5,214 research outputs found
Using Density Functional Theory to Model Realistic TiO2 Nanoparticles, Their Photoactivation and Interaction with Water
Computational modeling of titanium dioxide nanoparticles of realistic size is
extremely relevant for the direct comparison with experiments but it is also a
rather demanding task. We have recently worked on a multistep/scale procedure
to obtain global optimized minimum structures for chemically stable spherical
titania nanoparticles of increasing size, with diameter from 1.5 nm (~300
atoms) to 4.4 nm (~4000 atoms). We use first self-consistent-charge density
functional tight-binding (SCC-DFTB) methodology to perform thermal annealing
simulations to obtain globally optimized structures and then hybrid density
functional theory (DFT) to refine them and to achieve high accuracy in the
description of structural and electronic properties. This allows also to assess
SCC-DFTB performance in comparison with DFT(B3LYP) results. As a further step,
we investigate photoexcitation and photoemission processes involving
electron/hole pair formation, separation, trapping and recombination in the
nanosphere of medium size by hybrid DFT. Finally, we show how a recently
defined new set of parameters for SCC-DFTB allows for a proper description of
titania/water multilayers interface, which paves the way for modeling large
realistic nanoparticles in aqueous environment
Anisotropic Effects of Oxygen Vacancies on Electrochromic Properties and Conductivity of -Monoclinic WO
Tungsten trioxide (WO) is a paradigmatic electrochromic material, whose
peculiar optical properties in the presence of oxygen vacancies or intercalated
alkali atoms have been observed and investigated for a long time. In this paper
we propose a rationalization of experiments based on first-principles
calculations of optical and electrical properties of oxygen deficient (reduced)
WO. Our approach is based on a parameter-free dielectric-dependent hybrid
density functional methodology, used in combination with the charge transition
levels formalism, for studying excitation mechanisms in the presence of
defects. Our results indicate that oxygen vacancies lead to a different physics
in -monoclinic WO, depending on the orientation of the W-O-W chain
where the vacancy is created, thus evidencing strong anisotropic effects rooted
in the peculiar structural properties of the original nondefective monoclinic
cell. Different types of oxygen vacancies can hence be classified on the basis
of the calculated ground state properties, electronic structure, and
excitation/emission energies, giving a satisfactory explanation to a range of
experimental observations made on oxygen deficient WO.Comment: Accepted for publication in J. Phys. Chem.
Business Processes as Social Entities – A Use Case Driven Approach
Social media represents one of the most rapidly growing trends in information technology. Up to now, however, this trend has been mainly driven by private users, although in organizational context social media technologies would provide a multiplicity of benefits. This paper shows – based on a state of the art analysis – application potentials of social media in the field of business process management (BPM). The focus lies on the interplay between the subjects and objects of BPM and social media. The goal is to turn business processes into social entities with the capability to communicate and to take on identity. Based on a holistic idea finding process, several potential use cases are arranged within the phases of the BPM cycle. The results show that unexploited potentials exist particularly in the phases of “Business Process Design” and “Business Process Controlling”.
Available at: https://aisel.aisnet.org/pajais/vol5/iss3/5
Are Renewables Profitable in 2030? A Comparison between Wind and Solar across Europe
The European Union has set ambitious targets for emission reduction and the penetration of renewable energy, including the electricity generation sector as one of the major emitters of CO2. After a period of subsidy-driven investments, the costs of renewables decreased strongly making investments more attractive. Since European countries differ strongly in terms of natural resources, we analyse the profitability of wind onshore and offshore and solar PV across Europe to determine where it is optimal to invest in the future and to understand which factors drive the profitability of the investments. We use a power systems model to simulate the whole European electricity market in 2030. Using the renewable revenues determined by the model, we calculate the internal rate of return to analyse how profitable each technology is in each country. We find that investments in the considered technologies are not homogeneously profitable across Europe. This suggests that cooperation between European countries can be expected to achieve the overall targets at lower costs than nationally-driven approaches. We also find that in many countries, wind onshore and solar PV are profitable by 2030 in absence of any financial support. Wind offshore does not seem to be profitable without financial support
Electronic structure and phase stability of oxide semiconductors: Performance of dielectric-dependent hybrid functional DFT, benchmarked against band structure calculations and experiments
We investigate band gaps, equilibrium structures, and phase stabilities of
several bulk polymorphs of wide-gap oxide semiconductors ZnO, TiO2,ZrO2, and
