Density of Deep Trap States in Oriented TiO₂ Nanotube Arrays

Correlations between the population of deep trap states in an array of TiO₂ nanotubes (NT) and the dynamic photocurrent responses under supra-band-gap illumination are investigated. Ordered arrays of TiO₂ NT of 10 μm length, 125 nm inner diameter, and 12 nm wall thickness featuring strong anatase character were obtained by anodization of Ti in ethylene glycol solution containing NH₄F. Cyclic voltammograms at pH 10 show the characteristic responses for nanostructured TiO₂ electrodes, in particular a sharp cathodic peak as the electron density in the film increases. These responses are associated with the population of deep trap states with an average value of 5 × 10⁴ electrons per NT. Dynamic photocurrent measurements clearly show that the characteristic rise time of the photocurrent increases as the potential is increased above the onset region for charging deep trap states. At potentials in which deep trap states are fully depopulated in the dark, photocurrent rise time approaches values just below 1 s, which is more than 3 orders of magnitude slower than the estimated <i>RC</i> time constant. The occupancy of the deep trap states under photostationary conditions is a fraction of the density of states estimated from voltammetric responses. These findings are discussed in the context of current views about trap states in high surface area TiO₂ electrodes

Fast One-Pot Synthesis of MoS₂/Crumpled Graphene p–n Nanonjunctions for Enhanced Photoelectrochemical Hydrogen Production

Aerosol processing enables the preparation of hierarchical graphene nanocomposites with special crumpled morphology in high yield and in a short time. Using modular insertion of suitable precursors in the starting solution, it is possible to synthesize different types of graphene-based materials ranging from heteroatom-doped graphene nanoballs to hierarchical nanohybrids made up by nitrogen-doped crumpled graphene nanosacks that wrap finely dispersed MoS₂ nanoparticles. These materials are carefully investigated by microscopic (SEM, standard and HR TEM), diffraction (grazing incidence X-ray diffraction (GIXRD)) and spectroscopic (high resolution photoemission, Raman and UV−visible spectroscopy) techniques, evidencing that nitrogen dopants provide anchoring sites for MoS₂ nanoparticles, whereas crumpling of graphene sheets drastically limits aggregation. The activity of these materials is tested toward the photoelectrochemical production of hydrogen, obtaining that N-doped graphene/MoS₂ nanohybrids are seven times more efficient with respect to single MoS₂ because of the formation of local p–n MoS₂/N-doped graphene nanojunctions, which allow an efficient charge carrier separation