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Solar-driven photocatalytic CO reduction in water utilizing a ruthenium complex catalyst on p-type FeO with a multi-heterojunction 2

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Keita Sekizawa, Shunsuke Sato, Takeo Arai, and Takeshi Morikawa ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03244 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 8, 2018

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ACS Catalysis

Solar-driven photocatalytic CO2 reduction in water utilizing a ruthenium complex catalyst on p-type Fe2O3 with a multi-heterojunction Keita Sekizawa,*† Shunsuke Sato,† Takeo Arai, † Takeshi Morikawa† †

Toyota Central R&D Labs., Inc., 41-1 Yokomichi, Nagakute, Aichi 480-1192, Japan

ABSTRACT

A hybrid photocathode that consists of a ruthenium complex catalyst and a p-type semiconductor composed of earth-abundant elements, N,Zn-codoped Fe2O3, and with a multi-heterojunction structure (TiO2/N,Zn-Fe2O3/Cr2O3) was developed for the reduction of CO2 in aqueous solution with application of an electrical bias under simulated solar light irradiation. The TiO2 layer prevents contact between N,Zn-Fe2O3 and the electrolyte, so that dissolution of N,Zn-Fe2O3 by photoelectrochemical (PEC) self-reduction cannot occur. Both TiO2 and Cr2O3 significantly enhanced the cathodic photocurrent by tuning the band-bending in N,Zn-Fe2O3. The use of a Ru complex with an electronic network provided by polypyrrole improved the performance, and resulted in a stable photocurrent of 150 µA cm-2 for the production of HCOOH, CO and a small amount of H2 under 1 sun irradiation with application of 0.1 V vs. reversible hydrogen electrode

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(RHE). The total amount of generated HCOOH, CO and H2, two electron reduction products, was equal to half the amount of photogenerated electrons. The functional combination of the hybrid iron-based photocathode with a reduced SrTiO3 (SrTiO3-x) photoanode realized stoichiometric solar CO2 reduction coupled with the water oxidation reaction without an external electrical bias. The solar-to-chemical energy conversion efficiency was 0.15%, which is comparable to a reported tandem system using a Ru complex/single-crystalline InP photocathode.

KEYWORDS

Metal complex-semiconductor hybrid, CO2 reduction, Hematite, Ru complex catalyst, Multiheterojunction

INTRODUCTION The conversion of CO2 into transportable fuels or commodity chemicals is an increasingly important research area to address the shortage of energy and carbon resources, and the rising atmospheric concentrations of CO2. Harvesting the energy of solar light on a large scale and reducing CO2 into chemical fuels is one promising approach to realize a future energy supply. After the seminal work for CO2 reduction using a metal complex by Lehn and Ziessel in 1982,1 metal complexes have attracted significant attention as photocatalysts2 or electrocatalysts3-5 due to their controllable properties by tuning of the ligands. The use of some metal complexes has realized high quantum or current efficiencies and high selectivity. However, metal complex


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ACS Catalysis

photocatalysts require a sacrificial electron donor because of their low oxidation power. The durability of metal complex photoabsorbers is also generally low because of photoinitiated decomposition that arises from population of the low-lying d-d states.6 On the other hand, semiconductors are known to have photocatalytic ability for the reduction of CO2. They have relatively high durability due to their rigid crystal structures. Combination with an n-type semiconductor for water oxidation in a photoelectrochemical (PEC) cell or in a powder suspension enables coupling with O2 evolution via two-step photoexcitation.7 However, the selectivity of the semiconductor with metallic cocatalyst for the reduction reaction is low because the metal catalyzes H2 evolution by the reaction of H2O prior to CO2 reduction due to the thermodynamic stability of CO2. To utilize the advantages of a metal complex and semiconductor, a hybrid photocatalytic system for CO2 reduction was reported by Sato et al. with a [Ru(N^N)2(CO)2]2+ (N^N = diimine ligand) complex immobilized on N-doped Ta2O5 (N-Ta2O5), which accomplished CO2 reduction in acetonitrile (MeCN) containing triethanolamine.8 The number of reported hybrid photocatalysts8-28 and photocathodes29-38 that combine a metal complex catalyst and semiconductor

photosensitizer

[Ru(N^N)(CO)2Cl2],8-13,

has

29-31

increased.

Various

[Re(N^N)(CO)3Cl],14-17,

metal 32-33

complexes,

such

as

[Ni(terpyridine)2]2+,18

[Mn(CO)3(CH3CN)]+,34 binuclear complexes of RuRu12-13, 21-26, RuRe27-28, 35-37 and ZnRe38, and various semiconductors such as C3N4,10-13, 19, 23 InP29-30, Cu2ZnSnS4,31 crystalline-Si,32, 34 Cu2O,33 TiO2,15-17,

20

CdS,18 NiOx,35-36,

38

and CuGaO237 have been utilized to prepare hybrid

photocatalysts and photoelectrodes. Recently, some of these have been employed to drive CO2 reduction in aqueous solution without the inclusion of an organic solvent but with the addition of a sacrificial electron donor18,23,25 or application of a bias.29-31, 36-37 However, a critical issue is the


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cost of materials; cheaper light absorbers are necessary to collect sufficient amounts of solar light energy, which requires large amounts of light absorber material that cover a widespread area due to the low energy density of sunlight. Ultimately, iron oxide is the best candidate light absorber from a cost perspective. Hematite (α-Fe2O3), also known as red rust, is one of the most abundant and low-cost semiconductor materials that can absorb a substantial amount of solar light (2.1 eV bandgap). ntype Fe2O3 has been studied as a photocatalyst or photoanode for water oxidation. In addition, the doping of Fe2O3 with ions such as Mg2+, Zn2+, Cu2+ and N3- induces p-type conduction.39-43 Morikawa and colleagues44-45 have previously reported that N3- and Zn2+-codoped Fe2O3 exhibits a negative band edge shift of 0.9 V with respect to undoped Fe2O3 due to the surface dipole moment generated by nitrogen doping.46-47 Therefore, p-type Fe2O3 has the potential for application as a photocathode for artificial photosynthesis. However, challenges in the utilization of p-type Fe2O3 as a photocathode are poor stability, i.e., photochemical dissolution under reductive conditions,48 and inefficient charge separation properties due to low carrier mobility.49 To overcome these issues, we have recently developed a Fe2O3-based multilayered photocathode, Pt/TiO2/N,Zn-Fe2O3/Cr2O3, for the water splitting reaction.45 A TiO2 layer applied to the surface of N,Zn-Fe2O3 protected the unstable Fe2O3 surface and improved electron transfer from N,Zn-Fe2O3 to a Pt catalyst by the formation of a p-n junction. A Cr2O3 layer applied under the N,Zn-Fe2O3 improved hole transfer from N,Zn-Fe2O3 to a conducting substrate through ohmic contact. As a result, stable H2 evolution was accomplished with the Pt/TiO2/N,ZnFe2O3/Cr2O3 photocathode. Although a higher input energy is required for CO2 reduction than H2 evolution, if the electron transfer from Fe2O3 to an appropriate metal complex catalyst via the


