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Photoelectrochemical CO2 Reduction Using a Ru(II)−Re(I) Supramolecular Photocatalyst Connected to a Vinyl Polymer on a NiO Electrode Ryutaro Kamata, Hiromu Kumagai, Yasuomi Yamazaki,† Go Sahara, and Osamu Ishitani* Department of Chemistry, School of Science, Tokyo Institute of Technology, O-okayama 2-12-1-NE-1, Meguro-ku, Tokyo 152-8550, Japan

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ABSTRACT: A Ru(II)−Re(I) supramolecular photocatalyst and a Ru(II) redox photosensitizer were both deposited successfully on a NiO electrode by using methyl phosphonic acid anchoring groups and the electrochemical polymerization of the ligand vinyl groups of the complexes. This new molecular photocathode, polyRuRe/NiO, adsorbed a larger amount of the metal complexes compared to one using only methyl phosphonic acid anchor groups, and the stability of the complexes on the NiO electrode were much improved. The poly-RuRe/NiO acted as a photocathode for the photocatalytic reduction of CO2 at E = −0.7 V vs Ag/AgCl under visible-light irradiation in an aqueous solution. The poly-RuRe/NiO produced approximately 2.5 times more CO, and its total Faradaic efficiency of the reduction products improved from 57 to 85%. KEYWORDS: CO2 reduction, molecular photocathode, photoelectrochemical cell, metal complex−semiconductor hybrid, vinyl polymer

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INTRODUCTION The reduction of CO2 by using sunlight as an energy source and water as an electron donor to produce clean carbon resources, so-called artificial photosynthesis, has attracted a lot of attention as a promising way to solve serious energy and environmental problems, i.e., shortage of energy resources and global warming. Metal complexes have been widely investigated both as redox photosensitizers, which initiate photochemical electron transfer reactions from the electron donor to a catalyst, and as the catalysts for CO2 reduction. Suitable combinations of photosensitizer and catalyst can photocatalyze CO2 reduction with high efficiency, selectivity, and stability under visible-light irradiation.1,2 Among them, supramolecular photocatalysts, where the photosensitizer unit and the catalyst unit are connected to each other via a bridging ligand, show higher photocatalytic abilities compared to a mixture of systems of the corresponding mononuclear complexes.1,3−8 In the case of Ru(dmb)2(bpyC2bpy)Re(CO)3Cl (dmb = 4,4′dimethyl-2,2′-bipyridine, bpyC2bpy = 1,2-bis(4′-methyl-[2,2′bipyridine-9−4-yl]ethane), for example, the turnover number for CO formation (TONCO) was approximately 2.7 times higher compared with the mixture system and the selectivity for CO was improved from 38% to 76%, where the main byproduct was HCOOH.6 Rapid intramolecular electron transfer from the photosensitizer unit to the catalyst unit in the supramolecular photocatalysts, as confirmed by transientIR spectroscopy,9 results in superior catalytic behavior. The supramolecular photocatalyst works well even in aqueous © XXXX American Chemical Society

solutions containing suitable water-soluble electron donors such as ascorbic acid7 or 2-(1,3-dimethyl-2,3-dihydro-1Hbenzimidazol-2-yl)benzoic acid.8 However, the oxidation power of all of the reported supramolecular photocatalysts is too low for water to be used as the reductant. Therefore, these systems require a sacrificial reductant for the photocatalytic reduction of CO2 to proceed. The utilization of water as the reductant is essential for developing the artificial photosynthesis as a practical technology. We recently reported the photoelectrocatalytic activities of molecular photocathodes consisting of the Ru(II)−Re(I) supramolecular photocatalyst (RuReP, Chart 1) on a p-type semiconductor electrode, i.e., NiO10,11 or CuGaO2.12 The methyl phosphonic acid groups within one of the three diimine ligands of the photosensitizer unit anchor the RuReP to the semiconductor surface. These hybrid photocathodes (RuReP/ NiO and RuReP/CuGaO2) can reduce CO2 to CO photocatalytically with relatively high selectivity under visible-light irradiation by applying a moderate external bias. Notably, even in an aqueous electrolyte solution, the RuReP/NiO displayed highly selective CO generation (91%). The photocatalytic Special Issue: Artificial Photosynthesis: Harnessing Materials and Interfaces for Sustainable Fuels Received: April 5, 2018 Accepted: June 6, 2018

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DOI: 10.1021/acsami.8b05495 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

