Crystal Surfaces and Fate of Photogenerated Defects in Shape-Controlled Anatase Nanocrystals: Drawing Useful Relations to Improve the H₂ Yield in Methanol Photosteam Reforming

We comprehensively explored the photocatalytic properties, in H₂ production by methanol photosteam reforming, of anatase nanocrystals with nearly rectangular (<i>RC</i>), rhombic (<i>R</i>), and nanobar (<i>NB</i>) shapes having exposed {001}, {101}, and {010} surfaces. The aim was to relate the reactivity both to the type of crystal facets and to the photogenerated defects. The electron spin resonance (ESR) spectra reveal that the amount of Ti³⁺ (electron traps) is parallel to the H₂ evolution rate and becomes a maximum for the <i>RC</i> nanocrystals, which display the highest area of {001} surfaces and the lowest {101} area but also involve a significant area of {010} facets. This points out that the H₂ production cannot be related only to the envisaged reducing {101} facets, but that the {010} facets also play a key role. We suggest that the contiguous {001}, {101}, and {010} facets form a highly effective “surface heterojunction” within a <i>RC</i> nanoparticle which drives the electrons photogenerated on {001} facets not just toward the {101} but also to the {010} facets, while the holes are driven toward the {001} facets. This transfer improves the charge separation, thus boosting the photoefficiency of <i>RC</i> nanocrystals compared to that of <i>NB</i> and <i>R</i> nanocrystals. The ESR spectra performed after ultraviolet excitation in the presence of MeOH show the partial annihilation of the Ti³⁺ features, mainly for highly reactive <i>RC</i> nanocrystals. Because H₂ production involves an electron transfer to the proton, a relevant role in H⁺ photoreduction of the Ti³⁺ centers present on the exposed {010} and {101} surfaces is suggested. These findings underline the importance of determining the relationship between the photogenerated defects and the exposed crystal surfaces to optimize the photocatalytic properties of anatase nanocrystals

Mineralogy and geochemistry of Devonian ultramafic minor intrusions of the southern Kola Peninsula, Russia: implications for the petrogenesis of kimberlites and melilitites

Rechargeable sodium-ion batteries are becoming a viable alternative to lithium-based technology in energy storage strategies, due to the wide abundance of sodium raw material. In the past decade, this has generated a boom of research interest in such systems. Notwithstanding the large number of research papers concerning sodium-ion battery electrodes, the development of a low-cost, well-performing anode material remains the largest obstacle to overcome. Although the well-known anatase, one of the allotropic forms of natural TiO₂, was recently proposed for such applications, the material generally suffers from reduced cyclability and limited power, due to kinetic drawbacks and to its poor charge transport properties. A systematic approach in the morphological tuning of the anatase nanocrystals is needed, to optimize its structural features toward the electrochemical properties and to promote the material interaction with the conductive network and the electrolyte. Aiming to face with these issues, we were able to obtain a fine tuning of the nanoparticle morphology and to expose the most favorable nanocrystal facets to the electrolyte and to the conductive wrapping agent (graphene), thus overcoming the intrinsic limits of anatase transport properties. The result is a TiO₂-based composite electrode able to deliver an outstandingly stability over cycles (150 mA h g^–1 for more than 600 cycles in the 1.5–0.1 V potential range) never achieved with such a low content of carbonaceous substrate (5%). Moreover, it has been demonstrated for the first time than these outstanding performances are not simply related to the overall surface area of the different morphologies but have to be directly related to the peculiar surface characteristics of the crystals

Interplay between Composition, Structure, and Properties of New H₃PO₄‑Doped PBI₄N–HfO₂ Nanocomposite Membranes for High-Temperature Proton Exchange Membrane Fuel Cells

Polybenzimidazole (PBI) has become a popular polymer of choice for the preparation of membranes for potential use in high-temperature proton exchange membrane polymer fuel cells. Phosphoric acid-doped composite membranes of poly­[2,2′-(<i>m</i>-phenylene)-5,5′-bibenzimidazole] (PBI4N) impregnated with hafnium oxide nanofiller with varying content levels (0–18 wt %) have been prepared. The structure–property relationships of both the undoped and acid-doped composite membranes are studied using thermogravimetric analysis, modulated differential scanning calorimetry, dynamic mechanical analysis, wide-angle X-ray scattering, infrared spectroscopy, and broadband electrical spectroscopy. Results indicate that the presence of nanofiller improves the thermal and mechanical properties of the undoped membranes and facilitates a greater level of acid uptake. The degree of acid dissociation within the acid-doped membranes is found to increase with increasing nanofiller content. This results in a conductivity, at 215 °C and a nanofiller level <i>x</i> ≥ 0.04, of 9.0 × 10^–2 S cm^–1 for [PBI4N­(HfO₂)_<i>x</i>]­(H₃PO₄)_<i>y</i>. This renders nanocomposite membranes of this type as good candidates for use in high temperature proton exchange membrane fuel cells (HT-PEMFCs)