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ACS Catalysis

conduction band (CB) of TiO2 was efficient, then CO2 reduction in water could be accomplished with a hybrid photocathode comprised of a metal complex and TiO2/N,Zn-Fe2O3/Cr2O3. In such a hybrid photocatalyst, as illustrated in Scheme 1, the photoinduced electron transfers from a semiconductor in the excited state to a metal complex in the ground state, and the electron-acceptor metal complex then reduces CO2. Thus, the metal complex acts as both an electron acceptor from the CB of TiO2 and as an active site for CO2 reduction. Therefore, the molecular structure of the metal complex catalyst influences the efficiency for CO2 reduction. Various Ru complexes have been previously designed for CO2 reduction.4-5, 11, 29, 50 Herein, to add functionality, six different Ru complexes were prepared, the structures and abbreviations of which are shown in Scheme 1, and these were hybridized with TiO2/N,Zn-Fe2O3/Cr2O3. The hybrid photocathodes were evaluated with respect to CO2 reduction in aqueous solution under simulated solar irradiation. In addition, the role of the heterojunction structure in CO2 reduction was investigated. Unassisted (without application of a bias voltage) and stoichiometric CO2 reduction coupled with the water oxidation reaction was also demonstrated by connection with a SrTiO3-x photoanode.


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Scheme 1. Mechanism for CO2 reduction over a Ru complex/TiO2/N,Zn-Fe2O3/Cr2O3 photocatalyst and the structures of the Ru complexes used. EXPERIMENTAL SECTION Materials. A tin (IV) oxide/indium tin oxide (ITO) double layered transparent conducting glass (TCO; Geomatec Co., Ltd.) was used as a substrate for the electrodes. Commercially available Fe2O3, ZnO, TiO2 and Cr sputtering targets were obtained from Kojundo Chemical Laboratory Co., Ltd.


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Preparation of semiconductor photoelectrodes. N,Zn-Fe2O3 cathode.44-45 A N,Zn-Fe2O3 ﬁlm with a thickness of ca. 190 nm was deposited by radio frequency (RF) reactive magnetron sputtering of Fe2O3 and ZnO targets in Ar/N2 (4:1 v/v) plasma. The input power for Fe2O3 was 600 W and that for ZnO was 35 W. The substrate used for sputtering was SnO2/ITO layered glass, with or without a 10 nm thick Cr layer deposited by RF magnetron sputtering in Ar plasma. After deposition of the N,Zn-Fe2O3 layer, the electrodes were annealed at 823 K under N2/O2 (4:1 v/v) gas flow for 2 h. Surface TiO2 layer. A 60 nm thick TiO2 layer was deposited on the surface of the N,Zn-Fe2O3 thin film by RF reactive magnetron sputtering of a TiO2 target in Ar/O2 (4:1 v/v) plasma. The deposited electrodes were then annealed at 823 K for 2 h. n-SrTiO3-x photoanode. The n-SrTiO3 photoanode was fabricated using the method described in previous studies.30, 51 A (100)-oriented SrTiO3 crystal (Shinkosha) was reduced by heating for 2 h at 1073 K in a quartz tube furnace under N2/H2 (97:3 v/v) gas flow, and then allowed to cool for 7 h. The color of the transparent crystal became dark bluish. Preparation of Ru complexes. Four trans(Cl)-[Ru(bpyX2)(CO)2Cl2] (bpyX2=2,2’-bipyridine with X substituents in the 4,4’-positions, X=H, CO2H, CO2CH3, or CO2C3H6-1H-pyrrolyl) monomers (Scheme 1, Ru(CO)X) were synthesized according to the report by Anderson et al.52 trans(Cl)-[Ru{4,4’-di(1H-pyrrolyl-3-propylcarbonate)-bpy}(CO)(CH3CN)Cl2]

(Scheme

1,

Ru(MeCN)CO2C3Py)) was synthesized by the photochemical ligand substitution reaction of Ru(CO)CO2C3Py under irradiation from a white fluorescent light for 14 h in MeCN solution at room temperature.29 Deposition of Ru complexes on semiconductor electrodes. Deposition of the Ru complexes was conducted by the drop-casting method. An MeCN solution containing 0.70 mM of a Ru


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complex monomer was dropped (50 µL cm-2) on the surface of the semiconductor and dried at 333 K for 7 min. The coating procedure was repeated once. The hybrid photoelectrode was then rinsed with pure water. The total amount of Ru(II) complex deposited on each semiconductor was 70 nmol cm-2. Drop-casting of the Ru complex polymers (Ru(CO)CO2C3Py-P and Ru(MeCN)CO2C3Py-P) was conducted using an MeCN solution mixture of 0.70 mM Ru complex monomer, 2.9 µM pyrrole and 4.0 mM FeCl3. Photoelectrochemical (PEC) measurements. PEC measurements were conducted using an electrochemical analyzer (ALS612e, BAS). The PEC properties of the photocathode materials were evaluated with a three-electrode configuration. The photocathode, a silver-silver chloride (Ag/AgCl) electrode and a platinum electrode were used as working, reference, and counter electrodes, respectively. The exposed TCO layer of the photocathode was covered with silicone rubber. All potentials were converted to the reversible hydrogen electrode (RHE) reference scale using E (V vs. RHE) = E (V vs. Ag/AgCl) + 0.20 V + 0.059 V × pH. A sealed Pyrex® glass cell was used as a reactor. CO2saturated 0.1 M KHCO3 (pH 6.6) aqueous solutions was used as electrolytes. Irradiation of the electrode with 1 sun (AM 1.5; 100 mW cm−2) was conducted using a solar simulator (HAL-320, Asahi Spectra) after the intensity was adjusted using a silicon photodiode (CS-20, Asahi Spectra). The irradiation area was limited to 10×10 mm2 using a slit. Linear sweep voltammetry was conducted with a scan rate of 50 mV s−1 under chopped light irradiation. The PEC water splitting reaction with application of an electrical bias was conducted by measuring the photocurrent produced under continuous light irradiation at a fixed electrode potential of −0.5 V vs. Ag/AgCl. The gaseous reaction products were analyzed using gas chromatography (GC; GC2014, Shimadzu) with a thermal conductivity detector (TCD), an active carbon column (GL


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ACS Catalysis

Sciences) and Ar as the carrier gas. The amount of formate was determined using ion chromatography (IC; ICS-2000, Dionex) with IonPacAS15 and IonPacAG15 columns, and 10 mM KOH aqueous solution as the eluent. Electrochemical measurements. Cyclic voltammograms of the Ru complexes were measured in N,N-dimethylacetamide (DMA) containing 0.1 M tetraethylammonium tetrafluoroborate (Et4NBF4) as the supporting electrolyte and using an electrochemical analyzer (ALS612e, BAS) with a conventional threeelectrode type cell at room temperature under an Ar or CO2 atmosphere. A 3 mm diameter glassy carbon electrode, platinum wire electrode, and Ag/AgNO3 (0.01 M) electrode were employed as the working, counter, and reference electrodes, respectively. The concentration of dissolved Ru in the electrolyte solution was 0.5 mM, and the scan rate was 10 mV s–1. Isotope tracer experiment 13

CO2 isotope tracer analysis was conducted to verify the carbon source of formate and CO

generated by photoelectrolysis using Ru(MeCN)CO2C3Py-P/TiO2/N,Zn-Fe2O3/Cr2O3. The three-electrode configuration electrochemical cell was purged with 13CO2 gas (99%, ISOTEC) by bubbling for 30 min and was then sealed with a septum. To avoid carbon exchange between CO2 and HCO32-, 0.1 M Na2HPO4−NaH2PO4 (1:1) buffer electrolyte (after CO2 saturation; pH 6.4), instead of 0.1 M KHCO3, was used as an electrolyte. After photoelectrolysis for 1 h at +0.1 V vs. RHE with AM 1.5 irradiation, the gaseous reaction products were analyzed using gas chromatography-mass spectroscopy (GC-MS; 5973-6890, Agilent Technologies) with an HPMolesieve column. The formate generated was analyzed using IC interfaced with time-of-flight mass spectrometry (TOF-MS; JMS-T100LP, JEOL) using methanol as the mobile phase. PEC CO2 reduction with a tandem cell.


