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Metal-based semiconductor nanomaterials for thin-film solar cells 177

8.4.3 Hierarchical TiO 2 microspheres

Three main structural features, large surface area, scattering effect, and effective elec-
tron transport, are crucial for optimizing the structure of TiO 2 photoanodes. However,
these features are usually in conflict with each other; that is, it is difficult to integrate
all the features into a single photoanode material. To balance the surface area and the
scattering effect of the photoanode, numerous research groups have used hierarchical
TiO 2 microspheres (HTMs) as photoanodes. These HTMs exhibited not only large
particle sizes and high surface areas but also an increased scattering effect and effec-
tive electron transport.

8.4.3.1 Two-step self-template method

In this method’s first step, amorphous Ti-organic complex spheres are fabricated and
then are transformed into well-crystallized TiO 2 HMSs. Xu et al. prepared titanium
diglycolate spheres by using diethylene glycol as a coordinating reagent, after which
well-crystallized TiO 2 HMSs were obtained through hydrothermal conversion of
the precursors. The diameter of the TiO 2 HMSs ranged from 100 nm to 300 nm and
could be controlled by adjusting the amount of H 2 O. As shown in the TEM images in
Fig. 8.19, the HMSs consisted of ~8 nm nanocrystals and possessed a specific surface
2 −1
area of 193 m g and a main pore size of 5 nm. A high PCE of 8.20% was achieved,
indicating a 40% increase in the PCE compared to the standard Degussa P25 photo-
anode. The enhancement may be attributed to fact that the mesoporous TiO 2 HMSs
can enhance light harvesting within the photoanodes for DSSCs; moreover, their good
crystallinity and better interparticle connections caused by partially oriented attach-
ment of primary particles result in effective charge transport through the film of mes-
oporous TiO 2 spheres.

8.4.3.2 Titanium precursor transformation method
In this strategy, nanostructured TiO 2 nanomaterials are employed as titanium sources
to synthesize TiO 2 HMSs.
Li et al. developed a two-step hydrothermal method to achieve large-scale, urchin-
like Na-titanate microspheres (Na-UTMSs) assembled by NTs with an ultrahigh sur-
face area (Fig. 8.20) [85]. In this work, Na-titanate nanotubes (Na-TNT) were first
obtained by hydrothermal reacting P25 in a concentrated NaOH solution and then
transformed into urchin-like Na-UTMSs by adding H 2 O 2 under basic conditions.
Furthermore, Na-UTMSs could be converted to H-titanate microspheres (H-UTMS)
by proton exchange and then transferred to the anatase phase by annealing without
destroying the morphology.

8.4.3.3 One-pot hydrothermal method
In this strategy, TiO 2 HMSs with controlled sizes and morphologies can be fabri-
cated straightforwardly by intentionally adopting different precursors and solvents
or by capping agents into the reaction system [86,87]. In addition, the one-pot