We demonstrate, for the first time, the synthesis of titania nanotubes with ultrathin (3-5 nm) wall thickness. As revealed by the incident photon-to-current collection efficiency (IPCE) and electrochemical impedance spectroscopy measurements, the ultrathin walls, less than the charge carrier diffusion length, were essential to ensure fast and efficient charge carrier collection.One-dimensional metal oxide nanoarchitectures have demonstrated great performance in many technologies including solar energy conversion. [1][2][3] In particular, TiO 2 nanotube arrays formed by electrochemical anodization have demonstrated outstanding performance in solar fuel generation and solar cell applications. 4,5 Although the use of nanotubular form decouples the light absorption and the transfer of charge carriers, enhancing the dynamics of charge carriers in TiO 2 is still a challenge. 6,7 In this regard, many studies have been devoted to improve the dynamics including passivation of defects, 8 the use of co-catalysts among others. 9,10 As many studies have shown the dependence of the functionality of the material on its physical dimensions, the best way to improve the transport and collection of charge carriers is to optimize the inherent intrinsic properties of the material. 7,11,12 To this end, controlling the length and the diameter of TiO 2 nanotubes has shown tremendous positive effects on the performance of the materials in solar energy conversion. [12][13][14] However, the effect of wall thickness was poorly discussed in the literature, despite the fact that it is one of the determinant factors controlling the dynamics of charge carriers, especially in photoelectrochemical water splitting systems. 7,11,14,15 Most of the published articles deal with wall thicknesses that are greater than the diffusion length of charge carriers in titania. 7 Herein, we report the first demonstration of the fabrication of vertically aligned titania nanotube arrays with very thin wall thickness (3-5 nm) and their use in solar water splitting. The thin-walled nanotubes facilitate the diffusion of the photogenerated holes to the semiconductor/ electrolyte interface during water splitting, allowing for efficient separation of charge carriers.The diffusion length of charge carriers in titania is around 10 nm, 7 and until now there has been no reproducible method to produce titania nanotubes with a wall thickness that is considerably lower than the diffusion length. Inspired by the work of Amer et al., 16 who were able to fabricate thin-walled ZrO 2 nanotubes, we used a mixture of non-aqueous (glycerol) and aqueous (water) electrolytes to anodize titanium in order to achieve titania nanotubes with ultrathin walls. The detailed experimental setup and conditions are summarized in the ESI †. In order to show the effect of the wall thickness, two sets of samples were fabricated; namely thick-walled nanotubes (NT1) using the conventional anodization method and thin-walled nanotubes (NT2) using our modified fabrication method, see the ESI † for more d...

No abstract

Magnesium, the most abundant divalent cation in cells, catalyzes RNA cleavage but also promotes RNA folding. Because folding can protect RNA from cleavage, we predicted a ‘Goldilocks landscape’, with local maximum in RNA lifetime at Mg2+ concentrations required for folding. Here, we use simulation and experiment to discover an innate and sophisticated mechanism of control of RNA lifetime. By simulation we characterized RNA Goldilocks landscapes and their dependence on cleavage and folding parameters. Experiments with yeast tRNAPhe and the Tetrahymena ribozyme P4–P6 domain show that structured RNAs can inhabit Goldilocks peaks. The Goldilocks peaks are tunable by differences in folded and unfolded cleavage rate constants, Mg2+ binding cooperativity, and Mg2+ affinity. Different folding and cleavage parameters produce Goldilocks landscapes with a variety of features. Goldilocks behavior allows ultrafine control of RNA chemical lifetime, whereas non-folding RNAs do not display Goldilocks peaks of protection. In sum, the effects of Mg2+ on RNA persistence are expected to be pleomorphic, both protecting and degrading RNA. In evolutionary context, Goldilocks behavior may have been a selectable trait of RNA in an early Earth environment containing Mg2+ and other metals.