Departamento de Física, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Cuidad Universitaria, Buenos Aires 1428, Argentina.
Phys Chem Chem Phys. 2019 Jan 30;21(5):2743-2754. doi: 10.1039/c8cp07192f.
This work reports the fabrication of redox-active films of oligomeric and molecular viologens and mesoporous silica via the infiltration method. Pore-ellipsometry and UV-vis confirm that low-molecular-weight poly(viologens) in solution are able to enter the mesoporous structure, in contrast to high-molecular weight polymers that adsorb only on top of the film. Cyclic voltammetry shows that viologens are able to reach the bottom of the pores and access the electrode/film interface. However, the number of viologen sites that can be accessed by cyclic voltammetry at 50 mV s-1 is only a tenth of the total viologen population determined by UV-vis and pore-ellipsometry. The effect is ascribed to the very small apparent diffusion coefficient for charge transport within the film (Dapp < 10-12 cm2 s-1). A theoretical model is put forward to describe charge transport via the electron-hopping mechanism for redox sites randomly adsorbed on the inner walls of the pores. Our model predicts that the threshold of charge percolation occurs for viologen surface coverages close to those observed in our experiments; therefore, the low fraction of electrochemically addressable viologens is ascribed to inefficient charge percolation via the electron-hopping mechanism.
本文报道了通过浸渍法制备氧化还原活性的低聚和分子型联吡啶盐以及介孔二氧化硅薄膜。孔椭圆测量法和紫外-可见光谱证实，溶液中的低分子量聚联吡啶盐能够进入介孔结构，而高分子量聚合物仅吸附在薄膜的顶部。循环伏安法表明，联吡啶盐能够到达孔的底部并到达电极/薄膜界面。然而，在 50 mV s-1 的循环伏安法中可以访问的联吡啶盐位点数仅为紫外-可见光谱和孔椭圆测量法确定的总联吡啶盐数目的十分之一。这种现象归因于薄膜内电荷传输的表观扩散系数非常小（Dapp < 10-12 cm2 s-1）。提出了一个理论模型来描述通过随机吸附在孔内壁上的氧化还原位点的电子跳跃机制进行电荷输运。我们的模型预测，电荷渗流的阈值发生在联吡啶盐表面覆盖率接近实验观察到的那些情况，因此，电化学可寻址联吡啶盐的低分数归因于通过电子跳跃机制的有效电荷渗流。
