15 Yet to Be Challenged: TiO 2 as the Photo-Anode Material …
305
focused on the effect of morphologies such as nanofibre network mats (Kim et al.
2007), nano-sheets (Li et al. 2012) and among others (Giannouli and Spiliopoulou
2012; Zhang et al. 2009). While others have investigated synthesis procedures (Zhao
et al. 2008).
After much study, the maximum reported efficiency for pure ZnO based DSC,
using a liquid electrolyte is 7.5% (Memarian et al. 2011), which is much lower than
that of TiO 2 based DSC. The low conversion efficiencies of ZnO-based systems
are most likely due to the dissolution of ZnO to Zn
2+ by the adsorbed acidic dye,
followed by the formation of an insulating layer of Zn
2+ and dye molecules, blocking
the injected electrons from the dye molecules to the semiconductor by the insulating
layer (Hiroaki Horiuchi et al. 2003). As ZnO is more basic than TiO 2 , it is more
prone to be attacked by acidic dyes. In order to avoid the formation of the insulating
layer, many core-shell structures have been developed by coating a buffer layer on
the ZnO surface. SiO 2 has been demonstrated to be a very effective shell material
on ZnO, which prevents the formation of aggregates through a strong interaction
between Si
4+ and O
2− ions (Shin et al. 2007). Many core–shell structures have been
developed using ZnO nanocrystals and ZnO nanowires, by coating a buffer layer of
Al 2 O 3 (Matt Law et al. 2006), TiO 2 (Matt Law et al. 2006), and even ZnO (Guillén
et al. 2013) on their surfaces, to prevent the formation of the insulating Zn
2+ /dye
complex. ZnO has also been studied with CuO, with and without TiO 2 blocking
layer, in the presence of Co
2+ /Co
3+ redox electrolyte (Habibi et al. 2014).
Most nanostructures of the semiconductor material, have been synthesized so
as the final product would give a large surface area for the dye adsorption. ZnO
and Fe 2 O 3 have been taken into consideration as they are both suitable to prepare
thin films, have good electrical conductivity, are inexpensive, and have a very good
chemical stability. These reasons have led to use these materials in energy storage
and photoelectrochemistry (Livage and Ganguli 2001). Reda et al. have used ZnO
and Fe 2 O 3 as photoanode materials, and studied the photovoltaic characteristics
by changing the annealing temperature, in order to obtain a higher incident light to
electrical energy conversion and has obtained a maximum efficiency of 2.2% for ZnO
and 1.2% for Fe 2 O 3 (Reda 2010). Moreover, due to the suitable band gap of about
2.2 eV Fe 2 O 3 , various nanostructures have been synthesized, including nanocubes
(Ozaki et al. 1984), nano-rings (Jia et al. 2008), dendrites (Liang et al. 2010) and
polyhedron (Lv et al. 2010). Fe 2 O 3 has also been studied as a photoanode material in
DSCs, using a one-pot, low temperature synthesis method (Manikandan et al. 2014).
Another possible alternative to TiO 2 is Nb 2 O 5 , due to the larger band gap of
3.49 eV, leading to a higher conduction band edge of −1.32 eV compared to that of
TiO 2 (Ghosh et al. 2011), which could lead to higher V OC . Moreover, in terms of
IPCE Nb 2 O 5 has the second value of 18%, after that of TiO 2 , which has 45% (Jose
et al. 2009). Ghosh et al. synthesized a nanoforest of Nb 2 O 5 using laser ablation,
under several gas compositions and pressure conditions, which was quite similar
to vertically aligned nanocrystals to Gratzel’s TiO 2 nanoforest. This study has also
encouraged the growth of photoanode material, with high conduction band levels
such as Ta 2 O 5 and SrTiO 3 , in a similar manner, for future studies in DSCs (Ghosh
et al. 2011).
305
focused on the effect of morphologies such as nanofibre network mats (Kim et al.
2007), nano-sheets (Li et al. 2012) and among others (Giannouli and Spiliopoulou
2012; Zhang et al. 2009). While others have investigated synthesis procedures (Zhao
et al. 2008).
After much study, the maximum reported efficiency for pure ZnO based DSC,
using a liquid electrolyte is 7.5% (Memarian et al. 2011), which is much lower than
that of TiO 2 based DSC. The low conversion efficiencies of ZnO-based systems
are most likely due to the dissolution of ZnO to Zn
2+ by the adsorbed acidic dye,
followed by the formation of an insulating layer of Zn
2+ and dye molecules, blocking
the injected electrons from the dye molecules to the semiconductor by the insulating
layer (Hiroaki Horiuchi et al. 2003). As ZnO is more basic than TiO 2 , it is more
prone to be attacked by acidic dyes. In order to avoid the formation of the insulating
layer, many core-shell structures have been developed by coating a buffer layer on
the ZnO surface. SiO 2 has been demonstrated to be a very effective shell material
on ZnO, which prevents the formation of aggregates through a strong interaction
between Si
4+ and O
2− ions (Shin et al. 2007). Many core–shell structures have been
developed using ZnO nanocrystals and ZnO nanowires, by coating a buffer layer of
Al 2 O 3 (Matt Law et al. 2006), TiO 2 (Matt Law et al. 2006), and even ZnO (Guillén
et al. 2013) on their surfaces, to prevent the formation of the insulating Zn
2+ /dye
complex. ZnO has also been studied with CuO, with and without TiO 2 blocking
layer, in the presence of Co
2+ /Co
3+ redox electrolyte (Habibi et al. 2014).
Most nanostructures of the semiconductor material, have been synthesized so
as the final product would give a large surface area for the dye adsorption. ZnO
and Fe 2 O 3 have been taken into consideration as they are both suitable to prepare
thin films, have good electrical conductivity, are inexpensive, and have a very good
chemical stability. These reasons have led to use these materials in energy storage
and photoelectrochemistry (Livage and Ganguli 2001). Reda et al. have used ZnO
and Fe 2 O 3 as photoanode materials, and studied the photovoltaic characteristics
by changing the annealing temperature, in order to obtain a higher incident light to
electrical energy conversion and has obtained a maximum efficiency of 2.2% for ZnO
and 1.2% for Fe 2 O 3 (Reda 2010). Moreover, due to the suitable band gap of about
2.2 eV Fe 2 O 3 , various nanostructures have been synthesized, including nanocubes
(Ozaki et al. 1984), nano-rings (Jia et al. 2008), dendrites (Liang et al. 2010) and
polyhedron (Lv et al. 2010). Fe 2 O 3 has also been studied as a photoanode material in
DSCs, using a one-pot, low temperature synthesis method (Manikandan et al. 2014).
Another possible alternative to TiO 2 is Nb 2 O 5 , due to the larger band gap of
3.49 eV, leading to a higher conduction band edge of −1.32 eV compared to that of
TiO 2 (Ghosh et al. 2011), which could lead to higher V OC . Moreover, in terms of
IPCE Nb 2 O 5 has the second value of 18%, after that of TiO 2 , which has 45% (Jose
et al. 2009). Ghosh et al. synthesized a nanoforest of Nb 2 O 5 using laser ablation,
under several gas compositions and pressure conditions, which was quite similar
to vertically aligned nanocrystals to Gratzel’s TiO 2 nanoforest. This study has also
encouraged the growth of photoanode material, with high conduction band levels
such as Ta 2 O 5 and SrTiO 3 , in a similar manner, for future studies in DSCs (Ghosh
et al. 2011).