Two-step process for programmable removal of oxygen functionalities of graphene oxide: functional, structural and electrical characteristics

Abstract

Here we report a two-step programmable reduction of graphene oxide (GO) which was synthesized by oxidation of graphite. X-Ray photoelectron spectroscopic (XPS) analysis confirmed the synthesis of exfoliated graphene oxide (GO) by introduction of oxygen as carboxylic (–COOH), epoxy (C–O–C) and hydroxyl (–OH) groups. The first step of GO reduction was achieved separately by (i) hydrazine (rGO₁₁) and (ii) sodium borohydride (rGO₂₁). Soda lime was used in the second-stage reduction of (a) hydrazine reduced GO (rGO₁₂) and (b) sodium borohydride reduced GO (rGO₂₂) to remove most of the remaining carboxylic functionalities from the rGO₁₁ and rGO₂₁ surface. XPS spectra of rGO₂₁ showed a decrease (38 to 30%) in the oxygen whereas the further reduction of rGO₂₁ with soda lime can further reduce the oxygen content. Quantitative analysis of C(=O)OX in GO shows about 43% of carbon atoms (C 1s signal) as carboxylic functionalities whereas the reduction of the GO with sodium borohydride reduced this signal to about 10%. The use of soda lime for both rGO₁₁ and rGO₂₁ further reduced the amount of carboxylic functionalities. An increase in the proportion of carbon atoms as sp² and decrease in the oxygen functionalities were controlled in the two-step reduction. A good correlation in the conductivity of reduced GO with the percentage proportion of sp² carbon was observed.

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1. Introduction

Since the discovery of the graphene, a two-dimensional carbonic material, there has been huge interest because of its distinct properties and potential applications.^1–3 Several researchers had reported the potential use of graphene in ballistic transport at room temperature,⁴ its high electron and hole mobility,⁵ in supercapacitors,⁶ and in thin film transistors.⁷ Functional groups at the surface of chemically synthesized graphene and tunable optical properties have advantages in biosensing and optoelectronics applications.^8–10 Several methods have been used to synthesize graphene such as (i) zip removal from carbon nanotubes,¹¹ (ii) micromechanical method to exfoliate graphite^12,13 and (iii) chemical vapor deposition.^14–18 The method used for production of graphene by zip removal from carbon nanotubes can provide high purity samples but there are significant challenges with regard to commercial production. The unzipping of single-walled carbon nanotubes can provide higher purity samples but again it is not a commercially viable method. The use of micromechanical methods to exfoliate graphite leads to graphene crystallite plane sizes of the order of 1 mm but such samples are only on the research scale at the moment.¹² Despite the several advantages of graphene, the large scale chemical synthesis of graphene with high purity is still a challenging task.

Graphene samples synthesized by chemical vapor deposition methods have been used in several applications such as photonics, nanoelectronics, transparent conductive layers, sensors, biomedical applications. Similarly chemically reduced graphene oxide samples with crystallite plane size of 100 μm are also used for coatings, paint/ink, composites, transparent conductive layers, energy storage devices and biological applications. Extensive studies haver been carried out for graphene synthesis by chemical routes.^19–24 Studies showed that the reduction of GO via chemical methods removes most of the oxygen functionalities from the surface of graphene oxide.^25–28

Graphene production at the large scale is possible by the chemical exfoliation of graphite. This includes, oxidation of graphite powder followed by reduction by strong reducing reagents such as hydrazine,^29,30 hydroquinone,³¹ sodium borohydride and its derivatives,^32,33 lithium aluminium hydride,³⁴ ascorbic acid,^35,36 saccharides,³⁷ norepinephrine,³⁸ KOH,³⁹ ethylenediamine,⁴⁰ polyelectrolyte,⁴¹ protein,⁴² sodium citrate,⁴³ plant extracts,^44–46 metal/acid,^47–54 melatonin,⁵⁵ amino acids,^56–59 bacterial respiration,^60–65 thermal treatment,⁶⁶ photocatalytic,^67–71 sonochemical,⁷² laser,^73–76 plasmas,⁷⁷ lysozyme,⁷⁸ electrochemical^79–81 electric current.⁸² However, the complete removal of oxygen functionalities at the surface of graphene oxide has not achieved via such reduction methods of GO. There have been reports assessing the potential of two-step reduction for removal of selective functional groups, but still these process are poorly understood.^83,84 The graphene synthesized via chemical routes contains several impurities and is substantially disordered. Thus, the synthesis of large surface area sheets of graphene with high purity via a chemical route is also very challenging.

