Nitrogen-doped graphene-supported copper complex: a novel photocatalyst for CO₂ reduction under visible light irradiation

Abstract

A copper(II) complex grafted to nitrogen-doped graphene (GrN₇₀₀–CuC) was synthesized and then demonstrated as an efficient photocatalyst for CO₂ reduction into methanol under visible light irradiation using a DMF/water mixture. The chemical and microstructural features of GrN₇₀₀–CuC nanosheets were studied by FTIR, XPS, XRD and HRTEM analyses. Owing to its truly heterogeneous nature, GrN₇₀₀–CuC could be easily recovered after the photocatalytic reaction and showed efficient recyclability for subsequent runs.

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Introduction

Increasing atmospheric concentration of CO₂ has drawn large attention because of global warming.¹ Anthropogenic global emission of CO₂ from combustion of fossil fuels, industrial sources, various power plants etc. has increased significantly. Considering the global climate changes, worldwide stringent regulations to curb CO₂ emission and conversion of CO₂ to chemicals/energy products are gaining large interest.^2–4 The use of CO₂ as a source for carbon materials and its conversion into various chemicals by photocatalysis have been considered as strategic approaches to mitigate the increasing atmospheric CO₂.^5–7 In this context, a number of semiconductor based materials such as metal doped TiO₂, noble metals, mixed metal oxides and dye sensitized semiconductors have been emerged to be effective catalysts for the photoreduction of CO₂, albeit lower quantum yields and poor product selectivity are the major limitations.^8–10 Recently, transition metal coordinated compounds such as cobalt macrocycles, rhenium(I) complexes, ruthenium(II) polypyridine and iron porphyrins have been recognized as promising photocatalysts for reduction of carbon dioxide to high value chemicals under visible light illumination.^11,12 Recently, various enzymatic catalysts have been reported for photoreduction of CO₂ to value added chemicals.^13,14 However, a very significant limitation in the utilization of these photocatalysts is the use of sacrificial reducing agents, typically trialkylamines, trialkanolamines etc. Furthermore, difficult recovery and non-recycling ability of a homogeneous complex make these systems less attractive from both environmental as well as economical viewpoints. Immobilization of homogeneous transition metal complexes to heteroatom such as N-, or P-doped photoactive supports constitutes a logical approach, which may provide the facile recovery and recycling of the catalyst as well as avoids the use of additional sacrificial donors.

Environmentally friendly and economically viable nanomaterials exhibiting novel photocatalytic properties, and are gaining large interest. Graphene, atomic thick planar sheet of sp² bonded carbon in honeycomb crystal lattice, exhibits remarkable optical and electronic properties along with high specific surface area. The graphene-based semiconductor photocatalysts have shown immense potential for environment and energy applications including photocatalytic degradation of pollutants, hydrogen generation, disinfection and conversion of CO₂.^15,16 Doping of heteroatom such as boron, sulphur, nitrogen and phosphorus in graphene leads to significant changes and improves the semiconducting properties.^17–22 The nitrogen-doped graphene is gaining large interest and currently, several approaches have been developed to prepare by simple mixing of graphene with nitrogen precursor and then high temperature annealing, thermal exfoliation of graphite oxide in ammonia atmosphere, post-synthesis plasma doping, chemical vapour deposition method etc.^23–26 The presence of lone-pair electron on nitrogen interacts with transition metal-complex and forms the composite materials. Herein, we report a novel approach for grafting of copper complex onto nitrogen-doped graphene. The photocatalytic activity of developed material was demonstrated for conversion of CO₂ to methanol under the visible light irradiation.

