TiO 2-SiO 2 NEW INSIGHTS IN SUPER-HYDROPHILICITY OF TiO 2-SiO 2 NANOCOMPOSITE FILMS

Enhanced hydrophilicity effects arising from TiO2-SiO2 granular interfaces in composite films deposited via sol-gel routes have been studied before. Results obtained so far have shown that sol-gel formulations yielding less reactive sols lead to enhanced super-hydrophilicity persistence of composite films, i.e. a water contact angle less than 10 o after aging for more than 8 weeks (aging under ambient conditions without UV radiation). Taking into account the morphology of derived composite films and different surface thermodynamics models, we have suggested that a more rough/porous structure of the composite film might even increase this enhanced hydrophilicity effect. To verify this hypothesis, we have chosen to study more reactive TiO2-SiO2 composite sols with different compositions (0, 20, 60 and 100mol% of SiO2). This greater sol-gel reactivity is expected to reduce the natural superhydrophilicity of composite films in order to better assess eventual effects of the morphology. Then, in order to artificially create rough/porous morphologies, we have used polystyrene (PS) beads with average diameter of 0.6μm. Different parameters of spin-coating deposition method were tested to define the best conditions to obtain 2D layers of closely packed PS beads. Further, two experiments with different rotation speeds (1000rpm and 500rpm) were performed. Other spin-coating conditions were fixed as follows: acceleration of 5000rpm/s, rotation time of 1s, concentration of PS beads 1wt% in EtOH, volume of PS solution 100μL. Such 2D PS layers were then impregnated with TiO2-SiO2 composite sols. Using PS beads, we have obtained TiO2-SiO2 composite films with a roughness that is almost 100 times higher than composite films without PS beads. These morphology features are confirmed by optical microscopy, AFM and FEG-SEM measurements. Water contact angle measurements show in turn that the hydrophilicity effects are increased by morphologic modifications, and in the best cases (samples with 20-60mol% of SiO2) water contact angles are close to 5 o after 6 weeks of aging under ambient conditions without UV radiation. This study also shows that, in these cases, there is some relation between: 1) the surface coverage of PS beads, 2) the surface roughness/porosity of S1-X+PS films, and 3) their superhydrophilicity persistence. List of Figures and Tables Fig. 1: Illustration of the photo-induced super-hydrophilicity of a TiO2 coating .................. 3 Fig. 2: Kinetics of evolution of the water contact angle on three different ........................... 4 Fig. 3: Water contact angle measured after eight weeks of aging ......................................... 5 Fig. 4: FEG-SEM and AFM images of the optimized composite film ................................. 6 Fig. 5: Critical flow to detach oil drop as a function of water contact angle ........................ 7 Fig. 6: SEM micrograph and TEM image of macro-meso-microporous structure of SiO2 obtained with template of polystyrene latex spheres ............................................................. 8 Fig. 7: Schematic representation of the macro-pores formation ........................................... 8 Fig. 8: Schematic representation of two PS spheres partially immersed in a liquid layer convective flux toward the ordered phase ............................................................................. 9 Fig. 9: Schematic representation of spin-coating method ................................................... 10 Fig. 10: Schematics of contact angle θ formed between a liquid drop and a solid surface . 13 Fig. 11: Schematics of water contact angle θ formed between liquid drop and solid surface as a function of its hydrophobic/hydrophilic characteristics ............................................... 13 Fig. 12: Schematics of water contact angle formed between a liquid drop and a surface with an open porosity .......................................................................................................... 14 Fig. 13: Schema of the device for water contact angle measurements ................................ 15 Fig. 14: Formation of thin films via sol-gel approach ......................................................... 16 Fig. 15: Optical and FEG-SEM images representing the influence of solvent by using a standard deposition protocol ................................................................................................ 21 Fig. 16: Optical images and graphics represent results of different rotation speeds and rotation times ....................................................................................................................... 22 Fig. 17: Optical images representing the influence of the surface coverage when different accelerations are applied ...................................................................................................... 23 Fig. 18: Reproducibility of the samples represented by optical and FEG-SEM images ..... 24 Fig. 19: Influence of the duration of the ultrasound (US) and magnetic agitation (MA) represented by optical images and graphics ........................................................................ 25 Fig. 20: Optical and FEG-SEM images representing the influence of a rotation speed of 500rpm and 1000rpm tested on different surfaces .............................................................. 26 Fig. 21: Influence of the PS solution concentration represented by optical and FEG-SEM images .................................................................................................................................. 28 Fig. 22: Influence of the deposited volume of the solution (on Si substrate) observed in microscopic scale with OM and comparison of two samples in macroscopic scale ........... 29 Fig. 23: Optical images characterizing samples before and after impregnation with hydrophilic TiO2-SiO2 composite S1-60. The FEG-SEM images illustrate morphology of a composite coating after post-impregnation ......................................................................... 31 Fig. 24: Optical images representing different composite impregnation volumes and FEGSEM images representing the surface morphology after impregnation of the composite sol (top view and tilted sample view) ....................................................................................... 32 Fig. 25: FEG-SEM and AFM images to represent the morphology and roughness of composite films S1-0, S1-20, S1-60 and S1-100 directly deposited on Si substrates ......... 33 Fig. 26: Kinetics of water contact angle increase over aging for four different composite materials directly deposited on silicon without PS beads ................................................... 35 Fig. 27: Optical, AFM and FEG-SEM images represent general and more detailed structures of S1-0 composite film ........................................................................................ 37 Fig. 28: Optical, AFM and FEG-SEM images represent general and more detailed structures of S1-20 composite film ...................................................................................... 38 Fig. 29: Optical, AFM and FEG-SEM images represent general and more detailed structures of S1-60 composite film ...................................................................................... 39 Fig. 30: Optical, AFM and FEG-SEM images represent general and more detailed structures of S1-100 composite film .................................................................................... 40 Fig. 31: Quantitative comparison of film surface roughness as a function of the surface coverage rate ........................................................................................................................ 31 Fig. 32: Kinetics of water contact increase over aging for S1-X composite films deposited on silicon and S1-X+PS films ............................................................................................. 44 Fig. 33: Water contact angle measured after 6 weeks of aging under ambient atmosphere without UV irradiation as a function of the SiO2 sol molar composition ........................... 45 Fig. 34: Influence of the film surface roughness on the water contact angle values measured after 6 weeks of aging under ambient atmosphere without UV radiation for S120(+PS) and S1-60(+PS) films ............................................................................................ 46 Table 1: Summary of model conditions to obtain PS 2D layers ......................................... 30 Table 2: RMS roughness values of composite films and composite + PS films of two experiments with different rotations speeds (1000rpm and 500rpm) .................................. 41