We surround ourselves with plenty of materials in our daily lives. This includes (but not limited to) clothes made from woven fibres, cutlery made from malleable metals, and toothbrush made from plastics. Especially silicon has had a profound impact on our society in the form of transistors in everyday electronics (TVs, cell phones, computers etc.). It is difficult to imagine a world without these helpful items and common for these materials is they are built in all three spatial directions with small building blocks. This is commonly referred to as 3D materials. A new class of materials are currently under heavy investigation by researchers referred to as 2D materials that are materials thinned down to the ultimate thinnest possible thickness. Not only heavily influences the dimensionality these materials chemically, but also electronically. This opens up for numerous wonderful possibilities for applications in catalysis, electronics, photonics etc. My project involves the synthesis and characterisation of the highly promising candidate MoS2, but also its heavier brother WS2. Both belonging to the family of transition metal dichalcogonides (TMDCs), which share similar structure and offers a wide variety of properties and both exist as 2D materials.These materials are synthesised on the versatile perovskite SrTiO3 widely used as substrate for growth of various thin films. I use various surface-sensitive techniques such as scanning tunnelling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) to investigate the structural and electronic properties of these materials.

Photocatalysts are materials capable of absorbing sunlight and using it to drive certain chemical reactions. Some of these reactions can be useful for producing carbon-neutral fuels, such as the splitting of water into H2 gas and the reduction of CO2. 

Layered, two-dimensional materials such as bismuth oxyhalides and inorganic metal disulfides are promising photocatalysts, and their catalytic performance can be further enhanced using techniques such as heterojunction formation, nano-texturing, and element doping. MoS2 is the second most widely studied 2D material after graphene, exhibiting notable photocatalytic properties, particularly when incorporated into heterostructures with other 2D materials such as graphene and carbon nitride.

An alternative approach to improving the photocatalytic abilities of MoS2 is chemical functionalisation, which involves the intercalation of cationic organic molecules within exfoliated layers of MoS2. The resulting hybrid material can adopt the desired properties of both the organic and inorganic components, and the functionality and properties of the unique material can therefore be tailored to improve photocatalytic performance, making this a promising area for future research.

The primary focus of the project is the development of hybrid MoS2 compounds with cationic organic molecules intercalated inside and outside of chemically exfoliated MoS2 layers. Additionally, relevant photocatalytic and electrocatalytic studies will be conducted on MoS2 and other 2D hybrid materials, to explore the intriguing properties and capabilities of these materials.

The intrinsic properties of crystals are determined by their atomic structure, which for most materials has long-range order – they are crystals. However, when like atoms do not have exactly the same position in each unit cell, the crystal is disordered. It is increasingly clear that subtle disorder controls many of the essential properties of advanced materials such as magnetism. A good example of disorder dictating properties is a frustrated magnet, where the symmetry of the crystal lattice prevents long range ordering of the magnetic moments in the material.

We recently derived the underlying equations for interpreting 3D diffuse magnetic neutron scattering on frustrated magnets independent on atomic order, and also provided the first direct reconstruction of the magnetic correlations, in this case in the mineral Bixbyite. [1]

This PhD project focuses on initially synthesizing single crystals of two classes of magnetic materials: i) Bixbyite (Fe_xMn_1-x)₂O₃ and related oxides and ii) Spinels such as Ni_xZn_1-xFe₂O₄.

Bixbyite exhibits both structural and magnetic disorder, whereas the end members Mn₂O₃ and Fe₂O₃ both have ordered magnetic phases suggesting that disorder determines the magnetism in this compound. If samples suitable for 3D-PDF measurements are produced these will be studied using both synchrotron X-ray and neutron scattering.

Spinels are one of the most important families of magnetic materials, yet proper single crystal crystallographic studies of these are lacking in the literature. They have the formula AB₂O₄ where the A and B atoms are distributed across tetrahedral and octahedral sites in the ccp lattice of the oxygen atoms, where the exact distribution depends on the atomic species and is quantified by the so-called degree of inversion. For the grown single crystals, initial focus will be advanced multi-temperature structural studies to identify overall disorder. Secondly, potential correlated disorder, both atomic and magnetic, will be studied using 3D-PDF methods.

[1] N. Roth et al., IUCr-J 2018, 5, 410–416

Structure-property relations lie at the very foundation of functional materials development. However, often the structural knowledge of disordered crystalline materials is limited by the techniques available for structure characterization. Techniques such as single crystal X-ray diffraction rely on the assumption of perfect periodicity – an assumption which breaks down in the case of highly disordered crystalline materials. This calls for the use of supplementary techniques which take into account more than just the Bragg-scattering from the average crystal structure, for instance the newly developed 3D-ΔPDF method. This method exploits the local structure information available in the single crystal X-ray diffuse scattering intensities, which enables us to obtain a detailed view of the real structure of disordered materials. This structural knowledge can be used to explain e.g. why certain materials have a much lower thermal conductivity than expected from their average structures. This is useful in the case of thermoelectric (TE) materials as the thermal conductivity is an important parameter when it comes to optimizing the efficiency of the interconversion between thermal and electrical energy. The goal of my project is to use this new method to obtain structural knowledge on a range of systems of interest for TE applications.

With the success of renewable sources of energy such as solar and wind energy, there is a general hope that we can halt climate change and drop carbon emissions. However, the current sources of renewable energy suffer from a production-consumption mismatch, that is, we are making the energy at another time than we use it. One way to solve this is batteries, another way is to simply use more energy when more is available. One usage for the surplus energy is electrocatalysis. Here reactants (such as water or carbon dioxide) are converted into valuable chemicals such as methanol and formic acid. These reactions are driven by electricity.

The amount of electricity spent can vary greatly. The presence of an electrocatalyst will make the reaction much cheaper in terms of energy. Currently most commercial electrocatalysts are based on expensive noble metals. This is driving up cost and preventing mass implementation due to the scarcity of these substances. The project aims to produce novel electrocatalysts based on earth-abundant materials such as carbon or some transition metals such as molybdenum. Such electrocatalysts are already starting to appear, but the search for them is mostly guided by trial-and-error. Here the aim is to use surface science tools to gain a deeper understanding of the materials at hand, which may guide the search for new sustainable electrocatalysts. Towards this goal the project will be using a variety of techniques such as x-ray photoelectron spectroscopy, electron microscopy and a custom built electrochemical cell will allow characterization of a model system under operating conditions.

In order to increase the use of sustainable energy sources like solar and wind, new solutions in terms of energy storage are needed to overcome their highly intermittent nature. For battery technologies to be relevant for grid-scale storage, the electrode materials must be low cost, environmentally benign, have high energy efficiency and long cycle life. Because of the requirement for cheap electrode materials, the choice of elements are limited to earth abundant transition metals such as iron, manganese, zinc and copper.

I am interested in research into new, environmentally compatible, aqueous battery technologies with low cost potential. My work is focused on Prussian blue analogues (PBA) as cathode material in aqueous batteries. PBAs are a large family of transition metal hexacyanoferrates with the general structural formula A_XMFe(CN)₆, where A is a cation and M is a transition metal. PBAs are practically insoluble and are structurally stable towards insertion/extraction of a wide range of ions, providing good cycling capabilities.