However, the fabrication of these photoelectrochemical systems remains problematic; the immobilization of the metal complex(es) onto the semiconductor electrode, weak adsorption of the methyl phosphonic acid anchors, and the small amounts of the molecular photocatalyst adsorbed onto the surface of the electrode, are issues that need attention. In the case of RuReP/NiO, the amount of the adsorbed complex on the electrode was only around 4 nmol cm−2 at the maximum, and even when using mesoporous NiO the absorbance of incident photons by this photoelectrode was still inefficiently.11 This could be one of the reasons why the apparent quantum efficiency for CO2 reduction was low even though the [Ru(N^N)3]2+ (N^N = diimine ligand) type of photosensitizer has a relatively high molar extinction coefficient in the visible region. In addition, a previous study revealed that about the half of the RuReP desorbed from the electrode surface during the photocatalytic reaction over 5 h in an aqueous solution, which again reduces the production yield of CO.11 Recently, various hybrid photocatalytic systems consisting of metal complexes and solid materials have been developed not only for CO 2 reduction but also for H 2 and O 2 evolution.10−12,14−32 Most of these systems also used anchoring groups, such as phosphonic acid and carboxylic acid, and had similar problems, i.e., low adsorption amounts and weak adsorption of the metal complexes to the surface of the solid materials. Therefore, to improve the activity of the photocathodes for CO2 reduction and the potential of various hybrid photocatalytic systems, new loading methods of molecular photocatalysts onto the semiconductor electrodes with more stable and dense adsorption are desired. Modification of electrode surfaces with polymer films containing metal complexes have been studied to improve electron transfer between the electrode and the metal complexes in heterogeneous systems,33−45 including photoelectrochemical systems for energy conversion.46−52 Sato, Morikawa and their co-workers reported a p-type Zn doped InP (InP:Zn) electrode modified with a Ru(II) complex polymer ([Ru(N^N)(CO)2Cl2]n), which consists of polypyrrole in the N^N ligand and has Ru−Ru bonds in the polymer.46,47 This Ru polymer could work as a catalyst but not as a photocatalyst, which would accept electrons from the InP:Zn semiconductor photoelectrode under irradiation to reduce CO2 to HCOOH. Meyer et al. reported a photoanode for water oxidation, where the TiO2 electrode is modified by a mixed vinyl polymer using [Ru(dvb)2((PO3H2)2bpy)]2+ (dvb = 5,5′-divinyl-2,2′-bipyridine; (PO3H2)2bpy = [2,2′-bipyridine]-4,4′-diylbis(phosphonic acid)) and Ru(bda)(4-vinylpridine)2 (bda = 2,2′-bipyridine-6,6′-bis(carboxylic acid)) as the monomers.52 These examples showed clearly that

Chart 1. Structure and Abbreviation of Metal Complexes

reduction of CO2 using water as the reductant proceeded by connecting these hybrid photocathodes with a CoOx/TaON semiconductor photoanode for water oxidation.13 This setup produced CO and O2 simultaneously via a step-by-step photon absorption by the photosensitizer unit in the supramolecular photocatalyst and the TaON in the photoanode, i.e., a Zscheme type electron transfer (Scheme 1).11 The RuReP/ Scheme 1. Photoelectrochemical CO2 Reduction System Using H2O as a Reductanta

a

Electron moves via the Z-scheme type electron transfer.

CuGaO2 photocathode was also used successfully for constructing another cell for CO2 reduction with water as the reductant. To the best of our knowledge, these are the sole instances of visible-light driven photocatalytic systems consisting of a molecular photocatalyst for CO2 reduction using water as the reductant.

Scheme 2. Preparation of the Electrode by Electropolymerization

B


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ACS Applied Materials & Interfaces polymerization of the metal complexes can increase the amount of the metal complex attached to the electrode, and also decrease any desorption from the electrode compared to adsorption using only anchoring groups, e.g., phosphonic acid. We herein report a novel immobilization method of the Ru(II)−Re(I) supramolecular photocatalyst on a NiO electrode by using both the methyl phosphonic acid anchors and the electrochemical polymerization of the vinyl groups introduced into the ligands of the supramolecular photocatalyst (Scheme 2). This improved the photocatalysis of the molecular photocathode for CO2 reduction drastically.

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Figure 2. UV−vis absorption spectra of (a) poly-Ru/NiO and (b) poly-RuRe/NiO (blue line), RuVP/NiO (red line), and the NiO electrode (green line). FTO electrode was used as the background.

RESULTS AND DISCUSSION Preparation of the Poly-Ru/NiO Photocathode. To confirm how the polymerization adsorption method works, we modified the p-NiO electrodes with mononuclear [Ru(N^N)3]2+-type complexes as models of the photosensitizer unit in the supramolecular photocatalysts. Two kinds of Ru complexes were used (Chart 1); one was RuV (Chart 1) consisting of two 4-methyl-4′-vinyl-2,2′-bipyridine (vbpy) ligands and one dmb ligand, and another was RuVP consisting of two vbpy ligands and one 4,4′-di(methyl phosphonic acid)2,2′-bipyridine ligand ((H2O3PCH2)2bpy). First, a NiO electrode was dipped into a MeCN solution containing RuVP where the methyl phosphonic acid groups in the (H2O3PCH2)2bpy should work as the anchors. The amount of RuVP adsorbed on the NiO electrode was estimated from the UV−vis absorption spectrum in MeCN to be 10.4 ± 1.0 nmol over 2.5 cm2, i.e., 4.2 ± 0.4 nmol cm−2. This RuVP/NiO was used as a working electrode for the electrochemical polymerization of RuV. Cyclic voltammetric sweeps of the RuVP/NiO in a solution containing 0.5 mM of RuV in MeCN were conducted between 0 V and −1.9 V vs Ag/AgNO3. Figure 1a

