Nanopipes

Most wurtzite GaN films have been grown on either 6H-SiC(0001) (see Datareview A7.8) or sapphire (A1203) substrates. The orientation of sapphire most frequently used is C-plane (0001) although there have been some structural characterisation studies made for growth on A-plane (1120) [1-4] and R-plane (0112) [1,2,5-7] substrates. Other defects found in the a-phase include inversion domain boundaries, prismatic faults, nanopipes, pits, voids and cracks. The limited structural information available on bulk single crystals of a-GaN shows that they contain a low density of line dislocations and stacking faults near inclusions [12] (see Datareview A7.5). 

FIGURE 1 Cross-section TEM micrograph of nanopipes in a GaN film grown on sapphire (0001) by HVPE taken near the [1120] zone with g-3g, g = 0002. Voids are present along dislocations with Burgers vector b = [0001] (L.T. Romano et al [7]). 

CMK-5 is the first example of the ordered tube-type mesoporous carbons that can be characterized with well-defined Bragg diffractions by ordinary XRD instrument [6]. The XRD pattern of the CMK-5 carbon is distinguished from that of CMK-3 by the much lower intensity of the (100) diffraction. The structure of CMK-5 may be described by the substitution of the carbon nanorods in CMK-3 with nanopipes. The CMK-5 carbon is synthesized using SBA-IS, similar to CMK-3, but the carbon source and synthesis condition are somewhat different from those for CMK-3. The synthesis method for the tube-type carbon can be extended to the SBA-16 mesoporous template. The resultant CMK-7 carbon has a bicontinuous mesoporous structure [15]. 

It is noteworthy that the pore-size distribution curve obtained by the N2 adsorption has exhibited two sharp peaks with the maxima corresponding to the inside diameter of the carbon nanopipes (typically, 5.5 nm) and the pores formed between the adjacent pipes (4.2 nm), respectively [6]. It is reported that the outside diameter of the nanopipes is tailored by the pore diameter of the template SBA-15, while the wall thickness of the carbon nanopipes are also controllable to a certain degree. The specific BET surface area of the CMK-5 varies from 1500 to 2200 mV depending on wall thickness [17]. 

Later, other mesoporous silicas were used as templates for periodic mesoporous carbons. It is noteworthy that two types of carbon with SBA-15 as template but different framework configurations, rod- and tube-type, respectively, were obtained. The rod-type p6mm mesoporous carbon CMK-3 was synthesized by a complete filling of mesoporous cylindrical channels of SBA-15 silica. The CMK-5 carbon resulted from an incomplete filling of the SBA-15 channels which, after silica dissolution, gave interconnected nanopipes. 

M. Kruk, M. Jaroniec, T.W. Kim, and R. Ryoo, Synthesis and Characterization of Hex-agonally Ordered Carbon Nanopipes. Chem. Mater., 2003, 15, 2815-2823. 

First preliminary variants of DDM were applied in the full-profile X-ray diffraction structure analysis of a series of new silica mesoporous materials and ordered nanopipe mesostructured carbons. DDM allowed stable back-ground-independent full-profile refinement of the structure parameters of these advanced nanomaterials, a result that was unattainable by any other method. To date, DDM has been applied to many various mesoporous and mesostructured substances. The structural parameters of a series of face-centred cubic (Fm3m), body-centred cubic Im3m), and two-dimensional hexagonal (pGmm) mesoporous silicates were determined by DDM from synchrotron XRD. A comprehensive structural analysis of mesoporous silicates SBA-16 (cage-type cubic Irriim), their carbon replicas, and silica/carbon composites was performed by applying DDM. The structure of MCM-48 mesoporous silicate materials was analysed in detail by DDM from different laboratory and synchrotron XRD data. The pore wall thickness of both as-made and... 

