To examine the effect of elapsed time on the nanoporosity, subnanoporous silica thin films, fabricated by plasma-enhanced chemical vapor deposition (PECVD), were investigated by means of lowenergy positron annihilation lifetime spectroscopy and vapor-adsorption ellipsometric porosimetry. The structural change of the subnanoscaled pores was elucidated by comparison of the as deposited and 6-month-old films. It is expected that the change in the subnanoscaled pores of the present films, after exposure to air for half a year, is due to the adsorption of water molecules from air, followed by the filling up the nanoscaled pores as well as partial polycondensations between silanol groups at the silica grain boundaries of the films.
Recent progresses in inorganic membranes used for water treatment in reverse osmosis (RO) and nanofiltration (NF) are reviewed. Inorganic membranes, which show robust properties, can be used in harsh conditions such as high temperatures, chlorine-containing and acidic solutions. In this review, robust properties required for use in RO/NF are proposed, and then, inorganic RO/NF membranes, including zeolite, sol-gel derived organosilica, carbon-based materials (carbon nanotube, graphene oxide), and CVD-derived silica, are summarized. In addition, possible applications, including reverse osmosis at high temperatures, acid recovery in acid/metal mixture, and chlorine tolerance are introduced.
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