photochromic and electrochromic devices always use these oxides in the hypostoichiometric state; therefore, the exact formula is MeO 3Àx . As the deviation from ideal
stoichiometry is small, for reasons of brevity, in the following text, generally the
shorter version MeO 3 is used.) The photochromic or electrochromic behavior
performance of these oxides is related to electron/hole pairs intimately connected
to deviations from the perfect stoichiometry. When these oxides are excited,
positively charged holes h
þ and free electrons are formed. The holes react with
adsorbed water to produce protons:
H 2 O þ 2h
þ ) 2H
þ þ O
ð9:12aÞ
By reaction of these protons with the oxide:
MeO 3 þ xH
þ þ xe
À ) H x Me 1Àx
6þ Me x
5þ O 3
ð9:12bÞ
the material changes color to blue or green, while the oxygen radicals either occupy
vacant sites inside the sample or escape from the particle. An immediate
recombination of the charge carriers, h
þ þ e
À
) heat, reduces the performance
of the material. The reaction described above always require exchange with the
surrounding atmosphere and, therefore, the performance is difficult to control.
When considering the rate-controlling steps, it can be assumed that the reaction
according to Eq. (9.12a) occurs at the surface, whereas diffusion processes control
the step according to Eq. (9.12b). The effects observed with protons are also obtained
with alkaline metal doping. For alkaline metal-doped materials, Eq. (9.12b) becomes:
MeO 3 þ xA
þ þ xe
À ) A x Me 1Àx
6þ Me x
5þ O 3
ð9:13Þ
where A
þ is a monovalent ion (in most cases an alkaline metal). Again, the number
of free electrons that can react with the metal Me is most important for the efficiency
of the system.
A typical example of the absorption spectra of MoO 3 nanoparticles indicating
photochromic behavior is shown in Figure 9.59. This illustrates the absorption of
MoO 3 in the bleached state (0 s illumination time) and after two different times, 360
and 2160 s, of illumination with a pulsed laser emitting 72 mW per pulse at 308 nm.
Although it is clear that the intensity of coloration increases with time, the most
interesting point is the comparison of the time-dependent coloration of nanoparticulate and coarse-grained MoO 3 at a wavelength of 750 nm, as shown in
Figure 9.60. Here, the advantage of applying nanoparticles instead of coarse-grained
material is clearly visible. This is because the reaction mechanisms indicated in Eqs.
(9.12a) and (9.12b) require the transport of ions through the particle and, as the
diffusion time is indirectly proportional to the square of the particle size (see
Chapter 2), this acceleration is to be expected.
Although a decrease in particle size by a factor of only 10 should increase the rate
of coloration by a factor of 100, this is not the case as, experimentally; an increase in
the coloration rate only by a factor of 13 was verified. Clearly, diffusion is no longer
the rate-controlling step; rather, the rate-controlling process is more likely the
surface reaction according to Eq. (9.12a).
258j 9 Optical Properties of Nanoparticles
stoichiometry is small, for reasons of brevity, in the following text, generally the
shorter version MeO 3 is used.) The photochromic or electrochromic behavior
performance of these oxides is related to electron/hole pairs intimately connected
to deviations from the perfect stoichiometry. When these oxides are excited,
positively charged holes h
þ and free electrons are formed. The holes react with
adsorbed water to produce protons:
H 2 O þ 2h
þ ) 2H
þ þ O
ð9:12aÞ
By reaction of these protons with the oxide:
MeO 3 þ xH
þ þ xe
À ) H x Me 1Àx
6þ Me x
5þ O 3
ð9:12bÞ
the material changes color to blue or green, while the oxygen radicals either occupy
vacant sites inside the sample or escape from the particle. An immediate
recombination of the charge carriers, h
þ þ e
À
) heat, reduces the performance
of the material. The reaction described above always require exchange with the
surrounding atmosphere and, therefore, the performance is difficult to control.
When considering the rate-controlling steps, it can be assumed that the reaction
according to Eq. (9.12a) occurs at the surface, whereas diffusion processes control
the step according to Eq. (9.12b). The effects observed with protons are also obtained
with alkaline metal doping. For alkaline metal-doped materials, Eq. (9.12b) becomes:
MeO 3 þ xA
þ þ xe
À ) A x Me 1Àx
6þ Me x
5þ O 3
ð9:13Þ
where A
þ is a monovalent ion (in most cases an alkaline metal). Again, the number
of free electrons that can react with the metal Me is most important for the efficiency
of the system.
A typical example of the absorption spectra of MoO 3 nanoparticles indicating
photochromic behavior is shown in Figure 9.59. This illustrates the absorption of
MoO 3 in the bleached state (0 s illumination time) and after two different times, 360
and 2160 s, of illumination with a pulsed laser emitting 72 mW per pulse at 308 nm.
Although it is clear that the intensity of coloration increases with time, the most
interesting point is the comparison of the time-dependent coloration of nanoparticulate and coarse-grained MoO 3 at a wavelength of 750 nm, as shown in
Figure 9.60. Here, the advantage of applying nanoparticles instead of coarse-grained
material is clearly visible. This is because the reaction mechanisms indicated in Eqs.
(9.12a) and (9.12b) require the transport of ions through the particle and, as the
diffusion time is indirectly proportional to the square of the particle size (see
Chapter 2), this acceleration is to be expected.
Although a decrease in particle size by a factor of only 10 should increase the rate
of coloration by a factor of 100, this is not the case as, experimentally; an increase in
the coloration rate only by a factor of 13 was verified. Clearly, diffusion is no longer
the rate-controlling step; rather, the rate-controlling process is more likely the
surface reaction according to Eq. (9.12a).
258j 9 Optical Properties of Nanoparticles