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R. Cerc Korošec, P. Bukovec / Thermochimica Acta 410 (2004) 65–71
3.4. In situ optical and cyclovoltammetric results
Before isothermal treatment, the optical response shows
irreversibility for the process Ni2+ ↔ Ni3+. But different
electrochromic behaviors were observed for sulfate and
acetate thin films isothermally treated to the similar extent.
In the case of a sulfate precursor, the best properties up
to 100th cycle are possessed by a thin film, in which ther-
mal decomposition of acetate groups is complete (60 min
at 270 ◦C). The change in transmittance at λ = 480 nm
for this film in the 100th cycle is 46%. For the acetate
precursor optical reversibility is already achieved in a film
thermally treated at 230 ◦C for 15 min (25% decomposi-
tion). The monochromatic transmittance change is 40% in
the 100th cycle, but greater differences between these two
films are observed at the beginning of the cycling, where
the optical response of the latter is very small (11%). The
initial response in case of the sulfate precursor is 26%. The
electrochemical mechanism at the beginning of the cycling
process is most likely different for thin films prepared from
NiSO4 or Ni(CH3COO)2 precursors and further studies
should be made in order to elucidate this difference.
The grain size of the xerogel residues after thermal de-
composition is nearly the same for both precursors (from 8
to 9 nm), so we exclude that grain size causes the observed
differences. Most probably the anions are responsible for
them. IR analysis shows that in case of sulfate precursor,
sulfate ions remain monodentately bonded to nickel after
thermal decomposition of acetates. On the other hand, the
carbonate ions formed during thermal decomposition of the
mentioned species bind bidentately to nickel, or remain free
in the structure in the case of an acetate precursor.
The optical transmittance changes at λ = 480 nm during
cyclovoltammetric measurements for a thermally untreated
film and for films kept at 230 ◦C for 15 or 60 min are pre-
sented in Fig. 6 (1st cycle) and Fig. 7 (100th cycle). In the
thermally untreated film, oxidation of Ni2+ ions causes a
large change in transmittance, i.e. from 0.90 at the begin-
ning of the cycle to 0.43 at the end of the anodic scan, but
the reduction process does not bleach the layer to the initial
value. A decrease in the transmittance value of the film in its
bleached state of 4.1% is observed. Both thermally treated
films exhibit very little color change during the oxidation
process in the first cycle. The values are given in Table 1.
According to the results obtained from in situ optical mea-
surements the current density of the thermally untreated film
is large compared with the other two films. Up to the 100th
cycle, the anodic current density of the thermally untreated
film diminishes and the cathodic current density becomes a
little larger, but the transmittance is only 0.74 in the bleached
state. For both thermally treated films, current densities be-
come approximately three times larger, and the observed
transmittance change during the oxidation/reduction process
is 40%. The initial transmittance value for the film thermally
treated at 230 ◦C for 60 min is even 6% larger than at the
beginning of cycling. Reversibility of the optical process for
these films is achieved before the thermal decomposition of
acetate groups is finished (in the film thermally treated at
230 ◦C for 15 min, this means only 25% decomposition en-
sures stability up to 100th cycle).
The role of dynamic and isothermal TG analysis in the
optimization procedure of the described thin films is that
it enables the determination of the processing tempera-
ture. Optimal time of isothermal heat-treatment can be
determined using additional sprectoelectrochemical mea-
surements. From the shape of isothermal TG curves (time
dependence) one can propose the duration of heat-treatment
at which makes is essential to test electrochromic response.
4. Conclusions
In the conclusion also the results of Part I [8] are summa-
rized together with the results of this article.
Time dependence of the isothermal weight loss of thin
films prepared from nickel sulfate or nickel acetate precursor
via sol–gel route helps us to prepare thin films with different
stoichiometry between thermally undecomposed amorphous
phase and nanosized nickel oxide. Isothermal temperature
can be chosen on the basis of dynamic measurements. It is
270 ◦C for thin films prepared from NiSO4 precursor [8] and
230 ◦C in case of Ni(CH3COO)2 precursor. Results obtained
for the corresponding xerogels could not be used because
higher decomposition temperature was determined in the
latter case.
Acknowledgements
This work was supported by Grant PO-0508-0103 from
the Ministry of Education, Science and Sport of the Republic
of Slovenia.
During isothermal treatment, the combustion of acetate
groups, which are present in both samples (Ni-sulfate,
Ni-acetate precursor) due to peptisation of the gel with
acetic acid, takes place and nickel oxide is being formed.
It is evident from in situ spectroelectrochemical mea-
surements that the ratio between thermally undecomposed
amorphous phase and nickel oxide is decisive for the elec-
trochromic response and stability of these films during cy-
cling in alkaline electrolyte.
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