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Fig. 8.5 Formation mechanism of the rectangular bunched TiO 2 NR arrays and NT arrays.
SEM images of (A) TiO 2 NR arrays, (B) bunched TiO 2 NR arrays, and (C) TiO 2 NT arrays are
shown, and the corresponding schematic diagrams represent the growth mechanism of TiO 2
nanostructures on CFs.
Reprinted with permission from W. Guo et al., Rectangular bunched rutile TiO 2 nanorod arrays
grown on carbon fiber for dye-sensitized solar cells. J. Am. Chem. Soc. 134 (2012) 4437.

crystalline facet. This reaction resulted in the dissolution rate in the long-axis direction
being much faster than that in the side-wall direction, which explains precisely why
the formed NWs possessed a rectangular hollow structure.

8.2.2 Zinc oxide (ZnO)
Nanoscale ZnO exhibits outstanding carrier mobility, photocatalysis, and absorb and
scatter light abilities, which makes it a good choice for the fabrication of solar cells
(e.g., PSCs and DSSCs). However, in most cases, the power conversion efficiencies
(PCEs) of the solar cells based on ZnO are less than those of the devices based on
TiO 2 , which may be attributed to the instability of ZnO in acidic dye and slow elec-
tron injection kinetics (high charge recombination) [15]. The fabrication techniques
of various ZnO nanostructures including ZnO NPs, NRs/NWs, and NTs are discussed
in detail next.

8.2.2.1 Fabrication of ZnO NPs
Wet chemical synthesis is a widely used technique for growing ZnO NP solutions, in
which zinc nitrate (Zn(NO 3 ) 2 ) and sodium hydroxide (NaOH) are generally used as
the precursor reactants and deionized water as the reactive solvent. In the first step,
the aforementioned mixed solution was stirred to obtain Zn(OH) 2 and then heated at