In-situ electron loss spectroscopy reveals surface dehydrogenation of hydrated ceria nanoparticles at elevated temperatures

Ceria (CeO2) exhibits high reversible oxygen storage capacity at intermediate temperatures (500–800 °C) related to an extraordinary and not fully understood reduction of its surfaces. We have investigated pristine and alcohol-dispersed commercially available ceria nanoparticles by in-situ scanning transmission electron microscopy with electron energy loss spectroscopy (STEM-EELS) to examine the dynamic changes during the initial redox reaction process of ceria nanoparticles in an ultra-high vacuum atmosphere using an in-situ heating holder. High spatially resolved EELS was used to estimate the amounts of Ce3+ and Ce4+ in the nanoparticles as a function of temperature, based on the white-line ratios M5/M4 of the EELS spectra. The results show a nm-range thick surface layer rich in Ce3+ on pristine particles prior to heating. During heating, this oxidises to Ce4+. Heating in high vacuum should normally not lead to oxidation, but the observed results can be understood if the surface layer has an oxyhydroxide composition such as CeOOH, which by heating in the vacuum dehydrogenates and hence oxidises to CeO2, a process that requires diffusion of hydrogen only. This process occurred for all samples, but was more pronounced for the particles that were previously dispersed in ethanol. Thermogravimetric analysis (TGA) by heating the pristine powder in dry atmosphere yielded a considerable weight loss confirming the content of hydroxide and probably water in and on the CeO2 particles. The results suggest that CeO2 surfaces are reduced to a layer of oxyhydroxide by hydrogen-containing molecules like water vapour or alcohols.publishedVersio

The Role of Grain Boundary Precipitates during Intergranular Fracture in 6xxx Series Aluminium Alloys

Ringdalen, I.G.; Jensen, I.J.T.; Marioara, C.D.; Friis, J. The Role of Grain Boundary Precipitates during Intergranular Fracture in 6xxx Series Aluminium Alloys. Metals 2021, 11, 894. https://doi.org/ 10.3390/met11060894During ageing, 6xxx aluminium alloys will develop a microstructure characterised by needle-shaped Mg/Si-rich precipitates in the bulk, precipitate-free zones along the grain boundaries and larger Mg/Si-rich precipitates on the grain boundary. Depending on, among other things, the size of the precipitate-free zone, these alloys are prone to intergranular fracture. The role of the grain boundary precipitates during the initiation and propagation of the intergranular fracture is still not fully understood. Transmission Electron Microscopy has been used to characterise the grain boundaries and grain boundary precipitates. The precipitates were found to be of the β′ type surrounded by a layer of U2 structure. The atomic details of relevant interfaces of Al-β′ were characterised for further investigation. Density Functional Theory simulations were performed on the bulk precipitate structures and on the interfaces obtained experimentally. The decohesion energy of these interfaces was calculated and compared to bulk values. In addition, simulated tensile tests were performed in order to find values for the tensile strength σt. The dependence of the interfacial energy and tensile strength of β′ grain boundary precipitates were found to depend on the orientation and type of interface in addition to the amount of defects on the interfacepublishedVersio

Enhanced gas sensing response for 2D α-MoO3 layers: thickness-dependent changes in defect concentration, surface oxygen adsorption, and metal-metal oxide contact

In this study, α-MoO3 two-dimensional (2D) layers and thin films were synthesized using pulsed laser deposition technique. X-ray diffraction and Raman spectroscopy confirm the formation of anisotropic α-MoO3. Atomic Force Microscopy images show the evolution of morphology from 2D layers to oriented crystallite growth with increase in film thickness. Temperature and gas concentration dependent, gas sensing response has been observed to be quite different in 2D layers in comparison to thin film sample. 2D α-MoO3 layers (∼ 6 nm) show higher response of about 25 % at a lower temperature (100 °C); They exhibit lower detection limits (up to 100 ppb) and selectivity towards NO2 gas. Also, 2D layers show p-type gas sensing response and nonlinear current-voltage (I–V) characteristics of metal- metal oxide junctions, in complete contrast to thin film sample, which shows an n-type gas sensing response and linear I–V characteristics. The results have been explained on the basis of lower oxygen defect concentration, enhanced oxygen adsorption at the surface, and metal- α-MoO3 contact favoring hole conduction.acceptedVersio

Plasmonic properties of aluminium nanowires in amorphous silicon

Plasmonic structures can help enhance optical activity in the ultraviolet (UV) region and therefore enhancing photocatalytic reactions and the detection of organic and biological species. Most plasmonic structures are composed of Ag or Au. However, producing structures small enough for optical activity in the UV region has proved difficult. In this study, we demonstrate that aluminium nanowires are an excellent alternative. We investigated the plasmonic properties of the Al nanowires as well as the optoelectronic properties of the surrounding a − Si matrix by combining scanning transmission electron microscopy imaging, electron energy loss spectroscopy and electrodynamic modelling. We have found that the Al nanowires have distinct plasmonic modes in the UV and far UV region, from 0.75 eV to 13 eV. In addition, simulated results found that the size and spacing of the Al nanowires, as well as the embedding material were shown to have a large impact on the type of surface plasmon energies that can be generated in the material. Using electromagnetic modelling, we have identified the modes and illustrated how they could be tuned further.publishedVersio

