The first order Raman spectrum of isotope

labelled nitrogen-doped reduced graphene oxide

Tobias Dahlberg

Tobias Dahlberg February 19, 2016

i

Abstract

The topic of this thesis is the study of nitrogen functionalities in nitrogen-dopedreduced graphene oxide using Raman spectroscopy. Specifically, the project setout to investigate if the Raman active nitrogen-related vibrational modes ofgraphene can be identified via isotope labelling. Previous studies have usedRaman spectroscopy to characterise nitrogen doped graphene, but none hasemployed the method of isotope labelling to do so. The study was conducted byproducing undoped, nitrogen-doped and nitrogen-15-doped reduced grapheneoxide and comparing the differences in the first-order Raman spectrum of thesamples. Results of this study are inconclusive. However, some indicationslinking the I band, a band previously speculated to be nitrogen or sp3 carbonrelated, to nitrogen functionalities are found. Also, a hypothetical Raman banddenoted I* possibly related to sp3 hybridised carbon is introduced in the samespectral area as I. This indication of a separation of the I band into two bands,each dependent on one of these factors could bring clarity to this poorly under-stood spectral area. As the results of this study are highly speculative, furtherresearch is needed to confirm them and the work presented here serves as apreliminary investigation.

Sammanfattning

Amnet som behandlas i denna avhandling ar studien av kvavefunktionaliteter ikvavedopat reducerat grafenoxid med hjalp av Ramanspektroskopi. Specifikt savar malet med projektet att undersoka om Ramanaktiva kvaverelaterade vib-rationsmoder i grafen kunde identifieras via isotopmarkning. I tidigare studierhar Ramanspektroskopi anvants for att karakterisera kvavedopat grafen, menisotopmarkning har aldrig tllampats for detta andamal. Studien genomfordesgenom framstallning av odopat, kvave-dopat och kvave-15-dopat reducerat gra-fenoxid vilkas Ramanspektra sedan jamfordes. Resultaten av denna studie kaninte ses som avgorande da da datamangden var for liten. Men vissa indikatio-ner som forbinder I-bandet, ett band som tidigare spekluerats som relaterat tillkvave eller sp3 hybridiserat kol, till kvavefunktionaliteter hittades. Dessutomintroducerades ett hypotetisk Raman band, I*, i samma spektrala region somI. Detta band anses mojligen vara relaterade till sp3 hybridiserat kol. Dennaindikation av en separation av I bandet till tva band, vardera beroende av enav dessa faktorer kan bringa klarhet till detta tidigare daligt forstadda spektra-la omrade. Eftersom resultaten av denna studie ar hogst spekulativa, behovsdet ytterligare studier for att bekrafta dem och det arbete som presenteras harfungerar enbart som en forstudie for eventuellt fortsatt arbete.

Tobias Dahlberg February 19, 2016

ii

Contents

1 Introduction 3

2 Theory 52.1 Graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.1.1 Electronic Properties . . . . . . . . . . . . . . . . . . . . . 62.1.2 Vibrational Properties . . . . . . . . . . . . . . . . . . . . 62.1.3 Nitrogen Doping . . . . . . . . . . . . . . . . . . . . . . . 82.1.4 Reduced Graphene Oxide . . . . . . . . . . . . . . . . . . 9

2.2 Characterization Methods . . . . . . . . . . . . . . . . . . . . . . 102.2.1 XPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.2.2 Raman Spectroscopy . . . . . . . . . . . . . . . . . . . . . 12

2.2.2.1 Raman Spectra of Graphene . . . . . . . . . . . 132.2.2.2 Isotope Labelling . . . . . . . . . . . . . . . . . . 15

3 Experimental 173.1 Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.1.1 Synthesis of GO . . . . . . . . . . . . . . . . . . . . . . . 173.1.2 Synthesis of N-rGO, N15-rGO and rGO . . . . . . . . . . 17

3.2 Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . 183.2.1 XPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183.2.2 Raman Spectroscopy . . . . . . . . . . . . . . . . . . . . . 18

4 Results and Discussion 194.1 XPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194.2 Raman Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.2.1 D-Band . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204.2.2 I-Band . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224.2.3 I*-Band . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244.2.4 D”-Band . . . . . . . . . . . . . . . . . . . . . . . . . . . 264.2.5 G-Band . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284.2.6 D’-Band . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

5 Summary and Outlook 31

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2

Chapter 1

Introduction

One of the most talked about and hyped groups of materials of the last decadeare the sp2 nanocarbons which contains well-known names such as graphene,carbon nanotubes, fullerenes and graphite. The arguably most famous mem-ber of this group is graphene, which is a one-dimensional sheet comprised ofcarbon atoms arranged in hexagonal rings. Graphene, which was first isolatedin 2004 by Novoselov et al. [21], is the youngest member of the sp2 nanocar-bon family. Although being the youngest member of the family graphene issometimes referred to as the mother of all sp2 nanocarbons. The reason be-hind this moniker is that all other members of this family can be seen as beingconstructed using graphene as a basic building block. For example, carbon nan-otubes, being nanometer scale (diameter wise) tubes of hexagonal carbon rings,can simply be seen as rolled up sheets of graphene. During the last decade, theresearch community has seen a figurative explosion of graphene-related articlesbeing published [14]. What has drawn researchers to this area are the extraordi-nary properties of this material. For example, graphene exhibits both excellentchemical, mechanical and electrical properties, making it attractive for manydifferent potential applications including; electronics [13], hydrogen storage [16]and fuel cells [17]. However, for graphene to be properly useful, its propertiesneed to be carefully tuned to the particular application. One way of modify-ing both the chemical and electronic qualities of graphene is through doping,the intentional inclusion of impurities. Being next to carbon in the periodictable nitrogen is one of the most suitable dopant atoms for carbon structuresdue to their similarities in size and electronic characteristics [18]. The resultingproperties of nitrogen-doped carbon depend on the exact nature and amount ofnitrogen inclusions present. As of yet, the only precise method of analysing thestructure of the nitrogen functionalities present in doped sp2 nanocarbon hasbeen through the use of X-Ray Photoelectron Spectroscopy (XPS) which is apowerful but difficult and time-consuming method [19].

