Morphology and physical-chemical properties of baked nanoporous frustules

We investigated the morphology and physical-chemical properties of baked and unbaked nanoporous frustules. Scanning electron microscopy (SEM) observations showed that the nanoporous structures of frustules unchanged at 400 degrees C even after baking for 6 h. During baking at 800 degrees C, the frustule structures changed dramatically. On the other hand, Fourier transform infrared spectroscopy (FTIR) of bulk frustule samples indicated that physical-chemical properties of the frustules had clearly changed after baking at not only 800 degrees C but also 400 degrees C. These results showed that the reconstruction of the structures had occurred inside the frustules, even though the morphology of the frustules had not apparently changed at 400 degrees C. In order to characterize the exact shape of the frustules, living diatom cells were grown on a functionalized mica surface, and then baked without any chemical treatment for SEM study. This 'direct baking' technique is effective for comparing minute structures of the frustules, because completed combination of every part of the frustules can be observed.     

厘米级盘形硅藻微粒单层密排研究

硅藻土较好的保持了天然硅藻的多级微纳孔阵列、大比表面积等特征, 将其分选应用于硅藻研究具有低成本、大量、可重复等优势。本研究采用物理沉降结合机械筛选的方式从硅藻土中提取出盘形硅藻壳微粒, 尺寸分布在40~80 μm之间。借助空气-水界面颗粒漂浮自组装原理, 探究疏水处理的硅藻土壳片快速大面积密排工艺可行性。结果显示通过控制硅藻壳微粒在水面的浓度以及微粒自身形态, 可以获得厘米级单层密排结构, 且实验制得最大硅藻壳微粒漂浮单层密排面积达到23.7 cm², 捞取干燥完整硅藻微粒单层密排面积达到5.0 cm²。     

Synthesis and characterization of nanoporous hydroxyapatite using cationic surfactants as templates

Nanoporous hydroxyapatite was synthesized utilizing cationic surfactants as templates. The effects of cetyltrimethylammonium bromide and reaction temperatures on the phase and morphology of HA were investigated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and transmission electron microscopy (TEM). The thermal stability of nanoporous structures was studied by XRD and thermal analyzers (TGA/DTA), while the pore structure of HA was observed using high resolution TEM. It was found that the pore size was about 1 nm, and the pore structure of HA was thermally stable up to 700 °C and the pore size did not change with reaction temperature and CTAB:PO₄^3? ratio. The possible formation mechanism of nanoporous structure was proposed.     

Pore architecture of diatom frustules: potential nanostructured membranes for molecular and particle separations

Diatoms produce diverse three-dimensional regular silica structures with nanometer to micrometer dimensions and hold considerable promise for biological and biomimetic fabrication of nanostructured materials and devices. In the present work, we describe the ultrastructural characterization of porous structures in diatom biosilica and discuss their potential as membrane filters for diffusion based separations. The frustules of two centric diatom species, Coscinodiscus sp. and Thalassiosira eccentrica, were investigated using scanning electron microscopy and atomic force microscopy. Their morphological features, including pore size, shape, porosity, and pore organization, are described. We observed that although pore organization in frustules of Thalassiosira eccentrica and Coscinodiscus sp. is in reverse order, a striking commonality is the size range of the smallest pores in both species (around 40 nm). The consensus lower pore size suggests that frustule valves have a common function at this size of excluding viruses or other deleterious particles, and the pore size and organization is optimized for this purpose. We suggest and implement an experimental approach to study the potential of diatom frustules for diffusive separation of molecular or nanoparticular components in microfluidic or lab-on-a-chip environments.     

Template synthesis of nanophase mesocarbon

Templating techniques are used increasingly to create carbon materials with precisely engineered pore systems. This article presents a new templating technique that achieves simultaneous control of pore structure and molecular (crystal) structure in a single synthesis step. With the use of discotic liquid crystalline precursors, unique carbon structures can be engineered by selecting the size and geometry of the confining spaces and selecting the template material to induce edge-on or face-on orientation of the discotic precursor. Here mesophase pitch is infiltrated by capillary forces into a nanoporous glass followed by slow carbonization and NaOH etching. The resulting porous carbon material exhibits interconnected solid grains about 100 nm in size, a monodisperse pore size of 60 nm, 42% total porosity, and an abundance of edge-plane inner surfaces that reflect the favored edge-on anchoring of the mesophase precursor on glass. This new carbon form is potentially interesting for a number of important applications in which uniform large pores, active-site-rich surfaces, and easy access to interlayer spaces in nanometric grains are advantageous.     