WO3. We are particularly concerned with assessing the performance of hybrid
functionals built with the fraction of Hartree-Fock exact exchange obtained
from the computed electronic dielectric constant of the material. We provide
comparison with more standard density-functional theory and GW methods. We
finally analyze the chemical reduction of TiO2 into Ti2O3, involving a change
in oxide stoichiometry. We show that the dielectric-dependent hybrid functional
is generally good at reproducing both ground-state (lattice constants, phase
stability sequences, and reaction energies) and excited-state (photoemission
gaps) properties within a single, fully ab initio framework.Comment: Minor changes in the final published versio
Defect calculations in semiconductors through a dielectric-dependent hybrid DFT functional: the case of oxygen vacancies in metal oxides
We investigate the behavior of oxygen vacancies in three different
metal-oxide semiconductors (rutile and anatase TiO2, monoclinic WO3, and
tetragonal ZrO2) using a recently proposed hybrid density-functional method in
which the fraction of exact exchange is material-dependent but obtained ab
initio in a self-consistent scheme. In particular, we calculate
charge-transition levels relative to the oxygen-vacancy defect and compare
computed optical and thermal excitation/emission energies with the available
experimental results, shedding light on the underlying excitation mechanisms
and related materials properties. We find that this novel approach is able to
reproduce not only ground-state properties and band structures of perfect bulk
oxide materials, but also provides results consistent with the optical and
electrical behavior observed in the corresponding substoichiometric defective
systems.Comment: Accepted for publication in J. Chem. Phy
Reactive molecular dynamics simulations of hydration shells surrounding spherical TiO 2 nanoparticles: Implications for proton-transfer reactions
In many potential applications, nanoparticles are typically in an aqueous medium. This has strong influence on the stability, optical properties and reactivity, in particular for their functionalization. Therefore, the understanding of the chemistry at the interface between the solvent and the nanoparticle is of utmost importance. In this work, we present a comparative ReaxFF reactive molecular dynamics investigation on spherical TiO2 nanoparticles (NSs) of realistic size, with diameters from 2.2 to 4.4 nm, immersed in a large drop of bulk water. After force field validation for its use for a curved anatase TiO2 surface/water interface, we performed several simulations of the TiO2 nanoparticles of increasing size in a water drop. We found that water can be adsorbed jointly in a molecular and dissociative way on the surface. A Langmuir isotherm indicating an adsorption/desorption mechanism of water on the NS is observed. Regarding the dissociative adsorption, atomistic details reveal two different mechanisms, depending on the water concentration around the NS. At low coverage, the first mechanism involves direct dissociation of a single water molecule, whereas, at higher water coverage, the second mechanism is a proton transfer reaction involving two water molecules, also known as Grotthuss-like mechanism. Thermal annealing simulations show that several water molecules remain on the surface in agreement with the experimental reports. The capacity of adsorption is higher for the 2.2 and 3.0 nm NSs than for the 4.4 nm NS. Finally, a comparative investigation with flat surfaces indicates that NSs present a higher water adsorption capacity (undissociated and dissociated) than flat surfaces, which can be rationalized considering that NSs present many more low-coordinated Ti atoms available for water adsorption. This journal is.Fil: Soria, Federico Ariel. Universita Di Milano Bicocca; Italia. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - CĂłrdoba. Instituto de Investigaciones en FĂsico-quĂmica de CĂłrdoba. Universidad Nacional de CĂłrdoba. Facultad de Ciencias QuĂmicas. Instituto de Investigaciones en FĂsico-quĂmica de CĂłrdoba; ArgentinaFil: Di Valentin, Cristiana. UniversitĂ Di Milano Bicocca; Itali
Communication: Hole localization in Al-doped quartz SiO2 within ab initio hybrid-functional DFT
We investigate the long-standing problem of the hole localization at the Al
impurity in quartz SiO, using a relatively recent DFT hybrid-functional
method in which the exchange fraction is obtained \emph{ab initio}, based on an
analogy with the static many-body COHSEX approximation to the electron
self-energy. As the amount of the admixed exact exchange in hybrid functionals
has been shown to be determinant for properly capturing the hole localization,
this problem constitutes a prototypical benchmark for the accuracy of the
method, allowing one to assess to what extent self-interaction effects are
avoided. We obtain good results in terms of description of the charge
localization and structural distortion around the Al center, improving with
respect to the more popular B3LYP hybrid-functional approach. We also discuss
the accuracy of computed hyperfine parameters, by comparison with previous
calculations based on other self-interaction-free methods, as well as
experimental values. We discuss and rationalize the limitations of our approach
in computing defect-related excitation energies in low-dielectric-constant
insulators.Comment: Accepted for publication in J. Chem. Phys. (Communications
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