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S U M M A RY
The past decades have seen a rapid development of systems capable
to transform solar energy to heat or electricity, which was triggered by
a growing need for covering the increasing energy demand in cheap
and completely sustainable way. However, the development of solarto-chemical energy conversion systems seems to be more attractive approach because energy can be directly stored in chemical bonds, e.g.
of dihydrogen molecules.
One of the most appealing methods for direct conversion of sunlight energy into chemical energy is the use of photoelectrochemical cells in which simultaneous oxidation and reduction of water
to hydrogen and oxygen takes place. Although impressive solar-tohydrogen efficiencies reaching far beyond the 10 % threshold required
for commercial implementation have been demonstrated repeatedly,
all these high-efficiency cells have been based on very expensive materials and cannot be easily scaled-up. In this context it is important to
realize that the major challenge in photoelectrochemical water splitting devices is the development of low-cost, efficient and stable photoanodes. This is particularly because of the complex chemistry involved in four-electron oxidation of water to dioxygen, which typically translates into high overvoltages needed to drive the reaction,
which, in turn, enhances the risk of oxidative photocorrosion.
Traditionally, the attention in scope of water photooxidation is drawn
mostly to pristine or doped metal oxides like TiO2 , WO3 , and Fe2 O3 .
Whereas these materials have relatively high inherent stability, the
major problems are: too positive position of CB or too large bandgap,
and limited flexibility of their structure, which brings about increased
overpotentials for oxygen evolution. Consequently, significant external bias application is a prerequisite to drive water photooxidation at
pristine metal oxides.
A different approach is to use dye-sensitized photoanodes in which
nanocrystalline layer of a stable wide-bandgap oxide sensitized by
a visible light absorbing dye is coupled to the oxygen-evolving catalyst. Despite many advantages, like visible light absorption, flexibility,
and kinetic charge separation, such materials often suffer because of
instability of the dye in harsh water oxidation conditions.
An alternative way, which might combine advantages of pristine
metal oxides (stability) and dye-sensitized materials (flexibility, kinetic charge separation) is represented by photoanodes based on a novel
class of visible-light photoactive hybrid materials: nanocrystalline TiO2
modified at the surface by a thin (< 1–3 nm) layer of polyheptazine
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(TiO2 -PH). One of the most attractive features of these inorganic/organic hybrids is the high thermal (up to 550 °C in air) and chemical stability of polyheptazine-type compounds as compared to conventional organic dyes. It was shown that polyheptazine and TiO2
form an interfacial charge-transfer complex, which effectively shifts
the optical absorption edge of the TiO2 -polyheptazine hybrids into
the visible (2.3 eV; ∼ 540 nm) as compared to the bandgaps of both of
the single components – TiO2 (3.2 eV; ∼ 390 nm) and polyheptazine
(bandgap of 2.9 eV; ∼ 428 nm). [44] In other words, the direct optical
charge transfer is expected to produce electrons with a relatively negative potential in the conduction band of TiO2 , while the photoholes
are located in the surface polyheptazine layer. Those properties makes
TiO2 -PH an excellent platform for development of photoanodes for
use in photoelectrochemical applications.
Difficulties arise, however, when an attempt is made to use simply
TiO2 -PH photoanode to oxidize water. TiO2 -PH is a very good visible
light absorber and charge separating material, but complete photooxidation of water to dioxygen it feasible only after coupling with additional metal oxide sites (co-catalyst) able to catalyze complicated hole
transfer to water molecule.
The present study was designed to determine the effect of loading
of TiO2 -PH with various co-catalysts by careful evaluation of evolved
dioxygen concentration measurements results obtained by means of
the home-build photoelectrochemical experimental setup. In particular, this dissertation sought to address the following questions:
• Which type of co-catalysts can efficiently mediate hole transfer
from TiO2 -PH hybrid to water molecules?
• How the photooxidation of water depends on the method, by
which the co-catalyst was introduced to the light-absorbing photoanode material?
• What is the influence of conditions, e.g. external bias, pH, type
of an electrolyte at the activity of TiO2 -PH loaded with a cocatalyst in water oxidation reaction?
The content of the present thesis can be summarized as follows:
chapter 2 has introductory character and brings basic informations
about photoelectrochemical cells, design of photoanodes, principles of water oxidation reaction, and strategies which can be
employed to improve system efficiency.
chapter 3 compiles experimental details about photoelectrochemical measurements, design of experimental setup for dissolved
oxygen measurements, instrumental characterization techniques,
and synthesis of TiO2 -PH photoanodes.
summary
chapter 4 begins with concise literature review introducing recent
developments regarding iridium-based catalysts in context of water oxidation, followed by presentation of the author’s investigation results. In the first part, nanocrystalline TiO2 -polyheptazine (TiO2 -PH) hybrid photoelectrodes were loaded with iridium oxide nanoparticles, acting as oxygen evolving co-catalyst,
by the colloidal deposition method (TiO2 -PH/CD). Significantly,
visible-light irradiation (λ = 420 nm) led to dioxygen evolution.
In the second part of Chapter 4, iridium oxide co-catalyst was
photodeposited onto TiO2 -PH photoelectrodes from solution containing [Ir(OH)6 ]2- anions. Iridium oxide clusters were formed
in-situ under the irradiation of monochromatic visible light. It
was found that TiO2 -PH photoanodes with photodeposited IrOx
yield 2.5 times higher amount of evolved dioxygen than IrOx
loaded using colloidal deposition method mainly due to the
better coupling to holes photogenerated in TiO2 -PH absorber.
