Pore size and surface charge control in mesoporous TiO2 using post-grafted SAMs

Abstract

Two types of TiO2 are used as mesoporous scaffolds, one (i) randomly sintered yielding an average pore size of 15-20 nm including bottlenecks of 1-3 nm (s-TiO2), the other (ii) prepared by evaporation-induced self-assembly with a pore size of 7-9 nm (t-TiO2). The pore walls of these materials were post-grafted with phosphonic acids bearing one or two pyridinium or sulfonate head groups via 6, 10 or 14 methylene groups, in order to tune the free pore diameter and the surface charge over a broad range. The modification was characterized by FTIR spectroscopy. Charge transport through the modified pores was investigated by cyclic voltammetry using [Fe(CN)(6)](4-/3-), [IrCl6](2-/3-) [Ru(NH3)(6)](3+/2+), and (ferrocenylmethyl) trimethylammonium as electroactive tracer ions and La3+ or naphthalene trisulfonate as non-electroactive species. The Faradaic current through the pores is controlled by the combination of surface charge, tracer ion charge, charge of the non-electroactive ions present, as well as the pore diameter. High currents due to strong preconcentration are observed, e. g. a partitioning coefficient value of 7 x 10(3) for [Fe(CN)(6)](4-/3-) on a modified electrode making it a candidate for ion-exchange voltammetry. Other phenomena presented are: (i) electrostatic closure of the porous system due to overlapping Debye layers, (ii) charge inversion of the pore walls, and (iii) the mode of charge propagation along the pore walls. Interestingly s-TiO2 is more effective at building up an electrostatic barrier compared to t-TiO2, probably because of narrow bottlenecks which interconnect the pores in s-TiO2 only.