A simple synthesis of sulfur-doped graphene using sulfur powder by chemical vapor deposition

Abstract

We wish to report the growth of S-doped graphene (SG) with high sulfur content (5 at%, determined by XPS) using chemical vapor deposition (CVD). The nano-sized Fe/CaCO₃ was employed as a catalyst, flow rate-controlled acetylene gas was introduced into the CVD furnace as a carbon resource and the solid sulfur powder was placed in the first zone of CVD as a sulfur resource. The structures, sizes and specifications of the prepared SGs were determined and confirmed by field emission scanning electron microscopy, transmission electron microscopy, X-ray energy dispersive spectroscopy, elemental mapping, powder X-ray diffraction, Raman spectroscopy and X-ray photoelectron spectroscopy analyses. The surface area and porosity of SG were obtained via a low-temperature N₂ physisorption. The presented method could be employed as an efficient, fast and cheap technique for the preparation of SG.

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Introduction

Carbon nanostructures, especially graphene, have attracted worldwide interest based on their significant physical, chemical and mechanical properties as well as their potential applications in electronics, sensors, supercapacitors and batteries.^1–5 However, it is possible to change their band gaps to improve the on/off ratio properties of their applications in electronic devices.^6,7 Recently, doping of carbon materials such as graphene, CNTs and porous carbon materials with various heteroatoms (such as N, B, P and S) has been a promising approach to tailor the electronic properties of the sp² carbon structures and enhance the electrochemical and physical properties of the product.^8–12 Generally, B-doped carbon nanomaterials could act as p-type and N- or S-doped carbon nanomaterials could act as n-type conductors.^13–17 Theoretical studies showed that the substitutional doping can change the energy gap of carbon nanostructures, leading to change their conductivity and chemical reactivity.^18–20 Our recent theoretical studies showed that S-doped carbon nanostructures such as fullerenes, CNTs and graphenes are dramatically sensitive to commonly used substances such as CH₃OH, CH₃SH, H₂O, H₂S, and molecular halogens.^21–24 During the past decades, an increasing interests have been observed in doping of various atoms (like sulfur) into carbon frameworks.^25,26 In this way, insertion of sulfur atoms during the growth of CNTs could be occurred by chemical vapor deposition (CVD) method using liquid sulfur-containing materials such as carbon disulfide and dimethyl sulfide as precursors.^27,28 In addition, different methods (such as thermal annealing of graphene oxide) were demonstrated to produce S-doped graphene (SG).²⁶ For example, Poh et al. synthesized SG via thermal exfoliation of graphite oxide in hydrogen sulfide, sulfur dioxide, or carbon disulfide. They found that SGs could be applied as metal-free electrocatalysts for an oxygen reduction reaction.²⁹ Yang et al. have prepared SG by direct annealing of graphene oxide (GO) with benzyl disulfide (BDS) under argon atmosphere. They found that SG could exhibit excellent catalytic activity, long-term stability and high methanol tolerance in alkaline media. In addition, they employed SG as an efficient metal-free cathode catalyst for oxygen reduction.³⁰ Gao et al. fabricated SG on copper substrate by CVD at 950 °C, using liquid organic material precursors.³¹ Liang and co-workers has prepared SG via an annealing method at 1000 °C with induced H₂S gas flow (as sulfur source) on Si/SiO₂ substrate. They demonstrated that sulfur atom plays acceptor role in doped graphene, leading to a p-type behavior of SG.³² In 2014, Zhang et al. reported a low-temperature approach to prepare large area N- and S-doped graphene films on copper foil. By using these methods, the percentage of S content in graphene was obtained in the range of 0.55–1.54 at%.³³ Recently, SG has been used for detection of chemical compounds such as hydrogen peroxide and dopamine.^34,35 It has been well established that SG could exhibit good catalytic activity in the aerobic oxidation of hydrocarbons.³⁶ Moreover, some reports have been found on the synthesis of sulfur-doped microporous carbons via pyrolysis of appropriate precursor at high temperatures^37–39 or heat treatment of carbon adsorbents with hydrogen sulfide.^40,41 Xia et al. recently reported sulfur-containing porous carbon material, generated via infiltration of the carbon precursors into zeolite pores via impregnation, combined with CVD method.⁴² Vidic et al. prepared sulfur functionalized carbon structures from sulfurization of carbon's surface in presence of hydrogen sulfide and employed the product in the adsorption of mercury.⁴³ However, there is a big challenge for the synthesis of S-doped carbon nanomaterials with high atomic percentage of sulfur impurities and the higher specific surface area. In this line, innovation of cheaper techniques, using more available materials and preparing the product at lower temperature will be more welcomed. This should be noticed that because of the larger atomic radius of sulfur atom versus carbon atom and the weaker energy of the C–S bond (272 kJ mol⁻¹) versus C–C bond (346 kJ mol⁻¹), it is difficult to dope S atom into the graphene lattice⁴⁴ and many attempts have been failed to produce SG. Herein, we have demonstrated a facile single-step method for fabricating SG powder with a high content of doped S atoms (5 at%, determined by XPS), which was achieved by CVD at 600 °C. The general process involves Fe/CaCO₃ as catalyst, sulfur powder as sulfur precursor and acetylene gas as carbon precursor. The removal of the support, impurities and the catalysts was easily done by mild acid treatment and washing with distilled water, which enable this method as simple and low-cost method for production of SG.

