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Abstract

Aerosols can act as cloud condensation nuclei and/or ice nucleating particles, influencing cloud properties. In particular, ice nucleating particles show a variety of different and complex mechanisms when interacting with water during the freezing process. To gain a fundamental understanding of the heterogeneous freezing mechanisms, studies with proxies for atmospheric ice nucleating particles must be performed. Graphene and its derivatives offer suitable model systems for soot particles, which are ubiquitous aerosols in the atmosphere. In this work, we present an investigation of the ice nucleation activity of different types of graphene and graphene oxides. Immersion droplet freezing experiments as well as additional analytical analyses, such as X-ray photoelectron spectroscopy, Raman spectroscopy and transmission electron microscopy, were performed. We show within a group of samples that a highly ordered graphene lattice (Raman G band intensity >50%) can support ice nucleation more effectively than a lowly ordered graphene lattice (Raman G band intensity <20%). Ammonia-functionalized graphene revealed the highest ice nucleation activity of all samples. Atmospheric ammonia is known to play a primary role in the formation of secondary particulate matter, forming ammonium-containing aerosols. The influence of functionalization on interactions between the particle interface and water molecules, as well as on hydrophobicity and agglomeration processes, is discussed.
Ice Nucleation Activity of Graphene and Graphene Oxides
Thomas Hä
usler,
Paul Gebhardt,
Daniel Iglesias,
Christoph Rameshan,
Silvia Marchesan,
Dominik Eder,
and Hinrich Grothe
,
*
Institute of Materials Chemistry, TU Wien, 1060 Vienna, Austria
Department of Chemical and Pharmaceutical Sciences, Universitàdegli studi di Trieste, 34127 Trieste, Italy
*
SSupporting Information
ABSTRACT: Aerosols can act as cloud condensation nuclei
and/or ice-nucleating particles (INPs), inuencing cloud
properties. In particular, INPs show a variety of dierent and
complex mechanisms when interacting with water during the
freezing process. To gain a fundamental understanding of the
heterogeneous freezing mechanisms, studies with proxies for
atmospheric INPs must be performed. Graphene and its
derivatives oer suitable model systems for soot particles,
which are ubiquitous aerosols in the atmosphere. In this work,
we present an investigation of the ice nucleation activity (INA)
of dierent types of graphene and graphene oxides. Immersion
droplet freezing experiments as well as additional analytical
analyses, such as X-ray photoelectron spectroscopy, Raman
spectroscopy, and transmission electron microscopy, were performed. We show within a group of samples that a highly ordered
graphene lattice (Raman G band intensity >50%) can support ice nucleation more eectively than a lowly ordered graphene
lattice (Raman G band intensity <20%). Ammonia-functionalized graphene revealed the highest INA of all samples. Atmospheric
ammonia is known to play a primary role in the formation of secondary particulate matter, forming ammonium-containing
aerosols. The inuence of functionalization on interactions between the particle interface and water molecules, as well as on
hydrophobicity and agglomeration processes, is discussed.
INTRODUCTION
At temperatures below 35 °C, ice forms via the homogeneous
nucleation of supercooled droplets or heterogeneous nucleation
on ice-nucleating particles (INPs).
13
At temperatures above
35 °C, heterogeneous nucleation is considered to be the
dominant mechanism.
3,4
Therefore, INPs play a major role in
the ice forming process in clouds. The composition and origin
of INPs have been studied intensively over the past few
decades, as summarized, for example, by Murray et al.
5
However, the microscopic and molecular mechanisms of
heterogeneous ice nucleation are complex and remain poorly
understood. Necessary attributes, such as the ice-like structure
or hydrophobicity of the INP, can rarely be applied.
3
Factors
controlling heterogeneous processes have been investigated
intensively, but the knowledge remains fragmentary.
5
To
answer the major question of ice nucleationwhat makes an
eective ice-nucleating site?investigations need to focus on
simple INP proxies. Consequently, more laboratory and
atmospheric data are required.
6
In this work, we focus on surrogates of soot particles. A series
of studies demonstrated that black carbon as well as organic
soot components forms a large fraction of urban aerosols.
79
The ice nucleation activity (INA) of soot has been well-
investigated, for example, by DeMott
10
and Dymarska et al.
11
Nevertheless, the complexity of the atmospheric soot makes
forecasts of their ice-nucleating behavior rather dicult. Several
laboratory and eld studies showed that an increase in the INA
of aged soot results from an increase in the hydrophilicity of the
surface upon oxidation.
1216
However, oxidation also impacts
the nanostructure of the soot, making it dicult to assess the
separate eects of soot nanostructure and hydrophilicity via
experiments. Therefore, graphene, a simple proxy substance
with chemical and structural similarities to soot,
17,18
was
investigated to conrm or correct the established rules
regarding the necessary ice nucleation characteristics. Graphene
is a single two-dimensional layer of carbon atoms bound in a
hexagonal lattice structure.
17
It was rst isolated and identied
in 1962 by Böhm.
19
Since then, graphene has been the focus of
extensive studies, primarily because of its exceptional electrical,
thermal, and mechanical properties.
20
Lupi and Molinero
21
used molecular dynamics simulations to
investigate the eect of changes in the hydrophilicity of model
graphitic surfaces on the freezing temperature of ice. Their
results indicate that the ordering of liquid water in contact with
the surface plays an important role in the heterogeneous ice
nucleation mechanism and that the hydrophilicity of the surface
Received: October 28, 2017
Revised: February 23, 2018
Published: March 1, 2018
Article
pubs.acs.org/JPCC
Cite This: J. Phys. Chem. C 2018, 122, 81828190
© 2018 American Chemical Society 8182 DOI: 10.1021/acs.jpcc.7b10675
J. Phys. Chem. C 2018, 122, 81828190
This is an open access article published under a Creative Commons Attribution (CC-BY)
License, which permits unrestricted use, distribution and reproduction in any medium,
provided the author and source are cited.
is not generally a good predictor of the INA. These ordered
water molecule domains of bilayer hexagons are necessary for
the INA of graphene but have not been observed on
hydrophobic or hydrophilic atomically rough surfaces. They
suggested that knowledge of the actual nanostructure and
spatial distribution of chemical groups in soot and other
atmospheric carbon particles is needed for an accurate
prediction of the INA of these aerosols. A molecular dynamics
study by Bi et al.
22
revealed that a crystalline graphitic lattice
with an appropriate hydrophilicity may indeed template ice and
thus signicantly enhance its INA. Their calculations
demonstrated that the templating eect is found to transit
from within the rst contact layer of water to the second as the
hydrophilicity increases, yielding an oscillating distinction in the
INA of crystalline and amorphous graphitic surfaces.
