In situ growth of nanoparticles through control of non-stoichiometry

Transcription

1 DOI: /NCHEM.1773 In situ growth of nanoparticles through control of non-stoichiometry Dragos Neagu 1 *, George Tsekouras 1, David N. Miller 1, Hervé Ménard 2 and John T.S. Irvine 1 * 1 University of St Andrews, St. Andrews, KY16 9ST, Scotland, United Kingdom. 2 Sasol Technology (UK) Ltd. St. Andrews, KY16 9ST, Scotland, United Kingdom. Figure S 1 Gibbs free energy of reduction of oxides to either metals or selected oxides, at 900 C in H 2. NATURE CHEMISTRY 1

2 (a) (b) (c) Figure S 2 Accommodation of nonstoichiometry with respect to the ideal perovskite structure: (a) the ideal perovskite structure; (b) deficiency through vacancies denoted by red hollow spheres (c), excess through intergrowths (marked with red dotted lines). In the case where only oxygen excess is present, the intergrowth comprises of the region where perovskite slabs are offset in such a way to allow accommodation of extra oxygen ions (e.g. the A n B n O 3n+2 series, La x Sr 1-x TiO 3+x/2 ) 16,17. A-site super-stoichiometry is incorporated by intergrowing perovskite slabs with other crystal structures such as the rock-salt lattice, leading to the well-known Ruddlesden-Popper structures (e.g. in the Ruddlesden-Popper phases, A n+1 B n O 3n+1 ) 19. Structure determination The procedure employed here for the identification of the structure is based on indexing the perovskite on a double cubic cell and analyzing the splitting (or broadening) of the relevant cubic primitive peaks and presence and type of the super reflections 1 4. A close inspection of the (400) P and (444) P peaks (the latter shown in Figure S 4; subscript P refers to indexing on a NATURE CHEMISTRY 2

3 cubic primitive cell) coupled with the fact that we only observe R-type 3,4 super reflections indicative of out-of-phase tilting, suggest the space group for the undoped and the Mn-doped samples is tetragonal I4/mcm, whereas for the remaining B-site doped samples the rhombohedral R 3 c, orthorhombic Imma or monoclinic I2/a seem reasonable choices. Generally the structure can be confirmed by a Rietveld refinement in which the collected diffraction pattern is matched with a model of the diffraction pattern derived from the proposed model of the structure. By employing this procedure, we found that the undoped and the Mn-doped share an I4/mcm space group, while the other B-site doped samples are best described by an I2/a space group with R 3 c being a close match (see, for example the refinement in Figure S 6). The compositions La 0.8 Ce 0.1 Ni 0.4 Ti 0.6 O 3 composition exhibits a significantly more distorted structure (super reflections more intense and in larger number) with both in phase and out of phase tilting, but a very good model was found in the Pbnm space group, thus confirming its perovskite-like structure (see the Rietveld refinement in Figure S 5). Figure S 3 Ionic radii vs. coordination number for some cations typically encountered in perovskites. The domains of this plot that are characteristic for A and B-site cations are emphasized and labelled. Ionic radii from Shannon 5. NATURE CHEMISTRY 3

4 Figure S 4 Peak (444) as indexed on a double cell from room temperature XRD patterns of selected as-prepared compositions (6% B-site doping, x = 0.06): (1) La 0.4 Sr 0.4 TiO 3 (2) La 0.4+x Sr 0.4-x Fe x Ti 1-x O 3, (3) La 0.4 Sr 0.4 Mn x Ti 1- xo 3-γ, (4) La 0.4 Sr 0.4 Fe x Ti 1-x O 3- γ, (5) La 0.4+2x Sr 0.4-2x Ni x Ti 1-x O 3, (6) La 0.4 Sr 0.4 Ni x Ti 1-x O 3- γ, (7) La 0.4 Sr 0.4 Cu x Ti 1-x O 3- γ. Rietveld refinement of La. Ce.ଵ Ni.ସ Ti. O ଷ Crystal structure ( ) Perspective view Projection along ݖ Projection along ݕ Figure S 5 Rietveld refinement and crystal structure of as-prepared La. Ce.ଵ Ni.ସ Ti. O ଷ. NATURE CHEMISTRY 4

