BCN–Co₃O₄ hybrid – a highly efficient catalyst for the oxygen evolution reaction and dye degradation

Abstract

A simple one-step procedure for the preparation of nanosheets of crystalline 2D boron carbon nitride containing cobalt oxide (Co₃O₄–BCN) involving glycine–nitrate combustion chemistry is reported. The as-prepared hybrid is a highly active catalyst for dye degradation, and for the water oxidation reaction as compared to pristine Co₃O₄. Co₃O₄–BCN shows enhanced catalytic activity for the oxygen evolution reaction (OER) in alkaline electrolyte. The onset potential towards the OER is 0.406 V (vs. Ag/AgCl) in 1 M KOH. A current density of 10 mA cm⁻² has been achieved at an overpotential of 394 mV in 1 M KOH. The as-prepared hybrid exhibits highly-efficient degradation ability for methyl orange and methyl blue dyes under direct sunlight irradiation.

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The electrochemical splitting of water is one of the most promising approaches for producing clean and sustainable energy.^1,2 Splitting of water via an electrocatalyst is a process involving water oxidation and reduction. The main issue with water splitting is the oxygen evolution reaction (OER) which requires a large overpotential. Ruthenium oxide (RuO₂) or iridium oxide (IrO₂) are known to be the most efficient catalysts for the OER with low overpotentials and high current densities.³ However, the main issues with RuO₂ or IrO₂ based catalysts are their high cost and limited availability. Hence much of the current research effort is aimed at synthesising non-noble metal catalysts with low overpotentials and high current densities. In the past few years, considerable efforts have been dedicated towards the development of transition metal oxides (MO) as electrocatalysts for the OER. Among transition MO, Co₃O₄ has attracted extensive attention owing to high activity and stability. For example, H. Tuysuz et al. prepared mesoporous cobalt oxide (Co₃O₄) that showed high catalytic activity for OER compared to that of bulk Co₃O₄.⁴ J. Rosen et al.⁵ prepared ordered mesoporous cobalt oxide showing excellent activity for OER. J. A. Koza et al.⁶ studied OER catalytic activity of both crystalline and amorphous Co₃O₄ via electro deposition. But issues like low conductivity and poor stability in alkaline solution resulted in the development of hybrid catalyst involving carbon nanotube (CNT), graphene (G) and mesoporous carbon. B. Suryanto et al.⁷ reported layer-by-layer assembly of Co₃O₄/graphene for OER. Wu, J. et al.⁸ prepared Co₃O₄ decorated single-walled carbon nanotubes as a highly efficient oxygen-evolving catalyst. Recently, two-dimensional nanosheets containing boron (B), nitrogen (N) and carbon (C) have shown tremendous potential in applications ranging from electronics to catalysis.^9–11 But to the best of our knowledge, no efforts have been made to combine BCN and Co₃O₄ for OER. Therefore, the combination of cobalt oxide and BCN can be an excellent catalyst for OER. In this work, for the first time BCN was used as a support material to encapsulate Co₃O₄ via thermal treatment involving glycine, copper nitrate and B₂O₃. The obtained hybrid shows enhanced catalytic activity for OER in alkaline electrolyte as compared to pristine Co₃O₄. The onset potential towards the OER is 0.405 V (vs. Ag/AgCl) in 1 M KOH. The current density of 10 mA cm⁻² has been achieved at the overpotential of 394 mV in 1 M KOH. The as-prepared hybrid also exhibits high-efficient degradation ability for methyl orange (MO) and methyl blue (MB) dyes under direct sunlight irradiation as compared to pristine Co₃O₄. The catalyst has a long lifetime and can be recovered and reused with a slight loss of reactivity.

