Graphene metal organic framework composites and their potential applications 2

Chapter 2: Experimental Methods Introduction This chapter presents the principles of the analytical techniques used in the characterization of the physical and chemical properties of graphene and graphene-based hybrid material. The principles of materials characterization techniques using Fourier-transform Infrared spectroscopy (FTIR), UV-Vis absorbance spectroscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) are briefly introduced here. Techniques for checking applications of the composites such as cyclic voltammetry (CV), linear sweep voltammetry (LSV), rotating disk electrode (RDE), rotating ring disk electrode (RRDE), GC/MS, BET surface area are also discussed. 2.1 Materials Characterization 2.1.1 UV-Vis spectroscopy An ultraviolet-visible (UV-Vis) Spectrophotometer is used to determine the absorption or transmission of UV-Vis light (180 to 820 nm) by a sample.1 In this region of electromagnetic spectrum, molecules undergo electronic transitions. UV-Vis spectrophotometer is used for quantitative concentration measurement of absorbing materials based on the calibration curves of the material. The concentration of an analyte in solution can be determined by measuring the absorbance at a particular wavelength and applying Beer-Lambert’s law: A=log10 (I0/I) =. c. L where A is the measured absorbance, I0 is the intensity of the incident light at a given wavelength, I is the transmitted intensity, L the pathlength through the sample, and c the concentration of the absorbing species. For each species and wavelength, ε is a constant known 27 as the molar absorptivity or extinction coefficient. This constant is a fundamental molecular property in a given solvent, at a particular temperature and pressure. The Beer-Lambert Law is useful for characterizing many compounds but does not hold as a universal relationship for the concentration and absorption of all substances. When white light passes through or is reflected by a colored substance, a characteristic portion of the mixed wavelengths is absorbed. The remaining light will then assume the complementary color to the wavelength(s) absorbed. This relationship is demonstrated by the color wheel shown on the right. Here, complementary colors are diametrically opposite each other. A common feature of all these colored compounds, displayed below, is a system of extensively conjugated pi-electrons (Figure 2.1).  Violet: 400 - 420 nm  Indigo: 420 - 440 nm  Blue: 440 - 490 nm  Green: 490 - 570 nm  Yellow: 570 - 585 nm  Orange: 585 - 620 nm  Red: 620 - 780 nm Figure 2.1 the color wheel spectrum covered UV- vis –IR region Absorption of ultraviolet and visible radiation in organic molecules is restricted to certain functional groups (chromophores) that contain valence electrons of low excitation energy. The spectrum of a molecule containing these chromophores is complex. This is because the superposition of rotational and vibrational transitions on the electronic transitions gives a 28 combination of overlapping lines. This appears as a continuous absorption band. Possible electronic transitions of p, s, and n electrons are as shown5 in the Figure 2.2. Figure 2.2 electronic transitions of species containing p, s, and n electrons.5 Image reproduced from reference 5. 2.1.2 Fourier-transform Infra-red Spectroscopy (FTIR) The mid infra-red (IR) spectral range (2.5-25 μm) is the most accessible and the richest in providing structural information. The absorption bands in this frequency domain form a molecular fingerprint, thereby allowing the detection of compounds and the deduction of structural details.6 This is important for this thesis work to confirm the success of functionalization of graphene-based materials. The initial IR instrument is based on dispersive spectrometers that functions in a sequential mode. Subsequently, Fourier – transform infra-red (FTIR) spectrophotometer emerged to overcome the limitations encountered with dispersive instruments. FTIR is capable of simultaneous analysis of the full spectral range using interferometry, the interferometer produces a unique type of signal which has all of the infrared frequencies “encoded” into it. Interferometers employ a beam splitter which takes the incoming infrared beam and divides it into two optical beams. One beam reflects off of a flat mirror which is fixed in place. The other beam reflects off of a flat mirror which allows this mirror to move a 29 very short distance (~ few mm) away from the beam splitter.7 The two beams reflect