Universidade de Aveiro
Ano 2018 Departamento de Química
Adriana Filipe
Bernardes
Esfoliação em fase Líquida de Grafite
Pirolítica Altamente Orientada e a Sua
Oxidação por Atomização com uma Mistura
de Ar-Ozono
Liquid-phase Exfoliation of Highly Oriented
Pyrolytic Graphite and Its Oxidation by
Air-Ozone Atomization
Universidade de Aveiro
Ano 2018 Departamento de Química
Adriana Filipe
Bernardes
Exfoliação em Fase Líquida de Grafite
Pirolílica
Altamente
Orientada
e
Sua
Oxidação por Atomização com uma Mistura
de Ar-Ozono
Liquid-phase Exfoliation of Highly Oriented
Pyrolytic Graphite and Its Oxidation by
Air-Ozone Atomization
Dissertação apresentada à Universidade de Aveiro para cumprimento dos requisitos necessários à obtenção do grau de Mestre em Engenharia Química, realizada sob a orientação científica do Doutor Bruno Reis Figueiredo, especialista em grafeno, co-fundador e administrador científico da Graphenest, S.A., e da Doutora Ana Luísa Daniel da Silva, Investigadora Auxiliar do Departamento de Química da Universidade de Aveiro.
Dissertation presented to University of Aveiro in order to get the requirements necessary to obtain a master’s degree in chemical engineering, performed under the scientific guidance of Dr. Bruno Reis Figueiredo, co-founder, Chief Science Officer and graphene’s specialist at Graphenest, S.A., and of Dr. Ana Luísa Daniel da Silva, Auxiliary Researcher of the Chemistry Department at the University of Aveiro.
o júri
presidente Prof. Doutor Carlos Manuel Santos da Silva
professor associado do Departamento de Química da Universidade de Aveiro
Doutor Jérôme Borme
investigador na equipa 2D Materials and Devices do Laboratório Ibérico Internacional de Nanotecnologia
Doutor Bruno Reis Figueiredo
agradecimentos Ao meu orientador, Doutor Bruno Figueiredo, por todo a dedicação, disponibilidade, e ensinamentos prestados.
À minha orientadora, Prof. Doutora Ana Silva, pela orientação e partilha de conhecimentos.
Ao Eng. Rui Silva, diretor tecnológico da Graphenest, pelo apoio, dedicação, partilha de conhecimentos e acompanhamento na operação dos equipamentos da empresa.
Ao Doutor Paulo Ferreira e à equipa do Laboratório Ibérico Internacional de Nanotecnologia (INL), pelos ensaios de microscopia eletrónica de transmissão (TEM), assim como ao Doutor José Fernandes (do Departamento de Física da Universidade de Aveiro), pela realização dos ensaios de Raman, e à técnica de laboratório Maria Celeste Azevedo (do Departamento de Química da Universidade de Aveiro) pela ajuda na preparação dos ensaios de XPS
Aos meus amigos e família por todo o apoio e motivação ao longo deste período.
Aos meus avós, que estão sempre presentes, quaisquer as circunstâncias.
À minha irmã, que é como uma segunda mãe para mim! Obrigada pela preocupação, apoio e amizade.
Aos melhores pais, um obrigado especial por tudo. Sem vocês nada disto seria possível!
palavras-chave Grafite pirolítica altamente orientada, HOPG, nanoplaquetas de grafeno, GNP, óxido de grafeno, GO, exfoliação, ozono, oxidação, atomização
resumo Com vista a responder à procura de um método de produção de grafeno altamente rentável, versátil e amigo do ambiente, a Graphenest desenvolveu uma metodologia baseada numa exfolição em fase líquida que foi agora testada com recurso ao uso de uma matéria-prima diferente: grafite pirolitica altamente orientada (HOPG). Após a exfoliação, a dispersão de grafeno de multicamadas passou por uma etapa de atomização utilizando uma mistura de ar-ozono, por forma a se obter um material com um nível de oxidação superior. Para tal, o processo de exfoliação foi realizado, efetuando um desenho de experiências (DoE) que permitisse compreender o efeito de quatro variáveis distintas no rendimento da produção de grafeno: 1) temperatura; 2) densidade de potência do equipamento de ultrassons; 3) frequência do equipamento de ultrassons; e 4) concentração inicial de grafite dispersa.
Todas as amostras foram caracterizadas por espectroscopia Raman e por Dispersão Dinâmica de Luz (DLS) com o objetivo de determinar as condições processuais que permitem a obtenção de particulas com tamanho lateral e espessura mais pequenas. Adicionalmente, a concentração de grafeno disperso após cada uma das corridas de exfoliação foi determinada por espectroscopia UV-Vis, após centrifugação com diferentes velocidades (1000, 2000 e 4000 rpm). Antes da etapa de atomização, as amostras com as caracteristicas pretendidas (menor dimensão lateral e espessura) foram caracterizadas por microscopia eletrónica de trasmissão (TEM).
Relativamente às condições processuais, o DoE revelou que a combinação do nível mais baixo de cada variável em análise permitiu a produção de maior quantidade (maior rendimentos) e melhor qualidade (menor dimensão lateral e espessura) de partículas de grafeno. As amostras cristalinas de grafeno manifestaram natureza ultrafina e boa flexibilidade.
De modo a se obter uma forma rápida e eficiente para a funcionalização deste nanomaterial, a produção de óxido de grafeno foi testada, recorrendo a uma mistura de gás ar-ozono durante o processo de atomização. Para avaliar a oxidação, uma determinada selecção de amostras foi atomizada com ar e paralelamente com uma mistura de ar-ozono. Essas amostras foram, de seguida, caracterizadas por espectroscopia de fotoeléctrones excitados por raios-X (XPS) e os resultados, embora, de certa forma, inconclusivos, revelaram uma oxidação residual.
keywords Highly oriented pyrolytic graphite, HOPG, graphene nanoplatelets, GNP, graphene oxide, GO, exfoliation, ozone, oxidation, atomization
abstract In order to meet the demand for a highly profitable, versatile and environmentally friendly graphene production method, Graphenest developed a methodology based on liquid phase exfoliation that was now tested using a different raw material: highly oriented pyrolytic graphite (HOPG). After the exfoliation, the obtained multilayer graphene dispersion underwent to a step of atomization using a mixture of air-ozone, in order to achieve a material having a higher oxidation level. To accomplish this, the exfoliation process was carried out applying a Design of Experiments (DoE) that allowed to understand the effect of four different variables on the yield of graphene’s production: 1) temperature; 2) power density of ultrasound equipment; 3) frequency of ultrasound equipment; and 4) initial concentration of dispersed graphite.
All the samples were characterized by Raman spectroscopy and Dynamic Light Scattering (DLS) in order to determine which processual conditions allow the obtaining of graphene particles with the smallest lateral size and thickness.
