Graphite Mediated Oxidation of Coronene Adsorbates: A UHV Study

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Graphite Mediated Oxidation of Coronene Adsorbates: A UHV Study Jürgen Weippert, Vincent Gewiese, Artur Böttcher, and Manfred M. Kappes J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08637 • Publication Date (Web): 07 Nov 2018 Downloaded from http://pubs.acs.org on November 10, 2018

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The Journal of Physical Chemistry

A) TRMS maps obtained for a monolayer of Coronene deposited on top of an HOPG surface pretreated with various atomic oxygen doses [C24H12+ O*HOPG]. All panels have the same temperature scale and temperature range (with the exception of topmost panel which extends to higher temperatures but retains the same temperature scale). B) Yield of desorbable lactones (O1) and dilactones (OIII) vs. oxygen dose (based on the TRMS maps shown in A)). C) Yield of desorbable Coronene (open squares) and oxide products (sum of OI and OIII signals; open circles) vs. oxygen exposure (based on the TRMS maps shown in A)). 195x276mm (150 x 150 DPI)

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A) O-dose dependency of the thermal desorption spectra of Coronene (from TRMS maps shown in fig. 6 A). Two well-defined desorption bands,  and  are observed reflecting two distinct desorption sites. B) Coronene thermal desorption spectra versus Coronene coverage for fixed oxygen pre-exposures of D= 1.8 × 1016 O/cm2. A reference TD spectrum for 1 ML C24H12 is also shown without oxygen pretreatment (thin line, α0). The inset shows the activation barriers for Coronene desorption as derived from the C24H12TD spectra assuming first order desorption kinetics with a preexponential factor of 1016 s-1. 190x141mm (150 x 150 DPI)


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1

Oxidation of Coronene Adsorbates Mediated by Graphite Oxides: a UHV Study Jürgen Weippert1, Vincent Gewiese1, Artur Böttcher1* and Manfred M. Kappes1,2* 1Institute

of Physical Chemistry, Karlsruhe Institute of Technology (KIT), Fritz-Haber-Weg 2, 76131 Karlsruhe, Germany 2Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT), Postfach 3630, D-76021 Karlsruhe, Germany

Abstract Thin Coronene films on HOPG were oxidized by exposing them to atomic oxygen under UHV conditions. The products were probed by mass-resolved thermal desorption spectroscopy (TDS) and Xray photoelectron spectroscopy (XPS). Whereas the species subliming from thick oxidized films comprise predominantly

epoxides C24H12On (n≤7), the oxidation of monolayers yields different

sublimable products, notably lactones and dilactones – thus providing clear evidence for a surface mediated reaction mechanism. After on-top oxidation, roughly 7% of the Coronene monolayer can be sublimed as oxidized derivatives (limited by competing surface reactions which lead to non-desorbable products). The desorption yield can be raised to 10% by depositing the Coronene monolayer onto preoxidized HOPG. Based on our observations and literature results concerning HOPG oxidation, we argue that lactones and dilactones can be generated by reaction of Coronene adsorbates with epoxy and ether species formed on the graphite support. Our results obtained for the O/Coronene/HOPG model system may be relevant for the mechanism of carbon-based oxidation catalysis in general. (*) corresponding authors: [email protected] and [email protected]

1. Introduction Graphene oxides (GOs) are of interest for applications in nanoelectronic devices (e.g. 1,2,3). The preferred bulk-scale fabrication method uses the Hummers-Offeman procedure4, a rather harsh oxidative treatment of graphite involving sulfuric acid and potassium permanganate. According to the LerfKlinovski model, GO comprises randomly shaped and substantially defected graphene flakes which are oxidized and hydrated to differing degrees.5,6 Thus, GOs differ substantially from pristine planar graphene flakes. Despite their polydispersity, GOs can function as stable quantum dot emitters due to localized intrinstic states.7 Their performance could likely be improved if shape, size and chemical composition of the corresponding GOs could be better controlled. This publication relates to one of the approaches by which this goal can be achieved.