In this thesis, we try to expand this tool-kit also to include Raman spec-troscopy. Raman spectroscopy is a fast, non-destructive and easy to use method,and it is often used to characterise nanocarbons as they have a strong Raman

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activity [15]. Because of this, the Raman spectra of graphene has been in-tensely researched and studies have shown it to hold a great deal of structuralinformation [29, 30, 31, 32]. One method of singling out the effects of cer-tain functionalities on the Raman spectra is through isotope labelling. In thismethod instead of using, for example, the common earth abundant nitrogen-14,the heavier nitrogen-15 is used. As Raman spectroscopy probes the vibrationalproperties of a material, this mass change modifies the vibrations related to thatelement and they can thus, easily be identified and characterised. Isotope la-belling and Raman spectroscopy have been used before to study the vibrationalmodes of sp2 carbon [33, 34, 12]. The results of one of these studies were thatthe Raman modes related to the isotope labelled element were down-shifted infrequency. The frequency down-shift was found to follow the same behaviour asa classical harmonic oscillator [12]. This study observed these results in carbon-13 labelled carbon nanotubes and similar effects should be observed while usingthe heavier nitrogen-15. This method has not been employed before to inves-tigate the nitrogen inclusion in sp2 carbon structures. We expect to be ableto distinguish the nitrogen related Raman vibrations in a more straightforwardmanner by employing isotope-labelling during the synthesis process.

The study was conducted by systhesising undoped, nitrogen-doped and nitrogen-15-doped reduced graphene oxide, a form of sp2 carbon similar to graphene withthe main difference being a higher degree of defectiveness, and then comparingtheir respective Raman spectra. XPS was used to characterise the elemental andchemical compositions of the materials, to act as a support for the observationsmade in Raman spectroscopy. Raman spectra are often measured as single spotmeasurements. This type of measurement provides spectral information that isvery spatially limited, making it unideal for some projects. Instead, the Ramanspectra in this study were measured as overview maps. These maps containedaround 1000 spectra spatially separated over large areas for each sample. Thismethod provides a greater amount of data points allowing for stronger statis-tical analysis while simultaneously ensuring that genuine representations of thematerials bulk properties are measured.

This thesis studied nitrogen inclusion in reduced graphene oxide by isotopelabelling and Raman spectroscopy. With the aim to expand the understandingof vibrational modes related to nitrogen included in sp2 carbon.

4

Chapter 2

Theory

2.1 Graphene

The structure of the graphene lattice can be seen in Figure 2.1a. As we cansee, the structure is comprised of a hexagonal network of carbon atoms withtwo unique atomic sites A and B. The atoms are separated by the C-C bonddistance ac−c = 0.142nm. In this figure, we can also see the unit cell (dottedregion) and the unit vectors of the cell. The unit vectors a1 and a2 are describedby Equation (2.1) [2, section 2.2.1]. This basic geometry also serves as the fun-damental building blocks of other sp2 nanocarbons, such as carbon nanotubes,leading to them having similar properties [2, section 1.1].

Figure 2.1: The real space unit cell of graphene showing unit vectors a1 and a2and atomic sites A and B.[1]

a1 =

(√3

2a,a

2

), a2 =

(√3

2a,−a

2

)(2.1)

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Tobias Dahlberg February 19, 2016

2.1.1 Electronic Properties

In Figure 2.2 the electron dispersion relation, the relationship between momen-tum and energy of electrons, for the 1BZ (first Brillouin zone) of graphene isshown, the high symmetry points K,K’,Γ and M are marked. One importantfeature to note here is the degeneracy of the valence and conduction bands, thelower and upper surfaces shown in Figure 2.2, at the K and K ′ high symmetrypoints. With the Fermi level of graphene also located at these points, this makesgraphene a zero band gap semiconductor with properties similar to metals. Ifwe zoom in at the K point, another interesting feature is revealed about the elec-tronic properties of graphene. Near these points, the dispersion relationship islinearly dependent on the wave vector. This linear dependency is something usu-ally seen in massless relativistic particles, and it gives the electrons in graphenesimilar properties. From this, electrons and holes in graphene are effectivelymassless, resulting in a material with remarkable transport properties. Thesepoints are known as the Dirac points. [2, section 2.2]

Figure 2.2: The electron dispersion relationship of the 1BZ of graphene with highsymmetry points Γ, K, K ′ and M marked. Inset shows dispersion relationshipover directions KΓ, ΓM and MK. [2, p. 30]

2.1.2 Vibrational Properties

The phonon dispersion relation is shown in Figure 2.3. There we can see thatgraphene has six phonon branches corresponding to different lattice vibrations.Lattice vibrations or molecular vibrations are the oscillations of a material dueto the molecular bonds present bending or stretching, essentially acting as acomplex system of harmonic oscillators. The abbreviations in the figure standfor iLO (in-plane longitudinal optical), iTO (in-plane transversal optical), oTO(out-of-plane transversal optical), iLA (in-plane longitudinal acoustic), iTA (in-plane transversal acoustic) and oTA (out-of-plane transversal acoustic). Theseabbreviations explain how the atoms in the vibrations moves relative to the lat-tice. If we take a closer look at, for example, the iLO phonons the corresponding

