A spectroscopic study of the mechanism of Au(III) (hydro-)oxides in promoting plasmon-mediated photoelectrochemical water-oxidation.

To understand the roles of Au(III) (hydro-)oxides in promoting plasmon-mediated photoelectrochemical (PMPEC) water-oxidation, we employed in situ microphotoelectrochemical surface-enhanced Raman spectroscopy and ambient-pressure x-ray photoelectron spectroscopy to elucidate the correlations between the amount of surface Au(III) (hydro-)oxides and the photocurrent of PMPEC water-oxidation on Au. By applying preoxidation potentials, we made surface Au(III) (hydro-)oxides on a plasmonic Au photoanode. According to the charge of reductively stripping surface oxygenated species before and after PMPEC water-oxidation, we found that a negative shift of an onset potential, increase in photocurrent, and much less growth of surface (hydro-)oxides were correlated with each other as a result of the increase in the coverage of Au (III) (hydro-)oxides. These results suggest that the surface Au(III) (hydro-)oxides kinetically promoted water-oxidation by restricting the growth of surface (hydro-)oxides.

Direct observation of hydration of a Nafion membrane using APXPS and AIMD simulation.

The hydration of perfluorinated sulfonic-acid ionomers is the most important phenomenon that determines their transport and electrical properties. To bridge the gap between the macroscopic electrical properties and the microscopic water-uptake mechanism, we investigated the hydration process of a Nafion membrane using ambient-pressure x-ray photoelectron spectroscopy (APXPS) from vacuum up to ∼90% relative humidity at room temperature. The O 1s and S 1s spectra provided a quantitative analysis of the water content (λ) and the transformation of the sulfonic acid group (-SO3H) to its deprotonated type (-SO3 -) during the water-uptake process. Taking advantage of a specially designed two-electrode cell, the conductivity of the membrane was determined by electrochemical impedance spectroscopy before APXPS measurements with the same conditions, thereby establishing the connection between the electrical properties and the microscopic mechanism. By means of ab initio molecular dynamics simulations based on density functional theory, the core-level binding energies of O- and S-containing species in the Nafion + H2O system were obtained.

In Situ Spectroscopic Identification of the Electron-Transfer Intermediates of Photoelectrochemical Proton-Coupled Electron Transfer of Water Oxidation on Au.

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.

Origin of the Liquid/Gaseous Water Binding Energy Splitting Measured via X-ray Photoelectron Spectroscopy.

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.