Z-Schematic and visible-light-driven CO₂ reduction using H₂O as an electron donor by a particulate mixture of a Ru-complex/(CuGa)_1−xZn_2xS₂ hybrid catalyst, BiVO₄ and an electron mediator

Abstract

Visible-light-driven Z-schematic CO₂ reduction using H₂O as an electron donor was achieved using a simple mixture of a metal-sulfide/molecular hybrid photocatalyst for CO₂ reduction, a water oxidation photocatalyst and a redox-shuttle electron mediator. This is the first demonstration of a highly selective particulate CO₂ reduction system accompanying O₂ generation utilizing a semiconductor/molecular hybrid photocatalyst.

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Photocatalytic CO₂ reduction into useful energy-rich chemicals using water as an electron donor with a particulate system has attracted attention for sustainable and scalable artificial photosynthesis to mitigate global warming and generate useful fuels.^1–5 Particulate systems are recognized as cost-effective and could be the ultimate tool for CO₂ fixation and solar fuel generation.^6,7 A photocatalytic Z-scheme (or two-step photoexcitation) system that connects photocatalysts for CO₂ reduction with H₂O oxidation is considered to be a promising approach; however, few particulate Z-scheme systems for CO₂ reduction using water as an electron donor have been reported. Visible-light-driven conversion of CO₂ into CO over a combined system of CoO_x-loaded BiVO₄ and metal sulfides with a reduced graphene oxide electron mediator has yet to reach 1% CO selectivity. This is due to competitive H₂ generation and low CO₂ selectivity at metal sulfide surfaces.^8,9

A hybrid catalyst of a semiconductor^10–12 linked with a metal-complex catalyst^13–15 is promising for visible-light-driven selective CO₂ reduction with regard to high selectivity, depending on the selective coordination of CO₂ molecules to the metal centers of the complexes.^13,16–22 It is essential that photoexcited electrons in the conduction band (CB) of the semiconductor transfer to the metal-complex catalyst within the picosecond region, which leads to two-electron reduction of CO₂ at the complex.²³ We previously demonstrated solar formate generation from CO₂ and H₂O using a photocathode of a Ru-complex catalyst linked with Zn-doped InP or N,Zn-codoped Fe₂O₃ combined with TiO₂ or SrTiO_3−x photoanodes,^24–26 and confirmed the potential of the system with a solar-to-chemical conversion efficiency of 4.6% over a monolithic Ru-complex-polymer/Si–Ge/IrO_x electrode.²⁷ Ishitani and colleagues reported photoelectrochemical CO₂ reduction to CO via H₂O oxidation using a photocathode composed of a Ru(II)–Re(I) supramolecular metal complex immobilized on a NiO or CuGaO₂ semiconductor.^28,29 In contrast, a particulate system for Z-schematic CO₂ reduction using a metal complex combined with a H₂O oxidation reaction has not yet been demonstrated. Efficient electron transfer from the H₂O oxidation site to the CO₂ reduction site without degradation of the catalytic activity for both reactions should be implemented.

To realize Z-schematic CO₂ reduction with a particulate system, we focused our attention on metal sulfides as the CO₂ reduction semiconductor because they generally possess long photoexcited carrier lifetimes and narrow band gaps (BGs) originating from the S 3p state, which allows absorption of a substantial amount of visible light. Reisner et al. reported photocatalytic conversion of CO₂ into CO in water with >90% selectivity over a Ni–cyclam catalyst anchored with CdS.¹⁹ We reported CO₂ photoconversion into HCOOH over ZnS:Ni and (AgIn)_0.22Zn_1.56S₂ linked with a Ru–bipyridine catalyst [Ru(4,4′-diphosphonate-2,2′-bipyridine)(CO)₂Cl₂] ([Ru(dpbpy)]).³⁰ These Cd- and Zn-based sulfides are considered to be beneficial when combined with molecular catalysts because of their negative CB minimum (E_CBM) positions originating from Cd 5s5p and Zn 4s4p orbitals, respectively, which facilitate fast electron transfer from the CB in a photoexcited state to a metal-complex catalyst due to the greater energy difference (ΔG) between the E_CBM and CO₂ reduction potential of the metal-complex, which promotes the CO₂ reduction reaction. However, these systems conducted half reactions that required triethanolamine as a strong sacrificial electron donor. Here, we demonstrate the first particulate-based visible-light-driven Z-scheme system that harmonizes both reactions of CO₂ reduction and H₂O oxidation over a hybrid photocatalyst of a semiconductor/molecular-catalyst.

