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Deposited in DRO: 28 May 2020 Version of attached le: Accepted Version Peer-review status of attached le: Peer-reviewed Citation for published item: Halukeerthi, Siriney O. and Shephard, Jacob J. and Talewar, Sukhpreet K. and Evans, John S. O. and Rosu-Finsen, Alexander and Salzmann, Christoph G. (2020) 'Amorphous mixtures of ice and C60 fullerene.', Journal of physical chemistry A., 124 (24). pp. 5015-5022. Further information on publisher's website: https://doi.org/10.1021/acs.jpca.0c03439

Publisher's copyright statement: This document is the unedited Author's version of a Submitted Work that was subsequently accepted for publication in Journal of Physical Chemistry A, copyright c American Chemical Society after peer review. To access the nal edited and published work see https://doi.org/10.1021/acs.jpca.0c03439

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Durham University Library, Stockton Road, Durham DH1 3LY, United Kingdom Tel : +44 (0)191 334 3042 | Fax : +44 (0)191 334 2971 https://dro.dur.ac.uk Subscriber access provided by UNIV OF DURHAM A: Environmental, Combustion, and Atmospheric Chemistry; Aerosol Processes, Geochemistry, and Astrochemistry Amorphous Mixtures of Ice and C60 Fullerene Siriney O. Halukeerthi, Jacob J. Shephard, Sukhpreet K. Talewar, John S. O. Evans, Alexander Rosu-Finsen, and Christoph G Salzmann J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.0c03439 • Publication Date (Web): 22 May 2020 Downloaded from pubs.acs.org on May 27, 2020

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1 2 3 4 5 6 7 Amorphous Mixtures of Ice and C60 Fullerene 8 9 10 11 12 Siriney O. Halukeerthi,a Jacob J. Shephard,a,b Sukhpreet K. Talewar,a John S. O. 13 14 Evans,b Alexander Rosu-Finsen,a Christoph G. Salzmanna* 15 16 a 17 Department of Chemistry, University College London, 20 Gordon Street, London 18 WC1H 0AJ, United Kingdom. 19 20 21 b Department of Chemistry, Durham University, South Road, Durham DH1 3LE, United 22 23 Kingdom. 24 25 26 27 28 Corresponding Author 29 30 * email: [email protected], tel: +44 7679 8864 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 1 60 ACS Paragon Plus Environment The Journal of Physical Chemistry Page 2 of 33

1 2 3 Abstract 4 5 6 Carbon and ice make up a substantial proportion of our Universe. Recent space exploration has 7 8 shown that these two chemical species often coexist including on comets, asteroids and in the 9 10 11 interstellar medium. Here we prepare mixtures of C60 fullerene and H2O by vapor co-deposition at 12 13 90 K with molar C60:H2O ratios ranging from 1:1254 to 1:5. The C60 percolation threshold is found 14 15 between the 1:132 and 1:48 samples, corresponding to a transition from matrix-isolated C60 16 17 18 molecules to percolating C60 domains that confine the H2O. Below this threshold, the 19 20 crystallization and thermal desorption properties of H2O are not significantly affected by the C60, 21 22 whereas the crystallization temperature of H2O is shifted towards higher temperatures for the C60- 23 24 rich samples. These C -rich samples also display exotherms corresponding to the crystallization 25 60 26 27 of C60 as the two components undergo phase separation. More than 60 volume percent C60 is 28 29 required to significantly affect the desorption properties of H2O. A thick blanket of C60 on top of 30 31 pure amorphous ice is found to display large cracks due to water desorption. These findings may 32 33 34 help understand the recently observed unusual surface features and the H2O weather cycle on the 35 36 67P/Churyumov–Gerasimenko comet. 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 2 60 ACS Paragon Plus Environment Page 3 of 33 The Journal of Physical Chemistry

1 2 3 Introduction 4 5 6 Vapor-deposited amorphous ice, traditionally called amorphous solid water (ASW),1-2 is the 7 8 3-7 9 most abundant form of H2O in the Universe. The reactive accretion of H and O atoms onto 10 11 12 dust grain surfaces is the dominant formation pathway for ice in the interstellar medium.8 Such 13 14 grain surfaces are mainly silicate- or carbon-based as evidenced from astronomical spectra.3 15 16 Polycyclic aromatic hydrocarbons (PAHs) and fullerenes are common carbon species in space.3, 17 18 9-11 19 While PAHs can undergo complicated dehydrogenation, fragmentation and isomerization 20 3, 12 21 reactions, C60 and other fullerenes are considered to be stable end members of astrochemical 22 23 reaction cascades involving more reactive carbon species such as PAHs.13 The evidence for 24 25 14 15-16 26 carbon species in space is based on heavy-element depletion and spectroscopic studies. The 27 28 Long Duration Space Exploration mission observed fullerene residues on the panels of the 29 30 spacecraft17 and they have also been found in the Allende meteorite.18 31 32 33 Carbon species and ice have been found to coexist with H2O in a variety of settings. 34 35 Notably, NASA’s deep impact mission proved the coexistence of PAHs and ice in the Tempel 1 36 37 comet.19 As part of ESA’s Rosetta mission, the Philae probe landed on the surface of the 38 39 67P/Churyumov–Gerasimenko comet. The dark appearance of 67P is attributed to a ‘carbon 40 41 20-22 42 blanket’ on top of a nucleus composed of ices, organic materials and minerals. The presence 43 44 of unique surface features such as ‘pits’ and ‘goosebumps’ was attributed to complex weather 45 46 cycles arising from consecutive water desorption and re-condensation cycles.23-24 Future 47 48 49 developments will include sample-return missions by the OSIRIS-REx and Hayabusa2 50 51 spacecrafts from the carbon-rich asteroids Bennu and Ryugu, respectively.25-26 Such asteroids 52 53 could provide an interesting window into the past of the solar system. In addition to future 54 55 56 advances in space exploration, lab-based studies into the physical properties of carbon / ice 57 58 59 3 60 ACS Paragon Plus Environment The Journal of Physical Chemistry Page 4 of 33

