Though the field of photoelectrochemical (PEC) solar fuel production focuses on a broad variety of materials, metal oxides are probably the most commonly employed materials in state-of-the-art photoelectrodes. Metal oxides such as bismuth vanadate and hematite are currently intensely studied as photoanode materials in our group and by the PEC water oxidation community.^1-4 Stable metal oxides such as TiO₂ have been successfully employed as protection layers in, for example, Cu₂O photocathodes and are now advancing the field of organic photocathodes for hydrogen evolution.^{5, 6} Especially metal oxides grown by atomic layer deposition (ALD) offer many benefits to solar fuel research such as i) precise growth control on the atomic scale ii) conformal and pinhole-free coatings and ii) formation of high-quality interfaces. In our studies of recombination losses, we benefit from high-quality ALD hematite films and are able to gain new insights from electrochemical impedance spectroscopy that will be reported here. Furthermore, we could demonstrate stable organic photocathodes for hydrogen evolution and will report on new organic photocathode designs employing ALD TiO₂ electron selective layers.⁶ Finally, I will discuss the importance of ALD surface treatments in the science of CO₂ reduction.⁷

1. Le Formal, F.; Pastor, E.; Tilley, S. D.; Mesa, C. A.; Pendlebury, S. R.; Gratzel, M.; Durrant, J. R. Rate Law Analysis of Water Oxidation on a Hematite Surface. Journal of the American Chemical Society 2015, 137, 6629-6637.
2. Mesa, C. A.; Kafizas, A.; Francas, L.; Pendlebury, S. R.; Pastor, E.; Ma, Y. M.; Le Formal, F.; Mayer, M. T.; Gratzel, M.; Durrant, J. R. Kinetics of Photoelectrochemical Oxidation of Methanol on Hematite Photoanodes. Journal of the American Chemical Society 2017, 139, 11537-11543.
3. Ma, Y. M.; Mesa, C. A.; Pastor, E.; Kafizas, A.; Francas, L.; Le Formal, F.; Pendlebury, S. R.; Durrant, J. R. Rate Law Analysis of Water Oxidation and Hole Scavenging on a Bivo4 Photoanode. Acs Energy Letters 2016, 1, 618-623.
4. Ma, Y. M.; Kafizas, A.; Pendlebury, S. R.; Le Formal, F.; Durrant, J. R. Photoinduced Absorption Spectroscopy of Copi on Bivo4: The Function of Copi During Water Oxidation. Advanced Functional Materials 2016, 26, 4951-4960.
5. Luo, J. S.; Steier, L.; Son, M. K.; Schreier, M.; Mayer, M. T.; Gratzel, M. Cu2o Nanowire Photocathodes for Efficient and Durable Solar Water Splitting. Nano Letters 2016, 16, 1848-1857.
6. Steier, L.; Bellani, S.; Rojas, H. C.; Pan, L. F.; Laitinen, M.; Sajavaara, T.; Di Fonzo, F.; Gratzel, M.; Antognazza, M. R.; Mayer, M. T. Stabilizing Organic Photocathodes by Low-Temperature Atomic Layer Deposition of Tio2. Sustainable Energy & Fuels 2017, 1, 1915-1920.
7. Schreier, M.; Heroguel, F.; Steier, L.; Ahmad, S.; Luterbacher, J. S.; Mayer, M. T.; Luo, J. S.; Gratzel, M. Solar Conversion of Co2 to Co Using Earth-Abundant Electrocatalysts Prepared by Atomic Layer Modification of Cuo. Nature Energy 2017, 2.