Additionally, the concentration of graphene dispersed obtained in each of the exfoliation Runs was determined by UV-Vis spectroscopy after a centrifugation step at different speeds (1000, 2000 and 4000 rpm). Before the atomization step, the samples with the desired characteristics (smaller lateral size and thickness) were characterized by transmission electron microscopy TEM.
Regarding the processual conditions, the DoE revealed that the combination of the lowest level of each variable under analysis allowed the production of more quantity (higher yield) and better quality (smaller lateral size and thickness) of graphene particles. The crystalline graphene samples showed ultrafine nature and good flexibility.
In order to obtain a fast and efficient way for the functionalization of this nanomaterial, the production of graphene oxide was tested relying on the usage of an air-ozone gas mixture during the atomization process. To verify the validity of the oxidation, a selected sample was atomized exclusively with air and in paralell with a mixture of air-ozone. These samples were then characterized by X-ray photoelectron spectroscopy (XPS) and the results, although to some extent inconclusive, revealed a residual oxidation.
Table of Contents
1.
Framework, motivation and proposed objectives, and outline ... 1
1.1. Framework ... 1
1.2. Motivation and proposed objectives ... 3
1.3. Outline ... 4
2.
Introduction ... 6
2.1. Graphite ... 6 2.2. Graphene ... 7 2.3. Graphene oxide ... 10 2.4. Properties of graphene ... 102.5. Graphene production methods ... 14
2.5.1. Epitaxial growth on an insulator (SiC) ... 14
2.5.2. Chemical vapor deposition (CVD) ... 15
2.5.3. Graphite exfoliation ... 16
3.
Design of Experiments (DoE) and Response Surface Methodology
(RSM) ... 21
4.
Experimental procedure ... 24
4.1. HOPG exfoliation according to the Graphenest’s procedure ... 25
4.2. Graphite and graphene nanoplatelets dispersibility ... 26
4.3. Atomizing and Oxidation of the most prominent samples ... 28
4.4. Characterization of the samples ... 29
4.4.1. Raman Spectroscopy ... 30
4.4.2. Dynamic Light Scattering (DLS) ... 30
4.4.3. UV-vis Spectroscopy ... 30
4.4.4. Transmission Electron Microscopy (TEM) ... 31
4.4.5. X-ray Photoelectron Spectroscopy (XPS) ... 31
5.
Presentation and discussion of results ... 32
5.1. Pre- and post-exfoliation samples’ characterization ... 32
5.2. Transmission electron microscopy (TEM) characterization ... 47
6.
Conclusions ... 53
7.
Future work ... 55
8.
Bibliography ... 56
Tables index
Table 1 - Examples of defects existing in multiple layers and single-layer graphene. ... 9
Table 2 - Comparison of graphene production methods. ... 20
Table 3 - Factorial design standard order explication. ... 22
Table 4 - Coded full factorial design worksheet used. ... 26
Table 5 - Results obtained for the I2D/IG with different speeds of centrifugation. ... 35
Table 6 - I2D/IG model equations, respective r2 and P-value obtained for the different centrifugal speeds. ... 40
Table 7 - Size of graphene particles (L) obtained for h=3nm and different centrifugal speeds. ... 41
Table 8 - L model equations, respective r2 and P-value obtained for the different centrifugal speeds. ... 44
Table 9 - Values of A and B that provide smaller particles, in function of C and D and also the centrifugal speed, for DLS and Raman techniques. ... 45
Table 10 – Mean percentage error obtained for the 5 central points and different centrifugal speeds. ... 45
Table 11 – Average percentage of graphene nanoplatelets (GNP (%)) based on the initial concentration in function of the centrifugal speed. ... 46
Table 12 - Relative atomic percentage of the C 1s and O 1s peaks obtained from the XPS analysis, using Air or a mixture of Air/Ozone gas in the atomization process. ... 50
Figures index
Figure 1 - Summary of the applications of graphene in different sectors [1]. ... 1 Figure 2 - Graphite structural scheme [10]. ... 6 Figure 3 - Production scheme of fullerenes, nanotubes and graphite [12]. ... 7 Figure 4 - Representation of the Brillouin zone and Dirac points in graphene mesh [12]. 11 Figure 5 - Common stacking types for a bilayer graphene [12]. ... 12 Figure 6 - Stable stacking configurations for a tri-layer graphene. ABA stacking is represented in the left scheme and ABC in the right one [23]. ... 12 Figure 7 - Reactive spots in graphene, namely, defects armchair, zigzag and combined edges [25]. ... 13 Figure 8 - Different types of exfoliation methods [30]. ... 18 Figure 9 -Principle of ultrasound cavitation [36]. ... 19 Figure 10 - Factorial design for 3 factors, X1, X2 e X3, where each factor has two levels [41].
... 22 Figure 11 - Scheme of Graphenest’s exfoliation method (GRX-a20). ... 25 Figure 12 - Ra in function of the volume fraction of the organic solvent in water. ... 27 Figure 13 - Atomizing equipment diagram. In the figure, TC means temperature controller. ... 28 Figure 14 - Raman spectrum of HOPG. ... 33 Figure 15 - Dispersion of slightly exfoliated HOPG or graphene submitted to 3 hours of sonication in aqueous media containing variable volume% of acetone. ... 34 Figure 16 - Raman spectrum for the Runs most and less prominent, for different centrifugal speeds. ... 37 Figure 17 - Raman spectrum for HOPG and Run 16 centrifuged with 4000 rpm. ... 38 Figure 18 - Normalized intensity ratio I2D/IG as function of the coded factors A and B. .... 39
Figure 19 - Particle size distribution curves (individual Runs and average) obtained by DLS for Run 13 centrifuged with 4000 rpm. ... 41 Figure 20 - Calculated lateral size (L) as function of the coded factors A and B. ... 43 Figure 21 - Average percentage of graphene nanoplatelets GNP (%) based on the initial concentration in function of the centrifugal speed for Run 8, 12 and 14. ... 47
Figure 22 - TEM images and the selected area diffraction pattern of Run 2 centrifuged at 1000 rpm and 4000 rpm and of Run 14 centrifuged at 1000 rpm. ... 48 Figure 23 - XPS scan of Run 19A (a)) and 19B (d)), the deconvoluted C 1s (b)) and O 1s (c)) of Run 19A and the deconvoluted C 1s (e)) and O 1s (f)) of Run 19B. ... 52
Abbreviations and Symbols
At (%) Relative Atomic Percentage AVG Average
CA Carbon Adatoms
CVD Chemical Vapor Deposition d Average spherical dimension DLS Dynamic Light Scattering DMF Dimethylformamide
DNA Deoxyribonucleic Acid DoE Design of Experiments GNP Graphene Nanoplatelets
GO Graphene Oxide
h Height of the parallelepiped particles HOPG Highly Oriented Pyrolytic Graphite
HSN Hanson Solubility Parameters
Ii Raman spectrum intensity of the band i
K and K’ Dirac points
L Size of graphene particles NMP N-methylpyrrolidone
QHE Quantum Hall Effect
r2 Coefficient of determination
Ra Hansen solubility parameters distance rpm Rotations per minute
RSM Response Surface Methodology SEM Scanning Electron Microscopy
SiC Silicon Carbide SW Stone-Wales defects
Xi Factorial design inputs or factors
XPS X-ray Photoelectron Spectroscopy Y Factorial design yield
0D Non-dimensional 1D One-dimensional 2D Two-dimensional 3D Three-dimensional
bi Factorial design model coefficients
ϵ Uncontrolled sources of variance affecting Xi e Absorption coefficient
1. Framework, motivation and proposed objectives, and outline
1.1. FrameworkThe exceptional properties of graphene, such as its electrical and thermal conductivity and mechanical strength, make it a material with a growing number of applications and uses, such as in electronic devices, spintronic devices, composites, sensors and flexible electrical devices, energy storage, power conversion, photonics, optoelectronics and biomedical devices. Figure 1[1] shows a summary of possible applications of graphene in different fields. Some of these applications are briefly described in the following paragraphs [1].
