Synthesis of Ideal AM‐6 and Elucidation of V⁴⁺‐to‐O Charge Transfer in Vanadate Quantum Wires

Abstract

The synthesis and characterization of one-dimensional (1D) semiconductor quantum-confined materials are important, since they have great potential as building blocks for nanoscale electronic devices and other applications. [1][2][3][4][5][6][7][8][9] Among the known 1D semiconductor materials, molecular wires or quantum wires [1][2][3][4][5] are the thinnest 1D quantum-confined materials. However, examples of such quantum wires are rare. Recently, we elucidated the interesting quantum-confinement properties of titanate (TiO 3 2À ) quantum wire [10] regularly placed within a titanosilicate molecular sieve known as ETS-10. [11][12][13] It shows a length-dependent quantum-confinement effect even at length scales longer than 50 nm.[10] Its estimated effective reduced exciton mass along the wire m z is lower than 0.0006 m e (m e = rest mass of electron), which is much lower than the lowest reported values (InSb: 0.014 m e , single-walled carbon nanotubes (SWNT): 0.019 m e ) and indicate much higher exciton mobility along the quantum wire than in InSb and SWNTs. The nature of the electronic absorption of the titanate quantum wire was reportedly oxide-to-Ti IV charge transfer or ligand-to-metal charge transfer (LMCT). [14][15][16] The stretching frequency of the titanate wire increases with increasing electron density of the wire. [14] After elucidation of such important properties of titanate quantum wire, it would be of interest to determine the physicochemical properties of the closely related vanadate (VO 3 2À ) quantum wire. In this regard, the discovery of vanadosilicate AM-6 by Rocha, Anderson, and co-workers in 1997 [17] [designated AM-6-(RA)] is important, since it adopts the ETS-10 structure but with VO 3 2À quantum wires replacing TiO 3 2À quantum wires. Unfortunately, however, ETS-10 crystals were required as seeds to induce ETS-10 structure in the vanadosilicate. Accordingly, AM-6-(RA) inevitably contains ETS-10 crystals within AM-6. Thus, AM-6-(RA) should more strictly be defined as ETS-10 core/AM-6 shell. Furthermore, Lobo, Doren, and co-workers revealed that VO 3 2À quantum wires in AM-6-(RA) are composed of both V 4+ and V 5+. [18][19][20] As a result, it is intrinsically impossible to elucidate the physicochemical properties of the pure V IV O 3 2À quantum wire. Furthermore, their procedure always simultaneously produces substantial amounts of quartz. Hence, methods to prepare ETS-10-free, pure AM-6 have long been awaited.Twelve years after the report of AM-6-(RA), Sacco, Jr. and co-workers finally developed a method of synthesizing ETS-10-free AM-6.[21] However, they had to use tetramethylammonium ion (TMA + ) as structure-directing agent. Accordingly, this AM-6 contains TMA + ions within the channels. We designate this AM-6 as AM-6-(S)-TMA. We found that AM-6-(S)-TMA also contains both V 4+ and V 5+ (see below). Furthermore, the TMA + ions are tightly encapsulated within and hence completely block the silica channels. As a result, ion exchange of the pristine cations (Na + and K + ) with other cation...

Accessibility of Lithium Cations in VSH‐2 Zeotype: Structural Effects and Formation of Protonated Water Clusters

“…This reveals the occurrence of V 4+ ‐to‐framework O 2− charge transfer or metal‐to‐ligand charge transfer (MLCT) and, simultaneously, a ligand‐to‐metal charge‐transfer (LMCT) between O 2− and M n+ countercation. This phenomenon was shown also in vanadosilicates AM‐6 [18] and VSH‐1 K [19] …”

“…This reveals the occurrence of V 4 + -toframework O 2À charge transfer or metal-to-ligand charge transfer (MLCT) and, simultaneously, a ligand-to-metal charge-transfer (LMCT) between O 2À and M n + countercation. This phenomenon was shown also in vanadosilicates AM-6 [18] and VSH-1 K. [19] Our investigation is aiming to explore the influence of Li substitution on the host VSH-2 structure. Understanding the structural changes induced by the Li ion-exchange in vanadosi-licate frameworks is indeed of great interest since the physicochemical properties of these materials are potentially enhanced.…”

DFT Calculated Structures and Electronic Properties of ETS-10, AM-6, and SGU-29: Structure Stabilization through Periodic Distortions

“…25−28 AM-6, which is isostructural with ETS-10, contains VO 6 instead of TiO 6 octahedrons in the 1D quantum wires (Figure 1A, B). 29,30 SGU-29 differs from both ETS-10 and AM-6 by having 1D arrays of CuO 4 square planes instead of 1D quantum wires (Figure 1A, C). 1 From a broader view, SGU-29 is isostructural with ETS-10 and AM-6.…”

“…ETS-10 is a microporous titanosilicate having one-dimensional (1D) quantum wires of TiO 6 octahedrons aligned along the (110) and (1–10) directions within their frameworks (Figure A, B). − AM-6, which is isostructural with ETS-10, contains VO 6 instead of TiO 6 octahedrons in the 1D quantum wires (Figure A, B). , SGU-29 differs from both ETS-10 and AM-6 by having 1D arrays of CuO 4 square planes instead of 1D quantum wires (Figure A, C) . From a broader view, SGU-29 is isostructural with ETS-10 and AM-6.…”