The present work brings together the concepts of defect chemistry and photoelectrochemistry in order to consider TiO2-based photosensitive oxide semiconductors as photocatalysts for water purification. This paper reports the most recent progress in the defect chemistry of TiO2 and its solid solutions with aliovalent ions forming donors and acceptors. The relationship between the defect-related properties, such as electrical and photocatalytic properties, are outlined. It is shown that reactivity, photoreactivity, and the related charge transfer of photocatalysts based on TiO2 are determined by defect disorder and the related chemical potential of electrons. Therefore, defect chemistry may be used as a framework for the processing of well-defined TiO2-based photocatalysts. The photoreactivity of TiO2 with water and its solutes is considered in terms of the effect of both collective and local properties. The effect of noble metals attached to TiO2 as a separate phase, such as platinum, on photoelectrochemical properties and the related photocatalytic performance of TiO2 is discussed. The key functional properties, which are responsible for the efficient conversion of solar energy into chemical energy (required for water purification), are outlined. The effect of TiO2 doping with aliovalent ions on properties is considered in terms of the doping mechanisms and the related semiconducting properties. It is argued that comparison of the experimental data reported in the literature on the photocatalytic properties of TiO2 dictates the need to establish standards for photocatalysts, which are well-defined. This paper reports the processing conditions of well-defined TiO2. It is argued that knowledge of the mass transport kinetic data, such as chemical and self-diffusion coefficients, is needed for selecting the optimal processing conditions.

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A fibrous herringbone-modified helicoidal architecture is identified within the exocuticle of an impact-resistant crustacean appendage. This previously unreported composite microstructure, which features highly textured apatite mineral templated by an alpha-chitin matrix, provides enhanced stress redistribution and energy absorption over the traditional helicoidal design under compressive loading. Nanoscale toughening mechanisms are also identified using high load nanoindentation and in-situ TEM picoindentation. A Sinusoidally-Architected Helicoidal BiocompositeBy Nicholas A. Yaraghi, Nicolás Guarín-Zapata, Lessa K. Grunenfelder, Eric Hintsala, Sanjit Bhowmick, Jon M. Hiller, Mark Betts, Edward L. Principe, Jae-Young Jung, Leigh Sheppard, Richard Wuhrer, Joanna McKittrick, Pablo D. Zavattieri Keywords: (Composites, Toughness, Impact, Biomineral, Ultrastructure) Submitted to 3 Biologically mineralized composites offer inspiration for the design of next generation structural materials due to their low density, high strength and toughness currently unmatched by engineering technologies. [1][2][3][4][5][6][7][8][9] Such properties are based on the ability for the organism to utilize structural organics and acidic proteins to guide and control the mineralization process to yield hierarchical architectures with well-defined compositional gradients.One notable example is the highly developed raptorial appendage, or dactyl, of the stomatopods, a group of aggressive marine crustaceans that use these structures for feeding upon hard-shelled and soft-bodied prey. [10][11][12][13][14] The dactyls of the "smashers", those that feed primarily on hard-shelled prey, (see Figure 1A) takes the form of a bulbous club ( Figure 1B), which is used to smash through mollusk shells, crab exoskeletons, and other tough mineralized structures with tremendous force and speed. [11][12][13][14][15][16] Achieving accelerations over 10,000g and reaching speeds of 23 m/s from rest, the dactyl strike is recognized as one of the fastest and most powerful impacting events observed in Nature. [11,12] The club is capable of delivering and subsequently enduring repetitive impact forces up to 1500 N and cavitation stresses without catastrophically failing, demonstrating its utility as an exceptionally damage-tolerant natural material.The origins of such a mechanical response lie in the structural design. Previous work identified the club as a multi-regional composite material containing an organic matrix composed of alpha-chitin fibers mineralized by amorphous forms of calcium carbonate and calcium phosphate as well as crystalline apatite. [17,18] These investigations revealed mechanisms responsible for providing damage-tolerance and impact-resistance to the club, which were largely attributed to the interior of the club (periodic region), identified as the primary energy-absorbing layer. [17,18] The combination of soft polymeric nanofibers and stiffer mineral provides a periodic modulus mismatch leading to crack deflection, which in co...

The mechanism of photoreactivity between the TiO(2) surface and H(2)O, and the related charge transfer, is considered in terms of both collective and local properties. It is shown that the effective charge transfer between TiO(2) and water requires the presence of surface active sites that are able to provide electron holes to adsorbed water molecules. Titanium vacancies located at or near the surface are identified as the active sites for water adsorption leading to the formation of an active complex and resulting, in consequence, in water splitting. A model of the photoreactivity between the TiO(2) surface and water is proposed. This model indicates that the photoreactivity of the TiO(2)-based photoelectrode may be enhanced through imposition of the surface active sites (Ti vacancies) in a controlled manner by surface engineering.