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The Ru(MeCN)CO2C3Py-P/TiO2/N,Zn-Fe2O3/Cr2O3 cathode and SrTiO3-x anode were immersed in an aqueous solution of 0.1 M KHCO3 in a sealed Pyrex® cell. The electrolyte was purged with CO2 prior to the experiments. Both of the electrodes were connected to a digital multimeter (PC7000, Sanwa Electric Instrument) to record the current in a two-electrode conﬁguration without application of an external potential bias. The Ru(MeCN)CO2C3PyP/TiO2/N,Zn-Fe2O3/Cr2O3 electrode was irradiated with simulated solar light (1 sun) transmitted through SrTiO3-x. The illuminated area was 10×10 mm2. The gaseous products and formate in solution were analyzed using TCD-GC with an active carbon column (Shincarbon ST, Shinwa Chemical Industries) and IC, respectively. Characterization. Scanning electron microscopy (SEM) observations were conducted using an S5500 microscope (Hitachi High-Tech). Scanning transmission electron microscopy (STEM) with energy dispersive X-ray spectroscopy (EDX) was conducted using a JEM-2100F (JEOL) microscope after cutout samples were produced using a focused ion beam (FIB; NB5000, Hitachi High-Tech). Prior to cutout, the surfaces of the films were coated with C and W layers to protect from the FIB. The crystal structures of the films were analyzed using X-ray diffraction (XRD; Ultima IV, Rigaku) with Cu Kα radiation. The optical properties of the films were measured using UV-vis absorption spectroscopy (UV-3600, Shimadzu) with the transmission method. The amount of Fe dissolved into the reaction solution after the PEC water splitting reaction was analyzed using inductively coupled plasma optical emission spectroscopy (ICP-OES; CIRIOS 120EOP, Rigaku). RESULTS AND DISCUSSION Electrochemical properties of Ru complexes and N,Zn-Fe2O3 in organic media


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ACS Catalysis

To investigate the electrochemical properties of Ru complexes, cyclic voltammetry of the monomeric Ru complexes in a DMA solution containing 0.1 M Et4NBF4 as a supporting electrolyte were measured under an Ar atmosphere (Figure 1, blue line). The potentials of the reduction curves are summarized in Table 1. The first irreversible reduction waves were attributed to reduction of the bipyridine ligand involving the desorption of Cl−.53 The first reduction potentials of Ru(CO)CO2C3Py, Ru(CO)CO2H were more positive than that of Ru(CO)H due to the electron withdrawing properties of the carbonyl substituents. That of Ru(CO)CO2C3Py was more positive than Ru(CO)CO2H, because the order for a negative mesomeric effect is as follows: carboxylate ester (-COOR) ≈ carboxylic acid (-COOH) > carboxylate ion (-COO-). Partial deprotonation from the carboxylic acid moiety of Ru(CO)CO2H may induce a negative shift of the reduction potential. The potential of Ru(CO)CO2C3Py was more positive than that of Ru(MeCN)CO2C3Py due to π-electron backdonation from the metal center to the CO ligand. Under a CO2 atmosphere, the catalytic wave for CO2 reduction was observed at a more negative potential (Figure 1, red line). The order for the onset potential of the CO2 reduction wave was as follows: Ru(MeCN)CO2C3Py ≈ Ru(CO)H > Ru(CO)CO2H > Ru(CO)CO2C3Py. Although the CO2 reduction wave of Ru(CO)H rose on the first reduction wave, that of Ru(CO)CO2H and Ru(CO)CO2C3Py rose on the second reduction wave, which is attributed to the reduction of desorbed Cl− species The CO2 reduction wave of Ru(MeCN)CO2C3Py rose between the first and second reduction wave, which was caused by the difference in electron density at the Ru center in the desorbed Cl− species. Thus, the higher electron density on the Ru center would give a more positive CO2 onset potential. To consider the polymerization effect, Ru(CO)CO2C3Py and Ru(MeCN)CO2C3Py were polymerized by the addition of FeCl3 and pyrrole in DMA containing 0.1 M Et4NBF4 and


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heating at 60 °C for 15 h.29 The polypyrrole chain should be formed, which resulted in a color change to darker one. Here, the polypyrrole structure is considered to remain on the working electrode because the C-C bond of polypyrrole in the working electrode is known to be stable under these electrochemical conditions.54 However, the reduction potentials were not shifted compared with the monomers (Figure 1), which indicated that the inherent electrochemical properties of the monomeric Ru complex were not affected by pyrrole polymerization, because the electronic interaction via a propyl chain between bpy and polypyrrole is very weak.

Figure 1. (a-d) Cyclic voltammograms of Ru complexes (0.5 mM) in DMA solution containing 0.1 M Et4NBF4 as a supporting electrolyte under an Ar (blue line) or CO2 (red line) atmosphere. (e,f) FeCl3 (3.0 mM) and pyrrole (2.2 µM) were added as a polymerizing agent.


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ACS Catalysis

Table 1. Electrochemical data for the monomeric Ru complexes.a Ru complex

Epc b (V)

ECO2onset c (V)

Ru(CO)H

-1.55, -1.65

-1.48

Ru(CO)CO2H

-1.40, -1.60

-1.56

Ru(CO)CO2C3Py

-1.28, -1.62, -1.93

-1.78

Ru(MeCN)CO2C3Py

-1.44, -1.72, -1.84

-1.45

Ru(CO)CO2C3Py-Pd

-1.27, -1.65, -1.88

-1.78

Ru(MeCN)CO2C3Py-Pd

-1.45, -1.74, -1.88

-1.45

a

Versus Ag/AgNO3, determined by cyclic voltammetry at 10 mVs-1 in DMA containing Et4NBF4 on a glassy carbon electrode. b Potential of cathodic peak. c Onset potential of catalytic current for CO2 reduction. d The monomeric Ru complex were polymerized by the addition of FeCl3 and pyrrole. The potential of the CB minimum (CBM) of N,Zn-Fe2O3 was determined to be located at −0.6 V vs. RHE in aqueous solution.45 To compare the CBM with the reduction potential of Ru complexes in DMA, a Mott-Schottky (M-S) plot for N,Zn-Fe2O3 in DMA was investigated (Figure 2a). The slope of the M-S plot indicates that N,Zn-Fe2O3 is a p-type semiconductor, and the flat-band potential of N,Zn-Fe2O3 was determined to be +0.4 V vs. Ag/AgNO3 in DMA from the intercept of the tangent on the potential axis. The valence band maximum (VBM) is considered to be close to the flat-band potential in p-type semiconductors with high carrier densities.55 N,Zn-Fe2O3 has a high carrier density (4.9×1018 cm-3);44 therefore, the energy gap between the VB edge and the flat-band potential is considered to be small. The energy gap of N,Zn-Fe2O3 was determined to be 0.1 eV by comparison of the flat-band potential estimated from M-S plot and the VBM estimated from photoelectron spectroscopy measurements in air.45 Therefore, the VBM of N,Zn-Fe2O3 would be +0.5 V vs. Ag/AgNO3 in DMA. The bandgap of