Due to its electric, optical and high transmittance characteristics, graphene can be applied to obtain flexible equipment, high-frequency and logic transistors, among others. Since its electrons are confined into a two dimensions (2D) electron gas with null mass, graphene nanomaterials are used in photonic applications as photo detectors, optical modulators and optical polarizers’ controllers [2]. Curiously, graphite flakes were already
1. Framework, motivation, objectives and outline
applied in plasma displays, opening a great opportunity to graphene to be applied in this field, since it can significantly improve the emission characteristics [3].
Since the production of graphene powder can be easily scaled up, its use in the production of composite materials is one of the most promising industrial applications. This process allows to obtain conductive plastics, even by adding a small percentage of graphene [3]. Despite this fact, its application is still not very usual in the industry because graphene-based composites’ electrical resistance still not match the ones observed using carbon nanotubes [1][3]. In addition to conductive plastics, conductive paints and coatings can also be obtained using graphene [2].
Properties such as wide surface area per volume ratio and high conductivity make the powder form of graphene a strong candidate to substitute carbon nanofillers in electric batteries and supercapacitors, leading to improvements in their efficiency and electrical current density [1][3]. These graphene-based energy storage systems can be very useful in the production of electrical vehicles, and portable electronic equipment [1]. In bio-applications, graphene is a promising material to be used as carrier for drug transport owing to its wide specific surface area, chemical purity, and easy surface functionalization that might be favorable to the release of the active principle of the drug. Its use in regenerative medicine can also be exploited due to its mechanical properties allowing, for instance, the production of conductive tissues [2]. The electronically low noise of graphene, make it also an excellent alternative to the production of solid-state gas sensors [4], field-effect superconducting transducers [3], and DNA sequencing [2]. Its use as sensors is possible due to its extremely high environmental sensitivity [2]. Finally, graphene is being studied as an absorption material of hydrogen [3].
Graphene can be also used in advanced characterization techniques, such as electron microscopy, due to its thickness, high electrical and thermal conduction, high strength and easy functionalization [2]. In order to observe supercritical screening, detect local magnetic moments and map wave function in quantizing fields, graphene can be used in scanning probe microscopy [5].
Although great improvements and discoveries have been made with graphene-related materials in recent years, there are still some concerns which make the possible applications not yet fully materialized [6]. The most prominent limitation is the
production of large quantities of high-quality graphene, due to the difficulty to preserve the structural integrity and quality upon the scale-up from methods tested on a smaller scale. Consequently, other concern is the high cost of graphene production related to its scarcity and good quality. Despite a recent decrease on the processes’ cost, the majority is still not yet sufficient cheap to make graphene commercially viable.
1.2. Motivation and proposed objectives
In order to produce cheap, high-quality graphene nanoplatelets and distribute it worldwide, Graphenest developed and patented a fast, highly cost-effective, and environmentally friendly production method based on a liquid-phase exfoliation, where the solvent is an aqueous-organic solution. Graphenest is now betting on the development of groundbreaking applications and its mission consists now on provide high-value graphene-based solutions to drive the next generation products [7].
The motivation of this dissertation is related with the intention to certify the confidence in Graphenest’s production method by using a different graphite, namely highly oriented pyrolytic graphite (HOPG). Additionally, Graphenest is also implementing a possible method to produce graphene oxide (GO) and use it as a new product, since it enables a rapid and efficient functionalization due to the presence of oxygen-containing groups that increase its dispersibility in a plethora of solvents, and hence, the reduction of the polar-polar interactions strength [8].
The existence of different production techniques and consequent grades (few- and multi-layer graphene) and forms (graphene nanoplatelets, graphene oxide, etc.) of graphene makes their production and functionalization industry very competitive. A constant research and development on new processes, methods or techniques is imperative to produce and functionalize this class of materials in different, reliable and cost-effective ways. The objectives of the present dissertation were idealized having this in mind, along with the exceptional properties of these graphene-related materials and Graphenest’s mission to drive the next generation of graphene-based products. Hence, three objectives were established as the following:
1. Framework, motivation, objectives and outline
1. Determine how effective is the liquid-phase exfoliation method developed and patented by Graphenest when using HOPG. Due to the experimental number of variables a Design of Experiments (DoE) was conducted. The influence of each variable was interpreted by a Response Surface Methodology (RSM), studying their relationship with the size and thickness of the exfoliated particles, which were determined by Dynamic Light Scattering (DLS) and Raman Spectroscopy, respectively. The samples were also characterized by UV-vis spectroscopy in order to estimate the concentration of material dispersed in the samples;
2. Install an Ozone-generator into an existing atomizing system at Graphenest in order to produce graphene oxide thought an air-ozone atomization. This form of graphene is not being currently produced by the company, and thus will allow new functionalization processes and increase the products’ portfolio;
3. Perform and evaluate the ozone-oxidation treatment through atomization of high-quality exfoliated graphene. The samples where characterized by X-Ray Photoelectron Spectroscopy (XPS), with and without the use of ozone in the process, allowing the determination of the presence of oxidized and non-oxidized states. Previous to the atomization process, transmission electron microscopy (TEM) images of the samples were also recorded;
1.3. Outline
The following paragraphs slightly describe the structure of the present dissertation, which is divided in seven different chapters.
Chapter 2 starts by introducing some basic but important notions to understand the work here presented. This includes enlightenments about what is graphite and how it is formed or produced, some clarifications about graphene, its properties and methods of production and also a brief overview of graphene oxide.
Information about DoE and RSM is provided in chapter 3. This chapter is focused on how to proceed in order to design a worksheet and execute it during a set of experiments based on a full factorial design.
Chapter 4 describes the experimental methods taken in this dissertation. It is explained how to proceed with the exfoliation of HOPG and the following atomization and oxidation of the most notable samples. Elucidations about some basic concepts on how to install and operate an ozone generator in a pre-existing atomizer are given. The description of the characterization techniques used in the different samples are also presented in this chapter.
