1. Carlos Larriba-Andaluz1, Santiago Ruiz-Valdepenas1,
Hui Ouyang1, Takuya Nakazawa1 , Michel Attoui*, Christopher J. Hogan Jr1
Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN 55455
Overview
• Clusters of metal aggregates and metal nanoparticles can
be orchestrated through the evaporation and subsequent
nucleation and coagulation of supersaturated metal vapors.
• Despite the arising potential that gas-phase synthesis
provides for nanoparticle generation, there is little
understanding of the physics involved in material generation,
namely nucleation
• Predicted particle and cluster growth nucleation rates
founded on principles of classical nucleation theory (CNT)
disagree, in general, with experimental nucleation rates by
orders of magnitude.
• This failure to predict cluster growth correctly is becoming a
fundamental issue not only in gas-phase material production,
but also in our general understanding of metallic and non-
metallic materials in the nanometer range.
• This study allows us to examine several theoretically
predicted and yet experimentally dissenting variations in
metal cluster properties which would be expected to occur for
clusters in the sub-5-nm size range, namely the Kelvin, the
Tolman, and the Thomson Effect.
Methods
Introduction
Conclusions and Future Work
Results
•The evaporation rate, kb, can be determined from particle
parameters and temperature. For simplification purposes,
this equation for spheres can be written in terms of its vapor
pressure as:
•The surface vapor pressure differs from the saturation
vapor pressure for particles of small size. This is due to the
Kelvin and Thomson effect. The Kelvin effect accounts for
the influence of the particle surface curvature in the surface
pressure and the Thomson effect accounts for the influence
of the charge in the cluster surface pressure. In general,
•Theoretically, at constant temperature, the evaporation rate
can be analytically solved (Figure 3).
I S. L. Girshick, C. P. Chiu -- Kinetic nucleation theory: A
new expression for the rate of homogeneous nucleation
from an ideal supersaturated vapor -- The Journal of
chemical physics 1990, 93, (2), 1273-1277
II A. W. Castleman, R. G. Keesee -- Small Clusters - Aerosol
Precursors -- Aerosol Science and Technology 1983, 2,
(2), 145-152
III C. Peineke, M.B. Attoui, A. Schmidt-Ott -- Using a
glowing wire generator for production of charged,
uniformly sized nanoparticles at high concentrations --
Journal of aerosol science 2006, 37, (12), 1651-1661
•We have performed tandem DMA evaporation experiments
on charged silver nanoclusters between 2 and 5 nm
generated using a glowing wire generator.
•Stable distributions are being formed and subsequently
studied on the tandem DMA furnace system, one selected
diameter at a time.
•Theory predicts that complete evaporation of the silver
particles, up to the limiting Thomson size should occur for
the studied temperatures.
•Experiments do not show such steep evaporation
processes. The reason for it is most likely due to the lower
gas temperature achieved inside the furnace in comparison
to the wall temperature.
•Lower aerosol flowrates as well as higher signal output are
required to verify if the experiments are consistent and
Kelvin and Thomson effects do not apply as continuum bulk
effects.
Tandem DMA Measurement of the Evaporation of
Sub 5nm Metal Nanoparticles
Experimental Setup
•Here we propose a technique where a tandem set of
furnaces (wire generator and tube furnace) intercalated with
a tandem set of DMAs are used to predict dissociation rates
of silver nanoparticles in the sub 5nm range.
•To our knowledge, this is the first time that such a task has
been undertaken for such small sizes where a combined
experimental and theoretical effort is used to quantitatively
monitor growth and dissociation kinetics of gas-phase neutral
and ionized metal clusters.
•Considering the size and properties of these nanoparticles,
the experiments provide a unique opportunity to examine the
thermodynamic properties of metallic materials at the
nanometer scale.
•The Kelvin, the Tolman, and the Thomson Effect are
predicted based on bulk scale metal properties (surface
tension and vapor pressure), which—as stated above—are
ambiguously defined for clusters, but their use in predicting
cluster formation and growth is common. We will therefore
compare experimental and theoretical results for these
predicted phenomena.
•The first DMA with an electrometer is used to characterize
the silver particles being formed.
• A distribution of particle diameters as a function of the
collected electrometer signal for several operation conditions
for the wire can be obtained.. There is a noticeable increase
in signal as the power is increased. See Figure 2.
•Fixing a voltage on the first DMA allows to collect a
monodispersed size distribution on a TEM grid and studied in
electron microscopy. Figure 1 shows 3.8nm particles where
the close up shows atom fringes unveiling the crystallinity of
such particles
•In our case, we use the temperature profile (Figure 4) in the
furnace to calculate numerically the theoretical evaporation
as a function of time assuming the temperature is that
calculated at the wall. (Figures 4 and 5)
•We can compare experimental results with theoretical
predictions. Figure 6 shows an example of the recorded
experimental variation of the particle diameter (4.7nm) as a
function of the furnace temperature set at the PID.
•Unexpectedly there is some variation of size at the smallest
temperature which we relate to slight sintering of the
crystalline particles.
•At higher temperatures T>800, the particles start to
evaporate as can be observed from the widening of the
distribution profiles.
•The evaporation recorded is too small for the assigned
furnace temperature. This probably occurs due to the air
temperature being lower than that assigned at the furnace.
•A comparison between the expected theoretical and the
experimental shifts in diameter are shown below (Figures 7
and 8) for several diameters, either assigning the peak
diameter at the crest or at the lower 5% of the maximum
assigned intensity of a given diameter distribution.
•The experimental setup consists of a closed circuit where a
flow of nitrogen between 2-5lpm is pushed through the
system. The electrospray is only used for calibration
purposes.
Figure 1 Figure 2
Figure 4 Figure 5
Figure 3
1000 K
1010 K
1020 K
1030 K
1040 K
1050 K
KELVIN
EFFECT
THOMSON
EFFECT
Figure 7 Figure 8
Figure 6
5% of the maximum intensity curve
IV C. Peineke, A. Schmidt-Ott -- Explanation of charged
nanoparticle production from hot surfaces -- Journal of
aerosol science 2008, 39, (3), 244-252.
V K. K. Nanda, A. Maisels, F. E. Kruis -- Surface Tension and
Sintering of Free Gold Nanoparticles -- The journal of
physical chemistry C 2008, 112, (35), 13488-13491
VI M. Blackman, J.R. Sambles -- Melting of very small
particles during evaporation at constant Temperature. --
Nature 1970, 226, 938