Mn oxidation state). The equilibrium oxygen adsorbate concentration was found to be two orders of magnitude larger for (La,Sr)O, but on the other hand, the surface oxygen vacancy concentration is smaller by nearly six orders of magnitude. Therefore, the oxygen reduction rate on (La,Sr)O termination is expected to be significantly lower than on MnO2 [3]. [1] Y.Mastrikov,R.Merkle, E.Heifets, E.A. Kotomin, J. Maier, J.Phys.Chem.C 114, 3017 (2010). [2]. S. Piskunov, E. Heifets, T. Jacob, E.A. Kotomin, E. Spohr, Phys. Rev.B 78, 121406 (2008). [3] Yu. Mastrikov, R.Merkle, E.A.Kotomin, M.M.Kuklja, J.Maier, J. Mater. Chem. A 6, 11929 (2018)

Authors : Kiran George, Matthijs van Berkel, Xueqing Zhang, Rochan Sinha, Anja Bieberle-Hütter
Affiliations : Dutch Institute for Fundamental Energy Research (DIFFER), Eindhoven, 5612AJ, The Netherlands

Resume : Photoelectrochemical (PEC) cells can split water directly into hydrogen and oxygen. We study the oxygen evolution reaction (OER) at the Fe2O3-water interface in PEC to identify limiting processes at the interface and to improve performance. Recently, we have developed a general approach to relate electrochemical measurements to the kinetics of multistep reactions at the semiconductor-electrolyte interface.[1] In this approach, the set of microkinetic equations for OER is formulated as a state-space model. By solving the model for steady state, j-V plots and impedance spectra are simulated and are compared to in-house experiments.[1] In this talk, we show how to simulate the time-dependent behavior of the interface using the same framework. Simulated linear sweep voltammetry curves, cyclic voltammetry curves, and chop light measurements will be presented. The sensitivity of these measurements to the reaction rates, voltage-scan-rate, semiconductor parameters, and other interface parameters are investigated. Moreover, the effect of potential drop across the back-contact resistance on these measurements will be discussed. The method is generic and can be easily extended to other materials and interfaces. (1) George, K.; van Berkel, M.; Zhang, X.; Sinha, R.; Bieberle-Hütter, A. J. Phys. Chem. C 2019, 123, 9981–9992.

Authors : Ángel Morales-García, Antoni Macià, Stefan T. Bromley, Francesc Illas
Affiliations : Departament de Ciència de Materials i Químia Física & Institut de Química Teórica i Computacional (IQTCUB), Universitat de Barcelona, c/ Martí i Franquès 1-11, 08028 Barcelona, Spain; Departament de Ciència de Materials i Químia Física & Institut de Química Teórica i Computacional (IQTCUB), Universitat de Barcelona, c/ Martí i Franquès 1-11, 08028 Barcelona, Spain; Departament de Ciència de Materials i Químia Física & Institut de Química Teórica i Computacional (IQTCUB), Universitat de Barcelona, c/ Martí i Franquès 1-11, 08028 Barcelona, Spain. Institució Catalana de Recerca i Estudis Avançats (ICREA) Passeig Lluis Companys 23, 08010 Barcelona, Spain; Departament de Ciència de Materials i Químia Física & Institut de Química Teórica i Computacional (IQTCUB), Universitat de Barcelona, c/ Martí i Franquès 1-11, 08028 Barcelona, Spain

Resume : Hydrogen constitutes an ideal green fuel but so far it is mainly obtained from fossil resources. Water splitting using TiO2 as a photocatalyst constitutes an alternative although its practical use under visible sunlight is hindered from the too large ?band gap? of the known forms of this oxide. Suitable modifications have been proposed including doping and/or nanostructuring. Nevertheless, finding a practical solution without a deep knowledge of how the properties of TiO2 nanoparticles evolve with size, shape and morphology seems to be an insurmountable task. To make progress in this direction, a systematic study has been carried out regarding the relative stability and properties of differently shaped realistic (TiO2)n nanoparticles containing explicitly up to 1785 atoms or 595 formula units.1 Using all electron, relativistic, density functional theory-based calculations with accurate numerical orbital centered basis sets, we investigated the relative stability of spherical and faceted nanoparticles as a function of size. A crossover between faceted and spherical morphology is found at ~100 TiO2 units where the faceted nanoparticles become more stable than the spherical ones. The present study provides compelling evidence that for stoichiometric nanoparticles of realistic size, a faceted morphology is preferred although spherical shaped ones tend to exhibit smaller optical gap. Therefore, the successful synthesis of spherical nanoparticles, often showing better photocatalytic activity, has to be attributed to the particular route used able to stabilize these energetically metastable structures. The present study suggests that controlling kinetic of particle growth rather than thermodynamic stability is the key towards TiO2 nanoparticles with better photocatalytic activity under sunlight. References: 1. Á. Morales-García, A. Macià, F. Illas, S. T. Bromley, Nanoscale 2019, 11, 9032-9041.

10:30 Coffee break    
Session 2 : .
Authors : Bartłomiej M. Szyja
Affiliations : Division of Fuels Chemistry and Technology, Wrocław University of Technology

Resume : Due to the excessive CO2 emission to the atmosphere and inevitable depletion of the fossil fuels, an alternative method for environmentally friendly energy generation is needed. One of the viable methods is the conversion of CO2 to energy carrier molecules making use of the solar energy. This method, however, poses significant challenges to overcome significant thermodynamic stability of the CO2 molecule. This work follows analogous approach to a natural photosynthesis process, where water oxidation and CO2 reduction reactions occur separately. Modified TiO2 (anatase) or Fe2O3 (hematite) are used as the photoanode. These materials are characterized by the optimal photon absorption abilities and low overpotential in water splitting reaction. As a cathode, a graphene supported organic ruthenium complexes are used. Several complexes have been tested with respect to the optimal overpotentnial and product selectivity. In 2-electron reduction process, a Pidko-type Ru-CNC complex displays the best selectivity towards formic acid and appears resistant to deactivation due to interactions with the carbonyl. Ru-porphyrins on the other hand are selective towards methane in relatively low overpotential. Graphene support play here an important role of the charge repolarization, leading to the optimal interaction of intermediates with the Ru site.

Authors : Cristiana Di Valentin
Affiliations : Dipartimento di Scienza dei Materiali Università di Milano Bicocca, Italy

Resume : In this talk I will present an overview on the activity of the group devoted to the design of 0D and 2D nanocatalysts for the photo and the electrocatalysis of water in water, through the combination of quantum mechanics with molecular mechanics and with molecular dynamics or by means of continuum models.

