Density Functional Based Tight Binding Research Articles

A theoretical study of (9, 0) Singlewalled Carbon Nanotubes using quantum mechanical techniques

First principles simulation studies using the density functional theory have been performed on (9, 0) Zigzag Singlewalled Carbon Nanotube (SWCNT) to investigate its electronic, optical and thermodynamic properties using CASTEP (Cambridge Sequential Total Energy Package) and DFTB (Density Functional based Tight Binding) modules of the Material Studio Software version 7.0. Various functionals and sub-functionals available in the CASTEP Module (using Pulay Density Mixing treatment of electrons) and various eigen-solvers and smearing schemes available in the DFTB module (using smart algorithm) have been tried out to chalk out the electronic structure. The analytically deduced values of the band gap obtained were compared with the experimentally determined value reported in the literature. By comparison, combination of Anderson smearing scheme and standard diaogonalizer produced best results in DFTB module while in the CASTEP module, GGA (General Gradient approximation) functional with RPBE (Revised-perdew-Burke-Ernzerh) as Sub-functional was found to be the most consistent. These optimized parameters were then used to determine various electronic, optical and thermodynamic properties of (9, 0) Singlewalled Nanotube. (9, 0) Singlewalled Nanotube, which is extensively being used for sensing NH3, CH4 & NO2, has been picked up in particular as it is reported to exhibit a finite energy band gap in contrast to its expected metallic nature. The study is of utmost significance as it not only probes and validates the simulation route for predicting suitable properties of nanomaterials but also throws light on the comparative efficacy of the different approximation and rationalization quantum mechanical techniques used in simulation studies.

Dynamical simulation of collision-induced dissociation of pyrene dimer cation

We report a theoretical investigation of the collision-induced dissociation of pyrene dimer cation, as recently investigated in the experimental work by Zamith et al. (J. Chem. Phys. 153, 054311 (2020)). Molecular dynamics simulations using potential energies and forces computed at the self-consistent charge density functional-based tight binding level were conducted for different collision energies between 2.5 and 30 eV. It appears that most of the dissociation occurs on a short timescale (less than 3 ps). The dynamical simulations allow to visualize the dissociation processes. At low collision energies, the dissociation cross section increases with collision energies, whereas it remains almost constant for collision energies greater than 10-15 eV. The analysis of the kinetic energy partition is used to get insights into the collision/dissociation processes at the atomic scale. The simulated time-of-flight mass spectra of parent and dissociation products are obtained from the combination of molecular dynamics simulations and phase space theory to address the short and long timescales dissociation, respectively. The agreement between the simulated and measured mass spectra suggests that the main processes are captured by this approach.

Exploration of Sunflower Oil As a Renewable Biomass Source to Develop Scalable and Highly Effective Corrosion Inhibitors in a 15% HCl Medium at High Temperatures.

The feasibility study of utilizing sunflower oil as renewable biomass source to develop highly effective inhibitors for mild steel corrosion (MS) in the 15% HCl medium was done by weight loss, potentiodynamic polarization (PDP), dynamic electrochemical impedance spectroscopy (DEIS), and electrochemical impedance spectroscopy (EIS), supported with energy-dispersive X-ray (EDX), atomic force microscopy (AFM), and field-emission scanning electron microscope (FESEM) techniques. Moreover, a complementary theoretical investigation was carried out to clarify the inhibition mechanism of inhibitors by density functional theory (DFT), density functional based tight-binding (DFTB), and molecular dynamics (MD) simulation approaches. The obtained results confirm that sunflower-oil-based corrosion inhibitor (SFOCI) has a significant anticorrosion property toward the dissolution of MS in 15% HCl solution in the temperature range 20-80 °C. In addition, the results show that SFOCI could provide an inhibition efficiency of 98 and 93% at 60 and 80 °C, respectively. The inhibition mechanism of SFOCIs was mixed-type and their adsorption on the surface of MS was mainly chemisorption. The FESEM and EDX studies proved the presence of SFOCI molecules on the surface of MS. In addition, the adsorption energy of SFOCI indicated an intense interaction between the inhibitor and surface of Fe. The results of this study could open a new window for the design and development of scalable and effective eco-friendly vegetable-oil-based corrosion inhibitors for highly corrosive solutions at high temperatures.

