Work function modifications of graphite surface via oxygen plasma treatment

Accepted Manuscript Title: Work function modifications of graphite surface via oxygen plasma treatment Authors: J. Duch, P. Kubisiak, K.H. Adolfsson, M. Hakkarainen, M. Gołda-C˛epa, A. Kotarba PII: DOI: Reference:

S0169-4332(17)31309-0 http://dx.doi.org/doi:10.1016/j.apsusc.2017.05.007 APSUSC 35942

To appear in:

APSUSC

Received date: Revised date: Accepted date:

24-2-2017 11-4-2017 2-5-2017

Please cite this article as: J.Duch, P.Kubisiak, K.H.Adolfsson, M.Hakkarainen, M.Gołda-C˛epa, A.Kotarba, Work function modifications of graphite surface via oxygen plasma treatment, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.05.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Work function modifications of graphite surface via oxygen plasma treatment J. Duch a,*, P. Kubisiak a, K. H. Adolfsson b, M. Hakkarainen b, M. Gołda-Cępa a, A. Kotarba a,* a

Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland

b

Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, SE-100

44 Stockholm, Sweden * Corresponding authors. Tel.: +48 12 663 2017, +48 12 663 2246. E-mail addresses: [email protected] (J. Duch), [email protected] (A. Kotarba) Graphical abstract

Highlights  Substantial modifications of graphite surface by oxygen plasma treatment  Correlation between work function and wettability established  Surface coverage of OH groups rationalized in terms of Helmholtz model

1

Abstract The surface modification of graphite by oxygen plasma was investigated experimentally (X-ray diffraction, nanoparticle tracking analysis, laser desorption ionization mass spectrometry, thermogravimetry, water contact angle) and by molecular modelling (Density Functional Theory). Generation of surface functional groups (mainly –OHsurf) leads to substantial changes in electrodonor properties and wettability gauged by work function and water contact angle, respectively. The invoked modifications were analyzed in terms of Helmholtz model taking into account the theoretically determined surface dipole moment of graphite—OHsurf system (μ = 2.71 D) and experimentally measured work function increase (from 0.75 to 1.02 eV) to determine the –OH surface coverage (from 0.70 to 1.03 × 1014 groups cm-2). Since the plasma treatment was confined to the surface, the high thermal stability of the graphite material was preserved as revealed by the thermogravimetric analysis. The obtained results provide a suitable quantitative background for tuning the key operating parameters of carbon electrodes: electronic properties, interaction with water and thermal stability. Key words: carbon materials, basal plane, oxygen functional groups, surface dipole, work function, wettability

2

1. Introduction Carbon materials due to the diversity in structures, dimensionalities (0-3D, such as quantum dots, nanotubes, graphene, diamond, respectively) and properties are applied in numerous industrial branches. Among their unique properties, the most important include: chemical stability under acidic and basic conditions, good electrical and thermal conductivity, thermal resistance (in a non-oxidizing atmosphere), light weight, lubricity, mechanical strength and gas, liquid adsorption [1]. Carbon materials are used in energy storage, conversion systems [2–7], and catalytic processes [8], where they successfully play the roles of sensors [9,10], absorbers [11] and catalyst supports [8], respectively. It is worth mentioning that the biocompatibility and antibacterial properties of carbon materials (especially nanotubes [12] and more recently graphene [13]) are the most intensively investigated nowadays. While designing carbon materials for specific applications (fuel cells oxygen electrodes, catalytic supports, pollutant adsorbers, chemical sensors, bioactive surfaces), the following crucial properties are most often considered: electrodonor properties of the surface, bulk electric conductivity, wettability and thermal stability [2,14,15]. Each application, however, requires precise adjustment of process conditions to tune these properties for the optimal performance. There are numerous reports on tuning these properties via chemical and physical modifications, recently reviewed in [16]. In general, carbon material surfaces, such as graphite, graphene, nanotubes etc. can be readily modified by the introduction of different heteroatoms, such as sulfur, nitrogen, phosphorus and most frequently – oxygen [17–20]. Mild and controlled oxidation of carbon surfaces leads to the formation of various oxygencontaining functional groups. The most popular oxygen functional groups include hydroxyls, carboxyls, phenols, carbonyls and lactones covalently attached to the surface (out-of-plane position) [21]. The modifications of the carbon surface result in increased surface 3

hydrophilicity and polarity, change of the acid-base properties, and formation of active centers for adsorption and/or catalytic reactions [22]. The surface functional groups can also be used as anchors for further functionalization, such as deposition of magnetic particles [23], catalytically active nanoparticles [24] and complexes of noble metals e.g. Pt or Rh [21]. Depending on the chemical nature and surface coverage of the introduced groups, electronic properties of carbon can be substantially modified. For example changes in the work function of multiwalled carbon nanotubes were identified as indicative for the extent of modification in their electronic properties [18]. X. Li et al. showed that surface –OH or – COOH groups changed the carbon black surface from hydrophobic to hydrophilic and played a role as active centers for water adsorption [14]. An increase in wettability has also further improved electrochemical oxygen reduction on carbon electrocatalyst. Enhance of carbon surface hydrophilicity improves a dispersion of metal-related active sites, as well as increase the accessibility of reactants to active centers [25]. This may be the reason for increased mass utilization efficiency of catalysts. Functionalization of carbon surfaces with oxygen can be achieved by a number of methods [26]. The easiest way comprises controlled oxidizing processes, such as flow of oxidizing agent (oxygen, air, ozone, nitrous oxide), treatment with nitric and/or sulfuric acid, ammonium persulfate aqueous solution or exposure to oxidizing plasma [17]. Oxygen plasma modification is advantageous compared with classic chemical methods: significantly shorter functionalization time, safety for the environment and the precise process control by adjusting plasma parameters (power, treatment time and oxygen partial pressure) [27]. During the oxygen plasma treatment, the carbon surface is bombarded by a number of excited species (electrons, photons, ions and free radicals) which interact with the exposed surface. As a result, functionalization of carbon surface via oxygen plasma is the combination of physical and chemical processes, where the carbon-carbon bonds are broken. This allows the

