Graphene metal organic framework composites and their potential applications 1

The Japanese scientist, Sumio Iijima, discovered carbon nanotubes in 1991 with the help of the transmission electron microscope.3 In 2004, two physicists at the University of Manchester

Chapter 1: Introduction

1.1 History and properties of graphene

Human beings have been using carbon and its allotropes for a long time In 1960s, nanodiamond was first synthesized in Russia using a detonation method.1 Carbon research was given a new impetus with the discovery of fullerene,2 C60, in 1985 The Japanese scientist, Sumio Iijima, discovered carbon nanotubes in 1991 with the help of the transmission electron microscope.3 In 2004, two physicists at the University of Manchester first isolated individual graphene planes using adhesive tape4 (Figure 1.1(a))

Graphene is an allotrope of carbon with one-atom-thick planar sheets of sp2-bonded carbon

atoms that are densely packed in a honeycomb crystal lattice The term graphene was coined as a

combination of graphite and the suffix -ene by Hanns-Peter Boehm,5 who described single-layer carbon foils in 1962 Graphene is most easily visualized as an atomic-scale thick wire made of carbon atoms and their bonds The crystalline or "flake" form of graphite have many graphene sheets stacked together The carbon-carbon bond length in graphene is about 0.142 nanometers.6Graphene sheets stack to form graphite with an interplanar spacing of 0.335 nm, which means that

a stack of three million sheets would be only one millimeter thick Graphene is the basic structural element of some carbon allotropes including graphite, charcoal, carbon nanotubes and fullerenes

(Figure 1.1(b)) It can also be regarded as an indefinitely large aromatic molecule, the limiting

case of the family of flat polycyclic aromatic hydrocarbons Graphene is distinguished by its еxcеptіonаl physіcаl propеrtіеs, such аs exceptional mobility,7

excellent thеrmаl stаbіlіty аnd mеchаnіcаl strеngth.8

Othеr forms of grаphеnе-rеlаtеd mаtеrіаls can be derived from graphite, іncludіng grаphеnе oxіdе, rеducеd grаphеnе oxіdе, аnd еxfolіаtеd multilayer graphene Thе

multifunctional propеrtіеs, togеthеr wіth thе еаsе of procеssіbіlіty, render grаphеnе-bаsеd mаtеrіаls highly useful for іncorporаtіon іnto а vаrіеty of functіonаl mаtеrіаls Grаphеnе аnd іts composites hаvе bееn used for various аpplіcаtіons, such аs еlеctronіc аnd photonіc dеvіcеs,9еnеrgy storage and conversion devices10

аnd catalysts.11

Figure 1.1 (a) Timeline of carbon nanostructure discovery (b) Schematic representation of graphene,

which is the fundamental starting material for a variety of fullerene materials; bucky balls, carbon nanotubes, and graphite.12 Image reproduced from reference 12.

1.2 Synthetic method to produce Graphene

Graphite is made up of adjacent graphene layers that are bound by weak van der Waals forces13 Graphene can be obtained by mechanical exfoliation of graphite using adhesive tapes. This method was discovered by A.K Geim and K.S Novoselov, who have been awarded the Nobel Prize in Physics for their work on graphene14 (Figure 1.2(a)) However, an economically

viable method for large scale production is needed to be used in industry and mechanical exfoliation is clearly unsuitable Therefore, other methods for the synthesis of graphene have been developed These methods can be grouped into two major categories: bottom-up synthesis and solution-processed synthesis

Figure 1.2 (a) Mechanical exfoliation of graphite by using adhesive tapes (b) CVD method of growing

Graphene sheet.15 Image reproduced from reference 15

The bottom-up synthesis of graphene typically uses chemical vapor deposition (CVD)16

method (Figure 1.2(b)) In this method few-layer graphene sheets are deposited on copper foil or

metal catalyst-coated surfaces such as silicon Such type of graphene typically show good electrical properties as can be judged from the presence of quantum hall states, and are used mainly in bench top experiments by physicists to probe the behavior of Dirac electrons, however they are not amenable to solution-processing Unlike the bottom up synthesis, the solution route typically produces graphene oxide (GO) using the exfoliation and oxidation of graphite, followed

by chemical reduction to convert it to reduced graphene oxide (r-GO).17 GO can be produced by

the oxidative treatment of graphite via one of these three methods: Brodie, Hummers, and Staudenmaier Brodie’s method18

involves the addition of potassium chlorate (KClO3) to a slurry

of graphite in fuming nitric acid Staudenmaier19 improved Brodie’s method by adding the chlorate in multiple aliquots over the course of reaction instead of a single addition as Brodie had done This resulted in a similar degree of oxidation compared to Brodie’s multiple oxidation approach Thereafter, Hummer and Offeman20 developed an alternate oxidation method by reacting graphite with a mixture of KMnO4 and concentrated sulfuric acid (H2SO4), and achieved similar oxidation levels as well The formed GO has a basal plane decorated with epoxide and

hydroxyl groups, while its edges are decorated with carboxyl and carbonyl group (Figure 1.3)

