Probing the Structure and Binding Mode of EDTA on the Surface of Mn3O4 Nanoparticles for Water Oxidation by Advanced Electron Paramagnetic Resonance Spectroscopy

Zinc tungstate encapsulated into a scarce Zn(II)-viologen framework with photochromic, electrochromic and chemochromic properties

Smart chromic materials reacting to physicochemical stimuli are widely applied in optical switches, smart windows, and chemical sensors. Currently, most materials only respond to a single stimulus, but those that respond to multiple external stimuli are still in the minority. Herein, we report a novel porous zinc tungstate@metaloxoviologen framework [Zn3(Bcbpy)6(H2O)2]-[ZnW12O40]·6H2O (ZnW12@MV, H2BcbpyCl2 = 1,1'-bis(3-carboxybenzyl)-4,4'-bipyridinium dichloride), which shows multiple stimulus-responsive properties due to a combination of different functional motifs, namely, viologen electron acceptors, luminescent zinc-oxygen-clusters, porous cationic frameworks, and ZnW12O406- electron donors. Generally, the large-sized polyoxometalate (POM) anions serving as structure-directing agents can easily direct the formation of the oligomeric metaloxoviologen cations, mainly because POMs may break down some linkages leaving larger spaces for themselves. The large ZnW12O406- anions in ZnW12@MV are encapsulated into three-dimensional (3D) metaloxoviologen frameworks built up from the linkages of trinuclear zinc-oxygen clusters and Bcbpy viologens, which offer the first example of a 3D metaloxoviologen framework induced by large-sized POM anions. ZnW12@MV shows a reversible chromic response to X-ray/UV and electricity via different stimulus-induced electron transfers between electron-rich POM anions and electron-deficient metaloxoviologen frameworks, whereas the coloration changes are ascribed to the formation of radical and mixed-valence colored state ZnW12O406- species. The photochromic behavior is accompanied by photoluminescence quenching. The discriminative response to different-sized amines is attributed to the formation of viologen radicals through host-guest electron transfer. These results indicate that the multi-stimulus response ZnW12@MV can be applied in electrochromic devices, inkless erasable printing, and the detection of amines.

Surface Structures of Mn3O4 and the Partition of Oxidation States of Mn

The Mn(III) ions at Mn₃O₄ surface are hypothesized to contribute to catalytic activity in oxygen reduction reaction. However, the surface structure and stability of Mn₃O₄ are far less understood. Here, the atomic structures of the widespread (101) and (001) surfaces of Mn₃O₄ are determined by combining aberration-corrected transmission electron microscopy and DFT calculations. The surface stabilization mechanisms and the oxidation states of Mn are revealed and correlated to the catalytic activity of the surfaces. The results show that the (101) surface undergoes a subsurface reconstruction, forming a rock-salt-type surface layer. The Mn(III) ions are in the outermost layer of the (001) surface but in the subsurface of the (101) surface. The surface partition of the Mn(III) ions provides a microscopic understanding to the observed higher catalytic activity of the (001) surface relative to the (101) surface and would contribute to further development of novel catalysts based on Mn₃O₄.