Thesis defended

Mateusz Trochowski  has got his PhD degree. Congratulation!

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

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

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

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.

Thesis defended

Rafał Sadowski has got his PhD degree. Congratulation!

Photocatalytic coatings for medicine and food industry

The main goal of the doctoral thesis was to obtain stable and photoactive upon visible light irradiation semiconductor coatings on various polymers. A further purpose of this study was to understand the nature of semiconductor particles binding to polymer surface and determination of physical and chemical properties of the photocatalytic coatings. Additionally, the most active coatings were subjected to microbiological tests based on a modified ISO standard. The goal has been achieved by three steps procedure: activation, immobilization and sensitization. The crucial step was activation of the polymer surface by a low temperature oxygen plasma. It was shown that plasma pre-treatment is essential for oxygen groups formation which contribute to titanium dioxide binding to the polymer surface. Unmodified TiO2 can only be used as a photocatalyst active under ultraviolet (UV) light, which limits future applications. For this reason coatings were photosensitized by titanium(IV) surface charge transfer complexes formed by impregnation with organic ligands (catechol, 2,3-napthalenediol, pyrogallol, salicylic acid, ascorbic acid and rutin). Moreover, the efficacy of such coatings against Escherichia coli and Staphylococcus aureus was demonstrated.

Thesis defended

Tomasz Baran has got his PhD degree. Congratulation!

Photocatalytic reduction of carbon dioxide in hybrid systems

Carbon dioxide utilization may contribute to controlling its atmosphere level, reducing the emission and recycling carbon. Various methods of CO2 reduction are under intensive study, including photocatalysis that is promising, environmentally friendly technique of solar to chemical energy conversion. Three strategies of CO2 reduction to useful chemicals and fuels were investigated: photocatalytic carboxylation of organic compounds with CO2 reducing coupling, photocatalytic CO2 reduction to C1 compounds and bioinspired systems for CO2 reduction to methanol by an enzymatic way with photocatalytic regeneration of coenzyme. Carboxylation of acetylacetone and 2,3-dihydrofuran with carbon dioxide has been performed by using ZnS-based photocatalysts. The formation of carboxylic acid, as proven by 13C NMR, GC-MS and IR, was observed. The reaction efficiency was enhanced after modification of ZnS with ruthenium nanoparticles. The reaction involves one-electron reduction of CO2 to CO2.-with photogenerated electrons from the conduction band of ZnS and one-hole oxidation of organic substrates to the relevant radical. Coupling of formed radicals leads to the formation of carboxylic acids through C-C bound formation. Multielectron CO2 reduction to C1 molecules has been studied using various neat and modified materials: ZnS, CuI, NiO, CuS. HCOOH and CO were found as the main reduction products. The amount and ratio of products were influenced by the type of materials, the presence of deposited ruthenium(0) nanoparticles at the surface of materials and solvent polarity. Last approach of CO2 utilization based on photochemical regeneration of NADH for the enzymatic CO2 to CH3OH reduction through the enzymes cascade: FateDH, FaldDH and ADH. A series of photocatalysts was employed as photocatalyst of reduction of NAD+ to NADH under visible light irradiation. TiO2 photosensitized with organic dye and inorganic chromium complexes, narrow band gap photocatalysts such as Cu2O and InVOas well as ZnS doped with iron or cobalt cations seems to be the most promising photocatalysts. Thorough investigations led to the development of a photocatalytic systems constructed of photocatalyst and electron mediator that yield in selective and efficient regeneration of 1,4-NADH isomer.studied processes can find applications for the solar-light-driven green synthesis of fuels or Cn+1 carboxylic compounds from Csubstrates by utilizing carbon dioxide.

Thesis defended

Elżbieta Świętek has got her PhD degree. Congratulation!