Experimental

Fe(NO₃)₃·9H₂O (96% purity) and CaCO₃ (99.95% purity) was purchased from Merck company (Germany) and sulfur powder (industrial grade) was obtained from Pars chemical Co (Isfahan, Iran). Acetylene (99.99% purity) and argon (99.99% purity) gases were bought from Tebgas Co. (Isfahan, Iran). All of these materials were used without further purification. The growth step were performed using a hand-made, double-zone electric tube furnace (designed by Nano sat Co, Semnan, Iran; http://www.nanosatco.com). This instrument is consisted of a quartz tube with 110 cm length and 10 cm internal diameter. Field emission scanning electron microscopy (FESEM), X-ray energy dispersive spectrometry (EDS) and elemental mapping analyses have been performed using Mira 3-XMU FE-SEM (Tescan Co, Brno, Czech Republic). Transmission electron microscopy (TEM) analysis was performed using EM208S TEM microscope (Philips Co, The Netherland). In addition, powder X-ray diffraction (XRD) spectrum was recorded by XPERT instrument (Philips Co, The Netherland) using Cu as a radiation source and X-ray photoelectron spectroscopy (XPS) analysis was performed by Specs model EA10 plus (Bestec Co, Germany) using Al as a radiation source. Moreover, Raman spectra were obtained using Senterra (Bruker Co, USA) instrument using 514 nm laser beam and the low-temperature N₂ physisorption was performed using Finesorb 3010 (Finetec Co, China) instrument at −196 °C. More details about the characterization techniques have been provided in the ESI.†

Preparation of catalyst

Each synthesis of nanostructured materials using CVD is consisted of three separate steps, preparation of the catalyst, the growth (synthesis) step and purification of the product. In the present work, Fe/CaCO₃ has been selected as catalyst because during our preliminary experiments, by employing Fe/CaCO₃ the higher yields have been obtained for the final product. In addition, by employing Fe/CaCO₃ as catalyst, the use of H₂ gas is unnecessary for the growth step and removing the catalyst in the purification step will be easier. In the first step, to prepare Fe/CaCO₃ nanocatalyst, the sol–gel method was applied as it was described in the reported studies.^45,46 First; Fe(NO₃)₃·9H₂O was weighted and dissolved in distilled water and the solution was added drop-wise to CaCO₃ support. The total metal loading was about 5 wt% relative to the carbonate.⁴⁷ Then, during 5 h heating at 90 °C with continuous stirring, the solvent was evaporated. The pH of the suspension was kept constantly at 7.2 by adding ammonia solution in order to reduce the release of CO₂ when the carbonate react with acid. The prepared product was dried at 130 °C (overnight) and then calcinated at 400 °C in air for 16 h.⁴⁸