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lattice vibration is shown in Figure 2.4a. As we can see, iLO corresponds toatoms moving parallel to the graphene plane in the longitudinal direction (trans-lates to up and down in the figure) with each atom moving out of phase to itsneighbours, which is the definition of an optical vibrational mode (with acousticmodes refers to neighbours being in phase). Some other examples of vibrationalmodes can also be seen in Figure 2.4 where vibrations near Γ Figure 2.4(a) andK Figure 2.4(b) are shown. [2, section 3]

Figure 2.3: The phonon dispersion relationship of graphene over directions KΓ,ΓM and MK. [2, p. 53]

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Figure 2.4: Vibrations in graphene and their related phonons near a) Γ b) K.[4]

2.1.3 Nitrogen Doping

As stated before, nitrogen doping is a commonly used method for tuning theproperties of graphene. For example, the inclusion of nitrogen in graphene in-troduces chemically active sites into this otherwise highly inert structure. Theseactive sites give catalytic properties to graphene as they can participate in chem-ical reactions, such as the breaking of O-O bonds which is useful for fuel cell ap-plications [10]. Nitrogen doping also alters the electronic properties of graphene,due to nitrogen having one more valence electron than carbon. This introduc-tion of donor atoms then shifts the Fermi level of the graphene, changing theordinarily conductive material into a semiconductor. The exact nature of the in-duced changes depends on the total number and types of nitrogen functionalitiespresent in the sample. Usually four types of nitrogen inclusions are considered inNitrogen-doped rGO (N-rGO) and these are called Npyridinic, Npyrrolic, Quater-nary center Nitrogen (NQcenter

) and NQvalley. The Npyridinic arises when nitrogen

shares one p-electron to the π-system next to a vacancy, it can be seen in Fig-ure 2.5 marked in blue. Npyrrolic which originates from nitrogen atoms thatshare two p-electrons with the π system of graphene, resulting in five memberrings or six member rings bonded to hydrogen next to a vacancy, this is shownin Figure 2.5 marked in green. NQcenter

, also known as graphitic nitrogen, re-sults from sp2 coordinated nitrogen that forms a direct in-plane substitution ofa carbon atom. Another variation of graphitic nitrogen is when nitrogen formsa graphitic substitution near an edge or vacancy and it is known as NQvalley

.These inclusions are shown in Figure 2.5 marked in black and red, respectively.

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Figure 2.5: Illustration of different nitrogen inclusions in graphene. Npyrrolic

(green), Npyridinic (blue), N-Qcenter (black) and N-Qvalley (red). [8]

2.1.4 Reduced Graphene Oxide

Reduced graphene oxide (rGO) are reduced and exfoliated flakes of graphiteoxide (GO); this process is shown in Figure 2.6. The GO precursor used isoften synthesised using a modified Hummer’s method where graphite is mixedwith sulfuric acid, potassium permanganate and sodium nitrate [27]. Theseflakes are similar to graphene in structure, with the primary difference being thedegree of defectiveness. As rGO is subjected to heavy oxidisation, the resultingstructures are massively defective and corrugated even after reduction. Despitebeing defective, this material retains some of the properties of graphene whilealso being much easier to synthesise with a high yield. It is worth emphasisingthat the possibility to upscale this method is one of its most promising featuresas it makes it suitable for industrial use. Also, the high degree of defectivenessof rGO also makes it easier to functionalise than pristine graphene, anotherattractive property. These two properties factor in to make rGO a promisingmaterial for future applications. [20]

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Tobias Dahlberg February 19, 2016

Figure 2.6: Illustration of the transformation of graphite to reduced grapheneoxide.[22]

2.2 Characterization Methods

2.2.1 XPS

XPS is a spectroscopic technique that is used to measure the elemental andchemical composition of a material. The basic principle of this technique is tomeasure the number and energy of photoelectrons emitted by a sample dur-ing x-ray irradiation. As each element and each chemical state have electronswith unique binding energies, the energy of emitted photoelectrons works as afingerprint indicating the composition of the material. Counting the numberof emitted electrons and correlating the count to their binding energy gives ameasure of the relative amount of that chemical species present. XPS is limitedto detecting surface properties as only the electrons emitted close to the surfacecan easily make it to the detector. Electrons emitted further into the samplehave a significant chance of being recaptured or scattered by the material.

The relevant bands for the analysis of rGO and N-rGO are the N1s, O1sand C1s bands shown in Figure 2.7. From these spectral regions the nitrogen,oxygen and carbon content and their chemical states can be estimated.

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Tobias Dahlberg February 19, 2016

Figure 2.7: Example XPS spectra of N-rGO showcasing the relevant bands O1s,N1s and C1s used to calculate oxygen, nitrogen and carbon content respectively.

The N1s band used for the analysis of nitrogen inclusion appear at a bindingenergy of 400 eV. This band is composed of a varying number of sub-peaks.These sub-peaks correspond to the different types of nitrogen functionalitiespresent in the sample as shown in Figure 2.8a. The first relevant band appeararound 398 eV and it is assigned to Npyridinic. Next a band appear around 399eV and it is assigned to Npyrrolic. Then a band at 401 eV appear which originatesfrom N-Qcenter. Lastly, a band at 402 eV is observed and it is assigned to N-Qvalley.[8]

Appearing around 285 eV the C1s band is related to the types of carbonfound in a structure. A high-resolution spectrum of this band is shown inFigure 2.8b where the sub-bands relevant to this study is highlighted. The firstimportant sub-band appears at 284.5 eV and is related to carbon bonded tocarbon in an sp2 configuration. At 285.5 eV the next relevant sub-band appears

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(a) (b)

Figure 2.8: A high resolution XPS spectra of a) N1s and b) C1s band used tocharacterise the amount and types of carbon and nitrogen present.

which is also related to carbon-carbon bonds but this time, to sp3 coordinatedcarbon, which is the same type of carbon found in diamond. [9]