Furthermore, it was evident that crystalline graphene becomes
up to 105times more ecient within certain hydrophilicity
ranges, suggesting that the crystallinity is also a key factor for
ice nucleation under such hydrophilicity. The experimental
investigation of Zheng et al.
23
showed that a sprinkle of
graphene oxide nanoakes is eective for condensing water
nanodroplets and seeding ice epitaxy on graphite under
ambient conditions. They discovered that ice nucleation and
growth can be inuenced by modifying the functional groups of
graphene oxide nanoakes and by intermolecular hydrogen
bonding between nanoakes. Carboxylate groups introduced by
base treatment play a key role in the INA of graphene oxide
nanoakes. So-called charge-assisted hydrogen bonding
24
allows interaction with water molecules, thus increasing the
INA. Reduction via acid or ammonium treatment decreases the
INA signicantly. Furthermore, by arranging graphene oxide
nanoakes in one dimension, an ice-like structure can be
induced, leading to an increase in the INA. A phenyl ring
structure, however, reduces the number of possible hydrogen-
bonding carbonyl sites for ice nucleation and decreases the
INA.
Whale et al.
25
investigated dierent kinds of carbon
nanomaterials in laboratory experiments using an immersion
freezing technique. Their results agreed with the calculations
done by Lupi and Molinero
21
and Lupi et al.
26
and showed that
materials with a lower oxidation state nucleate ice more
eciently than materials with a higher oxidation state. Any
oxidation, roughness or curvature was found to decrease the
observed nucleation temperature. The result that oxidized
surfaces nucleate ice less well than atomically at surfaces is
somewhat in contradiction with the commonly stated chemical
bondingrequirement for ice nucleation.
3
Oxides or other polar
groups on the surface of INPs are meant to oer so-called
functional sites that are able to interact with water molecules
and nucleate ice.
Biggs et al.
27
modied graphene oxides by means of thiol
epoxy chemistry, resulting in materials with increased INA.
They revealed that hydrophobic chains increased the
heterogeneous nucleation temperature from 22.5 to 12.5
°C. Hydrophilic surface modications did not promote the
activity but also did not reduce the underlying INA of the
graphene oxide. They suggested that due to the functionaliza-
tion and increase in hydrophobicity, an increased degree of
aggregation occurred, with larger aggregates potentially leading
to more nucleation. Ammonia is highly relevant in the
atmosphere, as it has been shown to play a primary role in
the formation of secondary particulate matter, forming
ammonium-containing aerosols.
28
Ammonium-containing aero-
sols constitute the major fraction of PM2.5 aerosols in the
atmosphere.
28
Anthropogenic ammonia originates, for example,
from soil because of agriculture activities and from industrial
and trac emissions.
29,30
Therefore, ammonia-functionalized
graphene should be investigated.
Because of the dierences in approaches concerning the
required features of an INP, investigations of simple and closely
related INPs are important to gain more information on the
characteristics of a functional site. Therefore, graphene was
chosen to be a good proxy substance to fundamentally
understand heterogeneous ice nucleation. The goal of our
study was to investigate the immersion-mode ice nucleation
characteristics of dierent kinds of graphene and graphene
modications. Moreover, the graphene samples were analyzed
regarding their chemistry and structure by means of X-ray
photoelectron spectroscopy (XPS), Raman spectroscopy, and
transmission electron microscopy. Combining these data, we
propose possible explanations as to why some graphene
samples are good INPs while others are not.
METHODS
The INA was determined by an oil-immersion freezing
technique, which was used in recent publications, for example,
by Hauptmann et al.,
31
Pummer et al.,
32
and Zolles et al.
33
Supplementary analytical techniques were applied to character-
ize the chemical properties, surface and bulk properties, and
morphology of the INPs.
Cryomicroscopy. The measurement setup consisted of a
custom-built freezing cell that was placed directly underneath a
light microscope. This experimental setup has been used in
previous studies, in which a detailed description can be
found.
32,33
Here, only a short description of the technique is
given. The experimental setup consisted of a light microscope
to observe the freezing experiment, a freezing cell to cool the
sample, and a computer to control the cell temperature and
cooling rate. The main part of the custom-built freezing cell is a
thermoelectric cooler (TEC, Peltier element Quick-Cool QC-
31-1.4-3.7). The TEC enables cooling rates between 0.1 and 10
°C/min down to a temperature of 40 °C. Two gas connectors
on the shell of the freezing cell allow ushing with dry nitrogen.
This is done before every experiment to remove humidity and
to establish a neutral atmosphere. The freezing process is
observed via a glass window on top of the freezing cell using the
light microscope. The INA of the graphene samples was
determined in the immersion freezing mode using a wateroil
emulsion technique. A stable suspension of the INPs in
ultrapure water was achieved by sonication for 5 min with an
operating frequency of 40 kHz. Graphene is considered to
disperse poorly in water.
34,35
Nevertheless, the oxidation of
graphene together with the use of sonication allows graphene
and graphene oxides to be dispersed in a larger number of
solvents, including water.
34,35
The samples used in this work
revealed a sucient degree of oxidation to achieve stable
suspensions. Images of the selected aqueous graphene and
graphene oxide suspensions immediately after sonication and
after settling for 30 min are provided in the Supporting
Information (see Figure S1). To avoid any subsequent phase
separation, further preparation steps were carried out
immediately after sonication. Graphene samples from Sigma-
Aldrich were purchased directly as an aqueous suspension. The
suspensions were emulsied into an oil matrix (80 wt %
paran, 20 wt % lanolin), producing droplets in the
micrometer range with a diameter of 2080 μm. Only droplets
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with diameters between 35 and 45 μm were used for evaluation.
The wateroil emulsion was put on a glass slide and placed on
the TEC. The migration of individual particles into the oil
phase cannot be excluded. To minimize migration, the time
between emulsication and the start of the cooling process was
intentionally kept short. A refreezing experiment was
performed to demonstrate that no migration into the oil
phase occurs during the freezing experiment and is provided in
the Supporting Information (see Figure S2).
The TEC was cooled at a constant cooling rate of 2 °C/min.
The temperature accuracy was ±0.5 °C. The freezing process
was recorded with a microscope camera and then analyzed. The
frozen droplets can be easily distinguished from the liquid
droplets, as they appear darker because of their dierent light-
scattering properties. The so-called ice nucleation active surface
site density ns
3639
was used to describe ice nucleation because
the total surface area of INPs per droplet is important to their
activity, as predicted by the classical nucleation theory and
conrmed by Edwards et al.