Authors : Maria Peressi (1), Virginia Carnevali (1), German Soldano (2), Marcelo M. Mariscal (2), Mirko Panighel (3), Alessandro Sala (1,3), Zhiyu Zou (3,4), Cinzia Cepek (3), Giovanni Comelli (1,3), Cristina Africh (3)
Affiliations : (1) Department of Physics, University of Trieste, Italy; (2) INFIQC, CONICET and Universidad Nacional de Córdoba, Argentina; (3) IOM-CNR Laboratorio TASC, Italy; (4) Abdus Salam International Centre for Theoretical Physics, Trieste, Italy

Resume : Ni(100) micrograins appear to be a promising substrate to finely tailor the electronic properties of graphene at the nanoscale, with relevant perspective applications in electronics and catalysis. Extended moiré patterns originate from the interface symmetry mismatch. In particular, a stripe moiré pattern is observed when a zigzag direction of graphene is oriented along the [110] direction of the substrate, with parallel physi- and chemisorbed regions of regular width alternating along the graphene armchair direction [1]. Besides characterizing and rationalizing by density functional theory (DFT) the structure and the bonding of this moiré superlattice, we addressed the problem of its stability and evolution. To this aim, a Kinetic Monte Carlo code has been developed on the basis of DFT ground states energies and energy barriers of about fifty single and cooperative elementary processes involving carbon atoms at graphene/substrate interface. Local detachments in the chemisorbed regions experimentally observed upon cooling are explained by peculiar diffusion mechanisms of exceeding carbon segregating from the bulk at the interface region [2]. [1] Z. Zou et al., Carbon 2018, 130, 441-447 [2] V. Carnevali, in preparation

Authors : Federico Calle-Vallejo
Affiliations : Departament de Ciència de Materials i Química Fisica & Institut de Química Teòrica i Computacional. Carrer de Marti i Franques 1, 08028, Barcelona, Spain

Resume : The scaling relation between *OOH and *OH supposedly slows down the oxygen evolution reaction (OER) [1, 2]. Whereas the ideal separation between the adsorption energies of *OOH and *OH is 2.46 eV, it is around 3.20 eV on most catalysts. Hence, it is currently believed that breaking such scaling relation is crucial to enhance OER electrocatalysts. In this talk, I will test this widespread idea using a large collection of data [3]. The results suggest that breaking the OOH-OH scaling relation is a necessary yet insufficient condition to optimize OER electrocatalysis. As an alternative, I will present the “electrochemical-step symmetry index” (ESSI), the minimization of which is connected to low overpotentials [3, 4]. Finally, I will show how to use ESSI in conjunction with experiments to rationalize activity trends and provide guidelines for the optimization of electrocatalysts [5, 6]. [1] M.T.M. Koper, J. Electroanal. Chem., 660 (2011) 254-260. [2] I.C. Man, H.-Y. Su, F. Calle-Vallejo, H.A. Hansen, J.I. Martinez, N.G. Inoglu, J. Kitchin, T.F. Jaramillo, J.K. Norskov, J. Rossmeisl, ChemCatChem, 3 (2011) 1159-1165. [3] N. Govindarajan, J.M. García-Lastra, E.J. Meijer, F. Calle-Vallejo, Current Opinion in Electrochemistry, 8 (2018) 110-117. [4] N. Govindarajan, M.T.M. Koper, E.J. Meijer, F. Calle-Vallejo, ACS Catal., (2019). [5] M. Retuerto, F. Calle-Vallejo, L. Pascual, P. Ferrer, Á. García, J. Torrero, D. Gianolio, J.L.G. Fierro, M.A. Peña, J.A. Alonso, S. Rojas, Journal of Power Sources, 404 (2018) 56-63. [6] M. Retuerto, L. Pascual, F. Calle-Vallejo, P. Ferrer, D. Gianolio, A.G. Pereira, Á. García, J. Torrero, M.T. Fernández-Díaz, P. Bencok, M.A. Peña, J.L.G. Fierro, S. Rojas, Nat. Commun., 10 (2019) 2041.

12:30 Lunch break    
Session 3 : .
Authors : Michael Nolan
Affiliations : Tyndall National Institute, Lee Maltings, UCC, T12 R5CP, Cork, Ireland

Resume : Replacing liquid fossil fuels with renewable fuels produce from water or CO2 is a key challenge for the 21st century. This is linked with both the increase in world energy demand and the climate emergency. Two potential technologies than will alleviate these issues are replacing liquid fuels with hydrogen for transport and capturing & converting CO2 produce in industrial process to useful chemicals. If this is driven by, e.g. solar energy then the processes will be fully renewable. In this presentation we present density functional theory simulations of modifications to TiO2 rutile and anatase by nanoclusters of metal oxides and chalcogenides and explore the possibility for these heterostructures to promote hydrogen evolution (water oxidation) and CO2 activation/reduction. We show that modifying TiO2 with metal chalcogenides (sulfide, selenide) or bimetallics based on Cu can promote hydrogen adsorption to make these heterostructures viable for hydrogen evolution. We also discuss initial work on deposition of these nanoclusters on TiO2 supports. For CO2 conversion we identify a number of modifiers to TiO2 that can activate or dissociate CO2 to CO. This includes bismuth oxide, ceria and very low loadings of copper. Key aspects appear to be the presence of active sites as a result of oxide reduction, the presence of reduced cations and the electronic structure of the modifiers. Comparison with experimental results will also be presented.

Authors : Mariarosaria Tuccillo 1, Ana B. Muñoz García 2, Michele Pavone 3
Affiliations : 1 ISC-CNR UOS, Sapienza 2 Physic Department, University of Naples Federico II 3 Chemistry Department, University of Naples Federico II

Resume : In this work we present first-principles periodic DFT+U calculations on Mn and Fe co-doped BaZrO3 (BaZr0.5Fe0.25Mn0.25O3-δ, BZFM), a promising cathode material for proton-conducting solid oxide fuel cells (PC-SOFCs). Our aims is to individuate perovskite-based material with good mixed proton and electron conductor (MPEC) properties and good electrocatalytic capabilities toward oxygen reduction reaction (ORR) so to be applied as single phase electrode in PC-SOFCs. We have studied BZFM (001) surface for ORR mechanism, identifying the potential-determining steps (PDS) of the ORR using the theoretical standard hydrogen electrode (TSHE) proposed by Nørskov. In particular, we focused on the surface oxygen vacancies as ORR active reaction sites, namely Mn-VO-Fe, Fe-VO-Zr and Mn-VO-Zr. For the low-energy Mn-VO-Fe surface oxygen vacancy, we have studied two possible O2 adsorption pattern: one vertical respect to TMs, and one bridge where the O2 molecule is placed along and above the Mn-VO-Fe line; for Fe-VO-Zr and Mn-VO-Zr, we have evaluated the ORR mechanisms only in the case where O2 adsorption occurs in the vertical configuration. From our calculations on the electrocatalytic activity, for all reaction paths, the PDS is the formation of *OOH intermediate, starting from adsorbed O2. Overpotential for the bridge configuration is on the medium-to-high range ( 0.98 V), while for the vertical configuration is as low as 0.20 V. Still, final overpotentials are on the same range as those calculated for well-known good ORR catalysts such as Pt, for this reason we can assess BZMF as a very promising cathode for PC-SOFCs.