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N,Zn-Fe2O3 was estimated to be 2.1–2.2 eV from the intercept of the Tauc plot tangent on the hν axis (Figure 2b). Therefore, the CBM is determined to be in the range of −1.6 to −1.7 V vs. Ag/AgNO3. For N,Zn-Fe2O3 coated with TiO2, the CBM at the interface side with TiO2 should be fixed at this potential, while that at the bulk side should be shifted to a negative potential due to the p-n junction.45The energy differences (∆G) between the CBM of N,Zn-Fe2O3 and the onset potential for CO2 reduction with the Ru complexes were as follows; Ru(MeCN)CO2C3Py (−0.2 V), Ru(CO)H (−0.2 V), Ru(CO)CO2H (−0.1 V), and Ru(CO)CO2C3Py (+0.1 V).

Figure 2. (a) M-S plot for the N,Zn-Fe2O3 electrode measured in CO2-saturated DMA solution containing 0.1 M Et4NBF4 at 50 (green circles), 100 (blue circles) and 200 Hz (red circles). (b) Tauc plots for calculation of the direct (blue line) and indirect (green line) optical bandgap of N,Zn-Fe2O3. PEC reduction of CO2 with various Ru complexes loaded on TiO2/N,Zn-Fe2O3/Cr2O3. Ru complexes were loaded on the TiO2/N,Zn-Fe2O3/Cr2O3 photocathode by the drop-casting method. Figure 3 shows an STEM image and STEM-EDX elemental maps for the Ru(MeCN)CO2C3Py-P/TiO2/N,Zn-Fe2O3/Cr2O3 electrode. A 110 nm thick Ru complex layer was coated on the TiO2/N,Zn-Fe2O3/Cr2O3 multilayer structure. XRD analysis revealed the


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ACS Catalysis

crystalline phase of each layer to be TiO2 (anatase), α-Fe2O3 and α-Cr2O3, respectively (Figure S1).

Figure 3. Cross-sectional STEM image and STEM-EDX elemental maps for the Ru(MeCN)CO2C3Py-P/TiO2/N,Zn-Fe2O3/Cr2O3 electrode. TiO2 and Cr2O3 absorb λ < 360 nm light, and N,Zn-Fe2O3 absorbs λ < 600 nm light (Figure S2); therefore, irradiated solar simulated light is absorbed by the N,Zn-Fe2O3 layer. As shown in Figure S3, monomeric Ru complex catalysts have a weak absorption band between 400 and 600 nm, and the polymeric complex was dark brown due to the absorption of polypyrrole. Forward irradiation (from the Ru complex side) of the Ru complex loaded TiO2/N,Zn-Fe2O3/Cr2O3 generated a much higher photocurrent than backward irradiation (from the Cr2O3 side), despite interference of photoabsorption by the Ru complexes at the N,Zn-Fe2O3 layer (Figure S4), the so-called inner filter effect. This was due to the difference in the electron diffusion length; backward irradiation requires a longer electron diffusion length than forward irradiation. The major carrier of a p-type semiconductor is holes; therefore, electron transfer is not as efficient as


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hole transfer. As such, the various Ru complexes loaded on the TiO2/N,Zn-Fe2O3/Cr2O3 photocathodes were evaluated using forward irradiation. PEC CO2 reduction activities for the 4 monomeric and 2 polymeric Ru complexes loaded on TiO2/N,Zn-Fe2O3/Cr2O3 photoelectrodes were investigated in CO2-saturated 0.1 M KHCO3 aqueous electrolyte at a constant potential of +0.1 V vs. RHE under 1 sun (AM 1.5, 100 mW cm2

) irradiation. Figure 4 shows time courses of the photocurrents during photoirradiation for 1 h,

which represents the activity and stability of the photocathode for CO2 reduction reaction after photoexcitation of N,Zn-Fe2O3, ultrafast electron transfer from N,Zn-Fe2O3 to the Ru complexes, and charge accumulation in the photocathode and electric double layer.23, 56-58 Table 2 lists the PEC reaction activity of TiO2/N,Zn-Fe2O3/Cr2O3 in combination with the various Ru complexes for HCOOH, CO and H2 generation. All of these hybrid electrodes generated CO2-reduction products, i.e., HCOOH and CO, and the amounts were larger than that of loaded Ru complexes (0.07 µmol cm-2), although the performance was different depending on the Ru complex used. The order for the total amount of CO2 reduction products after irradiation for 1 h was as follows: Ru(MeCN)CO2C3Py-P> Ru(CO)CO2C3Py-P> Ru(MeCN)CO2C3Py ≈ Ru(CO)CO2C3Py > Ru(CO)H > Ru(CO)CO2H.


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ACS Catalysis

Figure 4. Photocurrent transients of (a) monomeric (Ru(CO)H: black line, Ru(CO)CO2H: green line, Ru(CO)CO2C3Py: blue line, Ru(MeCN)CO2C3Py: red line) and (b) polymeric Ru complexes (Ru(CO)CO2C3Py-P: blue line, Ru(MeCN)CO2C3Py-P: red line) loaded on TiO2/N,Zn-Fe2O3/Cr2O3 at +0.1 V vs. RHE in CO2-saturated 0.1 M KHCO3 electrolyte (pH 6.6) under 1 sun (100 mW cm-2, AM 1.5) irradiation.

