M. Hoppe

Synthesis, characterization and DFT studies of zinc-doped copper oxide nanocrystals for gas sensing applications

Due to their unique properties, p-type copper oxide nanostructures have demonstrated promising potential for various applications, especially for the detection of ethanol vapour and other volatile organic compounds (VOCs). In this work a simple and cost-effective synthesis from chemical solutions (SCS) at low temperatures (≤80 °C) and rapid thermal annealing (RTA) process were used to grow zinc-doped copper oxide (ZnxCu1−xOy) nanostructures. The structural, morphological, vibrational, chemical, electronic and sensorial characteristics of ZnxCu1−xOy nanocrystallite layers obtained by using such an efficient approach based on both, the SCS and RTA processes, have been studied. The investigations demonstrated the possibility to tune sensitivity from VOC to H2, as well as an improved response and high selectivity with respect to hydrogen gas for ZnxCu1−xOy nano-crystalline thin films with x = 0.03. Density functional theory calculations showed that the charge transfer together with changes in the Fermi level facilitate H2 gas sensing, which is further enhanced by Zn doping. Hydrogen gas sensing with a high response and selectivity using p-type hybrid semiconductor nanostructures has been reported. An improved stability in humid air was observed by exposure of doped samples to rapid thermal annealing process for the first time. The experimental and calculation results provide an alternative to sensitive and selective detection of ethanol and hydrogen gases, which would be of particular benefit in the area of public security, industrial and environmental applications.

H2 gas sensing properties of a ZnO/CuO and ZnO/CuO/Cu2O Heterostructures

The most important parameters of gas sensors are sensitivity and especially high selectivity to specific chemical species. To improve these parameters we developed sensor structures based on layered semiconducting oxides, namely CuO/Cu2O, CuO:Zn/Cu2O:Zn, NiO/ZnO. In this work, the ZnO/CuxO (where x = 1, 2) bi-layer heterostructure were grown via a simple synthesis from chemical solution (SCS) at relatively low temperatures (< 95 °C), representing a combination of layered n-type and p-type semiconducting oxides which are widely used as sensing material for gas sensors. The main advantages of the developed device structures are given by simplicity of the synthesis and technological cost-efficiency. Structural investigations showed high crystallinity of synthesized layers confirming the presence of zinc oxide nanostructures on the surface of the copper oxide film deposited on glass substrate. Structural changes in morphology of grown nanostructures induced by post-grown thermal annealing were observed by scanning electron microscopy (SEM) investigations, and were studied in detail. The influence of thermal annealing type on the optical properties was also investigated. As an example of practical applications, the ZnO/CuxO bi-layer heterojunctions and ZnO/CuO/Cu2O three-layered structures were integrated into sensor structures and were tested to different types of reducing gases at different operating temperatures (OPT), showing promising results for fabrication of selective gas sensors.

Functional Ecofriendly Coatings for Marine Applications

The worldwide increasing number of off-shore wind turbines brings along the need for innovative polymer coatings which on the one hand can withstand the extreme forces acting on the rotor blades and on the other hand can provide adequate antifouling properties to counter biocorrosion of the basement. The development of environmentally friendly and commercially realizable coating systems is most important since the prohibition of tributyltin (TBT)-based antifouling coatings in 2008. In this study an alternative polymer/particle composite coating based on polythiurethan (PTU) with tetrapodal shaped ZnO (t-ZnO) as additive is investigated with respect to chemical, mechanical and antifouling properties. Overall, four different composites with filler amounts of 0 wt% t-ZnO, 1 wt% t-ZnO, 5 wt% t-ZnO, 10 wt% t-ZnO were tested. Preliminary antifouling experiments were carried out under artificial habitat conditions at an aquarium, imitating Pacific Sea water. For mechanical and chemical characterization tensile tests, adhesion tests and contact angle measurements were performed. An optimum of mechanical and antifouling properties were found for a filler amount of 5 wt% t-ZnO in PTU.

Detectors based on Pd-doped and PdO-functionalized ZnO nanostructures

In this work, zinc oxide (ZnO) nanostructured films were grown using a simple synthesis from chemical solutions (SCS) approach from aqueous baths at relatively low temperatures (< 95 °C). The samples were doped with Pd (0.17 at% Pd) and functionalized with PdO nanoparticles (NPs) using the PdCl2 aqueous solution and subsequent thermal annealing at 650 °C for 30 min. The morphological, micro-Raman and optical properties of Pd modified samples were investigated in detail and were demonstrated to have high crystallinity. Gas sensing studies unveiled that compared to pure ZnO films, the Pd-doped ZnO (ZnO:Pd) nanostructured films showed a decrease in ethanol vapor response and slight increase in H2 response with low selectivity. However, the PdO-functionalized samples showed excellent H2 gas sensing properties with possibility to detect H2 gas even at room temperature (gas response of ~ 2). Up to 200 °C operating temperature the samples are highly selective to H2 gas, with highest response of ~ 12 at 150 °C. This study demonstrates that surface functionalization of n-ZnO nanostructured films with p-type oxides is very important for improvement of gas sensing properties.