1 2 3 mixtures are needed to fully understand the properties of these primordial materials. 4 5 6 The vast majority of studies into mixtures of ASW with other materials to date have 7 8 focused on so-called “gas-laden amorphous ices”.27-30 Guest species with lower volatilities have 9 10 included PAHs, and a variety of studies exploring the spectroscopic and photochemical 11 12 properties of PAH / ASW mixtures have been reported.31-36 Benzene has served as a PAH 13 14 15 analogue in a variety of studies.37-40 The aggregation of pyrene as a consequence of the 16 17 crystallization of the ASW matrix was investigated spectroscopically41 and matrix-isolated 2- 18 19 naphthol was used as a spectroscopic probe for investigating the molecular mechanism of the 20 21 42 43-44 22 glass transition of ASW which is governed by reorientation dynamics. 23 24 Here we prepare mixtures of amorphous ice and C fullerene, the guest species with the 25 60 26 27 lowest volatility investigated so far. C60 is used as a ‘dust’ analogue thought to resemble the dark 28 29 carbon species identified, for example, on the surfaces of comets and asteroids. Using 30 31 calorimetry, X-ray diffraction and temperature-programmed desorption experiments, the effect 32 33 34 of C60 on the crystallization and desorption properties of amorphous ice is investigated across a 35 36 large composition range. 37 38 39 40 Experimental Methods 41 42 43 Cryogenic co-deposition of C60 fullerene and water vapor 44 45 46 The experimental setup for cryogenic vapor deposition was based on a 12×12×24-inch box 47 48 vacuum chamber from Kurt Lesker as shown schematically in Figure 1a. Full details on the setup 49 50 are given in refs 30, 43, 45. For this study, the vacuum chamber was additionally fitted with an 51 52 53 LTE-10 point source evaporator deposition source from Kurt Lesker equipped with a 54 55 pneumatically controlled shutter. For each experiment, about one gram of C60 fullerene powder 56 57 58 59 4 60 ACS Paragon Plus Environment Page 5 of 33 The Journal of Physical Chemistry

1 2 3 3 from Tokyo Chemical Industry UK Ltd was placed inside the 15 cm Al2O3 crucible of the 4 5 6 evaporation source and gently compacted with a piston. The temperature of the evaporation 7 8 source was measured with a K-type thermocouple and controlled with a Eurotherm 2408 PID 9 10 unit which is part of a MAPS power supply from Kurt Lesker. 11 12 13 14 (a) z-positioner 15 liquid-nitrogen reservoir & pump 16 17 mass spectrometer pressure sensor 18 cold trap 5 6 (cold cathode & 19 diffusion pump Pirani) 20 1 21 2 1 3 22 23 8 24 leak valve 25 7 4 26 27 28 water reservoir & 29 pressure sensor (Pirani) 30 (b) 31 1:17 (C60:H2O) y t 32 i s 1:48 n

33 e t n 34 i 1:132

t

35 o

o 1:452 r 36 - e

r 1:1254

37 a u pure H O 38 q 2 s 39 ice Ih C 40 60 41 10 20 30 40 50 42 (c) 2 /  43 44 1:132 45 46 47 1:48 48 49 50 Figure 1. (a) Schematic illustration of the experimental setup for cryogenic co-deposition of C 51 60 52 53 54 fullerene and water vapor. A range of accessories are connected to the ports of the vacuum 55 56 57 58 59 5 60 ACS Paragon Plus Environment The Journal of Physical Chemistry Page 6 of 33