Fig. 1(a) shows an illustration of the Z-schematic system for photoconversion of CO₂ to CO and HCOO⁻ over [Ru(dpbpy)]-modified (CuGa)_1−xZn_2xS₂ as a CO₂ reduction hybrid photocatalyst, [Co(tpy)₂]^3+/2+ (tpy: 2,2′:6,2′′-terpyridine) as an electron mediator, and BiVO₄ as a water oxidation photocatalyst. Fig. 1(b) shows that the simple mixture in an aqueous solution produced CO and HCOO⁻ at almost linear rates together with O₂ generation under visible light irradiation after hours of pre-irradiation in a CO₂ flow reactor,^8,9i.e., the Z-scheme reaction. Although BiVO₄ can photogenerate O₂, E_CBM for BiVO₄ is located at an unfavorably deep position for the reduction reaction of CO₂ and protons.^8,31 Thus, the CO₂ reduction reaction could occur at the [Ru(dpbpy)]/(CuGa)_0.3Zn_1.4S₂ photocatalyst, which suggests a Z-scheme mechanism with H₂O as an electron donor, which is discussed later. Although H₂ evolution by proton reduction also proceeded, the CO₂ reduction selectivity to total reductive products (CO, HCOO⁻, H₂) for 9 h reached 64 mol%, significantly higher than the ca. 1% of previous non-molecular particulate Z-scheme systems.^8,9 The modification of [Ru(dpbpy)] led to the production of HCOO⁻ and the turnover number (TON) of HCOO⁻ evolution was calculated to be 17, assuming that all [Ru(dpbpy)] remained on (CuGa)_1−xZn_2xS₂ during the photocatalytic reaction.

Fig. 1 (a) Visible-light-driven Z-schematic system for CO₂ reduction consisting of particulate [Ru(dpbpy)] modified (CuGa)_1−xZn_2xS₂ hybrid photocatalysts, a BiVO₄ photocatalyst, and a [Co(tpy)₂]^3+/2+ redox shuttle electron mediator. (b) Z-schematic CO₂ reduction in an aqueous solution with a CO₂ flow system. Conditions: [Ru(dpbpy)]/(CuGa)_0.3Zn_1.4S₂ (0.4 g) and BiVO₄ (0.2 g); 0.02 mM [Co(tpy)₂]²⁺ containing 0.1 M NaHCO₃ aqueous solution (150 mL) and visible-light (λ > 420 nm).

The key to designing the visible-light-driven Z-schematic system is the hybrid photocatalyst for CO₂ reduction. Thus, a sufficient energy difference ΔG to facilitate fast electron transfer from the semiconductor to the Ru-complex catalyst plays a decisive role.^16,23 Electron transfer from the water oxidation photocatalyst to the CO₂ reduction photocatalyst is also indispensable. First, the optimal combination of Ru-complex and semiconductor was investigated. A solid solution of CuGaS₂ and ZnS ((CuGa)_1−xZn_2xS₂)³² was selected as the semiconductor for the hybrid photocatalyst because their BG and band position, particularly E_CBM, are dominated by Zn 4s4p + Ga 4s4p and are tunable by changing x. This could be beneficial to control the electron transfer rate from the CB to the molecular catalysts. XRD measurements of (CuGa)_1−xZn_2xS₂ synthesized by a solid-state reaction³² revealed that a single chalcopyrite phase (x ≤ 0.2) or zincblende structure (x ≥ 0.5) was obtained (Fig. S1 and Table S1, ESI†). The BG of CuGaS₂ and (CuGa)_1−xZn_2xS₂ (x < 1.0) was within the range of 2.24–2.54 eV, which enables absorption of visible light (Fig. S2 and Table S1, ESI†). Hybrid catalysts of 0.03–0.08 wt% [Ru(dpbpy)] with the phosphonate ligand (CO₂ reduction potential of ca. −1.0 V vs. NHE)³⁰ linked with (CuGa)_1−xZn_2xS₂ were prepared using an adsorption method³⁰ (Table S2, ESI†).

