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Heterogeneous Freezing of Carbon Nanotubes: A Model System for Pore Condensation and Freezing in the Atmosphere Valerie J. Alstadt, Joseph Nelson Dawson, Delanie J. Losey, Sarah K. Sihvonen, and Miriam Arak Freedman J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b06359 • Publication Date (Web): 27 Sep 2017 Downloaded from http://pubs.acs.org on September 28, 2017

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The Journal of Physical Chemistry

Heterogeneous Freezing of Carbon Nanotubes: A Model System for Pore Condensation and Freezing in the Atmosphere Valerie J. Alstadt1,#, Joseph Nelson Dawson1, Delanie J. Losey1, Sarah K. Sihvonen1,&, Miriam Arak Freedman1* 1) Department of Chemistry, The Pennsylvania State University, University Park, PA 16802 Submission for the Journal of Physical Chemistry A June 27, 2017 * To whom all correspondence should be addressed: [email protected], 814-867-4267 # Present address: US Army Edgewood Chemical and Biological Center, Aberdeen Proving Ground, MD 21005 & Present address: Syngenta, Greensboro, NC 27419

Abstract Heterogeneous ice nucleation is an important mechanism for cloud formation in the upper troposphere. Recently, pores on atmospheric particles have been proposed to play a significant role in ice nucleation. To understand how ice nucleation occurs in idealized pores, the immersion freezing activity of various sizes of carbon nanotubes was characterized. Carbon nanotubes are used both as a model for pores and a proxy for soot particles. We determined that carbon nanotubes with inner diameters between 2-3 nm exhibit the highest ice nucleation activity. Implications for the freezing behavior of porous materials and nucleation on soot particles will be discussed.

Introduction Heterogeneous ice nucleation is an important means of cloud formation in the upper troposphere. Although the freezing point of water is 0° C, homogeneous freezing of aqueous droplets only occurs at temperatures less than approximately -36 °C.1 Between -36 °C and 0 °C,

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heterogeneous freezing can occur in the atmosphere when a particle on which ice can nucleate (also known as an ice nucleus; IN) is present. A variety of particle types can serve as IN, such as mineral dust, volcanic ash, organic aerosol, salts, soot, and biological particles.1, 2 Heterogeneous freezing can proceed through a variety of different mechanisms, including deposition mode nucleation, immersion/condensation freezing, and contact freezing. In deposition mode nucleation, water vapor forms an ice nucleus on the surface of the particle, while in immersion/condensation freezing, an aqueous solution on or surrounding the IN freezes. In contact freezing, the collision of an IN with a droplet induces the freezing process.1, 3, 4 Studies of these modes provide insight into how the surface of the IN may influence the ice nucleation activity. However, recent research suggests that these modes are not always distinct from one another.3,

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For example, a recent paper proposed that for some systems, deposition mode

freezing may actually be freezing of water inside the pores of the IN.6 This mechanism is known as pore condensation freezing (PCF).6 Many studies have found differences in the freezing temperatures of the same material with different pore sizes, thereby demonstrating that pore size can play a significant role in the freezing activity. Morishige and Nobuoka used X-ray diffraction to determine that freezing in pores of MCM-41, a mesoporous silica, occurred at higher temperatures for pores with a diameter of 4.2 nm than for pores with a diameter of 2.4 nm.7 Note that freezing at higher temperatures is indicative of a more efficient IN. Kittaka et al. found similar results for SBA-15, also a mesoporous silica material, as the freezing and melting temperature was higher for the 10.4 nm pores than the 4.6 nm pore.8 However, the hysteresis between freezing and melting indicates that freezing in SBA-15 may not occur in the pore itself. Specifically, Findenegg et al. theorized that ice freezing at the opening of the pore initiates freezing inside the pore and

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constrictions in pore size explain the hysteresis, since as pore size decreases, the water and ice phases become more disordered due to the smaller space.9 In the field of atmospheric chemistry, PCF has been proposed to be relevant for mineral dust particles, as these particles tend to have pores of varying sizes and shapes. Marcolli used the theory of PCF to fit experimental data for heterogeneous ice nucleation to examine the possible role of PCF in the deposition mode freezing process.6 Marcolli determined that freezing in pores at temperatures lower than -38°C corresponded to homogeneous freezing.6 Freezing in pores at temperatures greater than -38°C was thought to be immersion freezing, meaning that freezing resulted due to heterogeneous nucleation on a site on the pore wall.6 In kaolinite, for example, pores arise from the interleaving of the plate layers, or the mismatch of the aluminum layers.10 Kaolinite pores are typically 30 nm or greater in diameter and fill with water at low relative humidity (RH), as monolayers of water are known to form on the kaolinite surface at 10% RH.11, 12

