development of mild methods for selective covalent...
TRANSCRIPT
ACTAUNIVERSITATIS
UPSALIENSISUPPSALA
2017
Digital Comprehensive Summaries of Uppsala Dissertationsfrom the Faculty of Science and Technology 1593
Development of Mild Methodsfor Selective CovalentFunctionalization of Graphene
ANNA LUNDSTEDT
ISSN 1651-6214ISBN 978-91-513-0135-8urn:nbn:se:uu:diva-332004
Dissertation presented at Uppsala University to be publicly examined in A1:107a, Husargatan3, Uppsala, Thursday, 14 December 2017 at 09:15 for the degree of Doctor of Philosophy.The examination will be conducted in English. Faculty examiner: Professor Bo WeggeLaursen (University of Copenhagen).
AbstractLundstedt, A. 2017. Development of Mild Methods for Selective Covalent Functionalizationof Graphene. Digital Comprehensive Summaries of Uppsala Dissertations from theFaculty of Science and Technology 1593. 48 pp. Uppsala: Acta Universitatis Upsaliensis.ISBN 978-91-513-0135-8.
This thesis discusses methods for the comparatively mild covalent functionalization ofgraphene. Several graphene models were investigated: polycyclic aromatic hydrocarbons(PAHs), chemical vapor deposition (CVD)-graphene on SiO2/Si substrate, graphite foil, graphiteflakes, kish graphite and highly oriented pyrolytic graphite. The PAHs were viewed as grapheneedge analogs with the following molecules representing different edge motifs: pyrene, perylene,benzo[a]pyrene, benzo[e]pyrene, triphenylene, acenapthylene, and anthracene.
Ozone was used in combination with different solvents to functionalize PAHs, graphite, andCVD-graphene on SiO2/Si. Ozonation in water or methanol resulted in trapping of the carbonyloxide intermediate that was formed in the reaction, producing a variety of functional groups.Ozonation in hydrogen peroxide solution with sonication promoted radical formation, possiblyresulting in edge-oxidation of graphite. The regioselectivity for addition reactions (ozonolysis)and electrophilic aromatic substitution reactions with graphene edges is discussed.
To achieve functionalization of the basal plane of graphite or graphene, white light irradiationwas used in combination with several transfer hydrogenation reagents. Formic acid treatmentunder irradiation resulted in the expected hydrogenation, whereas iso-propanol treatmentresulted in iso-propanol attachment to the graphene.
The developed methods provide opportunities for graphene functionalization without the needfor metal based reagents or harsh conditions.
Keywords: Polycyclic aromatic hydrocarbons, graphene models, graphite, local ionizationenergy surfaces, graphene functionalization
Anna Lundstedt, Department of Chemistry - BMC, Box 576, Uppsala University, SE-75123Uppsala, Sweden.
© Anna Lundstedt 2017
ISSN 1651-6214ISBN 978-91-513-0135-8urn:nbn:se:uu:diva-332004 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-332004)
List of Papers
This thesis is based on the following papers, which are referred to in the text by their Roman numerals.
I Lundstedt, A., Webb, J. M., Grennberg, H. Ozonolysis of poly-
cyclic aromatic hydrocarbons with participating solvents. RSC Advances, 2017, 7, 6152 - 6159
II Lundstedt, A., Grennberg, H., Ozonolysis of triphenylene in acetone. Manuscript
III Lundstedt, A., Nordlund, M., Ahlberg, P., Grennberg, H. Edge oxidation of graphite using a combined hydrogen peroxide - ozone treatment under sonication conditions. Manuscript
IV Papadakis, R., Li, H., Bergman, J., Lundstedt, A., Jorner, K., Ayub, R., Haldar, S., Jahn, B., Denisova, A., Zietz, B., Lindh, R., Sanyal, B., Grennberg, H., Leifer, K., Ottosson, H. Metal-Free Photochemical Silylations and Transfer Hydrogenations of Benzenoid Hydrocarbons and Graphene. Nature Communica-tions, 2016, 7, 12962.
V Lundstedt, A.†, Papadakis, R.†, Li, H.†, Han, Y., Jorner, K., Bergman, J., Leifer, K., Grennberg, H., Ottosson, H. White-light photoassisted covalent functionalization of graphene using 2-propanol. Small Methods, 2017, 1700214 †These authors contributed equally to this work
Appendix: Treating polycyclic aromatic hydrocarbons with deuterium chloride
Reprints were made with permission from the respective publishers.
Contribution Report
The author's contributions to Papers I-V.
I Contributed to the research idea and formulation. Planned, per-formed, and evaluated all experiments, analysis, and calcula-tions. Wrote the paper.
II Contributed with the original research idea. Planned, performed, and evaluated all experiments, analysis, and calculations. Wrote the paper.
III Contributed with the original research idea. Contributed signifi-cantly to the planning, performing, and evaluation of all expe-riments, analysis, and writing of the paper.
IV Contributed to the research idea and formulation. Designed some of the experiments with graphene and some with PAHs, conducted some of the experiments with PAHs, analysis with GC-MS, and NMR spectroscopy.
V Contributed to the research idea and formulation. Contributed significantly to the planning, performing, and evaluation of all experiments and writing of the paper.
Appendix: Planned, performed, and evaluated all experiments.
