Use carbon nanotubes and graphene oxide absorbed copper ion

The study using carbon nanotubes and graphene oxide nanomaterial to absorb copper oxide ion in water is absolutely necessary.. The last result of the absorption of the formula we use to

THAI NGUYEN UNIVERSITY UNIVERSITYOF AGRICULTURAL AND FORESTRY

GIANG NAM KHÁNH

TOPIC TITLE:USE CARBON NANOTUBES AND GRAPHENE

OXIDE ABSORBED COPPER ION

BACHELOR THESIS

Study Mode : Full-time

Major : Environmental Science And Management

Faculty : International Training and Development Center

Thai Nguyen, 21/01/2015

ACKNOWLEDGMENT

First of all, I would like to express sincere thanks to the school board Thai Nguyen University of Agriculture and Forestry, Faculty of International Training and Development; advanced program, thank the teachers who has imparted to me the knowledge and valuable experience during the process of learning and researching here

In the process of implementing and completing thesis, I have received the enthusiastic help of the teachers of National Tsing Hua University I would like to express my special thanks to Prof Ruey An Doong who has spent a lot of time, created favorable conditions, enthusiastic to guide me to complete this thesis

I sincerely thank my friends in the laboratory facilitated, and provided the information and data necessary for my implementation process and helped me finish this thesis

In the process of implementing the project, due to time, financial and research levels of myself is limited so this project is inevitable shortcomings So, I would like

to receive the attention and feedback from teachers and friends to this thesis is more complete

I sincerely thank you!

Taiwan, 2014

Students perform

Giang Nam Khanh

Table of Contents

Abstract 1

1 INTRODUCTION 3

1.1 Rationale of study 3

1.2 Aim of the study 5

1.3 Research questions 5

1.4 Scope of the study 5

2 LITERATURE REVIEW 5

2.1 Carbon nanotube (CNTs) 6

2.1.1 Structure of Carbon nanotubes 6

Single-walled 7

Multi-walled 9

2.1.2 Properties adsorption of Carbon Nanotubes (CNTs) 10

2.1.3 Applications of Carbon Nanotubes 13

2.2 Graphene oxide (GO) 15

2.2.1 Structure of Graphene oxide (GO) 15

2.2.2 Properties adsorption of Graphene oxide (GO) 19

2.2.3 Applications of Graphene oxide 21

2.3 Atomic Absorption Spectrometric machine (AAS) 23

3 MATERIALS AND METHODOLOGY 26

3.1 MATERIALS 26

3.1.1 Carbon nanotubes (CNTs) 26

3.1.2 Graphene oxide (GO) 27

3.1.3 Solution Cu2+ 29

3.2 METHODOLOGY 30

3.2.1 Carbon nanotubes (CNTs) 30

3.2.2 Graphene oxide (GO) 30

4 RESULTS AND DISCUSSION 32

4.1 RESULTS 32

4.1.1 Results of solution Cu2+ 32

4.1.2 Carbon nanotubes (CNTs) 33

4.1.3 Graphene oxide (GO) 36

4.2 Discussion 39

4.2.1 Carbon nanotubes (CNTs) 39

4.2.2 Graphene oxide (GO) 40

5 CONCLUSION 42

REFERENCES 43

LIST OF ABBREVIATIONS

DSC Differential scanning calorimetry

F-AAS Flame atomic absorption spectrometry

LIST OF TABLES

Table 3.1 Stock solution……… ……… 30

Table 4.1 The volume of the stock solution based on the concentration of the final

volume………32

Table 4.2 Concentration before add CNTs and GO……….……….…33

Table 4.3 The concentration after use Atomic Absorption Spectrometric of CNTs… …34

Table 4.4 The adsorption capacity q (mg/g CNTs) of CNTs ……….……… 35

Table 4.5 The concentration after use Atomic Absorption Spectrometric of GO … …36