40
N0is the total number of
droplets in the experiment, NFis the number of frozen droplets
at temperature T, and sis the particle surface per droplet. The
fraction of frozen droplets f(T) is given by
==×
f
TNT
NnT s() () 1exp[()
]
ice
F
0
s
(1)
The surface area was determined via nitrogen adsorption and
assumed to be equal to the available surface area in suspension.
By increasing the size of droplets from cloud droplet size
(diameters of approximately 10 μm) to approximately 40 μm,
the surface area per droplet for a constant mass fraction of INPs
in water was increased. According to eq 1, this allows the
quantication of nsto smaller values and the determination of
nucleation eciencies over a wider range of temperatures than
that is possible by using cloud-sized droplets.
41
X-ray Photoelectron Spectroscopy. XPS was carried out
using the facilities of the Analytical Instrumentation Center
AIC at the TU Wien. Measurements were performed with a
SPECS XPS spectrometer equipped with an Al KαX-ray source
(μFocus 350) and a hemispherical WAL-150 analyzer. The
excitation energy was set to 1486.6 eV, the pass energy was 30
eV, and the resolution was 50 meV. The lower detection limit
of quantication was 0.1 at. % with an accuracy of 1020%,
depending on the element. For preparation, the samples were
drop-coated on a silicon single crystal.
Raman Spectroscopy. The Raman microscope system
(Jobin Yvon, LabRAM HR) consisted of a light microscope
(Olympus BX) coupled to a Raman spectrometer. A 20-fold
objective and a grating with 300 g/mm were used.
Furthermore, 60 scans with an exposure of 5 s each were
collected to obtain a sucient signal-to-noise ratio using a laser
with a wavelength of 633 nm. The interpretation and evaluation
of the Raman spectra were performed according to Sadezky et
al.
42
By applying and tting potential vibrational bands (see
Table 1) to the measured Raman spectra, qualitative and
quantitative descriptions of the graphene lattice and its disorder
can be made. The appearance of individual vibrational bands
provides a qualitative description, while the proportional
integrated intensities provide information on the contribution
of each vibration. The band intensities are expressed as a
fraction of the total intensity.
Transmission Electron Microscopy. TEM measurements
were performed on FEI Tecnai F20 with an acceleration voltage
of 200 kV. The samples were prepared by adding dropwise an
aqueous or ethanolic solution on Lacey(R)-coated copper grids.
Nitrogen Adsorption. The surface areas of the samples
were measured using a commercial liquid nitrogen adsorption
system (ASAP 2020, Micromeritics). Data evaluation was based
on the model by Brunauer, Emmett, and Teller (BET).
43
Description of Materials. We investigated a variety of
functionalized and nonfunctionalized graphene and graphene
oxide materials with dierent chemical and structural character-
istics to evaluate the dependence of the INA on the surface
chemistry, micromorphology, and nanomorphology. Four
graphene oxides (GO) were chosen for investigation: (i) GO
synthesized by our group (GO-DE), (ii) GO purchased from
Sigma-Aldrich (GO-SA), (iii) ammonia-functionalized GO
(GO-NH2), and (iv) nanosized colloidal GO (GO-nano).
Moreover, three graphene samples were analyzed: (i) non-
functionalized graphene (G-non) and (ii) and (iii) covalently
functionalized graphene G-NPr3+X(X = I and OH). Table 2
summarizes the material characteristics, including the source of
synthesis and the specic surface area according to BET. The
representative chemical structures are given in Figure 1. The
synthesis and further characterization are provided in the
Supporting Information (see Scheme S1 and S2, Figures S3
S16).
RESULTS
The INA of the entire set of samples is shown in Figure 2. For
clarity, the samples were divided into two groups according to
their chemical characteristics: (i) graphene, including G-non,
G-NPr3+I, and G-NPr3+OH, and (ii) graphene oxides,
including GO-DE, GO-SA, GO-nano, and GO-NH2(see
Table 2).
Graphene. Covalently functionalized graphene (G-NPr3+I
and G-NPr3+OH) show a similar INA with slightly increased
nsvalues of G-NPr3+OHbetween 30 and 36.5 °C
compared to G-NPr3+I. In contrast, nonfunctionalized
graphene (G-non) shows an increased INA once the
temperature is below 28 °C compared to the functionalized
samples. XPS measurements were performed to determine the
elemental composition and the sp2-hybridized carbon (C-sp2)
proportion. The C-sp2proportions are representative for the
graphitic carbon ratio (see Table 3). The detailed list of XPS-
determined carbon components is given in the Supporting
Table 1. First-Order Raman Bands and Vibrational Modes of
Soot and Graphite According to Sadezky et al.
42
for
Interpretation of the Obtained Raman Spectra (vs = Very
Strong, s = Strong, m = Medium, and w = Weak)
a
Raman shift [cm1]
band soot disordered
graphite
c
vibrational mode
b
G1580, s 1580, s ideal graphitic lattice (E2g symmetry)
D1 1350, vs 1350, m disordered graphitic lattice (graphene layer
edges, A1g symmetry)
D2 1620, s 1620, w disordered graphitic lattice (surface
graphene layers, E2g symmetry)
D3 1500, m amorphous carbon (Gaussian line shaped)
D4 1200, w disordered graphitic lattice (A1g symmetry),
polyenes, and ionic impurities
a
Minor changes in the Raman shifts may occur because of the dierent
measurement parameters.
b
Lorentzian line shaped unless otherwise
mentioned.
c
Polycrystalline graphite (<100 nm) and boron-doped
highly oriented polycrystalline graphite (HOPG).
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Information (see Table S1). The composition of the three
samples is similar and consists of approximately 9192 at. %
carbon and 49 at. % oxygen. G-NPr3+Iand G-NPr3+OH
show a nitrogen proportion of 34 at. % as well as 1 at. %
iodine for G-NPr3+Ioriginating from the functionalization
(see Figure 1). The C-sp2proportions are between 72 at. % for
G-NPr3+OHand 91 at. % for G-NPr3+I.
Raman spectroscopy was performed and analyzed according
to Sadezky et al.
42
to distinguish the dierences in the
microstructure, lattice disorder, and short-range order. The
Raman spectra (see Table 4 and Figure 3) reveal that
nonfunctionalized graphene (G-non) has fewer structural
defects, shown by the lower intensity of the disordered band
D1 (layer edge disorder) of 27% and the higher intensity of the
ideal graphite band G of 69% compared to those of covalently
functionalized graphene. Covalently functionalized graphene
exhibits an integrated intensity of D1 of up to 40% for G-
NPr3+OHand 44% for G-NPr3+I. The intensity of D2 is
shown to be similar, within a ±1% range for all three samples.