The oxidation of graphite via chemical method introduces mainly carboxylic, aldehyde and ketonic functional groups at the edges and epoxide and hydroxyl groups at the basal planes of graphene oxide.^85–90 N₂H₄ and NaBH₄ are the most commonly used reducing agents for reduction of graphene oxide. NaBH₄ has ability to reduce aldehyde, ketone and carboxylic groups to hydroxyl groups whereas N₂H₄ can reduce epoxide and hydroxyl groups.^91,92 The above reported reducing reagents in the literature can remove most of the oxygen from the surface of GO but still a large proportion of carboxylic and hydroxyl groups may remain at the surface of graphene oxide.

Here we report a two-step programmable reduction of graphene oxide which was synthesized by oxidation of graphite. The main objective of the study was to target the removal of carboxylic acids from the surface of reduced graphene oxide. The uniqueness of this study was the use of soda lime for removing carboxylic functional groups from the surface of reduced GO by decarboxylation.⁹³ The first stage of reduction of GO was obtained via a chemical route by the separate use of (i) hydrazine and (ii) sodium borohydride. The physico-chemical nature of the synthesized graphene oxide and hydrazine and sodium borohydride reduced GO were characterized by different spectroscopic techniques. Subsequent effects of the soda lime on removal of carboxylic acid from the (i) hydrazine and (ii) sodium borohydride reduced GO were evaluated by X-ray photoelectron spectroscopy. In this study, we further quantified the proportion of carbon as sp² and sp³ in reduced GO and GO by XPS.

2. Materials and methods

2.1 Materials

NaBH₄, hydrazine hydrate and soda lime were purchased from Sigma Aldrich. Graphite flakes were obtained from CDH. H₂SO₄ and hydrochloric acid (HCl) were obtained from RANKEM. KMnO₄ and H₂O₂ were purchased from MERCK and S D Fine-Chem Limited (SDFCL), respectively. Milli-Q water (18 M ohm) was used as a solvent for all the experiments. All other chemicals were of analytical grade and purchased from local suppliers.

2.2 Synthesis of exfoliated graphite oxide sheets

Here, we have modified Hummer’s method for the synthesis of graphene oxide.^20,94 H₂SO₄ (46 ml) was added to a mixture of graphite flakes (2 g) and NaNO₃ (1 g) and stirred at 0–4 °C using an ice-bath until the solution became homogeneous. Gradually 6 g of KMnO₄ was added to the homogeneous graphite solution in 7 h at ∼20 °C by carrying out the reaction in an ice-bath during the reaction period. Further to this mixture, 6 g of KMnO₄ was added to the graphite homogeneous solution in 4 h at 35–40 °C and stirred for another 8 h. This reaction mixture was allowed to cool to room temperature (25 °C) and poured onto ice prepared from ∼260 ml of Milli-Q water. Finally 6 ml of 30% H₂O₂ was added to complete the reaction. Then the mixture was filtered with Whatman paper and the filtrate was collected. The filtrate was washed with 10% HCl and ethanol. Finally the product was thoroughly washed with Milli-Q water. The wet graphite oxide was dried by vacuum at room temperature for 5 days.

2.3 Reduction of GO

2.3.1 Reduction of GO with NaBH₄ and hydrazine hydrate. Synthesized graphite oxide (200 mg) was added to 200 ml water and ultrasonicated for 3 h while maintaining the pH at 8–9. NaBH₄ (1.600 g, 62.2 mM) was added to well dispersed graphene oxide solution and stirred at 70–80 °C for 2 h. The reaction mixture was allowed to cool down to room temperature and was filtered by Whatman paper. The filtrate was washed with methanol and MQ water and dried in vacuum at room temperature for 3 days. For reduction of GO by hydrazine, 200 mg of GO was dispersed in 200 ml water by sonicating for 3 h and 2 ml of 64.2 mM of hydrazine was added and the solution was continuously stirred at 95 °C for 24 h. The reaction mixture was allowed to cool down to room temperature and was filtered by Whatman paper. The filtrate was washed with methanol and MQ water and dried in vacuum at room temperature for 3 days.

