NNANO D Supplementary Information

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1 NNANO D Supplementary Information Borosilicate nanoparticles prepared by exothermic phase separation Virendra K. Parashar, Jean-Baptiste Orhan, Abdeljalil Sayah, Marco Cantoni, and Martin A.M. Gijs Video microscopy Video microscopy reveals that, when exposing a sol droplet using a micropipette to moisture in air, a solid film is instantaneously formed on the surface of the droplet (see Video 1). Exposing the sol via the micropipette to a reservoir filled with water induces a vigorous exothermic reaction (see Video 2), during which the sol transforms into a solid phase and a colloid of nanoparticles. Scanning Electron microscopy Fig. S1a shows a Scanning Electron Microscope (SEM) photograph of nanoparticles collected from the colloid. The dispersion of the particle size is represented in the histogram of Fig. S1b, showing a majority of nanoparticles in the nm range. The size distribution of the nanoparticles is determined using ImageJ freeware utilizing the Analyze Particles function (ImageJ 1.37v, Wayne Rasband, National Institutes of Health, USA). BET surface area measurements indicate these nanoparticles are non-porous having a total surface area in the range of m 2 /g. Examination of the solid phase by SEM reveals a surface with crater-like hemispherical cavities that are comparable in size to the nanoparticles (see Fig. S1c). This suggests the upsurge of nanoparticles from the surface, a hypothesis which is confirmed by SEM analysis of a sample where the reaction of the borosilicate sol with water is quenched with liquid nitrogen (Fig. S1d). Temperature measurements and energy calculations Boroxine rings as well as B-O bonds are known to be highly reactive toward hydrolysis 1 and are easily cleaved by water under neutral, acidic or basic conditions 2. Hence, we expect an exothermic reaction when mixing our binary sol with water. Using a miniaturized Pt temperature sensor 3, we in situ probe the temperature rise during mixing of the sol with water in a 5 ml glass beaker. When 1 ml of sol is gently 1

2 mixed with 1 ml of water under magnetic stirring conditions, we notice an instantaneous temperature rise of (3.7±0.1) C from an initial temperature of 23.9 C upon mixing (see Figure S2). The observed temperature rise can be well understood by a theoretical analysis of the hydrolysis energies of the B-O and Si-O bonds. As each TMB molecule contributes three B atoms, the number of B atoms in 1 ml of sol is equal to the number of moles of TMB in 1 ml of sol 3 N A = x 10 21, with N A Avogadro s number. When all B atoms are bonded to oxygen, the number of B-O bonds in 1 ml of sol = x The energy released by hydrolysis of one B-O bond per TMB molecule in 1 ml of sol = (E B-O / N A ) x (3.246 x ) = J, with E B-O = kj/mole 4 the B-O bond energy per mole. The number of Si-O bonds, using all Si originating from TEOS, in 1 ml of sol = x The energy consumed by hydrolysis of one Si-O bond per TEOS molecule in 1 ml of sol = (E Si-O / N A ) x (1.606 x ) = J, with E Si-O = kj/mole 4 the Si-O bond energy per mole. Hence, the total energy release by the mentioned hydrolysis reactions in 1 ml of sol is J J = J E. hyd As our sol has 20 wt% borosilicate in isopropanol solvent, the specific heat of 1 gram of our sol-water mixture is C p,mix = 0.1 C p,b C p,i C p,w = 3.2 J/gK, with C p,b = 0.70 J/gK, C p,i = 2.60 J/gK, and C p,w = 4.18 J/gK, the specific heat of borosilicate, isopropanol and water, respectively 5, and taking a density of 1 g/ml for both water and the sol. The provided hydrolysis energy results in a theoretical temperature rise ΔT = E /( mc ) = 7.32 K hyd p, with m = 2 g for the 2 ml of the solwater mixture. The experimental temperature rise is (3.7±0.1) C, corresponding to a liberated energy of (24±1) J, about half of the calculated value. We can perfectly understand this by the fact that not all B-O bonds are hydrolyzed during the nanoparticles formation. Our analysis gives strong evidence that heat is generated only at the sol-water contact interface, and especially in boron-rich regions of the sol. Nanoparticle formation mechanism All our experimental data converge to proposing a nanoparticle generation process via exothermic phase separation (see Fig. S3). Due to the breaking of B-O bonds at boron-rich regions of the sol-water contact interface, heat will be locally generated. Heat and surface-tension drive the disconnected matter as spherical nanoparticles away from the interface. The broken B-O bonds on the nanoparticle surface will be 2