Authors : Maytal Caspary Toroker
Affiliations : Department of Materials Science and Engineering, Technion - Israel Institute of Technology

Resume : Photoelectrochemical cells containing iron(III) oxide (Fe2O3) have attracted extensive investigations due to their ability to convert solar energy into chemical energy by water splitting. Recently, fabrication of nanoscaled Fe2O3 has been adopted for photoelectrochemical cells to increase solar energy absorption and reduce slow diffusion length of charge carriers. To understand how nanoscaled confinement influences catalytic efficiency, we performed density functional theory + U calculations of water oxidation on a thin slab of Fe2O3(0001). We considered possible hydrogen vacancies that may appear at high pH and voltage and find that promoting hydrogen vacancy formation improves catalytic efficiency. We conclude that nano-Fe2O3 should be grown on a substrate with a similar lattice constant to reduce strain and improve catalytic efficiency. References: 1. J. Phys. Chem. C, 121(11), 6120 (2017). 2. Advanced Materials, 1706577 (2018). 3. Phys. Chem. Chem. Phys. 19, 17278 (2017).

15:15 Coffee break    
Session 4 : -
Authors : Adriana Pecoraro (1), Ana B. Muñoz-Garcia (2), Eduardo Schiavo (1), Felice Gesuele (2), Pasqualino Maddalena (2), Michele Pavone(1)
Affiliations : (1) Università degli Studi di Napoli Federico II, Dipartimento di Scienze Chimiche, via Cintia 26, 80126, Napoli, Italia (2) Università degli Studi di Napoli Federico II, Dipartimento di Fisica “E. Pancini”, via Cintia 26, 80126, Napoli, Italia

Resume : Starting from the isolation of Graphene, alternative 2D materials have been manufactured and widely studied due to their promising applications in different technological areas, from nanoelectronics to sensing, from electrochemistry to photovoltaics. Among them, Transition Metal Dichalcogenides present such peculiar optical and catalytic properties that have emerged as electro-catalyst of choice for the hydrogen evolution reaction (HER) in photo-electrochemical device for water splitting. Moreover, recent experiments on vertically stacked van der Waals heterostructures made of MoS2 and WS2 provide new and promising results that combine an efficient inter-layer photo-assisted charge transfer and subsequent catalysis of both HER (on the MoS2 side) and water oxidation to molecular oxygen (on the WS2). To further develop these systems, it is crucial to understand the connection between the chemical structure and electronic properties. To this end, ab initio tools are pivotal in unveiling these structure-property-function relationships and provide new rational design principles. Unfortunately, there is not a unique first-principle method that is accurate enough and computationally feasible in order to address both the optical properties and the surface chemistry of 2D heterojunction. We report a benchmark study on the electronic structure of the MoS2:WS2 heterojunction. We compare semi-local and hybrid DFT results and subsequent GW calculations by varying several numerical parameters, taking in account the spin-orbit coupling term. Our aim is to set a definite scale to assess the efficiency of each level of theory taking as reference the best possible accuracy at the least computational burden.

Authors : Harald Oberhofer
Affiliations : Chair for Theoretical Chemistry, Technical University Munich

Resume : The importance of solvation effects on electrochemical reactions is by now a widely accepted fact. On the other hand, sampling all solvent degrees of freedom at potentially expensive levels of electronic structure is often not a feasible approach either. For this reason, implicit solvation models, first pioneered over 80 years ago, are currently undergoing a strong renaissance. Such models generally treat the electrostatic response and possibly dissolved electrolytes on the level of a polarised continuum and the ions' Boltzmann distributions, respectively. The downside to this is that the accuracy of such models generally depend very strongly on a number of effective parameters which, in most cases, cannot be derived from first principles, but rather have to be fitted to a suitable training set. In my talk I present two implicit solvation models, differing in computational cost and complexity, which we implemented in the full-potential numeric atomic orbital code FHI-AIMS. Thereby, I will focus on the parametrisation of the models based on the recently presented Solv@TUM database of experimental solvation free energies. I will highlight some of the intricacies involved the application of popular non-electrostatic correction terms, generally applied to include interactions other than pure electrostatics and to compensate for errors in the solvation model itself. I will also present a new descriptor which to a certain degree can account for structuring of the solvent otherwise absent in pure continuum models. Finally, I will discuss first steps towards new non-electrostatic corrections derived from a compressed sensing machine learning approach used to identify cheaply accessible descriptors for the missing interactions.

Authors : Patrick Gono, Francesco Ambrosio, Alfredo Pasquarello
Affiliations : Chaire de Simulation à l’Echelle Atomique (CSEA) Ecole Polytechnique Fédérale de Lausanne (EPFL)

Resume : We investigate the solvation effect of water on the overpotentials of the oxygen evolution reaction on rutile TiO2 by applying the thermodynamic integration method on atomistic model interfaces with and without the water molecules. We compare the results at the vacuum interface with the commonly used computational hydrogen electrode method, finding overall good agreement. The effect of the solvent is found to be twofold. First, the explicit treatment of the solvent can lead to equilibrium configurations differing from the relaxed structures without solvent. Second, the overpotentials can be affected by up to 0.5 eV. The energetics are subject to electrostatic effects at the interface rather than to modifications in the hydrogen bond network. These results provide a promising perspective on the use of implicit models for treating the solvent.