Table 4.6 The adsorption capacity q (mg/g CNTs) of GO… ……….….…….38

LIST OF FIGURES

Figure 2.1 The (n,m) nanotube naming scheme can be thought of as a vector (Ch) in an infinite graphene sheet that describes how to "roll up" the graphene sheet to make the nanotube T denotes the tube axis, and a1 and a2 are the unit vectors of graphene in real

space ……… ………8

Figure 2.2 Structure of the GO……….……….19

Figure 2.3 Diagram systems AAS Atomic Absorption……….24

Figure 3.1 Heat to appropriate temperature (35oc) for 10 hour……….27

Figure 3.2 Rotary – Vacuum – Evaporate……….………28

Figure 4.1 Concentration adsorption of CNTs……… ……….…… 34

Figure 4.2 Adsorption capacity of CNTs.……… ……… …… 36

Figure 4.3 Concentration adsorption of GO……… ….……37

Figure 4.4 Adsorption capacity of GO……… ……….38

ABSTRACT

Thai Nguyen University of Agriculture and Forestry

graphene oxides

Assoc.Prof Dam Xuan Van Abstract:

Copper is an element with atomic number of 29 and is heavy metal ion in the water which is not only harmful to the environment but also human health The study using carbon nanotubes and graphene oxide nanomaterial to absorb copper oxide ion in water is absolutely necessary Prepare a quantity carbon nanotubes and graphene oxide required like in the calculation and clean them with as required of the experiment (pH, drying, and ensure it is unique)

Prepare a solution containing Cu2+ by means of synthetic compounds Cu(NO3)2 + 2 H2O in

5 tubes test at concentrations of was calculated from the previous

To absorb Cu2+, we used direct absorption from the nanomaterials already used for solution into 5 tubes test containing Cu2+ ion solution with different concentrations Then, apply the appropriate conditions for the best absorption After the appropriate time 2 hour, 4 hour, 6

hour, we will absorb the sample to measure the concentration of Cu2+ in solution by Atomic Absorption Spectrometric machine The last result of the absorption of the formula we use to calculate the adsorption capacity of the carbon nanotubes and graphene oxide Concluded: volume, solution concentration, time, conditions similar in laboratory, we identified graphene oxide Cu2+ ion absorption better than carbon nanotubes

Keywords: Absorb, Atomic Absorption Spectrometric, Carbon nanotubes, Graphene oxide, Copper(II)

Date of Submision :

1 INTRODUCTION

1.1 Rationale of study

The 21st century is the reign of nanotechnology Nanotechnology has brought to the world many astonishing applications for life Scientists, industrialists and manufacturers always pay attention to every detail of the development of this technology, also the nature, characteristics and applications of it The new properties of nanotechnology are results of the reducing in the size of materials to nanometers; this changes the mechanism

of quantum interference People often call this phenomenon is the size effect or the confinement effect As a result, nano-materials are used a lot in practice, becoming super hard, super durable, and superconductivity products

Heavy metals in water have been a major preoccupation for many years because of their toxicity towards aquatic-life, human beings and the environment As they do not degrade biologically like organic pollutants, their presence in drinking water or industrial effluents

is a public health problem due to their absorption and therefore possible accumulation in organ-isms Several processes have been used and developed over the years to remove metal ions, such as chemical precipitation, reverse osmosis, electrolytic recovery, ion exchange or adsorption The latter has been studied for both mineral and organic materials