2.2.2 Raman Spectroscopy

Raman spectroscopy is a spectroscopic technique that is used to analyse themolecular vibrational modes of a material. This method has its strengths inthat it is non-destructive, fast and requires little sample preparation. Relyingon the Raman effect, this method measures the inelastic scattering of photonsin a material to create a vibrational spectrum characteristic of that material. InFigure 2.9, schematic energy level diagrams depicting the scattering processesare shown. Firstly we have the elastic Rayleigh scattering that is not relevant toRaman spectroscopy as it does not change the photon energy. In this process,a photon excites the molecule to a virtual state with energy denoted as hv0, ashort-lived excited state not related to any eigenfunction of the system. Fromthis virtual state, the molecule then relaxes back to the initial state, emitting aphoton. Then we have the first of the Raman scattering processes, Stokes scat-tering. In this process, the photon again excites the system to a virtual statefrom the ground state E0. The molecule then emits a phonon of energy hvmbecoming vibrationally excited before radiatively relaxing to E0 − hvm. Thisphonon interaction causes a difference in energy of hvm between the absorbedand emitted photon. This energy difference is the measured quantity in Ramanspectroscopy, known as the Raman shift. Lastly, we have the anti-stokes scat-

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Figure 2.9: Illustration of Rayleigh and Raman scattering processes for photons.[23]

tering. In this process, the system, instead of generating a phonon, interactswith an already excited phonon increasing the energy of the resulting photon.Raman processes are not limited to these examples and can contain multiplephonon and defect related scattering events. The number of scattering eventstaking place in Raman processes indicates its order. As those listed in Figure 2.9contain only one, they are considered first order processes. If the excited virtualstate coincides with a real electronic state, the process is called resonant andthe likelihood of the transitions occurring is greatly increased. [2, section 4]

The shape of Raman bands is often described using Gaussian or Lorentzianfunctions. When deconvoluting Raman spectra containing many bands theVoigt functions is often used instead. This function is a convolution of theGaussian and Lorentzian functions and, therefore, it can describe both shapessimultaneously. [28]

2.2.2.1 Raman Spectra of Graphene

Raman spectroscopy is a commonly used technique to characterise both elec-tronic and structural properties of graphene. To give a more detailed descriptionof the Raman processes in graphene, they are usually not illustrated as can beseen in Figure 2.9. Instead, they are commonly shown at their respective lo-cation in the electron band structure, as can be seen in Figure 2.10. In thissection the Raman active bands present in graphene and their properties will

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be discussed. In Figure 2.11 an example of the first order Raman spectrum ofgraphene is shown. There we can see the measured data (blue dots in the figure)and its deconvolution; an estimation of the bands making up the spectrum). Inthe following sections, a brief overview of the I, D, D”, G and D’ bands arepresented. Commonly the spectral area 1000-1200 cm−1is only assigned to oneband, the I band. It should be noted that the regions 1000-1200 cm−1and 1350-1550 cm−1are poorly researched and there is no real consensus on their exactnature. The I* band presented in the figure will be discussed in Section 4.2.3,as it is unique to this study.

Figure 2.10: Illustration of Raman processes of different bands where the conesshow the shape of the electron dispersion relationship close to the Dirac pointsof graphene. a) G band, b) D band, c) D’ band. [2, p. 55]

I BandThe I band appears in the 1100-1200 cm−1region, and little is known about thisfeature. It has been reported that this band originates in C-C, C=C or sp2-sp3

carbon bonds [3]. Alternatively, the band is seen as related to functionalisedgraphene (such as nitrogen-doped graphene) or heavily disordered carbon [10].

G BandNamed so after the fact that it occurs in all graphene allotropes and the existenceof this band is a clear indicator of the presence of sp2 carbon. [2, p. 161] The Gband is a first order Raman band where a virtually excited electron inelasticallyscatters with an iTO or iLO phonons near the Γ point; this process can be seenin Figure 2.10a [4]. The G band is usually located around 1585 cm−1.

D BandThe D band is named after its close relation to defects in graphenes sp2 structure.

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Tobias Dahlberg February 19, 2016

Figure 2.11: An example of the first order Raman spectrum of graphene (blue)showcasing a deconvolution of its sub-bands.

As we can see in Figure 2.10b, this is a second order process where a virtuallyexcited electron near the K point scatters elastically from a defect to the K ′

point where it is inelastically scattered by an iTO phonon back to the initialstate near the K point. The D band is usually observed in the spectral regionaround 1300 cm−1and it intensifies and broadens with increased defectiveness.Due to this relationship, the relative intensity of the D band is often used toestimate the crystallinity of sp2 nanocarbons. [2, p. 215]

D” BandThe D” band is a broad peak observed in the region 1400-1550 cm−1and isanother poorly understood band. This spectral feature is believed to be defectrelated [11].

D’ BandThe D’ band is another defect related Raman band [3], it usually appears around1615 cm−1, the underlying mechanics can be seen in Figure 2.10c.

2.2.2.2 Isotope Labelling

Isotope labelling of a structure is to exchange some elements with heavier iso-topes. Doing this induces changes in the vibrational properties of the material,leaving the electronic characteristics unchanged. Molecular vibrations can, ina simplified manner, be seen as harmonic oscillators whose frequency ω is de-

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Tobias Dahlberg February 19, 2016

scribed by Equation (2.2)

ω =

√k

m, (2.2)

where m is the mass of the oscillator and k is the spring constant (correspondsto the bond strength in a molecule). This means that, if we assume the springconstant to remain unchanged during isotope labelling, the change in frequencyafter a mass change can be expressed as Equation (2.3)

ω2 = ω

√m

m(1− x) +m2x(2.3)

where m2, ω2 the isotope labelled mass and frequency and x is ration of atomsin the structure which have been isotope exchanged (x = 1 all atoms are iso-topes, x = 0 no atoms are isotopes). As we can see from the equation, theoscillator frequency down-shifts with increased mass. The relationship shown inEquation (2.3) have been shown to accurately describe the down-shift of Ramanbands in carbon-13 isotope labelled carbon nanotubes [12].