Figure 1.3 Scheme showing the chemical route to the synthesis of aqueous graphene dispersions (1)

Oxidation of graphite to graphite oxide, GO, with greater interlayer distance (2) Exfoliation of graphite oxide in water by sonication to obtain GO colloids (3) Controlled conversion of GO colloids to conducting graphene colloids by hydrazine reduction.21 Image reproduced from reference 21

Solution processed graphene can be scaled up industrially, thus it has the potential for cost-effective applications In this thesis, solution-processed graphene is the main ingredient used for the preparation of composites with Metal Organic Framework (MOF)

1.3 Reactions of Graphene

Graphene can be functionalized covalently or non-covalently to form chemically modified graphene GO platelets have chemically reactive oxygen functionalities, such as carboxylic acid at the periphery and epoxy and hydroxyl groups on the basal planes The carboxylic acid of GO sheets can react with thionyl chloride (SOCl2),22 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC),23 or N,N’-dicyclohexylcarbodiimide (DCC)24 Their nucleophilic species like amines or hydroxyls can be added to carboxylic groups to form amide or ester bonds with GO Ring-opening reactions activate the epoxy group of GO which involves nucleophilic attack at the α-carbon For example, octadecylamine can be attached to GO to make dispersible colloidal suspension of graphene in organic solvents.25 Non-covalent functionalization of GO can occur by hydrogen bonding andelectrostatic interaction on its oxygen functionalities, or π-π stacking, van der Waals interactions and cation- π interaction on the aromatic rings on GO.26 Reduced GO (rGO) platelets have partially recovered π-conjugation.27 rGO can undergo covalent interaction by its residual

functional groups after reduction, for instance through diazonium reaction In order to prepare fuctionalized graphene in this thesis, rGO is linked to functional ligands by diazonuim reaction.28

In Diels-Alder reaction, graphene can act as diene and dienophile via covalent interaction.29

Indeed, graphene-based derivative can be modified readily by covalent route to tune the chemical

structure for specific applications (Figure 1.4)

Figure 1.4 Selection of currently available non-covalent and covalent functionalized graphenes.30 Image reproduced from reference 30.

1.4 Graphene-based composites

Graphene has attracted enormous research interest in recent years, due to its interesting properties Graphene derivatives such as graphene oxide (GO) and reduced graphene oxide (rGO) offer various possibilities to synthesize graphene-based functional materials for different

applications such as the Li-ion batteries,31 supercapacitors,32 fuel cells,33 photovoltaic devices,34photocatalysis,35 as well as Raman enhancement36 (Figure 1.5)

Figure 1.5 Schematic naming some grapheme-based composites and their potential applications37 Image reproduced from reference 37

1.4.1 Graphene- polymer composites

Carbon-based materials, such as amorphous carbon and carbon nanotubes (CNTs), are conventional fillers for enhancing the electronic, mechanical and thermal properties of polymer matrices.38 CNT has been rendered as one of the most useful filler materials, although it is relatively costly Graphene-based fillers are often promoted as favorable replacement or supplement to CNTs

To use lower amount of graphene filler, the dispersity and its bonding with the polymer matrix are important in order to have desirable properties of the composites Therefore, graphene-filled polymer composites are usually prepared by solution mixing, melt blending, and in situ polymerization.37 Graphene fillers can be dispersed in the polymer matrixes in layered structures, which are used in specific applications, such as the directional load-bearing membranes, and thin films for photovoltaic applications For example, GO can be deposited onto poly(allylamine hydrochloride) (PAH) or

poly(sodium 4-styrene sulfonate) (PSS) to form layer-by-layer (LbL) assembling via the Langmuir–

Blodgett (LB) method (Figure 1.6(a) The resulting composited membrane shows enhanced directional elastic modulus with 8 vol% loading of the graphene (Figure 1.6(b)).40

Figure 1.6 (a) Schematic illustration of fabrication and assembly of the free-standing GO-LbL film (b)

Plot showing the variation of elastic modulus calculated theoretically (under parallel and random orientation) and that obtained experimentally (using buckling and bulging measurements) with the volume fraction of GO.40Image reproduced from reference 40