The XRD pattern of as-prepared Co₃O₄–BCN (Fig. 1a) exhibits a broad peak between 22 and 26° and at 42°, which is similar to (002) and (100) lattice planes of BCNs reported previously with h-BN structures.¹² Interestingly, the XRD pattern of Co₃O₄–BCN does not show any peak originating from cobalt species in line with the previously reported cobalt-containing carbon nitride compounds.¹³ But XRD pattern of Co(NO₃)₂ thermally treated at 500 °C for two hours showed characteristic cobalt oxide peaks (Fig. 1a). This is an indication that the cobalt species is chemically coordinated to the BCN host.¹³ Fig. 1b shows FTIR spectra of the Co₃O₄ and Co₃O₄–BCN hybrid materials. The Co₃O₄ spectra showed two peaks at 576 cm⁻¹ and 665 cm⁻¹ for metal oxide (Co–O) stretching vibrations. Co₃O₄–BCN hybrid materials shows a large absorption band at 1380 cm⁻¹ which is due to stretching mode of B–N in the hexagonal boron nitride. The band at 702.04 cm⁻¹ is due to stretching of Co–O and is shifted to higher wavelength as compared to Co₃O₄ due to interaction between cobalt oxide between BCN host. Absorption bands due to C–N and cubic-BN are assigned to be at 1618 cm⁻¹ and 1043 cm⁻¹, respectively.^14,15 Based on the above FTIR result it may be said that Co₃O₄–BCN hybrid structures are achieved. The Raman spectra of Co₃O₄–BCN (Fig. 1c) exhibited two peaks corresponding to the D band (at 1355 cm⁻¹) and G band (at 1581 cm⁻¹), along with a Co–O band at 743 cm⁻¹.¹⁶ Hence, the Raman data indicates the presence of both BCN structures and cobalt oxide. In Fig. 2, the absorption properties of BCN–Co₃O₄ are compared with those of the pristine Co₃O₄. The as-prepared BCN–CO₃O₄ is characterized by a significantly improved absorption in the visible light range than the Co₃O₄.

Fig. 1 (a) XRD, (b) FT-IR and (c) Raman spectra of Co₃O₄–BCN and Co₃O₄.

Fig. 2 UV-visible spectra of Co₃O₄ and Co₃O₄–BCN.

Fig. 3 shows the XPS spectra of as-prepared hybrid. Survey spectra (Fig. 3a) indicate the presence of B, C, N, O and Co species. The high resolution C 1s spectra (Fig. 3b) show a major peak at 284.3 eV and a small shoulder at 285.7 eV corresponding to C=C bonds, and C=N respectively.¹⁷ It also shows a tail in the higher binding energy region which indicates the presence of small amount of oxygen containing groups. High resolution B 1s spectra (Fig. 3c) of the as-prepared sample show a major peak at 190.9 eV, which indicates the presence of boron in the form of B–N.¹⁸ This suggests that the main bonding configuration for B in our films is similar to that of h-BN. The N 1s spectra (Fig. 3d) show a major peak with a binding energy of 398.2 eV which correspond to N in h-BN.¹⁹ Both the B 1s and the N 1s spectrum indicate that the main configuration for the B and N atoms is the B–N bond, implying that h-BN domains exist in the film. High resolution XPS spectrum of cobalt (Fig. 3e) showed doublet spectral lines at binding energies of 778.7 eV (Co 2p3/2) and 794.9 eV (Co 2p1/2) with energy separation of 15.6 eV, which indicates the presence of cobalt in divalent form.²⁰ The smaller at around 800 eV correspond to satellite peaks, resulting from shake excitations.²¹ The high resolution O 1s spectrum (Fig. 3f) shows a peak at 530.1 eV, which correspond to the oxygen species from Co₃O₄.²⁰

Fig. 3 Survey (a) and high resolution spectra of carbon (b), boron (c), nitrogen (d), cobalt (e) and oxygen (f).

