The Fermi Level in Hematite Doping, Band Alignment, and Charge Transitions Zur Erlangung des akademischen Grades Doktor-Ingenieur (Dr.-Ing.) genehmigte Dissertation von Christian Lohaus aus Seeheim-Jugenheim Tag der Einreichung: 05.10.2018, Tag der Prüfung: 07.02.2019 Darmstadt — D 17 1. Gutachten: Prof. Dr. W. Jaegermann 2. Gutachten: Ass.-Prof. Dr. O. Clemens Materials Science Department Surface Science Division The Fermi Level in Hematite Doping, Band Alignment, and Charge Transitions Genehmigte Dissertation von Christian Lohaus aus Seeheim-Jugenheim 1. Gutachten: Prof. Dr. W. Jaegermann 2. Gutachten: Ass.-Prof. Dr. O. Clemens Tag der Einreichung: 05.10.2018 Tag der Prüfung: 07.02.2019 Darmstadt — D 17 Bitte zitieren Sie dieses Dokument als: URN: urn:nbn:de:tuda-tuprints-85416 URL: http://tuprints.ulb.tu-darmstadt.de/8541 Dieses Dokument wird bereitgestellt von tuprints, E-Publishing-Service der TU Darmstadt http://tuprints.ulb.tu-darmstadt.de [email protected] Die Veröffentlichung steht unter folgender Creative Commons Lizenz: Namensnennung – Keine kommerzielle Nutzung – Keine Bearbeitung .0 International http://creativecommons.org/licenses/by-nc-nd/4.0/deed.de/ "If in doubt, Meriadoc, always follow your nose." Gandalf, Lord of the Rings Contents 1 Motivation 5 2 Fundamentals 9 2.1 Hematite - an overview over basic properties and applications . 9 2.2 Physics of transition metal oxides - a short introduction . 13 2.2.1 Conventional semiconductors in a nutshell . 13 2.2.2 Semiconducting properties in transition metal oxides . 18 2.2.3 Fermi levelpinningand charge transition points . 23 2.3 Methods.......................................... 25 2.3.1 Thin film deposition by magnetron sputtering . 25 2.3.2 Photoelectron Spectroscopy on transition metal oxides . 32 2.3.3 Raman Spectroscopy on iron oxides . 43 2.3.4 Brief introduction to X-ray diffraction . 46 2.3.5 Conductivity measurements . 48 3 Experimental Procedure 51 3.1 Thelab .......................................... 51 3.2 Samplepreparation.................................. 53 3.2.1 Sputter deposition and doping . 53 3.2.2 Sampletreatments............................... 54 3.3 SampleCharacterization............................. 57 3.3.1 PhotoelectronSpectroscopy . 57 3.3.2 Interfaceexperiments ............................. 62 3.3.3 RamanSpectroscopy ............................. 64 3.3.4 X-RayDiffraction................................ 64 3.3.5 Thickness Determination . 65 3.3.6 OpticalSpectroscopy ............................. 65 3.3.7 Conductivity measurements . 66 4 Setting the baseline - Phase verification 67 4.1 Oxygenpartialpressuredependencies . 68 4.2 Theinfluenceofthetemperature . 73 4.2.1 Depositionatheatedsubstrates . 73 4.2.2 In-situ heating . 76 4.2.3 Ex-situheating ................................. 82 4.3 Epitaxialhematitethinfilms .......................... 89 3 4.4 Surfacepotentialsofhematite . 98 4.5 Summary .........................................100 5 The electronic structure of the valence band of hematite 103 6 Fermi level manipulation of the bulk 109 6.1 Magnesium doping . 110 6.2 Silicon doping . 119 6.3 Zirconium doping . 126 6.4 Surfacepotentialsofdopedhematite . 132 6.5 Core-level to valence band maximum distance of doped hematite . 135 6.6 Opto-electronicpropertiesofdopedhematite . 137 6.7 Ondopingmechanismsandeffectsinhematite . 145 6.8 Summary .........................................147 7 Fermi level manipulation of the surface 149 7.1 Surface treatment by oxygen plasma and exposure to water . 150 7.1.1 Oxygenplasmatreatmentofhematite . 150 7.1.2 Exposureofhematitetowater. 153 7.2 Surface modification by atomic layer deposited alumina . 157 7.3 Interfaceexperiments ............................. 160 7.3.1 Interface to RuO2 ................................160 7.3.2 Interface to NiO . 162 7.3.3 Interface to ITO - Effect on Fe2O3 and Sn : In2O3 . 166 7.3.4 Interface to STO . 178 7.4 Summary .........................................180 8 Polarons in hematite and their implication for the band gap 183 9 Summary of results and Outlook 193 Bibliography 224 Abbreviations 225 Additional Figures and Information 227 List of Figures 245 List of Tables 247 Publications & Résumé 249 4 Contents 1 Motivation Why Water Splitting? In the wake of the 21st century mankind faces many challenges among which the closely- related growing energy demand and climate change are only two but of eminent impor- tance. Globally the temperatures are rising and the ten warmest years since 1880 have all appeared since 1998.[1] Scientifically, it is out of question that the climate becomes warmer. The consequences on the environment, economy, global conflict potential, and migration, however, are still under debate within society and politics with different opin- ions on the topics. While still denied by certain groups, the majority of scientists agree that the cause for climate change is to be found in human activities.