Photocatalyst-biocatalyst integrated artificial photosynthesis system for formic acid production from CO 2

Figure 2 depicts the photocatalyst-biocatalyst integrated artificial photosynthesis system for formic acid production from CO 2 . The light-harvesting DdIC chromophore (as an electron donor) collects incident photons as an electronic transition between localized orbital around it (HOMO to LUMO) and conducts via graphene (as a multi electron acceptor) to reduce the rhodium complex Rh ([Cp*Rh(bpy)H 2 O]2+;Cp* = pentamethylcyclopentadienyl, bpy = 2,2′-bipyridine). Upon reduction, the rhodium complex abstracts a proton from aqueous solution and transfers hydride ion to NAD+, which gets converted into NADH, thus completing the photocatalytic cycle. In this way, Rh behaves as an electron mediator between the graphene film photocatalyst and NAD+ leading to regeneration of NADH cofactor. Finally the enzymatic (Formate Dehydrogenase) conversion of CO 2 substrate to formic acid consumes NADH. The NAD+ thus released again acts as a substrate for photocatalytic cycle, leading to photoregeneration of NADH. The photocatalytic-enzymatic cycles thus couple integrally, leading to exclusive formic acid production from CO 2 3.

Preparation and characterization of GFPC 1 photocatalyst

The GFPC 1 photocatalyst for this research work was obtained by coupling graphene film (hereinafter GF) with 1,3-Dioxo-1H-dibenzo[de,h]isoquinoline-2[3 H]-carbaldehyde (hereinafter DdIC chromophore) via 1,3-dipolar cycloaddition. More details are provided in the experimental section. The integration of GFPC 1 photocatalyst with biocatalyst (formate dehydrogenase enzyme) afforded the integrated artificial photosynthesis system for carrying out highly selective formic acid production from CO 2 (Fig. 2).

The existence of DdIC in GFPC 1 was confirmed by UV-vis spectroscopy. A strong soret band at 433 nm was observed in the absorption spectrum of DdIC chromophore in DMF (Fig. 3a). On the other hand, the absorption spectrum of GFPC 1 exhibited a broad peak from 400–470 nm with a blue shifted maxima of DdIC unit (415 nm)18. This blue shift in absorption maxima can be attributed to the 1,3-dipolar cycloaddition of DdIC to GF. This observation indirectly suggested the covalent attachment of DdIC chromophores on the monolayer graphene.

Figure 3 (a) Absorption spectrum of GFPC 1 and DdIC chromophore. (b) FTIR spectra of DdIC chromophore, GFPC 1 and GF. Full size image

The Fourier Transform Infrared (FTIR) data provided clear evidence for the coupling of chromophore to graphene film (Fig. 3b). The GF spectrum was nearly featureless, while in GFPC 1 some features of DdIC chromophore were observed in the fingerprint region (1500–550 cm−1)19. Moreover in GFPC 1, the C=O stretching was observed at 1772 and 1707 cm−1 while C=C stretching was observed at 1670 and 1597 cm−1 (Fig. S1). In comparison the C=O stretching was observed at 1775 and 1710 cm−1 while C=C stretching was observed at 1597 cm−1 in case of DdIC chromophore. The coupling was further confirmed from the C-H stretching vibration of aldehyde group at 2783 cm−1 in DdIC. The absence of this peak in GFPC 1 provided decisive evidence for the coupling of DdIC chromophore with GF.

The AFM analysis (Fig. 4) of GF revealed monolayer graphene with thickness of 0.9344 nm with identical values reported in literature (0.6–0.9 nm)19. An increased thickness of 1.4681 nm in GFPC 1 indicated functionalization on the GF surface. Apart from increased thickness, the presence of DdIC on the GF surface was further supported by the bloated appearance and roughness of the sample. The difference in the high resolution transmission electron microscopy (HRTEM; Fig. 4) and scanning electron microscopy (SEM; Fig. 4) images of GF and GFPC 1 further support the AFM data. Clearly evident morphological changes between GF and GFPC 1 were observed which can be attributed to the 1,3-dipolar cycloaddition of DdIC chromophore on GF surface affording the GFPC 1 photocatalyst.

