Characterization of Carbon Surface Chemistry by Combined

Article pubs.acs.org/ac

Characterization of Carbon Surface Chemistry by Combined Temperature Programmed Desorption with in Situ X-ray Photoelectron Spectrometry and Temperature Programmed Desorption with Mass Spectrometry Analysis Patrice Brender, Roger Gadiou,* Jean-Christophe Rietsch, Philippe Fioux, Joseph Dentzer, Arnaud Ponche, and Cathie Vix-Guterl Institut de Science des Matériaux de Mulhouse LRC CNRS 7228, 15, rue Jean Starcky BP 2488, 68057 Mulhouse cedex, France S Supporting Information *

ABSTRACT: The analysis of the surface chemistry of carbon materials is of prime importance in numerous applications, but it is still a challenge to identify and quantify the surface functional groups which are present on a given carbon. Temperature programmed desorption with mass spectrometry analysis (TPD-MS) and X-ray photoelectron spectroscopy with an in situ heating device (TPD-XPS) were combined in order to improve the characterization of carbon surface chemistry. TPD-MS analysis allowed the quantitative analysis of the released gases as a function of temperature, while the use of a TPD device inside the XPS setup enabled the determination of the functional groups that remain on the surface at the same temperatures. TPD-MS results were then used to add constraints on the deconvolution of the O1s envelope of the XPS spectra. Furthermore, a better knowledge of the evolution of oxygen functional groups with temperature during a thermal treatment could be obtained. Hence, we show here that the combination of these two methods allows to increase the reliability of the analysis of the surface chemistry of carbon materials.

C

kinds of complexes: quinones, phenols, ethers, lactones, anhydrides, and carboxylic acids.9,18 A large number of experimental techniques can be used for the characterization of these oxygen containing functional groups; the most widely used are Fourier transformed infrared spectroscopy (FT-IR),12,13,15−22 temperature programmed desorption (TPD),5,12,13,16,19,21,23−25 X-ray photoelectron spectroscopy (XPS),13,15,16,19,21,24,26−28 Raman spectroscopy,29,30 and chemical titration.23 XPS analysis is a powerful technique for the chemical analysis of surfaces, and it has been widely used for the study of oxygen atoms present on the external surface of carbon materials. Nevertheless, the interpretation of the high resolution spectra is not obvious, and the results depend strongly on the model used for the O1s peak fitting. For example, this peak has been decomposed using two,31 three,26,32 four,33 and even five16,18,34 components. A major advantage of TPD is that this method provides a quantitative description of the amount of surface groups. Therefore, it can be used to establish correlations with application performances of carbon materials. Nevertheless, contradictions arise from the attribution of the desorption temperature of oxygenated groups, and significant differences can be

arbon materials are used as components for composites or as materials for energy and environmental applications. For all these applications, the surface chemistry of these carbon materials is a key point for their performances.1−3 Depending on the presence of functional groups on the surface of carbons, they can exhibit completely different chemical, hydrophilic/hydrophobic, or acido-basic properties.4 Moreover, specific interactions between surface groups and the environment can occur in some applications like adsorption.5,6 The surface chemistry of carbon material has been widely studied for decades, and several reviews are available.7,8 The main surface groups are oxygen containing functional groups like carboxyls, ethers, or phenols, and it has been shown that these oxygenated groups can affect the performances of carbon materials when they are used as catalyst supports,9 sorbents,5,10 or reinforcement materials in composites.11 In order to improve their performances, the control of the surface chemistry of carbon materials has been a challenge during the last decades. It is possible to implant oxygen containing functional groups on the surface of carbon materials by chemical5,12,13 or electrochemical14 processes, whereas heat treatment can enable the selective removal of some functional groups.15 Gas phase reactions with air,16 O2, or N2O17 can also be used to oxidize the surface of carbon materials. Nevertheless, these oxidative treatments lead to the implantation of several © 2012 American Chemical Society

