Lisandist ja valmistamistingimustest sõltuvad õhukeste TiO2 kilede omadused

Väitekirja elektrooniline versioon ei sisalda publikatsioone.TiO2 on laialt kasutatav nii tööstuslikult kui ka igapäevaelus, seda kasutatakse palju valge pigmendina ja paakumisvastase ainena värvides, plastikutes, paberis ja teistes toodetes. Samuti kasutatakse TiO2-te päikesekreemides UV-kiirgust blokeeriva ainena, samuti on TiO2 kasutusel toiduvärvina (E171). Lisaks on TiO2 juba leidnud või alles leidmas rakendust fotokatalüütilise materjalina, pooljuhtmaterjalina päikesepatareides, bioloogiliselt ühilduva materjalina ja isepuhastuvate katete baasmaterjalina. Nanostruktuursus, kristallstruktuur, puhtus ja paljud teised omadused mõjutavad TiO2 kasutamist erinevates rakendustes. Käesolevas töös uuriti põhjalikult nii erinevate lisandite kui ka aluse eeltöötluse mõju TiO2 õhukeste sool-geel meetodil valmistatud kilede struktuurile ja kristallilisele koostisele. Näidati, et kilede kristalliline koostis ja faasisiirde temperatuur anataasist rutiiliks sõltub tugevalt konkreetsest lisandist ja ka aluse eeltöötlusest. Samuti näidati, et lisandiaatomid segregeeruvad kilede kuumutamise käigus TiO2 kiledest välja ja nii tekivad lisandirikkad piirkonnad kile pinnal. Näidati, et kilede fotokatalüütiline aktiivsus sõltub nii nende nanostruktuursusest kui ka lisandite valikust ja mõjust. Lisaks õhukestele sool-geel metoodikaga valmistatud kiledele valmistati eelnevalt sünteesitud nanoosakestes koosnevaid TiO2 kilesid ning uuriti nende fotokatalüütilisi omadusi. Näidati, et nanoosakestest koosnevad TiO2 kiled lagundavad väga efektiivselt orgaanilisi ühendeid ja omavad tugevat antibakteriaalset toimet. Nanoosakestest koosnevaid kilesid on lihtne valmistada, mistõttu on nad potentsiaalselt kasutatavad ka tööstuslikes rakenduste.Titania (TiO2) is widely used in industry and our everyday life. It finds use as white pigment and anticaking agent in paints, plastic, paper and other consumer products. It also finds application in sunscreens as UV blocker and in foodstuff as colouring agent (E171). Titania has been considered also as a perspective material for many advanced applications. It has been studied extensively as a promising photocatalyst, solar cell material, biocompatible material, material for anti-fogging and self-cleaning coatings. Nanostructure, crystal structure, purity and other qualities all play important roles in many of titania applications. Influence of the different dopants and the substrate pretreatment to the sol-gel film structure and crystal phase composition was thoroughly investigated. It was demonstrated that exact crystal phase transition temperature and extent from anatase to rutile are highly dependent on the doping element and the substrate pre-treatment. Also during annealing the impurity element tends to segregate out of the titania matrix and form dopant rich regions on the surface of the film. It was shown that the photoactivity of the TiO2 film depends on the choice of the impurity element and the nanostructure of the films. Beside titania sol-gel thin films also titania thin films consisting of premade nanoparticles were prepared and their photoactive properties were investigated. It was shown that thin titania films consisting of small premade nanoparticles exhibited enhanced photocatalytic and antibacterial properties. The films consisting of premade nanoparticles are simple to prepare and they are also efficient photocatalysts which means that they are potentially scalable to industrial level

Aqueous synthesis of Z-scheme photocatalyst powders and thin-film photoanodes from earth abundant elements