5 Rietveld refinement of La.ହଶ Sr.ଶ Ni. Ti.ଽସ O ଷ ( ) / 2 ܫ structure Crystal Perspective view ݔ Projection along ݖ Projection along Figure S 6 Rietveld refinement and visualisation of associated crystal structure for La.ହଶ Sr.ଶ Ni. Ti.ଽସ O ଷ. The fitting of the (444) peak and projections showing out-of-phase tilting are highlited. R p = 4.8, R wp = 6.3, R e = 4.7, χ 2 = NATURE CHEMISTRY 5

6 Figure S 7 Room temperature XRD pattern of the as-prepared La 0.3 Sr 0.7 TiO 3.15 (1) and La 0.3 Sr 0.7 Ni 0.06 Ti 0.94 O 3.09 (2). Peaks denoting weak oxygen excess ordering are indicated by (*). (a) La 0.4 Sr 0.4 Fe x Ti 1-x O 3-γ (x = 0.06, dry, 930 C, 20 h) (b) La 0.4+x Sr 0.4-x Fe x Ti 1-x O 3 (x = 0.09, dry, 930 C, 20 h) (c) La 0.4 Sr 0.4 Fe x Ti 1-x O 3-γ (x = 0.06, wet, 1000 C, 20 h) (d) La 0.4 Sr 0.4 Ni x Ti 1-x O 3-γ (x = 0.06, dry, 930 C, 20 h) (f) La 0.4 Sr 0.4 Mn x Ti 1-x O 3-γ (x = 0.06, dry, 1000 C, 20 h) (i) La 0.4 Sr 0.4 Cu x Ti 1-x O 3-γ (x = 0.06, dry, 1000 C, 20 h) Figure S 8 Exsolutions formed in different systems after reduction performed in dry or humidified (~3%H 2 O) 5%H 2 /Ar, at various temperatures (specific parameters indicated in brackets). NATURE CHEMISTRY 6

7 Figure S 9 Room temperature XRD patterns of (1) La 0.46 Sr 0.34 Fe 0.06 Ti 0.94 O 3 and (2) La 0.52 Sr 0.28 Ni 0.06 Ti 0.94 O 3 reduced at 1000 C in 5%H 2 /Ar showing broad peaks of metallic Fe and Ni, respectively. (a) (b) Figure S 10 The key role of the innate perovskite surface structuring in the formation of exsolutions. (a) La 0.4 Sr 0.4 TiO 3 reduced at 1100 C (20 h) in 5%H 2 /Ar (b). La 0.3 Sr 0.7 TiO 3.15, reduced at 1100 C (20 h) in 5%H 2 /Ar; NATURE CHEMISTRY 7

8 (a) A ଵ BO ଷ α = 0.2 La ௫ Sr ଵ ଷ௫Ȁଶ TiO ଷ ͲǤͶ ݔ (b) A ଵ BO ଷ α = 0.1 La ௫ Sr ଵ ଷ௫Ȁଶ TiO ଷ ͲǤʹ ݔ (c) A ଵ BO ଷ α = 0.05 La ௫ Sr ଵ ଷ௫Ȁଶ TiO ଷ ͲǤͳ ݔ (d) ABO ଷ α = 0 La ௫ Sr ଵ ଷ௫Ȁଶ TiO ଷ ) ଷ ؠͲሺ ݔ Figure S 11 Samples from the specified nonstoichiometry class and composition cleaved and reduced in ʹ. ݔ ʹ) ݔ ͳ ݔ) 1 = α 5%H 2 /Ar at 1100 C (20 h). α is the A-site deficiency, NATURE CHEMISTRY 8