[2] Especially the burning of fossil fuels results in the emission of greenhouse gases, such as e.g. CO2. In addition, the pollution in cities around the world from fine particulate air, which can be associated with the burn- ing of fossil fuels, has become an evident health concern.[3] These facts alone justify the investment and research in alternative energy solutions. Among the different possibilities of sustainable energies the conversion of solar power into a usable energy form is of high interest. On a global scale most investments in the area of sustainable energy are in photovoltaics.[4] Even though this technique offers many advantages over the conventional burning of fossil fuels, such as oil or coal, there are also some challenges to be faced. Of these challenges the storability of electrical power combined with the regional un- availability of power from photovoltaics due to the day-and-night-cycle are unquestion- able of huge concern. One possible (and very elegant) solution is the direct conversion of solar power into chemical fuels without a further need of an external bias. On a first glance, the easiest process seems to be the production of hydrogen (and oxygen) from water with the help of sunlight. This unassisted photocatalytic process would result in a clean and sustainable product which could be directly used for the generation of electrical power in night-times, to fuel mobility, or as a reagent in further reactions to create fuels which are easier to use and store.[5–9] 5 The Water Splitting Process - Mechanism and Requirements Since the early work of Fujishima and Honda it is known that the unassisted light-driven splitting of water can occur at the interface between an illuminated semiconductor and water.[10] The processes which need to take place in order to convert light and water into usable hydrogen are depicted schematically in Figure 1.1. This sketch is simplified, a more detailed description of an actual device can be found e.g. in Refs. [5] and [11]. Figure 1.1: Schematic representation of the photocatalytic splitting of water at the inter- face of a semiconductor to an electrolyte. The first step in order to split water is the absorption of photons by the semiconductor and the associated generation of electron-hole-pairs. These charge carriers need then to be transported to the semiconductor/electrolyte interface. During this charge transport recombination is a huge concern and needs to be prevented. At the interface a charge transfer from the semiconductor to the water molecules needs to take place. During this process it is again crucial to prevent recombination by e.g. surface states. In an ideal situation the energy which is needed to split water into hydrogen and oxy- gen is given by the difference of their respective redox potentials of 1.23 eV. Due to overpotentials η the actual driving force which is required is at about 1.8 eV. This en- ergy has to be provided by the semiconductor in form of a photovoltage of this value or larger. The photovoltage on the other hand is in a first assumption given by the splitting of the quasi-Fermi levels ∆µ, which results from the non-equilibrium situation within the semiconductor due to illumination. It is, therefore, a key requirement for the unassisted light-driven water splitting process that the splitting of the quasi-Fermi levels is larger than the required energy of 1.8eV. As a consequence of these considerations the band gap of the semiconductor has to be in the range of 2.4 eV as the band edges are considered to be a natural limit for the posi- tion of the quasi-Fermi level. Another limitation can be found in any energy states in the 6 1 Motivation bulk or at the surface that can prevent the movement of the Fermi level. This mechanism is referred to as Fermi level pinning and it is of major concern for any photoactive device. In addition to a suitable band gap the positions of the band edges with respect to the redox potentials of the water splitting process have to be suitable as well. It is necessary that -under working conditions- they encompass the redox potentials as it is shown in Figure 1.1. This is crucial as for a successful charge transfer the quasi-Fermi lev- els have to be below (hole transfer) or above (electron transfer) the respective redox level. Hematite as active material in a water splitting device - Challenges and Limitations After the first proof of concept on TiO2 many other transition metal oxides (TMOs) have been investigated in order to be utilized as the active material in a water splitting device.[5, 12] The choice for this materials class is