Applications of density functional theory for modeling metal-semiconductor contacts, reaction pathways, and calculating oxidation states

Abstract Density functional theory (DFT) is a well-established tool for calculating the properties of materials. The volume of DFT-related publications doubles every 5–6 years, which has resulted in the appearance of continuously growing open material databases, containing information on millions of compounds. Furthermore, the results of DFT computations are frequently coupled with experimental ones to strengthen the computational findings. In this thesis, several applications of DFT related to physics and chemistry are discussed. The conductivity between MoS₂ and transition metal nanoparticles is evaluated by calculating the electronic structure of two different models for the nanoparticles. Chemical bonding of Ni to the MoS₂ host is proven by the system’s band alignment. To meet the demand for cleaner fuel, the applicability of the (103) edge surface of molybdenum disulfide in relation to the early stages of the hydrodesulfurization (HDS) reaction is considered. The occurrence of the (103) edge surface of molybdenum disulfide in the XRD patterns is explained. A method for calculating oxidation states based on partial charges using open materials databases is suggested. We estimate the applicability of the method in the case of mixed valence compounds and surfaces, showing that DFT calculations can be used for the estimation of oxidation states.Tiivistelmä Tiheysfunktionaaliteoria (density functional theory, DFT) on yleisesti käytetty työkalu laskennallisessa materiaalitutkimuksessa. DFT:llä tuotettujen julkaisujen määrä kaksinkertaistuu 5–6 vuoden välein, minkä johdosta käytettävissä on jatkuvasti kasvava määrä avoimia materiaalitietokantoja, joihin on talletettu miljoonien yhdisteiden ominaisuuksia. DFT-laskujen tuloksia täydennetään myös usein kokeellisilla tuloksilla. Tässä työssä tarkastellaan tiheysfunktionaaliteorian sovelluksia fysiikassa ja kemiassa. MoS₂:n ja metallisten nanopartikkelien välistä johtavuutta on tutkittu mallintamalla erilaisia nanopartikkeleita. Nikkelin ja MoS₂:n välinen kemiallinen sidos selittyy systeemin energiavöiden kohdistumisella. MoS₂:n (103)-pinnan soveltuvuutta rikinpoistoreaktion varhaisissa vaiheissa on tutkittu tarkoituksena löytää uusia menetelmiä puhtaan polttoaineen tuottamiseksi. Myös (103)-pinnan esiintyminen röntgendiffraktiokuvissa selitetään. Työssä on myös esitetty menetelmä hapetustilojen laskemiseksi tietokannoista löytyvien laskettujen varausjakaumien avulla. Menetelmän soveltuvuutta on tarkasteltu erilaisille yhdisteille ja pinnoille. Tämä tarkastelu osoittaa, että DFT-tuloksia voidaan käyttää hapetustilojen laskemiseen

Surface morphology and sulfur reduction pathways of MoS₂ Mo edges of the monolayer and (100) and (103) surfaces by molecular hydrogen:a DFT study

Abstract We have performed a density functional theory study of the MoS₂ monolayer and the MoS₂ (100) and (103) surfaces in relation to the early stages of the hydrodesulfurization reaction. In many X-ray diffraction (XRD) results, the (103) surface exhibits a higher peak than the (100) surface, yet one of the most frequently occurring surface has not been studied extensively. By analyzing experimental studies, we conclude that the (103) surface of MoS₂ is the most frequently occurring edge surface when the sample size is thicker than ∼10–15 nm. Herein, we report the first comparison of reaction paths for the formation of a sulfur vacancy on the (103) surface of MoS₂, monolayer, and (100) surface of MoS₂. The reason for the occurence of the (103) surface in the XRD patterns has been established. We point out the similarity in the reaction barriers for the monolayer and (100) and (103) surfaces and discuss the reason for it. Moreover, we found a more energetically favorable step in the reaction pathway for the formation of a sulfur vacancy, which allowed us to refine the previously established pathway

A DFT study of the effect of SO₄ groups on the properties of TiO₂ nanoparticles

Abstract We present a study of the optical, electronic, and structural properties of TiO₂ anatase-structured nanoparticles upon adsorption of SO₄ groups, which are always present on the surface of the particles during the sulfate manufacturing method. Structural and electronic properties were studied using the density functional theory method (DFT), and optical properties were obtained by time-dependent DFT. It was found that SO₄ groups alter both the geometric and electronic structure of TiO₂ nanoparticles and change the photoabsorption characteristics. In particular, we find that ɳ²-O₂ type O–O moieties are formed due to the adsorption of 3 and 4SO₄ groups

Oxidation States of Binary Oxides from Data Analytics of the Electronic Structure

A connection between the oxidation state (OS) and Bader charge has been missing so far. To our knowledge, all previous work tried to connect OS with Bader charges only with few compounds. The aim of this work was to find a dependency between OS and Bader charge, using a large number of compounds from an open database. We show that a correlation indeed exists between OSs and Bader charges using the simplest machine learning algorithm, linear regression. The applicability of determining OS by Bader charges in mixed-valence compounds and surfaces is considered.</p