The morphology of the as-prepared Co₃O₄–BCN was characterized by field emission scanning electron microscopy (FE-SEM). FE-SEM image (Fig. 4b) reveals porous layered morphology as compared to spherical shaped pristine Co₃O₄ particles (Fig. 4a). EDX spectra (Fig. S1†) confirm the presence of B, C, N, O, and Co species. The elemental mapping (Fig. S2†) indicates homogeneous distribution of B, C, N, Co, and O species. Sonication was used as a top–down method to exfoliate layered Co₃O₄–BCN sheets. Representative TEM images (Fig. 4c) show that the exfoliated CoO–BCN sheet morphology resembles that of highly crumbled and randomly aggregated graphene-like sheets. SAED pattern (Fig. 4d) indicates the crystalline nature of the as-prepared Co₃O₄ hybrid. The Co₃O₄ samples were also investigated by nitrogen adsorption and desorption measurement at 77 K. As shown in Fig. 3d, the nitrogen adsorption–desorption curves demonstrate a Type IV isotherm and the specific surface areas of Co₃O₄ and BCN–Co₃O₄ have been 24.17 and 107.19 m² g⁻¹, respectively. The BET result is consistent with the SEM observations discussed above, showing porous structure with relatively high interfacial area. The structural feature gives implications of the applications in catalysis.

Fig. 4 FE-SEM of (a) Co₃O₄; (b) Co₃O₄–BCN and TEM image (c) and SAED pattern (d) of Co₃O₄–BCN.

The electrochemical performances of the prepared materials were analysed using cyclic voltammetry (CV). Fig. 5a shows the CV curves of Co₃O₄ and BCN–Co₃O₄ electrodes at a scan rate of 50 mV s⁻¹ between 0 and +0.6 V (vs. SCE) in 1 M KOH aqueous electrolyte. A typical single redox peak for cobalt oxide has been observed in the CV curves.¹⁶ On the other hand, BCN containing cobalt oxide exhibit a larger CV area than that of pristine Co₃O₄, indicating a higher specific capacitance compared with the Co₃O₄ without BCN. Fig. 5b shows the cyclic voltammograms as a function of scan rate for BCN–Co₃O₄. With the increase of the scan rate, the current response increases. A quasi-linear relationship is observed between the redox peak current and the scan rate, indicating a diffusion-controlled process and dominant surface redox reactions.

Fig. 5 (a) Cyclic voltammetry curves of Co₃O₄ and BCN–Co₃O₄; (b) CV of BCN–Co₃O₄ at different scan rates; (c) LSV curves of Co₃O₄ and BCN–Co₃O₄ for OER.

