Nanocrystalline TiO 2 thin films on conductive glass electrodes have been modified with monolayers of different electrochromic moieties (mono-, di- and trimeric
N,N
0 -dialkyl- or diphenyl-4,4
0 -bipyridinium salts) functionalized with TiO 2 anchoring groups such as benzoate, salicylate, phosphonates [159, 160]. The type of anchoring group and the viologen moiety influenced the electrochemically active coloring centers on TiO 2 . The transmittance changes from 79 to 11% in less than 1 s
indicating the fast response time of these electrodes [160]. An attempt has also
been made to prepare electrochromic material by electropolymerizing N,N
0 -bis[p(trimethoxysilyl)benzyl]-4,4
0 -bipyridinium dichloride (BPQ
2þ Á2Cl
À ) on top of a reductively conducting film of tungsten trioxide [161]. The resulting composite film
responds to a potential step from 0.9 to À0.65 V by turning from colorless to blue
in two steps: the absorbance due to one-electron reduction of each monomer
unit in poly(BPQ
2þ ) increases quickly (in <4 s), and is followed by a slower development of the absorbance due to the WO 3 reduction (> 1 min). Metal hexacyanoferrates constitute another class of redox species that can be coupled with
semiconductor nanostructures for designing electrochromic hybrid assemblies. For
example TiO 2 –metal hexacyanoferrate has recently been shown to possess excellent electrochromic properties [162].
19.4.3
Photocurrent Generation
The nanostructured films of semiconductor oxides (SnO 2 , TiO 2 , ZnO) when used
as photoanodes in a photoelectrochemical cell, respond to the UV light by generating photocurrent. One way to extend their photoresponse is to modify the
semiconductor surface with sensitizing dyes that absorb strongly in the visible.
This approach of light energy conversion is similar to plant photosynthesis, in
which chlorophyll act as light absorbing antenna molecules. In other words, the
dye modified semiconductor films (see for example, Figure 19.9) provide an efficient method to mimic the photosynthetic process. The high porosity and strong
surface bonding property of the nanostructured semiconductor films facilitate surface modification with organic dyes and organometallic complexes such as bis(2,2
0 -
bipyridine)(2,2
0 -bipyridine-4,4
0 dicarboxylic acid)ruthenium(II). The nanostructured
TiO 2 films modified with a ruthenium complex exhibit photoconversion efficiencies of about 10% [163], which is comparable to that of amorphous silicon-based
photovoltaic cells.
When the dye modified semiconductor electrode is illuminated with visible light
the sensitizer molecules absorb light and inject electrons into the semiconductor
(e.g., TiO 2 ) particles. These electrons are then collected at the conducting glass
surface to generate anodic photocurrent. The redox couple (e.g., I 3
À
/I
À ) present in
the electrolyte quickly regenerates the sensitizer. By choosing an appropriate sensitizer it is possible to tune the photoresponse of these nanostructured semiconductor films. For example, sensitizing dyes such as chlorophyll a and chlorophyll
b, [164, 165] squaraines [166] and oxazines [156] can extend the photoresponse of
SnO 2 films to the red–infrared region. Ru(II) polypyridyl complex modified TiO 2
19 Photochemistry and Electrochemistry of Nanoassemblies
630
N,N
0 -dialkyl- or diphenyl-4,4
0 -bipyridinium salts) functionalized with TiO 2 anchoring groups such as benzoate, salicylate, phosphonates [159, 160]. The type of anchoring group and the viologen moiety influenced the electrochemically active coloring centers on TiO 2 . The transmittance changes from 79 to 11% in less than 1 s
indicating the fast response time of these electrodes [160]. An attempt has also
been made to prepare electrochromic material by electropolymerizing N,N
0 -bis[p(trimethoxysilyl)benzyl]-4,4
0 -bipyridinium dichloride (BPQ
2þ Á2Cl
À ) on top of a reductively conducting film of tungsten trioxide [161]. The resulting composite film
responds to a potential step from 0.9 to À0.65 V by turning from colorless to blue
in two steps: the absorbance due to one-electron reduction of each monomer
unit in poly(BPQ
2þ ) increases quickly (in <4 s), and is followed by a slower development of the absorbance due to the WO 3 reduction (> 1 min). Metal hexacyanoferrates constitute another class of redox species that can be coupled with
semiconductor nanostructures for designing electrochromic hybrid assemblies. For
example TiO 2 –metal hexacyanoferrate has recently been shown to possess excellent electrochromic properties [162].
19.4.3
Photocurrent Generation
The nanostructured films of semiconductor oxides (SnO 2 , TiO 2 , ZnO) when used
as photoanodes in a photoelectrochemical cell, respond to the UV light by generating photocurrent. One way to extend their photoresponse is to modify the
semiconductor surface with sensitizing dyes that absorb strongly in the visible.
This approach of light energy conversion is similar to plant photosynthesis, in
which chlorophyll act as light absorbing antenna molecules. In other words, the
dye modified semiconductor films (see for example, Figure 19.9) provide an efficient method to mimic the photosynthetic process. The high porosity and strong
surface bonding property of the nanostructured semiconductor films facilitate surface modification with organic dyes and organometallic complexes such as bis(2,2
0 -
bipyridine)(2,2
0 -bipyridine-4,4
0 dicarboxylic acid)ruthenium(II). The nanostructured
TiO 2 films modified with a ruthenium complex exhibit photoconversion efficiencies of about 10% [163], which is comparable to that of amorphous silicon-based
photovoltaic cells.
When the dye modified semiconductor electrode is illuminated with visible light
the sensitizer molecules absorb light and inject electrons into the semiconductor
(e.g., TiO 2 ) particles. These electrons are then collected at the conducting glass
surface to generate anodic photocurrent. The redox couple (e.g., I 3
À
/I
À ) present in
the electrolyte quickly regenerates the sensitizer. By choosing an appropriate sensitizer it is possible to tune the photoresponse of these nanostructured semiconductor films. For example, sensitizing dyes such as chlorophyll a and chlorophyll
b, [164, 165] squaraines [166] and oxazines [156] can extend the photoresponse of
SnO 2 films to the red–infrared region. Ru(II) polypyridyl complex modified TiO 2
19 Photochemistry and Electrochemistry of Nanoassemblies
630