Page 277 - Multifunctional Photocatalytic Materials for Energy

Multidimensional TiO 2 nanostructured catalysts for sustainable H 2 generation 259

kinds of organic compounds, such as triethylamine, urea, thiourea, and hydrazine hy-
drate, were used as nitrogen sources to prepare the N-doped TiO 2 . Absorption studies
revealed a shift in the absorption edge to a lower energy and to a stronger absorption in
the visible light region [206]. Recently, Wang et al. utilized hydrothermal processing
and subsequent heating in a NH 3 environment for the facile synthesis of N-doped TiO 2
photocatalysts with different doping levels. They synthesized N-doped TiO 2 photocat-
alysts according to the density functional theory, and the process was easy because the
level of N-doping in a TiO 2 crystal can be highly controlled by adjusting the reaction
system [259]. Since then other nonmetals, such as C, B, S, F, and P, have also been
incorporated into the TiO 2 lattice by different approaches, such as hydrothermal meth-
ods, thermal treatment in a gas atmosphere (N 2 , Ar, etc.), plasma ion implantation or
sputtering in a special atmosphere, and Ti alloy anodization. Similar to nitrogen dop-
ing, these dopants can also narrow the band gap of TiO 2 as well as enhance the visible
light absorption [210–213]. However, they suffer from the energy accelerators, rough
reaction conditions, and limited doping depth [214,215]. Ti alloy anodization is used
only for TiO 2 NTAs, which limits applications in other higher dimensional structures
[216,217]. Heat treatments in N 2 , H 2 , or Ar atmospheres are recognized as facile and
widely used doping techniques. Apart from the N-doping technology, C and P doping
introduce deep states in the gap [218]. Because of the large ionic radius, it is diffi-
cult to incorporate S into the TiO 2 crystal because significantly more formation en-
ergy is required for the substitution of S than for the substitution of N [209,219,220].
Additionally, the intrinsic shortcomings of TiO 2 , including reduced Ti species and
oxygen vacancies after calcination, also lower the energy excitation pathway. Zhang's
group obtained F-doped TiO 2 NSs with widths ranging from 3 to 6 μm and with a
thicknesses of 200 nm (Fig. 11.11A–D) [221]. Compared with the pristine TiO 2 NSs,
the as-prepared F-doped TiO 2 NSs exhibited markedly more enhanced hydrogen pro-
duction efficiency because of the absorption in the visible light region and fast transfer
of charge carriers. Behara et al. co-doped hydrogenated TiO 2 NPs with N and S under
annealing in a hydrogen atmosphere [226]. The introduction of N and S shifted the
3 +
valance band upward. The formed Ti states and oxygen vacancies lowered the CB,
resulting in a narrow band gap and faster transfer of charge carriers. Additionally, the
dopants and vacancies extended the visible light absorption. Therefore N and S co-
doped hydrogenated TiO 2 exhibited a high solar-to-hydrogen efficiency (6.6%) and
enhanced photoelectrocatalytic hydrogen production. Binary or ternary doping of hy-
drogenated TiO 2 with noble metals and other semiconductors is also being widely in-
vestigated for improving photocatalytic and photoelectrocatalytic activity [227–230].
TiO 2 doped with transition metal cations such as Fe, Cu, V, Co, and Mn has also
demonstrated the ability to widen the visible light absorption range, suppress the re-
combination of photogenerated electron-hole pairs, and improve photoelectric per-
formance [231–238]. Zhao's group synthesized Fe-doped TiO 2 photocatalysts by a
modified hydrothermal method [239]. These as-prepared Fe-doped TiO 2 photocatalysts
exhibited a smaller crystallite size and higher specific surface area than pristine TiO 2
photocatalysts. Additionally, first-principle density theory revealed that Fe can induce
the formation of impurity levels near the VB, which causes a reduction of the band
gap, electron-hole separation, a high electron transfer efficiency, and improvement of