Comparison of computational chemistry methods for the discovery of quinone-based electroactive compounds for energy storage

High-throughput computational screening (HTCS) is a powerful approach for the rational and time-efficient design of electroactive compounds. The effectiveness of HTCS is dependent on accuracy and speed at which the performance descriptors can be estimated for possibly millions of candidate compounds. Here, a systematic evaluation of computational methods, including force field (FF), semi-empirical quantum mechanics (SEQM), density functional based tight binding (DFTB), and density functional theory (DFT), is performed on the basis of their accuracy in predicting the redox potentials of redox-active organic compounds. Geometry optimizations at low-level theories followed by single point energy (SPE) DFT calculations that include an implicit solvation model are found to offer equipollent accuracy as the high-level DFT methods, albeit at significantly lower computational costs. Effects of implicit solvation on molecular geometries and SPEs, and their overall effects on the prediction accuracy of redox potentials are analyzed in view of computational cost versus prediction accuracy, which outlines the best choice of methods corresponding to a desired level of accuracy. The modular computational approach is applicable for accelerating the virtual studies on functional quinones and the respective discovery of candidate compounds for energy storage.

Theoretical Analysis of Photo- and Electrochemical Properties of Anatase-Type TiO2 Nanoparticles

Oxide nanoparticles, like TiO2 nanoparticles are often applied in electrodes and photocatalytic systems. The morphology and shape of these nanoparticles can be easily tuned with experimental conditions, which raises the question how much the electro- and photochemical properties are affected by the nanoparticle geometry.We studied the electronic states around the bandgap in anatase-type TiO2 nanoparticles of different shapes with (101) and (001) facets using Density Functional-based Tight Binding (DFTB) method. Previous calculation has already shown that most orbitals of the valence and conduction bands forms delocalized band-like orbitals, however there are a significant number localized states at the lower at edge of conduction band, too. These trapping sites may play important role in electrochemistry when located on the surface of the nanoparticle.Photochemical properties however depend on optical transitions between electronic states with proper symmetry, and mostly dominated by delocalized states of atoms in the bulk. In this talk we analyze the optical spectra (see Figure 1) on the nanoparticles and compare it to the electrochemically important surface trapping Figure 1

Detoxification and Recovery of Spent Carbon Cathodes via NaOH–Na2CO3 Binary Molten Salt Roasting–Water Leaching: Toward a Circular Economy for Hazardous Solid Waste from Aluminum Electrolysis

The conversion of solid waste into secondary resources has become a prerequisite for the sustainable development of the industry. This study focused on the development of the detoxification process for spent carbon cathode (SCC) without environmental pollution risks and the exploration of the recycling direction of recovered carbonaceous materials. First, the mechanism of thermodynamic conversion and decomposition of toxic substances was explored. Parameters of NaOH–Na₂CO₃ binary molten salt roasting were optimized through an orthogonal design and a single-factor experiment, and gas release behavior during detoxification treatment was investigated. Results showed that most of the fluoride and cyanide were dissociated and decomposed, and the leaching concentration of fluoride and cyanide was as low as 7.58 and 0.12 mg/L after detoxification treatment, respectively. Then, a multistage recovery process of fluorine from fluorine-containing leachate was designed, and the comprehensive utilization of exhaust gas and the recycling of the reaction reagent were realized simultaneously. The possibility of recovering graphitized carbon blending to prepare carbon anodes was discussed in detail, and the catalytic mechanism of main impurity elements on the O₂/CO₂ reactivity of carbon anodes was investigated via density-functional-based tight binding (DFTB). The results showed that the Boudouard reaction and carbon–O₂ reaction were affected because of the presence of Ca or Na, and the recovered graphitized carbon with a low blending ratio was more suitable for the preparation of carbon anodes. Finally, an economic benefit analysis was conducted to evaluate the industrial application potential of the proposed detoxification treatment. Compared with landfills, the proposed process reduced the disposal cost while eliminating environmental pollution risks and realized the on-site recovery and reuse of valuable components, showing considerable environmental and economic benefits.

Solid-State 17O NMR Studies of Sulfonate Jump Dynamics in Crystalline Sulfonic Acids: Insights into the Hydrogen Bonding Effect.