1 2 3 4 chamber as indicated. The components inside the chamber are (1) K-type thermocouple 5 6 7 temperature sensors, (2) cryostat and removable 8-inch diameter cryogenic deposition plate, (3) 8 9 10 quartz crystal microbalance sensor, (4) inlet tube for water vapor, (5) feedthrough connected to 11 12 13 a liquid nitrogen reservoir, (6) feedthrough connected to a rotary-vane pump, (7) C60 14 15 16 evaporation source with pneumatic shutter and (8) metal mesh above the water inlet. (b) 17 18 19 20 Powder X-ray diffraction patterns (Cu Kα) at 95 K of pure ASW and C60 / H2O mixtures with the 21 22 23 indicated C60:H2O molar ratios. Tickmarks indicate the expected positions of ice Ih and C60 24 25 26 Bragg peaks. Minor contaminations with ice Ih can arise during the sample transfer in a humid 27 28 29 atmosphere. Bragg peaks in the gray-shaded areas are due to the brass sample holder. (c) 30 31 32 Photographic images of the 8-inch deposition plate under liquid nitrogen containing the 1:132 33 34 35 36 (left) and 1:48 deposits (right). 37 38 39 40 41 The pressure of the chamber was monitored with a combination Pirani / cold cathode pressure 42 43 gauge (PenningVac PTF 90, Oerlikon Leybold Vacuum). The water-inlet system contained a 44 45 46 needle valve (EV 016 DOS AB, Oerlikon Leybold Vacuum) to adjust the amount of dosed water 47 48 vapor and a Pirani pressure sensor (Thermovac TTR91, Oerlikon Leybold Vacuum) was used to 49 50 measure the water inlet pressure. Before each deposition, the C60 powder was degassed by 51 52 heating to 250°C and waiting until the chamber reached base pressure (~2×10–6 mbar). The 53 54 55 56 57 58 59 6 60 ACS Paragon Plus Environment Page 7 of 33 The Journal of Physical Chemistry

1 2 3 liquid water in the water reservoir was carefully degassed using at least three freeze-pump-thaw 4 5 6 cycles. 7 8 9 The cryogenic deposition plate, which can be cooled to ~85 K with liquid nitrogen, was 10 11 positioned 15 cm above the water inlet tube and the C60 evaporation source. The temperatures of 12 13 the cryostat and the deposition plate were measured with K-type thermocouples using a home- 14 15 16 made instrument built with an Adafruit Feather 32u4 Basic Proto microcontroller and two 17 18 Adafruit MAX31856 Universal Thermocouple Amplifiers. The partial pressures of water were 19 20 recorded using a mass spectrometer (Hiden HALO 100) containing a Faraday cup. For quartz- 21 22 23 crystal microbalance (QCM) measurements, gold-plated AT-cut 6 MHz plano-convex quartz 24 25 crystals (Sycon Instruments) were placed inside an Allectra 710-SH sensor firmly attached to the 26 27 center of the deposition plate. The QCM sensor was connected to a reflection bridge and a 0.5 – 28 29 30 60 MHz N2PK vector network analyzer through coaxial cables. The deposition rates were 31 32 monitored by recording the changes in the fundamental resonance frequency in the electrical 33 34 conductance data over time using the myVNA and QTZ softwares.46 35 36 37 Different compositions of the C / H O deposits were realized by either adjusting the 38 60 2 39 40 temperature of the C60 deposition source or the flow of water vapor into the chamber. Full details 41 42 on the calibration of the deposition rates based on QCM measurements are given in the 43 44 Supporting Information. The deposited C / H O mixtures had molar C :H O ratios of 1:1254, 45 60 2 60 2 46 47 1:452, 1:132, 1:48, 1:17 and 1:5. 48 49 50 To prepare samples for characterization outside the vacuum chamber, three-hour 51 52 deposition times were used. Following the deposition, the deposition plate was heated to ~125 K 53 54 55 under vacuum to reduce the surface area of the sample and thus avoid the formation of nitrogen 56 57 58 59 7 60 ACS Paragon Plus Environment The Journal of Physical Chemistry Page 8 of 33

1 2 3 47 4 clathrate hydrates upon exposure to liquid nitrogen. After cooling back to ~ 90 K, the sample 5 6 was extracted from the chamber under liquid nitrogen for further analysis as described in ref. 30. 7 8 9 10 Powder X-ray diffraction 11 12 13 After extraction from the vacuum chamber, the samples were transferred into custom-made 14 15 sample holders with Kapton windows under liquid nitrogen and mounted on a Stoe Stadi-P X-ray 16 17 18 transmission diffractometer (Cu Kα1, 40 kV, 30 mA, Ge(111)-monochromator) with a Mythen 19 20 1K linear detector. The data was collected from 2° – 60° in 5° steps (100 s per step). The 21 22 temperature of the samples was maintained initially at 95 K and then increased from 100 to 270 23 24 25 K in 10 K steps with a CryojetHT from Oxford Instruments. 26 27 28 Differential scanning calorimetry 29 30 31 The recovered samples were transferred into stainless-steel DSC pans. These were quickly 32 33 transferred into a pre-cooled Perkin Elmer DSC 8000 Advanced Double-Furnace Differential 34 35 36 Scanning Calorimeter and heated from 93 to 263 K at 10 K min–1 thus forming bulk ice Ih and 37 38 39 40 C60. A second heating scan was then recorded which was used for background subtraction. 41 42 43 Finally, the ice was melted at 0°C. From the enthalpy of melting, the number of moles of H2O 44 45 were determined using a molar enthalpy of melting of 6012 J mol–1.48 The background-corrected 46 47 DSC signal was then divided by the moles of H O and the heating rate, which yields a quantity 48 2 49 –1 –1 50 in J mol K . 51 52 53 Temperature-programmed desorption (TPD) experiments 54 55 56 57 58 59 8 60 ACS Paragon Plus Environment Page 9 of 33 The Journal of Physical Chemistry

