Unravelling the photo-excited chlorophyll-a assisted deoxygenation of graphene oxide: formation of a nanohybrid for oxygen reduction

Abstract

We attempted to expand the horizons of graphene oxide (GO) reduction methods using chlorophyll-a (CHL-a), which is inclined to react favourably with GO by virtue of photo-excited electron transfer from the singlet excited CHL-a LUMO (−0.7 V) to GO (−0.4 V) in aqueous media assisted by the interactive affinity between CHL-a and reduced graphene oxide (RGO), which is ensured through π–π interaction between the CHL-a macro-cycle and the GO surface. This results in the formation of a CHL-a⁺ radical cation which might favor the oxidation of water with oxygen evolution. The formation of RGO after photo-exposure can also be confirmed via TEM and Raman spectroscopy. Gradual restoration of sp² hybridisation in the GO framework with increasing CHL-a concentration can be correlated with the enhanced contribution of the conformation in which electron transfer is efficient from CHL-a to GO (also supported by XPS and XRD data). This fact corroborates the faster component (a₁) augmentation with increasing CHL-a concentration towards overall excited state lifetime. The applicability of this RGO/CHL-a nano-hybrid as a possible electro-catalyst, to be used for oxygen reduction in energy conversion systems such as fuel cells, has also been explored through cyclic voltammetry. All these results cumulatively highlight the effective, environmentally friendly mechanism of the photo-excited CHL-a assisted deoxygenation of GO in aqueous media, which eventually gives rise to a RGO/CHL-a nano-hybrid as a potential electro-catalyst in next generation bio-fuel cells.

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1. Introduction

The preparation of high-quality 2D graphene sheets with the synergistic combination of versatile properties like high electrical/thermal conductivity,^1–5 flexible but strong mechanical properties,^6,7 high thermal/chemical stability,⁸ and extremely large surface area within the 2D interface is the first and most crucial step for the successful realization of real-life applications in nanoelectronics,^9,10 sensors,^11,12 nanocomposites, batteries,¹² supercapacitors, hydrogen storage, and energy harvesting.

Recent interest in understanding the properties of graphene has led to many theoretical and experimental efforts to explore the possibility of graphene exfoliation through a combination of oxidation and sonication procedures, followed by reduction through various chemical methods;^13–16 for example Stankovich et al.^13,14 and Wang et al.¹⁶ used hydrazine-hydrate and hydroquinone for the reduction of exfoliated GO sheets, and Kamat et al. proposed the photocatalytic reduction of graphite oxide with TiO₂, whereas Kundu et al. used positively charged cysteamine capped CdTe quantum dots to reduce GO under photoexposure.^17a,b

However, the electronic properties may be interfered with by these excessive reducing agents and further limit applications requiring precise controllability.¹⁸ In addition, reduced graphene oxide (RGO) produced in suspension often aggregates due to van der Waals interactions. This led us to venture into the exploration of using photo-excited CHL-a molecules as reducing as well as dispersive agents. In the photosynthetic reaction, energy is harvested through self-organised CHL-a molecules where it is transferred via a sequence of quantum mechanical energy-transfer processes across a total distance of ∼20–100 nm with near-unit quantum efficiency^19a towards a reaction centre,^19b where electrons are donated from the photo-excited CHL-a molecules to a closely packed pheophytin. The faster forward reaction^19c of a few picoseconds time window is ensured by the geometric arrangement of closely packed pigments. Our aim is to use this CHL-a photo-generated electron to reduce GO in water, which will eventually give rise to tuneable functionalised graphene by controlling the reduction degree. Due to its high work function (4.42 eV),^19d GO can accept photo-generated electrons from the lowest unoccupied molecular orbitals (LUMOs) of CHL-a and can potentially contribute to the formation of functionalised graphene nanosheets through reduction (Fig. 1). Khaderbad et al. used metallo-porphyrin for the photo-catalytic reduction of graphene in ethanolic solvent.^19e We propose to reduce graphene in aqueous solvent, where after being oxidised, the photo-chemically generated CHL-a radical will have the potential to acquire an electron through water splitting by virtue of returning to the ground state. In our previous publication we reported the synthesis of a RGO/CHL-a nanohybrid through CHL-a exfoliation and showed that CHL-a can effectively tune the Fermi velocity in the extended molecular framework of graphene, eventually making it highly functional for advanced molecular electronics. The data suggested that the π-conjugated planar tetra-pyrrole structure of the CHL-a can be easily π-stacked over the surface of the sp² hybridized RGO, giving rise to the RGO/CHL-a nano-hybrid.^19f