(Bailey et al, 1999; Ricordel et al, 2001) Moreover, one of the important properties of

solid matrices explored is related to the adsorption of trace elements taking preconcentration or separation into account, where from a complex mixture, a single

element or a group of elements can be separated and quantitatively determined (Pyrzynska

et al, 1999; Camel et al, 2003) Carbon nanotubes (CNTs), a member in carbon family,

are relatively new adsorbents that have been proven to possess great potential for

removing many kinds of pollutants such as chlorobenzenes (Peng et al, 2003), herbicides

as well as lead and cadmium ions (Li et al,2003 ) The hexagonal arrays of carbon atoms

in graphite sheets of CNTs surface have strong interactions with other molecules or atoms The study of adsorption properties of carbon nanotubes is important in both fundamental and practical point of view The studies on the adsorption of heavy metals with CNTs presented in the literature are limited to a few examples, thus, in this work, the analytical potential of CNTs as an adsorbent was examined for wider range of metal ions The effect of solution conditions such as pH value and metal ion concentration on the adsorption behavior was investigated Moreover, the kind of preliminary treatment processing of CNTs resulting the nanotube surface status was checked as it could significantly influence their adsorption efficiency

Graphene oxide (GO) is a super adsorbent with a great potential for removing cationic heavy-metal contaminants from water The adsorption capacities of graphene oxide for lead (Pb2+), cobalt (Co2+), copper (Cu2+), and cadmium (Cd2+) are over an order of magnitude greater than the adsorption capacities of conventional adsorbents such as activated carbon and iron-oxide nanoparticles Even for uranium (UO22+), which is bulky and subject to complexation with hydroxide and carbonate anions in aqueous solution, graphene oxide has exhibited an adsorption capacity surpassing any known natural and

engineering materials (Willner,2012) Graphene oxide is a two-dimensional nanomaterial

(i.e., material with size in one dimension being negligible compared to the sizes in the other two dimensions) that can be readily produced by exfoliating naturally occurring

graphite under oxidation (Li et al, 2008)

1.2 Aim of the study

The pollution of the environment in general and water pollution in particular becomes more and more serious in Vietnam Accessing to the mass media every day, we can easily come across the image and information of water pollution Hence, the development of technology in water treatment is increasingly urgent Also, research of using the compounds of carbon (nano carbontubes and graphene oxide) to remove metal ions and water pollutants is absolutely necessary The study mentions using carbon nanotubes and graphene oxide to absorb ion copper (Cu2+) that contaminates water sources, simultaneously comparing efficiency of those compounds to find out which one has better ion copper absorbance

1.3 Research questions

What are the efficiency of carbon nanotubes and graphene oxide in absorption of ion copper? And which one is better in comparison?

1.4 Scope of the study

Due to experiment using nanomaterial with high cost and need to used computer for checking, so the experiment was take place in the laboratory

2 LITERATURE REVIEW

2.1 Carbon nanotube (CNTs)

2.1.1 Structure of Carbon nanotubes

The carbon nanotubes (English: Carbon nanotubes - CNTs) are allotropes of carbon forms A single-layer carbon nanotube is a graphite sheet thickness-a-cause death rolled into a cylindrical instant, with a nanometer in diameter This happened in the nano-structure in which the ratio between length and diameter in excess of 10,000 These cylindrical carbon molecules have interesting properties that make them potentially useful

in many applications of nanotechnology, industrial electronics, optics, and some other school They show incredible strength and unique electrical properties, and thermal conductivity effect Inorganic nanotubes have also been synthesized

Nanotubes are a fullerene structure, which also includes buckyballs While the spherical buckyballs, nanotubes have a cylindrical shape, with at least one head is covered by a hemispherical buckyball structure Their names were placed in their shape, the diameter

of the nanotube due to the size of a few nanometers (approximately 50,000 times smaller than a human hair), while their length can be up to several millimeters Researchers at the University of Cincinnati (UC) have developed a process to build networks of carbon nanotubes aligned extremely long They were able to produce carbon nanotubes 18mm long and can fold into carbon nanofibers There are two main types of nanotubes: single-layer nanotubes (SWNT) and multi-layer nanotubes (MWNT) The nature of the link in the carbon nanotube is explained by quantum chemistry, namely the orbital overlap