16

Chapter 3

Experimental

3.1 Synthesis

3.1.1 Synthesis of GO

The GO was synthesised using a modified Hummer’s method. This methodhas more recently become known as Tours method and involves oxidation ina concentrated sulfuric and phosphoric acid mixture together with potassiumpermanganate. This method is safer and faster, yet yields similar products asthe conventional Hummers method. The starting graphite used was Alfa aesar-100mesh natural flake graphite, briquetting grade.[25]

3.1.2 Synthesis of N-rGO, N15-rGO and rGO

rGO was chosen for this study because of it being easily functionalised andsynthesised. The three samples N-rGO, Nitrogen-15 doped rGO (N15-rGO)and rGO were prepared using a method of simultaneous thermal reduction andexfoliation similar to [26]. For the nitrogen-doped and isotope labelled samples,100 mg of ammonium nitrate (Sigma-Aldrich > 99%) and 100 mg nitrogen-15enriched ammonium nitrate (Sigma-Aldrich 98% N15) was mixed with 100 mgof GO by stirring for 30 min in 10 ml of ethanol 99.5%. rGO was prepared usingthe same method omitting the addition of ammonium nitrate. The mixture wasthen dried while stirred at 60 ◦C on a hotplate. After drying, the sample wascalcinated in an oven at 350 ◦C for 1 h and then washed with ethanol and withmilli-Q water with repeated centrifugation. Finally, the samples were dried in90 ◦C until thoroughly dry (< 2 h).

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3.2 Characterization

3.2.1 XPS

The measurements were carried out using a Kratos Axis Ultra DLD electronspectrometer using a monochromatic Al Kα source operating at 120 W .

3.2.2 Raman Spectroscopy

Three samples of rGO, N-rGO and N15-rGO, all in powder form, were flattenedonto a glass slide to ease the analysis. The samples were analysed using aRenishaw inVia confocal Raman microscope with an excitation wavelength of633nm. An overview map (an image of where each pixel contains a Ramanspectrum) of more than 1000 spectra were recorded for each of N15-rGO, N-rGO and rGO respectively. A high amount of spatially separated measurementpoints was acquired to ensure that the data obtained were representative of thematerials bulk characteristics. The acquired spectra were noise filtered using amultivariate noise filter and baseline corrected using an asymmetric least squarefit. The resulting processed spectra were batch de-convoluted in MATLABTM

via a custom script. The script used 14 randomised starting guesses for eachspectrum to minimise any bias introduced via the choice of starting parametersand all bands were fitted with Voigt functions. During the deconvolution, anadditional Raman band was added to the region 1100-1300 cm−1as the dataindicated its existence. This hypothetical band will in the subsequent sectionsbe referred to as the I* band. Statistical analysis was conducted on the dataresulting from the deconvolution. The statistical analysis was used to estimatethe mean differences between the three samples. A one-way ANOVA (AnalysisOf Variance) test at a 5% significance level was used to achieve this.

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Chapter 4

Results and Discussion

4.1 XPS

Table 4.1 and Table 4.2 shows a compilation of the relevant information ob-tained from the XPS measurements. There we can see the measured atomiccontent of the different nitrogen inclusion and also the total nitrogen and oxy-gen content of the three samples. The most important result shown here is thatthe two nitrogen doped samples, N-rGO and N15-rGO, were similar in totaland specific nitrogen inclusion content. We can also observe that the N15-rGOsample contained a lower amount of sp2 carbon (peak at 284.5 eV) comparedto N-rGO. While both samples had almost identical sp3 carbon (peak at 285.5eV) content. These results indicate that these samples are suitable for Ramancomparison, as the main differences are those introduced via isotope labelling.

Table 4.1: The results of the XPS analysis of the carbon content of the threesamples.

Sample Csp2 [at%] Csp3 [at%]

N-rGO 43.14 18.36N15-rGO 38.85 18.45rGO 48.54 15.3

Table 4.2: The results of the XPS study of the nitrogen content of the threesamples. The atomic percentages of the different nitrogen inclusions and thetotal nitrogen content are shown.

Sample Npyridinic

[at%]Npyrrolic

[at%]NQcenter

[at%]NQvalley

[at%]Ntotal

[at%]N-rGO 3.66 4.38 0.84 0.51 9.31N15-rGO 3.93 4.61 0.89 0.61 10.04

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4.2 Raman Analysis

In this section, the results of the Raman analysis is presented. All peak heightsmentioned in this section are normalized with respect to the G band

4.2.1 D-Band

The results of the measurements of the D band parameters of the three samplesare shown in Figure 4.1 and the results of the statistical analysis are shown inTable 4.3. Firstly we can note that the mean center position of the D-bandin N-rGO and N15-rGO is down-shifted relative to rGO by 8.2 cm−1and 7.5cm−1, for N-rGO and N15-rGO respectively. The results also show that thereis a statistically significant but weak up-shift of 0.7 cm−1in the mean centerposition of N15-rGO, compared to N-rGO. This result indicates that this banddoes not directly originate in phonons related to nitrogen inclusions. Instead,this can be explained by the increased defectiveness introduced by the nitrogeninclusion in the structure. Knowing that the D band has been observed asclosely linked with defective sp2 structures this result is expected.

