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2. INTRODUCTION
Since the discovery, in 1972, of the water photochemical splitting by means of titanium dioxide
(TiO
2
) electrodes
1
, there has been an exponential growth of studies regarding this promising
material with special regards to photocatalytic applications (Figure 1.1).
Figure 1.1 Number of pubblications devoted to TiO
2
and TiO
2
–photocatalyst from 1995 to 2003
(Source: Ref. 2).
As reported in a recent review
2
, in the last 30 years the number of publications devoted to TiO
2
has more than doubled, revealing growing attention of the scientific community to titanium
dioxide. In fact the large variety of applications of TiO
2
has attracted the interest of many
researchers: from physicists and chemists to material engineers
3
.
TiO
2
, also known as titania, is very strong, light, refractory (mp 1800 °C) and resistant to
corrosion and drastic chemical conditions; it has high photocatalytic activity (PCA), a relatively
low cost and it is non toxic
4
.
As a pure substance it is a bright and brilliant white powder with
high scattering power which makes it an important pigment so that it is widely used in the
industry of paints. Titanium dioxide pigment is the single greatest use of titanium worldwide:
almost 95% of the total production is consumed with this purpose. However, TiO
2
is commonly
used also in papers, cements, plastics, cosmetics and food industry. TiO
2
is important in earth
science processes, it plays a key role in the biocompatibility of bone implants and is being
1
A. Fujishima, K. Honda, Nature 238 (1972) 37–38.
2
O. Carp, C.L. Huisman, A. Reller, Prog. Solid State Chem. 32 (2004) 33–177.
3
A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem Photobiol. C: Photochem Rew. 1 (2000) 1–21.
4
L. Pauling, General Chemistry, Dover Publications, INC., New York.
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discussed either as a gate insulator for the new generation of MOSFET
5
or for spacing materials
in magnetic spin–valve system. It finds applications in nanostructured form in Li–based batteries
and electrochromic devices. Besides, nanoscale particles of anatase (one of the TiO
2
polymorphs) are transparent to visible light but are highly effective in the absorption of UV–
light. The UV absorption of nano–sized anatase particles is blue–shifted (i.e. wavelength
absorption occurs at 70–80 nm lower than bulk anatase), so that higher energy UV–light is
absorbed by this type of nanoparticles. Hence, they are used in sunscreens to protect against UV
induced skin damage.
In recent years TiO
2
has also played a major role in fine photocatalytic applications. Possible
uses of TiO
2
as photocatalyzer are innumerable (see Table 1.1).
Among these purification of
wastewaters
6,7
protective coatings of ancient marbles and materials
8
,
degradation of organic
pollutants (some species can be completely mineralized to CO
2
and ions generally harmless for
humans and environment)
6,9
, sterilization
10,11,12
and self–cleaning materials. These range from
paints to cements, from glasses to ceramics and asphalts. Nowadays even photocatalytic artificial
flowers are known
13
.
Table 1.1 Some photocatalytic applications of TiO
2
(source: Ref. 3).
5
The metal–oxide–semiconductor field–effect transistor (MOSFET) is a device used to amplify or switch electronic
signals. It is by far the most common field–effect transistor in both digital and analog circuits. The MOSFET is
composed of a channel of n–type or p–type semiconductor material, and is accordingly called an nMOSFET or
pMOSFET (Adapted from http://en.wikipedia.org/wiki/MOSFET).
6
J. Zhao, C. Chen, W. Ma, Top. Catal. 35(3–4) (2005) 269–278.
7
N.S. Prakash, E.T. Puttaiah, B.R. Kiran, K. Harish Babu, K. M. Mahadevan, Res. J. of Chem. and Env. 11(4)
(2007) 73–77.
8
I. Poulios, P. Spathis, A. Grigoriadou, K. Delidou, P. Tsoumparis, J. Environ. Sci. Health, Part A: Environ. Sci.
Eng. A34 (7) (1999) 1455–1471.
9
D. Chatterjee, S. Dasgupta, J. Photochem Photobiol. C: Photochem Rew. 6(2–3), (2005), 186–205.
10
C.H. King, Appl. Environ. Microbiol 54 (1988) 3023.