Table 2. Reaction products from photoelectrolysis for 1 h at TiO2/N,Zn-Fe2O3/Cr2O3 electrodes loaded with various Ru complexes. a Charge Products / µmol cm-2 (current efficiency) Ru complex amount / HCOOH CO H2 HCOOH+CO mC cm-2 0.17 0.37 0.03 0.57 107 Ru(CO)H (31%) (66%) (6%) (97%) 0.06 0.08 0.19 0.14 60 Ru(CO)CO2H (19%) (25%) (62%) (44%) 0.26 0.46 0.07 0.71 155 Ru(CO)CO2C3Py (33%) (57%) (8%) (90%) 0.38 0.39 0.14 0.77 179 Ru(MeCN)CO2C3Py (41%) (42%) (15%) (83%) 1.15 0.83 0.12 1.22 410 Ru(CO)CO2C3Py-P (54%) (39%) (6%) (93%) 1.51 0.73 0.16 2.24 454 Ru(MeCN)CO2C3Py-P (63%) (30%) (7%) (93%) a Bias voltage applied at +0.1 V vs. RHE in CO2 saturated 0.1 M KHCO3 electrolyte (pH 6.6) under 1 sun (100 mW cm-2, AM 1.5) irradiation


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Among the monomeric Ru complexes, Ru(MeCN)CO2C3Py exhibited the highest activity. The difference in activity with Ru(CO)CO2H is explainable in terms of ∆G. Thus, a larger driving force for electron transfer from N,Zn-Fe2O3 to CO2 via a Ru complex induces efficient CO2 reduction. Moreover, Ru(CO)CO2H could also act as a H2 evolution catalyst rather than a CO2 reduction catalyst. On the other hand, the difference between Ru(MeCN)CO2C3Py and Ru(CO)H cannot be explained by the energy gap, because the ∆G values were similar. The higher activity of Ru(MeCN)CO2C3Py is induced by the carboxylate group on the bipyridine ligand, because a carboxylate group can adsorb onto the TiO2 layer on top of the semiconductor, which creates an efficient electron transfer pathway between the TiO2 layer and Ru complex.59 In the case of Ru(CO)CO2C3Py, the photocurrent curve was different from the other Ru complexes. The photocurrent just after irradiation was only 17 µA cm-2, but this increased to 60 µA cm-2 after irradiation for 0.18 h, and then decayed (Figure 4a, blue line). The small photocurrent in the first stage is attributed to Ru(CO)CO2C3Py reduction but not CO2 reduction, because ∆G for CO2 reduction was endergonic (+0.1 V). The increase of the photocurrent would then be induced by a photochemical ligand substituent reaction from Ru(CO)CO2C3Py to Ru(MeCN)CO2C3Py because the photochemical reaction proceeds even under weak fluorescent lamp irradiation. After photoelectrolysis for 1 h, a partial conversion from Ru(CO)CO2C3Py to Ru(MeCN)CO2C3Py was observed using FT-IR spectroscopy (Figure S5). Therefore, the actual catalyst would be Ru(MeCN)CO2C3Py. However, all of these monomeric Ru complexes decayed within 1 h when irradiated. We consider the reason for the decay is not the Ru complexes but N,Zn-Fe2O3; hard X-ray photoelectron spectroscopy measurements have previously confirmed the generation of Fe2+ species at the surface of N,Zn-Fe2O3 accompanied


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ACS Catalysis

by deactivation of the PEC process.45 The reduced Fe species generates electron trap states, which leads to lower PEC performance. In contrast, the polymeric Ru(CO)CO2C3Py-P- and Ru(MeCN)CO2C3Py-P-loaded electrodes exhibited stable photocurrents under irradiation for 1 h (Figure 4b). As a result, the polymeric Ru complexes consumed much larger amounts of electrons and exhibited higher catalytic activity for CO2 reduction than the monomeric Ru complexes (Table 2), even though the inner filter effect by the dark-brown polymeric Ru complexes lowered the efficiency of the photocathode more than that by the monomeric Ru complexes (Figure S3). The electrochemical properties in DMA solution were not changed by the polymerization of pyrrole; therefore, the catalytic activity of the respective Ru complex components in the polymer for CO2 reduction should be equivalent with the monomeric Ru complexes. The stabilization of the photocurrent is thus considered to be induced by the formation of electron transfer pathways between the Ru complexes via the construction of polypyrrole chains, so that Ru complexes distant from the TiO2 layer would be able to accept electrons via the 110 nm thick Ru complex layer. As a result, more Ru complexes can act as catalysts, and most of the photogenerated electrons in N,Zn-Fe2O3 are then consumed by the Ru complexes for CO2 reduction without the reduction of Fe species in N,Zn-Fe2O3, which causes deactivation. The initial increase of the photocurrent for an electrode loaded with a polymeric Ru complex can be speculated to be related to Cl- desorption at the first catalytic cycle because the Cl- ligand should desorb from the Ru complex after acceptance of the first electron, as indicated by the irreversible reduction wave in the cyclic voltammetry measurements (Figure 1).53 The photoelectrode with Ru(CO)CO2C3Py-P did not exhibit an induction period, in contrast with that of the Ru(CO)CO2C3Py photoelectrode. The reason for this may be the outer Ru(MeCN)CO2C3Py species generated by the photoabsorption of


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Ru(CO)CO2C3Py just after irradiation, which can accept electrons from TiO2 via polypyrrole chains. To investigate the PEC stability of Ru(MeCN)CO2C3Py-P/TiO2/N,Zn-Fe2O3/Cr2O3, which exhibited the highest activity in this study, electrolysis was conducted for 13 h. Figure 5 shows the time course for the generation of products and a half amount of photogenerated electrons by 1 sun (AM 1.5) irradiation at an applied potential of +0.1 V vs. RHE. The photocurrent was stable at ca. 150 µA cm-2 for 3 h irradiation and the amounts of reduction products increased linearly. The total amount of HCOOH, CO, and H2 (black diamonds in Figure 5) generated during the initial 3 h was equivalent with the half amount of photogenerated electrons (black line in Figure 5); therefore, all of the photogenerated electrons were consumed for CO2 and H2O reduction without side-reactions such as the self-redox reaction of Fe2O3. After irradiation for 13 h, 12 µmol cm-2 of HCOOH, 9.2 µmol cm-2 of CO, and 1.9 µmol cm-2 of H2 were generated when the turnover number for CO2 reduction (TONHCOOH+CO) based on the loaded Ru(MeCN)CO2C3Py-P electrode was 304. The layer structure of the electrode before and after the PEC reaction was compared by STEM observation (Figure S6a,b). There was no exfoliation of the Ru complex layer from the surface of TiO2, which strongly suggested that Ru(MeCN)CO2C3Py-P would adsorb onto the surface of TiO2 via the carboxylate group. The thickness of the Ru complex layer was not almost decreased after the PEC reaction, which was speculated to be due to rigid structure of polypyrrole. However, the grained contrast in the Ru complex layer disappeared, which indicates that the Ru center gradually dissociated from the polymerized ligand; the EDX peak for Ru decreased after the PEC reaction (Figure S6c,d). Therefore, the slight decrease in the production rate shown in Figure 5 is speculated to correlate with the decrease of EDX peak height of Ru. In our future work, dissociation should be


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ACS Catalysis

prevented by improved molecular design to form stronger chemical bonding between the Ru center and the ligand.