From the analysis of the peak height, all of which have been normalizedwith respect to the G-band intensity, we can see that both N-rGO and N15-rGO shows an intensity increase of 85% and 56% (relative rGO), respectively.Also, it is apparent that the D band in the N15-rGO is significantly lower thanthat of N-rGO, being decreased by 28.3%. From this, it can be reasoned thatthe decline in intensity can be linked to the lower sp2 content in N15-rGO, asrevealed by the XPS analysis. Reports studying nanodiamond powders havealso related a reduction of D band intensity to increased sp3 content [7, p. 104].This reason may also be an explanation of this behaviour as the N15-rGO samplecontains a higher sp3 to sp2 ratio than the N-rGO.

Widthwise this band seems to broaden with nitrogen doping as a widthincrease (relative rGO) of −10.1 cm−1and −5.3 cm−1is observed for N-rGO andN15-rGO. This behaviour is expected as the D bandwidth has been reported toincrease with increased defectiveness due to changes in the lifetime of relatedphonons.

To summarise these results the D-band intensifies, broadens and down-shiftswith nitrogen doping while being almost unaffected by isotope labelling. Thesefindings indicate that the band is related to the degree of defectiveness presentin the sample and not necessarily nitrogen inclusion directly. As the D band iswell studied and has been confirmed to originate in defect activated vibrationalmodes of sp2 carbon this behaviour is theoretically expected.

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Figure 4.1: D band center position, height relative to G band and width withmean values marked by a star and the 95% confidence interval marked by solidlines. The colors correspond to rGO (red), N-rGO (blue) and N15-rGO (black).

Table 4.3: Results of ANOVA on D band parameters for rGO, N-rGO and N15-rGO. Results should be read as the difference between samples means i.e. rGOvs N-rGO is equivalent to mean(rGO)-mean(N-rGO).

Sample Mean center shift [cm−1] p valuerGO vs N-rGO 8.2± 0.1 1 ∗ 10−9

rGO vs N15-rGO 7.5± 0.1 1 ∗ 10−9

N-rGO vs N15-rGO −0.7± 0.1 1 ∗ 10−9

Sample Mean height shift [-] p valuerGO vs N-rGO −0.85± 0.01 1 ∗ 10−9

rGO vs N15-rGO −0.56± 0.01 1 ∗ 10−9

N-rGO vs N15-rGO 0.283± 0.009 1 ∗ 10−9

Sample Mean width shift [cm−1] p valuerGO vs N-rGO −10.1± 0.2 1 ∗ 10−9

rGO vs N15-rGO −5.3± 0.2 1 ∗ 10−9

N-rGO vs N15-rGO 4.8± 0.2 1 ∗ 10−9

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4.2.2 I-Band

The most important result shown here can be seen in Figure 4.2 and Table 4.4. Itis the significant (p-value << 0.05) down-shift of 11.7 cm−1of the I band centerin N15-rGO relative N-rGO. As this change is both statistically significant andlarger than 10 cm−1it is not unreasonable to correlate this down-shift to theisotopic labelling. Knowing that isotope labelling a chemical species with heavierisotopes should down-shift the related Raman bands, this correlation is furthersupported. As mentioned before, the down-shift upon isotope labelling can bedescribed by Equation (2.3). By using this equation, and setting m1 = 14, m2 =15 and x = 0.99 the expected down-shift can be estimated to 40 cm−1; fourtimes the measured value. This discrepancy between theory and experimentalresults indicates that while this equation describes the behaviour of carbon-13labelled sp2 carbon well, it does not accurately represent the more complicatedcase of nitrogen-15 labelled carbon. As a nitrogen doped structures are moredefective and nitrogen inclusions form more complex and varied structures thancarbon-13 inclusions, this might serve as an explanation for this deviation. Itcan also be noted that the mean I band center position up-shifts by 1.2 cm−1inN-rGO compared to rGO. However, as this change is relatively weak, it cannotbe correlated to nitrogen doping with any certainty.

If we move on to how the mean height of the peak differs between the sampleswe can see that it is increased by 18.5% in N-rGO and 9.2% in N15-rGO,compared to rGO. As this increase is strongly pronounced for both N-rGO andN15-rGO, it can be related to nitrogen doping. Interestingly, we can notice thatthe mean height of the peak in N15-rGO is lower than that in N-rGO. Thisresult can again be correlated to the fact that the N15-rGO, as revealed by theXPS study, contained less sp2 carbon than the other samples.

Another interesting feature is that the width decreases by 5.7 cm−1and by15.0 cm−1relative rGO, in N-rGO and N15-rGO respectively. This observationshows that the band narrows with isotope labelling that again could indicate alink to nitrogen functionalities.

To sum up these results they indicate that the I band is firmly related tonitrogen doping in graphene as we can see that it both intensifies and narrowsafter nitrogen doping. This connection is further supported by the significantdown-shift of the band after isotope labelling.

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Figure 4.2: I band center position, height relative to G band and width withmean values marked by a star and the 95% confidence interval marked by solidlines. The colors correspond to rGO (red), N-rGO (blue) and N15-rGO (black).

Table 4.4: Results of ANOVA on I band parameters for rGO, N-rGO and N15-rGO. Results should be read as the difference between samples means i.e. rGOvs N-rGO is equivalent to mean(rGO)-mean(N-rGO).