Fig. 1 Potential energy diagram showing the chemical processes possibly involved in the cathodic and anodic currents of the RGO/CHL-a nano-hybrid.

Therefore, it may be postulated that the transfer of an electron from the energy level of the excited CHL-a unit to the energy level of GO results in the healing of the defects of GO and thus reduces GO to RGO. The photo-excited CHL-a can be readily π-stacked over the RGO surface as well as be protonated. This results in the formation of a CHL-a⁺ radical cation, which might favor the oxidation of water with oxygen evolution, a process that proceeds with a decrease in Gibbs enthalpy. The ground state redox potentials of the CHL-a/CHL-a⁺ and CHL-a/CHL-a⁻ couples are −0.94 V and −0.80 V (SHE), obtained from cyclic voltammetry measurements in different solvents.²⁰ The excited-state redox potentials of CHL-a were obtained by ascribing 1.85 and 1.33 eV to the singlet and triplet excitation energies.²¹

Water photo-oxidation with oxygen evolution catalyzed by hydrated oligomers of CHL-a immobilized at the interface between two immiscible liquids, on a bilayer lipid membrane or on an electrode, has often been reported.²² Hence we also explore the possibility of using this photochemically prepared RGO/CHL-a nanohybrid as a possible electrocatalyst through cyclic voltammetry, to be used for oxygen reduction in energy conversion systems.

2. Experimental

Preparation of graphene oxide (GO)

We used exfoliated GO prepared by oxidizing graphite flakes in acidic medium as the starting material for the preparation of graphene, according to the modified Hummer’s method.²³

A wide range of oxygen groups bonded onto the surface of GO increases the charge concentration, thereby increasing the dispersion of GO in water to a greater extent. CHL-a molecules were added to the GO suspension with increasing concentrations of 10⁻⁷, 10⁻⁶, and 10⁻⁵ M, along with 30 min of xenon lamp irradiation from a distance of 45 cm. The intensity measured was 80 mW cm². GO reduction proceeds with the appearance of black dispersed material from brownish yellow, suggesting the restoration of electronic conjugation. The characterization of morphology and the detailed structural features were investigated using high-resolution transmission electron microscopy (HRTEM; JEOL 2100). Raman spectra of the samples were recorded at room temperature in the solution phase.

The light irradiation dependence study was done after different irradiation times with a white light source combined with a <420 nm cut off filter. Room temperature optical absorption spectra were taken with a UV-vis spectrophotometer (Perkin Elmer). Room temperature photoluminescence studies were carried out using a Fluoro Max-P (Horiba Jobin Yvon) luminescence spectrometer. For time correlated single photon counting (TCSPC) measurements, the samples were excited at 375 nm by a picosecond NANO-LED IBH-375. The fluorescence decay measurements were collected on a Hamamatsu MCP photomultiplier. The following expression was used to analyze the experimental time-resolved fluorescence decays, p(t):

here, n is the number of emissive species, b is a baseline correction (“dc” offset), and α_i and τ_i are the pre-exponential factors and excited-state fluorescence lifetimes associated with the ith component, respectively. For multiexponential decays (n), the average lifetime, 〈τ〉, was calculated from eqn 2:

3. Results and discussion

TEM analysis

The formation of GO has been confirmed through TEM, which shows a crystalline lattice surrounded by an amorphous zone (Fig. 2A(a) and (b)), as well as the formation of RGO after steady state photoexposure of the CHL-a/GO homogenous aqueous mixture which clearly shows the evolution of a crystalline graphene lattice at the highest CHL-a concentration (Fig. 2B(a) and (b)) in comparison with the bare GO counterpart. The interactive affinity between CHL-a and RGO is ensured through π–π interactions, and can create a platform for GO to be reduced via CHL-a molecules in a closely packed environment. The SAED pattern with the final CHL-a concentration clearly shows a bright hexagonal inner circle, which gradually fades towards the outer region. However, a few bright spots in the outer region, signifying the folded RGO or stacked macrocycles of CHL-a over the RGO surface, are clearly visible.²⁴

