Fabrication, characterization, electrochemistry, and redox-induced electrochromism of viologen-functionalized silica core-shell nano-composites

- Viologen-functionalized core-shell silica nano-composites were prepared.
- Nano-composites can be strongly adsorbed on the electrode surface.
- Nano-composites showed reversible viologen redox and electrochromic behaviors.
- Violet viologen nano-composites could be separated by centrifugation.

Electroactive viologen-functionalized silica core-shell nano-composites have been prepared and characterized using thermogravimetry, field emission transmission electron microscopy, and infrared, UV-vis absorption, and X-ray photoelectron spectroscopy. The viologen monolayer functionalized nano-silica core (nano-SiO2VBen) was obtained via a substitution reaction of propylamine−modified silica nanoparticles (nano-SiO2NH2) with N,N'-di(bromomethylbenzyl)-4,4'-bipyridium dibromide (VBenBr2). The nano-SiO2VBen core was positively charged and could thus act as a support for the construction of the shell layer of viologen multilayers through, alternatively, the electrostatic adsorption of poly(sodium-p-styrenesulfonate) (PSS) and VBenBr2. Dynamic light scattering and morphological studies revealed that the dynamic sizes of the nanoparticles increased in the following order: nano-SiO2NH2, nano-SiO2VBen, and nano-SiO2VBen@(PSS/VBen)n. This was due to the assembly of organic multilayers around the nanoparticles and formation of aggregates. Cyclic voltammograms of nano-SiO2VBen- and nano-SiO2VBen@(PSS/VBen)n-modified electrodes showed two couples of redox waves in the potential range from 0 to −1.2 V (vs Hg/Hg2Cl2), corresponding to the redox processes of viologens in the nano-composites. Compared to their precursor of VBenBr2, the nano-SiO2VBen and nano-SiO2VBen@(PSS/VBen)n composites could be more strongly adsorbed on the electrode surface without co-adsorption of the polymeric supporter (Nafion), which can be attributed to a stronger molecular interaction between the present nano-composites and electrode surfaces. Finally, the redox-induced electrochromic properties of VBenBr2 and nano-composites were investigated in the solutions or polymer gel electrolytes, and the results suggest that both viologen organic electrolyte and nano-composites could act as potential candidates for the development of redox-based electrochromic indicators and displays.

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