Page 25 - Multifunctional Photocatalytic Materials for Energy

14 Multifunctional Photocatalytic Materials for Energy

2.5.3.1 Superhydrophilic

When the surface of a photocatalytic layer is exposed to light, the contact angle of the
photocatalyst surface with H 2 O is decreased slowly. After sufficient exposure to light,
the surface reaches a super-hydrophilic state; i.e., it does not repel water at all, so wa-
ter cannot exist in the shape of a drop, but spreads flatly on the surface of the substrate.
Finally, the water acquires the form of a highly uniform thin film, which acts optically
like a clear sheet of glass. The hydrophilic nature of metal oxides, coupled with grav-
ity, will enable the dust particles to be cleaned away following the water stream, thus
making the product self-cleaning.
TiO 2 can degrade organic contaminants at the surface level with the assistance of
UV light. This characteristic has led to the application of TiO 2 photocatalysis in a “self-
cleaning” technique where TiO 2 -coated surfaces maintain cleanliness under UV light, that
is, by utilizing readily available sunlight or ultraviolet emissions from fluorescent lamps,
which in turn saves maintenance costs and reduces the need to rely on detergents for clean-
ing [24]. Fujishima et al. confirmed this self-cleaning concept on a titania-coated ceramic
tile in 1992 [25]. The first commercialized product using this method was the self-cleaning
cover glass for highway tunnel lamps [24,26]. An example is the sodium lamp in Japan that
emits UV light through the cover glass, which is mainly used to decompose the contam-
inants from automobile exhaust because the UV light is not useful for lighting purposes.
Therefore the cover glass maintains its transparency for long-term use.
Also, Wang et al. added that the self-cleaning effect could be aided with flowing
water (e.g., rainfall) on the TiO 2 surfaces [27]. This enhancing phenomenon of flowing
water such as rainfall was ascribed to the super-hydrophilic property of TiO 2 surfaces;
i.e., water was able to penetrate the molecular-level space between the stain and the
super-hydrophilic TiO 2 surface [27–30]. Outdoor materials benefit from the combined
self-cleaning effect of TiO 2 photocatalytic and superhydrophilic nature.
Apart from lamps, other materials on the road, such as spray coatings for cars,
rearview mirrors and windshields, tunnel walls, reflectors, and traffic signs, have uti-
lized TiO 2 surface coating to achieve self-cleaning and antifogging advantages. Also,
application could be found in materials for residential and office buildings, such as
exterior tiles, kitchen and bathroom components, interior furnishings, plastic surfaces,
aluminum siding, tent material, building stone and curtains, glass windows, and win-
dow blinds [26].

2.5.4 Antibacterial effect
The advantage of using photocatalysts as antibacterial materials is that they not only
kill bacterial cells but also completely decompose the cells. The TiO 2 photocatalyst
has been found to be more active than any other antibacterial agent, because the pho-
tocatalytic process acts even when the cells' surfaces are covered and bacteria are
actively propagating. The end toxins produced after the death of the cells are also
•
•
expected to be decomposed by photocatalytic action, i.e., by the use of OH and O 2
radicals. TiO 2 does not deteriorate and demonstrates a long-term antibacterial effect.
According to researchers, decontamination by TiO 2 is three-fold greater than that of