## 3 Results and Discussion
The dispersion characteristic of TiO\({}_{2}\), which can be evaluated by the degree of nanoparticles aggregation in water, is of crucial importance to the aqueous based suspension fabrication. As shown in Figure 4, the median size of secondary particles is about 0.28 \(\upmu\)m, and the narrow distribution ranging between 0.1 and 0.6 \(\upmu\)m implies a high degree of dispersion for the prepared TiO\({}_{2}\) in water. The surface morphology of the TiO\({}_{2}\) nanoparticles with super-hydrophobic modification coated on a silicon wafer is visualized in Figure 5. From the AFM topographic image and the sectional view of the height variation, it is estimated that the diameter of a single TiO\({}_{2}\) particle is about 50 nm on average, while the clusters range on the sub micrometer scale, in agreement with the secondary particle size distribution presented in Figure 4. The aerated brick was dipped into the TiO\({}_{2}\) based aqueous  
Figure 4: Particle size distribution of TiO\({}_{2}\) dispersed in water. Figure 3: Schematic diagram of the surface modification. super-hydrophobic suspension, introducing the micro- and nanostructure on the outmost surface of the brick. Along with the micrometer-sized pores in the brick, a certain rough surface modified with super-hydrophobic groups was built, as shown in Figure S3 in the Supplementary Materials. As displayed in Figure 6, FT-IR was employed to characterize the functional groups and the molecular bonding in the modified TiO\({}_{2}\) nanoparticles with PFOTS. The upper blue curve in Figure 6 shows the absorption peaks of PFOTS (including TEOS), in which the existence of C-F bonds in the form of CF, CF\({}_{2}\) and CF\({}_{3}\) are situated at 520, 739, 954, 1120 and 1205 cm\({}^{-1}\)[24]. The peaks at 1078 cm\({}^{-1}\) and 800 cm\({}^{-1}\) are associated with the asymmetric stretching vibration and bending mode of Si-O-Si bonds formed by dehydrate reaction of the silane coupling agent, respectively [25; 26]. The peak located at 897 cm\({}^{-1}\) is assigned to the C-H bonds [25; 26]. The Si-O-C bond appears at 1142 cm\({}^{-1}\), which confirms that the functional groups from PFOTS are linked to the silica network [27]. The middle red curve in Figure 6 shows the infrared absorption of the PFOTS modified nano-TiO\({}_{2}\), while the bottom black line gives the spectrum of nano-TiO\({}_{2}\) before treatment. It can clearly be seen  
Figure 5: AFM image of TiO\({}_{2}\) based aqueous super-hydrophobic suspension coated on a silicon wafer:  
(a) Top view; (b) Sectional height difference along the white line in (a). Figure 6: FT-IR spectra of the PFOTS modified nano-TiO\({}_{2}\) particles. that the middle spectrum is a superposition of the upper and bottom ones, which implies that the PFOTS is attached to the TiO\({}_{2}\) nanoparticles. The hydrophobicity of the surface can be determined by its contact angle. As displayed in Figure 7, a contact angle of 146\({}^{\circ}\) on average demonstrates an excellent water-repellency performance of the aerated brick surface with hierarchical structure and super-hydrophobic material modification. To further investigate the water-proofing performance of the bricks with a hydrophobic surface, a water immersion experiment was carried out, as shown in Figure 8. The sample with surface modification is shown in the left in Figure 8a,b), while the right in Figure 8a,b displays the bare brick. It is apparently observed that the brick with surface modification was floating on the water like a boat, while the control brick sank to the bottom of the water. The water-repellent surface dams the water into the pores of the brick, shielding the porous interior and forming relatively lower density than that of the water. By contrast, the water can easily permeate into the bare brick through the pores. To be more specific, the weight variations of the two brick samples were recorded before and after immersed into the water.