Nitrided galloaluminophosphates `AlGaPONʹ: influence of the nitridation time on the nature and the stability of surface nitrogenous species

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to monitor the changes induced by nitridation on the surface of a mixed galloaluminophosphate Al0.5Ga0.5PO4. The spectrum of the phosphate was compared to those recorded on four nitrided `AlGaPONʹ (5.3, 11.0, 15.9, 23.3 wt% N) obtained by activating the precursor under ammonia flow at 750 °C for increasing periods of time. NH4+, co-ordinated NH3, –NH2 and –NH– species were identified on the surface of the `AlGaPONʹ solids. Their relative concentrations depend on the nitridation time. This study reveals that the concentration of amido (–NH2) and imido (–NH–) species increases with the nitridation time. Simultaneously, the number of surface acid sites decreases significantly. The thermal stability of the nitrogen species was studied by conjugating DRIFTS experiments and mass spectrometry thermally programmed desorptions (TPD-MS). The conjunction of those two techniques reveals that the removal of –NH2 species starts below 300 °C but that some groups are stable up to 600 °C.