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 deposition with SiH4 and O-2. The silica surface was cleaned and analyzed 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 temperature. The thermal stability of the hydroxyl groups was monitored by two different monolayer sensitive experimental methods. In the primary method, methanol (CH3OH) was used to titrate the surface SiOH species 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 desorption (TPD) signal after saturation CH3OH exposures represented the total SIGH surface coverage. In contrast, the LITD H2O signal appeared to originate only from neighboring (vicinal) SiOH groups. Both the CH3OH 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 measurements of hydroxyl species versus thermal annealing on high surface area silica powders. The H2O LITD measurements also indicated that the dehydroxylation proceeded quickly at each temperature and reached a fairly constant coverage in less than 1 min.