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Knowledge of the electronic properties of aqueous solutions is crucial to understanding water-based chemistry. Here, a quantity of particular interest is the vertical ionization energy (VIE) of solvents and embedded solutes, especially of valence electrons which are directly related to the chemical reactivity. Liquid-jet photoelectron spectroscopy provides direct access to the electronic structure, but the accurate determination of absolute-scale electronic energetics remained elusive as so-far standard gas-phase referencing schemes suffered from experiment-dependent extrinsic potentials. Our work addresses these shortcomings by implementing a novel energy-referencing method where we additionally measure the low-energy cutoff, Ecut. Ecut corresponds to the low-energy limit for electrons to escape the solution, thus determining the absolute zero on the kinetic energy scale, and allows us to accurately determine VIEs of liquids on an absolute scale[1]. Furthermore, by measuring the Fermi edge of a metallic reference electron under precisely controlled, field-free conditions, we are able to establish VIEs with respect to the Fermi-level, which yields the solutions work function as the difference of the vacuum-referenced and Fermi-referenced energies.
Applying this novel approach, we determine the absolute VIEs of liquid water and quantify solute-induced effects on the water’s electronic energetics as well as on the solute VIEs as a function of salt concentration. We study two prototypical aqueous salt solutions, NaI(aq) and the strong surfactant tetrabutylammonium, TBAI(aq). The lowest vacuum-referenced VIE of the 1b1 (HOMO) orbital of neat liquid water was determined as 11.33 ± 0.03 eV[1], and we observed opposing concentration-dependent energy shifts of this feature, and the spectrum as a whole, induced by above solutes[2]. NaI(aq) leads to an overall shift towards lower VIEs of up to 0.3 eV at 8 M, while electronic structure changes in the solvent are revealed by changes in the split 3a1 orbital of water. Larger shifts in the opposite direction were observed for TBAI(aq) solutions with an VIE increase by about 0.7 eV at saturation concentration. These trends are associated with a change of water’s electronic structure in the bulk and changes of the solution’s interface in the case of NaI and TBAI, respectively. Similar observations are made when referencing these solutions to the Fermi level, and we discuss these observations in terms of work-function changes arising from surface molecular dipoles and band-bending close to the interface. These techniques mark a major advance in our ability to quantify electronic–structure interactions and chemical reactivity in liquid water, which now explicitly extends to the measurement of absolute-scale bulk and interfacial solution energetics, including those of relevance to aqueous electrochemical processes.
1 S. Thürmer, S. Malerz, F. Trinter, U. Hergenhahn, C. Lee, D. M. Neumark, G. Meijer, B. Winter and I. Wilkinson, Chem. Sci., 2021, 12, 10558-10582.
2 B. Credidio, M. Pugini, S. Malerz, F. Trinter, U. Hergenhahn, I. Wilkinson, S. Thürmer and B. Winter, Phys. Chem. Chem. Phys., 2021, DOI: 10.1039/d1cp03165a.