Additionally, TEM measurements were performed to
determine the morphology of the particles. The images reveal
akes with diameters of up to 400 nm and with up to 7 layers
(see Figure 4) for all three graphene samples. Individual sheets
of G-non, however, show additional accumulation, forming
larger uyaggregates.
Graphene Oxides. Graphene oxides show a broader variety
of INA than graphene (see Figure 2). The nsvalues of the
Table 2. List of Samples Investigated, Including the Sources
of Each Sample, BET-Determined Surface Areas and a Short
Description of the Samples, Including TEM-Determined
Particle Thickness and Shape
material
surface
areas
[m2/g] description
Graphene Oxides
GO-DE
a
112 large single-layer graphene oxide sheets
(>1 μm)
GO-SA
b
<10 large 27 multi-layer graphene oxide sheets
(>1 μm)
GO-NH2
b
<10 large 27 multi-layer graphene oxide sheets
(>1 μm), ammonia functionalized
GO-nano
b
176 graphene oxide nanocolloids with varying
particle size/shape up to 200 nm and
thickness of >3 nm
Graphene
G-non
a
<10 nonfunctionalized graphene akes with a
diameter of 400 nm and up to 7 layers;
precursor for G-NPr3+I/OH
G-NPr3+I
a
<10 covalently functionalized graphene with Ias
the counter ion; same form and shape as that
of G-non
G-NPr3+OH
a
<10 covalently functionalized graphene with OHas
the counter ion; same form and shape as that
of G-non
a
Synthesized by our workgroup.
b
Acquired from Sigma-Aldrich
Chemistry.
Figure 1. Representative chemical structures of the samples investigated: (a) nonfunctionalized graphene G-non,
44
(b) covalently functionalized
graphene G-NPr3+X(X = I and OH), (c) graphene oxide nanocolloids GO-nano, (d) graphene oxide GO-DE and GO-SA,
45
and (e) ammonia-
functionalized graphene oxide GO-ammo. According to the provided datasheet of the purchased material from Sigma-Aldrich Chemistry.
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chemically most similar graphene oxide samples GO-DE and
GO-SA show a deviation of 2 orders of magnitude but an
increase at a similar rate over the entire temperature window.
The nsvalue of GO-SA increases from 3 ×103cm2at 25 °C
to 105cm2at 35 °C and that of GO-DE increases from 20
cm2at 25 °Cto7×102cm2at 35 °C. GO-nano, on the
other hand, initiates the ice nucleation process at a range similar
to GO-DE at 27.5 °C, with a nsvalue of 130 cm2. The ns
value, however, rises more steeply until it reaches the same
value as GO-SA of 3 ×104cm2at 32.5 °C. The
functionalized graphene oxide sample (GO-NH2) shows the
highest INA of all samples investigated over the entire freezing
process, revealing nsvalues between 104cm2at 25 °C and 3
×105cm2at 35 °C.
XPS measurements show a similar composition among all
graphene oxide samples, that is, of approximately 6773 at. %
carbon and 2333 at. % oxygen (see Table 5). GO-nano is
composed of an increased amount of oxygen (33 at. %) and a
consequently decreased proportion of carbon (67%). GO-NH2
additionally consists of approximately 3 at. % nitrogen due to
the ammonia functionalization. The proportion of sp2-
hybridized carbon lies between 19 at. % for GO-nano and 45
at. % for GO-DE.
Furthermore, Raman spectroscopy reveals distinctions in
lattice orders (see Table 6 and Figure 5). In contrast to
graphene, the graphene oxide samples show a more complex
composition of lattice disorders. The D/G ratio is a suitable
indicator of the degree of disorder, which increases from GO-
SA (4.82) to GO-NH2(9.0). The D value summarizes the
integral intensities of all D-bands (D1D4): edge (D1) and
surface (D2) disorders, amorphous graphene oxide features
(D3), and ionic impurities/polyene disorders (D4). GO-DE
exhibits an increased ratio of integrated intensities of D3 (7%)
and D4 (4%) compared to the chemically most similar sample,
GO-SA. The Raman spectrum of GO-nano reveals a disordered
lattice with an integrated intensity of amorphous disorder of
11%, making this sample the most amorphous of all samples.
Nevertheless, GO-NH2exhibits the more intense disorder,
which is evident in the lowest integrated intensity of the ideal
lattice of 10%.
TEM analysis of GO-SA, GO-DE, and GO-NH2reveal
similar large akes of several μm in diameter and a thickness of
27 layers. GO-nano, on the other hand, consists of particles of
varying shapes and sizes of up to 200 nm and thicknesses of at
least 3 nm (see Figure 6).
DISCUSSION
Graphene. In Figure 2, the functionalized graphene samples
show a similar INA, with slightly increased nsvalues for G-
NPr3+OHbetween 30 and 36.5 °C. In contrast, non-
functionalized graphene (G-non) shows an increased INA
above 28 °C compared to the functionalized samples. TEM
analyses indicate that the ake and layer size of functionalized
graphene stay nearly the same as that of nonfunctionalized
graphene. Dierences in composition revealed by XPS might
inuence the INA because of the introduction of additional
functional sites that are able to interact with water molecules
and trigger ice formation. This would be consistent with the
classical chemical-bonding requirement stated by Pruppacher
and Klett.
3
The Raman data revealed that covalently function-
alized graphene (G-NPr3+Iand G-NPr3+OH) exhibits an
increased integrated intensity of lattice disorders. With a
decrease in the D/G ratio of functionalized graphene, the INA
decreases at a similar rate (see Table 4). The functionalization
process seems to have a major inuence on the INA. The
ordering process of water molecules at the watergraphene
interface supports heterogeneous ice nucleation and depends
Figure 2. (a) Ratio of frozen droplets fice and (b) the ice nucleation
active surface site density nsat a given temperature for all investigated
samples. The nsvalue, the freezing temperature range, and the nstrend
provide key information on the characteristics of the ice-nucleation-
active samples of graphene or graphene oxides. Some droplets nucleate
at about 36 °C, meaning that they do not contain INPs.
Experimental uncertainty in the nsvalue was calculated by changes
in the weight and droplet size.
Table 3. Elemental Composition and sp2-Hybridized Carbon
Proportion of All Investigated Graphene Samples
Determined via XPS
element [at. %]
sample C O N I C-sp2[at. %]
G-non 91 9 86
G-NPr3+I92 4 3 1 91
G-NPr3+OH92 4 4 72
Table 4. Proportional Intensities of Fitted First-Order
Raman Impulses (G, D1, and D2) According to Sadezky et
al.