Fig. 2 TEM & SAED image of (A(a) and (b)) GO (the yellow circles show the crystalline zones within the amorphous domain) & (B(a) and (b)) RGO/CHL-a at the highest CHL-a concentration.

XRD analysis

The sharp (002) diffraction peak at 2θ = 10.8° and the broad diffraction peak at 2θ = 22.8° appearing in the XRD spectra of GO and RGO/CHL-a respectively (as shown in Fig. 3) signifies GO reduction in the presence of CHL-a, corroborating evidence in the literature.^24b The appearance of a broad peak in the spectra for the nanohybrid indicates that its sheets are poorly ordered.^24e In addition, the effective CHL-a functionalization of graphene was verified by the disappearance of the appropriate intense diffraction peak in the XRD pattern. This clearly demonstrates the creation of fully π stacked CHL-a/RGO sheets.

Fig. 3 XRD spectra of GO & the RGO/CHL-a nano-hybrid with 10⁻⁵ M CHL-a concentration.

XPS analysis

The high-resolution carbon 1s XPS spectrum of the GO sheets shows a sharp peak at around 284.6 eV that corresponds to the C–C bonds of carbon atoms in the conjugated honey-comb lattice. The peaks at 286.7, 288.4 and 290.1 eV can be attributed to different sp³ bonding configurations due to the harsh oxidation and destruction of the sp² atomic structure (Fig. 4(A)).^24f After reduction with CHL-a, the intensities of all of the related oxygen peaks except C=O were profoundly decreased in the RGO/CHL-a sample (conc. of 10⁻⁵ M) compared to GO, indicating that the delocalized π conjugation was restored in our RGO/CHL-a sample (Fig. 4(B)). The intensity of the C=O peak increased due to the contribution of the CHL-a carbonyl group. Based on the XPS analyses, the as-prepared GO had a very low delocalized π conjugation percentage of 14.58%. In contrast, the RGO/CHL-a produced by CHL-a reduction has a percentage of 57.2% (Table 1). We concluded that the RGO from our process contained far less oxygen, confirming its high quality. The main peak of the high-resolution nitrogen 1s XPS spectrum N-1, at a binding energy of 398.1–398.9 eV in CHL-a (Fig. 4(C)) is characteristic of the pyrrolidine nitrogen atoms of the porphyrin macrocycle of CHL-a. A second peak (N-2), a shoulder, in the XPS spectrum of CHL-a alone can also be seen at 400 eV. It is most likely due to the protonated nitrogen produced as a result of a small degree of demetallation of CHL-a during its exposure to X-rays in the course of the experiment.^24g In the RGO/CHL-a hybrid we found that the most intense peak N-3 was at around 408.4 eV, indicating that most of the nitrogen of CHL-a remains in the oxidised form or positively charged over the graphene surface, whereby the expulsion of the core electrons from the CHL-a nitrogen has become more difficult, thereby needing a comparatively higher energy, i.e. 408.4 eV. The positively charged nitrogen evolved is due to the transfer of non-bonding electrons on the nitrogen of CHL-a to GO after photoexposure; as a consequence reduced RGO is being formed.

Fig. 4 (A and B) Carbon 1s XPS spectra of GO & the RGO/CHL-a nano-hybrid with 10⁻⁵ M CHL-a concentration. (c) Nitrogen 1s XPS spectra of RGO/CHL-a nano-hybrid & CHL-a (10⁻⁵ M CHL-a concentration).