Figure 5. Time courses for the formation of products (red circles: HCOOH, blue circles: CO, green circle: H2, black diamonds: HCOOH+CO+H2) and a half amount of photogenerated electrons (black line) by Ru(MeCN)CO2C3Py-P/TiO2/N,Zn-Fe2O3/Cr2O3 with a three-electrode configuration at +0.1 V vs. RHE in 0.1 M KHCO3 aqueous electrolyte (pH 6.6) under 1 sun (100 mW cm-2, AM 1.5) irradiation. Isotope tracer experiments using 13CO2 and unlabeled CO2 (13C content: 1%) were performed to confirm the carbon source of HCOOH and CO generated by photoelectrolysis at Ru(MeCN)CO2C3Py-P/TiO2/N,Zn-Fe2O3/Cr2O3 (Figure 6). IC-MS analysis of the reaction solution under a

13

CO2 atmosphere showed the formation of H13COO- (m/z = 46) without

H12COO- (m/z = 45). In contrast, when unlabeled CO2 was used, the H13COOH peak was not detected. GC-MS analysis of the gas phase under a evolved CO was content in

13

13

13

CO2 atmosphere showed that 97% of the

CO (m/z = 29) and the remaining 3% was

12

CO (m/z = 28), where the

CO2 was 99%. Although a very small amount of

12

13

C

CO would originate from


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Page 22 of 41

detachment of the CO ligand by light-induced decarbonylation reaction,60 this result indicates that most of the evolved CO originated from CO2 reduction.

Figure 6. Ion chromatograms for the reaction solution using TOF-MS [(a) m/z 46, (b) m/z 45]. Gas chromatograms of the gas phase using MS for detection [(c) m/z 29, (d) m/z 28]. The Ru(MeCN)CO2C3Py-P/TiO2/N,Zn-Fe2O3/Cr2O3 photoelectrode was irradiated with 1 sun (100 mW cm-2, AM 1.5) for 1 h with application of a bias (+0.1 V vs. RHE) in 0.1 M Na2HPO4−NaH2PO4 (1:1) buffer electrolyte (pH 6.4) under

13

CO2 (13C content: 99%, red line)

and unlabeled CO2 (13C content: 1%, blue line). Effect of Multi-layered Structure. We have previously reported that a TiO2 overlayer and Cr2O3 underlayer for N,Zn-Fe2O3 significantly improved PEC hydrogen evolution with a Pt co-catalyst. In this hybrid photocathode for CO2 reduction, the multilayer structure has an important role. To demonstrate the effect of the TiO2 and Cr2O3 layers on iron-based photocathodes for CO2 reduction, the PEC


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properties of the Ru(MeCN)CO2C3Py-P/N,Zn-Fe2O3, Ru(MeCN)CO2C3Py-P/TiO2/N,ZnFe2O3, Ru(MeCN)CO2C3Py-P/N,Zn-Fe2O3/Cr2O3, and Ru(MeCN)CO2C3Py-P/TiO2/N,ZnFe2O3/Cr2O3 photoelectrodes were compared.

Figure 7. (a, b) J-V characteristics and (c, d) photocurrent transients at +0.1 V vs. RHE in CO2 saturated 0.1 M KHCO3 electrolyte (pH 6.6) under 1 sun (100 mW cm-2, AM 1.5) irradiation with Ru(MeCN)CO2C3Py-P/TiO2/N,Zn-Fe2O3/Cr2O3 (red line), Ru(MeCN)CO2C3Py-P/N,ZnFe2O3

(black

line),

Ru(MeCN)CO2C3Py-P/TiO2/N,Zn-Fe2O3

(blue

line),

Ru(MeCN)CO2C3Py-P/N,Zn-Fe2O3/Cr2O3 (green line), and TiO2/N,Zn-Fe2O3/Cr2O3 (orange line). Figure

7a

(black

line)

shows

a

current

density-voltage

(J-V)

curve

for

the

Ru(MeCN)CO2C3Py-P/N,Zn-Fe2O3 electrode without the junction. Although a cathodic


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photocurrent was generated at more negative voltages than +1.1 V vs. RHE, the photocurrent was much smaller and a large dark current was also observed at more negative voltages than those of Ru(MeCN)CO2C3Py-P/TiO2/N,Zn-Fe2O3/Cr2O3 (Figure 7a, red line). Figure 7c (black line) shows the time course of the photocurrent measured at a constant potential of +0.1 V vs. RHE, where a small photocurrent of 5 µA cm-2 decayed immediately after irradiation, and then the dark current increased. After photoelectrolysis for 1 h, HCOOH (0.01 µmol cm-2) and H2 (0.03 µmol cm-2) were determined to have been generated with Faradaic efficiencies of 6% and 12%, respectively (Table 3). After a stability test for 1 h, the orange color became pale, as shown by the UV/vis absorption spectrum in Figure S7a. The dissolution of Fe ions was also confirmed by ICP-OES analysis of the electrolyte (Figure 8, black line). The dissolution of Fe could be caused by the generation of surface Fe2+ species as a result of the self-reduction of N,Zn-Fe2O3.45 Therefore, most of the electrons generated are consumed for the electrochemical dissolution of N,Zn-Fe2O3, which results in deterioration of the catalytic reaction. Table 3 Reaction products at the Ru(MeCN)CO2C3Py-P loaded photocathode after photoelectrolysis for 1 ha Products/µmol cm-2 [Faradaic Eff./%] Photocathodes HCOOH

CO

H2

Ru(MeCN)CO2C3Py-P/TiO2/N,Zn-Fe2O3/Cr2O3

1.80 [63]

0.87 [30]

0.191 [7]

Ru(MeCN)CO2C3Py-P/N,Zn-Fe2O3

0.01 [6]

n.d.

Ru(MeCN)CO2C3Py-P/TiO2/N,Zn-Fe2O3

0.04 [80]

n.d.

Ru(MeCN)CO2C3Py-P/N,Zn-Fe2O3/Cr2O3

0.02 [9]

n.d.

TiO2/N,Zn-Fe2O3/Cr2O3

n.d.

b

n.d.

b b

0.027 [12] n.d.

c

b

0.019 [8]

b

0.125 [68]

a

Bias voltage applied at +0.1 V vs. RHE in CO2 saturated 0.1 M KHCO3 electrolyte (pH 6.6) under 1 sun (100 mW cm-2, AM 1.5) irradiation. b < 0.01 µmol cm-2. c < 0.001 µmol cm-2.


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ACS Catalysis

Figure 8. ICP-OES spectra for Fe in CO2 saturated 0.1 M KHCO3 aqueous solution, before (blue line) and after photoelectrolysis for 1 h with the Ru(MeCN)CO2C3Py-P/N,Zn-Fe2O3 electrode (black line), and after the 1 h PEC stability test with the Ru(MeCN)CO2C3Py-P/TiO2/N,ZnFe2O3 electrode (green line). The PEC stability tests were performed under 1 sun (AM1.5, 100 mW cm-2) irradiation at +0.1 V vs. RHE. The J-V curve for Ru(MeCN)CO2C3Py-P/N,Zn-Fe2O3/Cr2O3 showed enhanced cathodic photocurrents at voltages more negative than +1.4 V vs. RHE by insertion of the Cr2O3 layer (Figure 7b, green line), which was more positive than that for Ru(MeCN)CO2C3Py-P/N,ZnFe2O3 (+1.1 V vs. RHE). As previously discussed,45 a positive shift of the onset potential of the cathodic photocurrent would be induced by carrier diffusion between p-type N,Zn-Fe2O3 and ptype Cr2O3 to match both Fermi levels and form a heterojunction, because the Fermi level of ptype Cr2O3 is more positive than that of p-type N,Zn-Fe2O3. Enhancement of the photocurrent is induced by the improvement of hole transfer that results from formation of the heterojunction. As an additional effect, the lattice parameters of the Cr2O3 layer may match those of N,ZnFe2O3; several previous reports have attributed the relatively poor PEC behavior of n-type Fe2O3 to the lattice mismatch between Fe2O3 and SnO2.61-63 The lattice mismatch may be eliminated by