2.3.2 Reduction of hydrazine and NaBH₄ reduced GO by soda lime. The reaction of soda lime with rGO can be described in terms of eqn (1):

R–COOH + 2(NaOH/CaO) → RH + Na₂CO₃ + H₂O (1)

where R–COOH represents rGO with remaining COOH functionalities.⁹³ The decarboxylation of rGO by soda lime removes carboxylic functionalities and generates reduced species “RH”.. 100 mg of NaBH₄ reduced graphene oxide were dissolved in 100 ml water by ultrasonication for 2 h. 60 mg of 6.2 mM soda lime at pH (7 to 8) was added and stirred at 45–50 °C for 1 h. To adjust the pH of the solution, 1 M NaOH and 1 M HCl was used. The solution was filtered by Whatman filter papers and the filtrate was subsequently washed with 1 M HCl, methanol and MQ water. The material was dried by vacuum at room temperature for 3 days. Alternatively, 100 mg of hydrazine reduced graphene oxide were dissolved in 100 ml water by ultrasonication for 2 h. 30 mg of 3.1 mM soda lime at pH (7 to 8) was added and stirred at 45–50 °C for 1 h. To adjust the pH of the solution, 1 M NaOH and 1 HCl was used in the same way discussed above. The solution was filtered by Whatman filter papers and the filtrate was subsequently washed with 1 M HCl, methanol and MQ water. The material was dried by vacuum at room temperature for 3 days.

Using the above methods for synthesis of graphene oxide and reduced graphene oxides, we have synthesized five different types of carbonic materials as: (i) graphene oxide (GO), (ii) N₂H₄ reduced GO (rGO₁₁), (iii) NaBH₄ reduced GO (rGO₂₁), (iv) soda lime reduced rGO₁₁ (rGO₁₂) and (v) soda lime reduced rGO₂₁ (rGO₂₂). The above materials were dissolved in three different solvents (water, tetrahydrofuran (THF) and methanol) and photographs of these are shown in Fig. 1. After sonication of 10 min, hydrazine reduced graphene oxide showed relatively less solubility as compared to the NaBH₄ reduced GO. For both the soda lime reduced GO samples very good solubility in all three solvents was observed. The GO and reduced GO dissolved in water were further characterized by Raman spectroscopy, UV-visible spectroscopy, ATR-FTIR spectroscopy, TGA, XPS and XRD.

Fig. 1 Optical images of graphene oxide (GO), N₂H₄ reduced GO (rGO₁₁), NaBH₄ reduced GO (rGO₂₁), soda lime reduced rGO₁₁ (rGO₁₂) and soda lime reduced rGO₂₁ (rGO₂₂) in three different solvents (water, THF and methanol).

2.4 Characterization

The extent of disorder in the crystal structures of the carbonic materials (graphite, graphene oxide and reduced graphene oxide) were characterized by Raman spectroscopy. Raman spectra of GO and reduced GO were measured by a RENISHAW System at 532 nm laser. Absorption spectra of GO and reduced GO were measured by a UV-2600 SHIMADZU spectrophotometer. ATR-FTIR spectroscope (Alpha-e Bruker System) was used to obtain information about the surface functionalities of GO.

The crystallite structure of GO and reduced GO was characterized from the XRD patterns. The XRD spectra were obtained by using a XRD-6000 (Japan) X-ray diffractometer in the diffraction angle range 5–80° with Cu-Kα radiation (λ = 1.54060 Å). Electrical characteristics of GO and reduced GO was characterized by taking the same amount of the materials and solution casting between gold electrodes. The current was measured at different voltages and the current–voltage relationship plotted. X-Ray photoelectron spectra of GO and reduced GO were obtained by MultiLab200 with standard Mg-Kα radiation to quantify elemental composition, surface carbon and oxygen functionalities. All spectra were taken at a working pressure of ∼10⁻⁹ mbar. Wide scan XPS survey was used for elemental proportion quantification and high-resolution spectra of C 1s was used for characterization of surface functionalities. The different surface states were obtained in the high resolution C 1s spectra by specifying a line shape, relative sensitivity factor, peak position, full width at half maxima, and area constraints.

