Deposição de nanocristais coloidais: da síntese à aplicação na fotoeletroquímica da oxidação da água
Abstract
An excellent and potentially efficient route towards storing solar energy is
to convert light into chemical energy in the form of chemical bonds, which is a form
of artificial photosynthesis. Considering the abundance of H2O on the planet, water
splitting is a natural pathway for artificial photosynthesis. Hematite is an n-type
semiconductor with high chemical stability in alkaline media and promising material
for photoelectrochemical water splitting. This Thesis describes critical parameters
involved in the Colloidal Nanocrystals Deposition (CND) process to produce hematite
photoanodes with high efficiency for solar-to-hydrogen conversion.
In chapter 2, a fundamental study reveals that the interface solid-solid is a
parameter that has strong influence on the performance of the photoanode. The gap
between the FTO substrate and hematite thin film was observed by HRTEM image
and it can be overcome during a sintering stage. In the same chapter, the solid-solid
interface analysis was correlated with the photoresponse and it has showed that
hematite thin film treated at 1000 oC also improved the response of this photoande.
This result was explained based on the grain growth and associated with the mass
distribution on the FTO surface.
In chapter 3, the CND process was improved using the magnet to assist the
nanocrystals deposition and also oxidation of magnetite (Fe3O4) to maghemite (γ-
Fe2O3) to avoid the presence of Fe2+. In this approach, Sn4+ was used as a doping
element and has showed a significant improve on the photoresponse of the hematite.
The STEM-EDS analysis has showed that Sn has ability to segregate on the hematite
grain boundary during sintering process, blocking grain growth process.
The results showed in chapter 4 were essential to understand the thickness
effect on the photocurrent of thin film produced by CND process. In this case,
changing the nanocrystals concentration has direct effect on the thickness of the
hematite thin film. The FTO roughness also showed significant influence on the
orientation of hematite grain along the direction <110>. In this study, it was possible
to calculate the maximum theoretical efficiency for the hematite photoanode obtained
by this method. The thickness control and homogeneity of the thin film give a great
perspective for technological application of this process.
The in situ heating TEM demonstrated that nanocrystals has abnormal grain
growth and also a superplastic phenomenon, as revealed in chapter 5. In this chapter,
Sn was deposited on γ-Fe2O3 impeding atom dislocation on the grain boundary and
consequently inhibits the growth process. This experiment was an approach to
simulate the sintering process performed in the CND process.
The electrocatalyst described in chapter 6, showed low overpotential for OER.
The strategy to use a Prussian blue analogue to deposit a thin layer of nickel-iron
hexacyanoferrate and convert into oxyhydroxide achieved excellent homogeneity and
low overpotential for OER. This result is comparable with IrO2 and RuO2 that are
electrocatalysts with high electrochemical performance. Catalyst supports were also
evaluated, such as FTO, palladium and PGS. The PGS substrate showed an excellent
performance as catalytic support for OER, with similar results of palladium foil.