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

On the dynamical nature of the active center in a single-site photocatalyst visualized by 4D ultrafast electron microscopy

Understanding the dynamical nature of the catalytic active site embedded in complex systems at the atomic level is critical to developing efficient photocatalytic materials. Here, we report, using 4D ultrafast electron microscopy, the spatiotemporal behaviors of titanium and oxygen in a titanosilicate catalytic material. The observed changes in Bragg diffraction intensity with time at the specific lattice planes, and with a tilted geometry, provide the relaxation pathway: the Ti^(4+)=O^(2−) double bond transformation to a Ti^(3+)−O^(1−) single bond via the individual atomic displacements of the titanium and the apical oxygen. The dilation of the double bond is up to 0.8 Å and occurs on the femtosecond time scale. These findings suggest the direct catalytic involvement of the Ti^(3+)−O^(1−) local structure, the significance of nonthermal processes at the reactive site, and the efficient photo-induced electron transfer that plays a pivotal role in many photocatalytic reactions

Pricing Decision of Closed-Loop Supply Chain to Improve Service Level under Patent Protection

This paper constructs a two-level closed-loop supply chain system consisting of original parts manufacturers and parts distributors. Based on the different preferences of consumers for remanufactured parts and new parts, four combination models of patent protection and service improvement are constructed. Through comparative analysis, the impact of implementing patent protection policies by original parts manufacturers and improving service levels by parts distributors on the pricing decisions of the closed-loop supply chain is explored.Through the comparison between related models and the verification of calculation examples, it is found that (1) a manufacturer prevents the price of new products from being affected by the price of remanufactured products and upgrading of service level by introducing royalties, which reduces its loss of profit; (2) in the absence of patent protection, the manufacturerꞌs profit decreases as the level of service increases; in the presence of patent protection, the manufacturerꞌs profit increases as the level of service increases; (3) retailersꞌ profits decrease after the manufacturer introduces royalties, which discourages them to improve service levels for remanufactured products; (4) as retailers raise the service level of the remanufactured products, the profits of the manufacturer and third-party manufacturers keep increasing, while the profits of the retailers first increase and then decrease

Hydrothermal growth of highly oriented single crystalline Ta2O5 nanorod arrays and their conversion to Ta3N5 for efficient solar driven water splitting

We grow vertically aligned single crystalline Ta2O5 nanorod arrays that can be converted to Ta3N5 nanorod arrays by nitridation. Combined with cobalt phosphate (Co-Pi) as a co-catalyst, such Ta3N5 nanorod photoanodes can yield photocurrent densities of ∼3.6 mA cm−2 at 1.23 VRHE and ∼8.2 mA cm−2 at 1.59 VRHE under AM 1.5G (100 mW cm−2) irradiation

Self-organized cobalt fluoride nanochannel layers used as a pseudocapacitor material

Aligned CoF2 nanochannel layers have been formed by self-ordering electrochemical anodization. In voltammograms these layers provide multiple oxidation states, an almost ideal rectangular pseudocapacitive behavior, a high specific capacitance and good capacitance retention. These layers may thus be promising for supercapacitor applications

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

Porous anodic metal oxides

An equifield strength model has been established to elucidate the formation mechanism for the highly ordered alumina pore arrays and titanium oxide nanotubular arrays prepared via a common electrochemical methodology, anodisation. The fundamentals of the equifield strength model was the equilibrium between the electric field driven oxidation rate of the metal and electric field enhanced dissolution rate of oxide. During the anodic oxidation of metal, pore initiation was believed to generate based on dissolution rate difference caused by inhomogeneity near the metal/oxide interface. The ionic nanoconvection driven by the electric force exerted on the space charge layer in the vicinity of electrolyte/oxide interface is established to be the main driving force of the pore ordering at the early stage of the anodisation. While the equifield strength requirement governs the following formation of the single pore and the self-ordering of random distributed pore arrays during the anodisation process. Hexagonal patterned Al2O3 nanopore arrays and TiO2 nanotubular arrays have been achieved by anodisation of corresponding metal substrates in proper electrolytes. The two characteristic microstructural features of anodic aluminium oxide (AAO) and anodic titanium oxide (ATO) were investigated using scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). The observations of the hemispherical electrolyte/oxide and oxide/metal interfaces, uniform thickness of the oxide layer, as well as self-adjustment of the pore size and pore ordering can be well explained by the equifield strength model. Field enhanced dissociation of water is extremely important in determination of the porosity of anodic metal oxide. The porosity of AAO and ATO films was found to be governed by the relative dissociation rate of water which is dependent on anodisation conditions, such as electrolyte, applied voltage, current density and electric field strength. Using an empirical method, the relations between the porosity of the AAO (ATO) films and the anodisation parameters, such as electric field strength, current density and applied voltage, have been established. Besides, the extent that an external electric field can facilitate the heterolytic dissociation of water molecule has been estimated using quantum-chemical model computations combined with the experimental aspect. With these achievements, the fabrication of anodic metal oxide films can be understood and controlled more precisely. Additionally, the impacts of other factors such as the electrolyte type and the temperature effect on the morphology of the anodic products were also investigated. Some important experimental evidences on the pore diameters variation with applied voltage in the anodisation of aluminium and the titanium were obtained for future investigation of the anodic metal oxide formation processes

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