Structure Model for Anion-Disordered Photochromic Gadolinium Oxyhydride Thin Films

Rare-earth (RE) oxyhydride thin films prepared by reactive magnetron sputtering followed by air-oxidation show a color-neutral photochromic effect at ambient conditions within a wide composition range (REOxH3-2x where 0.5 ≤ x < 1.5). Due to the high degree of anion sublattice disorder present in these thin films, the structure models proposed for the related bulk materials are not directly applicable. Instead we use a combination of EXAFS analysis and lattice energy calculations to establish a fcc-based model linking the oxyhydrides to the related binary compounds. The oxide anions are found to occupy predominantly the tetrahedral sites in the fcc structure, which is attributed to electrostatic lattice energy minimization.ChemE/Materials for Energy Conversion & Storag

Metal-hydrogen systems with an exceptionally large and tunable thermodynamic destabilization

Hydrogen is a key element in the energy transition. Hydrogen-metal systems have been studied for various energy-related applications, e.g., for their use in reversible hydrogen storage, catalysis, hydrogen sensing, and rechargeable batteries. These applications depend strongly on the thermodynamics of the metal-hydrogen system. Therefore, tailoring the thermodynamics of metal-hydrogen interactions is crucial for tuning the properties of metal hydrides. Here we present a case of large metal hydride destabilization by elastic strain. The addition of small amounts of zirconium to yttrium leads to a compression of the yttrium lattice, which is maintained during (de)hydrogenation cycles. As a result, the equilibrium hydrogen pressure of YH2 → YH3 can be rationally and precisely tuned up to five orders of magnitude at room temperature. This allows us to realize a hydrogen sensor which indicates the ambient hydrogen pressure over four orders of magnitude by an eye-visible color change.ChemE/Materials for Energy Conversion & Storag

Operando EXAFS study reveals presence of oxygen in oxide-derived silver catalysts for electrochemical CO₂ reduction

Electrocatalysis of carbon dioxide can provide a valuable pathway towards the sustainable production of chemicals and fuels from renewable electricity sources. One of the main challenges to enable this technology is to find suitable electrodes that can act as efficient, stable and selective CO2 reduction catalysts. Modified silver catalysts and in particular, catalysts electrochemically derived from silver-oxides, have shown great promise in this regard. Here, we use operando EXAFS analysis to study the differences in surface composition between a pure silver film and oxide-derived silver catalysts-a nanostructured catalyst with improved CO2 reduction performance. The EXAFS analysis reveals the presence of trace amounts of oxygen in the oxide-derived silver samples, with the measured oxygen content correlating well with experimental studies showing an increase in CO2 reduction reactivity towards carbon monoxide. The selectivity towards CO production also partially scales with the increased surface area, showing that the morphology, local composition and electronic structure all play important roles in the improved activity and selectivity of oxide-derived silver electrocatalysts. Earlier studies based on X-ray photoelectron spectroscopy (XPS) were not able to identify this oxygen, most likely because in ultra-high vacuum conditions, silver can self-reduce to Ag0, removing existing oxygen species. This operando EXAFS study shows the potential for in situ and operando techniques to probe catalyst surfaces during electrolysis and aid in the overall understanding of electrochemical systems.Accepted Author ManuscriptChemE/Materials for Energy Conversion & Storag

Chemisorption of Anionic Species from the Electrolyte Alters the Surface Electronic Structure and Composition of Photocharged BiVO₄

Photocharging has recently been demonstrated as a powerful method to improve the photoelectrochemical water splitting performance of different metal oxide photoanodes, including BiVO4. In this work, we use ambient-pressure X-ray Raman scattering (XRS) spectroscopy to study the surface electronic structure of photocharged BiVO4. The O K edge spectrum was simulated using the finite difference method near-edge structure program package, which revealed a change in electron confinement and occupancy in the conduction band. These insights, combined with ultraviolet-visible spectroscopy and X-ray photoelectron spectroscopy analyses, reveal that a surface layer formed during photocharging creates a heterojunction with BiVO4, leading to favorable band bending and strongly reduced surface recombination. The XRS images presented in this work exhibit good agreement with soft X-ray absorption near-edge structure spectra from the literature, demonstrating that XRS is a powerful tool to study the electronic and structural properties of light elements in semiconductors. Our findings provide direct evidence of the electronic modification of a metal oxide photoanode surface as a result of the adsorption of electrolyte anionic species under operating conditions. This work highlights that the surface adsorption of these electrolyte anionic species is likely present in most studies on metal oxide photoanodes and has serious implications for the photoelectrochemical performance analysis and fundamental understanding of these materials.ChemE/Materials for Energy Conversion & Storag

