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 TiO(2) 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 TiO(2) 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.

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Selecting the proper doping strategy is essential to controlling the photocatalytic activity of TiO 2 -based nanomaterials. In this work, we compare impregnation and bulk synthesis methods for feeding titania nanocrystals with either Nb or Ta as transition metal dopant and N as nonmetal codopant.The resulting photocatalytic efficiency was tested toward ethanol degradation under either UV or simulated solar irradiation. Microstructure, morphology, and electronic properties at various length scales were deeply investigated and compared with DFT simulations. Instead, under UV irradiation, impregnated samples performed better than bulk synthesis ones, with Ta-doped powders being more efficient than Nb-doped and undoped TiO 2 . Under simulated solar irradiation, bulk synthesis Nbdoped materials were the most active ones, while all the impregnated samples were even less performing than the undoped TiO 2 reference. On the basis of XPS, EPR, DRS, and XRPD results, such differences were attributed to the bulk synthesis approach producing a more homogeneous distribution of guest dopants within the grains, in conjunction with a higher amount of intrinsic defects (such as O vacancies). Implications of these findings on the engineering of efficient titania photocatalysts are discussed.

Pair distribution function analysis up to tens of nanometres allows probing of the structural changes in Ce1−xGdxO2−x/2 solid solutions at varying gadolinium concentrations. Dopant ions and oxygen vacancies form extended Gd2O3-like clusters (droplets) and nanodomains which, on increasing the Gd concentration, percolate and cause a long-range phase transformation. A general crystallographic rationale is presented which could be adopted to describe phase transformations in highly doped materials.

Three SrTiO 3±δ catalysts have been compared. One of them (T3) was prepared by the traditional sol-gel procedure, the other two through the flame hydrolysis (FH) technique, in the presence of either 1.5 mol excess of citric acid (sample T2) or 3 mol excess of tartaric acid (sample T1) as complexing agent in the precursor solution. All the catalysts possessed the perovskite-like structure, with some SrCO 3 as impurity. T1 was more crystalline than the other two samples and showed the most active as catalyst for the flameless combustion of methane. Isolated Ti 3+ ions were found in T3 by EPR analysis, while different EPR features were noticed with T1 and T2 and attributed to Ti 4+ /O 3 − . A high concentration of isolated Ti 3+ ions was observed with T1, accompanied by Ti 4+ /O 3 − and by other paramagnetic species, probably Ti 4+ /O − and Sr 2+ /O − , not observed with the other samples. This difference between the samples T1 and T2, likely connected with the different temperature attained during the FH preparation process with the two different complexing agents, can explain the higher oxidising activity of the T1 sample.

Several samples of Sr1-xAgxTiO3 (x = 0, 0.1) perovskite-structured catalysts have been prepared and tested for the title reaction under lean-burn conditions. For the same overall nominal composition, catalyst activity and durability depend strongly on the method of preparation and, for the samples prepared by the same method, mainly on temperature and on residence time at such a temperature during preparation. In Sr0.9Ag0.1TiO3 catalysts Ag is present either as intra-crystalline Ag 2+ , likely substituting for Sr 2+ in the perovskite lattice, or as inter-crystalline metallic Ag. The catalytic activity of Sr0.9Ag0.1TiO3 is connected with the presence of O2 -/Ti 4+ , Ox -/Ag and Ox -/Ag 2+ or of O3 -/Ti 4+ , O -/Sr 2+ and of O -/Ti 4+ species, depending on preparation method and conditions. Furthermore, intercrystalline metallic Ag plays an important role in improving catalyst resistance to sintering, the most durable samples being those calcined for a sufficient residence time at the highest temperature during preparation, i.e. under conditions better favouring the segregation of this metal from the perovskite lattice.

We use electron paramagnetic resonance spectroscopy (EPR) and synchrotron radiation x-ray powder diffraction to study the temperature-induced insulator-to-metal transition (IMT) for GdBaCo 2 O 5+δ samples in the δ range 0.54(1) δ 0.63(1). The EPR linewidth markedly changes across IMT and its temperature evolution can be explained considering spin state transition involving Co ions. The temperature dependences of the EPR linewidth and of the a lattice parameter fairly overlap each other suggesting spin-lattice interaction along the same crystallographic direction of the reported Ising-like spin anisotropy [A. A. Taskin et al., Phys. Rev. Lett. 90, 227201 (2003)]. A possible mechanism describing the interplay between this strong spin-lattice interaction and IMT is proposed.