High-energy resolution X-ray absorption and emission spectroscopy reveals insight into unique selectivity of La-based nanoparticles for CO₂

Abstract

The lanthanum-based materials, due to their layered structure and f-electron configuration, are relevant for electrochemical application. Particularly, La 2 O 2 CO 3 shows a prominent chemoresistive response to CO 2 . However, surprisingly less is known about its atomic and electronic structure and electrochemically significant sites and therefore, its structure-functions relationships have yet to be established. Here we determine the position of the different constituents within the unit cell of monoclinic La 2 O 2 CO 3 and use this information to interpret in situ high-energy resolution fluorescencedetected (HERFD) X-ray absorption near-edge structure (XANES) and valence-to-core X-ray emission spectroscopy (vtc XES). Compared with La(OH) 3 or previously known hexagonal La 2 O 2 CO 3 structures, La in the monoclinic unit cell has a much lower number of neighboring oxygen atoms, which is manifested in the whiteline broadening in XANES spectra. Such a superior sensitivity to subtle changes is given by HERFD method, which is essential for in situ studying of the interaction with CO 2 . Here, we study La 2 O 2 CO 3 -based sensors in real operando conditions at 250°C in the presence of oxygen and water vapors. We identify that the distribution of unoccupied La d-states and occupied O p-and La d-states changes during CO 2 chemoresistive sensing of La 2 O 2 CO 3 . The correlation between these spectroscopic findings with electrical resistance measurements leads to a more comprehensive understanding of the selective adsorption at La site and may enable the design of new materials for CO 2 electrochemical applications.C O 2 has become a challenge for our society and we have to develop new materials for its photo/electrocatalysis, chemoresistive sensing, and storage (1-8). Particularly, for the variety of electrochemical applications the selective interaction of CO 2 and charge transfer with solids is in the foreground. At the same time, the interaction of CO 2 with solids in the electrochemical cell or sensing device is rather complex, thus it remains challenging to experimentally identify the key elements determining their selectivity and efficiency. X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) provide complementary information on the electronic structure of materials (9, 10) and on the orbitals participating in the interaction with absorbing molecules (11). High-energy resolution fluorescence-detected (HERFD) XAS probes unoccupied states with a spectral resolution higher than regular XAS. Furthermore, with the same experimental setup XES can be measured, which allows one to probe the occupied states within the valence band (12). In situ HERFD XAS or XES experiments have been previously carried out to study the catalytic reaction at the surface of noble metals (11, 13-16), zeolites (17), and metal organic frameworks (18). Thus far, no such in situ experiments have been performed to directly track the changes of the electronic structure of a solid and its electrochemical activity toward CO 2...

Structure-Dependent CO2 Gas Sensitivity of La2O2CO3 Thin Films

“…The current through the sample was monitored at the applied constant voltage of 3 V. Previously, it is reported that the sensor structures based on La and Nd oxycarbonate nanorods have high electrical resistivity and that restrict their use in commercial sensors [2][3][4]. In our sensor structures, the resistivity followed the Arrhenius law with an activation energy of 0.56 eV between 250 and 450 ∘ C. It shows that the conductivity is determined by deep trap levels, as the bandgap of the material, as concluded from X-ray absorption spectroscopy and X-ray emission spectroscopy studies by Hirsch et al [6], is approximately 3.7 eV in the case of La 2 O 2 CO 3 nanoparticles. At 250 ∘ C, the resistivity was at the limit of our detection capability as the detected current was slightly below 1 nA.…”

“…According to Hirsch et al [6], CO 2 adsorbing onto monoclinic La 2 O 2 CO 3 interacts directly with La site on the surface and forms surface carbonate at the site of absorption. Adsorbing CO 2 acts as an electron acceptor, thus reducing the concentration of free electrons and causing an increase of resistivity of the sensor material.…”

“…Adsorbing CO 2 acts as an electron acceptor, thus reducing the concentration of free electrons and causing an increase of resistivity of the sensor material. Also, recent studies indicate that surface hydroxyl groups play important role in CO 2 adsorption at La 2 O 2 CO 3 , but exact mechanism and the role of OH groups are not known [4,6].…”

“…Current industrially available CO 2 sensors are relatively complex and expensive, so it is desirable to develop simple and low-cost methods for monitoring CO 2 level [1]. La and Nd oxycarbonates have recently been studied as promising candidates for chemiresistive CO 2 sensor materials [2][3][4][5][6]. However, in these reports, La and Nd oxycarbonates have been studied in a form of nanorods and there are no investigations of thin film deposition of these promising materials for CO 2 gas sensor application.…”

Determination of Conduction and Valence Band Electronic Structure of LaTiOxNyThin Film

“…The Xrays, delivered by bending magnets, were monochromatized with double Si(111)c rystals, and focused down to as ize of 100 100 mm 2 by aP tm irror.T he XAS spectra were measured around the Ti Ka bsorption edge at 4966 eV in total fluorescence mode by as ilicon drift detector.Afixed incidence X-ray energy at 5000 eV was used for measurements of the XES spectra for Ti valence-tocore transitions and by ad ispersive-type von Hamos spectrometer [25] with curvature radius of 25 cm. [28,29] The simulations retrieve the electronic orbital contribution for occupied and unoccupied states. The X-rays diffracted by the crystal, were registered with a2 DP ilatus detector having ap ixel size of 175 mm.…”