The presentation and discussion of the results related with the exfoliation of the HOPG, as well as the ones obtained for the oxidation/atomization of graphene is done in chapter 5.
The conclusions and some future work are taken in chapter 6 and chapter 7, respectively.
2. Introduction
2. Introduction
2.1. GraphiteGraphite is one of the natural forms of carbon. It has an enlarged importance in industry due to its properties, such as chemical and thermal resistances, high electric conductivity and spectral and reflective characteristics. Natural graphite can be found in ore deposits and, according to the characteristics of the deposit, it can be classified in different forms: crystalline flake graphite, amorphous graphite and lump or vein graphite. In terms of quality level, it depends on the type of graphite and thus, its application diverges accordingly. Due to its importance in high-tech applications, a high demand of high purity graphite is being witnessed [9].
Figure 2[10] shows a scheme of the structure of graphite, where the blue dots correspond to carbon atoms, the fulfilled lines relate to covalent bonds and the dashed lines represent Van der Waals interactions [9].
Due to the limited amounts of high purity
natural graphite, efforts to produce synthetic high purity graphite have been made. One method to produce synthetic graphite consists in the pyrolytic deposition of hydrocarbons in vapor phase, which is performed at high temperature and pressure and implemented in large temporal scales in order to re-annealing the structure. The produced graphite is denominated highly oriented pyrolytic graphite (HOPG), which despite having characteristics very similar to those of natural graphite cannot be considered a lubricant. Its production process is extensive, hard and have low yield which make it expensive [9]. Note that, a high purity natural HOPG, can be also found in ore deposits, but in very small amount which make it very expensive.
The term synthetic graphite is not only used for HOPG. In fact, there are other ways to produce synthetic graphite, like the thermal treatment of calcined petroleum coke and coal tar pitch. Furthermore, highly pure graphite can be obtained by chemical purification of natural graphite, where the usual processes are hydrofluoric acid leaching, hydrochloric acid caustic leach and thermal treatment with high temperatures. Although
these processes are viable, the level of purity obtained falls shortly when compared to the same for synthetic graphite [9].
2.2. Graphene
The study of graphene and its different variations has been increasing due to recent uncover of certain properties that make it a very interesting material. These nanomaterial’s notable properties are related to its crystalline hexagonal lattice made of carbon atoms [11]. The latter are bonded to each other by the sp2 hybridization between
a s orbital and two p orbitals causing the formation of s bonds amongst the atoms and providing the necessary stabilization [12]. The set of s bonds generate a band of closed rings, establishing the robustness of the lattice. The p orbitals, whose orientation is perpendicular to the planar surface, interact with the p orbitals of the adjacent carbons forming covalent bonds (p bond) [12]. Each carbon atom is connected with three others forming an angle of 120° and a bonding length of 1,42 Å [12].
Graphene can be considered the base form of many other allotropic forms of carbon [11]. Its two-dimensional structure (planar structure with atomic thickness)[11] can be wrapped up into fullerenes (non-dimensional, 0D)[12], rolled into nanotubes (one-dimensional, 1D) and stacked into graphite (three-(one-dimensional, 3D)[12]. Figure 3 shows a scheme of the structures mentioned before [12].
2. Introduction
Graphene disorder causes imperfections and defects on the lattice, being very important for the mechanical, electronic, optical and thermal properties of the material, ultimately influencing the applications where it can be used [12].
Defects are classified as intrinsic or extrinsic depending on whether the modification of the lattice’s order is caused by internal or external atoms to the system, respectively. Besides, those lattice variations have dimensions that depend on the graphene type. For example, for a single-layer graphene the defects can be 0D or 1D and can migrate, i.e., they can move through the lattice. The activation barriers is the triggering effect to the movement because it depends on the type of defect and the temperature [12].
The energy of formation of graphene’s defects is high, and thus its pristine form occurs as naturally as possible. However, defects have to be accounted because they are always present. There are many types of defects and they can occur simultaneously. Note that it is also possible to induce defects and imperfections in these materials to obtain the desired properties and, consequently, use them in different practical applications. Table 1 presents a brief description of some typical defects and its corresponding observations [12].
Type of defect Brief description Observations Scheme
Stone-Wales Defects (SW)
It consists in the 90° rotation of the C-C bond, rearranging the
graphene lattice
4 hexagons give origin to 2 pentagons and 2 heptagons
Carbon vacancies
One or more carbon atom is/are lacking in the graphene
It depends on the number of carbon atoms missing
For example, from left to right: a single vacancy; double vacancy; double vacancy
Carbon Adatoms (CA)
Absorption of one or more carbon atoms forming a stable configuration (sp3 hybridization)
1-carbon: bridge or dumbbell configuration; 2-carbon: SW configuration (has a curvature)
From left to right: bridge CA; dumbbell CA
Grain boundary loops
A part of the lattice is extracted and is then reintroduced after a rotation with a certain angle
Depends on the number of atoms in the extracted section and rotation
angle; SW contained in this group
For example: the 13-atom core with C3 symmetry
2. Introduction
2.3. Graphene oxide
One of the most important derivatives of graphene is its oxidized version: graphene oxide (GO). GO has been under the spotlight because it is frequently used as precursor for the production of graphene through the reduction of its oxide groups by reducing agents, such as hydrazine, ascorbic acid, sodium borohydride, etc [12]. This graphene derivative has a high density of oxygen and functional groups, both in the basal plane (hydroxyl and epoxy group) and in the edges (carboxyl groups) [13]. The presence of these groups increases the solubility in water and turns GO easily functionalized and processed [13], permitting it to be used as the base substrate for grafting reactions [14].
The oxidation can be performed under different circumstances, although the key point is the utilization of an economically viable, environmentally friendly and scalable method [13]. Normally the synthesis of GO is done via chemical oxidation of natural graphite but there are other routes to do it. The most used process to produce GO is called the Hummers method, and it uses the dissolution of NaNO3 and KMnO4 in
concentrated H2SO4 to oxidize graphene. This process has suffered various modifications
through the years because of the production of toxic gases, residual nitrate and its low yield [13].
A recent study reported a new way of oxidizing graphene nanoplatelets which is capable of manufacturing materials with high-performance without using high temperatures or pressures. In this process, bonds between carbon and oxygen are formed by using ultrasonic spray deposition to induce a chemical reaction with oxygen. The oxygen is fixed into the nanomaterial by using ultrasonic energy and atomization. This also can be applied for nitrogen instead of oxygen. This method can be useful in the production of high-performance carbon-nanomaterial-based supercapacitor electrodes [15].
2.4. Properties of graphene
Graphene displays unique characteristics, such as good electric properties, high absorption of white light (optical transmittance)[16], high elasticity, non-usual magnetic properties, high surface area, gas absorption (permeability), high thermal conductivity
and charge-transfer interaction with molecules [17]. In this section some of the most important properties of graphene are described, assuming that it has inexistent or low structural defects.