polymerization at around 464 nm, which is attributed to the singlet metal-to-ligand-charge-transfer (1MLCT) absorption band of the Ru complex. This absorption increased upon polymerization indicating that the Ru complex was fixed to the electrode and the [Ru(N^N)3]2+ structure remained intact after adsorption via both the methyl phosphonic acid and the vinyl polymer units to give a new electrode polymerized-RuVRuVP/NiO (poly-Ru/NiO). Comparison of Poly-Ru/NiO and RuP/NiO. The amount of electroactive Ru complex on the poly-Ru/NiO and the RuP/ NiO surfaces were estimated from the oxidation wave of the Ru complexes (RuII/RuIII, E1/2 = +0.84 V vs Ag/AgNO3) in the cyclic voltammograms (Figure 3). The poly-Ru/NiO (after

Figure 3. Cyclic voltammograms of (a) poly-Ru/NiO and (b) RuP/ NiO (in both cases, the electrode area was 2.5 cm−2). Blue lines show the results using as-prepared electrodes and red lines show the electrochemical behavior after soaking the electrodes in a 50 mM NaHCO3 (aq) overnight. All cyclic voltammograms were recorded in an Ar-saturated MeCN solution containing Et4NBF4 electrolyte (0.1 M), and the potential was scanned between 0 V and +1.2 V at a scan rate of 10 mV s−1.

Figure 1. Cyclic voltammograms of the (a) RuV and (b) RuReV on RuVP/NiO (electrode area: 2.5 cm−2) in an Ar-saturated MeCN solution containing (a) RuV or (b) RuReV (0.5 mM) and Et4NBF4 as electrolyte (0.1 M). The applied voltage was repeatedly scanned (20 times) from 0 to −1.9 V with a scan rate of 100 mV s−1.

20 sweeps) showed a wave corresponding to 24.8 nmol of the electroactive Ru complex, while RuP/NiO was only 9.0 nmol, where the geometric area of the NiO electrode was 2.5 cm−2 in both cases. This indicates clearly that electropolymerization is a useful method to adsorb electroactive metal complexes onto the surface of an electrode.53 Desorption of the Ru complexes from the electrodes was investigated in an aqueous electrolyte solution as follows. Even after soaking these electrodes overnight in a 50 mM NaHCO3 aqueous solution, which is a typical reaction solution for the photoelectrochemical reduction of CO2, 20.1 nmol of the active Ru complex remained on the electrode in the case of the poly-Ru/NiO, whereas only 0.9 nmol of the Ru complex was maintained on the RuP/NiO. This higher stability against desorption could be due to the hydrophobicity of the vinyl

shows the cyclic voltammograms during the electropolymerization. The current which corresponds to the redox reaction of the Ru(II) complex is evident between −1.3 and −1.9 V vs Ag/AgNO3 and increased with the number of sweep cycles. This augmentation of the current suggests that immobilization of the electroactive Ru complexes on the electrode was successful; the anion radical generated by the reduction of RuV should attack another vinyl group and then polymerization proceeded. Figure 2a shows the UV−vis absorption spectrum of the NiO electrode, RuVP/NiO, and the same electrode after the polymerization. A characteristic absorption maximum appeared in the spectrum of the RuVP/NiO before and after C


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ACS Applied Materials & Interfaces polymer containing the Ru complex. These results indicate strongly that, from the viewpoint of both the amount and the stability of the adsorbed metal complex, the polymerization adsorption method is superior to only adsorption by the interaction between the methyl phosphonic acid anchors and NiO. Preparation and Characterization of Poly-RuRe/NiO Photocathode and Its Characterization. The polymerization adsorption method was applied successfully to synthesize a hybrid photocathode for CO2 reduction from the Ru(II)−Re(I) supramolecular photocatalyst. A polymerized-RuReV-RuVP/NiO (poly-RuRe/NiO) was prepared by a similar method for poly-Ru/NiO, by simply changing the precursor monomer from RuV to RuReV, which consists of both the Ru(II) photosensitizer and the Re(I) catalyst units. The current during electropolymerization increased gradually with the number of cycles (Figure 1b), indicating polymerization of the vinyl groups. It is noteworthy that the increase in current during the potential sweeps was much larger than that observed for RuV (Figure 1a). One of the main reasons for this increase in current is probably due to the formation of a Re−C bond (Scheme 3), which has been previously reported in the

Figure 4. FT-IR spectra of poly-RuRe/NiO (blue line) and Ru(dmb)2(bpyC2bpy)Re(CO)3Br on (RuRe) on a NiO electrode (red line). A diffuse reflectance unit was used for the measurements. Background was measured using the bare NiO electrode and the resolution was 2 cm−1. RuRe on the NiO electrode was prepared by dropping a CH2Cl2 solution containing RuRe and drying the solution.

be estimated by ICP measurement reproducibly because of the volatility of Ru oxidized during dissolving into acid solution for ICP measurement. Scanning electron microscopy (SEM) was carried out to investigate the macro structure of the poly-RuRe/NiO (Figure 5). The SEM images of the NiO electrode showed a porous

