Photocatalytic approaches, that is the reaction of light-produced charge carriers at a semiconductor surface with their environment, currently attract an extremely wide scientific interest. This is to a large extent due to the high expectations: i) to convert sunlight directly into an energy carrier (H(2)), ii) to stimulate chemical synthetic reactions, or iii) to degrade unwanted environmental pollutants. Since the early reports in 1972, TiO(2) has been the most investigated photocatalytic material by far; this originates from its outstanding electronic properties that allow for a wide range of applications. Not only the material, but also its structure and morphology, can have a considerable influence on the photocatalytic performance of TiO(2). In recent years, particularly 1D (or pseudo 1D) structures such as nanowires and nanotubes have received great attention. The present Review focuses on TiO(2) nanotube arrays (and similar structures) that grow by self-organizing electrochemistry (highly aligned) from a Ti metal substrate. Herein, the growth, properties, and applications of these tubes are discussed, as well as ways and means to modify critical tube properties. Common strategies are addressed to improve the performance of photocatalysts such as doping or band-gap engineering, co-catalyst decoration, junction formation, or applying external bias. Finally, some unique applications of the ordered tube structures in various photocatalytic approaches are outlined.

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.

All-inorganic lead halide perovskites demonstrate improved thermal stability over the organic-inorganic halide perovskites, but the cubic α-CsPbI with the most appropriate bandgap for light harvesting is not structurally stable at room temperature and spontaneously transforms into the undesired orthorhombic δ-CsPbI. Here, we present a new member of black-phase thin films of all-inorganic perovskites for high-efficiency photovoltaics, the orthorhombic γ-CsPbI thin films with intrinsic thermodynamic stability and ideal electronic structure. Exempt from introducing organic ligands or incorporating mixed cations/anions into the crystal lattice, we stabilize the γ-CsPbI thin films by a simple solution process in which a small amount of HO manipulates the size-dependent phase formation through a proton transfer reaction. Theoretical calculations coupled with experiments show that γ-CsPbI with a lower surface free energy becomes thermodynamically preferred over δ-CsPbI at surface areas greater than 8600 m/mol and exhibits comparable optoelectronic properties to α-CsPbI. Consequently, γ-CsPbI-based solar cells display a highly reproducible efficiency of 11.3%, among the highest records for CsPbI thin-film solar cells, with robust stability in ambient atmosphere for months and continuous operating conditions for hours. Our study provides a novel and fundamental perspective to overcome the Achilles' heel of the inorganic lead iodide perovskite and opens it up for high-performance optoelectronic devices.

Photocatalytic reactions on TiO 2 have recently gained an enormous resurgence because of various new strategies and findings that promise to drastically increase efficiency and specificity of such reactions by modifications of the titania scaffold and chemistry. In view of geometry, in particular, anodic TiO 2 nanotubes have attracted wide interest, as they allow a high degree of control over the separation of photogenerated charge carriers not only in photocatalytic reactions but also in photoelectrochemical reactions. A key advantage of ordered nanotube arrays is that nanotube modifications can be embedded site specifically into the tube wall; that is, cocatalysts, doping species, or junctions can be placed at highly defined desired locations (or with a desired regular geometry or pattern) along the tube wall. This allows an unprecedented level of engineering of energetics of reaction sites for catalytic and photocatalytic reactions, which target not only higher efficiencies but also the selectivity of reactions. Many recent tube alterations are of a morphologic nature (mesoporous structures, designed faceted crystallites, hybrids, or 1D structures), but a number of color-coded (namely, black, blue, red, green, gray) modifications have attracted wide interest because of the extension of the light absorption spectrum of titania in the visible range and because unique catalytic activity can be induced. The present Perspective gives an overview of TiO 2 nanotubes in photocatalysis with an emphasis on the most recent advances in the use of nanotube arrays and discusses the underlying concepts in tailoring their photocatalytic reactivity.

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...