Parts of section 2.2 and an earlier version of Paper I in this thesis are in-cluded in:
Lundstedt, A. Edge Reactivity and Selectivity for Electrophilic Aromatic Substitution and Ozonolysis of Polycyclic Aromatic Hydrocarbons, Licentiate thesis, 2014
Contents
1 Introduction ................................................................................................ 11 1.1 The graphene family ........................................................................... 11 1.2 Analysis methods ............................................................................... 17 1.3 Graphene fabrication methods ............................................................ 18 1.4 Functionalization of graphene ............................................................ 19 1.5 Aims of the research and structure of this thesis ................................ 20 1.6 Chemistry in this thesis ...................................................................... 21
2 Ozone as reagent for edge-selective functionalization of graphene (Papers I - III).............................................................................................. 23
2.1 Ozonation of graphite and graphene .................................................. 23 2.2 Addition and substitution reactivity of PAHs (I-II) ........................... 24 2.3 Ozonation of graphite and graphene (III) .......................................... 29 2.4 Summary and conclusions .................................................................. 32
3. Photoassisted basal plane functionalization of graphene using transfer hydrogenation reagents (Papers IV and V) ................................................. 33
3.1 Basal plane functionalization of graphene ..................................... 33 3.2 White light irradiation of graphite with iso-propanol .................... 36
3.4 Summary and conclusions .................................................................. 37
4 Concluding remarks and future outlook ..................................................... 38
5 Grafen, modifierad grafen och polycykliska aromatiska kolväteföreningar .......................................................................................... 39
6 Acknowledgements .................................................................................... 42
7 References .................................................................................................. 44
Abbreviations
AFM Atomic force microscopy AOP Advanced oxidation process CVD Chemical vapor deposition DCl Deuterium chloride DCM Dichloromethane EAS Electrophilic aromatic substitution FT Fourier-transform GC-MS Gas chromatography-mass spectrometry GNR Graphene nanoribbon GO Graphene oxide HCl Hydrochloric acid HOPG Highly oriented pyrolytic graphite HR High resolution IE Ionization energy IR Infrared LC-MS Liquid chromatography-mass spectrometry LOM Light optical microscopy MeCN Acetonitrile MeOH Methanol MS Mass spectroscopy NMR Nuclear magnetic resonance PAH Polycyclic aromatic hydrocarbon rGO Reduced GO SEM Scanning electron microscopy TFA Trifluoroacetic acid TGA Thermogravimetric analysis UV Ultraviolet WCA Water contact angle XPS X-ray photoelectron spectroscopy
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1 Introduction
The rapid technological advances in society put new and ever increasing demands on further development of materials. Graphene, a newly isolated and characterized material, has the potential to cause a paradigm shift in how technological devices are built. Graphene is lightweight, strong, almost transparent, and conductive, all desirable properties for materials in fields like electronics, medicine, biomaterials, construction, and energy storage. One challenge in order to realize the potential of graphene lies in the produc-tion and modification of cost-efficient, processable graphene.
The main aim of the work presented in this thesis was to expand the toolbox for making graphene-derived materials by developing methods for covalent functionalization of graphene. The selectivity and reactivity of different types of chemical reactions have been investigated using spectroscopy and microscopy. The analytical methods used depended on the system under investigation. These graphene systems included: polycyclic aromatic hydro-carbons (PAHs), different types of graphite, and chemical vapor deposition (CVD)-grown graphene on silicon oxide on silicon (SiO2/Si).
A brief introduction to the graphene family1 and its fabrication and modifica-tion, the chemistry explored and the aims of this work are given in the first chapter.
1.1 The graphene family Graphite, graphene and graphene nanoribbons Graphite consist of stacked sheets of graphene that interacts through non-covalent π-π interactions with a distance of 0.335 nm. Although graphite contains mostly carbons atoms, there are many different types of graphitic materials, e.g. natural graphite flakes, highly oriented pyrolytic graphite (HOPG), kish graphite and graphite foil. (Figure 1)
12
Figure 1. Graphite and graphene and photos of ~11 cm samples of natural graphite flakes, highly oriented pyrolytic graphite (HOPG), graphite foil, kish graphite and CVD-graphene on SiO2/Si.
Graphite has a high melting point, is nearly inert and conducts both heat and electricity well. The various types of graphite differ according to the amounts and types of impurities, the alignment and spacing of the sheets, and the grain size (size of the sheets). For example, synthetic graphite (e.g. pyrolytic graphite and kish graphite) usually contains almost perfect gra-phite. Graphite flakes, a natural form of graphite, are found and mined in various locations in the world. The grain size, and amount and type of im-purities vary according to the mine site. Graphite flakes can be used to make the commonly used graphite foil (also named flexible graphite). The graphite market is large; graphite-derived materials have a broad range of uses, e.g. as insulators, lubricants, heat sinks, flame retardants, gas adsorbers, seals, gaskets and in batteries.2-4
The structure of graphene, a honeycomb lattice, arises from the sp2 hybri-dized carbons in graphene which are arranged in a flat hexagonal pattern where each carbon atom is bonded to three other carbons. This 'infinite' sp2 carbon network results in the unique mechanical, optical and electronic properties of graphene. These properties are suppressed when graphene is stacked together to form graphite.5 Since its isolation and characterization by Geim and Novoselov in 2004,6, 7 graphene-like materials have been tested in applications such as sensors,8 programmable self-folding paper,9 corrosion-free paint,10 flexible touch screens,11, 12 with a multitude of further applica-tions on the horizon.13-15
Graphene nanoribbons (GNRs) are more well-defined than graphene but still not soluble like nanographenes (discussed in section 1.1.2). Control of the shape, edge type and also edge termination of GNRs has been demon-strated, and GNRs can be prepared by lithography or bottom-up fabrication.16, 17
13
One of the challenges with realizing the potential of graphene is its lack of solubility, which makes modifying, cleaning, processing and handling diffi-cult.18
Nanographenes - polycyclic aromatic hydrocarbons (PAHs) and the edge structures and defects of graphene Nanographenes or PAHs consist, like graphene, of flat aromatic hexagons comprised of carbon and have therefore the same structural features as gra-phene. (Figure 2)
Figure 2. Photo of CVD-graphene on SiO2/Si substrate and illustration of the basal plane of graphene and the structural similarity to nanographenes.
Regardless of the graphene quality, each sheet has specific structural features such as edges and defects. Dangling bonds, holes and grain boundaries are some commonly discussed defects. The amount and type of defects and the grain size of the graphene vary according to how it was produced.17, 19
14
Several types of defects have been observed by high resolution electron mi-croscopy in a single layer of graphene. Some examples are Stone Wales defect, single vacancies and double vacancies (Figure 3). Other types of defects include point defects, grain boundary defects, carbon adatoms, for-eign adatoms, and holes. 20,21,17
Figure 3. Examples of observed defects in the basal plane of graphene, with the number of atoms in each ring indicated. A) Stone Wales, B) single vacancy and C-E) double vacancies. 21
The edges of the graphene may be folded or terminated by various entities, depending on the source of the material.22 The termination of the edges of a single layer of graphene can be controlled during CVD preparation of gra-phene; for example, the edges can be terminated by hydrogen atoms (Figure 4).23
Figure 4. Graphene edge terminated by hydrogen atoms.