Scheme 3. Possible Structures of the Re−C Bonds

electropolymerization of [Re(vbpy)(CO)3(MeCN)]+.33 The FT-IR spectrum of the poly-RuRe/NiO showed two pairs of νco peaks attributable to the tricarbonyl structures of the two Re complexes. One of which should be from Ru(polyvbpy)2(bpyC2bpy)Re(CO)3Br (poly-vbpy = polymerized vbpy ligand), where the νco of the totally symmetric vibration was 2018 cm−1, and another possibly had a Re(N^N)(CO)3(CH2X) type structure, which had the totally symmetric vibration at νco = 1990 cm−1 (Figure 4). These results indicate that the simultaneous formation of not only the methylene chain but also the Re−C bond within the polymer networks, result in a more rapid and dense adsorption of RuReV onto the electrode surface compared to RuV. Figure 2b shows the UV−vis absorption spectra of the polyRuRe/NiO, RuVP/NiO, and the bare NiO electrode. Although the 1MLCT absorption band derived from the Ru photosensitizer unit was observed in the case of poly-Ru/NiO, a larger amount of the Ru unit was adsorbed in the case of the poly-RuRe/NiO, which agrees well with the increased current observed in the CV. The adsorbed amount of the RuReV in the case of the poly-RuRe/NiO was estimated as 31.4 ± 7.3 nmol from the absorption spectrum, where uniform immobilization of the complex on the electrode was assumed. These values agreed with the elemental analysis data of Re on the electrode by ICP measurement (32.5 ± 3.8 nmol). We note that the amount of Ru complex in the polymer was not able to

Figure 5. SEM images of the NiO electrode ((a) top image and (b) sectional image) and the poly-RuRe/NiO ((c) top image and (d) sectional image) (50 000×)

structure composed of nanoparticles with a diameter of approximately 40 nm (Figures 5a and 5b for top and sectional views, respectively). On the other hand, a flat smooth polymer layer was found on the electrode in the case of poly-RuRe/NiO (Figure 5c). From its sectional image, shown in Figure 5d, a layered structure of the poly-RuRe/NiO/FTO was observed and the thicknesses of each layer of the RuRe polymer and the NiO film were approximately 140 and 650 nm, respectively. Time-of-Flight secondary ion mass spectrometry (TOF-SIMS) indicated the detailed distribution of the metal complexes D


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ACS Applied Materials & Interfaces down through the depth of the poly-RuRe/NiO (Figure 6). The sputter times on the horizontal axis in the figure

the electrode. Thus, the following experiments were conducted using illumination from the back of the electrode. Figure 8

Figure 6. TOF-SIMS result of poly-RuRe/NiO. Figure 8. Wavelength dependence of the IPCE of the poly-RuRe/NiO at −0.5 V vs Ag/AgCl in a CO2 purged 50 mM NaHCO3 (aq) (pH 6.6) (red dot). The blue line shows the UV−vis absorption spectrum of the poly-RuRe/NiO, where the absorbance of the NiO electrode was subtracted.

correspond to the components from the surface of the electrode. While the Ru ones were distributed not only in the polymer layer but also inside the NiO layer, most of the Re species were located at the top of the polymer layer. These results suggest that the polymerized RuReV was mainly localized in the polymer layer on the outer surface of the NiO electrode. On the other hand, the mononuclear RuVP adsorbed into the interior of the NiO layer, which had mesoporous structure. The larger size of RuReV, and the previously adsorbed RuVP might prevent polymerization in the mesoporous NiO layer. This is also confirmed by the TOFSIMS analysis of the RuVP/NiO (Figure S9), where the Ru complexes were widely distributed throughout the NiO layer. Photoelectrochemical Properties of Poly-RuRe/NiO. Figure 7 shows the current−potential curves of the poly-RuRe/

shows the action spectrum of the incident photon to current conversion efficiency (IPCE) at E = −0.5 V vs Ag/AgCl. A good correlation was obtained between the action spectrum and the UV−vis absorption spectrum of the poly-RuRe/NiO. It is noteworthy that this absorption spectrum was obtained as the difference in the absorption spectrum of the poly-RuRe/ NiO and that of the bare NiO electrode. Since the absorption band was observed between 400 and 600 nm, it is attributed mostly to the 1MLCT absorption of the Ru complexes immobilized on the NiO electrode. Therefore, it was concluded that the cathodic photocurrent was generated by photoexcitation of the Ru complexes. The maximum IPCE of the poly-RuRe/NiO was 0.93% at E = −0.5 V vs Ag/AgCl under irradiation at λex = 480 nm. Photoelectrochemical CO2 Reduction Using the polyRuRe/NiO. Photoelectrochemical CO2 reduction using polyRuRe/NiO was conducted under irradiation of visible-light (λex > 460 nm) at −0.7 V vs Ag/AgCl in a CO2 saturated aqueous solution (50 mM NaHCO3, pH 6.6). Figure 9 shows the time courses of the products and half amounts of electrons passed through the electrode. The main product was CO (507 nmol), with H2 (120 nmol) and HCOOH (151 nmol) detected as byproducts. The total Faradaic efficiency of

Figure 7. Current−potential curves of (a) poly-RuRe/NiO and (b) RuReP/NiO as the working electrodes in CO2 purged 50 mM NaHCO3 (aq) (pH 6.6); scan rate = 10 mV s−1. Light at λex > 460 nm was irradiated from the top (blue line) or the back (red line) of the electrode.

