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The solvothermal method is also an effective wet method to synthesis N-doped
TiO 2 . Dai et al. used urea as nitrogen source in the hydrolysis of TTIP [ 44 ]. The
precipitation solution was treated in an autoclave at 200 C for 10 h. They
observed that the (101) peak positions of N-doped TiO 2 showed a shift compared
with the undoped ones. This is also reported in Jagadale's work on N-doped TiO 2
by the sol-gel method [ 45 ].
Wet methods are the first choice to be employed to determine the suitable
nitrogen dopants, and it is also a simple N-doping method. Therefore, it is nec-
essary to seek an appropriate wet method, nitrogen sources, and N dopant amount
for the large-scale production of N-doped TiO 2 nanomaterials.
2.3 Other Techniques
There are some other approaches for preparation of N-doping TiO 2 materials such
as combustion, ion-implantation, and sputtering techniques.
Recently, Ogale and Gopinath et al. reported a disordered mesoporous frame-
work of N-doped TiO 2 consisting of nanoparticles. They used a simple combustion
synthesis method to prepare N-doped TiO 2 using Ti(NO 3 ) 4 as Ti precursor and
urea as fuel. They found that urea/Ti(NO 3 ) 4 molar ratio of27 leads to a biphasic
(anatase and rutile) titania. A high ratio of urea/Ti(NO 3 ) 4 (39) leads to exclusive
anatase phase TiO 2 . The pseudo-3D nature of mesoporous N-doped TiO 2 con-
sisting of mesoporosity and electrically interconnected nanosized crystalline par-
ticles lead to a higher efficiency in DSCs [ 46 ].
Kang et al. reported an ion-implantation technique combination with electro-
static spray to prepare hierarchical nanostructured TiO 2 clumps doped by nitrogen-
ion [ 47 ]. The ion-implantation could be a straightforward tool to implant foreign
atoms into the lattice. This ion doping intrinsically modifies the lattice structure
and consequently the properties of host counterparts [ 48 ].
Magnetron sputtering deposition method is also a widely used technique to
prepare N-doped TiO 2 thin films. We can obtain the films by depositing Ti in
plasma of argon, oxygen, and nitrogen. By varying the nitrogen contents in the
flow, we can get a different nitrogen concentration within TiO 2 lattice from 2.0 to
16.5 % [ 49 ]. Early in 2003, Lindquist et al. used DC magnetron sputtering to
prepare nanocrystalline porous N-doped TiO 2 thin films [ 50 ]. These films dis-
played a porous and rough surface. The crystal structure of N-doped TiO 2 thin
films varying from rutile to anatase varied with the nitrogen content. However, the
thickness of films only reached to several hundred nanometers.
Therefore, many methods can be used to synthesize nanocrystalline N-doped
TiO 2 . However, the crystal structure, surface property, and optical property of
N-doped TiO 2 are all related to the synthesis methods.
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