3 sealed by formation of B-O-Si linkages. This explains the measured borosilicate composition of the nanoparticles. Instant sealing of the broken Si-O bond on the solid phase leads to a Si-O-Si bond and explains why the hemispherical cavities are nearly free of boron. Stability of the borosilicate nanoparticles We also have evaluated the chemical inertness of the borosilicate nanoparticles in comparison with pure silica nanoparticles. The pure silica nanoparticles are prepared as follows. In total, 9 ml of TEOS and 20 ml of hydrolyzing mixture containing 40 vol. % water, 12 vol % ethanol and 48 vol % aqueous ammonia (29.5 wt.% NH 3 ) are used. First 1 ml TEOS and 2 ml hydrolyzing mixture are mixed in a 100 ml beaker at 25 C by magnetic stirring at 300 rpm. The same quantities of TEOS and hydrolyzing mixture are added after an interval of 5, 4, 3, 2, 2 minutes, respectively, and the rest of the TEOS and hydrolyzing solution is added after 5 minutes of the last addition. Then the beaker is transferred to a water bath maintained at C. The stirring speed at this point is increased to 600 rpm for 30 minutes under open conditions. The resultant silica nanoparticles are dried under vacuum at 180 C and further calcined at 350 C. The chemical inertness of both types of nanoparticles is evaluated as follows. Thermal treatment of the nanoparticles is done at 121 C for 30 minutes in de-ionized water (autoclave sterilization conditions) in an acid digestion bomb (type 4744 from Parr Instrument Company, Moline, IL, USA). Fig. S4a,b show that the treatment of the silica nanoparticles induces silanol groups/gel layer formation on the surface, leading to agglomerated nanoparticles after drying. As expected, due to the stable chemical bonds in silicon-boron binary oxide, we do not observe this phenomenon for the borosilicate nanoparticles (Fig. S4c,d). 3

4 Figure S1: Microscopic characterization of the nanoparticles formation process. a SEM photograph of borosilicate nanoparticles, b histogram of the size distribution of the nanoparticles, c SEM photograph of the solid phase showing hemispherical cavities, d SEM photograph of a fractured sample showing a nanoparticle under formation, obtained by quenching the reaction with liquid nitrogen. Figure S2: Temperature measurement during mixing of sol with water. 4

5 Figure S3: Illustration of the proposed mechanism of nanoparticle generation. 5

6 Figure S4: Characterization of the stability of the borosilicate nanoparticles. SEM photographs of: a as-made silica nanoparticles, b gel layer formation and gel agglomerated silica nanoparticles after autoclave treatment at 121 C, c as-made borosilicate nanoparticles, d discrete borosilicate nanoparticles after autoclave treatment at 121 C. 6

7 References 1. Hall, D. G. in Boronic Acids (ed. Hall, D. G.) 1-99 (2006). 2. Irwin, A. D., Holmgren, J. S., Zerda, T. W. & Jonas, J. Spectroscopic investigations of borosiloxane bond formation in the sol-gel process. Journal of Non-Crystalline Solids 89, (1987). 3. Moser, Y. & Gijs, M. A. M. Miniaturized flexible temperature sensor. Journal of Microelectromechanical Systems 16, (2007). 4. Lee, S. K., Musgrave, C. B., Zhao, P. D. & Stebbins, J. F. Topological disorder and reactivity of borosilicate glasses: Quantum chemical calculations and O-17 and B-11 NMR study. Journal of Physical Chemistry B 105, (2001). 5. CRC Handbook of Chemistry and Physics, 63rd edition, pp. D-178, D-180 (CRC Press, Inc., 1983). 7