NiO and RuReP/NiO in a CO2-purged aqueous solution containing NaHCO3 (50 mM). The electrodes were illuminated at λex > 460 nm perpendicular to the surface of the polymer layer either from the top, or from the back, i. e., from the FTO glass side. In all cases, the cathodic photocurrent was observed from E ≈ +0.3 V vs Ag/AgCl, and increased with scanning to a more negative potential. In the case of poly-RuRe/NiO, the larger photocurrent was observed under irradiation from the back, compared with from the top. This indicates that photoexcitation of the Ru complexes localized on the side of the NiO film were more efficient for producing the photocurrent. The photocurrent of poly-RuRe/NiO was approximately twice that of RuReP/NiO. These results reflect the increased amount of Ru complex on

Figure 9. Time courses of CO (red triangle), H2 (blue circle), HCOOH (green square), and half amounts of electrons (black line) passed through the poly-RuRe/NiO (2.5 cm−2) at E = −0.7 V vs Ag/ AgCl under irradiation at λex > 460 nm. CO2 purged 50 mM NaHCO3 (aq), (pH 6.6) was used as electrolyte. E


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ACS Applied Materials & Interfaces formation of these reduction products was 85% after 5 h irradiation. TONCO of this reaction based on the estimated amount of Re by ICP was approximately 16, indicating that the photocatalytic CO formation did proceed in this reaction. Isotopic labeling experiments using 13CO2 and NaH13O3 were conducted to evaluate the carbon sources of the CO produced during the photoelectrochemical reaction. Figure 10

Figure 12. FT-IR spectra of poly-RuRe/NiO before reaction (blue line) and poly-RuRe/NiO after reaction (red line). Diffuse reflectance unit was used for the measurements. Background was measured using the bare NiO electrode and the resolution was 2 cm−1.

(CO)3(CH2X) and Re(N^N)(CO)3Br disappeared completely and a new set of νCO bands were observed at 2014, 1900, and 1873 cm−1. This indicates that a Re complex with a tricarbonyl structure remains on the photocathode even after the photoelectrochemical reaction, and the produced Re species probably had an anionic ligand because of relatively low νCOs. The ligation of an anionic ligand to the central Re increases the electric density in the d orbitals and π-back-donation to π* orbitals of the CO ligands, which lowers νCOs. The possible candidates for X should be hydroxide ion and bicarbonate ion; it was reported that the νCOs of Re(4,4′-dimethyl-bpy)(CO)3(OCO2H) are 2017, 1912, and 1886 cm−1, which were measured in N,N-dimethylformamide.54 Comparison of the XPS spectra of the poly-RuRe/NiO before and after the reaction also supports the observation that the Ru and Re complexes remain on the electrode after the photoelectrochemical reaction (Figure S10). Table 1 summarizes the results of the photoelectrochemical CO2 reduction using various electrodes and conditions. The amount of CO generated from the poly-RuRe/NiO (entry 1) was approximately 2.5 times higher than previously reported

Figure 10. GC-MS chromatograms of the gas phase in the reaction cells after irradiation, showing the peaks at m/z = 28 for 12CO and 29 for 13CO. The poly-RuRe/NiO at E = −0.7 V vs Ag/AgCl was irradiated at λex > 460 nm in a CO2 purged aqueous solution containing 50 mM NaHCO3 (pH 6.6) for 5 h using (a) 13CO2 and NaH13O3 or (b) ordinary CO2 and NaHCO3.

shows the GC-MS chromatograms of peaks with m/z = 28 (12CO) and 29 (13CO), of the gas phase in the reaction cells after irradiation for 5 h. Almost all the CO, which produced under a 13CO2 atmosphere in a NaH13O3 aqueous solution, was 13CO. A trace amount of 12CO was detected, which could originate from the three CO ligand in the Re complex, and/or from 12CO2 impurities in the 13CO2 gas, (99% pure). On the other hand, only 12CO was detected when ordinary CO2 and NaHCO3 were used. These results indicate clearly that CO was produced photocatalytically from the CO2 in the photoelectrochemical reaction. Figures 11 and 12 show the UV−vis absorption and FT-IR spectra of the poly-RuRe/NiO after the photoelectrochemical

Table 1. Photoelectrochemical Reactions Using Various Electrodes under Various Conditionsa products (nmol) entry 1 2 3 4 5c 6

Figure 11. UV−vis absorption spectra of the poly-RuRe/NiO (blue line), poly-RuRe/NiO after 5 h reaction (green line), and bare NiO electrode (red line). FTO electrode was used as the background.