Experimental section

Synthesis of N-doped graphene-supported copper complex

Graphene oxide (GO), a precursor to N-doped graphene, was prepared by harsh oxidation of graphite powder using NaNO₃, H₂SO₄ and KMnO₄ as strong oxidizing reagents. The oxidized product was washed with 30% H₂O₂ and 5% HCl solution in subsequent order to remove the undigested content of oxidizing reagents. This is followed by several washings of oxidized product with distilled water. In the subsequent step, oxidized product in water was exfoliated into GO using ultrasonic probe. The exfoliated dispersion was centrifuged at 5000 rpm for 30 minutes and supernatant containing fine content of GO was separated and dried in oven at 80 °C. The ethylenediamine was used as a nitrogen source to prepare N-doped graphene. The 40 mL aqueous dispersion of GO (100 mg) and 40 mL ethanolic solution of ethylenediamine (1 mL) were mixed and refluxed overnight. The developed black colour product was washed with distilled water and then separated using membrane filtration, connected to a vacuum line. The product was dried in oven at 200 °C for one hour. The N-doped graphene was obtained by annealing of dried solid material in a reactor at 700 °C under argon flow for two hours. The prepared N-doped graphene coins as GrN₇₀₀ throughout the manuscript. The copper complex [Cu(bpy)₂(H₂O)₂]Cl₂·2H₂O, was synthesized by following the reported procedure with some modifications.²⁷ In a typical procedure, a 15 mL of alcoholic solution of bipyridine (0.702 g, 4.5 mmol) was added drop-wise to a 10 mL aqueous solution of CuCl₂·2H₂O (0.34 g, 2.0 mmol) under uninterrupted stirring for two hours at room temperature. The developed bluish-green solution was kept for 4–5 days for precipitation of the complex. The obtained precipitate was collected through filtration and washed with diethyl ether, dried under vacuum at 50 °C for 12 h. Yield: 42%. C₂₀H₂₄N₄CuO₄Cl₂, elemental analysis calculated (found) C%: 46.28 (45.27), H%: 4.62 (4.78), N%: 10.79 (10.64), Cu% (ICP-AES): 12.25 (12.17). In the subsequent step, the synthesized copper complex was immobilized on GrN₇₀₀, targeting the lone pair electrons of nitrogen. The 25 mg of copper complex in mixture of 10 mL water and ethanol (1 : 1) was thoroughly mixed with 20 mL aqueous suspension of 20 mg GrN₇₀₀ and uninterrupted stirred for overnight. The developed copper complex immobilized-GrN₇₀₀ (GrN₇₀₀–CuC) was then washed with water and ethanol in subsequent order. Copper content of GrN₇₀₀–CuC by ICP-AES was found to be 1.2 wt% or 0.19 mmol CuC g⁻¹ cat or 9.8 g CuC/100 g cat.

Chemical and structural characterizations

Powder X-ray diffraction (XRD) of all samples were carried out using a Bruker D8 Advanced diffractometer at 40 kV and 40 mA with Cu K_α radiation (λ = 0.15418 nm). The chemical features of GO and GrN₇₀₀–CuC were examined by Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS). FTIR spectrum of each sample was recorded using a Thermo-Nicolet 8700 research spectrometer at a resolution of 4 cm⁻¹. The XPS measurement of these samples was conducted using JPS-9010TRX (JEOL Ltd.) spectrophotometer and Mg K_α line was used as the X-ray source. The Peak-fitting of the C 1s and N 1s spectra were carried out using a Gaussian–Lorentzian function after performing a Shirley background correction. UV-visible absorption spectra of these samples were recorded by using Perkin-Elmer lambda-19 UV/vis-NIR spectrophotometer. Field emission scanning electron microscopy (FESEM) analysis of GrN₇₀₀–CuC was carried out using an FEI Quanta 200 F. The elemental mapping of GrN₇₀₀–CuC was probed using energy dispersive X-ray spectroscopy (EDS) coupled with FESEM. High-resolution transmission electron microscopic (HRTEM) images of GrN₇₀₀–CuC were captured using JEOL 3010 electron microscope at 300 kV. The quantitative determination of photocatalytic products was done by injecting samples in GC-FID (Varian CP3800 by using 30 m long Stabilwax® w/Integra-Guard® column, flow rate: 0.5 mL min⁻¹, injector temp.: 250 °C, FID detector temp.: 275 °C). Identification of methanol was carried out using HPLC (Shimadzu UFLC, using Oyster BDS Premium C18 250 × 4.6 mm, 5 μm column mobile phase acetonitrile: acetone 60 : 40, flow rate 0.5 mL min⁻¹ at wavelength-205 nm).

