Nanostructured Nitrogen Doping TiO Nanomaterials for Photoanodes of Dye-Sensitized Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 58

nitrogen sources can be also chosen to adjust the nitrogen doping process. Inor-
ganic and organic nitrogen dopants (such as aqueous ammonia, urea, ammonium
chloride, triethylamine, and diethylamine) are widely used in the synthesis of N-
doped TiO 2 .
Our group focused on the sol-gel wet method to investigate the types of nitrogen
dopants and amount on the performance of the N-doped TiO 2 , and thereby their
photovoltaic performance of the DSCs. The sol-gel method usually involves two
steps: (1) hydrolysis of titanium alkoxide in solvent (water or ethanol) containing
nitrogen sources; and (2) sintering the obtained precipitate under 400-500 C for a
certain amount of time. However, we found that the nitridation process and doping
amount varies with each N-doping method and type of nitrogen sources. We used
ammonia, triethylamine, and urea nitrogen dopants to synthesize N-doped TiO 2
nanocrystals, which were denoted as N-A, N-U, and N-T, respectively [ 38 - 41 ].
By varying initial molar ratios of N/Ti, a series of N-doped TiO 2 with different N
dopant amounts can be also synthesized according to the N-A method [ 42 ]. As the
previous literature reported, we also found that the nitrogen doping process differs
for the N-A, N-U, and N-T powders. During the preparation of N-A, nitrogen
doping proceeded simultaneously with the hydrolysis of the titanium alkoxide. The
hydrolysis of the titanium isopropoxide (TTIP) consisted of two steps [ 43 ]:
hydrolysis and concentration. Titanium hydroxide was formed in the hydrolysis and
was called titanic acid, which exhibits acidity. The titanic acid then reacted with
NH 4 OH to form ammonium titanate, which when heated, dehydrated and desorbed
to NH 3 and allowed N-doping to occur. Ammonia in the doped samples becomes
oxidized by the lattice oxygen, and this oxidation allows for the uptake of nitrogen.
During the preparation of N-U, simultaneous N-doping with phase formation
occurred by heating a mixture of titanium hydroxide and urea. When the mixture
was heated, the urea was decomposed into NH 3 and CO 2 , and the generated NH 3
reacted with the oxygen of the TiO 2 to form the N-doped TiO 2 . The N-T sample
was formed by direct nitridation of the anatase TiO 2 nanostructures with alky-
lammonium salt. In this case, triethylamine was used as the alkylammonium salt,
and the N-T nanocrystals were obtained by controlling hydrolysis rate of the TTIP
and the pH value of the solution. As a side note, some amine groups can coordinate
to the central Ti ion early during the N-doping process, and these amine linkages
can be hydrolyzed by the addition of a dilute solution of acid or base, but this
addition in turn adjusts the pH of the reaction mixture. Therefore, high pH values
are required. By using high pH values, the Ti-bound amine groups can be easily
substituted by OH - during the hydrolysis process, which results in the formation of
N-doped TiO 2 nanoparticles. [ 40 ] Furthermore, the obtained different phases and
crystallite sizes of N-doped TiO 2 can be ascribed to the different types of nitrogen
dopants and to the hydrolysis of titanium alkoxide under controlled conditions.
In the sol-gel wet method, the type of nitrogen sources not only influences the
nitridation process but also the particle size and nitrogen concentration. The N
dopant amounts were calculated using the XPS results and were found to be 2.77,
0.29, and 0.47 % for N-A, N-U, and N-T, respectively.