Grundlagenstudien zur photoelektrochemischen Wasseroxidation an Hämatit-Nanostrukturen

Während des vergangenen Jahrzehnts gewann die Synthese neuartiger Materialien für die Anwendung bei der photoelektrochemischen Wasserspaltung zunehmende Bedeutung. Das Verständnis der zugrunde liegenden Reaktionsmechanismen ist für die Entwicklung und Effizienzverbesserung solcher Materialien von großer Wichtigkeit. Hämatit ist eines der meist-untersuchten und best-bekannten Materialien für die photoelektrochemische Wasseroxidation und seine Reaktions- mechanismen sind zunehmend gut untersucht. Trotzdem gibt es noch offene Fragen in Bezug auf die Natur der bei der Bestrahlung entstehenden Ladungsträger und ihre Rekombinationskinetik. Transiente Absorptionsspektroskopie mittels Laser Flash Photolyse ist eine verbreitete und potente Methode zur Untersuchung der Ladungsträgerdynamiken. In der vorliegenden Arbeit wurde die Natur der Ladungsträger in kommerziellen sowie selbst erstellten Hämatit-Pulvern mittels Nanosekunden-Laser Flash Photolyse in diffuser Reflexion untersucht. Die kinetische Abhängigkeit der Transienten von der Sauerstoff-Konzentration an der Oberfläche wurde durch Variation der umgebenden Atmosphäre (Argon, Stickstoff, Luft, Sauerstoff, Ozon) untersucht. Eine steigende Sauerstoff-Konzentration führt dabei zu kürzeren Lebensdauern der getrappten Ladungsträger. Ein Vergleich mit thermisch vorbehandelten Proben zeigt eine Korrelation der Sauerstoff-Konzentration mit der Anzahl der Fehlstellen an der Kristallit-Oberfläche. Eine Rekonstruktion des transienten Absorptionsspektrums ermöglicht die Zuordnung der Signale im Wellenlängenbereich von 550-850 nm zu verschiedenen Spezies getrappter Löcher. Aufgrund der Laserenergie während der Bestrahlung wurden irreversible strukturelle Veränderungen der Pulverproben festgestellt. Eine Charakterisierung mittels Röntgendiffraktometrie zeigt einen Phasenübergang von Hämatit zu Magnetit. Dieser konnte als durch einen Überschuss an Fehlstellen und die damit verbundene Gitterverzerrung verursachte Strukturänderung beschrieben werden. Die Aufnahme von transienten Absorptionsspektren an Magnetit-Pulvern bestätigt die Rolle der Defekte, erzeugt durch die Lasereinwirkung. Schließlich konnten zwei Arten des Ladungstransfers in Abhängigkeit von der Sauerstoff-Konzentration beobachtet und über mathematische Auflösung des Spektrums den einzelnen Banden zugeordnet werden. Die Übergänge entsprechen dabei dem Lochtrapping an Eisen-Kationen sowie an Sauerstoff-Anionen. Ein dritter Übergang entspricht den von der Sauerstoff-Konzentration abhängigen Fehlstellen.During the last decade, the synthesis of new materials for photoelectrochemical water splitting has gained increased attention. Understanding on the fundamental reaction mechanisms is of high importance for efficiency enhancement and design of such materials. Hematite is among the most well known materials for photoelectrochemical water splitting and its reaction mechanisms are well researched. However, there are still open questions concerning the nature of the charge carriers created upon irradiation and its recombination kinetics. Transient absorption spectroscopy via laser flash photolysis is a well known and widely used technique to investigate fundamental charge carrier dynamics. In the present work, the nature of photogenerated charge carriers in commercial and self prepared hematite powders was investigated via Nanosecond Laser Flash Photolysis Spectroscopy in diffuse reflectance mode. The kinetic dependency of the transients from oxygen concentration on the surface was studied via variation of surrounding atmosphere (argon, nitrogen, air, oxygen, ozone). Increasing oxygen concentration was found to lead to shorter lifetimes of the trapped char- ge carriers. A comparison with thermally pre-treated samples revealed a correlation of oxygen concentration and the number of defects on the cristallite surface. A reconstruction of the transient absorption spectrum allowed to assign signals in the wavelength region of 550-850 nm to trapped holes of different species. With high output energy during laser irradiation, irreversible structural changes could be observed on powdered samples. Characterization via X-ray diffractometry reveals phase change of hematite to magnetite. This change could be described as structural change caused by an excess of defects and resulting lattice distortion. Transient absorption spectra recorded from magnetite powdered samples confirmed the role of defects created by laser exposure. Finally, two different ways of charge transfer, dependent on oxygen concentration were observed and could be assigned to signals at specific wavelengths via mathematical deconvolution of the absorption spectrum. These correspond to hole trapping at iron-kations and oxygen-anions, respectively. A third charge transfer signal corresponds to oxygen dependent defects