1 2 3 A deposition time of 2 hours was generally used for TPD experiments. After the deposition, the 4 5 6 samples were maintained at ~90 K until the chamber base pressure was reached. The samples 7 8 were then heated up to ~180 K while recording the partial pressure of H2O (m/z = 18) with the 9 10 mass spectrometer. The heating rates changed from ~2.5 K min–1 at 90 K to 1.0 K min–1 at 180 11 12 K. Upon reaching ~180 K, the upper pressure limit of the mass spectrometer was typically 13 14 15 reached (1×10–4 mbar). In addition to the co-deposition experiments, a ‘blanket’ experiment was 16 17 conducted for TPD analysis where a pure ASW film was deposited first for two hours (0.100 18 19 mbar inlet pressure) followed by two hours of C deposition on top of the ice (evaporation 20 60 21 22 source at 525°C). This corresponds to a bulk composition of 1 C60:132 H2O. 23 24 25 26 Scanning electron microscopy (SEM) 27 28 29 The C film from the blanket experiment was recovered after melting the sample and the ice- 30 60 31 32 facing side of the film was imaged using a Jeol JSM-6700F scanning electron microscope. 33 34 Additionally, the same amount of C60 was deposited directly onto an aluminum SEM stub firmly 35 36 attached to the deposition plate. 37 38 39 40 41 Results and Discussion 42 43 The low-temperature X-ray diffraction (XRD) patterns shown in Figure 1b demonstrate the 44 45 46 amorphous nature of the pure ASW and the C60 / H2O mixtures. Due to a very finely powdered 47 48 nature and the associated difficulties transferring the sample into the sample holder, it was 49 50 unfortunately not possible to record an XRD pattern of the 1:5 sample. Pure ASW displays a 51 52 53 characteristic first strong diffraction peak at ~24° and a weaker feature around 43°.2, 49-50 Upon 54 55 56 increasing the C60 content, the diffraction patterns of the C60 / H2O mixtures remain similar to 57 58 59 9 60 ACS Paragon Plus Environment The Journal of Physical Chemistry Page 10 of 33

1 2 3 pure ASW up to the 1:132 sample. An additional broad diffraction feature at ~29° emerges for 4 5 6 the 1:48 sample. The diffraction pattern of the 1:17 sample is quite different to that of pure 7 8 ASW, with a very broad feature around 21° and the ~29° feature, previously observed for the 9 10 1:48 sample, has grown more intense. It is important to note that none of these samples display 11 12 Bragg peaks characteristic of bulk C (see Figure S6).51 Upon increasing the C content, the 13 60 60 14 15 color of the samples changed from light pink for the sample with a molar C60:H2O ratio of 16 17 1:1254 to pink (1:452), red (1:132), dark red (1:48) and finally black for the two most C60-rich 18 19 samples (1:17 and 1:5). Photographic images of the 1:132 and 1:48 samples are shown in Figure 20 21 22 1c. Based on the diffraction data and the color changes, it can be concluded that C60 fullerene 23 24 and H2O have been successfully mixed over a large composition range using our cryogenic vapor 25 26 co-deposition technique. This would be very difficult to achieve by any other method since the 27 28 –9 –1 10 29 solubility of C60 in liquid water is ~1×10 mol L corresponding to a 1:6×10 molar C60:H2O 30 31 ratio.52-53 32 33 To investigate the effect of C60 on the crystallization properties of ASW, the various C60 / 34 35 H O mixtures were heated in a differential scanning calorimeter (DSC) at 10 K min–1. The 36 2 37 38 resulting DSC scans, which were normalized with respect to the moles of H2O present in each of 39 40 the samples, are shown in Figure 2a. Consistent with the similarities in the XRD patterns, the 41 42 DSC scans of the 1:1254, 1:452 and 1:132 samples resemble those of pure ASW with exotherms 43 44 –1 45 starting just above 150 K. The exotherm of pure ASW was found to be 1285 ± 93 J mol , in 46 47 good agreement with literature values.50, 54-58 The onset temperatures and area of the exotherms 48 49 were not found to change significantly upon increasing the C60 content from pure ASW to the 50 51 52 1:132 sample. This means that the presence of C60 does not have a significant effect on the 53 54 crystallization of ASW in this composition range. 55 56 57 58 59 10 60 ACS Paragon Plus Environment Page 11 of 33 The Journal of Physical Chemistry