Dangling bonds are often mentioned in the context of graphene defects. Dangling bonds, which can be described as unpaired electrons or radicals on immobilized atoms, can form during CVD growth of graphene. They are highly reactive and unstable, and are thus unlikely to survive under ambient conditions.24, 25
A B
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15
The two shapes of graphene edges (zigzag and armchair) are also found in PAHs. The armchair or zigzag configurations can be combined into different regions: coves, bays, fjords, or K-regions. (Figure 5). These different struc-tural features result in different properties, e.g. armchair edges are more thermodynamically stable than zigzag edges. Cove and fjord regions distort the structure because of steric repulsion between attached hydrogens, e.g. helicenes (PAHs) are chiral because of this distortion.26 The combination of both solubility and structural similarities found in PAHs make them ideal models for studying graphene edge reactivity. Furthermore, due to their li-mited molecular size they are more easily handled then graphene. 27 The selection criteria for the PAHs investigated in this thesis involved solubility and edge type. Seven PAHs were investigated: pyrene (1), perylene (2), ben-zo[a]pyrene (3), benzo[e]pyrene (4), triphenylene (5), acenapthylene (6) and anthracene (7) (Figure 5).
Figure 5. The seven PAHs investigated in the thesis and the edge types they repre-sent. Pyrene (1), perylene (2), benzo[a]pyrene (3), benzo[e]pyrene (4), triphenylene (5), acenapthylene (6) and anthracene (7). The PAHs are color coded for the respec-tive edge types.
Another benefit obtained by studying PAHs as graphene edge models is that they are not as computationally challenging as graphene itself, and thus higher level calculations can be utilized.
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Other, more recently developed, methods for preparing GO use treatment with: chromium trioxide, concentrated sulfuric acid and potassium perman-ganate,34 or titanium dioxide, and hydrogen peroxide under ultraviolet (UV) irradiation,35 or potassium ferrate and concentrated sulfuric acid.36, 37
The identity of the oxygen-containing functional groups and the characte-rization of the GO with regard to impurities, grain size, and holes depends on the method of preparation and the quality of the graphite.36-41
Since the isolation of graphene in 2004, GO has been investigated as a precursor of graphene. GO far surpasses graphene both in scalability and processability. Graphene-like materials, such as rGO, can be obtained from GO by thermal, chemical or photochemical reduction. The properties of rGO depend on the quality of the graphite starting material, the oxidation method for obtaining the GO and the method of reduction to rGO.42-44
1.2 Analysis methods The analysis of graphene and graphite usually only gives averaged indica-tions of the structure. In contrast, using PAHs as graphene edge models pro-vide reactivity information on the atomic level.
Analysis of graphite and graphene When analyzing graphene and graphite, analytical techniques commonly used for small organic molecules such as chromatography, nuclear magnetic resonance (NMR) spectroscopy, liquid chromatography-mass spectrometry (LC-MS) or gas chromatography-mass spectrometry (GC-MS) cannot be used. Therefore, information is difficult to obtain at an atomic level for gra-phene and graphite, which are non-homogeneous materials. Usually it is only a cumulative effect of the measured range of the analyzed material than can be observed.
Information about the surface of graphite and graphene can be obtained by a variety of microscopic techniques, i.e. light optical microscopy (LOM), atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Sometimes, if high resolution instruments are available, information down to atomic levels can be achieved.45
Raman spectroscopy can show how much the material deviated from gra-phene or graphite and can, for example, be used to detect the amounts of defects introduced, such as holes, edges, sp3-carbons, aggregation and fold-ing. Upon analysis of graphene and graphite with Raman spectroscopy, the D and G peaks are integrated and the ratio between them calculated (ID/IG ratios), this value indicate the defect content.38, 46
18
X-ray photoelectron spectroscopy (XPS) or electron spectroscopy for chemi-cal analysis (ESCA), show what elements and bonds there are and are po-werful techniques for the analysis of surfaces.47
Thermogravimetric analysis (TGA) is a bulk method that can provide in-formation about the presence of functional groups. More information is achieved through coupling with an IR spectrometer or also a GC-MS instru-ment.48
Another method for bulk characterization of materials regarding the pres-ence of functional groups is FT-IR spectroscopy. This method provides the identity of functional groups, and is useful for characterizing GO and other materials where graphene has been extensively functionalized.49
It is also possible to follow successful functionalization by observing changes in macroscopic properties. One method is to measure the change in the wettability of graphite by measuring the water contact angle (WCA). This method allows changes in the polarity of the material introduced by certain functional groups to be observed.50, 51
Analysis of PAHs The methods used for analyzing graphene and graphite are also possible for PAHs, but due to their solubility additional analytic techniques are available. Since analysis of PAHs provides structural information on the atomic level they provide better understanding of the details of the functionalizations.
Information about the number of different compounds in a mixture and their molecular mass can be obtained by GC-MS or LC-MS. NMR spectros-copy, the most powerful analytical technique, can be used to reveal the mo-lecular structure of the compound. Some changes to the structure of conju-gated systems can also be revealed by UV/vis spectroscopy.
1.3 Graphene fabrication methods Graphene and graphene-like materials can be obtained by chemical reduction of graphene oxide (GO) to reduced graphene oxide (rGO),19, 43, 52 chemical vapor deposition to CVD-graphene on a substrate,53 heating silicon carbide,54 the scotch tape method on graphite, or liquid exfoliation of graphite.55 The quality (e.g. grain size, number of layers, impurities, and defects), properties, edge-termination, scale, environmental impact and reactivity of the material varies according to how the graphene was obtained and supported (in a liq-uid or on a solid substrate).
19
1.4 Functionalization of graphene As discussed in section 1.1.1, the reactivity differs between the edges, de-fects, and basal plane. This difference in reactivity can be used for selective chemical functionalization. Graphene can be covalently and non-covalently functionalized, and the functionalization can be edge-selective or non-selective (Figure 8). 56-61
Functionalization of graphene is important for two main reasons: to change the properties and to improve the processability (e.g. solubility). It is important to understand the reactivity of graphene's many different sites (e.g. edges, defects and holes) and the origin of the material in order to success-fully functionalize graphene with high reproducibility.
