Synthesis and crystal chemistry of the hybrid perovskite (CH3NH3)PbI3 for solid-state sensitised solar cell applications

High-quality Zn(x)Cd(1-x)Se nanocrystals have been successfully prepared at high temperature by incorporating stoichiometric amounts of Zn and Se into pre-prepared CdSe nanocrystals. With increasing Zn content, a composition-tunable emission across most of the visible spectrum has been demonstrated by a systematic blue-shift in emission wavelength. The photoluminescence (PL) properties for the obtained Zn(x)Cd(1-x)Se nanocrystals (PL efficiency of 70-85%, fwhm = 22-30 nm) are comparable to those for the best reported CdSe-based QDs. In particular, they also have good PL properties in the blue spectral range. Moreover, the alloy nanocrystals can retain their high luminescence (PL efficiency of over 40%) when dispersed in aqueous solutions and maintain a symmetric peak shape and spectral position under rigorous experimental conditions. A rapid alloying process was observed at a temperature higher than "alloying point". The mechanism of the high luminescence efficiency and stability of Zn(x)Cd(1-x)Se nanocrystals is explored.

This article describes a variable temperature solid-state NMR and single crystal X-ray/neutron diffraction study of the hybrid perovskites (CH3NH3)PbX3 (X = I, Br and Cl).

The crystal structures of the [A(1)(2)][A(2)(3)](BO(4))(3)X apatites and the related compounds [A(1)(2)][A(2)(3)](BO(5))(3)X and [A(1)(2)][A(2)(3)](BO(3))(3)X are collated and reviewed. The structural aristotype for this family is Mn(5)Si(3) (D8(8) type, P6(3)/mcm symmetry), whose cation array approximates that of all derivatives and from which related structures arise through the systematic insertion of anions into tetrahedral, triangular or linear interstices. The construction of a hierarchy of space-groups leads to three apatite families whose high-symmetry members are P6(3)/m, Cmcm and P6(3)cm. Alternatively, systematic crystallographic changes in apatite solid-solution series may be practically described as deviations from regular anion nets, with particular focus on the O(1)-A(1)-O(2) twist angle phi projected on (001) of the A(1)O(6) metaprism. For apatites that contain the same A cation, it is shown that phi decreases linearly as a function of increasing average ionic radius of the formula unit. Large deviations from this simple relationship may indicate departures from P6(3)/m symmetry or cation ordering. The inclusion of A(1)O(6) metaprisms in structure drawings is useful for comparing apatites and condensed-apatites such as Sr(5)(BO(3))(3)Br. The most common symmetry for the 74 chemically distinct [A(1)(2)][A(2)(3)](BO(4))(3)X apatites that were surveyed was P6(3)/m (57%), with progressively more complex chemistries adopting P6(3) (21%), P3; (9%), P6 (4.3%), P2(1)/m (4.3%) and P2(1) (4.3%). In chemically complex apatites, charge balance is usually maintained through charge-coupled cation substitutions, or through appropriate mixing of monovalent and divalent X anions or X-site vacancies. More rarely, charge compensation is achieved through insertion/removal of oxygen to produce BO(5) square pyramidal units (as in ReO(5)) or BO(3) triangular coordination (as in AsO(3)). Polysomatism arises through the ordered filling of [001] BO(4) tetrahedral strings to generate the apatite-nasonite family of structures.

Low-temperature CO oxidation over a compositional series of Pd-Au nanoalloy catalysts supported on silica fume was studied. Except for the pure metals, these materials invariably showed biphasic separation into palladium- and gold-rich components. Performance was optimal for a catalyst of bulk composition Pd(4)Au(1), a mixture of Pd(90)Au(10) (72.5 at. %) and Pd(31)Au(69) (27.5 at. %), that was remarkably active at 300 K and more stable than a pure Au catalyst. For bulk materials dominated by Pd (Pd:Au = 16:1; 8:1; 4:1), the palladium-rich alloy fraction frequently adopted hollow sphere or annular morphology, while the gold-rich crystals were often multiply twinned. Quantitative powder X-ray diffraction (XRD) showed that under the synthesis conditions used, the Au solubility limit in Pd crystals was approximately 12 at. %, while Pd was more soluble in Au (approximately 31 at. %). This was consistent with X-ray photoelectron spectroscopy (XPS), which revealed that the surfaces of Pd-rich alloys were enriched in gold relative to the bulk composition. In situ Fourier transform infrared spectra collected during CO oxidation contained a new band at 2114 cm(-1) (attributed to linear CO-Au/Au-Pd bonds) and reduced intensity of a band at 2090 cm(-1) (arising from a linear CO-Pd bond) with escalating Au content, indicating that the Pd sites became increasingly obscured by Au. High-resolution electron micrographs (HRTEM) of the Pd-rich alloys revealed atomic scale surface defects consistent with this interpretation. These results demonstrate that gold-containing biphasic Pd nanoalloys may be highly durable alternatives for a range of catalytic reactions.