Nanostructured Nitrogen Doping TiO Nanomaterials for Photoanodes of Dye-Sensitized Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 56

2 Synthesis and Characterizations of Nanocrystalline
N-Doped TiO 2
Various methods have been reported for the synthesis of N-doped TiO 2 since the
study by Asahi et al. in 2001 [ 22 ]. The methods are generally classified as:
(1) sintering TiO 2 at high temperatures under an N-containing atmosphere (NH 3
gas or mixeds), which we called dry methods [ 23 , 27 , 28 ]; (2) chemical wet
methods, which involve sol-gel and solvothermal methods [ 29 - 32 ], some chem-
ical nitrogen sources are added into water or alcohol during the hydrolysis of
titanium alkoxide; and (3) sputtering and implantation deposition techniques, [ 33 ,
34 ] that were mainly used to prepare single crystalline or polycrystalline N-doped
TiO 2 thin films. Herein, we emphasize the development of the former two methods
that were used to fabricate N-doped TiO 2 photoanodes of DSCs.
2.1 Dry Methods
Dry method involves a high temperature sintering and doping process. This
method can be easily controlled by adjusting the N-containing atmospheres and
starting materials.
In 2005, our group reported the synthesis of N-doped TiO 2 employing the dry
method. The starting pristine TiO 2 was commercial anatase powders (ST-01,
Ishihara Sangyo Kaisha, Ltd.), which were treated at 550 C for 3 h under a dry N 2
and NH 3 flow [ 23 ]. Interestingly, we obtained needle-like N-doped TiO 2 crystals
with excellent thermostability. Afterwards, we also used P25 (Degussa) as the
starting materials to obtain N-doped P25. These N-doped ST-01 and P25 materials
show good performance over pristine TiO 2 photoanodes. Additionally, we also
found that the starting materials apparently affect the N-doping effect from
N-doping amount and optical properties. As reported earlier, the phase transition
of anatase into rutile can occur at a high sintering temperature [ 35 ]. However, our
nitridation process did not affect the crystal structure of pristine TiO 2 , as shown in
Fig. 1 . Moreover, the obtained N-doped ST-01 showed excellent thermal stability.
We can see in Fig. 2 the UV-Vis absorption spectra of N-doped ST-01 powders,
treated under different conditions, which suggest that after being sintered sepa-
rately in air, N 2 , Ar, or at high temperature up to 700 C, the N-doped ST-01 still
shows visible light absorption, which is a signal for successful N-doping.
In 2009, Yang et al. developed a set of reaction devices for the process of
thermal doping treatment [ 36 ]. The TiO 2 samples were treated with NH 3 under
middle pressures and controlled conditions. This synthesis route is an effective
approach to adjust the nitrogen concentration and band gap of N-doped TiO 2 . They
obtained a series of N-doped TiO 2 materials with different nitrogen doping amount
by adjusting the temperature, pressure, and time. Moreover, the anatase type
N-doped TiO 2 can be obtained at the sintering temperature of 400-500 C. When
the temperature increased to 600 C, the rutile phase can be observed.