Received: August 19, 2011 Accepted: January 12, 2012 Published: January 12, 2012 2147

dx.doi.org/10.1021/ac102244b | Anal. Chem. 2012, 84, 2147−2153

Analytical Chemistry

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software35 was used for data treatment. The preparation chamber of the XPS setup was equipped with a heating device which enables to heat the sample up to 900 °C under vacuum. The heating rate was set to 5 K/min, every 100 °C a 45 min dwell time was done, and the XPS analysis was performed. As pointed out in the introduction, the choice of the peak fitting model of the XPS spectra has a great influence on the results. For the O1s region we used a model based on eight components as presented in Table 1. The components (A) to (D) are presented in Figure 1. They are associated to oxygen

found in the literature: carboxylic groups may desorb from 200 °C19 up to 400 °C,18 lactones from 350 °C19 up to 660 °C,18 and phenolic groups from 550 °C33 up to 800 °C.17 Moreover, if we assume that some functional groups are stabilized by delocalization because of the aromatic character of graphite, CO2 and CO desorption peaks should not be decomposed by Gaussian/Lorentzian symmetric functions. Therefore, a drawback of XPS analysis is that it needs information on the nature of the groups present on the surface of the carbon materials in order to improve the quality of the deconvolution. On the other side, TPD analysis provides a very quantitative analysis of the amount of desorbed species, but the links between the desorbed species and the nature of the pristine functional groups are doubtful. The aim of the present work was to combine TPD and XPS in order to improve the reliability of the analysis of the surface chemistry of carbon materials and to propose a new model for the deconvolution of the O1s envelope. To achieve this, the surface oxygenated groups of a ballmilled graphitic carbon have been investigated by combining temperature-programmed desorption with quantitative mass spectrometry analysis (TPD-MS) and temperature programmed desorption with in situ X-ray photoelectron spectrometry (TPD-XPS). The aim was to obtain a direct correlation between the surface groups remaining on the surface (analyzed by XPS) and the ones released (analyzed by TPD) as a function of temperature. The second objective of this work was to give new insights on desorption temperatures of oxygenated groups.

Table 1. Components Used for O1s Peak Fittinga

a

peak

attribution

(A) (B) (C) (D) (E) (F) (G) (H)

quinones carbonyl O in carboxyls ether noncarbonyl O in carboxyls, phenols water water in microporosity satellite of carbonyls satellite of carboxyles

binding energy (eV) (A)+0,6 (A)+1,5 (A)+2,2 (A)+3,8 (A)+5,3 (A)+5,9 (A)+7,1

531.2 531.8 532.7 533.4 535.0 536.5 537.1 538.3

Binding energy are given ±0.1 eV.

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EXPERIMENTAL SECTION Sample Preparation. SLX 50 Graphite (TIMCAL, Switzerland) was milled in a planetary ball miller at 650 rpm during 4 cycles of 15 min under argon. Between each cycle, a break of 5 min was done to avoid excessive heating of the jar. The powder was then treated with a 8 M HNO3 solution at boiling temperature during 8 h. The carbon material obtained after the milling exhibits a significant amount of reactive sites, and the subsequent chemical treatment allowed to add oxygenated functional groups on its surface. A transmission electron microscopy analysis of the material was done (Philips CM200 at 200 kV); it showed that the material results from the agglomeration of particles which have a size ranging between 10 and 20 nm (Figure 1 in the Supporting Information). It must be noticed that the size of the carbon particles of the same order than the analysis depth of XPS. This allows a direct quantitative comparison between the amount of oxygen analyzed by XPS and by TPD. Moreover, this size is very small compared to the XPS beam size. XPS analysis are therefore representatives of a large number of carbon particles. X-ray Photoelectron Spectroscopy. XPS spectra were obtained on a SCIENTA 200 X-ray photoelectron spectrometer equipped with a conventional hemispherical analyzer. The latter was operated at constant pass energy of 100 eV in the fixed transmission mode. The incident radiation used was generated by a monochromatic Al Kα anode (1486.6 eV) operating at 420 W (14 kV; 30 mA). The analysis was performed using a takeoff angle of 90°, and the base pressure in the analysis chamber was about 10−9 mbar. The analyzed surface area was approximately 3 mm2. The spectrometer energy scale was calibrated using the Ag3d5/2, Au4f 7/2, and Cu2p3/2 core level peaks set respectively at binding energies of 368.2, 84.0, and 932.7 eV. The surface oxygen groups were identified by decomposing O1s spectra. The CasaXPS

