Probing Complex Disorder in Ce_1‑<i>x</i>Gd_<i>x</i>O_{2‑<i>x</i>/2} Using the Pair Distribution Function Analysis

In this work the first Pair Distribution Function (PDF) study on Ce_1‑<i>x</i>Gd_<i>x</i>O_{2‑<i>x</i>/2} (CGO) electrolytes for solid oxide fuel cells is presented, aiming to unveil the complex positional disorder induced by gadolinium doping and oxygen vacancies formation in these materials. The whole range of Gd concentration <i>x</i>_Gd (0 ≤ <i>x</i>_Gd ≤ 1) of the CGO solid solutions was investigated through high resolution synchrotron radiation powder diffraction. The reciprocal space Rietveld analysis revealed in all the solid solutions the presence of positional disorder, which has been explicitly mapped into the real space. The <i>average</i> structural models, as obtained by the Rietveld method, fit well the experimental PDF data only for a spatial range <i>r</i> > ∼10 Å. The same models applied at lower <i>r</i> values fails to reproduce the experimental curves. A clear improvement of the fit quality in the 1.5 < <i>r</i> < ∼6 Å range was obtained for all the CGO samples applying a <i>biphasic</i> model encompassing both a fluorite CeO₂-like and a C-type Gd₂O₃-like phases. This provides evidence that extended defects at local scale exist in the CGO system. Gd-rich and Ce-rich droplets coexist in the subnanometric range

Defect Structure of Y‑Doped Ceria on Different Length Scales

An exhaustive structural investigation of a Y-doped ceria (Ce_1–<i>x</i>Y_<i>x</i>O_{2–<i>x</i>/2}) system over different length scales was performed by combining Rietveld and Pair Distribution Function analyses of X-ray and neutron powder diffraction data. For low doping amounts, which are the most interesting for application, the local structure of Y-doped ceria can be envisaged as a set of distorted CeO₂- and Y₂O₃-like droplets. By considering interatomic distances on a larger scale, the above droplets average out into domains resembling the crystallographic structure of Y₂O₃. The increasing spread and amount of the domains with doping forces them to interact with each other, leading to the formation of antiphase boundaries. Single phase systems are observed at the average ensemble level

Phase Transformations in the CeO₂–Sm₂O₃ System: A Multiscale Powder Diffraction Investigation

The structure evolution in the CeO₂–Sm₂O₃ system is revisited by combining high resolution synchrotron powder diffraction with pair distribution function (PDF) to inquire about local, mesoscopic, and average structure. The CeO₂ fluorite structure undergoes two phase transformations by Sm doping, first to a cubic (C-type) and then to a monoclinic (B-type) phase. Whereas the C to B-phase separation occurs completely and on a long-range scale, no miscibility gap is detected between fluorite and C-type phases. The transformation rather occurs by growth of C-type nanodomains embedded in the fluorite matrix, without any long-range phase separation. A side effect of this mechanism is the ordering of the oxygen vacancies, which is detrimental for the application of doped ceria as an electrolyte in fuel cells. The results are discussed in the framework of other Y and Gd dopants, and the relationship between nanostructuring and the above equilibria is also investigated

Hierarchical Hematite Nanoplatelets for Photoelectrochemical Water Splitting

A new nanostructured α-Fe₂O₃ photoelectrode synthesized through plasma-enhanced chemical vapor deposition (PE-CVD) is presented. The α-Fe₂O₃ films consist of nanoplatelets with (001) crystallographic planes strongly oriented perpendicular to the conductive glass surface. This hematite morphology was never obtained before and is strictly linked to the method being used for its production. Structural, electronic, and photocurrent measurements are employed to disclose the nanoscale features of the photoanodes and their relationships with the generated photocurrent. α-Fe₂O₃ films have a hierarchical morphology consisting of nanobranches (width ∼10 nm, length ∼50 nm) that self-organize in plume-like nanoplatelets (350–700 nm in length). The amount of precursor used in the PE-CVD process mainly affects the nanoplatelets dimension, the platelets density, the roughness, and the photoelectrochemical (PEC) activity. The highest photocurrent (<i>j</i> = 1.39 mA/cm² at 1.55 V_RHE) is shown by the photoanodes with the best balance between the platelets density and roughness. The so obtained hematite hierarchical morphology assures good photocurrent performance and appears to be an ideal platform for the construction of customized multilayer architecture for PEC water splitting

Effect of Nature and Location of Defects on Bandgap Narrowing in Black TiO₂ Nanoparticles

The increasing need for new materials capable of solar fuel generation is central in the development of a green energy economy. In this contribution, we demonstrate that black TiO₂ nanoparticles obtained through a one-step reduction/crystallization process exhibit a bandgap of only 1.85 eV, which matches well with visible light absorption. The electronic structure of black TiO₂ nanoparticles is determined by the unique crystalline and defective core/disordered shell morphology. We introduce new insights that will be useful for the design of nanostructured photocatalysts for energy applications