Ultrafast elemental and oxidation-state mapping of hematite by 4D electron microscopy

This work was supported by the Air Force Office of Scientific Research (FA9550-11-1-0055) in the Gordon and Betty Moore Center for Physical Biology at the California Institute of Technology.We describe a new methodology that sheds light on the fundamental electronic processes that occur at the subsurface regions of inorganic solid photocatalysts. Three distinct kinds of microscopic imaging are used that yield spatial, temporal and energy-resolved information. We also carefully consider the effect of photon-induced near-field electron microscopy (PINEM), first reported by Zewail et al. in 2009. The value of this methodology is illustrated by studying afresh a popular and viable photocatalyst, hematite, α-Fe2O3, that exhibits most of the properties required in a practical application. By employing high-energy electron-loss signals (of several hundred eV), coupled to femtosecond temporal resolution as well as ultrafast energy-filtered transmission electron microscopy in 4D, we have, inter alia, identified Fe4+ ions that have a lifetime of a few picoseconds, as well as associated photoinduced electronic transitions and charge transfer processes.PostprintPeer reviewe

Highly graphitized nitrogen-doped porous carbon nanopolyhedra derived from ZIF-8 nanocrystals as efficient electrocatalysts for oxygen reduction reactions

Nitrogen-doped graphitic porous carbons (NGPCs) have been synthesized by using a zeolite-type nanoscale metal–organic framework (NMOF) as a self-sacrificing template, which simultaneously acts as both the carbon and nitrogen sources in a facile carbonization process. The NGPCs not only retain the nanopolyhedral morphology of the parent NMOF, but also possess rich nitrogen, high surface area and hierarchical porosity with well-conducting networks. The promising potential of NGPCs as metal-free electrocatalysts for oxygen reduction reactions (ORR) in fuel cells is demonstrated. Compared with commercial Pt/C, the optimized NGPC-1000-10 (carbonized at 1000 °C for 10 h) catalyst exhibits comparable electrocatalytic activity via an efficient four-electron-dominant ORR process coupled with superior methanol tolerance as well as cycling stability in alkaline media. Furthermore, the controlled experiments reveal that the optimum activity of NGPC-1000-10 can be attributed to the synergetic contributions of the abundant active sites with high graphitic-N portion, high surface area and porosity, and the high degree of graphitization. Our findings suggest that solely MOF-derived heteroatom-doped carbon materials can be a promising alternative for Pt-based catalysts in fuel cells

Formation, morphology control and applications of anodic TiO₂ nanotube arrays

Anodic titanium dioxide films, especially anodic TiO2 nanotube arrays, have attracted extensive interest in the past decade. A number of electrolytes, either aqueous or non-aqueous, fluoride containing or fluoride free, have been chosen to grow anodic titanium oxide films. With great improvements in the morphology control on porosity, pore size, nanotube length and pore ordering, anodic titanium oxide films have been widely applied in photochemical water splitting, hydrogen sensing, dye-sensitized solar cells, templating for low dimensional nanomaterials and biomedical research. This article presents a brief review of the progress to date in the formation mechanism, morphology control and some applications of these smart materials.</p

Formation, microstructures and crystallization of anodic titanium oxide tubular arrays

Formation of highly ordered TiO2 nanotubular arrays during anodization of titanium can be elucidated by using the equifield strength model and a double-layer structure. The two characteristic microstructural features of anodic titanium oxide (ATO) in comparison with anodic aluminium oxide (AAO), a thin titanium hydroxide layer and an O-ring like surface pattern, were investigated using scanning electron microscopy and high resolution transmission electron microscopy (HRTEM). Field-enhanced dissociation of water is extremely important in the formation of the nanotubes with a double-layer wall and an O-ring-like pattern, and in the determination of porosity. The relations between porosity of the ATO films and the anodization conditions, such as current density and electric field strength, have been established. Crystallization of the anodic TiO2 nanotubular arrays was also achieved and the microstructures were studied by using HRTEM.</p

Pore diameter control in anodic titanium and aluminium oxides

A nonlinear relation between the pore diameter and anodising voltage is established for nanoporous anodic titanium oxide (ATO) and anodic aluminium oxide (AAO). The pore diameters of both ATO and AAO have been found to increase with the anodising voltage and drop down when the voltage exceeds a critical value. The origin for the existence of this maximum value of pore diameter in AAO and ATO is discussed.</p

Formation, morphology control and applications of anodic TiO₂ nanotube arrays

Anodic titanium dioxide films, especially anodic TiO2 nanotube arrays, have attracted extensive interest in the past decade. A number of electrolytes, either aqueous or non-aqueous, fluoride containing or fluoride free, have been chosen to grow anodic titanium oxide films. With great improvements in the morphology control on porosity, pore size, nanotube length and pore ordering, anodic titanium oxide films have been widely applied in photochemical water splitting, hydrogen sensing, dye-sensitized solar cells, templating for low dimensional nanomaterials and biomedical research. This article presents a brief review of the progress to date in the formation mechanism, morphology control and some applications of these smart materials.</p

Pore diameter control in anodic titanium and aluminium oxides

A nonlinear relation between the pore diameter and anodising voltage is established for nanoporous anodic titanium oxide (ATO) and anodic aluminium oxide (AAO). The pore diameters of both ATO and AAO have been found to increase with the anodising voltage and drop down when the voltage exceeds a critical value. The origin for the existence of this maximum value of pore diameter in AAO and ATO is discussed.</p