Self-cleaning tiles and glasses for eco-efficient buildings - Nanotechnology in Eco-efficient Construction - page 329

of organic contaminants require high processing temperatures and hence
high-temperature stability.2. 4,17 High consumer demand is projected for
these materials.
The second area to investigate is the stability of the anatase at high tem-
perature. This can be investigated in many ways. Previous X-ray photoelec-
tron spectroscopy (XPS) studies by the authors' group have indicated that
only 0.3 at% F of doping has achieved when 16 mole time (1:16) F precur-
sors trifl uoroacetic acid (TFA). This 0.3% F doped sample showed stability
up to 900°C and a small increase in dopant content is expected to give even
higher temperature stability. Adding dopants above 0.5% by addition of
high-temperature stable dopant precursors should be developed. Therefore
both the levels of dopants (e.g., N-F, S-F, S-N, C-F, C-N, C-F, etc.) and various
precursors (e.g., TFA, TiCl 4 ) should be looked at and incorporated into the
titania matrix.
The solid state chemistry at high temperature can also be tuned by chang-
ing precursors or employing various annealing schedules such as step
heating without grain growth. Step annealing was previously employed suc-
cessfully to sinter Y 2 O 3 materials without signifi cant grain growth. A similar
heating strategy can be designed to produce anatase materials with less or
little grain growth at higher temperatures ( 1000°C) thus preventing rutile

formation. New sintering techniques such as ramp-sustain-decay can be
applied. The development of photocatalysts (with lower band gap) which
can be activated under visible light (

≥

400 nm) is desired in order to make
use of the main part of the solar spectrum, and to extend their applications
to room interiors where there is relatively poor lighting illumination.
The anatase-to-rutile phase transformation in TiO 2 is an area of both
scientifi c and technological interest. 44,45 The anatase-to-rutile transforma-
tion (ART) is kinetically defi ned and the reaction rate is determined by
parameters such as particle shape/size, 46 purity, 47 source effects, 48 atmo-
sphere 49 and reaction conditions. 50 It is agreed that the mechanism for phase
transformation of titania is one of nucleation and growth. 51,52 Anatase nano-
crystals coarsen, grow and then transform to rutile only when a critical size
is reached. 53 Therefore, phase transformation is dominated by effects such
as defect concentration, 54 grain boundary concentration 55 and particle
packing. 49 Rutile is the thermodynamically stable phase, while anatase and
brookite are both metastable, transferring to rutile under heat treatment at
temperatures typically ranging between 600 and 700°C. 56 Anatase is widely
regarded as the most photocatalytically active of the three crystalline
structures. 57,58
The generally accepted theory of phase transformation is that two Ti-O
bonds break in the anatase structure, allowing rearrangement of the Ti-O
octahedra, which leads to a smaller volume, forming a dense rutile phase.
The removal of oxygen ions, which generate lattice vacancies, accelerates

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