Figure 8. Side view illustration of pristine graphene and different types of graphene functionalization.
When the basal plane is covalently functionalized, the carbon hybridization is changed from sp2 to sp3. This gives longer carbon-carbon bonds and a tetrahedral arrangement of bonds around the carbon atom that has reacted. In contrast, the edges of graphene can react without affecting the main sp2 car-bon network and the overall geometry. This is the main reason for the much higher reactivity observed at the edges and grain boundaries of graphene compared to the basal plane. Most of the properties of graphene will not be affected since the sp2 network of the basal plane is left intact. 62-64
Edge-selective functionalization of graphene Functionalizing the edges can provide points of attachment,64-67 and improve solubility or dispensability.68, 69
Several approaches for achieving covalently functionalized edges have been reported,61 including construction of graphene from smaller fragments (bottom-up), cutting with a directed beam of highly focused electrons or ions or etching with reactive plasma (lithography), and solution-based methods. Solution-based methods for edge functionalization, such as Birch reduction,70, 71 and diazonium chemistry69 have been reported. Several edge selective methods for graphite or graphene oxidation have also been re-ported. These use successive treatments with potassium permanganate, con-
20
centrated sulfuric acid, ammonia and hydrogen peroxide68 or ozone,72 or treatment with ozone under UV irradiation,73 or treatment with ozone in water under sonication.74
Regioselectivity between the zigzag and armchair edges can be expected for most edge-selective reactions. Several studies report that the armchair-edge structures are less reactive than the zigzag-edge structures.,75,25
Basal plane functionalization of graphene Basal plane functionalization is another type of graphene reaction. However, covalent functionalization is normally not possible without simultaneous reaction of the edges and defects.
Basal plane functionalization changes the properties of the material more drastically than edge functionalization. The sp2 network is disturbed and the bandgap and other properties arising from the sp2 network will be affected. The extent of the effects on the properties of the sp2 network depends on the extent of the functionalization, the properties of the functional groups, and whether holes or other types of defects are also introduced. For example, hydrogenation of graphene either to fully hydrogenated graphane or partially reduced graphene has been suggested to introduce properties such as band gap tuneability and fluorescence and to have potential for hydrogen storage. 76-78 However, methods for reducing graphene to graphane usually involve harsh reaction conditions.
Other basal plane reactions for graphene include radical reactions,79, 80 fluorination by atomic fluorine,81 and reductive alkylation.48
1.5 Aims of the research and structure of this thesis The aims of the projects presented here were:
1. To develop reactions for covalent functionalization of graphene re-
sulting in materials that require no subsequent processing and gener-ate little waste.
2. To understand the selectivity of the developed reactions.
The topics in this thesis is as follows. Chapter 2 outlines the experimental and computational evaluation of the edge selectivity of the addition (reagent: ozone) and substitution (reagent: deuterium chloride; DCl) reactions of sev-eral PAHs. The ozonation reaction was further investigated graphite and CVD-graphene which lead to proposed edge-oxidation under comparatively mild conditions. Chapter 3 discusses the hydrogenation and hydroxypropyla-tion of CVD-graphene immersed in formic acid or iso-propanol upon irradia-tion with white-light.
21
1.6 Chemistry in this thesis Ozonation Using ozone as an oxidative reagent has many advantages: at ambient condi-tions it is a gas, it is considered an environmentally friendly reagent, and it has been well studied.82-85 As early as 1905, Harries published a paper on the investigation of oxidative cleavage of unsaturated compounds using ozone.86
Today, ozone is a commonly used reagent for oxidative cleavage (ozono-lysis) of alkenes and PAHs. In synthetic organic chemistry, ozonolysis of PAHs can be performed to obtain functionalized PAHs.82, 87-96
As proposed by Criegee,84 the mechanism of ozonolysis proceeds in three steps. Firstly, an intermediate primary ozonide forms by cycloaddition. Since primary ozonides are only stable at low temperatures (usually below -78 °C), the primary ozonide decomposes into carbonyl and carbonyl oxide. Lastly, the carbonyl oxide and the carbonyl react to form the final ozonide (Figure 9).
1 2 3
primary ozonide carbonyl oxide carbonyl ozonide
Step 1: 1,3-cycloadditionStep 2: retro 1,3-cycloadditionStep 3: flip and 1,3-cycloaddition
Figure 9. The three steps of ozonolysis. In the first step, a primary ozonide is formed via 1,3-cycloaddition; in the second step, a retro 1,3-cycloaddition forms a carbonyl oxide and a carbonyl; and, in step three, an ozonide forms after a 180° rotation (flip) of the carbonyl.
In the first two steps of the ozonolysis (steps 1 and 2 in Figure 9), the inter-mediates formed (primary ozonide and carbonyl oxide) can be trapped in situ, yielding products other than the ozonide. It has been demonstrated that the carbonyl oxide formed during ozonolysis of both alkenes and PAHs may be trapped with a variety of nucleophils.97-99 The carbonyl oxide can also be trapped by a second, more reactive carbonyl, forming a cross-ozonide.84, 100-
103
Over the last 30 years, several efficient methods for oxidative removal of PAHs from soil and water have been developed. The procedures employs ozone in combination with other reagents that promotes formation of hy-droxy radicals.104-107
22
Electrophilic aromatic substitution (EAS) reaction An EAS is a reaction where a hydrogen (usually) bonded to an aromatic carbon is replaced by another entity. (Figure 10)
Here, 2HCl (DCl) was selected as the reagent to study the selectivity of the reaction with PAHs 1-6, as deuterium incorporations can be observed by 1H NMR spectroscopy. While reaction of alkenes with aqueous hydrochloric acid usually results in addition of water, a corresponding water addition to PAHs under the conditions explored herein is unlikely.
Figure 10. Mechanism of electrophilic aromatic substitution (EAS).