Characterisation of pore structures in nanoporous materials for advanced bionanotechnology

Porous materials are potential candidates for applications in various fields, such as bionanotechnology, gas separation, catalysts and micro-electronics. In particular, their applications in bionanotechnology include biosensors, biomedical implants and microdevices, biosupporters, bio-encapsules, biomolecule separations and biomedical therapy. All these bionanotechnology applications utilise the shape, size and size distribution of pores in porous materials. Therefore the controlled creation of pores with desired shape, size and size distribution is most important in the development of nanoporous materials. Accordingly, the accurate evaluation of pore structure is necessary in the development of nanoporous materials and their applications. This article reviews recent developments in analytical techniques to characterise the pore structures of nanoporous materials.     

On the shell theory on the nanoscale with surface stresses

Below we discuss the derivation of the governing nonlinear shell equations taking into account the surface stresses. The surface effects are significant for the modeling of some structures as nanofilms, nanoporous materials and other nano-size structures. In particular, the surface stresses are responsible for the size effect, i.e. dependence of the material properties on the specimen size. The theory of elasticity with surface stresses is applied to the modeling of shells with nano-scaled thickness. It will be shown that the resultant stress and couple stress tensors can be represented as a sum of two terms. The first term in the sum depends on the stress distribution in the bulk material while the second one relates to the surface stresses. Hence, the resultant stress and couple stress tensors are linear functions with respect to the surface stresses. As an example the effective stiffness properties of a linear elastic Cosserat shells taking into account the surface stresses are presented.     

Transport modulation in Ge/Si core/shell nanowires through controlled synthesis of doped Si shells

Appropriately controlling the properties of the Si shell in Ge/Si core/shell nanowires permits not only passivation of the Ge surface states, but also introduces new interface phenomena, thereby enabling novel nanoelectronics concepts. Here, we report a rational synthesis of Ge/Si core/shell nanowires with doped Si shells. We demonstrate that the morphology and thickness of Si shells can be controlled for different dopant types by tuning the growth parameters during synthesis. We also present distinctly different electrical characteristics that arise from nanowire field-effect transistors fabricated using the synthesized Ge/Si core/shell nanowires with different shell morphologies. Furthermore, a clear transition in the modification of device characteristics is observed for crystalline shell nanowires following removal of the shell using a unique trimming process of successive native oxide formation/etching. Our results demonstrate that the preferred transport path through the nanowire structure can be modulated by appropriately tuning the growth conditions.     

Nanoporous polymeric nanofibers based on selectively etched PS-b-PDMS block copolymers

One-dimensional nanoporous polymeric nanofibers have been fabricated within an anodic aluminum oxide (AAO) membrane by a facile approach based on selective etching of poly(dimethylsiloxane) (PDMS) domains in polystyrene-block-poly(dimethylsiloxane) (PS-b-PDMS) block copolymers that had been formed within the AAO template. It was observed that prior to etching, the well-ordered PS-b-PDMS nanofibers are solid and do not have any porosity. The postetched PS nanofibers, on the other hand, had a highly porous structure having about 20-50 nm pore size. The nanoporous polymeric fibers were also employed as a drug carrier for the native, continuous, and pulsatile drug release using Rhodamine B (RB) as a model drug. These studies showed that enhanced drug release and tunable drug dosage can be achieved by using ultrasound irradiation.     

Bi-stable pore size during electrochemical etching of n-type silicon during a thermal ramp

We have investigated the change in size and density of pores during electrochemical etching of n-type (5 Ωcm) silicon under backside illumination and subjected to a thermal ramp. The pore structure was allowed to self organize, and for the parameters reported here this results in macropores with diameters in the ∼ μm order of magnitude range. As the etching progressed under constant current conditions, the electrolyte and the sample heated slowly up in the temperature range 20-60 °C. The resulting pore structure was obtained by scanning electron microscope examination of cross sections of cleaved samples. The temperature ramp caused the pore diameters and pore densities to change abruptly rather than continuously. The change can thus phenomenologically be described as a transition between two stable pore size configurations; bi-stable switching or phase transition. This phenomenon is observed for a range of parameters yielding macropores: current densities between 2-20 mA/cm² and varying light intensity. The transition is between known configurations.     