42
and Ratios of the Raman Band Intensity of the
Disordered to the Ideal Graphitic Lattice (D/G) of All
Investigated Graphene Samples
proportional intensity of tted bands [%]
sample G D1 D2 D/G
G-non 69 27 4 0.45
G-NPr3+I51 44 4 0.94
G-NPr3+OH57 40 3 0.75
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on the lattice features. Disorientations of the lattice disturb the
interaction with liquid water and therefore lower the INA of
graphene. Thus, the graphene lattice is a signicant parameter
inuencing the INA of graphene and is also responsible for the
INA of soot particles. This is consistent with calculations of
Lupi et al.,
26
Lupi and Molinero,
21
and Bi et al.
22
and the
laboratory work of Whale et al.,
25
which revealed that the
ordering of liquid water on an ideal graphitic lattice plays an
important role in the heterogeneous ice nucleation mechanism
and any oxidation, roughness, or curvature was found to
decrease the observed nucleation temperature. Nevertheless,
accumulations of G-NPr3+Iand G-NPr3+OHakes into
larger aggregates, as demonstrated in Figure 4, may reduce the
INA by reducing the available active surface area, which needs
to be considered. According to the data obtained in this study,
it can be stated that the lattice conditions have an impact on the
INA of graphene, yet the inuence of agglomeration and
functionalization cannot be excluded. However, more graphene
species need to be investigated to state a signicant trend and
distinguish the relevance of each inuence.
Graphene Oxides. The INA of graphene oxide appears to
be more complex. In contrast to graphene, the graphene oxide
samples not only show signicant dierences in composition
but also experience additional lattice disorder. Because of
oxidation, the C-sp2proportions of the graphene oxides were
cut by one-half compared to the graphene samples because of
an increased amount of carbon-containing contaminants (see
Tables 3 and 5). The nsvalues of the chemically most similar
graphene oxide samples, GO-DE and GO-SA, show a deviation
of 2 orders of magnitude but increase at a similar rate over the
entire temperature window. Furthermore, a consistent chemical
composition and particle form was shown for both samples. In
contrast, the lattice of GO-DE features an increased portion of
structural disorder (D/G of 6.21) compared to GO-SA (D/G
Figure 3. Raman spectra of (a) all investigated graphene samples and (b) nonfunctionalized graphene (G-non); rst-order curve tted with band
combination according to Table 1 (λ0= 633 nm).
Figure 4. TEM images of graphene samples investigated: (a) nonfunctionalized graphene (G-non) indicates akes with diameters of up to 400 nm
and with up to 7 layers
46
and (b) covalently functionalized graphene (G-NPr3+Iand G-NPr3+OH) shows the same layer size and thickness as its
nonfunctionalized precursor G-non but also shows individual sheets accumulating into larger uyaggregates.
Table 5. Elemental Composition and Proportion of sp2-
Hybridized Carbon (C-Sp2) of All Investigated Graphene
Oxide Samples Determined via XPS
element [at. %]
sample C O N C-sp2[at. %]
GO-SA 71 28 <1 29
GO-DE 72 27 <1 45
GO-nano 67 33 <1 19
GO-NH273 23 3 42
Table 6. Proportional Intensities of Fitted First-Order
Raman Impulses (G, D1, and D2) According to Sadezky et
al.
42
and Ratios of the Raman Band Intensity of the
Disordered to the Ideal Graphitic Lattice (D/G) of All
Investigated Graphene Oxide Samples
proportional intensity of tted bands [%]
sample G D1 D2 D3 D4 D/G
GO-SA 17 68 7 5 2 4.82
GO-DE 14 69 7 7 4 6.21
GO-nano 14 64 6 11 5 6.14
GO-NH210 70 8 8 4 9.0
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J. Phys. Chem. C 2018, 122, 81828190
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of 4.82). Because of the similarity in composition and particle
shape, the signicant dierences in lattice order of GO-DE and
GO-SA may account for the activity dierence observed for the
graphene samples. However, the observed dependence of INA
on the lattice order cannot be applied to all graphene oxide
samples investigated. In particular, GO-nano initiates ice
nucleation in the same range as GO-DE, but nucleation
increases substantially more steeply until it reaches the same
value as GO-SA at 32.5 °C (see Figure 2). The degree of
graphitization, however, is in the same range as that of GO-DE
and GO-SA, with an amorphous proportion of 11% being the
highest of all samples (see Table 6). GO-nano shows an
increased amount of oxygen (33 at. %) and a corresponding
decreased proportion of carbon (see Table 5). Nevertheless,
GO-nano consists of signicantly smaller particles of varying
shapes and sizes of up to 200 nm and thicknesses of at least 3
nm (see Figure 6). On the basis of the data, three features may
account for the increased INA of GO-nano: (i) the particle
shape of GO-nano may cause an increase in the INA because of
the benecial arrangement of functional sites on the surface, (ii)
additional oxygen groups act as functional sites and improve the
interaction of the graphene oxide interface with water
molecules because of a possible increase in hydrogen bonds,
and (iii) the increased proportion of oxygen increases the
hydrophilicity of graphene, reduces agglomeration, and hence
increases the surface area. However, the inuence of hydro-
philicity and the resulting agglomeration is not clear. Biggs et
al.
27
reported an increase in the INA due to a decrease in
hydrophilicity. The resulting agglomeration may have led to a
favorable positioning of the functional site and therefore to an
increase in the INA, even though a decrease in the surface area
occurs.
XPS measurements of GO-NH2revealed a composition of 73
at. % carbon, 23 at. % oxygen, and 3 at. % nitrogen due to
functionalization. GO-NH2exhibits the highest integrated
intensity of disorder (G <10%) and the highest INA of all
samples investigated. Amines are known to be more hydrophilic
than comparable organic hydrocarbons because of their polarity
and basicity.
47
They therefore interact more easily with other
polar groups, such as water molecules, via hydrogen bonds and
may act as functional sites, increasing the INA. However, very
few proteins show INA, despite containing lysine, an amino-
group-containing amino acid, and instead are known to act as
antifreeze.
48,49
The inuence of increased hydrophilicity and
therefore reduced agglomeration of G-NH2akes in aqueous
suspension may lead to an increased INA and cannot be
excluded. Exfoliation can lead to an increase in the surface area
and hence may increase the INA.
SUMMARY
In this work, we investigated the INA of dierent types of
graphene and graphene oxides. Immersion drop freezing
experiments as well as additional analytical analyses such as
X-ray photoelectron spectroscopy, Raman spectroscopy, and
transmission electron microscopy were performed to gain
insight into the surface chemistry, micromorphology, and
Figure 5. Raman spectra of (a) all investigated graphene oxide samples and (b) GO-NH2;rst-order curve tted with band combination according
to Table 1 (λ0= 633 nm).