7 8d

reaction for 5 h, respectively. The 1MLCT absorption band of the Ru complex was mostly maintained even after irradiation, therefore, most of the Ru complexes remained as [Ru(N^N)3]2+ and did not leave from the photoelectrode. The FT-IR spectrum of the irradiated electrode showed a critical change after irradiation, the νCO bands of Re(N^N)-

electrodes poly-RuRe/ NiO RuReP/NiO NiO poly-Ru/NiO poly-Re/NiO poly-RuRe/ NiO poly-RuRe/ NiO poly-RuRe/ NiO

Fred (%)

potential (V)b

CO

H2

−0.7

507

151

120

85

−0.7 −0.7 −0.7 −0.7 −0.3

210 n. d. n. d. n. d. 238

7.0 5.0 383 6.4 86

n. d. n. d. 177 n. d. 174

57 2.3 62 1.6 76

0

n. d.

14

trace

7

85

91

n. d.

19

−0.7

HCOOH

a

Electrodes in CO2 purged aqueous solutions containing NaHCO3 (50 mM, pH 6.6) were irradiated at λex > 460 nm using a 300 W Xe lamp for 5 h. bVersus Ag/AgCl (sat. KCl). cThe poly-Re/NiO was prepared by electropolymerization of ReV on a NiO electrode with a similar method for poly-Ru/NiO. dUnder an Ar atmosphere. Fred: Faradaic efficiency for reduction reaction. n.d.: not detected. F


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electrode at 0 V was consumed for reduction of Ni3+ or some products below the detection limit. Under an Ar atmosphere, a small amount of CO was produced, which was 2.5 times more than that from the adsorbed Re complex (entry 8). This CO formation was probably caused by the decomposition of the Re unit to release its carbonyl ligands, as discussed already in the labeling experiments.

for RuReP/NiO, which was prepared by absorption via phosphonic acid anchors of RuReP on the NiO electrode (entry 2).11 The larger amount of CO formation by using polyRuRe/NiO clearly indicates that the electropolymerization method improved the photocatalytic CO2 reduction ability of the molecular photocathode. This method also increased the Faradaic efficiency of the reduction products (Fred = 85%) compared to the RuReP/NiO (Fred = 57%). The two main reasons for the improvement in photocatalysis when using poly-RuRe/NiO are (1) the increase in the amount of the active RuRe photocatalyst on the electrode and (2) the suppression of desorption of the metal complexes from the electrode. Additional adsorption of the mononuclear Ru(II) complex probably had a positive effect on the photocatalytic reaction because of its role as a redox photosensitizer. It has been reported that the reduction of contaminated Ni3+ in the NiO film to Ni2+ during the photoreaction lowers the Faradaic efficiency of the photoelectrochemical reduction of CO2 by using RuReP/NiO.10,11 The increase in the yield of the reduction products should suppress this negative effect and, consequently, the Faradaic efficiency should increase. No CO formation was observed when using a bare NiO electrode (entry 3), the electropolymerized Ru complex (polyRu/NiO, entry 4) or the Re complex (poly-Re/NiO, entry 5). Both the Ru-complex and Re-complex units are necessary for the photocatalytic reduction of CO2 to CO. Relatively larger amounts of HCOOH and H2 were produced in the cases using poly-RuRe/NiO and poly-Ru/NiO. This is probably caused by the catalysis of the decomposed products from the Ru complexes, which have [Ru(N^N)2(solvent)2]2+ structures, on the electrode. Photocatalytic systems consisting of the residual Ru-complex redox photosensitizers, with [Ru(N^N) 3 ] 2+ structures, and the [Ru(N^N) 2 (solvent) 2 ]2+ catalysts should mainly produce HCOOH and H2. Such decomposition processes of Ru(II) redox photosensitizers and the catalysis of the decomposition products have been reported previously in various photocatalytic systems.6,8 When the applied potential was set at more positive potentials, i.e., E = −0.3 V (entry 6) and 0 V (entry 7), the photocurrents were lower (Figure 13), and the CO generation and Fred also decreased compared to those at −0.7 V (entry 1), (Figure 9 and Figure S11). This correlation suggests that the electron used for CO2 reduction was supplied from the NiO electrode to the immobilized metal complexes on the NiO electrode. It is thought that electrons passed through the

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CONCLUSION A novel preparation method for a stable and efficient molecular photocathode bearing Ru(II)−Re(I) supramolecular photocatalyst units for CO2 reduction, poly-RuRe/NiO, was developed successfully by using the reductive electropolymerization of the vinyl groups in the diimine ligand combined with methyl phosphonic acid anchors. This method increased the adsorbed amount of metal complexes on the NiO electrode, and suppressed desorption of the metal complexes from the electrode. The poly-RuRe/NiO showed a much higher activity of visible-light-driven photoelectrochemical CO2 reduction in an aqueous solution. Poly-RuRe/NiO produced a larger amount of CO with a higher Faradaic efficiency than previously reported molecular photocathodes, where the Ru(II)−Re(I) supramolecular photocatalyst was adsorbed onto the electrode only by using methyl phosphonic acid anchors.

