Identifying Ionic and Electronic Charge Transfer at Oxide Heterointerfaces

The ability to tailor oxide heterointerfaces has led to novel properties in low-dimensional oxide systems. A fundamental understanding of these properties is based on the concept of electronic charge transfer. However, the electronic properties of oxide heterointerfaces crucially depend on their ionic constitution and defect structure: ionic charges contribute to charge transfer and screening at oxide interfaces, triggering a thermodynamic balance of ionic and electronic structures. Quantitative understanding of the electronic and ionic roles regarding charge-transfer phenomena poses a central challenge. Here, the electronic and ionic structure is simultaneously investigated at the prototypical charge-transfer heterointerface, LaAlO3/SrTiO3. Applying in situ photoemission spectroscopy under oxygen ambient, ionic and electronic charge transfer is deconvoluted in response to the oxygen atmosphere at elevated temperatures. In this way, both the rich and variable chemistry of complex oxides and the associated electronic properties are equally embraced. The interfacial electron gas is depleted through an ionic rearrangement in the strontium cation sublattice when oxygen is applied, resulting in an inverse and reversible balance between cation vacancies and electrons, while the mobility of ionic species is found to be considerably enhanced as compared to the bulk. Triggered by these ionic phenomena, the electronic transport and magnetic signature of the heterointerface are significantly altered

No signs of inbreeding despite long-term isolation and habitat fragmentation in the critically endangered Montseny brook newt (Calotriton arnoldi)

Endemic species with restricted geographic ranges potentially suffer the highest risk of extinction. If these species are further fragmented into genetically isolated subpopulations, the risk of extinction is elevated. Habitat fragmentation is generally considered to have negative effects on species survival, despite some evidence for neutral or even positive effects. Typically, non-negative effects are ignored by conservation biology. The Montseny brook newt (Calotriton arnoldi) has one of the smallest distribution ranges of any European amphibian (8 km2) and is considered critically endangered by the International Union for Conservation of Nature. Here we apply molecular markers to analyze its population structure and find that habitat fragmentation owing to a natural barrier has resulted in strong genetic division of populations into two sectors, with no detectable migration between sites. Although effective population size estimates suggest low values for all populations, we found low levels of inbreeding and relatedness between individuals within populations. Moreover, C. arnoldi displays similar levels of genetic diversity to its sister species Calotriton asper, from which it separated around 1.5 million years ago and which has a much larger distribution range. Our extensive study shows that natural habitat fragmentation does not result in negative genetic effects, such as the loss of genetic diversity and inbreeding on an evolutionary timescale. We hypothesize that species in such conditions may evolve strategies (for example, special mating preferences) to mitigate the effects of small population sizes. However, it should be stressed that the influence of natural habitat fragmentation on an evolutionary timescale should not be conflated with anthropogenic habitat loss or degradation when considering conservation strategies

CO oxidation on Pt/CeO2 monitored by NAP-XPS and operando DRIFTS

+ING+LCS:FRMInternational audienceContentThe electronic properties of Pt and ceria-based catalysts control the activity for CO conversion at low temperatures for applications relating to energy production (hydrogen fuel cells, H2 being formed through the water gas shift reaction CO + H2O → CO2 + H2) [1] and pollutant removal (by combustion in the presence of O2). Under CO and O2 mixtures a competitive adsorption process take place on Pt/CeO2 determined by the electronic nature of the surface sites [2,3]. Pt single atoms and Pt nanoparticles (NP), oxidized or metallic with varying electronic interaction with ceria have been reported. In order to get meaningful insights into the nature of actives sites it is crucial to keep relevant reaction conditions in terms of CO and O2 pressures by using states-of-the-art in situ characterization methods. We have obtained in situ NAP-XPS and operando diffuse reflectance FT-IR spectroscopy (DRIFTS) data collected over two 5% Pt/CeO2 samples synthetized by two different techniques. NAP-XPS analyses carried out on both samples using Pt 4f core level shows the presence of three different Pt species under reaction conditions (Fig. 1a). One of the species, likely Pt0 associated with Pt atoms located at the core of Pt NP, Pt2+ probably single atoms and oxidised NP corresponding to Pt4+. The DRIFTS data was consistent with NAP-XPS results, by evidencing reversible conversion of oxidized Pt NP (exhibiting no IR band) into reduced Pt NP (characterized by a CO band at 2075 cm-1) upon addition/removal of CO in an oxygen stream (Fig. 1c). An inert species at ca. 2100 cm-1 was also observed, corresponding to CO adsorbed on Pt2+ single atoms. A spectrokinetic analysis indicates that the 2075 cm-1 species was a reaction intermediate quantitatively leading to CO2.References[1]“Quantitative DRIFTS investigation of possible reaction mechanisms for the water–gas shift reaction on high-activity Pt- and Au-based catalysts”, F.C. Meunier, A. Goguet, C. Hardacre, R. Burch, D. Thompsett, Journal of Catalysis, 252 (2007) 18-22.[2]“Velocity-resolved kinetics of site-specific carbon monoxide oxidation on platinum surfaces”, J. Neugebohren, D. Borodin, H. W. Hahn, J. Altschäffel, et al.Nature, 558 (2018).[3]“Tuning Pt-CeO2 interactions by high-temperature vapor-phase synthesis for improved reducibility of lattice oxygen”, X. Isidro Pereira-Hernández, A. DeLaRiva, Valery Muravev, et al. Nature Communications (2019) 10:135