Controlled morphological and structural variations of LaTiO2N and CoO x nanoparticles loaded as a cocatalyst increased the photocatalytic O2 evolution. Four different LaTiO2N samples were prepared from two varied La2Ti2O7 precursors by thermal ammonolysis with and without flux. The most effective transformation from La2Ti2O7 into LaTiO2N in terms of crystallinity and nitrogen contents was obtained in the following two cases: flux-assisted ammonolysis of La2Ti2O7 characterized by a comparatively small particle size and a large surface area and ammonolysis without flux of highly crystalline La2Ti2O7. In both cases, high activity for O2 evolution under visible-light illumination (λ ≥ 420 nm) (∼50 μmol h–1) was obtained. Loading LaTiO2N with CoO x enhanced the photocatalytic activity for O2 evolution, although the effect of CoO x depended on the morphologies of LaTiO2N as a support. The promotional effect was most pronounced for LaTiO2N with a skeletal morphology, because such LaTiO2N contain most unsaturated bonds that act as nucleation centers that in turn generate small and highly dispersed CoO x nanoparticles.

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Valence electron energy loss spectroscopy ͑VEELS͒ and high resolution transmission electron microscopy ͑HRTEM͒ are performed on three different HfO 2 thin films grown on Si ͑001͒ by chemical vapor deposition ͑CVD͒ or atomic layer deposition ͑ALD͒. For each sample the band gap ͑E g ͒ is determined by low-loss EELS analysis. The E g values are then correlated with the crystal structure and the chemical properties of the films obtained by HRTEM images and VEELS line scans, respectively. They are discussed in comparison to both experimental and theoretical results published in literature. The HfO 2 ALD film capped with poly-Si exhibits the largest band gap ͑E g = 5.9± 0.5 eV͒, as a consequence of its nanocrystallized orthorhombic structure. The large grains with a monoclinic structure formed in the HfO 2 ALD film capped with Ge and the carbon contamination induced by the precursors in the HfO 2 CVD film capped with Al 2 O 3 are identified to be the main features responsible for lower band gap values ͑E g = 5.25± 0.5 and 4.3± 0.5 eV respectively͒.

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Two series of oxide precursors for perovskite-type (La,Ca)Ti(O,N) 3 were prepared by adding Ca 2+ to A-site deficient LaTiO 3.5 heterogeneously (Ca 2+ -backfilling) or by substituting Ca 2+ for La 3+ in stoichiometric LaTiO 3.5 homogeneously (Ca 2+ -substitution). Activity of the resultant (La,Ca)Ti(O,N) 3 for photocatalytic O 2 evolution was tested in the presence of an electron acceptor (Ag + ) and a superior activity of Ca 2+ -backfilled LaTiO 2 N compared to Ca 2+ -substituted LaTiO 2 N and unsubstituted LaTiO 2 N was demonstrated. X-ray diffraction patterns of the precursor oxides revealed a higher degree of crystallinity in La 1Àx TiO 3.5À3x/2 compared to La 1Àx Ca x TiO 3.5Àx/2 . The higher crystallinity in La 1Àx TiO 3.5À3x/2 resulted in lower Ti 3+ defect formation during the ammonolysis reaction. This was evidenced by the lower background absorption of the diffuse reflectance spectra in Ca 2+ -backfilled compared to Ca 2+ -substituted LaTiO 2 N. Structural refinement of the diffraction patterns revealed growing crystal sizes with the Ca 2+ content, which was more pronounced for Ca 2+ -backfilled than for Ca 2+ -substituted LaTiO 2 N. Thus, the enhanced photocatalytic activity of Ca 2+ -backfilled LaTiO 2 N was related to the quality of the precursor oxides, which influenced the defect concentrations and crystallite sizes of the resulting oxynitrides.

Mesoporosity in photocatalytically active oxynitride single crystals and single-crystalline zones has been investigated by transmission electron microscopy techniques including nanobeam diffraction, electron energy loss spectroscopy, electron tomography, and high-resolution imaging. Several particle morphologies of the perovskite-related oxynitride LaTiO2N were synthesized by solid-state and polymer complex synthesis of the La2Ti2O7 precursor followed by thermal ammonolysis. A detailed analysis of pore sizes, pore shapes, and lattice defects and the local analysis of oxidation states allowed correlation between morphology, synthesis procedures, chemical and crystal defects, and photocatalytic activity. A pore formation mechanism via lattice condensation is proposed, which is simultaneously linked to lattice defect formation processes. On the basis of mechanistic understanding of the transformation from oxide to oxynitride, mesoporosity, and hence the photocatalytic or photoelectrochemical properties of the material, can be tuned