Chemical bonding of nanotubes is composed entirely by linking sp2, similar to graphite This link structure, stronger links in the sp3 diamond creates molecules with exceptional durability The nanotubes are typically self-organized into "ropes" held together by Van der Waals forces Under high pressure, nanotubes can merge together, exchange some links sp2 to sp3 link, creating the ability to produce strong rope, the length is not limited

by the pressure nanotube linking high

Single-walled

Most single-walled nanotubes (SWNTs) have a diameter of close to 1 nanometer, with a tube length that can be many millions of times longer The structure of a SWNT can be conceptualized by wrapping a one-atom-thick layer of graphite called graphene into a seamless cylinder The way the graphene sheet is wrapped is represented by a pair of indices (n, m) The integer’s n and m denote the number of unit vectors along two directions in the honeycomb crystal lattice of graphene If m = 0, the nanotubes are called zigzag nanotubes, and if n = m, the nanotubes are called armchair nanotubes Otherwise, they are called chiral The diameter of an ideal nanotube can be calculated from its (n,m) indices as follows

where a=0.246nm

SWNTs are an important variety of carbon nanotube because most of their properties change significantly with the (n,m) values, and this dependence is non-monotonic (see

Kataura plot) In particular, their band gap can vary from zero to about 2 eV and their electrical conductivity can show metallic or semiconducting behavior Single-walled nanotubes are likely candidates for miniaturizing electronics The most basic building block of these systems is the electric wire, and SWNTs with diameters of an order of a nanometer can be excellent conductors One useful application of SWNTs is in the development of the first intermolecular field-effect transistors (FET) The first

intermolecular logic gate using SWCNT FETs was made in 2001(Dalton, 2013) A logic

gate requires both a p-FET and an n-FET Because SWNTs are p-FETs when exposed to oxygen and n-FETs otherwise, it is possible to protect half of an SWNT from oxygen exposure, while exposing the other half to oxygen This results in a single SWNT that acts

as a not logic gate with both p and n-type FETs within the same molecule

Figure 2.1 The (n,m) nanotube naming scheme can be thought of as a vector (Ch) in an infinite graphene sheet that describes how to "roll up" the graphene sheet to make the nanotube T denotes the tube axis, and a1 and a2 are the unit vectors of graphene in real space

Multi-walled

Multi-walled nanotubes (MWNTs) consist of multiple rolled layers (concentric tubes) of graphene There are two models that can be used to describe the structures of multi-walled nanotubes In the Russian Doll model, sheets of graphite are arranged in concentric cylinders, a (0,8) single-walled nanotube (SWNT) within a larger (0,17) single-walled nanotube In the Parchment model, a single sheet of graphite is rolled in around itself, resembling a scroll of parchment or a rolled newspaper The interlayer distance in multi-walled nanotubes is close to the distance between graphene layers in graphite, approximately 3.4 Å The Russian Doll structure is observed more commonly Its individual shells can be described as SWNTs, which can be metallic or semiconducting Because of statistical probability and restrictions on the relative diameters of the individual tubes, one of the shells, and thus the whole MWNT, is usually

a zero-gap metal Double-walled carbon nanotubes (DWNTs) form a special class of nanotubes because their morphology and properties are similar to those of SWNTs but their resistance to chemicals is significantly improved This is especially important when functionalization is required (this means grafting of chemical functions at the surface of the nanotubes) to add new properties to the CNT In the case of SWNTs, covalent functionalization will break some C=C double bonds, leaving "holes" in the structure on the nanotube and, thus, modifying both its mechanical and electrical properties In the case of DWNTs, only the outer wall is modified DWNT synthesis on the gram-scale was first proposed in 2003 by the CCVD technique, from the selective reduction of oxide solutions in methane and hydrogen

The telescopic motion ability of inner shells and their unique mechanical properties will permit the use of multi-walled nanotubes as main movable arms in coming nanomechanical devices Retraction force that occurs to telescopic motion caused by the

Lennard-Jones interaction between shells and its value is about 1.5 nN (Zavalniuk, 2011)