We report variable-temperature (VT) 17O solid-state nuclear magnetic resonance (NMR) spectra for three crystalline sulfonic acids: l-cysteic acid monohydrate (CA), 3-pyridinesulfonic acid (PSA), and p-toluenesulfonic acid monohydrate (TSA). We were able to analyze the experimental VT 17O NMR spectra to obtain the activation barriers for SO3- jumps in these systems. Using the density functional-based tight-binding (DFTB) method, we performed potential energy surface scans for SO3- jumps in the crystal lattice of CA, PSA, and TSA, as well as for three related crystalline sulfonic acids (taurine, homotaurine, and 4-aminobutane-1-sulfonic acid) for which relevant 17O solid-state NMR data are available in the literature. The calculated activation barriers are in reasonable agreement with the experimental values. On the basis of the DFTB results, we hypothesized that activation barriers for SO3- jumps in the crystal lattice depend largely on the hydrogen bonding energy difference between the ground state and the transition state.

Simulation of Structural Evolution Using Time-Dependent Density-Functional Based Tight-Binding Method.

A faster and more efficient quantum mechanical simulation method for application to complicated issues of real systems beyond model cases has long been sought after. The density-functional based tight-binding (DFTB) method has successfully explained the atomistic and electronic properties of semiconductors, surfaces, and nanostructures. In addition, the time-dependent formalism implemented in DFTB showed high efficiency in terms of computational cost. In this study, we demonstrated the structural and electronic evolution of small molecules induced by a laser pulse using the time-dependent DFTB (TD-DFTB) method. We identified the critical fluence of the input laser for structural dissociations in carbon chains and fullerenes, which related to the structural stability. The excitation energies of several molecules calculated by TD-DFTB agreed with the experimental values.

Propyne Gas Adsorption in a Cyclic Hexapeptoid: A Combined In Situ XRPD and DFTB Study*.

Cyclic peptoids are macrocyclic N-substituted oligoglycines, with remarkable structural, chemical and physical properties. The gas adsorption properties of a permanently porous hexameric cyclopeptoid decorated with four propargyl and two methoxyethyl side chains were monitored by in situ X-ray powder diffraction (XRPD). High-resolution XRPD data together with Rietveld and density functional based tight binding (DFTB) method allowed us to locate propyne guest molecules inside the host channels, even though the powder sample contains more than one phase. We were able to characterize the host-guest interactions, providing useful information on the host recognition sites and discuss host adaptiveness and host-guest chemical affinity in comparison with analogous compounds.

Effect of pore-size disorder on the electronic properties of semiconducting graphene nanomeshes

Graphene nanomeshes (GNMs) are novel materials that recently raised a lot of interest. They are fabricated by forming a lattice of pores in graphene. Depending on the pore size and pore lattice constant, GNMs can be either semimetallic or semiconducting with a gap large enough (∼ 0.5 eV) to be considered for transistor applications. The fabrication process is bound to produce some structural disorder due to variations in pore size. Recent electronic transport measurements in GNM devices (ACS Appl. Mater. Interfaces 10, 10 362, 2018) show a degradation of their bandgap in devices having pore-size disorder. It is therefore important to understand the effect of such variability on the electronic properties of semiconducting GNMs. In this work we use the density functional-based tight binding formalism to calculate the electronic properties of GNM structures with different pore sizes, pore densities, and with hydrogen and oxygen pore edge passivations. We find that structural disorder reduces the electronic gap and the carrier group velocity, which may interpret recent transport measurements in GNM devices. Furthermore, the trend of the bandgap with structural disorder is not significantly affected by the change in pore edge passivation. Our results show that even with structural disorder, GNMs are still attractive from a transistor device perspective.

Ab-initio search for efficient red thermally activated delayed fluorescence molecules for organic light emitting diodes.

In this work, we present a computational study on 105 selected organic molecules in order to find suitable candidates for using as thermally activated delayed fluorescent (TADF) emitters in organic light emitting diodes (OLEDs), in the emission range of red light. Based on time-dependent density functional theory (TD-DFT) computations, three promising candidates were found, predicted to have low singlet-triplet splittings, lower than 0.06eV, and TADF rates of 0.124, 0.154 and 0.231 1/μs. Then, using an experimental-theory calibration approach, the emission wavelength of the molecules were estimated to be 570, 476, and 623nm, respectively. For the molecule whose emission wavelength (623nm) is predicted to be in our desired range, we measured the photoluminescence (PL) spectrum and find out that its emission peak is within the predicted accuracy of the employed method. Moreover, we benchmarked the performance of density functional based tight-binding (DFTB) method for future screening works and find out that, this method is an efficient pre-screening tool, useful in searching for molecules with desired emission wavelengths.