Figure 4 (a) AFM roughness image of GF. (b) AFM roughness image of GFPC 1. (c) AFM 3D image of GF. (d) AFM 3D image of GFPC 1. (e) HRTEM image of GF. (f) HRTEM image of GFPC 1. (g) SEM image of GF. (h) SEM image of GFPC 1. Full size image

Raman studies further confirmed the coupling of DdIC chromophore to GF (Fig. S3). The G and 2D bands of GF were observed at 1580 and 2670 cm−1, respectively20. Upon coupling of DdIC chromophore to GF, the G and 2D bands in GFPC 1 shifted to 1569 and 2686 cm−1, respectively. This shift in G and 2D bands clearly result from functionalization of GF by 1,3-dipolar cycloaddition of DdIC chromophores21,22. Besides this the appearance of peak at 1341 cm−1 in case of GFPC 1 due to D band of graphene can be further attributed to covalent bond formation between graphene film (GF) and DdIC chromophore3,23.

While there was no peak in N1s XPS spectrum of GF, following 1,3-dipolar cycloaddition coupling24 of DdIC a single new peak at 397 eV ascribed to N atoms of the C-N bond appeared (Fig. S4)25. Moreover changes were also observed in C1s XPS spectra of GFPC 1 when compared to GF (Fig. S5). These XPS data along with the various spectroscopic and structural data confirmed the grafting of DdIC on GF thereby resulting in GFPC 1.

Following the above analysis, the thermal behavior of CVD grown graphene film (GF) and GFPC 1 was also studied by thermogravimetric analysis (TGA) (Fig. S6). TGA was performed at a heating rate of 10 °C min−1 under nitrogen in the temperature range of 50–900 °C. The TGA curve for GF exhibited slow weight loss with an increase in temperature up to 900 °C. On the other hand, 24.63% weight loss due to the DdIC chromophore attached to GF was observed in GFPC 1 from 200 to 600 °C. Accordingly, the loading of DdIC chromophore in GFPC 1 was also determined by TGA and estimated to be one DdIC group per 70 carbon atoms of graphene in GFPC 121,26.

Photocatalytic NADH Regeneration and Formic Acid Production from CO 2

The most critical part of the work was to evaluate the photocatalytic performance of the GFPC 1 in comparison to the DdIC chromophore, graphene-DdIC powder photocatalyst and its spin coated film sample (hereinafter GFPC 2; details in experimental section). Therefore, visible light-driven 1,4-NADH photo-regeneration ability of all these were examined along with GF. While GF failed to carry out any NADH regeneration, DdIC chromophore afforded 21.8% NADH regeneration over a period of 120 minutes. On the other hand, graphene-DdIC powder photocatalyst and GFPC 2 afforded 48.9 and 40.1% of NADH regeneration over a period of 120 minutes, respectively. Since GFPC 2 was obtained by spin coating the graphene-DdIC powder, a lower NADH regeneration by GFPC 2 can be clearly attributed to irregular orientation of the photocatalyst species on the polyimide sheet. In comparison to this 91.8% NADH regeneration was observed when GFPC 1 was evaluated for the photocatalytic activity (Fig. 5a). In other words, more than 225% higher NADH regeneration was carried out by GFPC 1 in the same time period when compared to spin coated GFPC 2.

Figure 5 Photocatalytic activity of GF, DdIC chromophore, graphene-DdIC powder, GFPC 1 and GFPC 2. (a) NADH regeneration [β–NAD+ (1.24 μmol), Rh (0.62 μmol), AsA (0.1 mmol) and photocatalyst (1 × 1 cm2 film of GF, GFPC 1 and GFPC 2 or 0.5 mg of DdIC and graphene-DdIC powder) in 3.1 mL of sodium phosphate buffer (100 mM, pH 7.0)]. (b) Selective production of formic acid from CO 2 (flow rate: 0.5 mL/min) under visible light [β–NAD+ (1.24 μmol), Rh (0.62 μmol), AsA (0.1 mmol), formate dehydrogenase (3 units) and photocatalyst (1 × 1 cm2 film of GF, GFPC 1 and GFPC 2 or 0.5 mg of DdIC and graphene-DdIC powder) in 3.1 mL of sodium phosphate buffer (100 mM, pH 7.0)]. Full size image

