Here we report that TiO2 nanotube (NT) arrays, converted by a high pressure H2 treatment to anatase-like "black titania", show a high open-circuit photocatalytic hydrogen production rate without the presence of a cocatalyst. Tubes converted to black titania using classic reduction treatments (e.g., atmospheric pressure H2/Ar annealing) do not show this effect. The main difference caused by the high H2 pressure annealing is the resulting room-temperature stable, isolated Ti(3+) defect-structure created in the anatase nanotubes, as evident from electron spin resonance (ESR) investigations. This feature, absent for conventional reduction, seems thus to be responsible for activating intrinsic, cocatalytic centers that enable the observed high open-circuit hydrogen generation.

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...

Abstract:In the present work we show, how a high pressure hydrogenation of commercial anatase or anatase/rutile powder can create a photocatalyst for hydrogen evolution that is highly effective and stable without the need of any additional co-catalyst. This activation effect can not be observed for rutile. For anatase/rutile mixtures, however, a strong synergistic effect is found (similar to findings commonly observed for noble metal decorated TiO 2 ). ESR measurements indicate the intrinsic co-catalytic activation of anatase TiO 2 to be due to specific defect centers formed during hydrogenation. 2Ever since the groundbreaking work of Fujishima and Honda in 1972 [1], TiO 2 is considered as a promising photocatalyst for the splitting of water into H 2 and O 2 . In the original experiment, Fujishima et al. used a TiO 2 photoanode, connected via an external circuit to a platinum counter electrode -the latter was needed to successfully evolve H 2 from water. Due to the simplicity of the concept, illumination of a cheap and abundant semiconductor to create photoexcited charge carriers that can be transferred directly to water to form a high density energy fuel (H 2 ), the report found a tremendous scientific resonance. Meanwhile, more than 10000 papers have been published on using TiO 2 in a large palette of morphologies and modifications, to trigger a wide range of photocatalytic ractions (for overviews see e.g. refs.[2-8]). While numerous photoelectrochemical studies (i.e., using an illuminated TiO 2 electrode in an electrochemical circuit) where performed, still the most direct and economic approach is the use of TiO 2 in the form of particle suspensions -thus using the photocatalytic system without an external applied voltage. However, under these so called open-circuit conditions (OCP), TiO 2 alone is not efficient for the photoproduction of hydrogen without the use of a co-catalyst -mostly this is a noble metal (M), such as Pt, Pd or Au -for overviews see e.g. refs. [9][10][11]. These combined photocatalytic M@TiO 2 systems have therefore been widely investigated in view of optimizing their efficiency towards H 2 generation from water (with or without using sacrificial agents such as ethanol) [9,12].In general, the function of the noble metal co-catalyst has been described in terms of i) providing an electron acceptor that mediates electron transfer to the electrolyte, ii) forming solid state junctions (metal/semiconductor), or iii) acting as a hydrogen recombination center that strongly promotes H 2 formation [9][10][11]. In M@TiO 2 catalysts it has been generally observed that the crystalline phase of TiO 2 is a very important factor for the performance of such photocatalytic H 2 -generation systems [6-9, 13, 14]. Anatase and rutile are the most commonly used polymorphs in photoactivated TiO 2 applications. In photocatalytic water splitting, generally M@anatase combinations are found to be more efficient than M@rutile. 3This difference in photocatalytic activity is commonly attributed to a higher charge recom...

The high-pressure hydrogenation of commercially available anatase or anatase/rutile TiO 2 powder can create a photocatalyst for H 2 evolution that is highly effective and stable without the need for any additional co-catalyst. This activation effect cannot be observed for rutile; however, for anatase/rutile mixtures, a strong synergistic effect can be found (similar to results commonly observed for noble-metal-decorated TiO 2 ). EPR and PL measurements indicated the intrinsic co-catalytic activation of anatase TiO 2 to be due to specific defect centers formed during hydrogenation. These active centers can be observed specifically for high-pressure hydrogenation; other common reduction treatments do not result in this effect.

Anodic aluminum oxide (AAO) ceramic membranes with macroscopically aligned and hexagonally packed nanopore architecture are attractive substrates for forming nanotubular lipid bilayers as well as sorption and catalytic media because of a tunable pore diameter, robust pore structure, and low fabrication cost. Here we employed continuous wave X-band (9 GHz) EPR of two pH-sensitive nitroxide radicals to assess acid−base properties AAO membranes prepared from low-cost commercial grade aluminum and compared those with commercial Anodisc membranes from Whatman, Ltd. The AAO membranes with pore diameters ≥58 ± 8 nm showed essentially the same pH inside the pores, pH int , as the bulk external solution, pH ext , over the 0.1−3.0 M range of ionic strength. However, the apparent pK a of nitroxide probes inside the pores deviated from the bulk values for the nanopores of smaller diameters of ca. 29 and 18 nm. Specifically, for the latter nanopores the values of pH int were found to be 0.5−0.8 pH unit lower than the bulk pH ext . An increase in acidity of the bulk solution led to a steady decrease of the negative charge on inner surface of the 38 nm nanopores and its recharge from a negative to a positive value at pH 4.7 ± 0.1, corresponding to the point of zero charge (pzc). Overall, the EPR titration method described here could assist in characterization of meso-and nanoporous membranes for catalytic and sorption applications as well as act a support medium for self-assembled biomembrane systems.