The water oxidation catalytic activities of the as-synthesized nanocomposites were first investigated in alkaline solution (1 M KOH) in a standard three-electrode setup. Linear sweep voltammetry (LSV) curves of BCN–Co₃O₄ catalyst is shown in Fig. 5c. BCN–Co₃O₄ catalyst exhibits significantly higher anodic current and lower onset potential than that of the Co₃O₄ catalyst, in 1 M KOH solutions (Fig. 5c). In 1 M KOH solution, BCN–Co₃O₄ catalyst shows a sharp onset potential at 0.406 V (vs. Ag/AgCl), and achieves the current density of 10 mA cm⁻² at the overpotential of 397 mV, which is much better than that of the Co₃O₄, Co₃O₄/SWNTs (593 mV)⁴ and mesoporous Co₃O₄ catalysts (525 mV)⁸ in the same alkaline solution. This shows that BCN can be used as support for Co₃O₄. Through our work we have shown that BCN can be used as a potential material for supporting Co₃O₄ towards OER. The excellent electrochemical performance of BCN–Co₃O₄ catalyst suggests its promising application towards OER both in alkaline solution. A series of controlled experiments, shown in Fig. 5, have demonstrated that the obtained BCN–Co₃O₄ catalyst displayed better OER activity than those of pure Co₃O₄ in 1 M KOH conditions. The strong interaction of Co₃O₄ nanoparticles with BCN nanosheets and the unique architecture have strong effect on the catalytic property of the composite. This further illustrates that the strong chemical coupling and good interaction between the Co₃O₄ and BCN can significantly improve the electron transport and reaction kinetics during the OER. In order to further investigate the high performance of BCN–Co₃O₄, the effective surface areas were estimated, which was determined by electrochemical capacitance measurements from static cyclic voltammetry. The capacitance for BCN–Co₃O₄ is much higher than Co₃O₄ (Fig. 5a). Therefore, the high performance could be also associated with the high electro active surface area. In addition, the unique and intimate contact between the BCN and Co₃O₄ contributes to the superior catalytic activity. The photocatalytic activity of BCN–Co₃O₄ catalyst was tested by examining the degradation of MB and MO dye solution as a function of time under direct sunlight irradiation. Fig. 6a shows the photocatalytic activities of the sample under direct sunlight irradiation. A blank experiment was performed with MO dye in water under the same conditions, but without any catalyst, to exclude the possible photolysis of the dye under direct sunlight irradiation. No or very low MO degradation was observed in the absence of catalyst. Only 19% of the MO was degraded by pure Co₃O₄ (control) in 2 h (data not shown here). Using BCN–Co₃O₄ sheets leads to 89.13% photo degradation of MO in about 4 min (Fig. 6a). The MO and MB dye exhibits a characteristic absorption peak at 473 nm and 664.^22,23 Fig. 6b shows that the degradation of MO dye (the quenching of the MO absorbance peak by 90.33%) and a decrease in colour compared with the original solution was clearly observed. These results demonstrate that the BCN–Co₃O₄ showed excellent photocatalytic activity compared with pristine Co₃O₄. The BCN–Co₃O₄ also degrades MB in 4 minutes with 92.1% efficiency under same experimental condition. Thus, the as-prepared catalyst degrades both the methylene blue and methyl orange dye effectively with above 90% efficiency in less than 5 minutes without any accelerators, which is much better than that of the Co₃O₄-g-C₃N₄ (complete degradation occurs in 3 hour),²⁴ Co₃O₄ nanoparticles (4 hour in presence of H₂O₂ accelerator)²⁵ and Co₃O₄/GO (6 minutes in presence of peroxymonosulfate accelerator).²⁶ The rate of the reaction is also determined by the calculating rate constant value for MO and MB degradation (Fig. S3†). Further to understand the effect of visible range, a control experiment was done under sunlight with UV filter. With UV filter the degradation efficiency decreases only about 3–4% (Fig. S4†). The stability and reusability of the Co₃O₄–BCN catalyst was studied in repeated degradation reaction of MO and MB. The catalyst was reused (three times) after each reaction by simple filtration and washing with water. The reused catalyst displayed consistent reactivity with less than 3% decrease in efficiency (Fig. S5a†). The stability of catalyst after dye degradation is analysed by comparing the FTIR spectra of samples before and after dye degradation. It is observed from the Fig. S5† that there are no changes in bonding characteristics of the spectra. This confirms there is no dislocation of bonding or modification of bonding upon recycling which proves the material is highly stable.

Fig. 6 UV spectra of degradation of methylene blue (a) and methyl orange (b).

In summary, for the first time we have shown that BCN can be used as a support material to encapsulate Co₃O₄ via thermal treatment involving glycine, copper nitrate and B₂O₃. The obtained hybrid shows enhanced catalytic activity for OER in alkaline electrolyte as compared to pristine Co₃O₄. The onset potential towards the OER is 0.405 V (vs. Ag/AgCl) in 1 M KOH. The current density of 10 mA cm⁻². The as-prepared hybrid also exhibits high-efficient degradation ability for methyl orange (MO) and methyl blue (MB) dyes under direct sunlight irradiation as compared to pristine Co₃O₄.

Acknowledgements

This work was supported by SERB Young Scientist Reserach Grant GAP 11/14. The authors thank CSIR-CECRI Central Instrumentation Facility for the analytical support and Er A. Rathishkumar, Er V. Prabhu and Er J. Kennady for TEM, FE-SEM and XPS analysis.

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, reusability of the Co₃O₄–BCN for dye degradation. See DOI: 10.1039/c6ra16058a

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