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31 1 32 33 Figure 2. (a) DSC scans of pure ASW and C60 / H2O mixtures with the indicated C60:H2O molar 34 35 36 ratios heated at 10 K min–1. The DSC signals are normalized per mole H O. (b) Onset- 37 2 38 39 40 temperatures of crystallization of the various samples. 41 42 43 44 45 In the case of the more C60-rich samples, an impact on the crystallization properties of H2O can 46 47 be seen. Firstly, the initial exotherm shifts by more than 10 K towards higher temperatures, as 48 49 50 shown in Figure 2b, and decreases in area as the C60:H2O ratio changes from 1:48 and 1:17 to 51 52 1:5. Secondly, a second exothermic peak emerges at ~200 K for the 1:48 sample which increases 53 54 in area and shifts towards lower temperatures for the 1:17 and 1:5 samples. 55 56 57 58 59 11 60 ACS Paragon Plus Environment The Journal of Physical Chemistry Page 12 of 33

1 2 3 In addition to the crystallization of the H2O, C60 can also be expected to crystallize upon 4 5 6 heating either as it is expelled as the ASW matrix crystallizes or as amorphous C60 clusters 7 8 crystallize. Judging from the DSC data alone, it is not possible to tell at which temperatures the 9 10 various processes take place. However, it is tempting to assign the low- and high-temperature 11 12 exotherms to the crystallization of H O and C , respectively. The 1:48 sample was the first 13 2 60 14 15 significantly darker sample, as shown in Figure 1c, and, based on computational studies, the first 16 59-61 17 hydration shell of C60 is expected to contain 60 water molecules. This means that the 18 19 percolation threshold of C , corresponding to the transition from fully matrix-isolated C 20 60 60 21 22 molecules to the formation of C60 aggregates with long-range connectivity, is reached between 23 24 the 1:132 and 1:48 samples. Reassuringly, this agrees with theoretical estimates of contact 25 26 networks of hard spheres, where the critical-volume fraction for percolation was determined to 27 28 62 29 be 18.3%. The volume percentages of C60 for the 1:132 and 1:48 mixtures are 14 and 31%, 30 31 –3 respectively, using molecular volumes of H2O and C60 of 32.34 and 687.35 Å calculated from 32 33 34 the bulk densities. 35 36 37 It is important to emphasize that both exotherms were irreversible and therefore not 38 39 observed upon second heating. Bulk C60 is known to display two reversible phase transitions: a 40 41 glass transition-like transition around 90 K and a first-order transition at 260 K corresponding to 42 43 63 64-65 44 the unfreezing of rotational motion. The two phase transitions are endothermic upon 45 46 heating and not observed in our data which we attribute to the C60 being poorly crystalline with 47 48 small domain sizes. 49 50 To investigate the crystallization processes upon heating in more detail, XRD patterns of 51 52 53 the various samples were recorded as a function of temperature. The corresponding contour plots 54 55 are shown in Figure 3a-c for selected samples. The complete set of data are shown in Figure S7 56 57 58 59 12 60 ACS Paragon Plus Environment Page 13 of 33 The Journal of Physical Chemistry

1 2 3 in the Supporting Information. The XRD data of pure ASW shown in Figure 3a display the well- 4 5 6 known sequence of irreversible phase transitions upon heating starting with the crystallization of 7 8 ASW to give stacking disordered ice (ice Isd) at ~140 K which is followed by its gradual 9 10 66 conversion to the stable ice Ih above 180 K. Consistent with the DSC data, increasing the C60 11 12 content from pure ASW to 1:132 does not lead to significant differences in the XRD data upon 13 14 15 heating as shown in Figure S7. This includes the cubicities (i.e. the percentage of cubic 16 17 stacking)67 of the ice Isd samples right after crystallization at 150 K as shown in Figure S8. 18 19 Specifically, the cubicities of the ice Isd obtained from pure ASW and the 1:1254, 1:452 and 20 21 22 1:132 samples at 150 K took values between 62.2 and 63.0% which is within the margins of 23 24 error.67 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 13 60 ACS Paragon Plus Environment The Journal of Physical Chemistry Page 14 of 33

1 2 3 4 40 (a) 5 pure 6 H2O 7 30 8 9 10 20 11 square-root

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45 o 100 120 140 160 180 200 220 240 260 46 n T / K 47 Figure 3. X-ray diffraction data (Cu Kα) of pure ASW and C60 / H2O mixtures upon heating. (a-c) 48 49 50 Contour plots of X-ray diffraction patterns recorded upon heating from ~95 to 270 K in steps of 51 52 53 10 K. The molar C :H O ratios are indicated in (b,c). The diffraction intensities are shown on a 54 60 2 55 56 57 58 59 14 60 ACS Paragon Plus Environment Page 15 of 33 The Journal of Physical Chemistry

