Two Sharp Phase Change Processes of Diphenyl Viologen at a Au(111) Electrode Surface: Non-Faradaic Transition with Interplay of Ionic Adsorption of Chloride and Bromide and Faradaic One

Two phase change processes of diphenyl viologen (dPhV) on a Au(111) electrode in KCl and KBr aqueous solutions were described using the results of voltammetric, electroreflectance (ER), and electrochemical scanning tunneling microscopic (EC-STM) measurements. Both processes exhibited sharp spikelike voltammetric responses. In KCl solution, the phase change at 0.30 V versus Ag/AgCl/saturated KCl was found to be a nonfaradaic order–disorder phase transition, from an ordered adlayer of dPhV dication (dPhV²⁺) with coadsorbed Cl^– at more positive potentials than 0.30 V to a gaslike phase at less positive potentials. The faradaic reaction at −0.09 V was found to be the transition from the gaslike phase to a condensed monolayer of dPhV^•+. The EC-STM images of the condensed monolayer showed stripe patterns of rows of π–π stacked dPhV^•+. Almost the same set of two processes was observed in KBr solution but not in KF solution. In KF solution, although two voltammetric responses were observed, the peaks were small and broad, indicative of sluggish adsorption state changes of individual dPhV cations. Taken together, specific adsorption of coexistent anions is of critical importance for the occurrence of the sharp nonfaradaic phase transition

CdTe-Based Photoanode for Oxygen Evolution from Water under Simulated Sunlight

This study investigated the properties of a photoanode fabricated by depositing a p-type CdTe thin film on a CdS-coated FTO substrate (CdTe/CdS/FTO) via close-space sublimation. This CdTe/CdS/FTO electrode was found to work as a photoanode with a long absorption edge wavelength of 830 nm. In a CdTe-based photoanode such as this, the p–n junction formed at the CdTe/CdS interface promotes charge separation of photoexcited carriers and forces photogenerated holes to move toward the photoanode surface to promote oxidation reactions on the electrode surface. A MoO_<i>x</i> buffer layer was also found to play a crucial role in facilitating the transfer of photogenerated holes to surface reaction sites through decreasing the energy barrier at the interface between the CdTe and a surface protective layer. A biphotoelectrode photoelectrochemical cell composed of a CdTe-based photoanode and a CdTe-based photocathode exhibited a solar-to-hydrogen conversion efficiency of 0.22% without an external voltage in response to illumination by AM 1.5G light

Elucidating the Role of Surface Energetics on Charge Separation during Photoelectrochemical Water Splitting

Efficient photoelectrochemical (PEC) water splitting requires charge separation and extraction from a photoactive semiconductor. Such a charge transport process is widely believed to be dictated by the bulk energetics of the semiconductor. However, its dependence on surface energetics along the semiconductor/electrolyte interface remains an open question. Here, we elucidate the influence of surface energetics on the performance of a well-established Mo-doped BiVO4 photoanode whose surface energetics are regulated by the facet-selective cocatalyst loading. Surprisingly, photodeposition of RhOx and CoOx cocatalysts onto the {010} and {110} facets, respectively, strongly enhanced the charge-separation efficiency, in addition to improving the injection efficiency for water oxidation. Detailed optoelectrical simulations confirm that the synergistic enhancement of charge separation originates from the distinct effects of the cocatalyst loading on the surface energetics. This insight into the fundamental charge-separation mechanism in PEC cells provides a perspective for cell design and operation

Enhancement of Charge Separation and Hydrogen Evolution on Particulate La₅Ti₂CuS₅O₇ Photocathodes by Surface Modification

Particulate La₅Ti₂CuS₅O₇ (LTC) photocathodes prepared by particle transfer show a positive onset potential of 0.9 V vs RHE for the photocathodic current in photoelectrochemical (PEC) H₂ evolution. However, the low photocathodic current imposes a ceiling on the solar-to-hydrogen energy conversion efficiency of PEC cells based on LTC photocathodes. To improve the photocurrent, in this work, the surface of Mg-doped LTC photocathodes was modified with TiO₂, Nb₂O₅, and Ta₂O₅ by radio frequency reactive magnetron sputtering. The photocurrent of the modified Mg-doped LTC photocathodes was doubled because these oxides formed type-II heterojunctions and extended the lifetimes of photogenerated charge carriers. The enhanced photocathodic current was attributed to hydrogen evolution at a positive potential of +0.7 V vs RHE. This work opens up possibilities for improving PEC hydrogen evolution on particulate photocathodes based on surface oxide modifications and also highlights the importance of the band gap alignment