Comparison Mean center shift [cm−1] p valuerGO vs N-rGO −1.2± 0.5 8 ∗ 10−9

rGO vs N15-rGO 10.5± 0.5 1 ∗ 10−9

N-rGO vs N15-rGO 11.7± 0.5 1 ∗ 10−9

Comparison Mean height shift [-] p valuerGO vs N-rGO −0.18.5± 0.002 1 ∗ 10−9

rGO vs N15-rGO −0.092± 0.002 1 ∗ 10−9

N-rGO vs N15-rGO 0.093± 0.002 1 ∗ 10−9

Comparison Mean width shift [cm−1] p valuerGO vs N-rGO 5.7± 0.5 1 ∗ 10−9

rGO vs N15-rGO 15.0± 0.5 1 ∗ 10−9

N-rGO vs N15-rGO 9.2± 0.5 1 ∗ 10−9

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4.2.3 I*-Band

The results of the measured hypothetical I* band parameters of rGO, N-rGOand N15-rGO is shown in Figure 4.3 and Table 4.5. As we can see from theseresults, the mean center position of the I* band is significantly down-shifted by6.9 cm−1and 7.1 cm−1(relative to rGO) for N-rGO and N15-rGO respectively.Whereas, only a 0.2 cm−1mean difference is seen between N-rGO and N15-rGO. As this change is small and barely significant, having a p-value (strengthof evidence, should be below 0.05 for statistical significance) of 0.0248, thisband cannot be said to be affected by isotope labelling. From this, it can bespeculated that as this band down-shifts as a result of nitrogen doping but isnot affected by the isotope labelling, the band is probably defect related.

Figure 4.3: I* band center position, height relative to G band and width withmean values marked by a star and the 95% confidence interval marked by solidlines. The colors correspond to rGO (red), N-rGO (blue) and N15-rGO (black).

In the statistical analysis, we also see a significant increase in peak heightrelative rGO of 25.2% for N-rGO and 25.5% for N15-rGO. As the p-value of thetest between the mean heights of N-rGO and N15-rGO is 0.1395, no significantdifference in mean height can be established between the two. Following thesame reasoning as in Section 4.2.1 the height of all band linked to sp2 carbonshould be decreased in the N15-rGO sample. However, as this is not the casefor this band, it can be speculated to be associated with sp3 carbon. Drawing

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this conclusion is not unreasonable as this spectral region has been reported asconnected to sp3 carbon in previous studies [6]. As we saw in the XPS analysis,the sp3 carbon content in both N-rGO and N15-rGO are at similar levels. Thisresult would mean that a Raman band related to this species would likely havea comparable intensity in both samples, which further supports this conclusion.

Table 4.5: Results of ANOVA on I* band parameters for rGO, N-rGO and N15-rGO. Results should be read as the difference between samples means i.e. rGOvs N-rGO is equivalent to mean(rGO)-mean(N-rGO).

Sample Mean center shift [cm−1] p valuerGO vs N-rGO 6.9± 0.2 1 ∗ 10−9

rGO vs N15-rGO 7.1± 0.2 1 ∗ 10−9

N-rGO vs N15-rGO 0.2± 0.2 0.0248

Sample Mean height shift [-] p valuerGO vs N-rGO −0.252± 0.004 1 ∗ 10−9

rGO vs N15-rGO −0.255± 0.004 1 ∗ 10−9

N-rGO vs N15-rGO −0.003± 0.004 0.1395

Sample Mean width shift [cm−1] p valuerGO vs N-rGO 2.5± 0.4 1 ∗ 10−9

rGO vs N15-rGO −4.4± 0.4 1 ∗ 10−9

N-rGO vs N15-rGO −7.0± 0.3 1 ∗ 10−9

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4.2.4 D”-Band

The results relevant to the D” band are presented in Figure 4.4 and Table 4.6.We can see that this band displays a similar behaviour to that observed inthe D band. The mean height is strongly increased by nitrogen doping beingshifted relative rGO by 31.7% in N-rGO and 24.8% in N15-rGO. Again theband intensity is lowered in N15-rGO compared to N-rGO. Similarly to the D

Figure 4.4: D” band center position, height relative to G band and width withmean values marked by a star and the 95% confidence interval marked by solidlines. The colors correspond to rGO (red), N-rGO (blue) and N15-rGO (black).

band, the mean center position shows a dependence on nitrogen doping but notstrongly on isotope labelling, being down-shifted relative rGO by 8.2 cm−1in N-rGO and 10.3 cm−1in N15-rGO. The difference between N-rGO and N15-rGOhere is stronger than that observed for the D band but still relatively weak. Asstated before, changes of this magnitude cannot be assigned significance owingto the inherently inhomogeneous nature of reduced graphene oxide.

Interestingly it can be noted that these results indicate that the width ofthis band is not different between the N-rGO and N15-rGO. This result is notconsistent with the behaviour exhibited by the D-band and the reason behindthis is unclear.

The results presented here seems to indicate that this band is defect related,as it shows a behaviour similar to the D band. This observation is in line with

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the results of previous studies.

Table 4.6: Results of ANOVA on D” band parameters for rGO, N-rGO andN15-rGO. Results should be read as the difference between samples means i.e.rGO vs N-rGO is equivalent to mean(rGO)-mean(N-rGO).

Sample Mean center shift [cm−1] p valuerGO vs N-rGO 8.2± 0.5 1 ∗ 10−9

rGO vs N15-rGO 10.3± 0.5 1 ∗ 10−9

N-rGO vs N15-rGO 2.1± 0.4 1 ∗ 10−9

Sample Mean height shift [-] p valuerGO vs N-rGO −0.317± 0.005 1 ∗ 10−9

rGO vs N15-rGO −0.248± 0.005 1 ∗ 10−9

N-rGO vs N15-rGO 0.07± 0.004 1 ∗ 10−9

Sample Mean width shift [cm−1] p valuerGO vs N-rGO −8.0± 0.3 1 ∗ 10−9

rGO vs N15-rGO −7.7± 0.3 1 ∗ 10−9

N-rGO vs N15-rGO 0.3± 0.3 0.1008

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4.2.5 G-Band

As we can see from both Figure 4.5 and the statistical analysis in Table 4.7 theG band is weakly affected by both nitrogen doping and isotope labelling. Allchanges are on the order of 1 cm−1making these results hard to use to draw anyconclusions.

