PH dependent TiO 2– water interaction mechanism Those approaches had reported that water absorbs dissociatively on TiO 2 defect surface sites, and it is either molecularly or partially dissociated on defected free TiO 2 surfaces. This also implies that pH was not an experimental variable. Several previous approaches to examine this solid-liquid interfacial layer were limited to model systems in which only a few layers of water – too few to exhibit bulk solution properties – could be stabilized for photoelectron spectroscopy application. A critical aspect then is to keep the surface nanoparticle surface coverage sufficiently low as to provide free adsorption sites for water. This has been achieved by the addition of HNO 3 and HCl, on the one hand, yielding very acidic pH and by the addition of NH 4OH, which leads to slightly basic solutions. Stable liquid jet from TiO 2 nanoparticles dispersed in water requires to charge their surface as to avoid aggregation. In order to make photoelectron spectroscopy applicable to highly volatile water, a micron-sized vacuum liquid jet is generated. Such information is accessible by photoelectron spectroscopy, an ultra-high vacuum-based technique, which is the method exploited in the present study. Hence, it is essential to investigate the electronic structure of the TiO 2–water interface. However, the fast back-reaction of a proton (H +) and hydroxide (OH –) recombination into water molecules reduces the efficiency significantly. Many efforts have been made to improve the TiO 2-catalyzed energy conversion efficiency in water-splitting reactions. Up to now, solar energy conversion into H 2 fuel is not efficient enough and competitive enough for a viable industrial application. Hydrogen fuel production by decomposing water molecules at the TiO 2 surface (catalyzing material) under applied bias (photoelectrochemical cell conditions) was reported in the early '70s. Conversion of solar energy into H 2 fuel is one of the best candidates as a future energy source. Switching from fossil fuel to clean and renewable energy sources are essential to save our planet from climate change, environmental pollution, and global warming, and to cope with the increasing energy demand arising from the quickly developing industrial society. The nature of water interaction with TiO 2nanoparticle surfaces examined as a function of pH using liquid-jet photoelectron spectroscopy Variation of pH thus provides a means to control this interaction and amount of produced H 2 fuel production. In contrast, water interacts dissociative at slightly basic pH. By measuring core-level photoelectron as well as resonant photoemission spectra at the oxygen 1s edge, we find that in acidic aqueous solutions, water adsorbs molecularly at the Ti sites. Our system thus mimics the conditions within photoelectrochemical cells. One crucial aspect here is that the nanoparticles are fully dispersed in an aqueous solution, which allows aqueous ions to diffuse freely from the interface into the bulk solution. Using liquid-jet photoelectron spectroscopy, we can access the electronic structure of the TiO 2 nanoparticle–aqueous interface. Despite intense research on the nature of water interaction, it remains unclear whether water adsorbs dissociatively, associatively, or mixes at the TiO 2 surface. Characterizing the electronic structure of the titanium dioxide–aqueous interface is crucial for enhancing H 2 fuel production efficiency in photoelectrochemical cells.
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