4

incorporation of functional groups [28] which nature and concentration depend on the plasma parameters, however, typically C–O, O–C=O and C=O surface groups are formed [27,29,30]. It is worth mentioning here that due to mild oxidation conditions the reaction takes place on the material surface only, whereas the bulk properties remain intact [14]. One of the most common crystalline form of carbon materials is graphite. Due to its low price, exceptionally high thermal and electrical conductivities, graphite is commonly used in electrochemical applications [31] such as electrodes in capacitors [2,32], Li-ion batteries [33] and fuel cells [34]. On the other hand, graphite basal surfaces being flat and well-defined provide a suitable platform for both experimental and molecular modeling studies. The aim of this study was to investigate the effect of oxygen plasma treatment of graphite surface in terms of its key physicochemical characteristics: electrodonor property, wettability and thermal stability. The applied quantified approach takes advantage of combining experimental and theoretical methods. The surface modification of graphite by oxygen plasma was investigated by X-ray diffraction, nanoparticle tracking analysis, laser desorption ionization mass spectrometry, thermogravimetry and water contact angle. The DFT calculated surface dipole moment of plasma generated –OHsurf groups together with the experimentally determined changes in the work function allow to propose a model of functionalized graphite basal plane. The application of the Helmholtz equation allowed for the quantification of the surface coverage with –OHsurf functional groups. The results are discussed in the context of tuning the surface properties of carbon electrocatalysts and biomaterials.

2. Experimental 2.1. Samples preparation The graphite samples (Dor-chem, Krakow, Poland) were purchased in the powder form and characterized as received (X-Ray diffraction, nanoparticle tracking analysis, 5

thermogravimetry). The effect of plasma treatment on work function (Kelvin method) and wettability (water contact angle) were investigated on samples in the form of disks. The latter were prepared from the graphite powder by pressing under 1 MPa for 5 s, in a hydraulic press (Specac Ltd, UK) into the disc form of 10 mm in diameter and thickness of ~1 mm. 2.1.1. Oxygen plasma treatment The oxygen plasma modification of graphite surfaces was carried out using a Diener Electronic Femto plasma system (Diener Electronic GmbH, Nagold, Germany). The oxygen (Air Products, 99.9998% O2) partial pressure was precisely adjusted to 0.2 mbar, the generator frequency was 40 kHz and power was set as 60 W. The varied parameter was the exposure time of graphite surface to the oxygen plasma (0.1, 0.2, 1, 2 and 5 min). For each modification time, the new graphite disk (10 mm) was used. For further characterization, the modified samples were used immediately after the plasma treatment. The progress of graphite disk surface modification was monitored by changes in work function and water contact angle. Each measurement was repeated three times. 2.2. Methods 2.2.1. X-ray diffraction (XRD) The XRD measurements were recorded by a Rigaku MiniFlex diffractometer in reflection mode, using CuKα radiation with a wavelength λ = 1.540598 Å. The XRD patterns were collected in the region of 2Θ between 10° and 80° with a step scans of 0.02° and speed 2.5° min-1. 2.2.2. Nanoparticle tracking analysis (NTA) The graphite particles size distribution was determined using an LM10 Nanosight instrument (Malvern Instruments Ltd, Malvern, UK) equipped with a sCMOS camera (Hamamatsu Photonics, Hamamatsu, Japan) and a 450 nm blue laser. The recorded 6

nanoparticles tracking data were analzyed with NTA software version 3.1 Build 3.1.45. The powder graphite samples were dispersed in deionized water and sonicated for 15 min to avoid extended agglomeration of nanoparticles. The viscosity settings for water were automatically corrected for the used temperature in the range of 18.9-19.1 °C. The size distribution was measured at camera level 13 (shutter: 1232, gain: 219). A single experiment consisted of three movies, 60 s each at 25 frames second-1. Only measurements above 1000 completed tracks were analyzed. 2.2.3. Work function measurements (WF) The work function (Φ) values of graphite disks were determined based on contact potential difference (VCPD) measurements which were carried out by the dynamic condenser method of Kelvin with a KP6500 probe (McAllister Technical Services), installed in a vacuum chamber. The reference electrode was a standard stainless steel plate with a diameter of 3 mm (Φref ≈ 4.3 eV), provided by the manufacturer. During the measurements, the gradient of the peak-to-peak versus backing potential (BP) was set to 0.1, whereas the vibration frequency and the amplitude were set to 300 Hz and 40 a.u., respectively. A single VCPD value was obtained using five BPs, each being an average of 64 independent measurements. The final VCPD value was an average of 75 independent points. The work function values were obtained from a simple relation: eVCPD = Φref – Φsample. The work function of graphite disks was measured before and after oxygen plasma treatment. The change in work function is defined as the difference between the value of the modified graphite sample and the corresponding unmodified material.