Enabling Increased Delithiation Rates in Silicon-Based Anodes through Alloying with Phosphorus

The capability of battery materials to deliver not only high lithium storage capacity, but also the ability to operate at high charge/discharge rates is an essential property for development of new batteries. In the present work, the influence on the charge/discharge rate behaviour of substoichiometric concentrations of phosphorus (P) in silicon (Si) nanoparticles was studied. The results revealed an increase in rate capability as a function of the P concentration between 0 and 5.2 at %, particularly during delithiation. The stoichiometry of the nanoparticles was found to strongly affect the formation of the Li3.5Si phase during lithiation. Cyclic stability experiments demonstrated an initial increase in capacity for the SiPx materials. Galvanostatic intermittent titration technique and electrochemical impedance spectroscopy demonstrated the increased lithium diffusivity with inclusion of P. Density functional theory and ab initio molecular dynamics were deployed to provide a rationale for the electrochemical behaviour of SiPx.publishedVersio

Enabling Increased Delithiation Rates in Silicon-Based Anodes through Alloying with Phosphorus

The capability of battery materials to deliver not only high lithium storage capacity, but also the ability to operate at high charge/discharge rates is an essential property for development of new batteries. In the present work, the influence on the charge/discharge rate behaviour of substoichiometric concentrations of phosphorus (P) in silicon (Si) nanoparticles was studied. The results revealed an increase in rate capability as a function of the P concentration between 0 and 5.2 at %, particularly during delithiation. The stoichiometry of the nanoparticles was found to strongly affect the formation of the Li3.5Si phase during lithiation. Cyclic stability experiments demonstrated an initial increase in capacity for the SiPx materials. Galvanostatic intermittent titration technique and electrochemical impedance spectroscopy demonstrated the increased lithium diffusivity with inclusion of P. Density functional theory and ab initio molecular dynamics were deployed to provide a rationale for the electrochemical behaviour of SiPx.publishedVersio

Plasmonic properties of aluminium nanowires in amorphous silicon

Plasmonic structures can help enhance optical activity in the ultraviolet (UV) region and therefore enhancing photocatalytic reactions and the detection of organic and biological species. Most plasmonic structures are composed of Ag or Au. However, producing structures small enough for optical activity in the UV region has proved difficult. In this study, we demonstrate that aluminium nanowires are an excellent alternative. We investigated the plasmonic properties of the Al nanowires as well as the optoelectronic properties of the surrounding aSi matrix by combining scanning transmission electron microscopy (STEM) imaging, electron energy loss spectroscopy (EELS) and electrodynamic modelling. We have found that the Al nanowires have distinct plasmonic modes in the UV and far UV region, from 0.75 eV to 13 eV. In addition, the size and spacing of the Al nanowires, as well as the embedding material were shown to have a large impact on the type of surface plasmons energies that can be generated in the material. Using electromagnetic modelling, we have identified the modes and illustrated how they could be tuned further

Combined experimental and computational study of the meta-stable Mg-Ti-H system

Nanoteknologirevolusjonen er over oss, og i takt med at komponenter og systemer blir stadig mindre vokser behovet for å forstå materialegenskaper helt ned på atom- og elektronskala. Arbeidet i denne doktorgraden gir en smakebit på den kompleksiteten som møter oss når materialer skal skreddersys på denne skalaen. Systemet som ble studert bestod av magnesium (Mg) og titan (Ti) med og uten hydrogen (H). Kombinasjonen av Mg og Ti i tynne filmer har nemlig vist seg å gi materialer med interessante optiske og elektriske egenskaper: Alene er Mg-Ti metallisk med en blank reflekterende overflate, men tilsettes H skifter materialet til å være svart og lysabsorberende. Muligheten for å justere materialegenskapene ved å tilpasse hydrogeninnholdet er interessant i forhold til anvendelser, for eksempel som smarte overflatelag på solfangere. Hvis et materiale skal brukes må det imidlertid også forstås, og Mg-Ti reiser en rekke interessante spørsmål. Mg og Ti er nemlig uløselige grunnstoffer, som betyr at de ikke danner legeringer med hverandre og normalt ikke lar seg blande. De kan likevel tvinges sammen ved å bruke såkalte ikke-likevekststeknikker, som gjør det vanskelig å forutse hvordan Mg- og Ti-atomene er ordnet i forhold til hverandre. I dette prosjektet ble atomskalaberegninger basert på elektrontetthets-funksjonaler (DFT) kombinert med den eksperimentelle teknikken røntgenfotoelektron-spektroskopi (XPS) for å studere hvordan Mg og Ti interagerer både med hverandre og med H. To DFT-modeller ble sammenliknet, en der Ti var distribuert tilfeldig rundt i Mg og en der Ti var samlet i grupper på 10-20 atomer. Ved å sammenlikne dannelsesenergiene og elektronstrukturene til de to modellene fant vi ut at Ti helst ønsker å unngå Mg, men at dannelsen av de små sub-nanogruppene i modellen var tilstrekkelig for at Ti skulle føle at det var separert fra Mg. Mellom Ti-gruppene og Mg ble det dermed dannet en grenseflate, og ved hjelp av DFT-beregningene fant vi ut at denne grenseflaten virket som en felle for H atomer. H atomene fungerte som et elektronskjold mellom Mg og Ti og bidro til å stabilisere systemet. I de eksperimentelle spektrene fra XPS oppdaget vi nye detaljer som kunne forklares ved hjelp av DFT-resultatene og dermed fant vi støtte for at beregningene vi gjorde var relevante i forhold til virkeligheten