Synthesis and purification of SG

To prepare SG by CVD, all parameters of the growth step should be optimized first. Therefore, the best condition for the chemical structure of catalyst, reaction temperature, amount of sulfur powder, the growth time, acetylene and argon flow rates and temperature program (for increasing and decreasing the temperature) was determined by preliminary experiments and studying the reported studies. Based on these experiments, in each run for preparing SG, 0.2 g of prepared nanocatalyst (Fe/CaCO₃) were placed on the small area (5 × 1 cm) of an alumina boat (20 × 2 cm) in the second thermal zone and 1.4 g of sulfur powder were separately placed on the alumina boat (12 × 1.5 cm) in the first zone of our CVD instrument. Then, the heating of the second zone was started in 10 °C min⁻¹ heating rate to reach to 600 °C and the first zone was heated to reach its temperature to 450 °C at the same time while argon gas was flowing in 50 mL min⁻¹ rate. After that, the temperature of the second zone was fixed at 600 °C, argon flow rate was increased to 250 mL min⁻¹ and acetylene gas was introduced into the reaction chamber at a constant flow rate of 90 mL min⁻¹ for 60 min. After this step, the acetylene flow was stopped and the furnace was cooled to room temperature with the argon atmosphere flowing at 50 mL min⁻¹. The produced SG was collected from the alumina boat (in the second zone) and purified by stirring in hydrochloric acid solution (37% HCl, 15 min, r.t.) with sonication. The product then washed with distilled water and dried at 120 °C overnight. The quantity of final product was 0.5 g. Therefore, based on the total value of the acetylene, as a carbon resource of doped graphene, the final yield was 8.6 wt%.

Characterization of product

The synthesized SG was first analyzed by FESEM and TEM to ensure, its shape, characteristics and size. In addition, during FESEM analysis, X-ray energy dispersive spectrometry (EDS) analysis and elemental mapping were employed to determine its elemental composition. During FESEM characterization, the samples were mounted on the sample holder without needing further processing (like gold coating) due to its high conductivity. The surface area and porosity of SG were obtained via a low-temperature (−196 °C) N₂ physisorption, based on Brunauer–Emmett–Teller (BET) theory. The Brunauer–Emmett–Teller specific surface area was calculated from the nitrogen adsorption isotherm in the P/P₀ range of 0.06–0.24. To sum up, the whole nitrogen isotherm is collected in the P/P₀ values from 0.02 up 0.96, but the BET surface area is calculated only from part of the isotherm, usually in the range 0.02–0.24. The total pore volume is calculated from the last point of isotherm for P/P₀ = 0.96. The product was also analyzed using XRD, XPS and Raman spectroscopy.

Results and discussion

Morphology and chemical structure

The prepared product (SG) was first analyzed by FESEM. The FESEM images of different products, obtained at 600 °C, 650 °C, and 700 °C (at the optimum conditions) were shown in Fig. 1a–f. In the higher temperatures (than 700 °C), we have obtained the graphene with less sulfur content and less purity. The resulting materials in different temperatures were named as SG-600, SG-650 and SG-700. These SEM analyses revealed that various nano sheets (or SGs) with nearly similar morphologies could be observed via this method. However, in the lower temperature (600 °C), more uniform structures was obtained. In this product (SG-600), the graphene layers are more homogenous, have smaller sizes and have less wrinkles than SG-650 and SG-700. Therefore we have focused on this product in next analyses.

Fig. 1 FESEM images of prepared SGs at 600 °C (a and b), 650 °C (c and d) and 700 °C (e and f).

The observed size (the thickness of the graphene sheet) of the product (SG-600) was about 10 nm. Moreover, the elemental mapping, EDS spectrum and XRD analysis of SG-600 were shown in Fig. 2a–d, in addition to 2 μM (the largest) FESEM image of the product. The elemental composition analysis for SG-600 (Fig. 2c), obtained from the EDS spectrum (in FESEM instrument), confirmed successful doping of sulfur atoms in the graphene structures. The concentration of sulfur atom in SG-600, SG-650 and SG-700 (obtained from the EDS analyses) are respectively 9.12, 7.66 and 4.05 wt%, which show the maximum doping was observed at 600 °C.

Fig. 2 FESEM image (a), elemental mapping of S (b), EDS spectrum (c) and XRD pattern of SG-600 (d).