Figure 6. TEM images of the samples investigated: (a) nonfunctionalized (GO-SA and GO-DE) and ammonia-functionalized graphene oxides (GO-
NH2) reveal large akes of approximately 5 nm thickness and several micrometers in diameter, and the thickness of the sheets and number of signals
in the corresponding electron diraction pattern indicate multi-layer graphene composed of 27 layers, while (b) GO-nano consists of particles of
varying shapes with sizes of up to 200 nm and thickness of at least 3 nm.
The Journal of Physical Chemistry C Article
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J. Phys. Chem. C 2018, 122, 81828190
8188
nanomorphology of the INPs. The investigation of graphene
and graphene oxides show that the lattice order can have a
major impact on the INA. The introduction of dierent kinds
of disorders (layer edges, amorphousness, impurities, etc.) can
inuence the ability to perform heterogeneous ice nucleation.
The ordering of water molecules at the interface to perform
heterogeneous ice nucleation seems to depend on the graphitic
lattice. Disorders in the lattice disturb the interactions with
liquid water and therefore lower the INA of graphene.
However, the observed dependence of the INA on the lattice
order cannot be applied to all investigated graphene oxide
samples. In particular, GO-NH2exhibits the highest proportion
of disorder (G <10%) and revealed the highest INA of all
samples investigated. Functionalization with amines inuences
the INA by increasing the number of functional sites and/or by
increasing the hydrophilicity. In general, two other INP
characteristics in addition to the lattice order were shown to
inuence the INA of graphene and graphene oxides: (i) the
particle size, in particular that within the nanometer range, may
cause an increase in the INA due to the benecial arrangement
of functional sites on the surface and (ii) the degree of
oxidation, which inuences the hydrophilicity, reduces
agglomeration, and hence increases the surface area, as well
as generates functional sites.
On the basis of this work, the impact of the lattice order was
demonstrated. Additionally, dierences in structure, size, and
functionalization between the investigated samples are shown.
More species with closely controlled dierences need to be
investigated to state a rm experimental conclusion about the
eect of each feature on the INA. Therefore, declarations of the
most decisive ice nucleation feature cannot be made. A variety
of features relevant to ice nucleation were shown to be essential
when describing the ice-nucleating behavior of graphene and
graphene oxides.
ASSOCIATED CONTENT
*
SSupporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jpcc.7b10675.
Synthesis and characterization of functionalized graphene
samples (G-NPr3+X); visual comparison of graphene
and graphene oxide suspensions after sonication and
after settling for 30 min; refreezing experiments of G-
non; and XPS-determined C 1s components of all
samples (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: grothe@tuwien.ac.at. Phone: +43 (1) 58801 165122.
ORCID
Thomas Hä
usler: 0000-0001-6375-0886
Daniel Iglesias: 0000-0002-1998-0518
Silvia Marchesan: 0000-0001-6089-3873
Dominik Eder: 0000-0002-5395-564X
Hinrich Grothe: 0000-0002-2715-1429
Notes
The authors declare no competing nancial interest.
ACKNOWLEDGMENTS
We thank the Austrian Science Fund (FWF) for the nancial
support (project number P26040). XPS measurements were
carried out using facilities of the Analytical Instrumentation
Center, TU Wien, Austria. TEM measurements were
performed at the University Service Centre for Transmission
Electron Microscopy (USTEM, TU Wien).
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Tiny liquid or solid particles carried by air, atmospheric aerosols, are present everywhere we go and affect our lives in numerous ways. In addition to many commonly known effects, such as air quality and visibility, these aerosols are behind several physical processes that influence the upcoming weather in the short term and contribute to climate change in the long term. This thesis focuses on one such process, ice nucleation induced by those particles. The importance of atmospheric ice processes originates from their crucial contribution to cloud formation and lifetime which, in turn, determine many weather-related and climatic phenomena, such as precipitation and changes in Earth’s albedo. Interaction between atmospheric particles and ice processes within clouds is a fundamental physical process, yet the scientific understanding of which still includes large uncertainties. This thesis is a synthesis of three studies that focus on experimental work investigating particle-induced ice nucleation, and one on how to improve instrumentation to reach low sampling temperatures relevant to freezing conditions at the tropopause. The ice-nucleating abilities of emission particles from biomass and diesel fuel combustion, and secondary organic aerosol (SOA) particles were investigated using a commercial portable ice-nuclei counter, Spectrometer Ice Nuclei (SPIN). Additional investigation of the particles studied include their physicochemical characterisation, with the aim of exploring potential linkages between their physical and chemical properties and ice-nucleating abilities. Moreover, this characterisation of laboratory-generated SOA particles is important for experiment reproducibility in future studies. The study focusing on the modification of the SPIN system includes measurements using two well-documented test particles. In addition to test measurements related to SPIN modification, system performance and experiment reproducibility are evaluated and discussed. The findings of the studies included in this thesis increase our understanding of how different types of atmospheric aerosol particles contribute to ice crystal formation under freezing conditions at altitudes up to the topmost troposphere. This knowledge is important for further understanding cloud processes that play a significant role in precipitation formation, changes in Earth’s albedo, and consequently, the climate. Moreover, we found the SPIN modification successful. The discussion on the operational procedures of the ice-nuclei counter should also help new users find efficient methods to use their instruments.
Article
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Soot particles are important candidates for ice nucleating particles (INPs) in cirrus cloud formation which is known 10 to exert a warming effect on climate. Bare soot particles, generally hydrophobic and fractal, mainly exist near emission sources. Coated or internally mixed soot particles are more abundant in the atmosphere and have a higher probability to impact cloud formation and climate. However, the ice nucleation ability of coated soot particles is not as well understood as that of freshly produced soot particles. In this study, two samples, a propane (C3H8) flame soot and a commercial carbon black were coated with varying wt % of sulphuric acid (H2SO4). The ratio of coating material mass to the mass of bare soot particle was controlled 15 and progressively increased from less than 5 wt % to over 100 wt %. Both bare and coated soot particle ice nucleation activities were investigated with a continuous flow diffusion chamber operated at mixed-phase and cirrus cloud conditions. The mobility size and mass distribution of size selected soot particles with/without H2SO4 coating were measured by a scanning mobility particle sizer (SMPS) and a centrifugal particle mass analyser (CPMA) running in parallel. The mixing state and morphology of soot particles were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In 20 addition, the evidence for the presence of H2SO4 on coated soot particle surface is shown by Energy Dispersive X-ray spectroscopy (EDX). Our study demonstrates that H2SO4 coatings suppress the ice nucleation activity of soot particles to varying degrees depending on the coating thickness, but in a non-linear fashion. Thin coatings causing pore filling in the soot-aggregate inhibits pore condensation and freezing (PCF). Thick coatings promote particle ice activation via droplet homogeneous freezing. Overall, our findings reveal that H2SO4 coatings will suppress soot particle ice nucleation abilities in 25 the cirrus cloud regime, having implications for the fate of soot particles with respect to cloud formation in the upper troposphere. 30
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Understanding the formation of ice from supercooled water on a surface is a matter of fundamental importance and general use. Kinetics of ice nucleation and ice growth on solid surfaces are actively studied, based on which effective anti-icing surfaces are designed. This review introduces one major breakthrough of experimental estimation of the critical ice nucleus size in heterogenous ice nucleation (HIN). Besides that, targeted anti-icing strategies are summarized according to the icing steps: suppression of ice nucleation, regulation of ice growth, and reducing ice adhesion. Factors such as crystal lattice match, charge, and ions, etc., are found to have determined effects on the HIN in specific circumstances. This promotes the study of surfaces with ice nucleation inhibition properties. Recent research about distinct ice growth patterns on hydrophobic and hydrophilic surfaces provides a new insight of designing icephobic surfaces by regulating ice growth or spreading processes. At last, effective surfaces including lubricated surface, low interfacial toughness surface, and superhydrophobic surface are developed to significantly reduce ice adhesion strength. The robustness, cost, and manufacture complexity can be problems that need to be considered for the widespread practice of anti-icing surfaces.
