Thin films and nanostructured coatings for eco-efficient buildings - Nanotechnology in Eco-efficient Construction - page 173

pipe removes nanoparticles outside the growth region. The nanoparticles
are then deposited via a nozzle in order to gain momentum onto a substrate
that can be moved so that they form a uniform fi lm comprised of nanopar-
ticles. The technique can be implemented with multiple vapour sources and
transfer pipes in order to prepare materials consisting of mixed or layered
nanoparticles. Figure 8.12 shows some typical nanoparticle deposits for the
case of as-deposited tungsten oxide and WO 3 :Pd sintered at 600°C (Hoel
et al. , 2005). The particle size distribution is seen to be narrow. Evidently
the sintering has caused signifi cant grain growth.
A distinctive advantage of the AGD technique is that it separates
nanoparticle formation and growth from thin fi lm deposition. This feature
makes it possible to fi ne-tune particle interaction within the fi lm, at least to
some extent, which is benefi cial for devices that require well-controlled
electrical contact between adjacent nanoparticles such as conductometric
gas sensors for determining and surveying air quality. Gas phase synthesis
of nanoparticles also can use high-temperature processes, since particles can
form in a fl ame or plasma. The reader is referred to the topic by Granqvist
et al. (2004) for detailed coverage of this subject.
Chemical approaches are now widely used to grow and precipitate metal-
lic, inorganic and semiconducting nanoparticles from solution and have
been refi ned so as to limit size ranges, create elongated particles as well as
spheres, and to overcoat nanoparticles or microparticles with nanoshells in
'core-shell' structures which enable new or improved functionality. These
aspects have been discussed in detail in the literature (Cushing et al. , 2004).
Thick layers incorporating nanoparticles can effectively function as thin
ones provided that metal fl akes are included and serve as 'artifi cial sub-
strates' (Kuni ˇ et al. , 2009). Layers or coatings containing previously pre-
pared nanoparticles are usually made by fi rst dispersing them in a paint

￿ ￿ ￿ ￿ ￿ ￿

100 nm 100 nm

(a) (b)

8.12 Scanning electron micrographs of (a) an as-deposited fi lm of
WO 3 and (b) a WO 3 :Pd fi lm sintered at 600°C. Both fi lms were
prepared by advanced gas deposition. Horizontal bars are 100 nm in
length. From Hoel et al. (2005).