3. Results and discussion

Raman spectra of graphite, synthesized GO and reduced GO (rGO₁₁, rGO₁₂, rGO₂₁ and rGO₂₂) are shown in Fig. 2. The Raman spectra of graphite shows a sharp peak at 1576 cm⁻¹ as shown in Fig. 2. Normally two distinct peaks in Raman spectra of graphite materials are observed due to (i) breathing of sp² carbon atoms (known as D band at ∼1360 cm⁻¹) and (ii) graphitic carbonic sp² carbon atoms (known as G band at ∼1580 cm⁻¹).⁹⁴ The peak at 1576 cm⁻¹ corresponds to the G-band which represents stretching of the C–C bond. The conversion of graphite into graphene oxide induces several disorders in sp²-hybridized carbon sheets; therefore an increase in D-band peak intensity of Raman spectra in GO occurs.^29,95 The Raman spectra of GO shows a wide peak at 1597 cm⁻¹ due to stretching of the C–C bond present in aromatic rings of GO containing sp² carbons. The peak at 1358 cm⁻¹ is mainly associated with disorder introduced by addition of oxygen atoms at the surface of graphite by oxidation process in GO. The observed ratio of the peak intensities of the D-band (I_D) with the G-band (I_G) peaks were 0.30 and 0.85 for graphite and GO, respectively. The relative peak intensity of the D-band at 1358 cm⁻¹ was increased as compared to the G-band at 1597 cm⁻¹ in GO in relation with graphite.

Fig. 2 Raman spectra of graphite, graphene oxide (GO), N₂H₄ reduced GO (rGO₁₁), NaBH₄ reduced GO (rGO₂₁), soda lime reduced rGO₁₁ (rGO₁₂) and soda lime reduced rGO₂₁ (rGO₂₂).

The Raman peaks for the D- and G-bands are at 1349 and 1581 cm⁻¹, respectively, for rGO₁₁. The measured intensity ratio of I_D/I_G for rGO₁₁ was 1.17. The peak intensity of the D-band as compared to the G-band in the Raman spectrum of rGO₁₁ was relatively higher which is similar to previous findings.²⁹ A further reduction of rGO₁₁ by soda lime which additionally deoxygenates the surface of rGO₁₁ led to a decrease (I_D/I_G ∼ 1.08) in the intensity of the D-band (at 1333 cm⁻¹) as compared to the G band (at 1596 cm⁻¹) in the Raman spectra of rGO₁₂. The Raman spectrum of NaBH₄ reduced GO (rGO₂₁) had a similar pattern to GO, with D- and G-band peaks at 1355 and 1589 cm⁻¹, respectively. The intensity ratio of I_D/I_G (∼0.93) for rGO₂₁ was slightly decreased as compared to rGO₁₁. The Raman peak position for D band and G bands are at 1331 and 1599 cm⁻¹, respectively, for rGO₂₂ and the peak intensity ratio I_D/I_G was ∼1.14.

The Raman spectra of GO showed a large red shift in the G-band position after oxidation of graphite into GO and results are shown in ESI (SFig. 1†). A previously similar red shift was observed by Bo et al.⁹⁶ Gupta et al.⁹⁷ had explained the red shift of the G band in the Raman spectra due to an increase in the number of layers of graphene. The change in Raman peak position and shape were used to estimate the number of layers of graphene.⁹⁸ Thus, the observed red shift in the G-band position of GO Raman spectra in our finding indicated an increase in the thickness of the layered structures of graphene oxide sheets. Reduction of GO by either of the reducing agents (i) NaBH₄ and (ii) N₂H₄ led to a decrease in the red shift of the D-band. The oxidation of graphite had showed a red-shift in the D band whereas the subsequent reduction of GO caused a blue shift in D band of the Raman spectra. This change may be due to change in sp² hybridized cluster size by addition/removal of oxygen functional groups from the surface in the oxidation and reduction process. A further reduction with soda lime causes a large red shift in the spectra and the D-band positions are at 1596 and 1599 cm⁻¹ for rGO₁₂ and rGO₂₂. We do not understand the mechanism, but it could be due to multiple folding of highly reduced graphene oxide. A previous Raman peak at 1582–1600 cm⁻¹ corresponded to glassy carbon in carbonic materials.⁹⁹