off of their respective mirrors and are recombined when they meet back at the beam splitter (Figure 2.4). As the path of one beam is fixed and the other is constantly changing as its mirror moves, the signal which exits the interferometer is the result of these two beams “interfering” with each other. The resulting signal is called an interferogram which has information about every infrared frequency which comes from the source. As the measured interferogram signal cannot be interpreted directly, Fourier transformation is performed by the computer which presents the spectral information in a plot of absorbance (or transmission) versus the wave number (Figure 2.3). Figure 2.3 Schematic of processing of interferogram using Fourier-Transform (FFT) calculations to produce an IR spectrum.8 Image reproduced from reference 8. For the work presented in this thesis, FTIR samples were prepared by KBr pellet. The samples is mixed with KBr powder and pressed into a pellet using a 13mm die set with a force of 10-tonne exerted by a bench top hydraulic press. 30 Figure 2.4 Schematic diagram of an interferometer, configured for FTIR.9 Image reproduced from reference 9. 2.1.3 Atomic Force Microscopy (AFM) The AFM is a high-resolution type of scanning probe microscope, with a resolution of less than a nanometer. The image is gathered by scanning the sample surface with a sharp probe at the end of a micro-scale cantilever.10 Atomic resolution can be obtained by reducing the contact force to ~10 − N. This is less than most interatomic forces, limiting tip induced sample deformation and contact area, which allows the imaging of single atoms. Estimating the ionic bond energy ≤10 eV, a van der Waals bonding energy of ≤10 meV, and a repulsive force acting of a distance of Δ ≈ 0.2 Å. Figure 2.5 shows a schematic view of AMF device. When the tip is brought into sample surface, repulsion forces between the tip and the atomic shells of the sample lead to a deflection of the cantilever providing a true 3D surface profile. Samples viewed by AFM not require any special treatments that would irreversibly change or damage the sample and can work perfectly well without the need for vacuum. 31 Figure 2.5 Schematic of AFM.11 Image reproduced from reference 11. 2.1.4 Scanning Electron Microscopy (SEM) Scanning electron microscope (SEM) uses electron beam for imaging. 12 The schematic diagram of SEM instrumentation is shown in Figure 2.6. A beam of electrons is produced by heating of a metallic filament. The electron passes through the electromagnetic lenses which focus and direct the beam down towards the sample. As it hits the sample, photons and electrons are ejected from the sample. Detectors collect the secondary or backscattered electrons, and convert them to a signal. The most common detection mode, secondary electron imaging (SEI), can produce very high-resolution images of a sample surface in this thesis. SEM micrographs have a large depth of field because of the very narrow electron beam thereby has the capability to characterize three-dimensional structure of a sample. In this thesis for samples that are nonconductive, sputtering them with platinum improves the resolution of the image. 32 Figure 2.6 Schematic diagram of SEM instrumentation.12 Image reproduced from reference12. 2.1.5 Transmission electron microscopy (TEM) The first transmission electron microscopy (TEM) was built by Max Knoll and Ernst Ruska in 1931.13 While SEM imaging is due to the secondary or backscattered electrons, TEM imaging is based on the transmitted electrons that interact with the sample as it passes through. TEM is a microscopy technique whereby a beam of electrons is transmitted through an ultra-thin specimen, interacting with the specimen as it passes through. An image is formed from the interaction of the electrons transmitted through the specimen; the image is magnified and focused onto an imaging device, such as a fluorescent screen, on a layer of photographic film, or to be detected by a sensor such as a CCD camera (Figure 2.7). TEMs are capable of imaging at a significantly higher resolution than light microscopes, owing to the small de Broglie wavelength of electrons. 