Open tips

Fig. 2. A graphic of a nanotube showing a pulled-out atomic wire and several stabilizing spot-welds. Only two layers have been shown for clarity, although typical multiwalled nanotubes have I0-I5 layers. The spot-weld adatoms shown between layers stabilize the open tip conformation against closure. The atomic wire shown was previously part of the hexagonal lattice of the inner layer. It is prevented from pulling out further by the spot-weld at its base.

Fig. 14. Field emission patters from (a) closed-tip and (b) open-tip MWCNTs taken by a CCD camera [46],...

Fig. 15. Temperature-programmed desorption (TPD) spectra SWCNTs, (b) activated carbon and (c) open-tip SWCNTs [52].

One more structural dilferentiation that can influence functionalization is the tip vs. sidewall region. The tips are the two ending parts of the tube in these two regions, the curvature is increased and the shape more resembles that of a hemisphere, with reactivity expected to be similar to that of fullerenes. In contrast, sidewalls present reduced pyramidalization angles and therefore different behaviors towards functionalization. Reactions involving the use of harsh conditions can result in a fracture of the tubes, enabling production of shorter tubes with open tips, where the aromatic pattern is interrupted and carbon atoms are more reactive. 

Opening and shortening of the tubes is a sought-after event for certain applications and can be induced ad hoc by different techniques such as ball milling, mechanical cutting, electrochemically and chemically. The open tips can be closed again if required [14]. 

G Edwards, AH Weston. Structure-activity relationships of K+ channel openers. TIPS 11 417-422, 1990. 

Various methods have been reported to fabricate monolithic tips in polymeric materials for MS analysis [782], The tip fabricated in PC is shown in Figure 7.37 [290]. Other materials, such as parylene [783], PDMS [784,785], or SU-8 [786], were also used. For instance, an open tip was constructed on a PDMS chip. A lid was used to cover the whole chip except the microchannel tip [172,787],... 

Recently, open-tipped MoS2 nanotubes were prepared by the decomposition of ball-milled ammonium thiomolybdate powder under a H2 + thiophene atmosphere, and used as a catalyst for the methanation of CO with H2.143 The conversion of CO to CH4 was achieved at a much lower temperature compared to polycrystalline MoS2 particles, and there was no deterioration even after 50 h of consecutive catalyzing cycles. This observation is of importance in the context of energy conversion of global C02. 

Open-tip liquid ion-exchange microelectrodes can be fabricated by pulling borosilicate glass capillary tubing to an appropriate tip diameter (0.5 to 1 /im),... 

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