Figure 1. Representation of the different types of atoms which are taken into account by the components used for the peak fitting of the O1s peak of the XPS spectra.

atoms in quinones functional groups (A); carbonyl oxygen atoms in acids, anhydrides and lactones groups (B); oxygen atoms in ether groups (C); noncarbonyl oxygen atoms in acid, anhydride, and lactones and oxygen atoms in phenol functional groups (D). The binding energies for these peaks were adjusted after XPS analysis on model molecules and polymers (benzoic acid, terephtalic acid, maleic anhydride from plasma polymer deposition, PEEK). In addition, four supplementary peaks are attributed to physisorbed water (E); physisorbed water in microporosity (F); quinones satellites (G) and acid, anhydride and lactone satellites (H). All O1s envelopes were peak-fitted with Gaussian−Lorentzian convoluted functions with a ratio 70/30. It was assumed that the fwhm are the same for the peaks 2148



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(A) to (D) and that their values should be between 1.5 and 2 eV; the fwhm of the satellites were set to 3 eV. Compared to some models used in the literature, our model exhibits some differences; the main one is related to the functional groups assigned to binding energies around 535 eV. Zhou et al.16 assigned this peak to the two oxygen atoms of carboxyls. Nevertheless, spectra of benzoic acids and of polymers with carboxyls groups like poly(acrylic acid) exhibit two peaks close to 532 eV and 533.5 eV.36 Therefore, we included the carbonyl atom of carboxylic acids in the peak (B) and the noncarbonyl atom of this functional group to peak (D).32,37 Instead, the bonding energies between 534.5 and 536.5 eV were assigned to chemisorbed or adsorbed water.37−40 Temperature-Programmed Desorption (TPD-MS) Analysis. The surface chemistry of HNO3 treated powders was also investigated by TPD coupled with mass spectrometry (TPD-MS). During a TPD, the oxygen containing chemical groups are decomposed mainly to CO and CO2. Carboxylic acids groups are decomposed into CO2 at relatively low temperatures, whereas ethers and quinones are decomposed into CO at high temperature owing to their high thermal stability. However, as it was pointed out in the Introduction, the temperature ranges for the thermal decomposition of the different functional groups can vary depending on the authors. The TPD-MS analysis of the HNO3 treated ball-milled powder was performed in a custom-made experimental setup. The measurements were done in a vacuum system at a maximum pressure of 10−4 mbar. 10 mg of carbon powder was deposited in a fused silica tube and heated from room temperature to 950 °C with a heating rate of 5 °C/min. Two TPD-MS experiments were done; the first one was performed in a conventional way with a continuous heating rate to obtain the TPD profiles of the different gases. The second one was done following the procedure used in XPS, that is from 200 to 900 °C with 45 min temperature steps every 100 °C. This second experiment allowed a direct quantitative comparison between TPD-MS and TPD-XPS results. During the TPD-MS, the gas phase was continuously analyzed quantitatively by the mass spectrometer. Before the experiments, the mass spectrometer was calibrated using H2 (m/z = 2), H2O (m/z = 18), CO (m/z = 12 and 28), N2 (m/z = 14 and 28), O2 (m/z = 32), and CO2 (m/z = 44) gases. The quantitative determination of functional groups was based on H2, H2O, CO, and CO2, as the amount of O2 and N2 was found to be negligible. The total pressure of the gases released during the heat treatment was measured continuously as a function of temperature using a Bayard−Alpert gauge. This total gas pressure was then compared to the one calculated from the sum of the partial pressures of the gas species deduced from the quantitative analysis by mass spectrometry. This allows to verify that there are no additional species evolving from the sample. From the TPD-MS analysis, the desorption rate of each gas was determined, and the total amount of each gas released was computed by time integration of the TPD-MS curves.