2.1.2 Properties adsorption of Carbon Nanotubes (CNTs)

The properties of MWNTs are generally similar to those of regular polyaromatic solids (which may exhibit graphitic, turbostratic or intermediate crystallographic structure) Variations are mainly due to different textural types of the MWNTs considered (concentric, herringbone, bamboo) and the quality of the nanotexture, both of which control the extent of anisotropy Actually, for polyaromatic solids that consist of stacked graphenes, the bond strength varies significantly depending on whether the in-plane direction is considered (characterized by very strong covalent and therefore very short 0.142 nm bonds) or the direction perpendicular to it (characterized by very weak van der Waals and therefore very loose ≈ 0.34 nm bonds) Such heterogeneity is not found in single (isolated) SWNTs However, the heterogeneity returns, along with the related consequences, when SWNT sassociate into bundles Therefore, the properties, and applicability of SWNT s may also change dramatically depending on whether single SWNT or SWNT ropes are involved An important problem to solve when considering adsorption onto nanotubes is to identify the adsorption sites The adsorption of gases into

a SWNT bundle can occur inside the tubes (internal sites), in the interstitial triangular channels between the tubes, on the outer surface of the bundle (external sites), or in the

grooves formed at the contacts between adjacent tubes on the outside of the bundle Experimental adsorption studies on CNTs have confirmed the adsorption on internal, external and groove sites Modeling studies have pointed out that the convex surface of the CNTs is more reactive than the concave one and that this difference in reactivity increases as the tube diameter decreases Compared to the highly bent region in fullerenes, CNTs are only moderately curved and are expected to be much less reactive towards dissociative chemisorption Models have also predicted enhanced reactivity at the kink sites of bent CNTs Additionally, it is worth noting that unavoidable imperfections, such as vacancies, Stone–Wales defects, pentagons, heptagons and dopants, are believed

to play a role in tailoring the adsorption properties (Lu et al, 2005) Considering

closed-end SWNTs first, simple molecules can be adsorbed onto the walls of the outer nanotubes

of the bundle and preferably on the external grooves In the first stages of adsorption (corresponding to the most attractive sites for adsorption), it seems that adsorption or condensation in the interstitial channels of the SWNT bundles depends on the size of the molecule (and/or on the SWNT diameters) and on their interaction energies Opening the tubes favors gas adsorption (including O2 ,N2 within the inner walls (Fujiwara et al,

2001;Yang et al, 2002) It was found that the adsorption of nitrogen on open-ended

SWNT bundles is three times larger than that on closed-ended SWNT bundles The significant influence that the external surface area of the nanotube bundle has on the character of the surface adsorption isotherm of nitrogen (type I, II or even IV of the

IUPAC classification) has been demonstrated from theoretical calculations (Jiang et al;

2003) Additionally, it has been shown that the analysis of theoretical adsorption

isotherms, determined from a simple model based on the formalism of Langmuir and Fowler, can help to experimentally determined the ratio of open to closed SWNTs in a sample For hydrogen and other small molecules like CO, computational methods have shown that, for open SWNTs, the pore, interstitial and groove sites are energetically more

favorable than surface sites (Zhao et al, 2002) In the case of carbon monoxide, aside

from physisorbed CO, CO hydrogen bonds to hydroxyl functionalities created on the

SWNTs by acid purification have been identified (Matranga et al, 2005) FTIR and

temperature-programmed desorption (TPD) experiments have shown that NH3 or NO2adsorb molecularly and that NO2 is slightly more strongly bound than NH3 For NO2, the formation of nitrito (O-bonded) complexes is preferred to nitro (N-bonded) ones For ozone, a strong oxidizing agent, theoretical calculations have shown that physisorption occurs on ideal, defect-free SWNT, whereas strong chemisorption occurs on Stone–Wales defects, highlighting the key role of defective sites in adsorption properties

