Characterization of hydrogenated graphite powder by X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry

Abstract

Hydrogenated graphite powder was obtained through Birch reduction of graphite powder and characterized by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) at 500 °C. The sp³ carbons formed at the edges of the surface of the hydrogenated graphite powder exhibited an sp³ carbon peak in the XPS C1s spectrum. The sp³-to-sp² carbon ratio calculated from the XPS spectra increased from 0.08 to 1.19 after hydrogenation. Two sets of peaks, the C_x⁻ and C_xH⁻ ion series (where x = 1, 2, 3…), were identified in the ToF-SIMS spectra of both the graphite powder and hydrogenated graphite powder. The difference between these two spectra represented an increase in the normalized intensities of the H⁻ and C_xH⁻ ions in the spectrum of the hydrogenated graphite powder, indicating the formation of more sp³ carbons on the surface.

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1. Introduction

Graphite, which consists of stacked layers of sp² hybridized carbon, exhibits high electrical and thermal conductivity.^1,2 Defects exist in the form of zigzag or armchair edges on the graphite surface.^3,4 These defect sites can serve as reaction sites for chemical modification. Chemisorption of hydrogen converts the sp² carbons of graphite to sp³ carbons.⁵ One way to generate sp³ carbons on the surface of graphite is by using hydrogen plasma.^6–8 After hydrogenation, C–H bonds are formed on the surface, confirming the formation of sp³ carbons.^7,8 However, hydrogen plasma treatment can generate atomic vacancies,^8,9 leading to the formation of defects in the aromatic rings.⁹ Another way to generate sp³ carbons on the surface of graphite is through Birch reduction.^10–13 Birch reduction is a hydrogenation reaction which converts localized sp² carbons to sp³ carbons at the edges of the graphite surface without creating additional defects in the graphite structure.¹⁰ Birch reduction achieves a higher degree of hydrogenation in graphite than does hydrogen plasma treatment.¹⁴

Hydrogen plasma treatment of graphite produces sp³ carbons which could easily be identified in the X-ray photoelectron spectroscopy (XPS) C1s spectrum.^7,8 Ruffieux et al. analyzed the relative intensities of ions in the C1s spectra obtained at different take-off angles. They found that the reactions were confined to the surface of the graphite.⁸ Birch reduction mainly involves hydrogenation of the sp² carbons at the edges of the graphite surface without producing additional defects on that surface. Yang et al. produced hydrogenated graphite powder from graphite powder using Birch reduction and characterized the products by XPS.¹⁰ They found that the binding energy of the main C1s peak of the graphite powder, which was equal to 284.5 eV, was assigned to the localized sp² carbons. The main C1s peak of the hydrogenated graphite powder became broader, suggesting the presence of both sp² and sp³ carbons. However, in their work, the binding energy of the main C1s peak of the graphite powder stayed constant at 284.5 eV before and after hydrogenation. The binding energy should have been higher after hydrogenation as an increasing number of sp³ carbons is produced on the surface. Bouša et al. prepared hydrogenated graphite nanofibers using Birch reduction.¹² The products were characterized by XPS. They fitted the C1s spectra with a sp² carbon peak at the binding energy of 284.5 eV and a sp³ carbon peak at 285.5 eV. The intensity of the sp³ carbon peak was used as an indication of the hydrogenation degree of graphite. However, when the XPS measurements are performed at room temperature, adsorption of hydrocarbon and oxygen contaminants on the graphite surface is very likely. To prevent this problem, XPS experiments can be performed at high temperatures.¹⁵ The key disadvantage of using XPS in the analysis is that XPS cannot detect H and confirm the increase in hydrogen concentration on a hydrogenated graphite surface. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is extremely sensitive to hydrogen and can be used for this purpose instead.¹⁶

In this work, we used Birch reduction¹⁰ to produce hydrogenated graphite powder and characterized the product using a combination of XPS and ToF-SIMS at 500 °C. The binding energy of the C1s main peak of the hydrogenated graphite powder shifted by 0.4 eV toward the high binding energy. In addition, the ratio of sp³ to sp² carbons increased after hydrogenation. The normalized intensities of the H⁻ and C_xH⁻ ions (where x = 1, 2, 3…) increased after hydrogenation, indicating a higher concentration of hydrogen on the graphite surface.