14 This enables the instrument's user to examine fine detail even as small as a single column of atoms, which is tens of thousands times 33 smaller than the smallest resolvable object in a light microscope. TEM forms a major analysis method in a range of scientific fields, in both physical and biological sciences. TEMs find application in cancer research, virology, materials science as well as pollution, nanotechnology, and semiconductor research.15 At smaller magnifications TEM image contrast is due to absorption of electrons in the material, due to the thickness and composition of the material. At higher magnifications complex wave interactions modulate the intensity of the image, requiring expert analysis of observed images. Alternate modes of use allow for the TEM to observe modulations in chemical identity, crystal orientation, electronic structure and sample induced electron phase shift as well as the regular absorption based imaging. Figure 2.7 TEM Image of our COOH Functionalized Nanotubes COOH-MWNTS.16,17 Image reproduced from reference 16,17. 34 2.1.6 Raman Spectroscopy Raman spectroscopy is a spectroscopic technique based on inelastic scattering of monochromatic light from a laser source. 18 It is a spectroscopic technique used to observe vibrational, rotational, and other low-frequency modes in a system. It relies on inelastic scattering, or Raman scattering, of monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range. 19 The laser light interacts with molecular vibrations, phonons or other excitations in the system, resulting in the energy of the laser photons being shifted up or down. The shift in energy gives information about the vibrational modes in the system. The Raman effect occurs when light impinges upon a molecule and interacts with the electron cloud and the bonds of that molecule. 20 For the spontaneous Raman Effect, which is a form of light scattering, a photon excites the molecule from the ground state to a virtual energy state. When the molecule relaxes it emits a photon and it returns to a different rotational or vibrational state. The difference in energy between the original state and this new state leads to a shift in the emitted photon's frequency away from the excitation wavelength ( Figure 2.8). The Raman Effect, which is a light scattering phenomenon, should not be confused with absorption (as with fluorescence) where the molecule is excited to a discrete (not virtual) energy level. Figure 2.8 Energy level diagram showing the states involved in Raman signal. 35 2.1.7 X-ray Photoelectron Spectroscopy (XPS) X-ray photoelectron spectroscopy (XPS) is a quantitative spectroscopic technique 21 that measures the elemental composition, empirical formula, chemical state and electronic state of the elements that exist within a material. 22 XPS spectra are obtained by irradiating a material with a beam of X-rays while simultaneously measuring the kinetic energy and number of electrons that escape from the top to 10 nm of the material being analyzed. XPS requires ultra-high vacuum (UHV) conditions. XPS is one of the most versatile and generally applicable surface spectroscopic techniques used for a myriad of application, from catalyst characterization to fundamental physics of adsorbate ionization. XPS measures the elemental composition, empirical formula, chemical state and electronic state of the elements of a material. To obtain XPS spectra, the sample/material is irradiated with a beam of X-rays while simultaneously measuring the kinetic energy and number of electrons that escape from the material being analyzed. Figure 2.9 presents the schematic diagram of X-ray photoemission process. Figure 2.9 Schematic drawing of the X-ray photoemission process of core-level electrons 36 Figure 2.12 linear sweep voltammogram The voltage scan rate (v) is calculated from the slope of the line. Clearly by changing the time taken to sweep the range we alter the scan rate. The characteristics of the linear sweep voltammogram recorded depend on a number of factors including:  The rate of the electron transfer reaction(s)  The chemical reactivity of the electroactive species  The voltage scan rate In LSV measurements the current response is plotted as a function of voltage rather than time, unlike potential step measurements. For example in the Fe3+/Fe2+ system; The scan begins from the left hand side of the current/voltage plot where no current flows. As the voltage is swept further to the right (to more reductive values) a current begins to flow and eventually reaches a peak before dropping. To rationalise this behaviour we need to consider the influence of voltage on the equilibrium established at the electrode surface. If we consider the electrochemical reduction of Fe3+ to Fe2+, the rate of electron transfer is fast in 39 comparison to the voltage sweep rate. Therefore at the electrode surface equilibrium is established identical to that predicted by thermodynamics. We may consider