Nanometer-thick hematite films as photoanodes for solar water splitting. - In: ACS applied nano materials, ISSN 2574-0970, Bd. 5 (2022), 2, S. 2897-2905
Photoelectrochemical (PEC) water splitting is one of the most promising sustainable methods for feasible solar hydrogen production. However, this method is still impractical due to the lack of suitable photoanode materials that are efficient, stable, and cost-effective. Here, we present a surprisingly simple fabrication method for efficient, stable, and cost-effective nanometer-thick hematite films utilizing a rapid, ambient annealing approach. In the oxygen evolution reaction, the fabricated hematite films exhibit a Faradaic efficiency of 99.8% already at 1 V versus the reversible hydrogen electrode (RHE), a real photocurrent density of 2.35 mA cm-2 at 1.23 V versus RHE, and a superior photo-oxidation stability recorded for over 1000 h. Considering the active surface area, the measured photocurrent density is higher than any value achieved so far by hematite and other single-material thin-film photoanodes. Hence, we show for the first time that undoped hematite thin films can compete with doped hematite and other semiconductor materials.
https://doi.org/10.1021/acsanm.2c00095
Localized surface states influence in the photoelectrocatalytic performance of Al doped a-SiC:H based photocathodes. - In: Materials science in semiconductor processing, ISSN 1873-4081, Bd. 143 (2022), 106474, insges. 10 S.
https://doi.org/10.1016/j.mssp.2022.106474
An experimental test of the Mocikat-Herwig theory of local turbulent heat transfer measurements on cold objects. - In: Heat and mass transfer, ISSN 1432-1181, Bd. 58 (2022), 6, S. 1041-1055
Experimental results are presented of a test of the theory of local turbulent heat transfer measurements proposed by Mocikat and Herwig in 2007. A miniaturized multi-layer heat transfer sensor was developed and employed in this study. The new heat transfer sensor was designed to work in air and liquids, and this capability enabled the simultaneous investigation of different Prandtl numbers. Two basic configurations, namely the flow past a blunt plate and the flow past an inclined square cylinder, were investigated in test sections of wind and water tunnels. Convective heat transfer coefficients were obtained through conventional testing (i.e., employing thoroughly heated test objects) and using the new miniaturized sensor approach (i.e., utilizing cold test objects without heating). The main prediction of the Mocikat-Herwig theory that a specific thermal adjustment coefficient of the employed actual miniaturized heat transfer sensor should exist in the fully turbulent flow regime was proven for developed two-dimensional flow. The observed effect of the Prandtl number on this coefficient was in good agreement with the prediction of the asymptotic expansion method. The square cylinder results indicated the inherent limits of the local turbulent heat transfer measurement approach, as suggested by Mocikat and Herwig.
https://doi.org/10.1007/s00231-021-03158-y
Selective metallization of polymers: surface activation of polybutylene terephthalate (PBT) assisted by picosecond laser pulses. - In: Advanced engineering materials, ISSN 1527-2648, Bd. 24 (2022), 4, 2100933, S. 1-15
https://doi.org/10.1002/adem.202100933
Hematite nanowire and nanoflake-decorated photoelectrodes: implications for photoelectrochemical water splitting. - In: ACS applied nano materials, ISSN 2574-0970, Bd. 5 (2022), 1, S. 1016-1022
Hematite, a low-cost, nontoxic, and earth-abundant n-type semiconductor, is still an intriguing photoanode material for photoelectrochemical (PEC) water splitting. Nevertheless, the PEC performance of hematite is still hindered by ultrafast recombination rates or short diffusion lengths of charge carriers. Therefore, nanostructure implementation has been proposed in this and other cases to overcome this limitation, while simultaneously improving the photon harvesting efficiency. However, this approach must be critically reviewed. We show that both, hematite nanowire- and nanoflake-decorated photoelectrodes, show a low PEC performance in a NaOH (1 M) electrolyte. Reproducible nanostructure synthesis was achieved by the thermal oxidation of low-cost steel foils using only ambient air. Full absolute-energy reconstruction under ambient conditions of the electronic surface band structure of these nanostructured surfaces showed distinct Fermi-level pinning, resulting in high recombination rates. Based on our results, we can conclude that unmodified nanostructures hardly improve the performance but suffer from the lack of internal electrical splitting fields, which suppresses the electron-hole pair separation and can thus actually decrease the performance of PEC electrodes.
https://doi.org/10.1021/acsanm.1c03684
Ferrimagnetic large single domain iron oxide nanoparticles for hyperthermia applications. - In: Nanomaterials, ISSN 2079-4991, Bd. 12 (2022), 3, 343, S. 1-12
This paper describes the preparation and obtained magnetic properties of large single domain iron oxide nanoparticles. Such ferrimagnetic particles are particularly interesting for diagnostic and therapeutic applications in medicine or (bio)technology. The particles were prepared by a modified oxidation method of non-magnetic precursors following the green rust synthesis and characterized regarding their structural and magnetic properties. For increasing preparation temperatures (5 to 85 ˚C), an increasing particle size in the range of 30 to 60 nm is observed. Magnetic measurements confirm a single domain ferrimagnetic behavior with a mean saturation magnetization of ca. 90 Am2/kg and a size-dependent coercivity in the range of 6 to 15 kA/m. The samples show a specific absorption rate (SAR) of up to 600 W/g, which is promising for magnetic hyperthermia application. For particle preparation temperatures above 45 ˚C, a non-magnetic impurity phase occurs besides the magnetic iron oxides that results in a reduced net saturation magnetization.
https://doi.org/10.3390/nano12030343
Towards green 3D-microfabrication of Bio-MEMS devices using ADEX dry film photoresists. - In: International journal of precision engineering and manufacturing-green technology, ISSN 2198-0810, Bd. 9 (2022), 1, S. 43-57
Current trends in miniaturized diagnostics indicate an increasing demand for large quantities of mobile devices for health monitoring and point-of-care diagnostics. This comes along with a need for rapid but preferably also green microfabrication. Dry film photoresists (DFPs) promise low-cost and greener microfabrication and can partly or fully replace conventional silicon-technologies being associated with high-energy demands and the intense use of toxic and climate-active chemicals. Due to their mechanical stability and superior film thickness homogeneity, DFPs outperform conventional spin-on photoresists, such as SU-8, especially when three-dimensional architectures are required for micro-analytical devices (e.g. microfluidics). In this study, we utilize the commercial epoxy-based DFP ADEX to demonstrate various application scenarios ranging from the direct modification of microcantilever beams via the assembly of microfluidic channels to lamination-free patterning of DFPs, which employs the DFP directly as a substrate material. Finally, kinked, bottom-up grown silicon nanowires were integrated in this manner as prospective ion-sensitive field-effect transistors in a bio-probe architecture directly on ADEX substrates. Hence, we have developed the required set of microfabrication protocols for such an assembly comprising metal thin film deposition, direct burn-in of lithography alignment markers, and polymer patterning on top of the DFP.
https://doi.org/10.1007/s40684-021-00367-y
Effect of SiO2 interlayer thickness in Au/SiO2/Si multilayer systems on Si sources and the formation of Au-based nanostructures. - In: Advanced materials interfaces, ISSN 2196-7350, Bd. 9 (2022), 2, 2101493, insges. 9 S.
Si sources involved in the growth of Au-SiOx nanostructures are investigated through the rapid thermal annealing of gold thin films on SiO2/Si substrates with various SiO2 layer thicknesses (3, 25, 100, 500 nm) in a reducing atmosphere. This method reveals three Si sources whose involvement depends on the thickness of the SiO2 layers, i.e., Si diffusion from the substrate, and SiO from SiO2 decomposition and from Si active oxidation. Increasing thicknesses of the SiO2 layer hampers the Si diffusion and the decomposition of regions of the SiO2 layer, which decreases the concentrations of discovered regions weakening the Si active oxidation. These discovered regions appear in systems with a SiO2 layer of 25 or 100 nm, while they are absent for a 500 nm layer. Furthermore, Au-SiOx nanostructures of different shapes form in each system. Both behaviors indicate that the influence and transport mechanisms of the different Si sources are largely dependent on the thicknesses of the SiO2 layers and that they control the evolution of the Au-SiOx nanostructures. A clear understanding of the relationship between these thicknesses and the possible Si sources and their roles in the evolution of the nanostructures makes the tailored fabrication of nanostructures possible.
https://doi.org/10.1002/admi.202101493
Yttrium and rare-earth modified lithium orthoborates: glass formation and vibrational activity. - In: Journal of non-crystalline solids, ISSN 0022-3093, Bd. 575 (2022), 121152
Glass formation and structure-property relations were explored in highly modified borate glasses containing high loads of rare-earth elements, whose crystalline analogues display a trigonal to tetrahedral borate phase transition (BO33- -> BØ2O23-, where Ø and O- indicate bridging and non-bridging oxygen atoms). The resulting borate networks are completely depolymerized, where borate anions are crosslinked to rare-earth and modifier cations via ionic bonds. The borate structure was found to be based on a single structural unit, BO33- triangles, whose fundamental vibrations are all active in both the Raman and infrared. The local environment of the rare-earth ions in orthoborate glasses was studied with far infrared spectroscopy and, in some cases, by using terbium as a probe ion. A linear correlation was obtained between the effective force constant in the far infrared and the field strength of the rare-earth cation.
https://doi.org/10.1016/j.jnoncrysol.2021.121152
Luminescence-site symmetry correlations in Dy3+ doped alkali-alkaline earth orthoborates of the type XZBO3 with X = Li, Na, K and Z = Mg, Ca, Ba. - In: Journal of luminescence, ISSN 0022-2313, Bd. 241 (2022), 118429, S. 1-12
A systematic investigation of the luminescence properties of Dy3+ doped alkali-alkaline earth orthoborates of the stoichiometric composition XZBO3 with X = Li, Na, K and Z = Mg, Ca, Ba was conducted. XRD diffractograms show that the compounds LiMgBO3, LiCaBO3, LiBaBO3, NaMgBO3, NaCaBO3, NaBaBO3, and KMgBO3 could be produced in high purity. Relatively intense luminescence was observed only for the phases LiCaBO3, NaMgBO3, NaCaBO3 and NaBaBO3. Micro Raman investigations show that the Dy3+ luminesence mainly originates from the orthoborate phase in these samples. Photo-luminescence spectroscopy of LiCaBO3, NaCaBO3 and NaBaBO3 shows the typical Dy3+ emission with the prominent emission peak in the yellow spectral range around 575 nm. A second, but much less intense peak is observed at around 485 nm (cyan). The luminescence emission spectrum of Dy3+:NaMgBO3 is much different: here, the highest emission intensity is observed at about 485 nm. It is proposed that the Dy3+ ions occupy the Na positions in this crystal phase which has a much higher symmetry than the alkaline earth positions in the other examined crystal phases. This is supported by broadened and more split up peaks in the excitation and emission spectra of Dy3+:NaMgBO3 suggesting a much stronger local crystal field at the rare earth position in this compound. The results are additionally compared to spectroscopic data of different well known Dy3+ doped crystalline compounds and Dy3+ and Eu3+ doped alkali-alkaline earth orthoborates from other publications, which offer further insight into the relation between the crystallographic sites of the doped rare earth ions and their luminescence.
https://doi.org/10.1016/j.jlumin.2021.118429