Author Archives: Ageo Meier de Andrade

Symmetric Small-Molecules With Acceptor-Donor-Acceptor Architecture for Efficient Visible-Light Driven Hydrogen Production: Optical and Thermodynamic Aspects

Authors: Giane Benvinda Damas, Fredrik von Kieseritzky, Jonas Hellberg, Cleber Fabiano N. Marchiori, Carlos Moyses Araujo 

Small-molecules (SM) have attracted a great deal of attention in the field of solar energy conversion due to their unique properties compared to polymers, such as well-defined molecular weight and lack of regio-isomeric impurities. Furthermore, these materials can be synthesized in a variety of configurational architectures, representing an opportunity for tailoring chemical and optical properties that could lead to a better photocatalytic efficiency for hydrogen generation. Here, we evaluate by means of density functional theory (DFT) and time-dependent DFT methods a set of small-molecules with A-D-A architecture (A-acceptor; D- donor) based on well-known building blocks like thiophene (T), cyclopentadithiophene (CPT) and benzothiadiazole (BT) as potential candidates for photocatalytic hydrogen evolution reaction (HER). We also propose i) the replacement of the thiophene unit by 3,4-ethylenedioxythiophene (EDOT) to form with CPT unit an extended donor core ii) an additional acceptor unit, the 1,3,4-thiadiazole, in the extremities and iii) insertion of the difluoromethoxy (DFM) as substituent in the BT unit. Our outcomes reveal that these materials have a broad absorption spectrum with λ= 318-719 nm, being the most intense absorption peak originated from an electronic transition with charge-transfer nature, as the spatial distribution of LUMO is concentrated on the acceptor units for all materials. Moreover, these small-molecules not only present catalytic power or thermodynamic driving force to carry out the chemical reactions involved in the process of hydrogen production, but can be coupled in cooperative photocatalytic systems to promote intramolecular charge transfer that is expected to boost the overall photocatalytic efficiency of these materials.

J. Phys. Chem. C 2019,

π‐Conjugation Enables Ultra‐High Rate Capabilities and Cycling Stabilities in Phenothiazine Copolymers as Cathode‐Active Battery Materials

Authors: Pascal Acker, Luisa Rzesny, Cleber F. N. Marchiori, C. Moyses Araujo and Birgit Esser

In recent years, organic battery cathode materials have emerged as an attractive alternative to metal oxide–based cathodes. Organic redox polymers that can be reversibly oxidized are particularly promising. A drawback, however, often is their limited cycling stability and rate performance in a high voltage range of more than 3.4 V versus Li/Li+. Herein, a conjugated copolymer design with phenothiazine as a redox‐active group and a bithiophene co‐monomer is presented, enabling ultra‐high rate capability and cycling stability. After 30 000 cycles at a 100C rate, >97% of the initial capacity is retained. The composite electrodes feature defined discharge potentials at 3.6 V versus Li/Li+ due to the presence of separated phenothiazine redox centers. The semiconducting nature of the polymer allows for fast charge transport in the composite electrode at a high mass loading of 60 wt%. A comparison with three structurally related polymers demonstrates that changing the size, amount, or nature of the side groups leads to a reduced cell performance. This conjugated copolymer design can be used in the development of advanced redox polymers for batteries.

Adv. Funct. Mater.2019, 1906436

Tailoring the Electron-Rich Moiety in Benzothiadiazole-Based Polymers for Efficient Photocatalytic Hydrogen Evolution Reaction

Authors: Giane Benvinda Damas, Cleber Fabiano N. Marchiori, and Carlos Moyses Araujo

Polymeric materials containing an extended π-conjugated backbone have shown a wide range of applicability including photocatalytic activity for hydrogen evolution reaction (HER). The latter requires highly efficient materials with optimal light absorption and thermodynamic driving force for charge transfer processes, properties that are tailored by linking chemical units with distinct electron-affinity to form donor-acceptor architecture. Here, this concept is explored by means of ab initio theory in benzothiadiazole-based polymers with varying electron-rich moieties, viz. fluorene (PFO), cyclopentadithiophene (CPT), methoxybenzodithiophene (O-BzT), thiophenebenzodithiophene (T-BzT), thiophene- (T, VT) and thienethiophene (TT, VTT)-based units. All materials exhibit a red-shifted absorption spectrum with respect to the reference polymer (PFO-DT-BT), while keeping the catalytic power for hydrogen production almost unchanged. In particular, a displacement of ∆λ=167 nm in the first absorption maximum has been achieved upon combination of chemical units with high donating character in CPT-VTT-BT. Furthermore, the exciton binding energies (Eb) have been systematically investigated to unveil the effects of geometry relaxation, environment polarity and finite temperature contributions to the free energy. For instance, we show a significant change in Eb when going from the gas phase (Eb=1.43-1.85 eV) to the solvent environment (Eb=0.29-0.54 eV in 1-bromooctane with ε=5.02). Furthermore, we have found a linear correlation between the lowering of exciton binding energies and the increasing of the ratio between donor and acceptor contributions to the HOMO orbital. This is a consequence of increased donating ability and enhanced spatial separation of electron-hole pairs, which weakens their interaction. Finally, our findings reveal that the donor unit plays a crucial role on key properties that govern the photocatalytic activity of donor-acceptor polymers contributing to the development of a practical guideline to design more efficient photocatalysts for HER. This goes through a proper combination of electron-rich moieties to tune the optical gap, favor thermodynamic driving force for charge transfer and lower exciton binding energies.

