ABSTRACT
Significant charge separation and potential-dependent photocurrent polarity switching are reported at multilayers of polyacrylate-capped CdS quantum dots (Q-CdS, d = 3.6 +/-0.5 nm) assembled in poly(diallydimethylammonium chloride) with an alkaline sulfide solution interface. The films were deposited by dip self-assembly or dip-spin self-assembly, and photocurrents were enhanced up to 2-fold by the latter method and reached a maximum at 4-6 bilayers. The monochromatic incident-photon-to-current-conversion efficiency equalled 6.5% at 340 nm and 2.1% at 440 nm at a 6-bilayer film in the sulfide electrolyte. The photocurrent magnitude and direction were found to depend on the assembly method, number of bilayers, film history, electrode potential and solution redox species. While significant anodic and cathodic photocurrents were measured in sulfide, the film acted predominantly as a photocathode in the presence of another hole scavenger, ascorbic acid. Charge separation leading to a cathodic photocurrent in the presence of hole scavengers is possibly mediated by a photo-oxidized species in the multilayers, which facilitates net photogenerated hole transfer to the electrode at reducing potentials.
ABSTRACT
Atomic hydrogen electrosorption is reported at crystallite sites of polyacrylate-capped Pt nanoparticles (d = 2.5 +/- 0.6 nm), by assembling nanostructured electrodes of polyacrylate-Pt nanocrystallites layer-by-layer in a cationic polyelectrolyte, poly(diallyldimethylammonium chloride). Cyclic voltammetry in 1 M H2SO4 revealed a strongly adsorbed hydrogen state and a weakly adsorbed hydrogen state assigned to adsorption at (100) and (110) sites of the modified nanocrystallites, respectively. Resolving hydrogen adsorption states signifies that surface capping by the carboxylate groups is not irreversibly blocking hydrogen adsorption sites at the modified Pt nanoparticle surface. Adsorption peak currents increased with increasing the number of layers up to 16 bilayers, indicating the feasibility of nanoparticle charging via interparticle charge hopping and the accessibility of adsorption states within the thickness of the nanoparticle/polyelectrolyte multilayers. Despite similarity in hydrogen adsorption in the cyclic voltammorgrams in 1 M H2SO4, negative shifts in adsorption potentials were measured at the nanocrystallite Pt-polyelectrolyte multilayers relative to a polycrystalline bulk Pt surface. This potential shift is attributed to a kinetic limitation in the reductive hydrogen adsorption as a result of the Pt nanoparticle surface modification and the polyelectrolyte environment.