Water in pores leads to the onset of ice nucleation for kaolinite at temperatures above -23°C

with the relative humidity with respect to the saturation vapor pressure of ice (RHi) below 120%.6 Smaller particles of kaolinite lack larger sized pores, and therefore, are thought to need higher RHi to homogeneously nucleate ice, which illustrates the effect of pore size.6 However, using kaolinite to determine the effects of freezing on pore size can be problematic as kaolinite particles have pores of varying size. Additionally, a recent study has shown that freezing may take place on the edge of the kaolinite particles and not in pores.13 Alternately, studying pore size across different minerals is difficult because changes in composition and crystal structure across different mineral types can also affect ice nucleation activity. Using materials with the same composition and a range of monodisperse and well-characterized pore sizes would provide data on the trend in freezing temperature with pore size. Carbon nanotubes are a material with a

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controllable outer diameter, inner diameter, and length, and are therefore ideal for the study of freezing in pores. Carbon nanotubes can be single walled (SWCNT), double-walled (DWCNT), or multiwalled (MWCNT). Investigating the ice nucleation activity of carbon nanotubes is also motivated by continued interest of the atmospheric community in the ice nucleation of carbonaceous materials, such as soot aerosol, which is responsible for the nucleation of ice particles in contrails and mixed-phase arctic clouds.14-16 In addition, carbon nanotubes are not just an appropriate proxy material for soot, but a realistic test surface for combustion processes that produce carbon nanopolyhedra or carbon nanotubes.17 Two studies have observed multiwalled carbon nanotubes in soot produced from the burning of pine wood and as the products of burning natural gas or propane with air.18, 19 PCF has been proposed to be an important mechanism for the freezing of soot, as water could fill the space between the spherules of the soot via capillary action at low relative humidities and subsequently freeze.6 Soot enters the atmosphere from combustion processes such as emissions from power plants and vehicles as well as cooking processes and biomass burning.20 Soot is also emitted into the upper troposphere from aircraft, resulting in the nucleation and formation of contrails.14, 15 In contrails, water condenses on soot to form cloud condensation nuclei which subsequently freeze. Experimental studies of the ice nucleation activity of soot have varied results.21-23 Deposition nucleation studies of soot have not seen evidence of heterogeneous freezing of water on soot, and as a result, freezing occurs above water saturation.21-23 In contrast, studies of immersion freezing methods for ice nucleation observed heterogeneous ice nucleation.24-27 Additionally, studies vary regarding the effect of increased oxidation on the ice nucleation activity of soot as some studies have seen an increase in the activity,28,

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while other studies have seen no improvement.23,

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Further study of how ice

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nucleation occurs on the surface of carbonaceous materials in immersion freezing is therefore needed. Several experimental and computational studies have been performed on the freezing of water inside carbon nanotubes.31-33 Despite their graphitic, hydrophobic composition, water is drawn into the carbon nanotubes by capillary action.31,

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Using X-ray diffraction and

molecular modeling of ordered ice, Maniwa et al. determined that an increase in the inner diameter of a SWCNT from 1.09 to 1.52 nm resulted in a decrease of the melting/freezing temperature from 27°C to -83°C.31 Note that the results for carbon nanotubes show the opposite trend as the results presented for mesoporous silica, either due to the material itself or the number of water molecules involved in the freezing process. Specifically, from models of SWCNTs with diameters of 1 – 5 nm, Ohba determined that water molecules in carbon nanotubes with diameters of 2 or 3 nm could best align into an ice-like structure, different from ice I, inducing freezing at higher temperatures.33 These mixed results indicate that the freezing temperature of water in pores with defined inner diameters, such as carbon nanotubes, may not strictly increase as the diameter increases. Molecular dynamics simulations by Lupi et al. focused on the freezing of water on carbon surfaces.35 They determined that freezing on smooth, hydrophobic sheets occurred on average 13°C warmer than homogeneous freezing and the ice nuclei necessary for freezing only formed at the graphene surface, indicating that the freezing was heterogeneous and did not initiate in the bulk water.35 Additionally, as the radius of curvature of the surface increases, heterogeneous freezing occurred at increasingly warmer temperatures and when the radius of curvature was very small and equal to the radius of the critical ice nucleus, only homogeneous freezing occurred.35 This result implies that the warmest temperatures for heterogeneous freezing

