Titanophosphate glasses as lithium-free nonsilicate pH-responsive glasses-Compatibility between pH responsivity and self-cleaning properties

摘要

Lithium silicate-based glasses have been widely used as commercially available pH glass electrodes. It was revealed that Ti~(3+)-containing titanophosphate (TiO_2-P_2O_5, TP) glasses are pH-responsive as lithium-free nonsilicate glasses for the first time. The absorption coefficient at 532 nm, α_(532) as a measure of Ti~(3+) content in TP glasses increased with increasing melting temperature. TP glasses with large α_(532) tended to give low electrical resistivity, high pH sensitivity and the short pH response time. The first post-annealing (oxidation of Ti~(3+)) of TP glasses at 600-620℃ for 60-240 h resulted in the occurrence of the photo-induced hydrophilicity along with the disappearance of pH responsivity and the increase of electrical resistivity. The second post-annealing (reduction of Ti~(4+)) of the first post-annealed TP glasses at 600-620℃ for 48 h under vacuum recovered both pH responsivity and electrical resistivity to the level of the as-prepared TP glasses with maintaining the photo-induced hydrophilicity. Moreover, the second post-annealed TP glasses had photocatalytic activity for methylene blue (MB) comparable to commercially available self-cleaning glass. Thus, TP glasses with the compatibility between pH responsivity and self-cleaning properties were obtained by the sequential post-annealing (oxidation and reduction) of as-prepared glasses. From some circumstantial evidences, pH response of TP glasses was explained in terms of phase boundary potential model related to hopping conduction of electron from Ti~(3+) to Ti~(4+) via O~(2-) ion in TP glasses rather than diffusion potential model.