Page 274 - Multifunctional Photocatalytic Materials for Energy

256 Multifunctional Photocatalytic Materials for Energy

single crystalline phase also leads to fast recombination of the electrons and holes.
To surmount these obstacles, various synthetic strategies and structural modifications
have been explored to reinforce the photoactivities of TiO 2 nanomaterials, such as
metal and nonmetal doping, noble metal deposition, and hetero-coupling/sensitizing
with many narrow band gap semiconductors, which will be discussed in the following
sections [192–195].

11.3.2 Synthesis of pristine TiO 2 -based active photocatalysts

Because pure nanostructured TiO 2 materials exhibit relatively low hydrogen pro-
duction rates, ways to modify the intrinsic structure of TiO 2 are drawing attention,
including enlarging the photocatalytically active area, optimizing the crystallinity,
and exposing high-energy facets. Improvements in these areas have led to greater
solar-to-hydrogen efficiencies [196].

11.3.2.1 Enlargement of the photocatalytically active area
Semiconductor photocatalysis for the degradation of organic molecules or WS can
be carried out only on surfaces that contain enough photocatalytically active sites.
Therefore it is important to create a number of active sites sufficient enough to enlarge
the surface areas and thus absorb the pollutants. A common and effective approach
to enlarging surface areas is to construct multilevel surfaces with different structures
(NPs, nanobelts, NRs, nanoporous, etc.) on the surface of the primary TiO 2 nano-
structure. Tang and coworkers synthesized a complex TiO 2 nanoflake-NP hierarchical
structure that exhibited a relatively high photocatalytic activity toward the degradation
of organic compounds (Fig. 11.10E and F) [198]. However, some problems still need
to be resolved, such as accurate control of particle size and distribution and connec-
tivity between the particles and the TiO 2 nanobelt substrate. Looking at hydrothermal
acid corrosion as an effective way to enlarge the active surface area of TiO 2 materials,
researchers in another work created a chummy contact between the NPs and TiO 2
nanostructures and promoted separation and transfer of the charge carrier. Combining
the electrospinning process and hydrothermal methods, Meng et al. successfully grew
second phase TiO 2 NRs on as-prepared TiO 2 NFs with high densities in order to ob-
tain a larger active surface area on the hierarchical TiO 2 nanostructures [197]. As a
result, the photocatalytic activity of the compound was superior to that of pristine TiO 2
NFs (Fig. 11.10A–D). Moreover, Chen and coworkers obtained TiO 2 nanobelts by
traditional electrochemical anodization, and a maximum photoconversion efficiency
of 4.51% was achieved at an applied voltage of 0.1 V, which performed much higher
than the reported TiO 2 NTAs achieved through the same synthetic technology (2.43%
at 0.39 V) (Fig. 11.10G and H) [199].

11.3.2.2 Optimizing the crystallinity and exposed facets

Additionally, the chemical activity of TiO 2 is closely related to the different crystal
facets with specific surface energy. As has been reported, the surface formation ener-
− 2
− 2
− 2
gies of TiO 2 facets are 0.90 J m (001), 0.53 J m (100), and 0.44 J m (101) [200].