Photocatalytic reduction of CO₂ into methanol

The photocatalytic activity of developed catalyst was examined for CO₂ reduction under visible light irradiation. In a typical experiment, borosil cylindrical vessel (capacity: 100 mL, ϕ = 5 cm) was charged with 45 mL DMF and 5 mL water and degassed by purging with nitrogen for 15 min in order to remove dissolved gaseous. Subsequently, water and DMF mixture was purged with CO₂ for 30 min for saturating the solution with CO₂. A 100 mg of catalyst was added in the reaction vessel and sealed with a rubber septum. The reaction mixture was then irradiated to visible light using 20 W white cold LED flood light (Model no. HP-FL-20W-F-Hope LED Opto-Electric Co., Ltd) maintaining uninterrupted stirring. Intensity of light on the reaction vessel was found to be 75 W m⁻². After every two hours, sample was withdrawn from reaction vessel using a needle. The quantitative determination of each sample was done by GC-FID (Varian CP-3800 by using 30 m long Stabilwax® w/Integra-Guard® column, flow rate: 0.5 mL min⁻¹, injector temperature: 250 °C, FID detector temperature: 275 °C) for determination of liquid products. Prior to these quantitative determination of reaction product, a calibration was made by examining standard doses (50, 100, 200 and 500 ppm) of methanol in GC-FID. Furthermore, identification of methanol was carried out by HPLC. For confirming that formic acid was not formed during the reaction we have analyzed standard solutions (50, 100, 200 ppm) of formic acid with HPLC equipped with Oyster BDS Premium C18 250 × 4.6 mm, 5 μm column; mobile phase; acetonitrile: acetone 65 : 35, flow rate; 0.5 mL min⁻¹ and detector: RI and UV (wavelength 205 nm). We could able to detect formic acid in the standard solutions, however, it was not observed in the reaction mixture. The gaseous products were identified and quantified with the help of GC-TCD and GC-FID (Agilent 7890A GC system) using capillary column (RGA, refinery gas analyzer) at the flow rate (H₂: 35 mL min⁻¹, air: 350 mL min⁻¹, makeup flow: 27 mL min⁻¹, for TCD reference flow: 45 mL min⁻¹, helium flow: 2 mL min⁻¹), injector temperature: 220 °C, TCD detector temperature and FID detector temperature: 220 °C.

Results and discussion

The GO exhibits ample epoxide, hydroxyl and carboxylic functionalities, which are prone to interact with amines by (a) covalent grafting via nucleophilic substitution on the epoxy group of GO, (b) hydrogen-bonding linkages between amino groups and oxygen functionalities of GO and (c) charge-induced interactions between protonated amines and carboxylate groups of GO.^28,29 The aminated-GO was then annealed at 700 °C under argon to reduce the graphene oxide into graphene and simultaneously, nitrogen of interacted amines becomes integral part of graphene skeleton and afford the bright black powder of GrN₇₀₀. The copper complex was most likely attached to GrN₇₀₀ by the replacement of labile chloride ions through ionic interaction between CuC and pyridinic nitrogen of GrN₇₀₀ and CuC.

Structural and morphological features of GrN₇₀₀–CuC were examined by XRD and HRTEM measurements. Fig. 1a shows the powder XRD patterns of GO and GrN₇₀₀–CuC. The GO exhibits intense diffraction peak at 2θ = 11.73°, corresponding to interlayer distance of ∼0.75 nm between its lamella. The presence of oxygen functionalities along with trapped water molecules in the basal plane of GO laminates increased the interlayer distance.^30,31 In contrast, GrN₇₀₀–CuC shows broad diffraction peak (002) at ∼26.3° and is attributed to 0.34 nm interlayer distance, which is equal to characteristics value of multilayer graphene (0.335 nm).^31,32 As shown in Fig. 1b, exfoliated GrN₇₀₀–CuC sheets exhibited puffy appearance and nanosheets were distributed/staked very randomly. The presence of numerous wrinkles and crumpled features on the surfaces could be attributed to the doping of nitrogen in the graphene skeleton. Furthermore, broadness of (002) diffraction peak of GrN₇₀₀–CuC is attributed to randomly stacking of GrN₇₀₀ lamella as can be seen in HRTEM images.

Fig. 1 (a) Powder XRD pattern of GO and GrN₇₀₀–CuC. (b and c) low and high resolutions TEM images of GrN₇₀₀–CuC.