Authors : Sandipan Chattaraj, Sumit Basu
Affiliations : Indian Institute of Technology Kanpur, Indian Institute of Technology Kanpur

Resume : PEEK and PEKK are important polymers with varied applications including those in aerospace. With regard to the various applications of these polymers, it would be beneficial to understand their welding behaviour, especially the molecular aspects. Molecular dynamics (MD) simulations of these polymers provide useful insight into their various properties. The welding behaviour of PEEK and PEKK can be investigated through molecular dynamics simulations. These welding simulations would involve large systems that would require significant computational time and resource if all-atom samples are considered. Therefore, in order to simulate these large systems, it is necessary to coarse-grain the polymers. This work describes our coarse-graining scheme for PEEK and PEKK. The coarse-graining scheme involves 3 beads per monomer of PEEK or PEKK. Various properties of the detailed sample and coarse-grained sample have been examined. Static properties such as end-to-end length, radial distribution function, glass transition temperature and compressibility have been compared between the all-atom sample and the coarse-grained sample. Dynamic properties such as mean square displacement (msd) of the chain centres have also been investigated. As expected, the match between dynamic properties of the all-atom and coarse-grained samples is poor and scaling has been performed between them.

Poster Session : .
Authors : Denis Gryaznov, Eugene Kotomin
Affiliations : Institute of Solid State Physics, University of Latvia, 8 Kengaraga, LV-1063, Riga, Latvia

Resume : Major sources for human-made CO2 emission comprise the energy and the industrial sector including cement production. One of the most appropriate concepts to capture CO2 from fossil plant exhausts is the oxyfuel combustion. However, most of known highly permeable ceramic membrane materials show unwanted chemical instability against CO2 and other flue gas components. Here the first principles calculations could be helpful in order to understand adsorption of CO2 on atomistic level. We performed detailed theoretical study of the effect of CO2 gas on surface properties of the (La,Sr)FeO3 perovskite membranes based on the hybrid density functional (PBE0) calculations as implemented in CRYSTAL computer code [1]. The role of dispersion interactions (PBE0-D3 [2]) is also addressed, and, in particular, for the surfaces containing oxygen vacancies. We could analyze carefully the correlation between the adsorption enthalpy and stability of different atomic configurations for the CO2 molecule adsorption and membrane basic properties. Thus, the Sr-content influences the Fe average oxidation state which is reflected in the adsorption properties. The behavior of surface vacancies as well as formation of carbonate phases are analyzed through charge redistribution near surface, chemical bond lengths and bond populations. [1] ] R. Dovesi, A. Erba, R. Orlando, et al., WIREs Comp. Mol. Sci. 8 (4), (2018). doi: 10.1002/wcms.1360. [2] S. Grimme, J. Anthony, S. Ehrlich, H. Krieg, J. Chem. Phys. 132, 154104 (2010).

Authors : Leonid L. Rusevich, Guntars Zvejnieks, Eugene A. Kotomin
Affiliations : Institute of Solid State Physics, University of Latvia, Riga, Latvia

Resume : Modern progress in material engineering allows producing new complex materials with improved ferroelectric and piezoelectric properties. The ability of the ABO3 perovskite structures (e.g., BaTiO3 — BTO) to accept isovalent dopants on both A- and B-sites is the way of enhancement of the electromechanical properties of lead-free materials. For example, our recent calculations show that Ba(1-x)SrxTiO3 (BSTO) and Ba(1-x)CaxTiO3 (BCTO) solid solutions exhibit significantly enhanced piezoelectric properties, in a comparison with pure BTO [1–3]. In this work, we presented the results of ab initio (first-principles) computations and local structural analysis for BTO, SrTiO3 (STO), CaTiO3 (CTO) perovskites and (Ba,Sr/Ca)TiO3 perovskite solid solutions. Calculations were performed with the CRYSTAL14 computer code within the linear combination of atomic orbitals (LCAO) approximation, using PBE0, B1WC and B3LYP advanced hybrid functionals of the density-functional-theory (DFT). Different chemical compositions are considered for the ferroelectric (tetragonal) phase of solid solutions by means of supercell calculations. Both Sr and Ca off-centering within the dodecahedra and the Ti atom displacement within the TiO6 octahedra in solid solutions are studied. The relation of these structural distortions with piezoelectric properties of solid solutions is discussed. [1] L.L. Rusevich, G. Zvejnieks, A. Erba, R. Dovesi, E.A. Kotomin, J. Phys. Chem. A 2017, 121, 9409. [2] L.L. Rusevich, G. Zvejnieks, E.A. Kotomin, M. Maček Kržmanc, A. Meden, Š. Kunej, I.D. Vlaicu, J. Phys. Chem. C 2019, 123, 2031. [3] L.L. Rusevich, G. Zvejnieks, E.A. Kotomin, Solid State Ionics 2019, 337, 76.

Authors : Andrew Chesnokov [1], Denis Gryaznov [1], Eugene A. Kotomin [1,2]
Affiliations : [1] Institute of Solid State Physics, University of Latvia, Riga, Latvia; [2] Max Planck Institute for Solid State Research, Stuttgart, Germany

Resume : Atomic and electronic structures of undoped and Tb-doped CeO2-δ were calculated from first principles with inclusion of strong correlation effects based on the Hubbard model (PBE U) and hybrid PBE0 exchange-correlation functional. To correctly account for possible oxidation states of Tb and Ce (3 and 4 ), distinct values of Hubbard U-parameter were simultaneously applied to Ce and Tb ions. Multiple configurations were obtained, with electrons localizing on different number of cations in response to a presence of an oxygen vacancy (Vo) near a Tb or Ce ion. Site symmetry approach was combined with the group theory analysis to successfully identify the ground state configuration for localization of excess electrons on different number of cations. Our results indicate that in Tb-doped CeO2 the difference in total energy between the systems in which Tb ion has either 3 or 4 oxidation state is very small and thus we predict that, unlike Gd-doped ceria, these states can co-exist without Vo formation. In Tb-doped CeO2-δ, Gibbs formation energy of Vo is diminished by a factor close to four, compared to undoped CeO2-δ. The lowest formation energy for the small polaron corresponds to a configuration in which two Ce3 (undoped CeO2-δ) are the nearest neighbors to VO, or one Tb3 and one Ce3 ions (Tb-doped CeO2-δ) are, correspondingly, the nearest and the third nearest neighbor to Vo [1, 2]. Such configuration is consistent with optical measurements from literature. [1] R.A. Evarestov, D. Gryaznov, M. Arrigoni, E.A. Kotomin, A. Chesnokov, J. Maier, Phys. Chem. Chem. Phys. 19 (2017) 8340–8348. [2] A. Chesnokov, D. Gryaznov, E. Kotomin, Opt. Mater. 90 (2019) 76–83.