Table 1 High resolution XPS data of the C 1s peak area contributions of all samples^a

Sample	sp^2 C at 284.6 eV (%)	C–O at 286.7 eV (%)	C=O at 288.4 eV (%)	O–C=O at 290.1 eV (%)
a RGO/CHL-a = 10^−5 M CHL-a concentration.
GO	14.58	72.54	7.81	5.07
RGO/CHL-a	57.30	23.88	13.77	5.05

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UV-vis absorption spectroscopy

The signature of GO reduction can also be observed through UV-vis absorption spectroscopy. The gradual increase of the 270 nm absorbance peak intensity at the expense of the 230 nm GO peak with increasing CHL-a concentration, proves the restoration of the sp² network in GO (Fig. 5A). The ratio of the 230 nm to 270 nm peak intensities decreases from 1.41 to 1.02 for GO to the highest concentration CHL-a reduced GO, respectively (Fig. 5A inset). This phenomenon implies that the contribution of the 270 nm absorbance is more pronounced than the 230 nm absorbance due to the enhancement of GO reduction with increasing CHL-a concentration. An apparent colour change of the GO solution from light yellow to black was observed due to the progressive formation of RGO under photo-excited CHL-a reduction with increasing concentration (Fig. 5A inset). The UV-vis absorption spectra of varying conc. of CHL-a without GO is given in the ESI.†

Fig. 5 (A) UV-vis absorption spectra (inset showing the colour of the samples & ratios of the 230 nm to 270 nm peak intensities), (B) photoluminescence spectra, (C) TCSPC data and (D) Raman spectra (inset showing I_D/I_G ratios of the intensities) of the nanohybrid with 10⁻⁷, 10⁻⁶, and 10⁻⁵ M CHL-a concentration from (a) to (c), respectively.

Photoluminescence spectroscopy & TCSPC

The enhancement of GO reduction with increasing CHL-a concentration is further supported by the gradual quenching of the photoluminescence spectra at 675 nm with increasing CHL-a concentration in the GO after photoexposure. The presence of CHL-a enhances the sp² restoration process of GO under photo-excitation which further corroborates the probability of π stacking and leads to electron transfer from CHL-a to RGO Fig. 5(B).

The electron transfer phenomenon can be understood through the TCSPC measurement. The τ values of bi exponentially fitted decay curves given in Table 2 indicate the presence of increased conformational heterogeneity.²⁵ The profound decreases in fluorescence average lifetime 〈τ〉 with increasing CHL-a concentration may be correlated with macrocycle distortion (Fig. 5(C)) in conjunction with both internal conversion and intersystem crossing to the CHL-a porphyrin ring.²⁶

Table 2 Decay time components of all samples (excitation at 405 nm; emission at 673 nm)^a

Sample	τ1 (ps)	a1	τ2 (ns)	a2	〈τ〉 (ns)	χ^2	Quantum yield (±3%)	Rad. rate (krad) (ns^−1) (±6%)	Non-rad. rate (knrad) (ns^−1) (±6%)	krad/knrad
a (a), (b), (c) = RGO/CHL-a nanohybrids with 10^−7, 10^−6, and 10^−5 M CHL-a concentration respectively. χ^2 = curve fitting residual parameter.b Taken from ref. 32, krad = RGO/CHL-a radiative decay rate, krad = monomeric CHL-a radiative decay rate.
(a)	640.16	31.07	4.24	68.93	4.01	1.13	0.24	0.059	0.189	0.312
(b)	611.72	58.73	3.81	42.27	3.22	1.11	0.17	0.052	0.257	0.202
(c)	506.86	78.09	2.07	21.91	1.26	1.23	0.15	0.119	0.674	0.172
CHL-a	889.07	11.30	4.68	88.70	4.59	1.15	0.30^b	6.535	0.152	—

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This phenomenon leads us to conclude that the increased rates of internal conversion of π–π* to the ground state arise from an enhanced Franck–Condon factor associated with structural reorganization in the excited state, whereas the enhanced intersystem crossing from π–π* was attributed to increased spin–orbit coupling caused by the non-planarity of the CHL-a macrocycle. As a consequence, significantly lower fluorescence yields, large Stokes shifts, and shorter lifetimes of the lowest excited state are expected to be evident due to the combined effects of destabilization and stabilization of the energy levels, which ultimately result in a further reduction of the energy gap between the HOMO and LUMO.²⁷