from equilibrium electrochemistry that the Nernst equation: The relationship between concentration and voltage (potential difference): where E is the applied potential difference and Eo is the standard electrode potential. So as the voltage is swept from V1 to V2 the equilibrium position shifts from no conversion at V1 to full conversion at V2 of the reactant at the electrode surface. The exact form of the voltammogram can be rationalised by considering the voltage and mass transport effects. As the voltage is initially swept from V1 the equilibrium at the surface begins to alter and the current begins to flow: The current rises as the voltage is swept further from its initial value as the equilibrium position is shifted further to the right hand side, thus converting more reactant. The peak occurs, 40 since at some point the diffusion layer has grown sufficiently above the electrode so that the flux of reactant to the electrode is not fast enough to satisfy that required by the Nernst equation. In this situation the current begins to drop just as it did in the potential step measurements. In fact the drop in current follows the same behaviour as that predicted by the Cottrell equation. The voltammogram was recorded at a single scan rate. If the scan rate is altered the current response also changes. The Figure 2.13 shows a series of linear sweep voltammograms recorded at different scan rates for an electrolyte solution containing only Fe3+. Figure 2.13 a series of linear sweep voltammograms recorded at different scan rates 2.2.2 Cyclic Voltammetry Cyclic voltammetry (CV) is very similar to LSV. In this case the voltage is swept between two values (Figure 2.14, left) at a fixed rate, however when the voltage reaches V2 the scan is reversed and the voltage is swept back to V1 .27 41 Figure 2.14 (left) the voltage swept between two values at a fixed rate for CV. (right) A typical cyclic voltammogram A typical cyclic voltammogram recorded for a reversible single electrode transfer reaction is shown in Figure 2.14, right. The forward sweep produces an identical response to that seen for the LSV experiment. When the scan is reversed we simply move back through the equilibrium positions gradually converting electrolysis product (Fe2+ back to reactant (Fe3+). The current flow is now from the solution species back to the electrode and so occurs in the opposite sense to the forward seep but otherwise the behaviour can be explained in an identical manner. For a reversible electrochemical reaction the CV recorded has certain well defined characteristics. I) The voltage separation between the current peaks is 42 II) The positions of peak voltage not alter as a function of voltage scan rate III) The ratio of the peak currents is equal to one IV) The peak currents are proportional to the square root of the scan rate Figure 2.15 Cyclic voltammogram at different scan rates As with LSV the influence of scan rate is explained for a reversible electron transfer reaction in terms of the diffusion layer thickness. The CV for cases where the electron transfer is not reversible show considerably different behaviour from their reversible counterparts. The Figure 2.16 shows the voltammogram for a quasi-reversible reaction for different values of the reduction and oxidation rate constants. 43 Figure 2.16 cyclic voltammogram for different values of the reduction and oxidation rate constants The first curve shows where both the oxidation and reduction rate constants are still fast, however, as the rate constants are lowered the curves shift to more reductive potentials. Again this may be rationalised in terms of the equilibrium at the surface is no longer establishing so rapidly. In these cases the peak separation is no longer fixed but varies as a function of the scan rate. Similarly the peak current no longer varies as a function of the square root of the scan rate. 2.2.3 Rotating disc electrode (RDE) The RDE was the first widely-used (solid) hydrodynamic electrode and is still the most popular.28 A disc electrode is set in an insulating rod, which is rotated at a constant frequency in solution. The solution drag on the rotating surface results in a vortex as shown in Figure 2.17. The electrode and rod need to have perfect cylindrical symmetry so that the electrode does not wobble on its axis when it rotates. This does not present technical complications for electrodes with radii of several millimetres but would be a serious problem when fabricating rotating disc 44 microelectrodes. This means that the most practical RDEs have radii of