XRD spectra of synthesized GO and reduced GO (rGO₁₁, rGO₁₂, rGO₂₁ and rGO₂₂) are shown in Fig. 3. A sharp peak at 2θ ∼ 10°, corresponding to the reflection from the (002) plane, was observed in XRD spectra of GO.¹⁰⁰ A peak at 2θ ∼ 43° may correspond to the turbostratic band of disordered carbon materials. XRD spectra of N₂H₄ reduced GO showed a broad peak at 2θ ∼ 43° and the peak at 2θ ∼ 10° completely disappeared. Reduced graphene oxide has a peak around 2θ ∼ 23°. The broad diffraction peak of rGO indicates poor ordering of the sheets along the stacking direction. rGO₂₁ XRD had a broad peak at 2θ ∼ 10° with increased full width half maximum (FWHM). Further reduction of rGO₂₁ with soda lime shows a peak shift towards higher 2θ and an increase in the peak broadening. This change in peak position and FWHM of the peak could be due to the exfoliation of GO sheets after removal of the intercalated carboxylic groups.^101,102 These XRD results are closely related to the exfoliation and reduction processes of GO.

Fig. 3 XRD spectra of graphene oxide (GO), N₂H₄ reduced GO (rGO₁₁), NaBH₄ reduced GO (rGO₂₁), soda lime reduced rGO₁₁ (rGO₁₂) and soda lime reduced rGO₂₁ (rGO₂₂).

Fig. 4A shows UV-vis spectra of synthesized GO and reduced GO (rGO₁₁, rGO₁₂, rGO₂₁ and rGO₂₂) between the spectral range 200–700 nm. The absorbance peak of graphene oxide present at 225 nm corresponds to the π–π* transition and a peak at 303 nm is due to C=O.¹⁰³ GO reduced by NaBH₄ and further by soda lime shows a peak shift to 262.5 and 263.5 nm, respectively. Reduction of GO by hydrazine and further by soda lime leads to peaks at 256 and 266.5 nm, respectively. Previous studies also showed a red shift in chemically reduced graphene oxide^64,68,71 and our experimental results are also consistent with previous observations.

Fig. 4 (A) UV-vis spectra and (B) TGA of graphene oxide (GO), N₂H₄ reduced GO (rGO₁₁), NaBH₄ reduced GO (rGO₂₁), soda lime reduced rGO₁₁ (rGO₁₂) and soda lime reduced rGO₂₁ (rGO₂₂).

Thermal stability of synthesized GO and reduced GO (rGO₁₁, rGO₁₂, rGO₂₁ and rGO₂₂) was measured between 10 and 1000 °C by thermogravimetric analysis and results are shown in Fig. 4B. The thermal stability data of GO shows maximum wt% loss due to the higher number of oxygen functionalities and the pyrolysis of the labile oxygen functional groups at 150 °C. Further increase in the temperature shows a very small decrease in the remaining mass which may be due to release of CO and CO₂ gases. The comparative data for rGO₁₁ and rGO₁₂ show about 50% weight loss at 430 and 520 °C, respectively. These results suggest a lower number of oxygen groups present in rGO₁₂ as compared to rGO₁₁. Similar observations were recorded for rGO₂₁ and rGO₂₂.

