THERMAL-STABILITY OF HYDROXYL-GROUPS ON A WELL-DEFINED SILICA SURFACE

The thermal stability of hydroxyl groups was studied on a well-defined silica surface. The silica sample was a 5 mu m thick SiO2 layer grown on Si(100) by a combination of thermal oxidation and chemical vapor d eposition with SiH4 and O-2. The silica surface was cleaned and analyz ed under ultrahigh vacuum conditions. Hydroxylation (Si-O-Si + H2O --> 2SiOH) was achieved by a H2O plasma reaction in a small internal high -pressure chamber. The extent of dehydroxylation (2SiOH --> Si-O-Si H2O) of the silica surface was then investigated versus annealing temp erature. The thermal stability of the hydroxyl groups was monitored by two different monolayer sensitive experimental methods. In the primar y method, methanol (CH3OH) was used to titrate the surface SiOH specie s by hydrogen bonding between the hydroxyl groups. Secondarily, laser- induced thermal desorption (LITD) was used to desorb directly H2O from hydroxyl groups on the surface. The CH3OH temperature-programmed deso rption (TPD) signal after saturation CH3OH exposures represented the t otal SIGH surface coverage. In contrast, the LITD H2O signal appeared to originate only from neighboring (vicinal) SiOH groups. Both the CH3 OH TPD and H2O LITD experiments monitored the progressive decrease of the hydroxyl coverage versus thermal annealing from 100 to 900 degrees C. These thermal stability results are consistent with earlier measur ements of hydroxyl species versus thermal annealing on high surface ar ea silica powders. The H2O LITD measurements also indicated that the d ehydroxylation proceeded quickly at each temperature and reached a fai rly constant coverage in less than 1 min.