In our system no Stokes shift is evident; instead the intensity of the CHL-a emission gradually reduces with increasing CHL-a concentration (Fig. 5(B)). When GO is not added to CHL-a with the same concentration the lifetime is very close to monomeric CHL-a lifetime (4.42 ns). Thus it can be inferred that intersystem crossing and internal conversion are negligible in our system. In summary, excluding the above mentioned possibilities of internal conversion and intersystem crossing, we can attribute the decrease in luminescence intensity to the fact that with increasing CHL-a concentration (while other parameters like distance and exposure time are constant) more CHL-a participates in restoration of the sp² network of GO through photo-generated electron donation from CHL-a to GO. The GO/CHL-a interaction is mostly ensured through π stacking of the CHL-a macrocycle over the almost planar GO surface. The two lifetimes, observed for CHL-a with electron-withdrawing GO, can be described as resulting from two conformations. We attribute the shorter lifetime component to the conformation in which electron transfer is efficient and the longer component to the conformation in which electron transfer is relatively inefficient. Since the latter one is very close to that of monomeric CHL-a, we can assume that this longer lifetime component signifies the geometry of monomeric porphyrin molecules which remain unreacted with GO within the solution.

Raman spectroscopy

Raman spectra give a clear insight in support of the formation of RGO where photo-excited electrons can be donated from CHL-a to GO. The G band and D band are usually assigned to the E_2g phonon of C sp² atoms and the breathing mode of k-point phonons of A_1g symmetry, respectively, for the graphene.^28a The appearance of a prominent D band in the spectrum is also an indication of disorder in the graphene owing to the presence of oxide functional groups.^28b It has been well observed that the size of the defect-free sp² cluster regions is the inverse of the ratio of the D and G band integrated intensities (I_D/I_G).^28c

Here, the ratio of I_D/I_G exhibits a significant decrease with increasing CHL-a concentration in comparison to the previously reported RGO formation process after chemical reduction, where an increased I_D/I_G ratio has been reported.²⁹

In the presence of an increasing concentration (10⁻⁷, 10⁻⁶, and 10⁻⁵ M) of CHL-a molecules, the corresponding I_D/I_G ratios of the as-prepared RGO are 1.1, 1.07, 1.05 and 1.01, respectively (Fig. 5(D)). We conclude that the production of RGO from GO by reduction with CHL-a can have a healing effect;³⁰ the photo-excited CHL-a molecules can further recover the aromatic structures by repairing the defects of GO by donating electrons from the excited state.

Cyclic voltammetry measurement

In order to gain further insight into the reduction of GO through adsorbed or π-stacked CHL-a molecules we conducted preliminary electrochemical measurements with a glassy carbon (GC) electrode coated with RGO with increasing CHL-a concentration by drop casting, and the electrolyte pH was maintained around 7 (neutral) against a Ag/AgCl electrode.