Preprint
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We studied ice-nucleating abilities of particulate emissions from a modern heavy-duty diesel engine using three different types of fuel. The polydisperse particle emissions were sampled during engine operation and introduced to a continuous-flow diffusion chamber (CFDC) instrument at a constant relative humidity RHwater = 110 %, and temperature was ramped between −43 °C and −32 °C (T-scan). The tested fuels were EN 590 compliant low-sulfur fossil diesel, hydrotreated vegetable oil (HVO) and rapeseed methyl ester (RME), and all were investigated without blending. Sampling was carried out at different stages in the engine exhaust after-treatment system, with and without simulated atmospheric processing using an oxidation flow reactor. In addition to ice-nucleation experiments, we used supportive instrumentation to characterize the emission particles and present six different physical and chemical properties of them. We found that the studied emissions were poor ice-nucleators and substitution of fossil diesel with renewable fuels, using different emission after-treatment systems and photochemical aging of total exhaust had only little effect on their ice-nucleating abilities.
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Here we investigate the freezing and thawing properties of aqueous solutions in oil emulsions, with a particular focus on investigating the influence of the oil and surfactant and the stirring time of the emulsion. Specifically, we employ optical cryomicroscopy in combination with differential scanning calorimetry to study the phase behavior of emulsified 25 wt. % ammonium sulfate droplets in the temperature range down to 93 K. We conclude that the nucleation temperature does not vary with oil-surfactant combination, that is, homogeneous nucleation is probed. However, incomplete emulsification and non-unimodal size distribution of dispersed droplets very often result in heterogeneous nucleation. This in turn affects the distribution of freeze-concentrated solution and the concentration of the solid ice/ammonium sulfate mixture and, thus, the phase behavior at sub-freezing temperatures. For instance, the formation of letovicite at 183 K critically depends on whether the droplets have frozen heterogeneously or homogeneously. Hence, the emulsification technique can be a very strong technique, but it must be ensured that emulsification is complete, i.e., a unimodal size distribution of droplets near 15 μm has been reached. Furthermore, phase separation within the matrix itself or uptake of water from the air may impede the experiments.
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Real-time measurements of ambient concentrations of gas-phase ammonia were performed in Barcelona (NE Spain) in summer between May and September 2011. Two measurement sites were selected: one in an urban background traffic-influenced area (UB) and the other in the historical city centre (CC). Levels of ammonia were higher at CC (5.6 &pm; 2.1 μg m−3 or 7.5 &pm; 2.8 ppbv) compared with UB (2.2 &pm; 1.0 μg m−3 or 2.9 &pm; 1.3 ppbv). This difference is attributed to the contribution from non-traffic sources such as waste containers, sewage systems, humans and open markets more dense in the densely populated historical city centre. Under high temperatures in summer these sources had the potential to increase the ambient levels of ammonia well above the urban-background-traffic-influenced UB measurement station. Measurements were used to assess major local emissions, sinks and diurnal evolution of NH3. The measured levels of NH3, especially high in the old city, may contribute to the high mean annual concentrations of secondary sulfate and nitrate measured in Barcelona compared with other cities in Spain affected by high traffic intensity. Ancillary measurements, including PM10, PM2.5, PM1 levels (Particulate Matter with aerodynamic diameter smaller than 10 μm, 2.5 μm, and 1 μm), gases and black carbon concentrations and meteorological data, were performed during the measurement campaign. The analysis of specific periods (3 special cases) during the campaign revealed that road traffic was a significant source of NH3. However, its effect was more evident at UB compared with CC where it was masked given the high levels of NH3 from non-traffic sources measured in the old city. The relationship between SO42− daily concentrations and gas-fraction ammonia (NH3/(NH3+NH4+)) revealed that the gas-to-phase partitioning (volatilization or ammonium salts formation) also played an important role in the evolution of NH3 concentration in summer in Barcelona.
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The ice content of mixed phase clouds, which contain both supercooled water and ice, affects both their lifetime and radiative properties. In many clouds, the formation of ice requires the presence of particles capable of nucleating ice. One of the most important features of ice nucleating particles (INPs) is that they are rare in comparison to cloud condensation nuclei. However, the fact that only a small fraction of aerosol particles can nucleate ice means that detection and quantification of INPs is challenging. This is particularly true at temperatures above about −20 °C since the population of particles capable of serving as INPs decreases dramatically with increasing temperature. In this paper, we describe an experimental technique in which droplets of microlitre volume containing ice nucleating material are cooled down at a controlled rate and their freezing temperatures recorded. The advantage of using large droplet volumes is that the surface area per droplet is vastly larger than in experiments focused on single aerosol particles or cloud-sized droplets. This increases the probability of observing the effect of less common, but important, high temperature INPs and therefore allows the quantification of their ice nucleation efficiency. The potential artefacts which could influence data from this experiment, and other similar experiments, are mitigated and discussed. Experimentally determined heterogeneous ice nucleation efficiencies for K-feldspar (microcline), kaolinite, chlorite, Snomax®, and silver iodide are presented.