2.2.4. Water contact angle measurements (WCA) The impact of oxygen plasma treatment on the hydrophilicity of graphite surfaces was determined by WCA changes between unmodified graphite sample (disk form) and oxygen 7

plasma treated. The measurements were made using a goniometer (Surftens Universal Instrument, OEG GmbH, Frankfurt (Oder), Germany). Static contact angles were measured using windows image processing software (Surftens 4.3). The WCA value for each water drop (2.5 μl) was obtained as an average of 10 independent measurements.

2.2.5. Laser desorption/ionization time of flight mass spectrometry (LDI-TOF-MS) The nature of surface functional groups of unmodified and oxygen plasma treated graphite were determined by means of a laser desorption/ionization time-of-flight mass spectrometer (LDI-TOF-MS) with a SCOUT-MTP ion source (Bruker UltraFlex TOF-MS, Bruker Daltonics, Bremen, Germany) in reflector mode and equipped with a 337 nm nitrogen laser. The acceleration voltage was 25 kV, the reflector voltage was 26.3 kV and laser power 60%. Samples of graphite particles in DMF (1 mg/ml), were sonicated for 1 min and dropped (5 µl) on the stainless steel LDI plate and left to dry. The mass-to-charge (m/z) ratio range was set to scan from 60 m/z to 600 m/z.

2.2.6. Thermogravimetric – differential thermal analysis (TGA/DTA) The thermal stability of modified graphite samples in the oxidizing atmosphere was measured by thermogravimetric analysis method using a TGA/DTA Mettler-Toledo apparatus. The measurements were done on powder samples of 5 mg placed in an alumina pot and heated from room temperature to 1025 °C (a heating rate of 10 °C min-1) in the flow of 5% O2/ Ar (14 ml/min).

8

2.2.7. Molecular modelling (DFT calculations) In order to determine the dipole moment which was created upon incorporation of the – OHsurf group DFT calculations for the model system were performed. The system was comprised of 51 six carbon atoms rings forming a planar surface of 16 Å x 22 Å with the outermost carbon atoms saturated with hydrogen atoms. The complete structure can be seen in Figure 4A. The –OHsurf group was placed in the middle of the system close to one of the carbon atoms, which simulate its out-of-plane position on the graphite basal surface. Such model is justified by the previously reported domination of the electron transfer processes taking place on graphite basal plane [35]. All quantum-chemical calculations were conducted with Gaussian 09 program [36]. B3LYP functional with Grimme’s [37] dispersion correction was used with 6-31+G** basis set. The geometry of the system was optimized and atomic partial charges were determined according to the Merz-Singh-Kollman scheme [38,39] (fit to the electrostatic potential with charges constrained to also reproduce the dipole moment). The electric dipole moment of the hydroxyl adspecies was calculated based on the atomic partial charge accumulated on oxygen and adjacent carbon surface atoms.

3. Results and discussion In Figure 1A a diffractogram of parent graphite (unmodified material) shows the diffraction lines which were indexed using ICSD database. The peaks are located at 2Θ values of 26.6° and 54.7° related to (002) and (004) planes, respectively. All of the detected sharp maxima are characteristic for graphite, confirming its well-developed crystal structure. In Figure 1B the size distribution of unmodified graphite particles obtained with NTA is presented. The smallest particle hydrodynamic diameter was found about 50 nm. The supplementary maxima at positions corresponding to multiplication of 50 nm were also found,

9

indicating the nanoparticles tendency for agglomeration in the suspension. The highest number of particles was found for the hydrodynamic dimension of ~100 nm (corresponding to dimers) and then the maxima intensity decreases with the particle size. Both the XRD and NTA results for modified graphite samples (see Supplementary Material) show the same characteristic features, thus it can be concluded that oxygen plasma treatment does not influence in a noticeable way the structure and particle size of the parent graphite material. The work function (Φ) of unmodified graphite determined with the Kelvin method was 4.6 eV, which is in-line with the previously reported values of 4.6 - 4.7 eV [40–42]. However, this characteristic value for clean graphite surface is dramatically increased upon contact with oxygen plasma. It was found that at the beginning the work function rises linearly with the treatment time and then stabilizes at a critical value of ~5.6 eV, which is reached after 2 min of modification. Thus, the observed changes between unmodified and oxygen plasma treated samples (plotted as ΔΦ) varied in a non-monotonous way with the duration of the plasma oxidation as shown in Figure 2A. After this critical time, further exposure to oxygen plasma does not influence the electrodonor properties. Significant changes of the graphite surface invoked by the oxygen plasma are also observed in wettability, expressed as a water contact angle. In Figure 2B the influence of plasma treatment time on the hydrophilicity of graphite surface (blue line) is shown together with the representative images of water drops deposited on the surface (gray shapes). The WCA decreased from 74.0°, for unmodified samples, to 3.8° for the plasma treated one (t = 5 min). In Figure 2C the changes of WCA as a function of ΔΦ are shown. Although these two parameters probe different properties of the surfaces: electrodonor (ΔΦ) and wettability (WCA), the observed correlation can be described by a linear function (R2 > 0.99). This finding can be rationalized by the changes in surface composition (plasma cleaning of the