Redox Characteristics of Semiconducting Photocatalytic Materials

The aim of the studies was to develop the method based on spectroelectrochemical measurements, which allows for the characterization of redox properties of various types of semiconducting materials, including determination of the impact of structural modification and surface as well as the influence of the experimental conditions on the redox properties of the semiconductors. As a part of this work a concise description of the methods used for characterization of redox properties of semiconducting photocatalytic materials was carried out and a novel model of photoinduced electron transfer in nanocrystalline semiconductor electrodes was proposed. An important part of the dissertation was the elaboration of the spectroelectrochemical method. The extension of the method is based on the application of the reflectance spectroscopy and analysis of the experimental conditions influence on VON values. The determined potentials correspond to the process of electron trapping on the available energy states close to the edge of the conduction band. The influence of several parameters like oxygen concentration, potential sweep rate, pH and the type of electrolyte, were discussed. The developed spectroelectrochemical method was also used for characterisation of the energy states lying close to the edge of the conduction band. Additional energy states of TiO2.(P25), localized within the bandgap, were qualitatively characterized. The applicability of the method to determine deep and shallow electron traps was confirmed. A quantitative analysis of shallow electron traps was also conducted using a modification of the spectroelectrochemical method by designating the distribution of electron density of states for selected samples. A wide range of applications of the new measurement method was discussed on the basis of measurements carried out for different semiconductors.

Thesis defended

Marta Buchalska has got her PhD degree. Congratulation!

Processes of Photoinduced Charge and Energy Transfer in Presence of Nanocrystaline TiOPhotosensitized with TiIV Complexes

High photocatalytic activity upon irradiation with UV light is one of major advantage of TiO2. This material can also be easily modified and sensitized to be active in visible light range. Especially modification with organic compounds that contain O-donor bonding groups often leads to stable and colorful charge transfer complexes. The aim of this study was to determine the mechanisms of photoinduced charge and energy transfer processes, that occur in presence of colloidal, nanocrystalline TiOwith modified surface. An additional objective was to determine the influence of the structure of the modifier, crystalline form of TiO2 and the type of radiation (Uv, visible, NIR) on the relative efficiencies of these processes. Colloidal, nanocrystalline anatase and rutile (from NanoAmor) were used during this work. The surface of materials was modified with the derivatives of catechol, salicylic acid and phthalic acid. Absorption and emission spectra were measured for all the samples, as well as electrochemical and spectroelectrochemical studies. Photocatalytic activity of the materials was determined upon irradiation with various light ranges (UV, visible, UV-Vis, NIR) and with various substrates (p-chlorophenol, Azur B). Unexpectedly high photocatalytic activity of all materials was observed upon irradiation with UV-vis light. Degradation rates are higher than sums of the effects observed separately for UV and visible light. Upon excitation of both, the semiconductor (with UV light) and the surface complex (with visible light), the oxidized form of the ligand can be reduced back by electrons from the conduction band. Presumably, this process inhibits electron-hole recombination and leads to an increase of the lifetimes of valence band holes and TiIII-centre of the excited surface complex. Increased lifetimes improve efficiencies of interfacial electron transfer process. The possibility of singlet oxygen generation was confirmed in direct measurements of phosphorescence at = 1270 nm upon excitation at = 355 nm. In this conditions 1O2 was detected in presence of unmodified anatase and material modified with salicylic acid. The efficiency of this process is F = 1,2 ± 0,5 for modified material and is about 3 times lower for unmodified anatase. Presence of singlet oxygen was also determined by monitoring the process of photooxidation of SOSG. The oxidized form of this compound shows high emission in visible light range. Tests were performed upon irradiation with monochromatic (laser l = 405 nm) and bichromatic light (additional l = 355 nm or l = 785 nm laser). It was shown that upon irradiation with l = 405 nm, singlet oxygen is generated in so called modified Nosaka’s mechanism. In this process photogenerated superoxide radical is oxidized to 1Oby oxidized form of surface complex. Upon additional irradiation with l = 785 nm, singlet oxygen can be generated in the process of energy transfer from the high excited state of the electron in the conduction band. That contain TiO2 in anatase form show higher photocatalytic activity than materials that contain rutile. The highest efficiency of hydroxyl radical generation was determined for anatase modified with catechol. Salicylic acid modified anatase was the most active in the process of singlet oxygen generation.