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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.

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 μm for any exposure time ≥ 1 min.

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 NiII at TiO2. In this case NiII, which is not deposited on illuminated TiO2, can be reduced by the intermediate CO2•- produced from the anodic oxidation of oxalate ion (C2O42-) at irradiated TiO2 in water. Irradiation of an aqueous NiII solution containing 10 μg Ni/mL in the presence of TiO2 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 TiO2 electrode.

Selenide and polyselenide UV-vis absorption spectroscopic characteristics were investigated for aqueous polyselenide solutions. From these observations equilibria interrelationships between OH{sup {minus}}, H{sup +}, H{sub 2}Se, HSe{sup {minus}}, Se{sup {minus}2}, Se{sub 2}{sup {minus}2}, Se{sub 3}{sup {minus}2}, and Se{sub 4}{sup {minus}2} were probed. The standard cell potential for the reduction of 1/2 Se{sub 2}{sup {minus}2} to Se{sup {minus}2} was determined to be {minus}0.716 V. The equilibrium reactions of Se{sub 3}{sup {minus}2} and 2 Se{sub 4}{sup {minus}2} with Se{sup {minus}2} to 3 Se{sub 3}{sup {minus}2}, respectively, have pK{sub 23} = {minus}0.7 ({+-}0.1) and pK{sub 34} = {minus}4.0 ({+-}0.1) in 0.5 to 3M aqueous electrolytes. Consistent with these equilibria, the free energy of formation for Se{sub 2}{sup {minus}2}, Se{sub 3}{sup {minus}2}, and Se{sub 4}{sup {minus}2} have been determined as 121, 115, nd 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.

The effect of aqueous polyselenide solution modification on the photoelectrochemical behavior of n-GaAs/aqueous polyselenide photoelectrochemical cells (PECS) is investigated. Solution modification is achieved through pH control and changing the ratio of dissolved selenium to selenide. Numerical analysis of speciation in various polyselenide solutions is compared to PEC behavior. It is shown that PEC photocurrent, photovoltage, and fill factor are affected by the distribution of hydroselenide, selenide, and polyselenide in solution. There appears to be an optimum pH ([KOH] = 1 to 2M) providing sufficient concentrations of Se{sup {minus}2} to improve PEC response. A back-wall PEC was designed to investigate the high selenium concentration domain not normally accessible to conventional regenerative solar cells. Photopotential is enhanced by up to 50 mV in high [Se{sup 0}] electrolytes. However, these electrolytes can diminish photocurrent stability.

The determination of activity of highly concentrated sulfide solutions is important for the thermodynamic analysis of speciation. A novel evanescent method for solvent analysis and determination of mean solution activities is applied to measure the first activity coefficients of highly concentrated sulfide solutions. Another technique, submicrometer path length spectroscopy, provides in situ observations of the problematic alkaline sulfide equilibria in the domain of highly concentrated sulfide

Discrepancies by a factor of 105 have been reported for the fundamental S2− aqueous acid equilibrium constant which acts as a cornerstone for the foundation of aqueous metal sulphide thermochemistry. We report the conductometric determination of the Gibbs free energy and the acid equilibrium constant for S2−aq,K2: Uncertainty in K2 directly affects solubility, thermodynamic and redox potential values of each of the metal sulphides. Unlike previous investigations, the conductometric measurements determine the extent of free OH− in solution directly. This study is performed in unusually concentrated media, and makes conventional use of the anomalously high equivalent conductance of hydroxide compared with other anions. The differential conductometric measurements and conductometric titrations reported here demonstrate that hydroxide resists dehydrolysis in solutions containing less than 3 M hydrosulphide and hydroxide, and the equilibrium: lies substantially to the left, confirming that pK2 > 16. As the concentration is increased from 3 to 7 M, the relative conductivity diminishes, and free OH− is consumed increasingly. A solution containing 6.4 M KHS and 6.4 M KOH, a solution with a pH of approximately 17, is shown (i) to contain comparable concentrations of free S2−, HS− and OH− species, (ii) to have an ionic conductance for free sulphide of λ 1 2 s2−= 0.87 λHS−, and (iii) to affirm that aqueous sulphide salt thermodynamics, solubilities and rest potentials are limited by HS− and not S2− chemistry.