The OMC structure also depends on the polymerization step. As mentioned above, the polymerization of the precursor adsorbed in the pore system of the matrix is catalyzed by acids. Different synthesis procedures were developed. For example, an acid solution can be added to the reaction mixture. In this case, the polymerization will take place throughout the entire pore system of the matrix. The resulting OMC can be described as a three-dimensional network of interconnected carbon rods. An example is CMK-3, already presented in Fig. 18.1. In this OMC, parallel-arranged carbon rods with a diameter of approximately 5 nm are connected by narrower carbon rods. The narrow carbon rods were formed in micropores that connect the mesopores of the SBA-15 silica matrix [21]. In an alternative synthesis procedure, a matrix with acid sites on the pore walls (e.g., an aluminosilicate) can be used. In this case, the polymerization of the precursor takes place on the mesopore walls and a carbon film is formed there, whereas the much narrower micropores are entirely filled with the polymerization product. Thus, after pyrolysis and removal of the matrix, the OMC consists of interconnected nanopipes, as opposed to interconnected carbon rods. An example is CMK-5. This OMC is synthesized in an acid form of the matrix used for the synthesis of CMK-3. Thus, CMK-5 consists of interconnected carbon nanopipes, arranged in the same fashion as the carbon rods of CMK-3 (Fig. 18.2) [22]. However, the pore system of these two OMCs differs. The pore system of CMK-3 consists of the voids in between the carbon rods, whereas in addition to these pores CMK-5 also has pores inside the nanopipes. 

For completeness, we also consider cylindrical pores, which are appropriate for defects such as threading dislocations or nanopipes. Here the charge-conservation condition is n(w2 - rp2)LNp = 2jTrpLNss, where L is the pore length. By solving Poisson s equation in the cylindrical coordinate system [let r2 r, in Equation (9.5)], we get, again for rp [Pg.239]

Whitby M, Quirke N (2007) Fluid flow in carbon nanotubes and nanopipes. Nat Nanotech 2 87-94... 

Fibrous assemblies like nanofibers, nanopipes, and nanotubes may have applications in micro-/ nanofiuidic systems. Vertically aligned carbon... 

Whitby M, Quirke N (2007) Fluid flow in carbon nanotubes and nanopipes. Nat Nanotechnol 2 87 Wongmanerod C, Zangooie S, Arwin H (2001) Determination of pore size distribution and surface area of thin porous silicon layers by spectroscopic ellipsometry. Appl Surf Sci 172 117 Zhang XG (1991) Mechanism of pore formation on n-type silicon. J Electrochem Soc 138 3750... 

It was found that transition to nanocomposites could also improve mechanical properties and promote stabilization of the basic material s parameters (Konig 1987). For example, it was established that in CNTs-polymer composites, the presence of carbon nanotubes inside the polymeric matrix can provide a mechanical support to the polymeric chain s conformational rearrangement. CNTs are hollow nanopipes, and therefore the incorporation of CNTs in a metal oxide matrix can provide better gas permeability for sensing materials and thus enhance gas diffusion into the bulk film. Thus, combination of CNTs with metal oxide (see Fig. 12.1) can lead to development of gas sensors with improved rate of response. 

Kim, W. et al. 2009. Preparation of nitrogen-doped mesoporous carbon nanopipes for the electrochemical double-layer capacitor. Carbon, 47,407-1411. 

Several applications of nanopipes can be envisioned, including highthroughput sensing, analysis of molecular species, drug delivery systems, and fluidic interfaces to live cells. 

Kruk, M, et al.. Synthesis and characterization ofhexagonally ordered carbon nanopipes. Chemistry of materials, 2003, 15(14), 2815-2823. 

D. Mattia, H. H. Bau, and Y. Gogotsi, Wetting of CVD carbon films by polar and non-polar liquids and implications for carbon nanopipes, Langmuir, 22,1789 (2006]. 

H. J. Shah, A. K. Fontecchio, M. P. Rossi, D. Mattia, and Y. Gogotsi, Imaging of liquid crystals confined in carbon nanopipes, Appl. Phys. Lett., 89, 043123 (2006]. 

M. P. Rossi, H. Ye, Y. Gogotsi, S. Babu, P. Ndungu, and J. C. Bradley, Environmental scanning electron microscopy study of water in carbon nanopipes. Nano Lett., 4,989 (2004). 

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