Figure 2. TPD-MS analysis of the ball-milled graphite after HNO3 treatment. This experiment was done without temperature steps.

treated by nitric acid.34 Compared to carbons oxidized by gas phase treatments, the amount of CO is relatively low. The H2 desorption begins at about 700 °C and goes on at a temperature higher that 950 °C. A desorption of water is observed between 50 and 300 °C. Two peaks are observed on the CO2 profile: the first one is related to the decomposition of carboxylic acids, while the second shows the presence of lactones and anhydrides. The lack of CO2 release above 700 °C confirms that these groups are no longer present above this temperature. The CO profile exhibits a specific shape with two peaks at temperatures around 500 and 700 °C. The first one can be attributed to ethers and phenols and the second one to quinones.34 The decomposition of anhydrides can also contribute to the low temperature release of CO. The onset of the CO desorption at 300 °C indicates that the thermal decomposition of these functional groups also starts at this temperature. The water desorption between 50 and 300 °C can be due to physisorbed water but also to the dehydration of two neighbors carboxylic acids leading to the formation of anhydride and water (Figure 2 in the Supporting Information). In a similar way, the formation of lactones by dehydration of a carboxylic acid and a phenol or the formation of ethers by dehydration of two neighbors phenols lead to the emission of water molecules. The release of H2 is generally attributed to thermal annealing of the material at high temperature. However, in some cases, the water present in the background gas can react with carbon above 700 °C leading to the formation of H2 and CO (syngas). Another TPD-MS analysis was done with steps every 100 °C following the conditions used for the XPS experiments (see experimental procedure above). Table 2 summarizes the amount of gas released over each temperature range during this TPD experiment. These values will then be compared to the amount of chemical groups remaining on carbon surface and therefore to the data deduced from the peak fitting of the XPS spectra. XPS Analysis. Figure 3 shows the evolution of the O1s region as a function of the temperature. As expected, the heating of the carbon sample under vacuum in the XPS cell decreases the amount of oxygen chemical groups on the carbon surface. The amount of oxygen which remains on the carbon surface as a function of temperature can be calculated from the XPS spectra, whereas the amount of oxygen removed from the carbon surface can be obtained from the TPD-MS experiments.

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RESULTS TPD-MS Analysis. The TPD-MS analysis at a constant heating rate of the carbon material is presented in Figure 2, and the desorption profiles are typical of oxidized carbons.34 The CO2 desorption occurs in the temperature range 150−700 °C, while the CO desorption occurs at higher temperature between 320 and 950 °C. The ratio of the CO and CO2 peaks is similar to the one observed in previous studies on carbon materials 2149



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oxygen on the material before heat-treatment was determined from its XPS spectrum (15.1%mol). It can be observed that there is a very good agreement between the two methods. The continuous decrease of oxygen concentration and the fact that 10 to 20% of the pristine carbon amount remain in the material after a treatment at 900 °C are in agreement with previously published studies.41 XPS Analysis Taking into Account TPD-MS Results. To decompose the O1s peaks of the XPS spectra, some assumptions were made: (1) quinones functional groups are assumed to be stable for temperatures below 400 °C and (2) only two ways of desorption are considered in TPD: CO resulting from phenols, ethers, quinones, lactones, and anhydrides and CO2 resulting from carboxylic acids, anhydrides, or lactones. By coupling the information presented in Table 2 and these assumptions, some conclusions can be made: · as no more CO2 is desorbed above 700 °C, neither carboxylic acids nor lactones or anhydrides remain above this temperature. This implies that the contributions of these chemical functional groups, that is component (B), will be set to 0 in the XPS spectra recorded after a heat treatment at temperatures higher or equal to 700 °C. · As quinones are very stable for temperatures lower than 400 °C, it is assumed that the areas of the corresponding components (A) are constant over the temperature range 25 °C−400 °C. · In a similar way, the areas of the components related to ethers and phenols are constant between 25 and 300 °C since no CO is released over this temperature range. For all the deconvolutions, the fwhm of the components related to functional groups were set to a common value (determined during the deconvolution). The fwhm of the satellites, plasmons, and of water peaks were set to 3 eV. The peak fitting was first done at high temperature where the oxygen content is the lowest. As shown above by the TPDMS experiments, for temperatures higher than 700 °C, no more carboxylic acids, lactones, and anhydrides are remaining on the surface. XPS Spectra Recorded at 900 °C. Only five components are required to decompose the O1s peak recorded at this temperature (Figure 5). The carbonyl component (A) corresponds to quinones. The component (D) is only due to phenols (since no anhydrides, lactones, and carboxylic acids are remaining). The peak fitting leads to a null area for the component (C), meaning that at 900 °C there are no more ethers left on the carbon surface. The component (G) is the satellite of the carbonyls functional groups; it is related to π−π* transitions, and its area is proportional to the one of the associated component (A). The peaks (E) and (F) are due to water physisorption. The presence of water on a carbon material heated at 900 °C has no physical sense, and we could have set it to zero. Nevertheless, we observed that, as expected, the intensity of this peak decreases strongly with increasing temperature. The remaining small peak observed here is then only an artifact related to the difficulty to fit with wide, low intensity peaks. XPS Spectra Recorded at 700 and 800 °C. The peak fitting of the spectra recorded at 700 and 800 °C are similar to the ones obtained at 900 °C (Figure 5). Indeed, at these two temperatures, carboxylic acids, anhydrides, and lactones are still absent from the surface, and the intensity of the component