Figure 4.5: G band center position and width with mean values marked by astar and the 95% confidence interval marked by solid lines. The height has beenomitted as it has been normalized to 1 for all samples. The colors correspondto rGO (red), N-rGO (blue) and N15-rGO (black).

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Table 4.7: Results of ANOVA on G band parameters for rGO, N-rGO and N15-rGO. Results should be read as the difference between samples means i.e. rGOvs N-rGO is equivalent to mean(rGO)-mean(N-rGO).

Sample Mean center shift [cm−1] p valuerGO vs N-rGO 2.7± 0.2 1 ∗ 10−9

rGO vs N15-rGO 3.7± 0.2 1 ∗ 10−9

N-rGO vs N15-rGO 1.0± 0.1 1 ∗ 10−9

Sample Mean width shift [cm−1] p valuerGO vs N-rGO −6.2± 0.3 1 ∗ 10−9

rGO vs N15-rGO −7.5± 0.3 1 ∗ 10−9

N-rGO vs N15-rGO −1.3± 0.3 1 ∗ 10−9

4.2.6 D’-Band

In Figure 4.6 and Table 4.8 we see that this band exhibits similar behaviour tothe D band, intensifying, broadening and down-shifting with nitrogen doping.Also, we can see that the down-shift of this band in the N15-rGO relative N-rGO is quite small, being less than 5 cm−1which makes it hard to draw anyconclusions, as stated before. This result is in line with the expected behaviourof this band as it has been linked to defects. Again the trend of a loweredintensity of the band in N15-rGO can be observed.

Table 4.8: Results of ANOVA on D’ band parameters for rGO, N-rGO andN15-rGO. Results should be read as the difference between samples means i.e.rGO vs N-rGO is equivalent to mean(rGO)-mean(N-rGO).

Sample Mean center shift [cm−1] p valuerGO vs N-rGO −0.9± 0.2 1 ∗ 10−9

rGO vs N15-rGO 3.6± 0.2 1 ∗ 10−9

N-rGO vs N15-rGO 4.5± 0.2 1 ∗ 10−9

Sample Mean height shift [-] p valuerGO vs N-rGO −0.340± 0.009 1 ∗ 10−9

rGO vs N15-rGO −0.264± 0.009 1 ∗ 10−9

N-rGO vs N15-rGO 0.075± 0.009 1 ∗ 10−9

Sample Mean width shift [cm−1] p valuerGO vs N-rGO −8.8± 0.2 1 ∗ 10−9

rGO vs N15-rGO −8.2± 0.2 1 ∗ 10−9

N-rGO vs N15-rGO 0.6± 0.2 1 ∗ 10−9

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Figure 4.6: D’ band center position, height relative to G band and width withmean values marked by a star and the 95% confidence interval marked by solidlines. The colors correspond to rGO (red), N-rGO (blue) and N15-rGO (black).

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Chapter 5

Summary and Outlook

This study set out to investigate if isotope labelling in conjunction with Ramanspectroscopy could be used to characterise nitrogen functionalities in graphene.Raman spectroscopy is a fast, easy and non-destructive method that probesmaterials molecular vibrational modes. These qualities make it an attractivemethod for characterisation. A strong understanding of the effects that nitrogeninclusions have on the Raman spectrum of graphene could potentially enablethe characterisation of these functionalities through this method alone.

In this thesis, rGO doped with both nitrogen (N-rGO) and nitrogen-15 (N15-rGO) was synthesised, and their Raman spectra were analysed. rGO was cho-sen as it is easier to both functionalise and synthesise compared to ordinarygraphene. From the study, potential evidence linking the I band to function-alised graphene was found, as it responded strongly to isotope labelling. Also,tentatively a new band denoted I*, possibly related to sp3 carbon, was identi-fied. Before this, the spectral region 1000-1300 cm−1has only been thought tocontain the I band and it has been speculated to be connected to functionalisedgraphene or sp2-sp3 carbon bonds. As this study indicates a separation of the Iband into two bands, each dependent on one of these factors, it could bring clar-ity to this poorly understood area. If these results can be replicated, and backedup by more measurements, they could lead to a better future understanding ofthe Raman spectrum of graphene.

The results of this thesis are speculative and further studies would be neededto confirm them. Firstly, more data from more samples from each category (un-doped, N-doped and N15-doped) would be necessary to gather, as one samplefrom each class only gives an idea of the variations present in those specific sam-ples and not the variations between the different sample categories. Gatheringmore data would result in the possibility to use stronger statistical methods andalso, most importantly, the ability to accurately judge which observations canbe deemed significant.

Besides this, a study of the Raman spectra for N-rGO doped with differentlevels of nitrogen could be used to identify clearly the nitrogen dependency ofthe Raman bands. This process would likewise be interesting to conduct for

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varying degrees of isotope content, which would serve as further support of theobservation made in this thesis.

Further, the Raman spectrum of graphene is not only limited to the spectralfeatures discussed in this thesis. Thus additional information could be found bystudying, for example, the higher order Raman bands in the region 2300-3300cm−1.

To gain insight into the impact of specific nitrogen functionalities on the Ra-man spectrum, and possibly identify their vibrational modes. An experimentwhere samples containing varying amounts of particular types of nitrogen inclu-sion could be synthesised. Such an experiment could, for example, be carriedout via heat treatment of N-rGO which would transform less stable nitrogenfunctionalities in more stable ones [8]. Also, another possibility would be totweak synthesis parameters to promote different types of nitrogen inclusions, assuggested by Indrawirawan et al.[26].

To conclude, we can see that the results of this thesis are intriguing butvague. However, they clearly indicate what to expect in similar experimentsand they provide a sound foundation for further studies.

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