It was also shown that presence of photodeposited IrOx cocatalyst reduces the surface recombination rate. In addition, the
photodeposition method is very reproducible, highly controllable, and yields stable co-catalyst layers which allowed to investigate properties of TiO2 -PH photoelectrode loaded with IrOx
co-catalyst in the broad range of pH. A strong dependence of
activity on electrolyte properties (anion, pH) was found. TiO2 PH + IrOx /PD photoanode exhibit highest activity and stability
in Na2 SO4 at pH = 6. These results have shown that measurement conditions must be cautiously selected in order to assure
compatibility with all essential components of photoanode.
chapter 5 commences with brief literature review of cobalt-based
materials investigated in scope of water photooxidation, followed by presentation of investigation results. In the first part,
a cobalt oxide-based oxygen-evolving co-catalyst (Co-Pi) was
photodeposited by visible-light irradiation onto TiO2 -PH photoelectrodes from phosphate-buffered solution containing Co2+ .
In the second part, cobalt-based co-catalysts were introduced
by mixing three different oxides (Co3 O4 , NiCo2 O4 , and CoTiO3 )
with pristine TiO2 and subsequent modification with polyheptazine. Both mixtures of oxides and Co-Pi co-catalyst couple
effectively to photoholes generated in polyheptazine layer of
TiO2 -PH photoanode, as evidenced by complete photooxidation
of water to oxygen under visible-light (λ > 420 nm) irradiation
at moderate bias potentials. In addition, the presence of the
co-catalyst again reduces the recombination of photogenerated
charges, particularly at low bias potentials, which was ascribed
to better photooxidation kinetics. This suggests that further improvements of photoconversion efficiency can be achieved if
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more effective catalytic sites for water oxidation are introduced
to the surface structure of the hybrid photoanodes.
chapter 6 begins with short literature review of nickel-based materials investigated in context of water oxidation. The next Section
presents investigation results of TiO2 -PH loaded with a nickelbased oxygen evolving catalyst (NiOx ) photodeposited under
the visible light irradiation from borate-buffered solution containing Ni2+ cations. It was found, that the amount of dioxygen generated by TiO2 -PH + Ni-Bi and the IPCE values are
rather moderate, especially if compared with those of cobaltor iridium-based oxygen evolving co-catalysts created by the
photodeposition method.
chapter 7 starts with concise presentation of recent developments
regarding managanese-based materials in scope of water oxidation. In the following Sections attempts to interfaces Mn-based
oxides with TiO2 -PH by two methods: in-situ photodeposition
and mixing of metal oxides are presented. None of the samples exhibit capability to photoxodize water, as evidenced by
unchanged dissolved dioxygen concentration during irradiation
with visible light. It is probably due to the low, in comparison to
iridium, cobalt or nickel oxides, electrocatalytic activity of here
prepared manganese oxides acting in this case rather as a light
blocking layer.
chapter 8 describes investigation of visible (λ > 420 nm) light -driven
photooxidation of water at TiO2 -PH electrodes loaded with two
different metal oxide co-catalysts: Co-Pi and IrOx /CD. As compared with TiO2 -PH photoanodes loaded with iridium oxide by
colloidal deposition (IrOx /CD), photoelectrodes modified with
CoOx oxygen-evolving co-catalyst (Co-Pi) deposited by photoassisted deposition precipitation method showed both higher photocurrents and more efficient oxygen evolution under prolonged
irradiation. The minimum external electric bias needed to observe complete photooxidation of water to dioxygen at TiO2 -PH
photoanodes modified with Co-Pi was estimated to be ∼ 0.6 V
at pH 7. The key factor limiting the photoconversion efficiency
at low bias potentials is the fast primary recombination of photogenerated charges.
In conclusion, this thesis presents results of investigation of visible
light active TiO2 -PH photoanodes loaded with additional metal oxides acting as oxygen evolving centers. It is focused on evolved dioxygen measurements and photoelectrochemical characterization.
One of the more significant findings to emerge from this study is
that after loading with iridium, cobalt or nickel oxide species, TiO2 PH photoanode oxidizes water under irradiation with visible light
summary
(λ > 420 nm), as evidenced by dissolved dioxygen measurements. On
the contrary, it was also found, that manganese oxide species are
completely inactive as an oxygen evolving co-catalyst. The second
major finding is that photooxidation efficiency strongly depends on
a co-catalyst loading method, as illustrated by higher activity of IrOx
introduced by the photodeposition method than the colloidal deposition method, mostly due to the better coupling with absorber (TiO2 PH). It is important to emphasize that the photodeposition method
(particularly investigated for IrOx co-catalyst implementation) is very
promising for the further research. For example, it would be interesting to assess the possibility of simultaneous deposition of two or
more co-catalysts on the photoanode surface. The next conclusion
which can be drawn from the present study is that the presence of
the co-catalyst reduces accumulation of holes, thereby retarding recombination of photogenerated charges, particularly at low bias potentials. Finally, results of this thesis have also indicated importance
the electrolyte type and its pH for efficiency and stability of hybrid
photoanodes.
Taken together, the findings of this thesis add to our understanding about methods which can be used to implement catalytic sites
into semiconducting and hybrid (e.g. dye-sensitized) light absorbing
materials in order to use photogenerated holes to drive water oxidation. Further research aiming to improve electrocatalytic activity
of the co-catalyst, the visible-light photoactivity of absorber and coupling between those two, would be of great help in development of
stable and efficient photoanodes which are in the center of our efforts
devoted to create new solar-to-chemical energy conversion systems.
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