Hydrogenation Hydrogenations, leading to reductions of, e.g. alkenes, arenes, alkynes or carbonyls are usually achieved either by dihydrogen or transfer hydrogena-tion reagents. (Figure 11) Dihydrogen, is flammable, explosive, and hazard-ous 108 thus using transfer hydrogenation reagents e.g., iso-propanol, metha-nol and formic acid are preferred. Transfer hydrogenation reagents provide hydrogen species (protons and hydrides) in situ and, the reaction usually require a metal catalyst (e.g. Pd or Rh).109
H2
HCOOHCO2
[M]
[M]
Figure 11. General hydrogenation of benzene with dihydrogen or formic acid.
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2 Ozone as reagent for edge-selective functionalization of graphene (Papers I - III)
Previous work has demonstrated that addition reactions and electrophilic aromatic substitution (EAS) reactions with graphene are edge-selective. 61, 72-
74, 110-112 In this chapter, the regioselectivity and versatility of ozone with PAHs, and several graphitic materials from work reported in Papers I-III and the appendix are discussed.
2.1 Ozonation of graphite and graphene Full oxidation of graphene to GO is, among other things, useful from a proc-essability perspective, even if most of the properties associated with the basal plane of graphene are lost and not fully recovered upon reduction to rGO. If only the edges of graphene are oxidized, the properties of graphene are retained while the groups on the edges can contribute to better solubility than that of non-oxidized graphene, even if it is not as good as that of GO.68,
74, 113-116 Ozone reacts edge-selectively with graphene and it is important to know
the identity of the functional groups that can be incorporated at the edges in order to understand how this type of edge functionalization can be used. Ozone normally undergo addition reactions, but it can also undergo substitu-tion reaction with aromatic compounds.87 Ozone can also form radicals and ozonation under radical promoted conditions might result in graphene that is even more oxidized.
Here, the identity of functional groups that can be incorporated at the edges of graphene and GNRs by ozonation were investigated by studying PAHs with different edge configurations. Further, ozonation of triphenylene with various carbonyls lead to several new compounds. Also, the possibility to use ozone under conditions similar to those in Paper I and also ozone un-der radical promoted conditions to develop comparatively mild procedures to oxidize graphite and graphene were investigated. (Figure 12)
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Hydroperoxy acetal
Graphene edges:
AlcoholAlcoholAlcohol
Hydroperoxy acetal
Ozonide
Cross-ozonide
Figure 12. Illustrated here is the alkene as part of graphene edges and which functio-nalities to expect. These pathways are investigated in Papers I-III. In the first two steps of the ozonolysis, the intermediates formed (primary ozonide and carbonyl oxide) can be trapped in situ to yield different products. Ozone can also form hy-doxyl radicals and undergo substitution reaction.
2.2 Addition and substitution reactivity of PAHs (I-II) Ozonation of PAHs 1-7 When investigating the ozonation reactions with the PAHs, either the solvent system or the reagents were varied, keeping all other variables unchanged. Ozononation of PAHs 1-7 produced thirteen isolated products (Figure 13) whose structure were determined. The relative stability of PAHs 1, 2, 5 and 7 to ozone has previously been reported; PAH 5 was observed to be the most stable followed by 1, 2, lastly 7 being the most reactive PAH.117 The zigzag edges of acenes have been reported to be more reactive to oxidation than the armchair edges.26 This corresponds well with our observations, where both B[a]P 3 and anthracene 7 (zig zag) was observed to be more reactive towards ozone than pyrene 1 and B[e]P 4 (k region). Triphenylene 5 (bay region) is the least ozone reactive (Table 1).
25
Table 1. Ozonolysis of compounds 1-5 in dry DCM-d2 at 20°C and their proposed regioselectivity with ozone.
PAH time, s conversion of % Proposed regioselectivity
1 5 40 K region 1 10 91 K region 3 5 100 Zigzag 4 5 40 K region 4 10 98 K region 5 15 29 Bay region 5 30 63 Bay region 7 5 88 Zigzag 7 10 90 Zigzag
For PAH 3, it was difficult to obtain substantial amounts of products as simi-lar to previous reports by Fieser.91 However, the production of various qui-nones have been mentioned by others.87, 118
Triphenylene 5 slowly degraded to a mixture of minor products, in amounts below the detection limit of 1H NMR spectroscopy, irrespectively of using dry conditions, water or methanol. However, ozonolysis of PAH 5 with acetone, resulted in its conversion to the cross-ozonide 5a. Also, pyrene ozonated in fomaldehyde have been reported to form the cross-ozonides at the k region.103
See Figure 13 for all isolated products as well as PAHs 1-7 relative ozone reactivity.
26
4c 6a 6c
1d
2b
1a
2c
4a
1e
4b
2d
1b 2a
1c
7a
7
5a
5
34
61
2
Bay region K region zigzag edge defectGraphene motif:
Ozonation conditions
more reactive towards ozone
dry:
water:
methanol:
acetone:
Figure 13. The isolated products from ozonolysis of PAHs 1, 2, 4 and 6 using either water or methanol as solvent or under dry conditions in dichloromethane (DCM). The products 1d, 2a-d, 4a-c, 5a, 1f, and 2e have not previously been reported. Com-pounds 4a-c are not fully characterized.
27
From the results above, it is demonstrated that ozone is a versatile reagent, depending on solvent a variety of functional groups are accessible on the edges of graphene. Further we propose that for the zigzag edges, ketones or hydroxyl groups will be incorporated regardless of conditions and they will react faster than armchair edges. On the armchair edges, incorporation of aldehydes, ozonides, cross-ozonides, peroxides, hydroperoxy acetals, car-boxylic acid groups are possible and can be controlled by the choice of sol-vent.
A new compound from ozonolysis of triphenylene 5 Functionalized PAHs are a group of compounds with properties relevant to many fields, including organic dyes, ligands for asymmetric catalysis, phar-maceuticals, and agrochemicals, and they have the potential to be used as organic conductors or semiconductors.26, 119 The isolation of cross-ozonide 5a from triphenylene 5 was unexpected since no products from ozonolysis of triphenylene have previously been reported or been detected under other conditions. The isolated product 5a is potentially a useful compound and several sites for future functionalization have been introduced (Figure 14).