2. Experimental section

2.1 Preparation of hydrogenated graphite powder

Graphite powder (Nano24) was purchased from Asbury Carbons. The powder was 2.5 nm thick in the through-plane direction and had a surface area of 350 m² g⁻¹ with primary particles coming under 100 nm in size. A Birch reduction method similar to the one described in the literature was used.¹⁰ A mixture of dry ice and acetone (−78 °C) was used to cool a flask and a condenser. Ammonia (60 mL) was condensed into the flask, followed by the addition of lithium (3 g) and graphite powder (0.7 g). The mixture was stirred for 1.5 h. Then, t-butanol (10 mL) and methanol (10 mL) were added to the reaction and stirred for 3 h. The ammonia was then evaporated overnight. The product was washed with methanol and deionized water and dried in an oven at 60 °C for 10 h. The reaction mechanism is shown in Scheme 1. To prepare samples for surface analyses, the graphite powder and hydrogenated graphite powder were compressed on a 1 cm × 1 cm Cu grid (200 mesh) under 15 tons of pressure.

Scheme 1 Birch reduction of graphite powder.

2.2 XPS

An X-ray photoelectron spectrometer (Axis Ultra DLD system, Kratos Analytical, UK) equipped with a 150 watt monochromatic Al Kα X-ray source (1486.6 eV) was used for the analysis. A take-off angle of 20° was set with a spot size of 2 mm × 1 mm under the electrostatic mode. High-resolution C1s, O1s and Cu2p spectra were obtained at a step size of 0.1 eV and a pass energy of 20 eV. Ten scans were taken for each spectrum with acquisition time of 100 s per scan. All the spectra were obtained at 500 °C to prevent adsorption of hydrocarbons. The pressure inside the chamber during the experiment was kept to approximately 10⁻⁸ Torr.

In curve fitting, the C1s spectrum of a highly oriented pyrolytic graphite (HOPG) sample after annealing at 500 °C for 2 h was used as a reference spectrum for a defect-free surface, which only has delocalized sp² carbons.¹⁵ An asymmetric fitting function with a combination of Doniach–Sunjic and Gaussian–Lorentzian functions was used in the curve-fitting analysis of the C1s spectrum of the HOPG to determine the asymmetric parameter, which was determined to be 0.035. The binding energy of delocalized sp² carbons in the HOPG sample was centered at 284.6 eV.¹⁵ For other symmetric components, a Gaussian–Lorentzian fitting function was used. A Shirley background was subtracted in the curve-fitting analysis. The software used for peak fitting was CasaXPS.

2.3 ToF-SIMS

ToF-SIMS (ToF-SIMS V spectrometer, ION-TOF GmbH, Munster, Germany) spectra were acquired using a Bi₃⁺ beam operating at 25 keV. The scanning area was 200 μm × 200 μm with an acquisition time of 40 s. Spectra were collected at 500 °C. The pressure inside the chamber during the experiment was approximately 10⁻⁸ Torr. The software used for peak analysis was SurfaceLab 6.6 from ION-TOF.

2.4 Raman spectroscopy

Raman spectra were obtained at 25 °C using a micro-Raman spectrometer (Renishaw, UK) equipped with an Argon laser (wavelength 514 nm, powder 20 mW). An 1800 g mm⁻¹ grating and an acquisition time of 30 s were used. The silicon peak at 521 cm⁻¹ was used for calibration.

2.5 Scanning electron microscopy

The morphology and size distribution of graphite and hydrogenated graphite powders were measured using a scanning electron microscope (SEM, JSM-6390, JEOL, Japan). The SEM images were acquired at 20 kV accelerating voltage. The graphite and hydrogenated graphite powders were first dispersed in methanol and ultrasonicated at 25 °C for 3 h. The graphite powder and hydrogenated powder suspensions (about 0.05 mg mL⁻¹) were dropped on Si wafers, respectively, to prepare samples for the SEM experiment.

3. Results and discussion

The surface chemical composition of the graphite powder and hydrogenated graphite powder after annealing at 500 °C was determined by XPS. The results indicate that C, O, and Cu were present at 97.4, 2.3 and 0.3 at%, respectively, on the surface of the graphite powder and at 95.8, 3.9 and 0.3 at%, respectively, on the surface of the hydrogenated graphite powder. The almost nonexistent Cu in the samples demonstrates the negligible influence of the Cu grid on the results. The small presence of O on the surface suggests the existence of hydroxyl and carboxyl groups.