Elemental mapping image (Fig. 2b) shows that the S distribution in the plane of SG-600 is nearly uniform. It seems that S atoms were covalently incorporated into the carbon matrix rather than making aggregations or clusters on the surface of SG. The XRD pattern of SG-600 was also shown in Fig. 2d. The peaks around 2θ = 22.2 and 2θ = 44.5 are corresponding to graphitic planes (002) and (101), respectively.⁴⁹ The XRD pattern for pristine graphite exhibits a sharp (002) signal centered at around 26°, as observed in the literature.⁵⁰ The appearance of the (101) diffraction pattern is attributed to the presence of sulfur atoms (or defects) in the graphene layers, which is similar the recently reported results.^51,52

Transmission electron microscope (TEM) is more useful tools to observe the exact structure of nanomaterials. Our SG-600 product was analyzed by TEM and the result was shown in Fig. 3a and b. This figure shows that the doped graphene sheets are thin and highly transparent versus the electron beams. The TEM picture shows us a thin sheet, twisted on some places (like large crumpled paper) and some darker point, maybe because of wrinkle places, aggregation of carbon structures or existence of sulfur atoms. This type of rumpling was produced during the growth step and it has also been observed in the graphene and N-doped graphene by the previously reported studies.^53–55 Moreover, because of the high transparency of the graphene sheets in these pictures and comparison with the reported TEM images of graphene (simple or doped), they probably consisted of only 3–7 graphitic layers.^53–55

Fig. 3 TEM images of SG-600.

Raman spectroscopy has been mostly used to conclude the number of layers, strain, doping level, density of defects and functional groups of simple and doped graphenes.^56,57 The two main signals in the Raman spectra of carbon nanomaterials are commonly observed at 1350–1360 cm⁻¹ and 1585–1595 cm⁻¹, respectively corresponding to the D and G bands.^29,58 These values are similar in all forms of carbon structures, consisted of both simple and doped graphene because they refer to carbon–carbon bonds. The G band is related to the bond stretching of all pairs of sp² atoms in graphene layers (due to the E_2g vibration mode) and the D band is related to defect in sp² lattice (due to an A_1g breathing modes of sp² atoms in rings) and seen in disordered graphite. These defects that have sp³ nature are active sites in electrochemical reactions.^59,60 In Fig. 4, the Raman spectra of SG-600 and SG-700 were shown and compared. For SG-600, D band signal is observed at 1413 cm⁻¹ and G band at 1607 cm⁻¹. The intensity of the D band is higher than that of G band, not only because of S-doping in the graphene structure, but also low carbonization temperature is contributed to high D-band intensity. In comparing with the spectrum of SG-600, the D band in the Raman spectrum of SG-700 is located at 1304 cm⁻¹, which showed a downshift (109 cm⁻¹), while the G band of SG-700 located at 1603 cm⁻¹ without important change. It is well established that the presence of dopants or defects causes a slight upshift of the D band in the Raman spectrum.⁶¹ Moreover, both D and G bands in the presented Raman spectra are broadened, which shows some disordered structures in the prepared SGs.^62,63 The density of defects is related to I_D/I_G (the ratio of the intensities of the G and D bands, respectively) that show the degree of disordered structures in the carbon lattice.⁶⁴

Fig. 4 The Raman spectra of SG-600 and SG-700.

The I_D/I_G ratios of SG-600 (with the higher S content) and SG-700 are respectively 1.54 and 1.50, indicating that by decreasing the sulfur content, the amount of disordered structures decreases. In addition, it could be said that increase of the temperature yields grapheme wilt less defects. The in-plane crystallite size (La) of the graphene layers could be calculated by using I_D/I_G ratio (that was exactly calculated by integration of deconvoluted peaks using computer) in the following equation.⁶⁵

La = (2.4 × 10⁻¹⁰) × λ_laser⁴ × I_G/I_D (1)

λ_laser is the laser's wavelength used in the Raman experiment and I_D and I_G refer to the relative intensities of D and G bands. Therefore, the calculated La values are 10.9 nm for SG-600 and 11.2 nm for SG-700.

Sulfur content and type of S bonding in products

XPS analysis was employed to find the surface composition and the nature of binding between C and S in SG-600, as shown in Fig. 5a–c. Fig. 5a shows a total XPS spectrum that composed mainly of carbon (C 1s = 284.8 eV), sulfur (S 2p = 164.6 eV) and oxygen (O 1s = 533.1 eV), related to the oxidized surface of SG.⁶⁶

Fig. 5 Wide-range XPS spectrum (a) and high-resolution XPS spectra of C 1s and S 2p signals for SG-600 (b and c).