Aptamer-modified polymer nanoparticles for targeted drug delivery

The purpose of this study was to develop a model system for targeted drug delivery. This system should enable targeted drug release at a certain tissue in the body. In conventional drug delivery systems, drugs are often delivered unspecifically resulting in unwarranted adverse effects. To circumvent this problem, there is an increasing demand for the development of intelligent drug delivery systems allowing a tissue-specific mode of delivery. Within this study, nanoparticles consisting of two biocompatible polymers are used. Because of their small size, nanoparticles are well-suited for effective drug delivery. The small size affects their movement through cell and tissue barriers. Their cellular uptake is easier when compared to larger drug delivery systems. Paclitaxel was encapsulated into the nanoparticles as a model drug, and to achieve specific targeting an aptamer directed against lung cancer cells was coupled to the nanoparticles surface. Nanoparticles were characterized by dynamic light scattering (DLS), transmission electron microscopy (TEM), fourier transform infrared spectroscopy (FTIR), and nanotracking analysis (NTA). Also their surface charge was characterized from ζ-potential measurements. Their preparation was optimized and subsequently specificity of drug-loaded and aptamer-functionalized nanoparticles was investigated using lung cancer cells. © 2016 by De Gruyter

Photocatalytic reduction of Cr(VI) on hematite nanoparticles in the presence of oxalate and citrate

Hematite nanoparticles (nHm) were tested for Cr(VI) photocatalytic reduction (300 μM) in the presence of different electron donors such as citrate (Cit), oxalate (Ox), 2-propanol and methanol. At pH 3 and under irradiation at λ ≥ 310 nm, almost negligible reaction took place in the absence of donor or with the alcohols, while the reduction was very rapid in the presence of Cit (less than 25 min), and faster with Ox (15 min). Homogeneous experiments with FeCl3 instead of nHm showed a complete Cr(VI) reduction in the presence of both complexing agents in less than 10 min. Under irradiation at λ > 495 nm and with nHm at pH 3, a good Cr(VI) transformation took place with both donors, but at a considerably lower rate than under UV light (around 100% at 180 min), the decay being negligible in the homogeneous systems with Fe(III). Under irradiation at λ > 610 nm, no Cr(VI) transformation took place over nHm. Experiments at pH 6 under UV–Vis light with Cit in the presence of nHm gave a good Cr(VI) decay, faster with Fe(III) (60% and 90% in 105 min, respectively); some Cr(VI) transformation (around 35% in 180 min) was found with Cit under Vis light. Interestingly, at both wavelength ranges, the reaction was negligible when Ox was used. Mechanisms taking place under the different conditions were proposed, including the role of surface charge transfer complexes on nHm.Fil: Kretschmer, Imme. Leibniz Universitat Hannover; AlemaniaFil: Senn, Alejandro Marcelo. Comisión Nacional de Energía Atómica. Gerencia del Área de Seguridad Nuclear y Ambiente. Gerencia de Química (CAC); Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Meichtry, Jorge Martin. Comisión Nacional de Energía Atómica. Gerencia del Área de Seguridad Nuclear y Ambiente. Gerencia de Química (CAC); Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Custo, Graciela Susana. Comisión Nacional de Energía Atómica. Gerencia del Área de Seguridad Nuclear y Ambiente. Gerencia de Química (CAC); ArgentinaFil: Halac, Emilia Beatriz. Comisión Nacional de Energía Atómica. Gerencia de Área Investigaciones y Aplicaciones No Nucleares. Gerencia Física (CAC). Departamento de Física de la Materia Condensada; Argentina. Universidad Nacional de San Martín. Escuela de Ciencia y Tecnología; ArgentinaFil: Dillert, Ralf. Leibniz Universitat Hannover; AlemaniaFil: Bahnemann, Detlef W.. Leibniz Universitat Hannover; Alemania. Saint-Petersburg State University; RusiaFil: Litter, Marta Irene. Universidad Nacional de San Martín. Instituto de Investigación en Ingeniería Ambiental; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Seguridad Nuclear y Ambiente. Gerencia de Química (CAC); Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Enhanced Photoelectrochemical Water Oxidation on Nanostructured Hematite Photoanodes via p‑CaFe₂O₄/n-Fe₂O₃ Heterojunction Formation

