Polymorphism and Structural Distortions of Mixed-Metal Oxide Photocatalysts Constructed with α-U3O8 Types of Layers

Abstract

Abstract:A series of mixed-metal oxide structures based on the stacking of α-U 3 O 8 type pentagonal bipyramid layers have been investigated for symmetry lowering distortions and photocatalytic activity. The family of structures contains the general composition A m+ ((n+1)/m) B (3n+1) O (8n+3) (e.g., A = Ag, Bi, Ca, Cu, Ce, Dy, Eu, Gd K, La, Nd, Pb, Pr, Sr, Y; B = Nb, Ta; m = 1-3; n = 1, 1.5, 2), and the edge-shared BO 7 pentagonal pyramid single, double, and/or triple layers are differentiated by the average thickness, (i.e., 1 ≤ n ≤ 2), of the BO 7 layers and the local coordination environment of the "A" site cations. Temperature dependent polymorphism has been investigated for structures containing single layered (n = 1) monovalent (m = 1) "A" site cations (e.g., Ag 2 Nb 4 O 11 , Na 2 Nb 4 O 11 , and Cu 2 Ta 4 O 11 ). Furthermore, symmetry lowering distortions were observed for the Pb ion-exchange synthesis of Ag 2 Ta 4 O 11 to yield PbTa 4 O 11 . Several members within the subset of the family have been constructed with optical and electronic properties that are suitable for the conversion of solar energy to chemical fuels via water splitting.

A Metastable p-Type Semiconductor as a Defect-Tolerant Photoelectrode

“…The structure is comprised of layers of linearly-coordinated Cu(I) and octahedrally-coordinated Ta(V) cations that alternate with double layers of edge-and vertex-shared TaO 7 pentagonal bipyramids. The layers of pentagonal bipyramids, i.e., the TaO 7 in Figure 1b, are similar to those previously described in the structure of α-U 3 O 8 [24]. To probe the bulk and surface-level thermal stability of Cu3Ta7O19, the crystalli were heated in air at temperatures spanning the range of 350 °C to >550 °C.…”

Crystallography of Functional Materials

“…In the case of the binary oxide compound Ca 12 Al 14 O 33 , firing in a reducing atmosphere results in the removal of occluded oxygen atoms, replacing them with other atoms, molecules, or electrons, where the latter can alter the conduction from highly resistive to near-metallic. King et al [10] showed that mixed-metal oxide compounds, with the general formula A m+ ((n+1)/m) B (3n+1) O (8n+3) , phase transitions that occur from varying the temperature, the composition, or the chemical reactions, can result in structural and electronic distortions that impact the efficiency of the photocatalysis. Two of the papers reported the crystal structures of new compounds along with complementary property measurements.…”

“…In photoelectrochemical water splitting, the electricity used to produce the hydrogen is supplied by a renewable energy source (i.e., a photovoltaic system). In the paper presented by King et al [10], the authors review crystal structures having photocatalytical properties. This paper focuses on a series of mixed-metal oxide structures, based on the stacking of α-U 3 O 8 type pentagonal bipyramid layers, with the general formula A m+ ((n+1)/m) B (3n+1) O (8n+3) , where the A cation can be Ag, Bi, Ca, Cu, Ce, Dy, Eu, Gd K, La, Nd, Pb, Pr, Sr, or Y, the B cation is either Nb or Ta, and m = 1-3 and n = 1, 1.5, 2.…”

Does Symmetry Control Photocatalytic Activity of Titania-Based Photocatalysts?

“…However, though the structural symmetry of orthorhombic KNbO 3 is lower than that of tetragonal KNbO 3 , it presents a better performance, probably due to better morphology, that is, the exposed facets and the associated electronic structure. In contrast, King et al have found that structure distortions lowering the symmetry have an influence on the photocatalytic performance, for example, for PbTa 4 O 11 with an out-of-plane distortion of the d 0 transition metal cation, an internal electric dipole can facilitate the separation of charge carriers (electron/hole), hindering the recombination and back-reaction [32]. Finally, Huang et al have reviewed synthesis methods for symmetry-breaking in multicomponent NPs, concluding that NPs with broken symmetry could be beneficial for various photocatalytic and electrocatalytic reactions resulting from: (i) arrangement of one-direction of charge migration, (ii) spatial separation of domains (prevention of reverse reactions), and (iii) synergism at the interface of different domains [33].…”