Electronic structure and properties
The electronic properties of graphene depend on the number of layers. Curiously, at 10 layers they are similar to bulk graphite’s electronic properties [12].
In a single-layer graphene the charge carrier act as relativistic particles and their behavior is described more rigorously by the Dirac equation rather than the Schrödinger equation. Thus, even if these electrons are not really relativistic, their interaction with the periodical potential of the graphene lattice generate quasiparticles called massless Dirac fermions by moving around the carbon atoms, which are massless electrons or neutrinos with an electron charge [18].
The two conical points per Brillouin zone, K and K’, where the intersection of the conduction and valence bands occurs [18], are called Dirac points and when referring to a single-layer graphene, these points are located at the hexagon’s vertices of the Brillouin zone, as it can be seen in Figure 4[12].
Due to the energy linear behavior of the quasiparticles, it is expected that those particles
behave differently from the observed for electrons in metallic structures whose behavior is parabolic [18].
A single-layer of graphene shows a strong and ambipolar electric field effect [3], explaining the easy mobility of the charge carriers whose velocities are in the Fermi velocity range (ca., 106 m/s)[3][19]. High-mobility can be obtained even at high
concentrations and room temperatures [20]. Also, a single-layer graphene can support a maximum density current value some millions above the value found for copper [21].
A graphene bilayer no longer has a conical dispersion of its electric structure in the surroundings of the Dirac points, like it is observed in the single layer. Instead a parabolic
Figure 4 - Representation of the Brillouin zone and Dirac points in graphene mesh [12].
2. Introduction
Figure 6 - Stable stacking configurations for a tri-layer graphene. ABA stacking is represented in the left
dispersion is observed [12]. The gate voltage changes the concentration of charge carriers, inducing asymmetries between the layers of graphene and resulting in a semiconductor gap [18].
For a number of layers higher than two, if the number is even the electronic structure dispersion presents a conical geometry, if the number is odd the dispersion’s geometry is a mix of conical and linear. Because of having more than two layers, the electronic structure will have several carriers and the valence and conduction bands will have a greater overlap[12].
Additionally, a fractional Quantum Hall Effect (QHE) that differs by a factor of 0.5 from the original QHE can be observed at low temperatures [12]. Specifically, the Hall conductivity shows a ladder feature with a series of equally distributed plateau, which remain even in the Dirac points[12]. Thus, the conductivity of graphene is never less than a minimum value corresponding to the quantum unity of conductance, even when the charge carrier’s concentration tends to zero [12].
It is important to mention that the stacking order also has a drastic effect on the electrical properties of graphene, where the Dirac fermions can be formed as a consequence of the symmetry loss. The stacking order influences the Dirac points and consequently the band structure [12].
For a bilayer graphene, the most common types of stacking are AA and AB [12][22]. In AB stacking the carbons from layer B are above the center of the carbon hexagon in layer A, while in the AA stacking the carbons from each layer are directly on top of each other[22], as
shown in Figure 5[12]. Hence AB stacking is more stable than the AA, which is energetically unfavorable, because a metallic behavior with chiral parabolic dispersions appear in graphene near the K point[22].
For a tri-layer graphene, there are two stable configurations: ABA and ABC (see Figure 6 [23]). ABA stacking is just an extension of the AB structure from two to three layers. In ABC stacking it is added a third layer, C, to an AB
Figure 5 - Common stacking types for a bilayer graphene [12].
Figure 7 - Reactive spots in graphene, namely, defects armchair, zigzag and
stacking (explained before), where the corner of a hexagon in layer B is directly below a nonequivalent corner of a hexagon in layer C. ABC sequence opens a gap at the Dirac point, K, under an electric field. So, despite having different band structures, both configurations are semi-metallic[22].
Mechanical properties
Graphene and its derivatives are materials with high mechanical strength, as a consequence of the covalent sp2 hybridization between the carbon atoms [12].
Stiffness is fundamental to the utilization of materials in many and distinct applications, due to the stabilization and duration that those characteristics provide. In fact, values found in literature for the Young modulus (0.5-1.0 MPa) and for the tensile force (ca., 130 GPa), show that single-layer graphene is about 200 times more stiff than structural steel at microscopic levels. Note that its tensile force depends on the defects, on the gaps and also on the type of existing bonds, so the uncertainty of the defects present on the graphene make the measure of those mechanical properties difficult [12].
Magnetic Properties
Materials related to graphite have a ferromagnetic behavior at high temperatures due to different factors [17]. Some papers refer that the ferromagnetism shown by graphene comes from its structural defects, like the holes in the lattice or the functional groups connected to the surface or/and to the edge (e.g., by the chemical absorption of hydrogen)[17]. Furthermore, studies suggest that the
zigzag edges are responsible for the magnetic properties of graphene, because they have an antiferromagnetic ground state, where the edges have symmetrical spins (one up and one down), which doesn’t happen for the armchair edges [24]. The schematic representation of the reactive sites in graphene is shown in Figure 7[25]. Also,
2. Introduction
note that the magnetic properties depend on the measuring temperature [17][25].
2.5. Graphene production methods
In graphene production, the goal is always to obtain the minimum number of layers possible. Different production methods lead to different characteristics of the nanomaterial [26].
There are two major categories of nanomaterials production approaches, namely: top-down and bottom-up[11]. With regard to graphene, the bottom-up approach relates to the direct growth of graphene in an organic precursor, where techniques like epitaxial growth on silicon carbide and chemical vapor deposition (CVD) are included [26]. The top-down methods are based on the exfoliation of graphite, which may be mechanical, thermal, chemical processes or a combination of them, and can occur in a gas or in a liquid medium [26].
The production of graphene should also take into consideration the need for stabilization after isolation of a single-layer graphene, since it has a strong tendency to aggregate. In fact, one of the main obstacles in its production is the fact that the quality of the material differs due to its size polydispersity caused by aggregation[11].
On the following paragraphs both production approaches and corresponding most important methods are slightly revisited.
Bottom-up methods
2.5.1. Epitaxial growth on an insulator (SiC)
This production method was developed aiming the production of a large area graphene with low defects density in its layers settled upon a semiconductor substrate. The process is based on the faster kinetics of sublimation of the silicon atoms in relation to carbon atoms in the crystalline structure of the silicon carbide (SiC) under conditions of ultra-high vacuum and elevated temperatures. This leads to the formation of a carbon layer that can rearrange itself into the crystalline structure of graphene, guaranteeing the stability of the material [22].
Graphene is manufactured in a furnace with argon over pressure in order to improve the uniformity of the epitaxial layer. Note that, the silicon carbide is a polar material, that has two different terminations: the polar surface (contains the silicon) and the nonpolar carbon surface. Thus, depending on the referring phase, the kinetics mechanism of the graphene layer growth will differ and consequently the morphology and electric properties will be different [22].
