Page 255 - Multifunctional Photocatalytic Materials for Energy

238 Multifunctional Photocatalytic Materials for Energy

spectrum. Therefore, with the goal of achieving high hydrogen production rates and
stabilities, more efforts have been devoted to enlarging the effective photocatalytic
surface, forming Schottky junctions or heterojunctions, and engineering the band
structure to match particular energy levels [26–28].
In this chapter, the main areas of focus are as follows: (i) to introduce state-of-
the-art synthetic methods of well-defined 0D–3D TiO 2 nanomaterials, including
hydrothermal methods, solvothermal methods, sol-gel methods, template methods,
electrospinning, electrochemical anodic oxidation, and so on; (ii) to improve the de-
sign principles of TiO 2 -based heterostructures produced by combining different sur-
face engineering strategies, such as enlargement of photocatalytically active areas,
optimizing the crystallinity and exposed facets, development of visible light- sensitized
photocatalysts by doping or modifying TiO 2 with metals, nonmetals, and narrow band
gap semiconductors and other complex compounds to aim for higher solar WS effi-
ciency of the specific nanostructures; and (iii) to highlight recent challenges and future
directions of full solar light photocatalysis based on TiO 2 nanomaterials.

11.2 Preparations of multidimensional TiO
2
nanostructures

Nanostructures play an important role in the properties related to photocatalytic effi-
ciency of TiO 2 -based materials, and various synthetic approaches have been reported
for different TiO 2 nanostructures. The technological methods for the formation of
multidimensional TiO 2 nanostructured materials will be discussed in this section. We
will briefly recount various approaches reported to prepare multidimensional (0D,
1D, 2D, and 3D) TiO 2 materials of different sizes and morphologies, such as NPs,
nanotubes, nanorods (NRs), nanowires (NWs), nanofibers (NFs), porous films, porous
spheres, 3D hierarchical nanostructures, and so on [29–39].

11.2.1 Controlled growth of 0D TiO 2 nanostructures
Zero-dimension (OD) nanostructures refer to those with nearly spherical shapes
within a dimension of 100 nm [40]. Various recent approaches have been reported on
synthesizing TiO 2 NPs. Jiang's group produced a simple electrochemical method for
the deposition of TiO 2 NPs on multiwalled carbon nanotube arrays (NTAs). In brief,
10 mM H 2 O 2 , 3 M KCl, and 10 mM Ti(SO 4 ) 2 were mixed as the electrolyte, and the
carbon nanotubes were an Ag/AgCl electrode, a Pt sheet acting as the working elec-
trode, the reference electrode, and the counter electrode (CE), respectively. During the
process, ions dispersed in the migrating solution were deposited as NPs on the elec-
trode at − 0.10 V for 30 m [41]. They found that the morphology of the TiO 2 NPs could
be highly controlled so that the reaction time, temperature, and pH did not change.
To obtain advanced nanostructured materials, which covers the participations of ca-
talysis, electronics, ceramics and other portions, a hydrothermal method was taken
into consideration [42]. In a typical process, different phases of TiO 2 nanocrystals
were selectively obtained by Li et al. via a hydrothermal method. For anatase TiO 2 ,