3 research outputs found
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
Inhibition of Tafel Kinetics for Electrolytic Hydrogen Evolution on Isolated Micron Scale Electrocatalysts on Semiconductor Interfaces
Semiconductor–liquid junctions
are ubiquitous in photoelectrochemical approaches to artificial photosynthesis.
By analogy with the antennae and reaction centers in natural photosynthetic
complexes, separating the light-absorbing semiconductor and electrocatalysts
can improve catalytic efficiency. A catalytic layer can also impair
the photovoltage-generating energetics of the electrode without appropriate
microscopic organization of catalytically active area on the surface.
Here, we have developed a method using high-speed X-ray phase contrast
imaging to study <i>in situ</i> electrolytic bubble growth
on semiconductor electrodes fabricated with isolated, micron-scale
platinum electrocatalysts. X-rays are a nonperturbative probe by which
gas evolution dynamics can be studied under conditions relevant to
solar fuels applications. The self-limited growth of a bubble residing
on the isolated electrocatalyst was measured by tracking the evolution
of the gas–liquid boundary. Contrary to observations on macroscopic
electrodes, bubble evolution on isolated, microscopic Pt pads on Si
electrodes was insensitive to increasing overpotential. The persistence
of the bubble causes mass transport limitations and inhibits the expected
Tafel-like kinetics. The observed scaling of catalytic current densities
with pad size implies that electrolysis is occurring predominantly
on the perimeter of the active area
Quaternary core–shell oxynitride nanowire photoanode containing a hole-extraction gradient for photoelectrochemical water oxidation
A nanowire
photoanode SrTaO2N, a semiconductor suitable
for overall water-splitting with a band gap of 2.3 eV, was coated
with functional overlayers to yield a core–shell structure
while maintaining its one-dimensional morphology. The nanowires were
grown hydrothermally on tantalum, and the perovskite-related oxynitride
structure was obtained by nitridation. Three functional overlayers
have been deposited on the nanowires to enhance the efficiency of
photoelectrochemical (PEC) water oxidation. The deposition of TiOx protects the oxynitride from photocorrosion
and suppresses charge-carrier recombination at the surface. Ni(OH)x acts a hole-storage layer and decreases
the dark-current contribution. This leads to a significantly improved
extraction of photogenerated holes to the electrode–electrolyte
surface. The photocurrents can be increased by the deposition of a
cobalt phosphate (CoPi) layer as a cocatalyst. The heterojunction
nanowire photoanode generates a current density of 0.27 mA cm–2 at 1.23 V vs the reversible hydrogen electrode (RHE)
under simulated sunlight (AM 1.5G). Simultaneously, the dark-current
contribution, a common problem for oxynitride photoanodes grown on
metallic substrates, is almost completely minimized. This is the first
report of a quaternary oxynitride nanowire photoanode in core–shell
geometry containing functional overlayers for synergetic hole extraction
and an electrocatalyst