In this paper, nanostructured hematite p-CaFe₂O₄/n-Fe₂O₃ heterojunction photoanodes have been fabricated employing a facile template-less film processing technique by controlling the chemical bath. Anisotropic growth of a β-FeOOH akaganeite film on FTO conductive glass from an aqueous FeCl₃ solution containing CaCl₂ followed by a two-step thermal annealing at 550 and 800 °C induces the formation of a p-CaFe₂O₄/n-Fe₂O₃ heterojunction. The structural, morphological, electronic states, and electrochemical characteristics of the films have been investigated by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and impedance spectroscopy, respectively. The heterojunction photoanode showed 100% higher photocurrent response than that obtained using a bare hematite electrode under simulated 1-sun illumination (100 mW/cm²). The photocurrent enhancement is attributed to the enhanced charge carrier separation and the reduced resistance in the charge transfer across the electrode and the electrolyte as revealed by electrochemical impedance spectroscopy analysis. The modification of the p-CaFe₂O₄/n-Fe₂O₃ heterojunction photoanode with CoPi cocatalyst further facilitates the electron transfer at the electrode/electrolyte interface and thus enhances the photoelectrochemical water oxidation. Since cheap and abundant materials have been employed for the synthesis of the heterojunction photoanode via a simple route, the current results have great importance, both from a scientific and an economical point of view

A Facile Surface Passivation of Hematite Photoanodes with TiO₂ Overlayers for Efficient Solar Water Splitting

The surface modification of semiconductor photoelectrodes with passivation overlayers has recently attracted great attention as an effective strategy to improve the charge-separation and charge-transfer processes across semiconductor–liquid interfaces. It is usually carried out by employing the sophisticated atomic layer deposition technique, which relies on reactive and expensive metalorganic compounds and vacuum processing, both of which are significant obstacles toward large-scale applications. In this paper, a facile water-based solution method has been developed for the modification of nanostructured hematite photoanode with TiO₂ overlayers using a water-soluble titanium complex (i.e., titanium bis­(ammonium lactate) dihydroxide, TALH). The thus-fabricated nanostructured hematite photoanodes have been characterized by X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy. Photoelectrochemical measurements indicated that a nanostructured hematite photoanodes modified with a TiO₂ overlayer exhibited a photocurrent response ca. 4.5 times higher (i.e., 1.2 mA cm^–2 vs RHE) than that obtained on the bare hematite photoanode (i.e., 0.27 mA cm^–2 vs RHE) measured under standard illumination conditions. Moreover, a cathodic shift of ca. 190 mV in the water oxidation onset potential was achieved. These results are discussed and explored on the basis of steady-state polarization, transient photocurrent response, open-circuit potential, intensity-modulated photocurrent spectroscopy, and impedance spectroscopy measurements. It is concluded that the TiO₂ overlayer passivates the surface states and suppresses the surface electron–hole recombination, thus increasing the generated photovoltage and the band bending. The present method for the hematite electrode modification with a TiO₂ overlayer is effective and simple and might find broad applications in the development of stable and high-performance photoelectrodes

One-pot aqueous synthesis of highly strained CdTe/CdS/ZnS nanocrystals and their interactions with cells

In this work, a very simple one-pot synthetic approach was developed to generate aqueous CdTe/CdS/ZnS type-II/type-I red-emitting nanocrystals (NCs). Strain-induced optical properties of CdTe/CdS particles having core((small))/shell((thick)) structure with a maximum quantum yield (QY(max)) similar to 57% were further improved with the overgrowth of a ZnS shell, resulting in a core((small))/shell((thick))/shell((small)) structure (QY(max) similar to 64%). The spectral properties were tuned further to the near-infrared region as the ZnS shell grew in thickness. X-ray powder diffraction (XRD) analysis and high-resolution transmission electron microscope (HRTEM) images showed the crystalline structure of NCs proving the epitaxial growth of ZnS without crystalline defects. Under continuous UV-irradiation for 5 h, the NCs did not exhibit any photo-degradation but instead displayed a photo-annealing process. These extremely photostable NCs were further characterized in terms of their cytotoxicity and their cell labeling performances. The presence of a ZnS shell was found to reduce the toxicity of the CdTe/CdS NCs. Furthermore, aptamer-conjugated NCs were successfully utilized in targeted cell imaging. Promisingly, the aptamer-NCs bioconjugates were internalized by A549 cells within 2 hours of incubation and retained their fluorescence even after 24 hours of internalization.DAA