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occur for a flat surface. Note that these are open carbon surfaces, and therefore, formation of ice I is studied rather than polymorphs found in confined volumes. The computational results were identical if the surface was convex or concave,35 which suggests that results from immersion freezing for concave pores could be used to understand freezing in the convex surfaces provided in the spaces between soot spherules. A different study by Lupi et al. characterized the freezing temperatures of water on carbon surfaces that were made more hydrophilic by either increasing the carbon-water attraction or adding OH groups.36 The results suggest that the effect of oxidation is difficult to predict, as increasing the carbon-water attraction, i.e. increasing oxidation, resulted in increased freezing temperatures, whereas increasing oxidation through the addition of OH groups decreased the freezing temperatures.36 These results can be compared to the experimental work of Whale et al., which investigated the immersion freezing activity of four different kinds of carbon nanomaterials: carboxylated graphene flakes, graphene oxide, and oxidized SWCNT and MWCNTs.37 Whale et al. determined that the carbon nanotubes were more efficient ice nuclei than the graphene sheets, which differs from the result of Lupi et al., which determined that freezing occurred at higher temperatures as the radius of curvature increased.35, 37 This difference is likely due to the confinement of water in the carbon nanotubes in Whale et al., which resulted in a polymorph other than ice I upon freezing.37 In addition, Whale et al. determined that the carbon materials were more efficient ice nuclei if they were less oxidized.37 Whale et al. focused on only one example of each type of material; to explore the role of idealized pores, a more systematic study of one type of system is needed. In addition, the acid treatment used to oxidize the carbon nanotubes in Whale et al. may have impacted pore width, which could complicate the interpretation of their results.

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In this paper, we use carbon nanotubes with a range of lengths, outer diameters, and inner diameters to determine how these factors impact the immersion ice nucleation activity. The use of oxygen plasma to oxidize the samples keeps the pores intact, and enables the tubes to be suspended in water. From the range of samples used, we aim to determine where on the carbon nanotube nucleation occurs, how the size of the pore affects the freezing activity, and gain insight into how combustion aerosol nucleates ice.

Experimental Treatment, Preparation, and Freezing Activity of Carbon Nanotubes Carbon nanotube sample parameters can be found in Table 1. The carbon nanotube samples (US Research Nanomaterials, Houston, TX) will be considered below according to their length with two categories: short (0.5-2 µm) and normal length (10-50 µm). Samples are referred to by their outer diameter (OD) below (Table 1). These samples were selected due to their varying inner and outer diameters and represent all of the carbon nanotubes available for these length ranges. Additionally, these tubes were synthesized in the same way, which eliminates the influence of manufacturing technique on ice nucleation activity. To oxidize the commercial carbon nanotubes, approximately 30 mg of each carbon nanotube sample was plasma treated with oxygen using air for 10 minutes on the medium setting in a Harrick Plasma Cleaner (Model PDC-32G). The surface area of each carbon nanotube sample was determined using the BET method with N2 as the gas. We expect BET to provide an accurate measurement of the total surface area of the carbon nanotubes down to inner diameters of 2 nm; in other words, for all samples in this study with the possible exception of the DWCNTs. Note that this technique does not measure the space in between the walls of a MWCNT, but rather the surface area from the inner, outer, and pore edge surfaces. After this measurement, 1.8 mg of carbon nanotubes were