The elemental content in the surface layer could be obtained from the corresponding peak areas. The presence of sulfur atom in this spectrum suggests that S atoms are successfully incorporated into the structure of SG-600. To probe the chemical state of sulfur and carbon in the carbon lattice, we carried out the high resolution XPS spectra of C 1s and S 2p core level peaks. As shown in Fig. 5b, the C 1s spectrum can be deconvoluted into four different peaks (using Shirley backgrounds) at the binding energies of 284.8, 285.7, 286.4 and 288.1 eV, corresponding to C=C, C–C, C–S and C=O or O–C=O, respectively.^67–69 The main peak of C 1s at 284 eV is attributed to the sp² carbon, indicating that most of the C atoms are in the conjugated honeycomb lattice. The small peak at 285.7 eV is assigned to the sp³ hybridized carbon. The appearance of a C–S peak at 286.4 eV shows that sulfur atoms are covalently bonded to the carbon atoms in the lattice structure of the graphene. The spectrum also has a peak at 288.1 for C=O or O–C=O interactions, which is of a higher energy than the other interactions due to oxygen being more electronegative and therefore, have stronger charging behavior.²⁹

It is well known that some oxygen atoms could react with C atoms during the growth or purification (acid washing) step of simple or doped carbon nanomaterials.³¹ The high resolution XPS scan of S 2p (Fig. 5c) can be resolved into two main peaks at the binding energies of 164.01 and 165.17 overlapping with each other, which is similar to previous XPS analysis for S-doped carbon materials.^10,70,71 The peaks at 164.01 eV and 165.17 eV correspond to 2p_3/2 and 2p_1/2 positions of thiophene-S, respectively.⁴² As shown in Fig. 5c, the intensity ratio of 2p_3/2 and 2p_1/2 is 2 : 1 due to spin–orbit splitting of thiophen's sulfur atoms.^72,73 There are no signals around 168 eV, which shows that S exists only in reduced –C–S–C– state, without oxidized S=O moieties.^74,75 Therefore, the sulfur is mainly doped at the defects of carbon lattice in the form of thiophene-like structure.

Adsorption–desorption diagram

The surface area and porosity of SG-600 were characterized by adsorption–desorption diagram (for nitrogen) using BET theory. The whole nitrogen adsorption–desorption isotherm of SG-600 is given in Fig. 6 (in the P/P₀ range of 0.02–0.96). As it shown in Fig. 6, the isotherm for the product is of type III, according to the planar surface of prepared SG.^49,76 The isotherm exhibits no hysteresis, indicating the presence of blind cylindrical, cone-shaped and wedge-shaped pores.⁷⁷ The adsorption–desorption diagram also indicates that the specific surface area of the SG-600 is 771 m² g⁻¹ and the product is non-porous. These results confirm the expected structures from doped graphene sheets.

Fig. 6 The whole nitrogen adsorption–desorption diagram of SG-600.

Conclusion

In this work, a facile and cheap route was reported for preparation of SG using sulfur powder and acetylene gas by CVD instrument. Therefore, a series of SG with various sulfur content at different growth temperatures were prepared. The products were characterized by FESEM, EDS, elemental mapping, TEM, XRD, Raman, XPS and nitrogen adsorption–desorption diagram. The SG prepared at 600 °C has the highest sulfur content (about 5 at%, obtained by XPS) and more uniform shape. The elemental mapping confirmed that sulfur atoms are homogenously incorporated into SG-600. Raman and XPS results show that sulfur atoms are successfully incorporated into the carbon matrix of samples with covalent bonds. The high-resolution XPS scan of S 2p shows that sulfur atoms formed in the thiophene-like structures. The nitrogen gas adsorption–desorption measurement show high surface area (771 m² g⁻¹) for SG-600. The results reveal that our method can be used as an effective way for producing non-porous sulfur-doped graphenes with high surface areas and high content of sulfur atoms. We believe this synthetic route shows a new way for production of sulfur and other doped carbon nanomaterials.

Acknowledgements

This work was supported by the research affairs of Isfahan University of technology.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra02109c

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