Typical Z-schematic CO₂ reduction was performed by using [Ru(dpbpy)]/(CuGa)_1−xZn_2xS₂ and BiVO₄ in a test tube filled with [Co(tpy)₂]²⁺ containing aqueous NaHCO₃ solution saturated with CO₂ under visible light irradiation for 16 h. This method is useful to measure the tendency of the sample-dependent product for multiple runs.³³Fig. 2(a) shows that the CO₂ reduction activity was significantly dependent on the composition of (CuGa)_1−xZn_2xS₂. For bare (CuGa)_1−xZn_2xS₂, the amount of CO produced increased with x within the range of 0.0 ≤ x ≤ 0.5, which could be explained by the more negative E_CBM and narrower BG (Fig. 2(b) and Fig. S2, ESI†), which led to a more efficient electron transfer toward a higher CO₂ reduction rate. Further increase in x (x ≥ 0.7) decreased CO evolution due to a reduced amount of absorbed photons with a wider BG (≥2.36 eV). H₂ evolution induced by the competitive proton reduction increased with x within the range of 0.5 ≤ x ≤ 0.7. Replacement of the bare (CuGa)_1−xZn_2xS₂ with the [Ru(dpbpy)]/(CuGa)_1−xZn_2xS₂ hybrid photocatalyst not only enhanced the CO formation rate but also induced formate production; [Ru(dpbpy)] is known as a catalyst for the production of formate and CO.^30,34 [Ru(dpbpy)]/(CuGa)_1−xZn_2xS₂ at x = 0.7 showed the highest CO₂ reduction activity for CO and HCOO⁻ production, with TONs (based on product generated by the [Ru(dpbpy)] catalyst) of 214 and 70 after 16 h of reaction, respectively. In the Z-schematic water splitting system composed of metallic Ru-loaded (CuGa)_1−xZn_2xS₂ for H₂ generation, CoO_x-loaded BiVO₄ for water oxidation and [Co(tpy)₂]^2+/3+ for electron mediation, the highest water splitting rate was reported at x = 0.2.³² In contrast, [Ru(dpbpy)]/(CuGa)_0.8Zn_0.4S₂ (x = 0.2) showed a negligibly small CO₂ reduction activity. These results suggest that more negative E_CBM (with greater x) improves the electron transfer rate for CO₂ reduction at [Ru(dpbpy)].^16,17,23,30 The CO₂ reduction activity of the hybrid catalyst is in a trade-off relationship between the negative E_CBM to facilitate electron transfer to the complex catalyst and the BG to determine the number of photons absorbed; therefore, overall matching in the electron transfer process was successful in the present Z-schematic ([Ru(dpbpy)]/(CuGa)_1−xZn_2xS₂)–([Co(tpy)₂]^3+/2+)–(BiVO₄) system.

