PAR Ludmilla STEIER "Oui, mes amis, je crois que l'eau sera un jour employée comme combustible, que l'hydrogène et l'oxygène, qui la constituent, utilisés isolément ou simultanément, fourniront une source de chaleur et de lumière inépuisables et d'une intensité que la houille ne saurait avoir. Un jour, les soutes des steamers et les tenders des locomotives, au lieu de charbon, seront chargés de ces deux gaz comprimés, qui brûleront dans les foyers avec une énorme puissance calorifique. Ainsi

more »

... c, rien à craindre. Tant que cette terre sera habitée, elle fournira aux besoins de ses habitants, et ils ne manqueront jamais ni de lumière ni de chaleur, pas plus qu'ils ne manqueront des productions des règnes végétal, minéral ou animal. Je crois donc que lorsque les gisements de houille seront épuisés, on chauffera et on se chauffera avec de l'eau. L'eau est le charbon de l'avenir." Jules Vernes, L'île Mystérieuse, 1874 I A ABSTRACT Motivated to revolutionize our today's fossil fuels based energy production, my work concentrated on the investigation of promising low-cost materials for photoelectrochemical hydrogen production and photovoltaic electricity generation. Hydrogen presents a fully scalable energy storage solution while photovoltaics have the biggest potential for clean electricity generation. Both are combined in the hydrogenbased economy that will be introduced in Chapter 1. A clear way to achieve this revolutionary technological and societal goal is through fundamental understanding of the complex electronic properties of the most promising low-cost semiconductors offering strong visible light absorption. The modern fields of photoelectrochemical (PEC) water splitting and photovoltaics have a lot in common: materials, scientific concepts and theoretical background. In other words, their complementarity was a strong motivation for the interdisciplinary work presented in this thesis. The main focus in this thesis is on hematite (α-Fe 2 O 3 ), which is a promising low-cost material offering visible light absorption and the chemical robustness for photoelectrochemical water oxidation. However, it has two major drawbacks: firstly, for a semiconductor, hematite has extremely low electron and hole mobilities. This makes it challenging to collect charges that are photo-generated deep within the hematite layer and far away from the surface. Secondly, water oxidation appears to be limited by trap states located in the mid band gap region. Chapter 3 addresses these drawbacks showing that doping of hematite from the underlayer, surface passivation from annealing treatments and/or overlayers are all key parameters to consider for the design of more efficient iron oxide electrodes. By better understanding the underlying principles of over-and underlayers, I was able to design multilayered hematite photoanodes comprised of functional thin films to obtain a significant reduction in the water oxidation overpotential. Whereas hematite thin film electrodes were fabricated by ultrasonic spray pyrolysis in Chapter 3, I introduce a new atomic layer deposition (ALD) route towards crystalline, highly photoactive, phase pure and impurity-free hematite films in Chapter 4. With this thin film model system I could precisely demonstrate that only the 10 nm thick space charge region of hematite is photoactive, which presents a major challenge when IR Infrared region (700 nm -1 mm) ITO Tin-doped indium oxide (Sn:In 2 O 3 ) J Current density (in mA cm -2 ) J Ph Photocurrent (in mA cm -2 ) J SC Short circuit current density (in mA cm -2 ) k B Boltzmann constant (= 1.380 10 -23 J K -1 ) Wavelength (in nm or µm) m Mean free path LLW Low level (radioactive) waste R Resistance (in Ω) or Reflectance (in %) RC Resistor-capacitor unit RHE Reversible hydrogen electrode RoHS Restriction of Hazardous Substances σ Conductivity (in S cm -1 ) ΔS Entropy sat'd saturated SCLJ Semiconductor/liquid junction SEM Scanning electron microscopy STEM Scanning transmission electron microscopy STH Solar-to-hydrogen conversion efficiency X XI XII Equation Chapter 1 Section 1 solution. Evidently, an insecure electricity supply is inacceptable -especially in the industrial sector working 24/7 with high power demanding machines. Pumped-storage hydropower is a good strategy used nowadays to buffer the electricity grid. Especially in the United States, China and Europe pumped storage is well established and planned to be expanded in the future. To date, there are approximately 270 pumped-storage power 1.23 V (=ΔG/nF for n=2 electrons with F being the Faraday constant). However, in practice, this voltage is not enough to split water. Overpotential losses at the anode (η a ), cathode (η c ) and in solution due to ionic conductivity (η sol ) have to be considered such that the final applied voltage can be substantially higher than 1.23 V. Given the correct energetics, the following processes can occur depending on whether the reaction medium is alkaline or acidic: Under alkaline conditions Under acidic conditions Cathode (Reduction): Anode (Oxidation): Total: H 2 ½O 2 H 2 O Anode Cathode Membrane + _ e --H + Electricity 16. Luo, J. S.; Im, J. H.; Mayer, M. T.; Schreier, M.; Nazeeruddin, M. K.; Park, N. G.; Tilley, S. D.; Fan, H. J.; Gratzel, M. Water Photolysis at 12.3% Efficiency Via Perovskite Photovoltaics and Earth-Abundant Catalysts. Science 2014, Salvi, P.; Nelli, P.; Pesenti, R.; Villa, M.; Berrettoni, M.; Zangari, G.; Kiros, Y. Advanced Alkaline Water Electrolysis. Electrochimica Acta 2012, 82, 384-391. Photoelectrochemical Water Splitting with Mesoporous Hematite Prepared by a Solution-Based Colloidal Approach. . 56. Stefik, M.; Cornuz, M.; Mathews, N.; Hisatomi, T.; Mhaisalkar, S.; Gratzel, M. Transparent, Conducting Nb:Sno(2) for Host-Guest Photoelectrochemistry. Brillet, J.; Gratzel, M. Controlling Photo-Activity of Solution-Processed Hematite Electrodes for Solar Water Splitting. Solar Hydrogen and Nanotechnology V 2010, 7770. 6. Sivula, K.; Zboril, R.; Le Formal, F.; Robert, R.; Weidenkaff, A.; Tucek, J.; Frydrych, J.; Gratzel, M. Photoelectrochemical Water Splitting with Mesoporous Hematite Prepared by a Solution-Based Colloidal Approach. Journal of the American Chemical Society 2010, 132, 7436-7444. 7. Li, L.; Yu, Y.; Meng, F.; Tan, Y.; Hamers, R. J.; Jin, S. Facile Solution Synthesis of Alpha-Fef3.3h2o Nanowires and Their Conversion to Alpha-Fe2o3 Nanowires for Photoelectrochemical Application. Nano Letters 2012, 12, 724-731. 8. Sivula, K.; Le Formal, F.; Gratzel, M. Wo3-Fe2o3 Photoanodes for Water Splitting: A Host Scaffold, Guest Absorber Approach. Chemistry of Materials 2009, 21, 2862-2867. 9. Stefik, M.; Cornuz, M.; Mathews, N.; Hisatomi, T.; Mhaisalkar, S.; Gratzel, M. Transparent, Conducting Nb:Sno(2) for Host-Guest Photoelectrochemistry. Xu, Y.; Mayer, M. T.; Simpson, Z. I.; McMahon, G.; Zhou, S.; Wang, D. Growth of P-Type Hematite by Atomic Layer Deposition and Its Utilization for Improved Solar Water Splitting. Journal of the American Chemical Society 2012, 134, 5508-5511. 15. Klahr, B.; Gimenez, S.; Fabregat-Santiago, F.; Hamann, T.; Bisquert, J. Water Oxidation at Hematite Photoelectrodes: The Role of Surface States. Journal of the American Chemical Society 2012, 134, 4294-4302. 16. Badia-Bou, L.; Mas-Marza, E.; Rodenas, P.; Barea, E. M.; Fabregat-Santiago, F.; Gimenez, S.; Peris, E.; Bisquert, J. Water Oxidation at Hematite Photoelectrodes with an Iridium-Based Catalyst. Journal of Physical Chemistry C 2013, 117, 3826-3833. 17. Braun, A.; Sivula, K.; Bora, D. K.; Zhu, J. F.; Zhang, L.; Gratzel, M.; Guo, J. H.; Constable, E. C. Direct Observation of Two Electron Holes in a Hematite Photoanode During Photoelectrochemical Water Splitting.