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the incorporation of different metal oxides in an underlayer between Fe2O3 and SnO2.61 The Cr2O3 underlayer would thus promote the crystallization of Fe2O3, because Cr2O3 has a corundum-type structure similar to hematite. The photocurrent would thus be increased as a result of the effects of the Cr2O3 underlayer. However, the photocurrent at +0.1 V vs. RHE decayed within 20 min (Figure 7d, green line), and a negligible amount of reduction products was generated with low Faradaic efficiency (Table 3), which is consistent with the dissolution of N,Zn-Fe2O3 (Figure S7b). Ru(MeCN)CO2C3Py-P/TiO2/N,Zn-Fe2O3 exhibited a cathodic photocurrent at more negative voltages than +0.8 V vs. RHE (Figure 7a, blue line), the onset potential of which was more negative than that of Ru(MeCN)CO2C3Py-P/N,Zn-Fe2O3 (+1.1 V vs. RHE), due to carrier diffusion between TiO2 and N,Zn-Fe2O3 to match both Fermi levels to form the p-n junction. Therefore, the band edge of N,Zn-Fe2O3 would bend downward preferably for electron transfer to the catalyst;45 the photocurrent at +0.1 V vs. RHE was largely enhanced compared to Ru(MeCN)CO2C3Py-P/N,Zn-Fe2O3 (Figure 7c, blue line). After photoelectrolysis for 1 h, no dissolution of Fe ions was detected by ICP-OES analysis (Figure 8, green line) and UV/vis absorption measurement (Figure S7b). HCOOH (0.04 µmol cm-2) was generated as a chemical product with a Faradaic efficiency of ca. 80%, while no other products were detected (Table 3). However, the photocurrent gradually decayed during photoelectrolysis for 1 h. Therefore, both TiO2 and Cr2O3 are necessary for stable CO2 reduction with N,Zn-Fe2O3. Operating mechanism of the hybrid photoelectrode To assess the role of the Ru complex in CO2 reduction, the PEC CO2 reduction efficiency of the photocathode without a Ru complex, i.e., TiO2/N,Zn-Fe2O3/Cr2O3, was tested. As shown in Figure 7d, although the time course of the photocurrent measured at a constant potential of +0.1


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ACS Catalysis

V vs. RHE exhibited a photocurrent of 120 µA cm-2 just after irradiation, it decayed immediately within a few minutes. After photoelectrolysis for 1 h, only H2 was produced, while neither HCOOH nor CO was detected (Table 3). Therefore, the Ru complex, which was reduced by the acceptance of electrons from the TiO2 layer, should host the active site for CO2 reduction in the Ru complex loaded photocathodes. To investigate the contribution of a photoabsorbed Ru complex to the photocurrent, the PEC performance of Ru(MeCN)CO2C3Py-P/TCO and Ru(MeCN)CO2C3Py-P/TiO2/Cr2O3/TCO was tested (Figure S8). Ru(MeCN)CO2C3Py-P/TCO exhibited negligible photocurrent. Ru(MeCN)CO2C3Py-P/TiO2/Cr2O3/TCO exhibited only 3 µA cm-2 of photocurrent because only thin Cr2O3 layer can absorb visible light. The contribution of this photocurrent in that of Ru complex/TiO2/N,Zn-Fe2O3/Cr2O3

is

very

small

because

N,Zn-Fe2O3

fully

intercept

photoabsorption of Cr2O3 (Figure S2). Therefore, it was concluded that photoabsorption by the Ru complex did not contribute to the photocurrent. To confirm the origin of the cathodic photocurrent, the dependence of the incident photon-tocurrent efficiency (IPCE) of Ru(MeCN)CO2C3Py-P/TiO2/N,Zn-Fe2O3/Cr2O3 on the irradiation wavelength when the applied potential was 0.5, 0.3, 0.2, 0.1 V vs. RHE was measured (Figure 9). The photoabsorption below λ ≤ 560 nm was attributed to the bandgap of N,Zn-Fe2O3, which indicates that the enhanced cathodic photocurrent originated from the excitation of N,Zn-Fe2O3.


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Figure 9. IPCE spectra for Ru(MeCN)CO2C3Py-P/TiO2/N,Zn-Fe2O3/Cr2O3 with application of a bias at 0.5 V (red circles), 0.3 V (blue circles), 0.2 V (green circles), and 0.1 V (purple circles) vs. RHE with monochromatic light irradiation. The photoabsorption of N,Zn-Fe2O3 excites electrons to the CBM (-0.6 V vs. RHE).45 The flat-band potential of TiO2 was estimated to be -0.3 V vs. RHE from the M-S analysis in CO2saturated 0.1 M KHCO3 aqueous solution (Figure 10a). The CBM of TiO2 is considered to be close to the flat-band potential,64 because of the high carrier density (2.0 ×1020 cm-3); therefore, the excited electrons should be injected to the TiO2 layer and would relax to ~-0.3 V vs. RHE. The standard electrode potentials for CO and HCOOH production by CO2 reduction are -0.11 V and -0.20 V vs. RHE, respectively;65 therefore, the Ru complex should catalyze CO2 reduction at low overpotentials. The catalytic ability for CO2 reduction is thus dependent on the substrate that is directly connected to the catalyst;66-67 therefore, to investigate the operating potential for CO2 reduction on TiO2, electrochemical CO2 reduction tests for the Ru(MeCN)CO2C3Py-P/TiO2 electrode were conducted at various applied potentials from -0.1 V to -0.4 V vs. RHE without light irradiation. Figure 10b shows the amounts of products generated during electrolysis for 1 h. HCOOH and CO were generated by application of potentials more negative than -0.2 V and -0.3


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ACS Catalysis

V vs. RHE, respectively, which corresponds with the CBM of TiO2. Therefore, the CO2 reduction potential at Ru(MeCN)CO2C3Py-P on TiO2 is lower than the CBM of TiO2. During the PEC reaction, the photoelectrons injected into TiO2 from N,Zn-Fe2O3 can thus transfer to the Ru complex and reduce CO2, even after relaxation to the CBM of TiO2.