This process has some advantages, such as the fact that there is no need to transfer the graphene layers to a processing device, as well as that the sheet of graphene has the same size of the substrate [22]. However, as drawbacks is the silicon carbide high price and the high temperatures that the process requires making the consumption of energy elevated. Furthermore, the conditions of the process have to be delimitated very carefully in order to produce highly pure graphene, turning it difficult to produce [11].
2.5.2. Chemical vapor deposition (CVD)
Chemical deposition on metallic substrates, such as copper and nickel, in the vapor phase is another method to produce high-area graphene[27] (in a sample of 1 cm2, 95%
were single-layer graphene [28]). In this process, the gas species are supplied to a reactor, passing through a hot zone where the hydrocarbon precursors decompose into carbon radicals on a metallic surface and then are rearranged to form graphene layers[27]. During the reaction, the surface of the metallic subtract acts as a catalyst and also determines the kinetics and the mechanism by which the deposition takes place, influencing the quality of the graphene [27]. For instance, if the formation of non-six-membered ring structures is allowed, the occurrence of grain boundaries is possible and that decreases the quality of the graphene films formed [27].
Normally, the metallic substrate is previously subjected to a thermal treatment in order to achieve certain surface specifications. The quality of the metallic substrates is a parameter to take into account because it will influence the graphene nucleation mechanism, according to the anomalies present in the substrate. It is then necessary to use a gas precursor to remove of all unwanted carbon sources, preventing the uncontrolled nucleation of low-quality carbon layers and still removing the anomalies
2. Introduction
sustainability [20]. For the process to be considered sustainable, the precursor has to come from renewable sources and be environmental friendly [20]. This normally is difficult to obtain, since it usually comes from non-renewable sources, such as hydrogen and methane [20].
Overall, this method allows the production of graphene with a low quantity of defects that are mostly suitable for manufacturing electronic devices [11][29]. However, the high-cost and the very restricted temperature range where the synthesis occurs are the source of its technical limitations, and make the scale-up of this process very difficult [11][29]. Moreover, the price of precursors materials have a great impact on the total cost of processes, hence a cheap and commonly available precursor is needed [6].
Top-down methods
2.5.3. Graphite exfoliation
Currently, the exfoliation of graphite is the method that allows to produce the highest amount of graphene and it can have a chemical or a physical nature[30]. This method is both versatile and cost-effective, because it can be combined with other chemical treatments, such as chemical functionalization, and the majority of the graphite sources (raw-material) is inexpensive, respectively [11][30].
Although the graphite exfoliation nature may differ through different methods, the main objective is always the same: van der Waals bond breaking between adjacent layers of graphene. Even though these bonds are weak, they only allow the sliding in the direction perpendicular to the graphene surfaces making the individual layer exfoliation a more difficult process. Thus, it is necessary to overcome these van der Waals attractions between adjacent layers, in order to obtain a successful exfoliation [30].
One of the methods used to reduce van der Waals forces is based on the expansion of the distance between layers using oxidation and chemical intercalation reactions (e.g., producing graphite intercalated compounds), since the force is inversely proportional to the interatomic separation [30]. For example, in an oxidizing functionalization the functional groups (e.g., hydroxyls) are connected to the graphite layers resulting in a
disorganized stacking, and thus turning easier to exfoliate it [30]. Excitations can also be introduced to overcome the van der Waals interactions, being ultrasonic and thermal treatments the most used techniques. On the first, shear forces and cavitation act as peeling mechanics on graphite allowing for its exfoliation. On the latter, graphite oxide or other compounds resulting from intercalation reactions are exposed to increasing pressures exceeding the van der Waals forces, and thus resulting in its exfoliation [11][30].
The liquid immersion of graphite also results in the reduction of the Van der Waals forces [31], since the potential energy between layers is given by the interaction among London’s dispersive forces, which observes a decrease in the presence of a certain solvent under vacuum conditions [30]. Generally speaking, by making the solvent and the material refractive indices equal the potential energy will tend to zero [30]. In the case of graphite, the solvent should have a surface tension of ca., 40 mJ.m-2 [32], as, for example,
is the case of dimethylformamide (DMF)[11].
One drawback in using liquid phase exfoliation to produce efficiently hydrophobic graphene is the large quantities of solvent needed to produce a small amount of graphene, because of the poor dispersibility and the tendency of graphene sheets to suffer p-p stacking. This makes the process economically infeasible and also environmentally unfriendly [33]. To solve this problem, graphene exfoliation can also be performed using surfactants, polymers and organic molecules in order to obtain a stable dispersion of graphene in solvents [34]. Surfactants induce repulsive electric forces between graphite layers, allowing to produce a stable colloidal graphene suspension [30]. Polymers and/or organic molecules are used to prevent the aggregation and the re-association of graphene flakes by providing repulsive steric forces [30]. But, even using some additive, these approaches are not totally effective, due to the elevated ratio of area to thickness (>103) and the predisposition of graphene flakes to endure p-p stacking
[33]. Exfoliation using chemical intercalation was also studied to resolve this drawback, but it is required a designer ionic liquid, the scale up is difficult and the exfoliation is not complete (>10 nm in thickness) [33]. A recent study showed that, to reduce the use of solvent, a non-dispersion strategy can be applied, by using a flocculated slurry, to produce large quantities of high-quality graphene sheets [33]. This approach uses a viable
2. Introduction
water-phase exfoliation to produce loosely stalked, flocculated aggregates, due to the presence of adsorbed ions on the weakly oxidized surface, avoiding the use of destructive chemical oxidation processes and the use of large quantities of solvent, while addressing issues in the transportation and storage of graphene [33].
The type of graphite used as a raw-material is also a factor that should be taken into account, since the yield of the exfoliation process depends on whether the starting material is graphite, expanded graphite or graphite intercalated compounds [30].
The direct exfoliation of graphite is considered the best method to obtain graphene, since the resulting graphene has high quality, good conductivity and low density of defects although it usually presents a low yield [30]. Figure 8 shows a schematic of the different types of exfoliation and how do they can be related [30].
In the following lines, the mechanical, thermal and other exfoliation processes, are briefly described.
Mechanical exfoliation: This method is one of the most important exfoliation methods and includes the application of mechanical forces through agitation, vibration or sonication. Some of the new techniques associate carefully chosen solvents to these
Figure 8 - Different types of exfoliation methods [30].
Me chanical Micro wave Furnace He ating Oxidat ion Intercalation a nd
expansion Microw
ave Furnac e Heati ng Sonication Electrochemical exfoliation Graphite Graphene
processes with the aim of reducing the energy required for the exfoliation and avoid the re-aggregation of the exfoliated material [11][30].