(Picozzi et al, 2004) For MWNTs, adsorption can occur in the aggregated pores, inside

the tube or on the external walls In the latter case, the presence of defects, as incomplete graphene layers, must be taken into consideration Although adsorption between the graphenes (intercalation) has been proposed in the case of hydrogen adsorption in h-

MWNTs or platelet nanofibers (Chambers et al, 1998), it is unlikely to occur for many

molecules due to steric effects and should not prevail for small molecules due to the long diffusion paths involved In the case of inorganic fluorides (BF3, TiF4, NbF5 and WF6 ), accommodation of the fluorinated species into the carbon lattice has been shown to result

from intercalation and adsorption/condensation phenomena (Giraudet et al, 2003) Only a

few studies deal with adsorption sites in MWNTs, but it has been shown that butane adsorbs more onto MWNTs with smaller outside diameters, which is consistent with another statement that the strain on curved graphene surfaces affects sorption Most of the butane adsorbs to the external surface of the MWNTs while only a small fraction of the

gas condenses in the pores (Hilding et al, 2001) Comparative adsorption of krypton or of

ethylene onto MWNTs or onto graphite has allowed scientists to determine the dependence of the adsorption and wetting properties of the nanotubes on their specific morphologies Nanotubes were found to have higher condensation pressures and lower

heats of adsorption than graphite (Masenelli-Varlot et al, 2002) These differences are

mainly due to decreased lateral interactions between the adsorbed molecules, related to the curvature of the graphene sheets

2.1.3 Applications of Carbon Nanotubes

Near - Field Microscope Probes

The high mechanical strength of carbon nanotubes makes them almost ideal candidates for use as force sensors in scanning probe microscopy (SPM) They provide higher durability and the ability to image surfaceswith a high lateral resolution, the latter being a typicallimitation of conventional force sensors (based on ceramic tips) The idea was first

proposed and tested by Dai et al (Dai et al, 1996) using c-MWNTs It was extended to SWNTs by Hafner et al (Cassel et al, 1999), since small-diameter SWNTs were believed

to give higher resolution than MWNTs due to the extremely short radius of curvature of the tube end However, commercial nanotube-based tips (such as those made by

Piezomax, Middleton, WI, USA) use MWNTs for processing convenience It is also likely that the flexural modulus of a SWNT is too low, resulting in artifacts that affect the lateral resolution when scanning a rough surface On the other hand, the flexural modulus

of a c-MWNT is believed to increase with the number of walls, although the radius of curvature of the tip increases at the same time Whether based on SWNT or MWNT ,such SPM tips also offer the potential to be functionalized, leading to the prospect of selective imaging based on chemical discrimination in chemical force microscopy (CFM) Chemical function imaging using functionalized nanotubes represents a huge step forward

in CFM because the tip can be functionalized very specifically (ideally only at the very tip

of the nanotube, where the reactivity is the highest), increasing the spatial resolution The interaction between the chemical species present at the end of the nanotube tip and the surface containing chemical functions can be recorded with great sensitivity, allowing the

chemical mapping of molecules (Kitiyanan et al, 2000)

Field Emission-Based Devices

In a pioneering work by de Heer et al (Heer et al,1995), carbon nanotubes were shown to

be efficient field emitters and this property is currently being used several applications, including flat panel displays for television sets and computers (the first prototype of such

a display was exhibited by Samsung in 1999), and devices requiring an producing cathode, such as X-ray sources Briefly, a potential difference is set up between the emitting tips and an extraction grid so that electrons are pulled from the tips onto an electron-sensitive screen layer Replacing the glass support and protecting the screen

electron-using a polymer-based material should even permit the development of flexible screens Unlike regular (metallic) electron-emitting tips, the structural perfection of carbon nanotubes allows higher electron emission stability, higher mechanical resistance, and longer lifetimes Most importantly, using them saves energy since the tips operate at a lower heating temperature and require much lower threshold voltage than in other setups