Riga Technical University supported the preparation of this manuscript from the Scientific Research Project Competition for Young Researchers No. ZP 2017/8Solid-state narrow band gap semiconductor heterostructures with a Z-scheme charge-transfer mechanism are the most promising photocatalytic systems for water splitting and environmental remediation under visible light. Herein, we construct all-solid Z-scheme photocatalytic systems from earth abundant elements (Ca and Fe) using an aqueous synthesis procedure. A novel Z-scheme two-component Fe2O3/Ca2Fe2O5 heterostructure is obtained in a straightforward manner by soaking various iron-containing nanoparticles (amorphous and crystalline) with Ca(NO3)2 and performing short (20min) thermal treatments at 820°C. The obtained powder materials show high photocatalytic performances for methylene blue dye degradation under visible light (45 mW/cm2), exhibiting a rate constant up to 0.015min-1. The heterostructure exhibits a five-fold higher activity compared to that of pristine hematite. The experiments show that amorphous iron-containing substrate nanoparticles trigger the Fe2O3/Ca2Fe2O5 heterostructure formation. We extended our study to produce Fe2O3/Ca2Fe2O5 nanoheterostructure photoanodes via the electrochemical deposition of amorphous iron-containing sediment were used. The visible-light (15mW/cm2) photocurrent increases from 183μA/cm2 to 306μA/cm2 after coupling hematite and Ca2Fe2O5. Notably, the powders and photoanodes exhibit distinct charge-transfer mechanisms evidenced by the different stabilities of the heterostructures under different working conditions.Riga Technical University No. ZP 2017/8; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

Facile synthesis of magnetically separable CoFe2O4/Ag2O/Ag2CO3 nanoheterostructures with high photocatalytic performance under visible light and enhanced stability against photodegradation

Riga Technical University supported the preparation of this manuscript from the Scientific Research Project Competition for Young Researchers No. ZP-2016/7. The authors wish to kindly acknowledge the financial support of HZB, Estonian Research Council (PUT1096, PUT735 and IUT2-25) and Estonian Centre of Excellence in Research Project “Advanced materials and high-technology devices for sustain-able energetics, sensorics and nanoelectronics” TK141 (2014–2020.4.01.15-0011).We have developed magnetically separable and reasonably stable visible light active photocatalysts containing CoFe2O4 and mixture of Ag2O/Ag2CO3 nanoheterostructures. Obtained ternary nanoheterostructures outperform previously reported magnetically separable visible light photocatalysts, showing one of the highest visible light photocatalytic dye degradation activities in water by a magnetically separable photocatalyst. Photocatalytically active part is Ag2O/Ag2CO3 whereas the CoFe2O4 mainly has stabilizing and magnetic separation functions. The Ag2CO3 phase junction on Ag2O nanoparticle surface were obtained by straightforward phase transformation from silver oxide to silver carbonate in air due to ambient CO2. The phase transformation was followed using X-ray diffraction (XRD), and hard X-ray photoelectron spectroscopy (HAXPES) measurements.Riga Technical University No. ZP-2016/7; Estonian Research Council (PUT1096, PUT735 and IUT2-25); Estonian Centre of Excellence in Research TK141 (2014–2020.4.01.15-0011); Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

Solvothermal synthesis derived Co-Ga codoped ZnO diluted magnetic degenerated semiconductor nanocrystals

Authors kindly acknowledge to the Estonian Research Council ( PUT1096 , IUT2-25 , PUT735 ), the Estonian Centre of Excellence in Research project “Advanced materials and high-technology devices for sustainable energetics, sensorics and nanoelectronics (TK141), and the financial support of HZB. We are grateful to the staff of BESSY II for the assistance and co-operation during the synchrotron-based measurements.Here we are reporting solvothermal synthesis derived diluted magnetic and plasmonic Co-Ga co-doped ZnO nanocrystals with high magnetization values (from 1.02 to 4.88 emu/g) at room temperature. Co-Ga co-doped ZnO nanocrystals show up to 2 fold increase in saturation magnetization compared to Co doped ZnO nanocrystals at the same Co concentration, with the observed room temperature magnetization higher than previously reported values for multifunctional magnetic and plasmonic nanocrystals, and the effect of Ga suggesting some role of the correspondingly introduced itinerant charge. While at the lowest Ga content the nanoparticles appear homogeneously doped, we note that already a moderate Ga content of several percent triggers a fraction of Co to segregate in metallic form in the bulk of the nanoparticles. However, the amount of segregated Co is not sufficient to account for the total effect, whereas a dominating contribution to the observed magnetism has to be related to itinerant charge mediated exchange interactions.Eesti Teadusagentuur IUT2-25,PUT1096,PUT735; Estonian Centre of Excellence in Research TK141; HZB; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

ORR activity and stability of Co-N/C catalysts based on silicon carbide derived carbon and the impact of loading in acidic media

