No abstract

Porous silicon ͑PS͒ with a micro-nano-hybrid structure has been successfully fabricated with an electrochemical etching process. The micropores consist of one-dimensional tunnels, which vary from ca. 1 to 1.5 m in pore diameter and extend up to 15 m in depth. The walls of these micropores are covered with a nanoporous structure that consists of small spherical particles, the feature size of which is of the order of tens of nanometers or smaller. The as-prepared PS structures show cathodic/anodic peaks for lithiation/delithiation during cyclic voltammetry with minimal destruction of either the micropore or nanopore of its wall even after 50 cycles. Furthermore, the peak current, the cumulative charge increase, and the electrochemical impedance for electrode reactions consistently decreases with the surface area of the tunnel wall, indicating that processes at the tunnel wall govern the overall lithiation/delithiation reactions. A hybrid porous structure consisting of microtunnels with nanostructured surface layers appears to provide a viable and practical way to utilize silicon for anode materials in rechargeable microbatteries.One of the largest obstacles to the breakthrough of high-capacity lithium-ion batteries is the limited reversible capacity of the anode material, i.e., graphite, which is used commercially in most of the rechargeable lithium batteries. Much effort has recently been made to find a reasonable substitute to meet the rapid advancement of telecommunication devices toward digital multifunction systems. [1][2][3] Silicon has been widely studied as a promising candidate for the next-generation anode material, due to its high theoretical specific capacity ͑4200 mAh g −1 ͒ and its applicability for an on-chip microbattery. 4,5 A variety of structures and Si-based composites have been examined in order to reduce the lithiation-induced stress and suppress the structural destruction of silicon, which is believed to be the main cause for the loss of sustainability and the lack of capacity retention during charge/discharge cycling. 6-12 Nevertheless, finding an optimal structure/composition of silicon and/or siliconbased materials is still a great challenge to most of the researchers in this field.In a previous communication, 13 we have reported preliminary results on the use of porous silicon ͑PS͒ with a one-dimensional microtunnel structure as the anode material for rechargeable microbatteries. The results showed that lithiation/delithiation reactions readily take place in PS with minimal structural change. Nevertheless, PS has many scientific and practical issues still unexplored for battery applications. These include the effect of the area/structure of the tunnel wall on electrochemical activity, difference in the activity between the surface and the tunnel wall in the PS structure, and sustainability of the charge/discharge reactions. Here we report recent results on PS with a hybrid micro-nano-porous structure, prepared by a select electrochemical etching process. In particular, the structural chan...

With a goal of preparing silicon-based interactive filters in the micropore range, we have developed the first single-step etch-liftoff procedure ͑one-step separation͒ based on the formation and removal of macroporous silicon layers. With silicon wafers whose resistivities are in the range from 14 to 22 ⍀ cm, we are able to create, in a surprisingly controlled manner, films whose pore diameters range from 1-2 m and whose thickness ranges from 3 to 70 m. These silicon-based films ͑filters͒, which carry a polarizing negative charge, represent an alternative to porous alumina ͑films͒ filters ͑pore diameter not yet in the 1-2 m range͒ both in terms of their size range and potential interaction-reaction at elevated temperature. Preliminary experiments demonstrate that platinum and copper can be introduced to these filters using electroless solutions with a goal to creating an effective reductive surface. Using these and alternate material combinations, interactive-reactive filters in the 1 m size range that operate at temperatures well in excess of porous polymer films can be realized.

We study diffusion-controlled, solvent-mediated self-assembly and dendritic structure formation as anatase TiO2 is seeded with silver oxides and silver that are formed using silver nitrate (zinc-catalyzed). Anatase TiO2 nanocolloid solutions are seeded with silver nitrate over a concentration range extending to saturation as the zinc-catalyzed transformation of the nitrate is studied in a number of solvents used in the formation and study of porous silicon interfaces. In acetone alone and, to a lesser extent, butanone, the silver/silver-oxide-seeded titania nanoparticles assemble into needlelike arrays of significant extent. This assembly is greatly accentuated in benzene/acetone mixtures and, to a much lesser extent, in diethyl ether/acetone mixtures, due primarily to the significant vapor pressure of the ether. In contrast to these aprotic solvents, self-assembly cannot be induced in the protic solvents water, methanol, and ethanol or in the aprotic solvents acetonitrile, methylene chloride, and dimethylformamide, even in mixtures with acetone. These results are explained and attributed to several controlling factors:  the titania crystal structure and the nature of its subsequent silver seeding; the nature of diffusion through the considered solvents; the formation of silver acetate, propionate, and benzoate; the formation of strong cyano and chloride bonds. At saturated silver nitrate concentrations, distinctly different solvent-mediated dendrite growth in water and acetone solvents is explained from the perspective of a diffusion-limited aggregation (DLA) process.