11
U. Zszewsyk, Ann. Rev. Microbiol. (2000) 54–81.
12
L.V. Venczel, Appl Environ. Microbiol 63 (1997) 1598.
13
http://moriryohei.trustpass.alibaba.com/product/11894498/Artificial_Flower_Plant_Air_Purifier_.html
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The following images (Figure 1.2) are just some examples of goods and building that already
exploit the photocatalytic technology.
a b
c
Figure 1.2 Some possible applications of photocatalytic titania: a. A piece of photocatalytic asphalt; b. A
photocalytic tile (used in different either indoor or outdoor paving); c. Building entirely built with
photocatalytic cement (church of Dives in Misericordia–Rome).
Unfortunately TiO
2
has a low quantum yield for the photochemical conversion of solar energy.
Nevertheless the use of colloidal suspension with the addition of dye molecules has been shown
to improve efficiency of solar cells and has moved TiO
2
based photoelectrochemical converters
into the realm of economic competitiveness. These are known as dye–sensitized photovoltaic
cells
14,15,16,17
and have absorption in the visible light.
However, for all the mentioned purposes it is necessary to realize innovative synthetic routes that
permit to get specific phases, controlled surface area and crystals dimensions. The research
group at the University of Milan has been very active
18,19,20,21,22,23
in the tailored synthesis and
characterization of TiO
2
. This Thesis was performed in the context of these studies. The main
14
M. Gratzel, Nature 414 (2001) 338.
15
M. Gratzel, Curr. Opin. State Mater. Sci. 4 (1999) 314.
16
D. Wahrle, D. Meissner, Adv. Mater 3 (1991) 129.
17
http://www.g24i.com/
18
T. Boiadjieva, G. Cappelletti, S. Ardizzone, S. Rondinini, A. Vertova, Phys. Chem. Chem. Phys. 5 (2003) 1689–
1694.
19
T. Boiadjieva, G. Cappelletti, S. Ardizzone, S. Rondinini, A. Vertova, Phys. Chem. Chem. Phys. 6 (2004) 3535–
3539.
20
G. Cappelletti, C. Ricci, S. Ardizzone, C. Pirola, A. Anedda, J. Phys. Chem. B 109 (2005) 10.
21
G. Cappelletti, C.L. Bianchi, S. Ardizzone, Appl. Surf. Sc. 253 (2006) 519–524.
22
S. Ardizzone, C.L. Bianchi, G. Cappelletti, S. Gialanella, C. Pirola, V. Ragaini, J. Phys. Chem. C 111 (2007)
13222–13231.
23
G. Cappelletti, C.L. Bianchi, S. Ardizzone, App. Cat. B: Env. 78 (2008) 193–201.
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aims of the work were the synthesis of nanocrystalline titanium dioxide (sol–gel reaction) with
controlled properties (phase content, crystals dimensions, morphological aspects) exploring the
possibility to extend the absorption to the visible light by means of nitrogen doping and testing
the PCA against the conversion of NO
x
(one of the main pollutant in Milan metropolitan area).
The thesis focused on the role of the ions SO
4
2–
and NH
4
+
as a promoter of the anatase phase and
as a visible light shifter respectively. The coupled behaviour of the ions was also investigated.
Other parameters like the ratio of Ti/H
2
O or H
2
O/IPA used during the synthesis were not
investigated since data are available in the literature
22
. Characterizations of the structure of the
home–made samples were performed through different analytical techniques including XRPD,
XPS, BET, UV–reflectance and TEM.
It was also outlined a method to produce an efficient photocatalytic paint. It was hoped that a
final outcome of the thesis would have been the discovery of a way to formulate an efficient
photocatalytic paint (optimization of chemicals, mixing ratio, mixing of nanocrystalline
photocatalytic TiO
2
and testing of the photocatalytic activity). In this, we have succeeded
exploiting several commercial samples and tracing a future path to be followed to get the best
catalytic activity under visible (solar) irradiation.
