The Effect of Crystallinity and Crystal Structure on the Immersion

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The Effect of Crystallinity and Crystal Structure on the Immersion Freezing of Alumina Esther Chong, Megan King, Katherine Marak, and Miriam Arak Freedman J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b12258 • Publication Date (Web): 01 Mar 2019 Downloaded from http://pubs.acs.org on March 2, 2019

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The Journal of Physical Chemistry

The Effect of Crystallinity and Crystal Structure on the Immersion Freezing of Alumina Esther Chong1, Megan King2,+, Katherine E. Marak1, and Miriam Arak Freedman1,* 1) Department of Chemistry, The Pennsylvania State University, University Park, PA 16802 2) Department of Geology, State University of New York at New Paltz, New Paltz, NY 12561

Revised Submission to Journal of Physical Chemistry A Virtual Special Issue: Young Scientists February 26, 2019 *To whom all correspondence should be addressed: [email protected], 814-867-4267 + Present address: WCD Group, Poughkeepsie, NY 12603

ABSTRACT Determining the factors that constitute an efficient ice nucleus is an ongoing area of research in the atmospheric community. In particular, surface characteristics such as functional groups and surface defects impact the ice nucleation efficiency. Crystal structure has been proposed to be a possible factor that can dictate ice nucleation activity through the templating of water molecules on the surface of the aerosol particle. If the crystal structure of the surface matches that of the crystal structure of ice, it has been shown to increase ice nucleation activity. In this study, alumina was chosen as a model system because crystal structure and crystallinity can be tuned, and the effect on immersion freezing was explored. The nine alumina samples include polymorphs of AlOOH, Al(OH)3, Al2O3, which have a range of crystal structures and

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crystallinities. The samples were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and Brunauer-Emmett-Teller (BET). From the immersion freezing experiments, corundum [α- Al2O3] was shown to have the highest ice nucleation activity likely because of its high lattice match and high degree of crystallinity. Crystal structure alone did not show a strong correlation with ice nucleation activity, but a combination of a hexagonal crystal structure and a highly crystalline surface was seen to nucleate ice at warmer temperatures than the other alumina samples. This study provides experimental results in the study of ice nucleation of a range of alumina samples, which have possible implications for the alumina-based mineral dust particles. Our findings suggest that crystallinity and crystal structure are important to consider when evaluating the ice nucleation efficiency of aerosol particles in laboratory and modeling studies. INTRODUCTION Ice can nucleate by two mechanisms: homogeneously, without a solid surface, or heterogeneously, with a solid surface. To nucleate homogeneously, water must supercool to approximately -38 °C.1 In the atmosphere, heterogeneous ice nucleation is a preferred pathway to homogeneous ice nucleation because aerosol particles, which can serve as ice nuclei are

ubiquitous, and facilitate ice nucleation at warmer temperatures.2–6 However, the temperature at which this nucleation occurs depends strongly on the surface of the particle on which the ice nucleates.7 The characteristics of a surface that make it a good ice nucleus are still not well understood. In the atmosphere, a variety of aerosol particles can act as ice nuclei such as mineral dust, carbonaceous species (e.g., soot), bacteria, pollen, and other biological species.8 Although the chemical composition widely varies, some general criteria for a good ice nucleus have been

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laid out by Pruppacher and Klett.9 In particular, the following criteria were among those considered: (1) the ice nucleus must be water insoluble; (2) the ice nucleus needs to have a structure conducive for hydrogen bonding; and (3) the surface lattice structure should be close to that of ice to facilitate orienting the water layer.7 However, these parameters are only guidelines and do not necessarily indicate if a sample will be a good ice nucleus. An extensive effort has been made with both modeling and experimental studies to explore the interactions between ice nuclei and water. Specifically, the relationship of the surface and its activity towards nucleation has been studied using model systems such as AgI,10–13 kaolinite,14–18 and carbonaceous species.19–24 When studying ice nucleation, molecular dynamics (MD) simulations for heterogeneous ice nucleation have become prevalent in the last decade with studies on a diverse number of materials as the heterogeneous nucleus.11,25–30 Efforts made to study orientation of the water molecules have been done. Kaolinite is a popular representative mineral in ice nucleation studies. Using a kaolinite surface, Sosso et al. concluded that the templating of the hydroxylated surface promoted hexagonal ice formation through the relaxation of the first water layer into the hexagonal orientation.31 In other modeling studies done with kaolinite, lattice matching and hydrogen bonding are found to be important contributors but are not the sole conditions for the ice nucleation activity of kaolinite.32,33 In addition to kaolinite, Cox et al. used canonical Monte Carlo to observe the interaction of the first monolayer of water on simulated surfaces. They determined that the oxygen-oxygen distance of the nearest neighbor in the water layer was a better indicator of ice nucleation activity than the bulk lattice parameters of ice.34 Bi et al. modeled a graphitic surface and found that a surface with sufficient hydrophilicity and crystallinity would be able to template ice.28 However, it has been shown that even with good lattice match, surfaces can have poor ice activity such as for BaF2.35,36