The cyclic voltammetry profiles in Fig. 6 show that the photo-excited CHL-a molecules mediate electron transfer from the electrode to the oxygen functional groups on the GO sheet. The hole, on the other hand, is expected to be transferred to water which acts as a hole acceptor and is oxidized. The water reduction potential is around 0.5 V more positive than for the CHL/CHL⁺ pair.³¹ Thus the resulting CHL⁺ radical cation may oxidize water with oxygen evolution, a process that proceeds with a decrease in Gibbs enthalpy, as shown by the energy diagram in Fig. 1. Interestingly, very distinguished characteristics are observed in both the positive and negative shift of the potential. The GO cathodic peak at −0.62 V and anodic peak at 0.46 V reduces with increasing CHL-a concentration with the gradual evolution of a cathodic peak at −1.7 V and at 0.4 V. We propose that the reason for this is based upon the probability that CHL-a on the RGO surface is restored back to its neutral form by accepting an electron from the aqueous environment after photoexposure. At a positive electrode potential the electrostatic interaction between the positively charged GC electrode and the electron cloud of the RGO causes the macromolecules to get into a different orientation through the reduced involvement of π-electrons in the π-stacking of CHL-a and RGO. Thus the C=O of CHL-a cannot participate in the reduction process. The C=O reduction peak cannot be observed, hence the 0.46 V anodic peak which disappears in the second cycle (Fig. 6(B)) may be attributed to the C=O impurities present as defects in the GO. The 1st cycle cathodic peak at 0.4 V may be attributed to the reduction peak of the CHL-a carbonyl group. This group is likely to be oriented closer to the electrode during the reverse sweep and accepts an electron to get reduced,³¹ thus rendering the reduction reaction possible. This C=O peak is retained in the second cycle only for the CHL-a/RGO hybrids, signifying that the formation of RGO (after photoexposure) causes the planar CHL-a molecules to π-stack over the graphene surface more firmly and allows the electron to take part in the subsequent carbonyl reduction. Thus we could not find any peak in the bare GO and GO with CHL-a without photoexposure. If CHL-a molecules had desorbed from the RGO surface in the positive run we could not have found the 0.46 V anodic peak. Thus we attribute the appearance of the 0.46 V anodic peak to the reduction of the carbonyl group of CHL-a. The intensity of this peak decreases with the increasing CHL-a concentration and the gradual appearance of RGO. Interestingly, when we compare the GO profile with that of its CHL-a reduced counterpart, the major difference found is the appearance of the −1.7 V cathodic peak. This peak is also found in CHL-a reduced RGO, where the intensity increases towards higher CHL-a concentration. The potential of the CHL⁺/CHL²⁻ couple is −1.57 V, estimated from the cyclic voltammogram of a CHL-a solution in DMF.³¹ Thus it is highly unlikely that this peak is due to the oxidation of the macrocycle at such a higher negative potential. The disappearance of the porphyrin redox wave may be attributed to the stacking of the porphyrin macrocycles, which prevents the reduction of the ring at the cathode. The CHL-a assembly behaves differently in terms of its redox property than that of monomeric one as it is π-stacked over the graphene surface. Thus we assign this −1.7 V peak to the oxygen reduction in neutral pH in aqueous NaCl electrolyte which is retained in the second cycle, proving the reversibility. The cyclic voltammetry of CHL-a without GO is given in the ESI.†

Fig. 6 (A) Cyclic voltammetry profiles of the electrochemical reduction and oxidation of the nanohybrid with increasing CHL-a concentrations of 10⁻⁷, 10⁻⁶, and 10⁻⁵ M in (a), (b) and (c) respectively in the first cycle. (B) Cyclic voltammetry profiles of the electrochemical reduction and oxidation of the nanohybrid with increasing CHL-a concentration of 10⁻⁷, 10⁻⁶, and 10⁻⁵ M in (a), (b) and (c) respectively in the second cycle. Conditions: 50 mM NaCl, background electrolyte pH 7.2. Scan rate: 100 mV s⁻¹.

The oxygen reduction reaction (ORR) in aqueous solution occurs mainly by two pathways: the direct 4-electron reduction pathway from O₂ to H₂O, and the 2-electron reduction pathway from O₂ to hydrogen peroxide (H₂O₂). In proton exchange membrane (PEM) fuel cells, including direct methanol fuel cells (DMFCs), ORR is the reaction occurring at the cathode. In our experiment the overall reaction mechanism can be as follows: in the first step, the adduct between oxygen and the Mg center of the macrocyclic compound is formed, followed by an intra-adduct electron transfer from the Mg ion to the oxygen. Alt et al. explained³³ that in the interaction between O₂ and the Mg center electron, a transition occurs first from oxygen into the empty d_z² orbital, forming a σ bond, lowering the anti-bonding π orbitals and raising the energy of the d_xz and d_yz orbitals of the center magnesium, thus allowing the electron transition from these filled orbitals to the anti-bonding π orbital, and resulting in an enhanced interaction. The addition of protons from the electrolyte, together with the electron transfer, then produces H₂O₂ at 0.7 V. The H₂O₂ is not the final product and can be further reduced to produce water at a potential of 1.7 V, resulting in an overall 4-electron reduction of oxygen on the CHL-a/graphene surface. The mechanism can be summarized as follows:

[LMg^II] + O₂ ↔ [LMg^δ+⋯O₂^δ−]

[LMg^δ+⋯O₂^δ−] + H⁺ → [LMg^III⋯O₂H]⁺

[LMg^III⋯O₂H]⁺ + H⁺ + 2e⁻ → [LMg^II] + H₂O₂

H₂O₂ + 2H⁺ + 2e⁻ → 2H₂O

H₂O₂ → H₂O + 1/2O₂

where L represents the ligand and Mg is the magnesium centre.

