Ordered Structures and Morphology-Induced Phase Transitions at

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Ordered Structures and Morphology-Induced Phase Transitions at Graphite#Acetonitrile Interfaces Chengyi Wu, and Ding-Shyue Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b06440 • Publication Date (Web): 23 Aug 2019 Downloaded from pubs.acs.org on August 29, 2019

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The Journal of Physical Chemistry

J. Phys. Chem. C

08/22/2019 (revised)

Ordered Structures and Morphology-Induced Phase Transitions at Graphite‒Acetonitrile Interfaces

Chengyi Wu, Ding-Shyue Yang* Department of Chemistry, University of Houston, Houston, Texas 77204, United States

*To whom correspondence should be addressed. Tel. No. Email: [email protected]

1 Environment ACS Paragon Plus

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Abstract In this report, reflection high-energy electron diffraction as a direct structure probing method is used to reveal an unanticipated vertical ordering and long-range crystallinity in interfacial acetonitrile physisorbed on highly oriented pyrolytic graphite (HOPG), whose assembly structures are relevant to further development of batteries. Even without significant guiding forces in this solid‒molecule system, surface morphology and terraces of HOPG play a critical role in molecular ordering as a result of the lattice-matching template effect. More surprisingly, a phase transition between two acetonitrile polymorphs solely due to surface topography of HOPG is observed for the first time. By careful examination of the crystal structures, the interfacial ordering and the structural transition can be understood as the results of a competition between the kinetically controlled Ostwald process and thermodynamic energetics. Lastly, the present observations add to the mounting evidence for the need to explicitly take surface morphology of HOPG into consideration and the unique strength of reflection electron diffraction for interfacial studies.


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1. INTRODUCTION Visualization and understanding of solid‒adsorbate interfaces and interfacial assemblies are arguably the most important tasks in a significant portion of surface science research, including heterogeneous catalysis,1 corrosion,2 self-assembly,3 electrochemistry,4 tribology,5and even glass and phase transitions.6 In the last fifty years, a variety of experimental methods with surface sensitivity or specificity have been developed and now broadly used to gain molecular-level information at interfaces.7,8 For vapor-deposited molecules on solid supports, whose research has relevance to applications such as surface modifications, lubrication, and designs of electrodes in batteries and fuel cells,9–11

the typical techniques used include x-ray photoelectron

spectroscopy,12 infrared spectroscopy,13,14 sum-frequency generation spectroscopy,15 and temperature-programmed desorption.14,16 More direct structure-probing methods such as lowenergy electron diffraction, scanning tunneling microscopy, atomic force microscopy (AFM), and grazing incidence x-ray diffraction (XRD) have also been utilized frequently,1,17–19 providing structural details on different scales from subatomic to ensemble assembly levels. The advantages of using reflection high-energy electron diffraction (RHEED) to probe molecular assemblies and surface structures are worth noting, although its use has been mostly in materials sciences and industries for thin-film fabrication and characterization in recent decades. The surface specificity of RHEED due to the large electron‒matter scattering cross sections at grazing incidence allows non-contact, direct probing of structures for a wide variety of interfacial assemblies and their unique organizations or phase transformations. For instance, less anticipated but robust ordered structures of water and ionic liquids have been observed on graphite.20–22 More unexpectedly, the amorphous-to-crystalline transition of vapor-deposited methanol on hydrophobic smooth surfaces undergoes a two-step process, which is far from the general