a few millimetres. Moreover the rotation speed is also limited by this constraint - a maximum practical rotation speed is about 50Hz. Figure 2.17 Flow profile at a rotating disc electrode.29 Image reproduced from reference 29. One way around this problem would be to fabricate a rotating microring electrode. The diameter of the insulator inside the ring could be made large to reduce eccentricities in the rotation whist the ring itself could be made very thin to achieve high rates of mass transport. Although a microring (analogous to a microband) does not reach a steady-state under diffusiononly conditions, the convection due to rotation would ensure a steady-state response. The first mathematical treatment of convection and diffusion towards a rotating disk electrode was given by Levich. Considering the case where only the oxidized form of a molecule (or ion) of interest is initially present in the electrochemical cell, the cathodic limiting current (iLC) observed at a rotating disk electrode is given by the Levich equation : 30 45 iLC = 0.620 n F A (DO)2/3 ν-1/6 CO ω1/2 In terms of the concentration (CO) of the oxidized form in the solution, the Faraday constant (F = 96485 coulombs per mole), the electrode area (A), the kinematic viscosity of the solution (ν), the diffusion coefficient (DO) of the oxidized form, and the angular rotation rate (ω). Alternatively, when the solution initially contains only the reduced form, the Levich equation for the anodic limiting current (iLA) can be written as iLA = 0.620 n F A (DR)2/3 ν-1/6 CR ω1/2 where the concentration term (CR) and diffusion coefficient (DR) are for the reduced form rather than the oxidized form. 2.2.4 Rotating Ring-Disk Electrode (RRDE) Soon after the rotating disk electrode was developed, the idea of putting a ring electrode around the disk electrode was introduced, and the rotating ring-disk electrode was born.31 In this “ring-disk” geometry, the overall axial flow pattern initially brings molecules and ions to the disk electrode.32 Then, the subsequent outward radial flow carries a fraction of these molecules or ions away from the disk electrode and past the surface of the ring electrode. This flow pattern allows products generated (upstream) by the half reaction at the disk electrode to be detected as they are swept (downstream) past the ring electrode (Figure 2.18). Two of the key parameters which characterize a given ring-disk geometry are the collection efficiency and the transit time. 33 The collection efficiency is the fraction of the material from the disk which subsequently flows past the ring electrode, and can be expressed as 46 a fraction between 0.0 and 1.0 or as a percentage. Typical ring-disk geometries have collection efficiencies between 20% and 30%. The transit time is a more general concept indicating the average time required for material at the disk electrode to travel across the gap between the disk and the ring electrode. Obviously, the transit time is a function of both the gap distance and the rotation rate. Figure 2.18 (a) Rotating Ring-disk device image (b) Rotating ring-disk electrode (c) Rotating Ring-Disk Voltammograms at Various Rotation Rates.34 Image reproduced from reference 34. Once the collection efficiency value has been established empirically for a particular RRDE, it can be treated as a property of that particular RRDE, even if the RRDE is used to study a different half reaction in a different solution on a different day. Although the empirically measured collection efficiency (Nempirical) is a ratio of two currents with opposite mathematical signs (anodic and cathodic), the collection efficiency is always expressed as a positive number. Nempirical = - iLIMITING, RING / iLIMITING, DISK 47 2.2.5 BET surface area BET theory aims to explain the physical adsorption of gas molecules on a solid surface and serves as the basis for an important analysis technique for the measurement of the specific surface area of a material. In 1938,35 Stephen Brunauer, Paul Hugh Emmett, and Edward Teller published an article about the BET theory in a journal[1] for the first time; "BET" consists of the first initials of their family names. The concept of the theory is an extension of the Langmuir theory, which is a theory for monolayer molecular adsorption, to multilayer adsorption with the following hypotheses: (a) gas molecules physically adsorb on a solid in layers infinitely; (b) there is no interaction between each adsorption layer; and (c) the Langmuir theory can be applied to each layer. The resulting BET equation is expressed :36 p and p0 are the equilibrium and the saturation pressure of adsorbates at the temperature of adsorption, v is the adsorbed gas quantity (for example, in volume units), and vm is the monolayer adsorbed gas quantity. c is the BET constant. Considering multilayered gas molecule adsorption, where it is not required for a layer to be completed before an upper layer formation starts. Furthermore, we have five assumptions to calculate BET surface area: 1. Adsorptions occur only on well-defined sites of the sample surface (one per molecule) 2. The only considered molecular interaction is the following one: a molecule can act as a single adsorption site for a molecule of the upper layer. 48 3. The uppermost molecule layer is in equilibrium with the gas phase, i.e. similar molecule adsorption and desorption rates. 4. The desorption is a kinetically-limited process, i.e. a heat of adsorption must be provided: 4.1. these phenomenon are homogeneous, i.e. same heat of adsorption for a given molecule layer. 4.2. it is E1 for the first layer, i.e. the heat of adsorption at the solid sample surface 4.3. the other layers are assumed similar and can be represented as condensed species, i.e. liquid state. Hence, the heat of adsorption is E L is equal to the heat of liquefaction. 5. At the saturation pressure, the molecule layer number tends to infinity (i.e. equivalent to the sample being surrounded by a liquid phase) Figure 2.19 nitrogen adsorption and desorption diagram 2.2.6 Gas chromatography–mass spectrometry (GC-MS) GC-MS is a method that combines the features of gas-liquid chromatography and mass spectrometry to identify different substances within a test sample. 37 Applications of GC-MS include drug detection, fire investigation, environmental analysis, explosives investigation, and identification of unknown samples. GC-MS can also be used in airport security to detect 49 substances in luggage or on human beings. Additionally, it can identify trace elements in materials that were previously thought to have disintegrated beyond identification. GC-MS has been widely heralded as a "gold standard" for forensic substance identification because it is used to perform a specific test. A specific test positively identifies the actual presence of a particular substance in a given sample. A non-specific test merely indicates that a substance falls into a category of substances. Although a non-specific test could statistically suggest the identity of the substance, this could lead to false positive identification. Figure 2.20 (a) GC-MS schematic (b) The insides of the GC-MS, with the column of the gas chromatograph in the oven on the right. 38 Image reproduced from reference 38. The GC-MS is composed of two major building blocks: the gas chromatograph and the mass spectrometer (Figure 2.20).39 The gas chromatograph utilizes a capillary column which depends on the column's dimensions (length, diameter, film thickness) as well as the phase 50 properties (e.g. 5% phenyl polysiloxane). The difference in the chemical properties between different molecules in a mixture will separate the molecules as the sample travels the length of the column. The molecules are retained by the column and then elute (come off of) from the column at different times (called the retention time), and this allows the mass spectrometer downstream to capture, ionize, accelerate, deflect, and detect the ionized molecules separately. The mass spectrometer does this by breaking each molecule into ionized fragments and detecting these fragments using their mass to charge ratio. 40 51 References (1) Skoog, D. A.; West, D. M. Principles of instrumental analysis; Saunders College Philadelphia, Pa., 1980. (2) Shealy, D. B.; Lipowska, M.; Lipowski, J.; Narayanan, N.; Sutter, S.; Strekowski, L.; Patonay, G. Anal. Chem. 1995, 67, 247. (3) Clark, B.; Frost, T.; Russell, M. UV Spectroscopy: Techniques, instrumentation and data handling; Springer, 1993; Vol. 4. (4) Lobnik, A. Optical chem. sensors 2006, 77. (5) Lambert, J. B.; Shurvell, H. F.; Lightner, D. A.; Cooks, R. G. Organic structural spectroscopy; Prentice Hall Upper Saddle River, NJ, 1998. (6) Guy, R. 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Handbook of analytical derivatization reactions; Wiley- 732. Interscience, 1979. (39) Sáiz-Jiménez, C.; De Leeuw, J. J. Anal. Appl. Pyrolysis 1986, 9, 99. (40) Gross, J. H. Mass spectrometry: a textbook; Springer Verlag, 2004. 54 [...]