1 2 3 4 square-root scale to emphasize weaker features. Tick marks on the side of the panels indicate 5 6 7 the expected positions for ice Ih and C60 Bragg peaks. The complete set of diffraction data 8 9 10 (including additional compositions) is shown in Figure S7 in the Supporting Information. (d) 11 12 13 Intensities of the 40-degree ice I diffraction feature as function of temperature and normalized 14 15 16 with respect to the most intense feature. This quantity was used to follow the crystallization of 17 18 19 20 ASW. 21 22 23 24 25 Consistent with the previously-observed changes in the DSC data, the XRD data recorded upon 26 27 heating started to change significantly with the 1:48 sample as shown in Figure 3b. The 28 29 30 formation temperature of ice Isd shifts towards from ~130 K to ~160 K. Weak and broad Bragg 31 32 peaks appear above ~185 K at 10.8, 17.9 and 20.6° which indicate the formation of small 33 51 34 crystalline domains of C60 within the sample (see Figure S6). For the 1:17 sample, the C60 35 36 diffraction features are more intense and appear at slightly lower temperature compared to the 37 38 39 1:48 sample. In both cases, the appearance of C60 Bragg peaks is accompanied by the 40 41 disappearance of diffraction intensity below 5° which is only observed for the two C60-rich 42 43 samples (see Figure S7). This low-angle diffraction intensity is thought to arise from local 44 45 46 density differences between H2O and C60 regions on the length scale of nanometers. Its presence 47 48 therefore provides evidence for the existence of percolating nanodomains of H2O and C60. Upon 49 50 crystallization, the domain sizes increase significantly as the two components undergo phase 51 52 53 separation which leads to the disappearance of the low-angle diffraction intensity and the 54 55 appearance of C60 Bragg peaks. 56 57 58 59 15 60 ACS Paragon Plus Environment The Journal of Physical Chemistry Page 16 of 33

1 2 3 Based on these findings, it is possible to firmly assign the low- and high-temperature 4 5 6 exotherms in the DSC data to the crystallization of H2O and C60, respectively (see Figure 2a). 7 8 The decrease in temperature of the high-temperature exotherm is consistent with the appearance 9 10 of C60 Bragg peaks at lower temperatures in the more C60-rich samples. Obviously, if a sample 11 12 contains more C , it is easier from a kinetic point of view to form more extended regions of 13 60 14 15 crystalline C60. Equally, in highly diluted samples, the concentration of C60 is simply too small to 16 17 form large enough aggregates that can be detected with diffraction. As a side aspect of our study, 18 19 it should be possible to make C with very high surface areas upon freeze drying the samples 20 60 21 22 with matrix-isolated C60. The very small domain sizes of C60 obtained from the 1:17 sample at 23 24 270 K can be seen by comparing the half-widths of the Bragg peaks with those of bulk C60 (see 25 26 Figure S6). 27 28 41 29 Recently, pyrene was matrix-isolated in ASW at a 1:500 ratio. In this study, the 30 31 aggregation of pyrene was followed by detecting the excimer formation in fluorescence 32 33 spectroscopy. It was found that the aggregation of pyrene coincides with the crystallization of 34 35 ASW. The temperature difference between the H O and C crystallization observed here can be 36 2 60 37 38 explained by the sensitivity of X-ray diffraction vs. fluorescence spectroscopy for detecting 39 40 aggregation. While fluorescence spectroscopy can already detect the formation of dimeric 41 42 aggregates, much more extended and crystalline regions are needed to give rise to Bragg peaks in 43 44 45 diffraction experiments. 46 47 To follow the crystallization of H2O in the various samples more quantitatively, the 48 49 intensities of the (110) Bragg peaks at ~40° were analyzed as a function of temperature. Figure 50 51 52 3d shows the obtained intensities normalized with respect to the highest intensity for each given 53 54 sample which was typically observed at 220 K. Conveniently, the shape of this Bragg peak is not 55 56 57 58 59 16 60 ACS Paragon Plus Environment Page 17 of 33 The Journal of Physical Chemistry