Fig. 5 shows ATR-FTIR spectra of graphite, synthesized GO and reduced GO (rGO₁₁, rGO₁₂, rGO₂₁ and rGO₂₂). The absence of any functional groups in the ATR-FTIR spectrum of graphite was observed. Oxidation of graphite show peaks at 1035, 1390, 1635 and 1751 cm⁻¹ in the ATR-FTIR spectrum. The peak at 1751 cm⁻¹ corresponds to saturated carboxylic acids and the peak at 1635 cm⁻¹ corresponds to C=C bonds. Peaks at 1035 and 1390 cm⁻¹ may be due to the presence of C–O and C=O. The presence of different oxygen functional groups and an increase in D-band of Raman spectra confirms the conversion of graphite into graphene oxide by the oxidation process used in this study. The ATR-FTIR spectra of rGO₁₂ obtained after reduction of GO with N₂H₄ showed a very wide peak between 1300–1700 cm⁻¹. The disappearance of several functional peaks and the weak peak intensity between 1300–1700 cm⁻¹ confirms the removal of oxygen from the surface of GO by the reduction process. A small peak at 1255 cm⁻¹ is due to the presence of C–O. The FTIR result suggests that a large proportion of oxygen functionalities were removed from the surface of GO after reduction. The use of soda lime led to further removal of carboxylic functional groups from the surface of reduced GO (rGO₁₂).

Fig. 5 ATR-FTIR spectra of graphite, graphene oxide (GO), N₂H₄ reduced GO (rGO₁₁), NaBH₄ reduced GO (rGO₂₁), soda lime reduced rGO₁₁ (rGO₁₂) and soda lime reduced rGO₂₁ (rGO₂₂).

The strong appearance of the peak at 1645 cm⁻¹ in NaBH₄ reduced GO (rGO₂₁) ATR-FTIR spectrum indicates that the reduced GO contains a large number of C=C bonds. A peak at 905 in the spectrum was assigned to the alkenic functionalities in the reduced GO sample. Two small peaks at 1373 and 1108 cm⁻¹ could be associated with the C–O and –OH which indicate the presence of small proportion of oxygen groups at NaBH₄ reduced GO surface. ATR-FTIR spectra of soda lime deoxygenated NaBH₄ reduced GO (rGO₂₂) samples were very similar to the NaBH₄ reduced GO ATR-FTIR spectra as shown in Fig. 5. The peak intensity at 1373 cm⁻¹ was slightly reduced whereas a small increase in the peak intensity at 1645 cm⁻¹ was observed. XPS analysis was carried out for quantitative analysis of carbon functionalities of GO and rGO.

Wide scan XPS spectra of synthesized GO and reduced GO (rGO₁₁, rGO₁₂, rGO₂₁ and rGO₂₂) were obtained to further quantify the chemical nature of GO and reduced GO. Wide scan XPS spectra of these samples are shown in ESI (SFig. 2†). Oxygen/carbon elemental percentage proportions in synthesized GO were 38 and 62 atom%, respectively. XPS wide scan spectra of GO reduced with NaBH₄ showed a significant decrease (30 atom%) in the oxygen content at the surface. An even smaller value of 11 atom% oxygen was observed in N₂H₄ treated GO. The further reduction of rGO₁₁ and rGO₂₁ with soda lime, to produce rGO₁₂ and rGO₂₂, respectively, further reduces the oxygen content (to 4 and 28 atom%, respectively) at the surface of rGO.

High-resolution XPS spectra of C 1s of synthesized GO and reduced GO were obtained. The peak fitting for surface state quantification from C 1s was done as described in previous studies¹⁰⁴ and results are shown in Fig. 6. C 1s peak mainly fitted as hydrocarbon (CC), hydroxyl (COX), C=O/O–C–O and carboxylic functionality peaks.^105–107 In our analysis and peak fitting, we have separately fitted two peaks of hydrocarbons as C 1s (sp²) and C 1s (sp³) for a better representation of the XPS observations.¹⁰⁸ An additional peak at the tail of the spectra towards higher binding energy, which is the shake-up peak associated with carbon in aromatic rings, was also separately assigned during the peak fitting.¹⁰⁹ Thus, the higher resolution C 1s XPS spectra of GO was fitted with six peaks of different carbon environments as: hydrocarbon (C=C) at 283.5 eV, (C–C/C–H) at 285.7 eV, (C–OX) at 287.4 eV, (C=O/O–C–O) at 288.9 eV, (C(=O)OX) at 290.8 eV and a satellite peak at 293.5 eV due to π–π interactions. The positions of each peak associated with C–OX, (C=O/O–C–O) and (C(=O)OX) were fixed by assigning 1.5 ± 0.3 eV shift in the binding energy, respectively.¹¹⁰ Previously Chu et al.¹⁰⁸ had characterized amorphous and nanocrystalline carbon films and observed about ∼1.7 eV difference in the binding energy associated with C 1s (sp²) and C 1s (sp³) peaks of carbon. During peak fitting we have observed about ∼1.7 ± 0.3 eV difference in the binding energy for C 1s (sp²) and C 1s (sp³). The percentage proportion of different carbon environments in C 1s was 8.3, 17.7, 13.2, 17.6 and 42.1 corresponding to the C=C, C–C/C–H, C–OX, C=O/O–C–O and C(=O)OX, respectively.