Table 2. Amount of CO and CO2 Desorbed in the Different Temperature Ranges temperature range (C)

CO (mmol/g)

CO2 (mmol/g)

O released (mmol/g)

20−200 200−300 300−400 400−500 500−600 600−700 700−800 800−900 total

0 0 0.374 1.070 0.889 1.010 0.607 0.255 4.210

0.434 0.730 0.599 0.481 0.137 0.081 0 0 2.460

0.868 1.460 1.572 2.032 1.163 1.172 0.607 0.255 9.130

Figure 3. Influence of temperature on the O1s region of the XPS spectra.

It is not obvious to compare these two methods since XPS investigates the surface of materials, whereas TPD-MS leads to a decomposition of the bulk material. Nevertheless, since the size of the nanoparticles is similar to the analysis depth of the XPS, a quantitative comparison of the oxygen content can be done between the two methods. Figure 4 presents the evolution

Figure 4. Comparison between the fraction of oxygen desorbed during TPD-MS and the fraction of oxygen remaining on the carbon material measured by XPS.

of the oxygen functional groups which remain on the carbon surface as a function of increasing temperatures. The amount of 2150



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from TPD-MS experiments, the fitting would have led to a solution which is different from the one obtained by combining TPD-MS and TPD-XPS: the surface chemistry would be still free from ethers, but anhydrides, carboxylic acids, and lactones would be present. This shows that the peak fitting of the O1s peak can lead to significant errors if it is done without the information of TPD-MS experiments. This need for additional information when XPS analysis is performed has already been noticed,41 but the coupling of TPD-MS and TPD-XPS analysis provides much more information on the surface chemistry of carbon materials. XPS Spectra Recorded between 300 and 600 °C. Between 300 and 600 °C, a desorption of CO2 is observed during TPD-MS experiments, indicating that carboxylic acids, anhydrides, or lactones are present on the carbon surface. Therefore, the areas of the component (B) are no longer constrained for the peak fitting procedure (Figure 5). The XPS spectra recorded between 300 and 500 °C were decomposed following the same procedure as described above, except that an additional assumption was used: ketone functional groups are stable at temperature lower than 400 °C. Therefore, the areas of the peak (A) were kept constant for temperatures lower or equal to 400 °C. As expected, the areas of the peaks corresponding to carboxylic acids, anhydrides, and lactones increase as the temperature decreases from 500 to 300 °C. The area of the component (C) of the O1s region does not vary significantly when the temperature decreases from 500 to 300 °C. This is in agreement with the stability of ethers in this temperature range. The desorption of CO in this temperature range is therefore only related to the decomposition of anhydrides. XPS Spectra Recorded at 25 and 200 °C. The XPS spectra recorded in this temperature range are the most complex to decompose since they include all the components presented in Table 1. The TPD-MS experiments show that no CO desorption occurs between 25 and 300 °C. Therefore, the area of the components (A) and (C) corresponding to functional groups leading to CO desorption were kept constant with areas equal to the corresponding values obtained at 300 °C. The areas of components (B) and (D) are free of constraints. This allows to study accurately the thermal stability of carboxyl in the temperature range 25 °C−300 °C. As expected, the main components of the XPS spectrum are the ones corresponding to carboxyl functional groups (components (B) and (D)). The fact that the main functional groups present on the surface of carbon nanoparticles treated are carboxyl-like functional groups (carboxylic acids, anhydrides, and lactones) is in agreement with data found in the literature.2,6,7 A significant amount of phenol is also detected on the surface of the carbon, while only a few ethers and quinones are found.

