Henrik Haspel

Low-temperature conversion of titanate nanotubes into nitrogen-doped TiO2 nanoparticles

Hydrothermally synthesized protonated titanate nanotubes were doped with nitrogen using ammonia gas as the dopant. Thermal decomposition of urea, which served as the ammonia source, offered a low-temperature synthesis route for obtaining a potential visible-light photocatalyst. Nitrogen doping could be achieved at as low as 200 °C. The doped samples were calcined at different temperatures and changes in the morphology and crystalline phase were studied by transmission and scanning electron microscopy, selected area electron diffraction, energy-dispersive X-ray spectroscopy and X-ray diffraction. The nitrogen content and calcination temperature were found to affect the size and shape of the particles as well as their crystalline phase to a great extent. H-form trititanate was shown to transform into rutile TiO2 through the anatase phase in parallel with the collapse of the nanotube morphology and the production of rod-like nanoparticles first and then finally round nitrogen-doped nanoparticles. A phase map was constructed from the data to facilitate the rational design of N-doped trititanate nanotube-based nanostructures.

INCREASING CHEMICAL SELECTIVITY OF CARBON NANOTUBE-BASED SENSORS BY FLUCTUATION-ENHANCED SENSING

Nowadays gas detection in the ppm and sub-ppm domain is essential in terms of environmental protection as well as reducing sanitary risks. However, detecting systems to perform these measurements (e.g., gas chromatographs) are expensive and take up too much space, thus their use is not likely to become wide-spread. Small, cheap and easily mountable sensors, such as resistive sensors are more applicable for this purpose. But the main disadvantage of these sensors is the lack of chemical selectivity. Yet, a novel method called fluctuation-enhanced sensing (FES), which considers the sensor noise as the source of chemical information, can be used to improve selectivity. Since carbon nanotube (CNT)-based sensors are regarded as promising devices for FES measurements, we investigated whether stationary fluctuations in output signal (dc-resistance) of a CNT sensor could be used to increase chemical selectivity. In this work we prove that FES is applicable to increase selectivity of CNT sensors: air polluting gases (N2O, NH3 and H2S) and their mixtures can be distinguished. Furthermore, we also show that different concentrations of the same analyte can be differentiated and chemical selectivity can be extended into the sub-ppm region.

Water Types and Their Relaxation Behavior in Partially Rehydrated CaFe-Mixed Binary Oxide Obtained from CaFe-Layered Double Hydroxide in the 155–298 K Temperature Range

Heat-treated CaFe-layered double hydroxide samples were equilibrated under conditions of various relative humidities (11%, 43% and 75%). Measurements by FT-IR and dielectric relaxation spectroscopies revealed that partial to full reconstruction of the layered structure took place. Water types taking part in the reconstruction process were identified via dielectric relaxation measurements either at 298 K or on the flash-cooled (to 155 K) samples. The dynamics of water molecules at the various positions was also studied by this method, allowing the flash-cooled samples to warm up to 298 K.

Structure-Independent Proton Transport in Cerium(III) Phosphate Nanowires.

Understanding of water-related electrical conduction is of utmost importance in applications that utilize solid-state proton conductors. However, in spite of the vast amount of theoretical and experimental work published in the literature, thus far its mechanism remained unsolved. In this study, the structure-related ambient temperature electrical conduction of one-dimensional hydrophilic nanostructures was investigated. Cerium phosphate nanowires with monoclinic and hexagonal crystal structures were synthesized via the hydrothermal and ambient temperature precipitation routes, and their structural and surface properties were examined by using high-resolution transmission electron microscopy, X-ray diffractometry, nitrogen and water sorption, temperature-programmed ammonia desorption, and potentiometric titration techniques. The relative humidity (RH)-dependent charge-transport processes of hexagonal and monoclinic nanowires were investigated by means of impedance spectroscopy and transient ionic current measurement techniques to gain insight into their atomistic level mechanism. Although considerable differences in RH-dependent conductivity were first found, the distinct characteristics collapsed into a master curve when specific surface area and acidity were taken into account, implying structure-independent proton conduction mechanism in both types of nanowires.