Fig. 6 Peak fitted C 1s XPS spectra of graphene oxide (GO), N₂H₄ reduced GO (rGO₁₁), NaBH₄ reduced GO (rGO₂₁), soda lime reduced rGO₁₁ (rGO₁₂) and soda lime reduced rGO₂₁ (rGO₂₂).

The higher resolution C 1s XPS spectra of NaBH₄ reduced GO was fitted with six peaks of different carbon environments and the values for hydrocarbon (C=C) at 284.2 eV, (C–C/C–H) at 286.1 eV, (C–OX) at 287.6 eV, (C=O/O–C–O) at 289.1 eV, (C(=O)OX) at 290.6 eV and shake-up peak was at 293.2 eV. The percentage proportion of different carbon environments in C 1s was 22.9, 30.1, 24.2, 12.4 and 9.7, corresponding to C=C, C–C/C–H, C–OX, C=O/O–C–O and C(=O)OX, respectively. The higher resolution C 1s XPS spectra of soda lime reduced rGO₂₁ was also fitted with similar six peaks as: hydrocarbon (C=C) at 284.2 eV, (C–C/C–H) at 286 eV, (C–OX) at 287.4 eV, (C=O/O–C–O) at 288.9 eV, (C(=O)OX) at 290.5 eV and a shake-up peak at 293.1 eV due to aromatic carbon atoms. The percentage proportion of different carbon environments in C 1s was 43.9, 34.1, 12.6, 4.9 and 3.8 corresponding to C=C, C–C/C–H, C–OX, C=O/O–C–O and C(=O)OX, respectively.

The higher resolution C 1s XPS spectra of N₂H₄ reduced GO was fitted as: hydrocarbon (C=C) at 284.2 eV, (C–C/C–H) at 285.7 eV, (C–OX) at 287.1 eV, (C=O/O–C–O) at 288.6 eV, (C(=O)OX) at 290.3 eV and shake-up peak at 292.8 eV. The percentage proportion of different carbon environments in C 1s was 64.8, 17.2, 8.4, 3.2 and 5.6, corresponding to C=C, C–C/C–H, C–OX, C=O/O–C–O and C(=O)OX, respectively. The C 1s XPS spectra of soda lime reduced rGO₁₁ was fitted as: hydrocarbon (C=C) at 284.2 eV, (C–C/C–H) at 285.8 eV, (C–OX) at 287.2 eV, (C=O/O–C–O) at 288.3 eV, (C(=O)OX) at 290.5 eV and shake-up peak at 292.9 eV. The percentage proportion of different carbon environments in C 1s was 56.4, 26.9, 11.1, 1.9 and 3.7, corresponding to C=C, C–C/C–H, C–OX, C=O/O–C–O and C(=O)OX, respectively. The XPS spectra of GO showed a spectra shift towards lower binding energy which may be due to insulating nature of the sample.¹¹¹ However, there was no spectral shift observed in reduced GO, which indicates the conducting nature of reduced GO.

Fig. 7 shows quantitative analysis of C(=O)OX in synthesized GO and reduced GO (rGO₁₁, rGO₁₂, rGO₂₁ and rGO₂₂). The synthesized GO has about 43% proportion of carbon atoms in C 1s as carboxylic functionalities whereas the reduction with NaBH₄ reduced this level to about 10%. The use of soda lime further reduced the percentage proportion of carboxylic functionalities. The rGO₁₁ showed a very low percentage of carboxylic functionality as compared to NaBH₄ reduced GO. The remaining carboxylic functionalities of rGO₁₁ and rGO₂₁ were further reduced by soda lime and a low level of carboxylic functionalities were achieved. Soda lime reduced the carboxylic group amount significantly in both NaBH₄ and N₂H₄ reduced GO. An increase in the proportion of carbon atoms as sp² and decrease in the oxygen functionality was controlled in the two-step process in a much more precise way. Table 1 shows comparison of D and G band shifts in Raman spectra, peak intensity ratio of D to G Raman band and oxygen functionalities for different rGO samples.