Fig. 2 (a) Z-Schematic CO₂ reduction using the (CuGa)_1−xZn_2xS₂ or ([Ru(dpbpy)]/(CuGa)_1−xZn_2xS₂)–([Co(tpy)₂]^3+/2+)–(BiVO₄) systems under visible-light irradiation using the test tube method. Conditions: 8 mg of each photocatalyst; 0.02 mM [Co(tpy)₂]²⁺ containing 0.1 M NaHCO₃ aqueous solution (4 mL); visible-light (390 < λ ≤ 750 nm) for 16 h. (b) Estimated band structures of (CuGa)_1−xZn_2xS₂.

Experimental data used to determine the effective components for the Z-schematic CO₂ reduction are summarized in Table 1. A mixture of BiVO₄ and [Co(tpy)₂]²⁺ produced neither H₂, nor CO, nor HCOO⁻ due to the unfavorably deep E_CBM position (entry 1). The addition of (CuGa)_0.3Zn_1.4S₂ produced H₂ and CO (entry 2), and [Ru(dpbpy)] modification of (CuGa)_0.3Zn_1.4S₂ induced HCOO⁻ formation and further improved CO production by the effect of [Ru(dpbpy)] (entry 3, Fig. S3, ESI†). The absence of [Co(tpy)₂]²⁺ largely decreased CO and HCOO⁻ generation (entry 4), and removal of BiVO₄ resulted in the termination of both CO and HCOO⁻ formation (entry 5, Fig. S4, ESI†). These results indicate that BiVO₄ is necessary for high CO₂ reduction activity, and the presence of [Co(tpy)₂]²⁺ is essential for a high CO and HCOO⁻ formation rate. The absence of NaHCO₃ decreased the formation of both CO and HCOO⁻ (entry 6), and the formation of CO and HCOO⁻ was negligible in the absence of [Co(tpy)₂]²⁺ (entry 7). Among the aqueous electrolyte and Co-complexes, NaHCO₃ and [Co(tpy)₂]²⁺ were evaluated to be the best for the CO₂ reduction selectivity and production rate, respectively (Tables S3 and S4, ESI†). It was also confirmed that no reaction occurred without irradiation (entry 8), and that the formation of CO and HCOO⁻ was negligibly small when Ar was bubbled in the solution (entry 9).

Table 1 Z-Schematic CO₂ reduction activity of the ([Ru(dpbpy)]/(CuGa)_0.3Zn_1.4S₂)–([Co(tpy)₂]^3+/2+)–(BiVO₄) system under visible-light irradiation for 16 h^a

Entry	CO2 reduction photocatalyst		Mediator	O2 evolution photocatalyst	Salt	Gas	hν	Amount of products (μmol)
	(CuGa)0.3Zn1.4S2	[Ru(dpbpy)]	[Co(tpy)2]^2+	BiVO4	NaHCO3			H2	CO	HCOO^−
a Conditions: 8 mg of each photocatalyst; 0.02 mM [Co(tpy)2]^2+ containing 0.1 M NaHCO3 aqueous solution (4 mL); visible-light (390 < λ ≤ 750 nm) for 16 h; Pyrex test tube.
1			✓	✓	✓	CO2	✓	0.00	0.00	0.00
2	✓		✓	✓	✓	CO2	✓	1.82	0.69	0.02
3	✓	✓	✓	✓	✓	CO2	✓	1.98	1.64	0.37
4	✓	✓		✓	✓	CO2	✓	1.53	0.10	0.10
5	✓	✓	✓		✓	CO2	✓	1.06	0.79	0.28
6	✓	✓	✓	✓		CO2	✓	3.59	0.56	0.18
7	✓	✓		✓		CO2	✓	2.42	0.01	0.02
8	✓	✓	✓	✓	✓	CO2		0.00	0.00	0.00
9	✓	✓	✓	✓	✓	Ar	✓	0.96	0.01	0.01