wisdom of a single process.23 Incorporated with an ultrashort temporal resolution, time-resolved RHEED revealed a nonequilibrium phase transition and structural dynamics of interfacial water.20,24 These unique results demonstrate the capability of direct probing of structures via reciprocal-space imaging for a variety of interfacial studies. We also note that radiation-induced damages are largely mitigated because an extremely low rate of electron dose on the order of 0.1‒10 nA/cm2 (equivalently, an accumulated dose of 0.001‒0.1 e/Å2 in 4 hours) is used with the help of an image intensifier in the camera assembly. In this study, the structures of acetonitrile assemblies on highly oriented pyrolytic graphite (HOPG) and a few smooth surfaces (with differences in the root-mean-square roughness and without significant solid‒molecule interactions) are examined using RHEED. Graphite has been a common choice of the anode material in commercial Li-ion batteries because of the balance of its relatively low cost, abundance, support for moderate energy density, and cycle life.25 Although often omitted, surface topography of graphite may lead to prominent behavioral differences such as step-edge regions showing more catalytic activity compared to the basal plane;26,27 it may even influence or promote structural orders of interfacial assemblies.20,21,28 Acetonitrile (ACN) has been considered potential for high-voltage and fast-charging Li-ion and Li-air batteries29,30 given that it is the simplest aprotic organic nitrile with a high dielectric constant and a high dipole moment capable of dissolving various electrolytes to exhibit high ionic conductivities.29 The intermolecular forces between ACN molecules are mainly dipole‒ dipole interactions.30 Thin films of ACN vapor-deposited at a cryogenic temperature in ultrahigh vacuum (UHV) allow studies of solid‒ACN interfaces without the interference of water forming hydrogen bonds with ACN.31,32 Previous reports showed that the chemisorbed layer of ACN displayed an “end-on” or “side-on” orientations on metal33–36 and semiconductor surfaces.37–39


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With respect to physisorption, it was found by XRD that thick ACN films exhibit a hightemperature α phase at 115 K instead of the more stable β phase observed in the bulk.40,41 It is therefore important to conduct a thorough study of ACN on graphite due to the seeming inconsistency and the multiple structural possibilities as well as the relevance to potential applications. Here, we report the typical observation of the monoclinic α phase for interfacial ACN immobilized at cryogenic temperatures in UHV, instead of the orthorhombic β phase found in bulk below 216.4 K. Compared to the diffractions observed on smooth hydrogen-terminated Si(111) [H:Si(111)], sputtered InAs(111), and sapphire-supported monolayer MoS2, the apparent long-range 3-D orders of interfacial ACN on HOPG are unanticipated, likely as a result of lattice matching between the ACN and HOPG structures and hence a template effect for ordering. More surprisingly, a new morphology-induced α-to-β transition is found in thicker ACN films on HOPG surfaces with higher step densities, in contrast to the thermally induced α-to-β phase transition.41 These results indicate that the crystallization of vapor-deposited ACN at interfaces is more complex than being simply driven by the intermolecular dipole‒dipole interactions of ACN. Thus, the various examples including the current study collectively show the critical role of morphology-dependent ordering of interfacial assemblies even on substrates without strong guiding forces for adsorbate molecules.20–22

2. EXPERIMENTAL SECTION Preparation and Characterization of Substrate Surfaces. Each of the substrates used was carefully examined by RHEED before ACN deposition to ensure an appropriate surface condition. An n-type, Sb-doped single-crystalline Si(111) wafer was purchased from MTI Corporation. Typical wet chemical etching was used to produce smooth H-terminated Si(111).23



Afterward, the specimen was quickly loaded into the vacuum chamber for later use. Two HOPG substrates were employed in this work. The “smooth” HOPG (ZYA grade, GE Advanced Ceramics) with a mosaic spread angle as low as 0.4°±0.1° were obtained from SPI Supplies. Freshly cleaved surfaces were obtained by exfoliation for a total of less than 10 times to avoid surface degradation.42 The “rough” HOPG (also ZYA grade) with a mosaic spread angle of 0.8°±0.2° was purchased from Ted Pella; exfoliation was made for hundreds of times, resulting in more surface steps and smaller domain sizes.42 In either case, the HOPG substrate was quickly loaded into the vacuum chamber after fresh cleavage and no further treatment was made. The surface topography of both HOPG specimens was characterized ex situ by the tapping mode of AFM in air. An undoped, n-type single-crystalline InAs(111) wafer with a root-mean-square roughness of

Ordered Structures and Morphology-Induced Phase Transitions at

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