Fig. 7 Percentage proportion of carboxylic functionalists in C 1s XPS spectra of N₂H₄ reduced GO (rGO₁₁), NaBH₄ reduced GO (rGO₂₁), soda lime reduced rGO₁₁ (rGO₁₂) and soda lime reduced rGO₂₁ (rGO₂₂).

Table 1 D and G Raman band shifts and oxygen functionalities for different rGO

Sample	Raman D-band (cm^−1)	Raman G-band (cm^−1)	ID/IG^a	XPS COOX^b (%)
a Experimentally observed peak intensity ratio of D-band (ID) and G-band (IG) from Raman spectra.b Percentage of carboxylic functionalities in C 1s XPS spectra.
rGO11	1349	1581	1.17	5.6
rGO12	1333	1596	1.09	2.7
rGO21	1355	1589	0.94	9.7
rGO22	1331	1599	1.14	3.8

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Fig. 8 shows I–V characteristics of GO and reduced GO (rGO₁₁, rGO₁₂, rGO₂₁ and rGO₂₂). As expected, the GO did not show current conduction whereas the I–V response of hydrazine reduced GO (rGO₁₁) showed a linear response. Two-step reduction led to a change in the current response, with the response of rGO₂₁ being very distinct from that of rGO₁₁. Initially a low current conduction was observed up to the 2 V, but a further increase in the bias voltage showed a rapid increase in the current and this reached saturation at 4 V bias voltage.

Fig. 8 I–V response of GO (black squares) and reduced GO samples (green squares). (A) rGO₁₁, (B) rGO₁₂, (C) rGO₂₁ and (D) rGO₂₂.

Change in the relative conductivity of different types of reduced GO can be assessed by comparison of current at contact bias voltage in the I–V response curve. The conductivity of different types of reduced GO was observed according to the chemical and structural nature of the reduced GO. It is seen that the conductivity of the reduced GO decreases with the percentage of sp² carbon obtained from XPS analysis (ESI SFig. 3†). The two-step process of reduction described here may provide an improvement and better structural, functional and electrical properties control in the reduction of GO. The two-step process of reduction described here may also provide a better conversion of graphite into graphene.

4. Conclusions

Here we have described a method for a synthesis of chemically reduced graphene oxide which has a higher proportion of graphene. First we have prepared highly exfoliated graphene oxide from graphite which was reduced by hydrazine and sodium borohydride. The Raman spectroscopic and XPS analysis confirm the synthesis of exfoliated graphene oxide by chemically introduced oxygen as carboxylic groups (–COOH), hydroxyl (–OH) and epoxy groups (C–O–C). Two distinct peaks of graphene oxide and reduced graphene in Raman spectra are present due to the breathing mode of sp² atoms (graphitic carbonic sp² of carbon atoms). Raman spectra of hydrazine reduced GO showed relatively higher intensity of the D-band as compared to the G-band in the spectra. A strong red shift in the G-band position was observed after oxidation of graphite into GO due to an increase in the number of layers of graphene. The reduced GO by either reducing agent NaBH₄ and N₂H₄ showed a decrease in the red shift of the D-band due to decrease in the thickness of the reduced GO sheets. The synthesized GO has very high percentage of carbon atoms as carboxylic functionalities whereas reduction with NaBH₄ and particularly hydrazine reduces this level. Soda lime reduced the carboxylic group significantly further in both NaBH₄ and N₂H₄ reduced GO. The two step process of reduction described here provides an improvement in the reduction of GO, and therefore better conversion of graphite into graphene.

Acknowledgements

This work was supported by network project (NanoSHE) of Council for Scientific and Industrial Research (CSIR), Ministry of Science and Technology, Govt. of India. KD is a Project Student at CCMB from Centre for Converging Technologies, University of Rajasthan, Jaipur 302004, India.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra18880f

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