... Crystallogr 1977, 10, 73 (25 ) Ramachandran, V S.; Beaudoin, J J Handbook of analytical techniques in 115 20 03 concrete science and technology: principles, techniques and applications; William Andrew, 20 08 (26 ) Kalapathy, U.; Tallman, D E.; Hagen, S J Electroanal Chem 19 92, 325 , 65 (27 ) Vielstich, W Handbook of fuel cells-fundamentals, Technology and applications 20 03, 2, 155 (28 ) Filinovsky, V.; Pleskov,... Exploration 20 01, 1 (18) Al-Khanbashi, A.; Dhamdhere, M.; Hansen, M Appl Spectrosc Rev 1998, 33, (19) Schrader, B J Anal Chem 1996, 355, 23 3 (20 ) Furtak, T.; Reyes, J Surf Sci 1980, 93, 351 (21 ) Rao, C.; Biswas, K Annu Rev Anal Chem 20 09, 2, 435 (22 ) Hüfner, S Photoelectron spectroscopy: principles and applications; Springer, (23 ) Crist, B V Handbooks of monochromatic XPS spectra; XPS International, 20 04 (24 )... through the crystal (c) XRD-Diagram .25 Image reproduced from reference25 2. 2 Techniques used for application 2. 2.1 Linear Sweep Voltammetry In linear sweep voltammetry (LSV) a fixed potential range is employed much like potential step measurements .26 However in LSV the voltage is scanned from a lower limit to an upper limit as shown below (Figure 2. 12) 38 Figure 2. 12 linear sweep voltammogram The voltage... Infrared Gas Analyzer, DTIC Document, 20 09 (8&9) Malacara, D.; Servín, M.; Malacara, Z Interferogram analysis for optical testing; CRC, 20 05; Vol 84 (10) Jalili, N.; Laxminarayana, K Mechatronics 20 04, 14, 907 (11) Lang, K.; Hite, D.; Simmonds, R.; McDermott, R.; Pappas, D.; Martinis, J M Rev Sci Instrum 20 04, 75, 27 26 ( 12) Venables, J.; Harland, C Philos Mag 1973, 27 , 1193 (13) Brown, P.; McMullan, D.;... Plenum 1984, 29 3 (29 ) Riddiford, A Adv Electrochem Sci Eng 1966, 4, 47 53 (30) Bejerano, T.; Gileadi, E J Electroanal Chem Interfacial Electrochem 1977, 82, 20 9 (31) Johnson, D.; Napp, D.; Bruckenstein, S Electrochim Acta 1970, 15, 1493 ( 32) Paulus, U.; Schmidt, T.; Gasteiger, H.; Behm, R J Electroanal Chem 20 01, 495, 134 (33) Anderson, J L.; Coury Jr, L A.; Leddy, J Anal chem 20 00, 72, 4497 (34) Appleby,... Electroanal Chem Interfacial Electrochem 1978, 92, (35) Allen, T Particle Size Measurement: Surface Area and Pore Size Determination; 15 Springer, 1996; Vol 2 (36) Quirk, J Soil Science 1955, 80, 423 (37) Luedemann, A.; Strassburg, K.; Erban, A.; Kopka, J Bioinformatics 20 08, 24 , (38) Knapp, D R Handbook of analytical derivatization reactions; Wiley- 7 32 Interscience, 1979 (39) Sáiz-Jiménez, C.; De... liquid phase) Figure 2. 19 nitrogen adsorption and desorption diagram 2. 2.6 Gas chromatography–mass spectrometry (GC-MS) GC-MS is a method that combines the features of gas-liquid chromatography and mass spectrometry to identify different substances within a test sample 37 Applications of GC-MS include drug detection, fire investigation, environmental analysis, explosives investigation, and identification... current (iLA) can be written as iLA = 0. 620 n F A (DR )2/ 3 ν-1/6 CR ω1 /2 where the concentration term (CR) and diffusion coefficient (DR) are for the reduced form rather than the oxidized form 2. 2.4 Rotating Ring-Disk Electrode (RRDE) Soon after the rotating disk electrode was developed, the idea of putting a ring electrode around the disk electrode was introduced, and the rotating ring-disk electrode was... Society,[1966]-c2004.: 1996; 31, 161 (14) Nellist, P.; Pennycook, S Phys Rev Lett 1998, 81, 4156 52 (15) Sebolt-Leopold, J S.; Dudley, D T.; Herrera, R.; Van Becelaere, K.; Wiland, A.; Gowan, R C.; Tecle, H.; Bridges, A.; Przybranowski, S Nat Med.( 1999, 5, 810 (16) Zhang, Y.; Broekhuis, A A.; Stuart, M C A.; Fernandez Landaluce, T.; Fausti, D.; Rudolf, P.; Picchioni, F Macromolecules 20 08, 41, 6141... the standard electrode potential So as the voltage is swept from V1 to V2 the equilibrium position shifts from no conversion at V1 to full conversion at V2 of the reactant at the electrode surface The exact form of the voltammogram can be rationalised by considering the voltage and mass transport effects As the voltage is initially swept from V1 the equilibrium at the surface begins to alter and the . p, s, and n electrons are as shown 5 in the Figure 2. 2. Figure 2. 2 electronic transitions of species containing p, s, and n electrons. 5 Image reproduced from reference 5. 2. 1 .2 Fourier-transform. (c) XRD-Diagram. 25 Image reproduced from reference25. 2. 2 Techniques used for application 2. 2.1 Linear Sweep Voltammetry In linear sweep voltammetry (LSV) a fixed potential range is. concentration and voltage (potential difference): where E is the applied potential difference and E o is the standard electrode potential. So as the voltage is swept from V 1 to V 2 the equilibrium