Authors : Inta Isakovica, Sergei Piskunov
Affiliations : Institute of Solid State Physics, University of Latvia, 8 Kengaraga str., Riga LV-1063, Latvia

Resume : Engineering the electronic energy band structure of hybrid nanostructured semiconductor materials through judicious control of their atomic composition is a promising route to increase visible light photoresponse, which is necessary prerequisite for efficient photocatalytic hydrogen production from water. In our study we have carried out the first principle calculations to simulate the electronic structure of hybrid GaN/WS2 nanotubes. Ab initio modeling reported here have been performed within the formalism of hybrid Density Functional Theory and Hartree-Fock method when using HSE06 exchange-correlation functional properly adapted and verified relatively to properties of GaN and WS2 bulk and nanosheets. The calculation results show that the studied hybrid GaN/WS2 nanotube with the diameter of about 2 nm is a semiconductor with a band gap of about 2.2 eV. GaN/WS2 nanotubes found to be suitable for photocatalytic water splitting under influence of solar light. The edges of their band gaps correspond to the range of visible spectrum. The top of the valence band and the bottom of the conduction band of considered GaN/WS2 nanotubes have been properly aligned relative to the oxidation and reduction potentials necessary for water splitting under visible light irradiation. Funding from Latvian Council of Science fundamental and applied research project Nr. LZP-2018/2-0083 is greatly acknowledged.

Authors : Marcin Roland Zemła, Tomasz Wejrzawnoski
Affiliations : Faculty of Materials Science and Engineering, Warsaw University of Technology, Woloska 141, 02-507 Warsaw, Poland

Resume : Nowadays, scientific advancement has a growing impact on the natural environment. It is most significant in the fields of industry, transportation, and the energy production, where air pollution is most noticeable. Hence, more and environment-friendly and alternative energy sources are pursued. One of the most interesting and promising solutions are fuel cells, which directly convert the fuel (hydrogen or hydrocarbons) to electrical energy by the electrochemical reaction without a combustion process. The high-temperature fuel cells, such as Molten Carbonate (MCFC) and Solid Oxide Fuel Cell, are most prospective. Typically in MCFC the electrodes are made of Ni/NiO. Numerous of addition and changes in chemical composition of electrodes were studied and it is found that, the promising candidate is Bi2O3 addition. Due to that, in current studies, we used density functional theory (DFT) calculations to investigate the chemisorption and stability of O2 and CO2 at Bi2O3 surface. Moreover, the partial density of states combined with the effective bond order were used to analyse the bonding nature of adsorbed molecules. We applied the climbing image nudged elastic band method (CI-NEB) to estimate energy barriers in reactions taking place on Bi2O3 surface. The obtained results were compare with analogical reactions occurring on the Ni/NiO surfaces.

Authors : Stefano Falletta
Affiliations : EPFL, Switzerland

Resume : The search for semiconductor co-catalysts requires the calculation of the alignment of valence and conduction bands of the co-catalyst with respect to the redox levels of liquid water. In this context, we analyze two transition metal-based co-catalyst, namely NiO and Ni2P, which have been proven to exhibit high hydrogen evolution rates when used in conjunction with metal-organic frameworks, acting as photo-catalysts. We perform hybrid-functional calculations to compute the band structure of the two semiconductors. Both a molecular and a dissociative model of the water adsorbed at the semiconductor surfaces are considered. Finally, molecular dynamics simulations of the NiO/H2O and Ni2P/H2O interfaces are used to determine the band edge alignment. Our results aim at giving an explanation for the role of NiO and Ni2P as co-catalysts for the photo-catalyst MIL-125-NH2.

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Session 5 : .
Authors : Mariachiara Pastore
Affiliations : Laboratoire de Physique et Chimie Théoriques (LPCT) UMR 7019 CNRS - Université de Lorraine Nancy, France

Resume : In the context of solar energy exploitation, dye-sensitized solar cells (DSCs) and dye-sensitized photoelectrosynthetic cells (DSPECs) offer the promise of cost effective sunlight conversion and storage, respectively. Dye-functionalization of bot n- and p-type semiconductors (like TiO2 and NiO, respectively) can be either exploited to build active DS photoelectrodes or tandem DSC and DSPECs devices . Computational modelling has played a prominent role in the development of the DSC technology, whereas the understanding of the interfacial processes in DSPEC is still at its inception, especially for what concerns the electron and hole transfer phenomena, which are central to the efficient device functioning. Here I will discuss the recent advances concerning first principle modelling of materials, interfaces and processes of active photoelectrodes for solar water splitting. On the photoanode side, we will discuss the issues related to the modelling of TiO2/dye/catalyst multiple interfaces and the exploitation of alternative n-type semiconductor as WO3. On the photocathode side, particular emphasis will be put on the characterization of the electronic and structural properties of the complex NiO/solvent/dye/electrolyte interface, whose characterization is still poor when compared to the level of understanding reached for TiO2 sensitized photoanodes, from both the experimental and computational point of view We will discuss the main methodological limitations of state-of-the art DFT methodologies in predicting the energy level alignment across the dye/semiconductor interface and the challenging definition of a proper structural model needed to reliably capture the interface complexity.

Authors : Tomasz A. Wesolowski
Affiliations : Department of Physical Chemistry, University of Geneva, Switzerland

Resume : Frozen-Density Embedding Theory (FDET) [1] was formulated as a formal basis for multi-level simulations in which the embedded species is described by means of an embedded interacting N_A-electron wavefunction whereas its environment is represented by means of classical descriptors (charge densities). The use of these classical descriptors for the environment makes it possible to join quantum-mechanical level of description with different levels of description for the environment and for the embedded species including not only different quantum-mechanical models but also classical statistical mechanics based models [2] and even experimentally obtained charge densities [3]. In the original formulation of FDET, the embedded ground-state wavefunction is obtained from variational methods which guarantees satisfaction of the Hoheberg-Kohn theorems. We will review applications of the recently developed extension of FDET to excited states [4] guaranteeing the orthogonality of the embedded wavefunctions at different states. In the second part another extension of FDET, for which the Hohenberg-Kohn theorems are satisfed even for non-variationally obtained embedded wavefunctions, will be presented [5]. [1] T.A. Wesolowski, Phys. Rev.A. 77 (2008) 012504. [2] A.A. Laktionov, E. Chemineau-Chalaye,T.A. Wesolowski, Phys. Chem. Chem. Phys, (2016) 18 21069 and references therein [3] Ricardi et al., to be published [4] T.A. Wesolowski, J. Chem. Phys , 140, (2014) 18A530 [5] A. Zech, A. Dreuw, T.A. Wesolowski, J. Chem. Phys. 150 (2019) 121101