To further examine the photocatalytic ability, they were used as the photocatalyst in an artificial photosynthetic system for formic acid production from CO 2 (Fig. 2)27. The results obtained were similar to NADH regeneration. While as expected GF failed to carry out any formic acid formation, DdIC chromophore afforded 48.6 µmol of formic acid over a period of 120 minutes. On the other hand, graphene-DdIC powder photocatalyst and GFPC 2 afforded 129.8 and 95.2 µmol of formic acid over a period of 120 minutes, respectively. Similar to NADH regeneration results, GFPC 1 photocatalyst afforded 228.6 µmol of formic acid which is also 2.3 fold higher formic acid production compared to GFPC 2 in 120 minutes (Fig. 5b). Undoubtedly this remarkably higher photocatalytic activity of GFPC 1 can be attributed to the regular arrangement of light harvesting units with spatial orientation towards the visible light as well as improved delocalization of π electrons and direct electron transfer from covalently attached chromophore to monolayer graphene. At the same time it was also evident from these experiments that spin coated film sample show lower photocatalytic performance compared to powder type photocatalysts which can be attributed to irregular orientation of the photocatalyst species on the film substrate. Overall these experiments clearly verify the success of our new design and fabrication strategy for graphene film photocatalysts.

The stability and reusability of GFPC 1 photocatalyst were examined by subjecting it to 6 cycles of formic acid production from CO 2 . In comparison to the first cycle wherein the photocatalyst carried out 228.55 μmol of formic acid production, remarkably high 187.89 μmol of formic acid was obtained in the 6th cycle (Fig. S7). These results indicate that GFPC 1 is a stable and reusable film photocatalyst for practical use.

The high formic acid production led us to consider the possibility of photocatalytic decomposition of GFPC 1 photocatalyst to carbon residues that may subsequently produce additional formic acid photocatalytically without CO 2 consumption28. To discount this possibility, a control experiment in the absence of CO 2 was performed. A lack of formic acid formation in this control experiment confirmed photocatalytic NADH regeneration under visible light irradiation. In a related fashion, a control experiment without the GFPC 1 photocatalyst (but with formate dehydrogenase enzyme) and another one without visible light irradiation were also carried out which also failed to show any detectable amount of formic acid. Additionally, no formic acid formation was detected in another set of control experiments carried out in the absence of rhodium complex Rh and/or NAD+ which confirmed their vital role in the functioning of the photocatalyst/biocatalyst integrated artificial photosynthesis system. To evaluate the possibility of rhodium complex Rh functioning as the CO 2 reduction photocatalyst a control experiment consisting of rhodium complex and CO 2 was performed. However, no formic acid was detected in this case also. This indicated that rhodium complex does not carry out photocatalytic CO 2 reduction. Along with the high stability of GFPC 1 under visible light irradiation the various control experiments clearly point towards exclusive visible light driven photocatalytic NADH regeneration followed by enzymatic CO 2 reduction as the sole route for formic acid production. This was further confirmed by C-13 isotope labelling experiment which clearly indicated that CO 2 is the sole source of formic acid formation in our system (Fig. S8).

Cyclic Voltammetry Studies

To understand the electron transfer pathway, cyclic voltammetry (CV) measurements were carried out. The reduction potential at cathodic peak current of GFPC 1 was observed at around −1.2 V (Fig. S9). On mixing GFPC 1 with rhodium complex Rh ([Cp*Rh(bpy)H 2 O]2+; Cp* = pentamethylcyclopentadienyl, bpy = 2,2′-bipyridine) reduction was observed at −0.97 V (Fig. 6a). This is an anodic shift of GFPC 1 reduction which is clearly indicative of electron transfer from GFPC 1 to Rh23. Moreover, the GFPC 1-Rh system revealed reduction potential at −1.05 V with NAD+, implying that the system consisting of GFPC 1 and Rh catalyzed the reduction of NAD+ to NADH (Fig. 6a). From these CV measurements it may be concluded that following the photoexcitation of the GFPC 1 electron from HOMO (E = −5.40 eV) to LUMO (E = −3.25 eV), it cascades into Rh (E = −3.79 eV). The vicinity and potential gradient between the light harvesting GFPC 1 photocatalyst and Rh center enable this efficient electron transfer from former to latter. The resultant reduced Rh species [Cp*Rh(bpy)] upon chemical protonation carries out catalytic regeneration of enzymatically active 1,4-NADH from NAD+ (E = −4.20 eV). This photocatalytic pathway is schematically shown in Fig. 6b.