Figure 14. Cross-ozonide 5a from ozonolysis of triphenylene 5 in acetone.
Other carbonyls were investigated with PAH 5 forming other cross-ozonides, cross-ozonide 5a was also reduced to the corresponding aldehyde.
Evaluation of experimental and calculated regioselectivity for addition and substitution of PAHs 1-7 The regioselectivity for deuterium substitution in PAHs 1-6 was observed with 1H NMR spectroscopy. For acenaphthylene 6, deuterium oxide was added to the alkene. For PAHs 1-3 and 5, the selectivity observed was con-sistent with the results found by Dewar et al. in 1956120 when they deter-mined the selectivity for a nitration with PAHs.
The calculated and experimentally observed regioselectivity for addition and substitution reactions with PAHs 1-7 are summarized in Figure 15. Cal-culating the local IE surfaces has potential to be a good tool for predicting the regioselectivity for both addition and substitution reactions under kinetic control with graphene. The calculated regioselectivities were not completely consistent with our experimental results. It is possible that the main products from the ozonation reactions were not observed because of polymer forma-
28
tion (PAH 2) or subsequent oxidation leading to degradation (PAH 5). For the reaction with triphenylene, the observed product (cross-ozonide 5a) was only detected when thermodynamic conditions (high concentrations and temperatures) were used.
Figure 15. The experimentally determined regioselectivity of ozonolysis and deute-rium substitution of PAHs 1-7 and their calculated IE surfaces. The red arrows indi-cate ozonolysis and the blue arrows indicate deuterium substitution. For PAH 3, the location of the ozonolysis could not be determined.
The local IE surfaces for three hydrogen-terminated GRNs were calculated to demonstrate their reaction with ozone and to visualize how ozone can be used to introduce different functional groups depending on the edge configu-ration. (Figure 16) This type of chemical differentiation of the edges could potentially be used to connect GRNs with control of the edge alignment.
29
Figure 16. Calculated local IEs and suggested oxidation products for three hydrogen-terminated GNRs. Based on these calculations and previous experimental observa-tions, the reactive outcomes for functionalization of three larger PAHs (or GNRs), with ozone under dry conditions, are suggested.
2.3 Ozonation of graphite and graphene (III) In section 2.2, ozonation reactions of PAHs to introduce a variety of func-tional groups depending on edge type and solvent were discussed. This sec-tion presents ozonation reactions of graphite in water, ethanol or hydrogen peroxide solution under sonication.
Several graphitic materials were investigated: natural crystalline graphite flakes were chosen since they constitute the most abundant form of graphite; HOPG containing almost perfect graphite and very low levels of impurities, and graphite foil having a porous structure which is ideal for intercalation. Also CVD-graphene on SiO2/Si was investigated. The foil, flakes and HOPG
3
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30
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31
mally found for GO and rGO,135, 136 which could indicate edge-selective oxi-dation.
Figure 18. TGA of graphite flakes after treatment with hydrogen peroxide solution, ozone under sonication for 0 h, 24 h and 72 h.
The oxidized graphitic materials were analyzed by Raman spectroscopy (bulk measurement) and ID/IG ratios were calculated from the Raman spectra. Increasing the oxidative treatment and time resulted in increased ID/IG ratios. The highest observed ID/IG ratio of 0.3 is still considerably lower than ratios commonly obtained for GO and rGO.19 These results further support edge-selective oxidation.
Ozonation of CVD-graphene on SiO2/Si in hydrogen peroxide solution The SEM images in Figure 19 show that a short treatment time (10 minutes) in aqueous hydrogen peroxide solution saturated with ozone had little effect on the graphene sheet; however, after 24 h, the graphene sheet was exten-sively degraded. The circular pattern on the surface was probably caused by formation of reactive bubbles during the procedure. XPS analysis of the ma-terial left on the substrate showed it to be graphene.
Figure 19. SEM images of CVD-graphene treated with ozone and hydrogen perox-ide solution for A) 10 min and B) 24 h.
32
2.4 Summary and conclusions From the investigations of PAHs we conclude that substitution and addition reactions displayed complimentary regioselectivities. Substitution reactions preferred zigzag edges and addition reactions preferred armchair edges. The regioselectivity for both the ozonolysis and deuterium substitution reactions with PAHs was mostly predicted by local IE surface calculations.
The ozonation experiments with PAHs with different solvents demon-strated two things. Firstly, ozone is a versatile reagent that can result in the formation of many different functional groups, depending on solvent and edge type. Secondly, the zigzag edge is more reactive than the armchair edge.
Graphite in water, ethanol or hydrogen peroxide solution was treated with ozone under sonication. Full oxidation to GO was not achieved but it is rea-sonable to conclude that edge-oxidation occurred. The highest extent of oxi-dation was observed when the graphite were treated simultaneously with ozone and hydrogen peroxide solution, indicating a cooperative effect (Fig-ure 17).
Purification of the material obtained from treating graphite flakes with ozone under the above conditions is straightforward, as all reagents are ei-ther gases or have good solubility in water, and drying or filtering and rins-ing with water is sufficient. No metals or acids were used in the oxidation process described here and thus the methods are comparatively environmen-tally friendly.
Further studies are required to optimize and fully understand the reactions but also to study the properties, such as electrical conductivity and dispersi-bility, of the obtained materials.
33
3. Photoassisted basal plane functionalization of graphene using transfer hydrogenation reagents (Papers IV and V)
The methods discussed in chapter 2 are efficient for edge functionalization. This chapter discusses basal plane functionalization by increasing the reac-tivity of graphene by irradiation. We studied the photoreactions of CVD-graphene on SiO2/Si (Papers IV and V) and kish graphite (Paper V) with several transfer hydrogenation reagents.
3.1 Basal plane functionalization of graphene The key to basal plane functionalization is the ability of graphene to absorb visible light,122 which probably results in a photoexcited material. Thus, the use of light should allow for milder reactants (e.g. iso-propanol and formic acid) to react with graphene without adding metals or using high tempera-tures and pressures.
CVD-graphene on Si/SiO2, kish graphite, and HOPG were used as sub-strates and methanol, ethanol, iso-propanol, tert-butanol, and formic acid were investigated as reagents. With CVD-graphene and the graphitic mate-rials, white light emitting diode (LED) irradiation was used.
