Thesis defence

  • 28 February 2024

Kamil Urbanek has got his PhD degree with distinction. Congratulations!

Photocatalytic CO2 reduction in the presence of composites of semiconductor photocatalysts

The urgent need to address the economic and environmental challenges posed by carbon dioxide has led to a concerted effort to develop techniques for its atmospheric transformation. Photocatalysis, leveraging solar energy, emerges as a promising avenue for mitigating CO2 levels and achieving a closed-loop carbon economy. However, traditional semiconductor photocatalysts like titanium dioxide demonstrate limited efficacy in CO2 reduction, especially when water serves as the electron donor. As water oxidation represents a less environmentally harmful process, this thesis explores TiO2 composites with other semiconductor photocatalysts to enhance photocatalytic activity. The investigation focuses on gas-phase reactors, enabling the utilization of air as a feed gas, thus potentially facilitating widespread adoption. A range of materials, including tin-iron, zinc-iron, cobalt, zinc-cobalt spinels, cadmium, copper and copper-cadmium sulfides, copper, tin, cerium, and tungsten oxides, and cobalt and copper tungstates, were characterized and tested for photocatalytic CO2 reduction, either alone or in conjunction with TiO2. While certain materials exhibited promising activity, others proved inert, even after attempts to activate them with platinum or palladium nanoparticles. Further experiments employed spectroelectrochemical techniques, photocurrent measurements, surface photovoltage measurements, and complementary oxidation processes to elucidate the mechanisms governing photocatalytic activity. Key inquiries included the potential efficiency enhancement of individual materials via composite formation, the viability of achieving S-scheme electron transfer with n-type and p-type semiconductor combinations, and the stability of multicomponent systems during photocatalytic CO2 reduction. Results demonstrated a notable improvement in photocatalytic activity for certain composites relative to their constituent materials, with mechanisms such as heterojunctions, S-Schemes, and TiO2 sensitization confirmed. Additionally, while combining p-type and n-type semiconductors did not always yield S-schemes or enhanced activity, the appropriate selection of materials with suitable Fermi levels proved crucial. Moreover, factors such as electrical contact between semiconductors and surface engineering influenced activity. Stability assessments revealed that some materials experienced decreased activity due to photocorrosion or active centre poisoning with CO2 reduction products. These findings underscore the complexity of optimizing photocatalytic CO2 reduction and highlight the importance of material selection and engineering for achieving efficient and stable systems.

Thesis defended

  • 16 February 2024

Anna Jakimińska has got her PhD degree with distinction. Congratulations!

Processes of energy and electron transfer in photocatalytic systems containing metallic nanostructures

In this thesis, the possibility of energy or electron transfer in the systems containing titanium dioxide and various metallic nanostructures during photocatalytic processes was examined. Emphasis was placed on the elucidation of the mechanisms of plasmonic photocatalysis and the real role of metallic nanostructures in the commonly used photocatalytic systems. It was determined that metallic nanostructures, regardless of exhibiting surface plasmon resonance in visible light or not, are responsible for the decrease of the electronic states availability in the semiconductor during growth and, therefore, change in the redox properties of modified materials. Such a change might sometimes be very significant, determining the routes and nature of processes taking place in the system. Additionally, from the nucleation step, metallic nanostructures act as the electron sinks, decreasing the recombination rate and enhancing photocatalytic performance. Therefore, the universal nature of the modification of titanium dioxide with metallic nanostructures was recognized. Moreover, the extraordinary use of plasmonic modes in photocatalysis is presented, i.e., the remote photocatalysis with the propagating surface plasmon polaritons of single silver nanowires that enables excitation of the photocatalytic system without direct irradiation. The demonstration of such a possibility may open a new avenue for constructing photocatalytic systems with higher performance and plasmonic nanostructures.

Thesis defended

  • 22 September 2023

Kasidid Yaemsunthorn has got his PhD degree. Congratulations!