Figure 10. (a) M-S plot of a TiO2 electrode measured in CO2-saturated 0.1 M KHCO3 aqueous solution at 10 (green circles), 50 (blue circles) and 100 Hz (red circles). (b) Dependence of the amounts of products (blue diamonds: HCOOH, green triangles: CO, red squares: H2) generated on the applied potential for the Ru(MeCN)CO2C3Py-P/TiO2 electrode in CO2-saturated 0.1 M KHCO3 aqueous solution during electrolysis for 1 h without light irradiation. Electrical bias-free CO2 reduction coupling with H2O oxidation with a Z-scheme configuration The p-type Ru(MeCN)CO2C3Py-P/TiO2/N,Zn-Fe2O3/Cr2O3 photocathode has the potential to catalyze CO2 reduction coupled with water oxidation in combination with an n-type semiconductor photoanode. n-type SrTiO3-x was employed as the photoanode because it has a CBM of −0.4 V68 and unique surface properties that suppress re-oxidation of the formate to CO2.30 Therefore, it is reasonable to expect a Z-scheme (two-step photoexcitation) reaction with


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Page 30 of 41

this tandem configuration. In the present case, n-type SrTiO3-x absorbs mainly UV light due to bandgap excitation and partially visible light (Figure S2) due to the presence of reduced species (e.g., Ti3+) that are photocatalytically inert. However, SrTiO3-x is beneficial due to the relatively negative onset potential to facilitate electron transfer to the Fe2O3-based photocathode in the photoexcited state (Figure S9). A PEC tandem cell as an unassisted system was constructed by connecting p-type Ru(MeCN)CO2C3Py-P/TiO2/N,Zn-Fe2O3/Cr2O3 and n-type SrTiO3-x electrodes (Figure 11a). When simulated solar light (AM 1.5, 100 mW cm-2) was irradiated to the tandem electrodes, the direct light was absorbed by the forward SrTiO3-x electrode and transmitted light reached N,Zn-Fe2O3 at the backside. Photogenerated holes in SrTiO3-x oxidized water to form O2 and the photogenerated electrons in N,Zn-Fe2O3 reduced CO2 at Ru(MeCN)CO2C3Py-P. The system exhibited a stable photocurrent with an average value of 102 µA cm-2 during irradiation for 3 h without application of an external voltage (Figure 11b). Figure 11c shows the amounts of photoelectrons and products measured as a function of the irradiation time. The total amount of reduction products, i.e., HCOOH+CO+H2, and O2 were equal to one half and one quarter of the total number of electrons and holes calculated from the photocurrent, respectively. This indicates that the solar CO2 reduction reaction coupled with H2O oxidation was stoichiometric and all of the photogenerated electrons and holes were consumed for the reaction. The PEC reactions are given as: CO2 + H2O → HCOOH + 1/2O2

∆G° = +270 kJ mol-1

(1)

CO2 → CO + 1/2O2

∆G° = +257 kJ mol-1

(2)

H2O → H2 + 1/2O2

∆G° = +237 kJ mol-1

(3)

After irradiation for 1 h, HCOOH (1.55 µmol cm-2), CO (0.31 µmol cm-2) and H2 (0.12 µmol cm-2) were generated with Faradaic efficiencies of 79%, 16% and 6%, respectively. The total


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solar-to-chemical conversion efficiency was calculated to be 0.15%, which is comparable to that for a previously reported tandem system consisting of a Ru complex polymer/single-crystalline InP photocathode connected with a SrTiO3-x photoanode that demonstrated CO2 reduction with a solar-to-formate conversion efficiency of 0.14%.30 Taking into account the operation where the previous InP-based system was measured with both sides irradiated and which excluded photocatalytically inert absorption (λ > 400 nm) by SrTiO3-x, the present Zn,N-Fe2O3 system is more efficient. The efficiency of the present system was high, even with irradiation only from the SrTiO3-x side. This is considered to be due to the facilitated charge transfer between the photocathode and photoanode by the band-alignment effect, heterojunction formation with Cr2O3, and the improved electron transfer rate from N,Zn-Fe2O3 to Ru(MeCN)CO2C3Py-P by the heterojunction with TiO2.


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Figure 11. (a) Schematic for the PEC reduction of CO2 with a two-electrode configuration comprised of Ru(MeCN)CO2C3Py-P/TiO2/N,Zn-Fe2O3/Cr2O3 and SrTiO3-x. (b) Photocurrent transients with the two-electrode configuration in CO2 saturated 0.1 M KHCO3 electrolyte (pH 6.6) under 1 sun (100 mW cm-2, AM 1.5) irradiation with no electrical bias. (c) Time course for the production of HCOOH (red circles), CO (blue circles), H2 (green circles), O2 (purple squares), and the half and quarter-amounts of photoelectrons (solid and dotted lines, respectively) produced during photoelectrolysis in the two-electrode configuration cell. The total amounts of reduction products, i.e., HCOOH + CO + H2, are also shown (black diamonds).


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CONCLUSION Solar light induced PEC reduction of CO2 to HCOOH and CO was successfully achieved in aqueous media by utilizing the combination of an abundant p-type semiconductor, N,Zn-codoped Fe2O3 (hematite), with a multi-heterojunction structure by a TiO2 overlayer and a Cr2O3 underlayer, and Ru complexes as a reduction catalyst. Although monomeric Ru complexes loaded on the photocathode were deactivated within 1 h of photoelectrolysis, the stability was improved by introducing an electrical network of polypyrrole chains. A polymeric Ru complex, Ru(MeCN)CO2C3Py-P, loaded on the photocathode realized a stable photocurrent of 150 µA cm-2 that was used completely for the production of HCOOH, CO and a small amount of H2 under 1 sun irradiation at +0.1 V vs. RHE. For the CO2 reduction reaction, the TiO2 overlayer resolved the issue of Fe2O3 dissolution, and both the TiO2 overlayer and Cr2O3 underlayer significantly enhanced the cathodic photocurrent for CO2 reduction by the facilitation of bandbending in N,Zn-Fe2O3. The overpotential for CO2 reduction by the Ru complex is quite low; therefore, even the electrons relaxed into the CBM of TiO2 can reduce CO2. The combination of an n-type SrTiO3-x photoanode with the Ru(MeCN)CO2C3Py-P/TiO2/N,Zn-Fe2O3/Cr2O3 photocathode realized CO2 reduction and water oxidation without application of an external electrical bias. The reaction proceeded stoichiometrically and was stable for 3 h with a solar-tochemical energy conversion efficiency of 0.15%. The efficiency is comparable to a previously reported tandem system using a Ru complex polymer/single-crystalline InP photocathode.30 This is the first example of stoichiometric CO2 reduction using a Fe2O3-based photocathode conducted in an aqueous solution without waste of the photocurrent. ASSOCIATED CONTENT


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Supporting Information. XRD patterns, UV/visible and FT-IR absorption spectra, and current– potential characteristics. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT The authors thank K. Oh-ishi, S. Kosaka and M. Yamamoto for assistance with the experiments. This work was supported by Advanced Catalytic Transformation Program for Carbon Utilization (ACT-C) of the Japan Science and Technology Agency (JST). REFERENCES (1) Lehn, J.-M.; Ziessel, R., Proc. Natl. Acad. Sci. U.S.A. 1982, 79, 701-704. (2) Yamazaki, Y.; Takeda, H.; Ishitani, O., J. Photochem. Photobiol., C 2015, 25, 106-137. (3) Wang, W.-H.; Himeda, Y.; Muckerman, J. T.; Manbeck, G. F.; Fujita, E., Chem. Rev. 2015, 115, 12936-12973. (4) Chardon-Noblat, S.; Deronzier, A.; Ziessel, R.; Zsoldos, D., J. Electroanal. Chem. 1998, 444, 253-260. (5) Ishida, H.; Tanaka, K.; Tanaka, T., Organometallics 1987, 6, 181-186.


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