1.4.2 Other graphene-based composites

Other than inorganic nanostructures and polymers, materials such as organic crystals,41 metal–organic frameworks (MOF),42 biomaterials,43 and carbon nanotubes (CNTs)44 have also been mixed with graphene derivatives to target various applications For example, N,N’-dioctyl-3,4,9,10-perylenedicarboximide (PDI)–graphene core/shell nanowires used in organic solar cells have been formed through – interaction.45 MOF, a recently emerging material, is used for gas purification and storage applications, and has also been used to form composites with GO/rGO sheets.46 Moreover, biomaterials like DNA hybridized with GO or rGO are used in fluorescent sensing platforms based on the fluorescence resonance energy transfer (FRET).47 Graphene–CNT composites have also been

prepared via solution blending or in situ CVD growth48 to be applied in Li ion batteries, transparent conductors,44 and supercapacitors.49

1.5 Overview of Metal-Organic Frameworks (MOFs)

The design and synthesis of extended network materials has been an area of intense research over the past decade Specifically, the porous nature of many of these materials makes them attractive for numerous applications This section provides background information on many areas concerning the chemistry of MOFs including: (1) the design of MOFs from precursors, (2) various applications of MOF materials, (3) previous works concerning the use of MOF and carbon composites

1.5.1 The Design of MOFs

MOFs can be formed through a node and linker approach that was first reported by Yaghi, Robson,50 Fujita.51 This method uses metal ions as nodes and organic molecules as linkers The metal ion, with a preferred coordination number and geometry, in combination with divergent linker molecules, creates an extended network in one, two, or three dimensions The interactions

of the metal ion and the linker molecule vary widely and have included ionic, covalent, and coordinate interactions,52 as well as hydrogen bonding and π-π interactions.53 Often, the strengths

of these interactions directly influence the overall stability of the resulting framework As shown

in Figure 1.7, the principles of coordination complexes can be used to construct extended network

assemblies

Figure 1.7 Illustration of the paradigm shift from molecular coordination chemistry involving terminal

ligands to extended assemblies using diverging ligands

The single metal center used as a node can change the structure owing to the preference for

a specific geometry and coordination environment of the given metal For example, in the compound [Cu2(4,4’-bipy)4]·(D-HCam)·(4,4’-bipy)2·12H2O synthesized by Zhang, et.al., the Cu2+

metal centers adopt a tetrahedral coordination geometry and assemble into a network via

4,4’-bipyridine linkages54 (Figure 1.8)

Figure 1.8 The Cu2+ ion in [Cu 2 (4,4’-bipy) 4 ]·(D-HCam)·(4,4’- bipy) 2 ·12H 2 O adopts a tetrahedral geometry

and acts as the node for the extension of a network linked via 4,4’-bipyridine.54 Image reproduced from reference 54

In comparison, the compound [Cd(4,4'-bpy)2(H2O)2](ClO4)2.1.5(4,4'-bpy)], synthesized by Liu et al., contains Cd2+ centers that adopt an octahedral geometry.55 The axial positions of the octahedron are occupied by terminally water molecules The equatorial positions are assumed by four 4,4’-bipyridine which act as the linker molecules Therefore, a square network is formed as a

result of the geometry around the metal center (Figure 1.9)

Figure 1.9 The Cd2+ ion in [Cd(4,4'-bpy) 2 (H 2 O) 2 ](ClO 4 ) 2 1.5(4,4'-bpy)] adopts an octahedral geometry by binding to two water molecules (red = oxygen atoms) and four 4,4’-bipyridine molecules 55

Image reproduced from reference 55

Yaghi introduced the second approach of the MOF design through the use of secondary building units (SBUs).56 This method makes use of many common structural motifs known in molecular cluster chemistry by incorporating them as nodes for network extension Two examples

of this strategy are shown in Figure 1.10 The “paddlewheel” structure of copper(II) acetate is a

common example in this regime, where the acetate anions can be replaced by a variety of dicarboxylates to provide four points for network extension to form a square net.57

Figure 1.10 Common transition metal acetate clusters and the divergent linker benzene dicarboxylate

creating SBUs Top: the copper acetate paddlewheel becomes a square planar SBU Bottom: The [Zn 4 O]6+cluster becomes an octahedral SBU (Color scheme: Zn, green; Cu, light blue; C, black; N, blue, O, red.)57Image reproduced from reference 57

The “basic” zinc acetate structure, composed of a molecular [Zn4O]6+

cluster and six acetate anions, is another example of an SBU for framework extension The acetate anions in this oxo-centered cluster can be replaced by divergent ligands.58 This SBU serves as an octahedral node for the formation of a primitive cubic network Additionally, SBUs formed from metal clusters can also be combined with neutral, divergent Lewis basic ligands to provide further points

of extension for network growth Figure 1.11 shows that these Lewis bases can be replaced by

divergent molecules, such as 4,4’-bipyridine, to create an octahedral node

Figure 1.11 Replacement of the Lewis base, DMF, in the square planar SBU with divergent