Phase-Dependent Photocatalytic Activity of TiO2 – the Role of Intrinsic and Extrinsic Factors

The doctoral thesis delves into the phase-dependent photoactivity of anatase and rutile TiO2, explored through various photocatalytic reactions involving oxidation and reduction processes. The investigation of the influential factors involved in this dependency has led to the identification of two primary categories: intrinsic factors, which relate to the physicochemical and characteristic properties of the materials themselves, and extrinsic factors, which are associated with the nature of the specific reactions and their corresponding reaction conditions. The study demonstrates that the photoactivity of TiO2 is relative to both intrinsic and extrinsic factors, and sheds light on the mechanisms underlying photocatalysis. By optimizing these factors, it is possible to enhance the photoactivity of TiO2 and develop more efficient and practical photocatalytic systems. These findings reveal the fundamental mechanisms underlying photocatalysis and pave the way for the development of more suitable photocatalysts for particular reactions, which are crucial for optimizing and realizing the practical use of photocatalysis for addressing the energy crisis and climate change.

Thesis defended

  • 19 July 2022

Paweł Mikrut has got his PhD degree. Congratulations!

Solar energy utilisation in shape- and redox-engineered photocatalytic systems

Thesis defended

  • 18 February 2020

Magdalena Mikrut has got her PhD degree. Congratulations!

Redox activity of particulate matter in the context of autoimmune diseases development.

 

Thesis defended

  • 23 October 2019

Michał Pacia has got his PhD degree. Congratulations!

Activation of O2, H2O and H2O2 in photoinduced electron transfer processes.

As part of the dissertation research was carried to recognize various aspects of activation of the O2, H2O and H2O2 presence of TiO2 materials. In the case of anatase and rutile the influence of O2 and H2O2 presence on ROS generation in the irradiated system was determined. It was proven that rutile, compared to anatase, performs better in both O2 and H2O2 reduction. The influence of the exposed crystal facets on redox properties of materials were also investigated. The facets were put in an order (101) > (100) > (001) according to their activity in the H2O2 activation via reduction. Monocrystalline rutile materials, with exposed(110), (101) or (001) facets were also tested. The next step involved tests on surface modified TiO2 materials. Proper approach to determining the EBG in the case of surface modified materials was established. Activity of materials modified with organic compounds coordinating to the surface of TiO2 was tested. Additionally, a series of other semiconductors were investigated in terms of H2O and H2O2 activation properties. One of the crucial parts of this work was to verify the suitability of available ROS detection and determination methods in the systems containing heterogeneous photocatalysts. Weaknesses of methods based on reduction of XTT and chemiluminescence of luminol have been shown. Both high selectivity and sensitivity of the method to analyze production of hydroxyl radicals based on the oxidation of TA was confirmed. By means of EPR spectroscopy, EDTA has been confirmed to have very good holes quenching properties.

Thesis defended

  • 10 June 2019

Mateusz Trochowski  has got his PhD degree. Congratulations!

Diversity of the photosensitization mechanisms of surface-functionalized oxide semiconductors.

The doctoral thesis is focused on verification of four hypotheses related to the photosensitization mechanisms of titanium(IV) oxide and zinc oxide. The first hypothesis concerned the possibility of protection of photosensitizers bound to the TiO2 surface by covering them with a thin layer of titanium(IV) oxide to improve the photostability. The strategy worked only for a minority of the tested samples. The second hypothesis was the higher stability of dihydroxy derivatives of anthraquinone comparised to catechol adsorbed at TiO2. Long-term photocatalytic tests have shown that all obtained materials are more photostable than the reference catechol/TiO2 system and some of them are also more active. The third problem concerned investigation how insignificant modifications of photocatalysts can significantly change their photocatalytic activity. A significant effect on photocatalytic activity, of the studied materials, in particular upon visible light irradiation, was observed, although changes in the phase composition, spectroscopic properties and morphology of the samples tested were negligible. The fourth hypothesis concerned determining of the mechanisms of localized surface plasmon resonance interaction with ZnO modified with Au nanoparticles. The analysis of emission spectra has shown that photosensitization of ZnO occurs most probably via the transfer of “hot” electrons.

 

Thesis defended

  • 30 December 2018

Marcin Surówka  has got his PhD degree. Congratulation!

