TÉCNICAS SOBRE LOS RECURSOS HUMANOS

In order to obtain a clear image of the different pore systems that can be formed us- ing Brij 56 as template, a series of different samples was synthesized on cleaned glass cover-slips.166 As mentioned above in Section 2.2, it is a well known fact that variation of the surfactant/silica ratio in the precursor solution results in different pore topolo- gies in the mesoporous film. A broad range of different recipes with surfactant/silica ratios from low (0.03) to high (0.715) were used for the synthesis and the resulting structure was analysed using XRD and TEM (in Table 7.1 only those recipes of samples that are discussed here further are summarized).

To determine the film thickness by ellipsometry, additional samples were synthesized on silicon wafers. The measurements showed that all the films were about170−180nm thick.

X-ray diffraction

Figure 7.3a shows the different 1D X-ray diffractogrammes that were measured in Bragg-Brentano-geometry (θ−θ) on fresh samples a few hours after synthesis, i.e. im- mediately after the wide-field measurements, using the recipes B2, B3, and B5 from Table 7.1. Whereas sample B1 shows only a very weak and fairly broad reflexion at

2θ = 1.6° (data not shown), sample B2exhibits a distinct peak at about the same po- sition. Increasing the surfactant/silica ratio further (sampleB3) results in a diffraction pattern with two peaks at2θ = 1.45° and2θ = 1.60°, respectively. Samples with a sur- factant/silica ratio ≥0.27 show only one peak at about 2θ = 1.45°. The twod-values calculated for the different structures according to the Bragg equation (Equation 2.3.1) are d=5.5nm (2θ = 1.60°) and d=6.2nm (2θ = 1.45°), respectively. The d-spacing of the phase formed at comparatively low surfactant concentrations, B2, is by a factor

sin 120° smaller than that of the other structure,B5. This is indicative of the formation of a hexagonal and a lamellar phase. The pore-to-pore distance, which is identical to the unit cell parameter a for the hexagonal phase, can be calculated from the lattice plane distanced= (2_/√_{3)atoa=6.1nm. For a lamellar phase the layer-to-layer distance} corresponds to the spacing of the lattice planesd=6.2nm.

Figure 7.3: 1D XRD of the different samples, before and after ageing.(a) XRD of the freshly prepared samples. The left peak corresponds to the (100) reflex of the lamellar phase (B5, green), the right peak to the (100) hexagonal reflex (B2, blue). SampleB3(black line) shows two peaks at the same positions asB5andB2. Here a mixture of the two phases is present. (b) Shrinkage of the pore-to-pore distances for purely hexagonal (blue) and lamellar (green) phase, measured on a different sample as in (a). Solid lines: Freshly prepared samples. Dotted lines: after three weeks of aging.

ments presented above were obtained from samples a few hours up to one day after the synthesis. During this time no changes of the peak positions were observed. However, three weeks after the synthesis the peaks were shifted to higher2θvalues, which corre- sponds to a shrinkage of the pore-to-pore distances (see Figure 7.3b). The initial peaks at (2θ = 1.43°) and (2θ = 1.63°) were shifted towards (2θ = 1.58°) and (2θ = 1.70°), respectively. The latter values for 2θ correspond to a distance of lattice planes in the lamellar phase of dlam aged=5.6nm, thus a decrease of 0.6nm (10%). In the hexagonal phase, after ageing the lattice distance isdhex aged=5.2nm and the pore-to-pore distance ahex aged=6.0nm, which corresponds to a shrinkage of about 5%. The lamellar phase contracts thus about twice as much as the hexagonal phase.

This shrinkage is not necessarily caused by a shrinking of the pore diameter, and might instead result from condensation and thus contraction of the pore walls only. Diffusion measurements were done with the aged samples, and the trajectories and diffusion coefficients were similar to those of the fresh samples.