The CVD-graphene on Si/SiO2 was analyzed by Raman spectroscopy, SEM, AFM and XPS, and the graphite was analyzed by Raman spectrosco-py, LOM, AFM and WCA measurements.
White light irradiation of CVD-graphene with formic acid or iso-propanol CVD-graphene immersed in formic acid or iso-propanol and irradiated with white light resulted in changes to the material. These changes varied with the conditions. SEM imaging revealed that, under conditions involving white light and formic acid, parts of the material were missing, while for condi-tions involving white light and iso-propanol, the surface was intact (Figure 20).
34
Figure 20. SEM images of (A) CVD-graphene on SiO2; (B) CVD-graphene on SiO2 after 24 h irradiation in formic acid; C) CVD-graphene on SiO2; and (D) CVD-graphene on SiO2 after 24 h irradiation in iso-propanol.
Analysis with XPS and Raman spectroscopy showed partially hydrogenated graphene on the SiO2/Si surface after formic acid treatment. For the iso-propanol treated graphene, XPS analysis showed an increase in both oxygen content and sp3 carbons atoms on the surface. AFM measurements of the same sample indicated increased adhesion between the SiO2 tips. These ob-servations are not consistent with hydrogenation of graphene nor were they observed for formic acid-treated graphene, and we suggest that graphene reacts differently with the two reagents, with formic acid leading to hydro-genation of the surface, and iso-propanol leading to addition of iso-propanol to the surface (Figure 21).
Both of these functionalization methods are photoassisted, and do not proceed without light, although the mechanism behind is so far unknown. The XPS analysis show that the extent of functionalization is too large to be limited to the edges, and therefore basal plane functionalization is proposed. However, if the photoexcitation is spread evenly over the graphene surface, no reaction would happen; the photoexcitation is likely localized – but how this occur and where it localized is not known.
A
D
B
C
35
Figure 21. Proposed functionalization of graphene under white light irradiation with A) formic acid and B) iso-propanol.
Time-dependence of basal plane functionalization of CVD-graphene treated with iso-propanol XPS analysis indicated that reacting CVD-graphene with iso-propanol for different times (0h, 6h, 12h, 18h, 24h, and 48h) allowed the degree of func-tionalization to be controlled (Figure 22).
Figure 22. Plot of the integrated areas of sp2, sp3 and C-O in XPS peaks after de-convolution for CVD-graphene after irradiation in iso-propanol for various reaction times.
Secondary functionalization of isohydroxypropylated CVD-graphene Iso-propanol-treated graphene was chlorinated to investigate whether the material could be further functionalized and to confirm the incorporation of hydroxyl groups by iso-propanol (Figure 23). Substitution of a hydroxy group for a chloride using thionyl chloride is commonly used in organic synthetic procedures.123 XPS analysis revealed the reaction to be successful,
36
thus providing further support for iso-propanol fragments binding into the graphene.
Figure 23. Proposed reaction of iso-propanol treated graphene with thionyl chloride.
3.2 White light irradiation of graphite with iso-propanol When kish graphite and HOPG were treated under our iso-propanol condi-tions, a clear change in morphology was observed by LOM (Figure 24). An increase of the D band in the Raman spectra was also observed.
Figure 24. Light optical microscopy images of A) pristine HOPG (left) and iso-propanol-treated HOPG (right); and B) pristine kish graphite (left) and iso-propanol-treated kish graphite (right).
WCA measurements were performed to measure the wettability of the sam-ples before and after treatment. Before the reaction the WCA value was 89° and after the reaction the WCA values were 61°and 58° (Figure 25). Gra-phite oxidized with the well-established Hummers method33 or Brodie me-thod31 give reported WCA values of 43.8- 58°51, 124, 125 and 60.7°, respective-ly.126 Our WCA values around 60° indicate that our method introduces polar-ity, making graphite more hydrophilic.
Figure 25 Difference in the wettability of graphite before and after reaction with iso-propanol. A) Kish graphite and B) Kish graphite after reaction with iso-propanol for 7 days.
A
89° 61°
B
A B
A
37
3.4 Summary and conclusions The methods presented in Papers IV and V are promising. Covalently func-tionalized graphene was obtained by irradiation of graphene on SiO2/Si im-mersed in formic acid or iso-propanol. Formic acid treatment resulted in hydrogenation and iso-propanol treatment resulted in iso-propanol attach-ment to the graphene surface. The reactions are proposed to have led to basal plane functionalization.
The extent of functionalization is tuneable, as shown by changing the du-ration of the iso-propanol treatment of CVD-graphene. Further modifications of the material and the presence of hydroxyl groups were demonstrated using a chlorination reaction on iso-propanol-treated graphene.
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4 Concluding remarks and future outlook
In this thesis comparatively mild reactions for edge or basal plane covalent functionalization of graphene are presented. Ozone gas or other water-soluble reagents (hydrogen peroxide, formic acid and iso-propanol) were investigated.
Ozone was used to functionalize PAHs in water or methanol resulting in a variety of functional groups, with several new compounds isolated. Ozona-tion of graphite in different solvents during sonication were proposed to introduce oxygen containing functional groups to the edges of graphite.
White light irradiation was used to mediate basal plane functionalizations, and these reactions proceeded slowly enough to enable control over the de-gree of functionalization. As stated in chapter 1, the aims of the work presented were:
1. To develop reactions for covalent functionalization of graphene re-sulting in materials that require no subsequent processing and gener-ate little waste.
2. To understand the selectivity of the developed reactions.
All methods developed in this work have potential to be used to produce new materials either by edge or basal plane covalent functionalization of gra-phene. The obtained materials can be used either directly or as precursors in other reactions. The properties of these new materials, which will vary ac-cording to edge-selective functionalization, basal plane functionalization, functional groups and extent of functionalization, are yet to be explored.
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5 Grafen, modifierad grafen och polycykliska aromatiska kolväteföreningar
Med detta avhandlingsarbete har jag utvecklat metoder för att modifiera grafen samt ökat förståelsen för grafens reaktivitet.