10

surface and insertion of oxygen containing surface groups) and roughness (plasma etching) since both work function and water contact angle strongly depend on these both factors. For example, a linear correlation between the work function and roughness was observed for the Mg-Al alloys [43], whereas a complicated relation between wettability and roughness was observed for anthracite [44], coal [45] and various (metallic, ceramic and plastic) anisotropic surfaces [46]. Therefore, it is worth to underline here that such a strong linear correlation between the changes in work function and water contact angle is experimentally observed for the first time for oxygen plasma functionalized graphite surface. The impact of oxygen plasma modification on graphite electrodonor properties and wettability is caused by oxygen species formed on the graphite surface. The presence and chemical nature of oxygen functional groups introduced on the graphite surface via oxygen plasma modification were examined by LDI-TOF-MS method. In Figure 3 comparison of mass spectra for unmodified and oxygen plasma treated graphite samples (t = 5 min) are presented. For the unmodified samples only carbon clusters were detected (e.g. 168.361 m/z for C14+), while for plasma treated samples several additional mass-to-charge peaks were observed (marked by gray shadowing). Analysis of m/z ratio for the modified samples allowed for the identification of surface functional groups formed upon exposure to plasma. The characteristic m/z peaks with the assigned probable stoichiometric formula and the corresponding functional groups are presented in Table 1. The presence of sodium or potassium adducts is typical for LDI-MS method [47]. The results clearly indicate that with the applied plasma conditions –OHsurf groups are dominant among the surface oxygen species. As revealed by the work function measurements the presence of the –OHsurf groups on the graphite surface has a strong influence on its electrodonor properties. To quantify the relation between surface functional groups and the graphite work function changes as a first approach

11

we assume the Helmholtz model. In this model, if the well-separated adsorbed species are taken into consideration, the work function change after heteroatoms adsorption on the material surface can be given by the Helmholtz equation [48]: ∆𝛷 = 𝑁𝑒𝜇⁄𝜀0

(Eq. 1)

where ∆𝛷 is the change in the work function [J], 𝑁 is the number of adsorbed atoms per unit square, 𝑒 is the elementary charge, 𝜇 is the dipole moment [C·m] and 𝜀0 is the vacuum permittivity. Since the model is narrowed to non-interacting surface dipoles it corresponds to the plasma exposure time lower than 2 min (Figure 2A). After that time the surface coverage of –OHsurf dipoles is high enough for their mutual interactions leading to depolarization. As a result, further plasma treatment does not change the work function value. However, for the initial period of plasma treatment, it is possible to establish the number of –OHsurf dipoles based on the relation in Eq. 1. The work function value can be determined experimentally (Figure 2A), while the –OHsurf dipole moment is determined by molecular modelling. The cluster model of graphite basal plane functionalized with the hydroxyl group used for the DFT calculations is shown in Figure 4. The colors represent electrostatic potential superimposed on the isodensity surface. We can clearly observe the accumulation of the negative potential (red color) in the vicinity of the –OHsurf group, especially near the oxygen atom. The further from the hydroxyl group the potential is less negative. Due to the presence of the hydrogen atom in the –OHsurf adspecies a small, positively charged region is formed on the other side of the group. The electric dipole moment of the hydroxyl adspecies was calculated based on the formula 𝜇𝑛 = ∑𝑞𝑖 𝑟𝑖,𝑛 , where qi and ri,n are the atomic partial charge and the n-th coordinate of atom i, respectively. The calculated dipole moment was found to be µ = 2.71 D.

12

The dependence between work function changes after plasma treatment and the surface concentration of –OHsurf groups is shown in Figure 5. The calculated number of surface dipoles varies in the range of 0.7 – 1.03 × 1014 –OHsurf groups cm-2. The critical value for oxygen functional groups can be estimated from Figure 2A, where the work function change reaches a plateau after 2 min of plasma treatment. This point corresponds to the surface concentration of the –OHsurf groups of ~1.0 × 1014 cm-2 (the last point in the linear plot in Figure 5A). The graphical model of the calculated concentration of surface dipoles on graphite basal plane with size 40 Å × 40 Å is shown in Figure 5B. The red circle represents the area where the electrostatic potential of surface C atoms substantially changed upon formation of the isolated –OHsurf dipole. The effect of oxygen plasma functionalization of graphite surface on its thermal stability was evaluated by thermogravimetric – differential thermal analysis in oxidizing conditions. In Figure 6 the comparison of TG and DTA curves for unmodified (reference sample of parent graphite) and oxygen plasma treated graphite is presented. It was found that the oxidizing profiles for both materials are practically the same with the maximum heat flow around 820°C. The effect of oxygen plasma modification for other graphite samples, with shorter exposure time to plasma, was very similar - the oxidizing profiles exhibit the same shape. It may be thus concluded that the oxygen plasma modification of graphite is limited to the top surface and does not influence the bulk properties of the material. Furthermore, the results rationalized the model of the out-of-plane position of –OHsurf groups on the graphite basal surface used in the DFT calculations. Indeed, in more detailed DFT studies on coronene molecule (the epitome of graphite basal surface), the free edges are the energetically most demanding for prime dioxygen attack which triggers the total oxidation reaction cascade [49]. Therefore, the adsorption of oxygen on edge is a reactive one and lead to the destruction of the graphite material, which was not observed in our case. The obtained experimental results 13