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3. TITANIUM DIOXIDE – STRUCTURE AND PROPERTIES
3.1. Structure
TiO
2
is a crystalline solid with a dominant ionic character, present in nature in three different
forms: rutile, anatase and brookite. The structure of the three polymorphs can be described by
means of common octahedral units TiO
6
2–
whose distortion and supramolecular assembly vary
for the different polymorphs. In the anatase polymorph octahedron share vertex, in the rutile
polymorphs edges whereas both are shared in the brookite structure.
It is noticeable that starting from rutile and working at high pressure other polymorphs have been
synthetized like TiO
2
(II)
23
with a PbO
2
–like structure and TiO
2
(H)
24
with an hollandite–like
structure.
Several phase diagrams
25,26
have been determined as well for TiO
2
; they show the relationship in
term of temperature and pressure among the polymorphs. Nonetheless, it must be underlined that
solidus curves depend on many factors including the mean crystal size and the way TiO
2
has
been produced.
Thermodynamic data show that TiO
2
–rutile polymorph is the most stable phase at all the
temperatures if a maximum pressure of 60 kbar is considered
26,27
. Actually small differences in
terms of free energy (4–20 kJ/mol) between the polymorphs suggest that the metastable phases
are stable almost as rutile at normal values of pressure and temperature. However, the real
system is somehow more complicated since experiments have shown that the relative stability of
a phase changes as a function of the particles dimensions. Surface free energy and surface stress
depend on the size so that small crystallites (nano–crystals in the present study) are affected by
greater variation of the global free energy due to the increase in the surface/volume ratio. This
leads to different range of stability of the different phases as function of the particles dimensions.
As an example the [101] anatase surface
28
has the lowest surface energy so that it is the most
stable phase when particles dimensions are lower than 11 nm; but when dimensions are included
between 11 and 35 nm the most stable phase is brookite whereas rutile is the most stable for
crystal bigger than 35 nm
29
.
24
M. Latroche, L. Brohan, R. Marchand, M. Tournoux, J. Solid State Chem (1989) 79–78.
25
P.Y. Simon, F. Dachille, Acta Cryst. 23 (1967) 334.
26
J.C. Jemieson, B. Olinger, Amer. Miner. 54 (1969).
27
A. Narotsky, J. C. Jamieson, O. J. Kleppa, Science 158 (1967) 338.
28
For a detailed study of titanium dioxide surfaces see U. Diebold, Surf. Scienc. Rep. 48 (2003) 53–229.
29
A. Arbezzano, Influenza dell’adsorbimento di ioni inorganici sull’attività foto catalitica del biossido di titanio,
(2006) University of Turin, Matser Thesis (in Italian), not published.
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3.1.1. Rutile
Rutile is the most common phase and has the highest refractive index among of any known
mineral and also exhibits high dispersion. Natural rutile may contain up to 10% Fe and
significant amounts of Nb and Ta. Rutile is a common accessory mineral in high–temperature
and high–pressure metamorphic rocks and igneous rocks. Rutile is the preferred polymorph of
TiO
2
in such environments because it has the lowest molecular volume among the three
polymorphs. Rutile is also the most studied phase thanks to the possibility to easily get single
crystal.
Its structure is characterized by a hexagonal compact lattice of O
2–
ions whereas the Ti
4+
cations
occupy half of the octahedral holes. Coordination number for Ti
4+
is 6 and for O
2–
is 3. Rutile
structure can be described also as a cation centred cubic lattice; in this case the cell is highly
distorted. Rutile crystallizes in the tetragonal system, P4
2
/mnm space group. Cell constants are a
= b = 4.593 Å and c = 2.959 Å (Figure 2.1).
TiO
6
2–
units (distorted octahedron – Figure 2.2) share only two edges giving rows of octahedron
parallel to the [001] direction. Rows are connected along [110] directions through the vertex of
the octahedron. Neighbouring octahedrons are turned by 90°. Channels are created along [001]
directions (Figure 2.3).
It has a submetallic luster and a hardness of 6 in the Mohs scale; specific weight is 4.3 g cm
–3
.
Ti
O
a
b
c
Ti
O
Ti
O
a
b
c
a
b
c
Figure 2.1 The rutile unit cell. Figure 2.2 A TiO
6
2–
unit in rutile:
R
4
= 1.97Å, R
5
= 1.95Å.
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