We are always looking for motivated students that would like to learn more about computational chemistry. Enlisted in this document are a few timely projects that you could pursue either in the form of a degree project or as a research training. You are also free to propose other projects that fit with the research profile of the Ångström Computational Materials Chemistry Group (ÅCMC). ÅCMC is a research group at the department of chemistry – Ångström and is headed by Prof. Kersti Hermansson.

The focus of ÅCMC is e-science for materials chemistry, i.e. molecules + materials + chemistry + computers + software. We develop multi-scale models and methods to bring simulations closer to the complex, large-scale dynamical problems of the real world, in fields such as catalysis, nano-chemistry, and hydration phenomena. Thus, we explore and explain physical and chemical phenomena involving metal-oxide surfaces and nano-particles, both under high vacuum and in more realistic environments, such as aqueous media. Water and aqueous solutions themselves are also of particular interest to the group.

CONTACT AND WEBPAGE:e-mail: kersti@kemi.uu.se

webpage: http://www.teoroo.kemi.uu.se

Degree (X-jobb) and small research training projects in computational

materials chemistry

CM+ -

Comparison of F impurities at the CeO2 and TiO2 surface

A comparison of the electronic strucutre of common defects in ceria. The left panels show the Density of States including projections onto the Fluoride and Hydroxide defects. The right hand panels show the simulated STM appearance, very similar for all three defects, using a bias voltage, Eb, of 3 eV. In the current project similar electronic structure simulations will be performed for titania.

0-3 +3Energy (eV)

Den

sity

of

stat

es (

arb.

uni

ts)

EB = -3eV

Vacancy

Fluoride

Hydroxide

Distance (Å)

Hei

ght (

Å)

Hei

ght (

Å)

Hei

ght (

Å)

-8 -4 4 80

CeO2 (ceria) and TiO2 (Titania) are two prototypical reducible metal oxides with broad applications in redox catalysis. We have recently shown that fluorine impurities at the surface are very different to distinguish from vacancies in scanning tunneling microscopy (STM) experiments. This in turn calls for a reinterpretation of the reactiviy of the surface which, largely based on the STM observation, have been assumed to be controlled by oxygen vacancies. It was recently suggested that such halide impurities in ceria could increase its reactivity at lower temperatures. In a soon to be published manuscript, we propose a mechanism in terms of which this observation could be explained for the case of F impurities. In this project, you will investigate whether a similar mechanism holds for F impurities in TiO2.

Bandgap opening in graphene by chemical reactions

Graphene, a single layer of sp2 bonded carbon atoms packed in a hexagonal lattice, i.e. a single layer of graphite, has been regarded as one of the most fascinating materials due to its exceptionally high electronic mobility (10 million times greater than copper), ultimate mechanical strength (100 times stronger than the strongest steel) and etc. However, graphene is a zero-gap semiconductor which limits applications in electronic devices, that makes a bandgap engineering so important. In the current project, we will investigate how chemical reactions such as hydrogen adsorption on the graphene surface can be used to tune the bandgap. We will use density functional theory (DFT) and density functional based tight binding (DFTB) methods.

A simgle graphene sheet. Defects such as vacncies, hetero atoms and adsorbates have a huge impact on the electronic properties of graphene. Theoretical simulation can be used to investigate precisly how this happens and allow us to suggest new ways to tailor graphene by "defect engineering".

Properties of small anions in water

The OH- and F- ions are isoelectronic and the similarities and differences between their solvation properties are rather intriguing. The experimental hydration enthalpy of the OH- ion lies in between that of F- and Cl- ions. It is being debated in the literature (and explored by several experimental groups) to what extent, and in what way, the small H atom of the OH- ion perturbs its hydration shell compared to the “spherical” hydration shells of the F- and Cl- ions. We have previously explored the hydroxide ion's properties in aqueous solution using molecular dynamics simulation based on Density Functional Theory, but will now move the focus to its surroundings in a comparative study of the three ions.

Typical water enviorment around a OH- ion in liquid from our simulation. How does this local enviorment change when OH- is replaced for a spherical ion like F- and Cl-?

Reactive oxygen species in metal oxide nanoparticles

The O2 molecule, O2- (superoxide),O2

2- (peroxide ion), and O species are examples of Reactive Oxygen Species (ROS) and display an important and intriguing oxygen chemistry. Based on simulation of the type to be performed here, we have explained why small CeO2 nanoparticles (CNPs) show such remarkable properties allowing CNPs to be used in nano-medicine and low temperature catalysis. Our mechanism which is shown in the figure below has been confirmed experimentally and is all to do with formation of ROS which is promoted by the special geometric and electronic structure of the CNPs. The current project will explore ROS formation on other metal oxide NPs e.g PrO2, TiO2, MgO, Na2O.

An illustration of our mechanism for "oxygen activation" in small CNPs. In the figure cerium atoms are black and oxygen is white. When exposed to a recuding atmosphere, stochiometric ceria particles (a) can give away some of the oxygen ions in the lattice to form the reduced state shown in (b). Our simulations have shown that when exposed to O2 molecules it is not energetically favourable for the reduced states to be re-oxidized to it's stoichiometric form but instead the particle transforms into a very reactive supercharged state (c) in which O2 has been converted to O2

-.

(a)

(b)(c)

Red.

Red.

Ox.