(XRD, NTA, TGA/DTA, LDI-TOF-MS) consistently revealed that the oxygen plasma modification led to the insertion of –OHsurf groups on the basal plane instead of the edges. It is worth to underline that the oxygen plasma parameters were precisely adjusted to cause only the surface modification instead of destructive graphite oxidation (which was observed only upon prolonged treatment time). The obtained results have a practical implication in the context of potential applications of the carbon materials as components in energy storage and conversion systems. The positive effects of the surface modifications on the electrochemical performance of the carbonelectrodes (i.e. better energy efficiency of the cells) have been reported by several authors [50–52]. However, the mild oxidation is usually performed with laborious procedures (e.g.

chemical

oxidation)

which

require

environmentally

unfriendly

chemicals

(e.g. concentrated acids). Additionally, chemical oxidation is difficult to control and do not allow for precise adjustment. The treatment with chemical substances often leads to penetration of the whole material and formation of disordered carbon in the sample which decreases the thermal stability. In contrast to the wet chemistry methods, plasma treatment provides several practical advantages. As demonstrated here, it can be used for introduction of surface oxygen functional groups (for the adjusted plasma parameters preferentially hydroxyls) on graphite. The presence of these groups changes the natively hydrophobic carbon surface into hydrophilic one which results in better solubility in polar media extending their application to aqueous solutions due to reduced polarization. The interactions at watersurface interfaces are highly relevant for various scientific and practical problems, such as self-cleaning surfaces [53], oxygen reduction reaction [25], industrial catalysis [54], as well as biological applications including designing bioactive [55] and antimicrobial surfaces [56]. A precise control of –OHsurf concentration can be obtained by applying a simple quantitative Helmholtz model. Tuning of electronic properties and wettability can be achieved 14

due to the established correlation between change in work function and water contact angle. Adjusting the oxygen plasma treatment parameters allows for functionalization of the graphite surface alone and does not affect the material bulk properties, as revealed by TG/DTA analysis. Another novel feasible significance of the obtained results concerns biomedical applications, where the work function can be used as a direct parameter for tuning surface properties against bacterial infections. The monitoring of work function modification can be used for direct evaluation of the carbon-based biomaterials since the repulsion electrostatic forces between surface and bacteria cell walls play a key role in the irreversible adhesion of microorganisms, which is an initial step in bacterial colonization, as recently illustrated in [57].

4. Conclusions The oxygen plasma-generated –OH surface groups on graphite basal plane extensively influence its electrodonor properties (work function increase) and wettability (water contact angle decrease). The maximal increase in work function of 1.02 eV was obtained after 2 min of plasma treatment and corresponds to the practically hydrophilic material (gauged by a water contact angle of 8.5°). The extend of surface modifications (from 0.7 to 1.03 x 1014 groups cm-2) was quantified by application of Helmholtz formula taking into account the DFT calculated graphite—OHsurf dipole moment (μ = 2.71 D). It was shown that the oxygen plasma treatment of graphite is confined to the surface and changes its key parameters such as electronic properties and wettability, while the bulk structure and thermal stability of graphite are preserved. The obtained results pave the road for surface optimization of carbon materials

15

to facilitate their application as catalytic supports, chemical sensors and electrodes, as well as their evaluation as biomaterial surfaces against bacterial infections. Acknowledgment The study was financed by the Polish National Science Centre project awarded by decision number DEC-2016/21/B/ST8/00398. The research was carried out with the equipment purchased thanks to the financial support of the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (contract no. POIG.02.01.00-12-023/08). References [1]

M. Inagaki, F. Kang, Materials Science and Engineering of Carbon: Fundamentals, Butterworth-Heinemann, 2014. doi:10.1016/B978-0-12-800858-4.00002-4.

[2]

E. Frackowiak, F. Béguin, Carbon materials for the electrochemical storage of energy in capacitors, Carbon N. Y. 39 (2001) 937–950. doi:10.1016/S0008-6223(00)00183-4.

[3]

Y. Zhai, Y. Dou, D. Zhao, P.F. Fulvio, R.T. Mayes, S. Dai, Carbon materials for chemical capacitive energy storage, Adv. Mater. 23 (2011) 4828–4850. doi:10.1002/adma.201100984.

[4]

P. Trogadas, T.F. Fuller, P. Strasser, Carbon as catalyst and support for electrochemical energy conversion, Carbon N. Y. 75 (2014) 5–42. doi:10.1016/j.carbon.2014.04.005.

[5]

F. Jaouen, E. Proietti, M. Lefèvre, R. Chenitz, J.-P. Dodelet, G. Wu, H.T. Chung, C.M. Johnston, P. Zelenay, Recent advances in non-precious metal catalysis for oxygenreduction reaction in polymer electrolyte fuel cells, Energy Environ. Sci. 4 (2011) 114. doi:10.1039/c0ee00011f.

[6]

A.H.A.M. Videla, L. Osmieri, S. Specchia, Non-noble Metal (NNM) Catalysts for Fuel Cells: Tuning the Activity by a Rational Step-by-Step Single Variable Evolution, in: Electrochem. N4 Macrocycl. Met. Complexes Vol. 1 Energy, Second Ed., Springer International Publishing, 2016: pp. 69–101.

