Low-Cost Nanomaterials for Photoelectrochemical Water Splitting - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 267

Fig. 3 ZnO nanowire array
on a 4-inch (ca. 10 cm)
silicon wafer. At the center is
a photograph of a coated
wafer, surrounded by SEM
images of the array at
different locations and
magnifications. These images
are representative of the
entire surface. Scale bars,
clockwise from upper left 2,
1 mm. 500 and 200 nm.
Reproduced with permission
from [ 24 ]
the film. Mesoporous films heat-treated below 300 C appeared yellow, compared
to the red films obtained after annealing at higher temperatures. Prochazka et al.
synthesized the exceptionally dense TiO 2 films via dip-coating from a sol con-
taining poly(hexafluorobutyl methacrylate) as the structure-directing agent [ 72 ].
The TiO 2 films can be deposited on glass, F-doped SnO 2 , and crystalline silicon
(111) faces. Noriyuki et al. fabricated ZnO/Zn 0.85 Mg 0.15 O heterostructure thin
films by sol-gel spin-coating method on sapphire (0001) substrate [ 66 ]. The
bottom ZnO layer was prepared from zinc acetate dissolved in 2-methoxyethanol.
And, monoethanolamine was added to the solution until reaching the same molar
ratio with Zn ions. Its solution was deposited onto sapphire substrate by spin-
coating process, and then annealed in air at 280 C for 1 min to remove the
solvent. Likewise, Elfanaoui et al. used a very simple and low-cost sol-gel syn-
thesis technique to prepare granular TiO 2 film [ 12 ]. Thin films of TiO 2 were
deposited by the method of spin coating in air at room temperature, on corning
glass as substrate. The films were dried at 100 C for 1 h and annealed at 300, 350,
and 400 C in air during 1 h. They investigated the effect of the number of
coatings and the annealing temperature on the films as well.
In comparison to sol-gel spin-coating techniques, doctor-blading of thin films is
an even more simple and convenient method for creating ordered and mesoporous
nanostructures [ 1 , 34 , 75 , 83 ]. Films that are 10 lm thick can be fabricated with
relative ease using this method. For example, Kabre et al. prepared a Tin-doped
TiO 2 photoanodes with a thickness of 2 mm and a resistance of 8 X/sq on a
2.5 9 2.5 cm of FTO glass [ 34 ]. A TiO 2 slurry was prepared by grinding 2 g of
Degussa P25 TiO 2 powder in 3 mL of 10 % acetic acid for 30 min, followed by
addition of 1 drop of an aqueous 10 % Triton-X solution. The slurry was then
deposited on the conductive side of the glass plate using the doctor blade technique
with a mask made of 3 M tape applied to three sides of the plate. After air drying