J. Phys. Chem. C, 2019, 123, 42, 25531-25542


Molecular origin of efficient hole transfer from non-fullerene acceptors: insights from first-principles calculations

Authors: Leandro Benatto, Cleber F. N. Marchiori, C. Moyses Araujo and Marlus Koehler

Due to the strong exciton binding energy (Eb) of organic materials, the energy offset between donor (D) and acceptor (A) materials is essential to promote charge generation in organic solar cells (OSCs). Yet an efficient exciton dissociation from non-fullerene acceptors (NFAs) began to be observed in D/A blends even at very low driving force for hole transfer (ΔHh). The mechanism behind this efficient photoinduced hole transfer (PHT) remains unclear since current estimates from calculations of isolated molecules indicate that Eb > ΔHh. Here we rationalize these discrepancies using density functional theory (DFT), the total Gibbs free energy method and the extended Hückel theory (EHT). First, we employed DFT to calculate Eb for NFAs of three representative groups (perylene diimide derivatives, indacenodithiophene and subphthalocyanines) as well as for fullerene acceptors (FAs). Considering isolated molecules in the calculations, we verified that Eb for NFAs is lower than for FAs but still higher than the experimental ΔHh in which efficient PHT has been observed. Finding the molecular geometry of the excited state, we also obtain that the structural relaxation after photoexcitation tends to further decrease (increase) Eb for NFAs (FAs). This effect helps explain the delayed charge generation measured in some NFA systems. However, this effect is still not large enough for a significant decrease in Eb. We then applied EHT to quantify the decrease of Eb induced by energy levels coupling between stacked molecules in a model aggregate. We then estimated the number of stacked molecules so that Eb approaches ΔHh‘s. We found that small NFA aggregates, involving around 5 molecules, are already large enough to explain the experiments. Our results are justified by the low energy barrier to the generation of delocalized states in these systems (especially for the hole delocalization). Therefore, they indicate that molecular systems with certain characteristics can achieve efficient molecular orbital delocalization, which is a key factor to allow an efficient exciton dissociation in low-driving-force systems. These theoretical findings provide a sound explanation to very recent observations in OSCs.

J. Mat. Chem. C 2019, 2019,7, 12180-12193



Predicting Structure and Electrochemistry of Dilithium Thiophene-2,5-Dicarboxylate Electrodes by Density Functional Theory and Evolutionary Algorithms

Authors: Cleber F. N. Marchiori, Daniel Brandell, and C. Moyses Araujo

Organic electroactive materials are promising candidates to be used as lithium insertion electrodes in the next generation of environmentally friendly battery technologies. In this work, evolutionary algorithms at interplay with density functional theory calculations have been employed to predict the crystal structure for both delithiated and lithiated phases of dilithium thiophene dicarboxylate (Li2TDC). On the basis of the resulting crystals, electronic structure modifications and voltage profiles for the lithiation process have been calculated. The obtained structure for the delithiated phase showed a well-defined salt layer intercalating the organic components, forming a so-called lithium organic framework (LOF). Upon lithiation, new structures appear which deviate from the LOF as a consequence of the reduction of the S atoms, which coordinate with the additional Li ions. The calculated average potential of ∼1.00 V vs Li/Li+ is found to be in good agreement with experimental findings. An additional study at the molecular level has also been conducted aiming at gaining insight into the importance of the crystallographic environment on the structural and thermodynamics properties. This strategy is suitable for an initial assessment of the electrochemical process that underlies the lithiation mechanism of electrode materials. Moreover, the employed evolutionary algorithm emerges as a promising tool to predict crystal structures during lithiation, which are otherwise difficult to resolve experimentally.

J. Phys. Chem. C, 2019 123, 8, p. 4691-4700

Identification of High‐Performance Single‐Atom MXenes Catalysts for Low‐Temperature CO Oxidation

Authors: Cheng Cheng, Xilin Zhang, Zongxian Yang and Kersti Hermansson

On the basis of first‐principles calculations, Fe, Co, Ni, Cu, Zn, Ru, Rh, Ag, Ir, Pt, and Au decorated Mo2CO2δ monolayers are investigated as potential single‐atom catalyst (SAC) candidates for low‐temperature CO oxidation reaction. From a first screening based on intuitive criteria concerning metal sintering, CO poisoning, and O2 adsorption strength, the Zn/Mo2CO2δ system is selected for further scrutiny by means of reactivity calculations for different CO concentrations. A lower barrier is found for Eley–Rideal reaction mechanism than for the Langmuir–Hinshelwood mechanism. The low Eley–Rideal barrier (0.15 eV) is attributed to the fact that the Zn atom weakens the O‐O bond considerably and the electrophilic attack of CO weakens it further. The main conclusion is that this system is a promising low‐temperature SAC candidate with a lower energy barrier for CO oxidation than noble metal and other 2D SAC systems investigated.