Figure 6 (a) Cyclic voltammetry (CV) studies on Rh and GFPC 1 in the absence and presence of NAD+. The potential was scanned at 100 mVs−1 using glassy carbon (working), silver-silver chloride (reference) and platinum (counter) electrodes in sodium phosphate buffer (100 mM, pH 7.0). (b) Proposed photocatalytic pathway based on CV studies23,34,35. Full size image

Photocurrent studies

To further observe the behavior of the photogenerated electrons in GFPC 1, photocurrent measurements were carried out. The GFPC 1 film on FTO substrate exhibited a reversible photocurrent of 49 µA cm−2 in response to the on/off simulated sunlight (1 sun) illumination (Fig. 7). This value is nearly 10-times higher compared to that obtained for DdIC chromophore film on FTO substrate (5.1 µA cm−2) under identical conditions (Fig. 7). The high photocurrent is responsible for the photocatalytic activity of GFPC 1 under simulated solar light (one sun)29. Moreover photocurrent stability of GFPC 1 is evident from its reversible response on exposure to several on/off cycles. According to multiple reports this phenomenon indicates multi-electron transfer from DdIC to GF under one sun illumination3,23,30,31. It has also been experimentally demonstrated in a number of reports that graphene acts as an excellent multi electron transfer agent3,23. Thus, GFPC 1 can act as the photocatalytic conversion material for NADH regeneration and solar fuel formation32.

Figure 7 Photocurrent-time (I-T) profiles of FTO/GFPC 1 and FTO/DdIC electrodes under simulated solar light (1 sun) illumination (three electrodes; scan rate 50 mV/s; input power: 100 mW/cm2 and electrolyte: 0.1 M NaCl in water; bias potential: 0 to 0.1 V (vs Ag/AgCl)). Full size image

Density Functional Theory calculations

To elucidate the origin of the photocatalytic activity and activity enhancement of GFPC 1 theoretically, a series of first principles calculations using plane-wave based density functional theory (DFT) code, VASP were carried out33. At first, electronic structure calculations were performed separately on the DdIC chromophore and the graphene monolayer (an 8 × 8 unit composed of 128 carbon atoms). The Fermi level of graphene was calculated to be 4.20 eV lower than vacuum level, which lies around the center of bandgap of the chromophore, about 0.6 eV lower than the lowest unoccupied molecular orbital (LUMO) level of chromophore. Although this “divide-and-conquer” method is simple and powerful to deal with a large system without too much computational burden, the two components (DdIC chromophore and graphene monolayer) of the photocatalyst are tightly bonded via strong covalent bond. Thus the electron can be transferred spontaneously to make the Fermi levels equal. So, electronic structure calculations of the whole system were performed, secondly. The optimized atomic geometry and electronic band structure are shown in Fig. 8. The chromophore and graphene are tightly combined by four covalent bonds between two carbon atoms in chromophore and 4 atoms in graphene; the C-C bond lengths are within the range of 1.54–1.60 Å. The binding is further enhanced by π-π stacking between aromatic rings in chromophore and graphene. The band structure plot clearly shows that the Dirac point of graphene lies between the HOMO-LUMO energy-gap of the chromophore. The LUMO of chromophore lies ~0.3 eV higher than Dirac point. The localized molecular orbitals of chromophore are almost unaffected at Γ and M point but significantly hybridized with orbitals of graphene around K point. The incident light is absorbed at the chromophore, where occurs photoexcitation, and the created electrons move into the graphene via stable covalent bonds. The electron transfer efficiency could be estimated by the energy level alignment between the Fermi level of graphene and the LUMO of chromophore and also on the overlap of molecular orbitals. The existence of hybridized orbitals near Fermi level makes the ballistic transport of photoexcited electrons possible from the chromophore to graphene. The DFT calculation results clearly suggested that the photoexcited electrons transfer from chromophore finally to hydrogen reduction site via graphene. These simulation studies conform with the experimental results discussed above.