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Atmospheric dust rich in illite is transported globally from arid regions and impacts cloud properties through the nucleation of ice. We present measurements of ice nucleation in water droplets containing known quantities of an illite rich powder under atmospherically relevant conditions. The illite rich powder used here, NX illite, has a similar mineralogical composition to atmospheric mineral dust sampled in remote locations, i.e. dust which has been subject to long range transport, cloud processing and sedimentation. Arizona Test Dust, which is used in other ice nucleation studies as a model atmospheric dust, has a significantly different mineralogical composition and we suggest that NX illite is a better surrogate of natural atmospheric dust. Using optical microscopy, heterogeneous nucleation in the immersion mode by NX illite was observed to occur dominantly between 246 K and the homogeneous freezing limit. In general, higher freezing temperatures were observed when larger surface areas of NX illite were present within the drops. Homogenous nucleation was observed to occur in droplets containing low surface areas of NX illite. We show that NX illite exhibits strong particle to particle variability in terms of ice nucleating ability, with ~1 in 10<sup>5</sup> particles dominating ice nucleation when high surface areas were present. In fact, this work suggests that the bulk of atmospheric mineral dust particles may be less efficient at nucleating ice than assumed in current model parameterisations. For droplets containing ≤2 × 10<sup>−6</sup> cm<sup>2</sup> of NX illite, freezing temperatures did not noticeably change when the cooling rate was varied by an order of magnitude. The data obtained during cooling experiments (surface area ≤2 × 10<sup>−6</sup> cm<sup>2</sup>) is shown to be inconsistent with the single component stochastic model, but is well described by the singular model ( n <sub> s </sub>(236.2 K ≤ T ≤ 247.5 K) = exp(6.53043 × 10<sup>4</sup>− 8.2153088 × 10<sup>2</sup> T + 3.446885376 T <sup>2</sup> − 4.822268 × 10<sup>−3</sup> T <sup>3</sup>). However, droplets continued to freeze when the temperature was held constant, which is inconsistent with the time independent singular model. We show that this apparent discrepancy can be resolved using a multiple component stochastic model in which it is assumed that there are many types of nucleation sites, each with a unique temperature dependent nucleation coefficient. Cooling rate independence can be achieved with this time dependent model if the nucleation rate coefficients increase very rapidly with decreasing temperature, thus reconciling our measurement of nucleation at constant temperature with the cooling rate independence.
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Harnessing the exceptional physical properties of graphene often requires its dispersion into aqueous or organic media. Dispersion must be achieved at a concentration and stability appropriate to the final application. However, the strong interaction between graphene sheets means it disperses poorly in all but a few high boiling organic solvents. This review presents an overview of graphene dispersion applications and a discussion of dispersion strategies: in particular the effect of shear, solvent and chemical modification on the dispersion of graphene (including graphene oxide and reduced graphene oxide). These techniques are discussed in the context of manufacturing and commercialisation.
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The microscopic mechanisms controlling heterogeneous ice nucleation are complex and remain poorly understood. Although good ice nucleators are generally believed to match ice lattice and to bind water, counter examples are often identified. Here we show, by advanced molecular simulations, that the heterogeneous nucleation of ice on graphitic surface is controlled by the coupling of surface crystallinity and surface hydrophilicity. Molecular level analysis reveals that the crystalline graphitic lattice with an appropriate hydrophilicity may indeed template ice basal plane by forming a strained ice layer, thus significantly enhancing its ice nucleation efficiency. Remarkably, the templating effect is found to transit from within the first contact layer of water to the second as the hydrophilicity increases, yielding an oscillating distinction between the crystalline and amorphous graphitic surfaces in their ice nucleation efficiencies. Our study sheds new light on the long-standing question of what constitutes a good ice nucleator.
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Base-washed graphene-oxide which has been sequentially- modified by thiol-epoxy chemistry, results in materials with potent ice-nucleation activity. The role of hydro-philic/phobic grafts and polymers was evaluated with the most potent...
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We present results of experiments at the aerosol interactions and dynamics in the atmosphere (AIDA) chamber facility looking at the freezing of water by three different types of mineral particles at temperatures between −12°C and −33°C. The three different dusts are Asia Dust-1 (AD1), Sahara Dust-2 (SD2) and Arizona test Dust (ATD). The dust samples used had particle concentrations of sizes that were log-normally distributed with mode diameters between 0.3 and 0.5 μm and standard deviations, σg, of 1.6–1.9. The results from the freezing experiments are consistent with the singular hypothesis of ice nucleation. The dusts showed different nucleation abilities, with ATD showing a rather sharp increase in ice-active surface site density at temperatures less than −24°C. AD1 was the next most efficient freezing nuclei and showed a more gradual increase in activity than the ATD sample. SD2 was the least active freezing nuclei. We used data taken with particle counting probes to derive the ice-active surface site density forming on the dust as a function of temperature for each of the three samples and polynomial curves are fitted to this data. The curve fits are then used independently within a bin microphysical model to simulate the ice formation rates from the experiments in order to test the validity of parameterising the data with smooth curves. Good agreement is found between the measurements and the model for AD1 and SD2; however, the curve for ATD does not yield results that agree well with the observations. The reason for this is that more experiments between −20 and −24°C are needed to quantify the rather sharp increase in ice-active surface site density on ATD in this temperature regime. The curves presented can be used as parameterisations in atmospheric cloud models where cooling rates of approximately 1°C min−1 or more are present to predict the concentration of ice crystals forming by the condensation-freezing mode of ice nucleation. Finally a polynomial is fitted to all three samples together in order to have a parameterisation describing the average ice-active surface site density vs. temperature for an equal mixture of the three dust samples.
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Graphene has emerged as a material with a vast variety of applications. The electronic, optical and mechanical properties of graphene are strongly influenced by the number of layers present in a sample. As a result, the dimensional characterization of graphene films is crucial, especially with the continued development of new synthesis methods and applications. A number of techniques exist to determine the thickness of graphene films including optical contrast, Raman scattering and scanning probe microscopy techniques. Atomic force microscopy (AFM), in particular, is used extensively since it provides three-dimensional images that enable the measurement of the lateral dimensions of graphene films as well as the thickness, and by extension the number of layers present. However, in the literature AFM has proven to be inaccurate with a wide range of measured values for single layer graphene thickness reported (between 0.4 and 1.7 nm). This discrepancy has been attributed to tip-surface interactions, image feedback settings and surface chemistry. In this work, we use standard and carbon nanotube modified AFM probes and a relatively new AFM imaging mode known as PeakForce tapping mode to establish a protocol that will allow users to accurately determine the thickness of graphene films. In particular, the error in measuring the first layer is reduced from 0.1-1.3 nm to 0.1-0.3 nm. Furthermore, in the process we establish that the graphene-substrate adsorbate layer and imaging force, in particular the pressure the tip exerts on the surface, are crucial components in the accurate measurement of graphene using AFM. These findings can be applied to other 2D materials.