Ultrasounds technique is essentially the transmission of an elastic high-intensity sound wave through a liquid, creating alternating positive and negative pressure regions [35][36]. A cavity/bubble field with the liquid vapor is created in the negative pressure zones due to evaporation and it explodes when it reaches the critical size (resonant) in the contraction phase (positive pressure regions) [35][36]. This is known as cavitation and it is explained in Figure 9[36]. The energy released in the violent collapse is used to exfoliate the material [35][36].
Figure 9 -Principle of ultrasound cavitation [36].
Thermal exfoliation: This method of exfoliation is used when graphite has a high density of functional groups. During the process the functional groups of the graphite decompose and produce gases which increase the pressure between adjacent layers. The exfoliation happens when the pressure overcomes the van der Waals forces. Thermal exfoliation is faster than mechanical exfoliation and avoids the use of liquids [30].
Other methods: There are some other methods that although not yet widely used, are quite promising[30]. Some examples are the electrochemical exfoliation, thermal quenching of graphite and exfoliation using supercritical fluids. For instance, in the electrochemical exfoliation an electrical voltage is applied on a graphite bar/rod used as an electrode in a conductive solution. As the rod disintegrates, it also gives rise to graphene nanoplatelets that were functionalized by the solvent[11][37]. In the case of the
2. Introduction
allowing the exfoliation. Finally, in the case of supercritical fluids exfoliation, a certain fluid (e.g., carbon dioxide) is intercalated between the layers and its sudden expansion causes the separation of the graphene layers [30]. Exfoliation with a supercritical CO2-H2O
medium is one example, where the pairing of ultrasounds with supercritical fluids can enhance the exfoliation and also can promote intercalation [38].
In Table 2 a comparison of graphene production methods is shown.
Table 2 - Comparison of graphene production methods.
Method Advantages Disadvantages
Epitaxial Growth on an Insulator (SiC)
-High-area and low defects graphene production; -Size of the graphene sheets and
substrate is equal;
-No transfer of graphene layers to a processing device.
-Silicon Carbide is expensive; -Elevated consumption of
energy;
-Difficult delimitation of the process conditions.
Chemical Vapor Deposition (CVD)
-High-area and low defects graphene production.
-High cost;
-Graphene quality depends on the metallic subtract;
-Difficult to scale-up; -Precursor comes from
non-renewable sources. Mechanical
Exfoliation -Reduced energy required.
-The starting material has to be in solution;
-Some solvents can be harmful to the environment.
Thermal Exfoliation
-Near complete exfoliation; -Faster than the mechanical
exfoliation;
-Can be done in a gaseous medium.
-Starting material has interlayer functional groups.
3. Design of Experiments (DoE) and Response Surface Methodology (RSM)
In order to develop, improve and/or optimize processes, response surface methodology (RSM) is used by applying statistics and mathematics. This methodology uses graphical methods to explore how a certain response depends on the different factors. Because the true response function is unknown we have to approximate, normally with polynomials. The approximation accuracy will determine the RSM answer, so this is a very important point to consider [39].RSM can be used to fit and check the adequacy of a model, by using a Design of Experiments (DoE). For experiments involving several factors, design of experiments is very useful in a RSM because it allows to identify the important factors (screening experiments)[39].
A Design of Experiments allows the evaluation of the combined effects of two or more variables (factors) used simultaneously. Since it admits the evaluation of the interaction between the variables, the information obtained through this practical method is more complete than the one acquired from the elaboration of series of experiments with only one variable (factor). Additionally, the results permit to make a decision that can be used in a much wider range of applications [40].
DoE can be performed by applying a central composite design to fit the response surface. This design involves a run in each corner of a square, four runs at the center and one run in each axis, giving a total of 12 runs. For more than two factors, full factorial design is used, which is basically an expansion of central composite design [39].
In full factorial design, if a certain variable of interest or response, Y, depends upon three different factors, X1, X2 and X3, it is necessary to determine these factors’
combination that provides the desired effect on Y. If each factor contains two levels, then the number of possible combinations is determined by 2n, where n is the number of
factors. In this specific case the number of combinations equals 23 = 8 [41].
For a case of 3 levels, the number of combinations equals 27, and all possible interactions among the variables are given by three main effects, β"X", three combinations of two factors, β"$X"X$, and a combination of the tree factors, β"$%X"X$X%,
3. Design of Experiments (DoE) and Response Surface Methodology (RSM)
the factors, and ϵ is the uncontrolled sources of variance affecting all X" (see Equation 1)
[41].
Y = β)+ β+X++ β,X,+ β-X-+ β+,X+X,+ β+-X+X-+ β,-X,X-+ β+,-X+X,X-+ ϵ (1)
The numbers at the vertices of the cube depicted in Figure 10 refer to the standard order by which an experiment is constructed, and the sort code was elaborated according to Table 3, by replacing the two possible levels of the factor by -1 and 1 values. In order to increase the accuracy of the results replicates of the experience can be performed. However, it should be noted that in each complete experiment only an average response value should be considered along with its deviation (variability and consistency) [41].
Table 3 - Factorial design standard order explication.
Order X1 X2 X3 1 -1 -1 -1 2 +1 -1 -1 3 -1 +1 -1 4 +1 +1 -1 5 -1 -1 +1 6 +1 -1 +1 7 -1 +1 +1 8 +1 +1 +1
Figure 10 - Factorial design for 3 factors, X1, X2 e X3,
The dispersion of results should be assumed to be uniform in the experimental space, i.e., the variance is homogeneous. Thus, by replicating the experiments it is possible to verify the validity of this assumption and still understand how each combination of factors results in inconsistent characteristics. The experiments should be carried out in a random order in order to ensure the minimization of errors from external factors that may affect the results.[41] The addition of central points to the DoE guarantees better results, because they provide information about the curvature and also permit to measure the process stability and the intrinsic variability [42].
To check if the regression model is significant a F-test of overall significance can be used. This statistical test compares the model obtained with the intercept-only model (model with no independent variables) by comparing the fits of different linear models. This is possible due to the ability of the test to evaluate multiple model terms simultaneously [43].
The F-test for overall significance has two hypotheses: the null hypothesis which states that the intercept-only model fits the data as well the model obtained, and the alternative hypothesis that claims the model fits the data better than the model with no independent variables [43].
The comparison between the P-value and the significance level of the model is used to interpret this test. The regression model fits the data better than the intercept-only model if the P-value is smaller than the significance level. Recall that the P-value is the probability that a sample behavior is correspondent to the model, when the null hypothesis is true for the populations. The significance level, measures if the model rejects the null hypothesis, by saying that the effects are statistical significant [43].
The coefficient of determination (r2) can also be used to check if the model fits the
4. Experimental procedure
4. Experimental procedure
On the following lines it is briefly described the experimental procedure executed during this dissertation. First, the exfoliation of HOPG was performed according to the Graphenest’s procedure, allowing to understand the effect of 4 factors, temperature, ultrasonics power density and frequency, and graphite concentration on the yield of the process, i.e., the amount and quality of the graphene produced. To do that, a Design of Experiments (DoE) was putted in place leading to a set of 21 different experiments and corresponding collection of samples that were quickly characterized by Raman Spectroscopy and Dynamic Light Scattering (DLS).
