3.2. XPS analysis
To further confirm the composition and chemical state of a protective layer on nanoparticle surface, XPS measurements were performed for MT 500 SA nanoparticles (for comparison) and the SSL was subjected to 7 ppm of OCl−, relative amounts of CaSO4·2H2O, Ca2+, and SO42− ions (main SPW constituents) using Gaussian–Lorentzian fitting. The high resolution spectra of each region are shown in Fig. 4. The full scale patterns of MT 500 SA and SSL with and without chlorine treatment are given in Fig. SI 4 and SI 5. The atom% composition of MT 500 SA and SSL including standard deviations is summarized in Table SI 1. Undoubtedly, there was a difference in O (Osingle bondAl and Osingle bondTi in particular) and Ti atom% content in both MT 500 SA and SSL in the presence of 7 ppm chlorine in SPW. For instance, Osingle bondTi content decreased from 28.8 to 26.1% and from 10.3 to 9.8%, whereas Osingle bondAl increased from 28.2 to 35.6% and from 12.5 to 14.4% in the presence of chlorine in MT 500 SA and SSL samples, respectively. Such changes in atom% surface composition may indicate the redistribution or even a damage of a protective layer as supported by the TEM and SEM observations (Figs. 3, SI 2, SI 3).