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EXPERIMENTAL SECTION

Fabrication of NiO Electrode. The NiO electrode was prepared according to literature procedures.11 The outline of the preparation is as follows. The precursor solution of water-EtOH (4.5 g, 1:2, w/w), and a solution containing Ni(NO3)2·6H2O (1.0 g, 99.95%, KANTO chemicals) and Pluronic F-88 (0.5 g) were deposited onto clean FTO glass (AGC fabritech, 15 × 50 mm2, 12 Ω/sq) by the squeegee method. This sample was calcined at 773 K for 0.5 h in air. This deposition-calcination cycle was repeated four times. This electrode was cut in half before being used. Preparation of Metal Complex/NiO by Adsorbing through a Phosphonic Acid Anchor. The NiO electrode (electrode area: 2.5 cm2) was soaked in a MeCN solution (4 mL) containing RuP, RuVP, or RuReP (Chart 1) (5 μM) overnight. This electrode was washed with MeCN and dried under dark condition. The amount of the adsorbed metal complex on the NiO electrode was calculated from the difference of the absorbance of the solution at the absorption maximum of the 1MLCT adsorption band, before and after the adsorption procedures. Electropolymerization of RuV or RuReV onto RuVP/NiO. A MeCN solution (5 mL) containing the metal complex (0.5 mM) and Et4NBF4 (0.1 M) was introduced to a one-component electrochemical cell. The RuVP/NiO was utilized as the working electrode. An Ag/AgNO3 reference electrode (Ag wire immersed in MeCN containing 10 mM of AgNO3 and 0.1 M Et4NBF4 separated by vycol) and a platinum wire were used as the reference and the counter electrodes, respectively. After being purged by Ar for 10 min, the potential was swept between 0 and −1.9 V vs Ag/AgNO3 at 100 mV s−1. The obtained poly-metal complex/NiO was washed with pure MeCN and kept in the dark. Estimation of the Amount of Electroactive Metal Complex on the Electrode. Cyclic voltammetry was conducted by using a three-electrode setup for estimating the amount of electroactive metal complex on the electrode. An Ar-purged MeCN solution (10 mL) containing Et4NBF4 (0.1 M) was introduced to the electrochemical cell, where the working electrode was the modified NiO electrode adsorbed, a Ag/AgNO3 reference electrode and a platinum-wire counter electrode were also used. The potential was swept between 0 and +1.3 V vs Ag/AgNO3 at 10 mV s−1. The amount of the

Figure 13. Time courses of photocurrent using poly-RuRe/NiO (electrode area: 2.5 cm−2) at various potentials under light irradiation (λex > 460 nm). G



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electroactive metal complex on the electrode was calculated using eq 1. nCV = S /Fv

*E-mail: [email protected].

(1)

ORCID

Hiromu Kumagai: 0000-0003-2714-8577 Yasuomi Yamazaki: 0000-0002-8640-7367 Osamu Ishitani: 0000-0001-9557-7854 Present Address †

Y.Y. is currently at Faculty of Science and Technology, Department of Materials and Life Science, Seikei University, 33-1 Kichijoji-Kitamachi, Musashino-shi, Tokyo, 180-8633, Japan Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JST CREST Grant Number JPMJCR13L1 in “Molecular Technology,” JST Strategic International Collaborative Research Program (SICORP), and JSPS KAKENHI Grant Number JP17H06440 in Scientific Research on Innovative Areas “Innovations for Light-Energy Conversion.” H.K. thanks JSPS KAKENHI Grant Number JP16K21031 for Young Scientists (B). We thank Ookayama Materials Analysis Division, Technical Department, from Tokyo Institute of Technology for the SEM measurement and TOF-SIMS measurement.

(2)

Where nTS is the amount of metal complex (mol), Abefore and Aafter are absorbances of the electrode before and after adsorbing metal complex at the maximum wavelength (λ = 464 nm), S is the electrode area (2.5 cm2) and ελ is the molar extinction coefficient at the same wavelength, which was determined in a MeCN solution (in the case of RuReV, ε464 = 1.78 × 104 M−1 cm−1). Photoelectrochemical Measurements. An aqueous solution containing NaHCO3 (50 mM) was introduced to a one-component electrochemical cell. The metal-complex/NiO electrodes were set in the cell with an Ag/AgCl reference electrode (saturated KCl aqueous solution) and a platinum-wire counter electrode. A 300 W Xe lamp (Asahi Spectrum, MAX-302) equipped with an IR-blocking mirror module was utilized as a light source. A cutoff filter (HOYA Y48 for irradiation at λex > 460 nm) or a band-pass filter (Asahi Spectra, for IPCE measurements) was employed to controlling the irradiation wavelength. The dependence on the wavelength of the IPCE was calculated by eq 3.