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Few previous experimental systems have been used to examine surfaces with the detail of computation, due to the fact that the systems used were ones that are commonly found in the atmosphere. These systems can therefore have many variables to deconvolute such as composition, porosity, and surface groups. 32,37–39 In this study, select transition aluminas were used as a model system to investigate the effect of crystal structure and crystallinity on ice nucleation efficiency. The transition alumina system allows for similar compositions to atmospheric systems, while having control to vary the crystal structures and crystallinity. A series of transition aluminas are created from calcination of one alumina [AlOOH, Al(OH)3, or Al2O3], which causes dehydration, resulting in changes to the crystal structure to form the other phases of transition aluminas.40 Although there are slight changes in the stoichiometric ratio, the basic composition remain approximately the same. Through these systems, we have performed a systematic study of the effect of crystal structure and crystallinity on ice nucleation activity, which can inform heterogeneous ice nucleation in the atmosphere. EXPERIMENTAL METHODS A combination of heat treatments of purchased samples and chemical syntheses were used to produce the samples in this study. The heat treatments used to dehydrate the alumina samples followed the procedure of Wefers et al.41 Purchased samples included: several types of boehmite [AlOOH] (Disperal P2 and Dispal 23N4-80, Sasol), the γ- Al2O3 sample (Fisher Scientific, 99.997%), and gibbsite [Al(OH)3] (Rochester Distribution, 99%). It should be noted that the type of boehmite [AlOOH] used in the ice nucleation experiments was Disperal P2, while Dispal 23N4-80 was used to synthesize the η-γ- Al2O3 sample. The ice nucleation activities of these two boehmites did not have any differences in their ice nucleation activity. Figure S1 summarizes the samples prepared in this study, including the synthesis and calcination

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processes. The η- Al2O3 sample was made by firing Disperal P2 in a furnace for five hours at 550 °C.41 The η-γ- Al2O3 alumina sample was made by firing Dispal 23N4-80 for 12 hours at

600 °C.41 The χ-η- Al2O3 sample was prepared by firing gibbsite [Al(OH)3] and for 24 hours at

600 °C.41 The δ- Al2O3 and corundum [α- Al2O3] samples were made using the procedure from

Nampi et al. using Disperal P2 as the boehmite powder.42 First, the boehmite powder was

dispersed in distilled water. After lowering the solution to pH 3 with HNO3, the solution was heated to 30 °C for 24 hours and then concentrated into a viscous gel. It was dried in a furnace at 50 °C and then was further calcined at 250 °C to remove all excess water. The δ- Al2O3 phase

alumina was then formed by heating the solid for 3 hours at 900 °C. The procedure was repeated and the sample was fired at 1200 °C for corundum [α- Al2O3] .42 The bayerite [Al(OH)3] sample

was synthesized using the procedure of Jiao et al.43 In the procedure, NaAlO2 (Alfa Aesar, 99.9%) was dissolved in a mixture of hot distilled water and ethanol (Fisher Scientific). A mixture of diethyl adipate (Alfa Aesar, 99%) and ethanol was added to the NaAlO2 solution and stirred for 1 h. The resulting white solid was filtered, then washed with hot distilled water, and dried overnight.43 All samples were ground gently with a mortar and pestle before characterization and immersion freezing. An X-ray diffractometer (PANalytical Empryean) with Cu Kα radiation with a current of 40 mA and a voltage of 45 kV was used to characterize the structure and crystallinity of the samples. The powder samples were loaded onto a background silicon holder. The goniometer angles were set from 5 to 70° 2θ with a 0.0263° 2θ step size and a scan step time of 96.4 s. The XRD diffractograms were analyzed using Jade XRD libraries and whole pattern fitting. The SEM studies were carried out using an FEI Nova NanoSEM 630 SEM with samples dispersed on carbon tape. For TEM and EDS, the alumina powders were dispersed in hexane (Fisher

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Scientific), and a drop was deposited on continuous carbon coated copper mesh grids (CF200CU, Electron Microscopy Sciences, Hatfield, PA, USA) until the hexanes evaporated. Additionally, BET analysis with nitrogen gas was performed on the ASAP 2020 Automated Surface Area and Porosimetry System to determine the surface area of each sample.44 Immersion freezing data were taken using an environmental chamber described in detail at Alstadt et al.45 Briefly, 2 μL droplets (~7 mm diameter) of 0.02 wt. % of alumina in ultrapure water (Millipore Q, 18.2 Ω·cm2) were pipetted onto siliconized slides (Hampton Research). From the average diameter (50 µm) observed with SEM, approximately 100 alumina particles are within each drop. Then, the slides are placed into the chamber. Pictures are taken every 0.5 °C as

the chamber is cooled by liquid N2 at a cooling rate of 3 °C/min, until all droplets are frozen. In

the chamber, N2 is used as a purge gas; however, the volume of the droplets is sufficiently large that evaporation (

The Effect of Crystallinity and Crystal Structure on the Immersion

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