Locating and Controlling the Zn Content in In(Zn)P Quantum Dots

Zinc is routinely employed in the synthesis of InP quantum dots (QDs) to improve the photoluminescence efficiency and carrier mobility of the resulting In(Zn)P alloy nanostructures. The exact location of Zn in the final structures and the mechanism by which it enhances the optoelectronic properties of the QDs are debated. We use synchrotron X-ray absorbance spectroscopy to show that the majority of Zn in In(Zn)P QDs is located at their surface as Zn carboxylates. However, a small amount of Zn is present inside the bulk of the QDs with the consequent contraction of their lattice, as confirmed by combining high-resolution high-angle annular dark-field imaging scanning transmission electron microscopy with statistical parameter estimation theory. We further demonstrate that the Zn content and its incorporation into the QDs can be tuned by the ligation of commonly employed Zn carboxylate precursors: the use of highly reactive Zn acetate leads to the formation of undesired Zn3P2 and the final nanostructures being characterized by broad optical features, whereas Zn carboxylates with longer carbon chains lead to InP crystals with much lower zinc content and narrow optical features. These results can explain the differences between structural and optical properties of In(Zn)P samples reported across the literature and provide a rational method to tune the amount of Zn in InP nanocrystals and to drive the incorporation of Zn either as surface Zn carboxylate, as a substitutional dopant inside the InP crystal lattice, or even predominantly as Zn3P2.ChemE/Opto-electronic Material

Near-complete suppression of surface losses and total internal quantum efficiency in BiVO₄ photoanodes

Bismuth vanadate (BiVO4) is one of the most efficient light absorbing metal oxides for solar water splitting. BiVO4 photoanodes immersed in an electrolyte in an open circuit configuration and exposed to simulated solar illumination for prolonged time achieve superior photoelectrochemical (PEC) activity. This photocharging (PC) effect is capable of almost completely overcoming the surface and bulk limitations of BiVO4. Herein we show that alkaline conditions favor the PC effect; specifically BiVO4 photoanodes subjected to PC treatment at pH 10 achieve a record high photocurrent for undoped and uncatalyzed BiVO4 of 4.3 mA cm-2 @ 1.23 VRHE, an outstandingly low onset potential of 0.25 VRHE, and a very steep photocurrent onset. Alkaline conditions also facilitate excellent external and internal quantum efficiencies of 75 and 95% respectively (average in the 440 nm > λ > 330 nm range). Moreover, impedance spectroscopy and in situ XAS study suggest that electronic, structural and chemical properties of the bulk of these films remain unchanged during the PC treatment. However, appreciable changes in the surface-related properties take place. Ultimately, our results indicate that the improved activity of PC-BiVO4 is enhanced by surface reaction pathways of the semiconductor-liquid junction, which is directly correlated with the electrochemical environment in which it is modified.ChemE/Materials for Energy Conversion & StorageChemE/Transport Phenomen

Tuning and Probing the Distribution of Cu⁺ and Cu²⁺ Trap States Responsible for Broad-Band Photoluminescence in CuInS₂ Nanocrystals

The processes that govern radiative recombination in ternary CuInS2 (CIS) nanocrystals (NCs) have been heavily debated, but recently, several research groups have come to the same conclusion that a photoexcited electron recombines with a localized hole on a Cu-related trap state. Furthermore, it has been observed that single CIS NCs display narrower photoluminescence (PL) line widths than the ensemble, which led to the conclusion that within the ensemble there is a distribution of Cu-related trap states responsible for PL. In this work, we probe this trap-state distribution with in situ photoluminescence spectroelectrochemistry. We find that Cu2+ states result in individual "dark" nanocrystals, whereas Cu+ states result in "bright" NCs. Furthermore, we show that we can tune the PL position, intensity, and line width in a cyclic fashion by injecting or removing electrons from the trap-state distribution, thereby converting a subset of "dark" Cu2+ containing NCs into "bright" Cu+ containing NCs and vice versa. The electrochemical injection of electrons results in brightening, broadening, and a red shift of the PL, in line with the activation of a broad distribution of "dark" NCs (Cu2+ states) into "bright" NCs (Cu+ states) and a rise of the Fermi level within the ensemble trap-state distribution. The opposite trend is observed for electrochemical oxidation of Cu+ states into Cu2+. Our work shows that there is a direct correlation between the line width of the ensemble Cu+/Cu2+ trap-state distribution and the characteristic broad-band PL feature of CIS NCs and between Cu2+ cations in the photoexcited state (bright) and in the electrochemically oxidized ground state (dark).ChemE/Opto-electronic MaterialsApplied SciencesBN/Technici en Analiste