[7]

E.F. Holby, P. Zelenay, Linking structure to function: The search for active sites in 16

non-platinum group metal oxygen reduction reaction catalysts, Nano Energy. 29 (2016) 54–64. doi:10.1016/j.nanoen.2016.05.025. [8]

F. Rodríguez-Reinoso, The role of carbon materials in heterogeneous catalysis, Carbon N. Y. 36 (1998) 159–175. doi:10.1016/S0008-6223(97)00173-5.

[9]

E. Bekyarova, M. Davis, T. Burch, M.E. Itkis, B. Zhao, S. Sunshine, R.C. Haddon, Chemically functionalized single-walled carbon nanotubes as ammonia sensors, J. Phys. Chem. B. 108 (2004) 19717–19720. doi:10.1021/jp0471857.

[10] J. Li, Y. Lu, Q. Ye, M. Cinke, J. Han, and M. Meyyappan†, Carbon Nanotube Sensors for Gas and Organic Vapor Detection, Nano Lett. 3 (2003) 929–933. doi:10.1021/nl034220x. [11] Y.H. Li, C.W. Lee, B.K. Gullett, The effect of activated carbon surface moisture on low temperature mercury adsorption, Carbon N. Y. 40 (2002) 65–72. doi:10.1016/S0008-6223(01)00085-9. [12] B.S. Harrison, A. Atala, Carbon nanotube applications for tissue engineering, Biomaterials. 28 (2007) 344–353. doi:10.1016/j.biomaterials.2006.07.044. [13] H.M. Hegab, A. Elmekawy, L. Zou, D. Mulcahy, C.P. Saint, M. Ginic-Markovic, The controversial antibacterial activity of graphene-based materials, Carbon N. Y. 105 (2016) 362–376. doi:10.1016/j.carbon.2016.04.046. [14] X. Li, K. Horita, Electrochemical characterization of carbon black subjected to RF oxygen plasma, Carbon N. Y. 38 (2000) 133–138. doi:10.1016/S0008-6223(99)001086. [15] G.S. Szymanski, Z. Karpinski, S. Biniak, A. Świątkowski, The effect of the gradual thermal decomposition of surface oxygen species on the chemical and catalytic properties of oxidized activated carbon, Carbon N. Y. 40 (2002) 2627–2639. doi:10.1016/S0008-6223(02)00188-4. [16] A. Schaetz, M. Zeltner, W.J. Stark, Carbon modifications and surfaces for catalytic organic transformations, ACS Catal. 2 (2012) 1267–1284. doi:10.1021/cs300014k. [17] W. Shen, Z. Li, Y. Liu, Surface Chemical Functional Groups Modification of Porous Carbon, Recent Patents Chem. Eng. 1 (2008) 27–40. 17

doi:10.2174/2211334710801010027. [18] H. Ago, T. Kugler, F. Cacialli, W.R. Salaneck, M.S.P. Shaffer, A.H. Windle, R.H. Friend, Work Functions and Surface Functional Groups of Multiwall Carbon Nanotubes, J. Phys. Chem. B. 103 (1999) 8116–8121. doi:10.1021/jp991659y. [19] Y. Li, Y. Zhao, H.H. Cheng, Y. Hu, G.Q. Shi, L.M. Dai, L.T. Qu, Nitrogen-Doped Graphene Quantum Dots with Oxygen-Rich Functional Groups, J. Am. Chem. Soc. 134 (2012) 15–18. doi:10.1021/ja206030c. [20] T. Szabo, O. Berkesi, P. Forgo, K. Josepovits, Y. Sanakis, D. Petridis, I. Dekany, Evolution of surface functional groups in a series of progressively oxidized graphite oxides, Chem. Mater. 18 (2006) 2740–2749. doi:10.1021/cm060258+. [21] M.C. Román-Martínez, C. Salinas-Martínez de Lecea, Heterogenization of Homogeneous Catalysts on Carbon Materials, Elsevier B.V., 2013. doi:10.1016/B9780-444-53876-5.00003-9. [22] A.H.A. Monteverde Videla, S. Ban, S. Specchia, L. Zhang, J. Zhang, Non-noble FeNX electrocatalysts supported on the reduced graphene oxide for oxygen reduction reaction, Carbon N. Y. 76 (2014) 386–400. doi:10.1016/j.carbon.2014.04.092. [23] H. Zhou, C. Zhang, H. Li, Z. Du, Decoration of Fe3O4 nanoparticles on the surface of poly(acrylic acid) functionalized multi-walled carbon nanotubes by covalent bonding, J. Polym. Sci. Part A Polym. Chem. 48 (2010) 4697–4703. doi:10.1002/pola.24259. [24] Y. Zhou, K. Neyerlin, T.S. Olson, S. Pylypenko, J. Bult, H.N. Dinh, T. Gennett, Z. Shao, R. O’Hayre, Enhancement of Pt and Pt-alloy fuel cell catalyst activity and durability via nitrogen-modified carbon supports, Energy Environ. Sci. 3 (2010) 1437. doi:10.1039/c003710a. [25] G. Hao, N.R. Sahraie, Q. Zhang, S. Krause, M. Oschatz, A. Bachmatiuk, P. Strasser, S. Kaskel, Hydrophilic non-precious metal nitrogen-doped carbon electrocatalysts for enhanced efficiency in oxygen reduction reaction, Chem. Commun. 51 (2015) 17285– 17288. doi:10.1039/C5CC06256J. [26] K.A. Wepasnick, B.A. Smith, K.E. Schrote, H.K. Wilson, S.R. Diegelmann, D.H. Fairbrother, Surface and structural characterization of multi-walled carbon nanotubes 18

