Ambient-pressure X-ray photoelectron spectroscopy (APXPS) provides an effective way of tackling the challenge of detecting chemical states within complex systems. Here a fundamental understanding of the core-level shift (CLS) of water in the liquid/gas phase observed via APXPS is obtained with computational modeling at the molecular and electronic levels. The CLS value of ∼2 eV derived from experiments is reproduced by modeling in terms of the total shift and photon energy dependence. The contributions of collective electrical effects, including electrostatic potential, orbital deformation, and electronic polarization, to the CLS were further analyzed and discussed. Our results show that the CLS is dominated by the final state effect due to electronic polarization of the surrounding molecules following photoionization, while the peak broadening is mainly determined by the electrostatic potential, which belongs to an initial state effect. The physical insights and computational approaches could be further applied to study more complex molecules or materials.
Experimental elucidation of the decoupling of electron
and proton
transfer at a molecular level is essential for thoroughly understanding
the kinetics of heterogeneous (photo)electrochemical proton-coupled
electron transfer water oxidation. Here we illustrate the electron-transfer
intermediates of positively charged surface oxygenated species on
Au (Au–OH+) and their correlations with the rate
of water oxidation by in situ microphotoelectrochemical
surface-enhanced Raman spectroscopy (SERS) and ambient-pressure X-ray
photoelectron spectroscopy. At the intermediate stage of water oxidation,
a characteristic blue shift of the vibration of Au–OH species
in laser-power-density-dependent measurements was assigned to the
light-induced production of Au–OH+ in water oxidation.
The photothermal effect was excluded according to the vibrational
frequencies of Au–OH species as the temperature was increased
in a variable-temperature SERS measurement. Density functional theory
calculations evidenced that the frequency blue shift is from the positively
charged Au–OH species. The photocurrent-dependent frequency
blue shift indicated that Au–OH+ is the key electron-transfer
intermediate in water oxidation by decoupled electron and proton transfer.
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