This work was supported by the EU through the European Regional Development Fund under projects TK141 “Advanced materials and high-technology devices for energy recuperation systems” (2014-2020.4.01.15-0011), NAMUR ”Nanomaterials - research and applications” (3.2.0304.12-0397) and by the Estonian institutional research grant No. IUT20-13.A simple and facile synthesis method was used to produce two Co-N/C type oxygen reduction reaction (ORR) catalysts. The materials were initially characterized by utilizing a variety of physical methods. Most importantly, the XPS analysis revealed high amounts of pyridinic nitrogen and Co-Nx species in the case of both studied Co-N/C catalysts. The electrochemical characterization showed that both of the synthesized Co-N/C catalysts have a high ORR activity in acidic media, displaying a half-wave potential of 0.70 V vs RHE. Additionally, the effect of varying the catalyst loading was studied and it was found that increasing the catalyst loading from 0.1 to 1.8 mg cm−2 significantly improved the ORR activity and the electron transfer number. Finally, several catalysts were subjected to a week-long stability test in order to establish their activity degradation rates. It was found that increased degradation rates of the Co-N/C catalysts were established at decreased catalyst loadings.European Commission; ERDF 2014-2020.4.01.15-0011,TK141, NAMUR 3.2.0304.12-0397; Estonian institutional research grant No. IUT20-13; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

Dissolution of Silver Nanowires and Nanospheres Dictates Their Toxicity to Escherichia coli

Silver nanoparticles are extensively used in antibacterial applications. However, the mechanisms of their antibacterial action are not yet fully explored. We studied the solubility-driven toxicity of 100 × 6100 nm (mean primary diameter × length) silver nanowires (NWs) to recombinant bioluminescent Escherichia coli as a target representative of enteric pathogens. The bacteria were exposed to silver nanostructures in water to exclude the speciation-driven alterations. Spherical silver nanoparticles (83 nm mean primary size) were used as a control for the effect of NPs shape. Toxicity of both Ag NWs and spheres to E. coli was observed at similar nominal concentrations: the 4h EC50 values, calculated on the basis of inhibition of bacterial bioluminescence, were 0.42 ± 0.06 and 0.68 ± 0.01 mg Ag/L, respectively. Dissolution and bioavailability of Ag from NWs and nanospheres, analyzed with AAS or Ag-sensor bacteria, respectively, suggested that the toxic effects were caused by solubilized Ag + ions. Moreover, the antibacterial activities of Ag NWs suspension and its ultracentrifuged particle-free supernatant were equal. The latter indicated that the toxic effects of ∼80-100 nm Ag nanostructures to Escherichia coli were solely dependent on their dissolution and no shape-induced/related effects were observed. Yet, additional nanospecific effects could come into play in case of smaller nanosilver particles

Colorimetric gas detection by the varying thickness of a thin film of ultrasmall PTSA-coated TiO2 nanoparticles on a Si substrate

Financial support from the Estonian Research Council (IUT2-25, PUT170, PUT1096, PUT748, PUTJD680), the Estonian Centre of Excellence in Research Projects “Advanced materials and high-technology devices for sustainable energetics, sensorics and nanoelectronics” TK141 (2014-2020.4.01.15-0011), “Emerging orders in quantum and nanomaterials” TK134 and the Development Fund of the University of Tartu, are all gratefully acknowledged.Colorimetric gas sensing is demonstrated by thin films based on ultrasmall TiO2 nanoparticles (NPs) on Si substrates. The NPs are bound into the film by p-toluenesulfonic acid (PTSA) and the film is made to absorb volatile organic compounds (VOCs). Since the color of the sensing element depends on the interference of reflected light from the surface of the film and from the film/silicon substrate interface, colorimetric detection is possible by the varying thickness of the NP-based film. Indeed, VOC absorption causes significant swelling of the film. Thus, the optical path length is increased, interference wavelengths are shifted and the refractive index of the film is decreased. This causes a change of color of the sensor element visible by the naked eye. The color response is rapid and changes reversibly within seconds of exposure. The sensing element is extremely simple and cheap, and can be fabricated by common coating processes.Eesti Teadusagentuur PUT748,IUT2-25,PUT170,PUT1096,PUTJD680; Estonian Centre of Excellence in Research Projects 2014-2020.4.01.15-0011,TK134,TK141; University of Tartu; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

