Combined XPS and first principle study of metastable Mg–Ti thin films

Jensen, Ingvild Julie Thue; Løvvik, Ole Martin; Schreuders, Herman; Dam, B.; Diplas, Spyros

doi:10.1002/sia.4847

Cited by 6 publications

(4 citation statements)

References 10 publications

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“…Jensen et al have calculated the mixing enthalpies for Mg x Ti 1– x in the range of 0.0156 ≤ x ≤ 0.9844 both for quasirandom and nanocluster models by density functional theory (DFT) calculations. − They find that Ti segregates into nanometer-sized clusters in Mg-rich compositions while nanometer-sized Mg segregates in the Ti-rich ones. This has been verified for Mg-rich Mg x Ti 1– x thin films prepared by magnetron cosputtering. − In the as-deposited state, these alloys have an HCP structure, similar to the pure Mg and Ti metals. The crystal structure of Mg x Ti 1– x hydrides in the range of x ≥ 0.90 resembles that of the rutile-type MgH 2 BCT phase while for x ≤ 0.87 a fluorite-type TiH 2 face centered cubic (FCC) phase is found .…”

Section: Introductionmentioning

confidence: 99%

Destabilization of Mg Hydride by Self-Organized Nanoclusters in the Immiscible Mg–Ti System

et al. 2015

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Mg is an attractive hydrogen storage material not only because of its high gravimetric and volumetric hydrogen capacities but also because of it low material costs. However, the hydride of MgH 2 is too stable to release hydrogen under moderate conditions. We demonstrate that the formation of nanometer-sized clusters of Mg reduces the stability of MgH 2 by the interface energy effect in the immiscible Mg−Ti system. Ti-rich Mg x Ti 1−x (x < 0.5) thin films deposited by magnetron sputtering have a hexagonal close packed (HCP) structure, which forms a face-centered cubic (FCC) hydride phase upon hydrogenation. Positron Doppler broadening depth profiling demonstrates that after hydrogenation, nanometer-sized MgH 2 clusters are formed which are coherently embedded in an FCC TiH 2 matrix. The P (pressure)−T (optical transmission) isotherms measured by hydrogenography show that these MgH 2 clusters are destabilized. This indicates that the formation of nanometer-sized Mg allows for the development of a lightweight and cheap hydrogen storage material with a lower desorption temperature. ■ INTRODUCTIONMg is one of the typical lightweight metals with a density of 1.74 g cm −3 , which is much less than that of many transition and rare earth metals. 1 Mg has a hexagonal close-packed (HCP) structure, and that forms the hydride phase MgH 2 by hydrogenation. MgH 2 has a rutile-type body-centered tetragonal (BCT) structure (α-MgH 2 ) at moderate temperatures and pressures. 2,3 The gravimetric hydrogen capacity is 7.6 mass %, which is higher than most metal hydrides. The volumetric hydrogen capacity of 109 g-H 2 l −1 corresponds to 2.9 times the gaseous hydrogen concentration under a pressure of 70 MPa and 1.6 times the one of liquid hydrogen at 20 K. 4 Mg is one of the most attractive hydrogen storage materials not only because of the high capacity but also the low material costs. However, MgH 2 is too stable to desorb hydrogen at moderate temperatures and pressures and thus unsuited for practical applications. The enthalpy for hydride formation is −75 kJ mol −1 -H 2 , 2,3 which corresponds to a dehydrogenation temperature of around 550 K under a hydrogen pressure of 0.1 MPa. Furthermore, the slow reaction kinetics for hydrogenation and dehydrogenation is a disadvantage for using it as hydrogen storage material. The diffusion of hydrogen in both Mg and MgH 2 has been studied, 2,5,6 and the activation energy for hydrogen diffusion in MgH 2 of 140 kJ mol −1 is particularly high. 6 This value is around 2−5 times those in hydrides of LaNi 5 7 and V. 8 A possible way to destabilize MgH 2 and enhance the reaction kinetics is to reduce the size of Mg. It has been calculated that MgH 2 is destabilized by decreasing the size on a nanometer scale. 9−11 The destabilization is remarkable for a MgH 2 cluster with a diameter below 1.3 nm, corresponding to 19 Mg atoms or less. 11 On the basis of these calculations, the dehydrogenation temperature for a MgH 2 cluster of 0.9 nm would be more than 100 K lower than for bulk MgH 2 under a given hydr...