following different oxidative treatments, Carbon N. Y. 49 (2010) 24–36. doi:10.1016/j.carbon.2010.08.034. [27] C. Chen, B. Liang, A. Ogino, X. Wang, M. Nagatsu, Oxygen functionalization of multiwall carbon nanotubes by microwave-excited surface-wave plasma treatment, J. Phys. Chem. C. 113 (2009) 7659–7665. doi:10.1021/jp9012015. [28] M. Gołda, M. Brzychczy-Włoch, M. Faryna, K. Engvall, A. Kotarba, Oxygen plasma functionalization of parylene C coating for implants surface: Nanotopography and active sites for drug anchoring, Mater. Sci. Eng. C. 33 (2013) 4221–4227. doi:10.1016/j.msec.2013.06.014. [29] A. Felten, C. Bittencourt, J.J. Pireaux, G. Van Lier, J.C. Charlier, Radio-frequency plasma functionalization of carbon nanotubes surface O 2, NH 3, and CF 4 treatments, J. Appl. Phys. 98 (2005). doi:10.1063/1.2071455. [30] T. Takada, M. Nakahara, H. Kumagai, Y. Sanada, Surface Modification and Characterization Carbon Black With Oxygen Plasma of, Carbon N. Y. 34 (1996) 1087– 1091. doi:10.1016/0008-6223(96)00054-1. [31] M. Wissler, Graphite and carbon powders for electrochemical applications, J. Power Sources. 156 (2006) 142–150. doi:10.1016/j.jpowsour.2006.02.064. [32] H. Wang, M. Yoshio, Graphite, a suitable positive electrode material for high-energy electrochemical capacitors, Electrochem. Commun. 8 (2006) 1481–1486. doi:10.1016/j.elecom.2006.07.016. [33] J.B. Goodenough, K.S. Park, The Li-ion rechargeable battery: A perspective, J. Am. Chem. Soc. 135 (2013) 1167–1176. doi:10.1021/ja3091438. [34] C.A. Bessel, K. Laubernds, N.M. Rodriguez, R.T.K. Baker, Graphite nanofibers as an electrode for fuel cell applications, J. Phys. Chem. B. 105 (2001) 1115–1118. doi:10.1021/jp003280d. [35] G. Zhang, P.M. Kirkman, A.N. Patel, A.S. Cuharuc, K. Mckelvey, P.R. Unwin, Molecular Functionalization of Graphite Surfaces : Basal Plane vs Step Edge Electrochemical Activity Molecular Functionalization of Graphite Surfaces : Basal Plane vs Step Edge Electrochemical Activity, J. Am. Chem. Soc. 136 (2014) 11444– 19

11451. [36] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, Gaussian 09, Revision D.01; Gaussian, Inc., (n.d.). [37] S. Grimme, J. Antony, S. Ehrlich, H. Krieg, A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu, J. Chem. Phys. 132 (2010). doi:10.1063/1.3382344. [38] U.C. Singh, P. a Kollman, An approach to computing electrostatic charges for molecules, J. Comput. Chem. 5 (1984) 129–145. doi:10.1002/jcc.540050204. [39] B.H. Besler, K.M. Merz, P.A. Kollman, Atomic charges derived from semiempirical methods, J. Comput. Chem. 11 (1990) 431–439. doi:10.1002/jcc.540110404. [40] T. Takahashi, H. Tokailin, T. Sagawa, Angle-resolved ultraviolet photoelectron spectroscopy of the unoccupied band structure of graphite, Phys. Rev. B. 32 (1985) 8317–8324. doi:10.1103/PhysRevB.32.8317. [41] S. Suzuki, C. Bower, Y. Watanabe, O. Zhou, Work functions and valence band states of pristine and Cs-intercalated single-walled carbon nanotube bundles, Appl. Phys. Lett. 76 (2000) 4007. doi:10.1063/1.126849. [42] R. Iidiy, E. Tosatti, C.D. Chen, Voltage-dependent scanning-tunneling microscopy of a crystal surface: Graphite, Phys. Rev. B. 31 (1993) 2602–2605. [43] M. Xue, S. Peng, F. Wang, J. Ou, C. Li, W. Li, Linear relation between surface roughness and work function of light alloys, J. Alloys Compd. 692 (2017) 903–907. doi:10.1016/j.jallcom.2016.09.102. [44] W. Xia, Y. Li, Role of Roughness Change on Wettability of Taixi Anthracite Coal Surface before and after the Heating Process, Energy & Fuels. 30 (2016) 281–284. doi:10.1021/acs.energyfuels.5b02621. [45] W. Xia, C. Ni, G. Xie, The influence of surface roughness on wettability of natural/gold-coated ultra-low ash coal particles, Powder Technol. 288 (2016) 286–290. doi:10.1016/j.powtec.2015.11.029.

