Abstract:We apply high-energy proton ion-implantation to modify TiO 2 nanotubes selectively at their tops. In the proton-implanted region we observe the creation of intrinsic co-catalytic centers for photocatalytic H 2 -evolution. We find proton implantation to induce specific defects and a characteristic modification of the electronic properties not only in nanotubes but also on anatase single crystal (001) surfaces. Nevertheless, for TiO 2 nanotubes a strong synergetic effect between implanted region (catalyst) and implant-free tube segment (absorber) can be obtained.
Keywords:nanotubes; photocatalysts; water-splitting; titania; self-organization; ion-implantation 2 Ever since 1972, when Honda and Fujishima introduced photolysis of water using a single crystal of TiO 2 , photocatalytic water splitting has become one of the most investigated scientific topics of our century [1]. The concept is strikingly simple: light (preferably sunlight) is absorbed in a suitable semiconductor and thereby generates electron-hole pairs. These charge carriers migrate in valence and conduction bands to the semiconductor surface where they react with water to form O 2 and H 2 , respectively. Thus hydrogen, the energy carrier of the future, could be produced using just water and sunlight.Key factors for an optimized conversion of water to H 2 are i) as complete as possible absorption of solar light (small band gap) while ii) still maintaining the thermodynamic driving force for water splitting (sufficiently large band-gap), including suitable band-edge positions relative to the water red-ox potentials, and iii) possibly most challenging -a sufficiently fast carrier transfer from semiconductor to water to obtain a reasonable reaction kinetics as opposed to carrier recombination or photo-corrosion [2][3][4][5][6][7].In spite of virtually countless investigations on a wide range of semiconductor materials that in many respects are superior to titania (mostly in view of solar light absorption and carrier transport), TiO 2 still remains one of the most investigated photocatalysts. This is only partially due to suitable energetics but more so because of its outstanding (photo-corrosion) stability [2][3][4][5][6][7].In general, the main drawbacks of TiO 2 are on the one hand its too large band-gap of 3-3.2 eV that allow only for about 7% of solar light absorption, and on the other hand that although a charge transfer to aqueous electrolytes is thermodynamically possible, the kinetics of these processes at the TiO 2 /water interface are extremely slow if no suitable co-catalysts such as Pt, Au, Pd or similar are used [8][9][10]. Mao demonstrated a significantly increased photocatalytic activity for water splitting when black TiO 2 was loaded with a Pt co-catalyst and used under bias-free conditions (i.e. used directly as a nanoparticle suspension in an aqueous/methanol solution under sunlight (AM 1.5) conditions). The high catalyst activity was attributed to a thin amorphous TiO 2 hydrogenated layer that was formed under high pressure tre...
Low-dose nitrogen implantation induces an ion and damage profile in TiO2 nanotubes that leads to "co-catalytic" activity for photocatalytic H2 -evolution (without the use of any noble metal). Ion implantation with adequate parameters creates this active zone limited to the top part of the tubes. The coupling of this top layer and the underlying non-implanted part of the nanotubes additionally contributes to an efficient carrier separation and thus to a significantly enhanced H2 generation.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.