We used cationic surfactant, cetyltrimethyl ammonium bromide (CTAB), to produce graphene flakes in water by mechanical exfoliation of graphite via ultrasonication (Fig. 1)65. The surfactant gets adsorbed on the surface of so produced graphene via Van der Wall’s interactions and stabilizes the 2-dimensional material (I in Fig. 1). In the following step, a cobalt complex (Na 3 [Co(NO 2 ) 6 ]: CoL) was added whose anion ([Co(NO 2 ) 6 ]3-) electrostatically binds with the the positive head of the surfactant (II in Fig. 1). In third step, cobalt complex was reduced to produce Co/G NSs (III in Fig. 1). In past, stable graphene dispersions with an optimum yield have been achieved with a maximum surfactant concentration of 0.1 mg/ml70. Any higher concentration of surfactant leads to the agglomerations of graphene flakes due to the disruption of electric double layer by counter ions71.

Figure 1 Schematic presentation for the synthesis of Co/G NSs. (I) Mechanical exfoliation of graphite in the presence of CTAB; (II) Cobalt complex electrostatically binds with N-functions of the surfactant; (III) Reduction of Co ion to Co/G NSs. Full size image

UV-vis absorption spectrum of graphene dispersion was found featureless and flat (Fig. 2a) at and above 400 nm where absorption by surfactant molecules was negligible. Absorption at 660 nm could therefore be attributed to suspended graphene and was used to find the concentration using Beer-Lambert law (A = αCl)71. For the calculation of extinction coefficient (α), a graphene dispersion (500 ml) was prepared using graphite (3 mg/mL) and CTAB (0.1 mg/mL). After centrifugation and decantation, absorption spectrum was taken at 660 nm. 60 mL of this dispersion was filtered through pre-weighted filter units, dried and weighed to find the graphene concentration. The value of absorptivity constant was found to be 1256 mL/mg/l which is close to the reported value of 1390 mL/mg/l65.

Figure 2 (a) Absorption spectrum of CTAB-supported graphene. Tyndall effect shown by graphene dispersions is shown in inset. Surfactant absorption is dominated below 400 nm while any absorption above attributes to suspended graphene. (b) Graphene yield as the function of sonication time; CTAB concentration (C CTAB ): 0.1 mg/mL; initial graphite concentration (C G ) of 3 mg/mL. (c) Raman spectra (graphite, graphene and Co/G NSs) and zeta potential (d) of graphene synthesized by liquid-phase exfoliation of graphite. Full size image

Next, various parameters for instance, sonication time, surfactant and graphite concentration were optimized to improve graphene yield. It was found that graphene yield increases almost linearly with sonication time as presented in Fig. 2b. The yield (8 μg/mL) obtained after ten hours of sonication was used as the optimum value for the rest of the experiments. In the same way concentration of graphite (C G ) and CTAB (C CTAB ) were also optimized and optimum yield of graphene was found with C G = 3 mg/mL and C CTAB = 0.1 mg/mL [see Fig. S1 in supporting information (SI)]. Under optimized conditions, 500 mL of graphene dispersion was centrifuged (@15000 rpm for 1 hour) followed by vacuum filtration to remove undissolved graphite. Afterwards graphene concentration in the dispersion was calculated using uv-visible spectrophotometry65.

Raman spectroscopy66,72 was used to get further structural information about graphite, graphene and Co/G NSs materials. All the spectra as presented in Fig. 2c contain D (~1352 cm−1), G (~1587 cm−1) and 2D (~2710 cm−1) bands65. The D band corresponds to the breathing modes of sp2 atoms in rings73 while G band attributes to in-plane vibrational modes of sp2-bonded carbon atoms74. 2D band which is the second order of D band appeared at ~2710 cm−1 in all the spectra. Intensity ratio of G and 2D bands (I G /I 2D ) can be used to differentiate graphite from graphene and is probably the best way to decide layer-thickness75. I G /I 2D ratio in our case decreased from 1.42 in graphite to 1.2 in graphene and to 1.0 in Co/G NSs material. The latter corresponds roughly to a thickness of 5-layers76. Raman spectrum of Co/G NSs showed an additional intense band at ~520 cm−1 which was assigned to A 2u lattice mode of cobalt(II) hydroxide77,78,79.

Stability of graphene dispersions was tested by zeta potential analysis (Fig. 2d) as these colloids were expected to be stabilized by electrostatic repulsion between the surfactant-coated graphene sheets. Hydrophobic tail of CTAB gets adsorbed on graphene sheets due to Van der Waal forces and impart positive charge due to the positively charged head of surfactant80. The formation of so-called electric double layer will avoid possible agglomeration of graphene sheets81. It is reported that colloidal particles will be electrostatically stabilized if the zeta potential values fall between −15 mV and 15 mV82. Our graphene dispersion showed a zeta potential value of 47 mV which indicated stability of the system83.

