Environmental Engineering Reference
In-Depth Information
cells, because heterogeneous semiconductor electrodes are able to absorb a larger
fraction of the solar spectrum and thus generate more photoinduced electron-hole
pairs [ 344 , 345 ]. Moreover, the coupling of two different semiconductors with
proper conduction band potentials facilitates the transfer of electrons from an
excited small band gap semiconductor into neighboring semiconductors. This
facilitates charge separation and improves device performance.
Many metal oxides (such as Bi 2 O 3 [ 346 ], Al 2 O 3 [ 347 ], Cu 2 O[ 348 ], Fe 2 O 3
[ 349 ], MoO 3 [ 312 ], SnO 2 [ 350 ], SiO 2 [ 351 ], WO 3 [ 245 ], ZnO [ 352 ], and ZrO 2
[ 353 ]) and metal sulfides (such as Bi 2 S 3 [ 346 ], Cu 2 S[ 354 ], CdS [ 355 ], PbS [ 356 ],
and Ag 2 S[ 357 ]) have been reported to couple with TiO 2 to form heterostructured
photocatalysts with enhanced photocatalytic performance. TiO 2 surface modifi-
cation with an insulating layer, such as SrCO 3 [ 358 ], Al 2 O 3 [ 359 ], La 2 O 3 [ 302 ] and
MgO [ 361 ], or a higher conduction band edge semiconductor layer, such as SnO 2
[ 362 ], In 2 O 3 [ 314 ], Nb 2 O 5 [ 226 ], and ZnO [ 363 ], was proven effective in reducing
the recombination and increasing the DSSCs conversion efficiency. Bi 4 Ti 3 O 12 /
TiO 2 heterostructures composed of Bi 4 Ti 3 O 12 nanosheets on the surface of TiO 2
submicron fibers were prepared via a facile two-step synthesis route combining an
electrospinning method and hydrothermal process. These heterostructures showed
a higher degradation rate of rhodamine B (RhB) than the pure TiO 2 submicron
fibers under visible light. This is largely due to the extended absorption in the
visible light spectrum resulting from the Bi 4 Ti 3 O 12 nanosheets, and the effective
separation of photoexcited charges driven by the photoinduced potential difference
generated at the Bi 4 Ti 3 O 12 /TiO 2 interface [ 364 ]. TiO 2 nanotube arrays sensitized
with ZnFe 2 O 4 nanocrystals (Fig. 16 a) were successfully fabricated by a two-step
process of anodization and vacuum-assisted impregnation followed by annealing.
It has been shown that the ZnFe 2 O 4 sensitization enhanced the photoinduced
charge separation (Fig. 16 b, c) and extended the range of the photoresponse of
TiO 2 nanotube arrays from the UV to the visible region [ 365 ]. TiO 2 -multiwalled
carbon nanotube (MWCNT) nanocomposites (Fig. 16 d) synthesized by hydro-
thermal processes possess a 50 % enhancement in the conversion efficiency
(4.9-7.37 %) of DSSCs compared to hydrothermally synthesized TiO 2 without
MWCNTs and Degussa P25. Efficient charge transfer in the nanocomposites is a
possible reason for the enhancement (Fig. 16 e, f) [ 366 ].
As the most recently discovered carbonaceous material, graphene has attracted
extensive attention as a useful material for solar energy applications. With a
unique sp 2 hybrid carbon network, a large theoretical specific surface area
(2,630 m 2 g -1 ), a high thermal conductivity (5,000 Wm -1 K -1 ), a large intrinsic
electron mobility (200, 000 cm 2 V -1 s -1 ), and good mechanical stability, it has
applications in sensors, catalysts, and energy conversion [ 223 ]. Graphene can
serve as a strong electron collector and carrier in a TiO 2 /graphene composite
system because their energy levels and physical properties are compatible [ 367 ].
Graphene-TiO 2 composites are also highly desirable for their promising energy
and environmental remediation applications.
Search WWH ::




Custom Search