Fig. 2 shows the FTIR spectra of GO, CuC and GrN₇₀₀–CuC. The GO exhibits strong vibrational bands at 3350, 1720, 1625, 1220 and 1055 cm⁻¹ owing to the presence of hydroxyl, carboxylic, epoxy, carbonyl, ether functionalities and discrete units of sp² carbon (C=C).^33,34 The characteristics oxygen functionalities and amino groups vibrational peaks could not observed in FTIR spectrum of GrN₇₀₀–CuC, indicating that these groups were decomposed during high temperature annealing. The nitrogen doping in GrN₇₀₀–CuC was evidenced by the appearance of a broad vibrational mode at 1220 cm⁻¹ owing to C–N bond and is overlapped with residual C–O group vibrations.³⁵ Furthermore, GrN₇₀₀–CuC showed a broad vibrational modes at 1620 cm⁻¹ and was attributed to the overlapping of C=N and C=C stretches. However, FTIR spectrum of GrN₇₀₀–CuC could not show vibrational modes attributed to the copper complex. This could be attributed to high absorbance of GrN₇₀₀–CuC and low concentration of copper complex grafted on GrN₇₀₀–CuC. Therefore, XPS measurements were further performed to probe the chemical composition of GrN₇₀₀–CuC nanosheets.

Fig. 2 FTIR spectra of CuC, GO and GrN₇₀₀–CuC samples.

Fig. 3a and b shows high resolution C 1s spectra of GO and GrN₇₀₀–CuC. The C 1s spectrum of GO exhibits double peak structure and can be deconvoluted into three well-separated chemically shifted components at 284.8, 286.9 and 288.8 eV, corresponding to C=C/C–C (carbon skeleton), C–O (hydroxyl, epoxide, ether etc.) and COOH (carboxylic) functionalities, respectively. The GrN₇₀₀–CuC exhibits single peak structure centred at 284.7 eV along with a shoulder towards higher binding energy. This spectrum can be resolved into four components at 284.7, 285.8, 286.8 and 287.6 eV, representing C=C/C–C, C_sp2–N, C_sp3–N, C–O bonds, respectively. Furthermore, a peak component at higher binding energy (290.2 eV) was attributed to π–π* transition owing to π-conjugated network in the GrN₇₀₀–CuC. Fig. 3c depicts broad spectrum of N 1s for GrN₇₀₀–CuC. Further, this can be resolved into three components at 398.2, 399.8 and 401 eV, corresponding to pyridinic, pyrrolic and graphitic nitrogen atoms, respectively.^36,37 The presence of N-based linkage creates polarity and attract the metal complexes.³⁸ Herein, grafting of dichloro-bis(2,2′-bipyridine) copper complex on GrN₇₀₀ was confirmed by the appearance of Cu 2p and Cl 2p peaks (Fig. 3d and e). As the chloride ions of copper complex have labile in nature,³⁹ we assumed that the attachment of copper complex with support may involve the coordination of copper ions with the pyridinic nitrogen of N-doped graphene (GrN₇₀₀) through the removal of chloride ions. This assumption is further confirmed by XPS analysis where the atomic ratio of Cu : Cl has significantly increased to 3 vs. 0.5 after the immobilization (Table S1, ESI†). Furthermore, elemental mapping based on EDS measurements (Fig. S1, ESI†), shows regular distribution of copper and chlorine in the GrN₇₀₀–CuC, revealing uniform loading of copper complex on GrN₇₀₀ nanosheets.

Fig. 3 (a) High-resolution C 1s XPS spectrum of GO. High-resolution (b) C 1s (c) N 1s (d) Cu 2p and (e) Cl 2p XPS spectra of GrN₇₀₀–CuC.

Prior to examine the photocatalytic activity of GrN₇₀₀–CuC for CO₂ reduction, the UV-visible absorption characteristic of both GO and GrN₇₀₀–CuC was measured. As shown in Fig. 4a, GO exhibits a strong absorption band at 226 nm which is attributed to the π–π* transition of discrete domains of sp² carbon in GO. There is a red shift of absorption band from 226 in GO to 246 nm in GrN₇₀₀–CuC, revealed the conjugation of sp² carbon domains.^40,41 The strong and broad absorption band in the range of 290–320 nm and a moderate absorption band in the range of 720–760 nm (Fig. 4b) are attributed to the copper complex grafted on GrN₇₀₀ nanosheets.⁴²

Fig. 4 UV-visible spectra of (a) GO and (b) GrN₇₀₀–CuC complex.

