Nanostructured Nitrogen Doping TiO Nanomaterials for Photoanodes of Dye-Sensitized Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 60

2.4 Physical and Chemical Characterization
of N-doped TiO 2
To evaluate the optical properties of N-doped TiO 2 , UV-Vis spectrometry is the
most commonly used technique to examine the doping effects on the host metal
oxide matrix [ 51 ]. Generally, after N-doping treatment, the N-doped TiO 2
nanomaterials show a good visible light response between 400 and 500 nm. This
trend is observed by many works. In our previous work, compared with pure TiO 2
and P25 electrodes, the N-doped TiO 2 samples (N-doped ST-01, N-doped P25)
exhibited new absorption peaks in the visible light region between 400 and 550 nm
(Fig. 3 ). However, intensity of the absorption response peaks show much depen-
dence on the preparing conditions, such as N-doping amount as well as other
related factors.
The reasons for the visible light response origin of N-doped TiO 2 are still open
questions. Some work reported that the enhanced visible light absorption derived
from band gap narrowing [ 52 ] (Fig. 4 ): (1) the localized dopant levels near the VB
and the CB; (2) broadening of the VB; (3) localized dopant levels and electronic
transitions to the CB. Then it was found that the Ti 3+ defect or oxygen vacancies
can also induce the redshift absorption. Giamello et al. [ 30 ] reported that N-doped
TiO 2 electrodes contained N b centers that were responsible for visible light
absorption. Nevertheless, Serpone et al. [ 53 ] analyzed the DRS spectra (diffuse
reflectance spectra) of anion- and cation-doped TiO 2 electrodes. They concluded
that the absorption features in the visible light region originated from color centers
developed during the doping process or post-treatments rather than by narrowing
the intrinsic band gap for the TiO 2 electrode as originally proposed by Asahi and
co-workers [ 22 ]. Burda et al. recently studied the electronic origins of the visible
light absorption properties of C-, N-, and S-doped TiO 2 nanomaterials. They
revealed that additional electronic states above the valence band edge existed,
which could explain the redshift absorption of these materials [ 54 ]. On the basis of
the above discussion, a conclusion doping mechanism is still needed to further
understand the origin of visible light response.
X-ray photoelectron spectroscopy (XPS) is a powerful tool to get information
about the electronic structure and chemical environment of the elements on the
surface. So far, the XPS analysis is a surface characterization technique that could
be affected by testing environment. The XPS result can be considered as a ref-
erence. Especially for N-doped TiO 2 , XPS is the most reported technique to
analyze the nitrogen concentration and chemical environment. What we concern
most is three areas: the N 1s region, the Ti 2p region, and the O 1s region (Fig. 5 ).
For the N 1s region, the binding energy peaks ranged from 396 to 408 eV.
However, the N 1s binding energy is highly dependent on the method of prepa-
ration. The peaks at 396 eV were not always observed. According to an earlier
XPS study on the oxidation of pure TiN, the N 1s peak at 396 eV was assigned as
the b-N in TiN; the 397.5 eV peak was due to the a-N 2 , and the 400 eV and
405 eV peaks were assigned to the c-N 2 . In our previous work, we suggested to