Environmental Engineering Reference
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yolk@shell hierarchical sphere photoanodes (6.01 %) showed higher overall light
conversion efficiency (g) for DSSC applications mainly due to their large surface
area (up to 245.1 m
2
g
-1
) and their promotion of light scattering in the visible
region using submicrometer scale features [
113
]. Han et al. presented the fabrica-
tion of truncated tetragonal bipyramidal TiO
2
nanoparticles with 60 % exposed
(001) facets via a fluorine-free hydrothermal route, using potassium titanate
nanowires as a precursor and urea as surface regulator [
141
]. Notably, Miao et al.
prepared anatase TiO
2
microspheres with[90 % exposed (001) facets using TiF
4
as
capping and stabilizing agent without introducing any additional hydrofluoric acid.
Due to the high exposure of reactive (001) facets, the anatase TiO
2
microspheres
gave an efficient photocatalyst in degrading methyl orange (MO) and producing
hydrogen from water under UV light irradiation [
142
]. Similarly, Yang et al. fab-
ricated hierarchical TiO
2
spheres made of thin nanosheets with over 90 % exposed
(001) facets via a diethylene glycol-solvothermal route, which were used as
photoanodes for DSSCs, and gave an energy conversion efficiency of 7.51 % [
112
].
Along with hydrothermal/solvothermal methods, other strategies have been
investigated for the green preparation of highly reactive TiO
2
nanomaterials with
exposed (001) facets [
143
]. Amona et al. found that faceted decahedral single-
crystalline anatase particles with reactive (001) facets could be fabricated by a gas-
phase process using TiCl
4
as a titanium source and that such a structure possessed
comparable photocatalytic activity to that of P25 for reactions performed under
various conditions [
144
]. Jung et al. presented the preparation of TiO
2
NTs with
exposed (001) facets by electrochemical anodization of Ti foil using surfactant-
assisted processes with poly(vinyl pyrrolidone) (PVP) and acetic acid. The DSSC
assembled using a TiO
2
nanotube photoanode with 77 % exposed (001) facets
demonstrated an overall conversion efficiency of 3.28 % [
145
]. Xie et al. devel-
oped a solid-state precursor strategy for preparing a hollow anatase TiO
2
box-like
structure enclosed by six single-crystalline TiO
2
plates with highly exposed
reactive (001) facets by sintering a cubic TiOF
2
solid precursor at 500-600 C.
The formation of these particular nanostructures is attributed to the hard self-
template restriction and the adsorption of F
-
ions from the TiOF
2
. Due to the high
percentage of reactive (001) facets, such novel TiO
2
boxes exhibited good per-
formance in photocatalytic H
2
evolution (7.55 mmol g
-1
h
-1
)[
146
].
Furthermore, in addition to exposed (001) facets, TiO
2
nanocrystals with other
important active facets have also been studied recently. For example, single crys-
talline anatase TiO
2
rods with dominant reactive (010) facets (also with 100 % five-
coordinate Ti (Ti5c) atoms) were directly synthesized by hydrothermally treating
Cs
0.68
Ti
1.83
O
4
/H
0.68
Ti
1.83
O
4
particles. The nano-sized rods showed a 7.73 % con-
version efficiency in DSSCs. This was comparable to the 7.67 % benchmark for
P25 TiO
2
nanocrystals [
147
]. The preparation of tetragonal faceted-nanorods of
single-crystalline anatase TiO
2
with predominately exposed higher energy (100)
facets was performed by hydrothermal transformation of sodium titanate in alkaline
solution [
138
]. Significantly, anatase TiO
2
nanocuboids wholly exposed with high-
energy (001) and (100) facets were successfully synthesized by an environmentally
benign synthetic strategy using low-cost acid-delaminated vermiculite (DVMT)
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