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added to 8 mL of Millipore Q purified water (18.2 MΩ cm) and sonicated between experiments. The weight percent of the carbon nanotubes in the sonicated solution was determined by taking a known volume of solution, removing the water using a gentle boil (100°C) and measuring the mass of carbon nanotubes after drying. The immersed carbon nanotubes were sonicated for 30 min to disperse the material through the solution and then 3.3 µL droplets were pipetted onto silanized (hydrophobic) glass slides. The freezing spectrum of 80-100 droplets of each carbon nanotube solution was measured (Table 1). Slides with droplets were placed in an environmental chamber, as depicted in Figure 1, at room temperature. The new ice nucleation chamber described here is similar to other environmental chambers that are used to study immersion freezing activity.38-40 While picoliter volumes have been used by Atkinson et al. and Broadley et al., microliter volumes have been used by Atkinson et al., O’Sullivan et al., and Whale et al. and are not expected to influence the results.38, 40-42 The chamber was then cooled at a rate of 3 K/min using chilled nitrogen gas and temperature was monitored using a thermocouple. Images of the droplets were taken every 0.5 °C from 0 °C to -30 °C using a CCD camera. During the course of the experiment, dry nitrogen was flowed over the droplets to reduce chamber frost. Little change in particle size was observed during the time scale of the experiment. Because these carbon nanotubes fill with water, the ice nucleation activity can be studied using immersion freezing.

Carbon nanotubes cannot be

studied using deposition mode freezing because it is difficult to make a monolayer of carbon nanotubes when they are not in solution due to static. The freezing spectrum that is indicative of homogeneous freezing was determined by measuring the immersion freezing activity of ammonium sulfate droplets. The number of droplets frozen is recorded every 0.5 °C. As a result,

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fewer than 100 data points are shown in each figure. The ammonium sulfate solution was 5% by weight ammonium sulfate (EMD, ACS Grade) in Millipore Q purified water.

Figure 1: a) Schematic of immersion ice nucleation freezing chamber. b) Image of droplets of DWCNTs at -14.5 °C with some droplets frozen as indicated by their cloudy appearance.

The cumulative fraction of droplets frozen, n(T)/N, for each sample was determined by dividing the number of droplets frozen at a given temperature, n(T), by the total number of droplets, N. To determine the contribution of the homogeneous freezing of ultrapure water on our results for carbon nanotubes, freezing experiments were performed with ultrapure water droplets. The homogeneous freezing spectrum of 200 droplets was measured. Using the procedure outlined in O’Sullivan et al., the fraction of droplets frozen at a specific temperature, F(T), from

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each trial was used to calculate K(T), the number of nucleation sites per mL at that temperature for each water and carbon nanotube trial as follows: 𝐾 𝑇 =

$%& '$((*) ,

×𝑑

(1)

where V is the droplet volume in mL and d is the dilution factor. K(T) is calculated for each individual trial of the ultrapure water droplets and carbon nanotubes, which is used to find the average and standard deviation of K(T) at each temperature.43 The average K(T) at each temperature for the ultrapure water droplets is then subtracted from the average K(T) calculated at each temperature for each carbon nanotube experiment. The cumulative number of surface sites per unit area as a function of temperature, ns, is calculated from the K(T) using the following equation: 𝑛0 = 𝐾(𝑇)×𝐶 $'

(2)

where C is the total surface area of the carbon nanotubes in a given volume, which is calculated using the BET surface area and mass percent of carbon nanotubes in each experiment. The average and standard deviation of K(T) at each temperature for carbon nanotubes corrected for the homogeneous freezing of ultrapure water are used to calculate ns and its standard deviation at each temperature using Eqn. 2.43 Hiranuma et al. studied the ice nucleation activity of illite using a suite of varied instrumentation and determined that the variance in freezing temperature between the instruments was 8°C.44 In addition, measurements of ns as determined by the different instrumentation methods varied by three orders of magnitude.44 We expect that the chamber technique introduced here deviates in temperature and ns values in the manner reported by Hiranuma et al. which is common among instruments of this type.44 In addition, Hiranuma found few differences regarding the experimental technique in terms of droplet size, amount of particle

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mass in suspension, and the cooling rate during freezing, as the freezing efficiency expressed by ns is independent of these variables.44

Characterization of Carbon Nanotubes The ratio of carbon to oxygen in the carbon nanotubes was determined using X-ray photoelectron spectroscopy (XPS). The spectrum was collected using a Physical Electronics VersaProbe II spectrometer using an Al Kα X-ray source at 1486.7 eV and a concentric hemispherical analyzer. Charge neutralization was performed using both low energy electrons (

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