2.2 Graphene oxide (GO)

2.2.1 Structure of Graphene oxide (GO)

Graphite oxide, formerly called graphitic oxide or graphitic acid, is a compound

of carbon, oxygen and hydrogen in variable ratios, obtained by treating graphite with strong oxidizers The maximally oxidized bulk product is a yellow solid with C:O ratio between 2.1 and 2.9, that retains the layer structure of graphite but with a much larger and irregular spacing Graphene, is a thin layer of pure carbon; it is a single, tightly packed layer of carbon atoms that are bonded together in a hexagonal honeycomb lattice In more complex terms, it is an allotrope of carbon in the structure of a plane of sp2 bonded atoms with a molecule bond length of 0.142 nanometres Layers of graphene stacked on top of each other form graphite, with an interplanar spacing of 0.335 nanometres

While graphite is a 3 dimensional carbon based material made up of millions of layers of graphene, graphite oxide is a little different By the oxidation of graphite using strong oxidizing agents, oxygenated functionalities are introduced in the graphite structure which not only expand the layer separation, but also makes the material hydrophilic (meaning

that they can be dispersed in water) This property enables the graphite oxide to be exfoliated in water using sonication, ultimately producing single or few layer graphene, known as graphene oxide (GO) The main difference between graphite oxide and graphene oxide is, thus, the number of layers While graphite oxide is a multilayer system

in a graphene oxide dispersion a few layers flakes and monolayer flakes can be found

The structure and properties of graphite oxide depend on particular synthesis method and degree of oxidation It typically preserves the layer structure of the parent graphite, but the layers are buckled and the interlayer spacing is about two times larger (~0.7 nm) than that of graphite Strictly speaking "oxide" is an incorrect but historically established name Besides oxygen epoxide groups (bridging oxygen atoms), other functional groups experimentally found are: carbonyl (C=O), hydroxyl (-OH), phenol, for graphite oxides prepared using sulphuric acid (e.g Hummers method) also some impurity of sulphur is often found, for example in a form of organo sulfate groups There is evidence of

"buckling" (deviation from planarity), folding and cracking of graphene oxide sheets upon deposition of the layers on a choice of substrate The detailed structure is still not

understood due to the strong disorder and irregular packing of the layers (Lei, 2013)

Graphene oxide layers are about 1.1 ± 0.2 nm thick Scanning tunneling microscopy shows the presence of local regions where oxygen atoms are arranged in a rectangular pattern with lattice constant 0.27 nm × 0.41 nm The edges of each layer are terminated with carboxyl and carbonyl groups X-ray photoelectron spectroscopy shows presence of several C1s peaks, their number and relative intensity depending on

particular oxidation method used Assignment of these peaks to certain carbon functionalization types is somewhat uncertain and still under debates For example, one of interpretations goes as following: non-oxygenated ring contexts (284.8 eV), C-O (286.2 eV), C=O (287.8 eV) and O-C=O (289.0 eV) Another interpretation using density functional theory calculation goes as following: C=C with defects such as functional groups and pentagons (283.6 eV), C=C (non-oxygenated ring contexts) (284.3 eV), sp3C-

H in the basal plane and C=C with functional groups (285.0 eV), C=O and C=C with functional groups, C-O (286.5 eV), and O-C=O (288.3 eV)

Graphite oxide is hydrophilic and easily hydrated exposed to water vapor or immersed in liquid water, resulting in a distinct increase of the inter-planar distance (up to 1.2 nm in saturated state) Additional water is also incorporated into interlayer space due to high pressure induced effects Maximal hydration state of graphite oxide in liquid water corresponds to insertion of 2-3 water monolayers, cooling the graphite oxide/H2O samples results in "pseudo-negative thermal expansion" and below freezing point of water media results in de-insertion of one water monolayer and lattice contraction Complete removal

of water from the structure seems difficult since heating at 60–80 °C results in partial decomposition and degradation of the material

