After a two-step anodization process, in aluminum foils or films, it is possible to obtain fairly well-ordered membranes. Figure
shows a set of images corresponding to alumina membranes. Figure
a shows a SEM image of the surface of the porous alumina grown over a silicon substrate. Figure
b,c shows SEM images of top and side views of a self-supported porous alumina membrane, respectively. From these micrographs, it is possible to observe that silicon-supported and self-supported membranes exhibit a high grade of order especially in the regularity of pores and hexagonal distribution as in the perpendicularity to the substrate (Figure
c). If we compare both membranes, it is possible to observe that self-supported membranes exhibit a high degree of order. Figure
d,e shows the pore diameter distribution for both samples, silicon-supported and self-supported, respectively. The silicon-supported membrane has a mean diameter close to 53
nm, whereas the self-supported membrane exhibits a similar diameter of 54
nm, but this last one has a sharper diameter distribution and a lower standard deviation. This difference comes from the fact that for the preparation of silicon-supported membranes, we employ a lower time in the first anodization step. The first anodization is responsible for the formation of pore nucleus; for longer periods, the order increases. Because silicon-supported membranes are prepared from very thin aluminum films, we have a limited thickness, and not longer anodization periods can be used since this implies the total anodization of the aluminum. This fact finally affects the order of the membrane.
Figure 1 SEM images of silicon-supported and self-supported AAO membranes. (a) SEM image of the surface of silicon-supported AAO membrane. (b,c) SEM images of top and side views of a self-supported AAO membrane, respectively. (d,e) Pore diameter distribution for (more ...)
Both silicon-supported and self-supported membranes were used to grow CNTs and as templates in the CVD decomposition of acetylene. Figure
a shows a diagram of the formation of CNTs inside the AAO membranes. Figure
b corresponds to a SEM micrograph showing the surface of the AAO membranes after the CNTs have grown. CNTs occupy almost all the pores of the membrane, and their external diameter depends only on the AAO pore diameter. The tube lengths do not dependent on the synthesis time since they do not grow beyond the membrane thickness; they are limited by the extension of the pores. Figure
c shows the AAO-CNT membrane but with the alumina partially dissolved. From the exposed tips of the CNTs, it is possible to note that their growth is fairly well ordered inside the pores following the template array.
Synthesis scheme and SEM micrographs of CNTs. (a) Schematic representation of the CNT synthesis steps, (b) SEM micrograph of 30-min-grown CNTs inside an AAO template, and (c) SEM micrograph showing CNTs after partial dissolution of AAO by NaOH.
shows TEM images of the CNTs prepared using self-supported membrane at different synthesis times (10, 30, and 60
min). Those tubes grow by adopting the inverse morphology of the pores; if the pores have regular sizes, then they also grow regularly, and the resulting tubes always preserve the dimension of the pore. On the other hand, if we change the time of carbon deposition, the tubes grow with a thicker wall, making the inner diameter smaller. Figure
also shows the histograms of the wall thickness of these nanotubes. It is possible to observe how the wall thickness increases almost linearly from 7
nm to as close as 24
nm when the synthesis time changes from 10 to 60
Figure 3 TEM micrographs and variation of the CNT wall thickness. Micrographs corresponding to different synthesis times: (a) 10, (b) 30, and (c) 60min.
shows a set of TEM images of the Pd@CNT hybrid nanostructures prepared using chloride and nitrate palladium precursors. Figure
a,b shows the hybrids prepared by wet impregnation of CNT (30
min) and AAO membranes with the palladium chloride solution by periods of 24 and 72
h, respectively. Figure
c,d shows the hybrids prepared by drop casting 10 and 30 μL of palladium chloride solution directly on the CNT (30
min) and AAO membranes, respectively. On the other hand, Figures
e,f shows the samples prepared using palladium nitrate solution, immersing the CNT-AAO membrane for 116
h and by casting 30 μL of the same solution directly on a piece of CNT-AAO, respectively.
TEM images of Pd@CNT nanohybrids. These nanohybrids were prepared using palladium chloride and nitrate precursors by wet impregnation (a, b, d) and incipient wetness impregnation (c, d, f).
Comparatively, it is possible to note from these pictures that the palladium chloride precursor was introduced more efficiently than the palladium nitrate precursor since more particles were formed in the inner cavity of the nanotubes. For example, when we use the nitrate precursor in wet impregnation for periods as long as 116
h, almost no particles were found inside the tubes as compared with those shorter periods using chloride precursor. This exhibits an appreciable amount of palladium nanoparticles inside CNTs. This fact could be associated to the formation of a very soluble metal complex, the [PdCl4
, which could penetrate into the pores more easily than the palladium nitrate precursor which tends to precipitate in dissolution. Additionally, it is possible to observe that the spontaneous diffusion of ions (by wet impregnation) is less efficient in the introduction of palladium inside the tubes. An EDX analysis of those samples supports the TEM observations. These results show that only samples prepared with the chloride precursor exhibit a quantifiable amount of palladium using this technique. Figure
a shows a typical EDX spectrum of the sample with the higher amount of palladium and an inserted table with the quantification for the hybrids prepared with the chloride precursor by the drop casting technique.
EDX (a) and Raman spectra (b) of Pd@CNT nanohybrids prepared using palladium chloride and nitrate precursors.
b shows the Raman spectra of Pd@CNT samples prepared under different conditions and also the spectrum of pure CNTs. All spectra exhibit characteristic peaks observed in these kinds of CNTs [16
]. The G
-band generated by the E2g
vibrational mode and the -band associated to an active A1g
breathing mode of the six-member ring are both linked to vibrational modes of sp2
-bonded carbon atoms. For all our samples, including the pure CNTs, these vibrational modes appear in the same frequency: close to 1,596
for the G
-band and 1,322
for the D
-band. No significant shifts in these bands were appreciated for the Pd@CNT hybrids with respect to the pure CNT spectrum. Additionally, no variations were detected in the I
) ratio; this value was very close to 1.2 for all samples. These results indicate that the introduction of palladium nanoparticles inside the nanotubes does not cause appreciable damage or structural changes in the graphitic material.
In order to probe the use of CNT-AAO membranes with vapor phase precursor, we have explored the use of titanium isopropoxide to form TiO2
inside CNTs. Figure
shows TEM images of CNTs and TiO2
@CNT hybrid composites prepared using the silicon-supported membrane. Figure
a shows an image of a pure CNT sample after release from the AAO. The insert of Figure
a displays a high resolution image of the wall structure of CNTs wherein it is possible to observe that the tubes synthesized are of a multi-walled nature with a relatively high degree of disorder, as we can also observe in the Raman spectrum of pure CNTs prepared from a self-supported membrane (Figure
b,c exhibits micrographs of the samples prepared at 500°C, 200 sccm of Ar, whereas Figure
d,e shows the micrographs of the samples prepared at 400°C and 100 sccm. When we compare these images with the image of pure CNTs (Figure
a), the results show that TiO2
effectively covered the inner cavity of carbon nanotubes since an appreciable amount of material was deposited inside the cavity. Different variables have shown that the morphology of the deposit can be changed; for example, the deposits prepared at 500°C look more like a film inside the nanotubes as compared with the samples prepared at 400°C that exhibit a distribution of small nanoparticles embedded in a film.
Figure 6 TEM images of pure CNTs and TiO2@CNT nanohybrids. TEM images of (a) pure CNTs and TiO2@CNT nanohybrids (b to e) prepared by CVD of TTIP precursor using AAO-Si substrates. The insert in (a) shows an HRTEM image of the wall structure of a pure CNT. (b, (more ...)