The DoE was comprised by a full factorial design and was designed having in mind the available resources, the time necessary to perform the experiments and the number of runs needed to check the process stability [42]. The samples collected from exfoliation were also characterized by UV-vis spectroscopy in order to estimate the concentration of material dispersed in the samples.
Before the characterization, the samples were centrifuged at 1000, 2000 and 4000 rpm in order to study the effect of the centrifugal speed on the characteristics of the particles obtained in each sample. Moreover, smaller particles are expected for a higher centrifugal speed which results on a premium sample. This step is of great importance and typically applied at Graphenest, allowing it to sell graphene nanoplatelets at a higher price.
After the characterization of each sample, those better exfoliated were chosen to perform an oxidizing atomization step using a gas mixture of air and ozone. These samples were characterized with and without the oxidation process using X-ray photoelectron spectroscopy (XPS). The comparison of the results allowed to understand if the oxidation was efficiently performed. Also, the samples that were atomized were previously characterized by transmission electron microscopy (TEM).
4.1. HOPG exfoliation according to the Graphenest’s procedure
The exfoliation procedure developed by Graphenest (GRX-a20) comprises a 16 dm3
vessel where ultrasonic cavitation occurs (1) and a high-shear homogenizer (2) pumps the dispersion through and a refrigeration circuit (3). All are controlled by a computer. Figure 11 shows a scheme of Graphenest’s exfoliation method.
Figure 11 - Scheme of Graphenest’s exfoliation method (GRX-a20).
The DoE based on four factors at two levels was executed: 1) temperature of the medium; 2) power density; 3) frequency of the ultrasonic device; 4) concentration of HOPG. The responses analyzed were the ratio of intensities of the 2D and G (I2D/IG) bands
obtained by the analysis of Raman spectrum, that provides information about the number of layers present in the graphene sample and the size of graphene particles (L) obtained by DLS.
The experiments of this factorial design were performed in a random order, without replication of the experiments and with 5 central points, giving a total of 21 (24+5) trials.
Five central points were used because they can provide information about the stability of the process, while making the experiments executable in terms of time and resources
1
2
3
1: Cavitation
2: Shear
3: Thermal
GRX-a20
4. Experimental procedure
randomly codified with the letters A, B, C and D. The levels of each factor were also coded by the numbers -1, 0 and 1 to run the DoE. Table 4 shows the coded full factorial design worksheet used in this experiment.
Table 4 - Coded full factorial design worksheet used.
Run A B C D 1 1 1 1 -1 2 1 -1 1 1 3 1 -1 1 -1 4 0 0 0 0 5 1 -1 -1 1 6 1 1 -1 -1 7 1 1 1 1 8 0 0 0 0 9 -1 1 1 -1 10 1 -1 -1 -1 11 -1 1 -1 -1 12 -1 1 -1 1 13 -1 -1 1 1 14 -1 -1 -1 -1 15 0 0 0 0 16 -1 -1 1 -1 17 0 0 0 0 18 0 0 0 0 19 -1 -1 -1 1 20 -1 1 1 1 21 1 1 -1 1
4.2. Graphite and graphene nanoplatelets dispersibility
To guarantee a good exfoliation of HOPG it is necessary to use a solvent that provide a good dispersibility of graphite and ultimately of graphene, and this can be obtained by matching the Hansen solubility parameters (HSP) of the dispersed medium and both the materials [32].
In order to study which solvent should be used during the exfoliation, the HSP distance (the so-called Ra parameter) was calculated for mixtures of water with acetone, ethanol and N-methylpyrrolidone (NMP). Equation 2 [32] was used to calculate Ra, where
the ./,1, .2,1 and .3,1 are the Hansen parameter for the solvent (1) and the graphene (2) [32].
Ra = 74 ∙ :D .;,1−.;,2?,+ :.@,1−.@,2?,+ (.B,1−.B,2), (2)
Note that, each material or solvent has three Hansen parameters ./, .2 and .3 which are the coordinates of the 3D Hansen space and represent the influence of the three major types of interactions in common organic solvents in terms of solubility: dispersive (D), polar (P) and hydrogen bonding (H) [44]. If the solvent is a mixture, the Hansen parameters are proportional to the volume fractions of its pure components [32]. The values of the Hansen parameters of the pure components used to calculate the mixture parameter and later to determine the respective Ra are shown in Table A. 1 of the attachments[32][44]. Figure 12 shows the plots for the Ra in function of the volume fraction of different organic solvents in water.
4. Experimental procedure
In order to achieve a high dispersibility, Ra has to be small [32]. From Figure 12 it is possible to check that a higher dispersibility of graphene is obtained by using pure NMP. However, this solvent is now considered environmental unfriendly [45], and an acetone-water mixture with more than 70 % vol. of acetone provides almost a similar dispersibility without being harmful to the environment. Some trials were conducted to test this theoretical principle, and the results achieved, and respective discussion are presented in chapter 5.
4.3. Atomizing and Oxidation of the most prominent samples
The most prominent samples were atomized in two different ways in order to study the validity of the oxidation with ozone: one with air and the other with a mixture of air/ozone. The scheme of the atomization equipment used is depicted in Figure 13. In the following lines a brief explanation of the procedure is given.
The samples chosen to be atomized were fed to an atomizing nozzle by a peristaltic pump at the same time as air/ozone or air, depending on type of atomization. Note that, when the atomization is done with air, the ozone generator is kept turned off.
The atomizing nozzle sprays the sample and the particles are dried by hot air in the drying chamber. Bigger particles (coarse) are collected in this chamber, while the smaller ones are directed to the cyclone and then collected. However, the cyclone is not totally efficient in the removal of the particles, and some particles are dragged with the air out of the cyclone, being gathered in a bag filter.
The air flow is established by an air pump, placed at the end of the circuit of air, that works by sucking air from the atmosphere by an air filter. The air flow is then heated by two heating resistances at 300°C.
4.4. Characterization of the samples
Graphene characterization is very important to know the quality of this nanomaterial. On other words, characterization techniques allow the determination of the shape, number of layers, purity, and the presence (or absence) of defects, among others. There are a lot of different methods to analyse a certain characteristic, and thus the choice of the characterization technique should be appropriated.
The samples collected after exfoliation were characterized through DLS and Raman and UV-vis spectroscopy techniques.
The samples chosen to be atomized were previously characterized by transmission electron microscopy (TEM).
With the objective to determine the efficiency of the atomization, the quality of the material obtained and to compare the characteristics of the nanomaterial before and after oxidation, another characterization of the samples was necessary. So, the samples obtained from atomization, with and without the use of ozone, were characterized by X-ray photoelectron spectroscopy (XPS).