TiO2 Nanowire Dispersions in Viscous Polymer Matrix: Electrophoretic Alignment and Optical Properties

The changes in optical properties during TiO2 nanowire orientation in polydimethylsiloxane (PDMS) matrix under the influence of an electric field are strongly influenced by nanowire (NW) diameter. It was demonstrated for the first time that either positive or negative change in transmittance can be induced by NW alignment parallel to the electric field depending on the NW diameter. These effects can be explained by the interplay between scattering and reflectance. Experimental findings reported could be important for smart window applications for the regulation of visible or even infrared transparency, thus reducing the energy consumption by air conditioning systems in buildings and automobiles in the future

Solvothermal Synthesis of One-Dimensional Transition Metal Doped ZnO Nanocrystals and Their Applications in Smart Window Devices

Oxide semiconductor nanowire (NW) suspension based devices have been attracted growing interest in smart window applications due to their great controllability of light transmittance, simplicity and long term stability. Recently, we demonstrated smart window device using the suspension of electrospun TiO2 or solvothermally synthesized ZnO NWs in viscous polydimethylsiloxane (PDMS) matrix. The operating principle of the oxide semiconductor NW and PDMS device is based on the alterable orientation, alignment or spatial distribution of the NWs in an electric field, by changing light scattering cross-section and thus reversibly increasing or decreasing transmittance [1]. One-dimensional (1D) nanostructures such as NWs exhibit a good response to electric field due to the highly anisotropic shape. Herein we report transition metal doped ZnO NW and PDMS based smart window devices. Doping ZnO NWs with other chosen metal ions, such as transition metals, may lead to the emergence of new targeted material properties. For instance, high quality ZnO nanocrystals doped with transition metal cations lead to enhanced optical absorption of light in visible range. The pristine ZnO NWs used in our previous work did not exhibit visible light absorption and transmittance of ZnO NWs and PDMS based smart window device was regulated by changing scattering cross-section. The visible light absorption could increase efficiency of smart window device, because during electrophoretic alignment of NWs towards direction of electric field will change not only scattering, but also visible light absorption cross-section. One-dimensional ZnO nanostructures have been synthesized by wet chemical techniques including microemulsion hydrothermal synthesis, surfactant-assisted hydrothermal orientation growth and alcohol solution refluxing. However, synthesis of high quality transition metal doped ZnO nanowires with small diameter and high aspect ratio is still a challenge. Most wet chemical methods fail to produce high aspect ratio doped ZnO NWs in large quantities. Here we are demonstrating large-scale, single-step, direct solvothermal method and have successfully prepared high aspect ratio single crystalline transition metal (Co, Cu, Fe, Ni, Mn) doped ZnO NWs. The solvothermal synthesis process presented here can be scaled up to macro scale production and the fact that NWs need no further modification increases the technological potential of oxide semiconductor nanowire suspension based devices electro-optical smart window devices

Dissolution of Silver Nanowires and Nanospheres Dictates Their Toxicity to Escherichia coli

Silver nanoparticles are extensively used in antibacterial applications. However, the mechanisms of their antibacterial action are not yet fully explored. We studied the solubility-driven toxicity of  nm (mean primary diameter × length) silver nanowires (NWs) to recombinant bioluminescent Escherichia coli as a target representative of enteric pathogens. The bacteria were exposed to silver nanostructures in water to exclude the speciation-driven alterations. Spherical silver nanoparticles (83 nm mean primary size) were used as a control for the effect of NPs shape. Toxicity of both Ag NWs and spheres to E. coli was observed at similar nominal concentrations: the 4h EC50 values, calculated on the basis of inhibition of bacterial bioluminescence, were 0.42 ± 0.06 and 0.68 ± 0.01 mg Ag/L, respectively. Dissolution and bioavailability of Ag from NWs and nanospheres, analyzed with AAS or Ag-sensor bacteria, respectively, suggested that the toxic effects were caused by solubilized Ag+ ions. Moreover, the antibacterial activities of Ag NWs suspension and its ultracentrifuged particle-free supernatant were equal. The latter indicated that the toxic effects of ~80–100 nm Ag nanostructures to Escherichia coli were solely dependent on their dissolution and no shape-induced/related effects were observed. Yet, additional nanospecific effects could come into play in case of smaller nanosilver particles