Modifications of nanostructured TiO 2 for efficient photocatalytic and photoelectrochemical green hydrogen production Ewa Wierzbicka 1 , Chengxu Shen 2 , Thorsten Schultz 3,4 , Karolina Syrek 5 , 2 Institut fur Chemie and IRIS Adlershof, Humboldt-Universitat zu Berlin, Germany 3 Institut fur Physik and IRIS Adlershof, Humboldt-Universitat zu Berlin, Germany 4 Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin, Germany 5 Faculty of Chemistry, Jagiellonian University in Krakow, Poland Hydrogen is the most promising substitute for classic fossil fuels. It is an energy carrier that could show a zero- carbon footprint in ideal production and usage conditions. Developing new technologies for clean and sustainable hydrogen production is necessary for response to the growing demand for hydrogen in newly developing sectors such as transportation. Grzegorz Sulka 5 , Norbert Koch 3,4 , Nicola Pinna 2 1 Military University of Technology, Warsaw, Poland One strategy of sustainable hydrogen generation is based on photosensitive materials that can split water into hydrogen and oxygen using solar radiation. Since 1972, when Fujishima and Honda discovered the phenomenon of water photolysis from TiO 2 , it has become the benchmark semiconductor material tested for photocatalysis (PC) and photoelectrocatalysis (PEC). After these years, the PC/PEC hydrogen evolution from water still needs to become more efficient to meet the economic criteria of commercialization. Therefore, further modifications are necessary to improve the material's performance. TiO2 can form various nanostructures, such as nanopowders and nanotubes/nanoporous arrays, with much higher surface area than bulk materials. Nanostructurization of the TiO 2 -based material itself strongly increases PC/PEC performances. Moreover, such materials show great potential for further modifications. On the one hand, the most efficient and frequently used approach to TiO 2 modification is coupling it with noble metal cocatalysts such as Pt, Ag, and Au [1,2] . This modification brings much higher PEC and PC process efficiencies assigned to several effects, such as Schottky barrier formation, co-catalytic properties into hydrogen absorption, or surface plasmon resonance. The processes that might occur at the metal-semiconductor interface are complex depending on the type of metal used in heterojunction. On the other hand, noble-metal-free modification strategies, such as sensitization with other semiconductors [3] or so-called material self-doping, are becoming increasingly popular [4,5] . Here will be shown how adequately designed modification of nanostructured titania can improve PC/PEC water- splitting performance by creating suitable electronic pathways to enhance spatial separation of photogenerated charge carriers, improve reaction kinetics at the material/electrolyte interface, or extending absorption into visible light range. References 1. E. Wierzbicka, T. Schultz, K. Syrek, G.D. Sulka, N. Koch, N. Pinna, Mater. Horiz. 9, 2797-2808 (2022). 2. E, Wierzbicka, et al. ACS Appl. Energy Mater. 2, 8399–8404 (2019). 3. C. Shen, E. Wierzbicka*, T. Schultz, R. Wang, N. Koch, N. Pinna*, Adv. Mater. Interfaces 9, 2200643 (2022).
4. E. Wierzbicka, et al. J. Mater. Chem. A 9, 1168-1179 (2021). 5. E, Wierzbicka, et al. ChemSusChem 12, 1900-1905 (2019).
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