IPCE(%) = [(1240/λex )(Ilight − Idark)/Plight]100

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REFERENCES

(1) Yamazaki, Y.; Takeda, H.; Ishitani, O. Photocatalytic Reduction of CO2 Using Metal Complexes. J. Photochem. Photobiol., C 2015, 25, 106−137. (2) Morris, A. J.; Meyer, G. J.; Fujita, E. Molecular Approaches to the Photocatalytic Reduction of Carbon Dioxide for Solar Fuels. Acc. Chem. Res. 2009, 42, 1983−1994. (3) Tamaki, Y.; Ishitani, O. Supramolecular Photocatalysts for the Reduction of CO2. ACS Catal. 2017, 7, 3394−3409. (4) Tamaki, Y.; Watanabe, K.; Koike, K.; Inoue, H.; Morimoto, T.; Ishitani, O. Development of Highly Efficient Supramolecular CO2 Reduction Photocatalysts with High Turnover Frequency and Durability. Faraday Discuss. 2012, 155, 115−127. (5) Yamazaki, Y.; Ishitani, O. Synthesis of Os(II)−Re(I)−Ru(II) Hetero-Trinuclear Complexes and Their Photophysical Properties and Photocatalytic Abilities. Chem. Sci. 2018, 9, 1031−1041. (6) Kuramochi, Y.; Ishitani, O. Iridium(III) 1-Phenylisoquinoline Complexes as a Photosensitizer for Photocatalytic CO2 Reduction: A Mixed System with a Re(I) Catalyst and a Supramolecular Photocatalyst. Inorg. Chem. 2016, 55, 5702−5709. (7) Nakada, A.; Koike, K.; Nakashima, T.; Morimoto, T.; Ishitani, O. Photocatalytic CO2 Reduction to Formic Acid Using a Ru(II)−Re(I) Supramolecular Complex in an Aqueous Solution. Inorg. Chem. 2015, 54, 1800−1807. (8) Nakada, A.; Koike, K.; Maeda, K.; Ishitani, O. Highly Efficient Visible-Light-Driven CO2 Reduction to CO Using a Ru(II)−Re(I) Supramolecular Photocatalyst in an Aqueous Solution. Green Chem. 2016, 18, 139−143. (9) Koike, K.; Grills, D. C.; Tamaki, Y.; Fujita, E.; Okubo, K.; Yamazaki, Y.; Saigo, M.; Mukuta, T.; Onda, K.; Ishitani, O. Investigation of Excited State, Reductive Quenching, and Intramolecular Electron Transfer of Ru(II)-Re(I) Supramolecular Photocatalysts for CO2 Reduction Using Time-Resolved IR Measurements. Chem. Sci. 2018, 9, 2961−2974. (10) Sahara, G.; Abe, R.; Higashi, M.; Morikawa, T.; Maeda, K.; Ueda, Y.; Ishitani, O. Photoelectrochemical CO2 Reduction Using a Ru(II)−Re(I) Multinuclear Metal Complex on a p-Type Semiconducting NiO Electrode. Chem. Commun. 2015, 51, 10722−10725.

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Where λex and Plight are the wavelength and power density of the incident light (nm and μW cm−2), respectively, Ilight is the current density under irradiation (μA cm−2), and Idark is the current density in the dark (μA cm−2). Photocatalytic CO2 Reduction Using Poly-RuRe/NiO. The photoelectrochemical CO2 reduction was conducted using a Pyrex Hshaped cell which has two compartments separated by a Nafion film (Aldrich, Nafion 117). The volume of each compartment is ca. 28 mL, and a 15 mL of an aqueous solution of NaHCO3 (50 mM) was added to each compartment. The poly-RuRe/NiO and the Ag/AgCl reference electrodes were placed in the cathode compartment while the platinum-wire counter electrode was placed in the anode one. The three-electrode setup was used with a potentiostat (Hokuto Denko, HZ-7000) throughout the photocatalytic reactions. The reaction was conducted after purging CO2 into the cathode compartment for 15 min. The poly-RuRe/NiO was irradiated at λex > 460 nm using a 300 W Xe lamp with a cutoff filter (HOYA Y48). CO and H2 in the gas phase were analyzed by a micro-GC (Inficon, MGC3000A) and HCOOH in the solution was detected by a capillary electrophoresis apparatus (Otsuka electronics, Agilent 7100L).

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AUTHOR INFORMATION

Corresponding Author

where nCV is the amount of the electrochemically active Ru unit (mol), S is the area of the oxidation peak for Ru(II/III) (A V), F is the Faraday constant (96485 C mol−1), and v is the sweep rate (V s−1). Confirmation of Desorption of the Metal Complex(es) from the Electrode. The electrode was soaked into 50 mM NaHCO3 aqueous solution (4 mL) overnight. The amount of the nondesorbing electroactive metal complex was estimated by the same method described above and compared with the value from before soaking. Estimation of the Amount(s) of the Adsorbed Ru(II) Complexes on the Electrode by Using Visible Adsorption Spectrum. The visible absorption spectra were measured by using the bare FTO glass as background. The amount of metal complex on the electrode (nTS) was estimated from eq 2.

n TS = (A after − Abefore)S × 10−3/ελ

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b05495. General procedures, materials, synthesis of metal complexes, TOF-SIMS spectrum of RuVP/NiO, XPS spectrum of poly-RuRe/NiO, and supplementary results of photoelectrochemical reactions (PDF) H


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