1 2 3 affected by the presence of stacking disorder and its intensity does not depend significantly on 4 5 6 changes in the cubicity, at least within the 0 to 70% range as shown in Figure S9. For pure ASW 7 8 and the 1:1254, 1:452 and 1:132 samples, very similar trends were observed. The normalized 9 10 peak intensities gain significant intensity between 130 and 140 K, which corresponds to the 11 12 formation of ice Isd from ASW. After this, the peak intensities rise continuously up to ~210 K. 13 14 15 Upon heating from 150 to 220 K, the half-width decreases from 0.470 to 0.319°. This suggests 16 17 that the ice Isd domains increase continuously in size in this temperature range while also 18 19 transforming to ice Ih. It is noted that such changes in domain sizes have recently been predicted 20 21 68 22 on the basis of vapor pressure measurements of ASW. The much delayed intensity increases of 23 24 the two C60-rich samples can be explained by the presence of the percolating C60 domains that 25 26 hinder the coalescence of ice I (ice Isd or ice Ih) domains. Essentially, the ice I is confined by the 27 28 29 C60 domains. Owing to the presence of large amounts of C60 in those samples, it was not possible 30 31 to accurately determine the cubicities of the ice Isd. The final drops in peak intensity above ~250 32 33 K observed for all samples may be due to the increases in the atomic thermal displacement 34 35 parameters as the melting point is approached or simply to the fact that the samples drop in the 36 37 38 sample holder as the ice grains soften. 39 40 As a next step, the effect of C60 on the desorption properties of H2O vapor were 41 42 investigated. For this, temperature-programmed desorption (TPD) experiments were carried out 43 44 45 where the samples were heated in the vacuum chamber from ~85 to ~180 K. The partial pressure 46 47 of water of pure ASW and the C60 / H2O mixtures were recorded with a mass spectrometer (m/z 48 49 = 18) as shown in Figure 4a. In the case of pure ASW, the partial pressure increases rapidly 50 51 28, 30 52 above ~130 K. The absence of a commonly observed pre-peak in this region is attributed to 53 54 the macroscopic thickness of our films.30 As ASW crystallizes to ice Isd, the partial pressure 55 56 57 58 59 17 60 ACS Paragon Plus Environment The Journal of Physical Chemistry Page 18 of 33

1 2 3 decreases around ~150 K and then rises again above ~165 K as the vapor pressure of ice Isd 4 5 28, 30 6 increases with temperature. The TPD data of the 1:1254, 1:132 and 1:48 samples follow pure 7 8 ASW closely, which means that the presence of C60 has no significant effect on the desorption of 9 10 water in this composition range. 11 12 As mentioned in the Experimental section, the more C -rich samples were prepared by 13 60 14 15 keeping the C60 deposition rate constant while reducing the amount of water vapor dosed into the 16 17 chamber. Therefore, the partial pressures upon heating increase less compared to the previous 18 19 experiments because there is less H O present in the depositions. In order to make the various 20 2 21 22 experiments comparable, the desorption peaks around 150 K were integrated against time in 23 24 order to take variations in the heating rates into account and divided by the total mass of water 25 26 dosed into the chamber. The resulting quantity reflects the amount of water desorption in the 27 28 29 temperature range of ASW crystallization normalized with respect to the amount of dosed H2O. 30 31 Figure 4b shows these quantities from all TPD experiments plotted against the volume 32 33 percentage of C60 which is a convenient quantity for this comparison. The amounts of water 34 35 desorption only begin to drop above 60 volume percent C (1:17 ratio), which implies that 36 60 37 38 amounts of C60 significantly above the percolation threshold that are needed to suppress the 39 40 water desorption. 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 18 60 ACS Paragon Plus Environment Page 19 of 33 The Journal of Physical Chemistry

1 2 3 -5 4 3x10 (a) pure H2O r

5 r 1:1254 o

t -5 1:132

6

/ 2x10 1:48

7 O 2 1:17 8 H -5 p 1x10 1:5 9 10 0 1 11 - 100 120 140 160 180 g T / K

12 s

r

13 r o

t (b)

14 -2 /

1x10

15 n o i

16 t blanket p 17 r 5x10-3 experiment o

18 s e d

19 . 0

20 m r 0 20 40 60 80 100 21 o

n volume % C 22 60 -5 23 3x10 (c) 1:132 (C60:H2O) r

24 r

o mixture t -5

25

/ 2x10 H2O + C60 blanket

26 O 2

27 H -5 p 1x10 28 29 0 30 100 120 140 160 180 31 T / K 32 33 Figure 4. Thermal desorption of H O vapor from C / H O mixtures upon heating. (a) Partial 34 2 60 2 35 36 37 pressures of H2O (m/z = 18) of pure ASW and various C60 / H2O mixtures with the indicated 38 39 40 C60:H2O molar ratios recorded upon heating. (b) Water desorption features integrated against 41 42 43 time and normalized with respect to the mass of H2O administered into the chamber during the 44 45 46 vapor deposition experiment. (c) Comparison of desorption features of C60 / H2O mixtures and 47 48 49 pure ASW covered with a blanket of C . The nominal bulk compositions of the two samples are 50 60 51 52 53 1:132 C60:H2O. 54 55 56 57 58 59 19 60 ACS Paragon Plus Environment The Journal of Physical Chemistry Page 20 of 33