Figure 5. Peak fitting of the O1s region of the XPS spectra of the carbon material as a function of the temperature of desorption.

(B) is set to 0. Moreover, we assume that there are no recombination reactions between the surface groups. This assumption is used to transfer information from the XPS spectra recorded at 900 °C to the spectra obtained at 800 °C: the area of a component at a temperature T cannot be higher than the one at a temperature T-100 °C. As it was mentioned above, recombinations of carboxylics acids to form anhydrides can occur, but they are not present above 700 °C. Moreover, this reaction has only a slight influence on the different components because carboxyls and anhydrides are taken into account in the same components of the O1s spectra. Compared with the spectra recorded at 900 °C, a significant difference is that the component (C) related to ether functional groups is present on the O1s spectrum and is no longer equal to zero. Compared to 900 °C, we also observe an increase of the intensity of the component (D) on the O1s spectrum. This component is only due to phenols since no carboxylic acids, anhydrides, and lactones remain on the surface at this temperature. An increase of the amount of quinones is also observed (component (A)). Therefore, the surface chemistry of the sample treated at 800 °C is composed of phenols, quinones, and ethers functional groups. The same procedure was used for the XPS spectra recorded after a treatment at 700 °C. Compared to 800 °C, the peak fitting shows that the amounts of phenols and ether increase, whereas the amount of quinones remains constant. This shows that quinone functional groups are stable up to 800 °C and that below 700 °C the desorption of carbon monoxide is not due to decomposition of these quinones. It must be noticed that if the peak fitting of the O1s spectrum would have been done without adding the constraints obtained

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DISCUSSION In the previous section, we have shown that the combined use of TPD-MS and XPS allows to obtain a reliable peak fitting of the XPS spectrum of the carbon powder (Figure 5). This method can also give new insights on the evolution of the different functional groups with the thermal treatment’s temperature. For a carbon material, carboxylic acids, anhydrides, and lactones are indistinguishable in XPS spectra or the CO2 desorption profiles of TPD-MS experiments. The decomposition of anhydrides and lactones explains that a release of CO2 can be observed at high temperature. Anhydrides and 2151



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also shown in Figure 5. It must be noticed that the components related to quinones were not constrained for the spectra recorded above 300 °C; therefore, the XPS analysis demonstrates their stability up to 700 °C. Like ethers, quinones are stable at low temperature; their decomposition begins at 700 °C but is still incomplete at 900 °C. The TPD-MS experiments have shown that CO desorption occurs in a very wide temperature range, that is between 350 and 950 °C. The CO profile exhibits two main wide peaks (see Figure 2), the maximum of the second one is observed at 700 °C. From the XPS analysis, this high temperature peak can be explained by the decomposition of ethers and quinones. The low temperature desorption peak of CO with a maximum at 500 °C can be related to the decomposition of phenols, lactones, or anhydrides. Although anhydrides are relatively stable molecules, their decomposition can occur at moderate temperature when they are present on the surface of carbon materials. This decomposition proceeds through a radicalar mechanism which is initiated by the presence of significant amounts of native radicals in carbon materials.42 Anhydride decomposition leads to the emission of a mixture of CO and CO2, and it can explain the simultaneous emission of these two molecules between 350 and 500 °C. In a similar way, the decomposition of lactone can lead to the emission of carbon monoxide.43 This assumption is nevertheless difficult to verify from XPS analysis since these functional groups are included in the (D) component spectrum together with other C−O functions like carboxyls. If the XPS spectra recorded for temperatures between 25 and 900 °C are analyzed without using the assumptions presented above, significant errors are done on the amount of surface groups (see the Supporting Information). At high temperature, there is no difference since the decomposition is very simple; only quinones and phenols are present. Below 700 °C, significant differences are observed. If no constraints are used, then the amount of quinones is negligible at low temperature and increases with increasing temperature. Moreover, concentration of ethers fall down to zero at high temperature. The components (B) and (D) increase with increasing temperature. This increase would mean that the amount of oxygen atoms involved in carboxyls, carbonyls, and phenols groups increases with increasing temperature. Therefore, if the peak fitting is done without doing assumptions, the evolution of the surface functional groups with increasing temperature is not in agreement with the stability of the functional groups and with the results of temperature programmed desorption experiments (Figure 3 in the Supporting Information).

