534. Mesoporous materials as gas sensors

Fig. 1 Schematic of the nanocasting synthesis of ordered mesoporous metal oxides. The product is created inside the pores of a silica matrix (a) which is later removed (b); the mesoporous replica retains the nanostructural symmetry of the matrix. Alternatively, mesoporous carbon may be created first in order to serve as the matrix for a second replication process (c and d); this is useful when the metal oxide is unstable against the conditions (chemical etching) under which silica is removed (b).

Fig. 2 Model of the electron depletion layer in resistive sensors. In granular materials with grain sizes larger than ca. twice the depletion layer thickness (left) the core regions do not contribute much to an overall change in conductivity (i.e. change in depletion layer thickness) upon interaction with the target gas. Smaller grains (center) and, especially, mesoporous materials with thin pore walls (right) are fully depleted, which maximizes the sensitivity.

T. Wagner, et al, Chem. Soc. Rev. 42, 4036 (2013)
https://doi.org/10.1039/c2cs35379b

(1) Chemical synthesis of periodically ordered, uniform mesopores usually requires the utilization of a porogen, i.e. an auxiliary species that acts as a template or structure director.
(2) The ‘soft templating’ route has turned out to be successful mostly for such systems which have
a tendency to form amorphous phases at moderate (sol-gel chemistry) temperature.
(3) The SAW type is considered more sensitive than BAW (due to higher frequencies in the GHz range as compared to the MHz range for BAW).
(4) Mesoporous materials show improved properties in terms of their nanostructural dimensions, large specific surface areas, the option to finely disperse active species (dyes, nanoparticles, etc.), and specific gas diffusion phenomena within the pores.