Incorporation of pre-synthesized nanoparticles into titania network

As discussed in chapter 5, pre-synthesized titania nanoparticles have been incorporated successfully into the templated titania network structure. To make a comparison, the nanoparticles are also incorporated into a metal oxide phase which is derived from EGMT at low temperatures. The sol-gel process at low temperatures is described in section 4.2.2.

The detailed information of the incorporation of the nanoparticles is given in section 4.2.2.

After solution preparation, the titania/PS-b-PEO composite films are deposited with spray coating. The parameters for spray deposition are listed in section 4.2.3. The meso-porous titania films are obtained by the removal of polymer template via UV irradiation and solvent extraction. In the present work the sol-gel-synthesized TiO2 and TiO2 from pre-synthesized nanoparticles have a weight ratio of 1:1. This mesoporous titania sample is denoted as 50 % EGMT.

Figure 8.8: Plan-view of the 50 % EGMT film at a) low and b) high magnifications and b) corresponding cross-section SEM image.

The surface morphology of the 50 % EGMT film is investigated by SEM and the results are shown in figure 8.8. A rough surface is obtained as a result of the spray depostion.

Moreover, some tiny cracks are present in the surface (figure 8.8a). When zoomed in, a mesoporous nature of the sponge-like morphology is observed in figure 8.8b. The mesopore sizes are in the range of 10 nm to 20 nm. However, some pores are connected to form macropores. The cross-section SEM image gives a proof that the polymer template is extracted completely.

The inner film morphology is investigated with the time-of-flight grazing incidence small-angle neutron scattering (TOF-GISANS). The measurement was performed at the beamline REFSANS of the Helmholtz Zentrum Geesthacht at FRM II, Garching. An incident angle of 0.45 ^◦ and a sample-detector distance of 10.534 m were set for the experiment. The wavelengths of the neutrons ranges from 2.00 ˚A to 14.93 ˚A. Due to gravity for neutrons, the real incident angle needs to be corrected. Selected 2D GISANS data with various wavelengths are illustrated in figure 8.9. As noted, the position of the

Figure 8.9: Selected 2D GISANS data. The wavelengths are labeled in each image. The dashed line in the first image indicates the sample horizon. The specular peak, the Yoneda peak and the direct beam are labeled in the second image.

Yoneda peak moves upwards with increasing wavelength of the neutron beam. This is caused by the proportional relationship between the critical angle of the probed materials and the neutron wavelength. The vertical line cuts obtained from the 2D TOF-GISANS data are performed at qy = 0 and the results are shown in figure 8.10a for the probed wavelengths of 4.45 ˚A, 4.92 ˚A, 5.44 ˚A, 6.01 ˚A, 6.98 ˚A, 7.71 ˚A, 8.12 ˚A, 8.97 ˚A, 9.92 ˚A and 10.96 ˚A. The position of the specular peaks remain unchanged atα_i +α_f ≈0.9, whereas the Yoneda peaks of the titania and silicon shift towards higher α_i + α_f values with increasing neutron wavelengths. Determined by the Yoneda peak positions, the critical angles of the silicon substrate and the probed 50 % EGMT film are plotted in figure 8.10b as a function of neutron wavelengths. By a linear fitting function, both scattering length densities (SLDs) can be extracted via equation 2.28. The SLD of the silicon substrate is

Figure 8.10:a) Selected vertical line cuts obtained from 2D TOF-GISANS data. The curves are shifted along the intensity axis with increasing wavelength from bottom to top. The cor-responding wavelengths are 4.45 ˚A, 4.92 ˚A, 5.44 ˚A, 6.01 ˚A, 6.98 ˚A, 7.71 ˚A, 8.12 ˚A, 8.97 ˚A, 9.92 ˚A, 10.96 ˚A. The position-shifting trend of the Yoneda peaks of titania and silicon are labeled by dashed lines. The arrow indicates the specular peak. b) The wavelength-dependent critical angles of the silicon substrate (red triangles) and the 50 % EGMT film (black rect-angles) obtained from the vertical cuts in a). Red and black lines are their corresponding linear fits.

determined to be (2.06×10⁻⁶ ±2.68×10⁻⁷) ˚A⁻², which fits quite well to the theoretical value (2.07 × 10⁻⁶ ˚A⁻²). The SLD of the 50 % EGMT sample (ρ) is calculated to be (8.97 × 10⁻⁷ ± 1.76 × 10⁻⁷) ˚A⁻². According to previous studies, the SLD of a compact titania layer (ρ_c) is 2.41 × 10⁻⁶ ˚A⁻² [183]. Since the porosity (Φ) can be calculated via equation 2.34, a porosity of (62.8 ± 7.3) % is obtained for the 50 % EGMT sample.

Information about lateral structures can be obtained from horizontal line cuts which are performed at the position of the titania Yoneda peak. The selected horizontal line cuts are displayed in figure 8.11 with the same wavelengths as for the vertical line cuts.

It is noteworthy that, the whole film volume is probed when the neutron wavelengths are smaller than 9 ˚A in the present work. If they are larger than 9 ˚A the penetration depth of neutrons reduces exponentially and surface structures are probed instead. In order to parametrize the lateral structures in the 50 % EGMT film, the horizontal line

cuts are fitted with the same model for used for GISAXS measurements as described in section 5.1.2. From modeling, two characteristic structure sizes are extracted. In detail, lateral structure radii of (8.8 ±1.6) nm and (40.0 ±8.9) nm with center-to-center distances of (30.0±6.6) nm and (175.0±40.6) nm are obtained. Therefore, the pore sizes of (12.4±6.8) nm and (95.0±41.6) nm can be calculated via equation 5.1. Furthermore, it is noted that the horizontal line cuts for all wavelengths give the same information about the lateral structures, which means that surface structures are similar to structures in the volume of the film.

Figure 8.11:Selected horizontal line cuts obtained from 2D TOF-GISANS data. The curves are shifted along the intensity axis with increasing wavelength from bottom to top. The corresponding wavelengths are 4.45 ˚A, 4.92 ˚A, 5.44 ˚A, 6.01 ˚A, 6.98 ˚A, 7.71 ˚A, 8.12 ˚A, 8.97 ˚A, 9.92 ˚A, 10.96 ˚A. The red lines represent the fits to the data.