Adv. Theory Simul., 2019, 2: 1900006.

Anion-mediated electronic effects in reducible oxides: Tuning the valence band of ceria via fluorine doping

Authors:  Miroslav Kettner,  Tomáš Duchoň,  Matthew J. Wolf,  Jolla Kullgren,  Sanjaya D. Senanayake,  Kersti Hermansson,  Kateřina Veltruská, and  Václav Nehasil

Combining experimental spectroscopy and hybrid density functional theory calculations, we show that the incorporation of fluoride ions into a prototypical reducible oxide surface, namely, ceria(111), can induce a variety of nontrivial changes to the local electronic structure, beyond the expected increase in the number of Ce3+ ions. Our resonant photoemission spectroscopy results reveal new states above, within, and below the valence band, which are unique to the presence of fluoride ions at the surface. With the help of hybrid density functional calculations, we show that the different states arise from fluoride ions in different atomic layers in the near surface region. In particular, we identify a structure in which a fluoride ion substitutes for an oxygen ion at the surface, with a second fluoride ion on top of a surface Ce4+ ion giving rise to F 2p states which overlap the top of the O 2p band. The nature of this adsorbate F–Ce4+ resonant enhancement feature suggests that this bond is at least partially covalent. Our results demonstrate the versatility of anion doping as a potential means of tuning the valence band electronic structure of ceria.

J. Chem. Phys. 151, 044701 (2019)

Effects of H-bond asymmetry on the electronic properties of liquid water – An AIMD analysis

Authors: Imre Bakó, János Daru, Szilvia Pothoczk, László Pusztai and Kersti Hermansson

The effects of an asymmetric environment on the electronic properties of a water molecule in liquid water are in focus in this paper and were analysed from ab initio molecular dynamics simulations of liquid water at 300 and 350 K with the BLYP-D3 functional. We make the following observations. (1) The electronic DOS and the net molecular charge are more affected by the asymmetry of the water molecule's H-bond surroundings than by the number of H-bonded neighbours. The reverse is true for the dipole moment. (2) For all three properties, a 3-coordinated water molecule is more perturbed by accepting two H-bonds and donating one than by donating two and accepting one. (3) This order is not maintained in the calculated XES spectrum, which is less straightforward to interpret in terms of structure-property relationships than the DOS spectrum.

Journal of Molecular Liquids, Vol 293, 2019, p. 111579


From Ceria Clusters to Nanoparticles: Superoxides and Supercharging

Authors: Dou Du, J. Kullgren, K. Hermansson and P. Broqvist

Several studies have reported a dramatically increased oxygen storage capacity (OSC) for small ceria nanoparticles (∼5 nm). Both experiments and theory have correlated this effect with superoxide ion formation. In previous studies, density functional theory (DFT) calculations with the PBE+U density functional have been used, and the obtained results were only in qualitative agreement with the experimental observations. One severe problem is the underbinding of the O2 molecule upon superoxide ion formation, which suggests that such species should not exist above room temperature. In this work, we use hybrid DFT functional to resolve this problem. We find that the discrepancy between theory and experiment originates from an incorrect estimate of the energy associated with the localized f-electrons with respect to the oxygen p-levels. By using average O2 adsorption energies from hybrid DFT calculations, extrapolated to large nanoparticles (3−10 nm), in conjunction with first-order desorption kinetics, we find that superoxide ions are indeed stable on nanosized ceria well above room temperature, in accordance with experiments.

Multiscale Modeling of Agglomerated Ceria Nanoparticles: Interface Stability and Oxygen Vacancy Formation

Authors: Byung-Hyun Kim, Jolla Kullgren, Matthew J. Wolf, Kersti Hermansson and Peter Broqvist

The interface formation and its effect on redox processes in agglomerated ceria nanoparticles (NPs) have been investigated using a multiscale simulation approach with standard density functional theory (DFT), the self-consistent-charge density functional tight binding (SCC-DFTB) method, and a DFT-parameterized reactive force-field (ReaxFF). In particular, we have modeled Ce40O80 NP pairs, using SCC-DFTB and DFT, and longer chains and networks formed by Ce40O80 or Ce132O264 NPs, using ReaxFF molecular dynamics simulations. We find that the most stable {111}/{111} interface structure is coherent whereas the stable {100}/{100} structures can be either coherent or incoherent. The formation of {111}/{111} interfaces is found to have only a very small effect on the oxygen vacancy formation energy, Evac. The opposite holds true for {100}/{100} interfaces, which exhibit significantly lower Evac values than the bare surfaces, despite the fact that the interface formation eliminates reactive {100} facets. Our results pave the way for an increased understanding of ceria NP agglomeration.

Front. Chem., Vol. 7, article id 203,  22 May 2019