Transmission electron microscopy

In addition, cross-sections and scratches from the different samples were analysed us- ing TEM. Figure 7.4 shows the cross-sections that were obtained from the samplesB2, B3 and B5. On the left, the open hexagons are indicative for a hexagonal phase in sample B2, whereas the layered structure on the right can be assigned to a lamellar

Figure 7.4: Cross-section TEM of the different samples.(a) The cross-section TEM clearly show the openings of the hexagonally arranged pores (B2) and (b) the stacking of the lamellae (B5). (c) For the phase mixture it displays the different mesophases stacked on top of each other. Different stacking order can be observed in other areas of this sample. In all images the glass substrate is visible at the bottom and the silica-air interface at the top, the z-arrows point along the optical axis of the widefield microscope (z-direction), x and y mark the observation plane. The insets in (a) and (b) sketch the topology of the pores as in Figure 2.2. Courtesy of B. Platschek, Bein group, LMU Munich.

phase in sampleB5. The middle panel shows a cross-section TEM of sampleB3which exhibits two peaks in the XRD, where the two different phases are visible in the same image, with the lamellar phase on top of the hexagonal channel system.

In addition to the cross-sections, small pieces of the samples with low surfactant/silica ratio were scratched of the substrate using a razor blade, deposited on on a copper grid for elelctron microscopy and analysed with TEM. In Figure 7.5a a scratch of a sample with recipeB2is shown. Here, the hexagonally arranged pores are seen from top. The

Figure 7.5: Scratches of the different samples analysed with TEM.(a) Hexagonal structure obtained with recipeB2. (b) Worm-like structure obtained with recipeB2. (c) Worm-like struc- ture obtained with recipeB1, having a lower surfactant/silica ratio thanB2. Courtesy of B. Platschek, Bein group, LMU Munich.

image in Figure 7.5b shows a scratch from a sample that was made using the same recipe (B2), but in this special case the structure is much less developed (worm-like). This phase can be understood as having regulard-spacing but rods too short to arrange in a hexagonal way with respect to the substrate surface. Thus, the structure results in spherical intensity maxima in reciprocal space and cannot be distinguished from a hexagonal phase by 1D XRD. No diffusion was visible in this sample, showing that the pores have no connections that would let the dye molecules move in this worm-like structure. Even though the recipe was the same as for the pure hexagonal samples, in this special case a less ordered sample was developed. In fact, the recipe for the pure hexagonal samples is at the limit towards recipes that result at all times in amorphous or worm-like structures. On the right, Figure 7.5c, a TEM image of a scratch from such a sample made with an even lower surfactant silica ratio (B1) is depicted. This sample exhibited only a very weak and broad peak at2θ = 1.6° in the XRD. The TEM also shows that the structure is not very well developed here. Note that scratching off pieces of samples with a lamellar phase will only result in images of amorphous silica, i.e. the lamellae seen from top. Therefore no such images were taken from samples with higher surfactant/silica ratio.

Atomic force microscopy

The surface of three different samples was investigated using atomic force microscopy (AFM). The Surface Images were taken with a commercial AFM (Asylum Research

Figure 7.6: AFM Surface scans of the different samples.In all three images surface steps of about6nm are visible in the cross-section along the black line (bottom graphs). (a)B2, (b)B3, (c)B5. The scale bar in each of the images corresponds to10µm.

MFP3D) in tapping mode. The Olympus AC160 SiN Cantilever was driven 5% be- low its resonance frequency with a target amplitude of 1.2V. In the measurements a set point of0.85V was used to scan each surface with a resolution of512px ×512px and a scanning rate of 2Hz per line. The AFM images (Figure 7.6) show the surface structure of samples from B2, B3 and B5. In the pure hexagonal phase (B2) steps of about 6nm height are visible. This corresponds to the pore-to-pore distance that was calculated from the XRD patterns (Figure 7.3). In this image four different levels can be distinguished. The lamellar phase on the right shows a more plate-like structure on the surface. Here, the height differences fit well to multiples of the d-spacing between the silica planes. The middle image, taken from the sample with the phase mixture, however, does not show very clear surface steps, but the surface roughness is in about the same range as on the surface of the two pure phases.