Environmental Engineering Reference
In-Depth Information
ordered domains on the 5-50 nm length scale. This is driven by the incompati-
bility of its covalently linked macro-molecular blocks. The inorganic material is
typically selectively incorporated into one of the polymer domains in the form of a
nanoparticle sol. The structure-direction controlled by macro-molecular self-
assembly undergoes a high-temperature calcination step, resulting in an inorganic
material that resembles the polymer microphase morphology [
80
].
Commonly the used organic templates are amphiphilic poly(alkylene oxide)
block copolymers, composed of two different polymers covalently connected at one
end, such as triblock copolymers, HO(CH
2
CH
2
O)
20
(CH
2
CH(CH
3
)O)
70
(CH
2
CH
2
O)
20
H (designated EO
20
PO
70
EO
20
, called Pluronic P-123) [
81
] and
HO(CH
2
CH
2
O)
106
(CH
2
CH-(CH
3
)O)
70
(CH
2
CH
2
O)
106
H (designated EO
106
PO
70
EO
106
, called Pluronic F-127) [
79
,
82
]. Recently diblock polymers have also been
used as structure-directing agents. Some examples include polystyrene-block-
poly(4-vinylpyridine) (PS-b-P4VP) [
83
,
84
], polystyrene-block-poly(2-vinylpyri-
dine) (PS-b-P2VP) [
85
], polystyrene-block-poly(ethylene oxide) (PS-b-PEO) [
86
,
87
], poly(vinyl chloride)-g-poly(oxyethylene methacrylate) (PVC-g-POEM) [
67
],
and poly(isoprene-block-ethylene oxide) (PI-b-PEO) [
88
]. Moreover, other
organics including poly(dimethylglutarimide) (PMGI), hydroxyl styrene-based
cross-linkable polymers [
89
], cetyltrimethylammonium bromide (CTAB) micelles
[
90
], cellulose [
65
], sodium alginate [
91
] and polyethylene glycol (PEG) [
40
] are
also employed to direct the formation of mesoporous TiO
2
for solar energy
applications. Using such sol-gel process, electrodes can be prepared directly on Si
or FTO substrates. Furthermore, the process is highly scalable because it can be
performed at low temperatures without any expensive or complicated equipment.
Among the various new microstructured electrodes designed recently, three-
dimensional (3D) photonic crystal electrodes (Fig.
3
c) made from colloidal crystal
templates or inverse opals (Fig.
3
b) have been well investigated due to their
unique advantages [
92
,
93
]. Photonic crystal (PC) materials exhibit periodicities in
their refractive index on the order of the wavelength of light, and thus provide
many interesting possibilities for ''photon management'' [
94
,
95
]. Bragg diffrac-
tion in a periodic lattice, localization of heavy photons near the edges of a photonic
band gap, multiple scattering at disordered regions in the photonic crystal, and the
formation of multiple resonant modes are some of the phenomena that are
exhibited by photonic crystals and can greatly enhance the effective light path
within the active layer [
68
,
96
]. 3D photonic colloidal crystals are of particular
interest for enhancing light harvesting in DSSCs because these TiO
2
crystals can
be both porous and significantly enhance light-matter interactions on the long
wavelength side of the stop band. More importantly, the 3D ordered porous
electrode with relatively large porosity is beneficial in applications that include
polymeric electrolytes with high viscosities and relatively large molecular weight.
Furthermore, 3D connected TiO
2
networks can provide an organized electron path,
which may facilitate charge transport and thus enhance the collection efficiency of
back-contact electrodes [
72
,
92
]. In this regard, several hard templates, such as
polystyrene (PS) [
97
,
98
], poly(methyl methacrylate) (PMMA) [
73
], and SU8
photoresist [
72
] can be used to build the 3D backbone.
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