Nanoporous anodic aluminium oxide has been widely used for the development of various functional nanostructures. So far these self-organized pore structures could only be prepared within narrow processing conditions. Here we report a new oxalic-acid-based anodization process for long-range ordered alumina membranes. This process is a new generation of the so-called "hard anodization" approach that has been widely used in industry for high-speed fabrication of mechanically robust, very thick (>100 microm) and low-porosity alumina films since the 1960s. This hard anodization approach establishes a new self-ordering regime with interpore distances, (D(int))=200-300 nm, which have not been achieved by mild anodization processes so far. It offers substantial advantages over conventional anodization processes in terms of processing time, allowing 2,500-3,500% faster oxide growth with improved ordering of the nanopores. Perfectly ordered alumina membranes with high aspect ratios (>1,000) of uniform nanopores with periodically modulated diameters have been realized.
In recent years, nanoporous anodic aluminum oxide (AAO) has been intensively exploited as a template material for the preparation of multifunctional nanostructures, which have applications in various scientific and technological fields. [1] In template-based materials synthesis, it is desirable to use a template with long-range order, so that structurally well-defined materials can be subsequently produced. In a typical anodization process, a self-ordered close-packed array of oxide nanopores forms with domain size (ordering length) on a scale of a few micrometers. [2] To achieve a long-rangeordered pore arrangement over a larger area, Masuda and co-workers first developed a pretexturing process that uses a SiC mold to produce ordered arrays of dimples on the Al substrate by nanoindentation prior to anodization. [3] Shallow indentations on an Al substrate initiate pore nucleation during anodization and lead to a long-range-ordered pore arrangement within the stamped area (e.g., 4 4 mm). This work has sparked considerable interest within the growing community of research groups using porous alumina, which is evident from the several hundred citations of these publications within a few years. However, few groups have been able to fabricate large-area, long-range-ordered alumina membranes due to the high processing costs of the imprint stamps, which can be a few thousand US$ for a cm 2 pattern.Recently, alternative methods based on focused ion beams (FIB), [4] optical diffraction gratings, [5] and micro-A C H T U N G T R E N N U N G beads [6] were also used to achieve prepatterning of Al substrates, thus avoiding fabrication of the expensive SiC imprint stamp. More recently, Masuda and co-workers demonstrated the fabrication of ideally ordered AAO films with a [*] Dr.
To meet the demanding requirements in plug-in hybrid electric vehicles (PHEVs) or electric vehicles (EVs), higher energy density materials, such as the Li-rich, layered manganese-based oxides (LLOs) with the general formula xLi 2 MnO 3 ·(1-x)LiTMO 2 (TM = Mn, Ni, Co, etc.), are promising candidates as they possess higher reversible capacity (>250 mAh g −1 ), improved safety and much reduced cost. [4][5][6][7][8][9] Recent microscopic evidence reveals the intergrowth of rhombohedral LiTMO 2 (R-3m) and the monoclinic Li 2 MnO 3 -like layered structure (C/2m) at the atomic scale in the oxide grains. [10] The Li 2 MnO 3 component serves as an electrochemically active phase for Li storage when cycled above 4.5 V versus Li/Li + . [8,[11][12][13][14] Nevertheless, these LLO materials undergo steady voltage/capacity decay when cycled above 4.5 V, resulting in a substantial decrease of the cathode energy density. [15][16][17][18] The origin of voltage/capacity decay upon cycling stems from cation migration between TM layers and Li layers and subsequent phase transformation. [19,20] The cationic doping with other metallic cations (such as Mg, [21] Al, [22] Ti, [23] Sn, [24] Ru, [25] Y, [26] Zn, [27] etc.) and polyanion doping based on nonmetal elements, such as BO 4 5− , [28] SiO 4 4− , [29] PO 4 3-, [30] etc., have been employed to improve the cyclic durability by weakening the TM-O covalency in the oxygen closepacked structure. In addition, surface coatings using metal oxides, [31][32][33][34] fluorides and phosphates, [35][36][37] LiNiPO 4 and Li 3 VO 4 , [38][39][40] have been applied to protect the surface structure from side reactions with the electrolyte under high voltage and to restrain the layered-to-spinel transformation which occurs preferentially on the crystal surface and leads to capacity fading of LLO materials. However, the ionic dopants and coating materials are mostly electrochemically inactive, so the improved cycling stability is achieved at the expense of reduced specific capacity/energy density of the cathode. Moreover, a conformal and continuous coating on the surface of oxide particles is rather difficult to obtain practically. Hence, advancing the structural and cycling stability in both the bulk material and the surface structure through a simple way is highly desired for potential applications of LLO materials.Herein, we develop a novel LLO material with a nanoscaled spinel-like surface layer through gradient doping of polyanions Surface Structural Transition Induced by Gradient Polyanion-Doping in Li-Rich Layered Oxides: Implications for Enhanced Electrochemical PerformanceYing Zhao, Jiatu Liu, Shuangbao Wang, Ran Ji, Qingbing Xia, Zhengping Ding, Weifeng Wei,* Yong Liu, Peng Wang,* and Douglas G. Ivey Lithium-rich layered oxides (LLOs) exhibit great potential as high-capacity cathode materials for lithium-ion batteries, but usually suffer from capacity/ voltage fade during electrochemical cycling. Herein, a gradient polyaniondoping strategy is developed to initiate surface structural trans...
SummaryThe fabrication of precise 2D Au nanoparticle arrays over a large area is presented. The technique was based on pre-patterning of the substrate before the deposition of a thin Au film, and the creation of periodic particle arrays by subsequent dewetting induced by annealing. Two types of pre-patterned substrates were used: The first comprised an array of pyramidal pits and the second an array of circular holes. For the dewetting of Au films on the pyramidal pit substrate, the structural curvature-driven diffusion cooperates with capillarity-driven diffusion, resulting in the formation of precise 2D particle arrays for films within a structure dependent thickness-window. For the dewetting of Au films on the circular hole substrate, the periodic discontinuities in the films, induced by the deposition, can limit the diffusion paths and lead to the formation of one particle per individual separated region (holes or mesas between holes), and thus, result in the evolution of precise 2D particle arrays. The influence of the pre-patterned structures and the film thickness is analyzed and discussed. For both types of pre-patterned substrate, the Au film thickness had to be adjusted in a certain thickness-window in order to achieve the precise 2D particle arrays.
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