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[Co(tpy)₂]²⁺ was reported to act as a [Co(tpy)₂]^3+/2+ redox shuttle electron mediator connecting two semiconductors for H₂ and O₂ generation in Z-scheme water splitting.³² As inferred from the mechanism of the IO₃⁻/I⁻ shuttle redox mediator system,^35,36 it is speculated that [Co(tpy)₂]³⁺ may favorably adsorb onto BiVO₄, while [Co(tpy)₂]²⁺ adsorbs onto (CuGa)_0.3Zn_1.4S₂, mediating electrons between them as a redox shuttle. [Co(tpy)₂]²⁺ was reported to be an electrocatalyst for the reduction of CO₂ to CO in dimethylformamide/H₂O (90/10 v/v) when one terpyridine ligand was eliminated.^37,38 However, the intrinsic potential for the reaction is too negative (−2.0 V vs. Fc/Fc⁺) compared to the E_CBM of (CuGa)_0.3Zn_1.4S₂; therefore, [Co(tpy)₂]²⁺ is not a catalyst in the present case. [Co(tpy)₂]²⁺ did not catalyze CO₂ reduction in an aqueous solution when electrically biased in the E_CBM region under visible light irradiation.

Isotope tracer analyses using ¹³CO₂ confirmed that the carbon source of evolved CO and HCOO⁻ was dissolved CO₂ (Fig. S5 and S6, ESI†). O₂ was also confirmed to originate from water using H₂¹⁸O (Fig. S7, ESI†), in which the total amount of ¹⁸O₂ and ¹⁶O¹⁸O was more than 85% of the total dioxygen detected. These results explained that CO₂ was reduced to CO and HCOO⁻ using electrons extracted from H₂O molecules. The TON for O₂ evolution with the Co-complex was calculated to be 9 (9 h), which suggests that the Co-complex acted as an electron mediator. A slight deviation from stoichiometric O₂ evolution (4-electron reaction) compared with half of the total of CO + HCOO⁻ + H₂ (2-electron reactions) in Fig. 1(b) was similarly observed in all-inorganic Z-scheme systems for CO₂ reduction.^8,9 Self-photooxidation of (CuGa)_1−xZn_2xS₂ could partially supply electrons for the CO₂ reduction reaction. Further investigations will clarify the overall electron/hole stoichiometry. X-ray photoelectron spectroscopy (XPS) measurements before and after the Z-scheme reaction (in Fig. 1(b)) revealed no change in the chemical state of Ru and sulfur ions (Fig. S8, ESI†). Half of the amount of [Ru(dpbpy)] was eliminated from the (CuGa)_1−xZn_2xS₂ surface according to the change in the Ru/S and Ru/Zn ratios (Table S5, ESI†).

In conclusion, a visible-light-driven Z-schematic CO₂ reduction to CO and HCOO⁻ in an aqueous particulate suspension system was achieved using a simple mixture of [Ru(dpbpy)]/(CuGa)_1−xZn_2xS₂ hybrid, [Co(tpy)₂]²⁺ and BiVO₄. Adjustment of band alignment is essential to the Z-schematic CO₂ reduction reaction accompanying O₂ generation. The very high CO₂ reduction selectivity beyond 60% against competing H₂ generation strongly suggests that the particulate Z-schematic system is feasible to construct selective and efficient photocatalysts for CO₂ fixation and solar fuel generation.

This work was partially supported by the JST ACT-C Grant Number JPMJCR12ZA, Japan (T. M. S. and T. M.). The authors thank Ms Ayako Oshima, Mr Kosuke Kitazumi, Ms Naoko Takahashi, Mr Satoru Kosaka, Mr Ikoma Narita and Ms Saori Narita for experimental assistance. The authors also thank Dr Shunsuke Sato, Dr Keita Sekizawa, and Dr Hiromitsu Tanaka for fruitful discussions.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, XRD patterns, UV-vis diffuse reflectance spectra, Z-schematic CO₂ reduction using a hybrid photocatalyst, BiVO₄ and a Co-complex, GC-MS chromatograms, IC-TOFMS chromatograms, and XPS spectra. See DOI: 10.1039/c8cc05505j

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