title: On the Origin and Underappreciated Effects of Ion Doping in Silica.
abstract: The origin of selectivity in the hollowing of silica nanoparticles is investigated to further understand silica. It is realized that, during the synthesis, the silica precursors are essentially ion-paired polyelectrolytes, whose nucleation depends on the concentration of the counter ions, and most importantly, the size/length of the poly(silicic acid). Thus, the "silica" that nucleates out at the different stages of synthesis has different degrees of ion doping, which explains its solubility in water, its microporosity, and the selective etching phenomena. The etching of silica in water is shown to be a matter of silica solubility, which correlates to the relative amounts of solvent and to the solvent quality (the water/isopropanol ratio). Hollowing does not occur when the silica nanoparticles are incubated in solutions presaturated with "silica," ruling out surface reposition and Ostwald ripening as the hollowing mechanism. The embedded ions in silica are confirmed by elemental analysis (CHNS) and inductively coupled plasma-mass spectrometry. The high ionic doping ratios (N/Si = 2.3% for NH3 -catalyzed silica; Na/Si = 11.2% for NaOH-catalyzed silica) explain the unusual solubility of silica in neutral water. The new view of silica with the ionic impurities on the central stage allows for insights in silica properties and versatility in synthetic design.
authors.affiliation: Division of Chemistry and Biological Chemistry, Nangyang Technological University, 21 Nanyang Link, Singapore, 637371, Singapore.
authors.forename: Xiaohui
authors.initials: X
authors.lastname: Song
authors.affiliation: Division of Chemistry and Biological Chemistry, Nangyang Technological University, 21 Nanyang Link, Singapore, 637371, Singapore.
authors.forename: Tao
authors.initials: T
authors.lastname: Ding
authors.affiliation: Division of Chemistry and Biological Chemistry, Nangyang Technological University, 21 Nanyang Link, Singapore, 637371, Singapore.
authors.forename: Lin
authors.initials: L
authors.lastname: Yao
authors.affiliation: Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore, S117602, Singapore.
authors.forename: Ming
authors.initials: M
authors.lastname: Lin
authors.affiliation: Division of Chemistry and Biological Chemistry, Nangyang Technological University, 21 Nanyang Link, Singapore, 637371, Singapore.
authors.forename: Rachel Lee
authors.initials: RL
authors.lastname: Siew Tan
authors.affiliation: Division of Chemistry and Biological Chemistry, Nangyang Technological University, 21 Nanyang Link, Singapore, 637371, Singapore.
authors.forename: Cuicui
authors.initials: C
authors.lastname: Liu
authors.affiliation: Division of Chemistry and Biological Chemistry, Nangyang Technological University, 21 Nanyang Link, Singapore, 637371, Singapore.
authors.forename: Katarzyna
authors.initials: K
authors.lastname: Sokol
authors.affiliation: School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, Singapore, 637459, Singapore.
authors.forename: Le
authors.initials: L
authors.lastname: Yu
authors.affiliation: School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, Singapore, 637459, Singapore.
authors.forename: Xiong Wen David
authors.initials: XW
authors.lastname: Lou
authors.affiliation: Division of Chemistry and Biological Chemistry, Nangyang Technological University, 21 Nanyang Link, Singapore, 637371, Singapore.
authors.forename: Hongyu
authors.initials: H
authors.lastname: Chen
keywords: counter ions; ion pairing, doping, nanoparticles; polyelectrolytes; porosity; silica