Grafen består i princip endast av kolatomer som är sammanbundna på ett platt hexagonalt vis (hönsnät). Många lager grafen som interagerar med var-andra bildar grafit, och grafit av 1 mm tjocklek består av ca 3 miljoner lager grafen. Grafen är tvådimensionellt eftersom det enbart är uppbyggt med en atoms tjocklek, och har unika egenskaper. Geim och Novoselov isolerade grafen från grafit med den s.k. tejpmetoden år 20041 och redan 6 år senare (2010) fick de nobelpriset i fysik för sitt karakteriserings arbete. Grafen är ett tunt, starkt, nästan genomskinligt material som leder ström och värme bra samt är flexibelt. Applikationsmöjligheterna för grafen är många, allt ifrån byggnadsmaterial i hus, flygplan och sportutrustning till biosensorer, gas-sensorer och som elektronikkomponenter i framtidens datorer, har föresla-gits.2 För många applikationer är dock inte alla grafens egenskaper lämpliga. I till exempel elektronik är även halvledande och isolerande material viktiga. Grafen är heller inte lösligt vilket försvårar hanterandet av grafen vid fram-ställningen av många applikationer. Grafen kan ses som en enormt stor polycyklisk aromatisk kolväteförening (PAH) och har flera olika regioner som kan ha olika reaktivitet. Grafen har kanter, defekter samt ett inre plan. (Bild 1) För att förstå hur grafens kanter reagerar kan PAHer användas som små grafenkantmodeller och fördelarna med att studera grafens kanter på detta sätt är många. Analysmetoderna som blir möjliga, bland annat på grund av löslighet, leder till information på en högre detaljnivå än den nivå som är möjlig för grafit och grafen. Man kan i grafenkantmodellerna se var och vad som har hänt och få full inblick i hur olika reaktioner sker.
1 Novoselov K. S., et al. Electric Field Effect in Atomically Thin Carbon Films, Science, 306, 666-669, 2004 2 Grafen – den perfekta atomväven, nobelpriset i fysik 2010, Kungliga Vetenskapsakademien, 2010
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Bild 1. Illustration av: grafens inre plan, kanter och hål defekter, samt likheten med polycyklisk aromatisk kolväteförening (PAH), grafit och grafits uppbyggnad av grafenlager.
Nuvarande metoder som finns för att kemiskt modifiera grafen innebär ofta kraftiga reaktionsförhållanden såsom användande av höga temperaturer, högt tryck, starka syror, radikaler, plasma eller metaller.3 Materialen som erhålls kan vara svåra att rena upp, och mycket avfall produceras.
I denna avhandling har grafenkanters reaktivitet med två olika reaktions-typer, additions- och substitutionsreaktioner, samt grafenytans fotoreaktivi-tet undersökts. Flera olika grafitmaterial samt grafen på kiseloxid har under-sökts och PAHer har använts som grafenkantmodeller. För kantoxidering av grafit och grafen har som reagens, ozon tillsammans med olika lösningsme-del använts. (Bild 2) Beroende på vilket lösningsmedel som används under ozoneringen kan olika funktionella grupper på kanterna erhållas. För grafe-nytmodifiering undersöktes ett flertal hydrogeneringsreagens, och av de undersökta reagensen resulterade myrsyra i hydrogenering av grafen och isopropanol i att isopropanol bands till ytan.
3 Boukhvalov, D. W. & Katsnelson, M. I. Chemical functionalization of graphene, Journal of Physics-Condensed Matter, 21, 2009
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Bild 2. Grafitfolie före (höger) och efter (vänster) behandling med ozon och väte-peroxid.
Alla de nya metoderna beskrivna i denna avhandling har potential till att användas för att tillverka nya material under jämförbart milda förhållanden samt generera material som är lätta att rena upp, utan att producera mycket avfall. Alla egenskaper och potentiella användningsområden för dessa nya material är dock ännu i stort sett outforskade.
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6 Acknowledgements
First I would like to express my gratitude to my supervisor, Helena Grenn-berg, thank you for accepting me to the group, for allowing me freedom in my Phd studies and for interesting discussions and support during these years. Thank you, Henrik Ottosson, for interesting collaborations, discussions and good advice. Also, thank you, Adolf Gogoll, for always being available to help, you have been very good support over these years and a nice and useful second super-visor. A special thanks to you Matt for introducing me to my projects and teaching me a lot about research we had a lot of fun sharing an office and drinking beer at Buddy’s. Thank you, all other past and present members in the AGHG group, for some really great group activities (x-mas decoration, gingerbread NMR building, hot pot, surströmming, BBQ, game nights etc….) and it has been fun work-ing together. All nice ‘mini’ breaks, fikas, projects in the department, confe-rence trips, teaching together, have been memorable. I especially want to thank: Micke, you are great to collaborate with, always doing a bit more than your part, good humor and also thank you for fixing so much in the lab, San-dra, my latest office mate, I have enjoyed our intense discussions, coffee breaks and also our non-syncing parental leave, Hao, for some reason you always make me smile, I have enjoyed tasting your food at lunch and our discussions about the future, Xiao, always have been nice to discuss things and to share lab with you. Classe, Magnus and Sara, thank you for the fun we had together both inside and outside of BMC. The technical and administrative staff for good support, thank you, Gunnar, Bosse, Tomas, Johanna, Eva, Lina and Francoise. Also a thanks to my collaborators from outside chemistry - BMC; Raffaello, Hu and Patrick. Raffaello your positive outlook and fantastic lab journal is inspiring!
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Many thanks to all past and present people at the department for nice con-versations, a happy atmosphere and good company in the lunch room. Thank you Helena, Adolf, Christian, Micke, Rikard and Kjell for proof read-ing and commenting on this thesis, it is now much better. Thank you my closest friends; Cia, Renée, Tove, Johanna and Senait for all the fun and distractions so far, I am very grateful to have such wonderful, interesting, intelligent and funny friends! A big thank you to my brothers and parents for trying to understand what I have done these past few years, your support, and for all our nice family dinners and discussions. Rikard, Etna and Leone my own little family, you are fantastic and I Love you. Thank you Rikard, the best and most wonderful person ever, for all help, advice and patience during these years.
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