Characteristic molecular dynamics of reactant molecules confined in the space of the nanometer scale augments the frequency of collisions with the electrified surface so that a given faradaic reaction can be enhanced at nanoporous electrodes, the so-called nano-confinement effect. Since this effect is grounded on diffusion inside nanopores, it is predicted that adsorption onto the surface will seriously affect the enhancement by nano-confinement. We experimentally explored the correlation between adsorption and the confinement effect by examining the oxidation of butanol isomers at platinum and gold nanoporous electrodes. The results showed that electrooxidation of 2-butanol, which is a non-adsorption reaction, was enhanced more than that of 1-butanol, which is an adsorption reaction, at nanoporous platinum in acidic media. In contrast, the nanoporous gold electrode, on which 1-butanol is less adsorptive than it is on platinum, enhanced the electrooxidation of 1-butanol greatly. Furthermore, the electrocatalytic activity of nanoporous gold for oxygen reduction reaction was improved so much as to be comparable with that of flat Pt. These findings show that the nano-confinement effect can be appreciable for electrocatalytic oxygen reduction as well as alcohol oxidation unless the adsorption is extensive, and suggests a new strategy in terms of material design for innovative non-noble metal electrocatalysts.

For more efficient photoelectrochemical water splitting, there is a dilemma that a photoelectrode needs both light absorption and electrocatalytic faradaic reaction. One of the promising strategies is to deposit a pattern of electrocatalysts onto a semiconductor surface, leaving sufficient bare surface for light absorption while minimizing concentration overpotential as well as resistive loss at the ultramicroelectrodes for faradaic reaction. This scheme can be successfully realized by "maskless" direct photoelectrochemical patterning of electrocatalyst onto an SiO/amorphous Si (a-Si) surface by the light-guided electrodeposition technique. Electrochemical impedance spectroscopy at various pHs tells us much about how it works. The surface states at the SiO/a-Si interface can mediate the photogenerated electrons for hydrogen evolution, whereas electroactive species in the solution undergo outer-sphere electron transfer, taking electrons tunneling across the SiO layer from the conduction band. In addition to previously reported long-distance lateral electron transport behavior at a patterned catalyst/SiO/a-Si interface, the charging process of the surface states plays a crucial role in proton reduction, leading to deeper understanding of the operation mechanisms for photoelectrochemical water splitting.

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