Photoelectrochemistry of core–shell tandem junction n–p^+-Si/n-WO_3 microwire array photoelectrodes

Tandem junction (n–p^+-Si/ITO/WO_3/liquid) core–shell microwire devices for solar-driven water splitting have been designed, fabricated and investigated photoelectrochemically. The tandem devices exhibited open-circuit potentials of E_(∝) = −1.21 V versus E^0′(O_2/H_2O), demonstrating additive voltages across the individual junctions (n–p^+-Si E_(∝) = −0.5 V versus solution; WO_3/liquid E_(∝) = −0.73 V versus E^0′(O_2/H_2O)). Optical concentration (12×, AM1.5D) shifted the open-circuit potential to E_(∝) = −1.27 V versus E^0′(O_2/H_2O) and resulted in unassisted H_2 production during two-electrode measurements (anode: tandem device, cathode: Pt disc). The solar energy-conversion efficiencies were very low, 0.0068% and 0.0019% when the cathode compartment was saturated with Ar or H_2, respectively, due to the non-optimal photovoltage and band-gap of the WO_3 that was used in the demonstration system to obtain stability of all of the system components under common operating conditions while also insuring product separation for safety purposes

Molecular solvation dynamics from inelastic x-ray scattering measurements

This chapter contains sections titled: Introduction; Review of High-Resolution Inelastic X-Ray Scattering on Liquid Water: Theory and Experiment; Green's Function Imaging of Dynamics with Femtosecond Temporal and Angstrom Spatial Resolution; An excluded volume implementation for Green's Function Imaging of Dynamics; Conclusions and Outlook; References

Characterizing the Solvent‐Induced Inversion of Colloidal Aggregation During Electrophoretic Deposition

Abstract Electrophoretic deposition (EPD) of colloidal particles is a practical system for the study of crystallization and related physical phenomena. The aggregation is driven by the electroosmotic flow fields and induced dipole moments generated by the polarization of the electrode‐particle‐electrolyte interface. Here, the electrochemical control of aggregation and repulsion in the electrophoretic deposition of colloidal microspheres is reported. The nature of the observed transition depended on the composition of the solvent, switching from electrode‐driven aggregation in water to electrical field‐driven repulsion in ethanol for otherwise identical systems of colloidal microspheres. This work uses optical microscopy‐derived particles and a recently developed particle insertion method approach to extract model‐free, effective interparticle potentials to describe the ensemble behavior of the particles as a function of the solvent and electrode potential at the electrode interface. This approach can be used to understand the phase behavior of these systems based on the observable particle positions rather than a detailed understanding of the electrode‐electrolyte microphysics. This approach enables simple predictability of the static and dynamic behaviors of functional colloid‐electrode interfaces

Counterions between charged polymers exhibit liquid-like organization and dynamics

Current understanding of electrostatics in water is based on mean-field theories like the Poisson–Boltzmann formalism and its approximations, which are routinely used in colloid science and computational biology. This approach, however, breaks down for highly charged systems, which exhibit counterintuitive phenomena such as overcharging and like-charge attraction. Models of counterion correlations have been proposed as possible explanations, but no experimental comparisons are available. Here, collective dynamics of counterions that mediate like-charge attraction between F-actin filaments have been directly observed in aqueous solution using high-resolution inelastic x-ray scattering down to molecular length-scales. We find a previously undescribed acoustic-like phonon mode associated with correlated counterions. The excitation spectra at high wave-vector Q reveal unexpected dynamics due to ions interacting with their “cages” of nearest neighbors. We examine this behavior in the context of intrinsic charge density variations on F-actin. The measured speed of sound and collective relaxation rates in this liquid agree surprisingly well with simple model calculations

Boosting the Performance of WO3/n-Si Heterostructures for Photoelectrochemical Water Splitting: from the Role of Si to Interface Engineering

Metal oxide/Si heterostructures make up an exciting design route to high-performance electrodes for photoelectrochemical (PEC) water splitting. By monochromatic light sources, contributions of the individual layers in WO3/n-Si heterostructures are untangled. It shows that band bending near the WO3/n-Si interface is instrumental in charge separation and transport, and in generating a photovoltage that drives the PEC process. A thin metal layer inserted at the WO3/n-Si interface helps in establishing the relation among the band bending depth, the photovoltage, and the PEC activity. This discovery breaks with the dominant Z-scheme design idea, which focuses on increasing the conductivity of an interface layer to facilitate charge transport, but ignores the potential profile around the interface. Based on the analysis, a high-work-function metal is predicted to provide the best interface layer in WO3/n-Si heterojunctions. Indeed, the fabricated WO3/Pt/n-Si photoelectrodes exhibit a 2 times higher photocurrent density at 1.23 V versus reversible hydrogen electrode (RHE) and a 10 times enhancement at 1.6 V versus RHE compared to WO3/n-Si. Here, it is essential that the native SiO2 layer at the interface between Si and the metal is kept in order to prevent Fermi level pinning in the Schottky contact between the Si and the metal