Structural, electronic, and thermodynamic properties of TiO2/organic clusters: performance of DFTB method with different parameter sets

AbstractThe clusters of bare TiO2 and TiO2 with linked organic ligands modeling polyorganic composites used as photocatalytic materials were studied using the density functional based tight binding (DFTB) electronic structure method with three parameter sets (trans3d, tiorg, and matsci) in comparison with results of B3LYP/6‐31G(d,p) calculations, semiempirical methods PM6 and PM7, and available experimental data. It was found that the highly scalable DFTB method shows results that are close to the B3LYP/6‐31G(d,p) level of theory. The corrected version of the tiorg DFTB parameter set (tiorg‐smooth) has better performance for the estimations of structural parameters, whereas the trans3d set better reproduces energies of the composite material formation in polycondensation reactions. Performance of the matsci set is somehow in the middle of the tiorg‐smooth and trans3d sets. The tiorg‐smooth and matsci sets can be used for the studies of adsorption complexes of bare TiO2 clusters. All three DFTB parameter sets well estimate the electronic parameters of clusters (HOMO‐LUMO gap, ionization potential, and dipole moment). DFTB results are closer to the estimates made with DFT (B3LYP/6‐31G(d,p)) than the results of PM6 and PM7 methods. DFTB calculations of large (up to 448 atoms) bare TiO2 and TiO2/organic clusters (72 structures in total) confirm the robustness and computational efficiency of the method.

A theoretical and experimental study of castor oil-based inhibitor for corrosion inhibition of mild steel in acidic medium at elevated temperatures

The castor oil as a cheap and environmentally friendly source was used to prepare a novel green corrosion inhibitor for mild steel in acidic medium. The poor performance at high temperatures and low biodegradability are two important challenges of corrosion inhibitors that castor oil-based corrosion inhibitor (COCI) is designed to overcome them. The inhibition performance of COCI was evaluated by weight loss, open circuit potential (OCP), electrochemical impedance spectroscopy (EIS) and polarization techniques. The morphology of MS was examined by field emission scanning electron microscope (FESEM), energy dispersive X-ray (EDX) and atomic force microscopy (AFM). Furthermore, a comprehensive computational study was done to clarify the anticorrosive mechanism of COCI by molecular dynamics (MD) simulation, density functional based tight-binding (DFTB) approach and density functional theory (DFT). Maximum inhibition efficiency of 85 % and 91 % were achieved at 80 °C using 140 μM of COCI in EIS and polarization tests, respectively. COCI acted as a mixed type inhibitor and the adsorption of the inhibitor on the mild steel surface was chemisorption. The obtained results demonstrate that the COCI has a considerable ability to suppress acidic corrosion at all concentrations were studied here, particularly at high temperatures. Given the fact that several articles were reported a significant reduction in performance of corrosion inhibitors by increasing temperature, in this study was found out not only was there no decrease in inhibition efficiency of COCI at higher temperatures, but it depicted good performance at 80 °C. In addition, the high adsorption energy of COCI confirmed a strong interaction between Fe surface and inhibitor molecules and showed a significant effect of the urethane bonds and the triglyceride groups on molecular activity. The Fermi energy state indicated that the chemical adsorption occurred in the interaction of COCI-Fe surface by electron transition from inhibitor to Fe 3d band state. We expect the results of this work provide new opportunities for the design and synthesis of efficient corrosion inhibitors based on vegetable oils.