1 2 3 In an attempt to block the water vapor desorption more effectively, a ‘blanket’ of pure C60 was 4 5 6 deposited on top of a pure ASW film so that the bulk composition of the entire sample was 1:132 7 8 C60:H2O. Based on the molecular volumes used earlier, this means that the thickness of the C60 9 10 layer was about 14% of the entire film. A comparison of the TPD data of the ‘blanket’ 11 12 experiment with the corresponding 1:132 mixture is shown in Figure 4c. As can be seen from the 13 14 15 data point in Figure 4b, the presence of the film leads to a ~21% reduction of the water 16 17 desorption. The slight ‘wiggle’ in the TPD data of the blanket experiment at ~150 K could 18 19 indicate some sudden rupturing of the C blanket as increasing amounts of desorbing H O vapor 20 60 2 21 22 force their way through the C60 film. In any case, even if a C60 blanket is used, significant 23 24 amounts of C60 are needed in order to suppress the water desorption. After the TPD experiment, 25 26 the C60 film was recovered. A low-magnification scanning electron microscopy image of the ice- 27 28 29 facing side is shown in Figure 5. The film is significantly cracked most likely as a result of 30 31 water-vapor permeation. To test if the cracks are due to the thermal treatment, a C60 film of the 32 33 same thickness was deposited at 90 K directly onto a metal substrate and heated to room 34 35 temperature. The SEM image of this sample shows a smooth surface (see Figure S10). 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 20 60 ACS Paragon Plus Environment Page 21 of 33 The Journal of Physical Chemistry

1 2 3 4 Figure 5. Low-magnification scanning electron microscopy image of the ASW-facing side of the 5 6 7 C60 blanket after heating to ~180 K in vacuum. 8 9 10 11 12 The blanket experiment reminds of the previously reported ‘molecular volcano’ studies where 13 14 volatile species ‘erupted’ through layers of amorphous ice upon heating.69-70 In our experiment, 15 16 17 the roles have changed and amorphous ice has become the volatile species that forces its way 18 19 through the C60 film. 20 21 22 23 24 Conclusions 25 26 27 The presence of matrix-isolated C60 appears to have no significant impact on the crystallization 28 29 properties of ASW, including the cubicities of the ice Isd crystallization products. Upon reaching 30 31 the percolation threshold of C60, the onset temperature of crystallization of H2O shifts towards 32 33 34 higher temperatures by about 10 K. These findings illustrate the weak interaction between H2O 35 36 and C60. The shift of the crystallization temperature above the percolation threshold is probably 37 38 due to a physical confinement effect from the C60 which reduces the size of the H2O domains. 39 40 Small H O regions are physically separated, and they therefore crystallize independently. 41 2 42 43 44 The aggregation of C60 is seen as a separate process at higher temperature that shifts 45 46 towards lower temperatures as the C60 content of the samples increases. In general, all 47 48 crystallization processes of C / H O mixtures were found to be exothermic which is in stark 49 60 2 50 71 51 contrast to the endothermic crystallization of ASW containing volatile guest species. 52 53 54 55 56 57 58 59 21 60 ACS Paragon Plus Environment The Journal of Physical Chemistry Page 22 of 33

1 2 3 The desorption of H2O vapor in the temperature range of the crystallization of amorphous 4 5 6 H2O is not significantly affected by the presence of C60, even for C60-rich samples containing up 7 8 to 60 volume percent C60. This illustrates the force of water-vapor desorption that also seems to 9 10 produce large cracks into a blanket of C60 covering a film of ASW. 11 12 13 A general feature of the C / H O mixtures is that their physical properties change in a 14 60 2 15 16 highly non-linear fashion as a function of the composition. In fact, there are ‘jumps’ once certain 17 18 threshold values are reached such as the C60 percolation. In addition to the thermal stability and 19 20 water-vapor desorption studied here, other physical properties including heat and electrical 21 22 23 conductivity as well as optical absorbance properties can also be expected to display non-linear 24 25 behavior as a function of the C60 content. This means that even small variations in the C60 26 27 content can lead to composite materials with very different physical properties. Ultimately, this 28 29 30 behavior could well be the explanation for the ‘pits’ and ‘goosebumps’ that form as a 31 23-24 32 consequence of the H2O weather cycle on the 67P comet. 33 34 35 36 Supporting Information 37 38 39 The Supporting Information includes details on controlling the composition of the C60 / H2O 40 41 samples, additional X-ray diffraction patterns and a scanning electron microscopy image of a 42 43 44 pure C60 film. 45 46 47 48 Acknowledgment 49 50 51 Funding is acknowledged from the European Research Council under the European 52 53 Union’s Horizon 2020 research and innovation programme (grant agreement No 725271). 54 55 We thank the Department of Chemistry at Durham University for a PhD studentship 56 57 58 59 22 60 ACS Paragon Plus Environment Page 23 of 33 The Journal of Physical Chemistry

1 2 3 (JJS), J. Cockcroft for access to the Cryojet, D. Johannsmann for advice on the QCM 4 5 6 setup, M. Vickers for help with the X-ray diffraction experiments and S. Firth for help 7 8 with the SEM imaging. 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 23 60 ACS Paragon Plus Environment The Journal of Physical Chemistry Page 24 of 33

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1 2 3 TOC Graphic 4 5 6 7 8 9 10 11 12 13 amorphous ice 14 15 matrix-isolated C60 percolating C60 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 32 60 ACS Paragon Plus Environment Page 33 of 33 The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 TOC image 25 26 82x44mm (900 x 900 DPI) 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment