High yield and wide lateral size growth of α-Mo2C: exploring the boundaries of CVD growth of bare MXene analogues.

Synthesis of Mo2C bare MXenes, without surface terminations groups, via chemical vapor deposition (CVD) on metal foils is scientifically a very intriguing crystal growth process, and there are still challenges and limited fundamental understanding to overcome to obtain high yield and wide crystal size lateral growth. Achieving large area coverage via direct growth is scientifically vital to utilize the full potential of their unique properties in different applications. In this study, we sought to expand the boundaries of the current CVD growth approach for Mo2C MXenes and gain insights into the possibilities and limitations of large area growth, with a particular focus on controlling Mo concentration. We report a facile modification of their typical CVD growth protocol and show its influence on the Mo2C synthesis, with growth times spanning up to 3 h. Specifically, prior to initiating the CVD growth process, we introduced a holding step in temperature at 1095 °C. This proved to be beneficial in increasing the Mo concentration on the liquid Cu growth surface. We achieved an average Mo2C crystals coverage of approximately 50% of the growth substrate area, increased tendency of coalescence and merging of individual flakes, and lateral flake sizes up to 170µm wide. To gain deeper understanding into their CVD growth behavior, we conducted a systematic investigation of the effect of several factors, including (i) a holding step time on Mo diffusion rate through molten Cu, (ii) the Cu foil thickness over the Mo foil, and (iii) the CVD growth time. Phase, chemical and microstructural characterization by x-ray diffraction, x-ray photon spectroscopy, SEM and scanning/transmission electron microscopy revealed that the grown crystals are single phaseα-Mo2C. Furthermore, insights gained from this study sheds light on crucial factors and inherent limitations that are essential to consider and may help guide future research progress in CVD growth of bare MXenes.

Microstructure and Salt Fog Corrosion of Wrought Mg-Al-Zn and Mg-RE Alloys.

Wrought magnesium alloys have received attention due to their potential application as lightweight materials. However, their use is limited by their poor corrosion resistance. Rare earth additions have the potential to enhance corrosion resistance. The present work included a microstructural investigation and corrosion testing of the alloy WE-43, containing Nd and Y, which was compared against the more conventional compositions of AZ31 and AZ61 alloys. All three alloys exhibited a recrystallized equiaxed structure after hot rolling with the presence of second phases-precipitates. The WE-43 alloy exhibited a better corrosion resistance than AZ31 and AZ61 under salt fog testing, indicated by the lower depth of attack and lower weight loss. The second phases in the microstructure of AZ31 and AZ61 alloys determined their corrosion resistance. The second phases in the AZ31 and AZ61 alloys (based on Al-Mg and Al-Mn phases) were nobler than the Mg matrix and catholically acted, thus sacrificing the Mg matrix. The superior corrosion resistance of WE43 was due to the incorporation of Y in the oxide/hydroxide film. In addition, the second phases in the WE43 consisted of Nd and Y and were less noble than the Mg-matrix. Thus, they acted as anodic sites protecting the Mg-matrix. The above results show the beneficial effect of rare earth additions to wrought Mg alloys towards increased corrosion resistance.

Zn Redistribution and Volatility in ZnZrOx Catalysts for CO2 Hydrogenation.

ZnO-ZrO2 mixed oxide (ZnZrOx) catalysts are widely studied as selective catalysts for CO2 hydrogenation into methanol at high-temperature conditions (300-350 °C) that are preferred for the subsequent in situ zeolite-catalyzed conversion of methanol into hydrocarbons in a tandem process. Zn, a key ingredient of these mixed oxide catalysts, is known to volatilize from ZnO under high-temperature conditions, but little is known about Zn mobility and volatility in mixed oxides. Here, an array of ex situ and in situ characterization techniques (scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDX), transmission electron microscopy (TEM), powder X-ray diffraction (PXRD), X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), Infrared (IR)) was used to reveal that Zn2+ species are mobile between the solid solution phase with ZrO2 and segregated and/or embedded ZnO clusters. Upon reductive heat treatments, partially reversible ZnO cluster growth was observed above 250 °C and eventual Zn evaporation above 550 °C. Extensive Zn evaporation leads to catalyst deactivation and methanol selectivity decline in CO2 hydrogenation. These findings extend the fundamental knowledge of Zn-containing mixed oxide catalysts and are highly relevant for the CO2-to-hydrocarbon process optimization.

Galvanic Deposition of Pt Nanoparticles on Black TiO2 Nanotubes for Hydrogen Evolving Cathodes.

A galvanic deposition method for the in-situ formation of Pt nanoparticles (NPs) on top and inner surfaces of high-aspect-ratio black TiO2 -nanotube electrodes (bTNTs) for true utilization of their total surface area has been developed. Density functional theory calculations indicated that the deposition of Pt NPs was favored on bTNTs with a preferred [004] orientation and a deposition mechanism occurring via oxygen vacancies, where electrons were localized. High-resolution transmission electron microscopy images revealed a graded deposition of Pt NPs with an average diameter of around 2.5 nm along the complete nanotube axis (length/pore diameter of 130 : 1). Hydrogen evolution reaction (HER) studies in acidic electrolytes showed comparable results to bulk Pt (per geometric area) and Pt/C commercial catalysts (per mg of Pt). The presented novel HER cathodes of minimal engineering and low noble metal loadings (µg cm-2 range) achieved low Tafel slopes (30-34 mV dec-1 ) and high stability in acidic conditions. This study provides important insights for the in-situ formation and deposition of NPs in high-aspect-ratio structures for energy applications.

Valence charge distribution in homogenous silicon-aluminium thin-films.

Homogenous aSi1-x Al x H y alloyed thin films, made by magnetron sputtering, has been found to exhibit tunable band gap and dielectric constant depending on their composition. The optical properties of alloys are largely defined by their electronic structure, which is is strongly influenced by interatomic charge transfer. In this work we have quantified interatomic charge transfer between Si, Al and H in aSi1-x Al x H y thin-films, with [Formula: see text] and [Formula: see text]. Charge transfer was found experimentally using x-ray photoelectron spectroscopy, by incorporating Auger parameter data into the Thomas and Weightman model. Both the perfect and imperfect screening models were tested, and the results were compared to models calculated using density functional theory based molecular dynamics. Using imperfect screening properties of Si and Al resulted in an excellent agreement between the experimental and computational results. Alloying aSi with Al is associated with donation of electrons from Al to Si for y = 0. For y > 0 electrons are transferred away from both Al and Si. The change in Si valence charge increases linearly with increasing band gap and decreasing dielectric constant. These relationships can be used as a quick guide for the evaluation of the Si valence charge and subsequently optoelectronic properties, at specific Al/Si ratios.

Formation of nanoporous Si upon self-organized growth of Al and Si nanostructures.

Nanostructured materials offer unique electronic and optical properties compared to their bulk counterparts. The challenging part of the synthesis is to create a balance between the control of design, size limitations, up-scalability and contamination. In this work we show that self-organized Al nanowires in amorphous Si can be produced at room temperature by magnetron co-sputtering using two individual targets. Nanoporous Si, containing nanotunnels with dimensions within the quantum confinement regime, were then made by selective etching of Al. The material properties, film growth, and composition of the films were investigated for different compositions. In addition, the reflectance of the etched film has been measured.

Improving carrier transport in Cu2O thin films by rapid thermal annealing.

Cuprous oxide (Cu2O) is a promising material for large scale photovoltaic applications. The efficiencies of thin film structures are, however, currently lower than those for structures based on Cu2O sheets, possibly due to their poorer transport properties. This study shows that post-deposition rapid thermal annealing (RTA) of Cu2O films is an effective approach for improving carrier transport in films prepared by reactive magnetron sputtering. The as-deposited Cu2O films were poly-crystalline, p-type, with weak near band edge (NBE) emission in photoluminescence spectra, a grain size of ~100 nm and a hole mobility of 2-18 cm2 V-1 s-1. Subsequent RTA (3 min) at a pressure of 50 Pa and temperatures of 600-1000 °C enhanced the NBE by 2-3 orders of magnitude, evidencing improved crystalline quality and reduction of non-radiative carrier recombination. Both grain size and hole mobility were increased considerably upon RTA, reaching values above 1 µm and up to 58 cm2 V-1 s-1, respectively, for films annealed at 900-1000 °C. These films also exhibited a resistivity of ~50-200 Ω cm, a hole concentration of ~1015 cm-3 at room temperature, and a transmittance above 80%.

Electronic structure of thermoelectric Zn-Sb.

The electronic structures of the two main compounds of the binary zinc antimonides that are stable at room temperature, Zn(1)Sb(1) and ß-Zn(4)Sb(3), were probed with x-ray photoelectron spectroscopy. Additionally, electron energy loss measurements and density functional theory calculations are presented. The compounds are found to share a very similar electronic structure. They both feature only small charge transfers and differ moderately in their screening potentials. These results are in line with recent theoretical works on the Zn-Sb system and are discussed in light of the reported thermoelectric performance of the materials.