Photocatalytic activity of nanosized TiO2 on self cleaning surfaces
Libe Arzubiaga
Introduction
Titanium (IV) oxide (TiO2), also known as titania, is the natural occurring oxide of titanium.
The main reason for the popularity of this system in the research community is its wide range
of applications. TiO2 is used in heterogeneous catalysis, as a photocatalyst, in solar cells for the
production of hydrogen and electric energy, as gate oxide in electronics, as a gas sensor, as
white pigment, as sunscreen in cosmetics, for biocompatibility in bone implants, etc (1).
It must be noted that TiO2 occurs in nature as different mineral forms, anatasa, rutile and
brookite. Rutile is the only stable form in bulk (the others are metastable), but when paticlesize is decreased to about 13-14 nm, anatase becomes more stable. Each has its own surface
structure and chemistry, therefore it will be more suitable for certain applications (2).
TiO2 as a photocatalyst
By far, the most actively pursued applied research on TiO2 is its use in photo-assisted
degradation of organic molecules. The applications of this process range from purification of
wastewaters and air, sterilization of surfaces (based on its bactericidal properties), selfcleaning coatings, to protective coatings for marble statues against environmental damage. It
is even being studied as an agent for destroying tumor cells in cancer therapy, in the form of
subcutaneous injections followed by UV light illumination (1).
In particular, we are going to focus on the self-cleaning action consisting in the destruction of
organic stains on coatings containingTiO2 due to light irradiation. These reactions occur in the
presence of atmospheric oxygen (O2).
Fundamentals of photocatalysis with TiO2
TiO2 is a semiconductor with relatively wide bandap (about 3 eV). If it receives photons with
energy larger than the material’s bandgap (UV light, λ < 400 nm) an electron/hole pair is
generated. This holes are highly oxidizing (with redox potential of about +2.53V compared to
the standard H2 electrode at pH 7), thus being able to produce hydroxil radicals (•OH ) from
adsorbed moisture. The conduction band electrons have a redox potential of -0.52V, being
reducing enough to produce superoxide radical anions (•O2-) from atmospheric oxygen (O2)
(3). This very reactive radicals are responsible for the oxidation of the organic compounds
adsorbed to the TiO2 surface to eventually obtain CO2 and (Figure 1). The yield of this
conversion is mainly limited by the electron /hole recombination losses that are intrinsic to
each type of film, probably occurring at bulk defects.Hence the importance of the intrinsic
quality of the TiO2 particles, the quality of the nanostructurate film and the particle size.
Attempts to reduce recombination include doping with metal ions into the TiO2 lattice, due to
their ability to trap electrons (4). (See Figure 1).
Figure 1. left: Electron/ hole pair formation in TiO2 by UV light and assisted by Ag nanoparticle , followed
by radical formation from O2, water and related species and oxidation of organic pollutants into CO 2 and
water (4). Right: IR spectra qualitatively showing decrease of pollutant concentration as a fuction of
reaction time. (3)
Besides, the photocatalytic activity of TiO2 can be broadened by dye-sensitizing , often by a
pigment present in the stain we are trying to clean from the surface itself. This means that a
dye molecule can absorb light in a range outside the bandgap of TiO2 to promote an electron
and inject it in the TiO2 conduction band, starting the oxidative process (5). For example, this is
the case of the discoloration of a tomato stain (carotene or lycopene dye with absorption in
visible range). See figure 2.
Figure 2. The discoloration of carotene pigment is due to the generation of reactive radicals on the TiO2
surface. The reaction gets started thanks to the dye itself. (taken from ref .5)
Figure 3. Time evolution of red wine stain on cement with TiO2 coating. The process is also dye
sensitized by lycopene dye present in red wide.
Advantage of being “nano”
The above described photodegradation of organic matter is a series of reactions occurring in
active sites located on the surface of TiO2. From this fact we can conclude that a size reduction
of TiO2 to nanoscale particles leads to a higher efficiency of this catalytic activity, due to a
higher surface to volume ratio. On the other hand, particles with smaller sizes (5-7nm) show a
decrease in the band-gap energy (about 2.18 eV) compared to bigger particles (100-150 nm
with 3 eV bandgap). Thus, there is a red shift in the optical absorption spectrum (6).
Besides, nano-TiO2, typically produced via sol-gel process, have been made in form of nanorods, whiskers, wires, spheres,etc. This nanoparticles can at the same time form
nanostructurate coatings that are versatile enough for their application in a wide range of
surfaces. Also, we have seen that smaller particles could be less prone to electron hole
recombination, related to bulk defects (3).
Future challenges for TiO2 self cleaning coating
There are some issues to be solved: Several studies show that TiO2 coatings are still
energetically expensive and shortlived, therefore their use is restricted to special cases
(sculptures, valuable architectonic elements, etc) (7,8). Besides, they have been often found to
degrade the substrate they’re protecting or the matrix in which they are introduced (often an
organic polymer). This leads to free TiO2 particles which have been said to represent a hazard
themselves, as they create radicals that are carcinogenic (9).
(1) Diebold et al, Surface Science Reports 48 (2003) 53-229
(2) Lei et al, Modelling Simul. Mater. Sci. Eng. 18, 025004 (2010)
(3) Fujishima et al, J. Photochem. Photobiol. C: Photochemistry reviews 1, 1-21 (2000)
(4) Behnajady et al, Global NEST Journal, 10, No 1, 1-7 (2008)
(5)Yuranova et al, Journal of Photochemistry and Photobiology A: Chemistry 188 (2007) 334–
341
(6) Hedge et al, Pramana J. Phys., 65, 4 (2005)
(7) Mills et al., Research on Chemical Intermediates 31, 295-308 (2005)
(8) L. Reijnders, Polymer Degradation and Stability 94, 873-876 (2009)
(9) . Auvinen and L. Wirtanen, Atmospheric Environment 42 (2008) 4102-4112