9 (a) (b) (c) Figure S 12 Semi-coherent metallic Ni and fluorite-type (possibly CeO 2-δ ) nano-particles exsolved from La 0.8 Ce 0.1 Ni 0.4 Ti 0.6 O 3 upon reduction (5%H 2 /Ar, 20 h, 1000 C, microstructure after reduction shown in Figure 5 b). (a) XRD pattern of the reduced material together with metallic Ni (Fm3m), fluorite (Fm3m) and perovskite (Pbnm as-prepared and Pm3m reduced) reflections. After reduction, the following cell parameters were found: perovskite ~3.929 Å, Ni ~ Å, fluorite phase ~5.55 Å. The fluorite phase appears to be coherent to some extent to the perovskite since 5.55 Å Å 2 = Å and because if a CeO 2 sample is reduced in the same conditions as above its unit cell is significantly smaller, ~5.45 Å. The expanded unit cell of the fluorite phase obtained by in situ exsolution could be attributed to the large number of defects (Ce 3+, V ) expected to arise through this unique formation mechanism, or possibly to a small number of La 3+ ions doping it (r(la 3+ )>r(ce 3+ )). (b) TEM diffraction pattern along the perovskite direction [111] on an area of the grain show in Figure 5 c (same sample with the XRD diffraction pattern shown in this figure, part (a)). (c) interpretation of the TEM diffraction pattern shown in (b) indicating that Ni and perovskite [111] directions are parallel. NATURE CHEMISTRY 9

10 Sr + O SrO + V ᇱᇱ 1).ݍܧ) + V 2).ݍܧ) M +O M ᇱᇱ +V + ଵ ଶ O ଶ M ᇱᇱ M (௫௦௨௧) +V ᇱᇱ (3.ݍܧ) M +O MO + V +V ᇱᇱ (4.ݍܧ) 5).ݍܧ) MO M (௫௦௨௧) + ଵ ଶ O ଶ Figure S 13 Proposed point-defect reactions for the exsolution of e.g. M II B-site dopants from highly A-site deficient perovskites upon reduction. It is likely that the large number of V ᇱᇱ imposed through doping limits the number of intrinsic Schottky defects, pushing Eq. 1 to the left and thus decreasing the number of V O. For a given మ, Eq. 2 shifts right to oppose the change, facilitating the removal of lattice oxygen and reduction of the metal dopant (M II to M ᇱᇱ ᇱᇱ ) by the reducing gas. The metal atom M spontaneously exsolves from the oxide lattice leaving behind a cation vacancy, V ᇱᇱ, as expressed by Eq. 3. Alternatively, exsolutions could originate from the MO Schottky defects, as indicated in Eq. 4, which would subsequently convert to metal exsolutions upon reduction, Eq. 5. These point-defect reaction should, however, be interpreted with prudence since the defects discussed here diverge from the purest definition of point-defect given their high concentration and impact on the host lattice. Additionally, further experiments should be carried out in the future in order to establish the actual exsolution pathway. La 3d 5/2 O 1s Ti 2p 3/2, 1/2 Sr 3d 5/2, 3/2 Regular surface Cleaved surface Figure S 14 XPS spectra of the surface of a porous La 0.52 Sr 0.28 Ni 0.06 Ti 0.94 O 3 sample before and after cleaving (both cases before reduction). NATURE CHEMISTRY 10

11 References 1. Glazer, A. M. Simple ways of determining perovskite structures. Acta Crystallographica Section A 31, (1975). 2. Ball, C.., Begg, B.., Cookson, D.., Thorogood, G.. & Vance, E.. Structures in the System CaTiO 3 /SrTiO 3. Journal of Solid State Chemistry 139, (1998). 3. Howard, C. J. & Stokes, H. T. Structures and phase transitions in perovskites a grouptheoretical approach. Acta Crystallographica Section A Foundations of Crystallography 61, (2004). 4. Howard, C. J. & Stokes, H. T. Group-Theoretical Analysis of Octahedral Tilting in Perovskites. Acta Crystallogr B Struct Sci 54, (1998). 5. Shannon, R. D. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Cryst A 32, (1976). NATURE CHEMISTRY 11