lactones can be present on the surface, but they can be also a product of secondary reactions between surface functional groups as explained above. For example, the release of water which is observed up to 300 °C indicates the recombination of two neighbored carboxylic acids. Therefore, even if only carboxylic acids are present on the pristine carbon material, a release of CO2 due to secondary reactions can be observed at a temperature higher than 500 °C. The amount of carboxyl groups can be measured by XPS from the component (B). It can also be obtained from the amount of CO2 released during TPD-MS experiments. The evolution of the amount of carboxyl groups as a function of temperature determined by these two methods is presented in Figure 6. The good agreement between the different methods gives a validation of the procedure used to decompose XPS spectra.

Figure 6. Evolution of the amount of carboxyl functional groups with increasing temperature of heat treatment as determined by XPS and TPD-MS.

Figure 7 presents the evolution of the amount of ethers as a function of temperature. These functional groups begin to

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CONCLUSION The aim of this study was to show that by combining temperature programmed desorption and XPS, the reliability of the analysis of the surface chemistry of carbon materials could be improved. To achieve this goal, a model carbon material was synthesized by milling a graphite; oxygen functional groups were then added by a subsequent chemical treatment with nitric acid. Then, two complementary methods were used to study the surface chemistry of this material: temperature programmed desorption with quantitative mass spectrometry analysis performed between 25 and 900 °C (TPD-MS) and XPS analysis with in situ heating of the sample over the same range of temperature (TPD-XPS). A peak fitting model was proposed for the O1s peak, and the deconvolution was achieved by taking into account the information obtained from TPD-MS

Figure 7. Evolution of ethers and quinones as a function of temperature.

decompose at 500 °C and are completely eliminated between 600 and 900 °C. For component (A) linked to the carbonyl functional groups, the variation as a function of temperature is 2152



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experiments. This combined method allows to improve the reliability of the XPS model used for the curve fitting of the O1s envelope and also to give new insight on the thermal decomposition of the surface functional groups during TPDMS experiments. Since the size of the particles was in the nanometer range, TPD-MS and XPS were in good agreement for the evolution of the oxygen balance with the temperature of thermal treatment. Furthermore, the two analyses agreed well for the evolution of the carboxyl functional groups as a function of temperature. The behavior of the ethers and quinones with increasing temperatures could be studied. The main peak for the decomposition of these groups occurs above 700 °C and corresponds to the high temperature peak of CO desorption. The TPD-MS profiles showed that a simultaneous emission of CO and CO2 occurs between 350 and 600 °C. This is probably related to the formation and subsequent decomposition of anhydrides and lactones. These contributions are nevertheless difficult to quantify since none of these functional groups leads to specific peaks in the XPS spectra. Then, from this study, we were able to improve the reliability of the deconvolution of the O1s peak of carbon materials by adding information obtained from the TPD-MS analysis. Furthermore, the XPS analysis gave new insight on the peaks of emission of carbon oxides and water observed during TPDMS experiments on carbon materials. Further studies will include the extension of this method to other carbon materials and specific studies on anhydrides and lactones formation and decomposition.

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ASSOCIATED CONTENT

S Supporting Information *

Figures 1−3. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

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ACKNOWLEDGMENTS The authors thank Dr. Michael Spahr from Timcal SA for the graphite material.

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REFERENCES

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Characterization of Carbon Surface Chemistry by Combined

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