Photocatalytic Degradation of Herbicides at Titanium(IV) Oxide Modified with Metal Salts and Oxides

The doctoral thesis was focused on environmental problems related to contamination of water with herbicides: 2,4-D and 2,4,5-T. One of the most promising methods for water purification is heterogeneous photocatalysis. In this work a wide library of photocatalysts based on titanium(IV) oxide was synthetized. They were modified with different amounts (0.01-0.50%mol metal:Ti) of salts and oxides of following metals: Fe, Co, Ga, Bi, W, Mo, V and Ni. The materials were calcined at three chosen temperatures and characterised structurally and spectroscopically with XRD, UV-vis/DRS and photocurrent measurements. Their photocatalytic activity was investigated in the processes of terephthalic acid oxidation and herbicides and azure B degradation. Based on these studies the most favourable synthesis parameters were proposed: small modifier content and middle chosen calcination temperature, which result in both anatase and rutile presence in the final material. The most significant conclusion stated in this work is confirmation of indirect influence of modifiers on the photocatalytic activity of the materials. The role of modifiers is mostly observed during the photocatalyst synthesis – they influence the anatase to rutile ratio, being the main factor determining the overall photocatalytic activity of TiO2. Therefore, such modifiers are regarded as TiO2-synthesis-steering factors.

Thesis defended

  • 15 September 2016

Marcin Kobielusz has got his PhD degree. Congratulation!

Activation of C1 molecules in photocatalytic systems. The influence of electronic structure on the processes

The aim of the work was to determine the influence of the electronic structure of a semiconductor on its photocatalytic activity. In this doctoral thesis a modified spectroelectrochemical method is proposed, as a new technique to characterize the electronic states localized close to the edge of the conduction band. Distribution of additional electronic states localized within the bandgap can be qualitatively and quantitatively characterized using this approach. The applicability of the method in determination of deep and shallow electron traps was confirmed for selected semiconductors, such as TiO2, ZnO and ZnS.The proposed method has been applied to determine the influence of the energy states localized within the bandgap on processes of carbon dioxide reduction and methane oxidation. The work was focused on the study of multi- and one-electron reduction of carbon dioxide, and on photocatalytic steam reforming of methane. The studied materials involved bare and modified semiconductors. The influence of applied modifications including metal deposition, thermal treatment, atomic layer deposition, etc. on the photoactivity in the processes of photocatalytic oxidation and reduction reactions, as well as on the electronic structures of materials, has been analysed and discussed. Results and conclusions described in the thesis extend knowledge not only on mechanism of C1 activation, but also can be very useful in elucidating other photocatalytic processes, such as water splitting, hydroxyl radicals photogeneration, organic synthesis and many others.

Thesis defended

  • 15 September 2016

Szymon Wojtyła has got his PhD degree. Congratulation!

Reactive molecular oxygen species (1O2 , O3) in heterogenous catalysis

Numerous photocatalytic processes such as degradation of pollutants or organic synthesis occurs with reactive oxygen species, that are formed mainly as an effect of photoinduced charge transfer from excited semiconductor to O2 and H2O molecules. Photocatalytic activation of oxygen may lead to formation of two reactive oxygen species: superoxide radical anion and singlet oxygen. The former one is formed by one-electron reduction of oxygen while singlet oxygen can be generated on two pathways, involving either the electron transfer, generation of O2•- and its oxidation with a hole, or by a direct energy transfer to 3O2 molecule. The goal of this thesis was to study the mechanism and efficiency of singlet oxygen formation from molecular oxygen O2 and ozone O3 in photocatalytic heterogeneous systems. It is more and more discerned that alongside novel photocatalysts developments, understanding of the mechanism of primary photocatalytic processes is crucial for the technological development of photocatalysis. In this context, this thesis focuses on understanding of the mechanisms governing oxygen activation based on photoinduced charge or energy transfer, resulting in singlet oxygen formation. Moreover, a new method of singlet oxygen detection was developed with this work. A series of heterogeneous semiconducting materials has been prepared: neat and silylated or doped TiO2, silicon-based materials, wide band gap semiconductors, such as ZnS, ZnO, NiO, and semiconductors with a narrower band gap, e.g. CdS, Cu2O, Fe2O3. All materials have been characterized using spectroscopic and crystallographic methods. They were tested as photocatalysts of singlet oxygen formation from O2. Moreover, ozone was considered as an alternative substrate for singlet oxygen formation on a photocatalytic way. Obtained results prove that the prepared nanocrystalline materials are active photocatalysts and upon irradiation they are able to generate singlet oxygen based on one or both discussed mechanisms. Si, silylated TiO2, Fe2O3, Cu2O appeard particulary active oxygen photosensitizers. Formation of other reactive oxygen species, such as superoxide anion radical and hydroxyl radical from O2 and O3, has also been studied and discussed. Results and conclusions shine some new light on photocatalytic transformations involving 1O2 and O3.

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