20

[46] K.J. Kubiak, M.C.T. Wilson, T.G. Mathia, P. Carval, Wettability versus roughness of engineering surfaces, Wear. 271 (2011) 523–528. doi:10.1016/j.wear.2010.03.029. [47] N. Aminlashgari, W. Buchberger, J. Hacaloglu, M. Hakkarainen, P. Mischnick, M. Stiftinger, Mass Spectrometry of Polymers – New Techniques, Springer, 2012. doi:10.1007/978-3-642-28041-2. [48] G.A. Somorjai, Introduction to surface chemistry and catalysis, John Wiley & Sons, New York, 1994. [49] F. Zasada, W. Piskorz, P. Stelmachowski, P. Legutko, A. Kotarba, Z. Sojka, Density functional theory modeling and time-of-flight secondary ion mass spectrometric and Xray photoelectron spectroscopic investigations into mechanistic key events of coronene oxidation: Toward molecular understanding of soot combustion, J. Phys. Chem. C. 119 (2015) 6568–6580. doi:10.1021/jp512018z. [50] K.J. Kim, M.-S. Park, Y.-J. Kim, J.H. Kim, S.X. Dou, M. Skyllas-Kazacos, A technology review of electrodes and reaction mechanisms in vanadium redox flow batteries, J. Mater. Chem. A. 3 (2015) 16913–16933. doi:10.1039/C5TA02613J. [51] J.J. Park, J.H. Park, O.O. Park, J.H. Yang, Highly porous graphenated graphite felt electrodes with catalytic defects for high-performance vanadium redox flow batteries produced via NiO/Ni redox reactions, Carbon N. Y. 110 (2016) 17–26. doi:10.1016/j.carbon.2016.08.094. [52] W. Zhang, J. Xi, Z. Li, H. Zhou, L. Liu, Z. Wu, X. Qiu, Electrochemical activation of graphite felt electrode for VO 2+/VO2+ redox couple application, Electrochim. Acta. 89 (2013) 429–435. doi:10.1016/j.electacta.2012.11.072. [53] K. Guan, Relationship between photocatalytic activity, hydrophilicity and self-cleaning effect of TiO2/SiO2 films, Surf. Coatings Technol. 191 (2005) 155–160. doi:10.1016/j.surfcoat.2004.02.022. [54] E.L. Margelefsky, R.K. Zeidan, M.E. Davis, Cooperative catalysis by silica-supported organic functional groups, Chem. Soc. Rev. 37 (2008) 1118–1126. doi:10.1039/B710334B. [55] D. Rana, K. Ramasamy, M. Leena, C. Jiménez, J. Campos, P. Ibarra, Z.S. Haidar, M. 21

Ramalingam, Surface functionalization of nanobiomaterials for application in stem cell culture, tissue engineering, and regenerative medicine, Biotechnol. Prog. 32 (2016) 554–567. doi:10.1002/btpr.2262. [56] G. Gao, D. Lange, K. Hilpert, J. Kindrachuk, Y. Zou, J.T.J. Cheng, M. KazemzadehNarbat, K. Yu, R. Wang, S.K. Straus, D.E. Brooks, B.H. Chew, R.E.W. Hancock, J.N. Kizhakkedathu, The biocompatibility and biofilm resistance of implant coatings based on hydrophilic polymer brushes conjugated with antimicrobial peptides, Biomaterials. 32 (2011) 3899–3909. doi:10.1016/j.biomaterials.2011.02.013. [57] M. Golda-Cepa, K. Syrek, M. Brzychczy-Wloch, G.D. Sulka, A. Kotarba, Primary role of electron work function for evaluation of nanostructured titania implant surface against bacterial infection, Mater. Sci. Eng. C. 66 (2016) 100–105. doi:10.1016/j.msec.2016.04.079.

22

Figure 1. Characterization of the graphite parent material. (A) X-ray diffraction result with the indexed characteristic maxima, (B) size distribution of graphite particles determined by NTA measurements.

Figure 2. (A) Changes in the work function and (B) water contact angle of graphite as a function of oxygen plasma treatment time. (C) the corresponding correlation between hydrophilicity and electron properties of graphite.

23

Figure 3. LDI-TOF-MS spectra of unmodified and oxygen plasma treated graphite showing the formation of oxygen functional groups on the graphite surface upon oxygen plasma modification. The main oxygen species on the surface are –OHsurf groups.

Figure 4. (A) The top and (B) side views of graphite basal plane (16 Å x 22 Å) consisting of 51 units (six carbon atoms ring) functionalized with the hydroxyl group forming a surface dipole of 2.71 D. The colors represent electrostatic potential superimposed on the isodensity surface.

24

Figure 5. (A) The Helmholtz dependence between the number of surface dipoles (–OHsurf) per cm2 and the work function changes of graphite. (B) graphical visualization of the critical surface dipoles concentration.

Figure 6. The TG (upper panel) and DTA (lower panel) results showing the thermal stability of unmodified and representative plasma treated (t = 5 min) graphite sample in the flow of 5% O2/Ar.

25

Table 1. Suggested structures for the main oxygen containing species formed on graphite surface upon plasma modification (t = 5 min) based on the LDI-TOF-MS maxima. m/z formula

functional group(s)

151.275

[C8O2H + Na]+

–COOH

165.264

[C9O2H2 + Na]+

2x–OH

197.191

[C9O4H2 + Na]+

2x–COOH

213.177

[C13O2H2 + Na]+

2x–OH

304.409

[C22OH + Na]+

–OH

[C22OH + K]+

–OH

[C22O2H + Na]+

–COOH

340.351

[C25OH + Na]+

–OH

392.622

[C28O2H2 + Na]+

2x–OH

408.611

[C28OH2 + Na]+

–OH, –COOH

422.587

[C29O3H3 + Na]+

3x–OH

modified graphite

320.402

26