4,4’-bipyridine creates an octahedral node with two additional points of extension 58 Image reproduced from reference 58

1.5.2 Potential Applications of MOFs

MOFs have received considerable interest in recent times due to their potential applications in areas such catalysis, optics, electronics, small molecule storage, and separation science.59 There have been hundreds of reports of hydrogen storage in MOF materials.60 The interaction of H2 and the MOF surface is typically quite weak, being dominated by dispersion forces Various strategies for enhancing the H2-surface interaction have been explored, including systematically varying pore structure, minimizing pore size to increase van der Waals contacts with H2, and embedding coordinately unsaturated metal centers within the MOF structure to interact with H2 For example, Yaghi has reported many MOFs based on the [Zn4O]6+ SBU which are effective in storing H2 Probably the most popular MOF, MOF-5 or [Zn4O(BDC)3]∞ (BDC = benzene dicarboxylate), was shown to store 4.5 wt % H2 at 77K and 1.0 wt % at ambient temperature.61

MOFs have shown useful in the storage of other small molecules besides hydrogen gas For instance, the gas adsorption of N2, CO, CO2 and CH4 has been reported.62,63 MOFs can also be used in catalysis For example, Lin reported [Cd3Cl6L3] (L = (R)-6,6΄-dichloro-2,2΄-dihydroxy-

1,1΄-binapthyl-4,4΄-bipyridine) can be covalently modified with titanium isopropoxide This material was used to catalyze the enantioselective addition of diethylzinc to aromatic aldehydes

MOFs also have exciting potential as light-weight molecular selective sieves, due to their extremely high surface areas, low density, interconnected cavities and very narrow pore size distributions.64 Some frameworks are also adaptive materials which respond to external stimuli (for example, light, electrical field, presence of particular chemical species), promising new advanced practical applications However, in order to use MOF in next generation sensing, separation, catalysis and delivery devices, we may require the introduction of extrinsic functionality Incorporation of functional species in MOFs has to date been demonstrated through post-impregnation mostly of metal nanospecies by chemical vapour deposition and one-pot synthesis (adding either the functional species or its precursors directly into the MOF growing medium) Both these approaches cause the doping species to grow inside the MOF cavities and on the MOF outer surface The resulting lack of spatial control of the functional components within the MOF crystals compromises the molecular selectivity of the final composite

The investigation of MOFs in electrochemistry is quite recent Important applications of electrochemistry are energy storage and conversion65 (supercapacitors, batteries, fuel cells) The poor electron-conductive properties of most MOFs would limit them from being used as electrode Although MOFs have been successfully used as electrode materials for rechargeable batteries,66 we need some strategies to overcome their insulating nature The redox behavior of metal cations inside MOFs could provide a pathway for electrons Alternatively, the tuning of the linker structure may lead to better charge transfer inside the framework An efficient strategy is to mix MOFs with conductive phases (metal nanocrystals, carbon nanostructures, fuctionalized graphene, conductive polymers)67

1.6 Literature review on MOFs at carbon interfaces

Carbon-based materials such as activated carbon, fullerene (C60), carbon nanotubes (CNTs), graphite and graphene are of technological interests because of their mechanical strength, hydrophobicity, potential in adsorption and catalysis, and interesting electronic properties Thus, there have been various composite systems with MOF for myriad applications, ranging from energy storage to the production of catalyst

1.6.1 Composite of MOF and Activated carbon

Many studies have been done on MOF and activated carbon composites.68 As an example, Seung Jae Yang et al reported69 a facile method for the preparation of novel ZnO-based nanostructured architectures using a metal organic framework (MOF) as a precursor In this approach, ZnO nanoparticles and ZnO@C hybrid composites were produced under several heating and atmospheric (air or nitrogen) conditions The resultant ZnO nanoparticles formed hierarchical aggregates with a three-dimensional cubic morphology, whereas ZnO@C hybrid composites consisted of faceted ZnO crystals embedded within a highly porous carbonaceous species, as determined by several characterization methods The newly synthesized nanomaterials showed relatively high photocatalytic decomposition activity and significantly enhanced adsorption capacities for organic pollutants

1.6.2 Composite of MOF and Fullerene (C60)

Fullerene-MOF composites are very promising materials for gas storage applications A lot

of researches have been focused on the use of these materials for methane and hydrogen storage materials For instance the incorporation of magnesium-decorated fullerenes within metal−organic frameworks (MOFs) was reported by Aaron W Thornton.70 The system is modeled using a novel