Further, the CO₂ photoreduction experiments in visible light were carried out by using GO, GrN₇₀₀, homogeneous Cu complex and GrN₇₀₀–CuC as catalytic materials in DMF/water system by using 20 W LED as a light source. In order to check the methanol production, 1 μl liquid sample was withdrawn during the photoreduction and analyzed in a GC/FID. From GC chromatogram, it is observed that methanol is being formed as a major reaction product without any evidence for the formation of other product. From GC and HPLC analysis (Fig. S3†), it is observed that methanol is being formed as the major reaction product. Therefore, methanol yield is used to determine the performance of the catalyst. The methanol yield in μmol g⁻¹ cat by using homogeneous copper complex (using equivalent amount as presented in GrN₇₀₀–CuC), GO, GrN₇₀₀, physical mixture of GrN₇₀₀ : CuC (9 : 1) and GrN₇₀₀–CuC as a function of reaction time is given in Fig. 5. After 24 h of visible light irradiation, the methanol yield was found to be 285, 420, 780, 971 and 1600 μmol g⁻¹ cat for the Cu complex, GrN₇₀₀–CuC, GO, GrN₇₀₀, GrN₇₀₀ : CuC (9 : 1), and GrN₇₀₀–CuC respectively, and the corresponding quantum yield was found to be 0.003, 0.005, 0.009, 0.011, 0.021 respectively. As expected, the individual components such as GrN₇₀₀ and homogeneous Cu complex gave a very poor methanol yield, whereas the heterogeneous GrN₇₀₀–CuC catalyst afforded a highest methanol yield which is nearly five times higher in comparison to the individual components.

Fig. 5 Photocatalytic conversion of CO₂ into methanol using various catalytic materials under visible light irradiation. Light source: 20 W white cold LED (λ > 400 nm), solvent system-DMF : water (9 : 1) −50 mL.

The GC analysis of the gaseous reaction products did not show the formation of CO during the photoreduction reaction under described experimental conditions. Furthermore, we analyzed a standard gas mixture containing CO under identical conditions to ensure the detection ability of the instrument.

Gaseous products as analyzed by GC-FID/GC-TCD using RGA column showed a very small amount of hydrogen and oxygen. The yield of hydrogen by using Cu-complex, GrN₇₀₀–CuC, GO, physical mixture of GrN₇₀₀, GrN₇₀₀ : CuC (9 : 1) and GrN₇₀₀–CuC as catalysts was found to be 4.5, 6.5, 10.7, 14.8 and 18.5 μmol g⁻¹ cat respectively with a quantum yield of 1.4 × 10⁻⁴, 2.0 × 10⁻⁴, 3.4 × 10⁻⁴, 4.6 × 10⁻⁴ and 5.8 × 10⁻⁴ respectively. Furthermore, lower oxygen yield (24.6 μmol g⁻¹ cat) after 24 h of irradiation using GrN₇₀₀–CuC as catalyst suggested that water oxidation was not the major route for scavenging the holes in the system. Oxidation of DMF or of the graphene support was also responsible for scavenging the holes during the photoreduction of CO₂. In order to confirm it, we carried out photoreduction of CO₂ using DMF as solvent in place of DMF/water solution under identical conditions. After the visible light irradiation for 24 h we have analyzed gaseous as well as liquid product and obtained comparatively poor product yields i.e. 954 μmol g⁻¹ cat methanol, 7.6 μmol g⁻¹ cat hydrogen and 6.8 μmol g⁻¹ cat oxygen. These results confirmed that along with the water oxidation, the oxidation of DMF or of graphene support were also playing essential roles as holes scavenger during the reduction of CO₂ to methanol.