Similar to water, graphite oxide also easily incorporates other polar solvents, e.g alcohols However, intercalation of polar solvents occurs significantly different in Brodie and Hummers graphite oxides Brodie graphite oxide is intercalated at ambient conditions with by one monolayer of alcohols and several other solvents (e.g dimethylformamide

and acetone) when liquid solvent is available in excess Separation of graphite oxide layers is proportional to the size of alcohol molecule Cooling of Brodie graphite oxide immersed in excess of liquid methanol, ethanol, acetone and dimethylformamide results

in step-like insertion of additional solvent monolayer and lattice expansion The phase transition detected by X-ray diffraction and DSC is reversible; de-insertion of solvent monolayer is observed when sample is heated back from low temperatures Additional methanol and ethanol monolayer is reversibly inserted into the structure of Brodie graphite oxide also at high pressure conditions

Hummers graphite oxide is intercalated with two methanol or ethanol monolayers already

at ambient temperature The interlayer distance of Hummers graphite oxide in excess of liquid alcohols increases gradually upon temperature decrease, reaching 19.4 and 20.6 Å

at 140 K for methanol and ethanol, respectively The gradual expansion of the Hummers graphite oxide lattice upon cooling corresponds to insertion of at least two additional solvent monolayers

Graphite oxide exfoliates and decomposes when rapidly heated at moderately high temperatures (~280–300 °C) with formation of finely dispersed amorphous carbon,

somewhat similar to activated carbon (Talyzin et al, 2009; You, 2013)

Figure 2.2 Structure of the GO

In the report “Metal removal by sodium graphene oxide”, Marjorie R Willner stated that: The adsorption capacity of GO for Pb2+ ions was investigated from pH 2 to pH 8 The lead and GO loading varied across experiments but were kept between 5-100 ppm and around 0.1 g L-1 respectively The concentrations were selected for ease of experimental and analytical work with the main limitations being the filtering of the GO and the detection limit of the ICP-OES Moreover, the pursuit of a maximum adsorption capacity made the initial loading of the individual experiments irrelevant to the big picture

(Willner, 2012)

2.2.2 Properties adsorption of Graphene oxide (GO)

One of the advantages of the gaphene oxide is its easy dispersability in water and other organic solvents, as well as in different matrixes, due to the presence of the oxygen functionalities This remains as a very important property when mixing the material with ceramic or polymer matrixes when trying to improve their electrical and mechanical properties These tests showed that graphene sheets (with thicknesses of between 2 and 8 Nm) had spring constants in the region of 1-5 N/m and a Young’s modulus (different to

that of three-dimensional graphite) of 0.5 TPa Again, these superlative figures are based

on theoretical prospects using graphene that is unflawed containing no imperfections whatsoever and currently very expensive and difficult to artificially reproduce, though production techniques are steadily improving, ultimately reducing costs and complexity

On the other hand, in terms of electrical conductivity, graphene oxide is often described

as an electrical insulator, due to the disruption of its sp2 bonding networks (Gusynin and

Sharapov, 2005) In order to recover the honeycomb hexagonal lattice, and with it the

electrical conductivity, the reduction of the graphene oxide has to be achieved It has to be taken into account that once most of the oxygen groups are removed, the reduced graphene oxide obtained is more difficult to disperse due to its tendency to create

aggregates (Peres et al, 2006; Miller et al, 2009)

Functionalization of graphene oxide can fundamentally change graphene oxide’s properties The resulting chemically modified graphenes could then potentially become

much more adaptable for a lot of applications (Apalkov et al, 2006; Miller et al, 2009)

There are many ways in which graphene oxide can be functionalized, depending on the desired application For optoelectronics, biodevices or as a drug-delivery material, for example, it is possible to substitute amines for the organic covalent functionalization of graphene to increase the dispersability of chemically modified graphenes in organic

solvents (Toke and Jain, 2007) It has also been proved that porphyrin-functionalized

primary amines and fullerene-functionalized secondary amines could be attached to