Permeable silica shell through surface-protected etching

We describe a "surface-protected etching" strategy that allows convenient conversion of sol-gel derived silica into porous structures. Poly(vinyl pyrrolidone) is used to protect the near surface layer, and NaOH is used to selectively etch the interior of the silica spheres. Etching initially yields porous structures and eventually removes the core to leave behind hollow silica spheres with porous shells. This strategy is useful for constructing core-shell systems where active nanomaterials are embedded in silica shell for enhanced stability against aggregation. We experimentally demonstrate use of the surface-protected etching approach to create openings on silica shells; these openings allow dissolved chemical species to reach embedded catalytic particles to be chemically transformed while the porous shells continue to act as effective barriers against aggregation and loss of activity of the core particles. We also show that, by controlling the extent of etching, it is possible to control the permeation rate of the chemical species through the shells.     

单层柱面网壳结构的找形

该文研究刚性网壳结构的合理找形问题,将柔性结构中广泛应用的找形概念引入刚性结构,提出基于力密度法的单层柱面网壳结构的找形方法。通过在找形过程中引入自重,解决了高斯曲率为零的柱面网壳的找形问题,且通过合理找形可基本消除自重引起的弯矩和剪力。找形得到的网壳结构与传统柱面网壳在曲面外形上比较相近,但可大幅度提高单层柱面网壳结构的承载能力。该文提出的找形方法效率高、收敛性好,为单层柱面网壳结构的设计开拓了新的方法和思路。     

Processing and properties of nanoporous hydroxyapatite ceramics

Method for fabrication and properties of nanoporous hydroxyapatite (HA) ceramic were described in the present work. The nanoporous hydroxyapatite was derived from nano hydroxyapatite powder and polyvinyl alcohol (as a pore former). The HA nanopowder was obtained from vibro-milling for 4 h. The nanoporous ceramics were sintered at 1200 °C. Properties of the nanoporous ceramics were investigated using various methods. Average porosity of the final product was found to be 64.6 ± 1.4%. Open and interconnected pores were obtained with an average pore size less than 100 nm, confirming the nanoporous structure of this ceramic. A high bending strength of 14.7 ± 3.2 MPa for the nanoporous ceramic, shows significant promise as a potential bone repairing material.     

Photoluminescence of silica nanostructures from bioreactor culture of marine diatom Nitzschia frustulum

The marine diatom Nitzschia frustulum is a single-celled photosynthetic organism that uses soluble silicon as the substrate to fabricate intricately patterned silica shells called frustules consisting of 200 nm diameter pores in a rectangular array. Controlled photobioreactor cultivation of the N. frustulum cell suspension to silicon starvation induced changes in the nanostructure of the diatom frustule, which in turn imparted blue photoluminescence (PL) to the frustule biosilica. The photoluminescent properties were imbedded within a patterned substrate precisely ordered at the nano, submicron and microscales. The peak PL intensity increased by a factor of 18 from the mid-exponential to late stationary phase of the cultivation cycle, and the peak PL wavelength increased from 440 to 500 nm. TEM analysis revealed that the emergence of blue photoluminescence was associated with the appearance of fine structures on the frustule surface, including 5 nm nanopore arrays lining the base of the frustule pores, which were only observed at the late stationary phase when both silicon consumption and cell division were complete for two photoperiods. Photoluminescence was quenched by thermal annealing of diatom biosilica in air at 800 degrees C for 1.0 hr, commensurate with the loss of silanol (triple bond Si-OH) groups on the diatom biosilica, as confirmed by FT-IR. Consequently, the likely origin of blue photoluminescence in the diatom biosilica was from surface silanol groups and their distribution on the frustule fine structures.     