Structural studies of hydrides of Zr2Cu, Zr2Pd and LaPtIn

In this work hydrides formed from three different intermetallics compounds, Zr2Cu, Zr2Pd and LaPtIn, were investigated, with focus on hydrogen induced structural changes. The main methods were neutron (PND) and synchrotron powder diffraction (SR-PXD). Zr2Cu absorbs hydrogen up to Zr2CuD4.5, and the saturated hydride structure differs from the tetragonal \cu structure. In this work saturated \cu hydride was synthesized and studied by PND and high resolution SR-PXD. Desorption of saturated Zr2Cu hydride was investigated in situ by SR-PXD. The low decomposition temperature of the system complicated investigation of possible intermediate hydride phases, but the observed changes in unit cell parameters below the decomposition temperature might have indicated the existence of such a phase. A sudden change in behaviour of the cell parameters was observed, consistent with release of hydrogen from Zr3Cu2 coordinated sites (D5). A Zr4Cu2 oxide not previously reported in this system was discovered in the intermetallic samples. The kinetics of the system was found to change on cycling. Zr2Pd absorbs hydrogen up to Zr2PdD~3. Zr2PdD~2 is known to retain the tetragonal Zr2Pd structure, but for hydrogen contents both below and above this the structure was not clear. In this work three Zr2PdDx samples with x below 2, x~2 and x~3 were studied by PND. Saturated Zr2Pd hydride was also studied in situ under hydrogen pressure by PND. Desorption of saturated Zr2Pd hydride was investigated in situ by SR-PXD. Zr2PdDx, x2 the structure transforms. The saturated hydride type structure appeared to be close to the tetragonal intermetallics type structure. From calculations LaPtIn had been predicted to form hydrides with H—H separations shorter than ever seen before. To test this LaPtIn was hydrogenated at pressures up to 182 bar and temperatures up to 400 oC. LaPtIn did not form hydrides with extraordinary short H--H separations under these conditions. At pressure 182 bar and temperature 400 oC LaPtInH0.53 was formed. In this hydride hydrogen was found to occupy the tetrahedral 4h sites, in such a way that voids sharing a common face is avoided. Refined parameters for LaPtInH0.53 was a=7.8229(2) Å, c=4.1647(2) Å and V=220.73(1) Å3. Pt was also attempted partially substituted with Ni, but homogeneous intermetallics samples could not be obtained

The Role of Grain Boundary Precipitates during Intergranular Fracture in 6xxx Series Aluminium Alloys

During ageing, 6xxx aluminium alloys will develop a microstructure characterised by needle-shaped Mg/Si-rich precipitates in the bulk, precipitate-free zones along the grain boundaries and larger Mg/Si-rich precipitates on the grain boundary. Depending on, among other things, the size of the precipitate-free zone, these alloys are prone to intergranular fracture. The role of the grain boundary precipitates during the initiation and propagation of the intergranular fracture is still not fully understood. Transmission Electron Microscopy has been used to characterise the grain boundaries and grain boundary precipitates. The precipitates were found to be of the β′ type surrounded by a layer of U2 structure. The atomic details of relevant interfaces of Al-β′ were characterised for further investigation. Density Functional Theory simulations were performed on the bulk precipitate structures and on the interfaces obtained experimentally. The decohesion energy of these interfaces was calculated and compared to bulk values. In addition, simulated tensile tests were performed in order to find values for the tensile strength σt. The dependence of the interfacial energy and tensile strength of β′ grain boundary precipitates were found to depend on the orientation and type of interface in addition to the amount of defects on the interfac