THERMODYNAMIC CALCULATIONS OF ION-EXCHANGE SORBATION OF IRON IONS ON CLINOPTYLOLITE

Increased content of iron in water, impairs its quality and, in many cases, makes it impossible to use such water for home and industrial needs. Although iron is the basis of many biochemical processes in the human body, its excess content can have many negative effects. According to WHO standards, the permissible content of iron in drinking water is 0.3 mg/l. The high iron content is observed in the natural waters of many regions ofUkraineand other countries. This phenomenon is often not connected with anthropogenic influence but caused by the peculiarity of the geomorphological structure of the area. Therefore, the development of new and improvement of the existing methods of water desalination (iron removal) is a pressing issue.The methods of water desalination can be divided into four different categories: traditional techniques, biological methods, processes based on membrane technologies, and methods based on nanotechnologies. Among the traditional methods, one of the main approaches is ion-exchange.In this work, quantum-chemical calculations of iron ion exchange processes on clinoptilolite (structural type HEU) and its modified forms are carried out. The calculations used the software xTB. The semi-empirical GFN2-xTB method was chosen as the primary method, based on a modified density functional theory, namely the Density Functional based Tight Binding (DFTB) approach.Iron atoms were considered in oxidation states 2 and 3. Also, they considered the possible electronic states of each of the ions and sorption products (low spin state - multiplicity 1 and 2 in the case of Fe2+ and Fe3+, respectively, and high spin state - multiplicity 5 and 6 in the case of Fe2+ and Fe3+, respectively).Fe(H2O)6n+ + Zeo-X(H2O)a = X(H2O)6 + Zeo-Fe(H2O)b + y (H2O)6 (1) The thermodynamic calculations of the ion exchange (1) indicate the possibility of the forward reaction run (ΔG <0). In the case of monovalent sodium exchange ions, ion exchange is possible both Fe2+ and Fe3+. In the case of double-charge calcium exchange ions, ion exchange is possible for Fe3+ ions. Ion exchange involving Fe3 + ions is much more energetically favorable than Fe2+ ions.

Theoretical Analysis of Photo- and Electrochemical Properties of Anatase Nano-Particles

Oxide nanoparticles, like TiO2 nanoparticles are often applied in electrodes and photocatalytic systems. We studied the electronic states around the bandgap in anatase-type TiO2 nanoparticles of different shapes with (101) and (001) facets using Density Functional based Tight Binding (DFTB) method. Previous calculation has already shown that most orbitals of the valence and conduction bands forms delocalized band-like orbitals, however there are a significant number localized states at the lower at edge of conduction band, too.The energy and position of these unoccupied localized orbitals can be controlled by the shape of the nanoparticle and these orbitals can play important contribution to both electrochemical and photochemical properties. TD-DFTB calculations showed that the optical spectrum around the band gap is dominated by few transition with large oscillator strength (see Figure), while most of the transitions give only a weak contribution. Figure 1

DFTB+, a software package for efficient approximate density functional theory based atomistic simulations.

DFTB+ is a versatile community developed open source software package offering fast and efficient methods for carrying out atomistic quantum mechanical simulations. By implementing various methods approximating density functional theory (DFT), such as the density functional based tight binding (DFTB) and the extended tight binding method, it enables simulations of large systems and long timescales with reasonable accuracy while being considerably faster for typical simulations than the respective ab initio methods. Based on the DFTB framework, it additionally offers approximated versions of various DFT extensions including hybrid functionals, time dependent formalism for treating excited systems, electron transport using non-equilibrium Green's functions, and many more. DFTB+ can be used as a user-friendly standalone application in addition to being embedded into other software packages as a library or acting as a calculation-server accessed by socket communication. We give an overview of the recently developed capabilities of the DFTB+ code, demonstrating with a few use case examples, discuss the strengths and weaknesses of the various features, and also discuss on-going developments and possible future perspectives.

A theoretical study for the band gap energies of the most common silica polymorphs

Although numerous theoretical studies are available for the band gap energies of distinct silica (SiO2) polymorphs, some of the calculated results of these former studies still remain inconsistent and mostly disagree with the experimental data of the investigated polymorphs. To obtain more reasonable results, we have focused on the band gap energies of eleven different and more common silica polymorphs in this research by employing the self consistent charge (SCC) variant of the density functional based tight binding (DFTB) method. The surveyed eleven common silica polymorphs in this study are namely: α-quartz, β-quartz, α-tridymite, β-tridymite, α-cristobalite, β-cristobalite, α-moganite, β-moganite, stishovite, coesite, and keatite Our obtained band gap energy results for α-quartz and stishovite are undoubtedly reasonable when compared with the existing experimental data of these polymorphs. As well, the maximum band gap energy is found to be as 9.70 eV for the β-moganite polymorph, and the minimum band gap energy appears as 7.62 eV for the stishovite polymorph. Overall, the presently obtained results for the energy band gap of the considered silica polymorphs are satisfactory, and some of them are still waiting further experimental confirmation.