Growth of InAs/InP core-shell nanowires with various pure crystal structures

We have studied the epitaxial growth of an InP shell on various pure InAs core nanowire crystal structures by metal-organic vapor phase epitaxy. The InP shell is grown on wurtzite (WZ), zinc-blende (ZB), and {111}- and {110}-type faceted ZB twin-plane superlattice (TSL) structures by tuning the InP shell growth parameters and controlling the shell thickness. The growth results, particularly on the WZ nanowires, show that homogeneous InP shell growth is promoted at relatively high temperatures (～500?°C), but that the InAs nanowires decompose under the applied conditions. In order to protect the InAs core nanowires from decomposition, a short protective InP segment is first grown axially at lower temperatures (420-460?°C), before commencing the radial growth at a higher temperature. Further studies revealed that the InP radial growth rate is significantly higher on the ZB and TSL nanowires compared to WZ counterparts, and shows a strong anisotropy in polar directions. As a result, thin shells were obtained during low temperature InP growth on ZB structures, while a higher temperature was used to obtain uniform thick shells. In addition, a schematic growth model is suggested to explain the basic processes occurring during the shell growth on the TSL crystal structures.     

Preparation technique for copper-plating on Si nanoporous pillar array

Silicon nanoporous pillar array (Si-NPA) is prepared by hydrothermally etching method. The copper/silicon nanocomposite thin film (Cu/Si-NPA) is obtained utilizing a reductive deposition method where Si-NPA acts both as a reducing agent and as a substrate. Microstructural analysis indicates that Si-NPA is composed of large quantities of well-separated, quasi-identical silicon pillars which are uniform and perpendicular to the surface. The pillar surface is nanoporous and the pore wall is composed of silicon nanocrystallites with a size about 4 nm. Cu/Si-NPA inherits the morphological characteristic of Si-NPA. The density of the Cu nanoparticles changes alternatively with the geometrical sites of Si-NPA, which leads to the formation of crater-like structures. These results indicate that Si-NPA might be used as an ideal template for synthesizing metal/silicon nanocomposite systems.     

NiAl-W纳米多孔阵列制备方法研究

采用定向凝固与选择性腐蚀复合工艺,获得NiAl-W纳米多孔阵列。通过定向凝固速率可调节棒状NiAl-W共晶中棒状W相的间距和直径,电化学选择性腐蚀去除W相后,在NiAl基体表面上形成间距和孔径可调的多孔阵列。由于孔径尺度的限制,孔列在电势0.5V连续腐蚀2~3h后受到极大的阻力。     

Superfluidity of 4He Confined in a Porous Glass and Control of the Pore Size

⁴He confined in nanoporous media is one of the most interesting nano-sized systems. The aim of the present work is control of the pore size of nanoporous media in order to clarify the superfluid size effects. For the pore size control, monolayer adsorption of Kr on the pore wall is performed. The change in the pore size distribution by the monolayer adsorption are characterized by N₂ isotherms at 77 K. Superfluidity in the pores is observed with a torsional oscillator method. The pore diameter of the as-purchased Gelsil is estimated at 5.8 nm. The monolayer adsorption reduces the size by 1.1 nm, caused by the thickness of the Kr monolayer. The decrease in the pore size lowers the superfluid transition temperature by 40 mK.     

Nanoporous titania-coated alumina membranes: sol-gel synthesis and characterisation

In this work, nanoporous titania top layers were deposited by dip-coating process on microporous alpha-alumina substrates using the sol-gel process. The alumina substrates were synthesized by slip casting method using Taguchi optimising approach. The microporous substrate was then used to coat nanoporous titania layers by the sol-gel method. The thickness, pore size, structure and permeability of the membranes were characterised using SEM, XRD, STA and Hg-Porosimetry. The process conditions to achieve defect-free nanoporous titania layers with the average pore size of about 4 nm coated on the microporous alumina substrates with the average pore size of about 270 nm were determined.     

Hexagonal geometric patterns formed by radial pore growth of InP based on Voronoi tessellation

To fabricate ordered geometric patterns consisting of InP nanoporous structures, a photoresist mask with periodic opening arrays was prepared by sphere photolithography. The diameter and interval of the openings of the photoresist mask could be controlled independently by adjusting the diameter of silica spheres used as a lens and the exposure time. Through this resist mask with a two-dimensional (2D) hexagonal array of openings, the pore growth of InP during anodic etching was investigated. The isolated openings could act as initiation sites for the radial growth of pores, resulting in the formation of hexagonal geometric patterns based on Voronoi tessellation in 2D space. With further anodic etching, inside the substrate, the growth direction of the pores changed from radial to perpendicular relative to the substrate. Moreover, by removing domains consisting of nanopores by anisotropic chemical etching, the fabrication of InP microhole arrays with circular and triangular cross sections was also achieved.