15 Yet to Be Challenged: TiO 2 as the Photo-Anode Material …
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or electrospinning method. However, there are other methods of deposition possible,
namely; pulse laser deposition, immersing the final TiO 2 sample in a solution of the
dopant and applying a voltage, anodization and thermal oxidation.
Dopants of TiO 2 can be further categorized into four types: nonmetals, transition
metals, post-transition metals, lanthanides.
15.4.1 Nonmetals
Even though they are not studied much, nonmetals have been used as a dopant for
the improvement of TiO 2 . Studies have been conducted by doping nitrogen, sulfur
and fluorine, either by itself or as a combination of dopants.
TiO 2 is oxygen deficient, and produces species which can destroy both the dye
and redox electrolyte. Ma et al. doped TiO 2 with nitrogen to overcome this problem.
Nitrogen doped TiO 2 was synthesized by heating commercial anatase TiO 2 in the
presence of N 2 . The doped TiO 2 system showed an outstanding efficiency of 8%
compared to those of the other two types of commercially available TiO 2 nanoparticles [P25 and Solaronix, Ti-Nanoxide D (SL-D)] included in the study. As the V OC
among the three systems had no drastic change, it can be assumed the N doping had
not significantly affected the CB edge. However, N doping not only enhanced the
effective surface area for dye adsorption, leading to an efficient photoresponse in the
visible region, N doped TiO 2 was stable under extended thermal stress (Ma et al.
2005). It should be noted however that in this study the N doped TiO 2 was in the
shape of needles, while the structure of P25 and SL-D were different.
Further, Simya et al. have shown that sulfur can be used as a co-dopant with
N doped TiO 2 to decrease the band gap energy and improve light harvesting. DSCs
fabricated with S and N-doped TiO 2 gave an efficiency of 1.80% in the visible region,
which is 50% more when compared to pure TiO 2 (Simya et al. 2014). Fluorine has also
been used as a co-dopant along with Ho
3+ and Yb
3+ , to synthesize a doped TiO 2 ,
which exhibited upconversion properties (Yu et al. 2014). This would be further
discussed under lanthanide dopants.
15.4.2 Transition Metals
Transition metal dopants can be considered as the most commonly used type of
dopant with TiO 2 , as studies are still being conducted using these metals. The main
reason behind this could be the ability of most transition metals to shift the conduction
band edge to a more negative potential, for efficient electron transport.
Among all the considered transition metals, niobium has been studied the most.
Lee et al. coated Nb doped TiO 2 on the transparent conducting oxide and found that,
under optimal amounts of the dopant, the Nb doped TiO 2 layer acted as a blocking
layer, reducing the interfacial resistance compared to the undoped TiO 2 compact
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or electrospinning method. However, there are other methods of deposition possible,
namely; pulse laser deposition, immersing the final TiO 2 sample in a solution of the
dopant and applying a voltage, anodization and thermal oxidation.
Dopants of TiO 2 can be further categorized into four types: nonmetals, transition
metals, post-transition metals, lanthanides.
15.4.1 Nonmetals
Even though they are not studied much, nonmetals have been used as a dopant for
the improvement of TiO 2 . Studies have been conducted by doping nitrogen, sulfur
and fluorine, either by itself or as a combination of dopants.
TiO 2 is oxygen deficient, and produces species which can destroy both the dye
and redox electrolyte. Ma et al. doped TiO 2 with nitrogen to overcome this problem.
Nitrogen doped TiO 2 was synthesized by heating commercial anatase TiO 2 in the
presence of N 2 . The doped TiO 2 system showed an outstanding efficiency of 8%
compared to those of the other two types of commercially available TiO 2 nanoparticles [P25 and Solaronix, Ti-Nanoxide D (SL-D)] included in the study. As the V OC
among the three systems had no drastic change, it can be assumed the N doping had
not significantly affected the CB edge. However, N doping not only enhanced the
effective surface area for dye adsorption, leading to an efficient photoresponse in the
visible region, N doped TiO 2 was stable under extended thermal stress (Ma et al.
2005). It should be noted however that in this study the N doped TiO 2 was in the
shape of needles, while the structure of P25 and SL-D were different.
Further, Simya et al. have shown that sulfur can be used as a co-dopant with
N doped TiO 2 to decrease the band gap energy and improve light harvesting. DSCs
fabricated with S and N-doped TiO 2 gave an efficiency of 1.80% in the visible region,
which is 50% more when compared to pure TiO 2 (Simya et al. 2014). Fluorine has also
been used as a co-dopant along with Ho
3+ and Yb
3+ , to synthesize a doped TiO 2 ,
which exhibited upconversion properties (Yu et al. 2014). This would be further
discussed under lanthanide dopants.
15.4.2 Transition Metals
Transition metal dopants can be considered as the most commonly used type of
dopant with TiO 2 , as studies are still being conducted using these metals. The main
reason behind this could be the ability of most transition metals to shift the conduction
band edge to a more negative potential, for efficient electron transport.
Among all the considered transition metals, niobium has been studied the most.
Lee et al. coated Nb doped TiO 2 on the transparent conducting oxide and found that,
under optimal amounts of the dopant, the Nb doped TiO 2 layer acted as a blocking
layer, reducing the interfacial resistance compared to the undoped TiO 2 compact