Crystal Structure Exploration of Boron Nitride Polymorphs Using Anharmonic Downward Distortion Following Method with Potential Energy Surface Modified by the Inverse of Lattice Volume

The boron nitride polymorphs were explored using the anharmonic downward distortion following (ADDF) method with the energy value of the density functional-based tight binding approximation modified by the inverse of the volume of a unit cell. Twelve novel boron nitride polymorphs were discovered. Previously, it has been difficult to search reaction pathways connecting two-dimensional structures such as hexagonal boron nitride to three-dimensional structures using the ADDF method. This problem was partially resolved using this approach.

Transport properties of Ag decorated zigzag graphene nanoribbons as a function of temperature: a density functional based tight binding molecular dynamics study

The systemic study of the electronic transport (ET) properties of transition metal (TM) functionalized graphene was done with the aid of self-consistent charge density functional based tight binding (DFTB) method. Results show that among the TM considered, Silver metal adsorbed in the surface of graphene and its lower dimensional analogue zigzag graphene nanoribbon (ZGNR) can open its gapless bandstructure. This can be attributed to the breaking of bond and inversion symmetry. Further, the inherent effect of phonons (lattice vibrations) on the transport properties Ag-adsorbed ZGNR were investigated based on DFTB molecular dynamics (MD) simulation. Results show that excellent ET properties can be attributed to the Ag and ZGNR interaction. In addition, due to the unceasing lattice vibration of the Ag/ZGNR, the ET changes. Knowledge about the quantum of vibration at temperature T is quite important to elucidate its role governing the resulting ET. As the phonon having shorter wave length significantly increases at elevated temperatures, the corresponding forward bias voltage across the Ag/ZGNR increases. There is an increase in the conductance of vibrating Ag/ZGNR at elevated temperatures. A single-gated field effect transistor based on Ag-adsorbed ZGNR can act as a potential semiconductor for modern electronic applications.

Attaching titania clusters of various size to reduced graphene oxide and its impact on the conceivable photocatalytic behavior of the junctions—a DFT/D + U and TD DFTB modeling

DFT/D + U and density functional based tight binding (DFTB) molecular modeling was used to investigate the role of the structural, electronic and optical properties of reduced graphene oxide surface (r-GO), hybridized with hydrated TiO2 moieties of various size, ranging from small molecular Ti2O4 clusters into extended Ti43O86 rutile type nanocrystals of ~5 nm diameter. The calculated adhesion energies, varying from −5.048 eV (r-GO|Ti2O4), −12.159 eV (r-GO|Ti5O10), −18.499 eV (r-GO|Ti15O30) to −42.484 eV (r-GO|Ti43O86), indicate high stability of these composites. It was shown that electronic interactions at the r-GO|(1 1 0)TiO2 interface give rise to net charge flow from the r-GO substrate towards the TiO2 moieties, analyzed in terms of the partial charge density 3D plots and an interfacial dipole moment formation. The DOS structure of the composites was calculated by means of the time dependent DFTB approach, and the position and composition of the VB and CB edges, along with the presence of weak mid-gap 2p C states originating from the intact graphene-like patches in the r-GO substrate were discussed in detail in the context of conceivable photocatalytic activity of the composites. The constructed band alignment diagram implies formation of the staggered type II scheme, with the electric field offset that is sensitive to the titania cluster size. In the case of the nano-reticular TiO2, where only a fraction of the Ti atoms is engaged in the Ti–O–C linkers formation, recombination of the photogenerated charges is inhibited owing to favorable spatial separation effect. For small molecular TiO2 clusters with all Ti cations anchored to the r-GO layer fast cross-relaxation quenches the beneficial interfacial charge separation effect, since the strong hybridization of the oxygen and carbon states provides a convenient pathway for the efficient electronic coupling between the CB edge states of r-GO and the VB edge states of the TiO2 moieties. A phenomenological model of the molecular r-GO|Ti2O4 and the reticular r-GO|Ti43O86 composites was constructed in account for different photocatalytic behavior of both junctions.