Fig. 1a compares the C1s spectra of the HOPG, and the graphite powder before and after hydrogenation. The full width at half maximum (FWHM) of the C1s peak of the graphite powder was larger than that of the HOPG by 0.3 eV. The C1s main peak of the hydrogenated graphite powder shifted by 0.4 eV toward the high binding energy, possibly due to the conversion of the localized sp² carbons to sp³ carbons. This result is different from that of Yang et al.¹⁰ who showed that the binding energy of the graphite powder stayed at 284.5 eV before and after hydrogenation. In addition, the reduction in the intensity of the π–π* shake-up peak of the hydrogenated graphite powder further supports the notion of the conversion of sp² carbons to sp³ carbons. Fig. 1b shows the C1s curve-fitting results for the graphite powder. There are five components (see also Table S1†). An asymmetric sp² carbon peak at the binding energy of 284.6 eV represents the main structure of the graphite powder. The peaks centered at 286.5 and 288.0 eV came from hydroxyl and carboxyl groups, respectively.¹⁷ Another peak at 291.0 eV is related to the π–π* shake-up. We fitted the curve of the C1s peak of the hydrogenated graphite powder by fixing the sp² carbon peak at 284.6 eV with an FWHM of 0.8 eV and adding a peak representing the sp³ carbons. The binding energy of the sp³ carbons of graphite varies between 285.1 and 285.5 eV.^7,8,12,18,19 We varied the binding energy of the sp³ carbon peak between 285.1 and 285.5 eV and fixed its FWHM at 1.0 eV. The residual standard deviation of each set of curves/fitting representing a fit between experimental and curve-fitted data was calculated. The minimum residual standard deviation value was obtained when the sp³ carbon peak was at 285.2 eV (Table S2†). Fig. 1c shows the C1s curve-fitting results for the hydrogenated graphite powder. There are also five components (Table S1†). The sp³ carbon peak became the highest peak. We then calculated the sp³-to-sp² carbon ratio of the graphite powder before and after hydrogenation and found that it increased from 0.08 to 1.19 after hydrogenation. As discussed before, the conversion of sp² carbons to sp³ carbons via the Birch reaction occurs mainly at the edges of the graphite surface and the aromatic structure is not disturbed.

Fig. 1 (a) A comparison of the XPS C1s spectra of the HOPG, graphite powder and hydrogenated graphite powder after 500 °C annealing. XPS C1s curve-fitting results for (b) the graphite powder after 500 °C annealing and (c) the hydrogenated graphite powder also after 500 °C annealing. The black ([thick line, graph caption]) and green () lines represent the experimental and curve-fitted spectra, respectively. All spectra were obtained at 500 °C.

We further confirmed our results using ToF-SIMS. Fig. 2a shows the ToF-SIMS negative ion spectrum of the graphite powder annealed at 500 °C. Two sets of peaks can be found, namely the C_x⁻ and C_xH⁻ ion series (where x = 1, 2, 3…). Besides, peaks related to O and N, such as O⁻, CN⁻ and CNO⁻, can also be found. These peaks came from the impurities on the surface of the graphite powder. After hydrogenation, as shown in Fig. 2b, the C_x⁻ and C_xH⁻ ion series are still the dominant peaks in the spectrum, but the intensity of the C_xH⁻ ion peaks has increased significantly, indicating a high hydrogen concentration on the surface.

Fig. 2 ToF-SIMS negative ion spectra of (a) graphite powder after annealing at 500 °C and (b) hydrogenated graphite powder after annealing at 500 °C. Both spectra were obtained at 500 °C.

The difference between the graphite powder and hydrogenated graphite powder can also be seen in their normalized intensities. The normalized intensity of an ion peak was calculated by dividing its intensity by the total ion intensity of the spectrum. We also compared the normalized ion intensities of the two types of powder with those of the HOPG, which has a nearly defect-free surface. Fig. 3 shows the normalized intensity of the H⁻ ion in the HOPG, graphite powder and hydrogenated graphite powder. The normalized intensity of the H⁻ ion in the HOPG was 0.002, indicating that a minute amount of hydrogen was present on its surface. The normalized intensity of the H⁻ ion increased from 0.02 to 0.18 after hydrogenation of the graphite powder.

