The adsorption kinetics and self-assembly of hexadecyl mercaptan on gold have been investigated by scanning electrochemical microscopy (SECM), chronoamperometry, and cyclic voltammetry. The developed methodology allows one to evaluate the surface coverage and the average size of the defects in the monolayer film from the effective rate constant of electron transfer. Two kinetic regimes of self-assembly were identified: a rapid initial adsorption of hexadecyl mercaptan onto a clean gold surface from 5 mM solution (more than 90% coverage obtained in 1 to 5 min), and a slower subsequent annealing of a thiol monolayer resulting in a more compact film. Typically, a long-chain-length thiol-treated gold surface acts as an electronically insulating surface after about 1 h. The SECM images of partially covered gold surfaces were always featureless, suggesting that the defects in the film were smaller than 0.5 urn for any exposure time a 1 min. SAMs containing a controlled number of single-molecule "gate sites" were used to control the reactivity of an electrode." The ability to control the kinetics of selfassembly, and knowledge of the size and density of active centers, are essential for such applications.
Selenide and polyselenide UV-vis absorption spectroscopic characteristics were investigated for aqueous polyse]enide solutions. From these observations equilibria interrelationships between OH-, H +, H~Se, HSe , Se =, Se~, Se~, and Se~ were probed. The standard cell potential for the reduction of 1/2 Se~ to Se = was determined to be -0.716 V. The equilibrium reactions of Se~ with Se = to 2 Se~ and 2 Se~ with Se = to 3 Se~, respectively, have P/(23 = -0.7 (_+0.i) and pK3~ = -4.0 (+-0.I) in 0.5 to 3M aqueous electrolytes. Consistent with these equilibria, the free energy of formation for Se~, Se~, and Se~ have been determined as 121, 115, and 121 kJ/mol, respectively. Polarization studies of these polyselenide solutions demonstrated Pt and CoSe as efficient catalytic surfaces for the oxidation and reduction of selenide species.
In a photoinduced reaction, an intermediate produced by the reaction of a reductant R with photogenerated holes can promote a reduction reaction that is not observed in the absence of R. We illustrate this scheme by considering the reduction of Ni II at TiO 2 . In this case Ni II , which is not deposited on illuminated TiO 2 , can be reduced by the intermediate CO 2 •produced from the anodic oxidation of oxalate ion (C 2 O 4 2-) at irradiated TiO 2 in water. Irradiation of an aqueous Ni II solution containing 10 µg Ni/mL in the presence of TiO 2 particles led to 85% removal of the Ni from the solution. The mechanism of the reaction was probed by measurement of current-doubling effects for oxalate solutions at a single-crystal TiO 2 electrode.
Here we demonstrate progress on electrodeposition of photoactive silicon films from an environmentally friendly molten CaCl 2 electrolyte, which is the first step of a new route to a practical low-cost silicon solar cell. We report electrodeposition of severalmicron thick silicon films on a graphite substrate in a bath of molten CaCl 2 containing SiO 2 nanoparticles. The best silicon deposits was obtained at 6 mA/cm 2 for 1 hour in molten CaCl 2 containing 0.3 M SiO 2 nanoparticles, at 850 • C. The main impurities are Al, Mg, Ca, Na. A photoelectrochemical method was demonstrated as a reliable and sensitive measurement for testing the quality of the silicon film. The as-deposited film exhibits 31% of the photocurrent response of a commercial p-type wafer. A comparison of graphite and silver substrates is presented, and the remaining problems are discussed.
High-resolution electrochemical deposition of silver
nanostructures on insulating atomically flat mica surfaces
in humid air was achieved with a scanning tunneling microscope (STM)
operating in a scanning electrochemical
microscopy mode. The current is faradaic and flows between the tip
and a surface contact on the mica
substrate through a thin water layer on the mica surface. The
thickness and conductivity of the water layer
governs the magnitude of the faradaic current. Modified low
current STM and conventional tungsten STM
tips were used for the silver deposition. The tip was held at a
negative bias of −1.2 to −1.4 V (tip vs
substrate), with setpoint currents of 1−1.7 pA. The sizes and
shapes of the nanostructures were controlled
by the tip scan rates and were restricted to the scan areas under the
tip.
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