Titania nanotube stabilized BiOCl nanoparticles in visible-light photocatalysis

Photocatalysis is a green approach in environmental organic pollutant decomposition. Lately, considerable improvement in the activity of photocatalysts has been achieved with the realization of p–n heterostructures due to the lifetime extension of the photogenerated charge carriers. Herein, we report a facile synthesis approach for decorating n-type titanate nanotubes with p-type V–VI–VII compound semiconductor BiOCl nanoparticles. It is well-known that BiOX (X = Cl, Br, I) materials form nanometer-thick platelets, which can eventually assemble into micrometer size flower-like 3D structures. Here, we demonstrate that the tubular titanate support can stabilize BiOCl on its surface in the form of nanoparticles measuring a few nanometers in diameter, instead of forming the well-known bismuth-oxyhalide nanoflowers. Subsequent calcination at 400 °C transforms the pristine titanate structures into one-dimensional anatase nanotubes, along with the formation of a heterojunction at the interface of the emerging Bi2Ti2O7 and anatase phases. The resulting nanocomposite shows activity in visible-light photocatalytic test reactions.

Water-Induced Changes in the Charge-Transport Dynamics of Titanate Nanowires

The temperature dependence of dielectric processes in humid titanate nanowires was investigated via broadband dielectric spectroscopy under quasi-isosteric conditions in the temperature range of 150-350 K. It was found that the dynamic parameters obtained from low-temperature measurements cannot describe the dielectric behavior of the system above 273 K, implying changes in the dynamics of the corresponding dielectric processes. The calculated activation energies and pre-exponential factors counterintuitively increase linearly with the amount of adsorbed water, and a compensation effect was also found to apply to all contributions in the TiONW spectra.

pH-regulated antimony oxychloride nanoparticle formation on titanium oxide nanostructures: a photocatalytically active heterojunction

Improving the catalytic activity of heterogeneous photocatalysts has become a hot topic recently. To this end, considerable progress has been made in the efficient separation of photogenerated charge carriers by e.g. the realization of heterojunction photocatalysts. V–VI–VII compound semiconductors, namely, bismuth oxyhalides, are popular photocatalysts. However, results on antimony oxyhalides [SbxOyXz (X = Br, Cl, I)], the very promising alternatives to the well-known BixOyXz photomodifiers, are scarce. Here, we report the successful decoration of titanium oxide nanostructures with 8–11 nm diameter SbxOyXz nanoparticles for the first time ever. The product size and stoichiometry could be controlled by the pH of the reactant mixture, while subsequent calcination could transform the structure of the titanate nanotube (TiONT) support and the prepared antimony oxychloride particles. In contrast to the ease of composite formation in the SbxOyXz/TiONT case, anatase TiO2 could not facilitate the formation of antimony oxychloride nanoparticles on its surface. The titanate nanotube-based composites showed activity in a generally accepted quasi-standard photocatalytic test reaction (methyl orange dye decolorization). We found that the SbxOyClz/TiONT synthesized at pH = 1 is the most active sample in a broad temperature range.

Improving the performance of functionalized carbon nanotube thin film sensors by fluctuation enhanced sensing

Thin films of functionalized single-wall carbon nanotubes were deposited on silicon chips by drop-coating and inkjet printing. These sensors were subjected to 1-100 ppm NOx, CO, H2S and H2O vapor in synthetic air. We have found that besides the expected changes in the electrical resistance of the film, there are also characterteristic differences in the noise pattern of the resistance vs. time function. This phenomenon is called fluctuation enhanced sensing and it can be used to increase the amount of information gathered from a carbon nanotube sensor device. The main advantage of fluctuation enhanced sensing is the improved selectivity of the sensor even if changes in electical resistance are rather low. Combined with differentiation based on modifying the adsorption characterstics of the nanotubes (e.g. by covalent functionalization), fluctuation enhanced sensing appears to be a very useful method for bringing cheap and reliable carbon nanotube based chemical sensors to the market.

Vibrational Spectroscopic Studies on the Formation of Ion‐exchangeable Titania Nanotubes

Ion‐exchangeable titanium‐oxide nanotubes have commanded considerable interest from the materials science community in the past five years. Synthesized under hydrothermal conditions from TiO2, typical nanotubes are 150–200 nm long and 8–20 nm wide. High resolution TEM images revealed that unlike multiwall carbon nanotubes which are made of coaxial single‐wall nanotubes, the titania tubes possess a spiral cross‐section. An interesting feature of the titania tubes is their considerable ion‐exchange capacity which could be utilized e.g. for enhancing their photocatalytic activity by doping the titania tubes with CdS nanoparticles. In this contribution we present a comprehensive TEM, FT‐Raman and FT‐farIR characterization study of the formation process.