The morphology of Co/G NSs was analysed by TEM. Figure 3a presents an overview image of the Co/G NSs material showing a sheet-like morphology. At a higher resolution (Fig. 3b) folded nanosheets (NSs) with a thickness of ~3.8 nm (inset) were observed. A SEM image of the material is presented in Fig. 3c where again folded NSs can be clearly seen. EDX maps of the selected region suggest the presence of Co, O and C with an atomic ratio of 52, 25 and 9 atomic percent. Therefore we conclude that NSs are made up of either oxides or hydroxides of Co. EDX spectrum of another region is presented in S2 (SI) provides further confirmation about the chemical nature of NSs.

Figure 3 Transmission Electron Microcopy (TEM) images of Co/G NSs catalyst. Inset in (b) shows a magnified view of a selected portion. SEM with EDX mapping of the selected region is presented in (c). Full size image

Co/G NSs material was further confirmed by X-ray photoelectron spectroscopy (XPS). In survey spectrum of the material presented in Fig. 4a, peaks at 284.0, 531.0 and 779 eV were indexed to C(1s), O(1s) and Co(2p) respectively. Magnified view of C(1s) spectrum is provided in Fig. 4b where a main peak at 284.0 was assigned to sp2 carbon and a small peak at higher binding energy (286.0 eV) to O-C-O and C-OH groups84. The oxygen (1 s) core spectrum (Fig. 4c) was deconvoluted into two peaks at 530.2 and 530.6 eV and were assigned to O(1 s) in cobalt hydroxide Ca(OH) 2 85.

Figure 4 XPS spectra of Co/G NSs material. (a) Survey spectrum and high resolution spectra for O1s (b), C1s (c) and Co2p (d). Full size image

High resolution XPS spectrum of Co 2p (Fig. 4d) showed a spin-orbit splitting into 2p 3 /2 and 2p 1/2 with an energy difference of ~16 eV typically known for cobalt(II) hydroxide86,87. 2p 3/2 peak was deconvoluted into two peaks centering at 782.6 and 787.6 eV and were assigned to Co(OH) 2 , the latter being the shake-up satellite of 2p 3/2 88. The peak at higher binding energy was broken down into two Gaussian peaks at 798.6 and 804.5 eV and were attributed to 2p 1/2 and its shake-up satellite for cobalt hydroxide respectively. Although Co 2p 3/2 peak of Co 3 O 4 also occurs in the same region (~779 eV), its presence in the sample was ruled out because of the fact that Co 3 O 4 does not show any satellite peak89.

Co/G NSs composite for oxygen evolution reaction (OER) was tested in 0.1 M KOH using a three-electrode system details of which are given in experimental part. Linear sweep voltammetry (LSV) of the material revealed an efficient OER catalytic activity (onset 1.51 V) with a low over potential of 280 mV to achieve 10 mA/cm2 current density (Fig. 5a). This OER activity (280 mV) of Co/G NSs is even better than the benchmark IrO 2 catalyst (297 mV) under the same experimental conditions90 and other cobalt-based catalysts reported to date (see Table S1).

Figure 5 LSV polarization curve (a), Tafel plot (b), chronoamerpmetric plot (c) and Nyquist plot (d) of Co/G NSs catalyst. Circuit diagram is shown in (d) that was applied over the EIS data. Full size image

For electrocatalytic performance, Tafel slope can be used for the kinetics study to determine the rate and rate determining step during OER reaction. We calculated a value of 79.2 mV/dec for Co/G NSs material (see Fig. 5b) which is though high as compared to other first-row transition metals, nevertheless provides valuable information about the kinetic parameters. Value of Tafel slope suggests that the formation of MO (where M denotes a free metal site) from the adsorbed OH− ions is the rate determining step91. The mass activity (583.3Ag−1) and TOF (0.089 s−1) of Co/G NSs catalyst were also found to be high (Table S2) at low mass loading (17 µg/cm2).

Stability of the catalysts was evaluated by controlled current electrolysis (CCE) experiment performed in 0.1 M KOH (pH = 13). Chronoamperometry was conducted at 1.65 V (vs RHE) while maintaining current density of 5.65 mA/cm2 for 3600 sec under steady state condition. From the stability plot (Fig. 5c), the tolerance of this electrocatalyst against intermediate species may be attributed the strong interaction of Co with graphene. Fig. S3 displays a plot under constant current density of 10 mA/cm2 between applied potential (E/V vs RHE) and time (t) and it shows that this catalyst is stable up to 1100 sec vs applied current density.

Electrochemical impedance spectroscopy (EIS) measurement was also carried out to investigate OER activity of Co/G NSs. Figure 5d shows the Nyquist plot at applied potential of 1.51 V (vs RHE) in the frequency range of 1 Hz to 100 kHz to determine the conductivity and resistivity of the catalysts. The semicircular portion of the plot provided solution resistance (Rs), charge transfer resistance (R ct ), and double layer capacitance (C dl ). R ct value (408.7Ω) was rather high for our catalyst system and could be attributed to the presence of surfactant. Solution resistance (R s ) was calculated to be 72.8Ω (Table S3).

Using C dl value we calculated electrochemically active surface area of Co/G NSs to be 27 cm2 which can be correlated to the presence of more active sites at electrolyte/analyte interface, and hence higher electrocatalytic activity92 of our electrocatalyst as compared to other electrocatalysts (Table S2). There can be error in the reported value of specific capacitance of metal electrodes as large as 7% in acidic and basic media11.