Page 112 - Handbook of Surface Improvement and Modification

7.1 Methods and mechanisms of surface tension change 107

Figure 7.11. Contact angles of the TiO 2 -treated wood surfaces obtained using precursor solutions having different
pH. [Adapted, by permission, from Gao, L; Zhan, X; Lu, Y; Li, J; Sun, Q, Mater. Lett., 142, 217-20, 2015.]

Figure 7.12. The mechanism of the effect of pH on surface free energy of deposited TiO 2 . [Adapted, by permis-
sion, from Gao, L; Zhan, X; Lu, Y; Li, J; Sun, Q, Mater. Lett., 142, 217-20, 2015.]

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50% of fluorine-based additive. The maximum water-repellency was observed at 4% the
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fluorine-based additive.
TiO thin films having different wettabilities were fabricated on the wood surfaces
2
o
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by a simple low-temperature hydrothermal method (90 C for 5 h in autoclave). The pre-
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cursor solution had pH adjusted by hydrochloric acid/sodium hydroxide. The morpholo-
gies of TiO films changed from sphere-like to film-like by adjusting pH of precursor
2
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solution. The TiO -treated wood surfaces had different wettability and the water contact
2
angles ranging from 9.61 to 132.71, when the pH was changed from 1 to 14 (Figure
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7.11). Figure 7.12 shows that the increase in pH caused elimination of hydrophilic
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groups resulting in increased contact angle.
The wettability of stainless steel surface possessed time-dependent transition from
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hydrophilic to hydrophobic state (in 72 h, the contact angle changed from 44 to 81 ).
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The presence of oxygen was the main reason for change.
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Repeated photo-illumination affected the wettability of rutile titanium dioxide. The
rate of change of hydrophilic properties was increased by the repeated UV illumination. 32
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The effect was more remarkable on (001) than (110) surface of rutile crystal. On (001)
surface, there is no bridging site oxygen, therefore, the formation of defect causes a large
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distortion (Figure 7.13). The replacement of in-plane oxygen with water results in break-
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down of microstructure. This structural change may be not so easy to return to the origi-