Fig. 3 Normalized intensity of the H⁻ ion in HOPG, graphite powder and hydrogenated graphite powder.

Fig. 4a compares the normalized intensity of the C_x⁻ ion in the HOPG, graphite powder and hydrogenated graphite powder. The C₂⁻ ion has the highest intensity among all the ions. Their normalized intensities of the C_x⁻ ion decreased as the carbon number increased. A possible reason for this observation is that the C_x⁻ ions with a small number of carbon atoms can form not only when the carbon structure of graphite breaks up directly, but also when larger ions break up indirectly, as has been observed experimentally and confirmed theoretically in the literature.^20,21

Fig. 4 Normalized intensity of the (a) C_x⁻ and (b) C_xH⁻ ions as a function of the number of carbon atoms.

Fig. 4b compares the normalized intensity of the C_xH⁻ ion in the HOPG, the graphite powder and the hydrogenated graphite powder. The normalized intensity of the C_xH⁻ ion in the graphite powder is higher than that in the HOPG, indicating a greater presence of sp³ carbons on the surface of the graphite powder. The normalized intensity of the C_xH⁻ ion in the hydrogenated graphite powder is much higher than that in the graphite powder, indicating an even greater presence of sp³ carbons on the surface of the hydrogenated graphite powder.

To further compare the results, we calculated the ratio of the normalized intensities of the C_xH⁻ ion to the C_x⁻ ion. This ratio could also reflect the degree of hydrogenation on the surface. As shown in Fig. 5, the ratio is higher for the graphite powder than for the HOPG. The hydrogenated powder has the highest ratio in all of the samples. The higher intensity of the H-related ion in the hydrogenated graphite powder further confirms the formation of sp³ carbons on its surface.

Fig. 5 Ratio of the normalized intensities of the C_xH⁻ ion to the C_x⁻ ion as a function of the number of carbon atoms.

Raman spectroscopy has been used to investigate the defects in graphite.^10,12 To investigate the defects in the graphite and hydrogenated graphite powders, their Raman spectra were obtained and the results are shown in Fig. 6. The D peak in the hydrogenated graphite powder became more intense, indicating a higher concentration of sp³ carbons in the sample. Besides, the decrease in the intensity and broadening of the 2D peak and the increase in the intensity of the D + D′ peak in the hydrogenated graphite powder also confirm an increase in its defect density. Similar results have also been observed by other researchers.^10,22–24 Fig. 7 shows the SEM images and plots of the size distribution of dispersed graphite and hydrogenated graphite powders. The particle sizes and number within an area of 25.6 μm × 19.2 μm of an SEM image were determined and the size distribution of each sample was calculated using three images. It can be seen that the size and morphology of the graphite powder did not change much after hydrogenation.

Fig. 6 Raman spectra of graphite and hydrogenated graphite powders at 25 °C.

Fig. 7 SEM images and plots of size distribution of dispersed graphite powder (a and b) and dispersed hydrogenated graphite powder (c and d).

4. Conclusions

We prepared hydrogenated graphite powder from graphite powder through Birch reduction. In the curve-fitted C1s spectrum of the hydrogenated graphite powder, the sp³ carbon peak at 285.2 eV was the highest peak and was accompanied by an asymmetric sp² carbon peak and a π–π* shake-up peak. ToF-SIMS negative ion spectrum of the hydrogenated graphite powder contained C_x⁻ and C_xH⁻ ions. The normalized intensity of the C_xH⁻ ion in the hydrogenated graphite powder is much higher than that in the graphite powder. The ToF-SIMS results confirm the increase in hydrogen concentration and the formation of sp³ carbons on the surface of the hydrogenated graphite powder.

Acknowledgements

The work described in this paper was fully supported by the Research Grants Council of the Hong Kong Special Administrative Region, China (grant nos 600513 and 16300314). The authors would like to thank Mr Nick Ho of the Materials Characterization and Preparation Facility of the Hong Kong University of Science and Technology for assistance with XPS measurements.

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Footnote

† Electronic supplementary information (ESI) available: XPS C1s peak synthesis results. See DOI: 10.1039/c6ra17954a

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