Flame based growth of ZnO nano- and microstructures for advanced optical, multifunctional devices, and biomedical applications (Conference Presentation)

The recent flame based growth strategy offers a simple and versatile fabrication of various (one, two, and three-dimensional) nano- and microstructures from different metal oxides (ZnO, SnO2, Fe2O3, etc.) in a desired manner.[1] ZnO structures ranging from nanoscales wires to macroscopic and highly porous 3D interconnected tetrapod networks have been successfully synthesized, characterized and utilized for various applications. The ZnO micro- and nanoneedles grown at walls in silicon trenches showed excellent whispering gallery mode resonances and photocatalytic properties.[2] Using the same strategy, large polycrystalline micro- and nanostructured ZnO platelets can be grown with grains interconnected together via grain boundaries and these grain boundaries exhibit a higher conductivity as compared to individual grains.[3] This flame transport synthesis (FTS) approach offers the growth of a large amount of ZnO tetrapods which have shown interesting applications because of their 3D spatial shape and micro-and nanoscale size, for example, interconnected tetrapods based devices for UV-detection and gas sensing.[4-5] Because of their complex 3D shape, ZnO tetrapods can be used as efficient filler particles for designing self-reporting,[6] and other interesting composites. The nanostructured materials exhibit an important role with respect to advanced biomedical applications as grown ZnO structures have shown strong potentials for antiviral applications.[7] Being mechanically strong and micro-and nanoscale in dimensions, these ZnO tetrapods can be easily doped with other elements or hybridized with various nanoparticles in form of hybrid ZnO tetrapods which are suitable for various multifunctional applications, for example, these hybrid tetrapods showed improved gas sensing properties.[8] The sacrificial nature of ZnO allows the for growth of new tetrapods and 3D network materials for various advanced applications, for example, highly porous and ultra light carbon based Aerographite materials[9] and hollow silicon tetrapods.[10] These carbon based highly porous network can be further utilized for growth of new hybrid 3D nanomaterials, for example, Aerographite- GaN[11] and Aerographite-ZnO[12] for advanced optical and other applications.

Photocatalytic Applications of Doped Zinc Oxide Porous Films Grown by Magnetron Sputtering

Undoped, Al-doped and Sn-doped ZnO was sputter-deposited on glass substrates and on trenched Si substrates and some properties of these samples were evaluated. It was shown that the photocatalytic performance of the thin layers is improved by doping with Sn and Al. The samples doped with Al-doping provided a more pronounced effect compared to the control sample, as well as compared to the greater volume of undoped ZnO deposited on Si substrates. Morphology of produced particles was studied using transmission electron microscopy. Electron diffraction patterns were taken to confirm lattice parameters of the materials. Size and shape variations were monitored for un-doped, Al-doped and Sn-doped ZnO. EDX spectroscopy was utilized to assay chemical composition for doped and un-doped samples.

Properties of ZnO:Fe nanostructured films grown by successive chemical synthesis

Rapid progress of nanotechnology in recent years and continuous minimization of portable devices created the necessity of multifunctional devices elaboration. In this work, the morphological, structural and sensing properties of ZnO : Fenanostructured films are presented, namelyultraviolet and ethanol gas sensing properties, demonstrating multifunctionality of the fabricated devicestructures. The Fedoped ZnO nanostructured films were synthesized by a simple and cost-efficient method from aqueous solutions at relatively low temperatures (<; 90 °C), which allowsFe-doping of ZnO nanostructures directly in a synthesis process. Presented results demonstrate the perspectives to integrate the ZnO : Fe nanostructured films into multifunctional devices.

Micro-nano-technologies of zinc and copper oxides for sensor and medicine applications

The combination of sensors and biosensors with medicine and life science promises to yield extremely innovative and revolutionary advances in healthcare. In this work we report on micro- and nano-technologies for copper and zinc oxides crystalline structures. The detailed morphological study of Sn-doped ZnO nanostructured films and CuO nanowire (NW) networks for gas sensing and medicine applications are presented. ZnO based devices demonstrated good hydrogen response (Rair/Rgas ~ 3.4 to 50 ppm) with fast response and recovery times (2.7 s and 6.1 s, respectively) at operating temperature of 250 °C. In the case of CuO NW networks was observed an ethanol response (Rgas/Rair ~ 2.8 to 50 ppm) at the same operating temperature. Both sets of samples showed excellent repeatability and stability with complete recovery to initial baseline. Reported results serves as the basis for further investigations in field of biosensors and integration in biochips.