1. Results and Discussion Results and Discussion
Synthesis and characterization of silica nanospheres
Griffin Elbert, Robert Vittoe, and Roderick M. Macrae
Institute for Green and Sustainable Science (IGSS)
Marian University, Indianapolis, IN 46222
Introduction
Results and Discussion
Results and Discussion
Several methods were explored to characterize the size and uniformity of the silica nanospheres. The first method considered was dynamic light scattering (DLS). Light from a 532 nm diode-pumped solid state laser was passed through a clean dilute solution of nanoparticles in ethanol, and the perpendicularly-scattered light was measured using a Hamamatsu S1133 silicon photodiode coupled to a Vernier Instruments Universal Laboratory Interface. The Stokes-Einstein equation may be used to calculate the hydrodynamic radius of the particles from the translational diffusion coefficient (in turn related to the correlation time). [2]
The purpose of this study is to synthesize and characterize monodisperse nanoscale-sized silica spheres. The silica spheres are subsequently to be used as a scaffolding to fabricate a uniform, mesoporous, cellulose-based “green” carbon catalyst.
Methods
The silica nanospheres were synthesized using a process developed by Stöber, Fink and Bohn.[1] Several different molar concentrations of tetraethyl orthosilicate (TEOS) were reacted with different molar concentrations of ammonia in a solution of water and ethanol. The reaction was allowed to run for 24 hours, following which the silica nanoparticle product was worked up by repeated washing and centrifugation. Varying the concentrations of TEOS and ammonia alters the rate at which the reaction proceeds and thus the size of the silica nanospheres formed.
Optical microscopy of silica nanospheres spray-deposited onto a microscope slide indicated the formation of larger aggregates. Some evidence of finer structure near the diffraction limit (size order 1 µm) can be seen at the highest magnifications.
Stokes-Einstein Equation
where:
r = hydrodynamic radius
D = translational diffusion coefficient
k = Boltzmann’s constant
h = dynamic viscosity
Photon count for 90o scattering as silica nanospheres precipitate out of solution
A more straightforward measurement of particle size can be obtained via the sedimentation velocity, determined by the rate at which particles clear the optical window of the laser beam.[3] With an estimated average beam diameter of 540 µm, a clearing time of 90 min. corresponds to a particle diameter of about 300 nm.
For monodisperse spherical particles the intensity autocorrelation function is:
n= refractive index
l = laser wavelength
t = correlation time
Stokes Equation
vs = terminal settling velocity;
h = dynamic viscosity;
rf = mass density of fluid;
rp = mass density of particle;
g = gravitational acceleration;
r = hydrodynamic radius
Optical micrographs of silica nanospheres spray-deposited onto a glass slide
(a) A B
Because silica nanospheres have been known to deposit in an fcc arrangement, Bragg diffraction at optical wavelengths was also explored as a characterization option, with inconclusive results. [4]
References
(b) C D
[1] W. Stöber, A. Fink, E. Bohn, “Controlled growth of monodisperse silica spheres in the micron size range,” J. Colloid Interface Sci. 26 (1968) 62.
[2] B. J. Frisken “Revisiting the method of cumulants for the analysis of dynamic light scattering data,” Appl. Optics, 40 (2001) 4087.
[3] Y. C. Agrawal, H. C. Pottsmith, “Instruments for particle size and settling velocity observations in sediment transport,” Marine Geology 168 (2000) 89.
[4] H. Míguez et al., “Photonic crystal properties of packed submicrometric SiO2 spheres,” Appl. Phys. Lett. 71 (1997) 1148.
Acknowledgements
(a) Experimental DLS data obtained from a nanoparticle solution, together with (b) model data corresponding to an exponentially-correlated one-dimensional random walk. The experimental data appear to be dominated by an artifact of the acquisition electronics. Future measurements will probe a shorter timescale.
The authors wish to thank the EPA Environmental Education Grant Program for funding of the 2013 IGSS Summer Program through grant number
NE-00E
Silica deposit at low magnification B) Silica deposit at 40x
C) Silica deposit at 400x D) Silica deposit at 1000x