Environmental Engineering Reference
In-Depth Information
2.2 Modifications of TiO 2
As a promising semiconductor material, TiO 2 has attracted significant attention
over the past several decades. However, the practical photovoltaic and photoca-
talysis applications of TiO 2 are limited by its wide band gap and the serious
recombination of photogenerated electrons and holes [ 33 , 34 ]. Because of its large
band gap (3.2 eV for anatase, 3.0 eV for rutile), pure TiO 2 only absorbs ultraviolet
light shorter than 387.5 nm for the anatase and 413.3 nm for the rutile [ 33 ].
Unfortunately, UV light constitutes less than 5 % of the solar energy that reaches
the surface of the earth. This reduces the effective use of sunlight since visible light
(k = 400-700 nm) accounts for about 50 % of solar energy. Thus, it is necessary
to develop titania-based photocatalysts which are active under visible light (i.e.,
broad spectrum). Furthermore, recombination of photogenerated charge carriers is
another major limitation in TiO 2 semiconductor materials since it reduces the
overall quantum efficiency of devices. In photocatalysis applications, recombina-
tion occurs when the excited electron reverts to the valence band without reacting
with adsorbed species and the energy, non-radioactively or radioactively, dissi-
pates as either light or heat [ 34 ]. However, in DSSC applications, photoexcited
electron recombination in the electron transport process, including electron
injection from the excited dye to the TiO 2 conduction band and electron transport
from the conduction band to the conductive substrate, is regarded as one of the
major obstacles to achieving high solar-to-electricity conversion efficiencies [ 255 ].
Recombination may occur either on the surface or in the bulk and is generally
made worse by the presence of impurities, defects, and all other factors which
introduce bulk or surface imperfections into the crystal.
To solve these problems, extensive efforts have been devoted to creating TiO 2 -
based visible-light-active photocatalytic materials and modifying nanostructured
TiO 2 photoanodes to alleviate electron recombination [ 33 , 256 ]. Currently,
research interests focus mainly on modifying TiO 2 materials via (1) doping with
cations (e.g., Fe [ 257 - 259 ], Cr [ 260 ], Eu [ 261 ], La [ 262 ], V [ 263 ], Mg [ 255 ], In
[ 264 ]) or anions (e.g., S [ 265 ], C [ 266 ], F [ 267 ], B [ 268 ] and N [ 269 ]); (2)
sensitization with organic dyes (e.g. N3, N719) [ 270 ], conducting polymers (e.g.,
poly(3-hexylthiophene) (P3HT) [ 271 ], nafion (perfluorinated polymer with sulfo-
nate groups) [ 272 , 273 ], polyaniline (PANI) [ 274 ], and carbon nitride polymer
[ 275 ]), organic-inorganic hybrid dyes (e.g. copper(II) phthalocyanine) [ 276 ], or
other semiconductors that absorb visible light (e.g. CdS [ 277 - 279 ], Cu 2 O[ 280 ],
Ag 2 O[ 281 ], CdSe [ 282 ], PbZr 0.52 Ti 0.48 O 3 (PZT) [ 283 ], Bi 2 O 3 [ 284 ], BiOI [ 285 ],
Bi 2 WO 6 [ 286 ], CdTe [ 287 ], PbS [ 288 ], CuInS 2 [ 289 ], SnS [ 290 ], SnS 2 [ 291 ]); (3)
decoration with noble metals (e.g. Au [ 292 - 294 ], Ag [ 65 , 295 - 297 ], Pd [ 298 - 300 ],
Pt [ 301 - 303 ]); (4) combination with other semiconductors (e.g., SiO 2 [ 304 , 305 ],
Al 2 O 3 [ 100 , 306 ], MgO [ 307 ], Fe 2 O 3 [ 308 ], SrTiO 3 [ 309 ], Nb 2 O 5 [ 310 ], SnO 2
[ 311 ], WO 3 [ 312 ], ZnO [ 313 ], and ZrO 2 [ 314 ]); and (5) synthesis of reduced TiO 2
(TiO 2-x , containing Ti 3+
or O vacancies) [ 235 , 315 - 318 ].
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