Correlation between Electrical Conductivity, Relative Humidity, and Pore Connectivity in Mesoporous Silica Monoliths

Incipient wetness was used to load cobalt nitrate into the pores of a set of silica monoliths with pores arranged in a hierarchal fashion, in the form of either a bimocal pore structure containing only macropores and textural mesopores, or trimodal pore structures with additional mesopores as a result of surfactant templating. The volume and diameter of the surfactant templated mesopores were controlled by varying concentration and using surfactants of different chain length, tetradecyl-, hexadecyl-, and octadecyltrimethylammonium bromide (C(14)TAB, C(16)TAB, and C(18)TAB, respectively). Platinum replication of samples made with C(14)TAB and C(18)TAB was performed. The native and the cobalt nitrate impregnated monoliths were characterized with nitrogen physisorption, water sorption, and electron microscopy. The electrical conductivity of the cobalt nitrate impregnated monoliths was measured as a function of relative humidity. The presence of the surfactant templated mesopores resulted in significantly higher conductivity at all relative humidities, and generally the conductivity increased as a function of surfactant chain length (going from C(14)TAB to C(18)TAB surfactants) and with increasing surfactant concentration in the precursor sols. The findings are suggested to originate from differences in the macroscopic mesopore connectivity throughout the monoliths, a property which is otherwise difficult to assess by other characterization techniques.