To ensure that methanol was the photoreduction product of CO₂, various blank experiments were carried out in the absence of photocatalyst, in the dark reaction as well as using N₂ instead of CO₂. In all blank experiments the methanol yield was found to be negligible even after prolonged period of visible light irradiation (Table 1). Furthermore, we have carried out the recycling experiments to check the stability as well as reusability of the developed photocatalyst. After the reaction, the catalyst was collected with centrifugation and reused for subsequent five runs. In each recycling experiment the methanol yield was found to be nearly similar to that of freshly synthesized catalyst as shown in Fig. 6. These results indicate that the synthesized catalyst is quite stable and can be efficiently used for subsequent runs (Fig. 7).

Table 1 Photocatalytic reduction of CO₂ into methanol under controlled experimental conditions^a

Entry	Catalyst	Reaction precursor	Cat. mg^−1	Visible light illumination	T h^−1	Methanol yield (μmol g^−1 cat^−1)
a Reaction conditions: visible light source-20 W white cold LED (λ > 400 nm), solvent-DMF : water (9 : 1) −50 mL.b Equimolar to GrN700–CuC.
1	Nil	CO2	Nil	Yes	24	Nil
2^b	CuC	CO2	9.8	Yes	24	285
3	GO	CO2	100	Yes	24	420
4	GrN700	CO2	100	Yes	24	780
5	GrN700 : CuC (9 : 1) physical mixture	CO2	100	Yes	24	971
6	GrN700–CuC	CO2	100	Yes	24	1600
		N2	100	Yes	24	Nil
		CO2	100	No	24	Nil

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Fig. 6 Methanol yield from the photoreduction of CO₂ under visible irradiation using fresh and recycled GrN₇₀₀–CuC catalyst. After each reaction, the catalyst was recovered and used for the subsequent run in order to examine the recyclability of the developed catalyst.

Fig. 7 Plausible mechanism of CO₂ reduction by using GrN₇₀₀–CuC catalyst.

Furthermore, we have proposed plausible mechanism for the reduction of CO₂ over the GrN₇₀₀–CuC catalyst. It is well reported that pristine graphene has zero theoretical band gap and electron can move without any resistance. In nitrogen doped graphene, nitrogen atom make bonding with three neighbouring carbons by sp² hybridization and therefore influence the charge distribution of neighbouring carbon atoms. The sp² hybridized nitrogen contributes two p orbital electrons in aromatic π cloud of N-doped graphene and generate electron rich character in the resulting system, so the Fermi level shifts above the Dirac point.^43–45 This shifting break the symmetry of graphene sub-lattice and a band gap is produced between conduction and valance band. By this way N-doped graphene worked like a semiconductor.^46–48 Copper complex has good visible light absorbance and works like a photosensitizer. Immobilized copper bipyridine complex after absorption of light get excited and transfer the electrons from its HOMO to LUMO and then to the conduction band of N-doped graphene.^49–52 The electrons in the conduction band of GrN₇₀₀ facilitates the reduction of CO₂ adsorbed on sheets. DMF works as hole scavenger and provides necessary electrons for the reduction of CO₂. Importantly, N-doped graphene can transfer multiple electrons to convert CO₂ into methanol.^53,54 Furthermore, increase in activity was assumed due to N-doped graphene prevent recombination of photogenerated charges, provide surface for the attachment of complex and CO₂.⁵⁵

Conclusions

We have developed a novel copper(II) complex immobilized to N-doped graphene oxide to be an efficient heterogeneous photocatalyst for the reduction of CO₂ to methanol under visible light irradiation without using sacrificial agent. After the photoreduction, the catalyst was easily recovered and reused for subsequent runs. Importantly, the developed catalytic system did not require additional sacrificial donor for the photoreduction of CO₂, which is a significant finding and is one of the major challenges for their practical exploitation. Furthermore, facile recovery and efficient recycling of the photocatalyst make the developed protocol more attractive from economical and environmental viewpoints.

Acknowledgements

We kindly acknowledge the Director CSIR-IIP for his kind permission to publish these results. Authors are thankful to Analytical Science Division of CSIR-IIP Dehradun; DST unit of Nanoscience, IIT Madras and Kyoto University, Japan for providing their helps in various measurements. HPM and PK are thankful to UGC and CSIR India, respectively for financial support.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: XRD, FESEM, elemental mapping and TGA results. See DOI: 10.1039/c5ra05319f

‡ Both authors have equally contributed.

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