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Section: Introductionmentioning

confidence: 99%

Destabilization of Mg Hydride by Self-Organized Nanoclusters in the Immiscible Mg–Ti System

et al. 2015

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“…This is because it has already been reported in the case of MgTi thin films produced by sputtering and supported by DFT calculations that Ti segregates in the form of nanoclusters in an Mg rich composition. 23 These clusters are also distributed throughout the Mg matrix and Ti does not completely phase separate. Moreover, we reported earlier that Ti act as nuclei for the formation of Mg NPs, since Mg has nucleation difficulty and prefers always to grow on Ti.…”

Section: Resultsmentioning

confidence: 99%

Strategies to initiate and control the nucleation behavior of bimetallic nanoparticles

et al. 2017

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In this work we report strategies to nucleate bimetallic nanoparticles (NPs) made by gas phase synthesis of elements showing difficulty in homogeneous nucleation. It is shown that the nucleation assisted problem of bimetallic NP synthesis can be solved via the following pathways: (i) selecting an element which can itself nucleate and act as a nucleation center for the synthesis of bimetallic NPs; (ii) introducing H or CH as an impurity/trace gas to initiate nucleation during the synthesis of bimetallic NPs. The latter can solve the problem if none of the elements in a bimetallic NP can initiate nucleation. We illustrate the abovementioned strategies for the case of Mg based bimetallic NPs, which are interesting as hydrogen storage materials and exhibit both nucleation and oxidation issues even under ultra-high vacuum conditions. In particular, it is shown that adding H in small proportions favors the formation of a solid solution/alloy structure even in the case of immiscible Mg and Ti, where normally phase separation occurs during synthesis. In addition, we illustrate the possibility of improving the nucleation rate, and controlling the structure and size distribution of bimetallic NPs using H/CH as a reactive/nucleating gas. This is shown to be associated with the dimer bond energies of the various formed species and the vapor pressures of the metals, which are key factors for NP nucleation.

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“…In these conditions and if there is reasonable agreement between XPS and DOS spectra, attention is paid for the shape of PDOS. 25 PDOS spectra were obtained in the PAW formalism from relaxed geometries at the PBE GGA level of theory, 26 and a gaussian broadening of 0.005 Ry was used. There are two irreversible anodic peaks at 1.30 V and 1.75 V for both concentrations since no cathodic peak is observed in the reverse scans.…”

Section: Computational Detailsmentioning

confidence: 99%

“…Following the Koopmans’ theorem, XPS binding energies were approximated by calculated eigenvalues of the neutral system. In these conditions and if there is reasonable agreement between XPS and DOS spectra, attention is paid for the shape of PDOS . PDOS spectra were obtained in the PAW formalism from relaxed geometries at the PBE GGA level of theory, and a Gaussian broadening of 0.005 Ry was used.…”

Section: Experimental Sectionmentioning

confidence: 99%

Electrosynthesis of Poly(alanine)-Like Peptides in Concentrated Alanine Based Electrolytes, Characterization Coupled to DFT Study and Application to pH Proton Receptor

et al. 2014

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The anodic oxidation of concentrated L-alanine on smooth electrodes such as platinum and glassy carbon electrodes was studied. Contrary to the previous studies performed up to now with diluted L-alanine, the electrochemical process generated here results in a completely different situation. The oxidation on smooth platinum was carried out by electrochemical quartz crystal microbalance (EQCM) coupled to cyclic voltammetry technique. The effects of concentration, scan rate and pH (zwitterion at pH=6 and alkaline media at pH=13) on potential values were examined. Glassy carbon and smooth gold electrodes showed the same behavior as on smooth platinum electrode. Spectroscopic analysis such as attenuated total reflectance FT infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS) were used to characterize the resulting thin film coatings. The presence of peptide bonds is clearly highlighted. DFT modelization of poly(alaninate) strand on Pt(001) coupled to spectroscopic measurements are in favor of L-alanine electropolymerization into poly-L-alanine in an irreversible way. Due to its proton receptor behavior, electrosynthesized poly-L-alanine on smooth platinum electrode was tested as a whole solid transducer for pH sensing.

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Combined XPS and first principle study of metastable Mg–Ti thin films

Cited by 6 publications

References 10 publications

Destabilization of Mg Hydride by Self-Organized Nanoclusters in the Immiscible Mg–Ti System

Destabilization of Mg Hydride by Self-Organized Nanoclusters in the Immiscible Mg–Ti System

Strategies to initiate and control the nucleation behavior of bimetallic nanoparticles

Electrosynthesis of Poly(alanine)-Like Peptides in Concentrated Alanine Based Electrolytes, Characterization Coupled to DFT Study and Application to pH Proton Receptor

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