2 Müllen and Rabe have shown that large tailored polyaromatic hydrocarbons, PAHs, have chemical and physical properties much like nanostructured flakes and ribbons of graphene (nanographene).8,9 We have recently described a facile and controllable route to nanographene oxides (nanoGOs), which makes use of the UHV-based oxidation of such large PAHs. Specifically, we have exposed thick Coronene films to a flux of atomic oxygen.10 Interestingly, a large fraction of the reaction products obtained in this way can subsequently be sublimed intactly as C24H12On (n≤7). We have inferred that the corresponding “nanoGO” molecules comprise primarily rim-epoxides, i.e. Coronene molecules with perimeter epoxide functionalization. To generate the C24H12On nanoGOs, we typically deposited thick Coronene multilayers  5ML (ML=monolayers) onto graphite surfaces prior to oxidation. For Coronene films of one monolayer or less we unexpectedly observed different oxidation products. As we will outline below, a closer examination of this phenomenon led us to conclude that the substrate is involved in this change of reactivity – and may even act as a catalyst for the new reaction channels. Several recent reports have demonstrated catalytic action of carbon based surfaces.11,12 In fact, over the last decade significant effort has been devoted to replacing inorganic supports and metal catalysts with “greener” all-carbon materials (e.g. activated graphite, carbon nanotubes).13,14 For example, carbon materials have been shown to catalyze various reduction reactions as well as the dehydrogenation of ethylbenzene15 and the oxidation of cyclohexane.16 Recently, graphitic materials in contact with solution were demonstrated to efficiently catalyze a Baeyer-Villiger-like oxidation of cyclic ketones into lactones (in the presence of O2 as the oxidant and benzaldehyde as a sacrificial agent).17 It has been suggested (somewhat counter-intuitively), that the main contribution to such carbon catalyzed conversions originates not from defected surface sites but from graphitic grains with exposed basal planes, i.e. planar carbon networks with delocalized  electrons.18 This idea is supported by the observation that the yield of lactones found for a graphite catalyst is significantly higher than that found for activated graphite (high density of defects).17 However, an activation of molecular oxygen as the key step in the ketones-to-lactones transformation could not be clearly demonstrated. Also, while the facile formation of mobile peroxide dianions and the enhancement of the Lewis basicity of C=O bonds are often invoked as specific consequences of the interaction of reagents with the -electrons of the graphite surface (electron transfer from the -system to impinging O2 to form O22- is thought to trigger the reaction O22- 2O- 11,13,14), the associated energetics are still subject to question. So far, the complexity of the many coupled elementary steps in reactorbased carbon-catalysed oxidation processes has prevented unambiguous identification of key mechanistic steps.17 This situation has motivated us to use a reductionist surface science approach to study a model graphite-mediated oxidation process under UHV conditions. Specifically, we probe the oxidation of Coronene sub- and monolayers supported on nearly defect free HOPG using near-thermal oxygen atoms


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3 as the oxidant. The progressing oxidation reaction is monitored by means of established surface science spectroscopic tools: mass-resolved TDS and XPS.

2. Methods The approach used here is analogous to that applied in our study of the O-atom oxidation of thick Coronene films and has been described in detail in Ref. 19. All experiments were performed under ultra-high vacuum conditions (UHV, base pressure lower than 5x10-10 mbar). The UHV apparatus consists of four interconnected vacuum compartments. (1) An ion beam section in which Coronene molecules effusing from the Knudsen cell are ionized by electron impact (70 eV electron kinetic energy). The resulting cationic beam passes through a quadrupole mass filter (Extrel Merlin) tuned to transmit M=300±2 u; (2) The deposition chamber, in which the Coronene cations are gently deposited onto a clean HOPG substrate (sample temperature T=300K, soft landing conditions: Ekin≈ 6 eV per cation); (3) The reaction chamber in which the Coronene/HOPG samples created in the deposition chamber are oxidized by exposing them to a constant flux of near-thermal oxygen atoms (source: Gen2, TECTRA); (4) The analysis chamber in which the adsorbate system is characterized before and after oxidation using (in this study) in situ X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and mass-resolved thermal desorption spectroscopy (MS-TDS). Nearly defect-free HOPG substrates (7x7x1 mm, SPI, ZYB quality) were prepared by tapecleaving in air, transferring into the reaction chamber and subsequently heating in vacuum up to 1100 K until XPS (especially O1s and C1s) and UPS no longer showed impurities in the surface region. This procedure was applied in between every experiment. Coronene was obtained from Sigma-Aldrich (99%) and perdeuterated Coronene from CDN Isotopes (98%). Soft landing of the cations onto the room temperature surface was ensured by applying a retarding potential to the substrate (Uret≈29 V) resulting in an incident kinetic energy Ekin≈6 eV (low energy cluster beam deposition method, LECBD). We typically used LECBD rather than simple sublimation for two reasons: (1) mass selection excludes impurities always present in the sublimating raw Coronene material and (2) mass-selected ions enable to control the resulting coverage exactly by measuring the neutralization current at the deposition target. The only deficiency of the LECBD method derives from electron-impact induced dissociation of the terminating C-H bonds in Coronene cations. Under typical ionization conditions, mass spectra of the incident cationic beam manifest a dominant C24H12+ parent ion component but also significant amounts of dehydrogenated Coronene fragment ions, primarily C24H10+. For typical settings, codeposited fragments amounted to less than 15% of the parent ion intensity. To check that the presence of codeposited fragment species did not significantly affect the outcome of our experiments, we performed two kinds of reference measurements: (i) comparison to thin films prepared by direct Knudsen Cell evaporation and (ii) comparison to deposits obtained from Coronene ion beams ionized with 10eV ACS Paragon Plus Environment


4 electrons (no fragmentation, but significantly depleted ion current). Both reference experiments resulted in oxidation products identical to those observed for LECBD with Eel≈70 eV (see supplement). The Coronene surface coverage  in ML was determined from the neutralized charge Q by measuring the neutralization current integrated over the deposition time and relating Q to the number of Coronene molecules constituting a monolayer on HOPG (9.1 x 1013 molecules/cm2)20. The incident C24H12+ beam has a roughly elliptical cross section. The deposited spot has a mean diameter of 2-3 mm and exhibits a Gaussian-like coverage distribution. Coronene deposits (held at room temperature) were exposed to an oxygen beam generated by a plasma source. The source operates as a low-temperature and low-pressure flow reactor and is optimized for partial oxygen pressures of 10-5 mbar (16O 99.998%: Messer Griesheim GmbH and 18O Linde). The source output is equipped with an ion-deflecting high voltage electrode which efficiently prevents ions from reaching the sample. The incident beam comprised only oxygen atoms and oxygen molecules in a ratio N(O)/N(O2)

Graphite Mediated Oxidation of Coronene Adsorbates: A UHV Study

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