Authors : Yuval Elbaz, Maytal Caspary-Toroker
Affiliations : Department of Materials Science and Engineering, Technion - Israel Institute of Technology, Haifa 3200003, Israel

Resume : Production of electric power by solar splitting of water and the reveres process in a fuel cell is a major source of renewable energy. One of the best known catalysts for water splitting is nickel hydroxide doped with iron. Although NiOOH is a widely researched material, the hydrogen atoms position in it are not fully known. We carried out Nudged Elastic Band (NEB) calculations of hydrogen diffusion in the bulk of several related phases, including β-Ni(OH)2, β-NiOOH and α-Ni(OH)2. We considered vacancies and interstitial diffusion mechanisms and found the activation energy and minimum energy path for diffusion. Our results show that hydrogen diffusion is plausible in between the sheets of each phase. Furthermore, results suggests that as the material become charged (β-NiOOH, 50% hydrogen ) the formation of interstitial hydrogen atoms is spontaneous and the activation energy become as of the activation energy of vacancy in the discharged phase (β-Ni(OH)2, 100% hydrogen). We conclude that hydrogen diffusion is more effective in the charged state. Additionally, we calculated the bulk modulus and concluded that the presence of water and hydrogen molecules lower the bulk modulus, with the latter possessing a more significant influence.

Authors : Shih-Wei Liang, and Te-Hua Fang*
Affiliations : Department of Mechanical Engineering, National Kaohsiung University of Science and Technology, Kaohsiung 80778, Taiwan

Resume : In this study, the bending and mechanical properties of amorphous alloys by using in-situ compression experiments and molecular dynamics. The susceptibility of the Cu-doped Ni-Ti shape memory alloys nano-cantilevers to crack investigated at nano-scale fracture and strain-induced structure variation. The experimental results shows that the size effect enhanced the nanomechanical properties of the specimens. We also discuss the effects of Cu-doped content on the deformation response of the Ni–Ti–Cu nano-cantilevers. The results are important for understanding the mechanical properties, elastic behavior, and local bending fracture results of Ni-Ti-Cu nano-cantilevers could provide useful information.

10:30 Coffee break    
Session 6 : .
Authors : Joachim Sauer
Affiliations : Humboldt University

Resume : Density functional theory is applied to solve the structures of ultrathin silica and zeolite films on metal substrates based on experimental information including LEED, IR-RAS, and STM. Specifically we discuss crystalline films consisting of one or two layers of corner-sharing TO4-tetrahedra (T=Si, Al) on Mo(112) and Ru(0001) surfaces. We also discuss the formation of amorphous phases that can be directly imaged in real space by STM, and the substitution of Si with Ti and Fe which is not isomorphous but leads to new structure types. Joined computational – surface science studies on thin film zeolite models provide information for the limiting case of a flat surface corresponding to an infinitely large pore diameter. Whereas the non-specific adsorption energy is smaller on flat surfaces than inside zeolite pores, the acidity is significantly higher as OH frequency shifts on adsorption of molecules indicate. The consequences for understanding the acidity of layered powder materials (ZSM-5 layers) is discussed.

Authors : Patrick Rinke
Affiliations : Department of Applied Physics, Aalto University, Helsinki, Finland

Resume : Quantum mechanical accuracy is required to simulate materials for clean energy generation. However, accurate calculations are costly and limited in the tractable systems size (i.e. number of atoms in the calculation). Machine learning methods can help to overcome this size-accuracy conundrum. Here I will present two different machine learning methods for two different materials science applications. For halide perovskites (ABX3) alloys, we have combined the kernel ridge regression (KRR) approach with the many-body tensor [1,2] as materials descriptor. Trained on density-functional theory (DFT) supercell calculations for different alloy compositions, KRR becomes a fast energy predictor for exploring alloy space. For hybrid organic-inorganic interfaces, we have developed a smart-data active-learning approach. To promote unbiased studies into molecular surface structures and phenomena, we have combined atomistic simulations with Bayesian optimisation - an artificial intelligence (AI) technique designed for complicated optimisation tasks [3]. We demonstrate how the AI was adapted to learn surface and property landscapes of molecules on surface with minimal computational sampling [4], delivering most stable surface structures with favorable designer properties. [1] Huo and Rupp, arXiv:1704.06439 (2017). [2] A. Stuke, M. Todorović, M. Rupp, C. Kunkel, K. Ghosh, L. Himanen, and P. Rinke, arXiv:1812.08576 (2018). [3] M. U. Gutmann, J. Corander, J. Mach. Learn. Res. 17, 1 (2016). [4] M.Todorović, M. U. Gutmann, J. Corander and P. Rinke, npj Comp. Mat. 5, 35 (2019).

Authors : Ángel Morales-García 1, Antoni Macià Escatllar 1, Francesc Illas 1, Stefan T. Bromley 1,2
Affiliations : 1. Departament de Ciència de Materials i Química Física & Institut de Química Teòrica i Computacional (IQTCUB), Universitat de Barcelona, c/ Martí i Franquès 1-11, 08028 Barcelona, Spain 2. Institució Catalana de Recerca i Estudis Avançats (ICREA), Passeig Lluis Companys 23, 08010 Barcelona, Spain

Resume : Small TiO2 nanoparticles (NPs) (<5 nm diameter) typically have spherical morphologies, with enhanced photoactivity for hydrogen fuel production from water splitting. For such NPs it is difficult to experimentally determine their thermodynamic stability and internal atomic structure, to help rationalise their higher photoactivities. Using accurate electronic structure calculations, we establish the relative stability of spherical and faceted stoichiometric TiO2 NPs with 1-3.4 nm diameters. Mirroring experiment, simulated thermal annealing is found to significantly stabilise relaxed spherical cut anatase NPs. We find that the smallest spherical NPs become amorphized by annealing, but, for diameters >2 nm, annealing yields NPs with anatase-cores and amorphous-shells. Our core-shell NPs are metastable relative to faceted anatase NPs but have significantly smaller optical gaps, rationalising their higher photoactivities. Our calculated gaps are in excellent agreement with experimental data, strongly supporting the validity of our NP models. Gap narrowing in our core-shell NPs is due to valence band broadening induced by their amorphous shells, analogous to the mechanism proposed for larger "black TiO2" NPs. Our stoichiometric NPs also show that band narrowing does not require non-stoichiometric disordered shells or for incorporating other atom types, but mainly arises from 4-coordinated Ti atoms in the amorphous shell. Á. Morales-García, A. Macià Escatllar, F. Illas and S. T. Bromley, Nanoscale 11, 9032 (2019).

Authors : Stephen Rhatigan, Gerardo Colón, Michael Nolan
Affiliations : Tyndall National Institute, University College Cork; Instituto de Ciencia de Materiales de Sevilla, CSIC Sevilla; Tyndall National Institute, University College Cork;

Resume : We have studied surface modification of rutile TiO2 with dispersed alkaline earth oxide (AEO) nanoclusters using density functional theory corrected for on-site Coulomb interactions (DFT+U), focussing on the impact of MgO and CaO surface modifiers on key properties which dictate the photocatalytic activity towards the water oxidation reaction. Experiments show that MgO modification promotes the oxygen evolution reaction (OER), particularly at low coverages. In our theoretical models we have considered nanoclusters of sizes M4O4, M8O8 and M12O12 (M = Mg, Ca) modifying the rutile (110) surface. Our results indicate that for low coverages, small MgO clusters will be widely dispersed at the surface whereas CaO clusters will be present in clusters of various sizes. As coverage increases, the modifiers will aggregate to form larger nanoclusters. The modified surfaces are highly reducible, with moderate energy costs to produce oxygen vacancies and this cost increases with nanocluster size. After oxygen vacancy formation, Ti ions of the rutile substrate are reduced to Ti3+. Low-coordinated oxygen ions of the supported nanoclusters lead to an extension of the valence band edge to higher energies. However, for the smaller nanoclusters, this effect is lost after oxygen vacancy formation so that we predict a negligible impact on the light absorption properties at low coverages. Photoexcited electrons and holes localize on subsurface Ti sites and nanocluster O sites, respectively, so that modification promotes charge separation. The interaction of H2O at the modified surfaces is favourable and leads to the spontaneous dissociation to surface bound hydroxyls. We follow proton-coupled electron transfer steps involving adsorbed hydroxyls and identify pathways towards oxygen evolution with low overpotentials.

12:45 Lunch break    
Session 7 : -
Authors : Henrik Grönbeck
Affiliations : Competence Centre for Catalysis and Department of Physics, Chalmers University of Technology, Sweden

Resume : A major challenge in heterogeneous catalysis research is the determination of dominating reaction paths and kinetic bottlenecks. One reason for the challenge is the dynamic character of the kinetics, where the active sites may change with reaction conditions. Nevertheless, it is atomic scale information that allow for catalyst development beyond trial-and-error approaches. Kinetic modeling based on first principles calculations have over the past decade grown into an important tool for investigating the importance of different catalyst phases and reaction paths. In this contribution, I will discuss work where we have used density functional theory in combination with kinetic modeling to investigate catalytic reactions over metal surface and nanoparticles. The examples cover different aspects of kinetic modeling including determination of adsorbate entropies, importance of adsorbate-adsorbate interactions and the complexity of many types of active sites. Reactions over platinum nanoparticles are investigated using a recently developed scaling relation Monte Carlo technique [1,2]. Taking CO oxidation and acetylene hydrogenation as a model reactions, we find that the overall activity is determined by complex kinetic couplings. Effects of particle shape as well as internal and external strain will be discussed. [1] M. Jørgensen, H. Grönbeck, ACS Catalysis 7, 5054 (2017). [2] M. Jørgensen, H. Grönbeck, J. Chem. Phys. 149, 114101 (2018).

Authors : K. George, M. Van Berkel, X. Zhang, R. Sinha and Anja Bieberle-Hütter
Affiliations : Dutch Institute for Fundamental Energy Research (DIFFER), Eindhoven, 5612AJ, The Netherlands

Resume : Electrochemical devices are limited to large part by the performance of the electrochemical interfaces. However, the characterization of these interfaces and their analyses are difficult, since processes or surface species cannot be measured directly. We have therefore setup a multiscale modeling approach in which we combine atomistic and microkinetic modeling [1,2]. This enables us to directly trace the electrochemical data back to electrochemical quantities. In our presentation, we introduce our new multiscale modeling approach [1]. As case study, we use Fe2O3, an abundant, cheap, and non-toxic catalyst for water splitting. The micro-kinetic equations are formulated for the multiple reaction steps and are modeled in state-space form. As input to the state-space model we use reaction rates estimated from density functional theory [3]. We present simulated current-voltage curves and electrochemical impedance spectra and compare the simulations to our experimental data [4]. The model can also simulate the coverage of intermediate species as a function of applied potential which is highly demanded for identifying the limiting processes at the interface, but not available from experimental studies. The approach is generic and can be used for other electrochemical interfaces, like in fuel cells, electrolysers, or batteries. [1] George et al., J. Phys. Chem. C, (2019). [2] Zhang et al., ChemSusChem (2016). [3] Zhang et al., J. Phys. Chem. C (2016). [4] Sinha et al., ACS Omega (2019).

Authors : José C. Conesa
Affiliations : Instituto de Catálisis y Petroleoquímica, CSIC. Madrid, Spain

Resume : Band offsets between semiconductors are crucial to determine the direction of electron transfer at their interfaces, which is important in photovoltaics, photocatalysis/ photoelectrochemistry and in general in the semiconductor industry. Two methods are normally used to compute such offsets from first principles: alternating slabs of both materials put in contact, without empty spaces between them, and separate calculations of each material surface confronted with empty vacuum space. The first method has the risk of introducing distortions due to insufficient epitaxial match, which may lead to bandgap changes, and the second may neglect electron transfer at the interface, which may be important in systems having very different average electronegativities. In this work I will compare results of using both approaches for a number of interfaces: BiVO4/NiOOH (relevant for photoelectrochemistry), anatase TiO2/ZnO (used in some dye-sensitized solar cells), CuGaS2/CdS (relevant for thin film photovoltaics) and other systems like the rutile TiO2/PbTe and grey tin/diamond interfaces. The method will be based on using for the bulk phases hybrid or meta-GGA DFT methods providing bandgap values coincident with the experimental ones, and transferring to the interfaces the distances between the band positions and the profile of the electrostatic potential (J.C. Conesa, J. Phys. Chem. C 2012, 116, 18884). A critical analysis of any relevant differences found will be presented.

Authors : Eduardo Schiavo¹, Ana B. Muñoz-García², and Michele Pavone¹
Affiliations : ¹ Department of Chemical Sciences, University of Naples “Federico II”; Comp. Univ. Monte Sant’Angelo Via Cintia 21, 80126 Naples, Italy ² Department of Physics “Ettore Pancini”, University of Naples Federico II, Comp. Univ. Monte Sant’Angelo Via Cintia 21, 80126 Naples, Italy

Resume : Graphene Nanostructures (GNS) have attracted great interest over the years and found many applications due to their peculiar structural and electronic properties. In this contribution, we focus on GNS as possible catalysts for Oxygen Reduction Reaction (ORR): a fundamental process in fuel cells. The electro-catalytic application of graphene and its derivatives in fuel cells involves the presence of direct graphene-metal contact . The metallic substrate has several roles: it provides mechanical support and works as electric connector, but it can also actively participate in the electronic processes that occur during the ORR catalysis. Hence, the interactions between the GNS and the metal and the features of their interface deserve a detailed investigation, possibly supported by reliable computational models. In this contribution, we present a detailed analysis of graphene and doped graphene and their interaction with Ag (111), with a particular focus on graphene adsorption on the metal and on the modifications of its electronic structure. Such system poses great challenges to current standard DFT methods and dispersion corrections are needed to study the van der Waals interactions between the metal and the graphene moiety. In this work, we validate a recently proposed modification of the DFT-D2 approach for describing hybrid molecule-metal systems. We investigate the adsorption of molecular oxygen on these systems. This process is the first step for the ORR catalysis in GNS-based cathodes for fuel cells. We dissect the effect of the dopants from that of the metal on graphene. From our results, we show that the silver support provides an extra electronic density that can boost the performance of the already effective boron-doped graphene.

15:30 Coffee break    
Session 8 : .
Authors : Ricardo Grau-Crespo [a], Alex Aziz [a], Norge C. Hernández [b], A. Rabdel Ruiz-Salvador [c], Said Hamad [c]
Affiliations : [a] Department of Chemistry, University of Reading, UK; [b] Department of Applied Physics I, Escuela Técnica Superior de Ingeniería Informática, Universidad de Sevilla, Spain; [c] Department of Physical, Chemical and Natural Systems, Univ. Pablo de Olavide, Seville, Spain.

Resume : Tuning the electronic structure of metal organic frameworks (MOFs) is the key to extending their functionality, beyond gas adsorption, to photocatalytic chemical conversion of the absorbed gases. We have used density functional theory calculations to investigate how to engineer the excitation gaps and band edge positions in zeolitic imidazolate frameworks and porphyrin-based MOFs, with different compositions and dimensionalities. The key to obtain the optimal electronic structure is the mixing of different linkers and/or different metals within the same structure. We propose new stable materials with ideal band gap and band alignment, as well as stability and other properties that make them excellent candidates for the solar-driven production of solar fuels.

Authors : Gabriela Ben-Melech Stan, Maytal Caspary Toroker
Affiliations : Department of Materials Science and Engineering, Technion – Israel Institute of Technology, Haifa, Israel; Department of Materials Science and Engineering, Technion – Israel Institute of Technology, Haifa, Israel, and The Nancy and Stephen Grand Technion Energy Program, Technion – Israel Institute of Technology, Haifa, Israel

Resume : Molybdenum disulfide (MoS2) is a common two-dimensional semiconductor that has been intensively studied for catalysis and electronics. Fabrication of two-dimensional MoS2 induce point defects, typically sulfur vacancies, which highly affect the electronic and chemical characteristic of the material. In this study, we attempt to evaluate the effect of different types of sulfur vacancies on the electronic properties of a monolayer MoS2. The carrier transport around the vacancy was determined using a combined scheme based on Density Functional Theory (DFT) and quantum dynamics. The dynamic charge transport was evaluated via an effective Hamiltonian approach derived from Kohn Sham potential energy obtained from DFT calculations. According to our model, we find that sulfur vacancies create trap states in the original band gap of monolayer MoS2 that disrupt charge transmission through the monolayer.

Authors : David Abbasi-Pérez1, Hongqian Sang1,2, Lluïsa Pérez-García3, Andrea Floris4, David B. Amabilino5, Rasmita Raval6, J. Manuel Recio7, and Lev Kantorovich1
Affiliations : 1 Department of Physics, King’s College London, London, WC2R 2LS, (UK); 2 Institute for Interdisciplinary Research, Jianghan University, Wuhan 430056, (China); 3 School of Pharmacy, University of Nottingham, University Park, Nottingham, NG7 2RD, (UK); 4 School of Mathematics and Physics, University of Lincoln, Brayford Pool, Lincoln LN6 7TS, (UK); 5 School of Chemistry GSK Carbon Neutral Lab. for Sust. Chemistry, University of Nottingham, Triumph Road, NG7 2TU(UK); 6 Surface Science Research Centre, Department of Chemistry, University of Liverpool Liverpool L69 3BX (UK); 7 MALTA-Consolider Team and Department of Analytical and Physical Chemistry, Universidad de Oviedo, Oviedo, 33006, (Spain).

Resume : Molecular walkers standing on two or more “feet” on an anisotropic periodic potential of a crystal surface may perform a one-dimensional Brownian motion at the surface-vacuum interface along a particular direction in which their mobility is the largest. In thermal equilibrium the molecules move with equal probabilities both ways along this direction, as expected from the detailed balance principle, well-known in chemical reactivity and in the theory of molecular motors. For molecules that possess an asymmetric potential energy surface (PES), we propose a generic method based on the application of a time-periodic external stimulus that would enable the molecules to move preferentially in a single direction thereby performing as Brownian ratchets. To illustrate this method, we consider a prototypical synthetic chiral molecular walker, the 1,3-bis(imidazol-1-ylmethyl)-5(1- phenylethyl)benzene, diffusing on the anisotropic Cu(110) surface along the Cu rows. As unveiled by our kinetic Monte Carlo simulations based on the rates calculated using ab initio density functional theory, this molecule moves to the nearest equivalent lattice site via the so-called inchworm mechanism in which it steps first with the rear and then with the front foot. As a result, the molecule diffuses via a two-step mechanism, and due to its inherent asymmetry, the corresponding PES is also spatially asymmetric. Taking advantage of this fact, we show how the external stimulus can be tuned to separate molecules of different chirality, orientation and conformation. The consequences of these findings for molecular machines and the separation of enantiomers are also discussed.


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Symposium organizers
Florian LIBISCHVienna University of Technology

Institute for Theoretical Physics, Wiedner Hauptstraße 8-10/136, 1040 Vienna, Austria
Francesc ILLASUniversity of Barcelona

Department of Chemistry and Materials Science, Barcelona, Spain
Maytal CASPARY TOROKER Technion – Israel Institute of Technology

Department of Materials Science and Engineering, Technion City, Haifa 3600003, Israel
Michele PAVONEUniversity of Naples Federico II

Department of Chemical Sciences, Via Cintia 21, 80126 Naples, Italy