Characterization of LPD-derived TiO 2 coating on different substrates

different substrates

The morphology and adherence of the resultant film on substrate by LPD method was also affected by the surface functionality of the substrate [23]. In order to ensure uniform film growth using LPD method, surface treatments are usually required, which include the pre-deposition or self-assembly of a seed layer [29], and the surface formation of hydroxides [30]. In this chapter, pristine butyl rubber surface was treated by UV/Ozone for 20 min. Long-time exposure to UV/ozone would lead rubber age and degradation. The surface contact angle measurement results showed that the contact angle to water

droplets decrease to 40o compared to a value of 90o for pristine unmodified butyl rubber

surface, which revealed the surface was more hydrophilic after UV exposure due to the formation of oxygen-containing functional group. Comparing to UV/ozone treated butyl rubber surface, the fresh piranha solution treated silicon wafer exhibited superhydrophilic

property with the contact angle around 5o. Furthermore, piranha solution treated silicon

wafer and cotton sheet were chosen as the substrates to understand the formation of LPD-

derived TiO2 coating.

The surface morphology and grain size of the LPD derived TiO2 thin films on UV/Ozone

treated butyl rubber, silicon wafer and pristine cotton sheet were firstly characterized by

SEM. Fig. 4-1 presents the representative SEM images of the derived TiO2 coating from

the top views at different magnifications. Overall, smooth surface of the coated butyl

rubber without obvious aggregates of TiO2 was observed. The formation of visible cracks

appeared on the top of TiO2 films (Fig. 4-1a) was ascribed to the capillary stress as a

result of the shrinkage of films during dry process [31]. More visible cracks appeared on

the silicon wafer surface, while crack-free TiO2 coating mrophology was oberved on

cotton sheet surface. This phenomena indicated the stress mismatch of the LPD-coating

with the substrate had great effect on the surface morphology. In general, TiO2 particles

appeared larger domains were due to the surface roughness of butyl rubber and the small

TiO2 particles incorporated together regardless to the surface fluctuation in high

magnification 50,000×. The resultant film demonstrated a dense and defect-free morphology at high magnification (Fig. 4-1(a)-3). The energy dispersive x-ray (EDX) spectra of the surface showed in Fig. 4-1(a)-4 clearly indicate the existence of Ti and O. The appeared signal of C was from the substrate of butyl rubber and F was originated

from precursor in the LPD solution that might improve the photocatalytic activity of TiO2

Figure 4-1: SEM images of TiO2 coated on various substrates with different

magnification. (a) LPD-derived TiO2 coating on UV/Ozone treated butyl rubber;

(a)-2 zoom in picture in the white square area of (a); (b) LPD-derived TiO2 coating

on pristine cotton sheet; (b)-2 zoom in picture in the white square area of (b); (c) LPD-derived TiO2 coating on prianha solution treated silicon wafer; (c)-2 zoom in

picture in the white square area of (c); (a)-4 EDX result corresponding to (a)-2, carbon comes from the butyl rubber substrate, and fluorine comes from the LPD solution, Pt comes from the deposited condutive Pt layer.

The LPD-derived TiO2 coating presented good ahesion to the rubber substrate. The

optical microscopy image shwon in Fig. 4-2 displays the morphology changs after tape

test for two cycles. A large part of the TiO2 coating film showed integrity after tape test

Figure 4-2: Optical microscopy of TiO2 coated on butyl rubber substrates after tape

test. (a) LPD-derived TiO2 coating on UV/Ozone treated butyl rubber, (b) after tape

test once, (c) after tape test twice. The circle area showed the changes after tape test.

The XRD patterns of the TiO2 coating films on butyl rubber surface are presented in Fig.

4-3. As shown in Fig. 4-3 (a), the pattern presented well-defined peaks for both butyl

rubber and TiO2 coating film. The broad peak of 2θ around 17o was attributed to the

amorphous phase of butyl rubber [34]. The two diffraction peaks at 30o and 44.4o were

assigned to the anatase (101) facets and (004) facets, respectively, indicating that the

crystalline of TiO2 formed on the butyl rubber surface is in anatase structure [35]. To

confirm the crystal structure, the XRD pattern of precipitates collected from precursor solution after reaction also was given in Fig. 4-3 (b). The diffraction peaks of the

Figure 4-3: The XRD patterns of (a)TiO2 coating on butyl rubber surface and (b)

TiO2 powders collected from precipitates from LPD method.

Figure 4-4: AFM images of 3D morphology and height images of the TiO2 coating

on a butyl rubber surface deposited by the LPD method.

The film morphology was further characterized by AFM. 3D morphology images of the

densely packed sphere-like particles. It is consistent with the SEM results in higher magnification. The surface roughness, root mean square roughness was obtained from the AFM result. For the measured area of 1µm×1 µm, the Rq was as low as 3-9 nm. The

LPD derived TiO2 thin film by current deposition parameter showed quite smooth surface

morphology.