Preparation of nanoporous titania films by surface sol-gel process accompanied by low-temperature oxygen plasma treatment

Nanoporous ultrathin films of titania were prepared. First, a titania/poly(acrylic acid) (PAA) nanocomposite film was prepared via the surface sol-gel process, with the thickness of the titania gel layer to be 0.44 ± 0.10 nm/cycle. The organic species in the film were then removed by low-temperature oxygen plasma treatment, as monitored by quartz crystal microbalance frequency measurement and Fourier transform infrared and X-ray photoelectron spectroscopies. The plasma treatment was effective at least up to the depth of 10-12 nm. The resulting ultrathin titania film showed a UV-visible absorption with the edge at about 330 nm, which is considerably blue-shifted from that of bulk titania. The film surface was flat and uniform over a large area, and its density was as low as 0.54 g/cm³. Transmission electron microscopy revealed the presence of pores with diameters of ca. 2 nm. The pore existence was confirmed by the formation of silver nanoparticles inside the film by photoreduction of silver ions.