The −0.7 V cathodic wave in the second cycle (Fig. 6(B)) decreases for the first two concentrations due to the formation of RGO, and reappears in the highest CHL-a concentration accompanied by a concomitant increase in the peak current density, as with the first reduction wave of the above mentioned equations the trend remained the same in the following cycle.

All these data cumulatively suggest the presence of CHL-a over a graphene surface and indicate that photo-excited CHL-a in an aqueous environment can be restored back to its neutral stage by acquiring an electron from the surrounding water. Thus there is always a possibility of water splitting in due course. Although further study in this regard is necessary to prove water splitting, we can surely say that this one-pot process of preparing a RGO/CHL-a nanohybrid has paved the way for the architecture of a next generation electrode material where CHL-a over a graphene surface can effectively contribute to the oxygen reduction mechanism on the cathode and underpins clean-energy technology such as bio-fuel cells and metal–air-batteries.

4. Conclusions

We attempted to unravel the process of restoring sp² hybridization in a GO framework where photo-excited electrons can be transferred from the CHL-a LUMO to GO. This can be ensured via π interactive affinity between the CHL-a and RGO surfaces. The formation of a crystalline RGO lattice (evident from the TEM data) along with the gradual reduction of the I_D/I_G ratio in the Raman spectra points towards the evolution of RGO with increasing CHL-a concentration. This phenomenon further corroborates with the TCSPC data where the contribution of the faster decay component increases with CHL-a concentration, signifying the electron transfer process from CHL-a to graphene oxide and supported by C 1s XPS spectra showing an increased intensity of the sp³ binding energy at the expense of the C=O binding energy.The presence of an intense N 1s peak in the XPS spectra at a higher energy (408 eV) also supports the electron transfer from CHL-a non bonding electron to graphene surface. The ratio of radiative to non-radiative decay rate decreasing along with the CHL-a concentration also implies efficient electron transfer from CHL-a to GO. Interestingly when we compare the data of GO with that of its CHL-a reduced counterpart, through cyclic voltammetry in an aqueous NaCl environment, the major difference found is the appearance of a −1.7 V cathodic wave. This peak is also found in CHL-a reduced RGO where the intensity increases towards higher CHL-a concentration and can be attributed to a 4 electron oxygen reduction pathway. The 0.7 V anodic wave superimposes with the GO peak, thus we cannot differentiate it for the first two concentrations, from the first step of O₂ reduction leading to H₂O₂ production. Interestingly at final CHL-a concentration this peak is very intense along with the −1.7 V peak, in contradiction with the other data, and shows the highest possible reduction of GO. Thus it can be ascribed to the first reaction of oxygen reduction. The overall probable mechanism has been proposed. All these findings in combination bring a deeper understanding of RGO production in a simple bio-friendly route, by accepting electrons from photo-excited CHL-a, and simultaneously propose that this RGO CHL-a nanohybrid can be used as an effective electron transfer catalyst for next generation bio-fuel cells.

Acknowledgements

We gratefully acknowledge Prof. Siddhartha Das of Metallurgical & Materials Engineering, Indian Institute of Technology, Kharagpur who provided us with HRTEM and cyclic voltammetry facilities. The technical assistance rendered by Mr Subrata Das for TCSPC characterization & Mr Manash Kumar Gosh for Raman Spectroscopy characterization from Indian Association Cultivation of Science, Kolkata 700032, India, and financial support from University Grant Commission under BSR-Meritorious 2011–2012 scheme to Debmallya Das is also gratefully acknowledged.

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Footnotes

† The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

‡ Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra08593d

§ These authors contributed equally.

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